CN113337785A

CN113337785A - 1800 MPa-grade hot forming steel and hot stamping forming method and application thereof

Info

Publication number: CN113337785A
Application number: CN202110629706.4A
Authority: CN
Inventors: 韦习成; 宋宁宏; 倪雷; 郭亚洲; 林超; 孙浩; 王武荣
Original assignee: University of Shanghai for Science and Technology
Current assignee: University of Shanghai for Science and Technology
Priority date: 2021-06-07
Filing date: 2021-06-07
Publication date: 2021-09-03

Abstract

The invention provides 1800 MPa-grade hot forming steel, a hot stamping forming method and application thereof, and belongs to the technical field of hot forming steel. The 1800 MPa-grade hot forming steel provided by the invention comprises the following chemical components: 0.28-0.32% of C, 0.2-0.3% of Si, 1.5-1.7% of Mn, less than or equal to 0.015% of P, less than or equal to 0.01% of S, 0.002-0.0035% of B, 0.15-0.25% of Cr, 0.02-0.05% of Al, less than or equal to 0.03% of Ti, less than or equal to 0.005% of N, 0.04-0.055% of Nb0.15-0.25% of V, and the balance of Fe and impurities. The combined action of C, Si, Mn, P, S, B, Cr, Al, Ti, N, Nb and V with specific contents and tempering by utilizing waste heat after quenching ensure the tensile strength and the elongation of the 1800 MPa-grade hot forming steel member.

Description

1800 MPa-grade hot forming steel and hot stamping forming method and application thereof

Technical Field

The invention relates to the technical field of hot forming steel, in particular to 1800 MPa-level hot forming steel and a hot stamping forming method and application thereof.

Background

With the rapid development of the automobile industry, light weight and safety become the main direction of the development of the automobile industry. By adopting a large amount of hot-formed steel plates in the body-in-white, the thickness of the steel plates is reduced on the premise of ensuring the safety of the vehicle body, and the purpose of reducing the weight on the premise of ensuring the safety is achieved. At present, the most applied steel plates are low-carbon Mn-B series steel plates, the quenched structure of which is changed into complete martensite, the strength of which is between 1300 and 1500MPa, and the steel plates are mainly applied to components such as an A column, a B column, a vehicle door anti-collision beam and the like. In recent 10 years, higher grade hot forming steels (1800-2000 MPa) have been studied.

For example, chinese patent CN107354385A discloses a method for preparing ultra-high strength hot formed steel for automobiles, the hot formed steel comprises the following components: 0.5-0.6% of C, 0.5-2% of Mn, 1.5-2.5% of Si, 1-3% of Cr, 1-2% of Al, 0.01-0.03% of Nb, 0.001-0.005% of B and the balance of ironAnd no impurities, but the alloy elements are higher, so that the popularization in the automobile industry is more difficult. Chinese patent CN102296242A discloses a heat treatment method for a high strength and toughness hot formed steel plate for an automobile, which comprises the following components: 0.20-0.40% of C, 0.10-0.50% of Si, 1.0-2.05% of Mn, less than or equal to 0.02% of P, 0.02-0.06% of Nb, 0.01-0.05% of Ti, 0.1-0.5% of Cr, 0.001-0.005% of B, 0.01-0.1% of Al and less than or equal to 0.01% of N. Chinese patent CN103361560A discloses a cold-rolled hot-formed steel plate and a production method thereof, wherein the hot-formed steel plate comprises the following components: 0.22 to 0.25% of C, 0.20 to 0.30% of Si, 1.20 to 1.40% of Mn, and P<0.020％，S<0.030%, 0.10-0.30% Cr, 0.02-0.050% Ti, 0.025-0.060% Al, 0.002-0.004% B, less than or equal to 0.006% N, and the balance Fe, wherein the strength of the hot-formed steel plate is about 1500MPa, the elongation A is₈₀About 5.7 percent, the component design system is a system containing B and is a complete martensite structure, but the strength and the elongation rate of the component design system are both required to be improved and do not reach the use requirements of automobile parts (the strength is not less than 1800MPa and the elongation rate is not less than 6 percent). Chinese patent CN106119693A discloses a thin hot forming steel with tensile strength more than or equal to 2100MPa directly rolled by a thin slab and a production method thereof, wherein the hot forming steel comprises the following components: 0.41-0.50% of C, 0.45-0.65% of Si, 1.6-2.0% of Mn, less than or equal to 0.006% of P, less than or equal to 0.004% of S, 0.015-0.06% of Al, 0.50-0.65% of Cr, 0.004-0.005% of B, 0.046-0.060% of Ti, 0.046-0.060% of Nb, 0.046-0.060% of V, or a combination of more than two of the above, 0.036-0.60% of Mo, 0.21-0.35% of Ni, less than or equal to 0.004% of N, and the balance of Fe and no impurities; the carbon content of the alloy is high, and a large amount of alloy elements are added, so that the carbon equivalent is more than or equal to 0.78, and the weldability and the production cost are seriously influenced. From this, it is known that the steel sheet after hot forming has a completely martensitic structure, and although the strength of the formed article is improved, the elongation thereof is difficult to break through by 7%, so that the deformation of parts during collision is very likely to fail, and the safety of the vehicle is greatly reduced and the danger to passengers is increased. Therefore, on the premise of keeping the strength of the hot formed steel basically unchanged, the elongation rate of the hot formed steel is improved to become the ultrahigh-strength hot forming for automobilesThe difficult problem of shape steel.

