GALVANIZED STEEL SHEET AND MEMBER, AND METHOD FOR MANUFACTURING SAME

- JFE STEEL CORPORATION

Provided is a galvanized steel sheet having a TS of 980 MPa or more, high YS, excellent ductility, strain hardenability, and hole expansion formability. A base steel sheet has a defined chemical composition and a steel microstructure as follows: area ratio of ferrite: 65.0% or less (including 0%), area ratio of bainitic ferrite: 5.0% or more and 40.0% or less, area ratio of tempered martensite: 0.5% or more and 80.0% or less, area ratio of retained austenite: 3.0% or more, area ratio of fresh martensite: 20.0% or less (including 0%), SBF+STM+2×SMA: 65.0% or more, SMA1/SMA: 0.80 or less, and SMA2/SMA: 0.20 or more.

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Description
TECHNICAL FIELD

The present disclosure relates to a galvanized steel sheet, a member made from the galvanized steel sheet, and a method of producing same.

BACKGROUND

In recent years, enhancement of fuel efficiency of automobiles has become an important issue from the viewpoint of global environment protection. Therefore, there is a growing trend to reduce weight of automotive bodies by increasing strength and reducing thickness of steel sheets used as a material of automotive components.

Further, societal demand for improved crashworthiness of automobiles is becoming even higher. Therefore, developing steel sheets having high strength in addition to excellent anti-crash properties in the event of a collision while a vehicle is in motion (hereinafter also simply referred to as “anti-crash properties”) is desirable. In particular, in view of automotive body rust resistance, steel sheets used as material of automotive components are often galvanized. Therefore, developing galvanized steel sheets that have high strength as well as excellent anti-crash properties is desirable.

As an example of a steel sheet used as a material of automotive components, JP 3887235 B (PTL 1) describes:

    • “a high strength steel sheet having excellent stretch flangeability and collision resistance comprising: a chemical composition containing, in mass %, C: 0.04% to 0.22%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.01% to 0.1%, and N: 0.001% to 0.005%, with the balance being Fe and inevitable impurity; and a microstructure comprising a ferrite phase as a main phase and martensite as a secondary phase, a maximum grain size of the martensite phase being 2 μm or less, and an area ratio of the martensite phase being 5% or more.”

JP 5953693 B (PTL 2) describes:

    • “a high strength hot-dip galvanized steel sheet having excellent coating adhesion and formability, comprising: a cold-rolled steel sheet that has had a surface thickness of 0.1 μm or more ground off and been pre-coated with Ni: 0.2 g/m2 or more and 2.0 g/m2 or less, and a hot-dip galvanizing layer on a surface thereof, comprising:
    • a chemical composition containing, in mass %,
    • C: 0.05% or more and 0.4% or less,
    • Si: 0.01% or more and 3.0% or less,
    • Mn: 0.1% or more and 3.0% or less,
    • P: 0.04% or less,
    • S: 0.05% or less,
    • N: 0.010% or less,
    • Al: 0.01% or more and 2.0% or less,
    • Si+Al: more than 0.5%,
    • with the balance being Fe and inevitable impurity,
    • having a microstructure comprising, by volume fraction, ferrite: 40% or more as a main phase, retained austenite: 8% or more, and two or more of three types of martensite [1], [2], [3], as specified below, including martensite [3], bainite: 1% or more, and pearlite: 0% to 10%, where volume fractions of the martensite [1], [2], [3] are
    • martensite [1]: 0% or more and 50% or less,
    • martensite [2]: 0% or more and less than 20%,
    • martensite [3]: 1% or more and 30% or less,
    • the hot-dip galvanizing layer containing less than 7% Fe with the balance consisting of Zn, Al, and inevitable impurity, wherein
    • tensile strength TS (MPa), total elongation EL (%), and hole expansion rate λ (%) satisfy TS×EL=18,000 MPa·% or more, and TS×λ=35,000 MPa·% or more, and tensile strength is 980 MPa or more, wherein
    • martensite [1]: C concentration (CM1) is less than 0.8% and hardness Hv1 satisfies


Hv1/(−982.1×CM12+1676×CM1+189)≤0.60

    • martensite [2]: C concentration (CM2) is 0.8% or more and hardness Hv2 satisfies


Hv2/(−982.1×CM22+1676×CM2+189)≤0.60

    • martensite [3]: C concentration (CM3) is 0.8% or more and hardness Hv3 satisfies


Hv3/(−982.1×CM32+1676×CM3+189)≥0.80.”

JP 6052472 B (PTL 3) describes:

    • “a high strength hot-dip galvanized steel sheet comprising: a chemical composition containing, in mass %, C: 0.15% or more and 0.25% or less, Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less, P: 0.100% or less, S: 0.02% or less, Al: 0.01% or more and 2.5% or less, with the balance being Fe and inevitable impurity,
    • having a microstructure comprising, in area ratio, a tempered martensitic phase: 30% or more and 73% or less, a ferrite phase: 25% or more and 68% or less, a retained austenite phase: 2% or more and 20% or less, and other phases: 10% or less (including 0%), where the other phases include a martensite phase: 3% or less (including 0%) and a bainitic ferrite phase: less than 5% (including 0%), an average grain size of the tempered martensitic phase is 8 μm or less, and an amount of C in the retained austenite phase is less than 0.7 mass %.”

CITATION LIST Patent Literature

    • PTL 1: JP 3887235 B
    • PTL 2: JP 5953693 B
    • PTL 3: JP 6052472 B

SUMMARY Technical Problem

The current situation is that impact energy absorbing members of automobiles, such as front side members and rear side members, are limited to steel sheets having a tensile strength (hereinafter also referred to as TS) of 590 MPa grade.

In order to increase energy absorbed upon impact (hereinafter also referred to as impact absorption energy), increasing yield stress (hereinafter also referred to as YS) is effective. However, increasing the TS and YS of a steel sheet generally reduces formability, and particularly ductility, strain hardenability, and hole expansion formability. These properties correlate with a member's resistance to cracking in bending crush tests and axial crush tests that simulate crash tests. Therefore, when steel sheets having higher TS and YS are applied to the impact energy absorbing members of automobiles, the steel sheets are not only more difficult to form, but also crack in tests that simulate crash tests. In other words, actual impact absorption energy is not as high as might be assumed from the YS value. Therefore, the impact energy absorbing members are currently limited to steel sheets having a TS of 590 MPa grade. Strain hardenability and hole expansion formability are correlated with stretch formability and stretch flangeability, respectively.

In fact, the steel sheets described in PTL 1-3 have TS: 980 MPa or more, and cannot be said to have high YS, excellent ductility, strain hardenability, and hole expansion formability.

The present disclosure was developed in view of the current situation mentioned above, and it would be helpful to provide a galvanized steel sheet having TS: 980 MPa or more, high YS, excellent ductility, strain hardenability, and hole expansion formability, and an advantageous method of producing same.

It would also be helpful to provide a member made from the galvanized steel sheet and a method of producing same.

Here, high YS, excellent ductility, strain hardenability, and hole expansion formability are defined as follows,

    • YS measured by a tensile test in accordance with Japanese Industrial Standard JIS Z 2241 satisfies the following formula, depending on TS measured by the tensile test:
    • 550 MPa≤YS when 980 MPa≤TS<1,180 MPa
    • 700 MPa≤YS when 1,180 MPa≤TS<1,310 MPa
    • 800 MPa≤YS when 1,310 MPa≤TS
    • Total elongation (El) measured by a tensile test in accordance with JIS Z 2241 satisfies the following formula, depending on TS measured by the tensile test:
    • 13.0%≤El when 980 MPa≤TS<1,180 MPa
    • 12.0%≤El when 1,180 MPa≤TS<1,310 MPa
    • 10.000≤El when 1,310 MPa≤TS
    • n value/YR measured by a tensile test in accordance with JIS Z 2241 satisfies the following formula:
    • n value/YR≥0.070
    • Further, maximum hole expansion ratio (λ), as measured by a hole expanding test in accordance with JIS Z 2256, is 20% or more.

Solution to Problem

The inventors conducted intensive studies to solve the technical problem outlined above.

As a result, by appropriately adjusting a chemical composition of a base steel sheet of a galvanized steel sheet and microstructure of the base steel sheet of the galvanized steel sheet such that:

    • area ratio of ferrite: 65.0% or less (including 0%)
    • area ratio of bainitic ferrite: 5.0% or more and 40.0% or less,
    • area ratio of tempered martensite: 0.5% or more and 80.0% or less,
    • area ratio of retained austenite: 3.0% or more,
    • area ratio of fresh martensite: 20.0% or less (including 0%)
    • SBF+STM+2×SMA: 65.0% or more,
    • SMA1/SMA: 0.80 or less, and
    • SMA2/SMA: 0.20 or more,
    • the inventors found that a galvanized steel sheet having a TS of 980 MPa or more, high YS, excellent ductility, strain hardenability, and hole expansion formability may be obtained.

The present disclosure is based on these discoveries and further studies.

Primary features of the present disclosure are as follows.

1. A galvanized steel sheet comprising a base steel sheet and a galvanized layer on a surface of the base steel sheet, wherein the base steel sheet comprises:

    • a chemical composition containing (consisting of), in mass %,
    • C: 0.050% or more and 0.400% or less,
    • Si: 0.20% or more and 3.00% or less,
    • Mn: 1.00% or more and less than 3.50%,
    • P: 0.001% or more and 0.100% or less,
    • S: 0.0200% or less,
    • Al: 0.010% or more and 2.000% or less,
    • N: 0.0100% or less,
    • where carbon equivalent Ceq is 0.540% or more, and the balance is Fe and inevitable impurity,
    • further, the base steel sheet has a microstructure wherein
    • area ratio of ferrite: 65.0% or less (including 0%),
    • area ratio of bainitic ferrite: 5.0% or more and 40.0% or less,
    • area ratio of tempered martensite: 0.5% or more and 80.0% or less,
    • area ratio of retained austenite: 3.0% or more,
    • area ratio of fresh martensite: 20.0% or less (including 0%),
    • SBF+STM+2×SMA: 65.0% or more,
    • SMA1/SMA: 0.80 or less, and
    • SMA2/SMA: 0.20 or more, and
    • tensile strength is 980 MPa or more,
    • wherein
    • SBF is area ratio of the bainitic ferrite,
    • STM is area ratio of the tempered martensite,
    • SMA is area ratio of a hard secondary phase consisting of the retained austenite and fresh martensite,
    • SMA1 is a total area ratio of island regions among island regions constituting the hard secondary phase that have an equivalent circular diameter of 2.0 μm or more where 20% or less of the circumference is in contact with tempered martensite, and
    • SMA2 is a total area ratio of island regions among island regions constituting the hard secondary phase where 1% or more of the circumference is in contact with bainitic ferrite.

2. The galvanized steel sheet of aspect 1, wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of

    • Ti: 0.200% or less,
    • Nb: 0.200% or less,
    • V: 0.100% or less,
    • B: 0.0100% or less,
    • Cu: 1.000% or less,
    • Cr: 1.000% or less,
    • Ni: 1.000% or less,
    • Mo: 0.500% or less,
    • Sb: 0.200% or less,
    • Sn: 0.200% or less,
    • Ta: 0.100% or less,
    • W: 0.500% or less,
    • Mg: 0.0200% or less,
    • Zn: 0.0200% or less,
    • Co: 0.0200% or less,
    • Zr: 0.0200% or less,
    • Ca: 0.0200% or less,
    • Ce: 0.0200% or less,
    • Se: 0.0200% or less,
    • Te: 0.0200% or less,
    • Ge: 0.0200% or less,
    • As: 0.0200% or less,
    • Sr: 0.0200% or less,
    • Cs: 0.0200% or less,
    • Hf: 0.0200% or less,
    • Pb: 0.0200% or less,
    • Bi: 0.0200% or less, and
    • REM: 0.0200% or less.

3. The galvanized steel sheet of aspect 1 or 2, wherein SMA3/SMA is 0.05 or more in the microstructure of the base steel sheet,

    • wherein
    • SMA3 is a total area ratio of island regions among island regions constituting the hard secondary phase where 1% or more of the circumference is in contact with bainitic ferrite and more than 20% of the circumference is in contact with tempered martensite.

4. The galvanized steel sheet of any one of aspects 1 to 3, wherein an amount of diffusible hydrogen in the base steel sheet is 0.50 mass ppm or less.

5. The galvanized steel sheet of any one of aspects 1 to 4, further comprising a decarburized layer.

6. The galvanized steel sheet of any one of aspects 1 to 5, further comprising a metal coating or plating layer at least on one side between the base steel sheet and the galvanized layer.

7. The galvanized steel sheet of aspect 6, wherein the metal coating or plating layer is an Fe-based coating or plating layer.

8. The galvanized steel sheet of any one of aspects 1 to 7, wherein the galvanized layer is a hot-dip galvanized layer or a galvannealed layer.

9. A member made using the galvanized steel sheet of any one of aspects 1 to 8.

10. A method of producing a galvanized steel sheet, the method comprising:

    • a hot rolling process, in which a steel slab having the chemical composition of aspect 1 or 2 is hot rolled to make a hot-rolled steel sheet;
    • a cold rolling process, in which the hot-rolled steel sheet is cold rolled into a cold-rolled steel sheet;
    • an annealing process, in which the cold-rolled steel sheet is annealed at an annealing temperature of 760° C. or more and 900° C. or less for an annealing time of 20 s or more;
    • a first cooling process, in which the cold-rolled steel sheet is cooled to a first cooling stop temperature of 300° C. or more and 550° C. or less;
    • a holding process, in which the cold-rolled steel sheet is held at a temperature range of 300° C. or more and 550° C. or less for 3 s or more and 600 s or less;
    • a coating process, in which the cold-rolled steel sheet is subject to a galvanizing treatment to become the galvanized steel sheet;
    • a second cooling process, in which the galvanized steel sheet is cooled to a second cooling stop temperature of 100° C. or more and less than 300° C.; and
    • a reheating process, in which the galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature+50° C.) or more and 500° C. or less, and the galvanized steel sheet is held at a temperature range of (the second cooling stop temperature+50° C.) or more and 500° C. or less for 10 s or more and 2,000 s or less, wherein
    • a relationship between the first cooling stop temperature and a temperature of a galvanizing bath in the galvanizing treatment satisfies the following Formula (1),


−150° C.≤T0−T1≤50° C.  (1)

    • where T0 is the first cooling stop temperature (° C.) and T1 is the temperature of the galvanizing bath in the galvanizing treatment (° C.).

11. The method of producing a galvanized steel sheet of aspect 10, wherein a dew point of the annealing process is more than −30° C.

12. The method of producing a galvanized steel sheet of aspect 10 or 11, further comprising a metal coating or plating treatment process after the cold rolling process and before the annealing process, in which a metal coating or plating layer is formed on at least one surface of the cold-rolled steel sheet.

13. The method of producing a galvanized steel sheet of aspect 12, wherein the metal coating or plating layer is an Fe-based coating or plating layer.

14. The method of producing a galvanized steel sheet of any one of aspects 10 to 13, wherein the galvanizing treatment is a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.

15. A method of producing a member, wherein the galvanized steel sheet of any one of aspects 1 to 8 is subjected to at least one of a forming process and a joining process to make the member.

Advantageous Effect

According to the present disclosure, the galvanized steel sheet having a TS of 980 MPa or more, high YS, excellent ductility, strain hardenability, and hole expansion formability is obtainable. Further, the member made from the galvanized steel sheet of the present disclosure has high strength and excellent anti-crash properties, and is therefore extremely advantageous for use in impact energy absorbing members of automobiles.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1A illustrates an example of a microstructure image from a scanning electron microscope (SEM) used for microstructure identification, and FIG. 1B is a color-coded microstructure image of FIG. 1A made using Adobe Photoshop by Adobe Systems Inc.;

FIG. 2A illustrates an example of a microstructure image from a SEM used to identify island regions in a hard secondary phase, in particular including island regions identified as MA1, and FIG. 2B is a color-coded microstructure image of FIG. 2A made using Adobe Photoshop by Adobe Systems Inc.;

FIG. 3A illustrates an example of a microstructure image from a SEM used to identify island regions in the hard secondary phase, in particular including island regions identified as MA2, and FIG. 3B is a color-coded microstructure image of FIG. 3A made using Adobe Photoshop by Adobe Systems Inc.;

FIG. 4A illustrates an example of a microstructure image from a SEM used to identify island regions in the hard secondary phase, in particular including island regions identified as MA3, and FIG. 4B is a color-coded microstructure image of FIG. 4A made using Adobe Photoshop by Adobe Systems Inc.; and

FIG. 5A is a schematic diagram illustrating a method of evaluating resistance weld crack resistance at a welded portion, an upper part of FIG. 5B illustrates a top view of a sheet combination after the resistance spot welding of the evaluation, and a lower part of FIG. 5B illustrates a cross-section taken along the line A-A in the upper part.

 DETAILED DESCRIPTION

The following describes embodiments of the present disclosure.

[1] Galvanized Steel Sheet

First, a chemical composition of a base steel sheet of a galvanized steel sheet according to an embodiment of the present disclosure is described. Hereinafter, although the unit in all chemical compositions is “mass %”, this may be indicated simply as “%”, unless otherwise specified.

C: 0.050% or More and 0.400% or Less

C is an element effective for securing TS of 980 MPa or more and high YS by generating appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite and retained austenite. Here, when the C content is less than 0.050%, an area ratio of ferrite increases, making achieving a TS of 980 MPa or more difficult. This also leads to a decrease in YS. On the other hand, when the C content exceeds 0.400%, carbon concentration in the retained austenite increases excessively. Therefore, when a steel sheet is punched, hardness of fresh martensite formed from retained austenite increases significantly. As a result, crack propagation during hole expansion is accelerated in the steel sheet after the punching process (that is, leading to a reduction in hole expansion formability).

