Nitrogen-containing ultrahigh-strength steel and manufacturing method thereof

Abstract

The invention discloses nitrogen-containing ultra-high-strength steel which contains Fe and inevitable impurities and further comprises the following chemical elements in percentage by mass: 0.15-0.25% of C, 0.15-0.25% of Si, 0.15-0.25% of Cr, 0.15-0.25% of Mo, 0.15-0.25% of Mo, 0.15-0.25% of Ni, 0.15- The content of N is 0.005 to 0.015 percent; 0.7 to 1.4 percent of Si; mn: 1.5 to 2.5%; 0.3 to 1.0 percent of Al; 0.001 to 0.003 percent of Mg (magnesium); 0.03 to 0.07 percent of Ti; and Ca; wherein the content of Ca meets the condition that Mg + Ca is 0.002-0.004%; wherein the content of Si and Al is 1.5-1.8%; the N distribution coefficient in the nitrogen-containing ultrahigh-strength steel is that N solid solution / N precipitation is larger than or equal to 0.5.

Description

Technical Field

[0001] This invention relates to a steel plate and its manufacturing method, and more particularly to an ultra-high strength steel and its manufacturing method. Background Technology

[0002] The steel industry is a traditional high-energy-consuming, high-emission, and high-polluting industry, and its CO2 emissions have attracted widespread attention. By changing the proportion of raw materials in steelmaking, increasing the proportion of low-carbon raw materials such as scrap steel and reduced iron, and reducing the amount of molten iron used in blast furnaces, carbon emissions at the molten steel stage can be significantly reduced.

[0003] Under the background of low-carbon technology in iron and steel processes, it is expected that the scrap-electric furnace process and the blast furnace / converter-high scrap ratio process will be the mainstream carbon reduction technology routes in the future. These technologies will inevitably lead to the large-scale use of recycled materials, represented by scrap steel. The significant increase in the proportion of recycled materials will bring about significant changes in the composition of molten steel, mainly reflected in the increase of impurity elements such as N, Cu, Cr, Mo, and Ni, as well as residual elements. Among them, N is a strong austenite stabilizing element. N in steel is beneficial to improving the stability of residual austenite and improving the formability of high-formability ultra-high-strength steel. However, N easily forms nitride-type inclusions in steel, reducing the hot plasticity of the material and leading to continuous casting defects. In addition, large-sized and numerous nitride inclusions near the surface of the steel plate can act as crack initiation sources during bending deformation, leading to cracking failure. The increase in the content of residual elements such as Cu, Cr, Mo, Ni, and Sn will significantly change the phase transformation law and microstructure characteristics of high-strength steel, causing the original designed product to deviate significantly from the design specifications under the low-carbon manufacturing system.

[0004] Therefore, it is hoped that by redesigning the material composition, structure and properties, and increasing the proportion of recycled materials in product design and manufacturing, and by specially designing the material structure and processes, the residual elements can be efficiently controlled and utilized to obtain low-carbon emission, high-formability, ultra-high-strength steel.

[0005] Chinese patent document CN103361545A, published on October 23, 2013, entitled "A High-Nitrogen High-Strength Automotive Steel," discloses a high-nitrogen high-strength automotive steel with the following chemical composition by mass percentage: C 0.04%–0.08%, N 0.1%–0.4%, Si 0.2%–1.0%, Mn 2.0%–5.0%, P ≤0.01%, S ≤0.005%, Als 0.02%–0.04%, with the balance being Fe and unavoidable impurities. The microstructure of the steel contains 80%–92% martensite and 8%–20% austenite. This steel does not contain precious metals such as Nb, Ti, and Mo, resulting in low production costs; its high N content provides good corrosion resistance; and it retains a certain amount of residual austenite after deformation, which can effectively absorb energy during collisions. This invention provides a high-nitrogen steel for automobiles with a tensile strength of over 1000 MPa and an elongation of over 10%. Although the strength meets the requirements for ultra-high-strength steel, the elongation of the 1000 MPa grade product is only 15%, which cannot meet the requirements for high formability. Moreover, the material composition is designed as a medium-Mn system with a high Mn content, making production and processing extremely difficult. Summary of the Invention

[0006] One of the objectives of this invention is to provide a nitrogen-containing ultra-high strength steel that combines excellent strength and plasticity with a low carbon footprint, making it suitable for complex-shaped automotive safety structural components with high formability requirements.

[0007] To achieve the above objectives, the present invention provides a nitrogen-containing ultra-high strength steel, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages:

[0008] C: 0.15-0.25%; N: 0.005-0.015%; Si: 0.7-1.4%; Mn: 1.5-2.5%; Al: 0.3-1.0%; Mg: 0.001-0.003%; Ti: 0.03-0.07%; and Ca;

[0009] The Ca content satisfies the following condition: Mg+Ca: 0.002-0.004%;

[0010] Of which Si+Al: 1.5-1.8%;

[0011] The nitrogen distribution coefficient in the nitrogen-containing ultra-high strength steel is: N solid solution / N precipitation ≥ 0.5.

[0012] Furthermore, in the nitrogen-containing ultra-high strength steel described in this invention, the mass percentage content of each chemical element is as follows:

[0013] C: 0.15-0.25%; N: 0.005-0.015%; Si: 0.7-1.4%; Mn: 1.5-2.5%; Al: 0.3-1.0%; Mg: 0.001-0.003%; Ti: 0.03-0.07%; Ca; balance Fe and unavoidable impurities;

[0014] The Ca content satisfies the following condition: Mg+Ca: 0.002-0.004%;

[0015] Of which Si+Al: 1.5-1.8%;

[0016] The nitrogen distribution coefficient in the nitrogen-containing ultra-high strength steel is: N solid solution / N precipitation ≥ 0.5.

[0017] This invention overcomes the adverse effects of residual elements on the microstructure, properties, and manufacturing process of ultra-high strength steel by designing the elemental composition. At the same time, it makes full use of the strengthening effect of residual elements and can be combined with a manufacturing process that uses a large proportion of recycled materials to obtain ultra-high strength steel products with a low carbon footprint that have high formability and ultra-high strength.

