Low-cost 700mpa grade hot-rolled wheel steel and method of making same

Abstract

The application discloses a low-cost 700MPa-grade hot-rolled wheel steel and a preparation method thereof, and belongs to the field of materials and metallurgy technology. The low-cost 700MPa-grade hot-rolled wheel steel comprises the following components: C: 0.1%-0.15%, Si: 0.15%-0.25%, Mn: 1.45%-1.60%, P: 0.02% or less, S: 0.015% or less, Ti: 0.017%-0.0250%, Als: 0.015%-0.035%, V: 0.05%-0.08%, B: 0.0012%-0.0020%, N: 0.005%-0.0095%, Ti / N=3.4-3.7, 0.31<=C+{0.75+0.25tanh[20(C-0.12)]}x{Si / 24+Mn / 6+V / 5+5B}<=0.40, 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 700MPa-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 of use. With the development of vehicle lightweighting, high-strength thinning of wheels has become one of the important means for vehicle weight reduction. The demand for high-strength wheel steels from wheel manufacturers has significantly increased. At the present stage, 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, 700MPa level wheel steels have gradually become a new demand. In the production process of traditional high-strength alloy wheel steels, there are generally serious element segregation in 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, and poor fatigue resistance, etc. in the wheel steel processing process, and with the increase of strength, these defects will be more obvious. At present, the 700MPa level wheel steels generally adopt a low-carbon low-manganese composition system. In order to ensure the safety of the steel strip during service, steel plants require the steel plate to have a low yield ratio and high plasticity, and add expensive alloy elements such as niobium, chromium, molybdenum, and nickel to increase the strength. However, for wheel steels, this seems to be a quality surplus, 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 700MPa 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-toughness 700MPa 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 700MPa 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, the rolling mill load, steel plate temperature uniformity, shape control and other aspects are facing severe challenges, second, the improvement of steel plate performance 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 such as smelting and rolling in the production process. 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 700MPa 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 700MPa grade wheel steel.

[0004] So far, there are few reports on the manufacture of low-cost 700MPa grade hot-rolled wheel steel and its manufacturing method at home and abroad. A Chinese patent with application number 202011260989.1 discloses a 700MPa grade hot-rolled dual-phase steel plate and its manufacturing method. The chemical composition of the steel plate is as follows: C: 0.08-0.12%, Si: 0.10-0.15%, Mn: 1.30-1.50%, P≤0.015%, S≤0.003%, Cr: 0.50-0.70%, Ti: 0.015-0.025%, Mg: 0.0007-0.0020%, Alt: 0.020-0.050%, and the rest is Fe and inevitable impurities. The steel plate in the patent adds more Cr and Ti elements, and adopts a two-stage deep controlled rolling method, which results in low production efficiency and obviously increases the manufacturing cost. Meanwhile, the yield ratio and low-temperature impact performance are not specified. A Chinese patent with application number 202210448773.0 discloses a 700MPa grade hot-rolled wheel steel and its manufacturing method. The chemical composition of the hot-rolled wheel steel is as follows: C: 0.05%-0.08%, Si: 0.06%-0.10%, Mn: 1.65%-1.75%, S:≤0.005%, P:≤0.015%, Alt: 0.02%-0.05%, Nb: 0.055%-0.065%, Ti: 0.045%-0.055%, N:≤0.006%, and the rest is Fe and inevitable inclusions. The steel plate in the patent adds more Nb, Mn and Ti elements, and also adopts a two-stage controlled rolling method, which results in low production efficiency and high manufacturing cost. Meanwhile, the yield ratio, low-temperature impact and other performance indicators are not specified.

[0005] Although the steel plates disclosed in the above patent documents meet the strength requirements, their production cost is high, so they are not suitable for producing low-cost and excellent strength and toughness 700MPa grade wheel steel. SUMMARY

[0006] The present application aims to overcome the above-mentioned defects in the prior art and provide a low-cost 700MPa grade hot-rolled wheel steel and its manufacturing method, which realizes the production of a 700MPa grade wheel steel plate with a thickness of 4-10 mm and low cost from a continuous casting billet with a thickness of 150-200 mm. The steel plate has excellent low-temperature toughness and other characteristics, solves the problem of high alloy cost, and can ensure the service safety of the wheel steel plate.

