A high-efficiency low-cost thin-gauge 1000mpa-grade high-strength steel and a manufacturing method thereof

Abstract

This invention discloses a high-efficiency, low-cost, thin-gauge 1000MPa-grade high-strength steel and its manufacturing method, belonging to the field of low-carbon microalloyed steel production technology. By optimizing the casting, heating, rolling, controlled cooling, and straightening processes, a high-strength, high-toughness, and high-flatness 1000MPa-grade high-strength steel with a thickness of 6–15 mm is finally obtained. This solves the problems of plate warping and poor performance uniformity after online quenching of 1000MPa-grade high-strength steel, and eliminates the need for subsequent offline tempering and straightening treatments to meet the technical requirements of the plate.

Description

Technical Field

[0001] This invention belongs to the field of steel plate heat treatment technology, and specifically relates to a method for efficiently and cost-effectively manufacturing 1000MPa grade high-strength steel with a thickness of 6-15mm. Background Technology

[0002] Quenching of medium and heavy plates is an important technical means for producing high-performance, high-value-added products. A portion of the high-strength, high-grade medium and heavy plate products required for national economic development and construction, such as high-strength shipbuilding steel and engineering machinery steel, all require quenching processes to ensure their performance. However, offline quenching processes increase costs and delivery cycles, restricting the scale of high-strength steel production and the reduction of manufacturing costs. The new generation of thermomechanical controlled rolling (TMCP) technology, with ultra-fast cooling as its core, is gradually gaining attention from researchers. Thanks to the application and promotion of ultra-fast cooling, low-alloy high-strength steel is now widely produced using online quenching processes, offering advantages such as good mechanical properties, short delivery times, alloy savings, and energy conservation. Online quenching involves directly quenching the steel plate on the main rolling line, which not only saves energy but also shortens the delivery cycle. However, its disadvantages include uneven temperature after rolling, uneven cooling during quenching, uneven microstructure, poor plate shape, and negative impacts on the dynamic performance of subsequent materials, such as fracture toughness, severely affecting their application.

[0003] Numerous studies have shown that, by utilizing existing online ultra-fast cooling systems and rationally controlling the water flow rate, water ratio, and roller speed of the controlled cooling unit, steel plates thicker than 15mm can generally achieve excellent performance and good quenched plate shape. However, for thinner steel plates with a thickness not exceeding 15mm, the rapid temperature drop makes the factors affecting microstructure and post-quenching plate shape more complex. Firstly, the warping deformation of thin steel plates during online quenching is fundamentally caused by internal stress, which mainly consists of two types: thermal stress due to uneven cooling and structural stress due to phase transformation during quenching. The manifestation of these two stress forms within the quenched steel plate is related to various factors such as the steel's chemical composition, size, hardenability, quenching medium, and cooling method. Furthermore, online quenching of thin steel plates results in poor temperature uniformity along the length, leading to significant differences in microstructure and properties across different locations. As the steel plate thickness decreases to below 15mm, controlling the uniformity of quenched microstructure and the straightness of the plate shape becomes crucial for the development of online quenching processes for thin-gauge high-strength steel plates. Therefore, it is evident that solving the problem of uniformity in shape and properties of 1000MPa grade high-strength steel with thicknesses of 6-15mm during online quenching is key to the development of thin-gauge high-strength steel.

[0004] Compared with existing technologies:

[0005] To date, there has been very little research, both domestically and internationally, on online quenching manufacturing methods for thin-gauge (6-15mm) 1000MPa grade high-strength steel. Prior to this invention, patent CN 115011764A disclosed a "segmented quenching control process for ultra-thin high-hardness steel plates." This method employs segmented cooling control during the steel plate quenching process, using 50 to +150°C above Ms as the segmentation point to divide the steel plate into high-temperature and low-temperature sections. Different cooling methods are used for quenching, employing a combination of high restraint stress and different cooling rates. This ensures that the plate shape is straight when the phase transformation stress of the steel plate is large, and simultaneously achieves microstructure control of the steel plate. However, compared with online quenching processes, it has higher requirements for equipment investment and the precision and response time of cooling manifolds. Furthermore, it involves more processes, lower efficiency, and increased energy consumption due to secondary heating, resulting in higher manufacturing costs.

[0006] The journal article "Slab Shape Control in the Quenching Process of Thin Steel Plates" (Journal of Iron and Steel Research, 2011.9) mainly focuses on controlling the roller gap precision adjustment and correction of the quenching machine equipment, coupled with reasonable quenching process parameters, to stabilize the plate shape unevenness of thin steel plates within a controllable range after quenching. However, due to the use of offline production, the manufacturing cost is high.

