Bainite stainless steel based on laterite-nickel ore and manufacturing method of bainite stainless steel

Abstract

The bainite stainless steel comprises the following components in percentage by mass: 0.01-0.03% of C, 0.2-0.6% of Si, 0.5-1.5% of Mn, less than or equal to 0.045% of P, less than or equal to 0.015% of S, 10.50-11.8% of Cr, 0.5-1.0% of Ni, 0.01-0.1% of Mo, 0.01-0.05% of N and the balance of Fe and other inevitable impurities, and meets the following conditions: the corrosion resistance index is that Cr-16 * (C-0.05) + 3.3 * Mo + 16 * N is greater than or equal to 11.0; and the bainite isothermal transformation time TB is less than or equal to 1000min. The yield strength of the bainite stainless steel is larger than or equal to 750 MPa, the tensile strength is larger than or equal to 1000 MPa, the elongation is larger than or equal to 10%, the hardness HRC is 32-36, and the corrosion rate is smaller than or equal to 0.15 g / m < 2 > / h after 3.5% NaCl salt mist is used for 72 h.

Description

Technical Field

[0001] This invention relates to the field of stainless steel, and in particular to a bainitic stainless steel based on laterite nickel ore and its manufacturing method. Background Technology

[0002] Bainitic microstructures are highly complex and diverse, including upper bainite, lower bainite, carbide-free bainite, granular bainite, and columnar bainite. Due to the inherent strength and toughness of bainitic microstructure, bainitic steels possess excellent overall mechanical properties. Compared to non-quenched and tempered steels, they exhibit higher plasticity and toughness, further promoting the development and application of bainitic steels in production.

[0003] Currently available stainless steel varieties include austenitic stainless steel, ferritic stainless steel, duplex stainless steel (austenitic + ferritic), martensitic stainless steel, and sorbitic stainless steel. Bainitic stainless steel products have not been widely used. Austenitic stainless steel has excellent plasticity but lower strength and higher cost; ferritic stainless steel has lower cost but also lower strength; martensitic stainless steel has high strength but poor plasticity; sorbitic stainless steel has a complex production process, and duplex stainless steel is difficult to produce and has higher cost.

[0004] Chinese patent CN201910748215 discloses "A bainitic stainless steel and its preparation method," with the following chemical composition (wt%): C 0.03-0.10%, Cr 10-25%, Mn<0.8%, Si<0.8%, Ni 1-2%, P≤0.03%, S<0.020%, Al<0.02%, total O<30ppm, and the balance being Fe and unavoidable impurities. The technical route involves melting in an AOD furnace, induction furnace, or electric arc furnace, refining in an LF furnace, followed by continuous casting, hot rolling, and cooling. The hot rolling heating temperature is 1160-1220℃, the first-stage rolling temperature is 1070-1110℃, and the final rolling temperature is 1010-1030℃. The second-stage rolling temperature is 930-960℃, the second-stage final rolling temperature is 770-820℃, and the final cooling temperature is less than 300℃. It possesses a high yield strength of over 500 MPa, an elongation of over 18%, and an impact energy of over 40 J. It is well known that the formation of bainite requires isothermal or continuous cooling treatment, but this patent does not specify the cooling or heat treatment process for bainite, and therefore cannot demonstrate that bainitic stainless steel can be obtained under the stated chemical composition and process conditions.

[0005] Chinese patent CN202010075655 discloses "Stainless Steel Produced from Surface Low-Nickel Laterite Nickel Ore and Its Preparation Method," with the following chemical composition (wt%): C 0.03-0.16wt%, Si≤1.2wt%, Mn≤1.2wt%, Cr12.0-22.0wt%, Ni 0.6-1.5wt%, P≤0.06wt%, S≤0.020wt%, Al 0.01-0.05wt%, total O≤40ppm, and the balance being Fe and unavoidable impurities. The technical route involves smelting laterite nickel ore into molten iron or semi-molten steel using blast furnaces, RK-EF furnaces, and other smelting equipment, followed by refining in AOD and LF furnaces, and then continuous casting, hot rolling, and different cooling processes to obtain stainless steels with different microstructures. The process for obtaining bainite involves heating the steel billet to 1160℃, with a first-stage initial rolling temperature of 1070℃ and a first-stage final rolling temperature of 1010℃, resulting in a billet reduction of 50%. The second-stage initial rolling temperature is 930℃, and the second-stage final rolling temperature is 770℃. The billet is then tempered at 680℃ for at least 3 hours, followed by rapid water cooling to below 300℃. The properties of the bainitic stainless steel obtained by this patent are: yield strength ≥600MPa, fracture strength ≥750MPa, elongation ≥18%, and impact energy ≥30J. Typically, bainite formation occurs within the Ms point to 550℃ range, while the tempering temperature provided in this patent is between 680℃ and 720℃.

