Steel wire rod for B and V-containing medium carbon alloy sleeve and production method of steel wire rod

Abstract

The invention provides a steel wire rod for a B and V-containing medium carbon alloy sleeve and a production method of the steel wire rod. The steel wire rod comprises the following components: 0.27 to 0.36 percent of C, 0.05 to 0.10 percent of Si, 0.80 to 1.00 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.015 percent of S, 0.30 to 0.55 percent of Cr, 0.020 to 0.040 percent of A1, 0.0005 to 0.0035 percent of B, 0.025 to 0.055 percent of Ti, 0.10 to 0.20 percent of V, less than or equal to 0.0025 percent of O, less than or equal to 0.0060 percent of N and the balance of Fe and inevitable impurities. Compared with the prior art, chemical components are subjected to microalloying control, optimization of a continuous casting process and improvement of a rolling process, and the acid pickling quality of the continuous casting surface and the surface of a finished wire rod are guaranteed. And the produced high-speed wire rod is good in surface quality, and the mechanical property and hardenability of the finished sleeve meet the requirements.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal materials, and specifically relates to a steel wire rod for a B and V medium carbon alloy sleeve and a production method thereof. Background Art

[0002] Medium carbon microalloyed high strength sleeve wire rod is mainly used for processing sleeve tools. It is a Cr-V steel with B and Ti microalloy elements added. After high temperature quenching, it has good tensile strength and wear resistance. The alloy sleeve is composed of multiple sleeves with hexagonal or twelve-angle holes and equipped with handles, connecting rods and other accessories. It is suitable for turning bolts or nuts in very narrow spaces or deep recesses.

[0003] The production process of alloy sleeve is to use hot-rolled high-speed wire as raw material, and then go through spheroidizing annealing, pickling, phosphorus saponification, drawing into corresponding specifications of wire drawing materials, cold heading and cold extrusion into sleeve blanks, and then go through CNC machine tool processing, quenching, tempering, surface treatment and other processes. The alloy sleeve steel needs to have good cold heading and cold extrusion performance to ensure its formability. Generally, 1 / 3 of the hot-rolled wire rod needs to have good cold heading performance, and 1 / 4 of the cold heading after spheroidizing annealing does not crack. Good grinding and turning processing performance during machining is required. The surface of the wire rod needs to have high dimensional accuracy and excellent hardenability to ensure its hardness after high temperature quenching + medium temperature tempering heat treatment. Therefore, it is also required to have good composition control and microstructure.

[0004] At present, the domestic medium-carbon micro-alloyed high-strength sleeve wire rod has problems such as poor hardenability, high cracking rate, and surface pitting. The medium-carbon micro-alloyed high-speed wire rod for sleeves produced by the existing process is difficult to ensure that the surface quality of the finished coil and the performance of the final fastener product meet the standards and user requirements. The above factors have become the constraints for the production of medium-carbon micro-alloyed high-speed wire rod for sleeves. Summary of the invention

[0005] The purpose of the present invention is to provide a steel wire rod for sleeves containing medium carbon alloys of B and V and a production method thereof, wherein the chemical composition is micro-alloyed, the rolling process is optimized and improved by using new rollers for the entire line and cooling rolling, so as to ensure the surface pickling quality of the continuous casting medium carbon micro-alloyed steel for sleeves and the surface of the finished coil. The surface quality of the produced high-speed wire is good, and the mechanical properties and hardenability of the finished sleeves meet the requirements. Further optimization is performed by adding pulsed magneto-oscillation (PMO) solidification homogenization technology to the continuous casting process to reduce the cracking rate.

[0006] The specific technical solutions of the present invention are as follows:

[0007] A steel wire rod for a medium carbon alloy sleeve containing B and V, comprising the following components in percentage by mass:

[0008] C: 0.27~0.36%, Si: 0.05-0.10%, Mn: 0.80~1.00%, P≤0.015%, S≤0.015%, Cr: 0.30~0.55%, A1: 0.020~0.040%, B: 0.0005~0.0035%, Ti: 0.025~0.055%, V: 0.10~0.20%, O: ≤0.0025%, N: ≤0.0060%, the balance is Fe and unavoidable impurities.

