High-hardness easy-to-machine ultra-low-carbon stainless steel and method for manufacturing same

Abstract

The application provides a kind of high hardness easy-to-machine ultra-low carbon stainless steel, by mass percent, chemical composition includes: chromium 16-20%, nickel 8-14%, molybdenum 0-3%, niobium 0.15-0.20%, nitrogen 0.1-0.3%, carbon ≤0.03%, the balance is iron. It also includes its preparation method, including S1: raw material proportioning;S2: raw material smelting;S3: decarburization refining;S4: component adjustment;S5: preparation of ingot;S6: ingot hot deformation breakdown processing;S7: billet heat treatment;S8: surface treatment processing;S9: product detection into warehouse. The application adds 0.15~0.20 niobium (Nb) elements and 0.1~0.3% nitrogen (N) elements in molten steel, generates trace amounts of niobium nitride and niobium carbide hardening phase during solidification, and increases 850~930℃ stabilization treatment in subsequent heat treatment process, makes niobium nitride and niobium carbide precipitate and disperse, makes the hardness of castings reach HB180~220, thereby improves the machining cutting performance of castings.

Description

Technical Field

[0001] This invention relates to the field of stainless steel technology, and in particular to a high-hardness, easily machinable ultra-low carbon stainless steel and its preparation method. Background Technology

[0002] 316L and 304L stainless steel are ultra-low carbon austenitic stainless steels. Their metallographic structure consists of austenite with a small amount of ferrite and trace carbides. After solution treatment, their hardness is only HB120~150. The low hardness, coupled with the inherent work hardening phenomenon of austenitic structure, causes castings made from these stainless steels to stick and become entangled in the cutting tool during machining, resulting in high tool wear, poor surface finish, and low dimensional accuracy. Summary of the Invention

[0003] The purpose of this invention is to address the shortcomings of existing technologies by proposing a high-hardness, easily machinable ultra-low carbon stainless steel and its preparation method.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] A high-hardness, easily machinable ultra-low carbon stainless steel, by mass percentage, comprises: 16-20% chromium, 8-14% nickel, 0-3% molybdenum, 0.15-0.20% niobium, 0.1-0.3% nitrogen, ≤0.03% carbon, with the balance being iron.

[0006] Furthermore, by mass percentage, the chemical composition includes: 16-18% chromium, 10-14% nickel, 2-3% molybdenum, 0.15-0.20% niobium, 0.25-0.3% nitrogen, ≤0.03% carbon, and the balance being iron.

[0007] Furthermore, by mass percentage, the chemical composition includes: 18-20% chromium, 8-10.4% nickel, 0.15-0.20% niobium, 0.25-0.3% nitrogen, ≤0.03% carbon, and the balance being iron.

[0008] A method for preparing high-hardness, easily machinable ultra-low carbon stainless steel includes the following steps:

[0009] S1: Raw material ratio; weigh each raw material according to its mass percentage;

[0010] S2: Raw material smelting: The weighed raw materials are put into the smelting furnace and smelted into molten steel at 1640-1680℃;

[0011] S3: Decarburization refining: When molten steel enters the refining furnace, argon gas is blown into the molten steel to reduce the carbon content of the molten steel; nitrogen gas is also blown in to prevent oxidation and achieve nitrogen alloying of the molten steel; deoxidizer is added to reduce the oxygen content of the molten steel and remove steel slag.

[0012] S4: Composition Adjustment: Perform composition analysis on the refined molten steel and fine-tune the composition based on the results of the composition analysis;

[0013] S5: Ingot preparation: The finely adjusted molten steel is poured into the continuous casting machine and rapidly cooled to form an ingot;

[0014] S6: Hot deformation of ingots for billet processing; heating the ingot to 1100-1250℃ and forming a steel billet through multiple rolling passes, with the final rolling temperature controlled at 850-950℃.

[0015] S7: Heat treatment of steel billet;

[0016] S71: Solution treatment: Heat the steel billet to 1050-1100℃ and hold for 25-30 minutes, then water quench and cool rapidly.

[0017] S72: Stabilization treatment: Hold at 850-930℃ for 2-4 hours, then cool to room temperature;

[0018] S8: Surface treatment machining: Products obtained by surface pickling and polishing machining;

[0019] S9: Product inspection and warehousing.

