Hot-rolled steel sheet

Abstract

This hot-rolled steel sheet has an A value represented by formula (1) of 0.0150 mass% or more and an M value represented by formula (2) of 930 or less, and a metal structure at a position 1 / 4 of the sheet thickness from the surface contains, in area%, 50% or more but less than 80% of ferrite, more than 20% but 50% or less of bainite, 0-5% of pearlite, and 0-2% of MA, has an average crystal grain size of 10 [mu] m or less, and has a BH content of 45 MPa or more and a tensile strength of 780 MPa or more.

Description

Technical Field

[0001] The present invention relates to hot-rolled steel sheets.

[0002] This application claims priority based on Japanese Patent Application No. 2023-201058 filed in Japan on November 28, 2023, and incorporates its content herein. Background Art

[0003] In recent years, in order to reduce greenhouse gas emissions, each automobile manufacturer is promoting body weight reduction through component count reduction and wall thickness reduction. On the other hand, since automobile bodies also require collision safety, the application of high-strength steel sheets in automobile components is accelerating.

[0004] High-strength hot-rolled steel sheets applicable to automobiles and the like are required to have excellent stamping workability. There is known the following technique: in order to improve stamping workability in high-strength hot-rolled steel sheets, for example, in order to balance high strength and formability, ferrite is made 80% or more in the metal structure, and the remaining part is bainite and pearlite, and fine carbides are precipitated in the ferrite grains, thereby balancing high strength and ductility. In this technique, Ti, which is inexpensive per unit strength, is mainly used.

[0005] Although adding Ti can improve stamping workability, when cutting a high-strength hot-rolled steel sheet containing Ti into a blank shape (performing blanking), there is a tendency for poor conditions such as the shear end face to peel off in a two-layered manner. This peeling sometimes develops into cracks in the direction perpendicular to the plate thickness direction. The peeling of the end face generated during this blanking is called "peeling (original Japanese: 剥がれ)".

[0006] In high-strength hot-rolled steel sheets, it is required to prevent peeling generated during this blanking.

[0007] For example, Patent Document 1 discloses a technique for strengthening grain boundaries and improving blanking workability by leaving solid-solved carbon in the steel sheet.

[0008] Patent Document 2 discloses a method for improving blanking workability by adding B and causing B to segregate at grain boundaries to strengthen the grain boundaries.

[0009] Prior Art Documents Patent Documents Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-342684 Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-315857 Summary of the Invention

[0010] Problems to be Solved by the Invention However, in Patent Document 1, even in steel sheets with residual dissolved carbon, there is room for improvement from the viewpoint of suppressing minor peeling during punching. In the case of processing techniques where the sheared end face is subjected to plastic strain after punching, this minor peeling becomes the initiation point of cracks.

[0011] Furthermore, when B is added as described in Patent Document 2, the grain boundary strength increases, but from the viewpoint of controlling the metal structure and surface properties of the steel plate that are accompanied by increased hardenability, there is room for improvement.

[0012] This invention was made in view of the above-mentioned actual situation. The object of this invention is to provide a hot-rolled steel sheet with high strength and excellent stamping and blanking workability.

[0013] Methods for solving problems The inventors have discovered that by increasing the calculated amount of non-precipitated carbon, which is obtained from the C, Ti, N, Nb, and V contents in the chemical composition, the amount of dissolved carbon can be increased, thereby suppressing the occurrence of micro-peeling during punching.

[0014] Regarding the amount of dissolved carbon, Patent Document 1 states that if the non-fixed carbon (C-Ti / 4+N / 1.17) exceeds 0.015%, the strengthening of ferrite is insufficient. Due to the increase in carbon content in bainite, the hardness difference between bainite and bainite increases, thereby deteriorating the stamping processability. Furthermore, Patent Document 1 states that if the area ratio of ferrite is less than 80%, the ductility decreases. Therefore, the inventors have discovered that by increasing the area ratio of bainite in the metal structure, the increase in carbon content in bainite is suppressed, and the increase in hardness is suppressed, thereby improving the stamping processability.

[0015] The main points of the present invention, based on the above insights, are as follows.

[0016] [1] A hot-rolled steel plate, characterized in that its chemical composition, in mass % is: C: 0.04~0.10% Si: 0.11~0.30% Mn: 1.40~2.50%, P: below 0.05% S: Below 0.05% N: below 0.010% O: Below 0.010% Al: 0.001~0.050% Ti: 0.060~0.150%, Nb: 0.020~0.100% V: 0~0.200% Mo: 0~1.000% Ca: 0~0.0100% REM: 0~0.0100% Cu: 0~2.00% Ni: 0~1.50%, Cr: 0~2.00% Mg: 0~0.0200% Bi: 0~0.0200% Zr: 0~0.500%, Co: 0~1.000%, Zn: 0~0.200%, W: 0~0.200% Sb: 0~0.200% As: 0~0.050% Sn: 0~0.050% B: Less than 0.0005%, and Remaining components: Fe and impurities. The value of A expressed by the following formula (1) is 0.0150% by mass or more. The value of M, as expressed by the following equation (2), is 930 or less. The metallic microstructure at a location 1 / 4 of the plate thickness from the surface contains, by area %: Ferrite: 50% or more but less than 80% Bainite: More than 20% but less than 50% Pearlite: 0~5% MA: 0~2%, The average crystal grain size is less than 10 μm. For the aforementioned hot-rolled steel plates BH value is above 45MPa The tensile strength is above 780 MPa.

[0017] A value=C-Ti / 4+N / 1.17-Nb / 7.75-V / 4.25 (1) M value = 937 - 436.5 × C + 56 × Si - 19.7 × Mn - 16.3 × Cu - 26.6 × Ni - 4.9 × Cr + 38.1 × Mo + 124.8 × V + 136.3 × Ti - 19.1 × Nb + 198.4 × Al (2) In the above formulas (1) and (2), the element symbols represent the content of the element in terms of mass % and do not contain 0.

[0018] [2] The hot-rolled steel sheet according to [1] above is characterized in that the above chemical composition, in terms of mass%, contains one or more elements selected from the group consisting of: V: 0.001~0.200% Mo: 0.001~1.000% Ca: 0.0001~0.0100% REM: 0.0001~0.0100% Cu: 0.01~2.00% Ni: 0.01~1.50%, Cr: 0.01~2.00% Mg: 0.0001~0.0200% Bi: 0.001~0.0200% Zr: 0.001%~0.500% Co: 0.001~1.000%, Zn: 0.001~0.200% W: 0.001~0.200% Sb: 0.001~0.200% As: 0.001~0.050%, and Sn: 0.001~0.050%.

[0019] [3] According to the hot-rolled steel sheet described in [1] or [2] above, the characteristic is that, in the metal structure at a depth of 500 μm from the surface, the angle is Φ = 0~90°, measured in Euler angles. 1 = 0~90° The maximum value of the polar density of the orientation group (also known as the orientation cluster) exhibiting 2=45° is below 7.0.

[0020] [4] The hot-rolled steel plate according to any one of [1] to [3] above is characterized in that the above chemical composition, in mass%, is: B: less than 0.0002%.

