A large-diameter coiled easy-to-cut steel and a production process thereof

Abstract

The present application provides a kind of big coil easy cutting steel and its production process, component: C≤0.09%, Si≤0.005%, Mn:0.90~1.65%, P:0.04~0.09%, S:0.30~0.50%, Al≤0.030%, N≤0.0060%, O≤0.0140%, the balance is Fe and inevitable impurities.Compared with prior art, the present application controls LF furnace refining process activity oxygen range, cooperates with suitable big coil production process parameters, including soaking temperature, soaking time, opening rolling temperature, coiling temperature, controls coiling temperature range=J±15℃, J=(4×Mn / S+LF activity oxygen)+800℃.In this temperature interval, the final product sulfide spindle is properly controlled when coiling.The final surface quality is excellent, the product rod sulfide spindle rate is high, C-shaped scrap accounts for more than 90%, and the cutting performance is excellent.

Description

Technical Field

[0001] This invention belongs to the field of free-cutting steel technology, and particularly relates to a large coil of free-cutting steel and its production process. Background Technology

[0002] With the development of high-speed, precision, and automation in machining, especially the booming automotive, precision instrument, and home appliance industries, end users have an increasingly strong demand for reducing processing costs by improving the machinability of steel. High-quality free-cutting steel urgently needs development. Free-cutting steel is mainly used to manufacture non-load-bearing parts with lower stress requirements but strict requirements for dimensions and surface finish, such as instrument and watch parts, automotive parts, machine tools, and various other machines; and standard parts with strict requirements for dimensional accuracy and surface finish but relatively lower requirements for mechanical properties, such as gears, shafts, bolts, valves, bushings, pins, and pipe fittings.

[0003] Currently, the free-cutting steel on the market is mainly in the form of small-diameter wire rods, while large-coil free-cutting steel products with diameters exceeding 16mm are rarely involved. Besides constraints imposed by production line conditions, the main reason is that large-diameter coils, due to fewer rolling passes, have more difficult-to-heal internal defects compared to wire rods. Simultaneously, the reduced compression ratio results in less complete sulfide fragmentation, significantly impacting the hardness, microstructure, and machinability of the finished material. For sulfur-containing steel, sulfide morphology is defined as spindle-shaped sulfides with an aspect ratio ≤3. Finished products with a high proportion of spindle-shaped sulfides exhibit excellent machinability. However, the factors influencing sulfide morphology are complex. Besides the steelmaking process, the selection of continuous casting processes and the formulation of rolling conditions both affect the final morphology.

[0004] Chinese patent CN 117259432 A, published on December 22, 2023, discloses a rolling method for large coil free-cutting steel. By controlling process parameters such as heating temperature, heating time, initial rolling temperature, descaling pressure, pre-PSM temperature, coiling temperature, rolling speed, and the opening and closing of the walking beam insulation, it avoids the indentation of iron oxide scale generated by high-temperature heating in the furnace and secondary iron oxide scale formed during rolling into the finished product, and reduces the thickness of iron oxide scale on the large coil free-cutting steel. This satisfies the downstream customer's requirement for a finished product surface free of pitting and dents, meeting the requirements for machinability and surface quality stability at the downstream user's site, ensuring smooth drawing and cutting. However, this process is limited to controlling the rolling process and focuses primarily on controlling the surface quality of the large coil, without addressing the control of its machinability.

[0005] Chinese patent CN 113714281 A, published on November 30, 2021, discloses a method for producing Ф22mm large-diameter hot-rolled wire rod. The method primarily involves S1 and converter smelting, focusing on reducing impurities and their content in the molten iron to prevent insufficient flexibility in the wire rod due to excessive impurities. Simultaneously, appropriate billet heating and slow cooling processes are employed to ultimately produce Ф22mm large-diameter hot-rolled wire rod. While this process involves controlling the steelmaking and rolling processes, it mainly focuses on the performance improvement effect of raw material selection and does not address specific methods for controlling machinability.

