Hot-rolled steel sheet and method for producing same

Abstract

Provided are a hot-rolled steel sheet that has a yield strength (YS) of 680 MPa or greater, and is excellent in bendability and toughness, and a method for producing the same. The hot-rolled steel sheet has a chemical composition including C, Si, Mn, P, S, Al, N, Ti, and other optional components, has a metallographic structure where the area ratio of ferrite is 0% or more but 85% or less, the area ratio of residual austenite is 3% or less in total, the area ratio of a microstructure with a lath morphology is 5% or less, and the area ratio of a microstructure with a KAM value of 1.0 or greater is 15% or greater, includes Ti-containing carbides with an average grain size of 8 nm or less, and has a yield strength of 680 MPa or greater. The method for producing a hot-rolled steel sheet includes a rough-rolling step of rough-rolling a steel material having the foregoing chemical composition after heating it or without heating it, a finishing-rolling step of performing finishing rolling by setting the start temperature to 950°C or higher, setting the total rolling reduction for passes from a first pass to a fifth pass to 75% or greater, and setting a completion temperature to 860°C or higher but 910°C or lower, a cooling step, and a winding step.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet that has a yield strength of 680 MPa or greater, and is excellent in bendability and toughness, and a method for producing the same.Background Art

[0002] In recent years, from the perspective of global environmental protection, attempts have been made throughout the automobile industry to improve the fuel efficiency of automobiles for the purpose of regulating CO 2 emissions. To improve the fuel efficiency of an automobile, it is most effective to reduce the weight of the automobile by thinning parts used therefor. Thus, in recent years, there has been an increase in the volume of high-strength steel sheets that are used as the materials of automobile parts.

[0003] Typically, as the strength of a steel sheet increases, its formability and toughness tend to decrease. Therefore, to further promote the widespread use of high-strength steel sheets, it is essential to achieve high strength, high workability, and high toughness at the same time.

[0004] To solve the foregoing problems, various technologies for increasing the strength as well as the workability of steel sheets have been proposed.

[0005] For example, Patent Literature 1 discloses a hot-rolled steel sheet in which the area ratio of ferrite crystal grains is 95% or greater, and Ti carbides with an average grain size of less than 6 nm and TiS with an average grain size of 0.5 µm or less are dispersed in the ferrite crystal grains. Accordingly, it is presumably possible to obtain a high-tensile hot-rolled steel sheet having excellent bendability and a tensile strength of 780 MPa or greater but 900 MP or less.

[0006] Patent Literature 2 discloses a technology in which a steel slab containing one or more of Ti and Nb is heated and hot rough-rolled into a steel sheet, and then the steel sheet is joined to the rear end of a preceding rough-rolled steel sheet and is subjected to hot finishing rolling in the temperature range of Ar3 to Ar3+50°C. This makes it possible to obtain a hot-rolled steel sheet with excellent toughness for working.Citation ListPatent Literature

[0007] Patent Literature 1: International Publication No. WO 2013 / 099196 Patent Literature 2: JP-A-H09-227949 Summary of InventionTechnical Problem

[0008] However, the conventional technologies disclosed in the patent literatures have the following problems.

[0009] With the technology disclosed in Patent Literature 1, it would be impossible to obtain a microstructure required in the present invention, as shown, for example, in a steel sheet No. 5 of Example. Therefore, it would be impossible to achieve both excellent bendability and excellent toughness at a yield strength of 680 MPa or greater.

[0010] Meanwhile, with the technology disclosed in Patent Literature 2, it would be impossible to obtain high strength of 680 MPa or greater. Further, Patent Literature 2 fails to suggest the requirements for obtaining excellent bendability. Furthermore, controlling the temperature of hot rolling for obtaining high toughness in a narrow range would significantly decrease productivity, and it may be impossible to perform such control depending on the size of a hot-rolled steel sheet to be produced.

[0011] The present invention is developed in view of the foregoing problems of the conventional technologies, and it is an object of the present invention to provide a hot-rolled steel sheet that has a yield strength (YS) of 680 MPa or greater, and is excellent in bendability and toughness, and a method for producing the same.Solution to Problem

[0012] To solve the foregoing problems, the inventors made concentrated studies on the requirements for the production of a hot-rolled steel sheet that has a yield strength of 680 MPa or greater, high bendability, and high toughness. To obtain excellent bendability, it is necessary to provide high ductility, which is unfavorable for increasing strength. Nevertheless, the inventors have studied a process based on a high winding temperature that can obtain high total elongation. Specifically, the inventors have attempted to strengthen a hot-rolled steel sheet using ultrafine, nanosized T-containing carbides to obtain a yield strength of 680 MPa or greater at a winding temperature of 600°C or higher for the hot-rolled steel sheet.

[0013] However, it has been commonly considered that when a hot-rolled steel sheet, in which Ti-containing carbides are caused to precipitate, is wound at a temperature of 600°C or greater, a ferrite phase, which includes not so many dislocations, is formed in the steel sheet. The unit of fracture surface of such a ferrite microstructure that has a significant influence on toughness is considered to be equivalent to the ferrite grain size. As a result of studying a reduction in the grain size of the ferrite phase, the inventors have concluded that it is difficult to stably obtain target toughness.

[0014] The inventors conducted intensive studies on the possibility of forming a crystal structure other than ferrite when the hot-rolled steel sheet is wound at a temperature of 600°C or higher. As a result, the inventors found that a new microstructure, which is classified as neither ferrite nor bainite, is obtained, and such a microstructure is excellent in all aspects of strength, bendability, and toughness.

[0015] The inventors found that such a new microstructure is produced from fine austenite grains obtained through recrystallization during the process of finishing rolling in hot rolling.