Disclosure of Invention

In view of the above, the invention aims to provide 1800 MPa-grade hot forming steel and a hot stamping forming preparation method thereof, and the 1800 MPa-grade hot forming steel provided by the invention has a large elongation.

In order to achieve the above object, the present invention provides the following technical solutions:

the invention provides 1800 MPa-grade hot forming steel which comprises the following chemical components in percentage by mass: 0.28-0.32% of C, 0.2-0.3% of Si, 1.5-1.7% of Mn, less than or equal to 0.015% of P, less than or equal to 0.010% of S, 0.002-0.0035% of B, 0.15-0.25% of Cr, 0.02-0.05% of Al, less than or equal to 0.03% of Ti, less than or equal to 0.005% of N, 0.04-0.055% of Nb, 0.15-0.25% of V, and the balance Fe and no impurities.

The invention provides a hot stamping forming method of 1800 MPa-grade hot forming steel, which comprises the following steps:

providing a casting blank; the chemical composition of the casting blank is consistent with that of the 1800MPa grade hot forming steel of claim 1;

carrying out hot continuous rolling on the casting blank to obtain a hot rolled plate;

cold rolling the hot rolled plate to obtain a cold rolled plate;

transferring the cold-rolled sheet to a hot stamping die for hot stamping forming-quenching treatment after complete austenitizing to obtain a formed piece; the heating temperature for complete austenitizing is 880-950 ℃, and the heat preservation time is 3-7 min; the temperature of the formed piece is more than or equal to 150 ℃; the temperature of the hot stamping forming-quenching treatment is 830-870 ℃, and the cooling speed is 2-20 ℃/s;

stacking the formed parts, and tempering by using waste heat to obtain 1800 MPa-level hot formed steel members; the tempering temperature is 150-220 ℃, and the tempering time is 5-20 min.

Preferably, the heating temperature of the hot continuous rolling casting blank is 1200-1260 ℃, the initial rolling temperature is 1120-1180 ℃, and the final rolling temperature is 850-910 ℃.

Preferably, the thickness of the hot-rolled plate is 2.7-3.3 mm.

Preferably, the total reduction rate of the cold rolling is 45-60%.

Preferably, the thickness of the cold-rolled plate is 1.2-2.0 mm.

Preferably, the transfer time is ≦ 9 s.

The invention also provides application of the 1800 MPa-grade hot forming steel in the technical scheme or the 1800 MPa-grade hot forming steel component obtained by the hot stamping forming method in an automobile.

