High-corrosion-resistance austenitic stainless steel and preparation method thereof

Abstract

The invention discloses high-corrosion-resistance austenitic stainless steel and a preparation method thereof, and relates to the technical field of metallurgy. According to the high-corrosion-resistance austenitic stainless steel, the synergistic effect of the niobium, the rare earth and the boron is fully exerted, the inclusions penetrate through niobium armors, transmission and extension of pitting corrosion are blocked, in the high-corrosion-resistance austenitic stainless steel, the proportion of small-size inclusions smaller than 1 micrometer is larger than or equal to 75%, the proportion of large-size inclusions larger than 5 micrometers is smaller than or equal to 1%, the average diameter of the inclusions is smaller than or equal to 1.5 micrometers, and the corrosion resistance of the inclusions is greatly improved. The proportion of wrapped inclusions in total inclusions is larger than or equal to 80%, the average diameter of a niobium-containing phase without wrapped inclusions is smaller than 0.7 micrometer, compared with commercial austenitic stainless steel of the same brand, the corrosion rate is reduced by 1.5-3.6 mm.a <-1 > in the same environment, the pitting potential is improved by 500-800 mV, meanwhile, the mechanical property is guaranteed, and the service life of the stainless steel is prolonged. And the requirements on high corrosion resistance of austenitic stainless steel in the fields of ocean engineering, aerospace and the like are met.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a high corrosion-resistant austenitic stainless steel and a preparation method thereof. Background Art

[0002] Austenitic stainless steel is a type of iron-based alloy mainly composed of chromium and nickel, with a unique austenite crystal structure, in which the chromium content is generally above 16% and the nickel content is generally above 8%. The excellent corrosion resistance of austenitic stainless steel is one of its most outstanding characteristics. The chromium element will cause a dense Cr2O3 passivation film to form on its surface, effectively preventing the corrosive medium from corroding the matrix, thereby improving its corrosion resistance in corrosive media. At the same time, austenitic stainless steel also has good high temperature stability and mechanical properties. Therefore, austenitic stainless steel has been widely used in many fields such as chemical industry, marine engineering, medical equipment, and aerospace.

[0003] With the rapid development of modern industry, the service environment of high-end equipment is becoming increasingly harsh, and the demand for high-performance materials is increasing day by day. When faced with harsh corrosive environments such as high-concentration chloride ion media, high temperature, and strong acidity, traditional austenitic stainless steel often suffers from severe corrosion and fracture failure. Non-metallic inclusions are one of the main factors leading to corrosion. To this end, researchers have studied a variety of methods to reduce the hazards of inclusions, such as deep deoxidation, deep desulfurization, and modification treatment. However, the effects of these methods are limited, and corrosion may still occur in inclusions and their surrounding matrix. Summary of the invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art or related art.

[0005] To this end, the first aspect of the present invention provides a high corrosion resistant austenitic stainless steel, which includes the following elements, measured by mass percentage: C≤0.02%, Si≤1.00%, Mn≤2.50%, Cr: 16.00%-20.00%, Ni: 10.00%-15.00%, Mo: 2.00%-4.00%, Nb, RE: 0.009%-0.025%, B: 0.0015%-0.0055%, N: 0.10%-0.25%, O≤0.003%, P≤0.02%, S≤0.003%, Al≤0.03%, and the balance is Fe, wherein RE is one or more of Ce, La and Y;

[0006] The mass percentage of the Nb element in the highly corrosion-resistant austenitic stainless steel satisfies the following formula:

[0007] w[%Nb]=m×(w[%Cr]+w[%Mo]+w[%N]);

[0008] When 18.10≤(w[%Cr]+w[%Mo]+w[%N])≤21.16, m is 0.19% to 0.94%;

[0009] When 21.16<(w[%Cr]+w[%Mo]+w[%N])≤24.25, m is 0.15%~0.72%;

[0010] In the formula, w[%Nb] is the mass percentage of Nb in the high corrosion resistant austenitic stainless steel, w[%Cr] is the mass percentage of Cr in the high corrosion resistant austenitic stainless steel, w[%Mo] is the mass percentage of Mo in the high corrosion resistant austenitic stainless steel, and w[%N] is the mass percentage of N in the high corrosion resistant austenitic stainless steel.

[0011] Further, the highly corrosion-resistant duplex stainless steel includes the following elements in percentage by mass: C≤0.015%, Si≤0.5%, Mn 0.5%-2.0%, Cr: 17.00%-19.00%, Ni: 12.50%-14.00%, Mo: 2.50%-3.50%, Nb, RE: 0.012%-0.019%, B: 0.002%-0.004%, N: 0.15%-0.20%, O≤0.002%, P≤0.02%, S≤0.0015%, Al≤0.02%, and the balance is Fe, wherein RE is one or more of Ce, La and Y;

[0012] The mass percentage of the Nb element in the highly corrosion-resistant austenitic stainless steel satisfies the following formula:

[0013] w[%Nb]=m×(w[%Cr]+w[%Mo]+w[%N]);

[0014] When 19.65≤(w[%Cr]+w[%Mo]+w[%N])≤21.16, m is 0.17% to 0.94%;

[0015] When 21.16<(w[%Cr]+w[%Mo]+w[%N])≤22.70, m is 0.15%~0.77%;

[0016] In the formula, w[%Nb] is the mass percentage of Nb in the high corrosion resistant austenitic stainless steel, w[%Cr] is the mass percentage of Cr in the high corrosion resistant austenitic stainless steel, w[%Mo] is the mass percentage of Mo in the high corrosion resistant austenitic stainless steel, and w[%N] is the mass percentage of N in the high corrosion resistant austenitic stainless steel.

[0017] Furthermore, in the highly corrosion-resistant austenitic stainless steel, the proportion of small-sized inclusions less than 1 μm is ≥75%, the proportion of large-sized inclusions greater than 5 μm is ≤1%, the average diameter of the inclusions is ≤1.5 μm, the proportion of enclosed inclusions to total inclusions is ≥80%, and the average diameter of the niobium-containing phase of the unenclosed inclusions is less than 0.7 μm. Compared with the commercial austenitic stainless steel of the same grade, the corrosion rate in the same environment is reduced by 1.5 mm·a -1 ~3.6mm·a -1 , the pitting potential increased by 500mV to 800mV.

[0018] A second aspect of the present invention provides a method for preparing highly corrosion-resistant austenitic stainless steel, comprising:

[0019] Smelting, according to the chemical composition of the above-mentioned high corrosion resistant austenitic stainless steel, except for the niobium raw material, smelting to obtain molten steel;

[0020] Casting and niobium microalloying, controlling the casting temperature to 1480℃-1520℃, filling with high-purity nitrogen, performing protective casting and niobium microalloying, and controlling the cooling intensity during the casting process to ensure that the niobium-containing phase precipitates without excessive growth, to obtain an ingot or billet;

[0021] Forging or rolling; wherein, for the forging, the initial temperature is 1140°C-1200°C, and the final forging temperature is 1020°C-1060°C; for the rolling, the initial temperature is 1140°C-1200°C, and the final rolling temperature is 1020°C-1060°C;

[0022] Heat treatment, subjecting the hot-processed bars or rolled plates to a solid solution treatment at a temperature of 1040°C-1080°C.

