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

Abstract

The invention discloses a method for forming oxygen-sulfur composite inclusions in free-cutting steel and the free-cutting steel. The method comprises an electric furnace or converter working procedure, an LF refining working procedure and an RH refining working procedure. According to the method, the temperature of molten steel and the content of Al, O, S, Ca and Mg in the refining process are strictly controlled, a large number of stably existing Ca-Mg-Al-O oxides with small sizes and specific components are formed by means of temperature drop of the molten steel in the RH refining process, and the Ca-Mg-Al-O oxides serve as nucleation cores of MnS inclusions in the solidification process; a large number of evenly-distributed oxygen-sulfur composite inclusions exist, formation of large-size pure MnS is remarkably reduced, most of sulfide can be kept spherical or fusiform after rolling, no obvious aggregation phenomenon exists, and the mechanical property of the steel is improved.

Description

Technical Field

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

[0002] Adding a certain amount of S to steel can improve its cutting performance, but in actual production, due to the solidification segregation of Mn and S elements, large-sized pure MnS inclusions are often formed, which appear as long and thin strips after rolling, resulting in anisotropy of the mechanical properties of steel. Medium-carbon sulfur-containing steel needs to have both excellent cutting performance and good and uniform mechanical properties. The high S content and the use of Al deoxidation make it easier to form long and large-sized MnS in actual production, which seriously damages the mechanical properties of steel.

[0003] In order to reduce the formation of large-sized pure MnS in medium-carbon sulfur-containing steel, some researchers have tried high-temperature heat treatment and increased cooling rate during solidification, but the control effects of these two methods are very limited and cannot solve the problem of large deformation of MnS after rolling; some researchers add Ca, Mg, Zr, Ti or rare earth elements to steel to modify pure MnS into composite sulfides (Mn, X)S with higher hardness and less deformation to reduce the formation of long strips of MnS. The addition of Mg, Zr, Ti or rare earth will significantly reduce the cleanliness of molten steel. Adding Ca is an efficient and low-cost control method, but it is easy to have poor castability of molten steel during smelting, resulting in difficulty in continuous casting production; in addition, using oxides in steel to refine MnS is an important method. By using oxides as the nucleation core of MnS, a large number of uniformly distributed oxygen-sulfur composite inclusions are obtained in 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 steel, which is very difficult in actual smelting.

[0004] Patent CN104212942B discloses a process for improving the morphology of sulfides in sulfur-containing steel, and proposes to control the timing and amount of Ca addition, and to perform a Ca feeding operation before and after adjusting the S content to the target composition of 0.02-0.08%, so as to promote the formation of oxygen-sulfur composite inclusions. However, after increasing the S content to 0.02-0.08%, adding Ca again is easy to generate a large number of CaS inclusions, which reduces the castability of the molten steel and affects the continuous casting production. Patent CN113278762B discloses a Ca alloying method in a high-aluminum calcium-sulfur composite free-cutting steel, and attempts to obtain more oxygen-sulfur composite inclusions in the steel. However, in this process, Ca addition operations are performed twice after RH vacuum treatment, which easily produces more CaO-Al2O3 inclusions with high CaO content and large size, which are difficult to serve as effective nucleation cores for MnS, and are easy to remain in the steel to harm the mechanical properties of the finished steel. The above technology is difficult to ensure that a large number of uniformly distributed oxygen-sulfur composite inclusions are obtained in the steel.

[0005] Therefore, it is urgent to provide a solution for forming oxygen-sulfur composite inclusions in free-cutting steel. Summary of the invention

[0006] In order to solve the above problems, the technical solution of the present invention provides a method for forming oxygen-sulfur composite inclusions in free-cutting steel and free-cutting steel, which can form 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 technical solution of the present invention, there is provided a method for forming oxygen-sulfur composite inclusions in free-cutting steel using an RH process, comprising:

[0008] S1: Electric furnace or converter process: adding molten iron and scrap steel raw materials into the electric furnace or converter, melting and decarburizing the raw materials, and the C of the molten steel is not less than 0.08% by mass during steel tapping, and adding aluminum blocks, ferrosilicon, ferromanganese and slag during the steel 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%, and the rest are impurities. After the LF refining slag is formed, the composition of the molten steel is detected, and Al is added according to the detection result. The mass percentage of Al in the molten steel is 0.02-0.03%. The inclusions are floated and removed by low argon blowing. The molten steel is desulfurized to reduce the mass percentage of S to no more than 0.005%, and 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 from the vacuum chamber for re-pressing. During the vacuum treatment, the temperature of the molten steel is reduced by 70 to 100° C., and uniformly distributed oxygen-sulfur composite inclusions are formed in the molten steel. Then, the molten steel is continuously cast.

