HIGH STRENGTH AND HIGH PLASTICITY HOT-FORMING STEEL WITH OXIDATION RESISTANCE FOR AUTOMOBILES AND HOT-FORMING PROCESS

Abstract

Provided are a high-strength and high-plasticity hot-forming steel with oxidation resistance for automobiles and a hot-forming process thereof, and the hot-forming steel has chemical compositions in mass percentages as follows: C: 0.35%-50%, Si: ?0.20%, Mn: 1.50%-2.50%, P: 0.050%-0.10%, S?0.004%, Als: 0.02%-0.06%, Nb: 0.03%-0.07%, Ti: 0.020%-0.050%, V: 0.08%-0.15%, Cr: 1.50%-3.20%, Mo: 0.10%-0.30%, B: ?0.0040%, N?0.005%, the balance Fe and inevitable impurities. The hot-forming steel provided in the present invention has high oxidation resistance, high strength and plasticity, does not need atmosphere protection during hot forming, and does not need shot blasting treatment after hot forming.

Claims

1. A high-strength and high-plasticity hot-forming steel with oxidation resistance for automobiles, wherein the hot-forming steel has chemical compositions in mass percentages as follows: C: 0.35%-0.50%, Si: ?0.20%, Mn: 1.50%-2.50%, P: 0.050%-0.10%, S?0.004%, Als: 0.02%-0.06%, Nb: 0.03%-0.07%, Ti: 0.020%-0.050%, V: 0.08%-0.15%, Cr: 1.50%-3.20%, Mo: 0.10%-0.30%, B: ?0.0040%, N?0.005%, the balance Fe and inevitable impurities.

2. The high-strength and high-plasticity hot-forming steel with oxidation resistance for automobiles according to claim 1, wherein a microstructure of the hot-forming steel includes a ferrite, a martensite and a retained austenite.

3. The high-strength and high-plasticity hot-forming steel with oxidation resistance for automobiles according to claim 2, wherein the ferrite has a volume fraction of 4%-10%, the martensite has a volume fraction of 78%-90%, and the retained austenite has a volume fraction of 6%-12%.

4. The high-strength and high-plasticity hot-forming steel with oxidation resistance for automobiles according to claim 1, wherein the tensile strength of the hot-forming steel is 2000 MPa.

5. The high-strength and high-plasticity hot-forming steel with oxidation resistance for automobiles according to claim 1, wherein the oxidation resistance rate of the hot-forming steel is ?0.1 g/(m.sup.2.Math.h).

6. The high-strength and high-plasticity hot-forming steel with oxidation resistance for automobiles according to claim 1, wherein the yield strength of the hot-forming steel is 1400 MPa.

7. The high-strength and high-plasticity hot-forming steel with oxidation resistance for automobiles according to claim 1, wherein the elongation of the hot-forming steel is ?12.0%.

8. The high-strength and high-plasticity hot-forming steel with oxidation resistance for automobiles according to claim 1, wherein the surface of the hot-forming steel is not completely decarburized with a thickness of decarburized layer ?15 ?m.

9. The high-strength and high-plasticity hot-forming steel with oxidation resistance for automobiles according to claim 1, wherein the thickness of the hot-forming steel is 0.8 mm-12.0 mm;

10. A hot-forming process of high-strength and high-plasticity hot-forming steel with oxidation resistance for automobiles, comprising: (1) placing the hot-forming substrate containing the compositions according to claim 1 into a heating furnace at a temperature of A.sub.C3-A.sub.C3+30? C. for heating and heat preservation for a period of 180 s-300 s; and (2) taking the heated hot-forming steel out of the heating furnace for air cooling to a temperature of Ara and staying for 3 s-5 s before being put into a hot-forming mold for deformation and cooling, at a cooling rate of ?10? C./s, performing the pressure holding for 60 s-90 s after cooling to 250? C.-300? C., and taking out the formed parts for air cooling to room temperature after pressure holding to obtain the hot-forming steel.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] It should be noted that the embodiments of the present invention and features in the embodiments, under the condition of no conflict, can be combined with each other. The described embodiments are some, rather than all of the embodiments of the present disclosure. The following description of at least one example embodiment is merely illustrative in nature, and is in no way intended to limit the present disclosure, an application or use thereof. Based on the embodiments of the present disclosure, all other embodiments acquired by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present disclosure

[0039] The present invention provides a high-strength and high-plasticity hot-forming steel with oxidation resistance for automobiles, where the hot-forming steel has chemical compositions in mass percentages as follows: C: 0.35%-0.50%, Si: ?0.20%, Mn: 1.50%-2.50%, P: 0.050%-0.10%, S?0.004%, Als: 0.02%-0.06%, Nb: 0.03%-0.07%, Ti: 0.020%-0.050%, V: 0.08%-0.15%, Cr: 1.50%-3.20%, Mo: 0.10%-0.30%, B: ?0.0040%, N?0.005%, the balance Fe and inevitable impurities.

[0040] A microstructure of the hot-forming steel includes a ferrite, a martensite and a retained austenite.

[0041] The ferrite has a volume fraction of 4%-10%, the martensite has a volume fraction of 78%-90%, and the retained austenite has a volume fraction of 6%-12%.

[0042] The hot-forming steel has an oxidation resistance rate ?0.1 g/(m.sup.2.Math.h), an oxidation resistance up to Level 1, a tensile strength ?2000 MPa, a yield strength ?1400 MPa, and an elongation ?12.0%. The surface of the steel is not completely decarburized with a thickness of decarburized layer ?15 ?m, and the hot-forming steel has a thickness of 0.8-12.0 mm.