[0023] Furthermore, the smelting includes:

[0024] Melting: According to the chemical composition of the highly corrosion-resistant austenitic stainless steel, ingredients are prepared, and except for nitrided alloy, metallic aluminum, nickel-magnesium alloy, boron alloying raw material, rare earth raw material and niobium raw material, the remaining raw materials are melted in the furnace to obtain molten steel;

[0025] Nitrogen alloying: After the molten steel is melted, the temperature is controlled to 1520°C-1540°C, high-purity nitrogen is filled in and nitriding alloy is added for nitriding;

[0026] Boron microalloying: After nitrogen alloying, boron alloying raw materials are added to the molten steel;

[0027] Aluminum deoxidation: After boron microalloying, metallic aluminum is added for deoxidation;

[0028] Nickel-magnesium deoxidation and desulfurization: After aluminum deoxidation is completed, the temperature of the molten steel is lowered to 1420℃-1450℃, and nickel-magnesium alloy is added for deoxidation and desulfurization;

[0029] Rare earth microalloying: After the nickel-magnesium deoxidation and desulfurization are completed, the molten steel is heated to 1550℃-1570℃, and rare earth inoculants or rare earth metals are added to the molten steel, preferably rare earth inoculants; after rare earth alloying, nitrogen is blown from the bottom to allow large particles of rare earth inclusions to float up and be removed in the molten steel.

[0030] Furthermore,

[0031] The smelting includes: the material of the refractory is Al2O3, MgO-Al2O3, MgO-REO or REO, preferably REO;

[0032] The nitrogen alloying comprises: the nitriding alloy is chromium nitride, and the mass percentage of nitrogen in the chromium nitride is 8%-12%;

[0033] The boron microalloying comprises: the boron alloying raw material is a B-Fe alloy or metal B, the mass percentage of B in the B-Fe alloy is 15% to 20%, and the mass percentage of B in the metal B is ≥99.9%;

[0034] The aluminum deoxidation comprises: the mass percentage of Al in the metal aluminum is ≥ 99%; the amount of metal aluminum added is determined according to w[%Al]=2.5×(w[%O]-0.005%) / A1, wherein w[%Al] is the mass percentage of Al added to the molten steel, w[%O] is the mass percentage of initial O in the molten steel, and A1 is a coefficient, and the range of A1 is 85% to 95%;

[0035] The nickel-magnesium deoxidation and desulfurization method comprises: the mass percentage of Mg in the nickel-magnesium alloy is 18% to 22%; the amount of the nickel-magnesium alloy added is calculated according to w[%Mg]=1.6×(w[%O] Al +0.6w[%S] Al -0.003%) / A2, where w[%Mg] is the mass percentage of Mg added to the molten steel through the nickel-magnesium alloy, and w[%O] Al is the mass percentage of O in the molten steel after Al deoxidation, w[%S] Al is the mass percentage of S in the steel liquid after Al deoxidation, A2 is the coefficient, and the range of A2 is 15% to 25%;

[0036] The rare earth microalloying method comprises: the rare earth inoculant comprises Si: 35% to 40%, RE: 25% to 30%, Ca: 10% to 15%, and the balance is Fe and other inevitable impurities; the mass percentage of rare earth in the rare earth metal is ≥ 99.9%; the amount of rare earth added is calculated according to w[%RE]=(6.30×w[%O] Mg +1.26×w[%S] Mg-0.004%) / A3, where w[%RE] is the mass percentage of RE added to the molten steel, w[%O] Mg is the mass percentage of O in the molten steel after deoxidation of nickel-magnesium alloy, w[%S] Mg is the mass percentage of S in the molten steel after desulfurization of Ni-Mg alloy, A3 is the coefficient, and the range of A3 is 50% to 60%; after rare earth alloying, nitrogen is blown at the bottom for 4min-6min, and the nitrogen flow rate is 2NL·min -1 ·t -1 ~4NL·min -1 ·t -1 .

[0037] Furthermore, the casting includes die casting or continuous casting; wherein,

[0038] The die casting comprises: after casting molten steel into the mold, first cooling and second cooling are performed in sequence, the cooling method of the mold side wall is water cooling, the cooling water volume Q1 of the first cooling is determined according to Q1=(76S+7.94)(0.95T1-150) / B1, and the cooling water volume Q2 of the second cooling is determined according to Q2=(84S+7.94)T1 / B2, wherein Q1 is the cooling water volume of the first cooling (L·min -1 ), Q2 is the cooling water volume of the second cooling (L·min -1 ), S is the cross-sectional area of ​​the ingot (m 2 ), T1 is the casting temperature (°C), B1 is the coefficient, the range of B1 is 34 to 42, B2 is the coefficient, the range of B2 is 15 to 20;

[0039] The continuous casting comprises: controlling the water volume of the first section of the secondary cooling zone to be 0.05 L·m -2 ·s -1 ~0.07Lm -2 ·s -1 , control the water volume of the second last section of the secondary cooling zone to 0.34L·m -2 ·s -1 ~0.46L·m -2 ·s -1 .

[0040] Furthermore, the niobium microalloying comprises: feeding ferroniobium cored wire during casting, and controlling the cooling intensity to perform niobium microalloying in a temperature range where niobium-containing phases precipitate; wherein the mass percentage of niobium in the ferroniobium cored wire is 50% to 55%; and the feeding speed v of the ferroniobium cored wire is determined according to v=(d1+1220)T1 / C, wherein v is the feeding speed of the ferroniobium cored wire (m·min -1), d1 is the ingot diameter or the billet thickness (mm), T1 is the casting temperature (℃), C is the coefficient, and the range of C is 13500~15200.

[0041] Further, the forging, the initial temperature is 1140 ℃ -1200 ℃, the holding time is t1, the forging number is ≥3, the final forging temperature is 1020 ℃ -1060 ℃, and water cooling is used after the forging is completed; the holding time t1 is determined according to t1 = D (d1 + 85) / T2, wherein t1 is the holding time after reaching the temperature (min), T2 is the initial forging temperature (℃), d1 is the ingot diameter (mm), D is the coefficient, and the range of D is 520-700;

[0042] Furthermore, the rolling has an initial temperature of 1140°C-1200°C, a holding time of t2, no less than 4 rolling passes, a reduction rate of no less than 30% for each pass, a final rolling temperature of 1020°C-1060°C, and water cooling is used after rolling is completed; the holding time t2 is determined according to t2=E(d2+42) / T3, wherein t2 is the holding time after reaching the temperature (min), T3 is the initial rolling temperature (°C), d2 is the plate thickness (mm), and E is a coefficient, and the range of E is 1420~1660.