[0009] The composition of the steel wire rod for the medium carbon alloy sleeve containing B and V also satisfies that Ti / N is controlled at 6-10, thereby ensuring the cold working performance of the wire rod.

[0010] The composition of the steel wire rod for the medium-carbon alloy sleeve containing B and V also satisfies that the impurity elements Sn, Sb, As, Pb and Bi are all ≤0.01% each, and Pb+Sn+As+Sb+Bi≤0.050%.

[0011] The decarburization layer depth of the steel wire rod for the B and V medium carbon alloy sleeve is ≤0.02mm;

[0012] The properties of the steel wire rod containing B and V for the medium carbon alloy sleeve are as follows: tensile strength of 660-690MPa, yield strength of 440-460MPa, and section shrinkage of 40-50%.

[0013] The present invention provides a method for producing a steel wire rod for a medium-carbon alloy sleeve containing B and V, which comprises the following process flow: converter - LF refining - continuous casting of square billets - heating in a heating furnace - high-speed wire rolling - controlled cooling.

[0014] The continuous casting billet is 180mm×180mm;

[0015] In order to ensure the internal quality of the billet, the continuous casting process is fully protected, and the ladle is monitored by slag detection equipment, and slag is prohibited;

[0016] The continuous casting billet has a continuous casting superheat of 25-30°C and a crystallizer water volume of 110-120m 3 / h, specific water volume is 0.55-0.65L / kg; crystallizer stirring is adopted, electric stirring current is 300A, electric stirring frequency is 4.0HZ, terminal electromagnetic stirring current is 100-150A, electric stirring frequency is 10.0HZ, special protective slag is adopted in the crystallizer, online appearance inspection is carried out after the ingot is off the line, and the ingot is qualified.

[0017] Furthermore, pulsed magneto-oscillation (PMO) coagulation homogenization technology was used simultaneously, with a PMO voltage of 350 V and a PMO frequency of 10 Hz.

[0018] The heating furnace is used for heating, and the billet in the heating furnace is kept in the furnace for no less than 90 minutes. The steel is not allowed to stay in the soaking zone at a high temperature for a long time to avoid overheating and overburning of the steel in the furnace, which may cause decarburization of the final product. The residual oxygen content is no more than 3%;

[0019] After leaving the heating furnace, the water dephosphorization pressure is 22MPa;

[0020] The high-speed wire rolling adopts new rollers for the whole line, controls the starting rolling temperature to be 1000-1030°C, the finishing rolling inlet temperature to be 900-910°C, the reducing and sizing inlet temperature to be 870-890°C, and the spinning temperature to be 840-870°C;

[0021] The controlled cooling adopts the Stelmore air cooling line cooling method: two groups of insulation covers are turned on, and two groups of fans are turned on accordingly, with air volumes of 50% and 40% respectively, and the wire rod cooling rate is 2.5-3.0℃ / s, and the remaining fans and insulation covers are all closed, the air cooling roller speed is 0.18-0.27m / min, and the coils are air-cooled after leaving the covers.

[0022] The high-speed wire rolling rolls the continuous casting square billet 180mm×180mm into a wire rod with a diameter of φ6.5~24mm. No cracks shall be found in 1 / 3 of the wire rod after cold forging. The hot-rolled structure is uniform ferrite and pearlite with a grain size of about 10-20μm. No abnormal structures such as martensite, bainite, and widmanstattenite shall appear, and the hardness after quenching is greater than 56HRC.

[0023] The design ideas of the present invention are as follows:

[0024] B: Adding a trace amount of B to fastener steel can make up for the loss of strength and hardenability caused by carbon reduction, and can improve the cold deformation ability of steel. The main function of B in steel is to improve hardenability. A trace amount of B can significantly improve the performance of quenched and tempered steel. Using a small amount of B to replace alloy elements to produce fastener steel B reduces costs. Adding B element to steel has the best performance after quenching and tempering. The present invention controls B: 0.0005~0.0035%.

[0025] V: V exists in the form of carbides in steel. Its main function is to refine the structure and grain of steel and improve the strength and toughness of steel. When dissolved into solid solution at high temperature, V increases the tempering stability of quenched steel and produces a secondary hardening effect. V refines the grains in high-strength sleeve steel, reduces overheating sensitivity, increases tempering stability and wear resistance, thereby extending the service life of the high-strength sleeve.