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

[0008] A low-cost 700MPa-grade hot-rolled wheel steel comprises the following components by weight percentage: C: 0.1% to 0.15%, Si: 0.15% to 0.25%, Mn: 1.45% to 1.60%, P: ≤0.02%, S: ≤0.015%, Ti: 0.017% to 0.0250%, Als: 0.015% to 0.035%, V: 0.05% to 0.08%, B: 0.0012% to 0.0020%, N: 0.005% to 0.0095%, and Ti / N=3.4~3.7, 0.31≤C+{0.75+0.25tanh[20(C-0.12)]}×{Si / 24+Mn / 6+V / 5+5B}≤0.40, the balance being Fe and unavoidable impurities.

[0009] Optionally, the hot-rolled wheel steel has a transverse tensile yield strength of 600-670MPa, a tensile strength of 700-830MPa, an elongation of ≥18%, a transverse Charpy impact energy at -20℃ of ≥80J, and a yield strength ratio of ≤0.85.

[0010] Optionally, the hot-rolled wheel steel has a thickness of 4-10mm.

[0011] The application also discloses a preparation method of the low-cost 700MPa-grade hot-rolled wheel steel.

[0012] In the continuous casting, the overheat degree is 7-12℃, the strand-pulling rate is 1-1.3m / min, electromagnetic stirring is used in the control continuous casting stage two cooling zone, the equiaxial crystal ratio in the continuous casting billet obtained through the continuous casting is not less than 85%, meanwhile, in the fan-shaped section, strong cooling is used, the total cooling water quantity of the first to third sections is 600-900L / min, the total cooling water quantity of the fourth to eighth sections is 1000-1300L / min, meanwhile, the solidification end is put into light pressing, and the pressing amount of the continuous casting billet is 9-12mm.

[0013] In the billet heating, the billet is sequentially discharged after the preheating section, the heating section and the soaking section; the preheating section temperature interval is 800-900℃, the heating section temperature interval is 1260-1290℃, and the soaking section temperature interval is 1130-1150℃; the heating and soaking section time in the furnace is controlled to be 3-4h, the air-fuel ratio is controlled to be 1:1.7-1:2.1 by adjusting the opening degree of the upper and lower burners of the soaking section, the temperature difference between the upper and lower surfaces of the billet is ensured to be ≤15℃, and the soaking section time is ≥1h.

[0014] In the rolling, in the rough rolling stage, the reduction of each pass of the first three passes is 30-50%, the descaling water is sprayed in the first three passes of the rolling mill, the time of each pass is 0.5-1 min, the pressure is 10-15 MPa, the final rolling temperature in the rough rolling stage is 980-995 DEG C, the open rolling temperature in the finish rolling stage is 955-970 DEG C, the final rolling temperature in the finish rolling stage is 860-890 DEG C, and the total reduction in the finish rolling stage is greater than 80%, wherein the reduction of the last two passes in the finish rolling stage is greater than 15%;

[0015] In the cooling and coiling, the steel plate is controlled at a run-out speed of 5.5-7 m / s, the open cooling temperature range is controlled to be 720-735 DEG C, the final cooling temperature range is 520-550 DEG C, and the cooling speed is 25-40 DEG C / s.

[0016] Optionally, in the smelting of the molten steel, the smelting raw materials are configured according to the chemical composition, the molten iron is pretreated through KR, the content of S is controlled to be less than or equal to 0.015%, and the molten iron is poured into a converter after slagging; in the converter smelting, a 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 0.1%-0.15% at the end of the converter smelting, and argon is blown for 20-30 min when the molten steel is poured out.

[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 20-30 min.

[0018] Optionally, in the heating of the cast blank, the thickness of the cast blank is 150-200 mm.

[0019] Optionally, in the rolling, the cast blank is descaled by using high-pressure water for 1-2 min before open rolling, and the pressure of the descaling machine is 20-25 MPa; in the rough rolling stage, the total reduction is controlled to be 75%-85%, and the rough rolling passes are 4-5 passes; and the finish rolling passes are 4-6 passes.

[0020] Optionally, in the continuous casting, the stirring mode of the electromagnetic stirring is positive and negative reverse alternating stirring, wherein the positive stirring time is 20-30 s, the negative stirring time is 10-20 s, the current is 500-800 A, and the frequency is 30-50 Hz; and the cast blank is stacked and slowly cooled for 36-54 h after being discharged.