[0011] In the above scheme, in step S1, the steel tapping temperature is 1580-1630°C.

[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%, and the rest are Fe and impurities.

[0013] In the above scheme, when RH refining is completed, 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%, and the rest are Fe and impurities.

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

[0015] In the above scheme, the temperature of the molten steel after RH vacuum treatment is controlled to be 1550-1570°C.

[0016] In the above scheme, in step S3, the oxygen-sulfur composite inclusions include core oxides and peripheral sulfides, the core oxide composition and mass percentage are CaO: 5-30%, Al2O3: 60-90%, MgO: 5-20%, the peripheral sulfide composition and mass percentage are 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 / mm 2 .

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

[0019] In the above scheme, in step S3, FeS is added from the vacuum chamber and re-pressurization is performed after 5 minutes.

[0020] According to a second aspect of the technical solution of the present invention, there is provided a free-cutting steel, wherein the steel is made by the method for forming oxygen-sulfur composite inclusions in free-cutting steel as described in any one of the above aspects.

[0021] Beneficial effects of the present invention:

[0022] The invention discloses a method for forming oxygen-sulfur composite inclusions in free-cutting steel and the free-cutting steel. The temperature of molten steel and the contents of Al, O, S, Ca and Mg are strictly controlled during the refining process. During the RH refining process, a large number of stable Ca-Mg-Al-O type oxides with small size and specific composition are formed by utilizing the temperature drop of the molten steel. The oxides serve as nucleation cores of MnS inclusions during the solidification process. The obtained calcium-sulfur composite free-cutting steel contains a large number of uniformly distributed oxygen-sulfur composite inclusions, which significantly reduces the formation of large-sized pure MnS. Most of the sulfides can still maintain a spherical or spindle shape after rolling without obvious aggregation, thereby improving the mechanical properties of the steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

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

[0025] Figure 2 Schematic diagram of oxides obtained during the temperature reduction process of molten steel in Example 1 of the present invention;

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

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

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

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

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

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

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

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

[0034] Exemplary embodiments will be described in detail herein, examples of which are shown in the accompanying drawings. When the following description refers to the drawings, the same numbers in different drawings represent the same or similar elements unless otherwise indicated. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present disclosure. Instead, they are merely examples of devices and methods consistent with some aspects of the present disclosure as detailed in the appended claims.

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

[0036] In addition, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover non-exclusive inclusions. For example, a process, method, system, product, or apparatus that includes a series of steps or elements is not necessarily limited to those steps or elements explicitly listed, but may include other steps or elements not explicitly listed or inherent to such process, method, product, or apparatus.

[0037] Multiple includes two or more.

[0038] It should be understood that the term "and / or" used in this disclosure is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone.

[0039] like Figure 1 As shown, an 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: adding molten iron and scrap steel raw materials into the electric furnace or converter, melting and decarburizing the raw materials, and the C of the molten steel is not less than 0.08% by mass when tapping. 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 rest are impurities. After LF refining slag is formed, the composition of the molten steel is detected, and Al is added according to the test results. The mass percentage of Al in the molten steel is 0.02-0.03%. The inclusions are floated and removed by low argon blowing, 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, and then FeS is added from the vacuum chamber for re-pressing. During the vacuum treatment, the temperature of the molten steel is reduced by 70 to 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 embodiment, in step S1, the tapping temperature is preferably 1580-1630° C. If the tapping temperature is lower than 1580° C., the subsequent added deoxidizer, alloy and slag cannot be melted quickly. If the tapping temperature is higher than 1630° C., the initial dissolved O content in the molten steel is likely to be too high. Therefore, it is necessary to maintain a reasonable tapping temperature.

[0044] Preferably, 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%, and the rest are Fe and impurities.

[0045] Among them, when the mass percentage of Al is lower than 0.007%, the SiO2 content in the inclusions increases, and it is easy to form more CaO-SiO2-Al2O3 oxides with high CaO content and large size, which has a poor nucleation effect on MnS; when the mass percentage of Al is higher 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%, which ensures that 0.0005-0.0015% of Ca can be dissolved in the molten steel. The mass percentage of Ca is 0.0005-0.0015%, which ensures that CaO-Al2O3-type oxides with suitable components are precipitated during temperature drop. If the mass percentage of Ca is higher than 0.0015%, the CaO content in the formed CaO-Al2O3-type oxides is too high, which is not conducive to the subsequent growth of MnS thereon. Since the Ca element in the molten steel is mainly introduced from the slag in the LF refining process, in this embodiment, when the mass percentage of S before RH refining is not higher than 0.005%, the mass percentage of Ca can be controlled to be 0.0005-0.0015%.

