V-B-Ti microalloyed 690MPa-grade steel for engineering machinery

Abstract

The invention discloses a V-B-Ti microalloyed 690 MPa grade steel for engineering machinery, and belongs to the field of metal processing. The steel comprises the following chemical components in percentage by weight: 0.13%-0.17% of C, 0.15%-0.25% of Si, 1.55%-1.7% of Mn, less than or equal to 0.02% of P, less than or equal to 0.015% of S, 0.017%-0.0250% of Ti, 0.1%-0.2% of Cr, 0.015%-0.045% of Als, 0.05%-0.08% of V, 0.0015%-0.0020% of B, 0.005%-0.0095% of N, 3.4%-3.8% of Ti / N 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 steel plate toughness and plate shape unevenness are solved.

Description

Technical Field

[0001] The invention belongs to the field of metal processing, and in particular relates to a VB-Ti microalloyed 690MPa grade steel for engineering machinery with a thickness specification of 10-50mm. Background Art

[0002] In recent years, with the continuous advancement of steel materials, welding material manufacturing, and welding process technologies, the strength levels of structural steel have continued to rise. Offshore cranes, marine tanks, jack-up platform legs, coal mine hydraulic supports, and other applications have increasingly used 690 MPa high-strength steel. Especially for engineering machinery, 690 MPa high-strength steel has become a key steel grade. This type of steel is primarily based on carbon-manganese steel. Microalloying elements such as Nb, V, Cr, Mo, and Ni, as well as carbon and nitrides or high-hardenability elements, are added to the steel to enhance the strength and toughness of the steel through solid solution strengthening, precipitation strengthening, and grain refinement. According to statistics, medium and thick plates of this strength grade account for 5-10% of annual production. Therefore, developing low-cost manufacturing technologies for this type of steel is of great significance. Currently, research on in-line quenching processes for high-strength steel plates with thicknesses of 50 mm and below is increasing. The in-line thermomechanical control (TMCP) process, in which the steel plates are quenched directly on the main rolling line, not only saves energy but also shortens the lead time for finished steel plates. It is an environmentally friendly process for the production of quenched and tempered steel with a shortened process and reduced production volume. With the continuous advancement of post-rolling water cooling equipment and plate shape control capabilities, equipment such as ultra-fast cooling, pre-straightening machines, and warm straightening machines have been put into use, providing hardware support for process promotion. Major medium and thick plate mills have successively carried out process research and application based on new equipment, solving problems such as performance consistency. However, the product impact toughness and plate shape requirements are significantly lower than those of offline quenching production processes, which is one of the common problems faced by the industry. Many steel mills use the method of increasing the tempering temperature to improve product toughness and offline straightening to ensure plate shape, but the effect is not obvious and also increases production costs. Therefore, in response to the above-mentioned problems of conventional 690MPa high-strength steel plates, it is urgent to develop 690MPa-grade steel plates with low production equipment requirements, low cost, and high strength and toughness to meet the urgent demand of the rapidly developing manufacturing industry for high-strength steel with excellent performance and low cost.

[0003] Because the TMCP process produces high-strength and high-toughness steel without the addition of excessive alloying elements or the need for complex post-heat treatment, it is considered an alloy- and energy-saving, environmentally friendly process and has become an indispensable technology for the production of low-alloy steel plate. As market demand for TMCP steel continues to increase, the TMCP process itself has also continued to evolve in its applications. Recent research has focused on controlled cooling, particularly accelerated cooling. Accelerating the cooling rate after rolling not only inhibits grain growth but also achieves the ultrafine ferrite, bainite, or even martensite structures required for high strength and toughness. However, with the optimization of alloy costs and steelmaking processes for 690 MPa-grade steel plate, steel plates requiring alloy and process reductions must be produced using the TMCP process. As the alloy content decreases, the rolling and controlled cooling process window becomes increasingly narrow. Currently, direct cooling of the austenite uncrystallized zone is generally used. Steel plates produced using this process exhibit high strength but low impact toughness. The reason is that after rolling, the austenite grains of the steel plate are mainly flat. This form of austenite is easy to form martensite strips that penetrate the original austenite grains during the subsequent direct cooling process, and the orientation is relatively consistent, which is not conducive to preventing the expansion of cracks. Therefore, the toughness is low and cannot reach the level of offline quenching. In addition, the internal stress of the steel plate is large and the plate shape cannot be guaranteed, which limits the efficient and low-cost production and promotion of low-cost 690MPa grade engineering machinery steel with a thickness of 10-50mm.

