A tin-containing free-cutting steel ingot and a vacuum induction smelting method for preparing the same

Abstract

This invention discloses a method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting. The method involves preheating the alloy raw materials; vacuum melting the preheated alloy raw materials in a specific composition ratio; thoroughly mixing the vacuum-melted steel with graphite, ferrosilicon, ferromanganese, ferrophosphorus, ferromolybdenum, ferrisulfide, and metallic tin under an inert atmosphere to obtain a mixed steel liquid; pouring the mixed steel liquid into a mold and cooling it to room temperature to obtain the tin-containing free-cutting steel ingot. Through this method, the sulfide morphology in the free-cutting steel prepared by this invention is mainly spherical and spindle-shaped, with sulfides having an aspect ratio ≤3 accounting for more than 80%, thus possessing both good strength and machinability.

Description

Technical Field

[0001] This invention relates to the field of steel processing technology, and in particular to a tin-containing free-cutting steel ingot and its preparation method by vacuum induction smelting. Background Technology

[0002] Free-cutting steel refers to alloy steel in which one or more free-cutting elements such as sulfur, phosphorus, lead, calcium, selenium, and tellurium are added to improve machinability. Based on the different free-cutting elements contained, free-cutting steel can be divided into sulfur-based free-cutting steel, lead-based free-cutting steel, titanium-based free-cutting steel, and composite free-cutting steel, etc. Sulfur-based free-cutting steel is the earliest developed and, to date, the most widely used and consumed type of free-cutting steel, accounting for over 70% and 90% of the world's and my country's total free-cutting steel production, respectively. Sulfur-based free-cutting steel is mainly used in complex components such as bolts, nuts, pipe fittings, automotive brake parts, spring seats, and molds. These complex components require machining on CNC machine tools. To extend tool life, reduce machining costs, and improve production efficiency, the steel must possess excellent machinability. In sulfur-based free-cutting stainless steel, sulfur mainly exists in the form of manganese sulfide. Manganese sulfide inclusions can act as stress concentration sources, inducing numerous microcracks in the matrix, reducing cutting resistance, and causing the steel to easily break chips during turning.

[0003] Researchers initially classified manganese sulfide in steel castings into three categories based on its morphology and distribution: The first category consists of spherical composite inclusions, randomly distributed, found in steel without aluminum deoxidation; the second category consists of short rod-shaped inclusions, distributed in chains or networks along grain boundaries, found in steel deoxidized with a small amount of aluminum; and the third category consists of blocky inclusions, irregularly distributed, found in steel with high aluminum content and residual aluminum. Later, researchers added a fourth category: dendritic sulfides. Some researchers have studied the influence of alloying elements on the morphology of manganese sulfide in free-cutting steels and classified it as follows: the first category is spherical (Category I), formed by segregation reactions; the second category consists of short rods or dendritic inclusions (Category II), formed by eutectic reactions; and the third category consists of irregular shapes (Category III), generated by pseudo-eutectic reactions. The size, morphology, and distribution of manganese sulfide in sulfur-containing free-machining stainless steel have a significant impact on the mechanical properties of the steel. To obtain optimal machinability, spherical or spindle-shaped sulfide inclusions with small aspect ratios are desirable in production. These inclusions are less prone to deformation during rolling due to their low plasticity, and can maintain their spindle or ellipsoidal shape after matrix deformation, which is highly beneficial for improving machinability. Conversely, elongated manganese sulfide with an aspect ratio exceeding 4:1 not only disrupts the continuity of the matrix but also causes chip adhesion, reducing the surface quality of the workpiece. Free-machining stainless steel is prone to forming elongated manganese sulfide after forging deformation, causing anisotropy in the steel and reducing its overall mechanical properties.