[0046] Preferably, 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%, and the rest are Fe and impurities.

[0047] Before RH refining and during the end of RH refining, the mass percentage of O needs to be controlled to be 0.001% to 0.002%. If the mass percentage of O is lower than 0.001%, sufficient oxides cannot be obtained to disperse MnS. If the mass percentage of O is higher than 0.002%, it indicates that the cleanliness of the molten steel is poor and large-sized inclusions are easily formed. Under normal smelting conditions, there will be 0.0001 to 0.0005% Mg content in the molten steel, which can promote the oxides precipitated in the temperature drop process of step S3 to have a MgO content of 1 to 20%, ensuring that the endogenous oxides in the molten steel are not easy to aggregate.

[0048] In this embodiment, the temperature of the molten steel before RH refining is preferably controlled at 1640-1660° C., so as to ensure that the subsequent molten steel has a sufficiently large temperature drop to generate enough fine oxides.

[0049] In this embodiment, the temperature of the molten steel after the RH vacuum treatment is preferably controlled to be 1550-1570° C. This ensures that the RH vacuum treatment process produces a temperature drop of 70-100° C., and a large amount of fine oxides are precipitated and retained in the molten steel.

[0050] In step S3, the oxygen-sulfur composite inclusions include core oxides and peripheral sulfides, the core oxide composition and mass percentage are CaO: 5-30%, Al2O3: 60-90%, MgO: 5-20%, the peripheral sulfide composition and mass percentage are CaS: 2-40%, MnS: 60-98%.

[0051] In this embodiment, the size of the core oxide is preferably 0.5-3 μm, the size of the composite inclusion is 2-10 μm, and the number density of oxygen-sulfur composite inclusions is ≥ 60 / mm 2 .

[0052] In this embodiment, preferably, in step S3, the vacuum treatment time is 30 to 50 minutes in total, wherein FeS is added from the vacuum chamber and re-pressurization is performed 5 minutes later. That is, after the vacuum treatment for 25 to 45 minutes, FeS is added to the vacuum chamber and re-pressurization is performed 5 minutes later.

[0053] An embodiment of the technical solution of the present invention provides a free-cutting steel, which is made by the method for forming oxygen-sulfur composite inclusions in free-cutting steel provided by the present invention. A large number of evenly distributed oxygen-sulfur composite inclusions exist in the free-cutting steel, which significantly reduces the formation of large-sized pure MnS. Most of the sulfides can still maintain a spherical or spindle shape after rolling, and there is no obvious aggregation phenomenon, thereby improving the mechanical properties of the steel.

[0054] Example 1

[0055] Add molten iron and scrap steel raw materials into an electric furnace or a converter to achieve melting of the raw materials and efficient decarburization. The C content in the molten steel is 0.13% when tapping, and the tapping temperature is 1590°C. 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%, and the rest are inevitable impurities such as FeO. At the beginning of LF refining, Al wire is fed to increase the mass percentage of Al in the molten steel to 0.02%. During the refining process, alloys are added to make the C, Si, and Mn elements meet the composition requirements. Low argon blowing promotes the homogenization of the molten steel composition and the floating removal of inclusions to ensure high cleanliness of the molten steel. At the same time, desulfurization of the molten steel is achieved. The temperature of the molten steel is gradually increased to 1647°C 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 RH vacuum treatment time was 38 min in total, of which FeS was added from the vacuum chamber at 33 min. The temperature of the molten steel was 1560 °C at the end of RH refining, and then continuous casting was 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 oxide obtained during the temperature drop of the molten steel in Example 1 is as follows: Figure 2 As shown, it can be seen that these oxides are mainly Ca-Mg-Al-O, and are small in size, mainly in the range of 0.5 to 3 μm.

[0062] The typical morphology of oxygen-sulfur composite inclusions in the rolled material prepared in Example 1 is as follows Figure 3 As shown in the figure, it can be seen that the oxygen-sulfur composite inclusions contain two parts: core oxide and peripheral sulfide, and their compositions are as follows: Figure 4 and Figure 5 As shown in the figure, the composition of core oxides and peripheral sulfides are within the target range. The size of core oxides is mainly 0.5-3 μm, the size of composite inclusions is mainly 2-10 μm, and the number density of composite inclusions reaches 78 / mm 2 .

