Low-carbon-footprint ultrahigh-strength steel and manufacturing method thereof

Abstract

Disclosed in the present invention is a low-carbon-footprint ultrahigh-strength steel, comprising Fe and inevitable impurities, and further comprises the following chemical elements in percentage by mass: 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%, and N: 0.005-0.01%; and the distribution coefficient of N in the ultrahigh-strength steel is: Nsolid solution / N<sb / > precipitation≥4. The present invention further provides a manufacturing method for the ultrahigh-strength steel. The ultrahigh-strength steel of the present invention has the technical advantages of green and low-carbon characteristics as well as high strength and decreased weight.

Description

Ultra-high strength steel with low carbon footprint and manufacturing method thereof Technical Field

[0001] The present invention relates to a steel plate and a manufacturing method thereof, and in particular to an ultra-high strength steel and a manufacturing method thereof. Background Art

[0002] In recent years, in order to achieve vehicle body weight reduction, energy conservation and emission reduction, improve collision safety and reduce manufacturing costs, advanced high-strength steel for automobiles has been widely used. It increases the strength of steel plates to reduce the thickness of steel plates while maintaining excellent forming properties.

[0003] With the development of high-strength steel technology, we expect not only to focus on improving its mechanical properties, but also to achieve high strength, weight reduction, and green and low-carbon properties.

[0004] The Chinese patent document, publication number CN113215477A, ​​published on August 6, 2021, and titled "A Method for Preparing Low-Carbon Emission Cold-Rolled Base Steel Strip," discloses a method for preparing low-carbon emission cold-rolled base steel strip. The process involves top-bottom combined-blowing converter smelting, washing the converter slag to remove inclusions, continuous slab casting, and hot rolling to produce a low-carbon, low-silicon, boron-containing cold-rolled deep-drawn base steel coil. Although the nitrogen content in the steel is reduced by omitting the LF refining step, there is no electrode consumption during the production process, which reduces power consumption and CO2 emissions.

[0005] The Chinese patent document with publication number CN115058641A and publication date September 16, 2022, and titled "A method for smelting low-sulfur, low-aluminum, high-carbon wire rod steel in an all-scrap electric furnace" discloses a method for smelting low-sulfur, low-aluminum, high-carbon wire rod steel in an all-scrap electric furnace. The method belongs to the field of metallurgical technology, which includes electric furnace smelting, LF refining, RH vacuum refining, and continuous casting processes; the electric furnace smelting process uses all scrap steel as the charge, and the order of charging for steelmaking is ferrosilicon → carburizer → ferromanganese → other alloys → slag.

[0006] It can be seen that none of the above patent documents involve low carbon footprint technology.

[0007] Although the Chinese patent document, entitled "Aluminum Extrudate with a Low Carbon Footprint," with publication number CN 115433855A and publication date December 6, 2022, discloses an aluminum extrudate with a low carbon footprint and provides an alloy composition comprising approximately 60% aluminum scrap, this patent document pertains to aluminum alloys and does not address low-carbon steel materials.

[0008] Summary of the Invention

[0009] One of the objectives of the present invention is to provide an ultra-high-strength steel with a low carbon footprint. The ultra-high-strength steel has a unique composition design and a low carbon equivalent. It adopts a low-carbon emission steelmaking production process of electric furnace-LF furnace-RH-continuous casting, and uses all scrap steel as a smelting raw material to replace high-carbon molten iron. This allows the ultra-high-strength steel to meet certain forming and bending requirements while having the technical advantages of being green, low-carbon, and high-strength and light-weight.

[0010] To achieve the above objectives, the present invention provides an ultra-high strength steel with a low carbon footprint, which contains Fe and unavoidable impurities, and further contains the following chemical elements in the following mass percentages:

[0011] C: 0.06-0.12%, Si: 0.2-0.6%, Mn: 1.7-2.3%, Mo: 0.14-0.22%, Nb: 0.020-0.05 0%, Ti: 0.03-0.07%, Al: 0.02-0.06%, B: 0.0010-0.0030%, N: 0.0050-0.0100%;

[0012] Nitrogen distribution coefficient in the ultra-high strength steel: N 固溶 / N 析出 ≥4.00.

[0013] Accordingly, the present invention also provides an ultra-high strength steel with a low carbon footprint, wherein the mass percentage of each chemical element is:

[0014] C: 0.06-0.12%, Si: 0.2-0.6%, Mn: 1.7-2.3%, Mo: 0.14-0.22%, Nb: 0.020-0.050%, Ti: 0.03-0.07%, Al: 0.02-0.06%, B: 0.0010-0.0030%, N: 0.0050-0.0100%; the balance is Fe and unavoidable impurities;

[0015] Nitrogen distribution coefficient in the ultra-high strength steel: N 固溶 / N 析出 ≥4.00.

