HOT-ROLLED STEEL SHEET

Abstract

A hot-rolled steel sheet has a predetermined chemical composition, in which at a depth position of ¼ of a sheet thickness, a microstructure is formed of 20 area % or more of ferrite, 40 area % or more of pearlite, and 0 area % or more and 10 area % or less of a remainder in microstructure, and ferrite grains to be measured by an electron backscattering diffraction method satisfy a predetermined relationship.

Description

TECHNICAL FIELD

The present invention relates to a hot-rolled steel sheet.

Priority is claimed on Japanese Patent Application No. 2023-003920, filed Jan. 13, 2023, the content of which is incorporated herein by reference.

BACKGROUND ART

In so-called high carbon steels used for gears of various machines, transmission components of automobiles, seat recliners, and the like, there is a high carbon cold-rolled steel sheet manufactured through cold rolling. This steel sheet is worked in a desired shape by cold working and is then subjected to quenching treatment as necessary to ensure a desired hardness, so that the steel sheet is formed into a required member.

The strength (hardness), wear resistance, and the like, which are finally required for a member, of the high carbon cold-rolled steel sheet are adjusted by the chemical composition and the heat treatment conditions of the steel sheet, that is, basically carbon contained in a large amount and final quenching treatment. However, there is a problem that the cold rollability of the hot-rolled steel sheet is reduced with an increase in the amount of carbon. Therefore, elongation, that is, excellent cold rollability is required for a steel sheet (hot-rolled steel sheet) immediately before cold rolling in the process of manufacturing the high carbon cold-rolled steel sheet rather than strength (hardness).

As a steel sheet excellent in cold rollability, Patent Document 1 discloses a high-carbon hot-rolled steel sheet that includes, as a composition, by mass %, C: 0.10% to 0.33%, Si: 0.15% to 0.35%, Mn: 0.5% to 0.9%, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.0065% or less, Cr: 0.90% to 1.5%, and a remainder including Fe and unavoidable impurities, and includes a microstructure including ferrite and cementite, in which the density of the cementite is 0.25 pieces/μm² or less, a hardness is 110 to 160 in HV, and total elongation is 40% or more.

CITATION LIST

Patent Document

• • Patent Document 1: PCT International Publication No. WO2018/155254

SUMMARY OF INVENTION

Technical Problem

However, since the high-carbon hot-rolled steel sheet disclosed in Patent Document 1 is subjected to spheroidizing annealing, the cold rollability of the high-carbon hot-rolled steel sheet is excellent but there is a problem that the number of steps is increased due to spheroidizing annealing. For this reason, a hot-rolled steel sheet having higher productivity than the high-carbon hot-rolled steel sheet disclosed in Patent Document 1 is required.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-rolled steel sheet that is excellent in productivity and excellent in cold rollability.

Solution to Problem

The present inventors have conducted detailed studies on the cold rollability of a hot-rolled steel sheet as well as a chemical composition, a microstructure, and manufacturing conditions. The present inventors found that cold rollability is improved due to an increase in the ferrite fraction of an obtained hot-rolled steel sheet and a change in a grain size distribution in a case where a rolling reduction in a stage before hot rolling is increased and rolling is performed in the final stage with a small rolling reduction.

The present invention has been made in view of the above findings. The gist of the present invention is as follows.

• • <1> A hot-rolled steel sheet according to a first aspect of the present invention includes, as a chemical composition, by mass %, C: 0.20% to 0.70%, Si: 0.010% to 0.300%, Mn: 0.3% to 2.0%, Al: 0.001% to 0.100%, N: 0.0010% to 0.0100%, P: 0.008% to 0.030%, S: 0.010% or less, O: 0.0025% or less, Cr: 1.500% or less, B: 0.010% or less, Nb: 0.50% or less, Mo: 0.50% or less, V: 0.50% or less, Ti: 0.3000% or less, Cu: 0.500% or less, W: 0.500% or less, Ta: 0.500% or less, Ni: 0.500% or less, Mg: 0.003% or less, Ca: 0.003% or less, Y: 0.030% or less, Zr: 0.030% or less, La: 0.030% or less, Ce: 0.030% or less, Sn: 0.030% or less, Sb: 0.030% or less, As: 0.030% or less, and a remainder including Fe and impurities. At a depth position of ¼ of a sheet thickness, a microstructure is formed of 20 area % or more of ferrite, 40 area % or more of pearlite, and 0 area % or more and 10 area % or less of a remainder in microstructure, and the remainder in microstructure includes at least one of bainite and martensite, among ferrite grains to be measured by an electron backscattering diffraction method, ferrite grains excluding ferrite grains occupying 5% of a total number of the ferrite grains from a maximum grain size side and ferrite grains occupying 5% of the total number of the ferrite grains from a minimum grain size side of the ferrite grains are treated as evaluation ferrite grains, a minimum value of a grain size of the evaluation ferrite grains is set as a first grain size, a maximum value of the grain size of the evaluation ferrite grains is set as a second grain size, a grain size obtained by adding, to the first grain size, ⅓ of a difference between the second grain size and the first grain size is set as a third grain size, a grain size obtained by adding, to the first grain size, ⅔ of the difference between the second grain size and the first grain size is set as a fourth grain size, and when a range that is equal to or more than the first grain size and equal to or less than the third grain size is set as a first grain size range, a range that is more than the third grain size and equal to or less than the fourth grain size is set as a second grain size range, and a range that is more than the fourth grain size and equal to or less than the second grain size is set as a third grain size range, the number of the evaluation ferrite grains in the first grain size range is 2.5 times or more and 3.0 times or less the number of the evaluation ferrite grains in the second grain size range, and the number of the evaluation ferrite grains in the third grain size range is 2.0 times or more and 2.5 times or less the number of the evaluation ferrite grains in the second grain size range.
  • <2> According to a second aspect of the present invention, in the hot-rolled steel sheet according to the first aspect, an average grain size of the evaluation ferrite grains in the first grain size range may be in a range of 3 μm to 20 μm.
  • <3> According to a third aspect of the present invention, in the hot-rolled steel sheet according to the first or second aspect, an average grain size of the evaluation ferrite grains in the third grain size range may be in a range of 80 μm to 120 μm.
  • <4> According to a fourth aspect of the present invention, in the hot-rolled steel sheet according to any one of the first to third aspects, a Vickers hardness Hv may be 160 or less at the position having a depth of ¼ of the sheet thickness.
  • <5> According to a fifth aspect of the present invention, in the hot-rolled steel sheet according to any one of the first to fourth aspects, a total elongation may be 40% or more.

Advantageous Effects of Invention

According to the above-described aspects of the present invention, it is possible to provide a hot-rolled steel sheet that is excellent in productivity and excellent in cold rollability.

DESCRIPTION OF EMBODIMENTS

A chemical composition and a microstructure of a hot-rolled steel sheet according to an embodiment of the present invention (which may be simply referred to as a steel sheet according to the present embodiment hereinafter) and rolling conditions and the like in a manufacturing method capable of manufacturing the steel sheet will be described in detail below.

<Chemical Composition>

First, the chemical composition of the steel sheet according to the present embodiment will be described. “%” indicating the amount of each element in the chemical composition means “mass %” unless otherwise specified. In the present specification, a numerical range represented using “to” means a range that includes numerical values written in the front and rear of “to” as a lower limit and an upper limit.

[C: 0.20% to 0.70%]

C (carbon) is an essential element for the high-strengthening of the steel sheet. In a case where the C content is less than 0.20%, it is not possible to sufficiently obtain an effect of improving cold rollability through microstructure control. For this reason, the C content is set to 0.20% or more. The C content is preferably 0.25% or more.

On the other hand, in a case where the C content exceeds 0.70%, weldability is reduced and cold rollability deteriorates. For this reason, the C content is set to 0.70% or less. The C content is preferably 0.60% or less.

Meanwhile, in a case where the C content is within the above-described range, it is possible to ensure normal tensile properties required for a high carbon cold-rolled steel sheet subjected to cold rolling and heat treatment.

[Si: 0.010% to 0.300%]

Si (silicon) is a solid solution strengthening element and is an element that is generally contained in a high carbon cold-rolled steel sheet for the high-strengthening of a steel sheet. The Si content is set to 0.010% or more to obtain an effect of improving cold rollability through microstructure control. The Si content is preferably 0.100% or more.

On the other hand, in a case where an excessive amount of Si is contained, the embrittlement of a steel sheet is caused. For this reason, it is difficult to sufficiently ensure cold rollability even if microstructure control is applied. For this reason, the Si content is set to 0.300% or less. The Si content is preferably 0.150% or less.

