STEEL SHEET AND METHOD OF MANUFACTURING IT

Abstract

A steel sheet includes a predetermined composition, in which a microstructure at a position 1 / 4 of the thickness from a surface in the direction of the sheet thickness includes, in % by volume, ferrite: 80% or more, martensite: 2% or less and residual austenite: 2% or less, a proportion of non-recrystallized ferrite in the ferrite of 5% or less, and in the microstructure of the steel sheet drawn by 10% at the position 1 / 4 of the thickness from the surface in the direction of the sheet thickness, a numerical density of the voids having a maximum diameter of 1.0 µm or more is 1.0 × 109 pieces / m2 or less.

Description

STEEL SHEET AND METHOD OF MANUFACTURING IT Lznonn / zznz / E / YiAi Technical field of the invention [1] The present invention relates to a steel sheet and to a method of manufacturing the same. Priority is claimed in Japanese Patent Application No. 2020-001530, filed on January 8, 2020, the contents of which are incorporated herein by reference. Background of the invention [2] Recently, in vehicles, high-strength steel sheets have been widely used to reduce vehicle body weight, thereby improving fuel consumption, lowering carbon dioxide emissions, or absorbing collision energy to ensure passenger safety. However, in general, when a steel sheet has high strength, its deformation capacity (e.g., ductility or bending ability) deteriorates, and fracture is likely to occur in the locally large strain region generated by impact deformation. Therefore, steel sheets used in vehicles require excellent properties that minimize the likelihood of fracture, namely, impact resistance and resistance to fracture in the locally large strain region generated by impact deformation. [3] For example, Patent Document 1 describes a high-strength steel sheet having a tensile strength of 900 MPa or more, in which high strength and excellent formability can be simultaneously achieved. In Patent Document 1, a steel structure includes, by area ratio, 5% or more and 80% or less ferrite, 15% or more self-hardening martensite, 10% or less bainite, 5% or less residual austenite, and 40% or less hardened martensite; the average hardness of the self-hardening martensite is HV d 700; and the average number of precipitated iron-based carbide grains, each with a size of 5 nm or more and 0.5 pm or less in the self-hardening martensite, is 5 × 10⁴ or more per 1 mm². [4] Patent Document 2 describes a steel sheet having a tensile strength of 900 MPa or higher, excellent weldability, and excellent elongation. The steel sheet in Patent Document 2 includes, as a steel structure, by area ratio, 25% or more and 65% or less ferrite, 35% or more and 75% or less martensite having iron-based carbides precipitated on the martensite grains, and 20% or less (including 0%) in total of the remainder in a microstructure distinct from the ferrite and martensite, wherein the average grain size of each of the ferrites and the martensite is 5 pm or less, and a total atomic concentration of Si and Mn at an interface between the ferrite and the martensite is 5% or more. [5] Patent Document 3 describes a cold-rolled steel sheet comprising, as a steel structure, 60% or more total area of ​​ferrite and bainite and 3% or more and 20% or less area of ​​residual austenite, wherein the average grain size of the ferrite and bainite is 0.5 pm or more and 6.0 pm or less, the concentration of C in the residual austenite is 0.5% by mass or more and 1.2% by mass or less, the cold-rolled steel sheet has an element concentration distribution in which the average interval in a direction orthogonal to the rolling of each concentrated portion of Mn and a concentrated portion of Si extending in a rolling direction at a depth of 50 pm from a surface of the steel sheet is 1000 pm or less, and the cold-rolled steel sheet has surface properties in which the maximum depth The number of cracks on the surface of the steel sheet is 4.5 pm or less and the numerical density of cracks that have a width of 6 pm or less and a depth of 2 pm or more is 10 cracks / 50 pm or more, and the cold-rolled steel sheet has mechanical properties in which a tensile strength (TS) is 800 MPa or higher and 1200 MPa or lower, a work hardening coefficient (n3-8) in a plastic deformation region of 3% or more and 8% or less is 0.10 or more, and the bending capacity satisfies an Expression (R / t < 1.5). [6] However, as a result of an investigation carried out by the inventors hereof, it was found that the impact and fracture resistance is not sufficient with the techniques described in Patent Documents 1 to 3. Previous technique document Patent document [7] Patent Document 1: PCT International Publication No. WO2009 / 096596 Patent Document 2: International Publication PCT No. WO2018 / 030503 Patent Document 3: Japanese Patent No. 5659929 ίζηοηη / ζζηζ / Ε / γίΛΐ Description of the invention Problems that must be solved by the invention [8] The present invention has been considering that not only is an improvement in formability and strength required, but also an improvement in impact and fracture resistance for a high-strength steel sheet as described above. An object of the present invention is to provide a high-strength steel sheet (including a galvanized steel sheet, a zinc alloy-plated steel sheet, a galvanized-annealed steel sheet, and an alloy-annealed galvanized steel sheet) that has excellent formability, strength, and impact and fracture resistance, and a method for manufacturing the same. Lznonn / zznz / E / YiAi Means to solve the problem [9] As a result of an investigation to achieve the object, the inventors of the present made findings. (a) The fine voids formed in steel during formation are pathways where brittle and ductile fracture propagates during impact. Therefore, reducing the volume percentage of a hard structure (martensite and residual austenite) as the origin of these fine voids is effective in improving impact and fracture resistance. (b) Voids formed in the vicinity of hard cementite by cold rolling, or voids formed by cold rolling when a hot-rolled steel sheet has high hardness due to a carbon solid solution, appear to disappear after heat treatment (annealing). However, small voids formed during forming remain in the product. Therefore, controlling the cementite distribution and carbon solid solution before cold rolling is effective in improving impact and fracture resistance.

[10] The present invention has been made on the basis of the above findings, and its scope is as follows. [1] In accordance with one aspect of the present invention, a steel sheet is provided which includes, as a composition, in % by mass: C: 0.010% to 0.200%; Yes: 0.005% to 1.500%; Mn: 0.05% to 3.00%; Al: 0.005% to 1.000%; P: 0.100% or less; S: 0.0200% or less; N: 0.0150% or less; O: 0.0100% or less; Lznonn / zznz / E / YiAi Nb: 0% to 060% Ti : 0% a 100% V: 0% to 0500%; Cr: 0% to 00%; Ni : 0% to 00%; Cu: 0% to 00%; Here: 0% of 00%; W: 0% to 1•000%; B: 0% to 0 • 0100% Sn: 0% to 00%; Sb: 0% to 0.20%; one or two or more selected from the group consisting of Ca, Ce, Mg, Zr, La and REM: 0% to 0.0100% in total; and a remainder including Fe and impurities, wherein a microstructure at a position 1 / 4 of the thickness from a surface in the direction of the sheet thickness includes, in % by vol, ferrite: 80% or more, martensite: 2% or less, and residual austenite: 2% or less, has a proportion of non-recrystallized ferrite in the ferrite of 5% or less, and in the microstructure of the steel sheet drawn 10% at the position 1 / 4 of the thickness from the surface in the direction of the sheet thickness, a numerical density of voids with a maximum diameter of 1.0 pm or more is 1.0 × 109 pieces / m2 or less. [2] In the steel sheet in accordance with [1], the composition may also include, in % by mass, one or two or more selected from the group consisting of: Lznonn / zznz / E / YiAi Nb: 0.005% to 0.060% Ti: 0.015% to 0.100% V: 0.010% to 0.500%; Cr: 0.05% to 1.00%; Ni: 0.05% to 1.00%; Cu: 0.05% to 1.00%; Mo: 0.03% to 1.00%; W: 0.030% to 1.000%; B: 0.0005% to 0.0100%; Sn: 0.01% to 1.00%; Sb: 0.005% to 0.20%; and one or two or more selected from the group consisting of Ca, Ce, Mg, Zr, La and REM: 0.0001% to 0.0100% in total. [3] In the steel sheet in accordance with [1] or [2], an average ferrite grain size in the microstructure can be from 6.0 pm to 15.0 pm. [4] The steel sheet conforming to any of [1] to [3] may further include a galvanized surface layer. [5] The steel sheet conforming to any of [1] to [3] may include a zinc alloy plated layer on the surface. [6] In steel sheet conforming to [4] or [5], the Fe content in the galvanized layer or zinc alloy plated layer may be from 7.0% to 13.0% by mass. [7] Pursuant to another aspect of the present invention, a method is provided for manufacturing the steel sheet in accordance with any of [1] to [3], which includes: a hot rolling process of heating a piece of steel having the composition in accordance with [1] to 1150°C to 1320°C, completing the hot rolling in such a way that the hot rolling completion temperature is 850°C to 930°C, commencing cooling after 1.5 seconds or more, and cooling the piece of steel to a temperature range of 500°C or lower to obtain a hot-rolled steel sheet such that an average cooling rate in a temperature range from a cooling start temperature of 500°C is 20°C / s faster; a reheating process of heating the hot-rolled steel sheet to a temperature range of 500°C to 700°C; a cooling process of cooling the hot-rolled steel sheet to room temperature; a cold rolling process of cold rolling hot-rolled steel sheet to obtain a cold-rolled steel sheet such that the total rolling reduction is from 30% to 90% and the final cold rolling temperature is from 120°C to 250°C;and an annealing process of heating the cold-rolled steel sheet to an annealing temperature of 720°C to 850°C and cooling to a temperature range of 500°C or less, wherein, in the hot rolling process, Expression (1) is satisfied in a temperature range of 1000°C or less, in the reheating process, Expression (2) is satisfied in the temperature range of 500°C to 700°C, in the annealing process, a stress of 20 MPa or higher is applied and Expression (3) is satisfied in a temperature range of 720°C to the annealing temperature during heating to the annealing temperature, and Expression (4) is satisfied in a temperature range of 720°C to 500°C during cooling from the annealing temperature; Da- ΙΟ^νΤι'+^Τ^α-,)p' ^a^expVr^rñ) í “11 \ * = “10 (7^73) D, =D-1 ' ·exp(“4 (-) )+H(_| x 'Pt' |1 - exp (a4· + exp (ah· ')j + 10^; Λ . ji _exp Dn<12.5 • · · Expression (1) In Expression (1), Dn represents an index that represents a degree of advancement of the precipitation of a fine carbide in a temperature range of 1000°C or less of the hot rolling process, and the reference numbers in Expression (1) are the following 15, n: the number of rolling passes in the temperature range of 1000°C or less, T±: a rolling temperature in an i-th rolling pass, t±: an elapsed time [s] from the i-th rolling pass to an i+l-th rolling pass or an elapsed time [s] until the temperature of a steel sheet drops to 850°C from the i-th rolling pass, hi-i: a sheet thickness [mm] before the i-th rolling pass in the temperature range of 1000°C or less, hi: a sheet thickness [mm] after the i-th rolling pass in the temperature range of 1000°C or less, and aian: constants (ai = 2.54 * 10-6, a2 = 3.62 × 10~4, a3 = -6.38 × 10'1, a4 = -3.00 × 10'1, a5 = 8.50 × 10'1, a6 = -8.50 χ ΙΟ'4, a7 = 2.40 χ 10°, a8 = 7.83 χ ΙΟ'13, a9 = 2.80 χ 105, aio = 6.00 χ ΙΟ'12, and an = 2.80 χ 105), Lznonn / zznz / E / YiAi |-(1 Kn= (Tn+ 273) · {log10tn+ 20 · (1 + 0.08.YES)} / <20 > 1.50 x 104' ' Expression (2) In Expression (2), K20 represents an index that represents a degree of progress of fine carbide precipitation in a 20th period when a temperature history in the temperature range of 500°C to 700°C of the reheating process is divided into 20 periods with respect to time, and the reference numbers in Expression (2) are as follows, Tn: an average temperature [°C] in an nth period when a temperature history in the temperature range of 500°C to 700°C is divided into 20 periods with respect to time, Δίκ: a time [hr] in one of the 20 periods into which a total residence time is divided in the temperature interval from 500°C to 700°C, where ti = Δόκ, and Si: a Si content [% by mass], Σ^Ι Ό 5 -exp Μ ·ί < 20.0 ' T, / . . ι -1 · · Expression (?) the reference numbers in Expression (3) are as follows, K20: a value obtained by Expression (2), di and d2: constants (di = 9.67 × 1010 and d2 = 1.25 × 104) , Ti: an average heat treatment temperature [°C] in an i-th period when a temperature history in the temperature range of 720°C to the annealing temperature is divided into 10 periods with respect to time, yt' : 1 / 10 [s] of a residence time in the temperature range of 720°C to the annealing temperature, _ । jj and (5i + Λ · Nb° ·· + g3· Ti5) · (1 + 54 · Mo°*Γ' ~ ) (Δ, + gs· Δ,0 5) “ ' 'VI ' exp(- - - „ 1 · í'ur>> 1.0K\ Tt+ 273 / . .zx• · · Expression (4) the reference numbers in Expression (4) are the following, Ai: 750 - 18 χ Si - 17 χ Mn - 10 χ Cr - 8 χ Ni + 15 χ Al - Ti, where each of the elements represents a mass % content of the element, and when the element is not included, 0 is substituted as the content of the element, when a calculated value of Ai is a negative value, Ai is set to 0. gia6: constants (gi = 1.00 χ 10-1, g2 = 1.46 χ 10-1, g3 = 1.14 χ 10-1, g4 = 2.24 χ 10°, g5= 4.53 χ 10°, and g6= 4.83 x 103), Nb, Mo, Si, Mn, Cr, Ni and Al: a content [% by mass] of each of the elements, where when the element is not included, 0 is substituted as the content of the element, Ti*: an effective Ti content represented by Ti - 42 / 14 χ N, where Ti and N represent a content [% by mass] of each of the elements, when the element is not included, 0 is substituted as the content of the element, a minimum value is set to 0. Ti: an average heat treatment temperature [°C] in an i-th period when a temperature history in the temperature range of 720°C to 500°C is divided into 10 periods with respect to time, Here and Here: a transformation start temperature and a transformation end temperature [°C] during heating, Tmax: the highest heating temperature [°C] in a heat treatment process, and t': 1 / 10 [s] of a residence time in the temperature range of 720°C to 500°C. [8] In the method of manufacturing a steel sheet in accordance with [7], during cooling in the annealing process, hot-dip galvanizing can be performed on the cold-rolled steel sheet. [9] In the method of manufacturing a steel sheet in accordance with [7], during cooling in the annealing process, hot-dip zinc alloy plating can be performed on the cold-rolled steel sheet.

