A v-n-b microalloyed 550mpa grade engineering machinery steel and a preparation method thereof

Abstract

The application discloses a V-N-B microalloyed 550MPa grade engineering machinery steel and a preparation method thereof, and belongs to the technical field of low-carbon microalloyed steel production. The steel composition comprises, 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%, and the balance of Fe and inevitable impurities, wherein the total amount of other impurity elements is less than 0.05%, so that the problems of low toughness and uneven plate shape are solved.

Description

Technical Field

[0001] This invention belongs to the field of low-carbon microalloyed steel production technology, and specifically relates to a 550MPa grade VNB microalloyed engineering machinery steel with a thickness of 10-50mm and its preparation method. Background Technology

[0002] The requirements for the use of low-alloy high-strength steel are constantly increasing. Low-alloy high-strength steel is mainly used in marine engineering, lifting equipment, and construction machinery. The application of high-strength steel not only reduces material, transportation, and manufacturing costs, but also saves on welding material consumption and welding time, reduces post-weld stress, and lowers welding preheating temperatures and post-weld heat treatment requirements. With the continuous advancement of steel materials, welding material manufacturing, and welding technology, the strength levels of structural steel are constantly increasing. 550MPa-grade high-strength steel is widely used in marine cranes, marine liquid tanks, self-elevating offshore platform legs, and coal mine hydraulic supports. Currently, most high-strength steels with yield strengths above 550MPa, both domestically and internationally, adopt microalloying systems such as Nb-Ti, Nb-V, and Nb-Ti-Mo with low carbon content, and incorporate noble alloying elements such as Ni, Cr, and Cu. High strength and toughness are ensured through various means such as grain refinement strengthening, solid solution strengthening, phase transformation strengthening, precipitation strengthening, and dislocation strengthening.

[0003] Domestic steel companies have successfully developed 550MPa level Nb-Ti-Mo series composite microalloyed medium and heavy plates, but there are still production difficulties. The main difficulties are: (1) Difficulty in plate shape control. The main component of the steel after rolling is bainite. Due to the phase transformation characteristics of the rapid cooling process of this steel, the structural stress caused by the phase transformation leads to defects such as poor plate shape. Post-rolling heat treatment is required to achieve uniform structure, resulting in a long production cycle and high requirements for production equipment. (2) High production cost. Due to the use of a low carbon composition system, a large amount of expensive microalloying is required to compensate for the strength loss caused by the reduction in carbon content, resulting in excessively high production costs and wasted energy. (3) Difficulty in controlling plasticity and toughness. Due to the use of online cooling process, a rapid cooling process is adopted to ensure the formation of bainite phase, which leads to an increase in its brittleness ratio (M / A) and a decrease in plasticity. Therefore, in response to the aforementioned problems with conventional 550MPa high-strength steel plates, there is an urgent need to develop 550MPa grade steel plates that require less production equipment, have lower costs, and offer high strength and toughness, in order 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:

[0005] To date, there has been very little research, both domestically and internationally, on VNB microalloyed 550MPa grade engineering machinery steel with a thickness of 10-50mm and its preparation methods. Prior to this invention, the journal article "Analysis of the Reasons for the Unqualified Elongation of Low Alloy High-Strength Steel Q550D" (Shanxi Metallurgy, 2010.1) mainly enhanced the toughness of the steel plate by improving the purity of molten steel, modifying inclusions, optimizing rolling and controlled cooling processes, and using tempering heat treatment. However, due to the use of offline tempering heat treatment, its production cycle and cost increased significantly.

[0006] The above-disclosed analyses of the causes of elongation defects in low-alloy high-strength steel Q550D, while addressing the elongation discrepancies in some thicknesses, are not suitable for controlling and resolving the issue of using thermomechanical control (TMCP) to replace offline heat treatment processes for the efficient and low-cost production of engineering machinery steel plates with a yield strength of 550MPa and a thickness of 10-50mm. The technical solution provided by this invention effectively overcomes these shortcomings, solving the problems of low toughness and unevenness in the production of 10-50mm thick VNB microalloyed 550MPa grade hot-rolled steel plates from continuously cast billets with a thickness below 250mm using online cooling processes. Summary of the Invention

[0007] The purpose of this invention is to overcome the above-mentioned technical problems and deficiencies, and to provide a VNB micro-alloyed 550MPa grade engineering machinery steel and its preparation method, which has the characteristics of high efficiency and low cost. The method adopts optimized steelmaking, heating, rolling, controlled cooling and straightening processes, and finally solves the problems of low toughness and unevenness of 550MPa grade hot-rolled steel plates with a thickness of 10-50mm produced by online cooling process.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] A VNB microalloyed 550MPa grade engineering machinery steel plate, the chemical composition by 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%, 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%.

