Steel plate, steel member, and coated steel member

Abstract

In this steel member, a steel sheet base material, which comprises, by mass%, 0.10 to 0.65% of C, 0.10 to 2.00% of Si, 0.30 to 3.00% of Mn, 0.050% or less of P, 0.0100% or less of S, 0.010% or less of N, 0.010% or less of O, 0 to 0.100% of Ti, 0 to 0.0100% of B, 0 to 1.00% of Cr, 0 to 1.00% of Mo, 0 to 1.00% of Ni, 0 to 0.10% of Nb, 0 to 1.00% of Cu, 0 to 1.00% of V, 0 to 0.010% of Ca, 0 to 0.010% of Mg, 0 to 1.00% of Al, 0 to 1.00% of Sn, 0 to 1.00% of W, 0 to 1.00% of Sb, 0 to 1.00% of Zr, 0 to 1.00% of Co, 0 to 0.30% of REM, and the balance Fe and impurities, has a surface scale and a decarburized layer formed on the interface side with the scale, wherein the decarburized layer has an internal oxidized layer formed on the interface side with the scale, the depth of the decarburized layer from the interface between the steel sheet base material and the scale is 60 µm or greater, the depth of the internal oxidized layer from the interface is less than 40 µm, and the scale contains 70 mass% or more of Fe.

Description

Technical Field

[0001] The present invention relates to steel plates, steel members, and coated steel members. This application claims priority based on Japanese Patent Application No. 2021-043720 filed in Japan on March 17, 2021, and incorporates the content herein by reference.

Background Art

[0002] In the field of steel plates for automobiles, against the backdrop of recent environmental regulations and stricter collision safety standards, in order to improve both fuel efficiency and collision safety, the application of steel plates with high tensile strength (high-strength steel plates) has been expanding. However, as the strength increases, the press formability of the steel plate decreases, making it difficult to manufacture products with complex shapes.

[0003] Specifically, as the strength increases, the ductility of the steel plate decreases, resulting in a problem that it breaks at highly processed parts when processed into complex shapes. In addition, with the increase in the strength of the steel plate, springback and wall warping occur due to residual stress after processing, leading to a problem of deterioration in dimensional accuracy. Therefore, it is not easy to press-form a steel plate having a high strength, particularly a tensile strength of 780 MPa or more, into a product having a complex shape. If roll forming is used instead of press forming, it is easy to process high-strength steel plates, but its application is limited to parts having a uniform cross-section in the longitudinal direction.

[0004] Therefore, in recent years, hot stamping technology has been adopted as a technology for press-forming materials that are difficult to form, such as high-strength steel plates. The hot stamping technology is a hot forming technology in which the material to be formed is heated and then formed.

[0005] In this technology, the material is heated and then formed. Therefore, during forming, the steel material is soft and has good formability. As a result, even a high-strength steel plate can be accurately formed into a complex shape. In addition, in the hot stamping technology, quenching is performed simultaneously with forming by a press die, so the formed steel member has sufficient strength.

[0006] For example, according to Patent Document 1, it is disclosed that by hot stamping technology, it is possible to impart a tensile strength of 1400 MPa or more to a formed steel member.

[0007] On the other hand, automobiles are also required to have collision safety. The collision safety of an automobile is evaluated by the crushing strength and energy absorption in the collision test of the entire vehicle body or some members. In particular, since the crushing strength greatly depends on the material strength, the demand for ultra-high strength steel members as automobile members has increased dramatically. However, generally, steel members have a decrease in fracture toughness and deformability with the increase in strength. Therefore, it may break early during collision crushing, or break at a site where deformation is concentrated, and the crushing strength corresponding to the material strength cannot be exerted, and sufficient energy absorption may not be obtained. Therefore, in order to improve the collision safety of automobiles, not only the material strength but also the improvement of fracture toughness and deformability, that is, the improvement of toughness and bendability, are required for the steel members used. Therefore, in order to apply a high-strength steel member with a tensile strength exceeding 1.0 GPa to the vehicle body, a technology is required to provide a steel member having toughness and bendability higher than before and showing sufficient energy absorption even in the event of a collision accident.

[0008] Patent Document 2 discloses a hot press-formed product having excellent toughness and a tensile strength of 1.8 GPa or more. Patent Document 3 discloses a steel material having an extremely high tensile strength of 2.0 GPa or more and further having good toughness and ductility. Patent Document 4 discloses a steel material having a high tensile strength of 1.8 GPa or more and further having good toughness. Patent Document 5 discloses a steel material having an extremely high tensile strength of 2.0 GPa or more and further having good toughness. However, Patent Documents 1 to 5 do not describe bendability, and in the use of high-strength steel materials with a tensile strength exceeding 1.0 GPa as automobile members, they may not be able to fully meet higher requirements in some cases.

[0009] In addition, steel members are usually applied to automobile bodies by being assembled by welding. Therefore, when assuming use in automobile parts, in addition to the above-described high strength and bendability, excellent weldability is also required.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0011] The present invention has been made in view of the above problems. An object of the present invention is to provide a steel member having high strength, excellent bendability, and excellent weldability, and a steel sheet suitable as a material for the steel member.

Means for Solving the Problems

[0012] The inventors of the present invention conducted studies to obtain excellent bendability and weldability in a high-strength steel member obtained by hot stamping a steel sheet. As a result, in the steel sheet (material steel sheet) to be hot stamped, by increasing the Fe content of the scale formed on the surface and controlling the depths (thicknesses) of the internal oxide layer and the decarburized layer within an appropriate range, it was found that in the steel member obtained by hot stamping this steel sheet, high strength, excellent bendability, and excellent weldability can be obtained.