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

[0022] The embodiment of the application has the following beneficial effects:

[0023] 1. The smelting process employs KR hot metal pretreatment for deep desulfurization followed by thorough slag removal. The converter utilizes a double-slag method for P removal, resulting in lower P and S content in the billet. Controlling the argon blowing time and maintaining RH vacuum degassing time overcomes defects such as center segregation, inclusions, and excessive H and O content in the billet caused by high Mn and C content, thus improving the plasticity and toughness of the steel plate. Reducing superheat and continuous casting speed improves macroscopic segregation in the billet, decreases the spacing of secondary dendrite arms in the solidification structure, and helps reduce billet segregation and internal structural defects. Optimizing the electromagnetic stirring process during continuous casting significantly increases the equiaxed crystal ratio of the billet. Strong cooling ensures a temperature gradient along the billet thickness. Light pressure at the end of solidification effectively reduces central porosity and segregation, promoting core grain breakage and ensuring the strength and toughness of the subsequent steel plate. Furthermore, billet stacking after casting reduces residual H accumulation and inhibits the formation of microcracks within the billet.

[0024] 2. Billet Heating: The temperatures and times of the billet in the preheating, heating, and soaking zones are limited to ensure sufficient diffusion of elements and reduce the impact of component segregation on microstructure and properties. Simultaneously, the heating zone provides high-temperature heating and an optimal air-fuel ratio to ensure temperature uniformity across the billet and improve the uniformity of transverse and longitudinal metal flow on the steel plate surface. Based on the billet composition, energy consumption is reduced by lowering the soaking zone heating temperature. Controlling the temperatures and times of the heating and soaking zones inhibits excessive growth of the original austenite grains, increasing the contribution of fine-grain strengthening to the steel plate's strength. Furthermore, low-temperature heating in the soaking zone avoids the need for preheating during the finishing rolling stage of the steel plate.

[0025] 3. The chemical composition of this invention is reasonably designed, without adding any precious alloying elements (Nb, Mo, etc.), and adopts a low-temperature heating + hot rolling manufacturing mode, which greatly reduces the alloy cost and the resistance to high-temperature deformation in the roughing and finishing stages, which is conducive to increasing the reduction amount in each pass and ensuring the comprehensive performance of the super steel plate.

[0026] 4. Through the first three passes before the rough rolling stage, the rolling mill capacity is exerted as much as possible, a large reduction rate is adopted, the dynamic austenite is promoted to crystallize, the original austenite grains are refined, the V(C, N) phase is promoted to precipitate in the austenite grains by using the high temperature deformation induced effect, the nucleation sites are provided for the acicular ferrite, the formation of the intracrystalline acicular ferrite is promoted, the strength and toughness are improved, the TiN is induced to precipitate, and the BN is inhibited from precipitating. In addition, the high pressure water descaling of the rolling mill is adopted, the temperature gradient is generated from the surface to the center of the steel plate, the deformation of the center and the uniformity of the structure distribution are promoted in the subsequent finishing rolling process, and the strength and toughness of the steel plate are further improved. In addition, since the low temperature heating process is adopted, the intermediate blank waiting in the finishing rolling stage is cancelled, the last two passes are rolled in the non-recrystallization temperature range of the austenite, the large reduction rate is adopted in the last two passes, the dislocation density, vacancies and deformation bands in the austenite are ensured, the V(C, N) is promoted to precipitate, more nucleation sites are provided, the ferrite phase change is promoted to occur, the ferrite structure is refined, and the strength and toughness of the steel plate are ensured.

[0027] 5. Through calculation and experiment, the fastest temperature range of the V(C, N) precipitation is 750-770℃, the opening cooling speed is controlled, the V(C, N) nanometer precipitation density is ensured, the mechanical properties of the steel can be obviously improved through the precipitation strengthening, the steel plate is ensured to be in the ferrite + austenite after entering the water, in addition, the B addition can improve the hardenability of the steel, increase the stability of the austenite, improve the phase change driving force, promote the bainite transformation, in the cooling process, the cooling speed is controlled to ensure that the steel plate is in the ferrite + bainite + martensite after cooling, so as to ensure the coordination of the deformation, and further improve the strength and toughness of the steel plate. Through the side injection, the control of the shape of the steel plate is beneficial, and the uniformity of the performance of the steel plate is improved.