Therefore, the C content is 0.050% or more and 0.400% or less. The C content is preferably 0.100% or more. Further, the C content is preferably 0.300% or less.

Si: 0.20% or More and 3.00% or Less

Si suppresses carbide formation during annealing and promotes formation of retained austenite. In other words, Si is an element that affects an area ratio of retained austenite and carbon concentration in the retained austenite. Here, when Si content is less than 0.20%, an area ratio of retained austenite decreases and ductility is reduced. On the other hand, when the Si content exceeds 3.00%, the area ratio of ferrite increases excessively and achieving a TS of 980 MPa or more becomes difficult. This also leads to a decrease in YS. Further, the carbon concentration in the retained austenite increases excessively. Therefore, when a steel sheet is punched, hardness of fresh martensite formed from retained austenite increases significantly. As a result, crack propagation during hole expansion is accelerated in the steel sheet after the punching process (that is, leading to a reduction in hole expansion formability).

Therefore, the Si content is 0.20% or more and 3.00% or less. The Si content is preferably 0.40% or more. Further, the Si content is preferably 2.00% or less, since there is concern about a decrease in resistance weld crack resistance when the Si content exceeds 2.00%.

Mn: 1.00% or more and less than 3.50% Mn is an element that adjusts area ratios such as those of bainitic ferrite and tempered martensite. Here, when Mn content is less than 1.00%, the area ratio of ferrite increases excessively, making achieving a TS of 980 MPa or more difficult. This also leads to a decrease in YS. On the other hand, when the Mn content is 3.50% or more, the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite increases excessively. As a result, desired ductility is not achieved.

Therefore, the Mn content is 1.00% or more and less than 3.50%. The Mn content is preferably 1.80% or more. Further, the Mn content is preferably less than 3.20%.

P: 0.001% or More and 0.100% or Less

P is an element that acts as a solid solution strengthener and increases steel sheet strength. To achieve this effect, P content is 0.001% or more. On the other hand, when the P content exceeds 0.100%, P segregates to a prior austenite grain boundary and embrittles the grain boundary. Therefore, when steel sheets are punched, an amount of void formation increases, leading to a decrease in hole expansion formability.

Therefore, the P content is 0.001% or more and 0.100% or less. The P content is preferably 0.030% or less.

S: 0.0200% or Less

S is present in steel as sulfide. In particular, when S content exceeds 0.0200%, steel sheet ultimate deformability is reduced. Therefore, when steel sheets are punched, an amount of void formation increases, leading to a decrease in hole expansion formability.

The S content is therefore 0.0200% or less. The S content is preferably 0.0080% or less. Although a lower limit of S content is not particularly specified, the S content is preferably 0.0001% or more in view of production technology constraints.

Al: 0.010% or More and 2.000% or Less

Al suppresses carbide formation during annealing and promotes formation of retained austenite. In other words, Al is an element that affects the area ratio of retained austenite and the carbon concentration in the retained austenite. To obtain this effect, the Al content is 0.010% or more. On the other hand, when the Al content exceeds 2.000%, the area ratio of ferrite increases excessively, making achieving a TS of 980 MPa or more difficult. This also leads to a decrease in YS.

Therefore, the Al content is 0.010% or more and 2.000% or less. The Al content is preferably 0.015% or more. Further, the Al content is preferably 1.000% or less.

N: 0.0100% or Less

N is present in steel as nitride. In particular, when N content exceeds 0.0100%, steel sheet ultimate deformability is reduced. Therefore, when steel sheets are punched, an amount of void formation increases, leading to a decrease in hole expansion formability.

The N content is therefore 0.0100% or less. Further, the N content is preferably 0.0050% or less. Although a lower limit of N content is not particularly specified, the N content is preferably 0.0005% or more in view of production technology constraints.

Carbon Equivalent Ceq: 0.540% or More

The carbon equivalent Ceq affects TS. In particular, when the carbon equivalent Ceq is less than 0.540%, achieving a TS of 980 MPa or more becomes difficult. The carbon equivalent Ceq is therefore 0.540% or more.

Here, the carbon equivalent Ceq is defined by the following formula.


Carbon equivalent Ceq=[C %]+([Si %]/24)+([Mn %]/6)+([Ni %]/40)+([Cr %]/5)+([Mo %]/4)+([V %]/14)

The [element symbol %] in the above formula represents the content (mass %) of the element in the chemical composition of the base steel sheet. Elements not included in the chemical composition of the base steel sheet are calculated as 0.

Basic composition of the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure is described above. The base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure includes the basic composition above, with the balance being Fe (iron) and inevitable impurity. Here, the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure preferably has a chemical composition including the basic composition above, with the balance being Fe and inevitable impurity. In addition to the above basic composition, the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure may contain at least one of the components selected from the group listed below. The components listed below do not have a lower limit because the effect of the present disclosure is obtainable whenever content is equal to or less than the upper limit indicated below. When any of the following components are included below an appropriate lower limit described below, such a component is included as an inevitable impurity.

    • Ti: 0.200% or less,
    • Nb: 0.200% or less,
    • V: 0.100% or less,
    • B: 0.0100% or less,
    • Cu: 1.000% or less,
    • Cr: 1.000% or less,
    • Ni: 1.000% or less,
    • Mo: 0.500% or less,
    • Sb: 0.200% or less,
    • Sn: 0.200% or less,
    • Ta: 0.100% or less,
    • W: 0.500% or less,
    • Mg: 0.0200% or less,
    • Zn: 0.0200% or less,
    • Co: 0.0200% or less,
    • Zr: 0.0200% or less,
    • Ca: 0.0200% or less,
    • Ce: 0.0200% or less,
    • Se: 0.0200% or less,
    • Te: 0.0200% or less,
    • Ge: 0.0200% or less,
    • As: 0.0200% or less,
    • Sr: 0.0200% or less,
    • Cs: 0.0200% or less,
    • Hf: 0.0200% or less,
    • Pb: 0.0200% or less,
    • Bi: 0.0200% or less, and
    • REM: 0.0200% or less.

Ti: 0.200% or Less

Ti causes TS to increase due to formation of fine carbides, nitrides, and carbonitrides during hot rolling and annealing. To obtain this effect, Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, when the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Ti is included, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

Nb: 0.200% or Less

Nb, like Ti, causes TS to increase due to formation of fine carbides, nitrides, and carbonitrides during hot rolling and annealing. To obtain this effect, Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more. On the other hand, when the Nb content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Nb is included, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

V: 0.100% or Less

V, like Ti and Nb, causes TS to increase due to formation of fine carbides, nitrides, and carbonitrides during hot rolling and annealing. To obtain this effect, V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. On the other hand, when the V content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when V is included, the V content is preferably 0.100% or less. The V content is more preferably 0.060% or less.

B: 0.0100% or Less

B is an element that increases hardenability by segregating at an austenite grain boundary. Further, B is an element that suppresses ferrite formation and grain growth during cooling after annealing. To obtain this effect, B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, when the B content exceeds 0.0100%, cracking may occur inside the steel sheet during hot rolling, which may reduce the ultimate deformability of the steel sheet. Further, the reduction in the ultimate deformability of the steel sheet increases an amount of voids generated when the steel sheet is punched, leading to a reduction in hole expansion formability. Therefore, when B is included, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.

Cu: 1.000% or Less

Cu is an element that increases hardenability. In particular, Cu is an element that is effective for adjusting the area ratio of hard fresh martensite and the like to a more suitable range, and thereby adjusting TS to a more suitable range. To obtain this effect, Cu content is preferably 0.005% or more. The Cu content is more preferably 0.020% or more. On the other hand, when the Cu content exceeds 1.000%, the area ratio of fresh martensite increases excessively and TS becomes excessively high. Further, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during tensile testing, that is, hole expansion formability may decrease. Therefore, when Cu is included, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.

Cr: 1.000% or Less

Cr is an element that increases hardenability, and Cr is an element effective for formation of retained austenite and fresh martensite. To obtain these effects, the Cr content is preferably 0.0005% or more. In particular, from a viewpoint of achieving a more suitable range of TS, the Cr content is more preferably 0.010% or more. On the other hand, when the Cr content exceeds 1.000%, the area ratio of hard fresh martensite may increase excessively, leading to a decrease in hole expansion formability. Therefore, when Cr is included, the Cr content is preferably 1.000% or less. Further, the Cr content is more preferably 0.250% or less. The Cr content is even more preferably 0.100% or less.

Ni: 1.000% or Less

Ni is an element that increases hardenability. Further, Ni is an element that is effective for adjusting the area ratio of retained austenite and fresh martensite to a more suitable range, and thereby adjusting TS to a more suitable range. To obtain this effect, Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. On the other hand, when the Ni content exceeds 1.000%, the area ratio of fresh martensite may increase excessively, and may cause reduced ductility and dimensional accuracy during forming. Further, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Ni is included, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less.

Mo: 0.500% or Less

Mo is an element that increases hardenability. Further, Mo is an element effective for formation of hard fresh martensite and the like. To obtain this effect, Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more. On the other hand, when the Mo content exceeds 0.500%, the area ratio of fresh martensite may increase excessively, leading to a decrease in hole expansion formability. Therefore, when Mo is included, the Mo content is preferably 0.500% or less. The Mo content is more preferably 0.450% or less. The Mo content is even more preferably 0.400% or less.

Sb: 0.200% or Less

Sb is an element effective for suppressing diffusion of C in the vicinity of the steel sheet surface during annealing and for controlling the formation of a soft layer in the vicinity of the steel sheet surface. To obtain this effect, Sb content is preferably 0.002% or more. The Sb content is more preferably 0.005% or more. On the other hand, when the Sb content exceeds 0.200%, a soft layer is not formed in the vicinity of the steel sheet surface, which may lead to a decrease in hole expansion formability. Therefore, when Sb is included, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.020% or less.

Sn: 0.200% or Less

Sn, like Sb, is an element effective for suppressing the diffusion of C in the vicinity of the steel sheet surface during annealing and for controlling the formation of a soft layer in the vicinity of the steel sheet surface. To obtain this effect, Sn content is preferably 0.002% or more. The Sn content is more preferably 0.005% or more. On the other hand, when the Sn content exceeds 0.200%, a soft layer is not formed in the vicinity of the steel sheet surface, which may lead to a decrease in hole expansion formability. Therefore, when Sn is included, the Sn content is preferably 0.200% or less. The Sn content is more preferably 0.020% or less.

Ta: 0.100% or Less

Ta, like Ti, Nb, and V, causes TS to increase due to formation of fine carbides, nitrides, and carbonitrides during hot rolling and annealing. Further, Ta is partially solid-soluble in Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb, Ta) (C, N). This suppresses coarsening of precipitates and stabilizes strengthening by precipitation. This improves TS and YS. To obtain this effect, Ta content is preferably 0.001% or more. On the other hand, when the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Ta is included, the Ta content is preferably 0.100% or less.

W: 0.500% or Less

W is an element effective for increasing hardenability and adjusting TS to a more suitable range. To obtain this effect, W content is preferably 0.001% or more. The W content is more preferably 0.030% or more. On the other hand, when the W content exceeds 0.500%, the area ratio of hard fresh martensite may increase excessively, leading to a decrease in hole expansion formability. Therefore, when W is included, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less. The W content is even more preferably 0.400% or less.

Mg: 0.0200% or Less

Mg is an element effective for sphericalizing the shape of inclusions such as sulfides and oxides to improve steel sheet ultimate deformability and hole expansion formability. To obtain this effect, Mg content is preferably 0.0001% or more. On the other hand, when the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Mg is included, the Mg content is preferably 0.0200% or less.

Zn: 0.0200% or Less

Zn is an element effective for sphericalizing the shape of inclusions to improve steel sheet ultimate deformability and hole expansion formability. To obtain this effect, Zn content is preferably 0.0010% or more. On the other hand, when the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Zn is included, the Zn content is preferably 0.0200% or less.

Co: 0.0200% or Less

Co, like Zn, is an element effective for sphericalizing the shape of inclusions to improve steel sheet ultimate deformability and hole expansion formability. To obtain this effect, Co content is preferably 0.0010% or more. On the other hand, when the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Co is included, the Co content is preferably 0.0200% or less.

Zr: 0.0200% or Less

Zr, like Zn and Co, is an element effective for sphericalizing the shape of inclusions to improve steel sheet ultimate deformability and hole expansion formability. To obtain this effect, Zr content is preferably 0.0010% or more. On the other hand, when the Zr content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Zr is included, the Zr content is preferably 0.0200% or less.

Ca: 0.0200% or Less

Ca is present in steel as inclusions. Here, when Ca content exceeds 0.0200%, a large amount of coarse inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Ca is included, the Ca content is preferably 0.0200% or less. The Ca content is more preferably 0.0020% or less. Although a lower limit of Ca content is not particularly limited, the Ca content is preferably 0.0005% or more. Further, the Ca content is more preferably 0.0010% or more, in view of production technology constraints.

Ce: 0.0200% or Less, Se: 0.0200% or Less, Te: 0.0200% or Less, Ge: 0.0200% or Less, as: 0.0200% or Less, Sr: 0.0200% or Less, Cs: 0.0200% or Less, Hf: 0.0200% or Less, Pb: 0.0200% or Less, Bi: 0.0200% or Less, and REM: 0.0200% or Less

Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all elements effective for improving steel sheet ultimate deformability and hole expansion formability. To achieve this effect, content of each of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is preferably 0.0001% or more. On the other hand, when the content of any one of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM exceeds 0.0200%, a large amount of coarse precipitates and inclusions may form. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when at least one of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is included, the content thereof is preferably 0.0200% or less.

That is, the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure comprises:

    • a chemical composition containing, in mass %,
    • C: 0.050% or more and 0.400% or less,
    • Si: 0.20% or more and 3.00% or less,
    • Mn: 1.00% or more and less than 3.50%,
    • P: 0.001% or more and 0.100% or less,
    • S: 0.0200% or less,
    • Al: 0.010% or more and 2.000% or less,
    • N: 0.0100% or less,
    • where the carbon equivalent Ceq is 0.540% or more,
    • and optionally contains at least one selected from the group consisting of
    • Ti: 0.200% or less,
    • Nb: 0.200% or less,
    • V: 0.100% or less,
    • B: 0.0100% or less,
    • Cu: 1.000% or less,
    • Cr: 1.000% or less,
    • Ni: 1.000% or less,
    • Mo: 0.500% or less,
    • Sb: 0.200% or less,
    • Sn: 0.200% or less,
    • Ta: 0.100% or less,
    • W: 0.500% or less,
    • Mg: 0.0200% or less,
    • Zn: 0.0200% or less,
    • Co: 0.0200% or less,
    • Zr: 0.0200% or less,
    • Ca: 0.0200% or less,
    • Ce: 0.0200% or less,
    • Se: 0.0200% or less,
    • Te: 0.0200% or less,
    • Ge: 0.0200% or less,
    • As: 0.0200% or less,
    • Sr: 0.0200% or less,
    • Cs: 0.0200% or less,
    • Hf: 0.0200% or less,
    • Pb: 0.0200% or less,
    • Bi: 0.0200% or less, and
    • REM: 0.0200% or less.
    • with the balance being Fe and inevitable impurity.

Steel microstructure of the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure is described below.

The steel microstructure of the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure has a steel microstructure wherein

    • area ratio of ferrite: 65.0% or less (including 0%),
    • area ratio of bainitic ferrite: 5.0% or more and 40.0% or less,
    • area ratio of tempered martensite: 0.5% or more and 80.0% or less,
    • area ratio of retained austenite: 3.0% or more,
    • area ratio of fresh martensite: 20.0% or less (including 0%)
    • SBF+STM+2×SMA: 65.0% or more,
    • SMA1/SMA: 0.80 or less, and
    • SMA2/SMA: 0.20 or more,
    • wherein
    • SBF is area ratio of the bainitic ferrite,
    • STM is area ratio of the tempered martensite,
    • SMA is area ratio of a hard secondary phase consisting of the retained austenite and fresh martensite,
    • SMA1 is a total area ratio of island regions among island regions constituting the hard secondary phase that have an equivalent circular diameter of 2.0 μm or more where 20% or less of the circumference is in contact with tempered martensite, and
    • SMA2 is a total area ratio of island regions among island regions constituting the hard secondary phase where 1% or more of the circumference is in contact with bainitic ferrite.

The reasons for each of these limitations are described below.

Area Ratio of Ferrite: 65.0% or Less (Including 0%)

Soft ferrite is a phase that improves ductility and strain hardenability. However, from the viewpoint of securing TS above 980 MPa, high YS, and good hole expansion formability, the area ratio of ferrite is preferably 65.0% or less. The area ratio of ferrite is preferably 35.0% or less. The area ratio of ferrite is more preferably 25.0% or less. A lower limit of the area ratio of ferrite is not particularly limited, and may be 0%. In particular, when 980 MPa≤TS<1,180 MPa is required, the area ratio of ferrite is preferably 5.0% or more.

Area Ratio of Bainitic Ferrite: 5.0% or More and 40.0% or Less

Bainitic ferrite has an intermediate hardness between soft ferrite and hard fresh martensite and the like, and is an important phase for securing good hole expansion formability. Further, bainitic ferrite is a useful phase for obtaining an appropriate amount of retained austenite by utilizing C diffusion from bainitic ferrite to untransformed austenite. Therefore, the area ratio of bainitic ferrite is 5.0% or more. Further, the area ratio of bainitic ferrite is preferably 10.0% or more. On the other hand, an excessive increase in the area ratio of bainitic ferrite decreases hole expansion formability. Therefore, the area ratio of bainitic ferrite is 40.0% or less. Further, the area ratio of bainitic ferrite is preferably 35.0% or less.

