Large-coil free-cutting steel and production process thereof

Abstract

The invention provides large-coil free-cutting steel and a production process thereof. The large-coil free-cutting steel comprises the following components: less than or equal to 0.09% of C, less than or equal to 0.005% of Si, 0.90-1.65% 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 inevitable impurities. Compared with the prior art, the method has the advantages that the active oxygen range in the LF refining process is controlled, appropriate large coil production process parameters including soaking temperature, soaking time, rolling temperature and coiling temperature are matched, the coiling temperature range is controlled to be J + / -15 DEG C, and J is equal to (4 * Mn / S + LF active oxygen) + 800 DEG C. And the fusiform control of the final finished product sulfide coiled in the temperature interval is proper. And finally, the obtained steel wire rod is excellent in surface quality, the finished steel wire rod sulfide spindle ratio is high, the C-shaped chips account for more than 90%, and the cutting performance is excellent.

Description

Technical Field

[0001] The invention belongs to the technical field of free-cutting steel, and in particular relates to a large coil 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, the free-cutting steel on the market is mainly small-sized wire rods, while large coils with a diameter of more than 16mm are rarely involved. In addition to being constrained by production line conditions, the main reason is that due to the reduction in rolling passes, the defects inside the billet of large-sized coils are more difficult to heal than those of wire rods. At the same time, the reduction in compression ratio will also cause the sulfide fragmentation to be not thorough enough, and the material hardness, microstructure and cutting performance of the finished material are greatly affected. For the sulfide morphology of sulfur-containing steel, it is determined that the sulfide aspect ratio is ≤3, which is a spindle-shaped sulfide. The finished product with a high proportion of spindle-shaped sulfides has excellent cutting performance, and the factors affecting the sulfide morphology are relatively complex. In addition to the steelmaking process, the selection of continuous casting process and the formulation of rolling conditions will affect its final form.

[0004] The Chinese patent with publication number CN 117259432 A published on December 22, 2023, discloses a rolling method for free-cutting steel in large coils. By controlling the heating temperature, heating time, rolling temperature, descaling pressure, pre-PSM temperature, coiling temperature, rolling speed, walking beam insulation opening and closing and other process parameters, the iron oxide scale generated by the high temperature heating of the heating furnace and the secondary iron oxide scale formed during the rolling process are prevented from being pressed into the finished product and the thickness of the free-cutting steel iron oxide scale of the large coil is reduced, which satisfies the downstream customers that the surface of the finished product has no pitting and pit defects after processing, and satisfies the cutting performance and surface quality stability of the downstream users, ensuring that users can smoothly draw and cut. However, this process is limited to the control of the rolling process, and focuses on the control of the surface quality of the large coil, and does not involve the control of its cutting performance.

[0005] The Chinese patent with publication number CN 113714281 A published on November 30, 2021, discloses a method for producing large-size hot-rolled wire rods with a diameter of 22 mm, which is mainly smelted through S1 and converter, mainly by reducing the impurities and content in the smelting molten iron to avoid the late wire rods containing more impurities and resulting in insufficient flexibility. At the same time, appropriate billet heating and stacking slow cooling processes are adopted to finally produce large-size hot-rolled wire rods with a diameter of 22 mm. Although this process involves the control of steelmaking and rolling processes, it mainly focuses on the role of the selection of steelmaking raw materials in improving performance, and does not involve specific easy-to-cut control methods.

[0006] The Chinese patent with publication number CN 105234170 A published on January 13, 2016, discloses a large-size free-cutting steel rolling method, which finally obtains large-size free-cutting steel with fewer surface defects and no shrinkage cavity in the center by constraining the preheating section, soaking section and rolling temperature of the heating section, and formulating processes such as high-pressure water dephosphorization, flying shear, finishing rolling temperature and controlled cooling. However, the process also does not involve the influence of the control factor of sulfide.

