hot-rolled steel sheets

Abstract

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

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet. This application claims priority based on Japanese Patent Application No. 2023-201058, filed on November 28, 2023, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, in order to reduce greenhouse gas emissions, automobile manufacturers have been working to reduce the weight of their vehicles by reducing the number of parts and thinning the materials used. At the same time, automobile bodies are also required to have crashworthiness, so the use of high-strength steel sheets in automobile parts is accelerating.

[0003] High-strength hot-rolled steel sheets used in automobiles and other applications are required to have excellent press formability. To improve the press formability of high-strength hot-rolled steel sheets, for example, to achieve both high strength and formability, a known technique is to use a metal structure containing 80% or more ferrite, with the remainder consisting of bainite and pearlite, and to precipitate fine carbides within the ferrite grains, thereby achieving both high strength and ductility. In this technique, Ti, which has a low unit price for strength, is mainly used.

[0004] Although adding Ti can improve press workability, when high-strength hot-rolled steel sheets containing Ti are cut into blank shapes (punching), defects such as peeling into two layers at the sheared edge are likely to occur. This peeling can sometimes progress to cracks perpendicular to the thickness direction. This peeling at the edge that occurs during punching is called "peeling." In high-strength hot-rolled steel sheets, it is necessary to prevent peeling that occurs during punching.

[0005] For example, Patent Document 1 discloses a technique for strengthening grain boundaries and improving punching workability by leaving solute carbon in a steel sheet.

[0006] Patent Document 2 clearly discloses a method of adding B to segregate B at grain boundaries to strengthen the grain boundaries and improve punching workability. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2003-342684 [Patent Document 2] Japanese Patent Publication No. 2004-315857 Summary of the Invention [Problem to be solved by the invention]

[0008] However, even in the steel sheet in which solute carbon is retained as described in Patent Document 1, there is room for improvement in terms of suppressing minute peeling during punching. In a processing process in which the sheared edge surface is subjected to plastic strain after punching, the minute peeling can become the starting point of cracks.

[0009] Furthermore, although adding B as in the technology described in Patent Document 2 increases the grain boundary strength, there is room for improvement in terms of controlling the metal structure and surface properties of the steel sheet in association with improved hardenability.

[0010] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent press workability and punching workability. [Means for solving the problem]

[0011] The present inventors have found that by increasing the amount of uncalculated precipitated carbon, which is obtained by calculation from the C content, Ti content, N content, Nb content, and V content in the chemical composition, the amount of dissolved carbon can be increased, and the occurrence of minute peeling during punching can be suppressed.

[0012] Regarding the amount of solute carbon, Patent Document 1 describes that if the amount of free carbon (C-Ti / 4+N / 1.17) exceeds 0.015%, the strengthening of ferrite becomes insufficient, and the increase in the carbon content in bainite increases the difference in hardness with bainite, resulting in deterioration of press workability. Patent Document 1 also describes that if the area fraction of ferrite is less than 80%, ductility decreases. Therefore, the present inventors have discovered that press workability can be improved by increasing the area fraction of bainite in the metal structure, suppressing the increase in the carbon content in bainite, and suppressing the increase in hardness.

[0013] The gist of the present invention, which was made based on the above findings, is as follows. [1] Chemical composition, in mass%, C: 0.04 to 0.10%, Si: 0.11 to 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 to 0.050%, Ti: 0.060~0.150%, Nb: 0.020~0.100%, V: 0 to 0.200%, Mo: 0 to 1.000%, Ca: 0 to 0.0100%, REM: 0 to 0.0100%, Cu: 0-2.00% Ni: 0-1.50% Cr: 0~2.00%, Mg: 0 to 0.0200%, Bi: 0 to 0.0200%, Zr: 0 to 0.500%, Co: 0 to 1.000%, Zn: 0 to 0.200%, W: 0 to 0.200%, Sb: 0 to 0.200%, As: 0~0.050%, Sn: 0 to 0.050% B: Less than 0.0005%, and The balance is Fe and impurities. The A value represented by the following formula (1) is 0.0150 mass% or more, The M value represented by the following formula (2) is 930 or less, The metal structure at 1 / 4 of the plate thickness from the surface is, in area%, Ferrite: 50% or more, less than 80% Bainite: over 20% and not more than 50% Perlite: 0-5% MA: 0-2% The average crystal grain size is 10 μm or less, The BH amount is 45 MPa or more, A hot-rolled steel sheet characterized by having a tensile strength of 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) However, each element symbol in the formulas (1) and (2) indicates the content of the element in mass %, and 0 is substituted when the element is not contained. [2] The chemical composition is, 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 to 2.00%, Mg: 0.0001 to 0.0200%, Bi: 0.001 to 0.0200%, Zr: 0.001 to 0.500%, Co: 0.001 to 1.000%, Zn: 0.001 to 0.200%, W: 0.001 to 0.200%, Sb: 0.001 to 0.200%, As: 0.001 to 0.050%, and The hot-rolled steel sheet according to the above [1], characterized in that it contains at least one selected from the group consisting of Sn: 0.001 to 0.050%. [3] The hot-rolled steel sheet according to [1] or [2] above, characterized in that in the metal structure from the surface to a depth of 500 μm from the surface, the maximum value of the pole density of the orientation group expressed by Euler angles Φ=0 to 90°, φ1=0 to 90°, and φ2=45° is 7.0 or less. [4] The hot-rolled steel sheet according to any one of the above [1] to [3], characterized in that the chemical composition contains, in mass %, B: 0.0002% or less. [Effects of the Invention]

[0014] According to the above-described aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent press workability and punching workability. DETAILED DESCRIPTION OF THE INVENTION

[0015] A hot-rolled steel sheet according to one embodiment of the present invention (hereinafter, sometimes referred to as the hot-rolled steel sheet according to this embodiment) will be described below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the present disclosure.

[0016] The individual constituent elements of the present invention will be described in detail below. First, the reasons for limiting the chemical composition of the hot-rolled steel sheet according to this embodiment will be described. Below, numerical ranges indicated with "to" include the lower and upper limits. Numerical values indicated as "less than" or "greater than" are not included in the numerical range. In the following description, percentages relating to chemical compositions are mass % unless otherwise specified.