[0007] The manufacturing methods for thin-gauge high-strength steel plates disclosed in the above-mentioned literature, while improving the quenching shape problem of thin-gauge high-strength steel plates, are not suitable for controlling the quenching shape and performance uniformity of 1000MPa grade high-strength steel plates with a thickness of 6-15mm. Using the technical solution provided by this invention, the above-mentioned shortcomings can be effectively overcome, and the problems of quenching shape and performance uniformity of 1000MPa grade high-strength steel plates with a thickness of 6-15mm can be solved in online production. Summary of the Invention

[0008] The purpose of this invention is to overcome the aforementioned technical problems and deficiencies, and to provide a high-efficiency, low-cost, thin-gauge 1000MPa-grade high-strength steel and its manufacturing method. This method employs optimized processes for billet smelting, heating, rolling, online cooling, and straightening, ultimately yielding 1000MPa-grade high-strength steel with a thickness of 6–15 mm, exhibiting high strength, toughness, and high flatness. This solves the problems of plate warping and poor performance uniformity after online quenching of 1000MPa-grade high-strength steel, and eliminates the need for subsequent offline tempering and cold straightening treatments, thus meeting the technical requirements of the plate material.

[0009] The objective of this invention is achieved through the following technical solution:

[0010] This invention provides a high-efficiency, low-cost, thin-gauge 1000MPa grade high-strength steel. The chemical composition of the steel plate, by weight percentage, includes: C 0.14%–0.19%, Si 0.2%–0.35%, Mn 1.3%–1.5%, Nb 0.010–0.040%, Cr 0.25–0.35%, P≤0.02%, S≤0.005%, Ti 0.015%–0.03%, B 0.0003%–0.0016%, Als 0.015%–0.045%, Ceq 0.41%–0.51%, Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%.

[0011] The roles of the main elements in the chemical composition of the steel plate of this invention are as follows:

[0012] C: Carbon is an important alloying element that ensures the hardenability of steel plates and is also a crucial factor in determining carbon equivalent. It has a significant impact on the strength, toughness, plasticity, and weldability of steel. Excessive carbon content affects the weldability of the steel plate. Insufficient carbon content affects the overall strength of the steel plate. To ensure the required strength of the steel plate of this invention, the carbon content ranges from 0.14% to 0.19%.

[0013] Si: an essential element for deoxidation, with a certain solid solution strengthening effect. Excessive levels will affect the toughness of steel, the uniformity of performance of extra-thick plates, and weldability. In this invention, the range of silicon is 0.2% to 0.35%.

[0014] Mn: An important solid solution strengthening element in steel, it can lower the phase transformation temperature, refine the microstructure, and improve toughness while strengthening the steel plate; at the same time, it can improve hardenability. If the manganese content is too high, it will cause segregation, which is detrimental to welding and toughness. The range of manganese in this invention is 1.3% to 1.5%.

[0015] Cr: increases the hardenability of steel, refines the microstructure, and lowers the ductile-brittle transition temperature; when used in combination with manganese, it can improve the hardenability of steel and enhance its mechanical properties; chromium has a smaller tendency to segregate than manganese, and replacing some manganese with chromium can reduce segregation in the core of the steel plate, improve the internal quality of the steel plate, and enhance the uniformity of mechanical properties; the chromium content range of this invention is 0.25% to 0.35%.

[0016] Nitrogen (Nb) is a commonly used element in modern microalloyed pipeline steel, exhibiting excellent grain refinement and precipitation strengthening effects; it also delays austenite recrystallization. However, excessive Nb increases production costs and the difficulty of controlling the continuous casting process. This invention selects an Nb content range of 0.01% to 0.04%, combined with a reasonable TMCP process, to achieve good toughness.

[0017] Ti is a strong solid nitrogen element, existing in the form of TiN in continuously cast billets. Fine TiN particles effectively inhibit austenite grain growth during reheating of the continuously cast billet and help improve the solid solubility of Nb in austenite, thus improving the impact toughness of the weld heat-affected zone. When the Ti content exceeds a certain value, the TiN particles coarsen, increasing the stress concentration level at the particle interface and the matrix. Therefore, this invention selects a Ti content range of 0.015% to 0.03%.

[0018] B: A relatively effective element for significantly improving the hardenability of steel. It readily segregates at grain boundaries, preventing carbon precipitation. Even trace amounts of boron can have a significant effect. However, excessive boron content can easily form boron carbonitrides, reducing toughness and causing hot brittleness. The reasonable range is 0.0003% to 0.0016%.

