High-thermoforming 980mpa-grade ultra-high-strength automobile steel and manufacturing method

Abstract

The application discloses a high-thermal-plastic 980MPa-grade super-high-strength automobile steel and a manufacturing method thereof. The high-thermal-plastic 980MPa-grade super-high-strength automobile steel comprises the following components in percentage by weight: C: 0.1-0.3%, Si: 0.2-0.7%, Mn: 1-3%, Ti: 0.04-0.07%, Al: 0.02-0.07%, Ca: 0.001-0.005%, Ce: 0.0010-0.0050%, Cr: 0.2-0.4%, P: less than or equal to 0.013%, S: less than or equal to 0.002%, and the rest Fe and inevitable impurities. The application optimizes the alloy component design and the manufacturing process, feeds rare earth wires in a crystallizer, modifies inclusions, promotes the smallness of inclusions in the steel, improves the thermal plasticity of the super-high-strength automobile steel, and reduces the risk of casting blank defects of the super-high-strength automobile steel.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel metallurgy and steel material, and particularly relates to a high-thermoforming 980MPa-grade ultra-high-strength automobile steel and a manufacturing method. BACKGROUND

[0002] With the increasing demand for automobile safety and the increasing emphasis on environmental protection, the development of ultra-high-strength automobile steel is also getting faster. Ultra-high-strength automobile steel is generally used in the B-pillar, control arm, floor support, connecting rod and various reinforcing members and chassis structure parts of the automobile, and plays an important role in the safety of the automobile. If ultra-high-strength steel is used, the weight of the whole vehicle will be greatly reduced on the premise of ensuring the strength, and high-strength automobile steel is an effective means to solve the problem of lightweight. Under the condition of ensuring the safety performance of the automobile, the design is optimized to thin the body parts, which not only can effectively reduce the weight of the body and reduce fuel consumption, but also can improve the comfort of the passengers.

[0003] Although the ultra-high-strength automobile steel has high strength, the alloy content of the ultra-high-strength automobile steel is high, which is easy to produce casting crack in the production process, thereby affecting the overall yield of the finished product, causing the increase of production cost and the reduction of production efficiency, and at the same time, the large-size inclusions in the steel also affect the quality of the casting. Therefore, how to reduce the generation of casting crack and reduce the large-size inclusions is still a problem to be solved in the production process of ultra-high-strength automobile steel.

[0004] Chinese patent CN 106609313 B provides a high-purity rare earth steel treatment method, in which different contents of rare earth are added in the LF refining, VD / RH vacuum treatment and soft blowing processes, so as to obtain small-sized rare earth inclusions; however, this technology needs to add rare earth in the LF refining, VD / RH vacuum treatment and soft blowing processes, which is complicated to operate and consumes a large amount of rare earth, and this technology cannot reduce the generation of casting crack, and is not suitable for ultra-high-strength automobile steel.

[0005] Chinese patent CN 110257590 B provides a method for refining inclusions in high-cleanliness rare earth electroslag steel, in which rare earth is added in the electroslag remelting process, so as to significantly reduce the oxygen content in the steel. This technology is only suitable for electroslag steel, and is not suitable for non-electroslag steel, and this technology cannot reduce the generation of casting crack.

[0006] There are also some technologies in the prior art that feed rare earth wires during the preparation process, such as a method for solving center segregation of cast billets disclosed in Chinese Patent Publication No. CN102310178A. According to the formation mechanism of the center segregation of the cast billets, the casting speed, the overheat degree of the molten steel in the tundish, and the opening degree of the casting machine are optimized, and a nucleating agent is added into the crystallizer, electromagnetic stirring is used in the secondary cooling section, and light pressing is used at the solidification end, so as to improve the center segregation of C, S, Mn and other elements caused by the selective crystallization of the solute element-rich molten steel which is blocked and cannot exchange with other liquid during the solidification process of the cast billet. Although the technology mentioned above feeds rare earth wires in the crystallizer, the purpose of feeding the rare earth wires is to improve the center segregation degree of the cast billet, and the ultra-high-strength automotive steel of the present application is not mentioned, nor is the specific wire feeding process involved. The rare earth addition process in the continuous casting tundish disclosed in Chinese Patent Publication No. CN1824430A relates to the addition process of rare earth in the production of a continuous casting machine. Although the technology feeds rare earth wires in the tundish, the purpose is to ensure the uniformity of the rare earth composition in the same furnace molten steel, and to solve the problem of producing rare earth steel by a one-machine multi-flow continuous casting machine, and the steel grade of the technology does not mention the ultra-high-strength automotive steel of the present application.

