A microalloyed nickel-saving austenitic stainless steel and a method for manufacturing the same

Abstract

The present application relates to a kind of microalloy nickel-saving austenitic stainless steel and its manufacturing method, it is related to metal material field, it includes the following chemical components by mass percentage: C:0.09-0.14%, Si:0.19-0.49%, Mn:7.1-7.4%, S≤0.0014%, P≤0.014%, Ni:1.1-1.9%, Cr:17.1-17.9%, Cu:0.49-0.99%, N:0.26-0.34%, 90×10 ‑4 %≤Zr+Mg≤130×10 ‑4 %, and the content of Zr, Mg satisfies the following formula: Zr / Mg=3-6, balance is Fe and inevitable impurities.The present application improves the microalloy nickel-saving austenitic stainless steel organization stability, warm hot plasticity, high temperature oxidation resistance, and resistance to localized corrosion performance and comprehensive mechanical properties, on the basis of further reducing raw material cost, meet the application field of wet, water medium, marine environment conditions such as resistance to localized corrosion performance and other use requirements.

Description

Technical Field

[0001] This invention relates to the field of metallic materials, specifically to a microalloyed nickel-saving austenitic stainless steel and its manufacturing method. Background Technology

[0002] Austenitic stainless steel is widely used in various sectors of the national economy due to its excellent mechanical properties and corrosion resistance. However, the dramatic fluctuations in nickel prices in recent years have brought significant risks to the production of high-nickel austenitic stainless steel, leading to considerable attention being paid to the development of low-nickel or even nickel-free austenitic stainless steel. Low-nickel austenitic stainless steel possesses excellent mechanical properties and certain corrosion resistance, meeting the requirements for use in weakly corrosive environments, such as in decoration and finished products. However, stainless steel used in decoration, finished products, and in humid and marine environments often requires better corrosion resistance, especially resistance to localized seawater corrosion, placing higher demands on the localized corrosion resistance of stainless steel. Therefore, the development of low-nickel austenitic stainless steel with excellent resistance to localized corrosion has significant practical application implications.

[0003] Currently, existing technologies disclose some advance research on austenitic stainless steel. CN1584098A discloses the chemical composition of a low-Ni austenitic stainless steel. Although this chemical composition employs an alloying method to further reduce Ni content (Ni≤1.2%) and increase Mn content (8.5%≤Mn≤10.0%), it does not consider adding a certain amount of Cu to improve cold working performance, nor does it consider compositional design and process measures to improve resistance to localized corrosion. CN1500894A and CN1704497A disclose the chemical composition of a low-Ni austenitic stainless steel. While these chemical compositions also employ an alloying method to further reduce Ni content (1.0%≤Ni≤5.0%) and increase Mn content (7.5%≤Mn≤10.5%), and CN1500894A also considers adding trace amounts of B to improve hot working, the Ni content remains high, and resistance to localized corrosion under humid, aquatic, and marine environmental conditions is not considered. CN1876882A describes the chemical composition of a low-Ni austenitic stainless steel. While this composition employs an alloying method to further reduce the Ni content (0.6% ≤ Ni ≤ 1.3%) and increase the Mn content (8.5% ≤ Mn ≤ 12.0%), and also considers adding trace amounts of rare earth element RE to improve hot workability, the Ni content remains high. Furthermore, rare earth elements are difficult to incorporate in large-scale industrial production, and their yield is unstable, which is detrimental to large-scale industrial production. CN1240839A describes the chemical composition of a low-Ni austenitic stainless steel. While this composition also employs an alloying method to further reduce the Ni content (1.0% ≤ Ni ≤ 4.0%) and increase the Mn content (5.0% ≤ Mn ≤ 11.0%), and also considers adding trace elements B and Ca, the Ni content remains high, and the absence of nitrogen reduces the austenitic stability of the product, making it unsuitable for cold forming. To address the aforementioned problems in the prior art, this invention provides a microalloyed nickel-saving austenitic stainless steel and its manufacturing method. Summary of the Invention

[0004] This invention provides a microalloyed nickel-saving austenitic stainless steel and its manufacturing method. The aim is to improve the microstructural stability, thermoplasticity, high-temperature oxidation resistance, localized corrosion resistance, and overall mechanical properties of microalloyed austenitic stainless steel. While further reducing raw material costs, it meets the application requirements in humid, aquatic, and marine environments, such as resistance to localized corrosion.

[0005] The present invention solves the above-mentioned technical problems. In its first aspect, it provides a microalloyed nickel-saving austenitic stainless steel, comprising the following chemical composition by mass percentage: C: 0.09-0.14%, Si: 0.19-0.49%, Mn: 7.1-7.4%, S≤0.0014%, P≤0.014%, Ni: 1.1-1.9%, Cr: 17.0-18.0%, Cu: 0.49-0.99%, N: 0.26-0.34%, 90×10 -4 %≤Zr+Mg≤130×10 -4 The percentage of Zr and Mg is 3%, and the content of Zr and Mg satisfies the following formula: Zr / Mg = 3-6, with the balance being Fe and unavoidable impurities.

[0006] The above components play the following roles in microalloyed nickel-saving austenitic stainless steel:

[0007] C: Carbon is a strong austenitic element that stabilizes austenite and expands the austenitic region in austenitic stainless steel. Carbon interstitial atoms are dissolved in austenite, and solid solution strengthening can significantly improve the strength of austenite. However, carbon is often considered a harmful element in austenite, mainly because its presence reduces the electrode potential of iron and decreases corrosion resistance during certain uses or processing. For example, heating to 450-850℃ or welding can lead to chromium depletion at grain boundaries, reducing the steel's corrosion resistance, especially its resistance to intergranular corrosion. Carbon also increases the pitting corrosion tendency of chromium-nickel austenitic stainless steel; therefore, the mass content of C in this invention is 0.09-0.14%.