[0013] The present invention has been made in view of the above problems. The gist of the present invention is as follows. [1] The steel sheet according to one aspect of the present invention has a base steel sheet and a scale formed on the surface of the base steel sheet. The base steel sheet has, in mass%, C: 0.10 to 0.65%, Si: 0.10 to 2.00%, Mn: 0.30 to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Ti: 0 to 0.100%, B: 0 to 0.0100%, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Nb: 0 to 0.10%, Cu: 0 to 1.00%, V: 0 to 1.00%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, Al: 0 to 1.00%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.00%, Zr: 0 to 1.00%, Co: 0 to 1.00%, and REM: 0 to 0.30%, and the balance consists of Fe and impurities. The base steel sheet has a decarburized layer formed on the interface side with the scale. The decarburized layer has an internal oxidized layer formed on the interface side with the scale. The depth of the decarburized layer from the interface between the base steel sheet and the scale is 90 μm or more, and the depth of the internal oxidized layer from the interface is less than 30 μm. The scale contains 80% or more of Fe by mass%. See that the surface of the scale has no coating . [2] In the steel sheet according to [1] above, the scale may include a first region containing 80% or more of Fe and 0.1% or more and less than 3.0% of Si by mass%, and a second region containing 65% or more and less than 80% of Fe and 0.8% or more and less than 7.5% of Mn by mass%. [3] The steel member according to one aspect of the present invention has a steel plate base material and a scale formed on the surface of the steel plate base material. The steel plate base material contains, by mass%, C: 0.10 to 0.65%, Si: 0.10 to 2.00%, Mn: 0.30 to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Ti: 0 to 0.100%, B: 0 to 0.0100%, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Nb: 0 to 0.10%, Cu: 0 to 1.00%, V: 0 to 1.00%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, Al: 0 to 1.00%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.00%, Zr: 0 to 1.00%, Co: 0 to 1.00%, and REM: 0 to 0.30%, and the balance consists of Fe and impurities, and has a chemical composition. The steel plate base material has a decarburized layer formed on the interface side with the scale, and the decarburized layer has an internal oxidized layer formed on the interface side with the scale. The depth of the decarburized layer from the interface between the steel plate base material and the scale is 60 μm or more, and the depth of the internal oxidized layer from the interface is less than 40 μm. The scale contains 70% or more of Fe by mass% See that the surface of the scale has no coating . [4] In the steel sheet according to [1] above, the depth of the internal oxide layer from the interface may be 18 μm or less. [Advantages of the Invention]

[0014] According to the above aspect of the present invention, it is possible to provide a steel member (including a coated steel member) having high strength and excellent bendability and weldability, and a steel plate suitable as a material for the steel member. [Brief Description of the Drawings]

[0015]

Figure 1

Figure 2

Figure 3

Best Mode for Carrying Out the Invention

[0016] Hereinafter, a steel sheet (the steel sheet according to this embodiment), a steel member (including a coated steel member) (the steel member according to this embodiment), and their preferred manufacturing methods according to an embodiment of the present invention will be described.

[0017] First, the steel sheet according to this embodiment will be described. As shown in FIG. 1, the steel sheet 10 according to this embodiment has a base steel sheet 11 having the following chemical composition and a scale 12 formed on the surface of the base steel sheet 11 and containing 80% by mass or more of Fe. Further, this base steel sheet 11 has a decarburized layer 13 with a predetermined depth on the interface side (the region in contact with the interface) with the scale 12, and this decarburized layer 13 has an internal oxidation layer 14 on the interface side between the base steel sheet 11 and the scale 12. In FIG. 1, only the scale on one side is shown, but the scale may be formed on both sides. In that case, the decarburized layer 13 and the internal oxidation layer 14 are formed in the regions of both sides of the base steel sheet 11 in contact with the interfaces with the scale. In this embodiment, the "scale side" means the "scale side in the thickness direction of the base steel sheet", and the "interface side with the scale" means the "interface side (the region in contact with the interface) between the base steel sheet and the scale in the thickness direction of the base steel sheet".

[0018] <Base steel sheet> [Chemical composition] In the numerical limit range sandwiched by "~" described below, the values at both ends are included in the range as the lower limit value and the upper limit value. However, the numerical values indicated by "exceeding" or "less than" are not included in the numerical range. "% " regarding the content of each element means "mass%" unless otherwise specified.

[0019] C: 0.10 to 0.65% C is an element that enhances the hardenability of steel and improves the strength of the steel member (obtained by hot stamping a steel sheet) after hot stamping. However, when the C content is less than 0.10%, it becomes difficult to ensure sufficient strength (exceeding 1.0 GPa) in the steel member after hot stamping (the steel member obtained by hot stamping a steel sheet). Therefore, the C content should be 0.10% or more. The C content is preferably 0.15% or more, and more preferably 0.26% or more. On the other hand, when the C content exceeds 0.65%, the strength of the steel member after hot stamping becomes too high, resulting in significant deterioration of bendability. Also, the weldability deteriorates. Therefore, the C content should be 0.65% or less. The C content is preferably 0.60% or less.

[0020] Si: 0.10 - 2.00% Si is an element that enhances the hardenability of steel and is effective in stably ensuring strength in the steel member after hot stamping. To obtain this effect, the Si content needs to be 0.10% or more. The Si content is preferably 0.35% or more. On the other hand, when the Si content in the steel sheet exceeds 2.00%, the heating temperature required for austenite transformation during heat treatment becomes extremely high. As a result, the cost required for heat treatment may increase. Furthermore, when the Si content exceeds 2.00%, the toughness of the hardened part deteriorates. Therefore, the Si content should be 2.00% or less. The Si content is preferably 1.60% or less.

[0021] Mn: 0.30 - 3.00% Mn is an element that is very effective in enhancing the hardenability of steel and stably ensuring strength in the steel member after hot stamping. Mn further lowers the Ac3 point and promotes the lowering of the hardening treatment temperature. Also, Mn has the effect of diffusing into the Al-Fe based coating to improve corrosion resistance. When the Mn content is less than 0.30%, these effects are not sufficient, so the Mn content should be 0.30% or more. The Mn content is preferably 0.40% or more. On the one hand, when the Mn content exceeds 3.00%, the above effects are saturated, and the toughness and bendability of the quenched part deteriorate. Therefore, the Mn content should be 3.00% or less. The Mn content is preferably 2.80% or less, and more preferably 2.50% or less.

[0022] P: 0.050% or less P is an element that deteriorates the toughness of the steel member after hot stamping. In particular, when the P content exceeds 0.050%, the deterioration of toughness becomes significant. Therefore, the P content is limited to 0.050% or less. The P content is preferably limited to 0.005% or less. Since it is preferable that the P content is small, it may be 0%, but from the viewpoint of cost, it may be 0.001% or more.

[0023] S: 0.0100% or less S is an element that deteriorates the toughness and bendability of the steel member after hot stamping. In particular, when the S content exceeds 0.0100%, the deterioration of toughness and bendability becomes significant. Therefore, the S content is limited to 0.0100% or less. The S content is preferably limited to 0.0050% or less. Since it is preferable that the S content is small, it may be 0%, but from the viewpoint of cost, it may be 0.0001% or more.

[0024] N: 0.010% or less N is an element that deteriorates the toughness of the steel member after hot stamping. In particular, when the N content exceeds 0.010%, coarse nitrides are formed in the steel, and the toughness deteriorates significantly. Therefore, the N content should be 0.010% or less. The lower limit of the N content does not need to be particularly limited and may be 0%, but setting the N content to less than 0.0002% causes an increase in steelmaking cost and is not economically preferable. Therefore, the N content may be 0.0002% or more, and may be 0.0008% or more.

[0025] O: 0.010% or less O is an element that deteriorates the toughness of the steel member after hot stamping. In particular, when the O content exceeds 0.010%, coarse oxides are formed in the steel, and the toughness deteriorates significantly. Therefore, the O content should be 0.010% or less. The lower limit of the O content does not need to be particularly limited and may be 0%, but setting the O content to less than 0.0002% will increase the steelmaking cost, which is not economically preferable. Therefore, the O content may be 0.0002% or more, or 0.0008% or more.