[0019] Als: Typically used as a deoxidizer in steel, it can also refine the microstructure if it forms AlN. When the Al content exceeds 0.05%, excessive alumina inclusions will reduce the cleanliness of the steel. If the Al content is too low, deoxidation will be insufficient, and easily oxidized elements such as Ti will form oxides. Therefore, the lower limit for Al content is set at 0.015%.

[0020] P and S are unavoidable impurity elements in steel. Excessive P and S levels can affect the toughness and weldability of steel plates, so lower levels are better. However, due to smelting process considerations, they cannot be kept indefinitely low. Therefore, this invention sets the upper limits for P and S content at 0.02% and 0.005%, respectively.

[0021] Furthermore, given that the present invention adopts a medium carbon design, the carbon equivalent Ceq of the present invention is controlled at 0.41% to 0.51%, which can ensure both the strength and toughness of the steel plate and give the steel plate suitable weldability.

[0022] The high-strength steel described in this invention has a thickness of 6 to 15 mm, and is produced on a medium-thick plate reciprocating rolling mill using a continuous casting billet with a thickness of less than 250 mm, with water as the cooling medium.

[0023] This invention provides a method for manufacturing high-strength steel of 1000MPa grade in thin sections at high efficiency and low cost, comprising: steelmaking → ladle refining (LF refining) + RH vacuum degassing → micro-Ti treatment → B alloying → continuous casting → slab heating → controlled rolling → pre-straightening → cooling and cold straightening; specifically including the following steps:

[0024] 1) Steel smelting to continuous casting: Smelting according to the following composition, with the following chemical composition by weight percentage: C 0.14%~0.19%, Si 0.2%~0.35%, Mn 1.3%~1.5%, Nb 0.010~0.040%, Cr 0.25~0.35%, P≤0.02%, S≤0.005%, Ti 0.015%~0.03%, B 0.0003%~0.0016%, Als0.015%~0.045%, Ceq 0.41%~0.51%, Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15, balance is Fe and unavoidable impurities, O≤0.0050%, and the total amount of other impurity elements is less than 0.05%; molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting; after RH vacuum degassing, Al is added according to Als requirements; after micro-titanium treatment with Ti-Fe, B-Fe alloy is added; ensure that it is added to the molten steel to avoid floating on the slag layer; then slab continuous casting is carried out, the superheat of continuous casting is 10~15℃, and the continuous casting speed is 0.8~1.2m / min (reducing the superheat and the continuous casting speed can improve the macroscopic segregation of the continuous casting slab and reduce the solidification of the continuous casting slab). The spacing between secondary dendrite arms in the solidified structure helps reduce billet segregation and internal structural defects. In the fan-shaped section, strong cooling is used, with a total cooling water flow of 1100-1200 L / min in the first half and 1000-1100 L / min in the second half. At the same time, light reduction is applied at the end of solidification, with a reduction of 8-15 mm for the continuously cast billet (strong cooling ensures the temperature gradient in the thickness direction of the billet, while light reduction helps reduce billet segregation and internal structural defects, and promotes core grain breakage, ensuring the strength and toughness of the subsequent steel plate). After finishing, the billet is slowly cooled in the pit at a temperature not lower than 600℃ for a time not lower than 48 hours (to promote the diffusion of elements such as Mn and Cr, and reduce their impact on the microstructure and properties due to component segregation).

[0025] 2) Billet Heating: The billet is fed into a walking beam furnace for heating. The billet passes through a preheating section, a heating section, and a soaking section before exiting the furnace. The temperature range of the preheating section is 950-1150℃ (to promote the rapid and complete dissolution of Ti carbides into the matrix and facilitate their diffusion), the temperature range of the heating section is 1220-1250℃, and the temperature range of the soaking section is 1180-1210℃. The total time spent in the furnace during the heating and soaking sections is controlled to be 3-4 hours, ensuring that the temperature difference between the upper and lower surfaces of the billet is within 15℃. (The heating section provides high-temperature heating to ensure temperature uniformity in all parts of the billet, further promoting the diffusion of elements Mn, Cr, and C, reducing their impact on microstructure and properties due to component segregation, and improving the uniformity of transverse and longitudinal metal flow on the steel plate surface. Combined with the billet composition, reducing the heating temperature of the soaking section reduces energy consumption and also suppresses the impact of coarsening of the original austenite grains on performance.)

