Low-cost weldable ultrahigh-strength steel plate and preparation method thereof

Abstract

The invention relates to a low-cost weldable ultrahigh-strength steel plate and a preparation method thereof, belongs to the technical field of ultrahigh-strength steel plates, and solves the problems of lower toughness, poorer welding performance and higher cost of a steel plate with the strength of greater than 2300MPa in the prior art. The steel plate comprises, by mass, 0.42%-0.47% of C, 0.20%-0.50% of Si, smaller than or equal to 0.20% of Mn, smaller than or equal to 0.010% of P, smaller than or equal to 0.003% of S, smaller than or equal to 0.50% of Cr, 0.80%-1.20% of Ni, 0.20%-0.40% of Mo, 0.02%-0.06% of Ti, smaller than or equal to 0.04% of Nb, smaller than or equal to 0.0040% of N, 0.001%-0.004% of B and the balance Fe. According to the steel plate, the microcosmic metallographic phase of the steel plate has a martensite matrix structure by regulating and controlling the types and proportions of all the component elements, and the strength of the steel plate is improved; by regulating and controlling the contents of elements Ti, Nb and N, MC type carbides (such as TiC, NbC and (Ti, Mo) C) are formed to precipitate a strengthening phase, martensite grains are refined, and the obdurability of the steel is improved; the content of Mn and Cr is reduced, the carbon equivalent is controlled to be smaller than or equal to 0.70%, and the welding performance is improved And by adopting a continuous casting and rolling process for manufacturing, the cost is obviously reduced, and the popularization and application of the ultrahigh-strength steel can be promoted.

Description

Technical Field

[0001] This invention relates to the field of ultra-high strength steel plate technology, and in particular to a low-cost weldable ultra-high strength steel plate and its preparation method. Background Technology

[0002] Ultra-high strength steel plates are widely used in the automotive, machinery, and equipment manufacturing industries. With increasingly stringent energy conservation and emission reduction requirements, using higher strength steel plates to improve the lightweighting and load-bearing capacity of vehicle equipment has become a major development trend. Currently, the main ultra-high strength steel plates used are wear-resistant steel (NM450, NM500, etc.) and hot-formed steel (MS1500, MS1800, etc.), with strengths reaching 1500MPa-1800MPa, while ultra-high strength steel plates with even higher strengths are used less frequently.

[0003] Currently, ultra-high strength steel plates used in the automotive and machinery industries are mainly medium- and low-alloy steels, manufactured using an industrial process of converter + continuous casting + continuous rolling. They are then quenched and tempered at low temperatures to obtain a low-temperature tempered martensitic structure, achieving high strength while maintaining good toughness. However, when the strength of the steel plate reaches above 2000 MPa, the above process route is insufficient to meet the toughness requirements and weldability. Ultra-high strength steels such as 300M and Aermet100 employ medium- and high-alloy compositions combined with advanced special smelting processes such as vacuum arc remelting, achieving high strength and high toughness. However, the strength of these materials only reaches 2000 MPa, and the overall cost is too high, limiting their application areas.

[0004] Therefore, there is an urgent need for an ultra-high strength steel plate with a strength greater than 2300MPa and its preparation method, in order to balance strength, toughness, weldability and overall cost, so as to promote its widespread application. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a low-cost weldable ultra-high strength steel plate and its preparation method, in order to solve the problems of low toughness, poor welding performance and high cost of steel plates with strength greater than 2300MPa in the prior art.

[0006] On one hand, embodiments of the present invention provide a low-cost weldable ultra-high strength steel plate. The steel plate, calculated by mass percentage, comprises: C 0.42-0.47%, Si 0.20-0.50%, Mn ≤0.20%, P ≤0.010%, S ≤0.003%, Cr ≤0.50%, Ni 0.80-1.20%, Mo 0.20-0.40%, Ti 0.02-0.06%, Nb ≤0.04%, N ≤0.0040%, B 0.001-0.004%, with the balance being Fe.