[0010] The roles of the main elements in the chemical composition of the steel of this invention are as follows:

[0011] Carbon (C): The most economical and basic strengthening element in steel. It has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. However, increasing the C content has a negative impact on the plasticity, toughness, and weldability of steel. Therefore, this invention sets the C content range to 0.06% to 0.15%.

[0012] Mn: Mn strengthens steel through solid solution treatment, compensating for the strength loss caused by the reduction in carbon content. Furthermore, it lowers the γ-α phase transformation temperature, thereby refining ferrite grains and contributing to finer low-temperature transformation products, thus improving toughness. However, increasing the Mn content exacerbates center segregation in the continuously cast billet, hindering the improvement of low-temperature toughness and compromising the uniformity of the cross-sectional microstructure. Therefore, the Mn content range in this invention is designed to be 1.35%–1.65%.

[0013] Si (Si) plays a role in deoxidation in steelmaking and improving the strength of the matrix. Increasing the Si content can purify ferrite and reduce the content of pearlite, which is beneficial for reducing the Bauschinger effect in the matrix material. However, excessive Si will reduce the toughness of the heat-affected zone in the weld. Therefore, the Si content is set at 0.20% to 0.35% in this invention.

[0014] 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 complicates continuous casting process control. This invention selects an Nb content range of 0.01% to 0.03%, combined with a suitable TMCP process, to obtain a uniform composite phase dominated by pearlite, polygonal ferrite, and acicular ferrite, resulting in good toughness.

[0015] N: In steel, nitrogen (N) has no other significant role besides forming fine VN particles to refine austenite grains. Therefore, it needs to be kept at a low content level. The N content range selected in this invention is 0.01% to 0.025%.

[0016] V (V): A strong solid nitrogen element, existing in the form of VN in continuously cast billets. Fine VN 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 V content exceeds a certain value, the VN particles coarsen, increasing the stress concentration level at the particle interface and the matrix. Therefore, this invention selects a V content range of 0.05% to 0.08%.

[0017] Al: Commonly used as a deoxidizer in steel, it can also refine the microstructure if it forms AlN. When the Al content exceeds 0.045%, 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%.

[0018] Cr: The main element that can effectively improve hardenability, inhibit ferrite formation and promote bainite formation. It plays an important role in controlling phase transformation structure and promotes the formation of polygonal ferrite, pearlite and acicular ferrite with a large number of dislocations in the grain in the medium and low temperature range, thus improving the strength, plasticity and toughness of steel plates. The Cr content range selected in this invention is 0.15% to 0.25%.

[0019] Ti: In steel, it mainly plays a core role in refining grains and preventing intergranular corrosion. It also has functions such as deoxidation, degassing, and improving welding performance. However, if the Ti content is too high, it is easy to form titanium carbonitrides, which reduces toughness and causes hot brittleness. Its reasonable range is controlled between 0.0015% and 0.0025%.

[0020] 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. Its reasonable range is controlled between 0.0012% and 0.0020%.

[0021] Ca: Trace amounts of calcium (Ca) in steel can act as a deoxidizing and desulfurizing agent, improving the morphology of non-metallic inclusions, and are widely used in calcium-treated clean steel. By adding trace amounts of calcium to carbon steel, dispersed, thermally stable second-phase calcium-containing oxide particles are formed. Research results indicate that these dispersed calcium-containing oxide particles pin the austenite grain boundary migration in the coarse-grained heat-affected zone (CGHAZ) during welding thermal cycling, limiting austenite grain growth and achieving a finer welded CGHAZ grain size, thereby improving the strength and toughness of the micro-calcium steel welded CGHAZ. However, excessively high levels of Ca can lead to grain boundary segregation, adversely affecting the hardenability and impact toughness of the steel. The optimal range is 0.005%–0.010%.