[0026] 3) Controlled rolling: The billet is rolled in two stages: the first stage is recrystallization rolling (rough rolling), with an initial rolling temperature ≥1100℃ and a final rolling temperature range of 985~1025℃. Rough rolling consists of no more than 6 passes, with the first two passes ensuring a reduction rate of over 20%. The thickness of the resulting intermediate billet is 2.5~4 times the thickness of the finished product. During the intermediate billet's warm-up process, descaling water is sprayed onto the mill for 0.5~0.8 min at a pressure of 20~25MPa. (The first two passes of rough rolling maximize the mill's capacity with a high reduction rate to promote dynamic recrystallization of austenite and refine the original austenite grains. The use of descaling water on the intermediate billet creates a temperature gradient from the surface to the core of the steel plate, promoting core deformation during subsequent finish rolling.) The first stage is non-recrystallization rolling (finish rolling). The initial rolling temperature range for finish rolling is 940–970℃, and the final rolling temperature range is 850–880℃. Finish rolling is limited to no more than six passes. The reduction rate for the first two passes is guaranteed to be above 25%, while the reduction rate for the last pass is less than 5%. The rolling length of the steel plate is controlled within 25m. (The first two passes in the finish rolling stage use a high reduction rate to further refine the austenite grains and improve the core structure. Due to the temperature drop in the later stages of rolling, the deformation resistance increases. The last pass uses a low reduction rate to flatten the plate shape, reduce internal stress, and control the length of the rolled steel plate to avoid excessively rapid temperature drop in the later stages, which would affect the uniformity of the steel plate structure after controlled cooling.)

[0027] 4) Pre-straightening: The rolled steel plate is pre-straightened at high temperature. The straightening force is between 800KN and 1000KN, the bending amount is set to 0.5 to 1.0 mm, and the tilting value is set to 2 to 3.5 mm (to promote the full release of internal stress, set appropriate bending amount and tilting value, ensure good flatness of the straightened plate, and ensure uniform temperature of the steel plate when entering the water).

[0028] 5) Rapid cooling: The pre-straightened steel plate is subjected to online rapid cooling (online quenching). The initial cooling temperature of the steel plate is 770-810℃, the roller speed in the controlled cooling zone is 2-2.5m / s, and the acceleration is 0.003-0.005m / s². 2 The final cooling temperature range is 150–200℃, and the cooling rate is controlled at 20–30℃ / s. During the cooling process of the steel plate, the water flow ratio of the upper and lower manifolds is 1:1.6–1:2.1. After the steel plate exits the controlled cooling system, the side spray is activated, with a side spray pressure of 5–10 MPa and a water flow rate of 25–40 m³ / s. 3 / h (The starting cooling temperature is limited to ensure that the austenite ratio of the steel plate before quenching is above 70%; the cooling rate is limited to ensure that the martensite ratio of the quenched steel plate is above 80%, while avoiding excessive internal stress and poor plate shape due to excessive cooling rate. Based on this, quenching is carried out to 150~200℃, which ensures that the quenched steel plate is fully hardened and reduces the accumulation of stress in the steel plate during the quenching process. At the same time, the speed of the controlled cooling roller is increased and matched with appropriate acceleration to ensure the temperature uniformity of the steel plate along its length. The water ratio of the upper and lower manifolds is optimized to ensure the plate shape of the steel plate after quenching. Side spraying is used to reduce the residual water in the steel plate after quenching, ensuring that the plate shape is controlled and the uniformity of steel plate performance is improved).

[0029] 6) Cold straightening: The steel plate after controlled cooling is cold straightened with a straightening force between 3500KN and 5000KN, a bending amount of 1.2 to 1.8 mm, and an inclination value of 4.0 to 5.5 mm (online cold straightening is performed on the steel plate after online quenching to ensure that the transverse and longitudinal internal stresses of the steel plate after quenching are fully released).

[0030] Furthermore, in step 1), the raw material is pretreated with KR molten iron to control the S content to be less than 0.005%, and then enters the converter after slag removal; during converter smelting, the P content is controlled to be ≤0.02%, and the C content is controlled to be 0.14-0.19% at the end of converter smelting; argon gas is blown for 25-30 minutes when tapping the steel (argon blowing and calming before continuous casting can promote the removal of inclusions in the molten steel and improve the uniformity of the steel composition); then LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes.

[0031] Furthermore, during the converter smelting process, a double-slag method is used for phosphorus removal.

[0032] Furthermore, in the continuous casting process, the first half is sections 1 to 4, and the second half is sections 5 to 8.

[0033] Furthermore, in step 3), before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, with the descaling machine pressure being 15-25 MPa.

[0034] Furthermore, in step 5), the number of controlled cooling manifold groups opened is 3 to 6, and the water flow rate of a single manifold is 120 to 150 m³. 3 / h.

