High-strength galvanized steel sheet and method for producing the same

Patent metadata

Field	Value
Publication number	US-10450642-B2
Application number	US-201515543062-A
Country	US
Kind code	B2
Filing date	Nov 24, 2015
Priority date	Jan 15, 2015
Publication date	Oct 22, 2019
Grant date	Oct 22, 2019

Abstract

A high-strength galvanized steel sheet that includes a chemical composition containing, by mass %, C: 0.15% or more and 0.25% or less, Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less, P: 0.100% or less, S: 0.02% or less, Al: 0.01% or more and 2.5% or less, and Fe and inevitable impurities. The steel sheet having a microstructure containing, by an area percentage basis, a tempered martensite phase: 30% or more and 73% or less, a ferrite phase: 25% or more and 68% or less, a retained austenite phase: 2% or more and 20% or less, and other phases: 10% or less (including 0%), the other phases containing a martensite phase: 3% or less (including 0%) and a bainitic ferrite phase: less than 5% (including 0%).

First claim

The invention claimed is:

1. A galvanized steel sheet, the steel sheet having a chemical composition comprising:
C: 0.15% or more and 0.25% or less, by mass %;
Si: 0.50% or more and 2.5% or less, by mass %;
Mn: 2.3% or more and 4.0% or less, by mass %;
P: 0.100% or less, by mass %;
S: 0.02% or less, by mass %;
Al: 0.01% or more and 2.5% or less, by mass %; and
Fe and inevitable impurities,
wherein the steel sheet has a microstructure containing, in terms of area fraction:
a tempered martensite phase: 30% or more and 73% or less, where the tempered martensite phase has an average grain size of 8 μm or less,
a ferrite phase: 25% or more and 68% or less,
a retained austenite phase: 2% or more and 20% or less, where the retained austenite phase has a C content of less than 0.7% by mass, and
other phases: 10% or less (including 0%),
of which other phases containing: (i) a martensite phase: 3% or less (including 0%), and (ii) a bainitic ferrite phase: less than 5% (including 0%).

2. The galvanized steel sheet according to claim 1 , wherein the chemical composition further comprises at least one group selected from the groups consisting of A, B, and C:
group A—at least one element selected from:
Cr: 0.01% or more and 2.0% or less, by mass %,
Ni: 0.01% or more and 2.0% or less, by mass %, and
Cu: 0.01% or more and 2.0% or less, by mass %,

group B—B: 0.0002% or more and 0.0050% or less, by mass %, and
group C—at least one element selected from:
Ca: 0.001% or more and 0.005% or less, by mass %, and
REM: 0.001% or more and 0.005% or less, by mass %.

3. The galvanized steel sheet according to claim 2 , wherein the galvanized steel sheet includes a galvannealed steel sheet.

4. The galvanized steel sheet according to claim 3 , wherein the galvanized steel sheet has a tensile strength of 1180 MPa or more.

5. The galvanized steel sheet according to claim 2 , wherein the galvanized steel sheet has a tensile strength of 1180 MPa or more.

6. The galvanized steel sheet according to claim 1 , wherein the galvanized steel sheet includes a galvannealed steel sheet.

7. The galvanized steel sheet according to claim 6 , wherein the galvanized steel sheet has a tensile strength of 1180 MPa or more.

8. The galvanized steel sheet according to claim 1 , wherein the galvanized steel sheet has a tensile strength of 1180 MPa or more.

9. The galvanized steel sheet according to claim 1 , wherein the area fraction of the ferrite phase is 33% or more and 55% or less.

10. A method for producing the galvanized steel sheet according to claim 1 , the method comprising:
a hot-rolling step including: (i) heating a slab to a temperature of 1100° C. or higher, (ii) hot-rolling the slab at a finish rolling temperature of 800° C. or higher to produce a hot-rolled steel sheet, and (iii) coiling the hot-rolled steel sheet at a coiling temperature of 550° C. or lower,
the slab having a chemical composition comprising:
C: 0.15% or more and 0.25% or less, by mass %,
Si: 0.50% or more and 2.5% or less, by mass %,
Mn: 2.3% or more and 4.0% or less, by mass %,
P: 0.100% or less, by mass %,
S: 0.02% or less, by mass %,
Al: 0.01% or more and 2.5% or less, by mass %, and
Fe and inevitable impurities;