[0006] Chinese patent CN 105234170 A, published on January 13, 2016, discloses a method for rolling large-size free-cutting steel. By constraining processes such as the preheating and soaking stages of the heating zone and the initial rolling temperature, and by defining processes such as high-pressure water descaling, flying shear, finishing rolling temperature, and controlled cooling, a method is ultimately obtained to produce large-size free-cutting steel with fewer surface defects and no central shrinkage cavities. However, this process also does not address the influence of sulfide control factors.

[0007] Due to different production paths, the key control points for large-size free-cutting steel are not entirely the same. At the same time, since the compression ratio is lower than that of wire rod, the fragmentation of sulfides will also change. Therefore, it is necessary to develop a production process for high-quality free-cutting steel in large coils. Summary of the Invention

[0008] The purpose of this invention is to provide a large-coil free-cutting steel and its production process. By controlling the composition and process, the coil production process is constrained to achieve large-size coil production and ensure that the surface quality and machinability of the final finished coil meet the requirements.

[0009] The specific technical solution of this invention is as follows:

[0010] A free-cutting steel in large coils comprises the following composition by weight percentage: C ≤ 0.09%, Si ≤ 0.005%, Mn: 0.90–1.65%, P: 0.04–0.09%, S: 0.30–0.50%, Al ≤ 0.030%, N ≤ 0.0060%, O ≤ 0.0140%, with the balance being Fe and unavoidable impurities.

[0011] The composition of the large coil free-cutting steel also meets the requirement of Mn / S: 3.5~6.0.

[0012] This invention provides a production process for large coils of free-cutting steel, comprising the following steps:

[0013] Converter smelting - LF furnace refining - continuous casting - large coil rolling.

[0014] The -LF furnace refining process controls the active oxygen range to 25-50 ppm.

[0015] The large coil rolling process involves heating and homogenizing at a temperature of 1240-1280℃ for a homogenization period of ≥70 minutes.

[0016] The large coil rolling process involves a re-rolling temperature of 1130-1180℃ after dephosphorization at the furnace.

[0017] The large coil rolling process is controlled at a coiling temperature of J ± 15℃; where J = (4 × Mn / S + LF active oxygen) + 800℃. The Mn / S ratio multiplied by 4 is mainly an empirically derived factor, and its temperature difference from the commonly used 800℃ ± 15℃ is determined based on 4 × Mn / S and LF active oxygen.

[0018] After winding, it is cooled slowly at a rate of 3-5℃ / s.

[0019] The products are large coils of free-cutting steel with diameters ranging from Φ16 to 50 mm.

[0020] The spindle-shaped sulfides in the finished free-cutting steel coil account for ≥45% of the total sulfides;

[0021] The large coils of free-cutting steel produced have a C-type chip ratio of ≥90% when the finished wire rods are cut with a feed rate f = 0.10 mm / r, a cutting speed of 2000 r / min, and a cutting depth of 0.5 mm.

[0022] To obtain a good sulfide inclusion morphology, it is necessary to ensure a suitable [Mn] / [S] ratio and oxygen content. Compared with other elements, sulfur in steel tends to combine with manganese to form MnS. Therefore, the Mn / S content ratio in steel is crucial and can affect the precipitation of MnS inclusions. The sulfide spindle shape is closely related to the manganese-sulfur ratio. At the same time, increasing the oxygen content can also make the sulfides short and thick, which is beneficial to the machinability of the steel. However, excessive oxygen content will lead to defects such as subcutaneous porosity in the final cast billet. This invention controls the oxygen content by controlling the LF activity oxygen, and the two work synergistically. Based on previous research, under the conditions specified in this invention, the spindle shape of the sulfides in the large coil is best when the manganese / sulfur ratio is in the range of 3.5-6.0 and the coiling temperature is controlled within the range of J ± 15℃ (where J = (4 × Mn / S + LF activity oxygen) + 800℃).

[0023] The steel of this invention is produced via a production path of converter smelting - LF furnace refining - continuous casting - large coil rolling. The active oxygen range during the LF furnace refining process is controlled, along with appropriate large coil production process parameters, including soaking temperature, soaking time, initial rolling temperature, and coiling temperature. The coiling temperature is controlled within the range of J ± 15℃, where J = (4 × Mn / S + LF active oxygen) + 800℃. Within this temperature range, the final finished product exhibits well-controlled sulfide spindle shape. This results in excellent surface quality, a high proportion of sulfide spindles in the finished coil, a C-shaped chip percentage exceeding 90%, and excellent machinability. Attached Figure Description

[0024] Figure 1 The image shows the microstructure of the finished product from Example 1.

