Low-cost 780MPa-grade hot-rolled wheel steel and preparation method thereof

Abstract

The invention discloses low-cost 780MPa-grade hot-rolled wheel steel and a preparation method thereof, and belongs to the technical field of materials and metallurgy, the low-cost 780MPa-grade hot-rolled wheel steel comprises 0.06%-0.09% of C, 0.10%-0.20% 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%-0.15% of Cr, 0.015%-0.045% of Als, 0.03%-0.05% of V, 0.0015%-0.0020% of B, 0.0055%-0.01% of N, Ti / N = 2.9-3.3, 0.23 < = C + {0.75 + 0.25 tanh [20 (C-0.12)]} * {Si / 24 + Mn / 6 + (Cr + V

Description

Technical Field

[0001] The present invention relates to the fields of materials and metallurgy, and more particularly to a low-cost 780MPa-grade hot-rolled wheel steel and a preparation method thereof. Background Art

[0002] Wheels are a key component of commercial vehicles, with widespread demand and high usage. With the trend toward lightweight vehicles, wheel strength reduction and thinning have become a key means of reducing vehicle weight. Wheel manufacturers are experiencing a significant increase in demand for high-strength wheel steel. Currently, the industry generally uses a higher-strength grade with a tensile strength of 590 MPa, with some companies using even higher-strength 650 MPa grades. However, with market changes and demand, a new demand is emerging for 780 MPa tensile strength wheel steel. The production process of traditional high-strength alloy wheel steel is plagued by shortcomings such as severe ingot element segregation, difficulty controlling inclusions, high alloy costs, and significant performance fluctuations. These shortcomings directly impact issues such as weld cracking and punching cracking during wheel steel processing, and these defects become more pronounced as strength increases. Currently, wheel steel with a tensile strength of 780MPa generally adopts a low-carbon, low-manganese composition system. To ensure the safety of steel strips during service, steel mills require steel plates to have a low yield strength ratio and high plasticity. Expensive alloy elements such as niobium, chromium, molybdenum and nickel are added to increase strength. However, this quality seems to be excessive for wheel steel, and it also has high costs and poor market competitiveness.

[0003] Faced with the challenging market conditions of the steel industry, reducing the manufacturing cost of 780MPa-grade wheel steel while ensuring product quality and maximizing production line capacity is crucial. Ultimately, achieving maximum efficiency and minimizing energy and alloy costs while maintaining the required performance and technical specifications for the steel plate is crucial, while ensuring production and equipment safety. Therefore, developing a low-cost, high-strength and tough 780MPa-grade wheel steel is crucial for improving steel mill profitability. Currently, to ensure the low-temperature toughness of 780MPa-grade wheel steel, high alloying elements (such as Mn, Nb, Cr, Mo, and Ni) are typically used, along with a thick intermediate billet thickness and a two- or even three-stage controlled rolling process. This results in a final rolling temperature of the steel plate below 800°C, or even lower, significantly increasing the complexity of rolling. Firstly, mill load, plate temperature uniformity, and flatness control all face significant challenges. Secondly, improving steel plate performance depends primarily on controlling the content of impurities such as phosphorus and sulfur in the steel, the content of alloying elements in the steel, and the control of key process parameters during smelting and rolling. It can be seen that how to systematically combine strengthening mechanisms such as phase transformation strengthening, solid solution strengthening and fine grain strengthening to solve the above-mentioned problems in the production of 780MPa grade wheel steel, reduce alloy costs, improve market competitiveness, and at the same time ensure its excellent strength and toughness is the key to the development of low-cost 780MPa grade wheel steel.

[0004] Chinese patent application number 202310949000.5 discloses a low-temperature-resistant 780 MPa high-strength wheel steel and its production method. The chemical composition and weight percentage of the wheel steel are: C: 0.06-0.08%, Si: 0.10-0.15%, Mn: 1.60-1.70%, P ≤ 0.018%, S ≤ 0.003%, Als: 0.010-0.040%, Nb: 0.050-0.060%, V: 0.030-0.040%, Ti: 0.080-0.090%, N ≤ 0.0040%, with the balance being Fe and unavoidable impurities. The steel plate in this patent contains a high amount of Nb and uses a two-stage controlled rolling process, resulting in low production efficiency and significantly increased manufacturing costs. The yield strength ratio is also unclear. Chinese patent application number 201811002601.0 discloses a 780MPa grade high fatigue and high strength steel and its manufacturing method, the weight percentage of which is: C 0.03-0.07%, Si≤0.1%, Mn 1.0-1.8%, P≤0.01%, S≤0.002%, Al 0.02-0.08%, N≤0.004%, Ti 0.07-0.13%, Mo 0.15-0.50%, O≤30ppm; the rest is Fe and unavoidable impurities; a large amount of Mo element is added to the steel plate, and a two-stage controlled rolling method is also adopted, resulting in low production efficiency and high manufacturing cost. At the same time, performance indicators such as yield ratio are not clear.

