A low-carbon, high-sulfur, bismuth-containing free-cutting steel and a method for manufacturing the same

Abstract

The invention discloses a low-carbon, high-sulfur, bismuth-containing free-cutting steel and a manufacturing method thereof. The free-cutting steel is prepared by a certain ratio of manganese, sulfur and bismuth and a certain content of oxygen. Beneficial inclusions are precipitated during solidification and dispersed in the steel. The inclusions mainly include a manganese sulfide soft core structure, a manganese sulfide oxygen composite inclusion hard core structure, and a manganese sulfide bismuth composite inclusion hard core structure. The inclusions are dispersed in the steel, and during the rolling process, the manganese sulfide soft core structure is prevented from being elongated and deformed, so that the morphology of the inclusions changes to a spindle shape or a spherical shape that is beneficial to cutting, thereby achieving the purpose of improving the cutting performance and surface quality of the material.

Description

Technical Field

[0001] The present disclosure relates to the technical field of free-cutting steel, and in particular to a low-carbon, high-sulfur, Bi-containing free-cutting steel for instruments and meters and a manufacturing method thereof. Background Art

[0002] With the rapid development of mechanical automation, precision and high speed in the advanced manufacturing industry, the shapes of parts and components of precision instrumentation industry are becoming more and more complex, and the requirements for machinability, surface quality and precision are becoming higher and higher. In order to adapt to high-speed processing while ensuring good surface finish, high-machining performance free-cutting steel must be used, which leads to an increasing demand for high-speed free-cutting steel.

[0003] At present, 12L14 is still one of the most widely used low-carbon leaded free-cutting steels for precision cutting and high finish. However, due to the lead vapor pollution during the smelting process and the harmful effects of lead in the scrap steel recycling smelting process, leaded free-cutting steels will gradually be eliminated. At present, there are two main design ideas for lead-free free-cutting steel: one is to maximize the role of S, by adding Ca, Mg or Ca, Mg complexes, controlling the morphology and composition of MnS and oxides, inhibiting tool wear and improving cutting performance, but this route has limitations. When the sulfur content in steel exceeds 0.35%, the hot brittleness problem of smelting, casting and subsequent deformation is prominent. At the same time, after the sulfur content exceeds 0.4%, continuing to increase the sulfur content does not contribute much to improving the cutting performance of steel; the second is to use low-melting-point elements with similar physical properties to Pb to replace the lead element in lead-containing free-cutting steel. A large number of studies have shown that Bi is the most promising metal element to replace lead in free-cutting steel. Bi is non-toxic and has similar physical properties to lead. It can significantly improve the cutting performance of free-cutting steel like lead, and even outperforms lead free-cutting steel in some aspects. Bi is lighter than Pb, closer to Fe, more evenly distributed, and less likely to segregate. Therefore, using Bi to replace lead to produce environmentally friendly free-cutting steel is the direction of research and development of new free-cutting steel.

[0004] Bi is very effective in improving the easy-cutting performance of steel. It is non-toxic and has similar physical and chemical properties to lead. In recent years, it has been used in the composition design of super-easy-cutting steel development and production, with obvious effects. Its main function is to block the elongation and deformation of manganese sulfide inclusions in steel during rolling by being wrapped as a nucleation base point, and promote the development of manganese sulfide inclusions into spherical or spindle shapes, thereby giving the steel good cutting performance. The addition of Bi can also form dispersed MnS-Bi composite inclusions in steel, which play a good lubricating role in the steel cutting process, improving the cutting performance and effectively protecting the tool, extending the tool life.