The invention provides 1800 MPa-grade hot forming steel which comprises the following chemical components in percentage by mass: 0.28-0.32% of C, 0.2-0.3% of Si, 1.5-1.7% of Mn, less than or equal to 0.015% of P, less than or equal to 0.010% of S, 0.002-0.0035% of B, 0.15-0.25% of Cr, 0.02-0.05% of Al, less than or equal to 0.03% of Ti, less than or equal to 0.005% of N, 0.04-0.055% of Nb, 0.15-0.25% of V, and the balance Fe and no impurities. In the invention, C is an important element for determining the strength of steel, Si is an element for inhibiting carbide precipitation, and the element plays a role in deoxidation in molten steel and reduces impurities in the steel; mn is a solid-solution strengthening element, can reduce the phase transformation driving force and improve the hardenability of steel, and is beneficial to obtaining a martensite structure by controlling the content of Mn; b is an element for improving the hardenability of the steel, and the strength of the steel can be reduced by preventing the transformation from austenite to ferrite in the steel in the hot forming process through the content of B; cr can obviously improve the hardenability of steel, and the invention can ensure that a steel plate can quickly form a martensite structure in the quenching process by controlling the Cr content, and simultaneously improve the strength and the hardness of the steel plate; si and Al play a role in deoxidizing in steel, the invention needs a certain amount of acid-soluble aluminum in steel to ensure that the deoxidizing effect of Al is fully exerted, and Si can also reduce impurities in steel; the harmful element N can be fixed in the form of nitride by controlling the Ti content, and the hardenability reducing element from B caused by N can be prevented; nb is a strong C, N compound forming element and can refine grains, and the invention forms a large amount of dispersed Nb C, N compounds in the steel by controlling the Nb content, thereby improving the strength and toughness of the hot forming steel; v can play a solid solution role in the heating process, and the invention can improve the V content by controlling the V contentThe hardenability of the high hot forming steel widens the quenching process window. The combined action of C, Si, Mn, P, S, B, Cr, Al, Ti, N, Nb and V with specific contents ensures the tensile strength and elongation of the 1800MPa grade hot forming steel, and as shown in the example results, the tensile strength of the hot forming steel is more than or equal to 1800MPa, and the elongation A is₂₅Not less than 8 percent, can meet the requirement of the ultrahigh strength hot forming steel for the automobile.

The invention provides a preparation method of 1800MPa grade hot forming steel, which comprises the following steps: providing a casting blank; the chemical components of the casting blank are consistent with those of the 1800MPa grade hot forming steel in the technical scheme; carrying out hot continuous rolling on the casting blank to obtain a hot rolled plate; cold rolling the hot rolled plate to obtain a cold rolled plate; transferring the cold-rolled sheet to a hot stamping die for hot stamping forming-quenching treatment after complete austenitizing to obtain a formed piece; the heating temperature for complete austenitizing is 880-950 ℃, and the heat preservation time is 3-7 min; the temperature of the formed piece is more than or equal to 150 ℃; the temperature of the hot stamping forming-quenching treatment is 830-870 ℃, and the cooling speed is 2-20 ℃/s; stacking the formed parts, and tempering by using waste heat to obtain 1800 MPa-level hot formed steel members; the tempering temperature is 150-220 ℃, and the tempering time is 5-20 min. According to the invention, the casting blank is subjected to hot continuous rolling and cold rolling to obtain the cold-rolled sheet with a smooth surface, low strength and good forming performance, so that the requirements of blanking and preforming before hot stamping are met; the cold-rolled sheet can be completely austenitized by keeping the temperature for 3-7 min after the temperature is 880-950 ℃, hot stamping forming-quenching treatment is carried out at the temperature of 830-870 ℃, the cooling speed is controlled to be 2-20 ℃/s, and the cold-rolled sheet is rapidly cooled by quenching through a die so as to finish the transformation from austenite to martensite; the method has the advantages that the thickness accumulation is realized by stacking the parts subjected to the hot stamping forming-quenching treatment, excessive surface heat loss of the parts subjected to the die stripping is reduced, waste heat after the die stripping can be fully utilized, the heat loss is reduced, the self-tempering effect is realized, the dislocation density is reduced in the process of tempering at 180-220 ℃ for 5-25 min, the martensite is decomposed to separate out transition carbide, the pinning effect on dislocation is realized, the reduction of the tensile strength and the yield strength is avoided, meanwhile, part of residual stress of quenched steel is released in the tempering process, the tissue uniformity and the elongation (plasticity) are improved, the tensile strength of the finally prepared 1800 MPa-grade hot forming steel is high, the elongation is high, and the requirements of the ultrahigh-strength hot forming steel for automobiles can be met. Moreover, the preparation method provided by the invention can be directly used for production on the traditional hot forming steel production line, no additional equipment is required, and the production process is stable.

