Method for manufacturing high-strength cold-rolled steel sheet with outstanding workability

Patent metadata

Field	Value
Publication number	US-9631266-B2
Application number	US-201314382450-A
Country	US
Kind code	B2
Filing date	Feb 28, 2013
Priority date	Mar 29, 2012
Publication date	Apr 25, 2017
Grant date	Apr 25, 2017

Abstract

Provided is a method that enables manufacture, with good productivity, of a high-strength cold-rolled steel with improved elongation (EL), stretch-flangeability (λ), bendability (R), and balance of these properties (TS×EL×λ/1000), outstanding composite workability as evaluated by Erichsen test, and a tensile strength of at least 980 MPa. A steel material satisfying a prescribed constituent composition is held and soaked for at least fifty seconds at a temperature at or above the Ac 3 point, after which the steel material is cooled at an average cooling rate of at least 15° C./sec to a discretionary temperature (T) that satisfies expression (1) below, held for 5-180 seconds in a temperature range that satisfies expression (1) below, then heated to a temperature region that satisfies expression (2) below, held for at least 50 seconds in this temperature region, and then cooled.

300° C.≦ T 1(° C.)<400° C.  (1)

400° C.≦ T 2(° C.)<540° C.  (2)

First claim

The invention claimed is:

1. A method of manufacturing a high-strength cold-rolled steel sheet having excellent workability, the steel sheet satisfying, by mass percent,
C: 0.10 to 0.3%,
Si: 1.0 to 3%,
Mn: 1.5 to 3%,
Al: 0.005 to 3%,
P: 0.1% or less, and
S: 0.05% or less,
the remainder comprising iron and inevitable impurities,
a microstructure of the steel sheet including bainite, retained austenite, and tempered martensite, wherein
(1) when the microstructure is observed by a scanning electron microscope,
the bainite is composed of a composite structure of
high-temperature-region-formed bainite, in which an average distance between adjacent retained-austenite grains and/or carbide particles is 1 μm or more, and
low-temperature-region-formed bainite, in which an average distance between adjacent retained-austenite grains and/or carbide particles is less than 1 μm, and
when an area fraction of the high-temperature-region-formed bainite in the entire microstructure is denoted as “a”, and
when a total area fraction of the low-temperature-region-formed bainite and the tempered martensite in the entire microstructure is denoted as “b”,
a: 20 to 80%, b: 20 to 80%, and a+b: 70% or more are satisfied, and
(2) a volume fraction of the retained austenite determined by saturation magnetization measurement is 3% or more with respect to the entire microstructure,
the method being characterized in that steel satisfying the above-described constituent composition is:
held for 50 sec or more at a temperature equal to or higher than a Ac 3 point so as to be soaked, and then
cooled to a temperature T1 satisfying Formula (1) at an average cooling rate of 20° C./sec or higher, and held in a temperature region satisfying Formula (1) for 5 to 180 sec, and then
heated into a temperature region satisfying Formula (2) and held in the temperature region for 50 sec or more, and then cooled;
310° C.≦ T 1(° C.)<400° C.  (1)

400° C.≦ T 2(° C.)≦540° C.  (2).

2. The manufacturing method according to claim 1 , wherein the steel further comprises Cr: 1% or less (not including 0%) and/or Mo: 1% or less (not including 0%).

3. The manufacturing method according to claim 1 , wherein the steel further comprises at least one element selected from the group consisting of Ti: 0.15% or less (not including 0%), Nb: 0.15% or less (not including 0%), and V: 0.15% or less (not including 0%).

4. The manufacturing method according to claim 1 , wherein the steel further comprises Cu: 1% or less (not including 0%) and/or Ni: 1% or less (not including 0%).

5. The manufacturing method according to claim 1 , wherein the steel further comprises B: 0.005% or less (not including 0%).

6. The manufacturing method according to claim 1 , wherein the steel further comprises at least one element selected from the group consisting of Ca: 0.01% or less (not including 0%), Mg: 0.01% or less (not including 0%), and rare earth elements: 0.01% or less (not including 0%).

