High-forming ultrathin-specification 960MPa-grade non-quenched and tempered high-strength steel and production method thereof

Abstract

The invention provides high-forming ultrathin-specification 960MPa-level non-quenched and tempered high-strength steel and a production method, and relates to the technical field of ferrous metallurgy. The non-quenched and tempered high-strength steel is obtained through specific chemical components and mechanical properties, the problems of plate shape defects and low production efficiency caused by quenching and tempering in the prior art are solved, and the non-quenched and tempered high-strength steel has the advantages that the production process is simplified, the production cycle and energy consumption are reduced, the plate shape defects are avoided, and the forming performance and the surface quality are improved.

Description

Technical Field

[0001] This application relates to the field of iron and steel metallurgy technology, and in particular to a high-strength, non-quenched and tempered 960MPa grade steel with high formability and ultra-thin specifications, and its production method. Background Technology

[0002] Currently, 960MPa high-strength steel for construction machinery is widely used in key components such as the boom or jib of truck cranes, loaders, and aerial work platforms. These products face extremely harsh environments and complex stress states in actual use, requiring the steel to possess high strength, good plastic deformation capacity, and excellent surface quality simultaneously. Traditional production processes generally employ a quenching and tempering process after hot rolling. This process involves multiple steps, including heating, quenching, and tempering, significantly extending the overall production cycle, greatly increasing energy consumption, and keeping production costs high. Especially for ultra-thin steel with a thickness of no more than 2 mm, the quenching and tempering process easily induces shape defects during cooling and tempering, such as edge waviness, center waviness, or localized warping. These defects cannot be eliminated by conventional straightening methods, directly resulting in products that cannot meet the requirements of precision assembly and high-intensity operation. With the increasing competition in the global steel industry, construction machinery manufacturers are increasingly demanding lightweight and high-performance materials, and the shape stability of ultra-thin high-strength steel has become a key bottleneck restricting product market competitiveness. Existing technologies have significant shortcomings in simplifying the process while ensuring the mechanical and formability properties of ultra-thin steel, and there is an urgent need to develop new production technologies to overcome this predicament.

[0003] To address the aforementioned issues, existing technologies urgently need improvement. Summary of the Invention

[0004] The purpose of this application is to provide a high-strength, ultra-thin 960MPa grade non-quenched and tempered steel and its production method, which has the advantages of simplifying the production process, reducing the production cycle and energy consumption, avoiding plate defects, and improving forming performance and surface quality.

[0005] This application provides a high-strength, ultra-thin 960MPa grade non-quenched and tempered steel with a high formability, as follows: The chemical composition by weight percentage is as follows: C: 0.085~0.100%, Si: 0.15~0.25%, Mn: 1.75~1.85%, P≤0.018%, S≤0.0015%, Mo: 0.22~0.30%, V: 0.01~0.02%, Ti: 0.15~0.17%, Cr: 0.22~0.30%, Nb: 0.052~0.060%, N≤0.005%, Als: 0.01~0.06%, with the balance being Fe and unavoidable impurities; the mechanical properties meet the following requirements: yield strength ≥960MPa, tensile strength ≥1000MPa, elongation after fracture A80≥5%, and 180° bending test D=2a, where D is the diameter of the bending indenter and a is the thickness of the steel without cracks.

[0006] Furthermore, this application also proposes the following steps: (1) Steelmaking and continuous casting: smelt steel according to the above chemical composition and continuously cast it into slabs with a thickness of 230-240 mm; (2) Heating: After the slab is cooled for ≥24 hours, it is sent into the heating furnace. The holding temperature is 1260~1290℃, the holding time is 20~100min, the total time in the furnace is 180~400min, and the furnace exit temperature is 1260~1290℃. (3) Rolling: Rough rolling and finish rolling are performed in sequence. After rough rolling, a 36-59 mm thick intermediate billet is obtained. After finish rolling, a combination of ultra-fast cooling and layer cooling is used for cooling. (4) Pickling: Remove oxides from the surface of the strip steel, followed by rinsing and drying; (5) Single-stand rolling: Rolled in 3 passes to a thickness ≤2mm; (6) Heat treatment: After tempering, the single board is air-cooled to room temperature.