[0008] Si: Silicon is a strong ferrite-forming element. In austenitic stainless steel, with increasing silicon content, δ-ferrite increases, and the formation of the intermetallic compound σ phase also accelerates and increases, thus affecting the steel's properties. To maintain the single austenitic structure of austenitic stainless steel, the contents of nickel and nitrogen must also be increased accordingly with increasing silicon content. As a common deoxidizing element in steel, silicon has a synergistic effect with elements such as Cu, Cr, Zr, and Mg, thereby improving the corrosion resistance of steel. Therefore, the mass content of Si in this invention is 0.19-0.49%.

[0009] Mn (manganese): Manganese is an important alloying element, primarily acting to form stable austenite with nitrogen and a certain amount of nickel. The strength of chromium-nickel austenitic stainless steel increases with increasing manganese content. Manganese can improve the thermoplasticity of chromium-nickel austenitic stainless steel, with a noticeable effect at a manganese content of 1.5%. Manganese has a strong affinity for sulfur, forming MnS, which helps eliminate the harmful effects of residual sulfur in steel. However, the formation of MnS often leads to a decrease in the resistance of chromium-nickel austenitic stainless steel to chloride pitting corrosion and crevice corrosion. When the sulfur content in the steel is reduced to a certain level, the adverse effects of manganese can be largely eliminated; therefore, the mass content of Mn in this invention is 7.1-7.4%.

[0010] S: Sulfur is considered a harmful impurity in austenitic stainless steel. The harmful effects of sulfur are mainly: reducing the hot plasticity of austenitic stainless steel and affecting the hot workability of the steel, which is related to the precipitation of MnS or (Fe,Mn)S along the grain boundaries at high temperatures; sulfur also reduces the corrosion resistance of austenitic stainless steel. MnS is easily soluble in acidic chloride solutions and often becomes a corrosion source, leading to a significant reduction in resistance to pitting corrosion and crevice corrosion. Therefore, in this invention, the mass content of S is controlled to be ≤0.0014%.

[0011] P: Phosphorus is generally considered a harmful impurity in austenitic stainless steel. The harmful effects of phosphorus are mainly: significantly reducing the resistance of chromium-nickel austenitic stainless steel to corrosion by nitric acid of various concentrations in both the solution and sensitized states; and significantly enhancing the resistance of chromium-nickel austenitic stainless steel to corrosion by concentrated nitric acid and chromium-containing nitric acid. 5+ To reduce the susceptibility of P to solid-solution intergranular corrosion in nitric acid and decrease corrosion resistance under these operating conditions, the mass content of P is controlled to be ≤0.0014% in this invention.

[0012] Ni: The main role of nickel in austenitic stainless steel is to form and stabilize austenite to obtain a fully austenitic structure, thereby giving the steel a good balance of strength, plasticity, and toughness, as well as a series of excellent processing properties. Within the range of nickel content in the steel where martensitic transformation can occur, as the nickel content increases, the strength of the steel decreases while its plasticity increases due to the reduction in martensite. For chromium-manganese-nitrogen austenitic stainless steel with a stable austenitic structure, the addition of nickel can further improve its low-temperature toughness. Nickel can significantly reduce the cold work hardening tendency of austenitic stainless steel, mainly because nickel increases the stability of austenite, reducing or even eliminating the martensitic transformation during cold working, while nickel has little effect on the cold work hardening of austenite itself. Nickel improves the passivation tendency and thermodynamic stability of austenitic stainless steel, thus improving the alloy's resistance to uniform corrosion and oxidizing media; with the increase of nickel content, the resistance to reducing media is further improved. In austenitic stainless steel, nickel is an important element for improving its resistance to transgranular stress corrosion in some media. Increasing the nickel content reduces the solubility of carbon in austenitic steel, which enhances the tendency for carbide precipitation and lowers the critical carbon content for intergranular corrosion, thus increasing the susceptibility to intergranular corrosion. Therefore, the mass content of Ni in this invention is 1.1-1.9%.

[0013] Cr: Chromium is the most abundant alloying element in austenitic stainless steel. Cr oxide can form a dense passivation film, preventing oxidation of the internal material. Furthermore, the presence of chromium increases the electrode potential of Fe, effectively inhibiting the oxidation of Cl. - The erosion of Fe is effectively inhibited. 3+ To Fe 2+The transformation of carbon into a stable oxide layer gives stainless steel its corrosion resistance. In austenitic stainless steel, chromium increases the solubility of carbon and reduces the chromium depletion. As the chromium content increases, the tendency for σ phase formation increases. When the steel contains molybdenum, the increase in chromium content also promotes the formation of χ phase. Therefore, the mass content of Cr in this invention is 17.1-17.9%.

[0014] Cu: Copper significantly reduces the cold work hardening tendency of austenitic stainless steel and improves its cold forming performance. The combination of copper and molybdenum can further improve the corrosion resistance of austenitic stainless steel in reducing media. The reason why the increase in dissolved copper reduces the strength and increases the plasticity of the steel is that copper significantly increases the stacking fault energy of chromium-nickel austenitic stainless steel and stabilizes the austenitic structure. The increase in stacking fault energy hinders the formation of partial dislocations, promotes cross-slip of dislocations, prevents dislocation accumulation, and improves the plasticity of the material. Copper significantly reduces the hot workability of steel, which is more pronounced when the nickel content in austenitic stainless steel is low; therefore, when the copper content in the steel is high, the nickel content should also be increased accordingly. Copper can significantly improve the corrosion resistance of austenitic stainless steel to reducing media such as sulfuric acid and phosphoric acid. The effect is even more pronounced when copper and molybdenum are alloyed together. The addition of copper accelerates the dissolution of molybdenum in the stainless steel, forming MoO4. 2- This strongly promotes the passivation of chromium in stainless steel and the enrichment of chromium into the surface film, resulting in improved corrosion resistance of the steel. Therefore, the mass content of Cu in this invention is 0.49-0.99%.

