Cerium-containing high-sulfur free-cutting steel and vacuum induction smelting preparation method thereof

Abstract

The present application relates to the technical field of cutting steel, in particular to a kind of high-sulfur free-cutting steel containing cerium and a vacuum induction smelting preparation method thereof.The method for preparing the ingot of the free-cutting steel containing cerium by vacuum induction smelting of the present application carries out surface treatment of drying and shot blasting on alloy raw materials to remove surface oxides and impurities;the alloy raw materials after drying and shot blasting are vacuum smelted with specific component proportion;the molten steel after vacuum smelting is fully mixed with industrial pure iron, graphite, ferrosilicon, ferromanganese, electrolytic nickel, metallic chromium, ferrous sulphur, molybdenum strip and rare earth cerium in inert atmosphere to obtain mixed molten steel;the molten steel is poured out with electric charge, the pouring speed is controlled well, the vacuum is broken, the mold is demoulded and air-cooled, and the free-cutting steel ingot containing cerium is obtained after cooling to room temperature.The sulfide in the free-cutting steel containing cerium prepared by the present application is mainly in the form of cerium-containing composite sulfide, in spherical and spindle shape, and the proportion of sulfide with length-width ratio ≤3 in the free-cutting steel is more than 86%, which has good strength and cutting performance.

Description

Technical Field

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

[0002] Free-cutting steel refers to alloy steel whose machinability is improved by adding a certain amount of one or more elements such as S, P, Pb, Ca, Se, Te, Ti, and Ce. Based on the different added elements, free-cutting steel can be classified into sulfur-based free-cutting steel, lead-based free-cutting steel, tellurium-based free-cutting steel, titanium-based free-cutting steel, and composite free-cutting steel. Sulfur-based free-cutting steel is the earliest developed, and to date, it has the largest volume and widest application, accounting for more than 70% of the world's total free-cutting steel production. Studies have found that when sulfides are spherical or spindle-shaped, they are most beneficial for improving the material's machinability. Slender, elongated sulfides with an aspect ratio exceeding 4:1 not only disrupt the continuity of the matrix but also cause chip adhesion, reducing the surface quality of the workpiece. Rare earth cerium is miscible with molten steel at high temperatures. Cerium has a strong affinity for sulfur and oxygen, and can improve the sulfides in molten steel into rare earth sulfides, thereby controlling and improving the morphology, size, aspect ratio, and distribution of the sulfides.

[0003] The production and use of cerium-containing free-cutting steel will not have an adverse impact on the ecology and is a "green and environmentally friendly" steel. Therefore, how to prepare cerium-containing free-cutting steel with good mechanical properties has become a key technology. Summary of the Invention

[0004] To address the above problems, this invention provides a cerium-containing high-sulfur free-cutting steel and its preparation method by vacuum induction smelting.

[0005] In a first aspect, the present invention provides a cerium-containing high-sulfur free-cutting steel.

[0006] The composition is as follows (by mass percentage): 0.1%–0.14% C, 0.001%–0.1% Si, 1.05%–1.2% Mn, 0.001%–0.6% Ni, 12.0%–14.0% Cr, 0.25%–0.45% S, 0.001%–0.6% Mo, 0.005%–0.01% O, 0.01%–0.015% N, 0.0005%–0.008% Ce, P ≤ 0.01%, with the balance being Fe and unavoidable impurities.

[0007] Furthermore, the mass percentage content of Ce is adjusted to 0.001% to 0.007%.

[0008] Furthermore, the mass percentage content of S is adjusted to 0.25% to 0.3%.

[0009] Secondly, the present invention provides a vacuum induction smelting method for preparing cerium-containing high-sulfur free-cutting steel, the method comprising:

[0010] The alloy raw materials are dried and shot blasted to remove oxide scale and impurities from the surface of the alloy raw materials;

[0011] The alloy raw materials that have undergone dry shot blasting surface treatment are then vacuum melted.

[0012] The molten steel after vacuum melting is thoroughly mixed with industrial pure iron, graphite, ferrosilicon, ferromanganese, electrolytic nickel, metallic chromium, ferrous sulfate, molybdenum bars and rare earth cerium under an inert atmosphere to obtain a mixed molten steel.

[0013] The mixed molten steel is poured into a mold and cooled to room temperature to obtain a cerium-containing free-cutting steel ingot.