[0004] Compared with existing technologies: To date, little research has been conducted domestically or internationally on efficient and cost-effective methods for producing 690 MPa-grade engineering structural steel with a thickness of 10-50 mm. Prior to this invention, a journal article titled "Effect of Tempering Temperature on the Microstructure and Properties of Q690D-Grade High-Strength Structural Steel" (Hot Working Technology, August 2014) primarily employed offline tempering heat treatment to enhance the toughness of steel plates. However, this approach significantly increased production cycle time and costs.

[0005] The influence of tempering temperature on the microstructure and properties of Q690D-grade high-strength structural steel disclosed in the above literature can address the toughness issue of steel plates, but is not suitable for controlling and resolving the problem of efficient and low-cost production of economical steel plates with a yield strength of 690 MPa and a thickness of 10-50 mm using TMCP instead of offline heat treatment processes. The technical solution provided by the present invention can effectively overcome the above-mentioned shortcomings and solve the problems of low toughness and uneven plate shape when producing 690 MPa-grade hot-rolled steel plates with a thickness of 10-50 mm using an online cooling process for continuous casting billets with a thickness of 250 mm or less. Summary of the Invention

[0006] The purpose of the present invention is to overcome the above-mentioned technical problems and shortcomings and provide a VB-Ti microalloyed 690MPa grade steel for engineering machinery, which has the characteristics of high efficiency and low cost. The method adopts optimized steelmaking, heating, rolling, controlled cooling and straightening processes, and ultimately solves the problems of low toughness and plate unevenness of 690MPa grade hot-rolled steel plates with a thickness of 10-50mm produced by online cooling process.

[0007] To achieve the above object, the technical solution of the present invention is: A VB-Ti microalloyed 690 MPa grade steel plate for engineering machinery, comprising the following chemical compositions by weight: C 0.13%-0.17%, Si 0.15%-0.25%, Mn 1.55%-1.7%, P≤0.02%, S≤0.015%, Ti 0.017%-0.0250%, Cr 0.1%-0.2%, Als 0.015%-0.045%, V 0.05%-0.08%, B 0.0015%-0.0020%, N 0.005%-0.0095%, Ti / N ratio of 3.4-3.8, the balance being Fe and unavoidable impurities, with 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 0.13% to 0.17%.

[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 continuously cast slab, hindering low-temperature toughness and ensuring uniform cross-sectional structure. Therefore, the Mn content in this invention is designed to be between 1.55% and 1.70%.

[0010] Si: It deoxidizes steel during steelmaking 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 of the base material. Therefore, the Si content in this invention is set at 0.15% to 0.25%.

[0011] Ti, N: In addition to forming fine TiN particles to refine austenite grains, the N element in steel is also easy to form BN with B, affecting the yield of free B and causing the hardenability of the steel plate to decrease. Therefore, the binding ability of TiN is due to BN. Therefore, the N content selected in the present invention needs to be in the range of 0.005% to 0.0095% to ensure that the Ti / N ratio is controlled at 3.4 to 3.8. The remaining Ti combines with C to form TiC, which plays a dispersion strengthening role.

[0012] V is a strong nitrogen-fixing 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 V particles coarsen, increasing stress concentration at the particle interface and matrix. Therefore, the V content in this invention is selected to be in the range of 0.05% to 0.08%.

[0013] 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%.

[0014] 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 0.1% to 0.2%.

[0015] 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 0.0015-0.0020%.

[0016] 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.

[0017] The VB-Ti microalloyed 690MPa 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.