Area Ratio of Tempered Martensite: 0.5% or More and 80.0% or Less

Tempered martensite has an intermediate hardness between soft ferrite and hard fresh martensite and the like, and is an important phase for securing good hole expansion formability. Therefore, the area ratio of tempered martensite is 0.5% or more. The area ratio of tempered martensite is preferably 40.0% or more. On the other hand, from the viewpoint of securing good ductility, the area ratio of tempered martensite is 80.0% or less. Further, the area ratio of tempered martensite is preferably 75.0% or less.

Area Ratio of Retained Austenite: 3.0% or More

From the viewpoint of obtaining good ductility, the area ratio of retained austenite is 3.0% or more. The area ratio of retained austenite is preferably 5.0% or more. Although an upper limit of the area ratio of retained austenite is not particularly limited, the area ratio of retained austenite is preferably 20.0% or less.

Area Ratio of Fresh Martensite: 20.0% or Less (Including 0%)

From the viewpoint of securing good hole expansion formability, the area ratio of fresh martensite is 20.0% or less. A lower limit of the area ratio of fresh martensite is not particularly limited, and may be 0%. From the viewpoint of securing a TS of 980 MPa or more, the area ratio of fresh martensite is preferably 3.0% or more.

Fresh martensite is martensite as quenched (not tempered).

The area ratio of residual microstructure other than the above is preferably 10.0% or less. The area ratio of the residual microstructure is more preferably 5.0% or less. Further, the area ratio of the residual microstructure may be 0%.

The residual microstructure is not particularly limited, and may include carbides such as lower bainite, pearlite, cementite, and the like. The type of residual microstructure can be confirmed, for example, by observation with a scanning electron microscope (SEM).

Here, the area ratio of ferrite, bainitic ferrite, tempered martensite, and hard secondary phase (retained austenite+fresh martensite) is measured at a ¼ position of the thickness of the base steel sheet, as follows.

A sample is cut from the base steel plate such that a thickness cross-section parallel to a rolling direction of the base steel plate becomes an observation plane. The observation plane of the sample is then mirror-polished using diamond paste. The observation plane of the sample is then polished using colloidal silica and etched with 3 vol % nital to reveal the microstructure.

Then, five views of a 25.6 μm×17.6 μm field of view of the observation plane of the sample are observed using a scanning electron microscope (SEM) at an accelerating voltage of 15 kV and a magnification of 5000 times.

From an obtained microstructure image (see, for example, FIG. 1A), ferrite, bainitic ferrite, tempered martensite, and hard secondary phase (retained austenite+fresh martensite) are identified as follows.

Ferrite: a black-colored area, blocky in morphology. Further, almost no iron-based carbides are encapsulated. However, when iron-based carbides are encapsulated, the area of ferrite also includes the area of the iron-based carbides. The same is also true for bainitic ferrite and tempered martensite, which are described below.

Bainitic ferrite: black to dark gray area, which may be blocky or irregular in morphology. Further, no, or relatively few iron-based carbides are encapsulated.

Tempered martensite: gray area, irregular in morphology. Further, a relatively large number of iron-based carbides are encapsulated.

Hard secondary phase (retained austenite+fresh martensite): white to light gray area, irregular in morphology. Further, iron-based carbides are not encapsulated. When size is relatively large, the color gradually darkens farther away from an interface with other microstructure, and an interior may be dark gray.

Residual microstructure: lower bainite, pearlite, cementite, and other carbides mentioned above have morphology and the like as known in the art.

In addition to the observation by SEM, observation of carbides under higher magnification, detailed microstructural analysis by electron backscatter diffraction (EBSD) in the same field of view, composition analysis by electron probe micro-analyzer (EPMA), local hardness measurement by microhardness tester, and the like may be added as appropriate. For example, when identifying the microstructure by the observation by SEM is difficult, adding these measurements as appropriate is effective.

For example, in EBSD analysis, ferrite has no substructure (no substructure is observed). In contrast, bainitic ferrite, tempered martensite, and fresh martensite have a substructure and a specific crystal orientation in relation to retained austenite. Further, austenite microstructure in an annealing process may be reproduced from these microstructures and confirmed, for example. Such points may be a factor in judging microstructure identification.

Further, in component analysis by EPMA, C concentration and Mn concentration differing depending on microstructure may be a factor in judging tissue identification. For example, the C concentration of ferrite and bainitic ferrite is lower than that of an area that is primarily tempered martensite (including fine hard secondary phase, carbides, and the like). Further, in ferrite, Mn concentration may be lower than in other microstructures.

In measuring hardness with a microhardness tester, hardness differing according to microstructure is a factor in judging tissue identification. For example, among ferrite, bainitic ferrite, tempered martensite, and hard secondary phase, ferrite is the least hard and hard secondary phase is the hardest. Further, bainitic ferrite and tempered martensite exhibit hardness between that of ferrite and hard secondary phase.

Next, the areas of the phases identified in the microstructure image are color-coded (converted to a 4-value image) using Adobe Photoshop by Adobe Systems Inc. (see, for example, FIG. 1), and the area of each phase is calculated. Then, values are calculated by dividing the area of each phase (the total area of each phase) by the area of the observation area (25.6 μm×17.6 μm), and multiplying by 100 for each of five fields of view. For each phase (ferrite, bainitic ferrite, tempered martensite, and hard secondary phase), the average of the values is then used as the area ratio. FIG. 1A is a partial image extracted from one field of view of an observation area (25.6 μm×17.6 μm) of a sample, as a reference for the above description.

Further, the area ratio of retained austenite is measured as follows.

The base steel sheet is machine ground in the thickness direction (depth direction) to a ¼ position of the sheet thickness, and then chemically polished with oxalic acid to prepare the observation plane. The observation plane is then observed by X-ray diffraction. CoKα radiation is used for incident X-rays to determine a ratio of diffraction intensity of the (200), (220) and (311) planes of fcc iron (austenite) to diffraction intensity of the (200), (211) and (220) planes of bcc iron, and a volume fraction of retained austenite is calculated from the ratio of diffraction intensity of each plane. Then, assuming that the retained austenite is homogeneous in three dimensions, the volume fraction of the retained austenite is taken as the area ratio of retained austenite.

Further, the area ratio of fresh martensite is obtained by subtracting the area ratio of retained austenite from the area ratio of hard secondary phase obtained as described above.


[area ratio of fresh martensite (%)]=[area ratio of hard secondary phase (%)]−[area ratio of retained austenite (%)]

Further, the area ratio of residual microstructure is obtained by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of hard secondary phase as determined above from 100%.


[area ratio of residual microstructure (%)]=100−[area ratio of ferrite (%)]−[area ratio of bainitic ferrite (%)]−[area ratio of tempered martensite (%)]−[area ratio of hard secondary phase (%)]

SBF+STM+2×SMA: 65.0% or More

From the viewpoint of securing a TS of 980 MPa or more, SBF+STM+2×SMA is 65.0% or more. An upper limit of SBF+STM+2×SMA is not particularly limited, but is preferably 130.0% or less.

Here,

    • SBF: area ratio of bainitic ferrite
    • STM: area ratio of tempered martensite
    • SMA: area ratio of hard secondary phase consisting of retained austenite and fresh martensite.

SMA1/SMA: 0.80 or Less

The hard secondary phase consisting of retained austenite and fresh martensite (hereinafter also referred to as MA) includes multiple island regions. Among such island regions, an island region having an equivalent circular diameter of 2.0 μm or more where 20% or less of the circumference is in contact with tempered martensite (hereafter also referred to as MA1) has a low concentration of solute C. In other words, stability of the retained austenite in MA1 is low. Therefore, MA1 does not contribute to securing good ductility. Further, the ratio of fresh martensite in MA1 is high, and therefore MA1 reduces hole expansion formability. Therefore, SMA1/SMA, the ratio of the area ratio of MA1 to the area ratio of the hard secondary phase, is 0.80 or less. In particular, when 980 MPa≤TS<1,180 MPa is required, SMA1/SMA is preferably 0.75 or less. When 980 MPa≤TS<1,180 MPa is required, SMA1/SMA is more preferably 0.40 or less. Further, when 1,180 MPa≤TS is required, SMA1/SMA is preferably 0.50 or less. When 1,180 MPa≤TS is required, SMA1/SMA is more preferably 0.30 or less. A lower limit of SMA1/SMA is not particularly limited and may be 0.

Each island region is separated from other island regions of the hard secondary phase by a phase other than the hard secondary phase (each island region is in contact with a phase other than the hard secondary phase around an entire circumference of the island region). Further, specific shape of each island region is not particularly limited, and may be any of circular, elliptical, polygonal, ameboid (a shape extending in a plurality of irregular directions), and the like.

SMA2/SMA: 0.20 or More

Among island regions constituting the hard secondary phase consisting of retained austenite and fresh martensite, an island region where 1% or more of the circumference is in contact with bainitic ferrite (hereinafter also referred to as MA2) has a high concentration of solute C. In other words, stability of the retained austenite in MA2 is high. Therefore, MA2 plays a crucial role in securing good strain hardenability and ductility.

That is, when bainitic ferrite is formed under appropriate conditions during cooling after annealing, the solute C that diffuses from the bainitic ferrite into surrounding untransformed austenite does not sufficiently diffuse into the interior of the untransformed austenite. That is, in the untransformed austenite, only an area around the bainitic ferrite may be locally high in solute C. Subsequently, from this state, reheating treatment under appropriate conditions produces a hard secondary phase having a high solute C concentration around the bainitic ferrite. In other words, MA2 is produced. Therefore, the stability of the retained austenite in MA2 is high, and MA2 plays a crucial role in securing good strain hardenability and ductility.

Based on the above, SMA2/SMA, the ratio of the area ratio of MA2 to the area ratio of the hard secondary phase, is 0.20 or more. SMA2/SMA is preferably 0.25 or more. SMA2/SMA is more preferably 0.30 or more. An upper limit of SMA2/SMA is not particularly limited and may be 1. Further, from the viewpoint of securing high YS and excellent hole expansion formability, when 980 MPa≤TS<1,180 MPa is required, SMA2/SMA is preferably 0.98 or less. Further, when 1,180 MPa≤TS is required, SMA2/SMA is preferably 0.70 or less.

Further, in the steel microstructure of the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure, SMA3/SMA is preferably 0.05 or more.

Here

    • SMA3: a total area ratio of island regions among island regions constituting the hard secondary phase where 1% or more of the circumference is in contact with bainitic ferrite and more than 20% of the circumference is in contact with tempered martensite.

SMA3/SMA: 0.05 or More

Among island regions constituting the hard secondary phase consisting of retained austenite and fresh martensite, an island region where 1% or more of the circumference is in contact with bainitic ferrite and more than 20% of the circumference is in contact with tempered martensite (hereinafter also referred to as MA3) has an even higher concentration of solute C than MA2.

That is, MA3 has a particularly high concentration of solute C because solute C diffuses from tempered martensite as well as bainitic ferrite. Therefore, MA3 contributes particularly effectively to securing good strain hardenability and ductility.

Accordingly, SMA3/SMA, the ratio of the area ratio of MA3 to the area ratio of the hard secondary phase, is preferably 0.05 or more. SMA3/SMA is more preferably 0.07 or more. SMA3/SMA is more preferably 0.10 or more. An upper limit of SMA3/SMA is not particularly limited and may be 1. Further, SMA3/SMA is preferably 0.70 or less.

SMA1, SMA2, and SMA3 are respectively measured as follows.

Ferrite, bainitic ferrite, tempered martensite, and hard secondary phase (retained austenite+fresh martensite) are identified in a microstructure image (see, for example, FIG. 2A, FIG. 3A, and FIG. 4A), as described above. Then, after color-coding (conversion to a 4-value image) using Adobe Photoshop by Adobe Systems Inc., the island regions of the hard secondary phase are extracted, and the equivalent circular diameter of each island region, the circumference of each island region, and the length of each island region in contact with bainitic ferrite and tempered martensite are determined using ImageJ, which is open source. Pixel density of the microstructure image when determining the circumference is 30 pixels/μm or more and 100 pixels/μm or less. Then, from the obtained values, each island region is identified as corresponding to MA1, MA2, and MA3, respectively, color-coded using Adobe Photoshop by Adobe Systems Inc. (see, for example, FIG. 2B, FIG. 3B, and FIG. 4B), and respective areas are calculated. Then, for each of MA1, MA2, and MA3, a total area for island regions identified as either MA1, MA2, or MA3 is divided by the area of the observation area (25.6 μm×17.6 μm), and multiplied by 100 (area ratio) for each of five fields of view. The average of the values (area ratio) for the five fields of view for each of MA1, MA2 and MA3 is then used as SMA1, SMA2, and SMA3. For island regions that fall under both MA1 and MA2, the area is counted as both MA1 and MA2. The same is true for MA1 and MA3, and MA2 and MA3. Further, FIG. 2A, FIG. 3A, and FIG. 4A are each partial images extracted from one field of view of an observation area (25.6 μm×17.6 μm) of a sample, as a reference for the above description.

Further, in the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure, an amount of diffusible hydrogen is preferably 0.50 mass ppm or less.

Diffusible Hydrogen Content of the Base Steel Sheet: 0.50 Mass Ppm or Less

From the viewpoint of obtaining better hole expansion formability, an amount of diffusible hydrogen of the base steel sheet is preferably 0.50 mass ppm or less. Further, the amount of diffusible hydrogen of the base steel sheet is more preferably 0.35 mass ppm or less. A lower limit of the amount of diffusible hydrogen of the base steel sheet is not particularly specified and may be 0 mass ppm. Further, the amount of diffusible hydrogen of the base steel sheet is preferably 0.01 mass ppm or more, in view of production technology constraints.

Here, the amount of diffusible hydrogen of the base steel sheet is measured as follows.

A test piece 30 mm long and 5 mm wide is taken from a galvanized steel sheet, and the galvanized layer is removed with alkali. Then, an amount of hydrogen released from the test piece is measured by a thermal desorption analysis method. Specifically, the test piece is continuously heated from room temperature to 300° C. at a rate of 200° C./h, and then cooled to room temperature. At this time, the amount of hydrogen released from the test piece in the temperature range from room temperature to 210° C. during the continuous heating is measured (cumulative hydrogen amount). The measured hydrogen amount is then divided by the mass of the test piece (after removal of the galvanized layer and before continuous heating), and a value converted to mass ppm units is the amount of diffusible hydrogen of the base steel sheet.

For a product (member) after forming or joining of galvanized steel sheets, a test piece is cut from the product under a general operating environment and the amount of diffusible hydrogen of the base steel sheet is measured as described above. When the value is 0.50 mass ppm or less, the amount of diffusible hydrogen of the base steel sheet of the galvanized steel sheet at a material stage before the forming or joining may also be considered to be 0.50 mass ppm or less.

Further, the galvanized steel sheet according to an embodiment of the present disclosure preferably has a decarburized layer. In particular, the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure preferably has a decarburized layer. Cracking due to liquid metal embrittlement (LME) during resistance spot welding may be a problem for steel sheets containing Si, especially for coated or plated steel sheets in which the base steel sheet has a high Si content. However, when the galvanized steel sheet has a decarburized layer, particularly in a surface layer of the base steel sheet, resistance weld crack resistance may be improved even when the base steel sheet has a high Si content.

Thickness of the decarburized layer, in other words, depth in the thickness direction from the surface of the base steel sheet, is preferably 30 μm or more. Thickness of the decarburized layer is more preferably 40 μm or more. Although an upper limit of the thickness of the decarburized layer is not particularly limited, the thickness of the decarburized layer is preferably 130 μm or less in order to keep the tensile strength within a good range. Here, the decarburized layer is defined as a region where the C concentration of the base steel sheet is 80% or less of that of the C content of the chemical composition of the base steel sheet according to analysis of the C concentration of the base steel sheet in the thickness direction from the surface of the base steel sheet. The thickness of the decarburized layer is defined as the thickness of the region.

Further, the thickness of the decarburized layer is measured by surface or line analysis of element distribution in the vicinity of the surface layer of the base steel sheet using an electron probe microanalyzer (EPMA) on a cross-sectioned sample. First, a resin-embedded galvanized steel sheet is polished and a vertical section in the rolling direction is finished for observation, and then removed from the resin to be used as a sample for measurement. The accelerating voltage is 7 kV and the irradiation current is 50 nA. Surface analysis or line analysis of the sample cross-section is performed in 1 μm steps over a 300 μm×300 μm area including a topmost surface layer of the base steel sheet to measure C intensity. In order to suppress contamination, hydrocarbons on and around the sample surface are removed by a plasma cleaner in the measurement room and sample preparation room before the start of measurement. To suppress the accumulation of hydrocarbons during the measurement, the measurement is performed while the sample is heated to and held at a maximum sample temperature of 100° C. on the stage. The C intensity is converted to a C concentration (in mass %) using a calibration curve prepared by performing measurements on a standard sample separately. The next step is to confirm that the lower limit of C detection is sufficiently lower than 0.10 mass % due to the effect of contamination control. Details of equipment used and the method of contamination control are described in Reference 1 below.

  • Reference 1: Yamashita et al., “Distribution of Carbon in Low-Carbon Steel in the Early Phase of Proeutectoid Ferrite by High Precision FE-EPMA,” Iron and Steel, Vol. 103 (2017) No. 11., pp.14-20

However, the above configuration is not necessarily required because the necessity of contamination countermeasures during measurement depends on the machine model used and conditions. In other words, the measurement conditions are only required to confirm that sufficient accuracy has been obtained, and the measurement conditions are not intrinsically related to the effect of the present disclosure.

