Economical 550MPa grade hot-rolled steel plate and its elongation stability control method

Abstract

The invention relates to an economical 550MPa grade hot-rolled steel plate and a method for stabilizing elongation thereof. The chemical composition of the steel plate is as follows: C 0.13%-0.17%, Si 0.20%-0.40%, Mn 1.25%-1.55%, P≤0.02%, S≤0.015%, Nb 0.04%-0.06%, Ti 0.015%-0.0250%, Al 0.015%-0.035%, V 0.02%-0.05%, B 0.0004%-0.0006%, N 0.01%-0.02%, Cev 0.35%-0.43%, and the balance is Fe and impurities. By controlling each production process, the performance index requirements can be met without subsequent tempering heat treatment, which solves the problem of this type of economical steel plate being overcooled at the head and tail in the length direction due to the rapid temperature drop at the head and tail, the short length of the rolled steel plate, the inaccurate model tracking, and the narrowing of the cooling process window after alloy reduction, resulting in the elongation of the steel plate being lower than the technical standard.

Description

Technical Field

[0001] The present invention relates to the technical field of hot-rolled steel plate production, and in particular to an economical hot-rolled steel plate with a thickness of 40 to 80 mm and a yield strength of 550 MPa and a method for stabilizing the elongation thereof. Background Art

[0002] 550MPa grade medium and thick plate is the main type of medium and thick plate, typified by low-alloy grades such as Q550C / D / E. This type of steel is primarily based on carbon-manganese steel, with microalloying elements such as Nb, V, Ti, and Cr added as carbon and nitride-forming elements to enhance the steel's strength and toughness through solid solution strengthening, precipitation strengthening, and grain refinement. According to statistics, this strength grade accounts for 20% to 30% of my country's annual hot-rolled steel production. Therefore, the development and research of low-cost manufacturing technologies for this type of steel is of great significance. With increasingly stringent design specifications in engineering applications such as construction machinery and pipelines, the requirements for low-temperature toughness and other indicators are also increasing while ensuring the service economy and safety of the steel. For 550MPa grade structural steel with special requirements for low-temperature toughness, a "low-carbon" design with a carbon content below 0.10% is generally adopted, with a higher manganese content (1.6% to 1.8%), and alloying elements such as niobium, vanadium, titanium, chromium, and molybdenum are added to enhance performance. This results in relatively high production costs. If the "medium and high carbon" composition design is adopted instead of the "low carbon" composition design, and the amount of alloy elements added to the steel is reduced, and the steelmaking, rolling and controlled cooling processes are optimized, the alloy cost will be significantly reduced.

[0003] TMCP (Thermomechanical Control Process) is a general term for technologies that, during the hot rolling process, combine controlled rolling with controlled heating temperature, rolling temperature, and reduction, followed by air cooling, controlled cooling, and accelerated cooling. Because the TMCP process can produce high-strength and high-toughness steel without the addition of excessive alloying elements or the need for complex subsequent heat treatment, it is considered an alloy- and energy-saving, environmentally friendly process and has become an indispensable technology for the production of low-alloy steel plate. As market demand for TMCP steel continues to increase, the TMCP process itself is also evolving in its applications. Research in recent years has focused on controlled cooling, particularly accelerated cooling. By accelerating the cooling rate after rolling, not only can grain growth be suppressed, but the ultrafine ferrite or bainite structure, or even martensite, required for high strength and toughness can be obtained.

[0004] Economical 550MPa-grade steel plates, which achieve alloy cost reduction and process reduction, must be produced using the TMCP process. However, as the alloy content decreases, the rolling and controlled cooling process window also narrows. For economical thick-gauge (40-80mm) 550MPa-grade TMCP steel plates, due to their increased thickness, rapid temperature drop at the head and tail of the plates, and the narrowing cooling process window after alloy reduction, the plates are overcooled at the head and tail along their length, resulting in elongation below the technical standard, limiting the production and promotion of this grade of steel plates.

[0005] It can be seen that how to solve the problem of head and tail elongation lower than the technical standard, while reducing production costs and improving the steel plate performance qualification rate, is a key issue that needs to be solved urgently for the mass production of economical 550MPa grade hot-rolled steel plates with a thickness of 40 to 80mm.