[0026] V: 0.10~0.20%.

[0027] Ti: When smelting B-containing steel, in order to prevent B from combining with N to form boron nitride and reduce the hardenability of B, a certain amount of Ti needs to be added to fix N. Ti also plays a role in refining grains, so the residual Ti content in B steel will affect the austenite grain size of the steel. Residual Al and Ti in steel have a significant impact on the recovery rate of B in steel. In order to ensure the recovery rate of B, Al should be used for deoxidation and Ti should be used to fix N before adding B, and B should be added last. The better the deoxidation of the molten steel, the more fully the role of fixing N can be exerted, the recovery rate of B is stable, and the hardenability effect is better. The present invention controls Ti: 0.025-0.055%, and Ti / N is controlled at 6-10.

[0028] Cr: Cr is an element that stabilizes austenite and has a large solubility in austenite. During phase transformation, due to the slow movement of Cr, the incubation period of transformation will be prolonged, causing the C curve to shift to the right, thereby improving the permeability of the steel. While Cr increases hardenability, it does not significantly affect the cold working performance. Therefore, a certain amount of Cr is often added to boron steel to further improve the hardenability of the steel and expand the application size range of fasteners. Adding an appropriate amount of Cr element to the main component of boron microalloyed medium carbon cold steel can further improve the hardenability of the steel. It can be added in appropriate amounts according to customer needs. The present invention controls Cr: 0.30~0.55%.

[0029] Mn: Mn improves the hardenability of steel by stabilizing austenite. Mn is infinitely miscible with austenite and is the most effective element in improving hardenability among alloy elements. In order to obtain different quenching effects in B-containing cold heading steel, the content can be reasonably controlled according to customer requirements to meet different hardening diameter requirements. The present invention controls Cr: 0.30-0.55%.

[0030] Si: Si can dissolve in ferrite and austenite to improve the hardness and strength of steel. Its effect is second only to phosphorus and stronger than manganese, nickel, chromium, tungsten, molybdenum, vanadium and other elements. Si can improve the elastic limit, yield strength and yield ratio (σs / σb) of steel, as well as fatigue strength and fatigue ratio (σ-1 / σb). The present invention controls

[0031] Si: 0.05-0.10%.

[0032] C: C is the most important element that affects the deformation of cold heading steel. The higher the carbon content, the higher the strength of the steel, but the lower the plasticity. Research and practice have shown that for every 0.1% increase in C content, the yield strength σs increases by about 27.4Mpa; the tensile strength σb increases by 58.8~78.4Mpa; while the elongation δ decreases by 4.3%, and the cross-sectional shrinkage ψ decreases by 7.3%. It can be seen that the C content in steel has a great influence on the cold deformation performance of cold heading steel. In the absence of special requirements from customers, in order to ensure good cold heading performance of cold heading steel, the C content of cold heading and cold extrusion steel is usually C: 0.27~0.36%.

[0033] P, S: Except for easy-cutting steel, S in steel reduces the plasticity of steel and is a harmful element. S exists in the form of FeS with a lower melting point. The melting point of FeS is 1190℃, while steel is usually heated at 1100-1200℃. The FeS on the grain boundary will melt, greatly weakening the bonding force between the grains, resulting in hot brittleness of the steel. P has a strong effect of solid solution strengthening and cold working hardening in steel. The biggest harm is that it has serious segregation, increases temper brittleness, and significantly increases the plasticity and toughness of steel, making the steel easy to crack during cold working, which is the so-called "cold brittle" phenomenon. P and S are harmful elements and should be strictly controlled. The P content should be controlled within 0.015%, and the S content should be controlled within 0.015%.

[0034] O and N content: B is a very active element, and it is very easy to combine with gas elements such as O and N in steel to produce boron oxide and boron nitride, which will reduce the effect of B in improving hardenability. Therefore, O and N in steel should be controlled at a low level as much as possible. By adding aluminum deoxidation and suitable refining conditions, the O content can be controlled below 25ppm. The N element is greatly affected by the manufacturing process such as the added alloy material and the protective casting effect, and the fluctuation is also large. In order to eliminate the adverse effect of the N element on B, a certain amount of Ti is often added to the steel to fix N and inhibit the combination of N and B. Under normal circumstances, B easily reacts with N to form BN, and precipitates at the grain boundary. BN is continuously pinned at the deformation of austenite and matrix, reducing the fluidity of the grain boundary and increasing the recrystallization temperature. The present invention controls O:≤0.0025%, N:≤0.0060%.