[0025] Figure 2 Photograph of the finished sulfide product from Example 1. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] The present invention provides a large coil free-cutting steel comprising the following mass percentages: C≤0.09%, Si≤0.005%, Mn:0.90~1.65%, P:0.04~0.09%, S:0.30~0.50%, Al≤0.030%, N≤0.0060%, O≤0.0140%, with the balance being Fe and unavoidable impurities.

[0028] The composition of the large coil free-cutting steel also meets the requirement of Mn / S: 3.5~6.0.

[0029] This invention provides a production process for large coils of free-cutting steel, comprising the following steps:

[0030] Converter smelting - LF furnace refining - continuous casting - large coil rolling.

[0031] in:

[0032] The converter smelting process primarily involves deoxidation and alloying. The main alloying raw materials for the steel grades of this invention are lime, wollastonite, ferroaluminum, ferrophosphorus, low-carbon ferromanganese, and high-sulfur ferroalloys. During the converter tapping process, the composition of C, Si, Mn, P, and S is basically adjusted to the required level, with fine-tuning performed by the LF (sulfur-free furnace). This operation aims to reduce the additional refining time and heating steps that may be required for adjusting the composition via LF, thereby reducing the risk of increased oxygen content in the steel.

[0033] The LF furnace refining process involves fine-tuning the alloy composition and controlling the addition of slag to ensure appropriate basicity. The entire process controls the active oxygen range, indirectly ensuring the oxygen content in the final product. Specifically: after the molten steel is placed in the furnace, bottom-blown argon is used to stir it at a flow rate of 700–1000 NL / min. The temperature is increased according to the inlet temperature, alloy addition amount, and outlet temperature. The refining slag is a low-basicity slag with a basicity of 2.4–2.8 to ensure sulfur recovery. The key control point throughout the process is maintaining a final active oxygen range of 25–50 ppm. If the oxygen content exceeds this range, aluminum ferrometallurgy is used to deoxidize the molten steel, and silicon carbide is used for slag surface deoxidation to ensure a deoxidation balance between the molten steel and slag surface. The added alloys include ferrosulfide, ferrophosphorus, low-carbon ferromanganese, and high-sulfur ferrosulfide alloys.

[0034] The continuous casting process employs induction heating in the tundish, with the molten steel temperature controlled at 15–30°C above the liquidus temperature. A combination of electromagnetic stirring and light pressure is used to improve billet quality and address the center segregation phenomenon that is highly prevalent in this type of steel. The billet dimensions are 160mm × 160mm. 2 Specifically: Argon sealing is used for the entire continuous casting process; the steelmaking exit temperature is adjusted according to the tundish baking temperature; the superheat of the casting is controlled between 15 and 30°C; the corresponding casting speed is 1.6-1.8 m / min; the crystallizer water flow rate for primary cooling is 2400 L / min; the stopper rod curve is strictly controlled; the secondary cooling uses a cooling curve corresponding to the casting speed; and a combination of end electromagnetic stirring parameters and light pressure is used for straightening to reduce the center segregation phenomenon of this type of steel with relatively high sulfur content.

[0035] The large coil rolling process employs a high-temperature heating technique, involving roughing, finishing, a large coil reducing mill, and a coiler. After rolling, the coils are cooled via a vertical core-lift transport line and a PF line. Specifically, the heating and soaking temperature is controlled at 1240-1280℃, with a heating time ≥200 min. The air-fuel ratio in the heating furnace should be controlled at 0.4-0.7. The preheating zone temperature is 950℃, the first heating zone temperature is 950-1160℃, the second heating zone temperature is 1160-1260℃, and the soaking zone temperature is 1240-1280℃. The soaking zone time should be ≥70 min. Besides ensuring the uniformity of temperature inside and outside the steel billet, this heating range also promotes the melting and remelting of sulfides, resulting in larger, spindle-shaped sulfides, ensuring the correct sulfide morphology in the finished product. After descaling, the initial rolling temperature is 1130-1180℃, and the coiling temperature is controlled within the range of J ± 15℃. Wherein, J=(4×Mn / S+LF activity oxygen)+800℃, the cooling route adopts the large coil non-slow cooling material cooling channel, the cooling channel has no heat insulation cover, the first three fans are turned on, the fan air volume is determined according to the steel grade and specifications, so that the coil is cooled to below 550℃, the cooling rate is 3-5℃ / s.