[0004] Tin and lead belong to the same group and have similar physical and chemical properties, making them free-machining elements. Tin-containing steel exhibits a brittleness valley around 275°C. When machining tin-containing steel, the steel tends to fracture brittlely when the cutting temperature is near this valley, easily generating chips during the cutting process, thus improving the steel's machinability. Tin has a high boiling point, low vapor pressure, is not easily volatile, and is non-toxic. The production and use of tin-containing free-machining steel will not have adverse environmental impacts, making it a "green and environmentally friendly" new steel grade. Tin resources are abundant and reasonably priced, making it an ideal substitute for lead. However, low tin content makes it difficult to achieve high machinability, while high tin content can cause tin segregation at austenite grain boundaries during hot rolling, leading to material brittleness and adversely affecting processing performance. Therefore, how to prepare free-machining steel with high tin content has become a key technology. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing high-tin-content tin-containing free-machining steel ingots using vacuum induction smelting. The tin-containing free-machining steel ingots obtained by this method have a sulfide content of ≤3 (length-to-width ratio ≤3) of over 80%, exhibiting both good strength and machinability. This method can be applied to the manufacture of parts for industries such as automobiles, high-speed rail, home appliances, and office equipment to reduce machining costs, improve production efficiency, and enhance product competitiveness, demonstrating promising application prospects.

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

[0007] A method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting, the method comprising:

[0008] Preheating treatment is performed on the alloy raw materials;

[0009] The preheated alloy raw materials are vacuum melted in a specific composition ratio;

[0010] The molten steel after vacuum melting is thoroughly mixed with graphite, ferrosilicon, ferromanganese, ferrophosphorus, ferromolybdenum, ferrous sulfate, and metallic tin under an inert atmosphere to obtain a mixed molten steel.

[0011] The mixed molten steel is poured into a mold and cooled to room temperature to obtain a tin-containing free-cutting steel ingot.

[0012] As a further improvement of the present invention, the preheating treatment conditions are as follows: heating the alloy raw material to 150℃~250℃ and holding it at that temperature for 1h~2h.

[0013] As a further improvement of the present invention, the preheating conditions are as follows: the alloy raw material is heated to 250°C and held for 2 hours.

[0014] As a further improvement of the present invention, the composition ratio of the alloy raw material is as follows (by mass percentage): 0.01%–0.05% C, 0.1%–0.35% Si, 1.5%–2.5% Mn, 8.0%–9.0% Ni, 15.0%–18.0% Cr, 0.2%–0.5% S, 0.1%–0.25% Mo, 0.005%–0.01% O, 0.01%–0.015% N, 0.5%–0.6% Sn, P ≤ 0.01%, with the balance being Fe and unavoidable impurities.

[0015] As a further improvement of the present invention, the alloy raw material comprises:

[0016] The mass percentage of Sn to S is: 1.25 ≤ Sn / S ≤ 1.8.

[0017] As a further improvement of the present invention, the alloy raw material comprises:

[0018] The mass percentage content of Sn is 0.5% to 0.55%;

[0019] The mass percentage content of sulfur is 0.3% to 0.4%.

[0020] As a further improvement of the present invention, the step of vacuum melting the preheated alloy raw material with a specific composition ratio includes: controlling the vacuum degree during the vacuum melting to be ≤1Pa; the melting temperature to be 1550℃~1600℃; and refining at 1500℃~1550℃ after melting for 10min~20min.

[0021] As a further improvement of the present invention, the melting temperature is controlled at 1580°C, and after melting, refining is carried out at 1550°C for 20 minutes.

[0022] As a further improvement of the present invention, the inert atmosphere conditions are controlled to be an argon atmosphere at 20000-30000 Pa.

[0023] As a further improvement of the present invention, the pouring temperature is controlled at 1480℃~1550℃ and the pouring time is 3min~5min.

[0024] The present invention also provides a tin-containing free-cutting steel ingot, wherein the tin-containing free-cutting steel ingot is prepared by any of the preceding preparation methods.

[0025] The technical effects and advantages of this invention are as follows:

[0026] The present invention discloses a method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting. This method involves preheating the alloy raw materials; vacuum melting the preheated alloy raw materials in a specific composition ratio; thoroughly mixing the vacuum-melted steel with graphite, ferrosilicon, ferromanganese, ferrophosphorus, ferromolybdenum, ferrisulfide, and metallic tin under an inert atmosphere to obtain a mixed steel liquid; pouring the mixed steel liquid into a mold and cooling it to room temperature to obtain the tin-containing free-cutting steel ingot. Through this method, the sulfide morphology in the free-cutting steel prepared by the present invention is mainly spherical and spindle-shaped, with sulfides having an aspect ratio ≤3 accounting for more than 80%, thus achieving both good strength and machinability.