[0007] Furthermore, calculated by mass percentage, the constituent elements of the steel plate include: C 0.43-0.47%, Si 0.22-0.38%, Mn ≤0.19%, P ≤0.009%, S ≤0.002%, Cr ≤0.35%, Ni 0.80-1.20%, Mo 0.22-0.35%, Ti 0.025-0.058%, Nb ≤0.04%, N ≤0.0040%, B 0.0015-0.0031%, with the balance being Fe.

[0008] Furthermore, the contents of the elements Ti, Nb, and N in the steel plate satisfy the following relationship: 0.04%≤Ti+Nb-3.5N≤0.07%.

[0009] Furthermore, the original austenite grain size of the microstructure of the steel plate is ≤5.5μm.

[0010] Furthermore, the microstructure of the steel plate includes a martensitic matrix, retained austenite, and precipitated strengthening phases; wherein the precipitated strengthening phases include nanoscale TiC, NbC, (Ti, Mo)C, and (Nb, Mo)C precipitates.

[0011] On the other hand, embodiments of the present invention also provide a method for preparing the low-cost weldable ultra-high strength steel plate, the method comprising:

[0012] (1) Based on the target composition content of the steel plate, the raw materials are smelted, refined in a ladle, degassed in a vacuum and cast in sequence to obtain a billet; (2) The cast billet is heated, rolled, coiled and cooled, and leveled to obtain a rolled slab; (3) The slab is subjected to quenching and tempering treatment.

[0013] Furthermore, in step (2), the heating and heat preservation temperature is 1250-1300℃, and the time is 1.5-2.0 min / mm.

[0014] Furthermore, in step (2), the rolling process includes a roughing stage and a finishing stage. The temperature of the roughing stage is 1070-1220℃ and the total compression ratio is 60-70%. The initial rolling temperature of the finishing stage is 960-1000℃ and the final rolling temperature is 820-880℃, with a total compression ratio of 30-40%.

[0015] Furthermore, in step (3), the specific quenching process is as follows: at a temperature of 840-880℃, maintain for 2.0-3.0 min / mm, then cool at a rate of ≥100℃ / s to 50-100℃ below the martensitic transformation termination temperature (Mf point), and then air cool to room temperature.

[0016] Furthermore, in step (3), the tempering temperature is 150~180℃, and the holding time is calculated as 5~10min / mm based on the thickness of the steel plate.

[0017] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: 1. Compared with existing high-strength steel plates, the steel plate of this invention improves strength by controlling the types and proportions of each constituent element to create a martensitic matrix microstructure. Simultaneously, by controlling the content of Ti, Nb, and N, MC-type carbides (such as TiC, NbC, and (Ti,Mo)C) precipitate strengthening phases, refining the martensitic grains and improving the steel's strength and toughness. Reducing the Mn and Cr content minimizes their impact on the steel plate's weldability, and controlling the carbon equivalent to ≤0.70% improves weldability. Increasing the Ti content and decreasing the Nb content lowers costs. This invention, by controlling the chemical element composition and proportions of the steel plate, achieves a strength greater than 2300 MPa while simultaneously possessing excellent toughness, weldability, and lower cost, promoting the widespread application of ultra-high-strength steel.

[0018] 2. This invention regulates the grain size of the micro-martensite structure of the steel plate, keeping the original austenite grain size within 5μm, thereby improving the strength of the steel plate while also increasing its low-temperature impact toughness; it also limits the content of residual austenite structure, thereby reducing the strength reduction caused by austenite structure.

[0019] 3. In the steel plate preparation method, the protective atmosphere of vacuum degassing and casting process improves the cleanliness of the microstructure of the steel plate and reduces the impact of impurities on the performance of the steel plate; through the subsequent heat treatment process of the steel plate, the uniformity of the steel plate structure is improved, the internal stress of the steel plate is reduced, and the strength and toughness are improved.