In the C concentration map obtained, a line profile in the thickness direction is extracted from the surface of the base steel sheet and averaged over 300 points in the direction parallel to the base steel sheet surface to obtain a profile of C concentration in the thickness direction. The obtained profile of C concentration in the thickness direction is smoothed by a simple moving average method. In this case, the number of smoothing points is preferably about 21. The thickness of the decarburized layer is then determined by identifying the range in the thickness direction where the C concentration in the intensity profile after the smoothing treatment is 80% or less of that of the C content of the chemical composition of the base steel sheet.

Mechanical properties of the galvanized steel sheet according to an embodiment of the present disclosure are described below.

Tensile Strength (TS): 980 MPa or More

The tensile strength of the galvanized steel sheet according to an embodiment of the present disclosure is 980 MPa or more. The tensile strength of the galvanized steel sheet according to an embodiment of the present disclosure is preferably 1,180 MPa or more.

The yield stress (YS), total elongation (El), strain hardening index (n-value)/yield ratio (YR), and maximum hole expansion ratio (λ) of the galvanized steel sheet according to an embodiment of the present disclosure are as described above.

Further, tensile strength (TS), yield stress (YS), total elongation (El), and strain hardening index (n value)/yield ratio (YR) are measured by tensile testing in accordance with JIS Z 2241, as described later in reference to Examples. The maximum hole expansion ratio (λ) is measured by a hole expanding test in accordance with JIS Z 2256, as described later in reference to Examples.

Further, the galvanized layer of the galvanized steel sheet according to an embodiment of the present disclosure may be provided on only one surface of the base steel sheet, and may be provided on both surfaces.

The galvanized layer here refers to a coating layer in which Zn is the main component (Zn content of 50% or more), for example, a hot-dip galvanized layer or a galvannealed layer.

Here, for example, the hot-dip galvanized layer being composed of Zn, Fe: 20 mass % or less, and Al: 0.001 mass % or more and 1.0 mass % or less is appropriate. Further, the hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, totaling 0 mass % or more and 3.5 mass % or less. Further, the Fe content of the hot-dip galvanized layer is more preferably less than 7 mass %. Other than the above elements, the balance is inevitable impurity.

Further, for example, the galvannealed layer being composed of Fe: 20 mass % or less and Al: 0.001 mass % or more and 1.0 mass % or less is appropriate. Further, the galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, totaling 0 mass % or more and 3.5 mass % or less. The Fe content of the galvannealed layer is more preferably 7 mass % or more. The Fe content of the galvannealed layer is even more preferably 8 mass % or more. Further, the Fe content of the galvannealed layer is more preferably 15 mass % or less. The Fe content of the galvannealed layer is even more preferably 12 mass % or less. Other than the above elements, the balance is inevitable impurity.

Further, a coating weight per side of the galvanized layer is not particularly limited, but is preferably 20 g/m2 to 80 g/m2.

The coating weight of the galvanized layer is measured as follows.

A coating solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (“IBIT 700BK” (IBIT is a registered trademark in Japan, other countries, or both) manufactured by Asahi Chemical Co., Ltd.) to 1 L of a 10 mass % hydrochloric acid aqueous solution. Then, the galvanized steel sheet to be the test piece is immersed in the coating solution to dissolve the galvanized layer. Mass loss of the test piece before and after dissolving is measured, and the value is divided by the surface area of the base steel sheet (surface area of a coated portion) to calculate the coating weight (g/m2).

Further, the galvanized steel sheet according to an embodiment of the present disclosure may have a metal coating or plating layer other than the galvanized layer at least on one side between the base steel sheet and the galvanized layer. The metal coating or plating layer contributes to an improvement in resistance weld crack resistance. Formation of the metal coating or plating layer suppresses resistance weld cracking even when the Si content of the base steel sheet is high. Although a mechanism by which the metal coating or plating layer improves resistance weld crack resistance is not necessarily clear, the inventors understand that when the metal coating or plating layer is between the base steel sheet and the galvanized layer, or in other words on the surface of the base steel sheet, the metal coating or plating layer acts as a barrier layer that hinders the zinc in the galvanized layer from melting and penetrating into the base steel sheet during resistance spot welding, thereby making resistance weld cracking less likely to occur (zinc penetration suppression effect). When the galvanized layer is provided on both sides of the base steel sheet, the metal coating or plating layer may be between the base steel sheet and the galvanized layer on only one side, and may be between the base steel sheet and the galvanized layer on both sides.

Here, the coating weight of the metal coating or plating layer is preferably more than 0 g/m2. The coating weight of the metal coating or plating layer is more preferably 2.0 g/m2 or more. Although an upper limit of the coating weight of the metal coating or plating layer per side is not particularly limited, the coating weight of metal coating or plating layer is preferably 60 g/m2 or less, in view of cost. The coating weight of the metal coating or plating layer is more preferably 50 g/m2 or less. The coating weight of the metal coating or plating layer is even more preferably 40 g/m2 or less. The coating weight of the metal coating or plating layer is even more preferably 30 g/m2 or less. Here, the coating weight of the metal coating or plating layer is per side.

The coating weight of the metal coating or plating layer is measured as follows. A 10 mm×15 mm size sample is taken from the galvanized steel sheet and embedded in resin to make a cross-sectional embedded sample. The thickness of the metal coating or plating layer is measured at three arbitrary locations on the cross section of the sample using a scanning electron microscope (SEM) at an accelerating voltage of 15 kV and a magnification of 2,000 times to 10,000 times, depending on the thickness of the metal coating or plating layer, and the average value of the three locations is calculated. The calculated average value is then multiplied by the relative density of the metal constituting the metal coating or plating layer to convert to the coating weight per side of the metal coating or plating layer.

As a metal used for the metal coating or plating layer, a metal having a higher melting point than Zn is desirable, for example, Fe, Ni, and the like may be used. Further, in addition to the zinc penetration suppression effect, the metal coating or plating layer is preferably Fe-based, because a toughness reduction suppression effect described below may be expected.

When an amount of Si in the vicinity of the surface of the base steel sheet is high, this may reduce toughness of a weld and thereby decrease resistance weld crack resistance. On the other hand, when the Fe-based coating or plating layer is between the base steel sheet and the galvanized layer, that is, the surface of the base steel sheet has the Fe-based coating or plating layer, the Fe-based coating or plating layer acts as a solute Si deficient layer, and the amount of solute Si in a welded portion decreases. This may suppress the decrease in toughness of the welded portion and improve the resistance weld crack resistance in the welded portion (toughness reduction suppression effect). Further, the Fe-based coating or plating layer also functions as a soft layer and relaxes stress applied to the steel sheet surface during resistance spot welding. This may reduce residual stress in the welded portion and improve the resistance weld crack resistance (stress relaxation effect).

The Fe-based coating or plating layer may be an Fe (pure Fe) coating or plating layer, and may be an alloy coating or plating layer such as one formed from Fe—B alloy, Fe—C alloy, Fe—P alloy, Fe—N alloy, Fe—O alloy, Fe—Ni alloy, Fe—Mn alloy, Fe—Mo alloy, Fe—W alloy, and the like. The chemical composition of the Fe-based coating or plating layer is not particularly limited as long as the Fe content is 50 mass % or more, but in particular, a chemical composition consisting of Fe and inevitable impurity, and a chemical composition containing one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co totaling 10 mass % or less, with the balance being Fe and inevitable impurity, are preferable. When an element other than Fe is included, the total content of the one or more elements is preferably 10 mass % or less to help prevent a decrease in electrolytic efficiency and to form an Fe-based coating or plating layer, in particular an Fe-based electroplating layer, at low cost. In the case of Fe—C alloy, the C content is preferably 0.08 mass % or less.

Further, the galvanized steel sheet according to an embodiment of the present disclosure may have a metal coating or plating layer and a decarburized layer at the same time (that is, from the surface of the galvanized steel sheet, the galvanized layer, the metal coating or plating layer, and the decarburized layer (surface layer of the base steel sheet) in that order). This may further improve the resistance weld crack resistance. When there is a metal coating or plating layer, the thickness of the decarburized layer (depth in the thickness direction from the surface of the base steel sheet) may be evaluated by analyzing the C concentration from the surface of the metal coating or plating layer or an interface between the galvanized layer and the cold-rolled steel sheet in the direction of the sheet thickness by the method described above.

The thickness of the galvanized steel sheet according to an embodiment of the present disclosure is not particularly limited, but is preferably 0.5 mm or more. The thickness of the galvanized steel sheet is preferably 3.0 mm or less.

[2] Member

A member according to an embodiment of the present disclosure is described below.

The member according to an embodiment of the present disclosure is a member made using the galvanized steel sheet described above as a material. For example, the material, the galvanized steel sheet, is subjected to at least one of a forming process and a joining process to make the member.

Here, the galvanized steel sheet has a TS of 980 MPa or more, and has high YS, excellent ductility, strain hardenability and hole expansion formability. Therefore, the member according to an embodiment of the present disclosure has high strength and excellent anti-crash properties. Therefore, the member according to an embodiment of the present disclosure is particularly suitable for application as an impact energy absorbing member for use in the automotive field.

[3] Method of Producing Galvanized Steel Sheet

A method of producing the galvanized steel sheet according to an embodiment of the present disclosure is described below.

The method of producing the galvanized steel sheet according to an embodiment of the present disclosure comprises:

    • a hot rolling process, in which a steel slab having the chemical composition described above is hot rolled to make a hot-rolled steel sheet;
    • a cold rolling process, in which the hot-rolled steel sheet is cold rolled into a cold-rolled steel sheet;
    • an annealing process, in which the cold-rolled steel sheet is annealed at an annealing temperature of 760° C. or more and 900° C. or less for an annealing time of 20 s or more;
    • a first cooling process, in which the cold-rolled steel sheet is cooled to a first cooling stop temperature of 300° C. or more and 550° C. or less;
    • a holding process, in which the cold-rolled steel sheet is held at a temperature range of 300° C. or more and 550° C. or less for 3 s or more and 600 s or less;
    • a coating process, in which the cold-rolled steel sheet is subject to a galvanizing treatment to become the galvanized steel sheet;
    • a second cooling process, in which the galvanized steel sheet is cooled to a second cooling stop temperature of 100° C. or more and less than 300° C.; and
    • a reheating process, in which the galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature+50° C.) or more and 500° C. or less, and the galvanized steel sheet is held at a temperature range of (the second cooling stop temperature+50° C.) or more and 500° C. or less for 10 s or more and 2,000 s or less, wherein
    • a relationship between the first cooling stop temperature and a temperature of a galvanizing bath in the galvanizing treatment satisfies the following Formula (1).


−150° C.≤T0−T1≤50° C.  (1)

where T0 is the first cooling stop temperature (° C.) and T1 is the temperature of the galvanizing bath in the galvanizing treatment (° C.).

Unless otherwise specified, each of temperatures above refers to a surface temperature of the steel slab and the steel sheet.

First, a steel slab having the chemical composition described above is prepared. For example, steel raw material is melted to produce molten steel having the chemical composition described above. The steelmaking method is not particularly limited, and any known steelmaking method may be used, such as using a converter, electric furnace, and the like. Obtained molten steel is then solidified into a steel slab. The method of obtaining a steel slab from molten steel is not particularly limited. For example, continuous casting, ingot making, and thin slab casting methods may be used. A continuous casting method is preferred from the viewpoint of hindering macro-segregation.

[Hot Rolling Process]

The steel slab is then hot rolled to obtain a hot-rolled steel sheet.

An energy saving process may be applied to the hot rolling process. Energy saving processes include hot charge rolling (where a steel slab is charged into a furnace as a warm slab without cooling to room temperature and then hot rolled) and direct rolling (where a steel slab is hot rolled immediately after being subjected to heat retaining for a short period).

There are no particular limitations on hot rolling conditions. For example, hot rolling may be performed under the following conditions.

The steel slab is temporarily cooled to room temperature and then reheated before rolling. The slab heating temperature (reheating temperature) is preferably 1,100° C. or more in view of carbide dissolution and reduced rolling load. The slab heating temperature is preferably 1,300° C. or less, in order to prevent increased scale loss. The slab heating temperature is based on the temperature of the steel slab surface.

The steel slab is then subjected to rough rolling according to a conventional method to produce a rough-rolled sheet (hereinafter also referred to as a sheet bar). The sheet bar is then subjected to finish rolling to produce a hot-rolled steel sheet. When the slab heating temperature is low, heating the sheet bar using a bar heater or the like before finish rolling is preferable, from the viewpoint of preventing trouble during finish rolling. The rolling finish temperature is preferably the Ar3 transformation temperature or more, in order to reduce rolling load. Further, the rolling finish temperature is preferably the Ar3 transformation temperature or more because a high rolling reduction rate in an unrecrystallized state of austenite may result in the development of an abnormal microstructure elongated in the rolling direction, which may reduce the workability of the annealed sheet. The Ar3 transformation temperature is determined by the following formula.


Ar3(° C.)=868−396×[C %]+25×[Si %]−68[Mn %]

The [element symbol %] in the above formula represents the content (mass %) of the element in the chemical composition of the base steel sheet.

Sheet bars may be joined together during hot rolling, and finish rolling may be performed continuously. Further, the sheet bar may be rolled once before finish rolling. Further, at least part of finish rolling may be conducted as lubrication rolling to reduce rolling load in the hot rolling. Conducting lubrication rolling in such a manner is effective from the perspective of making the shape and material properties of the steel sheet uniform. In lubrication rolling, the coefficient of friction is preferably 0.10 or more. The coefficient of friction is preferably 0.25 or less.

In the hot rolling process, which includes rough rolling and finish rolling, a steel slab is generally turned into a sheet bar by rough rolling and into a hot-rolled steel sheet by finish rolling. However, depending on mill capacity and other factors, there is no need to be concerned with such classification as long as the defined size is obtained.

The rolling finish temperature is preferably 800° C. or more and 950° C. or less. The rolling finish temperature being 800° C. or more makes the steel microstructure at the hot-rolled steel sheet stage and, consequently, the steel microstructure of the final product more likely to be uniform. Uneven steel microstructure tends to reduce bendability. On the other hand, when the rolling finish temperature is more than 950° C., the amount of oxide (scale) formation increases. As a result, an interface between the steel substrate and oxide may be roughened, and the surface quality of the steel sheet after pickling and cold rolling may deteriorate. Further, coarse crystal grains may also cause a reduction in the strength and bendability of a steel sheet.

After finish rolling, the hot-rolled steel sheet is coiled. The coiling temperature is preferably 450° C. or more. The coiling temperature is preferably 750° C. or less.

[Pickling Process]

After the hot rolling process, the hot-rolled steel sheet is optionally pickled. Pickling may remove oxides from the steel sheet surface, securing good chemical convertibility and coating or plating quality. Pickling may be performed in one or more batches. Pickling conditions are not particularly limited, and a conventional method may be followed.

[Cold Rolling Process]

The hot-rolled steel sheet is then subjected to cold rolling to obtain a cold-rolled steel sheet. Cold rolling is performed by multi-pass rolling that requires two or more passes, for example, tandem-type multi-stand rolling, reverse rolling, and the like.

The rolling reduction of the cold rolling is not particularly limited. The rolling reduction of the cold rolling is preferably 20% or more. The rolling reduction of the cold rolling is preferably 80% or less. When the rolling reduction of the cold rolling is less than 20%, coarsening and non-uniformity of the steel microstructure is more likely to occur during the annealing process, which may result in reduced strength and workability in the final product. On the other hand, when the rolling reduction of the cold rolling exceeds 80%, the steel sheet may be prone to shape defects and the coating weight of the galvanized coating may become uneven.

Further, the cold-rolled steel sheet obtained after cold rolling may be subjected to pickling.

[Metal Coating or Plating Process]

In the method of producing the galvanized steel sheet according to an embodiment of the present disclosure, a metal coating or plating treatment to form a metal coating or plating layer on at least one surface of the cold-rolled steel sheet obtained as described above may optionally be applied after the cold rolling process and before the annealing process described below. A cold-rolled steel sheet that has a metal coating or plating layer on at least one surface before undergoing the annealing process described below may hereinafter be referred to as a metal coated or plated steel sheet. The metal coating or plating treatment method is not particularly limited, but electroplating is preferred from the viewpoint of manufacturability. A sulfuric acid bath, a hydrochloric acid bath, a mixed solution thereof, and the like may be used as a metal plating bath. In the case of electroplating, the coating weight of the metal plating layer may be adjusted by energization time and the like. As mentioned above, a metal coated or plated steel sheet means a steel sheet that has a metal coating or plating layer on at least one surface of the cold-rolled steel sheet before the annealing process described below, and does not exclude a cold-rolled steel sheet that is pre-annealed before the metal coating or plating treatment.

As a metal used in the metal coating or plating treatment, a metal having a higher melting point than Zn is desirable, for example, Fe, Ni, and the like may be used. Further, formation of the Fe-based coating or plating layer is a preferred result of the metal coating or plating treatment, as an improved effect of resistance weld crack resistance may be expected.

Further, in addition to Fe ions, the coating or plating bath to form the Fe-based coating or plating layer may contain one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. The total content of these elements in the coating or plating bath is preferably 10 mass % or less of the chemical composition of the metal coating or plating layer of the metal coated or plated steel sheet. Metallic elements may be included as metal ions, while non-metallic elements may be included as part of, for example, boric acid, phosphoric acid, nitric acid, or organic acid. Further, an iron sulfate plating solution may also contain conductivity aids such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.

As a pretreatment prior to the metal coating or plating treatment, degreasing and water washing may be performed to clean the surface of the cold-rolled steel sheet, and pickling and water washing may also be performed to activate the surface of the cold-rolled steel sheet. Following the pretreatment, the metal coating or plating process described above is performed. The methods of degreasing treatment and water washing are not particularly limited, and a conventional method may be used. Various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures of these acids may be used in the pickling. Among these, sulfuric acid, hydrochloric acid, or a mixture of sulfuric acid and hydrochloric acid are preferred. Although the acid concentration is not particularly limited, approximately 1 mass % to 20 mass % is preferable, in consideration of the ability to remove oxide coating and the prevention of rough skin (surface defects) due to excessive pickling. Further, the pickling solution may also contain, for example, a defoamer, a pickling promoter, a pickling inhibitor, and the like.

