A process for producing a wide and medium thick steel plate

Abstract

This invention discloses a production process for wide-width medium-thick steel plates. The process includes continuous casting, hot charging, two-stage controlled rolling, mist cooling straightening, two-stage quenching, and tempering. In the continuous casting process, the thickness of the chilled layer on the billet surface is controlled to 3-6 mm, and the thickness of the oxide layer is controlled to 4-7 mm during the heating process after hot charging. This ensures the chilled layer is removed during subsequent oxidation and descaling, thus preventing surface cracking during hot charging. Two-stage mist cooling straightening improves the flatness of the steel plate. Edge heating is applied during rolling, and the amount of water on the upper and lower surfaces is controlled during quenching to improve the uniformity of the steel plate's properties. The steel plate obtained by this invention has a flatness ≤3.5 mm / 2 m, and the strength difference between the center and edge in the width direction is ≤5%, and the low-temperature impact energy difference is ≤4%. This solves the problems of low flatness and uneven mechanical property distribution in wide-width medium-thick steel plates and effectively improves production efficiency.

Description

Technical Field

[0001] This invention relates to the field of metal material processing technology, and specifically to a production process for wide-width medium-thick steel plates. Background Technology

[0002] Hot charging and hot delivery technology can reduce energy consumption and improve production efficiency in steel production. However, the application of hot charging and hot delivery can easily cause surface star cracks in steel plates, especially microalloyed or low-alloy steels containing Ti and Nb, during subsequent rolling. Meanwhile, medium and heavy steel plates undergo high-temperature deformation and rapid cooling during controlled rolling, controlled cooling, and quenching. The formation of surface star cracks makes the steel plates more prone to problems such as difficulty in controlling plate shape quality and excessive performance fluctuations in different areas of the same plate. Especially for wide medium and heavy steel plates with a thickness of ≥20mm and a width ≥4000mm, simultaneously improving production efficiency through hot charging and hot delivery while avoiding poor plate shape and ensuring uniform performance distribution remains a pressing challenge.

[0003] CN 102228968 B discloses a method for directly feeding high-strength low-alloy steel continuously cast billets. This method rapidly cools the billet on a roller conveyor, lowering the surface temperature below the two-phase region. This avoids the nucleation of pseudo-grain boundary ferrite and other deposits on the surface during heating in the two-phase region, thereby preventing surface star cracks during steel plate rolling. However, this patented method's rapid cooling of the billet may cause other internal defects. Furthermore, the process of rapidly cooling the billet below the two-phase region and then reheating it for rolling also compromises the energy-saving effect of the hot-feeding and hot-charging process.

[0004] CN 103468902 B discloses a method for improving the thickness-direction properties of thick-gauge high-strength steel. This patent employs ACC cooling with head, tail, and edge shielding to control the uniform and rapid cooling of the steel plate after rolling, and uses no more than three hot straightening processes to control the unevenness of the steel plate, thereby obtaining thick-gauge steel plates with good thickness-direction properties. However, this preparation method has a slow cooling rate and a high final cooling temperature, which is not conducive to improving the strength of the steel plate. At the same time, the multi-pass hot straightening process requires reheating to 400~1000℃, reducing production efficiency. Furthermore, it does not address the problems of improving production efficiency through hot conveying and hot charging, or how to solve the problem of excessive performance fluctuations in different areas of the same plate.

[0005] CN 117900260 A discloses a method for producing high-straightness bridge steel. This patent involves pre-straightening after rolling, followed by hot straightening at the Mulpic exit to obtain high-straightness bridge steel with a thickness of 6-50 mm and a width of 1500-4200 mm. However, this method employs air cooling and slow cooling in the stack after rolling, resulting in steel plates with lower strength and difficulty in controlling the uniformity of plate properties.

[0006] Among the aforementioned existing technologies, the use of hot-feeding and hot-loading processes to improve the production efficiency of wide and medium-thick steel plates while simultaneously enhancing the control of plate shape and performance uniformity is an important topic for the further development of large-size equipment and pipeline manufacturing in various fields. Summary of the Invention

[0007] To address the shortcomings of the existing technology, this invention aims to provide a production process for wide-width medium-thick steel plates, producing wide-width medium-thick steel plates with a thickness of 20~40mm and a width of ≥4000mm, while optimizing the hot delivery and hot charging process and improving the flatness and performance uniformity of the wide-width medium-thick steel plates.