[0018] The design principles of each chemical element in the nitrogen-containing ultra-high strength steel described in this invention are as follows:

[0019] C: In the nitrogen-containing ultra-high strength steel described in this invention, carbon (C) is the most fundamental strengthening element in steel and also an austenite stabilizing element. A higher C content in austenite is beneficial for increasing the metastable austenite fraction and material properties. When the C content is below 0.15%, the stability of the metastable austenite does not meet design requirements. When the C content is high, C deteriorates the weldability of the steel. Therefore, in the nitrogen-containing ultra-high strength steel described in this invention, the mass percentage content of C is controlled between 0.15% and 0.25%. In some embodiments, the mass percentage content of C can be further controlled between 0.17% and 0.22%.

[0020] Nitrogen (N): Due to the use of a large proportion of recycled materials, the nitrogen content in the nitrogen-containing ultra-high strength steel described in this invention is relatively high. When nitrogen exists in the matrix in solid solution form, it improves the stability of austenite and the strength of the material. When nitrogen exists in precipitated form, it exists as inclusions such as TiN, and excessive inclusion content will affect the plasticity and bending properties of the material. Therefore, in the nitrogen-containing ultra-high strength steel described in this invention, the mass percentage content of nitrogen is controlled between 0.005-0.015%, and to ensure the solid solution nitrogen content, the nitrogen distribution coefficient (solid solution nitrogen / precipitated nitrogen) needs to be controlled ≥0.5.

[0021] Si: In the nitrogen-containing ultra-high strength steel described in this invention, Si is an element that inhibits carbide formation. Si has extremely low solubility in carbides, effectively inhibiting or delaying carbide formation, which is beneficial for suppressing austenite decomposition, thereby forming carbon-rich austenite during the partitioning process and retaining it as metastable austenite up to room temperature. However, when the Si content is too high, it reduces the plating and weldability of the material. Due to the high Si content design used in this invention, a special dew point control process must be employed during continuous annealing or hot-dip galvanizing of the steel plate to ensure the surface quality of the steel plate. Therefore, in the nitrogen-containing ultra-high strength steel described in this invention, the mass percentage content of Si is controlled between 0.7% and 1.4%. In some embodiments, the mass percentage content of Si can be further controlled between 0.8% and 1.1%.

[0022] Mn: In the nitrogen-containing ultra-high strength steel described in this invention, Mn is an austenite stabilizing element. The presence of Mn can lower the martensite transformation temperature Ms and increase the content of metastable austenite. Furthermore, Mn is a solid solution strengthening element, which is beneficial for improving the strength of the steel plate. However, excessively high Mn content can lead to excessively high hardenability of the steel, which is detrimental to the fine control of the material microstructure. In addition, high Mn content also reduces the plating versatility of the steel plate, requiring a special galvanizing process. Therefore, in the nitrogen-containing ultra-high strength steel described in this invention, the mass percentage content of Mn is controlled between 1.5% and 2.5%. In some embodiments, the mass percentage content of Mn can be further controlled between 1.8% and 2.2%.

[0023] Al: In the nitrogen-containing ultra-high strength steel described in this invention, Al mainly plays a role in solid solution strengthening and inhibiting carbide formation, and improves the stability of metastable austenite. The strengthening effect of Al is weaker than that of Si, and its impact on the welding and surface of the steel is relatively small. Therefore, the addition of Al can reduce the addition of Si, thereby improving the welding performance and surface quality of the material. Therefore, in the nitrogen-containing ultra-high strength steel described in this invention, the mass percentage content of Al is controlled between 0.3% and 1.0%. In some embodiments, the mass percentage content of Al can be further controlled between 0.5% and 0.9%.

[0024] Mg: In the nitrogen-containing ultra-high strength steel described in this invention, magnesium is a strong deoxidizing element with strong reducing properties, readily combining with oxygen and sulfur in the steel to form MgO and MgS. Adding a certain amount of Mg during steelmaking not only effectively deoxidizes and reduces the amount of oxide inclusions in the steel, but also allows the large number of fine MgO particles (0.1-0.4 μm in size) formed in the molten steel to become nuclei for the precipitation of inclusions such as TiN, forming MgO-TiN composite precipitates. MgO-TiN composite precipitates can effectively suppress the size of TiN inclusions, reducing the adverse effects of large-sized TiN inclusions on material properties. Therefore, in the nitrogen-containing ultra-high strength steel described in this invention, the mass percentage content of Mg is controlled between 0.001-0.003%. In some embodiments, the mass percentage content of Mg can be further controlled between 0.0015-0.0025%.

[0025] Ca: In the nitrogen-containing ultra-high strength steel described in this invention, Ca is an important element for controlling inclusions. During the smelting process, Ca can form Ca oxide particles, which become the precipitation nuclei for inclusions, thereby improving the shape, size, and distribution of inclusions. Ca can also combine with inclusions by forming low-melting-point calcium aluminates, etc., and enter the steel slag to improve the cleanliness of the steel through slag formation. In this invention, Ca acts as an auxiliary component with Mg, working together with Mg to achieve efficient control of inclusions. Therefore, in the nitrogen-containing ultra-high strength steel described in this invention, the mass percentage content of Ca is controlled at Mg+Ca: 0.002-0.004%.

[0026] Ti: In the nitrogen-containing ultra-high strength steel described in this invention, Ti can fix the nitrogen in the steel, forming stable compounds and improving the quality of the cast billet and corner cracks. Ti can also form fine carbides, inhibiting austenite grain growth and refining the grain size. Therefore, in the nitrogen-containing ultra-high strength steel described in this invention, the mass percentage content of Ti is controlled between 0.03% and 0.07%. In some embodiments, the mass percentage content of Ti can be further controlled between 0.04% and 0.06%.

[0027] Furthermore, in the nitrogen-containing ultra-high strength steel described in this invention, it also satisfies: 20C+60N+Mn: 5.0-8.0%, where each chemical element is substituted with its mass percentage content.

