High-strength hot-dip galvanized steel sheet and high-strength alloyed hot-dip galvanized steel sheet having excellent plating adhesion, formability, and hole expandability with tensile strength of 980 MPa or more and manufacturing method therefor

Patent metadata

Field	Value
Publication number	US-9783878-B2
Application number	US-201214347289-A
Country	US
Kind code	B2
Filing date	Sep 28, 2012
Priority date	Sep 30, 2011
Publication date	Oct 10, 2017
Grant date	Oct 10, 2017

Abstract

Provided is a high-strength hot-dip galvanized steel sheet having excellent plating adhesion, formability, and hole expandability with an ultimate tensile strength of 980 MPa or more, the hot-dip galvanized steel sheet comprising a hot-dip galvanized layer formed on a surface of a base steel sheet. The base steel sheet contains, by mass %, C: 0.05% to 0.4 %; Si: 0.01% to 3.0%; Mn: 0.1% to 3.0%; Al: 0.01 to 2.0%; in which Si+Al >0.5%, P: limited to 0.04% or less; S: limited to 0.05% or less; N: limited to 0.01% or less; and a balance including Fe and inevitable impurities, a microstructure of the base steel sheet contains 40% or more by total volume fraction of martensite and bainite, 8% or more by volume fraction of residual austenite, and a balance of the microstructure being ferrite or ferrite and 10% or less by volume fraction of pearlite. The martensite contains 10% or more by total volume fraction of two or more kinds of three kinds of martensites (1), (2), and (3), and the hot-dip galvanized layer contains less than 7 mass % of Fe.

First claim

The invention claimed is:

1. A high-strength alloyed hot-dip galvanized steel sheet having excellent plating adhesion, formability, and hole expandability with an ultimate tensile strength of 980 MPa or more, the alloyed hot-dip galvanized steel sheet comprising an alloyed hot-dip galvanized layer formed on a surface of a base steel sheet,
wherein the base steel sheet contains: by mass %,
C: 0.05% to 0.4%;
Si: 0.01% to 3.0%;
Mn: 0.1% to 3.0%;
Al: 0.01 to 2.0%; in which Si+Al>0.5%
P: limited to 0.04% or less;
S: limited to 0.05% or less;
N: limited to 0.01% or less; and
a balance including Fe and inevitable impurities,
a microstructure of the base steel sheet contains 40% or more by total volume fraction of martensite and bainite, 8% or more by volume fraction of residual austenite, and a balance of the microstructure being ferrite or ferrite and 10% or less by volume fraction of pearlite,
the martensite contains 10% or more by total volume fraction of two or more kinds of martensites (1), (2), and (3) below, and
the alloyed hot-dip galvanized layer contains 7 to 15 mass % of Fe,
the martensite (1): C concentration (when there is a cementite precipitation, also including C in cementite); carbon concentration in the martensite (CM1) is less than 0.8 mass %, and nano-indentation test hardness Hit1 satisfies Expression 1:
Hit1/{−982.1×(CM1) 2 +1676×CM1+189}≦0.50  Expression 1

the martensite (2): C concentration (when there is a cementite precipitation, also including C in cementite); carbon concentration in the martensite (CM2) is 0.8 mass% or more, and nano-indentation test hardness Hit2 satisfies Expression 2:
Hit2/{−982.1×(CM2) 2 +1676×CM2+189}≦0.50  Expression 2

the martensite (3): C concentration (when there is a cementite precipitation, also including C in cementite); carbon concentration in the martensite (CM3) is 0.8 mass % or more, and nano-indentation test hardness Hit3 satisfies Expression 3:
0.5<Hit3/{−982.1×(CM3) 2 +1676×CM3+189}≦0.80  Expression 3.

2. The high-strength alloyed hot-dip galvanized steel sheet having the excellent plating adhesion, formability, and hole expandability with the ultimate tensile strength of 980 MPa or more according to claim 1 , wherein the base steel sheet further contains one or more of: by mass %,
Cr: 0.05 to 1.0%;
Mo: 0.05 to 1.0%;
Ni: 0.05 to 1.0%; and
Cu: 0.05 to 1.0%.

3. The high-strength alloyed hot-dip galvanized steel sheet having the excellent plating adhesion, formability, and hole expandability with the ultimate tensile strength of 980 MPa or more according to claim 1 , wherein the base steel sheet further contains one or more of: by mass %,
Nb: 0.005 to 0.3%;
Ti: 0.005 to 0.3%; and
V: 0.01 to 0.5%.