[0017] The hot-rolled steel sheet according to this embodiment has a chemical composition, in mass %, of C: 0.04 to 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 to 0.050%, Ti: 0.060 to 0.150%, Nb: 0.020 to 0.100%, V: 0 to 0.200%, Mo: 0 to 1.000%, and Ca: 0 to 0.010%. 0%, 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 balance: Fe and impurities. Each element will be described in detail below.

[0018] C: 0.04 to 0.10% C is an element necessary for suppressing peeling at the end surfaces formed by shearing or punching by being present at the grain boundaries as solid solution C, and for precipitating as carbides to obtain the strength of the hot-rolled steel sheet. If the C content is less than 0.04%, it becomes difficult to obtain punching workability and the desired 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. On the other hand, if the C content exceeds 0.10%, the ductility decreases, which deteriorates the press workability of the hot-rolled steel sheet. Therefore, the C content is set to 0.10% or less. The C content is preferably 0.09% or less, more preferably 0.08% or less.

[0019] Si: 0.11 to 0.30% Si is an element effective as a deoxidizer and as a solid-solution strengthening element, which is effective in increasing strength. If the Si content is less than 0.11%, it becomes difficult to obtain the desired strength in the hot-rolled steel sheet. Therefore, the Si content is set to 0.11% or more. The Si content is preferably 0.15% or more. On the other hand, if the Si content exceeds 0.30%, striped Si scales are generated on the steel sheet surface, impairing the surface properties. Furthermore, the chemical conversion treatability of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 0.30% or less. The Si content is preferably 0.25% or less.

[0020] Mn: 1.40 to 2.50% Mn is an element that is effective in improving strength as a solid solution strengthening element. If the Mn content is less than 1.40%, it becomes difficult to obtain the desired strength in the hot-rolled steel sheet. Therefore, the Mn content is set to 1.40% or more. The Mn content is preferably set to 1.60% or more, and more preferably 1.80% or more. On the other hand, if the Mn content exceeds 2.50%, the ductility decreases, which deteriorates the press workability of the hot-rolled steel sheet. Therefore, the Mn content is set to 2.50% or less. The Mn content is preferably 2.30% or less, and more preferably 2.00% or less.

[0021] P:0.05% or less P is an element that segregates at grain boundaries and reduces ductility, thereby deteriorating the press workability of hot-rolled steel sheets. This effect becomes significant when the P content exceeds 0.05%, so the P content is set to 0.05% or less. Since the lower the P content, the better, it may be 0%. However, from the viewpoint of reducing refining costs, the P content may be 0.001% or more.

[0022] S: 0.05% or less S is an element that forms MnS, which acts as a fracture initiation point and deteriorates the hole expandability, thereby deteriorating the press workability of hot-rolled steel sheets. This effect becomes significant when the S content exceeds 0.05%, so the S content is set to 0.05% or less. Since the lower the S content, the better, it may be 0%. However, from the viewpoint of reducing refining costs, the S content may be 0.001% or more.

[0023] N: 0.010% or less N is an element that forms nitrides with Ti, reducing the amount of Ti that can bond with C, thereby reducing the strength of hot-rolled steel sheets. N also forms coarse TiN, which deteriorates punching workability. These effects become significant when the N content exceeds 0.010%, so the N content is set to 0.010% or less. Since the lower the N content, the better, it may be 0%. However, from the viewpoint of reducing refining costs, the N content may be 0.001% or more.

[0024] O: 0.010% or less O is an element that, when contained in large amounts in steel, forms coarse oxides that become the starting point of fracture, deteriorating press workability. This effect becomes significant when the O content exceeds 0.010%, so the O content is set to 0.010% or less. The O content is preferably 0.008% or less, or 0.006% or less. Since the lower the O content, the better, it may be 0%. However, from the viewpoint of reducing refining costs, the O content may be 0.001% or more.

[0025] Al: 0.001 to 0.050% Al is an element effective as a deoxidizer. If the Al content is less than 0.001%, the molten steel cannot be sufficiently deoxidized. Therefore, the Al content is set to 0.001% or more. The Al content is preferably set to 0.010% or more, and more preferably 0.020% or more. On the other hand, if the Al content exceeds 0.050%, the amount of nonmetallic inclusions increases, which reduces ductility and deteriorates press workability. Therefore, the Al content is set to 0.050% or less. The Al content is preferably 0.040% or less.

[0026] Ti: 0.060 to 0.150% Ti is an effective element for refining crystal grains and precipitating fine TiC to obtain strength in hot-rolled steel sheets. If the Ti content is less than 0.060%, it becomes difficult to obtain the desired strength. Therefore, the Ti content is set to 0.060% or more. The Ti content is preferably set to 0.080% or more, and more preferably 0.100% or more. On the other hand, if the Ti content exceeds 0.150%, a large amount of TiC is generated, reducing ductility and deteriorating press workability of the hot-rolled steel sheet. Furthermore, a large amount of TiN is generated, reducing punching workability. Therefore, the Ti content is set to 0.150% or less. The Ti content is preferably 0.145% or less, 0.140% or less, or 0.135% or less.

[0027] Nb: 0.020 to 0.100% Nb, like Ti, refines crystal grains and is an effective element for precipitating fine NbC to obtain strength. If the Nb content is less than 0.020%, it becomes difficult to obtain the desired strength in the hot-rolled steel sheet. Therefore, the Nb content is set to 0.020% or more. The Nb content is preferably 0.030% or more, 0.050% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of NbC is generated, which reduces ductility and deteriorates the press workability of the hot-rolled steel sheet. Therefore, the Nb content is set to 0.100% or less. The Nb content is preferably 0.090% or less, or 0.080% or less.

[0028] V: 0 to 0.200% V is an element that is effective in increasing the strength of hot-rolled steel sheets by precipitating fine VC. The V content may be 0%, but to fully obtain this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the V content exceeds 0.200%, a large amount of VC may be generated, resulting in a reduced amount of solute carbon. 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.

[0029] Mo: 0 to 1.000% Mo is an element that is effective in increasing the strength of hot-rolled steel sheets by improving the hardenability of steel. The Mo content may be 0%, but to fully obtain this effect, the Mo content is preferably 0.001% or more. The Mo content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Mo content exceeds 1.000%, the above effects saturate, 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.