[0006] To date, little research has been conducted domestically or internationally on control methods for ensuring that the head-to-tail elongation of economical steel plates with a thickness of 40-80 mm and a yield strength of 550 MPa or greater meets technical standards. A journal article, "Analysis of the Causes of Unqualified Elongation in Low-Alloy High-Strength Steel Q550D" (Shanxi Metallurgy, January 2010), focused on enhancing the toughness of the steel plates by improving molten steel purity, modifying inclusions, optimizing rolling and controlled cooling processes, and employing tempering heat treatment. However, the use of offline tempering heat treatment significantly increases production cycle time and costs.

[0007] Although the elongation control method for 550 MPa-grade steel plates disclosed in the above literature solves the problem of unqualified elongation of some thickness-specification steel plates, it is not suitable for controlling the elongation of economical steel plates with a yield strength of 550 MPa and a thickness of 40 to 80 mm produced by using TMCP instead of an offline heat treatment process. Summary of the Invention

[0008] The purpose of the present invention is to overcome the problems and shortcomings of the prior art and provide an economical hot-rolled steel plate with a thickness of 40 to 80 mm and a yield strength of 550 MPa and a stable elongation control method thereof; by controlling each production process, the performance index requirements of the steel plate can be met without subsequent tempering heat treatment, and the problem that such economical steel plates are overcooled at the head and tail in the longitudinal direction due to the rapid temperature drop at the head and tail and the short length of the rolled steel plates, and the cooling process window becomes narrower after alloy reduction, resulting in the elongation of the steel plate being lower than the technical standard and affecting the primary performance qualification rate of the steel plate is solved.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] An economical 550MPa grade hot-rolled steel plate comprises, by weight percentage, the following chemical compositions: C 0.13%-0.17%, Si 0.20%-0.40%, Mn 1.25%-1.55%, P≤0.02%, S≤0.015%, Nb 0.04%-0.06%, Ti 0.015%-0.0250%, Al 0.015%-0.035%, V 0.02%-0.05%, B 0.0004%-0.0006%, N 0.01%-0.02%, Cev 0.35%-0.43%, and the balance being Fe and unavoidable impurities.

[0011] Furthermore, the thickness of the finished steel plate is 40 to 80 mm.

[0012] Furthermore, the finished steel plate has a transverse tensile yield strength of ≥550 MPa, a tensile strength of 670-830 MPa, an elongation of ≥16%, and a transverse Charpy impact energy of ≥27 J at -20°C.

[0013] An economical method for stabilizing the elongation of 550MPa hot-rolled steel plates is disclosed. The steel plate production process includes molten steel smelting, continuous casting, billet heating, controlled rolling, cooling, and stacking and slow cooling. The following processes are controlled:

[0014] 1) Molten steel smelting: The raw materials are pretreated with KR hot metal to control the sulfur content to ≤0.02%, and then enter the converter after slag removal. During the converter smelting, the double slag method is used to remove phosphorus to control the phosphorus content to ≤0.030%. The carbon content at the converter smelting end point is controlled to be 0.15%-0.18%. Then LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 20 minutes.

[0015] 2) Continuous casting: Control the continuous casting superheat at 15-20°C, the continuous casting casting rate at 1.2-1.7 m / min, the electromagnetic stirring current intensity in the secondary cooling zone during the continuous casting stage at 250-300 A, and the secondary cooling water volume at 1.9-2.5 L / kg; under heavy pressure in the horizontal sector, i.e., at the end of solidification, the continuous casting billet is pressed down by 15-20 mm, and the billets are stacked for more than 12 hours after leaving the production line;

[0016] 3) Billet heating: The billet is heated with the preheating section temperature at 980-1150°C, the soaking section and the heating section heating temperatures at 1220-1250°C, the total heating time for the soaking section and the heating section at 3-4 hours, and the total furnace time at 5.5-6.5 hours; the air-fuel ratio is controlled at 1:2.1-1:2.5;

[0017] 4) Controlled rolling: Rolling is divided into two stages; the first stage is recrystallization rolling, with the rough rolling start temperature ≥ 1100℃, the rough rolling finish temperature 985-1020℃, and no more than six rough rolling passes. The pass reduction system is: at least the first two passes have a reduction rate of more than 15%; the second stage is non-recrystallization rolling, with the finishing rolling start temperature 870-930℃, the finishing rolling finish temperature 800-840℃, and no more than six finishing passes. The pass reduction system is: at least two of the first four passes have a reduction rate of more than 20%; pre-straightening is used;