[0022] P and S are unavoidable impurity elements in steel, and their content should be as low as possible. However, due to considerations of smelting costs and processes, their content cannot be infinitely low. Therefore, this invention sets the upper limits for P and S content at 0.020% and 0.015%, respectively.

[0023] The 550MPa grade engineering machinery steel microalloyed by this invention is a hot-rolled steel plate with a target thickness of 10-50mm. It is produced on a medium-thick plate reciprocating rolling mill using a billet with a thickness of less than 250mm, and the cooling medium is water.

[0024] The objective of this invention is achieved through the following technical solution:

[0025] This invention provides a method for preparing 550MPa grade engineering machinery steel with VNB microalloying, comprising: steelmaking → ladle refining (LF refining) + RH vacuum degassing → B alloying → continuous casting → billet heating → controlled rolling and cooling → hot straightening; specifically including the following steps:

[0026] 1) Steel smelting to continuous casting: Smelting according to the following composition, the chemical composition by weight percentage 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%, 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 molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting. During LF refining, the use of Si-Ca wire for ladle refining effectively reduces the oxygen and sulfur content in the molten steel, transforming sulfide and oxide inclusions into calcium-containing spherical inclusions, while also reducing the number of inclusions. This purifies the molten steel, improves billet quality, and enhances the low-temperature toughness of the rolled steel plate. After RH vacuum degassing, Al is added according to Als requirements; B-Fe alloy is added for B alloying; and the addition of B alloys ensures that the molten steel is not allowed to float on the slag layer. Slab continuous casting is then performed with a superheat of 15–25°C and a casting speed of 0.8–1.1 m / min (controlling the casting superheat and casting speed effectively reduces quality defects in the billet's core and corners). Light reduction of 10–15 mm is applied at the horizontal fan-shaped section, i.e., at the end of solidification (light reduction helps reduce billet segregation and refine austenite grains).

[0027] 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 900-1000℃ (to promote the rapid and complete solidification of Nb and V carbides or nitrides into the matrix and to facilitate their diffusion). The calorific value of the gas is controlled at 2000-2500 J / kg. The temperature range of the heating section is 1220-1250℃. The temperature range of the soaking section is 1210-1220℃. The total furnace time in the heating and soaking sections is not less than 3.5-4 hours (to ensure the calorific value of the gas and the time in the high-temperature sections (heating and soaking sections) to further promote the diffusion of alloying elements such as Mn, B, V, and N, reduce their impact on the microstructure and properties due to component segregation, and also avoid excessive furnace time to coarsen austenite grains and reduce the toughness of the steel plate).

[0028] 3) Control of rolling, cooling, and straightening: The billet is rolled in two stages. The first stage is recrystallization rolling (rough rolling). The initial rolling temperature of rough rolling is ≥1100℃, and the final rolling 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. Utilizing the pass locking function, the number of rolling passes in the rough rolling stage is less than four (utilizing the high-temperature deformation-induced effect to promote the precipitation of VN phase in the austenite grains, providing nucleation sites for acicular ferrite, thereby promoting the formation of intragranular acicular ferrite and improving strength and toughness). The thickness of the intermediate billet is 2 to 5 times the thickness of the finished product. During the intermediate billet's warming process, two passes of descaling water are sprayed, with a descaling time of 0.5 to 1.5 min and a descaling machine pressure of 15 to 20 MPa (to inhibit austenite grain growth and simultaneously generate temperature on the inner and outer surfaces of the billet). The first stage is gradient rolling, which promotes the penetration of rolling deformation into the thickness center, refines the grains at 1 / 2 thickness, and is beneficial to improving the core structure of thick steel plates and reducing the structural stress caused by structural inhomogeneity. The second stage is recrystallization rolling (finish rolling), with the initial rolling temperature range of 890-930℃ and the final rolling temperature range of 810-840℃. The finishing rolling is limited to no more than seven passes, and the reduction ratio of the first two passes is guaranteed to be above 20% (by controlling the final rolling temperature of the roughing stage, the intermediate billet is kept in the austenite recrystallization temperature range during the waiting period and finishing rolling process, ensuring the uniformity of the structure. At the same time, it increases the reduction ratio of the finishing rolling stage, ensures the dislocation density, vacancies and deformation bands in the austenite body, promotes VN precipitation, provides more nucleation sites, promotes ferrite phase transformation, refines the ferrite structure, and ensures the strength and toughness of the steel plate). Subsequently, laminar flow cooling was used for online temperature-controlled quenching, with an initial cooling temperature range of 750–810℃ and a final cooling temperature range of 580–610℃. The cooling rate was 10–15℃ / s. (The combination of N and V has a low-temperature precipitation effect. 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 the stability of austenite, increase the driving force of phase transformation, and promote the transformation of bainite. Based on this, a high final cooling temperature and a slow cooling rate were adopted to ensure the shape of the steel plate after cooling.) Then, hot straightening was performed at a straightening temperature below 500℃, followed by air cooling to room temperature.

