Preparation method of high-sulfur and selenium-containing free-cutting stainless steel

Abstract

The present application relates to a kind of preparation methods of high-sulfur selenium-containing free-cutting stainless steel, the stainless steel is by mass percentage: C≤0.10%, Si≤1.0%, Mn≤3.0%, P≤0.15%, S:0.25%~0.35%, Cr:17.0~19.0%, Ni:8.0%~10.0%, Cu:1.5%~3.5%, Se:0.01%~0.15%, activity oxygen [O]:0.001%~0.006%, N:0.015%~0.05%, the balance is Fe and inevitable impurities;Wherein, selenium sulfur mass ratio is 0.02~0.6, oxygen selenium mass ratio is 0.006~0.6;Above-mentioned free-cutting stainless steel preparation flow is: EAF→AOD refining→LF refining→ feed selenium cored wire line→continuous casting / mold casting→billet heating→rolling wire rod.The present application introduces free-cutting element selenium, controls selenium sulfur ratio and oxygen selenium ratio, weak cold system is used in continuous casting process, control low speed, multi-pass rolling is implemented in rolling process, and the strategy of "high temperature fast rolling, slow cooling after rolling" is followed.Compared with prior art, the present application solves the problem that free-cutting stainless steel sulfide is long strip distribution, improves sulfide shape as ellipsoidal or fusiform, and improves cutting performance stability.

Description

Technical Field

[0001] This invention relates to the field of free-machining stainless steel technology, and in particular to a method for preparing high-sulfur selenium-containing free-machining stainless steel. Background Technology

[0002] Free-cutting stainless steel is a special type of stainless steel with the addition of appropriate amounts of free-cutting elements. While maintaining good corrosion resistance and mechanical properties, it significantly improves machinability. It is widely used in precision mechanical parts, automatic lathe parts, valves, shaft parts, fasteners, and high-end equipment manufacturing. Based on the free-cutting elements added, free-cutting steels can be classified into: sulfur-based, lead-based, tellurium-based, bismuth-based, and calcium-based, etc. Sulfur-based free-cutting steels are the foundation of all free-cutting steels, and other free-cutting steels also contain varying amounts of sulfur. In Japan and other Asian regions, free-cutting stainless steel is commonly known as "free-machining steel." Compared to ordinary carbon free-cutting steel (commonly known as free-machining iron), it has higher corrosion resistance and is used in more critical applications.

[0003] Sulfur is commonly added to free-machining steels to improve their machinability. Sulfur combines with manganese in steel to form MnS. MnS, as a beneficial inclusion, also known as the free-machining phase, effectively disrupts the continuity of the steel matrix. During the cutting process, it acts as a "crack initiation point," promoting chip breakage through the "notch effect," thereby improving the steel's machinability. Simultaneously, MnS has a lubricating effect, effectively reducing cutting resistance and improving machining efficiency and tool life. However, the introduction of high sulfur content also has adverse effects on the steel: MnS, being highly malleable, easily deforms into elongated shapes during hot rolling, leading to significant anisotropy within the steel. Furthermore, MnS has a higher electrode potential relative to the steel matrix, often becoming a preferred site for pitting corrosion, resulting in reduced corrosion resistance. The contact points between elongated MnS and the steel matrix are most likely to become preferential sites for pitting and crevice corrosion initiation, significantly reducing the steel's corrosion resistance. Therefore, in order to solve such problems, the art has carried out a modification treatment on sulfide inclusions by adding modifiers to change the type of MnS, namely, reducing the proportion of eutectic cluster type II MnS and increasing the proportion of eccentric type I spherical MnS; thereby improving the sulfide morphology, reducing its aspect ratio, and improving its machinability.

[0004] Chinese patent CN112063924B discloses a high-carbon selenium-tin free-cutting steel and its production method. It is composed of the following chemical composition by weight percentage: C: 0.50%–0.80%, Si: 0.20%–0.40%, Mn: 0.50%–2.00%, P: 0.01%–0.10%, Se: 0.30%–0.50%, Sn: 0.36%–0.50%, O: 0.010%–0.015%, wherein the selenium-tin ratio (Se / Sn) is 0.6–1.35, the oxygen-selenium ratio (O / Se) is 0.02–0.05, the oxygen-tin ratio (O / Sn) is 0.02–0.04, and the balance is iron and unavoidable impurities. This application, by adding the free-machining elements selenium and tin, combined with high carbon, controls the distribution and morphology of beneficial inclusions in steel, significantly improving the machinability and mechanical properties of high-carbon structural steel, thus satisfying the requirements of improved machinability, efficient chip removal, and a good balance between mechanical and machinability in high-carbon free-machining steel. However, this application uses high-carbon carbon steel (without chromium, nickel, or other corrosion-resistant elements), adds selenium and tin, but does not add sulfur, which is significantly different from the composition system and technical characteristics of this invention. Furthermore, this invention improves the chip-breaking property during cutting by precisely controlling oxygen-sulfur-selenium to improve the type and morphology of sulfides, resulting in a spindle-shaped or ellipsoidal dispersed distribution of Mn(S,Se). The lack of sulfur, a key free-machining element, eliminates the notch brittle fracture effect and lubrication mechanism, significantly reducing machinability. Moreover, the comparison patent uses a carbon steel composition system, while this invention uses a stainless steel system, and their preparation processes are completely different.

[0005] Chinese patent CN115466908A discloses a selenium- and calcium-containing free-machining stainless steel and its manufacturing method. The free-machining stainless steel of this invention, by mass percentage, contains C≤0.08%, Si:≤1.00%, Mn≤3.0%, P≤0.10%, S:0.10~0.15%, Ni:8.5~9.0%, Cr:17.0~18.0%, Cu:2.0~3.0%, Ca:0.003~0.010%, Se:0.05~0.15%, with the remainder being iron and unavoidable impurities. This invention, by reducing the S content in the free-machining stainless steel and adding Ca and Se, obtains uniformly distributed ellipsoidal or spindle-shaped Ca, Se, and S composite inclusions, thereby improving the corrosion resistance and transverse mechanical properties of the free-machining stainless steel. During high-speed turning, it can improve the surface roughness of parts and reduce tool wear, making it suitable for manufacturing complex parts with high requirements for corrosion resistance and mechanical properties. However, the application essentially involves adding calcium, sulfur, and selenium to free-machining stainless steel, utilizing the adhesion of calcium-based inclusions to further improve tool life. Its machinability is not related to oxygen content, and the invention does not involve the O / Se technical feature. This patent is for cutting high-sulfur selenium-containing (S: 0.25%~0.35%, Se: 0.01-0.015) free-machining stainless steel. The Se / S ratio in this patent is completely different from that in the other patent, and their technical features are entirely different. The working principle of this patent, which improves the morphology of sulfides through the synergistic effect of Se, O, and S to enhance cutting performance, is significantly different from that of the other patent.

