High-hardness easy-to-process ultra-low carbon stainless steel and preparation method thereof

Abstract

The invention provides high-hardness easy-to-process ultra-low carbon stainless steel which comprises the following chemical components in percentage by mass: 16-20% of chromium, 8-14% of nickel, 0-3% of molybdenum, 0.15-0.20% of niobium, 0.1-0.3% of nitrogen, less than or equal to 0.03% of carbon and the balance of iron. The preparation method comprises the following steps: S1, proportioning the raw materials; s2, raw material smelting; S3, decarburization refining; S4, component adjustment; S5, ingot casting preparation; s7, steel billet heat treatment; s8, surface treatment machine processing; and S9, product detection and warehousing. 0.15-0.20% of niobium (Nb) and 0.1-0.3% of nitrogen (N) are added into the molten steel, trace niobium nitride and niobium carbide hardened phases are generated during solidification, and stabilization treatment at 850-930 DEG C is added in the subsequent heat treatment procedure, so that niobium nitride and niobium carbide precipitates are dispersed and separated out, the hardness of the casting reaches HB180-220, and the machining cutting performance of the casting is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of stainless steel, and in particular to ultra-low carbon stainless steel with high hardness and ease of processing and a preparation method thereof. Background Art

[0002] 316L and 304L stainless steels are ultra-low-carbon austenitic stainless steels with a microstructure composed of austenite, a small amount of ferrite, and trace amounts of carbides. After solution treatment, their hardness is only HB120-150. This low hardness, coupled with the inherent work hardening of austenite, can lead to tool sticking and tangling during machining of stainless steel castings, resulting in high tool wear, poor surface finish, and low dimensional accuracy. Summary of the Invention

[0003] The purpose of the present invention is to solve the shortcomings of the prior art and to propose an ultra-low carbon stainless steel with high hardness and easy processing and a preparation method thereof.

[0004] To achieve the above object, the present invention adopts the following technical solutions: A high-hardness, easy-to-process ultra-low carbon stainless steel, the chemical composition of which, by mass percentage, includes: 16-20% chromium, 8-14% nickel, 0-3% molybdenum, 0.15-0.20% niobium, 0.1-0.3% nitrogen, ≤0.03% carbon, and the balance being iron.

[0005] Furthermore, the chemical composition, calculated by mass percentage, includes: chromium 16-18%, nickel 10-14%, molybdenum 2-3%, niobium 0.15-0.20%, nitrogen 0.25-0.3%, carbon ≤0.03%, and the balance is iron.

[0006] Furthermore, the chemical composition includes, by mass percentage, chromium 18-20%, nickel 8-10.4%, niobium 0.15-0.20%, nitrogen 0.25-0.3%, carbon ≤0.03%, and the balance being iron.

[0007] A method for preparing high-hardness and easily machinable ultra-low carbon stainless steel comprises the following steps: S1: Raw material ratio; weigh each raw material according to mass percentage; S2: Raw material smelting: The weighed raw materials are placed into a smelting furnace and smelted into molten steel at 1640-1680°C; S3: Decarburization refining: The molten steel enters the refining furnace, and argon is blown into the molten steel to reduce the carbon content in the molten steel; nitrogen is also blown into the molten steel 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 slag; S4: Composition adjustment: Analyze the composition of the refined molten steel and fine-tune the composition according to the results of the composition analysis; S5: Ingot preparation: the finely adjusted molten steel is injected into the continuous casting machine for pouring and rapidly cooled to form an ingot; S6: Ingot hot deformation and billeting processing: the ingot is heated to 1100-1250℃ and formed into a steel billet through multiple rolling passes, and the final rolling temperature is controlled at 850-950℃; S7: heat treatment of steel billet; S71: Solution treatment: heat the billet to 1050-1100℃ and keep it at this temperature for 25-30 minutes, then quickly cool it down by water quenching; S72: Stabilization treatment: 850-930℃ for 2-4 hours, then cooling to room temperature; S8: Surface treatment machining: Surface pickling and polishing machine processing to obtain the product; S9: Product inspection and storage.