[0005] The above patent documents have high production costs and are therefore not suitable for producing low-cost, excellent strength and toughness 780 MPa grade wheel steel. Summary of the Invention

[0006] The purpose of the present invention is to overcome the above-mentioned defects of the prior art and provide a low-cost 780MPa grade hot-rolled wheel steel and a preparation method thereof, so as to realize the production of 4-10mm thick, low-cost 780MPa grade wheel steel plates from continuous casting billets with a thickness of 150-200mm, and the steel plates have excellent low-temperature toughness, low yield ratio and other characteristics.

[0007] To achieve the above object, the technical solution of the present invention is as follows: A low-cost 780MPa grade hot-rolled wheel steel comprises the following components in weight percentage: C: 0.06% to 0.09%, Si: 0.10% to 0.20%, Mn: 1.55% to 1.7%, P≤0.02%, S≤0.015%, Ti: 0.017% to 0.0250%, Cr: 0.1% to 0.15%, Als: 0.015% to 0.045%, V: 0.03%~0.05%, B: 0.0015%~0.0020%, N: 0.0055%~0.01%, and Ti / N=2.9~3.3, 0.23≤C+{0.75+0.25tanh[20(C-0.12)]}×{Si / 24+Mn / 6+(Cr+V) / 5+5B}≤0.29, the balance is Fe and unavoidable impurities.

[0008] Optionally, the thickness of the hot-rolled wheel steel is 4-10 mm.

[0009] The optional transverse tensile yield strength of the hot-rolled wheel steel is 700-800 MPa, the tensile strength is 780-900 MPa, the elongation is ≥16%, the transverse Charpy impact energy at -40°C is ≥150J, and the yield strength ratio is ≤0.9.

[0010] The present invention also discloses a method for preparing the above-mentioned low-cost 780MPa grade hot-rolled wheel steel, comprising the following steps: molten steel smelting, LF refining, RH vacuum degassing, continuous casting, billet heating, high-pressure water descaling, rolling, cooling and coiling; During the continuous casting, the continuous casting superheat is 10-15° C., and the casting rate is 0.7-0.9 m / min. Electromagnetic stirring is used in the secondary cooling zone during the continuous casting stage to continuously cast the molten steel into a continuous casting billet, wherein the equiaxed crystal ratio of the continuous casting billet is ≥85%. At the same time, strong cooling is used in the sector sections, with the total cooling water flow of the first to third sections being 700-1000 L / min, and the total cooling water flow of the fourth to eighth sections being 1200-1500 L / min. At the same time, heavy pressure is applied at the end of solidification, and the continuous casting billet is reduced by 12-16 mm. During the heating of the cast billet, the cast billet is sequentially passed through a preheating section, a heating section, and a soaking section before being taken out of the furnace; the temperature range of the preheating section is 800-950° C., the temperature range of the heating section is 1250-1275° C., the temperature range of the soaking section is 1130-1150° C., the furnace time of the heating section and the soaking section is 3.5-4.5 hours, and the soaking section time is 1-2.5 hours; the opening of the upper and lower burners in the soaking section is adjusted, and the air-fuel ratio is controlled to be 1:1.7-1:2.2, so as to ensure that the temperature difference between the upper and lower surfaces of the billet is ≤15° C.; During the rolling process, the final rolling temperature of the rough rolling stage is 980-995°C; the final rolling temperature of the fine rolling stage is 860-890°C; During the cooling and coiling, the rolling speed of the steel plate is controlled to be 2.5-4 m / s, the cooling temperature is 730-750°C, the roller speed in the cooling zone is 1.5-2.5 m / s, and the acceleration is 0.003-0.006 m / s. 2 During the cooling process of the steel plate, the water ratio of the upper and lower headers is 1:1.7~1:2.5, and the ultra-fast cooling and laminar cooling modes are adopted. The number of opening groups of the ultra-fast cooling area controlled cooling headers is 2~4 groups, and the water volume of a single header is 300~400m 3 / h, cooling speed is 50~60℃ / s, final cooling temperature range is 580~620℃, during laminar cooling, the number of cooling manifolds opened is 3~6 groups, and the water volume of a single manifold is 150~200m 3 / h, the cooling rate is 10~15℃ / s, and the final cooling temperature range is 450~480℃.

[0011] Optionally, during the molten steel smelting: smelting raw materials are configured according to the chemical composition, and the molten iron is pretreated by KR to control the S content to ≤0.015%, and the slag is removed before entering the converter; during the converter smelting, the double slag method is used to remove P to control the P content to ≤0.02%, and the C content is controlled to be 0.06% to 0.09% at the end point of the converter smelting, and argon gas is blown for 30 to 40 minutes when tapping.

