A low carbon footprint ultra-high strength steel and method of making same

Abstract

The application discloses a kind of low carbon footprint ultra-high strength steel, it contains Fe and inevitable impurities, it also contains the following chemical elements with mass percentage as follows: C:0.06-0.12%, Si:0.2-0.6%, Mn:1.7-2.3%, Mo:0.14-0.22%, Nb:0.02-0.05%, Ti:0.03-0.07%, Al:0.02-0.06%, B:0.001-0.003%, N:0.005-0.01%;N distribution coefficient in the ultra-high strength steel: N 固溶 / 析出 ≥4.The application also provides a manufacturing method of the ultra-high strength steel.The ultra-high strength steel of the application has the technical advantages of green low carbon and high strength weight reduction.

Description

TECHNICAL FIELD

[0001] The present application relates to a steel sheet and a manufacturing method thereof, and in particular to an ultra-high strength steel and a manufacturing method thereof. BACKGROUND

[0002] In recent years, in order to achieve the purpose of weight reduction of vehicle body, energy saving and emission reduction, improvement of collision safety and reduction of manufacturing cost, advanced high-strength steel is widely used in automobiles, which can reduce the thickness of steel sheet by improving the strength of steel sheet, while maintaining excellent forming performance.

[0003] With the development of high-strength steel technology, it is expected not only to focus on the improvement of mechanical properties, but also to have high strength and weight reduction and green low carbon.

[0004] The Chinese patent document with publication number CN113215477A, publication date August 6, 2021, and title "Preparation method of low-carbon emission cold-rolled base steel strip" discloses a preparation method of low-carbon emission cold-rolled base steel strip. Its process route is top and bottom combined blowing converter smelting, converter tapping slag washing to remove inclusions, slab continuous casting, and hot continuous rolling to produce a method of low-carbon, low-silicon boron-containing cold-rolled deep-drawing base strip steel coil. Although the nitrogen content in the steel is reduced due to the omission of the LF refining process, there is no electrode consumption in the production process, which reduces the power consumption and carbon dioxide emissions.

[0005] The Chinese patent document with publication number CN115058641A, publication date September 16, 2022, and title "Method for smelting low-sulfur low-aluminum high-carbon wire rod steel by full scrap electric furnace" discloses a method for smelting low-sulfur low-aluminum high-carbon wire rod steel by full scrap electric furnace. The method belongs to the field of metallurgy, which includes electric furnace smelting, LF refining, RH vacuum refining, and continuous casting process; the furnace charge of the electric furnace smelting process is all scrap steel, and the tapping and charging sequence is silicon iron→carburizing agent→manganese iron→other alloys→slag.

[0006] It can be seen that the above patent documents do not involve low-carbon footprint technology.

[0007] Although the Chinese patent document with publication number CN115433855A, publication date December 6, 2022, and title "Aluminum extrusion with low carbon footprint" discloses an aluminum extrusion with low carbon footprint, and provides the composition of an alloy composition, which is about 60% from Al scrap. However, this patent document belongs to the field of aluminum alloy and does not involve low-carbon footprint steel materials. SUMMARY

[0008] One of the purposes of the present application is to provide a low-carbon-footprint ultra-high-strength steel, which is uniquely designed in composition and has a low carbon equivalent, and is produced by a low-carbon-emission steelmaking production process of an electric furnace-LF furnace-RH-continuous casting, while using all scrap steel as smelting raw materials to replace high-carbon-consumption molten iron, so that the ultra-high-strength steel has the technical advantages of green low carbon and high strength weight reduction while having certain forming and bending requirements.

[0009] In order to achieve the above-mentioned purpose, the present application provides a low-carbon-footprint ultra-high-strength steel containing Fe and inevitable impurities, and further containing the following chemical elements in mass percentage:

[0010] C: 0.06-0.12%, Si: 0.2-0.6%, Mn: 1.7-2.3%, Mo: 0.14-0.22%, Nb: 0.02-0.05%, Ti: 0.03-0.07%, Al: 0.02-0.06%, B: 0.001-0.003%, N: 0.005-0.01%;

[0011] The N distribution coefficient in the ultra-high-strength steel: N 固溶 / N 析出 ≥4.

[0012] Correspondingly, the present application also provides a low-carbon-footprint ultra-high-strength steel, which contains the following chemical elements in mass percentage:

[0013] C: 0.06-0.12%, Si: 0.2-0.6%, Mn: 1.7-2.3%, Mo: 0.14-0.22%, Nb: 0.02-0.05%, Ti: 0.03-0.07%, Al: 0.02-0.06%, B: 0.001-0.003%, N: 0.005-0.01%; the balance is Fe and inevitable impurities;

[0014] The N distribution coefficient in the ultra-high-strength steel: N 固溶 / N 析出 ≥4.