[0006] Chinese patent CN111876689B discloses a low-carbon, selenium-containing free-cutting steel for instruments and meters, and its manufacturing method. It is composed of the following chemical components by weight percentage: C: 0.01%–0.10%, Si: 0.01%–0.10%, Mn: 0.50%–1.50%, P: 0.01%–0.10%, S: 0.25%–0.35%, Se: 0.18%–0.30%, O: 0.0040%–0.0060%, with a selenium-sulfur ratio (Se / S) of 0.2–2 and an oxygen-selenium ratio (O / Se) of 0.016–0.17. The balance is iron and unavoidable impurities. This application, through a specific manganese and selenium ratio and a specific combination of selenium and sulfur, achieves a lubricating effect on the cutting tool and obtains a good surface roughness. However, the selenium addition in this application is 0.18%~0.30%, which does not overlap with the selenium content control level (0.01%~0.15%) of this invention. The Se / S ratio of this patent is 0.02-0.6, while that patent's Se / S ratio is 0.2-2. They do not overlap at all and their technical features are completely different. Furthermore, this invention is made of stainless steel, while that patent is made of carbon steel, and their preparation processes are also completely different.

[0007] Chinese patent CN111187996B discloses a medium-carbon, sulfur- and selenium-containing free-cutting steel wire rod and its manufacturing method. The composition by weight percentage is as follows: C: 0.25%–0.34%, Si: 0.05%–0.28%, Mn: 1.65%–2.50%, P < 0.03%, S: 0.15%–0.24%, Se: 0.02%–0.20%, N: 0.008%–0.012%, TO: 0.0065%–0.010%; it also contains at least one of Ti, V, and Nb; at least two of Cr, N, and Mo, with the balance being Fe and unavoidable impurities; wherein the oxygen-to-sulfur ratio is controlled at (0.008–0.05):1, and the selenium-to-sulfur ratio is controlled at (0.08–1.5):1. However, the sulfur (0.15%~0.24%), oxygen (0.0065%~0.1%), and nitrogen (0.008%~0.012%) in the patent are completely different from the oxygen (0.001%~0.006%), sulfur (0.25%~0.35%), and nitrogen (0.015%~0.05%) in this invention, and their technical features are significantly different. The comparison patent is based on a carbon steel system, while this invention is based on a stainless steel system, and their preparation processes are also completely different.

[0008] Chinese patent CN120719225A discloses a free-machining steel bar with a sulfur-selenium composition and its manufacturing method. The composition of the free-machining steel bar is as follows: C: 0.35-0.50%, Si: 0.18-0.38%, Mn: 1.37-1.50%, P < 0.03%, S: 0.1-0.24%, Se: 0.02-0.20%, Cr: 0.16-0.23%, Mo: 0.15-0.20%, with the balance being Fe and unavoidable impurities. 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. This application, by adjusting the ratio of manganese, sulfur, and selenium elements and adding a certain amount of oxygen, precipitates beneficial inclusions during the steelmaking solidification process, which are then dispersed throughout the steel. This causes the inclusions to change morphology to be more favorable for cutting, thereby improving machinability. However, the oxygen content of the patented oxygen range of 0.0065% to 0.0100% is completely different from the oxygen control range of this patent, and the technical features are completely different. Furthermore, the patented oxygen range is based on a carbon steel system, while this invention is based on a stainless steel system, and the preparation processes of the two are also completely different.

[0009] In conclusion, developing a stainless steel that can optimize sulfide morphology, improve machinability, and enhance corrosion resistance is an urgent problem to be solved. Summary of the Invention