[0005] At present, the key to improving the cutting performance of free-cutting steel is to control the sulfide morphology and distribution. The Chinese invention patent application CN102330039A discloses a low-carbon bismuth-containing environmentally friendly free-cutting steel, which 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%, and the rest is Fe and impurities. The mass percentage ratio of Mn to S is Mn / S=1.79-6.52, the mass percentage of Bi to S is Bi / S=0.12-0.87, and T[O]: 0.0010-0.0250%. Bismuth-containing environmentally friendly free-cutting steel has low cost, good environmental protection and cutting performance that is not inferior to the corresponding carbon content of lead-containing free-cutting steel structure. However, this low-carbon bismuth-containing environmentally friendly free-cutting structural steel has obvious technical deficiencies, namely, first, T[O] is too high. Practice shows that when T[O] ≥ 150ppm, it not only seriously reduces the service life and increases the cost of steelmaking refractory materials, but also increases the number of unfavorable large-particle oxide inclusions in the steel; more fatally, such high oxygen content will cause serious defects such as "subcutaneous bubbles and surface pinholes" in the ingot, affecting the yield and quality; second, it also contains one or more elements among Se, Te, Zr, Ca, Mg, Zn, and Sb that can improve cutting performance, which will greatly increase the alloy cost and is not conducive to mass promotion. The Chinese invention patent application CN103255359A discloses "a bismuth-containing free-cutting steel", which is calculated by mass percentage as follows: C: 0.04% to 0.15%; Si: 0% to 0.15%; Mn: 0.9% to 1.6%; P: 0.02% to 0.11%; S: 0.25% to 0.45%; Cr: 0 to 0.2%; Ni: 0 to 0.2%; Cu: 0 to 0.2%; Bi: 0.05% to 0.2%; Ti: 0.005% to 0.05%; N: 0.005% to 0.05%; impurities: 0 to 0.2%; the remainder is Fe. The bismuth-containing free-cutting steel of the invention has excellent comprehensive mechanical properties and excellent cutting performance, but more than 10 alloy elements are added, which greatly increases the alloy cost while obtaining comprehensive mechanical properties and excellent cutting performance, which is not conducive to mass promotion.The Chinese invention patent application CN103911550A "An environmentally friendly low-carbon, high-sulfur, bismuth free-cutting steel with excellent thermoplasticity" contains, calculated by mass 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%, on this basis, the steel also contains B0.001-0.015% and Ti0.005-0.15%, and also contains one or more elements of Se, Te, Sn, Zr, Ca, Cr, Cu, Ni, Mg, Zn, Sb and rare earth: the weight percentage of the above components is 0.001-0.50%, and the balance is Fe and other inevitable trace elements. This invention has the characteristic of excellent thermoplasticity, but this technology cannot obtain the thermoplasticity of low-carbon high-sulfur free-cutting steel by combining only the four elements of bismuth, sulfur, carbon and oxygen. Instead, it needs to add B and Ti to achieve the purpose of improving plasticity, and needs to add rare earth elements for comprehensive matching. This makes it impossible to control the rare earth yield in the process, and the process compatibility is poor, and batch continuous casting production cannot be carried out. Summary of the invention

[0006] In view of this, the present invention provides a low-carbon, high-sulfur bismuth-containing free-cutting steel to solve the problems of high production cost, easy surface defects and limited mass production caused by adding a large amount of alloy elements to the existing bismuth-containing free-cutting steel to meet the cutting performance.

[0007] In the first aspect, the low-carbon, high-sulfur, bismuth-containing free-cutting steel has the following chemical composition by weight percentage:

[0008] C: 0.01%~0.10%, Si: 0.01%~0.10%, Mn: 0.50%~1.20%, P: 0.01%~0.10%, S: 0.20%~0.35%, Bi: 0.01%~0.20%, O: 0.0040%~0.0150%; Bi / S is 0.03~1, Mn / S is 1.43~6.0, and the balance is iron and unavoidable impurities.

