EP4632085A1

EP4632085A1 - Steel sheet and hot-stamp molded body

Info

Publication number: EP4632085A1
Application number: EP23900249.6A
Authority: EP
Inventors: Shota HAYASHIDA; Takuya MITSUNOBU; Hiroshi Takebayashi
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2022-12-09
Filing date: 2023-08-24
Publication date: 2025-10-15
Also published as: KR20250093407A; MX2025006474A; JPWO2024122122A1; CN120322579A; EP4632085A4; WO2024122122A1

Abstract

Provided is a steel sheet having a predetermined chemical composition wherein a depth from a surface in a sheet thickness direction with an area ratio of pearlite of 0 to 20% is 3 to 100 µm, and an area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with area ratio of pearlite of 0 to 20% is 0 to 30%. Further, provided is a hot-stamp formed body having a predetermined chemical composition wherein a mean C concentration from a surface down to 20 µm in a thickness direction is 0.20 mass%, and the hot-stamp formed body comprises, by area ratio, martensite at 90% or more.

Description

FIELD

The present invention relates to a steel sheet and a hot-stamp formed body, more specifically relates to a steel sheet and a hot-stamp formed body able to be obtained by hot-stamp forming the steel sheet.

BACKGROUND

Hot stamping (hot pressing) is known as a technique for press-forming a material, which is difficult to form, such as a high strength steel sheet. Hot stamping is a technique of hot forming which heats then forms a material to be formed. This technique heats then forms the material, and therefore at the time of forming, the steel material is soft and has good formability. Therefore, even a high strength steel material can be formed into a complex shape with a good precision. Further, it is hardened at the same time as being formed by the press dies, and therefore a formed steel material is known to have sufficient strength.
In relation to this, in the past, steel sheet for hot stamping use and plated steel sheet have been variously studied.
For example, PTL 1 describes galvanized steel sheet for hot stamping use provided with a base steel sheet and a plating layer provided on a surface of the base steel sheet, in which, the base steel sheet contains, by mass%, C: 0.10 to 0.5%, Si: 0.7 to 2.5%, Mn: 1.0 to 3 %, and Al: 0.01 to 0.5% and has a balance of iron and unavoidable impurities and the base steel sheet has inside it an internal oxide layer of a thickness of 1 µm or more including oxides of at least of Si and Mn and a decarburized layer of a thickness of 20 µm or less from an interface with the plating layer toward an inside direction of the base steel sheet. Further, PTL 1 teaches that by making the thickness of the internal oxide layer of the base steel sheet 1 µm or more, nonplating defects of galvanized steel sheet can be sufficiently suppressed and the adhesion between the plating layer formed and the base steel sheet can be made sufficiently high. In addition, PTL 1 teaches that the internal oxide layer is formed by high dew point annealing near the surface of the base steel sheet while the decarburized layer is formed by high dew point annealing at the surface of the base steel sheet and near the surface, the content of carbon is small at the decarburized layer, therefore the tensile strength becomes lower than the part which is not decarburized, and if the thickness of the decarburized layer is 20 µm or less, it is possible to keep the decarburized layer from affecting the strength of a galvanized steel sheet and a hot-stamp formed body produced using this.

[CITATIONS LIST]

[PATENT LITERATURE]

[PTL 1] Japanese Unexamined Patent Publication No. 2019-151883

SUMMARY

[TECHNICAL PROBLEM]

A conventional hot-stamp formed body produced by hardening a steel sheet by pressing is formed entirely by a hard structure (mainly martensite) in the sheet thickness direction, therefore there is the problem of generally poor deformation ability. If applying the hot-stamp formed body to an auto part, etc., obtaining an excellent impact resistance requires raising the ability to absorb impact energy. If considering the mode of deformation at the time of a collision, it is important in particular to raise the bendability even in the deformation ability. On the other hand, in the automobile industry, etc., further lighter weight of the steel material is being sought. To achieve such lighter weight, a need arises to make the steel material higher in strength even more than in the past. Therefore, there is a high need for a steel material having excellent bendability even if making the strength the same as or higher than in the past, more specifically a hot-stamp formed body and steel sheet for obtaining the same.
Therefore, the present invention has as its object to provide a steel sheet high in strength and able to achieve excellent bendability even when applied to hot-stamp forming and a hot-stamp formed body having such a high strength and excellent bendability.

[SOLUTION TO PROBLEM]

The inventors engaged in studies to achieve the above object and as a result discovered that by suitably modifying the surface layer part structure of the steel sheet, it is possible to achieve excellent bendability even when applied to hot-stamp forming and thereby completed the present invention.
The present invention able to achieve the above object is as follows:

(1) A steel sheet having a chemical composition comprising, by mass%, C: 0.27 to 0.60%,
- Si: 0.001 to 3.00%,
- Mn: 0.30 to 3.00%,
- Al: 0.0002 to 2.000%,
- P: 0.1000% or less,
- S: 0.1000% or less,
- N: 0.0100% or less,
- B: 0 to 0.0100%,
- Ti: 0 to 0.1500%,
- Nb: 0 to 0.150%,
- V: 0 to 0.150%,
- Mo: 0 to 1.00%,
- Cr: 0 to 1.0%,
- Cu: 0 to 1.000%,
- Ni: 0 to 1.00%,
- W: 0 to 1.000%,
- Hf: 0 to 0.050%,
- Mg: 0 to 0.050%,
- Zr: 0 to 0.050%,
- Ca: 0 to 0.0100%,
- REM: 0 to 0.3000%,
- Ir: 0 to 1.000%, and
- balance: Fe and impurities, wherein
- a depth from a surface in a sheet thickness direction with an area ratio of pearlite of 0 to 20% is 3 to 100 µm, and
- an area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with area ratio of pearlite of 0 to 20% is 0 to 30%.
(2) The steel sheet according to the above (1), wherein the depth with area ratio of pearlite of 0 to 20% is 10 to 100 µm.
(3) The steel sheet according to the above (2), wherein the depth with area ratio of pearlite of 0 to 20% is 30 to 100 µm.
(4) The steel sheet according to any one of the above (1) to (3), wherein the area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with area ratio of pearlite of 0 to 20% is 0 to 15%.
(5) A hot-stamp formed body having a chemical composition comprising, by mass%, C: 0.27 to 0.60%,
- Si: 0.001 to 3.00%,
- Mn: 0.30 to 3.00%,
- Al: 0.0002 to 2.000%,
- P: 0.1000% or less,
- S: 0.1000% or less,
- N: 0.0100% or less,
- B: 0 to 0.0100%,
- Ti: 0 to 0.1500%,
- Nb: 0 to 0.150%,
- V: 0 to 0.150%,
- Mo: 0 to 1.00%,
- Cr: 0 to 1.0%,
- Cu: 0 to 1.000%,
- Ni: 0 to 1.00%,
- W: 0 to 1.000%,
- Hf: 0 to 0.050%,
- Mg: 0 to 0.050%,
- Zr: 0 to 0.050%,
- Ca: 0 to 0.0100%,
- REM: 0 to 0.3000%,
- Ir: 0 to 1.000%, and
- balance: Fe and impurities, wherein
- a mean C concentration from a surface down to 20 µm in a thickness direction is 0.20 mass%, and
- the hot-stamp formed body comprises, by area ratio, martensite at 90% or more.
(6) The hot-stamp formed body according to the above (5), wherein the mean C concentration is 0.10 mass% or less.
(7) The hot-stamp formed body according to the above (5) or (6), wherein the mean C concentration is 0.05 mass% or less.

[ADVANTAGEOUS EFFECTS OF INVENTION]

According to the present invention, it is possible to provide a steel sheet high in strength and able to achieve excellent bendability even when applied to hot-stamp forming and a hot-stamp formed body having such a high strength and excellent bendability.

DESCRIPTION OF EMBODIMENTS

The steel sheet according to an embodiment of the present invention has a chemical composition comprising, by mass%,

C: 0.27 to 0.60%,
Si: 0.001 to 3.00%,
Mn: 0.30 to 3.00%,
Al: 0.0002 to 2.000%,
P: 0.1000% or less,
S: 0.1000% or less,
N: 0.0100% or less,
B: 0 to 0.0100%,
Ti: 0 to 0.1500%,
Nb: 0 to 0.150%,
V: 0 to 0.150%,
Mo: 0 to 1.00%,
Cr: 0 to 1.0%,
Cu: 0 to 1.000%,
Ni: 0 to 1.00%,
W: 0 to 1.000%,
Hf: 0 to 0.050%,
Mg: 0 to 0.050%,
Zr: 0 to 0.050%,
Ca: 0 to 0.0100%,
REM: 0 to 0.3000%,
Ir: 0 to 1.000%, and
balance: Fe and impurities, wherein
a depth from a surface in a sheet thickness direction with an area ratio of pearlite of 0 to 20% is 3 to 100 µm, and
an area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with area ratio of pearlite of 0 to 20% is 0 to 30%.

