A free-cutting steel bar with sulfur-selenium combination and a method for manufacturing the same

Abstract

The application discloses a sulfur-selenium matched free-cutting steel bar and a manufacturing method thereof. The free-cuting steel bar comprises the following components: C: 0.35-0.50%, Si: 0.18-0.38%, Mn: 1.37-1.50%, P: less than 0.03%, S: 0.1-0.24%, Se: 0.02-0.20%, Cr: 0.16-0.23%, Mo: 0.15-0.20%, and the balance of Fe and inevitable impurities; wherein, the O content is in the range of 0.0065-0.0100%, and the selenium-sulfur ratio is controlled in the range of 0.08-2.00. By adjusting the element ratio of manganese, sulfur and selenium, and matching a certain content of oxygen, beneficial inclusions are precipitated in the steel smelting and solidification process and are dispersedly distributed in the steel, the inclusions prevent the sulfide manganese soft core structure from being elongated and deformed in the rolling process, the morphology of the inclusions is changed to a shape beneficial to cutting, and the cutting performance is improved.

Description

Technical Field

[0001] This invention relates to the field of metal processing technology, and in particular to a free-cutting steel bar with sulfur and selenium complex and its manufacturing method. Background Technology

[0002] With the trend towards lightweighting and component integration in new energy vehicles, free-cutting steel, while maintaining its processing advantages, is developing towards higher strength and lead-free properties. For example, Tesla uses high-sulfur lead-free steel for some motor housings, achieving the dual goals of processing efficiency and environmental protection. Because free-cutting steel bars have better cutting performance, surface finish, and suitable mechanical properties than ordinary carbon steel bars, they are highly favored by automotive parts manufacturers. Currently, China has developed medium-carbon 40CrCaS and 53CaS for automotive engine connecting rods, and medium-carbon 40VS and 35MnVS for automotive crankshafts. However, their quality stability and cutting performance are inconsistent, and there is still a significant gap compared to foreign products. In the future, material development needs to further balance machinability, mechanical properties, and cost. This invention proposes, under medium-carbon design conditions, using sulfur as a basic cutting element and adding selenium for free-cutting strengthening, forming composite inclusions to improve the cutting performance of steel bars and compensate for the overall quality deficiencies of existing products.

[0003] Selenium significantly improves the machinability of steel. Belonging to the same group as sulfur, it shares similar physical and chemical properties. In recent years, it has been used in the composition design for the development and production of ultra-free-cutting steels with remarkable results. Its main function is to prevent the deformation of manganese sulfide inclusions during rolling, ensuring that these inclusions remain spherical or spindle-shaped to enhance the steel's machinability. The addition of selenium also allows for the formation of small amounts of dispersed MnSe in the steel, providing excellent lubrication during cutting. This improves machinability, effectively protects the cutting tool, and extends tool life. Furthermore, a certain amount of selenium contributes to the corrosion resistance of free-cutting steels. Selenium requires only a small amount to significantly improve the machinability of steel, and its price is relatively low.

[0004] CN107287504 A discloses a medium-carbon free-cutting non-quenched and tempered steel containing sulfur and tellurium and its production process. The process improves the free-cutting performance of the steel to a certain extent, but the price of tellurium is high, which is not conducive to reducing costs. Moreover, tellurium is toxic, and its compounds are all toxic. Tellurium vapor during smelting seriously damages human health.

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

[0006] CN119392101 A discloses a free-cutting steel for large coils. The composition of the free-cutting steel 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%, with the balance being Fe and unavoidable impurities. It can obtain excellent surface quality, a high sulfide spindle rate in the finished coil, and a C-shaped chip rate of over 90%, resulting in excellent cutting performance. However, this technology is limited to the field of low-carbon steel free-cutting coils, and its performance is insufficient for automotive structural parts.

[0007] CN119392103 A discloses a high-quality free-cutting steel and its production process. The composition of the free-cutting steel is as follows: 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. It also satisfies Mn / S = 3.6~5.0; Y value = 5×Mn / S + LF activity oxygen value) / 2. When 24≤Y value≤57.5, the final sulfide content is well controlled. However, this technology is only applicable to low-carbon steel free-cutting wire rods, and its performance is insufficient for 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 avoids the sulfide hot brittleness range, reducing energy consumption and preventing the deterioration of the finished product's machinability caused by the elongation of sulfide during rolling. However, this technology pertains to the field of wire rod and does not specify whether it can be applied to the field of medium carbon steel.