[0026] In order to improve the strength, toughness, bendability, corrosion resistance, and deoxidation property of the steel member according to this embodiment, in addition to the above elements, one or more elements selected from Ti, B, Cr, Mo, Ni, Nb, Cu, V, Ca, Mg, Al, Sn, W, Sb, Zr, Co, and REM shown below may be further contained. These elements are optional elements and do not necessarily need to be contained, so the lower limit is 0%.

[0027] Ti: 0 to 0.100% Ti is an element that has the effect of making austenite grains finer by suppressing recrystallization and forming fine carbides to suppress grain growth when the steel plate is heated to a temperature above the Ac3 point and heat-treated. Therefore, by containing Ti, the toughness of the steel member after hot stamping can be greatly improved. In addition, Ti preferentially binds to N in the steel to suppress the consumption of B due to the precipitation of BN, and promotes the effect of improving hardenability by B described later. Therefore, Ti may be contained. When fully obtaining the above effects, the Ti content is preferably 0.010% or more. The Ti content is more preferably 0.020% or more. On the other hand, when the Ti content exceeds 0.100%, the precipitation amount of TiC increases and C is consumed, so the strength of the steel member after hot stamping decreases. Therefore, the Ti content should be 0.100% or less. The Ti content is preferably 0.080% or less.

[0028] B: 0 to 0.0100% B is an element that has the effect of dramatically increasing the hardenability of steel even in trace amounts. Also, B is an element that segregates at grain boundaries, strengthens the grain boundaries, and increases toughness, and is an element that suppresses the grain growth of austenite during heating of the steel plate. Therefore, B may be contained. When sufficiently obtaining the above effects, the B content is preferably 0.0010% or more. The B content is more preferably 0.0020% or more. On the other hand, when the B content exceeds 0.0100%, a large amount of coarse compounds precipitate, and the toughness of the steel member after hot stamping deteriorates. Therefore, when contained, the B content should be 0.0100% or less. The B content is preferably 0.0080% or less.

[0029] Cr: 0 to 1.00% Cr is an element effective for increasing the hardenability of steel and stably ensuring the strength of the steel member after hot stamping. Therefore, Cr may be contained. When obtaining the above effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.05% or more, and even more preferably 0.08% or more. On the other hand, when the Cr content exceeds 1.00%, the above effects saturate and the cost increases. Also, since Cr has the effect of stabilizing iron carbide, when the Cr content exceeds 1.00%, coarse iron carbide may remain undissolved during heating of the steel plate, and the toughness of the steel member after hot stamping may deteriorate. Therefore, when contained, the Cr content should be 1.00% or less. The Cr content is preferably 0.80% or less.

[0030] Mo: 0 to 1.00% Mo is an element effective for increasing the hardenability of steel and stably ensuring the strength of the steel member after hot stamping. Therefore, Mo may be contained. When obtaining the above effects, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.05% or more. On the one hand, when the Mo content exceeds 1.00%, the above effects are saturated and the cost increases. In addition, since Mo has the effect of stabilizing iron carbide, when the Mo content exceeds 1.00%, coarse iron carbide remains undissolved during the heating of the steel plate, and the toughness of the steel member after hot stamping may deteriorate. Therefore, when adding, the Mo content should be 1.00% or less. The Mo content is preferably 0.80% or less.

[0031] Ni: 0 to 1.00% Ni is an element effective for increasing the hardenability of steel and stably ensuring the strength of the steel member after hot stamping. Therefore, Ni may be contained. When obtaining the above effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.10% or more. On the other hand, when the Ni content exceeds 1.00%, the above effects are saturated and the economic efficiency decreases. Therefore, when adding, the Ni content should be 1.00% or less.

[0032] Nb: 0 to 0.10% Nb is an element that forms fine carbides and has the effect of increasing the toughness of steel due to its grain refinement effect. Therefore, Nb may be contained. When sufficiently obtaining the above effects, the Nb content is preferably 0.02% or more. The Nb content is more preferably 0.03% or more. On the one hand, when the Nb content exceeds 0.10%, the carbides coarsen and the toughness of the steel member deteriorates. Therefore, the Nb content should be 0.10% or less. The Nb content is preferably 0.08% or less.

[0033] Cu: 0 to 1.00% Cu is an element effective for increasing the hardenability of steel and stably ensuring the strength of the steel member after hot stamping. Therefore, Cu may be contained. In addition, Cu is an element that has the effect of improving the corrosion resistance of the steel member. When obtaining the above effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more. On the one hand, when the Cr content exceeds 1.00%, the above effects will saturate and the cost will increase. Therefore, when adding, the Cu content should be 1.00% or less. The Cu content is preferably 0.80% or less.

[0034] V: 0 to 1.00% V is an element that forms fine carbides and enhances the toughness of steel due to its grain refinement effect. Therefore, V may be added. When obtaining the above effects, the V content is preferably 0.01% or more. The V content is more preferably 0.10% or more. On the other hand, when the V content exceeds 1.00%, the above effects will saturate and the economic efficiency will decrease. Therefore, when adding, the V content should be 1.00% or less.

[0035] Ca: 0 to 0.010% Ca is an element that has the effect of refining inclusions in steel and improving the toughness after hot stamping. Therefore, Ca may be added. When obtaining the above effects, the Ca content is preferably 0.001% or more. The Ca content is more preferably 0.002% or more. On the one hand, when the Ca content exceeds 0.010%, the effect will saturate and the cost will increase. Therefore, when adding, the Ca content should be 0.010% or less. The Ca content is preferably 0.005% or less, and more preferably 0.004% or less.

[0036] Mg: 0 to 0.010% Mg is an element that has the effect of refining inclusions in steel and improving the toughness of the steel member after hot stamping. Therefore, Mg may be added. When obtaining the above effects, the Mg content is preferably 0.001% or more. The Mg content is more preferably 0.002% or more. On the one hand, when the Mg content exceeds 0.010%, the effect will saturate and the cost will increase. Therefore, when adding, the Mg content should be 0.010% or less. The Mg content is preferably 0.005% or less, and more preferably 0.004% or less.

[0037] Al: 0 to 1.00% Al is an element commonly used as a deoxidizer for steel. Therefore, Al may be contained. When obtaining the above effects, the Al content is preferably 0.01% or more. On the other hand, when the Al content exceeds 1.00%, the above effects are saturated and the economy deteriorates. Therefore, when contained, the Al content should be 1.00% or less.

[0038] Sn: 0 to 1.00% Sn is an element that improves corrosion resistance in a corrosive environment. Therefore, Sn may be contained. When obtaining the above effects, the Sn content is preferably 0.01% or more. On the other hand, when the Sn content exceeds 1.00%, the grain boundary strength decreases and the toughness of the steel member after hot stamping deteriorates. Therefore, when contained, the Sn content should be 1.00% or less.

