Method for improving rolling rhythm of 315mpa grade steel plate

Abstract

The application discloses a method for improving rolling rhythm of a yield strength 315MPa 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%-0.22%, Si 0.1%-0.2%, Mn 0.7%-1.25%, Al 0.015%-0.04%, Ti 0.008%-0.014%, P≤0.035%, S≤0.035%, 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 25-55 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 315MPa grade steel plate with a thickness specification of 20-50mm. 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. In particular, the market competition for 315MPa grade plate products (AH32, etc.) 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 and Ti are added to the steel to form carbon and nitride, thereby improving 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, the production of plate products, especially 315MPa grade plate products, needs to be further optimized to reduce energy consumption, alloy cost and manufacturing cost, and improve the market competitiveness of the products.

[0003] Currently, for 315MPa grade steel plates with a thickness of 20-50mm, in order to reduce alloy cost, a medium-carbon, low-Mn composition design system is usually adopted, and the rolling and controlled cooling process path is optimized to reduce the performance fluctuations caused by the reduction of alloy cost. Generally speaking, TMCP (Thermal Mechanical Control Process) is a technology that controls the rolling temperature and reduction during rolling, and then implements air cooling or controlled cooling and accelerated cooling based on the 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 a green and reduced process that saves alloy elements, and has become an indispensable technology for producing plate. However, the steel plate after alloy reduction is usually produced by two-stage or even three-stage controlled rolling TMCP process, and the finish rolling temperature is as low as 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 it difficult to control the shape of the plate. 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 economic 315MPa grade steel plates.

[0004] Therefore, how to improve the rolling rhythm of 315MPa grade steel plates with a thickness specification of 20-50mm and reduce the manufacturing cost is a key problem that needs to be solved in the batch production process of economic 315MPa grade plate.

[0005] Compared with the prior art:

[0006] So far, there are few studies on the method of improving the rolling rhythm of 20-50mm thickness specification and 315MPa grade steel plate at home and abroad. Before the present application, the patent with publication number CN115537636A discloses a method for improving the rolling efficiency of A32 grade shipbuilding medium plate. The method adopts 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 and 315MPa grade steel plate, but it is not suitable for improving the rolling efficiency of 20mm and above economic type 315MPa grade 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 20-50mm thickness and 315MPa grade economic type medium plate is solved by using 250mm and below thickness continuous casting billet through heating, rolling and other process optimization. SUMMARY

[0008] The present application aims to overcome the above technical problems and deficiencies, and provides a method for improving the rolling rhythm of 20-50mm thickness specification and 315MPa grade economic type 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 plate shape one-pass rate are solved.

[0009] The present application provides a method for improving the rolling rhythm of 315MPa grade steel plate, the chemical composition of the steel plate includes: C 0.16%-0.22%, Si 0.1%-0.2%, Mn 0.7%-1.25%, Al 0.015%-0.04%, Ti 0.008%-0.014%, P≤0.035%, S≤0.035%, 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 purpose of the present application is achieved by the following technical solutions: the present application is a method for improving the rolling rhythm of a yield strength 315 MPa steel plate, which is used to improve the rolling efficiency of an economic medium plate with a thickness of 20-50 mm and a yield strength of 315 MPa, including slab heating, rolling, straightening (hot straightening), and air cooling to room temperature, and specifically comprising the following steps:

[0011] 1) Casting blank heating: the casting blank (thickness below 250 mm) is sent into a walking beam heating furnace for heating, and the casting blank is heated to 1050-1100℃, 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 in 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℃; (combined with the composition of the blank, the heating temperature of the blank is reduced to reduce energy consumption, while ensuring that the final rolling temperature is 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 of the blank and the core, 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 and subsequent shape of the steel plate), and at the same time, 2-4 spaces are left at the furnace head of the heating furnace (to prevent low furnace head temperature from affecting the uniformity of the blank temperature).