As explained above, a conventional hot-stamp formed body produced by hardening steel sheet by pressing is formed entirely by a hard structure (mainly martensite) in the sheet thickness direction, therefore there is the problem that generally the deformation ability is poor. As opposed to this, for example, by reducing the carbon concentration at the surface layer part of the steel sheet before hot-stamp forming by decarburization, etc., probably the surface layer part of the steel sheet is softened and thereby the bendability of the body after hot-stamp forming is improved. However, the inventors engaged in various studies and as a result discovered that from the viewpoint of improving bendability, even if reducing the carbon concentration at the surface layer part of the steel sheet before hot-stamp forming by decarburization, etc., the carbon contained in the bulk of the steel sheet diffuses to the surface layer part at the time of high temperature heating in hot-stamp forming and the effect of improvement of the bendability by the initial lowering of the carbon concentration at the surface layer part is lost or reduced due to such recarburization of the surface layer part. Therefore, the inventors engaged in further studies and discovered that building in a structure enabling suppression of such decarburization at the surface layer part of the steel sheet before hot-stamp forming, it is possible to sufficiently maintain the effect of improvement of the bendability by the initial lowering of the carbon concentration at the surface layer part to remarkably improve the bendability of the body obtained after hot-stamp forming. More specifically, the inventors discovered that by making the depth with an area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction 3 to 100 µm while forming a structure controlled in the area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with an area ratio of pearlite of 0 to 20% to 0 to 30%, it is possible to remarkably improve the bendability of the body obtained after hot-stamp forming.
While not intending to be bound to any specific theory, probably, in the steel sheet according to an embodiment of the present invention, the structure at the surface layer part of the steel sheet may act in the following way to suppress or reduce the carbon contained in the steel sheet from diffusing to the steel surface layer part and causing recarburization at the time of the high temperature heating in the hot-stamp forming. Explained in more detail, if the carbon concentration at the steel sheet is reduced by decarburization, etc., due in part to this lowering of the carbon concentration, the amount of pearlite formed in the microstructure at the surface layer part of the steel sheet becomes relatively smaller. Here, in the steel sheet according to an embodiment of the present invention, first it is important to lower the carbon concentration of the surface layer part of the steel sheet by decarburization, etc., so that the depth at which the area ratio of pearlite becomes 0 to 20% from the surface of the steel sheet in the sheet thickness direction, i.e., the depth of the region with a relatively low area ratio of pearlite, becomes 3 to 100 µm. Due to this, it becomes possible to sufficiently obtain the effect of improvement of the bendability based on the lowering of the carbon concentration. However, with just reduction of the area ratio of pearlite, if such pearlite precipitates along the grain boundaries, at the time of the high temperature heating in the hot-stamp forming, probably the pearlite transforms to austenite and routes for diffusion of carbon (i.e., routes for recarburization by the carbon) are formed by the austenite along the grain boundaries. At the time of the high temperature heating in the hot-stamp forming, the carbon in the bulk wants to diffuse to the surface side due to the gradient in concentration between the high carbon concentration of the bulk in the steel sheet and the low carbon concentration at the surface side. At that time, if there are routes for recarburization by the carbon due to the austenite running along the grain boundaries, the carbon in the bulk will diffuse to the surface side through the recarburization routes and recarburization to the steel surface layer part will be promoted. As a result, it will be no longer possible to sufficiently maintain the effect of improvement of bendability by the initial lowering of the carbon concentration of the steel sheet surface layer part. Explained in more detail, at the time of high temperature heating in hot-stamp forming, at the surface side of the steel sheet, decarburization also can arise, but since the above recarburization is dominant, the effect of improvement of bendability by the initial lowering of the carbon concentration of the steel sheet surface layer part can no longer sufficiently maintained. As opposed to this, according to the steel sheet according to an embodiment of the present invention, in the above depth region with a relatively low area ratio of pearlite, by controlling the area ratio of pearlite having a circle equivalent diameter of 5 µm or more to within a range of 0 to 30% to reduce the relatively large amount of pearlite, even at the time of the high temperature heating in the hot-stamp forming, it is possible to make the austenite transformed from the pearlite disperse on the grain boundaries and thereby reliably cut off the routes for recarburization due to the austenite.
Explained more specifically, if pearlite transforms to austenite at the time of the high temperature heating in the hot-stamp forming, a dual phase structure of ferrite and austenite is formed. In such a case, the austenite present at the interface of the different phases of the ferrite and austenite will extend to the surface side of the steel sheet causing the formation of routes for recarburization and as a result diffusion of carbon from the bulk of the steel sheet to the surface side will be promoted. In relation to this, in the steel sheet according to an embodiment of the present invention, it becomes important to reduce the pearlite at the surface layer part of the steel sheet, i.e., the 3 to 100 µm depth region from the surface of the steel sheet in the sheet thickness direction, to an area ratio of 0 to 20% and limit the relatively coarse pearlite at that depth region, i.e., pearlite having a circle equivalent diameter of 5 µm or more, to within a range of, by area ratio, 0 to 30%. By such a surface layer part structure, even at the time of the high temperature heating in the hot-stamp forming, it is possible to reduce the amount of austenite transformed from pearlite and further possible to make that austenite be present dispersed on the grain boundaries, therefore it becomes possible to reliably cut off the routes for recarburization due to the austenite. Therefore, according to the steel sheet according to an embodiment of the present invention, by remarkably suppressing or reducing recarburization at the time of high temperature heating in hot-stamp forming, it becomes possible to sufficiently maintain the effect of improvement of bendability due to initial lowering of the carbon concentration at the steel sheet surface layer part and remarkably improve the bendability of the body obtained after the hot-stamp forming. The fact that by suitably modifying the surface layer part structure of the steel sheet, it is possible to suppress or reduce the recarburization at the time of the high temperature heating in the hot-stamp forming was first made clear this time by the inventors. Further, according to the steel sheet according to an embodiment of the present invention, by improvement of the bendability due to the suppressions or reduction of such recarburization, it is possible to obtain a hot-stamp formed body excellent in impact resistance regardless of being high in strength. Therefore the steel sheet according to an embodiment of the present invention is particularly useful in use in the automobile industry.
Below, the steel sheet according to an embodiment of the present invention will be explained in more detail. In the following explanation, the "%" of the units of contents of the elements, unless otherwise indicated, means "mass%". Further, in this Description, the "to" showing a numerical range, unless otherwise indicated, is used in the sense of the numerical values described before and after the same being included as the lower limit value and the upper limit value.

[Chemical Composition of Steel Sheet (Common With Chemical Composition of Hot-Stamp Formed Body Explained Later)]

[C: 0.27 to 0.60%]

C is an element inexpensively making the tensile strength increase and an element important for controlling the strength of steel. To sufficiently obtain such an effect, the C content is 0.27% or more. The C content may also be 0.28% or more, 0.30% or more, 0.32% or more, 0.35% or more, 0.38% or more, or 0.40% or more. On the other hand, if excessively containing C, sometimes a drop in elongation will be invited. For this reason, the C content is 0.60% or less. The C content may also be 0.55% or less, 0.50% or less, 0.48% or less, or 0.45% or less.

[Si: 0.001 to 3.00%]

Si is an element acting as a deoxidizer and suppressing precipitation of carbides at the cooling process during annealing of the cold rolled sheet. To sufficiently obtain such an effect, the Si content is 0.001% or more. The Si content may also be 0.01% or more, 0.10% or more, 0.20% or more, 0.50% or more, 1.00% or more, or 1.20% or more. On the other hand, if excessively containing Si, sometimes a drop in elongation is invited along with an increase in the steel strength. For this reason, the Si content is 3.00% or less. The Si content may also be 2.50% or less, 2.00% or less, 1.70% or less, or 1.50% or less.

[Mn: 0.30 to 3.00%]

Mn is an element raising the hardenability of steel and an element effective for raising the strength. To sufficiently obtain such an effect, the Mn content is 0.30% or more. The Mn content may also be 0.50% or more, 1.00% or more, 1.20% or more, or 1.50% or more. On the other hand, if excessively containing Mn, sometimes a drop in elongation is invited along with an increase in the steel strength. For this reason, the Mn content is preferably 3.00% or less. The Mn content may also be 2.80% or less, 2.50% or less, 2.00% or less, or 1.70% or less.

[Al: 0.0002 to 2.000%]

Al is an element acting as a deoxidizer of steel and acting to make the steel sounder. To sufficiently obtain such an effect, the Al content is 0.0002% or more. The Al content may also be 0.001% or more, 0.010% or more, 0.040% or more, or 0.100% or more. On the other hand, if excessively containing Al, coarse Al oxides are formed and sometimes the toughness of the steel sheet falls. For this reason, the Al content is 2.000% or less. The Al content may also be 1.500% or less, 1.000% or less, 0.800% or less, or 0.500% or less.

[P: 0.1000% or Less]

P is an element segregating at the grain boundaries and promoting embrittlement of the steel. The smaller the content of P, the more preferable, therefore ideally it is 0%. However, excessive reduction of the P content sometimes invites a large increase in costs. For this reason, the P content may also be 0.0001% or more and may also 0.0005% or more, 0.0010% or more, or 0.0050% or more. On the other hand, if excessively containing P, as explained above, sometimes grain boundary segregation invites embrittlement of the steel. Therefore, the P content is 0.1000% or less. The P content may also be 0.0500% or less, 0.0300% or less, or 0.0100% or less.

[S: 0.1000% or Less]

S is an element forming MnS and other nonmetallic inclusions in the steel and inviting a drop in ductility of the steel material part. The smaller the content of S, the more preferable, therefore ideally it is 0%. However, excessive reduction of the S content sometimes invites a large increase in costs. For this reason, the S content may also be 0.0001% or more and may also be 0.0002% or more, 0.0010% or more, or 0.0050% or more. On the other hand, if excessively containing S, sometimes occurrence of cracks starting from the nonmetallic inclusions is invited at the time of cold shaping. Therefore, the S content is 0.1000% or less. The S content may also be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[N: 0.0100% or Less]

N is an element forming coarse nitrides in the steel sheet and lowering the workability of the steel sheet. The smaller the content of N, the more preferable, therefore ideally it is 0%. However, excessive reduction of the N content sometimes invites a large increase in the production costs. For this reason, the N content may also be 0.0001% or more and may also be 0.0005% or more or 0.0010% or more. On the other hand, if excessively containing N, as explained above, sometimes coarse nitrides are formed and the workability of the steel sheet is made to fall. Therefore, the N content is 0.0100% or less. The N content may also be 0.0080% or less or 0.0050% or less.
The basic chemical composition of the steel sheet according to an embodiment of the present invention is as explained above. Further, the steel sheet may also contain, in accordance with need, in place of part of the Fe of the balance, one or more elements selected from the group comprised of B: 0 to 0.0100%, Ti: 0 to 0.1500%, Nb: 0 to 0.150%, V: 0 to 0.150%, Mo: 0 to 1.00%, Cr: 0 to 1.0%, Cu: 0 to 1.000%, Ni: 0 to 1.00%, W: 0 to 1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.0100%, REM: 0 to 0.3000%, and Ir: 0 to 1.000% may be contained. Below, these optional elements will be explained in detail.

[B: 0 to 0.0100%]

B is an element raising the hardenability of steel and contributing to improvement of the strength. The B content may also be 0%, but to obtain such an effect, the B content is preferably 0.0001% or more. The B content may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if excessively containing B, the toughness and/or weldability sometimes falls. Therefore, the B content is preferably 0.0100% or less. The B content may also be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.

