High-quality free-cutting steel and production process thereof

Abstract

The invention provides high-quality free-cutting steel and a production process thereof, the high-quality free-cutting steel comprises the following components: less than or equal to 0.09% of C, less than or equal to 0.006% of Si, 0.90-1.60% of Mn, 0.04-0.09% of P, 0.30-0.50% of S, less than or equal to 0.030% of Al, less than or equal to 0.0060% of N, less than or equal to 0.0140% of O and the balance of Fe and other inevitable impurities, and Mn / S = 3.6-5.0; y value = 5 * Mn / S + LF activity oxygen value) / 2, and when the Y value is greater than or equal to 24 and less than or equal to 57.5, the final sulfide is properly controlled. The steel is produced through the production path of converter smelting, LF furnace refining, continuous casting and rolling, the LF and continuous casting process technologies are controlled, the appropriate rolling technology is matched, and finally the obtained casting blank is good in quality, the center segregation is stably controlled to be smaller than or equal to 1.0 level, the sulfide spindle ratio is larger than or equal to 45%, and the finished wire rod is excellent in cutting performance.

Description

Technical Field

[0001] The invention belongs to the technical field of free-cutting steel, and in particular relates to high-quality free-cutting steel and a production process thereof. Background Art

[0002] With the development of high-speed, precision and automation of mechanical processing, especially the prosperity of the automobile industry, precision instrument industry and household appliance industry, end users have an increasingly strong demand for reducing processing costs by improving the machining performance of steel materials, and high-quality free-cutting steel is in urgent need of development. Free-cutting steel is mainly used to make non-load-bearing parts with low force requirements but strict requirements on size and roughness, such as instruments, watch parts, automobiles, machine tools and other various machines; standard parts with strict requirements on dimensional accuracy and roughness, but relatively low requirements on mechanical properties, such as gears, shafts, bolts, valves, bushings, pins, pipe joints, etc.

[0003] At present, only a few steel mills can stably produce the mainstream sulfur-based free-cutting steel on the market, and the quality stability is poor, which is significantly different from foreign steel mills. In addition to production process control, the equipment conditions themselves are also a key factor limiting its quality improvement. In order to improve the quality and performance stability of free-cutting steel products, it is necessary to control the entire process from steelmaking-continuous casting process to rolling. While eliminating the segregation of sulfur elements, the sulfide morphology of the finished product is controlled to transform it into a spindle-shaped morphology that is beneficial to cutting performance. For the sulfide morphology of sulfur-containing steel, a sulfide aspect ratio of ≤3 is considered a spindle-shaped sulfide. Finished products with a high proportion of spindle-shaped sulfides have excellent cutting performance, and the factors affecting the sulfide morphology are relatively complex. In addition to the steelmaking process, the selection of the continuous casting process and the formulation of rolling conditions will affect its final morphology.

[0004] The Chinese patent with the publication number CN117089772A published on November 21, 2023, discloses a high-quality sulfur-containing cold heading steel wire rod and its preparation method, which controls the sulfur content in the raw material molten iron to obtain high-quality molten steel in the converter and refining process, reduces the number and size of oxide inclusions in the steel; combined with appropriate continuous casting and rolling processes, the composition and organization of the wire rod are homogenized, and the surface and internal quality are improved. The alloy composition of the developed steel is as follows by mass percentage: C: 0.45-0.55%, Si: 0.20-0.50%, Mn: 0.65-1.85%, Cr: 0.1-0.3%, S: 0.015-0.05%, Al: 0.010-0.060%, P≤0.015%, TO≤0.0025%, N≤0.0045%, and the rest is iron. However, this steel belongs to the category of cold heading steel and is not applicable to conventional free-cutting steel. Although the continuous casting process uses a light reduction process, it is mainly to solve the crack sensitivity of this type of steel during the cold heading process, and is used alone. The billet specifications are mainly 300mm×390mm, with a larger cross-section, and are not applicable to small cross-sections.

[0005] The Chinese patent with publication number CN116652136A published on August 29, 2023, discloses a production method for improving the segregation of continuous casting ingots of sulfur-containing steel grades. It mainly explores that the reduction amount in the continuous casting and straightening process of sulfur-containing steel is too small or the distribution amount is inappropriate, resulting in the segregation improvement is not obvious, and there are relatively serious light pressure cracks, which ultimately ended in failure. After systematic research on sulfur-containing steel grades, a production method for improving the segregation of continuous casting ingots of sulfur-containing steel grades has been successfully designed, which not only improves the segregation of the continuous casting ingots, but also does not produce light pressure cracks. However, its invention information is relatively general and cannot be applied to most production situations. Moreover, it only focuses on the continuous casting part, with a narrow scope, and cannot provide guidance for the production of a certain type of steel.