[0015] Nitrogen (N): Nitrogen can significantly improve the strength of austenitic stainless steel through solid solution strengthening without significantly impairing its plasticity and toughness. Simultaneously, nitrogen can improve the steel's resistance to uniform corrosion, pitting corrosion, crevice corrosion, and intergranular corrosion. This is because nitrogen, as an active element, preferentially accumulates along grain boundaries, reducing the diffusion ability of carbon and chromium atoms, thereby inhibiting carbide precipitation and delaying the formation of σ and χ phases. Nitrogen's ability to form austenite is comparable to carbon, approximately 30 times that of nickel. In austenitic stainless steel, it can replace some nickel, reducing the ferrite content and making austenite more stable, even preventing martensitic transformation. Therefore, the mass content of N in this invention is 0.26-0.34%.

[0016] Zr: Zirconium is a strong carbide-forming element, as well as a strong deoxidizing element and a complex oxysulfide-forming element. Adding a small amount of zirconium has degassing, purification, and grain refinement effects, which is beneficial for improving the low-temperature performance of stainless steel and its stamping properties. When dissolved in austenite, it significantly improves the hardenability of the steel. In austenitic steel, it can prevent intergranular corrosion by oxidizing media. Due to its carbon fixation and precipitation hardening effects, it can improve the high-temperature properties of heat-resistant steels, such as creep strength. Therefore, the mass content of Zr and Mg in this invention is 90 × 10⁻⁶. -4 %≤Zr+Mg≤130×10 -4The percentage of Zr and Mg is %, and the content of Zr and Mg satisfies the following formula: Zr / Mg = 3-6.

[0017] Mg: Magnesium is a strong deoxidizing element and a complex oxygen sulfide forming element. Magnesium can reduce the number, size, and uniform distribution of inclusions in steel, and improve their morphology. Trace amounts of magnesium can improve the size and distribution of carbides in stainless steel, resulting in fine and uniform carbide particles. The MgO inclusions formed have the effect of pinning austenite grain boundaries, thus having a good control effect on grain size. Therefore, the mass content of Zr and Mg in this invention is 90 × 10⁻⁶. -4 %≤Zr+Mg≤130×10 -4 The percentage of Zr and Mg is %, and the content of Zr and Mg satisfies the following formula: Zr / Mg = 3-6.

[0018] The beneficial effects of this invention are:

[0019] (1) This invention uses molten iron rich in chromium and nickel smelted from laterite nickel ore as raw material for smelting stainless steel, saving precious metal elements Cr and Ni; using industrial pure iron can reduce impurities such as P and S, which not only helps to reduce harmful impurity P in stainless steel, but also reduces the content of other impurities.

[0020] (2) The Cr and C required in this invention can be matched by ferrochrome. C can release heat through oxidation during smelting, which can raise the temperature and facilitate the smelting process. The microalloyed nickel-saving austenitic stainless steel prepared has strong structural stability, good high-temperature thermoplasticity and high-temperature oxidation resistance, and better resistance to local corrosion such as seawater and comprehensive mechanical properties.

[0021] Based on the above technical solution, the present invention can be further improved as follows.

[0022] Furthermore, it includes the following chemical composition by mass percentage: C: 0.09-0.10%, Si: 0.19-0.25%, Mn: 7.1-7.2%, S≤0.0011%, P≤0.011%, Ni: 1.1-1.4%, Cr: 17.0-17.5%, Cu: 0.49-0.59%, N: 0.26-0.30%, 90×10 -4 %≤Zr+Mg≤105×10 -4 The percentage of Zr and Mg is 3%, and the content of Zr and Mg satisfies the following formula: Zr / Mg = 3-6, with the balance being Fe and unavoidable impurities.

[0023] Furthermore, it includes the following chemical composition by mass percentage: C: 0.10-0.14%, Si: 0.25-0.49%, Mn: 7.2-7.4%, S≤0.0010%, P≤0.010%, Ni: 1.4-1.9%, Cr: 17.5-17.9%, Cu: 0.59-0.99%, N: 0.30-0.34%, 100×10 -4 %≤Zr+Mg≤110×10 -4 The percentage of Zr and Mg is 3%, and the content of Zr and Mg satisfies the following formula: Zr / Mg = 3-6, with the balance being Fe and unavoidable impurities.

[0024] Furthermore, it includes the following chemical components by mass percentage: C: 0.10%, Si: 0.25%, Mn: 7.2%, S: 0.0010%, P: 0.010%, Ni: 1.4%, Cr: 17.5%, Cu: 0.59%, N: 0.30%, Mg: 0.0025%, Zr: 0.0080%, with the balance being Fe and unavoidable impurities.

[0025] The second aspect provides a method for preparing microalloyed nickel-saving austenitic stainless steel, comprising the following steps:

[0026] Step 1: Take the following raw materials according to the following weight ratio: 30-35% ultra-low P industrial pure iron, 15-20% high carbon ferrochrome, and 50-60% molten iron from laterite nickel ore smelting; first, add the ultra-low P industrial pure iron and the high carbon ferrochrome to an induction furnace for melting, and then add the molten iron from laterite nickel ore smelting to the induction furnace to obtain raw materials for smelting microalloyed nickel-saving austenitic stainless steel;

[0027] Step 2: The smelting microalloyed nickel-saving austenitic stainless steel raw material obtained in Step 1 is sequentially refined, continuously cast, rolled and solution treated to obtain microalloyed nickel-saving austenitic stainless steel.

[0028] Furthermore, in step 1, the following raw materials are taken according to the following weight ratio: 30-34% ultra-low P industrial pure iron, 15-20% high carbon ferrochrome, and 52-60% molten iron smelted from laterite nickel ore.

[0029] Furthermore, in step 1, the weight contents of P and C in the ultra-low P industrial pure iron are: P≤0.0050%, C≤0.001%; ​​and the weight contents of Cr and C in the high carbon ferrochrome are: Cr≥60%, C≤9.5%.

[0030] Furthermore, in step 1, the weight contents of the chemical components Cr and P in the molten iron smelted from the laterite nickel ore are Cr≥60% and P≤0.03%, respectively.