[Annealing Process]

The cold-rolled steel sheet obtained as described above (including the case of the metal coated or plated steel sheet) is annealed at an annealing temperature of 760° C. or more and 900° C. or less, for an annealing time of 20 s or more. The number of annealing cycles may be two or more, but one is preferred from the viewpoint of energy efficiency.

Annealing Temperature: 760° C. or More and 900° C. or Less

When the annealing temperature is less than 760° C., the ratio of austenite formation during heating in the two-phase region of ferrite and austenite is insufficient. Thus, an excessive increase occurs in the area ratio of ferrite after annealing, leading to a decrease in YS. Hole expansion formability is also reduced. Further, achieving a TS of 980 MPa or more becomes difficult. On the other hand, when the annealing temperature exceeds 900° C., excessive austenite grain growth occurs and a subsequent formation rate of bainitic ferrite slows down. As a result, an appropriate amount of bainitic ferrite and retained austenite area ratio are not obtained. Therefore, SMA2/SMA decreases, resulting in lower ductility and strain hardenability. Therefore, the annealing temperature is 760° C. or more and 900° C. or less. The annealing temperature is preferably 780° C. or more. The annealing temperature is more preferably more than 790° C. Further, the annealing temperature is preferably 880° C. or less. The annealing temperature is the maximum arrival temperature during the annealing process.

Annealing Time: 20 s or More

When the annealing time is less than 20 s, the ratio of austenite formation during heating in the two-phase region of ferrite and austenite is insufficient. Thus, an excessive increase occurs in the area ratio of ferrite after annealing, leading to a decrease in YS. Hole expansion formability is also reduced. Further, achieving a TS of 980 MPa or more becomes difficult. Therefore, the annealing time is 20 s or more. An upper limit of the annealing time is not particularly limited, but is preferably 900 s or less. The annealing time is the holding time in a temperature range from (annealing temperature−40° C.) or more to the annealing temperature or less. That is, the annealing time includes not only the holding time at the annealing temperature, but also the time in the temperature range from (annealing temperature−40° C.) or more to the annealing temperature or less during heating and cooling before and after reaching the annealing temperature.

Dew Point: More than −30° C.

In the method of producing the galvanized steel sheet according to an embodiment of the present disclosure, the dew point of the annealing atmosphere in the annealing process is preferably more than −30° C. The dew point being more than −30° C. means the decarburization reaction is promoted and the C concentration in the surface layer of the cold-rolled steel sheet (base steel sheet) is reduced to form a decarburized layer. The dew point of the annealing atmosphere is preferably −20° C. or more. The dew point of the annealing atmosphere is more preferably −5° C. or more. The dew point of −5° C. or more may further enhance the resistance weld crack resistance in the welded portion. Although the upper limit of the dew point is not particularly limited, the dew point is preferably 30° C. or less from the viewpoint of suitably hindering oxidation on the surface of the cold-rolled steel sheet or metal coating or plating layer and securing good adhesion when a galvanized layer is applied.

[First Cooling Process]

The cold-rolled steel sheet annealed as described above is cooled to the first cooling stop temperature of 300° C. or more and 550° C. or less.

First Cooling Stop Temperature: 300° C. or More and 550° C. or Less

When the first cooling stop temperature is less than 300° C., the area ratio of tempered martensite increases excessively and the appropriate amount of bainitic ferrite and retained austenite area ratio are not obtained. Further, untransformed austenite may decompose into pearlite and carbides during the subsequent galvanizing process. Therefore, SMA2/SMA and SMA3/SMA decrease, resulting in lower ductility and strain hardenability. On the other hand, when the first cooling stop temperature exceeds 550° C., the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite increases excessively. Further, SMA2/SMA and SMA3/SMA decrease, resulting in lower ductility and strain hardenability. Therefore, the first cooling stop temperature is 300° C. or more and 550° C. or less. The first cooling stop temperature is preferably 350° C. or more. The first cooling stop temperature is preferably 510° C. or less.

[Holding Process]

The cold-rolled steel sheet is held at the temperature range from 300° C. or more to 550° C. or less (hereinafter also referred to as the holding temperature range) for 3 s or more to 600 s or less.

Holding time in holding temperature range: 3 s to 600 s.

In the holding process, bainitic ferrite is formed and C diffusion from the formed bainitic ferrite to the untransformed austenite adjacent to the bainitic ferrite occurs. As a result, the area ratio of a defined amount of retained austenite is secured and SMA2/SMA and SMA3/SMA increase.

Here, when the holding time in the holding temperature range is less than 3 s, the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite increases excessively. Further, SMA2/SMA and SMA3/SMA decrease, resulting in lower ductility and strain hardenability. On the other hand, when the holding time in the holding temperature range exceeds 600 s, the area ratio of bainitic ferrite may increase excessively and YS may decrease. Further, excessive C diffusion from bainitic ferrite to untransformed austenite may cause an increase in SMA1/SMA and reduced hole expansion formability. Further, excessive C diffusion inside the untransformed austenite occurs, such that a state where only the untransformed austenite around the bainitic ferrite has locally high solute C content is not obtained. As a result, SMA2/SMA and SMA3/SMA may decrease and ductility may decrease. Therefore, the holding time in the holding temperature range is 3 s or more and 600 s or less. The holding time in the holding temperature range is preferably 5 s or more. The holding time in the holding temperature range is more preferably 10 s or more. The holding time in the holding temperature range is preferably less than 200 s. The holding time in the holding temperature range is more preferably less than 80 s. The holding time in the holding temperature range includes the time the cold-rolled steel sheet remains in the temperature range until reaching the first cooling stop temperature in the first cooling process, and the time the cold-rolled steel sheet remains in the temperature range until the start of galvanizing in the coating process described below (for example, the time the cold-rolled steel sheet remains in the temperature range until it is dipped into the galvanizing bath). However, the holding time in the holding temperature range does not include the time in the temperature range of the galvanized steel sheet after hot-dip galvanizing treatment in the coating process.

[Coating Process]

The cold-rolled steel sheet is then subjected to a galvanizing treatment to produce the galvanized steel sheet. Examples of the galvanizing treatment include hot-dip galvanizing treatment and galvannealing treatment. In the coating process, the relationship between the first cooling stop temperature in the first cooling process and the temperature of the galvanizing bath in the galvanizing treatment (hereinafter also referred to as coating bath temperature) must satisfy the following Formula (1):


−150° C. T0−T1≤50° C.  (1)

where T0 is the first cooling stop temperature (° C.) and T1 is the temperature of the galvanizing bath in the galvanizing treatment (° C.).

That is, from the viewpoint of securing excellent strain hardenability, the difference between the first cooling stop temperature and the coating bath temperature must be appropriately controlled, and specifically, the relationship in Formula (1) must be satisfied. On the other hand, when T0−T1 is more than 50° C. or less than −150° C., SMA2/SMA and SMA3/SMA decrease, and strain hardenability and ductility decrease. T0−T1 is preferably −120° C. or more. T0−T1 is more preferably −100° C. or more. T0−T1 is preferably 45° C. or less. T0−T1 is more preferably 40° C. or less.

Conditions other than the above are not particularly limited, and a conventional method may be used.

For example, in the case of hot-dip galvanizing treatment, after the cold-rolled steel sheet is immersed in the galvanizing bath, adjusting the coating weight by gas wiping or the like is preferable. The coating bath temperature is 440° C. or more and 500° C. or less. Further, as a galvanizing bath, there is no particular limitation as long as the composition of the galvanized layer is as described above, but, for example, a galvanizing bath having an Al content of 0.10 mass % or more and 0.23 mass % or less with the balance being Zn and inevitable impurity is preferable.

Further, in the case of galvannealing treatment, after hot-dip galvanizing treatment is performed as described above, the galvanized steel sheet is preferably heated to an alloying temperature of 450° C. or more and 600° C. or less and subjected to an alloying treatment. When the alloying temperature is less than 450° C., the Zn—Fe alloying rate becomes slow and alloying may be difficult. On the other hand, when the alloying temperature exceeds 600° C., untransformed austenite may transform to pearlite, resulting in a decrease in TS and ductility. The alloying temperature is more preferably 470° C. or more. The alloying temperature is more preferably 570° C. or less.

The coating weight for both a hot-dip galvanized steel sheet (GI) and a galvannealed steel sheet (GA) is preferably 20 g/m2 to 80 g/m2 per side. The coating weight may be adjusted by gas wiping and the like.

In addition to the holding process described above (holding temperature of the cold-rolled steel sheet prior to the coating process), an additional holding process may be performed in which the galvanized steel sheet is held at a temperature range from 300° C. or more to 550° C. or less (hereinafter also referred to as the additional holding temperature range) for 3 s or more to 600 s or less. The additional holding process is a process that has a similar effect to the holding process. Further, the additional holding process may be performed after and/or during the coating process, as long as the additional holding process is performed before the second cooling process described below. Further, when the coating process is a galvannealing treatment, the additional holding process may be performed during the coating process. In other words, the coating process may also serve as the additional holding process. When the additional holding process is performed, the total holding time for the holding process and the additional holding process is preferably 3 s or more. The total holding time for the holding process and the additional holding process is preferably 600 s or less. The total holding time for the holding process and the additional holding process is more preferably less than 200 s.

[Second Cooling Process]

The galvanized steel sheet is then cooled to the second cooling stop temperature of 100° C. or more and less than 300° C.

Second cooling stop temperature: 100° C. or more and less than 300° C.

The second cooling process is necessary to control the area ratio of tempered martensite and retained austenite generated in the subsequent reheating process within a defined range. Here, when the second cooling stop temperature is less than 100° C., almost all of the untransformed austenite in the steel is transformed to martensite in the second cooling process. This results in an excessive increase in the area ratio of tempered martensite and decrease in the area ratio of retained austenite. As a result, ductility and strain hardenability are reduced. On the other hand, when the second cooling stop temperature is 300° C. or more, the area ratio of tempered martensite decreases and that of fresh martensite increases. With the increase in the area ratio of fresh martensite, the amount of diffusible hydrogen in the steel sheet increases and the hole expansion formability decreases. Further, an increase in SMA1/SMA also reduces hole expansion formability. Therefore, the second cooling stop temperature is 100° C. or more and less than 300° C. The second cooling stop temperature is preferably 120° C. or more. The second cooling stop temperature is preferably 280° C. or less.

[Reheating Process]

The galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature+50° C.) or more and 500° C. or less, and the galvanized steel sheet is held at the temperature range of (the second cooling stop temperature+50° C.) or more and 500° C. or less (hereinafter also referred to as the reheating temperature range) for 10 s or more and 2,000 s or less.

This tempers the martensite present in the steel at the end of the second cooling process. Further, by diffusing the supersaturated solute of C in martensite into untransformed austenite, stable austenite at room temperature, that is, retained austenite, is generated.

Reheating Temperature: (the Second Cooling Stop Temperature+50° C.) or More and 500° C. or Less

When the reheating temperature is less than (the second cooling stop temperature+50° C.), C diffusion from martensite present in the steel at the end of the second cooling process to untransformed austenite does not proceed sufficiently, and the defined area ratio of retained austenite is not obtained. This reduces ductility. Further, fresh martensite increases. Further, external release of hydrogen included in the base steel sheet becomes insufficient, resulting in an increase in the amount of diffusible hydrogen in the base steel sheet. This reduces hole expansion formability. On the other hand, when the reheating temperature exceeds 500° C., tempering of martensite present in the steel at the end of the second cooling process proceeds excessively, making achieving a TS of 980 MPa or more difficult. Further, the untransformed austenite present in the steel at the end of the second cooling process decomposes as carbides (pearlite), resulting in reduced ductility. Further, external release of hydrogen included in the base steel sheet becomes insufficient, resulting in an increase in the amount of diffusible hydrogen in the base steel sheet. This reduces hole expansion formability. Therefore, the reheating temperature is (the second cooling stop temperature+50° C.) or more and 500° C. or less. The reheating temperature is preferably (the second cooling stop temperature+70° C.) or more. The reheating temperature is preferably 450° C. or less. The reheating temperature is the maximum arrival temperature in the reheating process.

Holding Time in Reheating Temperature Range: 10 s or More and 2,000 s or Less

When the holding time in the reheating temperature range is less than 10 s, C diffusion from martensite present in the steel at the end of the second cooling process to untransformed austenite does not proceed sufficiently, and the defined area ratio of retained austenite is not obtained. This reduces ductility. Further, in addition to the increase in fresh martensite, external release of hydrogen included in the base steel sheet becomes insufficient, resulting in an increase in the amount of diffusible hydrogen in the base steel sheet. This may reduce hole expansion formability. On the other hand, when the holding time in the reheating temperature range exceeds 2,000 s, tempering of martensite present in the steel at the end of the second cooling process proceeds excessively, making achieving a TS of 980 MPa or more difficult. Further, the untransformed austenite present in the steel at the end of the second cooling process decomposes as carbides (pearlite), resulting in reduced ductility. Therefore, the holding time in the reheating temperature range is 10 s or more and 2,000 s or less. The holding time in the reheating temperature range is preferably 15 s or more. The holding time in the reheating temperature range is preferably 1,200 s or less. The holding time in the reheating temperature range includes not only the holding time at the reheating temperature, but also the time in the reheating temperature range during heating and cooling before and after reaching the reheating temperature.

Cooling conditions after holding in the reheating temperature range are not particularly limited, and a conventional method may be followed. For example, gas jet cooling, mist cooling, roll cooling, water cooling, air cooling, and the like may be applied as cooling methods. From the viewpoint of preventing surface oxidation, after holding the product in the reheating temperature range, cooling down to 50° C. or less is preferable. Cooling to about room temperature is more preferable. For example, an average cooling rate of 1° C./s or more and 50° C./s or less in cooling after holding in the reheating temperature range is suitable.

The galvanized steel sheet obtained as described above may be further subjected to temper rolling. When the reduction ratio of the temper rolling exceeds 2.00%, yield stress may increase and dimensional accuracy may decrease when forming the galvanized steel sheet into a member. Therefore, the reduction ratio of the temper rolling is preferably 2.00% or less. A lower limit of the reduction ratio of the temper rolling is not particularly limited, but is preferably 0.05% or more from the viewpoint of productivity. The temper rolling may be performed on equipment that is continuous (on-line) with the annealing equipment used to perform each of the aforementioned processes, and may be performed on equipment that is discontinuous (off-line) with the annealing equipment used to perform each of the processes. The number of rolling cycles for the temper rolling may be one, two, or more. Rolling by a leveler or the like is also acceptable, as long as the same elongation rate as temper rolling is provided.

Conditions other than those described above are not particularly limited, and a conventional method may be used.

[4] Method of Producing Member

The following describes a method of producing a member according to an embodiment of the present disclosure.

The method of producing a member according to an embodiment of the present disclosure includes a process of forming or joining at least one of the galvanized steel sheet (for example, the galvanized steel sheet produced by the method of producing the galvanized steel sheet) into a member by applying at least one of a forming process and a joining process.

Here, the method of the forming process is not particularly limited, and a general processing method such as press working may be used, for example. The method of the joining process is also not particularly limited, and for example, general welding such as spot welding, laser welding, arc welding, and the like, rivet joining, swaging joining, and the like may be used. Forming and joining conditions are not particularly limited and may follow a conventional method.

EXAMPLES Example 1

Steel having the chemical composition listed in Table 1 (the balance being Fe and inevitable impurity) was melted in a converter and made into steel slabs by a continuous casting method. The obtained steel slabs were heated to 1,250° C. After heating, the steel slabs were hot rolled, consisting of rough rolling and finish rolling, to produce hot-rolled steel sheets. The hot-rolled steel sheets were then pickled and cold-rolled (rolling reduction ratio: 50%) to produce cold-rolled steel sheets having the thicknesses listed in Table 3. The obtained cold-rolled steel sheets were then subjected to the annealing process, the first cooling process, the holding process, the coating process, the second cooling process, and the reheating process under the conditions listed in Table 2 to obtain galvanized steel sheets. The dew point during the annealing process was −35° C. to −30° C.

Here, in the coating process, hot-dip galvanizing treatment or galvannealing treatment was performed to obtain hot-dip galvanized steel sheets (hereinafter also referred to as GI) or galvannealed steel sheets (hereinafter also referred to as GA). In Table 2, the type of coating process is also indicated as “GI” and “GA”. In the case of the galvannealing treatment, except for No. 20, 27, and 28, the total of the holding time in the holding temperature range and the holding time in the temperature range of 300° C. or more and 550° C. or less in the alloying treatment was 3 s or more and 600 s or less.

Further, in the case of producing GI, a galvanizing bath was used that had a composition of Al: 0.20 mass % with the balance being Zn and inevitable impurity. In the case of producing GA, a galvanizing bath was used that had a composition of Al: 0.14 mass % with the balance being Zn and inevitable impurity.

The coating weight was 45 g/m2 to 72 g/m2 per side when producing GI and 45 g/m2 per side when producing GA.

The composition of the galvanized layer of the final galvanized steel sheet in the case of GI was Fe: 0.1 mass % to 1.0 mass %, Al: 0.2 mass % to 1.0 mass %, with the balance being Zn and inevitable impurity. In the case of GA, the composition was Fe: 7 mass % to 15 mass %, Al: 0.1 mass % to 1.0 mass %, with the balance being Zn and inevitable impurity.

Galvanized layers were formed on both sides of the base steel sheet.