[0010] The purpose of this invention is to address the problems of poor machinability and corrosion resistance in existing stainless steels by providing a method for preparing high-sulfur, selenium-containing free-machining stainless steel. By adding Se and maintaining the synergistic effect of Se, O, and S, the morphology of sulfides in the stainless steel is improved, resulting in a free-machining phase that is a manganese-selenium-sulfur composite inclusion (Mn(S,Se)) dispersed in a spindle-shaped or ellipsoidal pattern. This method improves the chip-breaking properties of the steel and extends tool life while ensuring its corrosion resistance, thus developing a novel free-machining stainless steel. The objective of this invention can be achieved through the following technical solutions: This invention provides a method for preparing a low-carbon, high-sulfur, selenium-containing free-machining stainless steel. The free-machining stainless steel, by mass percentage, comprises the following components: C≤0.10%, Si≤1.0%, Mn≤3.0%, P≤0.15%, S: 0.25%~0.35%, Cr: 17.0%~19.0%, Ni: 8.0%~10.0%, Cu: 1.5%~3.5%, Se: 0.01%~0.15%, active oxygen [O]: 0.001%~0.006%, N: 0.015%~0.05%, with the balance being Fe and unavoidable impurities; wherein the mass ratio of Se to S (Se / S): 0.02~0.6:1, and the mass ratio of active oxygen [O] to Se (O / Se): 0.006~0.6:1. The preparation process of this free-machining stainless steel is as follows: electric arc furnace smelting, argon-oxygen decarburization furnace refining, ladle refining furnace refining, selenium cored wire feeding, continuous casting / ingot casting, billet heating, and wire rod rolling, specifically including the following steps: (1) Electric arc furnace smelting: The raw materials for electric arc furnace smelting consist of low-phosphorus stainless steel return material and high-carbon ferrochrome alloy, wherein the phosphorus content meets P≤0.050%. According to the target composition, the primary smelting, melting and alloying are carried out, and the electric furnace tapping temperature is 1570-1590℃. (2) Argon-oxygen decarburization furnace refining: The early stage is the oxidation period, and mixed gases with different Ar and O ratios are blown in over time to carry out decarburization operation so that the carbon composition meets the composition control standard; the later stage is the reduction period, and active lime, fluorite, as well as ferrosilicon, ferromanganese silicon alloy, aluminum ingot, ferrochrome, ferronickel, and ferromolybdenum are added to adjust the chromium, nickel, molybdenum, silicon, and manganese composition of the molten steel to the required range; and ferrosulfite is added to adjust the sulfur content of the molten steel to the required range. The tapping temperature of the argon-oxygen decarburization furnace is ≥1600℃. (3) Ladle refining furnace refining: After the ladle refining furnace is in place, lime and submerged arc slag are added to the slag surface, and aluminum particles and silicon carbide are added to deoxidize the slag surface; the LF is energized and heated to raise the temperature of the molten steel to 1610-1630℃; after the energization is completed, the composition of the molten steel is tested to ensure that it meets the composition control window. Argon gas is blown from the bottom throughout the LF refining process to stir the molten steel and promote uniform composition and temperature. (4) Wire feeding: After the refining composition is controlled to the target window, when the temperature of the molten steel is 1540-1560 ℃, the selenium-containing cored wire is fed in through the wire feeding machine. The selenium-containing cored wire is an iron-coated cored wire with a diameter of 8-13 mm. The core material of the cored wire is a mixture of selenium powder and iron powder particles or selenium-iron alloy particles. The mass fraction of selenium is 10%-40%. In order to further ensure the selenium recovery rate and ensure that the slag layer thickness of the molten steel is 1-5 cm, the wire feeding is carried out in an intermittent manner, in 2-3 times, with an interval of 10-20 s each time. The wire feeding speed is controlled at 100-150 m / min. During the wire feeding process, the bottom blowing argon gas is stirred, and the flow rate is controlled at 100-300 NL / min. After the wire feeding, the soft blowing argon gas is continued for 10-30 min, and the flow rate is controlled at 100-180 NL / min. (5) Continuous casting / ingot casting: The cross-section of the continuous casting is 200×200 mm or 150×150 mm. High-sulfur special protective slag is used for casting during the continuous casting process; octagonal ingots of 1.0-3.0 tons are used for ingot casting, and bottom casting is used; the temperature of the molten steel in continuous casting / ingot casting is controlled at 1470℃~1520℃, the casting speed is controlled at 0.9~1.2 m / min, weak cooling method is used, and the water volume in the first cooling zone is 95-105 m³ / min. 3 / h, the water volume control ratio in the secondary cooling zone is 0.18~0.30 tons of water / ton of steel; the casting speed in the ingot casting process is 1.0~2.0 tons / min; (6) Billet heating: The continuously cast billet is hot-sent and heated in the heating furnace to 1150℃~1200℃. The total heating time is controlled according to the billet thickness of 0.95~1.05 min / mm, and the total heating time is 140-210 min. (7) Rolling: The initial rolling temperature is 1100℃~1150℃. The rolling steps include roughing, intermediate rolling, intermediate rolling, pre-finishing rolling, and finishing rolling. The rolling temperature is 850℃~1050℃. The maximum reduction during the rolling process is ≤20%. The final rolling temperature is 850-900℃. After rolling, the parts are air-cooled to ensure smooth rolling.

[0011] Further, by mass percentage, it contains the following components: C≤0.10%, Si≤0.6%, Mn≤1.5%, P≤0.03%, S: 0.26%~0.34%, Cr: 17.5%~18.5%, Ni: 8.5%~9.5%, Cu: 1.5%~2.5%, Se: 0.04%~0.14%, active oxygen [O]: 0.002%~0.005%, N: 0.02%~0.05%, with the balance being Fe and unavoidable impurities; wherein, the Se / S ratio is 0.12~0.54:1, and the O / Se ratio is 0.02~0.13:1.

[0012] Further, by mass percentage, it contains the following components: C≤0.030%, Si≤0.6%, Mn≤1.5%, P≤0.025%, S: 0.26%~0.30%, Cr: 17.5%~18.5%, Ni: 8.5%~9.5%, Cu: 1.5%~2.5%, Se: 0.06%~0.12%, active oxygen [O]: 0.003%~0.004%, N: 0.02%~0.04%, with the balance being Fe and unavoidable impurities; wherein, the Se / S ratio is 0.2~0.46:1, and the O / Se ratio is 0.025~0.07:1.

[0013] Furthermore, the selenium-containing free-machining phase in the steel is a manganese-selenium-sulfur composite inclusion that is dispersed in a spindle-shaped or ellipsoidal pattern: Mn(S,Se).

[0014] Furthermore, the selenium-containing cored wire is an iron-clad cored wire with a diameter of 10-13 mm, and its core powder is a mixture of selenium powder and industrial iron powder, or a selenium-iron alloy, with a selenium mass fraction of 20%-30%.

[0015] Furthermore, when the temperature of the molten steel at the end of the LF stage is 1540-1560 ℃, the selenium-containing cored wire is fed into the molten steel through a wire feeder, and the slag layer thickness of the molten steel is 1-2 cm. The wire feeding is carried out intermittently in 3 times, with an interval of 15 s between each feeding, and the wire feeding speed is 125 m / min. During the wire feeding process, argon gas is blown from the bottom at a flow rate of 150 NL / min. After the wire feeding is completed, argon gas is blown softly for 15-20 min at a flow rate of 150 NL / min.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention promotes the segregation reaction by synergistically controlling the Se / S mass ratio and the O / Se mass ratio, using active oxygen as the thermodynamic driving force. This results in the in-situ generation of ellipsoidal / spindle-shaped Mn(S,Se) composite inclusions during the solidification stage. Compared to existing technologies, this invention offers the following advantages: (1) Significant and stable improvement in cutting performance: This invention controls the Se / S mass ratio within the range of 0.02 to 0.6, ensuring that Se exists in MnS in a solid solution form without forming coarse MnS-MnSe composite inclusions. At the same time, the O / Se mass ratio is controlled within the range of 0.006 to 0.6. Utilizing the thermodynamic promoting effect of active oxygen [O] on the segregation reaction, the inclusions are uniformly dispersed in the matrix in an ellipsoidal or spindle shape. Compared with the long strip-shaped MnS inclusions extending along the machining direction in traditional high-sulfur free-machining stainless steel, the inclusions in this invention have a more regular morphology and a more uniform distribution. During the cutting process, the continuity of chips can be effectively interrupted, significantly improving the stability and consistency of cutting performance.

[0017] (2) The anisotropy problem is fundamentally improved: In traditional high-sulfur free-machining stainless steel, MnS inclusions extend in a long strip after hot deformation, resulting in significant differences in the transverse and longitudinal mechanical properties of the steel. The Mn(S,Se) ellipsoidal / spindle-shaped composite inclusions obtained by this invention maintain stable morphology and uniform dispersion during hot deformation, which greatly reduces the anisotropy of the inclusion morphology.