[0027] The method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting of the present invention can produce smaller tin-containing free-cutting steel ingots, reducing the processing difficulty of the material, shortening the preparation cycle, and improving the material quality. Furthermore, the smelting process of the present invention is simple, and the raw materials for steel are widely available, which can reduce the production cost of tin-containing free-cutting steel.

[0028] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures pointed out in the description and the drawings. Attached Figure Description

[0029] Figure 1 This is a flowchart of the method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting according to the present invention;

[0030] Figure 2 This is a low-magnification image of a tin-containing free-cutting steel ingot prepared according to an embodiment of the present invention;

[0031] Figure 3 This is a SEM image of manganese sulfide in tin-containing free-cutting steel prepared according to an embodiment of the present invention.

[0032] Figure 4 The stress-strain curve of tin-containing free-cutting steel obtained in an embodiment of the present invention is shown.

[0033] Figure 5 SEM image of the tensile fracture surface of tin-containing free-cutting steel obtained in an embodiment of the present invention;

[0034] Figure 6 These are cutting chips from tin-containing free-cutting steel prepared according to embodiments of the present invention. Detailed Implementation

[0035] 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 embodiments of the present invention, and not all embodiments. Based on the 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.

[0036] To address the shortcomings of existing technologies, this invention provides a method for preparing tin-containing free-cutting steel ingots through vacuum induction smelting, and also provides an environmentally friendly tin-containing free-cutting steel. To achieve sufficiently high cutting performance, this invention increases the tin content based on existing technologies, preparing a "super" free-cutting steel through vacuum smelting. Furthermore, by appropriately controlling the proportions of Mn, Sn, and S elements, both good mechanical properties and excellent cutting performance are ensured. A key feature of this invention is the effective control and improvement of the morphology, size, aspect ratio, and distribution of sulfides in the free-cutting steel through the rational addition of Sn. The sulfide morphology in the free-cutting steel prepared using this method is predominantly spherical and spindle-shaped, with sulfides having an aspect ratio ≤3 accounting for over 80%. The free-cutting steel exhibits tensile strength ≥570 MPa, yield strength ≥500 MPa, reduction of area ≥20%, elongation after fracture ≥20%, and impact toughness ≥15 J. The high-strength, high-machining-performance tin-containing free-cutting steel prepared by this invention is suitable for manufacturing parts for industries such as automobiles, high-speed rail, home appliances, and office equipment. It can effectively reduce machining costs, improve production efficiency, and enhance product competitiveness.

[0037] The technical solution of the present invention is as follows:

[0038] Please see Figure 1 As shown, this invention provides a method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting. The constituent elements and their mass percentages are as follows: 0.01%–0.05% C, 0.1%–0.35% Si, 1.5%–2.5% Mn, 8.0%–9.0% Ni, 15.0%–18.0% Cr, 0.2%–0.5% S, 0.1%–0.25% Mo, 0.005%–0.01% O, 0.01%–0.015% N, 0.5%–0.6% Sn, P ≤ 0.01%, with the remainder being Fe and unavoidable impurities.

[0039] Furthermore, the mass percentage of Sn to S satisfies 1.25 ≤ Sn / S ≤ 1.8;

[0040] Furthermore, the Sn content by mass percentage is preferably 0.5% to 0.55%.

[0041] Furthermore, the mass percentage content of S is preferably 0.3% to 0.4%.

[0042] A. Preparations before smelting:

[0043] Before smelting, all alloy raw materials required for smelting are baked to 150℃~250℃ and held at that temperature for 1h~2h. Before starting the furnace, all vacuum pumps, water cooling systems, rotating devices, electrical cabinet systems, and sealing devices are test-run to ensure they are operating normally.

[0044] Furthermore, the preferred baking temperature for the alloy raw materials in step a is 250°C, and the preferred holding time is 2 hours.

[0045] B. Weighing the materials:

[0046] Accurately weigh the raw materials according to the quantities specified in the batching instructions. Large furnace materials such as high-purity iron and metallic chromium are weighed using a 100kg electronic scale, while smaller furnace materials such as graphite, metallic silicon, metallic manganese, ferrophosphorus, ferrosulfur, ferromolybdenum, nickel plates, and metallic tin are weighed using a 200g electronic scale. Before weighing each furnace material, grind it with a grinding wheel to remove the oxide scale on its surface and dry it. Each furnace batch consists of 145kg of materials, and the batch is checked again before loading into the furnace.