[0007] Due to the different production paths, the control points of large-size free-cutting steel are also different. At the same time, since the compression ratio is lower than that of wire rod, the fragmentation of sulfides will also change. It is necessary to develop a production process for large-scale high-quality free-cutting steel. Summary of the invention

[0008] The purpose of the present invention is to provide a large coil free-cutting steel and its production process, through the composition and process control, the coil process is constrained, the large-scale coil production is achieved, and the surface quality and cutting performance of the final finished coil are ensured to meet the requirements.

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

[0010] A large coil free-cutting steel comprises the following components in mass 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%, and the balance is Fe and unavoidable impurities.

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

[0012] The present invention provides a production process for large coil free-cutting steel, comprising the following processes:

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

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

[0015] During the rolling of the large coil, the heating and soaking temperature is controlled at 1240-1280° C., and the soaking time is ≥70 minutes.

[0016] The large coil is rolled, and the rolling temperature is 1130-1180°C after being removed from the furnace and dephosphorized;

[0017] The coiling temperature of the large coil is controlled to be J±15°C, wherein J=(4×Mn / S+LF active oxygen)+800°C. The Mn / S ratio multiplied by 4 is mainly an empirical factor, and the temperature difference from the commonly used 800°C±15°C is determined by 4×Mn / S and LF active oxygen.

[0018] After coiling, it is slowly cooled with a cooling rate of 3-5℃ / s.

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

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

[0021] The large coil free-cutting steel produced has a cutting index of feed rate f=0.10mm / r, cutting speed 2000r / min, cutting depth 0.5mm, and the proportion of C-type chips is ≥90%.

[0022] 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 Mn and S content ratio 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 mutually synergistic. Based on the previous research, under the conditions specified in the present invention, when the manganese / sulfur ratio is in the range of 3.5-6.0 and the coiling temperature range is controlled = J±15°C (where J = (4×Mn / S+LF active oxygen)+800°C), the spindle-shaped sulfide effect of the finished large coil is the best.

[0023] The steel of the present invention is produced through the production path of converter smelting-LF furnace refining-continuous casting-large coil rolling, wherein the active oxygen range of the LF furnace refining process is controlled, and appropriate large coil production process parameters are matched, including soaking temperature, soaking time, start rolling temperature, and coiling temperature, wherein the coiling temperature range is controlled = J±15°C. Wherein, J = (4×Mn / S+LF active oxygen)+800°C. The spindle shape of the final sulfide product coiled in this temperature range is properly controlled. Finally, the surface quality is excellent, the spindle rate of sulfide in the finished wire rod is high, the proportion of C-shaped chips exceeds 90%, and the cutting performance is excellent. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a microstructure photo of the finished product of Example 1;

[0025] Figure 2 This is a photo of the finished sulfide of Example 1. DETAILED DESCRIPTION

[0026] 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.

[0027] The invention provides a large coil free-cutting steel, which comprises the following components in mass 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%, and the balance is Fe and unavoidable impurities.

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

[0029] The present invention provides a production process for large coil free-cutting steel, comprising the following processes:

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

[0031] in:

[0032] 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.

[0033] The LF furnace refining: fine-tune the alloy composition, control the addition of slag, ensure the appropriate basicity, control the active oxygen range in the whole process, and indirectly ensure the oxygen content in the final product. Specifically: after the molten steel is seated, 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, the basicity is 2.4-2.8, to ensure the sulfur recovery rate; wherein, the whole process is controlled by the final active oxygen range of 25-50ppm. When the oxygen content exceeds, aluminum iron is used to deoxidize the molten steel, and silicon carbide is used to deoxidize the slag surface to ensure the deoxidation balance of the molten steel-slag surface; the added alloy materials are ferrosulfide, ferrophosphorus, low-carbon ferromanganese, high-sulfur ferrosulfur alloy, etc.

[0034] The continuous casting: adopts induction heating in the tundish, the temperature of the molten steel in the tundish is controlled at 15-30°C above the liquidus temperature, and the electromagnetic stirring + light pressure combined process is adopted to improve the quality of the ingot, and solve the central segregation phenomenon that is very easy to occur in this type of steel. The specification of the continuous casting ingot is 160mm×160mm 2 . Specifically: 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 at 1.6-1.8m / min; the crystallizer water flow rate used for primary cooling is 2400L / min; the stopper rod curve is strictly controlled, and the secondary cooling adopts the cooling curve corresponding to the drawing speed. The combined method of end electromagnetic stirring parameters + light pressure is used for drawing and straightening to reduce the central segregation phenomenon of this type of steel with relatively high sulfur elements.