[0031] Furthermore, in step 2, the specific steps for refining, continuous casting, rolling, and solution treatment of the smelting microalloyed nickel-saving austenitic stainless steel raw material obtained in step 1 are as follows:

[0032] Step 2-1, Refining: The raw material of the smelting micro-alloyed nickel-saving austenitic stainless steel is refined outside the furnace in an LF furnace, and C, Si, Cr, Ni, Cu, P, S alloy fine-tuning and Zr, Mg, N micro-alloying treatment are carried out according to the chemical composition of the micro-alloyed nickel-saving austenitic stainless steel to obtain micro-alloyed stainless steel.

[0033] Step 2-2, continuous casting: The microalloyed stainless steel is continuously cast at a tundish temperature of 1465-1480℃ and a working casting speed of 1.05-1.20m / min to obtain a continuously cast billet;

[0034] Steps 2-3, rolling: The continuously cast billet is rolled at a temperature of 1150-1250℃ to obtain hot-rolled black coil;

[0035] Steps 2-4, solution treatment: The hot-rolled black steel coil is heated to a temperature of 1050-1150℃, and the solution treatment time is 0.6-1.0 mm / min. After cooling, microalloyed nickel-saving austenitic stainless steel is obtained.

[0036] Furthermore, in step 2-1, the refining time is 45-90 minutes, and the furnace exit temperature is ≥1530℃. Attached Figure Description

[0037] Figure 1 This is a morphology diagram of inclusions in austenitic stainless steel using conventional techniques of the present invention.

[0038] Figure 2 This is a morphology diagram of inclusions in the austenitic nickel-modified stainless steel of the present invention;

[0039] Figure 3 These are optical microscope images of microalloyed austenitic stainless steel using conventional techniques of this invention.

[0040] Figure 4 This is an optical microscope image of the microalloyed austenitic stainless steel prepared according to the present invention.

[0041] Figure 5 This invention relates to the constant electrode potential saturation current density of microalloyed austenitic stainless steel using conventional techniques.

[0042] Figure 6 The constant electrode potential saturation current density of the microalloyed nickel-saving austenitic stainless steel prepared in this invention;

[0043] Figure 7The present invention provides the potentiodynamic polarization curves of microalloyed austenitic stainless steel using conventional techniques.

[0044] Figure 8 Potentiodynamic polarization curves of the microalloyed nickel-saving austenitic stainless steel prepared according to the present invention;

[0045] Figure 9 AC impedance of the microalloyed austenitic stainless steel prepared by the present invention using conventional techniques. Figure I ;

[0046] Figure 10 AC impedance of the microalloyed austenitic stainless steel prepared by the present invention using conventional techniques. Figure II ;

[0047] Figure 11 This is a circuit diagram showing the fitting of the conventional technology of this invention and the microalloyed austenitic stainless steel prepared by this invention. Detailed Implementation

[0048] The principles and features of the present invention are described below. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0049] Example 1

[0050] This embodiment relates to a microalloyed nickel-saving austenitic stainless steel, which comprises the following chemical composition by weight percentage: 0.09wt% C, 0.19wt% Si, 7.1wt% Mn, 0.0010wt% S, 0.010wt% P, 1.1wt% Ni, 17.1wt% Cr, 0.49wt% Cu, 0.26wt% N, 0.0025wt% Mg, 0.0075wt% Zr, with the balance being Fe and unavoidable impurities.

[0051] This embodiment relates to a method for preparing a microalloyed nickel-saving austenitic stainless steel, comprising the following steps:

[0052] Step 1: Prepare the raw materials as follows: Use ultra-low P industrial pure iron as the smelting base material, accounting for 30% by mass; add high-carbon ferrochrome, accounting for 20% by mass; and then add molten iron smelted from laterite nickel ore, accounting for 50% by mass. The ultra-low P industrial pure iron contains P ≤ 0.0035 wt% and C ≤ 0.005 wt%. The high-carbon ferrochrome contains Cr ≥ 60 wt% and C ≤ 6.5 wt%. The molten iron smelted from laterite nickel ore contains Cr ≥ 60 wt% and P ≤ 0.01 wt%. Melt the ultra-low P industrial pure iron and high-carbon ferrochrome in a medium-frequency furnace, and then add molten iron smelted from laterite nickel ore (50% by mass) to obtain the raw material for smelting microalloyed nickel-saving austenitic stainless steel. After smelting, the stainless steel contains P ≤ 0.014 wt%.

[0053] Step 2: The smelting microalloyed nickel-saving austenitic stainless steel raw material obtained in Step 1 is sequentially refined, continuously cast, rolled and solution treated to obtain microalloyed nickel-saving austenitic stainless steel.

[0054] Preferably, in step 2, the specific steps for sequentially refining, continuously casting, rolling, and solution treatment of the smelting microalloyed nickel-saving austenitic stainless steel raw material obtained in step 1 are as follows:

[0055] Step 2-1: LF Ladle Refining: Before tapping, remove and skim the slag to clean it as thoroughly as possible; tap the steel with white slag, and use a small amount of lime and fluorite to adjust the slag's reducing properties and fluidity; tapping temperature 1530℃; first, melt the top slag by applying electricity, then perform alloy fine-tuning and micro-alloying to obtain a chemical composition by weight percentage including: 0.09wt% C, 0.19wt% Si, 7.1wt% Mn, 0.0010wt% S, 0.010wt% P, 1.1wt% Ni, 17.1wt% Cr, 0.49wt% Cu, 0.26wt% N, 0.0025wt% Mg, 0.0075wt% Zr, with the balance being Fe; the LF refining furnace smelting time is 45 min.

[0056] Step 2-2: Continuous casting: Tundish temperature 1465℃; Casting speed 1.05m / min; Water flow rate across the crystallizer width 175m³. 3 / h, narrow face water volume is 20m 3 / h, the inlet and outlet water temperature difference is 5℃; the crystallizer adopts non-sinusoidal vibration with a frequency of 140CPM and an amplitude of 3.0mm; the crystallizer protective slag is high-nitrogen protective slag.