[0006] Chinese patent CN120866746A discloses "A bainitic plastic mold steel based on laterite nickel ore and its manufacturing method," with the following composition by mass percentage: C 0.01~0.15%, Si 0.25~1.0%, Mn 0.5~2.0%, P≤0.045%, S≤0.015%, Cr 2.0~5.0%, Ni 1.01~2.0%, B 0.0010~0.0050%, N 0.004~0.008%, with the balance including Fe and other unavoidable impurities. This patent uses Cr and Ni, which are abundant in laterite nickel ore, to replace Mo. The mold steel has a tensile strength ≥800MPa, a hardness of 28~38HRC, a cross-sectional hardness difference of ≤2HRC for steel plates with a thickness of 15~300mm, a flatness ≤2mm / m, and its flaw detection quality meets the E / e level requirements of the SP1921-84 standard. This patented product exhibits excellent performance in terms of hardness uniformity and mechanical properties, but its corrosion resistance is poor, making it unsuitable for use in corrosive environments. Summary of the Invention

[0007] The purpose of this invention is to provide a bainitic stainless steel based on lateritic nickel ore and its manufacturing method. The bainitic stainless steel exhibits high strength, high ductility, and good corrosion resistance, with a yield strength ≥750 MPa, tensile strength ≥1000 MPa, elongation ≥10%, hardness HRC32~36, and a 3.5% NaCl salt spray corrosion rate ≤0.15 g / m³ after 72 hours. 2 / h; Moreover, no complex heat treatment process is required, resulting in high production efficiency; The bainitic stainless steel can be applied to plastic molds, building structures, vehicles, and other fields.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows: A bainitic stainless steel based on lateritic nickel ore, with the following chemical composition by mass percentage: C: 0.01~0.03%, Si: 0.20~0.60%, Mn: 0.50~1.50%, P≤0.045%, S≤0.015%, Cr: 10.50~11.8%, Ni: 0.50~1.00% Mo: 0.01~0.10%, N: 0.01~0.05%, The balance consists of Fe and other unavoidable impurities, and the following conditions must be met simultaneously: Corrosion resistance index: Cr-16×(C-0.05)+3.3×Mo+16×N≥11.0; Bain isothermal transition time T B ≤1000, unit: min T B = -7505+5370×C +1259×Si+586×Mn+604×Cr+560×Ni+597×N.

[0009] Furthermore, the chemical composition of the bainitic stainless steel also includes one or more of the following: Nb≤0.1%, V≤0.2%, Cu≤0.2%, Al≤0.02%, by mass percentage.

[0010] The bainitic stainless steel described in this invention has a bainitic microstructure, with a yield strength ≥750MPa, tensile strength ≥1000MPa, elongation ≥10%, hardness: HRC 32~36, and a 3.5% NaCl salt spray corrosion rate ≤0.15g / m³ after 72 hours. 2 / h.

[0011] In the chemical composition design of the bainitic stainless steel described in this invention: In the chemical composition design of the bainitic stainless steel described in this invention: Carbon (C) is a key element affecting the microstructure and properties of bainitic stainless steel. Its content directly influences the bainite transformation temperature, morphology, and carbide precipitation behavior. Under low-carbon conditions, granular bainite is more easily formed, contributing to good toughness. As the carbon content increases, it promotes the precipitation of carbides in either upper or lower bainite. Excessive carbon content lowers the maximum temperature at which lower bainite exists and may impair the material's ductility and impact toughness. Therefore, controlling the carbon content between 0.01% and 0.03% allows for the regulation of the strength-toughness balance in bainitic stainless steel.

[0012] Silicon (Si) is a key element in bainitic stainless steel that inhibits carbide precipitation and regulates its microstructure and properties. It is a good deoxidizer in steelmaking and can also inhibit the precipitation of brittle cementite, but it significantly slows down the bainitic phase transformation kinetics and reduces the bainite content. Therefore, its content is controlled at 0.20~0.60%.

[0013] Manganese (Mn) improves hardenability, strength, and toughness, and neutralizes the hot brittleness caused by sulfur. Lateritic nickel ore has a high manganese content; 0.5-1.5% manganese can fully utilize resources and reduce smelting costs. In addition, manganese can significantly delay the transformation of ferrite and pearlite in high-temperature zones, promote bainite formation, and ensure the acquisition of a uniform bainite structure.

[0014] Chromium (Cr) significantly improves corrosion resistance, wear resistance, and strength, forming a chromium oxide passivation film in steel to slow down rusting and corrosion. Therefore, the Cr content needs to be no less than 10.5%. However, Cr also significantly slows down the bainitic phase transformation kinetics. When the Cr content exceeds 11.8%, it becomes difficult for bainite to form.

[0015] Ni is a key element in bainitic stainless steel for improving low-temperature toughness and regulating phase transformation. It can significantly delay the ferrite transformation, reduce the bainitic phase transformation temperature range, and expand the cooling rate range for bainite formation, which is conducive to the formation of more lath bainite. Based on the content limitations in laterite nickel ore, the nickel content is controlled at 0.50~1.00%.