[Ti: 0 to 0.1500%]

Ti is an element forming carbonitrides in steel and contributing to improvement of the strength by precipitation strengthening. The Ti content may also be 0%, but to obtain such an effect, the Ti content is preferably 0.0001% or more. The Ti content may also be 0.0010% or more, 0.0100% or more, 0.0300% or more, or 0.0500% or more. On the other hand, even if excessively containing Ti, the effect becomes saturated and sometimes a rise in production costs is invited. Therefore, the Ti content is preferably 0.1500% or less. The Ti content may also be 0.1200% or less, 0.1000% or less, or 0.0800% or less.

[Nb: 0 to 0.150%]

Nb is an element forming carbides, nitrides, and/or carbonitrides in steel to contribute to refinement of the structure by a pinning effect. The Nb content may also be 0%, but to obtain such an effect, the Nb content is preferably 0.001% or more. The Nb content may also be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if excessively containing Nb, coarse carbides, etc., are formed in the steel and sometimes the toughness of the steel sheet is lowered. Therefore, the Nb content is preferably 0.150% or less. The Nb content may also be 0.120% or less, 0.100% or less, or 0.080% or less.

[V: 0 to 0.150%]

V is an element contributing to improvement of strength by precipitation strengthening, etc. The V content may also be 0%, but to obtain such an effect, the V content is preferably 0.001% or more. The V content may also be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if excessively containing V, a large amount of precipitates is formed and sometimes the toughness is made to fall. Therefore, the V content is preferably 0.150% or less. The V content may also be 0.120% or less, 0.100% or less, or 0.080% or less.

[Mo: 0 to 1.00%]

Mo is an element raising the hardenability of steel and contributing to improvement of the strength and an element contributing to improvement of the corrosion resistance. The Mo content may also be 0%, but to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content may also be 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, if excessively containing Mo, the deformation resistance at the time of hot working increases and sometimes the capital costs become greater. Therefore, the Mo content is preferably 1.00% or less. The Mo content may also be 0.80% or less, 0.50% or less, 0.20% or less, 0.10% or less, or 0.08% or less.

[Cr: 0 to 1.0%]

Cr is an element raising the hardenability of steel and contributing to improvement of the strength and/or corrosion resistance. The Cr content may also be 0%, but to obtain these effects, the Cr content is preferably 0.001% or more. The Cr content may also be 0.01% or more, 0.05% or more, or 0.1% or more. On the other hand, even if excessively containing Cr, the effect becomes saturated and sometimes a rise in production costs is invited. Therefore, the Cr content is preferably 1.0% or less. The Cr content may also be 0.8% or less, 0.5% or less, 0.3% or less, or 0.2% or less.

[Cu: 0 to 1.000%]

Cu is an element contributing to improvement of the strength and/or corrosion resistance. The Cu content may also be 0%, but to obtain these effects, the Cu content is preferably 0.001% or more. The Cu content may also be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if excessively containing Cu, sometimes deterioration of the toughness or weldability is invited. Therefore, the Cu content is preferably 1.000% or less. The Cu content may also be 0.800% or less, 0.700% or less, 0.500% or less, 0.300% or less, or 0.150% or less.

[Ni: 0 to 1.00%]

Ni is an element raising the hardenability of steel and contributing to improvement of the strength and/or corrosion resistance. The Ni content may also be 0%, but to obtain these effects, the Ni content is preferably 0.001% or more. The Ni content may also be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, even if excessively containing Ni, the effect becomes saturated and sometimes a rise in production costs is invited. Therefore, the Ni content is preferably 1.00% or less. The Ni content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[W: 0 to 1.000%]

W is an element raising the hardenability of steel and contributing to improvement of the strength. The W content may also be 0%, but to obtain such an effect, the W content is preferably 0.001% or more. The W content may also be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, if excessively containing W, sometimes the weldability falls. Therefore, the W content is preferably 1.000% or less. The W content may also be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[Hf: 0 to 0.050%]

[Mg: 0 to 0.050%]

[Zr: 0 to 0.050%]

Hf, Mg, and Zr are elements enabling control of the form of sulfides. The Hf, Mg, and Zr content may also be 0%, but to obtain such an effect, the contents of these elements are preferably respectively 0.0001% or more and may also be 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, even if excessively containing these elements, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in the production costs. Therefore, the Hf, Mg, and Zr contents are preferably respectively 0.050% or less and may also be 0.010% or less, 0.005% or less, or 0.003% or less.

[Ca: 0 to 0.0100%]

Ca is an element enabling control of the form of sulfides. The Ca content may also be 0%, but to obtain such an effect, the Ca content is preferably 0.0001% or more. The Ca content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if excessively containing Ca, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in the production costs. Therefore, the Ca content is preferably 0.0100% or less and may also be 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[REM: 0 to 0.3000%]

An REM is an element enabling control of the form of sulfides. The REM content may be 0%, but to obtain such an effect, the REM content is preferably 0.0001% or more. The REM content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if excessively containing a REM, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the REM content is preferably 0.3000% or less. The REM content may also be 0.1000% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. The "REM" in this Description is the general name of the 17 elements of atomic number 21 scandium (Sc), atomic number 39 yttrium (Y), and the lanthanoid atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu). The REM content is the total content of these elements.

[Ir: 0 to 1.000%]

Ir is an element forming refined oxides and contributing to improvement of the strength. The Ir content may also be 0%, but to obtain such an effect, the Ir content is preferably 0.001% or more. The Ir content may also be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, even if excessively containing Ir, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in the production costs. Therefore, the Ir content is preferably 1.000% or less. The Ir content may also be 0.500% or less, 0.100% or less, 0.080% or less, or 0.060% or less.
In the steel sheet, the balance besides the above elements is comprised of Fe and impurities. The "impurities" in the steel sheet are constituents, etc., entering due to the ore, scrap, or other raw materials and other various factors in the production process when industrially producing the steel sheet.
The chemical composition of the steel sheet may be measured by a general analysis method. For example, the chemical composition of the steel sheet may be measured at the chips based on JIS G 1201: 2014 using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Specifically, for example, a 35 mm square test piece is obtained from near the 1/2 position of the thickness of the steel sheet and is measured by an ICPS-8100, etc. (measuring device) made by Shimadzu Corporation under conditions based on calibration curves prepared in advance to thereby identify the composition. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, N can be measured using the inert gas melting-thermal conductivity method, and O can be measured using the inert gas melting-nondispersive type infrared absorption method. If the surface of the steel sheet is provided with a plating layer, mechanical grinding may be used to remove the plating layer, then the chemical composition analyzed.

[Depth With Area Ratio of Pearlite of 0 To 20% From Surface of Steel Sheet in Sheet Thickness Direction: 3 to 100 µm]

In an embodiment of the present invention, the depth with an area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction is 3 to 100 µm. This feature is related to the lowering of the carbon concentration at the surface layer part of the steel sheet. Therefore, by having this feature, the bendability of the body obtained after hot-stamp forming by the effect of improvement of bendability by lowering of the carbon concentration of the steel sheet surface layer part can be improved. In addition, it is possible to reduce the amount of pearlite at the surface layer part of the steel sheet to the above range to thereby reduce the amount of austenite transformed from pearlite at the time of high temperature heating in the hot-stamp forming. Therefore, this feature can be said to be an extremely important feature in preventing the formation of routes for recarburization of carbon due to the austenite running along the grain boundaries at the time of hot-stamp forming. From the viewpoint of further improving these effects, increasing the region of the surface layer part with the low pearlite is preferable. More specifically, the depth with an area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction is preferably 5 µm or more or 10 µm or more, more preferably 20 µm or more or 30 µm or more, most preferably 40 µm or more or 50 µm or more. The upper limit of the depth may also for example be 90 µm or 80 µm.

[Area Ratio of Pearlite Having Circle Equivalent Diameter of 5 µm or More at Depth With Area Ratio of Pearlite From Surface of Steel Sheet in Sheet Thickness Direction of 0 to 20%: 0 to 30%]

In an embodiment of the present invention, the area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with an area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction is 0 to 30%. By controlling the area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth region with a relatively low area ratio of pearlite as explained above to a range of 0 to 30% to reduce the relatively large amount of pearlite, even at the time of high temperature heating in hot-stamp forming, it is possible to make the austenite transformed from the pearlite be present dispersed on the grain boundaries and thereby reliably cut off the routes for recarburization of carbon by the austenite. Therefore, by reliably suppressing or reducing the recarburization at the time of high temperature heating in hot-stamp forming, it becomes possible to sufficiently maintain the effect of improvement of bendability due to the initial lowering of the carbon concentration of the steel sheet surface layer part to remarkably improve the bendability of the body obtained by the hot-stamp forming. From the viewpoint of further improving this effect, the area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with an area ratio of pearlite of 0 to 20% is preferably 25% or less or 20% or less, more preferably 15% or less or 12% or less, most preferably 10% or less or 8% or less. The lower limit of the area ratio of pearlite having a circle equivalent diameter of 5 µm or more may, for example, be 1% or 3%.

[Measurement of Depth of Area Ratio of Pearlite of 0 to 20% and Area Ratio of Pearlite Having Circle Equivalent Diameter of 5 µm or More]

The depth with an area ratio of pearlite of 0 to 20% in the microstructure at the surface layer part of the steel sheet and the area ratio of pearlite having a circle equivalent diameter of 5 µm or more are determined in the following way. First, five samples are taken from the surface of the steel sheet so that the cross-sections parallel to the rolling direction and sheet thickness direction can be examined. Next, these examined surfaces are polished to mirror surfaces, corroded by a picral corrosion solution, then examined for structure using a scan electron microscope (SEM). Regarding the measurement range for the samples, a rectangular range of 100 µm from the surface of the steel sheet (in case of steel sheet including plating layer, interface of steel sheet and plating layer) in the thickness direction and 500 µm in a direction perpendicular to the sheet thickness direction is deemed one field. Five fields are measured in total for the five samples. If the steel sheet includes a plating layer, the interface of the steel sheet and the plating layer can be discriminated by the difference in color between the steel sheet and the plating layer at the backscattered electron image (BSE image) of the SEM. For the pearlite, the area ratio is calculated using the point count method from a structural photograph of a power of for example 5000X or so. Here, a region surrounded by grain boundaries with a crystal orientation difference of ferrite of 15° or more and with a ferrite phase and cementite phase mixed and cementite of a layer and/or spherical shape is deemed pearlite and the area ratio of the same is calculated. In the samples, the depth positions from the surface of the steel sheet where the area ratio of pearlite has gradually increased and reached 20% are identified, the distances from the identified depth positions to the surface are calculated, and the arithmetic average of these is determined as the "depth of the area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction". Similarly, in the samples, at the depth regions from the interfaces where the area ratio of pearlite became 20%, image processing is used to calculate the area ratios of pearlite having a circle equivalent diameter of 5 µm or more, and the arithmetic average of these is determined as "the area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with an area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction".