[0008] Furthermore, the raw material is 316L or 304L stainless steel scrap; the process further comprises the following steps: A1: Raw material pretreatment: Stainless steel scrap is sorted and cleaned and pickled to remove grease and oxides from the surface of the stainless steel scrap. A2: Molten steel is produced by melting stainless steel scrap. The treated stainless steel scrap is weighed and placed into a melting furnace to be melted into 316L molten steel or 304L molten steel. Niobium and nitrogen are added to the 316L molten steel or 304L molten steel according to mass percentages, and the molten steel is completely melted to form molten steel.

[0009] Furthermore, the cooling rate of the ingot is controlled to be 50-100°C / s; Furthermore, in step S3, the nitrogen blowing pressure is 0.3-0.5 MPa and the duration is ≥15 min.

[0010] Furthermore, the temperature fluctuation during the stabilization treatment is ≤±10°C and is carried out under an inert atmosphere with a cooling rate of ≤50°C / h.

[0011] Furthermore, step S4 also includes supplementing trace elements, adding 0.03-0.05% Ti to the molten steel after the composition is adjusted.

[0012] Furthermore, the refining furnace is an argon oxygen decarburization furnace or a vacuum decarburization furnace.

[0013] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention adds 0.15-0.20% of niobium (Nb) and 0.1-0.3% of nitrogen (N) to 316L or 304L molten steel, so that a trace amount of niobium nitride and niobium carbide hardening phases are generated in the casting during solidification, and adds a stabilization treatment at 850-930°C in the subsequent heat treatment process, so that niobium nitride and niobium carbide are precipitated and dispersed, and the hardness of the casting reaches HB180-220, thereby improving the machining and cutting performance of the casting. It avoids tool sticking and tool wrapping during machining, and large tool loss, ensuring high surface finish and high dimensional accuracy of the machined surface. (2) The present invention reuses stainless steel waste, improves economic benefits, and avoids waste of resources. It saves the steps of weighing and proportioning, directly uses existing components for preparation, reduces the procurement cost of strategic metals such as nickel, chromium, and molybdenum, cooperates with subsequent refining, has a lower sulfur content, and removes grease on the surface of scrap steel to reduce the risk of carbon increase to 0.01-0.03%, ensuring ultra-low carbon control accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The present invention is a flowchart of the steps of a method for preparing ultra-low carbon stainless steel with high hardness and easy processing. DETAILED DESCRIPTION

[0015] In order to provide a further understanding of the purpose, structure, features, and functions of the present invention, the present invention is described in detail below with reference to the embodiments. Example 1

[0016] A high-hardness, easy-to-process ultra-low carbon stainless steel, the chemical composition of which, by mass percentage, includes: 16-20% chromium, 8-14% nickel, 0-3% molybdenum, 0.15-0.20% niobium, 0.1-0.3% nitrogen, ≤0.03% carbon, and the balance being iron.

[0017] The addition of chromium element forms a dense chromium oxide passivation film. The electrode potential of chromium is higher than that of iron. The passivation film can resist oxidation and acid-base corrosion through a self-repair mechanism, thereby improving corrosion resistance and oxidation resistance and maintaining the performance of stainless steel.

[0018] The addition of nickel expands the austenite phase region and lowers the critical temperature of phase transformation, allowing the steel to maintain a single-phase austenite structure in a wide temperature range, stabilize the austenite microstructure, inhibit crack propagation, improve the material's ductility, toughness and acid and alkali corrosion resistance, and reduce the risk of cracking under high temperature and stress environments.

[0019] The addition of molybdenum generates MoO4²⁻ to prevent Cl⁻ penetration, and cooperates with chromium to form a denser passivation film, inhibiting pitting and crevice corrosion (for chloride environments), forming molybdenum carbides to improve high temperature strength and creep resistance, and enhance corrosion resistance to reducing media.

[0020] The addition of niobium combines with carbon to form stable niobium carbide (NbC), which reduces the precipitation of chromium carbide, avoids intergranular corrosion caused by chromium depletion at grain boundaries, refines grains, and improves strength.