[0016] It should be noted that the ultra-high strength steel grade N 析出 The test method is: electrolytic extraction and separation of inclusions in the material, and then the N content in the inclusions is measured as N 析出 ; N 固溶 =N 总 -N 析出 , N 总 It is the total nitrogen content in the material measured according to GB / T 20124 inert gas fusion thermal conductivity method.

[0017] In the present invention, the use of an electric furnace smelting route results in a high nitrogen content. If nitrogen exists in the matrix in the form of solid solution, it can improve the strength of the material. However, if it exists in the form of precipitation, it will exist in the form of TiN inclusions. Excessive inclusions will affect the plasticity and bending properties of the material. Therefore, in the hot-rolled high-strength steel described in the present invention, while controlling the mass percentage of nitrogen to 0.0050-0.0100%, it is necessary to make nitrogen exist in the form of solid solution as much as possible, and control the nitrogen distribution coefficient N 固溶 / N 析出 ≥4.00. In some embodiments, N 固溶 / N 析出 In some embodiments, N 固溶 ≥ 0.0040, such as between 0.0040 and 0.0090 or between 0.0040 and 0.0085. In some embodiments, N 析出 ≤0.0020, such as between 0.0005 and 0.0015 or 0.0008 and 0.0015.

[0018] In addition, in the ultra-high strength steel of the present invention, the design principles of each chemical element are specifically described as follows:

[0019] C: In the ultra-high-strength steel described herein, carbon directly impacts the strength, weldability, and formability of the steel sheet / strip. A higher carbon content improves the steel sheet's strength. A carbon content below 0.06% will result in a lower strength than the target steel sheet / strip. A carbon content above 0.12% can lead to an excessively high carbon equivalent, impairing the steel sheet's weldability. Therefore, the carbon content in the ultra-high-strength steel described herein can be controlled between 0.06% and 0.12% by weight. In some embodiments, the carbon content can be controlled between 0.08% and 0.11% by weight.

[0020] Si: In the ultra-high strength steel described in the present invention, the Si element can dissolve in ferrite and play a role in solid solution strengthening. Moreover, the Si element can promote the enrichment of C and Mn elements in ferrite into austenite, reduce interstitial solid solution strengthening, and inhibit the formation of coarse carbides during cooling, thereby improving the ductility of the steel. However, the Si element is easily enriched and oxidized on the surface of the steel strip during annealing to form a SiO2 oxide film, which is difficult to be reduced by hydrogen and affects the surface quality. At the same time, a high Si element also has an adverse effect on the spot welding performance of the steel plate. Therefore, in the ultra-high strength steel described in the present invention, the mass percentage of silicon can be controlled between 0.2-0.6%. In some embodiments, the mass percentage of Si can be controlled between 0.3-0.6%.

[0021] Mn: In the ultra-high-strength steel of the present invention, Mn is a solid-solution strengthening element and a good desulfurizer, contributing to increased steel strength. Mn also accumulates in austenite, stabilizing the steel and thus improving its plasticity. Therefore, the mass percentage of Mn in the ultra-high-strength steel of the present invention can be controlled between 1.7% and 2.3%. In some embodiments, the mass percentage of Mn can be controlled between 1.9% and 2.1%.

[0022] Mo: In the ultra-high strength steel described in the present invention, the Mo element is a carbide element and has a promoting effect on improving the hardenability of austenite. It can cause the transformation areas of ferrite, pearlite and bainite to move to the right, and expand the austenite area. However, as the Mo content increases, problems such as cold rolling edge cracking are prone to occur, and the manufacturability of the material becomes more difficult. Therefore, in the ultra-high strength steel described in the present invention, the mass percentage of Mo can be controlled between 0.14-0.22%. In some embodiments, the mass percentage of Mo can be controlled between 0.16-0.20%.

[0023] Nb: In the ultra-high-strength steel of the present invention, Nb plays a significant role in grain refinement. The fine carbonitrides formed by Nb, combined with C and N, delay recrystallization and prevent grain growth, resulting in significant strengthening. Therefore, the mass percentage of Nb in the ultra-high-strength steel of the present invention can be controlled between 0.020% and 0.050%, and in some embodiments, between 0.025% and 0.045%.

[0024] Ti: In the ultra-high-strength steel described herein, Ti can stabilize nitrogen in the steel to form stable compounds, improving ingot quality and preventing edge cracking defects. It can also form fine carbides, inhibiting austenite grain growth and refining the grain size. Therefore, the mass percentage of Ti in the ultra-high-strength steel described herein can be controlled between 0.03% and 0.07%, and in some embodiments, between 0.04% and 0.06%.

[0025] Al: In the ultra-high strength steel described in the present invention, when Al exists in a solid solution state, it can inhibit the precipitation of cementite and improve the stability of austenite. In addition, Al can form finely dispersed insoluble particles with C and N, which can refine the grains. However, if the Al content in the steel is too high, a large number of oxide inclusions are easily formed, which is not conducive to the cleanliness of the molten steel. Therefore, in the ultra-high strength steel described in the present invention, the mass percentage of Al can be controlled between 0.02-0.06%. In some embodiments, the mass percentage of Al can be controlled between 0.02-0.05%.