[0012] Optionally, in the LF refining and RH vacuum degassing, the molten steel after converter smelting is subjected to LF refining and RH vacuum degassing, and the RH vacuum is maintained for 30 to 40 minutes.

[0013] Optionally, in the continuous casting: the electromagnetic stirring mode is alternating forward and reverse stirring, wherein the forward stirring time is 20 to 30 seconds, the reverse stirring time is 10 to 20 seconds, the current is 500 to 800A, and the frequency is 30 to 50Hz; the billet is put into the pit for slow cooling after coming off the line, and the billet is hot loaded into the slow cooling pit after coming off the line. When the billet temperature is greater than 700°C, the slow cooling pit is not heated, and the slow cooling time is 48 to 72 hours.

[0014] Optionally, during the heating of the ingot, the thickness of the ingot is 150-200 mm.

[0015] Optionally, during the high-pressure water descaling and rolling: before rolling, the cast ingot is descaled for 1 to 2 minutes using high-pressure water, and the descaling machine pressure is 20 to 25 MPa; in the rough rolling stage, the reduction rate of each of the first three passes of rough rolling is 30 to 50%, and the first three passes of rolling are sprayed with water for descaling, with each pass time being 1 to 1.5 minutes and a pressure of 20 to 25 MPa; the total reduction rate is controlled at 75% to 85%, and the rough rolling passes are 4 to 5 passes; the starting rolling temperature in the finishing rolling stage is 935 to 960°C, and the total reduction rate in the finishing rolling stage is >80%, of which the reduction rate of the last two passes of finishing rolling is >20%, and the finishing rolling passes are 4 to 6 passes.

[0016] Optionally, during the cooling and coiling, the side spray pressure and water volume are 2-5 MPa and 50-70 m 3 / h, the wind blowing pressure is 5~10MPa.

[0017] The implementation of the present invention will have the following beneficial effects: 1. The smelting process of the present invention uses KR hot metal pretreatment for deep desulfurization followed by clean slag removal. The converter uses a double slag method for P removal, resulting in low P and S contents in the ingot. The argon blowing time is controlled and the RH vacuum degassing time is maintained, thereby overcoming defects such as central segregation, inclusions, and excessive H and O contents in the ingot caused by high Mn and C contents, thereby improving the plasticity and toughness of the steel plate. Reducing superheat and the continuous casting speed can improve the macrosegregation of the ingot and reduce the spacing between secondary dendrite arms in the solidification structure of the ingot, which helps to reduce steel segregation and internal structural defects. By optimizing the electromagnetic stirring process during the continuous casting stage, the equiaxed crystal ratio of the ingot is significantly increased. Strong cooling is used to ensure a temperature gradient through the thickness of the ingot. At the end of solidification, the ingot is placed under heavy pressure to effectively reduce the central porosity and segregation of the ingot, promote core grain breakage, and provide a guarantee for the subsequent strength and toughness of the steel plate. In addition, the ingot is slowly cooled in the pit after it leaves the production line to promote the diffusion of elements such as Mn, Cr, C, B, and H, thereby reducing the impact of composition segregation on the structure and performance.

[0018] 2. Ingot heating: The temperature and time of the ingot in the preheating section, heating section and soaking section are limited to ensure the full diffusion of various elements and reduce the influence of composition segregation on the structure and performance. At the same time, the heating section provides high-temperature heating and air-fuel ratio to ensure the temperature uniformity of various parts of the ingot and improve the uniformity of metal flow in the horizontal and vertical directions on the surface of the steel plate. In combination with the composition of the steel billet, the heating temperature of the soaking section of the ingot is reduced to reduce energy consumption. The temperature and time of the heating section and soaking section are controlled to inhibit the excessive growth of the original austenite grains and increase the contribution of fine grain strengthening to the strength of the steel plate. In addition, low-temperature heating is adopted in the soaking section to avoid the steel plate waiting for temperature during the finishing rolling stage.

[0019] 3. The present invention adopts a low-temperature heating + hot rolling manufacturing mode, which greatly reduces alloy costs and resistance to high-temperature deformation in the roughing and finishing stages, is conducive to increasing the reduction per pass, and is conducive to ensuring the comprehensive performance of super steel plates. By maximizing the rolling mill capacity in the first three rolling passes of the roughing stage, a large reduction rate is adopted to promote the dynamic crystallization of austenite and refine the original austenite grains. At the same time, the high-temperature deformation induction effect is utilized to promote the precipitation of V(C,N) phase within the austenite crystals, providing nucleation sites for acicular ferrite, achieving the purpose of promoting the formation of acicular ferrite within the crystals, improving strength and toughness, and also inducing TiN precipitation and inhibiting BN precipitation. In addition, by using high-pressure water descaling in the rolling mill, a temperature gradient is generated from the surface to the core of the steel plate, promoting deformation and uniformity of the core structure distribution during the subsequent finishing rolling process, further improving the strength and toughness of the steel plate. In addition, due to the adoption of low-temperature heating process, the intermediate billet waiting for temperature in the finishing rolling stage is cancelled, ensuring that the last two passes of finishing rolling are in the austenite non-recrystallization temperature range, ensuring that the last two passes adopt a large reduction rate, ensuring the internal dislocation density, vacancies and deformation band quantity of the austenite, promoting V (C, N) precipitation, providing more nucleation sites, promoting ferrite phase transformation, refining the ferrite structure, and ensuring the strength and toughness of the steel plate.

