Cold-rolled steel sheet and process for production thereof

Patent metadata

Field	Value
Publication number	US-9435013-B2
Application number	US-201113818351-A
Country	US
Kind code	B2
Filing date	Aug 22, 2011
Priority date	Aug 23, 2010
Publication date	Sep 6, 2016
Grant date	Sep 6, 2016

Abstract

A cold-rolled steel sheet having a refined structure in which grain growth during annealing is suppressed has a chemical composition containing, in mass percent, controlled amounts of carbon, manganese, niobium, titanium, vanadium, sol. Aluminum, chromium, molybdenum, boron, calcium, and REM and a microstructure which contains at least 50% by area of ferrite as a main phase, a second phase containing at least 10% by area of a low temperature transformation phase and 0-3% by area of retained austenite and which satisfies the following Equations (1)-(3), in addition to a particular texture,

d m <2.7+10000/(5+300×C+50×Mn+4000×Nb+2000×Ti+400×V) 2   (1),

d m <4.0  (2),

and

d s ≦1.5  (3),

wherein d m is the average grain diameter (μm) of ferrite defined by a high angle grain boundary having a tilt angle of at least 15°, and d s is the average grain diameter (μm) of the second phase.

First claim

The invention claimed is:

1. A cold-rolled steel sheet characterized by having:
a chemical composition comprising, in mass %, C: 0.01-0.3, Si: 0.01-2.0, Mn: 0.5-3.5, P: at most 0.1, S: at most 0.05, Nb: 0-0.03, Ti: 0-0.06, V: 0-0.3, sol. Al: 0-2.0, Cr: 0-1.0, Mo: 0-0.3, B: 0-0.003, Ca: 0-0.003, REM: 0-0.003, and a remainder of Fe and impurities;
a microstructure having a main phase of ferrite which comprises at least 50% by area and a second phase containing a total of at least 10% by area of a low temperature transformation phase including one or more of martensite, bainite, pearlite, and cementite and 0-3% by area of retained austenite, and satisfying Equations (1), (2) and (3) below; and
a texture in which the average X-ray intensity for the [111]<145>, [111]<123>, and [554]<225> orientations at a depth of ½ of a sheet thickness is at least 4.0 times an average X-ray intensity of a random structure which does not have a texture;
d m <E 1 ;  Equation (1):

d m <4.0;  Equation (2):

d s ≦1.5;  Equation (3):

wherein
E 1 in Equation (1) represents a calculated value for the cold rolled sheet and calculated by E 1 =2.7+10000/(5+300×C+50×Mn+4000×Nb+2000×Ti+400×V) 2 , wherein C, Mn, Nb, Ti, and V indicate the contents in mass % of the respective elements;
in Equations (1) and (2), d m , is a physical value of the cold rolled sheet characterized by an average grain diameter in μm of ferrite defined by a high angle grain boundary having a tilt angle of at least 15 degrees; and
in Equation (3), d s , is a physical value of the cold rolled sheet characterized by an average grain diameter in μm of the second phase.

2. A cold-rolled steel sheet as set forth in claim 1 wherein
the chemical composition contains, in mass percent, one or more elements selected from the group consisting of Nb: 0.003-0.03, Ti: 0.005-0.06, and V: 0.01-0.3; and
the microstructure satisfies Equation (4) wherein
d m <3.5.  Equation (4):

3. A cold-rolled steel sheet as set forth in claim 1 wherein
the chemical composition contains, in mass percent, sol. Al: 0.1-2.0.

4. A cold-rolled steel sheet as set forth in claim 1 wherein
the chemical composition contains, in mass percent, one or more elements selected from the group consisting of Cr: 0.03-1.0, Mo: 0.01-0.3 and B: 0.0005-0.003.

5. A cold-rolled steel sheet as set forth in claim 1 wherein
the chemical composition contains, in mass percent, one or more elements selected from the group consisting of Ca: 0.0005-0.003 and REM: 0.0005-0.003.

6. A cold-rolled steel sheet as set forth in claim 1 which has a plating layer on the surface of the steel sheet.

7. A process for manufacturing a cold-rolled steel sheet characterized by comprising the following steps (A) and (B):
step (A) a cold rolling step in which a hot-rolled steel sheet having a chemical composition as set forth in any of claims 1 to 5 and having a microstructure which satisfies Equations (5) and (6) below is subjected to cold rolling to obtain a cold-rolled steel sheet; and
step (B) an annealing step in which the cold-rolled steel sheet obtained in step (A) is subjected to annealing by increasing the temperature of the steel sheet to a temperature range of at least (Ae 1 point+10° C.) to at most (0.95×Ae 3 point+0.05×Ae 1 point) under conditions such that the proportion of unrecrystallized ferrite is at least 30% by area when the temperature (Ae 1 point+10° C.) is reached and then holding the steel sheet in this temperature range for at least 30 seconds, wherein a rate of temperature increase at the time of annealing is at least 50° C. per second;
d n <E 5 ;  Equation (5):

d n <3.5;  Equation (6):

wherein
E 5 in Equation (5) represents a calculated value for the hot rolled sheet and calculated by E 5 =2.5+60001(5+300×C+40×Mn) 2 , wherein C and Mn indicate the contents in mass % of the respective elements;
in Equations (5) and (6), d n , is a physical value of the hot rolled sheet characterized by an average grain diameter of ferrite in μm defined by a high angle grain boundary having a tilt angle of at least 15 degrees;
wherein the hot-rolled steel sheet is obtained by a hot rolling step comprising performing hot rolling with a temperature at completion of rolling of at least the Ar 3 point on a slab having the above-described chemical composition and then performing cooling to a temperature range of 750° C. or below at an average cooling rate of at least 400° C. per second within 0.4 seconds after completion of rolling.

8. A process for manufacturing a cold-rolled steel sheet as set forth in claim 7 further having a step of carrying out plating on the cold-rolled steel sheet after step (B).

Assignees

• Hata Kengo
• Tomida Toshiro
• Imai Norio
• Nippon Steel & Sumitomo Metal Corp

Inventors

• Hata Kengo
• Tomida Toshiro
• Imai Norio

Classifications

• C22C38/005

  containing rare earths, i.e. Sc, Y, Lanthanides · CPC title

• C22C38/14

  containing titanium or zirconium · CPC title

• C21D8/0273

  Final recrystallisation annealing · CPC title

• C22C38/18 Primary

  containing chromium · CPC title

• C22C38/04

  containing manganese · CPC title