[0039] W: 0 to 1.00% W is an element that increases the hardenability of steel and enables the strength of the steel member after hot stamping to be stably ensured. Therefore, W may be contained. Also, W is an element that improves corrosion resistance in a corrosive environment. When obtaining the above effects, the W content is preferably 0.01% or more. On the other hand, when the W content exceeds 1.00%, the above effects are saturated and the economy deteriorates. Therefore, when contained, the W content should be 1.00% or less.

[0040] Sb: 0 to 1.00% Sb is an element that improves corrosion resistance in a corrosive environment. Therefore, Sb may be contained. When obtaining the above effects, the Sb content is preferably 0.01% or more. However, when the Sb content exceeds 1.00%, the grain boundary strength decreases and the toughness of the steel member after hot stamping deteriorates. Therefore, when contained, the Sb content should be 1.00% or less.

[0041] Zr: 0 to 1.00% Zr is an element that improves corrosion resistance in a corrosive environment. Therefore, Zr may be contained. In order to obtain the above effects, it is preferable that the Zr content is 0.01% or more. On the other hand, when the Zr content exceeds 1.00%, the grain boundary strength decreases, and the hydrogen embrittlement resistance of the steel member after hot stamping decreases. Therefore, when it is contained, the Zr content should be 1.00% or less.

[0042] Co: 0 to 1.00% Co is an element that improves corrosion resistance in a corrosive environment. Therefore, Co may be contained. When obtaining the above effects, it is preferable that the Co content is 0.01% or more. On the other hand, when the Co content exceeds 1.00%, the above effects are saturated and the economy decreases. Therefore, when it is contained, the Co content should be 1.00% or less.

[0043] REM: 0 to 0.30% REM is an element that has the effect of refining inclusions in steel and improving the toughness of the steel member after hot stamping, similar to Ca. Therefore, REM may be contained. When obtaining the above effects, it is preferable that the REM content is 0.01% or more. The REM content is more preferably 0.02% or more. On the other hand, when the REM content exceeds 0.30%, its effect is saturated and the cost increases. Therefore, when it is contained, the REM content should be 0.30% or less. The REM content is preferably 0.20% or less.

[0044] Here, REM refers to a total of 17 elements of scandium (Sc), yttrium (Y), and lanthanoids such as lanthanum (La) and neodymium (Nd), and the REM content means the total content of these elements. REM is added to the molten steel using, for example, an Fe-Si-REM alloy, and this alloy contains, for example, La, Nd, Ce, and Pr.

[0045] In the chemical composition of the base steel plate of the steel plate according to the present embodiment, other than the elements described above, that is, the balance is Fe and impurities. Here, the "impurities" are components that are mixed in due to raw materials such as ore and scrap, and various factors in the manufacturing process when the steel plate is industrially manufactured, and are those that are allowed within a range that does not adversely affect the characteristics of the steel plate and the steel member according to the present embodiment.

[0046] The chemical composition of the base steel plate can be determined by the following method. An analysis sample is cut out from the base steel plate and obtained by performing elemental analysis such as ICP (inductively coupled plasma) optical emission spectrometry. C and S may be measured using combustion-infrared absorption spectrometry, N using inert gas fusion-thermal conductivity method, and O using inert gas fusion-non-dispersive infrared line absorption spectrometry. As described in JIS G 0417:1999, the analysis sample is taken so as to obtain the average chemical composition of the entire thickness of the base steel plate. Specifically, avoiding the widthwise ends of the base steel plate, the analysis sample is taken from a position 1 / 4 of the thickness from the surface in the thickness direction.

[0047] [Decarburized layer] [Internal oxidation layer] As shown in FIG. 1, the base steel plate 11 of the steel plate 10 according to the present embodiment has a decarburized layer 13 on the scale 12 side (the interface side between the base steel plate 11 and the scale 12). That is, a part of the base steel plate 11 on the scale 12 side is the decarburized layer 13. Further, the decarburized layer 13 has an internal oxidation layer 14 on the scale 12 side. That is, a part of the decarburized layer 13 on the scale 12 side is the internal oxidation layer 14. The depth (the distance in the thickness direction from the interface) of the internal oxidation layer 14 in the cross section in the thickness direction from the interface between the base steel plate 11 and the scale 12 is less than 30 μm. The depth (the distance in the thickness direction from the interface) of the decarburized layer 13 in the cross section in the thickness direction from the interface between the base steel plate 11 and the scale 12 is 90 μm or more. In a steel member obtained by hot stamping a steel plate, in order to improve bendability, it is extremely effective to decarburize the surface layer to soften it. Since bending deformation causes greater stress and strain in the surface layer outside the bend, bendability can be improved by softening the surface layer to improve the fracture limit. In the steel sheet 10 according to the present embodiment, in order to form a decarburized layer on the surface layer of the steel member after hot stamping, a decarburized layer 13 having a depth (thickness) of 90 μm or more from the interface is formed on the interface side of the base metal steel sheet 11 with the scale 12 described later. If the depth (thickness) at which the decarburized layer 13 is formed is less than 90 μm, the decarburized layer is not formed to a sufficient depth in the steel sheet base material of the steel member after hot stamping, and the bendability of the steel member decreases. Although the surface of the base metal steel sheet is carburized again by hot stamping and the decarburized layer depth becomes smaller, by setting the decarburized layer depth of the base metal steel sheet 11 to 90 μm or more, the decarburized layer depth of the steel member after hot stamping can be set to 60 μm or more under normal hot stamping conditions.

[0048] As described above, as a technique for decarburizing the surface layer of a steel sheet, a method of annealing the steel sheet at a high dew point and decarburizing it with H2O in the atmosphere (high dew point annealing) is known. However, as a result of investigations by the present inventors, it has been found that when such high dew point annealing is performed, internal oxidation (oxidation of easily oxidizable elements such as Si and Mn in steel) that occurs simultaneously with decarburization causes various problems. Specifically, in a steel member obtained by hot stamping, a scale (internal scale) is generated inside the steel sheet starting from the internal oxidation layer during hot stamping, and it has been found that the scale generated from inside the steel sheet may inhibit weldability. As a result of further investigations by the present inventors, it has been found that in the steel sheet (base metal steel sheet 11) to be hot stamped, by making the depth of the internal oxidation layer 14 less than 30 μm, generation of scale inside the steel sheet during hot stamping can be suppressed. Therefore, in the steel sheet 10 according to the present embodiment, the depth (thickness) of the internal oxidation layer 14 of the base metal steel sheet 11 from the interface between the base metal steel sheet 11 and the scale 12 is made less than 30 μm. Preferably, the depth of the internal oxidation layer is less than 5 μm. In this case, the bendability becomes more excellent. In order to suppress the depth of the internal oxidation layer 14 while ensuring the depth of the decarburized layer 13, it is necessary to control the annealing conditions as described later.

