Steel sheet and plated steel sheet

Abstract

A steel sheet having a predetermined chemical composition and satisfying formulas (1) and (2).　(1): [C]30μm / [C]150μm≤0.80 (2): TSE-50≤TSA≤TSE+50 (3): TSE=(26.7+29.7×t+0.94×AGTS-0.4×D)

Description

Steel sheets and plated steel sheets

[0001] The present disclosure relates to a steel sheet and a plated steel sheet having a plating layer on its surface.

[0002] There has been a demand for improved occupant safety in vehicles, and for this purpose, the strength of vehicle body materials has been improved. On the other hand, against the backdrop of worsening issues such as global warming, efforts to improve automobile fuel efficiency are accelerating. It is known that reducing the weight of vehicle bodies is an effective way to improve fuel efficiency.

[0003] From the above viewpoints, steel sheets used for automobile body structural members are required to have high strength. On the other hand, since the steel sheets are formed and assembled into automobiles, it is also important that they have good bendability.

[0004] Patent Documents 1 and 2 disclose that a multilayer steel sheet obtained by welding a steel sheet (soft layer) having predetermined characteristics to one or both sides of a base material (hard layer) and hot rolling or annealing the steel sheet under specific conditions can most effectively improve bendability.

[0005] International Publication No. WO2018 / 151331 International Publication No. WO2018 / 151322

[0006] In recent years, automobile bodies have come to be required to have more accurate strength designs from the viewpoint of improving reliability. Steel sheets are sometimes required to have not only sufficient bendability but also a highly accurately predicted tensile strength. However, in the prior art such as those disclosed in Patent Documents 1 and 2, highly accurate prediction of the tensile strength of the resulting steel sheets has not been considered.

[0007] The present disclosure has been made in view of the above circumstances, and one of its objectives is to provide a steel sheet and a plated steel sheet that have sufficient bendability and a tensile strength that can be predicted with high precision.

[0008] A first aspect of the present invention is a steel sheet containing: C: 0.115 to 0.145 mass%, Si: 0.70 to 1.20 mass%, Mn: 2.2 to 2.5 mass%, P: 0.020 mass% or less (including 0 mass%), S: 0.0030 mass% or less (including 0 mass%), Al: 0.005 to 0.040 mass%, Cr: 0.10 to 0.70 mass%, Ti: 0.010 to 0.040 mass%, B: 0.0020 to 0.0040 mass%, N: 0.0100 mass% or less (including 0 mass%), and O: 0.0100 mass% or less (including 0 mass%), with the balance being iron and inevitable impurities, and satisfying the following formulas (1) and (2). [C] ３０μｍ / [C] １５０μｍ ≦0.80...(1) TS Ｅ -50≦TS Ａ ≦TS Ｅ +50...(2) TS Ｅ = (26.7 + 29.7 × t + 0.94 × AGTS − 0.4 × D) (3) In the above formula, [C] ３０μｍ is the carbon concentration (mass%) at a depth of 30 μm from at least one surface in the plate thickness direction, [C] １５０μｍ is the carbon concentration (mass%) at a depth of 150 μm from the at least one surface in the plate thickness direction, and TS Ｅ is the value (MPa) shown in formula (3), and TS Ａ is the tensile strength (MPa) of the steel plate, t is the plate thickness (mm), and D is the carbon concentration 0.9 × [C] １５０μｍ AGTS is the tensile strength (MPa) after grinding both surfaces in the thickness direction by 0.1 mm.

[0009] Aspect 2 of the present invention is Mo: 0.20 mass% or less (excluding 0 mass%), Cu: 0.20 mass% or less (excluding 0 mass%), Ni: 0.20 mass% or less (excluding 0 mass%), Ca: 0.0040 mass% or less (excluding 0 mass%), Nb: 0.040 mass% or less (excluding 0 mass%), V: 0.20 mass% or less (excluding 0 mass%), Ta: 0.010 mass% or less (excluding 0 mass%), W: 0.010 mass% or less (excluding 0 mass%), Co: 0.010 mass% or less (excluding 0 mass%), Sn: 0.010 mass% or less (excluding 0 mass%), Sb: 0.010 mass% or less (excluding 0 mass%), Mg: 0.010 mass% or less (excluding 0 mass%), REM: 0.010 mass% or less (excluding 0 mass%). The steel sheet according to aspect 1, further containing at least one selected from the group consisting of Zr: 0.010 mass% or less (excluding 0 mass%), Te: 0.010 mass% or less (excluding 0 mass%), Hf: 0.010 mass% or less (excluding 0 mass%), and Bi: 0.010 mass% or less (excluding 0 mass%).

[0010] A third aspect of the present invention is a plated steel sheet comprising the steel sheet according to the first or second aspect and a plating layer disposed on the surface of the steel sheet.

[0011] According to an embodiment of the present invention, it is possible to provide a steel sheet and a plated steel sheet having sufficient bendability and a tensile strength that can be predicted with high precision.

[0012] FIG. 1 is a diagram showing an example of a carbon concentration profile obtained in an example.

[0013] The present inventors have conducted research from various angles in order to realize a steel sheet having sufficient bendability and a tensile strength that can be predicted with high accuracy.

[0014] As a result of research by the inventors, it was found that by forming a predetermined decarburized layer on at least one surface of a steel sheet while maintaining a predetermined chemical composition (particularly the C and Mn contents within a specific, very narrow range), it is possible to design a thinner soft portion and impart sufficient bendability compared to, for example, a multi-layer steel sheet having a soft layer and a hardness transition zone (between the soft layer and the hard layer) as described in the prior art. Furthermore, since the decarburized layer can be designed to be thin, the steel sheet can achieve a tensile strength close to that of the bulk layer (excluding the surface decarburized layer). In the prior art, the tensile strength of the bulk layer can be controlled (predicted) to a desired value to some extent, so by forming the predetermined decarburized layer, it is easier to predict the tensile strength than in the prior art.

[0015] However, it was found that the tensile strength of the above-mentioned steel sheet varies slightly from the expected value (i.e., the tensile strength of the bulk layer) due to the presence of the predetermined decarburized layer. After further intensive research, the inventors discovered that this variation depends on the sheet thickness t, the predetermined decarburized layer depth D, and the predetermined bulk layer tensile strength ADTS. t, D, and ADTS can be controlled (i.e., predicted) to desired values ​​by the manufacturing conditions, and the inventors found that a steel sheet having a tensile strength that can be predicted with high accuracy can be realized using these t, D, and ADTS. Details of each requirement specified in the embodiments of the present invention are provided below.