[0026] B: In the ultra-high strength steel described in the present invention, the main function of the B element is to improve the hardenability of the steel, thereby increasing the strength of the steel. B tends to segregate at the austenite grain boundaries, delaying the transformation of austenite to ferrite. A lower content will have a significant effect. At the same time, B has a good grain boundary purification effect, which can hinder the segregation of harmful elements at the grain boundaries to a certain extent, thereby improving the deformation coordination of the material. However, an excessively 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 invention, the mass percentage of B can be controlled between 0.0010-0.0030%. In some embodiments, the mass percentage of B can be controlled between 0.0020-0.0030%.

[0027] Furthermore, the ultra-high strength steel of the present invention further contains one or more of Cr, Ni, Cu and V. When contained, the Cr content does not exceed 0.05 wt%, the Ni content does not exceed 0.05 wt%, the Cu content does not exceed 0.10 wt%, and the V content does not exceed 0.05 wt%.

[0028] In some embodiments, the ultra-high strength steel of the present invention further contains residual elements of at least one of the following:

[0029] Cr≤0.05wt%;

[0030] Ni≤0.05wt%;

[0031] Cu≤0.1wt%;

[0032] V≤0.05wt%.

[0033] In the present invention, since all scrap steel is used as the smelting raw material, residual elements Cr, Ni, Cu, and V, which are difficult to remove during steelmaking, may be introduced into the raw material. Excessive levels of Cr and Ni will alter the phase transformation behavior of the material, while excessive levels of Cu will cause hot-rolled copper embrittlement. Excessive levels of V will also generate more precipitates, increasing the material's strength. Therefore, it is necessary to control the Cr, Ni, Cu, and V elements within the above-mentioned ranges.

[0034] Furthermore, among the inevitable impurities of the ultra-high strength steel of the present invention, P≤0.015%, S≤0.0030%, and Sn≤0.010%.

[0035] In the present invention, the inevitable impurities mainly include phosphorus (P), sulfur (S), and tin (Sn). The lower the content of these impurities, the purer the steel and the better the performance. Among them, too high a P content will weaken the grain boundaries, increase the brittleness of the material, and deteriorate the welding performance. Therefore, in the ultra-high strength steel described in the present invention, the mass percentage of P can be controlled to be P≤0.015%. However, too high a S content in the steel will significantly deteriorate the plasticity of the material. Therefore, in the ultra-high strength steel described in the present invention, the mass percentage of S can be controlled to be S≤0.0030%. Sn is also a harmful element in steel. Segregation at the grain boundaries will have an adverse effect on the toughness and plasticity of the material. At the same time, too high a Sn content will also deteriorate the coating performance of the material. Therefore, in the ultra-high strength steel described in the present invention, the Sn content can be controlled to be Sn≤0.010%.

[0036] Furthermore, in the ultra-high strength steel of the present invention, the harmful element factor Hem is ≤ 0.0180%, wherein Hem = Sn + N / 2 + 2S, and each chemical element is substituted into its mass percentage.

[0037] In the present invention, the harmful element factor Hem is controlled to be ≤ 0.0180%, which can further ensure the bending and hole expansion performance of the material.

[0038] Furthermore, in the ultra-high strength steel of the present invention, its welding carbon equivalent C eq ≤0.280%, Ceq=C+Mn / 20+Si / 30+2P+4S, where each chemical element is substituted into its mass percentage.

[0039] In the present invention, when the mass percentages of C, Si, Mn, P and S meet the welding carbon equivalent C eq When the content of C is less than 0.280%, better welding performance can be obtained, and at the same time, the material solid solution and structure strengthening effect under this composition are better, and the material strength is higher. eq ≤0.250%. In some embodiments, C eq ≤0.240%. In some embodiments, C eq Between 0.220% and 0.240%.

[0040] Furthermore, in the ultra-high strength steel of the present invention, the mass percentage of each chemical element further satisfies at least one of the following:

[0041] C: 0.08-0.11%;

[0042] Si: 0.3-0.6%;

[0043] Mn: 1.9-2.1%;

[0044] Mo: 0.16-0.20%;

[0045] Nb: 0.025-0.045%;

[0046] Ti: 0.04-0.06%;

[0047] Al: 0.02-0.05%;

[0048] B: 0.0020-0.0030%.

[0049] Furthermore, in the ultra-high strength steel of the present invention, its microstructure includes ferrite and martensite.

[0050] Furthermore, in the ultra-high strength steel of the present invention, the volume phase ratio of martensite is 60-75%.