[10] In the method of manufacturing a steel sheet in accordance with [8] or [9], during cooling in the annealing process, the alloying can be carried out after hot-dip galvanizing or hot-dip zinc alloy plating. Effects of the invention

[11] In the aspects described above in accordance with the present invention, a steel sheet can be provided that has excellent formability, strength and resistance to impact and fracture and a method of manufacturing the same. Modalities of the invention

[12] A steel sheet conforming to a modality and its manufacturing conditions will now be described sequentially. First, the reason for limiting the composition (chemical composition) of the steel sheet according to the modality will be described. A limited numerical range described below, with ~ interposed between them, includes a lower limit value and an upper limit value. A numerical value shown together with "less than" or "greater than" is not included in a numerical range. All % in the composition represent % by mass.

[13] A steel sheet conforming to modality Lznonn / zznz / E / YiAi includes, as composition, in % by mass: C: 0.010% to 0.200%; Si: 0.005% to 1.500%; Mn: 0.05% to 3.00%; Al: 0.005% to 1.000%; P: 0.100% or less; S: 0.0200% or less; N: 0.0150% or less; O: 0.0100% or less; Nb: 0% to 0.060%; Ti: 0% to 0.100%; V: 0% to 0.500%; Cr: 0% to 1.00%; Ni: 0% to 1.00%; Cu: 0% to 1.00%; Mo: 0% to 1.00%; W: 0% to 1.000%; B: 0% to 0.0100%; Sn: 0% to 1.00%; Sb: 0% to 0.20%; one or two or more selected from the group consisting of Ca, Ce, Mg, Zr, La, and REM: 0% to 0.0100% in total; and a remainder including Fe and impurities. Each of the elements will be described below.

[14] C: 0.010% to 0.200% Carbon (C) is an element that significantly increases the strength of steel sheets. When the C content is 0.010% or more, sufficient tensile strength (maximum tensile strength) can be achieved. Therefore, the C content is typically set at 0.010% or more. To further increase the tensile strength of steel sheets, the C content is preferably 0.020% or more, and very preferably 0.030% or more. On the other hand, when the carbon content is 0.200% or less, the amount of ferrite after heat treatment can be controlled to the desired quantity. Therefore, impact and fracture resistance can be ensured. Thus, the carbon content is adjusted to 0.200% or less. In order to further improve impact and fracture resistance, the carbon content is preferably 0.180% or less, and very preferably 0.150% or less.

[15] Yes: 0.005% to 1.500% Silicon (Si) is an element that refines iron-based carbides and contributes to improving the balance between strength and formability. To improve this balance, the Si content is adjusted to 0.005% or more. Preferably, the Si content is 0.025% or more. Specifically, to increase strength, the Si content is highly preferable to be 0.100% or more. Furthermore, when the silicon content is 1,500% or less, the formation of a thick silicon oxide that acts as a source of fracture can be suppressed, cracking is unlikely, steel embrittlement and impact resistance are reduced, and fracture can be ensured. Therefore, the silicon content is adjusted to 1,500% or less. The silicon content is preferably 1,300% or less, and very preferably 1,000% or less.

[16] Mn: 0.05% to 3.00% Manganese (Mn) is an element that improves the hardenability of steel and contributes to increased strength. To achieve the desired strength, the Mn content is adjusted to 0.05% or more. The preferred Mn content is 0.15% or more. Furthermore, when the Mn content is 3.00% or less, the loss of macroscopic homogeneity in the steel sheet caused by Mn segregation during casting can be suppressed, the amount of ferrite can be controlled to the desired level, and the formability of the steel sheet can be ensured. Therefore, the Mn content is adjusted to 3.00% or less. To achieve more satisfactory formability, the Mn content is preferably 2.80% or less, and very preferably 2.60% or less.

[17] Al: 0.005% to 1.000% Aluminum (Al) acts as a deoxidizing agent. A sufficient deoxidizing effect can be achieved when the Al content is 0.005% or higher. Therefore, the Al content is adjusted to 0.005% or higher. Ideally, the Al content should be 0.010% or higher, and very preferably 0.020% or higher. Aluminum (Al) is also an element that forms a thick oxide, which can cause fracture and embrittle steel. When the Al content is 1.000% or less, the formation of a thick oxide as a cause of fracture can be suppressed, and the easy cracking of the steel part can be prevented. Therefore, the Al content is adjusted to 1.000% or less. The Al content is preferably 0.800% or less, and very preferably 0.600% or less.

[18] P: 0.100% or less Phosphorus (P) is an element that embrittles steel and a cast portion formed by spot welding. When the P content is 0.100% or less, the easy cracking of the steel sheet during the forming process caused by embrittlement can be suppressed. Therefore, the P content is adjusted to 0.100% or less. From a productivity standpoint, the P content is preferably 0.050% or less, and very preferably 0.030% or less. The lower limit for P content can be 0%. By adjusting the P content by 0.001% or more, manufacturing costs can be further reduced. Therefore, the lower limit for P content can be set at 0.001%.

[19] S: 0.0200% or less Sulfur (S) is an element that forms a manganese sulfide and impairs formability, such as ductility, hole expansion capacity, draw flanging, and bending capacity. When the S content is 0.0200% or less, significant impairment of the formability of the steel sheet can be suppressed. Therefore, the S content is adjusted to 0.0200% or less. The S content is preferably 0.0100% or less, and very preferably 0.0080% or less. The lower limit for sulfur content can be 0%. By adjusting the sulfur content to 0.0001% or less, manufacturing costs can be reduced even further. Therefore, the lower limit for sulfur content can be adjusted to 0.0001%. Lznonn / zznz / E / YiAi

[20] Ν: 0.0150% or less Nitrogen (N) is an element that forms nitrides and impairs formability, such as ductility, hole expansion capacity, draw flanging, and bending capacity. When the N content is 0.0150% or less, the impairment of the steel sheet's formability can be suppressed. Therefore, the N content is adjusted to 0.0150% or less. Furthermore, N also causes weld defects during welding and hinders productivity. Therefore, the N content is preferably 0.0120% or less, and very preferably 0.0100% or less. The lower limit for nitrogen content can be 0%. By adjusting the nitrogen content to 0.0005% or less, manufacturing costs can be reduced even further. Therefore, the lower limit for nitrogen content can be adjusted to 0.0005%.

[21] O: 0.0100% or less Oxygen (O) is an element that forms an oxide and impairs formability, such as ductility, hole expansion capacity, draw flanging, and bending capacity. When the O content is 0.0100% or less, significant deterioration in the formability of the steel sheet can be suppressed. Therefore, the O content is adjusted to 0.0100% or less. The O content is preferably 0.0080% or less, and very preferably Lznonn / zznz / E / YiAi 0.0050% or less. The lower limit of the content of 0 can be 0%. By adjusting the oxygen content by 0.0001% or less, manufacturing costs can be reduced even further. Therefore, the lower limit for oxygen content can be adjusted to 0.0001%.

[22] The remainder of the composition of the steel sheet conforming to the modality may include Fe and impurities. Examples of impurities include elements that are inevitably incorporated from raw materials or steel scrap and / or in the steelmaking process and are permitted within a range where the properties of the steel sheet conforming to the modality are not impaired. Examples of impurities include H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Te, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, and Po. The total impurity content may be 0.100% or less.

[23] The steel sheet conforming to the modality may include elements as optional elements in place of a Fe portion. When the steel sheet does not include the following optional elements, the content of the elements is 0%.

[24] Nb: 0% to 0.060% Nitrogen (Nb) is an element that contributes to improving the strength of steel sheets by strengthening through precipitation, enhancing grain refinement by suppressing ferrite crystal grain growth, and strengthening dislocations by suppressing recrystallization. Nitrogen does not need to be included. Therefore, the lower limit for Nitrogen content is 0%. To achieve a sufficient strength-enhancing effect from Nitrogen, the Nitrogen content is preferably 0.005% or more, and very preferably 0.015% or more. Furthermore, when the Nb content is 0.060% or less, the remaining unrecrystallized ferrite caused by the promotion of recrystallization can be suppressed, and the formability of the steel sheet can be ensured. Therefore, the Nb content is adjusted to 0.060% or less. The Nb content is preferably 0.050% or less, and very preferably 0.040% or less.

[25] Ti: 0% to 0.100% Titanium (Ti) reduces the amounts of sulfur (S), nitrogen (N), and oxygen (O), leading to the formation of a coarse inclusion that acts as a fracture origin. Furthermore, Ti refines the structure, improving the balance between strength and conformability. Nitrogen (Nb) contributes to the improved strength of the steel sheet by strengthening through precipitation, enhancing grain refinement by suppressing ferrite crystal grain growth, and strengthening dislocations by suppressing recrystallization. Ti does not need to be included. Therefore, the lower limit for Ti content is 0%. To achieve a sufficient effect from Ti, the Ti content is preferably 0.015% or more, and very preferably 0.025% or more. Furthermore, when the Ti content is 0.100% or less, the formation of coarse Ti sulfide, coarse Ti nitride, or coarse Ti oxide can be suppressed, and the formability of the steel sheet can be ensured. Therefore, the Ti content is adjusted to 0.100% or less. Ideally, the Ti content is 0.075% or less, and very preferably 0.060% or less.

[26] V: 0% to 0.500% Volatile (V) is an element that contributes to improving the strength of steel sheets by strengthening through precipitation, enhancing grain refinement by suppressing ferrite crystal grain growth, and strengthening dislocations by suppressing recrystallization. It is not necessary to include V. Therefore, the lower limit for V content is 0%. To sufficiently achieve the strength-enhancing effect of V, the V content is preferably 0.010% or more, and very preferably 0.030%. Lznonn / zznz / E / YiAi more . Furthermore, when the V content is 0.500% or less, the deterioration of the steel sheet's formability caused by the precipitation of a large amount of carbonitrides can be suppressed. Therefore, the V content is adjusted to 0.500% or less.

