High strength steel sheet and method of manufacturing the same

Patent metadata

Field	Value
Publication number	US-9816153-B2
Application number	US-201114345052-A
Country	US
Kind code	B2
Filing date	Sep 28, 2011
Priority date	Sep 28, 2011
Publication date	Nov 14, 2017
Grant date	Nov 14, 2017

Abstract

A high strength steel sheet is formed of steel having the composition containing by mass % over 0.015% and less than 0.100% C, less than 0.50% Si, over 1.0% and less than 2.0% Mn, 0.05% or less P, 0.03% or less S, 0.01% or more and 0.3% or less sol. Al, 0.005% or less N, less than 0.35% Cr, 0.0010% or more and 0.0050% or less B, less than 0.15% Mo, less than 0.030% Ti, and iron and unavoidable impurities as a balance, wherein the steel satisfies 2.1≦[Mneq]≦3.1, the microstructure of the steel includes a ferrite and a second phase, a volume fraction of the second phase is set to 2.0 to 12.0%, a total ratio of a volume fraction of martensite and a volume fraction of retained γ to the volume fraction of second phase is 60% or more, and the number of carbides which are present within ferrite particles, have an aspect ratio of 3.0 or less and have a diameter of 0.25 to 0.90 μm is set to 10000 pieces/mm 2 or less.

First claim

The invention claimed is:

1. A high strength steel sheet comprising:
steel having the composition containing by mass % over 0.015% and less than 0.100% C, less than 0.50% Si, over 1.0% and less than 2.0% Mn, 0.05% or less P, 0.03% or less S, 0.01% or more and 0.3% or less sol. Al, 0.005% or less N, less than 0.35% Cr, 0.0010% or more and 0.0050% or less B, less than 0.15% Mo, less than 0.030% Ti, and iron and unavoidable impurities as a balance,
wherein the steel satisfies 2.1≦[Mneq]≦3.1,
wherein the microstructure of the steel includes:
a ferrite and a second phase,
a volume fraction of the second phase is set to 2.0 to 12.0%,
a ratio of total volume fraction of martensite and retained γ to the volume fraction of second phase is 60% or more, and
the number of carbide particles which are present within ferrite particles, have an aspect ratio of 3.0 or less and have a diameter of 0.25 to 0.90 μm is set to 10000 pieces/mm 2 or less,

wherein:
[Mneq]=[% Mn]+1.3[% Cr]+3.3[% Mo]+8[% P]+150B*,
B*=[% B]+[% Ti]/48×10.8×0.9+[% Al]/27×10.8×0.025,

[% Mn], [% Cr], [% Mo], [% P], [% B], [% Ti] and [% Al] indicate contents of Mn, Cr, Mo, P, B, Ti and sol.Al respectively, and
B*=0.0022 when B* satisfies B*≧0.0022.

2. The high strength steel sheet according to claim 1 , wherein the steel further contains by mass % at least one component selected from a group consisting of 0.2% or less Sn, 0.2% or less Sb, 0.5% or less Cu, 0.5% or less Ni, 0.01% or less Ca, 0.01% or less Ce, 0.01% or less La and 0.01% or less Mg.

3. The high strength steel sheet according to claim 1 , wherein the steel further contains by mass % at least one component selected from a group consisting of less than 0.030% Nb, 0.2% or less V, 0.15% or less W and 0.1% or less Zr.

4. The high strength steel sheet according to claim 3 , wherein the steel further contains by mass % at least one component selected from a group consisting of 0.2% or less Sn, 0.2% or less Sb, 0.5% or less Cu, 0.5% or less Ni, 0.01% or less Ca, 0.01% or less Ce, 0.01% or less La and 0.01% or less Mg.

5. A method for manufacturing a high strength steel sheet, wherein in a step of hot-rolling a steel slab having the composition described in claim 1 ,
a coiling temperature CT is controlled within a range expressed by a formula (1) corresponding to sol. B and,
after performing cold rolling at a cold rolling reduction of 50 to 85%, and in a continuous galvanizing and galvannealing line (CGL) or continuous annealing line (CAL), annealing is performed by holding a cold-rolled sheet at an annealing temperature of 740° C. or above and 830° C. or below for 25 seconds or more, wherein
CT(° C.)≦670−50000×sol.B  formula (1)

sol.B=[% B]−{[% N]/14−[% Ti]/48×0.8−[% Al]/27×0.0005×(CT−560)}×10.8  formula (A),

[% B], [% N], [% Ti], and [% Al] indicate contents of B, N, Ti and sol. Al respectively, and CT indicates a coiling temperature (° C.),

when CT−560≦0, CT−560 is assumed as 0, and
the calculation is made by assuming sol. B as 0 when sol. B≦0.