[0009] CN113235019 A discloses a Fe-Mn-Al-NS system high-nitrogen, low-density free-machining steel bar and its preparation method. The composition of the free-machining steel bar is as follows: 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 technology produces products with advantages such as easy machining and low density, making it well-suited for the automotive industry, the high alloying elements (18.0-22.0% Mn, 5.0-10.0% Ni, 2.0-6.0% Al, 5.0-10.0% Cr) double the steelmaking time, resulting in extremely high costs, severe billet segregation, and low production efficiency, which is unfavorable for mass production. Summary of the Invention

[0010] There is currently no technology for combining sulfur and selenium elements to improve the overall performance of medium carbon free-cutting steel bars. Adding sulfur alone will significantly reduce the mechanical properties of medium carbon steel, while adding selenium alone will greatly increase production costs. This invention provides a sulfur-selenium combined free-cutting steel bar and its manufacturing method. The purpose is to overcome the shortcomings of existing technologies by using the synergistic effect of the two elements. By designing the sulfur-selenium ratio in medium carbon steel, the cutting performance and mechanical properties are balanced, meeting the processing and use requirements of medium carbon free-cutting structural steels such as automotive engine connecting rods and automotive crankshafts, while also being environmentally friendly.

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

[0012] The selection of the amount (by weight percentage) of each of the above elements and their functions are explained below:

[0013] C: Carbon is the most basic matrix strengthening element in steel materials. The addition of carbon in this invention ensures the matrix possesses a certain strength. When the carbon content is below 0.35%, the strength of the steel bar cannot be guaranteed; when the carbon content is above 0.50%, the surface hardness of the steel bar increases significantly, reducing its machinability. Therefore, the carbon content in the manufacturing method of the free-machining steel bar with sulfur and selenium formulation of this invention is controlled within the range of 0.35% to 0.50%.

[0014] Si: Silicon is the deoxidizer in this invention, and it undergoes co-deoxidation with Mn. When the silicon content is below 0.18%, the co-deoxidation effect is poor; when the silicon content is above 0.38%, the mechanical properties and machinability are significantly reduced. Therefore, the silicon content in the manufacturing method of the free-cutting steel bar with sulfur and selenium complex of this invention is controlled within the range of 0.18% to 0.38%.

[0015] Mn: Manganese is an important component element in this invention, such as MnS and MnSe, which is beneficial for improving machinability. When the manganese content is below 1.37%, the number of manganese sulfide and bismuth manganese sulfide composite inclusions is small, the aspect ratio is large, the proportion of spindle-shaped inclusions is very small, the machinability is reduced, and the mechanical properties are low. When the manganese content is above 1.50%, the surface hardness of the steel is high, making it difficult to cut, and the strength is too high, increasing the cost. Therefore, the manganese content in the manufacturing method of the sulfur-selenium-coated free-machining steel bar of this invention is controlled within the range of 1.37% to 1.50%.

[0016] P: To ensure the overall performance of this steel bar, the phosphorus content in the manufacturing method of the free-cutting steel bar with sulfur and selenium complex of this invention is controlled to be <0.03%.

[0017] S: Sulfur is one of the most beneficial elements for free cutting. Appropriate addition of sulfur forms manganese sulfide compounds with manganese and complex inclusions with manganese oxygen, significantly improving free cutting performance. When the sulfur content is below 0.10%, it cannot form complex compounds with manganese that contribute to cutting performance; when the sulfur content is above 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-coated free cutting steel bar of this invention, the sulfur content is controlled within the range of 0.10% to 0.24%.

[0018] Selenium (Se) is an effective free-machining element. Its main function is to synergistically interact with sulfur to prevent the deformation of manganese sulfide inclusions in steel during rolling, ensuring that the manganese sulfide inclusions are spherical or spindle-shaped to improve the machinability of the steel. The addition of selenium also allows for the formation of a small amount of dispersed MnSe in the steel, which provides good lubrication during steel cutting, improving machinability and effectively protecting the cutting tool, extending tool life. A certain amount of selenium also contributes to the corrosion resistance of free-machining steel. When the selenium content is below 0.02%, the above effects are not achieved; when the selenium content exceeds 0.20%, it is impossible to further improve the free-machining performance and increases smelting costs. Therefore, in the manufacturing method of the sulfur-selenium-coated free-machining steel bar of this invention, the selenium content is controlled within the range of 0.02% to 0.20%.