[0007] In view of the above, it is urgent to develop an ultra-high-strength automotive steel and a manufacturing method thereof, which can not only refine and modify the inclusions in the steel, refine the inclusions, and reduce the large-size inclusions in the steel, but also reduce the occurrence of cracks in the cast billet. SUMMARY

[0008] In view of the problems of the existing technology that the ultra-high-strength automotive steel is prone to produce cracks in the cast billet due to high alloy content, and the large-size inclusions in the steel affect the quality of the cast billet, the purpose of the present application is to provide a high-thermal-molding 980MPa-grade ultra-high-strength automotive steel and a manufacturing method thereof. By optimizing the alloy composition design and the manufacturing process, and feeding rare earth wires in the crystallizer, the inclusions are modified, the smallness of the inclusions in the steel is promoted, the thermal plasticity of the ultra-high-strength automotive steel is improved, and the risk of defects in the cast billet of the ultra-high-strength automotive steel is reduced.

[0009] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0010] The first aspect of the present application provides a high-thermal-molding 980MPa-grade ultra-high-strength automotive steel, which comprises the following components in terms of weight percentage: C: 0.1-0.3%, Si: 0.2-0.7%, Mn: 1-3%, Ti: 0.04-0.07%, Al: 0.02-0.07%, Ca: 0.001-0.005%, Ce: 0.0010-0.0050%, Cr: 0.2-0.4%, P≤0.013%, S≤0.002%, and the balance of Fe and unavoidable impurities.

[0011] Preferably, the components satisfy: Al / Ce = 11-40, Ce / S = 0.5-3, wherein Al, Ce, and S are the weight percentage contents of the corresponding elements.

[0012] Preferably, in the inevitable impurities, O is less than or equal to 0.0020% by weight.

[0013] Preferably, the inclusions in the high hot plastic 980 MPa grade ultra-high strength automobile steel are Al-Ca-Ce composite inclusions.

[0014] Preferably, in the high hot plastic 980 MPa grade ultra-high strength automobile steel, the density of inclusions greater than 5 microns is less than or equal to 0.05 per mm 2 , the density of inclusions greater than 1 micron is less than or equal to 25 per mm 2 , and the average diameter of the inclusions is less than or equal to 2 microns.

[0015] Preferably, in the high hot plastic 980 MPa grade ultra-high strength automobile steel, the proportion of acicular bainite structure is greater than 30%.

[0016] Preferably, the high hot plastic 980 MPa grade ultra-high strength automobile steel has a reduction of area of greater than or equal to 40%.

[0017] The second aspect of the present application provides a manufacturing method of the high hot plastic 980 MPa grade ultra-high strength automobile steel according to the first aspect of the present application, comprising the following steps:

[0018] S1, smelting, taking KR desulfurized molten iron and low-sulfur scrap steel as raw materials, using top and bottom combined blowing process in the converter, and controlling the oxygen content of the molten steel to be less than 600 ppm during tapping by pre-deoxidation;

[0019] S2, LF refining, adding aluminum slag during LF temperature rise, adding alloy to adjust the composition of the molten steel after reaching the target temperature, and stirring by bottom blowing argon;

[0020] S3, RH refining, vacuum composition adjustment in the RH furnace, first adding titanium alloy for deoxidation, then adding aluminum alloy for deoxidation after 3-5 minutes, and adding other alloys to adjust the composition of the molten steel to the target value, and feeding calcium wire for calcium treatment after breaking the vacuum;

[0021] S4, continuous casting wire feeding, controlling O to be less than or equal to 0.002% and S to be less than or equal to 0.002% in the molten steel in the tundish, and feeding rare earth wire in the continuous casting crystallizer.

[0022] Preferably, in the step S1,

[0023] The proportion of molten iron in the raw materials is greater than or equal to 88%;

[0024] The converter tapping temperature is above 1670-1680℃, and the tapping oxygen is controlled in 500-600ppm; and / or

[0025] The deoxidizer used in the pre-deoxidation process is ferrosilicon, and the addition amount of the ferrosilicon is 50-200kg / heat; and / or

[0026] 900-1100kg / heat of lime is added in the converter tapping process; and / or

[0027] In the step S2:

[0028] The addition amount of the aluminum slag is 100-200kg / heat;

[0029] The target temperature is 1620-1630℃;

[0030] The argon flow is 160-180L / min, and the stirring time is 4-6min; and / or

[0031] In the step S3:

[0032] In the vacuum component adjustment process, the vacuum degree is controlled to be ≤133Pa, and the pure degassing time is ≥10min;

[0033] The addition amount of the titanium alloy is 120-140kg / heat; and the addition amount of the aluminum alloy is 800-900kg / heat; and / or

[0034] In the step S4:

[0035] In the continuous casting wire feeding process, the wire feeding position is the intersection point of the distance of the nozzle from the narrow side being 1 / 2 and the distance of the nozzle from the narrow surface being 1 / 2, and single wire feeding is adopted;

[0036] In the continuous casting wire feeding process, the wire feeding speed is 4-5m / min;

[0037] The rare earth wire adopts La-Ce alloy with rare earth content being above 99% or rare earth wire with Ce content in the outer cladding iron shell being 55-65%.