[0051] Furthermore, in the ultra-high strength steel described in the present invention, its yield strength is 700-850MPa, the tensile strength is ≥980MPa, preferably the tensile strength is ≥1020MPa, the elongation at break is ≥10.0%, preferably the elongation at break is ≥13.0%, the 180-degree cold bending performance is guaranteed to be no cracking under 1T conditions (i.e., no cracking when the gasket is 1 times the plate thickness), and the carbon footprint is ≤1.2kgCO2eq / kg. In some embodiments, the tensile strength of the ultra-high strength steel described in the present invention is 980-1100MPa. In some embodiments, the uniform elongation of the ultra-high strength steel described in the present invention is ≥6.0%.

[0052] In addition, another object of the present invention is to provide a method for manufacturing ultra-high strength steel with a low carbon footprint. The manufacturing method is green and efficient, and the ultra-high strength steel produced has the advantages of being green and low carbon as well as high strength and light weight.

[0053] To achieve the above objectives, the present invention provides a method for manufacturing ultra-high strength steel with a low carbon footprint, comprising the following steps:

[0054] Use all scrap steel for electric furnace smelting, LF, RH and continuous casting; in the RH process, the vacuum treatment time is ≤10min, such as 3 to 10 minutes; after breaking the air, soft blowing is 15-25min;

[0055] hot rolling and coiling;

[0056] cold rolling;

[0057] Continuous annealing: first, keep the annealing temperature at 760-820℃ for 60-200s, then cool it to the rapid cooling start temperature of 650-690℃ at a first cooling rate of 3-10℃ / s, then cool it to the rapid cooling end temperature of 220-260℃ at a second cooling rate of 40-80℃ / s, then keep it for 200-400s for aging treatment, and finally cool it to 100℃ or below at a third cooling rate of 2-10℃ / s.

[0058] The smelting and casting process described above specifically includes: electric furnace - LF furnace - RH - continuous casting. Conventional blast furnace and converter manufacturing processes are lengthy, and the use of coke to reduce iron ore results in a significantly higher carbon footprint for the final product, generally ranging from 2.5-3.0 kgCO₂eq / kg. Ironmaking and steelmaking account for nearly 70% of the entire process, possessing the highest carbon footprint. Furthermore, conventional electric furnace smelting is primarily suitable for medium- and high-carbon, stainless steel, or specialty steel products, and rarely addresses low-carbon, automotive high-strength steels with controlled impurities such as low S and N.

[0059] The present invention utilizes an all-scrap electric furnace smelting method, significantly reducing carbon emissions from the steelmaking process. Furthermore, the present invention utilizes a short RH vacuum time combined with a long soft-blowing time. In some embodiments, vacuuming can be performed to ≤100 Pa, maintaining the vacuum for ≤10 minutes, followed by a 15-25 minute soft-blowing period. The soft-blowing effect should be sufficient to barely break the slag surface. The steel is then allowed to stand for a period of time, such as 10 minutes, before being hoisted to the continuous casting station for casting.

[0060] In the present invention, vacuum treatment can reduce the N content in molten steel, but the N content does not decrease significantly after the vacuum treatment time is too long, and long-term vacuum is not conducive to the plasticization treatment of inclusions; long-term soft blowing after breaking the air can remove inclusions (mainly N- and S-compound inclusions) or achieve plasticization treatment of inclusions. Therefore, the RH vacuum refining process adopts short vacuum time and long soft blowing time to effectively control the inclusion and gas content. Since the N content of scrap steel tapped from an electric furnace is significantly higher than that of a conventional converter process, the RH treatment is used to reduce the controllable lower limit of N to 50ppm. Through this treatment, the TiN inclusions in the molten steel can be largely removed so that they are basically retained in the slag.

[0061] Furthermore, in the method for manufacturing ultra-high-strength steel described herein, the deformed structure after cold rolling is subjected to two-phase annealing at an annealing holding temperature of 760-820°C for 60-200 seconds. The primary purpose of this process is to recrystallize the steel sheet structure after cold rolling. Excessively low temperatures result in incomplete recrystallization, while excessively high temperatures can easily cause coarsening of the steel sheet grains and decomposition of precipitates, thereby reducing the steel sheet strength. Therefore, in the present invention, the annealing holding temperature is controlled between 760-820°C. Similarly, controlling the duration of this process is crucial. Too short a time will result in incomplete recrystallization of the steel sheet structure and insufficient diffusion of austenite-stabilizing elements (C and Mn); while too long a time can easily lead to abnormalities such as abnormally coarse grains and decomposition of precipitates. Therefore, the annealing holding time is controlled between 60-200 seconds. During this process, the steel sheet structure is partially austenitized, while the cold-rolled deformed grains complete their recovery and recrystallization process.

[0062] Furthermore, the steel strip after holding is cooled slowly to 650-690°C at a first cooling rate of 3-10°C / s. This allows some of the austenite in the steel plate to be converted to ferrite first. This ferrite is finer than the recrystallized ferrite in the two-phase region. This facilitates the coordination of the soft and hard phases during final deformation and improves the material's plasticity.

