Method for improving rolling rhythm of 355mpa grade steel plate with yield strength

Abstract

The application discloses a method for improving rolling rhythm of a yield strength 355MPa grade 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 0.16%-0.19%, Si 0.15%-0.3%, Mn 0.9%-1.25%, Al 0.015%-0.04%, Ti 0.005%-0.012%, P≤0.025%, S≤0.02%, Ceq≤0.38%, wherein Ceq=C+Mn / 6, and the balance is 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 45-90 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 355 MPa grade steel plate with a thickness specification of 20-50 mm. 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 plate rolling rhythm 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. Especially for 355 MPa grade plate varieties (Q355B, AH36, etc.), this type of steel is mainly based on carbon manganese steel, and micro-alloying elements such as Al, Ti, Nb, V, etc. are added to the steel to form carbon and nitride, which improves the strength and toughness of the steel plate through solid solution strengthening, precipitation strengthening and fine grain strengthening. According to statistics, the annual output of this strength grade plate accounts for 25-35%. Therefore, in the future production of plate products, especially 355 MPa grade plate varieties, 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 products.

[0003] Currently, for 355 MPa grade steel plates with a thickness of 20-50 mm, in order to reduce alloy cost, a composition design system of medium carbon, medium Mn and adding trace amounts of Nb, V and Ti is usually adopted, and the rolling and controlled cooling process path is optimized to reduce the performance fluctuation caused by alloy cost reduction. Generally speaking, TMCP (Thermal Mechanical Control Process) is a technology that controls the rolling temperature and reduction in the rolling process based on the control of rolling, and then implements air cooling or controlled cooling and accelerated cooling. Since TMCP process produces high-strength and high-toughness steel without adding too much alloy element and without the need for complex subsequent heat treatment, it is considered 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 usually adopts two-stage or even three-stage controlled rolling TMCP process, and in order to ensure the cumulative reduction rate of the second stage, the intermediate blank waiting time is long, and the finish rolling temperature is at 820℃ or even lower. The lower temperature increases the deformation resistance of the rolled piece, thereby increasing the load of the rolling mill and making plate shape control more difficult. In addition, low-temperature rolling requires a longer intermediate waiting time, which affects the rolling rhythm and production efficiency, resulting in increased production cost of economic 355 MPa grade steel plates.

[0004] Therefore, how to improve the rolling rhythm of the 20-50mm thickness specification 355MPa grade steel plate and reduce its manufacturing cost is a key problem to be solved in the batch production process of the economic 355MPa grade medium plate.

[0005] Compared with the prior art:

[0006] So far, there is little research on the method of improving the rolling rhythm of the 20-50mm thickness specification 355MPa grade steel plate at home and abroad. Before the present application, the patent with publication number CN115446114A discloses a method for improving the rolling efficiency of A36 grade shipbuilding medium plate. The method uses a non-TMCP rolling process, controls the heating system of the billet, and makes the finishing rolling temperature hit the pre-set target range; but its alloy composition is high, and the production thickness is mainly below 16mm, and the production process for thickness 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 355MPa grade steel plate, but it is not suitable for improving the rolling efficiency of the 20mm and above thickness specification 355MPa grade economic steel plate. 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 the 20-50mm thickness specification 355MPa grade economic medium plate is solved by using a continuous casting billet with a thickness of 250mm and below, 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 the 20-50mm thickness specification 355MPa grade economic steel plate. Without subsequent long-time controlled rolling, the performance requirements of the steel plate can be met, and the problems such as the need to control the rolling temperature during the rolling process due to the low alloy composition of such steel plate, the reduction of the finishing rolling temperature, and the influence on the steel plate rolling efficiency and the one-time pass rate of the plate shape are solved.

[0009] The present application provides a method for improving the rolling rhythm of the 355MPa grade steel plate, and the chemical composition of the steel plate includes, by weight percentage: C 0.16%-0.19%, Si 0.15%-0.3%, Mn 0.9%-1.25%, Al 0.015%-0.04%, Ti 0.005%-0.012%, P≤0.025%, S≤0.02%, Ceq≤0.38%, wherein Ceq=C+Mn / 6, the balance is Fe and unavoidable impurities; the thickness of the steel plate is 20-50mm, and the continuous casting billet with a thickness of 250mm or below is used for production on a medium plate reciprocating mill.

[0010] The application aims at improving the rolling rhythm of the yield strength 355MPa steel plate by the following technical scheme.

[0011] 1) Casting blank heating: the casting blank (thickness below 250mm) is sent into the step-by-step heating furnace for heating, the casting blank is heated to 1050-1095 DEG C, the total heating time of the soaking section and the heating section is 2-3.5 hours, 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 1:1.8-1:2.3, and the temperature difference between the upper and lower surfaces of the blank is ensured to be within 15 DEG C; (combined with the composition of the blank, the blank heating temperature is reduced, the energy consumption is reduced, 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 blank, improve the uniformity of the transverse and longitudinal metal flow of the steel plate, effectively inhibit the excessive growth of the austenite grains, and ensure the performance of the steel plate and the subsequent rolling shape), and 4-6 spaces are left at the furnace head of the heating furnace (to prevent the low furnace head temperature from affecting the uniformity of the blank temperature).