[0016] <1. Chemical Composition of Steel Sheet> A steel sheet according to an embodiment of the present invention preferably contains 0.115 to 0.145 mass% C, 0.70 to 1.20 mass% Si, 2.2 to 2.5 mass% Mn, 0.020 mass% or less (including 0 mass%) of P, 0.0030 mass% or less (including 0 mass%) of S, 0.005 to 0.040 mass% of Al, 0.10 to 0.70 mass% of Cr, 0.010 to 0.040 mass% of Ti, 0.0020 to 0.0040 mass% of B, 0.0100 mass% or less (including 0 mass%) of N, and 0.0100 mass% or less (including 0 mass%) of O, with the balance being iron and inevitable impurities. Each of the components will be described in detail below.

[0017] (C: 0.115 to 0.145 mass%) C is an element necessary for ensuring the strength of the steel sheet. If the C content is insufficient, the tensile strength of the steel sheet will decrease. In order to ensure sufficient strength of the steel sheet, the C content is set to 0.115 mass% or more. The C content is preferably 0.120 mass% or more, and more preferably 0.125 mass% or more. However, if the C content is excessive, in addition to the tensile strength becoming excessively high, there is a risk of causing a decrease in bendability. Therefore, the C content is set to 0.145 mass% or less. The C content is preferably 0.140 mass% or less, and more preferably 0.135 mass% or less.

[0018] (Si: 0.70 to 1.20 mass%) Si is known as a solid-solution strengthening element and is an element that effectively acts to improve tensile strength while suppressing deterioration in bendability of steel sheet. To effectively exert these effects, the Si content is set to 0.70 mass% or more. The Si content is preferably 0.75 mass% or more. More preferably, it is 0.80 mass% or more, and even more preferably, it is 0.85 mass% or more. However, if the Si content is excessive, the volume fraction of retained austenite becomes excessive, which may lead to a decrease in the yield strength of the steel sheet. Furthermore, if the Si content is excessive, the effect saturates and only costs increase. Therefore, the Si content is set to 1.20 mass% or less. The Si content is preferably 1.15 mass% or less, and more preferably 1.10 mass% or less.

[0019] (Mn: 2.2 to 2.5 mass%) Mn is an element that contributes to increasing the strength of steel sheet. To effectively exert this effect, the Mn content is set to 2.20 mass% or more. The Mn content is preferably 2.25 mass% or more, and more preferably 2.30 mass% or more. However, an excessive Mn content may lead to slab breakage, an increase in cold rolling load, etc. Therefore, the Mn content is set to 2.50 mass% or less. The Mn content is preferably 2.45 mass% or less, and more preferably 2.40 mass% or less.

[0020] (P: 0.020% by mass or less (including 0% by mass)) P is an element that is inevitably contained in steel, and it is an element that segregates at the grain boundaries of steel and promotes grain boundary embrittlement. In order to avoid breakage and the like during processing of the steel sheet, it is preferable to reduce the P content as much as possible. Therefore, the P content is set to 0.020% by mass or less. The P content is preferably 0.015% by mass or less, and more preferably 0.012% by mass or less. In this specification, "including 0% by mass" means that it includes embodiments in which it is not intentionally added, that is, cases in which the content is below the unavoidable impurity level (it does not exclude cases in which it is intentionally added).

[0021] (S: 0.0030% by mass or less (including 0% by mass)) Like P, S is an element that is inevitably contained in steel. S forms inclusions together with other elements in the steel. These inclusions may cause breakage or the like of the steel sheet during processing. In order to avoid such breakage or the like of the steel sheet, it is preferable to reduce the S content as much as possible. Therefore, the S content is set to 0.0030% by mass or less. The S content is preferably 0.0025% by mass or less, and more preferably 0.0020% by mass or less.

[0022] (Al: 0.005 to 0.040 mass%) Al is an element that acts as a deoxidizer in steel. To effectively exert this function, the Al content is set to 0.005 mass% or more. The Al content is preferably 0.010 mass% or more, and more preferably 0.015 mass% or more. However, if the Al content is excessive, a large amount of inclusions such as alumina is generated in the steel sheet, which may cause fractures during processing of the steel sheet. Therefore, the Al content is set to 0.040 mass% or less. The upper limit of the Al content is preferably 0.035 mass% or less, and more preferably 0.030 mass% or less.

[0023] (Cr: 0.10 to 0.70 mass%) Cr is one of the important elements in the embodiment of the present invention. Cr is an element that contributes to increasing the strength of the steel sheet. Specifically, Cr is an element that improves the hardenability of the steel sheet, reduces the bainite that forms during quenching, and effectively acts to increase the strength of the steel sheet. To effectively exert these effects, the Cr content is set to 0.10 mass% or more. The Cr content is preferably 0.15 mass% or more, and more preferably 0.20 mass% or more. However, an excessive Cr content may lead to an increase in the cold rolling load, etc. Therefore, the Cr content is set to 0.70 mass% or less. The Cr content is preferably 0.65 mass% or less, and more preferably 0.60 mass% or less.

[0024] (Ti: 0.010 to 0.040% by mass) Ti is an element that forms carbides and / or nitrides to improve the strength of the steel sheet. Ti is also an effective element for effectively exhibiting the hardenability-improving effect of B, which will be described later. That is, Ti reduces the N content in the steel by forming nitrides. As a result, the formation of B nitrides is suppressed, B enters a solid solution state, and the hardenability-improving effect of B can be effectively exhibited. In this way, Ti contributes to increasing the strength of the steel sheet by improving the hardenability of the steel sheet due to B. To effectively exhibit this effect, the Ti content is set to 0.010% by mass or more. The Ti content is preferably 0.015% by mass or more, and more preferably 0.020% by mass or more. However, an excessive Ti content results in an excess of Ti carbides and / or Ti nitrides, which may cause cracks during processing of the steel sheet. Therefore, the Ti content is set to 0.040% by mass or less. The Ti content is preferably 0.035 mass % or less, and more preferably 0.030 mass % or less.

