Sulfur-selenium matched free-cutting steel bar and manufacturing method thereof

Abstract

The invention discloses a sulfur and selenium matched free-cutting steel bar and a manufacturing method thereof. The free-cutting steel bar comprises the following components: 0.35 to 0.50 percent of C, 0.18 to 0.38 percent of Si, 1.37 to 1.50 percent of Mn, less than 0.03 percent of P, 0.1 to 0.24 percent of S, 0.02 to 0.20 percent of Se, 0.16 to 0.23 percent of Cr, 0.15 to 0.20 percent of Mo and the balance of Fe and inevitable impurities. Wherein the content of O is in a range of 0.0065-0.0100%, and the ratio of selenium to sulfur is controlled to be 0.08-2.00. By adjusting the proportion of manganese, sulfur and selenium elements and matching with a certain content of oxygen, beneficial inclusions are separated out in the steelmaking solidification process and are dispersed and distributed in steel, the inclusions prevent a manganese sulfide soft core structure from being stretched and deformed in the rolling process, the shapes of the inclusions are changed to shapes beneficial to cutting, and the cutting performance is improved.

Description

Technical Field

[0001] The invention relates to the technical field of metal material processing, in particular to a sulfur-selenium compounded free-cutting steel bar and a manufacturing method thereof. Background Art

[0002] With the trend of lightweighting and component integration in new energy vehicles, free-cutting steel is developing towards high strength and lead-free while maintaining its processing advantages. For example, some of Tesla's motor housings are made of high-sulfur lead-free steel, achieving the dual goals of processing efficiency and environmental protection. Because free-cutting steel bars have better cutting performance, surface finish and appropriate mechanical properties than ordinary carbon steel bars, they are deeply favored by automotive parts manufacturers. At present, China has developed medium-carbon 40CrCaS and 53CaS for automobile engine connecting rods, and medium-carbon 40VS and 35MnVS for automobile crankshafts. However, the quality stability and cutting performance are mixed, and there is still a large gap with foreign products. In the future, material development needs to further balance cutting properties, mechanical properties and cost. The present invention proposes to use sulfur as the basic cutting element and add selenium to strengthen the free-cutting under medium-carbon design conditions to form composite inclusions to improve the cutting performance of steel bars and make up for the overall quality deficiencies of existing products.

[0003] Selenium significantly improves the free-cutting properties of steel. As an element in the same main group as sulfur, it shares similar physical and chemical properties. In recent years, it has been used effectively in the development and production of super-free-cutting steels, primarily preventing manganese sulfide inclusions from deforming during rolling, ensuring their spherical or spindle-shaped form, thereby enhancing the steel's cutting performance. The addition of selenium also creates a small amount of dispersed MnSe in the steel, which acts as a lubricant during cutting, enhancing cutting performance while effectively protecting the tool and extending tool life. A certain amount of selenium also contributes to the corrosion resistance of free-cutting steels. Even with a small amount of selenium, it significantly improves the cutting performance of steel, while remaining affordable.

[0004] CN107287504 A discloses a medium-carbon free-cutting non-quenched and tempered steel containing sulfur and tellurium and a production process method thereof. The process method improves the free-cutting performance of the steel to a certain extent, but the price of the tellurium element is relatively high, which is not conducive to cost reduction. In addition, tellurium has a certain degree of toxicity, and its compounds are all toxic. Tellurium vapor seriously damages human health during smelting.

[0005] CN117821833 A discloses a method for producing an ultra-high sulfur free-cutting steel bar. The composition of the bar is as follows: C ≤ 0.08%, Si ≤ 0.05%, Mn 2.00% to 2.80%, P 0.04% to 0.09%, S 0.50% to 0.70%, and the remainder is Fe and unavoidable impurity elements. The bar has a tensile strength Rm of 400 to 540 MPa, a cross-sectional reduction rate of ≥ 45%, and an elongation of ≥ 22%. However, the bar belongs to the field of low-carbon steel, and excessively high manganese and sulfur content easily leads to increased microcracks in the billet and severe segregation.