[0063] In addition, the second cooling rate is controlled at 40-80℃ / s. The steel strip is quickly cooled to 220-260℃ after slow cooling by blowing air, and then kept warm for 200-400s for aging treatment. This is because if the cooling rate is too slow during this process, pearlite will appear, reducing the performance of the steel plate; while too fast a cooling rate will lead to increased production difficulties and manufacturing costs. Therefore, the second cooling rate is controlled at 40-80℃ / s. The remaining austenite in the rapid cooling process is basically transformed into martensite, thereby greatly improving the strength. During the aging process, the quenched martensite will undergo a low-temperature tempering reaction, and the lamellar martensite obtained by quenching will undergo a small amount of decomposition and carbide precipitation, which is beneficial to improving the toughness of the material and avoiding cold brittle fracture.

[0064] In addition, the steel strip that has undergone aging treatment is finally cooled to 100° C. or below at a third cooling rate of 2-10° C. / s, below which the structure of the steel basically no longer changes.

[0065] Furthermore, in the electric furnace smelting process of the ultra-high strength steel manufacturing method described in the present invention, the scrap steel used includes three types of light scrap steel, medium scrap steel and heavy scrap steel, and the weight ratio of them is light scrap steel: medium scrap steel: heavy scrap steel = (3-3.5): (1.5-2): 1.

[0066] In this article, light scrap refers to lump or irregularly shaped finished steel scrap, trimmings, and similar items. Lumpiness requirements: <300×800×1800mm, unit weight ≤0.5 ton. Medium scrap refers to lump or irregularly shaped scrap of various machine parts, components, and similar items. Lumpiness requirements: <300×500×1500mm, unit weight ≤1.0 ton. Heavy scrap refers to lump ingots, slabs, and similar items. Lumpiness requirements: <300×500×1500mm, unit weight ≤2.0 ton.

[0067] In the electric furnace smelting process of this article, clean and clear scrap steel is selected. When adding scrap steel charge, 1-2 tons of light scrap steel is first laid on the bottom of the electric furnace as a substrate to prevent the impact and damage to the furnace lining when a large amount of scrap steel (especially heavy scrap steel) is poured in. During the smelting process, a large flow oxygen lance is used to blow oxygen to assist in fluxing and dephosphorization. The end point composition of the electric furnace is controlled as follows: C≤0.05wt%, P≤0.01wt%, S≤0.03wt%, and T is controlled at the same time. 出钢 ≥1630℃. After tapping, the steel goes into the slag removal station for slag removal, and the amount of slag left is controlled to be ≤40mm.

[0068] In some embodiments, 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%, recarburizer, ferrosilicon, and ferromanganese are added for carbon supplementation and deoxidation alloying treatment, and slag is added at the same time for slag desulfurization. Herein, the slag can be lime and fluorite, the amount of lime added can be 6-8kg / t steel, and the amount of fluorite added can be 1-2kg / t steel, so that the slag alkalinity is preferably controlled to 1.8-2.5, so that the S content is removed to within 30ppm. After desulfurization is completed, the required Nb, Ti, and B alloys are added. After all the alloy components are added in place, prepare to leave the station, and at the same time remove excess desulfurization slag and control the remaining slag amount to ≤30mm.

[0069] Furthermore, in the continuous casting process of the method for manufacturing ultra-high strength steel of the present invention, the superheat is controlled to be ≤30°C, the crystallizer taper is controlled to be 1.0-1.3%, and the continuous casting casting speed is controlled to be 1.0-1.5 m / min.

[0070] In the continuous casting process of the present invention, in order to ensure the smooth continuous pouring of molten steel, the molten steel needs to have a certain degree of superheat. When the superheat is higher than 30°C, it is detrimental to the quality of the center of the ingot, and defects such as segregation and shrinkage are likely to occur. Therefore, in the present invention, the superheat can be controlled to ≤30°C, such as 15 to 30°C. At the same time, in order to better control the surface quality of the ingot and prevent defects such as small longitudinal cracks, a chamfered crystallizer is selected to improve the stress distribution and control the crystallizer taper between 1.0-1.3%; in order to take into account both production efficiency and the surface quality of the ingot, the continuous casting drawing speed is controlled between 1.0-1.5m / min. Dynamic soft pressure and electromagnetic stirring are used during the casting process to better improve the segregation degree of the ingot.

[0071] Furthermore, in the hot rolling step of the method for manufacturing ultra-high strength steel of the present invention, the slab heating temperature is controlled to be 1200-1260°C, and the finishing rolling temperature is controlled to be 850-950°C.

