A method for preparing high-sulfur manganese free-cutting steel by modifying MnS inclusions

Abstract

The application discloses a high-sulfur manganese free-cutting steel MnS inclusion modification preparation method and belongs to the technical field of steel metallurgy. The application forms fine and dispersed rare earth manganese sulfur oxides in the steel through a strict smelting process, and the rare earth manganese sulfur oxides serve as the nucleation core of MnS inclusions in the solidification process, so that the MnS is attached to the surface of the nucleation core to generate composite inclusions. The spheroidization of the rare earth inclusions can reduce the length-width ratio of the MnS inclusions, reduce the anisotropy of the steel, realize the modification of the inclusions and further improve the performance. Through LaMn alloy modification treatment, the morphology of the MnS in the high-sulfur manganese medium-carbon free-cutting steel is improved, the MnS is changed from a strip shape or a chain shape into a fine and uniformly distributed spherical shape or a near-spherical shape, stress concentration is effectively reduced, the Mn content in the steel is also improved, the steel is not infinitely intermingled with the matrix, and the mechanical performance and the cutting performance of the steel are significantly improved.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy, and in particular to a method for preparing high-sulfur manganese free-cutting steel with MnS inclusion modification. Background Technology

[0002] Free-cutting steels are widely used in industries such as automobile manufacturing and machining due to their excellent machinability and economy. In these steels, MnS inclusions play a crucial role in their machinability. However, because MnS is a ductile inclusion, it is often irregular in shape and unevenly distributed, frequently deforming into elongated shapes during machining. These elongated MnS inclusions cause anisotropy in the properties of sulfur-based free-cutting steels, reducing their overall performance.

[0003] To improve the morphology and distribution of MnS in free-cutting steel and enhance its contribution to the steel's machinability, researchers have been exploring various modification methods. Invention patent CN116640979A discloses "A method for regulating the morphology of MnS in sulfur-based free-cutting steel." This patent proposes adding titanium alloy during the steelmaking process, controlling the mass ratios of Ti / Mn, Ti / N, and Ti / S at 0.01-15, 6-20, and 2-10, respectively. TiN precipitates on the MnS surface, forming a double-layered sulfide structure, inhibiting MnS growth and reducing its size. This produces numerous small MnS particles surrounded by a double-layered TiN inclusion, thus achieving the goal of modifying MnS. Invention patent CN117867209A discloses "A method for reducing inclusions in high-sulfur free-cutting steel." This patent controls inclusions through oxygen control and slag formation. The molten iron is melted to 1650-1700℃ using top-blown oxygen in a converter, with a final carbon content of 0.03-0.05wt%. Carbon powder is added during tapping, followed by ferrosulfide alloying, and then calcium carbide and quicklime slag washing. Afterward, LF refining is performed at 1596-1615℃, with ferrosilicon and aluminum particles added for diffusion deoxidation in the slag. Finally, the steel is continuously cast. Invention patent CN115889648A discloses "A forging method for improving the morphology of manganese sulfide in free-cutting steel." This patent utilizes a vacuum induction furnace to obtain free-cutting steel ingots with the required composition. The ingots are then subjected to homogenization heat treatment and three forging processes to improve key parameters such as the size, aspect ratio, and maximum chord length of MnS, thereby improving the performance of the free-cutting steel. Invention patent CN112176151A discloses "A method for controlling the morphology of MnS inclusions in free-cutting steel." This patent first performs preliminary refining of molten iron in an electric furnace or converter, followed by ladle refining (LF) to ensure that the alloy element composition in the molten steel meets the requirements for free-cutting steel. Then, bismuth-containing cored wire is added to the refined molten steel to control the morphology and size of MnS inclusions, achieving a modification effect. Invention patent CN118389773A discloses "A method for reducing Class A inclusions in high-sulfur free-cutting steel." This patent adds sulfur during tapping from the electric furnace to allow Class A inclusions to float to the surface. After LF refining, it can ultimately reduce Class A inclusions in the steel by 40%, improving steel quality, reducing the quantity and size of inclusions in free-cutting steel, and enhancing the performance of free-cutting steel. Invention patent CN113913676A discloses "a metallurgical method for improving the morphology of cast sulfides in medium-carbon, high-sulfur free-cutting steel". This patent describes a process involving primary refining in a converter or electric furnace, controlling the carbon content during tapping, adding alloying elements for alloying, and adding lime and low-basicity synthetic slag for slag formation. This is followed by LF refining, which is divided into early, middle, and late stages. The middle stage involves alloying to meet compositional requirements, while the late stage involves adding carbon wire to control the carbon content. After refining, soft stirring and continuous casting are performed. This method significantly improves the morphology, size, and distribution of MnS.