[0006] CN119392101 A discloses a large coil of free-cutting steel, the composition of which is as follows: C≤0.09%, Si≤0.005%, Mn: 0.90~1.65%, P: 0.04~0.09%, S: 0.30~0.50%, Al≤0.030%, N≤0.0060%, O≤0.0140%, and the remainder is Fe and unavoidable impurities. Excellent surface quality can be obtained, the finished wire rod has a high sulfide spindle rate, the C-shaped chips account for more than 90%, and the cutting performance is excellent. However, this technology is in the field of low-carbon steel free-cutting wire rod, and its performance is insufficient to be applicable to automotive structural parts.

[0007] CN119392103 A discloses a high-quality free-cutting steel and its production process. The free-cutting steel has the following composition: C ≤ 0.09%, Si ≤ 0.006%, Mn: 0.90-1.60%, P: 0.04-0.09%, S: 0.30-0.50%, Al ≤ 0.030%, N ≤ 0.0060%, O ≤ 0.0140%, with the remainder being Fe and other unavoidable impurities. Furthermore, the composition satisfies the following conditions: Mn / S = 3.6-5.0; and Y value = 5 × Mn / S + (LF active oxygen value) / 2. When 24 ≤ Y value ≤ 57.5, the final sulfide content is properly controlled. However, this technology is specific to the field of low-carbon steel free-cutting wire rod and its performance is insufficient for application in automotive structural parts.

[0008] CN119392102 A discloses a low-temperature rolled, high-quality free-cutting steel and its production process. The free-cutting steel has the following composition: C ≤ 0.10%, Si ≤ 0.006%, Mn: 0.90-1.60%, P: 0.30-0.90%, S: 0.30-0.50%, Al ≤ 0.030%, N ≤ 0.0060%, O ≤ 0.0140%, with the remainder being Fe and other unavoidable impurities. Low-temperature rolling is employed to avoid the sulfide hot brittleness zone, reducing energy consumption while also avoiding the deterioration in cutting performance of the finished product caused by elongation during the sulfide rolling process. However, this technology is intended for the wire rod and wire application, and it is not stated whether it can be applied to medium-carbon steel applications.

[0009] CN113235019 A discloses an Fe-Mn-Al-NS series high-nitrogen, low-density free-cutting steel bar and a preparation method thereof. The free-cutting steel bar has the following composition: 0.3-0.6% C, 18.0-22.0% Mn, 5.0-10.0% Ni, 2.0-6.0% Al, 5.0-10.0% Cr, 0.01-0.5% S, 0.35-0.65% N, P≤0.01%, with the remainder being Fe and unavoidable impurities. Although this technical product has the advantages of easy cutting and low density and can be well applied to the automotive field, such high alloying elements as 18.0-22.0% Mn, 5.0-10.0% Ni, 2.0-6.0% Al, and 5.0-10.0% Cr will double the steelmaking time, greatly increase the cost, cause serious billet segregation, and have low production efficiency, making it unfavorable for mass production. Summary of the Invention

[0010] There is no technology for adding sulfur and selenium to the existing medium-carbon free-cutting steel to improve the overall performance. The addition of sulfur alone will lead to a significant decrease in the mechanical properties of the medium-carbon steel, and the addition of selenium alone will greatly increase the production cost. The present invention provides a sulfur-selenium free-cutting steel bar and a manufacturing method thereof, aiming to address the shortcomings of the existing technology by synergistically adding the two elements. By designing the sulfur-selenium ratio in the medium-carbon steel, the cutting performance and mechanical properties are balanced, meeting the processing and use requirements of medium-carbon free-cutting structural steel such as automobile engine connecting rods and automobile crankshafts, and at the same time being beneficial to environmental protection.

