Sb-Bi complex high machinability bar and method for producing the same

Abstract

The application discloses a sulfur-bismuth combined high-cutting-property bar and a production method thereof. The chemical components of the sulfur-bismuth combined high-cutting-property bar are as follows: C: 0.07-0.12%, Si: 0.06-0.1%, Mn: 0.9-1.03%, P: 0.01-0.1%, S: 0.15-0.3%, Bi: 0.04-0.17%, O: 0.0055-0.0075%, Bi / S: 0.14-1.13, Mn / S: 3-6.86, and the balance is iron and inevitable impurities, and the content is in percentage by weight. The sulfur-bismuth combined high-cutting-property bar is beneficial to the precipitation of inclusions and the dispersion distribution of the inclusions in the steel in the steelmaking and solidification process by optimizing the ratio of manganese, sulfur and bismuth and matching a certain content of oxygen, the inclusions prevent the elongation deformation of the manganese sulfide soft core structure in the rolling process, the morphology of the inclusions is changed into spindle-like or spherical shape which is 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 high-machinability bar stock with bismuth-sulfur formulation and its production method. Background Technology

[0002] Machining is a key process in industrial manufacturing, and the machinability of materials directly affects machining efficiency, surface quality, and tool life. In the field of bar processing, lead-containing free-cutting steel bars remain one of the best cutting steel materials in terms of machinability and surface quality. However, due to lead vapor pollution during smelting, the market for lead-containing free-cutting steel bars is gradually shrinking. Alternatives include designing lead-free free-cutting bars using environmentally friendly cutting elements. Among these, Bi (Bi) is the most promising metallic element to replace lead in improving the machinability of bars. Bi is non-toxic and has similar physical properties to lead, is lighter than Pb, closer to Fe, has a more uniform distribution, and is less prone to segregation. Therefore, using Bi to replace lead in the production of environmentally friendly free-cutting steel is a new direction for the research and development of new free-cutting steel bars.

[0003] CN104245992 B discloses a free-cutting steel containing bismuth, wherein the carbon composition of the free-cutting steel is as follows: carbon: 0.10-0.17%; silicon: not more than 0.12%; manganese: 0.1-1.3%; sulfur: 0.15-0.30%; phosphorus: not more than 0.1%; lead: 0.15-0.30%; the remainder being iron and impurities, calculated by mass percentage. This free-cutting steel has good cutting performance, but contains the environmentally unfriendly element lead, which does not meet the technical requirements for environmentally friendly free-cutting steel.

[0004] CN105088106 B discloses a tin-bismuth composite free-cutting steel, the main components by weight percentage being: C: 0.06~0.09%, Si: ≤0.10%, Mn: 1.30~1.60%, P: 0.08~0.12%, S: 0.35~0.45%, Bi: 0.003%, Sn: 0.1%; the balance being Fe and residual trace elements. Such high manganese and sulfur content easily reduces the surface quality of the cast billet, causing microcracks on the surface of the rolled material, and significantly increases the hardness of the steel, leading to drawing difficulties and reduced machinability.

[0005] CN103255359 A discloses a bismuth-containing free-cutting steel, which, by mass percentage, contains: C: 0.04%–0.15%; Si: 0%–0.15%; Mn: 0.9%–1.6%; P: 0.02%–0.11%; S: 0.25%–0.45%; Cr: 0–0.2%; Ni: 0–0.2%; Cu: 0–0.2%; Bi: 0.05%–0.2%; Ti: 0.005%–0.05%; N: 0.005%–0.05%; impurities: 0–0.2%; balance: Fe. This bismuth-containing free-cutting steel possesses excellent comprehensive mechanical properties and superior machinability. However, the addition of more than 10 alloying elements significantly increases the alloy cost while achieving these properties, hindering mass production.