Meanwhile, in a case where the Si content is within the above-described range, it is possible to ensure normal tensile properties required for a high carbon cold-rolled steel sheet subjected to cold rolling and heat treatment.

[Mn: 0.3% to 2.0%]

Mn is an element that has an action of improving the hardenability of steel and is generally contained in a high carbon cold-rolled steel sheet. In a case where the Mn content is less than 0.3%, it is difficult to obtain an effect of improving cold rollability through microstructure control. Therefore, the Mn content is set to 0.3% or more. The Mn content is preferably 1.0% or more.

On the other hand, in a case where the Mn content exceeds 2.0%, the generation of ferrite in a hot-rolled steel sheet is suppressed. For this reason, desired cold rollability cannot be obtained. Therefore, the Mn content is set to 2.0% or less. The Mn content is preferably 1.5% or less.

Meanwhile, in a case where the Mn content is within the above-described range, it is possible to ensure normal tensile properties required for a high carbon cold-rolled steel sheet subjected to cold rolling and heat treatment.

[Al: 0.001% to 0.100%]

Al is an element having an action of deoxidizing steel. Therefore, Al may be contained in steel. In order to obtain the above-described effect, it is preferable that the Al content is set to 0.001% or more. The Al content is preferably 0.005% or more.

On the other hand, even if an excessive amount of Al is contained, not only an increase in cost is caused by the saturation of the above-described effect but also a load during hot rolling is increased due to an increase in the transformation temperature of steel. For this reason, the Al content is set to 0.100% or less. The Al content is preferably 0.090% or less. The Al content means a so-called total Al (T—Al) content.

[N: 0.0010% to 0.0100%]

N is an element that forms coarse nitrides in a steel sheet and deteriorates the cold rollability of the steel sheet. In a case where the N content exceeds 0.0100%, the above-described deterioration of the cold rollability becomes significant. Therefore, the N content is set to 0.0100% or less. The N content may be 0.0090% or less, 0.0080% or less, or 0.0070% or less.

Meanwhile, in a case where the N content is set to less than 0.0010%, manufacturing cost significantly increases. The N content is set to 0.0010% or more. The N content may be set to 0.0020% or more.

[P: 0.008% to 0.030%]

P is an element that is contained in steel as an impurity, and is an element that is segregated at grain boundaries to embrittle steel and to deteriorate cold rollability. For this reason, the P content is set to 0.030% v or less. The P content is preferably 0.020% or less and more preferably 0.010% or less.

The P content is preferably as low as possible, but is set to 0.008% or more in consideration of a time and a cost required to remove P.

[S: 0.010% or Less]

S is an element that is contained in steel as an impurity, and is an element that forms sulfide-based inclusions and deteriorates cold rollability. For this reason, the S content is set to 0.010% or less. The S content is preferably 0.009% or less and more preferably 0.007% or less. The S content is preferably as low as possible and may be 0%. However, the S content may be 0.001% or more in consideration of a time and a cost required to remove S.

[O: 0.0025% or Less]

O is an element that forms coarse oxides in steel and deteriorates cold rollability. In a case where the O content exceeds 0.0025%, cold rollability tends to significantly deteriorate. For this reason, the O content is set to 0.0025% or less. The O content may be 0.0020% or less or 0.0015% or less.

The O content is preferably low. However, setting the O content to less than 0.0001% is not economically preferable since cost is excessively increased. For this reason, the O content may be set to 0.0001% or more. The O content may be set to 0.0010% or more.

The steel sheet according to the present embodiment may contain the above-described elements and a remainder including Fe and impurities. Here, the impurities are elements that are mixed due to various factors of raw materials, such as ore and scrap, and manufacturing steps in a case where steel is industrially manufactured and that are allowed to be present in a range in which characteristics of the steel sheet according to the present embodiment are not impaired. Further, the impurities also include elements that are not components intentionally added to the steel sheet according to the present embodiment.

The steel sheet according to the present embodiment may further contain one or more elements (optional elements) selected from Cr, B, Nb, Mo, V, Ti, Cu, W, Ta, Ni, Mg, Ca, Y, Zr, La, Ce, Sn, Sb, and As. Since these elements do not necessarily need to be contained, the lower limits thereof in content are 0%.

[Cr: 1.500% or Less]

Cr is an element that is effective for the high-strengthening of a steel sheet through an increase in hardenability and is generally used in a high carbon cold-rolled steel sheet. Therefore, Cr may be contained in steel. In order to obtain an effect of improving cold rollability through microstructure control, it is preferable that the Cr content is 0.001% or more.

On the other hand, in a case where the Cr content exceeds 1.500%, Cr is segregated at a central portion of a steel sheet, so that coarse Cr carbides are formed. For this reason, cold rollability may be reduced. For this reason, the Cr content is set to 1.500% or less.

[B: 0.010% or Less]

B is an element that suppresses the formation of ferrite and pearlite in a cooling process from austenite, promotes the generation of a low temperature transformation structure, such as bainite or martensite, and is beneficial for the high-strengthening of a high carbon cold-rolled steel sheet. Therefore, B may be contained in steel. In order to obtain the above-described effect of B, it is preferable that the B content is 0.001% or more. On the other hand, B causes the formation of coarse B oxides or borides in steel and the coarse B oxides or borides serve as the origins of generation of voids during cold rolling, so that the cold rollability of a steel sheet may deteriorate. Further, B suppresses the formation of ferrite and pearlite in a cooling process from austenite, and inhibits an effect of improving cold rollability through microstructure control. For this reason, the B content is set to 0.010% or less.

[Nb: 0.50% or Less]

Nb is an element that is effective in controlling the morphology of carbides and is an element that is also effective in improving the toughness of a steel sheet since a structure is refined in a case where Nb is added. Therefore, Nb may be contained in steel. In order to obtain the above-described effect of Nb, it is preferable that the Nb content is set to 0.01% or more. Meanwhile, in a case where an excessive amount of Nb is added, a large number of coarse Nb carbides are precipitated and the coarse Nb carbides serve as the origins of generation of voids during cold rolling, so that the cold rollability of a steel sheet may deteriorate. For this reason, the Nb content is set to 0.50% or less.

[Mo: 0.50% or Less]

Mo is an element that is effective for the strengthening of a high carbon cold-rolled steel sheet. Therefore, Mo may be contained in steel. In order to highly strengthen a high carbon cold-rolled steel sheet using Mo, it is preferable that the Mo content is 0.01% or more. On the other hand, in a case where an excessive amount of Mo is added, cost increases and coarse Mo carbides are formed, so that the cold rollability of a steel sheet may be reduced. For this reason, the Mo content is set to 0.50% or less.

[V: 0.50% or Less]

V is an element that is effective in controlling the morphology of carbides and is an element that is also effective in improving the toughness of a steel sheet since a structure is refined in a case where V is added. Therefore, V may be contained in steel. In order to obtain the above-described effect of V, it is preferable that the V content is 0.01% or more. On the other hand, in a case where an excessive amount of V is added, a large number of fine V carbides are precipitated. Accordingly, the strength of a steel sheet is increased and the ductility of the steel sheet significantly deteriorates, so that the cold rollability of the steel sheet may be reduced. For this reason, the V content is set to 0.50% or less.

[Ti: 0.3000% or Less]

Ti is an element that is important in controlling the morphology of carbides and is an element that is contained in a high carbon cold-rolled steel sheet in a large amount to promote an increase in the strength of ferrite. Therefore, Ti may be contained in steel. However, in a case where the Ti content is 0.0001% or more, an effect of improving the strength of ferrite is obtained. On the other hand, in a case where an excessive amount of Ti is added, the cold rollability of a steel sheet may be reduced due to the presence of coarse Ti oxides or Ti carbonitrides in steel. For this reason, the Ti content is set to 0.3000% or less.

[Cu: 0.500% or Less]

Cu is an element that contributes to the improvement of the strength of a steel sheet and is contained in a high carbon cold-rolled steel sheet. Therefore, Cu may be contained in steel. In order to obtain the above-described effect, it is preferable that a Cu content is 0.001% or more. However, in a case where the Cu content is too high, hot shortness is caused. For this reason, there is a concern that productivity in hot rolling may be reduced. In addition, in a case where the Cu content is too high, there is a concern that cold rollability may be reduced due to the formation of coarse inclusions. For this reason, the Cu content is set to 0.500% or less. It is preferable that the Cu content is 0.300% or less.

[W: 0.500% or Less]

W is a carbide-forming element and is an element that is effective for the high-strengthening of a steel sheet, and is contained in a high carbon cold-rolled steel sheet. Therefore, W may be contained in steel. In order to obtain the above-described effect, it is preferable that the W content is 0.001% or more. It is more preferable that the W content is 0.005% or more. It is still more preferable that the W content is 0.010% or more.