[0021] Invention Effects According to the above-described solution of the present invention, it is possible to provide a hot-rolled steel sheet with high strength and excellent stamping and blanking properties. Detailed Implementation

[0022] The following describes a hot-rolled steel sheet according to one embodiment of the present invention (hereinafter, sometimes referred to as the hot-rolled steel sheet of this embodiment). However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the spirit of the present invention.

[0023] The various components of the present invention will be described in detail below. First, the reasons for limiting the chemical composition of the hot-rolled steel sheet in this embodiment will be explained.

[0024] In the numerical ranges enclosed in “~” below, the lower and upper limits are included. Values ​​expressed as “below” or “above” are not included in the numerical range. In the following explanations, percentages of chemical composition are by mass unless otherwise specified.

[0025] The hot-rolled steel sheet of this embodiment has the following chemical composition (by mass%): C: 0.04~0.10%, Si: 0.11~0.30%, Mn: 1.40~2.50%, P: 0.05% or less, S: 0.05% or less, N: 0.010% or less, O: 0.010% or less, Al: 0.001~0.050%, Ti: 0.060~0.150%, Nb: 0.020~0.100%, V: 0~0.200%, Mo: 0~1.000%, Ca: 0~0.0 100%, REM: 0~0.0100%, Cu: 0~2.00%, Ni: 0~1.50%, Cr: 0~2.00%, Mg: 0~0.0200%, Bi: 0~0.0200%, Zr: 0~0.500%, Co: 0~1.000%, Zn: 0~0.200%, W: 0~0.200%, Sb: 0~0.200%, As: 0~0.050%, Sn: 0~0.050%, B: less than 0.0005%, and the remainder: Fe and impurities. The following provides a detailed description of each element.

[0026] C: 0.04~0.10% Carbon (C) is an element required to suppress peeling of end faces formed during shearing or punching processes, existing as a solid solution at grain boundaries, and to obtain the strength of hot-rolled steel sheets as a carbide precipitation. When the C content is below 0.04%, it is difficult to obtain the desired punching workability and strength. Therefore, the C content is set to 0.04% or more. The C content is preferably 0.05% or more, or 0.06% or more.

[0027] On the other hand, when the carbon content exceeds 0.10%, the ductility decreases, thereby deteriorating the stamping processability of hot-rolled steel sheets. Therefore, the carbon content is set to 0.10% or less. The carbon content is preferably 0.09% or less and 0.08% or less.

[0028] Si: 0.11~0.30% Si is an effective element for deoxidation and for increasing strength as a solid solution strengthening element. When the Si content is below 0.11%, it is difficult to obtain the desired strength in hot-rolled steel sheets. Therefore, the Si content is set to 0.11% or more. Preferably, the Si content is 0.15% or more.

[0029] On the other hand, when the Si content exceeds 0.30%, a striped pattern of Si oxide scale forms on the surface of the steel sheet, damaging its surface properties. Furthermore, the chemical conversion treatment of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 0.30% or less. Preferably, the Si content is 0.25% or less.

[0030] Mn: 1.40~2.50% Mn is an element that effectively strengthens steel by acting as a solid solution strengthening element. When the Mn content is below 1.40%, it is difficult to obtain the desired strength in hot-rolled steel sheets. Therefore, the Mn content is set to 1.40% or higher. Preferably, the Mn content is 1.60% or higher, or 1.80% or higher.

[0031] On the other hand, when the Mn content exceeds 2.50%, the ductility decreases, thereby deteriorating the stamping processability of the hot-rolled steel sheet. Therefore, the Mn content is set to 2.50% or less. The preferred Mn content is 2.30% or less and 2.00% or less.

[0032] P: below 0.05% Phosphorus (P) is an element that segregates at grain boundaries, reducing ductility and thus deteriorating the stamping processability of hot-rolled steel sheets. This effect becomes significant when the P content exceeds 0.05%, therefore the P content is set below 0.05%.

[0033] The lower the phosphorus (P) content, the better; therefore, it can be set to 0%. However, from the perspective of controlling refining costs, the P content can also be set to 0.001% or higher.

[0034] S: below 0.05% Sulfur (S) is an element that deteriorates the workability of hot-rolled steel sheets by acting as a fracture initiation point through the formation of MnS, thereby worsening the porosity. This effect becomes significant when the S content exceeds 0.05%, therefore the S content is set below 0.05%.

[0035] The lower the sulfur content, the better; therefore, it can be set to 0%. However, from the perspective of controlling refining costs, the sulfur content can also be set to 0.001% or higher.

[0036] N: below 0.010% Nitrogen (N) is an element that forms nitrides with Ti, reducing the amount of Ti that can combine with C and thus decreasing the strength of hot-rolled steel sheets. Additionally, N forms coarse TiN deposits, which deteriorates the workability of the sheet. These effects become significant when the N content exceeds 0.010%, therefore the N content is set to 0.010% or less.

[0037] The lower the nitrogen content, the better; therefore, it can be set to 0%. However, from the perspective of controlling refining costs, the nitrogen content can also be set to 0.001% or higher.

[0038] O: below 0.010% O is an element that, when present in large quantities in steel, forms coarse oxides that become the initiation point for fracture and deteriorate the workability of the stamping process. This effect becomes significant when the O content exceeds 0.010%, therefore the O content is set to 0.010% or less. The O content is preferably 0.008% or less and 0.006% or less.

[0039] The lower the O content, the better; therefore, it can be set to 0%. However, from the perspective of controlling refining costs, the O content can also be set to 0.001% or higher.

[0040] Al: 0.001~0.050% Al is an effective element for deoxidation. When the Al content is below 0.001%, the molten steel cannot be sufficiently deoxidized. Therefore, the Al content is set to 0.001% or higher. Preferably, the Al content is 0.010% or higher, or 0.020% or higher.

[0041] On the other hand, when the Al content exceeds 0.050%, the amount of non-metallic inclusions increases, ductility decreases, and thus stamping processability deteriorates. Therefore, the Al content is set to 0.050% or less. Preferably, the Al content is 0.040% or less.

[0042] Ti: 0.060~0.150% Ti is an effective element for achieving strength in hot-rolled steel sheets by refining grain size and precipitating fine TiC. If the Ti content is below 0.060%, it is difficult to obtain the desired strength. Therefore, the Ti content is set to 0.060% or more. Preferably, the Ti content is 0.080% or more, or 0.100% or more.

[0043] On the other hand, when the Ti content exceeds 0.150%, a large amount of TiC is generated, which reduces ductility and deteriorates the stamping processability of hot-rolled steel sheets. Furthermore, due to the large amount of TiN generated, the blanking processability is reduced. Therefore, the Ti content is set to 0.150% or less. The preferred Ti content is 0.145% or less, 0.140% or less, or 0.135% or less.

[0044] Nb: 0.020~0.100% Like Ti, Nb is an effective element for refining grain size and precipitating fine NbC, thereby increasing strength. When the Nb content is below 0.020%, it is difficult to obtain the desired strength in hot-rolled steel sheets. Therefore, the Nb content is set to 0.020% or more. Preferably, the Nb content is 0.030% or more, or 0.050% or more.

[0045] On the other hand, when the Nb content exceeds 0.100%, a large amount of NbC is generated, which reduces ductility and deteriorates the stamping processability of hot-rolled steel sheets. Therefore, the Nb content is set to 0.100% or less. The Nb content is preferably 0.090% or less and 0.080% or less.