[0025] (B: 0.0020 to 0.0040 mass%) B is an element that improves hardenability and contributes to increasing the strength of steel sheet. To effectively exert this effect, the B content is set to 0.0020 mass% or more. The B content is preferably 0.0022 mass% or more, and more preferably 0.0025 mass% or more. However, if the B content is excessive, the effect saturates and only costs increase. Therefore, the B content is set to 0.0040 mass% or less. The B content is preferably 0.0038 mass% or less, and more preferably 0.0035 mass% or less.

[0026] (N: 0.0100% by mass or less (including 0% by mass)) N is an element that is inevitably present as an impurity element. N is an element that forms nitrides, and these nitrides become the starting points for cracks, deteriorating formability. Therefore, the less N there is, the better, and the N content is set to 0.0100% by mass. It is preferably 0.0080% by mass or less, and more preferably 0.0060% by mass or less.

[0027] (O: 0.0100% by mass or less (including 0% by mass)) O is an element that is inevitably present as an impurity element. O is an element that forms oxides, and these oxides become the starting points for cracks, deteriorating formability. Therefore, the less O there is, the better, and the O content is set to 0.0100% by mass. It is preferably 0.0080% by mass or less, and more preferably 0.0060% by mass or less.

[0028] The steel sheet according to the embodiment of the present invention preferably includes the above-described chemical composition, and in one embodiment of the present invention, the balance is preferably iron and unavoidable impurities. Elements introduced due to the conditions of raw materials, materials, manufacturing equipment, etc. are permitted as unavoidable impurities. Note that, for example, P, S, N, and O, which are typically present in lower amounts, are unavoidable impurities, but their composition ranges are separately specified as described above. Therefore, in this specification, the term "unavoidable impurities" constituting the balance excludes elements whose composition ranges are separately specified. Examples of such unavoidable impurities include Zn, Pb, and As. The total amount of unavoidable impurities may be, for example, 0.100 mass% or less.

[0029] Furthermore, at least one of the following optional additional elements may be contained alone or in combination: Mo: 0.20 mass% or less (excluding 0 mass%), Cu: 0.20 mass% or less (excluding 0 mass%), Ni: 0.20 mass% or less (excluding 0 mass%), Ca: 0.0040 mass% or less (excluding 0 mass%), Nb: 0.040 mass% or less (excluding 0 mass%), V: 0.20 mass% or less (excluding 0 mass%), Ta: 0.010 mass% or less (excluding 0 mass%), W: 0.010 mass% or less (excluding 0 mass%), Co: 0.010 mass% or less (excluding 0 mass%), Sn: 0.010 mass% or less (excluding 0 mass%), Sb: 0.010 mass% or less (excluding 0 mass%), Mg: 0.010 mass% or less (excluding 0 mass%). REM: 0.010% by mass or less (excluding 0% by mass), Zr: 0.010% by mass or less (excluding 0% by mass), Te: 0.010% by mass or less (excluding 0% by mass), Hf: 0.010% by mass or less (excluding 0% by mass), and Bi: 0.010% by mass or less (excluding 0% by mass). The preferred contents of each element when these optional additional elements are contained will be described below.

[0030] (Mo: 0.20 mass% or less (excluding 0 mass%)) Mo is an element that contributes to increasing the strength of steel sheet. This effect increases as the Mo content increases. To effectively exert this effect, the Mo content is preferably more than 0 mass%. The Mo content is more preferably 0.005 mass% or more, and even more preferably 0.010 mass% or more. However, if the Mo content is excessive, the effect saturates and costs increase. Therefore, the Mo content is preferably 0.20 mass% or less. The Mo content is more preferably 0.18 mass% or less, and even more preferably 0.16 mass% or less.

[0031] (Cu: 0.20 mass% or less (excluding 0 mass%)) Cu is an element effective in improving the corrosion resistance of steel sheet. This effect increases as the Cu content increases. To effectively exert this effect, the Cu content is preferably more than 0 mass%. The Cu content is more preferably 0.005 mass% or more, and even more preferably 0.010 mass% or more. However, if the Cu content is excessive, the effect saturates and costs increase. Therefore, the Cu content is preferably 0.20 mass% or less. The Cu content is more preferably 0.18 mass% or less, and even more preferably 0.16 mass% or less.

[0032] (Ni: 0.20 mass% or less (excluding 0 mass%)) Ni is an element effective in improving the corrosion resistance of steel sheet. This effect increases as the Ni content increases. To effectively exert this effect, the Ni content is preferably more than 0 mass%. The Ni content is more preferably 0.005 mass% or more, and even more preferably 0.010 mass% or more. However, if the Ni content is excessive, the effect saturates and costs increase. Therefore, the Ni content is preferably 0.20 mass% or less. The Ni content is more preferably 0.18 mass% or less, and even more preferably 0.16 mass% or less.

[0033] (Ca: 0.0040% by mass or less (excluding 0% by mass)) Ca is an element that is effective in spheroidizing sulfides in steel and improving the bendability of steel sheet. This effect increases as the Ca content increases. To effectively exert this effect, the Ca content is preferably more than 0% by mass. The Ca content is more preferably 0.0005% by mass or more, and even more preferably 0.0010% by mass or more. However, if the Ca content is excessive, the effect saturates and costs increase. Therefore, the Ca content is preferably 0.0040% by mass or less. The Ca content is more preferably 0.0035% by mass or less, and even more preferably 0.0030% by mass or less.

[0034] (Nb: 0.040 mass% or less (excluding 0 mass%)) Nb is an element that contributes to increasing the strength of steel sheet. This effect increases as the Nb content increases. To effectively exert this effect, the Nb content is preferably more than 0 mass%. The Nb content is more preferably 0.0005 mass% or more, and even more preferably 0.0010 mass% or more. However, if the Nb content is excessive, the effect saturates and costs increase. Therefore, the Nb content is preferably 0.040 mass% or less, more preferably 0.035 mass% or less, and even more preferably 0.030 mass% or less.

[0035] (V: 0.20 mass% or less (excluding 0 mass%)) V is an element that contributes to increasing the strength of steel sheet. This effect increases as the V content increases. To effectively exert this effect, the V content is preferably more than 0 mass%. The V content is more preferably 0.0005 mass% or more, and even more preferably 0.0010 mass% or more. However, if the V content is excessive, the effect saturates and costs increase. Therefore, the V content is preferably 0.20 mass% or less, more preferably 0.15 mass% or less, and even more preferably 0.10 mass% or less.