[0015] It should be noted that the N 析出 of the ultra-high-strength steel of the present application is tested by: electrolytic extraction separation of inclusions for the material, and then measuring the N content in the inclusions as N 析出 ; N 固溶 =N 总 -N 析出 , N 总 is the total N content in the material measured according to GB / T 20124 inert gas melting heat conduction method.

[0016] In the present application, the N content is high due to the electric furnace smelting route. N can improve the strength of the material when it exists in the form of solid solution, and can exist in the form of TiN inclusions when it exists in the form of precipitation, and too much inclusions can affect the plasticity and bending performance of the material. Therefore, in the hot-rolled high-strength steel described in the present application, the mass percentage of N is controlled to be 0.0050-0.010%, and N is made to exist in the form of solid solution as much as possible, and the distribution coefficient of N is controlled to be N 固溶 / N 析出 ≥4.

[0017] In addition, in the ultra-high-strength steel described in the present application, the design principles of each chemical element are as follows:

[0018] C: In the ultra-high-strength steel described in the present application, carbon directly affects the strength, weldability and formability of the steel plate / steel strip, and the higher the carbon content, the more beneficial to improve the strength of the steel plate. If the carbon content is lower than 0.06%, the strength of the steel plate / steel strip cannot meet the target requirements; if the carbon content is higher than 0.12%, it is easy to cause the carbon equivalent to be too high, thereby deteriorating the weldability of the steel plate. Therefore, in the ultra-high-strength steel described in the present application, the mass percentage of carbon can be controlled to be between 0.06-0.12%, and in some embodiments, the mass percentage of C can be controlled to be between 0.08-0.11%.

[0019] Si: In the ultra-high-strength steel described in the present application, Si element can dissolve in ferrite to play a role of solid solution strengthening, and Si element can promote the enrichment of C and Mn elements in ferrite to austenite, reduce the interstitial solid solution strengthening and inhibit the generation of coarse carbides during cooling, and improve the ductility of the steel. However, Si element is easy to form SiO2 oxide film on the surface of the steel strip during annealing, which is difficult to be reduced by hydrogen and affects the surface quality, and too high Si element also has adverse effects on the spot welding performance of the steel plate. Therefore, in the ultra-high-strength steel described in the present application, the mass percentage of silicon can be controlled to be between 0.2-0.6%, and in some embodiments, the mass percentage of Si can be controlled to be between 0.3-0.6%.

[0020] Mn: In the ultra-high-strength steel described in the present application, Mn element is a solid solution strengthening element and a good desulfurizer, which is beneficial to improve the strength of the steel, and Mn can also enrich in austenite to play a stabilizing role, thereby being beneficial to improve the plasticity of the steel. Therefore, in the ultra-high-strength steel described in the present application, the mass percentage of Mn can be controlled to be between 1.7-2.3%, and in some embodiments, the mass percentage of Mn can be controlled to be between 1.9-2.1%.

[0021] Mo: In the ultra-high strength steel described in the present application, the Mo element belongs to the carbide element, and at the same time has a promoting effect on improving the hardenability of austenite, can promote the ferrite, pearlite and bainite transformation region to move to the right, and expand the austenite region. However, with the increase of Mo content, problems such as cold rolling edge crack are easy to occur, and the manufacturability of the material is increased. Therefore, in the ultra-high strength steel described in the present application, the mass percentage of Mo can be controlled to be between 0.14-0.22%, and in some embodiments, the mass percentage of Mo can be controlled to be between 0.16-0.20%.

[0022] Nb: In the ultra-high strength steel described in the present application, the Nb element plays an important role in grain refinement, and the fine carbonitride formed by Nb combined with C and N can delay recrystallization and prevent grain growth, with significant strengthening effect. Therefore, in the ultra-high strength steel described in the present application, the mass percentage of Nb can be controlled to be between 0.02-0.05%, and in some embodiments, the mass percentage of Nb can be controlled to be between 0.025-0.045%.

[0023] Ti: In the ultra-high strength steel described in the present application, Ti can fix nitrogen in steel to form stable compounds, improve the quality of the casting blank and the corner crack defect, and also form fine carbides to prevent austenite grain growth and refine the grain. Therefore, in the ultra-high strength steel described in the present application, the mass percentage of Ti can be controlled to be between 0.03-0.07%, and in some embodiments, the mass percentage of Ti can be controlled to be between 0.04-0.06%.

[0024] Al: In the ultra-high strength steel described in the present application, when Al exists in solid solution state, it can inhibit cementite precipitation and improve austenite stability. And Al can form fine and dispersed insoluble points with C and N, which can refine the grain. However, if the Al content in the steel is too high, a large amount of oxide inclusions will be formed, which is not conducive to the cleanliness of the molten steel. Therefore, in the ultra-high strength steel described in the present application, the mass percentage of Al can be controlled to be between 0.02-0.06%, and in some embodiments, the mass percentage of Al can be controlled to be between 0.02-0.05%.