[0016] A hot-rolled steel sheet according to the present invention developed based on the foregoing findings has the following features. [1] A hot-rolled steel sheet that has a chemical composition including, in mass%: C: 0.035% or more but less than 0.110%; Si: 1.5% or less; Mn: 1.3% or less; P: 0.05% or less; S: 0.010% or less; Al: 0.005% or more but 0.080% or less; N: 0.0060% or less; and Ti: 0.08% or more but 0.20% or less, and further optionally including one or both of Groups A and B of components below: Group A: B: 0.0002% or more but 0.0050% or less; and Group B: one or more of Nb, V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se: 1% or less in total, with a balance being Fe and unavoidable impurities, characterized in that the hot-rolled steel sheet has a metallographic structure where an area ratio of ferrite is 0% or more but 85% or less, an area ratio of residual austenite is 3% or less, an area ratio of a microstructure with a lath morphology is 5% or less, and an area ratio of a microstructure with a KAM value of 1.0 or greater is 15% or greater, and the hot-rolled steel sheet includes Ti-containing carbides with an average grain size of 8 nm or less, and has a yield strength of 680 MPa or greater. [2] The hot-rolled steel sheet according to [1] above, further including a plating layer on a surface of the hot-rolled steel sheet.

[0017] A method for producing a hot-rolled steel sheet according to the present invention developed based on the foregoing findings has the following features. [3] A method for producing a hot-rolled steel sheet, including a rough-rolling step of rough-rolling a steel material having the chemical composition of [1] above after heating the steel material to 1200°C or higher or without heating the steel material that has been cast, thereby obtaining a sheet bar; a finishing-rolling step of finishing-rolling the sheet bar by setting a start temperature of rolling to 950°C or higher, setting a total rolling reduction for passes from a first pass through a fifth pass to 75% or greater, and setting a completion temperature of rolling to 860°C or higher but 910°C or lower, thereby obtaining a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet to a cooling stop temperature of 600°C or higher but 700°C or lower at an average cooling rate of 40°C / s or greater; and a winding step of winding the cooled hot-rolled steel sheet at a winding temperature of 600°C or higher but 700°C or lower. [4] The method for producing a hot-rolled steel sheet according to [3] above, further including a casting step of casting the steel material having the chemical composition of [1] and having a thickness of 35 mm or more but 200 mm or less before the rough-rolling step or the finishing-rolling step, in which the sheet bar is obtained by applying or not applying the rough-rolling step. [5] The method for producing a hot-rolled steel sheet according to [3] above, further including a joining step of joining the sheet bar obtained by the rough rolling to a preceding sheet bar at a temperature of 1050°C or higher, between the rough-rolling step and the finishing-rolling step, in which the finishing-rolling step includes finishing-rolling the joined sheet bars. [6] The method for producing a hot-rolled steel sheet according to any one of [3] to [5] above, further including a hot-band annealing step of annealing the hot-rolled steel sheet at an annealing temperature of 720°C or lower; and a plating step of plating the annealed hot-rolled steel sheet. [7] The method for producing a hot-rolled steel sheet according to [6] above, further including an alloying step of alloying the plated hot-rolled steel sheet at a temperature of 480°C or higher but 600°C or lower. Advantageous Effects of Invention

[0018] According to the present invention, it is possible to produce a hot-rolled steel sheet that has high strength, specifically, a yield strength (YS) of 680 MPa or greater, and is excellent in bendability and toughness. The hot-rolled steel sheet according to the present invention is suitable as a material of a suspension member for an automobile. Thus, when such a hot-rolled steel sheet is applied to an automobile part, the weight of the automobile part can be further reduced.Description of Embodiments

[0019] Hereinafter, a hot-rolled steel sheet according to the present embodiment will be described.<Chemical components of hot-rolled steel sheet>

[0020] The hot-rolled steel sheet has a chemical composition including, in mass%, C: 0.035% or more but less than 0.110%, Si: 1.5% or less, Mn: 1.3% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more but 0.080% or less, N: 0.0060% or less, and Ti: 0.08% or more but 0.20% or less. Each component will be described below. In the following description, "%" representing the content of each component means "mass%."C: 0.035% or more but less than 0.110%

[0021] C contributes to increasing the strength of the steel sheet, and forming a high dislocation structure during isothermal transformation, by bonding to Ti. The C content is set to 0.035% or greater to obtain a steel sheet with a yield strength of 680 MPa or greater. Meanwhile, if the C content is 0.110% or greater, coarse cementite will be precipitated, which results in an increased risk that bendability as well as toughness will be reduced. Therefore, the C content is set to 0.035% or more but less than 0.110%. Preferably, the C content is set to 0.035% or more but 0.10% or less.Si: 1.5% or less

[0022] Si is an element effective in enhancing workability as it increases the degree of elongation of the steel sheet and suppresses the precipitation of cementite. Meanwhile, if the Si content is over 1.5%, the effect of increasing bendability will be reduced, and surface quality as well as weldability will degrade, which results in increased adverse effects of the addition of Si in a large amount. Therefore, the Si content should be 1.5% or less. Preferably, the Si content should be 1.2% or less. Note that even when the Si content is 0%, the advantageous effects of the present embodiment are not impaired. However, to stably form a microstructure having no lath structure and thus having large crystal strain, the Si content is preferably set to 0.15% or greater.Mn: 1.3% or less

[0023] Mn increases hardenability and suppresses the formation of ferrite with small crystal strain during a cooling process following hot rolling. To stably produce a hot-rolled steel sheet, the Mn content should be 0.2% or greater. In addition, for stable recrystallization of austenite during hot rolling, it is preferable that hot-working strain be stably present. Thus, it is effective to control each of the contents of Si and Mn, which are substitutional solute elements, within a narrow range. To this end, Expression (1) below is preferably satisfied. 1.1 ≤ 0.8 % Si + % Mn ≤ 1.5 where [%Si] and [%Mn] represent the Si content and the Mn content, respectively, in mass%.