[0007] Furthermore, this application also proposes that the Ti content and N content in the chemical composition satisfy Ti / N≥30.

[0008] Furthermore, this application also proposes that the roughing in step (3) is carried out in 5 to 7 passes, with an initial rolling temperature of 1260 to 1290°C and a final cooling temperature of 1060 to 1100°C.

[0009] Furthermore, this application also proposes that in step (3), the finishing rolling adopts a 7-stand continuous rolling mill, the inlet descaling pressure is ≥18MPa, the initial rolling temperature is 1000~1080℃, the final pass reduction rate is ≥10%, and the final cooling temperature is 865~895℃.

[0010] Furthermore, this application also proposes that the specific cooling parameters in step (3) are: cooling rate of 80-200℃ / s in the ultra-fast cooling stage, cooling rate of ≥15℃ / s in the layer cooling stage, and coiling of the strip after cooling to 550-580℃.

[0011] Furthermore, this application also proposes that the pickling and post-treatment parameters in step (4) are: acid concentration 70~240g / L, pickling temperature 75~85℃, pickling speed ≤40m / min; rinsing temperature ≥50℃, pickling tank conductivity ≤650μs / cm, and drying hot air temperature ≥110℃.

[0012] Furthermore, this application also proposes that the thickness of the raw material rolled by a single stand in step (5) is 2.5 to 3 mm, the reduction rate per pass is 10 to 20%, the front tension is 4 to 12 KN / mm, and the back tension is 10 to 20 KN / mm.

[0013] Furthermore, this application also proposes that the tempering temperature in step (6) is 600-650℃ and the tempering time is 25-35min, and the tempering temperature and time satisfy the following: when the tempering temperature is 600-620℃, the tempering time is 32-35min; when the tempering temperature is 630-650℃, the tempering time is 25-30min.

[0014] Furthermore, this application also proposes that the ratio of Mo content to Cr content in the chemical composition is 1.0 to 1.36.

[0015] As can be seen from the above, the present application provides a high-formability, ultra-thin 960MPa grade non-quenched and tempered high-strength steel and its production method. Through specific chemical composition and mechanical properties, non-quenched and tempered high-strength steel is achieved, which solves the problems of plate shape defects and low production efficiency caused by quenching and tempering in the prior art. It has the advantages of simplifying the production process, reducing the production cycle and energy consumption, avoiding plate shape defects, and improving forming performance and surface quality. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the production method of the high-strength, ultra-thin 960MPa grade non-quenched and tempered high-strength steel of this application. Detailed Implementation

[0017] The technical solutions of this application will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are merely some embodiments of this application, and not all embodiments. The components of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0018] It should be noted that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. Furthermore, in the description of this application, terms such as "first," "second," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0019] Traditionally, 960MPa grade high-strength engineering machinery steel is produced using a quenching and tempering process, which involves many steps and high energy consumption. For ultra-thin steel with a thickness of 2mm or less, the traditional quenching and tempering method makes it difficult to control the sheet shape, fails to meet customers' high requirements for mechanical and formability properties, and is detrimental to improving market competitiveness.

[0020] To address this issue, this application proposes a high-strength, ultra-thin 960MPa grade non-quenched and tempered high-strength steel. This high-strength steel, through precise control of its chemical composition, achieves high yield strength, high tensile strength, good elongation after fracture, and excellent 180° bending performance even in its non-quenched and tempered state. This approach aims to overcome the limitations of traditional quenching and tempering processes, providing an ultra-thin high-strength steel that combines high strength, high formability, and good sheet shape.

[0021] For ease of understanding, the following explains some key terms in this embodiment: High-strength steel with high formability and ultra-thin specifications of 960MPa grade non-quenched and tempered refers to a type of steel that, without undergoing traditional quenching and tempering heat treatment processes, can achieve a yield strength of not less than 960MPa through optimized alloy design and rolling processes, and has good cold formability and a thickness of less than or equal to 2mm.

[0022] Chemical composition refers to the content of various alloying and impurity elements in steel, usually expressed as a weight percentage. The types and contents of these elements have a decisive influence on the microstructure and final properties of steel.