Using the galvanized steel sheets thus obtained, the steel microstructures of the base steel sheets were identified and the amounts of diffusible hydrogen were measured, as described above. Results are listed in Table 3. In Table 3, F is ferrite, BF is bainitic ferrite, TM is tempered martensite, RA is retained austenite, FM is fresh martensite, LB is lower bainite, P is pearlite, and θ is cementite.

Tensile strength (TS), yield stress (YS), total elongation (El), strain hardening index (n-value)/yield ratio (YR), and maximum hole expansion ratio (λ) were evaluated according to the following criteria by conducting tensile tests and hole expanding tests according to the following procedures.

    • TS
      • ∘ (Pass): 980 MPa or more
      • x (Fail): Less than 980 MPa
    • YS
      • ∘ (Pass):
      • when 980 MPa≤TS<1,180 MPa, 550 MPa≤YS
      • when 1,180 MPa≤TS<1,310 MPa, 700 MPa≤YS
      • when 1,310 MPa≤TS, 800 MPa≤YS
      • x (Fail):
      • when 980 MPa≤TS<1,180 MPa, 550 MPa>YS
      • when 1,180 MPa≤TS<1,310 MPa, 700 MPa>YS
      • when 1, 310 MPa≤TS, 800 MPa>YS
    • El
      • ∘ (Pass):
      • when 980 MPa≤TS<1,180 MPa, 13.0% El
      • when 1,180 MPa≤TS<1,310 MPa, 12.0% El
      • when 1,310 MPa≤TS, 10.0%≤El
      • x (Fail):
      • when 980 MPa≤TS<1,180 MPa, 13.0%>El
      • when 1,180 MPa≤TS<1,310 MPa, 12.0%>El
      • when 1,310 MPa≤TS, 10.0%>El
    • n value/YR
      • ∘ (Pass): n value/YR≥0.070
      • x (Fail): n value/YR<0.070
    • λ
      • ∘ (Pass): 20% or more
      • x (Fail): less than 20%

(1) Tensile Test

Tensile testing was performed in accordance with JIS Z 2241. That is, a JIS No. 5 test piece was taken from the obtained galvanized steel sheet such that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. The collected test piece was subjected to tensile testing at a crosshead velocity of 10 mm/min to measure TS, YS, El, and n value. Here, the n-value was calculated from elongation and strength at 0.4 times and 0.8 times uniform elongation (U-El). Further, the values of yield ratio YR (=YS/TS) and n value/YR were calculated from the measured YS, TS, and n value. The n value/YR represents the strain hardenability, which is a comprehensive evaluation index of the formability and anti-crash property of the steel sheet. The results are listed in Table 3.

(2) Hole Expanding Test

The hole expanding test was performed in accordance with JIS Z 2256. That is, a 100 mm×100 mm test piece was taken from the obtained galvanized steel sheet by shearing. A 10 mm diameter hole was punched through the test piece with a clearance of 12.5%. Then, a blank holding force of 9 tons (88.26 kN) was applied around the hole using a die having an inside diameter of 75 mm, and a conical punch having an apex angle of 60° was pressed into the hole to measure the diameter of the hole in the test piece at the crack initiation limit (when cracks occurred). The maximum hole expansion ratio: λ (%) was obtained by the following formula. Note that λ is an index for evaluating stretch flangeability. The results are listed in Table 3.


λ (%)={(Df−D0)/D0}×100

    • wherein
    • Df: diameter of hole in test piece at crack initiation (mm)
    • D0: diameter of hole in test piece at start (mm)

TABLE 1 Steel Chemical composition (mass %) Ceq sample ID C Si Mn P S Al N Other (%) A 0.112 1.47 2.52 0.017 0.0020 0.025 0.0029 — 0.593 B 0.115 0.91 2.46 0.010 0.0016 0.030 0.0056 — 0.563 C 0.102 1.52 2.31 0.013 0.0020 0.030 0.0022 — 0.550 D 0.114 0.88 2.39 0.015 0.0018 0.035 0.0030 — 0.549 E 0.178 1.49 2.79 0.014 0.0017 0.027 0.0038 — 0.705 F 0.196 0.89 2.82 0.009 0.0017 0.028 0.0051 — 0.703 G 0.153 1.50 2.65 0.009 0.0015 0.021 0.0044 — 0.657 H 0.166 0.89 2.73 0.011 0.0016 0.031 0.0039 — 0.658 I 0.217 1.51 2.19 0.014 0.0010 0.021 0.0034 — 0.645 J 0.133 0.91 2.72 0.017 0.0025 0.023 0.0034 — 0.624 K 0.110 0.83 2.64 0.017 0.0023 0.521 0.0050 — 0.585 L 0.225 0.03 2.85 0.017 0.0021 0.029 0.0034 — 0.701 M 0.117 3.56 1.90 0.013 0.0021 0.033 0.0028 — 0.582 N 0.391 0.57 0.79 0.017 0.0015 0.032 0.0046 — 0.546 O 0.095 0.52 3.92 0.022 0.0021 0.031 0.0043 — 0.770 P 0.112 1.22 1.97 0.014 0.0019 0.037 0.0045 — 0.491 Q 0.052 1.82 3.06 0.016 0.0019 0.028 0.0030 Ti: 0.028 0.638 R 0.098 0.43 3.02 0.021 0.0013 0.030 0.0040 Nb: 0.025 0.619 S 0.161 1.52 2.91 0.019 0.0019 0.031 0.0030 V: 0.032 0.712 T 0.390 0.91 1.17 0.009 0.0023 0.034 0.0033 Ti: 0.024, B: 0.0019 0.623 U 0.117 1.57 2.65 0.004 0.0015 0.030 0.0019 Cu: 0.18 0.624 V 0.153 1.43 1.81 0.021 0.0020 0.027 0.0039 Cr: 0.37 0.588 W 0.329 0.80 1.06 0.009 0.0026 0.036 0.0030 Ni: 0.72 0.557 X 0.116 1.43 2.95 0.005 0.0015 0.032 0.0038 Mo: 0.41 0.770 Y 0.104 1.60 3.47 0.011 0.0019 0.038 0.0033 Sb: 0.008 0.749 Z 0.186 1.36 2.74 0.008 0.0014 0.014 0.0029 Sn: 0.007 0.699 AA 0.190 1.99 2.53 0.009 0.0025 0.032 0.0037 Ta: 0.007 0.695 AB 0.189 1.72 2.63 0.008 0.0023 0.028 0.0043 W: 0.021 0.699 AC 0.170 2.97 2.40 0.014 0.0021 0.023 0.0031 Mg: 0.004 0.694 AD 0.212 1.19 2.58 0.012 0.0015 0.029 0.0037 Zn: 0.006 0.692 AE 0.162 1.62 2.92 0.005 0.0029 0.024 0.0042 Co: 0.009 0.716 AF 0.118 1.68 2.49 0.016 0.0018 0.032 0.0026 Zr: 0.002 0.603 AG 0.121 1.35 2.59 0.010 0.0019 0.034 0.0036 Ca: 0.002 0.609 AH 0.109 1.66 2.44 0.014 0.0030 0.036 0.0026 Ce: 0.001 0.585 AI 0.131 0.59 2.58 0.013 0.0021 0.028 0.0036 Se: 0.009 0.586 AJ 0.102 1.51 2.64 0.003 0.0027 0.027 0.0038 Te: 0.017 0.605 AK 0.177 1.53 2.81 0.008 0.0016 0.028 0.0040 Ge: 0.012 0.709 AL 0.298 1.01 1.94 0.010 0.0018 0.027 0.0031 As: 0.013 0.663 AM 0.165 1.57 2.93 0.019 0.0020 0.027 0.0034 Sr: 0.008 0.719 AN 0.172 1.28 2.97 0.013 0.0025 0.026 0.0035 Cs: 0.010 0.720 AO 0.157 1.38 3.05 0.027 0.0022 0.026 0.0038 Hf: 0.005 0.723 AP 0.191 1.40 2.72 0.007 0.0020 0.026 0.0040 Pb: 0.004 0.703 AQ 0.158 0.24 3.47 0.006 0.0022 0.030 0.0023 Bi: 0.003 0.746 AR 0.136 1.48 3.18 0.020 0.0014 0.024 0.0050 REM: 0.003 0.728

TABLE 2 Second First cooling Coating process cooling Annealing process process Holding Coating process Reheating process An- An- First cooling process bath Al- Second Hold- Steel nealing nealing stop temp. Holding temp. loying T0 − cooling Reheating ing sample temp. time T0 time T1 temp. T1 stop temp. temp. time No. ID (° C.) (s) (° C.) (s) (° C.) Type (° C.) (° C.) (° C.) (° C.) (s) Remarks 1 A 840 90 470 75 470 GA 530 0 180 400 50 Example 2 B 820 150 480 40 470 GA 500 10 250 420 80 Example 3 C 830 250 430 60 470 GA 540 −40 210 310 900 Example 4 D 870 100 460 70 470 GI — −10 260 320 100 Example 5 E 860 80 460 180 470 GA 520 −10 180 410 200 Example 6 F 840 80 400 20 470 GA 510 −70 230 420 40 Example 7 G 870 60 420 45 470 GI — −50 250 350 1000 Example 8 H 850 120 480 550 470 GA 490 10 180 400 60 Example 9 I 880 120 410 20 470 GA 510 −60 190 300 1100 Example 10 J 870 40 410 25 470 GI — −60 250 300 150 Example 11 K 890 70 460 40 470 GA 490 −10 230 390 90 Example 12 E 830 150 480 70 470 GA 530 10 200 340 60 Example 13 E 820 130 470 50 470 GI — 0 210 290 70 Example 14 K 720 250 400 25 470 GA 510 −70 110 410 400 Comparative Example 15 G 930 200 480 20 470 GI — 10 150 360 60 Comparative Example 16 K 780 3 430 150 470 GA 510 −40 120 380 800 Comparative Example 17 G 860 70 150 15 470 GA 550 −320 150 430 300 Comparative Example 18 G 870 140 580 10 470 GI — 110 230 330 200 Comparative Example 19 G 840 80 440 1 470 GI — −30 210 350 40 Comparative Example 20 G 870 100 450 900 470 GA 500 −20 230 350 900 Comparative Example 21 G 870 200 470 50 470 GA 530 0 30 350 180 Comparative Example 22 G 830 40 390 70 470 GI — −80 340 420 300 Comparative Example 23 C 860 120 480 60 470 GA 540 10 180 210 240 Comparative Example 24 C 870 150 480 25 470 GA 520 10 210 530 60 Comparative Example 25 C 820 300 440 20 470 GA 540 −30 260 400 2 Comparative Example 26 C 840 200 490 180 470 GA 510 20 200 410 3600 Comparative Example 27 G 890 200 470 800 470 GA 530 0 200 350 80 Comparative Example 28 I 870 110 500 750 470 GA 550 30 160 360 100 Comparative Example 29 I 830 100 480 30 470 GI — 10 190 400 60 Example 30 E 850 130 450 40 470 GA 520 −20 200 390 80 Example 31 L 820 80 480 10 470 GA 500 10 170 400 30 Comparative Example 32 M 880 100 400 50 470 GA 490 −70 220 410 60 Comparative Example 33 N 820 40 480 60 470 GA 540 10 130 390 180 Comparative Example 34 O 850 80 450 45 470 GA 530 −20 200 400 70 Comparative Example 35 P 880 100 480 30 470 GA 520 10 230 350 50 Comparative Example 36 Q 880 180 460 10 470 GA 520 −10 290 400 80 Example 37 R 860 100 440 40 470 GA 460 −30 200 390 90 Example 38 S 880 80 420 40 470 GA 550 −50 200 400 60 Example 39 T 820 200 350 5 470 GI — −120 220 380 100 Example 40 U 840 70 480 70 470 GA 530 10 200 400 1800 Example 41 V 850 90 480 45 470 GA 510 10 280 340 600 Example 42 W 790 150 320 15 470 GI — −150 150 370 200 Example 43 X 880 40 440 80 470 GA 580 −30 230 410 80 Example 44 Y 850 90 440 40 470 GA 520 −30 180 430 18 Example 45 Z 840 70 480 75 470 GI — 10 200 390 90 Example 46 AA 870 110 420 30 470 GA 510 −50 240 410 80 Example 47 AB 840 80 470 70 470 GA 540 0 160 400 60 Example 48 AC 890 100 400 75 470 GA 500 −70 160 400 180 Example 49 AD 830 90 400 30 470 GA 530 −70 220 380 200 Example 50 AE 840 120 370 15 470 GA 520 −100 250 420 50 Example 51 AF 840 90 410 1 470 GA 530 −60 200 400 80 Example 52 AG 800 130 540 10 470 GA 510 70 170 410 300 Example 53 AH 860 120 430 7 470 GA 500 −40 280 350 180 Example 54 AI 780 90 480 40 470 GA 550 10 150 400 210 Example 55 AJ 840 90 500 70 470 GA 520 30 240 420 30 Example 56 AK 870 100 490 25 470 GA 540 20 200 400 50 Example 57 AL 850 50 420 30 470 GI — −50 110 390 200 Example 58 AM 870 30 440 20 470 GA 510 −30 260 420 80 Example 59 AN 830 110 450 60 470 GA 530 −20 210 380 100 Example 60 AO 850 100 460 40 470 GI — −10 200 400 50 Example 61 AP 870 80 400 60 470 GA 500 −70 200 440 12 Example 62 AQ 770 200 430 25 470 GA 480 −40 140 410 90 Example 63 AR 860 40 430 60 470 GA 520 −40 200 400 60 Example 64 G 870 70 320 25 490 GA 530 −170 240 340 320 Comparative Example 65 G 860 120 530 70 450 GI — 80 260 360 110 Comparative Example 66 T 830 180 430 35 440 GA 520 −10 200 400 70 Example 67 W 820 160 390 20 480 GI — −90 140 390 80 Example 68 AE 830 150 420 40 500 GA 520 −80 230 380 90 Example