[0063] The overall distribution of sulfides in the rolled material of the material prepared in Example 1 is as follows Figure 6 As shown, it can be seen that there are a large number of evenly distributed oxygen-sulfur complex inclusions, most of the sulfides are spherical and spindle-shaped, and there is no obvious aggregation phenomenon.

[0064] The results show that, by using the technology of the present invention, a large number of evenly distributed oxygen-sulfur composite inclusions are obtained in the steel, which significantly reduces the formation of large-sized pure MnS. After rolling, most of the sulfides are spherical and spindle-shaped, and there is no obvious aggregation phenomenon.

[0065] Example 2

[0066] Add molten iron and scrap steel raw materials into an electric furnace or a converter to achieve melting of the raw materials and efficient decarburization. The C content in the molten steel is 0.11% when tapping, and the tapping temperature is 1602°C. 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%, and the rest are inevitable impurities such as FeO. At the beginning of LF refining, Al wire is fed to increase the mass percentage of Al in the molten steel to 0.025%. During the refining process, alloys are added to make the C, Si, and Mn elements meet the composition requirements. Low argon blowing promotes the homogenization of the molten steel composition and the floating removal of inclusions to ensure the high cleanliness of the molten steel. At the same time, the desulfurization of the molten steel is achieved. The temperature of the molten steel is gradually increased to 1654°C 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 min. FeS was added from the vacuum chamber at 39 min. The temperature of the molten steel was 1558 °C 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] Example 2 The compositions of the core oxide and the peripheral sulfide of the oxygen-sulfur composite inclusions 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, and the number density of the composite inclusions reaches 73 / mm 2 .

[0073] The overall distribution of sulfides in the rolled material of the material prepared in Example 2 is exactly the same as that of 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 are effectively obtained in the steel, and the formation of large-sized pure MnS is significantly reduced. After rolling, most of the sulfides are spherical and spindle-shaped, and there is no obvious aggregation phenomenon.

[0074] Example 3

[0075] Add molten iron and scrap steel raw materials into an electric furnace or a converter to achieve melting of the raw materials and efficient decarburization. The C content in the molten steel is 0.09% when tapping, and the tapping temperature is 1613°C. 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%, and the rest are inevitable impurities such as FeO. At the beginning of LF refining, Al wire is fed to increase the mass percentage of Al in the molten steel to 0.03%. During the refining process, alloys are added to make the C, Si, and Mn elements meet the composition requirements. Low argon blowing promotes the homogenization of the molten steel composition and the floating removal of inclusions to ensure high cleanliness of the molten steel. At the same time, desulfurization of the molten steel is achieved. The temperature of the molten steel is gradually increased to 1650°C 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 was 40 min. FeS was added from the vacuum chamber at 35 min. The temperature of the molten steel was 1562 °C at the end of RH refining, and then continuous casting was 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] Example 3 The compositions of the core oxide and the peripheral sulfide of the oxygen-sulfur composite inclusions in the prepared material are as follows: Figure 4 and Figure 5 As shown in Figure 2, the characteristics of the oxygen-sulfur composite inclusions in the material obtained in Example 1 are basically consistent, and the number density of the composite inclusions reaches 70 / mm 2 .

[0082] The overall distribution of sulfides in the rolled material of the material prepared in Example 3 is exactly the same as that of 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 are effectively obtained in the steel, and the formation of large-sized pure MnS is significantly reduced. After rolling, most of the sulfides are spherical and spindle-shaped, and there is no obvious aggregation phenomenon.

[0083] Comparative Example 1

[0084] The main difference between Comparative Example 1 and the present invention is the O content in the molten steel before RH refining, and the other operation processes meet the requirements of the present invention. The O content in the molten steel before RH refining in Comparative Example 1 is 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] The typical morphology and overall distribution of sulfides in the rolled material of the material prepared in Comparative Example 1 are as follows: Figure 7 As shown, the results show that since the O content in the molten steel before RH refining is lower than the required range of the present invention, a sufficient number of fine oxides cannot be obtained during the temperature drop process, and fewer oxygen-sulfur composite inclusions are formed in the steel, resulting in the formation of more large-sized pure MnS. Most of the sulfides in the rolled product are in the shape of long strips and have obvious aggregation.

[0088] Comparative Example 2

[0089] The main difference between Comparative Example 2 and the present invention is the Al content in the molten steel during the refining process, and the other operation processes meet the requirements of the present invention. In Comparative Example 2, the mass percentage of Al at the beginning of RH refining is less than 0.007%, and the mass percentage of Al during the RH treatment is always less than 0.007%. The chemical composition at the end of RH refining is shown in Table 5, wherein the Al content is 0.003%, which is lower than the 0.007-0.012% required by the present invention.