[0035] Compared with the prior art, the present invention improves the product qualification rate and the strength and hardness of the material through reasonable component design, optimization of continuous casting process and improvement of rolling process, thus meeting the standard requirements of customers. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The metallographic structure of Example 1;

[0037] Figure 2 is the decarburized layer depth of Example 1;

[0038] Figure 3 The metallographic structure of Example 2;

[0039] Figure 4 is the decarburized layer depth of Example 2;

[0040] Figure 5 The metallographic structure of Comparative Example 1;

[0041] Figure 6 It is the decarburized layer depth of Comparative Example 1. DETAILED DESCRIPTION

[0042] 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.

[0043] Example 1

[0044] A steel wire rod for a medium carbon alloy sleeve containing B and V, comprising the following components in percentage by mass:

[0045] C: 0.29%, Si: 0.09%, Mn: 0.85%, P: 0.015%, S: 0.0:05%, Cr: 0.38%, A1: 0.025%, B: 0.0020%, Ti: 0.032%, V: 0.12%, O: 0.0012%, N: 0.0034%, Sn, Sb, AS, Pb, Bi are all ≤0.005%, and Pb+Sn+As+Sb+Bi=0.00175%, the balance is Fe and unavoidable impurities.

[0046] The production method of the steel wire rod for medium carbon alloy sleeve containing B and V described in Example 1 includes the following process flow: converter - LF refining - continuous casting of square billet 180mm×180mm - heating in heating furnace - high speed wire rolling - controlled cooling; wherein:

[0047] In order to ensure the internal quality of the ingot, the continuous casting process is protected throughout the casting process. The ladle is monitored by slag detection equipment and slag is prohibited. The continuous casting overheating temperature is 25°C and the water volume of the crystallizer is 115m 3 / h, specific water volume is 0.6L / kg, crystallizer stirring is adopted, electric stirring current is 300A, electric stirring frequency is 4.0HZ, terminal electromagnetic stirring current is 150A, electric stirring frequency is 10.0HZ, special protective slag is used in the crystallizer, and pulsed magneto-oscillation (PMO) solidification homogenization technology is used at the same time, PMO voltage is 350V, PMO frequency is 10Hz, online appearance inspection is carried out after the ingot is off the line, and the ingot is qualified.

[0048] The actual time that the billet is in the heating furnace is 115 minutes, and the residual oxygen content is not more than 3%; the water dephosphorization pressure is 22MPa; the rolling start temperature is controlled to be 1010±10℃, the finishing rolling entrance temperature is 900-910℃, the sizing entrance temperature is 870-880℃, and the spinning temperature is 840-850℃; the cooling method of the Stelmore air-cooling line: turn on two sets of insulation hoods, and correspondingly turn on two sets of fans, with air volumes of 50% and 40% respectively, and the wire rod cooling rate is 2.7℃ / s, and the remaining fans and insulation hoods are all closed, the air-cooling roller speed is 0.18-0.27m / min, and the coils are air-cooled after leaving the hoods.

[0049] 1 / 3 of the rolled wire rods are qualified after cold heading, and the metallographic structure is ferrite + pearlite, the ferrite area accounts for 52.5%, the grain size is about 10-15μm, the decarburization layer depth is 0.01mm, and the surface quality of the coil pickling inspection is intact and has no defects. Figure 1 , the decarburization depth is Figure 2 .

[0050] Example 2

[0051] A steel wire rod for a medium carbon alloy sleeve containing B and V, comprising the following components in percentage by mass:

[0052] C: 0.30%, Si: 0.07%, Mn: 0.84%, P: 0.012%, S: 0.0:06%, Cr: 0.37%, A1: 0.026%, B: 0.0021%, Ti: 0.037%, V: 0.13%, O: 0.0022%, N: 0.0060%, Sn, Sb, As, Pb, Bi are all ≤0.005%, and Pb+Sn+As+Sb+Bi=0.002410%, the balance is Fe and unavoidable impurities.