[0019] Cr: Chromium is an element that improves the hardness of the matrix, and its role in this invention is to appropriately increase the strength of the steel bar. When the chromium content is below 0.16%, the improvement in mechanical properties is not significant; when the chromium content is above 0.23%, it will reduce the machinability. Therefore, in the manufacturing method of the free-machining steel bar with sulfur and selenium in this invention, the chromium content is controlled within the range of 0.16% to 0.23%.

[0020] Mo: Molybdenum is a fine-grained strengthening element. Combined with chromium, it can improve the strength and toughness of steel, enabling the steel bar to withstand certain stresses. When the molybdenum content is below 0.15%, this effect is not significant; when the molybdenum content is above 0.20%, it significantly increases the alloy cost. Therefore, in the manufacturing method of the free-cutting steel bar with sulfur and selenium composition of this invention, the molybdenum content is controlled within the range of 0.15% to 0.20%.

[0021] O: In this invention, oxygen provides the thermodynamic conditions for the dispersed distribution of sulfides, and requirements are placed on the total oxygen content. When the total oxygen content is below 0.0065%, fewer oxide particles are precipitated, and manganese selenide and manganese sulfide cannot be sufficiently encapsulated and preferentially nucleated. This causes the inclusions to be elongated during subsequent rolling, increasing their aspect ratio and negatively impacting machinability. When the total oxygen content is above 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, in the manufacturing method of the free-machining steel bar with sulfur and selenium in this invention, the total oxygen content is controlled within the range of 0.0065% to 0.0100%.

[0022] In another aspect, the present invention provides a method for manufacturing free-cutting steel bars, the method comprising the following steps:

[0023] (1) Converter smelting: The molten iron in the converter is not desulfurized, only decarburized and phosphorus-reduced. The active oxygen content is strictly controlled. Before being moved out, the active oxygen content is controlled at 125-145 ppm to prepare for oxygen level control in the refining furnace.

[0024] (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 tumble. Strictly control the order of adding selenium ferroalloy and sulfur ferroalloy. Add selenium ferroalloy, then add sulfur ferroalloy. That is, add selenium ferroalloy after white slag treatment. The addition method is wire feeding.

[0025] (3) Billet continuous casting: The continuous casting process uses medium carbon protective slag for protection and casting, and adopts a combination of crystallizer electromagnetic stirring and light pressure continuous casting process;

[0026] (4) Heating of billet: The billet is heated in the furnace by cold charging. The cross-sectional dimensions of the billet are not less than 180mm×180mm. Heating temperature and time: 1270~1300℃ for 2.0~3.0h;

[0027] (5) Rolling of billet and slow cooling after rolling: Rolling is carried out in the recrystallization zone with an initial rolling temperature of 1130-1160℃ and a final rolling temperature of 950-980℃ to obtain finished bars. The finished bars are cooled to 615-645℃ in the cooling bed and then enter the slow cooling pit. The cooling time to 200℃ is controlled at 8-10 hours. Then, they are air-cooled to room temperature to obtain free-cutting steel bars with sulfur and selenium complex.

[0028] Further, the selenium alloy mentioned in step (2) is selected from at least one of ferro-selenium alloy, manganese-selenium alloy, or selenium cored wire;

[0029] The specific addition method described in step (2) is to add 70-90m of cored wire, and then immediately add low phosphorus manganese iron and sulfur iron. 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.

[0030] Furthermore, the MnSe inclusions in the steel bars refined in the LF furnace in step (2) are distributed as fine, dispersed particles.

[0031] Further, 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 at the end of the light pressure to 13.2-15.0mm, the continuous casting billet enters the slow cooling pit for slow cooling, the temperature of 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.

[0032] Furthermore, the MnSe inclusions are in the form of encapsulated inclusions.

[0033] Furthermore, the finished bar material in step (5) has a size of φ30~φ80mm.

[0034] Furthermore, the proportion of C-type chips in the free-cutting steel bar with sulfur and selenium complex obtained in step (5) is 80-83% under the cutting conditions of feed rate f of 0.10 mm / r and rotation speed of 1000 r / min.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] (1) Under the design of the steelmaking process, the various elements form good non-metallic inclusions. That is, oxygen plays the role of adsorption and nucleation of selenides and sulfides. The encapsulated composite oxide inclusions do not undergo significant elongation changes in the morphology during the rolling of the bar, thus improving the overall cutting performance of the matrix.

