STEEL USED FOR HOT STAMPING, HOT STAMPING PROCESS AND FORMED COMPONENT

Abstract

A kind of steel is able to achieve a high elongation with the steel used for hot stamping by means of simple hot stamping process. The formed component has excellent yield strength, tensile strength and elongation. The steel used for hot stamping comprises by weight percent 0.1-0.19% of C, 5.09-9.5% of Mn, 0.11-0.4% of V, and 0-2% Si+Al, wherein the combination of C and V meets one of the following two requirements: 1) 0.1-0.17% of C and 0.11-0.4% of V; and 2) 0.171-0.19% of C and 0.209-0.4% of V.

Claims

1. A steel used for hot stamping, characterized in that the steel used for hot stamping comprises, by weight percent, the following components: 0.10.19% of C, 5.099.5% of Mn, 0.110.4% of V, and 02% Si+Al, wherein the combination of C and V also meets one of the following two requirements: 1) 0.10.17% of C and 0.110.4% of V; and 2) 0.1710.19% of C and 0.2090.4% of V.

2. The steel used for hot stamping according to claim 1, characterized in that the steel used for hot stamping also comprises at least one of the following components: 05% of Cr, 00.2% of Ti, 00.2% of Nb, 00.2% of Zr, 00.005% of B, 04% of Ni, 02% of Cu, 02% of Mo and 02% of W.

3. The steel used for hot stamping of claim 1, wherein the C content ranges from 0.12 to 0.17%, and the Mn content ranges from 5.09 to 8%.

4. The steel used for hot stamping of claim 1, wherein the steel used for hot stamping is provided on its surface with a coating selecting from the group comprising an AlSi coating, a galvanized coating and a high-temperature oxidization coating.

5. The steel used for hot stamping of claim 1, wherein the component ratio of the steel used for hot stamping meets the following requirement: the actual measured value of the martensitic transformation start temperature of the steel used for hot stamping after hot stamping is from 150 to 280 C.

6. A hot stamping process, wherein the hot stamping process comprises: Step A: heating the steel used for hot stamping of claim 1, or a preformed component obtained by preforming the steel used for hot stamping to a temperature ranging from 700 to 890 C. and maintaining the temperature for 0.1 to 10000 seconds; Step B: transferring the steel used for hot stamping or the preformed component processed in the Step A into a die for stamping so as to obtain a formed component; and Step C: cooling the formed component at an average cooling speed of 0.1 to 1000 C./s.

7. The hot stamping process according to claim 6, characterized in that in the Step A, the heating temperature ranges from 740 to 850 C.

8. The hot stamping process according to claim 7, characterized in that in the Step A, the heating temperature ranges from 740 to 780 C.

9. The hot stamping process according to claim 7, characterized in that in Step C, the average cooling speed is between 1 and 100 C./s.

10. A formed component, wherein the formed component is obtained by hot stamping the steel used for hot stamping of claim 1 or a preformed component made by preforming the steel used for hot stamping.

11. The formed component according to claim 10, characterized in that the formed component comprises, by volume, the following structures: 0.1 to 5% of vanadium carbide or composite carbonitride, 2 to 15% of retained austenite, 0 to 10% of ferrite, with the balance being martensite.

12. The formed component according to claim 10, characterized in that the formed component has an elongation of 6%.

13. The formed component of claim 10, wherein the formed component is heated and maintains the temperature within the temperature range from 140 to 220 C., and the time for the temperature maintaining lasts for 1 to 100000 seconds.

14. The formed component according to claim 13, characterized in that the formed component is used as a vehicle component, and the temperature maintaining is conducted for 5 to 30 minutes during the paint baking of the vehicle production procedure.

15. The formed component according to claim 13, characterized in that the formed component comprises, by volume, the following structures: 0.1 to 2% of vanadium carbide or composite carbonitride, 5 to 25% of retained austenite, 0 to 10% of ferrite, with the balance being martensite.