[0008] The technical solution of the present invention: A production process for wide-width medium-thick steel plates, producing plates with a thickness of 20-40mm and a width ≥4000mm, includes continuous casting, hot charging, reheating, two-stage controlled rolling, mist cooling straightening, two-stage quenching and tempering. Key process steps include: 1) Continuous casting: control the thickness of the quench layer on the surface of the billet to 3~6mm; control the superheat of the casting to 25~45℃; control the temperature difference between the inlet and outlet water of the crystallizer to 4~7℃; control the subsequent secondary cooling water volume to 0.5~0.7L / kg; use electromagnetic stirring to improve the center segregation of the billet, with a stirring frequency of 4~7Hz; and perform dynamic light reduction on the billet, with a reduction rate of 3%~6%.

[0009] 2) Hot feeding and charging: The cast billets are directly fed into the heating furnace via roller conveyors for reheating; the billet temperature upon entering the heating furnace is 500~950℃, the furnace inlet temperature is ≥950℃, the billet outlet temperature is ≥1200℃, and the furnace dwell time is 4.5~6.0h; the heating furnace uses mixed gas with a calorific value ≥8000kJ / m³. 3 The excess air coefficient is 1.2~1.4; the oxide layer thickness on the surface of the billet is controlled to be 4~7mm.

[0010] 3) Reheating: Zoned induction heaters are arranged at the outlet of the heating furnace to locally reheat both sides of the steel plate, so that the edge surface temperature of the steel plate is 10~30℃ higher than the center surface temperature when the rolling begins.

[0011] 4) Two-stage control: Rolling includes roughing and finishing. The roughing temperature is ≥1200℃, the finishing temperature is ≥1070℃, the number of rolling passes is ≤6, the pass reduction rate is 15~20%, and the number of descaling passes is ≥3. The finishing temperature is ≥1050℃, the finishing temperature is ≥860℃, the rolling passes are 7~9, and the pass reduction rate is 5~15%.

[0012] 5) Fog cooling straightening: This includes two-stage straightening and air mist controlled cooling. The first stage of straightening is carried out after finishing rolling, followed by air mist controlled cooling. The second stage of straightening begins after the self-tempering stage following the completion of air mist controlled cooling.

[0013] 6) Two-stage quenching and tempering: Two-stage quenching includes high-pressure zone quenching and low-pressure zone quenching; tempering is performed using stacked plate tempering; the steel plate is heated, and ultrasonic vibration is used during the holding process to promote the nucleation of austenite crystals in the steel plate microstructure; the ultrasonic frequency is 20~40Hz; the ratio of upper to lower water flow rate is 1:1.5~1.8, the roller speed is 0.32~0.42m / s, and the speed is 0.0015~0.0025m / s. 2 The acceleration is slightly accelerated; the quenching temperature is 880~930℃, the holding time is 25~50min, the tempering temperature is 200~250℃, and the tempering time is 30~60min.

[0014] Further, step 5) mist cooling straightening: The two-stage straightening includes high-temperature straightening and low-temperature straightening, wherein the high-temperature straightening temperature range is 830~860℃; after high-temperature straightening, the steel plate is cooled to 200~300℃, and low-temperature straightening begins; the mist-controlled cooling uses pressurized air to atomize the water flow into a mist, which evenly covers the surface of the rolled piece to form a water film that carries away heat, and the cooling rate is controlled at 40~60℃ / s.

[0015] Furthermore, step 6) two-stage quenching: the high-pressure zone quenching uses two sets of slit nozzles, with a water pressure of 5~8 bar and a water flow rate ≥8000 m³ / h. 3 The preferred water pressure is 5-6 bar per hour, and the water flow rate is 8500-9000 m³ / h. 3 The steel plate is cooled to 500~600℃ per hour; the low-pressure zone uses high-density nozzles with a water pressure of 2~4 bar and a water flow rate of ≥1500 m³ / h. 3 The preferred water pressure is 2-3 bar per hour, and the water flow rate is 1500-1600 m³ / h. 3 / h, cool the steel plate to 200~300℃; quenching temperature is 890~910℃, hold for 30~40min, tempering temperature is 220~240℃, tempering time is 40~50min.

