A method for forming oxygen-sulfur composite inclusions in free-cutting steel and free-cutting steel

Abstract

This invention discloses a method for forming oxygen-sulfur composite inclusions in free-cutting steel and the free-cutting steel itself, including an electric furnace or converter process, an LF refining process, and an RH refining process. This invention achieves this by strictly controlling the temperature of the molten steel and the contents of Al, O, S, Ca, and Mg during the refining process. During the RH refining process, the temperature drop of the molten steel is utilized to form a large number of stable, small-sized, specific-composition Ca-Mg-Al-O oxides. These oxides act as nucleation sites for MnS inclusions during solidification. The resulting calcium-sulfur composite free-cutting steel contains a large number of uniformly distributed oxygen-sulfur composite inclusions, significantly reducing the formation of large-sized pure MnS. Most of the sulfides retain their spherical or spindle-shaped shape after rolling, without significant aggregation, thus improving the mechanical properties of the steel.

Description

Technical Field

[0001] This invention relates to the field of steel smelting, and in particular to a method for forming oxygen-sulfur composite inclusions in free-cutting steel and the free-cutting steel itself. Background Technology

[0002] Adding a certain amount of sulfur (S) to steel can improve its machinability. However, in actual production, due to the solidification segregation of Mn and S, large-sized pure MnS inclusions are often formed. After rolling, these inclusions appear as long, thin strips, leading to anisotropy in the mechanical properties of the steel. Medium-carbon sulfur-containing steel needs to possess both excellent machinability and good, uniform mechanical properties. Higher S content and the use of Al deoxidation make it easier to form large, long, thin MnS inclusions in actual production, severely compromising the mechanical properties of the steel.

[0003] To reduce the formation of large-sized pure MnS in medium-carbon sulfur-containing steel, some researchers have attempted high-temperature heat treatment and increasing the cooling rate during solidification. However, these methods have limited control effects and cannot solve the problem of large MnS deformation after rolling. Some researchers have added Ca, Mg, Zr, Ti, or rare earth elements to the steel to modify pure MnS into composite sulfides (Mn,X)S with higher hardness and less deformation, thereby reducing the formation of elongated MnS. However, adding Mg, Zr, Ti, or rare earth elements can significantly reduce the cleanliness of the molten steel. Adding Ca is an efficient and low-cost control method, but it can easily lead to poor castability of the molten steel, making continuous casting difficult. In addition, using oxides in the steel to refine MnS is an important method. By using oxides as nucleation sites for MnS, a large number of uniformly distributed oxygen-sulfur composite inclusions can be obtained in the steel, which can improve the morphology and distribution of MnS. However, the key is to form a large number of fine oxides with suitable composition and uniform distribution in the steel, which is very difficult in actual smelting.

[0004] Patent CN104212942B discloses a process for improving the morphology of sulfides in sulfur-containing steel. It proposes to promote the formation of oxygen-sulfur composite inclusions by controlling the timing and amount of Ca addition, performing a Ca feed operation before and after adjusting the S content to the target composition of 0.02-0.08%. However, adding Ca again after increasing the S content to 0.02-0.08% easily generates a large amount of CaS inclusions, reducing the castability of the molten steel and affecting continuous casting production. Patent CN113278762B discloses a method for Ca alloying in high-alumina calcium-sulfur composite free-machining steel, attempting to obtain more oxygen-sulfur composite inclusions in the steel. However, this process involves two Ca addition operations after RH vacuum treatment, which easily generates many CaO-Al2O3 inclusions with high CaO content and large size. These inclusions are difficult to use as effective nucleation sites for MnS and are easily retained in the steel, harming the mechanical properties of the finished steel. The above technologies cannot guarantee the formation of a large number of uniformly distributed oxygen-sulfur composite inclusions in the steel.