[0025] B: In the ultra-high strength steel described in the present application, the main role of B element is to improve the hardenability of the steel, thereby improving the strength of the steel. B is easy to segregate at the grain boundary of austenite, delaying the transformation of austenite to ferrite, and a low content has obvious effect. At the same time, B has a good grain boundary purification effect, which can hinder the segregation of harmful elements on the grain boundary to a certain extent, thereby improving the deformation compatibility of the material. However, too high B content will cause the strength of the steel to increase, which is not conducive to obtaining good plasticity. Therefore, in the ultra-high strength steel described in the present application, the mass percentage of B can be controlled between 0.001-0.003%, and in some embodiments, the mass percentage of B can be controlled between 0.002-0.003%.

[0026] Further, in the ultra-high strength steel described in the present application, it also contains at least one of the following residual elements:

[0027] Cr≤0.05wt%;

[0028] Ni≤0.05wt%;

[0029] Cu≤0.1wt%;

[0030] V≤0.05wt%.

[0031] In the present application, since full scrap steel is used as the smelting raw material, residual elements such as Cr, Ni, Cu, and V that are difficult to remove by steelmaking may be introduced into the raw material. Too high Cr and Ni elements will change the phase transformation rule of the material, too high Cu element will cause hot rolling copper brittleness problem, and too high V element will also generate more precipitates to make the material strength increase, so it is necessary to control Cr, Ni, Cu and V elements within the range as described above.

[0032] Further, in the unavoidable impurities of the ultra-high strength steel described in the present application, P≤0.015%, S≤0.003%, Sn≤0.01%.

[0033] In the present application, the unavoidable impurities mainly include phosphorus (P), sulfur (S), and tin (Sn), and the less the content is, the purer the steel is and the better the performance is. Too high P content will weaken the grain boundary, increase the brittleness of the material, and deteriorate the welding performance. Therefore, in the ultra-high strength steel described in the present application, the mass percentage of P can be controlled to P≤0.015%. Too high S content in the steel will significantly deteriorate the plasticity of the material. Therefore, in the ultra-high strength steel described in the present application, the mass percentage of S can be controlled to S≤0.003%. Sn is also a harmful element in steel, which segregates at the grain boundary and has an adverse effect on the toughness and plasticity of the material, and too high Sn content will also deteriorate the coating performance of the material. Therefore, in the ultra-high strength steel described in the present application, the Sn content can be controlled to Sn≤0.01%.

[0034] Furthermore, in the ultra-high strength steel described in this invention, the harmful element factor Hem ≤ 0.018%, where Hem = Sn + N / 2 + 2S, and each chemical element is substituted with its mass percentage content.

[0035] In this invention, controlling the harmful element factor Hem to be ≤0.018% can further ensure the bending and pore-expanding properties of the material.

[0036] Furthermore, in the ultra-high strength steel described in this invention, its welding carbon equivalent C eq ≤0.28%, Ceq=C+Mn / 20+Si / 30+2P+4S, where each chemical element is substituted with its mass percentage content.

[0037] In this invention, the mass percentages of C, Si, Mn, P, and S satisfy the welding carbon equivalent C. eq When the content is ≤0.28%, better welding performance can be obtained, and the material with this composition has better solid solution and microstructure strengthening effect, resulting in higher material strength.

[0038] Furthermore, in the ultra-high strength steel described in this invention, the mass percentage content of each chemical element further satisfies at least one of the following conditions:

[0039] C: 0.08-0.11%;

[0040] Si: 0.3-0.6%;

[0041] Mn: 1.9-2.1%;

[0042] Mo: 0.16-0.20%;

[0043] Nb: 0.025-0.045%;

[0044] Ti: 0.04-0.06%;

[0045] Al: 0.02-0.05%;

[0046] B: 0.002-0.003%.

[0047] Furthermore, in the ultra-high strength steel described in this invention, its microstructure includes ferrite and martensite.

[0048] Furthermore, in the ultra-high strength steel described in this invention, the volume ratio of martensite is 60-75%.

[0049] Further, in the ultra-high strength steel described in the present application, the yield strength is 700-850 MPa, the tensile strength is ≥980 MPa, the fracture elongation is ≥10%, the 180-degree cold bending performance is guaranteed not to crack under the condition of 1T (i.e., the gasket is 1 times the thickness of the plate), and the carbon footprint is ≤1.2 kgCO2eq / kg.

[0050] In addition, another object of the present application is to provide a manufacturing method of an ultra-high strength steel with low carbon footprint, which is green and efficient, and the manufactured ultra-high strength steel has the advantages of green low carbon and high strength weight reduction.