[0012] 2) high pressure water descaling and rolling: descaling the cast slab after discharge for 0.5-1 min using high pressure water, descaling machine pressure 20-25 MPa, rolling reduction of each of the first three passes is greater than 20%, and the descaling water of the first three passes is sprayed before rolling, each pass time 0.2-0.5 min, pressure 15-20 MPa, before the last three passes of rolling, the intermediate slab is short-time temperature controlled for 10-20 s, the last pass reduction is controlled to be less than 5% (the first three passes of rolling are made to exert the rolling mill capacity as much as possible, the uniform distribution of the structure from the surface to the center of the steel plate is achieved by flexible and changeable high pressure water descaling process, the strength and toughness of the steel plate are improved, in the later stage of rolling, the last pass is rolled with small reduction to flatten the plate shape and reduce the internal stress of the steel plate, before the last three passes of rolling, the intermediate slab is short-time temperature controlled to make the surface fine grain zone static recrystallize, ensure the performance of the rolled steel plate, and also make the internal stress of the steel plate fully released); when the corresponding pass of rolling is performed, the pass table in the TCS jumps to the next pass to prepare for positioning in advance (the set roll gap of the second stage is received and the roll gap is distributed in advance; after the steel signal disappears in each pass, the equipment unrelated to rolling is allowed to position), at the same time, the speed of the transfer roller is increased from 1.0-2 m / s to 3-4 m / s, the final rolling temperature of the steel plate is 850-900 °C, the steel is thrown after rolling, the throwing speed is 5-6.5 m / s, then laminar cooling is adopted, the open cooling temperature range is 830-860 °C, the final cooling temperature interval is 630-660 °C, and the cooling speed is 25-40 °C / s (the open cooling temperature is controlled to ensure that the steel plate is in austenite 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 straightening is performed after hot straightening, the entry roll gap is -1.0 mm to -2.6 mm, the exit roll gap is -2.9 mm to -3.8 mm, and the straightening force is between 2000 KN and 2500 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 deep controlled rolling in the rolling process due to low alloy components, and affect 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 obtained by using the existing 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 355MPa, the tensile strength is between 490MPa and 630MPa, the elongation is greater than or equal to 22%, and the Charpy impact energy in the transverse direction at-20℃ is greater than or equal to 100J. The technical requirements of the user are met. According to the calculation, after the optimization of the components, process, and steel turning speed and throwing speed, the rolling rhythm of the 20-50mm thick plate with a yield strength of 355MPa level is significantly improved, and the rolling time of each steel plate is reduced by 45-90 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, in addition, the head of the soaking section of the heating furnace is left empty to ensure the uniformity of the steel blank temperature, and finally to ensure the uniformity of the transverse and longitudinal metal flow on the surface of the steel plate, which also effectively inhibits the excessive growth of austenite grains, and ensures the performance of the steel plate and the subsequent rolling shape.

[0018] 2. By using large reduction rate in the first three passes before rolling, and adopting flexible and variable high-pressure water descaling process, while also reducing the rolling passes; before the last three passes, the intermediate blank is short-time temperature holding, so that the surface fine grain zone occurs static recrystallization, to ensure the rolling plate performance and the uniform distribution of the structure from the surface to the core of the steel plate, while also inhibiting the excessive growth of grains, improving the strength and toughness of the steel plate, and also fully releasing the internal stress in the steel plate; 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 small reduction rate to flatten the plate shape and further reduce the internal stress of the steel plate; in addition, through the rolling model setting, when the rolling length of the current pass is 80%, the pass table in the TCS jumps to the next pass to perform positioning preparation in advance, so as to receive the setting roll gap of the second level and perform roll gap distribution, and after the disappearance of the steel content signal in each pass, the equipment unrelated to rolling is allowed to position; the throwing distance between the steel turning roller speed and the throwing air speed is increased, so that the rolling rhythm is significantly improved; the rolled steel plate is cooled by controlled cooling, the open cooling temperature is controlled, and the steel plate is ensured to be in the austenite state when entering the water, while the cooling rate and the red temperature are 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 structure, to avoid affecting the toughness of the steel plate due to the bainite or martensite phase change on the surface, and also to improve the strength.

[0019] 3. Adopting hot straightening, setting the position and pressure of the leading and trailing rollers to ensure the straightness and good shape of the straightened steel plate.

