Method for improving rolling rhythm of yield strength 235mpa grade steel plate

Abstract

The application discloses a method for improving rolling rhythm of a yield strength 235MPa steel plate, and belongs to the field of metal processing. The chemical components of the steel plate include the following in percentage by weight: C 0.17%-0.22%, Si 0.06%-0.1%, Mn 0.25%-0.49%, P≤0.035%, S≤0.035%, and the balance of Fe and inevitable impurities. The method comprises the following steps: slab heating, rolling, straightening (hot straightening), and air cooling to room temperature. The rolling rhythm of the steel plate is obviously improved, the average rolling time of each steel plate is reduced by 15-45 seconds, and the production efficiency is improved and the production cost is reduced.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of metal processing, and particularly relates to a method for improving rolling rhythm of a 235MPa grade steel plate with a thickness specification of 20-60mm and a width greater than 2200mm. BACKGROUND

[0002] In the face of the severe market situation of the steel industry, user requirements and manufacturing cost pressure, it is particularly important to improve the rolling rhythm and make the production line produce maximum capacity while ensuring product quality. The improvement of the rolling rhythm of the plate is to improve the output per unit time of the rolling line, i.e. the number of rolling blocks per hour, under the condition of ensuring production and equipment safety. Ultimately, high-efficiency production with maximum efficiency and minimum consumption can be realized while ensuring product quality. In particular, the market competition of the 235MPa grade plate has gradually become apparent in recent years. This type of steel is mainly based on carbon manganese steel, and micro-alloying elements such as Al are added to the steel as carbon and nitride forming elements to improve the strength and toughness of the steel plate through solid solution strengthening, precipitation strengthening and fine grain strengthening. According to statistics, the yield of this strength grade plate accounts for 20-30% of the total output each year. Therefore, in the future production of plate products, especially the 235MPa grade plate, it is urgent to further optimize the production process, further reduce energy consumption, alloy cost and manufacturing cost on the basis of the present, and improve the market competitiveness of the products.

[0003] Currently, for the 235MPa grade steel plate with a thickness of 20-60mm, in order to reduce the alloy cost, a medium-carbon and low-Mn composition design system is usually adopted, and the rolling and controlled cooling process path is optimized to reduce the performance fluctuation caused by the reduction of alloy cost. Generally speaking, TMCP (Thermal Mechanical Control Process) is a technology that controls the rolling temperature and reduction amount during rolling, and then implements air cooling or controlled cooling and accelerated cooling on the basis of controlled rolling. Since TMCP process can produce high-strength and high-toughness steel without adding too much alloy element and without the need for complex subsequent heat treatment, it is considered as a green and reduced process for saving alloy elements, and has become an indispensable technology for producing plate. However, the steel plate after alloy reduction is usually produced by TMCP process, and the finish rolling temperature is at 850℃ or even lower. The lower temperature increases the deformation resistance of the rolled piece, thereby increasing the load of the rolling mill and increasing the difficulty of shape control. In addition, low-temperature rolling requires a longer intermediate waiting time, which affects the rolling rhythm and production efficiency, resulting in an increase in the production cost of the 235MPa grade steel plate.

[0004] Therefore, how to improve the rolling rhythm of the 235MPa grade steel plate with a thickness specification of 20-60mm and a width greater than 2200mm and reduce its manufacturing cost is a key problem to be solved in the batch production process of the economic 235MPa grade plate.

[0005] Compared with the prior art:

[0006] So far, there are few studies on the method for improving the rolling rhythm of 235MPa grade steel plates with a thickness specification of 20-60mm and a width greater than 2200mm. Prior to the present application, a patent with publication number CN 115537524 A discloses a method for improving the rolling efficiency of A-grade shipbuilding medium plates. This method uses a non-TMCP rolling process, controls the billet heating system to make the final rolling temperature of the steel plate hit the pre-set target range; but its alloy composition is high, and the production thickness is mainly below 20mm, and the production process for thicknesses above 20mm is not involved. A patent with publication number CN 115446115 A discloses a method for improving the rolling efficiency of B-grade shipbuilding medium plates. This method uses a non-TMCP rolling process, controls the billet heating system to make the final rolling temperature of the steel plate hit the pre-set target range; but its alloy composition is high, and the production thickness is mainly below 18mm, and the production process for thicknesses above 20mm is not involved.