[0037] (2) MnSe inclusions are distributed in the steel bar as fine and 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 the chips more likely to break at this point (forming short C-shaped chips), avoiding long chips from wrapping around the tool or workpiece, and improving machining 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, reducing tool wear. At the high temperature of cutting, 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 the accumulation of cutting heat, delay the dulling of the tool edge, and significantly reduce the adhesive wear and diffusion wear of the tool.

[0038] (3) The synergistic effect of manganese selenide and manganese sulfide compensates for the deficiencies of manganese sulfide, namely: 1. Forming more stable inclusions. MnSe is more resistant to high-temperature oxidation than MnS and is not easily decomposed during rolling, thus maintaining the stability of cutting performance; 2. Compensating for the deficiencies of manganese sulfide. While retaining cutting performance, it reduces the insufficient transverse toughness and corrosion resistance of steel bars caused by sulfur, thereby improving the transverse toughness and corrosion resistance of steel bars; 3. Controlling the morphology of inclusions. The addition of selenium refines the size of inclusions and improves their distribution uniformity, avoiding anisotropy problems caused by coarse sulfides. 4. Inhibiting work hardening and improving the surface quality of steel bars, namely: A. Reducing 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. Improving the surface finish of the machined surface. The synergistic effect of chip breaking and lubrication can reduce cutting vibration and burr generation, resulting in a smoother workpiece surface.

[0039] (4) The final bar product has excellent surface quality and meets the user's cutting requirements. The cutting performance index is 80-83% of the C-type chips under the cutting conditions of f=0.10 mm / r and 1000r / min. The cutting efficiency of the steel bar is 32%-36% higher than that of the original medium carbon bar. Attached Figure Description

[0040] Figure 1 This image shows the morphology of sulfur-selenium composite inclusions in the free-cutting steel bar prepared in Example 14.

[0041] Figure 2 The image shows the C-shaped morphology of the sulfur-selenium composite bar chips from the free-cutting steel bar prepared in Example 14. Detailed Implementation

[0042] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0043] The present invention provides a free-cutting steel bar with sulfur and selenium composition in the specific embodiments section. The chemical composition of the free-cutting steel bar is as follows: C: 0.35%~0.50%, Si: 0.18%~0.38%, Mn: 1.37%~1.50%, P<0.03%, S: 0.10%~0.24%, Se: 0.02%~0.20%, Cr: 0.16%~0.23%, Mo: 0.15%~0.20%, with the balance being Fe and unavoidable impurities; wherein, with an O content of 0.0065%~0.0100%, the selenium-sulfur ratio is controlled at 0.08~2.00, by weight percentage.

[0044] The present invention provides a method for manufacturing free-machining steel bars with sulfur and selenium in the specific embodiments section. The process flow of the manufacturing method is as follows: converter smelting → LF furnace refining → billet continuous casting → billet slow cooling → heating furnace heating → rolling → slow cooling; wherein the free-machining element selenium is added at the end of LF furnace refining in the form of ferro-selenium alloy, manganese-selenium alloy or selenium cored wire, specifically including the following steps:

[0045] (1) Converter smelting: The molten iron in the converter is not desulfurized, only decarburized and phosphorus-reduced. The active oxygen content is strictly controlled. Before being moved out, the active oxygen content is controlled at 125-145 ppm to prepare for oxygen level control in the refining furnace.

[0046] (2) LF furnace refining: First, add refining slag and submerged arc slag, then add ferrosilicon and silicon carbide to deoxidize the molten steel surface. Argon blowing is used to ensure that the molten steel does not tumble. The order of adding selenium ferroalloy and sulfur ferroalloy is strictly controlled. The technical requirements of this invention are to add selenium ferroalloy first, then sulfur ferroalloy. That is, after white slag treatment, selenium alloy (selenium ferroalloy, manganese selenium alloy or selenium cored wire) is added. The addition method is wire feeding. The amount of cored wire added is 70-90m. Then, low phosphorus manganese ferroalloy and sulfur ferroalloy 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 a fine and dispersed particle distribution of MnSe inclusions in the steel bar. During the solidification process, each element forms a good non-metallic inclusion morphology. That is, oxygen plays the role of adsorption and nucleation of selenides and sulfides. The encapsulated composite oxide inclusions do not undergo significant elongation changes in morphology during the rolling of the bar, thus improving the overall machinability of the matrix.