[0028] In this invention, C, Mn, and N are all effective austenite stabilizing elements in steel. However, using these elements individually in large quantities can lead to negative effects such as reduced weldability, inclusions, and poor plating properties. Combining multiple elements can effectively improve the stability of residual austenite in the invented steel, enhance material properties, and mitigate the negative effects of a single element. Considering the metallurgical properties and cost of these elements, the nitrogen-containing ultra-high strength steel described in this invention will have a controlled composition of 20C + 60N + Mn: 5.0-8.0%, comprehensively ensuring the control of the microstructure and properties, manufacturability, and usability of the invented steel.

[0029] Furthermore, the nitrogen-containing ultra-high strength steel of the present invention also contains residual elements of at least one of the following:

[0030] Cr ≤ 0.05 wt%;

[0031] Ni ≤ 0.05 wt%;

[0032] Cu ≤ 0.1 wt%;

[0033] Mo ≤ 0.1 wt%;

[0034] And Cr+Ni+Cu+Mo: 0.05-0.2wt%.

[0035] In this invention, since all scrap steel is used as the smelting raw material, residual elements Cr, Ni, Cu, and Mo, which are difficult to remove during steelmaking, may be introduced into the raw material. Among these, Mo, Cr, Ni, and Cu promote the hardenability of austenite, reduce the critical cooling rate of martensitic transformation, promote martensite formation, and improve material strength. However, when the content of Mo, Cr, Ni, and Cu is too high, problems such as hot-rolled surface cracks, cold-rolled edge cracks, and increased rolling force are easily generated, leading to increased manufacturability difficulties. Therefore, in the nitrogen-containing ultra-high strength steel described in this invention, the mass percentage content of Cr can be controlled to Cr ≤ 0.05 wt%, Ni to Ni ≤ 0.05 wt%, Cu to Cu ≤ 0.1 wt%, and Mo to Mo ≤ 0.1 wt%. Furthermore, the total amount of Cr + Ni + Cu + Mo elements needs to be controlled between 0.05-0.2 wt%.

[0036] Furthermore, in the nitrogen-containing ultra-high strength steel described in this invention, among its unavoidable impurities, P ≤ 0.015%, S ≤ 0.003%, and Sn ≤ 0.01%.

[0037] In this invention, unavoidable impurities mainly include phosphorus (P), sulfur (S), and tin (Sn). The lower the content of these impurities, the purer and better the steel's performance. Among them:

[0038] P: In the nitrogen-containing ultra-high strength steel described in this invention, excessive phosphorus (P) content weakens grain boundaries, increases material brittleness, and deteriorates weldability. Therefore, in the nitrogen-containing ultra-high strength steel described in this invention, the mass percentage content of P can be controlled to P ≤ 0.015%.

[0039] S: In the nitrogen-containing ultra-high strength steel described in this invention, an excessively high sulfur content will significantly deteriorate the material's plasticity. Therefore, in the nitrogen-containing ultra-high strength steel described in this invention, the mass percentage content of sulfur can be controlled to S≤0.0030%.

[0040] Sn: In the nitrogen-containing ultra-high strength steel described in this invention, Sn is also a harmful element in steel. Its segregation at grain boundaries can adversely affect the toughness and plasticity of the material. Furthermore, excessive Sn content can also deteriorate the coating performance of the material. Therefore, in the nitrogen-containing ultra-high strength steel described in this invention, the mass percentage content of Sn can be controlled to Sn ≤ 0.01%.

[0041] Furthermore, in the nitrogen-containing ultra-high strength steel described in this invention, its impurity element factor I impurity ≤0.018%, where I impurity =Sn+N / 2+2S, where each chemical element is substituted with its mass percentage.

[0042] In this invention, by controlling the impurity element factor I impurity A content of ≤0.018% can further ensure the bending and pore-expanding properties of the material.

[0043] Furthermore, in the nitrogen-containing ultra-high strength steel described in this invention, its room temperature microstructure consists of ferrite + partitioned martensite + metastable austenite.

[0044] In order to obtain steel plates that have both high strength and high formability, the present invention aims to incorporate a certain amount of metastable austenite into the material microstructure design.

[0045] Furthermore, in the nitrogen-containing ultra-high strength steel of the present invention, the volume proportion of ferrite is 5-50%, the volume proportion of distributed martensite is 30-85%, and the volume proportion of metastable austenite is 10-20%.

[0046] Furthermore, in the nitrogen-containing ultra-high strength steel of the present invention, the metastable austenite has a retention rate of more than 30% after undergoing cold deformation with a strain of 10%.

[0047] Furthermore, in the nitrogen-containing ultra-high strength steel described in this invention, the average particle size of its TiN inclusions is ≤2μm.

[0048] Furthermore, in the nitrogen-containing ultra-high strength steel described in this invention, the proportion of TiN inclusions with Mg / Ca oxides in the core is ≥70% of all TiN inclusions.

[0049] Furthermore, in the nitrogen-containing ultra-high strength steel described in this invention, the oxides of Si and Mn are distributed in the range of 0.5-5 μm below the surface of the steel plate.

[0050] Furthermore, in the nitrogen-containing ultra-high strength steel described in this invention, its yield strength is 600-1300 MPa, its tensile strength is 1000-1600 MPa, its elongation is ≥15%, and the product of tensile strength and elongation is ≥24 GPa.

[0051] Furthermore, in the nitrogen-containing ultra-high strength steel described in this invention, its carbon footprint CE≤1.4kgCO2eq / kg, and satisfies 12 / (1+20Rs)≤CE≤20 / (1+20Rs), where Rs represents the proportion of recycled materials (i.e., the proportion of scrap steel), and Rs≥40%.

[0052] Another objective of this invention is to provide a method for manufacturing nitrogen-containing ultra-high strength steel. This method, by changing the proportion of steelmaking raw materials, increasing the proportion of low-carbon raw materials such as scrap steel and reduced iron, and reducing the amount of blast furnace iron, can significantly reduce carbon emissions at the steelmaking stage and achieve a low-carbon footprint.