[0020] 5. Through calculation and experiment, the fastest temperature range for V(C,N) precipitation is 750~770℃. Controlling the steel throwing speed to ensure that the density of V(C,N) nano-precipitation is the largest before cooling, can significantly improve the mechanical properties of steel through precipitation strengthening, and also ensure that the steel plate enters the water structure for ferrite + austenite, ensure the proportion of soft phase and deformation coordination of the steel plate after rolling, ensure the yield strength ratio, and also increase the proportion of large-angle grain boundaries and the density of precipitation phase to ensure strength and toughness; 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 and martensite; Cooling During the process, a combination of UFC and ACC is adopted. UFC uses a fast cooling rate to promote the steel plate to quickly break through the film boiling zone for rapid cooling. Subsequently, a slow cooling rate is adopted to promote the gradual decrease of the surface temperature to the core temperature. Based on this, it is cooled to 450-480°C. While ensuring that the cooled steel plate is fully hardened, it also reduces the stress level of the steel plate during the cooling process to ensure the shape of the steel plate. At the same time, it also ensures that the structure of the steel plate after cooling is ferrite + bainite + martensite, so as to ensure the coordination of deformation and further improve the strength and toughness of the steel plate. The side spraying is also conducive to the control of the steel plate shape and improves the uniformity of the steel plate performance. This invention utilizes a simple composition design and low alloy additions, eliminating the addition of any precious alloying elements (such as Nb and Mo), thus reducing alloy costs. By controlling the steelmaking, continuous casting, heating, and rolling-controlled cooling processes, it achieves the low-cost production of 780 MPa-grade hot-rolled wheel steel plates with a thickness of 4-10 mm. This steel exhibits excellent low-temperature toughness. Specific properties include a transverse tensile yield strength of 700-800 MPa, a tensile strength of 780-900 MPa, an elongation of 16% or greater, a transverse Charpy impact energy of 150 J or greater at -40°C, and a yield strength ratio of 0.9 or less. DETAILED DESCRIPTION

[0021] The present invention will be further described below with reference to specific examples, but the present invention is not limited thereto in any way.

[0022] The present invention discloses a low-cost 780MPa grade hot-rolled wheel steel, comprising the following components in weight percentage: C: 0.06% to 0.09%, Si: 0.10% to 0.20%, Mn: 1.55% to 1.7%, P≤0.02%, S≤0.015%, Ti: 0.017% to 0.0250%, Cr: 0.1% to 0.15%, Als: 0.015% to 0.04 5%, V: 0.03%~0.05%, B: 0.0015%~0.0020%, N: 0.0055%~0.01%, and Ti / N=2.9~3.3, 0.23≤C+{0.75+0.25tanh[20(C-0.12)]}×{Si / 24+Mn / 6+(Cr+V) / 5+5B}≤0.29, the balance is Fe and unavoidable impurities.

[0023] Specifically, the functions of the main elements in the chemical composition of the hot-rolled wheel 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 0.09%.

[0024] 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, contributing to the production of fine low-temperature transformation products and improving toughness. However, increasing the Mn content exacerbates central segregation and the formation of lamellar martensite in the continuous casting ingot, hindering low-temperature toughness and affecting the plate's hole expansion performance. Therefore, the Mn content in the present invention is designed to be between 1.55% and 1.70%.

[0025] Si: It deoxidizes steel during steelmaking and improves matrix strength. However, excessive Si content can reduce the toughness of the base material's weld heat-affected zone. Increasing Si content can purify ferrite, reduce pearlite content, and help mitigate the Bauschinger effect in the base material. Therefore, the Si content in this invention is set at 0.10% to 0.20%.

[0026] Ti and 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 bonding ability of TiN is better than that of BN. Therefore, the N content selected in the present invention is required to be within the range of 0.0055% to 0.01% to ensure that the Ti / N ratio is controlled at 2.9 to 3.3. The remaining Ti combines with C to form TiC, which plays a dispersion strengthening role.

