High strength galvanized steel sheet and method for manufacturing the same

Patent metadata

Field	Value
Publication number	US-9297060-B2
Application number	US-201314104451-A
Country	US
Kind code	B2
Filing date	Dec 12, 2013
Priority date	Feb 2, 2009
Publication date	Mar 29, 2016
Grant date	Mar 29, 2016

Abstract

The high strength galvanized steel sheet contains C: more than 0.015% and lower than 0.100%, Si: 0.3% or lower, Mn: lower than 1.90%, P: 0.015% or more and 0.05% or lower, S: 0.03% or lower, sol.Al: 0.01% or more and 0.5% or lower, N: 0.005% or lower, Cr: lower than 0.30%, B: 0.0003% or more and 0.005% or lower, and Ti: lower than 0.014% in terms of mass %, and satisfies 2.2≦[Mneq]≦3.1 and 0.42≦8[% P]+150B*≦0.73. The steel microstructure contains ferrite and a second phase, in which the second phase area ratio is 3 to 15%, the ratio of the area ratio of martensite and retained γ to the second phase area ratio is more than 70%, and 50% or more of the area ratio of the second phase exists in the grain boundary triple point.

First claim

What is claimed:

1. A method for manufacturing a galvanized steel sheet, comprising hot rolling and cold rolling a steel slab to form a steel sheet, annealing the steel sheet at an annealing temperature of higher than 740° C. and lower than 840° C. in a continuous galvanizing and galvannealing line (CGL), cooling the steel sheet at an average cooling rate of 2 to 30° C./sec from the annealing temperature to immersion in a galvanizing bath, immersing the steel sheet in the galvanizing bath for galvanization, and cooling the steel sheet to 100° C. or lower at an average cooling rate of 5 to 100° C./sec after galvanization or further performing alloying treatment of plating after galvanization, and cooling the same to 100° C. or lower at an average cooling rate of 5 to 100° C./sec after the alloying treatment;
maintaining 2.2≦[Mneq]≦3.1 and 0.42≦8[% P]+150B*≦0.73;
maintaining ferrite and a second phase as a microstructure of the steel, the second phase area ratio being 3 to 15%, and the ratio of the area ratio of martensite and retained γ to the second phase area ratio being more than 70%, with 50% or more of the area ratio of the second phase existing in the grain boundary triple point; and
maintaining [Mneq]=[% Mn]+1.3[% Cr]+8[% P]+150B* and B*=[% B]+[% Ti]/48×10.8×0.9+[% Al]/27×10.8×0.025, wherein [% Mn], [% Cr], [% P], [% B], [% Ti], and [% Al] represent the content of each of Mn, Cr, P, B, Ti, and sol.Al, respectively, and in the case of B*≧0.0022, B*=0.0022 being established.

2. The method for manufacturing a galvanized steel sheet according to claim 1 , wherein the steel slab is cooled to 640° C. or lower after hot-rolling at an average cooling rate of 20° C./sec or more, and then coiled at 400 to 620° C.

3. A method for manufacturing a galvanized steel sheet, comprising:
hot rolling and cold rolling a steel slab having, as an ingredient composition of the steel, C: more than 0.015% and lower than 0.100%, Si: 0.3% or lower, Mn: lower than 1.90%, P: 0.015% or more and 0.05% or lower, S: 0.03% or lower, sol.Al: 0.01% or more and 0.5% or lower, N: 0.005% or lower, Cr: lower than 0.30%, B: 0.0003% or more and 0.005% or lower, and Ti: lower than 0.014% in terms of mass %, with the balance being iron and inevitable impurities;
maintaining 2.2≦[Mneq]≦3.1 and 0.42≦8[% P]+150B*≦0.73;
maintaining ferrite and a second phase as a microstructure of the steel, the second phase area ratio being 3 to 15%, and the ratio of the area ratio of martensite and retained γ to the second phase area ratio being more than 70%, with 50% or more of the area ratio of the second phase existing in the grain boundary triple point; and
maintaining [Mneq]=[% Mn]+1.3[% Cr]+8[% P]+150B* and B*=[% B]+[% Ti]/48×10.8×0.9+[% Al]/27×10.8×0.025, wherein [% Mn], [% Cr], [% P], [% B], [% Ti], and [% Al] represent the content of each of Mn, Cr, P, B, Ti, and sol.Al, respectively, and in the case of B*≧0.0022, B*=0.0022 being established.

Assignees

• Jfe Steel Corp

Inventors

• Ono Yoshihiko
• Takahashi Kenji
• Okuda Kaneharu
• Taira Shoichiro

Classifications

• Y10T428/12972

  Containing 0.01-1.7% carbon [i.e., steel] · CPC title

• C22C38/14

  containing titanium or zirconium · CPC title

• C22C38/001

  containing N · CPC title

• C23C2/06

  Zinc or cadmium or alloys based thereon · CPC title

• C22C38/22

  with molybdenum or tungsten · CPC title