V-N-B microalloyed 550MPa-grade steel for engineering machinery and preparation method of V-N-B microalloyed 550MPa-grade steel

Abstract

The invention discloses V-N-B microalloyed 550MPa-grade steel for engineering machinery and a preparation method of the V-N-B microalloyed 550MPa-grade steel, and belongs to the technical field of low-carbon microalloyed steel production. The steel comprises the following components in percentage by weight: 0.06%-0.15% of C, 0.2%-0.35% of Si, 1.35%-1.65% of Mn, less than or equal to 0.02% of P, less than or equal to 0.015% of S, 0.01%-0.03% of Nb, 0.015%-0.0250% of Ti, 0.15%-0.25% of Cr, 0.015%-0.045% of Als, 0.05%-0.08% of V, 0.0012%-0.0020% of B, 0.01%-0.025% of N, 0.005%-0.010% of Ca and the balance of Fe and inevitable impurities, the total amount of other impurity elements is less than 0.05%, and the problems of low toughness and plate shape unevenness are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of low-carbon microalloyed steel production, and particularly relates to a VNB microalloyed 550MPa grade engineering machinery steel with a thickness specification of 10-50mm and a preparation method thereof. Background Art

[0002] The demand for low-alloy, high-strength steel is constantly increasing. Low-alloy, high-strength steel is primarily used in marine engineering, lifting equipment, construction machinery, and other fields. The use of high-strength steel not only reduces material, transportation, and manufacturing costs, but also saves on welding consumables, welding man-hours, reduces post-weld stress, lowers welding preheat temperatures, and reduces post-weld heat treatment requirements. With the continuous advancement of steel materials, welding material manufacturing, and welding process technology, the strength level of structural steel continues to increase. Offshore cranes, marine liquid tanks, jack-up platform legs, coal mine hydraulic supports, and other applications now make extensive use of 550MPa-grade high-strength steel. Currently, high-strength steels with a yield strength above 550MPa, both domestically and internationally, mostly utilize microalloy systems such as Nb-Ti, Nb-V, and Nb-Ti-Mo in a low-carbon composition, combined with the addition of precious alloying elements such as Ni, Cr, and Cu. High strength and toughness are achieved through various means, including grain refinement, solid solution strengthening, phase transformation strengthening, precipitation strengthening, and dislocation strengthening.

[0003] Domestic steel companies have successfully developed 550MPa grade Nb-Ti-Mo series composite microalloyed medium and thick plates, but there are still difficulties in production. The main manifestations are: (1) It is difficult to control the plate shape. The main component of the currently used component system after rolling is bainite. Because of the phase transformation characteristics of this steel during the rapid cooling process, the structural stress caused by the structure leads to defects such as poor plate shape. Post-rolling heat treatment is required to achieve uniform structure, the production cycle is long, and the production equipment is high. (2) The production cost is high. Due to the use of a low-carbon component system, a large amount of expensive microalloys need to be added to compensate for the loss of strength caused by the reduction in carbon content, resulting in excessively high production costs and waste of energy. (3) It is difficult to control the plasticity and toughness. Due to the use of an online cooling process, in order to ensure the formation of the bainite phase, a rapid cooling process is adopted, which leads to an increase in the brittleness ratio (M / A) and a decrease in plasticity. It can be seen that in view of the above-mentioned problems existing in conventional 550MPa high-strength steel plates, there is an urgent need to develop 550MPa-grade steel plates with low production equipment requirements, low cost, high strength and toughness, so as to meet the urgent demand of the rapidly developing manufacturing industry for high-strength steel with excellent performance and low cost.

[0004] Compared with existing technologies: To date, little research has been conducted domestically and internationally on a VNB microalloyed 550MPa-grade engineering machinery steel with a thickness of 10-50mm and its preparation method. Prior to this research, a journal article titled "Analysis of the Causes of Unqualified Elongation Properties of Low-Alloy High-Strength Steel Q550D" (Shanxi Metallurgy, January 2010) focused on enhancing the toughness of the steel plate by improving molten steel purity, modifying inclusions, optimizing rolling and controlled cooling processes, and employing tempering heat treatment. However, the use of an offline tempering heat treatment significantly increased production cycle time and costs.

