Steel plate used for hot stamping forming, forming process of hot stamping and hot-stamped component

Abstract

A steel sheet used for hot stamping includes, by weight percent, 0.180.42% of C, 48.5% of Mn and 0.83.0% of Si+Al with the balance being Fe and unavoidable impurities. The alloy elements of the steel sheet enable the actual measured value of the martensitic transformation start temperature after hot stamping to be 280 C. The method for manufacturing the component includes: heating the material to 700850 C. and then stamping; cooling it to the temperature that is 150260 C. below the martensitic transformation start temperature by cooling in a die, cooling by air, water, or other methods; heating the component to a temperature ranging from 160 to 450 C. and maintaining the temperature for 1 to 100000 seconds for heat treatment, and then cooling the component to room temperature. The formed component has a yield strength of 1200 MPa, a tensile strength of 1600 MPa and a total elongation of 10%.

Claims

1. A hot-stamped formed component, characterized in that the formed component is manufactured of steel sheet by means of a hot stamping process, wherein the steel sheet comprises by weight percent 0.180.42% of C, 5.098.5% of Mn, and 0.83.0% of Si+Al with the balance being Fe and unavoidable impurities, wherein the alloy elements of the steel sheet enable the actual measured value of the martensitic transformation start temperature of the steel sheet after hot stamping to be 280 C., the hot stamping process comprises the steps of: a) providing the steel sheet or its preformed component; b) heating the steel sheet or its preformed component to a temperature ranging from 700 to 850 C.; c) transferring the heated steel sheet or its preformed component to a die for stamping so as to obtain a formed component; and d) cooling the formed component to a temperature that is 150 to 260 C. below the martensitic transformation start temperature point by any cooling method and under any cooling condition, wherein the hot-stamped formed component is also subjected to a tempering heat treatment step after the step d), in the tempering heat treatment step, the formed component is heated to a temperature ranging from 160 to 450 C. and then maintaining the temperature for 1 to 100000 seconds, and then cooling the formed component to room temperature by any cooling method and under any cooling condition, wherein the tempering heat treatment step is conducted immediately after the step d) or after the formed component that has been subjected to the step d) is placed for a period of time, wherein the microstructure of the formed component comprises, by volume, 7% to 32% of retained austenite and 10% or less of ferrite with the balance being martensite, and wherein the formed component has a yield strength of 1200 MPa or more, a tensile strength of 1600 MPa or more and an elongation of 10% or more.

2. The hot-stamped formed component according to claim 1, characterized in that the formed component is used as at least one of a safety structural component, a reinforcement structural component, a wheel component, and a structural component of land vehicles.

3. The hot-stamped formed component according to claim 2, characterized in that the formed component is used as at least one of a B column reinforcement, a bumper, a car door anti-collision beam and a wheel spoke.

4. The hot-stamped formed component according to claim 1, characterized in that the formed component further contains 2% or less of carbides.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A and FIG. 1B show the variation in the amount of retained austenite in a hot-rolled sheet of the steel of the present invention;

(2) FIG. 2A and FIG. 2B show the variation in the amount of retained austenite in a cold-rolled sheet of the steel of the present invention;

(3) FIG. 3 shows a microstructure of an embodiment of the steel of the present invention after the heat treatment of the present invention; and

(4) FIG. 4 shows a typical lath distribution microstructure of the steel of the present invention after the heat treatment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(5) The present invention will be described in detail with reference to the embodiments. The embodiments are intended to explain exemplary technical solutions and the present invention is not limited to these embodiments.

(6) The present invention provides a steel sheet which can be galvanized and directly hot stamped, and a formed component of the steel sheet, and provides a method for producing the formed component, and a heat treatment method for improving the strength-toughness of the hot stamped component. The formed component may have a yield strength of 1200 MPa or more, a tensile strength of 1600 MPa or more and an elongation of 10% or more. The method for producing the formed component requires a relatively low heating temperature, which can greatly save energy. The galvanized steel sheet can be directly used for hot stamping and remain sufficient strength. When being manufactured, the formed component is quenched to a temperature that is 150 to 260 C. below the martensitic transformation start temperature point, and may be cooled by air to room temperature or by warm water quenching, realizing uniform temperature and easy controllability.

(7) The chemical components (by weight percent) of the steel of the present invention are defined for the following reasons:

(8) C: 0.18% to 0.42%

(9) 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 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 0.18%. However, an excessively high carbon content may result in poor weldability of steel and cause a great increase in strength and decrease in toughness of the sheet, therefore the upper limit of carbon is 0.42%. A preferred value is between 0.22% and 0.38%.