[0028] The present application realizes the low-cost manufacturing of the 700MPa grade hot-rolled wheel steel plate with the thickness of 4-10mm by simple component design, reducing the alloy cost, and adjusting the steelmaking, continuous casting, heating and rolling controlled cooling process. The steel plate has good low temperature toughness. The specific performance is that the yield strength of the transverse tensile property is between 600-670MPa, the tensile strength is between 700-830MPa, the elongation is ≥18%, the transverse Charpy impact energy at-20℃ is ≥80J, and the yield strength ratio is ≤0.85. DETAILED DESCRIPTION

[0029] 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.

[0030] The application discloses a low-cost 700MPa-grade hot-rolled wheel steel, which comprises the following components in percentage by weight: C: 0.1% to 0.15%, Si: 0.15% to 0.25%, Mn: 1.45% to 1.60%, P: 0.02% or less, S: 0.015% or less, Ti: 0.017% to 0.0250%, Al: 0.015% to 0.035%, V: 0.05% to 0.08%, B: 0.0012% to 0.0020%, N: 0.005% to 0.0095%, and Ti / N=3.4~3.7, 0.31<=C+{0.75+0.25tanh[20(C-0.12)]}x{Si / 24+Mn / 6+V / 5+5B}<=0.40, and the balance is Fe and inevitable impurities.

[0031] Specifically, the roles of the main elements in the steel plate chemical composition are as follows:

[0032] C: the most economical and basic strengthening element in the steel, which has obvious effects on improving the strength of the steel through solid solution strengthening and precipitation strengthening, but increasing the C content has negative effects on the plasticity, toughness and weldability of the steel. Therefore, the C content range is set to be 0.1% to 0.15% in the application.

[0033] Mn: the strength of the steel is improved through solid solution strengthening, and the loss of the strength of the steel plate caused by the decrease of the C content is compensated, in addition, the Mn can also lower the gamma-alpha phase transition temperature, thereby refining the ferrite grains, which is helpful to obtain fine low-temperature phase transition products and improve the toughness. However, increasing the content of Mn can aggravate 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 1.45% to 1.60% in the application.

[0034] Si: has the effects of steelmaking deoxidization and base body strength improvement. However, excessive Si can reduce the toughness of the base material welding heat affected zone, and increasing the content of Si can purify the ferrite and reduce the content of pearlite, which is conducive to reducing the bainite effect of the base material. Therefore, the Si content is set to be 0.15% to 0.25% in the application.

[0035] Ti, N and B: in addition to forming fine TiN particles to refine the austenite grains, the N element in the steel is also easy to form BN with B, which affects the free B yield and causes the hardenability of the steel plate to decrease, therefore, the TiN binding capacity is affected by BN, so the N content range is selected to be 0.005% to 0.0095%, the B content range is 0.0012% to 0.0020%, Ti is 0.017% to 0.0250%, and the Ti / N is controlled to be 3.4~3.7, the remaining Ti combines with C to form TiC, which plays a dispersion strengthening role.

[0036] Als: usually as a deoxidizer in steel, if the formation of AlN also has the effect of refining the organization. When the content of Als exceeds 0.035%, the excess alumina inclusions will reduce the cleanliness of the steel. If the content of Als is too low, the deoxidation is insufficient, so the lower limit of the content of Als is set to 0.015%.

[0037] V: is a strong solid N element, exists in the form of V(C, N) in the continuous casting billet. Fine V(C, N) particles can effectively inhibit the austenite grain growth when the continuous casting billet is reheated, in addition, the addition of V will precipitate V(C, N) in austenite and ferrite, which can significantly improve the strength and toughness of the steel. When the V addition exceeds a certain value, the V(C, N) particles will be coarsened, and the stress concentration level of the particle interface and the matrix will be improved. Therefore, the content of V in the present application is selected in the range of 0.05% to 0.08%.

[0038] P, S: are inevitable impurity elements in steel, and 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%. The present application further controls 0.31≤C+{0.75+0.25tanh[20(C-0.12)]}×{Si / 24+Mn / 6+V / 5+5B}≤0.40, to ensure 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, which will further affect the welding performance of the wheel steel.

