High-strength hot-rolled steel plate

Abstract

Provided is a high-strength hot-rolled steel sheet having a predetermined chemical composition, wherein at all of the positions of 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W from the end in the width direction (W is the total width in the direction perpendicular to the rolling direction and the sheet thickness direction), the metal structure at 1 / 4 position of the sheet thickness contains, in area%, at least 95% of tempered martensite, 5% or less of fresh martensite, and 5% of less in total of at least one among ferrite, upper bainite, and pearlite, and the difference between the maximum and minimum tensile strengths at all of the positions in the width direction is 30 MPa or less.

Description

Technical Field

[0001] The present invention relates to a high-strength hot-rolled steel sheet.

Background Art

[0002] In recent years, in the automotive industry, the strength of steel sheets has been increasing from the viewpoints of reducing environmental impact and ensuring the safety of passengers. With the increase in the strength of steel sheets, the main structure constituting the metal structure of the steel sheet is becoming a martensite structure.

[0003] For example, in Patent Document 1, in mass%, C: 0.08% or more and less than 0.16%, Si: 0.01 to 1.0%, Mn: 0.8 to 2.0%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.002 to 0.006%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Cr: 0.01 to 1.0%, B: 0.0005 to 0.0050%, with the balance being Fe and inevitable impurities, and a martensite phase or tempered martensite phase as the main phase, the volume fraction of the main phase being 90% or more with respect to the entire structure, the average grain size of prior austenite grains being 20 μm or less in a cross-section parallel to the rolling direction and 15 μm or less in a cross-section perpendicular to the rolling direction, and the aspect ratio of prior austenite grains in a cross-section parallel to the rolling direction being 18 or less. A high-strength hot-rolled steel sheet excellent in low-temperature toughness is described. Further, in Patent Document 1, according to the above configuration, without containing expensive Mo, it has a high strength with a yield strength YS of 960 MPa or more and a high toughness with vE -40 of 40 J or more, and further has excellent bend formability and stress corrosion cracking resistance, and also has a surface hardness of 360 HB or more in Brinell hardness and excellent wear resistance, and a hot-rolled steel sheet suitable for structural members of construction machinery and industrial machinery can be easily manufactured, and it is described that it has an extremely remarkable industrial effect.

[0004] Patent Document 2 describes a material containing, by mass%, C: 0.05-0.14%, Si: 0.01-1.0%, Mn: 0.50-2.0%, P: 0.025% or less, S: 0.005% or less, Al: 0.005-0.10%, N: 0.002-0.006%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, Cr: 0.01-1.0%, B: 0.0005-0.0050%, with the remainder being Fe and unavoidable impurities. A high-strength hot-rolled steel sheet is described, characterized by having a component composition consisting of the above, the steel structure having a main phase consisting of at least one of a martensite phase and a tempered martensite phase with an area ratio of 95% or more to the entire steel structure, and containing cementite with an average particle size of 0.5 μm or less within the lath of the martensite phase and / or tempered martensite phase, with a cementite content of 0.01 to 0.08% by mass%. Furthermore, Patent Document 2 describes a high-strength hot-rolled steel sheet that, according to the above configuration, has high toughness and excellent punchability and punched bending fatigue strength characteristics even without containing the expensive alloying element Mo, specifically high strength with a tensile strength TS: 980 MPa or higher and absorbed energy vE in a Charpy impact test at a test temperature of -40°C. -40 It is stated that this method can provide a high-strength hot-rolled steel sheet that has a toughness of 40J or more, and also exhibits excellent punchability and punched bending fatigue strength characteristics.

[0005] Patent Document 3 describes a high-strength hot-rolled steel sheet with excellent strength uniformity in the width direction, characterized by a composition in mass%, containing C: 0.10-0.25%, Si: 0.10% or less, Mn: 1.0-2.0%, P: 0.025% or less, S: 0.005% or less, Al: 0.005-0.10%, Nb: 0.01-0.05%, Ti: 0.005-0.05%, Cr: 0.05-1.0%, and B: 0.0005-0.0050%, with the remainder being Fe and unavoidable impurities; having a structure in which the tempered martensite phase accounts for 95% or more by volume relative to the entire structure, and having an average grain size of prior austenite grains of 20 μm or less in a cross section parallel to the rolling direction and 15 μm or less in a cross section perpendicular to the rolling direction, and having a yield strength (YS) of 960 MPa or more. Furthermore, Patent Document 3 teaches that a high-strength hot-rolled steel sheet with a yield strength (YS) of 960 MPa or higher, uniformly in the width direction, can be obtained by having tempered martensite as the main phase throughout the width direction of the steel sheet, with an average particle size of prior austenite (γ) grains of 20 μm or less in a cross section parallel to the rolling direction, and an average particle size of prior austenite grains of 15 μm or less in a cross section perpendicular to the rolling direction. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2016-211073 [Patent Document 2] Japanese Patent Publication No. 2018-188675 [Patent Document 3] Japanese Patent Publication No. 2016-183414 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] In hot-rolled steel sheets as described in Patent Documents 1 to 3, when the metal structure is made to be single-phase martensite or closer to single-phase martensite in order to increase strength, the shape of the steel sheet may collapse due to uneven cooling during cooling or transformation plasticity during transformation. In such cases, it becomes difficult to maintain sufficient flatness in the resulting hot-rolled steel sheet. For example, Patent Document 3 examines a high-strength hot-rolled steel sheet with a uniform yield strength YS in the width direction, but no specific examination is conducted from the perspective of improving the flatness of the said high-strength hot-rolled steel sheet.

[0008] Therefore, the present invention aims to provide a high-strength hot-rolled steel sheet having improved flatness through a novel configuration. [Means for solving the problem]

[0009] To achieve the above objective, the inventors focused particularly on the microstructure in the width direction of the hot-rolled steel sheet. As a result, the inventors found that by making the microstructure of a hot-rolled steel sheet having a predetermined chemical composition a structure mainly composed of tempered martensite, high strength can be ensured, and by uniformly distributing this structure in the width direction, the variation in strength in the width direction can be reduced. In this way, by reducing the variation in strength in the width direction, it is possible to provide a hot-rolled steel sheet with significantly improved flatness in the width direction.

[0010] The present invention, which has achieved the above objectives, is as follows. (1) In mass%, C: 0.050~0.100%, Si: 0.010~0.200%, Mn: 1.00~2.50%, Ti: 0.001~0.120%, Al: 0.001~0.050%, B: 0.0005~0.0050%, P: 0.100% or less, S: 0.050% or less, N: 0.0050% or less, O: 0 to 0.0050%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Sn: 0 to 0.10%, Cr: 0 to 0.40%, Mo: 0 to 0.20%, Nb: 0 to 0.05%, V: 0 to 0.10%, As: 0 to 0.100%, Zr: 0 to 0.100%, Ca: 0 to 0.0050%, Mg: 0 to 0.100%, Bi: 0 to 0.020%, Co: 0 to 0.20%, W: 0 to 0.20%, Zn: 0 to 0.20%, REM: 0 to 0.1000%, and the balance: having a chemical composition consisting of Fe and impurities, when the total width in the direction perpendicular to the rolling direction and the plate thickness direction is W, at all positions of 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from the end in the width direction, the metallographic structure at 1 / 4 of the plate thickness is, in area%, annealed martensite: 95% or more, fresh martensite: 5% or less, and at least one of ferrite, upper bainite and pearlite: 5% or less in total, A high-strength hot-rolled steel sheet characterized in that the difference between the maximum value and the minimum value of the tensile strength at all positions in the width direction is 30 MPa or less. (2) The chemical composition is, in mass%, O: 0.0001 to 0.0050%, Cu: 0.001 to 0.20%, Ni: 0.001 to 0.20%, Sn: 0.001 to 0.10%, Cr: 0.001 to 0.40%, Mo: 0.001 to 0.20%, Nb: 0.001 to 0.05%, V: 0.001 to 0.10%, As: 0.001 to 0.100%, Zr: 0.0001 to 0.100%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.100%, Bi: 0.0001 to 0.020%, Co: 0.001 to 0.20%, W: 0.001 to 0.20%, Zn: 0.001 to 0.20%, and REM: 0.0001 to 0.1000% The high-strength hot-rolled steel sheet according to the above (1), characterized by containing at least one of them. (3) The high-strength hot-rolled steel sheet according to the above (1) or (2), characterized in that the prior austenite grain size in the metal structure is 40 μm or less.

