Hot-rolled steel sheet

Abstract

In this hot-rolled steel sheet, the A value represented by formula (1) is 0.0150 mass% or more, the M value represented by formula (2) is 930 or less, the metal structure at a position 1 / 4 of the sheet thickness from the surface comprises, in terms of area%, no less than 50% and less than 80% ferrite, more than 20% and no more than 50% bainite, 0-5% pearlite, and 0-2% MA, the average crystal grain size is 10 μm or less, the BH amount is 45 MPa or more, and the tensile strength is 780 MPa or more.

Description

hot-rolled steel sheets

[0001] This application claims priority from Japanese Patent Application No. 2023-201058, filed on November 28, 2023, the contents of which are incorporated herein by reference.

[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 the like are required to have excellent press workability. To improve the press workability of high-strength hot-rolled steel sheets, for example, to achieve both high strength and formability, a technique is known in which the metal structure contains 80% or more ferrite, the remainder being bainite and pearlite, and fine carbides are precipitated within the ferrite grains, thereby achieving both high strength and ductility. In this technique, Ti, which has an inexpensive unit cost for strength, is mainly used.

[0004] Although the addition of Ti can improve press workability, when Ti-added high-strength hot-rolled steel sheets 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." High-strength hot-rolled steel sheets are required to prevent this peeling that occurs during punching.

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

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

[0007] Japanese Patent Application Publication No. 2003-342684 Japanese Patent Application Publication No. 2004-315857

[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 end 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 technique 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.

[0011] The present inventors have found that by increasing the amount of unprecipitated 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%, 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 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%, Ca: 0 to 0.0100%, REM: 0 to 0.0100%, Cu: 0 to 2.00%, Ni: 0 to 1.50%, Cr: 0 to 2.00%, the metal structure at a position of 1 / 4 of the plate thickness from the surface is, in area %, composed of: 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 to 0.050%, Sn: 0 to 0.050%, B: less than 0.0005%, and the balance: 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; and the metal structure at a position of 1 / 4 of the plate thickness from the surface is, in area %, composed of: 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%; A hot-rolled steel sheet having an average grain size of 10 μm or less, a bake hardness of 45 MPa or more, and 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) where each element symbol in formulas (1) and (2) indicates the content of the element in mass%, and 0 is substituted if the element is not contained.[2] The chemical composition is, in mass%, V: 0.001 to 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%, The hot-rolled steel sheet according to [1] above, characterized in that it contains one or more elements selected from the group consisting of Sb: 0.001 to 0.200%, As: 0.001 to 0.050%, and Sn: 0.001 to 0.050%. [3] The hot-rolled steel sheet according to [1] above or [2] above, characterized in that in the metallographic structure from the surface to a depth of 500 μm from the surface, the maximum value of 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 hot-rolled steel sheet according to any one of [1] to [3] above, characterized in that the chemical composition, in mass%, is B: 0.0002% or less.

[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.

[0015] A hot-rolled steel sheet according to one embodiment of the present invention (hereinafter, sometimes referred to as a 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] Each constituent element 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, the numerical ranges described with "to" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" do not include the numerical range. In the following description, percentages related to the chemical composition 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 and punching by being present at 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, 0.06% or more. On the other hand, if the C content exceeds 0.10%, the ductility decreases, thereby deteriorating 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, 0.08% or less.

[0019] Si: 0.11 to 0.30% Si is an effective element as a deoxidizer and as a solid-solution strengthening element for 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 scale occurs 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, or 1.80% or more. On the other hand, if the Mn content exceeds 2.50%, the ductility decreases, thereby deteriorating 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 set to 2.30% or less, or 2.00% or less.

[0021] P: 0.05% or less P is an element that segregates at grain boundaries and reduces ductility, thereby degrading 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 a lower P content is preferable, it may be set to 0%. However, from the viewpoint of reducing refining costs, the P content may be set to 0.001% or more.