[0027] Als: Typically used as a deoxidizer in steel, it also refines the structure by forming AlN. When Als content exceeds 0.045%, excessive aluminum oxide inclusions can reduce steel cleanliness, so the upper limit of Als content is set at 0.045%. Lower Als content results in inadequate deoxidation, so the lower limit is set at 0.015%.

[0028] Cr: A major element that can effectively improve hardenability, inhibit ferrite formation, and promote bainite formation. It plays an important role in controlling phase transformation structure, while improving the strength, plasticity, and toughness of the steel plate. The present invention selects a Cr content range of 0.1% to 0.15%.

[0029] 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% to 0.0020%.

[0030] V: A strong nitrogen-binding element, it exists as V(C,N) in the continuous casting ingot. Fine V(C,N) particles effectively inhibit austenite grain growth during reheating of the continuous casting ingot. Furthermore, the addition of V precipitates in austenite and ferrite to form V(C,N), significantly improving the strength and toughness of the steel. However, when V addition exceeds a certain level, the V(C,N) particles coarsen, increasing stress concentration at the particle interface and matrix. Therefore, the present invention selects a V content range of 0.03% to 0.05%.

[0031] 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.

[0032] The present invention further controls the ratio 0.23 ≤ C + {0.75 + 0.25tanh[20(C - 0.12)]} × {Si / 24 + Mn / 6 + (Cr + V) / 5 + 5B} ≤ 0.29, ensuring good weldability of the wheel steel and relatively low hardness in the heat-affected zone, minimizing the likelihood of weld cracking. Outside this range, weld cracking can occur, further impacting the weldability of the wheel steel.

[0033] In a specific embodiment, the thickness of the hot-rolled wheel steel is 4-10 mm.

[0034] In a specific embodiment, the hot-rolled wheel steel has a transverse tensile yield strength of 700-800 MPa, a tensile strength of 780-900 MPa, an elongation ≥16%, a transverse Charpy impact energy at -40°C ≥150 J, and a yield strength ratio ≤0.9.

[0035] The present invention also discloses a method for preparing the above-mentioned low-cost 780MPa grade hot-rolled wheel steel, comprising the following steps: molten steel smelting, LF refining, RH vacuum degassing, continuous casting, billet heating, high-pressure water descaling, rolling, cooling and coiling; S1. During molten steel smelting: smelting raw materials are configured according to the chemical composition, and the molten iron is pretreated by KR to control the S content ≤0.015%. After slag removal, it enters the converter; during converter smelting, the double slag method is used to remove P, and the P content is controlled to ≤0.02%. The C content is controlled at 0.06% to 0.09% at the end of converter smelting. Argon is blown for 30 to 40 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.

[0036] S2. LF refining and RH vacuum degassing: The molten steel after converter smelting is subjected to LF refining and RH vacuum degassing, and the RH vacuum is maintained for 30~40min.

[0037] S3, continuous casting: continuous casting superheat is 10 ~ 15 ℃, the billet drawing rate is 0.7 ~ 0.9m / min, reducing superheat and continuous casting drawing rate can improve the macro segregation of continuous casting billet, reduce the secondary dendrite arm spacing in the solidification structure of continuous casting billet, help to reduce billet segregation and reduce internal structural defects; electromagnetic stirring is used in the second cooling zone during the continuous casting stage, and the stirring mode of electromagnetic stirring is forward and reverse alternating stirring, wherein the forward stirring time is 20 ~ 30s, the reverse stirring time is 10 ~ 20s, the current is 500 ~ 800A, and the frequency is 30 ~ 50Hz, so that the molten steel is continuously cast to obtain continuous casting billet, and the proportion of equiaxed crystals in the continuous casting billet is ≥85%. At the same time, strong cooling is used in the fan-shaped section, and the total cooling water volume of the 1st to 3rd section is 700 ~ 1000 L / min, the total cooling water volume of the 4th to 8th sections is 1200~1500L / min, and heavy pressure is put into the end of solidification at the same time. The reduction of the continuous casting billet is 12~16mm. By optimizing the electromagnetic stirring process in the continuous casting stage, the equiaxed crystal rate of the continuous casting billet is greatly improved. Strong cooling is used to ensure the temperature gradient in the thickness direction of the billet. At the same time, light pressure is put into use, which can help reduce the segregation of the steel billet and the internal structural defects. At the same time, it promotes the crushing of the core grains, providing a guarantee for the subsequent strength and toughness of the steel plate; the billet after coming off the line is put into the pit for slow cooling. After the billet is off the line, it is hot-loaded into the slow cooling pit. When the billet temperature is greater than 700℃, the slow cooling pit is not heated. The slow cooling time is 48~72h, which promotes the diffusion of elements such as Mn, Cr, C, and B, and reduces the influence of composition segregation on the structure and performance.