[0011] According to one aspect of the present invention, a sulfur-selenium-containing free-cutting steel bar is provided, wherein the composition of the free-cutting steel bar is as follows: C: 0.35% to 0.50%, Si: 0.18% to 0.38%, Mn: 1.37% to 1.50%, P < 0.03%, S: 0.10% to 0.24%, Se: 0.02% to 0.20%, Cr: 0.16% to 0.23%, Mo: 0.15% to 0.20%, and the balance is Fe and unavoidable impurities; wherein the O content is in the range of 0.0065% to 0.0100%, and the selenium-sulfur ratio is controlled at 0.08 to 2.00, calculated by weight percentage.

[0012] The selection of the addition amount (weight percentage) of the above elements and their functions are explained as follows: C: Carbon is the most fundamental matrix-strengthening element in steel. The addition of carbon in this invention ensures a certain strength in the matrix. A carbon content below 0.35% fails to guarantee the strength of the steel bar. A carbon content above 0.50% significantly reduces cutting performance due to increased surface hardness. Therefore, the carbon content of the sulfur-selenium free-cutting steel bar manufactured in this invention is controlled within the range of 0.35% to 0.50%.

[0013] Si: Silicon is a deoxidizer in the present invention, co-deoxidizing with Mn. When the silicon content is below 0.18%, the co-deoxidation effect is poor. When the silicon content is above 0.38%, mechanical and cutting properties are significantly reduced. Therefore, in the manufacturing method of the sulfur-selenium free-cutting steel bar of the present invention, the silicon content is controlled within the range of 0.18% to 0.38%.

[0014] Mn: Manganese is an important component element of MnS, MnSe, and other materials in the present invention, and is beneficial for improving free-cutting performance. When the manganese content is less than 1.37%, the number of manganese sulfide and manganese-bismuth sulfide composite inclusions is relatively small, the aspect ratio is relatively large, the proportion of spindle-shaped inclusions is very small, the cutting performance is reduced, and the mechanical properties are relatively low. When the manganese content is higher than 1.50%, the surface hardness of the steel is high, cutting is difficult, the strength is too high, and the cost is increased. Therefore, the manganese content of the sulfur-selenium-compounded free-cutting steel bar and its manufacturing method of the present invention is controlled within the range of 1.37% to 1.50%.

[0015] P: To ensure the comprehensive performance of the steel bar, the phosphorus content of the sulfur-selenium compounded free-cutting steel bar of the present invention is controlled to be less than 0.03% during its manufacturing process.

[0016] Sulfur (S): Sulfur is one of the most beneficial elements for free-cutting. Adding it in appropriate amounts forms manganese sulfide compounds with manganese and complex inclusions with manganese oxide, significantly improving free-cutting performance. When the sulfur content is below 0.10%, it fails to form a complex compound with manganese that contributes to cutting performance. When the sulfur content exceeds 0.24%, the synergistic effect with selenium weakens, reducing the hot workability of the steel bar. Therefore, in the manufacturing method of the sulfur-selenium free-cutting steel bar of the present invention, the sulfur content is controlled within the range of 0.10% to 0.24%.

[0017] Se: Selenium is an effective free-cutting element. Its main function is to form a synergistic effect with sulfur, blocking the deformation of manganese sulfide inclusions in the steel during rolling, ensuring that the manganese sulfide inclusions are spherical or spindle-shaped to improve the cutting performance of the steel. The addition of selenium can also form a small amount of dispersed MnSe in the steel, which plays a good lubricating role in the steel cutting process, improving the cutting performance and effectively protecting the tool, extending the tool life. At the same time, a certain content of selenium is also beneficial to the corrosion resistance of free-cutting steel. When the selenium content is lower than 0.02%, the above effect cannot be achieved; when the selenium content exceeds 0.20%, the free-cutting performance can no longer be improved and the smelting cost increases. Therefore, the selenium content in the manufacturing method of the free-cutting steel bar with sulfur and selenium of the present invention is controlled within the range of 0.02% to 0.20%.

[0018] Cr: Chromium enhances matrix toughness and, in this invention, contributes to a moderate increase in the strength of the steel bar. A chromium content below 0.16% yields insignificant mechanical property enhancement, while a chromium content above 0.23% degrades cutting performance. Therefore, the chromium content in the manufacturing method of the sulfur-selenium free-cutting steel bar of this invention is controlled within a range of 0.16% to 0.23%.

