Preparation method for high-nitrogen austenitic stainless steel

Abstract

A preparation method for high-nitrogen austenitic stainless steel. The high-nitrogen austenitic stainless steel comprises the following chemical components in percentages by weight: carbon: ≤0.02%; silicon: 0.4-0.8%; manganese: 10.2-12.5%; chromium: 24.5-26.5%; nickel: 13.5-16.5%; molybdenum: 3.2-4.8%; nitrogen: 0.75-0.95%; phosphorus: ≤0.03%; sulfur: ≤0.0025%; vanadium: 0.016-0.022%; niobium: 0.026-0.034%; copper: 0.2-0.35%; aluminum: 0.01-0.02%; cerium: 0.015-0.025%; and the balance of iron and inevitable impurities. The preparation method comprises the steps of raw material charging, vacuum melting, refining, nitrogen-added alloying, deoxidation and desulfurization, casting, electroslag remelting, and secondary casting. The preparation is implemented by using the processes of smelting in a pressurized induction furnace and electroslag remelting under a protective atmosphere; and nitrogen is added by using a method in which a nitrogen alloy and pressurized nitrogen permeation are combined.

Description

A method for preparing high-nitrogen austenitic stainless steel Technical Field

[0001] This invention relates to the field of metal smelting technology, and in particular to a method for preparing high-nitrogen austenitic stainless steel. Background Technology

[0002] High-nitrogen austenitic stainless steel is a special type of stainless steel that is widely used in petroleum, chemical, food, and pharmaceutical industries due to its excellent mechanical properties and corrosion resistance. The addition of nitrogen makes this stainless steel more environmentally friendly and economical while maintaining its excellent corrosion resistance and mechanical properties.

[0003] The microstructure of high-nitrogen austenitic stainless steel mainly consists of a face-centered cubic austenitic matrix and dispersed nitrides. The addition of nitrogen alters the lattice constant of the stainless steel, thus affecting its mechanical and physical properties. The diffusion and solid solution strengthening effects of nitrogen also contribute to the higher strength and toughness of high-nitrogen austenitic stainless steel. The corrosion resistance of high-nitrogen austenitic stainless steel is primarily due to the stabilizing effect of nitrogen on the austenitic matrix and its protective effect on chromium. In oxidizing environments, nitrogen can interact with chromium to form a stable oxide film, thereby improving the corrosion resistance of the stainless steel.

[0004] Nitrogen solubility in steel under normal pressure follows Sievts' law and is relatively low (usually below 0.3%). Therefore, how to further increase the nitrogen content in the preparation of high-nitrogen austenitic stainless steel has become the focus of research. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by proposing a method for preparing high-nitrogen austenitic stainless steel. The method involves smelting in a pressure induction furnace and electroslag remelting under a protective atmosphere. Nitrogen is added by combining nitriding alloying with pressurized nitrogen infiltration, thereby increasing the solubility of nitrogen in the molten steel and thus improving the overall performance of the high-nitrogen stainless steel.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for preparing a high-nitrogen austenitic stainless steel, wherein the chemical composition of the high-nitrogen austenitic stainless steel is as follows (weight percentage): carbon: ≤0.2%, silicon: 0.4-0.8%, manganese: 10.2-12.5%, chromium: 24.5-26.5%, nickel: 13.5-16.5%, molybdenum: 3.2-4.8%, nitrogen: 0.75-0.95%, phosphorus: ≤0.03%, sulfur: ≤0.0025%, vanadium: 0.016-0.022%, niobium: 0.026-0.034%, copper: 0.2-0.35%, aluminum: 0.01-0.02%, cerium: 0.015-0.025%, with the balance being iron and unavoidable impurities.