[0049] Also, it is preferable that the ratio of the depth (thickness) of the decarburized layer to the depth (thickness) of the internal oxidation layer satisfies the relationship of 3 or more (thickness of decarburized layer / thickness of internal oxidation layer ≧ 3). In this case, the bendability described later becomes more excellent. More preferably, the above ratio is 10 or more. Even more preferably, the above ratio is 20 or more.

[0050] Regarding the depth of the decarburized layer 13 from the interface between the base steel plate 11 and the scale 12, it can be obtained by the following method using GDS. Perform GDS (glow discharge optical emission spectrometry) in the thickness direction from the surface of the steel plate to obtain the depth of the decarburized layer. The GDS measurement is performed at a position 1 / 4 of the plate width (short side) from the end in the width direction of the steel plate, from the surface (the surface of the scale) in the thickness direction, at a pitch of 50 nm or less, to measure the C content and the Fe content. As a result of the measurement, the position where the Fe content first becomes 95% or more is defined as the interface between the base steel plate and the scale. Also, the position where the C content obtained by GDS analysis becomes the C content at a position 1 / 4 of the plate thickness from the surface of the base steel plate described above is defined as the deepest position of the decarburized layer. The distance from the interface between the base material and the scale to the deepest position of the decarburized layer is defined as the depth (thickness of the decarburized layer) of the decarburized layer from the interface between the base steel plate and the scale. However, the above measurement is performed 5 times by changing the location, and the average value of the 5 times is defined as the depth of the decarburized layer from the interface between the base steel plate and the scale of the steel plate according to the present embodiment (also referred to as the total decarburization depth). When the decarburized layer is so deep that it exceeds the measurement limit of GDS, the depth of the decarburized layer may be obtained from microscopic observation as described in JIS G 0558 (2007). In this case, a cross-sectional observation sample is taken from a position 1 / 4 of the plate width (short side) in the width direction from the end in the width direction of the steel plate, and this sample is subjected to nital etching and cross-sectional observation is performed with an optical microscope. The depth at which the structure becomes equivalent to the position 1 / 4 of the plate thickness from the surface (interface with the scale) of the base steel plate is measured, and the depth at that position is defined as the depth of the decarburized layer. However, the measurement is performed 5 times by changing the location, and the average value of the 5 times is defined as the decarburized layer depth.

[0051] Regarding the depth of the internal oxidation layer from the interface between the base steel plate and the scale, it is obtained by SEM (scanning electron microscope) observation of the cross-section of the steel plate. A sample for cross-sectional observation is taken from a position 1 / 4 of the plate width (short side) from the widthwise end of the steel plate, and COMPO image observation is performed by SEM. Since internal oxidation progresses deeper along grain boundaries than within grains and appears darker in the COMPO image than the steady part of the base steel plate (higher Fe content), the internal oxidation layer is identified by the color difference, and the depth of the deepest internal oxidation layer is measured from the interface between the base steel plate and the scale. However, the above measurement is performed 5 times at different locations, and the average value of the 5 times is taken as the depth from the interface with the internal oxidation layer scale of the steel plate according to this embodiment.

[0052] [Scale] The steel plate according to this embodiment has a scale formed on the surface of the base steel plate. As will be described later, the steel plate according to this embodiment utilizes O in the scale formed on the surface of the base steel plate by rolling or the like to decarburize the base steel plate. Therefore, in the scale after decarburization occurs, the O content is greatly reduced compared to the normal scale composed of FeO, Fe2O3, Fe3O4, etc. that is usually formed by hot rolling or the like, and in mass%, it contains 80% or more of Fe. That is, by performing decarburization under the condition that the Fe content of the scale is 80% or more, a steel plate having the internal oxidation layer and decarburized layer with the above-described depth can be obtained. In other words, although the scale may be removed or peeled off during processing or the like, it can also be considered that a steel plate having the depth of the internal oxidation layer and decarburized layer within the above-described range has a scale equivalent to the scale provided in the steel plate according to this embodiment. Also, in terms of supplying O for decarburization, the thickness of the scale is preferably 5 μm or more. More preferably, it is 8 μm or more, and even more preferably, it is 10 μm or more. In terms of the yield of the steel plate, the thickness of the scale is preferably less than 100 μm. More preferably, it is 50 μm or less, or 30 μm or less.

[0053] The scale included in the steel sheet according to this embodiment preferably includes a first region containing 80% or more of Fe and 0.1% or more and less than 3.0% of Si by mass%, and a second region containing 65% or more and less than 80% of Fe and 0.8% or more and less than 7.5% of Mn. Substantially, it preferably consists of the first region and the second region. However, as the "other region", oxides mainly composed of impurities, Cr, Si, etc., or simple substances of hardly oxidizable elements such as Cu may exist on the outermost surface of the scale. By having a scale with such a configuration, in spot welding when assembling the vehicle body, the current limit at which spatter occurs can be improved, and a steel sheet with a wide proper current range, that is, good weldability, can be obtained. The first region may contain C, Ni, Cr, Mo, etc. in addition to Fe, Si, and O. The second region may contain C, Ni, etc. in addition to Fe, Mn, and O.

[0054] When the first region and the second region exist, the second region often takes a form of existing in an island shape in the first region which is the matrix. The second region may be dispersed in an island shape, or several islands may be combined. In any case, the first region and the second region can be distinguished by the method described later.

[0055] The Fe content of the scale is determined by the following method. At a position 1 / 4 of the plate width (short side) from the end in the width direction of the steel sheet, GDS (glow discharge optical emission spectrometry) is performed in the thickness direction from the surface to obtain the Fe content and O content of the scale. The region where the O content is 0.1% or more is excluded as an impurity, and the average value of the Fe content in the region where the O content is less than 0.1% from the surface is measured. The measurement is performed 5 times by changing the location, and the average value of the 5 times is taken as the Fe content of the scale. The Fe content, Si content in the first region of the scale, and the Fe content, Mn content in the second region are determined using SEM (scanning electron microscope) and electron probe microanalyzer (EPMA). Samples are taken from a position 1 / 4 of the plate width (short side) in the width direction from the width direction end of the steel plate so that the cross-section in the thickness direction of the steel plate can be observed. For this sample, a COMPO image is obtained using a scanning electron microscope, and the presence of two types of structures with different contrasts that make up the scale is confirmed. Since the first region contains more Fe, which is a heavy element, than the second region, it appears brighter than the second region. Therefore, the relatively bright region is defined as the first region, and the relatively dark region is defined as the second region. For each of these two types of structures (the first region and the second region), spot elemental analysis (beam diameter of 1 μm or less) is performed using an electron probe microanalyzer (EPMA) to determine the Fe content, Si content contained in the first region of the scale, and the Fe content, Mn content contained in the second region. During the measurement, 10-point analysis is performed for each, and the average value is taken as the Fe, Si content contained in the first region of the scale and the Fe content, Mn content contained in the second region. The scale may include the "other region" as described above. Regions containing 10 mass% or more of Cr, Si, or Cu are defined as the above "other region".

