High-strength thin steel sheet with excellent drawability and bake hardenability, and method for manufacturing same

Patent metadata

Field	Value
Publication number	US-10704116-B2
Application number	US-201615579406-A
Country	US
Kind code	B2
Filing date	Jun 7, 2016
Priority date	Jun 5, 2015
Publication date	Jul 7, 2020
Grant date	Jul 7, 2020

Abstract

A high-strength thin steel sheet having excellent drawability and bake hardenability, and a method of manufacturing the same are provided. The high-strength thin steel sheet includes 0.0005 to 0.003 wt % of carbon (C); 0.5 wt % or less (excluding 0 wt %) of silicon (Si); 1.2 wt % or less (excluding 0 wt %) of manganese (Mn); 0.005 to 0.12 wt % of phosphorous (P); 0.008 wt % or less of sulfur (S); 0.005 wt % or less of nitrogen (N); 0.1 wt % or less (excluding 0 wt %) of acid-soluable aluminum (Al); 0.01 to 0.04 wt % of titanium (Ti); iron (Fe) as a remainder thereof; and unavoidable impurities. The high-strength thin steel sheet has BH of 4 MPa or more and P of 80% or more in Equation 1: P (%)={N in /(N in +N gb )}×100, N in being the number of carbides having a size of 20 nm or less within a crystal grain, and N gb being the number of carbides having a size of 20 nm or less at a grain boundary.

First claim

The invention claimed is:

1. A comprising:
0.0005 wt % to 0.003 wt % of carbon (C); 0.5 wt % or less excluding 0 wt % of silicon (Si); 1.2 wt % or less excluding 0 wt % of manganese (Mn); 0.005 wt % to 0.12 wt % of phosphorous (P); 0.008 wt % or less of sulfur (S); 0.005 wt % or less of nitrogen (N); 0.1 wt % or less excluding 0 wt % of acid-soluable aluminum (Al); 0.01 wt % to 0.04 wt % of titanium (Ti); iron (Fe) as a remainder thereof, and unavoidable impurities;
P (%) of 80% or more, P (%) being defined by Equation 1: P (%)=({N in /(N in +N gb )}×100, where N in denotes the number of carbides present within a crystal grain having a size of 20 nm or less, and N gb denotes the number of carbides present at a grain boundary of the crystal grain; and
a bake hardenability (BH) of 4 MPa or more.

2. The steel sheet of claim 1 , further comprising: 0.005 wt % to 0.04 wt % of niobium (Nb).

3. The steel sheet of claim 1 , further comprising: 0.002 wt % or less excluding 0 wt % of boron (B).

4. The steel sheet of claim 1 , further comprising: 0.2 or less of FeTiP precipitates per unit area (μm 2 ).

5. The steel sheet of claim 1 , wherein, in a region of the steel sheet from a steel sheet surface to t/4 (t: thickness of the steel sheet) in a thickness direction of the steel sheet, an average random strength ratio of {111}<011> to {111}<112> orientation groups with respect to an alpha (α)-fiber texture in which a <110> direction and a rolling direction are parallel to each other is 5 or more.

6. The steel sheet of claim 1 , further comprising: a lankford value (r value) of 2.2 or more.

7. The steel sheet of claim 1 , further comprising a hot-dip galvanized layer formed on a surface of the steel sheet.

8. The steel sheet of claim 1 , further comprising an alloyed hot-dip galvanized layer formed on a surface of the steel sheet.

9. A method of manufacturing a steel sheet, the method comprising:
hot-rolling a steel slab to obtain a hot-rolled steel sheet, the steel slab including 0.0005 wt % to 0.003 wt % of carbon (C), 0.5 wt % or less excluding 0 wt % of silicon (Si), 1.2 wt % or less excluding 0 wt % of manganese (Mn), 0.005 wt % to 0.12 wt % of phosphorous (P), 0.008 wt % or less of sulfur (S), 0.005 wt % or less of nitrogen (N), 0.1 wt % or less excluding 0 wt % of acid-soluable aluminum (Al), 0.01 wt % to 0.04 wt % of titanium (Ti), iron (Fe) as a remainder thereof, and unavoidable impurities, and obtaining a hot-rolled steel sheet;
winding the hot-rolled steel sheet at a temperature of 450° C. to 750° C. to obtain a coiled steel sheet;
cold-rolling the coiled steel sheet under conditions of a total reduction ratio of 75% to 85% and a ratio of a final rolling reduction ratio to the total reduction ratio of 6.5% to 14.1%, to obtain a cold-rolled steel sheet;
continuously annealing the cold-rolled steel sheet by heating the cold-rolled steel sheet to an annealing temperature of 830° C. to 880° C. at a rate of 7° C./sec or lower and then maintaining the annealing temperature for 30 sec. to 80 sec. to obtain an annealed steel sheet; and
cooling the annealed steel sheet to 650° C. at an average cooling rate of 2° C./sec to 10° C./sec thereby producing the steel sheet of claim 1 .

10. The method of claim 9 , wherein the steel slab further comprises: 0.005 wt % to 0.04 wt % of niobium (Nb).

11. The method of claim 9 , wherein the steel slab further comprises: 0.002 wt % or less excluding 0 wt % of boron (B).

12. The method of claim 9 , wherein the hot-rolling comprises: a hot-finish rolling at Ar3 or higher.

13. The method of claim 9 , further comprising: hot-dip galvanizing annealed steel sheet.

14. The method of claim 9 , further comprising:
hot-dip galvanizing the annealed steel sheet to obtain a galvanized steel sheet; and
heat-treating the galvanized steel sheet at a temperature of 450° C. to 600° C.

Assignees

• Posco

Inventors

• Han Sang-Ho

Classifications

• C21D8/02

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

• C22C38/004 Primary

  Very low carbon steels, i.e. having a carbon content of less than 0,01% · CPC title

• C23C2/06

  Zinc or cadmium or alloys based thereon · CPC title

• C21D8/0273

  Final recrystallisation annealing · CPC title

• C21D8/0263

  following hot rolling · CPC title