Deformed steel bar with yield strength larger than or equal to 830 MPa and production method

Abstract

The invention discloses deformed steel bar with yield strength larger than or equal to 830 MPa and a production method. The deformed steel bar comprises, by weight, 0.35%-0.50% of C, 0.50%-0.90% of Si, 0.40%-0.90% of Mn, smaller than or equal to 0.035% of P, smaller than or equal to 0.035% of S, 0.050%-0.120% of Al, 0.01%-0.07% of V, 0.0045%-0.010% of N and 0.40%-0.60% of Zr. The production method comprises the steps of molten iron pretreatment; smelting in a converter; refining in an LF furnace; heating the casting blank; performing rough rolling and intermediate rolling; finish rolling is conducted according to two sections; and cooling to room temperature. Under the conditions that the hot rolling yield strength Rel is larger than or equal to 830 MPa, the tensile strength Rm is larger than or equal to 980 MPa, the elongation A is larger than or equal to 6%, and the total elongation Agt under the maximum force is larger than or equal to 3.5%, the process is simple, the finish rolling deformed steel bar product performance is stable, the heating temperature is increased by 25%, the rolling mill load can be reduced by 20%, the roller consumption cost is reduced by about 18%, the production efficiency can be improved, and the process adaptability is high; and as the process window is wide, field operation and smooth production are facilitated.

Description

Technical Field

[0001] The present invention relates to a threaded steel and a production method, in particular to a threaded steel with a yield strength of ≥830MPa and a production method, which is particularly suitable for producing 830MPa fine-rolled threaded steel and a production method. Background Art

[0002] Finely rolled threaded steel bar, also known as prestressed concrete threaded steel bar, is a straight steel bar with discontinuous external threads and no longitudinal ribs on the entire surface of the steel bar. This steel bar is directly connected with nuts and spiral sleeves, without welding and machining screw buckles. It has the advantages of simple connection and anchoring, strong adhesion, safe and reliable tensioning and anchoring, convenient construction, fast speed, and material saving. Finely rolled threaded steel bar is widely used in large-scale water conservancy and hydropower projects, continuous beams and large frame structures in industrial and civil buildings, highways, high-speed railways, large bridge baskets, nuclear power plants, ground anchors, slope anchors, and other super-large buildings, frame structures, bridges and culverts and other projects.

[0003] Finished rolled steel products have extremely strict requirements on the grade and quality of their products. Since there is no need to consider the influence of welding performance, a variety of micro-alloying or heat treatment processes can be used to produce this steel. According to the characteristics of finished rolled threaded steel bars, the connection device and finished product pass of the finished rolling mill need to be redesigned, and high control accuracy is required during the rolling process.

[0004] Compared with the traditional general bar rolling production line, the high-speed bar production line adopts the roller ring rolling mill to obtain higher precision finished products. The single-bar torsion-free low-temperature controlled rolling technology is adopted in the finishing rolling area to achieve alloy reduction production. Placing a single bar for a single tooth groove on the cooling bed for cooling and single-bar alignment can obtain a higher yield rate. Based on these advantages, the high-speed bar production technology has developed rapidly in recent years.

[0005] There are currently two main types of high-speed bar production lines: one is a single-line high-speed bar production line, also referred to as a single-high bar production line, that is, the rough rolling, intermediate rolling, pre-finishing rolling, and finishing rolling units are all single-bar rolling, and after finishing rolling, they are sent to a cooling bed by a set of high-speed steel-loading system; the other is a double-line high-speed bar production line, also referred to as a double-high bar production line, that is, rough rolling and intermediate rolling are single-bar rolling, pre-finishing rolling is two-slice rolling, and after pre-finishing rolling, double lines enter two groups of finishing rolling units at the same time, each line rolls a single bar in each group of finishing rolling, and after finishing rolling, they are merged and sent to the cooling bed by a set of double high-speed steel-loading system that can meet the needs of double lines on the cooling bed at the same time.

[0006] In the national standard GB / T 20065-2016, the chemical composition (melting composition) of finished rolled threaded steel bars is not clearly specified except that the mass fraction of S and P is controlled to be ≤0.035%. The chemical composition and alloy elements are selected by the manufacturer to ensure that the finished steel bars processed by different methods meet the mechanical property requirements specified in the standard. Finished rolled threaded steel bars have added relaxation, fatigue, and non-metallic inclusion inspection items, and the quality requirements are significantly higher than those of ordinary threaded steel bars.