[0018] The object of the present invention is achieved through the following technical solutions: The present invention provides a method for preparing VB-Ti microalloyed 690 MPa-grade engineering machinery steel, which is characterized by high efficiency and low cost, comprising molten steel smelting → furnace refining (LF refining) + RH vacuum degassing → B alloying → continuous casting → heating of the ingot → 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 0.13%~0.17%, Si 0.15%~0.25%, Mn 1.55%~1.7%, P≤0.02%, S≤0.015%, Ti 0.017%~0.0250%, Cr0.1%~0.2%, Als 0.015%~0.045%, V 0.05%~0.08%, B 0.0015%~0.0020%, N 0.005%~0.0095%, Ti / N=3.4~3.8, 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. After the RH vacuum degassing, Al is added according to the Als requirement. Ti-Fe alloy is then added for micro-titanium treatment, followed by B-Fe alloy for B alloying. Ensure that the molten iron is added to avoid floating on the slag layer. Slab continuous casting is then carried out, with a continuous casting superheat of 15-20°C and a continuous casting billet drawing rate of 0.9-1.2 m / min (control of pouring superheat and continuous casting billet drawing speed can effectively reduce the casting temperature). Fewer quality defects in the center and corners of the billet); control the electromagnetic stirring current intensity of the secondary cooling zone in the continuous casting stage to 200-250A, and the secondary cooling water volume to 1.8L / kg-2.5L / kg (reduce the carbon segregation index and inhibit segregation, while limiting the secondary cooling intensity to inhibit the trend of worsening cracks in the center of the billet); in the horizontal sector, that is, at the end of solidification, apply light reduction, and the reduction of the continuous casting billet is 10-15mm (which helps to reduce billet segregation, refine austenite grains, and reduce internal structural defects, etc.).

[0019] 2) Ingot heating: The ingot is fed into a walking beam heating furnace for heating, wherein the temperature range of the preheating section is 900-1050°C (to promote the rapid and sufficient solid solution of Ti and V carbides or nitrides in the matrix and to fully diffuse), the temperature of the soaking section and the heating section is 1220-1245°C, the total time of the soaking section and the heating section is 3-3.5 hours, and the total furnace time of the preheating section, soaking section and the heating section is 4-5.5 hours (using a high heating temperature while ensuring the time of the soaking section and the heating section in the furnace, ensuring the total furnace time is to effectively promote the full diffusion of elements such as C, Mn, H, and B, and ensure the uniformity of the rolled steel plate structure); 3) Controlled rolling, cooling and straightening: The ingot 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 970-1020℃. 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 VC phase in the austenite grains, providing nucleation sites for acicular ferrite, thereby promoting the formation of acicular ferrite in the grains and improving strength and toughness; at the same time, it also induces TiN precipitation and inhibits BN precipitation). The thickness of the intermediate billet is 2-4 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 (to inhibit the growth of austenite grains and generate thermal conductivity on the inner and outer surfaces of the billet). The second stage is recrystallization rolling (finishing rolling), the starting temperature range of finishing rolling is 850-910℃, the finishing temperature range is 750-780℃, and the finishing rolling is not more than seven passes (by controlling the finishing temperature of the roughing stage, ensuring that the intermediate billet is in the austenite recrystallization temperature range during the waiting and finishing rolling processes, ensuring the uniformity of the structure, and at the same time, spraying the mill to remove scale water during the intermediate billet waiting for temperature, ensuring the permeability of the core in the finishing rolling stage, so as to increase the dislocation density, vacancies and deformation bands in the austenite, promote VC precipitation, provide more nucleation sites, promote the occurrence of ferrite phase transformation, refine the ferrite structure, and ensure the strength and toughness of the steel plate). Subsequently, laminar cooling is used for online temperature-controlled quenching, with the start cooling temperature range of 680-710°C, the final cooling temperature range of 500-530°C, and the cooling rate of 10-15°C / s (calculation shows that the fastest VC precipitation temperature range is 690-720°C. Controlling the start cooling temperature ensures the VC precipitation density, which 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 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 400°C, followed by air cooling to room temperature.