[0072] During the hot rolling process of the present invention, the ingot can first be heated at a high temperature in the full austenite region for a period of time so that the material is softened and the components are completely diffused and uniform. After rough rolling to the intermediate billet thickness specification, the coarse structure generated in the heating furnace is broken and refined, and after further rolling to the required thickness through finishing rolling, a uniform and fine recrystallized material structure is formed. When the finishing rolling temperature is less than 850°C, ferrite will precipitate before finishing rolling, resulting in a low hard phase content in the final structure, insufficient hot-rolled strength, and thus affecting the subsequent cold rolling annealing performance. Taking into account the upper limit control of the slab heating temperature and the temperature drop during rolling, the finishing rolling temperature generally does not exceed 950°C. Therefore, in the present invention, the finishing rolling temperature can be controlled to be 850-950°C.

[0073] Furthermore, in the coiling step of the method for manufacturing ultra-high strength steel of the present invention, the coiling temperature is controlled to be 580-660°C.

[0074] In the present invention, the hot-rolling coiling temperature also affects the hot-rolled properties. When the coiling temperature exceeds 660°C, internal oxidation of Si and Mn is likely to occur on the steel plate surface, resulting in a surface fracture layer during pickling, which affects the surface quality of the final product. When the coiling temperature is below 580°C, the hot-rolled strength is too high, which is detrimental to subsequent cold rolling and leads to excessive deformation resistance, resulting in cold-rolled edge cracking or poor plate shape. Therefore, in the present invention, the coiling temperature is preferably controlled within a range of 580-660°C.

[0075] Furthermore, in the cold rolling step of the method for manufacturing ultra-high strength steel of the present invention, the cold rolling reduction ratio is controlled to be 50-70%.

[0076] In the present invention, after the hot rolled coil is pickled to remove the surface iron oxide scale, a 50-70% cold rolling reduction is performed to enable the steel plate to reach the target thickness and accumulate a certain deformation energy, which is more conducive to subsequent annealing and recrystallization.

[0077] The low-carbon-footprint ultra-high-strength steel and its manufacturing method of the present invention have the following advantages and beneficial effects:

[0078] The ultra-high strength steel with low carbon footprint of the present invention has a low carbon equivalent through a specific composition design, and controls the nitrogen to exist in the solid solution form as much as possible, and controls the nitrogen distribution coefficient N 固溶 / N 析出 ≥4.00 to improve the strength of the material and ensure the plasticity and bending properties of the material.

[0079] The method for manufacturing ultra-high-strength steel with a low carbon footprint described in the present invention adopts an electric furnace all-scrap steel smelting technology route, which makes the prepared ultra-high-strength steel have the technical advantages of being green and low-carbon. By controlling the scrap steel raw materials and steelmaking technology, the impurities and residual elements in the product are controlled at a low level, thereby not affecting the performance of the material.

[0080] The method for manufacturing ultra-high-strength steel with a low carbon footprint described in the present invention addresses the characteristic of high nitrogen content in the electric furnace all-scrap steel smelting path. By creatively controlling the RH process, nitrogen is mostly present in the material in a solid solution form, thereby improving the material strength without compromising the local formability of the material.

[0081] In some embodiments, the yield strength of the ultra-high strength steel with a low carbon footprint described in the present invention is 700-850 MPa, the tensile strength is ≥980 MPa, the elongation at break is ≥10%, and the 180-degree cold bending performance ensures no cracking under 1T conditions (i.e., no cracking when the gasket is 1 times the plate thickness). At the same time, the carbon footprint is ≤1.2 kgCO2eq / kg, achieving a balance between green and low carbon and ultra-high strength performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0082] FIG1 shows the microstructure of the ultra-high strength steel with a low carbon footprint according to Example 7 of the present invention. DETAILED DESCRIPTION

[0083] The ultra-high strength steel with a low carbon footprint and the method for manufacturing the same according to the present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an undue limitation on the technical solution of the present invention.

[0084] Examples 1-20 and Comparative Examples 1-3

[0085] The ultra-high strength steels with low carbon footprints in Examples 1-20 of the present invention are all produced by the following steps:

[0086] (1) Using all scrap steel for electric furnace smelting, LF, RH and continuous casting:

[0087] In the electric furnace smelting process, all scrap steel is divided into three types: light, medium and heavy. Clean scrap steel is selected. When adding scrap steel charge, 1-2 tons of light scrap steel is first laid on the bottom of the electric furnace as a substrate to prevent the impact and damage to the furnace lining when a large amount of scrap steel (especially heavy scrap steel) is poured in. During the smelting process, a large flow oxygen lance is used to blow oxygen to assist in fluxing and dephosphorization. The end point composition of the electric furnace is controlled as follows: C≤0.05wt%, P≤0.01wt%, S≤0.03wt%, and T is controlled at the same time. 出钢 ≥1630℃. After tapping, the steel goes into the slag removal station for slag removal, and the amount of slag left is controlled to be ≤40mm.

[0088] 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.