[0007] Finished rolled threaded steel bars are classified according to the yield strength level, including five grades: PSB785, PSB830, PSB930, PSBl080, and PSB1200. Among them, PSB830 is the most widely used grade in finished rolled threaded steel bars. The commonly used process in production is the core self-tempering (temper core) post-rolling residual heat treatment process to improve the strength level. It is a special water-penetrating cooling system that quenches the surface of the steel bar into martensite after the steel bar comes out of the last stand of the rolling mill, while the core remains in the austenite state. In the subsequent cooling, the heat of the core is transferred to the surface of the steel bar, resulting in the tempering of the martensite on the surface.

[0008] After searching:

[0009] The document with Chinese patent application number CN201911119120.2 (publication number CN 110760735A) discloses "A PSB830 fine-rolled threaded steel bar and its production method", whose chemical composition and mass percentage are as follows: C: 0.43% to 0.48%, Si: 1.60% to 1.80%, Mn: 0.90% to 1.00%, (Cr+Ni+Cu)≤0.10%, Nb: 0.01% to 0.025%, V: 0.125% to 0.155%, P≤0.020%, S≤0.015%, N: 75 to 110ppm, electric furnace smelting-LF furnace refining treatment-VD treatment-continuous casting-slab heating-rolling-controlled cooling after rolling, Its process: Steelmaking: Electric furnace smelting, electric furnace endpoint control C ≥ 0.04%, P ≤ 0.013%; LF refining process uses high-purity silicon carbide for diffusion deoxidation, refining time is more than 30 minutes; VD vacuum treatment, vacuum degree ≤ 1mbar, holding time is more than 12 minutes; after vacuum treatment, feed manganese nitride wire to adjust nitrogen content, and then feed pure calcium rod core wire 50 to 150 meters, and then stir for more than 15 minutes; continuous casting: the first ladle temperature is 1535 to 1550℃, the continuous casting furnace large ladle temperature is 1525 to 1540℃, the liquidus temperature is 1480℃, and the superheat is 10 to 40℃; the cross section is 150*150mm 2, the pulling speed is 2.0~2.4m / min, the continuous casting adopts full protection casting, and electromagnetic stirring is performed to ensure the stability of the liquid level of the tundish and the crystallizer, and to ensure the centering and insertion depth of the invasive nozzle; rolling: the preheating section temperature of the heating furnace before rolling is ≤900℃, the heating section temperature is 1120~1180℃, and the soaking section temperature is 1120~1200℃; the rolling temperature is 1000~1050℃, the red return temperature is 600~630℃, and the high-pressure water descaling pressure is ≥15MPa; controlled cooling after rolling. The steel is smelted in an electric furnace, treated with VD vacuum, continuously cast into slabs, and then rolled into finished rebar bars. It has the disadvantages of complex process, high production cost, and is not conducive to promotion and application.

[0010] The document with Chinese patent application number CN202011293465.2 (publication number CN 112410669A) discloses "A Corrosion-Resistant PSB830 Finished Rolled Threaded Steel and Its Production Process". The process is as follows: first hot-rolled to PSB400MPa grade, then subjected to high-temperature quenching and medium-temperature tempering heat treatment, and finally obtained a corrosion-resistant PSB830MPa grade or above finished rolled threaded steel. Its composition and mass percentage are: C 0.10-0.15%, Si 0.50-0.80%, Mn 1.30-1.60%, P≤0.030%, S≤0.030%, Cr 0.50~0.80%, Ni 0.50-0.65%, Cu 0.50-0.65%, Nb 0.015~0.035%, the balance is Fe and inevitable impurities, the parameters of hot rolling process and heat treatment process are: starting rolling temperature: 1120℃; finishing rolling temperature: 1010℃; upper cooling bed temperature ≥950℃; quenching temperature: 925℃; tempering temperature: 440℃. The steel in this document is produced by online hot rolling + offline quenching and tempering heat treatment process. Compared with the production process of online controlled rolling and controlled cooling, it has the disadvantages of complex process and the need to configure a heat treatment production line or corresponding plant.