[0004] MnS inclusions in sulfur-based free-cutting steels can improve machinability, but they significantly impact mechanical properties. Since MnS is a ductile inclusion, it deforms during forming, becoming elongated, leading to anisotropy in the steel's mechanical properties and reducing its overall performance. Therefore, refining, spheroidizing, and dispersing MnS inclusions can reduce the deformation amplitude during steel forming, minimizing its impact on overall performance. Rare earth element La has the potential to transform MnS inclusions with large aspect ratios into spherical inclusions, while simultaneously refining and increasing the number of sulfur-containing inclusions. Studies show that rare earth La has a strong affinity for S and O, readily forming rare earth sulfide inclusions. Replacing LaFe with LaMn, in addition to the effect of La's transformation into inclusions, allows Mn to participate in sulfide formation during steel solidification, forming (La,Mn)S composite inclusions. This further refines the inclusion size and improves distribution uniformity, while simultaneously enhancing the interfacial bonding between the inclusions and the steel matrix, reducing stress concentration. Summary of the Invention

[0005] MnS inclusions in sulfur-based free-cutting steels are prone to deformation and elongation during forming, leading to anisotropy and reduced steel performance. To address this issue, this invention provides a LaMn alloy modification treatment method to optimize the morphology of MnS inclusions in high-sulfur, medium-carbon free-cutting steels. This method aims to form a large number of spherical, fine, and uniformly distributed composite rare-earth manganese-sulfur-oxygen inclusions in sulfur-containing free-cutting steels, improving the morphology and distribution of MnS inclusions. This enhances the mechanical and machinability of sulfur-based free-cutting steels while mitigating the adverse effects of MnS inclusions on the steel's mechanical properties. Ultimately, this meets the demand for high-performance steels in high-end manufacturing industries, bringing significant economic benefits and industry advancements.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] This invention discloses a method for preparing MnS inclusion-modified high-sulfur manganese free-cutting steel, comprising:

[0008] (1) Smelting process

[0009] Scrap steel, pig iron, ferroalloys, and raw materials are loaded into a crucible according to the specified proportions. A carbon raiser, FeS, and LaMn alloy are placed in a feeding hopper. The crucible is then placed in a vacuum induction furnace, and a vacuum is created between 50-300 Pa. Power is then gradually increased to reach a temperature between 1600-1650℃ to melt the alloy. During the steel refining stage, carbon raiser, FeS, and LaMn alloy are added separately to adjust the sulfur content in the molten steel to the target range of 0.20-0.25%, the manganese content to the target range of 0.20-1.85%, and the lanthanum content to the target range of 0.001-0.018%. Before tapping, the composition of the molten steel is tested and adjusted appropriately. Once the composition meets the standards, the steel can be tapped and cast into a billet. The billet is cooled naturally in air.

[0010] (2) Addition and composition of LaMn alloy

[0011] During the smelting process, LaMn alloy is added 5 minutes before tapping according to the designed ratio to ensure that the alloy can be completely dissolved; the composition of LaMn alloy by mass percentage is: La content 60-70%, Mn content 30-40%;

[0012] (3) Heat treatment

[0013] Annealing, normalizing, and tempering improve the microstructure of steel and the composition, size, quantity, and distribution of inclusions; the cast billet undergoes heat treatment:

[0014] The first step is the annealing process, with a temperature between 830-850℃ and a holding time of 3-4 hours. The heating rate is controlled at 100-120℃ / h, and the cooling method is furnace cooling with a cooling rate controlled at 80-100℃ / h, in order to ensure the uniform internal structure of the billet and eliminate stress concentration.

[0015] The second process is normalizing, with a temperature between 930-950℃ and a holding time of 1.5h. The heating rate is controlled at 130-160℃ / h to ensure uniform heating of the billet. The cooling method is air cooling, with a cooling rate controlled at 200-300℃ / h to refine the microstructure of the billet.

[0016] Finally, there is the tempering process, with a temperature of 580-600℃ and a holding time of 1.5h. The heating rate is controlled at 100-120℃ / h to ensure uniform heating of the billet. Air cooling is used, with a cooling rate controlled at 150-200℃ / h to eliminate the stress generated by normalizing.

[0017] Furthermore, the LaMn alloy should be added in two stages to avoid adding too much at once and causing local enrichment. After each addition, the mixture should be stirred thoroughly to ensure uniform distribution of the alloy.