[0006] The Chinese patent with publication number CN112342464A published on February 9, 2021, discloses a production method of a hot-rolled wire rod of free-cutting steel for OA shafts, which limits the process conditions in converter smelting, LF furnace refining, continuous casting, billet rolling, billet finishing, finished wire rod rolling, wire rod packaging and protection, and finally ensures that the produced hot-rolled wire rod of free-cutting steel for OA shafts can ensure qualified chemical composition, good surface quality, excellent cutting performance, and good product stability, so as to meet the cutting performance and surface quality stability of downstream users, and the final processed OA shaft product has good stability and meets the requirements of flaw detection. However, this invention belongs to the second-fired product, that is, 280×320mm 2 The blank is 160×160mm 2 , the production conditions are not suitable for direct production of small square billets.

[0007] The control of sulfides in sulfur-based free-cutting steel needs to be considered from the perspective of the entire process. At the same time, the impact of the segregation of sulfur elements on the quality of the ingot needs to be fully considered. To this end, it is necessary to develop a high-quality free-cutting steel production process that can also be suitable for the current mainstream small square billet production. Summary of the invention

[0008] The purpose of the present invention is to provide a high-quality free-cutting steel and a production process thereof. Through composition design and process optimization, the free-cutting steel ingot produced has lighter segregation and a higher proportion of sulfide spindle rate in the finished wire rod. In addition, the process is also suitable for the current mainstream production of small square billets.

[0009] The specific technical solutions of the present invention are as follows:

[0010] A high quality free-cutting steel comprising the following mass percentages:

[0011] C≤0.09%, Si≤0.006%, Mn: 0.90~1.60%, P: 0.04~0.09%, S: 0.30~0.50%, Al≤0.030%, N≤0.0060%, O≤0.0140%, and the rest are Fe and other inevitable impurities.

[0012] The composition of the high-quality free-cutting steel also satisfies Mn / S=3.6-5.0.

[0013] The structure of the high-quality free-cutting steel is ferrite + pearlite, in which the volume of ferrite accounts for 92-96%. This structural feature ensures that the hardness of the steel is not too high and is beneficial to cutting; the inclusions in the steel are mainly type A MnS formed by Mn and S, and MnS is also the key to the excellent cutting performance of this type of steel, and the spindle-shaped sulfide has good cutting performance.

[0014] The present invention provides a production process for high-quality free-cutting steel, comprising the following process flow:

[0015] Converter smelting - LF furnace refining - continuous casting - rolling;

[0016] The specific control of LF refining is as follows: the whole process is controlled with the final active oxygen range of 30-90ppm as the key point.

[0017] Preferably, in the LF furnace refining, the active oxygen range of the LF furnace refining process is controlled to meet 24≤Y value≤57.5; Y value=(5×Mn / S+LF active oxygen value) / 2, which is a sulfide control factor. When 24≤Y value≤57.5, the final sulfide is properly controlled.

[0018] Mn / S is the ratio of the mass percentage of Mn to the mass percentage of S in the composition of the above-mentioned high-quality free-cutting steel;

[0019] The LF active oxygen value is the active oxygen in the LF furnace refining process, in ppm;

[0020] When calculating the above Y value, substitute the value before the above unit into the formula.

[0021] The continuous casting adopts induction heating of the tundish and argon sealing to protect the casting throughout the whole process. The tension straightening is carried out by a combination of terminal electromagnetic stirring parameters + light pressure reduction, wherein the terminal electromagnetic stirring parameters are selected to have a current of 360-380A and a frequency of 3.0-6.0Hz; the total reduction rate of light pressure reduction is 15-20mm, and the reduction is carried out on the first five frames, with the reduction amounts being 3-5-5-4-3mm respectively.

[0022] The present invention adopts a combined process of electromagnetic stirring at the end of solidification and light pressure to improve the casting quality of 160mm×160mm small square billets, reduce the center segregation phenomenon, and cooperate with a suitable rolling process to finally obtain a casting with excellent quality, a high proportion of sulfide spindle rate in the finished wire rod, and excellent cutting performance.