[0018] 5) Cooling: Laminar cooling is adopted, the cooling start temperature is 775~820℃, the final cooling temperature is 400~450℃, and the cooling rate is 13~18℃ / s; fully automatic cooling control is adopted, the steel plate head and tail are shielded, the upper header opening correction value is -600~-800mm, and the lower header opening correction value is -1000~-1400mm; 3~6 groups of water are opened for both the upper and lower headers, and the opening water volume is 200~230m 3 / h, the water ratio between the upper header and the lower header is 2 to 2.4; the roller speed of the steel plate conveying roller is 2.0 to 3.0 m / s, and the acceleration is 0.005 m / s 2 After the steel plate is cooled, the side spray is turned on, the side spray pressure is 1-3 MPa, and the side spray water volume is 35-70 m 3 / h;

[0019] 4) Stacking and slow cooling: The cooled steel plates are stacked and slow cooled. The stacking and slow cooling temperature is 400-600℃. The number of stacking blocks is not less than 12 and the slow cooling time is more than 12 hours.

[0020] Furthermore, the continuous casting slab has a thickness of ≤300 mm and is rolled on a medium and heavy plate reciprocating rolling mill.

[0021] Furthermore, the thickness of the rolling intermediate billet is 2.0 to 4 times the thickness of the finished steel plate; during the temperature-keeping process of the rolling intermediate billet, at least two passes of mill descaling water are sprayed, each descaling time is 1 to 1.5 minutes, and the descaling pressure is 20 to 25 MPa.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1) Maintaining the RH vacuum degassing time overcomes defects such as central segregation, inclusions, and excessive H and O contents in the ingot caused by high Mn content, thereby improving the plasticity and toughness of the steel plate. By controlling the continuous casting superheat, billet drawing rate, electromagnetic stirring current intensity in the secondary cooling zone, and secondary cooling water volume, the average carbon segregation index can be effectively reduced, inhibiting the trend of segregation and the worsening of central cracks in the ingot. Stacking the billets for more than 12 hours after they are finished can reduce the accumulation of residual H and inhibit the generation of microcracks inside the billets.

[0024] 2) By controlling the preheating temperature, the carbides and nitrides of Nb are ensured to be quickly and fully dissolved in the matrix and fully diffused. By using a higher heating temperature, ensuring the time in the soaking and heating sections, and controlling the air-fuel ratio, the temperature difference between the surface and the core of the continuous casting billet is shortened, the uniformity of the metal flow in the horizontal and vertical directions on the surface of the steel plate is improved, and the temperature uniformity of the upper and lower surfaces of the steel plate during the subsequent rolling process is ensured. The purpose of controlling the total time in the furnace is to effectively inhibit the excessive growth of austenite grains and ensure the performance of the steel plate.

[0025] 3) The steel plate of the present invention has a rationally designed chemical composition and a low alloy addition amount. By increasing the C content, reducing the Mn content, and adding a small amount of microalloying elements Nb, Ti, and V, the alloy cost is greatly reduced. The use of TMCP rolling increases the start rolling temperature and reduces the high-temperature deformation resistance in the roughing and finishing stages, which is conducive to increasing the reduction per pass and ensuring the overall performance of the steel plate.

[0026] 4) A two-stage controlled rolling process is adopted to control the pass reduction of roughing and finishing rolling, optimize the thickness of the intermediate billet and adopt a flexible and variable intermediate billet cooling process, so that a temperature gradient exists between the surface and the core of the intermediate billet, which promotes uniform structure from the surface to the core of the steel plate, while also inhibiting grain growth and improving the plastic toughness of the steel plate; the purpose of pre-straightening is to ensure the uniformity of temperature in the head, middle and tail parts of the steel plate after entering the water; side spraying is conducive to the control of the steel plate shape, further improving the uniformity of steel plate performance and reducing the probability of buckled head and buckled tail shape problems; controlling the final cooling temperature and cooling rate of the steel plate can inhibit the formation of bainite and martensite phases on the steel plate surface and the amount of bainite phase in the core structure, avoid affecting the toughness of the steel plate due to bainite or martensite phase transformation on the surface, and ensure that the mechanical properties of the steel plate meet the requirements;