[0051] In order to achieve the above-mentioned object, the present application provides a manufacturing method of an ultra-high strength steel with low carbon footprint, which comprises the following steps:

[0052] Full scrap steel is used for electric furnace smelting, LF, RH and continuous casting; wherein in the RH process, the vacuum treatment time is ≤10 min, and soft blowing is performed for 15-25 min after breaking the vacuum;

[0053] Hot rolling and coiling;

[0054] Cold rolling;

[0055] Continuous annealing: first holding at an annealing holding temperature of 760-820℃ for 60-200s, then cooling to a fast cooling start temperature of 650-690℃ at a first cooling rate of 3-10℃ / s, then cooling to a fast cooling end temperature of 220-260℃ at a second cooling rate of 40-80℃ / s, then holding for 200-400s for aging treatment, and finally cooling to 100℃ or below at a third cooling rate of 2-10℃ / s.

[0056] The above-mentioned smelting and casting process of the present application specifically comprises: electric furnace-LF furnace-RH-continuous casting. The conventional blast furnace converter manufacturing process is long, and the carbon footprint of the final product is significantly high due to the use of coke reduction iron ore method, which is basically at the level of 2.5-3.0 kgCO2eq / kg. And the ironmaking and steelmaking process accounts for nearly 70% of the whole process, which is the highest link of carbon footprint. In addition, the conventional electric furnace smelting is mostly suitable for medium-high carbon and stainless steel or special steel products, and there are few involvements in low S\N impurity element control medium-low carbon automobile high strength steel.

[0057] And the full scrap steel electric furnace smelting method of the present application can greatly reduce the carbon emission level of the steel smelting process. In addition, the present application adopts RH short vacuum time+long soft blowing time, which can be vacuumized to ≤100Pa in some embodiments, and the vacuum is maintained for ≤10min after breaking the vacuum, and soft blowing is performed for 15-25min after breaking the vacuum, and the soft blowing effect is just not to blow open the slag surface, then stand for a period of time, for example, 10min, then hoist to continuous casting for casting.

[0058] In the present application, the vacuum treatment can reduce the N content in the molten steel, but the N content does not decrease obviously after the vacuum treatment for too long time, and the long time vacuum is not conducive to the plasticization treatment of inclusions; the long time soft blowing after the vacuum breaking can remove the inclusions (mainly N compound and S compound inclusions) or realize the plasticization treatment of inclusions. Therefore, the short vacuum time and long soft blowing time in the RH vacuum refining process can effectively control the inclusions and gas content. Since the N content of the electric furnace tapping of scrap steel is obviously higher than that of the conventional converter process, the lower limit of the controllable N through the RH treatment is at the level of 50 ppm, and the TiN inclusions in the molten steel can be basically removed through the treatment to basically remain in the slag.

[0059] In addition, in the manufacturing method of the ultra-high strength steel, the deformed structure after cold rolling is annealed at an annealing holding temperature of 760-820℃ for 60-200s to perform two-phase zone annealing. The main purpose of this process is to recrystallize the structure of the steel plate after cold rolling. If the temperature is too low, the recrystallization of the steel plate structure will not be complete, and if the temperature is too high, the grains of the steel plate structure will be coarse and the precipitates will be decomposed, thereby reducing the strength of the steel plate. Therefore, in the present application, the annealing holding temperature is controlled to be 760-820℃. Similarly, the time control of this process is also very important. If the time is too short, the recrystallization of the steel plate structure will not be complete, and the diffusion of austenite stabilizing elements (C, Mn) will not be sufficient; and if the time is too long, the grains of the steel plate structure will be abnormally coarse and the precipitates will be decomposed, etc. Therefore, the annealing holding time is controlled to be 60-200s. In this process, the steel plate structure will be partially austenitized, and the cold rolled deformed grains will complete the recovery and recrystallization process.

[0060] In addition, the steel strip after holding is controlled at a first cooling speed of 3-10℃ / s and slowly cooled to 650-690℃, which can allow part of the austenite in the steel plate structure to be converted into ferrite. The part of the ferrite is relatively finer than the recrystallized ferrite in the two-phase zone. This is beneficial to the coordination of the soft and hard phases and the improvement of the material plasticity during the final deformation.

[0061] In addition, the second cooling speed is controlled to be 40-80℃ / s to quickly cool the slowly cooled steel strip to 220-260℃ through air blowing rapid cooling, and then hold for 200-400s for aging treatment. This is because if the cooling speed is too slow in this process, pearlite will appear, which will reduce the performance of the steel plate; and if the cooling speed is too fast, it will increase the difficulty and cost of production, therefore the second cooling speed is controlled to be 40-80℃ / s. The remaining austenite is basically converted into martensite during the rapid cooling process, thereby greatly improving the strength. During the aging process, the quenched martensite will undergo low-temperature tempering reaction, a small amount of the lamellar martensite obtained by quenching will be decomposed and carbide will be precipitated, which is beneficial to the improvement of the toughness of the material and avoids the cold brittle fracture.

