Low-cost 780mpa grade hot-rolled wheel steel and method of making same

Abstract

The application discloses a low-cost 780MPa-grade hot-rolled wheel steel and a preparation method thereof, and belongs to the field of materials and metallurgical technologies. The low-cost 780MPa-grade hot-rolled wheel steel comprises the following components: C: 0.06%-0.09%, Si: 0.10%-0.20%, Mn: 1.55%-1.7%, P: 0.02% or less, S: 0.015% or less, 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%, Ti / N=2.9-3.3, 0.23<=C+{0.75+0.25tanh[20(C-0.12)]}x{Si / 24+Mn / 6+(Cr+V) / 5+5B}<=0.29, and the balance is Fe and inevitable impurities.

Description

TECHNICAL FIELD

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

[0002] As one of the important components of commercial vehicles, wheels have a wide demand and a large amount. With the development of lightweight vehicles, high-strength thinning of wheels has become one of the important means for weight reduction of vehicles. The demand for high-strength wheel steels by wheel manufacturers has significantly increased. At present, the commonly used high-strength level in the wheel industry is 590MPa, and some enterprises use 650MPa level wheel steels. However, with the changes in the market and demand, the tensile strength of 780MPa grade wheel steel has gradually become a new demand. In the production process of traditional high-strength alloy wheel steel, there are generally serious element segregation of the casting blank, difficult inclusion control, high alloy cost, and large performance fluctuation, etc. These shortcomings directly affect the welding cracking problem, punching cracking problem, etc. in the wheel steel processing process, and with the increase of strength, these defects will be more obvious. At present, the tensile strength of 780MPa grade wheel steel generally adopts a low-carbon low-manganese composition system. In order to ensure the safety of the steel strip during service, the steel plate is required to have a low yield ratio and high plasticity, and expensive alloy elements such as niobium, chromium, molybdenum and nickel are added to increase the strength. However, for wheel steels, the quality is somewhat excessive, and the cost is high, and the market competitiveness is poor.

[0003] In the face of the severe market situation of the steel industry, how to reduce the manufacturing cost of 780 MPa grade wheel steel, while ensuring product quality, so that the production line can maximize the production capacity becomes particularly important. Under the condition of ensuring production and equipment safety, the final goal is to achieve the maximum efficiency, the lowest energy and alloy cost consumption while ensuring the technical requirements of steel plate performance. Based on this, the development of a low-cost, high strength and toughness 780 MPa grade wheel steel is crucial to improve the profit level of the steel plant. At present, in order to ensure the low temperature toughness of 780 MPa grade wheel steel, higher alloy composition (Mn, Nb, Cr, Mo, Ni, etc.) is usually used, the intermediate blank is kept thick and the temperature is thick, and the two-stage or even three-stage controlled rolling process is used, which leads to the final rolling temperature of the steel plate controlled below 800℃, even lower; which leads to a significant increase in rolling difficulty. First of all, the rolling mill load, the uniformity of the steel plate temperature, the shape control and so on are all facing severe challenges, secondly, the improvement of the performance of the steel plate mainly depends on the content of impurities such as phosphorus and sulfur in the steel, the content of alloy elements in the steel and the control of key process parameters in the production process such as smelting and rolling. As can be seen, how to systematically combine the strengthening mechanisms such as phase transformation strengthening, solid solution strengthening and fine-grain strengthening, solve the above problems in the production of 780 MPa grade wheel steel, reduce the alloy cost, improve the market competitiveness, while ensuring its excellent strength and toughness, is the key to the development of low-cost 780 MPa grade wheel steel.

[0004] A Chinese patent with application number 202310949000.5 discloses a low-temperature-resistant 780MPa-grade high-strength wheel steel and a production method thereof. The chemical composition and mass percentage of the wheel steel are as follows: 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%, and the balance is Fe and inevitable impurities. The steel plate of the patent adds a large amount of Nb element and adopts a two-stage controlled rolling method, resulting in low production efficiency and obviously increasing the manufacturing cost. Meanwhile, the yield strength ratio is not specified. A Chinese patent with application number 201811002601.0 discloses a 780MPa-grade high-fatigue high-strength steel and a manufacturing method thereof. The composition weight percentage is as follows: 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 balance is Fe and inevitable impurities. The steel plate adds a large amount of Mo element and also adopts a two-stage controlled rolling method, resulting in low production efficiency and high manufacturing cost. Meanwhile, the yield strength ratio and other performance indicators are not specified.

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

[0006] The present application aims 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, realizing the production of a low-cost 780MPa-grade wheel steel plate with a thickness of 4-10mm from a continuous casting billet with a thickness of 150-200mm, and the steel plate has excellent low-temperature toughness, low yield strength ratio, and other characteristics.