[0043] Furthermore, the solution treatment includes: a solution temperature of 1040°C-1080°C, a solution time t3 determined according to t3=F(d3-4.5) / T4+10, wherein t3 is the solution time (min), T4 is the solution temperature (°C), d3 is the material diameter or thickness (mm), F is a coefficient, and the range of F is 900-1000.

[0044] The third aspect of the present invention provides an application of a highly corrosion-resistant austenitic stainless steel. The highly corrosion-resistant austenitic stainless steel is used in fields that have high requirements on the corrosion resistance of stainless steel materials, including the chemical industry, medical equipment, marine engineering and aerospace fields.

[0045] Compared with the prior art, the present invention has at least the following beneficial effects:

[0046] The high corrosion-resistant austenitic stainless steel provided by the present invention gives full play to the synergistic effect of the three elements of niobium, rare earth and boron, puts on "niobium armor" for inclusions, blocks the propagation and extension of pitting corrosion, and in the high corrosion-resistant austenitic stainless steel, the proportion of small-sized inclusions less than 1 μm is ≥75%, the proportion of large-sized inclusions greater than 5 μm is ≤1%, the average diameter of the inclusions is ≤1.5 μm, the proportion of the enclosed inclusions in the total inclusions is ≥80%, and the average diameter of the niobium-containing phase of the unenclosed inclusions is less than 0.7 μm. Compared with the commercial austenitic stainless steel of the same grade, the corrosion rate in the same environment is reduced by 1.5 mm·a -1 ~3.6mm·a -1The pitting potential is increased by 500mV to 800mV, which significantly improves the corrosion resistance of austenitic stainless steel while ensuring the mechanical properties. Compared with the same grade of commercial austenitic stainless steel, the strength and toughness are slightly improved. It effectively solves the corrosion failure problem caused by inclusions, thereby meeting the high corrosion resistance requirements of austenitic stainless steel in the fields of marine engineering, aerospace, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is a high-magnification morphology image of a typical niobium-containing phase inclusion in austenitic stainless steel provided in Example 2 of the present application;

[0048] Figure 2 This is a graph showing the immersion corrosion results of austenitic stainless steel provided in Example 3 of the present application;

[0049] Figure 3 This is a diagram of the electrochemical corrosion results of austenitic stainless steel provided in Example 4 of the present application. DETAILED DESCRIPTION

[0050] In order to better understand the above-mentioned technical scheme, the technical scheme of the embodiments of the present application is described in detail below through the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present application and the specific features in the embodiments are detailed descriptions of the technical scheme of the embodiments of the present application, rather than limitations on the technical scheme of the present application. In the absence of conflict, the embodiments of the present application and the technical features in the embodiments may be combined with each other.

[0051] In the examples of the present invention, unless otherwise specified, the raw materials and equipment used are commercially available.

[0052] The first aspect of the embodiment of the present invention comprises the following elements in terms of mass percentage: in terms of mass percentage, the following elements are included: C≤0.02%, Si≤1.00%, Mn≤2.50%, Cr: 16.00%-20.00%, Ni: 10.00%-15.00%, Mo: 2.00%-4.00%, Nb: content satisfies formula 1, RE: 0.009%-0.025%, B: 0.0015%-0.0055%, N: 0.10%-0.25%, O≤0.003%, P≤0.02%, S≤0.003%, Al≤0.03%, and the balance is Fe, wherein RE is one or more of Ce, La and Y;

[0053] The mass percentage of the Nb element in the highly corrosion-resistant austenitic stainless steel satisfies the following formula 1:

[0054] w[%Nb]=m×(w[%Cr]+w[%Mo]+w[%N]) (Formula 1);

[0055] When 18.10≤(w[%Cr]+w[%Mo]+w[%N])≤21.16, m is 0.19% to 0.94%;

[0056] When 21.16<(w[%Cr]+w[%Mo]+w[%N])≤24.25, m is 0.15%~0.72%;

[0057] Among them, w[%Nb] is the mass percentage of Nb in the highly corrosion-resistant austenitic stainless steel, w[%Cr] is the mass percentage of Cr in the highly corrosion-resistant austenitic stainless steel, w[%Mo] is the mass percentage of Mo in the highly corrosion-resistant austenitic stainless steel, and w[%N] is the mass percentage of N in the highly corrosion-resistant austenitic stainless steel.

[0058] Preferably, a highly corrosion-resistant austenitic stainless steel comprises the following elements by mass percentage:

[0059] C≤0.015%;

[0060] Si≤0.5%;

[0061] Mn 0.5% to 2.0%, more preferably Mn 1% to 1.5%;

[0062] Cr 17.00% to 19.00%, more preferably 18.00% to 19.00%;

[0063] Ni 12.50%~14.00%;

[0064] Mo 2.50% to 3.50%, more preferably 2.50% to 3.00%;

[0065] The Nb content satisfies formula 2;

[0066] RE 0.012%~0.019%; RE is one, two or three of Ce, La and Y;

[0067] B 0.002%~0.004%;

[0068] N 0.15%~0.20%;

[0069] O≤0.002%;

[0070] P≤0.01%;

[0071] S≤0.0015%;

[0072] Al≤0.02%;

[0073] Fe excess;

[0074] The mass percentage of Nb element in high corrosion resistant austenitic stainless steel satisfies the following formula 2:

[0075] w[%Nb]=m×(w[%Cr]+w[%Mo]+w[%N]) (Formula 2);

[0076] When 19.65≤(w[%Cr]+w[%Mo]+w[%N])≤21.16, m is 0.17% to 0.94%;

[0077] When 21.16<(w[%Cr]+w[%Mo]+w[%N])≤22.70, m is 0.15%~0.77%;

[0078] Among them, w[%Nb] is the mass percentage of Nb in the highly corrosion-resistant austenitic stainless steel, w[%Cr] is the mass percentage of Cr in the highly corrosion-resistant austenitic stainless steel, w[%Mo] is the mass percentage of Mo in the highly corrosion-resistant austenitic stainless steel, and w[%N] is the mass percentage of N in the highly corrosion-resistant austenitic stainless steel.

[0079] It can be understood that, as an impurity element, the lower the content of O, the better; as an impurity element, the lower the content of S, the better; as an impurity element, the lower the content of Al, the better; as an impurity element, the lower the content of P, the better.