[0005] Therefore, there is an urgent need to provide a method for forming oxygen-sulfur composite inclusions in free-cutting steel. Summary of the Invention

[0006] To address the above problems, the present invention provides a method for forming oxygen-sulfur composite inclusions in free-cutting steel and a free-cutting steel thereof, which can achieve the formation of a large number of uniformly distributed oxygen-sulfur composite inclusions in sulfur-containing free-cutting steel, thereby improving the morphology and distribution of sulfides in the steel.

[0007] According to a first aspect of the present invention, a method for forming oxygen-sulfur composite inclusions in free-cutting steel using the RH process is provided, comprising:

[0008] S1: Electric furnace or converter process: Molten iron and scrap steel raw materials are added to the electric furnace or converter to melt and decarburize the raw materials. When tapping the steel, the C content of the molten steel is not less than 0.08% by mass. Aluminum blocks, ferrosilicon, ferromanganese and slag are added during the tapping process.

[0009] S2: LF refining process: The composition and mass percentage of the slag are CaO: 45-55%, SiO2: 10%-20%, Al2O3: 15-25%, MgO: 3%-8%, with the remainder being impurities. After the LF refining slag is formed, the composition of the molten steel is tested. Based on the test results, Al is added, and the Al content in the molten steel is 0.02-0.03% by mass. Low-pressure argon gas is used to float and remove inclusions, and the molten steel is desulfurized to reduce the mass percentage of S to no more than 0.005%. The temperature of the molten steel is gradually increased in stages.

[0010] S3: RH refining process: The molten steel is placed in a vacuum chamber for vacuum treatment, and then FeS is added to the vacuum chamber for repressurization. During the vacuum treatment, the temperature of the molten steel is reduced by 70-100°C, and uniformly distributed oxygen-sulfur composite inclusions are formed in the molten steel. The molten steel is then continuously cast.

[0011] In the above scheme, the tapping temperature in step S1 is 1580~1630℃.

[0012] In the above scheme, before RH refining, the composition and mass percentage of the molten steel are: C: 0.3-0.5%, Si: 0.15%-0.35%, Mn: 1.35-1.65%, Al: 0.007-0.012%, O: 0.001%-0.002%, S: ≤0.005%, Ca: 0.0005-0.0015%, Mg: 0.0001%-0.0005%, with the remainder being Fe and impurities.

[0013] In the above scheme, at the end of RH refining, the composition and mass percentage of the molten steel are: C: 0.3-0.5%, Si: 0.15%-0.35%, Mn: 1.35-1.65%, Al: 0.007-0.012%, O: 0.001%-0.002%, S: 0.02%-0.04%, Ca: 0.0005-0.0015%, Mg: 0.0001%-0.0005%, with the remainder being Fe and impurities.

[0014] In the above scheme, the temperature of the molten steel before RH refining is controlled at 1640~1660℃.

[0015] In the above scheme, the temperature of the molten steel after RH vacuum treatment is controlled at 1550~1570℃.

[0016] In the above scheme, in step S3, the oxygen-sulfur composite inclusion includes a core oxide and peripheral sulfides. The core oxide has the following composition and mass percentage: CaO: 5-30%, Al2O3: 60-90%, MgO: 5-20%. The peripheral sulfides have the following composition and mass percentage: CaS: 2-40%, MnS: 60-98%.

[0017] In the above scheme, the size of the core oxide is 0.5–3 μm, the size of the composite inclusion is 2–10 μm, and the number density of the oxygen-sulfur composite inclusion is ≥60 inclusions / mm². 2 .

[0018] In the above scheme, the vacuum treatment time in step S3 is 30 to 50 minutes.

[0019] In the above scheme, in step S3, FeS is added to the vacuum chamber, and the pressure is repressurized after 5 minutes.

[0020] According to a second aspect of the present invention, a free-cutting steel is provided, wherein it is manufactured using a method for forming oxygen-sulfur composite inclusions in the free-cutting steel as described in any of the preceding aspects.

