HIGH-STRENGTH STEEL SHEET AND METHOD FOR PRODUCING THE SAME

Provided are a high-strength steel sheet and a method for producing the same. A high-strength steel sheet has a composition containing 0.10% to 0.18% C, more than 0.5% to 1.5% Si, 0.5% to 1.5% Mn, 0.05% or less P, 0.005% or less S, and 0.05% or less Al on a mass basis, the remainder being Fe and inevitable impurities and also has a microstructure containing ferrite and pearlite. The volume fraction of the ferrite is 70% to 97%. The volume fraction of the pearlite is 3% or more. The volume fraction of cementite present at grain boundaries of the ferrite is 2% or less. The sum of the volume fractions of phases other than the ferrite, the pearlite, and the cementite is less than 3%. The average grain size of the ferrite is 7 μm or less.

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Description
CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2012/007663, filed Nov. 29, 2012, which claims priority to Japanese Patent Application No. 2011-276997, filed Dec. 19, 2011, the disclosures of each of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

Aspects of the present invention relate to high-strength steel sheets, having excellent formability, applicable to automotive parts and particularly relates to a high-strength steel sheet having a tensile strength TS of 600 MPa to 700 MPa, an elongation El of 25% or more (in the case of a JIS #5 test specimen with a thickness of 1.6 mm), and a hole expansion ratio λ of 80% or more, the hole expansion ratio λ being an indicator for stretch flangeability, and a method for producing the same.

BACKGROUND OF THE INVENTION

In recent years, the improvement of automotive fuel efficiency by automotive weight reduction has become an important issue from the viewpoint of environmental conservation. Therefore, gauge reduction and weight reduction have been investigated by increasing the strength of steel sheets which are materials of automotive parts. However, the increase in strength of steel sheets generally causes the reduction in ductility thereof; hence, high-strength steel sheets having both high strength and good formability are strongly needed.

Hitherto, several proposals have been made for high-strength steel sheets having excellent formability.

Patent Literature 1 discloses a high-strength steel sheet, having an excellent strength-hole expansion ratio balance and excellent shape fixability, for forming. The steel sheet contains 0.02% to 0.16% C, 0.010% or less P, 0.003% or more S, 0.2% to 4% one or both of Si and Al in total, and 0.5% to 4% one or more of Mn, Ni, Cr, Mo, and Cu in total as chemical components on a mass basis, the remainder being Fe and inevitable impurities, C/(Si+Al+P) being 0.1 or less. The cross-sectional microstructure of the steel sheet contains one or both of martensite and retained austenite, the sum of the area fraction of martensite and the area fraction of retained austenite being less than 3%, and one or both of ferrite and bainite, the sum of the area fraction of ferrite and the area fraction of bainite being 80% or more, the remainder being pearlite. The maximum length of pearlite, martensite, and retained austenite is 10 microns or less. The number of inclusions, having a size of 20 microns or more, present in a cross section of the steel sheet is 0.3 or less per square millimeter.

Patent Literature 2 discloses a hot-rolled steel containing 0.05% to less than 0.15% C, 0.8% to 1.2% Mn, 0.02% to 2.0% Si, 0.002% to less than 0.05% sol. Al, and 0.001%% to less than 0.005% N on a mass basis, the remainder being Fe and impurities. Each of Ti, Nb, and V in the impurities is less than 0.005%. The hot-rolled steel has a microstructure containing ferrite with an average grain size of 1.1 μm to 5.0 μm as a primary phase and one or both of pearlite and cementite as a secondary phase and satisfies the inequality Mnθ/Mnα≦1, where Mnθ is the content of Mn in cementite in pearlite containing cementite and Mnα is the content of Mn in ferrite.

Patent Literature 3 discloses a method for producing a hot-rolled steel sheet in which the structural fraction of cementite with an equivalent circle radius of 0.1 μm or more is 0.1% or less and/or the structural fraction of martensite is 5% or less and which has a tensile strength of 50 kgf/mm2 or more, stretch flangeability corresponding to a hole expansion ratio of 1.8 or more, and excellent ductility.