(10) Mn: 4% to 8.5%, Cr: 5% or Less

(11) 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. Cr can improve oxidation resistance and corrosion resistance, and is an important alloy element in stainless steel. Cr is a moderate strong carbide forming element. It can not only improve the strength and hardness of steel by solid solution strengthening, but also improve the stability of austenite and increase the hardenability of steel as its diffusion rate in austenite is low and it can inhibit carbon diffusion. The increase in the Cr content can greatly improve the amount of retained austenite after quenching. 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. One or both of the two elements, Mn and Cr, can be added. In order to decrease the heating temperature during the heat treatment, the lower limit of Mn is set to be 4% 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 730 C. so as to ensure that the galvanized sheet can be formed 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 be 8.5%. The addition of Cr, together with Mn, may further reduce the martensitic transformation start temperature and the complete austenitizing temperature of the material, but Cr has a relatively weak capability in reducing the martensitic transformation start temperature and the complete austenitizing temperature as compared with that of Mn, and costs higher than Mn, therefore its upper limit is set to be 5%. Mn preferably ranges from 4.5 to 7.5%, and Cr is preferably not added due to its higher cost.

(12) Si+Al: 0.8% to 3.0%

(13) 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 from martensite to retained austenite to improve the stability of austenite and improve the strength-ductility of steel. The addition of too little Si and Al cannot sufficiently inhibit the precipitation of carbides in the course of hot stamping, therefore the lower limit of Si+Al is 0.8%. 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, therefore the upper limit of Al is set to be 1.5%. A high Si content will lead to more impurities in steel, therefore the upper limit of Si is set to be 2.5%, and the upper limit of Si+Al is set to be 3.0%. The preferred value of Si ranges from 0.8 to 2%, and the preferred value of Al is less than 0.5%.

(14) P, S and N Unavoidable Impurities

(15) 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 ductility and weldability of steel. N is an unavoidable element in steel. N is similar to carbon in terms of strengthening effect and is helpful in bake hardening.

(16) Mo and W: 2.0% or Less

(17) Mo and W can improve the hardenability of steel, and effectively increase the strength of steel. In addition, even if the steel sheet 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, there is preferably no need to add extra Mo and W for the sake of lowered costs.

(18) Ti, Nb, Zr and V: 0.2% or Less

(19) Ti, Nb, Zr and V refine the crystalline grains of steel, increase the strength of steel and render the steel a good heat treatment properties. The excessive low concentration of Ti, Nb, Zr and V 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 ductility because of a reasonable design of C and Mn, therefore there is preferably no need to add extra Ti, Nb, Zr and V for the sake of reduced costs.

(20) Cu: 2.0% or Less, Ni: 4% or Less

(21) 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. 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. The steel of the present invention can obtain a strength of more than 1600 MPa and good ductility because of a reasonable design of C and Mn, therefore there is preferably no need to add extra Cu and Ni for the sake of reduced costs.

(22) B: 0.005% or Less

(23) 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 in steel of the present invention has a high hardenability, there is preferably no need to add extra B for the sake of reduced costs.

(24) An object of the present invention is to produce a steel sheet with a yield strength of 1200 MPa or more, a tensile strength of 1600 MPa or more and an elongation of 10% or more. The steel sheet comprises a hot-rolled steel sheet, a cold-rolled steel sheet, and a galvanized steel sheet. The microstructure of the steel sheet before heat treatment comprises, by volume, 3% to 23% of retained austenite, 10% or less (inclusive of 0%) of ferrite, with the balance being martensite, or further containing 2% or less of carbides. The steel sheet can be galvanized and directly formed by hot stamping.

(25) The method for manufacturing the formed component will be described. The steel sheet is processed by stamping, and heated to a temperature ranging from 700 to 850 C., preferably from 730 to 780 C., before the hot stamping. As for the preformed component of the steel sheet, after the cold stamping, it is heated to a temperature ranging from 700 to 850 C., preferably from 730 to 780 C. Subsequently, the stamped steel sheet is cooled within a die, or by air, or by other cooling method, to a temperature that is 150 to 260 C. below the martensitic transformation start temperature, preferably cooled to a temperature from room temperature to 100 C. Then, the microstructure of the formed component comprises, by volume, 3% to 23% of retained austenite, 10% or less (inclusive of 0%) of ferrite, with the balance being martensite, or further containing 2% or less of carbides. Too much retained austenite will render it unstable, whereas excessively high martensite content will render the amount of retained austenite insufficient, and a high amount of formed carbides will reduce the carbon content in austenite rendering it unstable, in such a way that the requirement of the present invention for elongation cannot be met. Deformation induced ferrite may occur during the hot forming process, and the amount of ferrite should not exceed 10% in order to achieve the desired strength.