[0014] Furthermore, the drying conditions for the pre-alloyed raw materials are as follows: heat the alloy raw materials to 100℃~200℃ and hold for 2h~3h.

[0015] Furthermore, it also includes: preheating the alloy raw materials before vacuum melting, with the preheating conditions being: heating the alloy raw materials to 200°C and holding for 3 hours.

[0016] Furthermore, the preheated alloy raw materials are vacuum melted in a specific composition ratio, including: controlling the vacuum degree during vacuum melting to be ≤1Pa; the leakage rate to be ≤3Pa / min; the melting temperature to be 1500℃~1600℃; and refining at 1500℃~1550℃ after melting for 5min~10min.

[0017] Furthermore, the melting temperature is controlled at 1560℃, and after melting, refining is carried out at 1550℃ for 10 minutes.

[0018] Furthermore, the inert atmosphere conditions are controlled to be an argon atmosphere at 3000–6000 Pa.

[0019] Furthermore, the pouring temperature is controlled at 1500℃~1550℃, and the pouring time is 5min~10min.

[0020] Furthermore, the steel is poured while energized and tapped. After pouring, it is allowed to be broken into air after furnace cooling for 30 minutes. After mold cooling for ≥4 hours, it is demolded and air-cooled.

[0021] Thirdly, the present invention provides an application of cerium-containing high-sulfur free-cutting steel, which is used to manufacture parts for the automotive, high-speed rail, home appliance and office equipment industries.

[0022] This invention has at least the following beneficial effects:

[0023] The key feature of this patent lies in the effective control and improvement of the morphology, size, aspect ratio, and distribution of sulfides in free-cutting steel through the rational addition of cerium. The sulfides in the cerium-containing free-cutting steel of this invention are mainly cerium-containing composite sulfides, with sulfides having an aspect ratio ≤3 accounting for over 86%. These sulfides are primarily spherical and spindle-shaped, exhibiting excellent cutting performance during turning, producing short, uniform, and discontinuous chips. The cerium-containing free-cutting steel of this invention possesses good mechanical properties, with a tensile strength ≥575MPa, yield strength ≥500MPa, reduction of area ≥20%, elongation after fracture ≥40%, and impact toughness ≥15J. It is suitable for manufacturing parts for the automotive, high-speed rail, home appliance, and office equipment industries to reduce machining costs, improve production efficiency and product competitiveness, and has promising application prospects.

[0024] This invention targets cerium-containing free-cutting steel, controlling the morphology of sulfides in the steel to be predominantly spherical and spindle-shaped, with sulfides having an aspect ratio ≤3 accounting for over 86%, thus improving the material's machinability. Using the cerium-containing free-cutting steel of this invention can reduce production costs and improve economic efficiency.

[0025] 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

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 Metallographic micrograph of sulfides in cerium-containing free-cutting steel prepared according to an embodiment of the present invention;

[0028] Figure 2 This is a SEM image of sulfides in cerium-containing free-cutting steel obtained in an embodiment of the present invention.

[0029] Figure 3 This is a schematic diagram of the energy dispersive spectroscopy (EDS) analysis points of cerium-containing free-cutting steel prepared according to an embodiment of the present invention;

[0030] Figure 4 The above is an energy spectrum of cerium-containing composite sulfidation of cerium-containing free-cutting steel obtained in an embodiment of the present invention.

[0031] Figure 5 The stress-strain curve of cerium-containing free-cutting steel obtained in an embodiment of the present invention is shown.

[0032] Figure 6 The tensile fracture morphology of cerium-containing free-cutting steel obtained in an embodiment of the present invention is shown in the figure.

[0033] Figure 7 This is a statistical chart showing the aspect ratio data of sulfides in cerium-containing free-cutting steel prepared according to an embodiment of the present invention;

[0034] Figure 8 This is a schematic diagram of the cutting chips of cerium-containing free-cutting steel prepared according to an embodiment of the present invention. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, 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] The cerium-containing free-machining steel ingot prepared by this invention, after scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), shows that the proportion of sulfides with an aspect ratio ≤3 is as high as 86% or more. It also exhibits excellent mechanical properties, with a tensile strength ≥575MPa, yield strength ≥500MPa, reduction of area ≥20%, elongation after fracture ≥40%, and impact toughness ≥15J. It combines good strength and plasticity with excellent machinability. This steel can be used to manufacture parts for the automotive, high-speed rail, home appliance, and office equipment industries to reduce machining costs, improve production efficiency, and enhance product competitiveness, demonstrating promising application prospects.