The hot-rolled steel sheet is obtained in such a way that steel containing 0.07% to 0.18% C, 0.5% to 1.0% Si, 0.7% to 1.5% Mn, 0.02% or less P, 0.005% or less S, 0.0005% to 0.0050% Ca, and 0.01% to 0.10% Al on a weight basis, the remainder being Fe and inevitable impurities, is formed into a slab; the slab is heated to 1,000° C. to 1,200° C. and is hot-rolled; finish rolling is completed at a temperature of (Ar3 transformation temperature+60)° C. to 950° C.; cooling is performed at a rate of 50° C./s or more within 3 seconds from the completion of finish rolling; quenching is completed within the range of not lower than (T−70)° C. to not higher than a temperature (T° C.) calculated by the equation T=660−450×[% C]+40×[% Si]−60×[% Mn]+470×[% P]; air-cooling is performed; and coiling is then performed at higher than 350° C. to 500° C.

PTL 1: Japanese Unexamined Patent Application Publication No. 2004-68095

PTL 2: Japanese Unexamined Patent Application Publication No. 2004-137564

PTL 3: Japanese Unexamined Patent Application Publication No. 4-88125

SUMMARY OF THE INVENTION

However, in the high-strength steel sheet disclosed in Patent Literature 1 and the hot-rolled steel disclosed in Patent Literature 2, a TS of 600 MPa to 700 MPa is not achieved. In the high-strength hot-rolled steel sheet, an El of 25% or more is not achieved when the thickness thereof is 1.6 mm.

It is an object of aspects of the present invention to provide a high-strength steel sheet, excellent in formability, having a TS of 600 MPa to 700 MPa, an El of 25% or more (in the case of a JIS #5 test specimen with a thickness of 1.6 mm), and a λ of 80% or more and a method for producing the same.

SOLUTION TO PROBLEM

The inventors have investigated a high-strength steel sheet targeted as described above and have found that it is effective to form a microstructure which contains ferrite and pearlite and in which the volume fraction of ferrite is 70% to 97%, the volume fraction of pearlite is 3% or more, the volume fraction of cementite present at ferrite grain boundaries is 2% or less, the sum of the volume fractions of the other phases is less than 3% or less, and the average grain size of ferrite is 7 μm or less.

Aspects of the present invention have been made on the basis of this finding and provides a high-strength steel sheet having a composition containing 0.10% to 0.18% C, more than 0.5% to 1.5% Si, 0.5% to 1.5% Mn, 0.05% or less P, 0.005% or less S, and 0.05% or less Al on a mass basis, the remainder being Fe and inevitable impurities, and also having a microstructure containing ferrite and pearlite. The volume fraction of the ferrite is 70% to 97%. The volume fraction of the pearlite is 3% or more. The volume fraction of cementite present at grain boundaries of the ferrite is 2% or less. The sum of the volume fractions of phases other than the ferrite, the pearlite, and the cementite is less than 3%. The average grain size of the ferrite is 7 μm or less.

The high-strength steel sheet according to aspects of the present invention preferably further contains at least one selected from the group consisting of 0.01% to 1.0% Cr, 0.01% to 0.1% Ti, and 0.01% to 0.1% V on a mass basis.

The high-strength steel sheet according to aspects of the present invention preferably has a tensile strength TS of 600 MPa to 700 MPa. The high-strength steel sheet according to aspects of the present invention preferably has a hole expansion ratio λ of 80% or more. In the high-strength steel sheet according to aspects of the present invention, the volume fraction of the ferrite is preferably 80% to 95%.

In the high-strength steel sheet according to aspects of the present invention, the volume fraction of the pearlite is preferably 3% to 30%. In the high-strength steel sheet according to aspects of the present invention, the volume fraction of the pearlite is preferably 5% to 28%.

A method for producing a high-strength steel sheet according to aspects of the present invention includes a step of preparing a steel slab having a chemical composition containing 0.10% to 0.18% C, more than 0.5% to 1.5% Si, 0.5% to 1.5% Mn, 0.05% or less P, 0.005% or less S, and 0.05% or less Al on a mass basis, the remainder being Fe and inevitable impurities; a step of hot-rolling the steel slab into a hot-rolled sheet; and a step of annealing the hot-rolled sheet in such a way that the hot-rolled sheet is heated to a two-phase temperature range between the Ac1 transformation temperature and the Ac3 transformation temperature, is cooled to a temperature range of 450° C. to 600° C. at an average cooling rate of 5° C./s to 30° C./s, and is then held at this temperature range for 100 s or more.