[0019] Mo: Molybdenum is a grain-refining strengthening element. Combined with chromium, it improves the strength and toughness of steel, giving the steel bar a certain degree of stress resistance. When the molybdenum content is below 0.15%, this effect is not significant; when the molybdenum content exceeds 0.20%, it significantly increases the alloy cost. Therefore, in the manufacturing method of the sulfur-selenium free-cutting steel bar of the present invention, the molybdenum content is controlled within the range of 0.15% to 0.20%.

[0020] O: In the present invention, oxygen provides the thermodynamic conditions for the dispersed distribution of sulfides, placing requirements on the total oxygen content. When the total oxygen content is less than 0.0065%, the precipitation of oxidized species is limited, which prevents the preferential nucleation of manganese selenide and manganese sulfide inclusions. This results in the inclusions becoming elongated during subsequent rolling, increasing their aspect ratio and impairing cutting performance. When the total oxygen content is higher than 0.0100%, large oxide particles are easily formed, making it difficult for manganese sulfide and manganese selenide to adsorb and grow, resulting in internal defects in the bar. Therefore, the total oxygen content in the manufacturing method of the sulfur-selenium free-cutting steel bar of the present invention is controlled within the range of 0.0065% to 0.0100%.

[0021] Another aspect of the present invention provides a method for manufacturing a free-cutting steel bar, the method comprising the following steps: (1) Converter smelting: The converter molten iron is not desulfurized, only decarbonized and dephosphorized. The active oxygen content is strictly controlled. The active oxygen content is controlled at 125-145 ppm before being moved out to prepare for the oxygen level control of the refining furnace; (2) LF furnace refining: first add refining slag and submerged arc slag, then add ferrosilicon and silicon carbide to deoxidize the slag surface of the molten steel, blow argon to ensure that the molten steel does not roll, strictly control the order of adding ferroselenium alloy and ferrosulphur alloy, add ferroselenium alloy, and then add ferrosulphur alloy, that is, add selenium alloy after white slag treatment, and add it in the wire feeding method; (3) Billet continuous casting: The continuous casting process uses medium carbon protective slag for protection casting, and adopts the combination of crystallizer electromagnetic stirring and light pressure continuous casting process for continuous casting; (4) Billet heating: Billets are heated in a cold-charged furnace. The cross-sectional dimensions of the billets should not be less than 180 mm × 180 mm. The heating temperature and time should be 1270-1300 °C for 2.0-3.0 h. (5) Ingot rolling and post-rolling slow cooling: The recrystallization zone rolling is adopted, the starting rolling temperature is 1130-1160℃, the final rolling temperature is 950-980℃, and the finished bar is obtained. When the finished bar is cooled to 615-645℃ on the cooling bed, it enters the slow cooling pit and the cooling time to 200℃ is controlled within 8-10 hours. Then it is air-cooled to room temperature to obtain sulfur-selenium-compounded free-cutting steel bar.

[0022] Furthermore, the selenium alloy in step (2) is selected from at least one of ferroselenium alloy, manganese-selenium alloy or selenium cored wire; The specific adding method in step (2) is to add 70 to 90 m of cored wire, and then immediately add low-phosphorus ferromanganese and ferrosulfur. When adding selenium alloy, the surface of the molten steel is covered with lime fluorite to isolate the air and reduce the evaporation of selenium oxide.

[0023] Furthermore, the MnSe inclusions in the steel rod after LF furnace refining in step (2) are distributed in the form of fine, dispersed particles.

[0024] Furthermore, the specific steps of the continuous casting process in step (3) are as follows: adjust the electromagnetic stirring current of the crystallizer to 345~380A, the frequency to 5.0~5.8Hz, the total reduction amount of the end light pressure to 13.2~15.0mm, the continuous casting billet enters the slow cooling pit for slow cooling, the temperature entering the slow cooling pit is 615~645℃, the slow cooling time is 24~30 hours, and it is cooled to room temperature with the furnace.

[0025] Furthermore, the MnSe inclusions are in the form of encapsulated inclusions.