[0007] The plate shape control method disclosed in the above patent literature improves the rolling efficiency method of part of the thickness specification and 235MPa grade steel plates, but is not suitable for improving the rolling efficiency of economic 235MPa grade steel plates with a thickness specification of 20mm and above and a width greater than 2200mm. Using the technical solution provided by the present application, the above-mentioned deficiencies can be effectively overcome, and the problem of low rolling efficiency of 235MPa grade economic medium plates with a rolling thickness of 20-60mm and a width of 2200mm or more is solved by using a continuous casting billet with a thickness of 250mm or less, optimizing the heating and rolling processes. SUMMARY

[0008] The present application aims to overcome the above technical problems and deficiencies, and provides a method for improving the rolling rhythm of economic steel plates with a thickness specification of 20-60mm and a width greater than 2200mm and a yield strength of 235MPa, which does not require subsequent long-term controlled rolling to meet the performance requirements of the steel plate, and solves the problems of low rolling efficiency and one-pass plate shape pass rate of such steel plates due to low alloy composition, the need to control the rolling temperature during rolling, and the reduction of the final rolling temperature.

[0009] The application provides a method for improving rolling rhythm of a yield strength 235MPa grade steel plate, the chemical components of the steel plate include, by weight percentage, C 0.17%-0.22%, Si 0.06%-0.1%, Mn 0.25%-0.49%, P≤0.035%, S≤0.035%, and the balance of Fe and inevitable impurities; the thickness of the steel plate is 20-60mm, the width is greater than 2200mm, and the method is used for producing the steel plate on a medium plate reciprocating mill using a continuous casting billet with a thickness of less than 250mm.

[0010] The application aims to improve the rolling rhythm of a yield strength 235MPa grade steel plate, and the method is used for improving the rolling rhythm of a yield strength 235MPa grade economic medium plate with a thickness of 20-60mm and a width of more than 2200mm, and the method includes slab heating, rolling, straightening (hot straightening), and air cooling to room temperature, and specifically includes the following steps.

[0011] 1) Billet heating: the billet (with a thickness of less than 250mm) is sent into a walking beam heating furnace for heating, the billet is heated to 1100-1150℃, the total heating time of the soaking section and the heating section is 2-3.5 hours, and the total in-furnace time is 3-5 hours; the opening degree of the upper and lower burners of the soaking section is adjusted, the air-fuel ratio is controlled to be 1:1.7-1:2.2, and the temperature difference between the upper and lower surfaces of the billet is ensured to be within 15℃; (combined with the composition of the billet, the billet heating temperature is reduced to reduce energy consumption, and the finish rolling temperature is ensured to be in the austenite temperature range; in addition, the in-furnace time of the soaking section and the heating section is ensured, and the air-fuel ratio of the soaking section is adjusted to shorten the temperature difference between the upper and lower surfaces and the core of the continuous casting billet, improve the uniformity of the transverse and longitudinal metal flow of the steel plate surface, effectively inhibit the excessive growth of austenite grains, and ensure the performance of the steel plate and the plate shape in subsequent rolling).

[0012] 2) High pressure water descaling and rolling: descaling the cast slab after leaving the furnace for 0.5-0.8 min using high pressure water, the pressure of the descaling machine is 20-25 MPa, the reduction rate of each of the first three passes before rolling is greater than 20%, and the descaling water of the spray rolling machine before the first three passes, the time of each pass is 0.3-0.8 min, the pressure is 15-20 MPa; the reduction rate of the last pass is controlled to be less than 5% (the rolling mill capacity is exerted as much as possible in the first three passes before rolling, a large reduction rate is used, the microstructure distribution from the surface to the center of the steel plate is uniform through the flexible and variable high pressure water descaling process, the strength and toughness of the steel plate are improved, in the later stage of rolling, the steel plate temperature decreases and the deformation resistance increases, a small reduction rate is used in the last pass to flatten the plate shape and reduce the internal stress of the steel plate); when the rolling length of the corresponding pass is 50%, the pass table in the TCS jumps to the next pass to prepare for positioning (the set roll gap of the second stage is received and the roll gap is distributed; after the steel signal disappears in each pass, the equipment unrelated to rolling is allowed to position), the cooling water of the roller way before and after the rolling mill is reduced to 150-200 m 3 / h, at the same time, the speed of the roller way during the turning of the steel is increased from 1.0-2 m / s to 3-4 m / s (the roller way cooling water is reduced by half during the turning of the steel to ensure that the intermediate slab and the roller way slip during the turning of the steel to match the increased speed of the turning of the steel), the finishing temperature range is 970-920°C, the steel is thrown after rolling, the throwing speed is 4-6 m / s, then laminar cooling is used, the open cooling temperature range is 930-850°C, the final cooling temperature range is 770-830°C, and the cooling speed is 10-15°C / s (the open cooling temperature is controlled to ensure that the steel plate is in austenitic state when entering the water, during the cooling process, the cooling speed and the final cooling temperature are controlled to suppress the formation of bainite and martensite phases on the surface of the steel plate and the number of bainite phases in the core, to avoid affecting the toughness of the steel plate due to the bainite or martensite phase change on the surface).