[0007] To achieve the above-mentioned purpose, the technical solution of the present application is as follows:

[0008] A low-cost 780MPa-grade hot-rolled wheel steel, comprising the following components by 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% to 0.05%, B: 0.0015% to 0.0020%, N: 0.0055% to 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 being Fe and unavoidable impurities.

[0009] Optionally, the thickness of the hot-rolled wheel steel is 4~10mm.

[0010] Optionally, the transverse tensile yield strength of the hot-rolled wheel steel is 700~800MPa, the tensile strength is 780~900MPa, the elongation is ≥16%, the -40℃ transverse Charpy impact energy is ≥150J, and the yield strength ratio is ≤0.9.

[0011] The application also discloses a preparation method of the low-cost 780MPa-grade hot-rolled wheel steel, comprising the following steps: molten steel smelting, LF refining, RH vacuum degassing, continuous casting, casting blank heating, high-pressure water descaling, rolling, cooling and coiling.

[0012] In the continuous casting, the continuous casting superheat is 10~15℃, and the blank drawing rate is 0.7~0.9m / min; electromagnetic stirring is adopted in the control of the secondary cooling zone in the continuous casting stage, so that the molten steel obtains a continuous casting blank, the proportion of the intermediate crystal grains in the continuous casting blank is ≥85%, meanwhile, in the fan-shaped section, strong cooling is adopted, the total cooling water quantity of the first to third sections is 700~1000L / min, the total cooling water quantity of the fourth to eighth sections is 1200~1500L / min, meanwhile, the heavy pressure is thrown into the solidification end, and the continuous casting blank pressure is 12~16mm;

[0013] In the casting blank heating, the casting blank is sequentially discharged after the preheating section, the heating section and the soaking section; the preheating section temperature interval is 800~950℃, the heating section temperature interval is 1250~1275℃, the soaking section temperature interval is 1130~1150℃, the in-furnace time of the heating section and the soaking section is 3.5~4.5h, and the soaking section time is 1~2.5h; the opening degree of the upper and lower burners of the soaking section is adjusted, the air-fuel ratio is controlled to be 1:1.7~1:2.2, and the temperature difference between the upper and lower surfaces of the blank is ensured to be ≤15℃;

[0014] In the rolling, the finishing rolling temperature of the rough rolling stage is 980-995 DEG C, and the finishing rolling temperature of the finishing rolling stage is 860-890 DEG C.

[0015] In the cooling and coiling, the steel plate control speed after rolling is 2.5-4 m / s, the open cooling temperature is 730-750 DEG C, the roller way speed of the controlled cooling area is 1.5-2.5 m / s, and the acceleration is 0.003-0.006 m / s 2 , the upper and lower header water ratio during the steel plate cooling process is 1:1.7-1:2.5, the super-fast cooling and laminar cooling modes are adopted, the super-fast cooling area controlled cooling header opening group number is 2-4 groups, the single header water volume is 300-400 m 3 / h, the cooling speed is 50-60 DEG C / s, the final cooling temperature interval is 580-620 DEG C, in the laminar cooling process, the controlled cooling header opening group number is 3-6 groups, the single header water volume is 150-200 m 3 / h, the cooling speed is 10-15 DEG C / s, and the final cooling temperature interval is 450-480 DEG C.

[0016] Optionally, in the molten steel smelting, the smelting raw materials are configured according to the chemical composition, the molten iron is pretreated by KR, the content of S is controlled to be less than or equal to 0.015%, and the molten iron is poured into the converter after slagging.

[0017] Optionally, in the LF refining and RH vacuum degassing, the molten steel after the converter smelting is subjected to LF refining and RH vacuum degassing, and the RH vacuum is maintained for 30-40 min.

[0018] Optionally, in the continuous casting, the electromagnetic stirring mode is forward and reverse alternating stirring, wherein the forward stirring time is 20-30 s, the reverse stirring time is 10-20 s, the current is 500-800 A, and the frequency is 30-50 Hz; the cast blank after the line is cooled in the pit, the steel blank is hot charged into the slow cooling pit after the line, the slow cooling pit is not heated when the steel blank temperature is greater than 700 DEG C, and the slow cooling time is 48-72 h.

[0019] Optionally, in the cast blank heating, the thickness of the cast blank is 150-200 mm.