[0012] 2) High-pressure water descaling and rolling: the casting blank is descaled for 0.5-0.8 min after being discharged from the furnace by using high-pressure water, the descaling machine pressure 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 rolling machine before rolling is 0.3-0.6 min per pass, the pressure is 15-20 MPa, after the end of the three passes of rolling, the intermediate blank is short-time temperature waiting for 10-15s, and the reduction rate of the last pass is controlled to be less than 5% (the rolling mill capacity is used as much as possible in the first three passes, the high-pressure water descaling process is flexible and variable, the organization distribution from the surface to the core of the steel plate is uniform, the strength and toughness of the steel plate are improved, the deformation resistance increases in the later stage of rolling due to the temperature drop of the steel plate, the last pass is rolled with small reduction rate to flatten the plate shape and reduce the internal stress of the steel plate, after the end of the three passes of rolling, the intermediate blank is short-time temperature waiting to make the surface fine grain zone static recrystallize to ensure the performance of the rolled steel plate); when corresponding to the pass, the pass table in the TCS jumps to the next pass to prepare for positioning (the set roll gap of the second level is received in advance and the roll gap is distributed; after the steel signal disappears in each pass, the equipment unrelated to rolling is allowed to position), the roller way cooling water before and after the rolling mill is reduced to 150-200m 3 / h, and the turing roller speed is increased from 1.0-2 m / s to 3-4 m / s (the roller cooling water is halved in the turing stage to ensure the intermediate billet and the roller to slip in the turing stage to match the increase of the turing speed), the plate final rolling temperature is 870-930 ℃, the plate is tumbled after rolling, the tumbling speed is 4-6 m / s, then the laminar cooling is adopted, the open cooling temperature range is 850-900 ℃, the final cooling temperature interval is 650-700 ℃, and the cooling speed is 25-40 ℃ / s (the open cooling temperature is controlled to ensure the plate to be in the austenite state when entering water, and the cooling speed and the final cooling temperature are controlled to inhibit the formation of the bainite and the martensite phases on the surface of the plate and the quantity of the bainite phase in the core, so that the bainite or the martensite phase change on the surface is avoided to affect the toughness of the plate).

[0013] 3) Hot straightening: the hot straightening is performed again, the lead-in roller gap is -1.3 mm--2.8 mm, the lead-out roller gap is -3.1 mm--3.9 mm, and the straightening force is between 2000 KN and 2500 KN (the appropriate roller gap and the straightening force are set to ensure that the straightened plate is flat and has good shape).

[0014] 4) Air cooling to room temperature.

[0015] By adopting the above composition and heating, rolling and cooling process scheme, the problems in the prior art are overcome, the problems that the alloy composition is low, the rolling process needs to be deeply controlled, the rolling efficiency and the plate shape one-time pass rate of the plate are affected and the like are solved. The performance of the finally obtained plate is the same as that when the plate is produced by adopting the prior TMCP process, the flatness of the plate is below 5 mm / 2 m, the yield strength performance of the plate in the transverse direction is greater than or equal to 315 MPa, the tensile strength is between 450 MPa and 560 MPa, the elongation is greater than or equal to 23 %, and the Charpy impact energy in the transverse direction at -20 ℃ is greater than or equal to 120 J. The technical requirements of the user are met. According to the measurement and calculation, after the composition, the process and the turing speed and the tumbling speed are optimized, the rolling rhythm of the 20-50 mm thick plate with the yield strength of 315 MPa level is obviously improved, the rolling time of each plate is reduced by 30-90 seconds on average, and the production efficiency is improved and the production cost is reduced.

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

[0017] 1. The billet heating temperature is reduced to reduce energy consumption, and the final rolling temperature is ensured to be in the austenite temperature interval; in addition, the furnace time of the soaking section and the heating section is ensured, and the air-fuel ratio of the soaking section is regulated to shorten the temperature difference between the upper and lower surfaces and the core of the continuous casting billet, in addition, the furnace head of the soaking section of the heating furnace is left empty to ensure the uniformity of the billet temperature, and finally the uniformity of the transverse and longitudinal metal flow on the surface of the plate is ensured, which effectively inhibits the excessive growth of the austenite grains and ensures the performance of the plate and the plate shape in subsequent rolling.