Effects of the Invention

[0011] According to the present invention, a high-strength hot-rolled steel sheet having improved flatness can be provided. [[ID=3​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​Cr: 0~0.40%, Mo: 0~0.20%, Nb: 0~0.05%, V: 0~0.10%, As: 0~0.100%, Zr: 0~0.100%, Ca: 0~0.0050%, Mg: 0~0.100%, Bi: 0~0.020%, Co: 0~0.20%, W: 0~0.20%, Zn: 0~0.20%, REM: 0~0.1000%, and The remainder has a chemical composition consisting of Fe and impurities. When the total width in the direction perpendicular to the rolling direction and the thickness direction is W, at all positions from the edge in the width direction, specifically at positions 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W, the metallographic structure at the 1 / 4 thickness position is, in area %, Tempered martensite: 95% or more, Fresh martensite: 5% or less, and At least one of ferrite, upper bainite, and pearlite: totaling 5% or less. A key feature is that the difference between the maximum and minimum tensile strengths at all positions in the width direction is 30 MPa or less.

[0013] As mentioned earlier, when hot-rolled steel sheets are made with a single-phase martensite or a structure closer to single-phase martensite to increase their strength, the shape of the steel sheet may collapse due to uneven cooling or transformation plasticity during transformation. In such cases, it becomes difficult to maintain sufficient flatness in the resulting hot-rolled steel sheet. Specifically, regarding the collapse of the steel sheet shape, warping may occur in the width direction of the steel sheet (the direction perpendicular to the rolling direction and the thickness direction). When warping occurs in the width direction of the steel sheet, it can lead to shape defects or cracking during forming when forming longitudinal members using such steel sheets. An existing method to improve such warping is straightening (flattening treatment) using a leveler. However, when the strength of the steel sheet is high, levelers may not always be able to straighten it sufficiently. In addition, if pre-processing is performed by straightening with a leveler, some of the ductility inherent in the steel sheet is consumed by this pre-processing, and the residual ductility decreases. When the residual ductility decreases, forming defects are more likely to occur when pressing the steel sheet, and as a result, productivity decreases. Therefore, good flatness is required in the as-hot-rolled state of the steel sheet, without correction using levelers or the like. The main factor causing deformation of the steel sheet shape is a fast cooling rate after finish rolling. When the cooling rate after finish rolling is fast, the controllability of the amount of water used for cooling deteriorates, and uneven cooling becomes pronounced, such as localized overcooling. As a result, thermal stress is generated due to temperature unevenness in the width direction of the steel sheet, causing warping in the width direction of the steel sheet. Therefore, from the viewpoint of ensuring flatness, it is not always appropriate to make the cooling rate after finish rolling excessively fast. On the other hand, from the viewpoint of increasing strength, in order to obtain a single-phase martensite or a structure closer to a single-phase martensite, it is necessary to cool at a cooling rate above the critical cooling rate.

[0014] Therefore, in order to improve the flatness of the steel sheet while achieving high strength, the inventors selected an appropriate steel composition and, in particular, focused on the microstructure in the width direction of the hot-rolled steel sheet. First, the inventors found that high strength, more specifically high strength of 980 MPa or more, can be achieved by making the microstructure of a hot-rolled steel sheet having a predetermined chemical composition a microstructure in which tempered martensite is the main phase. Next, the inventors considered that it would be effective to uniformly distribute such a microstructure in which tempered martensite is the main phase in the width direction as well, and conducted further investigations. First, in order to make the microstructure uniform in the width direction, it is necessary to perform cooling after finish rolling appropriately, and in order to form a microstructure in which tempered martensite is the main phase, it is necessary to make good use of auto-tempering during cooling. Therefore, the present inventors have found that by limiting the content of Si in the steel sheet to 0.200% by mass or less, which can delay or suppress auto-tempering, auto-tempering during cooling is promoted, and by making the amount of cooling water sprayed onto the steel sheet during cooling equal on the upper and lower surfaces of the steel sheet from after finish rolling until a temperature corresponding to the end temperature of martensitic transformation, as will be explained in detail later regarding the manufacturing method of hot-rolled steel sheets, that is, by uniformly cooling the upper and lower surfaces of the steel sheet with cooling water from after finish rolling until approximately 200°C, a structure in which tempered martensite accounts for 95 area or more can be uniformly distributed throughout the width direction of the hot-rolled steel sheet. More specifically, the inventors have found that, in particular, by setting the Si content of the hot-rolled steel sheet to 0.200 mass% or less and appropriately controlling the cooling after finish rolling as described above, when the total width of the hot-rolled steel sheet in the direction perpendicular to the rolling direction and thickness direction is W, the metal structure at the 1 / 4 thickness position can be made to have a structure of tempered martensite: 95% or more in area percent at all positions from the edge in the width direction, namely at 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W.As a result, the inventors found that the strength variation in the width direction of the hot-rolled steel sheet can be sufficiently reduced to a level where the difference between the maximum and minimum tensile strengths at all positions in the width direction is 30 MPa or less. In relation to the uniformity of the metal structure throughout the width direction and this reduction in strength variation, the flatness of the hot-rolled steel sheet can be significantly improved despite its high strength.

[0015] Conventionally, during cooling after finish rolling, the amount of water sprayed onto the upper surface of the steel sheet is generally greater than the amount sprayed onto the lower surface. Therefore, by uniformly cooling the upper and lower surfaces of the steel sheet from after finish rolling to a temperature corresponding to the martensitic transformation completion temperature, it is possible to achieve uniformity of the metal structure and reduction of strength variation throughout the width direction, as described above. This fact, which significantly improves the flatness of the hot-rolled steel sheet despite its high strength, has been revealed for the first time by the present inventors. Furthermore, according to the high-strength hot-rolled steel sheet according to the embodiment of the present invention, since it has sufficient flatness in the as-rolled state, there is no need for pre-processing with a leveler or the like, and therefore, no part of the ductility inherent in the steel sheet is consumed by such pre-processing. In connection with this, the risk of forming defects occurring during pressing of the high-strength hot-rolled steel sheet can be reduced, and productivity can be significantly improved. Therefore, the high-strength hot-rolled steel sheet according to the embodiment of the present invention is not only particularly useful in the automotive field, but can also be used very effectively in other fields.

[0016] The high-strength hot-rolled steel sheet according to an embodiment of the present invention will be described in more detail below. In the following description, "%", which is the unit for the content of each element, means "mass%" unless otherwise specified. In this specification, "~", which indicates a numerical range, is used to mean that the numbers written before and after it are included as the lower and upper limits, respectively, unless otherwise specified.

[0017] [C:0.050~0.100%] Carbon (C) is an effective element for increasing the strength of steel plates. To obtain this effect fully, the C content should be 0.050% or more. The C content may also be 0.055% or more, 0.060% or more, 0.065% or more, or 0.070% or more. On the other hand, if the C content is excessive, it becomes difficult to control the variation in strength in the width direction within a predetermined range due to excessive strength increase. Therefore, the C content should be 0.100% or less. The C content may also be 0.095% or less, 0.090% or less, 0.085% or less, or 0.080% or less.

[0018] [Si: 0.010~0.200%] Si is an effective element for increasing strength as a solid solution strengthening element. To obtain this effect fully, the Si content should be 0.010% or more. The Si content may be 0.020% or more, 0.040% or more, 0.060% or more, 0.080% or more, or 0.100% or more. On the other hand, if the Si content is excessive, it may delay or suppress auto-tempering during the cooling of the steel sheet, and in such cases, it may not be possible to obtain a hot-rolled steel sheet with the desired metallic structure. Therefore, the Si content should be 0.200% or less. The Si content may be 0.180% or less, 0.160% or less, 0.140% or less, or 0.120% or less.

[0019] [Mn: 1.00~2.50%] Mn is an effective element for increasing strength as a hardenability and solid solution strengthening element. If the Mn content is low, the hardenability will be insufficient, and a relatively large amount of soft phases such as ferrite will be formed during cooling, making it impossible to uniformly distribute the structure, which is mainly tempered martensite, in the width direction. In addition, due to the expansion of the steel sheet caused by such transformation, warping may occur in the width direction, and the shape of the steel sheet may be distorted. Therefore, the Mn content should be 1.00% or more. The Mn content may be 1.20% or more, 1.40% or more, 1.60% or more, or 1.80% or more. On the other hand, if the Mn content is excessive, the martensite may not be sufficiently tempered even by auto-tempering during the cooling of the steel sheet due to the improved hardenability, and the fresh martensite may not be sufficiently reduced in the final metal structure. Therefore, the Mn content should be 2.50% or less. The Mn content may be 2.40% or less, 2.20% or less, 2.00% or less, or 1.90% or less.