[0016] Mo, in small amounts, can significantly delay the formation of pearlite and proeutectoid ferrite, shifting the high-temperature transformation zone in the CCT curve to the right, thus creating a wider cooling window for bainitic transformation. In carbide-free bainitic steels, the addition of molybdenum helps to obtain a more uniform carbide-free bainitic microstructure and inhibits the formation of coarse bainite. This is beneficial for improving the impact toughness of the material. In addition, molybdenum can synergistically improve corrosion resistance with chromium and nitrogen, but molybdenum has a significant impact on cost, so its addition is limited to 0.01~0.1%.

[0017] N, carbon and nitrogen can all play a significant role in solid solution strengthening, but carbon significantly inhibits bainite transformation, while nitrogen does not. Therefore, using some nitrogen to replace carbon can improve strength and corrosion resistance, and reduce the bainite transformation time. Excessive addition will lead to poor plasticity, so the nitrogen content is controlled at 0.01~0.05%.

[0018] In addition to the required elements mentioned above, the present invention can also selectively add one or more of Nb≤0.1%, V≤0.2%, Cu≤0.2%, and Al≤0.02%, by mass percentage, according to the desired characteristics.

[0019] Nitrogen (Nb) can form stable carbides, NbC, which refines the grains and stabilizes the microstructure, thereby improving the strength, hardness, and toughness of the steel. In ultra-low carbon bainitic steel, it promotes bainite transformation and produces a synergistic effect with elements such as boron (B) and molybdenum (Mo), further optimizing the overall performance.

[0020] V is a ferrite-forming element. Excessive V content is detrimental to the stability of austenite structure, so V must be controlled to an upper limit. A small amount of V can form fine VN precipitates with N, which can play a strengthening role and help improve the strength of the material. Therefore, the V content should be controlled to ≤0.2%.

[0021] When Cu is combined with Ni and Cr, Cu can improve corrosion resistance. However, the addition of Cu will inhibit bainitic phase transformation while promoting martensitic phase transformation. Therefore, the Cu content should be controlled to ≤0.2%.

[0022] Similar to Si, Al effectively inhibits the precipitation of brittle carbides, thereby improving the strength-toughness balance of materials. Simultaneously, Al also has beneficial effects such as reducing the hydrogen embrittlement sensitivity of steel, accelerating bainitic phase transformation, and refining the microstructure. The addition of Al increases the activation energy for hydrogen atom diffusion in α-Fe and γ-Fe grains, which helps to reduce the hydrogen diffusion rate and thus improve the material's resistance to hydrogen embrittlement.

[0023] This invention utilizes laterite nickel ore for smelting. Laterite nickel ore is composed of metal oxides such as NiO, Fe2O3, Cr2O3, and Al2O3. Its metallic elemental composition is characterized by Ni content of 0.6-1.0%, Cr content of 1.3-1.7%, and Fe content of 40-50%. The resulting nickel-iron molten steel after blast furnace smelting has a Ni content of 1-1.5%, a Cr content of 4-5.5%, and a Mn content of 1-2.0%. While the nickel-iron molten steel contains Ni and Mn alloying elements, the Cr content is significantly lower than the required Cr content for stainless steel (above 10.5%), making direct production of stainless steel impossible. Therefore, it is necessary to combine it with scrap steel from ferritic stainless steel smelting. Ferritic stainless steel scrap has a comprehensive Cr content between 15-18%, containing small amounts of Mn and Ni. Mixing these two materials allows for the formulation of raw material steel with a chemical composition close to the target. Further optimization and adjustment of the steel composition using high-carbon chromite ore can then be achieved to obtain the chemical composition meeting the design objectives.

[0024] The core feature of using laterite nickel ore to produce alloy steel lies in directly transforming resources traditionally considered "low-grade" or "abandoned" into steel products containing beneficial elements such as nickel and chromium. This is not simply a "replacement of iron ore," but an innovative model for the synergistic utilization of resources.

[0025] This invention primarily utilizes laterite nickel ore, which has a low nickel content (1-1.5%) but a high iron content. Traditional nickel extraction from this type of ore is economically unfeasible; its use as ordinary iron ore is limited by its nickel and chromium content. Therefore, this invention provides a high-value-added solution for this resource. The nickel, chromium, and trace amounts of copper, niobium, vanadium, and molybdenum naturally present in laterite nickel ore enter the steel during smelting, significantly improving the performance of low-alloy steel and maximizing its utilization. Nickel in laterite nickel ore enhances the strength and toughness of steel and refines the grain structure; chromium improves hardenability and strength. The synergistic effect of these elements allows the steel to achieve high strength while maintaining good ductility. Nickel and chromium are key elements for improving the resistance of steel to atmospheric and seawater corrosion. Low-alloy steel produced using laterite nickel ore exhibits superior weather resistance compared to traditional weathering steel and a service life far exceeding that of ordinary carbon steel. This makes it ideal for applications requiring corrosion resistance, such as plastic molds, construction, bridges, and marine engineering.