[0016] Furthermore, the chemical composition of the steel by weight percentage is: C = 0.14%~0.18%, Si = 0.20%~0.50%, Mn = 0.80%~1.30%, Al = 0.03%~0.10%, Cr = 0.10%~0.40%, Mo = 0.40%~0.70%, Ti = 0.01%~0.04%, V = 0.02%~0.06%, Nb = 0.01%~0.03%, B = 0.0008%~0.0015%, P ≤ 0.02%, S ≤ 0.005%, with the balance being Fe and other unavoidable impurities; the yield strength Rp at 1 / 4 of the width direction of the steel plate. 0.2 ≥1200MPa, tensile strength Rm≥1400MPa, impact energy value at -40℃≥60J, strength difference between the center and edge of the steel plate in the width direction ≤5%, low temperature impact energy difference ≤4%, unevenness ≤3.5mm / 2m.

[0017] The innovative points and beneficial effects of this invention are as follows: (1) Controlling the thickness of the chilled layer on the continuous casting billet allows the chilled layer on the billet surface to be transformed into an oxide layer during subsequent heating and removed during descaling, thereby avoiding the formation of surface star cracks during rolling and improving the surface quality of wide and medium-thick steel plates under hot delivery and hot charging processes. (2) Controlling the rolling process at a higher temperature helps to reduce deformation resistance, and controlling the surface temperature of the steel plate edge to be 10~30℃ higher than the surface temperature of the center during rolling promotes the deformation resistance of the edge and the center to be consistent, thereby improving the flatness of the steel plate and the uniformity of the microstructure properties in the width direction. (3) The mist cooling and straightening process uses water mist cooling, and straightening is carried out before and after the controlled cooling process. The uniform and rapid cooling of the steel plate, combined with the high-temperature straightening and low-temperature straightening processes, helps to improve the flatness of the steel plate shape. (4) The two-stage quenching process employs ultrasonic treatment during heat preservation to refine austenite grains and improve their uniformity; by adjusting the nozzle arrangement and the ratio of upper and lower water volume, the steel plate is rapidly cooled and the heat dissipation uniformity of the upper and lower surfaces is improved, thus optimizing the mechanical properties of the plate and improving the uniformity of its microstructure. The steel plate obtained by this invention has a flatness ≤3.5mm / 2m, and the strength difference between the center and the edge in the width direction is ≤5%, and the low-temperature impact energy difference is ≤4%. Detailed Implementation

[0018] The following detailed description will be provided in conjunction with embodiments and comparative examples. These embodiments and comparative examples are merely illustrative of the invention and should not be construed as limiting the invention.

[0019] Examples 1-4 and Comparative Examples 1-7 were used to produce high-strength steel for engineering machinery. The chemical composition of the steel (by weight percentage) was C=0.16%, Si=0.30%, Mn=1.25%, Al=0.06%, Cr=0.20%, Mo=0.60%, Ti=0.03%, V=0.04%, Nb=0.03%, B=0.0010%, P≤0.02%, S≤0.005%, with the balance being Fe and other unavoidable impurities. The process flow included continuous casting, hot charging, reheating, two-stage controlled rolling and mist cooling straightening, two-stage quenching and tempering. Key steps included: 1) Continuous casting: control the superheat of the casting to 40℃, the temperature difference between the inlet and outlet water of the crystallizer to 6℃, the secondary cooling water volume to 0.6L / kg, the electromagnetic stirring frequency to 6Hz, and the dynamic light reduction rate of the billet to 5%.

[0020] 2) Hot charging and delivery: The furnace inlet temperature is 950℃, the billet outlet temperature is 1220℃, the furnace dwell time is 5 hours, and the calorific value of the mixed gas is 8000 kJ / m³. 3 The excess air index is 1.2, the oxide layer thickness is controlled as shown in Table 1, the descaling water pressure is 200 bar, and the descaling speed is 1 m / s.

[0021] 3) Reheating: Control the edge surface temperature of the steel plate to be 20°C higher than the center surface temperature when tapping.