[0020] [Ti: 0.001~0.120%] Ti is an element that contributes to improving strength through precipitation strengthening, etc. Furthermore, Ti consumes dissolved nitrogen in steel by bonding with nitrogen to form titanium nitride (TiN), thereby suppressing the decrease in dissolved boron caused by the formation of BN. To fully obtain these effects, the Ti content should be 0.001% or higher. The Ti content may also be 0.010% or higher, 0.020% or higher, 0.040% or higher, or 0.060% or higher. On the other hand, since Ti is also an element that suppresses austenite recrystallization, excessive Ti content can increase the driving force for ferrite transformation during cooling due to the presence of unrecrystallized austenite containing a relatively large number of dislocations. This can easily lead to the formation of soft phases such as ferrite from the unrecrystallized austenite, making it impossible to obtain the desired metal structure. Therefore, the Ti content should be 0.120% or lower. The Ti content may also be 0.110% or lower, 0.100% or lower, 0.090% or lower, or 0.080% or lower.

[0021] [Al:0.001~0.050%] Al is an element that acts as a deoxidizing agent. To obtain this effect sufficiently, the Al content should be 0.001% or more. The Al content may be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if the Al content is excessive, coarse oxides may form, which may reduce toughness. Therefore, the Al content should be 0.050% or less. The Al content may be 0.045% or less, or 0.040% or less.

[0022] [B:0.0005~0.0050%] B is an element that enhances the hardenability of steel and contributes to improving its strength. To obtain these effects fully, the B content should be 0.0005% or more. The B content may also be 0.0008% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more. On the other hand, excessive B content may reduce toughness and / or weldability. Therefore, the B content should be 0.0050% or less. The B content may also be 0.0045% or less, 0.0040% or less, 0.0030% or less, or 0.0025% or less.

[0023] [P:0.100% or less] Excessive phosphorus (P) content can negatively affect weldability and other properties. Therefore, the P content should be 0.100% or less. The P content may also be 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the P content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0024] [S:0.050% or less] Excessive sulfur content can lead to the formation of large amounts of manganese sulfur (MnS), which can reduce toughness. Therefore, the Si content should be 0.050% or less. The S content may be 0.020% or less, 0.010% or less, or 0.005% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0025] [N:0.0050% or less] Excessive nitrogen (N) content can form coarse nitrides, reducing toughness. Furthermore, N can combine with boron in the steel to form boron nitride (BN), reducing the amount of solid-solution boron and diminishing the hardenability-improving effect of boron addition. Therefore, a lower N content is preferable, ideally 0.0050% or less. The N content may also be 0.0045% or less, 0.0040% or less, or 0.0035% or less. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0026] The basic chemical composition of the high-strength hot-rolled steel sheet according to the embodiment of the present invention is as described above. Furthermore, the high-strength hot-rolled steel sheet may, if necessary, contain at least one of the following optional elements in place of a portion of the remaining Fe. For example, high-strength hot-rolled steel sheets may contain at least one of the following elements: O: 0-0.0050%, Cu: 0-0.20%, Ni: 0-0.20%, Sn: 0-0.10%, Cr: 0-0.40%, Mo: 0-0.20%, Nb: 0-0.05%, V: 0-0.10%, As: 0-0.100%, Zr: 0-0.100%, Ca: 0-0.0050%, Mg: 0-0.100%, Bi: 0-0.020%, Co: 0-0.20%, W: 0-0.20%, Zn: 0-0.20%, and REM: 0-0.1000%. These optional elements will be described in detail below.

[0027] [O:0 ~0.0050%] O is an element that is introduced during the manufacturing process. The O content may be 0%. However, reducing the O content to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the O content is excessive, coarse inclusions may form, reducing the toughness of the steel sheet. Therefore, it is preferable that the O content be 0.0050% or less. The O content may also be 0.0040% or less, 0.0035% or less, or 0.0030% or less.

[0028] [Cu: 0~0.20%] Cu is an element that contributes to improving strength and / or corrosion resistance. The Cu content may be 0%, but to obtain these effects, it is preferable that the Cu content be 0.001% or more. The Cu content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, excessive Cu content may lead to deterioration of toughness and weldability. Therefore, it is preferable that the Cu content be 0.20% or less. The Cu content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.

[0029] [Ni: 0~0.20%] Ni is an element that enhances the hardenability of steel and contributes to improving its strength and / or corrosion resistance. The Ni content may be 0%, but to obtain these effects, it is preferable that the Ni content be 0.001% or more. The Ni content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if the Ni content is excessive, the effect will saturate, leading to an increase in manufacturing costs. Therefore, it is preferable that the Ni content be 0.20% or less. The Ni content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.

[0030] [Sn: 0~0.10%] Sn is an effective element for improving corrosion resistance. While the Sn content may be 0%, to obtain this effect, the Sn content is preferably 0.001% or more, and may be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, excessive Sn content may lead to a decrease in toughness. Therefore, the Sn content is preferably 0.10% or less. The Sn content may also be 0.08% or less, 0.06% or less, or 0.04% or less.

[0031] [Cr: 0~0.40%] Cr is an element that enhances the hardenability of steel and contributes to improving its strength and / or corrosion resistance. The Cr content may be 0%, but to obtain these effects, it is preferable that the Cr content be 0.001% or more. The Cr content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Cr content is excessive, the effect will saturate, leading to an increase in manufacturing costs. Therefore, it is preferable that the Cr content be 0.40% or less. The Cr content may be 0.30% or less, 0.20% or less, 0.15% or less, or 0.12% or less.

[0032] [Mo: 0~0.20%] Mo is an element that enhances the hardenability of steel, contributes to improved strength, and also contributes to improved corrosion resistance. While the Mo content may be 0%, it is preferable that the Mo content be 0.001% or more to obtain these effects. The Mo content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if the Mo content is excessive, the deformation resistance during hot working may increase, and the equipment load may increase. Therefore, it is preferable that the Mo content be 0.20% or less. The Mo content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[0033] [Nb: 0~0.05%] Nb is an element that contributes to the refinement of the microstructure and, consequently, the increased strength of steel sheets by forming carbides, nitrides, and / or carbonitrides in steel through a pinning effect. The Nb content may be 0%, but to obtain such an effect, it is preferable that the Nb content be 0.001% or more. The Nb content may be 0.005% or more, or 0.01% or more. On the other hand, if the Nb content is excessive, coarse carbides and the like may be formed in the steel, which may reduce the toughness of the steel sheet. Therefore, the Nb content should be 0.05% or less. The Nb content may be 0.04% or less, 0.03% or less, or 0.02%.

[0034] [V: 0~0.10%] V is an element that contributes to improving strength through precipitation strengthening, etc. The V content may be 0%, but to obtain such an effect, it is preferable that the V content be 0.001% or more. The V content may be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, if the V content is excessive, a large amount of precipitate may be formed, which may reduce toughness. Therefore, it is preferable that the V content be 0.10% or less. The V content may be 0.08% or less, 0.06% or less, or 0.04% or less.

[0035] [As: 0~0.100%] As is an effective element for improving corrosion resistance. While the As content may be 0%, to obtain such an effect, it is preferable that the As content be 0.001% or more, and may be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, if the As content is excessive, the effect will saturate, leading to an increase in manufacturing costs. Therefore, it is preferable that the As content be 0.100% or less. The As content may also be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0036] [Zr:0~0.100%] Zr is an element that can control the form of sulfides. The Zr content may be 0%, but to obtain such an effect, it is preferable that the Zr content be 0.0001% or more. The Zr content may be 0.0005% or more, 0.001% or more, or 0.010% or more. On the other hand, if the Zr content is excessive, the effect will saturate, leading to an increase in manufacturing costs. Therefore, it is preferable that the Zr content be 0.100% or less. The Zr content may be 0.050% or less, 0.030% or less, or 0.020% or less.

[0037] [Ca: 0~0.0050%] Ca is an element that can control the form of sulfides. The Ca content may be 0%, but to obtain such an effect, it is preferable that the Ca content be 0.0001% or more. The Ca content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the Ca content is excessive, the effect will saturate, leading to an increase in manufacturing costs. Therefore, it is preferable that the Ca content be 0.0050% or less. The Ca content may be 0.0040% or less, 0.0030% or less, or 0.0020% or less.

[0038] [Mg: 0~0.100%] Mg is an element that can control the form of sulfides. The Mg content may be 0%, but to obtain such an effect, the Mg content is preferably 0.0001% or more, and may be 0.001% or more, 0.005% or more, or 0.008% or more. On the other hand, if the Mg content is excessive, the effect will saturate, leading to an increase in manufacturing costs. Therefore, the Mg content is preferably 0.100% or less. The Mg content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0039] [Bi: 0~0.020%] Bi is an effective element for improving corrosion resistance. While the Bi content may be 0%, it is preferable that the Bi content be 0.0001% or higher to obtain such an effect. The Bi content may also be 0.0005% or higher, 0.001% or higher, or 0.003% or higher. On the other hand, excessive Bi content leads to saturation of the effect and increases manufacturing costs. Therefore, it is preferable that the Bi content be 0.020% or lower. The Bi content may also be 0.010% or lower, 0.008% or lower, or 0.005% or lower.

