LOW-YIELD-RATIO ULTRA-HIGH-STRENGTH HOT-ROLLED Q&P STEEL AND PRODUCTION METHOD THEREFOR

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.

Claims

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.sub.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.sub.nr or less to 800° C. to obtain a hot-rolled piece; wherein the non-recrystallization temperature T.sub.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.sub.nr=887+464C+(6445Nb−644Nb.sup.1/2)+(732V−230V.sup.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%.

Description

DESCRIPTION OF THE DRAWINGS

[0051] FIG. 1 is a flow chart of the process for production of the low yield ratio and superhigh-strength hot-rolled Q&P steel of the present invention.

[0052] FIG. 2 is a rolling process for the low yield ratio and superhigh-strength hot-rolled Q&P steel of the present invention.

[0053] FIG. 3 is a cooling process for the low yield ratio and superhigh-strength hot-rolled Q&P steel of the present invention after rolling.

[0054] FIG. 4 is a typical metallographic photo of a test steel in example 1 of the present invention.

[0055] FIG. 5 is a typical metallographic photo of a test steel in example 2 of the present invention.

[0056] FIG. 6 is a typical metallographic photo of a test steel in example 3 of the present invention.

[0057] FIG. 7 is a typical metallographic photo of a test steel in example 4 of the present invention.

[0058] FIG. 8 is a typical metallographic photo of a test steel in example 5 of the present invention.

PARTICULAR EMBODIMENTS

[0059] The invention will be further illustrated with reference to the following examples.

[0060] Referring to FIG. 1, a process for production of the low yield ratio and superhigh-strength hot-rolled Q&P steel of the present invention is: smelting using a rotary furnace or electric furnace.fwdarw.secondary refining using a vacuum furnace.fwdarw.casting slab (ingot).fwdarw.reheating cast slab (ingot).fwdarw.a process of hot rolling+stepped cooling.fwdarw.steel reel.

EXAMPLE

[0061] Referring to FIGS. 2 and 3, the process for production of low yield ratio and superhigh-strength hot-rolled Q&P steels in examples 1-5 specifically comprises the following steps:

[0062] 1) Smelting, Secondary Refining, and Casting:

[0063] according to the composition of each steel in table 1, the smelting is performed using a rotary furnace or electric furnace, the secondary refining is performed using a vacuum furnace, and the casting is performed to form a cast slab or cast ingot.

TABLE-US-00001 TABLE 1 Unit: weight percentage Ms T.sub.nr Example C Si Mn P S Al N Nb Ti O ° C. ° C. 1 0.20 1.57 1.77 0.008 0.004 0.66 0.0037 0.06 0.012 0.0028 392 899 2 0.26 1.55 1.93 0.008 0.003 0.87 0.0044 0.04 0.003 0.0027 353 902 3 0.30 1.16 1.52 0.007 0.004 0.75 0.0045 0.02 0.011 0.0022 353 932 4 0.23 1.84 2.48 0.009 0.005 0.98 0.0060 0.05 0.020 0.0029 346 889 5 0.25 1.25 2.26 0.007 0.003 0.52 0.0055 0.05 0.018 0.0023 351 940 T.sub.nr = 887 + 464C + (6445Nb − 644Nb.sup.1/2) + (732V − 230V.sup.1/2) + 890Ti + 363Al − 357Si

[0064] 2) Heating and Hot Rolling:

[0065] The cast slab or cast ingot obtained in step 1) is heated to 1100-1200° C. and undergoes heat preservation for 1-2 h, with the rolling starting temperature being 1000-1100° C., multi-pass great reduction is performed at T.sub.nr with the accumulative deformation amount being ≧50%, the main purpose being obtaining fine equiaxial austenite grains; and thereafter, the intermediate slab, after the temperature is between 800° C. and not higher than the T.sub.nr, undergoes final 3-5 passes of rolling with the accumulative deformation amount being ≧70%; the temperature parameters of Ms and T.sub.nr are as shown in table 1, and the hot rolling process is as shown in FIG. 2;

[0066] wherein the T.sub.nr is determined according to the following formula;

T.sub.nr=887+464C+(6445Nb−644Nb.sup.1/2)+(732V−230V.sup.1/2)+890Ti+363Al−357Si; and

[0067] 3) Stepped Cooling:

[0068] The hot-rolled piece is rapidly water-cooled to 600-700° C. at a cooling rate of >30° C./s above the starting temperature of ferrite precipitation, further air-cooled for 5-10 s, then continues to be cooled to a temperature between 150-300° C. (i.e., between Ms and Mf) at a cooling rate of >30° C./s to obtain a structure of 10-25% of a ferrite+65-85% of a martensite+5-10% of a residual austenite, and finally coiled and slowly cooled to room temperature to obtain a low yield ratio and superhigh-strength hot-rolled Q&P steel, wherein parameters of the rolling process (the cast slab thickness being 120 mm) are as shown in FIG. 2, the mechanical properties and microstructures are as shown in table 3, and the cooling process after rolling is as shown in table 3;

[0069] wherein the relational formula of the air-cooling time and the content of Al is:

t(s)=23.7−34.3Al+15.6Al.sup.2.

TABLE-US-00002 TABLE 2 Cooling Cooling Final stopping stopping Heating rolling Steel plate temperature Air cooling temperature Temperature Temperature Thickness in the 1st time in the in the 3rd Example  ° C. ° C. mm stage, ° C. 2nd stage, s stage, ° C. 1 1180 800 8 600 5 250 2 1130 850 12 650 9 190 3 1150 900 6 690 10 220 4 1100 880 3 660 6 160 5 1200 830 10 630 8 300

TABLE-US-00003 TABLE 3 Mechanical property Yield Tensile −20° C. Volume fraction of each phase, % strength strength Elongation Yield ratio Impact Residual Example Rp0.2 MPa Rm MPa A % Rp0.2/Rm work J Ferrite Martensite austenite 1 652 1379 11.5 0.47 49 ~25 ~70 5.46 2 661 1333 11.0 0.50 36 ~25 ~65 9.69 3 651 1328 10.5 0.49 30 ~20 ~70 9.97 4 666 1335 11.5 0.50 43 ~20 ~70 9.04 5 687 1374 10.0 0.50 38 ~15 ~75 9.34

[0070] By detection, the typical metallographic photos of the low yield ratio and superhigh-strength hot-rolled Q&P steels obtained in examples 1-5 are respectively as shown in FIGS. 4-8.

[0071] FIGS. 4-8 provide the typical metallographic photos of the test steels in examples. It can be clearly seen from the metallographic photos that the structure of the steel plate is mainly a small amount of proeutectoid ferrite+martensite+residual austenite. It can be seen according to the X-ray diffraction results that the contents of the residual austenites in the steel plates in examples 1-5 are respectively 5.46%, 9.69%, 9.97%, 9.04% and 8.34%.

[0072] The microstructure of the steel plate of the present invention strip-shaped proeutectoid ferrite+martensite+residual austenite. Due to the presence of the residual austenite, a transformation-induced plasticity (TRIP) effect occurs to the steel plate in the process of extension or wearing, thereby improving the wear resistance of the steel plate.