[0026] Furthermore, the finished rod size in step (5) is φ30 to φ80 mm.

[0027] Furthermore, the sulfur-selenium compounded free-cutting steel bar obtained in step (5) has a C-type chip ratio of 80-83% under the cutting conditions of a feed rate f of 0.10 mm / r and a rotation speed of 1000 r / min.

[0028] Compared with the prior art, the present invention has the following beneficial effects: (1) Under the design of the steelmaking process, each element forms a good non-metallic inclusion morphology, that is, oxygen plays the role of adsorption nucleation point of selenide and sulfide, and the formed wrapped composite oxide inclusion does not undergo significant elongation changes in morphology during the bar rolling process, thereby improving the overall cutting performance of the matrix.

[0029] (2) MnSe inclusions are distributed in the steel rod as small, dispersed particles. During the cutting process: 1. They play a stress concentration role. The shear stress in the contact area between the tool and the workpiece will make the MnSe inclusions become local stress concentration points, making it easier for the chips to break here (forming short C-shaped chips), avoiding long chips from wrapping around the tool or workpiece, and improving processing efficiency; 2. They reduce cutting force. The chip breaking effect reduces the continuous shear force required during cutting, thereby reducing energy consumption and tool wear; 3. They have a lubricating effect and reduce tool wear. Under high cutting temperature, the MnSe inclusions will partially soften or even melt, forming a lubricating film covering the tool surface. This film can reduce the direct friction between the tool and the chips and workpiece, inhibit cutting heat accumulation, delay tool edge passivation, and significantly reduce the adhesive wear and diffusion wear of the tool.

[0030] (3) Manganese selenide and manganese sulfide work synergistically to compensate for the shortcomings of manganese sulfide, namely: 1. Form more stable inclusions. MnSe is more resistant to high-temperature oxidation than MnS and is not easily decomposed during the rolling process, maintaining the stability of cutting performance; 2. Compensate for the shortcomings of manganese sulfide, while retaining cutting performance, reduce the effect of sulfur on the transverse toughness and corrosion resistance of the steel bar, and improve the transverse toughness and corrosion resistance of the steel bar; 3. Control the morphology of inclusions. The addition of selenium refines the size of inclusions and improves their distribution uniformity, avoiding the anisotropy problem caused by coarse sulfides. 4. Inhibit work hardening and improve the surface quality of the steel bar, namely: A. Reduce the hardness of the shear zone. The presence of MnSe inclusions will locally weaken the material matrix, reduce the tendency of work hardening during cutting, and make the chips easier to peel off; B. Improve the surface finish of the machined surface. The synergistic effect of chip breaking and lubrication can reduce cutting vibration and burr generation, and obtain a smoother workpiece surface.

[0031] (4) The final bar product has excellent surface quality and meets the user's cutting processing requirements. The cutting performance index is f = 0.10 mm / r, the speed is 1000 r / min, the proportion of C-type chips is 80% to 83%, and the steel bar cutting efficiency is 32% to 36% higher than that of the original medium carbon bar. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a morphology diagram of sulfur-selenium composite inclusions in the sulfur-selenium free-cutting steel bar prepared in Example 14; Figure 2 This is a C-type morphology of the chips of the sulfur-selenium composite rod in the sulfur-selenium free-cutting steel rod prepared in Example 14. DETAILED DESCRIPTION

[0033] The present invention is further described below with reference to specific examples, but is not intended to limit the present invention in any way.

[0034] The present invention provides a sulfur-selenium free-cutting steel bar in a specific embodiment. The chemical composition of the free-cutting steel bar is as follows: C: 0.35% to 0.50%, Si: 0.18% to 0.38%, Mn: 1.37% to 1.50%, P < 0.03%, S: 0.10% to 0.24%, Se: 0.02% to 0.20%, Cr: 0.16% to 0.23%, Mo: 0.15% to 0.20%, and the balance is Fe and unavoidable impurities; wherein, when the O content is 0.0065% to 0.0100%, the selenium-sulfur ratio is controlled at 0.08 to 2.00, calculated by weight percentage.