[0046] V: 0~0.200% V is an effective element for increasing the strength of hot-rolled steel sheets by precipitating fine VC particles. The V content can be 0%, but to fully obtain this effect, it is preferable to set the V content to 0.001% or more. The V content is more preferably 0.005% or more or 0.010% or more.

[0047] On the other hand, if the V content exceeds 0.200%, a large amount of VC will be generated, and the amount of dissolved carbon may decrease. Therefore, the V content is set to 0.200% or less. The V content is preferably 0.150% or less or 0.100% or less.

[0048] Mo: 0~1.000% Mo is an effective element for increasing the strength of hot-rolled steel sheets by improving the hardenability of the steel. The Mo content can be 0%, but to fully obtain this effect, it is preferable to set the Mo content to 0.001% or more. More preferably, the Mo content is 0.005% or more or 0.010% or more.

[0049] On the other hand, even if the Mo content exceeds 1.000%, the above-mentioned effects become saturated, which is not economically preferable. Therefore, the Mo content is set to 1.000% or less. The Mo content is preferably 0.800% or less or 0.600% or less.

[0050] Ca: 0~0.0100% Ca is an effective element for improving stamping workability by suppressing the morphology of non-metallic inclusions that cause deterioration in workability and become the starting point for fracture formation. The Ca content can be set to 0%, but to obtain the full effect, it is preferable to set the Ca content to 0.0001% or more. The Ca content is more preferably 0.0005% or more or 0.0010% or more.

[0051] On the other hand, even if the Ca content exceeds 0.0100%, the above-mentioned effect becomes saturated, which is not economically preferable. Therefore, the Ca content is set to 0.0100% or less. The Ca content is preferably 0.0080% or less or 0.0060% or less.

[0052] REM: 0~0.0100% Like Ca, REM is an effective element for improving stamping workability by suppressing the morphology of non-metallic inclusions that cause deterioration in workability and become the starting point for fracture formation. The REM content can be 0%, but to fully obtain this effect, it is preferable to set the REM content to 0.0001% or more. More preferably, the REM content is 0.0005% or more, or 0.0010% or more.

[0053] On the other hand, even if the REM content exceeds 0.0100%, the above-mentioned effect becomes saturated, which is not economically preferable. Therefore, the REM content is set to 0.0100% or less. The REM content is preferably 0.0080% or less or 0.0060% or less.

[0054] Here, REM refers to a total of 17 elements, including Sc, Y, and the lanthanides. The REM content mentioned above refers to the total content of these elements. In industry, lanthanides are added as a mixture of rare earth metals.

[0055] Cu: 0~2.00% Cu is an effective element for increasing the strength of hot-rolled steel sheets by improving the hardenability of the steel. The Cu content can be 0%, but to fully obtain this effect, it is preferable to set the Cu content to 0.01% or more. The Cu content is more preferably 0.05% or more or 0.10% or more.

[0056] On the other hand, even if the Cu content exceeds 2.00%, the above-mentioned effect becomes saturated, which is not economically preferable. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.80% or less or 1.60% or less.

[0057] Ni: 0~1.50% Ni is an effective element for increasing the strength of hot-rolled steel sheets by improving hardenability. The Ni content can be 0%, but to fully achieve this effect, it is preferable to set the Ni content to 0.01% or more. More preferably, the Ni content is 0.05% or more, or 0.10% or more.

[0058] On the other hand, even if the Ni content exceeds 1.50%, the above-mentioned effects become saturated, which is not economically preferable. Therefore, the Ni content is set to 1.50% or less. The Ni content is preferably 1.30% or less or 1.10% or less.

[0059] Cr: 0~2.00% Cr is an effective element for improving the strength of hot-rolled steel sheets by increasing hardenability. The Cr content can be 0%, but to fully achieve this effect, it is preferable to set the Cr content to 0.01% or more. More preferably, the Cr content is 0.05% or more, or 0.10% or more.

[0060] On the other hand, even if the Cr content exceeds 2.00%, the above-mentioned effects become saturated, which is not economically preferable. Therefore, the Cr content is set to 2.00% or less. The Cr content is preferably 1.80% or less or 1.60% or less.

[0061] Mg: 0~0.0200% Mg is an element that improves the yield ratio of hot-rolled steel sheets by controlling the shape of inclusions in steel to a preferred shape. The Mg content can be 0%, but to fully obtain this effect, it is preferable to set the Mg content to 0.0001% or more. More preferably, the Mg content is 0.0005% or more, or 0.0010% or more.

[0062] On the other hand, if the Mg content exceeds 0.0200%, excessive inclusions will form in the steel, reducing the yield ratio of the hot-rolled steel sheet. Therefore, the Mg content is set to 0.0200% or less. The Mg content is preferably 0.0150% or less or 0.0100% or less.

[0063] Bi: 0~0.0200% Bi is an element that increases the yield ratio of hot-rolled steel sheets by refining the solidification structure. The Bi content can be 0%, but to fully obtain this effect, it is preferable to set the Bi content to 0.001% or more. More preferably, the Bi content is 0.0005% or more or 0.0010% or more.

[0064] On the other hand, when the Bi content exceeds 0.0200%, the effects described above become saturated, which is not economically preferable. Therefore, the Bi content is set to 0.0200% or less. Preferably, the Bi content is 0.0150% or less or 0.0100% or less.

[0065] Zr: 0~0.500% Zr is an element that improves the strength of hot-rolled steel sheets through solid solution strengthening. The Zr content can be 0%, but to fully achieve this effect, it is preferable to set the Zr content to 0.001% or more. More preferably, the Zr content is 0.005% or more, or 0.010% or more.

[0066] On the other hand, when the Zr content exceeds 0.500%, the hole-expanding property of hot-rolled steel sheets deteriorates, thereby worsening their stamping workability. Therefore, the Zr content is set to 0.500% or less. The Zr content is preferably 0.400% or less or 0.300% or less.

[0067] Co: 0~1.000% Co is an element that improves the strength of hot-rolled steel sheets through solid solution strengthening. The Co content can be 0%, but to fully obtain this effect, it is preferable to set the Co content to 0.001% or more. More preferably, the Co content is 0.005% or more or 0.010% or more.

[0068] On the other hand, if the Co content exceeds 1.000%, the hole-expanding property of the hot-rolled steel sheet deteriorates, thereby deteriorating its stamping processability. Therefore, the Co content is set to 1.000% or less. The Co content is preferably 0.800% or less or 0.600% or less.

[0069] Zn: 0~0.200% Zn is an element that improves the strength of hot-rolled steel sheets through solid solution strengthening. The Zn content can be 0%, but to fully obtain this effect, it is preferable to set the Zn content to 0.001% or more. More preferably, the Zn content is 0.005% or more or 0.010% or more.

[0070] On the other hand, if the Zn content exceeds 0.200%, the hole-expanding property of the hot-rolled steel sheet deteriorates, thereby worsening its stamping processability. Therefore, the Zn content is set to 0.200% or less. The Zr content is preferably 0.150% or less or 0.100% or less.

[0071] W: 0~0.200% W is an element that enhances the strength of hot-rolled steel sheets through solid solution strengthening. The W content can be 0%, but to fully achieve this effect, it is preferable to set the W content to 0.001% or more. More preferably, the W content is 0.005% or more, or 0.010% or more.