[0006] CN103911550 A discloses an environmentally friendly, low-carbon, high-sulfur bismuth free-cutting steel with excellent thermoplasticity. The free-cutting steel contains, by weight percentage, C≤0.15%, Si≤0.10%, Mn0.80~1.6%, P0.02~0.10%, S0.20~0.45%, Bi0.03~0.20%, N≤0.010%, T[O]0.002~0.02%. In addition, the steel also contains B0.001~0.015% and Ti0.005~0.15%, as well as 11 cutting elements and one or more rare earth elements. The weight percentage of each of the above components is 0.001~0.50%, with the balance being Fe and other unavoidable trace elements. This invention has excellent thermoplasticity, but the technology cannot obtain the thermoplasticity of low carbon and high sulfur free-cutting steel by combining only the four elements of bismuth, sulfur, carbon and oxygen. Instead, it is necessary to add B and Ti to improve plasticity, and rare earth elements need to be added for comprehensive matching. This makes it impossible to control the rare earth yield in the process, and the process compatibility is poor, making it impossible to carry out batch continuous casting production.

[0007] CN102330039 A discloses a low-carbon, bismuth-containing, environmentally friendly free-cutting steel, which, by mass percentage, contains C: ≤0.15%, Mn: 0.75%~1.5%, S: 0.23%~0.42%, Si: ≤0.10%, P: ≤0.11%, Bi: ≤0.05%~0.2%, N: 0.001%~0.0200%, with the remainder being Fe and impurities. The mass percentage ratio of Mn to S is Mn / S = 1.79~6.52, the mass percentage ratio of Bi to S is Bi / S = 0.12~0.87, and T[O]: 0.0010~0.0250%. This low-carbon, bismuth-containing, environmentally friendly free-cutting steel has low cost, good environmental performance, and cutting performance comparable to lead-containing free-cutting steel structures with corresponding carbon content. However, this low-carbon, bismuth-containing, environmentally friendly free-cutting structural steel has significant technical shortcomings. It contains one or more of the seven cutting elements that can improve cutting performance, which will greatly increase the alloy cost and is not conducive to mass production. Summary of the Invention

[0008] This invention provides a sulfur-bismuth composite high-machinability bar and its production method. The aim is to manufacture a bar with better machinability and no pollution, addressing the environmental problems associated with existing free-machining steel bars that primarily rely on lead-containing designs. It also aims to solve the problems of high production costs, low Bi addition yield, and surface defects in billets associated with existing bismuth-containing free-machining steels. This invention utilizes a low-carbon + sulfur-bismuth alloy design system. Through a specific ratio of manganese, sulfur, and bismuth, combined with a certain amount of oxygen, beneficial inclusions precipitate and disperse throughout the steel during solidification. These inclusions mainly consist of manganese sulfide soft core structures, manganese sulfide-oxygen composite hard core structures, and manganese sulfide-bismuth composite hard core structures. During rolling, the soft core structures of manganese sulfide are prevented from being elongated and deformed, causing the inclusions to change into spindle-shaped or spherical shapes that are favorable for machinability, thus improving machinability.

[0009] According to one aspect of the present invention, a bismuth-sulfur compound high-machinability bar stock is provided, wherein the chemical composition of the bismuth-sulfur compound high-machinability bar stock is as follows: C: 0.07%–0.12%, Si: 0.06%–0.10%, Mn: 0.90%–1.03%, P: 0.01%–0.10%, S: 0.15%–0.30%, Bi: 0.04%–0.17%, O: 0.0055%–0.0075%; Bi / S ratio is 0.14–1.13, Mn / S ratio is 3.00–6.86, and the balance is iron and unavoidable impurities, by weight percentage.

[0010] The selection of the amount (by weight percentage) of each of the above elements and their functions are explained below:

[0011] C: Carbon is the most basic matrix strengthening element in steel materials. In this invention, the addition of carbon ensures a certain strength in the matrix and, in combination with Bi, achieves good machinability. When the carbon content is below 0.07%, the strength of the steel bar cannot be guaranteed; when the carbon content is above 0.12%, it cannot form a good combination with Bi, resulting in increased surface hardness and significantly reduced machinability. Therefore, in this invention, the carbon content of the bismuth-sulfur composite high-machinability bar is controlled within the range of 0.07% to 0.12%.

[0012] Si: Silicon is the deoxidizer in this invention, and it undergoes co-deoxidation with Mn. When the silicon content is below 0.06%, the co-deoxidation effect is poor; when the silicon content is above 0.10%, the machinability is significantly reduced. Therefore, the silicon content of the bismuth-sulfur compound high-machinability bar stock in this invention is controlled within the range of 0.06% to 0.10%.