On the other hand, even if an excessive amount of W is contained, cost is increased due to the saturation of the effect. Therefore, the W content is set to 0.500% or less in a case where W is to be contained. It is preferable that the W content is 0.400% or less.

[Ta: 0.500% or Less]

Ta is an element that is effective in controlling the morphology of carbides and improving the strength of a steel sheet, and is contained in a high carbon cold-rolled steel sheet. Therefore, Ta may be contained in steel. In order to obtain the above-described effect, it is preferable that the Ta content is 0.001% or more. On the other hand, in a case where the Ta content is too high, a large number of fine Ta carbides are precipitated, so that a reduction in the ductility of a steel sheet is caused. For this reason, there is a concern that the cold rollability of a steel sheet may be reduced. Accordingly, the Ta content is set to 0.500% or less. It is more preferable that the Ta content is 0.300% or less. It is still more preferable that the Ta content is 0.200% or less.

[Ni: 0.500%]

Ni is an element that is effective in improving the strength of a steel sheet and is contained in a high carbon cold-rolled steel sheet. Therefore, Ni may be contained in steel. In order to obtain the above-described effect, it is preferable that the Ni content is 0.001% or more. It is more preferable that the Ni content is 0.010% or more. On the other hand, in a case where the Ni content is too high, the ductility of a steel sheet is reduced. For this reason, there is a concern that cold rollability may be reduced. Accordingly, the Ni content is set to 0.500% or less. It is preferable that the Ni content is 0.400% or less.

[Mg: 0.003% or Less]

Mg is an element that controls the morphology of sulfides and oxides and contributes to the improvement of the bendability of a steel sheet, and is contained in a high carbon cold-rolled steel sheet. Therefore, Mg may be contained in steel. In order to obtain the above-described effect, it is preferable that the Mg content is 0.001% or more. However, in a case where the Mg content is too high, there is a concern that cold rollability may be reduced due to the formation of coarse inclusions. For this reason, the Mg content is set to 0.003% or less. It is preferable that the Mg content is 0.002% or less.

[Ca: 0.003% or Less]

Ca is an element that can control the morphology of sulfides with a very small amount. Therefore, Ca may be contained in steel. In order to obtain the above-described effect, it is preferable that the Ca content is 0.001% or more. However, in a case where the Ca content is too high, coarse Ca oxides may be formed and may serve as the origins of occurrence of cracking during cold working. For this reason, there is a concern that cold rollability may deteriorate. Accordingly, the Ca content is set to 0.003% or less. It is preferable that the Ca content is 0.002% or less.

[Y: 0.030% or Less]

Y is an element that effectively acts to control the morphology of sulfides even if the content is very low. Therefore, Y may be contained in steel. In order to obtain the above-described effect, it is preferable that the Y content is 0.001% or more. However, in a case where the Y content is too high, coarse Y oxides are formed. For this reason, there is a concern that cold rollability and fracture resistance may be reduced. Accordingly, the Y content is set to 0.030% or less. It is preferable that the Y content is 0.020% or less.

[Zr: 0.030% or Less]

Zr is an element that can control the morphology of sulfides with a very small amount. Therefore, Zr may be contained in steel. In order to obtain the above-described effect, it is preferable that the Zr content is 0.001% or more. However, in a case where the Zr content is too high, coarse Zr oxides are formed. For this reason, there is a concern that cold rollability may be reduced. Accordingly, the Zr content is set to 0.030% or less. It is preferable that the Zr content is 0.020% or less.

[La: 0.030% or Less]

La is an element that effectively acts to control the morphology of sulfides even if the content is very low. Therefore, La may be contained in steel. In order to obtain the above-described effect, it is preferable that the La content is 0.001% or more. However, in a case where the La content is too high, coarse La oxides are formed. For this reason, there is a concern that cold rollability and fracture resistance may be reduced. For this reason, the La content is set to 0.030% or less. It is preferable that the La content is 0.020% or less.

[Ce: 0.030% or Less]

Ce is an element that effectively acts to control the morphology of sulfides even if the content is very low. Therefore, Ce may be contained in steel. In order to obtain the above-described effect, it is preferable that the Ce content is 0.001% or more. However, in a case where the Ce content is too high, coarse Cc oxides are formed. For this reason, there is a concern that cold rollability and fracture resistance may be reduced. For this reason, the Ce content is set to 0.030% or less. It is preferable that the Ce content is 0.020% or less.

[Sn: 0.030% or Less]

Sn is an element that can be contained in a steel sheet in a case where scrap is used as a raw material of the steel sheet. Further, there is a concern that Sn may cause a reduction in the cold rollability of a steel sheet attributed to the embrittlement of ferrite. For this reason, the Sn content is preferably as low as possible. Therefore, the Sn content is set to be 0.030% or less. It is preferable that the Sn content is 0.020% or less. However, reducing the Sn content to less than 0.001% is not preferable since an excessive increase in refining cost is caused. For this reason, the Sn content may be set to 0.001% or more.

[Sb: 0.030% or Less]

Similar to Sn. Sb is an element that can be contained in a steel sheet in a case where scrap is used as a raw material of the steel sheet. There is a concern that Sb may be strongly segregated at grain boundaries to cause the embrittlement of the grain boundaries, a reduction in ductility, and a reduction in cold rollability. For this reason, the Sb content is preferably as low as possible. The Sb content is set to 0.030% or less. It is preferable that the Sb content is 0.020% or less. Reducing the Sb content to less than 0.001% is not preferable since an excessive increase in refining cost is caused. For this reason, the Sb content may be set to 0.001% or more.

[As: 0% to 0.030%]

Similar to Sn and Sb, As is an element that can be contained in a steel sheet in a case where scrap is used as a raw material of the steel sheet. There is a concern that As may be strongly segregated at grain boundaries to cause a reduction in cold rollability. For this reason, the As content is preferably as low as possible. The As content is set to 0.030% or less. It is preferable that the As content is 0.020% or less. Reducing the As content to less than 0.001% is not preferable since an excessive increase in refining cost is caused. For this reason, the As content may be set to 0.001% or more.

The chemical composition of the steel sheet according to the present embodiment can be obtained by the following method.

The chemical composition of the steel sheet described above may be measured using a general chemical composition. For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). In addition, the C content and the S content may be measured using a combustion-infrared absorption method, the N content may be measured using an inert gas fusion-thermal conductivity method, and the O content may be measured using an inert gas fusion-nondispersive infrared absorption method.

<Microstructure at Position Having Depth of ¼ of Sheet Thickness>

Next, a microstructure at a position having a depth of ¼ of a sheet thickness of the steel sheet according to the present embodiment (a ¼ position of a sheet thickness from a surface in a sheet thickness direction) will be described.

In the description of the microstructure of the steel sheet according to the present embodiment, a microstructural fraction is represented by an area ratio. Therefore, unless otherwise specified, “%” in the description of the microstructure indicates “area %”.

The microstructure of the steel sheet according to the present embodiment at a position having a depth of ¼ of the sheet thickness (a ¼ position of the sheet thickness from a surface in the sheet thickness direction) includes 20 area % or more of ferrite, 40 area % or more of pearlite, and 0 area % or more and 10 area % or less of a remainder in microstructure.

(Ferrite: 20 Area % or More)

Ferrite is a relatively soft phase in a high carbon steel sheet such as the steel sheet according to the embodiment of the present invention, and improves the ductility of the steel sheet to improve cold rollability in a case where ferrite is mixed with a hard phase such as martensite. For this reason, the area ratio of ferrite is 20 area % or more at the position having a depth of ¼ of the sheet thickness. It is preferable that the area ratio of ferrite is 35 area % or more. An upper limit of the area ratio of ferrite is not particularly limited, but is, for example, 60 area % or less in a high carbon steel sheet such as the steel sheet according to the embodiment of the present invention.

(Pearlite: 40 Area % or More)

Pearlite is a structure that includes a large amount of cementite in the structure and consumes C (carbon), which is contained in steel and contributes to an increase in strength, to soften the steel. For this reason, the area ratio of pearlite is 40 area % or more at the position having a depth of ¼ of the sheet thickness. The area ratio of pearlite is more preferably 50 area % or more.

An upper limit of the area ratio of pearlite is not particularly limited, and is, for example, 80 area % or less.