Drawings

FIG. 1 is a CCT diagram of a cast slab prepared in example 1;

FIG. 2 is an SEM image of the microstructure of a 1800MPa grade hot formed steel prepared in example 1;

FIG. 3 is an SEM image of the microstructure of a 1800MPa grade hot formed steel prepared in example 2;

FIG. 4 is an SEM image of the microstructure of a 1800MPa grade hot formed steel prepared in example 3.

Detailed Description

The 1800 MPa-grade hot forming steel provided by the invention comprises 0.28-0.32% of C, preferably 0.29-0.31%, and more preferably 0.3%. In the present invention, C is an important element determining the strength of steel, and when the content of C is too high, the strength of steel is too high and the toughness is lowered, and the toughness of steel is remarkably lowered by increasing the amount of carbide in steel. The invention limits the content in the range, and ensures that the steel obtains the tensile strength of more than 1800MPa after quenching and tempering.

The 1800MPa grade hot forming steel provided by the invention comprises 0.2-0.3% of Si, preferably 0.21-0.29%, more preferably 0.22-0.28%, and particularly preferably 0.23%, 0.24%, 0.25%, 0.26%, 0.27% or 0.28%. In the invention, Si is an element for inhibiting carbide precipitation, and can also play a role in deoxidation in molten steel, thereby reducing impurities in the steel; the Si content is too low to facilitate deoxidation; the surface quality of the steel is reduced due to the excessively high Si content; the present invention limits the content thereof to the above range to ensure the forming quality of the steel.

The 1800MPa grade hot forming steel provided by the invention comprises 1.5-1.7% of Mn, preferably 1.51-1.69%, more preferably 1.55-1.65%, and particularly preferably 1.55%, 1.56%, 1.57%, 1.58%, 1.59%, 1.60%, 1.61%, 1.62%, 1.63%, 1.64% or 1.65%. In the present invention, the Mn is a solid solution strengthening element; the Mn content is too low, and the solid solution strengthening effect is not obvious; the Mn content is too high, Mn segregation is easily formed in the steel plate, and the performance of the steel plate is uneven; the invention controls the Mn content in the range, can reduce the phase transformation driving force, improves the hardenability of the steel, and is beneficial to obtaining a martensite structure.

The 1800 MPa-grade hot forming steel provided by the invention comprises P less than or equal to 0.015 percent, preferably less than or equal to 0.012 percent, and more preferably less than or equal to 0.010 percent. In the invention, P is a harmful element, the lower the content of P is, the better the P content is, however, the lower the content requirement is, the high production cost of the hot-formed steel is caused; the invention controls the P content in the range, does not influence the performance of the hot forming steel and has low production cost.

The 1800 MPa-grade hot forming steel provided by the invention comprises S less than or equal to 0.010%, preferably less than or equal to 0.008%, and more preferably less than or equal to 0.005%. In the invention, S is a harmful element, the lower the content of S is, the better the S content is, however, the low content requirement can cause the production cost of the hot-formed steel to be high; the invention controls the S content in the range, does not influence the performance of the hot forming steel and has low production cost.

The 1800 MPa-grade hot forming steel provided by the invention comprises B0.002-0.0035%, preferably 0.0021-0.0030%, particularly preferably 0.0023-0.0028%, and most preferably 0.0024%, 0.0025%, 0.0026% or 0.0027%. In the present invention, B is an element that improves the hardenability of steel, and excess B causes no ferrite structure in the hot-formed steel sheet, improving the strength of the steel sheet, and reducing the elongation; the present invention controls the content of B within the above range to prevent the transformation of austenite to ferrite in the steel during hot forming to lower the strength of the steel.

The 1800 MPa-grade hot forming steel provided by the invention comprises 0.15-0.25% of Cr, preferably 0.16-0.24%, more preferably 0.15-0.23%, and particularly preferably 0.16%, 0.17%, 0.18%, 0.19%, 0.20%, 0.21% or 0.22%. In the invention, Cr can obviously improve the hardenability of steel, the Cr content is too low, the hardenability of the steel plate is not good, and a bainite structure is easily formed during quenching, so that the strength of the steel plate is reduced; the elongation of the steel plate is reduced due to the overhigh Cr content; the invention controls the Cr content in the range, can ensure that the steel plate can quickly form a martensite structure in the quenching process, and simultaneously improves the strength and the hardness of the hot forming steel.