[0056] The thickness of the scale is determined by SEM. A cross-section observation sample is taken from a position 1 / 4 of the plate width (short side) in the width direction from the width direction end of the steel plate, and COMPO image observation by SEM is performed. The scale thickness at 10 locations where the scale has not peeled off is measured, and the average value is taken as the scale thickness.

[0057] <Steel member> As shown in Fig. 2, the steel member 110 according to this embodiment includes a steel plate base material 111 having a predetermined chemical composition and a scale 112 formed on the surface of the steel plate base material 111 and containing 70 mass% or more of Fe. Further, the steel plate base material 111 has a decarburized layer 113 with a predetermined depth formed on the scale 112 side (the interface side with the scale 112), and this decarburized layer 113 has an internal oxidation layer 114 formed on the scale 112 side. Also, the steel member 110 according to this embodiment is obtained by subjecting the steel plate 10 according to the above-described embodiment to a heat treatment (and processing) such as hot stamping. In the figure, the steel member 110 according to this embodiment is shown in the form of a flat plate, but it is a member obtained by hot stamping and is not limited to a flat plate.

[0058] [Chemical Composition] Since the chemical composition of the steel plate does not substantially change by hot stamping, the chemical composition of the steel plate base material 111 of the steel member 110 according to this embodiment is the same as the chemical composition of the base steel plate 11 of the steel plate 10 according to this embodiment and can be measured by the same measurement method as the base steel plate.

[0059] [Decarburized Layer] [Internal Oxidation Layer] In the steel member 110 according to this embodiment, a decarburized layer exists on the interface side of the steel plate base material 111 with the scale 112, and an internal oxidation layer 114 exists on the interface side of the decarburized layer 113 with the scale 112. Further, the depth of the decarburized layer 113 from the interface between the steel plate base material 111 and the scale 112 is 60 μm or more, and the depth of the internal oxidation layer 114 from the interface between the steel plate base material 111 and the scale 112 is less than 40 μm. In a steel member, in order to improve bendability, it is extremely effective to decarburize and soften the surface layer. Since bending deformation generates greater stress and strain in the surface layer farther from the bend, bendability can be improved by softening the surface layer to improve the fracture limit. If the depth (thickness) of the decarburized layer 113 formed on the surface layer of the steel member 110 according to this embodiment is 60 μm or more, the bendability is improved. Therefore, the depth of the decarburized layer 113 from the interface between the steel plate base material 111 and the scale 112 is set to 60 μm or more. In addition, in the steel member, if the depth of the internal oxidation layer is 40 μm or more, the weldability deteriorates due to the scale generated inside the steel sheet during hot stamping. Therefore, the depth of the internal oxidation layer 114 is made less than 40 μm.

[0060] <scale> The steel member 110 according to the present embodiment has a scale 112 formed on the surface of the steel sheet base material 111. The steel member 110 according to the present embodiment is obtained by hot stamping a steel sheet having a scale containing 80 mass% or more of Fe. Although the oxidation of the surface layer proceeds due to hot stamping, the scale 112 of the steel member 110 according to the present embodiment contains 70% or more of Fe by mass.

[0061] The steel member according to another embodiment of the present invention is a coated steel member obtained by subjecting the steel sheet according to the above-described present embodiment to pickling or the like to remove the scale on the surface, and then forming a coating containing Al such as plating to form a coated steel sheet, and subjecting this coated steel sheet to a heat treatment such as hot stamping. In this case, as shown in FIG. 3, the steel member (coated steel member) 210 according to another embodiment of the present invention has a steel sheet base material 211 and a coating 215 containing Al and Fe formed on the surface of the steel sheet base material 211, and does not include a scale between the steel sheet base material 211 and the coating 215 containing Al and Fe. In addition, the steel sheet base material 211 has a decarburized layer 213 formed on the coating 215 side, the decarburized layer 213 has an internal oxidation layer 214 formed on the coating 215 side, the depth of the decarburized layer 213 from the interface between the steel sheet base material 211 and the coating 215 is 30 μm or more, and the depth of the internal oxidation layer 214 from the interface between the steel sheet base material 211 and the coating 215 is less than 20 μm. If the depth (thickness) of the decarburized layer 213 is 30 μm or more, the bendability is improved. Also, if the depth of the internal oxidation layer 214 is less than 20 μm, the weldability is improved. In the case of the coated steel member having a coating, since the surface oxidation state during heat treatment such as hot stamping is different from that of the above-described steel member having no coating, the depth of the decarburized layer and the depth of the internal oxidation layer are different.

[0062] <Manufacturing method> The steel sheet and the steel member according to this embodiment can obtain the effects as long as they have the above characteristics regardless of the manufacturing method. However, a manufacturing method including the following steps (steps (I) to (IV) for the steel sheet and steps (I) to (V) for the steel member) is preferable because it can be stably manufactured. (I) Steel sheet manufacturing step of manufacturing a steel sheet having a predetermined chemical composition (II) Hot rolling step of heating the steel sheet and hot rolling it to obtain a hot-rolled steel sheet (III) Coiling step of winding up the hot-rolled steel sheet to obtain a hot-rolled coil (IV) Annealing step of box annealing (BAF) the hot-rolled coil on which hot-rolled scale is formed (V) Heat treatment step of cutting out a blank of a predetermined size from the hot-rolled coil after the annealing step and performing heat treatment to obtain a steel member Hereinafter, each step will be described. Steps and conditions not described below can be performed by known methods as appropriate.

[0063] (I) Steel sheet manufacturing step In the steel sheet manufacturing step, a steel sheet such as a slab having the above-described preferable chemical composition is manufactured. Molten steel adjusted to a predetermined chemical composition may be made into a steel sheet by continuous casting or the like under known conditions.

[0064] (II) Hot rolling step In the hot rolling step, the obtained steel sheet is heated and hot rolled to obtain a hot-rolled steel sheet. In the hot rolling step, scale (hot-rolled scale) is formed on the surface of the steel sheet. The hot rolling conditions are not particularly limited and may be appropriately set within a known condition range according to the required characteristics of the steel sheet.

[0065] (III) Coiling step In the coiling step, the hot-rolled steel sheet obtained in the hot rolling step is wound into a coil shape to obtain a hot-rolled coil. Conditions such as the coiling temperature are not particularly limited.