[0037] This invention provides a method for preparing cerium-containing free-cutting steel ingots by vacuum induction smelting, and also provides an environmentally friendly cerium-containing free-cutting steel. To obtain sufficiently high cutting performance, this invention increases the cerium content based on existing technologies, preparing a "super" free-cutting steel through vacuum smelting. Furthermore, by appropriately controlling the ratio of cerium and sulfur elements, both good mechanical properties and excellent cutting performance are ensured. The 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 cerium. The sulfide morphology in the cerium-containing high-sulfur free-cutting steel prepared by this invention is mainly spherical and spindle-shaped. The proportion of sulfides with an aspect ratio ≤3 in the free-cutting steel reaches over 86%. The tensile strength of the free-cutting steel is ≥575MPa, the yield strength is ≥500MPa, the reduction of area is ≥40%, the elongation after fracture is ≥20%, and the impact toughness is ≥15J. The high-strength, high-machining-performance cerium-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.

[0038] The technical solution of the present invention is as follows:

[0039] By mass percentage, it contains 0.1%–0.14% C, 0.001%–0.1% Si, 1.05%–1.2% Mn, 0.001%–0.6% Ni, 12.0%–14.0% Cr, 0.25%–0.45% S, 0.001%–0.6% Mo, 0.005%–0.01% O, 0.01%–0.015% N, 0.0005%–0.008% Ce, and P ≤0.01%, with the balance being Fe and unavoidable impurities.

[0040] In one embodiment, the alloy raw material comprises:

[0041] The mass percentage content of Ce is 0.001% to 0.007%;

[0042] The mass percentage content of S is 0.25% to 0.3%.

[0043] A. Preparations before smelting:

[0044] Before smelting, all alloy raw materials required for smelting are baked to 100℃~200℃ and held at that temperature for 2h~3h. 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.

[0045] In one embodiment, the preferred baking temperature for the alloy raw materials in step a is 200°C, and the temperature is maintained for 3 hours.

[0046] B. Weighing the materials:

[0047] Accurately weigh the raw materials according to the quantities specified in the batching instructions. Large pieces of industrial pure iron are weighed using a 100kg electronic scale, while smaller quantities of graphite, ferrosilicon, ferromanganese, electrolytic nickel, metallic chromium, ferrous sulfate, molybdenum bars, and rare earth cerium are weighed using a 200g electronic scale. Before weighing, each batch of raw material is shot-blasted to remove the oxide scale on its surface. Each batch is 100kg, and the weight is checked again before loading into the furnace.

[0048] C. Loading:

[0049] The materials should be packed as tightly as possible to avoid bridging. The charging sequence is as follows: industrial pure iron and electrolytic nickel are loaded into the furnace together. Graphite, ferrosilicon, ferromanganese, metallic chromium, ferrous sulfate, molybdenum bars, and rare earth cerium are placed in separate hoppers within the vacuum induction furnace.

[0050] D. Melting:

[0051] When the vacuum degree of the melting chamber is ≤1Pa and the leakage rate is ≤3Pa / min, the vacuum induction furnace is started to heat the furnace charge and begin to melt. 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 industrial pure iron and electrolytic nickel are completely melted along with the furnace charge.

[0052] E. Refining:

[0053] 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.

[0054] In one embodiment, 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.

[0055] F. Adding alloys:

[0056] After refining, the mixture is purged with argon gas to 3000–6000 Pa, and then graphite, ferrosilicon, ferromanganese, ferrophosphorus, ferromolybdenum, ferrisulfide, and cerium are added sequentially. After each alloy is added, it is stirred with high-power electromagnetic stirring for 3–5 minutes, and then allowed to stand for 3–5 minutes.

[0057] In one embodiment, the argon gas is preferably introduced at 5000 Pa after refining;

[0058] G. Pouring:

[0059] The pouring temperature is 1500℃~1550℃, the pouring time is 5min~10min, and the pouring is completed by pouring... In a metal mold, molten steel solidifies in a furnace and cools to room temperature to obtain... The ingot is cast, and the steel is tapped while the pouring is under power. The pouring speed is well controlled. After pouring, the furnace is allowed to cool for 30 minutes before breaking the air. After the mold is cooled for ≥4 hours, the ingot is demolded and air-cooled.