The 1800 MPa-grade hot forming steel provided by the invention comprises 0.02-0.05% of Al, preferably 0.021-0.049%, more preferably 0.022-0.048%, particularly preferably 0.023%, 0.024%, 0.025%, 0.026%, 0.027%, 0.028%, 0.029%, 0.030%, 0.031%, 0.032%, 0.033%, 0.034%, 0.035%, 0.036%, 0.037%, 0.038%, 0.039%, 0.040%, 0.041%, 0.042%, 0.043%, 0.044%, 0.045%, 0.046% or 0.047%. In the invention, Al plays a role in deoxidation in steel, and the content of Al is too high, so that impurities such as alumina and the like are easily generated, and the performance of the steel plate is reduced; the invention controls the Al content in the range, can ensure that a certain amount of acid-soluble aluminum exists in the steel, and further fully exerts the deoxidation effect of the Al.

The 1800 MPa-grade hot forming steel provided by the invention comprises Ti which is less than or equal to 0.03%, preferably 0.001-0.03%, more preferably 0.005-0.025%, and most preferably 0.01-0.02%. In the present invention, Ti can fix a harmful element N in the form of nitride and can prevent the decrease in hardenability caused by N, B, and Ti content is too low to effectively suppress the decrease in hardenability caused by B; too high Ti content results in too coarse Ti-containing carbonitrides, which reduces the bendability of the hot-formed steel; the present invention controls the Ti content within the above range, and can restrict coarse inclusions formed by Ti and N, B to not more than 0.03%, thereby improving the hardenability of the hot formed steel.

The 1800 MPa-grade hot forming steel provided by the invention comprises less than or equal to 0.005% of N, preferably 0.001-0.004%, and more preferably 0.002-0.003%. In the present invention, N is a harmful element, and too high N content results in too coarse carbonitrides of Ti and C, N to reduce the bendability of the hot formed steel. In the present invention, the N content is controlled within the above range, and the bendability of the hot-formed steel can be improved.

The 1800 MPa-grade thermoformed steel provided by the invention comprises 0.04-0.055% of Nb, preferably 0.041-0.054%, more preferably 0.042-0.053%, and particularly preferably 0.043%, 0.044%, 0.045%, 0.046%, 0.047%, 0.048%, 0.049%, 0.050%, 0.051% or 0.052%. In the invention, the Nb strong C, N compound forming element can refine grains; the effect is not obvious when the Nb content is too low; the Nb content is too high, the grain refining effect of the steel plate is not increased any more, and the production cost of the hot-formed steel is increased; the invention can improve the strength and toughness of the hot forming steel by controlling the Nb content to form a large amount of dispersed and distributed Nb C, N compounds in the steel.

The 1800 MPa-grade hot forming steel provided by the invention comprises V0.15-0.25%, preferably 0.16-0.24%, more preferably 0.17-0.23%, and particularly preferably 0.18%, 0.19%, 0.20%, 0.21% or 0.22%. V can play a role in fine grain strengthening and precipitation strengthening in the hot stamping process; the content of V is too high, the alloy cost is too high, and smelting and wide application are not facilitated; the content of V is too low, VC is difficult to be separated out from a steel matrix structure, and the effects of pinning dislocation and improving strength are achieved; the invention controls V in the range, can improve the hardenability of the hot forming steel and broaden the quenching process window.

The 1800 MPa-grade hot forming steel provided by the invention comprises the balance Fe and no impurities.

The invention provides a production method of 1800 MPa-grade hot-formed steel, which comprises the following steps:

providing a casting blank; the chemical components of the casting blank are consistent with those of the 1800MPa grade hot forming steel in the technical scheme;

cold rolling the hot rolled plate to obtain a cold rolled plate;

In the present invention, all the raw material components are commercially available products well known to those skilled in the art unless otherwise specified.

Providing a casting blank; the chemical components of the casting blank are consistent with those of the 1800MPa grade hot forming steel in the technical scheme. In the invention, the casting blank is preferably obtained by smelting and forging, and the smelting is preferably vacuum induction furnace smelting. The specific process conditions of smelting and forging are not particularly limited, and the casting blank with chemical components consistent with the 1800MPa grade hot forming steel in the technical scheme can be obtained.

