Low-yield-ratio ultra-high-strength hot-rolled q&p steel and production method therefor

Patent metadata

Field	Value
Publication number	US-2017342523-A1
Application number	US-201515536787-A
Country	US
Kind code	A1
Filing date	Nov 24, 2015
Priority date	Dec 19, 2014
Publication date	Nov 30, 2017
Grant date	—

Abstract

Disclosed is a low yield ratio and superhigh-strength hot-rolled Q&P steel and a method for manufacturing the same, having the following chemical composition in weight percentage: C: 0.2-0.3%, Si: 1.0-2.0%, Mn: 1.5-2.5%, P: ≦0.015%, S: ≦0.005%, Al: 0.5-1.0%, N: ≦0.006%, Nb: 0.02-0.06%, Ti: ≦0.03%, O: ≦0.003%, and the balance being Fe and inevitable impurities. The manufacture method comprises stepped cooling process to finally obtain a three-phase structure containing a certain volume fraction of proeutectoid ferrite+a certain volume fraction of martensite+a certain volume fraction of residual austenite. By controlling the relative contents of the three different phases, a low yield ratio and superhigh-strength hot-rolled Q&P steel having an excellent comprehensive performance which has a yield strength of ≧600 MPa and a tensile strength of ≧1300 MPa as well as a good elongation and a low yield ratio, shows an excellent match of high plasticity and can be applied in the field of steels requiring easy deformabilities and wear-resistances, is obtained.

First claim

1 . A low yield ratio and superhigh-strength hot-rolled Q&P steel, having the
following chemical composition in weight percentage: C: 0.2-0.3%, Si: 1.0-2.0%, Mn: 1.5-2.5%, P: ≦0.015%, S: ≦0.005%, Al: 0.5-1.0%, N: ≦0.006%, Nb: 0.02-0.06%, Ti: ≦0.03%, O: ≦0.003%, and the balance being Fe and inevitable impurities, and Ti/N≦3.42.

2 . The low yield ratio and superhigh-strength hot-rolled Q&P steel according to claim 1 , characterized in that said chemical composition, the content of Si is in the range of 1.3-1.7%.

3 . The low yield ratio and superhigh-strength hot-rolled Q&P steel according to claim 1 , characterized in that the content of Mn is in the range of 1.8-2.2% in weight percentage.

4 . The low yield ratio and superhigh-strength hot-rolled Q&P steel according to claim 1 , characterized in that the content of Al is in the range of 0.6-0.8% in weight percentage.

5 . The low yield ratio and superhigh-strength hot-rolled Q&P steel according to claim 1 , characterized in that the content of N is in the range of ≦0.004% in weight percentage.

6 . The low yield ratio and superhigh-strength hot-rolled Q&P steel according to claim 1 , characterized in that said chemical composition, the content of Nb is in the range of 0.03-0.05%.

7 . The low yield ratio and superhigh-strength hot-rolled Q&P steel according to claim 1 , characterized in that the microstructure of said Q&P steel is a three-phase structure of 10-25% of proeutectoid ferrite+65-85% of martensite+5-10% of residual austenite.

8 . The low yield ratio and superhigh-strength hot-rolled Q&P steel according to claim 1 , characterized in that said Q&P steel has a yield strength of ≧600 MPa, a tensile strength of ≧1300 MPa, and a yield ratio of ≦0.5.

9 . A method for manufacturing the low yield ratio and superhigh-strength hot-rolled Q&P steel according to claim 1 , characterized by comprising the following steps:
1) smelting, secondary refining, and casting smelting by using a rotary furnace or electric furnace, secondary refining by using a vacuum furnace, and casting to form a cast slab or cast ingot, according to the following chemical composition in weight percentage: C: 0.2-0.3%, Si: 1.0-2.0%, Mn: 1.5-2.5%, P: ≦0.015%, S: ≦0.005%, Al: 0.5-1.0%, N: ≦0.006%, Nb: 0.02-0.06%, Ti: ≦0.03%, O: ≦0.003%, and the balance being Fe and inevitable impurities, with Ti/N ≦3.42; 2) heating and hot rolling heating the cast slab or cast ingot to 1100-1200° C. and holding for 1-2 h, with a rolling starting temperature of 1000-1100° C. and an accumulative deformation amount of ≧50% after multi-pass large reduction at a non-recrystallization temperature T nr or higher to obtain an intermediate slab containing fine equiaxed austenite grains; and then rolling for 3-5 passes with an accumulative deformation amount of ≧70% after the temperature of the intermediate slab reaches a temperature in the range of T nr or less to 800° C. to obtain a hot-rolled piece; wherein the non-recrystallization temperature T nr is determined according to the following formula, in which various element symbols respectively represent the corresponding contents in weight percentage of the elements;
T nr =887+464C+(6445Nb−644Nb 1/2 )+(732V−230V 1/2 )+890Ti+363Al−357Si; and

3) stepped cooling rapidly water-cooling the hot-rolled piece from a temperature above a starting temperature of ferrite precipitation to 600-700° C. at a cooling rate of >30° C./s, air-cooling for 5-10 s, and continuing the cooling to a temperature between 150-300° C. at a cooling rate of >30° C./s, then coiling and slowly cooling to room temperature to obtain said low yield ratio and superhigh-strength hot-rolled Q&P steel.

10 . A method for manufacturing the low yield ratio and superhigh-strength hot-rolled Q&P steel according to claim 9 , characterized in that the microstructure of the low yield ratio and superhigh-strength hot-rolled Q&P steel obtained by the manufacturing method is a three-phase structure of 10-25% of ferrite+65-85% of martensite+5-10% of residual austenite.

11 . The method for manufacturing the low yield ratio and superhigh-strength hot-rolled Q&P steel according to claim 9 , characterized in that the low yield ratio and superhigh-strength hot-rolled Q&P steel obtained by the manufacturing method has a yield strength of ≧600 MPa, a tensile strength of ≧1300 MPa, a yield ratio of ≦0.5, and an elongation of ≧10%.

Assignees

• Baoshan Iron & Steel

Inventors

• Wang Huanrong
• Yang Ana
• Wang Wei

Classifications

• C21D8/02

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

• C22C38/02

  containing silicon · CPC title

• C21D2211/001

  Austenite · CPC title

• C22C38/06

  containing aluminium · CPC title

• C21D6/005

  containing Mn · CPC title