[0021] The beneficial effects of this invention are:

[0022] This invention discloses a method for forming oxygen-sulfur composite inclusions in free-cutting steel and the free-cutting steel itself. By strictly controlling the temperature of the molten steel and the contents of Al, O, S, Ca, and Mg during the refining process, a large number of stable, small-sized Ca-Mg-Al-O oxides with specific compositions are formed during the RH refining process by utilizing the temperature drop of the molten steel. These oxides act as nucleation sites for MnS inclusions during solidification. The resulting calcium-sulfur composite free-cutting steel contains a large number of uniformly distributed oxygen-sulfur composite inclusions, significantly reducing the formation of large-sized pure MnS. After rolling, most of the sulfides can still maintain a spherical or spindle shape without obvious aggregation, thus improving the mechanical properties of the steel. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0024] Figure 1 A flowchart of a method for forming oxygen-sulfur composite inclusions in free-cutting steel provided by the present invention;

[0025] Figure 2 This is a schematic diagram of the oxides obtained during the temperature drop process of molten steel in Embodiment 1 of the present invention;

[0026] Figure 3 This is a schematic diagram of the oxygen-sulfur composite inclusions in the rolled material prepared in Example 1 of the present invention;

[0027] Figure 4 This is a schematic diagram showing the mass fractions of the three components CaO, Al2O3, and MgO in Examples 1-3 of the present invention;

[0028] Figure 5 This is a schematic diagram showing the mass fractions of the three components Mn, S, and Ca in Examples 1-3 of the present invention;

[0029] Figure 6 This refers to the overall distribution of sulfides in the rolled material prepared in Example 1 of the present invention;

[0030] Figure 7 This is a schematic diagram of the sulfides in the rolled material prepared in Comparative Example 1 of the present invention;

[0031] Figure 8 This is a schematic diagram of the sulfides in the rolled material prepared in Comparative Example 2 of the present invention;

[0032] Figure 9 This is a schematic diagram of the sulfides in the rolled material prepared in Comparative Example 3 of the present invention.

[0033] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0034] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numerals in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this disclosure. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this disclosure as detailed in the appended claims.

[0035] The terms "first," "second," etc., used in this disclosure are for distinguishing similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such use of data can be interchanged where appropriate so that the embodiments of this disclosure described herein can be implemented, for example, in orders other than those illustrated or described herein.

[0036] Furthermore, the terms “comprising” and “having”, and any variations thereof, are intended to cover non-exclusive inclusion, such that a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units that are explicitly listed, but may include other steps or units that are not explicitly listed or that are inherent to such process, method, product, or apparatus.

[0037] Multiple, including two or more.

[0038] And / or, it should be understood that, for the purposes of this disclosure, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone.

[0039] like Figure 1 As shown, one embodiment of the technical solution of the present invention provides a method for forming oxygen-sulfur composite inclusions in free-cutting steel, comprising:

[0040] S1: Electric furnace or converter process: Molten iron and scrap steel raw materials are added to the electric furnace or converter to melt and decarburize the raw materials. When tapping the steel, the C content of the molten steel is not less than 0.08% by mass. Aluminum blocks, ferrosilicon, ferromanganese and slag are added during the tapping process.

[0041] S2: LF refining process: The composition and mass percentage of the slag are CaO: 45-55%, SiO2: 10%-20%, Al2O3: 15-25%, MgO: 3%-8%, and the remainder is impurities. After the LF refining slag is formed, the composition of the molten steel is tested. Based on the test results, Al is added, and the Al content in the molten steel is 0.02-0.03% by mass. Low-pressure argon gas is used to float and remove inclusions, and the molten steel is desulfurized to reduce the mass percentage of S to no more than 0.005%. The temperature of the molten steel is gradually increased in stages.

[0042] S3: RH refining process: The molten steel is placed in a vacuum chamber for vacuum treatment. Then FeS is added to the vacuum chamber for repressurization. During the vacuum treatment, the temperature of the molten steel is reduced by 70-100°C, and uniformly distributed oxygen-sulfur composite inclusions are formed in the molten steel. The molten steel is then continuously cast.