[0090] Table 5

[0091]

[0092] The typical morphology and overall distribution of sulfides in the rolled material of the material prepared in Comparative Example 2 are as follows: Figure 8 As shown, the results show that since the mass percentage of Al in the molten steel during the RH refining process is lower than the required range of the present invention, the SiO2 content in the oxides precipitated by temperature reduction is relatively high, and it is easy to form CaO-SiO2-Al2O3 type oxides with high CaO content and large size, which have poor nucleation effect on MnS during solidification, and few oxygen-sulfur composite inclusions are formed, resulting in the formation of more large-sized pure MnS, and the sulfides in the rolled material are mainly in the form of long strips; and large-sized CaO-SiO2-Al2O3 oxides will also be retained in the rolled material, appearing as a round shape, reducing the mechanical properties of the steel.

[0093] Comparative Example 3

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

[0095] Table 6

[0096]

[0097] The typical morphology and overall distribution of sulfides in the rolled material of the material prepared in Comparative Example 3 are as follows: Fig. 9 As shown, the results show that since the temperature drop of the molten steel during the RH refining process is lower than the required range of the present invention, a sufficient number of fine oxides cannot be obtained to form a large number of evenly distributed oxygen-sulfur composite inclusions, resulting in the formation of more large-sized pure MnS. The sulfides in the rolled product are mainly in the form of long strips, and there is an obvious aggregation phenomenon.

[0098] It should be noted that, in this article, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprises a ..." does not exclude the existence of other identical elements in the process, method, article or device including the element.

[0099] The serial numbers of the above embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0100] Through the description of the above implementation methods, those skilled in the art can clearly understand that the above implementation method can be implemented by means of software plus a necessary general hardware platform, and of course, by hardware, but in many cases the former is a better implementation method. Based on such an 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, which is stored in a storage medium (such as ROM / RAM, magnetic disk, optical disk), and includes a number of instructions for a terminal (which can be a mobile phone, computer, server, air conditioner, or network device, etc.) to execute the methods described in each embodiment of the present invention.

[0101] The embodiments of the present invention are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation modes, which are merely illustrative rather than restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the present invention and the claims, all of which are within the protection of the present invention.

Claims

1. A method for forming oxygen-sulfur composite inclusions in free-cutting steel, characterized in that: include: S1: Electric furnace or converter process: adding molten iron and scrap steel raw materials into the electric furnace or converter, melting and decarburizing the raw materials, and the C of the molten steel is not less than 0.08% by mass during steel tapping, and adding aluminum blocks, ferrosilicon, ferromanganese and slag during the steel 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%, and the rest are impurities. After the LF refining slag is formed, the composition of the molten steel is detected, and Al is added according to the detection result. The mass percentage of Al in the molten steel is 0.02-0.03%. The inclusions are floated and removed by low argon blowing. The molten steel is desulfurized to reduce the mass percentage of S to no more than 0.005%, and 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 from the vacuum chamber for re-pressing. During the vacuum treatment, the temperature of the molten steel is reduced by 70 to 100° C., and uniformly distributed oxygen-sulfur composite inclusions are formed in the molten steel. Then, the molten steel is continuously cast.

2. The method for forming oxygen-sulfur composite inclusions in free-cutting steel according to claim 1, characterized in that: In step S1, the tapping temperature is 1580-1630°C.

3. The method for forming oxygen-sulfur composite inclusions in free-cutting steel according to claim 1, characterized in that: 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%, and the rest are Fe and impurities.

4. The method for forming oxygen-sulfur composite inclusions in free-cutting steel according to claim 1, characterized in that: When RH refining is finished, 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%, and the rest are Fe and impurities.

5. The method for forming oxygen-sulfur composite inclusions in free-cutting steel according to claim 1, characterized in that: The temperature of the molten steel before RH refining is controlled at 1640-1660°C.

6. 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°C.

7. 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 oxide composition and mass percentage are CaO: 5-30%, Al2O3: 60-90%, MgO: 5-20%, the peripheral sulfide composition and mass percentage are CaS: 2-40%, MnS: 60-98%.

8. The method for forming oxygen-sulfur composite inclusions in free-cutting steel according to claim 7, 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 / mm 2 .

9. 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 to 50 minutes in total, during which FeS is added from the vacuum chamber and re-pressurization is performed after 5 minutes.

10. A free-cutting steel, characterized in that: The invention is made by using the method for forming oxygen-sulfur composite inclusions in free-cutting steel as described in any one of claims 1 to 9.

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