[0029] Furthermore, in step 1), the raw material is pretreated with KR molten iron to control the S content to be below 0.015%, 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 between 0.06% and 0.15% at the end of converter smelting; argon gas is blown for 20 to 30 minutes during tapping (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, while the RH vacuum is maintained for more than 20 minutes.

[0030] Furthermore, during the converter smelting process, a double-slag method is used for phosphorus removal.

[0031] Furthermore, in step 3), the cooling medium for laminar flow cooling is water.

[0032] Furthermore, in step 3), the straightening force is between 2500KN and 4000KN, the position of the inlet roller is -1mm to -1.3mm, and the position of the outlet roller is -2.3mm to -2.8mm (to optimize the straightening process and promote the further full release of internal stress).

[0033] Using the above-mentioned composition and process scheme, an efficient and low-cost production method for VNB microalloyed steel plates with a thickness of 10-50mm and a strength of 550MPa was obtained. By optimizing the steelmaking, heating, rolling, cooling and straightening processes, the problems of low toughness and unevenness of hot-rolled steel plates with a thickness of 10-50mm produced by online cooling process were finally solved.

[0034] The beneficial effects of this invention are:

[0035] 1. By employing argon blowing, argon blowing and calming before continuous casting can promote the removal of inclusions in molten steel and improve the uniformity of steel composition. During ladle refining, using Si-Ca wire for ladle refining can effectively reduce the oxygen and sulfur content in the molten steel, transforming sulfide and oxide inclusions into calcium-containing spherical inclusions, while reducing the number of inclusions. This achieves the purpose of purifying the molten steel, improving the quality of the steel, and enhancing the low-temperature toughness of rolled steel plates. Applying light pressure can help reduce billet segregation, refine austenite grains, and utilize the precipitation of VN before the billet enters the heating process to capture H in the steel plate, reducing hydrogen embrittlement and helping to improve the toughness of the steel plate.

[0036] 2. By controlling the temperature of the preheating section, the heating section, the soaking section, and the time of the high-temperature section, the carbides and nitrides of Nb, V, and B are ensured to dissolve rapidly and fully in the matrix and diffuse sufficiently. This ensures the calorific value of the gas and the duration of the high-temperature section, further promoting the diffusion of alloying elements such as Mn, B, V, and N, reducing their impact on microstructure and properties due to compositional segregation. At the same time, it avoids excessive time in the furnace, which coarsens the austenite grains and reduces the toughness of the steel plate.

[0037] 3. The composition of this invention is reasonable and the amount of alloy added is low. By using VNB composite design and adding a small amount of micro-alloying element Nb, the cost of alloy and resistance to high-temperature deformation during roughing and finishing stages are greatly reduced, which is conducive to ensuring the comprehensive performance of the super steel plate.

[0038] 4. A two-stage controlled rolling process is adopted. By controlling the final rolling temperature of the roughing stage, the intermediate billet is kept within the austenite recrystallization temperature range during the warming process, ensuring microstructure uniformity. Simultaneously, the rolling reduction rate and intermediate billet thickness are limited in both stages. Utilizing the high-temperature deformation-induced effect, the precipitation of the VN phase within the austenite grains is promoted, providing nucleation sites for acicular ferrite, thereby promoting the formation of intragranular acicular ferrite and improving strength and toughness. Descaling water is sprayed to inhibit austenite grain growth and simultaneously generate a temperature gradient on the inner and outer surfaces of the billet, promoting rolling deformation. The penetration towards the center of thickness refines the grains at half the thickness, which is beneficial to improving the core structure of thick steel plates and reducing structural stress caused by uneven structure. The combination of N and V has 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 the stability of austenite, increase the driving force of phase transformation, and promote bainite transformation. Based on this, a high red-heat temperature and slow cooling rate are adopted to ensure the plate shape after cooling. At the same time, the straightening process is optimized to promote the full release of internal stress.