[0035] The present invention provides a method for manufacturing a free-cutting steel bar containing sulfur and selenium in a specific embodiment. The process flow of the manufacturing method is converter smelting → LF furnace refining → billet continuous casting → slow cooling of the billet → heating in a heating furnace → rolling → slow cooling; wherein the free-cutting element selenium is added in the form of ferroselenium alloy, manganese-selenium alloy or selenium cored wire at the end of LF furnace refining, and specifically comprises the following steps: (1) Converter smelting: The converter molten iron is not desulfurized, only decarbonized and dephosphorized. The active oxygen content is strictly controlled. The active oxygen content is controlled at 125-145 ppm before being moved out to prepare for the oxygen level control of the refining furnace; (2) LF furnace refining: First, add refining slag and submerged arc slag, then add ferrosilicon and silicon carbide to deoxidize the slag surface of the molten steel. Argon is blown to ensure that the molten steel does not roll. The order of adding ferroselenium alloy and ferrosulphur alloy is strictly controlled. The technical requirements of the present invention are to add ferroselenium alloy first and then add ferrosulphur alloy, that is, add selenium alloy (ferroselenium alloy, manganese-selenium alloy or selenium cored wire) after white slag treatment. The addition method is wire feeding method, and the amount of cored wire added is 70-90m. Then, low-phosphorus ferromanganese and ferrosulphur are added immediately. When adding selenium alloy, the surface of the molten steel is covered with lime fluorite to isolate the air and reduce the evaporation of selenium oxide. This process can achieve the distribution of MnSe inclusions in the steel bar as fine and dispersed particles. During the solidification process, each element forms a good non-metallic inclusion morphology. That is, oxygen plays the role of adsorption nucleation point of selenide and sulfide. The formed wrapped composite oxide inclusions do not undergo significant elongation changes during the bar rolling process, thereby improving the overall cutting performance of the matrix.

[0036] (3) Continuous casting of square billets: The continuous casting process uses medium carbon protective slag for protective casting, and adopts a combination of crystallizer electromagnetic stirring + light pressure continuous casting process. The crystallizer electromagnetic stirring current is 345~380A, the frequency is 5.0~5.8Hz, and the total light pressure reduction at the end is 13.2~15.0mm. The continuous casting billet is slowly cooled in the slow cooling pit after it comes off the line. The temperature of the slow cooling pit is 615~645℃, and the slow cooling time is 24~30 hours. It is then cooled to room temperature with the furnace.

[0037] (4) Billet heating: Billets are heated in a cold-charged furnace. The cross-sectional dimensions of the billets should not be less than 180 mm × 180 mm. The heating temperature and time should be 1270-1300 °C for 2.0-3.0 h. (6) Ingot rolling and post-rolling slow cooling: The recrystallization zone rolling is adopted, with the starting rolling temperature of 1130-1160℃ and the final rolling temperature of 950-980℃. The final finished bar size is φ30-φ80mm. The recrystallization zone rolling can increase the grain boundary density. The dense grain boundary can prevent the composite inclusions from being rolled and elongated, further improve the morphology and distribution of the inclusions, and improve the cutting performance of the steel bar. At the same time, rolling in this temperature zone can maintain the high-temperature oxidation resistance of MnSe and MnS, making them not easy to decompose during the rolling process, and maintaining the stability of the cutting performance. Finally, when the steel bar is cooled to 615-645℃ on the cooling bed, it enters the slow cooling pit. The cooling time to below 200℃ is controlled within 8-10 hours, and then air-cooled to room temperature. After finishing, flaw detection, sizing, inspection, and storage, the sulfur-selenium-compounded free-cutting steel bar is obtained.