16. The formed component according to claim 13, characterized in that the formed component has a yield strength of 1100 MPa, a tensile strength of 1400 MPa and an elongation of 10%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 illustrates an example of a heat treatment process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0032] The technical solutions of the present invention will be explained with reference to the embodiments.

[0033] Steel used for hot stamping of the present invention comprises the following components by weight percent: 0.10.19% of C, 5.099.5% of Mn, 0.110.4% of V, and 02% Si+Al. The steel used for hot stamping may also comprise at least one of the following components: 05% of Cr, 00.2% of Ti, 00.2% of Nb, 00.2% of Zr, 00.005% of B, 04% of Ni, 02% of Cu, 02% of Mo and 02% of W, the content of which is also calculated by weight percent. The component ratio of the steel used for hot stamping is made in such a way that the actual measured value of the martensitic transformation start temperature (Ms) of the steel used for hot stamping after hot stamping is from 150 to 280 C.

[0034] The chemical components of the steel used for hot stamping of the present invention are listed as follows for the above reasons:

[0035] C: 0.1% to 0.19%

[0036] Carbon is the cheapest strengthening element that can greatly increase the strength of steel by interstitial solid solution. And the increase in the carbon content will greatly reduce the complete austenitizing temperature (Ac3), thereby reducing the heating temperature and saving energy. Although carbon can greatly reduce the martensitic transformation start temperature, the requirements of the alloy design for the martensitic transformation start temperature being 280 C. and the requirements for the microstructure of the steel must be met, and carbon is the most important interstitial solid solution strengthening element, therefore the lower limit of the carbon content is 0.1%. However, an excessively high carbon content may greatly affect the mechanical performance of steel and cause a great increase in strength and decrease in toughness of the steel, therefore the upper limit of carbon is 0.19%, and the carbon content that is higher than the value may cause brittle cracking of steels under the hot stamping state. More preferably, the C content ranges from 0.12% to 0.17%.

[0037] Mn: 5.09% to 9.5%

[0038] Mn is an important element in the present invention. Mn is a good deoxidizer and desulfurizer. Mn is an austenite stabilizing element that can expand the austenite region and reduce the Ac3 temperature. Mn has a good effect on inhibiting the transformation of austenite to ferrite and improving hardenability of steel. In order to decrease the heating temperature during the heat treatment, the lower limit of Mn is set to 5.09% so as to ensure that the martensitic transformation start temperature is 280 C., and meanwhile the complete austenitizing temperature (Ac3) of the material is guaranteed to be 780 C. so as to facilitate the formation of the galvanized sheet by hot stamping. Addition of too much Mn may result in that the material after quenching forms a brittle martensite, therefore the upper limit of Mn is set to 9.5%. More preferably, the Mn content ranges from 5.09 to 8%.

[0039] V: 0.11% to 0.4%

[0040] Vanadium is precipitated as strong carbide. Precipitation of vanadium carbide can achieve the effect of grain refinement and strength improvement. Vanadium carbide is precipitated from Vanadium during the austenitizing phase and the hot stamping phase, which, on the one hand, refines the original austenite grains, and, on the other hand, reduces the carbon content in the matrix, thereby keeping the carbon content in martensite at a low level after hot stamping. The present invention controls the carbon content in martensite after hot stamping by adding vanadium element and precipitating vanadium carbide, in order to guarantee the elongation and the elongation stability of the hot stamped material. Less than 0.11% of V cannot achieve an obvious effect and fails to meet the material design requirement of the present invention. However, addition of a large amount of vanadium element will lead to an increase in size of VC, and in steel cost. In order to keep stable elongation of initial steel after hot stamping, the V content shall be not more than 0.4%.