[0036] The finished wire rod is delivered to the customer, whose processing steps typically involve: mechanical peeling / shot blasting → initial straightening → one-pass drawing → precision straightening → cutting (to length) → straightening (polishing) → chamfering → oil soaking → packaging. Finally, it is delivered to the downstream customer for machining.

[0037] The following are several specific embodiments of the present invention:

[0038] Examples 1-5

[0039] A free-cutting steel in large coils comprises the following composition by weight percentage as shown in Table 1, where the balance not shown in Table 1 is Fe and unavoidable impurities.

[0040] Comparative Examples 1-6

[0041] A free-cutting steel comprising the following mass percentage composition as shown in Table 1, where the balance not shown in Table 1 is Fe and unavoidable impurities.

[0042] Table 1. Alloy composition of each embodiment and comparative example (unit: O and N are in ppm, others are in wt%)

[0043]

[0044]

[0045] The cutting steels in the above embodiments and comparative examples are produced according to the following production process: converter smelting - LF furnace refining - continuous casting - large coil rolling.

[0046] in:

[0047] The converter smelting mainly involves deoxidation and alloying. The main alloying raw materials for the steel grades of this invention are lime, wollastonite, ferroaluminum, ferrophosphorus, low-carbon ferromanganese, and high-sulfur ferroalloy. During the converter tapping process, the composition of C, Si, Mn, P, and S is basically adjusted to the required level, and the LF process is used for fine-tuning.

[0048] The LF furnace refining process involves the following steps: After the molten steel is placed in the furnace, bottom-blown argon gas is used to stir it at a flow rate of 700–1000 NL / min. The temperature is increased according to the inlet temperature, the amount of alloy added, and the outlet temperature. The refining slag is a low-basicity slag with a basicity of 2.4–2.8 to ensure sulfur recovery. Throughout the process, the final oxygen activity range of 25–50 ppm is the key control point. If the oxygen content exceeds this range, aluminum ferrometallurgy is used to deoxidize the molten steel, and silicon carbide is used for deoxidation on the slag surface to ensure a deoxidation balance between the molten steel and the slag surface. The added alloy materials include ferrosulfide, ferrophosphorus, low-carbon ferromanganese, and high-sulfur ferrosulfide alloys.

[0049] The continuous casting process employs induction heating in the tundish, with the molten steel temperature controlled at 15-30°C above the liquidus temperature. A combination of electromagnetic stirring and light pressure reduction is used to improve billet quality and address the center segregation phenomenon that is highly prevalent in this type of steel. Specifically, argon sealing is used throughout the casting process for full protection. The steelmaking outlet temperature is adjusted based on the tundish baking temperature, with the pouring superheat controlled at 15-30°C, corresponding to a casting speed of 1.6-1.8 m / min. The primary cooling uses a crystallizer water flow rate of 2400 L / min. The stopper rod curve is strictly controlled, and the secondary cooling uses a cooling curve corresponding to the casting speed. A combination of end-stage electromagnetic stirring parameters and light pressure reduction is used for straightening to reduce center segregation in this type of steel with relatively high sulfur content. The continuous casting billet size is 160mm × 160mm.

[0050] The large coil rolling process involves heating at a homogenization temperature of 1240-1280℃ for a heating time of ≥200 min. The air-fuel ratio in the heating furnace should be controlled at 0.4-0.7. The preheating zone temperature is 950℃, the first heating zone temperature is 950-1160℃, the second heating zone temperature is 1160-1260℃, and the homogenization zone temperature is 1240-1280℃. The homogenization zone time should be ≥70 min. This heating range not only ensures the uniformity of temperature inside and outside the steel billet but also promotes the melting and remelting of sulfides, resulting in larger, spindle-shaped sulfides, thus guaranteeing the sulfide morphology in the finished product. After descaling, the initial rolling temperature is 1130-1180℃, and the coiling temperature is controlled within the range of J ± 15℃. Wherein, J=(4×Mn / S+LF activity oxygen)+800℃, the cooling route adopts the large coil non-slow cooling material cooling channel, the cooling channel has no heat insulation cover, the first three fans are turned on, the fan air volume is determined according to the steel grade and size, so that the wire rod is cooled down to below 550℃.