[0047] (3) Continuous casting of billets: The continuous casting process uses medium carbon protective slag for protection and casting. The process combines the crystallizer electromagnetic stirring + light reduction continuous casting process. The current of the crystallizer electromagnetic stirring is 345~380A and the frequency is 5.0~5.8Hz. The total reduction at the end of the light reduction is 13.2~15.0mm. The continuous casting billet enters the slow cooling pit for slow cooling. The temperature in the slow cooling pit is 615~645℃ and the slow cooling time is 24~30 hours. It is cooled to room temperature with the furnace.

[0048] (4) Heating of billet: The billet is heated in the furnace by cold charging. The cross-sectional dimensions of the billet are not less than 180mm×180mm. Heating temperature and time: 1270~1300℃ for 2.0~3.0h;

[0049] (6) Rolling of billets and slow cooling after rolling: Rolling is carried out in the recrystallization zone with an initial rolling temperature of 1130-1160℃ and a final rolling temperature of 950-980℃. The final finished bar size is φ30-φ80mm. Rolling in the recrystallization zone can increase the grain boundary density. Dense grain boundaries can prevent the composite inclusions from being rolled and elongated, further improving the morphology and distribution of inclusions and improving the machinability of the steel bar. At the same time, rolling in this temperature zone can maintain the high-temperature oxidation resistance of MnSe and MnS, which are not easily decomposed during the rolling process, thus maintaining the stability of machinability. The final steel bar is cooled to 615-645℃ on the cooling bed and then enters the slow cooling pit. The cooling time to below 200℃ is controlled within 8-10 hours. Then it is air-cooled to room temperature, and after finishing, flaw detection, length setting, inspection, and warehousing, free-machining steel bars with sulfur and selenium complex are obtained.

[0050] Example

[0051] The composition of the free-cutting steel bars with sulfur and selenium complexes described in the embodiments and comparative examples of this 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 dimensions of the cast billet are not less than 180mm × 180mm. The process parameters related to rolling are shown in Table 3. The statistical information on end cracking of the rolled billet, surface scabs, bubbles, and chip types is shown in Table 4. The morphology of the sulfur-selenium composite inclusions in the free-cutting steel bars with sulfur and selenium complexes prepared in Example 14 is shown in Table 4. Figure 1 As shown in the figure, the microstructure consists of ferrite and pearlite, with brown, diffusely distributed composite inclusions in granular and short rod-like shapes. The C-shaped morphology of the sulfur-selenium composite rod chips from the free-cutting steel bar prepared in Example 14 is as follows. Figure 2 As shown, the chips are primarily C-shaped, reflecting the material's excellent machinability.

[0052] Table 1. Composition of steel in the embodiments of the present invention and comparative steel.

[0053] .

[0054] Table 2. Process parameters related to converters, LF furnaces, and continuous casting described in the embodiments and comparative examples of the present invention.

[0055] .

[0056] Table 3 lists the process parameters related to rolling in the examples and comparative examples.

[0057] .

[0058] Table 4. Statistics on end cracking of billets, surface scars and bubbles of steel bars, and chip types.

[0059] .

[0060] This invention, through the aforementioned technical solution, adds selenium to medium carbon steel bars. Through the synergistic effect of oxygen, sulfur, and manganese in the steelmaking process design, it achieves the formation of favorable non-metallic inclusion morphologies. Specifically, oxygen acts as a nucleation point for the adsorption of selenides and sulfides, resulting in encapsulated composite oxide inclusions whose morphology does not undergo significant elongation changes during bar rolling, thus improving the overall machinability of the matrix. The synergistic effect of manganese selenide and manganese sulfide compensates for the deficiencies of manganese sulfide, namely: 1. Forming more stable inclusions; MnSe is more resistant to high-temperature oxidation than MnS and is less prone to decomposition during rolling, maintaining stable machinability; 2. Compensating for the deficiencies of manganese sulfide; while retaining machinability, it reduces the impact of sulfur on the transverse toughness and corrosion resistance of the steel bar, improving the transverse toughness and corrosion resistance; 3. Controlling inclusion morphology; the addition of selenium refines the size of inclusions and improves their distribution uniformity, avoiding anisotropy problems caused by coarse sulfides. 4. Suppressing work hardening and improving the surface quality of steel bars: a. Reducing the hardness of the shear zone; the presence of MnSe inclusions locally weakens the material matrix, reducing the tendency for work hardening during cutting and making chips easier to remove; b. Improving the surface finish; the synergistic effect of chip breaking and lubrication reduces cutting vibration and burr formation, resulting in a smoother workpiece surface. The final bar products have excellent surface quality, meeting the user's cutting requirements. Under cutting conditions of f=0.10 mm / r and a speed of 1000 r / min, the proportion of C-type chips is 80-83%, and the cutting efficiency of the steel bars is 32%-36% higher than that of the original medium carbon bars.