[0024] Meanwhile, if the Mn content exceeds 1.3%, the driving force for the phase transformation from austenite to ferrite will excessively decrease, so that a microstructure with small crystal strain will not be obtained. Therefore, the Mn content should be 1.3% or less. Preferably, the Mn content should be 1.2% or less.P: 0.05% or less

[0025] P is a detrimental element that reduces toughness by segregating at grain boundaries. Thus, the P content is preferably minimized. In the present embodiment, the acceptable upper limit of the P content is 0.05%. The P content should be 0.04% or less. It is preferably 0.02% or less when the hot-rolled steel sheet is used in an environment where higher toughness is required. Meanwhile, 0.002% P may be unavoidably mixed during production.S: 0.010% or less

[0026] S forms coarse sulfides in steel, and such sulfides are elongated during hot rolling so as to form wedge-shaped inclusions, which adversely affect toughness. It is therefore preferable to reduce the content of S, which is also a detrimental element. The acceptable upper limit of the S content is 0.010%. The S content should be 0.003% or less. Preferably, the S content should be 0.001% or less when the hot-rolled steel sheet is to be used in an environment where higher toughness is required. Meanwhile, 0.0001% S may be unavoidably mixed in producitonAl: 0.005% or more but 0.080% or less

[0027] When Al is added as a deoxidizing agent during a steelmaking process, the Al content should be 0.005% or greater. Al reduces bendability and toughness by forming oxide. Thus, the Al content should be 0.080% or less. Preferably, the Al content should be 0.010% or more but 0.070% or less.N: 0.0060% or less

[0028] N is a detrimental element that reduces strength, bendability, and toughness by bonding to Ti and thus forming coarse TiN. Therefore, the N content is preferably minimized. The acceptable upper limit of the N content is 0.0060%. Preferably, the N content should be 0.0050% or less. Approximately 0.0005% N may be unavoidably mixed during production.Ti: 0.08% or more but 0.20% or less

[0029] Ti contributes to increasing the strength of the steel sheet by bonding to C to form fine Ti-containing carbides. To achieve a yield strength of 680 MPa or greater, the Ti content should be 0.08% or greater. Meanwhile, when the Ti content exceeds 0.20%, it is difficult to melt coarse Ti-containing carbides in a heating step performed before hot rolling. Thus, the effect of increasing the strength is saturated and bendability and toughness are also adversely affected. Therefore, the Ti content should be 0.08% or more but 0.20% or less. Preferably, the Ti content should be 0.09% or more but 0.19% or less.

[0030] As described above, C contributes to the formation of a microstructure with large crystal strain, while it is also used to form Ti-containing carbides by bonding to Ti. Therefore, to stably obtain a metallographic structure required for the hot-rolled steel sheet according to the present embodiment, Expression (2) below is preferably satisfied. In particular, when Expression (2) is less than 1.4, the concentration of C that deposits at grain boundaries during isothermal transformation decreases, so that a microstructure with large crystal strain cannot be stably achieved. Therefore, Expression (2) is preferably 1.4 or greater.

[0031] Meanwhile, to obtain a steel sheet with a yield strength of 680 MPa or greater, it is necessary to strengthen the steel sheet using fine Ti-containing carbides on the order of nanometers. However, when Expression (2) is over 2.8, it will be difficult to melt coarse TiC when the slab is heated again, reducing the strength and the bendability of the steel sheet. Therefore, Expression (2) should preferably be 2.8 or less. 1.4 ≤ % C / 12 / % Ti * / 48 ≤ 2.8 where [%Ti*] = [%Ti] - 48[%N] / 14.

[0032] Herein, [%CN], [%Ti], and [%N] respectively represent the C content, the Ti content, and the N content in mass%.

[0033] The hot-rolled steel sheet according to the embodiment has the foregoing basic chemical composition, and may further optionally contain one or both of Groups A and B of components described below. Group A: B: 0.0002% or more but 0.0050% or less Group B: one or more of Nb, V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se: 1% or less in total B: 0.0002% or more but 0.0050% or less

[0034] B is an element effective in increasing hardenability. To obtain a microstructure with large crystal strain, it is necessary to secure high hardenability. Setting the B content to 0.0002% or greater can contribute to stably obtaining a desired microstructure. Meanwhile, if the B content exceeds 0.0050%, the effect of increasing the hardenability of steel will be saturated. Thus, the B content should be 0.0050% or less. Preferably, the B content should be 0.0004% or more but 0.0030% or less.One or more of Nb, V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se: 1% or less in total

[0035] The acceptable range of the total content of one or more of such elements is 1% or less as such a range has little influence on the properties of the hot-rolled steel sheet according to the present embodiment. Meanwhile, the content of each element is preferably limited to 0.03% or less.

[0036] The hot-rolled steel sheet according to the present embodiment has a chemical composition including the foregoing elements, with the balance being Fe and unavoidable impurities.<Metallographic structure of hot-rolled steel sheet>

[0037] Next, the metallographic structure of the hot-rolled steel sheet will be described.

[0038] The hot-rolled steel sheet of the present embodiment has a metallographic structure in which the area ratio of ferrite is 0% or more but 85% or less, the area ratio of residual austenite is 3% or less, the area ratio of a microstructure having a lath morphology is 5% or less, and the area ratio of a microstructure having a KAM value of 1.0 or greater is 15% or greater, and the hot-rolled steel sheet includes Ti-containing carbides with an average grain size of 8 nm or less.

[0039] In the following description, "%" used to represent the metallographic structure refers to the "area ratio."Area ratio of ferrite: 0% or more but 85% or less

[0040] Ferrite is a microstructure with inferior toughness because the unit of fracture surface of ferrite upon the occurrence of a brittle fracture is larger than that of the new microstructure having large crystal strain of the present embodiment. Since ferrite has small crystal strain within its grain, its KAM value is below 1.0. To obtain the desired toughness, the area ratio of ferrite needs to be limited to 85% or less. The area ratio of ferrite should be 80% or less and preferably 70% or less.Area ratio of residual austenite: 3% or less (including 0%)

[0041] Each of bainite and tempered martensite defined in the present embodiment is observed to have a lath structure within its grain. Martensite is a microstructure that is observed to appear white in contrast to others in an SEM but may be cementite. Thus, the two may be distinguished from each other based on their crystal structures through electron backscatter diffraction (EBSD) pattern analysis. For example, bainite, martensite, and tempered martensite, which satisfy the relationship between the parent phase and Kurdjumov-Sachs, can be identified by obtaining a (001) α pole figure of a single former γ-grain region. The area ratio of residual austenite can be determined by performing XRD analysis on a sample obtained by subjecting the surface of the steel sheet to grinding work to a depth of 1 / 4 from the surface in the thickness direction, and then applying chemical polishing thereto by 0.1 mm or more. Such microstructures reduce the strength, workability, and toughness of the hot-rolled steel sheet of the present embodiment. Thus, such microstructures should be minimized. The area ratio of residual austenite is set to 3% or less. The total area ratio of bainite, martensite, tempered martensite, and residual austenite should be 5% or less, and preferably, it should be 3% or less.Area ratio of microstructure having lath morphology: 5% or less, and area ratio of microstructure having KAM value of 1.0 or greater: 15% or greater