[0011] The document with Chinese patent application number CN202210329814.4 (publication number CN 114686764A) discloses "A low-relaxation ultra-high-strength threaded steel bar and its preparation method", and its chemical composition is calculated by mass percentage: C 0.24%-0.50%, Si 1.3-2.0%, Mn 0.5%-1.4%, V 0.07%-0.14%, Nb0.10-0.21%, B 0.01-0.04%, Ni0.1%-0.2%, Mo 0.10%-0.15%, Hf 0.1%-0.2%, Zr 0.02%-0.05%, P≤0.035%, S≤0.035%, the remainder is iron and unavoidable impurities, and the relationship is satisfied: 0.23%≤0.55[Ni]+[Mo]+0.27[Hf]+0.1[Zr]≤0.25%. The document's fine-rolled threaded steel bar can achieve both strength and toughness and other mechanical properties of the steel bar while effectively improving the corrosion resistance of the steel bar, and the yield strength is ≥785MPa, the tensile strength is ≥980MPa, the elongation after fracture is ≥12%, and the maximum force total elongation is ≥4.5%. After continuous rolling, it is not directly water-cooled, but first air-cooled to a specific 820-850℃, then water-cooled to 450-460℃, and finally air-cooled to room temperature. The steel bar maintains a certain austenite state through air cooling, and then the surface is quenched into martensite through water cooling. During the cooling process, the residual heat is transmitted from the core to temper the surface to form tempered martensite. In addition to conventional C, Si, and Mn, this document also adds V, Nb, B, Ni, Mo, Hf, Zr and other alloy elements in specific amounts. The chemical composition is complex, the hit rate of the smelting process components is not high, and the total amount of alloy is not low. The air cooling-water cooling-air cooling control process is adopted, and the strength of the developed steel bar is only 785MPa.

[0012] The Chinese patent application number is CN201110347552.6 (publication number is CN 102363832A), which discloses "A Production Process for Finished Rolled Threaded Steel Bars", and its process route is: billet heating → rolling → online controlled cooling → natural cooling on a cooling bed → sizing → stack cooling; the square billet is made of medium carbon steel with a size of 150mm×150mm; a three-stage heating method of preheating, heating and soaking is adopted in the heating process; in the rolling process, the starting rolling temperature is controlled at 1000℃-1100℃; in the online controlled cooling process, the water inlet temperature is controlled at 920-1000℃, and one-time cooling is adopted, and the self-tempering temperature of PSB785 and PSB830 is controlled at 480-550℃; natural cooling on a cooling bed, and the cooling rate is controlled at 1-5℃ / s; in the stack cooling process, offline stack cooling is performed at 150-350℃, and the stack cooling time is not less than 48 hours; it has the disadvantages of long heating time, high energy consumption, poor cooling effect, and low production efficiency. Summary of the invention

[0013] In view of the shortcomings of the prior art such as high alloy content of steel bars, high energy consumption, poor cooling effect and low production efficiency, the present invention provides a threaded steel bar with a yield strength of ≥830MPa and a production method, which can make the process simpler, achieve a product performance qualification rate of more than 98% and have stable performance while ensuring hot-rolled yield strength Rel≥830MPa, tensile strength Rm≥980MPa, elongation A≥6% and total elongation under maximum force Agt≥3.5%.

[0014] Measures to achieve the above objectives:

[0015] A threaded steel with a yield strength of 830 MPa or more, the components and weight percentage contents of which are: C: 0.35-0.50%, Si: 0.50-0.90%, Mn: 0.40-0.90%, P≤0.035%, S≤0.035%, Al: 0.050-0.120%, V: 0.01-0.07%, N: 0.0045-0.010%, Zr: 0.40-0.60%, and the remainder is Fe and unavoidable impurities.

[0016] Preferably, the weight percentage of C is 0.38-0.45%.

[0017] Preferably, the weight percentage of Si is 0.55-0.85%.

[0018] Preferably, the weight percentage of Mn is 0.50-0.80%.

[0019] Preferably, the weight percentage of Al is between 0.060% and 0.110%.

[0020] Preferably, the weight percentage of V is 0.01-0.06%.

[0021] Preferably, the weight percentage of N is 0.0055-0.009%.

[0022] Preferably, the weight percentage of Zr is 0.45-0.55%.