[Area Ratio of Martensite: Less Than 1%]

Regarding the microstructure of the steel sheet, as explained above, it is sufficient to form at the surface layer part of the steel sheet a structure with a depth with an area ratio of pearlite of 0 to 20% from the surface in the sheet thickness direction made 3 to 100 µm and with an area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with an area ratio of pearlite of 0 to 20% controlled to 0 to 30%. Therefore, while the rest of the structure is not particularly limited, for example, in a preferred embodiment of the present invention, the area ratio of martensite contained in the steel sheet is less than 1 %. Regarding the method of production of the steel sheet, as explained in detail later, to obtain the above pearlite structure, in the cooling step after the annealing step, in particular from 620 to 670°C in control temperature down to room temperature, it is preferable to cool by a relatively slow mean cooling speed of 10°C/s or less. In the case of such a relatively slow mean cooling speed, almost no martensite precipitates. Even if precipitating, the area ratio becomes less than 1%. The area ratio of martensite may also be 0.5% or less or 0%.

[Identification of Martensite and Calculation of Area Ratio]

The martensite is identified and the area ratio is calculated in the following way. First, a sample is taken so that the cross-section of the hot-stamp formed body parallel to the rolling direction and sheet thickness direction becomes the examined surface. Next, the examined surface is polished to a mirror surface and corroded by a Nital corrosion solution, then is examined for structure using a scan electron microscope (SEM). A 300 µm×300 µm range is captured at 1000X at a sheet thickness 1/2 depth position of the examined surface. The obtained microstructure photograph is binarized to white and black, then the image analyzed to identify the pearlite, bainite, and ferrite. The method based on "Steels - Micrographic determination of the apparent grain size" determined in JIS G 0551: 2020 is used to find the total of the area ratio of these. Retained austenite is difficult to differentiate from martensite by an SEM, therefore X-ray diffraction is used to measure the area ratio of the retained austenite. Finally, the total area ratio of the pearlite, bainite, ferrite, and retained austenite obtained by the above methods is subtracted from 100% to determine the area ratio of martensite.

[Sheet Thickness of Steel Sheet]

The sheet thickness of the steel sheet is not particularly limited, but for example is 0.2 mm or more and may also be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, or 2.0 mm or more. Similarly, the sheet thickness of the steel sheet is 6.0 mm or less and may also be 5.0 mm or less or 4.0 mm or less.

[Plating]

The steel sheet according to an embodiment of the present invention may further contain a plating layer at the surface for the purpose of improvement of the corrosion resistance. The plating layer may be any of a hot dip coated layer and electroplated layer. The hot dip coated layer includes for example a hot dip galvanized layer (GI), hot dip galvannealed layer (GA), hot dip aluminum coated layer, hot dip Zn-Al alloy coated layer, hot dip Zn-Al-Mg alloy coated layer, hot dip Zn-Al-Mg-Si alloy coated layer, etc. The electroplated layer includes for example an electrogalvanized layer (EG) and electro-Zn-Ni alloy plating layer, etc. Preferably, the plating layer is a hot dip galvanized layer, hot dip galvannealed layer, or electrogalvanized layer. The amount of deposition of the plating layer is not particularly limited, and may be a general amount of deposition. Further, the steel sheet according to an embodiment of the present invention need not include a plating layer at the surface. Specifically, it need not include a hot dip coated layer at the surface or need not include an electroplated layer at the surface.

[Mechanical Properties]

The steel sheet according to an embodiment of the present invention, more specifically the steel sheet before hot-stamp forming, is not particularly limited, but, for example, has a less than 980 MPa tensile strength. The tensile strength may also be 950 MPa or less, 900 MPa or less, 850 MPa or less, or 800 MPa or less. The lower limit is not particularly prescribed, but, for example, the tensile strength may also be 500 MPa or more, 550 MPa or more, or 590 MPa or more. According to an embodiment of the present invention, even if the tensile strength is less than 980 MPa in the steel sheet before hot-stamp forming, at the body after hot-stamp forming, the microstructure becomes a mainly martensite structure, so a 500HV or more Vickers hardness can be sufficiently achieved. The tensile strength is measured by conducting a tensile test based on JIS Z 2241: 2011 on a JIS No. 5 test piece taken from an orientation where the length direction of the test piece becomes parallel to the rolling perpendicular direction of the steel sheet.

<Hot-Stainp Formed Body>

In an embodiment of the present invention, in addition to the above steel sheet, a hot-stamp formed body able to be obtained by hot-stamp forming the steel sheet is further provided. The hot-stamp formed body has a chemical composition comprising, by mass%,

C: 0.27 to 0.60%,
Si: 0.001 to 3.00%,
Mn: 0.30 to 3.00%,
Al: 0.0002 to 2.000%,
P: 0.1000% or less,
S: 0.1000% or less,
N: 0.0100% or less,
B: 0 to 0.0100%,
Ti: 0 to 0.1500%,
Nb: 0 to 0.150%,
V: 0 to 0.150%,
Mo: 0 to 1.00%,
Cr: 0 to 1.0%,
Cu: 0 to 1.000%,
Ni: 0 to 1.00%,
W: 0 to 1.000%,
Hf: 0 to 0.050%,
Mg: 0 to 0.050%,
Zr: 0 to 0.050%,
Ca: 0 to 0.0100%,
REM: 0 to 0.3000%,
Ir: 0 to 1.000%, and
balance: Fe and impurities, wherein
a mean C concentration from a surface down to 20 µm in a thickness direction is 0.20 mass%, and
the hot-stamp formed body comprises, by area ratio, martensite at 90% or more.

The steel sheet explained previously is suitable for production of the hot-stamp formed body according to an embodiment of the present invention, therefore below, a hot-stamp formed body according to an embodiment of the present invention will be explained in more detail linked with the steel sheet explained previously. However, the following explanation is intended simply as illustration of a characteristic embodiment of the hot-stamp formed body according to the present invention and is not intended to limit the hot-stamp formed body to one obtained by hot-stamp forming the steel sheet explained previously.
When applying the steel sheet according to an embodiment of the present invention explained above to hot-stamp forming, even at the time of high temperature heating in hot-stamp forming due to the suitably modified surface layer part structure, it is possible to reduce the amount of austenite transformed from pearlite and further possible to make the austenite present dispersed on the grain boundaries, therefore it becomes possible to reliably cut off the routes for recarburization of carbon due to austenite. As a result, finally, in the obtained hot-stamp formed body, the mean C concentration from the surface down to 20 µm in the thickness direction can be suppressed to 0.20 mass% or less. Explained in more detail, in the case of steel sheet lowered in carbon concentration at the steel sheet surface layer part by the usual decarburization treatment, decarburization can occur at the surface side of the steel sheet at the time of high temperature heating in hot-stamp forming, but the recarburization where the carbon contained in the bulk of the steel sheet diffuses to the surface layer part becomes dominant and the state of the steel sheet surface layer part initially lowered in carbon concentration can no longer be maintained. In such a case, at the obtained hot-stamp formed body, the carbon concentration of the surface layer part becomes relatively high and the effect of improvement of bendability due to the initial lowering of the carbon concentration of the steel sheet surface layer part can no longer be sufficiently obtained. However, according to an embodiment of the present invention, for example, by using a steel sheet formed at the surface layer part with a structure with a depth with an area ratio of pearlite of 0 to 20% from the surface in the sheet thickness direction made 3 to 100 µm and with an area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with an area ratio of pearlite of 0 to 20% controlled to 0 to 30% to remarkably suppress or reduce recarburization at the time of high temperature heating in hot-stamp forming, it is possible to maintain the carbon concentration at the surface layer part of the obtained hot-stamp formed body relatively low. Specifically, as explained above, it becomes possible to suppress the mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction to 0.20 mass% or less. As a result, it become possible to sufficiently maintain the effect of improvement of bendability by the initial lowering of the carbon concentration of the steel sheet surface layer part and remarkably improve the bendability of the hot-stamp formed body. Further, according to a hot-stamp formed body according to an embodiment of the present invention, despite being high in strength, it is possible to achieve excellent impact resistance by improvement of the bendability. Therefore, a hot-stamp formed body according to an embodiment of the present invention is particularly useful in use in the automotive field. Below, a hot-stamp formed body according to an embodiment of the present invention will be explained in more detail.

[Chemical Composition of Hot-Stamp Formed Body]

The chemical composition of the hot-stamp formed body is, as explained above relating to the steel sheet according to an embodiment of the present invention, comprised of not only the basic constituents of C, Si, Mn, Al, P, S, and N, but also the optional elements of B, Ti, Nb, V, Mo, Cr, Cu, Ni, W, Hf, Mg, Zr, Ca, REM, and Ir. Further, in the hot-stamp formed body, the balance besides these elements is comprised of Fe and impurities. The "impurities" in the hot-stamp formed body are constituents, etc., entering due to the ore, scrap, or other raw materials and other various factors in the production process when industrially producing the hot-stamp formed body.
The chemical composition of the hot-stamp formed body may be measured by a general analysis method. For example, the chemical composition of the hot-stamp formed body may be measured at the chips based on JIS G 1201: 2014 using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Specifically, for example, a 35 mm square test piece is obtained from near the 1/2 position of the thickness of the steel sheet and is measured by an ICPS-8100, etc. (measuring device) made by Shimadzu Corporation under conditions based on calibration curves prepared in advance to thereby identify the composition. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, N can be measured using the inert gas melting-thermal conductivity method, and O can be measured using the inert gas melting-nondispersive type infrared absorption method. If the surface of the steel sheet is provided with a plating layer, mechanical grinding may be used to remove the plating layer, then the chemical composition analyzed.