[0072] On the other hand, if the W content exceeds 0.200%, the hole-expanding property of the hot-rolled steel sheet deteriorates, thereby worsening its stamping processability. Therefore, the W content is set to 0.200% or less. The W content is preferably 0.150% or less or 0.100% or less.

[0073] Sb: 0~0.200% Sb is an element that improves the porosity of hot-rolled steel sheets by inhibiting the formation of oxides that become fracture initiation points. The Sb content can be 0%, but to fully obtain this effect, it is preferable to set the Sb content to 0.001% or more. The Sb content is more preferably 0.005% or more or 0.010% or more.

[0074] On the other hand, even with a large amount of Sb, the above-mentioned effect is saturated, so the Sb content is set to 0.200% or less. The Sb content is preferably 0.150% or less or 0.100% or less.

[0075] As: 0~0.050% As is an element that improves the porosity of hot-rolled steel sheets by refining the original austenite grains by lowering the austenite single-phase conversion temperature. The As content can be 0%, but to fully obtain this effect, it is preferable to set the As content to 0.001% or more. More preferably, the As content is 0.005% or more or 0.010% or more.

[0076] On the other hand, even with a large amount of As, the above effect becomes saturated, so the As content is set to 0.050% or less. The As content is preferably 0.030% or less or 0.020% or less.

[0077] Sn: 0~0.050% Sn is an element that improves the porosity of hot-rolled steel sheets by inhibiting the formation of oxides that become fracture initiation points. The Sn content can be 0%, but to fully obtain this effect, the Sn content is preferably set to 0.001% or more. More preferably, the Sn content is 0.005% or more or 0.010% or more.

[0078] On the other hand, even with a large amount of Sn, the above effect becomes saturated, so the Sn content is set to 0.050% or less. The Sn content is preferably 0.030% or less or 0.020% or less.

[0079] B: Less than 0.0005% In the chemical composition of the hot-rolled steel sheet of this embodiment, there is essentially no B, so the B content is set to be less than 0.0005%. From the viewpoint of ensuring the strength of the hot-rolled steel sheet, the lower the B content, the better. Therefore, it is preferred to be less than 0.0004%, less than 0.0004%, less than 0.0003%, less than 0.0002%, or less than 0.0002%.

[0080] A value: ≥0.0150% by mass In the chemical composition, if the A value (calculated amount of undeferred carbon) expressed by the following formula (1) is less than 0.0150% by mass, the amount of dissolved carbon is insufficient, the amount of BH cannot be sufficiently increased, and the desired stamping processability cannot be obtained in hot-rolled steel sheets. Therefore, the A value is set to 0.0150% by mass or more. The preferred A values ​​are 0.0200% by mass or more, 0.0250% by mass or more, and 0.0300% by mass or more.

[0081] There is no specific upper limit for the A value; it can be below 1.0000% mass, below 0.0800% mass, or below 0.0750% mass.

[0082] A value=C-Ti / 4+N / 1.17-Nb / 7.75-V / 4.25 (1) In the above formula (1), each element symbol represents the content of the element in terms of mass % and is 0 when it does not contain any.

[0083] M value: below 930 If the M value expressed by equation (2) below exceeds 930, the ferrite formation capacity increases, and the desired amount of bainite cannot be obtained. As a result, the porosity deteriorates, thereby deteriorating the stamping processability of the hot-rolled steel sheet. Therefore, the M value is set to 930 or less. The M value is preferably 920 or less, 910 or less, or 900 or less.

[0084] M value = 937 - 436.5 × C + 56 × Si - 19.7 × Mn - 16.3 × Cu - 26.6 × Ni - 4.9 × Cr + 38.1 × Mo + 124.8 × V + 136.3 × Ti - 19.1 × Nb + 198.4 × Al (2) In the above formula (2), each element symbol represents the content of the element in terms of mass % and is 0 when it does not contain any.

[0085] It should be noted that when the chemical composition of the hot-rolled steel plate contains 0.0005% or more of B, it is preferable to use the following formula (2') with the addition of B to replace the above formula (2). It should also be noted that when the chemical composition of the hot-rolled steel plate contains 0.0001% or more, 0.0002% or more, 0.0003% or more, or 0.0004% or more of B as required, the following formula (2') can also be used to replace the above formula (2).

[0086] M value = 937 - 436.5 × C + 56 × Si - 19.7 × Mn - 16.3 × Cu - 26.6 × Ni - 4.9 × Cr + 38.1 × Mo + 124.8 × V + 136.3 × Ti - 19.1 × Nb + 198.4 × Al + 3315 × B (2') In the above formula (2), each element symbol represents the content of the element in terms of mass % and is 0 when it does not contain any.

[0087] The hot-rolled steel sheet of this embodiment may also contain the aforementioned elements, with the remainder comprising Fe and impurities. In this embodiment, impurities refer to substances mixed in from the ore, scrap iron, or manufacturing environment used as raw materials, and / or substances permissible within a range that do not adversely affect the properties (strength, stamping workability, and blanking workability) of the hot-rolled steel sheet of this embodiment.

[0088] The chemical composition of the aforementioned hot-rolled steel sheet can be analyzed using a spark discharge emission spectrometer or similar device. It should be noted that C and S are determined by infrared absorption spectroscopy after combustion in an oxygen stream using a gas composition analyzer. N is determined by thermal conductivity spectroscopy after melting a sample taken from the steel sheet in a helium stream. O is determined by infrared absorption spectroscopy after melting a sample taken from the steel sheet and placed in a graphite crucible in a helium stream.

[0089] When hot-rolled steel sheets have coatings or films on their surface, the coatings or films are removed by mechanical grinding or other means as needed, and then the chemical composition is analyzed.

[0090] Next, the metal structure of the hot-rolled steel sheet of this embodiment will be described.

[0091] The metal microstructure of the hot-rolled steel sheet in this embodiment, located at 1 / 4 of the sheet thickness from the surface, comprises, by area %: ferrite: 50% or more and less than 80%, bainite: more than 20% and less than 50%, pearlite: 0-5%, MA: 0-2%, and the average grain size is less than 10 μm.

[0092] The following is an explanation of each requirement.

[0093] It should be noted that, in this embodiment, the metal structure at a position 1 / 4 of the plate thickness from the surface (a region from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface) is defined. In other words, the aforementioned position is the region starting from 1 / 8 of the plate thickness from the surface and ending at 3 / 8 of the plate thickness from the surface.

[0094] The reason for specifying the metal structure at the above-mentioned locations is that the metal structure at the above-mentioned locations represents the representative metal structure of the steel plate.

[0095] It should be noted that when hot-rolled steel sheets have coatings or films on their surface, the surface referred to here is the interface between the steel sheet and the coatings or films.

[0096] Ferrite: 50% or more but less than 80% In the hot-rolled steel sheet of this embodiment, fine TiC is precipitated within the ferrite grains to obtain the desired strength. To achieve this effect, the ferrite area fraction is set to 50% or more. Preferably, the ferrite area fraction is 55% or more, 60% or more, or 65% or more.