[0005] While the analysis of the reasons for the elongation failure of low-alloy, high-strength steel Q550D disclosed in the aforementioned literature addresses the issue of unacceptable elongation for some thickness specifications, it is not suitable for controlling the problem of low-cost, efficient, and cost-effective production of engineering machinery steel plates with a yield strength of 550 MPa and a thickness of 10-50 mm using thermomechanical control (TMCP) instead of offline heat treatment. The technical solution provided by the present invention effectively overcomes these shortcomings and addresses the low toughness and uneven flatness of VNB microalloyed 550 MPa hot-rolled steel plates with a thickness of 10-50 mm produced using an online cooling process from continuously cast billets less than 250 mm thick. Summary of the Invention

[0006] The present invention aims to overcome the above-mentioned technical problems and deficiencies by providing a VNB microalloyed 550 MPa grade steel for engineering machinery and a method for preparing the same, which are characterized by high efficiency and low cost. The method utilizes optimized steelmaking, heating, rolling, controlled cooling, and straightening processes to ultimately solve the problems of low toughness and uneven plate shape of 550 MPa grade hot-rolled steel plates with a thickness of 10-50 mm produced by an online cooling process.

[0007] To achieve the above object, the technical solution of the present invention is: A VNB microalloyed 550 MPa grade steel plate for engineering machinery, the chemical composition of which comprises, by weight percentage, the following: C 0.06%-0.15%, Si 0.2%-0.35%, Mn 1.35%-1.65%, P≤0.02%, S≤0.015%, Nb 0.01%-0.03%, Ti 0.015%-0.0250%, Cr 0.15%-0.25%, Als 0.015%-0.045%, V 0.05%-0.08%, B 0.0012%-0.0020%, N 0.01%-0.025%, Ca 0.005%-0.010%, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%.

[0008] The functions of the main elements in the chemical composition of the steel of the present invention are as follows: C: 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 0.06% to 0.15%.

[0009] 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 in the continuous casting ingot, hindering low-temperature toughness and ensuring uniform cross-sectional microstructure. Therefore, the Mn content in this invention is designed to be between 1.35% and 1.65%.

[0010] Si: It deoxidizes steel during production and improves matrix strength. Increasing Si content purifies ferrite, reduces pearlite, and helps mitigate the Bauschinger effect in the matrix material. However, excessive Si content can reduce the toughness of the weld heat-affected zone (HAZ) of the base material. Therefore, the Si content in this invention is set at 0.20% to 0.35%.

[0011] Nb is a common element in modern microalloyed pipeline steels, providing excellent grain refinement and precipitation strengthening effects. It also delays austenite recrystallization, but excessive Nb increases production costs and makes continuous casting process control more difficult. The present invention selects a Nb content range of 0.01% to 0.03%, combined with a suitable TMCP process, to produce a uniform composite phase dominated by pearlite, polygonal ferrite, and acicular ferrite, resulting in excellent toughness.

[0012] N: The N element in steel has no other significant function except forming fine VN particles to refine austenite grains. Therefore, the N content needs to be kept at a relatively low level. The N content selected in the present invention is in the range of 0.01% to 0.025%.

[0013] V is a strong nitrogen-binding element, present as VN in the continuous casting ingot. Fine VN 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. When V addition exceeds a certain level, the VN particles coarsen, increasing stress concentration at the particle interface and the matrix. Therefore, the V content in this invention is selected to be in the range of 0.05% to 0.08%.

[0014] Al: Typically acts as a deoxidizer in steel and, if formed into AlN, also refines the steel's structure. When the Al content exceeds 0.045%, excessive aluminum oxide inclusions can reduce the steel's cleanliness. If the Al content is too low, deoxidation is inadequate, and easily oxidizable elements like Ti can form oxides. Therefore, the lower limit for Al content is set at 0.015%.

[0015] Cr: A major element that can effectively improve hardenability, inhibit the formation of ferrite, and promote the formation of bainite. It plays an important role in controlling phase transformation structure, promoting the formation of polygonal ferrite, pearlite, and acicular ferrite with a large number of dislocations distributed within the crystal in the medium and low temperature regions, thereby improving the strength, plasticity, and toughness of the steel plate. The present invention selects the Cr content range to be 0.15% to 0.25%.

[0016] Ti: It plays a core role in steel in refining grains and preventing intergranular corrosion. It also has the functions of deoxidation, degassing, and improving welding performance. However, if the Ti content is too high, it will easily form titanium carbonitrides, reduce toughness and cause hot brittleness. The reasonable range is controlled at 0.0015% to 0.0025%.