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

[0054] For electric furnace or converter smelting, LF, RH and continuous casting;

[0055] Hot rolling and coiling: In the hot rolling step, the slab heating temperature Ts is controlled to be 1250-1280℃, the holding time ts≥90min, and (Ts+273)×lg(ts)≥3000;

[0056] Cold rolling;

[0057] Continuous annealing: Use a weak oxidizing atmosphere with a dew point of 0 to -10℃, anneal at 840-870℃, then cool to the rapid cooling start temperature of 710-730℃ at a first cooling rate of ≤10℃ / s, then cool to the rapid cooling end temperature of 220-320℃ at a second cooling rate of ≥50℃ / s, and then heat to 400-460℃ and hold for 60-180s.

[0058] The smelting and casting processes described above in this invention can be achieved through two pathways: one is electric furnace-LF furnace-RH-continuous casting; the other is blast furnace / converter-LF furnace-RH-continuous casting. The carbon footprint of products manufactured using conventional blast furnace / converter processes is generally 2.0-3.0 kg CO2. 2eqThe carbon footprint is between [value missing] kg. Furthermore, the iron and steelmaking processes account for nearly 70% of the entire process, making it the stage with the highest carbon footprint. This is because traditional blast furnaces use coke to reduce iron ore to obtain metallic iron, resulting in a high carbon footprint. However, in this invention, by adding a large amount of scrap steel to the raw materials, the proportion of iron ore used can be effectively reduced, thereby significantly lowering the carbon footprint of steel products.

[0059] In the hot rolling process of this invention, the slab is first heated at high temperature in the full austenitic region to soften the material and ensure complete and uniform diffusion of its components. After hot rolling to the required thickness, a uniform and fine recrystallized material structure is formed. The high-strength steel of this invention has a high content of elements such as Si and Al. Heating the slab at a high temperature above 1250℃ is beneficial for the full dissolution of C and N compounds, reducing the size and quantity of TiN inclusions, and avoiding the production of difficult-to-remove spinel-type oxide scale. However, excessively high heating temperatures not only lead to severe slab oxidation but also waste energy; therefore, the slab heating temperature Ts does not exceed 1280℃. When the heating temperature decreases, the holding time ts needs to be increased to ensure the nitride dissolution effect; therefore, (Ts+273)×lg(ts)≥3000 is controlled.

[0060] In the annealing process of this invention, continuous annealing is adopted, and a weak oxidizing atmosphere with a dew point of 0 to -10°C is used in the annealing section to control the oxidation reaction of easily oxidized elements such as Si and Mn in the steel plate within 0.5-5.0 μm of the surface layer, thereby preventing the enrichment of elements such as Si and Mn on the surface and inhibiting the formation of Si / Mn oxide films on the surface.

[0061] It should be noted that the aforementioned weakly oxidizing atmosphere refers to the effect of introducing a trace amount of water vapor into the annealing section to achieve a certain increase in oxidizing properties. It exhibits significant oxidizing properties for elements such as Si and Mn, but only a negligible oxidizing or reducing atmosphere for Fe.

[0062] Furthermore, controlling the annealing temperature to a relatively high level of 840–870℃ is to form a homogeneous austenitic structure, which is beneficial for improving the strength of the steel. Then, it is cooled at a rate of ≤10℃ / s to the "rapid cooling start temperature" of 710–730℃ to form partial ferrite and reduce the rapid cooling temperature difference to improve the plate shape. It is then cooled again at a rate of ≥50℃ / s to the "rapid cooling end temperature" of 220–320℃, causing the austenite to partially transform into martensite. Finally, it is reheated to the "reheating temperature" of 400–460℃ and held for 60–180 seconds. During this process, carbon elements redistribute within the martensite and austenite, not only increasing the carbon-rich stability of the austenite and thus obtaining more metastable austenite, which is beneficial for improving plasticity, but more importantly, reducing the carbon content in the martensite, thereby effectively reducing the hardness of the martensite without tempering.

[0063] Furthermore, the ferrite in the microstructure recovers during annealing, significantly reducing the high-density mobile dislocations generated by the volume expansion of martensitic phase transformation in the ferrite, thereby increasing the hardness of the ferrite. The decrease in martensitic hardness and the increase in ferrite hardness effectively reduce the hardness difference between the martensite and ferrite phases, improving the material's hole expansion and flanging performance.

[0064] Furthermore, in the manufacturing method described in this invention, when electric furnace smelting is used, 60-100% scrap steel is used. The scrap steel used includes three types: low-sulfur scrap steel, medium-sulfur scrap steel, and high-sulfur scrap steel, and their weight ratio is low-sulfur scrap steel: medium-sulfur scrap steel: high-sulfur scrap steel = (3-3.5): (1.5-2): (1.0-1.5).

[0065] By adopting the above-mentioned scrap steel-electric furnace process, it is possible to achieve 100% scrap steel as raw material and reduce the carbon footprint by more than 60%.

[0066] Furthermore, in the manufacturing method described in this invention, when using a converter for smelting, 40-50% of scrap steel is used. The scrap steel used includes three types: low-sulfur scrap steel, medium-sulfur scrap steel, and high-sulfur scrap steel, with a weight ratio of low-sulfur scrap steel: medium-sulfur scrap steel: high-sulfur scrap steel = (1.0-1.5): (1.0-1.5): (1.0-1.5).

[0067] Furthermore, in the manufacturing method described in this invention, in the LF process, the slag alkalinity is 1.8-2.5, so that the S content is reduced to less than 30 ppm.

[0068] Furthermore, in the manufacturing method described in this invention, in the RH process, the vacuum treatment time is ≤15min, and the soft blowing time after breaking the vacuum is 15-25min.

[0069] In the RH process of this invention, vacuum treatment can reduce the nitrogen content in molten steel. However, if the vacuum treatment time is too long, the decrease in nitrogen content is not significant, and prolonged vacuum is detrimental to the plasticization of inclusions. Therefore, the vacuum treatment time is controlled to be less than or equal to 15 minutes. Maintaining a long soft blowing time after vacuum breaking can remove inclusions (mainly nitrogen compounds and sulfur compounds) or achieve plasticization of inclusions. Therefore, the RH vacuum refining process, using a short vacuum time and a long soft blowing time, can effectively control the inclusion and gas content.