[0042] The main technical feature of the present embodiment is that a microstructure that has no lath structure and has large crystal strain is strengthened using Ti-containing carbides with a grain size of 8 nm or less. Ferrite has small crystal strain, that is, a KAM value of less than 1. Low-temperature transformation phases, such as bainite, martensite, and tempered martensite, have a lath structure. A microstructure that has no lath structure and has large crystal strain is a microstructure that cannot be classified as ferrite or bainite. Lath is a microstructure observed as a plate-like morphology within a grain through transmission electron microscope (TEM) or EBSD analysis. Such a microstructure having a lath structure is hard, but has poor workability and cannot achieve the desired bendability. A microstructure having large crystal strain of the present invention refers to one having a KAM value, as determined by EBSD analysis, of 1.0 or greater. The KAM value represents the degree of disorder of the crystal structure. With the disorder of crystal grains, the effective unit of fracture surface will become smaller, so that a microstructure that is tougher than a ferrite microstructure can be obtained. Accordingly, a steel sheet that is excellent in workability and toughness can be obtained with such a microstructure. Thus, a microstructure having no lath structure refers to the one in which the area ratio of a microstructure having a lath morphology is 5% or less, while a microstructure having large crystal strain refers to the one in which the area ratio of a microstructure having a KAM value of 1.0 or greater is 15% or greater. Preferably, the area ratio of a microstructure having a KAM value of 1.0 or greater should be 20% or greater. Note that the KAM value is often 1.0 or greater at crystal grain boundaries, and even in a single ferrite phase microstructure, the area ratio of a microstructure having a KAM value of 1.0 or greater is not 0%, and such a microstructure is unavoidably included at about 3%. Since the area ratio of ferrite is determined based on the morphology within grains, and not on grain boundaries, there may be a case where the total area ratio of the microstructure having a KAM value of 1.0 or greater and ferrite may exceed 100%.Ti-containing carbides with average grain size of 8 nm or less

[0043] In the present embodiment, the steel sheet is strengthened with Ti-containing carbides. To obtain a hot-rolled steel sheet with high strength, specifically, a yield strength of 680 MPa or greater, it is necessary to set the average grain size of the Ti-containing carbides, which are dispersed in steel, to 8 nm or less. To stably obtain a yield strength of 680 MPa or greater, the average grain size of the Ti-containing carbides is preferably set to 5 nm or less.

[0044] Further, setting the winding temperature to 600°C or greater allows Ti to sufficiently disperse in steel even though it is a substitutional element. By dispersing and precipitating T based on such a property of Ti, it is possible to obtain a steel sheet with a yield strength of 680 MPa or greater even if the steel sheet includes small amounts of microstructures of bainite, martensite, and tempered martensite which are often used for high-strength steel sheets. To obtain a steel sheet with a yield strength of 680 MPa or greater, 80% or more of the Ti content is utilized for the precipitation. Preferably, 85% or more of the Ti content is utilized for the precipitation.

[0045] The hot-rolled steel sheet according to the present embodiment preferably has a plating layer formed on its surface. Even when a plating layer is formed, the functions of the hot-rolled steel sheet are not impaired. As the composition of the plating layer, one or more of Zn, Si, Al, Ni, and Mg are preferably selected.

[0046] Note that the plated steel sheet of the present embodiment may be any one of a steel sheet that has been subjected to a hot-dip galvanizing process (GI), a steel sheet that has been subjected to a hot-dip galvanizing process that is further followed by an alloying process (GA), and a steel sheet that has been subjected to an electrogalvanizing process (EG).

[0047] Next, a first aspect of a method for producing a hot-rolled steel sheet according to the present embodiment will be described.

[0048] Typically, a hot-rolled steel sheet is produced by putting a slab (i.e., steel material), which has been cast and cooled to a temperature of 1000°C or lower, into a heating furnace to heat the slab for a short time, and then reducing the thickness of the slab to a predetermined thickness through a hot-rolling line, and further winding the slab into a coil form. Alternatively, a hot-rolled steel sheet is produced by heating a slab (i.e., steel material), which has been cast and once cooled to room temperature, in a heating furnace for a long time, and then reducing the thickness of the slab to a predetermined thickness through a hot-rolling line, and further winding the slab into a coil form. There is also a production method that includes directly transferring a slab (i.e., steel material), which has been cast, to a hot-rolling line without heating it in a heating furnace, and then reducing the thickness of the slab to a predetermined thickness, and further winding the slab into a coil form.

[0049] The method for producing a hot-rolled steel sheet according to the present embodiment is applicable to not only a process of heating a steel material, which has been cast, but also a process of directly transferring a steel material, which has been cast, to a hot-rolling line without heating it.<Steel material of first aspect>

[0050] A method for melting to produce a steel material of the present embodiment is not limited to a particular method. For example, a known melting method that involves the use of a converter, an electric furnace, and the like may be adopted. Further, secondary refining may be performed in a vacuum degassing furnace. Molten steel adjusted to have the foregoing chemical composition in such a manner is preferably formed into a slab (i.e., steel material) by a continuous casting process, taking productivity and quality into consideration. Alternatively, a slab may be formed using an ingot making-blooming process or any other known casting processes.<Rough-rolling step of first aspect>

[0051] In the present embodiment, the steel material is heated to a temperature of 1200°C or higher, or not heated after being cast, and then the steel material is rough-rolled to obtain a sheet bar.<Finishing-rolling step of first aspect>

[0052] Next, hot rolling is performed as finishing rolling in such a manner that the start temperature of the finishing rolling is set to 950°C or higher, the total rolling reduction for passes from a first pass to a fifth pass is set to 75% or greater, and the completion temperature of the finishing rolling is set to 860°C or higher but 910°C or lower, whereby a hot-rolled steel sheet is obtained.<Cooling step of first aspect>