[0038] Preferably, in the La-Ce alloy with rare earth content being above 99%, the content of La is 25-35wt%, and the content of Ce is 65-85wt%.

[0039] Preferably, when the rare earth wire adopts La-Ce alloy with rare earth content being above 99%, the wire feeding speed is 4-5m / min, and the steel passing amount is ≥3t / min; or

[0040] The rare earth wire adopts the rare earth wire coated with an outer iron shell, the content of Ce is 55-65%, the wire feeding speed is 6-8 m / min, and the steel passing amount is greater than or equal to 3 t / min.

[0041] Compared with the existing production technology, the high-thermal plasticity 980MPa-grade ultra-high-strength automobile steel has the following beneficial effects:

[0042] 1. By optimizing the alloy component design, adding an appropriate amount of Ce element, adding silicon iron for pre-deoxidation in the converter tapping process, and controlling the appropriate free oxygen in the steel in the refining process, the Al-Ca-Ce composite inclusions are formed in the molten steel by optimizing the deoxidizing alloy and the alloy adding sequence, and the purity of the molten steel is improved (O in the steel is less than or equal to 0.0020%);

[0043] 2. By feeding the rare earth wire in the crystallizer for inclusion refining and modification treatment, there is no deterioration of the protective slag and no slab inclusion defects in the wire feeding process. In this process, the typical inclusion type in the casting blank is changed from calcium aluminate to Al-Ca-Ce composite inclusion. Such inclusions play a role in promoting nucleation and refining grains during the casting blank solidification process.

[0044] 3. The casting blank of the high-thermal plasticity 980MPa-grade ultra-high-strength automobile steel has small original austenite grain size, uniform black pearlite structure, and high acicular bainite content (more than 30%), and the thermal plasticity of the casting blank is improved.

[0045] 4. The high-thermal plasticity 980MPa-grade ultra-high-strength automobile steel is stably produced by using the process of feeding the rare earth wire in the crystallizer to refine and modify the inclusions in the steel. BRIEF DESCRIPTION OF DRAWINGS

[0046] Other features, objects, and advantages of the present application will become more apparent from the following detailed description of non-limiting embodiments, read in conjunction with the accompanying drawings:

[0047] Figure 1 Fig. 1 is a schematic diagram of the morphology of typical inclusions in the high-thermal plasticity 980MPa-grade ultra-high-strength automobile steel of embodiment 1 of the present application; (a) is an electron microscope scanning diagram of the inclusions; (b) is a distribution diagram of O element in the inclusions, (c) is a distribution diagram of Al element in the inclusions, (d) is a distribution diagram of S element in the inclusions, (e) is a distribution diagram of Ca element in the inclusions, (f) is a distribution diagram of Ce element in the inclusions, (g) is a distribution diagram of La element in the inclusions, and (h) is a distribution diagram of Fe element in the inclusions.

[0048] Figure 2Schematic diagram of typical inclusion morphology in the steel of Comparative Example 1; (a) is a scanning electron microscope image of the inclusion; (b) is a distribution diagram of O element in the inclusion, (c) is a distribution diagram of Ca element in the inclusion, (d) is a distribution diagram of Al element in the inclusion, (e) is a distribution diagram of S element in the inclusion, and (f) is a distribution diagram of Fe element in the inclusion;

[0049] Figure 3 Metallographic diagram of the cast blank in the embodiment of the present application; (a) is a metallographic diagram of the cast blank in Example 1; (b) is a metallographic diagram of the cast blank in Example 2; (c) is a metallographic diagram of the cast blank in Example 3; and (d) is a metallographic diagram of the cast blank in Example 4;

[0050] Figure 4 Metallographic diagram of the cast blank in the comparative example; (a) is a metallographic diagram of the cast blank in Comparative Example 1; and (b) is a metallographic diagram of the cast blank in Comparative Example 2;

[0051] Figure 5 Comparison diagram of high-temperature tensile reduction of area of Example 1 and Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0052] In order to better understand the above technical solutions of the present application, the technical solutions of the present application are further described below in combination with examples.

[0053] The high-thermal-molding 980MPa-grade ultra-high-strength automobile steel of the present application comprises the following components in percentage by weight: C: 0.1-0.3%, Si: 0.2-0.7%, Mn: 1-3%, Ti: 0.04-0.07%, Al: 0.02-0.07%, Ca: 0.001-0.005%, Ce: 0.0010-0.0050%, Cr: 0.2-0.4%, P≤0.013%, S≤0.002%, and the rest Fe and inevitable impurities.

[0054] The design principles of the components in the high-thermal-molding 980MPa-grade ultra-high-strength automobile steel of the present application are as follows:

[0055] C is an important element in the ultra-high-strength steel. Increasing the content of C in the steel can increase the strength of the material and reduce the production cost, and increase the strength grade of the ultra-high-strength automobile steel. In the present application, the content of C is not less than 0.1%. At the same time, the increase of the content of C in the steel will reduce the plasticity and deteriorate the ductility. Therefore, the content of C is not higher than 0.3%. Therefore, in order to achieve the desired effect, the content of C in the present application is 0.1-0.3%.