[0035] The large coil rolling adopts a high-temperature heating process, and the rolling is carried out through rough rolling, finishing rolling, large coil reducing mill, and coiling machine. After rolling, it is cooled through the vertical coil core frame transportation line and the PF line. Specifically, the heating and soaking temperature is controlled at 1240-1280℃, and the heating time is ≥200min; wherein, the air-fuel ratio of the heating furnace should be controlled to 0.4-0.7, the preheating section temperature is 950℃, the heating section temperature is 950-1160℃, the heating section temperature is 1160-1260℃, and the soaking section temperature is 1240-1280℃; wherein, the soaking section time should be guaranteed to be ≥70min. 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 sulfides is converted into a large spindle shape, which is a guarantee for the sulfide morphology in the finished product. After leaving the furnace and removing phosphorus, the rolling temperature is 1130-1180℃, and the coiling temperature range is controlled to be J±15℃. Among them, J = (4×Mn / S+LF active oxygen) + 800℃, the cooling route adopts a large coil non-slow cooling material cooling channel, the cooling channel has no insulation cover, the first three fans are turned on, and the fan air volume is determined according to the steel type and specification, so that the wire rod is cooled to below 550℃, and the cooling rate is 3-5℃ / s.

[0036] The finished wire rods are sent to customers, and the customer processing process is generally: mechanical shelling / shot blasting → initial straightening → one-pass drawing → fine straightening → cutting (sizing) → straightening (polishing) → chamfering → oil soaking → packaging. Finally, it is sent to downstream customers for cutting processing.

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

[0038] Example 1-Example 5

[0039] A large coil 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.

[0040] Comparative Example 1-Comparative Example 6

[0041] 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.

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

[0043]

[0044]

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

[0046] in:

[0047] The converter smelting is mainly carried out for deoxidation and alloying. The alloy raw materials of the steel of the present invention are mainly lime, wollastonite, aluminum iron, ferrophosphorus, 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 LF is fine-tuned.

[0048] The LF furnace refining: after the molten steel is seated, 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, and the basicity is 2.4-2.8 to ensure the sulfur recovery rate; wherein, the whole process is controlled by the final active oxygen range of 25-50ppm, when the oxygen content exceeds, the molten steel is deoxidized by aluminum iron, and the slag surface is deoxidized by silicon carbide to ensure the deoxidation balance of the molten steel-slag surface; the added alloy material is ferrosulfide, ferrophosphorus, low-carbon ferromanganese, high-sulfur ferrosulfur alloy, etc.

[0049] The continuous casting: adopts induction heating of the tundish, the temperature of the molten steel in the continuous casting tundish is controlled at 15-30°C above the liquidus temperature, and the process of electromagnetic stirring + light pressure is used to improve the quality of the ingot, and at the same time solve the central segregation phenomenon that is very easy to occur in this type of steel. Specifically: argon sealing is used to protect the casting throughout the continuous casting process, 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 1.6-1.8m / min; the crystallizer water flow rate used for primary cooling is 2400L / min; the stopper rod curve is strictly controlled, and the secondary cooling adopts the cooling curve corresponding to the drawing speed. The combined method of terminal electromagnetic stirring parameters + light pressure is used for drawing and correction, which reduces the central segregation phenomenon of this type of steel with relatively high sulfur elements. The specification of the continuous casting billet is 160mm×160mm.

[0050] The large coil rolling, heating and soaking temperature is controlled at 1240-1280℃, heating time ≥200min; wherein, the air-fuel ratio of the heating furnace should be controlled at 0.4-0.7, the preheating section temperature is 950℃, the heating section temperature is 950-1160℃, the heating section temperature is 1160-1260℃, soaking section temperature is 1240-1280℃; wherein, soaking section time should be guaranteed to be ≥70min, 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 sulfides is converted into a large spindle shape, which is a guarantee for the sulfide morphology in the finished product. The rolling temperature after dephosphorization is 1130-1180℃, and the coiling temperature range is controlled to be J±15℃. Among them, J = (4×Mn / S+LF active oxygen) + 800℃, the cooling route adopts the large coil non-slow cooling material cooling channel, the cooling channel has no insulation cover, the first three fans are turned on, and the fan air volume is determined according to the size of the steel grade to cool the wire rod to below 550℃.