[0020] Optionally, in the high-pressure water descaling and rolling, the high-pressure water is used to descale the cast blank after being discharged from the furnace for 1-2 min before rough rolling, the descaling machine pressure is 20-25 MPa; in the rough rolling stage, the reduction rate of each of the first three passes is 30-50%, and the descaling water is sprayed before the first three passes of rolling, the time of each pass is 1-1.5 min, and the pressure is 20-25 MPa; the total reduction rate is controlled to be 75%-85%, and the rough rolling passes are 4-5 passes; the open rolling temperature in the finish rolling stage is 935-960 ℃, the total reduction rate in the finish rolling stage is > 80%, the reduction rate of the last two passes in the finish rolling stage is > 20%, and the finish rolling passes are 4-6 passes.

[0021] Optionally, in the cooling and coiling, the side spraying pressure and water volume are 2-5 MPa and 50-70 m 3 / h, respectively, and the air blowing pressure is 5-10 MPa.

[0022] By implementing the embodiment of the present application, the following beneficial effects can be achieved:

[0023] 1. In the smelting process, the KR molten iron is pretreated by deep desulfurization and then slagging is clean, the double-slag method is used in the converter to remove P, so that the P and S contents of the cast blank are low, the argon blowing time is controlled, and the RH vacuum degassing time is maintained, thereby overcoming the defects of cast blank center segregation, inclusions and H and O contents exceeding the standard due to high Mn and C contents, and being beneficial to improving the plasticity and toughness of the steel plate. Reducing the superheat and the continuous casting speed can improve the macrosegregation of the continuous casting blank, reduce the secondary dendrite arm spacing in the solidification structure of the continuous casting blank, help to reduce the segregation of the steel blank and reduce internal organizational defects. By optimizing the electromagnetic stirring process in the continuous casting stage, the equiaxed crystal rate of the continuous casting blank is greatly improved, strong cooling is adopted to ensure the temperature gradient in the thickness direction of the cast blank, heavy reduction is applied at the solidification end to effectively reduce the center porosity level and segregation of the cast blank, and promote the breaking of the core grain, thereby providing a guarantee for the strength and toughness of the subsequent steel plate. In addition, the cast blank after being discharged is subjected to pit cooling, which promotes the diffusion of elements such as Mn, Cr, C, B and H, and reduces the influence of composition segregation on the structure and performance.

[0024] 2. The cast blank is heated: the temperature and time of the cast blank in the preheating section, the heating section and the soaking section are limited, which ensures the full diffusion of each element, reduces the influence of composition segregation on the structure and performance, and at the same time, the heating section provides high-temperature heating and air-fuel ratio to ensure the uniformity of the temperature of each part of the cast blank, and improve the uniformity of the metal flow in the transverse and longitudinal directions of the steel plate surface; combined with the composition of the steel blank, the heating temperature of the cast blank in the soaking section is reduced to reduce energy consumption; the temperature and time of the heating section and the soaking section are controlled to inhibit the excessive growth of the original austenite grains, 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 waiting for the temperature during the finish rolling stage of the steel plate.

[0025] 3、The application adopts low-temperature heating + hot rolling manufacturing mode, greatly reduces alloy cost and high-temperature deformation resistance in rough rolling and finish rolling stages, is favorable for improving each pass reduction, and is favorable for guaranteeing comprehensive performance of the super steel plate. Through three passes before rough rolling, rolling mill capacity is used as much as possible, large reduction rate is adopted, dynamic austenite crystallization is promoted, original austenite grains are refined, high-temperature deformation induced effect is used, V(C, N) phase is precipitated in austenite grains, nucleation sites are provided for acicular ferrite, the purpose of promoting intracrystalline acicular ferrite formation is achieved, and the strength and toughness are improved. Meanwhile, TiN is induced to precipitate, and BN is inhibited from precipitating. In addition, high-pressure water descaling is adopted for the rolling mill, so that a temperature gradient is generated from the surface to the center of the steel plate, deformation of the center and uniformity of the microstructure distribution are promoted in the subsequent finish rolling process, and the strength and toughness of the steel plate are further improved. In addition, due to the adoption of low-temperature heating process, the intermediate billet waiting in the finish rolling stage is cancelled, the last two passes are ensured to be rolled at the non-recrystallization temperature range of austenite, large reduction rate is ensured for the last two passes, dislocation density, vacancies and deformation bands in the austenite are ensured, V(C, N) is precipitated, more nucleation sites are provided, ferrite phase change is promoted, ferrite structure is refined, and the strength and toughness of the steel plate are ensured.