[0023] A method for producing a threaded steel bar having a yield strength ≥ 830 MPa, comprising the following steps:

[0024] 1) Pre-treat the molten iron. After desulfurization, the molten iron has P < 0.020%, S < 0.020%;

[0025] 2) Converter smelting, control the converter endpoint C at 0.06-0.30%; when the molten steel is 2 / 3 out, add the alloy and carburizer all at once;

[0026] 3) Carry out LF furnace refining, the refining time is not less than 25min, and argon is blown throughout the refining process; after the composition is adjusted, the molten steel is prohibited from being exposed;

[0027] 4) Casting into square billets, during which a large tank long nozzle and a crystallizer immersion nozzle are used to protect the pouring, and the superheat of the molten steel in the tundish is controlled at 20-35°C; the size of the square billet is 160-170mm;

[0028] 5) Heat the ingot, control the temperature of the soaking section at 1080-1200°C, and the total heating time at 60-90 minutes;

[0029] 6) performing rough rolling and intermediate rolling, and controlling the starting rolling temperature to be between 980°C and 1050°C, and controlling the cumulative deformation rate of rough rolling and intermediate rolling to be between 20% and 60%;

[0030] 7) Finish rolling is performed in two stages, during which: the temperature of the finishing rolling stage I is controlled at 810-850°C, the temperature of the finishing rolling stage II is controlled at 790-830°C, and the rolling speed of the final stand is controlled at 20-45 m / s;

[0031] 8) Cooling is performed, the temperature of the cooling bed is controlled at 570-620°C, and the cooling rate is 6-9°C / s to cool to room temperature.

[0032] Functions and mechanisms of the components and main processes in the present invention

[0033] C: C is the cheapest and most effective strengthening element in steel materials. It can play a role of solid solution strengthening when dissolved in the matrix. If the C content is lower than 0.35%, it is difficult to ensure the yield strength and tensile strength of the steel bar. If the C content is higher than 0.50%, the toughness of the steel will deteriorate. Therefore, the C content is controlled within the range of 0.35-0.50%, preferably 0.38-0.45%.

[0034] Si: Si exists in the steel in a solid solution state to improve the strength, and is also a deoxidizing element for steelmaking. Under the premise of ensuring the toughness and strength of the steel bar, as much cheap Si element as possible is used, but the content should not be too high to avoid reducing the toughness and plasticity of the steel. Therefore, it is controlled at 0.50-0.90%, preferably 0.55-0.85%.

[0035] Mn: Mn dissolves in ferrite to improve the strength of steel, reduce the supercooled austenite transformation temperature, refine ferrite grains, and is beneficial to improving the strength and toughness of steel bars. However, Mn is also an element that is prone to segregation. The higher the content, the more likely it is to segregate. Composition segregation will lead to the formation of bainite structure during continuous cooling of the steel bar, which is not conducive to ensuring the toughness of the steel bar. Therefore, Mn is controlled at 0.40-0.90%, preferably 0.50-0.80%.

[0036] Al: Al is the most efficient deoxidizer and can effectively improve the purity of molten steel. At the same time, AlN particles can refine grains and strengthen precipitation, but too high Al will deteriorate the mechanical properties of steel, so it is controlled at 0.05-0.12%, preferably 0.06-0.11%.

[0037] V: One of the most common microalloying elements. V is a strong nitride and carbide forming element. During heating or rolling, it can produce strong solid solution and precipitation strengthening effects when forming fine dispersed second phase particles, and can refine grains and improve the strength and toughness of steel. When the V content is too high, it will lead to an increase in the size of precipitates, which may worsen the above effects. Therefore, the content is controlled at 0.01-0.07%, preferably 0.01-0.06%.

[0038] N: N is a gaseous element that combines with V microalloying elements in steel to form second phase particles such as VN, which enhances precipitation strengthening and fine grain strengthening, significantly improves the strength of steel, and improves toughness. Adding too little is difficult to ensure the above-mentioned strengthening effect, while adding too much will reduce the toughness of the steel bar and increase the difficulty of smelting operation, so the content is controlled at 0.0045-0.0100%. The N content is preferably 0.0055-0.09%.

[0039] Zr: Zr can degas and refine grains, but too high a zirconium content will increase smelting difficulty and production costs, and cause local segregation, deteriorating steel properties, so the content is controlled at 0.40-0.60%, preferably 0.45-0.55%.

