FIELD

The present invention relates to a coated steel member, coated steel sheet, and methods for manufacturing the same.

BACKGROUND

In the field of steel sheets for automobile use, due to the recent increasing severity of environmental regulations and collision safety standards, applications of steel sheet having high tensile strength are increasing so as to achieve both fuel efficiency and collision safety. However, along with the higher strength, the press formability of steel sheet falls, so it has become difficult to manufacture products with complicated shapes.

Specifically, due to the drop in ductility of steel sheet accompanying higher strength, the problem arises of fracture of the highly worked portions. Further, due to the residual stress after working, the problems arise that springback and wall camber occur and the dimensional precision deteriorates. Therefore, it is not easy to press form steel sheet having a high strength, in particular a tensile strength of 780 MPa or more, into a product having a complicated shape. Note that, if using not press forming, but roll forming, high strength steel sheet is easily worked, but applications have been limited to parts having uniform cross-sections in the longitudinal direction.

Therefore, in recent years, for example, as disclosed in PTLs 1 to 3, hot stamping has been employed as art for press-forming materials which are difficult to shape such as high strength steel sheet. Hot stamping is a hot shaping technology for heating a material before shaping, then shaping the material.

In this art, the material is heated, then shaped, so at the time of shaping, the steel material is soft and has good shapeability. Due to this, even if a high strength steel material, it is possible to precisely form it into a complicated shape. Further, in hot stamping, a press die is used for hardening during shaping, so after the shaping, the steel material has sufficient strength.

For example, according to PTL 1, hot stamping can be used to impart a tensile strength of 1400 MPa or more to a steel material after shaping.

CITATIONS LIST

Patent Literature

[PTL 1] Japanese Unexamined Patent Publication No. 2002-102980

[PTL 2] Japanese Unexamined Patent Publication No. 2012-180594

[PTL 3] Japanese Unexamined Patent Publication No. 2012-1802

[PTL 4] Japanese Unexamined Patent Publication No. 2003-183802

[PTL 5] Japanese Unexamined Patent Publication No. 2004-124208

[PTL 6] Japanese Unexamined Patent Publication No. 2012-62500

[PTL 7] Japanese Unexamined Patent Publication No. 2003-268489

[PTL 8] Japanese Unexamined Patent Publication No. 2017-179589

[PTL 9] Japanese Unexamined Patent Publication No. 2015-113500

[PTL 10] Japanese Unexamined Patent Publication No. 2017-525849

[PTL 11] Japanese Unexamined Patent Publication No. 2011-122207

[PTL 12] Japanese Unexamined Patent Publication No. 2011-246801

[PTL 13] Japanese Unexamined Patent Publication No. 2012-1816

SUMMARY

Technical Problem

At the present time, along with the establishment of challenging fuel efficiency targets in various countries, further higher strength steel materials are being demanded for lightening the weight of car bodies. Specifically, high strength steel materials of over the general strength of 1.5 GPa in hot stamping are considered necessary.

In this regard, if applying high strength steel materials of over a strength of 1 GPa to automobiles, not only the above-mentioned shapeability and toughness after shaping, but also hydrogen embrittlement resistance is demanded. If the hydrogen embrittlement resistance of high strength steel sheet is not sufficient, after an automobile is shipped to the market, the steel will corrode during use by the general user and the hydrogen generated along with the corrosion reaction may cause embrittlement cracking.

In a region over a strength of 1.5 GPa, the sensitivity of a steel material to hydrogen embrittlement rapidly increases, so hydrogen embrittlement cracking is a concern even at a surface coated steel sheet having corrosion resistance. Therefore, to commercially apply high strength steel materials of over 1.5 GPa to car bodies, art is necessary for providing a coated steel member provided with the conventional or better corrosion resistance and excellent in hydrogen embrittlement resistance in a corrosive environment.

Regarding high strength steel materials of over 1.5 GPa, for example, in PTL 2, a press formed article excellent in toughness and having a tensile strength of 1.8 GPa or more which is formed by hot pressing is disclosed. However, the measures against hydrogen embrittlement in a corrosive environment are not sufficient. In use as an automobile member, sometimes greater safety demands are not answered.