[0053] Next, the hot-rolled steel sheet that has been obtained through hot rolling is cooled to a cooling stop temperature of 600°C or higher but 700°C or lower at an average cooling rate of 40°C / s or greater.<Winding step of first aspect>

[0054] The cooled hot-rolled steel sheet is then wound at a winding temperature of 600°C or higher but 700°C or lower.Heating of steel material: heated to 1200°C or higher, or not heated

[0055] Coarse carbides containing Ti, which have precipitated in the slab (i.e., steel material), are melted in a heating step performed before the hot rolling, so that fine Ti-containing carbides are precipitated after the hot rolling. To achieve Ti-containing carbides with an average grain size of 8 nm or less, the slab (i.e., steel material) is heated to 1200°C or higher. The slab (i.e., steel material) should be heated to 1220°C or higher. If the Ti content is 0.12% or greater, the slab (i.e., steel material) should be preferably heated to 1240°C or higher. While there is no specific upper limit to the heating temperature, it should not be over 1300°C in terms of production restrictions for avoiding thermal damage to the heating furnace.

[0056] In the case where the steel material held at 1200°C or higher after casting is transferred directly to a hot-rolling line, the cast steel material is not heated.

[0057] Start temperature of finishing rolling: 950°C or higher, total rolling reduction for passes from first pass to fifth pass: 75% or greater, and completion temperature of finishing rolling: 860°C or higher but 910°C or lower

[0058] To form a microstructure with large crystal strain that is a characteristic feature of the hot-rolled steel sheet according to the present embodiment, it is necessary to precisely control the hot-rolling conditions. Specifically, as austenite is recrystallized during the finishing rolling, fine austenite is formed. To this end, recrystallization of austenite is caused to occur within a finishing-rolling stand of the fifth pass or the subsequent passes by setting the start temperature of the finishing rolling to 950°C or higher, setting the total rolling reduction for passes from the first pass to the fifth pass to 75% or greater, and setting the chemical components to be within the range of the chemical components of the hot-rolled steel sheet according to the present embodiment.

[0059] Thus, the finishing rolling is performed through five or more passes. If the start temperature of the finishing rolling is below 950°C, austenite is recrystallized at an early stage of the finishing rolling, so that the recrystallized austenite is rolled again. This leads to the formation of ferrite, with the result that a microstructure with large crystal strain cannot be obtained.

[0060] If the start temperature of the finishing rolling is over 1100°C, there is an increased possibility that austenite may not be recrystallized within the finishing-rolling stand. It is therefore preferable to set the start temperature of the finishing rolling to 1100°C or lower.

[0061] If the completion temperature of the finishing rolling is below 860°C, there is an increased risk for the formation of ferrite during the rolling.

[0062] Meanwhile, if the completion temperature of the finishing rolling is over 910°C, austenite cannot cause recrystallization during the finishing rolling. Thus, the completion temperature of the finishing rolling is set to 860°C or higher or 910°C or lower. To achieve stable recrystallization of austenite, the completion temperature of the finishing rolling should be 890°C or lower.

[0063] To achieve the recrystallization of austenite during the finishing rolling, it is necessary to accumulate strain during a period from the first pass to the fifth pass of the finishing rolling. If the interval between each pass from the first pass to the fifth pass is long, strain provided by the rolling will be recovered, so that austenite cannot cause recrystallization stably during the finishing rolling. From the perspective of avoiding adverse effects of the recovery of austenite, therefore, the interval between each pass from the first pass to the fifth pass should be set to at least 1.5 seconds or less.Average cooling rate to cooling stop temperature of 600°C or higher but 700°C or lower after finishing rolling: 40°C / s or greater

[0064] After hot rolling, if the cooling rate to a temperature of 700°C or lower is slow, polygonal ferrite (i.e., ferrite), which is coarse at a high temperature and has small crystal strain within the grains is formed. To suppress the formation of such ferrite, it is necessary to perform cooling at an average cooling rate of 40°C / s or greater after hot rolling. The steel sheet should be cooled to 700°C or lower within 2 seconds after hot rolling at an average cooling rate of 50°C / s.

[0065] Meanwhile, if the cooling stop temperature is lower than 600°C, it will be difficult to obtain Ti-containing carbides, so that a steel sheet with a yield strength of 680 MPa or greater cannot be obtained.

[0066] Thus, the cooling stop temperature should be 600°C or higher but 700°C or lower. Preferably, it should be 610°C or higher or 690°C or lower. Note that the average cooling rate may be calculated as, b forced cooling other than natural cooling performed after the hot rolling, {(cooling start temperature) - (cooling completion temperature)} / (time required for the forced cooling other than natural cooling). Examples of the forced cooling include water-cooling.Winding temperature: 600°C or higher but 700°C or lower

[0067] The winding temperature is set to 600°C or higher but 700°C or lower for the same reason as the cooling stop temperature. Preferably, it is set to 610°C or higher but 690°C or lower. When winding is performed in such a temperature range, the formation of ferrite, bainite, martensite, and residual austenite can be minimized.

[0068] Next, a second aspect of a method for producing a hot-rolled steel sheet according to the present embodiment will be described. The present embodiment will describe the differences from the first aspect.<Casting step of second aspect>

[0069] The hot-rolled steel sheet according to the present embodiment can also be produced using a thin-slab continuous casting process. When a thin-slab continuous casting process is used, a steel material with a thickness of 35 mm or more but 200 mm or less is cast.<Rough-rolling step of second aspect>

[0070] The steel material is heated to 1200°C or higher after casting, or is not heated after casting, followed by rough rolling, as appropriate, to form a sheet bar.

[0071] The finishing-rolling step and the subsequent steps are similar to those of the first aspect.

[0072] Hereinafter, the thickness of the slab (i.e., steel material) that is specific to the thin-slab continuous casting process will be described.Thickness of slab (i.e., steel material): thickness of 35 mm or more but 200 mm or less

[0073] In the thin-slab continuous casting process, different from the continuous casting process, the working degree of austenite during hot rolling is low because the slab before hot rolling is thin. If the thickness of the slab is less than 35 mm, it is difficult to achieve the desired total rolling reduction for passes from a first pass to a fifth pass. Meanwhile, if the thickness of the slab exceeds 200 mm, the casting speed becomes slow and productivity advantage of the thin-slab continuous casting process over the continuous casting process is lost. From such perspectives, the thickness of the slab in the thin-slab continuous casting process is set to 35 mm or more but 200 mm or less.

