STEEL PLATE FOR TORSION BEAM AND MANUFACTURING METHOD THEREFOR, AND TORSION BEAM AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed are a steel plate for a torsion beam and a manufacturing method therefor, and a torsion beam and a manufacturing method therefor. The steel plate for the torsion beam has the following chemical components in percentages by mass: 0.04-0.085% of C, 0.02-0.5% of Si, 1.3-1.8% of Mn, 0.15-0.5% of Cr, 0.12-0.30% of Mo, 0.058% or less of Nb, 0.15% or less of V, 0.02% or less of Ti, 0.02-0.1% of Al, 0.02% or less of P, 0.005% or less of S, 0.005% or less of N, and the balance being Fe and inevitable impurities. The steel plate has one or two of Nb and V, and the amount of Nb and V satisfies 0.096%<2Nb+V<0.17%. The steel plate for the torsion beam of the present invention has an excellent elongation and excellent cold bending properties while ensuring high strength, and meets the requirement for producing lightweight torsion beams.

Claims

1. A steel plate for a torsion beam, comprising the following chemical components in percentage by mass: 0.04-0.085% of C, 0.02-0.5% of Si, 1.3-1.8% of Mn, 0.15-0.5% of Cr, 0.12-0.30% of Mo, 0.058% or less of Nb, 0.15% or less of V, 0.02% or less of Ti, 0.02-0.1 % of Al, 0.02% or less of P, 0.005% or less of S, and 0.005% or less of N, the balance being Fe and inevitable impurities, wherein the steel plate comprises one or two of Nb and V, and the amount of Nb and V satisfies the following formula: 0.096%≤2Nb+V≤0.17%.

2. The steel plate for the torsion beam of claim 1, wherein the chemical components of the steel plate satisfy 0.3%≤0.5Cr+Mo≤0.55%.

3. The steel plate for the torsion beam of claim 1, wherein the chemical components of the steel plate satisfy: a carbon equivalent CE.sub.||w≤0.50, wherein CE.sub.||w=%C+%Mn/6+%(Cr+Mo+V)/5+%(Ni+Cu)/15.

4. The steel plate for the torsion beam of claim 1, wherein a microstructure of the steel plate contains bainite and ferrite, wherein a total volume fraction of the bainite and the ferrite is 90% or more, and a volume fraction of the bainite is greater than 50%.

5. The steel plate for the torsion beam of claim 4, wherein the microstructure of the steel plate further contains pearlite and/or martensite.

6. The steel plate for the torsion beam of claim 1, wherein the steel plate has a longitudinal yield strength of 620 MPa or more, a tensile strength of 760 MPa or more, an A50 elongation of 16% or more, and a 180° cold bending property R/T of 1.05 or more.

7. A manufacturing method of a steel plate for a torsion beam, wherein the steel plate comprises the following chemical components in percentage by mass: 0.04-0.085% of C, 0.02-0.5% of Si, 1.3-1.8% of Mn, 0.15-0.5% of Cr, 0.12-0.30% of Mo, 0.058% or less of Nb, 0.15% or less of V, 0.02% or less of Ti, 0.02-0.1 % of Al, 0.02% or less of P, 0.005% or less of S, and 0.005% or less of N, and the balance being Fe and inevitable impurities, wherein the steel plate comprises one or two of Nb and V, and the amount of Nb and V satisfies the following formula: 0.096%≤2Nb+V≤0.17%; and the manufacturing method comprises smelting, continuous casting, hot rolling and pickling, wherein the hot rolling comprises hot rolling heating, rolling, and cooling coiling, wherein in the hot rolling heating, a slab obtained after smelting and continuous casting is heated to 1200-1260° C. and maintained for 1-3 hr.; the rolling comprises rough rolling and finish rolling, wherein an outlet temperature of the rough rolling is controlled at 1020-1100° C., an outlet temperature of the final rolling is controlled at 840-920° C., and a total reduction ratio is controlled to be 80% or more; and in the cooling coiling, the rolled steel plate is subjected to laminar flow cooling at a rate of 30-70° C./s to 500-620° C., and then coiled.

8. The manufacturing method of the steel plate for the torsion beam of claim 7, wherein the manufacturing method further comprises air cooling between the rolling and the cooling coiling, wherein the air cooling time is 1-8 s.