[0040] [Co: 0~0.20%] Co is an element that contributes to improving hardenability and / or heat resistance. The Co content may be 0%, but to obtain these effects, it is preferable that the Co content be 0.001% or more. The Co content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if the Co content is excessive, the hot workability may decrease, and this can lead to an increase in raw material costs. Therefore, it is preferable that the Co content be 0.20% or less. The Co content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[0041] [W: 0~0.20%] W is an element that enhances the hardenability of steel and contributes to improving its strength. While the W content may be 0%, it is preferable that the W content be 0.001% or more to obtain such effects. The W content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, excessive W content may reduce weldability. Therefore, it is preferable that the W content be 0.20% or less. The W content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[0042] [Zn: 0~0.20%] Zn is an effective element for controlling the shape of inclusions. To obtain such an effect, the Zn content is preferably 0.001% or more. The Zn content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if the Zn content is excessive, the effect will saturate, leading to an increase in manufacturing costs. Therefore, the Zn content is preferably 0.20% or less. The Zn content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[0043] [REM:0~0.1000%] Rare earth metals (REMs) are elements that can control the form of sulfides. While the REM content may be 0%, it is preferable that the REM content be 0.0001% or more to obtain such an effect. The REM content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, excessive REM content leads to saturation of the effect and increases manufacturing costs. Therefore, it is preferable that the REM content be 0.1000% or less. The REM content may also be 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. In this specification, REM refers to the collective term for 17 elements including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.

[0044] In the high-strength hot-rolled steel sheet according to an embodiment of the present invention, the remainder of the elements other than those mentioned above consists of Fe and impurities. Impurities are components that are mixed in during the industrial production of high-strength hot-rolled steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap.

[0045] The chemical composition of the high-strength hot-rolled steel sheet according to the embodiment of the present invention can be measured by general analytical methods. For example, the chemical composition of the high-strength hot-rolled steel sheet can be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive infrared absorption method.

[0046] [Metal structure] In the high-strength hot-rolled steel sheet according to the embodiment of the present invention, when the total width of the high-strength hot-rolled steel sheet in the direction perpendicular to the rolling direction and the thickness direction is W, at all positions from the edge in the width direction, specifically at the 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W positions, the metal structure at the 1 / 4 thickness position is, in area percent, 95% or more of tempered martensite (tM), 5% or less of fresh martensite (fM), and at least one of ferrite (α), upper bainite (B), and pearlite (P) in total of 5% or less. In the present invention, "total width" refers to the length of the steel sheet in the direction perpendicular to the rolling direction and the thickness direction of the high-strength hot-rolled steel sheet (for example, a coiled high-strength hot-rolled steel sheet).

[0047] If the rolling direction of the hot-rolled steel sheet is not clear, it can be determined by the following method. After finishing the thickness cross section of the hot-rolled steel sheet with mirror polishing, the S concentration is measured using an electron probe microanalyzer (EPMA). The measurement conditions are an acceleration voltage of 15kV and a measurement pitch of 1μm, and a distribution image is measured in a 500μm square area in the center of the thickness. At this time, the stretched area with a high S concentration is determined to be an inclusion such as MnS. Multiple fields of view may be observed during the observation. Next, using the thickness cross section initially observed using the above method as a reference, the planes parallel to the planes rotated in 5° increments within the range of 0° to 180° around the thickness axis are observed in cross-section using the above method. The average length of the major axis of multiple inclusions in each obtained cross-section is calculated for each cross-section, and the cross-section with the maximum average length of the major axis of the inclusions is identified. The direction parallel to the major axis of the inclusions in that cross-section is identified as the rolling direction of the hot-rolled steel sheet. The following provides a more detailed explanation of each organization.

[0048] [Tempered martensite: 95% or more] By ensuring that the tempered martensite in the metal structure at the 1 / 4 thickness position accounts for 95% or more area percentage at all positions in the width direction, specifically at the 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W positions from the edge, high strength can be achieved due to the martensite-dominant structure, while reliably controlling the difference between the maximum and minimum tensile strengths at all these width directions to 30 MPa or less, thus significantly reducing strength variation in the width direction. At all of the above width directions, the area percentage of tempered martensite may be 96% or more, 97% or more, or 98% or more. The upper limit of the area percentage of tempered martensite is not particularly limited and may be 100%.

[0049] [Fresh martensite: 5% or less] In the high-strength hot-rolled steel sheet according to the embodiment of the present invention, it is necessary to control the area ratio of fresh martensite to 5% or less at all positions in the width direction. If the area ratio of fresh martensite exceeds 5% at any one position, the strength at that position may become too high, and it may not be possible to sufficiently reduce the strength variation in the width direction. Therefore, from the viewpoint of reducing strength variation, it is preferable that the area ratio of fresh martensite is as low as possible at all positions in the width direction, for example, 4% or less, 3% or less, 2% or less, or 1% or less. The lower limit of the area ratio of fresh martensite is not particularly limited and may be 0%.

[0050] [At least one of the following: ferrite, upper bainite, and perlite: total of 5% or less] In the high-strength hot-rolled steel sheet according to the embodiment of the present invention, the remaining structure other than tempered martensite and fresh martensite is composed of at least one of ferrite, upper bainite, and pearlite. Similarly, it is necessary to control the total area ratio of at least one of ferrite, upper bainite, and pearlite to 5% or less at all positions in the width direction. If the total area ratio of at least one of ferrite, upper bainite, and pearlite exceeds 5% at any one position, the strength at that position may become too low, and it may not be possible to sufficiently reduce the strength variation in the width direction. Therefore, from the viewpoint of reducing strength variation, it is preferable that the area ratio of at least one of ferrite, upper bainite, and pearlite is as low as possible at all positions in the width direction, for example, it may be 4% or less, 3% or less, 2% or less, or 1% or less in total. The lower limit of the area ratio of at least one of ferrite, upper bainite, and pearlite is not particularly limited and may be 0% in total. For example, the total area ratio of fresh martensite, ferrite, upper bainite, and pearlite may be 5% or less, 4% or less, 3% or less, 2% or less, or 1% or less. Similarly, the total area ratio of fresh martensite, ferrite, upper bainite, and pearlite may be 0%.

[0051] [Identification of metallographic structure and calculation of area ratio] The identification of the metallographic structure and calculation of its area ratio are performed using FE-SEM (Field Emission Scanning Electron Microscope), optical microscope, and X-ray diffraction after etching with Nital reagent or Repera solution. Microstructure observation using FE-SEM and optical microscope is performed at a magnification of 1,000 to 50,000 times for a 100 μm × 100 μm area in the cross-section of the steel sheet parallel to the rolling direction and perpendicular to the sheet surface. For each metallographic structure, three measurement points are taken at each of the following positions from the edge in the width direction (perpendicular to the rolling direction and the thickness direction): 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W. The area ratio at each position is determined by calculating the average of these three measurements.

[0052] The area ratio of ferrite can be determined by observing a 100 μm × 100 μm region within the range of 1 / 8 to 3 / 8 of the plate thickness, centered at the 1 / 4 position, in an electron channeling contrast image obtained using a FE-SEM (field emission scanning electron microscope). More specifically, within the above region, areas that appear with uniform contrast can be identified as ferrite, and their area ratio can be calculated using the image analysis software Image J.

[0053] The area ratio of fresh martensite (as-quenched martensite) is determined by the following procedure. First, the observation surface of the sample is etched with Repeller solution, and then a 100 μm × 100 μm area is observed with FE-SEM in the range of 1 / 8 to 3 / 8 of the plate thickness, centered at the 1 / 4 position of the plate thickness. In Repeller etching, fresh martensite and retained austenite are not etched, so they appear as a flat area with brighter contrast than other parts in the SEM image. The area ratio of the unetched area corresponds to the total area ratio of fresh martensite and retained austenite, if present. The area ratio of fresh martensite is calculated by subtracting the area ratio of retained austenite measured by the X-ray diffraction method, which will be explained later, from this area ratio of the unetched area.

[0054] The area fraction of retained austenite is calculated by X-ray diffraction. First, the sample is polished mechanically and chemically from the surface to a depth of 1 / 4 of the way down the plate thickness. Next, at the 1 / 4-thickness point, the microstructure fraction of retained austenite is calculated from the integral intensity ratio of the diffraction peaks (200) and (211) of the bcc phase and (200), (220), and (311) of the fcc phase obtained using MoKα radiation. The common 5-peak method is used for this calculation. The calculated microstructure fraction of retained austenite is determined as the area fraction of retained austenite.