[0030] Ca: 0 to 0.0100% Ca is an element that is effective in improving press workability by controlling the morphology of nonmetallic inclusions, which act as fracture initiation points and cause deterioration of workability. Although the Ca content may be 0%, to fully obtain this effect, the Ca content is preferably 0.0001% or more. The Ca content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, if the Ca content exceeds 0.0100%, the above effects will saturate and it 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.

[0031] REM: 0 to 0.0100% Like Ca, REM is an element that is effective in improving press workability by controlling the morphology of nonmetallic inclusions that act as fracture initiation points and cause workability to deteriorate. The REM content may be 0%, but to fully obtain this effect, the REM content is preferably 0.0001% or more. The REM content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, if the REM content exceeds 0.0100%, the above effects will saturate and it 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. REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of misch metal.

[0032] Cu: 0 to 2.00% Cu is an element that is effective in increasing the hardenability of steel and thereby increasing the strength of hot-rolled steel sheets. The Cu content may be 0%, but to fully obtain this effect, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more or 0.10% or more. On the other hand, if the Cu content exceeds 2.00%, the above effects will saturate and it 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.

[0033] Ni: 0 to 1.50% Ni is an element that is effective in increasing the strength of hot-rolled steel sheets by improving hardenability. The Ni content may be 0%, but to fully obtain this effect, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.05% or more or 0.10% or more. On the other hand, if the Ni content exceeds 1.50%, the above effects saturate, 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.

[0034] Cr: 0 to 2.00% Cr is an element that is effective in increasing the strength of hot-rolled steel sheets by improving hardenability. The Cr content may be 0%, but to fully obtain this effect, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.05% or more or 0.10% or more. On the other hand, if the Cr content exceeds 2.00%, the above effects will saturate and it 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.

[0035] Mg: 0 to 0.0200% Mg is an element that controls the shape of inclusions in steel to a preferred shape, thereby increasing the yield ratio of hot-rolled steel sheets. Although the Mg content may be 0%, to fully obtain this effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, if the Mg content exceeds 0.0200%, excessive inclusions are formed in the steel, resulting in a decrease in 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.

[0036] Bi: 0 to 0.0200% Bi is an element that refines the solidification structure and thereby increases the yield ratio of hot-rolled steel sheets. The Bi content may be 0%, but to fully obtain this effect, the Bi content is preferably 0.001% or more. The Bi content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, if the Bi content exceeds 0.0200%, the above effects will saturate, which is economically undesirable. Therefore, the Bi content is set to 0.0200% or less. The Bi content is preferably 0.0150% or less or 0.0100% or less.

[0037] Zr: 0 to 0.500% Zr is an element that increases the strength of hot-rolled steel sheets through solid solution strengthening. The Zr content may be 0%, but to fully obtain this effect, the Zr content is preferably 0.001% or more. The Zr content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Zr content exceeds 0.500%, the hole expandability of the hot-rolled steel sheet deteriorates, resulting in deterioration of press 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.

[0038] Co: 0 to 1.000% Co is an element that increases the strength of hot-rolled steel sheets through solid solution strengthening. The Co content may be 0%, but to fully obtain this effect, the Co content is preferably 0.001% or more. The Co content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Co content exceeds 1.000%, the hole expandability of the hot-rolled steel sheet deteriorates, resulting in deterioration of press workability. 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.

[0039] Zn: 0 to 0.200% Zn is an element that increases the strength of hot-rolled steel sheets through solid solution strengthening. The Zn content may be 0%, but to fully obtain this effect, the Zn content is preferably 0.001% or more. The Zn content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Zn content exceeds 0.200%, the hole expandability of the hot-rolled steel sheet deteriorates, resulting in deterioration of press workability. 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.

[0040] W: 0 to 0.200% W is an element that increases the strength of hot-rolled steel sheets through solid solution strengthening. The W content may be 0%, but to fully obtain this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the W content exceeds 0.200%, the hole expandability of the hot-rolled steel sheet deteriorates, resulting in deterioration of press workability. 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.

[0041] Sb: 0 to 0.200% Sb is an element that suppresses the generation of oxides that serve as fracture initiation sites, thereby improving the hole expandability of hot-rolled steel sheets. The Sb content may be 0%, but to fully obtain this effect, the Sb content is preferably 0.001% or more. The Sb content is more preferably 0.005% or more or 0.010% or more. On the other hand, even if Sb is contained in a large amount, the above effects are saturated, so the Sb content is set to 0.200% or less, and preferably 0.150% or less or 0.100% or less.

[0042] As: 0 to 0.050% As is an element that reduces the austenite single-phase temperature, thereby refining prior austenite grains and improving the hole expandability of hot-rolled steel sheets. The As content may be 0%, but to fully obtain this effect, the As content is preferably 0.001% or more. The As content is more preferably 0.005% or more or 0.010% or more. On the other hand, since the above effects are saturated even when As is contained in a large amount, the As content is set to 0.050% or less, and preferably 0.030% or less or 0.020% or less.

[0043] Sn: 0 to 0.050% Sn is an element that suppresses the generation of oxides that are the starting points for fracture, thereby improving the hole expandability of hot-rolled steel sheets. The Sn content may be 0%, but to fully obtain this effect, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.005% or more or 0.010% or more. On the other hand, even if Sn is contained in a large amount, the above effect saturates, so the Sn content is set to 0.050% or less, and preferably 0.030% or less or 0.020% or less.

[0044] B: Less than 0.0005% In the chemical composition of the hot-rolled steel sheet according to this embodiment, B is substantially not contained, and therefore the B content is set to less than 0.0005%. From the viewpoint of ensuring the strength of the hot-rolled steel sheet, the lower the B content, the better, and therefore the B content is preferably 0.0004% or less, less than 0.0004%, 0.0003% or less, 0.0002% or less, or less than 0.0002%.

[0045] A value: 0.0150% by mass or more In the chemical composition, if the A value (calculated unprecipitated carbon content) expressed by the following formula (1) is less than 0.0150 mass%, the amount of solute carbon will be insufficient, the BH amount will not be sufficiently increased, and the desired press workability will not be obtained in the hot-rolled steel sheet. Therefore, the A value is set to 0.0150 mass% or more. The A value is preferably set to 0.0200 mass% or more, 0.0250 mass% or more, or 0.0300 mass% or more. The upper limit of the A value is not particularly limited, but may be 1.0000 mass % or less, 0.0800 mass % or less, or 0.0750 mass % or less.