Further, in PTL 3, a steel material having an extremely high tensile strength of 2.0 GPa or more and further having excellent toughness and ductility is disclosed. However, the measures against hydrogen embrittlement in a corrosive environment are not sufficient. In use as an automobile member, sometimes greater safety demands are not answered.

Regarding corrosion resistance, for example in PTL 4, high strength Al-plated steel sheet excellent in corrosion resistance after coating is disclosed. However, there is no description regarding hydrogen embrittlement resistance. This is unsuitable for actual use of a high strength material of over 1.5 GPa.

Further, in PTL 5, high strength surface treated steel sheets having Ni, Cu, Cr, and Sn layers excellent in corrosion resistance after coating are disclosed and in PTL 6, Ni, Cr, Cu, and Co plated coated steel materials are disclosed. However, there are no descriptions regarding hydrogen embrittlement resistance. These are unsuitable for actual use of a high strength material of over 1.5 GPa.

Regarding hydrogen embrittlement resistance, for example, in PTLs 7, 8, and 9, hot stamped materials excellent in hydrogen embrittlement resistance in a hydrochloric acid immersion environment are disclosed. However, the hydrogen embrittlement resistance in an air corrosive environment and the hydrogen embrittlement resistance in a solution immersion environment of hydrochloric acid etc. often do not match. These materials are unsuitable for practical use of high strength materials of over 1.5 GPa.

Further, in PTL 10, a hot stamping material with the Ni in the steel concentrated at the surface layer is disclosed and it is described that this has the effect of suppressing hydrogen penetration at the time of heating in the hot stamping process. However, there is no description relating to the hydrogen embrittlement resistance in a corrosive environment at the time of actual use. This is unsuitable for actual use of a high strength material of over 1.5 GPa.

Further, in PTLs 11, 12, and 13, hot stamping materials in which Ni diffuses from an Ni-based plating layer to the surface layer of the steel sheet is disclosed and it is described that this has the effect of suppressing hydrogen penetration in a corrosive environment. However, the hydrogen embrittlement resistance of the steel material is not sufficient, so this is unsuitable for actual use of a high strength steel material of over 1.5 GPa.

The present invention was made so as to solve the above problem and has as its object the provision of a coated steel member and coated steel sheet having high tensile strength and excellent in hydrogen embrittlement resistance in a corrosive environment and methods for manufacturing the same.

Solution to Problem

The present invention has as its gist the following coated steel member, coated steel sheet, and methods for manufacturing the same. Below, steel sheet which is not coated and is used as a material of coated steel sheet will be simply referred to as “steel sheet”.

(1) A coated steel member comprising an Al—Fe-based coating on its surface, the Al—Fe-based coating containing Cu and one or more of Mo, Ni, Mn, and Cr in a total by mass % of 0.12% or more, the contents of Cu, Mo, Ni, Mn, and Cr satisfying, by mass %, Cu+0.01×63.5(Mo/95.9+Ni/58.7+Mn/54.9+Cr/52.0)≥0.12%.

(2) The coated steel member of (1) wherein a thickness of the Al—Fe-based coating is 10 to 100 μm and the chemical composition of the Al—Fe-based coating satisfies, by mass %, an average value of Al content in a thickness direction: 20.0% or more, an average value of Fe content in the thickness direction: 50.0% or more, a minimum value of Cu content in the thickness direction: 0.06% or more, and a ratio of a maximum value and minimum value of Cu content in the thickness direction: 1.4 or more.

(3) The coated steel member of (1) or (2) wherein a chemical composition of the part of the coated steel member other than the Al—Fe-based coating comprises, by mass %, C: 0.25 to 0.60%, Si: 0.25 to 2.00%, Mn: 0.30 to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010 to 0.100%, B: 0.0005 to 0.0100%, Mo: 0.10 to 1.00%, Cu: 0.01 to 1.00%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, V: 0 to 1.00%, Ca: 0 to 0.010%, Al: 0 to 1.00%, Nb: 0 to 0.10%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.00%, REMs: 0 to 0.30%, and balance: Fe and impurities.