[0041] Si+Al: 0% to 2%

[0042] Si and Al can both inhibit the formation of carbides. When the steel is maintained at a temperature range below the Ac1 temperature after being quenched to room temperature, Si and Al can both inhibit precipitation of carbides in martensite and partition carbon to retained austenite to improve the stability of austenite and improve the product of strength and elongation of steel. In the industrial production, too much Al may block the nozzle in the continuous casting, increasing the difficulty in continuous casting, and Al may increase the martensitic transformation start temperature and the complete austenitizing temperature of the material, which does not meet the requirement of structure temperature control of the steel of the present invention. A high Si content will lead to more impurities in steel. The present invention adopts carbon-partitioning at a low temperature ranging from 140 to 220 C. During the low-temperature range, the formation of cementite will be inhibited, and only a portion of transitional carbides may be formed, but the portion of carbides will not significantly affect the toughness of the material. Addition of a large amount of Si and Al cannot inhibit the production of transitional carbides, so the present invention does not depend on the addition of Si+Al. The content of Si+Al in the present invention is not more than 2%.

[0043] Cr: 0% to 5%

[0044] Cr is also an element that can improve hardenability of a material and reduce the martensitic transformation start temperature. Thus, the percentage of Mn and Cr in steel is determined according to the requirements of the alloy design for the martensitic transformation start temperature and the carbon content in steel. Mn and Cr are added either alone or both. Preferably, Cr is not added due to high cost.

[0045] Ti, Nb, Zr: 0% to 0.2%

[0046] Ti, Nb and Zr refine the crystalline grains of steel, increase the strength of steel and render the steel a good heat treatment property. The excessive low concentration of Ti, Nb and Zr does not work, but more than 0.2% thereof will increase unnecessary costs. The steel of the present invention can obtain a strength of more than 1600 MPa and good elongation because of a reasonable design of C and Mn, so preferably it does not need to add extra Ti, Nb and Zr for the sake of cost reduction.

[0047] B: 0% to 0.005%

[0048] The segregation of B at austenite grain boundaries prevents the nucleation of ferrite, which may greatly improve the hardenability of steel, and significantly improve the strength of steel after the heat treatment. The B content of more than 0.005% cannot obviously make improvement. Since the design of high Mn content in steel of the present invention has a high hardenability, preferably it does not need to add extra B for the sake of cost reduction.

[0049] Ni: 0% to 4%; Cu:0% to 2%

[0050] Ni can increase the strength of steel and maintain the good plasticity and toughness of steel. If the concentration of Ni is more than 4.0%, the costs will be increased. Cu can increase the strength and toughness, especially atmospheric corrosion resistance. When the Cu content is greater than 2%, the processability may be deteriorated, and a liquid phase may be formed during hot rolling, which results in cracking. The high Cu content may also cause an increase in unnecessary costs. The steel of the present invention can obtain a strength of more than 1600 MPa and good elongation because of a reasonable design of C and Mn, so preferably it does not need to add extra Ni and Cu for the sake of cost reduction.

[0051] Mo and W: 0% to 2%

[0052] Mo and W can improve the hardenability of steel, and effectively increase the strength of steel. In addition, even if the steel is not sufficiently cooled due to its unstable contact with the die during the high-temperature forming process, the steel may still have a suitable strength due to the increased hardenability resulting from Mo and W. In the case of Mo and W being greater than 2%, no additional effects can be achieved, and costs will rise instead. Since the design of high Mn content in steel of the present invention has high hardenability, preferably it does not need to add extra Mo and W for the sake of cost reduction.

[0053] Unavoidable Impurities Such as P, S and N

[0054] In general, P is a harmful element in steel, which may increase the cold brittleness of steel, worsen the weldability, reduce the plasticity and deteriorate the cold bending property. Generally speaking, S is also a harmful element, which may cause hot brittleness of steel, and reduce the elongation and weldability of steel. N is an unavoidable element in steel. N is similar to carbon in terms of function and is helpful in bake hardening.

[0055] The steel used for hot stamping or its performed component of the present invention is hot stamped.