9. The manufacturing method of the steel plate for the torsion beam of claim 7, wherein the chemical components of the steel plate satisfy 0.3%≤0.5Cr+Mo≤0.55%.

10. The manufacturing method of the steel plate for the torsion beam of claim 7, wherein the chemical components of the steel plate satisfy: a carbon equivalent CE.sub.||w≤0.50, wherein CE.sub.||w=%C+%Mn/6+%(Cr+Mo+V)/5+%(Ni+Cu)/15.

11. The manufacturing method of the steel plate for the torsion beam of claim 7, wherein a microstructure of the steel plate for the torsion beam contains bainite and ferrite, a total volume fraction of the bainite and the ferrite is 90% or more, and a volume fraction of the bainite is greater than 50%.

12. The manufacturing method of the steel plate for the torsion beam of claim 11, wherein the microstructure of the steel plate further contains pearlite and/or martensite.

13. The manufacturing method of the steel plate for the torsion beam of claim 7, wherein the steel plate has a longitudinal yield strength of 620 MPa or more, a tensile strength of 760 MPa or more, an A50 elongation of 16% or more, and a 180° cold bending property R/T of 1.05 or more.

14. A torsion beam, made of the steel plate for the torsion beam of claims 1.

15. The torsion beam of claim 14, wherein the torsion beam has a longitudinal yield strength of 680 MPa or more, a tensile strength of 800 MPa or more, and a bench fatigue of 0.5-1.8 million times.

16. A manufacturing method for a torsion beam, comprising the following steps: welding into a tube: welding the steel plate for the torsion beam of claims 1 into a round tube; forming: hydroforming or press forming the round tube into a shaped tube, the shaped tube being U-shaped or V-shaped and having an internal fillet, wherein a ratio of the internal fillet R to a thickness T of the shaped tube satisfies R/T≥1.05; and then preforming stress relief annealing and/or shot peening to form the torsion beam.

17. The manufacturing method for the torsion beam of claim 16, wherein in the stress relief annealing step, the shaped tube is heated, maintained at 475-610° C. for 20-90 min, then air cooled after cooling to 300° C.

18. The manufacturing method for the torsion beam of claim 16, wherein in the shot peening step, the inner or outer surface of the internal fillet of the shaped tube is subjected to shot peening.

19. The manufacturing method for the torsion beam of claim 16, wherein a core hardness at the internal fillet is 260 HV or more, and a microhardness at 0.05 mm from the inner or outer surface of the internal fillet is 30-80 HV higher than the core hardness.

20. The manufacturing method for the torsion beam of claim 16, wherein the torsion beam has a longitudinal yield strength of 680 MPa or more, a tensile strength of 800 MPa or more, and a bench fatigue of 0.5-1.8 million times.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] FIG. 1 shows a metallographic microstructure diagram of a steel plate in Example 2 of the present invention;

[0058] FIG. 2 shows a schematic diagram of a cross-sectional profile of an internal fillet of a torsion beam in an example of the present invention; and

[0059] FIG. 3 shows a schematic diagram of the change in microhardness of a torsion beam in Example 1, Example 11, and a torsion beam that is not subjected to stress relief annealing according to the present invention.

DETAILED DESCRIPTION

[0060] Embodiments of the present invention will be described below with reference to specific embodiments, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in the description. Although the present invention will be described in connection with preferred embodiments, it is not intended that the features of the present invention be limited to this embodiment. On the contrary, the description of the present invention in connection with the embodiments is intended to cover other alternatives or modifications that may be extended based on the claims of the present invention. The following description contains numerous specific details to provide a thorough understanding of the present invention. The present invention may also be implemented without these details. In addition, some specific details will be omitted from the description to avoid confusing or obscuring the focus of the present invention. It should be noted that the embodiments of the present invention and the features of the embodiments can be combined with each other without conflict.

[0061] It should be noted that in the present description, like reference signs and letters represent like items in the following drawings, and therefore, once a certain item is defined in one drawing, it need not be further defined and explained in the following drawings.

[0062] In order to make the objects, technical solutions and advantages of the present invention more clear, the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0063] Table 1 shows the mass percentage of each chemical element in Examples 1-10 and Comparative examples 1-7 of the present invention. Table 2 shows the corresponding steel plate manufacturing method in Examples 1-10 and Comparative examples 1-7, and the properties of the manufactured steel plates.