[0051] The finished wire rods are sent to customers, and the customer processing process is generally: mechanical shelling / shot blasting → initial 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] The main production process parameters of the above embodiments and comparative examples are shown in Table 2 below.

[0053] Table 2 Main parameters of each embodiment and comparative example

[0054]

[0055]

[0056] The aspect ratios of the finished wire rod sulfide particles produced in the above embodiments and comparative examples are shown in Table 3. According to the GB-T 10561-2005 standard for the determination of non-metallic inclusions in steel, the sulfide ratio is the ratio of the number of characteristic sulfides to the total sulfides.

[0057] Table 3 Aspect ratio of sulfide particle size in each embodiment and comparative example (% ratio)

[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, as shown in Table 4.

[0060] Table 4 Comparison of cutting performance of various embodiments and comparative examples

[0061]

[0062]

[0063] The production methods of each embodiment and comparative example are rolled into a finished product with a specification of 25mm large coil. By default, 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 a ratio of >9 is closer to a long strip, and its anisotropy is greater, resulting in a greatly increased tendency of lateral cracking. Table 4 is a comparison of the cutting performance of each embodiment and comparative example, in which the cutting conditions and cutting amount are the same, and the proportion of C-shaped chips in the chip shape after cutting is counted. The proportion of C-shaped chips represents excellent cutting performance of the material and less wear on the tool.

[0064] The Mn / S ratio of Comparative Example 1 exceeds the range of 3.5-6.0, the LF active oxygen range of Comparative Example 2 exceeds the range of 25-50ppm, and the final total oxygen content exceeds 0.0140%; the soaking temperature of Comparative Example 3 is lower than the range of 1240-1280°C, the holding time of Comparative Example 4 is less than 70min, the rolling temperature of Comparative Example 5 exceeds the range of 1130-1180°C, and the coiling temperature of Comparative Example 6 exceeds the range, all of which result in a decrease in the proportion of spindle-shaped sulfides when the finished product aspect ratio is ≤3, and a decrease in cutting performance.

[0065] Figure 1 This is a microstructure photo of Example 1. Figure 2 Example 1 Sulfide photos. Statistics of sulfide morphology 20 photos were taken at different locations, and the aspect ratios of the sulfides were compared using software for statistics.

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

[0067] 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 large coil free-cutting steel, characterized in that: The free-cutting steel in large coils includes the following components in mass 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%, and the balance is Fe and unavoidable impurities.

2. The large coil free-cutting steel according to claim 1, characterized in that: The composition of the large coil free-cutting steel also satisfies Mn / S: 3.5-6.

0.

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

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

5. The production process according to claim 3, characterized in that: During the rolling of the large coil, the heating and soaking temperature is controlled at 1240-1280° C., and the soaking time is ≥70 minutes.

6. The production process according to claim 3, characterized in that: The large coil is rolled at a starting temperature of 1130-1180°C.

7. The production process according to claim 3 or 6, characterized in that: During the large coil rolling, the coiling temperature is controlled to be =J±15°C; wherein J=(4×Mn / S+LF active oxygen)+800°C.

8. The production process according to any one of claims 3 to 7, characterized in that: The products are free-cutting steel in large coils with diameters of Φ16 to 50 mm.

9. The production process according to any one of claims 3 to 8, characterized in that: The spindle-shaped sulfide content of the large coil free-cutting steel product is ≥45%.

10. The production process according to any one of claims 3 to 9, characterized in that: The large coil free-cutting steel produced has a cutting index of feed rate f=0.10mm / r, cutting speed 2000r / min, cutting depth 0.5mm, and the proportion of C-type chips is ≥90%.

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