[0035] High-strength, high-toughness, and high-straightness 1000MPa grade high-strength steel with a thickness of 6-15mm, produced using the above-mentioned composition and process scheme, solves the problems of plate warping and poor performance uniformity after online quenching of 1000MPa grade high-strength steel. Furthermore, it meets the technical requirements of the plate without the need for subsequent offline tempering and straightening. The mechanical properties and plate shape of the produced plate are as follows: yield strength ≥1050MPa, tensile strength ≥1350MPa, elongation ≥10%, overall plate performance uniformity (the difference in yield strength and tensile strength between the longitudinal head, middle, and tail ends of the steel plate is within 35MPa), transverse Charpy impact energy at -20℃ ≥54J, and straightness below 5mm / 2m.

[0036] The beneficial effects of this invention are:

[0037] 1. After deep desulfurization pretreatment of KR molten iron, the slag is thoroughly removed. The converter employs a double-slag method for P removal, resulting in lower P and S content in the billet. Controlling the argon blowing time and maintaining the RH vacuum degassing time overcomes defects such as center segregation, inclusions, and excessive H and O content in the billet caused by high Mn, Cr, and C content, thus improving the plasticity and toughness of the steel plate. Reducing superheat and continuous casting speed can improve macroscopic segregation in the continuously cast billet, reducing the spacing of secondary dendrite arms in the solidification structure, which helps reduce billet segregation and internal structural defects. Strong cooling is used in the fan-shaped section to ensure a temperature gradient along the billet thickness, while light pressure promotes core grain breakage, ensuring the subsequent strength and toughness of the steel plate. Simultaneously, the billet is slowly cooled in the pit after finishing, with controlled cooling temperature and time, which helps promote the diffusion of Mn and Cr elements, mitigating their impact on microstructure and properties due to component segregation.

[0038] 2. Billet Heating: The temperatures and times of the billet in the preheating, heating, and soaking zones are limited to ensure that Nb, Cr, and Ti carbides and nitrides dissolve rapidly and fully in the matrix and diffuse sufficiently. Simultaneously, the heating zone provides high-temperature heating to ensure temperature uniformity across the billet, further promoting the diffusion of Mn, Cr, and C, mitigating their impact on microstructure and properties due to component segregation, and improving the uniformity of transverse and longitudinal metal flow on the steel plate surface. Combined with the billet composition, reducing the soaking zone heating temperature decreases energy consumption and also suppresses the impact of primary austenite grain coarsening on performance.

[0039] 3. A two-stage controlled rolling process is adopted. By controlling the final rolling temperature of the roughing stage, the intermediate slab is kept within the austenite recrystallization temperature range during the warming process, ensuring microstructure uniformity. Simultaneously, the reduction rate and intermediate slab thickness are limited in both stages of rolling. Descaling water is used in the intermediate slab spraying mill to create a temperature gradient from the steel plate surface to the core, promoting core deformation and microstructure uniformity during subsequent finish rolling, improving the steel plate's strength and toughness, and ensuring the uniformity of the microstructure in the thickness direction after rolling. The first two passes of the finish rolling stage use a high reduction rate to further refine the austenite grains and improve the core microstructure. Due to the temperature drop in the later stages of rolling, the deformation resistance increases, so the final pass uses a low reduction rate to flatten the plate shape and reduce internal stress. At the same time, the length of the rolled steel plate is controlled to avoid excessively rapid temperature drop in the later stages of rolling, which could affect the microstructure uniformity of the steel plate after controlled cooling. By incorporating pre-straightening, the internal stress of the steel plate is fully released. By setting appropriate bending roller amount and tilting value, the straightened plate has good flatness and the water temperature of the steel plate is uniform.

[0040] 4. Limit the initial cooling temperature to ensure the austenite content is above 70% before quenching; limit the cooling rate to ensure the martensite content of the quenched steel plate is 80% or above, avoiding excessive internal stress and poor plate shape due to excessively rapid cooling. Based on this, quench to 150~200℃ to ensure sufficient hardening of the quenched steel plate while reducing the accumulation of stress during the quenching process; simultaneously increase the speed of the controlled cooling roller conveyor and match an appropriate acceleration to ensure temperature uniformity along the length of the steel plate; optimize the water ratio of the upper and lower manifolds to ensure the plate shape after quenching; reduce residual water in the steel plate after quenching by side spraying to ensure controlled plate shape and improve the uniformity of steel plate performance; perform online cold straightening on the steel plate after online quenching to ensure that the transverse and longitudinal internal stresses of the steel plate are fully released.