[27] Cr: 0% to 1.00% Chromium (Cr) is an element that improves the hardenability of steel and contributes to the increased strength of steel sheets. It can be substituted for a portion of manganese (Mn). Cr does not need to be included. Therefore, the lower limit for Cr content is 0%. To achieve a sufficient strength-enhancing effect from Cr, the Cr content is preferably 0.05% or more, and very preferably 0.20% or more. Furthermore, when the Cr content is 1.00% or less, the formation of a coarse Cr carbide, which can act as a source of fracture, can be suppressed. Therefore, the Cr content is adjusted to 1.00% or less.

[28] Ni: 0% to 1.00% Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet, and it can be substituted for a portion of Mn. Ni does not need to be included. Therefore, the lower limit of the Ni content includes ίζηοηη / ζζηζ / E / γίΛΐ 0%. In order to obtain sufficiently the effect of improving resistance through Ni, the Ni content is preferably 0.05% or more and very preferably 0.20% or more. Furthermore, when the Ni content is 1.00% or less, the deterioration of the weldability of the steel sheet can be suppressed. Therefore, the Ni content is adjusted to 1.00% or less.

[29] Cu: 0% to 1.00% Copper (Cu) is an element present in steel as fine grains and contributes to improving the strength of the steel sheet. It can be substituted with a portion of carbon (C) and / or manganese (Mn). Copper does not need to be included. Therefore, the lower limit for copper content is 0%. To sufficiently achieve the strength-enhancing effect of copper, the copper content is preferably 0.05% or more, and very preferably 0.15% or more. Furthermore, when the copper content is 1.00% or less, the deterioration of the weldability of the steel sheet can be suppressed. Therefore, the copper content is adjusted to 1.00% or less.

[30] Mo: 0% to 1.00% Molybdenum (Mo) is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets, or it can be replaced by a portion of manganese (Mn). Mo does not need to be included. Therefore, the lower limit for Mo content is 0%. To sufficiently achieve the strength-enhancing effect of Mo, the Mo content is preferably 0.03% or more, and very preferably 0.06% or more. Furthermore, when the Mo content is 1.00% or less, the productivity decline caused by the deterioration of hot workability can be suppressed. Therefore, the Mo content is adjusted to 1.00% or less.

[31] W: 0% to 1,000% W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets, or it can be substituted for a portion of C and / or Mn. It is not necessary to include W. Therefore, the lower limit for W content is 0%. To sufficiently achieve the strength-enhancing effect of W, the W content is preferably 0.030% or more, and very preferably 0.100% or more. Furthermore, when the W content is 1,000% or less, the productivity decline caused by the deterioration of hot workability can be eliminated. Therefore, the W content is adjusted to 1,000% or less.

[32] B: 0% to 0.0100% Boron (B) is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. It can be substituted for a portion of manganese (Mn). Boron does not need to be included. Therefore, the lower limit for the boron content is 0%. To sufficiently achieve the strength-enhancing effect of boron, the boron content is preferably 0.0005% or more, and very preferably 0.0010% or more. Furthermore, when the B content is 0.0100% or less, the deterioration of the steel sheet strength caused by the formation of a B precipitate can be suppressed. Therefore, the B content is adjusted to 0.0100% or less.

[33] Sn: 0% to 1.00% Tin (Sn) is an element that suppresses the thickening of crystal grains and contributes to improving the strength of steel sheets. Tin does not need to be included. Therefore, the lower limit for tin content is 0%. To achieve a sufficient effect from tin, the tin content is preferably 0.01% or higher. Furthermore, when the Sn content is 1.00% or less, cracking during rolling caused by embrittlement of the steel sheet can be suppressed. Therefore, the Sn content is adjusted to 1.00% or less.

[34] Sb: 0% to 0.20% Antimony (Sb) is an element that suppresses crystal grain coarsening and contributes to improving the strength of steel sheets. Sb does not need to be included. Therefore, the lower limit for Sb content is 0%. To achieve a sufficient effect, the Sb content is preferably 0.005% or higher. Furthermore, when the Sb content is 0.20% or less, cracking during rolling caused by embrittlement of the steel sheet can be suppressed. Therefore, the Sb content is adjusted to 0.20% or less.

[35] One or two or more selected from Ca, Ce, Mg, Zr, La and REM: 0% to 0.0100% total The composition of the steel sheet in accordance with the modality may optionally include one or two or more selected from Ca, Ce, Mg, Zr, La and REM. Ca, Ce, Mg, Zr, La, and REM are elements that contribute to improving the formability of steel sheet. The lower limit of the total content of one or two selected elements of Ca, Ce, Mg, Zr, La, and REM includes 0%. In order to obtain a sufficient formability-enhancing effect, the total content is preferably 0.0001% or more, and very preferably 0.0010% or more. Furthermore, when the total content of one or two selected elements (Ca, Ce, Mg, Zr, La, and REM) is 0.0100% or less, the deterioration of the steel sheet's ductility can be suppressed. Therefore, the total content of these elements is adjusted to 0.0100% or less. The total content is preferably 0.0050% or less.

[36] REM (Rare Earth Metal) refers to a group Lznonn / zznz / E / YiAi of elements other than La and Ce that can be individually specified in the group of elements belonging to the lanthanides. In most cases, these elements are aggregated in the form of mischmetals. However, lanthanide-based elements other than La and Ce may inevitably be included.

[37] The microstructure of the steel sheet will now be described in accordance with the modality. In the steel sheet conforming to the modality, a microstructure at a position 1 / 4 of the thickness from a surface in the sheet thickness direction includes, in % by volume, ferrite: 80% or more, martensite: 2% or less, and residual austenite: 2% or less, and has a proportion of non-recrystallized ferrite in the ferrite of 5% or less. In this model, the reason for limiting the microstructures to the 1 / 4 thickness position in the thickness direction of the sheet from the surface is that the microstructures at this position are representative of the steel sheet and have a strong correlation with the mechanical properties of the steel sheet. All proportions of structures in the microstructure are volume percentages (% by volume).

[38] Ferrite: 80% or more Ferrite is a structure that has a Lznonn / zznz / E / YiAi Excellent conformability. When the ferrite volume percentage is 80% or more, the desired conformability can be obtained. Therefore, the ferrite volume percentage is adjusted to 80% or more. The ferrite volume percentage is preferably 85% or more, and very preferably 90% or more. A large amount of ferrite is preferable. Therefore, the ferrite volume percentage can be 100%. The ferrite described here also includes non-recrystallized ferrite. Lznonn / zznz / E / YiAi

[39] Proportion of non-recrystallized ferrite in ferrite: 5% or less Non-recrystallized ferrite is ferrite in which the deformation introduced by cold rolling or similar processes remains, and it has higher strength but lower ductility than typical ferrite. Therefore, in steel sheet conforming to this modality, the proportion of non-recrystallized ferrite in the ferrite is limited to 5% or less. The proportion of non-recrystallized ferrite in the ferrite is preferably adjusted to 3% or less and very preferably to 1% or less. To improve the formability of the steel sheet, it is even more preferable that no non-recrystallized ferrite be included. Therefore, the proportion of non-recrystallized ferrite in the ferrite may be 0%.

[40] Martensite: 2% or less Martensite is a structure that increases strength but acts as a source where small voids initiate during formation. When fine voids are initiated during formation, the desired impact and fracture strength cannot be achieved. To suppress the formation of fine voids during formation, the volume percentage of martensite is adjusted to 2% or less. The volume percentage of martensite is preferably 1% or less and very preferably 0%. In this modality, the volume percentage of martensite in the MA (region that includes both martensite and residual austenite) includes the volume percentage of martensite.

[41] Residual austenite: 2% or less Residual austenite is a structure that improves the balance between strength and ductility in steel sheets, but it acts as a source of fine voids during forming. To suppress the formation of fine voids during forming, the volume percentage of residual austenite is adjusted to 2% or less. The volume percentage of residual austenite is preferably 1% or less, and very preferably 0%. Lznonn / zznz / E / YiAi

[42] Remainder in Microstructure: 20% or less Examples of residues in the microstructure include pearlite and bainite. By setting the volume percentage of residues in the microstructure to 20% or less, the desired impact and fracture resistance can be achieved. Therefore, the total volume percentage of the structures can be 20% or less. The volume percentage of residues in the microstructure is as small as possible and can be 10% or less, 5% or less, or 0%.

[43] A method for measuring the volume percentage in the microstructure will now be described. Test pieces with a cross-section parallel to the rolling direction of the steel sheet and perpendicular to its surface are collected from the steel sheet. The cross-section of each test piece is polished and then etched with nital. In a region of t / 8 to 3t / 8 (t representing the sheet thickness) from the surface, where the position of 1 / 4 of the thickness from the surface in the sheet thickness direction is the center, an area of ​​2.0 x 10⁻⁹ m² or more in total was observed using a field emission scanning electron microscope (FE-SEM) in one or more fields of view. Various structures are identified based on microstructural morphology (e.g., a crystal grain shape, a subgrain boundary in the crystal). Lznonn / zznz / E / YiAi grains, or a carbide formation state), and their area ratios (% area) are measured. The area ratio of each of the resulting structures can be considered as the volume percentage. As a result, the volume percentages of ferrite, unrecrystallized ferrite, martensite, and MA (region that includes martensite and residual austenite) are obtained.

[44] When observing multiple fields of view, each of the analyzed areas within the fields is 4.0 x 10⁻¹⁰ m² or larger. Furthermore, the area ratios are measured using a point-counting method in each field of view. Fifteen lines parallel to the rolling direction and fifteen lines perpendicular to the rolling direction are drawn, and structures are identified at 225 points of intersection between these lines. Specifically, a massive region where cementite and a subgrain boundary are absent is identified as ferrite, and a massive region where a subgrain boundary is present but cementite is absent is identified as unrecrystallized ferrite. Additionally, martensite and MA, which contain a large amount of carbon solid solution, appear white due to their greater brightness compared to other structures and can therefore be distinguished from them.Using the method described above, the volume percentage of ferrite, the volume percentage of unrecrystallized ferrite, and the total volume percentage of martensite and MA are obtained. Calculating the sum of the obtained volume percentage of ferrite and the obtained volume percentage of unrecrystallized ferrite yields the volume percentage of ferrite. Furthermore, dividing the obtained volume percentage of unrecrystallized ferrite by the volume percentage of ferrite yields the proportion of unrecrystallized ferrite in the ferrite.

[45] The volume percentage of residual austenite is analyzed using an X-ray diffraction method. In the region from t / 8 to 3t / 8 (t represents the thickness of the sheet) from the surface of the test piece, a surface parallel to the surface of the steel sheet is given a mirror finish, and the volume percentage of FCC steel is analyzed using the X-ray diffraction method. Furthermore, by subtracting the obtained volume percentage of residual austenite from the total volume percentage of martensite and MA obtained by observation with the FE-SEM, the volume percentage of martensite is obtained. Furthermore, by subtracting the volume percentage of ferrite, the volume percentage of martensite, and the volume percentage of residual austenite from 100%, the volume percentage of the remainder in the microstructure is obtained.

[46] In the microstructure of the steel sheet stretched 10% at the position of 1 / 4 of the thickness from the surface in the direction of the sheet thickness, a numerical density of voids with a maximum diameter of 1.0 pm or more is 1.0 × 109 pieces / m2 or less. In steel sheets conforming to the specified method, the number of voids in the microstructure of the steel sheet stretched by 10% at the 1 / 4 thickness point from the surface in the thickness direction is 1.0 × 10⁹ pieces / m² or less. These voids collapse in the microstructure of the steel sheet before formation and are not visible. However, they form after formation and become visible. In steel sheets conforming to the specified method, the number of voids is reduced. Therefore, the number of voids is also low in the pre-formation stage. However, as described above, the voids collapse and are not visible in the pre-formation stage. Therefore, in this modality, the numerical density of voids is defined after the formation of voids in the steel sheet stretched to 10%.

[47] When the number density of voids having a maximum diameter of 1.0 pm or more is 1.0 × 10⁹ pieces / m² or less, the desired impact and fracture resistance can be obtained. The number density of the voids is preferably 0.7 × 10⁹ pieces / m² or less and very preferably 0.5 × 10⁹ pieces / m² or less. The maximum diameter refers to the maximum diameter of the voids, and when the voids are flat, the maximum diameter refers to the length of the major axis.

