Process for manufacturing cold-rolled steel sheet

Patent metadata

Field	Value
Publication number	US-10407749-B2
Application number	US-201715407347-A
Country	US
Kind code	B2
Filing date	Jan 17, 2017
Priority date	Feb 22, 2012
Publication date	Sep 10, 2019
Grant date	Sep 10, 2019

Abstract

A hot-rolled steel sheet having a composition comprising, in mass %, C: 0.06 to 0.3%, Si: 0.4 to 2.5%, Mn: 0.6 to 3.5%, P: at most 0.1%, S: at most 0.05%, Ti: 0 to 0.08%, Nb: 0 to 0.04%, a total content of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0 to 1%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0.003%, REM: 0 to 0.003%, the remainder Fe and impurities, is cold rolled, then heated at an average heating rate of at least 15° C./sec so that a proportion of an unrecrystallization of a region not transformed to austenite at a time of reaching (Ac 1 point+10° C.) is at least 30 area %, and then held for 30 seconds at temperature of at least (0.9×Ac 1 point+0.1×Ac 3 point) and at most (Ac 3 point+100° C.).

First claim

The invention claimed is:

1. A process for manufacturing a cold-rolled steel sheet comprising:
(A) a cold rolling step in which a hot-rolled steel sheet having a chemical composition comprising, in mass %, C: 0.06 to 0.3%, Si: 0.4 to 2.5%, Mn: 0.6 to 3.5%, P: at most 0.1%, S: at most 0.05%, Ti: 0 to 0.08%, Nb: 0 to 0.04%, a total content of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0 to 1%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0.003%, REM: 0 to 0.003% and the remainder of Fe and impurities, is cold-rolled to obtain a cold-rolled steel sheet, wherein the hot-rolled steel sheet is a steel sheet in which average grain diameter of a BCC phase defined by high angle grain boundaries having a tilt angle of at least 15° is at most 6 μm, the steel sheet being obtained by a hot rolling step of cooling at a cooling rate (Crate) satisfying following Equation (5) for a temperature range from a temperature at the completion of rolling to a temperature at the completion of rolling—100° C. after completion of hot rolling in which hot rolling is completed at at least an Ar 3 point:

IC
⁡

(
T
)

=

0.1
-

3
×

10

-
3

·
T

+

4
×

10

-
5

·

T
2

-

5
×

10

-
7

·

T
3

+

5
×

10

-
9

·

T
4

-

7
×

10

-
11

·

T
5

⁢

⁢

⁢

∫
0

-
100

⁢

-
d

⁢

⁢
T

Crate
⁡

(
T
)

·

IC
⁡

(
T
)

<
4

(
5
)

where Crate (T) is a cooling rate (° C./s) (positive value),
T is a relative temperature (° C., negative value) with the temperature at the completion of rolling as zero, and
if there is a temperature at which Crate is zero, a value obtained by dividing a detention period (Δt) at the temperature by IC (T) is added as an integral for the section; and
(B) an annealing step in which the cold-rolled steel sheet obtained in the step (A) is annealed under conditions that the cold-rolled steel sheet is heated at an average heating rate of at least 15° C./sec so that a proportion of an unrecrystallization of a region not transformed to austenite at a time of reaching (Ac 1 point+10° C.) is at least 30 area %, and is then held in a temperature range of at least (0.9×Ac 1 point+0.1×Ac 3 point) and at most (Ac 3 point+100° C.) for at least 30 seconds.

2. The process for manufacturing cold-rolled steel sheet as set forth in claim 1 , wherein the hot-rolled steel sheet, after completion of hot rolling, is coiled at a temperature of at most 300° C. and subsequent heat treatment in a temperature range of 500° C. to 700° C.

3. The process for manufacturing cold-rolled steel sheet as set forth in claim 1 , wherein the cooling for the temperature range includes starting cooling at a cooling rate of at least 400° C./sec and cooling at the cooling rate for a temperature range of at least 30° C.

4. The process for manufacturing cold-rolled steel sheet as set forth in claim 1 ,

Assignees

• Nippon Steel & Sumitomo Metal Corp
• Nippon Steel Corp

Inventors

• Hata Kengo
• Tomida Toshiro
• Imai Norio
• Haga Jun
• Nishio Takuya

Classifications

• C21D8/02

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

• C22C38/00

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

• C22C38/18

  containing chromium · CPC title

• C21D2211/002

  Bainite · CPC title

• C22C38/005

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