[0043] In this preferred embodiment, the tapping temperature in step S1 is 1580–1630°C. If the tapping temperature is below 1580°C, the subsequent addition of deoxidizer, alloy, and slag cannot be guaranteed to melt rapidly. If the tapping temperature is above 1630°C, the initial dissolved oxygen content in the molten steel is likely to be too high. Therefore, it is necessary to maintain a reasonable tapping temperature.

[0044] In this preferred embodiment, before RH refining, the composition and mass percentage of the molten steel are: C: 0.3-0.5%, Si: 0.15%-0.35%, Mn: 1.35-1.65%, Al: 0.007-0.012%, O: 0.001%-0.002%, S: ≤0.005%, Ca: 0.0005-0.0015%, Mg: 0.0001%-0.0005%, with the remainder being Fe and impurities.

[0045] When the mass percentage of Al is less than 0.007%, the SiO2 content in the inclusions increases, easily forming a large number of CaO-SiO2-Al2O3 oxides with high CaO content and large size, resulting in poor nucleation effect on MnS. When the mass percentage of Al is greater than 0.012%, the dissolved O content in the steel is too low, and a sufficient number of fine oxides cannot be obtained. The mass percentage of S is not higher than 0.005%, ensuring that 0.0005% to 0.0015% of Ca can be dissolved in the molten steel. The mass percentage of Ca is 0.0005–0.0015% to ensure that CaO-Al2O3 oxides with appropriate composition are precipitated during temperature drop. If the mass percentage of Ca is higher than 0.0015%, the CaO content in the formed CaO-Al2O3 oxides will be too high, which is not conducive to the subsequent growth of MnS on it. Since the Ca element in the molten steel is mainly introduced by slag in the LF refining process, in this embodiment, the mass percentage of Ca can be controlled to be 0.0005–0.0015% when the mass percentage of S before RH refining is not higher than 0.005%.

[0046] In this preferred embodiment, at the end of RH refining, the composition and mass percentage of the molten steel are: C: 0.3-0.5%, Si: 0.15%-0.35%, Mn: 1.35-1.65%, Al: 0.007-0.012%, O: 0.001%-0.002%, S: 0.02%-0.04%, Ca: 0.0005-0.0015%, Mg: 0.0001%-0.0005%, with the remainder being Fe and impurities.

[0047] During RH refining, both before and after the refining process, the mass percentage of O needs to be controlled between 0.001% and 0.002%. If the mass percentage of O is below 0.001%, a sufficient amount of oxides cannot be obtained to disperse MnS. If the mass percentage of O is above 0.002%, it indicates poor cleanliness of the molten steel, making it prone to forming large inclusions. Under normal smelting conditions, molten steel contains 0.0001% to 0.0005% Mg, which promotes the precipitation of oxides during the cooling process in step S3 to have a MgO content of 1% to 20%, ensuring that the endogenous oxides in the molten steel do not easily aggregate.

[0048] In this preferred embodiment, the temperature of the molten steel before RH refining is controlled at 1640–1660°C. This ensures that the subsequent temperature drop of the molten steel is large enough to generate a sufficient amount of fine oxides.

[0049] In this preferred embodiment, the temperature of the molten steel after RH vacuum treatment is controlled at 1550–1570°C. This ensures a temperature drop of 70–100°C during the RH vacuum treatment process, allowing a large amount of fine oxides to precipitate and remain in the molten steel.

[0050] In step S3, the oxygen-sulfur complex inclusions include core oxides and peripheral sulfides. The core oxides have the following composition and mass percentages: CaO: 5-30%, Al2O3: 60-90%, MgO: 5-20%. The peripheral sulfides have the following composition and mass percentages: CaS: 2-40%, MnS: 60-98%.

[0051] In this preferred embodiment, the core oxide has a size of 0.5–3 μm, the composite inclusions have a size of 2–10 μm, and the number density of oxygen-sulfur composite inclusions is ≥60 inclusions / mm². 2 .