[0036] (Ta: 0.010 mass% or less (excluding 0 mass%)) Ta is an element that contributes to increasing the strength of steel sheet. This effect increases as the Ta content increases. To effectively exert this effect, the Ta content is preferably more than 0 mass%. The Ta content is more preferably 0.005 mass% or more, and even more preferably 0.006 mass% or more. However, if the Ta content is excessive, the effect saturates and costs increase. Therefore, the upper limit of the Ta content is preferably 0.010 mass% or less, more preferably 0.009 mass% or less, and even more preferably 0.008 mass% or less.

[0037] (W: 0.010 mass% or less (excluding 0 mass%)) W is an element that contributes to increasing the strength of steel sheet. This effect increases as the W content increases. To effectively exert this effect, the W content is preferably more than 0 mass%. The W content is more preferably 0.005 mass% or more, and even more preferably 0.006 mass% or more. However, if the W content is excessive, the effect saturates and costs increase. Therefore, the W content is preferably 0.010 mass% or less, more preferably 0.009 mass% or less, and even more preferably 0.008 mass% or less.

[0038] (Co: 0.010 mass% or less (excluding 0 mass%)) Co is an element that contributes to increasing the strength of steel sheet. This effect increases as the Co content increases. To effectively exert this effect, the Co content is preferably more than 0 mass%. The Co content is more preferably 0.005 mass% or more, and even more preferably 0.006 mass% or more. However, if the Co content is excessive, the effect saturates and costs increase. Therefore, the Co content is preferably 0.010 mass% or less, more preferably 0.009 mass% or less, and even more preferably 0.008 mass% or less.

[0039] (Sn: 0.010 mass% or less (excluding 0 mass%)) Sn is an element that contributes to increasing the strength of steel sheet. This effect increases as the Sn content increases. To effectively exert this effect, the Sn content is preferably more than 0 mass%. The Sn content is more preferably 0.005 mass% or more, and even more preferably 0.006 mass% or more. However, if the Sn content is excessive, the effect saturates and costs increase. Therefore, the upper limit of the Sn content is preferably 0.010 mass% or less, more preferably 0.009 mass% or less, and even more preferably 0.008 mass% or less.

[0040] (Sb: 0.010 mass% or less (excluding 0 mass%)) Sb is an element that contributes to increasing the strength of steel sheet. This effect increases as the Sb content increases. To effectively exert this effect, the Sb content is preferably more than 0 mass%. The Sb content is more preferably 0.005 mass% or more, and even more preferably 0.006 mass% or more. However, if the Sb content is excessive, the effect saturates and costs increase. Therefore, the upper limit of the Sb content is preferably 0.010 mass% or less, more preferably 0.009 mass% or less, and even more preferably 0.008 mass% or less.

[0041] (Mg: 0.010% by mass or less (excluding 0% by mass)) Mg is an element that contributes to improving the formability of a steel sheet. To effectively exert this effect, the Mg content is preferably more than 0% by mass, and more preferably 0.0010% by mass or more. However, if the Mg content is excessive, the pickling properties, weldability, hot workability, and economic efficiency of the steel sheet deteriorate. Therefore, the Mg content is preferably 0.010% by mass or less, and more preferably 0.0040% by mass or less.

[0042] (REM: 0.010 mass% or less (excluding 0 mass%)) REM is an element that contributes to improving the formability of a steel sheet. To effectively exert this effect, the REM content is preferably more than 0 mass%, and more preferably 0.0010 mass% or more. However, if the REM content is excessive, the pickling properties, weldability, hot workability, and economic efficiency of the steel sheet deteriorate. Therefore, the REM content is preferably 0.010 mass% or less, and more preferably 0.0040 mass% or less.

[0043] (Zr: 0.010% by mass or less (excluding 0% by mass)) Zr is an element that contributes to improving the formability of steel sheet. To effectively exert this effect, the Zr content is preferably more than 0% by mass, and more preferably 0.0010% by mass or more. However, an excessive Zr content increases coarse precipitates and inclusions, which in turn reduces formability. Therefore, the Zr content is preferably 0.010% by mass or less, and more preferably 0.0040% by mass or less.

[0044] (Te: 0.010% by mass or less (excluding 0% by mass)) Te is an element that contributes to improving the formability of steel sheet. To effectively exert this effect, the Te content is preferably more than 0% by mass, and more preferably 0.0010% by mass or more. However, an excessive Te content increases coarse precipitates and inclusions, which in turn reduces formability. Therefore, the Te content is preferably 0.010% by mass or less, and more preferably 0.0040% by mass or less.

[0045] (Hf: 0.010 mass% or less (excluding 0 mass%)) Hf is an element that contributes to improving the formability of steel sheet. To effectively exert this effect, the Hf content is preferably more than 0 mass%, and more preferably 0.0010 mass% or more. However, an excessive Hf content increases coarse precipitates and inclusions, which in turn reduces formability. Therefore, the Hf content is preferably 0.010 mass% or less, and more preferably 0.0040 mass% or less.

[0046] (Bi: 0.010% by mass or less (excluding 0% by mass)) Bi is an element that contributes to improving the formability of steel sheet. To effectively exert this effect, the Bi content is preferably more than 0% by mass, and more preferably 0.0010% by mass or more. However, an excessive Bi content increases coarse precipitates and inclusions, which in turn reduces formability. Therefore, the Bi content is preferably 0.010% by mass or less, and more preferably 0.0040% by mass or less.

[0047] <2. Decarburized Layer> The steel sheet according to the embodiment of the present invention satisfies the following formula (1): [C] ３０μｍ / [C] １５０μｍ ≦0.80 (1) In formula (1), [C] ３０μｍ is the carbon concentration (mass%) at a depth of 30 μm from at least one surface in the thickness direction (i.e., one surface or both surfaces in the thickness direction), [C] １５０μｍ is the carbon concentration (mass%) at a depth of 150 μm from the at least one surface in the sheet thickness direction. The carbon concentration is measured by GD-OES (Glow Discharge Optical Emission Spectrometry) as described in the Examples below.

[0048] In formula (1), [C] １５０μｍ can correspond to the carbon concentration of the bulk layer, whereas [C] ３０μｍ is low enough to satisfy formula (1) (i.e., the steel sheet is sufficiently decarburized at a depth of 30 μm from the surface), a sufficiently soft layer is formed in the surface layer of the steel sheet, which alleviates the tensile stress and compressive stress that occur in the steel sheet surface during bending, thereby achieving sufficient bendability.