TABLE 3 Steel microstructure Steel Sheet Area ratio of each phase Residual sample thickness F BF TM RA FM SMA SMA1 SMA2 SMA3 micro- No. ID (mm) (%) (%) (%) (%) (%) (%) (%) (%) (%) structure 1 A 1.4 11.6 24.9 55.6 5.2 2.7 7.9 0.8 3.4 1.4 — 2 B 1.2 15.1 11.4 64.0 5.6 3.9 9.5 0.6 3.8 1.6 — 3 C 1.6 20.1 31.0 40.3 3.6 4.5 8.1 1.4 4.1 1.6 LB 4 D 1.2 1.2 20.6 69.1 3.6 5.0 8.6 0.2 6.2 2.7 LB 5 E 1.4 0.6 19.5 63.2 11.2 5.5 16.7 1.0 8.2 6.4 — 6 F 0.8 1.3 18.2 67.2 7.9 5.4 13.3 0.1 7.1 3.9 — 7 G 1.0 0.4 23.5 63.6 6.8 5.3 12.1 0.3 7.0 4.3 LB 8 H 1.2 0.5 37.5 50.4 7.3 4.3 11.6 5.1 7.3 3.1 — 9 I 1.4 4.3 21.4 60.5 7.5 4.3 11.8 0.7 6.9 3.7 LB 10 J 1.2 0.0 21.5 66.3 7.0 3.6 10.6 0.8 5.7 3.0 LB 11 K 1.6 19.0 17.9 55.0 4.6 3.5 8.1 0.5 2.9 0.7 — 12 E 1.4 8.9 10.3 67.1 8.6 4.6 13.2 0.3 5.8 2.3 LB 13 E 1.2 10.1 12.2 63.1 6.8 6.7 13.5 0.1 6.3 2.6 LB 14 K 1.6 71.1 5.2 12.8 4.2 6.7 10.9 4.2 0.7 0.1 — 15 G 1.4 0.0 2.8 91.1 2.8 3.1 5.9 0.0 0.5 0.4 LB 16 K 0.8 67.2 6.1 14.5 4.8 7.1 11.9 3.6 0.9 0.2 LB 17 G 2.0 2.9 1.1 89.6 1.9 2.4 4.3 0.0 0.2 0.2 θ, P 18 G 1.4 0.1 1.5 86.7 5.2 5.1 10.3 0.0 0.4 0.3 LB 19 G 1.2 4.2 1.0 84.4 5.4 4.9 10.3 0.1 0.2 0.1 LB 20 G 1.8 0.0 47.2 17.8 9.9 24.2 34.1 28.5 15.4 0.7 LB 21 G 1.0 0.1 8.5 90.1 0.9 0.2 1.1 0.0 0.5 0.4 LB 22 G 1.8 19.2 32.6 0.3 4.0 43.9 47.9 41.9 17.4 2.5 — 23 C 0.8 10.8 32.3 29.0 1.2 23.7 24.9 0.3 9.7 5.9 LB 24 C 1.4 5.5 12.6 67.4 1.0 4.5 5.5 0.1 1.5 0.5 θ, P 25 C 1.2 25.0 6.7 51.4 1.9 15.0 16.9 0.2 4.4 1.8 — 26 C 1.4 17.5 17.8 53.3 0.9 2.7 3.6 0.0 1.1 0.2 θ, P 27 G 1.6 2.6 38.2 36.8 4.1 18.2 22.3 19.0 17.2 1.6 LB 28 I 1.2 6.2 7.1 77.4 3.9 5.2 9.1 0.2 1.3 1.1 LB 29 I 1.4 61.2 9.9 2.2 14.2 12.5 26.7 13.8 9.1 0.2 — 30 E 1.6 4.1 20.4 61.5 9.4 4.6 14.0 0.4 7.5 4.7 — 31 L 1.4 0.2 14.9 73.3 2.1 9.5 11.6 0.1 4.8 3.3 — 32 M 1.2 50.2 7.9 31.6 6.1 4.2 10.3 7.8 2.3 0.8 — 33 N 1.6 55.0 10.7 25.3 3.9 5.1 9.0 3.7 1.9 0.5 — 34 O 1.0 0.4 2.5 82.2 7.5 7.4 14.9 0.0 1.2 1.0 — 35 P 1.8 25.1 18.5 45.7 5.8 4.5 10.3 0.1 3.2 1.6 LB 36 Q 1.2 4.5 9.7 77.1 3.1 5.6 8.7 0.8 2.9 2.1 — 37 R 2.0 9.0 21.5 59.8 5.7 4.0 9.7 0.9 3.7 2.4 — 38 S 1.6 0.4 23.2 61.4 10.1 4.9 15.0 0.7 6.8 4.3 — 39 T 0.8 17.9 6.7 57.0 11.8 6.3 18.1 0.3 4.4 1.3 LB 40 U 1.6 11.5 34.6 44.0 6.5 3.4 9.9 3.9 3.9 1.7 — 41 V 1.8 12.4 18.3 56.7 5.8 6.3 12.1 0.9 4.6 1.6 LB 42 W 1.0 17.5 6.5 66.3 5.4 4.0 9.4 0.4 2.3 0.9 LB 43 X 1.2 0.0 16.1 69.8 9.3 4.8 14.1 0.6 6.0 4.8 — 44 Y 1.2 2.2 20.4 66.0 7.2 4.1 11.3 0.7 5.6 4.5 LB 45 Z 1.4 17.4 11.2 57.4 10.5 3.5 14.0 0.5 4.1 3.3 — 46 AA 1.6 6.8 16.6 60.9 11.3 4.4 15.7 1.6 5.0 3.7 — 47 AB 1.4 13.7 12.7 61.1 8.2 4.3 12.5 0.6 3.5 1.7 — 48 AC 1.4 8.5 17.8 56.0 12.4 5.3 17.7 0.7 6.7 3.2 — 49 AD 1.2 17.4 15.5 51.7 9.9 4.9 14.8 0.4 4.2 1.8 LB 50 AE 1.6 7.6 19.2 59.5 9.0 4.7 13.7 0.6 5.8 3.4 — 51 AF 1.2 14.6 14.4 62.6 4.8 3.5 8.3 0.3 2.9 1.5 LB 52 AG 0.8 23.6 8.5 56.9 6.7 4.3 11.0 0.2 2.8 1.0 — 53 AH 1.0 6.3 15.1 65.1 7.1 5.8 12.9 0.8 5.1 3.6 LB 54 AI 1.4 33.4 6.2 48.5 8.4 3.5 11.9 0.6 2.6 0.7 — 55 AJ 1.0 12.3 8.3 68.1 6.6 4.7 11.3 0.4 3.2 1.1 — 56 AK 1.2 1.3 5.9 79.4 7.8 5.6 13.4 0.3 3.7 2.8 — 57 AL 1.4 0.5 24.7 63.2 8.1 3.5 11.6 0.7 5.3 3.6 — 58 AM 1.6 0.7 18.6 62.0 11.5 7.2 18.7 0.7 8.4 6.5 — 59 AN 1.2 14.6 10.7 60.8 9.5 4.3 13.8 0.4 4.0 2.2 LB 60 AO 1.0 2.1 19.4 63.1 10.1 5.3 15.4 0.8 5.5 3.7 — 61 AP 1.2 0.1 27.1 55.2 11.2 6.4 17.6 4.2 8.2 5.8 — 62 AQ 1.4 19.5 13.4 55.1 8.0 3.7 11.7 0.4 3.6 1.4 LB 63 AR 1.2 1.3 8.5 74.3 9.5 6.4 15.9 0.5 4.4 3.2 — 64 G 1.4 0.1 5.8 79.1 3.6 5.7 9.3 0.1 1.0 0.3 LB, θ, P 65 G 1.6 3.3 5.9 79.9 4.2 6.7 10.9 0.1 1.1 0.4 — 66 T 1.2 17.1 16.7 47.9 12.3 5.3 17.6 0.4 5.8 1.9 LB 67 W 1.4 7.3 12.5 68.0 7.4 4.8 12.2 0.3 3.7 2.1 — 68 AE 1.8 13.6 18.3 53.2 9.8 4.8 14.6 0.7 6.3 3.9 LB Steel microstructure Amount of SBF + STM + diffusible 2 × SMA SMA1/ SMA2/ SMA3/ hydrogen No. (%) SMA SMA SMA (mass ppm) Remarks 1 96.3 0.10 0.43 0.18 0.09 Example 2 94.4 0.06 0.40 0.17 0.25 Example 3 87.5 0.17 0.51 0.20 0.03 Example 4 106.9 0.02 0.72 0.31 0.08 Example 5 116.1 0.06 0.49 0.38 0.14 Example 6 112.0 0.01 0.53 0.29 0.27 Example 7 111.3 0.02 0.58 0.36 0.02 Example 8 111.1 0.44 0.63 0.27 0.13 Example 9 105.5 0.06 0.58 0.31 0.01 Example 10 109.0 0.08 0.54 0.28 0.04 Example 11 89.1 0.06 0.36 0.09 0.15 Example 12 103.8 0.02 0.44 0.17 0.19 Example 13 102.3 0.01 0.47 0.19 0.20 Example 14 39.8 0.39 0.06 0.01 0.14 Comparative Example 15 105.7 0.00 0.08 0.07 0.08 Comparative Example 16 44.4 0.30 0.08 0.02 0.05 Comparative Example 17 99.3 0.00 0.05 0.05 0.11 Comparative Example 18 108.8 0.00 0.04 0.03 0.19 Comparative Example 19 106.0 0.01 0.02 0.01 0.07 Comparative Example 20 133.2 0.84 0.45 0.02 0.41 Comparative Example 21 100.8 0.00 0.45 0.36 0.18 Comparative Example 22 128.7 0.87 0.36 0.05 0.73 Comparative Example 23 111.1 0.01 0.39 0.24 0.83 Comparative Example 24 91.0 0.02 0.27 0.09 0.75 Comparative Example 25 91.9 0.01 0.26 0.11 0.91 Comparative Example 26 78.3 0.00 0.31 0.06 0.19 Comparative Example 27 119.6 0.85 0.77 0.07 0.24 Comparative Example 28 102.7 0.02 0.14 0.12 0.11 Comparative Example 29 65.5 0.52 0.34 0.01 0.15 Example 30 109.9 0.03 0.54 0.34 0.59 Example 31 111.4 0.01 0.41 0.28 0.28 Comparative Example 32 60.1 0.76 0.22 0.08 0.22 Comparative Example 33 54.0 0.41 0.21 0.06 0.18 Comparative Example 34 114.5 0.00 0.08 0.07 0.10 Comparative Example 35 84.8 0.01 0.31 0.16 0.11 Comparative Example 36 104.2 0.09 0.33 0.24 0.15 Example 37 100.7 0.09 0.38 0.25 0.12 Example 38 114.6 0.05 0.45 0.29 0.27 Example 39 99.9 0.02 0.24 0.07 0.22 Example 40 98.4 0.39 0.39 0.17 0.18 Example 41 99.2 0.07 0.38 0.13 0.27 Example 42 91.6 0.04 0.24 0.10 0.24 Example 43 114.1 0.04 0.43 0.34 0.23 Example 44 109.0 0.06 0.50 0.40 0.34 Example 45 96.6 0.04 0.29 0.24 0.26 Example 46 108.9 0.10 0.32 0.24 0.27 Example 47 98.8 0.05 0.28 0.14 0.26 Example 48 109.2 0.04 0.38 0.18 0.20 Example 49 96.8 0.03 0.28 0.12 0.21 Example 50 106.1 0.04 0.42 0.25 0.30 Example 51 93.6 0.04 0.35 0.18 0.29 Example 52 87.4 0.02 0.25 0.09 0.24 Example 53 106.0 0.06 0.40 0.28 0.15 Example 54 78.5 0.05 0.22 0.06 0.21 Example 55 99.0 0.04 0.28 0.10 0.33 Example 56 112.1 0.02 0.28 0.21 0.26 Example 57 111.1 0.06 0.46 0.31 0.28 Example 58 118.0 0.04 0.45 0.35 0.31 Example 59 99.1 0.03 0.29 0.16 0.17 Example 60 113.3 0.05 0.36 0.24 0.30 Example 61 117.5 0.24 0.47 0.33 0.43 Example 62 91.9 0.03 0.31 0.12 0.31 Example 63 114.6 0.03 0.28 0.20 0.07 Example 64 103.5 0.01 0.11 0.03 0.02 Comparative Example 65 107.6 0.01 0.10 0.04 0.03 Comparative Example 66 99.8 0.02 0.33 0.11 0.29 Example 67 104.9 0.02 0.30 0.17 0.27 Example 68 100.7 0.05 0.43 0.27 0.30 Example

TABLE 4 Steel YS TS El λ No. sample ID (MPa) (MPa) (%) (%) YR n-value n/YR Type Remarks 1 A 844 1042 18.2 63 0.81 0.109 0.135 GA Example 2 B 781 1023 17.1 61 0.76 0.138 0.181 GA Example 3 C 712 1058 18.4 22 0.67 0.141 0.210 GA Example 4 D 927 1075 16.0 53 0.86 0.121 0.140 GI Example 5 E 938 1211 16.4 68 0.77 0.105 0.136 GA Example 6 F 935 1230 15.5 54 0.76 0.116 0.153 GA Example 7 G 974 1243 15.4 61 0.78 0.137 0.175 GI Example 8 H 813 1214 12.3 28 0.67 0.102 0.152 GA Example 9 I 952 1238 14.8 42 0.77 0.133 0.173 GA Example 10 J 881 1244 14.1 36 0.71 0.122 0.172 GI Example 11 K 802 1027 15.3 52 0.78 0.123 0.158 GA Example 12 E 967 1326 13.1 40 0.73 0.139 0.191 GA Example 13 E 942 1459 10.5 24 0.65 0.152 0.235 GI Example 14 K 516 932 14.7 15 0.55 0.151 0.273 GA Comparative Example 15 G 1155 1269 10.3 55 0.91 0.050 0.055 GI Comparative Example 16 K 573 964 12.4 8 0.59 0.174 0.293 GA Comparative Example 17 G 1041 1102 11.2 82 0.94 0.059 0.062 GA Comparative Example 18 G 1144 1275 11.3 68 0.90 0.052 0.058 GI Comparative Example 19 G 1149 1263 10.8 41 0.91 0.054 0.059 GI Comparative Example 20 G 662 1184 11.0 3 0.56 0.159 0.284 GA Comparative Example 21 G 1194 1276 10.2 48 0.94 0.049 0.052 GA Comparative Example 22 G 733 1289 12.5 1 0.57 0.167 0.294 GI Comparative Example 23 C 744 1293 9.8 13 0.58 0.135 0.235 GA Comparative Example 24 C 633 879 10.7 19 0.72 0.102 0.142 GA Comparative Example 25 C 849 1132 10.2 11 0.75 0.096 0.128 GA Comparative Example 26 C 689 942 11.2 62 0.73 0.098 0.134 GA Comparative Example 27 G 727 1252 12.2 5 0.58 0.153 0.263 GA Comparative Example 28 I 1152 1269 10.9 67 0.91 0.058 0.064 GA Comparative Example 29 I 573 1045 23.4 21 0.55 0.162 0.295 GI Example 30 E 906 1220 15.7 23 0.74 0.137 0.184 GA Example 31 L 1110 1287 10.1 62 0.86 0.088 0.102 GA Comparative Example 32 M 487 942 12.8 2 0.52 0.156 0.302 GA Comparative Example 33 N 477 918 13.4 30 0.52 0.136 0.262 GA Comparative Example 34 O 1156 1261 10.7 61 0.92 0.047 0.051 GA Comparative Example 35 P 652 914 18.3 49 0.71 0.133 0.186 GA Comparative Example 36 Q 771 1038 14.4 44 0.74 0.102 0.137 GA Example 37 R 821 1041 19.3 63 0.79 0.123 0.156 GA Example 38 S 1025 1247 14.2 60 0.82 0.138 0.168 GA Example 39 T 987 1260 12.5 38 0.78 0.145 0.185 GI Example 40 U 906 1045 16.0 32 0.87 0.129 0.149 GA Example 41 V 733 1108 16.4 42 0.66 0.140 0.212 GA Example 42 W 903 1213 13.5 55 0.74 0.126 0.169 GI Example 43 X 1025 1249 14.8 59 0.82 0.120 0.146 GA Example 44 Y 1064 1294 12.7 47 0.82 0.125 0.152 GA Example 45 Z 971 1260 15.4 43 0.77 0.143 0.186 GI Example 46 AA 1010 1236 15.6 37 0.82 0.127 0.155 GA Example 47 AB 1078 1283 14.0 49 0.84 0.138 0.164 GA Example 48 AC 969 1241 15.0 51 0.78 0.141 0.181 GA Example 49 AD 906 1242 16.1 43 0.73 0.151 0.207 GA Example 50 AE 964 1237 16.2 54 0.78 0.134 0.172 GA Example 51 AF 867 1064 17.9 77 0.81 0.120 0.147 GA Example 52 AG 632 991 15.2 40 0.64 0.132 0.207 GA Example 53 AH 809 1085 16.2 53 0.75 0.110 0.148 GA Example 54 AI 745 1092 14.0 32 0.68 0.141 0.207 GA Example 55 AJ 762 1072 14.8 51 0.71 0.134 0.189 GA Example 56 AK 1140 1253 12.7 66 0.91 0.065 0.071 GA Example 57 AL 1028 1276 14.3 58 0.81 0.136 0.169 GI Example 58 AM 1060 1239 16.2 44 0.86 0.126 0.147 GA Example 59 AN 1011 1252 14.6 49 0.81 0.141 0.175 GA Example 60 AO 1016 1268 14.0 58 0.80 0.129 0.161 GI Example 61 AP 931 1223 16.8 29 0.76 0.144 0.189 GA Example 62 AQ 753 1250 14.4 35 0.60 0.133 0.221 GA Example 63 AR 1124 1277 13.4 46 0.88 0.081 0.092 GA Example 64 G 1072 1191 11.1 71 0.90 0.060 0.067 GA Comparative Example 65 G 1092 1237 10.7 77 0.88 0.057 0.065 GI Comparative Example 66 T 908 1242 14.9 40 0.73 0.139 0.190 GA Example 67 W 1003 1239 14.5 58 0.81 0.127 0.157 GI Example 68 AE 931 1241 16.9 47 0.75 0.141 0.188 GA Example

As indicated in Table 3, tensile strength (TS), yield stress (YS), total elongation (El), strain hardening index (n value)/yield ratio (YR), and maximum hole expansion ratio (λ) were all judged to pass for all of the Examples.

In contrast, at least one of the following was not sufficient in the Comparative Examples: tensile strength (TS), yield stress (YS), total elongation (El), strain hardening index (n value)/yield ratio (YR), and maximum hole expansion ratio (λ).

Further, it was found that members obtained by a forming process or joining process using the steel sheet of the present disclosure have excellent properties in terms of tensile strength (TS), yield stress (YS), total elongation (El), strain hardening index (n value)/yield ratio (YR), and maximum hole expansion ratio (λ).

Example 2

Steel having the chemical composition listed in Table 1 (the balance being Fe and inevitable impurity) was melted in a converter and made into steel slabs by a continuous casting method. The obtained steel slabs were heated to 1,250° C. After heating, the steel slabs were hot rolled, consisting of rough rolling and finish rolling, to produce hot-rolled steel sheets. The hot-rolled steel sheets were then picked and cold rolled (rolling reduction ratio: 50%) to produce cold-rolled steel sheets having a thickness of 1.6 mm.

Then, among the cold-rolled steel sheets obtained, No. 8 to No. 10 were subjected to Fe-based electroplating as a metal plating process to form a metal plating layer (Fe-based plating layer) on the surfaces of the cold-rolled steel sheet. Specifically, the cold-rolled steel sheets were first degreased with alkali. The cold-rolled steel sheets were then subjected to electrolytic treatment using the cold-rolled steel sheet as the cathode under the following conditions to form a metal plating layer on the surfaces of the cold-rolled steel sheets.

[Electrolytic Conditions]

    • Bath temperature: 50° C.
    • pH: 2.0
    • Current density: 45 A/dm2
    • Plating bath: a sulfuric acid bath containing 1.5 mol/L of Fe2+ ions
    • Anode: iridium oxide electrode

The coating weight of the metal plating layer was controlled by the energizing time.

The obtained cold-rolled steel sheets (including metal plated steel sheets having a metal plating layer formed on the surfaces of the cold-rolled steel sheets) were then subjected to the annealing process, the first cooling process, the holding process, the coating process, the second cooling process, and the reheating process under the conditions listed in Table 5 to obtain galvanized steel sheets.

In the coating process, galvannealing treatment was applied to obtain galvannealed steel sheets (GA). Treatment conditions other than those listed in Table 5 were the same as in Example 1. Galvanized layers were formed on both sides of the base steel sheet.

Using the galvanized steel sheets thus obtained, the steel microstructures of the base steel sheets were identified, and the thicknesses of the decarburization layer, the coating weights of the metal plating layers, and the amount of diffusible hydrogen were measured, as described above. Results are listed in Table 6. In Table 6, F is ferrite, BF is bainitic ferrite, TM is tempered martensite, RA is retained austenite, FM is fresh martensite, LB is lower bainite, P is pearlite, and θ is cementite. In Table 6, “-” for decarburization layer thickness and metal plating layer coating weight means no decarburization layer and no metal plating layer, respectively.

Tensile strength (TS), yield stress (YS), total elongation (El), strain hardening index (n value)/yield ratio (YR), and maximum hole expansion ratio (λ) were evaluated by conducting tensile tests and hole expanding tests using the same criteria as in Example 1. The results are listed in Table 7.