(26) Then, the stamped component is cooled to room temperature after the heat treatment in which the stamped component is maintained at a temperature ranging from 160 to 450 C. for 1 to 10000 seconds. The microstructure of the heated formed component at this time comprises, by volume, 7% to 32% of retained austenite, 10% or less (inclusive of 0%) of ferrite, with the balance being martensite, or further containing 2% or less of carbides. During the heat treatment, carbon is partitioned from martensite to austenite to stabilize austenite, such that the component in the final state of use has a reasonable austenite volume fraction in steel and stability in order to obtain high strength-toughness. It should be noted that according to the heat treatment process of the present invention, the volume percentage of austenite in steel can be increased by 2% or more as compared with that before the heat treatment.

(27) The design on the alloy component in the steel of the present invention shall meet the requirement that the actual measured value of the martensitic transformation start temperature of the steel is 280 C. Addition of alloy elements will obviously reduce the austenitizing temperature of the steel. The steel sheet or the preformed component is formed by stamping after being heated to a temperature ranging from 700 to 850 C., preferably 730 to 780 C., wherein the steel sheet is maintained at the temperature range for 1 to 10000 seconds. It is then cooled to a temperature that is 150 to 260 C. below the martensitic transformation start temperature point, preferably cooled below 100 C. to room temperature or even a lower temperature. The cooling method comprises cooling in a die, cooling by air, hot water or cold water, or other cooling methods, with a cooling rate being 0.1 to 1000 C./s. The stamped and cooled component is heated again to a temperature range lower than or equal to Ac1 for heat treatment, and the steel sheet is maintained at the temperature range for 1 to 10000 seconds. It is then cooled to room temperature by any cooling method and under any cooling condition. If the maintenance time is less than 1 second, carbon may not be sufficiently diffused into retained austenite; and if it is larger than 10000 seconds, the austenite may be overly softened and the strength of the steel sheet may be decreased to an extent that cannot meet the requirement of the design.

(28) During the heat treatment, carbon is partitioned from martensite to austenite to stabilize austenite, which can improve the strength-toughness of steel. In a preferable case, after a low-temperature heat treatment, the volume percentage of retained austenite in steel will obviously increase by 2% or more as compared with that before the heat treatment. The newly generated austenite will apparently increase the plasticity of steel and is conducive to preventing cracks from expansion, thereby greatly enhancing the strength-elongation product of steel.

(29) The experiments based on the steel sheet of the present invention will be described. The steel ingot having the elements as determined in Table 1 shall be homogenized by maintaining temperature for 10 hours at 1200 C. and then maintained for 1 hour at a temperature between 1000 to 1200 C. and then hot rolled into a hot-rolled sheet. The hot-rolled sheet or hot-rolled pickling sheet is maintained for 5 to 32 hours at a temperature ranging from 600 to 700 C., and simulated batch annealing is performed to reduce the strength of the hot-rolled sheet and is advantageous to cold rolling. Then the hot-rolled picking sheet or hot-rolled pickling annealing sheet is cold rolled to 1.5 mm. In Table 1, No.IS1 to IS11 are the steels of the present invention, and No. CS1 to CS5 are contrast steels containing components recorded in the prior art.

(30) TABLE-US-00001 TABLE 1 Chemical Components of Steel Chemical components (weight %) No. C Mn Si others IS1 0.3 6.66 1.05 IS2 0.18 8.5 1.72 IS3 0.28 6.22 1.57 IS4 0.42 5.3 1.75 0.5Al, 0.05Ti, 0.05V, 0.05Nb, IS5 0.27 5.75 1.05 IS6 0.3 6.71 1.65 IS7 0.33 5.09 1.63 IS8 0.28 4.0 1.75 IS9 0.40 6.95 2Al IS10 0.3 6.5 1.7 2.9Cr, 0.5Cu, 1Ni IS11 0.3 6.5 1.7 0.5Mo, 0.5W CS1 0.31 3.03 1.6 CS2 0.11 3.03 1.6 CS3 0.11 4.84 CS4 0.2 4.92 1.7 CS5 0.22 1.2 0.2