[0049] <3. Prediction of Tensile Strength> The steel sheet according to the embodiment of the present invention satisfies the following formula (2): TS Ｅ -50≦TS Ａ ≦TS Ｅ +50...(2) TS Ｅ = (26.7 + 29.7 × t + 0.94 × AGTS - 0.4 × D) ... (3) In the formulas (2) to (3), TS Ｅ is the value (MPa) related to the predicted value of tensile strength shown in equation (3), and TS Ａ is the tensile strength (MPa) of the steel plate, t is the plate thickness (mm), and D is the carbon concentration 0.9 × [C] １５０μｍand AGTS is the tensile strength (MPa) after both surfaces in the thickness direction (i.e., the at least one surface and its back surface) are ground by 0.1 mm. In this specification, the depth D is sometimes referred to as the decarburized layer depth D, and the AGTS is sometimes referred to as the bulk layer tensile strength ADTS. Here, Equation (3) is calculated by using a large amount of data including the examples described later, with the objective variable being TS. Ａ , the equation obtained with explanatory variables t, AGTS and D.

[0050] The satisfaction of the above formula (2) is due to the TS shown in formula (3) found by the present inventors. Ｅ This allows for highly accurate (within the range of ±50 MPa) tensile strength TS. Ａ It is preferable that the steel sheet according to this embodiment further satisfies the following formula (4): TS Ｅ -30≦TS Ａ ≦TS Ｅ +30...(4)

[0051] TS Ａ It is preferable that the tensile strength is 1150 MPa or more. This makes it possible to obtain a high-strength steel sheet. Ａ The upper limit of the stress is not particularly limited, but from the above-mentioned composition of components, it can be, for example, 1400 MPa or less.

[0052] The plate thickness t is not particularly limited, but considering the use in automobile bodies, it may be 0.8 to 2.6 mm.

[0053] The decarburized layer depth D is not particularly limited, but may be more than 30 μm and less than 150 μm in consideration of the above formula (1).

[0054] The bulk layer tensile strength ADTS is the desired TS Ａ The upper limit of ADTS is not particularly limited, but may be, for example, 1400 MPa or less in view of the above-mentioned component composition.

[0055] <4. Plated Steel Sheet> A plated steel sheet according to an embodiment of the present invention includes a steel sheet and a plating layer disposed on the surface of the steel sheet. The plating layer may be a zinc-based plating layer such as a hot-dip galvanized layer, a galvannealed layer, or an electrogalvanized layer, or an aluminum plating layer such as a hot-dip aluminum plating layer. The zinc-based plating layer may be, for example, a zinc-based alloy plating layer such as zinc-Ni, zinc-Fe, or zinc-Al. The plating layer may be formed on at least one surface of the steel sheet on which the decarburized layer has been formed. Specific examples of plated steel sheets including the zinc-based plating layer include hot-dip galvanized steel sheets (GI), galvannealed steel sheets (GA), and electrogalvanized steel sheets (EG). In this embodiment, since the base material (steel sheet portion) generally has a strong influence on strength and formability, the influence of the presence or absence of a plating layer on bendability and tensile strength is negligible. The thickness of the plating layer may be, for example, 0.001 to 0.020 mm per side.

[0056] In the plated steel sheet according to the embodiment of the present invention, [C] in the above formulas (1) to (3) ３０μｍ , [C] １５０μｍ When measuring "D", "at least one surface" of the steel sheet can be a position where the Zn concentration measured by GD-OES is 50 mass %. Note that the plating layer is sufficiently thin, and even if the plating layer is included, if a measurement is made at a depth of 150 μm, the C concentration will be equivalent to that of the bulk. Therefore, in the plated steel sheet according to the embodiment of the present invention, [C] １５０μｍ may be the carbon concentration at a depth of 150 μm from at least one surface (i.e., the surface of the plating layer) in the thickness direction of the plated steel sheet, and even when measured in this way, [C] １５０μｍ Furthermore, since the plating layer is sufficiently thin compared to the steel sheet, in the plated steel sheet according to the embodiment of the present invention, the sheet thickness t in the above-mentioned formula (3) may be the sheet thickness of the plated steel sheet.

[0057] In the plated steel sheet according to the embodiment of the present invention, when measuring the AGTS in the above-mentioned formula (2), the coating layer can be ground and then the "tensile strength (MPa) after grinding both surfaces in the sheet thickness direction by 0.1 mm" of the steel sheet can be measured. Note that the coating layer is sufficiently thin, and even if the tensile strength (MPa) is measured after grinding both surfaces, including the coating layer, by 0.1 mm, it will be a similar value equivalent to the bulk. Therefore, in the plated steel sheet according to the embodiment of the present invention, the AGTS may be the tensile strength (MPa) after grinding both surfaces in the sheet thickness direction by 0.1 mm, and it is considered that the value of AGTS will not change even if measured in this manner.

[0058] 5. Method for manufacturing steel sheet The steel sheet according to this embodiment is obtained by hot rolling a slab having the above-described chemical composition, coiling it at a predetermined temperature, unwinding the coiled hot-rolled steel sheet and cold-rolling it, annealing the cold-rolled steel sheet obtained by cold rolling, and cooling the annealed cold-rolled steel sheet. Each step will be described below.

[0059] (Hot Rolling Step) A slab having the above-described component composition is hot rolled. If the heating temperature of the slab before hot rolling is low, carbides such as TiC may be difficult to dissolve in austenite. Therefore, the heating temperature of the slab before hot rolling is preferably 1100°C or higher. The heating temperature of the slab before hot rolling is more preferably 1200°C or higher. However, if the heating temperature before hot rolling is too high, costs will increase. Therefore, the upper limit of the heating temperature before hot rolling is preferably 1350°C or lower, more preferably 1300°C or lower.

[0060] If the finish rolling temperature in hot rolling is low, the deformation resistance of the slab during rolling may increase, making operation difficult. Therefore, the finish rolling temperature is preferably 850°C or higher, more preferably 870°C or higher. However, if the finish rolling temperature is too high, the strength of the hot-rolled steel sheet may become excessively high. Therefore, the finish rolling temperature is preferably 980°C or lower, more preferably 950°C or lower.