[0038] Example The composition of the sulfur-selenium free-cutting steel bars in the embodiments and comparative examples of the present invention is shown in Table 1. The process parameters related to the converter, LF furnace, and continuous casting are shown in Table 2. The cross-sectional size of the ingot is not less than 180 mm × 180 mm; the process parameters related to rolling are shown in Table 3. The statistics of the cracking of the ingot end, the scarring and bubble defects on the surface of the steel bar, and the chip type are shown in Table 4. The morphology of the sulfur-selenium composite inclusions in the sulfur-selenium free-cutting steel bars prepared in Example 14 is as follows: Figure 1 As shown in the figure, the structure is composed of ferrite and pearlite, the composite inclusions are brown, dispersed, and granular and short rod-shaped; the sulfur-selenium composite bar chip C-shaped morphology of the free-cutting steel bar prepared in the sulfur-selenium combination of Example 14 is as follows Figure 2 As shown, the chips are basically C-type, which reflects the good cutting performance of the material.

[0039] Table 1 Composition of the steel according to the present invention and the comparative example steel .

[0040] Table 2 Process parameters related to converter, LF furnace and continuous casting in the embodiments of the present invention and comparative examples .

[0041] Table 3 shows the process parameters related to rolling of the examples and comparative examples. .

[0042] Table 4 Statistics of cracks on the end of rolled billets, scarring and bubble defects on the surface of steel bars, and chip types .

[0043] The present invention, through the above-mentioned technical solution, adds selenium to medium-carbon steel bars. Through the synergistic coordination of oxygen, sulfur, and manganese, under the design of the steelmaking process, each element forms a well-defined non-metallic inclusion morphology. Specifically, oxygen acts as an adsorption nucleation point for selenides and sulfides, resulting in encapsulated composite oxide inclusions that do not undergo significant elongation changes during bar rolling, thereby improving the overall cutting performance of the matrix. The synergistic effect of manganese selenide and manganese sulfide is utilized to compensate for the shortcomings of manganese sulfide. Specifically, 1. more stable inclusions are formed. MnSe is more resistant to high-temperature oxidation than MnS and is less susceptible to decomposition during rolling, maintaining stable cutting performance. 2. The shortcomings of manganese sulfide are compensated by reducing the effects of sulfur on the transverse toughness and corrosion resistance of the steel bar while maintaining cutting performance, thereby improving the transverse toughness and corrosion resistance of the steel bar. 3. The inclusion morphology is controlled. The addition of selenium refines the inclusion size and improves its distribution uniformity, avoiding anisotropy problems caused by coarse sulfides. 4. Inhibit work hardening and improve the surface quality of the steel bar. This includes: a. Reducing shear zone hardness. The presence of MnSe inclusions locally weakens the material matrix, reducing the tendency toward work hardening during cutting and making chips easier to peel; b. Improving surface finish. The synergistic effect of chip breaking and lubrication reduces cutting vibration and burr generation, resulting in a smoother workpiece surface. The resulting bar product has excellent surface quality, meeting user cutting requirements. Under cutting performance conditions of f = 0.10 mm / r and a rotational speed of 1000 r / min, the proportion of C-shaped chips is 80-83%, and the cutting efficiency of the steel bar is 32-36% higher than that of the original medium-carbon bar.

[0044] The MnSe inclusions are distributed in the steel rod as small, dispersed particles. During the cutting process: a. They act as stress concentration points. The shear stress in the contact area between the tool and the workpiece causes the MnSe inclusions to become local stress concentration points, making it easier for chips to break at these points (forming short C-shaped chips), preventing long chips from wrapping around the tool or workpiece and improving processing efficiency; b. They reduce cutting forces. The chip breaking effect reduces the continuous shear force required during cutting, thereby reducing energy consumption and tool wear; c. They act as a lubricant, reducing tool wear. Under high cutting temperatures, the MnSe inclusions partially soften or even melt, forming a lubricating film covering the tool surface. This film can reduce direct friction between the tool, the chips, and the workpiece, inhibit cutting heat accumulation, delay tool edge blunting, and significantly reduce adhesive wear and diffusion wear of the tool.

