HOT ROLLED STEEL SHEET

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure, in % area, ferrite is less than 15.0%, residual austenite is less than 3.0%, L52 / L7, which is a ratio of a length L52 of a grain boundary that has a crystal orientation difference of 52° to a length L7 of a grain boundary that has a crystal orientation difference of 7° around one direction <110> It is from 0.10 to 0.18, a standard deviation of a Mn concentration is 0.60% by mass or less, and a tensile strength is 980 MPa or more.

Description

HOT ROLLED STEEL SHEET ccn l Ln / zznz / E / YiAi Technical field of the invention [1] The present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a hot-rolled steel sheet that is formed into various shapes by press working or similar means for use, and particularly relates to a hot-rolled steel sheet that has high strength and excellent ductility and shear workability. Priority is claimed in Japanese Patent Application No. 2020-041524, filed on March 11, 2020, the contents of which are incorporated herein by reference. Related technique [2] In recent years, from the perspective of global environmental protection, efforts have been made to reduce the amount of carbon dioxide gas emitted in many sectors. Vehicle manufacturers are also actively developing techniques to reduce the weight of vehicle bodies in order to decrease fuel consumption. However, reducing the weight of vehicle bodies is not easy, as there is a strong emphasis on improving collision resistance to ensure occupant safety. [3] In order to achieve both a reduction in vehicle body weight and improved collision resistance, research has been conducted to create a thinner component using a high-strength steel sheet. Therefore, a steel sheet with both high strength and excellent formability is highly desirable. Several techniques, derived from the related technique, have been proposed to meet these requirements. [4] Since there are several ways of working for vehicle members, the formability required for a steel sheet differs depending on the members to which it is applied, but among these, ductility stands out as an important index for formability. [5] In addition, vehicle members are formed by press forming, and the press-formed blank is often manufactured by highly productive shearing work. [6] For example, Patent Document 1 describes a high-strength steel sheet for a vehicle that has excellent safety and collision-resistant formability, in which residual austenite having an average crystal grain size of 5 pm or less is dispersed in ferrite having an average crystal grain size of 10 microns or less. In the steel sheet containing residual austenite in the microstructure, while the austenite transforms into martensite during working and exhibits high elongation due to transformation-induced plasticity, the formation of fully hard martensite impairs hole expansion capability. Patent Document 1 reveals that not only is ductility improved, but also hole expansion capability, by refining the ferrite and residual austenite. [7] Patent Document 2 describes a high-strength steel sheet having excellent elongation and elongation capacity and a tensile strength of 980 MPa or more, wherein a second phase consisting of residual austenite and / or martensite is finely dispersed in crystal grains. [8] With respect to a technique for improving shear workability, for example, Patent Document 3 describes a technique for controlling the burr height after die cutting by controlling a ds / db ratio of the ferrite grain size ds of the surface layer to the ferrite crystal grain db of an interior to 0.95 or less. [9] Patent Document 4 describes a technique for improving gaps or burrs on the end surface of a sheet by reducing the P content. Documents of the previous technique Patent document

[10] Patent Document 1: Unexamined Japanese Patent Application, First Publication No. Hll-61326 Patent Document 2: Unexamined Japanese Patent Application, First Publication No. 2005-179703 Patent Document 3: Unexamined Japanese Patent Application, First Publication No. H10-168544 Patent Document 4: Unexamined Japanese Patent Application, First Publication No. 2005-298924 ccn l Ln / zznz / E / YiAi Description of the invention Problems that must be solved by the invention

[11] The techniques described in Patent Documents 1 through 4 are all techniques for improving ductility or an extreme surface property after shearing. However, Patent Documents 1 through 3 do not describe a technique for achieving both properties. Patent Document 4 describes achieving both shear workability and press formability. However, since the strength of a steel sheet described in Patent Document 4 is less than 850 MPa, it may be difficult to apply the steel sheet to a member with a high strength of 980 MPa or more.

[12] Furthermore, particularly for a steel sheet with a high strength of 980 MPa or more, the load required for post-treatment such as coining after shearing is large, and therefore, it is desirable to control the height difference of an end surface after shearing with particularly high precision. When not only the shape of the end surface varies after shearing, but also the damage to the end surface after shearing, a deterioration of formability can occur due to stress concentration at a significantly damaged site.

[13] The present invention has been made in view of the prior problems of the related art, and an object of the present invention is to provide a hot-rolled steel sheet that has high strength and excellent ductility and workability under shear.Preferably, an object of the present invention is to provide a hot-rolled steel sheet having the various above properties and, in addition, excellent workability of an end surface after shearing work. ccn l Ln / zznz / E / YiAi Means to solve the problem

[14] In view of the above problems, the inventors of the present invention have carried out intensive studies on the chemical composition of a hot-rolled steel sheet and the relationship between its microstructure and mechanical properties. As a result, the following findings (a) to (i) were obtained, and the present invention was completed.

[15] Furthermore, the expression "excellent shear workability" indicates that the difference in height of an extreme surface after shearing is small. Additionally, the expression "excellent strength" or "high strength" indicates that the tensile strength is 980 MPa or higher. Furthermore, the expression "excellent extreme surface workability after shearing" indicates that the variation in extreme surface hardness after shearing in the thickness direction of the sheet is small.

[16] (a) In order to obtain excellent (maximum) tensile strength, the primary phase structure of a microstructure is preferably completely hard. That is, it is preferable that the soft microstructural fraction of ferrite or similar be as small as possible.

[17] (b) However, excellent shear workability cannot be ensured by forming a microstructure that mainly contains a completely hard structure.

[18] (c) In order to provide the high-strength hot-rolled steel sheet with extreme surface workability after cutting as well, it is effective to make the structure contained in the steel sheet uniform.

[19] (d) To make the structure completely hard and uniform, it is effective to adjust the cooling rate so that the precipitation of a soft structure such as ferrite can be suppressed during cooling after final rolling.

[20] (e) A completely hard structure generally forms in a phase transformation at 600°C or less, but in this temperature range, a large number of grain boundaries having a crystal orientation difference of 52° and grain boundaries having a crystal orientation difference of 7° form around a direction <110> .

[21] (f) When the grain boundary is formed which has a crystal orientation difference of 7° around the direction <110> In a completely hard phase, dislocations are less likely to accumulate. Therefore, in a microstructure where such a grain boundary is uniformly dispersed at a high density (i.e., the total length of the grain boundary having a crystal orientation difference of 7° around the direction), dislocations are less likely to accumulate. <110> (is large), the introduction of dislocations into the microstructure through shear work is easy, and distortion of a material is promoted during shear work. As a result, the height difference of the extreme surface is suppressed after shear work.

[22] (g) To evenly disperse the grain boundary that has a crystal orientation difference of 7° around the direction <110> To adjust the standard deviation of a Mn concentration to a certain value or less, it is necessary to hold a plate at a temperature between 700°C and 850°C for 900 seconds or more during plate heating, hold the plate at a temperature of 1100°C or higher for 6000 seconds or more, and perform hot rolling so that the sheet thickness is reduced by a total of 90% or more within a temperature range of 850°C to 1100°C.

[23] (h) To increase the length L? of the grain boundary that has a crystal orientation difference of 7o around the direction <110> and decrease the L52 length of the grain boundary that has a crystal orientation difference of 52° around the direction <110> It is effective to adjust a winding temperature to a predetermined or higher temperature.

[24] (i) In order to suppress extreme surface hardness variation along the sheet thickness direction after shearing, it is effective to suppress residual austenite formation and suppress the Vickers hardness standard deviation. Furthermore, to suppress the Vickers hardness standard deviation, it is effective to reduce the amount of ferrite and control the average cooling rate within a predetermined temperature range after rolling.

[25] The essence of the present invention based on the above findings is as follows. (1) A hot-rolled steel sheet according to an aspect of the present invention includes, ccn L ίη / ZZΖΠZ / Β / YΥΙΛΙ as its chemical composition, in % by mass: C: 0.100% to 0.250%, Si: 0.05% to 2.00%, Mn: 1.00% to 4.00%, In the sun: 0.001% to 2 P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, 0: 0.0100% or less, Ti: 0% to 0.300%, Nb: 0% to 0.100%, V: 0% to 0.500%, Cu: 0% to 2.00%, Cr: 0% to 2.00%, Mo: 0% to 1.00%, Ni: 0% to 2.00%, 0.000% B: 0% to 0.0100%, Ca: 0% to 0.0200%, Mg: 0% to 0.0200%, REM: 0% to 0.1000%, Bi: 0% to 0.020%, one or two or more of Zr, Co, Zn and W: 0% to 1.00% in total, Sn: 0% to 0.050%, and a remainder consisting of Fe and impurities, wherein, in a microstructure, in % area, ferrite is less than 15.0%, residual austenite is less than 3.0%, L52 / L7, which is a ratio of a length L52 of a grain boundary having a crystal orientation difference of 52° to a length L7 of a grain boundary having a crystal orientation difference of 70° around one direction <110> is from 0.10 to 0.18, a standard deviation of a Mn concentration is 0.60% by mass or less, and a tensile strength is 980 MPa or more. (2) Hot-rolled steel sheet in accordance with (1), wherein, in the microstructure, in % area, the ferrite may be 10.0% or less, and a standard deviation of the Vickers hardness may be 20 HV0.01 or less. (3) Hot-rolled steel sheet conforming to (1) or (2) may further include, as a chemical composition, in % by mass, one or two or more selected from a group consisting of Ti: 0.005% to 0.300%, Nb: 0.005% to 0.100%, V: 0.005% to 0.500%, Cu: 0.01% to 2.00%, Cr: 0.01% to 2.00%, Mo: 0.01% to 1.00%, Ni: 0.02% to 2.00%, B: 0.0001% to 0.0100%, Ca: 0.0005% to 0.0200%, Mg: 0.0005% to 0.0200%, REM: 0.0005% to 0.1000%, and Bi: 0.0005% to 0.020%. Effects of the invention