[0023] In the rolling, the heating and soaking temperature is controlled at 1220-1270°C, and the soaking time should be guaranteed to be ≥60min. In addition to ensuring the uniformity of the temperature inside and outside the steel billet, heating in this range also promotes the melting and remelting of the sulfide, so that the size of the sulfide is transformed into a large spindle shape, which ensures the morphology of the sulfide in the finished product.

[0024] The final ingot has excellent quality, with central porosity ≤1.0 level, central segregation ≤1.0 level, shrinkage ≤0.5 level, central crack ≤1.0 level, sulfide spindle rate ≥45%, and excellent cutting performance of the finished wire rod.

[0025] In order to obtain a good sulfide inclusion morphology, it is necessary to ensure a suitable [Mn] / [S] ratio and oxygen content. Compared with other elements, the S element in steel tends to form MnS with the Mn element. Therefore, the content ratio of Mn and S in steel is crucial and can affect the precipitation of MnS inclusions. The sulfide spindle rate is closely related to the manganese-sulfur ratio. At the same time, the increase in oxygen content can also make the sulfide present a thick and short spindle shape, which is beneficial to the cutting performance of the steel, but too high an oxygen content will cause defects such as subcutaneous pores to form in the final ingot. The present invention controls the oxygen content by controlling the LF active oxygen method, and the two are synergistic. Based on previous research, under the conditions defined by the present invention, when the manganese / sulfur ratio is in the range of 3.6 to 5.0 and when 24≤Y value≤57.5, the spindle-shaped effect of the finished sulfide is the best.

[0026] Compared with the prior art, the steel composition of the present invention is optimized, and the steel is produced through the production path of converter smelting-LF furnace refining-continuous casting-rolling. The LF and continuous casting process are controlled, and a suitable rolling process is coordinated to control the LF active oxygen value in the LF, so that Y=(5×Mn / S+LF active oxygen value) / 2) is the sulfide control factor. When 24≤Y≤57.5, the final sulfide is properly controlled, and the final ingot quality is excellent, with center porosity ≤1.0 level, center segregation ≤1.0 level, shrinkage cavity ≤0.5 level, center crack ≤1.0 level, sulfide spindle rate ≥45%, and the finished wire rod has excellent cutting performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a low-magnification photograph of Example 1;

[0028] Figure 2 This is a photo of the sulfide morphology of Example 1. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] The present invention provides a high-quality free-cutting steel, comprising the following components in percentage by mass:

[0031] C≤0.09%, Si≤0.006%, Mn: 0.90~1.60%, P: 0.04~0.09%, S: 0.30~0.50%, Al≤0.030%, N≤0.0060%, O≤0.0140%, and the rest are Fe and other inevitable impurities.

[0032] The composition of the high-quality free-cutting steel also satisfies Mn / S=3.6-5.0.

[0033] The microstructure of the high-quality free-cutting steel is ferrite+pearlite, wherein the volume of ferrite accounts for 92-96%.

[0034] The present invention provides a production process for high-quality free-cutting steel, comprising the following process flow:

[0035] Converter smelting - LF furnace refining - continuous casting - rolling;

[0036] The converter smelting is mainly for deoxidation and alloying. The alloy raw materials of the steel of the present invention are mainly lime, wollastonite, aluminum iron, phosphorus iron, low carbon ferromanganese and high sulfur ferrosulfur alloy. The components C, Si, Mn, P and S are basically adjusted in place during the converter steelmaking process, and the LF is fine-tuned. The purpose of this operation is to reduce the refining time and heating steps that may be added due to the adjustment of the LF components, and reduce the risk of increased oxygen content in the steel.

[0037] The LF refining: fine-tunes the alloy composition, controls the addition of slag, controls the basicity, controls the active oxygen range in the whole process, and indirectly ensures the oxygen content in the final product.

[0038] Furthermore, the LF refining is specifically controlled as follows: after the molten steel settles, the molten steel is stirred by bottom blowing argon, and the argon flow rate is 700-1000NL / min; the temperature is increased according to the inlet temperature, the amount of alloy added and the outlet temperature; the refining slag is low-basicity slag (basicity 2.4-2.8) to ensure the sulfur recovery rate; wherein, the whole process is controlled with the final active oxygen range of 30-90ppm as the key point, when the oxygen content exceeds the limit, the molten steel is deoxidized by aluminum iron, and the slag surface is deoxidized by silicon carbide to ensure the deoxidation balance between the molten steel and the slag surface; the added alloy materials are ferrosulfide, ferrophosphorus, low-carbon ferromanganese, high-sulfur ferrosulfur alloy, etc.

