Method for manufacturing high strength galvanized steel sheet with excellent formability

Patent metadata

Field	Value
Publication number	US-9028626-B2
Application number	US-201313849734-A
Country	US
Kind code	B2
Filing date	Mar 25, 2013
Priority date	Jan 31, 2008
Publication date	May 12, 2015
Grant date	May 12, 2015

Abstract

A method of manufacturing a high-strength galvanized steel sheet includes hot-rolling a slab to form a steel sheet; during continuous annealing, heating the steel sheet to a temperature of 750° C. to 900° C. at an average heating rate of at least 10° C./s at a temperature of 500° C. to an A 1 transformation point; holding that temperature for at least 10 seconds; cooling the steel sheet from 750° C. to a temperature of (Ms point—100° C.) to (Ms point—200° C.) at an average cooling rate of at least 10° C./s; reheating the steel sheet to a temperature of 350° C. to 600° C.; holding that temperature for 10 to 600 seconds; and galvanizing the steel sheet.

First claim

What is claimed is:

1. A method of manufacturing a high-strength galvanized steel sheet comprising:
hot-rolling a slab comprising on the basis of mass percent, C: 0.05% to 0.3%, Si: 0.01% to 2.5%, Mn: 0.5% to 3.5%, P: 0.003% to 0.100% or less, S: 0.02% or less, and Al: 0.010% to 1.5%, the total of Si and Al being 0.5% to 2.5%, the remainder being iron and incidental impurities, to form a steel sheet;
during continuous annealing, heating the steel sheet to a temperature of 750° C. to 900° C. at an average heating rate of at least 10° C./s at a temperature of 500° C. to an A 1 transformation point;
holding that temperature for at least 10 seconds;
cooling the steel sheet from 750° C. to a temperature of (Ms point—100° C.) to (Ms point—200° C.) at an average cooling rate of at least 10° C./s;
reheating the steel sheet to a temperature of 350° C. to 600° C.;
holding that temperature for 10 to 600 seconds to form a microstructure in the steel sheet that includes 20% or more of ferrite phase, 10% or less of martensite phase, and 10% to 60% of tempered martensite phase, on the basis of area percent, and 3% to 10% of retained austenite phase on the basis of volume percent, and the retained austenite phase has an average grain size of 2.0 μm or less; and
galvanizing the steel sheet.

2. A method of manufacturing a high-strength galvanized steel sheet comprising:
hot-rolling and cold-rolling a slab comprising on the basis of mass percent, C: 0.05% to 0.3%, Si: 0.01% to 2.5%, Mn: 0.5% to 3.5%, P: 0.003% to 0.100% or less, S: 0.02% or less, and Al: 0.010% to 1.5%, the total of Si and Al being 0.5% to 2.5%, the remainder being iron and incidental impurities, to form a steel sheet;
during continuous annealing, heating the steel sheet to a temperature of 750° C. to 900° C. at an average heating rate of at least 10° C./s at a temperature of 500° C. to an A 1 transformation point;
holding that temperature for at least 10 seconds;
cooling the steel sheet from 750° C. to a temperature of (Ms point—100° C.) to (Ms point—200° C.) at an average cooling rate of at least 10° C./s:
reheating the steel sheet to a temperature of 350° C. to 600° C.;
holding that temperature for 10 to 600 seconds to form a microstructure that includes 20% or more of ferrite phase, 10% or less of martensite phase, and 10% to 60% of tempered martensite phase, on the basis of area percent, and 3% to 10% of retained austenite phase on the basis of volume percent, and the retained austenite phase has an average grain size of 2.0 μm or less; and
galvanizing the steel sheet.

3. The method according to claim 1 , wherein the holding time after reheating to 350° C. to 600° C. ranges from t to 600 seconds as determined by the following formula (1):
t ( s )=2.5×10 −5 /Exp(−80400/8.31/( T+ 273))  (1)

wherein T denotes the reheating temperature (° C.).

4. The method according to claim 1 , wherein the galvanizing is followed by alloying.

5. The method according to claim 2 , wherein the holding time after reheating to 350° C. to 600° C. ranges from t to 600 seconds as determined by the following formula (1):
t ( s )=2.5×10 −5 /Exp(−80400/8.31/( T+ 273))  (1)

wherein T denotes the reheating temperature (° C.).

6. The method according to claim 2 , wherein the galvanizing is followed by alloying.

Assignees

• Jfe Steel Corp

Inventors

• Nakagaito Tatsuya
• Matsuoka Saiji
• Kaneko Shinjiro
• Kawasaki Yoshiyasu
• Suzuki Yoshitsugu

Classifications

• C23C2/02

  Pretreatment of the material to be coated, e.g. for coating on selected surface areas (C23C2/30 takes precedence) · CPC title

• Y10T428/12799

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

• C21D1/25

  Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten") · CPC title

• C21D2211/008

  Martensite · CPC title

• C22C38/02

  containing silicon · CPC title