(4) The coated steel member of any of (1) to (3) wherein the Al—Fe-based coating contains, by mass %, Si in 1 to 20%.

(5) A coated steel sheet having a layer where Cu is concentrated at a surface of a steel sheet, having a degree of Cu surface concentration at the layer where Cu is concentrated of 1.2 or more, further having an Al-based coating on the layer where Cu is concentrated, and having an average crystal grain size of the steel sheet of 30 μm or less. Here, the “degree of Cu surface concentration” shows the ratio of (maximum content of Cu in range from surface of steel sheet to depth of 0 to 30 μm)/(average content of Cu from surface of steel sheet to depth of 200 μm), while the “surface of steel sheet” means the position of a depth where the Fe content becomes 90% when performing GDS from the surface of the coated steel sheet in the thickness direction.

(6) A coated steel sheet comprised of a steel sheet, an intermediate layer positioned on a surface of the steel sheet, and an Al-based coating positioned on a surface of the intermediate layer, the intermediate layer containing Cu, the intermediate layer containing Cu and one or more of Mo, Ni, Mn, and Cr in a total by mass % of 30% or more.

(7) A coated steel sheet comprised of a steel sheet and an Al-based coating positioned on a surface of the steel sheet, the Al-based coating containing Cu, the Al-based coating containing Cu and one or more of Mo, Ni, Mn, and Cr in a total by mass % of 1.0% or more.

(8) The coated steel sheet of any of (5) to (7) wherein a chemical composition of the steel sheet comprises, by mass %, C: 0.25 to 0.60%, Si: 0.25 to 2.00%, Mn: 0.30 to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010 to 0.100%, B: 0.0005 to 0.0100%, Mo: 0.10 to 1.00%, Cu: 0.01 to 1.00%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, V: 0 to 1.00%, Ca: 0 to 0.010%, Al: 0 to 1.00%, Nb: 0 to 0.10%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.00%, REMs: 0 to 0.30%, and balance: Fe and impurities.

(9) A method for manufacturing the coated steel sheet of (5), the method comprising the steps of: heating a slab at 1100 to 1350° C., hot rolling the heated slab to obtain hot rolled steel sheet under conditions where, when defining a time t1(hr) from an end of rough rolling to a start of finish rolling and an average temperature T1(° C.) of a rough bar from the end of rough rolling to the start of finish rolling, (T1+273)×(log t1+20)≥20000 and a finish rolling end temperature becomes an Ar3point to 1000° C., cooling the above hot rolled steel sheet by an average cooling rate of 10° C./s, coiling the steel sheet after cooling at 700° C. or less, and pickling the steel sheet after coiling.

(10) The method of (3) wherein in the step of pickling hydrochloric acid or sulfuric acid is used, a pickling temperature is 80 to 90° C., and an acid concentration a (%) and pickling time “t” (s) satisfy 6≤α<14, 0<t≤420−30×α.

(11) A method for manufacturing the coated steel member of any of (1) to (4), the method comprising the steps of: heating the coated steel sheet of any of (5) to (8) under conditions where, when defining a peak temperature as T2(° C.) and a time from when a temperature of the coated steel sheet reaches a temperature lower by 10° C. from T2(° C.) as t2(hr), (T2+273−10)×(log t2+20)≥19000, an Ac3point≤T2≤(Ac3point+300)° C., and an average rate of temperature rise 5 to 1000° C./s, and cooling the heated coated steel sheet down to an Ms point by an average cooling rate made an upper critical cooling rate or more, then cooling from the Ms point down to 100° C. or less by an average cooling rate of 5° C./s or more.

(12) The method of (11), wherein the coated steel sheet is hot shaped during cooling down to the Ms point.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a coated steel member and coated steel sheet having high tensile strength and excellent in hydrogen embrittlement resistance in a corrosive environment and methods for manufacturing the same.