[0046] A value=C-Ti / 4+N / 1.17-Nb / 7.75-V / 4.25 (1) However, each element symbol in the formula (1) indicates the content of the element in mass %, and 0% is substituted when the element is not contained.

[0047] M value: 930 or less If the M value represented by the following formula (2) exceeds 930, the ferrite generating ability increases, making it impossible to obtain a desired amount of bainite. As a result, the hole expandability deteriorates, and the press workability of the hot-rolled steel sheet deteriorates. 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.

[0048] 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) However, each element symbol in the formula (2) indicates the content of the element in mass %, and 0% is substituted when the element is not contained. When the chemical composition of the hot-rolled steel sheet contains 0.0005% or more of B, it is preferable to use the following formula (2') instead of the above formula (2), which adds a term for B. When the B content is 0.0001% or more, 0.0002% or more, 0.0003% or more, or 0.0004% or more, the following formula (2') may also be used instead of the above formula (2), as necessary. 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 + 3315 x B (2') However, each element symbol in the formula (2) indicates the content of the element in mass %, and 0% is substituted when the element is not contained.

[0049] The hot-rolled steel sheet according to this embodiment may contain the above elements, with the balance being Fe and impurities. In this embodiment, the term "impurities" refers to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment, and / or substances that are allowed to exist within a range that does not adversely affect the properties (strength, press workability, and punching workability) of the hot-rolled steel sheet according to this embodiment.

[0050] The chemical composition of the hot-rolled steel sheet described above can be analyzed using a spark discharge optical emission spectrometer or the like. Values for C and S are determined by burning the steel sheet in an oxygen stream using a gas composition analyzer or the like and measuring by infrared absorption. Values for N are determined by melting a test piece taken from the steel sheet in a helium stream and measuring it by thermal conductivity. Values for O are determined by placing a test piece taken from the steel sheet in a graphite crucible, melting it in a helium stream, and measuring it by infrared absorption. When the hot-rolled steel sheet has a plating layer or a coating film on the surface, the plating layer or the coating film is removed by mechanical grinding or the like as necessary before analyzing the chemical composition.

[0051] Next, the metal structure of the hot-rolled steel sheet according to this embodiment will be described. The hot-rolled steel sheet according to this embodiment has a metal structure at a position from the surface to 1 / 4 of the sheet thickness, which is composed of, in area percentages, ferrite: 50% or more and less than 80%, bainite: more than 20% and 50% or less, pearlite: 0 to 5%, and MA: 0 to 2%, and has an average crystal grain size of 10 μm or less. Each requirement is explained below.

[0052] In this embodiment, the metal structure is defined at a position 1 / 4 of the plate thickness from the surface (a region from 1 / 8 of the plate thickness depth from the surface to 3 / 8 of the plate thickness depth from the surface). In other words, this position is a region starting from 1 / 8 of the plate thickness depth from the surface and ending at 3 / 8 of the plate thickness depth from the surface. The reason for specifying the metal structure at the above positions is that the metal structure at the above positions represents a typical metal structure of the steel plate. In addition, when the hot-rolled steel sheet has a plating layer, a coating film, or the like on the surface, the surface here refers to the interface between the steel sheet and the plating layer, the coating film, or the like.

[0053] Ferrite: 50% or more, less than 80% In the hot-rolled steel sheet according to this embodiment, fine TiC is precipitated within ferrite grains to obtain the desired strength. To achieve this effect, the area ratio of ferrite is set to 50% or more. The area ratio of ferrite is preferably 55% or more, 60% or more, or 65% or more. On the other hand, if the area ratio of ferrite is 80% or more, the area ratio of bainite decreases, which increases the carbon content in bainite, increases the hardness of bainite, deteriorates the hole expandability, and deteriorates the press workability of the hot-rolled steel sheet. Therefore, the area ratio of ferrite is set to less than 80%. The area ratio of ferrite is preferably 75% or less, and more preferably 70% or less.

[0054] Bainite: Over 20% and 50% or less If the area fraction of bainite is 20% or less, the carbon content in bainite increases, the hardness of bainite increases, and the hole expandability deteriorates, making it impossible to obtain the desired press workability in the hot-rolled steel sheet. Therefore, the area fraction of bainite is set to be more than 20%. The area fraction of bainite is preferably 25% or more, or 30% or more. In addition, in relation to the area ratio of ferrite, the area ratio of bainite is set to 50% or less, preferably 45% or less, 40% or less, or 35% or less.

[0055] Perlite: 0-5% Pearlite is a structure with poor strength. If the area ratio of pearlite exceeds 5%, the strength of the hot-rolled steel sheet decreases. Therefore, the area ratio of pearlite is set to 5% or less. The area ratio of pearlite is preferably 4% or less, 3% or less, 2% or less, or 1% or less. The smaller the area ratio of pearlite, the better, and therefore it is preferably 0%.

[0056] MA: 0-2% MA is a structure consisting of martensite and austenite. Because MA has high hardness, if the area ratio of MA exceeds 2%, the hole expandability deteriorates, and the press workability of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of MA is set to 2% or less. The area ratio of MA is preferably 1% or less. Since MA may not be included, the area ratio of MA may be 0%.

[0057] The area ratio of each structure is measured by the following method.

[0058] Ferrite area ratio measurement method A test piece is taken from the hot-rolled steel plate so that a cross section perpendicular to the plate width direction can be observed. The taken test piece is wet-polished to a mirror finish on the observation surface, and then the observation surface is polished for 8 minutes at room temperature using colloidal silica that does not contain alkaline solution to remove strain introduced into the observation surface. Next, the structure is observed using a SEM-EBSD device at a position 1 / 4 of the plate thickness from the surface (the region from 1 / 8 of the plate thickness depth from the surface to 3 / 8 of the plate thickness depth from the surface). The SEM-EBSD device used is a device consisting of a field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 type detector). At this time, the vacuum level inside the device is 9.6 x 10 -5 Pa or less, acceleration voltage is 15 kV, probe current level is 13, and electron beam irradiation level is 62. The observation area is 500 μm x 500 μm, with a step size of 0.3 μm, and crystal orientation information is obtained. At least three observation areas are used. The obtained crystal orientation information and OIM data analysis are used to determine the crystal orientation misorientation with adjacent measurement points, and areas surrounded by boundaries with a crystal orientation misorientation of 15° or more are considered crystal grains. Next, the average value of the misorientation with adjacent measurement points (Grain Average Misorientation, GAM) is calculated for each crystal grain. Crystal grains with a GAM of 1.0° or less are defined as ferrite, and the area ratio of ferrite is obtained by calculating the area ratio of these crystal grains. The ferrite defined by this method does not include ferrite within pearlite.