[0018] Furthermore, the chemical composition of the high-sulfur manganese medium-carbon free-cutting steel by mass percentage is as follows: C: 0.35-0.55 wt%; Si ≤ 0.30 wt%; Mn ≤ 1.85 wt%; P ≤ 0.020 wt%; S: 0.20-0.25 wt%; O: 0.005-0.007 wt%; La: 0.001-0.018 wt%; the remainder being Fe and other unavoidable impurities.

[0019] Furthermore, the chemical composition of the high-sulfur manganese medium-carbon free-cutting steel by mass percentage is as follows: C 0.39wt%, Si 0.25wt%, Mn 1.74wt%, P 0.018wt%, S 0.29wt%, O 0.0050wt%, La 0.012wt%; the remainder is Fe and other unavoidable impurities.

[0020] Furthermore, the melting temperature was 1600℃, the LaMn alloy addition was 0.12wt%, and the holding time was 5min;

[0021] Cooling method: Air cooling;

[0022] Heat treatment: Annealing temperature 850℃, holding for 3.5h, normalizing temperature 950℃, holding for 1.5h, tempering temperature 600℃, holding for 1.5h.

[0023] Furthermore, the chemical composition of the high-sulfur manganese medium-carbon free-cutting steel by mass percentage is as follows: C 0.38wt%, Si 0.25wt%, Mn 1.74wt%, P 0.019wt%, S 0.23wt%, O 0.0068wt%, La 0.015wt%; the remainder is Fe and other unavoidable impurities.

[0024] Furthermore, the melting temperature was 1600℃; the LaMn alloy addition was 0.24wt%; and the holding time was 5min.

[0025] Cooling method: Air cooling;

[0026] Heat treatment: Annealing temperature 850℃, holding for 3.5h; normalizing temperature 950℃, holding for 1.5h; tempering temperature 600℃, holding for 1.5h.

[0027] Furthermore, the chemical composition of the high-sulfur manganese medium-carbon free-cutting steel by mass percentage is as follows: C 0.38wt%, Si 0.25wt%, Mn 1.79wt%, P 0.018wt%, S 0.25wt%, O 0.0067wt%, La 0.016wt%; the remainder is Fe and other unavoidable impurities.

[0028] Furthermore, the melting temperature was 1600℃; the LaMn alloy addition was 0.36wt%; and the holding time was 5min.

[0029] Cooling method: Air cooling;

[0030] Heat treatment: Annealing temperature 850℃, holding for 3.5h; normalizing temperature 950℃, holding for 1.5h; tempering temperature 600℃, holding for 1.5h.

[0031] Furthermore, the LaMn alloy has the following composition by mass percentage: La 70wt%, Mn 30wt%.

[0032] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0033] This invention patent proposes a method for generating rare earth manganese-sulfur-oxygen composite inclusions in modified high-sulfur manganese medium-carbon free-cutting steel using LaMn alloys, controlling the morphology, size, quantity, and distribution of MnS. Through a rigorous smelting process, fine, uniformly distributed rare earth manganese-sulfur oxides are formed in the steel. During cooling, these oxides act as nucleation sites for MnS inclusions, causing MnS to adhere to their surfaces and form composite inclusions. The spheroidizing effect of the rare earth inclusions reduces their aspect ratio, decreases the anisotropy of the steel, and achieves the goal of modifying inclusions and thus improving performance.

[0034] This invention improves the morphology of MnS in high-sulfur manganese medium-carbon free-cutting steel by modifying LaMn alloys to form rare earth manganese sulfide oxides, which serve as nucleation centers for MnS during the solidification of molten steel. This transforms the MnS from elongated or chain-like structures into fine, uniformly distributed spherical or near-spherical structures, effectively reducing stress concentration. At the same time, it increases the bonding strength between the steel matrix and inclusions, significantly enhancing the mechanical and machinability of the steel.

[0035] The LaMn alloy modification treatment method used in this invention is easy to operate, has a simple process, and is low in cost, making it suitable for large-scale industrial production. Attached Figure Description

[0036] The present invention will be further described below with reference to the accompanying drawings.