[0097] On the other hand, when the ferrite area ratio is above 80%, the bainite area ratio decreases, the carbon content in bainite increases, the hardness of bainite increases, the porosity deteriorates, and the stamping processability of hot-rolled steel sheets deteriorates. Therefore, the ferrite area ratio is set to be below 80%. The preferred ferrite area ratio is 75% or less and 70% or less.

[0098] Bainite: More than 20% but less than 50% When the bainite area ratio is below 20%, the carbon content in the bainite increases, the hardness of the bainite increases, and the porosity deteriorates, but the desired stamping processability cannot be obtained in hot-rolled steel sheets. Therefore, the bainite area ratio is set to be above 20%. The preferred bainite area ratio is 25% or more and 30% or more.

[0099] Furthermore, based on the relationship with the area ratio of ferrite, the area ratio of bainite is set to 50% or less. Preferably, the area ratio of bainite is 45% or less, 40% or less, or 35% or less.

[0100] Pearlite: 0~5% Pearlite is a microstructure that results in decreased strength. When the area fraction of pearlite exceeds 5%, the strength of hot-rolled steel sheet decreases. Therefore, the area fraction of pearlite is set to 5% or less. Preferably, the area fraction of pearlite is 4% or less, 3% or less, 2% or less, or 1% or less.

[0101] The lower the area ratio of pearlite, the better; therefore, 0% is preferred.

[0102] MA: 0~2% Martensite (MA) is a microstructure composed of martensite and austenite. Due to its high hardness, if the area fraction of MA exceeds 2%, its porosity deteriorates, and the stamping processability of hot-rolled steel sheets also deteriorates. Therefore, the area fraction of MA is set to be below 2%. Preferably, the area fraction of MA is below 1%.

[0103] MA can also be excluded, therefore the area ratio of MA can also be 0%.

[0104] The area ratio of each tissue was determined by the following method.

[0105] Method for determining the area fraction of ferrite Test specimens were collected from hot-rolled steel plates to allow observation of sections perpendicular to the plate width. For each specimen, the observation surface was polished to a mirror finish using wet grinding. The observation surface was then ground for 8 minutes at room temperature with colloidal silica (without an alkaline solution) to remove strain introduced into the observation surface. Next, tissue observation was performed using a SEM-EBSD apparatus at a depth of 1 / 4 of the plate thickness from the surface (from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness). The SEM-EBSD apparatus consisted of a field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector). The vacuum level within the apparatus was set to 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage is set to 15kV, the irradiation current level is set to 13, and the electron beam irradiation level is set to 62.

[0106] The observation area was set to 500 μm × 500 μm with a step size of 0.3 μm to obtain crystal orientation information. At least three observation areas were used. Using the obtained crystal orientation information and OIM data analysis, the crystal orientation difference with adjacent measurement points was calculated. Regions enclosed by boundaries with an orientation difference greater than 15° were considered grains. Next, the average grain average misorientation (GAM) for each grain with an orientation difference greater than or equal to that of adjacent measurement points was calculated. For GAM, grains with an orientation difference less than 1.0° were defined as ferrite, and the area fraction of these grains was calculated, thus obtaining the area fraction of ferrite.

[0107] Furthermore, the ferrite defined by this method does not contain ferrite within pearlite.

[0108] Method for determining the area ratio of pearlite Test specimens were collected using the same method as described above. The observation surface was etched with nitric acid and ethanol to reveal the metal structure, which was then photographed using an optical microscope. The photographing position was set at 1 / 4 of the plate thickness from the surface (a depth ranging from 1 / 8 to 3 / 8 of the plate thickness), and the photographing area was set to 500 μm × 500 μm, with at least three such areas. Image analysis was performed on the obtained tissue photographs, and the darker etched grains were defined as pearlite. The area ratio of these grains was calculated to obtain the area ratio of pearlite.

[0109] In addition, the pearlite etched in a darker manner by this method does not include ferrite as defined by the SEM-EBSD device described above.

[0110] Methods for determining the area ratio of MA Test specimens were collected using the same method as described above. LePera etching was performed on the observation surface to reveal the metal microstructure, which was then photographed using an optical microscope. The photographing location and area were the same as described above. Image analysis was performed on the obtained microstructure photographs. The whiter etched grains were defined as MAs, and the area ratio of these grains was calculated to obtain the area ratio of MAs.

[0111] Methods for determining the area ratio of bainite Bainite is obtained by subtracting the area ratios of ferrite, pearlite, and MA from 100%.

[0112] Average crystal grain size: below 10 μm In this embodiment, in addition to effectively utilizing precipitation strengthening caused by the fine precipitation of TiC, the increased strength resulting from grain refinement is also effectively utilized, thereby obtaining the desired tensile strength. When the average crystal grain size exceeds 10 μm, the strengthening effect of grain refinement decreases, and the desired strength cannot be obtained. Therefore, the average crystal grain size is set to 10 μm or less. The average crystal grain size is preferably 8 μm or less, 7 μm or less, or 6 μm or less.

[0113] There is no particular limit to the lower limit of the average crystal grain size; it can be set to 2 μm or 3 μm or more.

[0114] The average grain size of the metal structure was determined by the following method.

[0115] Analysis was performed using crystal orientation information obtained when measuring the area fraction of ferrite. A grain was defined as a region enclosed by a boundary with an orientation difference greater than 15°, and the equivalent circle diameter of that grain was calculated. The equivalent circle diameter was calculated for all grains within the observation region, and their average value was calculated to obtain the average grain size.

[0116] In addition, the equivalent circle diameter refers to the diameter of a circle that has the same area as the area of ​​the defined grain.

[0117] BH level: above 45MPa In this embodiment, carbon, in addition to generating precipitates necessary for achieving the desired strength, also improves workability by remaining in the steel in a solid solution state. The inventors have discovered a correlation between the amount of dissolved carbon and the amount of bake hardening (BH). When the BH amount is below 45 MPa, the amount of dissolved carbon decreases, and the desired workability is not achieved. Therefore, the BH amount is set to 45 MPa or more. Preferably, the BH amount is 55 MPa or more, 60 MPa or more, or 70 MPa or more.

[0118] The hot-rolled steel sheet of this embodiment is a precipitation-strengthened steel sheet with a tensile strength of 780 MPa or more. In this embodiment, this strength is achieved by dispersing Ti-based carbides in the steel. Increasing the amount of dissolved carbon in the steel increases the BH content, but on the other hand, it reduces the amount of Ti-based carbides precipitated, resulting in a decrease in strength. Detailed research by the inventors revealed that by keeping the BH content below 100 MPa, the strength of the hot-rolled steel sheet is further improved. Therefore, the BH content can be set to below 100 MPa. The BH content can be set to 95 MPa or less, 90 MPa or less, 80 MPa or less, or 75 MPa or less.

[0119] BH levels were determined using the following method.

[0120] Test piece No. 5 was collected from hot-rolled steel sheet according to JIS Z 2241:2022. Using the collected test piece No. 5, the BH (bake hardening) amount was evaluated according to Appendix A of JIS G 3135:2018.

[0121] More specifically, the BH quantity (σ) BH The value can be obtained using the following formula.

[0122] σ BH =R SA -R WH Here, R SA The value (N / mm) is obtained by dividing the test force at the yield point of the heat-treated specimen under the following conditions, after being given a total elongation of 2%, by the original cross-sectional area of ​​the parallel portion of the specimen before pre-straining. 2 ).