[0013] Mn is a key component of the MnS and MnS-Bi composite inclusions in this invention, which is beneficial for improving machinability. When the manganese content is below 0.90%, the number of manganese sulfide and manganese bismuth sulfide composite inclusions is small, the aspect ratio is large, and the proportion of spindle-shaped inclusions is very low, which reduces machinability. When the manganese content is above 1.03%, the surface hardness of the steel is high, making cutting difficult and reducing the tool life. Therefore, the manganese content of the bismuth sulfide combined high-machinability bar stock in this invention is controlled within the range of 0.90% to 1.03%.

[0014] P: Appropriate phosphorus content improves machinability, while content above 0.10% leads to "cold brittleness" and reduces machinability. Therefore, the phosphorus content of the bismuth-sulfur compound high-machinability bar stock of this invention is controlled at 0.01% to 0.10%.

[0015] S: Sulfur is one of the most beneficial elements for free cutting. Appropriate addition of sulfur forms manganese sulfide compounds with manganese and composite inclusions with manganese-bismuth, significantly improving free cutting performance. When the sulfur content is below 0.15%, it cannot form good composite compounds with manganese and bismuth that contribute to cutting performance; when the sulfur content is above 0.30%, the hot workability of the steel decreases. Therefore, in this invention, the sulfur content of the sulfur-bismuth composite high-cutting-performance bar stock is controlled within the range of 0.15% to 0.30%.

[0016] Bismuth is an effective free-machining element and is considered the most suitable substitute for lead. It can form bismuth manganese sulfide complex inclusions with manganese and sulfur, altering the morphology and distribution of these inclusions. Combined with sulfur, it significantly improves the machinability, surface quality, and chip morphology of free-machining steels. When the bismuth content is below 0.04%, the above effects are not achieved; when the bismuth content exceeds 0.17%, the machinability cannot be further improved, and smelting costs increase. Therefore, in this invention, the bismuth content of the sulfur-bismuth combined high-machinability bar stock is controlled within the range of 0.04% to 0.17%, and the bismuth-sulfur ratio (Bi / S) is 0.14 to 1.13.

[0017] O: In this invention, oxygen provides the thermodynamic conditions for the diffuse distribution of sulfides. When the oxygen content is below 0.0055%, fewer oxides are precipitated, which does not allow for sufficient nucleation of manganese sulfide, resulting in inclusions that are mostly distributed in elongated strips with increased aspect ratio. When the oxygen content is above 0.0075%, there are more oxides, which can easily cause internal defects in the material. Therefore, the oxygen content in this invention is controlled within the range of 0.0055% to 0.0075%.

[0018] This invention adds bismuth to low-carbon steel bars. Through the synergistic effect of oxygen, sulfur, and manganese, and under the design of the steelmaking process, the elements form good non-metallic inclusion morphology. During the bar rolling process, the inclusion morphology does not undergo significant elongation changes, improving the overall machinability of the matrix and obtaining an easy-to-machine bar product. The product has excellent surface quality, meeting the user's machining requirements. The cutting performance index is as follows: under the cutting conditions of f=0.09 mm / r and a rotation speed of 800 r / min, the proportion of C-shaped chips is 84-87%, the average surface roughness of the steel bar is 4.5-4.8 μm, and the cutting efficiency of the steel bar is 20%-30% higher than that of the original bar cutting efficiency.

[0019] According to another aspect of the present invention, a method for producing bismuth sulfide compounded high-machinability bars is provided, the method comprising the following steps:

[0020] (1) Converter smelting: The molten iron in the converter is not desulfurized, only decarburized and phosphorus-reduced. The oxygen content is strictly controlled. Before being moved out, the oxygen content is controlled at 125-140 ppm to prepare for oxygen level control in the refining furnace.

[0021] (2) LF furnace refining: First, add refining slag and submerged arc slag, then add ferrosilicon and silicon carbide to deoxidize the molten steel surface, blow argon to ensure that the molten steel does not roll, strictly control the order of adding bismuth ferroalloy and sulfur ferroalloy, add bismuth ferroalloy first, then sulfur ferroalloy, and finally bismuth ferroalloy, and then alloy refine.