[0047] C. Loading:

[0048] The materials should be packed as tightly as possible to avoid bridging. The charging sequence is as follows: high-purity iron, ferrochrome, and nickel plates are loaded into the furnace along with the main charge. Graphite, metallic silicon, metallic manganese, ferrophosphorus, ferrosulfur, ferromolybdenum, and metallic tin are placed in separate hoppers within the vacuum induction furnace.

[0049] D. Melting:

[0050] When the vacuum degree in the melting chamber is ≤1Pa, start the vacuum induction furnace to heat the furnace charge and begin melting. The melting and heating process steps are as follows: 30% power, for 15-20 minutes; 60% power, for 10-20 minutes; 75% power, for 10-20 minutes; 90%-100% power, continue melting until all the pure iron, ferrochrome, and nickel plates are completely melted along with the furnace charge.

[0051] E. Refining:

[0052] After the furnace charge is completely melted, the temperature is further increased to 1550℃~1600℃. The power supply is then appropriately reduced to maintain the molten steel temperature at 1500℃~1550℃ for refining, which takes 10min~20min. During the refining process, the vacuum level in the melting chamber is maintained at ≤1Pa.

[0053] Furthermore, after the furnace charge is melted and cleared, the molten steel is heated to a preferred temperature of 1580°C, and the power supply is appropriately reduced to maintain the temperature of the molten steel at a preferred temperature of 1550°C for refining. The refining time is preferably 20 minutes.

[0054] F. Adding alloys:

[0055] After refining, argon gas is introduced to 20,000–30,000 Pa. Graphite, ferrosilicon, ferromanganese, ferrophosphorus, ferromolybdenum, ferrisulfide, and metallic tin are added sequentially. After each alloy is added, it is subjected to high-power electromagnetic stirring for 3–5 minutes, followed by standing for 3–5 minutes.

[0056] Furthermore, the preferred argon charge level after refining is 20000 Pa;

[0057] G. Pouring:

[0058] The pouring temperature is 1480℃~1550℃, the pouring time is 3min~5min, and the steel is poured into a φ200mm×400mm metal mold. After the molten steel solidifies in the furnace and cools to room temperature, a φ200mm ingot is obtained.

[0059] Furthermore, the preferred casting temperature is 1550℃, and the preferred casting time is 3 minutes.

[0060] The principle of this invention:

[0061] The effects of elemental composition on the machinability and mechanical properties of tin-containing free-cutting steel in this invention:

[0062] C: Carbon is one of the key elements for improving the strength and hardness of free-machining stainless steel. The carbon content directly affects the strength, plasticity, toughness, and weldability of the steel. A carbon content of 0.01% to 0.05% is most beneficial to the machinability and mechanical properties of free-machining stainless steel. When the carbon content in the steel is too low, the ferrite content increases, and the strength and hardness of the steel decrease. Because the steel is too soft and tough, the cutting chips tend to stick to the tool, resulting in a decrease in its machinability. Conversely, when the carbon content in the steel is too high, its hardness increases, which also causes tool wear and reduces the machinability of the steel. Therefore, the carbon content in this invention is controlled within the range of 0.01% to 0.05%.

[0063] Silicon (Si) primarily functions as a deoxidizer in free-machining stainless steel, controlling its oxygen content and improving its yield strength and work hardening rate. It also affects sulfide inclusion deformation and machinability. Excessive silicon content can lead to silicate inclusions, causing tool wear and reducing cutting performance. Furthermore, high silicon content can result in the formation of a large amount of oxide scale during hot working, reducing the surface quality of the steel and promoting phosphorus precipitation from the matrix, forming phosphide films at grain boundaries and worsening the steel's hot workability. Therefore, it is necessary to rationally control the silicon content in free-machining stainless steel. In this invention, the Si content is controlled within the range of 0.1% to 0.35%.

[0064] Mn: Manganese is one of the important elements for improving the machinability of free-machining stainless steel. Manganese readily combines with sulfur to form manganese sulfide, which is the most important free-machining phase in free-machining stainless steel. Excessive manganese content increases the strength and toughness of the steel but reduces its machinability, while insufficient manganese content results in less manganese sulfide formation, which is detrimental to improving machinability. Therefore, in this invention, the Mn content is controlled within the range of 1.5% to 2.5%.

