High-strength steel sheet and method for manufacturing the same

Patent metadata

Field	Value
Publication number	US-10801085-B2
Application number	US-201615577989-A
Country	US
Kind code	B2
Filing date	May 26, 2016
Priority date	May 29, 2015
Publication date	Oct 13, 2020
Grant date	Oct 13, 2020

Abstract

A high-strength steel sheet having high yield ratio, excellent stretch flange formability, and resistance to secondary working embrittlement. The steel sheet has a composition containing C: 0.02% to less than 0.10%, Si: less than 0.3%, Mn: less than 1.0%, P: 0.10% or less, S: 0.020% or less, Al: 0.01% to 0.10%, N: 0.010% or less, and Nb: 0.003% to less than 0.070% on a mass basis, the remainder being Fe and inevitable impurities. A steel microstructure of the steel sheet contains ferrite: 90% or more and a total of pearlite, martensite, retained austenite, and cementite: 0% to 10% on an area fraction basis, in which the average grain size of the ferrite is 15.0 μm or less, and in which the average aspect ratio of the ferrite is 1.2 or more; and a tensile strength of 500 MPa or less.

First claim

The invention claimed is:

1. A high-strength steel sheet having a composition comprising:
C: 0.02% to less than 0.10%, by mass %,
Si: less than 0.3%, by mass %,
Mn: less than 1.0%, by mass %,
P: 0.10% or less, by mass %,
S: 0.020% or less, by mass %,
Al: 0.01% to 0.10%, by mass %,
N: 0.010% or less, by mass %,
Nb: 0.003% to less than 0.070%, by mass %, and
Fe and inevitable impurities;
wherein:
the steel sheet has a steel microstructure that contains:
ferrite: 90% or more on an area fraction basis, and
a total of pearlite, martensite, retained austenite, and cementite: 0% to 10% on an area fraction basis,

the average grain size of the ferrite is 15.0 μm or less,
the average aspect ratio of the ferrite, as defined by the formula (average grain length of the ferrite in a rolling direction of the steel sheet)/(average grain length of the ferrite in a through-thickness direction of the steel sheet), is 1.2 or more,
an amount of oxides of Fe, Mn, Si, and Cr present on a surface of the steel sheet is provided so that when the steel sheet is subjected to a chemical conversion treatment with a phosphate conversion coating, an area fraction of a micro-region on a surface of the steel sheet that is free from phosphates is 5% or less, and
the steel sheet has a tensile strength of 500 MPa or less.

2. The high-strength steel sheet according to claim 1 , wherein the chemical composition further comprises one or more of:
Cr: less than 0.5%, by mass %,
Mo: 0.3% or less, by mass %,
B: 0.005% or less, by mass %,
Cu: 0.3% or less, by mass %, and
Ni: 0.3% or less, by mass %.

3. The high-strength steel sheet according to claim 2 , having a galvanized layer on a surface thereof.

4. The high-strength steel sheet according to claim 3 , wherein the galvanized layer is a galvannealed layer.

5. The high-strength steel sheet according to claim 2 , having an electrogalvanized layer on a surface thereof.

6. The high-strength steel sheet according to claim 1 , having a galvanized layer on a surface thereof.

7. The high-strength steel sheet according to claim 6 , wherein the galvanized layer is a galvannealed layer.

8. The high-strength steel sheet according to claim 1 , having an electrogalvanized layer on a surface thereof.

9. The high-strength steel sheet according to claim 1 , wherein if the composition further comprises Cr, the Cr is included in the composition in an amount of 0.2% or less, by mass %.

10. The high-strength steel sheet according to claim 1 , having a yield ratio of 0.70 or more.

11. A method for manufacturing the high-strength steel sheet according to claim 1 , the method comprising:
annealing a cold-rolled steel sheet in a box annealing furnace,
wherein:
a concentration of hydrogen in the atmosphere in the furnace is 5% to 100% by volume,
during the annealing, the cold-rolled steel sheet is heated to 550° C. at 120° C./h or less, and
during the annealing, the cold-rolled steel sheet is soaked in a temperature range from 550° C. to 750° C.

12. The method for manufacturing the high-strength steel sheet according to claim 11 , wherein after the annealing, galvanizing is performed.

13. The method for manufacturing the high-strength steel sheet according to claim 12 , wherein after the galvanizing is performed, the galvanizing is subjected to an alloying treatment.