[0022] S: 0.05% or less S is an element that forms MnS and acts as a fracture origin, degrading hole expandability and thereby degrading 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 set to 0%. However, from the viewpoint of reducing refining costs, the S content may be set to 0.001% or more.

[0023] N: 0.010% or less N is an element that forms nitrides with Ti and reduces the amount of Ti that can bond with C, thereby reducing the strength of the hot-rolled steel sheet. 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. The lower the N content, the better, so it may be set to 0%. However, from the viewpoint of reducing refining costs, the N content may be set to 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 points for fracture, thereby 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, 0.006% or less. The lower the O content, the better, so it may be set to 0%. However, from the viewpoint of reducing refining costs, the O content may be set to 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 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, 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 degrading the 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 set to 0.145% or less, 0.140% or less, or 0.135% or less.

[0027] Nb: 0.020 to 0.100% Nb, like Ti, is an effective element for refining crystal grains and 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, reducing ductility and deteriorating 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, 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, which may reduce the 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 effective in increasing the hardenability of steel and thereby increasing the strength of hot-rolled steel sheets. 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, even if the Mo content exceeds 1.000%, the above effect saturates and 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 effective in improving press workability by controlling the morphology of non-metallic inclusions, which become fracture initiation points and cause workability to deteriorate. 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, even if the Ca content exceeds 0.0100%, the above effect saturates, which is economically undesirable. 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 effective element for improving press workability by controlling the morphology of nonmetallic inclusions, which can become fracture initiation sites and cause poor workability. While the REM content may be 0%, to fully obtain this effect, a REM content of 0.0001% or more is preferred. The REM content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, a REM content exceeding 0.0100% saturates the above-mentioned effect and is therefore economically undesirable. 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. Lanthanides 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 strength of hot-rolled steel sheets by improving the hardenability of steel. 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, even if the Cu content exceeds 2.00%, the above effect saturates and 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, even if the Ni content exceeds 1.50%, the above effect saturates and 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, even if the Cr content exceeds 2.00%, the above effect saturates and 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 increases the yield ratio of hot-rolled steel sheets by controlling the shape of inclusions in steel to a preferred shape. While 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, reducing the yield ratio of the hot-rolled steel sheets. 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 effect of the above action saturates, 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, thereby deteriorating 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, thereby deteriorating 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, thereby deteriorating 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, thereby deteriorating 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 in order 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 a large amount of Sb is contained, the above effect saturates, 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.

[0042] As: 0 to 0.050% As is an element that refines prior austenite grains by lowering the austenite single-phase temperature, thereby 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, even if a large amount of As is contained, the above effect saturates, 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.

[0043] Sn: 0 to 0.050% Sn 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 Sn content may be 0%, but in order 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 a large amount of Sn is contained, the above effect saturates, 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.

[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, so the B content is less than 0.0005%. From the viewpoint of ensuring the strength of the hot-rolled steel sheet, the lower the B content, the better, so it 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 mass% or more In the chemical composition, if the A value (calculated unprecipitated carbon amount) represented 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 set to 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) In the formula (1), each element symbol 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 if the element is not contained. Note that 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') with the term for B added instead of the formula (2). Note that, if necessary, the following formula (2') may also be used instead of the formula (2) when the B content is 0.0001% or more, 0.0002% or more, 0.0003% or more, or 0.0004% or more. 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') 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 may 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 the results by infrared absorption spectroscopy. Values ​​for N are determined by melting a test piece taken from the steel sheet in a helium stream and measuring the result by a thermal conductivity method. Values ​​for O are determined by placing a test piece taken from the steel sheet in a graphite crucible, melting the test piece in a helium stream, and measuring the result by infrared absorption spectroscopy. If the hot-rolled steel sheet has a plating layer or a coating film on its surface, the plating layer or coating film may be removed by mechanical grinding or the like, as necessary, before analyzing the chemical composition.