[0080] The highly corrosion-resistant austenitic stainless steel provided in the embodiment of the present invention can form a highly corrosion-resistant niobium-containing Z phase during the solidification process by adding Nb elements to the N-containing austenitic stainless steel. According to the heterogeneous nucleation theory, the low-melting-point precipitated phase can precipitate with the high-melting-point inclusion as the core, thereby forming a structure in which the niobium-containing phase encapsulates the inclusion. This encapsulation structure can isolate the inclusions from the corrosive environment, effectively block the corrosion caused by the inclusions, and improve the corrosion resistance of the stainless steel. The addition of rare earths forms fine dispersed rare earth inclusions in the molten steel during deep deoxidation and deep desulfurization, providing more nucleation sites for the heterogeneous nucleation of the niobium-containing phase, and promoting the formation of the niobium-containing phase encapsulating the inclusion structure. The rare earth cations released by the dissolution of a few rare earth inclusions that are not encapsulated by the niobium-containing phase in the corrosive environment can also inhibit the combination of corrosive ions with the matrix, thereby slowing down the expansion and extension of pitting corrosion. However, the hot working and heat treatment processes may promote the precipitation and growth of the niobium-containing phase at the grain boundary, which has an adverse effect on the mechanical properties. To solve this problem, the present invention adds a trace amount of B element to the steel, utilizes the B element to preferentially segregate at the grain boundaries and occupy the grain boundaries, reduces the precipitation sites of the niobium-containing phase at the grain boundaries, and inhibits the precipitation of the niobium-containing phase at the grain boundaries, thereby achieving the purpose of ensuring good mechanical properties.

[0081] The high corrosion-resistant austenitic stainless steel provided in the embodiment of the present invention gives full play to the synergistic effect of the three elements of niobium, rare earth and boron, puts on "niobium armor" for inclusions, and blocks the propagation and extension of pitting corrosion. In the high corrosion-resistant austenitic stainless steel, the proportion of small-sized inclusions less than 1 μm is ≥75%, the proportion of large-sized inclusions greater than 5 μm is ≤1%, the average diameter of the inclusions is ≤1.5 μm, the proportion of enclosed inclusions in the total inclusions is ≥80%, and the average diameter of the niobium-containing phase of the unenclosed inclusions is less than 0.7 μm. Compared with the commercial austenitic stainless steel of the same grade, the corrosion rate in the same environment is reduced by 1.5 mm·a -1 ~3.6mm·a -1 The pitting potential is increased by 500mV to 800mV, which significantly improves the corrosion resistance of austenitic stainless steel while ensuring that the mechanical properties are slightly improved in strength and toughness compared with the same grade of commercial austenitic stainless steel. It effectively solves the corrosion failure problem caused by inclusions, thereby meeting the high corrosion resistance requirements of austenitic stainless steel in the fields of marine engineering, aerospace, etc.

[0082] A second aspect of an embodiment of the present invention provides a method for preparing highly corrosion-resistant austenitic stainless steel, comprising:

[0083] Smelting, according to the chemical composition of the above-mentioned high corrosion resistant austenitic stainless steel, except for the niobium raw material, smelting to obtain molten steel;

[0084] Casting and niobium microalloying, controlling the casting temperature to 1480℃-1520℃, filling with high-purity nitrogen, performing protective casting and niobium microalloying, and controlling the cooling intensity during the casting process to ensure that the niobium-containing phase precipitates without excessive growth, to obtain an ingot or billet;

[0085] Forging or rolling; wherein, for the forging, the initial temperature is 1140°C-1200°C, and the final forging temperature is 1020°C-1060°C; for the rolling, the initial temperature is 1140°C-1200°C, and the final rolling temperature is 1020°C-1060°C;

[0086] Heat treatment, subjecting the hot-processed bars or rolled plates to a solid solution treatment at a temperature of 1040°C-1080°C.

[0087] In some embodiments, the smelting includes:

[0088] Melting: According to the chemical composition of the highly corrosion-resistant austenitic stainless steel, ingredients are prepared, and except for nitrided alloy, metallic aluminum, nickel-magnesium alloy, boron alloying raw material, rare earth raw material and niobium raw material, the remaining raw materials are melted in the furnace to obtain molten steel;

[0089] Nitrogen alloying: After the molten steel is melted, the temperature is controlled to 1520°C-1540°C, high-purity nitrogen is filled in and nitriding alloy is added for nitriding;

[0090] Boron microalloying: After nitrogen alloying, boron alloying raw materials are added to the molten steel;

[0091] Aluminum deoxidation: After boron microalloying, metallic aluminum is added for deoxidation;

[0092] Nickel-magnesium deoxidation and desulfurization: After aluminum deoxidation is completed, the temperature of the molten steel is lowered to 1420℃-1450℃, and nickel-magnesium alloy is added for deoxidation and desulfurization;

[0093] Rare earth microalloying: After the nickel-magnesium deoxidation and desulfurization, the molten steel is heated to 1550℃-1570℃, and rare earth inoculants or rare earth metals are added to the molten steel, preferably rare earth inoculants; after rare earth alloying, nitrogen is blown from the bottom to fully float and remove large particles of rare earth inclusions in the molten steel;

[0094] In some embodiments, smelting includes: the material of the refractory is Al2O3 or MgO-Al2O3 or MgO-REO or REO; preferably MgO-REO or REO, further preferably REO, using REO refractory to reduce rare earth element burnout and improve rare earth recovery rate.

[0095] In some embodiments, nitrogen alloying includes: the nitriding alloy is chromium nitride, and the mass percentage of nitrogen in the chromium nitride is 8%-12%, more preferably 11-12%. Through alloy nitriding, the nitrogen content in the austenitic stainless steel is ensured to reach the target composition.

[0096] In some embodiments, boron microalloying is performed on the nitrided molten steel, and the boron microalloying includes: the boron alloying raw material is a B-Fe alloy or metal B, the mass percentage of B in the B-Fe alloy is 15% to 20%, preferably 17.5% to 20%, and the mass percentage of B in the metal B is ≥99.9%; through the boron microalloying, the precipitation of the niobium-containing phase at the grain boundary is suppressed during the hot working and heat treatment process, and the adverse effect of excessive growth of the niobium-containing phase on the mechanical properties is prevented.

[0097] In some embodiments, metal aluminum particles are added to the boron microalloyed molten steel for deoxidation, and aluminum deoxidation includes: the mass percentage of Al in the metal aluminum is ≥99%; the amount of metal aluminum added is determined according to w[%Al]=2.5×(w[%O]-0.005%) / A1, wherein w[%Al] is the mass percentage of Al added to the molten steel, w[%O] is the mass percentage of initial O in the molten steel, and A1 is a coefficient, and the range of A1 is 85% to 95%, preferably A1 is 95%; by adding metal aluminum for deoxidation, the O content in the molten steel can be reduced, and the subsequent deoxidation efficiency of the nickel-magnesium alloy can be improved.