[0020] 4. The method of the present invention promotes martensitic phase transformation by controlling the quenching temperature, time and cooling rate, and using ultra-fast cooling to control the microstructure of the steel plate to be a high-strength martensitic structure and reduce the content of austenite structure. The present invention controls the rolling process parameters, coiling temperature and quenching conditions to control the amount and size of MC type carbide precipitation, effectively improving the strength and toughness of the steel plate.

[0021] 5. Compared with existing ultra-high strength steel plates, the steel plate of this invention can combine ultra-high strength and high toughness. At room temperature, the tensile strength is ≥2300MPa, the yield strength is ≥1500MPa, and the elongation after fracture is ≥10%. The low-temperature impact absorption energy KV2 (-40℃) is ≥15J, and it has excellent weldability. At the same time, the Ni, Mo and Nb content in the material composition is relatively low, which significantly reduces the overall cost compared with high alloy ultra-high strength steel, which is conducive to its widespread application.

[0022] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0023] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0024] Figure 1 is a metallographic grain structure image of the steel plate obtained in Example 1 of the present invention; Figure 2 This is a photograph of a 6mm thick steel plate after welding, obtained in Embodiment 1 of the present invention. Detailed Implementation

[0025] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0026] Because the strength, toughness, and weldability of steel materials are mutually restrictive, as the strength of steel plates continues to increase, the problems of insufficient toughness and decreased weldability become increasingly prominent. This leads to insufficient structural impact resistance and reduced subsequent welding and processing performance, greatly limiting the practical application of ultra-high strength steel plates. Therefore, obtaining good toughness reserves and weldability while improving the strength of steel plates is both a hot topic and a challenge in ultra-high strength steel plate research.

[0027] Currently, although some research is being conducted on high-strength steel, the results are not ideal. The high-strength steel produced cannot simultaneously possess ultra-high strength (≥2300MPa), high toughness, and excellent weldability, or may have high manufacturing costs.

[0028] Therefore, this embodiment of the invention provides an ultra-high strength and high toughness steel plate. The steel plate, calculated by mass percentage, comprises: C 0.42-0.47%, Si 0.20-0.50%, Mn ≤0.20%, P ≤0.010%, S ≤0.003%, Cr ≤0.50%, Ni 0.80-1.20%, Mo 0.20-0.40%, Ti 0.02-0.06%, Nb ≤0.04%, N ≤0.0040%, B 0.001-0.004%, with the balance being Fe and unavoidable impurities.

[0029] Carbon equivalent is a quantitative measure of the weldability of steel plates; excessively high carbon equivalent results in poor weldability. According to the carbon equivalent calculation formula proposed by the International Welding Association, CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, it can be seen that, in addition to carbon, the contents of elements such as Mn, Cu, Ni, Cr, Mo, and V also affect the carbon equivalent value. Therefore, this invention, while ensuring the strength and toughness of the steel, adopts a low carbon equivalent composition design to achieve good weldability.

[0030] The roles and proportions of each element in this invention are based on the following: Carbon (C): As a major interstitial solid solution strengthening element, it plays a decisive role in the strength of quenched martensitic steel. It can also combine with Ti and Nb to form TiC and NbC precipitates, thus improving the steel's properties. Excessive carbon content will deteriorate the toughness of the steel plate. Considering the strength and toughness targets of the invented steel, the C content is controlled at 0.42-0.47 wt%.

[0031] Si: A common solid solution element in steel, it can inhibit carbide precipitation and improve the steel's resistance to tempering softening. However, excessive Si can lead to decarburization of steel plates during heat treatment, so it should be controlled at 0.20~0.50wt%.

[0032] Mn: Its main function is to improve the hardenability of steel, but excessive Mn content will exacerbate the formation of banded structure, thereby affecting toughness and reducing the weldability of steel. Considering that the steel plate of this invention is relatively thin, the content is kept as low as possible and controlled within 0.20 wt%.