[48] ​​It is assumed that when there are a large number of voids with a maximum diameter of 1.0 pm or more, the voids combine to form cracks during forming and are likely to break. When the maximum diameter of the voids is less than 1.0 pm, it is difficult for the voids to combine. Therefore, it is assumed that the impact strength and fracture resistance are not affected. Therefore, in this modality, the numerical density of voids with a maximum diameter of 1.0 pm or more is defined.

[49] In the microstructure of the steel sheet stretched 10% at the position 1 / 4 of the thickness from the surface in the direction of the sheet thickness, the numerical density of voids having a maximum diameter of 1.0 pm or more is measured using the following method. A 5-inch test piece is prepared according to JIS Z 2241:2011, and the rolling direction of the steel sheet is established as the tension axis for the tensile test. A 10% plastic strain is applied and unloaded. A small piece is cut from the center of a parallel section of the test piece, and an observation piece is collected with a cross-section parallel to the rolling direction and perpendicular to the surface of the steel sheet. The observation section of the observation piece is polished and subsequently etched with nital. In a region from t / 8 to 3t / 8 (t representing the sheet thickness) from the surface, where the position of 1 / 4 of the thickness from the surface in the sheet thickness direction is the center, an area of ​​2.0 × 10⁻⁹ m² or more in total using a field emission scanning electron microscope (FE-SEM) in one or more fields of view, and the number of holes having a maximum diameter of 1.0 pm or more is counted. Dividing the obtained number of holes by the area of ​​the observed section gives the numerical density of holes having a maximum diameter of 1.0 pm or more. Lznonn / zznz / E / YiAi

[50] Average ferrite grain size: 6.0 pm to 15.0 pm In the microstructure at the 1 / 4 thickness position from the surface of the steel sheet, in accordance with the sheet thickness direction, the average ferrite grain size is preferably from 6.0 pm to 15.0 pm. By adjusting the average ferrite grain size to 6.0 pm to 15.0 pm, the balance between strength and ductility can be further improved; that is, high strength and excellent ductility can be achieved simultaneously.

[51] A method for measuring the average grain size of ferrite will now be described. The average ferrite grain size is obtained using a linear analysis method. In the fields of view where the volume percentages of ferrite, unrecrystallized ferrite, martensite, and MA are measured, one or more straight lines with a total length of 200 pm or more are drawn in the rolling direction. One is added to the number of intersection points between the straight line and a ferrite grain boundary, and the resulting value is divided by the length of the straight line. The resulting value is established as the average grain size.

[52] Steel sheet conforming to this modality may include a galvanized layer or a zinc alloy-plated layer on one or both surfaces of the steel sheet. In addition, steel sheet conforming to this modality may include a galvanized-annealed layer or a galvanized-annealed layer with an alloy obtained by alloying a galvanized layer or a zinc alloy-plated layer.

[53] The plated layer formed on one or both surfaces of the steel sheet in accordance with this modality is preferably a galvanized layer or a zinc alloy plated layer comprising zinc as the principal component. It is preferable that the zinc alloy plated layer comprises nickel as an alloy component.

[54] The galvanized layer and the zinc alloy-plated layer are formed using a hot-dip plating method, an electroplating method, or a deposition plating method. When the Al content in the galvanized layer is 0.5% by mass or less, adhesion between the surface of the steel sheet and the galvanized layer can be ensured. Therefore, the Al content in the galvanized layer is preferably 0.5% by mass or less. When the galvanized layer is a hot-dip galvanized layer, to improve the adhesion between the surface of the steel sheet and the galvanized layer, the Fe content in the hot-dip galvanized layer is preferably 3.0% by mass or less. When the galvanized layer is an electrogalvanized layer, the Fe content in the plated layer is preferably 0.5% by mass or less from the point of view of improving corrosion resistance.

[55] The galvanized layer and the zinc alloy-plated layer may include one or two or more selected from the group consisting of Al, Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Ge, Hf, Zr, I, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, Pb, Rb, Sb, Si, Sn, Sr, Ta, Ti, V, W, Zr, and REM within a range where the corrosion resistance or formability of the steel sheet is not impaired. In particular, Ni, Al, and Mg are effective in improving corrosion resistance.

[56] The galvanized layer or the zinc alloy-plated layer on the surface of the steel sheet, in accordance with the modality, may be a galvanized-annealed layer or an alloyed galvanized-annealed layer obtained by alloying a galvanized layer or a zinc alloy-plated layer. When the hot-dip galvanized layer or the hot-dip zinc alloy-plated layer is alloyed, from the standpoint of improving the adhesion between the surface of the steel sheet and the alloyed layer, the Fe content in the alloyed hot-dip galvanized layer (galvanized-annealed layer) or the alloyed hot-dip zinc alloy-plated layer (galvanized-annealed layer) is preferably from 7.0% to 13.0% by mass.By alloying the steel sheet, which includes a hot-dip galvanized layer or a hot-dip zinc alloy-plated layer, iron is incorporated into the plated layer, increasing its iron content. As a result, the iron content can reach 7.0% by mass or higher. That is, a galvanized layer with an iron content of 7.0% by mass or higher is either an annealed galvanized layer or an alloyed annealed galvanized layer.

[57] The Fe content in the alloyed hot-dip galvanized layer (galvanized-annealed layer) or the alloyed hot-dip zinc alloy plated layer (galvanized-annealed alloyed layer) can be obtained using this method. Only the plated layer is removed by dissolving it in a 5% v / v aqueous solution of HCl to which an inhibitor is added. By measuring the Fe content in the resulting solution using inductively coupled plasma atomic emission spectrometry (ICP-AES), the Fe content (% by mass) in the galvanized layer is obtained.

[58] The thickness of the steel sheet conforming to the modality is not limited to a specific range, but considering versatility or manufacturability, it is preferably from 0.2 mm to 5.0 mm. When the sheet thickness is 0.2 mm or more, the shape of the steel sheet can be easily maintained flat, and dimensional accuracy and form can be improved. Therefore, the sheet thickness is preferably 0.2 mm or more. The sheet thickness is very preferably 0.4 mm or more. On the other hand, when the sheet thickness is 5.0 mm or less, appropriate tensioning and temperature control can be easily achieved during the manufacturing process, resulting in a homogeneous structure. Therefore, a sheet thickness of 5.0 mm or less is preferable. Lznonn / zznz / E / YiAi 4.5 mm or less.

[59] In the steel sheet conforming to the modality, the tensile strength is preferably 340 MPa or higher. The tensile strength is very preferably 400 MPa or higher. The upper limit is not particularly restricted and may be, for example, 700 MPa or lower or 500 MPa or lower. Tensile strength is measured by preparing a 5-inch test piece in accordance with JIS Z 2241:2011 and establishing the rolling direction of the steel sheet as the tension axis for performing the tensile test.

[60] A method for manufacturing the steel sheet in accordance with the modality will now be described. Provided the steel sheet conforming to the specified method has the characteristics described above, its effects can be achieved regardless of the manufacturing method. A manufacturing method that includes multiple processes is preferable because the steel sheet conforming to the specified method can be manufactured consistently. In the following manufacturing method, by controlling the processes in a complex and indivisible manner, a steel sheet with the desired characteristics can be manufactured. (I) a hot rolling process of heating a piece of steel having a predetermined composition from 1150°C to 1320°C, completing the hot rolling of such Lznonn / zznz / E / YiAi so that the completion temperature of hot rolling is from 850°C to 930°C, starting cooling after 1.5 seconds or more, and cooling the steel piece to a temperature range of 500°C or lower to obtain a hot-rolled steel sheet such that an average cooling rate in a temperature range from a cooling start temperature of 500°C is 20°C / s faster; (II) a reheating process of heating the hot-rolled steel sheet to a temperature range of 500°C to 700°C; (III) a cooling process of cooling the hot-rolled steel sheet to room temperature; (IV) a cold rolling process of cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet such that the total rolling reduction is from 30% to 90% and the final cold-rolling temperature is from 120°C to 250°C; and (V) an annealing process of heating the cold-rolled steel sheet to an annealing temperature of 720°C to 850°C and cooling to a temperature range of 500°C or lower. The preferred conditions for each of the processes will be described below.

[61] Hot rolling process First, the steel blank, which has the composition of the steel sheet described above, is heated to between 1150°C and 1320°C. When the heating temperature is 1150°C or higher, the carbides dissolve sufficiently. Therefore, the remaining coarse carbide in the hot-rolled steel sheet can be eliminated. Furthermore, when the heating temperature of the steel blank is 1320°C or lower, the thickening of the crystal grains can be suppressed, and the structure can be sufficiently homogenized by hot rolling. From a manufacturing cost perspective, it is preferable for the steel blank to be produced by continuous casting. However, the steel blank can be produced using another casting method (e.g., an ingot casting method).

[62] After heating the steel part, hot rolling is performed such that the final rolling temperature is between 850°C and 930°C. When the final rolling temperature is 850°C or higher, rolling is performed in a single-phase range. This suppresses the anisotropy of the metallographic structure. Therefore, the final rolling temperature is set to 850°C or higher. Furthermore, when the final rolling temperature is 930°C or lower, excessive thickening of the primary austenite structure can be suppressed, and the structure can be homogenized. Therefore, the final rolling temperature is set to 930°C or lower.

[63] In the hot rolling process, Expression (1) must be satisfied within a temperature range of 1000°C or less. By controlling a roll program such that Expression (1) is satisfied within a temperature range of 1000°C or less, recrystallization progresses uniformly and the carbides precipitate finely and homogeneously in the steel. By satisfying Expression (1) within a temperature range of 1000°C or lower, carbide segregation can be suppressed, as can the formation of voids in a region where carbides segregate.

[64] D()= / «9\p'=a^exp[r^273) φ = α10· exp *27J Λ, / ZÍAUS\ ( r \D' =D<~ia4- - AAί \ 'Pt'—1 £)'j * w . (^)“ j|+10<-ν . (±)“ jj Dn< 12.5 • · · Expression (1) ίζηοηη / ζζηζ / Ε / γίΛΐ

[65] Dn represents an index that represents a degree of advancement of the precipitation of a fine carbide in a temperature range of 1000°C or lower in the hot rolling process. The reference numbers in Expression (1) are as follows. n: the number of rolling passes in the temperature range of 1000°C or lower Ti: a rolling temperature in an i-th rolling pass ti: an elapsed time [s] from the i-th rolling pass to an i+l-th rolling pass or an elapsed time [s] until the temperature of a steel sheet drops to 850°C from the i-th rolling pass Here, i represents a natural number from 1 to n. Tη-ι: a sheet thickness [mm] before the i-th rolling pass in the temperature range of 1000°C or lower hi: a sheet thickness [mm] after the i-th rolling pass in the temperature range of 1000°C or lower aian: constants (ai = 2.54 χ 10-6, a2 = 3.62 χ 10-4, a3 = -6.38 χ 10-1, a4 = -3.00 χ 10'1, a5 = 8.50 χ 10'1, a6 = -8.50 χ 10'4, a7 = 2.40 χ 10°, a8 = 7.83 χ 10'13, a9 = 2.80 χ 105, aio = 6.00 χ IO'12, and an = 2.80 χ 105)

[66] The steel piece begins to cool after 1.5 seconds or more after the completion of hot rolling, and is cooled to a temperature range of 500°C or less, such that the average cooling rate over a temperature range from the start of cooling to 500°C is 20°C / s faster. As a result, a hot-rolled steel sheet is obtained. Ensuring that the time from the end of hot rolling to the start of cooling is 1.5 seconds or more allows recrystallization and results in a homogeneous structure. Setting the cooling start time to 5.0 seconds or less can suppress abnormal crystal grain growth and grain size variations in the steel sheet, which is preferable.

[67] By adjusting the average cooling rate in the temperature range starting at 500°C to be 20°C / s faster, a carbide such as cementite can be made to precipitate finely in the steel. When the average cooling rate in the temperature range is 20°C / s faster, the formation of a coarse carbide can be suppressed, and a desired microstructure can be obtained in the final steel sheet. The upper limit of the average cooling rate is not particularly restricted. Since a special cooling medium is required to achieve a cooling rate greater than 200°C / s, a lower average cooling rate is preferable from a production cost perspective.