[0051] The finished wire rod is delivered to the customer, whose processing steps typically involve: mechanical peeling / shot blasting → initial straightening → one-pass drawing → precision straightening → cutting (to length) → straightening (polishing) → chamfering → oil soaking → packaging. Finally, it is delivered to the downstream customer for machining.

[0052] The main production process parameters of the above embodiments and comparative examples are shown in Table 2 below.

[0053] Table 2. Main production parameters for each embodiment and comparative example.

[0054]

[0055]

[0056] The aspect ratio of sulfide particles in the finished wire rods produced in the above embodiments and comparative examples is shown in Table 3. According to the microscopic inspection method of the rating chart for the determination of non-metallic inclusions in steel in GB-T 10561-2005, the sulfide ratio is the proportion of this characteristic sulfide to the total amount of sulfides.

[0057] Table 3. Aspect ratio of sulfide particles in each example and comparative example (percentage %)

[0058] Serial Number Less than or equal to 3 Greater than 3 and less than or equal to 6 Greater than 6 and less than or equal to 9 Greater than 9 Example 1 47.5 31.3 12.6 8.6 Example 2 46.6 34.4 11.2 7.8 Example 3 47.1 32.4 12.3 8.2 Example 4 47.6 31.7 11.6 9.1 Example 5 46.4 34.1 9.2 10.3 Comparative Example 1 32.4 32.1 19.8 15.7 Comparative Example 2 29.2 36.7 18.6 15.5 Comparative Example 3 25.5 32.2 23.4 18.9 Comparative Example 4 30.1 34.1 21.2 14.6 Comparative Example 5 25.8 34.8 25.5 13.9 Comparative Example 6 38.2 33.8 17.4 10.6

[0059] The cutting performance of the finished wire rods produced in the above embodiments and comparative examples is compared in Table 4.

[0060] Table 4 Comparison of cutting performance of each embodiment and comparative example

[0061]

[0062]

[0063] In all embodiments and comparative examples, the rolled product is a 25mm large coil. It is assumed that when the aspect ratio of the sulfide is ≤3, it is a spindle-shaped sulfide, resulting in better machinability. Sulfides with an aspect ratio >9 are more elongated, exhibiting greater anisotropy and a significantly increased tendency for transverse cracking. Table 4 compares the machinability of each embodiment and comparative example. Under the same cutting conditions and cutting amounts, the proportion of C-shaped chips in the chip shape after cutting is statistically analyzed. A higher proportion of C-shaped chips indicates superior material machinability and less tool wear.

[0064] The Mn / S ratio of Comparative Example 1 exceeded the range of 3.5 to 6.0, the LF activity oxygen range of Comparative Example 2 exceeded the range of 25-50 ppm, and the final total oxygen content exceeded 0.0140%. The soaking temperature of Comparative Example 3 was lower than the range of 1240-1280℃, the holding time of Comparative Example 4 was less than 70 min, the rolling temperature of Comparative Example 5 exceeded the range of 1130-1180℃, and the coiling temperature of Comparative Example 6 exceeded the range. All of these factors led to a decrease in the proportion of spindle-shaped sulfides when the aspect ratio of the finished product was ≤3, and a decrease in machinability.

[0065] Figure 1 This is a microscopic tissue photograph of Example 1. Figure 2 Example 1: Sulfide photographs. Twenty photographs of different parts of the sulfide morphology were taken, and the aspect ratios of the sulfides were compared using software for statistical analysis.

[0066] The data underlined above do not meet the requirements of this invention.

[0067] The above description of the embodiments is intended to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any 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 invention should be within the protection scope of the present invention.