[Remainder in Microstructure]

The area ratio of the remainder in microstructure is 0 area % or more and 10 area % or less. The remainder in microstructure refers to a structure that remains after pearlite and ferrite are removed from the microstructure. The remainder in microstructure includes at least one of bainite and martensite. It is preferable that the remainder in microstructure is a structure including at least one of bainite and martensite as a main component. Here, the “structure including at least one of bainite and martensite as a main component” means that a ratio of a total area of bainite and martensite to a total area of the remainder in microstructure is 60 area % or more. More preferably, a ratio of the total area of bainite and martensite to the total area of the remainder in microstructure is 80 area % or more. An upper limit of a ratio of the total area of bainite and martensite to the total area of the remainder in microstructure may be 100 area %. Since bainite is softer than martensite, it is preferable to perform a control such that bainite is formed instead of martensite in the steel sheet according to the present embodiment in which ductility is emphasized rather than strength. Here, martensite refers to fresh martensite and tempered martensite, but it is preferable to perform a control such that tempered martensite is formed instead of fresh martensite in the steel sheet according to the present embodiment in which ductility is emphasized rather than strength. For example, the area ratio of bainite is set to 5 area % or more. The area ratio of the bainite is preferably 7 area % or more. The area ratio of martensite is set to, for example, 5 area % or less. The area ratio of martensite is preferably 3 area % or less.

Next, a method of identifying each microstructure and calculating the area ratio of each microstructure at the position having a depth of ¼ of the sheet thickness will be described.

The identification of each microstructure and the calculation of the area and area ratio of each microstructure can be performed by electron back scattering diffraction (EBSD), X-ray measurement, etching using a Nital reagent or a LePera solution, and observing a cross section of the steel sheet, which is parallel to a rolling direction and perpendicular to a sheet surface, at a magnification of 1000 to 50000 with a scanning electron microscope. Meanwhile, even in the measurement of an area ratio of any of the structures, area ratios are measured at three points and an average value thereof is calculated. At this time, for example, software “OIM DataCollection™ (ver. 7)” manufactured by TSL Solutions, Inc., or the like is used as software for acquiring crystal orientation data in the present invention.

The area and area ratio of ferrite can be measured by the following method. That is, an observed section is finished by colloidal silica polishing or electrolytic polishing, and an area and area ratio are measured at intervals (pitch) of 0.2 μm in a square region (a square having a side having a length of ⅛ to ⅜ of the sheet thickness in the sheet thickness direction) in a range having a depth of ⅛ to ⅜ of the sheet thickness, which is centered on the ¼ position of the sheet thickness from the surface of the steel sheet, by EBSD associated with a scanning electron microscope. Meanwhile, the sample preparation conditions and the like are set within the ranges of the conditions recommended in “standard for measurement of crystal misorientation for material evaluation using an electron backscatter diffraction (EBSD) method” of the Japan Institute of Metals and Materials. The value of grain average misorientation (GAM) is calculated from measured data. Further, a region in which the value of grain average misorientation is less than 0.5° is treated as ferrite, and the area and area ratio of ferrite are measured. Here, boundaries at which crystal misorientation is 15° or more are determined as grain boundaries, and a region surrounded by the grain boundaries is determined as crystal grains. Grain average misorientation is a value obtained by calculating misorientation between adjacent measurement points and averaging the calculated values for all the measurement points in crystal grains.

The area and area ratio of bainite are calculated by collecting a sample in a state where a sheet thickness cross section parallel to the rolling direction of the steel sheet is set as an observed section, polishing the observed section, etching the observed section with a Nital solution, observing a square region having a depth of ⅛ to ⅜ of the sheet thickness, which is centered on the position having a depth of ¼ of the sheet thickness, with a field emission scanning electron microscope (FE-SEM), and using publicly known image analysis software. Meanwhile, the area ratio can be calculated using, for example, “ImageJ” as the image analysis software. Here, “ImageJ” is open source, is public domain image processing software, and is widely used by those skilled in the art.

Meanwhile, in the observation with the FE-SEM, for example, structures in the square observed region are distinguished as follows. The bainite is an aggregate of lath-shaped crystal grains, and is a region that does not contain iron-based carbides having a major axis of 20 nm or more in the lath structure. Alternatively, in a case where iron-based carbides having a major axis of 20 nm or more are contained in the lath structure, the bainite is a region in which the iron-based carbides belong to a single variant, that is, an iron-based carbide group elongated in the same direction. Here, the iron-based carbide group elongated in the same direction means a group in which a difference in an elongation direction of the iron-based carbide group is within 5°.

As in the case of bainite, the area ratio of martensite is calculated by collecting a sample in a state where a sheet thickness cross section parallel to the rolling direction of the steel sheet is set as an observed section, polishing the observed section, etching the observed section with a Nital solution, observing a square region having a depth of ⅛ to ⅜ of the sheet thickness, which is centered on the position having a depth of ¼ of the sheet thickness, with a field emission scanning electron microscope (FE-SEM), and using publicly known image analysis software. Meanwhile, the area ratio can be calculated using, for example, “ImageJ” as the image analysis software. Here, “ImageJ” is open source, is public domain image processing software, and is widely used by those skilled in the art. Martensite is distinguished from other microstructures in an electron channeling contrast image using a scanning electron microscope since having a high dislocation density and includes substructures, such as blocks or packets, in grains.

The area ratio of pearlite can be calculated by collecting a sample in a state where a sheet thickness cross section parallel to the rolling direction of the steel sheet is set as an observed section, polishing the observed section, etching the observed section with a Nital reagent, and observing a square region in a range having a depth of ⅛ to ⅜ of the sheet thickness, which is centered on the ¼ position of the sheet thickness from the surface of the steel sheet, using a secondary electron image of a scanning electron microscope. A region in which portions having a bright contrast and portions having a dark contrast in the secondary electron image have a lamellar shape is determined as pearlite, and the area ratio of pearlite is calculated using the above-described image analysis software “ImageJ”. The determination of pearlite using the contrast of the secondary electron image is generally performed by those skilled in the art and can be easily performed by those skilled in the art. In a case where a total area ratio of each structure obtained using the above-described evaluation method is different from 100%, a value obtained by multiplying the area ratio of each structure by “100/(total area ratio of each structure)” is used as the area ratio of each structure.

(Evaluation Ferrite Grains)

In the steel sheet according to the present embodiment, a distribution state of ferrite grains in the sheet thickness direction is set in a predetermined range to ensure cold rollability. Specifically, fine ferrite grains are relatively increased, and coarse ferrite grains are relatively increased. The reason why cold rollability can be ensured in a case where the distribution state of ferrite grains in the sheet thickness direction is set in a predetermined range is not clear, but the inventor presumes as follows. Since fine grains and coarse grains are mixed with each other, the distribution of strain applied to each of the crystal grains is generated. In a case where the grains have a distribution, it is presumed that large cold rollability can be ensured as compared to a case where grains have a normal distribution since strain can be released from grains having a high strain to grains having a low strain (strain dispersion) during rolling. Meanwhile, in order to more accurately evaluate the distribution state of the grain sizes of the ferrite grains, ferrite grains excluding 5% of the total ferrite grains on a minimum grain size side and 5% of the total ferrite grains on a maximum grain size side are defined as evaluation ferrite grains, and the distribution state of the grain sizes is evaluated.

Specifically, among the ferrite grains measured at the position having a depth of ¼ of the sheet thickness by the electron backscatter diffraction (EBSD) method described above, ferrite grains excluding ferrite grains occupying 5% of the total number of the ferrite grains from the maximum grain size side and ferrite grains occupying 5% of the total number of the ferrite grains from the minimum grain size side of the ferrite grains are defined as evaluation ferrite grains. In this case, in a case where the number of n crystal grains from the maximum grain size side or the minimum grain size side is less than 5% of the total number of crystal grains and the number of n+1 crystal grains therefrom exceeds 5% of the total number of crystal grains, the n+1 crystal grains are excluded. Here, the minimum value of the grain sizes of the evaluation ferrite grains is set as a first grain size, the maximum value of the grain sizes of the evaluation ferrite grains is set as a second grain size, a grain size that is obtained by adding ⅓ of a difference between the second grain size and the first grain size to the first grain size is set as a third grain size, and a grain size that is obtained by adding ⅔ of the difference between the second grain size and the first grain size to the first grain size is set as a fourth grain size. Further, a range that is equal to or larger than the first grain size and equal to or less than the third grain size is set as a first grain size range, a range that exceeds the third grain size and is equal to or less than the fourth grain size is set as a second grain size range, and a range that exceeds the fourth grain size and is equal to or less than the second grain size is set as a third grain size range.