[0065] Ni: Nickel is an excellent corrosion-resistant material and an important alloying element for free-cutting steels. Nickel can both improve the strength of steel and maintain its good ductility and toughness. Nickel is an austenite-forming element in steel, enabling the material to achieve a pure austenitic structure. In this invention, the Ni content is controlled within the range of 8.0% to 9.0%.

[0066] Cr: The main role of chromium in steel is to improve its corrosion resistance. Chromium also improves the hardenability of steel, giving it better overall mechanical properties after quenching and tempering. Chromium also increases the strength of steel, especially when other alloying elements are added. However, excessive chromium content in steel increases its hardness, which can negatively impact its machinability. Therefore, in this invention, the Cr content is controlled within the range of 15% to 18%.

[0067] Sulfur (S) is the most abundant free-machining element in free-machining stainless steel. It mainly exists in the steel matrix as sulfides, and the size, content, morphology, and distribution of these sulfides directly affect the steel's machinability. When the sulfur content is below 0.1%, sufficient sulfides cannot be formed in the steel, failing to meet the requirements for high machinability. Conversely, when the sulfur content exceeds 0.6%, its hot workability decreases, and it easily leads to sulfur segregation in the center of the ingot. Therefore, in this invention, the S content is controlled within the range of 0.2% to 0.5%.

[0068] Mo: Molybdenum can refine the grain size of steel, improving hardenability and hot strength. When molybdenum coexists with chromium and manganese, it can reduce or suppress temper brittleness caused by other elements. Therefore, the Mo content in this invention is controlled within the range of 0.1% to 0.25%.

[0069] Tin (Sn) is miscible with iron in the liquid state at high temperatures, with a maximum solubility of 17.7% in solid α-Fe. However, the solubility of tin in steel decreases with decreasing temperature. Below 200℃, the solid solubility of Sn in α-Fe decreases sharply, theoretically leading to the formation of FeSn. However, Sn diffusion is very slow, and Sn exists in solid solution form without significant macroscopic segregation. Only a small portion forms FeSn, which is practically difficult to detect. Tin-containing steel has a brittleness valley around 275℃. During machining, the cutting temperature is near this valley, and the steel tends to fracture brittlely, easily generating chips during cutting, thus improving the machinability of the steel. The United States has developed a free-machining steel that uses tin instead of lead and has put it on the market. The mechanical properties of this cutting steel are basically the same as those of lead-containing free-machining steel, making it easier to cut. The US patent for "Tin-bearing free-machining steel" (US 5961747, Oct. 5, 1999) requires a Sn content of 0.04% to 0.08% in its chemical composition. The main characteristic of the steel provided in this invention compared to the US patent is a significantly increased tin content, using tin as the primary additive element to improve the steel's machinability. Therefore, the Sn content in this invention is controlled within the range of 0.5% to 0.6%.

[0070] O: In free-machining stainless steel, oxygen mainly plays a role in regulating the morphology of sulfides. Under normal circumstances, when the oxygen content is higher than 0.02%, type I sulfides (spherical, irregularly distributed, with single-phase or two-phase inclusions, often found in steels without aluminum deoxidation, which can significantly improve the machinability of the steel) are formed in the steel. When the oxygen content in the steel is between 0.004% and 0.01%, type II sulfides (short rod-shaped, distributed in a chain or network pattern along the grain boundaries, often found in steels deoxidized with a small amount of aluminum) are easily formed. When the oxygen content in the steel is less than 0.004%, type III sulfides (massive, irregularly distributed, often found in steels with high aluminum content and residual aluminum) are easily formed. The three types of sulfides often appear simultaneously in the steel, including spherical, massive, and short rod-shaped sulfides. Among them, type I sulfides are the most beneficial to the machinability of the steel. Oxygen in sulfides can form (Mn, Fe) and (S, O) complex inclusions with other elements. These inclusions have low plasticity and are not easily deformed during hot working, maintaining a spindle or spherical shape, which is beneficial for improving machinability. However, excessive oxygen content can also affect the surface quality of free-machining stainless steel, causing subcutaneous bubbles and severe compositional segregation in the center of the billet. High oxygen content in molten steel can also cause nozzle blockage during continuous casting, and excessive hard oxide inclusions can adversely affect tool wear, ultimately impacting the steel's machinability. Increased oxygen content also reduces the high-temperature mechanical properties of the test steel. By rationally controlling the oxygen content to form oxide nuclei of sulfur-containing inclusions in the steel, the size, quantity, distribution, and morphology of sulfides can be regulated and improved. Therefore, considering the composition, the O content in this invention is easily controlled within the range of 0.005% to 0.01%.