[0023] Mechanical properties refer to the properties exhibited by steel under different loads, including strength, plasticity, and toughness. These properties are important indicators for evaluating the suitability of steel. Yield strength refers to the stress value of steel when it undergoes plastic deformation during tensile testing and no further stress is added. This indicator reflects the steel's ability to resist plastic deformation.

[0024] Tensile strength refers to the maximum stress that steel can withstand in a tensile test. This indicator reflects the steel's ability to resist fracture.

[0025] Elongation after fracture (A80) refers to the ratio of the increase in gauge length to the original gauge length after tensile fracture of a steel specimen, expressed as a percentage. A80 indicates a gauge length of 80 mm. This indicator reflects the plastic deformation capacity of the steel.

[0026] The 180° bending test (D=2a) involves bending a steel sample 180 degrees with a bending indenter diameter D twice the steel thickness a, and observing whether cracks appear in the bent area. This test is used to evaluate the cold forming properties and toughness of steel.

[0027] The high-strength, ultra-thin 960MPa grade non-quenched and tempered steel provided in this embodiment is designed to achieve a combination of high strength and good formability through precise alloy element proportioning, without relying on traditional quenching and tempering processes. The steel can be produced using conventional smelting, continuous casting, hot rolling, and subsequent cold working processes. For example, by controlling rolling and cooling parameters, the steel can directly obtain the required microstructure and properties after hot rolling, thus avoiding additional quenching and tempering processes.

[0028] Specifically, the chemical composition of this high-strength steel, by weight percentage, is as follows: Carbon (C) content is typically in the range of 0.085% to 0.100%, serving to provide solid solution strengthening and refine grain size. Silicon (Si) content is typically in the range of 0.15% to 0.25%, acting as a deoxidizer and contributing to solid solution strengthening. Manganese (Mn) content is typically in the range of 1.75% to 1.85%, used to improve the steel's strength and hardenability, and to refine grain size. Phosphorus (P) content is controlled to be no higher than 0.018%, and sulfur (S) content is controlled to be no higher than 0.0015%, to minimize the adverse effects of harmful impurities on the steel's toughness and plasticity. Furthermore, the molybdenum (Mo) content is typically in the range of 0.22% to 0.30%, the vanadium (V) content is typically in the range of 0.01% to 0.02%, the titanium (Ti) content is typically in the range of 0.15% to 0.17%, the chromium (Cr) content is typically in the range of 0.22% to 0.30%, and the niobium (Nb) content is typically in the range of 0.052% to 0.060%. These alloying elements are used to form carbides or nitrides, thereby achieving precipitation strengthening and grain refinement, while improving the strength and toughness of the steel. The nitrogen (N) content is controlled to be no higher than 0.005% to avoid the formation of coarse nitrides that negatively impact performance. The acid-soluble aluminum (Als) content is typically in the range of 0.01% to 0.06%, acting as a deoxidizer and grain refiner. The balance is iron (Fe) and unavoidable impurities from the production process. Therefore, the precise proportions and synergistic effects of these elements ensure that the steel can achieve the expected comprehensive performance in its non-quenched and tempered state.

[0029] The mechanical properties of this high-strength steel are set to meet specific high standards. The yield strength is required to be no less than 960 MPa, and the tensile strength is required to be no less than 1000 MPa, indicating that the steel has excellent load-bearing capacity. The elongation after fracture (A80) is required to be no less than 5%, reflecting that the steel retains a certain degree of plastic deformation capacity while maintaining high strength, which is crucial for cold forming applications. Furthermore, the steel must pass a 180° bending test with D=2a without cracking, where D is the diameter of the bending indenter and a is the thickness of the steel. This bending test is a key indicator for evaluating the cold forming performance of the steel, requiring no cracks to appear under harsh bending conditions, ensuring the reliability of the steel in practical applications. For example, during bending, conventional bending equipment can be used, and by adjusting the bending die and pressure, macroscopic cracks can be prevented from forming in the steel plate during the bending process, thus meeting the forming requirements.