[0026] Based on the study of the influence of different alloying elements on the salt spray corrosion resistance of bainitic stainless steel, and building upon the traditional PREN formula, the corrosion resistance index of bainitic stainless steel was determined as: Cr - 16 × (C - 0.05) + 3.3 × Mo + 16 × N. This invention innovatively incorporates the influence of carbon (C) compared to the traditional PREN formula; however, when the carbon content exceeds 0.05%, a large amount of Cr will form. 23C6 content reduces corrosion resistance. When carbon content is below 0.05%, it can dissolve in the matrix, improving corrosion resistance. This is because carbon atoms dissolved in the crystal lattice inhibit the active dissolution rate of the metal matrix. Pitting corrosion begins with the local rupture of the passivation film and the rapid dissolution of the metal. Carbon effectively inhibits the growth and expansion of pits by slowing down this dissolution kinetics. To obtain good corrosion resistance, the corrosion resistance index should be controlled as follows: Cr - 16 × (C - 0.05) + 3.3 × Mo + 16 × N ≥ 11.0.

[0027] Compared to 4Cr13 martensitic stainless steel (C 0.40%, Mn 0.4%, Cr 13.5%, Ni 0.1%), although its Cr content is much higher than that of this invention, the C content readily combines with Cr to form Cr2. 23 The precipitation of C6 carbides significantly reduces the Cr content in the matrix, thereby drastically decreasing the corrosion resistance of 4Cr13. This invention innovatively replaces high-carbon martensite with low-carbon bainite. On one hand, the Ni contained in laterite nickel ore enhances the impact toughness of bainite, giving low-carbon bainite a better balance of ductility and toughness. On the other hand, the ultra-low C content allows for complete dissolution within the bainite, improving the corrosion resistance of bainitic stainless steel.

[0028] This invention innovatively proposes the Bain isothermal transition time T. B (T) B ≤1000 (unit: min), T B =-7505+5370×C+1259×Si+586×Mn+604×Cr+560×Ni+597×N.

[0029] This index is a kinetic quantification model used to accurately predict the time required to complete a full bainitic transformation within a given bainitic transformation temperature range. T B A larger value indicates a longer phase transformation time and a slower transformation kinetics. Each coefficient in this formula reflects the combined influence of a specific alloying element on the thermodynamic driving force and atomic diffusion rate of the bainitic phase transformation: The core hindering effect of carbon (C): The coefficient of carbon (5370) in the formula is much higher than that of other elements, which indicates that carbon is the most critical factor in delaying the bainite transformation. Its mechanism of action is: (1) Suppressing ferrite nucleation: Ferrite is the leading phase in bainite transformation. As a strong austenite stabilizing element, the increase in carbon content will significantly reduce the phase transformation driving force, making it more difficult for supercooled austenite to transform into ferrite, and reducing the ferrite nucleation rate; (2) Reduced diffusion efficiency: Bainite transformation involves the diffusion and redistribution of carbon atoms. An increase in carbon content means a significant increase in the total amount of carbon atoms that need to diffuse out of the ferrite nucleation region, which directly prolongs the diffusion time required for the phase transformation, thereby slowing down the entire phase transformation process.

[0030] Synergistic effects of alloying elements (Cr, Ni, Mn, N, Si): Cr, Ni, Mn, and N all exhibit positive coefficients, indicating that they all delay the bainite transformation to varying degrees. Their combined mechanisms include: (1) Reduce phase transformation driving force: Most of these elements can improve the stability of austenite, thereby reducing the chemical driving force of bainite transformation, making the transformation difficult to start and prolonging the incubation period; (2) Inhibition of carbon diffusion: Some elements (such as Cr and Mn) can hinder the diffusion of carbon atoms in austenite. Carbon diffusion is a key link in the growth of bainitic ferrite lamellars and the control of carbide precipitation. Impeded diffusion naturally delays the phase transformation kinetics. Silicon (Si) has a high coefficient. In addition to the above effects, it can also effectively inhibit the precipitation of carbides in steel. This makes the carbon-rich residual austenite more stable and indirectly delays the phase transformation towards the completion stage.

[0031] In summary, the Bain isothermal transition time T B Essentially, it quantifies the comprehensive impact of chemical composition on the thermodynamics and kinetics of phase transformation. It not only provides a key theoretical basis for optimizing the heat treatment process window (such as isothermal time) of bainitic stainless steel, but also serves as a powerful tool for achieving precise design of material microstructure and properties.

[0032] The method for manufacturing bainitic stainless steel based on laterite nickel ore according to the present invention includes the following steps: 1) Smelting and casting Laterite nickel ore is smelted in a blast furnace to produce nickel-iron molten steel, which is then mixed with ferritic stainless steel scrap through an electric arc furnace. The nickel-iron molten steel is mixed in a specific ratio of 55% to 65 wt%, and the scrap steel is mixed in a specific ratio of 35% to 45 wt%. The mixture then enters the AOD refining stage. During the AOD refining stage, high-carbon ferrochrome is added according to the chemical composition of the molten steel to adjust the chemical composition to the specified range. The mixture is then further refined in an LF furnace to obtain molten steel with the specified chemical composition of bainitic stainless steel. Finally, the molten steel is continuously cast into slabs. 2) Hot rolling The continuously cast slab is heated, held at a high temperature, descaled, and hot rolled into a steel plate. The slab is heated at a temperature of 1150~1270℃ and held at a high temperature for 180~240 minutes. After heating, the slab is descaled under high pressure and then enters the hot rolling stage. The initial rolling temperature is 1100~1200℃ and the final rolling temperature is 950~1050℃. 3) Straightening and cooling After hot rolling, the steel plate is cooled to 600~750℃ by air cooling or water mist cooling and then hot straightened. After hot straightening, it is cooled to the bainite transformation initiation temperature by air cooling, which is within the range of 400℃~420℃. Then, it is immediately hoisted into a heat-insulating pit for slow cooling to room temperature. The holding time in the temperature range of 300℃~420℃ must be ≥T. B T B The time is the Bausch isothermal transition time.