[0018] (3) Simultaneous improvement of corrosion resistance: Elongated MnS inclusions are the preferred sites for pitting corrosion in stainless steel, and the difference in electrochemical activity at the interface between them and the matrix easily induces the formation and expansion of corrosion channels. This invention effectively reduces the stress concentration at the ends of the inclusions and the active interface area between them and the matrix by changing the morphology of the inclusions from elongated to ellipsoidal / spindle-shaped, thus making up for the inherent defect of the decline in corrosion resistance of traditional high-sulfur stainless steel due to a large number of MnS inclusions.

[0019] (4) The dual-parameter synergistic control mechanism achieves precise thermodynamic regulation: The active oxygen [O] in the steel is precisely maintained in the range of 10 to 60 ppm. The O / Se mass ratio (0.006 to 0.6) and the Se / S mass ratio (0.02 to 0.6) are simultaneously controlled in high-sulfur selenium-containing austenitic stainless steel. The synergistic effect of OS-Se provides a stable thermodynamic driving force for the segregation reaction, which fundamentally solves the problem of difficult precise matching of selenium addition and unstable batch-to-batch morphology of inclusions in traditional processes, and gives the product higher compositional uniformity and quality consistency.

[0020] (5) Advanced preparation process, suitable for large-scale industrial production: While designing the composition of selenium-containing cored wire, this invention optimizes the feeding process of selenium-containing cored wire, adopts a batch-interval feeding process (feeding in 2 to 3 batches, with a batch interval of 10 to 20 s and a feeding speed of 100 to 150 m / min). By dispersing the feeding method, the local enrichment and oxidation loss of selenium elements are avoided, effectively improving the selenium yield and the uniformity of selenium in molten steel. The overall process route is mature and controllable, and is suitable for large-scale industrial promotion and application. Attached Figure Description

[0021] Figure 1 The images show SEM scans of selenium dissolved in MnS and EDS surface scans of Mn, S, and Se in Example 1. Figure 2 Metallographic photograph of the longitudinal section of the wire rod in Example 1; Figure 3 Metallographic photograph of the longitudinal section of the wire rod in Example 2; Figure 4 Metallographic photograph of the longitudinal section of the wire rod in Example 3; Figure 5 Metallographic photograph of the longitudinal section of the wire rod in Example 4; Figure 6 Metallographic photograph of the longitudinal section of the wire rod in Example 5; Figure 7 Metallographic photograph of the longitudinal section of the wire rod in Example 6; Figure 8 Metallographic photograph of the longitudinal section of the wire rod in Example 7; Figure 9 Metallographic photograph of the longitudinal section of the wire rod in Example 8; Figure 10 Metallographic photograph of the longitudinal section of the wire rod in Example 9; Figure 11 Metallographic photograph of the longitudinal section of the wire rod in Comparative Example 1; Figure 12 The cutting machine tool and cutting force testing system used in this invention. Detailed Implementation

[0022] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0023] For simplicity, this application only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form a range not explicitly stated; and any lower limit can be combined with other lower limits to form a range not explicitly stated, just as any upper limit can be combined with any other upper limit to form a range not explicitly stated. Furthermore, although not explicitly stated, each point or individual value between the endpoints of a range is included within that range. Therefore, each point or individual value can be used as its own lower or upper limit and combined with any other point or individual value, or combined with other lower or upper limits, to form a range not explicitly stated. In the description of this application, it should be noted that, unless otherwise stated, "above" includes the stated number, and "multiple" in "one or more" means two or more.

[0024] The foregoing description of this application is not intended to describe every disclosed implementation or method. Instead, the following description provides more specific examples of exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments that can be used in various combinations. The examples listed are representative only and should not be construed as exhaustive.

[0025] Numerous details are explored in the following description to provide a more thorough explanation of embodiments of this application; however, it will be apparent to those skilled in the art that embodiments of this application may be practiced without these specific details.

[0026] Selenium, like sulfur, belongs to Group VI elements and has similar physical and chemical properties. It can effectively improve the morphology of sulfides and enhance the machinability of steel. Selenium can dissolve in MnS to form Mn(S,Se) composite inclusions. Studies have shown that selenium is a strong morphological element. Adding selenium to stainless steel can reduce the proportion of eutectic cluster type II MnS and increase the proportion of morphological type I spherical MnS. This promotes the transformation of MnS inclusions from elongated strips to spherical, short rod-shaped, or spindle-shaped inclusions, achieving spheroidization and dispersion of inclusions, thereby improving the machinability of the material and optimizing its corrosion resistance.

[0027] Based on the composition of low-carbon stainless steel, this invention promotes the occurrence of segregation reaction by controlling the composition and ratio of selenium, sulfur, and oxygen, i.e., Se / S=0.02~0.6, O / Se=0.006~0.6, thereby controlling the morphology of the free-machining phase and forming spindle-shaped or ellipsoidal manganese, selenium, and sulfur composite inclusions Mn(S,Se), optimizing the morphology and distribution of sulfides, thereby improving the brittle fracture resistance of free-machining stainless steel and enhancing the machinability of the steel.

[0028] The effects of each alloy component composition on the stainless steel production process and its machinability in this invention are as follows: C: Carbon is a stabilizing element in austenitic stainless steel. Carbon can dissolve in austenite, and solid solution strengthening can improve the strength of stainless steel. When the carbon content is too high, carbon (C) tends to precipitate at grain boundaries. 23 C6-type carbides cause chromium depletion near grain boundaries, reducing the stainless steel's resistance to intergranular corrosion and worsening its hot working properties. At the same time, excessively high carbon content increases the difficulty of cold working. The carbon content of this invention is controlled at ≤0.10%, resulting in low-carbon or ultra-low-carbon stainless steel.

[0029] S: Sulfur is a key element in free-machining stainless steel, mainly existing as MnS inclusions. MnS acts as a chip-breaking source, improving the chip-breaking properties of stainless steel during machining, significantly enhancing its machinability, and extending tool life. When sulfur is added to steel, the flexible MnS inclusions easily deform during the rolling process, forming elongated sulfides. This severely reduces the steel's corrosion resistance, as well as its mechanical properties such as plasticity, toughness, and fatigue resistance. It also affects the steel's hot workability, causing high-temperature cracking. The sulfur content control range of this invention is 0.25%~0.35%.

[0030] Nitrogen (N) can significantly improve the strength and corrosion resistance of steel, especially in high-nitrogen stainless steel, where appropriate amounts of nitrogen help improve mechanical properties and pitting corrosion resistance. However, excessive nitrogen content can easily lead to the formation of pores or nitride precipitation, reducing the plasticity, toughness, and weldability of the steel, increasing its work hardening properties, and affecting its subsequent cold working ability. In this invention, the nitrogen content is controlled at 0.015%~0.05%.