[0009] In a second aspect, the method for preparing the low-carbon, high-sulfur, bismuth-containing free-cutting steel described in the first aspect comprises converter smelting, LF furnace refining, continuous casting, slow cooling of the ingot, heating of the ingot, rolling of the ingot and slow cooling after rolling, characterized in that:

[0010] In the present disclosure and possible embodiments, in the LF furnace refining process, the molten steel is subjected to slag surface deoxidation, and argon is blown into the molten steel during the deoxidation process to prevent the molten steel from rolling; after the white slag treatment, the first stage of oxygen control is entered, ferrosilicon is added, and when the oxygen content is controlled at 80-100 ppm, low-phosphorus ferromanganese and ferrosulphur are added to the molten steel, and then the second stage of oxygen control is entered, and when the oxygen content is controlled at 40-80 ppm, bismuth-iron cored wire and / or bismuth-iron alloy block are added to the molten steel; the alloying refining time is controlled to be 35-40 min, and the white slag refining time is maintained at 14-18 min.

[0011] In the converter smelting process, the converter molten iron is not subjected to desulfurization treatment, but only subjected to decarburization and phosphorus treatment; deoxidation treatment is carried out when 1 / 5 to 1 / 4 of the steel is tapped, and the oxygen content is controlled at 80 to 150 ppm.

[0012] In the present disclosure and possible embodiments, in the continuous casting process, low-carbon protective slag is used for protective casting, the tundish temperature is controlled at 1530-1545°C, and the continuous casting speed is controlled at 1.2-2.0 m / min.

[0013] In the present disclosure and possible embodiments, in the slow cooling process of the cast slab, the secondary cooling water adopts weak cooling, and the continuous casting slabs are slowly cooled to room temperature after being stacked off the production line.

[0014] In the present disclosure and possible embodiments, the surface of the continuous casting billet is cleaned by powder spraying or peeling.

[0015] In the present disclosure and possible embodiments, in the ingot heating process, the ingot is cold loaded, the cross-sectional size of the ingot is 120mm×120mm~180mm×180mm, the heating temperature is 1200℃~1300℃, and the heating time is 2.0~3.0h.

[0016] In the present disclosure and possible embodiments, in the casting billet rolling process, recrystallization zone rolling is adopted, the starting rolling temperature is 1030-1080°C, and the finishing rolling temperature is 860-900°C.

[0017] In the present disclosure and possible embodiments, in the post-rolling slow cooling process, the temperature is cooled to 430-500° C. at an average rate of 0.1° C. / s to 1° C. / s.

[0018] The present invention has the following beneficial effects:

[0019] The low-carbon, high-sulfur, bismuth-containing free-cutting steel and the preparation method thereof of the present invention, on the one hand, by organically combining five elements of carbon, sulfur, manganese, bismuth and oxygen, it is not necessary to add additional precious metal elements and thermoplasticity-enhancing elements, and the obtained free-cutting steel has significantly improved sulfide morphology and distribution compared with 1215, and the cutting performance and surface quality of the free-cutting steel are significantly improved. The tensile performance shows that the strength is between 450-550MPa, the cross-sectional shrinkage is between 45%-55%, and the strength and toughness are well matched; on the second hand, the manganese-sulfur ratio and the bismuth-sulfur ratio in the present invention are organically combined with the oxygen content to effectively improve the sulfide morphology and distribution, wherein the total oxygen content does not exceed 150ppm, and the free oxygen content is preferably not higher than 100ppm. When the manganese-sulfur ratio and the bismuth-sulfur ratio are combined, the obtained spindle-shaped or spherical manganese sulfide soft core structure + manganese sulfide oxygen composite inclusion hard core structure + manganese bismuth sulfide composite inclusion hard core structure has a composite inclusion content of 30%-45%, which is mainly distributed at the grain boundaries; especially when the free oxygen content is controlled at 40-80ppm, the composite inclusion content of 30%-45% is obtained. The obtained composite inclusion content of spindle-shaped or spherical manganese sulfide soft core structure + manganese sulfide oxygen composite inclusion hard core structure + manganese bismuth composite inclusion hard core structure is 45%-60%, and the composite inclusion content is mainly distributed at the grain boundary and in the grain, so as to achieve the best comprehensive performance of the free-cutting steel of the present invention, realize the purpose of improving the cutting performance and surface quality of the material, and reduce the cutting resistance and cutting heat, which has the effect of protecting the tool; thirdly, the free-cutting steel of the present invention has less alloy elements in the chemical composition, so as to effectively reduce the production cost, has good environmental protection, and is easy to control the manufacturing process, meeting the conditions for mass production; finally, the cutting performance index of the free-cutting steel is f=0.09mm / r, and under the cutting condition of a rotation speed of 800r / min, the proportion of C-type chips is ≥85%, and the average surface roughness of the workpiece is ≤4.5μm; effectively solving the problems of high production cost, easy surface defects and limited mass production caused by adding a large amount of alloy elements to meet the cutting performance of the existing bismuth-containing free-cutting steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above and other objects, features and advantages of the present disclosure will become more apparent through the following description of the embodiments of the present disclosure with reference to the accompanying drawings, in which:

[0021] Figure 1 is a morphology diagram of non-metallic inclusions in free-cutting steel according to Example 1 of the present disclosure;

[0022] As shown in the figure: it is mainly a manganese sulfide bismuth composite inclusion structure, and the manganese sulfide soft tissue wraps the pen element hard core to form a spherical shape, which is beneficial to improving the cutting performance and surface finish. DETAILED DESCRIPTION

[0023] The present disclosure is described below based on embodiments, but it is worth noting that the present disclosure is not limited to these embodiments. In the detailed description of the present disclosure below, some specific details are described in detail. However, for the parts that are not described in detail, those skilled in the art can also fully understand the present disclosure.

[0024] At the same time, unless the context clearly requires otherwise, the words "include", "comprises" and similar words throughout the specification and claims should be interpreted as inclusive rather than exclusive or exhaustive; that is, the meaning is "including but not limited to".

[0025] The following embodiments of the present invention adopt the following composition design and steelmaking and rolling process design to manufacture the low-carbon, high-sulfur, bismuth-containing free-cutting steel of the present invention, as follows:

[0026] 1. Ingredient Design

[0027] The low-carbon, high-sulfur, bismuth-containing free-cutting steel of the following embodiment of the present invention has the following chemical composition by weight percentage:

[0028] C: 0.01% to 0.10%, Si: 0.01% to 0.10%, Mn: 0.50% to 1.20%, P: 0.01% to 0.10%, S: 0.20% to 0.35%, Bi: 0.01% to 0.20%, O: 0.0040% to 0.0150%; Bi / S ratio of bismuth to sulfur is 0.03 to 1, Mn / S ratio of manganese to sulfur is 1.43 to 6.0, and the remainder is iron and unavoidable impurities. Among them: the functions of each chemical component are:

[0029] C: Carbon is the most basic matrix strengthening element of steel materials. The addition of carbon in the present invention is to ensure that the matrix has a certain strength and to cooperate with Bi to obtain good cutting performance. When the carbon content is lower than 0.01%, the strength index of the wire rod cannot be guaranteed; when the carbon content is higher than 0.10%, it cannot form a good combination with Bi, and the increase in surface hardness significantly reduces the cutting performance. Therefore, the carbon content of the low-carbon, high-sulfur, Bi-containing free-cutting steel of the present invention is controlled within the range of 0.01% to 0.10%.

[0030] Si: Silicon is a deoxidizer in the present invention, and performs deoxidation together with Mn. When the silicon content is lower than 0.01%, the deoxidation effect is poor; when the silicon content is higher than 0.10%, the free-cutting performance is significantly reduced. Therefore, the silicon content of the low-carbon, high-sulfur, Bi-containing free-cutting steel of the present invention is controlled within the range of 0.01% to 0.10%.

[0031] Mn: It is an important component element of MnS and MnS-Bi composite inclusions in the present invention, which is beneficial to improving the free-cutting performance. When the manganese content is lower than 0.50%, 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 small, which reduces the cutting performance; when the manganese content is higher than 1.20%, the surface hardness of the steel is high, cutting is difficult, and the service life of the turning tool is reduced. Therefore, the manganese content of the low-carbon, high-sulfur, Bi-containing free-cutting steel of the present invention is controlled within the range of 0.50% to 1.20%.