[0033] Preferably, in step 1), the molten nickel-iron from laterite nickel ore smelted in a blast furnace and the molten steel from ferritic stainless steel scrap smelted in an electric arc furnace are mixed in a specific ratio according to the target chemical composition. The main raw material for smelting is molten nickel-iron from laterite nickel ore smelted in a blast furnace, containing 1-1.5% Ni, 4-5.5% Cr, and 1-2.0% Mn; supplemented by molten steel from ferritic stainless steel scrap smelted in an electric arc furnace, with a Cr content of 15-18%. The molten nickel-iron and the scrap steel are mixed, with the molten nickel-iron mixture ratio being 55%-65% and the molten steel mixture ratio being 35%-45%. Subsequently, in the AOD refining stage, according to the chemical composition of the AOD steel, high-carbon ferrochrome with a Cr content of 60-70 wt% is added, and the addition ratio of high-carbon ferrochrome does not exceed 10% of the total mass of the molten nickel-iron and scrap steel. Finally, the alloy is refined and finely adjusted in the LF furnace to meet the alloy composition design of this invention and to meet the corrosion resistance index: Cr-16×(C-0.05)+3.3×Mo+16×N≥11.0.

[0034] Preferably, in step 3), after hot rolling, the steel plate is cooled to 600~750℃ using air cooling or water mist cooling. If the air cooling rate reaches 40℃ / min or higher, air cooling is sufficient; otherwise, water mist spraying is required to assist cooling. Water mist is sprayed onto the upper and lower surfaces of the steel plate on the roller conveyor, with the water volume controlled at 2~6 ml. 3 The steel plate moving speed is 1.0~2.0 m / s, so that the average cooling rate of the steel plate after rolling is controlled at 40~70℃ / min.

[0035] Preferably, in step 3), the cooling rate of the air cooler after straightening is controlled at 10~30℃ / min.

[0036] In the manufacturing method described in this invention: Step 1) During the steelmaking process, molten nickel-iron (mixing ratio 55%~65%) provides basic nickel, chromium, and manganese, but the chromium content (4~5.5%) is insufficient to produce stainless steel. Scrap steel (mixing ratio 35%~45%) provides the core element chromium (15~18%), and utilizes the residual value of scrap steel, significantly reducing the initial dependence on expensive pure chromium or high-carbon ferrochrome. In the AOD stage, high-carbon ferrochrome (≤10% of the total mass of molten nickel-iron and scrap steel) is added quantitatively according to the composition of the mixed molten steel. The design concept of this step is to control carbon increment. Using high-carbon ferrochrome is less costly, but the amount added is strictly limited to avoid placing an excessive burden on subsequent decarburization, reflecting a balance between cost and process difficulty. Highly efficient chromium extraction is achieved by adding ferrochrome in the active environment of strong stirring and oxygen blowing decarburization in AOD, resulting in high alloy yield and uniform melting. The final composition fine-tuning of the LF furnace is responsible for finely adjusting C, N and trace elements at the ppm level to ensure that the composition falls completely within the narrow window designed in this invention, and to make the chemical composition meet the corrosion resistance index: Cr-16×(C-0.05)+3.3×Mo+16×N≥11.0, thus ensuring excellent corrosion resistance.

[0037] In step 2), the final rolling temperature is 950~1050℃. On the one hand, the hot rolling process can promote continuous dynamic recrystallization, and the rolled material remains within the austenitizing temperature range, allowing the deformed structure to continue to recover and recrystallize, thus promoting the equiaxing of austenite grains at high temperatures. On the other hand, the high final rolling temperature can effectively eliminate the residual strain stress and anisotropy of the structure generated during the hot rolling process, eliminating the need for further heat treatments such as normalizing or annealing.

[0038] In step 3), after hot rolling, air cooling or water mist cooling can be flexibly combined according to the production rhythm to cool to 600~750℃, with the average cooling rate controlled at 40~70℃ / min during this stage. This ensures production efficiency, avoids the precipitation of ferrite and pearlite in the high-temperature zone, and also prevents excessively rapid cooling, which could cause plate warping. Straightening within the 600~750℃ range prevents excessively high plate temperature, which could cause oxide scale to stick to the straightening rollers and damage the surface quality of the steel plate. It also prevents excessively low plate temperature, which could lead to excessively high steel plate resistance and difficulty in straightening.