[0098] In some embodiments, after the aluminum deoxidation operation is completed, Ni-Mg alloy is added to the aluminum deoxidized molten steel for deoxidation and desulfurization. Before adding the Ni-Mg alloy, the temperature of the molten steel is reduced to 1420°C-1450°C, and the deoxidation and desulfurization efficiency of the Ni-Mg alloy is improved by appropriately reducing the temperature at which the Ni-Mg alloy is added. The nickel-magnesium deoxidation and desulfurization process comprises: the mass percentage of Mg in the nickel-magnesium alloy is 18% to 22%; the amount of the nickel-magnesium alloy added is calculated according to w[%Mg]=1.6×(w[%O] Al +0.6w[%S] Al -0.003%) / A2, where w[%Mg] is the mass percentage of Mg added to the molten steel through the nickel-magnesium alloy, and w[%O] Al is the mass percentage of O in the molten steel after Al deoxidation, w[%S] Al is the mass percentage of S in the steel liquid after Al deoxidation, A2 is a coefficient, the range of A2 is 15% to 25%, and preferably A2 is 20%. By adding nickel-magnesium alloy for deoxidation, the content of O and S in the steel liquid can be reduced, and the subsequent recovery rate of rare earth can be improved.

[0099] In some embodiments, after the nickel-magnesium deoxidation and desulfurization operation is completed, a rare earth inoculant is added to the steel liquid after the nickel-magnesium deoxidation and desulfurization for rare earth alloying. The rare earth inoculant composition is Si: 35% to 40%, RE: 25% to 30%, Ca: 10% to 15%, and the balance is Fe and other unavoidable impurities; the addition temperature of the rare earth inoculant is 1550°C-1570°C. The amount of rare earth added is calculated according to w[%RE]=(6.30×w[%O] Mg +1.26×w[%S] Mg -0.004%) / A3, where w[%RE] is the mass percentage of RE added to the molten steel, w[%O] Mg is the mass percentage of O in the molten steel after deoxidation of nickel-magnesium alloy, w[%S] Mg is the mass percentage of S in the molten steel after desulfurization of Ni-Mg alloy, A3 is the coefficient, the range of A3 is 50% to 60%, and A3 is preferably 55%; by adding rare earth inoculants or rare earth metals for rare earth microalloying, the O and S contents in the molten steel can be reduced, and the inclusions can be finely dispersed, providing nucleation sites for the subsequent nucleation of niobium-containing phases. After rare earth alloying, bottom blowing nitrogen for 4min-6min, the nitrogen flow rate is 2NL·min -1 ·t -1 ~4NL·min -1 ·t -1 By blowing nitrogen at the bottom and stirring, large rare earth inclusions can be fully floated and removed in the molten steel.

[0100] In some embodiments, after rare earth microalloying is completed, niobium microalloying is performed by feeding ferroniobium cored wire during the casting process. Under nitrogen protection, the casting is performed while controlling the appropriate cooling intensity. By controlling the cooling rate during the casting process, the niobium-containing phase can precipitate within the precipitation temperature range without excessive growth, and the average diameter of the niobium-containing phase not encapsulating inclusions is ≤0.7 μm.

[0101] Casting includes mold casting or continuous casting; wherein,

[0102] The die casting comprises: after casting molten steel into the mold, first cooling and second cooling are performed in sequence, the cooling method of the mold side wall is water cooling, the cooling water volume Q1 of the first cooling is determined according to Q1=(76S+7.94)(0.95T1-150) / B1, and the cooling water volume Q2 of the second cooling is determined according to Q2=(84S+7.94)T1 / B2, wherein Q1 is the cooling water volume of the first cooling (L·min -1 ), Q2 is the cooling water volume of the second cooling (L·min -1 ), S is the cross-sectional area of ​​the ingot (m 2 ), T1 is the casting temperature (°C), B1 is the coefficient, the range of B1 is 34 to 42, preferably B1 is 38, B2 is the coefficient, the range of B2 is 15 to 20, preferably B2 is 15;

[0103] The continuous casting comprises: controlling the water volume of the first section of the secondary cooling zone to be 0.05 L·m -2 ·s -1 ~0.07Lm -2 ·s -1 , control the water volume of the second last section of the secondary cooling zone to 0.34L·m -2 ·s -1 ~0.46L·m -2 ·s -1 .

[0104] Furthermore, during the casting process, the ferroniobium cored wire is fed, and within the temperature range where the niobium phase precipitates, the cooling intensity is controlled to perform niobium microalloying; wherein the mass percentage of niobium in the ferroniobium cored wire is 50% to 55%; the feeding speed v of the ferroniobium cored wire is determined according to v=(d1+1220)T1 / C, wherein v is the feeding speed of the ferroniobium cored wire (m·min -1 ), d1 is the ingot diameter or the billet thickness (mm), T1 is the casting temperature (℃), C is the coefficient, and the range of C is 13500-15200, preferably C is 14500. By feeding the ferroniobium core wire, the niobium recovery rate is improved, and the premature formation of niobium-containing phase in the molten steel is avoided, which is conducive to promoting the precipitation of niobium during the solidification process.

[0105] In some embodiments, the forging, initial temperature is 1140℃-1200℃, holding time t1, forging number ≥3, final forging temperature is 1020℃-1060℃, water cooling is used after forging is completed; the holding time t1 is determined according to t1=D(d1+85) / T2, wherein t1 is the holding time after reaching the temperature (min), T2 is the initial forging temperature (℃), d1 is the ingot diameter (mm), D is the coefficient, D ranges from 520 to 700, preferably D is 630. Through a suitable forging process, the unencapsulated niobium-containing phase is broken and the grains are refined.

[0106] In some embodiments, the rolling, the initial temperature is 1140 ℃ -1200 ℃, the holding time t2 is not less than 4 times, the reduction rate of each time is not less than 30%, the final rolling temperature is 1020 ℃ -1060 ℃, and water cooling is used after the rolling is completed; the holding time t2 is determined according to t2 = E (d2 + 42) / T3, wherein t2 is the holding time after reaching the temperature (min), T3 is the initial rolling temperature (℃), d2 is the plate thickness (mm), E is a coefficient, and the range of E is 1420 ~ 1660, preferably E is 1580. Through a suitable rolling process, the unwrapped niobium-containing phase is broken and the grains are refined.

[0107] In some embodiments, the heat treatment includes: a solution treatment temperature of 1040°C-1080°C, a solution time t3 determined according to t3=F(d3-4.5) / T4+10, wherein t3 is the solution time (min), T4 is the solution temperature (°C), d3 is the material diameter or thickness (mm), F is a coefficient, and the range of F is 900-1000, preferably F is 950. Through a suitable solution treatment system, further precipitation of niobium-containing phases is promoted to encapsulate inclusions, ensuring that the grains recover completely without excessive growth.