[0020] 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 0.13-0.17% at the end of converter smelting. Argon 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.

[0021] Furthermore, during the converter smelting process, a double slag method is used to remove phosphorus.

[0022] Furthermore, in step 2), the calorific value of the coal gas is controlled at 2000-2500 J / kg.

[0023] Furthermore, in step 2), the temperature difference between the upper and lower surfaces of the ingot is ensured to be within 15°C.

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

[0025] Furthermore, in step 3), the straightening force of the hot straightening is between 2800KN and 3600KN, the position of the lead-in roller is -0.8mm to -1.5mm, and the position of the lead-out roller is -2.1mm to -2.6mm (to optimize the straightening process and promote further and full release of internal stress).

[0026] The above composition and process scheme are used to obtain an efficient and low-cost production method for VB-Ti microalloyed steel plates with a grade of 690MPa 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 690MPa 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 blowing calming before continuous casting can promote the removal of inclusions in molten steel and improve the uniformity of molten steel composition; controlling the continuous casting superheat and the billet drawing rate can effectively reduce the quality defects in the center and corners of the billet; optimizing the electromagnetic stirring current intensity in the secondary cooling zone and the secondary cooling water ratio can reduce the average carbon segregation index and inhibit segregation. At the same time, limiting the secondary cooling intensity can inhibit the trend of worsening cracks in the center of the billet; applying soft reduction can help reduce billet segregation, refine austenite grains, and reduce internal structural defects.

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

[0029] 3. The present invention features a rational composition and low alloy addition. The VB-Ti composite design significantly reduces alloy cost and high-temperature deformation resistance during roughing and finishing, facilitating increased pass reduction and ensuring the overall performance of super steel plates. The effect of boron on austenite stability is related to the amount of solid-solution boron in the steel. To increase the solid-solution boron content in boron-containing steel, titanium (Ti), which has a stronger bond than boron to nitrogen, should be added. The vanadium content in the steel has little effect on the solid-solution boron content. Calculations show that a Ti content of 3.4 to 3.8 times the nitrogen content in the steel ensures that boron is present in a free form.

[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 VC phase in the austenite crystal, providing nucleation sites for acicular ferrite, thereby promoting the formation of acicular ferrite in the crystal and improving the toughness. At the same time, TiN precipitation is also induced and BN precipitation is suppressed. Descaling water is sprayed to suppress 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 the penetration of rolling deformation to the thickness center. Through, refine the grains at 1 / 2 of the thickness, which is beneficial to improve the core structure of thick-gauge steel plates and reduce the structural stress caused by uneven structure; through calculation, the fastest temperature range of VC precipitation is 690-720℃, and the cooling temperature is controlled to ensure the VC precipitation density. VC 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 phase transformation driving force, and promote bainite transformation. Based on this, a high red-returning temperature and slow cooling rate are adopted to ensure the plate shape of the steel plate after cooling, while optimizing the straightening process and promoting the full release of internal stress.

[0031] 5. Utilizing the aforementioned composition and process, a highly efficient and cost-effective method for producing 690 MPa-grade VB-Ti microalloyed hot-rolled steel plate with a thickness of 10-50 mm was developed. By optimizing the composition design, steelmaking, heating, rolling, cooling, and straightening processes, the low toughness and uneven flatness of 690 MPa-grade hot-rolled steel plate with a thickness of 10-50 mm, produced using an online cooling process, were addressed. Specific performance characteristics of the 10-50 mm thick 690 MPa steel plate are: a transverse tensile yield strength of 700-770 MPa, a tensile strength of 890-905 MPa, an elongation of 17% or greater, a transverse Charpy impact energy of 100 J or greater at -40°C, and a flatness of less than 5 mm / 2 m. 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 VB-Ti microalloyed 690 MPa grade steel for engineering machinery, comprising, by weight, the following: C 0.13%-0.17%, Si 0.15%-0.25%, Mn 1.55%-1.7%, P ≤ 0.02%, S ≤ 0.015%, Ti 0.017%-0.0250%, Cr 0.1%-0.2%, Als 0.015%-0.045%, V 0.05%-0.08%, B 0.0015%-0.0020%, N 0.005%-0.0095%, with a Ti / N ratio of 3.4-3.8, the balance being Fe and unavoidable impurities, with the total amount of other impurity elements being less than 0.05%. The steel plate has a thickness of 10-50 mm and is produced on a medium and heavy plate reciprocating mill using continuously cast slabs with a thickness of less than 250 mm, using water as the cooling medium.