The number of the evaluation ferrite grains of the steel sheet according to the present embodiment in the first grain size range is 2.5 times or more and 3.0 times or less the number of the evaluation ferrite grains in the second grain size range. Further, in the steel sheet according to the present embodiment, the number of the evaluation ferrite grains in the third grain size range is 2.0 times or more and 2.5 times or less the number of the evaluation ferrite grains in the second grain size range. In a case where the number of the evaluation ferrite grains in the first grain size range is set to 2.5 times or more and 3.0 times or less the number of the evaluation ferrite grains in the second grain size range and the number of the evaluation ferrite grains in the third grain size range is set to 2.0 times or more and 2.5 times or less the number of the evaluation ferrite grains in the second grain size range as described above, the evaluation ferrite grains in each grain size range have a clear distribution, so that strain dispersion is significant. As a result, the cold rollability of the steel sheet can be improved. It is preferable that the number of the evaluation ferrite grains of the steel sheet according to the present embodiment in the first grain size range is 2.6 times or more and 3.0 times or less the number of the evaluation ferrite grains in the second grain size range. It is preferable that the number of the evaluation ferrite grains of the steel sheet according to the present embodiment in the third grain size range is 2.2 times or more and 2.5 times or less the number of the evaluation ferrite grains in the second grain size range.

It is preferable that the number of the evaluation ferrite grains of the steel sheet according to the present embodiment in the first grain size range is 1.1 times or more and 1.4 times or less the number of the evaluation ferrite grains in the third grain size range.

More preferably, the number of the evaluation ferrite grains of the steel sheet according to the present embodiment in the first grain size range is 1.2 times or more and 1.4 times or less the number of the evaluation ferrite grains in the third grain size range.

The number of the evaluation ferrite grains of the steel sheet according to the present embodiment in the first grain size range is 1.1 times or more and 1.4 times or less the number of the evaluation ferrite grains in the third grain size range, so that the cold rollability of the steel sheet can be further improved.

It is preferable that the average grain size of the evaluation ferrite grains in the first grain size range is in a range of 3 μm to 20 μm. The more preferable average grain size of the evaluation ferrite grains in the first grain size range is in a range of 3 μm to 10 μm. In a case where the average grain size of the evaluation ferrite grains in the first grain size range is in a range of 3 μm to 20 μm, fine crystal grains can accumulate a large amount of strain, so that strain can be efficiently dispersed to larger crystal grains. Therefore, the cold rollability of the steel sheet can be further improved.

It is preferable that the average grain size of the evaluation ferrite grains in the second grain size range exceeds 20 μm and is less than 80 μm. The more preferable average grain size of the evaluation ferrite grains in the second grain size range is in a range of 30 μm to 70 μm and more preferably in a range of 40 μm to 60 μm. In a case where the average grain size of the evaluation ferrite grains in the second grain size range is in a range of 30 μm to 70 μm, the cold rollability of the steel sheet can be further improved.

It is preferable that the average grain size of the evaluation ferrite grains in the third grain size range is in a range of 80 μm to 120 μm. The more preferable average grain size of the evaluation ferrite grains in the third grain size range is a range of 100 μm to 120 μm. In a case where the average grain size of the evaluation ferrite grains in the third grain size range is in a range of 80 μm to 120 μm, the cold rollability of the steel sheet can be further improved.

Next, a method of measuring ferrite grains at the position having a depth of ¼ of the sheet thickness will be described.

The identification of ferrite and the calculation of the grain size of ferrite can be performed by electron back scattering diffraction (EBSD) and observing a cross section of the steel sheet, which is parallel to a rolling direction and perpendicular to a sheet surface, at a magnification of 1000 to 50000 with a scanning electron microscope. At this time, for example, software “OIM DataCollection™ (ver. 7)” manufactured by TSL Solutions, Inc., or the like is used as software for acquiring crystal orientation data in the present invention.

An observed section is finished by colloidal silica polishing or electrolytic polishing, and an area and area ratio are measured at intervals (pitch) of 0.2 μm in a square region (a square having a side having a length of ⅛ to ⅜ of the sheet thickness in the sheet thickness direction) in a range having a depth of ⅛ to ⅜ of the sheet thickness, which is centered on the ¼ position of the sheet thickness from the surface of the steel sheet, by EBSD associated with a scanning electron microscope. Meanwhile, the sample preparation conditions and the like are set within the ranges of the conditions recommended in “standard for measurement of crystal misorientation for material evaluation using an electron backscatter diffraction (EBSD) method” of the Japan Institute of Metals and Materials. A value of grain average misorientation (GAM) is calculated from measured data. Further, a region in which the value of grain average misorientation is less than 0.5° is determined as ferrite grains. Here, boundaries at which crystal misorientation is 15° or more are determined as grain boundaries, and a region surrounded by the grain boundaries is determined as crystal grains. Grain average misorientation is a value obtained by calculating misorientation between adjacent measurement points and averaging the calculated values for all the measurement points in crystal grains. The grain sizes of the obtained ferrite grains are measured. The grain size is an equivalent circle diameter. Here, the equivalent circle diameter of a crystal grain means the diameter of a circle having an area equal to the area of a crystal grain.

<Vickers Hardness at Position Having Depth of ¼ of Sheet Thickness>

It is preferable that the Vickers hardness Hv of the steel sheet according to the present embodiment at the position having a depth of ¼ of the sheet thickness is 160 or less from the viewpoint of cold rollability. More preferably, the Vickers hardness Hv at the position having a depth of ¼ of the sheet thickness is 150 or less. In a case where the Vickers hardness Hv at the position having a depth of ¼ of the sheet thickness is 160 or less, cold rollability can be further improved.

The Vickers hardness of the steel sheet at the position having a depth of ¼ of the sheet thickness can be measured by the following procedure. First, a sheet thickness cross section, which is parallel to the rolling direction and the sheet thickness direction of the steel sheet, is finished as a mirror surface by mechanical polishing. In the polished section, the Vickers hardness (HV) is measured using an indentation load of 20 gf at 12 points on a straight line parallel to the rolling direction at a distance (depth) of ¼ of the sheet thickness from the surface of the steel sheet toward the inside of the sheet thickness. An average value of the Vickers hardnesses at 10 points excluding the lowest value and the highest value among these measured Vickers hardnesses at the 12 points is defined as the Vickers hardness Hv at the position having a depth of ¼ of the sheet thickness of the steel sheet. Meanwhile, it is preferable that an interval between the measurement points is set to a distance of 4 times or more the length of an indentation. The distance of 4 times or more the length of an indentation, which has been mentioned here, is a distance obtained by multiplying a numerical value of 4 times or more by the length of a diagonal line of an indentation formed by a diamond indenter during the measurement of the Vickers hardness.

[Total Elongation is 40% or More]

It is preferable that the total elongation of the steel sheet according to the present embodiment is 40% or more from the viewpoint of cold rollability. More preferably, the total elongation is 50% or more.

The total elongation can be obtained by collecting a JIS No. 5 tensile test piece from the steel sheet in a direction perpendicular to the rolling direction and the sheet thickness direction and performing a tensile test according to JIS Z 2241: 2011.

<Sheet Thickness>

The sheet thickness of the steel sheet according to the present embodiment is not limited. The sheet thickness of the steel sheet according to the present embodiment may be in a range of, for example, 1.5 mm to 5.0 mm.

<Manufacturing Method>

The steel sheet according to the present embodiment can be manufactured by a manufacturing method that includes the following steps (I) to (IV).

• • (I) A heating step of heating a slab having the above-described composition to a temperature equal to or higher than 1100° C. and lower than 1350° C.
  • (II) A finish rolling step of causing the slab, which has been subjected to the heating step, to continuously pass through a plurality of rolling stands to perform rolling such that a determined by Expression (1) satisfies “40≤σ≤80” at each of the first four rolling stands and satisfies “5≤σ≤10” at the last stand
  • (III) A cooling step of starting cooling a steel sheet at an average cooling rate of 30° C./s or higher within 0.2 seconds to 2.0 seconds after the end of the finish rolling step and cooling the steel sheet to a temperature range of 550° C. to 650° C.
  • (IV) A winding step of winding the steel sheet after the cooling step such that a winding temperature is in a range of 550° C. to 650° C.

Each step will be described below. Each temperature to be described below is a surface temperature of the slab or the steel sheet.

[Heating Step]

In the heating step, it is preferable that a slab having the same chemical composition as the steel sheet according to the present embodiment described above is heated to a temperature equal to or higher than 1100° C. and lower than 1350° C. In a case where a heating temperature is lower than 1100° C., the homogenization of materials is likely to be insufficient. Further, in a case where the heating temperature is set to 1350° C. or higher, the number of the evaluation ferrite grains in the first grain size range, the number of the evaluation ferrite grains in the second grain size range, and the number of the evaluation ferrite grains in the third grain size range are difficult to satisfy a predetermined relationship.