[0066] (IV) Annealing step In the annealing process, box annealing (BAF) is performed on the hot-rolled coil with hot-rolled scale formed on its surface without removing the scale (in the so-called black skin state). During annealing, the annealing atmosphere is an inert gas atmosphere (such as N2 atmosphere, H2 atmosphere, etc.), and annealing is performed at 650 - 900 °C for 4 - 16 hours. In normal decarburizing annealing, high dew point annealing is performed, and H2O in the atmosphere is used as the decarburization source. In contrast, in this embodiment, by annealing the hot-rolled coil with hot-rolled scale attached, decarburization is performed using O in the scale as the decarburization source. Specifically, the C in the outermost layer of the base metal steel plate reacts with O in the scale to become CO gas, resulting in decarburization. Subsequently, the insufficient C is supplied from the inside of the base metal steel plate to the outermost layer, and the C becomes CO gas, further promoting the decarburization reaction. At this time, O in the scale is consumed, and the Fe content in the scale increases. However, if the annealing temperature is less than 650 °C or the annealing time is less than 4 hours, sufficient decarburization does not proceed. On the other hand, if the annealing temperature exceeds 900 °C or the annealing time exceeds 16 hours, the reduction reaction of the scale is completed, and the supply of C from the inside of the steel plate to the surface layer continues thereafter, so the decarburization becomes shallow. Also, if the generated CO gas stays around the scale, further decarburization reaction does not proceed, and a deep decarburized layer cannot be obtained. Therefore, in the manufacturing method of the steel plate according to this embodiment, it is important to move the gas in the annealing furnace so that the generated CO gas does not stay around the scale. Specifically, a fan or the like is installed in the annealing furnace, and by setting the air volume to 250 m 3 / Hr or more, the flow rate in the annealing furnace can be ensured, and the decarburization reaction can proceed. If the air volume is less than 250 m 3 / Hr, the retention of CO gas around the scale cannot be sufficiently suppressed, and the decarburization becomes insufficient. The air volume is the air volume around the hot-rolled coil, and a plurality of fans or the like may be installed according to the size of the annealing furnace to obtain a predetermined air volume. The size of the hot-rolled coil is preferably a plate thickness of 9 mm or less, a plate width of 2100 mm or less, an outer dimension of 2000 mm or less, and a weight of 1 coil of 30 tons or less. When O in the scale is used as the decarburization source compared to the case where H2O in the atmosphere is used as the decarburization source in high dew point annealing, the O of the decarburization source is less likely to penetrate into the inside of the base steel plate, and as a result, internal oxidation is less likely to progress. That is, as described above, for the hot-rolled coil in the black skin state, in an inert gas atmosphere, the air volume is 250 m 3 / Hr or more and blowing is performed to perform box annealing, so that the desired decarburized layer depth, internal oxidation layer depth, and Fe content in the scale described for the steel plate according to this embodiment can be obtained.

[0067] (V) Heat treatment process In the heat treatment process, a blank of a predetermined size is cut out from the hot-rolled coil after the annealing process, and this blank is heat-treated to obtain a steel member. The heat treatment is preferably performed under the condition of heating from the Ac3 point to (Ac3 point + 300) °C at an average heating rate of 1.0 to 1000 °C / second and cooling at an average cooling rate equal to or higher than the upper critical cooling rate to a temperature below the Ms point (°C). If the heating rate is less than 1.0 °C / second, the productivity of the heat treatment decreases, which is not preferable. On the other hand, if the heating rate exceeds 1000 °C / s, a mixed grain structure is formed and the limiting hydrogen amount decreases, which is not preferable. Also, if the heat treatment temperature is less than the Ac3 point (°C), ferrite remains after cooling and the strength decreases, which is not preferable. On the other hand, if the heat treatment temperature exceeds Ac3 point + 300 °C, the structure coarsens and the limiting hydrogen amount decreases, which is not preferable. The upper critical cooling rate is the minimum cooling rate that does not precipitate ferrite or pearlite in the structure and supercools austenite to form martensite. Cooling at an average cooling rate below the upper critical cooling rate generates ferrite or pearlite and results in insufficient strength. During heating, holding may be performed for 1 to 300 seconds within a range of ±10 °C of the heating temperature. Also, after cooling to a temperature below the Ms point, tempering treatment may be performed in a temperature range of about 100 to 600 °C to adjust the strength of the steel material. In this heat treatment, processing may be performed simultaneously. That is, so-called hot stamping may be performed. Moreover, the steel member (including the coated steel member) according to the present embodiment may be a steel member having regions with different strengths, which is obtained by performing hot forming or heat treatment on a part of a steel plate serving as a material.

[0068] (VI) Pickling, cold rolling, and coating When the steel member is made into a coated steel member, between the annealing process and the heat treatment process, the hot-rolled coil may be pickled, cold-rolled, and further, a coating containing Al may be formed on the surface. In this case, pickling, cold rolling, and coating may be performed under known conditions. When the scale is not sufficiently peeled off during pickling, shot blasting may be performed before pickling to mechanically promote scale peeling. For example, shot grit #60 may be used.

Example

[0069] First, steel having the chemical components shown in Table 1 was melted to obtain a slab for hot rolling.

[0070] <Example 1> The obtained slab was hot-rolled to obtain a hot-rolled steel plate with a thickness of 3.2 mm and a plate width of 1000 mm. This hot-rolled steel plate was wound up at a temperature of 800°C or lower to obtain a hot-rolled coil with an outer diameter of 1700 mm and a weight of 14 tons per coil. The obtained hot-rolled coil was box annealed under the conditions (temperature, time, air volume) described in Tables 2-1 to 2-3. The annealing atmosphere was a nitrogen atmosphere.

[0071]

Table 1

[0072]

Table 2-1

[0073]

Table 2-2

[0074]

Table 2-3

[0075] A steel sheet (blank) of a predetermined size was cut out from the obtained hot-rolled coil after annealing, and GDS (glow discharge optical emission spectroscopy), SEM observation, EPMA analysis, and optical microscope observation were carried out in the manner described above to evaluate the decarburized layer depth, internal oxidation layer depth, scale thickness, and Fe content of the scale. Further, the Fe content, Si content in the first region constituting the scale, Fe content, and Mn content in the second region were evaluated in the manner described above. The evaluation results are shown in Tables 2-1 to 2-3. Also, the chemical composition at the position 1 / 4 of the plate thickness from the surface in the plate thickness direction of the steel sheet was the same as that of the slab.

[0076] As shown in Tables 2-1 to 2-3, for Invention Examples B1 to B60 that satisfy the scope of the present invention, steel sheets having a predetermined chemical composition and structure were obtained. On the other hand, Comparative Examples b1 to b23 that do not satisfy the scope of the present invention resulted in not satisfying at least one of the target structures. For example, in b7 to b19, the first region was not observed.

[0077] <Example 2> The steel sheets shown in the above Tables 2-1 to 2-3 were heat-treated under the conditions shown in Tables 3-1 to 3-3 to obtain steel members.

[0078] The obtained steel members were cut out, and GDS (glow discharge optical emission spectroscopy), SEM observation, and optical microscope observation were carried out in the manner described above to determine the decarburized layer depth, internal oxidation layer depth, and Fe content of the scale. The results are shown in Tables 3-1 to 3-3.

