High strength cold rolled steel sheet with excellent deep drawability and material uniformity in coil and method for manufacturing the same

Patent metadata

Field	Value
Publication number	US-9297052-B2
Application number	US-201214351264-A
Country	US
Kind code	B2
Filing date	Aug 23, 2012
Priority date	Oct 13, 2011
Publication date	Mar 29, 2016
Grant date	Mar 29, 2016

Abstract

A high strength cold rolled steel sheet includes a chemical composition containing, by mass %, C: 0.010% or more and 0.060% or less, Si: more than 0.5% and 1.5% or less, Mn: 1.0% or more and 3.0% or less, P: 0.005% or more and 0.100% or less, S: 0.010% or less, sol.Al: 0.005% or more and 0.500% or less, N: 0.0100% or less, Nb: 0.010% or more and 0.100% or less, Ti: 0.015% or more and 0.150% or less and the balance comprising Fe and inevitable impurities. The microstructure includes, in area fraction, 70% or more of a ferrite phase and 3% or more of a martensite phase. The tensile strength is 440 MPa or more and an average r value is 1.20 or more.

First claim

The invention claimed is:

1. A high strength cold rolled steel sheet with excellent deep drawability and uniformity of mechanical property in a coil, the steel sheet having a chemical composition containing, by mass %, C: 0.010% or more and 0.060% or less, Si: more than 0.5% and 1.5% or less, Mn: 1.0% or more and 3.0% or less, P: 0.005% or more and 0.100% or less, S: 0.010% or less, sol.Al: 0.005% or more and 0.500% or less, N: 0.0100% or less, Nb: 0.010% or more and 0.100% or less, Ti: 0.015% or more and 0.150% or less, and the balance comprising Fe and inevitable impurities, wherein expressions (1), (2), and (3) below are satisfied, a microstructure includes, in area fraction, 70% or more of a ferrite phase and 3% or more of a martensite phase, and a tensile strength is 440 MPa or more and an average r value is 1.20 or more:
(Nb/93)/(C/12)<0.20  (1),

0.005≦C*≦0.025  (2),

(Nb/93+Ti*/48)/(C/12)≧0.150  (3),

where, atomic symbol M represents the content (mass %) of the chemical element represented by the symbol M in relational expressions (1), (2), and (3), where C*=C−(12/93)Nb−(12/48)Ti* and where Ti*=Ti−(48/14)N−(48/32)S, in which Ti−(48/14)N−(48/32)S=0 in the case where Ti−(48/14)N−(48/32)S≦0.

2. The high strength cold rolled steel sheet according to claim 1 , wherein the chemical composition further contains, by mass %, at least one chemical element selected from among Mo, Cr, and V in a total amount of 0.50% or less.

3. The high strength cold rolled steel sheet according to claim 1 , wherein the chemical composition further contains, by mass %, one or two selected from Cu: 0.30% or less and Ni: 0.30% or less.

4. A method of manufacturing a high strength cold rolled steel sheet with excellent deep drawability and uniformity of mechanical property in a coil, comprising:
hot rolling a steel material having a chemical composition according to claim 1 ;
cold rolling; and
annealing,
wherein a rolling reduction ratio of a last pass of finish rolling in the hot rolling is 10% or more and a rolling reduction ratio of a second to last pass is 15% or more, and the cold rolled steel sheet is heated up to a temperature of 800° C. to 900° C. at an average heating rate of less than 3° C./sec. in a temperature range from 700° C. to 800° C. and then cooled down to a cooling stop temperature of 500° C. or lower at an average cooling rate of 5° C./sec. or more in the annealing.

5. The method according to claim 4 , further comprising cooling that starts within three seconds after the finish rolling has been finished in the hot rolling, cooling the hot rolled steel sheet down to a temperature of 720° C. or lower at an average cooling rate of 40° C./sec. or more, coiling the cooled steel sheet at a temperature of 500° C. to 700° C., and cold rolling the coiled steel sheet with the rolling reduction ratio of 50% or more.