[0051] Next, the metallographic structure of the hot-rolled steel sheet according to this embodiment will be described. In the hot-rolled steel sheet according to this embodiment, the metallographic structure at a position from the surface to 1 / 4 of the sheet thickness is composed of, in area %, 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 the average crystal grain size is 10 μm or less. Each requirement will be described below.

[0052] In this embodiment, the metallographic structure is defined at a position 1 / 4 of the sheet thickness from the surface (a region from 1 / 8 of the sheet thickness depth from the surface to 3 / 8 of the sheet thickness depth from the surface). In other words, this position is a region starting from 1 / 8 of the sheet thickness depth from the surface and ending at 3 / 8 of the sheet thickness depth from the surface. The reason for defining the metallographic structure at this position is that the metallographic structure at this position represents a typical metallographic structure of a steel sheet. In addition, when a hot-rolled steel sheet has a plating layer, a coating film, or the like on its surface, the surface referred to here refers to the interface between the steel sheet and the plating layer, 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 and increases the hardness of bainite, deteriorating the hole expandability and 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, or 70% or less.

[0054] Bainite: More than 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 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 more than 20%. The area fraction of bainite is preferably 25% or more, or 30% or more. Furthermore, in relation to the area fraction of ferrite, the area fraction of bainite is set to 50% or less. The area fraction of bainite is preferably 45% or less, 40% or less, or 35% or less.

[0055] Pearlite: 0 to 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, so 0% is preferable.

[0056] MA: 0 to 2% MA is a structure consisting of martensite and austenite. Since 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 does not need to be contained, the area ratio of MA may be 0%.

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

[0058] Method for Measuring Ferrite Area Fraction Test specimens are taken from hot-rolled steel sheets so that a cross section perpendicular to the sheet width direction can be observed. The sampled test specimens are wet-polished to a mirror finish on the observation surface, and then polished at room temperature for 8 minutes using colloidal silica that does not contain an 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 sheet thickness from the surface (a region from 1 / 8 of the sheet thickness depth from the surface to 3 / 8 of the sheet thickness depth from the surface). The SEM-EBSD device used is a device consisting of a field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL). The degree of vacuum in the device is 9.6 x 10 －５ The pressure is set to 100 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. The observation area is set to 500 μm x 500 μm, the step size is set to 0.3 μm, and crystal orientation information is obtained. At least three observation areas are set. Using the obtained crystal orientation information and OIM data analysis, the crystal orientation difference with adjacent measurement points is calculated, and areas surrounded by boundaries with a crystal orientation difference of 15° or more are considered crystal grains. Next, the average value (Grain Average Misorientation, GAM) of the orientation difference with adjacent measurement points for each crystal grain is calculated. 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. Note that the ferrite defined by this method does not include ferrite within pearlite.

[0059] Measurement method of pearlite area ratio Test pieces are collected in the same manner as described above, and the observation surface is etched with nital to reveal the metal structure, and a microstructure photograph is taken using an optical microscope. The photograph is taken 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), 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. Note that the pearlite that is black-etched using this method does not include ferrite, which is 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 method described above. 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 Area Fraction of Bainite Bainite is obtained by subtracting the area fractions of ferrite, pearlite, and MA from 100%.

[0062] Average crystal grain size: 10 μm 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 limitation on the lower limit of 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 area ratio of ferrite is measured. The region surrounded by a boundary with a crystal orientation difference of 15° or more is defined as a crystal grain, and the equivalent circle diameter of the crystal grain is calculated. The equivalent circle diameters of all crystal grains in the observation region are calculated, and the average value of these is calculated to obtain the average crystal grain size. The equivalent circle diameter is the diameter of a circle with the same area as the area of ​​the identified crystal grain.