[0056] Si element is an important deoxidizer in smelting process, and can also improve the strength of steel, the composition needs to add Si content. But the content of Si is too high, which will lead to the appearance of string-shaped Al-Si inclusions in steel, causing stamping cracking, the content of Si in the present application is 0.2% to 0.7%.

[0057] Mn is an important deoxidizer in steelmaking process, which can improve the hardenability and improve the toughness, and can also react with S in steel to generate MnS, eliminating the harmful effect of S in steel. Mn plays an important role in maintaining the strength and toughness of ultra-high strength automobile steel, so the content of Mn is 1% to 3%.

[0058] Ti can combine with nitrogen and oxygen to form Ti-containing oxides and nitrides, and Ti plays a part of deoxidizing effect in the present process, therefore, the content of Ti is not less than 0.04%. But too high Ti will promote the generation of large size TiN, causing stamping defects, so the upper limit of Ti content is 0.07%.

[0059] S will generate MnS with Mn in steel, in rare earth treated steel, after adding rare earth into steel, because rare earth has strong metal properties, its affinity with sulfur is much greater than that with iron and sulfur, therefore, the sulfur element in the molten steel quickly forms rare earth sulfide with rare earth element, because the density of rare earth sulfide is less than that of molten steel, under the action of buoyancy, the rare earth sulfide floats to the protective slag, affecting the melting of the protective slag, at the same time, the generation of rare earth sulfide affects the modification effect of rare earth on aluminum oxide, so the content of S in steel should be reduced as much as possible. In the present application, the reaction between rare earth and S is reduced to avoid the generation of rare earth sulfide. But too low S content requirement will greatly affect the cost of steelmaking, which will greatly increase the cost of steelmaking, so the upper limit of S content is 0.002%.

[0060] P has strong solid solution strengthening effect, which can significantly improve the strength and hardness of steel, therefore, the upper limit of P content is 0.013%.

[0061] Al is a commonly used grain refining element and a commonly used deoxidizer. In order to improve the purity of molten steel and reduce the total oxygen in steel, the lower limit of Al is 0.02%, and too high Al content will increase the cost, and increase the acid-soluble aluminum content in steel, which increases the risk of secondary oxidation, therefore, the upper limit of Al is 0.07%.

[0062] Ca is an element for modifying inclusions, which can react with Al2O3 inclusions in steel to generate low-melting-point calcium aluminate inclusions, reducing the clogging of the nozzle and improving the stability of pouring.

[0063] Cr can increase the hardenability of the steel and has the effect of secondary hardening. In the quenching and tempering process, the hardenability can be improved, so that the steel has good comprehensive mechanical properties after quenching and tempering. The lower limit of Cr is 0.2%. With the increase of the content of Cr, the tendency of high-temperature tempering brittleness increases, so the upper limit of Cr is 0.4%. Therefore, in order to achieve the expected effect, the content of Cr in the present application is 0.2-0.4%.

[0064] Ce is a rare earth element, which is an important element for refining inclusions in the steel in the present application. Ce is fed into the steel through a crystallizer wire feeding, which plays a role in refining inclusions. However, too high content of Ce will generate large size rare earth oxide inclusions, so the content of Ce in the present application is 0.0010-0.0050%.

[0065] In the component design of the present application, Al, Ti and Ce three elements need to be controlled in the range, because these three elements are the key to the formation of inclusions, and different component combinations and deoxidation sequences will have a great influence on the type of oxide in the steel. Too high content of Al will form large size Al2O3 inclusions, affecting the quality of the casting blank. Too high content of Ti will promote the generation of TiN. At the same time, the content of Ti cannot be too low, too low content of Ti cannot reduce the oxygen in the steel to a suitable level, and too high oxygen in the steel will increase the content of aluminum oxide after adding Al. Too high content of Ce will generate large size rare earth oxide inclusions in the steel, which will float up in the crystallizer for a short time, reducing the purity of the steel.

[0066] Under the premise of controlling the components, the components in the steel also need to meet: Al / Ce=11-40, Ce / S=0.5-3, in which Al, Ce and S are the weight percentage content of the corresponding elements. Among them, controlling Al / Ce=11-40: the purpose is to avoid the generation of a large amount of large size Al-Ce composite oxides and large size rare earth oxides alone in the steel, which are difficult to remove in the molten steel and have large size, which can easily cause defects in the steel. At the same time, in this proportion range, the modification reaction of rare earth to calcium-aluminum composite inclusions can be carried out. Controlling Ce / S=0.5-3: because rare earth has strong metallic properties, its affinity with sulfur is much greater than that with iron and sulfur, so when the sulfur element in the molten steel is high, the S element will quickly form rare earth sulfide with the rare earth element, making it difficult for the free state of the rare earth element in the steel to modify the calcium-aluminum composite inclusions, and cannot form effective Al-Ce composite inclusions, refine the grain and strengthen the hot plasticity of the casting blank.