[0020] Furthermore, the raw material is 316L or 304L stainless steel scrap; the process also includes the following steps:

[0021] A1: Raw material pretreatment: Stainless steel waste is sorted and cleaned and acid-washed to remove grease and oxides from the surface of stainless steel waste.

[0022] A2: Smelting stainless steel scrap to obtain molten steel; Weigh the treated stainless steel scrap, put the treated stainless steel scrap into the smelting furnace and melt it into a molten state to form 316L molten steel or 304L molten steel, add niobium and nitrogen to the 316L molten steel or 304L molten steel according to the mass percentage, and smelt completely to form molten steel.

[0023] Furthermore, the cooling rate of the ingot is controlled at 50-100℃ / s;

[0024] Furthermore, in step S3, the nitrogen blowing pressure is 0.3-0.5 MPa, and the duration is ≥15 min.

[0025] Furthermore, the stabilization treatment involves temperature fluctuations ≤ ±10℃ and is carried out under an inert atmosphere with a cooling rate ≤ 50℃ / h.

[0026] Furthermore, step S4 also includes supplementing trace elements by adding 0.03-0.05% Ti to the molten steel after composition adjustment.

[0027] Furthermore, the refining furnace is an argon-oxygen decarburization furnace or a vacuum decarburization furnace.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention adopts the addition of 0.15~0.20% niobium (Nb) element and 0.1~0.3% nitrogen (N) element to 316L or 304L molten steel, so that trace amounts of niobium nitride and niobium carbide hardening phases are generated during the solidification of the casting, and a stabilization treatment of 850~930℃ is added in the subsequent heat treatment process, so that niobium nitride and niobium carbide precipitate and disperse, and the hardness of the casting reaches HB180~220, thereby improving the machinability of the casting. It avoids tool sticking and entanglement during machining, large tool wear, and ensures high surface finish and high dimensional accuracy. (2) The present invention reuses stainless steel scrap, improves economic efficiency, and avoids waste of resources. It eliminates the step of weighing and proportioning, directly utilizes existing components for preparation, reduces the procurement cost of strategic metals such as nickel, chromium, and molybdenum, and, in conjunction with subsequent refining, results in lower sulfur content. The removal of grease from the surface of scrap steel reduces the risk of carbon increase to 0.01-0.03%, ensuring the precision of ultra-low carbon control. Attached Figure Description

[0029] Figure 1 This is a flowchart illustrating the steps of a method for preparing a high-hardness, easily machinable ultra-low carbon stainless steel according to the present invention. Detailed Implementation

[0030] To provide a further understanding of the purpose, structure, features, and functions of the present invention, detailed descriptions are provided below with reference to specific embodiments. Example 1

[0031] A high-hardness, easily machinable ultra-low carbon stainless steel, by mass percentage, comprises: 16-20% chromium, 8-14% nickel, 0-3% molybdenum, 0.15-0.20% niobium, 0.1-0.3% nitrogen, ≤0.03% carbon, with the balance being iron.

[0032] Adding chromium forms a dense chromium oxide passivation film. Chromium has a higher electrode potential than iron, and the passivation film can resist oxidation and acid and alkali corrosion through a self-repair mechanism, thereby improving corrosion resistance and oxidation resistance and maintaining the performance of stainless steel.

[0033] Adding nickel expands the austenite phase region and lowers the critical temperature of phase transformation, enabling the steel to maintain a single-phase austenite structure over a wide temperature range. This stabilizes the austenite microstructure, inhibits crack propagation, improves the material's ductility, toughness, and resistance to acid and alkali corrosion, and reduces the risk of cracking under high temperature and stress conditions.

[0034] The addition of molybdenum generates MoO4²⁻, which prevents Cl⁻ penetration and works synergistically with chromium to form a denser passivation film, inhibiting pitting and crevice corrosion (for chloride environments). The formation of molybdenum carbides improves high-temperature strength and creep resistance, and enhances corrosion resistance to reducing media.

[0035] Adding niobium combines with carbon to form stable niobium carbide (NbC), reducing chromium carbide precipitation, avoiding intergranular corrosion caused by chromium depletion at grain boundaries, refining grains, and improving strength.