[68] In this modality, the average cooling rate refers to a value obtained by dividing a temperature difference between an initial point and a final point by an interval that will be established by the time elapsed from the initial point to the final point. Lznonn / zznz / E / YiAi

[69] Reheating process In the reheating process, the hot-rolled steel sheet is heated to a temperature range of 500°C to 700°C. By adjusting the maximum reheating temperature (the highest temperature reached during the reheating process) between 500°C and 700°C, a desired microstructure can be achieved, and impact and fracture resistance can be ensured.

[70] Furthermore, during the reheating process, a temperature history within the 500°C to 700°C range must satisfy Expression (2). This heating allows for the uniform precipitation of fine carbide within the steel. By satisfying Expression (2) within the 500°C to 700°C temperature range, fine carbide precipitates in the steel. As a result, the amount of carbon solid solution can be reduced, thereby lowering the strength of the hot-rolled steel sheet. ίζηοηη / ζζηζ / Ε / γίΛΐ

[71] i + 273 / t„ = 10 ^11^1'(' tOÍl-rO.OB.SO I„ + Z / 3 / Kn= (Tn+ 273) · {log10tn+ 20 · (1 + 0.08.YES)} / <20 > 1-50 x 104• · · Expression (2)

[72] In Expression (2), K20 represents an index that represents a degree of progress of fine carbide precipitation in a 20th period when a temperature history in the temperature range of 500°C to 700°C of the reheating process is divided into 20 periods with respect to time. The reference numbers in Expression (2) are as follows. Tn: an average temperature [°C] in an nth period when a temperature history in the temperature range of 500°C to 700°C is divided into 20 periods with respect to time ALr: a time [hr] in one of the 20 periods into which a total residence time is divided in the temperature interval from 500°C to 700°C, where ti = AtK Si: a Si content [% by mass] Here, loglO is a common logarithm with a base of 10. Lznonn / zznz / E / YiAi

[73] Cooling process After the reheating process, the hot-rolled steel sheet is cooled to room temperature. At this point, the cooling rate is not particularly limited, and examples of cooling methods include air cooling. For example, if the room temperature is 25°C, the average cooling rate to room temperature during air cooling after reheating is 10°C / s slower.

[74] Cold rolling process The cooled, hot-rolled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet such that the total rolling reduction is 30% to 90% and the cold-rolling completion temperature is 120°C to 250°C. This results in a cold-rolled steel sheet. When the total rolling reduction is 30% or more, recrystallization during subsequent heat treatment can progress sufficiently to suppress the remaining unrecrystallized ferrite and ultimately achieve the desired microstructure. Therefore, the total rolling reduction during cold rolling is adjusted to 30% or more. The total rolling reduction is preferably 45% or more, and very preferably 60% or more.Furthermore, when the total rolling reduction in cold rolling is 90% or less, an increase in the anisotropy of the steel sheet can be suppressed, the numerical density of voids can be reduced, and formability can be ensured. Therefore, the total rolling reduction during cold rolling is set at 90% or less. To further improve formability, the total rolling reduction is preferably 85% or less.

[75] When the cold rolling completion temperature is 120°C or higher, the void density can be reduced, and the desired microstructure can finally be achieved. Therefore, the cold rolling completion temperature is set at 120°C or higher. The cold rolling completion temperature is preferably 150°C or higher, and very preferably 170°C or higher. Furthermore, when the cold rolling completion temperature is 250°C or lower, sufficient recrystallization can be allowed to progress, ensuring formability. To ensure efficient recrystallization and formability, the cold rolling completion temperature is set to 250°C or lower. The final temperature of cold rolling is preferably 230°C or lower or 200°C or lower.

[76] Annealing process Heating Next, the cold-rolled steel sheet undergoes heat treatment (annealing). First, the cold-rolled steel sheet is heated to an annealing temperature of 720°C to 850°C. During this heating, within the temperature range of 720°C to the annealing temperature (720°C to 850°C), a stress of 20 MPa or more must be applied, and a temperature history is required to satisfy Equation (3). By applying a stress of 20 MPa or more within the temperature range of 720°C to the annealing temperature, the voids formed during cold rolling are sufficiently blocked, and any voids present in the region are blocked after formation. When the stress is less than 20 MPa, void formation during cold rolling can be sufficiently suppressed. Therefore, the applied stress is preferably 25 MPa or higher.Due to the temperature history in the temperature range from 720°C to the annealing temperature that satisfies Expression (3), recrystallization and cementite dissolution are promoted. As a result, it is finally possible. Lznonn / zznz / E / YiAi to obtain a desired microstructure.

[77] ίζηοηη / ζζηζ / Ε / γίΛΐ Σίί 1 / ^2 \ · Ο 5 τΛ-exp Μ ·ί0< 20.0κζο \ι,' ι -1 • · · Expression (3)

[78] The reference numbers in Expression (3) are as follows. K20: a value obtained by Expression (2) di and d2: constants (di = 9.67 × 1010 and d2 = 1.25 * 104) Ti: an average heat treatment temperature [°C] in an i-th period when a temperature history in the temperature range of 720°C to the annealing temperature is divided into 10 periods with respect to time t': 1 / 10 [s] of a residence time in the temperature range of 720°C to the annealing temperature

[79] . The annealing temperature in the annealing process is set to 720°C or higher. When the annealing temperature is 720°C or higher, dissolution and the remaining coarse cementite can be suppressed, recrystallization can be made sufficiently advanced, and the desired microstructure can be obtained. The annealing temperature is preferably 750°C or higher and very preferably 780°C or higher. Furthermore, when the annealing temperature is 850°C or lower, an excessive reduction in the ferrite volume percent can be suppressed. Consequently, the annealing temperature is set to 850°C or less.In order to increase the volume percentage of ferrite to further improve formability, the annealing temperature is preferably 830°C or lower and very preferably 810°C or lower.

[80] Retention The holding time at the annealing temperature, i.e., the time required for the annealing temperature to reach 720°C again from the 720°C or higher range to the 720°C to 850°C range during heating, is preferably 3 seconds or more. By setting the holding time to 3 seconds or more, sufficient cementite can be dissolved, ensuring formability. The holding time is preferably 10 seconds or more, and very preferably 25 seconds or more. The upper limit of the holding time is not particularly restricted, but even when the holding time exceeds 200 seconds, there is no influence on the properties of the steel sheet. Therefore, from a production cost perspective, the upper limit of the holding time is preferably 200 seconds or less.

[81] Cooling After heating to the annealing temperature and Lznonn / zznz / E / YiAi to maintain that temperature during the holding time, the steel sheet cools. During the cooling of the steel sheet to a temperature range of 500°C or less, a temperature history within the range of 720°C to 500°C must satisfy Expression (4). By performing cooling such that the temperature history within the range of 720°C to 500°C satisfies Expression (4), the formation of a hard phase (martensite and residual austenite) is suppressed. As a result, a desired microstructure can ultimately be obtained.

[82] _ । jjy (5, + 92 Nb° ' + g. · Ti5) (1 + g. · ~ ) (Δ, + gsΔ,05) “ ' ^3 'Vl ' • exp(~9*} ' í'ur' > 1.0 \ T¡+ 273 / . .zs• · · Expression (4)

[83] The reference numbers in Expression (4) are as follows. Aí: 750 - 18 x Si - 17 x Mn - 10 x Cr - 8 x Ni + 15 x Al - Ti where each of the elements represents a mass % content of the element, and when the element is not included, 0 is substituted as the content of the element. Furthermore, when a calculated value of Ai is a negative value, Ai is set to 0. Lznonn / zznz / E / YiAi giae: constants (gi = 1.00 χ 101, g2 = 1.46 χ ΙΟ-1, g3 = 1.14 χ 10-1, g4 = 2.24 χ 10u, g5= 4.53 χ 10°, and g6= 4.83 χ 103) Nb, Mo, Si, Mn, Cr, Ni, and Al: a content [% by mass] of each of the elements, where, when the element is not included, 0 is substituted as the content of the element Ti*: an effective Ti content represented by Ti - 42 / 14 χ N, where Ti and N represent a content [% by mass] of each of the elements, when the element is not included, 0 is substituted as the content of the element, a minimum value is set to 0 Ti: an average heat treatment temperature [°C] in an i-th period when a temperature history in the temperature range of 720°C to 500°C is divided into 10 periods with respect to time Aci and Ac3: a transformation start temperature and a transformation end temperature [°C] during heating Tmax: the highest heating temperature [°C] in a heat treatment process t': 1 / 10 [s] of a residence time in the temperature range of 720°C to 500°C.

[84] After the annealing process, hot-dip galvanizing or plating can be carried out with Lznonn / zznz / E / YiAi hot-dip zinc alloy is applied to steel sheets at a temperature of 500°C or lower. The steel sheet can then be reheated before being immersed in a plating bath. The plated steel sheet can also be heated to alloy the plating layer. Aci and AC3 in this mode are obtained by heating a small piece cut from a cold-rolled steel sheet provided in the annealing process and obtaining a change in the thermal expansion of the small piece during heating.

[85] By performing electrolytic plating or deposition on the steel sheet after the annealing process to form a galvanized layer on one or both surfaces of the steel sheet, a galvanized steel sheet including the galvanized layer can be manufactured. The atmosphere in the annealing process can be controlled to reshape the surface of the steel sheet. For example, heating in a decarburizing atmosphere produces a steel sheet with excellent bending capacity where a portion of the surface layer is properly decarburized.

[86] Tempering rolling process After the annealing process, quench rolling can be performed to achieve a total reduction of 0.05% to 2.00%. During quench rolling, the surface shape can be flattened and the surface roughness adjusted. Examples

[87] Examples of the present invention will now be described, but the conditions of the examples are merely illustrative to confirm the operability and effects of the present invention. The present invention is not limited to these example conditions. The present invention may adopt various conditions within a range that does not depart from the scope of the present invention, provided that the object of the present invention can be achieved under those conditions.

[88] Examples of the present invention will now be described, but the conditions of the examples are merely illustrative to describe the operability and effects of the present invention. The present invention is not limited to these example conditions. The present invention may adopt various conditions within a range that does not depart from the scope of the present invention, provided that the object of the present invention can be achieved under those conditions.

[89] The cast steels having the chemical compositions shown in Table 1 were cast to Lznonn / zznz / E / YiAi manufacture steel parts. Next, hot-rolled steel sheets were obtained by hot rolling the steel parts under the conditions shown in Table 2. Table 2 shows the Dn obtained from the hot rolling conditions in a temperature range of 1000°C or less in the hot rolling process and Expression (1). Next, the steel pieces were reheated under the conditions shown in Table 2. Table 2 shows the K20 obtained from the temperature history over a temperature range of 500°C to 700°C during the reheating process and Equation (2). After reheating, the hot-rolled steel sheets were cooled to room temperature (25°C) at an average cooling rate 10°C / s slower.

[90] Next, steel sheets were obtained by performing cold rolling, heat treatment (annealing), and quench rolling on hot-rolled steel sheets under the conditions shown in Tables 3-1 and 3-2. For annealing, the steel sheets were heated to the annealing temperatures shown in Tables 3-1 and 3-2 and held at those temperatures for 3 seconds to 200 seconds (the time required until the annealing temperature reached 720°C again from the interval of 720°C or higher through holding). Lznonn / zznz / E / YiAi steel at room temperature to obtain an alloyed galvanized-annealed steel sheet. GA: a process of cooling the steel sheet to a temperature range of 500°C or lower in the annealing process, immersing the steel sheet in a molten zinc bath and reheating the steel sheet to 560°C to alloy and cooling the steel sheet to room temperature to obtain a hot-dip galvanized-annealed (GA) steel sheet. GI: a process of cooling the steel sheet to a temperature range of 500°C or lower in the annealing process, immersing the steel sheet in a molten zinc bath and cooling the steel sheet to room temperature to obtain a hot-dip galvanized steel sheet (IG). Deposition: a process of performing deposition plating after the annealing process to obtain a galvanized steel sheet. EG: a process of performing electrogalvanizing after the annealing process to obtain an electrogalvanized steel sheet (EG).