[0057] Steps 2-3: Rolling: The continuously cast billet is first heated and then hot-rolled to obtain hot-rolled black coils; the hot rolling temperature is 1150℃.

[0058] Steps 2-4: Solution treatment: After hot rolling, solution treatment is performed at a temperature of 1050℃ and a time of 0.6 mm / min. The air cooling + water cooling time is 60 min. After air cooling, water cooling is performed at room temperature to obtain microalloyed nickel-saving austenitic stainless steel.

[0059] Example 2

[0060] This embodiment relates to a microalloyed nickel-saving austenitic stainless steel, which comprises the following chemical composition by weight percentage: 0.10wt% C, 0.25wt% Si, 7.2wt% Mn, 0.0011wt% S, 0.011wt% P, 1.4wt% Ni, 17.5wt% Cr, 0.59wt% Cu, 0.30wt% N, 0.0025wt% Mg, 0.0080wt% Zr, with the balance being Fe and unavoidable impurities.

[0061] This embodiment relates to a method for preparing a microalloyed nickel-saving austenitic stainless steel, comprising the following steps:

[0062] Step 1: Prepare the raw materials as follows: Use ultra-low phosphorus (UP) industrial pure iron as the smelting base material, accounting for 34% by mass; add high-carbon ferrochrome, accounting for 14% by mass; and then add molten iron smelted from laterite nickel ore, accounting for 52% by mass. The UP industrial pure iron contains P ≤ 0.0050 wt% and C ≤ 0.01 wt%. The high-carbon ferrochrome contains Cr ≥ 60 wt% and C ≤ 9.5 wt%. The molten iron smelted from laterite nickel ore contains Cr ≥ 60 wt% and P ≤ 0.03 wt%. Melt the UP industrial pure iron and high-carbon ferrochrome in a medium-frequency furnace, and then add the molten iron smelted from laterite nickel ore to obtain the raw material for smelting microalloyed nickel-saving austenitic stainless steel. After smelting, the stainless steel contains P ≤ 0.015 wt%.

[0063] Step 2: The smelting microalloyed nickel-saving austenitic stainless steel raw material obtained in Step 1 is sequentially refined, continuously cast, rolled and solution treated to obtain microalloyed nickel-saving austenitic stainless steel.

[0064] Preferably, in step 2, the specific steps for sequentially refining, continuously casting, rolling, and solution treatment of the smelting microalloyed nickel-saving austenitic stainless steel raw material obtained in step 1 are as follows:

[0065] Step 2-1: LF Ladle Refining: Before tapping, remove and skim the slag to clean it as thoroughly as possible; tap the steel with white slag, and use a small amount of lime and fluorite to adjust the slag's reducing properties and fluidity; tapping temperature 1630℃; first, melt the top slag by applying electricity, then perform alloy fine-tuning and micro-alloying to obtain a chemical composition by weight percentage including: 0.10wt% C, 0.25wt% Si, 7.2wt% Mn, 0.0011wt% S, 0.011wt% P, 1.4wt% Ni, 17.5wt% Cr, 0.59wt% Cu, 0.30wt% N, 0.0025wt% Mg, 0.0080wt% Zr, with the balance being Fe; the LF refining furnace smelting time is 60 min.

[0066] Step 2-2: Continuous casting: Tundish temperature 1470℃; Casting speed 1.10 m / min; Water flow rate across the crystallizer width 185 m³ / min 3 / h, narrow face water volume is 24m³ 3 / h, the inlet and outlet water temperature difference is 6℃; the crystallizer adopts non-sinusoidal vibration with a frequency of 140CPM and an amplitude of 3.0mm; the crystallizer protective slag is high-nitrogen protective slag.

[0067] Steps 2-3: Rolling: The continuously cast billet is first heated and then hot-rolled to obtain hot-rolled black coils; the hot rolling temperature is 1200℃.

[0068] Steps 2-4: Solution treatment: After hot rolling, solution treatment is performed at a temperature of 1100℃ and a time of 0.9 mm / min. The air cooling + water cooling time is 100 min. After air cooling, water cooling is performed at room temperature to obtain microalloyed nickel-saving austenitic stainless steel.

[0069] Example 3

[0070] This embodiment relates to a microalloyed nickel-saving austenitic stainless steel, which comprises the following chemical composition by weight percentage: 0.14wt% C, 0.49wt% Si, 7.4wt% Mn, 0.0014wt% S, 0.014wt% P, 1.9wt% Ni, 17.9wt% Cr, 0.89wt% Cu, 0.34wt% N, 0.0020wt% Mg, 0.0090wt% Zr, with the balance being Fe and unavoidable impurities.

[0071] This embodiment relates to a method for preparing a microalloyed nickel-saving austenitic stainless steel, comprising the following steps:

[0072] Step 1: Prepare the raw materials as follows: Use ultra-low P industrial pure iron as the smelting base material, accounting for 35% by mass; add high-carbon ferrochrome, accounting for 15% by mass; then add molten iron smelted from laterite nickel ore, accounting for 60% by mass. The ultra-low P industrial pure iron contains P ≤ 0.0050 wt% and C ≤ 0.01 wt%. The high-carbon ferrochrome contains Cr ≥ 60 wt% and C ≤ 9.5 wt%. The molten iron smelted from laterite nickel ore contains Cr ≥ 60 wt% and P ≤ 0.03 wt%. Melt the ultra-low P industrial pure iron (35% by mass) and high-carbon ferrochrome (15% by mass) in a medium-frequency furnace, then add the molten iron smelted from laterite nickel ore (60% by mass) to obtain the raw material for smelting microalloyed nickel-saving austenitic stainless steel. After smelting, the stainless steel contains P ≤ 0.012 wt%.

[0073] Step 2: The smelting microalloyed nickel-saving austenitic stainless steel raw material obtained in Step 1 is sequentially refined, continuously cast, rolled and solution treated to obtain microalloyed nickel-saving austenitic stainless steel.