[0036] The addition of nitrogen enhances the dislocation pinning effect by solidifying nitrogen atoms in the austenite lattice, partially replacing nickel to stabilize the austenite phase, while simultaneously improving strength, hardness, and resistance to localized corrosion (such as pitting), and optimizing the material's work hardening characteristics. Furthermore, it forms trace amounts of niobium nitride, causing the formation of trace niobium nitride and niobium carbide hardening phases during solidification of molten steel. Under subsequent stabilization treatment, niobium nitride and niobium carbide precipitate and disperse, increasing the hardness of stainless steel to HB180~220. This improves the machinability of castings, preventing tool sticking and entanglement during machining, reducing tool wear, and ensuring high surface finish and dimensional accuracy.

[0037] Ultra-low carbon (≤0.03%) reduces Cr 23 C6 precipitation inhibits carbide formation, avoids excessive carbide generation, preferentially forms niobium carbide (NbC) and niobium nitride (NbN), avoids intergranular corrosion-sensitive areas, and improves welding and hot working performance.

[0038] like Figure 1 It also includes a method for preparing high-hardness, easily machinable ultra-low carbon stainless steel, comprising the following steps:

[0039] S1: Raw material ratio; weigh each raw material according to its mass percentage; accurately calculate and weigh the raw materials of each component to ensure the accurate distribution ratio of each group.

[0040] S2: Raw material smelting: The weighed raw materials are put into the smelting furnace and smelted into molten steel at 1640-1680℃; the raw materials are fully smelted into liquid steel. The smelting process requires constant stirring to ensure that the components are smelted evenly and to reduce burn-off.

[0041] S3: Decarburization Refining: Molten steel enters the refining furnace, and argon gas is blown into the molten steel to reduce its carbon content. The molten steel is then smelted using a mixture of argon and oxygen gases, blown into the molten pool from the side of the furnace bottom. During the smelting process, the partial pressure of CO is reduced by the dilution of argon gas, which is beneficial for decarburization and chromium retention in stainless steel smelting. Nitrogen gas is also blown in to prevent oxidation, achieving nitrogen alloying of the molten steel and stabilizing the nitrogen content. Deoxidizers, such as ferrosilicon and limestone slag, are added. Under the action of argon gas and stirring, desulfurization is achieved, causing oxides in the molten steel to float to the surface, forming slag and reducing the oxygen content. Slag is removed through skimming and other operations. This ensures low inclusion content and high cleanliness in the molten steel, avoiding defects such as particulate inclusions and porosity in later ingot casting.

[0042] S4: Composition Adjustment: Perform composition analysis on the refined molten steel and fine-tune the composition based on the analysis results; ensure the accuracy of the composition in the molten steel and guarantee the quality of the ingots.

[0043] S5: Ingot preparation: The finely adjusted molten steel is poured into the continuous casting machine and rapidly cooled to form an ingot; the mechanized process reduces the workload of workers, shortens the production cycle, realizes continuous production, and the rapid cooling (such as solidification within 30 seconds) refines the grains, increases the tensile strength of the billet by 15%-20%, and reduces component segregation.

[0044] S6: Hot deformation of ingots for billet processing; heating the ingot to 1100-1250℃ and forming a steel billet through multiple rolling passes, gradually reducing the thickness and improving the internal microstructure, with the final rolling temperature controlled at 850-950℃; avoiding the precipitation of σ phase that affects machinability.

[0045] S7: Heat treatment of steel billet;

[0046] S71: Solution Treatment: Heat the steel billet to 1050-1100℃ and hold for 25-30 minutes, then rapidly cool by water quenching; ensure that the alloying elements are fully dissolved in the austenite. After solution treatment, rapidly cool the material to room temperature using water quenching or other rapid cooling methods. This restores the single-phase austenite structure, prevents carbide precipitation along grain boundaries, and improves the toughness of the material. It also dissolves residual Nb(C,N) phases, balancing hardness and machinability.

[0047] S72: Stabilization treatment: Hold at 850-930℃ for 2-4 hours, then cool to room temperature; the temperature range is the stable precipitation window of NbC / NbN, which promotes the uniform dispersion of precipitates from the matrix, and allows niobium nitride and niobium carbide to precipitate in a dispersed manner, refining the particle size of niobium nitride and niobium carbide to the nanoscale, so that the hardness of the casting reaches HB180~220, thereby improving the machinability of the casting.