[0052] In this preferred embodiment, the vacuum treatment time in step S3 is 30-50 minutes, during which FeS is added to the vacuum chamber and repressurized after 5 minutes. That is, after vacuum treatment for 25-45 minutes, FeS is added to the vacuum chamber and repressurized after 5 minutes.

[0053] One embodiment of the technical solution of the present invention provides a free-cutting steel, which is made by forming oxygen-sulfur composite inclusions in the free-cutting steel provided by the present invention. This free-cutting steel contains a large number of uniformly distributed oxygen-sulfur composite inclusions, which significantly reduces the formation of large-sized pure MnS. After rolling, most of the sulfides can still maintain a spherical or spindle shape and there is no obvious aggregation phenomenon, thus improving the mechanical properties of the steel.

[0054] Example 1

[0055] Molten iron and scrap steel are added to an electric furnace or converter to melt the raw materials and achieve efficient decarburization. The carbon content in the molten steel is 0.13% when tapping, and the tapping temperature is 1590℃. Aluminum blocks, ferrosilicon, ferromanganese and slag are added during the tapping process.

[0056] The composition and mass percentage of the slag in the LF refining process are: CaO: 45-55%, SiO2: 10-20%, Al2O3: 15-25%, MgO: 3-8%, with the remainder being unavoidable impurities such as FeO. At the beginning of LF refining, an Al wire is fed in to increase the mass percentage of Al in the molten steel to 0.02%. During the refining process, alloys are added to ensure that the C, Si, and Mn elements meet the composition requirements. Low-pressure argon gas promotes the homogenization of the molten steel composition and the removal of inclusions by flotation, ensuring high cleanliness of the molten steel and simultaneously achieving desulfurization. The temperature of the molten steel is gradually increased to 1647℃ in stages. At the end of LF refining, the composition and mass percentage of the molten steel include: S: 0.005%, Al: 0.008%, O: 0.0016%, and Ca: 0.001%.

[0057] The total RH vacuum treatment time is 38 minutes, with FeS added from the vacuum chamber at 33 minutes. The temperature of the molten steel is 1560℃ at the end of RH refining, after which continuous casting is carried out.

[0058] Table 1 shows the chemical composition analysis of the steel sample after RH refining in Example 1:

[0059] Table 1

[0060]

[0061] The typical morphology of the oxides obtained during the cooling process of molten steel in Example 1 is as follows: Figure 2 As shown, these oxides are mainly Ca-Mg-Al-O, with small sizes, primarily in the range of 0.5–3 μm.

[0062] Typical morphology of oxygen-sulfur composite inclusions in the rolled material prepared in Example 1 is as follows: Figure 3 As shown, the oxygen-sulfur complex inclusions consist of two parts: a core oxide and a peripheral sulfide, with compositions as follows: Figure 4 and Figure 5 As shown, the composition of both the core oxide and the peripheral sulfides is within the target range. The core oxide size is mainly 0.5–3 μm, and the composite inclusion size is mainly 2–10 μm. The number density of the composite inclusions reaches 78 inclusions / mm². 2 .

[0063] The overall distribution of sulfides in the rolled material prepared in Example 1 is as follows: Figure 6 As shown, there are a large number of uniformly distributed oxygen-sulfur complex inclusions. Most of the sulfides are spherical and spindle-shaped, and there is no obvious aggregation.

[0064] The results show that, using the technology of this invention, a large number of uniformly distributed oxygen-sulfur composite inclusions were obtained in steel, which significantly reduced the formation of large-sized pure MnS. After rolling, most of the sulfides exhibited spherical and spindle-shaped shapes and no obvious aggregation phenomenon.

[0065] Example 2

[0066] Molten iron and scrap steel are added to an electric furnace or converter to melt the raw materials and achieve efficient decarburization. The carbon content in the molten steel is 0.11% when tapping, and the tapping temperature is 1602℃. Aluminum blocks, ferrosilicon, ferromanganese and slag are added during the tapping process.