[0064] BH amount: 45 MPa or more. In this embodiment, carbon not only generates precipitates necessary to obtain the desired strength, but also remains in solid solution in the steel, thereby improving punching workability. The inventors have discovered that there is a correlation between the amount of solute carbon and the BH amount (bake hardening amount), and that a BH amount of less than 45 MPa results in a low amount of solute carbon, making it difficult to obtain the desired punching workability. 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. Increasing the amount of solute carbon in the steel improves the BH amount, but also reduces the amount of Ti-based carbide precipitation, thereby decreasing the strength. As a result of detailed studies, the present inventors have found that the strength of the hot-rolled steel sheet is further improved by setting the bake hardness to less than 100 MPa. Therefore, the bake hardness may be set to less than 100 MPa. The bake hardness may also be set to 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 sheet in accordance with JIS Z 2241:2022. The BH amount (amount of bake hardening) is evaluated using the No. 5 test pieces taken 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 Here, RＳＡ 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: heated to 170°C for 20 minutes, then cooled in air. ＷＨ is the value (N / mm) obtained by dividing the test force when the total elongation is 2% in the tensile test by the original cross-sectional area of ​​the parallel part of the test piece before pre-straining. ２ The tensile test is carried out 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 pole density of the orientation group expressed by Euler angles Φ = 0 to 90°, φ1 = 0 to 90°, and φ2 = 45°: 7.0 or less. The 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. As a result, they found that the end face properties of a plane parallel to the rolling direction and the sheet thickness direction (L cross section) were inferior to those of a plane perpendicular to the rolling direction (C cross section). The inventors believed that this was due to the anisotropy of the crystal orientation of the hot-rolled steel sheet and conducted extensive research on the relationship between the texture during hot rolling and the end face properties. As a result, it was found that, in the metal structure from the surface to a depth of 500 μm from the surface, deterioration of the end face properties of punching can be suppressed by setting 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° to 7.0 or less. Therefore, in order to further improve punching workability, it is preferable to set the maximum value of the pole density to 7.0 or less. The maximum value of the pole density is more preferably 6.7 or less or 6.5 or less. There is no particular limitation on the lower limit of the maximum value of the pole density, but it 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 a cross section (thickness direction × rolling direction) of a 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 may be, for example, a rectangular parallelepiped with 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 for 8 minutes at room temperature using colloidal silica that does not contain an alkaline solution to remove 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 an OIM Analysis (registered trademark) manufactured by TSL are used. The sample is analyzed using the EBSD (Electron Backscattering Diffraction) method. From the obtained data, a crystal orientation distribution function (ODF) is calculated. From the obtained crystal orientation distribution function, the maximum pole density of the orientation group expressed by 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. Note that the total elongation refers to the "total elongation at break" as defined in JIS Z 2241:2022. The yield point (YP) may also 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 specimen is taken from an arbitrary position 50 mm or more away from the end of the hot-rolled steel sheet so that the thickness cross section can be observed. The thickness cross section of the taken test specimen is mirror-polished and then observed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x. Depending on the size of the inclusion, an appropriate magnification at which the inclusion dimensions can be measured is selected. The observation range is 500 μm or more in width and across the entire thickness of the sheet, 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 by the above method as a reference, planes parallel to the planes rotated in 5° increments in the range of 0° to 180° around the thickness direction 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 where the average value of the length of the major axis of the inclusions obtained is greatest is identified, and the direction parallel to the major axis of the inclusions in that 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 a 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] Punchability The hot-rolled steel sheet according to this embodiment preferably has excellent punchability as evaluated by the following method. A test piece taken from the hot-rolled steel sheet is punched at three locations using a 12 mmφ punch with a clearance of 20%. The length of cracks (peeling) occurring in the direction perpendicular to the sheet thickness direction is measured around the entire circumference of each punched hole. The sum of the lengths of cracks exceeding 2 mm (total crack length) is determined for the three punched holes. If this sum is divided by the total circumference length of the three punched holes (113 mm) (total crack length / total circumference length×100) and is 20% or less, the steel sheet can be determined to have excellent punchability.