[Mean C Concentration Down to 20 µm From Surface of Hot-Stamp Formed Body in Thickness Direction: 0.20 Mass% or Less]

In a hot-stamp formed body according to an embodiment of the present invention, the mean C concentration from the surface down to 20 µm in the thickness direction is 0.20 mass% or less. By controlling the mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction to 0.20 mass%. or less to soften the surface layer part of the hot-stamp formed body, it is possible to remarkably improve the bendability of the hot-stamp formed body. From the viewpoint of improvement of the bendability of the hot-stamp formed body, the lower the mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction, the more preferable. For example, it may be 0.18 mass% or less, 0.15 mass% or less, 0.12 mass% or less, 0.10 mass% or less, 0.08 mass% or less, 0.06 mass% or less, or 0.05 mass% or less. The lower limit is not particularly prescribed, but, for example, the mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction may also be 0.001 mass% or more, 0.005 mass% or more, or 0.01 mass% or more.

[Measurement of Mean C Concentration From Surface of Hot-Stamp Formed Body Down to 20 µm in Thickness Direction]

The mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction is determined in the following way using high frequency glow discharge spectrometry (GDS). Specifically, the method is used of rendering the surface of the hot-stamp formed body an Ar atmosphere, applying voltage to generate glow plasma, and in that state sputtering the surface of the hot-stamp formed body while analyzing it in the depth direction. Further, the elements contained in the material are identified from the emission spectral wavelengths unique to the elements generated by excitation of atoms in the glow plasma and the emission intensities of the identified elements are estimated. Depth direction data can be estimated from the sputter time. Specifically, by using standard samples in advance to find the relationship between the sputter time and the sputter depth, it is possible to convert the sputter time to the sputter depth. Therefore, the sputter depth converted from the sputter time can be defined as the depth from the surface of the material. The obtained emission intensities are converted to mass% by creating calibration curves. The mean C concentration at the region from the surface down to 20 µm in the thickness direction measured in this way is determined as "the mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction". In GDS measurement, peaks believed to be related to contamination at the hot-stamp formed body surface, etc., are ignored when calculating the mean C concentration.
As explained above, in the case of steel sheet lowered in carbon concentration at the steel sheet surface layer part by the usual decarburization treatment, decarburization can occur at the surface side of the steel sheet at the time of high temperature heating in hot-stamp forming, but the recarburization where the carbon contained in the bulk of the steel sheet diffuses to the surface layer part becomes dominant and the state of the steel sheet surface layer part initially lowered in carbon concentration can no longer be maintained. For this reason, even if the surface layer part of the steel sheet is sufficiently lowered in carbon concentration by decarburization, etc., before hot-stamp forming, after hot-stamp forming, sometimes the C concentration at the surface layer part of the steel sheet becomes a value closer to the C concentration at the bulk. However, according to an embodiment of the present invention, diffusion of C from the bulk to the surface layer part is suppressed due to suppression of recarburization, therefore compared with a conventional hot-stamp formed body, the C concentration at the surface layer part of the hot-stamp formed body can be sufficiently reduced compared with the C concentration of the bulk. From the viewpoint of improvement of the bendability, the lower the mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction compared with the C content of the hot-stamp formed body (bulk), the more preferable. More specifically, the mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction is preferably 0.90 time or less of the C content of the hot-stamp formed body, for example, may be 0.85 time or less, 0.80 time or less, 0.78 time or less, 0.75 time or less, 0.70 time or less, 0.60 time or less, 0.50 time or less, or 0.40 time or less. The lower limit is not particularly prescribed, but, for example, the mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction may be 0.01 time or more of the C content of the hot-stamp formed body, 0.03 time or more, 0.05 time or more, or 0.10 time or more. In the present invention, the "C content of the hot-stamp formed body" means the value measured for chips by ICP-AES (inductively coupled plasma-atomic emission spectrometry) based on JIS G 1201: 2014 using a test piece obtained from near the 1/2 position of thickness of the hot-stamp formed body.

[Martensite Area Ratio of Hot-Stamp Formed Body: 90% or More]

The hot-stamp formed body according to an embodiment of the present invention comprises, by area ratio, martensite at 90% or more. The balance structure is not particularly limited, but may comprise 10% or less of at least one of bainite, ferrite, retained austenite, and pearlite. Martensite is an extremely hard structure, therefore by including martensite in the hot-stamp formed body in an area ratio of 90% or more, high strength, specifically 400HV Vickers hardness, can be achieved. On the other hand, if the area ratio of martensite is low and the ratio of ferrite and other soft structures becomes high, sometimes a 400HV Vickers hardness cannot be achieved. Therefore, the greater the area ratio of martensite, the more preferable. For example, it may be 92% or more, 94% or more, 96% or more, or 98% or more. The upper limit of the area ratio of martensite is not particularly prescribed and may also be 100%.

[Identification of Martensite and Calculation of Area Ratio]

The martensite is identified and the area ratio is calculated in the following way. First, a sample is taken so that the cross-section of the hot-stamp formed body parallel to the rolling direction and sheet thickness direction becomes the examined surface. Next, the examined surface is polished to a mirror surface and corroded by a Nital corrosion solution, then is examined for structure using a scan electron microscope (SEM). A 300 µm×300 µm range is captured at 1000X at a sheet thickness 1/4 depth position of the examined surface. The obtained microstructure photograph is binarized to white and black, then the image analyzed to identify the pearlite, bainite, and ferrite. The method based on "Steels - Micrographic determination of the apparent grain size" determined in JIS G 0551: 2020 is used to find the total of the area ratio of these. Retained austenite is difficult to differentiate from martensite by an SEM, therefore X-ray diffraction is used to measure the area ratio of the retained austenite. Finally, the total area ratio of the pearlite, bainite, ferrite, and retained austenite obtained by the above methods is subtracted from 100% to determine the area ratio of martensite.

[Plating]

The hot-stamp formed body according to an embodiment of the present invention may further contain a plating layer at the surface for the purpose of improvement of the corrosion resistance. The plating layer may be any of a hot dip coated layer and electroplated layer. The hot dip coated layer includes for example a hot dip galvanized layer (GI), hot dip galvannealed layer (GA), hot dip aluminum coated layer, hot dip Zn-Al alloy coated layer, hot dip Zn-Al-Mg alloy coated layer, hot dip Zn-Al-Mg-Si alloy coated layer, etc. The electroplated layer includes for example an electrogalvanized layer (EG) and electro-Zn-Ni alloy plating layer, etc. Preferably, the plating layer is a hot dip galvanized layer, hot dip galvannealed layer, or electrogalvanized layer. The amount of deposition of the plating layer is not particularly limited, and may be a general amount of deposition. Further, the hot-stamp formed body according to an embodiment of the present invention need not include a plating layer at the surface. Specifically, it need not include a hot dip coated layer at the surface or need not include an electroplated layer at the surface.

[Mechanical Properties]

According to the hot-stamp formed body according to an embodiment of the present invention, excellent mechanical properties, for example, a 500HV or more Vickers hardness, more specifically a 500HV or more Vickers hardness at the 1/2 position of thickness of the hot-stamp formed body, can be achieved. The Vickers hardness is preferably 530HV or more, more preferably 550HV or more or 600HV. The upper limit is not particularly prescribed, but, for example, the Vickers hardness may also be 700HV or less or 650HV or less.

[Measurement of Hardness]

The Vickers hardness is determined in the following way. First, a test piece is cut out from any position except the end parts of the hot-stamp formed body to enable a cross-section vertical to the surface (thickness cross-section) to be examined. The sheet thickness cross-section of the test piece is polished using #600 to #1500 silicon carbide paper, then is finished to a mirror surface using a solution of particle size 1 to 6 µm diamond powder dispersed in alcohol or other diluent or pure water and the thickness cross-section is made the measured surface. Next, a micro-Vickers hardness tester is used to measure the Vickers hardness by a load of 1 kgf at intervals of 3 times or more of the indentations. A total of 20 points are randomly measured near the 1/2 position of thickness of the hot-stamp formed body so as to not include the surface layer part with the low carbon concentration and the arithmetic average of these is determined as the hardness of the hot-stamp formed body.

Next, a preferable method of production of the steel sheet according to an embodiment of the present invention will be explained. The following explanation is intended to illustrate the characteristic method for production of the steel sheet according to an embodiment of the present invention and is not intended to limit the steel sheet to one produced by the method of production explained below.
The steel sheet according to an embodiment of the present invention can be produced by, for example, a casting step of casting molten metal adjusted in chemical composition to form a steel slab, a hot rolling step of hot rolling the steel slab to obtain hot rolled steel sheet, a coiling step of coiling the hot rolled steel sheet, a cooling step of cold rolling the coiled up hot rolled steel sheet to obtain cold rolled steel sheet, and, in accordance with need, a plating step of forming a plating layer on the obtained steel sheet. Alternatively, the steel sheet need not be coiled up after the hot rolling step, but may be pickled and sent on to the cold rolling step as is. Below, the steps will be explained in detail.

[Casting Step]

The conditions of the casting step are not particularly limited. For example, after smelting by a blast furnace, electric furnace, etc., various secondary refining operations may be performed, then the steel may be cast by the usual continuous casting, casting by the ingot method, or other method.

[Hot Rolling Step]

The cast steel slab can be hot rolled to obtain hot rolled steel sheet. The hot rolling step is performed by hot rolling the cast steel slab directly or after cooling once, then reheating. If reheating, the heating temperature of the steel slab may, for example, be 1100 to 1250°C. In the hot rolling step, usually rough rolling and finish rolling are performed. The temperatures and rolling reductions of the rolling operations can be suitably determined in accordance with the desired metallostructure and sheet thickness. For example, the end temperature of the finish rolling may be 900 to 1050°C and the rolling reduction of the finish rolling may be 10 to 50%.

[Coiling Step]

The hot rolled steel sheet can be coiled up at a predetermined temperature. The coiling temperature can be suitably determined in accordance with the desired metallostructure, etc. For example, it may be 500 to 800°C. Before coiling or after coiling, then uncoiling, the hot rolled steel sheet may be subjected to predetermined heat treatment. Alternatively, the coiling step need not be performed, but the steel sheet may be pickled after the hot rolling step and then the cold rolling step performed.