[0031] Se: Selenium is the core modifying element in this invention, mainly used for the modification treatment of sulfide inclusions. Selenium exists in MnS in two forms: solid solution and composite inclusions, namely: Mn(S,Se) solid solution phase and MnS-MnSe composite inclusions, as shown in the appendix. Figure 1 As shown. Se is a strong eccentric element. Se addition promotes eccentricity reactions in MnS during solidification and precipitation, forming spherical type I sulfides and reducing the proportion of eutectic type II clustered MnS. In solid solution form, selenium, together with Mn and S, forms the Mn(S,Se) phase. Solid solution increases the hardness of the composite inclusions, making them less prone to deformation during rolling. The original elongated MnS inclusions transform into spherical or spindle-shaped inclusions, improving the morphology and distribution of the free-machining phase, reducing anisotropy, and enhancing plasticity, toughness, and corrosion resistance, while maintaining excellent chip-breaking performance. Further increasing the Se content leads to saturation of selenium solubility in MnS, initiating the formation of MnS-MnSe composite inclusions. Excessive selenium content not only increases production costs but also causes coarsening of the MnS-MnSe composite inclusions, affecting the overall performance of the product and reducing the corrosion resistance, strength, plasticity, and other mechanical properties of stainless steel. The selenium content of this invention is controlled within the range of 0.01% to 0.15%, ensuring that it exists mainly in solid solution form and minimizing the formation of MnS-MnSe composite inclusions.

[0032] Selenium-sulfur ratio: In this invention, the selenium content is controlled at 0.01%~0.15% by mass, and the sulfur content is controlled at 0.25%~0.35%, resulting in a Se / S ratio of 0.02~0.6. The purpose is to ensure that selenium exists in a solid solution form and to minimize the formation of MnS-MnSe composite inclusions. The Se / S ratio has a significant impact on the morphology of sulfides in free-machining stainless steel and even its machinability. Based on experimental research, this invention controls the selenium-sulfur ratio at 0.02~0.6. This selenium-sulfur ratio can promote the formation of the Mn(S,Se) solid solution phase, such as... Figure 1 As shown, it significantly improves the chip-breaking ability and tool life of the material, while maintaining good hot workability and surface finish. When Se / S > 0.6, MnS-MnSe composite inclusions will form, causing coarsening of the composite inclusions, reducing corrosion resistance and mechanical properties, and adversely affecting stainless steel; when Se / S < 0.02, the amount of selenium added is too small, and the effect on the morphology and type of sulfides is limited, which is not conducive to improving cutting performance.

[0033] Active oxygen (atomic oxygen) is a strong morphological element. Active oxygen in molten steel promotes the morphological reaction of MnS during solidification, forming spherical type I sulfides. During oxygen smelting, atomic oxygen provides the thermodynamic conditions for the dispersed distribution of sulfides. When the active oxygen is below 0.001% (10 ppm), fewer spherical sulfides precipitate, and they cannot form a dispersed distribution. When the active oxygen is above 0.006%, although it is beneficial for sulfide morphology control, deoxidation during smelting is difficult, which is not conducive to improving the cleanliness control of steel and increases the risk of continuous casting defects. Based on experimental research, this invention controls the active oxygen at 0.001%~0.006%, i.e., at 10-60 ppm.

[0034] Oxygen-Se ratio (O / Se) plays a crucial role in controlling the machinability and inclusion morphology of free-machining stainless steel. In this invention, the mass fraction of active oxygen is controlled at 0.001%~0.006%, the mass fraction of selenium at 0.01%~0.15%, and the O / Se ratio at 0.006~0.60. When O / Se > 0.6, the upper limit of active oxygen is 0.006%, indicating that the amount of selenium added is too low, limiting its effect. When O / Se < 0.006, the Se content is too high, leading to the formation of MnS-MnSe composite inclusions, resulting in sulfide coarsening and negatively impacting the overall performance of the steel.

[0035] An appropriate oxygen-selenium ratio can promote the formation of spherical or spindle-shaped sulfur-selenium composite oxide inclusions, which is beneficial to improving the chip-breaking properties, surface quality, and tool life of the material, while maintaining good thermoplasticity and cold working ability.

[0036] The free-machining stainless steel containing selenium of the present invention controls the mass percentage content of each component within the following range: by mass percentage, it contains the following components: C≤0.10%, Si≤1.0%, Mn≤3.0%, P≤0.15%, S: 0.25%~0.35%, Cr: 17.0%~19.0%, Ni: 8.0%~10.0%, Cu: 1.5%~3.5%, Se: 0.01%~0.15%, active oxygen [O]: 0.001%~0.006%, N: 0.015%~0.05%, with the balance being Fe and unavoidable impurities; wherein, the mass ratio of Se to S, Se / S, is 0.02~0.6, and the mass ratio of active oxygen [O] to Se, O / Se, is 0.006~0.6.