[0060] In one embodiment, the casting temperature is preferably 1520°C, and the casting time is preferably 8 minutes.

[0061] The principle of this invention:

[0062] The effects of elemental composition on the machinability and mechanical properties of cerium-containing free-cutting steel in this invention:

[0063] 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. 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 easily stick to the tool, and its cutting performance decreases. Conversely, when the carbon content in the steel is too high, its hardness increases, which also causes tool wear and reduces the cutting performance of the steel. Therefore, the C content in this invention is controlled within the range of 0.1% to 0.14%.

[0064] 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 results in the formation of a large amount of oxide scale during hot working, reducing surface quality and promoting phosphorus precipitation from the matrix, forming phosphide films at grain boundaries and worsening hot workability. Therefore, it is crucial to rationally control the silicon content in free-machining stainless steel. In this invention, the Si content is controlled within the range of 0.001% to 0.1%.

[0065] 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 1.05% to 1.2%.

[0066] 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. In this invention, the Ni content is controlled within the range of 0.001% to 0.6%.

[0067] 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 12.0% to 14.0%.

[0068] Sulfur 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 machinability of the steel. Therefore, in this invention, the sulfur content is controlled within the range of 0.25% to 0.45%.

[0069] 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, in this invention, the Mo content is controlled within the range of 0.001% 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] Ce: Rare earth cerium is miscible with molten steel at high temperatures. Cerium has a strong affinity for sulfur and oxygen, which can improve sulfides in molten steel into rare earth sulfides, thereby controlling and improving the morphology, size, aspect ratio, and distribution of sulfides. Therefore, in this invention, the Ce content is controlled at 0.0005% to 0.008%.

[0074] The method for preparing cerium-containing free-cutting steel ingots by vacuum induction smelting provided by the present invention will be illustrated below with specific examples:

[0075] The composition of cerium-containing free-cutting steel is as follows: C: 0.1%, Si: 0.005%, Mn: 1.1%, Cr: 12%, Ni: 0.5%, S: 0.3%, O: 0.01%, Mo: 0.1%, Ce: 0.007%, N: 0.013%, O: 0.009%, P: 0.004% (wt%); the remainder is Fe and unavoidable impurities.

[0076] (1) Preparation before smelting: Before smelting, bake all alloy raw materials required for smelting to 200℃ and keep them at that temperature for 3 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.

[0077] (2) Weighing: Weigh the raw materials accurately according to the quantity specified in the batching. Large pieces of industrial pure iron are weighed using a 100Kg electronic scale, while smaller amounts of graphite, ferrosilicon, ferromanganese, electrolytic nickel, metallic chromium, ferrosulfite, molybdenum bars, and rare earth cerium are weighed using a 200g electronic scale. Before weighing, each batch of materials is shot-blasted to remove the oxide scale on its surface. Each batch is 100Kg, and the weight is checked again before loading into the furnace.

[0078] (3) Charging: The materials should be packed as tightly as possible to avoid bridging. Charging sequence: Industrial pure iron and electrolytic nickel are loaded into the furnace together. Graphite, ferrosilicon, ferromanganese, metallic chromium, ferrous sulfate, molybdenum bars, and rare earth cerium are placed in separate bins of the vacuum induction furnace.

[0079] (4) Melting: When the vacuum degree of the melting chamber is 0.8 Pa and the leakage rate is ≤3 Pa / min, 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 all industrial pure iron and electrolytic nickel are melted.

[0080] (5) Refining: After the furnace charge is melted and cleared, continue to raise the temperature to 1560℃. 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.

[0081] (6) Adding alloys: After refining, purge with argon gas to 5000 Pa, and then add graphite, ferrosilicon, ferromanganese, ferrophosphorus, ferromolybdenum, ferrisulfide, and cerium in sequence. After each alloy is added, perform high-power electromagnetic stirring for 5 minutes, and then let it stand for 5 minutes.

[0082] (7) Casting: Casting with the steel being tapped while energized, the casting temperature is 1520℃, the casting time is 8 minutes, and the steel is poured to... In the metal mold, the pouring speed is carefully controlled. After pouring, the furnace is allowed to cool for 30 minutes before voiding. After the mold cools for 5 hours, the steel is demolded and air-cooled. The molten steel solidifies in the furnace and cools to room temperature to obtain... The chemical composition of the cerium-containing free-cutting steel obtained from the ingots is shown in Table 3.