[0056] In one embodiment, the steel used for hot stamping or its preformed component is heated to a temperature ranging from 700 to 890 C. and maintains the temperature for 0.1 to 10000 seconds (Step A). In a process used in the experiment, the heating temperature ranges from 750 to 840 C. and the temperature is maintained for 5 minutes. As shown in FIG. 1, the heating temperature may be 780 C. and maintaining the temperature for 5 minutes. Then, the steel used for hot stamping or its preformed component is transferred into a die for hot stamping (Step B), and the formed component is cooled by air or other means to a temperature below 100 C. at an average cooling speed of 0.1 to 1000 C./s (Step C). After a period of time, the processed component is heated and maintains the temperature within a temperature range from 140 to 220 C. for 1 to 100000 seconds for a carbon-partitioning treatment and then cooled to room temperature. The cooling media may include, but are not limited to, air, water, oil and die surface. In a process used in the experiment, the heating and temperature maintaining is conducted within a temperature from 150 to 210 C. for 5 to 30 minutes. As shown in FIG. 1, the heating and temperature maintaining can be conducted during the paint baking of the vehicle production procedure.

[0057] Table 1 shows the components of steel used in an embodiment. The steel can be made into a sheet by the following processes, namely, a cast blank is maintained at the temperature of 1200 C. for 3 hours and then forged into a sheet blank, the sheet blank is maintained at the temperature of 1200 C. for 10 hours before undergoing a homogenization treatment and ground to make its superficial decarburized layer off, and then is heated to 1200 C. and maintains the temperature for 1 hour before being hot-rolled at a temperature ranging from 800 C. to 1200 C. to form a hot-rolled sheet. The hot-rolled pickled sheet is maintained at the temperature of 600 C. for 10 hours to simulate hooded annealing so as to reduce the strength of the hot-rolled sheet for the sake of cold rolling, the hot-rolled, pickled and annealed sheet is cold-rolled to be, e.g., 1.5 mm thick, and the cold-rolled sheet is annealed to simulate industrial cold-rolled sheet continuous annealing or coated sheet production process to obtain a steel sheet used for hot stamping.

[0058] In the tables, BT series is the steel of the present invention, and CT series is the compared steel, and the components of the CT series steel extend beyond the scope of the present invention.

[0059] Table 2 shows the processes adopted, and Table 3 shows the properties of the formed component obtained by treating the steel of Table 1 by means of the process shown in Table 2.

TABLE-US-00001 TABLE 1 Main Chemical Components of Steel C Mn Si V BT1 0.15 7.5 0.2 0.15 BT2 0.15 7.5 0.2 0.25 BT3 0.17 6.4 0.19 0.34 BT4 0.12 7.43 0.21 0.18 CT1 0.22 8 1.26 CT2 0.24 7.3 1.21 0.25 CT3 0.17 7.2 0.2

TABLE-US-00002 TABLE 2 Heat Treatment Processes of Steel austentizing temperature ( C.) and time for temperature baking No. maintaining quenching tem- baking Steel Process (min) in Step temperature perature Time Type No. A ( C.) ( C.) (min) BT1 1-1 780; 5 room temperature 20 1-2 850; 15 room temperature 20 1-1-200 780; 5 room 200 30 temperature 20 1-2-200 800; 5 room 200 30 temperature 20 1-3 800; 5 room temperature 20 1-4 760; 5 room temperature 20 1-5 780; 5 room temperature 20 1-5-170 780; 5 room 170 25 temperature 20 BT2 2-1 800; 5 room temperature 20 2-2 850; 15 room temperature 20 2-3 800; 5 room temperature 20 2-4 780; 5 room temperature 20 2-4-180 780; 5 room 180 30 temperature 20 2-4-200 780; 5 room 200 20 temperature 20 BT3 3-1 780; 5 room temperature 20 3-1-200 780; 5 room 200 20 temperature 20 BT4 4-1 780; 5 room temperature 20 4-1-200 780; 5 room 200 20 temperature 20 CT1 CT1-1 800; 6 room temperature 20 CT1-2 780; 5 room temperature 20 CT1-1-200 800; 6 room 200 30 temperature 20 CT2 CT2-1 780; 5 room temperature 20 CT2-2 780; 5 room temperature 20 CT2-2-170 780; 5 room 170 30 temperature 20 CT3 CT3-1 780; 5 room temperature 20 CT3-2 780; 5 room temperature 20 CT3-2-170 780; 5 room 170 30 temperature 20