[0062] In addition, the steel strip after aging treatment is cooled to 100°C or below at a third cooling rate of 2-10°C / s, and the microstructure of the steel does not change substantially below this temperature.

[0063] Further, in the electric furnace smelting process of the manufacturing method of the ultra-high strength steel, scrap steel includes light scrap steel, medium scrap steel and heavy scrap steel, and the weight ratio of the light scrap steel, the medium scrap steel and the heavy scrap steel is (3-3.5):(1.5-2):1.

[0064] The light scrap steel refers to block or irregular shape finished scrap steel, cut edges and the like, and the block size requirement is <300x800x1800mm single weight ≤0.5 tons, the medium scrap steel refers to block or irregular shape various machine scrap steel parts, components and the like, and the block size requirement is <300x500x1500mm single weight ≤1.0 tons, and the heavy scrap steel refers to block scrap steel ingot, roughing scrap blank and the like, and the block size requirement is <300x500x1500mm single weight ≤2.0 tons.

[0065] Further, in the LF process of the manufacturing method of the ultra-high strength steel, the slag basicity is 1.8-2.5 to remove the S content to within 30ppm.

[0066] Further, in the continuous casting process of the manufacturing method of the ultra-high strength steel, the superheat is controlled to be ≤30°C, the mold taper is controlled to be 1.0-1.3%, and the continuous casting withdrawal speed is controlled to be 1.0-1.5m / min.

[0067] In the continuous casting process of the present application, in order to ensure the smooth continuous casting process of the molten steel, the molten steel needs to have a certain superheat. When the superheat is higher than 30°C, it is not good for the center quality of the casting blank, and segregation, shrinkage and other defects are easily produced. Therefore, in the present application, the superheat can be controlled to be ≤30°C. At the same time, in order to better control the surface quality of the casting blank and prevent small longitudinal cracks and other defects, a chamfered mold is selected to improve the stress distribution and control the mold taper to be between 1.0-1.3%, and in order to balance the production efficiency and the surface quality of the casting blank, the continuous casting withdrawal speed is controlled to be between 1.0-1.5m / min. Dynamic soft reduction and electromagnetic stirring are used in the pouring process to better improve the segregation degree of the casting blank.

[0068] Further, in the hot rolling step of the manufacturing method of the ultra-high strength steel, the slab heating temperature is controlled to be 1200-1260°C, and the finish rolling temperature is controlled to be 850-950°C.

[0069] In the hot rolling process of the present application, the casting blank can be first heated at a high temperature in the full austenite zone for a period of time to make the material soften and the composition fully diffuse uniformly. After rough rolling to the intermediate blank thickness specification, the coarse organization generated in the heating furnace is broken and refined, and after further rolling to the required thickness by finish rolling, a uniform and fine recrystallized material organization is formed. When the finish rolling final rolling temperature is less than 850℃, ferrite will be precipitated before finish rolling, resulting in a low content of hard phase in the final organization, which will cause insufficient hot rolling strength and further affect the subsequent cold rolling and annealing performance. Considering the upper limit control of the slab heating temperature and the temperature drop during rolling, the finish rolling final rolling temperature is generally not more than 950℃. Therefore, in the present application, the finish rolling final rolling temperature can be controlled to be 850-950℃.

[0070] Further, in the coiling step of the manufacturing method of the ultra-high strength steel according to the present application, the coiling temperature is controlled to be 580-660℃.

[0071] In the present application, the hot rolling coiling temperature also affects the hot rolling performance. When the coiling temperature is higher than 660℃, the surface of the steel plate is prone to Si and Mn internal oxidation, and through pickling, a surface broken layer is generated to affect the surface quality of the final product; and when the coiling temperature is lower than 580℃, the strength of the hot rolling state is too high, which will be not conducive to the subsequent cold rolling, and will cause the problem of excessive deformation resistance and thus produce cold rolling edge cracks or poor plate shape. Therefore, preferably, in the present application, the coiling temperature can be controlled to be 580-660℃.

[0072] Further, in the cold rolling step of the manufacturing method of the ultra-high strength steel according to the present application, the cold rolling reduction is controlled to be 50-70%.

[0073] In the present application, after pickling to remove the surface iron oxide scale of the hot rolling coil, through a cold rolling reduction of 50-70%, the steel plate can reach the target thickness size and accumulate a certain deformation energy, thereby being more conducive to the subsequent annealing recrystallization.