[0061] The average cooling rate of the hot-rolled steel sheet obtained by hot rolling from finish rolling to coiling is preferably 10°C / sec or more, more preferably 20°C / sec or more, in consideration of productivity. On the other hand, if the average cooling rate is too fast, the equipment cost increases. Therefore, the average cooling rate is preferably 100°C / sec or less, more preferably 50°C / sec or less.

[0062] (Coiling process of hot-rolled steel sheet) The hot-rolled steel sheet obtained by hot rolling is coiled at 630°C or higher. If the coiling temperature of the hot-rolled steel sheet is less than 630°C, the strength of the hot-rolled steel sheet becomes too high, making cold rolling difficult. The coiling temperature of the hot-rolled steel sheet is preferably 640°C or higher, more preferably 650°C or higher. On the other hand, if the coiling temperature of the hot-rolled steel sheet is too high, the pickling properties for removing scales deteriorate. Therefore, the coiling temperature is preferably 800°C or lower, more preferably 750°C or lower.

[0063] (Cold Rolling Step) The coiled hot-rolled steel sheet is subjected to cold rolling, and if necessary, pickling is performed on the hot-rolled steel sheet to remove scale.

[0064] The lower limit of the rolling ratio during cold rolling is preferably 10% or more. The rolling ratio in this embodiment is synonymous with "rolling reduction." Specifically, when the thickness of the steel sheet before rolling is h1 and the thickness of the steel sheet after rolling is h2, the rolling ratio (%) is "(h1-h2) / h1×100." If the rolling ratio during cold rolling is less than 10%, the thickness of the hot-rolled steel sheet must be thinned in the hot-rolling process to obtain a steel sheet of a predetermined thickness. Thinning the hot-rolled steel sheet increases the length of the hot-rolled steel sheet, which requires more time for pickling and reduces productivity. The lower limit of the rolling ratio during cold rolling is more preferably 25% or more. On the other hand, if the rolling ratio during cold rolling exceeds 70%, a high-capacity cold rolling mill is required. Therefore, the upper limit of the rolling ratio during cold rolling is preferably 70% or less, more preferably 65% ​​or less.

[0065] The method for producing a steel sheet according to this embodiment includes, after the cold rolling step, (a) a soaking step, (b) a cooling step, (c) a holding step after cooling, and (d) a final cooling step. In order to obtain the steel sheet according to this embodiment, it is important to appropriately adjust the conditions of each of the steps (a) to (d).

[0066] (a) Soaking Step In the soaking step, the cold-rolled steel sheet is heated and held at a temperature of 830°C or higher for 100 to 300 seconds. Furthermore, to impart a decarburized layer (soft structure) to the surface, the dew point in the atmosphere is controlled to -25°C to -5°C. If the soaking temperature is lower than 830°C, soft ferrite may remain, which reduces the tensile strength of the steel sheet. Therefore, the lower limit of the holding temperature in the soaking step is 830°C. The lower limit of the holding temperature in the soaking step is preferably 930°C. On the other hand, if the holding temperature in the soaking step is too high, productivity may deteriorate or economic efficiency may decrease due to increased furnace fuel consumption. From the viewpoint of productivity and the like, the upper limit of the holding temperature in the soaking step is preferably 900°C or lower. In the soaking step, the temperature of the cold-rolled steel sheet may be kept constant, or the temperature may vary within the above holding temperature range. If the dew point is lower than -25°C, decarburization will be insufficient, the above formula (1) will not be satisfied, and bendability will deteriorate. Therefore, the lower limit of the dew point is set to -25°C. The lower limit of the dew point is preferably -20°C. On the other hand, if the dew point is higher than -5°C, the soft structure in the surface layer will be excessive, and the desired tensile strength may not be obtained. Therefore, the upper limit of the dew point is set to -5°C. The upper limit of the dew point is preferably -10°C.

[0067] In the heating region up to the soaking in step (a), oxidation may be performed at an air ratio of 1.0 or more and 1.2 or less at 300°C to 720°C in order to ensure plating properties. In addition, in the heating region, the dew point may be controlled to -35°C to -5°C at 670°C to 930°C in order to ensure plating properties.

[0068] If the hydrogen concentration in step (a) is in the range of 2 to 15% by volume, the surface layer oxidized in the previous step will be reduced, so the hydrogen concentration may be adjusted appropriately.

[0069] Furthermore, if the holding time in the temperature range of 830°C or higher is less than 100 seconds, the amount of elements that dissolve in the steel sheet during the soaking process from the carbides present in the cold-rolled steel sheet before heating and the elements that were solid-solved in the carbides will be insufficient, resulting in poor hardenability. Therefore, the holding time is set to 100 seconds or more. The lower limit of the holding time in the temperature range of 830°C or higher is preferably 120 seconds or more, more preferably 140 seconds. The upper limit of the holding time in the temperature range of 830°C or higher is 300 seconds or less. Since an excessively long holding time deteriorates productivity, the holding time is preferably less than 280 seconds. Note that a person skilled in the art can control the decarburized layer depth D by appropriately adjusting the temperature, holding time, and dew point in the soaking process.

[0070] (b) Cooling Step In the cooling step, the cold-rolled steel sheet after the soaking step is cooled to a temperature range of 490 to 540°C at an average cooling rate of 2°C / sec or more. If this average cooling rate is less than 2°C / sec, there is a high possibility that ferrite will be generated, making it difficult to ensure the desired tensile strength. Therefore, the average cooling rate must be 2°C / sec or more, preferably 2.5°C / sec or more, and more preferably 3.0°C / sec or more. On the other hand, although there is no particular upper limit, if the average cooling rate exceeds 50°C / sec, it becomes difficult to control the steel sheet temperature and equipment costs increase. Therefore, the upper limit of the average cooling rate is 50°C / sec or less, preferably 40°C / sec or less.