[0067] Among the unavoidable impurities, O≤0.0020%, in terms of weight percentage.

[0068] The inclusion type of the high hot plastic 980 MPa grade ultra-high strength automobile steel is Al-Ca-Ce composite inclusion; the density of inclusions greater than 5 microns is ≤0.05 per / mm 2 The density of inclusions greater than 1 micron is ≤25 per / mm 2 The average diameter of inclusions is ≤2 microns. In the cast slab structure, the proportion of acicular bainite structure is greater than 30%, so the high hot plastic 980 MPa grade ultra-high strength automobile steel has excellent hot plasticity.

[0069] The high hot plastic 980 MPa grade ultra-high strength automobile steel has a reduction of area ≥40%.

[0070] The manufacturing method of the high hot plastic 980 MPa grade ultra-high strength automobile steel described above comprises the following steps:

[0071] S1, smelting, taking KR desulfurized molten iron and low-sulfur scrap steel as raw materials, using top and bottom combined blowing process in the converter, and controlling the oxygen content of tapping to be below 600 ppm through pre-deoxidation during tapping in the converter;

[0072] Specifically, according to the composition of the high hot plastic 980 MPa grade ultra-high strength automobile steel, KR desulfurized molten iron and low-sulfur scrap steel are taken as raw materials, and the molten iron ratio of the raw materials is controlled to be ≥88%; in order to ensure the bottom blowing effect of the converter, top and bottom combined blowing process is used; during tapping in the converter, the oxygen content of tapping is controlled to be below 600 ppm, and in the preferred embodiment, the oxygen content of tapping is controlled to be 500-600 ppm; the tapping temperature is controlled to be 1670-1680℃, and fast tapping is performed, and silicon iron is added for pre-deoxidation during the tapping process, wherein the addition amount of silicon iron is determined according to the oxygen content of tapping, and the specific addition amount is 50-200 kg / heat; lime is added for slagging during the tapping process, and the addition amount of lime is 900-1100 kg / heat.

[0073] S2, LF refining, adding aluminum slag during LF heating, adding alloy to adjust the composition of the molten steel after reaching the target temperature, and simultaneously blowing argon for stirring;

[0074] Specifically, the LF is heated, 100-200 kg / heat of aluminum slag is added to deoxidize the slag surface, and through desulfurization by slagging, the sulfur in the molten steel is maintained within a relatively low range; after the LF heating reaches the target temperature (such as 1620-1630℃), alloy is added to the molten steel to adjust the Al, Cr, Si, Mn and other alloy compositions to the target value; after adding the alloy, argon is blown for stirring to ensure the uniformity of the composition of the molten steel; wherein the argon flow is 160-180 L / min, and the stirring time is 4-6 min.

[0075] S3, RH refining, vacuum composition adjustment in the RH furnace, first add titanium alloy for deoxidation, interval 3-5 minutes, then add aluminum alloy for deoxidation, and add other alloys, adjust the composition of the molten steel to the target value, after breaking the vacuum, feed in calcium wire for calcium treatment;

[0076] Specifically, vacuum composition adjustment is performed in the RH furnace, alloying process is performed after controlling the vacuum degree <100 Pa, first add titanium alloy for deoxidation to reduce the oxygen content in the steel, then interval 3-5 minutes, then add aluminum alloy for deoxidation, and add other alloys, adjust the composition of the molten steel to the target value, during the vacuum composition adjustment process, the pure degassing time ≥10 min; after breaking the vacuum, feed in calcium wire for calcium treatment, wherein the wire feeding speed is 160-180 m / min. During the vacuum composition adjustment process, the addition amount of titanium alloy is 120-140 kg / heat, and the addition amount of aluminum alloy is 800-900 kg / heat.

[0077] In the above smelting, LF refining and RH refining processes, under the premise of controlling the composition of the steel, the deoxidation process is optimized, the deoxidation process (Si→Ti→Al), that is, silicon iron is added for pre-deoxidation during converter tapping, titanium alloy and aluminum alloy are added in the RH refining. Silicon iron is added for pre-deoxidation according to different tapping free oxygen during converter tapping, to ensure that the oxygen before Ti addition in the refining is at a suitable level, while reducing the deoxidation cost; the silicon-based deoxidizing alloy is used for pre-deoxidation during converter tapping, to reduce the Al2O3 generated by the subsequent addition of Al deoxidizing alloy, while optimizing the oxygen level before RH refining treatment, to reduce the generation of large-size cluster-shaped Al2O3. From the perspective of inclusion control, Si deoxidation first can effectively reduce free oxygen, from the perspective of thermodynamics, Al as a stronger deoxidizer than Si, can reduce the SiO2 inclusions in the steel after subsequent Al addition, to ensure the purity of the molten steel.