[0039] After thermal straightening, according to Figure 1 The CCT curve of the bainitic stainless steel described in this invention shows that the bainitic transformation range is 300~420℃, and a long isothermal treatment is required. When air-cooled to 420℃, the steel plate enters the bainitic transformation region and must be immediately hoisted into an insulated pit for controlled cooling to room temperature. During the controlled cooling process in the insulated pit, the holding time within the temperature range of 300℃~420℃ must be ≥T. B T BTo ensure the completion of the bainitic isothermal transformation, it is crucial to ensure that the bainitic isothermal transformation is achieved within the bainitic transformation temperature range and to suppress the martensitic transformation. This is a key process for obtaining a fully bainitic microstructure.

[0040] The innovation of the manufacturing method described in this invention compared with the prior art lies in: 1. An innovative chemical composition organization system.

[0041] Bainitic steels are mostly used in structural components requiring high strength and toughness (such as wear-resistant steel and offshore platform steel), while stainless steel is mainly composed of austenitic, ferritic, and martensitic structures, with bainitic stainless steel being relatively rare. This invention successfully combines the two characteristics of "bainitic structure" and "corrosion resistance of stainless steel." This is not only an innovation in material design but also provides a new material selection option for downstream applications.

[0042] This invention employs an ultra-low carbon design, which effectively suppresses the precipitation of brittle carbides, forming the basis for good toughness and weldability. It also promotes the formation of granular bainite, improving the strength-toughness balance. Setting the chromium content above 10.5% ensures the basic corrosion resistance of stainless steel, distinguishing it from ordinary bainitic structural steel and giving it the property of being "stainless." The innovative use of nitrogen to partially replace the solid solution strengthening effect of carbon, while nitrogen having a lower inhibitory effect on bainite transformation, provides a new approach to further optimize phase transformation and microstructure while ensuring strength.

[0043] 2. Innovate stainless steel smelting methods.

[0044] Traditional stainless steel smelting methods typically employ either molten iron + alloy or molten nickel + alloy, resulting in high raw material costs. This invention innovatively and strategically couples two traditionally independent metallurgical raw materials (intermediate products from laterite nickel ore smelting and stainless steel scrap), maximizing resource value and achieving complementary compositional advantages. Ferritic stainless steel addresses the issue of insufficient chromium content (Cr: 4.0~5.5%) in laterite nickel ore, virtually eliminating the need for additional manganese, nickel, or other alloying materials during steelmaking. The mixture of the two materials forms a pre-formed composition before AOD refining, clarifying subsequent alloying tasks, reducing energy consumption, and balancing raw material costs.

[0045] 3. Innovative integrated offline heat treatment process for controlling bainitic microstructure.

[0046] The manufacturing method described in this invention abandons the quenching and tempering (Q&T) or offline isothermal quenching processes commonly used in traditional high-strength steel production. Instead, it utilizes a precise online three-stage controlled cooling process to directly obtain a full bainitic microstructure and finished product properties online, achieving a significant simplification of the process and a substantial reduction in energy consumption. Figure 2 As shown. The innovation of this invention lies in the introduction of a Bainstein isothermal transition time T based on chemical composition.B The process uses a model to guide production. The final rolling temperature is 950~1050℃ to ensure the post-rolling microstructure is fully recrystallized homogeneous austenite, laying the foundation for subsequent phase transformation. Precise control of the temperature at which the steel plate enters the bainitic transformation zone and the subsequent isothermal transformation holding time ensures that the pearlite and ferrite transformation zones are effectively avoided during continuous cooling, and martensitic transformation is suppressed, resulting in a fully bainitic microstructure. This process completely eliminates the need for offline reheating for quenching and prolonged high-temperature tempering. This not only significantly shortens the production cycle and improves efficiency but also significantly reduces energy consumption and carbon emissions by avoiding secondary heating, aligning with the development direction of green manufacturing.

[0047] Compared to Chinese patent CN201910748215, the inventiveness of this invention lies primarily in the significant differences in its composition design and performance positioning. The comparative patent employs traditional smelting raw materials and a two-stage controlled rolling process, with a high carbon content and no added molybdenum or nitrogen, aiming to achieve a balance between high strength and high toughness. In contrast, this invention, through the use of an ultra-low carbon composition system with added molybdenum and nitrogen, and an innovative process of co-smelting laterite nickel ore nickel-iron molten steel and stainless steel scrap, shifts its technological focus to the synergistic improvement of corrosion resistance and ultra-high strength. This results in a yield strength (≥750MPa) far exceeding the 500MPa level of the comparative patent, and through molybdenum-nitrogen alloying, it achieves excellent salt spray corrosion resistance (3.5% NaCl salt spray corrosion rate ≤0.15g / m³ over 72 hours). 2 / h).

[0048] Compared to Chinese patent CN202010075655, although both utilize laterite nickel ore resources, their technical approaches and microstructure control mechanisms differ. The comparative patent employs a complex process of "two-stage rolling + high-temperature tempering + rapid cooling" to obtain bainitic microstructure. This invention, however, innovatively adopts a simplified process of "stacked controlled cooling within the bainitic transformation temperature range after hot straightening," eliminating the need for specialized tempering treatment and directly promoting the full transformation of bainite through slow cooling in an insulating pit. This process differentiation, combined with the lower carbon content and molybdenum-nitrogen alloying design of this invention, results in superior strength (yield strength ≥ 750 MPa) and corrosion resistance (3.5% NaCl salt spray corrosion rate ≤ 0.15 g / m³ after 72 hours). 2 It has established a clear advantage in / h) and achieved a breakthrough in differentiated performance.