[0037] Its preparation process is as follows: EAF → AOD refining → LF refining → selenium-fed cored wire → continuous casting / in-situ casting → billet heating → rolled wire rod, specifically: The preparation process of this free-machining stainless steel is as follows: EAF → AOD refining → LF refining → selenium cored wire feeding → continuous casting / ingot casting → billet heating → wire rod rolling, specifically: (1) Electric arc furnace smelting (EAF): The raw materials for electric arc furnace smelting consist of low-phosphorus stainless steel return material and high-carbon ferrochrome alloy, wherein the phosphorus content meets P≤0.050%. According to the target composition, the primary smelting, melting and alloying are carried out, and the tapping temperature of the electric furnace is 1580℃. (2) Argon-Oxygen Decarburization Furnace (AOD) Refining: The early stage is the oxidation period, in which mixed gases with different Ar and O ratios are blown in over time to carry out decarburization operations so that the carbon composition meets the composition control standards; the later stage is the reduction period, in which active lime, fluorite, as well as ferrosilicon, ferrosilicon-manganese alloy, aluminum ingots, ferrochrome, ferronickel, and ferromolybdenum are added to adjust the chromium, nickel, molybdenum, silicon, and manganese composition of the molten steel to the required range; and ferrosulfite is added to adjust the sulfur content of the molten steel to the required range. The AOD tapping temperature is ≥1600℃. (3) Ladle refining furnace (LF): After the LF is in place, lime and submerged arc slag are added to the slag surface, and aluminum particles and silicon carbide are added to deoxidize the slag surface; the LF is energized and heated to raise the temperature of the molten steel to 1620℃; after the energization is completed, the composition of the molten steel is tested to ensure that it meets the composition control window. Argon gas is blown from the bottom throughout the LF refining process to stir the molten steel and promote uniform composition and temperature. (4) Wire feeding: After the refining composition is controlled to the target window, the selenium-containing cored wire is fed into the steel at a temperature of 1550 ℃ through a wire feeder. The selenium-containing cored wire is an iron-coated cored wire with a diameter of 8~13 mm. The core material of the cored wire is a mixture of selenium powder and iron powder particles or selenium-iron alloy particles. The mass fraction of selenium is 10%~40%. In order to further ensure the selenium yield and ensure that the slag layer thickness of the molten steel is 1-5 cm, the wire feeding is carried out in an intermittent manner, in 3 times, with an interval of 15 s between each time. The wire feeding speed is controlled at 125 m / min. During the wire feeding process, the bottom blowing argon gas is stirred and the flow rate is controlled at 200 NL / min. After the wire feeding, the soft blowing argon gas is continuously blown for 20 min and the flow rate is controlled at 140 NL / min. (5) Continuous casting / ingot casting: The cross-section of the continuous casting is 200×200 mm, and a high-sulfur special protective slag is used for casting during the continuous casting process; the ingot casting uses 2-ton octagonal ingots and is cast by bottom casting; the temperature of the molten steel in continuous casting / ingot casting is controlled at 1490℃, the casting speed is controlled at 1 m / min, a weak cooling method is adopted, and the water volume in the first cooling zone is 100m³. 3 / h, the water volume control ratio in the secondary cooling zone is 0.24 tons of water / ton of steel; the casting speed during the ingot casting process is 1.5 tons / min; (6) Billet heating: The continuously cast billet is hot-sent and heated in the heating furnace to 1175℃. The total heating time is controlled according to the billet thickness of 1 min / mm, and the total heating time is 180 min. (7) Rolling: The initial rolling temperature is 1125℃. The rolling steps include roughing, intermediate rolling, intermediate rolling, pre-finishing rolling, and finishing rolling. The temperature of the rolled piece during the rolling process is 850℃~1050℃. The maximum reduction during the rolling process is ≤20%. The final rolling temperature is 875℃. After rolling, the piece is air-cooled to ensure smooth rolling.

[0038] To further understand the present invention, the following embodiments are provided. It is worth noting that, unless otherwise specified, all raw materials used in the present invention are commercially available; and all methods and equipment employed are common in the art.

[0039] In Examples 1-10 and Comparative Examples 1-6, the elemental compositions of Cr, Ni, Cu, Si, Mn, and P were controlled within the following ranges: Cr: 17.5%~18.5%, Ni: 8.5%~9.5%, Cu: 1.5%~2.5%, Si: 0.35%~0.55%, Mn: 0.80%~1.20%, P≤0.025%, with the balance being Fe and unavoidable impurities. The fluctuations in the above-mentioned fixed compositions between examples do not exceed 10% of the specified range and do not constitute a source of performance differences in the present invention.

[0040] Specifically, the percentage content of the fixed basic components in each embodiment and comparative example is as follows: Cr: 18.0%, Ni: 9.0%, Cu: 2.0%, Si: 0.4%, Mn: 1%, P≤0.025%, and the remaining core components are shown in Table 1 below: The components in the above embodiments all meet the stainless steel composition control range of C ≤ 0.10%, S: 0.25%~0.35%, Se: 0.01%~0.15%, O: 0.001%~0.006%, and N: 0.015%~0.05%; wherein, the selenium-sulfur ratio Se / S is 0.02~0.6, and the oxygen-selenium ratio O / Se is 0.006~0.6.

[0041] All embodiments and comparative examples were produced according to the above preparation method. The process route was EAF → AOD refining → LF refining → feeding selenium-coated wire → continuous casting → billet heating → rolling. The differences between the embodiments are as follows: First, by adjusting the total amount of selenium-coated wire fed, the mass fraction of Se in the finished steel was controlled, so that the mass ratio of Se to S (Se / S) reached the target range corresponding to each embodiment. Second, by adjusting the type and amount of deoxidizer (aluminum particles, silicon carbide) on the slag surface during the LF refining stage, the active oxygen [O] of the molten steel was controlled at different levels, so that the mass ratio of active oxygen to Se (O / Se) in the finished steel fell within the control range set in each embodiment.

[0042] The comparative example was prepared according to the same process route, but the process of feeding selenium cored wire was omitted, that is, no selenium-containing materials were added to the molten steel, and the finished steel did not contain Se. The remaining process parameters were the same as those of each example, and it served as a benchmark control for evaluating the effect of selenium modification.

[0043] Machinability tests were performed on the above embodiments and comparative examples using a test system as follows: Figure 12 As shown, the cutting tool was a 6 mm diameter stainless steel cutting tool, with a spindle speed of 5000 rev / min, a feed rate of 70 mm / min, a depth of cut of 2 mm, and a side depth of cut of 0.5 mm. Cutting performance was tested on a V-8L machining center, and the cutting forces during milling were collected using a Kistler 9257B force gauge. The cutting performance evaluation parameters for each embodiment and comparative example are shown in Table 2 below.

[0044] Table 2. Cutting performance evaluation parameters for Examples 1-10 and Comparative Examples 1-6 As can be seen from the test results in Table 2, the cutting performance was significantly improved after adjusting the selenium-sulfur ratio. Specifically, the proportion of the ellipsoidal free-machining phase in Examples 1-10 increased significantly, reaching 55%. In contrast, Comparative Example 2 had a lower selenium content (0.005%), and its ellipsoidal free-machining phase ratio was only 23%, indicating that the selenium addition was low and the morphology of the free-machining phase was not good. The ellipsoidal phase ratio was only 40%, and the cutting force during the machining process increased (68.0 N), the C-type chip ratio was low (35%), and the roughness value was high (2.55). Comparative Example 3 has a higher selenium content (0.20%), resulting in better morphology of the free-machining phase. The proportion of ellipsoidal phases is only 70%, and the cutting force during processing is significantly reduced (40.0 N). The proportion of C-type chips is high (78%), and the roughness value is low (1.56). However, the high selenium content leads to excessively coarse sulfides, resulting in a significant decrease in the mechanical properties of the product. Furthermore, the production cost is too high, which is unacceptable to steel mills.