[0026] 5、Through calculation and experiment, the fastest temperature range of V(C, N) precipitation is 750-770℃, the throwing speed is controlled to ensure that the V(C, N) nanometer precipitation density is maximum before cooling, the mechanical properties of the steel can be obviously improved through precipitation strengthening, the ferrite + austenite state of the steel plate is ensured during water cooling, the proportion of soft phase and deformation compatibility of the rolled steel plate are ensured, the yield strength ratio is ensured, the proportion of large-angle grain boundaries and the density of precipitated phase are increased, so as to ensure the strength and toughness. In addition, the addition of B and Cr can improve the hardenability of the steel, increase the stability of austenite, improve the phase change driving force, and promote the transformation of bainite and martensite. During the cooling process, UFC + ACC combination mode is adopted, UFC adopts fast cooling speed to promote the steel plate to quickly break through the film boiling zone for rapid cooling, and then slow cooling speed is adopted to gradually reduce the surface temperature to the center temperature, so as to cool to 450-480℃. The steel plate is fully hardened during cooling, the stress level of the steel plate during the cooling process is reduced, the plate shape of the steel plate is ensured, the microstructure of the steel plate after cooling is ferrite + bainite + martensite, the deformation compatibility is ensured, and the strength and toughness of the steel plate are further improved. Through side injection, the control of the plate shape of the steel plate is also beneficial to the improvement of the performance uniformity of the steel plate;

[0027] The present application realizes low-cost manufacturing of a 780MPa-grade hot-rolled wheel steel plate with a thickness of 4-10mm by simple component design, low alloy addition amount, no addition of any noble alloy elements (Nb, Mo, etc.), reduction of alloy cost, and regulation of steelmaking, continuous casting, heating, and controlled cooling processes during rolling. The steel plate has good low-temperature toughness, and the specific performance is as follows: the yield strength of the transverse tensile property is between 700-800MPa, the tensile strength is between 780-900MPa, the elongation is greater than or equal to 16%, the -40℃ transverse Charpy impact energy is greater than or equal to 150J, and the yield strength ratio is less than or equal to 0.9. DETAILED DESCRIPTION

[0028] The present application is further described below in combination with specific embodiments, but the present application is not limited in any way by the embodiments.

[0029] The present application discloses a low-cost 780MPa-grade hot-rolled wheel steel, which comprises the following components by 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, and the balance is Fe and unavoidable impurities.

[0030] Specifically, the roles of the main elements in the chemical composition of the hot-rolled wheel steel of the present application are as follows:

[0031] C: the most economical and basic strengthening element in steel, which has a significant effect on improving the strength of the steel through solid solution strengthening and precipitation strengthening, but increasing the C content has a negative impact on the plasticity, toughness, and weldability of the steel. Therefore, the C content range is set to 0.06%-0.09% in the present application.

[0032] Mn: which improves the strength of the steel through solid solution strengthening, and compensates for the loss of the strength of the steel plate caused by the reduction of the C content. In addition, it can also reduce the γ-α phase transition temperature, thereby refining the ferrite grains, which helps to obtain fine low-temperature phase transformation products and improve the toughness. However, increasing the Mn content will exacerbate the center segregation of the continuous casting billet and the formation of lamellar martensite, which is not conducive to the improvement of the low-temperature toughness of the steel plate and also affects the hole expansion performance of the steel plate. Therefore, the Mn content range is designed to be 1.55%-1.70% in the present application.

[0033] Si: has the effect of steelmaking deoxidation and improving the strength of the matrix. However, excessive Si will reduce the toughness of the base material welding heat affected zone, increasing the Si content can purify ferrite and reduce the content of pearlite, which is beneficial to reduce the bainite effect of the base material. Therefore, the Si content in the present application is set to 0.10%~0.20%.

[0034] Ti, N: In steel, N element is not only to form fine TiN particles to refine austenite grains, but also easy to form BN with B, affect the yield of free B, cause the hardenability of the steel plate to decrease, therefore TiN binding capacity is superior to BN, so the N content range selected by the present application is 0.0055%~0.01%, ensure Ti / N control in 2.9~3.3, the remaining Ti combines with C to form TiC, which plays a dispersion strengthening role.

[0035] Als: usually as a deoxidizer in steel, if AlN is formed, it also has the effect of refining the organization. When the content of Als exceeds 0.045%, the excessive alumina inclusions will reduce the cleanliness of the steel, therefore the upper limit of the content of Als is set to 0.045%; if the content of Als is too low, the deoxidization is insufficient, therefore the lower limit of the content of Al is set to 0.015%.

[0036] Cr: can effectively improve the hardenability, inhibit the formation of ferrite, promote the formation of bainite, is the main element for controlling phase change organization, and can improve the strength, plasticity and toughness of the steel plate, the content of Cr selected by the present application is 0.1%~0.15%.

[0037] B: can significantly improve the hardenability of the steel, and is a very effective element, which is easy to segregate at the grain boundary and prevent the precipitation of carbon. A small amount of boron can have a significant effect. However, excessive boron content is easy to form boron carbonitride, which reduces the toughness and causes thermal embrittlement. The reasonable range is 0.0015%~0.0020%.