[0039] In a specific embodiment, the transverse tensile yield strength of the hot-rolled wheel steel is 600-670 MPa, the tensile strength is 700-830 MPa, the elongation is ≥18%, the transverse Charpy impact energy at-20℃ is ≥80J, and the yield strength ratio is ≤0.85.

[0040] In a specific embodiment, the thickness of the hot-rolled wheel steel is 4-10 mm.

[0041] The present application also discloses a preparation method of the low-cost 700MPa-grade hot-rolled wheel steel as described above, which comprises the following steps: molten steel smelting, LF refining, RH vacuum degassing, continuous casting, billet heating, high-pressure water descaling, rolling, cooling and coiling.

[0042] S1, in the molten steel smelting: the smelting raw materials are configured according to the chemical composition, KR hot metal pretreatment is carried out, the content of S is controlled to be ≤0.015%, and after slagging, it enters the converter; the 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 0.1%-0.15%, and argon gas is blown for 20-30 minutes when tapping. The argon 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.

[0043] 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 20-30 min.

[0044] S3, continuous casting: the overheat degree of continuous casting is 7-12°C, and the withdrawal rate is 1-1.3 m / min. Reducing the overheat degree and the continuous casting 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 reduce the segregation of the billet, and reduce internal structural defects. Electromagnetic stirring is used in the control of the secondary cooling zone in the continuous casting stage. The proportion of the equiaxed crystal in the continuous casting billet obtained through continuous casting is not less than 85%, and strong cooling is used in the fan-shaped section. The total cooling water quantity of the first to third sections is 600-900 L / min, and the total cooling water quantity of the fourth to eighth sections is 1000-1300 L / min. Meanwhile, the end of solidification is subjected to light pressing, and the pressing amount of the continuous casting billet is 9-12 mm. Through optimization of 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, and light pressing is used, which can help reduce the segregation of the billet, reduce internal structural defects, and promote the breaking of the core grain, thereby providing a guarantee for the strength and toughness of the subsequent steel plate.

[0045] In a specific embodiment, in the continuous casting: the stirring mode of the electromagnetic stirring 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; and the billet is stacked and slowly cooled for 36-54 h after being discharged. This reduces the aggregation of residual H, inhibits the generation of internal micro-cracks in the billet, and ensures the toughness of the steel plate.

[0046] S4, slab heating: the slab with thickness of 150-200 mm is sequentially discharged from the furnace after preheating section, heating section and soaking section; wherein, the temperature range of the preheating section is 800-900℃, which promotes the homogenization of the microstructure in the slab, the carbide or nitride of V and Ti is fully solid-solved in the matrix, and fully diffused, and the full diffusion of the elements is promoted, the temperature range of the heating section is 1260-1290℃, and the temperature range of the soaking section is 1130-1150℃; the time in the furnace of the heating and soaking section is controlled to be 3-4h, the opening degree of the upper and lower burners of the soaking section is adjusted, the air-fuel ratio is controlled to be 1:1.7-1:2.1, the temperature difference between the upper and lower surfaces of the slab is ensured to be ≤15℃, and the time of the soaking section is ≥1h. The heating temperature is increased, and the time in the furnace is prolonged, which further promotes the diffusion of the elements Mn, C, V, N and B, and reduces the influence of the composition segregation on the microstructure and performance; at the same time, the high-temperature heating of the heating section and the control of the air-fuel ratio ensure the uniformity of the temperature of each part of the slab, and improve the uniformity of the metal flow in the transverse and longitudinal directions of the slab surface; combined with the composition of the slab, the heating temperature of the soaking section of the slab is reduced, the energy consumption is reduced, 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 the contribution of the fine-grain strengthening to the strength of the slab is ensured, in addition, the low-temperature heating of the soaking section reduces the rolling waiting time in the finishing rolling stage, and also ensures that the finishing rolling stage is in the non-recrystallization zone of austenite.