After a casting blank is obtained, the casting blank is subjected to hot continuous rolling to obtain a hot rolled plate. In the invention, the heating temperature of the hot continuous rolling casting blank is preferably 1200-1260 ℃, more preferably 1210-1250 ℃, further preferably 1220-1240 ℃ and most preferably 1230 ℃; the initial rolling temperature of the hot continuous rolling is preferably 1120-1180 ℃, more preferably 1130-1170 ℃, further preferably 1140-1160 ℃, and most preferably 1150 ℃; the finishing temperature of the hot continuous rolling is preferably 850-910 ℃, more preferably 860-900 ℃, even more preferably 870-890 ℃, and most preferably 880 ℃. The invention has no special limitation on the pass of the hot continuous rolling, and can obtain hot rolled plates with the thickness of 2.7-3.3 mm; the thickness of the hot-rolled plate is more preferably 2.8 to 3.2mm, still more preferably 2.9 to 3.1mm, and most preferably 3 mm. The invention carries out hot continuous rolling under the conditions, can ensure that a casting blank forms a ferrite and pearlite structure after hot rolling, and is beneficial to subsequent cold rolling production. After the hot continuous rolling, the invention preferably also comprises pickling the steel plate obtained by the hot continuous rolling to obtain a hot rolled plate; the present invention is not particularly limited to the above pickling, and the scale on the surface of the steel sheet may be removed.

After the hot rolled plate is obtained, the hot rolled plate is subjected to cold rolling to obtain the cold rolled plate. In the invention, the total reduction rate of the cold rolling is preferably 45-60%, more preferably 48-62%, and most preferably 50-55%. In the present invention, the thickness of the cold-rolled sheet is preferably 1.2 to 2.0mm, more preferably 1.4 to 1.8mm, still more preferably 1.5 to 1.7mm, and most preferably 1.6 mm. In the present invention, the structure of the cold-rolled sheet is mainly ferrite and pearlite.

After a cold-rolled sheet is obtained, the cold-rolled sheet is completely austenitized and then transferred to a hot stamping die for hot stamping forming-quenching treatment to obtain a formed piece; the heating temperature for complete austenitizing is 880-950 ℃, and the heat preservation time is 3-7 min; the temperature of the formed piece is more than or equal to 150 ℃; the temperature of the hot stamping forming-quenching treatment is 830-870 ℃, and the cooling speed is 2-20 ℃/s.

In the invention, the heating temperature for complete austenitizing is 880-950 ℃, preferably 890-940 ℃, more preferably 900-930 ℃, and further preferably 910-920 ℃; the heat preservation is carried out for 3-7 min, preferably for 4-6 min; the complete austenitization is preferably carried out in a furnace. In the invention, the cooling mode in the transferring process is air cooling, and the transferring time is preferably less than or equal to 9s, more preferably 1-8 s, further preferably 2-7 s, and most preferably 3-5 s; the overlong transfer time can reduce the temperature of the steel plate, so that the subsequent hot stamping forming is difficult and the crack is easy to generate, the abrasion of a hot stamping die is accelerated, the power of a stamping press is improved, the energy consumption is obviously increased, and in addition, bainite structures are easy to appear in the structures of the hot stamped components, so that the strength of the steel plate is reduced, the steel plate is easy to be lower than 1800MPa, and the design requirement is not met. In the invention, the hot stamping forming-quenching treatment temperature is 830-870 ℃, preferably 840-860 ℃ and further preferably 850-860 ℃; the time for the hot stamping forming-quenching treatment is preferably 5 to 20s, and more preferably 10 to 15 s. In the invention, the cooling speed of the hot stamping forming-quenching treatment is preferably 2-20 ℃/s, more preferably 5-18 ℃/s, and further preferably 10-15 ℃/s; the invention carries out hot stamping forming and quenching integrated treatment under the conditions, controls the cooling speed to be 2-20 ℃/s, and rapidly cools to be more than 150 ℃ through die quenching, thereby finishing the transformation from austenite to martensite. In the invention, the temperature of the formed part is preferably not less than 150 ℃, more preferably 150-220 ℃, and further preferably 180-200 ℃.