[26] According to the preceding aspect of the present invention, it is possible to obtain a hot-rolled steel sheet having excellent strength, ductility, and shear workability. Furthermore, according to the preferred aspect of the present invention, it is possible to obtain a hot-rolled steel sheet having the aforementioned properties and, in addition, excellent end surface workability after shearing. The hot-rolled steel sheet according to the preceding aspect of the present invention is suitable as an industrial material used for vehicle components, mechanical structural members, and building members. Brief description of the drawing

[27] Figure 1 is a view to describe a method for measuring a height difference of an extreme surface after shearing work. Modalities of the invention

[28] The chemical composition and microstructure of a hot-rolled steel sheet according to the present embodiment (hereinafter sometimes referred to simply as steel sheet) will be described more specifically below. However, the present invention is not limited solely to a configuration described herein, and various modifications may be made without departing from the scope of the essence of the present invention.

[29] The numerical limit interval described below, with 'a' in the middle, includes the lower and upper limits. For numerical values ​​indicated by 'ccn l Ln / zznz / E / YiAi' being less than or greater than, the value is not within the numerical interval. In the following description, the % with respect to the chemical composition of the steel sheet is % by mass unless otherwise specified. ccn l Ln / zznz / E / YiAi

[30] 1. Chemical composition Hot-rolled steel sheet in accordance with the present specification includes, in % by mass, C: 0.100% to 0.250%, Si: 0.05% to 2.00%, Mn: 1.00% to 4.00%, I1 sol.: 0.001% to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and a remainder consisting of Fe and impurities. Each element will be described in detail below.

[31] (1-1) C: 0.100% to 0.250% Carbon increases the proportion of a hard phase. When the carbon content is less than 0.100%, it is difficult to achieve the desired strength. Therefore, the carbon content is adjusted to 0.100% or higher. The carbon content is preferably 0.120% or higher, and very preferably 0.150% or higher. On the other hand, when the carbon content is greater than 0.250%, the transformation rate slows, the formation of a hard phase is easy, it becomes difficult to obtain a structure with uniform strength, and the difference in height of the extreme surface after shearing becomes large. Therefore, the carbon content is adjusted to 0.250% or lower. The carbon content is preferably 0.220% or lower.

[32] (1-2) If: 0.05% to 2.00% Silicon (Si) retards cementite precipitation. This action allows a large amount of carbon to remain in solid solution within the hardened phase, preventing cementite thickening and consequently increasing the strength of the steel sheet. Furthermore, Si itself contributes to the increased strength of the steel sheet through solid solution hardening. Additionally, Si improves the steel by deoxidizing it (preventing the formation of defects such as holes). When the Si content is below 0.05%, this effect is not achievable. Therefore, the Si content is adjusted to 0.05% or higher. Ideally, the Si content should be 0.50% or higher, or 0.80% or higher. However, when the Si content is greater than 2.00%, cementite precipitation is significantly delayed and the residual austenite area fraction increases to 3.0% or more, which is not preferable.Furthermore, when the Si content exceeds 2.00%, the surface properties, chemical convertibility, ductility, and weldability of the steel sheet deteriorate significantly, and the A3 transformation point increases significantly. Therefore, stable hot rolling can be difficult. Thus, the Si content is adjusted to ccn l Ln / zznz / E / YiAi. 2.00% or less. The Si content is preferably 1.70% or less or 1.50% or less.

[33] (1-3) Mn: 1.00% to 4.00% Manganese (Mn) suppresses ferritic transformation, increasing the strength of steel sheets. When the Mn content is less than 1.00%, a tensile strength of 980 MPa or higher cannot be achieved. Therefore, the Mn content is adjusted to 1.00% or higher. The preferred Mn content is 1.50% or higher, and ideally 1.80% or higher. Conversely, when the Mn content exceeds 4.00%, the grain angle difference in the hard phase becomes non-uniform due to Mn segregation, resulting in a large difference in the extreme surface height after shearing. Therefore, the Mn content is adjusted to 4.00% or lower. The preferred Mn content is 3.70% or lower, or 3.50% or lower.

[34] (1-4) In the sun: 0.001% to 2.000% Like silicon, aluminum retards cementite precipitation. This action allows a large amount of carbon solid solution to remain in the hard phase, preventing cementite thickening and consequently increasing the strength of the steel sheet. Furthermore, aluminum deoxidizes the steel, improving its properties. When the aluminum content is reduced... If the concentration is less than 0.001%, no effect can be achieved. Therefore, the Al sol content is adjusted to 0.001% or higher. The Al sol content is preferably 0.010% or higher. On the other hand, when the Al sol content exceeds 2.000%, cementite precipitation is significantly delayed, and the residual austenite area fraction increases to 3.0% or higher, which is not economically desirable. Therefore, the Al sol content is adjusted to 2.000% or lower. The Al sol content is preferably 1.500% or lower, or 1.300% or lower. Al sol. in the present form means acid-soluble Al and refers to a solid solution of Al present in steel in a solid solution state.

[35] (1-5) P: 0.100% or less Phosphorus (P) is an element that is generally present as an impurity and also increases strength by strengthening the solid solution. Therefore, P can be present in positive quantities, but it is also an element that segregates easily. When the P content exceeds 0.100%, the deterioration of ductility becomes significant due to segregation. Therefore, the P content is adjusted to 0.100% or less. Preferably, the P content is 0.030% or less. It is not necessary to specify a lower limit for the P content, but it is preferably adjusted to... 0.001% or more from the point of view of refining cost.

[36] (1-6) S: 0.0300% or less Sulfur (S) is an element present as an impurity in steel, forming sulfide-based inclusions that degrade the ductility of hot-rolled steel sheet. When the S content exceeds 0.0300%, the ductility of the steel sheet deteriorates significantly. Therefore, the S content is adjusted to 0.0300% or less. Preferably, the S content is 0.0050% or less. While a lower limit for the S content does not need to be specified, it is preferably adjusted to 0.0001% or more from a refining cost perspective.

[37] (1-7) N: 0.1000% or less Nitrogen (N) is an element present in steel as an impurity and degrades the ductility of the steel sheet. When the N content exceeds 0.1000%, the ductility of the steel sheet deteriorates significantly. Therefore, the N content is adjusted to 0.1000% or less. The N content is preferably 0.0800% or less, and most preferably 0.0700% or less. Although it is not necessary to specify the lower limit for the N content, as will be described later, if one or more of Ti, Nb, and V are present to refine the microstructure, the N content is preferably adjusted to 0.0010% or more, and most preferably adjusted to 0.0020% or more to promote carbonitrid precipitation.

[38] (1-8) 0: 0.0100% or less When a large amount of oxygen is present in steel, it forms a coarse oxide that becomes the source of fracture, leading to brittle fracture and hydrogen-induced cracking. Therefore, the oxygen content is adjusted to 0.0100% or less. Preferably, the oxygen content is 0.0080% or less, or 0.0050% or less. The oxygen content can be adjusted to 0.0005% or more, or 0.0010% or more, to disperse a large amount of fine oxides when deoxidizing molten steel.

[39] The remainder of the chemical composition of the hot-rolled steel sheet conforming to this modality consists of Fe and impurities. In this modality, impurities means a substance that is incorporated from the ore as raw material, scrap, manufacturing environment, or the like, or a substance that is intentionally added and a substance that is permitted to the extent that the hot-rolled steel sheet conforming to this modality is not adversely affected.

[40] In addition to the above elements, hot-rolled steel sheet conforming to this specification may contain Ti, Nb, V, Cu, Cr, Mo, Ni, B, Ca, Mg, REM, Bi, Zr, Co, Zn, W, and Sn as optional elements. If the above optional elements are not present, the lower limit of their content shall be 0%. The above optional elements are described in detail below.

[41] (1-9) Ti: 0.005% to 0.300%, Nb: 0.005% to 0.100% and V: 0.005% to 0.500% Since all Ti, Nb, and V precipitate as a carbide or nitride in steel and refine the microstructure through a fixing effect, one, two, or more of these elements may be present. To reliably achieve this effect, it is preferable to adjust the Ti content to 0.005% or more, the Nb content to 0.005% or more, or the V content to 0.005% or more. However, even when these elements are excessively present, the effect becomes saturated, which is not economically desirable. Therefore, the Ti content is adjusted to 0.300% or less, the Nb content to 0.100% or less, and the V content to 0.500% or less.