[0071] (c) Holding Step After Cooling After step (b), the cold-rolled steel sheet is cooled to a temperature range of 490 to 540°C and held at that temperature range for 200 to 500 seconds. If the holding temperature is less than 490°C / second, there is a high possibility that excessive bainite will be formed, making it difficult to ensure the desired tensile strength. Therefore, the holding temperature must be 490°C or higher, preferably 500°C or higher. On the other hand, if the holding temperature exceeds 540°C, there is a high possibility that ferrite will be formed, making it difficult to ensure the desired tensile strength. Therefore, the holding temperature must be 540°C or lower, preferably 530°C or lower. If the holding time is less than 200 seconds, there is a possibility that bainite will be reduced too much, resulting in excessively high strength, making it difficult to ensure the desired tensile strength. Therefore, the holding time must be 200 seconds or longer, preferably 250 seconds or longer, and more preferably 300 seconds or longer. On the other hand, if the holding time exceeds 500 seconds, there is a high possibility that bainite will be excessively formed, making it difficult to ensure the desired tensile strength. Therefore, the holding time must be 500 seconds or less, preferably 450 seconds or less, and more preferably 400 seconds or less.

[0072] (d) Final Cooling Step In the final cooling step, the average cooling rate to 200°C or less must be 10°C / s or more. Temperatures above 200°C may result in excessive self-tempering of martensite, which may lead to a decrease in tensile strength. If the average cooling rate is less than 10°C / s, excessive self-tempering of martensite occurs, making it difficult to ensure the desired tensile strength. This average cooling rate must be 10°C / s or more, preferably 12°C / s or more, and more preferably 14°C / s or more. On the other hand, although there is no particular upper limit, if the average cooling rate exceeds 100°C / s, it becomes difficult to control the steel sheet temperature, resulting in increased equipment costs. Therefore, the upper limit of the average cooling rate is 100°C / s or less, preferably 50°C / s or less.

[0073] (Other Steps) After step (d), the product may be cooled to room temperature and subjected to temper rolling and / or processing using a leveler.

[0074] 6. Method for manufacturing plated steel sheet The plated steel sheet according to this embodiment is obtained by forming a plating layer on the surface of the steel sheet obtained as described above. The conditions for forming the plating layer are not particularly limited, and a conventional plating process can be used.

[0075] An example of a method for producing a hot-dip galvanized steel sheet is as follows: In the above-described method for producing a steel sheet, after the holding step (c), the steel sheet may be cooled to about 460°C, immersed in hot-dip galvanizing, and then subjected to step (d) to produce a hot-dip galvanized steel sheet.

[0076] Furthermore, an example of a method for producing a galvannealed steel sheet will be given below. In the above-described method for producing a steel sheet, after the holding step (c), the steel sheet may be cooled to about 460°C, immersed in a hot dip galvanizing bath, heated to 450°C or higher to perform an alloying treatment, and then subjected to step (d) to produce a galvannealed steel sheet.

[0077] The method for manufacturing a plated steel sheet according to an embodiment of the present invention may include other steps without departing from the scope of the present disclosure.

[0078] The following examples are provided to more specifically describe the embodiments of the present invention. The embodiments of the present invention are not limited to the following examples, and may be modified as appropriate within the scope of the above-described and below-described aims, and all such modifications are within the technical scope of the embodiments of the present invention.

[0079] Steel (steel slab) having the chemical composition shown in Table 1 was heated to 1100°C or higher and then hot-rolled to a finish rolling temperature of 880 to 960°C to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was then coiled at a coiling temperature of 630°C or higher. The average cooling rate from finish rolling to coiling was 10 to 100°C / sec. The sheet was then cooled to room temperature and pickled to remove surface scale. A cold-rolled sheet having a thickness of 1.0 to 2.3 mm (rolling reduction of 50% or less) was then obtained. After the holding step (c), the cold-rolled sheet was immersed in hot-dip galvanizing and subjected to an alloying treatment at 430°C to 550°C. A final cooling step was then performed to obtain galvannealed steel sheets of Test Nos. 1 to 129. In the heating region up to the soaking step in step (a), the air ratio was set to 1.0 to 1.2 at 300°C to 720°C to ensure coatability. In addition, in the heating region, the dew point was controlled between -35°C and -5°C at 670°C and 930°C to ensure plating performance. The plating layer thickness was within the range of 0.007 to 0.012 mm on one side. Tables 2 to 4 show the respective conditions. Note that although the O in Table 1 was not measured, it is known from prior knowledge to be 0.0100% by mass or less (more specifically, approximately 0.0010% by mass).

[0080]

[0081]

[0082]

[0083]

[0084] The plated steel sheets of Test Nos. 1 to 129 described above were subjected to the following evaluations.

[0085] <Measurement of Carbon Profile by GD-OES> Carbon profile was measured by GD-OES (Glow Discharge Optical Emission Spectrometry) as described below to investigate the decarburization behavior.

[0086] (Sample Preparation) A material measuring 30 mm x 30 mm x plate thickness was collected. It was then degreased to prepare a sample. Using this sample, the mass % concentration of each element was measured using GD-OES under the following conditions.

[0087] (Measurement conditions) Equipment used: High frequency glow discharge optical emission surface analyzer GDA-750 manufactured by Rigaku Electric Industrial Co., Ltd. Sputtering method: Normal sputtering Measurement range: φ4 mm Type of gas: Ar Elements to be analyzed: B, C, O, Al, Si, Ti, Cr, Mn, Fe, Zn, P, S (In this example, the evaluation was performed on these elements, but if elements other than those mentioned above are contained in, for example, the plating layer and / or the steel sheet, the elements other than those mentioned above will also be analyzed.)

[0088] (Measurement Method) GD-OES measurement was carried out in the thickness direction on the plated surface of the sample.

[0089] (Analysis Method) Since the sputtering rate of the above-mentioned device is almost constant, the sputtering crater depth of the sample was measured after the analysis was completed, and the value (sputtering depth) was plotted on the horizontal axis.

[0090] The details of the calibration curve method for converting the measured emission intensity of each element into a concentration are shown below.

[0091] Sputter weight W of element i per unit time ｉ (g / sec) and emission intensity I ｉ The relationship between W and W is expressed by the following formula (5) using the slope a and intercept b of the calibration curve. ｉ = aI ｉ + b (5) Sputter weight W of the element i per unit time ｉ is the concentration C ｉ (mass%), density ρ (g / cm ３ ), and for a reference sample with a known sputtering rate Δd (cm / sec), the sputtering area S (cm ２ ) and is calculated using the following formula (6): ｉ =C ｉ ×ρ×Δd×S (6)

[0092] W ｉ Using two or more reference samples with known emission intensities I ｉThe slope a and intercept b of the above formula (5) were determined, and a calibration curve was created with the horizontal axis representing the emission intensity and the vertical axis representing the sputter weight. Using the created calibration curve, the sputter weight was determined from the emission intensity of each target element, and the weight ratio was converted into a concentration.