[0038] V: is a strong solid N element, which exists in the form of V(C, N) in continuous casting billet. Fine V(C, N) particles can effectively inhibit the growth of austenite grains during reheating of continuous casting billet. In addition, the addition of V can form V(C, N) in austenite and ferrite, which can significantly improve the strength and toughness of the steel. When the addition amount of V exceeds a certain value, the V(C, N) particles will coarsen, and the stress concentration level of the particle interface and the matrix will increase. Therefore, the content of V selected by the present application is 0.03%~0.05%.

[0039] P, S: are inevitable impurity elements in steel, which should be as low as possible. However, due to the consideration of smelting cost and process, it cannot be unlimitedly low. Therefore, the upper limit of the content of P and S in the present application is set to 0.020% and 0.015%.

[0040] The application further controls 0.23 <= C + {0.75 + 0.25tanh[20(C-0.12)]}x{Si / 24 + Mn / 6 + (Cr+V) / 5 + 5B} <= 0.29, so that the welding performance of the wheel steel is good, the hardness of the heat-affected zone is relatively low, and the possibility of welding crack is low. If it is not in this range, welding cracks will occur, further affecting the welding performance of the wheel steel.

[0041] In a specific embodiment, the thickness of the hot-rolled wheel steel is 4-10 mm.

[0042] In a specific embodiment, the transverse tensile yield strength of the hot-rolled wheel steel is 700-800 MPa, the tensile strength is 780-900 MPa, the elongation is greater than or equal to 16%, the transverse Charpy impact energy at -40 DEG C is greater than or equal to 150 J, and the yield strength ratio is less than or equal to 0.9.

[0043] The application also discloses a preparation method of the low-cost 780 MPa-grade hot-rolled wheel steel.

[0044] S1, in the molten steel smelting, smelting raw materials are configured according to chemical composition, KR hot metal pretreatment is performed, the content of S is controlled to be less than or equal to 0.015%, and slag is removed before entering the converter; in the converter smelting, double-slag method is used to remove P, the content of P is controlled to be less than or equal to 0.02%, the content of C is controlled to be 0.06%-0.09% at the end of the converter smelting, argon gas is blown for 30-40 min when tapping, and the argon gas blowing and settling before continuous casting can promote the removal of inclusions in the molten steel and improve the composition uniformity of the molten steel.

[0045] S2, in the LF refining and RH vacuum degassing, the molten steel after the converter smelting is subjected to LF refining and RH vacuum degassing, and the RH vacuum is maintained for 30-40 min.

[0046] S3, in continuous casting: the superheat of continuous casting is 10-15℃, the withdrawal rate is 0.7-0.9m / min, reducing the superheat and the withdrawal rate can improve the macrosegregation of the continuous casting billet, reduce the secondary dendrite arm spacing in the solidification structure of the continuous casting billet, help to reduce the segregation of the billet and reduce internal structure defects; in the control of the secondary cooling zone in the continuous casting stage, the electromagnetic stirring mode is positive and negative rotation alternating stirring, the positive rotation stirring time is 20-30s, the negative rotation stirring time is 10-20s, the current is 500-800A, and the frequency is 30-50Hz, so that the molten steel obtains the continuous casting billet after passing through the continuous casting, and the proportion of the equiaxed crystal in the continuous casting billet is greater than or equal to 85%, at the same time, in the fan-shaped section, strong cooling is adopted, the total cooling water quantity of the first to third sections is 700-1000L / min, the total cooling water quantity of the fourth to eighth sections is 1200-1500L / min, and the solidification end is put into heavy pressing at the same time, the pressing amount 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 adopted to ensure the temperature gradient in the thickness direction of the billet, and light pressing is put into at the same time, which can help to reduce the segregation of the billet and reduce internal structure defects, promote the grain breakage in the core, and provide guarantee for the strength and toughness of the subsequent steel plate; the billet after being discharged is subjected to pit slow cooling, the billet is hot charged into the slow cooling pit after being discharged, the slow cooling pit is not heated when the billet temperature is greater than 700℃, the slow cooling time is 48-72h, the diffusion of elements such as Mn, Cr, C and B is promoted, and the influence of composition segregation on the structure and performance is reduced.