[0059] Method for measuring the area ratio of pearlite A test piece is collected in the same manner as described above, and the observation surface is etched with nital to reveal the metal structure. A microstructure photograph is then taken using an optical microscope. The photograph is taken from a position 1 / 4 of the plate thickness from the surface (a region from 1 / 8 of the plate thickness depth from the surface to 3 / 8 of the plate thickness depth from the surface), and the photographed area is a 500 μm x 500 μm region, with at least three regions. Image analysis is performed on the obtained microstructure photograph, and the black etched crystal grains are defined as pearlite. The area ratio of these crystal grains is calculated to obtain the pearlite area ratio. It should be noted that the pearlite that is etched black by this method does not include ferrite as defined using the SEM-EBSD device described above.

[0060] Measurement method for the area ratio of MA A test piece is taken in the same manner as described above, and the observation surface is subjected to REPELLA etching to reveal the metal structure, and a structural photograph is taken using an optical microscope. The photographing position and area are the same as in the above method. Image analysis is performed on the obtained structural photograph, and the white-etched crystal grains are defined as MA. The area ratio of these crystal grains is calculated to obtain the MA area ratio.

[0061] Method for measuring the area fraction of bainite Bainite is obtained by subtracting the area fractions of ferrite, pearlite, and MA from 100%.

[0062] Average grain size: 10μm or less In this embodiment, the desired tensile strength is obtained by utilizing not only precipitation strengthening due to fine precipitation of TiC but also high strength due to refined crystal grains. If the average crystal grain size exceeds 10 μm, the amount of strengthening due to refined crystal grains 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. There is no particular lower limit to the average crystal grain size, but it may be 2 μm or more or 3 μm or more.

[0063] The average crystal grain size of the metal structure is measured by the following method. Analysis is performed using crystal orientation information obtained when the ferrite area ratio is measured. Regions surrounded by boundaries with a crystal orientation difference of 15° or more are defined as crystal grains, and the circle-equivalent diameter of the crystal grains is calculated. The circle-equivalent diameters of all crystal grains in the observation region are calculated, and the average of these is calculated to obtain the average crystal grain size. The equivalent circle diameter is the diameter of a circle having the same area as the area of the identified crystal grain.

[0064] BH amount: 45MPa or more In this embodiment, carbon not only forms precipitates necessary to obtain the desired strength, but also remains in solid solution in the steel, thereby improving punching workability. The inventors have found that there is a correlation between the amount of solute carbon and the bake hardening amount (BH amount), and that if the BH amount is less than 45 MPa, the amount of solute carbon becomes small and the desired punching workability cannot be obtained. Therefore, the BH amount is set to 45 MPa or more. The BH amount is preferably 55 MPa or more, 60 MPa or more, or 70 MPa or more. The hot-rolled steel sheet according to this embodiment is a precipitation-strengthened steel sheet having a tensile strength of 780 MPa or more. In this embodiment, the strength is achieved by dispersing Ti-based carbides in the steel. An increase in the amount of solute carbon in the steel improves the BH amount, but causes a decrease in the amount of Ti-based carbide precipitation and simultaneously decreases the strength. As a result of detailed studies, the inventors have found that the strength of the hot-rolled steel sheet is further improved by setting the BH amount to less than 100 MPa. Therefore, the BH amount may be less than 100 MPa. The BH amount may also be 95 MPa or less, 90 MPa or less, 80 MPa or less, or 75 MPa or less.

[0065] The BH amount is measured by the following method. No. 5 test pieces are taken from the hot-rolled steel plate in accordance with JIS Z 2241:2022. Using the No. 5 test pieces taken, the BH amount (amount of bake hardening) is evaluated in accordance with Appendix A of JIS G 3135:2018.

[0066] More specifically, the BH amount (σ BH ) is calculated using the following formula: σ BH =R SA -R WH where R SA is the value (N / mm) obtained by dividing the test force at the yield point of a test piece that was heat-treated under the following conditions after giving it a total elongation of 2% by the original cross-sectional area of the parallel part of the test piece before pre-straining. 2 ) Heat treatment conditions: 170°C for 20 minutes, then air cooled Also, R WHis the value (N / mm) obtained by dividing the test force when the total elongation is 2% in a tensile test by the original cross-sectional area of the parallel part of the test piece before pre-straining. 2 The tensile test is performed in accordance with JIS Z 2241:2022.

[0067] In the metal structure from the surface to a depth of 500 μm from the surface, the maximum value of the pole density of the orientation group expressed by Euler angles Φ = 0 to 90°, φ1 = 0 to 90°, φ2 = 45°: 7.0 or less The present inventors performed punching using a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, and observed the end faces of the punched holes. They found that the end face properties were inferior in a plane parallel to the rolling direction and the sheet thickness direction (L cross section) compared to a plane perpendicular to the rolling direction (C cross section). The inventors attributed this to the anisotropy of the crystal orientation of the hot-rolled steel sheet and conducted extensive research into the relationship between the texture during hot rolling and the end face properties. As a result, they found that the deterioration of the end face properties of punched steel sheets can be suppressed by setting the maximum pole density of the orientation group expressed by Euler angles Φ = 0 to 90°, φ1 = 0 to 90°, and φ2 = 45° in the metallographic structure from the surface to a depth of 500 μm from the surface to 7.0 or less to 7.0. Therefore, to further improve punching workability, it is preferable that the maximum pole density be 7.0 or less. The maximum pole density is more preferably 6.7 or less or 6.5 or less. The lower limit of the maximum pole density is not particularly limited, but may be 5.5 or more, or 6.0 or more.