[0070] Furthermore, in the manufacturing method described in this invention, during the continuous casting process, the superheat can be controlled at 15-30°C, and the secondary cooling water volume can be 0.8-1.2 m³. 3 / ton of steel.

[0071] In this invention, during continuous casting, the molten steel needs a certain degree of superheat to ensure smooth continuous pouring. When the superheat is below 15°C, it is detrimental to the stable continuous pouring process; when the superheat is above 30°C, it is unfavorable to the internal quality of the billet center, easily leading to defects such as segregation and shrinkage cavities. Furthermore, to better control the surface quality of the billet and prevent defects such as small longitudinal cracks, a chamfered mold can be used to improve stress distribution, and the mold taper can be controlled between 1.0% and 1.3%. The area from the billet exiting the mold to the straightening section is called the secondary cooling zone. Cooling in this zone directly affects the microstructure and the size and morphology of nitrides in the billet. Therefore, the regulation of the cooling water (secondary cooling water) in the secondary cooling zone is crucial for controlling nitride precipitation and the nitrogen distribution coefficient. When the secondary cooling water flow rate is below 0.8 m³ / min... 3 When the water volume is less than 1.2 m³, it can lead to the formation of a large number of coarse nitride particles in the billet, severely affecting material properties; when the water volume is higher than 1.2 m³, it can also cause this problem. 3 When the temperature is reduced to a certain level (e.g., 1 ton of steel), it can lead to excessive cooling of the billet, reduced plasticity, and an increased risk of transverse cracks after entering the straightening zone.

[0072] Furthermore, in the manufacturing method described in this invention, in the hot rolling step, the finishing rolling temperature is controlled to be 900-960°C; in the coiling step, the coiling temperature is controlled to be 450-550°C.

[0073] In the hot rolling step of this invention, when the final rolling temperature is below 900°C, excessive deformation resistance can lead to production problems such as excessive rolling load and poor plate shape. However, considering the upper limit control of the slab heating temperature and the temperature drop during rolling, the finishing rolling temperature generally does not exceed 960°C.

[0074] In the winding step of this invention, using a winding temperature not exceeding 550°C helps reduce the surface quality degradation caused by the eutectoid reaction of oxide scale, thus avoiding reduced pickling efficiency and impacting the final surface quality. When the winding temperature is below 450°C, it will result in excessively high hot-rolling strength, deterioration of the sheet shape, and affect subsequent pickling and cold rolling production.

[0075] The nitrogen-containing ultra-high strength steel and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0076] In conventional processes, tempered martensite is typically generated to reduce its hardness. This involves the dissolution of supersaturated carbon in the martensite at the tempering temperature, forming carbides. This process generates a large amount of carbides, which cannot be used to stabilize the retained austenite, leading to a decrease in the effective carbon content of the material. This invention achieves a reduction in martensite hardness through compositional design and manufacturing processes, while avoiding martensite tempering and carbide formation. It fully utilizes the alloying elements in the material, representing a low-cost and high-efficiency design solution.

[0077] The present invention employs a high-Si design in its composition design, which ensures that the metastable austenite in the steel does not decompose during the galvanizing process, thereby guaranteeing the final desired microstructure.

[0078] The method for manufacturing nitrogen-containing ultra-high strength steel described in this invention adopts an electric furnace all-scrap steel smelting technology path, which enables the prepared ultra-high strength steel to have green and low-carbon technical advantages. Furthermore, by controlling the scrap steel raw materials and steelmaking technology, the impurities and residual elements of the product are controlled at a low level, thereby not affecting the performance of the material.

[0079] The method for manufacturing nitrogen-containing ultra-high strength steel described in this invention addresses the issue of high N content in the electric arc furnace scrap steel smelting path. It creatively controls the Mg treatment during steelmaking and the RH process to make the TiN precipitates uniform and fine in size, which can effectively improve the impact on the local formability of the material. The remaining N exists in the material in a solid solution form, which can improve the strength of the material.

[0080] The nitrogen-containing ultra-high strength steel described in this invention has the characteristics of excellent strength and plasticity and low carbon footprint. Its room temperature microstructure has a certain content of metastable austenite; and after undergoing cold deformation with a strain of 10%, the ratio of the proportion of metastable austenite to the proportion of the original metastable austenite is greater than 0.3.

[0081] The ultra-high strength steel of this invention has a yield strength of 600-1300 MPa, a tensile strength of 1000-1600 MPa, an elongation of 15-28%, and a tensile strength-to-elongation product ≥24 GPa%, exhibiting excellent mechanical properties. Simultaneously, its recycled material ratio (Rs) ≥40% and its carbon footprint (CE) ≤1.4 kg CO2 2eq / kg.

[0082] The nitrogen-containing ultra-high strength steel described in this invention is suitable for complex-shaped automotive safety structural components with high formability requirements. Attached Figure Description

[0083] Figure 1 The microstructure of the nitrogen-containing ultra-high strength steel of Example 7 of the present invention is shown.

[0084] Figure 2 The TiN morphology of the nitrogen-containing ultra-high strength steel of Example 7 of the present invention is shown.

[0085] Figure 3 The TiN morphology of the nitrogen-containing ultra-high strength steel of Comparative Example 1 of the present invention is shown.

[0086] Figure 4 The diagram schematically illustrates the TiN size distribution of the nitrogen-containing ultra-high strength steel of Example 13 of the present invention.

[0087] Figure 5The TiN size distribution of the nitrogen-containing ultra-high strength steel of Comparative Example 1 of the present invention is schematically shown.

[0088] Figure 6 An image of the oxide layer of the nitrogen-containing ultra-high strength steel of Example 7 of the present invention is shown. Detailed Implementation

[0089] The following will provide further explanation and description of the nitrogen-containing ultra-high strength steel and its manufacturing method according to the present invention with reference to specific embodiments. However, such explanation and description do not constitute an improper limitation on the technical solution of the present invention.