[0033] P and S: Harmful elements in steel that can form harmful inclusions and segregate at grain boundaries, severely reducing the ductility and toughness of steel. Therefore, they should be eliminated as much as possible, with P ≤ 0.010 wt% and S ≤ 0.003 wt%.

[0034] Cr: Its main function is to improve hardenability and reduce segregation tendency, but too much Cr will affect welding performance. Therefore, the amount added should be within 0.50 wt%.

[0035] Ni can improve the hardenability of steel and significantly improve its low-temperature impact performance, but too much Ni will increase the cost of steel plates and worsen the weldability. Therefore, the addition amount is 0.80-1.20wt%.

[0036] Mo can effectively improve the hardenability of steel and can also combine with elements such as Ti and Nb to precipitate MC-type carbides, increasing precipitation strengthening and inhibiting the growth of precipitated phases. However, excessive Mo will increase costs and affect the weldability of the steel. Therefore, the Mo content in the invented steel is controlled at 0.20-0.40 wt%. Ti and Nb are both strong carbonitride forming elements. Ti has a high binding energy with C and N, which can inhibit the combination of B and N to form BN and enhance the effect of B. However, a high Ti content can easily lead to the formation of sharp, large-sized TiN inclusions, affecting the toughness and plasticity of the steel. Nb can form fine, dispersed MC-type carbon or carbonitrides with C, resulting in precipitation strengthening and refining austenite grains, thus improving the strength and toughness of the steel. However, excessive Nb content significantly increases the cost. Therefore, the Ti content in the invented steel is controlled within 0.02~0.06 wt%, and the Nb content is controlled within 0.04 wt%, while satisfying 0.40%≤Ti+Nb-3.5N≤0.70% to ensure a sufficient amount of precipitated phases.

[0037] B: It can significantly improve the hardenability of steel and purify grain boundaries. The effect is not obvious when the content is below 0.001%, and the increase in effect is not significant when it is above 0.004%. Therefore, the B content should be controlled within the range of 0.001-0.004 wt%.

[0038] Nitrogen (N) combines with elements such as Ti and Nb to form nitrides, which refine austenite grains. However, excessive N content can lead to the formation of large-sized TiN, reducing the toughness of the steel. Therefore, the N content should be controlled below 0.0040%.

[0039] Furthermore, calculated by mass percentage, the constituent elements of the steel plate include: C 0.43-0.47%, Si 0.22-0.38%, Mn ≤0.19%, P ≤0.009%, S ≤0.002%, Cr ≤0.35%, Ni 0.80-1.20%, Mo 0.22-0.35%, Ti 0.025-0.058%, Nb ≤0.04%, N ≤0.0040%, B 0.0015-0.0031%, with the balance being Fe.

[0040] It should be noted that in order to refine the microstructure of the steel plate and reduce the formation of large-sized TiN precipitates in the steel, the Ti content (number of atoms) in the steel should be higher than the N content in the steel, but not too high, to prevent the formation of large-sized TiN; at the same time, the excess small amount of Ti and Nb elements combine with C to form carbides, which refines the grains and thus improves the strength and toughness of the steel plate.

[0041] Specifically, the contents of elements Ti, Nb, and N in the steel plate satisfy the following relationship: 0.04% ≤ Nb + Ti - 3.5N ≤ 0.07%. If the value of Nb + Ti - 3.5N is too small, it will lead to insufficient total amount of precipitates and unsatisfactory refining effect. If the value is too large, it will lead to the growth of precipitates and reduced strengthening effect.

[0042] Specifically, the thickness of the steel plate is ≤15 mm. If the thickness is too large, the core cannot be fully quenched, resulting in a decrease in the strength and toughness of the steel plate.

[0043] It should be noted that, in order to obtain high strength, high toughness and excellent weldability, the present invention needs to define the composition of the microstructure of the steel plate.

[0044] Specifically, the microstructure of the steel plate includes a martensitic matrix, a retained austenitic structure, and precipitated strengthening phases.