[0021] The addition of nitrogen enhances the dislocation pinning effect by dissolving nitrogen atoms in the austenite lattice, partially replacing nickel to stabilize the austenite phase, while also improving strength, hardness, and resistance to localized corrosion (such as pitting), optimizing the material's work-hardening properties. It also forms trace amounts of niobium nitride, which creates trace hardening phases of niobium nitride and niobium carbide during solidification of the molten steel. During the subsequent stabilization treatment, these niobium nitride and niobium carbide precipitate and disperse, increasing the hardness of the stainless steel to HB180-220. This improves the machining performance of the casting, prevents tool sticking and tangling during machining, and reduces tool wear, ensuring high surface finish and dimensional accuracy.

[0022] Ultra-low carbon (≤0.03%) reduces Cr 23 C6 precipitates, 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 properties.

[0023] like Figure 1 , also includes, a method for preparing ultra-low carbon stainless steel with high hardness and easy processing, comprising the following steps: S1: Raw material ratio; weigh each raw material according to the mass percentage; accurately calculate and weigh the raw materials of each component to ensure the accurate distribution ratio of each group.

[0024] S2: Raw material smelting: The weighed raw materials are placed into the smelting furnace and smelted into molten steel at 1640-1680℃; the raw materials are fully smelted into liquid molten steel. The smelting process requires continuous stirring to ensure uniform melting of each component and reduce burn-off.

[0025] S3: Decarburization refining: The molten steel enters the refining furnace, and argon is blown into the molten steel to reduce the carbon content in the molten steel. The molten steel is blown with argon and oxygen gases, and the mixed gas is blown into the molten pool from the side of the furnace bottom. During the blowing process, the CO partial pressure is reduced by dilution of the argon gas, which is beneficial for decarburization and chromium preservation during stainless steel smelting. Nitrogen is blown in to prevent oxidation, achieve nitrogen alloying of the molten steel, and stabilize the nitrogen content in the composition. Deoxidizers such as ferrosilicon and limestone slag are added. Under the action of argon and stirring, desulfurization is achieved, causing oxides in the molten steel to float to form slag, reducing the oxygen content of the molten steel. The slag is removed through operations such as slag skimming, ensuring that the molten steel has a low content of inclusions and high cleanliness, avoiding defects such as particle inclusions and pores in the ingots in the later stage.

[0026] S4: Composition adjustment: Analyze the composition of the refined molten steel and fine-tune the composition according to the results of the composition analysis; ensure the accuracy of the composition of the molten steel and the quality of the ingot.

[0027] S5: Ingot preparation: The finely adjusted molten steel is injected into the continuous casting machine for pouring and rapid cooling to form an ingot; the mechanized process reduces the workload of workers, shortens the production cycle, and realizes continuous production. Rapid cooling (such as solidification within 30 seconds) refines the grains, increases the tensile strength of the ingot by 15%-20%, and reduces component segregation.

[0028] S6: Ingot hot deformation and blanking processing; the ingot is heated to 1100-1250℃, and formed into a steel billet through multiple rolling passes, gradually reducing the thickness and improving the internal structure. The final rolling temperature is controlled at 850-950℃ to avoid the precipitation of σ phase affecting the processability.

[0029] S7: heat treatment of steel billet; S71: Solution Treatment: Heat the billet to 1050-1100°C, hold for 25-30 minutes, and rapidly cool with water quenching to ensure full dissolution of the alloying elements in the austenite. After solution treatment, rapidly cool the material to room temperature using water quenching or other rapid cooling methods. This restores the austenite single-phase structure, prevents carbide precipitation along grain boundaries, and improves toughness. It also dissolves residual Nb(C,N) phases to balance hardness and processability.

[0030] S72: Stabilization treatment: 850-930℃ for 2-4 hours, followed by cooling to room temperature; the temperature range is the stable precipitation window of NbC / NbN, which promotes the uniform dispersion of the precipitated phase from the matrix, the precipitation and dispersion of niobium nitride and niobium carbide, and the refinement of the particle size of niobium nitride and niobium carbide to the nanometer level, so that the hardness of the casting reaches HB180~220, thereby improving the machining performance of the casting.

[0031] S8: Surface Treatment Machining: Surface pickling and polishing are performed to produce the product. Pickling removes surface oxide scale. Common polishing methods include mechanical polishing, chemical polishing, and electrolytic polishing. These methods improve the surface finish of stainless steel.

[0032] S9: Product inspection and storage. The product can be inspected for appearance to avoid defects such as pores or cracks.