[0030] The high-strength, ultra-thin 960MPa grade non-quenched and tempered steel provided in this embodiment achieves a combination of high strength and high formability by optimizing the alloy element ratio without employing traditional quenching and tempering processes. This steel not only possesses a yield strength of no less than 960MPa and a tensile strength of no less than 1000MPa, but also maintains good plasticity, with an elongation after fracture (A80) of no less than 5%, and can successfully pass a 180° bending test (D=2a) without cracking. Therefore, this solution effectively solves the problems of numerous steps, high energy consumption, and difficulty in controlling the shape of ultra-thin steel sheets in traditional quenching and tempering processes, providing the engineering machinery field with a high-performance, high-efficiency, and lower-cost solution for ultra-thin high-strength steel.

[0031] In the production of high-strength, ultra-thin 960MPa grade non-quenched and tempered high-strength steel, a key technical challenge is to precisely control the microstructure of the steel without traditional quenching and tempering processes, thereby simultaneously achieving high strength, excellent formability, and ultra-thin dimensions. Traditional production processes often struggle to effectively control the microstructure uniformity and surface quality of ultra-thin strips while ensuring the steel's mechanical properties, easily leading to performance fluctuations, low production efficiency, or failure to meet stringent mechanical performance requirements.

[0032] In response, this application proposes a method for producing high-strength, ultra-thin 960MPa grade non-quenched and tempered high-strength steel with high formability, such as... Figure 1 As shown, the method includes the following steps: Step (1): Steelmaking and continuous casting are carried out. Molten steel is smelted according to the chemical composition and continuously cast into slabs with a thickness of 230-240 mm.

[0033] Step (2): Heating is carried out. After the slab has been cooled for at least 24 hours, it is sent into the heating furnace and kept at a holding temperature of 1260-1290℃ for 20-100 minutes. The total time in the furnace is 180-400 minutes, and the furnace exit temperature is controlled at 1260-1290℃.

[0034] Step (3): Rolling is carried out, and rough rolling and finish rolling are completed in sequence. After rough rolling, an intermediate billet with a thickness of 36-59mm is obtained. After finish rolling, ultra-fast cooling and layer cooling are combined for cooling.

[0035] Step (4): Pickling is performed to remove oxides from the surface of the strip steel, followed by rinsing and drying.

[0036] Step (5): Perform single-stand rolling, and reduce the strip thickness to 2 mm or less through 3 rolling passes.

[0037] Step (6) involves heat treatment, where the single board is tempered and then air-cooled to room temperature.

[0038] Specifically, in the steelmaking and continuous casting processes, the precise control of the content of alloying elements such as carbon, silicon, manganese, molybdenum, vanadium, titanium, chromium, and niobium in the molten steel, along with strict control of harmful impurities such as phosphorus, sulfur, and nitrogen, ensures that the steel possesses the foundation for achieving high strength and good toughness. Continuous casting produces slabs 230–240 mm thick, providing suitable initial dimensions and internal structure for subsequent rolling. In the heating process, after sufficient heat treatment, the slab is fed into a heating furnace and held at a temperature range of 1260–1290°C for an extended period. This aims to fully austenitize the internal structure of the slab, completely dissolve carbides, and eliminate inhomogeneities in the as-cast structure, providing uniform and fine austenite grains for subsequent rolling while preventing excessive grain coarsening or overheating.

[0039] In the rolling process, the roughing stage mainly involves plastic deformation of the slab through a large reduction, effectively breaking down and refining the as-cast microstructure, and forming an intermediate slab with a thickness of 36–59 mm. The finishing stage further thins the strip to near its final thickness, and further refines the austenite grains by precisely controlling the rolling temperature and reduction rate. The combination of ultra-rapid cooling and laminar cooling after finishing rolling is key to achieving high strength without tempering. Ultra-rapid cooling can rapidly reduce the strip temperature, effectively suppressing the precipitation of proeutectoid ferrite and creating conditions for subsequent bainitic or martensitic phase transformations. Laminar cooling, based on ultra-rapid cooling, further precisely controls the cooling rate to ensure that the strip obtains a uniform and stable bainitic or martensitic microstructure before coiling, thereby achieving high strength without traditional tempering treatment.