[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 making the thickness 8.0 mm or less, it is possible to stably obtain the above-mentioned metal structure after hot rolling.

[0076] The hot-rolled steel sheet according to this embodiment may be provided with a plating layer on the surface to improve corrosion resistance or the like, thereby forming 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 plating and electrolytic Zn—Ni alloy plating. Examples of hot-dip plated layers include hot-dip galvanized plating, alloyed hot-dip galvanized plating, hot-dip aluminum plating, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, and hot-dip Zn—Al—Mg—Si alloy plating. The coating weight is not particularly limited and may be the same as conventional coating weights. Furthermore, corrosion resistance can be further improved by performing 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 manufacturing a hot-rolled steel sheet according to this embodiment, a slab having the above-described chemical composition is heated to a temperature range of 1230°C or higher, hot rolling is performed so that the finish rolling end temperature is in the temperature range of 880 to 980°C, cooling is performed to a temperature range of 650 to 750°C at an average cooling rate of 50°C / s or higher, after said cooling, air-cooling is performed in said temperature range of 650 to 750°C for 3.0 to 10.0 seconds, after said air-cooling, cooling is performed to a temperature range of 470 to 530°C at an average cooling rate of 50°C / s or higher, and after said cooling, coiling is performed in a temperature range of 470 to 530°C. Each step will be explained below.

[0079] In order to dissolve the coarse TiC precipitated in 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, etc., and then performing continuous casting can be used. Instead of continuous casting, an ingot casting method, thin slab casting method, etc. may also be used.

[0080] In order to obtain the desired average crystal grain size and ferrite area ratio and 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 generation ability increases excessively, and coarse ferrite is generated in the high temperature range, which may prevent the desired average crystal grain size from being obtained and reduce the strength of the hot-rolled steel sheet. On the other hand, if the finish rolling end temperature is more than 980°C, the ferrite generation ability decreases and ferrite generation during air cooling is suppressed, which may reduce 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 and end temperature of the finish rolling, the total reduction in true strain in the temperature range of 930 ° C or higher, and the total reduction in true strain in all stages of the finish rolling. More specifically, it is preferable that the start temperature (entrance temperature of the first stage) of the finish rolling be in a temperature range of 1000 ° C or higher, the end temperature (exit temperature of the final stage) be in a temperature range of 880 ° C or higher, the total reduction in true strain in the temperature range of 930 ° C or higher be 1.5 or higher, and the total reduction in true strain in all stages of the finish rolling be 3.0 or lower. The true strain reduction ε is expressed by the following formula. When rolling is performed in multiple stages, the total reduction can be obtained by calculating the sum of ε for each stage. ε = |ln(h １ / h ２ ) | In the above formula, ln is the natural logarithm with base e, and h １ is the plate thickness before rolling (mm), and h ２ 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 value obtained by dividing the temperature difference between the start point and the end point of the set range by the elapsed time from the start point to the end point.

[0083] After the cooling, it is preferable to perform air cooling for 3.0 to 10.0 seconds in a temperature range of 650 to 750°C. By performing air cooling under these conditions, the desired amount of ferrite can be generated. If the air cooling temperature is less than 650°C, a large amount of pearlite is generated, and the strength of the hot-rolled steel sheet may be reduced. On the other hand, if the air cooling temperature is more than 750°C, the rate of ferrite generation slows, the desired amount of ferrite cannot be obtained, and the strength of the hot-rolled steel sheet may be reduced. 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 is more than 10.0 seconds, ferrite may be excessively generated, and the desired amount of bainite may not be obtained.

[0085] After the air cooling, it is preferable to cool 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 generated during cooling, which may reduce the strength of the hot-rolled steel sheet. In order to make the BH amount less than 100 MPa, it is preferable to set the average cooling rate after air cooling to a temperature range of 470 to 530°C to 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 precipitate state after coiling and 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 become coarse, reducing the amount of solute carbon in the steel and the BH amount, which may make it impossible to obtain the desired punching workability.