[0074] The low-carbon-footprint ultra-high strength steel and the manufacturing method thereof according to the present application have the following advantages and beneficial effects:

[0075] The low-carbon-footprint ultra-high strength steel according to the present application is designed by a specific composition to make the carbon equivalent low, and the N is controlled to exist in the solid solution as much as possible, and the N partition coefficient N 固溶 / N 析出 ≥4 to improve the strength of the material and ensure the plasticity and bending performance of the material.

[0076] The manufacturing method of the low-carbon footprint ultrahigh-strength steel has the technical advantages of green low carbon, and the impurities and residual elements of the product are controlled at a low level through the control of the scrap steel raw material and the steelmaking technology, so as to not affect the performance of the material.

[0077] The manufacturing method of the low-carbon footprint ultrahigh-strength steel has the technical advantages of green low carbon, and the impurities and residual elements of the product are controlled at a low level through the control of the scrap steel raw material and the steelmaking technology, so as to not affect the performance of the material.

[0078] In some embodiments, the yield strength of the low-carbon footprint ultrahigh-strength steel is 700-850 MPa, the tensile strength is ≥980 MPa, the elongation at break is ≥10%, the 180-degree cold bending performance is guaranteed without cracking under the condition of 1T (i.e., without cracking when the gasket is 1 times the plate thickness), and the carbon footprint is ≤1.2 kgCO2eq / kg, realizing the consideration of green low carbon and ultrahigh strength performance. BRIEF DESCRIPTION OF DRAWINGS

[0079] Figure 1 The microstructure morphology of the low-carbon footprint ultrahigh-strength steel of Example 7 of the present application is shown. DETAILED DESCRIPTION

[0080] The low-carbon footprint ultrahigh-strength steel and the manufacturing method thereof described in the present application will be further explained and described below in conjunction with specific examples, however, the explanation and description do not constitute undue limitations on the technical solutions of the present application.

[0081] Examples 1-20 and Comparative Examples 1-3

[0082] The low-carbon footprint ultrahigh-strength steel of Examples 1-20 of the present application is prepared by the following steps:

[0083] (1) Full scrap steel is used for electric furnace smelting, LF, RH and continuous casting:

[0084] Among them, in the electric furnace smelting process, the types of scrap steel are light, medium and heavy, and clean and clean materials are selected for scrap steel. When adding scrap steel furnace charge, 1-2 tons of light scrap steel is laid on the bottom of the electric furnace as a substrate to prevent the impact and damage of a large amount of scrap steel (especially heavy scrap steel) when pouring. Large flow oxygen lance is used for oxygen blowing and dephosphorization during smelting, and the end point composition of the electric furnace is controlled according to the following control, C≤0.05wt%, P≤0.01wt%, S≤0.03wt%, and T 出钢 ≥1630℃. After tapping, the slag is removed in the slag removal station, and the amount of residual slag is controlled to be ≤40mm.

[0085] In a preferred embodiment, the weight ratio of the three types of scrap steel is light scrap steel: medium scrap steel: heavy scrap steel = (3-3.5):(1.5-2):1.

[0086] In the LF process, after the ladle enters the LF station, Al is first added for deep deoxidation. When the sampling analysis shows that Al≥0.03wt%, a carbonizer, ferrosilicon, and ferromanganese are added for carbon supplement and deoxidation alloying treatment, and slag material is added for slagging and desulfurization. The slag material is lime and fluorite, the addition amount of lime can be 6-8 kg / t of steel, and the addition amount of fluorite can be 1-2 kg / t of steel, so as to preferably control the slagging basicity to be 1.8-2.5, so that the S content is removed to within 30 ppm. After S removal is completed, the required Nb, Ti, and B alloys in the composition are added. After the alloying components are all added in place, the outlet is prepared, and at the same time, the excess desulfurization slag is removed, and the amount of residual slag is controlled to be≤30 mm.

[0087] In the RH process: after entering the RH station, vacuum is extracted to≤100 Pa, and then vacuum is maintained for≤10 min before breaking the vacuum. After breaking the vacuum, soft blowing is performed for 15-25 min, and the soft blowing effect is just not to blow off the slag surface. Then, stand for about 10 min, and then hoist to continuous casting for casting.

[0088] In the continuous casting process: the superheat is controlled to be 15-30℃, the mold taper is controlled to be between 1.0-1.3%, and the continuous casting speed is controlled to be between 1.0-1.5 m / min. The chemical composition of the obtained casting blank is shown in Table 1-1, Table 1-2, and Table 1-3.

[0089] (2) Hot rolling and coiling: the slab heating temperature is 1200-1260℃, rough rolling is followed by finish rolling, and the finish rolling final rolling temperature is 850-950℃. After rolling, water cooling is performed to the coiling temperature of 580-660℃.