[0033] Furthermore, the cooling rate of the ingot is controlled to be 50-100°C / s; the initial nucleation of the NbC / NbN nanoscale precipitation phase is promoted, dendrite segregation is reduced, and the grain size is small.

[0034] Furthermore, in step S3, the nitrogen blowing pressure is 0.3-0.5 MPa and the duration is ≥15 min. This increases the nitrogen recovery rate to above 92%, increases the nitrogen content in the stainless steel component, and increases the hardness of the stainless steel, thereby improving the machinability and preventing tool sticking.

[0035] Furthermore, during stabilization, the temperature fluctuation is ≤±10°C and is carried out in an inert atmosphere to prevent surface oxidation and coarsening of the precipitated phase, thereby improving product quality. A slow cooling rate of ≤50°C / h ensures stable carbide precipitation, helps reduce residual stress, improves the yield of finished products, balances microstructure evolution and stress release, and enhances corrosion resistance and processability.

[0036] Furthermore, step S4 includes supplementing trace elements by adding 0.03-0.05% Ti to the molten steel after the composition adjustment. This refines the grains (average grain size ≤ 10μm) by forming TiN as heterogeneous nucleation sites. Grain refinement enhances material strength and improves cutting performance, while also increasing grain boundary area to hinder crack propagation. Ti preferentially combines with residual carbon to form TiC, preventing the precipitation of Cr23C6 at grain boundaries, eliminating intergranular corrosion tendencies, and significantly improving corrosion resistance. The addition of Ti delays the precipitation temperature of the σ phase, allowing the material to maintain a single austenitic structure during long-term service at 400-800°C, improving structural stability and making it suitable for use under high-temperature conditions.

[0037] Furthermore, the refining furnace is an argon oxygen decarburization furnace or a vacuum decarburization furnace, which can be used alone or in combination to reduce the carbon content, achieve the production of ultra-low carbon stainless steel, and reduce the formation of carbides. Example 2

[0038] Furthermore, the chemical composition, calculated 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, and the balance iron. This composition, similar to 316L stainless steel, facilitates the direct addition of 0.15-0.20% niobium (Nb) and 0.1-0.3% nitrogen (N) to the molten steel during conventional stainless steel production processes. This avoids changes to the production process and eliminates the need for additional production lines, ensuring the high universality of this stainless steel. By designing a higher nitrogen content, the hardness can be increased by approximately 3-5 HV for every 0.01% nitrogen, directly increasing the strength and hardness of the stainless steel and improving the machining performance of the casting.

[0039] Furthermore, the chemical composition, calculated by mass percentage, includes: chromium 18-20%, nickel 8-10.4%, niobium 0.15-0.20%, nitrogen 0.25-0.3%, carbon ≤0.03%, and the balance iron. This composition is the composition of 304L stainless steel and meets the requirements of 304L stainless steel.

[0040] like Figure 1 , the raw material is 316L or 304L stainless steel scrap; and further comprising the following steps: A1: Raw material pretreatment: Stainless steel scrap is cleaned and sorted, then cleaned and pickled to remove surface grease and oxides. The scrap stainless steel is then recycled and reused to avoid resource waste. Removing surface grease and oxides controls gas emissions, preventing the formation of gases like H2 and CO during grease smelting, which could lead to excessive hydrogen content in the molten steel and the formation of subcutaneous bubbles. This reduces porosity and avoids the generation of VOCs and other pollutants that could contaminate the atmosphere. Removing surface oxides reduces non-metallic inclusions formed during smelting, improving the purity of the molten steel and ensuring the quality of the stainless steel.

[0041] A2: Molten steel is obtained by melting stainless steel scrap; the treated stainless steel scrap is weighed, and the treated stainless steel scrap is put into a melting furnace to be melted into a molten state to form 316L molten steel or 304L molten steel, and niobium and nitrogen are added to the 316L molten steel or 304L molten steel according to mass percentage, and the molten steel is completely melted to form molten steel. The remaining steps are the same as in Example 1. It is convenient to reuse stainless steel scrap, improve economic efficiency, and avoid waste of resources. It saves the step of weighing and proportioning, and directly uses existing components for preparation, reducing the procurement cost of strategic metals such as nickel, chromium, and molybdenum, coordinating with subsequent refining, and having a lower sulfur content. The removal of grease on the surface of the scrap steel reduces the risk of carbon increase to 0.01-0.03%, ensuring ultra-low carbon control accuracy.