4. The high-strength alloyed hot-dip galvanized steel sheet having the excellent plating adhesion, formability, and hole expandability with the ultimate tensile strength of 980 MPa or more according to claim 1 , wherein the base steel sheet further contains B: 0.0001 to 0.1% by mass %.

5. The high-strength alloyed hot-dip galvanized steel sheet having the excellent plating adhesion, formability, and hole expandability with the ultimate tensile strength of 980 MPa or more according to claim 1 , wherein the base steel sheet further contains one or more of: by mass %,
Ca: 0.0005 to 0.01%;
Mg: 0.0005 to 0.01%; and
REM: 0.0005 to 0.01%.

6. A manufacturing method of a high-strength alloyed hot-dip galvanized steel sheet having excellent plating adhesion, formability, and hole expandability with an ultimate tensile strength of 980 MPa or more, the manufacturing method comprising:
with respect to a steel billet containing: by mass %,
C: 0.05% to 0.4%;
Si: 0.01% to 3.0%;
Mn: 0.1% to 3.0%;
Al: 0.01 to 2.0%; in which Si+Al>0.5%
P: limited to 0.04% or less;
S: limited to 0.05% or less;
N: limited to 0.01% or less; and
a balance including Fe and inevitable impurities,
heating to 1200° C. or higher and performing hot rolling at an Ar3 transformation temperature or higher;
performing cold rolling on a base steel sheet after the hot rolling at a reduction ratio of 40 to 70%;
annealing the base steel sheet after the cold rolling at 730 to 900° C.;
cooling the base steel sheet after the annealing to a temperature of 650 to 750° C. at an average cooling rate of 0.1 to 200° C/second, and cooling the base steel sheet to 450° C. or lower from the temperature of 650 to 750° C. at an average cooling rate of 20° C/second or faster;
holding the base steel sheet cooled to the 450° C. or lower in a range of 350 to 450° C. for 120 seconds or longer;
cooling the base steel sheet held in the range of 350 to 450° C. to 70° C. or lower at an average cooling rate of 5° C./second or faster;
rolling the base steel sheet cooled to the room temperature at an elongation percentage of 0.2 to 2%;
heating the rolled base steel sheet to “temperature of hot-dip galvanizing bath −40”° C. to “temperature of hot-dip galvanizing bath +50”° C. at an average temperature rising rate of 10° C./second or faster;
dipping and hot-dip galvanizing the base steel sheet heated to the “temperature of hot-dip galvanizing bath −40”° C. to “temperature of hot-dip galvanizing bath +50”° C. into a hot-dip galvanizing bath and performing alloying-heating treatment at “temperature of hot-dip galvanizing bath −40”° C. or higher and 560° C. or lower within 40 seconds; and
cooling the alloyed hot-dip galvanized steel sheet, which is subjected to the alloying-heating treatment, to 70° C. or lower at an average cooling rate of 5° C/second or faster.

7. The manufacturing method of the high-strength alloyed hot-dip galvanized steel sheet having the excellent plating adhesion, formability, and hole expandability with the ultimate tensile strength of 980 MPa or more according to claim 6 , wherein a hot-dip galvanizing bath flows at a flow rate of 10 m/min or more and 50 m/min or less at the time of the hot-dip galvanizing.

8. The manufacturing method of the high-strength alloyed hot-dip galvanized steel sheet having the excellent plating adhesion, formability, and hole expandability with the ultimate tensile strength of 980 MPa or more according to claim 6 , wherein before being heated to the “temperature of hot-dip galvanizing bath −40”° C. to “temperature of hot-dip galvanizing bath +50”° C., the base steel sheet is subjected to pickling, and then a surface of the base steel sheet is polished and removed to a depth of 0.1 μm or more and is pre-plated with 0.2 to 2 g/m 2 of Ni.

Assignees

• Nippon Steel & Sumitomo Metal Corp

Inventors

• Wakabayashi Chisato
• Azuma Masafumi
• Fujita Nobuhiro
• Sano Kohichi

Classifications

• C21D2211/008

  Martensite · CPC title

• Y10T428/12799

  Next to Fe-base component [e.g., galvanized] · CPC title

• C22C18/00

  Alloys based on zinc · CPC title

• C21D8/0473

  Final recrystallisation annealing · CPC title

• C23C2/06 Primary

  Zinc or cadmium or alloys based thereon · CPC title