[0022] (3) Large billet continuous casting: Low carbon protective slag is used for protection during the continuous casting process. The continuous casting speed is controlled at 0.6 to 1.0 m / min. The secondary cooling water is weakly cooled with a specific water volume of 1.0 to 1.4 L / kg. The continuous casting process is carried out by electromagnetic stirring in the crystallizer and electromagnetic stirring at the end of solidification under light pressure.

[0023] (4) Surface cleaning of continuous casting billet: The surface of the continuous casting billet is cleaned to ensure that there are no defects such as microcracks on the surface of the billet;

[0024] (5) Heating of billet: The billet is heated in the furnace by cold charging. The cross-sectional dimensions of the billet are 180mm~280mm×180mm~380mm. Heating temperature and time: 1240~1260℃ for 3.0~4.0h;

[0025] (6) Rolling of billets and slow cooling after rolling: Rolling is carried out in the recrystallization zone with an initial rolling temperature of 1050-1100℃ and a final rolling temperature of 880-920℃ to obtain finished bars. Rolling in the recrystallization zone can increase the grain boundary density. Dense grain boundaries can hinder the elongation of sulfur-bismuth composite inclusions during rolling, further improving the morphology and distribution of inclusions and improving the machinability of the steel bars. The steel bars are cooled to 650-710℃ on the cooling bed and then enter the slow cooling pit. The cooling time to below 200℃ is controlled within 8-10 hours. Then, they are air-cooled to room temperature. After finishing, flaw detection, length setting, inspection, and warehousing, high machinability bars with sulfur-bismuth composite are obtained.

[0026] Further, in step (2), the alloy addition sequence in the LF furnace refining is as follows: after white slag treatment, 50-60 m of bismuth-iron cored wire is fed first, followed by the immediate addition of low-phosphorus ferromanganese and ferrous sulfate. After the slag is melted evenly, 30-40 m of bismuth-iron cored wire is fed in. The alloying refining time is controlled at 26-33 min, and the oxygen content is controlled at 55-75 ppm.

[0027] Further, the specific steps of the large billet continuous casting in step (3) are as follows: adjust the electromagnetic stirring current of the crystallizer to 300-340A and the frequency to 4.0-4.8Hz, the electromagnetic stirring current at the end of solidification to 350-430A and the frequency to 2.5-4.0Hz, the total reduction at the end of light pressing to 11.0-13.0mm, the maximum reduction of each section to 2.5-4.5mm, the continuous casting billet enters the slow cooling pit for slow cooling, the temperature of the slow cooling pit is 580-610℃, the slow cooling time is 20-24 hours, and it is cooled to room temperature with the furnace.

[0028] Furthermore, the surface treatment method described in step (4) is powder spraying for flaw detection or peeling.

[0029] Furthermore, under cutting conditions with a feed rate f of 0.09 mm / r and a rotational speed of 800 r / min, the proportion of C-shaped chips is 84-87%, and the average surface roughness of the steel bar cutting surface is 4.5-4.8 μm.

[0030] Furthermore, the finished bar obtained in step (6) has a size of φ30~φ80mm.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] (1) Through the above-mentioned composition and smelting process design, plus the rolling process design, the composite inclusion morphology of the present invention is achieved as follows: a small amount of white inclusions are distributed on the boundary of MnS inclusions. The white inclusions contain a large amount of pure Bi and a small amount of Fe elements. Bi wraps the MnS inclusions to form typical Bi-MnS two-phase inclusions. At the same time, Bi particles are distributed. That is, the composite inclusion control structure is achieved, with one MnS inclusion located in the central region and Bi located on the outer boundary of the MnS inclusion. The wrapping form is similar to that of Pb. That is, the composite inclusion control technology that replaces Pb is achieved, which significantly improves the cutting performance of steel bars and reflects environmental protection.

[0033] (2) The test billet obtained by the technical solution of the present invention has good surface quality without hot brittle cracks and supercooled cracks, and the steel bar has good surface quality without cracks. The surface finish after cutting is between 4.5 and 4.8 μm, which is excellent.

[0034] (3) The cutting efficiency of steel bars (bars) obtained by the technical solution of the present invention is 20% to 30% higher than that of the original bar cutting efficiency.