[0017] Boron: An element that significantly improves the hardenability of steel. It easily 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 ideal range is 0.0012% to 0.0020%.

[0018] Ca: Trace amounts of calcium (Ca) act as a purifier for deoxidation and desulfurization in steel, improving the morphology of non-metallic inclusions and is widely used in calcium-treated clean steel. By adding trace amounts of Ca to carbon steel, dispersed, thermally stable second-phase calcium oxide particles form within the steel. Research results indicate that these dispersed Ca oxide particles pin the austenite grain boundaries in the coarse-grained heat-affected zone (CGHAZ) during the weld thermal cycle, limiting austenite grain growth and resulting in a finer CGHAZ grain size. This, in turn, improves the strength and toughness of the CGHAZ of Ca-containing steels. However, excessive Ca concentrations can segregate at grain boundaries, negatively impacting the steel's hardenability and impact toughness. A reasonable Ca content range is 0.005% to 0.010%.

[0019] P and S are unavoidable impurity elements in steel and should be as low as possible. 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 0.020% and 0.015%, respectively.

[0020] The VNB microalloyed 550MPa grade engineering machinery steel of the present invention is a hot-rolled steel plate with a target thickness of 10-50mm. It is produced on a medium and heavy plate reciprocating rolling mill using a casting with a thickness of less than 250mm and water as the cooling medium.

[0021] The object of the present invention is achieved through the following technical solutions: The present invention provides a method for preparing VNB microalloyed 550MPa grade engineering machinery steel, comprising molten steel smelting → furnace refining (LF refining) + RH vacuum degassing → B alloying → continuous casting → billet heating → controlled rolling and cooling → hot straightening; specifically comprising the following steps: 1) Molten steel smelting to continuous casting: smelting according to the following chemical composition, the weight percentage of which includes C 0.06%~0.15%, Si 0.2%~0.35%, Mn 1.35%~1.65%, P≤0.02%, S≤0.015%, Nb 0.01%~0.03%, Ti0.015%~0.0250%, Cr 0.15%~0.25%, Als 0.015%~0.045%, V 0.05%~0.08%, B 0.0012%~0.0020%, N 0.01%~0.025%, Ca 0.005%~0.010%, the balance is Fe and unavoidable impurities, and the total amount of other impurity elements is less than 0.05%. The molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting. During the LF refining process, Si-Ca wire is used to refine the molten steel outside the furnace, effectively reducing the oxygen and sulfur content in the molten steel. This converts sulfide and oxide inclusions into calcium-containing spherical inclusions, while also reducing the number of inclusions, thereby purifying the molten steel, improving the quality of the ingot, and enhancing the low-temperature toughness of the rolled steel plate. After the RH vacuum degassing, Al is added according to Als requirements. B-Fe alloy is added for B alloying, ensuring that the added Al does not float on the slag layer. Slab continuous casting then proceeds, with a superheat of 15-25°C and a casting speed of 0.8-1.1 m / min. (Controlling the pouring superheat and casting speed can effectively reduce defects in the center and corners of the ingot.) Soft reduction of 10-15 mm is applied in the horizontal sector, i.e., at the end of solidification. (Soft reduction helps reduce ingot segregation and refine the austenite grains.) 2) Ingot Heating: The ingot is fed into a walking beam furnace for heating. The ingot passes through the preheating section, heating section, and soaking section in sequence before exiting the furnace. The temperature range of the preheating section is 900-1000°C (to promote the rapid and complete solid solution of Nb and V carbides or nitrides in the matrix and their sufficient diffusion). The calorific value of the gas is controlled at 2000-2500 J / kg. The temperature range of the heating section is 1220-1250°C, and the temperature range of the soaking section is 1210-1220°C. The total furnace time of the heating and soaking sections is not less than 3.5-4 hours (to ensure the calorific value of the gas and the time of the high temperature sections (heating and soaking sections), further promote the diffusion of alloying elements such as Mn, B, V, and N, and reduce the impact of their composition segregation on the structure and properties. At the same time, it also avoids coarsening of austenite grains and reduced toughness of the steel plate due to excessive furnace time). 3) Controlled rolling, cooling and straightening: The billet is rolled in two stages. The first stage is recrystallization rolling (rough rolling). The starting temperature of rough rolling is ≥1100℃, and the finishing temperature range of rough rolling is 1010-1060℃. The reduction rate of the first two passes in the rough rolling stage is not less than 30% per pass. By using the pass locking function, the number of rolling passes in the rough rolling stage is less than four (using the high temperature deformation induction effect to promote the precipitation of VN phase in the austenite crystal, providing nucleation sites for acicular ferrite, so as to promote the formation of acicular ferrite in the crystal and improve strength and toughness). The thickness of the intermediate billet is 2-5 times the thickness of the finished product. During the intermediate billet waiting for temperature, descaling water is sprayed twice, the descaling time is 0.5-1.5min, and the descaling machine pressure is 15-20MPa (to inhibit the growth of austenite grains and generate temperature on the inner and outer surfaces of the billet). Gradient, promotes the penetration of rolling deformation to the center of the thickness, refines the grains at 1 / 2 of the thickness, is beneficial to improve the core structure of thick-gauge steel plates, and reduces the structural stress caused by structural unevenness); the second stage is recrystallization rolling (finishing rolling), the finishing rolling starting temperature range is 890-930℃, the final rolling temperature range is 810-840℃, the finishing rolling is not more than seven passes, and the pass reduction system is to ensure that the reduction rate in the first two passes is above 20% (by controlling the final rolling temperature in the roughing stage, ensuring that the intermediate billet is in the austenite recrystallization temperature range during the warming and finishing rolling processes, ensuring structural uniformity, and at the same time increasing the reduction rate in the finishing rolling stage, ensuring the dislocation density, vacancies and deformation bands in the austenite, promoting VN precipitation, providing more nucleation sites, promoting ferrite phase transformation, refining the ferrite structure, and ensuring the strength and toughness of the steel plate). Subsequently, laminar cooling is used for online temperature-controlled quenching, with the starting cooling temperature range of 750-810°C, the final cooling temperature range of 580-610°C, and the cooling rate of 10-15°C / s (N and V combined have a low-temperature precipitation effect, and VN precipitation can significantly improve the mechanical properties of steel through precipitation strengthening. In addition, the addition of B and Cr can improve the hardenability of steel, increase austenite stability, increase the phase transformation driving force, and promote bainite transformation. Based on this, a high final cooling temperature and slow cooling rate are adopted to ensure the plate shape after cooling); hot straightening is then carried out, and the straightening temperature of hot straightening is below 500°C, followed by air cooling to room temperature.