[0047] S4, in billet heating: the billet with a thickness of 150-200mm is sent into a walking beam furnace for heating, and the billet is sequentially discharged after passing through a preheating section, a heating section and a soaking section. The temperature interval of the preheating section is 800-950℃, which promotes the homogenization of the structure in the billet, the rapid and sufficient solid solution of carbides or nitrides of V, Ti and Cr in the matrix, and the full diffusion of the elements, and promotes the full diffusion of the elements; the temperature interval of the heating section is 1250-1275℃, the temperature interval of the soaking section is 1130-1150℃, the in-furnace time of the heating section and the soaking section is 3.5-4.5h, the opening degree of the upper and lower burners of the soaking section is adjusted, the air-fuel ratio is controlled to be 1:1.7-1:2.2, the temperature difference between the upper and lower surfaces of the billet is ensured to be less than or equal to 15℃, and the soaking time is 1-2.5h, the heating temperature is increased, the in-furnace time is prolonged, the diffusion of elements Mn, C, Cr, V, N and B is further promoted, and the influence of composition segregation on the structure and performance is reduced; at the same time, the heating section provides high temperature heating and controls the air-fuel ratio to ensure the uniformity of the temperature of each part of the billet and improve the uniformity of the metal flow in the transverse and longitudinal directions of the steel plate; combined with the composition of the billet, the heating temperature of the soaking section of the billet is reduced to reduce energy consumption, the temperature and time of the heating section and the soaking section are limited to inhibit the excessive growth of the original austenite grains, ensure the contribution of fine grain strengthening to the strength of the steel plate, in addition, low temperature heating is adopted in the soaking section, the rolling standby time in the finishing rolling stage is reduced, and at the same time, it is ensured that the finishing rolling stage is carried out in the non-recrystallization zone of austenite.

[0048] S5, in high-pressure water descaling and rolling: before rough rolling, the cast slab after being discharged from the furnace is descaled by high-pressure water for 1-2 min, and the descaling machine pressure is 20-25 MPa; in the rough rolling stage, the reduction rate of each of the first three passes is greater than 30%, and the descaling water is sprayed in the first three passes before rolling, and the time of each pass is 1-1.5 min, and the pressure is 20-25 MPa; the rolling mill capacity is exerted as much as possible in the first three passes before rolling, a large reduction rate is adopted, the original austenite grains are refined by promoting the dynamic crystallization of austenite, the V(C, N) phase is precipitated in the austenite grains by utilizing the high-temperature deformation-induced effect, the nucleation sites for acicular ferrite are provided, the formation of intragranular acicular ferrite is promoted, the strength and toughness are improved, in addition, the temperature gradient from the surface to the center of the steel plate is generated by adopting high-pressure water descaling of the rolling mill, the core deformation and the uniformity of the microstructure distribution are promoted in the subsequent finishing rolling process, the strength and toughness of the steel plate are further improved, and TiN is also precipitated, BN is inhibited, and the quenching effect of B is ensured; the final rolling temperature in the rough rolling stage is 980-995℃; the total reduction rate is controlled to be 75%-85%, and the rough rolling passes are 4-5 passes; the opening rolling temperature in the finishing rolling stage is 935-960℃, the total reduction rate in the finishing rolling stage is >80%, the reduction rate of the last two passes in the finishing rolling stage is >20%, and the finishing rolling passes are 4-6 passes; and the final rolling temperature in the finishing rolling stage is 860-890℃. Due to the adoption of the low-temperature heating process, the inter-billet standing time in the finishing rolling stage is reduced, the last two passes are ensured to be in the austenite non-recrystallization temperature range, a large reduction rate is ensured to be adopted in the last two passes, the dislocation density, vacancies and deformation bands in the austenite are ensured, V(C, N) is precipitated, more nucleation sites are provided, the ferrite phase transformation is promoted, the ferrite structure is refined, and the strength and toughness of the steel plate are ensured.