[0067] The composition and mass percentage of the slag in the LF refining process are: CaO: 45-55%, SiO2: 10-20%, Al2O3: 15-25%, MgO: 3-8%, with the remainder being unavoidable impurities such as FeO. At the beginning of LF refining, an Al wire is fed in to increase the mass percentage of Al in the molten steel to 0.025%. During the refining process, alloys are added to ensure that the C, Si, and Mn elements meet the composition requirements. Low-pressure argon gas promotes the homogenization of the molten steel composition and the removal of inclusions by flotation, ensuring high cleanliness of the molten steel and simultaneously achieving desulfurization. The temperature of the molten steel is gradually increased to 1654℃ in stages. At the end of LF refining, the composition and mass percentage of the molten steel include: S: 0.004%, Al: 0.009%, O: 0.0015%, and Ca: 0.0012%.

[0068] The total RH vacuum treatment time was 44 minutes. FeS was added from the vacuum chamber at 39 minutes. The temperature of the molten steel was 1558℃ at the end of RH refining, and then continuous casting was carried out.

[0069] Table 2 shows the chemical composition analysis of the steel sample after RH refining in Example 2:

[0070] Table 2

[0071]

[0072] In Example 2, the compositions of the core oxide and peripheral sulfide of the oxygen-sulfur composite inclusion in the prepared material are as follows: Figure 4 and Figure 5 As shown, the characteristics of the oxygen-sulfur composite inclusions in the material obtained in Example 1 are basically consistent, with the number density of composite inclusions reaching 73 inclusions / mm². 2 .

[0073] The overall distribution of sulfides in the rolled material prepared in Example 2 was exactly the same as that in the material obtained in Example 1, and will not be repeated here. The results also show that, using the technology of the present invention, a large number of uniformly distributed oxygen-sulfur composite inclusions were effectively obtained in the steel, significantly reducing the formation of large-sized pure MnS. After rolling, most of the sulfides exhibited spherical and spindle-shaped shapes, and there was no obvious aggregation phenomenon.

[0074] Example 3

[0075] Molten iron and scrap steel are added to an electric furnace or converter to melt the raw materials and achieve efficient decarburization. The carbon content in the molten steel is 0.09% when tapping, and the tapping temperature is 1613℃. Aluminum blocks, ferrosilicon, ferromanganese and slag are added during the tapping process.

[0076] The composition and mass percentage of the slag in the LF refining process are: CaO: 45-55%, SiO2: 10-20%, Al2O3: 15-25%, MgO: 3-8%, with the remainder being unavoidable impurities such as FeO. At the beginning of LF refining, an Al wire is fed in to increase the mass percentage of Al in the molten steel to 0.03%. During the refining process, alloys are added to ensure that the C, Si, and Mn elements meet the composition requirements. Low-pressure argon gas promotes the homogenization of the molten steel composition and the removal of inclusions by flotation, ensuring high cleanliness of the molten steel and simultaneously achieving desulfurization. The temperature of the molten steel is gradually increased to 1650℃ in stages. At the end of LF refining, the composition and mass percentage of the molten steel include: S: 0.003%, Al: 0.011%, O: 0.0012%, and Ca: 0.0014%.

[0077] The total RH vacuum treatment time is 40 minutes. FeS is added from the vacuum chamber at 35 minutes. The temperature of the molten steel is 1562℃ at the end of RH refining, and then continuous casting is carried out.

[0078] Table 3 shows the chemical composition analysis of the steel sample after RH refining in Example 3:

[0079] Table 3

[0080]

[0081] The compositions of the core oxide and peripheral sulfides of the oxygen-sulfur composite inclusions in the material prepared in Example 3 are as follows: Figure 4 and Figure 5 As shown, the characteristics of the oxygen-sulfur composite inclusions in the material obtained in Example 1 are basically consistent, and the number density of composite inclusions reaches 70 inclusions / mm². 2 .