[0090] Examples 1-15 and Comparative Examples 1-3

[0091] The nitrogen-containing ultra-high strength steels in Examples 1-15 of this invention were all prepared using the following steps:

[0092] (1) Perform electric furnace or converter smelting, LF, RH and continuous casting;

[0093] When using electric arc furnace smelting, the sulfur content of low-sulfur scrap steel should be ≤0.015%, medium-sulfur scrap steel ≤0.030%, and high-sulfur scrap steel ≤0.050%. Clean and hygienic materials should be selected. The weight ratio of the three types of scrap steel is: low-sulfur scrap steel: medium-sulfur scrap steel: high-sulfur scrap steel = (3-3.5):(1.5-2):(1-1.5). Furthermore, when adding scrap steel charge, 1-2 tons of scrap steel can be laid on the furnace bottom as a lining to prevent impact and damage to the furnace lining when a large amount of scrap steel is poured in. Simultaneously, during the smelting process, a high-flow-rate oxygen lance can be used to assist in melting and dephosphorization. After tapping, the steel is removed from the furnace at the slag removal station.

[0094] When using a converter for smelting, the blast furnace charge should consist of 40%-50% scrap steel, with the remainder being iron ore. The scrap steel should be particle steel with a particle size of 5-100mm, and the molten iron should reach a temperature ≥1320℃. The converter charge should consist of 20%-40% scrap steel, with the remainder being molten iron. The scrap steel should be of three types: low-sulfur, medium-sulfur, and high-sulfur. The sulfur content of low-sulfur scrap steel should be ≤0.015%, medium-sulfur scrap steel ≤0.030%, and high-sulfur scrap steel ≤0.050%. Clean and hygienic scrap steel should be selected. The weight ratio of the three types of scrap steel should be: low-sulfur scrap steel : medium-sulfur scrap steel : high-sulfur scrap steel = (1.0~1.5):(1.0~1.5):(1.0~1.5).

[0095] In the LF process, Al is first added for deep deoxidation, and samples are taken for analysis. When Al ≥ 0.03 wt%, carburizing agents, ferrosilicon, ferromanganese, etc., are added for carbon replenishment and deoxidation alloying treatment. At the same time, slag is added for slag formation and desulfurization. The slag consists of lime and fluorite. The amount of lime added can be 6-8 kg / t steel, and the amount of fluorite added can be 1-2 kg / t steel. The slag basicity is controlled at 1.8-2.5 to remove the sulfur content to below 30 ppm. After desulfurization is completed, the required Nb, Ti, B, and other alloys are added. After all the alloy components are added, the excess desulfurization slag is removed, and the amount of slag left is controlled to be ≤ 30 mm.

[0096] In the RH process, after entering the station, a vacuum is drawn to ≤100Pa and maintained for ≤10min before the vacuum is broken. Following this, soft blowing is performed for 15-25min, with the aim of just barely blowing away the slag surface. The mixture is then allowed to stand for approximately 10min before being hoisted to the continuous casting plant for casting. Because the nitrogen content in steel from the electric arc furnace is significantly higher than in conventional converter processes, RH treatment ensures that the controllable lower limit of nitrogen is at 50ppm. After the vacuum treatment, Ca and Mg are added via a Ca-Mg alloy wire to modify inclusions, significantly refining the size of TiN inclusions in the molten steel. Larger inclusions are removed by being fed into the slag.

[0097] In the continuous casting process, the superheat can be controlled at 15-30℃, and the secondary cooling water flow rate is 0.8-1.2m³. 3 / ton of steel. Meanwhile, dynamic light pressing and electromagnetic stirring are employed during the casting process to better improve the segregation of the cast billet.

[0098] (2) Hot rolling and coiling: In the hot rolling step, the slab heating temperature Ts is controlled to be 1250-1280℃, the holding time ts≥90min, and (Ts+273)×lg(ts)≥3000, and the finishing rolling temperature is controlled to be 900-960℃.

[0099] (3) Cold rolling;

[0100] (4) Continuous annealing: A weak oxidizing atmosphere with a dew point of 0 to -10℃ is used. The annealing temperature is 840-870℃, then cooled to the rapid cooling start temperature of 710-730℃ at a first cooling rate of ≤10℃ / s, and then cooled to the rapid cooling end temperature of 220-320℃ at a second cooling rate of ≥50℃ / s. Then, it is heated to 400-460℃ and held for 60-180s. The weak oxidizing atmosphere refers to the introduction of a trace amount of water vapor in the annealing section to achieve a certain increase in the oxidizing property of the annealing atmosphere. This atmosphere is significantly oxidizing for elements such as Si and Mn, and has a non-significant oxidizing or reducing effect on elements such as Fe.

[0101] Tables 1-1 and 1-2 list the chemical composition ratios of the nitrogen-containing ultra-high strength steel of Examples 1-15 and the comparative steel of Comparative Examples 1-3 of the present invention.

[0102] It should be noted that in the nitrogen-containing ultra-high strength steel described in this invention, N 析出 The testing method is as follows: Electrolytic extraction and separation of inclusions are performed on the material, and then the nitrogen content in the inclusions is measured as the nitrogen content. 析出 Among them, N 固溶 =N 总 -N 析出 N 总 The total nitrogen content in the material is determined according to GB / T 20124 inert gas melting thermal conductivity method.

[0103] Table 1-1. (wt%, balance Fe and other unavoidable impurities besides P, S, and Sn)

[0104]

[0105]

[0106] Table 1-2. (wt%, balance Fe and other unavoidable impurities besides P, S, and Sn)

[0107]

[0108] Tables 2-1, 2-2, and 2-3 list the specific process parameters of the nitrogen-containing ultra-high strength steel of Examples 1-15 and the comparative steel of Comparative Examples 1-3 in the above steps.

[0109] Table 2-1.

[0110]

[0111] Note: Comparative Examples 2 and 3 do not use scrap steel for smelting.

[0112] Table 2-2.

[0113]

[0114] Table 2-3.

[0115]

[0116] Samples from Examples 1-15 were taken and their microstructures were observed. The observation results are listed in Table 3. Among them:

[0117] Oxide depth detection: The depth of the oxide layer was measured under a scanning electron microscope using metallographic methods.