[0020] 4. The above heating, rolling and cooling process scheme solves the problems of the low alloy content of the steel plate, the need for deep control during rolling, the influence on 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 using the existing TMCP process, the flatness of the steel plate is less than 5 mm / 2 m, the cost of the subsequent cold straightening process is saved, the yield strength of the steel plate in the transverse direction is greater than or equal to 355 MPa, the tensile strength is between 490-630 MPa, the elongation is greater than or equal to 22%, and the Charpy impact energy in the transverse direction at -20°C is greater than or equal to 100 J. The technical requirements of the user are met. According to the calculation, after the optimization of the composition, process and steel transfer speed and casting speed, the rolling rhythm of the 20-50 mm thick plate with a yield strength of 355 MPa is significantly improved, and the average rolling time of each steel plate is reduced by 45-90 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, and these examples 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 slab and the high-pressure water descaling process before rolling of the continuous casting slab 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; and 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 are P≤0.025%, S≤0.02%.

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

[0028]

[0029] Table 3 Rolling method of the example steel

[0030]

[0031]

[0032] Table 4 Cooling and straightening process of example steels

[0033] Example Start cooling temperature / °C Finish cooling temperature / °C Cooling rate / °C Entry roll gap / mm Exit roll gap / mm Straightening force / KN 1 831 645 30 -1.3 -3.2 2200 2 860 652 35 -1.7 -3.6 2500 3 850 660 40 -2.2 -3.8 2000 4 855 632 26 -2.5 -3.6 2100 5 842 650 33 -1.9 -3.3 2300 6 839 651 35 -1.5 -2.9 2450

[0034] Table 5 Example steel dimensions, properties and flatness

[0035]

[0036] Therefore, compared with the prior art, the method for improving the rolling rhythm of the 355MPa grade steel plate provided by the present application. The technical scheme provided by the present 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 rolling efficiency and the one-time pass rate of the plate shape of the steel plate. According to the calculation, after the composition and process are optimized, the rolling time of each steel plate of the 355MPa grade economic medium plate with a thickness of 20-60mm is reduced by 45-90 seconds, and the flatness of the steel plate is below 5mm / 2m, which has a significant effect on improving the production efficiency and reducing the production cost.

[0037] The above examples are only for illustrating the technical concept and characteristics of the present application, and the purpose is to enable those skilled in the art to understand the content of the present application and to implement it, and cannot limit the protection scope of the present application. Any equivalent changes or modifications made according to the spirit and essence of the present application shall be covered within the protection scope of the present application.

Claims

1. A method of improving the rolling pace of a yield strength 355 MPa grade steel sheet, characterized in that, The method comprises the following steps: 1) billet heating: the billet is sent into a walking beam heating furnace for heating, the billet is heated to 1050-1095℃, the total heating time of the soaking section and the heating section is 2-3.5 hours, the total in-furnace time is 3-5 hours; the air-fuel ratio of the soaking section is controlled to be 1:1.8-1:2.3, so that the temperature difference between the upper and lower surfaces of the billet is ensured to be within 15℃; meanwhile, 4-6 spaces are left at the furnace head of the soaking section of the heating furnace; 2) high-pressure water descaling and rolling: the billet after being discharged from the furnace is descaled by high-pressure water for 0.5-1 min before rolling, 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 first three passes before rolling is sprayed, the time of each pass is 0.2-0.5 min, the pressure is 15-20 MPa, the intermediate billet is short-time temperature-kept for 10-20 s before the last three passes, and the reduction rate of the last pass is controlled to be less than 5%; when corresponding passes are rolled, the pass table in the TCS jumps to the next pass to prepare for positioning in advance when the rolling length of the current pass reaches 80%, meanwhile, the speed of the steel transfer roller is increased to 3-4 m / s, the final rolling temperature of the steel plate is 850-900℃, the steel plate is thrown after rolling, the throwing speed is 5-6.5 m / s, then laminar cooling is adopted, the open cooling temperature is 830-860℃, the final cooling temperature is 630-660℃, and the cooling speed is 25-40℃ / s; 3) hot straightening: the entry roller gap is -1.0 mm to -2.6 mm, the exit roller gap is -2.9 mm to -3.8 mm, and the straightening force is 2000 KN to 2500 KN; 4) air cooling to room temperature; The steel plate comprises the following chemical components in percentage by weight: C 0.16%-0.19%, Si 0.15%-0.3%, Mn 0.9%-1.25%, Al 0.015%-0.04%, Ti 0.005%-0.012%, P≤0.025%, S≤0.02%, Ceq≤0.38%, wherein Ceq=C+Mn / 6, and the balance is Fe and inevitable impurities.

2. The method of claim 1, wherein, In step 2), the air-fuel ratio is controlled to be 1:1.8-1:2.3 by adjusting the opening degree of the upper and lower burners of 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 20-50 mm.

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

6. The method of claim 1, wherein, The steel plate is obtained by rolling the billet 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 355 MPa, the tensile strength is 490-630 MPa, the elongation is greater than or equal to 22%, the Charpy impact energy at -20℃ in the transverse direction is greater than or equal to 100 J, and the flatness is less than 5 mm / 2 m.

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