[0022] Furthermore, in step 1), the raw materials are pretreated with KR molten iron to control the S content to be less than 0.015%, and then enter 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 0.06-0.15% at the end of converter smelting. Argon gas is blown for 20-30 minutes during tapping (argon blowing before continuous casting can promote the removal of inclusions in the molten steel and improve the uniformity of the molten steel composition); then LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 20 minutes.

[0023] Furthermore, during the converter smelting process, a double slag method is used to remove phosphorus.

[0024] Furthermore, in step 3), the cooling medium for laminar cooling is water.

[0025] Furthermore, in step 3), the straightening force is between 2500KN and 4000KN, the position of the lead-in roller is -1mm to -1.3mm, and the position of the lead-out roller is -2.3mm to -2.8mm (to optimize the straightening process and promote further and full release of internal stress).

[0026] The above-mentioned composition and process scheme are used to obtain an efficient and low-cost production method for VNB microalloyed steel plates with a grade of 550MPa and a thickness specification of 10-50mm. By optimizing the steelmaking, heating, rolling, cooling and straightening processes, the problems of low toughness and uneven plate shape in the production of 550MPa grade hot-rolled steel plates with a thickness of 10-50mm using an online cooling process are finally solved.

[0027] Beneficial effects of the present invention: 1. By adopting argon blowing, argon calming before continuous casting can promote the removal of inclusions in molten steel and improve the uniformity of molten steel composition; during the refining process, using Si-Ca wire to refining the molten steel can effectively reduce the oxygen and sulfur content in the molten steel, and transform the sulfide and oxide inclusions into calcium-containing spherical inclusions, while reducing the number of inclusions, thereby achieving the purpose of purifying the molten steel, improving the quality of steel, and improving the low-temperature toughness of rolled steel plates; applying soft reduction can help reduce billet segregation, refine austenite grains, and at the same time utilize VN precipitation before the billet enters the heating process to capture H in the steel plate, reduce hydrogen embrittlement, and help improve the toughness of the steel plate.