[0049] S6, in cooling and coiling: the steel plate is controlled to be thrown at a speed of 2.5-4 m / s after rolling, the opening cooling temperature is controlled to be 730-750℃, the roller speed in the controlled cooling area is 1.5-2.5 m / s, and the acceleration is 0.003-0.006 m / s 2 , the ratio of the upper and lower header water is 1:1.7-1:2.5 during the cooling process of the steel plate, the UFC (ultra-fast cooling) + ACC (laminar flow) cooling mode is adopted, the number of groups of the UFC area cooling header opened is 2-4 groups, the single header water volume is 300-400 m 3 / h, the cooling speed is 50-60℃ / s, the final cooling temperature interval is 580-620℃, the number of groups of the ACC area cooling header opened is 3-6 groups, the single header water volume is 150-200 m 3 / h, the cooling speed is 10-15℃ / s, and the final cooling temperature interval is 450-480℃; the side spraying pressure and water volume are 2-5 MPa and 50-70 m 3h, and the wind purge pressure is 5-10 MPa, in order to reduce the surface scale defects of the steel plate, through calculation and experiment, the fastest temperature interval of V(C, N) precipitation is 750-770 DEG C, the speed of the steel plate is controlled, the maximum density of V(C, N) nano-precipitation is ensured before cooling, the mechanical properties of the steel can be obviously improved through precipitation strengthening, meanwhile, the structure of the steel plate is ensured to be ferrite + austenite, the proportion of the soft phase of the structure after rolling and the deformation compatibility are ensured, the yield ratio is ensured, meanwhile, the proportion of the large-angle grain boundary and the density of the precipitated phase are increased, so that the strength and toughness are ensured; in addition, the addition of B and Cr can improve the hardenability of the steel, increase the stability of austenite, improve the phase transformation driving force, promote the transformation of bainite and martensite, in the cooling process, the UFC+ACC combination mode is adopted, the fast cooling speed is adopted in the UFC, the steel plate is promoted to quickly break through the film boiling zone for rapid cooling, then the slow cooling speed is adopted, the surface temperature is gradually reduced to the core temperature, on this basis, the steel plate is cooled to 450-480 DEG C, while ensuring that the cooled steel plate is fully hardened, the stress level of the steel plate in the cooling process is also reduced, the shape of the steel plate is ensured, and the structure of the steel plate after cooling is ensured to be composed of ferrite, bainite and martensite, so that the deformation compatibility is ensured, and the strength and toughness of the steel plate are further improved; through side injection, the control of the shape of the steel plate is also beneficial to the improvement of the performance uniformity of the steel plate.

[0050] Specifically, by adopting the above-mentioned composition, the steelmaking, continuous casting, heating, controlled rolling and controlled cooling scheme, the shortcomings of the prior art are overcome, a 780 MPa grade hot-rolled wheel steel plate with a thickness of 4-10 mm is low-cost manufactured, the problems of high alloy cost and the like are solved, and the steel plate has low cost, high strength and toughness, and can ensure the service safety of the wheel steel plate.

[0051] The following is a specific embodiment

[0052] Table 1 is the chemical composition of the example steel, Table 2 is the smelting process system of the example steel, Table 3 is the heating system of the cast blank and the high-pressure water descaling process before rolling of the continuous casting blank; Table 4 is the rolling parameter of the example steel; Table 5 is the controlled cooling and coiling process parameter of the example steel; and Table 6 is the performance index of the example steel plate.

[0053] Table 1 is the chemical composition of the example steel, Table 2 is the smelting process system of the example steel, Table 3 is the heating system of the cast blank and the high-pressure water descaling process before rolling of the continuous casting blank; Table 4 is the rolling parameter of the example steel; Table 5 is the controlled cooling and coiling process parameter of the example steel; and Table 6 is the performance index of the example steel plate.

[0054]

[0055] Note: P≤0.02% in the steel; S≤0.015%.

[0056] Table 2 is the smelting process system of the example steel

[0057]

[0058] Table 3 Heating schedule of casting billet of example steel and high pressure water descaling process of continuous casting billet before rolling

[0059]

[0060] Table 4 Rolling parameters of example steel and part cooling process of steel plate

[0061]

[0062] Table 5 Controlled cooling and coiling process parameters of example steel of the present application

[0063]

[0064] Table 6 Microstructure and performance index of steel plate of example of the present application

[0065]

[0066] It can be seen that, compared with the prior art, the component design, the steelmaking and continuous casting, the heating and the controlled rolling and controlled cooling scheme of the present application overcome the deficiencies of the prior art, and provide a 780MPa grade hot-rolled wheel steel with a thickness specification of 4-10mm and a manufacturing method thereof. The problems of high alloy cost and the like are solved, and the steel plate has low cost, high strength and toughness, and can ensure the service safety of the wheel steel plate.

[0067] The above-described embodiments only express several embodiments of the present application, and the description is relatively specific and detailed, but it should not be understood as a limitation on the scope of the application. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are all within the scope of the present application. Therefore, the protection scope of the present application patent should be subject to the appended claims.