14. The method for manufacturing the high-strength steel sheet according to claim 11 , wherein after the annealing, electro-galvanizing is performed.

15. A method for manufacturing the high-strength steel sheet according to claim 2 , the method comprising:
annealing a cold-rolled steel sheet in a box annealing furnace,
wherein:
a concentration of hydrogen in the atmosphere in the furnace is 5% to 100% by volume,
during the annealing, the cold-rolled steel sheet is heated to 550° C. at 120° C./h or less, and
during the annealing, the cold-rolled steel sheet is soaked in a temperature range from 550° C. to 750° C.

16. The method for manufacturing the high-strength steel sheet according to claim 15 , wherein after the annealing, galvanizing is performed.

17. The method for manufacturing the high-strength steel sheet according to claim 16 , wherein after the galvanizing is performed, the galvanizing is subjected to an alloying treatment.

18. The method for manufacturing the high-strength steel sheet according to claim 15 , wherein after the annealing, electro-galvanizing is performed.

19. A high-strength steel sheet having a composition comprising:
C: 0.02% to less than 0.10%, by mass %,
Si: less than 0.3%, by mass %,
Mn: less than 1.0%, by mass %,
P: 0.10% or less, by mass %,
S: 0.020% or less, by mass %,
Al: 0.01% to 0.10%, by mass %,
N: 0.010% or less, by mass %,
Nb: 0.003% to less than 0.070%, by mass %, and
Fe and inevitable impurities;
wherein:
the steel sheet has a steel microstructure that contains:
ferrite: 90% or more on an area fraction basis, and
a total of pearlite, martensite, retained austenite, and cementite: 0% to 10% on an area fraction basis,

the average grain size of the ferrite is 15.0 μm or less,
the average aspect ratio of the ferrite, as defined by the formula (average grain length of the ferrite in a rolling direction of the steel sheet)/(average grain length of the ferrite in a through-thickness direction of the steel sheet), is 1.2 or more,
an amount of oxides of Fe, Mn, Si, and Cr present on a surface of the steel sheet is provided so that when the steel sheet is subjected to a treatment that comprises surface modifying the steel sheet with a solution of zinc phosphate at 25° C. for 20 s and then immersing the steel sheet in a solution of zinc phosphate at 40° C. for 120 s, an area fraction of a micro-region on a surface of the steel sheet that is free from phosphates is 5% or less, and
the steel sheet has a tensile strength of 500 MPa or less.

20. A high-strength steel sheet having a composition consisting of:
C: 0.02% to less than 0.10%, by mass %,
Si: less than 0.3%, by mass %,
Mn: less than 1.0%, by mass %,
P: 0.10% or less, by mass %,
S: 0.020% or less, by mass %,
Al: 0.01% to 0.10%, by mass %,
N: 0.010% or less, by mass %,
Nb: 0.003% to less than 0.070%, by mass %,
Cr: less than 0.5%, by mass %,
Mo: 0.3% or less, by mass %,
B: 0.005% or less, by mass %,
Cu: 0.3% or less, by mass %,
Ni: 0.3% or less, by mass %,
inevitable impurities selected from the group consisting of Mg, Ca, Sr, Sn, Sb, Bi, Ta, and W: 1% or less, by mass, and
Fe: balance;
wherein:
the steel sheet has a steel microstructure that contains:
ferrite: 90% or more on an area fraction basis, and
a total of pearlite, martensite, retained austenite, and cementite: 0% to 10% on an area fraction basis,

the average grain size of the ferrite is 15.0 μm or less,
the average aspect ratio of the ferrite, as defined by the formula (average grain length of the ferrite in a rolling direction of the steel sheet)/(average grain length of the ferrite in a through-thickness direction of the steel sheet), is 1.2 or more, and
the steel sheet has a tensile strength of 500 MPa or less.

Assignees

• Jfe Steel Corp

Inventors

• Honda Yuma
• Funakawa Yoshimasa
• Harada Kozo

Classifications

• C21D8/04

  to produce plates or strips for drawing, e.g. for deep-drawing · CPC title

• C23C2/28

  Thermal after-treatment, e.g. treatment in oil bath · CPC title

• C23C2/29

  Cooling or quenching · CPC title

• C23C2/024

  by cleaning or etching · CPC title

• C23C2/022

  by heating · CPC title