[0083] Table 1. Composition of cerium-containing free-cutting steel (wt%)

[0084]

[0085] Figure 1 The image shows the metallographic microscopy morphology of sulfides in the cerium-containing free-cutting steel prepared according to an embodiment of the present invention. The sulfides are relatively small and distributed in clusters.

[0086] Figure 2 The image shows the SEM morphology of sulfides in the cerium-containing free-cutting steel prepared according to an embodiment of the present invention. As can be seen from the image, the sulfides in the microstructure are mainly spindle-shaped and elliptical, and this morphology of sulfides is very helpful in improving the machinability of the material.

[0087] Figure 3 This is a schematic diagram of the energy dispersive spectroscopy (EDS) analysis points of cerium-containing composite sulfide in the cerium-containing free-cutting steel prepared according to an embodiment of the present invention. Figure 4 This is a schematic diagram of the energy dispersive spectroscopy (EDS) analysis results of cerium-containing free-cutting steel prepared according to an embodiment of the present invention. It can be seen that cerium plays a modifying role on the sulfides in the steel. The addition of cerium adjusts the type, quantity and morphology of the sulfides, which can effectively improve the cutting performance of the material.

[0088] Figure 5 The stress-strain curve of the cerium-containing free-cutting steel prepared in the embodiment of the present invention shows that there is no obvious yield plateau during the tensile process, and it exhibits continuous yield deformation.

[0089] Figure 6 The tensile fracture morphology of the cerium-containing free-cutting steel prepared according to the embodiment of the present invention can be seen as a mixed morphology of dimples and cleavage, indicating that the material has good plasticity and toughness.

[0090] Figure 7 The figure shows that the sulfide aspect ratio data of the cerium-containing free-cutting steel prepared in the embodiments of the present invention is as follows: the proportion of sulfides with aspect ratio ≤3 is as high as 86.9%, which has excellent cutting performance.

[0091] Figure 8 The figure shows the cutting chips (400 rpm, feed rate 0.05 mm) of cerium-containing free-cutting steel obtained in an embodiment of the present invention. As can be seen from the figure, the cutting chips are evenly distributed and fine. Cerium-containing free-cutting steel has excellent cutting performance.

[0092] The mechanical properties of this invention were determined by room temperature tensile and Charpy impact tests using a 5mm diameter tensile test bar and a 10mm×10mm×55mm standard V-notch Charpy impact specimen, conducted on an MTS Landmark 370 electro-hydraulic servo universal testing machine and an MTS falling weight impact testing machine. The tensile strength, yield strength, elongation after fracture, reduction of area, and impact toughness of the embodiment are shown in Table 2. As can be seen from Table 2, its tensile strength ≥ 575MPa, yield strength ≥ 500MPa, reduction of area ≥ 20%, elongation after fracture ≥ 40%, and impact toughness ≥ 15J, exhibiting good strength and machinability.

[0093] Table 2 Mechanical properties of cerium-containing free-cutting steel in the examples

[0094]

[0095] In summary, the method for preparing cerium-containing free-cutting steel ingots by vacuum induction smelting of the present invention produces 200mm cerium-containing free-cutting steel ingots with a smaller size, reducing the processing difficulty, shortening the preparation cycle, and improving material quality. Since cerium-containing free-cutting steel does not contain lead, it will not pollute the environment and effectively promotes energy conservation and emission reduction. Furthermore, the method has a simple smelting process and a wide range of raw material sources, reducing the production cost of cerium-containing free-cutting steel. In addition, the cerium-containing free-cutting steel of the present invention has a sulfide content of ≤3 of over 86%, a tensile strength ≥575MPa, a yield strength ≥500MPa, a reduction of area ≥20%, an elongation after fracture ≥40%, and an impact toughness ≥15J, exhibiting excellent strength and machinability. Therefore, the high-strength and high-machinability cerium-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, reducing machining costs, improving production efficiency and product competitiveness, and has good application prospects.