[0089] 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%, recarburizer, ferrosilicon, and ferromanganese are added for carbon supplementation and deoxidation alloying treatment, and slag is added at the same time for slag desulfurization. The slag is lime and fluorite. The amount of lime added can be 6-8kg / t steel, and the amount of fluorite added can be 1-2kg / t steel, so that the slag alkalinity is preferably controlled to 1.8-2.5, so that the S content is removed to within 30ppm. After desulfurization is completed, the required Nb, Ti, and B alloys are added. After all the alloy components are added in place, it is ready to leave the station, and at the same time, the excess desulfurization slag is removed and the remaining slag is controlled to be ≤30mm.

[0090] In the RH process: after the RH enters the station, it is vacuumed to ≤100Pa and the vacuum is maintained for ≤10 minutes before breaking the air. After breaking the air, it is soft-blown for 15-25 minutes. The soft-blowing effect is based on not blowing open the slag surface. Then it is left to stand for about 10 minutes, and then it is hoisted to the continuous casting for casting.

[0091] In the continuous casting process: the superheat is controlled at 15-30°C, the crystallizer taper is controlled between 1.0-1.3%, and the continuous casting billet speed is between 1.0-1.5m / min. The chemical composition content of the obtained billets is shown in Table 1-1, Table 1-2 and Table 1-3.

[0092] (2) Hot rolling and coiling: The slab is heated to 1200-1260℃, and then rough rolled and finished at a temperature of 850-950℃. After rolling, the slab is water-cooled to a coiling temperature of 580-660℃.

[0093] (3) Pickling to remove iron oxide scale.

[0094] (4) Cold rolling: using a reduction of 50-70%.

[0095] (5) Continuous annealing: After holding at an annealing holding temperature of 760-820°C for 60-200s, cool to the rapid cooling start temperature of 650-690°C at a first cooling rate of 3-10°C / s to obtain a certain proportion of ferrite; then rapidly cool to the rapid cooling end temperature of 220-260°C at a second cooling rate of 40-80°C / s; then hold for 200-400s for aging treatment; finally cool to below 100°C at a third cooling rate of 2-10°C / s.

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

[0097] Table 1-2 (wt%, the balance is Fe and other inevitable impurities except P, S, Sn)

[0098] Table 1-3: Composition Control Coefficients

[0099] Note: In Table 3, Hem=Sn+N / 2+2S, Ceq=C+Mn / 20+Si / 30+2P+4S, where each chemical element is substituted into its mass percentage.

[0100] The steel grades AE in Table 1-1, Table 1-2 and Table 1-3 are the steel grade numbers used in the embodiments of the present invention, and ac are the steel grade numbers used in the comparative examples.

[0101] Table 2-1 and Table 2-2 list the specific process parameters in the above process steps of each embodiment of the present invention and the comparative example.

[0102] Table 2-1

[0103] Note: The “-” in the scrap steel weight ratio column of Comparative Example 1 indicates that it does not adopt the electric furnace all-scrap steel smelting process, but still adopts the converter steelmaking process and uses conventional blast furnace molten iron.

[0104] Table 2-2

[0105] The obtained samples of Examples 1-20 and Comparative Examples 1-3 were corroded with a 4% nitric acid alcohol solution on the polished, bright surface without obvious scratches. The corrosive solution was then wiped with a clean laboratory cotton ball for about 15 seconds to observe the microstructure. The bright white area in the microstructure was ferrite, and the fine, dark structure was martensite. The observation results are listed in Table 3.

[0106] In addition, Figure 1 also shows the microstructure of the ultra-high strength steel with a low carbon footprint according to Example 7 of the present invention. As shown in Figure 1 , the microstructure of the ultra-high strength steel with a low carbon footprint is ferrite + martensite.

[0107] In addition, the samples of Examples 1-20 and Comparative Examples 1-3 were subjected to ISO 6892:1998 (Metallic Materials Room Temperature Tensile Test Methods) and P14 (A 50 ) tensile specimens were subjected to mechanical property testing and carbon footprint calculations, and the results are listed in Table 3. The product carbon footprint calculation boundary is cradle-to-gate, and the specific calculation method and product classification comply with ISO 14067 "Greenhouse Gases - Product Carbon Footprint - Quantification Requirements and Guidelines" and the China Iron and Steel Association EPD Platform PCR 2022:01 "Common Steel Products and Special Steel Products."

[0108] Table 3

[0109] As can be seen from Table 3, Examples 1-20 of the present invention achieve ideal microstructure characteristics through reasonable chemical element composition design combined with optimized process parameters. While ensuring strength, the plasticity performance is very excellent, with a yield strength between 700-850 MPa, a tensile strength greater than or equal to 980 MPa, a uniform elongation greater than 6.0%, an elongation at break greater than or equal to 10.3%, and excellent 180-degree cold bending performance, all meeting the requirement of no cracking under 1T conditions.