[0013] 3) Hot straightening: one pass of straightening is performed after hot straightening, the entry roll gap is -1.2 mm to -2.5 mm, the exit roll gap is -2.6 mm to -3.9 mm, and the straightening force is between 1500 KN and 1800 KN (by setting appropriate roll gap and straightening force, the straightened steel plate is ensured to be flat and have good shape).

[0014] 4) Air cooling to room temperature.

[0015] The above components and heating, rolling and cooling process scheme overcome the deficiencies in the prior art, solve the problems that the steel plate needs to be controlled in the rolling process due to low alloy components, and affects the rolling efficiency and the one-time pass rate of the plate shape. The performance of the final obtained steel plate is the same as that produced by using the original TMCP process, the flatness of the steel plate is below 5mm / 2m, the yield strength of the steel plate in the transverse direction is greater than or equal to 235MPa, the tensile strength is between 380-450MPa, the elongation is greater than or equal to 26%, and the Charpy impact energy at-20℃ in the transverse direction is greater than or equal to 60J. The user's technical requirements are met. According to the calculation, after the optimization of the components, process, and steel turning and throwing speeds, the rolling rhythm of the 235MPa grade economic medium plate with a thickness of 20-60mm and a width of more than 2200mm is significantly improved, and the rolling time of each steel plate is reduced by 15-45 seconds on average, which has a significant effect on improving the production efficiency and reducing the production cost.

[0016] The beneficial effects of the present application are:

[0017] 1. By reducing the heating temperature of the blank, the energy consumption is reduced, while ensuring that the final rolling temperature is in the austenite temperature range; in addition, the soaking and heating sections are ensured in the furnace time, while the air-fuel ratio of the soaking section is adjusted to shorten the temperature difference between the upper and lower surfaces and the core of the continuous casting blank, improve the uniformity of the metal flow in the transverse and longitudinal directions of the steel plate, effectively inhibit the excessive growth of austenite grains, and ensure the performance of the steel plate and the shape of the subsequent rolled plate.

[0018] 2. By exerting the rolling mill capacity as much as possible in the first three passes, a large reduction rate is adopted, and a flexible and variable high-pressure water descaling process is used to make the microstructure distribution of the steel plate from the surface to the core uniform, while also inhibiting the excessive growth of grains, improving the strength and toughness of the steel plate, and reducing the rolling passes; due to the temperature drop of the steel plate in the later rolling stage, the deformation resistance increases, and the last pass is rolled with a small reduction rate to flatten the plate shape and reduce the internal stress of the steel plate; in addition, by setting the rolling model, when the rolling length of the current pass is 50%, the pass table in the TCS jumps to the next pass to perform positioning preparation in advance to receive the secondary setting roll gap and perform roll gap distribution, and after the steel containing signal disappears in each pass, the equipment unrelated to rolling is allowed to position; the throwing distance between the steel turning roller way speed and the throwing air passing speed is increased to significantly improve the rolling rhythm; the rolled steel plate is weakly cooled to control the open cooling temperature, so that the steel plate is in the austenite state when entering the water, and the cooling speed is controlled to inhibit the formation of bainite and martensite phases on the surface of the steel plate and the number of bainite phases in the core, so as to avoid the influence of bainite or martensite phase change on the toughness of the steel plate, and also improve the strength.

[0019] 3. Hot straightening is adopted, the guide-in and guide-out roller positions and the pressure are set to ensure that the straightened steel plate is flat and has good plate shape.

[0020] 4. The above heating, rolling and cooling process solves the problems of low alloy content, deep control during rolling, affecting the rolling efficiency and the one-time pass rate of the plate shape, etc. The performance of the final steel plate is the same as that produced by the original TMCP process, the flatness of the steel plate is below 5 mm / 2 m, the subsequent cold straightening process cost is saved, the yield strength of the steel plate is ≥235 MPa, the tensile strength is between 380-450 MPa, the elongation is ≥26%, the -20℃ transverse Charpy impact energy is ≥60J, meeting the user's technical requirements. According to the calculation, after the process optimization, the rolling rhythm of the 20-60mm thick, 2200mm wide and 235MPa grade economic medium plate is significantly improved, the average rolling time of each steel plate is reduced by 15-45 seconds, which has a significant effect on improving the production efficiency and reducing the production cost. DETAILED DESCRIPTION

[0021] The following examples are used to specifically illustrate the content of the present application, which are only a general description of the content of the present application and do not limit the content of the present application.

[0022] Table 1 is the chemical composition of the example steel, Table 2 is the heating system of the cast blank and the high-pressure water descaling process before rolling of the continuous casting blank of the example steel; Table 3 is the rolling method of the example steel; Table 4 is the cooling and steel straightening process of the example steel; Table 5 is the size, performance and flatness of the example steel.