[0078] Converter tapping with silicon iron and LF refining with aluminum slag deoxidation can reduce the deoxidation cost; in the RH refining process, first add Ti for deoxidation to reduce the oxygen content in the steel, then add Al for deoxidation and alloying. After adopting the Ti deoxidation process first, a certain amount of submicron TiN inclusions will be generated in the molten steel, which can refine the grains during the solidification of the casting billet, thereby improving the corner toughness of the casting billet and reducing the occurrence of casting billet cracks.

[0079] S4, continuous casting wire feeding, control O≤0.002% and S≤0.002% in the molten steel in the tundish, and feed in rare earth wire in the continuous casting crystallizer.

[0080] Specifically, in the continuous casting process, the O≤0.0020% and S≤0.001% of the tundish is controlled, because the rare earth added into the steel will react with the oxygen in the steel, if the oxygen content in the steel is too high, large size of rare earth oxide will be generated, and if the sulfur content is too high, a large amount of rare earth sulfide will be generated, both of which are not desirable, so the oxygen content and sulfur content of the molten steel before feeding the rare earth wire into the crystallizer should be controlled. In the continuous casting crystallizer, two types of rare earth wires are fed: the first type is La-Ce alloy with rare earth content of more than 99%, wherein the content of La is 25-35wt%, and the content of Ce is 65-85wt%; the second type is Ce rare earth wire with external iron shell and Ce content of 55-65%.

[0081] The specific wire feeding process is: the wire feeding position is the geometric center of the single side of the nozzle, that is, the intersection point of the distance of the nozzle from the narrow side and the narrow surface by 1 / 2 and 1 / 2, and single wire feeding is adopted. Among them, the wire feeding speed is 4-8m / min, and the diameter of the rare earth wire can be 2.5mm.

[0082] When the first type of rare earth wire, i.e. La-Ce alloy with rare earth content of more than 99%, is used, the steel flow rate is≥3t / min, single wire feeding is adopted, the wire feeding speed is 4-5m / min, the yield is 40-60%, and the rare earth content in the steel is 25-49ppm.

[0083] When the second type of rare earth wire, i.e. rare earth wire with external iron shell and Ce content of 55-65%, is used, the steel flow rate is≥3t / min, single wire feeding is adopted, the wire feeding speed is 6-8m / min, the yield is 80-90%, and the rare earth content in the steel is 25-49ppm.

[0084] In the above continuous casting wire feeding process, the rare earth content of the steel grade can be adjusted according to the different cross sections of the slab to reach the target value. In the wire feeding process of the crystallizer, no agglomeration of the protective slag is found, and no sticking and cracks are found in the slab. The analysis of the protective slag of the rare earth feeding and non-rare earth feeding furnace shows that there is no obvious difference in composition. In the process, attention should be paid to the wire feeding speed and the steel flow rate, and the change of the protective slag and the thickness of the molten layer should be paid attention to during the casting process; in the continuous casting wire feeding process, increasing the steel flow rate and improving the temperature of the molten steel are beneficial to the melting speed of the rare earth wire in the molten steel, and are also beneficial to the melting of the protective slag, so it is inclined to choose the largest steel flow rate as possible.

[0085] The inclusions of the above prepared high thermal forming 980MPa grade ultra-high strength automobile steel are Al-Ca-Ce composite inclusions; the density of inclusions greater than 5 microns is≤0.05 / mm 2 , and the density of inclusions greater than 1 micron is≤25 / mm 2The average diameter of the inclusions is less than or equal to 2 microns. In the structure of the cast blank, the proportion of acicular bainite structure is greater than 30%, and therefore the high-thermal-plasticity 980MPa-grade ultra-high-strength automobile steel has excellent thermal plasticity.

[0086] The present application modifies and refines the inclusions by designing the alloy composition and optimizing the preparation process, and finally the high-thermal-plasticity 980MPa-grade ultra-high-strength automobile steel has a cross-section shrinkage rate of greater than or equal to 40% at high temperature, effectively improving the high-temperature plasticity and reducing the risk of cast blank defects of the ultra-high-strength automobile steel.

[0087] The high-thermal-plasticity 980MPa-grade ultra-high-strength automobile steel and the manufacturing method thereof will be further described below with specific examples.

[0088] Embodiment

[0089] In the embodiments 1-4, the high-thermal-plasticity 980MPa-grade ultra-high-strength automobile steel is manufactured by the method of the present application, and the specific process is as follows:

[0090] (1) Smelting: KR desulfurized molten iron and scrap steel are used as raw materials, and the sticking water ratio is controlled to be greater than or equal to 88%. In order to ensure the bottom blowing effect of the converter, a top and bottom combined blowing process is adopted, the tapping oxygen is controlled to be less than or equal to 600ppm, and the tapping temperature is controlled to be 1670-1680℃. Fast tapping is adopted, and silicon iron is added for pre-deoxidization during the tapping process. The amount of silicon iron added is determined according to the tapping oxygen; lime is added during the tapping process of the converter.