[0032] P: Appropriate phosphorus will improve the cutting performance, but above 0.10% will cause "cold brittleness" and reduce the cutting performance. Therefore, the phosphorus content of the low-carbon high-sulfur Bi-containing free-cutting steel of the present invention is controlled at 0.01% to 0.10%.

[0033] S: Sulfur is one of the most beneficial elements for free cutting. When added in appropriate amount, it forms manganese sulfide compounds with manganese and forms composite inclusions with manganese and bismuth, which can significantly improve the free cutting performance. When the sulfur content is lower than 0.20%, it cannot form a composite compound with manganese and bismuth that is conducive to cutting performance; when the sulfur content is higher than 0.35%, the hot workability of the steel is reduced. Therefore, the sulfur content of the low-carbon, high-sulfur, Bi-containing free cutting steel of the present invention is controlled within the range of 0.20% to 0.35%.

[0034] Bi: Bismuth is an effective free-cutting element and is also considered to be the most suitable free-cutting element to replace lead. It can form manganese-bismuth sulfide composite inclusions with manganese and sulfur, change the morphology and distribution of inclusions, and can significantly improve the cutting performance, surface quality and chip morphology of free-cutting steel when combined with sulfur. When the selenium content is lower than 0.01%, the above effect cannot be achieved; when the selenium content exceeds 0.20%, the free-cutting performance can no longer be improved and the smelting cost increases. Therefore, the selenium content of the low-carbon, high-sulfur Bi-containing free-cutting steel of the present invention is controlled within the range of 0.01% to 0.20%, and the bismuth-sulfur ratio Bi / S is 0.03 to 1.

[0035] O: In the present invention, oxygen provides the thermodynamic conditions for the dispersed distribution of sulfides. When the oxygen content is lower than 0.0040%, the oxide material is less precipitated, which cannot fully allow the manganese sulfide to wrap and preferentially nucleate. The inclusions are distributed in multi-directional elongated strips, and the aspect ratio increases. When the oxygen content is higher than 0.0150%, there are more oxides, which easily cause internal defects in the material. Therefore, the oxygen content of the bismuth-sulfur ratio Bi / S of the present invention is controlled within the range of 0.0040% to 0.0150%.

[0036] In the present invention, a certain ratio of manganese, bismuth and sulfur is used to form non-metallic inclusions, such as manganese sulfide and manganese bismuth sulfide composites, and a certain oxygen content is used to achieve nucleation and precipitation during refining and dispersed distribution in the steel, so that manganese sulfide is nucleated on oxide and bismuth particles, and will not be elongated and deformed during the rolling process after being wrapped, so that the inclusion morphology is basically spindle-shaped, thereby improving the cutting performance and surface quality of the material, and at the same time, it is easy to break chips during the cutting process to protect the tool.

[0037] 2. Steelmaking and rolling process

[0038] The following embodiments of the present disclosure specifically adopt the following process flow: converter smelting → LF furnace refining → continuous casting → slow cooling of ingot → heating of ingot → rolling of ingot → slow cooling after rolling; wherein the free-cutting element Bi is added in the form of bismuth-iron cored wire at the end of LF furnace refining, specifically comprising the following steps:

[0039] 1) Converter smelting: The converter molten iron is not desulfurized, only carbonized and phosphorus removed; deoxidation is carried out when 1 / 5 to 1 / 4 of the steel is tapped, and the oxygen content is controlled at 80 to 150 ppm. The purpose is to ensure that the oxygen content is stably controlled within the required range during LF refining.