[0071] P: Phosphorus can improve the strength and hardenability of steel, but excessive P content will reduce the machinability of steel and lead to a decrease in hot working properties. Therefore, in this invention, the P content is controlled to be below 0.01%.

[0072] Nitrogen (N) can combine with titanium to form nitrides and carbonitrides. These nitrides and carbonitrides often nucleate and precipitate at grain boundaries, which helps refine grains and improve the strength of steel. During the solidification of molten steel, nitrides and carbonitrides can also act as nucleation sites for sulfides, which helps improve the distribution of sulfides and thus improves the machinability of free-machining stainless steel. However, if the nitrogen content is too high, large inclusions are easily formed, reducing the machinability of the steel. Therefore, in this invention, the N content is controlled within the range of 0.01% to 0.015%.

[0073] The method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting provided by the present invention will be illustrated below with specific examples:

[0074] The composition of tin-containing free-cutting steel is as follows: C: 0.04%, Si: 0.3%, Mn: 1.9%, Cr: 17.2%, Ni: 8.1%, S: 0.32%, O: 0.01%, Mo: 0.2%, Sn: 0.5%, N: 0.013%, O: 0.009%, P: 0.004% (wt%); the remainder is Fe and unavoidable impurities. According to the technical solution of this invention, high-purity iron, ferrochrome, nickel plate, graphite, metallic silicon, metallic manganese, ferrophosphorus, ferrosulfur, ferromolybdenum, and metallic tin are used as alloy raw materials, and the material list of each alloy raw material is shown in Table 1.

[0075] Table 1 Composition of each raw material

[0076] Al C Ca Cr Ni Fe Mn P S Si Mo P Sn metallic manganese 0 0.053 0 0 2.8 97.17 ＜0.1 0.021 0.4 0 0 0 Metallic silicon 0.3 0.023 0.7 ＜0.1 0 3.6 0 0.8 ＜0.003 95.44 0 0 0 Ferromolybdenum 0 0.07 0 0 0 0 0 0 0 0.242 56.07 0 0 metallic chromium 0.024 0.032 0 99.2 0 0.568 0 ＜0.01 0.0044 0.127 0 0 0 Iron Pyrite 0 0.12 0 0 0 36.75 0.2 ＜0.1 26 0.5 0 0 0 Phosphorus iron 0 0 0 0 0 75 0 0 0 0 0 25 0 Metallic tin 0 0 0 0 0 0.01 0 0 0 0 0 0 99.9 graphite 0 97.5 0 0 0 0 0 0 0 0 0 0 0 Nickel plate 0 0 0 0 99.98 0 0 0 0 0 0 0 0 High-purity iron 0 0.003 0.01 0 99.94 0.04 0.004 0.001 0 0 0 0

[0077] (1) Preparation before smelting: Before smelting, bake all alloy raw materials required for smelting to 250°C and keep them at that temperature for 2 hours. Before starting the furnace, test run all vacuum pumps, water cooling systems, rotating devices, electrical cabinet systems, and sealing devices, and ensure that they are operating normally.

[0078] (2) Weighing: Weigh the raw materials accurately according to the quantity specified in the batching. Large furnace materials such as high-purity iron and metallic chromium are weighed using a 100kg electronic scale, while smaller furnace materials such as graphite, metallic silicon, metallic manganese, ferrophosphorus, ferrosulfur, ferromolybdenum, nickel plates, and metallic tin are weighed using a 200g electronic scale. Before weighing each furnace material, grind it with a grinding wheel to remove the oxide scale on its surface and dry it. Each furnace batch is 145Kg, and the batching is checked again before loading the furnace. The batching list of alloy raw materials is shown in Table 2.