[0091] (2) LF furnace refining: LF is used for heating, and 100-200kg / heat of aluminum slag is added for deoxidization; after the LF is heated to the target temperature (such as 1620-1630℃), the alloy is added to the molten steel to adjust the Al, Cr, Si, Mn and other alloy components to the target value; after the alloy is added, argon gas is blown at the bottom to stir, so as to ensure the uniformity of the molten steel composition; the argon gas flow is 160-180L / min, and the stirring time is 4-6min.

[0092] (3) RH refining: vacuum composition adjustment is carried out in the RH furnace, and after the vacuum degree is controlled to be less than 100Pa, the alloying process is carried out, sponge titanium is first added for deoxidization, and the sponge titanium plays a part of the deoxidization role, at the same time, submicron TiN is produced, then after 3-5 minutes, aluminum alloy is added for deoxidization, and other alloys are added, and the molten steel composition is adjusted to the target value. During the vacuum composition adjustment process, the pure degassing time is guaranteed to be more than 10min; after breaking the vacuum, calcium wire is fed for calcium treatment, and the wire feeding speed is 160m / min.

[0093] (4) Continuous casting wire feeding: during the continuous casting process, the O in the tundish is controlled to be less than or equal to 0.0020% and the S is controlled to be less than or equal to 0.002%, and rare earth wire is fed in the continuous casting crystallizer;

[0094] Examples 1 and 2 use the first type of rare earth wire, La-Ce alloy with rare earth content of 99% or more, La content of 25-35 wt%, Ce content of 65-85 wt%, wire passing rate of 3 t / min or more, single wire feeding, yield of 40-60%, single wire feeding, wire feeding speed of 4-5 m / min, and rare earth content in steel of 49 ppm. Examples 3 and 4 use the second type of rare earth wire, i.e. rare earth wire with an outer iron shell and Ce content of 55-65%, wire passing rate of 3 t / min or more, single wire feeding, yield of 80-90%, single wire feeding, wire feeding speed of 6-8 m / min, and rare earth content in steel of 49 ppm.

[0095] Comparative Examples

[0096] Comparative Examples 1 and 2 use the same process as Example 1, except that the chemical composition is different and no rare earth wire feeding treatment is performed in the mold during continuous casting.

[0097] Table 1 shows the chemical composition of the steel of Examples 1-4 and Comparative Examples 1-2, and Table 2 shows the process parameters during the manufacture of Examples 1-4.

[0098] Table 1 Chemical composition of Examples and Comparative Examples (wt%)

[0099]

[0100] Table 2 Process parameters during the manufacture of Examples

[0101]

[0102] In combination Figure 1 , Figure 2 The inclusion morphology of the Examples and Comparative Examples shows that the typical inclusion in the Examples of the present application is Al-Ca-Ce composite inclusion. This type of inclusion promotes nucleation and grain refinement during the solidification of the casting billet.

[0103] In combination Figure 3 , Figure 4 The microstructure of the casting billet of the Examples and Comparative Examples shows that the original austenite grains of the casting billet of the Examples of the present application are fine, the black pearlite structure is uniformly distributed, and the proportion of acicular bainite is high (more than 30%). The original austenite grains of the casting billet of the Comparative Examples are coarse, the banded pearlite structure is more, and the acicular bainite structure is less. This type of casting billet structure of the Examples results in an increase in thermal plasticity.

[0104] Figure 5For the high temperature tensile reduction of area of the example and the comparative example material, it can be seen from the figure that the high temperature tensile reduction of area of the high hot plastic 980MPa grade ultra-high strength automobile steel in the example is obviously greater than that of the comparative example, and the high temperature plasticity of the high hot plastic 980MPa grade ultra-high strength automobile steel prepared in the example is obviously improved.

[0105] In summary, the composition design of the high hot plastic 980MPa grade ultra-high strength automobile steel is optimized, and a rare earth element is added, in the converter tapping process, ferrosilicon is added for pre-deoxidization, in the refining process, the free oxygen in the steel is controlled, the deoxidizing alloy and the alloy addition sequence are optimized, and the time is ensured, so that the Al-Ce composite inclusions are formed in the molten steel, and the O in the steel is less than or equal to 0.0020%. The rare earth wire is fed into the crystallizer to modify the inclusions, and the protective slag is not deteriorated during the wire feeding process, and the slab slag inclusion defect does not occur, and finally the high hot plastic 980MPa grade ultra-high strength automobile steel is manufactured.

[0106] It should be noted that those skilled in the art should recognize that the above examples are only used to illustrate the present application, and are not used as a limitation of the present application, as long as the changes and modifications of the above described examples are within the scope of the present application.