[0123] Heat treatment conditions: Heat at 170℃ for 20 minutes, then air cool. Additionally, R WH The value (N / mm) is obtained by dividing the test force (with a total elongation of 2%) in the tensile test by the original cross-sectional area of ​​the parallel portion of the test piece before pre-straining. 2 It should be noted that the tensile test was conducted in accordance with JIS Z 2241:2022.

[0124] In the metallic microstructure at a depth of 500 μm from the surface, using Euler angles Φ = 0~90°, 1 = 0~90° The maximum value of the polar density of the orientation group exhibiting 2=45° is below 7.0. The inventors discovered that when hot-rolled steel sheets with a tensile strength of 780 MPa or higher are punched, the end faces of the punched holes show poorer characteristics compared to the face perpendicular to the rolling direction (section C). The end faces are parallel to the rolling direction and the thickness direction (section L). The inventors believe this is due to the anisotropy of the crystal orientation of the hot-rolled steel sheet and conducted an in-depth study on the relationship between the texture and end face characteristics during hot rolling. The results showed that in the metal microstructure from the surface to a depth of 500 μm, by adjusting the Euler angles from Φ=0 to 90°, 1 = 0~90° Setting the maximum value of the extreme density of the orientation group exhibiting 2=45° to 7.0 or less can suppress the deterioration of the end face characteristics during punching. Therefore, in order to further improve the punching processability, it is preferable to set the maximum value of the above-mentioned extreme density to 7.0 or less. The maximum value of the above-mentioned extreme density is more preferably 6.7 or less or 6.5 or less. The lower limit of the maximum value of the above-mentioned extreme density is not particularly limited, and can be set to 5.5 or more or 6.0 or more.

[0125] The maximum value of the above-mentioned extreme density was determined by the following method.

[0126] Samples are collected in a manner that allows observation of the microstructure of a cross-section (thickness direction × rolling direction) with the width direction of the hot-rolled steel sheet as the normal direction. The sample size also depends on the measuring apparatus; for example, a cuboid with the full thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction can be used. Next, the observation surface of the sample is mirror-polished and then polished for 8 minutes at room temperature using colloidal silica without an alkaline solution to remove strain introduced into the observation surface of the sample. Measurements are taken at 5.0 μm intervals from the surface of the polished sample (the surface of the steel sheet) to a depth of 500 μm in the thickness direction and at any position in the rolling direction exceeding 2000 μm.

[0127] In the measurements, an EBSD apparatus combining a scanning electron microscope and an EBSD analysis device, along with an OIM Analysis (registered trademark) manufactured by TSL Corporation, was used. The samples were analyzed using EBSD (Electron Back Scattering Diffraction). Based on the obtained data, the crystal orientation distribution function (ODF) was calculated. From the obtained crystal orientation distribution function, the values ​​of Φ=0~90° were calculated using Euler angles. 1 = 0~90° The maximum value of the polar density of the orientation group represented by 2=45°.

[0128] Tensile strength (TS): 780 MPa or higher The hot-rolled steel sheet of this embodiment has a tensile strength of 780 MPa or higher. If the tensile strength is lower than 780 MPa, the effect of vehicle body lightweighting is small, and it cannot be properly applied to the running gear of automobiles. A tensile strength of 800 MPa or higher, or 820 MPa or higher, is more preferable.

[0129] There is no special limit to the upper limit of tensile strength; it can be set to below 1000MPa or below 900MPa.

[0130] Total elongation (E1): 14% or more Hole expansion ratio (λ): 40% or more The total elongation can be set to 14% or higher, and the hole expansion rate can be set to 40% or higher. If the total elongation is 14% or higher and the hole expansion rate is 40% or higher, then the stamping processability is considered excellent.

[0131] It should be noted that the total elongation refers to the "total elongation at fracture" as stated in JIS Z 2241:2022.

[0132] Alternatively, the yield point (YP) can be set to 700 MPa or higher.

[0133] Tensile strength, total elongation, and yield point were obtained by collecting test piece No. 5 according to JIS Z 2241:2022 and conducting tensile tests according to JIS Z 2241:2022. In the tensile test piece, the direction perpendicular to the rolling direction was taken as the length direction.

[0134] In this embodiment, unless the rolling direction is known in advance, the rolling direction is determined by the following method.

[0135] Test pieces were collected from any position at least 50 mm from the end of the hot-rolled steel sheet, allowing for observation of the sheet thickness section. After mirror polishing of the collected test pieces' sheet thickness section, observations were performed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x. An appropriate magnification was selected based on the size of the inclusions to determine their dimensions. The observation range was set to a width of at least 500 μm and the full thickness of the sheet, with darker areas identified as inclusions. Multiple fields of view could also be used during observation. Next, using the sheet thickness section initially observed as a reference, cross-sectional observations were performed on surfaces parallel to planes rotated every 5° within a range of 0° to 180° about the sheet thickness direction, using the same method. The average length of the major axes of multiple inclusions in each cross-section was calculated. The cross-section with the largest average length of the major axes of the inclusions was determined. The direction parallel to the major axis of the inclusions in this cross-section was identified as the rolling direction.

[0136] The hole expansion rate was determined by a hole expansion test according to JIS Z 2256:2020. More specifically, firstly, a hole was punched in the center of the test piece using a 10 mm diameter punch, and then a 60° conical punch was pressed into the hole. The 60° conical punch was pressed in until a crack generated at the edge of the hole in the test piece penetrated through the thickness direction. The hole expansion rate was obtained by calculating the ratio of the increase in hole diameter when the crack penetrated through the thickness direction to the initial hole diameter (10 mm diameter punch).

[0137] punching processability The hot-rolled steel sheet of this embodiment preferably exhibits excellent punching processability as evaluated below.

[0138] For test pieces collected from hot-rolled steel plates, 12mm was used. The punch was used to perform three punching operations with a 20% clearance. The length of cracks (stripping) generated in a direction perpendicular to the sheet thickness was measured along the entire circumference of each punch. For the three punches, the total length of cracks exceeding 2 mm was calculated (total crack length). If the value of this total divided by the total circumference length of the three punches (113 mm) (total crack length / total circumference length × 100) is less than 20%, the punching processability is considered excellent.

[0139] The thickness of the hot-rolled steel sheet in this embodiment is not particularly limited, but it can be set to 2.0 to 8.0 mm. By making the thickness of the hot-rolled steel sheet 2.0 mm or more, it is possible to suppress the excessive rolling load and the difficulty of hot rolling.

[0140] Furthermore, by making the plate thickness less than 8.0 mm, the aforementioned metal structure can be stably obtained after hot rolling.

[0141] To improve corrosion resistance, the hot-rolled steel sheet of this embodiment can be surface-treated by having a coating on its surface. The coating can be an electroplated coating or a hot-dip galvanized coating. Examples of electroplated coatings include electroplating with zinc and electroplating with a Zn-Ni alloy. Examples of hot-dip galvanized coatings include hot-dip galvanizing, alloyed hot-dip galvanizing, hot-dip aluminizing, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. The amount of coating is not particularly limited and can be the same as in conventional methods.

[0142] Alternatively, appropriate chemical conversion treatments (e.g., coating and drying of silicate-based chromium-free chemical conversion solutions) can be performed after plating to further improve corrosion resistance.