[0039] 5. A high-efficiency, low-cost production method for VNB microalloyed hot-rolled steel plates with a thickness of 10-50mm and a strength of 550MPa was obtained using the above-mentioned composition and process scheme. By optimizing the composition design, steelmaking, heating, rolling, cooling, and straightening processes, the problems of low toughness and unevenness in 10-50mm thick 550MPa hot-rolled steel plates produced by online cooling processes were ultimately solved. Specific properties include: the yield strength (R0) of 10-50mm thick 550MPa steel plates under transverse tensile stress. t0.5 The performance ranges from 565 to 610 MPa, with a tensile strength (R) of... m The strength is between 670 and 770 MPa, the elongation (A) is ≥19%, the transverse Charpy impact energy (VN2) at -40℃ is ≥120 J, and the flatness is below 5 mm / 2 m. Detailed Implementation

[0040] 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.

[0041] A VNB microalloyed 550MPa grade engineering machinery steel has the following chemical composition 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%, 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 10-50mm. It is produced on a medium-thickness reciprocating rolling mill using continuously cast billets with a thickness of less than 250mm, and the cooling medium is water.

[0042] The preparation method of the above-mentioned VNB microalloyed 550MPa grade engineering machinery steel includes steelmaking → ladle refining (LF refining) + RH vacuum degassing → B alloying → continuous casting → billet heating → controlled rolling and cooling → hot straightening; specifically including the following steps:

[0043] 1) Steelmaking and Continuous Casting: Smelting is carried out according to the above composition. Raw materials undergo KR hot metal pretreatment to control the sulfur content below 0.015%, and after slag removal, they enter the converter. In converter smelting, a double-slag method is used to remove phosphorus (P), controlling the P content to ≤0.02%. At the end of converter smelting, the carbon content is controlled between 0.06% and 0.15%. Argon gas is blown for 20-30 minutes during tapping. This is followed by LF refining and RH vacuum degassing. During LF refining, using a Si-Ca wire for ladle refining effectively reduces the oxygen and sulfur content in the molten steel, transforming sulfide and oxide inclusions into calcium-containing spherical inclusions. Simultaneously, the number of inclusions is reduced, thereby purifying the molten steel, improving the quality of the billet, and enhancing the low-temperature toughness of the rolled steel plate; at the same time, RH vacuum is maintained for more than 20 minutes; after degassing, Al is added according to Als requirements; B-Fe alloy is added for B alloying; ensure that it is added to the molten steel to avoid floating on the slag layer; then slab continuous casting is carried out, with a continuous casting superheat of 15-25℃ and a continuous casting drawing speed of 0.8-1.1m / min. Light reduction is applied in the horizontal fan-shaped section, that is, at the end of solidification, with a reduction of 10-15mm;

[0044] 2) Billet heating: Billets with a thickness of less than 250mm are fed into a walking beam furnace for heating. The billets pass 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℃, and the calorific value of the gas is controlled at 2000-2500J / kg. The temperature range of the heating section is 1220-1250℃, and the temperature range of the soaking section is 1210-1220℃. The total time spent in the furnace in the heating section and soaking section is not less than 3.5-4 hours.