[0035] (4) The cutting performance test results of the bar obtained by the technical solution of the present invention are excellent, that is, the proportion of C-shaped chips is 84% ​​to 87%, which shows good cutting performance. Attached Figure Description

[0036] Figure 1 The image shows the morphology of the composite inclusions in the rods prepared in Example 6 of this invention (where (a) is a Bi-MnS composite inclusion, and (b) is a Bi-encapsulated (BiMn) inclusion). x S y );

[0037] Figure 2 The energy spectrum of the Bi-MnS composite inclusions in the rods prepared in Example 6 of this invention;

[0038] Figure 3 The (BiMn) coated in the rod prepared in Example 6 of this invention. x S y Energy spectrum of composite inclusions;

[0039] Figure 4 This is a diagram showing the cutting effect of the bar obtained in Embodiment 6 of the present invention. Detailed Implementation

[0040] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0041] The present invention provides a bismuth-sulfur compounded high-machinability bar stock in the specific embodiments section. Its chemical composition by weight percentage is as follows: C: 0.07%–0.12%, Si: 0.06%–0.10%, Mn: 0.90%–1.03%, P: 0.01%–0.10%, S: 0.15%–0.30%, Bi: 0.04%–0.17%, O: 0.0055%–0.0075%; Bi / S ratio is 0.14–1.13, Mn / S ratio is 3.00–6.86, and the balance is iron and unavoidable impurities.

[0042] The present invention provides a method for producing high-machinability bars with bismuth-sulfur composites in the specific embodiments section. The process flow of the method includes: converter smelting → LF furnace refining → continuous casting → slow cooling of the billet → heating in a heating furnace → rolling → slow cooling; wherein the free-machining element Bi is added in the form of bismuth-iron cored wire at the end of the LF furnace refining stage, specifically including the following steps:

[0043] (1) Converter smelting: The molten iron in the converter is not desulfurized, only decarbonized and phosphorusized. The oxygen content is strictly controlled. The oxygen content should not be too high before it is moved out. The oxygen content is controlled at 125-140 ppm to prepare for oxygen level control in the refining furnace.

[0044] (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 bismuth ferroalloy and ferrosulfur alloy is strictly controlled. The technical requirements of this invention are to add bismuth ferroalloy first, then ferrosulfur alloy, and finally bismuth ferroalloy. That is, after white slag treatment, feed 50~60m / 100 tons of bismuth ferro-core wire first, then immediately add low phosphorus manganese ferrosulfur and ferrosulfur. After melting evenly, feed 30~40m / 100 tons of bismuth ferro-core wire. Control the alloying refining time to 26~33min and control the oxygen content to 55~75ppm.

[0045] (3) Large billet continuous casting: Low carbon protective slag is used for protection during continuous casting. The casting speed is controlled at 0.6-1.0 m / min. The secondary cooling water is weakly cooled with a specific water volume of 1.0-1.4 L / kg. The continuous casting process combines crystallizer electromagnetic stirring and solidification end electromagnetic stirring with light reduction. The crystallizer electromagnetic stirring current is 300-340 A and the frequency is 4.0-4.8 Hz. The solidification end electromagnetic stirring current is 350-430 A and the frequency is 2.5-4.0 Hz. The total reduction at the end is 11.0-13.0 mm, and the maximum reduction per section is 2.5-4.5 mm. The continuous casting billet enters the slow cooling pit for slow cooling. The temperature in the slow cooling pit is 580-610℃, and the slow cooling time is 20-24 hours. It is then cooled to room temperature with the furnace.

[0046] (4) Surface cleaning of continuous casting billet: The surface of the continuous casting billet is cleaned by powder spraying or peeling to ensure that there are no defects such as microcracks on the surface of the billet.

[0047] (5) Heating of billet: The billet is heated in the furnace by cold charging. The cross-sectional dimensions of the billet are 180mm~280mm×180mm~380mm. Heating temperature and time: 1240~1260℃ for 3.0~4.0h;

[0048] (6) Rolling of billets and slow cooling after rolling: Rolling in the recrystallization zone is adopted, with an initial rolling temperature of 1050-1100℃ and a final rolling temperature of 880-920℃. The final finished bar size is φ30-φ80mm. Rolling in the recrystallization zone can increase the grain boundary density. Dense grain boundaries can hinder the elongation of sulfur-bismuth composite inclusions during rolling, further improving the morphology and distribution of inclusions and improving the machinability of the steel bar. The steel bar is cooled to 650-710℃ 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. After finishing, flaw detection, length setting, inspection, and warehousing, sulfur-bismuth composite high machinability bars are obtained.