[0028] 2. By controlling the temperature of the preheating section, the heating section, the soaking section and the high-temperature section time, it is ensured that the carbides and nitrides of Nb, V and B are quickly and fully dissolved in the matrix and fully diffused, ensuring the calorific value of the gas and the high-temperature section time, further promoting the diffusion of alloying elements such as Mn, B, V and N, reducing the impact of component segregation on the structure and performance, and also avoiding the coarsening of austenite grains due to too long furnace time and reducing the toughness of the steel plate.

[0029] 3. The composition of the present invention is reasonable and the alloy addition amount is low. By using VNB composite design and adding a small amount of microalloying element Nb, the alloy cost and high-temperature deformation resistance in the rough rolling and finishing rolling stages are greatly reduced, which is conducive to ensuring the comprehensive performance of super steel plates.

[0030] 4. A two-stage controlled rolling process is adopted. By controlling the final rolling temperature in the rough rolling stage, the intermediate billet is kept in the austenite recrystallization temperature range during the waiting process to ensure the uniformity of the structure. At the same time, the two-stage rolling reduction rate and the thickness of the intermediate billet are restricted. The high-temperature deformation induction effect is used to promote the precipitation of VN phase in the austenite crystal, providing nucleation sites for acicular ferrite, so as to achieve the purpose of promoting the formation of acicular ferrite in the crystal and improve the strength and toughness. Descaling water is used to inhibit the growth of austenite grains. At the same time, a temperature gradient is generated on the inner and outer surfaces of the billet to promote rolling deformation. The shape of the steel plate penetrates toward the center of the thickness, refining the grains at 1 / 2 of the thickness, which is beneficial to improving the core structure of the thick steel plate and reducing the structural stress caused by uneven structure. The combination of N and V has a low-temperature precipitation effect. VN precipitation can significantly improve the mechanical properties of the steel through precipitation strengthening. In addition, the addition of B and Cr can improve the hardenability of the steel, increase the stability of austenite, increase the driving force of phase transformation, and promote bainite transformation. Based on this, a high red-returning temperature and slow cooling rate are adopted to ensure the plate shape after cooling, while optimizing the straightening process and promoting the full release of internal stress.

[0031] 5. Using the above composition and process scheme, a high-efficiency and low-cost production method for VNB microalloyed hot-rolled steel plates with a thickness of 10-50mm and a 550MPa grade is obtained. By optimizing the composition design, steelmaking, heating, rolling, cooling and straightening processes, the low toughness and uneven plate shape problems of 550MPa grade hot-rolled steel plates with a thickness of 10-50mm in the online cooling process are finally solved. The specific performance is as follows: the transverse tensile yield strength (R t0.5 ) performance is between 565 and 610 MPa, and the tensile strength (R m ) is between 670 and 770 MPa, elongation (A) ≥ 19%, -40℃ transverse Charpy impact energy (VN2) ≥ 120J, and straightness is below 5mm / 2m. DETAILED DESCRIPTION

[0032] The following examples are used to specifically illustrate the present invention. These examples are only general descriptions of the present invention and do not limit the present invention.

[0033] A VNB microalloyed 550 MPa grade steel for engineering machinery, the chemical composition of which comprises, by weight percentage, the following: C 0.06%-0.15%, Si 0.2%-0.35%, Mn 1.35%-1.65%, P≤0.02%, S≤0.015%, Nb 0.01%-0.03%, Ti 0.015%-0.0250%, Cr 0.15%-0.25%, Als 0.015%-0.045%, V 0.05%-0.08%, B 0.0012%-0.0020%, N 0.01%-0.025%, Ca 0.005%-0.010%, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%. The target thickness of the steel plate is 10-50 mm, and the steel plate is produced on a medium and heavy plate reciprocating rolling mill using continuous casting billets with a thickness of less than 250 mm, and the cooling medium is water.