[0039] Preferably, in the LF furnace refining, the active oxygen range of the LF furnace refining process is controlled to meet 24≤Y value≤57.5; Y value=(5×Mn / S+LF active oxygen value) / 2, (the formula mainly highlights the combined effect of two factors, the Mn / S ratio multiplied by 5 is mainly a factor obtained from experience, and the final average value of the sum of the two factors affects the factor Y value) is the sulfide control factor, and when 24≤Y value≤57.5, the final sulfide is properly controlled.

[0040] Mn / S is the ratio of the mass percentage of Mn to the mass percentage of S in the composition of the above-mentioned high-quality free-cutting steel;

[0041] The LF active oxygen value is the active oxygen in the LF furnace refining process, in ppm;

[0042] When calculating the above Y value, substitute the value before the above unit into the formula.

[0043] The continuous casting adopts induction heating of the tundish, and the temperature of the molten steel in the tundish is controlled at 15-30°C above the liquidus temperature. The specification of the continuous casting billet is 160mm×160mm 2 .

[0044] Furthermore, the entire continuous casting process uses argon sealing to protect the casting. The steelmaking exit temperature is adjusted according to the baking temperature of the tundish. The pouring superheat is controlled at 15-30°C, and the corresponding drawing speed is selected as 1.6-2.0m / min. The crystallizer water flow rate used for primary cooling is 2400-2600L / min. The plug rod curve is strictly controlled, and the secondary cooling adopts the cooling curve corresponding to the drawing speed. The end electromagnetic stirring parameters + light pressure are used for pulling and straightening. Among them, the end electromagnetic stirring parameters use a current of 360-380A; a frequency of 3.0-6.0Hz; the total light pressure reduction rate is 20mm, and the first five racks are pressed down, with a reduction of 3-5-5-4-3mm respectively. By using the end electromagnetic stirring + light pressure method, the 160mm×160mm 2 The casting quality of small square billets can be improved, while reducing the center segregation phenomenon of this type of steel with relatively high sulfur content.

[0045] The present invention adopts a combined process of electromagnetic stirring at the end of solidification and light pressure to improve the casting quality of 160mm×160mm small square billets, reduce the center segregation phenomenon, and cooperate with a suitable rolling process to finally obtain a casting with excellent quality, a high proportion of sulfide spindle rate in the finished wire rod, and excellent cutting performance.

[0046] In the rolling, the heating and soaking temperature is controlled at 1220-1270°C, and the heating time is ≥120min; wherein, the air-fuel ratio of the heating furnace should be controlled at 0.4-0.7, the preheating section temperature is 900°C, the heating section temperature is 1000-1100°C, the heating section temperature is 1150-1250°C, and the soaking section temperature is 1220-1270°C; wherein, the soaking section time should be guaranteed to be ≥60min. In addition to ensuring the uniformity of the temperature inside and outside the steel billet, heating in this range also promotes the melting and remelting of sulfides, so that the size of the sulfides is transformed into a large spindle shape, which ensures the morphology of the sulfides in the finished product.

[0047] The rolling also includes that the temperature after leaving the furnace and removing phosphorus is not lower than 1160°C, the wire drawing temperature is 820-910°C, all the insulation covers of the Stelmor cooling line are opened, the first three fans are turned on, and the fan air volume is determined according to the size of the steel grade, so that the wire rod is quickly cooled to below 550°C, and the cooling rate is 3-5°C / s.

[0048] The invention adopts a high-temperature heating process for rolling, and the rolling process includes rough rolling, finish rolling and wire laying, and the steel is cooled by a Stelmor cooling line after rolling.

[0049] The final ingot is of excellent quality, the center segregation is stably controlled at ≤1.0 level, the sulfide spindle rate accounts for ≥45%, and the finished wire rod has excellent cutting performance.

[0050] Further, it is processed into Φ5.5~20mm hot-rolled wire rod.

[0051] The finished wire rods produced above are sent to customers. The customer's processing technology is generally: mechanical shelling / shot blasting → primary straightening → one-pass drawing → fine straightening → cutting (sizing) → straightening (polishing) → chamfering → oil soaking → packaging. Finally, it is sent to downstream customers for cutting processing.

[0052] Several specific embodiments and comparative examples of the present invention are as follows:

[0053] Example 1-Example 5

[0054] A high quality free-cutting steel comprises the following compositions in mass percentage: as shown in Table 1, the remainder not shown in Table 1 is Fe and unavoidable impurities.