[0068] The maximum pole density is measured by the following method. A sample is taken so that the metal structure of the cross section (thickness direction × rolling direction) of the hot-rolled steel sheet can be observed, with the sheet width direction as the normal direction. The size of the sample depends on the measurement device, but for example, it may be a rectangular parallelepiped measuring the full thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction. Next, the observation surface of the sample is mirror-polished, and then polished at room temperature for 8 minutes using colloidal silica without alkaline solution to remove any strain introduced into the observation surface of the sample. After the polishing, a region from the surface of the sample (the surface of the steel sheet) to a depth of 500 μm in the thickness direction and a region of 2000 μm or more at any position in the rolling direction are measured at measurement intervals of 5.0 μm. For the measurements, an EBSD device combining a scanning electron microscope and an EBSD analyzer, and OIM Analysis (registered trademark) manufactured by TSL, are used. The above sample is analyzed using the EBSD (Electron Backscattering Diffraction) method. From the obtained data, the orientation distribution function (ODF) is calculated. From the obtained orientation distribution function, the maximum pole density of the orientation group expressed in Euler angles Φ = 0 to 90°, φ1 = 0 to 90°, and φ2 = 45° is obtained.

[0069] Tensile strength (TS): 780 MPa or more The hot-rolled steel sheet according to this embodiment has a tensile strength of 780 MPa or more. If the tensile strength is less than 780 MPa, the effect of reducing the vehicle body weight is small, and the steel sheet cannot be suitably applied to automobile suspension parts. The tensile strength is more preferably 800 MPa or more or 820 MPa or more. There is no particular need to limit the upper limit of the tensile strength, but it may be 1000 MPa or less or 900 MPa or less.

[0070] Total elongation (El): 14% or more Hole expansion ratio (λ): 40% or more The total elongation may be 14% or more and the hole expansion ratio may be 40% or more. If the total elongation is 14% or more and the hole expansion ratio is 40% or more, it can be determined that the press workability is excellent. The total elongation is the "total elongation at break" as defined in JIS Z 2241:2022. The yield point (YP) may be 700 MPa or more.

[0071] The tensile strength, total elongation, and yield point are obtained by taking a No. 5 test piece in accordance with JIS Z 2241: 2022 and conducting a tensile test in accordance with JIS Z 2241: 2022. In the tensile test piece, the direction perpendicular to the rolling direction is defined as the longitudinal direction.

[0072] In this embodiment, unless the rolling direction is known in advance, the rolling direction is determined by the following method. A test piece is taken from any position at least 50 mm away from the end of the hot-rolled steel plate so that the thickness cross section can be observed. The thickness cross section of the taken test piece is mirror-polished and then observed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x. An appropriate magnification at which the inclusion dimensions can be measured is selected depending on the size of the inclusion. The observation range is 500 μm or more in width and across the entire thickness of the plate, and areas with dark brightness are determined to be inclusions. Observation may be performed from multiple fields of view. Next, using the thickness cross section initially observed using the above method as a reference, the plate is rotated in 5° increments in the range of 0° to 180° around the thickness direction, and the cross sections parallel to the plane are observed using the same method as above. The average length of the major axes of the multiple inclusions in each cross section is calculated for each cross section. The cross section with the largest average value of the major axis length of the inclusions obtained is identified. The direction parallel to the longitudinal axis of the inclusions in the cross section is determined to be the rolling direction.

[0073] The hole expansion ratio is measured by conducting a hole expansion test in accordance with JIS Z 2256:2020. More specifically, a hole is first punched in the center of the test piece with a 10 mm diameter punch, and a 60° conical punch is then pressed into the hole. The 60° conical punch is pressed until a crack that occurs at the edge of the hole in the test piece penetrates through the thickness direction. The hole expansion ratio is obtained by calculating the ratio of the expansion amount of the hole diameter when the crack penetrates through the thickness direction to the initial hole diameter (10 mm diameter punch hole).

[0074] Punching processability The hot-rolled steel sheet according to this embodiment is preferably excellent in punching workability as evaluated below. Test pieces taken from hot-rolled steel plates are punched in three places with a 12mm diameter punch at a clearance of 20%. The length of cracks (peeling) that occur in the direction perpendicular to the plate thickness is measured around the entire circumference of each punched hole. The sum of the lengths of cracks exceeding 2mm (total crack length) for the three punched holes is calculated. If this total is divided by the total circumference of the three punched holes (113mm) (total crack length / total circumference length x 100) and is 20% or less, the plate can be judged to have excellent punching workability.

[0075] The thickness of the hot-rolled steel sheet according to this embodiment is not particularly limited, but may be 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 prevent the rolling load from becoming excessive and making hot rolling difficult. Furthermore, by setting the plate thickness to 8.0 mm or less, the above-mentioned metal structure can be stably obtained after hot rolling.

[0076] The hot-rolled steel sheet according to this embodiment may be provided with a plating layer on the surface for the purpose of improving corrosion resistance or the like, to form a surface-treated steel sheet. The plating layer may be an electroplated layer or a hot-dip plated layer. Examples of electroplated layers include electrogalvanized coating and electrolytic Zn-Ni alloy coating. Examples of hot-dip plated layers include hot-dip galvanized coating, alloyed hot-dip galvanized coating, hot-dip aluminum coating, hot-dip Zn-Al alloy coating, hot-dip Zn-Al-Mg alloy coating, and hot-dip Zn-Al-Mg-Si alloy coating. The coating weight is not particularly limited and may be the same as conventional coatings. Furthermore, it is possible to further enhance corrosion resistance by carrying out an appropriate chemical conversion treatment after plating (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying it).

[0077] Next, a preferred method for manufacturing the hot-rolled steel sheet according to this embodiment will be described. According to the manufacturing method described below, the hot-rolled steel sheet according to this embodiment can be stably manufactured. Note that the temperature of the slab and the temperature of the steel sheet in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.

[0078] In a preferred method for producing a hot-rolled steel sheet according to this embodiment, A slab having the above-mentioned chemical composition is heated to a temperature range of 1230°C or higher, Hot rolling is performed so that the finishing temperature is in the range of 880 to 980°C. Cool to a temperature range of 650-750°C at an average cooling rate of 50°C / s or more. After the cooling, air cooling is performed in the temperature range of 650 to 750°C for 3.0 to 10.0 seconds, After the air cooling, the material is cooled to a temperature range of 470°C to 530°C at an average cooling rate of 50°C / s or more, After the cooling, the sheet is wound up in a temperature range of 470°C to 530°C. Each step will be described below.