[0008] The preparation method of the high-nitrogen austenitic stainless steel includes:

[0009] Step S1: Raw material loading: Prepare raw materials according to the set ratio, load the large materials into the crucible of the pressure induction furnace, and load the small materials into the feeding bin of the pressure induction furnace;

[0010] Step S2: Vacuum melting: After closing the furnace, vacuum is applied to achieve a vacuum level below 4 Pa; initial power of 30 kW is used for heating to melt the molten steel, and the power is increased by 20 kW every 20 minutes until it reaches 70-75 kW;

[0011] Step S3: Refining: Refining temperature 1520-1580 degrees Celsius, refining time 25-30 minutes;

[0012] Step S4: Nitrogen alloying: At a power of 70-75 kW, nitrogen is introduced into the furnace at a pressure of 0.35-0.40 MPa; manganese nitride and chromium nitride are added in batches. After the nitride alloy is completely melted, electromagnetic stirring and mechanical stirring are performed to make the steel composition uniform and to carry out nitrogen alloying treatment.

[0013] Step S5: Deoxidation and desulfurization: Add aluminum granules and calcium silicon to carry out deoxidation and desulfurization;

[0014] Step S6: Casting: Steel is tapped and cast at a temperature of 1520–1580 degrees Celsius, with a casting pressure of 0.8–1.6 MPa;

[0015] Step S7: Electroslag Remelting: The ingot is placed into an atmosphere-protected electroslag furnace. Before the electroslag is powered, a gas pipe is inserted to the bottom of the crystallizer, and nitrogen gas is introduced at a flow rate of 200 liters / minute for 8.5 to 12 minutes. During the melting process, the melting rate is 4.0 to 4.5 kg / minute, the current intensity is 3050 to 3215 A, and the voltage is 20 to 25 V.

[0016] Step S8: Secondary casting: Casting is carried out at a steel temperature of 1530-1580 degrees Celsius.

[0017] Preferably, the raw materials in step S1 include industrial pure iron, metallic nickel, metallic chromium, ferromolybdenum granules, electrolytic manganese, electrolytic copper, aluminum granules, manganese nitride, chromium nitride, carbon powder, calcium silicon, ferroniobium, and ferrovanadium.

[0018] Preferably, the ratio of nitrogen added to manganese nitride and chromium nitride is 3:2; the mass fraction of manganese in manganese nitride is 70-75%, and the mass fraction of nitrogen is 5.85-6.05%; the mass fraction of chromium in chromium nitride is 53.5-56.5%, and the mass fraction of nitrogen is 3.15-3.21%.

[0019] Preferably, step S7 uses a five-element slag system of calcium fluoride-calcium oxide-aluminum oxide-magnesium oxide-cerium oxide, wherein w(calcium fluoride) / w(aluminum oxide) = 1.36~1.43, w(cerium oxide) = 18%~20%, w(magnesium oxide) = 4%~5%, and w(calcium fluoride) = 52%~60%.

[0020] Preferably, the chemical composition of the high-nitrogen austenitic stainless steel is as follows (by weight percentage): carbon: ≤0.2%, silicon: 0.45-0.75%, manganese: 10.8-11.8%, chromium: 24.8-25.6%, nickel: 13.8-15.5%, molybdenum: 3.7-4.5%, nitrogen: 0.82-0.95%, phosphorus: ≤0.02%, sulfur: ≤0.0025%, vanadium: 0.018-0.022%, niobium: 0.026-0.034%, copper: 0.25-0.32%, aluminum: 0.01-0.02%, cerium: 0.015-0.022%, with the balance being iron and unavoidable impurities.

[0021] Preferably, the chemical composition of the high-nitrogen austenitic stainless steel is as follows by weight percentage: carbon: 0.008%, silicon: 0.65%, manganese: 11.6%, chromium: 25.3%, nickel: 14.5%, molybdenum: 4.1%, nitrogen: 0.83%, phosphorus: 0.015%, sulfur: 0.0012%, vanadium: 0.018%, niobium: 0.031%, copper: 0.028%, aluminum: 0.015%, cerium: 0.016%, with the balance being iron and unavoidable impurities.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention optimizes the composition ratio and process parameters of high-nitrogen austenitic stainless steel, and prepares it through pressurized induction furnace smelting and protective atmosphere electroslag remelting process. Nitrogen element is added by combining nitriding alloy and pressurized nitrogen infiltration, thereby improving the solubility of nitrogen element in molten steel and thus improving the comprehensive performance of high-nitrogen stainless steel. Attached Figure Description

[0023] Figure 1 is a flowchart of a method for preparing high-nitrogen austenitic stainless steel according to some embodiments of the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the described embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. It is understood that, without conflict, some technical means of the various embodiments described herein can be substituted for or combined with each other.