(31) Then, the steel sheet containing the above components is formed by hot stamping using the process parameters shown in Table 2. To be specific, the steel sheet or its preformed component of the present invention is heated in a furnace to a temperature ranging from 700 to 850 C. (AT) and maintained at the temperature for 10 minutes, and then transformed to a die for hot stamping, and the formed component is cooled by air or by other method to a temperature below 100 C. (QT). After a time period, the processed formed component is heated to a temperature ranging from 180 to 500 C. (TT) and maintained at the temperature for a time period for heat treatment, and then cooled to room temperature. In addition, the contrast steel sheet is formed and heat treated according to the parameters of the hot stamping process in the prior art as shown in Table 3. It shall be noted that in Tables 2 and 3, IS is the steel of the present invention, AT is the austentizing temperature, TT is a heat treatment temperature, Ms is the martensitic transformation start temperature. The balance temperatures Ae1 and Ae3 in the Tables are calculated according to the components of steel by thermodynamical software Thermal-cal.

(32) TABLE-US-00002 TABLE 2 No. heat treatment conditions steel sample Ae3/ Ae1/ Ms/ AT/ one-step cooling TT/ Heat Treatment type No. C. C. C. C. temperature/ C. C. Time,/min IS1 ISP1 690 495 201 850 0 C. 250 30 ISP2 850 0 C. 250 60 ISP3 850 0 C. 300 5 ISP4 850 0 C. 300 10 ISP5 760 0 C. 250 10 ISP6 760 0 C. 250 30 ISP7 760 0 C. 250 60 ISP8 760 0 C. 300 5 ISP9 760 0 C. 300 10 IS2 ISP10 682 323 204 780 room 250 30 temperature 15 C. ISP11 750 room 300 10 temperature 15 C. IS3 ISP12 704 524 219 760 30 C. 250 30 ISP13 760 30 C. 300 10 IS4 ISP14 723 609 219 780 40 C. 250 30 ISP15 780 40 C. 300 10 IS5 ISP16 708 531 242 760 80 C. 250 30 ISP17 760 80 C. 300 10 IS6 ISP18 692 511 196 700 room 300 10 temperature 15 C. ISP19 740 room 300 10 temperature 15 C. ISP20 760 room 250 10 temperature 15 C. IS7 ISP21 722 592 232 760 60 C. 300 30 ISP22 760 60 C. 300 60 IS8 ISP23 744 605 255 850 100 C. 250 30 ISP24 850 100 C. 300 10 IS9 ISP25 782 525 217 850 room 300 10 temperature 15 C. IS10 ISP26 750 490 178 760 room 250 30 temperature 15 C. ISP27 760 room 300 10 temperature 15 C. IS11 ISP28 758 517 200 760 room 250 30 temperature 15 C. ISP29 760 300 10

(33) TABLE-US-00003 TABLE 3 No. heat treatment conditions steel sample Ae3/ Ae1/ Ms/ AT/ quenching partitioning partitioning type No. C. C. C. C. temperature/ C. temperature/ C. time/min CS1 CSP1 804 680 293 900 201 423 10 s CSP2 900 198 423 100 s CSP3 900 176 419 500 s CS2 CSP4 852 669 377 900 269 421 10 s CS3 CSP5 750 150 cooling by air ro room temperature CS4 CSP6 750 150 cooling by air ro room temperature CSP7 750 200 250 10 CSP8 750 200 300 10 CS5 CSP9 930 room temperature

(34) After the above hot forming and heat treatment process, the mechanical properties of different steel and corresponding heat treatment process at room temperature are analyzed, the result of which is shown in Table 4. IS in Table 4 still represents the steel of the present invention, while CS indicates the contrast steel. In addition, YS indicates yield strength, TS indicates tensile strength, TE indicates total elongation, HR is hot-rolled steel, and CR is cold-rolled steel. In addition, tensile specimens in Table 4 are ASTM standard specimens having a 50 mm gauge length, and the strain rate of tensile mechanical properties tests is 510.sup.4.