[0037] Figure 1 MnS inclusions without LaMn alloy (Comparative Example 1);

[0038] Figure 2 MnS inclusions in a LaMn (La-0.012wt%) alloy (Example 1);

[0039] Figure 3 MnS inclusions in a LaMn (La-0.015wt%) alloy (Example 2);

[0040] Figure 4 MnS inclusions in a LaMn (La-0.016wt%) alloy (Example 3). Detailed Implementation

[0041] Comparative Example 1:

[0042] Raw material: High-sulfur manganese medium-carbon free-cutting steel, composition: C 0.38wt%, Si 0.28wt%, Mn 0.25wt%, P 0.021wt%, S 0.22wt%, O 0.0063wt%, La 0wt%, the remainder being Fe and unavoidable impurities;

[0043] Melting temperature: 1600℃, LaMn alloy addition: 0wt%.

[0044] Cooling method: Air cooling.

[0045] Heat treatment: Annealing temperature 850℃, holding for 3.5h, normalizing temperature 950℃, holding for 1.5h, tempering temperature 600℃, holding for 1.5h.

[0046] Example 1:

[0047] Raw material: High-sulfur manganese medium-carbon free-cutting steel, composition: C 0.39wt%, Si 0.25wt%, Mn 1.74wt%, P 0.018wt%, S 0.29wt%, O 0.0050wt%, La 0.012wt%, the remainder being Fe and unavoidable impurities;

[0048] LaMn alloy: La 70wt%, Mn 30wt%.

[0049] Melting temperature: 1600℃, LaMn alloy addition: 0.12wt% (0.12wt% of the total weight of raw materials), holding time: 5min.

[0050] Cooling method: Air cooling.

[0051] Heat treatment: Annealing temperature 850℃, holding for 3.5h, normalizing temperature 950℃, holding for 1.5h, tempering temperature 600℃, holding for 1.5h.

[0052] Example 2:

[0053] Raw material: High-sulfur manganese medium-carbon free-cutting steel, composition: C 0.38wt%, Si 0.25wt%, Mn 1.74wt%, P 0.019wt%, S 0.23wt%, O 0.0068wt%, La 0.015wt%, the remainder being Fe and unavoidable impurities;

[0054] LaMn alloy: La 70wt%, Mn 30wt%.

[0055] Melting temperature: 1600℃, LaMn alloy addition: 0.24wt% (0.24wt% of the total weight of raw materials), holding time: 5min.

[0056] Cooling method: Air cooling.

[0057] Heat treatment: Annealing temperature 850℃, holding for 3.5h, normalizing temperature 950℃, holding for 1.5h, tempering temperature 600℃, holding for 1.5h.

[0058] Example 3:

[0059] Raw material: High-sulfur manganese medium-carbon free-cutting steel, composition: C 0.38wt%, Si 0.25wt%, Mn 1.79wt%, P 0.018wt%, S 0.25wt%, O 0.0067wt%, La 0.016wt%, the remainder being Fe and unavoidable impurities;

[0060] LaMn alloy: La 70wt%, Mn 30wt%.

[0061] Melting temperature: 1600℃, LaMn alloy addition: 0.36wt% (0.36wt% of the total weight of raw materials), holding time: 5min.

[0062] Cooling method: Air cooling.

[0063] Heat treatment: Annealing temperature 850℃, holding for 3.5h, normalizing temperature 950℃, holding for 1.5h, tempering temperature 600℃, holding for 1.5h.

[0064] Table 1 Impact test data results (room temperature, J)

[0065]

[0066] Table 2. Hardness test data results (Rockwell hardness: HRC)

[0067]

[0068]

[0069] Table 3. Data results for tensile strength

[0070]