a cold-rolling step of cold-rolling the hot-rolled steel sheet at a cumulative rolling reduction of more than 20% to produce a cold-rolled steel sheet;
a pre-annealing step including: (i) heating the cold-rolled steel sheet to an annealing temperature of 800° C. or higher and 1000° C. or lower, (ii) holding the cold-rolled steel sheet at the annealing temperature for 10 s or more, and (iii) cooling the cold-rolled steel sheet to a cooling stop temperature of 550° C. or lower at an average cooling rate of 5° C./s or more;
a final-annealing step including: (i) heating the cold-rolled steel sheet to an annealing temperature of 680° C. or higher and 790° C. or lower at an average heating rate of 10° C./s or less, (ii) holding the cold-rolled steel sheet at the annealing temperature for 30 s or more and 1000 s or less, (iii) cooling the cold-rolled steel sheet to a cooling stop temperature of 460° C. or higher and 550° C. or lower at an average cooling rate of 1.0° C./s or more, and (iv) holding the cold-rolled steel sheet at the cooling stop temperature for 500 s or less;
a galvanization step of galvanizing the cold-rolled steel sheet subjected to the final-annealing step and cooling the galvanized cold-rolled steel sheet to room temperature; and
a tempering step of tempering the galvanized cold-rolled steel sheet at a tempering temperature of 50° C. or higher and 400° C. or lower.

11. The method for producing the galvanized steel sheet according to claim 10 , wherein the galvanization step further includes, after the galvanization, galvannealing the galvanized cold-rolled steel sheet at a temperature range of 460° C. or higher and 580° C. or lower for 1 s or more and 120 s or less and then cooling the galvannealed cold-rolled steel sheet to room temperature.

12. The method for producing the galvanized steel sheet according to claim 11 , further comprising, before and/or after the tempering step, a temper-rolling step of temper rolling the galvannealed cold-rolled steel sheet at an elongation rate of 0.05% or more and 1.00% or less.

13. The method for producing the galvanized steel sheet according to claim 10 , further comprising, before and/or after the tempering step, a temper-rolling step of temper rolling the galvanized cold-rolled steel sheet at an elongation rate of 0.05% or more and 1.00% or less.

14. The method for producing the galvanized steel sheet according to claim 10 , wherein the chemical composition further comprises at least one group selected from the groups consisting of A, B, and C:
group A—at least one element selected from:
Cr: 0.01% or more and 2.0% or less, by mass %,
Ni: 0.01% or more and 2.0% or less, by mass %, and
Cu: 0.01% or more and 2.0% or less, by mass %,

group B—B: 0.0002% or more and 0.0050% or less, by mass %, and
group C—at least one element selected from:
Ca: 0.001% or more and 0.005% or less, by mass %, and
REM: 0.001% or more and 0.005% or less, by mass %.

15. The method for producing the galvanized steel sheet according to claim 14 , wherein the galvanization step further includes, after the galvanization, galvannealing the galvanized cold-rolled steel at a temperature range of 460° C. or higher and 580° C. or lower for 1 s or more and 120 s or less and then cooling the galvannealed cold-rolled steel sheet room temperature.

16. The method for producing the galvanized steel sheet according to claim 15 , further comprising, before and/or after the tempering step, a temper-rolling step of temper rolling the galvannealed cold-rolled steel sheet at an elongation rate of 0.05% or more and 1.00% or less.

17. The method for producing the galvanized steel sheet according to claim 14 , further comprising, before and/or after the tempering step, a temper-rolling step of temper rolling the galvanized cold-rolled steel sheet at an elongation rate of 0.05% or more and 1.00% or less.

Assignees

• Jfe Steel Corp

Inventors

• Hasegawa Hiroshi
• Kimura Hideyuki
• Funakawa Yoshimasa
• Makimizu Yoichi
• Suzuki Yoshitsugu

Classifications

• C21D8/02

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

• C21D6/001

  containing Ni (C21D6/004 takes precedence) · CPC title

• C22C38/12

  containing tungsten, tantalum, molybdenum, vanadium, or niobium · CPC title

• C21D8/0226

  Hot rolling · CPC title

• C22C38/14

  containing titanium or zirconium · CPC title