[0040] The pickling step aims to thoroughly remove the iron oxide scale formed on the strip surface during rolling, ensuring a clean strip surface and preventing the scale from being pressed into the steel during subsequent processing, thus affecting product quality and surface finish. Rinsing and drying further ensure the cleanliness and dryness of the strip surface. The single-stand rolling step, through three precise passes, further reduces the strip thickness to 2mm or less, achieving ultra-thin specifications. Simultaneously, moderate work hardening is introduced through cold deformation, further enhancing the steel's strength. The final heat treatment step, single-plate tempering, aims to eliminate internal stresses introduced during single-stand rolling, restore and optimize the steel's ductility and toughness, and stabilize the microstructure. This significantly improves the steel's formability and elongation at fracture while maintaining high strength. After tempering, air cooling to room temperature avoids additional stress or microstructural inhomogeneity that might result from rapid cooling.

[0041] Through the above technical solution, this application effectively solves the production challenge of high-strength steel with ultra-thin 960MPa specifications without traditional quenching and tempering treatment. Specifically, the precisely controlled steelmaking continuous casting process ensures the accuracy of chemical composition; the optimized heating process provides a uniform austenitic structure for subsequent rolling; the synergistic effect of roughing and finishing rolling during the rolling process, especially the combination of ultra-fast cooling and layer cooling, effectively refines the grains and induces the formation of the required bainite or martensite non-quenched and tempered structures, thus laying the foundation for high strength in the hot rolling stage. The subsequent pickling step ensures the surface quality of the strip and avoids the generation of defects. Single-stand rolling achieves precise control of ultra-thin specifications and introduces appropriate work hardening. The final heat treatment (single-plate tempering) step effectively eliminates internal stress without significantly reducing strength, restores and optimizes the plasticity and toughness of the steel, and ensures that the final product has excellent yield strength, tensile strength, elongation after fracture, and good 180° bending performance. This holistic and collaborative process enables the production of ultra-thin high-strength steel to maintain high strength while also possessing excellent formability and dimensional accuracy, significantly improving product quality and production efficiency.

[0042] The high-strength steel is designed to achieve excellent mechanical properties, including high strength and good formability, through a specific chemical composition. However, in achieving these properties, if the interaction between certain microalloying elements, particularly titanium (Ti) and nitrogen (N), is not precisely controlled, precipitation behavior may be unsatisfactory. This can result in insufficient grain refinement or inadequate precipitation strengthening, thus hindering the full realization of the desired high strength and formability, especially for ultra-thin applications.

[0043] To this end, this application further proposes that the Ti / N content in the chemical composition of the high-strength steel meets the requirement of Ti / N ≥ 30. This technical feature aims to optimize the precipitation behavior of titanium (Ti) and nitrogen (N) elements in steel by precisely controlling the ratio of these elements. Titanium and nitrogen mainly form stable carbonitrides in steel, especially nitrides (TiN), which play an important role in the solidification and hot working of steel. When the Ti / N ratio is ≥ 30, sufficient titanium can be ensured in the steel to fully fix nitrogen, forming fine, dispersed TiN particles. These TiN particles can effectively pin austenite grain boundaries, inhibit grain growth, and thus achieve grain refinement. Simultaneously, the fine TiN precipitates can also provide precipitation strengthening during subsequent heat treatment. By precisely controlling the Ti / N ratio, the synergistic effect of titanium and nitrogen in grain refinement and precipitation strengthening can be maximized, laying the microstructural foundation for obtaining ultra-thin steel with high strength and good toughness.

[0044] By controlling the Ti and N content in the chemical composition within the range of Ti / N ≥ 30, this application effectively promotes the formation of stable and dispersed TiN precipitates from titanium and nitrogen elements in steel. These fine TiN particles strongly inhibit the growth of austenite grains during the austenitization process of steel, thereby obtaining even finer austenite grains. During subsequent cooling and phase transformation, the fine austenite grains can transform into even finer ferrite or bainite structures, significantly improving the strength and toughness of the steel. In addition, these TiN precipitates can also play an effective precipitation strengthening role in the matrix, further improving the yield strength and tensile strength of the steel. This precise control of the Ti / N ratio ensures the optimal utilization efficiency of microalloying elements and avoids performance fluctuations that may be caused by insufficient fixation of nitrogen elements. As a result, high-strength, ultra-thin 960MPa grade non-quenched and tempered steel maintains high strength while still possessing excellent plasticity and bending forming ability, meeting stringent application requirements.