[0041] 5. A high-strength, high-toughness, and high-straightness 1000MPa grade high-strength steel with a thickness of 6-15mm was prepared using the above-mentioned composition and process scheme. This solves the problems of plate warping and poor performance uniformity after online quenching of 1000MPa grade high-strength steel. Furthermore, it eliminates the need for subsequent offline tempering and straightening treatments, thus meeting the technical requirements for the plate. The mechanical properties and plate shape of the prepared plate are as follows: yield strength ≥1050MPa, tensile strength ≥1350MPa, elongation ≥10%, overall plate performance uniformity (the difference in yield strength and tensile strength between the longitudinal head, middle, and tail of the steel plate is within 35MPa), transverse Charpy impact energy at -20℃ ≥54J, and straightness below 5mm / 2m. Detailed Implementation

[0042] The following examples are used to illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0043] This invention provides a high-efficiency, low-cost, thin-gauge 1000MPa grade high-strength steel, with the following chemical composition by weight percentage: C 0.14%–0.19%, Si 0.2%–0.35%, Mn 1.3%–1.5%, Nb 0.010–0.040%, Cr 0.25–0.35%, P ≤0.02%, S ≤0.005%, Ti 0.015%–0.03%, B 0.0003%~0.0016%, Als 0.015%~0.045%, Ceq 0.41%~0.51%, Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15, the balance being Fe and unavoidable impurities, O≤0.0050%, and the total amount of other impurity elements is less than 0.05%; the thickness of the high-strength steel is 6~15mm, and it is produced on a medium-thick plate reciprocating rolling mill using continuously cast billets with a thickness of less than 250mm.

[0044] The aforementioned method for manufacturing high-efficiency, low-cost, thin-gauge 1000MPa grade high-strength steel includes steelmaking → ladle refining (LF refining) + RH vacuum degassing → micro-Ti treatment → B alloying → continuous casting → slab heating → controlled rolling → pre-straightening → cooling and cold straightening; specifically, it includes the following steps:

[0045] 1) Steelmaking to continuous casting: Smelting according to the above composition. Raw materials undergo KR hot metal pretreatment to control the S content below 0.005%, and after slag removal, they enter the converter. During converter smelting, a double-slag method is used to remove P, controlling the P content ≤0.02%. At the end of converter smelting, the C content is controlled between 0.14% and 0.19%. Argon gas is blown for 25-30 minutes during tapping. This is followed by LF refining and RH vacuum degassing, with RH vacuum maintained for at least 30 minutes. After degassing, sufficient Al is added according to Al requirements. After adding sufficient Ti-Fe for micro-titanium treatment, B-Fe alloy is added. The steel is added to prevent it from floating on the slag layer; then, slab continuous casting is carried out with a superheat of 10-15℃ and a casting speed of 0.8-1.2m / min; in the fan-shaped section, strong cooling is used, with a total cooling water volume of 1100-1200L / min for the first 4 sections and 1000-1100L / min for the fifth 8 sections. At the same time, light reduction is applied at the end of solidification, with a reduction of 8-15mm for the continuously cast slab. After the slab is removed from the line, it is slowly cooled in the pit with a slow cooling temperature of not less than 600℃ and a slow cooling time of not less than 48h.

[0046] 2) Billet heating: The billet (thickness less than 250mm) is fed into a walking beam furnace for heating. The billet passes through the preheating section, heating section and soaking section in sequence before exiting the furnace. The temperature range of the preheating section is 950-1150℃, the temperature range of the heating section is 1220-1250℃, and the temperature range of the soaking section is 1180-1210℃. The total time in the furnace for the heating and soaking sections is controlled at 3-4 hours to ensure that the temperature difference between the upper and lower surfaces of the billet is within 15℃.

[0047] 3) High-pressure water descaling and controlled rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, with a descaling machine pressure of 15-25 MPa; the billet is rolled in two stages: the first stage is recrystallization rolling (rough rolling), with a rough rolling start temperature ≥1100℃ and a rough rolling finish temperature range of 985-1025℃. The rough rolling is not more than 6 passes, and the reduction rate of the first two passes is guaranteed to be above 20%. The thickness of the intermediate billet is 2 times the thickness of the finished product. 5 to 4 times; during the intermediate billet warming process, spray descaling water in the rolling mill is applied for 0.5 to 0.8 minutes at a pressure of 20 to 25 MPa; the second stage is non-recrystallization rolling (finishing rolling), with the starting rolling temperature range of 940 to 970℃ and the finishing rolling temperature range of 850 to 880℃. The finishing rolling process has no more than six passes, and the reduction rate for the first two passes is guaranteed to be above 25%, while the reduction rate for the last pass is less than 5%, and the longitudinal rolling length of the steel plate is controlled within 25m.