[0064] The manufacturing method of the steel plate for the torsion beam of the present invention includes the steps of smelting, continuous casting, hot rolling, and pickling. In the following examples and comparative examples, first converter smelting is performed, and the molten steel is subjected to RH vacuum degassing treatment and LF furnace desulfurization treatment, and a specific method can refer to the existing smelting method. Continuous casting is then performed. The level of center segregation and inclusions of the continuous casting slab can be controlled by, for example, controlling the degree of superheat, controlling the secondary cooling water, and using appropriate soft reduction during the continuous casting process, and the inclusion grade can be controlled to less than 1.5. The inclusion grade in this description refers to GB/T 10561-2005.

[0065] The obtained slab is hot rolled after continuous casting. The hot rolling step includes hot rolling heating, rolling, and cooling coiling. The heating temperature and holding time of the hot rolling heating in the following examples and comparative examples are shown in Table 2. The rolling process includes rough rolling and finish rolling, and Table 2 also lists the outlet temperature of rough rolling and final rolling temperature in the examples. A steel plate is formed after rolling, the steel plate is then subjected to cooling coiling, cooling adopts laminar flow cooling, and the coiling temperature in Table 2 represents the temperature at which the steel plate is coiled. The air-cooling time represents the time during which the slab is air cooled after rolling, and when the air-cooling time is 0, it means that in this example, laminar flow cooling is directly conducted without air cooling.

[0066] The steel plates formed in the following Examples 1-10 and Comparative examples 1-7 are subjected to property tests. In this description, mechanical property tests are carried out according to the standard GB/T228.1-2010, and the 180° cold bending property R/T is carried out according to the standard GB/T232-2010, and the results of the longitudinal yield strength, tensile strength, uniform elongation, A50 elongation, and 180° cold bending property are obtained accordingly. In Table 2, in the column of the 180° cold bending property, no wrinkle on the cold bending surface is expressed as “OK” and having wrinkle on cold bending surface is expressed as “NG”.

[0067] FIG. 1 is a metallographic microstructure diagram of a steel plate in Example 2. It can be seen from the figure that the microstructure of the obtained steel plate includes bainite and ferrite, as well as a small amount of pearlite. The sum of the volume fractions of bainite+ferrite in this example is greater than 90%.

[0068] Table 3 shows the calculation parameters and properties of torsion beams made of the steel plates in the above examples. The manufacturing method for the torsion beam of the present invention uses cold forming to manufacture the torsion beam, including the steps of welding into a tube, forming, and stress relief annealing and/or shot peening to form the torsion beam.

[0069] The steel plates in the above examples are welded to form round tubes, and then the round tubes are subjected to hydroforming or press forming to form shaped tubes which are U-shaped or V-shaped and have internal fillets, and the structure of the formed torsion beam is shown in FIG. 2.

[0070] The radius R of the internal fillet of the torsion beam and the thickness T of the torsion beam are shown in FIG. 2. In the present invention, the internal fillet with R/T greater than or equal to 1.05 is manufactured during manufacturing of the torsion beam, e.g., the internal fillet has a radius R of 4.61 and the thickness T is 3, resulting in R/T=1.537 in FIG. 2. For example, the round tube is placed in a lower die and is subjected to press forming by closing an upper die, or a hydroforming method in which a tube is pressure expansion molded by liquid after closing a die is adopted. In Examples 11-21 and Comparative examples 8-14, steel plates are formed into torsion beams in different ways, respectively. Table 3 lists the R/T values at the internal fillets formed in different examples and comparative examples, the process after forming, that is, the temperature and time of stress relief annealing, the position of shot peening and other specific parameters.

[0071] The torsion beams formed in Examples 11-20 and Comparative examples 8-14 described below are subjected to performance tests. The mechanical performance tests in this description are carried out in accordance with the standard GB/T 228.1-2010. The longitudinal yield strength and tensile strength at the internal fillet cannot be measured, and the core hardness at the internal fillet and a microhardness difference between the surface and the core are measurable. The microhardness tests are carried out according to GB/T 4342-1991.

[0072] The test conditions for a torsion beam bench include a displacement control of ± 50 mm and a frequency of 1.5 HZ.

[0073] Detailed description is given below in conjunction with the examples and comparative examples in Tables 1, 2 and 3:

[0074] In Examples 1-10 in Tables 1 and 2 and Examples 11-21 in Table 3, by adopting the design method of the present invention, the steel plate with high strength and high formability and the torsion beam with high strength and high fatigue performance are obtained.