[0082] The overall distribution of sulfides in the rolled material prepared in Example 3 was exactly the same as that in the material obtained in Example 1, and will not be repeated here. The results also show that, using the technology of the present invention, a large number of uniformly distributed oxygen-sulfur composite inclusions were effectively obtained in the steel, significantly reducing the formation of large-sized pure MnS. After rolling, most of the sulfides were spherical and spindle-shaped, and there was no obvious aggregation phenomenon.

[0083] Comparative Example 1

[0084] The main difference between Comparative Example 1 and the present invention lies in the O content in the molten steel before RH refining; other operating processes comply with the requirements of the present invention. In Comparative Example 1, the O content in the molten steel before RH refining was 0.0007%, which is lower than the requirements of the present invention. The chemical composition of the steel sample at the end of RH refining is shown in Table 4.

[0085] Table 4

[0086]

[0087] Typical morphology and overall distribution of sulfides in the rolled material prepared in Comparative Example 1 are as follows: Figure 7 As shown, the results indicate that because the O content in the molten steel before RH refining was lower than the range required by this invention, a sufficient number of fine oxides were not obtained during the temperature drop process, resulting in fewer oxygen-sulfur composite inclusions formed in the steel, leading to the formation of more large-sized pure MnS. Most of the sulfides in the rolled product are elongated and show obvious aggregation.

[0088] Comparative Example 2

[0089] The main difference between Comparative Example 2 and the present invention lies in the Al content in the molten steel during the refining process; other operating procedures comply with the requirements of the present invention. In Comparative Example 2, the mass percentage of Al was less than 0.007% at the beginning of RH refining, and the mass percentage of Al remained below 0.007% throughout the RH treatment process. The chemical composition at the end of RH refining is shown in Table 5, where the Al content was 0.003%, which is lower than the 0.007–0.012% required by the present invention.

[0090] Table 5

[0091]

[0092] Typical morphology and overall distribution of sulfides in the rolled material prepared in Comparative Example 2 are as follows: Figure 8 As shown, the results indicate that, due to the lower mass percentage of Al in the molten steel during RH refining than the range required by this invention, the SiO2 content in the oxides precipitated during temperature drop is relatively high. This easily leads to the formation of CaO-SiO2-Al2O3 oxides with high CaO content and large size. The nucleation effect on MnS during solidification is poor, resulting in very few oxygen-sulfur composite inclusions. This leads to the formation of a large number of large-sized pure MnS, and the sulfides in the rolled material are mainly elongated. Moreover, the large-sized CaO-SiO2-Al2O3 oxides are also retained in the rolled material, appearing as round shapes, which reduces the mechanical properties of the steel.

[0093] Comparative Example 3

[0094] The main difference between Comparative Example 3 and the present invention lies in the temperature drop of the molten steel during the RH vacuum treatment process; other operating procedures comply with the requirements of the present invention. In Comparative Example 3, the temperature of the molten steel before RH refining was 1609°C, lower than the 1640–1660°C required by the present invention; the temperature of the molten steel after RH vacuum treatment was 1557°C; and the temperature drop of the molten steel during the RH refining process was 52°C, lower than the 70–100°C required by the present invention. The chemical composition at the end of RH refining is shown in Table 6.

[0095] Table 6

[0096]

[0097] Typical morphology and overall distribution of sulfides in the rolled material prepared in Comparative Example 3 are as follows: Figure 9 As shown, the results indicate that because the temperature drop of the molten steel during the RH refining process is below the range required by this invention, a sufficient number of fine oxides are not obtained to form a large number of uniformly distributed oxygen-sulfur composite inclusions, resulting in the formation of a large number of large-sized pure MnS. The sulfides in the rolled product are mainly elongated and exhibit obvious aggregation.

[0098] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0099] The sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0100] Through the above description of the embodiments, those skilled in the art can clearly understand that the above implementation methods can be implemented by means of software plus necessary general-purpose hardware platforms. Of course, they can also be implemented by hardware, but in many cases the former is a better implementation method. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product is stored in a storage medium (such as ROM / RAM, magnetic disk, optical disk) and includes several instructions to cause a terminal (which may be a mobile phone, computer, server, air conditioner, or network device, etc.) to execute the methods described in the various embodiments of the present invention.