[0049] Example

[0050] The chemical composition and dimensions of the steels in the embodiments and comparative examples of this invention are shown in Table 1. The relevant process parameters for converter smelting and LF refining in the corresponding embodiments and comparative examples are shown in Table 2. The cross-sectional dimensions of the cast billets are 180mm~280mm×180mm~380mm. The relevant process parameters for continuous casting in the corresponding embodiments and comparative examples are shown in Table 3. The rolling process parameters in the corresponding embodiments and comparative examples are shown in Table 4. The end cracking, surface scale, bubble defects, and chip types of the rolled billets in the corresponding embodiments and comparative examples are shown in Table 3. The morphology of the composite inclusions in the steel bar (bar stock) prepared in Example 6 under a scanning electron microscope is shown in Table 3. Figure 1 As shown (where (a) is a Bi-MnS composite inclusion and (b) is a Bi-encapsulated (BiMn) inclusion), x S yThe composite inclusions exhibit a diffuse spindle-shaped distribution, without elongated sulfides, and show an increased aspect ratio (i.e., a small amount of white inclusions are distributed at the boundary of the MnS inclusions, containing a large amount of pure Bi and a small amount of Fe; B encapsulates the MnS inclusions to form a typical Bi-MnS dual-phase inclusion, while Bi particles are also distributed, thus achieving the controllable structure of the composite inclusions, with one MnS inclusion located in the central region and Bi located at the outer boundary of the MnS inclusions, with an encapsulation form similar to Pb); the energy dispersive spectroscopy (EDS) results of the Bi-MnS composite inclusions in the rods prepared in Example 6 are as follows. Figure 2 As shown in the figure, the element that combines with the sulfide is Bi, forming a composite Bi-MnS inclusion; the rod prepared in Example 6 contains Bi-encapsulated (BiMn). x S y Energy dispersive spectroscopy (EDS) results of composite inclusions are as follows: Figure 3 As shown; the chipping effect of the bismuth-sulfur composite rod prepared in Example 6 is as follows. Figure 4 As shown, the cutting pattern is clearly C-shaped, accounting for 87%, indicating a significant improvement in the material's cutting performance.

[0051] Table 1 shows the composition and dimensions of the bismuth-sulfur composite rods described in Examples 1-20 and Comparative Example 1 of the present invention.

[0052] .

[0053] Table 2 Process parameters for converter smelting and LF refining

[0054] .

[0055] Table 3. Control process parameters for continuous casting process

[0056] .

[0057] Table 4 Rolling process parameters

[0058] .

[0059] Table 5. Statistics on end cracking of billets, surface scars and bubbles of steel bars, and chip types.

[0060] .

[0061] The present invention achieves the composite inclusion morphology (such as...) through the above technical solution. Figure 1As shown, a small amount of white inclusions are distributed on the boundary of MnS inclusions. These white inclusions contain a large amount of pure Bi and a small amount of Fe. Bi encapsulates the MnS inclusions, forming a typical Bi-MnS two-phase inclusion, while Bi particles are also distributed. This achieves composite inclusion control, where one MnS inclusion is located in the central region, and Bi is located at the outer boundary of the MnS inclusion, with an encapsulation pattern similar to Pb. This achieves composite inclusion control technology that replaces Pb, significantly improving the cutting performance of steel bars and demonstrating environmental friendliness. The experimental steel billet surface quality is good, with no hot brittle cracks or undercooling cracks. The steel bar surface quality is good, without cracks, and the surface finish after cutting is between 4.5 and 4.8 μm, which is excellent. The cutting efficiency of the steel bars obtained by this invention is 20% to 30% higher than that of the original bar cutting efficiency. The cutting performance test results are excellent. The proportion of C-type chips is 84% ​​to 87%, exhibiting good cutting performance.