[0047] S5, in rolling: before opening, the cast blank after tapping is descaled for 1-2 min by using high-pressure water, 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 is 30-50%, and the descaling water of the first three passes is sprayed before rolling, and the time of each pass is 0.5-1 min, and the pressure is 10-15 MPa; the rolling mill capacity is used as much as possible in the first three passes before rolling, a large reduction rate is adopted, the dynamic recrystallization of austenite is promoted, the original austenite grains are refined, the high-temperature deformation induced effect is utilized, the V(C, N) phase is precipitated in the austenite grains, the nucleation sites for acicular ferrite are provided, the formation of intracrystalline acicular ferrite is promoted, the strength and toughness are improved, in addition, the high-pressure water descaling of the rolling mill can produce a temperature gradient from the surface to the center of the steel plate, promote the deformation of the core and the uniformity of the microstructure distribution in the subsequent finishing rolling process, further improve the strength and toughness of the steel plate, and also induce TiN precipitation and suppress BN precipitation; the final rolling temperature in the rough rolling stage is 980-995℃; the total reduction rate is controlled at 75%-85%, and the rough rolling passes are 4-5 passes; the opening rolling temperature in the finishing rolling stage is 955-970℃; the total reduction rate in the finishing rolling stage is >80%, the reduction rate of the last two passes is >15%, and the finishing rolling passes are 4-6 passes; the final rolling temperature in the finishing rolling stage is 860-890℃. Due to the adoption of low-temperature heating process, the inter-billet waiting time in the finishing rolling stage is reduced, the last two passes are ensured to be in the non-recrystallization temperature range of austenite, a large reduction rate is ensured in the last two passes, the dislocation density, vacancies and deformation bands in the austenite are ensured, the V(C, N) precipitation is promoted, more nucleation sites are provided, the ferrite phase transition is promoted, the ferrite structure is refined, and the strength and toughness of the steel plate are ensured.

[0048] S6, in cooling and coiling: the steel plate is controlled to have a throw-off speed of 5.5-7 m / s, the opening cooling temperature range is controlled to be 720-735℃, and the final cooling temperature range is 520-550℃; the cooling speed is 25-40℃ / s, the side spraying pressure and water quantity are 2-5 MPa and 50-70 m 3 / h respectively. Through calculation and experiment, the fastest temperature range of V(C, N) precipitation is 750-770℃, the throw-off speed is controlled, the opening cooling temperature is ensured, the V(C, N) nanometer precipitation density is ensured, the mechanical properties of the steel can be obviously improved through precipitation strengthening, at the same time, the structure of the steel plate is ensured to be ferrite + austenite when entering the water, in addition, the addition of B and Cr can improve the hardenability of the steel, increase the stability of austenite, improve the phase transition driving force, promote the bainite transformation, and in the cooling process, the cooling speed is controlled to ensure that the structure of the steel plate after cooling is composed of ferrite, bainite and martensite, so as to ensure the coordination of deformation and further improve the strength and toughness of the steel plate; through side spraying, the plate shape of the steel plate is beneficial to control, and the performance uniformity of the steel plate is improved.

[0049] The following is a specific embodiment

[0050] Examples 1-6

[0051] 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 casting blank of the example steel 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.

[0052] Table 1 Chemical composition of the example steel (wt, %)

[0053]

[0054] Note: P≤0.02% and S≤0.015% in the steel.

[0055] Table 2 Smelting process system of the example steel

[0056]

[0057] Table 3 Heating system of the casting blank of the example steel and high-pressure water descaling process before rolling of the continuous casting blank

[0058]

[0059] Table 4 Rolling parameter of the example steel and partial cooling process of the steel plate

[0060]

[0061] Table 5 Controlled cooling and coiling process parameter of the example steel

[0062]

[0063] Table 6 Performance index of the example steel plate

[0064]

[0065] As can be seen, 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 shortcomings of the prior art, and provide a 700MPa grade hot-rolled wheel steel with a thickness specification of 4-10mm and a manufacturing method thereof. The steel plate has low cost, high strength and toughness, and can ensure the service safety of the wheel steel plate.