[0048] S8: Surface treatment machining: Products obtained through surface pickling and polishing; pickling removes surface oxide scale. Common polishing methods include mechanical polishing, chemical polishing, and electrolytic polishing. This improves the surface finish of stainless steel.

[0049] S9: Product Inspection and Warehousing. Visual inspection can be performed on products to avoid defects such as pores or cracks.

[0050] Furthermore, the cooling rate of the ingot is controlled at 50-100℃ / s; this promotes the initial nucleation of NbC / NbN nanoscale precipitates, reduces dendrite segregation, and results in small grain size.

[0051] Furthermore, in step S3, the nitrogen blowing pressure is 0.3-0.5 MPa, and the duration is ≥15 min. This increases the nitrogen yield to over 92%, increases the nitrogen content in the stainless steel composition, increases the hardness of the stainless steel, thereby improving machinability and preventing issues such as tool sticking.

[0052] Furthermore, the stabilization treatment involves temperature fluctuations ≤ ±10℃ and is carried out under an inert atmosphere to prevent surface oxidation and coarsening of precipitated phases, thereby improving product quality. A cooling rate ≤ 50℃ / h, with slow cooling, ensures stable carbide precipitation, helps reduce residual stress, increases the yield of finished products, balances microstructure evolution and stress release, and enhances corrosion resistance and processability.

[0053] Furthermore, step S4 also includes supplementing trace elements by adding 0.03-0.05% Ti to the molten steel after composition adjustment. This forms TiN as a heterogeneous nucleation site, refining the grains (average grain size ≤10μm). Grain refinement improves material strength and machinability, while increasing grain boundary area to hinder crack propagation. Ti preferentially combines with residual C to form TiC, preventing Cr23C6 precipitation at grain boundaries, eliminating intergranular corrosion tendency, and significantly improving corrosion resistance. The addition of Ti delays the σ-phase precipitation temperature, allowing the material to maintain a single austenitic structure during long-term service at 400-800℃, improving structural stability and making it suitable for use under high-temperature conditions.

[0054] Furthermore, the refining furnace is either an argon-oxygen decarburization furnace or a vacuum decarburization furnace. It can be used alone or in combination to reduce carbon content, achieve the production of ultra-low carbon stainless steel, and reduce carbide formation. Example 2

[0055] Furthermore, by mass percentage, the chemical composition includes: 16-18% chromium, 10-14% nickel, 2-3% molybdenum, 0.15-0.20% niobium, 0.25-0.3% nitrogen, ≤0.03% carbon, with the balance being iron. This composition is typical for 316L stainless steel, allowing for the direct addition of 0.15-0.20% niobium (Nb) and 0.1-0.3% nitrogen (N) to molten steel in conventional stainless steel manufacturing processes. This avoids altering the manufacturing process, eliminates the need for additional production lines, and ensures the high versatility of this stainless steel product. By designing higher nitrogen levels, each 0.01% N can increase hardness by approximately 3-5 HV, directly enhancing the strength and hardness of the stainless steel and improving the machinability of castings.

[0056] Furthermore, by mass percentage, the chemical composition includes: 18-20% chromium, 8-10.4% nickel, 0.15-0.20% niobium, 0.25-0.3% nitrogen, ≤0.03% carbon, with the balance being iron. This composition is consistent with the requirements for 304L stainless steel.

[0057] like Figure 1 The raw material is 316L or 304L stainless steel scrap; it also includes the following steps:

[0058] A1: Raw Material Pretreatment: Stainless steel scrap is sorted and cleaned, then pickled to remove surface grease and oxides. Recycling and reusing scrap stainless steel avoids resource waste. Removing surface grease and oxides controls gas emissions, preventing the formation of gases such as H2 and CO during grease smelting, which can lead to excessive hydrogen content in the molten steel, subcutaneous bubbles, and reduced porosity. It also prevents the generation of VOCs and other pollutants that pollute the atmosphere. Removing surface oxides reduces non-metallic inclusions formed during smelting, improves the purity of the molten steel, and ensures the quality of the stainless steel.