[0049] Compared to the Chinese patent CN120866746A, which uses Cr and Ni rich in laterite nickel ore to replace Mo in the preparation of bainitic plastic mold steel, this invention uses laterite nickel ore and scrap ferritic stainless steel as raw materials. Its composition is characterized by ultra-low carbon and the addition of molybdenum and nitrogen elements. It has a corrosion resistance index ≥11.0 and exhibits excellent corrosion resistance (corrosion rate ≤0.15g / m³ in 72 hours of 3.5% NaCl salt spray).2 The comparative patent has a lower chromium content (Cr: 2.0~5.0%), resulting in poorer corrosion resistance and making it difficult to meet the requirements for application under corrosive conditions.

[0050] The beneficial effects of this invention are: 1. This invention uses laterite nickel ore, ferritic stainless steel scrap, and chromium ore as raw materials to directly smelt bainitic stainless steel without the need for additional alloying materials, which significantly reduces alloy costs compared to traditional smelting methods.

[0051] 2. This invention innovatively proposes an integrated heat-free bainitic microstructure control process, which eliminates the need for offline reheating quenching and long-term high-temperature tempering, significantly shortening the production cycle (by 24-48 hours), improving efficiency, and significantly reducing energy consumption and carbon emissions, in line with the development direction of green manufacturing.

[0052] 3. The bainitic stainless steel described in this invention achieves an excellent balance between strength, plasticity and hardness (yield strength ≥750MPa, tensile strength ≥1000MPa, elongation ≥10%, hardness HRC 32~36).

[0053] 4. The bainitic stainless steel described in this invention can be applied to corrosive plastic product molds, coastal engineering construction, and mechanical equipment. Compared with traditional mold steels and weathering steels, it has significantly improved corrosion resistance, with a corrosion rate of ≤0.15g / m³ in 3.5% NaCl salt spray over 72 hours. 2 / h. Attached Figure Description

[0054] Figure 1 The CCT curve of the bainitic stainless steel described in this invention; Figure 2 This is a process flow diagram of the hot rolling and heat treatment process of the bainitic stainless steel described in this invention. Figure 3 This is a metallographic diagram of Example 1 of the bainitic stainless steel described in this invention. Detailed Implementation

[0055] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0056] The chemical composition of the embodiments and comparative examples of this invention is shown in Table 1, with the balance including Fe and other unavoidable impurity elements. The manufacturing process parameters of the embodiments and comparative examples are shown in Table 2. The performance results of the embodiments and comparative examples are shown in Table 3.

[0057] Examples 1-9 were produced according to the chemical composition and process of the present invention, and their performance met the requirements. The metallographic structure of Example 1 is as follows: Figure 3 As shown, it is granular bainite.

[0058] Compared with Examples 1-9, Comparative Example 1, T B Reaching 1300.8, the holding time within the 300~420℃ temperature range is less than T. B The value is not completely transformed into bainite, and some martensite is present, resulting in high strength, excessive hardness, and poor plasticity.

[0059] Compared with Examples 1-9, Comparative Example 2 had a lower final rolling temperature and exhibited hot-rolled deformation structure, resulting in lower plasticity.

[0060] Compared with Examples 1-9, Comparative Example 3 had a corrosion index of 10.6, which was lower than 11.0, and a higher carbon content, which led to carbide precipitation and poor corrosion resistance.

[0061] Compared with Examples 1-9, Comparative Example 4 experienced faster air cooling after straightening, failed to enter the insulation pit in time, and had insufficient insulation time. Its microstructure consisted of martensite + bainite, resulting in higher strength and hardness but insufficient plasticity.

[0062] Compared with Examples 1-9, Comparative Example 5 had an excessively high post-rolling cooling rate, resulting in greater residual stress in the steel plate and causing plate warping.

[0063] Compared with Examples 1-9, Comparative Example 6 showed that the initial temperature of the stack cooling entering the insulation pit was too high, reaching 616°C, which is much higher than 420°C, resulting in ferrite precipitation and failure to meet the requirements for strength and hardness.

[0064] Compared with Examples 1-9, Comparative Example 7 showed that the hot straightening temperature was too low, only 524°C, and the steel plate could not be straightened.

[0065] As illustrated by the above embodiments and comparative examples, qualified bainitic stainless steel products can only be obtained by following the chemical composition design and process parameters described in this invention.

[0066] The bainitic stainless steel described in this invention exhibits excellent mechanical properties: In the examples, the stainless steel, under the specified chemical composition and process parameters, achieves a bainitic structure with a yield strength ≥750 MPa, tensile strength ≥1000 MPa, elongation ≥10%, hardness HRC32~36, and a 3.5% NaCl salt spray corrosion rate ≤0.15 g / m³ after 72 hours. 2 / h.