[0022] 4) Two-stage controlled rolling: including roughing and finishing rolling. The initial rolling temperature of roughing is 1200℃, the final rolling temperature is 1180℃, and the number of rolling passes is 5. The reduction rate of 4 passes is 18%, the reduction rate of 1 pass is 20%, and the thickness of the intermediate billet is 60mm. The initial rolling temperature of the finishing mill is 1050℃, and the final rolling temperature is shown in Table 1. The mill is rolled in 9 passes with a reduction rate of 10% per pass. A zoned induction heater is arranged at the outlet of the heating furnace to locally supplement the heating on both sides of the steel plate, so that the edge surface temperature of the steel plate is higher than the center surface temperature when the rolling begins. The specific values ​​of the difference are shown in Table 1.

[0023] 5) Fog cooling straightening: The fog cooling straightening includes two-stage straightening and air mist controlled cooling. The first stage of straightening (i.e., high temperature straightening) is carried out after finishing rolling, followed by air mist controlled cooling. The second stage of straightening, i.e. low temperature straightening, begins in the self-tempering stage after the air mist controlled cooling is completed. The first stage of straightening is high-temperature straightening, and the temperature is shown in Table 1. After the first stage of straightening is completed, air mist cooling is carried out. Pressurized air is used to atomize the water flow into an air mist, which is evenly covered on the surface of the rolled piece to form a water film that carries away the heat. The cooling rate is controlled, and the specific cooling rate is shown in Table 1. When the temperature is cooled to 250°C, the cooling is stopped to allow the steel plate to produce a self-tempering effect, and then low-temperature straightening is carried out.

[0024] 6) Two-stage quenching and tempering: After mist-cooling straightening, the steel plate is heated to 890℃ and held for 50 minutes, supplemented with ultrasonic treatment (ultrasonic frequency of 30Hz), followed by two-stage quenching. During the two-stage quenching process, high-pressure quenching is performed first, followed by low-pressure quenching. The ratio of upper to lower water flow rates during quenching is shown in Table 1. The roller speed is 0.34m / s, and the speed is 0.002m / s. 2 The acceleration is slightly accelerated. In actual operation, the steel plate quenched in the high-pressure zone reaches 550℃ and then undergoes low-pressure zone quenching to 250℃.

[0025] The high-pressure zone uses two sets of slit nozzles, with a water pressure of 6 bar and a water flow rate of 8500 m³ / h. 3 / h; High-density nozzles are used in the low-pressure zone, with a water pressure of 3 bar and a water flow rate of 3000 m³ / h. 3 / h.

[0026] After the two-stage quenching is completed, the steel plates are tempered by stacking, with a stack thickness of ≤160mm, a tempering temperature of 230℃, and a tempering holding time of 40min.

[0027] Table 1 lists a comparison of some process parameters between Examples 1-4 and Comparative Examples 1-7 for high-strength steel used in engineering machinery. Other parameters not listed are consistent. The finished steel plate has a size of 20mm × 4000mm, and its performance indicators are shown in Table 2.

[0028] Table 1. Main hot working parameters for producing high-strength steel for engineering machinery in the examples and comparative examples. .

[0029] Table 2 Performance Indicators of High-Strength Steel for Engineering Machinery Produced in the Examples and Comparative Examples .