[0061] The MnSe inclusions in the steel bar are distributed as fine, dispersed particles. During cutting: a. They act as stress concentration points, where the shear stress in the tool-workpiece contact area makes the MnSe inclusions local stress concentration points, causing the chips to break more easily at these points (forming short C-shaped chips), preventing long chips from entangled in the tool or workpiece and improving machining efficiency; b. They reduce cutting forces, as the chip-breaking effect reduces the continuous shear force required during cutting, thereby reducing energy consumption and tool wear; c. They act as lubricants, reducing tool wear. At high cutting temperatures, the MnSe inclusions partially soften or even melt, forming a lubricating film covering the tool surface. This film reduces direct friction between the tool and the chips and workpiece, inhibits the accumulation of cutting heat, delays tool edge dulling, and significantly reduces adhesive and diffused wear of the tool.

[0062] The above description is merely a few embodiments of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any modifications or alterations made by those skilled in the art without departing from the scope of the technical solution of the present invention using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A free-cutting steel bar with sulfur and selenium composition, characterized in that, The composition of the free-cutting steel bar, by weight percentage, is as follows: C: 0.35%–0.50%, Si: 0.18%–0.38%, Mn: 1.37%–1.50%, P < 0.03%, S: 0.10%–0.24%, Se: 0.02%–0.20%, Cr: 0.16%–0.23%, Mo: 0.15%–0.20%, with the balance being Fe and unavoidable impurities; wherein the O content is in the range of 0.0065%–0.0100%, and the selenium-sulfur ratio is controlled between 0.08 and 2.

00. The manufacturing method includes the following steps: (1) Converter smelting: The molten iron in the converter is not desulfurized, only decarburized and phosphorus-reduced. The active oxygen content is strictly controlled. Before being moved out, the active oxygen content is controlled at 125-145 ppm to prepare for oxygen level control in 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 tumble. Strictly control the order of adding selenium ferroalloy and sulfur ferroalloy. Add selenium ferroalloy, then add sulfur ferroalloy. That is, add selenium ferroalloy after white slag treatment. The addition method is wire feeding. (3) Billet continuous casting: The continuous casting process uses medium carbon protective slag for protection and casting, and adopts the continuous casting process of electromagnetic stirring in the crystallizer and light pressure. (4) Heating of billet: The billet is heated in the furnace by cold charging. The cross-sectional dimensions of the billet are not less than 180mm×180mm. Heating temperature and time: 1270~1300℃ for 2.0~3.0h; (5) Rolling of billet and slow cooling after rolling: Rolling is carried out in the recrystallization zone with an initial rolling temperature of 1130-1160℃ and a final rolling temperature of 950-980℃ to obtain finished bars. The finished bars are cooled to 615-645℃ in the cooling bed and then enter the slow cooling pit. The time for cooling to below 200℃ is controlled within 8-10 hours. Then, they are air-cooled to room temperature to obtain free-cutting steel bars with sulfur and selenium complex.

2. The free-cutting steel bar according to claim 1, characterized in that, The selenium alloy mentioned in step (2) is selected from at least one of selenium-iron alloy cored wire, manganese-selenium alloy cored wire, or selenium cored wire; In step (2), the amount of cored wire added is 70-90m, and then low phosphorus manganese iron and sulfur iron 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.

3. The free-cutting steel bar according to claim 1, characterized in that, In step (2), the MnSe inclusions in the steel bars after refining in the LF furnace are distributed as fine, dispersed particles.

4. The free-cutting steel bar according to claim 1, 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 at the end of the light pressure to 13.2-15.0mm, and the continuous casting billet enters the slow cooling pit for slow cooling. The temperature in 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.

5. The free-cutting steel bar according to claim 1, characterized in that, The finished bar material in step (5) has a size of φ30~φ80mm.

6. The free-cutting steel bar according to claim 3, characterized in that, The MnSe inclusions are in the form of encapsulated inclusions.

7. The free-cutting steel bar according to claim 1, characterized in that, The free-cutting steel bar with sulfur and selenium complex obtained in step (5) has a C-type chip ratio of 80-83% under the cutting conditions of feed rate f of 0.10 mm / r and rotation speed of 1000 r / min.

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