[0053] The production method of the steel wire rod for medium carbon alloy sleeve containing B and V described in Example 2 includes the following process flow: converter - LF refining - continuous casting of square billet 180mm×180mm - heating in heating furnace - high speed wire rolling - controlled cooling; wherein:

[0054] In order to ensure the internal quality of the ingot, the continuous casting process is protected throughout the casting process. The ladle is monitored by slag detection equipment and slag is prohibited. The continuous casting overheating temperature is 27°C and the water volume of the crystallizer is 114m 3 / h, specific water volume is 0.65L / kg, crystallizer stirring is adopted, electric stirring current is 300A, electric stirring frequency is 4.0HZ, terminal electromagnetic stirring current is 100A, electric stirring frequency is 10.0HZ, PMO is not used, special protective slag is used in the crystallizer, online appearance inspection is carried out after the ingot is off the line, and the ingot is qualified.

[0055] The actual time that the billet is in the heating furnace is 108 minutes, the residual oxygen content is not more than 3%, and the water dephosphorization pressure is 22MPa; the rolling start temperature is controlled to be 1020±10℃, the finishing rolling entrance temperature is 900-910℃, the sizing entrance temperature is 880-890℃, and the wire laying temperature is 860-870℃; the cooling method of the Stelmore air-cooling line: turn on two sets of insulation hoods, and correspondingly turn on two sets of fans, with air volumes of 50% and 40% respectively, and the wire rod cooling rate is 2.9℃ / s. The remaining fans and insulation hoods are all closed, and the air-cooled roller speed is 0.18-0.27m / min. After leaving the hood, the coil is air-cooled.

[0056] 1 / 3 of the rolled wire rods are qualified after cold heading, and the metallographic structure is ferrite + pearlite, the ferrite area accounts for 49.4%, the grain size is about 12-18μm, the decarburization layer depth is 0.02mm, and the surface quality of the wire rods is intact after pickling inspection without defects. Figure 3 , the decarburization depth is Figure 4 .

[0057] Comparative Example 1

[0058] A steel wire rod for a medium carbon alloy sleeve containing B and V, comprising the following components in percentage by mass:

[0059] C: 0.29%, Si: 0.09%, Mn: 0.78%, P: 0.015%, S: 0.0:05%, Cr: 0.34%, A1: 0.025%, B: 0.0019%, V: 0.12%, O: 0.0022%, N: 0.0060%, Sn, Sb, As, Pb, Bi are all ≤0.010%, and Pb+Sn+As+Sb+Bi=0.0179%, the balance is Fe and unavoidable impurities.

[0060] The production method of the steel wire rod for medium carbon alloy sleeve containing B and V described in Comparative Example 1 includes the following process flow: converter - LF refining - continuous casting of square billet 180mm×180mm - heating in heating furnace - high-speed wire rolling - controlled cooling; wherein:

[0061] In order to ensure the internal quality of the ingot, the continuous casting process is protected throughout the casting process. The ladle is monitored by slag detection equipment and slag is prohibited. The continuous casting overheating temperature is 28°C and the water volume of the crystallizer is 118m 3 / h, The specific water volume is 0.7L / kg, The crystallizer is stirred with an electric stirring current of 300A and a stirring frequency of 4.0HZ. The terminal electromagnetic stirring current is 150A and a stirring frequency of 10.0HZ. The crystallizer uses special protective slag. After the ingot is off the line, an online appearance inspection is carried out and the ingot is qualified.

[0062] The actual time of the billet in the heating furnace is 127 minutes, the residual oxygen content is not more than 3%, and the water dephosphorization pressure is 22MPa; the starting rolling temperature is controlled to be 1015±10℃, and the finishing rolling entrance temperature is 910~ 930℃， Sizing inlet temperature 890~ 900℃ , spinning temperature 870~ 880℃； Stelmore air cooling line cooling method: The fans and heat preservation covers are all turned off. The air-cooled roller speed is 0.18-0.27m / min, and the rolls are air-cooled after leaving the hood.