[0079] Table 2 Alloy Raw Material Batching List

[0080] alloy material Weight (kg) pure iron 101.098 graphite 0.048 Metallic silicon 0.416 metallic manganese 2.901 Phosphorus iron 0.178 Iron Pyrite 1.879 Ferromolybdenum 0.528 metallic chromium 25.267 Nickel plate 11.923 Metallic tin 0.763

[0081] (3) Charging: The charging should be as tight as possible to avoid bridging. Charging sequence: Pure iron, ferrochrome, and nickel plates are charged with the furnace. Graphite, metallic silicon, metallic manganese, ferrophosphorus, ferrosulfur, ferromolybdenum, and metallic tin are placed in separate bins in the vacuum induction furnace.

[0082] (4) Melting: When the vacuum degree of the melting chamber is 0.5 Pa, start the vacuum induction furnace to heat the furnace charge and start melting. The melting and heating process steps are as follows: 30% power, for 20 min; 60% power, for 20 min; 75% power, for 20 min; 90%~100% power, continue melting until pure iron, ferrochrome, and nickel plates are completely melted along with the furnace charge;

[0083] (5) Refining: After the furnace charge is melted and cleared, continue to raise the temperature to 1580℃. Reduce the power supply appropriately to keep the temperature of the molten steel at 1550℃ for refining. The refining time is 20 minutes. During the refining period, always maintain the vacuum degree of the melting chamber ≤1Pa.

[0084] (6) Adding alloys: After refining, purge with argon gas to 20000 Pa, and add graphite, ferrosilicon, ferromanganese, ferrophosphorus, ferromolybdenum, ferrisulfide, and metallic tin in sequence. After each alloy is added, perform high-power electromagnetic stirring for 5 minutes, and then let it stand for 5 minutes.

[0085] (7) Casting: The casting temperature is 1550℃ and the casting time is 3min. The steel is poured into a φ200mm×400mm metal mold. After the molten steel solidifies in the furnace and cools to room temperature, a φ200mm ingot is obtained. The chemical composition of the tin-containing free-cutting steel is shown in Table 3.

[0086] Table 3. Composition of tin-containing free-cutting steel (wt%)

[0087] C Si Si P S Ni Cr Mo O Sn N Fe 0.034 0.252 1.9 0.0041 0.361 8.06 16.83 0.211 0.0094 0.54 0.013 margin

[0088] Figure 2 The image shown is a low-magnification image of a tin-containing free-cutting steel ingot prepared according to an embodiment of the present invention. The ingot has no obvious shrinkage porosity or shrinkage cavities in the center, indicating good ingot quality. Figure 3 This is a SEM image of manganese sulfide in tin-containing free-cutting steel prepared according to an embodiment of the present invention. Figure 3 It can be seen that the manganese sulfide in the structure is spindle-shaped and elliptical, with very little content in the long strip-shaped manganese sulfide. The morphology of manganese sulfide is very helpful in improving the machinability of the material. Figure 4 This is a stress-strain curve diagram of tin-containing free-cutting steel obtained in an embodiment of the present invention. Figure 4 It can be seen that there is no obvious yield plateau during the stretching process, and it exhibits continuous yield deformation. Figure 5 This is a SEM image of the tensile fracture surface of tin-containing free-cutting steel obtained in an embodiment of the present invention. Figure 5 It can be seen that the fracture surface is a mixture of dimples and cleavage, indicating that the material has good plasticity and toughness. Figure 6 The cutting chips (500 rpm, feed rate 0.06 mm) of tin-containing free-cutting steel prepared according to embodiments of the present invention are produced by... Figure 6 It can be seen that tin-containing free-cutting steel with uniform and fine chip distribution has excellent cutting performance.

[0089] The manganese sulfide in the as-cast steel prepared in the examples was analyzed using ASPEX scanning electron microscopy and energy dispersive spectroscopy. The area of ​​the selected sample for statistical analysis was 8.348872 mm². 2 Table 4 shows the statistical results of the aspect ratio of manganese sulfide in the sample of the example. In the free-machining stainless steel of the example, the proportion of manganese sulfide with an aspect ratio ≤3 is as high as 88.49%, mainly in spherical and spindle shapes, which is beneficial to improving the machinability of the steel.

[0090] Table 4. Statistical analysis of the length-to-width ratio of manganese sulfide in tin-containing free-cutting steel.