[42] (1-10) Cu: 0.01% to 2.00%, Cr: 0.01% to 2.00%, Mo: 0.01% to 1.00%, Ni: 0.02% to 2.00% and B: 0.0001% to 0.0100% All the elements Cu, Cr, Mo, Ni, and B have the effect of improving the hardenability of steel sheet. In addition, Cr and Ni have the effect of stabilizing residual austenite, and Cu and Mo have the effect of precipitating a carbide in the steel to increase its strength. Furthermore, when Cu is present, Ni has the effect of effectively suppressing grain boundary cracking in a plate caused by Cu. Therefore, one, two, or more of these elements may be present.

[43] Copper (Cu) improves the hardenability of steel sheets and precipitates as a carbide in low-temperature steel to increase its strength. To reliably achieve this effect, the Cu content is preferably adjusted to 0.01% or more, and very preferably to 0.05% or more. However, when the Cu content exceeds 2.00%, grain boundary cracking may occur in some cases. Therefore, the Cu content is adjusted to 2.00% or less. The Cu content is preferably 1.50% or less, or 1.00% or less.

[44] As described above, Cr has the effect of improving the hardenability of the steel sheet and stabilizing the residual austenite. In order to obtain the effect of this action more reliably, the Cr content is preferably adjusted to 0.01% or more or 0.05% or more. However, when the Cr content exceeds 2.00%, the chemical convertibility of the steel sheet deteriorates significantly. Therefore, the Cr content is adjusted to 2.00% or less.

[45] As described above, Mo has the action of improving the hardenability of the steel sheet and the action of precipitating a carbide in the steel to increase its strength. In order to obtain the effect of this action more reliably, the Mo content is preferably adjusted to 0.01% or more or 0.02% or more. However, even when the Mo content is adjusted to more than 1.00%, the effect of this action becomes saturated, which is not economically preferable. Therefore, the Mo content is adjusted to 1.00% or less. The Mo content is preferably 0.50% or less and 0.20% or less.

[46] As described above, Ni has the effect of improving the hardenability of the steel sheet. In addition, when Cu is present, Ni effectively suppresses grain boundary cracking in the plate caused by Cu. To obtain this effect more reliably, the Ni content is preferably set to 0.02% or more. Since Ni is an expensive element, it is not economically preferable to contain a large amount of Ni. Therefore, the Ni content is set to 2.00% or less.

[47] As described above, boron (B) has the effect of improving the hardenability of steel sheet. In order to obtain this effect more reliably, the B content is preferably adjusted to 0.0001% or more, or 0.0002% or more. However, when the B content exceeds 0.0100%, the formability of the steel sheet deteriorates significantly, and therefore the content of B is adjusted to 0.0100% or less. The content of B is preferably 0.0050% or less.

[48] ​​(1-11) Ca: 0.0005% to 0.0200%, Mg: 0.0005% to 0.0200%, REM: 0.0005% to 0.1000% and Bi: 0.0005% to 0.020% All the elements Ca, Mg, and REM have the effect of improving the formability of steel sheet by adjusting the shape of inclusions to a preferable form. Additionally, Biscorus (Bi) has the effect of improving the formability of steel sheet by refining the solidification structure. Therefore, one, two, or more of these elements may be present. To obtain the effect more reliably, it is preferable to have 0.0005% or more of one or more of Ca, Mg, REM, and Bi. However, when the Ca or Mg content exceeds 0.0200%, or when the REM content exceeds 0.1000%, excessive inclusions form in the steel, and therefore, the ductility of the steel sheet may be degraded in some cases. Furthermore, even when the Bi content is adjusted to more than 0.020%, the aforementioned effect becomes saturated, which is not economically desirable. Therefore, the Ca content and the Mq content are adjusted to 0.0.200% or less, the REM content is adjusted to 0.1000% or less, and the Bi content is adjusted to 0.020% or less. The Bi content is preferably 0.010% or less.

[49] Here, REM refers to a total of 17 elements consisting of Se, Y, and lanthanoids, and the REM content refers to a total amount of these elements. In the case of the lanthanoids, the lanthanoids are added industrially in the form of a metal misch.

[50] (1-12) One or two or more of Zr, Co, Zn or W: 0% to 1.00% total and Sn: 0% to 0.050% With respect to Zr, Co, Zn, and W, the inventors hereof have confirmed that even when these elements are present in a total of 1.00% or less, the effect of the hot-rolled steel sheet according to the hereof is unaffected. Therefore, one, two, or more of Zr, Co, Zn, or W may be present in a total of 1.00% or less. Furthermore, the inventors of the present confirm that, even when it contains a small amount of Sn, the effect of the hot-rolled steel sheet in accordance with the present embodiment is not affected; however, a defect may occur during hot rolling and therefore the Sn content is adjusted to 0.050% or less.

[51] The chemical composition of the above hot-rolled steel sheet can be measured using a general analytical method. For example, inductively coupled plasma atomic emission spectrometry (ICP-AES) can be used for the measurement. Al₂ can be measured with ICP-AES using a filtrate after a sample is decomposed with an acid by heating. C₂ and S₂ can be measured using an infrared combustion-absorption method, and N₂ can be measured using the inert gas fusion thermal conductivity method. O₂ can be measured using an inert gas fusion non-dispersive infrared absorption method. ccn l Ln / zznz / E / YiAi

[52] 2. Microstructure of hot-rolled steel sheet The microstructure of hot-rolled steel sheet in accordance with this modality will now be described. In hot-rolled steel sheet according to the present modality, in a microstructure, in % area, ferrite is less than 15.0%, residual austenite is less than 3.0%, L52 / L7, which is a ratio of a length L52 of a grain boundary having a crystal orientation difference of 52° to a length L7 of a grain boundary having a crystal orientation difference of 7° around one direction <110> The concentration of Mn is 0.10 to 0.18, and the standard deviation of the Mn concentration is 0.60% by mass or less. Therefore, hot-rolled steel sheet produced according to this modality can achieve excellent strength, ductility, and shear workability. In this modality, the microstructure is specified at a position 1 / 4 of the sheet thickness from a surface and at a central position in the width direction of the sheet in a cross-section parallel to the rolling direction.The reason for this is that the microstructure at this position indicates a typical microstructure of the steel sheet. The 1 / 4 thickness position is an observation point for specifying the microstructure and is not strictly limited to 1 / 4 depth. A microstructure obtained by observing somewhere within a range of 1 / 8 to 3 / 8 thickness of the sheet can be considered the microstructure at the 1 / 4 position.

[53] (2-1) Ferrite area fraction: Less than 15.0% Ferrite is a structure formed when face-centered cubic (fcc) metals transform into body-centered cubic (bcc) metals at relatively high temperatures. Since ferrite has low strength, an excessive ferrite area fraction prevents the desired tensile strength from being achieved. Furthermore, an excessive ferrite area fraction results in a high Vickers hardness standard deviation. Therefore, the ferrite area fraction is typically kept below 15.0%. Ideally, the ferrite area fraction should be 10.0% or less, and very preferably less than 5.0%. When the ferrite area fraction is kept below 10.0% and the Vickers hardness standard deviation is controlled as described below, it is possible to improve the workability of the extreme surface of hot-rolled steel sheet after shearing. Since ferrite is preferably the smallest possible, the ferrite area fraction can be 0%.

[54] The ferrite area fraction is measured using the following method. The cross-section perpendicular to the rolling direction is mirror-finished and further polished at room temperature with colloidal silica containing no alkali solution for 8 minutes, thus eliminating the stress introduced into a surface layer of the sample. At a random position in the longitudinal cross-section of the sample, a region 50 pm long and between 1 / 8 of the sheet thickness from the surface and 3 / 8 of the sheet thickness from the surface is measured by electron backscatter diffraction at a measurement interval of 0.1 pm to obtain information about the crystal orientation.For the measurement, an EBSD analyzer configured with a thermal field emission scanning electron microscope (JSM-7001F, manufactured by JEOL) and an EBSD detector (DVC5 type detector, manufactured by TSL) is used. The vacuum level inside the EBSD analyzer is set to 9.6 × 10⁻⁵ Pa or less, the accelerating voltage to 15 kV, the irradiation current to 13, and the electron beam irradiation level to 62. A region where the average grain misorientation value is 1.0° or less is identified as ferrite, using the obtained crystal orientation information and an average grain misorientation function installed in the OIM Analysis software (registered trademark) attached to the EBSD analyzer. The area fraction of the region identified as ferrite is then calculated, thus obtaining the ferrite area fraction.

[55] (2-2) Residual austenite area fraction: less than 3.0% Residual austenite is a microstructure present as a face-centered cubic lattice even at room temperature. Residual austenite enhances the ductility of hot-rolled steel sheet through transformation-induced plasticity (TRIP). Furthermore, during shearing, residual austenite transforms into high-carbon martensite (hereafter also referred to as high-carbon martensite), thus inhibiting the formation of stable cracks and localizing damage to the cut edge. Shear damage is distributed across the worked face, and the varying degrees of damage result in areas where the austenite transforms into high-carbon martensite and areas where it does not.As a result, in the more significantly damaged portion of the damage distribution, the fully hardened, high-carbon martensite generated acts to promote further damage, and thus the damage on the cut end surface becomes even more localized. When the residual austenite area fraction is 3.0% or more, this effect is amplified, and the workability of the cut end surface on the hot-rolled steel sheet deteriorates. Therefore, the residual austenite area fraction is adjusted to less than 3.0%. The residual austenite area fraction is preferably less than 1.0%. Since the residual austenite is preferably as small as possible, the residual austenite area fraction can be 0%.