[0045] 3) Control of rolling, cooling and straightening: The billet is rolled in two stages. The first stage is recrystallization rolling (rough rolling), with an initial rolling temperature of ≥1100℃ and a final rolling temperature range of 1010~1060℃. The reduction rate of the first two passes in the rough rolling stage is not less than 30% per pass. Utilizing the pass locking function, the number of rolling passes in the rough rolling stage is less than four. The thickness of the intermediate billet is 2 to 5 times the thickness of the finished product. During the intermediate billet's warming process, two passes of descaling water are sprayed, with a descaling time of 0.5~1.5min and a descaling machine pressure of 15~20MPa. The second stage is recrystallization rolling (finish rolling), with an initial rolling temperature range of 890~930℃ and a final rolling temperature range of 810~840℃. The finish rolling has no more than seven passes, and the reduction rate of the first two passes is guaranteed to be above 20%. Subsequently, laminar flow cooling was used for online temperature-controlled quenching, with an initial cooling temperature range of 750–810℃ and a final cooling temperature range of 580–610℃, and a cooling rate of 10–15℃ / s. After that, hot straightening was performed, with a straightening temperature below 500℃, a straightening force between 2500KN and 4000KN, an inlet roller position of -1mm to -1.3mm, and an outlet roller position of -2.3mm to -2.8mm. Then, it was air-cooled to room temperature.

[0046] Examples 1-6

[0047] Table 1 shows the chemical composition of the steel in the examples; Table 2 shows the smelting and continuous casting process of the steel in the examples; Table 3 shows the heating process of the billet of the steel in the examples; Table 4 shows the rolling parameters of the steel in the examples; Table 5 shows the main process parameters for cooling and straightening of the steel in the examples; and Table 6 shows the performance and quality indicators of the steel plates in the examples.

[0048] Table 1 Chemical composition (wt, %) of the examples

[0049]

[0050] Note: Impurity elements in steel: P≤0.02%; S≤0.015%; total amount of other impurity elements less than 0.05%.

[0051] Table 2. Smelting and continuous casting process of steel in the examples

[0052]

[0053] Table 3 Heating process regime for steel billets in the examples

[0054]

[0055] Table 4 Rolling parameters of steel in embodiments of the present invention

[0056]

[0057] Table 5. Main process parameters for cooling and straightening of the steel in the examples.

[0058]

[0059] Table 6. Performance and quality indicators of steel plates in the examples

[0060]

[0061] Therefore, compared with the existing technology, the composition design and steelmaking continuous casting, heating and controlled rolling and cooling scheme of this invention overcomes the shortcomings of the existing technology and realizes an efficient and low-cost production method for manufacturing VNB microalloyed hot-rolled steel plates with a thickness of 10-50mm and a strength of 550MPa. It solves the problems of low toughness and unevenness of 550MPa hot-rolled steel plates with a thickness of 10-50mm produced by online cooling process.

[0062] 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 VNB microalloyed 550MPa grade engineering machinery steel, characterized in that, Its chemical composition by weight percentage 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%, 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 method for preparing the VNB microalloyed 550MPa grade engineering machinery steel includes the following steps: 1) Steelmaking to continuous casting: Molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting; during LF refining, Si-Ca wire is used for ladle refining; after RH vacuum degassing, Al is added according to Al requirements, and B-Fe alloy is added for B alloying; then slab continuous casting is carried out, with a superheat of 15-25℃ and a casting speed of 0.8-1.1m / min. Light reduction is applied in the horizontal fan-shaped section, i.e., at the end of solidification, with a reduction of 10-15mm; 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 900-1000℃, the temperature of the heating section is 1220-1250℃, and the temperature of the soaking section is 1210-1220℃. The total time in the furnace for the heating section and soaking section is 3.5-4 hours. 3) Control of rolling, cooling and straightening: The billet is rolled in two stages. The first stage is rough rolling. The initial rolling temperature of rough rolling is ≥1100℃, and the final rolling temperature 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. The number of rolling passes in the rough rolling stage is less than four. The thickness of the intermediate billet is 2 to 5 times the thickness of the finished product. During the waiting period of the intermediate billet, two passes of descaling water are sprayed. The descaling time is 0.5~1.5min, and the pressure of the descaling machine is 15~20MPa. The second stage is finishing rolling, with an initial rolling temperature of 890–930℃ and a final rolling temperature of 810–840℃. The finishing rolling process consists of no more than seven passes. The reduction rate for the first two passes is guaranteed to be above 20%. Subsequently, laminar flow cooling is used for online temperature-controlled quenching, with an initial cooling temperature of 750–810℃ and a final cooling temperature of 580–610℃, at a cooling rate of 10–15℃ / s. After that, hot straightening is performed at a straightening temperature below 500℃, followed by air cooling to room temperature.

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

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

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

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

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

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

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

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

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