[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 bismuth-sulfur compounded high-machinability bar stock, characterized in that, The chemical composition of the bismuth-sulfur compounded high-machinability bar stock, by weight percentage, is as follows: C: 0.07%–0.12%, Si: 0.06%–0.10%, Mn: 0.90%–1.03%, P: 0.01%–0.10%, S: 0.15%–0.30%, Bi: 0.04%–0.17%, O: 0.0055%–0.0075%; Bi / S ratio is 0.14–1.13, Mn / S ratio is 3.00–6.86, and the balance is iron and unavoidable impurities. The method for producing the bismuth-sulfur compounded high-machinability bar stock includes the following steps: (1) Converter smelting: The molten iron in the converter is not desulfurized, only decarburized and phosphorus-reduced. The oxygen content is strictly controlled. Before being moved out, the oxygen content is controlled at 125-140 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 molten steel surface, blow argon to ensure that the molten steel does not roll, strictly control the order of adding bismuth ferroalloy and sulfur ferroalloy, add bismuth ferroalloy first, then sulfur ferroalloy, and finally bismuth ferroalloy, and then alloy refine. (3) Large billet continuous casting: Low carbon protective slag is used for protection during the continuous casting process. The continuous casting speed is controlled at 0.6 to 1.0 m / min. The secondary cooling water is weakly cooled with a specific water volume of 1.0 to 1.4 L / kg. The continuous casting process is carried out by electromagnetic stirring in the crystallizer and electromagnetic stirring at the end of solidification under light pressure. (4) Surface cleaning of continuous casting billet: The surface of the continuous casting billet is cleaned to ensure that there are no micro-cracks or defects on the surface of the billet; (5) Heating of billet: The billet is heated in the furnace by cold charging. The cross-sectional dimensions of the billet are 180mm~280mm×180mm~380mm. It is heated at 1240~1260℃ for 3.0~4.0h. (6) Rolling of billet and slow cooling after rolling: Rolling in the recrystallization zone is adopted. The initial rolling temperature is 1050-1100℃ and the final rolling temperature is 880-920℃ to obtain finished bars. Rolling in the recrystallization zone can increase the grain boundary density. Dense grain boundaries can hinder the rolling and elongation of sulfur-bismuth composite inclusions, further improve the morphology and distribution of inclusions, and improve the cutting performance of steel bars. When the steel bars are cooled to 650-710℃ in the cooling bed, they enter the slow cooling pit. The time to cool to below 200℃ is controlled within 8-10 hours. Then, they are air-cooled to room temperature. After finishing, flaw detection, length setting, inspection, and warehousing, sulfur-bismuth composite high-cutting-performance bars are obtained.

2. The bismuth-sulfur composite high-machinability bar stock according to claim 1, characterized in that, In step (2), the specific order of alloy addition in the LF furnace refining process is as follows: after the white slag is treated, 50-60 m of bismuth-iron cored wire is fed first, followed by the immediate addition of low-phosphorus ferromanganese and ferrous sulfate. After the slag is melted evenly, 30-40 m of bismuth-iron cored wire is fed in. The alloying refining time is controlled at 26-33 min, and the oxygen content is controlled at 55-75 ppm.

3. The bismuth-sulfur composite high-machinability bar stock according to claim 1, characterized in that, The specific steps of the large billet continuous casting in step (3) are as follows: adjust the electromagnetic stirring current of the crystallizer to 300-340A and the frequency to 4.0-4.8Hz, the electromagnetic stirring current at the end of solidification to 350-430A and the frequency to 2.5-4.0Hz, the total reduction at the end of light pressing to 11.0-13.0mm, the maximum reduction of each section to 2.5-4.5mm, the continuous casting billet enters the slow cooling pit for slow cooling, the temperature of the slow cooling pit is 580-610℃, the slow cooling time is 20-24 hours, and it is cooled to room temperature with the furnace.

4. The bismuth-sulfur compounded high-machinability bar stock according to claim 1, characterized in that, The surface cleaning method described in step (4) is powder spraying for flaw detection or peeling.

5. The bismuth-sulfur composite high-machinability bar stock according to claim 1, characterized in that, Under cutting conditions with a feed rate f of 0.09 mm / r and a rotational speed of 800 r / min, the proportion of C-shaped chips is 84-87%, and the average surface roughness of the steel bar cutting surface is 4.5-4.8 μm.

6. The bismuth-sulfur composite high-machinability bar stock according to claim 1, characterized in that, The finished bar obtained in step (6) has a size of φ30~φ80mm.

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