The steel slab preferably further contains at least one selected from the group consisting of 0.01% to 1.0% Cr, 0.01% to 0.1% Ti, and 0.01% to 0.1% V on a mass basis.

The annealing step preferably includes heating to the two-phase temperature range between the Ac1 transformation temperature and the Ac3 transformation temperature, cooling to a temperature range of 450° C. to 600° C. at an average cooling rate of 10° C./s to 20° C./s, and then holding at this temperature range for 100 s to 300 s.

According to aspects of the present invention, a high-strength steel sheet, excellent in formability, having a TS of 600 MPa to 700 MPa, an El of 25% or more, and a λ of 80% or more can be produced.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Reasons for limiting a high-strength steel sheet according to aspects of the present invention and a method for producing the same are described below in detail.

(1) Composition

The unit “%” for the content of an element component hereinafter refers to mass percent.

C: 0.10% to 0.18%

C forms a secondary phase such as pearlite, microstructure, or cementite to contribute to increasing the strength of the steel sheet. In order to achieve a TS of 600 MPa or more, the content of C needs to be 0.10% or more. However, when the C content is more than 0.18%, the amount of the secondary phase is too large; hence, TS exceeds 700 MPa or El or λ is reduced. Therefore, the C content is 0.10% to 0.18%. The C content is preferably 0.12% to 0.16%.

Si: more than 0.5% to 1.5%

Si is an element contributing to solid solution hardening. In order to achieve a TS of 600 MPa or more, the content of Si needs to be more than 0.5%. However, when the Si content is more than 1.5%, surface properties of the steel sheet are impaired by scaling. Therefore, the Si content is more than 0.5% to 1.5%. The Si content is preferably 0.7% to 1.2%.

Mn: 0.5% to 1.5%

Mn is an element contributing to solid solution hardening. In order to achieve a TS of 600 MPa or more, the content of Mn needs to be 0.5% or more. However, when the Mn content is more than 1.5%, TS exceeds 700 MPa or a reduction in λ is caused by segregation. Therefore, the Mn content is 0.5% to 1.5%. The Mn content is preferably 1.1% to 1.5%.

P: 0.05% or Less

P is an element contributing to solid solution hardening. However, when the content of P is more than 0.05%, a reduction in El is caused by segregation. Therefore, the P content is 0.05% or less. The P content is preferably 0.03% or less.

S: 0.005% or Less

When the content of S is more than 0.005%, S segregates at prior-austenite grain boundaries or Mn precipitates in the steel sheet to cause a reduction in λ. Therefore, the S content is 0.005% or less and is preferably low.

Al: 0.05% or Less

Al is added to steel as a deoxidizer and is an element effective in enhancing the cleanliness of steel. However, when the content of Al is more than 0.05%, a large number of inclusions are caused, thereby causing surface defects of the steel sheet. Therefore, the Al content is 0.05% or less. The Al content is preferably 0.03% or less.

The remainder is Fe and inevitable impurities. At least one selected from the group consisting of 0.01% to 1.0% Cr, 0.01% to 0.1% Ti, and 0.01% to 0.1% V may be contained. This is because Cr, Ti, and V have a function of suppressing the recrystallization and recovery of austenite in a hot-rolling temperature range, promoting the grain refining of ferrite, forming a carbide, or strengthening ferrite in a solid solution state. Incidentally, Nb is an element for achieving a similar effect. The addition of these elements does not significantly reduce the elongation (El) as compared to the addition of the same amount of Nb. It is preferred that C is 0.02% to 0.5%, Ti is 0.02% to 0.05%, and V is 0.02% to 0.05%.

Incidentally, the inevitable impurities are, for example, O, which is 0.003% or less, Cu, Ni, Sn, and Sb, which are 0.05% or less.

(2) Microstructure

In order to increase the strength and formability of the steel sheet, a microstructure containing ferrite and pearlite is formed.