TABLE-US-00003 TABLE 3 Mechanical Properties of Formed Component yield strength tensile strength elongation No. (MPa) (MPa) (%) BT1 1-1 970 1889 6.7 1-2 985 1826 7.1 1-1-200 1269 1630 12.2 1-2-200 1182 1611 10.2 1-3 989 1860 8.4 1-4 944 1810 9.6 1-5 930 1827 9.5 1-5-170 1148 1722 12.4 BT2 2-1 1001 1804 10.1 2-2 1024 1722 6.8 2-3 1016 1753 8.9 2-4 1020 1776 9.9 2-4-180 1210 1625 11.2 2-4-200 1234 1594 11.8 BT3 3-1 1083 1566 7.8 3-1-200 1220 1505 10.5 BT4 4-1 984 1545 9.1 4-1-200 1158 1458 10.8 CT1 CT1-1 1145 1.7(brittle failure) CT1-2 1211 1.9(brittle failure) CT1-1-200 1020 1844 8.8 CT2 CT2-1 1355 2.2(brittle failure) CT2-2 1327 1.9(brittle failure) CT2-2-170 1095 1941 10.2 CT3 CT3-1 1418 3.5(brittle failure) CT3-2 1390 2.8(brittle failure) CT3-2-170 1180 1729 10.6

[0060] The formed component which has not undergone the heating and temperature maintaining treatment (baking treatment) comprises, by volume, the following structures: 0.1 to 5% of vanadium carbide or a composite carbonitride, 2 to 15% of retained austenite, 0 to 10% of ferrite, with the balance being martensite. As known from 1-1, 1-2, 1-3, 1-4, 1-5, 2-1, 2-2, 2-3, 2-4, 3-1 and 4-1 of Table 3, all of these formed components have an elongation of more than 6%.

[0061] The formed component which has undergone the heating and temperature maintaining treatment comprises, by volume, the following structures: 0.1 to 2% of vanadium carbide or a composite carbonitride, 5 to 25% of retained austenite, 0 to 10% of ferrite, with the balance being martensite. As known from 1-1-200, 1-2-200, 1-5-170, 2-4-180, 3-1-200 and 4-1-200 of Table 3, all of these formed components have a yield strength of more than 1100 MPa, a tensile strength of more than 1400 MPa, and an elongation of more than 10%.

[0062] In contrast, irrespective of the heat treatment processes, the steels CT1, CT2, CT3 in the compared examples all fail to meet the four properties of the steel of the present invention: an elongation of 6% under hot stamping state (before carbon-partitioning); a yield strength of 1100 MPa, a tensile strength of 1400 MPa, and an elongation of 10% after carbon-partitioning (such as paint baking). In particular, as known from CT1-1, CT1-2, CT2-1, CT2-2, CT3-1, CT3-2, the steels CT1, CT2 and CT3 in the compared examples are very likely to suffer from brittle cracking before carbon-partitioning, whereas the steel of the present invention has an elongation of 6% before carbon-partitioning, which helps to avoid brittle cracking and can meet the requirement for weld assembling.

[0063] The formed component of the present invention can be used as a high-strength component for land vehicles, including, but not limited to, B-post reinforcers, bumpers, vehicle door anti-collision beams and wheel spokes.

[0064] The above embodiments and experimental data are intended to exemplarily explain the present invention. Those skilled in the art shall understand that the present invention is not limited to these embodiments, and can be changed without departing from the protection scope of the present invention.