[0108] In summary, the highly corrosion-resistant austenitic stainless steel provided in the embodiment of the present invention utilizes rare earth to refine and modify inclusions, promoting the corrosion-resistant niobium-containing phase to encapsulate inclusions; a small amount of rare earth inclusions not encapsulated by the niobium-containing phase form rare earth ions after dissolution, which can reduce the carrier density of chloride ions and inhibit the nucleation of pitting corrosion; at the same time, niobium and rare earth can also participate in the formation of passivation film. Adding boron elements that are easily segregated at grain boundaries to steel can inhibit the precipitation and growth of niobium-containing phases at grain boundaries, while improving corrosion resistance and ensuring good mechanical properties. Thereby meeting the requirements for high corrosion resistance of key stainless steel materials in the fields of chemical industry (corrosion-resistant storage tanks, heat exchangers, etc.), medical equipment (surgical instruments, disinfection instrument housings, etc.), marine engineering (submarine oil and gas pipelines, key structural components of ships, etc.), aerospace (turbine blades, fuel nozzles, etc.).

[0109] In the method for preparing high corrosion-resistant austenitic stainless steel provided by the embodiment of the present invention, Al is first used for pre-deoxidation, which can improve the deoxidation efficiency of subsequent Mg deoxidation, is beneficial to improve the cleanliness of molten steel, and improve the rare earth recovery rate. Rare earth is preferably added to the molten steel in the form of rare earth inoculant, which is more conducive to the formation of fine dispersed rare earth inclusions, and provides more effective nucleation sites for the subsequent heterogeneous nucleation of niobium-containing phases. It is preferred to feed niobium iron cored wire for niobium microalloying, and control the appropriate cooling intensity during the cooling process, while ensuring that the niobium-containing phase fully wraps the inclusions, while avoiding the deterioration of the mechanical properties caused by the excessive growth of the niobium-containing phase. During the casting process, in the first half of the high-temperature interval when the niobium-containing phase is precipitated and the harmful phase has not yet precipitated, a lower cooling intensity is used so that the niobium-containing phase precipitates and wraps the inclusions; in the second half of the low-temperature section, the cooling intensity is rapidly increased to accelerate the cooling to prevent the precipitation of harmful phases and the excessive growth of the niobium-containing phase. Subsequent heat processing and heat treatment processes can fully refine the grains, promote the precipitation of niobium-containing phases to further wrap inclusions, break up large-sized niobium-containing phases that are not wrapped in inclusions, and improve the corrosion resistance and mechanical properties of the material.

[0110] The high corrosion-resistant austenitic stainless steel and the preparation method thereof provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0111] Example 1 Preparation of microalloyed corrosion-resistant austenitic stainless steel

[0112] This embodiment provides a method for improving the corrosion resistance of austenitic stainless steel. Examples 1# to 4# are steel grades smelted by the present invention, and comparative examples 5# to 8# are commercial austenitic stainless steels available on the market. Examples 1# and 4# are compared with examples 5# and 8#, and examples 2# and 3# are compared with examples 6# and 7#.

[0113] Step 1: Metallurgy of microalloyed corrosion-resistant austenitic stainless steel

[0114] (1) According to the target composition shown in Table 1, the remaining raw materials except chromium nitride, metal aluminum, nickel-magnesium alloy, boron alloying raw material, niobium-iron alloy and rare earth are smelted in the furnace. After the molten steel is completely melted, the temperature is raised to 1520° C.-1540° C., nitrogen is charged and chromium nitride (N content is 11.78%) is added to carry out nitriding operation.

[0115] Table 1 Composition of steel grades in 1# to 4# examples and 5# to 8# comparative examples (wt.%)

[0116]

[0117] (2) After the nitriding process is completed, the temperature of the molten steel is controlled to be 1530-1550°C, and B-Fe alloy (B content is 17.58%) is added.

[0118] (3) After B microalloying, metallic aluminum particles are added for deoxidation (Al content is 99.7%); after aluminum deoxidation, the molten steel temperature is controlled at 1420°C-1450°C, and Ni-Mg alloy (Mg content is 22.74%) is added for deoxidation and desulfurization; the deoxidation and desulfurization process parameters are shown in Table 2.

[0119] (4) After the nickel-magnesium deoxidation and desulfurization, the molten steel is heated to 1550°C-1570°C, and rare earth inoculants (Si: 38%, RE: 29%, Ca: 10%, the balance is Fe and other inevitable impurities) are added to the molten steel. After the rare earth inoculants are completely melted, the temperature of the molten steel is kept stable, and nitrogen is blown from the bottom for 2-4 minutes to fully float and remove large-sized inclusions. The nitrogen purity is ≥99.999%. After the aluminum deoxidation, nickel-magnesium deoxidation and desulfurization, and rare earth microalloying are completed, samples are taken to test the O and S contents in the molten steel, and the test results are shown in Table 3.

[0120] Table 2 Deoxidation, desulfurization and rare earth microalloying process parameters of Examples 1# to 4#

[0121]

[0122] Table 3 Oxygen and sulfur content in molten steel after nickel-magnesium deoxidation and desulfurization and rare earth microalloying in Examples 1# to 4#

[0123]

[0124] (5) After rare earth alloying, the casting temperature is controlled to be 1480℃-1520℃, and high-purity nitrogen is charged into the molten steel, and the purity of nitrogen is ≥99.999%; the feeding speed of ferroniobium core wire is controlled, and the casting parameters are controlled to obtain 1#~2# ingots and 3#~4# billets. The specific niobium microalloying and casting process are shown in Table 4.

[0125] Table 4 Niobium microalloying and casting process parameters for Examples 1# to 4#

[0126]

[0127]

[0128] Step 2: Hot working and heat treatment of microalloyed corrosion resistant austenitic stainless steel

[0129] (1) The 1#~2# ingots are heated to the furnace temperature and kept warm. After the holding time is reached, the ingots are taken out for surface descaling. The start forging temperature and the final forging temperature are controlled to fully break up the unwrapped inclusions and refine the grains. After forging, they are water-cooled. The specific forging process is shown in Table 5.

[0130] (2) The 3#~4# ingots are heated to the start rolling temperature and kept warm. After the holding time is reached, the start rolling temperature, reduction ratio and final rolling temperature are controlled to fully break up the unwrapped inclusions and refine the grains. After rolling is completed, water cooling is used. The specific rolling process is shown in Table 5.

[0131] (3) Heat the heating furnace to the solution treatment temperature. After the furnace temperature stabilizes, load the 1# to 4# plates into the heating furnace, strictly control the insulation time, and immediately water cool after the insulation time. The specific solution treatment process is shown in Table 5.

[0132] Table 5 Forging and rolling process parameters of Examples 1# to 4#

[0133]

[0134]

[0135] Example 2 Statistics of inclusions and niobium-containing phases in austenitic stainless steel

[0136] 1#~4# example steels and commercially available 5#~8# comparative steels were prepared into 10 mm×10 mm metallographic specimens by wire cutting, ground and polished, and the inclusions and niobium-containing phases were analyzed and counted by metallographic microscope, scanning electron microscope and IPP6.0 software. The results are shown in Table 6 and Figure 1 As shown. It can be seen that the average diameter of inclusions in the austenitic stainless steel of the present invention is significantly smaller than that of the commercial grade, and the inclusions are significantly refined; at the same time, the proportion of niobium-containing phases encapsulating inclusions is also more than 80%, and the average size of niobium-containing phases not encapsulating inclusions is also less than 0.7μm. While fully encapsulating inclusions to improve corrosion resistance, it reduces the deterioration of mechanical properties.