[56] As a method for measuring the area fraction of the residual austenite, methods such as X-ray diffraction, electron backscatter diffraction (EBSP) image analysis, and magnetic measurement and similar methods can be used, and the measured values ​​may vary depending on the measurement method. In the present embodiment, the area fraction of the residual austenite is measured by X-ray diffraction. In measuring the area fraction of residual austenite by X-ray diffraction in the present modality, first, the integrated intensities of a total of 6 peaks of «(110), «(200), «(211), y( 111), γ(200) yy(220) are obtained in the cross section parallel to the rolling direction at the position 1 / 4 of the thickness of the steel sheet and the central position in the direction of the width of the sheet using Co-Kα rays and the area fraction of residual austenite is obtained by calculation using the strength average method.

[57] (2-3) Bainite, martensite and self-hardening martensite: more than 82.0% and 100.0% or less in total In hot-rolled steel sheet according to the present embodiment, a low-temperature structure is contained as a microstructure distinct from ferrite and residual austenite. The low-temperature structure in the present embodiment consists of martensite, bainite, and self-hardening martensite in a total area fraction of more than 82.0% and 100.0% or less. When the total area fraction of bainite, martensite, and self-hardening martensite is 82.0% or less, there is a concern that the desired strength may not be achievable. Therefore, the total area fraction of bainite and martensite is preferably adjusted to more than 82.0%. The total area fraction is very preferably 85.0% or more. The total area fraction of bainite, martensite, and self-hardening martensite is preferably as large as possible and can therefore be adjusted to 100.0%.

[58] As a low-temperature structure, one of the ccn l Ln / zznz / E / YiAi bainite, martensite and self-hardening martensite may be contained in an area fraction of more than 82.0% and 100.0% or less or two or more of the bainite, martensite and self-hardening martensite may be contained in a total area fraction of more than 82.0% and 100.0% or less.

[59] In the microstructure of hot-rolled steel sheet according to the present modality, ferrite is less than 15.0% of the area, residual austenite is less than 3.0%, and the aforementioned low-temperature structure is contained as the remainder of the microstructure. That is, since the microstructure other than ferrite and residual austenite is the low-temperature structure consisting of one or more of the bainite, martensite, and self-hardening martensite, its area fraction can be obtained by subtracting the total area fraction of ferrite and residual austenite from 100.0%. Incidentally, as a method for measuring the area fraction of the low-temperature structure, the following method can be performed using a thermal field emission scanning electron microscope.

[60] In the low-temperature structure, an area ratio of the martensite can be obtained by the following procedure.A cross-section parallel to the rolling direction at one-quarter of the steel sheet thickness and centered along the sheet width is designated as the observed section, and this observed section is etched with LePera fluid. The observed section is considered a cross-section of the sheet thickness parallel to the rolling direction of the steel sheet. A secondary electron image of a 100 pm x 100 pm region within a range of one-eighth to three-eighth of the sheet thickness, centered at one-quarter of the sheet thickness, is observed in the observed section using a thermal field emission scanning electron microscope (JSM-7001F, manufactured by JEOL). Since the martensite and residual austenite are not corroded by LePera corrosion, the area ratio of the uncorroded regions can be considered as the total area ratio of the martensite and residual austenite.The martensite area ratio can be calculated by subtracting the residual austenite area ratio measured by the above method from the area ratio of these uncorroded regions.

[61] Furthermore, in the low-temperature structure, a bainite-to-self-hardening martensite area ratio, similar to the previous method for measuring the martensite area fraction, can be determined from a secondary electron image obtained by observing with a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL). An observed section is polished and etched with Nital, and a 100 pm x 100 μπ region is observed within a range of 1 / 8 to 3 / 8 of the sheet thickness, with 1 / 4 of the sheet thickness centered in the observed section. A plurality of notches are left around the region observed by the LePera corrosion above, thus confirming the same region as the region observed by the LePera corrosion. Self-hardening martensite is a lattice-like crystal grain aggregate and is a structure in which an iron carbide has two or more extension directions. Incidentally, bainite is also a lattice-like crystal grain aggregate, but it is a structure in which an iron-based carbide with a major axis of 20 nm or more is not contained, or a structure in which an iron-based carbide with a major axis of 20 nm or more is contained and the carbide is a single variant, that is, it has one extension direction of the iron-based carbide group. Self-hardening martensite can be distinguished from bainite by the fact that the cementite in the structure has multiple variants.

[62] The area fractions of bainite, martensite, and self-hardening martensite, which are the low-temperature structure, can be obtained by the method described above using a thermal field emission scanning electron microscope.

[63] As described above, in the microstructure of hot-rolled steel sheet according to this modality, less than 15.0% ferrite and less than 3.0% residual austenite are contained. The remainder of the microstructure consists substantially of the low-temperature structure and, in addition to these structures, may contain pearlite. Pearlite is a lamellar microstructure in which cementite precipitates in layers between the ferrite and is a soft microstructure compared to bainite and martensite. Pearlite is a structure that has low strength and degrades ductility and is therefore preferably not contained in hot-rolled steel sheet according to this modality. Furthermore, even when pearlite is present, its area fraction is preferably 5% or less to ensure strength and ductility.The area fraction is very preferably 3% or less. Since perlite is preferably as small as possible, the perlite area fraction can be 0%.

[64] The area fraction of pearlite can be measured using the following method. A sample is taken from a test piece of the steel sheet such that the microstructure of a cross-section one thickness of the sheet parallel to the rolling direction at a depth of 1 / 4 of the sheet thickness from the surface (a region from a depth of 1 / 8 of the sheet thickness from the surface to a depth of 3 / 8 of the sheet thickness from the surface) can be observed. The one-thickness cross-section is then polished, the polished surface is etched with Nital, and the structures of at least three 30 µm × 30 µm regions are observed using an optical microscope and a scanning electron microscope (SEM). The area ratio of pearlite is obtained by performing image analysis on a photograph of the structure obtained from this observation. When pearlite is present, the ferrite area fraction measurement is performed on crystal grains, excluding those identified as pearlite. Specifically, a region where the average grain misorientation value is 1.0° or less is identified as ferrite, using the obtained crystal orientation information and an average grain misorientation function installed in the OIM Analysis software (registered trademark) attached to the EBSD analyzer. The area fraction of the region identified as ferrite is then calculated, thus yielding the ferrite area fraction.

[65] (2-4) L52 / L7 ratio of the L52 length of the grain boundary that has a crystal orientation difference of 52° to the L7 length of the grain boundary that has a crystal orientation difference of 7° around the direction <110> : 0.10 to 0.18 To achieve a high strength of 980 MPa or more, the primary phase must have a fully hard structure. A fully hard structure typically forms during a phase transformation at 600°C or lower, but within this temperature range, a significant number of grain boundaries exhibit a crystal orientation difference of 52° and a crystal orientation difference of 7° along the grain boundary direction. <110> They form. When the grain boundary forms, it has a crystal orientation difference of 7° around the direction <110> , dislocations are less likely to accumulate in a hard phase.Therefore, in a microstructure where the grain boundary is uniformly dispersed at a high density (i.e., the total length of the grain boundary is large), the introduction of dislocations into the microstructure by shear work is easy, and distortion of the material is promoted during shear work. As a result, the difference in height of the extreme surface is suppressed after shear work.

[66] On the other hand, at the grain boundary which has a crystal orientation difference of 52° around the direction <110> Dislocations are likely to accumulate in the hard phase. Therefore, introducing dislocations into the microstructure through shearing is difficult, and the material fractures immediately during shearing. Consequently, the difference in height of the extreme surface after shearing becomes large. Therefore, the length of a grain boundary with a crystal orientation difference of 52° is denoted by L52, and the length of a grain boundary with a crystal orientation difference of 7° around the direction is denoted by L52. <110> Indicated by L7, the height difference of the extreme surface after shearing work is dominated by L52 / L7. When L52 / L7 is less than 0.10. Since dislocation accumulation in the hard phase is extremely difficult, it is not possible to adjust the base metal strength to 980 MPa or higher. Furthermore, when L52 / L7 exceeds 0.18, the extreme surface height difference after shearing becomes significant. Therefore, to reduce this difference, L52 / L7 is adjusted to between 0.10 and 0.18. Ideally, L52 / L7 should be 0.12 or higher, or 0.13 or higher. Additionally, Ls2 / L7 should preferably be 0.16 or lower, or 0.15 or lower. ccn l ίη / ζζηζ / Ε / γίΛΐ

[67] A grain boundary having a crystal orientation difference of Xo around the direction <110> It refers to a grain boundary that has a crystallographic relationship in which the crystal orientations of a crystal grain A and a crystal grain B are the same when rotating a crystal grain B by Xoa along the axis <110> When two adjacent crystal grains (crystal grain A and crystal grain B) are specified at a certain grain boundary, an orientation difference of ±4° with respect to the matched orientation relationship is allowed, considering the accuracy of the crystal orientation measurement.

[68] In the present embodiment, the length L52 of the grain boundary having a crystal orientation difference of 52° and the length L7 of the grain boundary having a crystal orientation difference of 7° around the direction <110> They are measured using the electron backscatter diffraction pattern orientation imaging microscopy (EBSP-OIM) method.