[0038] S4. In the process of heating the ingot: the ingot with a thickness of 150~200mm is sent into the step-beam heating furnace for heating. The ingot is taken out of the furnace after passing through the preheating section, heating section and soaking section in turn. The temperature range of the preheating section is 800~950℃, which promotes the homogenization of the structure in the ingot, and the carbides or nitrides of V, Ti and Cr are quickly and fully dissolved in the matrix and fully diffused, and promotes the full diffusion of elements; the temperature range of the heating section is 1250~1275℃, the temperature range of the soaking section is 1130~1150℃, the time in the furnace of the heating section and the soaking section is 3.5~4.5h, the opening degree of the upper and lower burners in the soaking section is adjusted, and the air-fuel ratio is controlled to 1:1.7~1:2.2 to ensure that the temperature difference between the upper and lower surfaces of the billet is ≤15℃, and the soaking section time is 1~2.5h. Increasing the heating temperature and extending the time in the furnace further promotes The diffusion of the elements Mn, C, Cr, V, N, and B is carried out to reduce the influence of their composition segregation on the structure and performance; at the same time, the heating section provides high-temperature heating and controls the air-fuel ratio to ensure the temperature uniformity of each part of the ingot and improve the uniformity of the transverse and longitudinal metal flow on the surface of the steel plate; based on the composition of the steel billet, the heating temperature of the ingot soaking section is lowered to reduce energy consumption, and the temperature and time of the heating section and the soaking section are limited to inhibit the excessive growth of the original austenite grains and ensure the contribution of fine grain strengthening to the strength of the steel plate. In addition, the soaking section adopts low-temperature heating to reduce the waiting time for the steel plate in the finishing rolling stage, while also ensuring that the finishing rolling stage is carried out in the austenite non-recrystallization zone.

[0039] S5. High-pressure water descaling and rolling: Before rolling, high-pressure water is used to descale the ingot after it is discharged from the furnace for 1 to 2 minutes, and the descaling machine pressure is 20 to 25 MPa; in the rough rolling stage, the reduction rate of each of the first three passes of rough rolling is greater than 30%, and the first three passes of rolling are sprayed with water for descaling, each pass time is 1 to 1.5 minutes, and the pressure is 20 to 25 MPa. In the first three passes of rolling, the capacity of the rolling mill is exerted as much as possible, and a large reduction rate is used to promote the dynamic crystallization of austenite and refine the original austenite grains. At the same time, the high-temperature deformation induction effect is used to promote the precipitation of V (C, N) phase in the austenite crystal, provide nucleation sites for acicular ferrite, and achieve the purpose of promoting the formation of acicular ferrite in the crystal. In addition, by using high-pressure water descaling in the rolling mill, a temperature gradient is generated from the surface of the steel plate to the core, which promotes the deformation of the core and the uniformity of the microstructure distribution during the subsequent finishing rolling process, further improving the strength and toughness of the steel plate. At the same time, it also induces the precipitation of TiN, inhibits the precipitation of BN, and ensures the hardenability of B. The final rolling temperature in the roughing stage is 980~995℃; the total reduction rate is controlled at 75%~85%, and the roughing passes are 4~5 passes; the starting rolling temperature in the finishing stage is 935~960℃, the total reduction rate in the finishing stage is >80%, of which the reduction rate of the last two passes of finishing rolling is >20%, and the finishing passes are 4~6 passes; the final rolling temperature of finishing rolling is 860~890℃. Due to the adoption of low-temperature heating process, the waiting time of the intermediate billet in the finishing rolling stage is reduced, ensuring that the last two passes of finishing rolling are in the austenite non-recrystallization temperature range, ensuring that the last two passes adopt a large reduction rate, ensuring the dislocation density, vacancies and deformation band quantity in the austenite, promoting V (C, N) precipitation, providing more nucleation sites, promoting ferrite phase transformation, refining the ferrite structure, and ensuring the strength and toughness of the steel plate.