[0027] 5) The stacking slow cooling process is adopted, and the slow cooling temperature and time are strictly controlled, which effectively reduces the content of core banded structure and surface martensite structure or metastable lath bainite structure, ensuring the plastic toughness of the steel plate; the performance of the finished steel plate is: transverse tensile yield strength ≥550MPa, tensile strength between 670 and 830MPa, elongation ≥16%, and transverse Charpy impact energy at -20℃ ≥27J. DETAILED DESCRIPTION

[0028] The economical 550MPa grade hot-rolled steel plate of the present invention comprises, by weight percentage, the following chemical compositions: C 0.13% to 0.17%, Si 0.20% to 0.40%, Mn 1.25% to 1.55%, P ≤ 0.02%, S ≤ 0.015%, Nb 0.04% to 0.06%, Ti 0.015% to 0.0250%, Al 0.015% to 0.035%, V 0.02% to 0.05%, B 0.0004% to 0.0006%, N 0.01% to 0.02%, Cev 0.35% to 0.43%, and the balance being Fe and unavoidable impurities.

[0029] Furthermore, the thickness of the finished steel plate is 40 to 80 mm.

[0030] Furthermore, the finished steel plate has a transverse tensile yield strength of ≥550 MPa, a tensile strength of 670-830 MPa, an elongation of ≥16%, and a transverse Charpy impact energy of ≥27 J at -20°C.

[0031] The present invention discloses an economical method for stabilizing the elongation of a 550MPa grade hot-rolled steel plate. The steel plate production process includes molten steel smelting, continuous casting, billet heating, controlled rolling, cooling, and stacking and slow cooling. The following processes are controlled:

[0032] 1) Molten steel smelting: The raw materials are pretreated with KR hot metal to control the sulfur content to ≤0.02%, and then enter the converter after slag removal. During the converter smelting, the double slag method is used to remove phosphorus to control the phosphorus content to ≤0.030%. The carbon content at the converter smelting end point is controlled to be 0.15%-0.18%. Then LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 20 minutes.

[0033] 2) Continuous casting: Control the continuous casting superheat at 15-20°C, the continuous casting casting rate at 1.2-1.7 m / min, the electromagnetic stirring current intensity in the secondary cooling zone during the continuous casting stage at 250-300 A, and the secondary cooling water volume at 1.9-2.5 L / kg; under heavy pressure in the horizontal sector, i.e., at the end of solidification, the continuous casting billet is pressed down by 15-20 mm, and the billets are stacked for more than 12 hours after leaving the production line;

[0034] 3) Billet heating: The billet is heated with the preheating section temperature at 980-1150°C, the soaking section and the heating section heating temperatures at 1220-1250°C, the total heating time for the soaking section and the heating section at 3-4 hours, and the total furnace time at 5.5-6.5 hours; the air-fuel ratio is controlled at 1:2.1-1:2.5;

[0035] 4) Controlled rolling: Rolling is divided into two stages; the first stage is recrystallization rolling, with the rough rolling start temperature ≥ 1100℃, the rough rolling finish temperature 985-1020℃, and no more than six rough rolling passes. The pass reduction system is: at least the first two passes have a reduction rate of more than 15%; the second stage is non-recrystallization rolling, with the finishing rolling start temperature 870-930℃, the finishing rolling finish temperature 800-840℃, and no more than six finishing passes. The pass reduction system is: at least two of the first four passes have a reduction rate of more than 20%; pre-straightening is used;

[0036] 5) Cooling: Laminar cooling is adopted, the cooling start temperature is 775~820℃, the final cooling temperature is 400~450℃, and the cooling rate is 13~18℃ / s; fully automatic cooling control is adopted, the steel plate head and tail are shielded, the upper header opening correction value is -600~-800mm, and the lower header opening correction value is -1000~-1400mm; 3~6 groups of water are opened for both the upper and lower headers, and the opening water volume is 200~230m 3 / h, the water ratio between the upper header and the lower header is 2 to 2.4; the roller speed of the steel plate conveying roller is 2.0 to 3.0 m / s, and the acceleration is 0.005 m / s 2 After the steel plate is cooled, the side spray is turned on, the side spray pressure is 1-3 MPa, and the side spray water volume is 35-70 m 3 / h;

[0037] 4) Stacking and slow cooling: The cooled steel plates are stacked and slow cooled. The stacking and slow cooling temperature is 400-600℃. The number of stacking blocks is not less than 12 and the slow cooling time is more than 12 hours.