[69] In the EBSP-OIM method, a highly inclined sample is first irradiated with electron beams in a scanning electron microscope (SEM), and a backscattering Kikuchi pattern is photographed with a high-sensitivity camera. The resulting photographed image is then processed by a computer, allowing the crystal orientation to be measured from a point of irradiation over a short period of time.

[70] The EBSP-OIM method is performed using an analyzer The EBSP-OIM method was configured with a scanning electron microscope (JSM-7001F, manufactured by JEOL) and an EBSP-OIM detector (registered trademark) manufactured by AMETEK, Inc. In the EBSP-OIM method, because the fine structure of the sample surface and the crystal orientation can be analyzed, the length of the grain boundary with a specific crystal orientation difference can be quantitatively determined. Furthermore, the analyzable area of ​​the EBSP-OIM method is a region observable with the SEM. The EBSP-OIM method allows analysis of a region with a minimum resolution of 20 nm, which varies depending on the SEM resolution.

[71] L52 of the present modality is calculated using the following method. The length of the grain boundary that has a crystal orientation difference of 52° around the direction <110> It is measured at a position 1 / 4 of the sheet thickness from the surface of the steel sheet and at the center in the direction of the sheet width in a cross-section parallel to the rolling direction. In this measurement, the analysis is performed in at least 5 fields of view in a 40 pm * 30 pm region with a magnification of 1200 times, and the average value of the lengths of the grain boundaries that have a crystal orientation difference of 52° around <110> The direction is calculated, thus obtaining L52. Similarly, an average value is calculated for the lengths of grain boundaries that have a crystal orientation difference of 7° around the direction <110> to obtain L-. As described above, when calculating L52 and L?, an orientation difference of ±4° is allowed. Ferrite is a soft phase and has little influence on the dislocation accumulation effect within the hard phase. Furthermore, residual austenite is not a structure formed by a phase transformation at 600°C or lower and has no dislocation accumulation effect. Therefore, in the present measurement method, ferrite and residual austenite are not included as targets in the analysis. Ferrite can be specified and excluded from the analysis target using the same method as the ferrite area fraction measurement method. In the EBSP-OIM method, residual austenite with an fcc crystal structure can be excluded from the analysis target.

[72] (2-5) Standard deviation of Mn concentration: 0.60% by mass or less The standard deviation of the Mn concentration at the 1 / 4 thickness position from the surface of the hot-rolled steel sheet, in accordance with the present method, and the center position in the width direction of the sheet, is 0.60% by mass or less. Consequently, the grain boundary has a crystal orientation difference of 7° around the width direction. <110> It can be dispersed uniformly. As a result, the height difference of the extreme surface after shearing can be reduced. The standard deviation of the Mn concentration is preferably 0.55% by mass or less, 0.50% by mass or less, or 0.40% by mass or less. From the perspective of suppressing extreme surface irregularities after shearing, the standard deviation of the Mn concentration is desirable to be as small as possible. However, from the perspective of manufacturing process constraints, the practical lower limit of the standard deviation of the Mn concentration can be adjusted to 0.10% by mass or more.

[73] The standard deviation of the Mn concentration of the present modality is calculated using the following method. After mirror-polishing an L-shaped cross-section (cross-section parallel to the rolling direction) of the hot-rolled steel sheet, the concentration of Mn is measured at a point 1 / 4 of the sheet thickness from the surface and at the center of the sheet width using an electronic probe microanalyzer (ERMA) to determine the standard deviation of the Mn concentration. The measurement conditions are set to an acceleration voltage of 15 kV and a magnification of 5000x. The measurement interval is set to 20 pm in both the rolling and thickness directions, and a distribution image is measured. More specifically, the measurement interval is set to 0.1 pm, and Mn concentrations are measured at 40,000 or more points. The standard deviation is then calculated based on the Mn concentrations obtained from all measurement points.Therefore, the standard deviation of the Mn concentration is obtained.

[74] (2-6) Standard deviation of Vickers hardness: 20 HV0.01 or less When the Vickers hardness standard deviation at the center position in the width direction of the sheet is set to 20 HV0.01 or less, and the ferrite area fraction is set to 10.0% or less, as described above, in a sheet thickness cross-section parallel to the rolling direction of the hot-rolled steel sheet, it is possible to improve the workability of the extreme surface of the hot-rolled steel sheet after shearing. The workability of the extreme surface after shearing is significantly degraded by the damage to the extreme surface caused by shearing.Specifically, the end surface damage caused by shearing is distributed along the thickness of the sheet, and the extent of the damage is localized to a portion along the thickness of the sheet; that is, a portion along the thickness of the sheet is significantly damaged. In particular, when further work is performed on the end surface after shearing, the significantly damaged portion is expected to become a source of cracking and lead to fracture.

[75] The inventors hereof discovered that as the amount of ferrite and the standard deviation of the Vickers hardness decrease, the localization of damage in the thickness direction of the sheet on the end surface after shearing decreases, and the workability of the end surface after shearing is further improved. This is thought to be because the structure of the hot-rolled steel sheet becomes more uniform, thereby suppressing void formation during shearing and reducing the localization of damage. To achieve the above, the standard deviation of the Vickers hardness distribution of the hot-rolled steel sheet is preferably set to 20 HV0.01 or less. More preferably, the standard deviation is 18 HV0.01 or less and 17 HV0.01 or less.

[76] The standard deviation of Vickers hardness is obtained by the following method. In the microstructure at the central position in the width direction of the sheet in a cross-section of the sheet thickness parallel to the rolling direction, the Vickers hardness is measured at equal intervals at 300 or more measurement points within a sheet thickness interval of 1 mm. The measured load is set to 10 gf. Based on the measurement results, the standard deviation of the Vickers hardness (HV0.01) is calculated. ccn l Ln / zznz / E / YiAi

[77] 3. Tensile strength properties In hot-rolled steel sheet conforming to this specification, the tensile strength (maximum) is 980 MPa or higher. When the tensile strength is less than 980 MPa, the applicable component is limited, and the contribution to reducing the vehicle body weight is small. The upper limit need not be particularly restricted and can be set at 1780 MPa to minimize die wear. Tensile strength is measured in accordance with JIS Z 2241:2011 using test piece No. 5 of JIS Z 2241:2011. The sampling position of the tensile test piece can be a 1 / 4-quarter portion from the end in the width direction of the sheet, and the tensile test piece can be sampled such that a direction perpendicular to the rolling direction becomes the longitudinal direction.

[78] 4. Sheet thickness The thickness of the hot-rolled steel sheet according to this modality is not particularly limited and can be from 0.5 to 8.0 mm. By adjusting the thickness of the hot-rolled steel sheet to 0.5 mm or more, it becomes easy to ensure a rolling completion temperature, and the rolling force can be reduced, thus facilitating hot rolling. Therefore, the thickness of the hot-rolled steel sheet according to this modality can be adjusted to 0.5 mm or more. The sheet thickness is preferably 1.2 mm or more and 1.4 mm or more. Furthermore, when the sheet thickness is adjusted to 8.0 mm or less, refining the microstructure becomes easy, and the microstructure described above can be readily ensured. Therefore, the sheet thickness can be adjusted to 8.0 mm or less. The sheet thickness is preferably 6.0 mm or less.

[79] 5. Other (5-1) Veneer layer Hot-rolled steel sheet conforming to this specification, having the chemical composition and microstructure described above, may be surface-treated steel sheet provided with a plating layer on the surface to improve corrosion resistance and the like. The plating layer may be an electrolytic plating layer or a hot-dip plating layer. Examples of electrolytic plating layers include electrogalvanizing, electrolytic Zn-Ni alloy plating, and the like. Examples of hot-dip plating layers include hot-dip galvanizing, hot-dip galvanized-annealed, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, hot-dip Zn-Al-Mg-Si alloy plating, and the like.The degree of adhesion of the plating is not particularly limited and can be the same as before. Furthermore, it is also possible to further improve corrosion resistance by performing an appropriate chemical conversion treatment (for example, applying and drying a chromium-free, silicate-based chemical conversion treatment liquid) after plating. ccn l Ln / zznz / E / YiAi

[80] 6. Manufacturing conditions A suitable method for manufacturing hot-rolled steel sheet in accordance with the present modality having the chemical composition and microstructure described above is as follows.

[81] To obtain hot-rolled steel sheet in accordance with the present modality, it is effective to perform hot rolling after heating a plate under predetermined conditions, perform accelerated cooling to a predetermined temperature range after hot rolling, and monitor the cooling history after rolling.

[82] In the appropriate method for manufacturing hot-rolled steel sheet in accordance with the present modality, the following steps (1) to (7) are performed sequentially. The plate temperature and the steel sheet temperature in the present modality refer to the plate surface temperature and the steel sheet surface temperature.