[0101] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.

Claims

1. A method of forming an oxysulfide complex inclusion in a free-cutting steel, characterized in that, include: S1: Electric furnace or converter process: Molten iron and scrap steel raw materials are added to the electric furnace or converter to melt and decarburize the raw materials. When tapping the steel, the C content of the molten steel is not less than 0.08% by mass. Aluminum blocks, ferrosilicon, ferromanganese and slag are added during the tapping process. S2: LF refining process: The composition and mass percentage of the slag are CaO: 45~55%, SiO2: 10%~20%, Al2O3: 15~25%, MgO: 3%~8%, with the remainder being impurities. After the LF refining slag is formed, the composition of the molten steel is tested. Based on the test results, Al is added, and the Al content in the molten steel is 0.02~0.03% by mass. Low-pressure argon gas is used to float and remove inclusions, and the molten steel is desulfurized to reduce the mass percentage of S to no more than 0.005%. The temperature of the molten steel is gradually increased in stages. S3: RH refining process: The molten steel is placed in a vacuum chamber for vacuum treatment, and then FeS is added to the vacuum chamber for repressurization. During the vacuum treatment, the temperature of the molten steel is reduced by 70~100℃, and uniformly distributed oxygen-sulfur composite inclusions are formed in the molten steel. Then the molten steel is continuously cast. Before RH refining, the composition and mass percentage of the molten steel are as follows: C: 0.3~0.5%, Si: 0.15%~0.35%, Mn: 1.35~1.65%, Al: 0.007~0.012%, O: 0.001%~0.002%, S: ≤0.005%, Ca: 0.0005~0.0015%, Mg: 0.0001%~0.0005%, with the remainder being Fe and impurities; At the end of RH refining, the composition and mass percentage of the molten steel are as follows: C: 0.3~0.5%, Si: 0.15%~0.35%, Mn: 1.35~1.65%, Al: 0.007~0.012%, O: 0.001%~0.002%, S: 0.02%~0.04%, Ca: 0.0005~0.0015%, Mg: 0.0001%~0.0005%, with the remainder being Fe and impurities; During the RH refining process, the temperature drop of the molten steel is used to form Ca-Mg-Al-O oxides.

2. The method of forming oxysulfide complex inclusions in free machining steel according to claim 1, characterized in that, In step S1, the tapping temperature is 1580~1630℃.

3. The method of forming oxysulfide complex inclusions in free machining steel according to claim 1, characterized in that, The temperature of the molten steel before RH refining is controlled at 1640~1660℃.

4. The method for forming oxygen-sulfur composite inclusions in free-cutting steel according to claim 1, characterized in that, After RH vacuum treatment, the temperature of the molten steel is controlled at 1550~1570℃.

5. The method for forming oxygen-sulfur composite inclusions in free-cutting steel according to claim 1, characterized in that, In step S3, the oxygen-sulfur composite inclusions include core oxides and peripheral sulfides. The core oxides have the following composition and mass percentages: CaO: 5-30%, Al2O3: 60-90%, MgO: 5-20%. The peripheral sulfides have the following composition and mass percentages: CaS: 2-40%, MnS: 60-98%.

6. The method for forming oxygen-sulfur composite inclusions in free-cutting steel according to claim 5, characterized in that, The size of the core oxide is 0.5-3 μm, the size of the composite inclusion is 2-10 μm, and the number density of the oxygen-sulfur composite inclusion is ≥60 pieces / mm 2 .

7. The method for forming oxygen-sulfur composite inclusions in free-cutting steel according to claim 1, characterized in that, In step S3, the vacuum treatment time is 30-50 minutes in total, during which FeS is added to the vacuum chamber and repressurized after 5 minutes.

8. A free-cutting steel, characterized in that, It is prepared by the method of forming oxygen-sulfur complex inclusions in free-cutting steel as described in any one of claims 1-7.

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