[0143] Next, a preferred manufacturing method for the hot-rolled steel sheet of this embodiment will be described. According to the manufacturing method described below, the hot-rolled steel sheet of this embodiment can be manufactured stably. Furthermore, in this embodiment, the temperatures of the slab and the steel sheet refer to the surface temperatures of the slab and the steel sheet, respectively.

[0144] In the preferred manufacturing method of the hot-rolled steel sheet of this embodiment, The slab with the above chemical composition is heated to a temperature range of 1230°C or higher. Hot rolling is performed with the finishing temperature set in the range of 880~980℃. Cooled to a temperature range of 650~750℃ at an average cooling rate of over 50℃ / s. After the above cooling process, air cooling is performed for 3.0 to 10.0 seconds within the aforementioned temperature range of 650 to 750°C. After air cooling, the temperature is reduced to the range of 470℃ to 530℃ at an average cooling rate of 50℃ / s or higher. After cooling, the product is wound up in a temperature range of 470℃ to 530℃.

[0145] The following is a description of each process.

[0146] To achieve solid solution treatment of the coarse TiC precipitated during the casting process, it is preferable to heat the slab to a temperature range above 1230°C. If the heating temperature is below 1230°C, the amount of coarse TiC increases, and a sufficient amount of fine TiC is not obtained, which may reduce the strength of the hot-rolled steel sheet.

[0147] Furthermore, there are no particular limitations regarding the heated slab, except that it must have the aforementioned chemical composition. For example, slabs manufactured by continuously casting can be produced using steel with the aforementioned chemical composition smelted in a converter or electric furnace. Alternatively, ingot casting or thin slab casting can be used instead of continuous casting.

[0148] To obtain the desired average grain size and ferrite area ratio, and thus the desired strength, the finishing rolling end temperature (the exit temperature of the final section of finishing rolling) is preferably set in the temperature range of 880~980℃. When the finishing rolling end temperature is below 880℃, the ferrite formation capacity increases excessively, resulting in the formation of coarse ferrite in the high-temperature region. Consequently, the desired average grain size cannot be obtained, and sometimes the strength of the hot-rolled steel plate decreases.

[0149] On the other hand, when the finishing temperature exceeds 980°C, the ability to form ferrite decreases, and the formation of ferrite during air cooling is suppressed, which sometimes reduces the strength of hot-rolled steel plates.

[0150] In the metal microstructure at a depth of 500 μm from the surface, to preferably control the maximum value of the extreme density, it is preferable to strictly control the total reduction under true strain in the temperature range above 930°C during finishing rolling, the starting temperature, the ending temperature, and the temperature range above 930°C, as well as the total reduction under true strain throughout the entire finishing rolling section. More specifically, it is preferable to set the starting temperature (entry temperature of the first section) in the finishing rolling range to a temperature range above 1000°C, the ending temperature (exit temperature of the final section) to a temperature range above 880°C, the total reduction under true strain in the temperature range above 930°C to 1.5 or more, and the total reduction under true strain throughout the entire finishing rolling section to 3.0 or less.

[0151] The reduction rate ε under true strain is expressed by the following formula. The total reduction rate when multiple rolling stages are performed can be obtained by summing the ε of each stage.

[0152] ε = |ln(h1 / h2)| In the above formula, ln is the natural logarithm with base e, h1 is the plate thickness before rolling (mm), and h2 is the plate thickness after rolling (mm).

[0153] After finishing rolling, it is preferable to cool to a temperature range of 650~750℃ at an average cooling rate of 50℃ / s or higher. If the average cooling rate in the above temperature range is lower than 50℃ / s, coarse ferrite will be formed during cooling, and the desired average grain size will not be obtained, which may reduce the strength of the hot-rolled steel plate.

[0154] Furthermore, in this embodiment, the average cooling rate refers to the value obtained by dividing the temperature difference between the start and end points of the set range by the elapsed time from the start to the end point.

[0155] After the above cooling, air cooling for 3.0 to 10.0 seconds is preferably performed in the temperature range of 650 to 750°C. Air cooling under these conditions allows for the formation of the desired amount of ferrite. When the air cooling temperature is below 650°C, a large amount of pearlite is formed, sometimes resulting in a decrease in the strength of the hot-rolled steel sheet.

[0156] On the other hand, when the air cooling temperature exceeds 750°C, the ferrite formation rate slows down, and the desired amount of ferrite cannot be obtained, sometimes resulting in a decrease in the strength of the hot-rolled steel plate.

[0157] In addition, in this embodiment, air cooling refers to cooling with an average cooling rate of less than 10°C / s.

[0158] When the air cooling time is less than 3.0 seconds, ferrite cannot be fully generated, and sometimes the strength of hot-rolled steel plates decreases.

[0159] On the other hand, when the air cooling time exceeds 10.0 seconds, excessive ferrite is generated, and sometimes the desired amount of bainite cannot be obtained.

[0160] After air cooling, it is preferable to cool to a temperature range of 470~530℃ at an average cooling rate of 50℃ / s or higher. When the average cooling rate after air cooling is lower than 50℃ / s, a large amount of pearlite may be generated during cooling, resulting in a decrease in the strength of the hot-rolled steel plate.

[0161] To keep the BH value below 100MPa, the average cooling rate in the temperature range of 470~530℃ after air cooling is preferably set to below 85℃ / s.

[0162] To control the phase transformation and precipitate state after winding and obtain the desired stamping and blanking workability, the winding temperature is preferably set in the range of 470~530℃. When the winding temperature is below 470℃, a large amount of MA is generated after winding, and sometimes the desired stamping workability cannot be obtained.

[0163] On the other hand, when the coiling temperature exceeds 530°C, the carbides coarsen, thereby reducing the amount of dissolved carbon in the steel and the amount of BH, which sometimes makes it impossible to obtain the desired punching processability.

[0164] Example Next, the effects of one aspect of the present invention will be described more specifically through embodiments. However, the conditions in the embodiments are merely examples used to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to these single examples. Various conditions can be employed to achieve the purpose of the present invention without departing from its spirit.

[0165] By performing converter smelting and continuous casting, slabs with the chemical compositions shown in Tables 1A to 2B are obtained. Based on the conditions shown in Tables 3A and 3B, hot-rolled steel sheets with a thickness of 2.6–3.2 mm are obtained from the obtained slabs.

[0166] In addition, the “total reduction rate above 930°C” in Tables 3A and 3B is the total reduction rate of the temperature range above 930°C expressed in true strain, while the “total reduction rate” is the total reduction rate of the entire finishing rolling section expressed in true strain.

[0167] For the obtained hot-rolled steel sheet, the microstructure, BH content, maximum value of extreme density, tensile strength, total elongation, yield point, expansion ratio, and punching workability are evaluated using the methods described above. Furthermore, in this embodiment, since the rolling direction is determined beforehand, the aforementioned determination of the rolling direction is not performed.

[0168] The results are shown in Tables 4A and 4B.

[0169] Additionally, the "maximum value of extreme density" in Tables 4A and 4B represents the value of the maximum density in the metallic microstructure at a depth of 500 μm from the surface, expressed in Euler angles from Φ=0 to 90°. 1 = 0~90° The maximum value of the polar density of the orientation group represented by 2=45°.