[Cold Rolling Step]

The hot rolled steel sheet may be pickled, etc., then the hot rolled steel sheet may be cold rolled to obtain cold rolled steel sheet. The rolling reduction of the cold rolling can be suitably determined in accordance with the desired metallostructure and sheet thickness. For example, it may be 20 to 80%. After the cold rolling step, for example, the steel sheet may be air cooled to cool it down to room temperature.

[Annealing Step]

Next, the obtained cold rolled steel sheet is annealed. The annealing step includes heating the cold rolled steel sheet in an atmosphere with a dew point of -20 to 10°C to 730 to 900°C in temperature and holding it there for 10 to 300 seconds. By performing the annealing step under such relatively high dew point conditions, it is possible to suitably decarburize the surface layer part of the cold rolled steel sheet. Therefore, at the finally obtained steel sheet, it becomes possible to control the depth with an area ratio of pearlite of 0 to 20% from the surface in the sheet thickness direction to a range of 3 to 100 µm. By forming a structure with such a relatively small amount of pearlite at the surface layer part of the steel sheet, at the finally obtained hot-stamp formed body obtained at the subsequent hot-stamp forming, it becomes possible to reliably suppress the mean C concentration from the surface thereof down to 20 µm in the thickness direction to 0.20 mass% or less. If the dew point is less than-20°C, the heating temperature is less than 730°C, and/or the holding time is less than 10 seconds, the decarburization at the surface layer part of the cold rolled steel sheet becomes insufficient. As a result, at the finally obtained steel sheet, it becomes no longer possible to make the depth with an area ratio of pearlite of 0 to 20% from the surface in the sheet thickness direction at the 3 µm or more. On the other hand, if the dew point is more than 10°C, the heating temperature is more than 900°C, and/or the holding time is more than 300 seconds, sometimes the steel sheet surface is formed with an external oxide layer, the plateability falls, and excessive decarburization causes the finally obtained steel sheet and in turn the hot-stamp formed body to fall in strength. The dew point is preferably -10 to 5°C, more preferably -5 to 5°C. Further, the atmosphere at the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a hydrogen 1 to 10% reducing atmosphere (for example, hydrogen 4% and nitrogen balance).

[Cooling Step]

The cold rolled steel sheet with the surface layer part decarburized in the annealing step has to be suitably cooled in the next cooling step in order to obtain the desired surface layer part structure. Specifically, the cooling step includes cooling from the heating temperature of the annealing step to a control temperature of 620 to 670°C by a mean cooling speed of 20°C/s or more (primary cooling) and cooling from the control temperature to room temperature by a mean cooling speed of 10°C/s or less (secondary cooling). Below, the primary cooling and secondary cooling will be explained in more detail.

[Primary Cooling]

In the primary cooling, it is important to suppress precipitation of pearlite at a high temperature. Explained more specifically, pearlite precipitating at a high temperature of the 730 to 900°C heating temperature at the annealing step to the 620 to 670°C control temperature quickly disperses, therefore pearlite dispersed at the grain boundaries and running along the grain boundaries after precipitation is easily formed. The pearlite formed along the grain boundaries transforms to austenite at the time of hot-stamp forming, therefore routes for recarburization of carbon are formed by the austenite running along the grain boundaries and recarburization by carbon in the bulk to the steel surface layer part is promoted. Therefore, in the temperature region from the heating temperature of the annealing step to the control temperature, it becomes extremely important to suppress precipitation of pearlite at a high temperature at the steel surface layer part by cooling the cold rolled steel sheet by a relatively fast mean cooling speed of 20°C/s or more. If the mean cooling speed is less than 20°C/s and/or the control temperature is more than 670°C, pearlite precipitates at the fast dispersion high temperature, therefore formation of pearlite running along the grain boundaries is promoted. As a result, at the finally obtained steel sheet, the area ratio of the circle equivalent diameter 5 µm or more coarse pearlite at a depth with an area ratio of pearlite of 0 to 20% from the surface in the sheet thickness direction becomes more than 30% and recarburization at the time of high temperature heating in the hot-stamp forming can no longer be sufficiently suppressed or reduced.

[Secondary Cooling]

On the other hand, in the secondary cooling after the primary cooling, making the pearlite precipitate at a low temperature where dispersion is relatively slow is important. More specifically, it is possible to make the pearlite precipitate by cooling from the 620 to 670°C control temperature to room temperature by a mean cooling speed of 10°C/s or less. Pearlite precipitating at such a control temperature or less low temperature region is relatively slow to disperse, therefore does not form strings along the grain boundaries. The pearlite can remain present dispersed on the grain boundaries. In the case of such a structure, even at the time of the high temperature heating in the hot-stamp forming, the austenite transformed from pearlite at the A_{c 1} point or more can similarly be made present dispersed on the grain boundaries, therefore the routes for recarburization of carbon by austenite can be reliably cut off. On the other hand, if the mean cooling speed is more than 10°C/s and/or the control temperature is less than 620°C, not pearlite, but mainly martensite or bainite precipitates and the depth with an area ratio of pearlite of 0 to 20% from the surface in the sheet thickness direction at the finally obtained steel sheet can no longer be made 100 µm or less. Martensite and bainite are faster in speed of transformation to austenite compared with pearlite and immediately transform to austenite right above the A_{c 1} point. For this reason, compared with the case of pearlite, at the time of hot-stamp forming, the time exposed to a high temperature at the dual phase structure of ferrite and austenite becomes longer. In the same way as such a case, routes for recarburization become easily formed at the grain boundaries, therefore recarburization can no be sufficiently suppressed or reduced.

[Plating Step]

For the purpose of improving the corrosion resistance, etc., the surface of the obtained cold rolled steel sheet may also be plated. The plating treatment may also be hot dip coating, alloyed hot dip coating, electroplating, etc. For example, as the plating treatment, the steel sheet may be treated by hot dip galvanization and may be alloyed after the hot dip galvanization. The specific conditions of the plating treatment and alloying are not particularly limited. They may also be any suitable conditions known to persons skilled in the art. For example, further, the plating treatment may be hot dip galvanization, electroplating, vapor deposition plating, thermal spraying, cold spraying, etc. The other conditions of the plating step may be suitably set considering the thickness, amount of deposition, etc., of the plating layer. For example, by dipping the cold rolled steel sheet in a plating bath, then pulling it up and immediately blowing N₂ gas or air for gas wiping, then cooling, it is possible to adjust the amount of deposition of the plating layer to within a predetermined range, for example, to within a range of 20 to 200 g/m² per surface.
The steel sheet produced by the present method of production can have formed at the surface layer part of the steel sheet a structure with a depth with an area ratio of pearlite of 0 to 20% from the surface in the sheet thickness direction made 3 to 100 µm and with an area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with an area ratio of pearlite of 0 to 20% controlled to 0 to 30%. For this reason even if exposed to a high temperature such as at the time of hot-stamp forming, it is possible to remarkably suppress or reduce the recarburization of the carbon in the bulk to the steel surface layer part, therefore it becomes possible to sufficiently maintain the effect of improvement of bendability by the initial lowering of the carbon concentration of the steel sheet surface layer part and remarkably improve the bendability of the body obtained after hot-stamp forming.

The hot-stamp formed body according to an embodiment of the present invention can be produced by performing a hot-stamp forming step of hot-stamp forming the steel sheet obtained by the method explained above. In particular, from the viewpoint of obtaining the desired hard structure, the steel sheet is preferably loaded into a 800 to 1000°C furnace and held in the furnace for 60 to 600 seconds after the temperature of the steel sheet reaches a predetermined temperature, for example, the internal furnace temperature -10°C. If the heating temperature is less than 800°C and/or the holding time is less than 60 seconds, the austenization becomes insufficient, the desired area ratio of the hard structure (i.e., a martensite area ratio of 90% or more) cannot be obtained, and at the finally obtained hot-stamp formed body, sometimes a 500HV or more Vickers hardness cannot be achieved. The heating atmosphere is not particularly limited and may be the usual conditions. For example, it may be the air, a gas combustion atmosphere controlled in ratio of air and fuel, or a nitrogen atmosphere. The dew point may also be controlled in these gases. After being heated and held in the furnace, the steel sheet can be taken out from the furnace, then hot-stamp formed under usual conditions after the steel sheet reaches a predetermined temperature, for example a predetermined temperature of 850°C or less. After the hot-stamp forming, while not particularly limited to this, for example the body may be cooled down to the 250°C or less temperature region by a mean cooling speed of 20°C/s or more.
The hot-stamp formed body produced by the present method of production can have a mean C concentration from the surface down to 20 µm in the thickness direction controlled to 0.20 mass%, or less and can contain, by area ratio, martensite at 90% or more. Therefore, compared with a conventional hot-stamp formed body, despite being high in strength, it becomes possible to realize a better impact resistance by improvement of the bendability due to softening of the surface layer part. For this reason, the body is particularly useful in use in the automobile field and can contribute to the development of industry.
Below, examples will be used to explain the present invention in more detail, but the present invention is not limited to these examples in any way.

EXAMPLES

In the following examples, steel sheets and hot-stamp formed bodies according to an embodiment of the present invention were produced under various conditions and the properties of the produced steel sheets and hot-stamp formed bodies were investigated.

[Production of Steel Sheet]

First, molten steel was cast by the continuous casting method for form steel slabs having the chemical compositions shown in Table 1. Each steel slab was cooled once, then reheated to 1200°C and hot rolled, then was coiled up at a 600°C or less temperature. The hot rolling was performed by rough rolling and finish rolling. The end temperature of the finish rolling was 900 to 1050°C and the rolling reduction of the finish rolling was 30%. Next, the obtained hot rolled steel sheet was pickled, then was cold rolled by a rolling reduction of 50% to obtain cold rolled steel sheet having a 1.6 mm sheet thickness. Next, the obtained cold rolled steel sheet was subjected to an annealing step in an oxygen concentration 20 ppm or less furnace in a mixed gas atmosphere of hydrogen 4% and a nitrogen balance under the conditions shown in Table 1, then was similarly subjected to a cooling step under the conditions shown in Table 1 to produce a steel sheet.