[0048] 4) Pre-straightening: The rolled steel plate is straightened at high temperature with a straightening force between 800KN and 1000KN, a bending amount of 0.5 to 1.0 mm, and an inclination value of 2 to 3.5 mm (to promote the full release of internal stress, set appropriate bending amount and inclination value, ensure good flatness of the straightened plate, and ensure uniform temperature of the steel plate when entering the water).

[0049] 5) Rapid cooling: The pre-straightened steel plate is rapidly cooled. The initial cooling temperature of the steel plate is 770–810℃, the roller speed in the controlled cooling zone is 2–2.5 m / s, and the acceleration is 0.003–0.005 m / s². 2 The final cooling temperature range is 150–200℃, the number of controlled cooling manifolds open is 3–6, and the water flow rate per manifold is 120–150 m³. 3 The cooling rate is controlled at 20~30℃ / s per hour. During the steel plate cooling process, the water flow ratio between the upper and lower manifolds is 1:1.6~1:2.1. After the steel plate exits controlled cooling, the side spray is activated, with a side spray pressure of 5~10MPa and a water flow rate of 25~40m³ / h. 3 / h (limiting the starting cooling temperature to ensure that the austenite proportion of the steel plate is above 70% before controlled cooling; limiting the cooling rate to ensure that the martensite proportion of the steel plate is 80% or above after controlled cooling).

[0050] 6) Cold straightening: The steel plate after controlled cooling is cold straightened. The straightening force is between 3500KN and 5000KN, the bending amount of the roller is set to 1.2 to 1.8mm, and the tilting value is set to 4.0 to 5.5mm.

[0051] Examples 1-6

[0052] Table 1 shows the chemical composition of the embodiment steel, Table 2 shows the smelting and stacking process of the embodiment steel, Table 3 shows the heating process of the billet of the embodiment steel and the high-pressure water descaling process before rolling of the continuous casting billet, Table 4 shows the rolling parameters of the embodiment steel, Table 5 shows the main process parameters of straightening and controlled cooling of the embodiment steel, and Table 6 shows the performance and shape index of the embodiment steel.

[0053] Table 1 Chemical composition (wt, %) of the examples

[0054]

[0055] Note: Impurity elements in steel: P≤0.02%, S≤0.005%.

[0056] Table 2. Smelting and stacking process regulations for steel in the examples.

[0057]

[0058] Table 3 Heating regime and high-pressure water descaling process before rolling of steel billets in the examples

[0059]

[0060] Table 4 Rolling parameters of the steel in the examples

[0061]

[0062] Table 5. Main process parameters for straightening and controlled cooling of steel in the examples.

[0063]

[0064] Table 6 Main performance parameters and plate shape of the steel in the examples.

[0065]

[0066] Therefore, compared with the prior art, the present invention provides a high-strength, high-toughness, and high-flatness 1000MPa grade high-strength steel with a thickness of 6-15mm, which solves the problems of plate warping and poor performance uniformity after online quenching of 1000MPa grade high-strength steel. At the same time, it can meet the technical requirements of the plate without the need for subsequent offline tempering and straightening treatment.