After a formed part is obtained, stacking the formed part, and tempering by using waste heat to obtain an 1800 MPa-level hot formed steel member; the tempering temperature is 150-220 ℃, and the tempering time is 5-20 min.

In the invention, the temperatures of the stacked lower bedding heat insulation plate and the stacked upper covering steel plate are preferably 150-220 ℃, more preferably 180-210 ℃ and further preferably 190-200 ℃; the stacking aims to reduce excessive surface heat loss of the part after the part is demoulded through stacking, and can fully utilize waste heat after the part is demoulded to reduce heat loss so as to realize subsequent tempering. In the invention, the tempering temperature is 150-220 ℃, preferably 180-210 ℃, and more preferably 190-200 ℃; the tempering time is 5-20 min, preferably 10-18 min, and more preferably 15-16 min. In an embodiment of the present invention, the preferred time of the low temperature tempering hold is used to simulate stack tempering in an actual industrial process. The invention carries out tempering within the temperature range, utilizes the residual heat after the hot stamping forming and the die stripping to carry out tempering for proper time to ensure that the martensite is tempered, releases partial residual stress of the quenched steel, improves the uniformity of the structure, improves the elongation (plasticity) to a certain extent, and has little change of the tensile strength of the hot forming steel under the comprehensive influence of the reduction of the dislocation density and the pinning effect of the transitional carbide on the dislocation during the tempering.

The invention provides application of the 1800 MPa-grade hot forming steel in the technical scheme or the 1800 MPa-grade hot forming steel component obtained by the hot stamping forming method in an automobile.

The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. It is to be understood that the described embodiments are merely exemplary of the invention, and not restrictive of the full scope of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Example 1

Smelting and forging the raw materials in a vacuum induction furnace to obtain a casting blank with the thickness of 30mm and chemical components shown in the table 1;

the CCT diagram of the casting blank is shown in figure 1, wherein Ac₃The final temperature at which the steel transforms into austenite when heated; ac of₁The temperature at which pearlite transforms to austenite when heated; ms is the martensite start temperature, M_fF is ferrite, P is pearlite, and B is bainite. The critical cooling rate and austenitizing temperature of the 1800MPa grade hot forming steel can be obtained from the graph 1, and a basis is provided for hot stamping forming technological parameters of the 1800MPa grade hot forming steel.

Carrying out hot continuous rolling and acid pickling on the casting blank to obtain a hot rolled plate with the thickness of 3.0 +/-0.3 mm; wherein the heating temperature of the hot continuous rolling casting blank is 1230 ℃, the initial rolling temperature is 1150 ℃, the heat preservation time is 1h, and the final rolling temperature is 880 ℃;

cold rolling the hot rolled plate to obtain a cold rolled plate with the thickness of 1.6 +/-0.4 mm;

heating the cold-rolled sheet to 930 ℃, preserving heat for 4min, and completely austenitizing the cold-rolled sheet, and then transferring the cold-rolled sheet to a die for hot stamping forming-quenching treatment to obtain a formed sheet with the thickness of 1.34 mm; wherein the transfer time is 3-5 s, and the temperature of the forming plate is 250 ℃; the temperature of the hot stamping forming-quenching treatment is 830 ℃, and the time is 15 min;

stacking the forming plates, and tempering by using waste heat to obtain an 1800 MPa-level hot forming steel member with the thickness of 1.34 mm; wherein the temperature ranges of the lower bedding heat insulation plate and the upper covering steel plate at the stacking position are 180-220 ℃, the tempering temperature is 200 ℃, and the heat preservation time is 5 min.

The chemical compositions of the cast slab and the 1800MPa grade hot forming steel are shown in Table 1:

TABLE 1 chemical composition of ingot and 1800MPa grade Hot Forming Steel

Examples 2 to 4

1800MPa grade hot-formed steels were prepared according to the method of example 1, the preparation conditions of examples 2 to 3 are shown in Table 2, and the other preparation conditions are the same as those of example 1.

Comparative examples 1 to 9

1800MPa grade hot formed steels were prepared according to the method of example 1, the preparation conditions of comparative examples 1 to 9 are shown in Table 2, and the other preparation conditions are the same as those of example 1.

The thickness, tensile strength, yield strength and elongation after fracture of the 1800MPa grade hot formed steels prepared in examples 1 to 4 and comparative examples 1 to 9 are shown in Table 2.