[0075] Examples 1 and 3-10 adopt an air-cooling mode in the steel plate manufacturing process. In the examples, when the coiling temperature is 500-620° C. and the carbon equivalent is less than 0.50, relatively good formability and elongation can be achieved.

[0076] The high carbon content in Comparative Example 1 results in poor bending properties, further resulting in microcracks on the surface of the fillet R during the hydroforming process in Comparative Example 8 in Table 3, and cracks at the weld seam, ultimately resulting in the bench fatigue not reaching 0.5 million times. The steel plates in Example 4 and Comparative Example 1 are produced by similar process parameters (e.g., the coiling temperature and the air-cooling time are the same), while in Comparative Example 1, a carbon equivalent is higher, and in Example 4, a carbon equivalent is lower, so that the steel plate in Example 4 has better weldability.

[0077] The relatively low Cr and Mo content in Comparative Example 2 results in relatively low tempering resistance (i.e., lower strength). In Comparative Examples 2 and 3, the requirement of 0.096%≤2Nb+V≤0.17% is not satisfied, resulting in relatively low yield strength and lower tensile strength of the steel plate. Accordingly, the steel plates in Comparative Examples 2 and 3 are used to produce torsion beams, i.e., the strength of the torsion beams obtained in Comparative Examples 9 and 10 is also low, eventually resulting in relatively weak fatigue resistance of the material, and the effect of weight reduction cannot be achieved.

[0078] In Comparative Example 4, the carbon content is relatively low, while the hot rolling coiling temperature is too high, which results in a relatively low plate strength, does not reach the requirement of the tensile strength of the steel plate of 760 MPa or more, and finally results in the condition that the tensile strength of the torsion beam in Comparative Example 11 in Table 3 cannot reach 800 MPa, and the effect of weight reduction cannot be achieved.

[0079] The microalloy content in Comparative Example 5 is relatively high, which does not satisfy the condition of 0.096%≤2Nb+V≤0.17%, resulting in high strength of the steel plate and a bending property that does not satisfy R/T≥1.05, further resulting in microcracks on the surface of the fillet during hydroforming in Comparative Example 12 in Table 3, eventually resulting in a bench fatigue that cannot reach 0.5 million times.

[0080] The relatively low Cr content in Comparative Example 6 results in a lower strength of the plate and ultimately in a relatively poor bench fatigue capacity of the torsion beam in Comparative Example 13. Due to the low Cr content and the high annealing temperature during the manufacturing of the torsion beam, the strength is reduced, and the effects of high strength and weight reduction cannot be achieved.

[0081] The strength of the plate and the strength of the final torsion beam (Comparative Example 14) can reach 800 MPa or more by microalloy strengthening with high Ti content in Comparative Example 7, but the torsion beam in Comparative Example 14 has a final bench fatigue performance of only 0.29 million times, mainly due to the coarse TiN precipitates, without Nb and V precipitations that are beneficial to the fatigue performance. In addition, the coiling temperature in Comparative Example 7 is only 480° C., resulting in poor formability of the material.

[0082] In Examples 11-21, the fatigue resistance of the torsion beam is improved by using stress relief annealing or shot peening step during forming the torsion beam, wherein the longitudinal yield strength of the torsion beam is maintained or even improved by using the stress relief annealing process and/or the shot peening process.

[0083] The increase in yield strength of the torsion beam after the stress relief annealing is more pronounced in Example 11 compared with Example 21.

[0084] FIG. 3 shows the change in microhardness along the thickness direction of a steel plate in Example 1, i.e., the change in microhardness of the steel plate in the figure. According to the present invention, the steel plate in Example 1 is subjected to welding into a tube and forming to form a shaped tube, and the change in microhardness along the thickness direction of the fillet without the stress relief annealing step, i.e., the change in microhardness at the fillet without annealing in FIG. 3 is detected. The steel plate in Example 1 is also subjected to welding into a tube, forming, and stress relief annealing in the present invention to form the torsion beam, i.e., Example 11 of the present invention, and the change in microhardness at the fillet thereof, i.e., the change in microhardness at the fillet subjected to annealing in FIG. 3 is detected. After the stress relief annealing, the microhardness of the surface in Example 11 is relatively high and the core hardness at the fillet is relatively low, achieving the effect of the outside having high strength and the inside having high toughness.