(35) TABLE-US-00004 TABLE 4 No. sample steel plate steel type No. type YS/Mpa TS/Mpa TE/% IS1 ISP1 HR 1274 1784 14.2 ISP2 HR 1263 1754 15.1 ISP3 HR 1290 1741 16.2 ISP4 HR 1230 1690 16.1 ISP5 CR 1250 1830 15 ISP6 CR 1247 1800 15.8 ISP7 CR 1279 1794 17.1 ISP8 CR 1297 1748 14.7 ISP9 CR 1292 1729 15.5 IS2 ISP10 CR 1245 1634 15.6 ISP11 CR 1210 1593 16.4 IS3 ISP12 CR 1316 1795 15 ISP13 CR 1284 1776 14.6 IS4 ISP14 CR 1520 2040 10.9 ISP15 CR 1490 1998 11.6 IS5 ISP16 CR 1256 1790 14 ISP17 CR 1248 1763 13.2 IS6 ISP18 CR 1317 1731 12.8 ISP19 CR 1374 1765 15.2 ISP20 CR 1340 1854 15.5 IS7 ISP21 CR 1386 1894 12.4 ISP22 CR 1364 1863 13.6 IS8 ISP23 CR 1258 1793 14.9 ISP24 CR 1217 1734 16.6 IS9 ISP25 HR 1210 1620 14.5 IS10 ISP26 HR 1356 1830 16.8 ISP27 HR 1374 1812 14.5 IS11 ISP28 HR 1380 1857 16.5 ISP29 HR 1391 1849 15.6 CS1 CSP1 CR 971 1594 14.8 CSP2 CR 1063 1479 16.8 CSP3 CR 1013 1445 17.7 CS2 CSP4 CR 1003 1194 13.7 CS3 CSP5 1030 1450 11 CS4 CSP6 1100 1880 12 CSP7 1008 1530 16 CSP8 1020 1460 16 CS5 CSP9 1200 1500 7

(36) It can be known from the mechanical properties data shown in Table 4, a formed component with an excellent combination of strength and elongation can be made of the steel sheet having the components of the present invention by the hot stamping process of the present invention. To be specific, it can make a formed component with a yield strength of 1200 MPa, a tensile strength of 1600 MPa and a total elongation of 10%. In contrast, the formed component made of the steel sheet having the components in the prior art by the hot stamping process in the prior art has a lower comprehensive performance, and the yield strength thereof is lower than 1200 MPa when the elongation is greater than 10%. Because the yield strength is an important parameter to evaluate the performance of vehicle safety structural components, the formed component made of the steel sheet of the present invention by the hot stamping process of the present invention achieves a comprehensive performance much better than the existing technology.

(37) Moreover, it can be known by analysing the microstructure of the steel of the present invention that the microstructure of the steel without being subject to the heat treatment comprises, by volume, 3% to 23% of retained austenite, 10% or less of ferrite, with the balance being martensite, or further containing 2% or less of carbides. After being subject to the heat treatment, the microstructure of the formed component comprises, by volume, 7% to 32% of retained austenite, 10% or less of ferrite, with the balance being martensite, or further containing 2% or less of carbides. FIG. 1A shows a tendency of retained austenite in the hot-rolled steel sheet of the present invention that varies with different heat treatment time at the same temperature, i.e., 250 C. FIG. 1B shows the tendency of retained austenite in the hot-rolled steel sheet of the present invention that varies with different heat treatment time at the same temperature, i.e., 300 C. FIG. 2A shows the variation in the amount of retained austenite in the cold-rolled steel sheet of the present invention at 250 C. under different heat treatment processes. FIG. 2B shows the variation in the amount of retained austenite in the cold-rolled steel sheet of the present invention at 300 C. under different heat treatment processes. As these figures show, under different heat treatment processes, the amount of retained austenite in the steel sheet of the present invention generally increases with time.

(38) A small fraction of retained austenite is not good to improving the ductility of a component, whereas a high volume fraction of retained austenite will cause austenite to form into coarse blocks, which will transform into brittle blocky martensite by TRIP effect during tensile deformation or collision deformation, which is bad to improving the ductility of the component. Thus, the present invention controls the martensitic transformation start temperature point to be not more than 280 C. and the quenching temperature to be 150 to 260 C. below the martensitic transformation start temperature point, so as to guarantee a reasonable volume fraction of austenite and a lath (or film) like morphology. FIG. 3 shows the microstructure after being subjected to the heat treatment for 5 minutes at 300 C. after austenitizing treatment. And FIG. 4 shows a typical lath distribution microstructure.

(39) The above embodiments are typical embodiments of the present invention. Without departing from the inventive concept disclosed herein, those skilled in the art can make any modifications to the above embodiments that still fall within the scope of the present invention.