6. The high strength cold rolled steel sheet according to claim 2 , wherein the chemical composition further contains, by mass %, one or two selected from Cu: 0.30% or less and Ni: 0.30% or less.

7. A method of manufacturing a high strength cold rolled steel sheet with excellent deep drawability and uniformity of mechanical property in a coil, comprising:
hot rolling a steel material having a chemical composition according to claim 2 ;
cold rolling; and
annealing,
wherein a rolling reduction ratio of a last pass of finish rolling in the hot rolling is 10% or more and a rolling reduction ratio of a second to last pass is 15% or more, and the cold rolled steel sheet is heated up to a temperature of 800° C. to 900° C. at an average heating rate of less than 3° C./sec. in a temperature range from 700° C. to 800° C. and then cooled down to a cooling stop temperature of 500° C. or lower at an average cooling rate of 5° C./sec. or more in the annealing.

8. A method of manufacturing a high strength cold rolled steel sheet with excellent deep drawability and uniformity of mechanical property in a coil, comprising:
hot rolling a steel material having a chemical composition according to claim 3 ;
cold rolling; and
annealing,
wherein a rolling reduction ratio of a last pass of finish rolling in the hot rolling is 10% or more and a rolling reduction ratio of a second to last pass is 15% or more, and the cold rolled steel sheet is heated up to a temperature of 800° C. to 900° C. at an average heating rate of less than 3° C./sec. in a temperature range from 700° C. to 800° C. and then cooled down to a cooling stop temperature of 500° C. or lower at an average cooling rate of 5° C./sec. or more in the annealing.

9. A method of manufacturing a high strength cold rolled steel sheet with excellent deep drawability and uniformity of mechanical property in a coil, comprising:
hot rolling a steel material having a chemical composition according to claim 6 ;
cold rolling; and
annealing,
wherein a rolling reduction ratio of a last pass of finish rolling in the hot rolling is 10% or more and a rolling reduction ratio of a second to last pass is 15% or more, and the cold rolled steel sheet is heated up to a temperature of 800° C. to 900° C. at an average heating rate of less than 3° C./sec. in a temperature range from 700° C. to 800° C. and then cooled down to a cooling stop temperature of 500° C. or lower at an average cooling rate of 5° C./sec. or more in the annealing.

10. The method according to claim 7 , further comprising cooling that starts within three seconds after the finish rolling has been finished in the hot rolling, cooling the hot rolled steel sheet down to a temperature of 720° C. or lower at an average cooling rate of 40° C./sec. or more, coiling the cooled steel sheet at a temperature of 500° C. to 700° C., and cold rolling the coiled steel sheet with the rolling reduction ratio of 50% or more.

11. The method according to claim 8 , further comprising cooling that starts within three seconds after the finish rolling has been finished in the hot rolling, cooling the hot rolled steel sheet down to a temperature of 720° C. or lower at an average cooling rate of 40° C./sec. or more, coiling the cooled steel sheet at a temperature of 500° C. to 700° C., and cold rolling the coiled steel sheet with the rolling reduction ratio of 50% or more.

12. The method according to claim 9 , further comprising cooling that starts within three seconds after the finish rolling has been finished in the hot rolling, cooling the hot rolled steel sheet down to a temperature of 720° C. or lower at an average cooling rate of 40° C./sec. or more, coiling the cooled steel sheet at a temperature of 500° C. to 700° C., and cold rolling the coiled steel sheet with the rolling reduction ratio of 50% or more.

Assignees

• Kimura Hideyuki
• Nagataki Yasunobu
• Okuda Kaneharu
• Kawamura Kenji
• Jfe Steel Corp

Inventors

• Kimura Hideyuki
• Nagataki Yasunobu
• Okuda Kaneharu
• Kawamura Kenji

Classifications

• C21D8/04

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

• C21D9/48 Primary

  deep-drawing sheets · CPC title

• C22C38/00

  Ferrous alloys, e.g. steel alloys (cast-iron alloys C22C37/00) · CPC title

• C22C38/24

  with vanadium · CPC title

• C22C38/001 Primary

  containing N · CPC title