[0055] The identification and calculation of the area fraction of upper bainite and tempered martensite are performed using the following procedure. First, the observation surface of the sample is etched with nital reagent, and then a 100 μm × 100 μm area is observed using FE-SEM in the range of 1 / 8 to 3 / 8 of the plate thickness, centered on 1 / 4 of the plate thickness. From the position and arrangement of cementite contained within the microstructure in this observation area, upper bainite and tempered martensite are identified as follows. In upper bainite, cementite or retained austenite exists at the interface of the lath-like bainite ferrite. Based on these characteristic points, upper bainite is identified, and the area fraction of upper bainite is calculated by dividing the identified bainite area by the area of ​​the observation field. On the other hand, in tempered martensite, cementite exists inside the martensite lath, but there are two or more crystal orientations of martensite lath and cementite, and cementite has multiple variants, so tempered martensite can be identified. The area ratio of tempered martensite is calculated by dividing the region of tempered martensite identified in this way by the area of ​​the observation field.

[0056] The identification of perlite and the calculation of its area percentage are performed using the following procedure. First, the observation surface of the sample is etched with Nital reagent, and then the area from 1 / 8 to 3 / 8 of the plate thickness, centered at 1 / 4 of the plate thickness, is observed with an optical microscope. In the optical microscope image, the region where carbides and ferrite are present in layers is identified as perlite, and the area percentage of perlite is calculated by dividing this region by the area of ​​the observation field.

[0057] [The difference between the maximum and minimum tensile strengths at all positions from the end in the width direction, specifically at 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W, is 30 MPa or less.] In the high-strength hot-rolled steel sheet according to the embodiment of the present invention, as described above, at all positions from the edge in the width direction, specifically at the 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W positions, the tempered martensite in the metal structure at the 1 / 4 thickness position accounts for 95% or more by area percentage. This achieves high strength due to a structure with martensite as the main phase, while reliably controlling the difference between the maximum and minimum tensile strengths at all these positions in the width direction to 30 MPa or less, thus significantly reducing strength variation in the width direction. Furthermore, in relation to the uniformity of the metal structure throughout the width direction and this reduction in strength variation, the flatness of the hot-rolled steel sheet can be significantly improved despite its high strength. From the viewpoint of improving the flatness of the hot-rolled steel sheet, a smaller difference between the maximum and minimum tensile strengths is preferable, for example, 28 MPa or less, 25 MPa or less, 22 MPa or less, 20 MPa or less, 17 MPa or less, or 15 MPa or less. The lower limit is not particularly limited, but for example, the difference between the maximum and minimum values ​​of the tensile strength may be acceptable if it is 5 MPa or more, 8 MPa or more, or 10 MPa or more.

[0058] The difference between the maximum and minimum tensile strengths is determined as follows: First, tensile test specimens of type 5 according to JIS Z2241:2011 are taken from the ends of the hot-rolled steel sheet in the width direction at positions 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W, with the test direction parallel to the rolling direction. Next, tensile tests are performed using these specimens in accordance with JIS Z2241:2011 to obtain five tensile strength values, and finally, the difference between the maximum and minimum values ​​is calculated.

[0059] [Tensile strength] The minimum of the five tensile strength values ​​described above is determined as the tensile strength of the high-strength hot-rolled steel sheet according to the embodiment of the present invention. The high-strength hot-rolled steel sheet according to the embodiment of the present invention, having the chemical composition and metal structure described above, can achieve a high tensile strength, specifically a tensile strength of 980 MPa or higher. The tensile strength is preferably 1000 MPa or higher, 1050 MPa or higher, or 1100 MPa or higher. According to the high-strength hot-rolled steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, it can achieve excellent flatness in relation to the uniformity of the metal structure and reduction of strength variation throughout the width direction. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the high-strength hot-rolled steel sheet may be 1300 MPa or less, 1250 MPa or less, 1200 MPa or less, or 1180 MPa or less.

[0060] [Prior austenite particle size in the metal structure: 40 μm or less] In a preferred embodiment of the present invention, the prior austenite grain size in the metal structure is 40 μm or less. As previously mentioned, the high-strength hot-rolled steel sheet according to the embodiment of the present invention can achieve excellent flatness in relation to the uniformity of the metal structure and the reduction of strength variation throughout the width direction. In addition, by controlling the prior austenite grain size within such a fine range, it is possible to further improve additional properties such as toughness. From the viewpoint of improving toughness, the smaller the prior austenite grain size, the better, and it may be, for example, 37 μm or less, 35 μm or less, 32 μm or less, 30 μm or less, 27 μm or less, or 25 μm or less. The lower limit is not particularly limited, but for example, the prior austenite grain size may be 10 μm or more, 12 μm or more, 15 μm or more, 18 μm or more, or 20 μm or more.

[0061] The prior austenite grain size in a metal microstructure is determined as follows. First, a 200 μm × 200 μm region in the L-section of a steel billet taken from the surface of a hot-rolled steel sheet at a position 1 / 4 of the sheet thickness is analyzed by SEM / EBSD (scanning electron microscope / backscattered electron diffraction). More specifically, the crystal orientation data obtained by SEM / EBSD is subjected to a predetermined crystal orientation transformation (see "Study toward improving the accuracy of the reconstruction method of austenite microstructure of steel", Kengo Hata, Masayuki Wakita, Tomoya Fujiwara, Kaori Kawano, Nippon Steel & Sumitomo Metal Technical Report No. 404 (2016), pp. 24-30) to obtain an image of reconstructed prior austenite grains. The diameter of a circle with the same area, i.e., the equivalent circle diameter, is determined from the prior austenite grains in this image. This operation is performed for a total of 10 prior austenite grains, and the prior austenite grain size is determined by averaging the obtained 10 equivalent circle diameters.

[0062] [Full width W] The high-strength hot-rolled steel sheet according to the embodiment of the present invention can have any total width W. While not particularly limited, for example, in the case of a hot-rolled steel sheet (coil) in a wound state, the total width W may be 700 mm or more, 800 mm or more, 900 mm or more, or 1000 mm or more. There is no particular upper limit, but from the viewpoint of more reliably improving flatness, the total width is preferably 2500 mm or less, and may be 2200 mm or less, 2000 mm or less, 1800 mm or less, 1600 mm or less, 1500 mm or less, 1400 mm or less, or 1300 mm or less.

[0063] [plate thickness] The high-strength hot-rolled steel sheet according to the embodiment of the present invention is not particularly limited, but generally has a thickness of 1.0 to 6.0 mm. For example, the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or 5.0 mm or less, 4.0 mm or less, or 3.0 mm or less.

[0064] <Manufacturing method for high-strength hot-rolled steel sheets> Next, preferred manufacturing methods for high-strength hot-rolled steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing high-strength hot-rolled steel sheets according to embodiments of the present invention, and is not intended to limit the high-strength hot-rolled steel sheets to those manufactured by the manufacturing methods described below.

[0065] A method for manufacturing a high-strength hot-rolled steel sheet according to an embodiment of the present invention is as follows: A hot rolling process relating to high-strength hot-rolled steel sheets, comprising heating a slab having the chemical composition described above to a temperature of 1220 to 1300°C and performing rough rolling and finish rolling, wherein the exit temperature of the rough rolling is 1100 to 1200°C, the entry temperature (F0) of the finish rolling is 1000 to 1100°C, the exit temperature (FT) of the finish rolling is 940 to 1000°C, and the total reduction ratio of the finish rolling is 85 to 95%. A cooling process comprising: first cooling a finish-rolled steel sheet in the temperature range from the exit temperature (FT) of the finish rolling to the martensitic transformation onset temperature Ms+50°C at a critical cooling rate Vc+10°C / s or more and an average cooling rate of 60°C / s or less; and second cooling in the temperature range from Ms+50°C to 200°C at an average cooling rate of 50 to 120°C / s, wherein the vertical cooling ratio of the upper surface to the lower surface of the steel sheet in the first cooling is 0.8 to 1.2, and the vertical cooling ratio of the upper surface to the lower surface of the steel sheet in the second cooling is 0.8 to 1.2; and A winding process in which the second-cooled steel sheet is wound up at 50-100°C. It is characterized by including [specific features]. The following describes each process in detail.