[0096] The present invention discloses a method for preparing cerium-containing free-cutting steel ingots by vacuum induction smelting. This method involves drying and shot-blasting the alloy raw materials to remove surface oxides and impurities; then, vacuum melting the dried and shot-blasted alloy raw materials in a specific composition ratio; finally, thoroughly mixing the vacuum-melted steel with industrial pure iron, graphite, ferrosilicon, ferromanganese, electrolytic nickel, metallic chromium, ferrous sulfate, molybdenum bars, and rare earth cerium under an inert atmosphere to obtain a mixed steel liquid; then, casting the steel under electric current, controlling the casting speed, allowing furnace cooling for 30 minutes after casting, followed by degassing, mold cooling for ≥4 hours, and air cooling to room temperature to obtain the cerium-containing free-cutting steel ingot. Through this method, the sulfide morphology in the cerium-containing free-cutting steel prepared by the present invention is mainly cerium-containing composite sulfides, exhibiting spherical and spindle-shaped forms. The proportion of sulfides with an aspect ratio ≤3 in the free-cutting steel reaches over 86%, resulting in a steel that combines good strength and machinability.

[0097] The method for preparing cerium-containing free-cutting steel ingots by vacuum induction smelting of the present invention can produce cerium-containing free-cutting steel ingots with smaller dimensions, reducing the processing difficulty of the material, shortening the preparation cycle, and improving the material quality. Furthermore, the vacuum melting process of the present invention is simple, and the alloy raw materials are widely available, which can reduce the production cost of cerium-containing free-cutting steel.

[0098] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing cerium-containing high-sulfur free-cutting steel by vacuum induction smelting, characterized in that, The method includes: The alloy raw materials are dried and shot blasted to remove oxide scale and impurities from the surface of the alloy raw materials; Before vacuum melting, the alloy raw materials are preheated. The preheating conditions are: heat the alloy raw materials to 200℃ and hold for 3 hours. The alloy raw materials that have undergone dry shot blasting surface treatment are then vacuum melted. The molten steel after vacuum melting is thoroughly mixed with industrial pure iron, graphite, ferrosilicon, ferromanganese, electrolytic nickel, metallic chromium, ferrous sulfate, molybdenum bars and rare earth cerium 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 cerium-containing free-cutting steel ingot. The composition of cerium-containing high-sulfur free-cutting steel is as follows (by mass percentage): 0.1%–0.14% C, 0.001%–0.1% Si, 1.05%–1.2% Mn, 0.001%–0.6% Ni, 12.0%–14.0% Cr, 0.25%–0.45% S, 0.001%–0.6% Mo, 0.005%–0.01% O, 0.01%–0.015% N, 0.0005%–0.008% Ce, P ≤ 0.01%, with the balance being Fe and unavoidable impurities. The preheated alloy raw materials are vacuum melted in a specific composition ratio, including: controlling the vacuum degree during vacuum melting to be ≤1 Pa; the leakage rate to be ≤3 Pa / min; the melting temperature to be 1500℃~1600℃; and refining at 1500℃~1550℃ after melting for 5min~10min. The inert atmosphere conditions were controlled as argon atmosphere at 3000~6000 Pa; The steel is poured while energized and tapped. After furnace cooling for 30 minutes, it is allowed to break the air. After mold cooling for ≥4 hours, it is demolded and air-cooled.

2. The method for preparing cerium-containing high-sulfur free-cutting steel by vacuum induction smelting according to claim 1, characterized in that, The drying conditions for pre-alloyed raw materials are as follows: heat the alloy raw materials to 100 ℃~200 ℃ and hold for 2 h~3 h.

3. The method for preparing cerium-containing high-sulfur free-cutting steel by vacuum induction smelting according to claim 1, characterized in that, The melting temperature was controlled at 1560℃, and after melting, the refining was carried out at 1550℃ for 10 minutes.

4. The method for preparing cerium-containing high-sulfur free-cutting steel by vacuum induction smelting according to claim 1, characterized in that, The pouring temperature is controlled at 1500℃~1550℃, and the pouring time is 5 min~10 min.

5. The method for preparing cerium-containing high-sulfur free-cutting steel by vacuum induction smelting according to claim 1, characterized in that, The Ce content is 0.001% to 0.007% by mass.

6. The method for preparing cerium-containing high-sulfur free-cutting steel by vacuum induction smelting according to claim 1, characterized in that, The mass percentage content of S is 0.25% to 0.3%.

7. An application of a cerium-containing high-sulfur free-cutting steel, characterized in that, The cerium-containing high-sulfur free-cutting steel prepared by the vacuum induction smelting method according to any one of claims 1-6 is used to manufacture parts for automobiles, high-speed trains, home appliances and office equipment.

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