[0038] Furthermore, the continuous casting slab has a thickness of ≤300 mm and is rolled on a medium and heavy plate reciprocating rolling mill.

[0039] Furthermore, the thickness of the rolling intermediate billet is 2.0 to 4 times the thickness of the finished steel plate; during the temperature-keeping process of the rolling intermediate billet, at least two passes of mill descaling water are sprayed, each descaling time is 1 to 1.5 minutes, and the descaling pressure is 20 to 25 MPa.

[0040] The chemical composition design principle of the economical 550MPa grade hot-rolled steel plate described in the present invention is as follows:

[0041] C is the most economical and fundamental strengthening element in steel. It significantly increases steel strength through solid solution strengthening and precipitation strengthening. However, increasing C content negatively impacts the steel's plasticity, toughness, and weldability. Therefore, the present invention sets the C content range to 0.13% to 0.17%.

[0042] Mn: It increases steel strength through solid solution strengthening, compensating for the loss of steel plate strength caused by reduced carbon content. It also lowers the γ-α transformation temperature, thereby refining ferrite grains and contributing to the production of fine low-temperature transformation products, thereby improving toughness. However, increasing the Mn content exacerbates central segregation and microstructural deterioration in the continuously cast slab, hindering low-temperature toughness improvement and preventing cross-sectional structural uniformity. Therefore, the present invention sets the Mn content range to 1.25% to 1.55%.

[0043] Si: It deoxidizes steel and improves matrix strength. Increasing Si content purifies ferrite and reduces pearlite, which helps mitigate the Bauschinger effect in the matrix material. However, excessive Si can reduce the toughness of the base material's weld heat-affected zone. Therefore, the present invention sets the Si content range to 0.20% to 0.40%.

[0044] N: The N element in steel has no other significant function except forming fine TiN particles to refine austenite grains. Therefore, the N content needs to be kept at a relatively low level. The present invention sets the N content range to 0.01% to 0.02%.

[0045] Al: Typically acts as a deoxidizer in steel and, if formed into AlN, also refines the steel's structure. When the Al content exceeds 0.035%, excessive aluminum oxide inclusions can reduce the steel's cleanliness. However, if the Al content is too low, deoxidation will be inadequate, and easily oxidizable elements like Ti will form oxides. Therefore, the present invention sets the lower limit of the Al content at 0.015%.

[0046] Nb is a common element in modern microalloyed pipeline steels. It has excellent grain refinement and precipitation strengthening effects, and also delays austenite recrystallization. However, excessive Nb increases production costs and makes continuous casting process control more difficult. The present invention selects a Nb content range of 0.04% to 0.06%, combined with a suitable TMCP process, to produce a uniform composite phase dominated by acicular ferrite or pearlite, resulting in excellent toughness in the steel plate.

[0047] V: is a strong carbide and hardenability forming element, which can improve the low-temperature toughness of steel, improve the hardenability of steel, and thus improve the strength and structural uniformity of steel, while improving the hot brittleness of steel. The present invention selects the V content range as 0.02% to 0.05%.

[0048] B: A hardenability-enhancing element. Its beneficial effect on hardenability is due to its delayed ferrite nucleation process, but it does not affect the thermodynamic properties of the austenite or ferrite matrix. Furthermore, dissolved B promotes the formation of fine bainite after controlled rolling and controlled cooling. This fine bainite contains numerous stable dislocations, which inherit deformation dislocations generated within the austenite during formation. Simultaneously, through precipitation strengthening from Nb, Ti, and V, the steel plate achieves a yield strength of 500-900 MPa, significantly exceeding the toughness of conventional low-alloy high-strength steel. The B content in this invention is selected to be between 0.0004% and 0.0006%.

[0049] Ti: A strong nitrogen-binding element, it exists as TiN in the continuous casting ingot. Fine TiN particles effectively inhibit austenite grain growth during reheating of the continuous casting ingot, help increase the solid solubility of Nb in austenite, and improve the impact toughness of the weld heat-affected zone. However, when the Ti addition exceeds a certain level, the TiN particles coarsen, increasing stress concentration at the particle interface and the matrix. Therefore, the Ti content in this invention is selected to be within the range of 0.015% to 0.025%.

[0050] P and S are unavoidable impurity elements in steel. Theoretically, the lower the better. However, due to smelting costs and process considerations, they cannot be kept indefinitely low. Therefore, the present invention sets the upper limits of P and S content at 0.02% and 0.015%, respectively.