[0074] Next, a third aspect of a method for producing a hot-rolled steel sheet according to the present embodiment will be described. The present embodiment will describe the differences from the first aspect and the second aspect. The third aspect may employ a hot continuous rolling technology.<Joining step of third aspect>

[0075] The sheet bar obtained in the first aspect or the second aspect is joined to a preceding sheet bar at a temperature of 1050°C or higher, before finishing rolling. If the temperature is below 1050°C, it will be difficult to perform finishing rolling at a finishing-rolling start temperature of 950°C or higher. The heating temperature for the sheet bars when joining them should be 1070°C or higher.

[0076] The cooling step and the following steps are similar to those of the first aspect.

[0077] The method for producing a hot-rolled steel sheet according to the present embodiment may include an annealing step of performing annealing in a continuous annealing line at an annealing temperature of 720°C or lower, and a plating step of performing plating in a continuous plating line. Further, the method may also include an alloying step of performing an alloying process by heating the plated hot-rolled steel sheet to 480°C or higher but 600°C or lower. Even when such an annealing process or a plating process is performed, the quality of the materials of the hot-rolled steel sheet according to the present embodiment is not influenced. Therefore, the surface of the hot-rolled steel sheet may be further subjected to a plating process to form a plating layer on the surface of the steel sheet.

[0078] Further, as described above, neither the plating process nor the composition of the plating bath has any influence on the quality of the materials of the hot-rolled steel sheet according to the present embodiment. Thus, any one of a hot-dip galvanizing process, an alloying hot-dip galvanizing process, and an electrogalvanizing process is applicable as the plating process. The composition of the plating bath may include one or more of Zn, Al, Mg, Si, and Ni. That is, the composition of the plating layer formed on the surface of the hot-rolled steel sheet during the plating process may include one or more of Zn, Si, Al, Ni, and Mg.Example

[0079] The embodiment of the present invention will be described in further detail by way of Example. Note that the present invention is not limited to the production conditions or the product performance described below as Example. The desired performance can be achieved as long as the embodiment is within the range of the present invention.<First aspect involving continuous casting process>

[0080] Steel materials each having a thickness of 250 mm and having chemical compositions shown in Table 1 were hot-rolled under the conditions of rough rolling and finishing rolling shown in Table 2. Each steel material was subjected to temper rolling with an elongation ratio of 0.1 to 0.5% and then to pickling, thus producing a steel sheet to be evaluated.<Second aspect involving thin-slab continuous casting process>

[0081] Thin slabs of steel having the chemical compositions shown in Table 1 were each hot-rolled under the conditions shown in Table 3. The hot-rolled thin slabs were then subjected to temper rolling with an elongation ratio of 0.1 to 0.5% and then to pickling, thus producing a steel sheet to be evaluated.<Third aspect involving hot continuous rolling process>

[0082] Sheet bars of steel with the chemical composition shown in Table 1 were joined under the conditions shown in Table 4, and the joined sheet bars were then hot-rolled, followed by being subjected to temper rolling with an elongation ratio of 0.1 to 0.5% and then to pickling, thus producing a steel sheet to be evaluated.< Production method involving forming plating layer on hot-rolled steel sheet >