[0090] (3) Pickling to remove iron oxide scale.

[0091] (4) Cold rolling: a reduction of 50-70% is adopted.

[0092] (5) Continuous annealing: after holding at the annealing holding temperature of 760-820℃ for 60-200 s, cooling is performed at a first cooling rate of 3-10℃ / s to the fast cooling start temperature of 650-690℃ to obtain a certain proportion of ferrite; then fast cooling is performed at a second cooling rate of 40-80℃ / s to the fast cooling end temperature of 220-260℃; then holding is performed for 200-400 s for aging treatment; finally, cooling is performed at a third cooling rate of 2-10℃ / s to below 100℃.

[0093] Table 1-1. (wt%, the balance is Fe and other unavoidable impurities except P, S, and Sn)

[0094]

[0095]

[0096] Table 1-2. (wt%, the balance being Fe and other inevitable impurities except P, S, Sn)

[0097]

[0098] Table 1-3. Ingredient control coefficient

[0099]

[0100] Note: Hem = Sn + N / 2 + 2S, Ceq = C + Mn / 20 + Si / 30 + 2P + 4S in Table 3, in which each chemical element is substituted by its mass percentage.

[0101] The steel grades A-E in Table 1-1, Table 1-2 and Table 1-3 are the steel grade numbers used in the embodiments of the present application, and a-c are the steel grade numbers used in the comparative examples.

[0102] Table 2-1 and Table 2-2 list the specific process parameters in the above process steps for the embodiments of the present application and the comparative examples.

[0103] Table 2-1.

[0104]

[0105]

[0106] Note: the column of scrap weight ratio of Comparative Example 1 is "-" indicating that it does not use the electric furnace full scrap melting process, but still uses the converter steelmaking process and uses conventional blast furnace molten iron.

[0107] Table 2-2.

[0108]

[0109]

[0110] The samples of Examples 1-20 and Comparative Examples 1-3 obtained are polished and have no obvious scratches on the bright surface, and then the polished surface is corroded with 4% nitric acid alcohol solution, clean experimental cotton ball is used for wiping the corrosion liquid, the wiping time is about 15s to observe the microstructure, the slightly white area in the microstructure is ferrite structure, and the small dark structure is martensite structure, and the observation results are listed in Table 3.

[0111] In addition Figure 1 The microstructure morphology of the low-carbon footprint ultrahigh-strength steel of Example 7 of the present application is also shown. As Figure 1As shown, the microstructure of the low-carbon footprint ultrahigh-strength steel is ferrite + martensite.

[0112] In addition, the obtained samples of Examples 1-20 and Comparative Examples 1-3 were subjected to mechanical property testing according to ISO 6892:1998 (Metallic materials-tensile testing at ambient temperature) and P14 (A 50 ) tensile specimen standard, and the carbon footprint was calculated, and the test results are shown in Table 3. The product carbon footprint calculation boundary is from the cradle to the door, and the specific calculation method and product classification comply with ISO 14067 "Greenhouse gases-Product carbon footprint-Quantification requirements and guidelines" and China Steel Association EPD platform PCR 2022:01 "General steel products and special steel products".

[0113] Table 3.

[0114]

[0115]

[0116] As can be seen from Table 3, by reasonable chemical element composition design and combined with optimized process parameters, the ideal microstructure characteristics of Examples 1-20 of the present application are obtained. While ensuring the strength, the plasticity is very excellent, the yield strength is between 700-850 MPa, the tensile strength is greater than or equal to 982 MPa, the uniform elongation is greater than 6%, the elongation at break is greater than or equal to 10.3%, and the 180-degree cold bending performance is very excellent, and can meet the requirement of no cracking under 1T condition.

[0117] At the same time, it can also be seen that the carbon footprint of Examples 1-20 of the present application is less than or equal to 1.2 kg CO2 eq / kg, and the carbon emission level is low, which meets the technical requirements of green and low carbon. The process path of Comparative Example 1 still uses the conventional converter path using blast furnace molten iron, so its carbon footprint is high.

[0118] Unlike the present application, each comparative example cannot achieve the technical effects that can be achieved by the present application because the composition or process parameters do not meet the design requirements of the present application.

[0119] It should be noted that the combination of technical features in the present case is not limited to the combination of technical features in the claims or the combination of technical features in the specific embodiments. All technical features described in the present case can be freely combined or combined in any way, unless contradictory to each other.

[0120] It should also be noted that the above-mentioned embodiments are only specific embodiments of the present application. Obviously, the present application is not limited to the above-mentioned embodiments, and similar changes or modifications made in accordance with the disclosure of the present application are directly derived or easily thought of by those skilled in the art, and should all belong to the protection scope of the present application.