[0034] The preparation method of the VB-Ti microalloyed 690 MPa grade engineering machinery steel comprises molten steel smelting → furnace refining (LF refining) + RH vacuum degassing → B alloying → continuous casting → ingot reheating → controlled rolling and cooling → hot straightening; specifically, the following steps are included: 1) Molten steel smelting to continuous casting: smelting according to the above composition. The raw materials are pre-treated by KR molten iron to control the S content to less than 0.015%, and then enter the converter after slag removal; the double slag method is used to remove P in the converter to control the P content ≤ 0.02%, and the C content is controlled at 0.13-0.17% at the end of the converter smelting. Argon is blown for 20-30 minutes when tapping; then LF refining and RH vacuum degassing are carried out; at the same time, the RH vacuum is maintained for more than 20 minutes; after the degassing treatment, Al is added according to the Als requirement; then Ti-Fe alloy is added for After micro-titanium treatment, 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, the continuous casting overheat is 15-20°C, and the continuous casting billet drawing rate is 0.9-1.2m / min; the electromagnetic stirring current intensity of the secondary cooling zone in the continuous casting stage is controlled to be 200-250A, the secondary cooling water volume is 1.8L / kg-2.5L / kg, and light reduction is applied in the horizontal sector section, that is, at the end of solidification, and the continuous casting billet reduction is 10-15mm.

[0035] 2) Billet heating: Billets (less than 250mm) are fed into a walking beam furnace for heating. They pass through the preheating section, heating section, and soaking section before exiting the furnace. The temperature range of the preheating section is 900-1050°C, and the temperatures of the soaking and heating sections are 1220-1245°C. The total time in the soaking and heating sections is 3-3.5 hours, and the total time in the furnace for the preheating, soaking, and heating sections is 4-5.5 hours. The calorific value of the gas is controlled at 2000-2500 J / kg. 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 970-1020℃. The reduction rate of the first two passes of rough rolling 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 2-4 times the thickness of the finished product. During the waiting process of the intermediate billet, descaling water is sprayed twice, the descaling time is 0.5-1.5min, and the pressure of the descaling machine is 15-20MPa. The second stage is recrystallization rolling (finishing rolling). The starting temperature range of finishing rolling is 850-910℃, the finishing temperature range of finishing rolling is 750-780℃, and the finishing rolling is not more than seven passes. Subsequently, laminar cooling is used for online temperature-controlled quenching, with the starting cooling temperature range of 680-710°C, the final cooling temperature range of 500-530°C, and the cooling rate of 10-15°C / s; then hot straightening is carried out, with the straightening temperature below 400°C, the straightening force between 2800KN and 3600KN, the position of the lead-in roller from -0.8mm to -1.5mm, and the position of the lead-out roller from -2.1mm to -2.6mm, followed by air cooling to room temperature.

[0036] 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 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 for cooling and straightening of the example steel, and Table 6 shows the performance and quality indicators of the example steel plate.

[0037] Table 1 Chemical composition of example steel (wt, %)

[0038] Note: Impurity elements P in steel are ≤ 0.02%, S ≤ 0.015%, and the total amount of other impurity elements is less than 0.05%.

[0039] Table 2 Process system from smelting to continuous casting of example steel

[0040] Table 3 Heating schedule for the cast steel of Example

[0041] Table 4 Rolling parameters of example steel

[0042] Table 5 Main process parameters of cooling and straightening of example steel

[0043] Table 6 Performance and quality indicators of the steel plates in the examples

[0044] It can be seen that compared with the prior art, the purpose of the present invention is to overcome the above-mentioned technical problems and shortcomings, and to provide an efficient and low-cost production method for 690MPa grade VB-Ti microalloyed hot-rolled steel plates with a thickness specification of 10-50mm. By optimizing the composition design, steelmaking, heating, rolling, cooling and straightening processes, the low toughness and uneven plate shape problems of 690MPa grade hot-rolled steel plates with a thickness of 10-50mm produced by online cooling process are finally solved.