[Production of Hot-Stamp Formed Body]

First, the produced steel sheet was loaded in a 900°C atmospheric heating furnace. After the temperature of the steel sheet reached the internal furnace temperature-10°C, it was held there for 100 seconds. Next, the steel sheet was taken out from the furnace. The steel sheet was clamped between a flat plate die set of a temperature of about room temperature to rapidly cool it and obtain a hot-stamp formed body.

[Evaluation of Hardness After Hot Stamping (HS)]

A test piece was cut out from any position of the obtained hot-stamp formed body other than the end parts so that a cross-section vertical to the surface (sheet thickness cross-section) could be examined. The sheet thickness cross-section of the test piece was polished using #600 to #1500 silicon carbide paper, then finished to a mirror surface using a solution of particle size 1 to 6 µm diamond powder dispersed in alcohol or another diluent or pure water and the sheet thickness cross-section was made the measured surface. Next, a micro-Vickers hardness tester was used to measure the Vickers hardness by a load of 1 kgf at intervals of 3 times or more of the indentations. A total of 20 points were randomly measured near the 1/2 position of thickness of the hot-stamp formed body so as to not include the surface layer part with the low caron concentration and the arithmetic average of these was determined as the hardness after hot stamping (HS). This was evaluated as follows:

AAA: hardness after HS of more than 600HV
AA: hardness after HS of more than 530 to 600HV
A: hardness after HS of 500 to 530HV
B: hardness after HS of less than 500HV

[Evaluation of Bendability]

The bendability was evaluated by a bending test based on VDA (Verband der Automobilindustrie) standard, 238-100: 2017-04. More specifically, the displacement at the time of the maximum load obtained by the bending test was converted to angle by the VDA standard to find the maximum bending angle α (°) and the bendability was evaluated in the following way:

AAA: maximum bending angle α of more than 80°
AA: maximum bending angle α of more than 70 to 80°
A: maximum bending angle α of 60 to 70°
B: maximum bending angle α of less than 60°

Cases where the hardness after HS was evaluated as AAA, AA, and A and the bendability was evaluated as AAA, AA, and A were evaluated as hot-stamp formed bodies high in strength and having excellent bendability. The results are shown in Table 1. In the hot-stamp formed bodies shown in Table 1, the balance structure besides martensite was comprised of bainite, ferrite, retained austenite, and/or pearlite.

[Table 1-1]

Table 1-1

No.	Class											Production conditions
		C	Si	Mn	Al	P	S	N	B	Ti	Others	Annealing step			Cooling step
		C	Si	Mn	Al	P	S	N	B	Ti	Others	Dew point (°C)	Heating temp. T1 (°C)	Holding time (s)	T1→T2 mean cooling speed (°C/s)	Control temp. T2 (°C)	T2→room temp. mean cooling speed (°C/s)
1	Ex.	0.27	0.20	1.20	0.040	0.0001	0.0006	0.0010	0.0009	0.0003		0	800	10	20	650	10
2	Ex.	0.35	0.20	1.20	0.040	0.0080	0.0009	0.0006	0.0008	0.0002		0	800	15	20	650	10
3	Ex.	0.27	0.20	1.20	0.040	0.0080	0.0007	0.0003	0.0007	0.1500	Hf:0.001	0	800	60	20	650	10
4	Ex.	0,27	0.20	1.20	0.040	0.0700	0.0005	0.0007	0.0007	0.0006	Ir:0.100	0	800	60	30	650	10
5	Ex.	0.27	0.20	1.20	0.300	0.0020	0.0010	0.0005	0.0001	0.0005		0	800	60	30	650	10
6	Ex.	0.27	0.20	1.20	0.500	0.0100	0.0008	0.0006	0.0100	0.0003	Mg:0.001	0	800	60	30	650	10
7	Ex.	0.35	0.20	1.20	0.400	0.0020	0.1000	0.0002	0.0005	0.0008	Zr:0.015	0	800	60	30	650	10
8	Ex.	0.35	0.20	1.20	0.700	0.0008	0.0007	0.0010	0.0002	0.0008		0	800	60	30	650	10
9	Ex.	0.35	0.20	1.20	0.500	0.0017	0.0007	0.0007	0.0008	0.0006	Cr:0.1	0	800	80	30	650	10
10	Ex.	0.45	0.90	1.20	0.400	0.0011	0.0002	0.0005	0.0002	0.0004	Cu:0.0007	0	800	100	30	650	5
11	Ex.	0.45	1.00	1.20	0.500	0.1000	0.0009	0.0009	0.0009	0.0003		0	800	100	30	650	5
12	Ex.	0.60	0.20	2.20	0.500	0.0065	0.0004	0.0003	0.0007	0.0007	Ni:0.08	0	800	100	30	650	5
13	Ex.	0.27	1.00	0.30	0.500	0.0012	0.0008	0.0009	0.0005	0.0001	Nb:0.010	0	800	120	30	650	10
14	Ex.	0.27	1.00	2.20	0.500	0.0700	0.0002	0.0010	0.0003	0.0004	V:0.009	0	800	120	30	650	10
15	Ex.	0.27	0.80	3.00	0.700	0.0099	0.0005	0.0003	0.0010	0.0009		0	800	120	30	650	10
16	Ex.	0.27	0.80	0.30	0.700	0.0110	0.0008	0.0002	0.0001	0.0002		0	800	120	30	650	10
17	Ex.	0.27	0.80	1.50	0.700	0.0092	0.0008	0.0002	0.0003	0.0009	Mo:0.09	0	800	120	30	650	10
18	Ex.	0.27	1.00	2.20	0.500	0.0091	0.0005	0.0006	0.0008	0.0007	REM:0.0008	0	800	120	30	650	10
19	Ex.	0.27	0.20	2.20	0.500	0.0045	0.0009	0.0003	0.0003	0.0004		0	800	140	30	650	10
20	Ex.	0.27	1.00	3.00	0.500	0.0035	0.0009	0.0004	0.0010	0.0004	W:0.005	0	800	140	30	650	10
21	Ex.	0.27	1.00	2.20	0.500	0.0082	0.0001	0.0009	0.0007	0.0004		0	800	170	30	650	10
22	Ex.	0.35	0.80	2.20	0.700	0.0080	0.0008	0.0006	0.0002	0.0009	Ca:0.0005	0	900	210	30	650	10
23	Ex.	0.35	1.00	2.00	0.010	0.0004	0.0003	0.0009	0.0003	0.0001	Mo:0.1	0	850	220	30	650	10
24	Ex.	0.35	1.00	1.50	0.300	0.0100	0.0003	0.0009	0.0009	0.0007		0	800	230	30	650	10
25	Ex.	0.35	0.50	2.00	0.300	0.0100	0.0004	0.0002	0.0003	0.0006		0	800	240	30	650	10
26	Ex.	0.35	2.20	1.70	0.400	0.0100	0.0006	0.0010	0.0010	0.0003		0	800	220	30	650	10
27	Ex.	0.35	1.00	1.70	1.000	0.0100	0.0005	0.0001	0.0007	0.0008	Cu:0.001	0	730	210	30	650	10
28	Ex.	0.45	0.70	1.70	0.300	0.0100	0.0005	0.0001	0.0005	0.0005		0	800	300	30	650	5
29	Ex.	0.45	1.00	1.70	0.010	0.0100	0.0006	0.0007	0.0006	0.0007	Ca:0.0005	0	800	300	30	650	5
30	Ex.	0.45	1.00	1.70	0.100	0.0100	0.0010	0.0004	0.0009	0.0008		0	800	300	30	650	5
31	Ex.	0.60	3.00	1.70	0.400	0.0100	0.0008	0.0009	0.0010	0.0004		0	800	300	30	650	5
32	Ex.	0.60	1.00	2.00	0.400	0.0100	0.0007	0.0007	0.0002	0.0003		0	800	300	30	650	5
33	Ex.	0.35	0.50	2.00	0.300	0.0100	0.0004	0.0002	0.0003	0.0006		0	800	240	30	670	10
34	Ex.	0.35	0.50	2.00	0.300	0.0100	0.0004	0.0002	0.0003	0.0006		0	800	240	30	620	10
35	Comp. ex.	0.26	1.00	1.20	0.500	0.0100	0.0002	0.0001	0.0002	0.0005		0	800	120	20	650	5
36	Comp. ex.	0.27	0.20	1.20	0.500	0.0100	0.0006	0.0007	0.0004	0.0006		0	710	120	20	650	5
37	Comp. ex.	0.27	0.20	1.20	0.500	0.0100	0.0005	0.0009	0.0006	0.0004		0	800	8	20	650	5
38	Comp. ex.	0.27	0.20	1.20	0.500	0.0100	0.0005	0.0010	0.0002	0.0010		-30	800	100	20	650	5
39	Comp. ex.	0.27	0.20	1.20	0.040	0.0001	0.0007	0.0003	0.0005	0.0004		0	800	100	15	650	5
40	Comp. ex.	0.27	0.20	1.20	0.040	0.0080	0.0003	0.0005	0.0004	0.0006		0	800	100	20	700	5
41	Comp. ex.	0.27	0.20	1.20	0.040	0.0080	0.0008	0.0008	0.0002	0.0002		0	800	100	20	600	5
42	Comp. ex.	0.27	0.20	1.20	0.040	0.0700	0.0006	0.0009	0.0003	0.0006		0	800	10	20	650	13

[Table 1-2]