[0023] Table 1 Chemical composition of the example steel (wt, %)

[0024]

[0025]

[0026] Note: Impurity elements in the steel P≤0.035%, S≤0.035%

[0027] Table 2 Heating system of the cast blank and high-pressure water descaling process before rolling of the continuous casting blank of the example steel

[0028]

[0029] Table 3 Rolling method of the example steel

[0030]

[0031] Table 4 Cooling and steel straightening process of the example steel

[0032] Example Start cooling temperature / °C Finish cooling temperature / °C Cooling rate / °C Entry roll gap / mm Exit roll gap / mm Straightening force / KN 1 910 820 12 -1.3 -2.6 1700 2 870 805 15 -1.7 -2.7 1800 3 880 795 10 -2.2 -3.1 1550 4 915 810 11 -2.3 -3.6 1600 5 905 825 13 -1.6 -3.3 1700 6 892 785 15 -1.4 -3.2 1800

[0033] Table 5 Size, performance and flatness of the example steel

[0034]

[0035] Therefore, compared with the prior art, the method for improving the rolling rhythm of the yield strength 235MPa steel plate provided by the application. The technical scheme provided by the application solves the problems that the steel plate needs to be controlled in the rolling process due to the low alloy composition, which affects the steel plate rolling efficiency, plate shape one-pass rate and the like. According to the calculation, after the composition and process are optimized, the rolling time of each steel plate of the yield strength 235MPa economic medium plate with a thickness of 20-60mm and a width of 2200mm or more is reduced by 15-45 seconds, and the flatness of the steel plate is less than 5mm / 2m, which has a significant effect on improving the production efficiency and reducing the production cost.

[0036] The above examples are only for illustrating the technical concept and characteristics of the application, and the purpose is to enable those skilled in the art to understand the content of the application and implement it, and cannot limit the protection scope of the application. Any equivalent changes or modifications made according to the spirit and principle of the application shall be covered within the protection scope of the application.

Claims

1. A method for improving the rolling pace of a yield strength 235 MPa grade steel sheet, characterized in that, The method comprises the following steps: 1) slab heating: the slab is sent into a walking beam furnace for heating, the slab is heated to 1100-1150℃, the total heating time of the soaking section and the heating section is 2-3.5 hours, and the total in-furnace time is 3-5 hours; the air-fuel ratio of the soaking section is controlled to be 1:1.7-1:2.2, and the temperature difference between the upper and lower surfaces of the slab is ensured to be within 15℃; 2) high-pressure water descaling and rolling: the slab after being discharged from the furnace is descaled by high-pressure water for 0.5-0.8 minutes, the pressure of the descaling machine is 20-25 MPa, the reduction rate of each pass of the first three passes before rolling is greater than 20%, and the descaling water of the spray descaling machine before rolling is used for 0.3-0.8 minutes per pass, and the pressure is 15-20 MPa; the reduction rate of the last pass is controlled to be less than 5%; when the rolling length of the current pass reaches 50%, the pass table in the TCS jumps to the next pass to prepare for positioning in advance; The cooling water of the roller tables before and after the rolling mill is reduced to 150-200 m 3 / h, while the speed of the roller tables during the turning of the steel is increased to 3-4 m / s, the finishing temperature is 970-920 °C, the steel is thrown after rolling at a speed of 4-6 m / s, then laminar cooling is applied, the open cooling temperature is 930-850 °C, the final cooling temperature is 770-830 °C, and the cooling speed is 10-15 °C / s; 3) hot straightening: the entry roll gap is -1.2mm to -2.5mm, the exit roll gap is -2.6mm to -3.9mm, and the straightening force is 1500KN-1800KN; 4) air cooling to room temperature; The chemical composition of the steel plate comprises, by weight percentage: C 0.17%-0.22%, Si 0.06%-0.1%, Mn 0.25%-0.49%, P≤0.035%, S≤0.035%, and the balance is Fe and inevitable impurities.

2. The method of claim 1, wherein, In step 1), the air-fuel ratio is controlled to be 1:1.7-1:2.2 by adjusting the opening degree of the upper and lower burners in the soaking section.

3. The method of claim 1, wherein, In step 3), the hot straightening is one straightening.

4. The method of claim 1, wherein, The thickness of the steel plate is 20-60mm, and the width is greater than 2200mm.

5. The method of claim 1, wherein, The thickness of the slab is less than 250mm.

6. The method of claim 1, wherein, The steel plate is obtained by rolling the slab on a medium plate reciprocating mill.

7. The method of claim 1, wherein, The yield strength of the steel plate in the transverse direction is greater than or equal to 235MPa, the tensile strength is 380-450MPa, the elongation is greater than or equal to 26%, the Charpy impact energy at -20℃ in the transverse direction is greater than or equal to 60J, and the flatness is less than 5mm / 2m.

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