[0045] Specifically, in the steel plate of the present invention, the volume fraction of the retained austenite structure is less than 3%, which can be 1%, 2%, or 3%. If the content is increased, the strength of the steel will decrease.

[0046] Specifically, the microstructure of the steel plate, i.e. the average grain size of the original austenite, is within 5.5 μm. If the grains are too large, it will lead to reduced strength and insufficient low-temperature impact toughness.

[0047] Specifically, of the microalloying elements (Ti, Nb) added in this invention, more than 90% of the elements are precipitated in the form of precipitated phases.

[0048] It should be noted that, in order to refine the grains and improve the strength, toughness and weldability of the steel, this invention needs to limit the type and size of the precipitated phases in the steel plate.

[0049] Specifically, the precipitated phases include nanoscale TiC, NbC, (Ti,Mo)C, (Nb,Mo)C, and other MC-type precipitates, as well as micron-sized TiN precipitates. MC-type precipitates with a size of less than 10 nm account for more than 30% of the total precipitate volume, MC-type precipitates with a size of 10-30 nm account for more than 50% of the total precipitate volume, and TiN precipitates have a size not greater than 10 μm.

[0050] Compared with existing ultra-high strength steel plates, the steel plate of this invention can combine ultra-high strength, high toughness, and excellent weldability. At room temperature, its tensile strength is ≥2300MPa (e.g., 2310-2430MPa), yield strength is ≥1500MPa (e.g., 1590-1680MPa), and elongation after fracture is ≥10% (e.g., 11.0-12.5%). Its low-temperature impact absorption energy KV2 (-40℃) is ≥15J (e.g., 18-23J). Its carbon equivalent CE is ≤0.70%, and it has excellent weldability. At the same time, the material composition contains less Ni, Mo, and Nb, which significantly reduces the overall cost compared with high alloy ultra-high strength steel, which is conducive to its widespread application.

[0051] On the other hand, embodiments of the present invention also provide a method for preparing the ultra-high strength and high toughness steel plate, the method comprising: (1) Based on the target composition content of the steel plate, the raw materials are smelted, refined in a ladle, degassed in a vacuum and cast in sequence to obtain a billet; (2) The cast billet is heated, rolled, coiled and cooled, and leveled to obtain a rolled slab; (3) The slab is subjected to quenching and tempering treatment.

[0052] It should be noted that in step (1), the smelting can be carried out using a converter or an electric furnace.

[0053] Specifically, in step (1), when performing vacuum degassing, RH vacuum degassing or VD vacuum degassing can be used to deoxidize, dehydrogenate, desulfurize, etc. of molten steel. When performing RH or VD vacuum degassing, the treatment time under high vacuum shall not be less than 30 minutes, and can be 30 minutes, 35 minutes, 40 minutes, etc.

[0054] Specifically, in step (1), the content of element S in the molten steel from the electric furnace / converter is controlled to be less than 0.003%.

[0055] Specifically, in step (1), the casting is carried out under a protective atmosphere, which can be argon or helium.

[0056] During the smelting process, the content of impurity elements is controlled, and during the casting process, an atmosphere protection is used, and the molten pool is cooled rapidly to obtain a high-purity and high-uniformity billet.

[0057] It should be noted that, in order to reduce the deformation resistance of the billet during subsequent processing and to facilitate subsequent rolling and other processes, the present invention first performs homogenization heat treatment on the billet by heating and holding it at a certain temperature.

[0058] Specifically, in step (2), the heating and heat preservation temperature is 1250-1300℃, which can be 1250℃, 1270℃, 1290℃, or 1300℃. If the heating temperature is too low, it is easy to cause excessive deformation resistance during rolling, affecting the rolling process and deformation force; if the heating temperature is too high, it is easy to cause excessive growth of the steel structure.

[0059] Specifically, in step (2), the heating and holding time is 1.5-2.0 min / mm. The holding time can ensure the uniformity of temperature, composition, and solubility of the continuous casting billet, and avoid the uneven internal temperature and large composition segregation of the continuous casting billet caused by the holding time being too short, and the abnormally coarse structure of the continuous casting billet caused by the holding time being too long.