Table 1-2

No.	Class	Plating layer	Steel sheet	Cross-sectional structure of steel sheet			Cross-sectional structure of hot-stamp formed body			Performance
			Tensile strength before HS (MPa)	Pearlite area ratio 0 to 20% depth (µm)	Circle equivalent diameter 5 µm or more pearlite area ratio (%)	Martensite area ratio (%)	Martensite area ratio (%)	x	X/HS body C content	Hardness after HS	Bendability
			Tensile strength before HS (MPa)	Pearlite area ratio 0 to 20% depth (µm)		Martensite area ratio (%)	Martensite area ratio (%)	Surface to 20 µm mean C conc. (mass%)	X/HS body C content	Hardness after HS	Bendability
1	Ex.	No	590	3	30	0	95	0.20	0.74	A	A
2	Ex.	No	640	8	25	0	94	0.20	0.57	AA	A
3	Ex.	No	590	10	24	0	91	0.12	0.44	A	A
4	Ex.	No	590	11	22	0	93	0.11	0.41	A	A
5	Ex.	No	590	11	23	0	99	0.11	0.44	A	A
6	Ex.	No	590	12	21	0	93	0.11	0.41	A	A
7	Ex.	No	640	13	26	0	94	0.15	0.43	AA	A
8	Ex.	No	640	10	14	0	97	0.06	0.17	A	AA
9	Ex.	No	640	19	14	0	96	0.09	0.26	AA	AA
10	Ex.	No	730	22	13	0	98	0.09	0.20	AAA	AA
11	Ex.	No	730	25	13	0	96	0.09	0.20	AAA	AA
12	Ex.	No	920	29	13	0	94	0.10	0.17	AAA	AA
13	Ex.	No	590	30	8	0	93	0.05	0.19	A	AAA
14	Ex.	No	590	33	7	0	96	0.03	0.11	A	AAA
15	Ex.	No	590	31	8	0	91	0.03	0.11	A	AAA
16	Ex.	No	590	33	6	0	90	0.03	0.11	A	AAA
17	Ex.	No	590	40	6	0	93	0.03	0,11	A	AAA
18	Ex.	No	590	50	8	0	92	0.01	0.04	A	AAA
19	Ex.	No	590	61	7	0	95	0.03	0.11	A	AAA
20	Ex.	No	590	82	8	0	96	0.02	0.07	A	AAA
21	Ex.	No	590	99	8	0	99	0.03	0.11	A	AAA
22	Ex.	No	640	100	7	0	95	0.02	0.06	AA	AAA
23	Ex.	No	640	60	9	0	95	0.02	0.06	AA	AAA
24	Ex.	No	640	35	6	0	93	0.03	0.09	AA	AAA
25	Ex.	No	640	37	7	0	94	0.01	0.03	AA	AAA
26	Ex.	No	640	34	8	0	92	0.02	0.06	AA	AAA
27	Ex.	No	640	55	6	0	92	0.03	0.09	AA	AAA
28	Ex.	No	730	43	11	0	94	0.03	0.07	AAA	AAA
29	Ex.	No	730	35	10	0	93	0.02	0.04	AAA	AAA
30	Ex.	No	730	35	10	0	97	0.03	0.07	AAA	AAA
31	Ex.	No	920	41	15	0	98	0.01	0.02	AAA	AAA
32	Ex.	No	920	35	11	0	92	0.02	0.03	AAA	AAA
33	Ex.	No	630	35	8	0	90	0.02	0.06	AA	AAA
34	Ex.	No	640	37	7	0	91	0.01	0.03	AA	AAA
35	Comp. ex.	No	500	35	8	0	96	0.03	0.12	B	AAA
36	Comp. ex.	No	590	0	24	0	99	0.26	0.96	A	B
37	Comp. ex.	No	590	2	23	0	92	0.25	0.93	A	B
38	Comp. ex.	No	590	2	14	0	93	0.25	0.93	A	B
39	Comp. ex.	No	590	22	36	0	91	0.24	0.89	A	B
40	Comp. ex.	No	590	15	50	0	95	0.25	0.93	A	B
41	Comp. ex.	No	590	>100	0	7	96	0.26	0.96	A	B
42	Comp. ex.	No	590	>100	0	8	97	0.25	0.93	A	B

Referring to Table 1, in Comparative Example 35, the C content was low, therefore the hardness after HS fell. In Comparative Example 36, the heating temperature of the annealing step was low, therefore probably the decarburization at the surface layer part of the cold rolled steel sheet was insufficient. As a result, the depth with an area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction could not be made 3 µm or more, the mean C concentration down to 20 µm from the surface of the hot-stamp formed body obtained from the steel sheet in the thickness direction also became higher, and the bendability fell. In Comparative Example 37, the holding time of the annealing step was short, therefore similarly probably the decarburization at the surface layer part of the cold rolled steel sheet was insufficient. As a result, the depth with an area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction could not be made 3 µm or more, the mean C concentration from the surface of the hot-stamp formed body obtained from the steel sheet down to 20 µm in the thickness direction also became higher, and the bendability fell. In Comparative Example 38, the dew point of the annealing step was low, therefore similarly probably the decarburization at the surface layer part of the cold rolled steel sheet was insufficient. As a result, the depth with an area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction could not made 3 µm or more, the mean C concentration down to 20 µm from the surface of the hot-stamp formed body obtained from the steel sheet in the thickness direction also became higher, and the bendability fell. In Comparative Example 39, the mean cooling speed of the primary cooling at the annealing step was low, therefore pearlite precipitated at a high temperature and probably this was formed along the grain boundaries. As a result, the area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with an area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction became more than 30%. Further, in relation to this, probably recarburization at the time of high temperature heating in hot-stamp forming was promoted, the mean C concentration from the surface of the hot-stamp fanned body down to 20 µm in the thickness direction also became higher, and the bendability fell. In Comparative Example 40, the control temperature of the primary cooling at the cooling step was high, therefore similarly probably pearlite precipitated in a high temperature and was formed along the grain boundary. As a result, the area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with an area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction became more than 30%. Further, in relation to this, probably recarburization at the time of high temperature heating in hot-stamp forming was promoted, the mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction also became higher, and the bendability fell. In Comparative Example 41, the control temperature of the secondary cooling at the cooling step was low, therefore not pearlite, but bainite mainly precipitated. As a result, the depth with an area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction could not be made the desired depth. Further, in relation to this, probably recarburization at the time of high temperature heating in hot-stamp forming was promoted, the mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction also became higher, and the bendability fell. In Comparative Example 42, the mean cooling speed of the secondary cooling in the cooling step was fast, therefore similarly not pearlite, but bainite mainly precipitated. As a result, the depth with an area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction could not be made the desired depth. Further, in relation to this, probably recarburization at the time of high temperature heating in hot-stamp forming was promoted, the mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction also became higher, and the bendability fell.
In contrast to this, in each of the steel sheets according to all of the examples, by having the predetermined plating chemical composition, having a depth with an area ratio of pearlite of 0 to 20% from the surface of the steel sheet in the sheet thickness direction of 3 to 100 µm, and controlling the area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with an area ratio of pearlite of 0 to 20% to 0 to 30%, even when exposed to a 900°C high temperature by hot-stamp forming, it was possible to control the mean C concentration from the surface down to 20 µm in the thickness direction at the obtained hot-stamp formed body to 0.20 mass% or less. As a result, it was possible to achieve a high strength and high bendability. In particular, in each of Examples 8 to 12 with a depth with an area ratio of pearlite of 0 to 20% made 10 to 100 µm and with an area ratio of pearlite having a circle equivalent diameter of 5 µm or more at that depth controlled to 0 to 15%, the mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction was reduced to 0.10 mass% or less and, as a result, the bendability was evaluated as AA and the bendability was improved more. Further, in each of Examples 13 to 34 with a depth with an area ratio of pearlite of 0 to 20% made 30 to 100 µm and with an area ratio of pearlite having a circle equivalent diameter of 5 µm or more at that depth controlled to 0 to 15%, the mean C concentration from the surface of the hot-stamp formed body down to 20 µm in the thickness direction was reduced to 0.05 mass% or less and, as a result, the bendability was evaluated as AAA and the bendability was improved more.

Claims

A steel sheet having a chemical composition comprising, by mass%,
C: 0.27 to 0.60%,

Si: 0.001 to 3.00%,

Mn: 0.30 to 3.00%,

Al: 0.0002 to 2.000%,

P: 0.1000% or less,

S: 0.1000% or less,

N: 0.0100% or less,

B: 0 to 0.0100%,

Ti: 0 to 0.1500%,

Nb: 0 to 0.150%,

V: 0 to 0.150%,

Mo: 0 to 1.00%,

Cr: 0 to 1.0%,

Cu: 0 to 1.000%,

Ni: 0 to 1.00%,

W: 0 to 1.000%,

Hf: 0 to 0.050%,

Mg: 0 to 0.050%,

Zr: 0 to 0.050%,

Ca: 0 to 0.0100%,

REM: 0 to 0.3000%,

Ir: 0 to 1.000%, and

balance: Fe and impurities, wherein

a depth from a surface in a sheet thickness direction with an area ratio of pearlite of 0 to 20% is 3 to 100 µm, and

an area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with area ratio of pearlite of 0 to 20% is 0 to 30%.
The steel sheet according to claim 1, wherein the depth with area ratio of pearlite of 0 to 20% is 10 to 100 µm.
The steel sheet according to claim 2, wherein the depth with area ratio of pearlite of 0 to 20% is 30 to 100 µm.
The steel sheet according to any one of claims 1 to 3, wherein the area ratio of pearlite having a circle equivalent diameter of 5 µm or more at the depth with area ratio of pearlite of 0 to 20% is 0 to 15%.
A hot-stamp formed body having a chemical composition comprising, by mass%,
C: 0.27 to 0.60%,

Si: 0.001 to 3.00%,

Mn: 0.30 to 3.00%,

Al: 0.0002 to 2.000%,

P: 0.1000% or less,

S: 0.1000% or less,

N: 0.0100% or less,

B: 0 to 0.0100%,

Ti: 0 to 0.1500%,

Nb: 0 to 0.150%,

V: 0 to 0.150%,

Mo: 0 to 1.00%,

Cr: 0 to 1.0%,

Cu: 0 to 1.000%,

Ni: 0 to 1.00%,

W: 0 to 1.000%,

Hf: 0 to 0.050%,

Mg: 0 to 0.050%,

Zr: 0 to 0.050%,

Ca: 0 to 0.0100%,

REM: 0 to 0.3000%,

Ir: 0 to 1.000%, and

balance: Fe and impurities, wherein

a mean C concentration from a surface down to 20 µm in a thickness direction is 0.20 mass%, and

the hot-stamp formed body comprises, by area ratio, martensite at 90% or more.
The hot-stamp formed body according to claim 5, wherein the mean C concentration is 0.10 mass% or less.
The hot-stamp formed body according to claim 5 or 6, wherein the mean C concentration is 0.05 mass% or less.