Volume fraction of ferrite: 70% to 97%

When the volume fraction of ferrite in the microstructure is less than 70%, TS exceeds 700 MPa or a λ of 80% or more is not achieved. In contrast, when the volume fraction thereof is more than 97%, a TS of 600 MPa is not achieved because the amount of pearlite is reduced. Therefore, the volume fraction of ferrite is 70% to 97%. The volume fraction of ferrite is preferably 95% or less and more preferably 80% to 90%.

Volume Fraction of Pearlite: 3% or More

When the volume fraction of pearlite is 3% or more, λ is increased. The volume fraction of pearlite is preferably 5% or more. This is probably because pearlite is soft as compared to cementite, martensite, and retained austenite and therefore the number of voids caused at the interface between ferrite and pearlite is small as compared to the number of voids caused at the interface between ferrite and martensite and the interface between ferrite and retained austenite after forming.

Volume Fraction of Cementite Present at Ferrite Grain Boundaries: 2% or Less

The steel sheet according to aspects of the present invention may possibly contain cementite, martensite, and the like in addition to ferrite and pearlite. When the volume fraction of cementite, particularly cementite present at ferrite grain boundaries, in the microstructure is more than 2%, the number of voids caused at the interface between ferrite and cementite during hole expansion is increased and therefore a reduction in λ is caused. Thus, the volume fraction of the cementite present at the ferrite grain boundaries is 2% or less. Incidentally, the volume fraction thereof may be 0%.

Volume Fractions of Phases Other than Ferrite, Pearlite, and the Cementite Present at Ferrite Grain Boundaries: Less than 3% in Total

Phases other than ferrite, pearlite, and the cementite present at the ferrite grain boundaries are martensite, retained austenite, and the like. When the sum of the volume fractions of these phases in the microstructure is less than 3%, required properties of the steel sheet are not significantly affected. Therefore, the sum of the volume fractions of the phases other than ferrite, pearlite, and the cementite present at the ferrite grain boundaries is less than 3%. The sum thereof is preferably 2.5% or less and may be 0%.

Average Grain Size of Ferrite: 7 μm or Less

When the average grain size of ferrite is more than 7 μm, a reduction in strength is caused and therefore a TS of 600 MPa or more is not achieved. Therefore, the average grain size of ferrite is 7 μm or less. The average grain size of ferrite is preferably 5 μm or less.

Herein, the volume fraction of each of ferrite, pearlite, cementite, martensite, and retained austenite in the microstructure is determined in such a way that a thickness-wise cross-section of the steel sheet that is parallel to the rolling direction of the steel sheet is polished and is subsequently corroded with nital, three fields of view are photographed at 1,000 times magnification using an optical microscope, and the types of structures are identified by image processing. Furthermore, the average grain size of ferrite is also calculated by an intercept method. Herein, in the determination of the average grain size of ferrite, orthogonal line segments are drawn so as to longitudinally divide an image (corresponding to 84 μm in the rolling direction and 65 μm in the thickness direction) photographed at 1,000 times magnification using the optical microscope into 20 parts and so as to laterally divide the image into 20 parts, a value obtained by dividing the sum of the lengths of ferrite grains cut by one of the line segments by the number of the ferrite grains is defined as the cut length, and the average intercept length L is calculated for each line segment. The average grain size d is determined by the following equation:


d=1.13×L.

The volume fraction of the cementite present at the ferrite grain boundaries in the microstructure is determined in such a way that three fields of view are photographed at 3,000 times magnification using a scanning electron microscope and the cementite present at the ferrite grain boundaries is extracted by image processing.

(3) Production Method

Steel slab: A steel slab used is preferably produced by a continuous casting process for the purpose of preventing the macro-segregation of components of molten steel, produced by a known process using a converter or the like, having the above composition and may be produced by a ingot-casting process.

Hot rolling: The steel slab produced as described above is reheated in a furnace after being cooled to room temperature or without being cooled to room temperature or is held at high temperature without being fed through a furnace and is then hot-rolled. Hot-rolling conditions are not particularly limited. It is preferred that after the steel slab is heated to a temperature of 1,100° C. to 1,300° C., hot rolling (finish rolling) is completed at 850° C. to 950° C. and the steel slab is coiled at 720° C. or lower. This is due to reasons below. That is, when the heating temperature is lower than 1,100° C., the deformation resistance of steel is high and therefore hot rolling may possibly be difficult. When the heating temperature is higher than 1,300° C., crystal grains become coarse and therefore TS may possibly be reduced. When the finishing delivery temperature is lower than 850° C., ferrite is produced during rolling; hence, extended ferrite is formed and a reduction in λ may possibly be caused. When the finishing delivery temperature is higher than 950° C., crystal grains become coarse and therefore TS may possibly be reduced. Furthermore, the coiling temperature is higher than 720° C., the formation of an internal oxidation layer is significant and therefore chemical treatability and post-painting corrosion resistance may possibly be deteriorated.