[0034] The method for preparing the VNB microalloyed 550MPa grade engineering machinery steel comprises molten steel smelting → furnace refining (LF refining) + RH vacuum degassing → B alloying → continuous casting → slab heating → controlled rolling and cooling → hot straightening. The method specifically comprises the following steps: 1) Molten steel smelting and continuous casting: Smelting is carried out according to the above composition. The raw materials are pre-treated with KR molten iron to control the S content to less than 0.015%, and then enter the converter after slag removal. During converter smelting, the double slag method is used to remove P to control the P content to ≤ 0.02%. The C content is controlled at 0.06-0.15% at the end of converter smelting. Argon is blown for 20-30 minutes during tapping. LF refining and RH vacuum degassing are then carried out. During the LF refining process, Si-Ca wire is used to refining the molten steel outside the furnace to effectively reduce the oxygen and sulfur content in the molten steel, converting sulfide and oxide inclusions into calcium-containing spherical inclusions. At the same time, the number of inclusions is reduced, thereby purifying the molten steel, improving the quality of the cast billet, and improving the low-temperature toughness of the rolled steel plate; at the same time, the RH vacuum is maintained for more than 20 minutes; after the degassing treatment is completed, Al is added according to the Als requirement; B-Fe alloy is added for B alloying; and it is ensured that it is added to the molten steel to avoid floating on the slag layer; then the slab is continuously cast, with a continuous casting superheat of 15-25°C and a continuous casting billet speed of 0.8-1.1m / min. In the horizontal sector, that is, at the end of solidification, a soft reduction is applied, with a reduction of 10-15mm; 2) Billet heating: Billets with a thickness of less than 250mm are fed into a walking beam furnace for heating. The billets are sequentially passed through the preheating section, heating section, and soaking section before being discharged from the furnace. The temperature range of the preheating section is 900-1000°C, the calorific value of the gas is controlled at 2000-2500J / kg, the temperature range of the heating section is 1220-1250°C, and the temperature range of the soaking section is 1210-1220°C. The total time in the heating and soaking sections in the furnace is not less than 3.5-4 hours. 3) Controlled rolling, cooling and straightening: The billet is rolled in two stages. The first stage is recrystallization rolling (rough rolling). The starting rolling temperature of rough rolling is ≥1100℃, and the finishing rolling temperature range is 1010-1060℃. The reduction rate of the first two passes in the rough rolling stage is not less than 30% per pass. By using the pass locking function, the number of rolling passes in the rough rolling stage is less than four, and the thickness of the intermediate billet is 2-5 times the thickness of the finished product. During the intermediate billet waiting for temperature, descaling water is sprayed twice, the descaling time is 0.5-1.5min, and the descaling machine pressure is 15-20MPa. The second stage is recrystallization rolling (finishing rolling). The starting rolling temperature range of finishing rolling is 890-930℃, the finishing rolling temperature range is 810-840℃, and the finishing rolling is no more than seven passes. The pass reduction system is to ensure that the reduction rate of the first two passes is above 20%. Subsequently, laminar cooling is used for online temperature-controlled quenching, with the starting cooling temperature range of 750-810°C, the final cooling temperature range of 580-610°C, and the cooling rate of 10-15°C / s; then hot straightening is carried out, with the straightening temperature below 500°C, the straightening force between 2500KN and 4000KN, the position of the lead-in roller -1mm to -1.3mm, and the position of the lead-out roller -2.3mm to -2.8mm, followed by air cooling to room temperature.

[0035] Examples 1-6 Table 1 shows the chemical composition of the example steel, Table 2 shows the smelting to continuous casting process system of the example steel, Table 3 shows the heating process system of the cast steel of the example steel, Table 4 shows the rolling parameters of the example steel, Table 5 shows the main process parameters of cooling and straightening of the example steel, and Table 6 shows the performance and quality indicators of the example steel plate.

[0036] Table 1 Chemical composition of the examples (wt, %)

[0037] Note: Impurity elements P in steel ≤ 0.02%; S ≤ 0.015%; the total amount of other impurity elements is less than 0.05%.

[0038] Table 2 Process system from smelting to continuous casting of example steel

[0039] Table 3 Heating process of the steel ingot of Example

[0040] Table 4 Rolling parameters of steel according to the present invention

[0041] Table 5 Main process parameters for cooling and straightening of example steel

[0042] Table 6 Performance and quality indicators of steel plates in the examples

[0043] It can be seen that compared with the existing technology, the composition design and steelmaking continuous casting, heating and controlled rolling and cooling scheme of the present invention overcome the shortcomings of the existing technology, and realize an efficient and low-cost production method for manufacturing 550MPa grade VNB micro-alloyed hot-rolled steel plates with a thickness of 10-50mm. It solves the problems of low toughness and plate unevenness in the production of 550MPa grade hot-rolled steel plates with a thickness of 10-50mm using an online cooling process.