Claims

1. A low cost 780 MPa grade hot rolled wheel steel, characterized in that, Comprise the following weight percentage of components: 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 inevitable impurities; The transverse tensile yield strength of the hot-rolled wheel steel is 700~800MPa, the tensile strength is 780~900MPa, the elongation is ≥16%, the transverse Charpy impact energy at-40℃ is ≥150J, and the yield strength ratio is ≤0.9; The preparation method of the low-cost 780MPa grade hot-rolled wheel steel comprises the following steps: molten steel smelting, LF refining, RH vacuum degassing, continuous casting, casting blank heating, high-pressure water descaling, rolling, cooling and coiling; In the continuous casting: the continuous casting superheat is 10~15℃, and the strand pulling rate is 0.7~0.9m / min; electromagnetic stirring is used in the control of the secondary cooling zone in the continuous casting stage, so that the molten steel obtains a continuous casting blank, and the intermediate crystal grain ratio in the continuous casting blank is ≥85%, and at the same time, in the fan-shaped section, strong cooling is used, the total cooling water quantity of the 1~3 sections is 700~1000L / min, and the total cooling water quantity of the 4~8 sections is 1200~1500L / min, and at the same time, the heavy pressure is poured into the solidification end, and the continuous casting blank pressure is 12~16mm; In the casting blank heating: the casting blank is sequentially discharged after the preheating section, the heating section and the soaking section; the preheating section temperature interval is 800~950℃, the heating section temperature interval is 1250~1275℃, the soaking section temperature interval is 1130~1150℃, the in-furnace time of the heating section and the soaking section is 3.5~4.5h, and the soaking section time is 1~2.5h; the air-fuel ratio is controlled to be 1:1.7~1:2.2 by adjusting the opening degree of the upper and lower burners of the soaking section, so as to ensure that the temperature difference between the upper and lower surfaces of the blank is ≤15℃; In the rolling: the final rolling temperature in the rough rolling stage is 980~995℃; the opening rolling temperature in the finish rolling stage is 935~960℃, and the finish rolling final rolling temperature is 860~890℃; In the cooling and coiling: the steel plate after rolling controls the speed of 2.5-4 m / s, the open cooling temperature is 730-750℃, the roller speed of the controlled cooling area is 1.5-2.5 m / s, and the acceleration is 0.003-0.006 m / s 2 , the water ratio of the upper and lower headers during the cooling of the steel plate is 1:1.7-1:2.5, the super-fast cooling and laminar cooling modes are adopted, the number of the opened groups of the controlled cooling header in the super-fast cooling area is 2-4, the single header water volume is 300-400 m 3 / h, the cooling speed is 50-60℃ / s, the final cooling temperature interval is 580-620℃, in the laminar cooling process, the number of the opened groups of the controlled cooling header is 3-6, the single header water volume is 150-200 m 3 / h, the cooling speed is 10-15℃ / s, and the final cooling temperature interval is 450-480℃.

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

3. The low cost 780 MPa grade hot rolled wheel steel of claim 1, wherein, In the molten steel smelting: the smelting raw materials are configured according to the chemical composition, the KR hot metal pretreatment is carried out, the content of S is controlled to be ≤0.015%, and after slagging, it is poured into the converter; double-slag method is used for P removal in the converter smelting, the content of P is controlled to be ≤0.02%, the content of C at the end of the converter smelting is controlled to be 0.06%~0.09%, and argon gas is blown for 30~40min when tapping.

4. The low cost 780 MPa grade hot rolled wheel steel of claim 1, wherein, In the LF refining and RH vacuum degassing, the molten steel after the converter smelting is subjected to LF refining and RH vacuum degassing, and the RH vacuum is maintained for 30-40 min.

5. The low cost 780 MPa grade hot rolled wheel steel of claim 1 wherein, In the continuous casting, the stirring mode of electromagnetic stirring is positive and negative rotation alternating stirring, wherein the positive rotation stirring time is 20-30 s, the negative rotation stirring time is 10-20 s, the current is 500-800 A, and the frequency is 30-50 Hz; the cast blank after being discharged is subjected to pit entry slow cooling, the steel blank after being discharged is hot charged into the slow cooling pit, the slow cooling pit is not heated when the steel blank temperature is greater than 700 ℃, and the slow cooling time is 48-72 h.

6. The low cost 780 MPa grade hot rolled wheel steel of claim 1, wherein, In the cast blank heating, the thickness of the cast blank is 150-200 mm.

7. The low cost 780 MPa grade hot rolled wheel steel of claim 1 wherein, In the high-pressure water descaling and rolling, the cast blank after being discharged is descaled by high-pressure water for 1-2 min before rough rolling, and the descaling machine pressure is 20-25 MPa; in the rough rolling stage, the reduction rate of each pass of the first three passes before rough rolling is 30-50%, and the descaling water is sprayed before the first three passes of rolling, the time of each pass is 1-1.5 min, and the pressure is 20-25 MPa; the total reduction rate is controlled to be 75-85%, and the rough rolling passes are 4-5 passes; the total reduction rate in the finish rolling stage is >80%, wherein the reduction rate of the last two passes of finish rolling is >20%, and the finish rolling passes are 4-6 passes. The side blowing pressure and water quantity are respectively 2~5MPa and 50~70m 3 / h, and the air blowing pressure is 5~10MPa.

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