[0051] The economical 550MPa hot-rolled steel plate described in this invention has a thickness of 40 to 80mm and is produced on a medium and heavy plate reciprocating mill using continuously cast slabs less than 300mm thick. The production process includes molten steel smelting → continuous casting → slab heating → controlled rolling → cooling → stacking and slow cooling. To improve the elongation of the finished steel plate, the following process steps are controlled:

[0052] 1) Molten steel smelting: smelting according to the set chemical composition, that is, the chemical composition percentage by weight in the steel is C 0.13%-0.17%, Si 0.20%-0.40%, Mn 1.25%-1.55%, P≤0.02%, S≤0.015%, Nb 0.04%-0.06%, Ti 0.015%-0.0250%, Al 0.015%-0.035%, V 0.02%-0.05%, B 0.0004%-0.0006%, N 0.01%-0.02%, Cev 0.35%-0.43%, and the balance is Fe and unavoidable impurities. The raw materials are pretreated with KR molten iron to control the S content below 0.02%, and then enter the converter after slag removal. During the converter smelting, the double slag method is used to remove P to control the P content to ≤ 0.030%, and the C content is controlled at 0.15% to 0.18% at the end of the converter smelting. LF refining and RH vacuum degassing are then carried out, and the RH vacuum is maintained for more than 20 minutes.

[0053] 2) Continuous Casting: Control the continuous casting superheat to 15-20°C, the continuous casting casting rate to 1.2-1.7 m / min, the electromagnetic stirring current intensity in the secondary cooling zone during the continuous casting process to 250-300 A, and the secondary cooling water volume to 1.9-2.5 L / kg. These measures reduce the average carbon segregation index to inhibit segregation. Simultaneously, the secondary cooling intensity is limited to suppress the tendency of central cracks in the slab to worsen. In the horizontal sector, i.e., at the end of solidification, the slab is pressed down by 15-20 mm under heavy pressure to reduce the central porosity and segregation of the slab. After leaving the production line, the slabs are stacked for at least 12 hours to reduce the accumulation of residual hydrogen and inhibit the formation of microcracks within the slab.

[0054] 3) Ingot Heating: Ingots (thickness under 300mm) are heated in a walking beam furnace. The preheating temperature range is 980-1150°C to promote rapid and thorough dissolution and diffusion of Nb carbides and nitrides in the matrix. The soaking and heating sections are heated to 1220-1250°C. The total heating time for the soaking and heating sections is 3-4 hours, and the total furnace time is 5.5-6.5 hours. The opening of the upper and lower burners in the heating furnace is adjusted to maintain an air-fuel ratio of 1:2.1 to 1:2.5. The use of higher heating temperatures and a guaranteed soaking and heating section furnace time are intended to minimize the temperature difference between the surface and core of the continuous casting ingot and improve the uniformity of metal flow both horizontally and vertically across the steel plate surface. Controlling the total furnace time effectively inhibits excessive austenite grain growth and ensures steel plate performance.

[0055] 4) Controlled rolling: Rolling is divided into two stages; the first stage is recrystallization rolling (rough rolling), the starting rolling temperature of rough rolling is ≥1100℃, the finishing rolling temperature range of rough rolling is 985~1020℃, the rough rolling is not more than six passes, and the pass reduction system is preferably that the reduction rate of the first two passes is above 15%.

[0056] The thickness of the rolling intermediate billet is 2.0 to 4 times the thickness of the finished steel plate; during the temperature-keeping process of the rolling intermediate billet, it is preferably sprayed with two passes of mill descaling water, with each pass taking 1 to 1.5 minutes and a descaling pressure of 20 to 25 MPa.

[0057] The second stage is non-crystallizing rolling (finishing rolling), the starting temperature range of finishing rolling is 870-930℃, the final temperature range of finishing rolling is 800-840℃, the finishing rolling is no more than six passes, and the pass reduction system is preferably that the reduction rate of two of the first four passes is above 20%.

[0058] The two-stage rolling process refines the grain structure at different stages, and the intermediate billet cooling method makes the structure uniform from the surface to the core of the steel plate, thereby improving the low-temperature toughness of the steel plate. The purpose of pre-straightening is to prevent the steel plate from tilting during rolling, which would cause water to accumulate on the steel plate surface during the subsequent controlled cooling process, affecting the uniformity of performance.