[83] (1) The plate is held in a temperature range of 700°C to 850°C for 900 seconds or more, then heated further and held in a temperature range of 1100°C or more for 6000 seconds or more. (2) Hot rolling is carried out in a temperature range of 850°C to 1100°C, so that the thickness of the sheet is reduced by 90% or more. (3) Hot rolling is completed at a temperature TI (°C), which is represented by the following formula <1> , or higher. (4) Cooling begins within 1.5 seconds after completion of hot rolling, and accelerated cooling is carried out at an average cooling rate of 50°C / second or faster to a temperature T2 (°C), which is represented by the following formula <2> , or lower. (5) Cooling is carried out from the accelerated cooling stop temperature to the winding temperature at an average cooling rate of 10°C / second or faster. (6) The winding is carried out at a temperature T3 (°C), which is represented by the following formula <3> , or higher. (7) In cooling after coiling, the cooling is carried out such that, in the predetermined temperature intervals of the outermost part in the width direction of the sheet and the central part in the width direction of the hot-rolled steel sheet, the lower limit of the holding time after coiling satisfies a condition I (one or more of more than 2000 seconds at 450°C or more, more than 8000 seconds at 400°C or more, and more than 30000 seconds at 350°C or more). Very preferably, the average cooling rate in a temperature interval from the coiling temperature to the coiling temperature - 10°C is set to 0.010°C / second or slower.

[84] TI (°C) = 868 - 396 * [C] - 68.1 * [Mn] + 24.6 * [Si] - 36.1 x [Ni] - 24.8 x [Cr] - 20.7 χ [Cu] + 250 χ [Sol.]... <1>

[85] T2 (°C) = 770 - 270 χ [C] - 90 χ [Mn] - 37 χ [Ni] 70 χ [Cr] - 83 χ [Mo] ... <2>

[86] T3 (°C) = 591 - 474 χ [C] - 33 χ [Mn] - 17 χ [Ni] 17 χ [Cr] - 21 χ [Mo] ... <3> However, the [element symbol] in each formula indicates the amount (% by mass) of each element in the steel. When the element is not present, the substitution is made with 0.

[87] (6-1) Plate, plate temperature when subjected to hot rolling, holding time and retention As a plate to be subjected to hot rolling, a plate obtained by continuous casting, a plate obtained by casting and roughing, and similar plates may be used. If necessary, a plate obtained by additional hot working or cold working of the plate described above may be used.

[88] It is effective that the plate to be subjected to hot rolling be held in a temperature range of 700°C to 850°C during heating for 900 seconds or more, then further heated and held in a temperature range of 1100°C or more for 6000 seconds or more. During holding in the temperature range of 700°C to 850°C, the temperature of the steel sheet may fluctuate or remain constant within this temperature range. Furthermore, during holding in the temperature range of 1100°C or higher, the temperature of the steel sheet may fluctuate or remain constant at 1100°C or higher.

[89] In the austenite transformation in the temperature range of 700°C to 850°C, when Mn diffuses between the ferrite and austenite and the transformation time is lengthened, Mn can diffuse into the ferrite region. Consequently, the unevenly distributed microsegregation of Mn in the plate can be eliminated, and the standard deviation of the Mn concentration can be significantly reduced. By reducing the standard deviation of the Mn concentration, it is possible to uniformly disperse grain boundaries that have a crystal orientation difference of 7° around the direction. <110> in the final microstructure and reduce the height difference of the extreme surface after shearing. In addition, to ensure uniformity of the austenite grains during plate heating, it is effective to heat the plate at a temperature of 1100°C or higher for 6000 seconds or more.

[90] In hot rolling, it is preferable to use a reverse rolling mill or a tandem rolling mill for rolling in several passes. Particularly from the point of view of industrial productivity, it is more preferable that at least the final several stages be subjected to hot rolling using a tandem rolling mill.

[91] (6-2) Hot-rolled reduction: Reduction of the total thickness of the sheet of 90% or more in the temperature range of 850°C to 1100°C Hot rolling is performed at a temperature range of 850°C to 1100°C, reducing the sheet thickness by 90% or more. This primarily refines the recrystallized austenite grains. Furthermore, it promotes the accumulation of strain energy in the unrecrystallized austenite grains, thereby promoting austenite recrystallization and the atomic diffusion of manganese (Mn). Consequently, the standard deviation of the Mn concentration can be reduced. Therefore, hot rolling at a temperature range of 850°C to 1100°C is effective in reducing the sheet thickness by 90% or more.In other words, in the present modality, the standard deviation of the Mn concentration cannot be sufficiently suppressed solely by precise control of the plate heating, but can be suppressed by controlling the reduction of the hot rolling so that it is within the above range.

[92] The reduction of sheet thickness in a temperature range of 850°C to 1100°C can be expressed as (to ti) / tox100 (%) when the thickness of the incoming sheet before a first step in rolling in this temperature range is to and the thickness of the outgoing sheet after a final step in rolling in this temperature range is ti.

[93] (6-3) Hot rolling completion temperature: TI (°C) or higher The hot rolling termination temperature is preferably set to Ti (°C) or higher. By setting the hot rolling termination temperature to Ti (°C) or higher, an excessive increase in the number of ferrite nucleation sites in the austenite is suppressed. Furthermore, this suppresses ferrite formation in the final structure (the microstructure of the hot-rolled steel sheet after manufacturing), resulting in a high-strength steel sheet.

[94] (6-4) Accelerated cooling after completion of hot rolling: initial cooling in 1.5 seconds and accelerated cooling to T2 (°C) or less at an average cooling rate of 50°C / second or faster To suppress the growth of hot-rolled refined austenite crystal grains, it is preferable to perform accelerated cooling to T2 (°C) or lower within 1.5 seconds after completion of hot rolling at an average cooling rate of 50°C / second or faster.

[95] By performing accelerated cooling to T2 (°C) or lower within 1.5 seconds after the completion of hot rolling at an average cooling rate of 50°C / second or faster, ferrite and pearlite formation can be suppressed. Consequently, the strength of the steel sheet is improved. The average cooling rate referred to here is a value obtained by dividing the width of the temperature drop of the steel sheet from the start of accelerated cooling (when the steel sheet is introduced into the cooling equipment) to the completion of accelerated cooling (when the steel sheet is removed from the cooling equipment) by the time required from the start of accelerated cooling to the completion of accelerated cooling. In accelerated cooling after the completion of hot rolling, when the time to start cooling is set to 1.Within 5 seconds or less, the average cooling rate is set to 50°C / second or faster, and the cooling stop temperature is set to T2 (°C) or lower. This suppresses ferritic and / or pearlitic transformations within the steel sheet, and a tensile strength (TSh) of 980 MPa can be achieved. Therefore, within 1.5 seconds after the completion of hot rolling, accelerated cooling is performed at T2 (°C) or lower at an average cooling rate of 50°C / second or faster. The upper limit of the average cooling rate is not specified, but increasing the cooling rate increases the size of the cooling equipment and its cost. Therefore, considering equipment cost, the average cooling rate is preferably 300°C / second or lower, very preferably less than 200°C / second, and most preferably still less than 150°C / second.In addition, the cooling stop temperature of the accelerated cooling can be set to T3 (°C) or higher.

[96] (6-5) Average cooling rate from the cooling stop temperature of accelerated cooling to the winding temperature: 10°C / second or faster To suppress the pearlite area fraction and obtain a tensile strength of 980 MPa or higher, the average cooling rate from the accelerated quenching stop temperature to the rolling temperature is adjusted to 10°C / second or faster. In such a case, the primary phase structure can be fully hardened. The average cooling rate referred to here is a value obtained by dividing the temperature drop width of the steel sheet from the accelerated quenching stop temperature to the rolling temperature by the time required from the accelerated quenching stop temperature to the rolling temperature. By adjusting the average cooling rate to 10°C / second or faster, it is possible to reduce the pearlite area fraction and ensure strength and ductility.Therefore, the average cooling rate from the accelerated cooling stop temperature to the winding temperature is set to 10°C / second or faster.

[97] (6-6) Winding temperature: T3 (°C) or higher The winding temperature is set to T3 (°C) or higher. By setting the winding temperature to T3 (°C) or higher, it is possible to decrease the driving force for the austenite-to-bcc transformation and also to decrease the austenite's resistance to deformation. Therefore, during the bainitic or martensitic transformation, L52 / L7 can be adjusted to 0.18 or less by reducing the L52 length of the grain boundary that has a crystal orientation difference of 52° around the direction. <110> or by increasing the L7 length of the grain boundary that has a crystal orientation difference of 7o around the direction <110> As a result, the height difference of the extreme surface after shearing can be reduced. Therefore, the winding temperature is set to T3 (°C) or higher.

[98] (6-7) Cooling after coiling: After coiling of hot-rolled steel sheets, cooling is carried out in a predetermined temperature range for the lower limit of the holding time to satisfy the following condition I Condition I: One or more of more than 2000 seconds at 450°C or more, more than 8000 seconds at 400°C or more, and more than 30000 seconds at 350°C or more

[99] In post-winding cooling, the cooling is carried out so that the lower limit of the holding time in a predetermined temperature range satisfies condition I, i.e., the cooling is carried out with a holding time that satisfies one or more of more than 2000 seconds at 450°C or more, more than 8000 seconds at 400°C or more, and more than 30000 seconds at ccn l Ln / zznz / E / YiAi 350°C or higher is ensured, so the transformation progresses sufficiently. As the transformation advances, the austenite may stabilize and the transformation may stop; however, if this retention time is met, the transformation resumes and the residual austenite area fraction can be reduced. As a result, it is possible to adjust the residual austenite area fraction to less than 3.0%.