[0025] In the description of this invention, the terms "first," "second," etc., are used only to distinguish the described objects and have no sequential or technical meaning. Therefore, objects specified with "first," "second," etc., may explicitly or implicitly include one or more of those objects. Furthermore, the words "one" or "a" do not indicate a quantity limitation, but rather indicate the presence of at least one, while "multiple" indicates at least two.

[0026] In the description of this invention, references to "one embodiment" or "some embodiments" mean that one or more embodiments of the invention include a particular feature, structure, or characteristic described in connection with that embodiment. Therefore, the phrases "one embodiment," "some embodiments," "other embodiments," "and other embodiments," etc., appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized.

[0027] According to a first aspect of the present invention, a high-nitrogen austenitic stainless steel is provided, wherein the chemical composition of the high-nitrogen austenitic stainless steel is as follows by weight percentage: carbon: ≤0.02%, silicon: 0.4-0.8%, manganese: 10.2-12.5%, chromium: 24.5-26.5%, nickel: 13.5-16.5%, molybdenum: 3.2-4.8%, nitrogen: 0.75-0.95%, phosphorus: ≤0.03%, sulfur: ≤0.0025%, vanadium: 0.016-0.022%, niobium: 0.026-0.034%, copper: 0.2-0.35%, aluminum: 0.01-0.02%, cerium: 0.015-0.025%, and the balance being iron and unavoidable impurities.

[0028] In the aforementioned high-nitrogen austenitic stainless steel, controlling the carbon content below 0.02% enhances its strength and hardness, and improves its wear resistance and tensile strength without affecting its corrosion resistance. Controlling the nitrogen content between 0.75% and 0.95% significantly improves the steel's strength and hardness, while also enhancing its corrosion resistance. Nitrogen atoms combine with chromium atoms to form chromium nitride, a compound that prevents intergranular corrosion and localized corrosion, thus improving the steel's corrosion resistance. Chromium reacts with oxygen to form a dense chromium oxide layer, preventing further oxidation and further improving corrosion resistance. High chromium content also increases the steel's strength and hardness. Nickel is used to improve the steel's toughness and impact resistance, reduce its hardness and strength, and improve its machinability. Furthermore, nickel improves the steel's corrosion resistance, particularly against strong acids and alkalis. The addition of molybdenum enhances the corrosion resistance of high-nitrogen austenitic stainless steel and effectively inhibits nitrogen precipitation during processing. Manganese can form a reinforcing phase with nitrogen, further improving the strength and hardness of steel, and can also improve the heat resistance and corrosion resistance of high-nitrogen austenitic stainless steel.

[0029] In some embodiments, the chemical composition of high-nitrogen austenitic stainless steel by weight percentage is as follows: carbon: ≤0.2%, silicon: 0.45-0.75%, manganese: 10.8-11.8%, chromium: 24.8-25.6%, nickel: 13.8-15.5%, molybdenum: 3.7-4.5%, nitrogen: 0.82-0.95%, phosphorus: ≤0.02%, sulfur: ≤0.0025%, vanadium: 0.018-0.022%, niobium: 0.026-0.034%, copper: 0.25-0.32%, aluminum: 0.01-0.02%, cerium: 0.015-0.022%, with the balance being iron and unavoidable impurities.

[0030] According to a second aspect of the present invention, a method for preparing high-nitrogen austenitic stainless steel is provided. Referring to FIG1, the method for preparing high-nitrogen austenitic stainless steel includes:

[0031] Step S1: Raw material loading: Prepare raw materials according to the set ratio, load the large materials into the crucible of the pressure induction furnace, and load the small materials into the feeding bin of the pressure induction furnace;

[0032] Step S2: Vacuum melting: After closing the furnace, vacuum is applied to achieve a vacuum level below 4 Pa; initial power of 30 kW is used for heating to melt the molten steel, and the power is increased by 20 kW every 20 minutes until it reaches 70-75 kW;

[0033] Step S3: Refining: Refining temperature 1520-1580 degrees Celsius, refining time 25-30 minutes;

[0034] Step S4: Nitrogen alloying: At a power of 70-75 kW, nitrogen is introduced into the furnace at a pressure of 0.35-0.40 MPa; manganese nitride and chromium nitride are added in batches. After the nitride alloy is completely melted, electromagnetic stirring and mechanical stirring are performed to make the steel composition uniform and to carry out nitrogen alloying treatment.