Further, resistance weld crack resistance of the welded portions was evaluated according to the following procedure.

<Evaluation of Resistance Weld Crack Resistance of Welded Portions>

As a sheet combination, a test piece 2 that was cut from the obtained galvanized steel sheet into 150 mm (long direction)×50 mm (short direction) with the transverse direction (direction orthogonal to the rolling direction) (TD) as the long direction and the rolling direction as the short direction, was overlapped with overlapped with a test galvannealed steel sheet 1 (thickness: 1.6 mm, TS: 980 MPa grade). The test galvannealed steel sheet 1 had a coating weight of 50 g/m2 per side of the galvannealed layer and was cut to the same size as the test piece 2. The sheet combination was assembled so that the surface to be evaluated of the test piece 2 (in a case where the galvanized layer and the metal plating layer is on only one side, the galvanized layer on the one side) and the galvanized layer of the test galvannealed steel sheet 1 faced each other. The sheet combination was fixed to a fixing stand 4 via spacers 3 having a thickness of 2.0 mm. The spacers 3 were a pair of steel sheets, each measuring 50 mm (long direction)×45 mm (short direction)×2.0 mm thick. As illustrated in FIG. 5A, the long direction end faces of the pair of steel sheets are aligned with the short direction end faces of the sheet combination. Thus, the distance between the pair of steel sheets was 60 mm. The fixing stand 4 was a single plate with a hole in the center.

Then, using a single-phase AC (50 Hz) resistance welding machine of servomotor pressure type, the sheet combination was subjected to resistance spot welding at a welding current that resulted in a nugget diameter r of 5.9 mm while being deflected by applying pressure with a pair of electrodes 5 (tip diameter: 6 mm) under the conditions of an electrode force of 3.5 kN, a holding time of 0.12 s, 0.18 s, or 0.24 s, and a welding time of 0.36 s, to form a sheet combination with a welded portion. The pair of electrodes 5 applied pressure to the sheet combination from above and below in the vertical direction, with the lower electrode applying pressure to the test piece 2 through the hole in the fixing stand 4. In applying pressure, the lower electrode of the pair of electrodes 5 and the fixing stand 4 were fixed so that the lower electrode was in contact with a plane that was an extension of a plane where the spacers 3 were in contact with the fixing stand 4, and the upper electrode was movable. Further, the upper electrode was in contact with the center of the test galvannealed steel sheet 1. The sheet combination was welded with an inclination of 5° in the long direction of the sheet combination with respect to the horizontal direction. The holding time refers to the time between the end of passage of the welding current and the beginning of electrode release. As illustrated in the lower part of FIG. 5B, the nugget diameter r means the distance between the ends of a nugget 6 in the long direction of the sheet combination.

Then, each sheet combination with a welded portion was cut along the A-A line indicated in the upper part of FIG. 5B to include the center of the welded portion including the nugget 6, and the cross-section of the welded portion was observed under an optical microscope (200×) to evaluate the resistance weld crack resistance at the welded portion using the following criteria. When the result was A+, A, or B, the sheet combination was judged to have satisfactory resistance weld crack resistance at the welded portion. When the result was C, the sheet combination was judged to have poor resistance weld crack resistance at the welded portion. The results are listed in Table 7.

    • A+: no cracks of length 0.1 mm or more were observed at holding times of 0.12 s, 0.18 s, and 0.24 s.
    • A: cracks of length 0.1 mm or more were observed at the holding time of 0.12 s, but no cracks of length 0.1 mm or more were observed at the holding times of 0.18 s and 0.24 s.
    • B: cracks of length 0.1 mm or more were observed at the holding times of 0.12 s and 0.18 s, but no cracks of length 0.1 mm or more were observed at the holding time of 0.24 seconds.
    • C: cracks of length 0.1 mm or more were observed at the holding times of 0.12 s, 0.18 s, and 0.24 s.

A crack in the test piece 2 is schematically illustrated in the lower part of FIG. 5B, as indicated by reference numeral 7. When a crack forms in the counterpart steel sheet (test galvannealed steel sheet), the stress in the steel sheet to be evaluated (any of the steel sheets in the Examples and Comparative Examples) is distributed, and an appropriate evaluation is not obtained. For this reason, the data in which no cracking occurred in the counterpart steel sheet was used as Examples.

TABLE 5 Second First cooling Coating process cooling Annealing process process Holding Coating process An- An- First cooling process bath Second Reheating process Steel nealing nealing Dew stop temp. Holding temp. Alloying T0 − cooling Reheating Holding sample temp. time point T0 time T1 temp. T1 stop temp. temp. time No. ID (° C.) (s) (° C.) (° C.) (s) (° C.) Type (° C.) (° C.) (° C.) (° C.) (s) Remarks 1 B 820 150 −32 480 40 490 GA 500 −10 250 420 80 Example 2 C 830 250 −31 430 60 460 GA 540 −30 210 310 900 Example 3 G 870 60 −35 420 45 470 GA 510 −50 250 350 1000 Example 4 I 880 120 −32 410 20 470 GA 510 −60 190 300 1100 Example 5 AA 870 110 −33 420 30 450 GA 510 −30 240 410 80 Example 6 G 870 60 7 420 45 470 GA 510 −50 250 350 1000 Example 7 I 880 120 16 410 20 480 GA 510 −70 190 300 1100 Example 8 B 820 150 −26 480 40 460 GA 490 20 250 420 900 Example 9 C 830 250 6 430 60 460 GA 500 −30 210 310 900 Example 10 AA 870 110 18 420 30 470 GA 490 −50 240 410 80 Example

TABLE 6 Steel microstructure Steel Sheet Area ratio of each phase Residual SBF + STM + sample thickness F BF TM RA FM SMA SMA1 SMA2 SMA3 micro- 2 × SMA No. ID (mm) (%) (%) (%) (%) (%) (%) (%) (%) (%) structure (%) 1 B 1.6 14.5 11.1 65.6 5.2 3.6 8.8 0.5 3.6 1.7 — 94.3 2 C 1.6 19.5 32.4 39.4 3.7 4.4 8.1 1.5 4.0 1.6 LB 88.0 3 G 1.6 0.4 22.5 65.1 6.5 4.9 11.4 0.3 7.1 4.4 LB 110.4 4 I 1.6 2.8 23.3 59.4 8.0 4.6 12.6 0.5 6.9 3.9 LB 107.9 5 AA 1.6 8.2 14.8 61.2 11.0 4.8 15.8 1.5 4.9 3.5 — 107.6 6 G 1.6 0.6 22.4 64.9 6.6 5.0 11.6 0.3 6.7 4.3 LB 110.5 7 I 1.6 5.7 23.2 56.5 7.8 4.6 12.4 0.8 7.0 3.6 LB 104.5 8 B 1.6 15.9 11.0 63.1 5.9 4.1 10.0 0.7 3.6 1.5 — 94.1 9 C 1.6 19.6 31.8 39.3 4.1 4.9 9.0 1.6 4.0 1.6 LB 89.1 10 AA 1.6 6.1 15.0 62.9 11.8 4.2 16.0 1.6 5.2 3.7 — 109.9 Metal Amount of Steel microstructure Decarburized plating layer diffusible SMA1/ SMA2/ SMA3/ layer thickness coating weight hydrogen No. SMA SMA SMA (μm) (g/m2) (mass ppm) Remarks 1 0.06 0.41 0.19 — — 0.24 Example 2 0.19 0.49 0.20 — — 0.03 Example 3 0.03 0.62 0.39 — — 0.02 Example 4 0.04 0.55 0.31 — — 0.02 Example 5 0.09 0.31 0.22 — — 0.26 Example 6 0.03 0.58 0.37 38 — 0.02 Example 7 0.06 0.56 0.29 52 — 0.01 Example 8 0.07 0.36 0.15 10 17.6 0.25 Example 9 0.18 0.44 0.18 42 6.8 0.03 Example 10 0.10 0.33 0.23 69 13.4 0.27 Example

TABLE 7 Steel Resistance sample YS TS El λ weld crack No. ID (MPa) (MPa) (%) (%) YR n-value n/YR resistance Type Remarks 1 B 802 1038 18.7 60 0.77 0.135 0.175 A GA Example 2 C 713 1054 18.3 23 0.68 0.142 0.210 B GA Example 3 G 972 1256 14.8 57 0.77 0.137 0.177 B GA Example 4 I 966 1231 13.7 44 0.78 0.131 0.167 B GA Example 5 AA 1003 1230 16.2 37 0.82 0.129 0.158 B GA Example 6 G 958 1253 16.2 41 0.76 0.138 0.180 A GA Example 7 I 967 1239 15.7 65 0.78 0.132 0.169 A GA Example 8 B 771 1014 17.4 60 0.76 0.139 0.183 A+ GA Example 9 C 717 1068 18.9 25 0.67 0.143 0.213 A+ GA Example 10 AA 993 1216 16.7 46 0.82 0.129 0.158 A GA Example

As indicated in Table 7, tensile strength (TS), yield stress (YS), total elongation (El), strain hardening index (n value)/yield ratio (YR), and maximum hole expansion ratio (λ) were all judged to pass for all of the Examples. Resistance weld crack resistance at the welded portion was also excellent.

Further, No. 1 and No. 6 to No. 10, in particular No. 8 and No. 9, had excellent resistance weld crack resistance at the welded portion.

Further, it was found that steel sheets of the present disclosure have excellent properties in terms of tensile strength (TS), yield stress (YS), total elongation (El), strain hardening index (n value)/yield ratio (YR), maximum hole expansion ratio (λ), and resistance weld crack resistance at the welded portion.

REFERENCE SIGNS LIST

    • 1 Test galvannealed steel sheet
    • 2 Test piece
    • 3 Spacers
    • 4 Fixing stand
    • Electrodes
    • 6 Nugget
    • 7 Crack

Claims

1. A galvanized steel sheet comprising a base steel sheet and a galvanized layer on a surface of the base steel sheet, wherein

the base steel sheet comprises:
a chemical composition containing, in mass %,
C: 0.050% or more and 0.400% or less,
Si: 0.20% or more and 3.00% or less,
Mn: 1.00% or more and less than 3.50%,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
Al: 0.010% or more and 2.000% or less,
N: 0.0100% or less,
optionally at least one selected from the group consisting of
Ti: 0.200% or less,
Nb: 0.200% or less,
V: 0.100% or less,
B: 0.0100% or less,
Cu: 1.000% or less,
Cr: 1.000% or less,
Ni: 1.000% or less,
Mo: 0.500% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Mg: 0.0200% or less,
Zn: 0.0200% or less,
Co: 0.0200% or less,
Zr: 0.0200% or less,
Ca: 0.0200% or less,
Ce: 0.0200% or less,
Se: 0.0200% or less,
Te: 0.0200% or less,
Ge: 0.0200% or less,
As: 0.0200% or less,
Sr: 0.0200% or less,
Cs: 0.0200% or less,
Hf: 0.0200% or less,
Pb: 0.0200% or less,
Bi: 0.0200% or less, and
REM: 0.0200% or less,
where carbon equivalent Ceq is 0.540% or more, and the balance is Fe and inevitable impurity,
further, the base steel sheet has a microstructure wherein
area ratio of ferrite: 65.0% or less (including 0%),
area ratio of bainitic ferrite: 5.0% or more and 40.0% or less,
area ratio of tempered martensite: 0.5% or more and 80.0% or less,
area ratio of retained austenite: 3.0% or more,
area ratio of fresh martensite: 20.0% or less (including 0%),
SBF+STM+2×SMA: 65.0% or more,
SMA1/SMA: 0.80 or less, and
SMA2/SMA: 0.20 or more, and
tensile strength is 980 MPa or more,
wherein
SBF is area ratio of the bainitic ferrite,
STM is area ratio of the tempered martensite,
SMA is area ratio of a hard secondary phase consisting of the retained austenite and fresh martensite,
SMA1 is a total area ratio of island regions among island regions constituting the hard secondary phase that have an equivalent circular diameter of 2.0 μm or more where 20% or less of the circumference is in contact with tempered martensite, and
SMA2 is a total area ratio of island regions among island regions constituting the hard secondary phase where 1% or more of the circumference is in contact with bainitic ferrite.

2. (canceled)

3. The galvanized steel sheet of claim 1, wherein SMA3/SMA is 0.05 or more in the microstructure of the base steel sheet,

wherein
SMA3 is a total area ratio of island regions among island regions constituting the hard secondary phase where 1% or more of the circumference is in contact with bainitic ferrite and more than 20% of the circumference is in contact with tempered martensite.

4. The galvanized steel sheet of claim 1, wherein the galvanized steel sheet satisfies at least one selected from following (a), (b), (c), (d) and (e),

(a) an amount of diffusible hydrogen in the base steel sheet being 0.50 mass ppm or less,
(b) the galvanized steel sheet further comprising a decarburized layer,
(c) the galvanized steel sheet further comprising a metal coating or plating layer at least on one side between the base steel sheet and the galvanized layer,
(d) the galvanized steel sheet further comprising a metal coating or plating layer at least on one side between the base steel sheet and the galvanized layer, the metal coating or plating layer being an Fe-based coating or plating layer, and
(e) the galvanized layer being a hot-dip galvanized layer or a galvannealed layer.

5-8. (canceled)

9. A member made using the galvanized steel sheet of claim 1.

10. A method of producing a galvanized steel sheet,

the method comprising:
a hot rolling process, in which a steel slab having the chemical composition of claim 1 is hot rolled to make a hot-rolled steel sheet;
a cold rolling process, in which the hot-rolled steel sheet is cold rolled into a cold-rolled steel sheet;
an annealing process, in which the cold-rolled steel sheet is annealed at an annealing temperature of 760° C. or more and 900° C. or less for an annealing time of 20 s or more;
a first cooling process, in which the cold-rolled steel sheet is cooled to a first cooling stop temperature of 300° C. or more and 550° C. or less;
a holding process, in which the cold-rolled steel sheet is held at a temperature range of 300° C. or more and 550° C. or less for 3 s or more and 600 s or less;
a coating process, in which the cold-rolled steel sheet is subject to a galvanizing treatment to become the galvanized steel sheet;
a second cooling process, in which the galvanized steel sheet is cooled to a second cooling stop temperature of 100° C. or more and less than 300° C.; and
a reheating process, in which the galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature+50° C.) or more and 500° C. or less, and the galvanized steel sheet is held at a temperature range of (the second cooling stop temperature+50° C.) or more and 500° C. or less for 10 s or more and 2,000 s or less, wherein
a relationship between the first cooling stop temperature and a temperature of a galvanizing bath in the galvanizing treatment satisfies the following Formula (1), −150° C. T0−T1≤50° C.  (1)
where T0 is the first cooling stop temperature (° C.) and T1 is the temperature of the galvanizing bath in the galvanizing treatment (° C.).

11. The method of producing a galvanized steel sheet of claim 10, wherein the method satisfies at least one selected from following (f), (g), (h) and (i),

(f) a dew point of the annealing process being more than −30° C.,
(g) the method further comprising a metal coating or plating treatment process after the cold rolling process and before the annealing process, in which a metal coating or plating layer is formed on at least one surface of the cold-rolled steel sheet,
(h) the method further comprising a metal coating or plating treatment process after the cold rolling process and before the annealing process, in which a metal coating or plating layer is formed on at least one surface of the cold-rolled steel sheet and the metal coating or plating layer is an Fe-based coating or plating layer, and
(i) the galvanizing treatment being a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.

12-14. (canceled)

15. A method of producing a member, wherein the galvanized steel sheet of claim 1 is subjected to at least one of a forming process and a joining process to make the member.

16. The galvanized steel sheet of claim 3, wherein the galvanized steel sheet satisfies at least one selected from following (a), (b), (c), (d) and (e),

(a) an amount of diffusible hydrogen in the base steel sheet being 0.50 mass ppm or less,
(b) the galvanized steel sheet further comprising a decarburized layer,
(c) the galvanized steel sheet further comprising a metal coating or plating layer at least on one side between the base steel sheet and the galvanized layer,
(d) the galvanized steel sheet further comprising a metal coating or plating layer at least on one side between the base steel sheet and the galvanized layer, the metal coating or plating layer being an Fe-based coating or plating layer, and
(e) the galvanized layer being a hot-dip galvanized layer or a galvannealed layer.

17. A member made using the galvanized steel sheet of claim 3.

18. A member made using the galvanized steel sheet of claim 4.

19. A member made using the galvanized steel sheet of claim 16.

20. A method of producing a member, wherein the galvanized steel sheet of claim 3 is subjected to at least one of a forming process and a joining process to make the member.

21. A method of producing a member, wherein the galvanized steel sheet of claim 4 is subjected to at least one of a forming process and a joining process to make the member.

22. A method of producing a member, wherein the galvanized steel sheet of claim 16 is subjected to at least one of a forming process and a joining process to make the member.

Patent History
Publication number: 20240124964
Type: Application
Filed: Mar 18, 2022
Publication Date: Apr 18, 2024
Applicant: JFE STEEL CORPORATION (Chiyoda-ku, Tokyo)
Inventors: Sho HIGUCHI (Chiyoda-ku, Tokyo), Yoshiyasu KAWASAKI (Chiyoda-ku, Tokyo), Tatsuya NAKAGAITO (Chiyoda-ku, Tokyo), Tomomi KANAZAWA (Chiyoda-ku, Tokyo), Shunsuke YAMAMOTO (Chiyoda-ku, Tokyo)
Application Number: 18/546,428
Classifications
International Classification: C23C 2/06 (20060101); C21D 8/02 (20060101); C21D 9/46 (20060101); C22C 38/00 (20060101); C22C 38/02 (20060101); C22C 38/06 (20060101); C23C 2/40 (20060101);