[0118] Microstructure analysis: Metallographic observation was employed, involving cutting samples from the steel plate cross-section along the rolling thickness direction, grinding and polishing, etching with a 4% nitric acid alcohol solution, and magnifying observation using a scanning electron microscope (SEM). The volume fractions of ferrite, martensite, and metastable austenite, as well as the size and quantity of TiN inclusions, were statistically analyzed. The energy dispersive spectroscopy (EDS) function of the SEM was used to determine the number of TiN inclusions containing Mg / Ca oxides in the core.

[0119] Table 3 lists the microstructure observation results of the nitrogen-containing ultra-high strength steel of Examples 1-15 of the present invention and the comparative steel of Comparative Examples 1-3.

[0120] Table 3.

[0121]

[0122] As can be seen from Table 3 above, the room temperature microstructure of the nitrogen-containing ultra-high strength steels of Examples 1-15 of this invention consists of ferrite + partitioned martensite + metastable austenite. The volume fraction of ferrite is between 5% and 50%, the volume fraction of partitioned martensite is between 30% and 85%, and the volume fraction of metastable austenite is between 10% and 20%. Furthermore, the metastable austenite retains at least 30% of its original size after undergoing cold deformation with a strain of 10%.

[0123] also, Figure 1 The microstructure morphology of the nitrogen-containing ultra-high strength steel of Example 7 of the present invention is also shown. From Figure 1 As can be seen from the data, the room temperature microstructure of this nitrogen-containing ultra-high strength steel consists of ferrite (F), distributed martensite (M), and metastable austenite (A).

[0124] Figure 2 The TiN morphology of the nitrogen-containing ultra-high strength steel of Example 7 of the present invention is shown. Figure 3 The TiN morphology of the nitrogen-containing ultra-high strength steel of Comparative Example 1 of the present invention is shown.

[0125] like Figure 2 and Figure 3 As shown, the TiN particle size of the nitrogen-containing ultra-high strength steel in Example 7 of the present invention is significantly smaller than that of the TiN particle size of the nitrogen-containing ultra-high strength steel in Comparative Example 1 of the present invention, and the angular morphology of the particles is not significant, exhibiting a certain irregular shape. The TiN particles of the nitrogen-containing ultra-high strength steel in Comparative Example 1 of the present invention exhibit a significantly square morphology, which is very detrimental to the bending and other properties of the material.

[0126] Figure 4 The diagram schematically illustrates the TiN size distribution of the nitrogen-containing ultra-high strength steel of Example 13 of the present invention. Figure 5 The TiN size distribution of the nitrogen-containing ultra-high strength steel of Comparative Example 1 of the present invention is schematically shown.

[0127] like Figure 4 and Figure 5 As shown, the average TiN size of the nitrogen-containing ultra-high strength steel of Example 13 of the present invention is 2.5 μm, which is significantly smaller than the average TiN size of 5.5 μm of the nitrogen-containing ultra-high strength steel of Comparative Example 1 of the present invention.

[0128] Figure 6 An image of the oxide layer of the nitrogen-containing ultra-high strength steel of Example 7 of the present invention is shown.

[0129] like Figure 6 As shown, oxide depth detection reveals that in this embodiment, the oxides of both Si and Mn are distributed within a range of 0.5-5 μm below the surface of the steel plate.

[0130] To further verify the mechanical properties of the nitrogen-containing ultra-high strength steels of Examples 1-15 and the comparative steels of Comparative Examples 1-3, the mechanical properties of the nitrogen-containing ultra-high strength steels of Examples 1-15 and the comparative steels of Comparative Examples 1-3 were resampled and tested. The test results are recorded in Table 4. Wherein:

[0131] Tensile testing method: Use ASTM standard 50mm gauge length tensile specimens, with the tensile direction perpendicular to the rolling direction. For specific test methods, refer to: ASTM E8 / E8M-13 "Standard Test Methods For Tension Testing of Metallic Materials".

[0132] Table 4 lists the mechanical property test results of the nitrogen-containing ultra-high strength steel of Examples 1-15 of the present invention and the comparative steel of Comparative Examples 1-3.

[0133] Table 4.

[0134]

[0135]

[0136] As can be seen from Table 4 above, the yield strength of the nitrogen-containing ultra-high strength steels in Examples 1-15 of the present invention is between 600-1280 MPa, the tensile strength is between 1020-1600 MPa, the elongation is greater than 15%, and the product of tensile strength and elongation is greater than or equal to 24 GPa%, which has good mechanical properties.

[0137] Furthermore, the carbon footprints of the nitrogen-containing ultra-high strength steels in Examples 1-15 and the comparative steels in Comparative Examples 1-3 were calculated using a material carbon footprint calculation method, and the calculation results are listed in Table 5. Among them:

[0138] Material carbon footprint calculation method: The calculation boundary is from cradle to gate. The specific calculation method and product classification comply with ISO14067 "Greenhouse gases - Carbon footprint of products - Quantification requirements and guidelines" and China Iron and Steel Association EPD platform PCR 2022:01 "Ordinary steel products and special steel products".

[0139] Table 5 lists the material carbon footprints of the nitrogen-containing ultra-high strength steels of Examples 1-15 and the comparative steels of Comparative Examples 1-3 of the present invention.

[0140] Table 5.

[0141]

[0142]

[0143] As can be seen from Tables 1 to 5 above, by adopting a high-proportion scrap steel smelting technology path and controlling the scrap steel raw materials and steelmaking technology, the impurities and residual elements of the product can be controlled at a low level, so as not to affect the performance of the material. At the same time, an extremely low carbon footprint level of the product can be obtained, ultimately resulting in an ultra-high strength steel with both excellent strength and plasticity and low carbon footprint.