6. A method for manufacturing a high strength steel sheet, wherein in a step of hot-rolling a steel slab having the composition described in claim 2 ,
a coiling temperature CT is controlled within a range expressed by a formula (1) corresponding to sol. B and,
after performing cold rolling at a cold rolling reduction of 50 to 85%, and in a continuous galvanizing and galvannealing line (CGL) or continuous annealing line (CAL), annealing is performed by holding a cold-rolled sheet at an annealing temperature of 740° C. or above and 830° C. or below for 25 seconds or more,

wherein
CT(° C.)≦670−50000×sol.B  formula (1)

sol.B=[% B]−{[% N]/14−[% Ti]/48×0.8−[% Al]/27×0.0005×(CT−560)}×10.8  formula (A),

[% B], [% N], [% Ti], and [% Al] indicate contents of B, N, Ti and sol. Al respectively, and CT indicates a coiling temperature (° C.),
when CT−560≦0, CT−560 is assumed as 0, and
the calculation is made by assuming sol. B as 0 when sol. B≦0.

7. A method for manufacturing a high strength steel sheet, wherein in a step of hot-rolling a steel slab having the composition described in claim 3 ,
a coiling temperature CT is controlled within a range expressed by a formula (1) corresponding to sol. B and,
after performing cold rolling at a cold rolling reduction of 50 to 85%, and in a continuous galvanizing and galvannealing line (CGL) or continuous annealing line (CAL), annealing is performed by holding a cold-rolled sheet at an annealing temperature of 740° C. or above and 830° C. or below for 25 seconds or more,

wherein
CT(° C.)≦670−50000×sol.B  formula (1)

sol.B=[% B]−{[% N]/14−[% Ti]/48×0.8−[% Al]/27×0.0005×(CT−560)}×10.8  formula (A),

[% B], [% N], [% Ti], and [% Al] indicate contents of B, N, Ti and sol. Al respectively, and CT indicates a coiling temperature (° C.),
when CT−560≦0, CT−560 is assumed as 0, and
the calculation is made by assuming sol. B as 0 when sol. B≦0.

8. A method for manufacturing a high strength steel sheet, wherein in a step of hot-rolling a steel slab having the composition described in claim 4 ,
a coiling temperature CT is controlled within a range expressed by a formula (1) corresponding to sol. B and,
after performing cold rolling at a cold rolling reduction of 50 to 85%, and in a continuous galvanizing and galvannealing line (CGL) or continuous annealing line (CAL), annealing is performed by holding a cold-rolled sheet at an annealing temperature of 740° C. or above and 830° C. or below for 25 seconds or more,

wherein
CT(° C.)≦670−50000×sol.B  formula (1)

sol.B=[% B]−{[% N]/14−[% Ti]/48×0.8−[% Al]/27×0.0005×(CT−560)}×10.8  formula (A),

[% B], [% N], [% Ti], and [% Al] indicate contents of B, N, Ti and sol. Al respectively, and CT indicates a coiling temperature (° C.),
when CT−560≦0, CT−560 is assumed as 0, and
the calculation is made by assuming sol. B as 0 when sol. B≦0.

Assignees

• Ono Yoshihiko
• Takahashi Kenji
• Okuda Kaneharu
• Fushiwaki Yusuke
• Sakurai Michitaka
• Jfe Steel Corp

Inventors

• Ono Yoshihiko
• Takahashi Kenji
• Okuda Kaneharu
• Fushiwaki Yusuke
• Sakurai Michitaka

Classifications

• Y02P10/20

  Recycling · CPC title

• B32B15/013

  one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium · CPC title

• C21D8/0263 Primary

  following hot rolling · CPC title

• C21D8/0284

  Application of a separating or insulating coating · CPC title

• C22C38/32

  with boron · CPC title