[0035] Step S5: Deoxidation and desulfurization: Add aluminum granules and calcium silicon to carry out deoxidation and desulfurization;

[0036] Step S6: Casting: Steel is tapped and cast at a temperature of 1520–1580 degrees Celsius, with a casting pressure of 0.8–1.6 MPa;

[0037] Step S7: Electroslag Remelting: The ingot is placed into an atmosphere-protected electroslag furnace. Before the electroslag is powered, a gas pipe is inserted to the bottom of the crystallizer, and nitrogen gas is introduced at a flow rate of 200 liters / minute for 8.5 to 12 minutes. During the melting process, the melting rate is 4.0 to 4.5 kg / minute, the current intensity is 3050 to 3215 A, and the voltage is 20 to 25 V.

[0038] Step S8: Secondary casting: Casting is carried out at a steel temperature of 1530-1580 degrees Celsius.

[0039] Studies have shown that under high-temperature conditions, the solubility of nitrogen in molten slag decreases with increasing temperature. In the initial stage of electroslag remelting, the slag pool has just formed, the temperature is low, and the nitrogen solubility is high. Therefore, raising the slag temperature as early as possible can reduce nitrogen loss. Taking all factors into consideration, this invention employs the electroslag remelting process in step S7 above, increasing the nitrogen partial pressure in the initial melting stage and improving the melting power to achieve early slag formation, rapidly increase the slag temperature, and reduce the nitrogen solubility in the molten slag, thereby ensuring the nitrogen content of the finished steel.

[0040] This invention optimizes the composition ratio and process parameters of high-nitrogen austenitic stainless steel. It is prepared through pressurized induction furnace smelting and protective atmosphere electroslag remelting. During the smelting process, nitrogen is increased by combining alloy nitriding with pressurized nitrogen infiltration, thereby enhancing the solubility of nitrogen in the molten steel and improving the overall performance of the high-nitrogen stainless steel. The increased nitrogen partial pressure during smelting suppresses the formation of subcutaneous bubbles; after protective atmosphere electroslag remelting, inclusions are generally reduced.

[0041] Preferably, the raw materials in step S1 include industrial pure iron, metallic nickel, metallic chromium, ferromolybdenum granules, electrolytic manganese, electrolytic copper, aluminum granules, manganese nitride, chromium nitride, carbon powder, calcium silicon, ferroniobium, and ferrovanadium.

[0042] Furthermore, the ratio of nitrogen added to manganese nitride and chromium nitride is 3:2; the mass fraction of manganese in manganese nitride is 70-75%, and the mass fraction of nitrogen is 5.85-6.05%; the mass fraction of chromium in chromium nitride is 53.5-56.5%, and the mass fraction of nitrogen is 3.15-3.21%.

[0043] In some embodiments, step S7 employs a pentagonal slag system of calcium fluoride-calcium oxide-aluminum oxide-magnesium oxide-cerium oxide (CaF2-CaO-Al2O3-MgO-Ce2O3), wherein w(calcium fluoride) / w(aluminum oxide) = 1.36–1.43, w(cerium oxide) = 18%–20%, w(magnesium oxide) = 4%–5%, and w(calcium fluoride) = 52%–60%. Studies have shown that as the amount of cerium oxide added to the slag system increases from 0 to 25%, the melting point of the slag system initially decreases and then increases. Adding a small amount of cerium oxide can promote a decrease in the melting point of the slag system, but excessive cerium oxide will lead to the formation of more high-melting-point phase 2CeAlO3 in the slag system, thereby increasing the melting point of the slag system. The reasonable proportion of cerium oxide in this invention can promote a decrease in the melting point of the slag system and reduce the loss of cerium elements during ignition.