[100] Furthermore, in post-rolling cooling, as a preferable condition, the average cooling rate over a temperature range from the rolling temperature to the rolling temperature - 10°C is set to 0.010°C / second or slower. In such a case, it is possible to make the transformation temperature uniform throughout the microstructure. As a result, it is possible to adjust the standard deviation of the Vickers hardness of the hot-rolled steel sheet to 20 HV0.01 or less and improve the workability of the extreme surface after shearing.

[101] The cooling rate of hot-rolled steel sheet after coiling can be controlled with a heat-insulating cover or edge mask, by mist cooling or similar. In this method, the temperature of the hot-rolled steel sheet is measured with a contact or non-contact thermometer at the widest point of the sheet. For portions other than the widest point of the hot-rolled steel sheet, the temperature is measured with a thermocouple or calculated using heat transfer analysis. ccn l Ln / zznz / E / YiAi Examples

[102] The effects of one aspect of the present invention will now be described more specifically by way of examples, but the conditions in the examples are illustrative of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these illustrative conditions. The present invention may employ various conditions, provided that the object of the present invention is achieved without departing from the essence of the present invention.

[103] Steels having the chemical composition shown in Steels Nos. A to T in Table 1 and Table 2 were continuously melted and cast to manufacture plates 240 to 300 mm thick. The resulting plates were used to produce hot-rolled steel sheets shown in Table 5 under the manufacturing conditions shown in Table 3 and Table 4. The plates to be subjected to hot rolling were held at a temperature range of 700°C to 850°C for the holding time shown in Table 3, then further heated to the holding temperature shown in Table 3 and held.

[104] For the hot-rolled steel sheets obtained, the ferrite and residual austenite area fractions, L52 / L-, the standard deviations of the Mn concentrations, and the standard deviations of the Vickers hardness were obtained using the methods described above. The measurement results obtained are shown in Table 5. In the microstructure of the examples of the present invention, as a result of confirmation by a method in which the above thermal field emission scanning electron microscope was used, the structure distinct from ferrite and residual austenite consisted of one or more of bainite, martensite, and tempered martensite. ccn l Ln / zznz / E / YiAi

[105] Method for evaluating the properties of hot-rolled steel sheets (1) Tensile strength and total elongation Among the mechanical properties of the hot-rolled steel sheets obtained, tensile strength and total elongation were evaluated in accordance with JIS Z 2241: 2011. One test piece was a test piece No. 5 from JIS Z 2241: 2011. The sampling position of the tensile test piece was a portion of 1 / 4 from the end portion in the direction of the sheet width, and samples were taken from the tensile test piece so that a direction perpendicular to the rolling direction became the longitudinal direction.

[106] In a case where the tensile strength TS b 980 MPa and the tensile strength TS χ total elongation El b 14000 (MPa-%), were satisfied, the hot-rolled steel sheet was determined to be acceptable as a hot-rolled steel sheet with excellent strength and ductility. On the other hand, in the case where neither the tensile strength TS b 980 MPa and the tensile strength TS χ total elongation El > 14000 (MPa-%), were satisfied, the hot-rolled steel sheet was determined to be unacceptable for not having excellent strength and ductility. ccn l ίη / ζζηζ / Ε / γίΛΐ

[107] (2) Shear workability and end surface workability after shear work The shear workability of the hot-rolled steel sheet and the workability of the cut end surface were evaluated by a punching test. Five punched holes were prepared with a hole diameter of 10 mm, a clearance of 10%, and a punching speed of 3 m / s.

[108] First, in the evaluation of shear workability, the cross-sections of the five holes drilled perpendicular to the rolling direction were embedded in a resin, and the shapes of the cross-sections were photographed using a scanning electron microscope. In the resulting photographs, the machined end surfaces could be observed, as shown in Figure 1.In the observation photograph, a straight line that was perpendicular to a top surface and a bottom surface of the hot-rolled steel sheet and passed through a vertex of a burr (a point A on a portion of burr furthest from the bottom surface of the hot-rolled steel sheet in the direction of the sheet thickness) (straight line 1 in Figure 1) and a straight line that was perpendicular to the top surface and the bottom surface of the hot-rolled steel sheet and passed through a position B in the cross section nearest to the punched hole (farthest from straight line 1) (straight line 2 in Figure 1) were plotted, and the distance between these two straight lines (d in Figure 1) was defined as the difference in height of the end surface.For 10 end surfaces obtained from the 5 drilled holes, the height differences of the end surfaces were measured and, when the maximum value of the height differences of the end surfaces was 18% or less of the thickness of the sheet (maximum value of the height differences of the end surfaces (mm) / thickness of the sheet (mm) x 100 i 18), the hot-rolled steel sheet was determined to be acceptable as a hot-rolled steel sheet that had excellent shear workability.On the other hand, when the maximum value of the height differences of the extreme surfaces was greater than 18% of the sheet thickness (maximum value of the height differences of the extreme surfaces (mm) / sheet thickness (mm) χ 100 > 18), the hot-rolled steel sheet was determined to be unacceptable as a hot-rolled steel sheet that had poor shear workability.

[109] Next, in the evaluation of the workability of the extreme surface after shearing, the Vickers hardness was measured for the 10 extreme surfaces whose cross-sections were photographed. The load was set to 100 gf, and the Vickers hardness (HV0.1) was measured at a position 80 pm from the extreme surface (a position 80 pm from straight line 2 toward the side of straight line 1 in Figure 1) from the top surface to the bottom surface of the hot-rolled steel sheet at 100 pm intervals in the direction of the sheet thickness. When the difference between a maximum and a minimum Vickers hardness value obtained was 85 HV0.1 or less, the hot-rolled steel sheet was determined to have excellent extreme surface workability after shearing.

[110] The measurement results obtained are shown in Table 5. ccn l Ln / zznz / E / YiAi

[111] Table ccn l Ln / zznz / E / YiAi

[112] Table 2 Steel No. % by mass, the remainder Fe and impurity TI (“CJ T2 (°C) T3 Í“C) Note Ca Mg REM Bi Zr Co Zn w Sn A 0.0018 0.0018 720 552 473 Example of the invention B 683 518 429 Example of the invention C 0.0012 658 511 404 Example of the invention D 0.002 653 501 442 Example of the invention E 695 524 419 Example of the invention F 759 617 462 Example of the invention G 785 388 371 Example of the invention H 958 403 383 Example of the invention I 0.06 729 564 4 66 Example of the invention J 685 536 435 Example of the Invention K 0.03 710 545 459 Example of the invention L 0.05 716 557 459 Example of the invention M 667 500 419 Example of the invention N 0.025 716 540 459 Example of the invention O 687 541 444 Example of the invention P 0.16 699 540 444 Example of the invention Q 734 566 487 Comparative example R 657 502 390 Comparative example S 719 530 435 Comparative example T 794 656 496 Comparative example

[113] Table Note Example of the invention Example of the invention Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Cooling | Average cooling rate from cooling stop temp. of accelerated cooling to winding temp. I °C / sec I CN Csl O- £ co co OJ CN cc CxJ O co LO CN CSI CSI Temp. of cooling stop of accelerated cooling 522 iñ 490 506 501 508 488 499 or LO 565 516 499 490 CSI 1— 552 518 518 518 518 518 518 518 518 518 518 518 518 Average cooling rate of accelerated cooling στ Φ O 00 2 3 O» o LO $ 5 O Ό SI LO in £ Λ LO Time until cooling start στ φ ω CN o O o CN O - - a 00 o oo co o - O Hot rolling | Temp.finalization of hot rolling ω S88 891 c? 904 896 908 678 907 894 885 872 881 co co 720 683 683 683 683 683 683 683 683 683 683 683 683 Reduction of foil thickness at 850°C at 1100°C CM CM a O 51 51 O co CO Plate heating | Storage time στ φ ω 6615 8194 7035 6855 5320 6730 7099 8137 7605 7300 7612 8028 6842 Heating temperature ω 1157 1298 1199 1238 1296 1183 1281 1264 1285 1248 1250 1225 1281 Maintenance time στ ω ω 1187 1068 834 850 1135 995 1219 1131 1245 1166 1032 1136 1079 ó Ó Φ Jr Z Ό S Π] < 02 m 02 02 m 02 02 02 02 02 02 02 No. de fabricación. - en in Ό 00 OO - CN CO. ccn l Ln / zznz / E / YiAi Comparative Example Comparative Example Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention m CM LO in co CM in CM CM co - co C4 co co CM o CM o» CM 505 04 íñ 491 499 487 471 CM íñ 605 382 399 552 525 523 534 479 íñ 04 íñ 518 00 íñ 00 LO 518 íñ LO? 524 Ό 388 403 564 536 545 557 500 540 541 m Ό CM LO CM O s O 103 CO OO 04 120 105 103 00 00 O *o CO CO o* o OO oooo* o O o CM OO 00 oo co OO o - 04 CO OO 912 902 905 51 LO 00 00 00 co 894 O co 965 915 914 806 895 920 co OO 901 OO co 00 co 00 00 CO 5 co lo 695 759 m co 00 £ 729 685 O Ό 667 o 687 co oo 04 04 04 o cT 04 o* 5* 04 5= O θ' CO θ' 7674 7512 8073 8342 8492 7524 7543 8101 8079 7652 7909 9045 8592 8052 7848 8679 8763 1168 12511162 1152 1260 1241 1218 1298 1216 1220 1215 1265 1296 1190 1243 1192 1166 1157 1134 1246 997 956 1149 1106 1076 1017 1204 1109 1089 1004 966 1049 1007 953 m en en ω QL±J U- OT - -) z O 2 in £ co o· O CM CM CM CM co CM CM LO CM CM CM 00 CM 04 o co ccn l in / zz / E / yLi ccn l in / zz / E / yLi