[0067] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A high efficiency low cost thin gauge 1000 MPa grade high strength steel, characterized in that, The chemical composition of the high-strength steel, by weight percentage, includes: C 0.14%–0.19%, Si 0.2%–0.35%, Mn 1.3%–1.5%, Nb 0.010–0.040%, Cr 0.25–0.35%, P≤0.02%, S≤0.005%, Ti 0.015%–0.03%, B 0.0003%–0.0016%, Als 0.015%–0.045%, Ceq 0.41%–0.51%, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%. The method for manufacturing high-efficiency, low-cost, thin-gauge 1000MPa grade high-strength steel is characterized by comprising the following steps: 1) Steelmaking to continuous casting: Molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting; after RH vacuum degassing, Al is added according to Als requirements; after Ti-Fe micro-titanium treatment, B-Fe alloy is added; then slab continuous casting is carried out, with a superheat of 8-15℃ and a casting speed of 0.7-1.0m / min; in the fan-shaped section, strong cooling is used, with a total cooling water volume of 1100-1200L / min in the first half and 1000-1100L / min in the second half. At the same time, light reduction is applied at the end of solidification, with a reduction of 8-15mm for the continuous casting slab. After the slab is removed from the line, it is slowly cooled in the pit, with a slow cooling temperature of not less than 600℃ and a slow cooling time of not less than 48h. 2) Billet heating: The billet is fed into a walking beam furnace for heating. The billet passes through the preheating section, heating section and soaking section in sequence before exiting the furnace. The temperature of the preheating section is 950-1150℃, the temperature range of the heating section is 1220-1250℃, and the temperature of the soaking section is 1180-1210℃. The total time in the furnace for the heating and soaking sections is controlled at 3-4 hours to ensure that the temperature difference between the upper and lower surfaces of the billet is within 15℃. 3) Controlled rolling: The billet is rolled in two stages: The first stage is rough rolling, with an initial rolling temperature ≥1100℃ and a final rolling temperature of 985~1025℃. The rough rolling process has no more than 6 passes, and the reduction ratio for the first two passes is 20%~25%. The thickness of the intermediate billet is 2.5~4 times that of the finished product. During the intermediate billet's warming process, descaling water is sprayed onto the rolling mill for 0.5~0.8 min at a pressure of 20~25MPa. The second stage is finish rolling, with an initial rolling temperature of 940~970℃ and a final rolling temperature of 850~880℃. The finish rolling process has no more than 6 passes, and the reduction ratio for the first two passes is 25%~28%, while the final pass has a reduction ratio of less than 5%. The rolling length of the steel plate is controlled within 25m. 4) Pre-straightening: The rolled steel plate is pre-straightened at high temperature. The straightening force is between 800KN and 1000KN, the bending amount is set to 0.5 to 1.0 mm, and the tilting value is set to 2 to 3.5 mm. 5) Rapid cooling: The pre-straightened steel plate is rapidly cooled. The initial cooling temperature of the steel plate is 770–810℃, the roller speed in the controlled cooling zone is 2–2.5 m / s, and the acceleration is 0.003–0.005 m / s². 2 The final cooling temperature is 150–200℃, and the cooling rate is controlled at 20–30℃ / s. During the cooling process of the steel plate, the water flow ratio of the upper and lower manifolds is 1:1.6–1:2.

1. After the steel plate exits the controlled cooling system, the side spray is activated, with a side spray pressure of 5–10 MPa and a water flow rate of 25–40 m³ / s. 3 / h; 6) Cold straightening: The steel plate after controlled cooling is cold straightened. The straightening force is between 3500KN and 5000KN, the bending amount of the roller is 1.2 to 1.8mm, and the tilt value is set to 4.0 to 5.5mm.

2. The high efficiency low cost thin gauge 1000 MPa grade high strength steel of claim 1, wherein, The thickness of the high-strength steel is 6-15 mm.

3. The high efficient low cost thin gauge 1000 MPa grade high strength steel as claimed in claim 1, wherein, The high-strength steel has a yield strength ≥1050MPa, tensile strength ≥1350MPa, elongation ≥10%, longitudinal yield strength difference and tensile strength difference between the head, middle and tail of the steel plate are within 35MPa, transverse Charpy impact energy at -20℃ is ≥54J, and straightness is less than 5mm / 2m.

4. The high efficiency low cost thin gauge 1000 MPa grade high strength steel as claimed in claim 1, wherein, In step 1), the raw material is pretreated with KR molten iron to control the S content to be less than 0.005%, and then enters the converter after slag removal. During converter smelting, the P content is controlled to be ≤0.02%, and the C content is controlled to be 0.14-0.19% at the end of converter smelting. Argon gas is blown for 25-30 minutes when tapping the steel. Then, LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes.

5. The high-efficiency, low-cost, thin-gauge 1000MPa grade high-strength steel according to claim 1, characterized in that, During the converter smelting process, the double slag method is used for phosphorus removal.

6. The high efficiency low cost thin gauge 1000 MPa grade high strength steel as claimed in claim 1, wherein, During the intensive cooling process, the first half consists of stages 1 to 4, and the second half consists of stages 5 to 8.

7. The high efficiency low cost thin gauge 1000 MPa grade high strength steel as claimed in claim 1, wherein, The thickness of the cast billet is less than 250 mm.

8. The high-efficiency, low-cost, thin-gauge 1000MPa grade high-strength steel according to claim 1, characterized in that, In step 3), before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, and the descaling machine pressure is 15-20 MPa.

9. The high efficiency low cost thin gauge 1000 MPa grade high strength steel as claimed in claim 1, wherein, In step 5), the number of controlled cooling manifold groups opened is 3 to 6, and the water flow rate of a single manifold is 120 to 150 m³. 3 / h; the austenite content of the steel plate before controlled cooling is above 70%; the martensite content of the steel plate after controlled cooling is above 80%.

Images