Claims

1. A low carbon footprint ultra-high strength steel, characterized in that, The mass percentage of each chemical element is: C: 0.06-0.12%, Si: 0.2-0.6%, Mn: 1.7-2.3%, Mo: 0.14-0.22%, Nb: 0.02-0.05%, Ti: 0.03-0.07%, Al: 0.02-0.06%, B: 0.001-0.003%, N: 0.005-0.01%; the balance is Fe and inevitable impurities; The N distribution coefficient in the ultra-high strength steel: N 固溶 / N 析出 ≥4; The low-carbon footprint ultra-high strength steel is prepared by the following steps: Electric furnace smelting with full scrap steel, LF, RH and continuous casting; wherein in the RH process, the vacuum treatment time is ≤10 min, and soft blowing is performed for 15-25 min after breaking the vacuum; Hot rolling and coiling; Cold rolling; Continuous annealing: first annealing at an annealing holding temperature of 760-820℃ for 60-200s, then cooling at a first cooling rate of 3-10℃ / s to a fast cooling start temperature of 650-690℃, then cooling at a second cooling rate of 40-80℃ / s to a fast cooling end temperature of 220-260℃, then holding for 200-400s for aging treatment, and finally cooling at a third cooling rate of 2-10℃ / s to below 100℃.

2. The ultra-high strength steel according to claim 1, wherein It also contains at least one of the following residual elements: Cr≤0.05wt%; Ni≤0.05wt%; Cu≤0.1wt%; V≤0.05wt%.

3. The ultra-high strength steel of claim 1, wherein, Among the inevitable impurities, P≤0.015%, S≤0.003%, Sn≤0.01%.

4. The ultra-high strength steel according to claim 3, wherein The harmful element factor Hem of it is ≤0.018%, wherein Hem=Sn+N / 2+2S, and in the formula, each chemical element is substituted by its mass percentage.

5. The ultra-high strength steel according to claim 3, wherein The welding carbon equivalent Ceq of it is ≤0.28%, Ceq=C+Mn / 20+Si / 30+2P+4S, and in the formula, each chemical element is substituted by its mass percentage.

6. The ultra-high strength steel of claim 1, wherein, The mass percentage of each chemical element further satisfies at least one of the following: C：0.08-0.11％； Si: 0.3-0.6%; Mn: 1.9-2.1%; Mo: 0.16-0.20%; Nb: 0.025-0.045%; Ti: 0.04-0.06%; Al：0.02-0.05％； B：0.002-0.003％。 7. The ultra-high strength steel of claim 1, wherein, The microstructure thereof includes ferrite and martensite.

8. The ultra-high strength steel according to claim 7, wherein, The volume ratio of martensite is 60-75%.

9. The ultra-high strength steel of claim 1, wherein, The yield strength is 700-850MPa, the tensile strength is ≥980MPa, the fracture elongation is ≥10%, the 180-degree cold bending performance is guaranteed not to crack under the condition of 1T, and the carbon footprint is ≤1.2kgCO2eq / kg.

10. The method of manufacturing a low carbon footprint ultra-high strength steel according to any one of claims 1-9, wherein, It includes the following steps: Electric furnace smelting with full scrap steel, LF, RH and continuous casting; wherein in the RH process, the vacuum treatment time is ≤10 min, and soft blowing is performed for 15-25 min after breaking the vacuum; Hot rolling and coiling; Cold rolling; Continuous annealing: first holding at annealing holding temperature 760-820℃ for 60-200s, then cooling to fast cooling start temperature 650-690℃ at first cooling rate 3-10℃ / s, then cooling to fast cooling end temperature 220-260℃ at second cooling rate 40-80℃ / s, then holding for 200-400s for aging treatment, and finally cooling to below 100℃ at third cooling rate 2-10℃ / s.

11. The production method according to claim 10, wherein In the electric furnace smelting process, scrap steel includes three types of light scrap, medium scrap and heavy scrap, and the weight ratio is light scrap: medium scrap: heavy scrap = (3-3.5): (1.5-2):

1.

12. The production method according to claim 10, wherein In the LF process, the slag basicity is 1.8-2.5 to remove S content to within 30ppm.

13. The production method according to claim 10, wherein In the continuous casting process, the superheat is controlled to be ≤30℃, the mold taper is controlled to be 1.0-1.3%, and the continuous casting withdrawal speed is controlled to be 1.0-1.5m / min.

14. The production method according to claim 10, wherein In the hot rolling step, the slab heating temperature is controlled to be 1200-1260℃, and the finish rolling temperature is controlled to be 850-950℃.

15. The production method according to claim 10, wherein In the coiling step, the coiling temperature is controlled to be 580-660℃.

16. The production method according to claim 10, wherein In the cold rolling step, the cold rolling reduction is controlled to be 50-70%.

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