[0059] 5) Cooling: Laminar cooling is adopted, the cooling temperature range is 775~820℃, the final cooling temperature range is 400~450℃, and the cooling rate is 13~18℃ / s. Fully automatic cooling control is adopted, the head and tail of the steel plate are shielded, the upper header opening correction value is -600~-800mm, the lower header opening correction value is -1000~-1400mm: 3~6 groups of water are opened on the header, and the opening water volume is 200~230m 3 / h, the water ratio between the upper header and the lower header is 2 to 2.4; the roller speed of the steel plate conveyor is 2.0 to 3 m / s, and the acceleration is 0.005 m / s 2 After the steel plate is cooled, the side spray is turned on, the side spray pressure is 1~3MPa, and the side spray water volume is 35~70m 3 / h. Controlling the cooling rate, red-hot temperature, and shielding the steel plate's head and tail during cooling effectively regulates the formation of bainite and martensite at the head, middle, and tail of the steel plate, as well as the amount of bainite in the core structure, ensuring the steel's toughness. Side spraying facilitates plate shape control, improves plate performance uniformity, and reduces the occurrence of shape issues such as buckled heads and tails.

[0060] 6) Stacking and slow cooling: After cooling, the steel plates are stacked and slow cooled at a temperature of 400-600°C. The number of stacked blocks should be no less than 12, and the slow cooling time should be more than 12 hours. The use of low-temperature stacking reduces the content of banded structure, martensite structure or metastable lath-shaped bainite structure, ensuring the plastic toughness of the steel plate.

[0061] The present invention, in combination with the above-mentioned chemical composition and production process, solves the problem that the economical 550MPa grade hot-rolled steel plate is overcooled at the head and tail in the length direction due to the rapid temperature drop at the head and tail and the narrowing of the cooling process window after alloy reduction, resulting in the elongation of the steel plate being lower than the technical standard and affecting the primary performance qualification rate of the steel plate.

[0062] The following examples are implemented on the premise of the technical solution of the present invention, and provide detailed implementation methods and specific operating processes, but the protection scope of the present invention is not limited to the following examples.

[0063] [Example]

[0064] Table 1 shows the chemical composition of the steel of each embodiment, Table 2 shows the smelting process parameters of the steel of each embodiment, Table 3 shows the heating process parameters of the ingot of each embodiment; Table 4 shows the rolling process parameters of the steel of each embodiment; Table 5 shows the reduction amount of each pass of the steel of each embodiment at different rolling stages; Table 6 shows the controlled cooling process parameters of the steel plate of each embodiment; Table 7 shows the stacking slow cooling process parameters of the steel plate of each embodiment and the properties of the finished steel plate.

[0065] Table 1 Chemical composition of steel (wt, %)

[0066] Example C Si Mn N Al Nb Ti V B Cev 1 0.16 0.35 1.52 0.016 0.026 0.041 0.016 0.038 0.0004 0.41 2 0.17 0.29 1.30 0.019 0.018 0.058 0.019 0.043 0.0006 0.39 3 0.15 0.38 1.45 0.020 0.029 0.049 0.025 0.025 0.0004 0.39 4 0.13 0.21 1.55 0.012 0.033 0.058 0.022 0.032 0.0005 0.39 5 0.14 0.29 1.52 0.018 0.028 0.043 0.018 0.041 0.0004 0.39 6 0.16 0.32 1.49 0.019 0.023 0.053 0.017 0.029 0.0006 0.41

[0067] Note: Impurity elements P in steel ≤ 0.02%, S ≤ 0.015%.

[0068] Table 2 Steel smelting process parameters

[0069]

[0070] Table 3 Heating process parameters of the casting

[0071]

[0072] Table 4 Steel rolling process parameters

[0073]

[0074] Table 5 Reduction of steel in each pass at different rolling stages

[0075]

[0076] Table 6 Controlled cooling process parameters of steel

[0077]

[0078] Table 7 Steel stacking slow cooling process parameters and finished steel plate properties

[0079]

[0080] It can be seen that after adopting the economical 550MPa grade hot-rolled steel plate elongation stabilization control method described in the present invention, the performance index requirements of the steel plate can be met without subsequent tempering heat treatment, which solves the problem that such economical steel plates are overcooled at the head and tail in the length direction due to the rapid temperature drop at the head and tail and the narrowing of the cooling process window after alloy reduction, resulting in the elongation of the steel plate being lower than the technical standard, thereby affecting the primary performance qualification rate of the steel plate.