[0045] In Comparative Example 4, the oxygen content was significantly low (0.0008%), resulting in poor fluidity of the molten steel during the production process, blockage during casting, and interruption of continuous casting production.

[0046] Compared with Example 5, the sulfur and selenium composite requirements significantly increased the oxygen content (0.008%), resulting in a better morphology of the free-machining phase, with an ellipsoidal proportion of only 65%, and a significant reduction in cutting force during machining (45.0 N). The proportion of C-type chips was high (75%), and the roughness value was low (1.35). However, the high oxygen content significantly increased the pinhole and bubble defects on the surface of the billet, resulting in a significant increase in the product defect rate, which was unacceptable to steel mills.

[0047] Comparative Example 6 had a low sulfur content (only 0.12%), a relatively small number of easily machinable phases, and an ellipsoidal proportion as high as 85%, resulting in poor overall machinability. The machining process involved a large cutting force (60.2 N), a low proportion of C-type chips (55%), and a high surface roughness value (2.30).

[0048] Figure 1 The images shown are SEM scans of selenium powder dissolved in MnS and EDS scans of Mn, S, and Se in Example 1. Figure 1 As can be seen, selenium is effectively dissolved in MnS inclusions to form Mn(S,Se) solid solution inclusions.

[0049] Figure 2 The image shows a metallographic photograph of the longitudinal section of the wire rod in Example 1. Figure 2 As can be seen, the proportion of the ellipsoidal free-machining phase increased significantly, reaching 60%, indicating that the amount of selenium added according to the technical features of this embodiment can improve the morphology of the free-machining phase and improve machinability (low cutting force, high proportion of C-type chips, and low roughness value).

[0050] Figure 3 Metallographic photographs of the longitudinal section of the wire rod in Example 2; from Figure 3 It can be seen that the proportion of ellipsoidal free-machining phase has increased significantly, reaching 63%, indicating that the amount of selenium added according to the technical characteristics of this embodiment can improve the morphology of the free-machining phase and improve machinability (low cutting force, high proportion of C-type chips, and low roughness value).

[0051] Figure 4 Metallographic photographs of the longitudinal section of the wire rod in Example 3; from Figure 4 It can be seen that the proportion of the ellipsoidal free-machining phase has increased significantly, reaching 64%, indicating that the amount of selenium added according to the technical characteristics of this embodiment can improve the morphology of the free-machining phase and improve machinability (low cutting force, high proportion of C-type chips, and low roughness value).

[0052] Figure 5 Metallographic photographs of the longitudinal section of the wire rod in Example 4; from Figure 5 It can be seen that the proportion of the ellipsoidal free-machining phase has increased significantly, reaching 66%, indicating that the amount of selenium added according to the technical characteristics of this embodiment can improve the morphology of the free-machining phase and improve machinability (low cutting force, high proportion of C-type chips, and low roughness value).

[0053] Figure 6 Metallographic photographs of the longitudinal section of the wire rod in Example 5; from Figure 6 It can be seen that the proportion of the ellipsoidal free-machining phase has increased significantly, reaching 55%, indicating that the amount of selenium added according to the technical characteristics of this embodiment can improve the morphology of the free-machining phase and improve machinability (low cutting force, high proportion of C-type chips, and low roughness value).

[0054] Figure 7 Metallographic photographs of the longitudinal section of the wire rod in Example 6; from Figure 7 It can be seen that the proportion of the ellipsoidal free-machining phase has increased significantly, reaching 66%, indicating that the amount of selenium added according to the technical characteristics of this embodiment can improve the morphology of the free-machining phase and improve machinability (low cutting force, high proportion of C-type chips, and low roughness value).

[0055] Figure 8 Metallographic photographs of the longitudinal section of the wire rod in Example 7; from Figure 8 It can be seen that the proportion of the ellipsoidal free-machining phase increases to 55%, indicating that the amount of selenium added according to the technical characteristics of this embodiment can improve the morphology of the free-machining phase and improve machinability (low cutting force, high proportion of C-type chips, and low roughness value).

[0056] Figure 9 Metallographic photographs of the longitudinal section of the wire rod in Example 8; from Figure 9 It can be seen that the proportion of ellipsoidal free-machining phase has increased significantly, reaching 61%, indicating that the amount of selenium added according to the technical characteristics of this embodiment can improve the morphology of the free-machining phase, resulting in lower cutting force, higher proportion of C-type chips, and lower roughness value.

[0057] Figure 10 Metallographic photograph of the longitudinal section of the wire rod in Example 9; from Figure 10 It can be seen that the proportion of ellipsoidal free-machining phase has increased significantly, reaching 70%, indicating that the amount of selenium added according to the technical characteristics of this embodiment can improve the morphology of the free-machining phase, resulting in lower cutting force, higher proportion of C-type chips, and lower roughness.

[0058] Figure 11 This is a metallographic photograph of the longitudinal section of the wire rod in Comparative Example 1; this example does not contain selenium, from Figure 11 It can be seen that the proportion of elongated free-machining phases has increased significantly, while the proportion of ellipsoidal free-machining phases is only 23%. This indicates that without selenium, the morphology of the free-machining phases is not good, and the cutting force during the machining process is increased (71.0 N), the proportion of C-type chips is low (22%), and the roughness value is high.

[0059] Figure 12 The cutting machine tool and cutting force testing system used in this invention are shown in the figure. The rightmost spectrogram in the figure is the Ra value of the surface roughness of the cutting surface.

[0060] Salt spray, pitting, and intergranular corrosion tests were also conducted on the corrosion performance of the materials corresponding to the various embodiments of the present invention. The test results all met the requirements for material corrosion resistance, so they will not be described in detail here.