[0091] Aspect Ratio percentage X≤3 88.49 3＜X≤5 8.41 5＜X≤10 2.86 10＜X≤30 0.23 X＞30 0.01

[0092] Referring to the current national standards GB / T228.1-2010 and GB / T19748-2005, tensile test bars with a diameter of 5 mm and standard V-notch Charpy impact specimens of 10 mm × 10 mm × 55 mm were prepared. Room temperature tensile and Charpy impact tests were conducted using an MTS Landmark 370 electro-hydraulic servo universal testing machine and an MTS falling weight impact testing machine to determine the mechanical properties of the embodiment of the invention. The tensile strength, yield strength, elongation after fracture, reduction of area, and impact toughness of the embodiment are shown in Table 5. As can be seen from Table 5, its tensile strength ≥ 570 MPa, yield strength ≥ 500 MPa, reduction of area ≥ 20%, elongation after fracture ≥ 20%, and impact toughness ≥ 15 J, exhibiting good strength and machinability.

[0093] Table 5 Mechanical properties of tin-containing free-cutting steel in the examples

[0094]

[0095] In summary, the method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting of the present invention produces φ200mm tin-containing free-cutting steel ingots with smaller dimensions, reducing material processing difficulty, shortening the preparation cycle, and improving material quality. Since tin-containing free-cutting steel does not contain lead, it will not cause environmental pollution and conforms to the national energy conservation and emission reduction development strategy. Furthermore, the method has a simple smelting process and a wide range of raw material sources, reducing the production cost of tin-containing free-cutting steel. In addition, the tin-containing free-cutting steel of the present invention has a sulfide content of ≤3 of over 80%, tensile strength ≥570MPa, yield strength ≥500MPa, reduction of area ≥20%, elongation after fracture ≥20%, and impact toughness ≥15J, exhibiting excellent strength and machinability. Therefore, the high-strength and high-machinability tin-containing free-cutting steel prepared by the present invention can be used to manufacture parts for industries such as automobiles, high-speed rail, home appliances, and office equipment to reduce machining costs, improve production efficiency and product competitiveness, and has good application prospects.

[0096] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting, characterized in that, The method includes: Preheating treatment is performed on the alloy raw materials; The preheated alloy raw materials are vacuum melted in a specific composition ratio. The composition ratio of the alloy raw materials is as follows (by mass percentage): 0.01%–0.05% C, 0.1%–0.35% Si, 1.5%–2.5% Mn, 8.0%–9.0% Ni, 15.0%–18.0% Cr, 0.3%–0.4% S, 0.1%–0.25% Mo, 0.005%–0.01% O, 0.01%–0.015% N, 0.5%–0.55% Sn, P ≤ 0.01%, with the balance being Fe and unavoidable impurities. The mass percentage of Sn to S is 1.25 ≤ Sn / S ≤ 1.

8. The molten steel after vacuum melting is thoroughly mixed with graphite, ferrosilicon, ferromanganese, ferrophosphorus, ferromolybdenum, ferrous sulfate, and metallic tin under an inert atmosphere to obtain a mixed molten steel. The mixed molten steel is poured into a mold and cooled to room temperature to obtain a tin-containing free-cutting steel ingot.

2. The method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting according to claim 1, characterized in that, The preheating conditions are as follows: the alloy raw material is heated to 150 ℃~250 ℃ and held for 1 h~2 h.

3. The method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting according to claim 2, characterized in that, The preheating conditions are as follows: the alloy raw material is heated to 250 °C and held for 2 h.

4. The method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting according to claim 1, characterized in that, The process of vacuum melting the preheated alloy raw materials in a specific composition ratio includes: The vacuum degree during the vacuum melting is controlled to be ≤1 Pa; the melting temperature is 1550℃~1600℃; after the melting is completed, the refining is carried out at 1500℃~1550℃ for 10min~20min.

5. The method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting according to claim 4, characterized in that, The melting temperature was controlled at 1580℃, and after melting, refining was carried out at 1550℃ for 20 minutes.

6. The method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting according to claim 1, characterized in that, The inert atmosphere conditions are controlled to be an argon atmosphere at 20000~30000 Pa.

7. The method for preparing tin-containing free-cutting steel ingots by vacuum induction smelting according to claim 1, characterized in that, The pouring temperature is controlled at 1480℃~1550℃, and the pouring time is 3 min~5 min.

8. A tin-containing free-cutting steel ingot, characterized in that, The tin-containing free-cutting steel ingot is prepared by the method described in any one of claims 1 to 7.

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