Claims

1. A large coil free-cutting steel, characterized in that, The large coil free-cutting steel comprises the following components in percentage by mass: C≤0.09%, Si≤0.005%, Mn: 0.90~1.65%, P: 0.04~0.09%, S: 0.30~0.50%, Al≤0.030%, N≤0.0060%, O≤0.0140%, and the balance of Fe and inevitable impurities; The components of the large coil free-cutting steel also satisfy Mn / S: 3.5~6.0; In the production of the large coil free-cutting steel, the coiling temperature is =J±15℃; wherein, J=(4×Mn / S+LF activity oxygen)+800℃; The large coil free-cutting steel is a large coil free-cutting steel with a diameter of Φ16~50mm. The proportion of spindle sulfides with a length-width ratio of the finished product of the large coil free-cutting steel is ≥45%.

2. A process for the production of the large coil easy cutting steel according to claim 1, characterized in that, The production process comprises converter smelting-LF furnace refining-continuous casting-large coil rolling.

3. The production process according to claim 2, characterized in that, The LF furnace refining controls the activity oxygen range to be 25-50ppm.

4. The production process according to claim 2, characterized in that, The large coil rolling controls the heating soaking temperature to be 1240-1280℃, and the soaking time is ≥70min.

5. The production process according to claim 2, characterized in that, The large coil rolling controls the opening rolling temperature to be 1130-1180℃.

6. The production process according to claim 2 or 5, characterized in that, The large coil rolling controls the coiling temperature to be =J±15℃; wherein, J=(4×Mn / S+LF activity oxygen)+800℃.

7. The production process according to any one of claims 2 to 6, characterized in that, The large coil free-cutting steel produced is a large coil free-cutting steel with a diameter of Φ16~50mm.

8. The production process according to any one of claims 2 to 7, characterized in that, The proportion of spindle sulfides with a length-width ratio of the finished product of the large coil free-cutting steel is ≥45%.

9. The production process according to any one of claims 2-8, characterized in that, The large coil free-cutting steel produced is a large coil free-cutting steel with a diameter of Φ16~50mm. The proportion of spindle sulfides with a length-width ratio of the finished product of the large coil free-cutting steel is ≥45%. The large coil free-cutting steel produced is a large coil free-cutting steel with a diameter of Φ16~50mm. The proportion of spindle sulfides with a length-width ratio of the finished product of the large coil free-cutting steel is ≥45%. The large coil free-cutting steel produced is a large coil free-cutting steel with a diameter of Φ16~50mm. The proportion of spindle sulfides with a length-width ratio of the finished product of the large coil free-cutting steel is ≥45%. The large coil free-cutting steel produced is a large coil free-cutting steel with a diameter of Φ16~50mm. The proportion of spindle sulfides with a length-width ratio of the finished product of the large coil free-cutting steel is ≥45%. The large coil free-cutting steel produced is a large coil free-cutting steel with a diameter of Φ16~50mm. The proportion of spindle sulfides with a length-width ratio of the finished product of the large coil free-cutting steel is ≥45%. The large coil free-cutting steel produced is a large coil free-cutting steel with a diameter of Φ16~50mm. The proportion of spindle sulfides with a length-width ratio of the finished product of the large coil free-cutting steel is ≥45%. The large coil free-cutting steel produced is a large coil free-cutting steel with a diameter of Φ16~50mm. The proportion of spindle sulfides with a length-width ratio of the finished product of the large coil free-cutting steel is ≥45%. The large coil free-cutting steel produced is a large coil free-cutting steel with a diameter of Φ16~50mm. The proportion of spindle sulfides with a length-width ratio of the finished product of the large coil free-cutting steel is ≥45%. The large coil free-cutting steel produced is a large coil free-cutting steel with a diameter of Φ16~50mm. The proportion of spindle sulfides with a length-width ratio of the finished product of the large coil free-cutting steel is ≥45%. The large coil free-cutting steel produced is a large coil free-cutting steel with a diameter of Φ16~50mm. The proportion of spindle sulfides with a length-width ratio of the finished product of the large coil free-cutting steel is ≥45%. The large coil free-cutting steel produced is a large coil free-cutting steel with a diameter of Φ16~50mm. The proportion of spindle sulfides with a length-width ratio of the finished product of the large coil free-cutting steel is ≥45%. The large coil free-cutting steel

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