[0079] In order to dissolve the coarse TiC precipitated during the casting process, it is preferable to heat the slab to a temperature range of 1230°C or higher. If the heating temperature is lower than 1230°C, the amount of coarse TiC increases, and a sufficient amount of fine TiC cannot be obtained, which may result in a decrease in the strength of the hot-rolled steel sheet. The slab to be heated is not particularly limited except that it has the above-mentioned chemical composition. For example, a slab produced by melting molten steel having the above-mentioned chemical composition using a converter or an electric furnace and then continuous casting the slab can be used. Instead of continuous casting, an ingot casting method, a thin slab casting method, or the like may also be used.

[0080] In order to obtain the desired average grain size and ferrite area ratio, and thus the desired strength, the finish rolling end temperature (the temperature at the outlet of the final stage of finish rolling) is preferably set in the temperature range of 880 to 980° C. If the finish rolling end temperature is less than 880° C., the ferrite formation ability increases excessively, and coarse ferrite is formed in the high temperature range, which may prevent the desired average grain size from being obtained and reduce the strength of the hot-rolled steel sheet. On the other hand, if the finish rolling temperature exceeds 980°C, the ferrite formation ability decreases, and ferrite formation during air cooling is suppressed, which may result in a decrease in the strength of the hot-rolled steel sheet.

[0081] In order to favorably control the maximum pole density in the metal structure from the surface to a depth of 500 μm from the surface, it is preferable to strictly control the start temperature, end temperature, total reduction in true strain in a temperature range of 930° C. or higher, and total reduction in true strain in all stages of finish rolling. More specifically, it is preferable to set the start temperature (entrance temperature of the first stage) in finish rolling to a temperature range of 1000° C. or higher, the end temperature (exit temperature of the final stage) to a temperature range of 880° C. or higher, the total reduction in true strain in a temperature range of 930° C. or higher to 1.5 or higher, and the total reduction in true strain in all stages of finish rolling to 3.0 or lower. The reduction ratio ε at true strain is expressed by the following formula: When rolling is performed in multiple stages, the total reduction ratio can be obtained by calculating the sum of ε for each stage. ε = |ln(h1 / h2)| In the above formula, ln is the natural logarithm with base e, h1 is the plate thickness (mm) before rolling, and h2 is the plate thickness (mm) after rolling.

[0082] After the finish rolling is completed, it is preferable to cool the steel sheet to a temperature range of 650 to 750° C. at an average cooling rate of 50° C. / s or more. If the average cooling rate to the above temperature range is less than 50° C. / s, coarse ferrite is generated during cooling, the desired average grain size cannot be obtained, and the strength of the hot-rolled steel sheet may decrease. In this embodiment, the average cooling rate is the temperature difference between the start point and the end point of the set range divided by the elapsed time from the start point to the end point.

[0083] After the cooling, air cooling is preferably performed in a temperature range of 650 to 750°C for 3.0 to 10.0 seconds. By performing air cooling under these conditions, a desired amount of ferrite can be formed. If the air cooling temperature is less than 650°C, a large amount of pearlite is formed, which may reduce the strength of the hot-rolled steel sheet. On the other hand, if the air-cooling temperature exceeds 750°C, the rate of ferrite formation slows down, the desired amount of ferrite cannot be obtained, and the strength of the hot-rolled steel sheet may decrease. In this embodiment, air cooling refers to cooling at an average cooling rate of less than 10° C. / s.

[0084] If the air cooling time is less than 3.0 seconds, ferrite cannot be sufficiently generated, and the strength of the hot-rolled steel sheet may decrease. On the other hand, if the air cooling time exceeds 10.0 seconds, ferrite is generated in excess, and the desired amount of bainite may not be obtained.

[0085] After the air-cooling, it is preferable to cool the steel sheet to a temperature range of 470 to 530° C. at an average cooling rate of 50° C. / s or more. If the average cooling rate after air-cooling is less than 50° C. / s, a large amount of pearlite is formed during cooling, which may result in a decrease in the strength of the hot-rolled steel sheet. In order to make the bake hardness less than 100 MPa, it is preferable that the average cooling rate to the temperature range of 470 to 530°C after air cooling is 85°C / s or less.

[0086] The coiling temperature is preferably set in the temperature range of 470 to 530° C. in order to control the transformation and precipitation state after coiling and to obtain the desired press workability and punching workability. If the coiling temperature is less than 470° C., a large amount of MA is generated after coiling, and the desired press workability may not be obtained. On the other hand, if the coiling temperature exceeds 530°C, the carbides will become coarse, reducing the amount of dissolved carbon in the steel and the bake-hardening amount, which may make it impossible to obtain the desired punching workability. [Example]

[0087] Next, the effects of one embodiment of the present invention will be explained in more detail using examples, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0088] Slabs having the chemical compositions shown in Tables 1A to 2B were obtained by converter melting and continuous casting. Hot-rolled steel sheets having thicknesses of 2.6 to 3.2 mm were obtained from the obtained slabs under the conditions shown in Tables 3A and 3B. In Tables 3A and 3B, "total reduction at 930°C or higher" is the total reduction in the temperature range of 930°C or higher, expressed in true strain, and "total reduction" is the total reduction in all stages of finish rolling, expressed in true strain.

[0089] The obtained hot-rolled steel sheets were evaluated for metal structure, bake hardening amount, maximum pole density, tensile strength, total elongation, yield point, hole expansion ratio, and punching workability by the above-mentioned methods. Note that in the examples, the rolling direction was known in advance, so the above-mentioned determination of the rolling direction was not performed. The results obtained are shown in Tables 4A and 4B. In addition, the "maximum pole density" in Tables 4A and 4B indicates the maximum pole density of the orientation group expressed by Euler angles Φ=0 to 90°, φ1=0 to 90°, and φ2=45° in the metal structure from the surface to a depth of 500 μm from the surface.

[0090] When the tensile strength was 780 MPa or more, the specimen was judged to have high strength and pass, whereas when the tensile strength was less than 780 MPa, the specimen was judged to not have high strength and fail.