Claims

1. A production process for wide-width medium-thick steel plates, producing wide-width medium-thick steel plates with a thickness of 20~40mm and a width ≥4000mm, characterized in that... Key process steps include: 1) Continuous casting: Control the thickness of the quench layer on the surface of the billet to 3~6mm; control the superheat of the casting to 25~45℃; control the temperature difference between the inlet and outlet water of the crystallizer to 4~7℃; control the subsequent secondary cooling water flow rate to 0.5~0.7L / kg; use electromagnetic stirring to improve the center segregation of the billet, with a stirring frequency of 4~7Hz; perform dynamic light reduction on the billet, with a reduction rate of 3%~6%; 2) Hot feeding and charging: The cast billets are directly fed into the heating furnace via roller conveyors for reheating; the billet temperature upon entering the heating furnace is 500~950℃, the furnace inlet temperature is ≥950℃, the billet outlet temperature is ≥1200℃, and the furnace dwell time is 4.5~6.0h; the heating furnace uses mixed gas with a calorific value ≥8000kJ / m³. 3 The excess air coefficient is 1.2~1.4; the oxide layer thickness on the surface of the billet is controlled to be 4~7mm; before rough rolling, the oxide layer on the surface of the billet is dephosphorized, with a dephosphorization water pressure of 200~240bar and a dephosphorization speed of 0.5~1m / s; 3) Reheating: Zoned induction heaters are arranged at the outlet of the heating furnace to locally reheat both sides of the steel plate, so that the edge surface temperature of the steel plate is 10~30℃ higher than the center surface temperature when the rolling begins. 4) Two-stage control: Rolling includes roughing and finishing. The roughing temperature is ≥1200℃, the finishing temperature is ≥1070℃, the number of rolling passes is ≤6, the pass reduction rate is 15~20%, and the number of descaling passes is ≥3. The finishing temperature is ≥1050℃, the finishing temperature is ≥860℃, the rolling passes are 7~9, and the pass reduction rate is 5~15%. 5) Fog cooling straightening: It includes two-stage straightening and air mist controlled cooling. The first stage of straightening is carried out after finishing rolling, followed by air mist controlled cooling. The second stage of straightening begins in the self-tempering stage after the air mist controlled cooling is completed. 6) Two-stage quenching and tempering: Two-stage quenching includes high-pressure zone quenching and low-pressure zone quenching; tempering is performed using stacked plate tempering; the steel plate is heated, and ultrasonic vibration is used during the holding process to promote the nucleation of austenite crystals in the steel plate microstructure; the ultrasonic frequency is 20~40Hz; the ratio of upper to lower water flow rate is 1:1.5~1.8, the roller speed is 0.32~0.42m / s, and the speed is 0.0015~0.0025m / s. 2 The acceleration is slightly accelerated.

2. The production process of a wide-width medium-thick steel plate according to claim 1, characterized in that... 5) Fog cooling straightening: The two-stage straightening includes high-temperature straightening and low-temperature straightening, wherein the high-temperature straightening temperature range is 830~860℃; after high-temperature straightening, the steel plate is cooled to 200~300℃ and low-temperature straightening begins; the mist-controlled cooling uses pressurized air to atomize water into a mist, which is evenly covered on the surface of the rolled piece to form a water film that carries away heat, and the cooling rate is controlled at 40~60℃ / s.

3. The production process of a wide-width medium-thick steel plate according to claim 1, characterized in that... Step 6) Two-stage quenching: The high-pressure zone quenching uses two sets of slit nozzles, with a water pressure of 5~8 bar and a water flow rate ≥8000 m³ / h. 3 The preferred water pressure is 5-6 bar per hour, and the water flow rate is 8500-9000 m³ / h. 3 The steel plate is cooled to 500~600℃ per hour; the low-pressure zone uses high-density nozzles with a water pressure of 2~4 bar and a water flow rate of ≥1500 m³ / h. 3 The preferred water pressure is 2-3 bar per hour, and the water flow rate is 1500-1600 m³ / h. 3 / h, cool the steel plate to 200~300℃; quenching temperature is 880~930℃, hold for 25~50min, tempering temperature is 200~250℃, tempering time is 30~60min.

4. The production process of a wide-width medium-thick steel plate according to claim 1, characterized in that: The chemical composition of the steel by weight percentage is C=0.14%~0.18%, Si=0.20%~0.50%, Mn=0.80%~1.30%, Al=0.03%~0.10%, Cr=0.10%~0.40%, Mo=0.40%~0.70%, Ti=0.01%~0.04%, V=0.02%~0.06%, Nb=0.01%~0.03%, B=0.0008%~0.0015%, P≤0.02%, S≤0.005%, with the balance being Fe and other unavoidable impurities; Yield strength Rp at 1 / 4 position in the width direction of the steel plate 0.2 ≥1200MPa, tensile strength Rm≥1400MPa, impact energy value at -40℃≥60J, strength difference between the center and edge of the steel plate in the width direction ≤5%, low temperature impact energy difference ≤4%, unevenness ≤3.5mm / 2m.