[0059] A2: Smelting stainless steel scrap to produce molten steel; weigh the treated stainless steel scrap and put it into a smelting furnace to melt it into 316L or 304L molten steel. Add niobium and nitrogen to the 316L or 304L molten steel according to the mass percentage, and smelt completely to form molten steel. The remaining steps are the same as in Example 1. This method facilitates the reuse of stainless steel scrap, improves economic efficiency, and avoids waste of resources. It saves the step of weighing and proportioning, directly uses existing components to prepare the steel, reduces the procurement cost of strategic metals such as nickel, chromium, and molybdenum, and, in conjunction with subsequent refining, results in lower sulfur content. The removal of grease from the scrap steel surface reduces the risk of carbon increase to 0.01-0.03%, ensuring the precision of ultra-low carbon control.

[0060] 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 high-hardness, easily machinable ultra-low carbon stainless steel, characterized in that: The chemical composition, by mass percentage, includes: chromium 16-20%, nickel 8-14%, molybdenum 0-3%, niobium 0.15-0.20%, nitrogen 0.1-0.3%, carbon ≤0.03%, and the balance being iron. The preparation method includes the following steps: S1: Raw material ratio; weigh each raw material according to its mass percentage; S2: Raw material smelting: The weighed raw materials are put into the smelting furnace and smelted into molten steel at 1640-1680℃; S3: Decarburization refining: When molten steel enters the refining furnace, argon gas is blown into the molten steel to reduce the carbon content of the molten steel; nitrogen gas is also blown in to prevent oxidation and achieve nitrogen alloying of the molten steel; deoxidizer is added to reduce the oxygen content of the molten steel and remove steel slag. S4: Composition Adjustment: Perform composition analysis on the refined molten steel and fine-tune the composition based on the results of the composition analysis; S5: Ingot preparation: The finely adjusted molten steel is poured into the continuous casting machine and rapidly cooled to form an ingot; S6: Hot deformation of ingots for billet processing; heating the ingot to 1100-1250℃ and forming a steel billet through multiple rolling passes, with the final rolling temperature controlled at 850-950℃. S7: Heat treatment of steel billet; S71: Solution treatment: Heat the steel billet to 1050-1100℃ and hold for 25-30 minutes, then water quench and cool rapidly. S72: Stabilization treatment: Hold at 850-930℃ for 2-4 hours, then cool to room temperature; S8: Surface treatment machining: Products obtained by surface pickling and polishing machining; S9: Product inspection and warehousing.

2. The high-hardness, easily machinable ultra-low carbon stainless steel as described in claim 1, characterized in that: The chemical composition, by mass percentage, includes: 16-18% chromium, 10-14% nickel, 2-3% molybdenum, 0.15-0.20% niobium, 0.25-0.3% nitrogen, ≤0.03% carbon, with the balance being iron.

3. The high-hardness, easily machinable ultra-low carbon stainless steel as described in claim 1, characterized in that: The chemical composition, by mass percentage, includes: 18-20% chromium, 8-10.4% nickel, 0.15-0.20% niobium, 0.25-0.3% nitrogen, ≤0.03% carbon, and the balance being iron.

4. The high-hardness, easily machinable ultra-low carbon stainless steel as described in any one of claims 1-3, characterized in that: The raw material is 316L or 304L stainless steel scrap; the process also includes the following steps: A1: Raw material pretreatment: Stainless steel waste is sorted and cleaned and acid-washed to remove grease and oxides from the surface of stainless steel waste. A2: Smelting stainless steel scrap to obtain molten steel; Weigh the treated stainless steel scrap, put the treated stainless steel scrap into the smelting furnace and melt it into a molten state to form 316L molten steel or 304L molten steel, add niobium and nitrogen to the 316L molten steel or 304L molten steel according to the mass percentage, and smelt completely to form molten steel.

5. The high-hardness, easily machinable ultra-low carbon stainless steel as described in claim 4, characterized in that: The cooling rate of the ingot is controlled at 50-100℃ / s.

6. The high-hardness, easily machinable ultra-low carbon stainless steel as described in claim 4, characterized in that: In step S3, the nitrogen blowing pressure is 0.3-0.5 MPa and the duration is ≥15 min.

7. The high-hardness, easily machinable ultra-low carbon stainless steel as described in claim 4, characterized in that: Step S4 also includes supplementing trace elements by adding 0.03-0.05% Ti to the molten steel after the composition has been adjusted.

8. The high-hardness, easily machinable ultra-low carbon stainless steel as described in claim 4, characterized in that: The refining furnace is either an argon-oxygen decarburization furnace or a vacuum decarburization furnace.

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