[0110] It can also be seen that the carbon footprint levels of Examples 1-20 of the present invention are all less than or equal to 1.2 kgCO2eq / kg, with low carbon emission levels, meeting the technical requirements of green and low-carbon development. However, the process path of Comparative Example 1 still uses the conventional converter path using blast furnace molten iron, so its carbon footprint is relatively high.

[0111] Unlike the present invention, the comparative examples cannot achieve the technical effects that the present invention can achieve because the ingredients or process parameters do not meet the design requirements of the present invention.

[0112] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0113] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. An ultra-high strength steel plate with a low carbon footprint, comprising Fe and unavoidable impurities, characterized in that: It also contains the following chemical elements in the following mass percentages: C: 0.06-0.12%, Si: 0.2-0.6%, Mn: 1.7-2.3%, Mo: 0.14-0.22%, Nb: 0.020-0.05 0%, Ti: 0.03-0.07%, Al: 0.02-0.06%, B: 0.0010-0.0030%, N: 0.0050-0.0100%; The nitrogen distribution coefficient in the ultra-high strength steel satisfies: 固溶 / N 析出 ≥4.

00.

2. The ultra-high strength steel plate according to claim 1, 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: 0.020-0.050%, Ti: 0.03-0.07%, Al: 0.02-0.06%, B: 0.0010-0.0030%, N: 0.0050-0.0100%; the balance is Fe and inevitable impurities.

3. The ultra-high strength steel plate according to claim 1 or 2, characterized in that: It also contains residual elements of at least one of the following: Cr≤0.05wt%; Ni≤0.05wt%; Cu≤0.10wt%; V≤0.05wt%.

4. The ultra-high strength steel plate according to claim 1 or 2, characterized in that: Among its inevitable impurities, P≤0.015%, S≤0.003%, and Sn≤0.01%.

5. The ultra-high strength steel plate according to claim 4, characterized in that: Its harmful element factor Hem≤0.018%, wherein Hem=Sn+N / 2+2S, wherein each chemical element is substituted into its mass percentage; and / or its welding carbon equivalent Ceq≤0.28%, Ceq=C+Mn / 20+Si / 30+2P+4S, wherein each chemical element is substituted into its mass percentage.

6. The ultra-high strength steel plate according to claim 1 or 2, characterized in that: The mass percentage of each chemical element further satisfies at least one of the following items: 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 plate according to claim 1 or 2, characterized in that: Its microstructure includes ferrite and martensite; preferably, the volume phase ratio of martensite is 60-75%.

8. The ultra-high strength steel plate according to claim 1 or 2, characterized in that: Its yield strength is 700-850MPa, tensile strength is ≥980MPa, elongation at break is ≥10.0%, 180 degree cold bending performance ensures no cracking under 1T condition, carbon footprint is ≤1.2kgCO2eq / kg; preferably, its tensile strength is ≥1020MPa; preferably, its uniform elongation is ≥6.0%.

9. The method for manufacturing an ultra-high strength steel plate with a low carbon footprint according to any one of claims 1 to 8, characterized in that: It includes the steps of: Use all scrap steel for electric furnace smelting, LF, RH and continuous casting; in the RH process, the vacuum treatment time is ≤10min, and the soft blowing after breaking the air is 15-25min; Hot rolling and coiling; Cold rolling; Continuous annealing: first keep the annealing temperature at 760-820℃ for 60-200s, then cool to the rapid cooling start temperature of 650-690℃ at the first cooling rate of 3-10℃ / s, then cool to the rapid cooling end temperature of 220-260℃ at the second cooling rate of 40-80℃ / s, then keep for 200-400s for aging treatment, and finally cool to 100℃ or below at the third cooling rate of 2-10℃ / s.

10. The manufacturing method according to claim 9, characterized in that: In the electric furnace smelting process, the scrap steel used includes three types of light scrap steel, medium scrap steel and heavy scrap steel, and the weight ratio of them is light scrap steel: medium scrap steel: heavy scrap steel = (3-3.5): (1.5-2):

1.

11. The manufacturing method according to claim 9, characterized in that: In the LF process, the slag basicity is 1.8-2.5 to remove the S content to less than 30ppm.

12. The manufacturing method according to claim 9, characterized in that: In the continuous casting process, the superheat is controlled to be ≤30°C, the taper of the crystallizer is controlled to be 1.0-1.3%, and the continuous casting billet drawing speed is controlled to be 1.0-1.5m / min.

13. The manufacturing method according to claim 9, characterized in that: In the hot rolling step, the slab heating temperature is controlled to be 1200-1260°C, and the finishing rolling temperature is controlled to be 850-950°C.

14. The manufacturing method according to claim 9, characterized in that: In the coiling step, the coiling temperature is controlled to be 580-660°C.

15. The manufacturing method according to claim 9, characterized in that: In the cold rolling step, the cold rolling reduction ratio is controlled to be 50-70%.

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