TABLE-US-00001 Chemical composition (wt%, the balance is Fe and other inevitable impurities besides P, S and N) No. C Si Mn Cr Mo Nb V Ti Al CE.sub.IIW 2Nb+V 0.5Cr+Mo P S N Example 1 0.071 0.15 1.56 0.42 0.25 0.015 0.12 0 0.042 0.489 0.15 0.46 0.010 0.003 0.0045 Example 2 0.061 0.22 1.56 0.48 0.3 0 0.15 0 0.024 0.507 0.15 0.54 0.005 0.005 0.0050 Example 3 0.078 0.35 1.42 0.42 0.25 0.055 0 0 0.038 0.449 0.11 0.46 0.020 0.002 0.0040 Example 4 0.085 0.05 1.31 0.30 0.15 0.022 0.1 0 0.096 0.413 0.144 0.3 0.013 0.004 0.0042 Example 5 0.082 0.07 1.48 0.45 0.12 0.022 0.12 0 0.056 0.467 0.164 0.345 0.012 0.003 0.0045 Example 6 0.052 0.48 1.78 0.37 0.26 0.015 0.12 0 0.07 0.499 0.15 0.445 0.011 0.002 0.0025 Example 7 0.081 0.26 1.62 0.36 0.25 0.017 0.1 0 0.033 0.493 0.134 0.43 0.009 0.004 0.0036 Example 8 0.04 0.15 1.66 0.15 0.25 0.015 0.12 0 0.042 0.421 0.15 0.325 0.016 0.001 0.0023 Example 9 0.068 0.35 1.42 0.42 0.25 0.048 0 0.015 0.038 0.439 0.096 0.46 0.008 0.002 0.0042 Example 10 0.061 0.22 1.56 0.48 0.3 0 0.15 0.01 0.024 0.507 0.15 0.54 0.007 0.002 0.0038 Comparative Example 1 0.13 0.48 1.88 0.35 0.15 0.022 0.12 0 0.033 0.567 0.164 0.325 0.010 0.003 0.0045 Comparative Example 2 0.071 0.15 1.56 0.10 0.11 0 0.07 0 0.042 0.387 0.07 0.16 0.013 0.002 0.0038 Comparative Example 3 0.082 0.15 1.56 0.30 0.16 0.035 0 0 0.042 0.434 0.07 0.31 0.012 0.001 0.0036 Comparative Example 4 0.03 0.15 1.56 0.42 0.25 0.015 0.12 0.02 0.042 0.448 0.15 0.46 0.011 0.004 0.0045 Comparative Example 5 0.078 0.25 1.63 0.15 0.26 0.036 0.15 0 0.033 0.462 0.222 0.335 0.009 0.005 0.0023 Comparative Example 6 0.082 0.45 1.32 0.10 0.12 0.025 0.11 0.01 0.082 0.368 0.16 0.17 0.016 0.004 0.0050 Comparative Example 7 0.078 0.25 1.42 0.42 0.15 0.022 0.06 0.10 0.033 0.441 0.104 0.36 0.007 0.004 0.0045

TABLE-US-00002 Manufacturing process and mechanical properties of steel plate No. Heating temperature /°C Holding time /min Outlet temperature of rough rolling /°C Final rolling temperature/°C Air cooling time/s Laminar flow cooling rate/(°C/s) Coiling temperature/°C Total reduction ratio/% Longitudinal yield strength/Mpa Tensile strength/Mpa Uniform elongation/% A50 elongation/% 180° cold bending (R/T≥1.05) Example 1 1230 122 1060 880 3 50 580 98.4 676 842 10.0 21.5 OK Example 2 1250 79 1070 920 0 30 500 99.0 645 788 7.0 16.5 OK Example 3 1220 109 1020 840 8 70 520 97.3 655 782 7.5 17.5 OK Example 4 1230 125 1050 880 3 50 540 97.5 715 855 8.5 17.5 OK Example 5 1260 65 1100 900 5 50 610 99.2 645 808 11.5 24.0 OK Example 6 1230 180 1050 860 5 50 560 98.2 655 778 8.5 18.0 OK Example 7 1250 132 1080 900 2 40 600 98.5 658 813 10.5 23.0 OK Example 8 1230 96 1060 880 4 30 620 98.4 655 785 11.0 24.0 OK Example 9 1260 80 1020 880 3 50 560 97.3 628 773 9.5 21.5 OK Example 10 1260 79 1020 880 5 50 500 97.3 632 786 7.5 16.5 OK Comparative Example 1 1230 122 1060 880 3 50 540 98.4 801 926 5.0 12.0 NG Comparative Example 2 1230 122 1060 880 3 50 500 98.4 583 698 7.0 17.5 OK Comparative Example 3 1230 122 1060 880 3 50 500 98.4 602 725 7.0 17.0 OK Comparative Example 4 1230 122 1060 880 3 50 650 98.4 602 716 10.0 23.5 OK Comparative Example 5 1230 122 1060 880 3 50 500 98.4 802 953 5.5 14.0 NG Comparative Example 6 1230 122 1060 880 3 50 620 98.4 623 736 10.0 25.0 OK Comparative Example 7 1230 122 1060 880 3 50 480 98.4 702 843 5.5 14.5 NG