7. The manufacturing method according to claim 1 , wherein when the microstructure has a MA mixed phase including quenched martensite compound with retained austenite, a ratio of the number of grains of the MA mixed phase, each grain having a circle-equivalent diameter d satisfying more than 3 μm on a viewing section, in the total number of grains of the MA mixed phase, is less than 15% (including 0%).

8. The manufacturing method according to claim 1 , wherein the average circle-equivalent diameter D of prior austenite grains is 20 μm or less (not including 0 μm).

9. The manufacturing method according to claim 1 , wherein the steel sheet is held in the temperature region satisfying Formula (2) and then cooled, and is then subjected to one of electrogalvanizing, hot-dip galvanizing, and hot-dip galvannealing.

10. The manufacturing method according to claim 1 , wherein the steel sheet is subjected to one of hot-dip galvanizing and hot-dip galvannealing in the temperature region satisfying Formula (2).

11. The manufacturing method according to claim 1 , wherein the steel is cooled to a temperature T1 satisfying 310° C.≦T1 (° C.)<400° C. at an average cooling rate of 25° C./sec or higher.

12. The manufacturing method according to claim 1 , wherein the steel is cooled to a temperature T1 satisfying 310° C.≦T1 (° C.)<400° C. at an average cooling rate of 20° C./sec or higher and held at T1 for 5 to 180 sec.

13. A method of manufacturing a high-strength cold-rolled steel sheet having excellent workability, the steel sheet satisfying, by mass percent,
C: 0.10 to 0.3%,
Si: 1.0 to 3%,
Mn: 1.5 to 3%,
Al: 0.005 to 3%,
P: 0.1% or less, and
S: 0.05% or less,
the remainder comprising iron and inevitable impurities,
a microstructure of the steel sheet including bainite, retained austenite, and tempered martensite, wherein
(1) when the microstructure is observed by a scanning electron microscope,
the bainite is composed of a composite structure of
high-temperature-region-formed bainite, in which an average distance between adjacent retained-austenite grains and/or carbide particles is 1 μm or more, and
low-temperature-region-formed bainite, in which an average distance between adjacent retained-austenite grains and/or carbide particles is less than 1 μm, and
when an area fraction of the high-temperature-region-formed bainite in the entire microstructure is denoted as “a”, and
when a total area fraction of the low-temperature-region-formed bainite and the tempered martensite in the entire microstructure is denoted as “b”,
a: 20 to 80%, b: 20 to 80%, and a+b: 70% or more are satisfied, and
(2) a volume fraction of the retained austenite determined by saturation magnetization measurement is 3% or more with respect to the entire microstructure,
the method being characterized in that steel satisfying the above-described constituent composition is
held for 50 sec or more at a temperature equal to or higher than a Ac 3 point so as to be soaked, and then
cooled to a temperature T1 satisfying Formula (1) at an average cooling rate of 25° C./sec or higher, and held in a temperature region satisfying Formula (1) for 5 to 180 sec, and then
heated into a temperature region satisfying Formula (2) and held in the temperature region for 50 sec or more, and then cooled;
300° C.≦ T 1(° C.)<400° C.  (1)

400° C.≦ T 2(° C.)<540° C.  (2).

14. The manufacturing method according to claim 13 , wherein the steel is cooled to a temperature T1 satisfying 300° C.≦T1 (° C.)<400° C. at an average cooling rate of 25° C./sec or higher and held at T1 for 5 to 180 sec.

Assignees

• Kobe Steel Ltd

Inventors

• Futamura Yuichi
• Nakaya Michiharu

Classifications

• C22C38/06

  containing aluminium · CPC title

• C21D2211/001

  Austenite · CPC title

• C22C38/14

  containing titanium or zirconium · CPC title

• C23C2/02

  Pretreatment of the material to be coated, e.g. for coating on selected surface areas (C23C2/30 takes precedence) · CPC title

• C21D1/20

  Isothermal quenching, e.g. bainitic hardening · CPC title