[0170] A tensile strength of 780 MPa or higher is considered high strength and thus deemed acceptable. Conversely, a tensile strength below 780 MPa is considered insufficient strength and thus deemed unacceptable.

[0171] If the total elongation is 14% or more and the hole expansion rate is 40% or more, it is deemed to have excellent stamping workability and is therefore considered acceptable. On the other hand, if either condition is not met, it is deemed to have poor stamping workability and is therefore considered unacceptable.

[0172] In the above evaluation of workability, the evaluation is based on the value of (total crack length / total circumference length) × 100, according to the following criteria. If the evaluation is Fair or higher (i.e., the above value is less than 20%), it is considered to have excellent workability and is deemed acceptable. If the evaluation is Poor, it is considered to have poor workability and is deemed unacceptable.

[0173] Less than 1%: Excellent 1% or more but less than 10%: Good 10% or more but less than 20%: Fair 20% or more: Poor Nos. 1 to 20 and 47 to 49 in the table are examples of inventions, which are hot-rolled steel sheets with high strength and excellent stamping and blanking properties.

[0174] On the other hand, Nos. 21-46 and 50 are comparative examples, showing that there are differences in more than one characteristic.

[0175] No. 21 has lower strength due to its low C content.

[0176] No. 22 has a high carbon content, resulting in a lower total elongation and deteriorated stamping processability.

[0177] No. 23 has lower strength due to its low Si content.

[0178] No. 24 has reduced strength due to its low Mn content.

[0179] No. 25 has a high Mn content, resulting in a lower total elongation and deteriorated stamping processability.

[0180] No. 26 has a high phosphorus content, resulting in a lower total elongation and deteriorated stamping processability.

[0181] No. 27 has a high sulfur content, which reduces the hole expansion rate and deteriorates the stamping processability.

[0182] No. 28 has a high Al content, resulting in a lower total elongation and deteriorated stamping processability.

[0183] No. 29 has reduced strength due to its low Ti content.

[0184] Due to its high Ti content, No. 30 has a lower total elongation, resulting in poorer stamping and blanking workability.

[0185] No. 31 has reduced strength due to its low Nb content.

[0186] No. 32 has a high Nb content, resulting in a lower total elongation and deteriorated stamping processability.

[0187] No. 33 has a high N content, which reduces its strength and forms coarse TiN particles, resulting in poor workability during punching.

[0188] No. 34 has a low A value (calculated amount of carbon not released), resulting in a reduced BH content and deteriorated punching processability.

[0189] Due to its high M value, No. 35 has insufficient bainite volume, resulting in reduced hole expansion rate and deteriorated stamping processability.

[0190] No. 36 has reduced strength due to low heating temperature.

[0191] No. 37 Due to the low finishing temperature, coarse ferrite is formed, resulting in reduced strength.

[0192] No. 38 Due to the high finishing temperature, the amount of ferrite is reduced, resulting in lower strength.

[0193] No. 39 Due to the slow average cooling rate before air cooling, coarse ferrite is formed, resulting in reduced strength.

[0194] No. 40 has a lower initial temperature due to air cooling, resulting in a higher pearlite volume and a lower intensity.

[0195] No. 41 has a high initial air-cooling temperature, resulting in less ferrite and reduced strength.

[0196] No. 42 has a shorter air cooling time, resulting in less ferrite and reduced strength.

[0197] No. 43 Due to the long air cooling time, the bainite volume decreases, the hole expansion rate decreases, and the stamping processability deteriorates.

[0198] No. 44 Due to the slow average cooling rate after air cooling, the pearlite volume increases and the strength decreases.

[0199] No. 45 Due to the low winding temperature, the MA amount increases, the hole expansion rate decreases, and the stamping processability deteriorates.

[0200] No. 46 Due to the high winding temperature, the BH amount is reduced, and the punching processability deteriorates.

[0201] No. 50 has a high boron content, resulting in less ferrite and reduced strength.

[0202] Industrial availability According to the above-described solution of the present invention, it is possible to provide a hot-rolled steel sheet with high strength and excellent stamping and blanking properties.

Claims

1. A hot-rolled steel sheet characterized by, its chemical composition is, in mass %: C：0.04~0.10%、 Si: 0.11 to 0.30 %, Mn: 1.40 to 2.50 %, P: 0.05 % or less, S: 0.05 % or less, N: 0.010 % or less, O: 0.010 % or less, Al：0.001~0.050%、 Ti: 0.060 to 0.150 %, Nb: 0.020 to 0.100 %, V：0~0.200%、 Mo: 0 to 1.000 %, Ca: 0 to 0.0100 %, REM: 0 to 0.0100 %, Cu: 0 to 2.00 %, Ni: 0 to 1.50 %, Cr：0~2.00%、 Mg: 0 to 0.0200 %, Bi: 0 to 0.0200 %, Zr：0~0.500%、 Co: 0 to 1.000 %, Zn: 0 to 0.200 %, W：0~0.200%、 Sb: 0 to 0.200 %, As: 0 to 0.050 %, Sn: 0 to 0.050 %, B: less than 0.0005 %, and the remainder: Fe and impurities, an A value represented by the following (1) formula is 0.0150 mass % or more, an M value represented by the following (2) formula is 930 or less, a metal structure at a position 1 / 4 of the plate thickness from the surface contains, in area %: ferrite: 50 % or more and less than 80 %, bainite: more than 20 % and 50 % or less, pearlite: 0 to 5 %, MA: 0 to 2 %, an average crystal grain diameter is 10 μm or less, for the hot-rolled steel sheet, a BH amount is 45 MPa or more, a tensile strength is 780 MPa or more, A value = C - Ti / 4 + N / 1.17 - Nb / 7.75 - V / 4.25 (1) M value = 937 - 436.5 x C + 56 x Si - 19.7 x Mn - 16.3 x Cu - 26.6 x Ni - 4.9 x Cr + 38.1 x Mo + 124.8 x V + 136.3 x Ti - 19.1 x Nb + 198.4 x Al (2) wherein each of the element symbols in the formulas (1) and (2) represents the content of the element in mass %, and is not included in 0.

2. Hot-rolled steel sheet according to claim 1, characterized in that, the chemical composition contains, in mass %, V：0.001~0.200%、 Mo: 0.001 to 1.000 %, Ca: 0.0001 to 0.0100 %, REM: 0.0001 to 0.0100 %, Cu: 0.01 to 2.00 %, Ni: 0.01 to 1.50 %, Cr：0.01~2.00%、 Mg: 0.0001 to 0.0200 %, Bi: 0.001 to 0.0200 %, Zr：0.001%~0.500%、 Co: 0.001 to 1.000 %, Zn: 0.001 to 0.200 %, W：0.001~0.200%、 Sb: 0.001 to 0.200 %, As: 0.001 to 0.050 %, and Sn: 0.001 to 0.050 %.

3. Hot-rolled steel sheet according to claim 1 or 2, characterized in that, The maximum value of the pole density of the orientation group expressed by Φ = 0 ~ 90°, 1 = 0 ~ 90°, 2 = 45° in the metal structure of the surface at a depth of 500 μm from the surface is 7.0 or less.

4. The hot-rolled steel sheet according to any one of claims 1 to 3, characterized by, the chemical composition is, in mass %: B: 0.0002 % or less.

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