[0061] The foregoing has shown and described the basic process, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing low-carbon, high-sulfur, selenium-containing free-machining stainless steel, characterized in that, This free-machining stainless steel contains the following components by mass percentage: C≤0.10%, Si≤1.0%, Mn≤3.0%, P≤0.15%, S: 0.25%~0.35%, Cr: 17.0%~19.0%, Ni: 8.0%~10.0%, Cu: 1.5%~3.5%, Se: 0.01%~0.15%, active oxygen [O]: 0.001%~0.006%, N: 0.015%~0.05%, with the balance being Fe and unavoidable impurities; wherein, the mass ratio of Se to S is Se / S: 0.02~0.6:1, and the mass ratio of active oxygen [O] to Se is O / Se: 0.006~0.6:1; The preparation process of this free-machining stainless steel is as follows: electric arc furnace smelting, argon-oxygen decarburization furnace refining, ladle refining furnace refining, selenium cored wire feeding, continuous casting / ingot casting, billet heating, and wire rod rolling, specifically including the following steps: (1) Electric arc furnace smelting: The raw materials for electric arc furnace smelting consist of low-phosphorus stainless steel return material and high-carbon ferrochrome alloy, wherein the phosphorus content meets P≤0.050%. According to the target composition, the primary smelting, melting and alloying are carried out, and the electric furnace tapping temperature is 1570-1590℃. (2) Argon-oxygen decarburization furnace refining: The early stage is the oxidation period, and mixed gases with different Ar and O ratios are blown in over time to carry out decarburization operation so that the carbon composition meets the composition control standard; the later stage is the reduction period, and active lime, fluorite, as well as ferrosilicon, ferromanganese silicon alloy, aluminum ingot, ferrochrome, ferronickel, and ferromolybdenum are added to adjust the chromium, nickel, molybdenum, silicon, and manganese composition of the molten steel to the required range; and ferrosulfite is added to adjust the sulfur content of the molten steel to the required range. The tapping temperature of the argon-oxygen decarburization furnace is ≥1600℃. (3) Ladle refining furnace refining: After the ladle refining furnace is in place, lime and submerged arc slag are added to the slag surface, and aluminum particles and silicon carbide are added to deoxidize the slag surface; the LF is energized and heated to raise the temperature of the molten steel to 1610-1630℃; after the energization is completed, the composition of the molten steel is tested to ensure that it meets the composition control window. Argon gas is blown from the bottom throughout the LF refining process to stir the molten steel and promote uniform composition and temperature. (4) Wire feeding: After the refining composition is controlled to the target window, when the temperature of the molten steel is 1540-1560 ℃, the selenium-containing cored wire is fed in through the wire feeding machine. The selenium-containing cored wire is an iron-coated cored wire with a diameter of 8-13 mm. The core material of the cored wire is a mixture of selenium powder and iron powder particles or selenium-iron alloy particles. The mass fraction of selenium is 10%-40%. In order to further ensure the selenium recovery rate and ensure that the slag layer thickness of the molten steel is 1-5 cm, the wire feeding is carried out in an intermittent manner, in 2-3 times, with an interval of 10-20 s each time. The wire feeding speed is controlled at 100-150 m / min. During the wire feeding process, the bottom blowing argon gas is stirred, and the flow rate is controlled at 100-300 NL / min. After the wire feeding, the soft blowing argon gas is continued for 10-30 min, and the flow rate is controlled at 100-180 NL / min. (5) Continuous casting / ingot casting: The cross-section of the continuous casting is 200×200 mm or 150×150 mm. High-sulfur special protective slag is used for casting during the continuous casting process; octagonal ingots of 1.0-3.0 tons are used for ingot casting, and bottom casting is used; the temperature of the molten steel in continuous casting / ingot casting is controlled at 1470℃~1520℃, the casting speed is controlled at 0.9~1.2 m / min, weak cooling method is used, and the water volume in the first cooling zone is 95-105 m³ / min. 3 / h, the water volume control ratio in the secondary cooling zone is 0.18~0.30 tons of water / ton of steel; the casting speed in the ingot casting process is 1.0~2.0 tons / min; (6) Billet heating: The continuously cast billet is hot-sent and heated in the heating furnace to 1150℃~1200℃. The total heating time is controlled according to the billet thickness of 0.95~1.05 min / mm, and the total heating time is 140-210 min. (7) Rolling: The initial rolling temperature is 1100℃~1150℃. The rolling steps include roughing, intermediate rolling, intermediate rolling, pre-finishing rolling, and finishing rolling. The rolling temperature is 850℃~1050℃. The maximum reduction during the rolling process is ≤20%. The final rolling temperature is 850-900℃. After rolling, the parts are air-cooled to ensure smooth rolling.

2. The method for preparing low-carbon, high-sulfur, selenium-containing free-machining stainless steel according to claim 1, characterized in that, By mass percentage, it contains the following components: C≤0.10%, Si≤0.6%, Mn≤1.5%, P≤0.03%, S: 0.26%~0.34%, Cr: 17.5%~18.5%, Ni: 8.5%~9.5%, Cu: 1.5%~2.5%, Se: 0.04%~0.14%, active oxygen [O]: 0.002%~0.005%, N: 0.02%~0.05%, with the balance being Fe and unavoidable impurities; wherein, the Se / S ratio is 0.12~0.54:1, and the O / Se ratio is 0.02~0.13:

1.

3. The method for preparing low-carbon, high-sulfur, selenium-containing free-machining stainless steel according to claim 2, characterized in that, By mass percentage, it contains the following components: C≤0.030%, Si≤0.6%, Mn≤1.5%, P≤0.025%, S: 0.26%~0.30%, Cr: 17.5%~18.5%, Ni: 8.5%~9.5%, Cu: 1.5%~2.5%, Se: 0.06%~0.12%, active oxygen [O]: 0.003%~0.004%, N: 0.02%~0.04%, with the balance being Fe and unavoidable impurities; wherein, the Se / S ratio is 0.2~0.46:1, and the O / Se ratio is 0.025~0.07:

1.

4. The method for preparing low-carbon, high-sulfur, selenium-containing free-machining stainless steel according to claim 1, characterized in that, The selenium-containing free-machining phase in steel is a manganese-selenium-sulfur complex inclusion that is dispersed in a spindle-shaped or ellipsoidal pattern: Mn(S,Se).

5. The method for preparing low-carbon, high-sulfur, selenium-containing free-machining stainless steel according to claim 1, characterized in that, The selenium-containing cored wire is an iron-clad cored wire with a diameter of 10-13mm. Its core powder is a mixture of selenium powder and industrial iron powder, or a selenium-iron alloy, with a selenium mass fraction of 20%-30%.

6. The method for preparing low-carbon, high-sulfur, selenium-containing free-machining stainless steel according to claim 5, characterized in that, When the molten steel temperature is 1540-1560 ℃ at the end of the LF stage, selenium-containing cored wire is fed into the molten steel through a wire feeder, and the slag layer thickness of the molten steel is 1-2 cm. The wire feeding is carried out intermittently in 3 times, with an interval of 15 s between each time, and the wire feeding speed is 125 m / min. During the wire feeding process, argon gas is blown from the bottom at a flow rate of 150 NL / min. After the wire feeding is completed, argon gas is blown softly for 15-20 min at a flow rate of 150 NL / min.

Images