[0081] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. An economical method for stabilizing the elongation of a 550MPa grade hot-rolled steel plate, characterized in that: The chemical composition of the steel plate, calculated by weight, includes: C 0.16% to 0.17%, Si 0.29% to 0.40%, Mn 1.25% to 1.49%, P ≤ 0.02%, S ≤ 0.015%, Nb 0.041% to 0.06%, Ti 0.022% to 0.025%, Al 0.015% to 0.018%, V 0.02% to 0.038%, B 0.0004% to 0.0006%, N 0.012% to 0.02%, Cev 0.35% to 0.43%, with the balance being Fe and unavoidable impurities. The finished steel plate has a thickness of 40 to 80 mm. The finished steel plate has a transverse tensile yield strength of ≥ 550 MPa, a tensile strength of 670 to 830 MPa, an elongation of ≥ 16%, and a transverse Charpy impact energy of ≥ 27 J at -20°C. The steel plate production process includes molten steel smelting, continuous casting, billet heating, controlled rolling, cooling and stacking slow cooling; the following processes are controlled: 1) Molten steel smelting: The raw materials are pretreated with KR hot metal to control the sulfur content to ≤0.02%, and then enter the converter after slag removal. During the converter smelting, the double slag method is used to remove phosphorus to control the phosphorus content to ≤0.030%. The carbon content at the converter smelting end point is controlled to be 0.15%-0.18%. Then LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 20 minutes. 2) Continuous Casting: The continuous casting slab thickness is ≤ 300mm and is rolled on a medium and heavy plate reciprocating rolling mill. The continuous casting superheat is controlled at 15-19°C, the continuous casting casting rate is 1.2-1.7m / min, the electromagnetic stirring current intensity in the secondary cooling zone during the continuous casting stage is controlled at 250-300A, and the secondary cooling water volume is controlled at 1.9-2.5L / kg. The continuous casting slab is pressed down by 15-20mm under heavy pressure in the horizontal sector, i.e., at the end of solidification. The slabs are stacked for more than 12 hours after leaving the production line. 3) Billet heating: The billet is heated with the preheating section temperature at 980-1150°C, the soaking section and the heating section heating temperatures at 1225-1250°C, the total heating time for the soaking section and the heating section at 3-4 hours, and the total furnace time at 5.5-6.5 hours; the air-fuel ratio is controlled at 1:2.1-1:2.5; 4) Controlled rolling: Rolling is divided into two stages; the first stage is recrystallization rolling, with the rough rolling start temperature ≥1100℃ and the rough rolling finish temperature of 985-995℃. The rough rolling is performed in no more than six passes, and the pass reduction system is as follows: the reduction rate of at least the first two passes is above 15%; the thickness of the rolling intermediate bar is 2.0-4 times the thickness of the finished steel plate; during the warming process of the rolling intermediate bar, at least two passes of mill descaling water are sprayed, with each pass taking 1-1.5 minutes and a descaling pressure of 20-25MPa; the second stage is non-recrystallization rolling, with the finishing rolling start temperature of 880-930℃ and the finishing rolling finish temperature of 800-840℃. The finishing rolling is performed in no more than six passes, and the pass reduction system is as follows: the reduction rate of at least two of the first four passes is above 20%; pre-straightening is input; 5) Cooling: Laminar cooling is adopted, the cooling start temperature is 805~820℃, the final cooling temperature is 400~450℃, and the cooling rate is 13~18℃ / s; fully automatic cooling control is adopted, the steel plate head and tail are shielded, the upper header opening correction value is -600~-800mm, and the lower header opening correction value is -1000~-1400mm; 3~6 groups of water are opened for both the upper and lower headers, and the opening water volume is 200~230m 3 / h, the water ratio between the upper header and the lower header is 2 to 2.4; the roller speed of the steel plate conveying roller is 2.0 to 3.0 m / s, and the acceleration is 0.005 m / s 2 After the steel plate is cooled, the side spray is turned on, the side spray pressure is 1-3 MPa, and the side spray water volume is 35-70 m 3 / h; 6) Stacking and slow cooling: The cooled steel plates are stacked and slow cooled. The stacking and slow cooling temperature is 400-600°C. The number of stacking blocks is not less than 12, and the slow cooling time is more than 12 hours.

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