[0083] Hot-rolled coils produced under the conditions of Table 2 were pickled. The resulting hot-rolled steel sheets were then galvanized in a continuous galvanizing line (CGL) under the conditions shown in Table 5. Accordingly, a continuous galvanized steel sheet (GI) and an alloyed galvanized steel sheet (GA) were produced. [Table 1]Steel No.Chemical composition (mass%)Expression(1)Expression(2)RemarksCSMnPSAlNTiOthersA0.0820.251.050.010.00120.020.00410.140-1.32.6Invention ExampleB0.0681.050.650.020.00130.040.00270.152-1.51.9Invention ExampleC0.0320.201.040.030.00180.040.00280.160-1.20.9Comparative ExampleD0.0560.151.520.010.00260.050.00280.100-1.62.5Comparative ExampleE0.0810.750.770.030.00310.040.00500.072-1.45.9Comparative ExampleF0.0850.321.010.020.00340.030.00240.235-1.31.5Comparative ExampleG0.0740.370.910.030.00210.030.00460.157B:0.00181.22.1Invention ExampleH0.0770.520.900.030.00260.060.00400.168V:0.031.32.0Invention ExampleMo:0.01Sb:0.01REM:0.002Ca:0.002I0.0700.471.030.030.00250.040.00400.157Nb:0.021.42.0Invention ExampleHf:0.01Pb:0.01Cs:0.001As0.001J0.0710.720.770.010.00110.030.00420.165Ni:0.021.31.9Invention ExampleCu:0.02W:0.01Ta:0.001Zr:0.01K0.0700.540.920.030.00280.030.00450.150Cr:0.021.42.1Invention ExampleCo:0.002Mg:0.001Expression (1): 0.8 [%Si] + [%Mn], Expression (2): ([%C] / 12) / ([%Ti*] / 48) [%Ti*] = [%Ti] -48[%N] / 14 [Table 2] Steel sheet No.Steel No.Heating temperature (°C)Finishing-rolling conditionsCooling stop temperature (°C)Winding temperature (°C)RemarksRolling start temperature (°C)Total rolling reduction (%) for first to fifth passesFinishing-rolling temperature (°C)Cooling rate (°C / s)1A125298778878142624615Invention Example212429677888779637629Invention Example3123996376869151625622Invention Example412499418288861641635Comparative Example5123610257786399665655Comparative Example612419566688293685682Comparative Example712419747792077645635Comparative Example812359737686921655652Comparative Example912489668187098425420Comparative Example1012389647686896712709Comparative Example11C12439837686169660654Comparative Example12D12389857587597636632Comparative Example13E12489807588088664655Comparative Example14F12479827786091628619Comparative Example15G12529598085894679673Invention Example16H124897177869142641634Invention Example17I12489727587299612604Invention Example18J12389607688654645636Invention Example19K124297680860142619611Invention Example [Table 3] Steel sheet No.Steel No.Slab thickness (mm)Finishing-rolling conditionsCooling stop temperature (°C)Winding temperature (°C)Rolling start temperature (°C)Total rolling reduction (%) for first to fifth passesRolling end temperature (°C)Average cooling rate (°C / s)20B859707888269625622 [Table 4] Steel sheet No.Steel No.Heating temperature (°C)Temperature (°C) for joining slabsFinishing-rolling conditionsCooling stop temperature (°C)Winding temperature (°C)Rolling start temperature (°C)Total rolling reduction (%) for first to fifth passesRolling end temperature (°C)Average cooling rate (°C / s)21A124910729818087177612610 [Table 5] Steel sheet No.Plated steel sheet No.SurfaceCGL threading conditionsHeating temperature(°C)Alloying temperature (°C)222GA681482233GI692- [Table 6] Steel sheet No.Analysis of microstructuresMechanical propertiesRemarksArea ratio 1 (%) *1Area ratio(%) of ferriteComposition of other microstructures *2Area ratio (%) of lath microstructureDiameter (nm) of Ti-containing carbidesRatio (%) of Ti precipitation *3Yield strength (MPa)Tensile strength (MPa)Evaluation of bendabilityEvaluation of toughness15224θ0389749814GoodGoodInvention Example25248-0290707785GoodGoodInvention Example36733-0389703790GoodGoodInvention Example4595θ0389722839GoodPoorComparative Example5696θ0290770837GoodPoorComparative Example6696θ0390746802GoodPoorComparative Example7598-0391735835GoodPoorComparative Example8695θ01188480615GoodGoodComparative Example9980B, M, TM94No precipitation32875951PoorGoodComparative Example10595θ01095520658GoodGoodComparative Example115100-0489534621GoodGoodComparative Example12595θ0396740841GoodPoorComparative Example133763-0394531618PoorGoodComparative Example146832-0288646769PoorGoodComparative Example153664-0296726835GoodGoodInvention Example167030-0288807858GoodGoodInvention Example176535-0391753827GoodGoodInvention Example185347-0293774850GoodGoodInvention Example194951-0395779847GoodGoodInvention Example205050-0290725824GoodGoodInvention Example217525-1290793844GoodGoodInvention Example224258-0288756822GoodGoodInvention Example233862-0393793844GoodGoodInvention Example *1: Area ratio of microstructure having KAM value of 1 or greater *2: Microstructures other than ferrite: bainite (B), martensite (M), tempered martensite (TM), and cementite (θ) *3: Ratio of precipitation of Ti = (amount of precipitation of Ti-containing carbides) / (Ti content) × 100

[0084] The hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5 were evaluated in terms of the metallographic structure, tensile properties, bendability, and toughness, using the following methods. Table 6 shows the results.(i) Area ratio of metallographic structure

[0085] A specimen was cut out from each hot-rolled steel sheet such that a cross-section parallel with the rolling direction was observed. The central portion of the specimen in the thickness direction was corroded with 1% nital so as to reveal the microstructure. The microstructure was then magnified 2000x with a scanning electron microscope (SEM), and a portion of 1 / 4t of the specimen in the thickness direction was imaged for 10 visual fields at an accelerating voltage of 15 kV.

[0086] Ferrite is a crystal grain that is observed to have no corrosion marks therein and is observed to be less bright (which appears gray in an SEM) than martensite. Each of bainite and tempered martensite is a crystal grain that is observed to have three or more adjacent lath-like corrosion marks each having a width of 500 nm or less therein. Martensite is a crystal grain that is observed to have no corrosion marks therein, and is observed to be brighter (which appears white in an SEM) than ferrite. The area ratio of the thus separated microstructure in the metallographic structure was determined using image analysis software (Photoshop elements and Image J).

[0087] The surface of each specimen was ground to reach 3 / 4 of the total thickness of the specimen, followed by chemical polishing by 0.1 mm or more, and the polished surface was measured by the X-ray diffractometry to measure residual austenite. The volume fraction of residual austenite was measured from, with MoKα rays used as an incident ray source, peaks at (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ. The thus measured volume fraction of the residual austenite phase was determined as the area ratio of residual austenite.

[0088] The area ratio of a microstructure having no lath structure and having large crystal strain was measured using the SEM and EBSD methods. A spot is marked on each specimen before the observation using, for example, a Vickers testing machine, such that the observation can be performed with the same visual field for the SEM and EBSD methods. A microstructure having no lath structure and having large crystal strain is observed to have corrosion marks within its grain, when observed by an SEM. At this time, depending on the shapes of the corrosion marks, some corrosion marks may appear as lath, although such corrosion marks are not actually lath. In such a case, to distinguish between a microstructure that appears as lath and a lath microstructure, a rectangular microstructure in which the width of a crystal grain on the shorter side is over 500 nm and two or less corrosion marks are generated within adjacent grains is not regarded as a lath structure. Meanwhile, a microstructure in which the width of a crystal grain on the shorter side is 500 nm or less and three or more corrosion marks are generated within adjacent grains is regarded as a lath structure observed in bainite or tempered martensite. Such a lath structure can be more clearly distinguished when observed with a transmission electron microscope (TEM). In addition, EBSD analysis was performed with OIM Analysis software (produced by TSL Solutions K. K.). The KAM value was analyzed under the first-nearest-neighbor condition.

