High-strength plated steel sheet having excellent plating properties, workability, and delayed fracture resistance, and method for producing same

Patent metadata

Field	Value
Publication number	US-2017369965-A1
Application number	US-201615541918-A
Country	US
Kind code	A1
Filing date	Jan 5, 2016
Priority date	Jan 9, 2015
Publication date	Dec 28, 2017
Grant date	—

Abstract

A high-strength plated steel sheet has a plated layer on the surface of a base steel sheet and contains predetermined steel components. The steel sheet includes, in the order from the interface of the base steel sheet and the plated layer towards the base steel sheet: a soft layer having a Vickers hardness that is 90% or less of the Vickers hardness at a portion t/4 of the base steel sheet, where t is a sheet thickness of the base steel sheet; and a hard layer containing martensite and bainite, and ferrite in predetermined ranges. The average depth D of the soft layer is 20 μm or greater, and the average depth d of an internal oxide layer is 4 μm or greater and smaller than D.

First claim

1 . A high-strength plated steel sheet having a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of a base steel sheet, wherein
(1) the base steel sheet comprises, in mass %,
C: 0.08 to 0.30%;
Si: 0.25 to 3%;
Mn: 1.5 to 4%;
P: more than 0% to 0.1% or less;
S: more than 0% to 0.05% or less;
Al: 0.005 to 1%; and
N: more than 0% to 0.01% or less,
(2) the high-strength plated steel sheet sequentially has, from an interface of the base steel sheet and the hot-dip galvanized layer or galvannealed layer, towards the base steel sheet:
an internal oxide layer comprising at least one oxide selected from the group consisting of Si and Mn;
a soft layer comprising the internal oxide layer and having a Vickers hardness of 90% or less of a Vickers hardness at a portion t/4 of the base steel sheet, where t is a sheet thickness of the base steel sheet; and
a hard layer comprising martensite and bainite: 20 area % or more to less than 60 area %, and polygonal ferrite: more than 40 area % to 80 area % or less, wherein
the high-strength plated steel sheet satisfies:
an average depth D of the soft layer being 20 μm or greater; and
an average depth d of the internal oxide layer being 4 μm or greater and smaller than the D; and
a tensile strength being 980 MPa or higher.

2 . The high-strength plated steel sheet according to claim 1 , wherein the base steel sheet further comprises, in mass %, at least one of (a) to (c) below:
(a) at least one selected from the group consisting of Cr: more than 0% to 1% or less, Mo: more than 0% to 1% or less and B: more than 0% to 0.01% or less; (b) at least one selected from the group consisting of Ti: more than 0% to 0.2% or less, Nb: more than 0% to 0.2% or less and V: more than 0% to 0.2% or less; and (c) at least one selected from the group consisting of Cu: more than 0% to 1% or less and Ni: more than 0% to 1% or less.

3 . The high-strength plated steel sheet according to claim 1 , wherein the average depth d of the internal oxide layer and the average depth D of the soft layer satisfy the relationship D>2d.

4 . The high-strength plated steel sheet according to claim 2 , wherein the average depth d of the internal oxide layer and the average depth D of the soft layer satisfy the relationship D>2d.

5 . A method for producing a high-strength plated steel sheet according to claim 1 , the method comprising, in the following order:
hot rolling by coiling, at a temperature of 600° C. or higher, a steel sheet having the steel components of the said base steel sheet; pickling and cold rolling the steel sheet such that there remain the internal oxide layer with the average depth d of 4 μm or more; oxidizing the steel sheet at an air ratio in a range of 0.9 to 1.4 in an oxidation zone; soaking the steel sheet by keeping within a range of (Ac 1 point+30° C.) to Ac 3 point, in a reduction zone; and after the soaking, cooling the steel sheet at an average cooling rate of 5° C./sec or higher over a range down to a cooling stop temperature.

6 . A method for producing a high-strength plated steel sheet according to claim 1 , the method comprising, in the following order:
hot rolling by coiling, at a temperature of 500° C. or higher, a steel sheet having the steel components of the said base steel sheet; keeping the steel sheet in a temperature region of 500° C. or higher for 80 minutes or longer; pickling and cold rolling the steel sheet such that there remain the internal oxide layer with the average depth d of 4μm or more; oxidizing the steel sheet at an air ratio in the range of 0.9 to 1.4, in an oxidation zone; soaking the steel sheet by keeping within a range of (Ac 1 point+30° C.) to Ac 3 point, in a reduction zone; and after the soaking, cooling the steel sheet at an average cooling rate of 5° C./sec or higher over a range down to a cooling stop temperature.

Assignees

• Kobe Steel Ltd

Inventors

• Ikeda Muneaki
• Futamura Yuichi
• Nakaya Michiharu

Classifications

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C22C38/18

  containing chromium · CPC title

• C21D8/0226

  Hot rolling · CPC title

• C23C2/06

  Zinc or cadmium or alloys based thereon · CPC title

• C21D8/0278

  involving a particular surface treatment  (C21D8/0294 takes precedence) · CPC title