[0074] Preferably, in step 2, the specific steps for sequentially refining, continuously casting, rolling, and solution treatment of the smelting microalloyed nickel-saving austenitic stainless steel raw material obtained in step 1 are as follows:

[0075] Step 2-1: LF Ladle Refining: Before tapping, remove and skim the slag to clean it as thoroughly as possible; tap the steel with white slag, and use a small amount of lime and fluorite to adjust the slag's reducing properties and fluidity; tapping temperature is 1730℃; first, melt the top slag by applying electricity, then perform alloy fine-tuning and micro-alloying to obtain a chemical composition by weight percentage including: 0.14wt% C, 0.49wt% Si, 7.4wt% Mn, 0.0014wt% S, 0.014wt% P, 1.9wt% Ni, 17.9wt% Cr, 0.99wt% Cu, 0.34wt% N, 0.0020wt% Mg, 0.0090wt% Zr, with the balance being Fe; the LF refining furnace smelting time is 90 min.

[0076] Step 2-2: Continuous casting: Tundish temperature 1480℃; Casting speed 1.20 m / min; Water flow rate across the crystallizer width 195 m³ / min 3 / h, narrow face water volume is 25m 3 / h, the inlet and outlet water temperature difference is 7℃; the crystallizer adopts non-sinusoidal vibration with a frequency of 140CPM and an amplitude of 3.0mm; the crystallizer protective slag is high-nitrogen protective slag.

[0077] Steps 2-3: Rolling: The continuously cast billet is first heated and then hot-rolled to obtain hot-rolled black coils; the hot rolling temperature is 1250℃.

[0078] Steps 2-4: Solution treatment: After hot rolling, solution treatment is performed at a temperature of 1150℃ and a time of 1.0 mm / min. The air cooling + water cooling time is 120 min. After air cooling, water cooling is performed at room temperature to obtain microalloyed nickel-saving austenitic stainless steel.

[0079] The following tests were conducted on the inclusion characteristics and corrosion resistance of the microalloyed nickel-saving austenitic stainless steel prepared in Example 1, and the results are as follows:

[0080] (1) Characteristics of inclusions and their formation mechanism

[0081] The morphology of inclusions in the test samples was observed using SEM, and the results are as follows: Figure 1 and 2 .Depend on Figure 1 It is known that inclusions in conventional austenitic stainless steel are elongated or irregular in shape, with a size of approximately 2-5 μm. Figure 2 As can be seen, the inclusions in the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention are spherical or equiaxed, with an inclusion size of approximately 1-3 μm. These small and spherical inclusions play an important positive role in improving the plasticity and toughness of austenitic stainless steel.

[0082] According to the basic principles of metallurgical thermodynamics, both Zr and Mg are strong oxide-forming elements. Using Zr and Mg for combined deoxidation is beneficial for removing free oxygen from molten steel. The density of ZrO2 is 5.68 g / cm³. 3 The density is greater than that of Al₂O₃ (3.97 g / cm³). 3 ), especially the density of ZrO2 and molten steel (7.15 g / cm³). 3 The ZrO2 particles are more closely related to the Al2O3 particles, so once stable oxides are formed at high temperatures, ZrO2 floats uniformly in the molten steel, while Al2O3 collides and aggregates on the surface of the molten steel, becoming part of the steel slag. The portion of Al2O3 that fails to float remains in the steel as large, clustered inclusions. The electrical conductivity of the oxides is a key factor in their movement in the molten steel. According to existing research, the driving force for the movement of Al2O3 in molten steel is greater than that for ZrO2. During electrorefining, ZrO2 particles tend to repel each other and are difficult to aggregate, while Al2O3 easily collides with each other to form large particles that float to the surface of the molten steel and are absorbed by the surface coating agent. Therefore, compared with conventional Al and Si deoxidation, Zr and Mg composite deoxidation can form fine, dispersed composite oxides, in which… Figure 2 The experimental results prove this point.

[0083] Furthermore, MnS and ZrO2 have extremely similar lattice constants, as shown in Table 1. Due to the excellent lattice matching between MnS and ZrO2, the interfacial energy between them is reduced. Lower interfacial energy results in better adhesion between grains at different interfaces. This further explains why no strip-like or string-like sulfides formed in the experimental samples. This is because MnS tends to precipitate on pre-formed ZrO2 particles, thus refining, spheroidizing, and dispersing the sulfides. This is beneficial for improving the plasticity and toughness of Zr / Mg composite deoxidized ferritic stainless steel.

[0084] Table 1. Lattice constants of MnS and ZrO2

[0085]

[0086] (2) Density of corrosive inclusions

[0087] Corrosion-active inclusions were examined in conventional austenitic stainless steel and the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention. Microscopic images of the examined microstructures are shown below. Figure 3 and 4 As shown. Statistical analysis was performed on the density of corrosion-active inclusions in conventional austenitic stainless steel and the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention.

[0088] Black spots appear on the surface of conventional austenitic stainless steel, which are pitting corrosion pits caused by the dissolution of corrosive inclusions. However, the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention has virtually no corrosive inclusions on its surface. The density of corrosive inclusions on the sample surface was statistically analyzed. The density of corrosive inclusions in conventional austenitic stainless steel is 0.56 inclusions / mm². 2 The density of corrosion-active inclusions in the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention is 0.23 inclusions / mm². 2 Statistical analysis shows that the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention, using laterite nickel ore for smelting, exhibits a significantly lower density of corrosion-active inclusions compared to conventional austenitic stainless steel after treatment with elements such as Zr. According to literature reports, MnS inclusions, either directly or in their surrounding area, readily dissolve in sodium chloride solution, inducing pitting corrosion and exhibiting high corrosion activity. After Zr-based composite deoxidation, the originally highly corrosive MnS inclusions in the steel are modified into composite inclusions, such as ZrO2 and MgO, which possess higher stability and greater corrosion resistance in sodium chloride solution. Therefore, Zr and Mg composite deoxidation reduces the density of corrosion-active inclusions in the steel.