[0060] Specifically, in step (2), the rolling process includes a roughing stage and a finishing stage, which can greatly improve product quality, production flexibility and uniformity of structure. By controlling the rolling process, the continuous casting billet undergoes dynamic recrystallization, which can ensure that the size of most of the precipitated strengthening phases in the obtained steel plate is 10~30nm, and ensure a sufficient number of precipitated phases, reduce the probability of large-sized precipitated phases, and ensure the grain refinement effect.

[0061] More specifically, the temperature of the rough rolling stage is 1070-1220℃, and it is divided into 3 to 5 passes with a total compression ratio of 60-70%. By controlling the rough rolling process conditions, the precipitation of precipitates is effectively controlled, and the original coarse dendrite structure of the slab is broken up by a large amount of deformation.

[0062] More specifically, the initial rolling temperature of the finishing rolling stage is controlled at 960-1000℃, the final rolling temperature is controlled at 820-880℃, the rolling is carried out in 5-9 passes, and the total compression ratio is 30-40% in order to control the particle size of the precipitated phase.

[0063] It should be noted that, in order to ensure that the rolling compression ratio and the internal structure of the steel are fully broken and welded, the present invention needs to control the slab thickness at the roughing mill exit, i.e. the finishing mill entrance.

[0064] Specifically, in step (2), the thickness of the slab at the roughing mill exit, i.e. the finishing mill entrance, should be ≥ 5 times the target slab thickness to facilitate the refinement of the steel structure.

[0065] It should be noted that in order to obtain finely dispersed carbide precipitates, the present invention requires limiting the rolling temperature of the finished steel plate.

[0066] Specifically, in step (2), the rolled steel plate is cooled to a temperature range of 540~600℃ and then coiled, so that the steel coil is slowly cooled to room temperature.

[0067] According to some preferred embodiments of the present invention, the temperature at which the steel plate is rolled can be any value within the temperature range of 540~600℃, such as 540℃, 560℃, 580℃, and 600℃. If the rolling temperature is too high, it will lead to an increase in the size of the precipitated phase and reduce the refining effect. If the rolling temperature is too low, it will lead to incomplete precipitation of the precipitated phase and insufficient refining effect.

[0068] It should be noted that, in order to enable the steel plate to obtain a unique microstructure and excellent properties, the present invention employs an ultra-fast cold quenching process to heat treat the rolled steel plate.

[0069] Specifically, in step (3), the quenching process is as follows: at a temperature of 840-880℃, maintain for 2.0-3.0 min / mm, then cool at a rate of ≥100℃ / s to 50-100℃ below the martensitic transformation termination temperature (Mf point), and then air cool to room temperature.

[0070] More specifically, the quenching temperature can be any value within the temperature range of 840-880℃, such as 840℃, 850℃, 860℃, 870℃, or 880℃. If the temperature is too high, it will cause micro-grain growth, and if the temperature is too low, the elements will not dissolve sufficiently and the structure will not be completely homogenized.

[0071] More specifically, the heating and holding time can be any value within the range of 2.0-3.0 min / mm, such as 2.0 min / mm, 2.2 min / mm, 2.4 min / mm, 2.6 min / mm, 2.8 min / mm, and 3.0 min / mm. If the holding time is too short, the core will not transform and the hardness will be insufficient; if the holding time is too long, the precipitated phase will dissolve back, the grains will grow excessively, decarburization will be severe, and the toughness will decrease.

[0072] More specifically, the steel should first be cooled at a rate of ≥100℃ / s, preferably 500-1000℃ / s, to 50-100℃ below the martensitic transformation termination temperature (Mf point), followed by air cooling to room temperature. If the cooling rate is too low, the martensitic transformation of the steel plate will be incomplete, resulting in insufficient strength. If the cooling rate is too high, the steel plate will crack due to excessive quenching stress. Simultaneously, the cooling termination temperature should be controlled to be 50-100℃ below the Mf point. If the termination temperature is too high, the martensitic transformation will be incomplete, resulting in increased retained austenite and insufficient strength; if the termination temperature is too low, the quenching internal stress of the steel plate will be too high, easily leading to cracking.