[0067]

[0068]

[0069]

Claims

1. A bainitic stainless steel based on lateritic nickel ore, characterized in that, Its chemical composition by mass percentage is: C：0.01~0.03%， Si: 0.20~0.60%, Mn: 0.50~1.50%, P≤0.045%， S≤0.015%， Cr：10.50~11.8%， Ni: 0.50~1.00%, Mo: 0.01~0.10%, N：0.01~0.05%， The balance consists of Fe and other unavoidable impurities, and the following conditions must be met simultaneously: Corrosion resistance index: Cr-16×(C-0.05)+3.3×Mo+16×N≥11.0; Bain isothermal transition time T B ≤1000, unit: min; T B =-7505+5370×C+1259×Si+586×Mn+604×Cr+560×Ni+597×N。 2. The bainitic stainless steel based on laterite nickel ore as described in claim 1, characterized in that, The chemical composition of the bainitic stainless steel also includes one or more of the following: Nb≤0.1%, V≤0.2%, Cu≤0.2%, Al≤0.02%, by mass percentage.

3. The bainitic stainless steel based on laterite nickel ore as described in claim 1 or 2, characterized in that, The bainitic stainless steel has a bainitic microstructure, a yield strength ≥750MPa, a tensile strength ≥1000MPa, an elongation ≥10%, a hardness of HRC 32~36, and a corrosion rate of ≤0.15g / m² in 3.5% NaCl salt spray for 72 hours. 2 / h.

4. The method for manufacturing bainitic stainless steel based on laterite nickel ore as described in claim 1, 2, or 3, characterized in that, Includes the following steps: 1) Smelting and casting Laterite nickel ore is smelted in a blast furnace to produce nickel-iron molten steel, which is then mixed with ferritic stainless steel scrap through an electric arc furnace. The nickel-iron molten steel is mixed in a ratio of 55wt% to 65wt%, and the scrap steel is mixed in a ratio of 35wt% to 45wt%. The mixture then enters AOD refining. During the AOD refining stage, high-carbon ferrochrome is added according to the chemical composition of the molten steel to adjust the chemical composition to the range of the composition in claim 1 or 2. The mixture is then refined in an LF furnace to obtain molten steel with the chemical composition of the bainitic stainless steel. Finally, it is continuously cast into slabs. 2) Hot-rolled The continuously cast slab is heated, held at a high temperature, descaled, and hot rolled into a steel plate. The slab is heated at a temperature of 1150~1270℃ and held at a high temperature for 180~240 minutes. After heating, the slab is descaled under high pressure and then enters the hot rolling stage. The initial rolling temperature is 1100~1200℃ and the final rolling temperature is 950~1050℃. 3) Straightening and cooling After hot rolling, the steel plate is cooled to 600~750℃ by air cooling or water mist cooling and then hot straightened. After hot straightening, it is cooled to the bainite transformation initiation temperature, i.e., 400℃~420℃, by air cooling. Then, it is immediately hoisted into a heat-insulating pit for slow cooling to room temperature, and the holding time in the temperature range of 300℃~420℃ is ≥T. B T B The time is the Bain isothermal transition time, in minutes.

5. The method for manufacturing bainitic stainless steel based on laterite nickel ore as described in claim 4, characterized in that, In step 1), the main raw material for smelting is laterite nickel ore, which contains 0.6~1.0wt% Ni, 1.3~1.7wt% Cr, and 40~50wt% Fe; molten nickel is obtained by blast furnace smelting, which contains 1~1.5wt% Ni, 4~5.5wt% Cr, and 1~2.0wt% Mn; molten steel from ferritic stainless steel scrap in electric furnace smelting has a Cr content of 15~18wt%.

6. The method for manufacturing bainitic stainless steel based on laterite nickel ore as described in claim 4, characterized in that, In step 1), during the AOD refining stage, high-carbon ferrochrome with a Cr content of 60-70 wt% is added according to the chemical composition of the AOD molten steel. The proportion of high-carbon ferrochrome added shall not exceed 10 wt% of the total mass of nickel-iron molten steel and waste steel.

7. The method for manufacturing bainitic stainless steel based on laterite nickel ore as described in claim 4, characterized in that, In step 3), after hot rolling, the steel plate is cooled to 600~750℃ using either air cooling or water mist cooling. Air cooling is used when the cooling rate reaches 40℃ / min or higher; otherwise, water mist spraying is required to assist cooling. Water mist is sprayed onto the upper and lower surfaces of the steel plate on the roller conveyor, with the water volume controlled at 2~6 ml. 3 The steel plate moving speed is 1.0~2.0 m / s, so that the average cooling rate of the steel plate after rolling is controlled at 40~70℃ / min.

8. The method for manufacturing bainitic stainless steel based on laterite nickel ore as described in claim 4, characterized in that, In step 3), the cooling rate of the air cooler after straightening is controlled at 10~30℃ / min.

9. The method for manufacturing bainitic stainless steel based on laterite nickel ore as described in claim 7, characterized in that, In step 3), the cooling rate of the air cooler after straightening is controlled at 10~30℃ / min.

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