[Finish Rolling Step]

In the finish rolling step, the slab subjected to the heating step is subjected to rough rolling as necessary and is then caused to continuously pass through a plurality of rolling stands and to be subjected to rolling. In the manufacturing method for the steel sheet according to the present embodiment, it is preferable to perform rolling on the slab subjected to the heating step such that σ (unit: kgf/mm²) determined by Expression (1) satisfies “40≤σ≤80” at each of the first four rolling stands and satisfies “5≤σ≤10” at the last rolling stand.

$\begin{array}{cc}\sigma =\exp \left(0.753+3000/T\right)\times {\epsilon }^{0.21}\times {\epsilon }^{\mathrm{\prime 0}\text{.13}}& \left(1\right)\end{array}$

Here, T is a temperature (K) of the slab immediately before the slab enters the stand, ε is equivalent plastic strain, and ε′ is a strain rate (/s).

In a case where α determined by Expression (1) is set to 40 or more at each of the first four rolling stands, the amount of strain accumulated in the steel sheet during and immediately after working can be increased, so that crystal grains to be formed by recrystallization during and after the working can be appropriately refined. In addition, in a case where the rolling conditions of the last rolling stand to be described later are controlled, the number of the evaluation ferrite grains in the first grain size range, the number of the evaluation ferrite grains in the second grain size range, and the number of the evaluation ferrite grains in the third grain size range after austenite-to-ferrite transformation are likely to satisfy a predetermined relationship.

It is difficult to perform rolling on the slab such that a determined by Expression (1) exceeds 80 at each of the first four rolling stands, and an excessively fine structure is formed in a case where rolling is performed on the slab such that a exceeds 80. For this reason, even if the rolling conditions of the last rolling stand to be described later are controlled, the number of the evaluation ferrite grains in the first grain size range, the number of the evaluation ferrite grains in the second grain size range, and the number of the evaluation ferrite grains in the third grain size range after austenite-to-ferrite transformation are difficult to satisfy a predetermined relationship.

In the finish rolling step, it is preferable to perform rolling on the slab such that a satisfies “5≤σ≤10” at the last rolling stand. In a case where rolling is performed on the slab such that a determined by Expression (1) satisfies “40≤σ≤80” at each of the first four rolling stands and satisfies “5≤σ≤10” at the last rolling stand, a difference occurs in the amount of strain applied to each crystal grain. Accordingly, selective grain growth occurs in the winding step, so that the duplex grain structure of ferrite grains characteristic in the present invention is formed after austenite-to-ferrite transformation. Further, in a case where rolling is performed on the slab such that a satisfies “5≤σ≤10” at the last rolling stand, some crystal grains are sufficiently grown and coarsened. Accordingly, the Vickers hardness Hv of a steel sheet after austenite-to-ferrite transformation at a position having a depth of ¼ of the sheet thickness can be set to 160 or less.

It is preferable that a finish rolling start temperature is set to 1000° C. or higher. Accordingly, appropriate strain is likely to be accumulated at each of the first four rolling stands. In a case where the finish rolling start temperature is 1000° C. or higher, appropriate strain is likely to be accumulated in austenite grains during and immediately after working at each of the first four rolling stands.

It is preferable that an interpass time between the rolling stands in the finish rolling step is 10.0 seconds or shorter. Accordingly, appropriate strain is likely to be accumulated in austenite grains during and immediately after working. A lower limit does not need to be particularly limited and may be set to be as short as possible, but is set to be about 0.1 seconds considering a practical equipment configuration. Furthermore, in a case where the interpass time between the rolling stands in the finish rolling step is set to a time equal to or longer than 0.2 seconds and equal to or shorter than 3.0 seconds, appropriate strain is likely to be accumulated. It is preferable that an average interpass time between the rolling stands is set to a time equal to or longer than 0.2 seconds and equal to or shorter than 3.0 seconds.

It is preferable that a temperature on an exit side of the last rolling stand in the finish rolling step is set to a temperature equal to or higher than 850° C. and equal to or lower than 1000° C. In a case where the temperature on the exit side of the last rolling stand is set to a temperature equal to or higher than 850° C. and equal to or lower than 1000° C., appropriate strain is likely to be accumulated in austenite grains during and immediately after working at the last rolling stand.

It is preferable that a cumulative rolling reduction in the finish rolling step is set to 60% or more. In a case where the cumulative rolling reduction is set to 60% or more, appropriate strain is likely to be accumulated in austenite grains during and immediately after working.

“Cooling Step”

In the cooling step, it is preferable to start cooling the steel sheet at an average cooling rate of 30° C./s or higher between 0.2 seconds and 2.0 seconds after the end of the finish rolling step. In a case where the steel sheet is cooled at 30° C./s or higher between 0.2 seconds and 2.0 seconds, the growth of austenite grains can be stopped. Accordingly, after austenite-to-ferrite transformation, the number of the evaluation ferrite grains in the first grain size range is likely to be 2.5 times or more and 3.0 times or less the number of the evaluation ferrite grains in the second grain size range and the number of the evaluation ferrite grains in the third grain size range is likely to be 2.0 times or more and 2.5 times or less the number of the evaluation ferrite grains in the second grain size range. An average cooling rate can be obtained by dividing a difference between a cooling start temperature and a cooling end temperature by a time has passed until the end of cooling from the start of cooling.

In the cooling step, it is preferable to cool the steel sheet to a temperature range of 550° C. to 650° C. (cooling stop temperature range). In a case where the steel sheet is cooled to a temperature range of 550° C. to 650° C., the formation of martensite and bainite can be suppressed and the formation of pearlite and ferrite can be promoted.

“Winding Step”

In the winding step, the steel sheet is wound after the cooling step such that a winding temperature is in a temperature range of 550° C. to 650° C. In a case where the steel sheet is wound in a temperature range of 550° C. to 650° C., the formation of martensite and bainite can be suppressed and the formation of pearlite and ferrite can be promoted.

EXAMPLES

The present invention will be described more specifically with reference to examples.

Slabs having chemical compositions shown in Tables 1 and 2 were cast. The slabs having cast were heated under conditions shown in Table 3 and were subjected to finish rolling. After the finish rolling, steel sheets were cooled and wound under conditions shown in Tables 2A and 2B. An average interpass time between the rolling stands shown in Table 3 is an average value of the interpass times between the rolling stands. Meanwhile, an interpass time between the rolling stands in each of the examples was 0.2 seconds or longer and 3.0 seconds or shorter.

	TABLE 1
	Remainder: Fe and impurities (mass %)
Component	C	Si	Mn	Al	N	P	S	O	Cr	B	Nb	Mo	V
a	0.21	0.020	0.4	0.003	0.0012	0.008	0.004	0.0011
b	0.35	0.130	0.6	0.021	0.0090	0.010	0.010	0.0017	0.500	0.009		0.20
c	0.50	0.240	1.5	0.057	0.0100	0.014	0.007	0.0007			0.07		0.47
d	0.63	0.290	1.9	0.098	0.0070	0.018	0.003	0.0010
e	0.68	0.200	1.6	0.027	0.0046	0.010	0.010	0.0022
f	0.82	0.120	1.2	0.090	0.0097	0.030	0.001	0.0001
g	0.12	0.190	1.4	0.035	0.0008	0.015	0.003	0.0018				0.10
h	0.40	0.400	1.3	0.086	0.0099	0.019	0.010	0.0019		0.004
i	0.42	0.005	0.8	0.014	0.0057	0.020	0.010	0.0002	0.100
j	0.55	0.070	2.3	0.057	0.0069	0.022	0.003	0.0018			0.10
k	0.39	0.090	0.1	0.078	0.0025	0.018	0.002	0.0001					0.10
l	0.42	0.130	0.9	0.200	0.0088	0.016	0.002	0.0015
m	0.57	0.070	0.4	0.039	0.0300	0.017	0.002	0.0009
n	0.64	0.040	1.9	0.071	0.0075	0.060	0.004	0.0018
o	0.61	0.140	0.7	0.072	0.0093	0.012	0.020	0.0017
p	0.26	0.220	0.5	0.092	0.0073	0.023	0.008	0.0052