[114] Tabla Note Example of the invention Example of the invention Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Cooling after winding | Average cooling rate within the temperature range from winding temperature to winding temperature - 10°C °C / sec 9000 9000 9000 0.006 0006 9000 9000 0006 9000 ¿000 0006 1000 ¿000 0006 0.013 Holding time at 350°C or higher 07 Φ tn 24500 31000 24000 17800 26500 23800 16700 15000 15500 31500 21400 27000 14900 29200 4200 Holding time at 400°C or higher | Bes 15400 19200 14600 8400 9300 14300 7400 5600 6400 22400 8400 8500 5400 5800 3800 Holding time at 450°C or higher σ> OI<!--) 5400 8900 3600 O O 4500 OI OI 3600 13600 3700 O 700 O 1900 Enrollamiento Temperatura de enrollamiento o 480 500 470 437 430 475 5 430 470 525 472 410 LO 435 474 09 1— 473 θ' 429 θ' CN 429 O* CN 429 429 429 429 O* csi CN O 429 No. de acero. < 03 en m OQ en en en en m en en m en en No. de fabricación. - CN 09 in Ό r- 00 o- o - CN 09 2 LO<br--> ccn l Ln / zznz / E / YiAi Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention 0.015 oo 9000 9000 9000 9000 0001 0.001 9000 ¿000 9000 0.006 9000 9000 9000 9000 25400 24800 14100 20600 28700 28800 30064 30015 33300 19000 21000 23200 19600 23000 20400 22300 16100 8200 8200 11300 11300 19300 OO 24100 8500 11900 13900 10100 13700 11300 13200 2500 OO 1500 O 0096 oo 14500 O 3200 5200 300 4000 1600 3200 484 430 425 458 435 503 375 385 524 438 co 479 452 472 458 468 429 429 O 442 419 462 371 383 Ό -O 435 459 459 419 459 5 444 QQ OQ UJ Ll_ O TE - “1 _l zo Q- o £ 00 í o CM CN ΓΌ in (N Ό CSI CN 00 CM o CN o co cñ ccn l ίη / ζζηζ / Ε / γίΛΐ ccn l Ln / zznz / E / YiAi 32800 30600 31200 51500 cation preferable. iF oj OOO en o co 0*1 O 1860 423( ¡ condition UJ 00Z91 o 6700 32500 ites do not follow aoco 537 395 478 550 res corres^ 487 390 435 496 the value Z3 a OI — Q — 100 lican cc in o CO CO CO in CO )ray< □ in and o

[115] Table Note Example of the invention Example of the invention Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Comparative example Difference between maximum and minimum Vickers hardness value L OLH £ m LO LO s CO 2 125 103 106 in 107 Maximum value of difference of extreme surface height / sheet thickness 0 CN 2a 9 0- LO LO LO 2 25 20 LU X ω 1— MPa% 16151 15912 15375 14839 15432 15399 17221 15485 17898 15456 12931 14944 17123 17552 Total elongation El 5? 16.2 3 15.0 14.2 14.6 14.5 CO co 16.3 00 6L 13.4 14.3 in 14.9 Tensile strength TS MPa ¿66 1105 1025 1045 1057 1062 3 950 942 960 965 1045 1134 1178 Standard deviation of Vickers hardness LOOAH cn £ 2= co 0- CN LO CN CN CN CN 00 CN 0- CN CN CN CN Standard deviation of Mn % by mass 0.44 0.40 0.71 0.67 0.74 990 CN O 0.40 0.41 0.40 0.42 0.43 0.41 0.41 L52 / L7 0.15 ZLO 0 ZIO 81O 0.13 O 0 0.12 LIO 9L0 0.23 0.14 LIO Residual Austenite % of area 0 0 £ 0 0 CN - CN CN in 0 0 LO O 0.8 ¡TS trz Ferrite % of area OLI 0 tn 0 CN 0 in 4.0 O to 16.0 23.0 16.0 06 7.0 0¿ OS Sheet thickness mm in CN in cn in ev in . <n en cn 2.3 no. de fabricación. - lo o co 0 q enccn l Ln / zznz / E / YiAi Comparative Example Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention Example of the invention 124 CM 103 lo in LO £ CM 09 CM ooo 2 LO £ π 2 θ' θ' θ' LO CM - θ' CM - co 16259 15388 14387 14481 14084 14177 14911 15100 19640 16091 15629 14467 15800 15362 15653 16325 15302 14.2 14.6 12.8 12.2 13.2 12.9 15.2 12 7 O CM 15.9 CO 13.7 LO 14.7 14.4 15.3 14.9 1145 1054 1124 1187 1067 1099 981 1189 982 1012 1056 1056 1026 1045 1087 1067 1027 CM cm £ θ' θ' co CO θ' θ' CO £ £ £ θ' θ' CO 0.41 0.41 0.42 045 95 0 0.45 0.23 09 0 0.46 0.40 0.43 044 040 0.45 0.43 039 LO o 016 0.15 0.15 OO 0.16 0.16 018 0.17 IOL ll 0 0.16 0.15 LLO 9L 0 O or O <r £0 00 cm co o co csí -o 0.0 0.4 4.0 o lo 13.0 -o 09 om oj 9 m £ o- 20 24 25 28 29 30 cñccn L ίη / 77Π7 / E / YΙΛΙ ccn l ίη / ζζηζ / Ε / γίΛΐ

[116] As can be seen in Table 5, in manufacturing numbers 1, 2, and 16 to 31, which were the examples of the invention, hot-rolled steel sheets were obtained that had excellent strength, ductility, and shear workability. Furthermore, among the examples of the present invention, in manufacturing numbers 2 and 18 to 31, according to the preferred aspect, hot-rolled steel sheets were obtained that had the aforementioned properties and, in addition, excellent end surface workability after shearing.

[117] Furthermore, in manufacturing numbers 3 to 15 and 32 to 35, where the chemical composition and microstructure were not within the ranges specified by the present invention, one or more of the properties (tensile strength, ductility, and shear workability) were poor. Additionally, in manufacturing number 11, the formation of ferrite, residual austenite, and a low-temperature structure (6% pearlite by area) was confirmed. Consequently, the tensile strength decreased. ccn l Ln / zznz / E / YiAi Industrial applicability

[118] According to the preceding aspect of the present invention, it is possible to provide a hot-rolled steel sheet having excellent shear strength, ductility, and workability. Furthermore, according to the preferred aspect of the present invention, it is possible to obtain a hot-rolled steel sheet having the aforementioned properties and, in addition, excellent workability of an end surface after shearing. The hot-rolled steel sheet of ccn l Ln / zznz / E / YiAi in accordance with the present invention is suitable as an industrial material used for vehicle elements, mechanical structural elements and construction elements.< / r> < / n>

Claims

1. A hot-rolled steel sheet comprising, as chemical composition, in % by mass: C: 0.100% to 0.250%, Si: 0.05% to 2.00%, Mn: 1.00% to 4.00%, Al: 0.001% to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Ti: 0% to 0.300%, Nb: 0% to 0.100%, V: 0% to 0.500%, Cu: 0% to 2.00%, Cr: 0% to 2.00%, Mo: 0% to 1. 00%, Ni: 0% to 2.00%, B: 0% to 0.0100%, Ca: 0% to 0.0200%, Mg: 0% to 0.0200%, REM: 0% to 0.1000%, Bi: 0% to 0.020%, one or two or more of Zr, Co, Zn and W: 0% to 1.00% in total, Sn: 0% to 0.050%, and a remainder consisting of Fe and impurities, wherein, in a microstructure, in % area, ferrite is less than 15.0%, residual austenite is less than 3.0%, L52 / L7, which is a ratio of a length L52 of a grain boundary having a crystal orientation difference of 52° to a length L? of a grain boundary having a crystal orientation difference of 7° around a direction <110> , is from 0.10 to 0.18, a standard deviation of a Mn concentration is 0.60% by mass or less, and a tensile strength is 980 MPa or more.

2. The hot-rolled steel sheet according to claim 1, wherein, in the microstructure, in % area, the ferrite is 10.0% or less, and a standard deviation of the Vickers hardness is 20 HV0.01 or less.

3. The hot-rolled steel sheet according to claim 1 or 2, further comprising, as a chemical composition, in % by mass, one or two or more selected from a group consisting of: Ti: 0.005% to 0.300%; Nb: 0.005%; V: 0.005%; Cu: 0.01%; Cr: 0.01%; Mo: 0.01%; Ni: 0.02% to 0.100%; Ln: 0.500%; Ln: 0.0001% to 0.0100%; Ca: 0.0005% to 0.0200%; Mg: 0.0005% to 0.0200%; 10 REM: 0.0005% to 0.1000%; and Bi: 0.0005% to 0.020%.