After hot rolling, a hot-rolled sheet is pickled for the purpose of removing scale formed on the surface of the steel sheet.

Annealing: The pickled hot-rolled sheet is annealed in such a way that the hot-rolled sheet is heated to a two-phase temperature range between the Ac1 transformation temperature and the Ac3 transformation temperature, is cooled to a temperature range of 450° C. to 600° C. at an average cooling rate of 5° C./s to 30° C./s, and is then held at this temperature range for 100 s or more. The reason for heating the hot-rolled sheet to the two-phase temperature range between the Ac1 transformation temperature and the Ac3 transformation temperature is to form the microstructure containing ferrite and pearlite is formed. Furthermore, the reason for cooling the hot-rolled sheet to a temperature range of 450° C. to 600° C. at an average cooling rate of 5° C./s to 30° C./s is as follows: when the cooling temperature is higher than 600° C., the volume fraction of the cementite present at the ferrite grain boundaries exceeds 2% and therefore target λ is not achieved; when the cooling temperature is lower than 450° C., the amount of martensite is increased and therefore TS exceeds 700 MPa or λ is reduced; when the average cooling rate is less than 5° C./s, the ferrite grains become coarse and therefore a TS of 600 MPa is not achieved; and when the average cooling rate is more than 30° C./s, the volume fraction of the cementite present at the ferrite grain boundaries exceeds 2% and therefore a λ of 80% or more is not achieved. Incidentally, the average cooling rate is preferably 10° C./s to 20° C./s. The reason for holding the hot-rolled sheet at a temperature of 450° C. to 600° C. for 100 s or more is that when the residence time is less than 100 s, the amount of pearlite is reduced and therefore λ is reduced. The residence time is more preferably 150 s or more. Incidentally, the residence time is preferably 300 s or less from the viewpoint of production efficiency because an effect due to residence for an excessively long time is saturated. Annealing can be performed using a continuous annealing line.

EXAMPLES OF THE INVENTION

Steels having a composition shown in Table 1 were produced and were then formed into slabs. Each slab was heated to 1,200° C., was hot-rolled at a finishing delivery temperature of 890° C., and was then coiled at 600° C., whereby a hot-rolled sheet with a thickness of 1.6 mm was obtained. After being pickled, the hot-rolled sheet was annealed in a continuous annealing line under annealing conditions shown in Table 2. Incidentally, the Ac1 transformation temperature and Ac3 transformation temperature of each steel shown in Table 1 were calculated by the following equations:


Ac1 transformation temperature (° C.)=723+29.1(% Si)−10.7(% Mn)+16.9(% Cr)


and


Ac3 transformation temperature (° C.)=910−203(% C)1/2+44.7(% Si)−30(% Mn)+700(% P)+400(% Al)−11(% Cr)+104(% V)+400(% Ti)

where (% M) represents the mass percent of an element M.

The steel sheet obtained as described above was investigated for microstructure by the above-mentioned method and was subjected to a tensile test using a JIS #5 test specimen in accordance with JIS Z 2241, whereby TS and El were determined. Furthermore, a hole expansion test was performed using a 100 mm square test specimen in accordance with The Japan Iron and Steel Federation standard JFST 1001-1996, whereby λ was determined.

Results are shown in Table 3.

It is clear that steel sheets of examples of the present invention all have a TS of 600 MPa to 700 MPa, an El of 25% or more, a λ of 80% or more and are high-strength steel sheets with excellent formability. However, steel sheets of comparative examples do not have a target TS or λ.