TABLE-US-00003 Production process and corresponding performance of torsion beam No Steel plate selected Forming process R/T at the internal fillet Stress relief annealing process Shot peening process Core hardness at the fillet/HV Microhardness difference between the position at 0.05 mm from the surface and the core/ΔHV Yield strength at non-fillet/MPa Tensile strength at non-fillet/MPa Bench fatigue life/million time Yield strength difference between the torsion beam and the steel plate used/ΔMPa Example 11 Example 1 Hydroforming 2.00 500° C.+60 min No 283 37 756 850 0.86 80 Example 12 Example 2 Hydroforming 1.25 550° C.+30 min No 275 53 732 806 0.95 87 Example 13 Example 3 Hydroforming 2.30 610° C.+20 min No 265 33 690 800 0.53 35 Example 14 Example 4 Hydroforming 1.25 575° C.+30 min Outer surface shot peening 280 64 782 863 0.93 67 Example 15 Example 5 Hydroforming 1.05 575° C.+30 min Inner and outer surfaces shot peening 263 80 724 832 1.80 79 Example 16 Example 6 Hydroforming 1.55 No Inner and outer surfaces shot peening 302 77 692 814 1.01 37 Example 17 Example 7 Hydroforming 1.50 575° C.+30 min Outer surface shot peening 263 49 721 835 0.98 63 Example 18 Example 8 Hydroforming 1.25 475° C.+30 min Inner and outer surfaces shot peening 262 64 742 838 1.38 87 Example 19 Example 9 Press forming 1.50 610° C.+20 min No 265 32 680 813 0.78 52 Example 20 Example 10 Press forming 1.50 575° C.+30 min Inner and outer surfaces shot peening 276 63 718 829 0.97 86 Example 21 Example 1 Hydroforming 2.00 No Inner and outer surfaces shot peening 286 69 703 869 1.03 27 Comparative Example 8 Comparative Example 1 Hydroforming 1.25 550° C.+30 min No 290 Weld seam cracking Fillet R cracking 821 902 0.15 20 Comparative Example 9 Comparative Example 2 Hydroforming 1.25 No Inner and outer surfaces shot peening 231 Cracking 602 732 0.31 19 Comparative Example 10 Comparative Example 3 Hydroforming 2.30 550° C.+30 min No 249 Cracking 626 765 0.43 24 Comparative Example 11 Comparative Example 4 Hydroforming 2.00 550° C.+30 min No 239 Cracking 638 753 0.45 36 Comparative Example 12 Comparative Example 5 Hydroforming 1.25 475° C.+30 min Inner and outer surfaces shot peening 296 Fillet R cracking 843 928 0.25 41 Comparative Example 13 Comparative Example 6 Press forming 1.50 630° C.+20 min No 245 35 618 742 0.48 -5 Comparative Example 14 Comparative Example 7 Hydroforming 2.00 550° C.+30 min No 264 Fillet R cracking 742 824 0.29 40

[0085] Although the present invention has been illustrated and described by referring to some preferred embodiments of the present invention, those of ordinary skill in the art should understand that the above content is a further detailed description of the present invention in combination with specific embodiments, and it cannot be determined that the specific embodiments of the present invention are limited to these descriptions. Those skilled in the art can make various changes in the form and details, including some simple deductions or replacements without departing from the spirit and scope of the present invention.