[0040] 2) LF furnace refining: first add refining slag and submerged arc slag, then add ferrosilicon and silicon carbide to deoxidize the slag surface of the molten steel. During the deoxidation process, weak argon is blown into the molten steel to prevent the molten steel from rolling. After the white slag is treated, it enters the first stage of oxygen control, adding ferrosilicon. When the oxygen content is controlled at 80-100ppm, low-phosphorus ferromanganese and ferrosulphur are added to the molten steel to ensure uniform composition, in order to form a part of oxidized material points for sulfide nucleation; then enter the second stage of oxygen control, when the oxygen content is controlled at 40-80ppm, Bismuth-iron cored wire and / or bismuth-iron alloy block are added to the molten steel, and the sulfide is modified by the bismuth element. On the one hand, the bismuth element is adsorbed on the surface of the sulfide inclusions, so that the inclusion shape changes to spherical or spindle-shaped, thereby improving the cutting performance; on the other hand, the bismuth element can directly serve as the nucleation point of the sulfide, so that the long strip of sulfide directly changes into a spherical or spindle-shaped shape, thereby increasing the proportion of composite inclusions of spindle-shaped or spherical manganese sulfide soft core structure + manganese sulfide oxygen composite inclusion hard core structure + manganese sulfide bismuth composite inclusion hard core structure. In this process, the alloying refining time is controlled to be 35 to 40 minutes, and the white slag refining time is maintained at 14 to 18 minutes. The above process can make the bismuth recovery rate reach 50% to 60%, and the proportion of spherical or spindle-shaped composite inclusions reaches 45 to 60%.

[0041] 3) Continuous casting: Low-carbon protective slag is used for protective casting during the continuous casting process, the tundish temperature is controlled at 1530-1545°C, and the continuous casting speed is controlled at 1.2-2.0 m / min.

[0042] 4) Slow cooling of ingots: The secondary cooling water adopts weak cooling, and the continuous casting ingots are slowly cooled to room temperature after being stacked off the production line.

[0043] Continuous casting billet surface cleaning: Use powder spraying or peeling processing to ensure the surface quality of the billet.

[0044] 5) Ingot heating: The ingot is cold loaded, the cross-sectional size of the ingot is 120mm×120mm~180mm×180mm, the heating temperature and time are 1200℃~1300℃ for 2.0~3.0h.

[0045] 6) Billet rolling: Recrystallization zone rolling is adopted, the starting rolling temperature is 1030~1080℃, and the final rolling temperature is 860~900℃.

[0046] 7) Slow cooling after rolling: After rolling, slowly cool to 430-500°C at an average cooling rate of 0.1°C / s to 1°C / s.

[0047] Example

[0048] 1. The composition design and steelmaking and rolling process design of Examples 1-20 and Comparative Examples of the present disclosure are shown in Table 1 and Table 2:

[0049] Table 1 Composition (wt%) of the steel of the present invention and the comparative example

[0050]

[0051]

[0052] Table 2 Manufacturing process parameters of the steel of the present invention and the comparative example steel

[0053]

[0054] It can be seen from Table 1 and Table 2 that the composition design and process design of Examples 1-20 of the present invention are well matched, which can ensure the smooth steel passing of the rolling stock between the stands during hot rolling, thereby obtaining good continuity and stability of the hot rolling process.

[0055] 2. Evaluation of surface quality and cutting performance of the steels of the embodiments and comparative examples of the present disclosure:

[0056] The present disclosure evaluates the surface quality of the rolled materials of the embodiments and comparative examples, and statistically analyzes whether the ends of the continuous casting billets of each embodiment have cracks, surface scars, bubble defects and chips during hot rolling. The results are shown in Table 3.

[0057] In terms of cutting performance evaluation, INDEX G200 CNC lathe was used to conduct cutting tests on each embodiment and comparative example, with feed rate 0.05mm / s and cutting depth 1mm kept unchanged, and cutting rate changed to 180r / min, 560r / min, 1120r / min and 1600r / min respectively. The size of the test sample was Φ8mm×100mm, the test tool was an uncoated carbide blade, and the cutting was carried out without using cutting fluid. The surface finish of the samples at different cutting speeds was measured, thereby obtaining the surface quality of the material; at the same time, the types of chips were counted, and the cutting performance was further compared. The results are shown in Table 3.