[93] Tables 4-1 and 4-2 show the properties of the steel sheets obtained under the manufacturing conditions shown in Tables 1 to 3-2. As a result of the Based on the structural observations made using the method described above, Tables 4-1 and 4-2 show the volume percent of ferrite, the proportion of unrecrystallized ferrite to ferrite, the volume percent of martensite, the volume percent of residual austenite, and the average ferrite grain size. The proportion of unrecrystallized ferrite to ferrite was measured using OIM Data Collection and OIM Data Analysis software manufactured by TSL. Tables 4-1 and 4-2 also show the number density of voids with a maximum diameter of 1.0 pm or more, measured using the method described above. The thickness of the steel sheet was the same as the thickness of the sheet after rolling, as shown in Tables 3-1 and 3-2.

[94] With respect to the alloyed steel sheet, the Fe content in the alloyed hot-dip galvanized layer (galvanized-annealed layer) or the alloyed hot-dip zinc alloy plated layer (alloyed galvanized-annealed layer) was measured using the method described above.

[95] The veneered layers in Tables 4-1 and 4-2 are as follows. Zinc alloy plated: zinc alloy plated layer Galvanized-annealed with alloy: galvanized-annealed layer with alloy GA: hot-dip galvanized-annealed layer formed by immersing the steel sheet in a molten zinc bath and alloying the steel sheet Gl: hot-dip galvanized layer formed by immersing the steel sheet in a molten zinc bath Deposited: galvanized layer formed by deposition plating EG: galvanized layer formed by electrogalvanizing

[96] Tables 5-1 and 5-2 show the properties of the steel sheets obtained under the manufacturing conditions of Tables 1 through 3-2. The yield strength (YS) and ultimate tensile strength were obtained by performing a tensile test. A 5-inch test piece was prepared in accordance with JIS Z 2241:2011, and the rolling direction of the steel sheet was established as the tension axis for performing the tensile test. A steel sheet in which the ultimate tensile strength in the tensile test was 340 MPa or higher was determined to have excellent strength and passed. On the other hand, a steel sheet in which the ultimate tensile strength in the tensile test was less than 340 MPa was determined to have poor strength and failed. In addition, a steel sheet in which the uniform elongation (uEl) obtained from the tensile test was 15% or more was determined to have excellent formability and passed.On the other hand, it was determined that a steel sheet in which the uniform elongation was less than 15% had poor formability and failed.

[97] A tensile test was performed under the same conditions as the tensile test described above, and a 15% stress was applied and unloaded. A semicircular notch having a radius of 1.0 mm was provided at both ends of the center of a parallel body of the test piece, and the tensile test was again performed until the test piece fractured at -40°C. As a result, a breaking strength σ2 at -40°C and a maximum stress σi before unloading were obtained.

[98] A Charpy impact test was then performed. When the thickness of the steel sheet was less than 2.5 mm, a stacked Charpy test piece was obtained by stacking the steel sheets until the total thickness exceeded 5.0 mm, securing the steel sheets with bolts, and providing a V-notch with a depth of 2 mm. The other conditions were determined according to JIS Z 2242:2018. As a result, a ductile-brittle transition temperature was obtained at which the surface ratio of brittle fracture was 50% or more.

[99] With respect to a steel sheet where the value (σ2 / σ1) obtained by dividing the breaking stress σ2 at -40°C by the maximum stress σA before unloading obtained using the method described above was 0.70 or less and the ductile-brittle transition temperature where a brittle fracture surface ratio was 50% or more was -40°C or lower, it was determined that this steel sheet had sufficiently high deformability during deformation by impact after forming (excellent impact resistance) and it was determined that it passed. On the other hand, with respect to a steel sheet where o2 / ol was greater than 0.70 and / or the ductile-brittle transition temperature where the surface ratio of brittle fracture was 50% or more was greater than -40°C, it was determined that this steel sheet had little impact resistance and failed.

[100] Table ίζηοηη / ζζηζ / Ε / γίΛΐ

[101] Table 2: Ιζονην / ζηζ / Ε / γίΛί Steel Hot-rolled steel sheet Hot rolling process Reheating process Note Heating temperature of the steel part Laminate completion Dn Time to start of cooling Average cooling rate from cooling start temperature to 500°C Maximum reheating temperature K20 °C °C °C / s °C »104 A Al 1185 907 11.9 3.2 69 534 1.68 Example A A2 1265 904 8.1 1.6 26 604 1.63 Example A A3 1286 915 14.5 3.1 41 644 1.60 Comparative example A A4 1306 882 7.2 2.9 65 478 - Comparative example B B1 1161 871 8.0 2.0 34 612 1.55 Example B B2 1236 904 8.7 2.3 51 734 - Comparative Example B B3 1216 885 11.7 2.2 41 589 167 Example C 01 1184 916 10.3 1.7 82 674 1.66 Example C C2 1278 925 8.9 2.3 46 597 1.78 Example C C3 1130 912 11.2 2.2 26 562 1.72 Comparative Example DD 1191 908 8.8 2.5 34 630 1.86 Example EE 1269 881 6.9 3.0 41 571 1.58 Example F F1 1216 920 10.4 1.9 78 685 1.69 Example F F2 1314 881 9.5 2.9 38 617 1.79 Example F F3 1261 905 8.3 2.9 14 520 1.55 Comparative Example GG 1246 898 10.2 2.7 40 630 1.65 Example HH 1250 857 7.5 3.2 42 538 1.52 Example 1 1 1302 913 10.0 2.6 47 663 1.64 Example JJ 1262 880 9.1 3.4 54 544 1.67 Example K K1 1169 901 11.0 3.1 39 522 1.74 Example K K2 1222 876 6.6 2.3 23 693 1.81 Example K K3 1278 839 - 2.2 37 620 1.90 Comparative Example LL 1283 887 6.9 3.1 47 561 1.72 Example MM 1224 874 9.0 2.0 56 623 1.81 Example NN 1250 915 10.9 2.6 44 680 1.61 Example 0 0 1260 885 5.5 2.4 47 608 1.64 Example P P1 1215 906 8.3 1.8 63 581 1.80 Example P P2 1209 911 7.3 3.1 35 506 1.65 Example P P3 1281 892 10.8 2.8 38 539 1.45 Comparative Example P P4 1247 906 10.5 0.8 32 601 1.52 Comparative Example QQ 1226 883 6.0 3.4 35 584 1.61 Example RR 1236 864 8.4 2.9 50 600 1.56 Example SS 1173 880 7.1 2.0 61 577 1.56 Example TT 1196 894 7.9 1.7 66 656 1.69 Example uu 1212 908 9.9 2.3 56 620 1.94 Example VV 1183 879 5.9 2.6 38 661 1.81 Example w Wl 1175 877 9.6 2.7 33 684 2.00 Example w W2 1183 888 9.3 2.4 50 520 1.52 Example. w WG 1217 948 8.1 2.4 34 565 1.80 Comparative example w W4 1348 902 8.9 2.9 27 568 1.59 Comparative example XX 1242 900 7.1 3.0 73 568 1.73 Example YY 1246 913 7.3 2.6 37 545 1.75 Example z Z 1272 907 9.4 2.5 47 566 1.74 Example AA AA 1243 905 7.8 2.3 52 629 1.93 Comparative example AB AB 1270 895 9.5 2.4 45 620 1.72 Comparative example AC AC 1233 893 9.5 3.0 41 595 1.86 Comparative example AD AD 1222 871 6.6 2.5 40 544 1.69 Comparative example The underline indicates that the value is outside the range of the present invention. Lznonn / zznz / E / YiAi 102] Table Note Example | Example | Comparative Example Comparative Example Example | Comparative Example Comparative Example Comparative Example Example | Comparative Example Example | Example | Comparative Example Example | Comparative Example Example | Example | Example | Plating Process < e ω e LU Deposition | < o Tempering Rolling Process Total Rolling Reduction 5? 0.45 o 0.59 on oo 035 0.46 040 o 0.36 0.33 0.32 0.23 un o 0 24 ooo > Annealing Process Cooling Left side of expression (4) in co un co *q - £ un 00 is 51 -o un Csi 00 csi un Heating Middle side of expression (3) í • 10 5 is oo un -o in oo» c* 00 00 <> o <i 14.7 Tension MPa or O CS 00 on OO 00 is or is an is is o* is o* in in o* on o O* 00 in is Annealing temperature or O CS ”1 col 00 oo 00 00 un co 00 oo uñ oo^ o is 00 on oo 805 un oo CO o oo Ό CO Rolling process in fine Rolling completion temperature ω -O un in co £ 00 is 00 eñ oo CSI un o is Ό 00 o· 00 in 00 o in in un o Csi Total rolling reduction is or SI 5 co un 00 un o un un O un g un o* un o un Sheet thickness after rolling EE p in pp un o Ό O is p -9 is a 00 ooor? - in Sheet thickness before rolling EE On in in o un o un OO is in o CO in csi or 9 rsi or is is in is csi in Hot rolled steel sheet < < < < is < < < mm| in CD ω OO 3 n Ld lZ Steel < < < < < < < < in in in ω ooo Q LU ll- Example - is on <r un o oo o o - es en 2 un £ 00. ίζηοηη / ζζηζ / Ε / γίΛΐ o Ε φ LU Example | Comparative example OE di LU OE φ LU OE Φ LU I Example I Example | Comparative example Example | Comparative example Example | Deposition | O LU o I νθ 0.36 0.48 IZO 0.84 670 0.40 I iL° I 0.44 0.26 0.64 0.13 670 00 Ό o- CM <r CM in 09 601 09 00 ID in 09 o O* O I I θ' Lí9 09 13.3 o Ο 09 09 CM O 09 CM Lí9 09 09 09 09 CM OM 00 09 ο ο 00 Ό 784 cb 986 LO θ' I 866 799 681 696 784 828 SOL 2 175 ID 5 00 09 I °θ1 I O 153 981 5 071 Ο ιη 5 09 U9 00 O LO o co in O CM CM -o CM CM r- o 09 in - en o in o 09 CM o CO CM O o O LÍ9 Ό CM 00 ÍL CM Ll e X - —) CM ll Ll ll o X - -» o* o CM CM CM CM 09 CM CM in CM Ό OM CM 00 OM o CM o 09 ίζηοηη / ζζηζ / Ε / γίΛΐ

[103] Table ίζηοηη / ζζηζ / Ε / γίΛΐ Comparative Example Example | Comparative Example Comparative Example Example | Example Example | Comparative Example Comparative Example Comparative Example Comparative Example Comparative Example Comparative Example Zinc alloy plating 0.42 0.35 0.42 O O CO m 0.67 0.30 0.70 0.70 0.70 0.70 0.25 0.31 <r CM CO co 00 co <r O CO CM 221 cm - co o 00 m in L 01 Ό CO 0 LL 17.3 LO CM o| O Ό CO CM CM CO fO O co O CM O CM 792 co co 774 782 908 776 1 EV£ 790 794 815 748 752 756 O m 6L2 216 138 £91 o- CM 961 178 CO so 163 $ 306 CO o 5 5 CO O LO 2 CO O LO 00 co £ LO t? <r co CM m o ID csi co CM CM CM o Ό O CM 00 o co CO O in 3 8 2.2 co o CO *o <r 2.2 3 4 co Wl s W4 X N $ AB AC AD < < 5 3 3 3 X 2- ΓΜ $ AB AC AD < < 5 CO O LO LO cm LO co LO in in in O LO LO co LO o LO φ φ φ ιη tn ιη φ φ ιη ι ζηοηη / 77η7 / Β / γΐΛΐ