TABLE 2

Compo-

Remainder: Fe and impurities (mass %)

nent

Ti

Cu

W

Ta

Ni

Mg

Ca

Y

Zr

La

Ce

Sn

Sb

As

a

b

0.2800

c

d

0.400

0.300

0.002

0.002

0.002

e

0.400

0.400

0.002

0.001

0.001

0.001

0.001

0.001

f

0.1000

g

h

i

j

k

l

0.002

m

0.020

n

0.002

o

0.020

p

0.020

	TABLE 3
	Finish rolling step
				σ of	σ of	σ of				Temper-
				second	third	fourth			Average	ature
			σ of	rolling	rolling	rolling	σ of	Finish	interpass	on exit
		Heating	first	stand	stand	stand	last	rolling	time	side of
		step	rolling	from	from	from	rolling	start	between	last	Cumulative
		Heating	stand	first	first	first	stand	temper-	rolling	rolling	rolling	Sheet
Test	Compo-	temper-	(kgf/	(kgf/	(kgf/	(kgf/	(kgf/	ature	stands	stand	reduction	thick-
No.	nent	ature	mm2)	mm2)	mm2)	mm2)	mm2)	(° C.)	(sec.)	(° C.)	(%)	ness	Note
1	a	1241	50	52	56	58	7	1171	0.7	919	94	1.8	Example
													of present
													invention
2	a	1263	51	59	56	58	8	1042	0.9	862	90	3.9	Example
													of present
													invention
3	a	1208	52	52	50	58	5	1032	0.9	853	93	3.7	Example
													of present
													invention
4	a	1206	53	52	44	62	4	969	1.0	907	96	3.2	Comparative
													example
5	a	1251	49	42	41	42	13	1178	0.8	824	94	5.0	Comparative
													example
6	b	1207	51	61	52	46	9	1147	2.8	904	95	3.6	Example
													of present
													invention
7	b	1221	50	44	57	65	10	1054	0.7	942	89	4.8	Example
													of present
													invention
8	b	1379	69	42	54	47	9	1074	1.0	888	92	2.3	Comparative
													example
9	b	1080	41	70	52	38	4	1004	0.6	853	88	3.6	Comparative
													example
10	c	1300	51	55	59	68	9	1157	0.3	852	92	1.0	Example
													of present
													invention
11	c	1249	43	69	42	55	7	1118	0.3	905	92	4.5	Example
													of present
													invention
12	c	1256	69	42	42	87	5	1185	1.0	888	92	3.5	Comparative
													example
13	c	1249	60	46	48	55	9	1027	1.0	888	92	4.5	Comparative
													example
14	d	1233	40	63	54	50	10	1110	0.9	920	97	4.4	Example
													of present
													invention
15	d	1220	66	67	53	61	9	1129	0.3	917	94	3.8	Example
													of present
													invention
16	d	1263	42	46	57	78	9	1185	4.2	929	96	2.1	Comparative
													example
17	d	1263	56	64	42	67	9	1131	0.5	929	96	4.2	Comparative
													example
18	e	1264	47	49	54	66	7	1082	0.7	897	93	1.5	Example
													of present
													invention
19	e	1264	55	42	41	42	8	1004	0.9	885	93	2.8	Example
													of present
													invention
20	e	1282	45	42	43	82	23	1162	0.4	805	93	1.5	Comparative
													example
21	f	1262	40	49	42	63	5	1073	0.5	920	94	2.6	Comparative
													example
22	g	1215	47	70	58	52	6	1012	0.2	855	92	1.5	Comparative
													example
23	h	1296	54	58	59	62	8	1084	1.2	840	89	3.8	Comparative
													example
24	i	1212	68	53	59	51	7	1082	0.2	912	95	4.3	Comparative
													example
25	j	1246	60	52	69	44	6	1109	2.0	897	94	4.7	Comparative
													example
26	k	1240	76	40	41	33	7	1078	0.9	832	91	4.7	Comparative
													example
27	l	1234	58	53	59	51	8	1146	0.2	834	88	4.1	Comparative
													example
28	m	1228	68	73	69	51	9	1141	0.5	912	85	4.2	Comparative
													example
29	n	1228	58	53	78	61	5	1128	0.7	908	85	4.5	Comparative
													example
30	o	1228	77	58	56	51	9	1056	0.8	804	85	1.5	Comparative
													example
31	p	1228	57	63	55	72	8	1078	1.2	809	85	3.0	Comparative
													example
32	a	1237	61	51	52	48	9	1147	2.8	904	95	4.4	Example
													of present
													invention
33	b	1221	65	45	57	69	10	1054	0.7	942	89	4.2	Example
													of present
													invention

	TABLE 4
	Cooling step	Winding
	Time until		Cooling	step
	start of	Cooling	stop	Winding
Test	cooling	rate	temperature	temperature
No.	(sec.)	(° C./s)	(° C.)	(° C.)	Note
1	1.4	167	632	626	Example of present invention
2	0.3	149	575	616	Example of present invention
3	0.2	77	589	645	Example of present invention
4	0.4	134	578	612	Comparative example
5	0.2	116	572	569	Comparative example
6	0.8	111	557	620	Example of present invention
7	0.6	34	590	641	Example of present invention
8	0.7	165	593	649	Comparative example
9	0.6	138	622	647	Comparative example
10	0.7	155	555	598	Example of present invention
11	0.8	136	586	623	Example of present invention
12	0.7	165	593	649	Comparative example
13	3.0	165	593	649	Comparative example
14	0.5	35	567	598	Example of present invention
15	0.8	52	646	624	Example of present invention
16	0.2	157	670	680	Comparative example
17	0.2	157	520	450	Comparative example
18	0.7	126	636	638	Example of present invention
19	0.8	165	603	580	Example of present invention
20	0.3	76	604	634	Comparative example
21	0.6	70	628	636	Comparative example
22	0.7	120	630	625	Comparative example
23	0.1	31	581	556	Comparative example
24	0.4	111	572	625	Comparative example
25	0.4	167	552	581	Comparative example
26	0.6	110	568	606	Comparative example
27	1.0	71	589	618	Comparative example
28	0.7	35	604	629	Comparative example
29	0.4	121	644	636	Comparative example
30	0.6	33	561	559	Comparative example
31	0.6	83	626	605	Comparative example
32	1.2	167	638	646	Example of present invention
33	0.3	149	565	636	Example of present invention

A test piece for SEM observation was collected from the obtained hot-rolled steel sheet as described above, a sheet thickness cross section parallel to a rolling direction was polished, and a microstructure at a position having a depth of ¼ of the sheet thickness was observed by the above-described method, so that area ratios of ferrite, pearlite, bainite, and martensite at the position having a depth of ¼ of the sheet thickness were obtained. Obtained results are shown in Table 5. Similarly, using the above-described method, the grain sizes of ferrite grains were measured, and the number N1 and an average grain size of the evaluation ferrite grains in the first grain size range, the number N2 and an average grain size of the evaluation ferrite grains in the second grain size range, and the number N3 and an average grain size of the evaluation ferrite grains in the third grain size range were evaluated. Obtained results are shown in Table 5. A ratio (N1/N2) of the number N1 of the evaluation ferrite grains in the first grain size range to the number N2 of the evaluation ferrite grains in the second grain size range, a ratio (N3/N2) of the number N3 of the evaluation ferrite grains in the third grain size range to the number N2 of the evaluation ferrite grains in the second grain size range, a ratio (N1/N3) of the number N1 of the evaluation ferrite grains in the first grain size range to the number N2 of the evaluation ferrite grains in the third grain size range, and a Vickers hardness are shown in Table 6.

Total elongation was obtained by collecting a JIS No. 5 tensile test piece from the hot-rolled steel sheet in a direction perpendicular to the rolling direction and performing a tensile test according to JIS Z 2241: 2011. Obtained results are shown in Table 6.

Further, with regard to the cold rollability of the hot-rolled steel sheet, 90° bending workability was evaluated by a 90° V-block test. In a case where the sheet thickness of the steel sheet was denoted by t and an inside minimum bend radius of a punch was denoted by R, the test piece was pushed into a 90-degree die using a 90-degree punch having a curvature at which a ratio R/t was 1 and was then taken out and an appearance of the bend was visually observed. As the results of visual observation, a case where cracking occurred was marked with X and a case where no abnormality was observed was marked with O. Meanwhile, a defect of which a maximum width of a gap is 1 mm or more was defined as “cracking”. Obtained results are shown in Table 6.

As seen from Table 6, all of the steels of the present invention were excellent in cold rollability. Further, since steels of the present invention were excellent in cold rollability without spheroidizing annealing, the steels of the present invention were excellent productivity.