[0055] Comparative Example 1-Comparative Example 6

[0056] A free-cutting steel comprises the following components in mass percentage: as shown in Table 1, the remainder not shown in Table 1 is Fe and inevitable impurities.

[0057] Table 1 Alloy composition of each embodiment and comparative example (unit: O, N in ppm, others in wt%)

[0058] Serial number C Si Mn P S Al N O Mn / S Example 1 0.06 0.003 1.45 0.05 0.38 0.03 25 106 3.8 Example 2 0.07 0.002 1.49 0.06 0.37 0.03 30 100 4.0 Example 3 0.06 0.003 1.46 0.05 0.39 0.02 26 105 3.7 Example 4 0.07 0.003 1.47 0.05 0.39 0.03 28 110 3.8 Example 5 0.06 0.003 1.48 0.05 0.38 0.02 26 102 3.9 Comparative Example 1 0.06 0.003 1.48 0.04 0.49 0.02 30 110 3.0 Comparative Example 2 0.07 0.002 1.47 0.05 0.39 0.02 26 168 3.8 Comparative Example 3 0.07 0.002 1.49 0.04 0.38 0.03 28 99 3.9 Comparative Example 4 0.06 0.001 1.48 0.06 0.39 0.02 25 95 3.8 Comparative Example 5 0.06 0.002 1.48 0.05 0.40 0.02 30 102 3.7 Comparative Example 6 0.06 0.002 1.48 0.05 0.39 0.02 26 98 3.8

[0059] The free-cutting steels of the above embodiments and comparative examples were produced according to the following process flow:

[0060] Converter smelting - LF furnace refining - continuous casting - rolling;

[0061] The specific control of LF refining is as follows: the whole process is controlled with the final active oxygen range of 30-90ppm as the key point.

[0062] Preferably, in the LF furnace refining, the active oxygen range of the LF furnace refining process is controlled to meet 24≤Y value≤57.5; Y value=(5×Mn / S+LF active oxygen value) / 2, which is a sulfide control factor. When 24≤Y value≤57.5, the final sulfide is properly controlled.

[0063] The continuous casting adopts induction heating of the tundish and argon sealing to protect the casting throughout the whole process, and the corresponding drawing speed is selected as 1.6-1.8m / min; the water flow rate of the crystallizer used for primary cooling is 2400L / min; the combined method of end electromagnetic stirring parameters + light pressure is adopted for drawing and straightening, wherein the end electromagnetic stirring parameters are selected as current 360A; frequency 3.0Hz; the total reduction rate of light pressure is 20mm, and the reduction is carried out on the first five frames, and the reduction amounts are 3-5-5-4-3mm respectively.

[0064] In the rolling, the heating and soaking temperature is controlled at 1220-1270°C, and the heating time is ≥120min; wherein, the air-fuel ratio of the heating furnace should be controlled at 0.4-0.7, the preheating section temperature is 900°C, the heating section temperature is 1000-1100°C, the heating section temperature is 1150-1250°C, and the soaking section temperature is 1220-1270°C; wherein, the soaking section time should be guaranteed to be ≥60min. In addition to ensuring the uniformity of the temperature inside and outside the steel billet, heating in this range also promotes the melting and remelting of sulfides, so that the size of the sulfides is transformed into a large spindle shape, which ensures the morphology of the sulfides in the finished product.

[0065] The rolling also includes that the temperature after leaving the furnace and removing phosphorus is not lower than 1160°C, the wire drawing temperature is 820-910°C, all the insulation covers of the Stelmor cooling line are opened, the first three fans are turned on, and the fan air volume is determined according to the size of the steel grade, so that the wire rod is quickly cooled to below 550°C, and the cooling rate is 3-5°C / s.

[0066] Table 2 Main parameters of each embodiment and comparative example

[0067]

[0068]

[0069] The free-cutting steels produced in the above embodiments and comparative examples were graded using the GBT1979-2001 structural steel macrostructure defect rating chart. The results are shown in Table 3.