[0040] S6, cooling and coiling: after rolling, the steel plate is controlled to throw the steel speed, the throwing speed is 2.5 ~ 4m / s, the cooling temperature is controlled to be 730 ~ 750℃, the roller speed in the cooling area is 1.5 ~ 2.5m / s, and the acceleration is 0.003 ~ 0.006m / s 2 During the cooling process of the steel plate, the water ratio of the upper and lower headers is 1:1.7~1:2.5, and the UFC (ultra-fast cooling) + ACC (laminar flow) cooling mode is adopted. The number of UFC regional controlled cooling headers opened is 2~4 groups, and the water volume of a single header is 300~400m 3 / h, cooling rate is 50~60℃ / s, final cooling temperature range is 580~620℃, the number of ACC regional cooling header opening groups is 3~6, ​​and the water volume of a single header is 150~200m 3 / h, cooling rate is 10~15℃ / s, final cooling temperature range is 450~480℃; side spray pressure and water volume are 2~5MPa and 50~70m 3 / h, the wind blowing pressure is 5~10MPa, which shows that in order to reduce the iron oxide scale defects on the steel plate surface, through calculation and experiment, the fastest temperature range of V(C,N) precipitation is 750~770℃, and the steel throwing speed is controlled to ensure that the V(C,N) nano-precipitation density is the largest before cooling. It can significantly improve the mechanical properties of steel through precipitation strengthening, and also ensure that the steel plate organization is ferrite + austenite, ensure the soft phase ratio and deformation coordination of the steel plate organization after rolling, ensure the yield strength ratio, and also increase the high-angle grain boundary ratio and precipitation phase density to ensure strength and toughness; in addition, the addition of B and Cr can improve the hardenability of steel, increase the stability of austenite, and increase the driving force of phase transformation. To promote the transformation of bainite and martensite, the UFC+ACC combination is adopted during the cooling process. UFC adopts a fast cooling rate to promote the steel plate to quickly break through the film boiling zone for rapid cooling, and then adopts a slow cooling rate to promote the gradual decrease of surface temperature to core temperature. On this basis, it is cooled to 450-480℃. While ensuring that the cooled steel plate is fully hardened, it also reduces the stress level of the steel plate during the cooling process to ensure the shape of the steel plate. At the same time, it also ensures that the structure of the steel plate after cooling is composed of ferrite, bainite and martensite to ensure the coordination of deformation and further improve the strength and toughness of the steel plate. Side spraying is also beneficial to the control of the steel plate shape and improves the uniformity of the steel plate performance.

[0041] Specifically, the above-mentioned composition, combined with a steelmaking, continuous casting, heating, controlled rolling, and controlled cooling scheme, overcomes the shortcomings of existing technologies and enables the low-cost production of 780MPa-grade hot-rolled wheel steel plates with a thickness of 4-10mm, resolving issues such as high alloy costs. This steel plate offers low cost, high strength and toughness, and can ensure the safe service life of wheel steel plates.

[0042] The following are specific embodiments Table 1 shows the chemical composition of the example steel, Table 2 shows the smelting process system of the example steel, Table 3 shows the heating system of the cast slab and the high-pressure water descaling process before continuous casting slab rolling of the example steel; Table 4 shows the rolling parameters of the example steel; Table 5 shows the controlled cooling and coiling process parameters of the example steel; Table 6 shows the performance indicators of the example steel plate.

[0043] Table 1 Chemical composition of the examples of the present invention (wt, %)

[0044] Note: P≤0.02% in steel; S≤0.015%.

[0045] Table 2 Smelting process system of example steel

[0046] Table 3 Heating system of the cast steel and high-pressure water descaling process before continuous casting

[0047] Table 4 Rolling parameters of example steel and steel plate partial cooling process

[0048] Table 5 Controlled cooling and coiling process parameters of steel according to the present invention

[0049] Table 6 Microstructure and performance indexes of steel plates according to embodiments of the present invention

[0050] As can be seen, compared with existing technologies, the composition design, steelmaking and continuous casting, heating, and controlled rolling and cooling schemes of this invention overcome the shortcomings of existing technologies, providing a 780 MPa hot-rolled wheel steel with a thickness of 4-10 mm and a manufacturing method. This solves the problem of high alloy costs, and the steel plate offers high strength and toughness at a low cost, ensuring safe service for wheel steel plates.

[0051] The above-described embodiments merely represent several implementation methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A low-cost 780MPa grade hot-rolled wheel steel, characterized in that: The composition comprises the following components in weight percentage: C: 0.06%~0.09%, Si: 0.10%~0.20%, Mn: 1.55%~1.7%, P≤0.02%, S≤0.015%, Ti: 0.017%~0.0250%, Cr: 0.1%~0.15%, Als: 0.015%~0.045%, V: 0.03%~0.05%, B: 0.0015%~0.0020%, N: 0.0055%~0.01%, and Ti / N=2.9~3.3, 0.23≤C+{0.75+0.25tanh[20(C-0.12)]}×{Si / 24+Mn / 6+(Cr+V) / 5+5B}≤0.29, the balance is Fe and unavoidable impurities.

2. The low-cost 780 MPa grade hot-rolled wheel steel according to claim 1, characterized in that: The thickness of the hot-rolled wheel steel is 4-10 mm.

3. The low-cost 780 MPa grade hot-rolled wheel steel according to claim 1, characterized in that: The hot-rolled wheel steel has a transverse tensile yield strength of 700-800 MPa, a tensile strength of 780-900 MPa, an elongation of ≥16%, a transverse Charpy impact energy of -40°C of ≥150 J, and a yield strength ratio of ≤0.

9.