[104] Table Note Example | or E 0) iu Comparative Example Comparative Example Example | Comparative Example Comparative Example Comparative Example Example | Comparative Example Example | Example | Comparative Example Example | Comparative Example or E <D Lu Ejemplo | Ejemplo | Ejemplo | Contenido de Fe en GA o capa galvanizada recocida aleada % en masa CO O o Capa enchapada ω e ω lli Depositada V9 Propiedades de la lámina de acero Densidad numérica de huecos que tienen un diámetro máximo de 1.0 pm o más 10B huecos / m2 CO o in 04 CO -O <3 16.0 14.5 28.0 7'7 m OO o £ 20.0 o o o 09 o Csi Tamaño de grano promedio de ferrita 1 ωΓΐ o OO Ό 00 O 00 00 7.5 13.9 09 CO ZLl 7.8 | 04 CO 10.4 CO £ CO d CO 1 7'6 Austenita residual 7. en vol. o o o col O o O O o O O - - o o o o o - Martensita % en vol. | o o o o o o o o o o o - o o - o o o Proporción de ferrita no recristalizada o o o o o o o o o o o o o o o o o o o Ferrita % en vol.OO CO CO CO CO oooo*· 00 in o- in o 00 o 00 co CO 00 04 CO ID 00 09 CO CO Lamina de acero laminada en caliente < < < < 04 < < <! m 62 CO OO ω ω o 3 o Lli lZ £ Acero < < < < < < < < m en m ω ω ω ω o lli U- U- Ejemplo - 04 091 in ΌΙ es COI o- 3 - 04 3 2 a 2 £ 00 £. Είζηοηη / ζζηζ / Ε / γίΛΐ Example | Comparative example Example | Example | Example | Example | Example | Comparative example Example | Comparative example o E φ o I OZL Depositada | 1 « <5 I νθ o ιη 3 in 00 O co CN oo 12.6 Ό m 12.0 in mo co csi co <> CO co CO co o co 13.0 I 0* ooooo O - oo O 1 te invention. oooooo O oooo 1 de la presen oooo cs oooooo 1 ?ra del intervalo Ό co m 00 m 00 co LO 00 CM CO co co 00 00 co 00 81 valor está fu< CN £3 e X - -) ss CS K3 L ísenta que el ll U- o X - “OL | do reprt o CN 3 CN CN co csi 5 ID CS Ό CS CS 00 CS a I 30 El subraya Είζηοηη / ζζηζ / Ε / γίΛΐ

[105] Table Note Example I Example | Example Example Comparative Example or E φ LU Comparative Example Comparative Example Comparative Example Example 1 Example | Example | OEO) LU Example | Example | Comparative Example Example | Comparative Example Fe content in GA or alloyed annealed galvanized layer % by mass Q 100 Ό oo co cz> CM co Clad layer e Clad with Zn alloy Galvanized-annealed with alloy Galvanized-annealed with alloy <5 1 93 1 va Steel sheet properties Numerical density of voids having a maximum diameter of 1.0 μm or more 108 voids / m2 OO co CM o cm 4.3 4.0 -o O 3 0 81 O oo 00 CM o ID CO CM Z8 £ 3 Average ferrite grain size co O CO 15.2 O θ' 16.2 Residual austenite | ]oa ua % o OO o O - O oooo O o O o OO oo Martensite o Φ o - o - ooooooooooo O - o Proportion of non-recrystallized ferrite oooo cm o LO CM oo co ooooo O oo Ferrite 00 co ID 00 θ' 00 s CM σ* CM 00 co co o θ' Ό 00 co co 00 Hot rolled steel sheet szo £ CM CM 3 2 σ CL > 3 3 3 3 Example with CM CO CO in coco CO 3 with o 5 CM 5 ID 5 $ OO ίζηοηη / ζζηζ / Ε / γίΛΐ Comparative example Example | Example Example | Comparative example Comparative example Comparative example Comparative example Zn alloy plated 3 3.0 | in I 8'4 O oa 80 25.0 12.0 50 O SZL 14.9 LO co in O 00 CN CO CO CO r- o OO ooo co <31 oooooooo - - oo CN ooooooo SI co m 00 co co DDL 00 00 ¿soo §1 χ >- AA AB AC ÜV < < 3 X rsi AA AB AC ÜV < < 50 íñ CN in co in LO $ O in SI ίζηοηη / ζζηζ / Ε / γίΛΐ

[106] Table 5-1 Lznonn / zznz / E / YiAi Example Steel Hot-rolled steel sheet Properties Note YS TS uEl ol o2 σ2 / σ! Transition Temperature MPa MPa % MPa MPa “C 1 A Al 215 408 18 388 147 0.38 -80 Example 2 A A1 210 392 20 378 183 0.48 -80 Example 3 A A1 227 479 14 467 268 0.57 -80 Comparative Example 4 A Al 254 437 18 422 222 0.53 -20 Comparative Example 5 A A2 246 412 20 391 229 0.59 -80 Example 6 A A2 206 410 21 396 305 0.77 -80 Comparative Example 7 A A3 239 388 20 369 265 0.72 -80 Comparative Example 8 A A4 212 408 20 390 380 0.97 -80 Comparative Example 9 B B1 229 366 22 351 150 0.43 -100 Example 10 B B2 227 366 23 349 278 0.80 -60 Comparative Example 11 B B3 217 393 19 374 211 0.56 -100 Example 12 C C1 276 489 16 464 219 0.47 -60 Example 13 C C1 278 499 20 486 302 0.62 -20 Comparative Example 14 C C2 250 435 17 413 180 0.44 -80 Example 15 C C3 269 440 16 425 306 0.72 -40 Comparative Example 16 DD 220 439 18 428 193 0.45 -40 Example 17 EE 234 391 19 377 160 0.42 -60 Example 18 F F1 281 454 18 435 208 0.48 -80 Example 19 F F1 251 473 17 462 151 0.33 -60 Example 20 F F2 286 462 18 443 184 0.42 -80 Example 21 F F3 291 468 17 452 330 0.73 -40 Comparative Example 22 GG 234 402 17 382 188 0.49 -100 Example 23 HH 214 438 16 422 120 0.28 -80 Example 24 1 I 268 449 18 430 175 0.41 -60 Example 25 JJ 205 417 16 406 134 0.33 -80 Example 26 K K1 268 474 16 464 209 0.45 -60 Example 27 K K1 248 408 24 396 288 0.73 -60 Comparative example 28 K K2 281 442 19 423 189 0.45 -80 Example 29 K K3 260 457 16 438 328 0.75 -80 Comparative example 30 LL 299 460 17 438 193 0.44 -60 Example. The underline indicates that the value is outside the range of the present invention or represents undesirable properties.

[107] Table 5-2 Example Steel Hot-rolled steel sheet Properties Note YS TS uEl ol o2 σ2 / σ! Transition Temperature MPa MPa % MPa MPa °C 31 MM 277 411 21 397 175 0.44 -80 Example 32 NN 226 448 16 429 252 0.59 -40 Example 33 0 0 209 409 18 389 184 0.47 -80 Example 34 P P1 232 450 18 431 211 0.49 -60 Example 35 P P1 325 442 14 424 220 0.52 -60 Comparative Example 36 P P2 270 458 18 444 215 0.48 -60 Example 37 P P2 342 474 12 455 212 0.47 -80 Comparative Example 3S P P3 263 437 17 426 302 0.71 -60 Comparative Example 39 P P4 257 423 17 409 308 0.75 -60 Comparative Example 40 QQ 206 379 19 364 185 0.51 -80 Example 41 RR 256 463 17 444 195 0.44 -80 Example 42 SS 266 406 20 395 188 0.48 -80 Example 43 TT 236 458 18 440 157 0.36 -80 Example 44 uu 242 476 17 461 179 0.39 -80 Example 45 VV 261 435 17 414 133 0.32 -80 Example 46 w W1 294 434 19 423 231 0.55 -80 Example 47 w W1 265 475 18 464 333 0.72 -60 Comparative example 48 w W2 286 497 17 476 256 0.54 -60 Example 49 w W3 272 418 20 407 295 0.72 -80 Comparative example 50 w W4 270 426 18 413 321 0.78 -60 Comparative example 51 XX 284 460 19 439 165 0.38 -100 Example 52 YY 199 443 16 421 169 0.40 -80 Example 53 zz 204 390 19 375 173 0.46 -80 Example 54 AA AA 179 314 24 349 99 0.28 -120 Comparative example 55 AB AB 268 519 13 500 354 0.71 -80 Example Comparative example 56 AC AC 321 566 17 547 257 0.47 20 Comparative example 57 AD AD 412 779 12 748 663 0.89 280 Comparative example 58 A Al 206 394 16 367 274 0.75 -60 Comparative example 59 A Al 285 424 14 408 205 0.50 -60 Comparative example. The underline indicates that the value is outside the range of the present invention or represents undesirable properties.

[108] Among the steels A to AD shown in Table 1, the steels AA to AD are comparative examples where the composition was outside the range defined by the present invention.

[109] In AA steel, the C content was below the range of the present invention. In the steel sheet obtained using this steel according to Experimental Example 54, the maximum tensile strength was low.

[110] In AB steel, the C content was higher than the range of the present invention. In the steel sheet according to Experimental Example 55 obtained using this steel, the amount of ferrite was small and the number density of voids was high. Therefore, the uniform elongation was low and the o2 / ol was high.

[111] In AC steel, the Si content was greater than the range of the present invention. In the steel sheet according to Experimental Example 56 obtained using this steel, the amount of residual austenite was large and the ductile-brittle transition temperature was high.

[112] In AD steel, the Mn content was higher than the range of the present invention. In the steel sheet obtained using this steel according to Experimental Example 57, the amount of ferrite was small, the amounts of martensite and residual austenite were large, and the number density of voids was high. Therefore, the uniform elongation was low, and the o2 / ol and ductile-brittle transition temperatures were high.

[113] Experimental Examples 7, 15, 21, 29, 39, 49 and 50 were comparative examples where the hot rolling process conditions were outside the range of the present invention.

[114] Example Experiment 7 was a comparative example in which Dn was high and Expression (1) in the temperature range of 1000°C or lower was not satisfied. Therefore, the number density of voids was high and o2 / ol was high.

[115] Experimental Example 15 was a comparative example in which the heating temperature of the steel part was low. Therefore, the number density of voids was high and o2 / ol was high.

[116] Experimental Example 21 was a comparative example in which the average cooling rate in the temperature range from the cooling start temperature up to 500°C was low. Therefore, the void number density was high and o2 / ol was high.

[117] Experimental Example 29 was a comparative example in which the hot rolling completion temperature was low. Therefore, the number density of voids was high and σ2 / σ1 was high.

[118] Example Experiment 39 was a comparative example in which the time required for the start of cooling after completion of hot rolling was short. Therefore, the number density of voids was high and o2 / ol was high.

[119] Experimental Example 49 was a comparative example in which the hot rolling completion temperature was high. Therefore, the number density of voids was high and o2 / ol was high.

[120] Experimental Example 50 was a comparative example in which the heating temperature of the steel part was high. Therefore, the number density of voids was high and o2 / ol was high.

[121] Experimental Examples 8, 10 and 38 were comparative examples in which the reheating process conditions were outside the range of the present invention.

[122] Experimental Example 8 was a comparative example in which the maximum reheating temperature in the reheating process was low. Therefore, the number density of voids was high and o2 / ol was high.

[123] Experimental Example 10 was a comparative example in which the maximum reheating temperature in the reheating process was high. Therefore, the number density of voids was high and o2 / ol was high.

[124] Experimental Example 38 was a comparative example in which K20 was low and Expression (2) was not satisfied in the temperature range of 500°C to 700°C. Therefore, the number density of voids was high and σ2 / σ1 was high.

[125] Experimental Examples 6, 37, 58 and 59 were comparative examples in which the cold rolling process conditions were outside the range of the present invention.

[126] Experimental Example 6 was a comparative example in which the total reduction of rolling in the cold rolling process was high. Therefore, the number density of voids was high and σ2 / σ1 was high.

[127] Experimental Example 37 was a comparative example in which the total reduction in the cold rolling process was low. Therefore, an excess of unrecrystallized ferrite remained, and the uniform elongation was low.

[128] Experimental Example 58 was a comparative example in which the rolling completion temperature in the cold rolling process was low. Therefore, the number density of voids was high and o2 / ol was high.

[129] Experimental Example 59 was a comparative example in which the rolling completion temperature in the cold rolling process was high. Therefore, an excess of unrecrystallized ferrite remained, and the uniform elongation was low.

[130] Experimental Examples 3, 4, 13, 27, 35 and 47 were comparative examples where the annealing process conditions were outside the range of the present invention.