[0093] Then, a carbon profile was obtained using the results of the carbon analysis, an example of which is shown in Figure 1. Figure 1 shows the carbon profile of Test No. 11. The dashed line indicates the position on the surface of the steel sheet (i.e., the position where the Zn concentration is 50 mass % when measured by GD-OES).

[0094] From the obtained carbon concentration profile in the plate thickness direction, [C] ３０μｍ / [C] １５０μｍ , and "D" were calculated. Here, [C] ３０μｍ is the carbon concentration (mass%) at a depth of 30 μm from the position where the Zn concentration is 50 mass% when measured by GD-OES in the sheet thickness direction. D is the carbon concentration at a depth of 0.9 × [C] from the position where the Zn concentration is 50 mass% when measured by GD-OES in the sheet thickness direction. １５０μｍ The depth (μm) to the position where [C] １５０μｍ is the carbon concentration at a depth of 150 μm from at least one surface (i.e., the surface of the plating layer) in the thickness direction of the plated steel sheet.

[0095] <Measurement of Tensile Strength> A JIS No. 5 test piece (plate-shaped test piece) was taken from the plated steel sheet so that the longitudinal direction of the test piece was perpendicular to the rolling direction in a plane parallel to the cold-rolled rolling surface of the steel sheet. A tensile test was performed using the test piece, and the tensile strength TS Ａ Since the contribution of the plating layer to the strength is extremely small, after the tensile test of the plated steel sheet, the thickness of the plating layer was subtracted from the sheet thickness of the plated steel sheet to obtain the TS. Ａ In addition, both surfaces of the plated steel sheet in the sheet thickness direction were ground by 0.1 mm, and then the tensile strength AGTS was measured.

[0096] <Bendability (R / t) Evaluation> The bendability of the plated steel sheets was evaluated by the following procedure. Test pieces with a width of 40 mm and a length of 100 mm were prepared with the major axis perpendicular to the rolling direction, and a bending test was performed using the V-block method in accordance with JIS Z 2248:2014. The bending radius was varied from 0 to 7 mm to determine the minimum bending radius at which the material could be bent without cracking. This was taken as the bending radius R (mm), and the ratio of bending radius R (mm) to sheet thickness t (mm) was calculated. A magnifying glass (approximately 5x magnification) was used to determine cracks. Examples in which the ratio of bending radius R (mm) to sheet thickness t (mm) was 4.0 or less were evaluated as having sufficient bendability.

[0097] The results are shown in Tables 5 to 7. Here, the sheet thickness t (mm) is the sheet thickness of the plated steel sheet, and TS in the above formula (3) Ｅ However, since the thickness of the plating layer is sufficiently thin and negligible, t was used as the thickness of the plated steel sheet in the calculation.

[0098]

[0099]

[0100]

[0101] The results of Tables 5 to 7 can be considered as follows: All of the plated steel sheets of Test Nos. 1 to 129 in Tables 5 to 7 satisfy the requirements defined in the embodiment of the present invention, have sufficient bendability, and satisfy the formula (2), and the TS of the formula (3) Ｅ The tensile strength of the plated steel sheet was predicted with high accuracy.

[0102] This application claims priority from Japanese Patent Application No. 2024-105288, filed June 28, 2024, and Japanese Patent Application No. 2025-080759, filed May 13, 2025. Japanese Patent Application Nos. 2024-105288 and 2025-080759 are incorporated herein by reference.

Claims

1. A steel sheet containing C: 0.115 to 0.145 mass%, Si: 0.70 to 1.20 mass%, Mn: 2.2 to 2.5 mass%, P: 0.020 mass% or less (inclusive of 0 mass%), S: 0.0030 mass% or less (inclusive of 0 mass%), Al: 0.005 to 0.040 mass%, Cr: 0.10 to 0.70 mass%, Ti: 0.010 to 0.040 mass%, B: 0.0020 to 0.0040 mass%, N: 0.0100 mass% or less (inclusive of 0 mass%), and O: 0.0100 mass% or less (inclusive of 0 mass%), with the balance being iron and inevitable impurities, and satisfying the following formulas (1) and (2): [C] ３０μｍ / [C] １５０μｍ ≦0.80...(1) TS Ｅ -50≦TS Ａ ≦TS Ｅ +50...(2) TS Ｅ = (26.7 + 29.7 × t + 0.94 × AGTS − 0.4 × D) (3) In the above formula, [C] ３０μｍ is the carbon concentration (mass%) at a depth of 30 μm from at least one surface in the plate thickness direction, [C] １５０μｍ is the carbon concentration (mass%) at a depth of 150 μm from the at least one surface in the plate thickness direction, and TS Ｅ is the value (MPa) shown in formula (3), and TS Ａ is the tensile strength (MPa) of the steel plate, t is the plate thickness (mm), and D is the carbon concentration 0.9 × [C] １５０μｍ AGTS is the tensile strength (MPa) after grinding both surfaces in the thickness direction by 0.1 mm.

2. Mo: 0.20 mass% or less (not including 0 mass%), Cu: 0.20 mass% or less (not including 0 mass%), Ni: 0.20 mass% or less (not including 0 mass%), Ca: 0.0040 mass% or less (not including 0 mass%), Nb: 0.040 mass% or less (not including 0 mass%), V: 0.20 mass% or less (not including 0 mass%), Ta: 0.010 mass% or less (not including 0 mass%), W: 0.010 mass% or less (not including 0 mass%), Co: 0.010 mass% or less (not including 0 mass%), Sn: 0.010 mass% or less (not including 0 mass%), Sb: 0.010 mass% or less (not including 0 mass%), Mg: 0.010 mass% or less (not including 0 mass%), REM: 0.010 mass% or less (not including 0 mass%) 2. The steel sheet according to claim 1, further comprising at least one selected from the group consisting of Zr: 0.010 mass% or less (excluding 0 mass%), Te: 0.010 mass% or less (excluding 0 mass%), Hf: 0.010 mass% or less (excluding 0 mass%), and Bi: 0.010 mass% or less (excluding 0 mass%).

3. A plated steel sheet comprising the steel sheet according to claim 1 or 2 and a plating layer disposed on the surface of the steel sheet.

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