[0045] 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 VB-Ti microalloyed 690 MPa grade steel for engineering machinery, characterized in that: Its chemical composition by weight percentage includes: C 0.13%-0.17%, Si 0.15%-0.25%, Mn 1.55%-1.7%, P≤0.02%, S≤0.015%, Ti 0.017%-0.0250%, Cr 0.1%-0.2%, Als 0.015%-0.045%, V 0.05%-0.08%, B 0.0015%-0.0020%, N 0.005%-0.0095%, Ti / N=3.4-3.8, 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 VB-Ti microalloyed 690 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. After the RH vacuum degassing treatment, Al is added according to the Als requirement, and then Ti-Fe is added for micro-titanium treatment. After that, B-Fe alloy is added for B alloying. Slab continuous casting is then carried out, with a continuous casting superheat of 15-20°C and a continuous casting casting rate of 0.9-1.2 m / min. The electromagnetic stirring current intensity in the secondary cooling zone during the continuous casting stage is controlled at 200-250A, the secondary cooling water volume is 1.8L / kg-2.5L / kg, and soft reduction is applied in the horizontal sector, i.e., at the end of solidification, with a continuous casting billet 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 being discharged from the furnace. The temperature in the preheating section is 900-1050°C, and the temperatures in the soaking and heating sections are 1220-1245°C. The total time in the soaking and heating sections is 3-3.5 hours. The total time in the preheating, soaking, and heating sections is 4-5.5 hours. 3) Controlled rolling, cooling and straightening: The billet is rolled in two stages. The first stage is rough rolling. The starting temperature of rough rolling is ≥1100℃ and the finishing temperature of rough rolling is 970~1020℃. The reduction rate of the first two passes of rough rolling is not less than 30% per pass. The number of rough rolling passes is less than four. The thickness of the intermediate billet is 2~4 times the thickness of the finished product. During the intermediate billet waiting for temperature, spray descaling water twice. The descaling time is 0.5~1.5min and the descaling machine pressure is 0.5~1.5min. The pressure is 15-20 MPa; the second stage is finishing rolling, the starting rolling temperature of finishing rolling is 850-910℃, the finishing rolling temperature is 750-780℃, and the finishing rolling is not more than seven passes; then laminar cooling is used for online temperature-controlled quenching, the starting cooling temperature is 680-710℃, the final cooling temperature is 500-530℃, and the cooling rate is 10-15℃ / s; then hot straightening is carried out, the straightening temperature of hot straightening is below 400℃, and then air cooling is carried out to room temperature.

2. The VB-Ti microalloyed 690 MPa grade engineering machinery steel according to claim 1, characterized in that: The thickness of the steel is 10-50 mm.

3. The VB-Ti microalloyed 690 MPa grade engineering machinery steel according to claim 1, characterized in that: The steel has a transverse tensile yield strength of 700-770 MPa, a tensile strength of 890-905 MPa, an elongation of ≥17%, a transverse Charpy impact energy of ≥100 J at -40°C, and a straightness of less than 5 mm / 2 m.

4. The VB-Ti microalloyed 690 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 0.13-0.17% at the converter smelting end point. Argon is blown for 20-30 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 VB-Ti microalloyed 690 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 VB-Ti microalloyed 690 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 VB-Ti microalloyed 690 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 VB-Ti microalloyed 690 MPa grade engineering machinery steel according to claim 1, characterized in that: In step 2), ensure that the temperature difference between the upper and lower surfaces of the ingot is within 15°C.

9. The VB-Ti microalloyed 690 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 2800KN and 3600KN, the position of the inlet roller is -0.8mm to -1.5mm, and the position of the outlet roller is -2.1mm to -2.6mm.

Images