4. A method for preparing low-cost 780 MPa grade hot-rolled wheel steel according to any one of claims 1 to 3, characterized in that: The following steps are involved: Molten steel smelting, LF refining, RH vacuum degassing, continuous casting, billet heating, high-pressure water descaling, rolling, cooling and coiling; During the continuous casting, the continuous casting superheat is 10-15° C., and the casting rate is 0.7-0.9 m / min. Electromagnetic stirring is used in the secondary cooling zone during the continuous casting stage to continuously cast the molten steel into a continuous casting billet, wherein the equiaxed crystal ratio of the continuous casting billet is ≥85%. At the same time, strong cooling is used in the sector sections, with the total cooling water flow of the first to third sections being 700-1000 L / min, and the total cooling water flow of the fourth to eighth sections being 1200-1500 L / min. At the same time, heavy pressure is applied at the end of solidification, and the continuous casting billet is reduced by 12-16 mm. During the heating of the cast billet, the cast billet is sequentially passed through a preheating section, a heating section, and a soaking section before being taken out of the furnace; the temperature range of the preheating section is 800-950° C., the temperature range of the heating section is 1250-1275° C., the temperature range of the soaking section is 1130-1150° C., the furnace time of the heating section and the soaking section is 3.5-4.5 hours, and the soaking section time is 1-2.5 hours; the opening of the upper and lower burners in the soaking section is adjusted, and the air-fuel ratio is controlled to be 1:1.7-1:2.2, so as to ensure that the temperature difference between the upper and lower surfaces of the billet is ≤15° C.; During the rolling process, the final rolling temperature in the rough rolling stage is 980-995°C; the starting rolling temperature in the finishing rolling stage is 935-960°C, and the final rolling temperature in the finishing rolling stage is 860-890°C. During the cooling and coiling, the rolling speed of the steel plate is controlled to be 2.5-4 m / s, the cooling temperature is 730-750°C, the roller speed in the cooling zone is 1.5-2.5 m / s, and the acceleration is 0.003-0.006 m / s. 2 During the cooling process of the steel plate, the water ratio of the upper and lower headers is 1:1.7~1:2.5, and the ultra-fast cooling and laminar cooling modes are adopted. The number of opening groups of the ultra-fast cooling area controlled cooling headers is 2~4 groups, and the water volume of a single header is 300~400m 3 / h, cooling speed is 50~60℃ / s, final cooling temperature range is 580~620℃, during laminar cooling, the number of cooling manifolds opened is 3~6 groups, and the water volume of a single manifold is 150~200m 3 / h, the cooling rate is 10~15℃ / s, and the final cooling temperature range is 450~480℃.

5. The preparation method according to claim 4, characterized in that In the molten steel smelting, smelting raw materials are configured according to the chemical composition, the molten iron is pretreated by KR to control the sulfur content to be ≤0.015%, and the molten iron is fed into the converter after slag removal; a double slag method is used for P removal in the converter to control the P content to be ≤0.02%, the carbon content is controlled to be 0.06%-0.09% at the end point of the converter smelting, and argon gas is blown for 30-40 minutes during tapping.

6. The preparation method according to claim 4, characterized in that In the LF refining and RH vacuum degassing, the molten steel after converter smelting is subjected to LF refining and RH vacuum degassing, and the RH vacuum is maintained for 30 to 40 minutes.

7. The preparation method according to claim 4, characterized in that During the continuous casting, the electromagnetic stirring is performed in a forward and reverse alternating stirring mode, wherein the forward stirring time is 20 to 30 seconds, the reverse stirring time is 10 to 20 seconds, the current is 500 to 800 A, and the frequency is 30 to 50 Hz. After the billet is off the line, it is put into the pit for slow cooling. After the billet is off the line, it is hot loaded into the slow cooling pit. When the billet temperature is greater than 700°C, the slow cooling pit is not heated, and the slow cooling time is 48 to 72 hours.

8. The preparation method according to claim 4, characterized in that During the heating of the cast billet, the thickness of the cast billet is 150-200 mm.

9. The preparation method according to claim 4, characterized in that In the high-pressure water descaling and rolling process, the cast slab is descaled for 1 to 2 minutes using high-pressure water before rolling, with a descaling machine pressure of 20 to 25 MPa. In the rough rolling stage, the first three rough rolling passes have a reduction rate of 30 to 50%, and the first three rolling passes are sprayed with water for descaling, with each pass time of 1 to 1.5 minutes and a pressure of 20 to 25 MPa. The total reduction rate is controlled at 75% to 85%, and the rough rolling passes are 4 to 5. In the finishing rolling stage, the total reduction rate is greater than 80%, wherein the reduction rate of the last two finishing passes is greater than 20%, and the finishing passes are 4 to 6. During the cooling and coiling, the side spray pressure and water volume are 2-5 MPa and 50-70 m 3 / h, the wind blowing pressure is 5~10MPa.

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