Claims

1. A high-thermoplasticity 980MPa grade ultra-high strength automotive steel, characterized in that, The composition includes the following components by weight percentage: C: 0.1–0.3%, Si: 0.2–0.7%, Mn: 1–3%, Ti: 0.04–0.07%, Al: 0.02–0.07%, Ca: 0.001–0.005%, Ce: 0.0010–0.0050%, Cr: 0.2–0.4%, P≤0.013%, S≤0.002%, with the remainder being Fe and unavoidable impurities, satisfying the following composition: Al / Ce = 11–40, Ce / S = 0.5–3, where Al, Ce, and S are the weight percentage contents of the corresponding elements. The inclusion type of the high thermoplastic 980MPa grade ultra-high strength automotive steel is Al-Ca-Ce composite inclusion; In the aforementioned high-thermoplasticity 980MPa grade ultra-high strength automotive steel, the proportion of acicular bainite microstructure is greater than 30%. The high thermoplasticity 980MPa grade ultra-high strength automotive steel has a section reduction rate of ≥40% at high temperatures of 750℃, 800℃, 850℃, and 900℃.

2. The high thermoplasticity 980MPa grade ultra-high strength automotive steel according to claim 1, characterized in that, Of the unavoidable impurities, O ≤ 0.0020% by weight.

3. The high thermoplasticity 980MPa grade ultra-high strength automotive steel according to claim 1, characterized in that, In the aforementioned high thermoplasticity 980MPa grade ultra-high strength automotive steel, the density of inclusions larger than 5 micrometers is ≤0.05 inclusions / mm². 2 The density of inclusions larger than 1 micrometer is ≤25 inclusions / mm². 2 The average diameter of the inclusions is ≤2 micrometers.

4. A method for manufacturing high thermoplasticity 980MPa grade ultra-high strength automotive steel as described in any one of claims 1-3, characterized in that, Includes the following steps: S1, smelting, uses KR desulfurized molten iron and low-sulfur scrap steel as raw materials, and adopts top and bottom blowing process in converter. During the steel tapping process, the oxygen content of the tapped steel is controlled below 600ppm through pre-deoxidation. S2, LF refining, aluminum slag is added during the LF heating process, and after the target temperature is reached, alloy is added to adjust the composition of the molten steel, while bottom blowing argon gas is used for stirring. S3, RH refining, vacuum composition adjustment is carried out in RH furnace. First, titanium alloy is added for deoxidation, and after 3 to 5 minutes, aluminum alloy is added for deoxidation. Other alloys are added to adjust the composition of the molten steel to the target value. After breaking the vacuum, calcium wire is fed in for calcium treatment. S4, continuous casting wire feeding, control the O in the molten steel in the tundish to be ≤0.002% and S to be ≤0.002%, and feed rare earth wire into the continuous casting mold.

5. The manufacturing method of the high thermoplasticity 980MPa grade ultra-high strength automotive steel according to claim 4, characterized in that, In step S1: The iron content in the raw materials is ≥88%; The tapping temperature of the converter is 1670-1680℃, and the oxygen content during tapping is controlled at 500-600ppm. The deoxidizer used in the pre-deoxidation process is ferrosilicon, and the amount of ferrosilicon added is 50-200 kg / furnace. During the tapping process of the converter, 900–1100 kg / heat of lime is added, and / or In step S2: The amount of aluminum slag added is 100-200 kg / furnace. The target temperature is 1620–1630°C; The argon flow rate is 160–180 L / min, the stirring time is 4–6 min, and / or In step S3: During the vacuum composition adjustment process, the vacuum degree is controlled to be ≤133Pa, and the pure degassing time is ≥10min; the amount of titanium alloy added is 120-140kg / furnace; the amount of aluminum alloy added is 800-900kg / furnace, and / or In step S4: During the continuous casting wire feeding process, the wire feeding position is the intersection of 1 / 2 of the distance from the nozzle to the narrow side and 1 / 2 of the distance from the narrow face, and single-line wire feeding is adopted; During the continuous casting wire feeding process, the wire feeding speed is 4-8 m / min; The rare earth wire is made of La-Ce alloy with a rare earth content of over 99% or rare earth wire with an outer iron shell and a Ce content of 55-65%.

6. The manufacturing method of the high thermoplasticity 980MPa grade ultra-high strength automotive steel according to claim 4, wherein in the La-Ce alloy with a rare earth content of more than 99%, the content of La is 25-35wt% and the content of Ce is 65-85wt%.

7. The manufacturing method of the high thermoplasticity 980MPa grade ultra-high strength automotive steel according to claim 5, characterized in that, When the rare earth wire is made of La-Ce alloy with a rare earth content of over 99%, the wire feeding speed is 4-5 m / min, and the steel throughput is ≥3 t / min; or When the rare earth wire is a rare earth wire with a Ce content of 55-65% and an outer iron shell, the wire feeding speed is 6-8 m / min and the steel throughput is ≥3 t / min.

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