[0042] The present invention has been described with reference to the above embodiments. However, the above embodiments are merely exemplary embodiments of the present invention. It should be noted that the disclosed embodiments do not limit the scope of the present invention. On the contrary, modifications and improvements that do not depart from the spirit and scope of the present invention are intended to be protected by the present invention.

Claims

1. A high-hardness, easily machinable ultra-low carbon stainless steel, characterized by: Calculated by mass percentage, the 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%, and the balance is iron.

2. The high-hardness, easy-to-process ultra-low carbon stainless steel according to claim 1, characterized in that: Calculated by mass percentage, the chemical composition includes: chromium 16-18%, nickel 10-14%, molybdenum 2-3%, niobium 0.15-0.20%, nitrogen 0.25-0.3%, carbon ≤0.03%, and the balance is iron.

3. The high-hardness, easy-to-process ultra-low carbon stainless steel according to claim 1, characterized in that: Calculated by mass percentage, the chemical composition includes: chromium 18-20%, nickel 8-10.4%, niobium 0.15-0.20%, nitrogen 0.25-0.3%, carbon ≤0.03%, and the balance is iron.

4. A method for preparing the high-hardness, easy-to-process ultra-low carbon stainless steel according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1: Raw material ratio; weigh each raw material according to mass percentage; S2: Raw material smelting: The weighed raw materials are placed into a smelting furnace and smelted into molten steel at 1640-1680°C; S3: Decarburization refining: The molten steel enters the refining furnace, and argon is blown into the molten steel to reduce the carbon content in the molten steel; nitrogen is also blown into the molten steel 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 slag; S4: Composition adjustment: Analyze the composition of the refined molten steel and fine-tune the composition according to the results of the composition analysis; S5: Ingot preparation: the finely adjusted molten steel is injected into the continuous casting machine for pouring and rapidly cooled to form an ingot; S6: Ingot hot deformation and billeting processing: the ingot is heated to 1100-1250℃ and formed into a steel billet through multiple rolling passes, and the final rolling temperature is controlled at 850-950℃; S7: heat treatment of steel billet; S71: Solution treatment: heat the billet to 1050-1100℃ and keep it at this temperature for 25-30 minutes, then quickly cool it down by water quenching; S72: Stabilization treatment: 850-930℃ for 2-4 hours, then cooling to room temperature; S8: Surface treatment machining: Surface pickling and polishing machine processing to obtain the product; S9: Product inspection and storage.

5. The method for preparing high-hardness and easily machinable ultra-low carbon stainless steel according to claim 4, characterized in that: The raw material is 316L or 304L stainless steel scrap; The following steps are also included: A1: Raw material pretreatment: Stainless steel scrap is sorted and cleaned and pickled to remove grease and oxides from the surface of the stainless steel scrap. A2: Molten steel is produced by melting stainless steel scrap. The treated stainless steel scrap is weighed and placed into a melting furnace to be melted into 316L molten steel or 304L molten steel. Niobium and nitrogen are added to the 316L molten steel or 304L molten steel according to mass percentages, and the molten steel is completely melted to form molten steel.

6. The method for preparing high-hardness and easily machinable ultra-low carbon stainless steel according to claim 4, characterized in that: The cooling rate of the ingot is controlled at 50-100°C / s.

7. The method for preparing high-hardness and easily machinable ultra-low carbon stainless steel according to claim 4, characterized in that: In step S3, the nitrogen blowing pressure is 0.3-0.5 MPa and the duration is ≥15 min.

8. The method for preparing high-hardness and easily machinable ultra-low carbon stainless steel according to claim 1, wherein: The temperature fluctuation during the stabilization treatment is ≤±10°C and is carried out under an inert atmosphere with a cooling rate of ≤50°C / h.

9. The method for preparing high-hardness and easily machinable ultra-low carbon stainless steel according to claim 1, wherein: Step S4 also includes supplementing trace elements, adding 0.03-0.05% Ti to the molten steel after the composition adjustment.

10. The method according to claim 1, wherein: The refining furnace is an argon oxygen decarburization furnace or a vacuum decarburization furnace.

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