[0045] The above descriptions are merely several embodiments of the present invention and do not constitute any form of limitation to the present invention. Although the present invention is disclosed as above in terms of preferred embodiments, they are not intended to limit the present invention. Any technician familiar with the present profession who, without departing from the scope of the technical solution of the present invention, makes slight changes or modifications using the technical contents disclosed above are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A sulfur-selenium compounded free-cutting steel bar, characterized in that: The composition of the free-cutting steel bar is as follows: C: 0.35% to 0.50%, Si: 0.18% to 0.38%, Mn: 1.37% to 1.50%, P < 0.03%, S: 0.10% to 0.24%, Se: 0.02% to 0.20%, Cr: 0.16% to 0.23%, Mo: 0.15% to 0.20%, and the balance is Fe and unavoidable impurities; wherein, the O content is in the range of 0.0065% to 0.0100%, and the selenium-sulfur ratio is controlled at 0.08 to 2.00, calculated by weight percentage.

2. The method for manufacturing a free-cutting steel bar according to claim 1, wherein: The manufacturing method comprises the following steps: (1) Converter smelting: The converter molten iron is not desulfurized, only decarbonized and dephosphorized. The active oxygen content is strictly controlled. The active oxygen content is controlled at 125-145 ppm before being moved out to prepare for the oxygen level control of the refining furnace; (2) LF furnace refining: first add refining slag and submerged arc slag, then add ferrosilicon and silicon carbide to deoxidize the slag surface of the molten steel, blow argon to ensure that the molten steel does not roll, strictly control the order of adding ferroselenium alloy and ferrosulphur alloy, add ferroselenium alloy, and then add ferrosulphur alloy, that is, add selenium alloy after white slag treatment, and add it in the wire feeding method; (3) Billet continuous casting: The continuous casting process uses medium carbon protective slag for protection casting, and adopts crystallizer electromagnetic stirring and light pressure continuous casting process for continuous casting; (4) Billet heating: Billets are heated in a cold-charged furnace. The cross-sectional dimensions of the billets should not be less than 180 mm × 180 mm. The heating temperature and time should be 1270-1300 °C for 2.0-3.0 h. (5) Ingot rolling and post-rolling slow cooling: The recrystallization zone rolling is adopted, the starting rolling temperature is 1130-1160℃, the final rolling temperature is 950-980℃, and the finished bar is obtained. When the finished bar is cooled to 615-645℃ on the cooling bed, it enters the slow cooling pit, and the cooling time to below 200℃ is controlled within 8-10 hours, and then air-cooled to room temperature to obtain sulfur-selenium-compounded free-cutting steel bar.

3. The manufacturing method according to claim 2, characterized in that The selenium alloy in step (2) is selected from at least one of ferroselenium alloy, manganese-selenium alloy or selenium cored wire; The specific adding method in step (2) is to add 70 to 90 m of cored wire, and then immediately add low-phosphorus ferromanganese and ferrosulfur. When adding selenium alloy, the surface of the molten steel is covered with lime fluorite to isolate the air and reduce the evaporation of selenium oxide.

4. The manufacturing method according to claim 2, characterized in that After the LF furnace refining in step (2), the MnSe inclusions in the steel rod are distributed in the form of fine, dispersed particles.

5. The manufacturing method according to claim 2, characterized in that The specific steps of the continuous casting process described in step (3) are as follows: adjust the electromagnetic stirring current of the crystallizer to 345~380A, the frequency to 5.0~5.8Hz, the total reduction amount of the end light pressure to 13.2~15.0mm, the continuous casting billet is put into the slow cooling pit for slow cooling, the temperature of the billet entering the slow cooling pit is 615~645℃, the slow cooling time is 24~30 hours, and the billet is cooled to room temperature with the furnace.

6. The manufacturing method according to claim 2, characterized in that The finished rod size in step (5) is φ30~φ80mm.

7. The manufacturing method according to claim 4, characterized in that The MnSe inclusions are in the form of encapsulated inclusions.

8. The manufacturing method according to claim 2, characterized in that The sulfur-selenium compounded free-cutting steel bar obtained in step (5) has a C-type chip ratio of 80-83% under the cutting conditions of a feed rate f of 0.10 mm / r and a rotation speed of 1000 r / min.

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