[0071] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing MnS inclusion-modified high-sulfur manganese free-cutting steel, characterized in that: include: (1) Smelting process Scrap steel, pig iron, ferroalloys, and raw materials are loaded into a crucible according to the specified proportions. A carbon raiser, FeS, and LaMn alloy are placed in a feeding hopper. The crucible is then placed in a vacuum induction furnace, and a vacuum level of 50-300 Pa is created. Power is then gradually increased to reach a temperature of 1600-1650℃ to melt the alloy. During the steel refining stage, carbon raiser, FeS, and LaMn alloy are added separately to adjust the sulfur content, manganese content, and lanthanum content to the target range of 0.20-0.25 wt%, 0.20-1.85 wt%, and 0.001-0.018 wt%. Before tapping, the steel composition is tested and adjusted appropriately. Once the steel composition meets the standards, it can be tapped and cast into a billet. The billet is cooled naturally in air. (2) Addition and composition of LaMn alloy During the smelting process, LaMn alloy is added according to the designed ratio 5 minutes before tapping to ensure that the alloy can be completely dissolved; the composition of LaMn alloy by mass percentage is: La content 60-70wt%, Mn content 30-40wt%; (3) Heat treatment Annealing, normalizing, and tempering improve the microstructure of steel and the composition, size, quantity, and distribution of inclusions; the cast billet undergoes heat treatment: The first step is the annealing process, with a temperature between 830-850℃ and a holding time of 3-4 hours. The heating rate is controlled at 100-120℃ / h, and the cooling method is furnace cooling with a cooling rate controlled at 80-100℃ / h, in order to ensure the uniform internal structure of the billet and eliminate stress concentration. The second process is normalizing, with a temperature between 930-950℃ and a holding time of 1.5h. The heating rate is controlled at 130-160℃ / h to ensure uniform heating of the billet. The cooling method is air cooling, with a cooling rate controlled at 200-300℃ / h to refine the microstructure of the billet. Finally, there is the tempering process, with a temperature of 580-600℃ and a holding time of 1.5h. The heating rate is controlled at 100-120℃ / h to ensure uniform heating of the billet. Air cooling is used, with a cooling rate controlled at 150-200℃ / h to eliminate the stress generated by normalizing. The chemical composition of the high-sulfur manganese free-cutting steel by weight percentage is as follows: C: 0.35-0.55wt%; Si≤0.30wt%; Mn≤1.85wt%; P ≤ 0.020 wt%; S: 0.20-0.25wt%; O: 0.005-0.007wt%; La: 0.001-0.018 wt%; the remainder is Fe and other unavoidable impurities.

2. The method for preparing high-sulfur manganese free-cutting steel with MnS inclusion modification according to claim 1, characterized in that: When adding LaMn alloy, add it in two stages to avoid adding too much at once and causing local enrichment. After each addition, stir thoroughly to ensure uniform alloy distribution.

3. The method for preparing high-sulfur manganese free-cutting steel with MnS inclusion modification according to claim 1, characterized in that, The chemical composition of the high-sulfur manganese free-cutting steel by weight percentage is as follows: C 0.39wt%, Si 0.25wt%, Mn 1.74wt%, P 0.018wt%, S 0.29wt%, O 0.0050wt%, La 0.012wt%; the remainder is Fe and other unavoidable impurities.

4. The method for preparing high-sulfur manganese free-cutting steel with MnS inclusion modification according to claim 3, characterized in that, Melting temperature: 1600℃, LaMn alloy addition: 0.12wt%, holding time: 5min; Cooling method: Air cooling; Heat treatment: Annealing temperature 850℃, holding for 3.5h, normalizing temperature 950℃, holding for 1.5h, tempering temperature 600℃, holding for 1.5h.

5. The method for preparing high-sulfur manganese free-cutting steel with MnS inclusion modification according to claim 1, characterized in that... The chemical composition of the high-sulfur manganese free-cutting steel by weight percentage is as follows: C 0.38wt%, Si 0.25wt%, Mn 1.74wt%, P 0.019wt%, S 0.23wt%, O 0.0068wt%, La 0.015wt%; the remainder is Fe and other unavoidable impurities.

6. The method for preparing high-sulfur manganese free-cutting steel with MnS inclusion modification according to claim 5, characterized in that, Melting temperature: 1600℃, LaMn alloy addition: 0.24wt%, holding time: 5min; Cooling method: Air cooling; Heat treatment: Annealing temperature 850℃, holding for 3.5h, normalizing temperature 950℃, holding for 1.5h, tempering temperature 600℃, holding for 1.5h.

7. The method for preparing high-sulfur manganese free-cutting steel with MnS inclusion modification according to claim 1, characterized in that, The chemical composition of the high-sulfur manganese free-cutting steel by weight percentage is as follows: C 0.38wt%, Si 0.25wt%, Mn 1.79wt%, P 0.018wt%, S 0.25wt%, O 0.0067wt%, La 0.016wt%; the remainder is Fe and other unavoidable impurities.

8. The method for preparing high-sulfur manganese free-cutting steel with MnS inclusion modification according to claim 7, characterized in that, Melting temperature: 1600℃, LaMn alloy addition: 0.36wt%, holding time: 5min; Cooling method: Air cooling; Heat treatment: Annealing temperature 850℃, holding for 3.5h, normalizing temperature 950℃, holding for 1.5h, tempering temperature 600℃, holding for 1.5h.

9. The method for preparing high-sulfur manganese free-cutting steel with MnS inclusion modification according to any one of claims 1-8, characterized in that: The LaMn alloy has the following composition by mass percentage: La 70wt%, Mn 30wt%.

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