[0089] (3) Corrosion test

[0090] The microalloyed nickel-saving austenitic stainless steel prepared in Example 1 was electrochemically tested using a Zahner Zennium electrochemical workstation. A standard three-electrode system was selected, with the invented steel as the working electrode, a platinum sheet as the auxiliary electrode, and a saturated calomel electrode (SCE) as the reference electrode. Electrostatic potential method: The stability against general corrosion at the maximum saturation current density was determined under electrostatic potential conditions in a simulated environment. Simultaneously, potentiostatic polarization testing was performed. The potential for potentiostatic polarization curve testing was 300 mV, and the testing time was 3600 s. After the test, the saturation current density Imax was recorded. Potentiodynamic polarization curve testing was performed at a scan rate of 0.5 mV / s, with a scan range of -0.6 to 1.2 V relative to the open circuit potential (OCP). The test solution used was a 3.5 wt.% NaCl solution.

[0091] The potentiostatic polarization corrosion tests of conventional austenitic stainless steel and the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention were performed using potentiostatic polarization testing in accordance with the corrosion assessment method. The saturated current density results of conventional austenitic stainless steel and the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention are as follows: Figure 5 and 6As shown. Under the same corrosion time, the smaller the saturation current density value, the better the corrosion resistance of the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention. The saturation current density value of conventional austenitic stainless steel is 2.77 mA·cm. 2 The saturated current density of the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention is 0.342 mA·cm. 2 The saturated current density of the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention is 2.428 mA·cm⁻¹ lower than that of conventional austenitic stainless steel. 2 Therefore, the development of microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention based on laterite nickel ore effectively changed the types and morphologies of inclusions through multi-component deoxidation with Zr, Mg, etc., significantly reducing the density of corrosion-active inclusions and the constant potential polarization saturation current density value. Its resistance to localized corrosion is significantly better than that of traditional austenitic stainless steel.

[0092] Figure 7 and 8 The potentiodynamic polarization curves of conventional austenitic stainless steel and the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention are shown in 3.5 wt.% NaCl solution. The self-corrosion potential and corrosion current density of the test samples can be obtained by testing the potentiodynamic polarization curves and fitting them with Tafel, which can be used to evaluate the trend of corrosion rate changes and the corrosion reaction mechanism.

[0093] The corrosion potentials and corrosion current densities of conventional austenitic stainless steel and the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention are shown in Table 2. The self-corrosion potentials of conventional austenitic stainless steel and the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention are -0.0065V and 0.013V, respectively. The corrosion current densities of conventional austenitic stainless steel and the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention are 8.71 × 10⁻⁶ V and 8.71 × 10⁻⁶ V, respectively. -5 mA·cm 2 and 7.67×10 -5 mA·cm 2As shown in Table 2, the self-corrosion potential of the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention is higher than that of conventional austenitic stainless steel, and the corrosion current density is lower than that of conventional austenitic stainless steel. A more positive self-corrosion potential indicates better thermodynamic stability and a lower tendency to corrosion. A lower corrosion current density indicates a slower rate of corrosion. The microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention has a lower corrosion current density and a more positive self-corrosion potential; therefore, its corrosion resistance is better than that of conventional austenitic stainless steel.

[0094] Table 2. Fitting results of potentiodynamic polarization of austenitic stainless steel using conventional and inventive techniques.

[0095]

[0096] (4) AC impedance

[0097] Electrochemical tests were performed on the samples in a 3.5 wt.% NaCl solution using a Zahner Zennium electrochemical workstation. A three-electrode system was used, with a platinum mesh as the counter electrode, the sample as the working electrode, and a saturated calomel electrode (SCE) as the reference electrode. Electrochemical impedance spectroscopy (EIS) was performed by applying an amplitude of 10 mV in the frequency range of 0.01 to 100,000 Hz, and the experimental data were fitted using Zsimpwin software.

[0098] Figure 9 and 10 The figures show the AC impedance diagrams of conventional austenitic stainless steel and the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention in 3.5 wt.% NaCl solution. The Nyquist plots show that the two stainless steels have similar curve shapes: both are incomplete capacitive arcs, indicating similar corrosion mechanisms. The equivalent circuit diagram Rs(Q1(R1(Q2R2))) is selected, as shown below. Figure 11As shown in the figure. In the experiment, Rs is the solution resistance, Q1 is the double-layer capacitance, R1 is the passivation film resistance, Q2 is the passivation film capacitance, and R2 is the charge transfer resistance. The diameter of the capacitive arc in the Nyquist plot reflects the corrosion rate of the stainless steel. Generally, the larger the diameter of the capacitive arc, the better the corrosion resistance. The two stainless steels were fitted using Zsimpwin software, and the fitting results are shown in Table 3. As can be seen from Table 2, the passivation film resistance R1 and polarization resistance R2 of the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention are higher than those of conventional austenitic stainless steel, indicating that the stability of the passivation film is significantly improved and it is not easy to thin in the corrosive medium. Therefore, the corrosion resistance of the microalloyed nickel-saving austenitic stainless steel prepared in Example 1 of this invention is better than that of conventional austenitic stainless steel.

[0099] Table 3. Fitting results of AC impedance of austenitic stainless steel using conventional and inventive techniques.

[0100]

[0101]

[0102] In summary, this invention utilizes chromium- and nickel-rich molten iron from laterite nickel ore smelting as a raw material, saving precious metal Ni, and achieving good mechanical properties using low-cost elements such as silicon, manganese, and chromium. The invention also incorporates elements such as zirconium and magnesium, improving both ductility and toughness while enhancing weldability. Compared to conventional austenitic stainless steel, the microalloyed nickel-saving austenitic stainless steel of this invention exhibits a significantly lower saturation current density at constant electrode potential and superior seawater corrosion resistance. This is due, on the one hand, to the significant modification of corrosion-active inclusions by the composite deoxidizing elements Zr and Mg, resulting in marked changes in their microstructure and morphology. The highly corrosive MnS is replaced by composite sulfur oxides, greatly reducing the density of corrosion-active inclusions and significantly improving resistance to localized seawater corrosion. On the other hand, the Ni and Cr elements from the laterite nickel ore enhance the corrosion resistance of the matrix, contributing to improved seawater corrosion resistance to a certain extent.