[0066] [Hot rolling process] [Slab heating] First, a slab having the chemical composition described above in relation to hot-rolled steel sheets is heated. From the viewpoint of productivity, the slab used is preferably cast by a continuous casting method, but it may also be manufactured by an ingot-making method or a thin-slab casting method. The slab used contains a relatively large amount of alloying elements in order to obtain a high-strength steel sheet. For this reason, it is necessary to heat the slab before subjecting it to hot rolling to solid-solve the alloying elements in the slab. If the heating temperature is less than 1220°C, the alloying elements will not sufficiently solid-solve in the slab, leaving coarse alloy carbides, which may cause brittle cracking during hot rolling. For this reason, the heating temperature should be 1220°C or higher, preferably 1230°C or higher. The upper limit of the heating temperature is not particularly limited, but from the viewpoint of heating equipment capacity and productivity, it is preferably 1300°C or lower.

[0067] [Rough rolling] In this method, a heated slab is subjected to rough rolling before finish rolling to adjust the plate thickness, etc. The rough rolling is performed at an exit temperature of 1100 to 1200°C, preferably 1150 to 1200°C, in order to secure the desired sheet bar dimensions and to adjust the total reduction ratio in the temperature range of 940°C or higher during finish rolling to a desired range. If the exit temperature of rough rolling is less than 1100°C, it becomes difficult to obtain an exit temperature of 940°C or higher in the finish rolling that follows rough rolling. Also, if the exit temperature of rough rolling exceeds 1200°C, the crystal grains may coarseen, and the toughness of the resulting hot-rolled steel sheet may decrease.

[0068] [Finishing Rolling] The roughly rolled slabs are then subjected to finish rolling. As mentioned above, the slabs used contain a relatively large amount of alloying elements, so a large rolling load is required during hot rolling. For this reason, hot rolling is carried out under high temperature and high pressure. Specifically, the entry temperature (F0) of the finish rolling is 1000-1100°C, the exit temperature (FT) of the finish rolling is 940-1000°C, and the total reduction ratio of the finish rolling is 85-95%. The exit temperature of the finish rolling is particularly important in terms of controlling the metal structure of the steel sheet. More specifically, if the exit temperature of the finish rolling is too low, the metal structure may become uneven, and the formability may decrease. For this reason, the exit temperature of the finish rolling should be 940°C or higher. On the other hand, in order to suppress the coarsening of austenite, the exit temperature of the finish rolling should be 1000°C or lower.

[0069] [Cooling process] [Primary cooling] In the subsequent cooling process, the finish-rolled steel sheet is first primary-cooled in the temperature range from the exit temperature (FT) of finish rolling to the martensitic transformation initiation temperature Ms+50°C at an average cooling rate of Vc+10°C / s or higher and 60°C / s or lower. By primary-cooling this temperature range at an average cooling rate of Vc+10°C / s or higher and 60°C / s or lower, the martensitic transformation is promoted, and the total amount of at least one of ferrite, upper bainite, and pearlite in the final metal structure can be reduced to 5 area % or less, and the metal structure can be made uniform in the width direction. If the average cooling rate of the primary cooling is less than the critical cooling rate Vc+10°C / s, the total amount of at least one of ferrite, upper bainite, and pearlite will exceed 5 area %, and the desired strength may not be achieved. On the other hand, if the average cooling rate of the primary cooling exceeds 60°C / s, the rapid cooling rate makes it difficult to cool the steel sheet uniformly in the width direction, resulting in uneven cooling in the width direction. In this case, the desired metal structure may not be obtained in the width direction of the final hot-rolled steel sheet, and / or the variation in tensile strength in the width direction may increase. Related to this, the shape of the hot-rolled steel sheet may be distorted, causing warping in the width direction of the steel sheet and making it impossible to achieve sufficient flatness. Therefore, the average cooling rate of the primary cooling should be between the critical cooling rate Vc + 10°C / s and 60°C / s, preferably between the critical cooling rate Vc + 12°C / s and 60°C / s.

[0070] In this manufacturing method, the Ms point (°C) is determined by the following formula 1. Ms=823-350[C]-40[Mn]-35[V]-20[Cr]-17[Ni]-10[Cu]-10[Mo]-10[W]+15[Co]+30[Al]-273...Formula 1 Here, [C], [Mn], [V], [Cr], [Ni], [Cu], [Mo], [W], [Co], and [Al] represent the mass %) content of each element in the steel, and 0 indicates the absence of an element. The critical cooling rate Vc (°C) is also an index of hardenability where the martensite area ratio is 90% or more, and can be expressed by the following equations 2 and 3. When the amount of solid solution B is ≥ 0.0005% by mass, logVc=2.94-0.75×(2.7[C]+0.4[Si]+[Mn]+0.45[Ni]+0.8[Cr]+2[Mo])...Equation 2 When the amount of solid solution B is < 0.0005% by mass, logVc=3.69-0.75×(2.7[C]+0.4[Si]+[Mn]+0.45[Ni]+0.8[Cr]+[Mo])...Equation 3 Here, [C], [Si], [Mn], [Ni], [Cr], and [Mo] represent the mass %) content of each element in the steel, and 0 if no element is present. The amount of solid-solution B (mass %) corresponds to the amount of B contained in the steel minus the amount of B consumed to form boron nitride (BN). On the other hand, the amount of solid-solution N (mass %) that can form BN can be reduced by incorporating Ti into the steel and fixing it as TiN. Therefore, the amount of solid-solution B can be calculated by the following equations 4 and 5. Solid solute B amount = 10.81×([B] / 10.81-solid solute N amount / 14.01)...Equation 4 However, when [N] / 14.01-[Ti] / 47.88>0, Solid solute N amount = 14.01×([N] / 14.01-[Ti] / 47.88)...Equation 5 When [N] / 14.01 - [Ti] / 47.88 ≤ 0, the amount of dissolved N is 0. Here, [B], [N], and [Ti] represent the mass %) content of each element in the steel, and 0 indicates that no element is present.

[0071] In primary cooling, in addition to controlling the average cooling rate, it is extremely important to cool the steel plate evenly on both its top and bottom surfaces. Such cooling is performed so that the upper-to-lower surface cooling ratio of the steel plate is 0.8 to 1.2. More specifically, the amount of cooling water sprayed onto the upper surface of the steel plate is 0.8 to 1.2 times the amount of cooling water sprayed onto the lower surface. By performing this even cooling on both the upper and lower surfaces of the steel plate, it is possible to significantly suppress or reduce the occurrence of uneven cooling. As a result, uniformity of the metal structure and reduction of strength variation can be achieved throughout the width direction, and in connection with this, it is possible to achieve sufficient flatness without causing warping in the width direction of the hot-rolled steel plate. If the top-to-bottom cooling ratio is less than 0.8 or greater than 1.2, uneven cooling occurs, making it impossible to uniformly distribute the microstructure in the width direction. Specifically, at all positions from the edge in the width direction—1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W—the microstructure at the 1 / 4 thickness position cannot be made of tempered martensite at least 95% by area. As a result, the strength variation in the width direction cannot be sufficiently reduced to a level where the difference between the maximum and minimum tensile strengths at all of the above positions in the width direction is 30 MPa or less.

[0072] Here, the above-mentioned upper-lower cooling ratio does not refer to the ratio of the total amount of cooling water on the upper surface to the total amount of cooling water on the lower surface in the FT~(Ms+50)℃ section. More specifically, in this manufacturing method, the FT~(Ms+50)℃ section is divided into 10m sections, and the upper-lower cooling ratio is calculated for each of these sections from the amount of cooling water on the upper surface and the amount of cooling water on the lower surface. The upper-lower cooling ratio of each section calculated in this way is controlled to be within the range of 0.8 to 1.2. Controlling the upper-lower cooling ratio of the entire section rather than each divided section makes it very difficult to sufficiently suppress the occurrence of cooling unevenness caused by, for example, localized undercooling. However, by realizing this control of the upper-lower cooling ratio for each section, it is possible to reduce localized undercooling and reliably suppress the occurrence of cooling unevenness. Furthermore, this control of the upper-lower cooling ratio for each section can be carried out by any appropriate means. While not particularly limited, for example, each section has multiple cooling water nozzles positioned above and below the steel plate along the direction of travel of the steel plate. By appropriately spraying these cooling water nozzles based on on / off control, it is possible to relatively easily control the upper-lower cooling ratio of each section within the range of 0.8 to 1.2.