[0018] 2. The surface fine grain zone is recrystallized statically by adopting large reduction rate, flexible and changeable high-pressure water descaling process and short-time waiting for temperature of the intermediate blank to ensure the rolling performance of the steel plate and make the structure distribution of the steel plate from the surface to the center uniform, meanwhile, the excessive grain growth is inhibited, the strength and toughness of the steel plate is improved, and the rolling passes are reduced. In the later stage of rolling, the steel plate temperature decreases and the deformation resistance increases, so small reduction rate is adopted in the last pass to flatten the plate shape and reduce the internal stress of the steel plate. In addition, 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, so as to receive the setting roll gap of the second stage and distribute the roll gap. After the steel signal disappears in each pass, the positioning of the equipment unrelated to rolling is allowed. The distance of the steel throwing between the speed of the steel transfer roller and the speed of the steel throwing is increased, so that the rolling rhythm is significantly improved. The steel plate after rolling is controlled to cool, the open cooling temperature is controlled, the steel plate is ensured to be in the austenite state when entering the water, the cooling speed and the red temperature are controlled, the formation of the bainite and martensite phases on the surface of the steel plate and the number of the bainite phases in the center structure are inhibited, the bainite or martensite phase change on the surface is avoided to affect the toughness of the steel plate, and the strength is also improved.

[0019] 3. Hot straightening is adopted to set the positions of the leading-in and leading-out rollers and the pressure, so as to ensure that the straightened steel plate is flat and has good plate shape.

[0020] 4. The above heating, rolling and cooling process schemes are adopted to solve the problems of the steel plate, such as the low alloy content, the need for deep control in the rolling process, the influence on the rolling efficiency and the plate shape one-time pass rate and the like. 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 subsequent cold straightening process cost is saved, the yield strength of the transverse tensile property of the steel plate is greater than or equal to 315 MPa, the tensile strength is between 450-560 MPa, the elongation is greater than or equal to 23%, and the transverse Charpy impact energy at-20°C is greater than or equal to 120 J. The technical requirements of the user are met. According to the calculation, after the composition and process are optimized, the rolling rhythm of the 20-50 mm thick plate of the yield strength 315 MPa level economic type is obviously improved, and the rolling time of each steel plate is reduced by 25-55 seconds on average, 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 blank and the high-pressure water descaling process of the continuous casting blank before rolling 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: P≤0.035%, S≤0.035%.

[0027] Table 2 Heating schedule 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] Table 4 Cooling and 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 1 855 680 35 -1.3 -3.0 2200 2 880 670 30 -1.9 -3.2 2500 3 850 695 40 -2.7 -3.8 2300 4 893 700 38 -2.3 -3.6 2250 5 885 660 32 -1.8 -3.5 2400 6 858 665 28 -1.5 -3.2 2300

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

[0034]

[0035] It can be seen that, compared with the prior art, the method for improving the rolling pace of the 315MPa grade steel plate provided by the present application. The technical scheme provided by the present application solves the problem that the low alloy composition of such steel plate needs to be controlled during rolling, 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 315MPa grade economic medium plate with a thickness of 20-50mm is reduced by 15-45 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.

[0036] 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 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 315 MPa grade steel sheet, characterized by, 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 1050-1100℃, the total heating time of the soaking section and the heating section is 2-3.5 hours, and the total furnace residence 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, 2-4 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 subjected to high-pressure water descaling for 0.5-0.8 min, 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 rolling machine before rolling is 0.3-0.6 min per pass, and the pressure is 15-20 MPa; after the end of the three passes of rolling, the intermediate billet is allowed to stand for 10-15 s, and 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 final rolling temperature of the steel plate is 870-930 °C, the steel plate is thrown after rolling at a speed of 4-6 m / s, then laminar cooling is used, the open cooling temperature is 850-900 °C, the final cooling temperature is 650-700 °C, and the cooling speed is 25-40 °C / s; 3) hot straightening: the entry roller position is -1.3 mm to -2.8 mm, the exit roller position is -3.1 mm to -3.9 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%-0.22%, Si 0.1%-0.2%, Mn 0.7%-1.25%, Al 0.015%-0.04%, Ti 0.008%-0.014%, P≤0.035%, S≤0.035%, 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 1:1.7-1:2.2 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 rolling 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 235 MPa, the tensile strength is 380-450 MPa, 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 60 J, and the flatness is less than 5 mm / 2 m.

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