[0137] Table 6 Statistical results of inclusions and niobium-containing phases in 1#~8# austenitic stainless steels

[0138]

[0139] Example 3 Immersion corrosion of austenitic stainless steel

[0140] The immersion corrosion experiment was carried out according to the national standard GB / T 17897-2016. The samples of Examples 1#~4# and Comparative Examples 5#~8# were polished to 2000# with sandpaper step by step to remove the oxide layer on the surface of the samples. The final size of the immersed samples was controlled to be 50mm×20mm×5mm, and then they were cleaned with deionized water and alcohol and dried. The samples of Examples 1#~4# and Comparative Examples 5#~8# were immersed in a 6% FeCl3 solution at 30°C for 72 hours to remove the corrosion products on the surface. After drying, they were weighed and the corrosion rate was calculated. The results are shown in the figure. Figure 2It can be seen that the corrosion rate of the steel grades of the embodiments 1# to 4# of the present invention is significantly lower than that of the steel grades of the comparative embodiments 5# to 8#, and the corrosion rate of the microalloyed austenitic stainless steel is reduced by 1.5 to 3.6 mm·a. -1 This indicates that microalloying with niobium, rare earth and boron can effectively inhibit the corrosion problem caused by inclusions in austenitic stainless steel and significantly reduce the immersion corrosion rate.

[0141] Example 4 Electrochemical Corrosion of Austenitic Stainless Steel

[0142] Electrochemical samples were prepared from steel grades 1# to 4# of the examples and 5# to 8# of the comparative examples, respectively. Potentiodynamic experiments were performed using a Gamry Reference 600 electrochemical workstation in a 3.5wt.% HCl solution environment at 25°C. The results are shown in the figure. Figure 3 As shown. Figure 3 It can be seen that compared with the comparative example steels 5# to 8#, the pitting potential of the embodiments 1# to 4# of the present invention is significantly increased by 500 to 800 mV, indicating that the microalloying developed by the present invention can significantly improve the pitting corrosion resistance of austenitic stainless steel.

[0143] In the description of this specification, the description of the terms "one embodiment", "some embodiments", "specific embodiments", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0144] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A highly corrosion-resistant austenitic stainless steel, characterized in that: In terms of mass percentage, it includes the following elements: C≤0.02%, Si≤1.00%, Mn≤2.50%, Cr: 16.00%~20.00%, Ni: 10.00%~15.00%, Mo: 2.00%~4.00%, Nb, RE: 0.009%~0.025%, B: 0.0015%~0.0055%, N: 0.10%~0.25%, O≤0.003%, P≤0.02%, S≤0.003%, Al≤0.03%, the balance is Fe, wherein RE is one or more of Ce, La and Y; The mass percentage of the Nb element in the highly corrosion-resistant austenitic stainless steel satisfies the following formula: w[%Nb]=m×(w[%Cr]+w[%Mo]+w[%N]); When 18.10≤(w[%Cr]+w[%Mo]+w[%N])≤21.16, m is 0.19% to 0.94%; When 21.16<(w[%Cr]+w[%Mo]+w[%N])≤24.25, m is 0.15%~0.72%; In the formula, w[%Nb] is the mass percentage of Nb in the high corrosion resistant austenitic stainless steel, w[%Cr] is the mass percentage of Cr in the high corrosion resistant austenitic stainless steel, w[%Mo] is the mass percentage of Mo in the high corrosion resistant austenitic stainless steel, and w[%N] is the mass percentage of N in the high corrosion resistant austenitic stainless steel.

2. The highly corrosion-resistant austenitic stainless steel according to claim 1, characterized in that: The highly corrosion-resistant duplex stainless steel comprises the following elements in terms of mass percentage: C≤0.015%, Si≤0.5%, Mn0.5%~2.0%, Cr:17.00%~19.00%, Ni:12.50%~14.00%, Mo:2.50%~3.50%, Nb, RE:0.012%~0.019%, B:0.002%~0.004%, N:0.15%~0.20%, O≤0.002%, P≤0.02%, S≤0.0015%, Al≤0.02%, the balance is Fe, wherein RE is one or more of Ce, La and Y; The mass percentage of the Nb element in the highly corrosion-resistant austenitic stainless steel satisfies the following formula: w[%Nb]=m×(w[%Cr]+w[%Mo]+w[%N]); When 19.65≤(w[%Cr]+w[%Mo]+w[%N])≤21.16, m is 0.17% to 0.94%; When 21.16<(w[%Cr]+w[%Mo]+w[%N])≤22.70, m is 0.15%~0.77%; In the formula, w[%Nb] is the mass percentage of Nb in the high corrosion resistant austenitic stainless steel, w[%Cr] is the mass percentage of Cr in the high corrosion resistant austenitic stainless steel, w[%Mo] is the mass percentage of Mo in the high corrosion resistant austenitic stainless steel, and w[%N] is the mass percentage of N in the high corrosion resistant austenitic stainless steel.

3. The highly corrosion-resistant austenitic stainless steel according to claim 1 or 2, characterized in that: In the highly corrosion-resistant austenitic stainless steel, the proportion of small-sized inclusions less than 1 μm is ≥75%, the proportion of large-sized inclusions greater than 5 μm is ≤1%, the average diameter of the inclusions is ≤1.5 μm, the proportion of enclosed inclusions to the total inclusions is ≥80%, and the average diameter of the niobium-containing phase of the unenclosed inclusions is less than 0.7 μm. Compared with the commercial austenitic stainless steel of the same grade, the corrosion rate in the same environment is reduced by 1.5 mm·a -1 ~3.6mm·a -1 , the pitting potential increased by 500mV to 800mV.

4. A method for preparing highly corrosion-resistant austenitic stainless steel, characterized in that: include: Smelting, according to the chemical composition of the highly corrosion-resistant austenitic stainless steel according to any one of claims 1 to 3, except for the niobium raw material, to obtain molten steel; Casting and niobium microalloying, controlling the casting temperature to 1480℃-1520℃, filling with high-purity nitrogen, performing protective casting and niobium microalloying, and controlling the cooling intensity during the casting process to ensure that the niobium-containing phase precipitates without excessive growth, to obtain an ingot or billet; Forging or rolling; wherein, for the forging, the initial temperature is 1140°C-1200°C, and the final forging temperature is 1020°C-1060°C; for the rolling, the initial temperature is 1140°C-1200°C, and the final rolling temperature is 1020°C-1060°C; Heat treatment, subjecting the hot-processed bars or rolled plates to a solid solution treatment at a temperature of 1040°C-1080°C.