[0144] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0145] 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 thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A nitrogen-containing ultra-high strength steel, comprising Fe and unavoidable impurities, characterized in that, It also contains the following chemical elements in the following mass percentages: C: 0.15-0.25%; N: 0.005-0.015%; Si: 0.7-1.4%; Mn: 1.5-2.5%; Al：0.3-1.0％； Mg: 0.001-0.003%; Ti: 0.03-0.07%; and Ca; The Ca content satisfies the following condition: Mg+Ca: 0.002-0.004%; Of which Si+Al: 1.5-1.8%; The nitrogen distribution coefficient in the nitrogen-containing ultra-high strength steel: N 固溶 / N 析出 ≥0.

5.

2. The nitrogen-containing ultra-high strength steel as described in claim 1, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.15-0.25%; N: 0.005-0.015%; Si: 0.7-1.4%; Mn: 1.5-2.5%; Al：0.3-1.0％； Mg: 0.001-0.003%; Ti: 0.03-0.07%; Ca; balance Fe and unavoidable impurities; The Ca content satisfies the following condition: Mg+Ca: 0.002-0.004%; Of which Si+Al: 1.5-1.8%; The nitrogen distribution coefficient in the nitrogen-containing ultra-high strength steel: N 固溶 / N 析出 ≥0.

5.

3. The nitrogen-containing ultra-high strength steel as described in claim 1, characterized in that, It also satisfies: 20C+60N+Mn: 5.0-8.0; where each chemical element is represented by its mass percentage value before the percentage sign.

4. The nitrogen-containing ultra-high strength steel as described in claim 1 or 2, characterized in that, It also contains a residual element of at least one of the following: Cr ≤ 0.05 wt%; Ni ≤ 0.05 wt%; Cu ≤ 0.1 wt%; Mo ≤ 0.1 wt%; And Cr+Ni+Cu+Mo: 0.05-0.2wt%.

5. The nitrogen-containing ultra-high strength steel as described in claim 1 or 2, characterized in that, In its unavoidable impurities, P ≤ 0.015%, S ≤ 0.003%, and Sn ≤ 0.01%.

6. The nitrogen-containing ultra-high strength steel as described in claim 5, characterized in that, Its impurity element factor I impurity ≤0.018%, where I impurity =Sn+N / 2+2S, where each chemical element is substituted with its mass percentage.

7. The nitrogen-containing ultra-high strength steel as described in claim 1 or 2, characterized in that, Its room temperature microstructure consists of ferrite + partitioned martensite + metastable austenite.

8. The nitrogen-containing ultra-high strength steel as described in claim 7, characterized in that, The volumetric proportion of metastable austenite is 10-20%.

9. The nitrogen-containing ultra-high strength steel as described in claim 7, characterized in that, The metastable austenite retained more than 30% of its mass after undergoing cold deformation with a strain of 10%.

10. The nitrogen-containing ultra-high strength steel as described in claim 1 or 2, characterized in that, The average particle size of its TiN inclusions is ≤2μm.

11. The nitrogen-containing ultra-high strength steel as described in claim 10, characterized in that, TiN inclusions containing Mg / Ca oxides account for ≥70% of all TiN inclusions in the core.

12. The nitrogen-containing ultra-high strength steel as described in claim 1 or 2, characterized in that, The oxides of Si and Mn are distributed in the range of 0.5-5 μm below the surface of the steel plate.

13. The nitrogen-containing ultra-high strength steel as described in claim 1 or 2, characterized in that, Its yield strength is 600-1300MPa, tensile strength is 1000-1600MPa, elongation is ≥15%, and the product of tensile strength and elongation is ≥24GPa.

14. The nitrogen-containing ultra-high strength steel as described in claim 1 or 2, characterized in that, Its carbon footprint CE≤1.4kgCO2eq / kg, and satisfies 12 / (1+20Rs)≤CE≤20 / (1+20Rs), where Rs represents the proportion of recycled materials, and Rs≥40%.

15. The method for manufacturing nitrogen-containing ultra-high strength steel according to any one of claims 1-14, characterized in that, It includes the following steps: For electric furnace or converter smelting, LF, RH and continuous casting; Hot rolling and coiling: In the hot rolling step, the slab heating temperature Ts is controlled to be 1250-1280℃, the holding time ts≥90min, and (Ts+273)×lg(ts)≥3000; Cold rolling; Continuous annealing: Use a weak oxidizing atmosphere with a dew point of 0 to -10℃, anneal at 840-870℃, then cool to the rapid cooling start temperature of 710-730℃ at a first cooling rate of ≤10℃ / s, then cool to the rapid cooling end temperature of 220-320℃ at a second cooling rate of ≥50℃ / s, and then heat to 400-460℃ and hold for 60-180s.

16. The manufacturing method as described in claim 15, characterized in that, When using electric arc furnace smelting, 60-100% scrap steel is used. The scrap steel used includes three types: low-sulfur scrap steel, medium-sulfur scrap steel, and high-sulfur scrap steel, with a weight ratio of low-sulfur scrap steel: medium-sulfur scrap steel: high-sulfur scrap steel = (3-3.5): (1.5-2): (1-1.5)。 17. The manufacturing method as described in claim 15, characterized in that, When using a converter for smelting, 40-50% of the scrap steel is used. The scrap steel used includes three types: low-sulfur scrap steel, medium-sulfur scrap steel, and high-sulfur scrap steel, with a weight ratio of low-sulfur scrap steel: medium-sulfur scrap steel: high-sulfur scrap steel = (1.0-1.5):(1.0-1.5): (1.0-1.5)。 18. The manufacturing method as described in claim 15, characterized in that, In the LF process, the slag basicity is 1.8-2.5 to remove the sulfur content to below 30 ppm.

19. The manufacturing method as described in claim 15, characterized in that, In the RH process, the vacuum treatment time is ≤15min, and the soft blowing after breaking the vacuum is 15-25min.

20. The manufacturing method as described in claim 15, characterized in that, In the continuous casting process, the superheat is controlled at 15-30℃, and the secondary cooling water flow rate is 0.8-1.2m³. 3 / ton of steel.

21. The manufacturing method as described in claim 15, characterized in that, In the hot rolling step, the finishing rolling temperature is controlled at 900-960℃; in the coiling step, the coiling temperature is controlled at 450-550℃.

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