[0044] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A VNB microalloyed 550 MPa grade steel for engineering machinery, characterized in that: Its chemical composition includes by weight percentage: C 0.06%-0.15%, Si 0.2%-0.35%, Mn 1.35%-1.65%, P≤0.02%, S≤0.015%, Nb 0.01%-0.03%, Ti 0.015%-0.0250%, Cr 0.15%-0.25%, Als 0.015%-0.045%, V 0.05%-0.08%, B 0.0012%-0.0020%, N 0.01%-0.025%, Ca 0.005%-0.010%, the balance is Fe and unavoidable impurities, and the total amount of other impurity elements is less than 0.05%; The method for preparing the VNB microalloyed 550 MPa grade engineering machinery steel comprises the following steps: 1) Molten steel smelting to continuous casting: The molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting. During the LF refining process, Si-Ca wire is used for off-furnace refining. After the RH vacuum degassing, Al is added according to the Als requirement, and B-Fe alloy is added for B alloying. Slab continuous casting is then carried out, with a continuous casting superheat of 15-25°C and a continuous casting casting rate of 0.8-1.1 m / min. Soft reduction is applied in the horizontal sector, i.e., at the end of solidification, with a reduction of 10-15 mm. 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 being discharged from the furnace. The temperature of the preheating section is 900-1000°C, the temperature of the heating section is 1220-1250°C, and the temperature of the soaking section is 1210-1220°C. The total time in the heating and soaking sections in the furnace is not less than 3.5-4 hours. 3) Controlled rolling, cooling and straightening: The billet is rolled in two stages. The first stage is rough rolling. The starting rolling temperature of rough rolling is ≥1100℃, and the finishing rolling temperature is 1010-1060℃. The reduction rate of the first two passes of rough rolling is not less than 30% per pass. The number of rolling passes in the rough rolling stage is less than four. The thickness of the intermediate billet is 2-5 times the thickness of the finished product. During the intermediate billet waiting for temperature, descaling water is sprayed twice. The descaling time is 0.5-1.5 minutes, and the descaling machine pressure is 15-20MPa. The second stage is finishing rolling, with the starting rolling temperature of 890-930℃ and the final rolling temperature of 810-840℃. The finishing rolling is not more than seven passes. The pass reduction system is that the first two passes ensure the reduction rate is above 20%. Then laminar cooling is used for online temperature-controlled quenching, with the starting cooling temperature of 750-810℃, the final cooling temperature of 580-610℃, and the cooling rate of 10-15℃ / s. After that, hot straightening is carried out, and the straightening temperature of hot straightening is below 500℃, followed by air cooling to room temperature.

2. The VNB microalloyed 550 MPa grade engineering machinery steel according to claim 1, characterized in that: The thickness of the steel is 10-50 mm.

3. The VNB microalloyed 550 MPa grade engineering machinery steel according to claim 1, characterized in that: The steel has a transverse tensile yield strength of 565-610 MPa, a tensile strength of 670-770 MPa, an elongation of ≥19%, a transverse Charpy impact energy of ≥120 J at -40°C, and a straightness of less than 5 mm / 2 m.

4. The VNB microalloyed 550 MPa grade engineering machinery steel according to claim 1, characterized in that: In step 1), the raw materials are pretreated with KR molten iron to control the S content to less than 0.015%, and then enter 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 between 0.06 and 0.15% at the end of converter smelting. Argon is blown for 20 to 30 minutes during tapping. LF refining and RH vacuum degassing are then carried out, and the RH vacuum is maintained for more than 20 minutes.

5. The VNB microalloyed 550 MPa grade engineering machinery steel according to claim 4, characterized in that: During the converter smelting process, the double slag method is used to remove P.

6. The VNB microalloyed 550 MPa grade engineering machinery steel according to claim 1, characterized in that: The thickness of the cast slab is less than 250 mm; the steel plate is obtained by rolling the cast slab on a medium and thick plate reciprocating rolling mill.

7. The VNB microalloyed 550 MPa grade engineering machinery steel according to claim 1, characterized in that: In step 2), the calorific value of the coal gas is controlled at 2000-2500 J / kg.

8. The VNB microalloyed 550 MPa grade engineering machinery steel according to claim 1, characterized in that: In step 3), the cooling medium for laminar cooling is water.

9. The VNB microalloyed 550 MPa grade engineering machinery steel according to claim 1, characterized in that: In step 3), the cooling medium for laminar cooling is water; the straightening force for hot straightening is between 2500 and 4000 kN, the position of the lead-in roller is -1 to -1.3 mm, and the position of the lead-out roller is -2.3 to -2.8 mm.

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