[0089] Through the EBSD analysis, among grains surrounded by a high-angle grain boundary with an angular difference of 15° or greater, a microstructure observed to have a KAM value of over 1.0 and have no lath structure was determined as a microstructure having no lath structure and thus having large crystal strain. The area ratio of such a microstructure was determined in a visual field of 1 mm 2< or greater.(ii) Average grain size of Ti-containing carbides

[0090] A thin film to be observed was collected from a position corresponding to 1 / 4 of the hot-rolled steel sheet in the thickness direction. Then, 300 or more Ti-containing carbides were imaged with a transmission electron microscope at a magnification of 600,000x. The equivalent circular radii of the imaged Ti-containing carbides were determined, and the mean value thereof was determined as the average grain size. The Ti-containing carbides may be identified based on the presence or absence of a peak derived from Ti, using EDX provided to the TEM.(iii) Analysis of amount of precipitation of Ti-containing carbides

[0091] Each of the front and rear surfaces of each specimen was ground by 25% in the thickness direction. Then, the specimen was dissolved in a 10% AA electrolyte, and the electrolyte was filtered through a filter with a mesh diameter of 0.2 µm, so that the concentration of Ti contained in the filtered electrolyte was analyzed with ICP-MS. Further, the amount of Ti to be precipitated as TiN was calculated as [the Ti content] × 48 / 14. Meanwhile, the amount of Ti to be precipitated as TiS was calculated as [the Ti content] × 48 / 32. The amount of precipitation of Ti-containing carbides was then calculated by subtracting, from the Ti content, the concentration of Ti contained in the electrolyte, the amount of Ti to be precipitated as TiN, and the amount of Ti to be precipitated as TiS.(iv) Tensile test

[0092] From each of the hot-rolled steel sheets obtained under the conditions illustrated in Tables 2 to 5, a tensile specimen of JIS No. 5 was produced in the direction perpendicular to the rolling direction. Then, a tensile test was performed five times in compliance with the provision of JIS Z 2241 (2011) so that the average yield strength (YS) and tensile strength (TS) were determined. The crosshead speed of the tensile test was set to 10 mm / min. In Table 6, specimens having a yield strength of 680 MPa or greater correspond to examples of the invention.(v) Bending test

[0093] From each of the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5, a specimen with a width of 35 mm and a length of 100 mm and having ground end faces was collected. Then, a bending test was performed five times using the V-block method described in JIS Z 2248. A specimen for which R / t was 0.5 or less was evaluated as "Good" as having the properties required in the present invention, while a specimen for which surface cracking was found once or more under the condition that R / t was 0.5 or less was evaluated as "Poor" as not having the properties required in the present invention.(vi) Charpy impact test

[0094] From each of the hot-rolled steel sheets obtained under the conditions illustrated in Tables 2 to 5, a V-notch specimen described in JIS Z2242 was collected such that its longitudinal direction coincided with the direction normal to the rolling direction. When the thickness of the hot-rolled steel sheet was less than 10 mm, a plurality of specimens were stacked on top of each other, and a hole was punched through the end portions of the stacked specimens to couple the stacked specimens with a bolt, so that the total thickness was adjusted to be 10±1 mm. After the specimen(s) was / were immersed in a bathtub with a temperature controlled to -40°C for 10 minutes or longer, a test was performed using a method compliant with JIS Z 2242. Table 6 shows the test results. At this time, a specimen with an absorption energy of 30 J / cm 2< or greater was evaluated as "Good" as having the properties required in the present invention, while a specimen with an absorption energy of less than 30 J / cm 2< was evaluated as "Poor" as not having the properties required in the present invention.

[0095] Each of the examples of the present invention was found to have a yield strength (YS) of 680 MPa or greater, and have excellent bendability as well as excellent toughness. Meanwhile, each of the comparative examples that are outside the range of the present invention was found to have a yield strength of less than 680 MPa, or have neither the bendability nor the toughness required in the present invention.

Claims

1. A hot-rolled steel sheet that has a chemical composition including, in mass%, C: 0.035% or more but less than 0.110%, Si: 1.5% or less, Mn: 1.3% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more but 0.080% or less, N: 0.0060% or less, and Ti: 0.08% or more but 0.20% or less, and further optionally including one or both of Groups A and B of components below: - Group A: B: 0.0002% or more but 0.0050% or less, and - Group B: one or more of Nb, V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se: 1% or less in total, with a balance being Fe and unavoidable impurities, characterized in that the hot-rolled steel sheet has a metallographic structure where an area ratio of ferrite is 0% or more but 85% or less, an area ratio of residual austenite is 3% or less, an area ratio of a microstructure with a lath morphology is 5% or less, and an area ratio of a microstructure with a KAM value of 1.0 or greater is 15% or greater, and the hot-rolled steel sheet includes Ti-containing carbides with an average grain size of 8 nm or less, and has a yield strength of 680 MPa or greater.

2. The hot-rolled steel sheet according to claim 1, further comprising a plating layer on a surface of the hot-rolled steel sheet.

3. A method for producing a hot-rolled steel sheet, comprising: a rough-rolling step of rough-rolling a steel material having the chemical composition of claim 1 after heating the steel material to a temperature of 1200°C or higher or without heating the steel material that has been cast, thereby obtaining a sheet bar; a finishing-rolling step of finishing-rolling the sheet bar by setting a start temperature of rolling to 950°C or higher, setting a total rolling reduction for passes from a first pass through a fifth pass to 75% or greater, and setting a completion temperature of rolling to 860°C or higher but 910°C or lower, thereby obtaining a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet to a cooling stop temperature of 600°C or higher but 700°C or lower at an average cooling rate of 40°C / s or greater; and a winding step of winding the cooled hot-rolled steel sheet at a winding temperature of 600°C or higher but 700°C or lower.

4. The method for producing a hot-rolled steel sheet according to claim 3, further comprising: a casting step of casting the steel material having the chemical composition of claim 1 and having a thickness of 35 mm or more but 200 mm or less before the rough-rolling step or the finishing-rolling step, wherein: the sheet bar is obtained by applying or not applying the rough-rolling step.

5. The method for producing a hot-rolled steel sheet according to claim 3, further comprising: a joining step of joining the sheet bar obtained by the rough rolling to a preceding sheet bar at a temperature of 1050°C or higher, between the rough-rolling step and the finishing-rolling step, wherein the finishing-rolling step includes finishing-rolling the joined sheet bars.

6. The method for producing a hot-rolled steel sheet according to any one of claims 3 to 5, further comprising: a hot-band annealing step of annealing the hot-rolled steel sheet at an annealing temperature of 720°C or lower; and a plating step of plating the annealed hot-rolled steel sheet.

7. The method for producing a hot-rolled steel sheet according to claim 6, further comprising an alloying step of alloying the plated hot-rolled steel sheet at a temperature of 480°C or higher but 600°C or lower.

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