[0044] In a preferred embodiment, the chemical composition of the high-nitrogen austenitic stainless steel is as follows (weight percentage): carbon: 0.008%, silicon: 0.65%, manganese: 11.6%, chromium: 25.3%, nickel: 14.5%, molybdenum: 4.1%, nitrogen: 0.83%, phosphorus: 0.015%, sulfur: 0.0012%, vanadium: 0.018%, niobium: 0.031%, copper: 0.028%, aluminum: 0.015%, cerium: 0.016%, with the balance being iron and unavoidable impurities. The preparation method of this high-nitrogen austenitic stainless steel is as follows:

[0045] Step S1: Raw material loading: Prepare raw materials according to the set ratio, load the large materials into the crucible of the pressure induction furnace, and load the small materials into the feeding bin of the pressure induction furnace;

[0046] Step S2: Vacuum melting: After closing the furnace, a vacuum is drawn to achieve a vacuum level of 3 Pa; the initial power is 30 kW for heating to melt the molten steel, and the power is increased by 20 kW every 20 minutes until it reaches 72 kW;

[0047] Step S3: Refining: Refining temperature 1560 degrees Celsius, refining time 28 minutes;

[0048] Step S4: Nitrogen alloying: Nitrogen is introduced into the furnace at a power of 72 kW and a pressure of 0.39 MPa. Manganese nitride and chromium nitride are added in batches. After the nitride alloy is completely melted, electromagnetic stirring and mechanical stirring are performed to make the steel composition uniform and to carry out nitrogen alloying treatment.

[0049] Step S5: Deoxidation and desulfurization: Add aluminum granules and calcium silicon to carry out deoxidation and desulfurization;

[0050] Step S6: Casting: Steel is tapped and cast at a temperature of 1540 degrees Celsius, with a casting pressure of 1.2 MPa;

[0051] Step S7: Electroslag Remelting: Place the ingot into the atmosphere-protected electroslag furnace. Before powering the electroslag furnace, insert a gas pipe to the bottom of the crystallizer and introduce nitrogen gas at a flow rate of 200 liters / minute for 10 minutes to ensure a 100% nitrogen atmosphere in the crystallizer. This increases the nitrogen partial pressure in the early stages of melting and prevents a decrease in nitrogen solubility. During melting, the melting rate is 4.5 kg / minute, the current intensity is 3150 amps, and the voltage is 22 volts.

[0052] Step S8: Secondary casting: Casting is carried out at a steel temperature of 1540 degrees Celsius.

[0053] Tests showed that the average yield strength of the high-nitrogen austenitic stainless steel prepared by the above method was about 1050 MPa, and the average tensile strength was about 1230 MPa.

[0054] This invention optimizes the composition ratio and process parameters of high-nitrogen austenitic stainless steel. It is prepared by pressurized induction furnace smelting and protective atmosphere electroslag remelting. During the smelting process, nitrogen is increased by combining nitriding alloying with pressurized nitrogen infiltration, thereby improving the solubility of nitrogen in the molten steel and thus improving the comprehensive performance of high-nitrogen stainless steel.

[0055] The present invention has been described in the above-described embodiments; however, these embodiments are merely examples for implementing the present invention. It must be noted that the disclosed embodiments do not limit the scope of the present invention. Conversely, any modifications and refinements made without departing from the spirit and scope of the present invention are within the scope of patent protection of the present invention.