[0091] If the total elongation was 14% or more and the hole expansion ratio was 40% or more, the specimen was judged to have excellent press workability and passed the test. On the other hand, if either of these criteria was not met, the specimen was judged to have poor press workability and failed the test.

[0092] In the above-mentioned evaluation of punching workability, evaluation was performed according to the following criteria depending on the value of total length of cracks / total circumference length x 100. When the evaluation was Fair or better, that is, when the value was 20% or less, the specimen was judged to have excellent punching workability and to have passed. When the evaluation was Poor, the specimen was judged to have poor punching workability and to have failed. Less than 1%: Excellent 1% or more, less than 10%: Good More than 10%, less than 20%: Fair Over 20%: Poor

[0093] [Table 1A]

[0094] [Table 1B]

[0095] [Table 2A]

[0096] [Table 2B]

[0097] [Table 3A]

[0098] [Table 3B]

[0099] [Table 4A]

[0100] [Table 4B]

[0101] Nos. 1 to 20 and 47 to 49 in the table are inventive examples, and it is clear that these hot-rolled steel sheets have high strength and excellent press workability and punching workability. On the other hand, Nos. 21 to 46 and 50 are comparative examples, and it is clear that at least one of the properties is inferior.

[0102] No. 21 had a low C content, resulting in a decrease in strength. No. 22 had a high C content, which resulted in a decrease in total elongation and poor press workability. No. 23 had a low Si content, resulting in a decrease in strength.

[0103] No. 24 had a low Mn content, which resulted in a decrease in strength. No. 25 had a high Mn content, which resulted in a decrease in total elongation and deterioration in press workability. No. 26 had a high P content, which resulted in a decrease in total elongation and deterioration in press workability.

[0104] No. 27 had a high S content, which reduced the hole expansion ratio and deteriorated press workability. No. 28 had a high Al content, which resulted in a decrease in total elongation and deterioration in press workability. No.29 had a low Ti content, so the strength was reduced.

[0105] No. 30 had a high Ti content, which reduced the total elongation, degraded press workability, and also degraded punching workability. No. 31 had a low Nb content, resulting in a decrease in strength. No. 32 had a high Nb content, which resulted in a decrease in total elongation and a deterioration in press workability.

[0106] No. 33 had a high N content, which resulted in a decrease in strength and the formation of coarse TiN, which deteriorated punching workability. No. 34 had a low A value (calculated unprecipitated carbon amount), which resulted in a decrease in bake hardening amount and poor punching workability. In No. 35, the M value was high, resulting in an insufficient amount of bainite, a decrease in the hole expansion ratio, and deterioration in press workability.

[0107] No. 36 had a low heating temperature, resulting in a decrease in strength. In No. 37, the finish rolling temperature was low, which resulted in the formation of coarse ferrite and a decrease in strength. In No. 38, the finish rolling temperature was high, resulting in a decrease in the amount of ferrite and a decrease in strength.

[0108] For No. 39, the average cooling rate before air cooling was slow, which resulted in the formation of coarse ferrite and reduced strength. No. 40 had a low air-cooling start temperature, which resulted in a large amount of pearlite and a decrease in strength. For No. 41, the air-cooling start temperature was high, which resulted in a decrease in the amount of ferrite and a decrease in strength.

[0109] No. 42 had a short cooling time, which resulted in a decrease in the amount of ferrite and a decrease in strength. For No. 43, the air cooling time was long, which resulted in a decrease in the amount of bainite, a decrease in the hole expansion ratio, and deterioration of press workability. For No. 44, the average cooling rate after air cooling was slow, resulting in a large amount of pearlite and a decrease in strength.

[0110] For No. 45, the coiling temperature was low, resulting in a large amount of MA, a lower hole expansion ratio, and poor press workability. For No. 46, the coiling temperature was high, which resulted in a decrease in bake hardening and a deterioration in punching workability. No. 50 had a high B content, which resulted in a decrease in the amount of ferrite and a decrease in strength. [Industrial Applicability]

[0111] According to the above-described aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent press workability and punching workability.

Claims

1. The chemical composition, in mass%, is C: 0.04-0.10%, Si: 0.11 to 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 to 0.150%, Nb: 0.020-0.100%, V: 0 to 0.200%, Mo: 0-1.000%, Ca: 0-0.0100%, REM: 0-0.0100%, Cu: 0-2.00%, Ni: 0 to 1.50%, Cr: 0-2.00%, Mg: 0 to 0.0200%, Bi: 0-0.0200%, Zr: 0 to 0.500%, Co: 0-1.000%, Zn: 0-0.200%, 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 balance is Fe and impurities. The A value represented by the following formula (1) is 0.0150 mass% or more, The M value represented by the following formula (2) is 930 or less, The metal structure at 1 / 4 of the plate thickness from the surface is, in area%, Ferrite: 50% or more and less than 80% Bainite: more than 20% and not more than 50%; Perlite: 0-5%, MA: 0 to 2%; The average crystal grain size is 10 μm or less, The BH amount is 45 MPa or more, A hot-rolled steel sheet having a tensile strength of 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) However, each element symbol in the formulas (1) and (2) indicates the content of the element in mass %, and 0 is substituted when the element is not contained.

2. The chemical composition is, in mass %, V: 0.001-0.200%, Mo: 0.001 to 1.000%, Ca: 0.0001-0.0100%, REM: 0.0001-0.0100%, Cu: 0.01-2.00%, Ni: 0.01 to 1.50%, Cr: 0.01-2.00%, Mg: 0.0001-0.0200%, Bi: 0.001-0.0200%, Zr: 0.001 to 0.500%, Co: 0.001 to 1.000%, Zn: 0.001-0.200%, W: 0.001-0.200%, Sb: 0.001-0.200%, As: 0.001 to 0.050%, and The hot-rolled steel sheet according to claim 1, further comprising at least one selected from the group consisting of Sn: 0.001 to 0.050%.

3. 3. The hot-rolled steel sheet according to claim 1, wherein in the metal structure from the surface to a depth of 500 μm from the surface, the maximum value of the pole density of an orientation group expressed by Euler angles Φ = 0 to 90°, φ1 = 0 to 90°, and φ2 = 45° is 7.0 or less.

4. The chemical composition is, in mass %, The hot-rolled steel sheet according to claim 1 or 2, characterized in that B: 0.0002% or less.

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