[0043] After smelting, hot rolling and cold rolling, the high-strength hot-forming steel with excellent oxidation resistance provided in this embodiment obtains a hot-forming substrate with a thickness of 0.8-12.0 mm. Then, a hot-forming process is performed, and the hot-forming process includes the following steps: [0044] (1) the hot-forming substrate containing the above compositions is placed into a heating furnace at a temperature of A.sub.C3-A.sub.C3+30? C. for heating and heat preservation for a period of 180 s-300 s, and the step is implemented to make the hot-forming substrate completely austenitic and make it have smaller original austenitic grain size; at the same time, lower austenitizing temperature is beneficial to reducing the surface oxidation of the hot-forming substrate; and [0045] (2) the heated hot-forming steel is taken out of the heating furnace for air cooling to a temperature of Ara and stayed for 3 s-5 s before being put into a hot-forming mold for deformation and cooling, at a cooling rate of ?10? C./s, the pressure holding is performed for 60 s-90 s after cooling to 250? C.-300? C., and the formed parts is taken out for air cooling to room temperature after pressure holding to obtain the hot-forming steel.

[0046] The compositions, hot-forming process parameters, and the microstructure and performance of the steel after hot forming of the embodiments of the present invention are shown in Tables 1 to 3.

TABLE-US-00001 TABLE 1 Composition of Embodiment of the Present Invention (wt, %) Embodiment C Si Mn P S Als Nb V Ti Cr Mo B N 1 0.35 0.20 2.30 0.10 0.003 0.036 0.058 0.15 0.045 3.0 0.20 0.0040 0.003 2 0.37 0.17 2.50 0.092 0.002 0.020 0.070 0.13 0.050 3.2 0.10 0.0033 0.005 3 0.39 0.12 1.90 0.085 0.003 0.035 0.065 0.11 0.040 2.5 0.30 0.0025 0.003 4 0.42 0.05 1.85 0.074 0.002 0.041 0.053 0.10 0.036 2.3 0.15 0.0020 0.002 5 0.44 0.08 1.78 0.065 0.004 0.055 0.045 0.090 0.028 1.80 0.25 0.0015 0.001 6 0.50 0.01 1.50 0.050 0.003 0.060 0.030 0.080 0.020 1.50 0.30 0.0005 0.001 7 0.46 0.10 2.10 0.080 0.003 0.032 0.040 0.14 0.030 2.8 0.25 0.0030 0.002 8 0.40 0.05 1.60 0.055 0.001 0.023 0.060 0.095 0.033 2.0 0.28 0.0001 0.002

TABLE-US-00002 TABLE 2 Hot-forming Process of Embodiment of the Present Invention Heat Resi- Pres- Pres- Pre- dence sure- sure- Heating serva- Time of Cool- holding hold- Em- Temper- tion temper- ing Temper- ing bodi- A.sub.C3, ature, Period, ature Rate, ature, Time, ment ? C. ? C. s A.sub.r3, s ? C./s ? C. s 1 821 850 180 5 25 300 90 2 811 840 220 4 23 280 80 3 809 835 250 3 20 260 75 4 796 825 190 4 17 250 60 5 796 825 280 5 11 270 70 6 785 815 300 3 10 265 85 7 799 825 260 4 16 285 65 8 803 830 290 5 17 275 78

TABLE-US-00003 TABLE 3 Microstructure and performance parameters of Embodiment of the Present Invention Oxida- Thick- Volume Volume tion ness A Volume fraction fraction resist- of (elon- fraction of of ance decar- Em- Thick- Yield Tensile ga- of marten- auste- rate, burized bodi- ness, strength, strength, tion), ferrite, site, nite, g/ layer, ment mm MPa MPa % % % % (m.sup.2 .Math. h) ?m) 1 0.8 1400 2010 20.5 10 78 12 0.10 9 2 1.2 1530 2030 18.0 7 86 7 0.07 13 3 1.4 1460 2045 15.5 9 83 8 0.06 7 4 7.0 1650 2060 16.5 8 83 9 0.05 13 5 12.0 1550 2080 14.0 5 85 10 0.06 15 6 10.0 1680 2120 13.0 4 90 6 0.03 4 7 2.0 1480 2070 17.5 7 85 8 0.09 11 8 5.0 1710 2180 12.0 5 86 9 0.08 2

[0047] Through the combination of new chemical composition and hot forming process provided in the embodiment of the present invention, a hot-forming steel with high-strength and high-plasticity can be obtained, the tensile strength of the steel is ?2000 MPa, and the elongation reaches or exceeds 12%; by adding Cr, Mo and other elements, the oxidation resistance of the steel is improved, the oxidation resistance rate of the steel is ?0.1 g/(m.sup.2.Math.h), the oxidation resistance level reaches Level 1, and the steel does not need atmosphere protection during hot forming, does not need shot blasting treatment after hot forming, and can be subjected to the subsequent processes directly. Moreover, the proposed hot-forming steel and hot-forming process is lower in the whole process cost than that of the existing hot-forming products, and can be implemented through the existing equipment, without equipment modification.

[0048] At last, it should be noted that the above various embodiments are merely intended to illustrate the technical solution of the present disclosure and not to limit the same; although the present disclosure has been described in detail with reference to the foregoing embodiments, it should be understood by those ordinary skilled in the art that the technical solutions described in the foregoing embodiments may be modified or equivalents may be substituted for some or all of the technical features thereof; and the modification or substitution does not make the essence of the corresponding technical solution deviate from the scope of the technical solution of each embodiment of the present disclosure.