TABLE 2 preparation conditions of examples 1 to 4 and comparative examples 1 to 9 and performance test results of prepared 1800MPa grade hot-formed steels

As can be seen from Table 2, the quenching performance of the 1800MPa grade hot formed steel produced by the chemical components designed according to the invention and the laboratory simulation hot stamping forming method is shown in comparative example 1, the strength reaches 1832MPa, and the elongation is 8.27%; the mechanical properties of the alloyed components are obviously improved, and the preparation of 1800 MPa-grade steel is realized. From examples 1 to 3, it is found that the strength of the 1800MPa grade hot-formed steel is 1792 to 1844MPa, and the strength is slightly lost and the elongation A is slight loss in comparison with the strength in the quenched state₂₅The quenching state is obviously improved compared with that of the comparative example 1 when the quenching rate is over 8.5 percent; the embodiment 2 reaches 8.91 percent, has almost the same strength and quenching state, meets the basic requirements on an 1800 MPa-grade hot forming steel workpiece, realizes the low-cost process design, and has great value for popularization and application of hot forming steel. In addition, compared with other tempering temperatures and times, the tempering strength at 150 ℃ in the comparative examples 3-6 is basically consistentThe elongation is low and is not obviously improved; in the comparative example 1, the tempering at 200 ℃ for 30min needs more time and energy, and has certain influence on the production period; in the tempering at 250 ℃ in the comparative examples 7-10, due to the rise of the tempering temperature, the martensite matrix starts to recover, the strength is obviously reduced, the elongation is lower than the quenching state, and the requirements of ultrahigh-strength hot forming steel for automobiles are not met. The heat treatment method in the preparation method provided by the invention is beneficial to eliminating the internal stress and uniform martensite structure of the steel plate quenching, the plasticity can be obviously improved under the condition of slightly sacrificing the strength, the strength and the toughness are well matched, compared with the prior art, the ductility and the toughness of the hot formed steel are improved by utilizing the self-tempering principle, the process control is simple and easy to implement, and the cost is low.

The microstructure of the 1800MPa grade hot formed steel prepared in example 1 is shown in FIG. 2, and it can be seen from FIG. 2 that the structure tempered at 200 ℃ for 5min is tempered martensite.

The microstructure of the 1800MPa grade hot formed steel prepared in example 2 is shown in FIG. 3, and it can be seen from FIG. 3 that the structure tempered at 200 ℃ for 10min is tempered martensite.

The microstructure of the 1800MPa grade hot formed steel prepared in example 3 is shown in FIG. 4. it can be seen from FIG. 4 that the structure tempered at 200 ℃ for 20min is tempered martensite.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.

Claims

1. The 1800 MPa-grade hot forming steel is characterized by comprising the following chemical components in percentage by mass: 0.28-0.32% of C, 0.2-0.3% of Si, 1.5-1.7% of Mn, less than or equal to 0.015% of P, less than or equal to 0.010% of S, 0.002-0.0035% of B, 0.15-0.25% of Cr, 0.02-0.05% of Al, less than or equal to 0.03% of Ti, less than or equal to 0.005% of N, 0.04-0.055% of Nb, 0.15-0.25% of V, and the balance Fe and no impurities.

2. A method of hot stamping a 1800MPa grade hot formed steel according to claim 1, comprising the steps of:

cold rolling the hot rolled plate to obtain a cold rolled plate;

3. The hot stamp forming method according to claim 2, wherein the hot continuous rolling is performed at a billet heating temperature of 1200 to 1260 ℃, a starting rolling temperature of 1120 to 1180 ℃, and a finishing rolling temperature of 850 to 910 ℃.

4. The hot press forming method according to claim 2 or 3, wherein the hot rolled plate has a thickness of 2.7 to 3.3 mm.

5. The hot press forming method according to claim 2, wherein the total reduction rate of the cold rolling is 45 to 60%.

6. The hot press forming method according to claim 2 or 5, wherein the cold rolled sheet has a thickness of 1.2 to 2.0 mm.

7. A hot press forming method according to claim 2, wherein the transfer time is 9s or less.

8. Use of the 1800MPa grade hot formed steel according to claim 1 or the 1800MPa grade hot formed steel component obtained by the hot stamping method according to any one of claims 2 to 8 in an automobile.