5. The preparation method according to claim 4, characterized in that: The smelting includes: Melting: According to the chemical composition of the highly corrosion-resistant austenitic stainless steel, ingredients are prepared, and except for nitrided alloy, metallic aluminum, nickel-magnesium alloy, boron alloying raw material, rare earth raw material and niobium raw material, the remaining raw materials are melted in the furnace to obtain molten steel; Nitrogen alloying: After the molten steel is melted, the temperature is controlled to 1520°C-1540°C, high-purity nitrogen is filled in and nitriding alloy is added for nitriding; Boron microalloying: After nitrogen alloying, boron alloying raw materials are added to the molten steel; Aluminum deoxidation: After boron microalloying, metallic aluminum is added for deoxidation; Nickel-magnesium deoxidation and desulfurization: After aluminum deoxidation is completed, the temperature of the molten steel is lowered to 1420℃-1450℃, and nickel-magnesium alloy is added for deoxidation and desulfurization; Rare earth microalloying: After the nickel-magnesium deoxidation and desulfurization are completed, the molten steel is heated to 1550℃-1570℃, and rare earth inoculants or rare earth metals are added to the molten steel, preferably rare earth inoculants; after rare earth alloying, nitrogen is blown from the bottom to allow large particles of rare earth inclusions to float up and be removed in the molten steel.

6. The preparation method according to claim 5, characterized in that: The smelting includes: the material of the refractory material is Al2O3, MgO-Al2O3, MgO-REO or REO, preferably REO; The nitrogen alloying comprises: the nitriding alloy is chromium nitride, and the mass percentage of nitrogen in the chromium nitride is 8%-12%; The boron microalloying comprises: the boron alloying raw material is a B-Fe alloy or metal B, the mass percentage of B in the B-Fe alloy is 15% to 20%, and the mass percentage of B in the metal B is ≥99.9%; The aluminum deoxidation comprises: the mass percentage of Al in the metal aluminum is ≥ 99%; the amount of metal aluminum added is determined according to w[%Al]=2.5×(w[%O]-0.005%) / A1, wherein w[%Al] is the mass percentage of Al added to the molten steel, w[%O] is the mass percentage of initial O in the molten steel, and A1 is a coefficient, and the range of A1 is 85% to 95%; The nickel-magnesium deoxidation and desulfurization method comprises: the mass percentage of Mg in the nickel-magnesium alloy is 18% to 22%; the amount of the nickel-magnesium alloy added is calculated according to w[%Mg]=1.6×(w[%O] Al +0.6w[%S] Al -0.003%) / A2, where w[%Mg] is the mass percentage of Mg added to the molten steel through the nickel-magnesium alloy, and w[%O] Al is the mass percentage of O in the molten steel after Al deoxidation, w[%S] Al is the mass percentage of S in the steel liquid after Al deoxidation, A2 is the coefficient, and the range of A2 is 15% to 25%; The rare earth microalloying method comprises: the rare earth inoculant comprises Si: 35% to 40%, RE: 25% to 30%, Ca: 10% to 15%, and the balance is Fe and other inevitable impurities; the mass percentage of rare earth in the rare earth metal is ≥ 99.9%; the amount of rare earth added is calculated according to w[%RE]=(6.30×w[%O] Mg +1.26×w[%S] Mg -0.004%) / A3, where w[%RE] is the mass percentage of RE added to the molten steel, w[%O] Mg is the mass percentage of O in the molten steel after deoxidation of nickel-magnesium alloy, w[%S] Mg is the mass percentage of S in the molten steel after desulfurization of Ni-Mg alloy, A3 is the coefficient, and the range of A3 is 50% to 60%; after rare earth alloying, nitrogen is blown at the bottom for 4min-6min, and the nitrogen flow rate is 2NL·min -1 ·t -1 ~4NL·min -1 ·t -1 .

7. The preparation method according to claim 4, characterized in that: The casting includes die casting or continuous casting; wherein, The die casting comprises: after casting molten steel into the mold, first cooling and second cooling are performed in sequence, the cooling method of the mold side wall is water cooling, the cooling water volume Q1 of the first cooling is determined according to Q1=(76S+7.94)(0.95T1-150) / B1, and the cooling water volume Q2 of the second cooling is determined according to Q2=(84S+7.94)T1 / B2, wherein Q1 is the cooling water volume of the first cooling (L·min -1 ), Q2 is the cooling water volume of the second cooling (L·min -1 ), S is the cross-sectional area of ​​the ingot (m 2 ), T1 is the casting temperature (°C), B1 is the coefficient, the range of B1 is 34 to 42, B2 is the coefficient, the range of B2 is 15 to 20; The continuous casting comprises: controlling the water volume of the first section of the secondary cooling zone to be 0.05 L·m -2 ·s -1 ~0.07Lm -2 ·s -1 , control the water volume of the second last section of the secondary cooling zone to 0.34L·m -2 ·s -1 ~0.46L·m -2 ·s -1 .

8. The preparation method according to claim 4 or 7, characterized in that: The niobium microalloying method comprises: feeding ferroniobium cored wire during casting, and controlling cooling intensity to perform niobium microalloying in a temperature range where niobium phase is precipitated; wherein the mass percentage of niobium in the ferroniobium cored wire is 50% to 55%; and the feeding speed v of the ferroniobium cored wire is determined according to v=(d1+1220)T1 / C, wherein v is the feeding speed of the ferroniobium cored wire (m·min -1 ), d1 is the ingot diameter or the billet thickness (mm), T1 is the casting temperature (℃), C is the coefficient, and the range of C is 13500~15200.

9. The preparation method according to claim 4, characterized in that: The forging has an initial temperature of 1140°C-1200°C, a holding time of t1, a forging number of ≥3, a final forging temperature of 1020°C-1060°C, and water cooling is used after forging is completed; the holding time t1 is determined according to t1=D(d1+85) / T2, wherein t1 is the holding time after reaching the temperature (min), T2 is the initial forging temperature (°C), d1 is the ingot diameter (mm), and D is a coefficient, and the range of D is 520-700; The rolling has an initial temperature of 1140°C-1200°C, a holding time of t2, and is performed for no less than 4 passes with a reduction rate of no less than 30% for each pass. The final rolling temperature is 1020°C-1060°C, and water cooling is used after the rolling is completed. The holding time t2 is determined according to t2=E(d2+42) / T3, wherein t2 is the holding time after reaching the temperature (min), T3 is the initial rolling temperature (°C), d2 is the plate thickness (mm), and E is a coefficient, and the range of E is 1420-1660.

10. The preparation method according to claim 4, characterized in that: The solution treatment includes: a solution temperature of 1040°C-1080°C, a solution time t3 determined according to t3=F(d3-4.5) / T4+10, wherein t3 is the solution time (min), T4 is the solution temperature (°C), d3 is the material diameter or thickness (mm), F is a coefficient, and the range of F is 900-1000.

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