Claims

1. A method for preparing high-nitrogen austenitic stainless steel, characterized in that: The chemical composition of the high-nitrogen austenitic stainless steel is as follows (weight percentage): carbon: ≤0.2%, silicon: 0.4-0.8%, manganese: 10.2-12.5%, chromium: 24.5-26.5%, nickel: 13.5-16.5%, molybdenum: 3.2-4.8%, nitrogen: 0.75-0.95%, phosphorus: ≤0.03%, sulfur: ≤0.0025%, vanadium: 0.016-0.022%, niobium: 0.026-0.034%, copper: 0.2-0.35%, aluminum: 0.01-0.02%, cerium: 0.015-0.025%, with the balance being iron and unavoidable impurities. The preparation method of the high-nitrogen austenitic stainless steel includes: Step S1: Raw material loading: Prepare raw materials according to the set ratio, load the large materials into the crucible of the pressure induction furnace, and load the small materials into the feeding bin of the pressure induction furnace; Step S2: Vacuum melting: After closing the furnace, vacuum is applied to achieve a vacuum level below 4 Pa; initial power of 30 kW is used for heating to melt the molten steel, and the power is increased by 20 kW every 20 minutes until it reaches 70-75 kW; Step S3: Refining: Refining temperature 1520-1580 degrees Celsius, refining time 25-30 minutes; Step S4: Nitrogen alloying: At a power of 70-75 kW, nitrogen is introduced into the furnace at a pressure of 0.35-0.40 MPa; manganese nitride and chromium nitride are added in batches. After the nitride alloy is completely melted, electromagnetic stirring and mechanical stirring are performed to make the steel composition uniform and to carry out nitrogen alloying treatment. Step S5: Deoxidation and desulfurization: Add aluminum granules and calcium silicon to carry out deoxidation and desulfurization; Step S6: Casting: Steel is tapped and cast at a temperature of 1520–1580 degrees Celsius, with a casting pressure of 0.8–1.6 MPa; Step S7: Electroslag Remelting: The ingot is placed into an atmosphere-protected electroslag furnace. Before the electroslag is powered, a gas pipe is inserted to the bottom of the crystallizer, and nitrogen gas is introduced at a flow rate of 200 liters / minute for 8.5 to 12 minutes. During the melting process, the melting rate is 4.0 to 4.5 kg / minute, the current intensity is 3050 to 3215 A, and the voltage is 20 to 25 V. Step S8: Secondary casting: Casting is carried out at a steel temperature of 1530-1580 degrees Celsius.

2. The method for preparing high-nitrogen austenitic stainless steel as described in claim 1, characterized in that: The raw materials in step S1 include industrial pure iron, metallic nickel, metallic chromium, ferromolybdenum granules, electrolytic manganese, electrolytic copper, aluminum granules, manganese nitride, chromium nitride, carbon powder, calcium silicon, ferroniobium, and ferrovanadium.

3. The method for preparing high-nitrogen austenitic stainless steel as described in claim 2, characterized in that: The nitrogen addition ratio in manganese nitride and chromium nitride is 3:2; the mass fraction of manganese in manganese nitride is 70-75%, and the mass fraction of nitrogen is 5.85-6.05%; the mass fraction of chromium in chromium nitride is 53.5-56.5%, and the mass fraction of nitrogen is 3.15-3.21%.

4. The method for preparing high-nitrogen austenitic stainless steel as described in claim 1, characterized in that: In step S7, a five-element slag system of calcium fluoride-calcium oxide-aluminum oxide-magnesium oxide-cerium oxide is adopted, wherein w(calcium fluoride) / w(aluminum oxide) = 1.36~1.43, w(cerium oxide) = 18%~20%, w(magnesium oxide) = 4%~5%, and w(calcium fluoride) = 52%~60%.

5. The method for preparing high-nitrogen austenitic stainless steel as described in claim 1, characterized in that: The chemical composition of the high-nitrogen austenitic stainless steel by weight percentage is as follows: carbon: ≤0.2%, silicon: 0.45-0.75%, manganese: 10.8-11.8%, chromium: 24.8-25.6%, nickel: 13.8-15.5%, molybdenum: 3.7-4.5%, nitrogen: 0.82-0.95%, phosphorus: ≤0.02%, sulfur: ≤0.0025%, vanadium: 0.018-0.022%, niobium: 0.026-0.034%, copper: 0.25-0.32%, aluminum: 0.01-0.02%, cerium: 0.015-0.022%, with the balance being iron and unavoidable impurities.

6. The method for preparing high-nitrogen austenitic stainless steel as described in claim 1, characterized in that: The chemical composition (by weight percentage) of the high-nitrogen austenitic stainless steel is as follows: carbon: 0.008%, silicon: 0.65%, manganese: 11.6%, chromium: 25.3%, nickel: 14.5%, molybdenum: 4.1%, nitrogen: 0.83%, phosphorus: 0.015%, sulfur: 0.0012%, vanadium: 0.018%, niobium: 0.031%, copper: 0.028%, aluminum: 0.015%, cerium: 0.016%, balance is iron and unavoidable impurities.

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