[0103] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0104] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a microalloyed nickel-saving austenitic stainless steel, characterized in that, Microalloyed nickel-saving austenitic stainless steel comprises the following chemical composition by mass percentage: C: 0.09-0.14%, Si: 0.19-0.49%, Mn: 7.1-7.4%, S≤0.0014%, P≤0.014%, Ni: 1.1-1.9%, Cr: 17.1-17.9%, Cu: 0.49-0.99%, N: 0.26-0.34%, 90×10 -4 %≤Zr+Mg≤130×10 -4 The percentage of Zr and Mg is %, and the content of Zr and Mg satisfies the following formula: Zr / Mg = 3-6, with the balance being Fe and unavoidable impurities; Includes the following steps: Step 1: Take the following raw materials according to the following weight ratio: 30-35% ultra-low P industrial pure iron, 15-20% high carbon ferrochrome, and 50-60% molten iron from laterite nickel ore smelting; first, add the ultra-low P industrial pure iron and the high carbon ferrochrome to the medium frequency furnace for melting, and then add the molten iron from laterite nickel ore smelting to the medium frequency furnace to obtain raw materials for smelting microalloyed nickel-saving austenitic stainless steel; Step 2: The smelting microalloyed nickel-saving austenitic stainless steel raw material obtained in Step 1 is sequentially refined, continuously cast, rolled and solution treated to obtain microalloyed nickel-saving austenitic stainless steel.

2. The method for preparing a microalloyed nickel-saving austenitic stainless steel according to claim 1, characterized in that, Microalloyed nickel-saving austenitic stainless steel comprises the following chemical composition by mass percentage: C: 0.09-0.10%, Si: 0.19-0.25%, Mn: 7.1-7.2%, S≤0.0011%, P≤0.011%, Ni: 1.1-1.4%, Cr: 17.1-17.5%, Cu: 0.49-0.59%, N: 0.26-0.30%, 90×10 -4 %≤Zr+Mg≤105×10 -4 The percentage of Zr and Mg is %, and the content of Zr and Mg satisfies the following formula: Zr / Mg = 3-6, with the balance being Fe and unavoidable impurities.

3. The method for preparing a microalloyed nickel-saving austenitic stainless steel according to claim 1, characterized in that, Microalloyed nickel-saving austenitic stainless steel includes the following chemical composition by mass percentage: C: 0.10-0.14%, Si: 0.25-0.49%, Mn: 7.2-7.4%, S≤0.0010%, P≤0.010%, Ni: 1.4-1.9%, Cr: 17.5-17.9%, Cu: 0.59-0.99%, N: 0.30-0.34%, 100×10 -4 %≤Zr+Mg≤110×10 -4 The percentage of Zr and Mg is %, and the content of Zr and Mg satisfies the following formula: Zr / Mg = 3-6, with the balance being Fe and unavoidable impurities.

4. The method for preparing a microalloyed nickel-saving austenitic stainless steel according to claim 1, characterized in that, Microalloyed nickel-saving austenitic stainless steel comprises the following chemical composition by mass percentage: C: 0.10%, Si: 0.25%, Mn: 7.2%, S: 0.0010%, P: 0.010%, Ni: 1.4%, Cr: 17.5%, Cu: 0.59%, N: 0.30%, Mg: 0.0025%, Zr: 0.0080%, with the balance being Fe and unavoidable impurities.

5. The method for preparing a microalloyed nickel-saving austenitic stainless steel according to claim 1, characterized in that, In step 1, the following raw materials are taken according to the following weight ratio: 30-34% ultra-low P industrial pure iron, 15-20% high carbon ferrochrome, and 52-60% molten iron smelted from laterite nickel ore.

6. The method for preparing a microalloyed nickel-saving austenitic stainless steel according to claim 1 or 5, characterized in that, In step 1, the weight contents of P and C in the ultra-low P industrial pure iron are: P≤0.0050%, C≤0.001%; ​​the weight contents of Cr and C in the high carbon ferrochrome are: Cr≥60%, C≤9.5%.

7. The method for preparing a microalloyed nickel-saving austenitic stainless steel according to claim 1 or 5, characterized in that, In step 1, the weight contents of the chemical components Cr and P in the molten iron produced by smelting laterite nickel ore are Cr≥60% and P≤0.03%, respectively.

8. The method for preparing a microalloyed nickel-saving austenitic stainless steel according to claim 1 or 5, characterized in that, In step 2, the specific steps for refining, continuous casting, rolling, and solution treatment of the smelting microalloyed nickel-saving austenitic stainless steel raw material obtained in step 1 are as follows: Step 2-1, Refining: The raw material of the smelting micro-alloyed nickel-saving austenitic stainless steel is refined outside the furnace in an LF furnace, and the chemical composition of the micro-alloyed nickel-saving austenitic stainless steel C, Si, Cr, Ni, Cu, P, S is finely adjusted and Zr, Mg, N are micro-alloyed to obtain micro-alloyed stainless steel. Step 2-2, continuous casting: The microalloyed stainless steel is continuously cast at a tundish temperature of 1465-1480℃ and a working casting speed of 1.05-1.20 m / min to obtain a continuously cast billet; Steps 2-3, rolling: The continuously cast billet is rolled at a temperature of 1150-1250℃ to obtain hot-rolled black coil; Steps 2-4, solution treatment: heat the hot-rolled black steel coil to a temperature of 1050-1150℃, and the solution treatment time is 0.6-1.0 mm / min. After cooling, microalloyed nickel-saving austenitic stainless steel is obtained.

9. The method for preparing a microalloyed nickel-saving austenitic stainless steel according to claim 8, characterized in that, In step 2-1, the refining time is 45-90 min and the furnace exit temperature is ≥1530℃.

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