[0073] [Secondary cooling] The steel sheet, after primary cooling, is then secondary cooled at an average cooling rate of 50-120°C / s in the temperature range from Ms+50°C to 200°C. By secondary cooling at an average cooling rate of 50-120°C / s in the temperature range from Ms+50°C to 200°C, which corresponds to the end temperature of martensitic transformation, sufficient residence time in the temperature range where auto-tempering proceeds can be ensured, thereby promoting auto-tempering. Consequently, the structure that underwent martensitic transformation by the primary cooling is sufficiently tempered, making it possible to uniformly distribute tempered martensite of 95 area percent or more in the width direction in the final metal structure. If the average cooling rate of secondary cooling is less than 50°C / s, the desired metal structure may not be obtained in the width direction of the steel sheet, and the strength variation in the width direction may not be sufficiently reduced. On the other hand, if the average cooling rate of secondary cooling exceeds 120°C / s, auto-tempering cannot be promoted, and fresh martensite may remain in the final metal structure at a rate of more than 5 area percent. In addition to or instead of this, such rapid cooling can lead to poor control over the amount of water used for cooling, resulting in uneven cooling in the width direction, such as localized overcooling. In this case, the resulting hot-rolled steel sheet will have large variations in tensile strength in the width direction, which in turn can cause the shape of the hot-rolled steel sheet to collapse, resulting in warping in the width direction of the sheet and making it impossible to achieve sufficient flatness.

[0074] In secondary cooling, as with primary cooling, it is extremely important to control the average cooling rate and to cool the steel plate evenly on both its top and bottom surfaces. This cooling is carried out in a manner similar to primary cooling, where the cooling ratio of the top surface to the bottom surface of the steel plate is 0.8 to 1.2. More specifically, the amount of cooling water sprayed onto the top surface of the steel plate is 0.8 to 1.2 times the amount of cooling water sprayed onto the bottom surface. By cooling the top and bottom surfaces of the steel plate evenly in this way, it is possible to significantly suppress or reduce the occurrence of uneven cooling. As a result, uniformity of the metal structure and reduction of strength variation can be achieved throughout the width direction, and in connection with this, it is possible to achieve sufficient flatness without causing warping in the width direction of the hot-rolled steel plate. If the top-to-bottom cooling ratio is less than 0.8 or greater than 1.2, uneven cooling occurs, making it impossible to uniformly distribute the microstructure in the width direction. Specifically, at all positions from the edge in the width direction—1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W—the microstructure at the 1 / 4 thickness position cannot be made of tempered martensite at least 95% by area. As a result, the strength variation in the width direction cannot be sufficiently reduced to a level where the difference between the maximum and minimum tensile strengths at all of the above positions in the width direction is 30 MPa or less.

[0075] Here, the above-mentioned upper-lower cooling ratio does not refer to the ratio of the total amount of cooling water on the upper surface to the total amount of cooling water on the lower surface in the (Ms+50)°C to 200°C range. More specifically, in this manufacturing method, the (Ms+50)°C to 200°C range is divided into sections of 10m each, and the upper-lower cooling ratio is calculated for each of these sections from the amount of cooling water on the upper surface and the amount of cooling water on the lower surface. The upper-lower cooling ratio of each section calculated in this way is controlled to be within the range of 0.8 to 1.2. Controlling the upper-lower cooling ratio of the entire section rather than each divided section makes it very difficult to sufficiently suppress the occurrence of cooling unevenness caused by, for example, localized undercooling. However, by realizing this control of the upper-lower cooling ratio for each section, it is possible to reduce localized undercooling and reliably suppress the occurrence of cooling unevenness. Furthermore, this control of the upper-lower cooling ratio for each section can be carried out by any appropriate means. While not particularly limited, as in the case of primary cooling, for example, each section has multiple cooling water nozzles positioned along the direction of travel of the steel plate on the upper and lower sides of the steel plate. By appropriately spraying these cooling water nozzles based on on / off control, it is possible to relatively easily control the upper-lower cooling ratio of each section within the range of 0.8 to 1.2.

[0076] [Winding process] The second-cooled steel sheet is finally wound at 50-100°C in the winding process. If the winding temperature is too low, the hot-rolled steel sheet may harden and become brittle, and excessive water cooling may be required, reducing productivity. Therefore, the winding temperature should be 50°C or higher, preferably 80°C or higher.

[0077] According to the hot-rolled steel sheet manufactured by the above manufacturing method, when the total width in the direction perpendicular to the rolling direction and the thickness direction is W, at all positions from the edge in the width direction, specifically at the 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W positions, the tempered martensite in the metal structure at the 1 / 4 thickness position accounts for 95% or more by area percentage. This results in a structure where martensite is the main phase, achieving high strength, more specifically a tensile strength of 980 MPa or more, while reliably controlling the difference between the maximum and minimum tensile strengths at all these positions in the width direction to 30 MPa or less. Therefore, it becomes possible to significantly reduce strength variation in the width direction, and in connection with this, it becomes possible to achieve sufficient flatness without causing warping in the width direction of the hot-rolled steel sheet. Consequently, the high-strength hot-rolled steel sheet manufactured by the above manufacturing method has high strength, yet possesses homogeneous properties in the width direction and has excellent flatness. Furthermore, since the above-mentioned high strength and flatness can be achieved even in hot-rolled steel sheets that have not undergone planarization treatment using a leveler, for example, hot-rolled steel sheets immediately after manufacturing, such planarization treatment (pre-processing) does not consume some of the ductility that the steel sheet inherently possesses. As a result, the risk of forming defects occurring during pressing of the steel sheet can be reduced, and productivity can be significantly improved. Therefore, while it is natural that this high-strength hot-rolled steel sheet is particularly useful in the automotive sector, it can also be used very effectively in other fields.

[0078] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these examples. [Examples]

[0079] In the following examples, high-strength hot-rolled steel sheets according to the embodiment of the present invention were manufactured under various conditions, and the properties in the width direction and the flatness of the obtained high-strength hot-rolled steel sheets were investigated.

[0080] First, molten steel was cast using a continuous casting method to form slabs with various chemical compositions as shown in Table 1. These slabs were then heated under the conditions shown in Table 2 and subsequently hot-rolled. Hot rolling was carried out by rough rolling and finish rolling, and the exit temperature for rough rolling, as well as the entry temperature (F0), exit temperature (FT), and total reduction ratio for finish rolling, were as shown in Table 2. Next, the finish-rolled steel sheets were subjected to primary cooling under the conditions shown in Table 2, first in the temperature range from the exit temperature (FT) of finish rolling to the martensitic transformation onset temperature Ms+50°C, and then in the temperature range from Ms+50°C to 200°C.

[0081] In the primary and secondary cooling processes, the FT~(Ms+50)°C and (Ms+50)°C~200°C sections were divided into 10m sections, respectively. For each section, the upper and lower cooling ratio was calculated from the amount of cooling water on the top and bottom surfaces, and cooling was carried out so that the upper and lower cooling ratio of each section calculated in this way was controlled to stay within a predetermined range. The upper and lower cooling ratios in Table 2 for primary and secondary cooling represent the upper and lower cooling ratio of each section in primary and secondary cooling that has the largest absolute value difference from a cooling ratio of 1. Finally, the secondary cooled steel plate was rolled up under the conditions shown in Table 2 to obtain a hot-rolled steel plate with a thickness of approximately 2.3~3.2 mm and a total width of 1200 mm.

[0082] [Table 1]

[0083] [Table 2]

[0084] The properties of the obtained hot-rolled steel sheets were measured and evaluated by the following method.

[0085] [Prior austenite grain size in metallographic structures] The prior austenite grain size in the metal microstructure was determined as follows. First, a 200 μm × 200 μm region in the L-section of a steel billet taken at a position 1 / 4 of the plate thickness from the surface of a hot-rolled steel sheet was analyzed by SEM / EBSD. More specifically, a predetermined crystal orientation transformation was performed on the martensite microstructure obtained by SEM / EBSD to reconstruct the prior austenite grains, and then the equivalent circle diameter was determined from the prior austenite grains in this image. This operation was performed for a total of 10 prior austenite grains, and the prior austenite grain size was determined by averaging the obtained 10 equivalent circle diameters.

[0086] [Strength variation in the width direction] First, tensile test specimens of type 5 according to JIS Z2241:2011 were taken from the ends of the hot-rolled steel sheet in the width direction at positions 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W, with the test direction parallel to the rolling direction. Next, tensile tests were performed on these specimens in accordance with JIS Z2241:2011 to obtain five tensile strength values, and finally, the strength variation in the width direction was determined by calculating the difference between the maximum and minimum values.

[0087] [Tensile strength of hot-rolled steel sheet] Of the five tensile strength values ​​listed above, the minimum value was determined to be the tensile strength of the hot-rolled steel sheet.

[0088] [Evaluation of flatness] The flatness was evaluated as follows: First, the obtained hot-rolled steel sheet was placed on a surface plate so that at least a portion of one of its surfaces (bottom surface) was in contact with the surface plate. Next, the distance from the surface plate to the bottom surface of the hot-rolled steel sheet at the highest point of the sheet was measured, and the obtained measurement was determined as the maximum warp height H (mm) of the hot-rolled steel sheet. For the evaluation of flatness, a maximum warp height H of 10 mm or less was considered a pass, and a maximum warp height H exceeding 10 mm was considered a fail.