[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 plates having thicknesses of 2.6 to 3.2 mm were obtained from the obtained slabs under the conditions shown in Tables 3A and 3B. Note that the "total reduction at 930°C or higher" in Tables 3A and 3B is the total reduction in the temperature range of 930°C or higher, expressed in true strain, and the "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 using the methods described above. In the examples, the rolling direction was known in advance, so the above-mentioned determination of the rolling direction was not performed. The results are shown in Tables 4A and 4B. 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 have low 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 to have 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 to have failed the test.

[0092] In the above-mentioned evaluation of punching workability, the evaluation was made 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 higher, 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 no excellent punching workability and to have failed. Less than 1%: Excellent 1% or more but less than 10%: Good 10% or more but 20% or less: Fair 20% or more: Poor

[0093]

[0094]

[0095]

[0096]

[0097]

[0098]

[0099]

[0100]

[0101] In the table, Nos. 1 to 20 and 47 to 49 are invention examples, and it is clear that they are hot-rolled steel sheets having 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 one or more of the properties are inferior.

[0102] No. 21 had a low C content, which resulted in a decrease in strength. No. 22 had a high C content, which resulted in a decrease in total elongation and deterioration in press workability. No. 23 had a low Si content, which resulted 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 a deterioration in press workability. No. 26 had a high P content, which resulted in a decrease in total elongation and a 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 reduced the total elongation and deteriorated press workability. No. 29 had a low Ti content, which reduced strength.

[0105] In No. 30, the high Ti content resulted in a decrease in total elongation, which deteriorated press workability as well as punching workability. In No. 31, the low Nb content resulted in a decrease in strength. In No. 32, the high Nb content resulted in a decrease in total elongation and deterioration in press workability.

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

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

[0108] In No. 39, the average cooling rate before air cooling was slow, which resulted in the formation of coarse ferrite and a decrease in strength. In No. 40, the air cooling start temperature was low, which resulted in an increase in the amount of pearlite and a decrease in strength. In 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] In No. 42, the air-cooling time was short, resulting in a small amount of ferrite and a decrease in strength. In No. 43, the air-cooling time was long, resulting in a small amount of bainite, a decrease in the hole expansion ratio, and deterioration in press workability. In No. 44, the average cooling rate after air-cooling was slow, resulting in a large amount of pearlite and a decrease in strength.

[0110] In No. 45, the coiling temperature was low, which resulted in a large amount of MA, a reduced hole expansion ratio, and poor press workability. In No. 46, the coiling temperature was high, which resulted in a reduced bake hardening amount and poor punching workability. In No. 50, the B content was high, which resulted in a reduced amount of ferrite, and poor strength.

[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. Chemical composition, by mass%, is: 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: 0-0.0100%, REM: 0-0.0100%, Cu: 0-2.00%, Ni: 0-1.50%, Cr: 0-2.00%, the metal structure at a position of 1 / 4 of the sheet thickness from the surface is composed of, in terms of 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 balance: Fe and impurities; A hot-rolled steel sheet having an average grain size of 10 μm or less, a BH amount of 45 MPa or more, and 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) Note that each element symbol in the formulas (1) and (2) indicates the content of the element in mass%, and 0 is substituted if the element is not contained.

2. The chemical composition is, in mass%, V: 0.001 to 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%, The hot-rolled steel sheet according to claim 1, further comprising at least one selected from the group consisting of Sb: 0.001 to 0.200%, As: 0.001 to 0.050%, and Sn: 0.001 to 0.050%.

3. The hot-rolled steel sheet according to claim 1 or 2, characterized in that in the metal structure from the surface to a depth of 500 μm from the surface, the maximum pole density of the orientation group expressed by Euler angles Φ = 0 to 90°, φ1 = 0 to 90°, φ2 = 45° is 7.0 or less.

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

Images