[0066] The above embodiments only express several implementation manners of the present application, and the description is relatively specific and detailed, but should not be understood as a limitation on the patent application scope. It should be noted that, for ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection 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 700 MPa grade hot rolled wheel steel, characterized in that, Comprise the following weight percentage of components: C: 0.1%~0.15%, Si: 0.15%~0.25%, Mn: 1.45%~1.60%, P≤0.02%, S≤0.015%, Ti: 0.017%~0.0250%, Als: 0.015%~0.035%, V: 0.05%~0.08%, B: 0.0012%~0.0020%, N: 0.005%~0.0095%, and Ti / N=3.4~3.7, 0.31≤C+{0.75+0.25tanh[20(C-0.12)]}×{Si / 24+Mn / 6+V / 5+5B}≤0.40, the balance being Fe and unavoidable impurities; The preparation method of the low-cost 700MPa-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 7~12℃, the strand pulling rate is 1~1.3m / min, electromagnetic stirring is used in the control of the secondary cooling zone in the continuous casting stage, the equiaxial crystal ratio in the continuous casting blank obtained is not less than 85%, meanwhile, in the fan-shaped section, strong cooling is used, the total cooling water quantity of the first to third sections is 600~900L / min, the total cooling water quantity of the fourth to eighth sections is 1000~1300L / min, and the light press-down is used at the solidification end, and the press-down amount of the continuous casting blank is 9~12mm; In the casting blank heating, the casting blank is sequentially discharged after the preheating section, the heating section and the soaking section; wherein, the preheating section temperature interval is 800~900℃, the heating section temperature interval is 1260~1290℃, and the soaking section temperature interval is 1130~1150℃; the heating and soaking section time in the furnace is controlled to be 3~4h, the air-fuel ratio is controlled to be 1:1.7~1:2.1 by adjusting the opening degree of the upper and lower burners of the soaking section, the temperature difference between the upper and lower surfaces of the blank is ensured to be ≤15℃, and the soaking section time is ≥1h; In the rolling, in the rough rolling stage, the reduction rate of each pass of the first three passes before rough rolling is 30~50%, the descaling water is sprayed in the first three passes of rolling, the time of each pass is 0.5~1min, the pressure is 10~15MPa, and the final rolling temperature in the rough rolling stage is 980~995℃; the open rolling temperature in the finish rolling stage is 955~970℃; the finish rolling temperature is 860~890℃; the total reduction rate in the finish rolling stage is >80%, and the reduction rate of the last two passes in the finish rolling stage is >15%; In the cooling and coiling, the steel plate is controlled to have a throwing speed after rolling, the throwing speed is 5.5~7m / s, the open cooling temperature range is controlled to be 720~735℃, the final cooling temperature interval is 520~550℃, and the cooling speed is 25~40℃ / s.

2. The low cost 700 MPa grade hot rolled wheel steel of claim 1, characterized in that, The transverse tensile yield strength performance of the hot-rolled wheel steel is 600~670MPa, the tensile strength is 700~830MPa, the elongation is ≥18%, the transverse Charpy impact energy at-20℃ is ≥80J, and the yield strength ratio is ≤0.

85.

3. The low cost 700 MPa grade hot rolled wheel steel of claim 1, wherein, The thickness of the hot-rolled wheel steel is 4~10mm.

4. The low cost 700 MPa grade hot rolled wheel steel of claim 1, wherein, In the smelting of the molten steel, smelting raw materials are configured according to a chemical composition, KR molten iron pretreatment is performed, the content of S is controlled to be less than or equal to 0.015%, and then the molten steel is introduced into a converter after slagging; in the converter smelting, a 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 0.1% to 0.15% at the end of the converter smelting, and argon is blown for 20 min to 30 min when the molten steel is tapped.

5. The low cost 700 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 20 min to 30 min.

6. The low cost 700 MPa grade hot rolled wheel steel of claim 1, wherein, In the casting slab heating, the thickness of the casting slab is 150 to 200 mm.

7. The low cost 700 MPa grade hot rolled wheel steel of claim 1, wherein, In the rolling, the casting slab after tapping is descaled by using high-pressure water for 1 to 2 min before rough rolling, the pressure of the descaling machine is 20 to 25 MPa, the total reduction rate is controlled to be 75% to 85% in the rough rolling stage, and the rough rolling passes are 4 to 5 passes; the finishing rolling passes are 4 to 6 passes.

8. The low cost 700 MPa grade hot rolled wheel steel of claim 1, wherein, In the continuous casting, the stirring mode of the electromagnetic stirring is forward-reverse alternating stirring, the forward stirring time is 20 to 30 s, the reverse stirring time is 10 to 20 s, the current is 500 to 800 A, and the frequency is 30 to 50 Hz; the slab is stacked and slowly cooled for 36 h to 54 h after being discharged.

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