[0079] Also, tensile tests, bending tests, and spot welding tests were carried out on the obtained steel members by the following methods to evaluate the tensile strength, bendability, and weldability (welding appropriate current range).

[0080] <Tensile strength> The tensile test was carried out in accordance with the provisions of ASTM Standard E8. After grinding the soaking part of the steel member to a thickness of 1.2 mm, a half-size plate-shaped test piece (parallel part length: 32 mm, parallel part plate width: 6.25 mm) of ASTM Standard E8 was taken so that the test direction was parallel to the rolling direction. Then, a room temperature tensile test was carried out at a strain rate of 3 mm / min, and the tensile strength (maximum strength) was measured. In this example, when the tensile strength exceeded 1000 MPa, it was evaluated as high strength.

[0081] <Bendability> The bending test was carried out in accordance with the provisions of VDA238-100. A bending test piece of 60 mm parallel to the rolling direction and 30 mm perpendicular to the rolling direction was taken from the soaking part of the steel member. The bending punch was aligned so as to be perpendicular to the rolling direction, and the bending angle at the maximum load was measured. Since the bending angle is correlated with the strength, in this example, when the tensile strength is less than 2100 MPa, a bending angle exceeding 55 degrees, and when the tensile strength is 2100 MPa or more, a bending angle exceeding 45 degrees, it was evaluated that the bendability was superior to the prior art.

[0082] <Appropriate current range> Spot welding was carried out in accordance with JIS Z 3001-6:2013. A single-phase alternating current of 60 Hz was used for the power supply, an electrode with a tip diameter of 8 mm was used, and the welding time was 10 cycles. A spot welding test piece of 40 mm parallel to the rolling direction and 30 mm perpendicular to the rolling direction was taken from the soaking part of the steel member. These were bonded together, and the range from the welding current at which the nugget diameter becomes 3√t to the upper limit current at which no spatter occurs was obtained as the appropriate current range. The nugget diameter was the peel diameter obtained from the peel test. Five welding test pieces were taken at each current, and the average value of those peel diameters was taken as the nugget diameter. If the appropriate current range was 2.5 kA or more, it was judged that the weldability was excellent.

[0083]

Table 3-1

[0084]

Table 3-2

[0085]

Table 3-3

[0086] As shown in Tables 3-1 to 3-3, Invention Examples C1 to C60 that satisfy the scope of the present invention have good results in both structure and properties. However, Comparative Examples c1 to c19 that do not satisfy the scope of the present invention have results where at least one of the structure and properties does not meet the target.

[0087] <Example 3> The steel plates shown in the above Tables 2-1 to 2-3 were pickled, cold-rolled, and molten Al-plated to obtain a coated steel plate with a thickness of 2.0 mm. The coated steel plate was heat-treated under the conditions shown in Tables 4-1 to 4-3 to obtain a coated steel member.

[0088] The obtained steel member was cut out, and GDS (glow discharge optical emission spectroscopy), SEM observation, and optical microscope observation were performed in the manner described above to determine the decarburized layer depth, internal oxidation layer depth, and Fe content in the scale. In addition, tensile tests, bending tests, and spot welding tests were performed on the obtained steel member in the same manner as in Example 2, and the tensile strength, bendability, and weldability were evaluated. The results are shown in Tables 4-1 to 4-3.

[0089]

Table 4-1

[0090]

Table 4-2

[0091]

Table 4-3

[0092] As shown in Tables 4-1 to 4-3, Invention Examples D1 to D60 that satisfy the scope of the present invention have good results in both organization and properties. However, Comparative Examples d1 to d23 that do not satisfy the scope of the present invention have a chemical composition and an organization (depth of internal oxidation layer and decarburized layer) outside the scope of the present invention, resulting in at least one of bendability, weldability, and tensile strength not meeting the target.

Industrial Applicability

[0093] According to the present invention, it is possible to obtain high-strength steel members and steel plates with excellent collision characteristics. The steel members according to the present invention are particularly suitable for use as automotive frame parts.

Explanation of Signs

[0094] 10 Steel plate 11 Base metal steel plate 12 Scale 13 Decarburized layer 14 Internal oxidation layer 110 Steel member 111 Steel plate base material 112 Scale 113 Decarburized layer 114 Internal oxidation layer 210 Steel member 211 Steel plate base material 213 Decarburized layer 214 Internal oxidation layer 215 Coating

Claims

1. It has a base metal steel plate and a scale formed on the surface of the base metal steel plate, the base metal steel plate, in mass %, C: 0.10 to 0.65%, Si: 0.10 to 2.00%, Mn: 0.30 to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Ti: 0 to 0.100%, B: 0 to 0.0100%, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Nb: 0 to 0.10%, Cu: 0 to 1.00%, V: 0 to 1.00%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, Al: 0 to 1.00%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.00%, Zr: 0 to 1.00%, Co: 0 to 1.00%, and REM: 0 to 0.30%, has a chemical composition consisting of the balance being Fe and impurities, the base metal steel plate has a decarburized layer formed on the interface side with the scale, the decarburized layer has an internal oxidation layer formed on the interface side with the scale, the depth of the decarburized layer from the interface between the base metal steel plate and the scale is 90 μm or more, the depth of the internal oxidation layer from the interface is less than 30 μm, the scale contains 80% or more of Fe by mass, having no coating on the surface of the scale A steel sheet characterized by this.

2. The scale includes a first region containing 80% or more of Fe and 0.1% or more and less than 3.0% of Si by mass%, and a second region containing 65% or more and less than 80% of Fe and 0.8% or more and less than 7.5% of Mn. The steel sheet according to claim 1, characterized by this.

3. A steel sheet base material, and a scale formed on the surface of the steel sheet base material, the steel sheet base material having, by mass%, C: 0.10 to 0.65%, Si: 0.10 to 2.00%, Mn: 0.30 to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Ti: 0 to 0.100%, B: 0 to 0.0100%, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Nb: 0 to 0.10%, Cu: 0 to 1.00%, V: 0 to 1.00%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, Al: 0 to 1.00%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.00%, Zr: 0 to 1.00%, Co: 0 to 1.00%, and REM: 0 to 0.30%, having a chemical composition consisting of the balance being Fe and impurities, The steel plate substrate has a decarburized layer formed on the interface side with the scale, The decarburized layer has an internal oxidation layer formed on the interface side with the scale, The depth of the decarburized layer from the interface between the steel plate substrate and the scale is 60 μm or more, The depth of the internal oxidation layer from the interface is less than 40 μm, The scale contains 70% or more of Fe by mass%, The surface of the scale has no coating, A steel member characterized by the above.

4. The depth of the internal oxidation layer from the interface is 18 μm or less, The steel plate according to claim 1, characterized by the above.

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