[131] Experimental Example 3 was a comparative example in which the annealing temperature was high. Therefore, the amount of ferrite was small and the uniform elongation was low.

[132] Experimental Example 27 was a comparative example in which the annealing temperature was low. Therefore, the void number density was high and σ2 / σ1 was high.

[133] Experimental Example 4 was a comparative example in which the value of the middle side of Expression (3) was high. Therefore, the amount of residual austenite was large and the ductile-brittle transition temperature was high.

[134] Experimental Example 13 was a comparative example in which the value on the left-hand side of Expression (4) was low. Therefore, the amount of martensite was large and the ductile-brittle transition temperature was high.

[135] Experimental Example 35 was a comparative example in which the value of the mean side of Expression (3) was low. Therefore, an excess of ferrite remained unrecrystallized and the uniform elongation was low.

[136] Experimental Example 47 was a comparative example in which the applied stress in the temperature range from 720°C to the annealing temperature was low. Therefore, the void number density was high and o2 / ol was high.

[137] Experimental Examples other than the Comparative Examples described above were Examples in accordance with the present invention. The steel sheets described as Examples were found to have been manufactured using the manufacturing method that satisfied the manufacturing conditions in accordance with the present invention and, therefore, had excellent formability, strength, and resistance to impact and fracture.

[138] Experimental Examples 2, 9, 12, 16, 18, 22, 24, 26, 30, 32, 33, 34, 40, 42, 44, 46 and 52 are examples in which the plated steel sheets in accordance with the present invention were obtained by plating.

[139] Experimental Examples 9, 26, 32 and 42 were examples in which a hot-dip galvanized (HD) steel sheet was obtained by cooling the steel sheet to 500°C in the annealing process, immersing the steel sheet in a molten zinc bath, and cooling the steel sheet to room temperature.

[140] Experimental Examples 2, 18, 30 and 46 were examples in which hot-dip galvanized (GA) annealed steel sheet was obtained by cooling the steel sheet to 500°C in the annealing process, immersing the steel sheet in a molten zinc bath and reheating the steel sheet to 560°C to alloy and cooling the steel sheet to room temperature.

[141] Experimental examples 33 and 52 were examples in which a zinc alloy-coated steel sheet was obtained by cooling the steel sheet to 500°C in the annealing process, immersing the steel sheet in a bath of molten zinc alloy, and cooling the steel sheet to room temperature.

[142] Experimental Examples 34 and 40 were Examples in which an alloyed galvanized-annealed steel sheet was obtained by cooling the steel sheet to 500°C in the annealing process, immersing the steel sheet in a molten zinc alloy bath, and reheating the steel sheet to 580°C to alloy and cooling the steel sheet to room temperature.

[143] Experimental Examples 16 and 22 were Examples in which a galvanized steel sheet was obtained by deposition plating in the annealing process prior to temper rolling.

[144] Experimental Examples 12, 24 and 44 were Examples in which an electrogalvanized (EG) steel sheet was obtained by performing electrogalvanizing after the annealing process.

[145] Industrial applicability As described above in accordance with Lznonn / zznz / E / YiAi With the present invention, a high-strength steel sheet can be provided that has excellent formability, impact and fracture resistance, and toughness. The steel sheet according to the present invention is suitable for significantly reducing the weight of a vehicle and ensuring passenger protection and safety. Therefore, the present invention is highly applicable to the steel sheet manufacturing industry and the automotive industry.

Claims

CLAIMS 1. A steel sheet comprising, as composition in % by mass: C: 0.010% to 0.200%; Si: 0.005% to 1.500%; Mn: 0.05% to 3.00%; Al: 0.005% to 1.000%; P: 0.100% or less; S: 0.0200% or less; N: 0.0150% or less; O: 0.0100% or less; Nb: 0% to 0.060%; Ti: 0% to 0.100%; Iznonn / zznz / E / YiAi V: 0% to 0.500%; Cr: 0% to 1.00%; Ni: 0% to 1.00%; Cu: 0% to 1.00%; Mo: 0% to 1.00%; W: 0% to 1.000%; B: 0% to 0.0100%; Sn: 0% to 1.00%; Sb: 0% to 0.20%; one or two or more selected from the group consisting of Ca, Ce, Mg, , Zr, La and REM: 0% to 0.0100% in total; and a remainder that includes Fe and impurities, wherein a microstructure at a position 1 / 4 of the thickness from a surface in the direction of the sheet thickness includes, in % by vol, ferrite: 80% or more, martensite: 2% or less, and residual austenite: 2% or less, has a proportion of non-recrystallized ferrite in the ferrite of 5% or less, and in the microstructure of the drawn steel sheet 10% at the position 1 / 4 of the thickness from the surface in the direction of the sheet thickness, a numerical density of voids with a maximum diameter of 1.0 pm or more is 1.0 × 109 pieces / m2 or less.

2. The steel sheet according to claim 1, wherein the composition further includes, in % by mass, one or two or more selected from the group consisting of: Nb: 0.005% to 0.060%; Ti: 0.015% to 0.100%; V: 0.010% to 0.500%; Cr: 0.05% to 1.00%; Ni: 0.05% to 1.00%; Cu: 0.05% to 1.00%; Mo: 0.03% to 1.00%; W: 0.030% to 1.000%; B: 0.0005% to 0.0100%; Sn: 0.01% to 1.00%; Sb: 0.005% to 0.20%; and one or two or more selected from the group consisting of Ca, Ce, Mg, Zr, La and REM: 0.0001% to 0.0100% in total.

3. The steel sheet according to claim 1 or 2, wherein an average ferrite grain size in the microstructure is from 6.0 pm to 15.0 pm.

4. The steel sheet according to any of claims 1 to 3, comprising a galvanized layer on the surface.

5. The steel sheet according to any of claims 1 to 3, comprising a zinc alloy coated layer on the surface.

6. The steel sheet according to claim 4 or 5, wherein the Fe content in the galvanized layer or the zinc alloy-plated layer is from 7.0% to 13.0% by mass.

7. A method for manufacturing the steel sheet according to any one of claims 1 to 3, comprising: a hot rolling process of heating a piece of steel having the composition according to claim 1 to 1150°C to 1320°C, completing the hot rolling such that the final temperature of the hot rolling is 850°C to 930°C, and starting the cooling after 1.5 seconds or more,and cooling the steel piece to a temperature range of 500°C or lower to obtain a hot-rolled steel sheet such that the average cooling rate over a temperature range from a cooling start temperature of 500°C is 20°C / s faster; a reheating process of heating the hot-rolled steel sheet to a temperature range of 500°C to 700°C; a cooling process of cooling the hot-rolled steel sheet to room temperature; a cold-rolling process of cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet such that the total rolling reduction is from 30% to 90% and the final cold-rolling temperature is from 120°C to 250°C; and an annealing process of heating the cold-rolled steel sheet to an annealing temperature of 720°C to 850°C and cooling to a temperature range of 500°C or less,where in the hot rolling process, Expression (1) is satisfied in a temperature range of 1000°C or less, in the reheating process, Expression (2) is satisfied in the temperature range of 500°C to 700°C, in the annealing process, a stress of 20 MPa or higher is applied and Expression (3) is satisfied in a temperature range of 720°C to the annealing temperature during heating to the annealing temperature, and Expression (4) is satisfied in a temperature range of 720°C to 500°C during cooling from the annealing temperature, / “n \ *=^-^(7^273) ht / r 7* Ί hi-ι ~ h¡ D, = D.-i ' exp · (-) ) + lOGit ----L 2η(_! \ \p, / / 2n,_! • h - M* · (£D++1 {»-«*(«. · (£))} Dn <12.5 • · · Expression (1) In Expression (1), Dn represents an index that represents a degree of advancement of the precipitation of a fine carbide in a temperature range of 1000°C or lower of the hot rolling process,and 15 the reference numbers in Expression (1) are the following, n: the number of rolling passes in the temperature range of 1000°C or less, T±: a rolling temperature at an i-th rolling pass, ti: an elapsed time [s] from the i-th rolling pass to an i+l-th rolling pass or a time [s] elapsed until the temperature of a steel sheet drops to 850°C from the i-th rolling pass, hi-i: a sheet thickness [mm] before the i-th rolling pass in the temperature range of 1000°C or less, hi: a sheet thickness [mm] after the i-th rolling pass in the temperature range of 1000°C or less, and ai an: constants (ai = 2.54 χ 10-6, a2 = 3.62 χ 10-4, a3 = -6.38 χ 10-1, a4 = -3.00 χ 10'1, a5 = 8.50 χ 10'1, a6 = -8.50 χ ΙΟ'4, a7 = 2.40 χ 10°, a8 = 7.83 χ 10'13, a9 = 2.80 χ 105, aio = 6.00 χ 10-12, and an = 2.80 χ 105),Lznonn / zznz / E / YiAi i i273. t„ = 10 Kn = (Tn + 273) · {log10 tn + 20 · (1 + 0.08.YES)} / <20 > 1-50 x 104 + Δίκ ' ' Expression (2) In Expression (2), K20 represents an index that represents a degree of progress of fine carbide precipitation in a 20th period when a temperature history in the temperature range of 500°C to 700°C of the reheating process is divided into 20 periods with respect to time, and the reference numbers in Expression (2) are as follows, Tn: an average temperature [°C] in an nth period when a temperature history in the temperature range of 500°C to 700°C is divided into 20 periods with respect to time, Δίκ: a time [hr] in one of the 20 periods into which a total residence time is divided in the temperature range of 500°C to 700°C, where ti = Δίκ, and Si: a Si content [% by mass],10 Σί / 1 / d? \ '0 5 -L-expM ·i°'<20.0 < -1 20 ' · · · Expression (3) The reference numbers in Expression (3) are as follows, K20: a value obtained by Expression (2), di and d2: constants (di = 9.67 × 1010 and di = 1.25 × 104) , Ti: an average heat treatment temperature [°C] in an i-th period when a temperature history in the temperature range of 720°C to the annealing temperature is divided into 10 periods with respect to time, and t': 1 / 10 [s] of a residence time in the temperature range of 720°C to the annealing temperature, and (5, + 92 · Nbn 5 + g, · Ti 5) + Mo05)-' ' ~ ' (Δ, + 9^ · Δ,05) ZJ \ Ac3 Ac\ / • exp (- ) · t'aΛ > 1.0 7)+ 273 / • · · Expression (4) The reference numbers in Expression (4) are as follows, Ai: 750 - 18 x Si - 17 x Mn - 10 x Cr - 8 x Ni + 15 x Al - Ti, where each of the elements represents a mass % content of the element, and when the element is not included,0 is substituted as the element content, when a calculated value of Ai is a negative value, Ai is set to 0. gi to 6: constants (gi = 1.00 χ 10-1, g2 = 1.46 χ 10-1, g3 = 1.14 χ 10-1, g4 = 2.24 χ 10°, g5 = 4.53 χ 10°, and g6 = 4.83 x 103) , Nb, Mo, Si, Mn, Cr, Ni and Al: a content [% by mass] of each of the elements, where when the element is not included, 0 is substituted as the element content, Ti*: an effective content of Ti represented by Ti - 42 / 14 χ N, where Ti and N represent a content [% by mass] ίζηοηη / ζζηζ / E / γίΛΐ of each of the elements, when the element is not included, 0 is substituted as the content of the element, a minimum value is set to 0. Ti: an average heat treatment temperature [°C] in an i-th period when a temperature history in the temperature range of 720°C to 500°C is divided into 10 periods with respect to time,Aci and Ac3: a transformation start temperature and a transformation end temperature [°C] during heating, Tmax: the highest heating temperature [°C] in a heat treatment process, and t': 1 / 10 [s] of a residence time in the temperature range of 720°C to 500°C.

8. The method of manufacturing a steel sheet according to claim 7, wherein during cooling in the annealing process, hot-dip galvanizing is performed on the cold-rolled steel sheet.

9. The method of manufacturing a steel sheet according to claim 7, wherein during cooling in the annealing process, a hot-dip zinc alloy coating is applied to the cold-rolled steel sheet.

10. The method of manufacturing a steel sheet according to claim 8 or 9, wherein during cooling in the annealing process, the alloying is carried out after hot-dip galvanizing or hot-dip zinc alloy plating.