[0058] Table 3: Cracks on the end of the rolled billet, scarring and bubble defects on the surface of the wire rod, and chip types in the embodiments and comparative examples

[0059]

[0060]

[0061] As shown in Table 3, the continuous casting billets and wire rods corresponding to the embodiments have no end cracking, scarring or bubble defects. Compared with high-sulfur 1215 free-cutting steel, it has obvious surface quality advantages and better surface finish. The cutting performance index is f = 0.09mm / r, speed 800r / min cutting conditions, the proportion of C-type chips ≥ 85%, and the average surface roughness of the workpiece ≤ 4.5μm.

[0062] The above-described embodiments are only embodiments of the present disclosure, and the descriptions thereof are relatively specific and detailed, but they cannot be construed as limiting the scope of the present disclosure. It should be noted that, for a person of ordinary skill in the art, without departing from the concept of the present disclosure, several variations, equivalent substitutions, improvements, etc. may be made, all of which fall within the scope of protection of the present disclosure. Therefore, the scope of protection of the present disclosure shall be subject to the attached claims.

Claims

1. A low-carbon, high-sulfur, bismuth-containing free-cutting steel, characterized in that: Its chemical composition by weight percentage is: C: 0.09% to 0.10%, Si: 0.06% to 0.10%, Mn: 1.03% to 1.14%, P: 0.01% to 0.03%, S: 0.26% to 0.35%, Bi: 0.10% to 0.15%, O: 0.0040% to 0.0150%; Bi / S is 0.29 to 0.44, Mn / S is 3.04 to 3.96, and the balance is iron and unavoidable impurities; The method for preparing the low-carbon, high-sulfur, bismuth-containing free-cutting steel comprises a converter smelting process, an LF furnace refining process, a continuous casting process, a billet slow cooling process, a billet heating process, a billet rolling process and a post-rolling slow cooling process; In the LF furnace refining process, the molten steel is subjected to slag surface deoxidation, and argon is blown into the molten steel during the deoxidation process to prevent the molten steel from rolling; after the white slag treatment, the first stage of oxygen control is entered, ferrosilicon is added, and when the oxygen content is controlled at 80-100 ppm, low-phosphorus ferromanganese and ferrosulphur are added to the molten steel, and then the second stage of oxygen control is entered, and when the oxygen content is controlled at 40-80 ppm, bismuth-iron cored wire and / or bismuth-iron alloy block are added to the molten steel; the alloying refining time is controlled to be 35-40 minutes, and the white slag refining time is maintained to be 14-18 minutes.

2. The method for preparing the low-carbon, high-sulfur, bismuth-containing free-cutting steel according to claim 1, characterized in that: In the converter smelting process, the converter molten iron is not subjected to desulfurization treatment, but only subjected to decarburization and phosphorus treatment; deoxidation treatment is carried out when 1 / 5 to 1 / 4 of the steel is tapped, and the oxygen content is controlled at 80 to 150 ppm.

3. The preparation method according to claim 2, characterized in that: In the continuous casting process, low-carbon protective slag is used for protective casting, the tundish temperature is controlled at 1530-1545° C., and the continuous casting speed is controlled at 1.9-2.0 m / min.

4. The preparation method according to claim 3, characterized in that: In the ingot slow cooling process, the secondary cooling water adopts weak cooling, and the ingots are slowly cooled to room temperature after being stacked off the production line.

5. The preparation method according to claim 4, characterized in that: The surface of the continuous casting billet is cleaned by powder spraying or peeling.

6. The preparation method according to claim 5, characterized in that: In the casting billet heating process, the casting billet is cold loaded, the cross-sectional size of the casting billet is 120mm×120mm~180mm×180mm, the heating temperature is 1200℃~1300℃, and the heating time is 2.0~3.0h.

7. The preparation method according to claim 6, characterized in that: In the casting slab rolling process, recrystallization zone rolling is adopted, the starting rolling temperature is 1067-1080°C, and the final rolling temperature is 897-900°C.

8. The preparation method according to claim 7, characterized in that: In the post-rolling slow cooling process, the steel is cooled to 481-491° C. at an average rate of 0.1° C. / s-1° C. / s.

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