[0070] Table 3 Results of low-power inspection of various embodiments and comparative examples (grade)

[0071] Serial number Loose center Center Segregation Generally loose Shrinkage Middle crack Center Crack Example 1 1.0 1.0 0 0 0 0.5 Example 2 0 1.0 0 0.5 0 0.5 Example 3 1.0 0.5 0 0.5 0 0 Example 4 0 0.5 0 0.5 0 0 Example 5 1.0 0.5 0 0 0 0 Comparative Example 1 1.5 1.5 0 0.5 0 1.0 Comparative Example 2 1.0 1.5 0 0.5 0 0.5 Comparative Example 3 2.0 2.0 0 1.0 0 1.0 Comparative Example 4 1.5 2.0 0 1.0 0 1.0 Comparative Example 5 1.5 1.5 0 0.5 0 0.5 Comparative Example 6 1.5 1.5 0 0.5 0 0.5

[0072] Sulfide detection sampling was taken from the 1 / 2 radius of the cross section of the ingot. 10×10 specimens were randomly selected from 3 locations for sulfide detection. The sulfide ratio was evaluated according to the microscopic inspection method of the rating chart for the determination of the content of non-metallic inclusions in steel in GB-T 10561-2005. The sulfide ratio is the ratio of the number of characteristic sulfides to the total sulfides. The results are shown in Table 4.

[0073] Table 4 Aspect ratio of sulfide particle size in each embodiment and comparative example (% ratio)

[0074]

[0075]

[0076] The hot-rolled wire rods rolled in the above embodiments and comparative examples are of a finished product specification of 9 mm. It is assumed that when the aspect ratio of the sulfide is ≤3, it is a spindle-shaped sulfide, and the cutting performance of the finished product is better. The shape of the sulfide with an aspect ratio of >9 is closer to a long strip, and its anisotropy is greater, resulting in a greatly increased transverse cracking tendency.

[0077] Figure 2 The sulfide morphology of Example 1 is shown in FIG. 2. The statistics of the sulfide morphology are as follows: 20 photos are taken at different locations, and the aspect ratios of the sulfides are compared using software for statistics.

[0078] Among them, the Mn / S ratio of comparative example 1 exceeds the range of 3.6-5.0, the LF active oxygen range of comparative example 2 exceeds the range of 30-90ppm, and the Y value exceeds the range of 24≤Y≤57.5; comparative example 3 does not use the light reduction process, and the reduction amount of comparative example 4 is less than 20mm; the heating temperature of comparative example 5 exceeds the range of 1220-1270℃, and the soaking section holding time of comparative example 6 is less than 60min. All of these lead to a significant reduction in the quality of the product casting, and do not meet the requirements of central porosity ≤1.0 level, central segregation ≤1.0 level, shrinkage ≤0.5 level, and central crack ≤1.0 level. And when the aspect ratio is ≤3, the proportion of spindle-shaped sulfides does not meet ≥45%, resulting in reduced cutting performance of the product.

[0079] The underlined data above do not meet the requirements of the present invention.

[0080] The description of the above embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the protection scope of the present invention.

Claims

1. A high quality free-cutting steel, characterized in that: The high quality free-cutting steel comprises the following components in percentage by mass: C≤0.09%, Si≤0.006%, Mn: 0.90~1.60%, P: 0.04~0.09%, S: 0.30~0.50%, Al≤0.030%, N≤0.0060%, O≤0.0140%, and the rest are Fe and other inevitable impurities.

2. The high-quality free-cutting steel according to claim 1, characterized in that: The composition of the high-quality free-cutting steel also satisfies Mn / S=3.6-5.

0.

3. A process for producing high-quality free-cutting steel according to claim 1 or 2, characterized in that: The production process includes the following process flow: converter smelting-LF furnace refining-continuous casting-rolling.

4. The production process according to claim 3, characterized in that: In the LF refining, the active oxygen range is controlled to be 30-90 ppm.

5. The production process according to claim 3, characterized in that: The LF furnace refining controls the active oxygen range of the LF furnace refining process to meet 24≤Y value≤57.5; Y value=(5×Mn / S+LF active oxygen value) / 2.

6. The production process according to claim 3, characterized in that: The continuous casting adopts induction heating of the tundish and argon sealing to protect the casting throughout the whole process.

7. The production process according to claim 3 or 6, characterized in that: The tension straightening is carried out by combining the terminal electromagnetic stirring parameters and light pressure, wherein the terminal electromagnetic stirring parameters are: current 360-380A, frequency 3.0-6.0Hz; the total pressure reduction rate of light pressure is 15-20mm.

8. The production process according to claim 3, characterized in that: During the rolling, the heating and soaking temperature is controlled at 1220-1270° C., and the soaking time is ≥60 min.

9. The production process according to any one of claims 3 to 8, characterized in that: The high-quality free-cutting steel produced has ingot center porosity ≤1.0, center segregation ≤1.0, shrinkage ≤0.5, center crack ≤1.0, and sulfide spindle rate ≥45%.

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