TABLE 1 (mass percent) Ac3 Ac1 transformation transformation Steel temperature temperature No. C Si Mn P S Al Others (° C.) (° C.) Remarks A 0.140 1.00 1.30 0.030 0.0020 0.035 — 875 738 Within the scope of the present invention B 0.105 1.05 1.15 0.020 0.0015 0.040 — 887 741 Within the scope of the present invention C 0.175 0.95 0.85 0.020 0.0035 0.030 — 868 742 Within the scope of the present invention D 0.125 0.65 1.25 0.045 0.0010 0.025 — 871 729 Within the scope of the present invention E 0.155 1.45 0.95 0.020 0.0025 0.035 — 894 755 Within the scope of the present invention F 0.165 0.70 0.55 0.010 0.0008 0.040 — 865 737 Within the scope of the present invention G 0.115 1.35 1.45 0.020 0.0040 0.035 — 886 747 Within the scope of the present invention H 0.185 1.05 0.85 0.020 0.0035 0.030 — 870 744 Outside the scope of the present invention I 0.085 1.05 0.85 0.020 0.0035 0.030 — 895 744 Outside the scope of the present invention J 0.105 0.40 1.15 0.020 0.0015 0.040 — 860 724 Outside the scope of the present invention K 0.105 1.05 0.40 0.020 0.0015 0.040 — 908 749 Outside the scope of the present invention L 0.120 1.40 0.85 0.020 0.0015 0.040 Cr: 0.04 907 755 Within the scope of the present invention M 0.105 0.80 1.20 0.021 0.0016 0.041 Ti: 0.04 875 733 Within the scope of the present invention N 0.110 0.90 1.30 0.021 0.0016 0.041 V: 0.03 875 735 Within the scope of the present invention

TABLE 2 Annealing conditions Cooling Residence time Cooling attained at a temperature Steel Heating Average temperature at average range Sheet Steel Temperature cooling rate cooling rate of 450° C. to No. No. (° C.) (° C./s) (° C.) 600° C. (s) Remarks  1 A 820 15 500 120 Inventive example  2 A 821 32 498 126 Comparative example  3 A 817  3 498 253 Comparative example  4 B 815 20 480 135 Inventive example  5 C 800 25 490 130 Inventive example  6 D 780 28 460 140 Inventive example  7 E 780 17 460 105 Inventive example  8 F 790 23 490 195 Inventive example  9 G 870 25 570 200 Inventive example 10 G 872 25 320 105 Comparative example 11 G 872 27 570 95 Comparative example 12 H 800 15 520 130 Comparative example 13 I 806 20 521 125 Comparative example 14 J 798 23 525 135 Comparative example 15 K 802 23 525 132 Comparative example 16 L 821 19 495 123 Inventive example 17 M 820 21 496 124 Inventive example 18 N 825 20 495 135 Inventive example

TABLE 3 Steel Volume fraction Volume fraction Volume fraction of Another phase* Average grain sheet Steel of ferrite of pearlite cementite at ferrite and volume size of ferrite TS El λ No. No. (%) (%) grain boundaries (%) fraction (%) (μm) (MPa) (%) (%) Remarks  1 A 86.0 11.5 0.5 M: 2.0% 3.0 631 27 85 Inventive example  2 A 81.6 12.2 3.2 M: 3.0% 3.0 635 28 74 Comparative example  3 A 94.0  5.3 0.7 — 8.2 510 34 80 Comparative example  4 B 92.0  7.8 0.2 — 5.0 610 33 98 Inventive example  5 C 72.0 27.6 0.4 — 4.5 620 26 83 Inventive example  6 D 87.0 11.5 1.5 — 6.0 609 32 102  Inventive example  7 E 75.0 24.8 0.2 — 6.5 625 28 80 Inventive example  8 F 87.0 11.7 1.3 — 2.5 612 31 108  Inventive example  9 G 83.9 12.3 1.7 M: 2.1% 3.5 625 27 87 Inventive example 10 G 80.0 12.5 1.5 M. 6 0% 3.2 704 26 69 Comparative example 11 G 80.0 12.1 1.4 M: 6.5% 3.7 695 26 70 Comparative example 12 H 64.6 32.0 0.4 M: 3.0% 3.5 682 26 72 Comparative example 13 I 97.7  2.0 0.3 — 4.5 575 34 110  Comparative example 14 J 93.5  6.0 0.5 — 6.3 585 30 85 Comparative example 15 K 93.0  6.0 1.0 — 5.1 582 31 95 Comparative example 16 L 85.5 14.0 0.5 — 2.3 623 32 95 Inventive example 17 M 86.3 13.0 0.7 — 2.5 625 31 94 Inventive example 18 N 84.5 15.0 0.5 — 2.5 625 32 96 Inventive example *M: martensite