[0040] The reason why the present invention controls the converter endpoint C at 0.06-0.30% and adds the alloy and the carburizer all at once when 2 / 3 of the molten steel is discharged is that the contents of Si, Mn, Al, V, Zr alloy elements and C element can be accurately controlled. If the alloy elements are added in advance, the alloy elements will be oxidized prematurely, resulting in a low yield. If the alloy elements are added later, the alloy blocks will not have time to melt in the molten steel, resulting in uneven composition and even the presence of foreign metals.

[0041] The reason why the temperature of the soaking section is controlled at 1080-1200° C. in the present invention is that it is beneficial to the uniform diffusion and full solid solution of each component in the steel, which is beneficial to the uniform microstructure and stable performance.

[0042] The reason why the present invention controls the start rolling temperature at 980-1050°C during rough and medium rolling, and controls the cumulative deformation rate of rough and medium rolling at 20-60% is that the use of this start rolling temperature can ensure that the steel billet is fully austenitized in the heating furnace and undergoes austenite rolling deformation in the subsequent rolling, which is beneficial to the uniformity of composition and structure. Controlling the cumulative deformation rate can ensure uniform and refined microstructure.

[0043] The reason why the temperature of entering the finishing rolling stage I is controlled at 810-850°C, the temperature of entering the finishing rolling stage II is controlled at 790-830°C, and the rolling speed of the final stand is controlled at 20-45m / s is that the finishing rolling still adopts rolling deformation in the austenite zone, which is beneficial to ensure the appropriate strength and plasticity of the finished steel product; the speed of the final stand is controlled at 20-45m / s because different rolling speeds are adopted for different specifications, which, on the one hand, exerts the deformation-induced strengthening effect, and on the other hand, ensures smooth production and increases hourly output.

[0044] The reason why the present invention controls the temperature entering the cooling bed at 570-620°C is to obtain appropriate strength and plasticity. If the temperature entering the cooling bed is too low, the strength of the steel will be too high and the plasticity will be insufficient. Otherwise, the strength of the steel will be insufficient and the plasticity will be too high.

[0045] Compared with the prior art, the present invention has a simple process and stable performance of the finished rolled threaded steel product while ensuring the hot rolled yield strength Rel≥830MPa, the tensile strength Rm≥980MPa, the elongation A≥6%, and the total elongation under maximum force Agt≥3.5%. In addition, by increasing the heating temperature by 25%, the rolling mill load can be reduced by 20%, and the roll consumption cost can be reduced by about 18%, which is beneficial to improving production efficiency and has strong process adaptability. In addition, due to the wide process window, it is beneficial to on-site operation and smooth production. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 This is a picture of the metallographic structure of the surface layer of the thread of the present invention being tempered martensite;

[0047] Figure 2 This is a picture of the metallographic structure of the thread core of the present invention being pearlite + ferrite. DETAILED DESCRIPTION

[0048] The present invention is described in detail below:

[0049] Table 1 is a list of chemical composition values ​​of various embodiments and comparative examples of the present invention;

[0050] Table 2 is a list of process parameters of the smelting process of various embodiments and comparative examples of the present invention;

[0051] Table 3 is a list of rolling process parameters of various embodiments of the present invention and comparative examples;

[0052] Table 4 shows the test results of various embodiments of the present invention and comparative examples.

[0053] Each embodiment of the present invention is produced according to the following steps

[0054] 1) Pre-treat the molten iron. After desulfurization, the molten iron has P < 0.020%, S < 0.020%;

[0055] 2) Converter smelting, control the converter endpoint C at 0.06-0.30%; when the molten steel is 2 / 3 out, add the alloy and carburizer all at once;

[0056] 3) Carry out LF furnace refining, the refining time is not less than 25min, and argon is blown throughout the refining process; after the composition is adjusted, the molten steel is prohibited from being exposed;

[0057] 4) Casting into square billets, during which a large tank long nozzle and a crystallizer immersion nozzle are used to protect the pouring, and the superheat of the molten steel in the tundish is controlled at 20-35°C; the size of the square billet is 160-170mm;

[0058] 5) Heat the ingot, control the temperature of the soaking section at 1080-1200°C, and the total heating time at 60-90 minutes;

[0059] 6) performing rough rolling and intermediate rolling, and controlling the starting rolling temperature to be between 980°C and 1050°C, and controlling the cumulative deformation rate of rough rolling and intermediate rolling to be between 20% and 60%;

[0060] 7) Finish rolling is carried out in two stages, during which the temperature of the finishing rolling stage I is controlled at 810-850°C, and the temperature of the finishing rolling stage II is controlled at

[0061] 790~830℃, the rolling speed of the last stand is 20~45m / s;

[0062] 8) Cooling is performed, the temperature of the cooling bed is controlled at 570-620°C, and the cooling rate is 6-9°C / s to cool to room temperature.