[0073] It should be noted that, in order to remove the residual stress in the quenched steel plate, the present invention requires tempering the quenched steel plate.

[0074] Specifically, in step (3), the tempering process involves holding the quenched steel plate at a temperature of 150-180℃ for 60-120 minutes.

[0075] This invention involves sequentially smelting, ladle refining, vacuum degassing, and protective atmosphere casting of steel plates according to their target composition. This achieves homogenization of the steel plate structure and harmlessness of inclusions at a lower cost, resulting in a high-cleanliness and high-uniformity billet. The steel plate obtained after rolling the billet undergoes ultra-fast cold quenching treatment with controlled final cooling temperature to promote dislocation growth in the steel, refine the martensitic substructure, and thus improve strength and toughness, achieving a strengthening heat treatment for the steel plate.

[0076] Compared with existing ultra-high strength steel plates, the ultra-high strength steel obtained by the method of this invention can combine ultra-high strength and high toughness. At room temperature, the tensile strength is ≥2300MPa (e.g., 2310-2430MPa), the yield strength is ≥1500MPa (e.g., 1590-1680MPa), and the elongation after fracture is ≥10% (e.g., 11.0-12.5%). The low-temperature impact absorption energy KV2 (-40℃) is ≥15J (e.g., 18-23J), and the carbon equivalent CE is ≤0.70%. It has excellent weldability. At the same time, the Ni, Mo, and Nb contents in the material composition are relatively low, which significantly reduces the overall cost compared with high alloy ultra-high strength steel, which is conducive to its widespread application and can meet the lightweight design and manufacturing requirements of vehicles and equipment.

[0077] The technical solution of the present invention will be further explained and illustrated below with specific embodiments and comparative examples.

[0078] Examples 1-4 A method for preparing low-cost weldable ultra-high strength steel plates specifically includes the following steps: (1) According to the target composition content of the steel plate, smelting, ladle refining and vacuum degassing are carried out in sequence to obtain a billet, wherein the central degassing time is 40 min and the casting is a 200 mm thick billet; (2) The billet obtained in step (1) is subjected to heating and heat preservation, rolling, quenching and tempering treatment in sequence. The heating and heat preservation temperature is 1250-1300℃ and the time is 5h; the rough rolling temperature is 1070-1220℃, divided into 5 passes, and the total compression ratio is 60-70%; the thickness of the steel plate at the rough rolling exit is 60-80mm; the initial rolling temperature of the finishing rolling stage is 960-1000℃; the final rolling temperature is 820-880℃, divided into 7 passes, and the total compression ratio is 30-40%, to obtain a plate with a thickness of 6mm; the plate after finishing rolling is cooled to 550~600℃ for coiling and then slowly cooled to room temperature; After the steel coil is leveled, it is heated and held at 860℃±10℃ for 15 minutes. The slab after exiting the heating furnace is then subjected to ultra-rapid cooling with high-pressure water at a cooling rate of 500-1000℃ / s to 140℃ (approximately 70℃ below the Mf point), followed by air cooling to room temperature. Then, it is tempered: held at 170℃ for 40 minutes to obtain the steel plate.

[0079] Comparative Examples 1-4 The same process steps as in the example were used, except that the process parameters in the steel plate preparation process were changed, namely, the alloy composition, rough rolling process parameters, coiling parameters and quenching cooling rate were changed to obtain the steel plate.

[0080] The chemical composition of the steel plates obtained in Examples 1-4 and Comparative Examples 1-4 is shown in Table 1, the preparation process parameters are shown in Table 2, and the performance parameters of the steel plates are shown in Table 3.