[0063] The cold heading cracking rate of 1 / 3 rolled wire rod is 0.5%.The metallographic structure is ferrite + pearlite + widmanstattenite, the structure is uneven, there is central pearlite aggregation, the decarburization layer depth is 0.07mm, the surface quality of the disc pickling inspection is intact and has no defects. Figure 5 , the decarburization depth is Figure 6 Comparative Example 1 has high spinning temperature and fast cooling rate, which leads to uneven organization of wire rod and also affects the comprehensive performance of the material.

[0064] According to customer feedback, the material of Example 1 has good formability, and the strength and hardness after heat treatment meet the standard requirements, and no waste occurs. The material of Example 2 has good formability, and the strength and hardness after heat treatment meet the standard requirements, but about 0.01% of cracked waste occurs. The material of Comparative Example 1 has good formability, and the strength and hardness after heat treatment do not meet the standard requirements by about 1%, and about 2% of waste occurs.

[0065] Table 1 below is the feedback from customers on the production of Example 1-Example 2 and the comparative example products under the same conditions.

[0066] Table 1 Customer feedback on the use of the examples and comparative examples

[0067]

[0068] The underlined data above do not meet the requirements of the present invention.

[0069] 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 steel wire rod for a medium carbon alloy sleeve containing B and V, characterized in that: The steel wire rod for the B and V medium carbon alloy sleeve comprises the following components in percentage by mass: C: 0.27~0.36%, Si: 0.05-0.10%, Mn: 0.80~1.00%, P≤0.015%, S≤0.015%, Cr: 0.30~0.55%, A1: 0.020~0.040%, B: 0.0005~0.0035%, Ti: 0.025~0.055%, V: 0.10~0.20%, O: ≤0.0025%, N: ≤0.0060%, the balance is Fe and unavoidable impurities.

2. The steel wire rod for B and V medium carbon alloy sleeve according to claim 1, characterized in that: The composition of the steel wire rod for the medium carbon alloy sleeve containing B and V also satisfies that Ti / N is controlled at 6-10.

3. According to claim 1 or 2, the composition of the B, V medium carbon alloy sleeve steel wire rod also satisfies that the impurity elements Sn, Sb, As, Pb, Bi are all ≤0.01%, and Pb+Sn+As+Sb+Bi≤0.050%.

4. A method for producing a steel wire rod for a B or V medium carbon alloy sleeve as claimed in any one of claims 1 to 3, characterized in that: The production method comprises the following process flow: converter - LF refining - continuous casting of square billets - heating in a heating furnace - high-speed wire rolling - controlled cooling.

5. The production method according to claim 4, characterized in that: The continuous casting billet has a continuous casting superheat of 25-30°C and a crystallizer water volume of 110-120m 3 / h, specific water volume is 0.55-0.65L / kg; crystallizer stirring is adopted, electric stirring current is 300A, electric stirring frequency is 4.0HZ, end electromagnetic stirring current is 100-150A, electric stirring frequency is 10.0HZ.

6. The production method according to claim 4 or 5, characterized in that: During continuous casting, pulse magneto-oscillation (PMO) solidification homogenization technology is used simultaneously, with a PMO voltage of 350V and a PMO frequency of 10Hz.

7. The production method according to claim 4, characterized in that: The heating furnace is used for heating, the blank is heated in the furnace for not less than 90 minutes, and the residual oxygen content is not greater than 3%.

8. The production method according to claim 4 or 7, characterized in that: After leaving the heating furnace, the water dephosphorization pressure is 22MPa.

9. The production method according to claim 4, characterized in that: The high-speed wire rolling is controlled to have a starting rolling temperature of 1000-1030°C, a finishing rolling inlet temperature of 900-910°C, a sizing inlet temperature of 870-890°C, and a spinning temperature of 840-870°C.

10. The production method according to claim 4, characterized in that: The controlled cooling adopts the Stelmore air cooling line cooling method: two groups of insulation covers are turned on, and two groups of fans are turned on accordingly, with air volumes of 50% and 40% respectively, and the wire rod cooling rate is 2.5-3.0℃ / s, and the remaining fans and insulation covers are all closed, the air cooling roller speed is 0.18-0.27m / min, and the coils are air-cooled after leaving the covers.

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