	TABLE 5
		Evaluation ferrite grains at position having depth of
	Microstructure at position having depth of ¼ of	¼ of sheet thickness
	sheet thickness (area %)	First grain	Second grain	Third grain
	remainder in microstructure	size range	size range	size range
					Total		Average		Average		Average
					remainder	Number	grain	Number	grain	Number	grain
Test					in micro-	of pieces	size	of pieces	size	of pieces	size
No.	Ferrite	Pearlite	Bainite	Martensite	structure	(pieces)	(μm)	(pieces)	(μm)	(pieces)	(μm)	Note
1	49	42	4	5	9	125	18	50	50	102	96	Example
												of present
												invention
2	47	48	2	3	5	117	17	40	30	95	115	Example
												of present
												invention
3	52	43	1	4	5	120	11	48	36	114	80	Example
												of present
												invention
4	14	83	2	1	3	32	5	125	60	256	119	Comparative
												example
5	9	86	5	0	5	55	5	146	52	365	82	Comparative
												example
6	51	44	1	4	5	105	19	36	47	74	83	Example
												of present
												invention
7	53	42	5	0	5	126	20	49	32	102	92	Example
												of present
												invention
8	47	47	2	4	6	103	4	179	67	410	95	Comparative
												example
9	13	77	8	2	10	123	6	124	68	306	111	Comparative
												example
10	49	45	4	2	6	113	18	42	42	91	96	Example
												of present
												invention
11	45	46	8	1	9	123	4	43	46	93	116	Example
												of present
												invention
12	9	83	4	4	8	53	15	138	32	345	102	Comparative
												example
13	9	83	4	4	8	126	14	145	48	309	80	Comparative
												example
14	48	43	6	3	9	130	13	47	57	106	116	Example
												of present
												invention
15	43	49	7	1	8	100	19	35	31	73	102	Example
												of present
												invention
16	74	22	3	1	4	107	13	123	54	290	100	Comparative
												example
17	19	41	26	14	40	109	19	146	61	356	91	Comparative
												example
18	50	44	1	5	6	123	11	44	63	103	108	Example
												of present
												invention
19	52	40	6	2	8	101	16	34	41	77	82	Example
												of present
												invention
20	49	44	2	5	7	115	17	101	57	237	95	Comparative
												example
21	52	44	0	4	4	105	13	99	59	247	83	Comparative
												example
22	50	42	8	0	8	107	6	138	61	302	113	Comparative
												example
23	35	60	0	5	5	120	15	100	33	217	100	Comparative
												example
24	47	44	9	0	9	100	8	93	50	227	119	Comparative
												example
25	58	40	2	0	2	101	4	144	48	340	89	Comparative
												example
26	41	50	9	0	9	111	16	139	47	286	99	Comparative
												example
27	40	47	8	5	13	130	7	144	46	331	104	Comparative
												example
28	34	52	10	4	14	115	8	104	49	253	80	Comparative
												example
29	44	42	12	2	14	112	10	122	33	265	112	Comparative
												example
30	28	59	9	4	13	115	5	149	36	337	84	Comparative
												example
31	47	42	10	1	11	114	10	108	43	244	87	Comparative
												example
32	49	46	1	4	5	126	19	44	27	103	102	Example
												of present
												invention
33	53	42	5	0	5	100	15	35	75	77	80	Example
												of present
												invention

TABLE 6
				Vickers hardness
				(Hv) at position
				having depth of	Total
Test				¼ of sheet	elongation	Cold
No.	N1/N2	N3/N2	N1/N3	thickness	(%)	rollability	Note
1	2.5	2.0	1.2	138	51	◯	Example
							of present
							invention
2	2.9	2.4	1.2	146	41	◯	Example
							of present
							invention
3	2.5	2.4	1.1	157	50	◯	Example
							of present
							invention
4	0.3	2.1	0.1	170	36	X	Comparative
							example
5	0.4	2.5	0.2	180	33	X	Comparative
							example
6	2.9	2.1	1.4	123	56	◯	Example
							of present
							invention
7	2.6	2.1	1.2	136	55	◯	Example
							of present
							invention
8	0.6	2.3	0.3	172	27	X	Comparative
							example
9	1.0	2.5	0.4	175	24	X	Comparative
							example
10	2.7	2.2	1.2	160	57	◯	Example
							of present
							invention
11	2.9	2.2	1.3	147	47	◯	Example
							of present
							invention
12	0.4	2.5	0.2	166	24	X	Comparative
							example
13	0.9	2.1	0.4	178	22	X	Comparative
							example
14	2.8	2.3	1.2	141	47	◯	Example
							of present
							invention
15	2.9	2.1	1.4	130	43	◯	Example
							of present
							invention
16	0.9	2.4	0.4	174	31	X	Comparative
							example
17	0.7	2.4	0.3	167	24	X	Comparative
							example
18	2.8	2.3	1.2	135	46	◯	Example
							of present
							invention
19	2.9	2.3	1.3	128	58	◯	Example
							of present
							invention
20	1.1	2.4	0.5	174	30	X	Comparative
							example
21	1.1	2.5	0.4	177	30	X	Comparative
							example
22	0.8	2.2	0.4	169	31	X	Comparative
							example
23	1.2	2.2	0.6	165	24	X	Comparative
							example
24	1.1	2.4	0.4	177	34	X	Comparative
							example
25	0.7	2.4	0.3	177	26	X	Comparative
							example
26	0.8	2.1	0.4	168	26	X	Comparative
							example
27	0.9	2.3	0.4	180	26	X	Comparative
							example
28	1.1	2.4	0.5	179	20	X	Comparative
							example
29	0.9	2.2	0.4	174	33	X	Comparative
							example
30	0.8	2.3	0.3	162	36	X	Comparative
							example
31	1.1	2.3	0.5	180	25	X	Comparative
							example
32	2.8	2.3	1.2	123	59	◯	Example
							of present
							invention
33	2.9	2.2	1.3	136	55	◯	Example
							of present
							invention

INDUSTRIAL APPLICABILITY

Since the hot-rolled steel sheet according to the embodiment of the present disclosure is excellent in cold rollability, the hot-rolled steel sheet has high industrial applicability.

Claims

1. A hot-rolled steel sheet comprising, as a chemical composition, by mass %:

C: 0.20% to 0.70%;

Si: 0.010% to 0.300%;

Mn: 0.3% to 2.0%;

Al: 0.001% to 0.100%;

N: 0.0010% to 0.0100%;

P: 0.008% to 0.030%;

S: 0.010% or less;

O: 0.0025% or less;

Cr: 1.500% or less;

B: 0.010% or less;

Nb: 0.50% or less;

Mo: 0.50% or less;

V: 0.50% or less;

Ti: 0.3000% or less;

Cu: 0.500% or less;

W: 0.500% or less;

Ta: 0.500% or less;

Ni: 0.500% or less;

Mg: 0.003% or less;

Ca: 0.003% or less;

Y: 0.030% or less;

Zr: 0.030% or less;

La: 0.030% or less;

Ce: 0.030% or less;

Sn: 0.030% or less;

Sb: 0.030% or less;

As: 0.030% or less; and

a remainder including Fe and impurities,

wherein at a depth position of ¼ of a sheet thickness, a microstructure is formed of 20 area % or more of ferrite, 40 area % or more of pearlite, and 0 area % or more and 10 area % or less of a remainder in microstructure,

the remainder in microstructure includes at least one of bainite and martensite,

among ferrite grains to be measured by an electron backscattering diffraction method, ferrite grains excluding ferrite grains occupying 5% of a total number of the ferrite grains from a maximum grain size side and ferrite grains occupying 5% of the total number of the ferrite grains from a minimum grain size side of the ferrite grains are treated as evaluation ferrite grains,

a minimum value of a grain size of the evaluation ferrite grains is set as a first grain size,

a maximum value of the grain size of the evaluation ferrite grains is set as a second grain size,

a grain size obtained by adding, to the first grain size, ⅓ of a difference between the second grain size and the first grain size is set as a third grain size,

a grain size obtained by adding, to the first grain size, ⅔ of the difference between the second grain size and the first grain size is set as a fourth grain size, and

when a range that is equal to or more than the first grain size and equal to or less than the third grain size is set as a first grain size range,

a range that is more than the third grain size and equal to or less than the fourth grain size is set as a second grain size range, and

a range that is more than the fourth grain size and equal to or less than the second grain size is set as a third grain size range,

the number of the evaluation ferrite grains in the first grain size range is 2.5 times or more and 3.0 times or less the number of the evaluation ferrite grains in the second grain size range, and

the number of the evaluation ferrite grains in the third grain size range is 2.0 times or more and 2.5 times or less the number of the evaluation ferrite grains in the second grain size range.

2. The hot-rolled steel sheet according to claim 1,

wherein an average grain size of the evaluation ferrite grains in the first grain size range is in a range of 3 μm to 20 μm.

3. The hot-rolled steel sheet according to claim 1,

wherein an average grain size of the evaluation ferrite grains in the third grain size range is in a range of 80 m to 120 m.

4. The hot-rolled steel sheet according to claim 1,

wherein a Vickers hardness Hv is 160 or less at the position having a depth of ¼ of the sheet thickness.

5. The hot-rolled steel sheet according to claim 1,

wherein a total elongation is 40% or more.