Claims

1. A high-strength steel sheet having a composition containing 0.10% to 0.18% C, more than 0.5% to 1.5% Si, 0.5% to 1.5% Mn, 0.05% or less P, 0.005% or less S, and 0.05% or less Al on a mass basis, the remainder being Fe and inevitable impurities, the high-strength steel sheet having a microstructure containing ferrite and pearlite, wherein the volume fraction of the ferrite is 70% to 97%, the volume fraction of the pearlite is 3% or more, the volume fraction of cementite present at grain boundaries of the ferrite is 2% or less, the sum of the volume fractions of phases other than the ferrite, the pearlite, and the cementite is less than 3%, and the average grain size of the ferrite is 7 μm or less.

2. The high-strength steel sheet according to claim 1, further containing at least one selected from the group consisting of 0.01% to 1.0% Cr, 0.01% to 0.1% Ti, and 0.01% to 0.1% V on a mass basis.

3. The high-strength steel sheet according to claim 1, further having a tensile strength TS of 600 MPa to 700 MPa.

4. The high-strength steel sheet according to claim 1, further having a hole expansion ratio λ of 80% or more.

5. The high-strength steel sheet according to claim 1, wherein the volume fraction of the ferrite is 80% to 95%.

6. The high-strength steel sheet according to claim 1, wherein the volume fraction of the pearlite is 3% to 30%.

7. The high-strength steel sheet according to claim 1, wherein the volume fraction of the pearlite is 5% to 28%.

8. A method for producing a high-strength steel sheet, comprising:

a step of preparing a steel slab having a chemical composition containing 0.10% to 0.18% C, more than 0.5% to 1.5% Si, 0.5% to 1.5% Mn, 0.05% or less P, 0.005% or less S, and 0.05% or less Al on a mass basis, the remainder being Fe and inevitable impurities;
a step of hot-rolling the steel slab into a hot-rolled sheet; and
a step of annealing the hot-rolled sheet in such a way that the hot-rolled sheet is heated to a two-phase temperature range between the Ac1 transformation temperature and the Ac3 transformation temperature, is cooled to a temperature range of 450° C. to 600° C. at an average cooling rate of 5° C./s to 30° C./s, and is then held at this temperature range for 100 s or more.

9. The method for producing the high-strength steel sheet according to claim 8, wherein the steel slab further contains at least one selected from the group consisting of 0.01% to 1.0% Cr, 0.01% to 0.1% Ti, and 0.01% to 0.1% V on a mass basis.

10. The method for producing the high-strength steel sheet according to claim 8, wherein the annealing step includes heating to the two-phase temperature range between the Ac1 transformation temperature and the Ac3 transformation temperature, cooling to a temperature range of 450° C. to 600° C. at an average cooling rate of 10° C./s to 20° C./s, and then holding at this temperature range for 100 s to 300 s.

Patent History
Publication number: 20140332123
Type: Application
Filed: Nov 29, 2012
Publication Date: Nov 13, 2014
Applicant: JFE STEELCORPORATION (Chiyoda-ku, Tokyo)
Inventors: Kouichi Nakagawa (Chiba), Kenji Kawamura (Tokyo), Takeshi Yokota (Chiba), Kazuhiro Seto (Chiba)
Application Number: 14/363,166
Classifications
Current U.S. Class: With Flattening, Straightening, Or Tensioning By External Force (148/645); Ferrous (i.e., Iron Base) (148/320); Chromium Containing, But Less Than 9 Percent (148/333); Three Percent Or More Manganese Containing Or Containing Other Transition Metal In Any Amount (148/337)
International Classification: C22C 38/18 (20060101); C22C 38/14 (20060101); C22C 38/02 (20060101); C22C 38/06 (20060101); C22C 38/04 (20060101); C21D 8/02 (20060101); C22C 38/12 (20060101);