[0081] Table 1. Chemical composition of the steel plates obtained in the examples and comparative examples.

[0082] Table 2. Process parameters for preparing steel plates in the examples and comparative examples.

[0083] Table 3. Properties of the steel plates obtained in the examples and comparative examples.

[0084] Table 1-3 and Figure 2 It can be seen that the steel plate of the present invention, by controlling the types and proportions of each constituent element and coordinating with the preparation process conditions, enables the microstructure of the steel plate to have a martensitic matrix, thereby improving the strength of the steel plate; at the same time, the precipitation of MC-type carbide (such as TiC, NbC, (Ti,Mo)C) reinforcing phases refines the martensitic grains and improves the strength and toughness of the steel; by reducing the content of elements Mn, Cr, Mo, and Ni, the carbon equivalent CE is made ≤0.70%, improving the weldability of the steel plate, while significantly reducing the overall cost. The steel plate has a strength greater than 2300MPa, while also possessing excellent toughness, weldability, and low cost, thereby promoting the widespread application of the high-strength steel.

[0085] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-cost weldable ultra-high strength steel plate, characterized in that, The steel plate comprises the following elements by mass percentage: C 0.42-0.47%, Si 0.20-0.50%, Mn ≤0.20%, P ≤0.010%, S ≤0.003%, Cr ≤0.50%, Ni 0.80-1.20%, Mo 0.20-0.40%, Ti 0.02-0.06%, Nb ≤0.04%, N ≤0.0040%, B 0.001-0.004%, with the balance being Fe.

2. The steel plate according to claim 1, characterized in that, The steel plate comprises the following elements by mass percentage: C 0.43-0.47%, Si 0.22-0.38%, Mn ≤0.19%, P ≤0.009%, S ≤0.002%, Cr ≤0.35%, Ni 0.80-1.20%, Mo 0.22-0.35%, Ti 0.025-0.058%, Nb ≤0.04%, N ≤0.0040%, B 0.0015-0.0031%, with the balance being Fe.

3. The steel plate according to claim 1, characterized in that, The contents of the elements Ti, Nb, and N in the steel plate satisfy the following relationship: 0.04%≤Ti+Nb-3.5N≤0.07%.

4. The steel plate according to claim 1, characterized in that, The original austenite grain size of the microstructure of the steel plate is ≤5.5μm.

5. The steel plate according to claim 1, characterized in that, The microstructure of the steel plate includes a martensitic matrix, retained austenite, and precipitated strengthening phases; wherein the precipitated strengthening phases include nanoscale TiC, NbC, (Ti, Mo)C, and (Nb, Mo)C precipitates.

6. A method for preparing a low-cost weldable ultra-high strength steel plate according to any one of claims 1-5, characterized in that, The method includes: (1) Based on the target composition content of the steel plate, the raw materials are smelted, refined in a ladle, degassed in a vacuum and cast in sequence to obtain a billet; (2) The cast billet is heated, rolled, coiled and cooled, and leveled to obtain a rolled slab; (3) The slab is subjected to quenching and tempering treatment.

7. The method according to claim 6, characterized in that, In step (2), the heating and heat preservation temperature is 1250-1300℃ and the time is 1.5-2.0 min / mm.

8. The method according to claim 6, characterized in that, In step (2), the rolling process includes a roughing stage and a finishing stage. The temperature of the roughing stage is 1070-1220℃ and the total compression ratio is 60-70%. The initial rolling temperature of the finishing stage is 960-1000℃ and the final rolling temperature is 820-880℃, with a total compression ratio of 30-40%.

9. The method according to claim 6, characterized in that, In step (3), the specific quenching process is as follows: at a temperature of 840-880℃, maintain for 2.0-3.0 min / mm, then cool at a rate of ≥100℃ / s to 50-100℃ below the Mf point, and then air cool to room temperature.

10. The method according to claim 6, characterized in that, In step (3), the tempering temperature is 150-180℃ and the holding time is 5~10 min / mm.

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