[0089] Hot-rolled steel sheets with a tensile strength of 980 MPa or higher and passing the flatness evaluation were evaluated as high-strength hot-rolled steel sheets with improved flatness. The results are shown in Table 3.

[0090] [Table 3]

[0091] Referring to Tables 1-3, in Comparative Examples 2 and 10, the average cooling rate of the secondary cooling was high, and the upper-to-lower cooling ratio of the secondary cooling was inappropriate. As a result, the martensite could not be sufficiently tempered by auto-tempering during cooling, leading to a high proportion of fresh martensite (fM). Furthermore, due to uneven cooling, the metal structure could not be uniformly distributed in the width direction, resulting in significant variation in tensile strength in the width direction. Consequently, sufficient flatness could not be achieved. In Comparative Example 8, the average cooling rate of the primary cooling was high, and the upper-to-lower cooling ratio of the primary and secondary cooling was inappropriate. As a result of uneven cooling, the desired metal structure could not be uniformly distributed in the width direction. Related to this, a tensile strength of 980 MPa or higher could not be achieved, and there was also significant variation in tensile strength in the width direction. Consequently, sufficient flatness could not be achieved. In Comparative Example 9, the average cooling rate of the primary cooling was high, and the upper-to-lower cooling ratio of the primary cooling was inappropriate. As a result of uneven cooling, the desired metal structure could not be uniformly distributed in the width direction, and the variation in tensile strength in the width direction became significant. Consequently, sufficient flatness could not be achieved. In Comparative Example 11, the upper-to-lower cooling ratio of the primary and secondary cooling was inappropriate, resulting in uneven cooling. As a result, the desired metal structure could not be uniformly distributed in the width direction, and the variation in tensile strength in the width direction also became significant. Consequently, sufficient flatness could not be achieved. In Comparative Example 13, the variation in tensile strength in the width direction could not be controlled within the predetermined range due to the high carbon content, and the flatness decreased. In Comparative Example 14, the desired tensile strength could not be achieved due to the low carbon content. In Comparative Example 15, it is thought that the auto-tempering during steel sheet cooling was suppressed due to the high Si content. As a result, the proportion of fresh martensite (fM) in the metal structure increased, and consequently, the variation in tensile strength in the width direction could not be sufficiently reduced, leading to a decrease in flatness. In Comparative Example 16, it is thought that the high Mn content resulted in insufficient tempering of the martensite even by auto-tempering during steel sheet cooling, due to improved hardenability.As a result, the proportion of fresh martensite (fM) in the microstructure increased, and consequently, the variation in tensile strength in the width direction could not be sufficiently reduced, resulting in a decrease in flatness. Comparative Example 17 could not achieve the desired tensile strength because of its low Mn content. Furthermore, it is thought that the low Mn content resulted in insufficient hardenability, leading to the formation of a relatively large amount of soft phases such as ferrite during cooling. As a result, it was not possible to uniformly distribute the microstructure, which mainly consists of tempered martensite, in the width direction, and the expansion of the steel sheet due to the transformation to ferrite caused the shape of the steel sheet to collapse, resulting in a decrease in flatness. Comparative Example 18 had a high Ti content, which resulted in the formation of a relatively large amount of soft phases such as ferrite from unrecrystallized austenite during the cooling process. Consequently, it was not possible to form the desired microstructure in the width direction, and as a result, the variation in tensile strength in the width direction could not be sufficiently reduced, resulting in a decrease in flatness.

[0092] In Comparative Example 19, the average cooling rate of the primary cooling was low, resulting in at least one of ferrite, upper bainite, and pearlite exceeding 5 area percent in total, thus failing to achieve the desired tensile strength. In Comparative Example 20, the average cooling rate of the primary cooling was high, leading to uneven cooling and preventing the uniform distribution of the desired microstructure in the width direction, resulting in significant variation in tensile strength in the width direction. As a result, sufficient flatness could not be achieved. In Comparative Examples 21 and 22, the upper-lower cooling ratio of the primary cooling was inappropriate, leading to uneven cooling and preventing the uniform distribution of the desired microstructure in the width direction, resulting in significant variation in tensile strength in the width direction. As a result, sufficient flatness could not be achieved. In Comparative Example 23, the average cooling rate of the secondary cooling was low, preventing the acquisition of the desired microstructure in the width direction of the steel sheet, resulting in significant variation in tensile strength in the width direction. As a result, sufficient flatness could not be achieved. In Comparative Example 24, the average cooling rate of the secondary cooling was high, which prevented sufficient tempering of the martensite by auto-tempering during cooling. This resulted in a high proportion of fresh martensite (fM), and uneven cooling prevented a uniform distribution of the microstructure in the width direction, leading to significant variation in tensile strength in the width direction. As a result, sufficient flatness could not be achieved. In Comparative Examples 25 and 26, the upper-lower cooling ratio of the secondary cooling was inappropriate, which resulted in uneven cooling and prevented a uniform distribution of the desired microstructure in the width direction. Consequently, significant variation in tensile strength in the width direction also occurred. As a result, sufficient flatness could not be achieved.

[0093] In contrast, in all the examples of the invention, the hot-rolled steel sheets had a predetermined chemical composition, and by appropriately controlling the conditions of the manufacturing method, particularly the cooling process, it was possible to achieve a tensile strength of 980 MPa or more at all positions from the edge in the width direction, specifically at the 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W positions, by making the tempered martensite in the metal structure at the 1 / 4 thickness position 95% or more by area percentage, resulting in a structure where tempered martensite is the main phase. In addition, it was possible to reliably control the difference between the maximum and minimum tensile strengths at all these positions in the width direction to 30 MPa or less, thus significantly reducing the strength variation in the width direction. In connection with this, it was possible to achieve sufficient flatness without causing warping in the width direction of the hot-rolled steel sheet. Table 3 specifically shows only the microstructure of the locations where the minimum and maximum tensile strength were obtained, among the positions 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W from the edge in the width direction. However, in Invention Examples 1, 3-7, and 12, at all of these locations, the microstructure at the 1 / 4 thickness position consisted of tempered martensite: 95% or more, fresh martensite: 5% or less, and at least one of ferrite, upper bainite, and pearlite: 5% or less in total, by area percentage.

Claims

1. In mass percent, C: 0.050-0.100%, Si: 0.010-0.200%, Mn: 1.00-2.50%, Ti: 0.001 to 0.120%, Al: 0.001-0.050%, B: 0.0005-0.0050%, P: 0.100% or less, S: 0.050% or less, N: 0.0050% or less, O: 0 to 0.0050%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Sn: 0 to 0.10%, Cr: 0-0.40%, Mo: 0 to 0.20%, Nb: 0 to 0.05%, V: 0-0.10%, As: 0 to 0.100%, Zr: 0 to 0.100%, Ca: 0-0.0050%, Mg: 0-0.100%, Bi: 0 to 0.020%, Co: 0 to 0.20%, W: 0-0.20%, Zn: 0-0.20%, REM: 0-0.1000%, and The remainder has a chemical composition consisting of Fe and impurities. When the total width in the direction perpendicular to the rolling direction and the thickness direction is W, at all positions from the edge in the width direction, specifically at 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W, the metallographic structure at the 1 / 4 thickness position is, in area %, Tempered martensite: 95% or more, Fresh martensite: 5% or less, At least one of ferrite, upper bainite, and pearlite: totaling 5% or less. A high-strength hot-rolled steel sheet characterized in that the difference between the maximum and minimum tensile strengths at all positions in the width direction is 30 MPa or less.

2. The aforementioned chemical composition is, in mass%, O: 0.0001 to 0.0050%, Cu: 0.001 to 0.20%, Ni: 0.001 to 0.20%, Sn: 0.001 to 0.10%, Cr: 0.001-0.40%, Mo: 0.001-0.20%, Nb: 0.001 to 0.05%, V: 0.001 to 0.10%, As: 0.001 to 0.100%, Zr: 0.0001 to 0.100%, Ca: 0.0001-0.0050%, Mg: 0.0001-0.100%, Bi: 0.0001-0.020%, Co: 0.001 to 0.20%, W: 0.001-0.20%, Zn: 0.001–0.20%, and REM: 0.0001~0.1000% The high-strength hot-rolled steel sheet according to claim 1, characterized in that it includes at least one of the following.

3. The high-strength hot-rolled steel sheet according to claim 1 or 2, characterized in that the prior austenite grain size in the metal structure is 40 μm or less.

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