[0063] Table 1 Chemical composition list of various embodiments and comparative examples of the present invention (wt%)

[0064]

[0065]

[0066] Table 2 List of process parameters of the smelting process of each embodiment of the present invention and the comparative example

[0067]

[0068] Table 3 List of rolling process parameters of various embodiments of the present invention and comparative examples

[0069]

[0070]

[0071] Note: The size accuracy of square billet is ±3mm.

[0072] Table 4 Experimental results of various embodiments of the present invention and comparative examples

[0073]

[0074] It can be seen from Table 4 that, compared with the comparative example, the steel of the embodiment of the present invention has higher plastic properties such as yield strength, tensile strength and more suitable elongation, and can reduce production costs and increase output.

[0075] This specific implementation is only the best example and is not a restrictive implementation of the technical solution of the present invention.

Claims

1. A threaded steel bar with a yield strength of ≥830 MPa, wherein the components and weight percentages thereof are: C: 0.35-0.50%, Si: 0.50~0.90%, Mn: 0.40~0.90%, P≤0.035%, S≤0.035%, Al: 0.05~0.12%, V: 0.01~0.07%, N: 0.0045~0.010%, Zr: 0.40~0.60%, and the rest are Fe and inevitable impurities.

2. A threaded steel bar with a yield strength ≥830 MPa as claimed in claim 1, characterized in that: The weight percentage of C is 0.38-0.45%.

3. A threaded steel bar with a yield strength ≥ 830 MPa as claimed in claim 1, characterized in that: The weight percentage of Si is 0.55-0.85%.

4. A threaded steel bar with a yield strength ≥ 830 MPa as claimed in claim 1, characterized in that: The weight percentage of Mn is 0.50-0.80%.

5. The threaded steel bar with a yield strength ≥ 830 MPa according to claim 1, characterized in that: The weight percentage of Al is between 0.06 and 0.11%.

6. The threaded steel bar with a yield strength ≥ 830 MPa according to claim 1, characterized in that: The weight percentage of V is 0.01-0.06%.

7. The threaded steel bar with a yield strength ≥ 830 MPa according to claim 1, characterized in that: The weight percentage of N is 0.0055-0.009%.

8. The threaded steel bar with a yield strength ≥ 830 MPa according to claim 1, characterized in that: The weight percentage of Zr is 0.45-0.55%.

9. The method for producing a threaded steel bar with a yield strength of ≥830 MPa as claimed in claim 1, comprising the steps of: 1) Pre-treat the molten iron. After desulfurization, the molten iron has P < 0.020%, S < 0.020%; 2) Converter smelting, control the converter endpoint C at 0.06-0.30%; when the molten steel is 2 / 3 out, add the alloy and carburizer all at once; 3) Carry out LF furnace refining, the refining time is not less than 25min, and argon is blown throughout the refining process; after the composition is adjusted, it is forbidden to store molten steel In the phenomenon of nudity; 4) Casting into square billets, during which a large tank long nozzle and a crystallizer immersion nozzle are used to protect the pouring, and the superheat of the molten steel in the tundish is controlled at 20-35°C; the size of the square billet is 160-170mm; 5) Heat the ingot, control the temperature of the soaking section at 1080-1200°C, and the total heating time at 60-90 minutes; 6) performing rough rolling and intermediate rolling, and controlling the starting rolling temperature to be between 980°C and 1050°C, and controlling the cumulative deformation rate of rough rolling and intermediate rolling to be between 20% and 60%; 7) Finish rolling is performed in two stages, during which: the temperature of the finishing rolling stage I is controlled at 810-850°C, the temperature of the finishing rolling stage II is controlled at 790-830°C, and the rolling speed of the final stand is controlled at 20-45 m / s; 8) Cooling is performed, the temperature of the cooling bed is controlled at 570-620°C, and the cooling rate is 6-9°C / s to cool to room temperature.

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