[0045] In some embodiments described above in this application, high-strength steel is rolled to obtain the desired intermediate billet. However, improper parameter control during the rough rolling process may lead to uneven internal structure of the steel and difficulty in effectively refining the grain size, thereby affecting the mechanical properties of subsequent finish rolling and the final product. This is especially critical when pursuing high formability and ultra-thin specifications.

[0046] In this regard, this application further proposes that in the above rolling step (3), the rough rolling process adopts 5 to 7 passes, the initial rolling temperature is controlled at 1260 to 1290℃, and the final cooling temperature is controlled at 1060 to 1100℃.

[0047] Specifically, roughing is the initial stage of hot rolling, and its main purpose is to break up the as-cast structure, refine the grains, and reduce the slab thickness to the size of an intermediate slab through large deformation. Using 5 to 7 passes means that sufficient and appropriate deformation is applied in the roughing stage. Increasing the number of rolling passes allows for more uniform deformation within the steel while ensuring that the reduction in each pass is not excessive. This promotes more complete dynamic recrystallization, effectively refines the grains, and completely eliminates macroscopic defects in the as-cast structure. This lays the microstructural foundation for subsequent finishing rolling and the superior performance of the final product. This range of passes can be achieved by using a reversible roughing mill or a multi-stand roughing mill, ensuring that the steel remains in an ideal state throughout the deformation process by precisely controlling the reduction and rolling speed of each pass.

[0048] The initial rolling temperature is set between 1260 and 1290°C to ensure the slab is in a fully austenitic state when entering the roughing mill. Within this high-temperature range, the steel exhibits good plasticity and low deformation resistance, which is beneficial for large reduction rolling. Simultaneously, the high initial rolling temperature facilitates the complete solution and homogenization of alloying elements and provides sufficient driving force for subsequent dynamic recrystallization, thereby achieving effective grain refinement during rolling. By precisely controlling the holding temperature and time in the furnace and optimizing the slab's transport path from the furnace to the mill, it can be ensured that the slab reaches this temperature range at the start of roughing. Typically, an infrared thermometer is used to monitor the slab surface temperature in real time to ensure it meets the initial rolling temperature requirements.

[0049] The final cooling temperature is controlled at 1060–1100℃, referring to the temperature of the intermediate billet when it leaves the roughing mill. Maintaining the final cooling temperature in this relatively high but controlled austenitic region ensures that the steel retains fine and uniform austenitic grains after roughing and has sufficient energy reserves to facilitate further grain refinement and microstructure homogenization in the subsequent finishing rolling stage. This temperature range avoids phase transformation caused by premature cooling and prevents excessive grain growth that may result from excessively high temperatures, thus providing a good microstructure prerequisite for the high strength and high toughness of the final product. This temperature control can be achieved by adjusting the rolling speed, the dwell time between passes, and, if necessary, by using auxiliary cooling measures.

[0050] By precisely controlling the number of roughing passes, initial rolling temperature, and final cooling temperature, the microstructure of the steel can be significantly optimized. A high initial rolling temperature and an appropriate number of rolling passes ensure sufficient fragmentation of the as-cast structure and effective grain refinement, while a controlled final cooling temperature guarantees that the intermediate billet has uniform and fine austenite grains before entering the finish rolling mill. This refined roughing process allows the steel to better form ultra-thin products with high strength, high toughness, and excellent formability in subsequent processing, effectively solving the problems of uneven microstructure and decreased mechanical properties caused by improper roughing control, thus ensuring the comprehensive performance of 960MPa grade non-quenched and tempered high-strength steel.

[0051] In the production of high-strength, ultra-thin 960MPa grade non-quenched and tempered steel, the rolling process is a crucial step in shaping the steel's microstructure and properties. Especially during finish rolling, improper control of rolling parameters can lead to uneven grain structure and large fluctuations in mechanical properties, making it difficult to simultaneously meet the requirements of high strength, high toughness, and ultra-thin dimensions, thus affecting the quality and stability of the final product.

[0052] In this regard, this application further proposes an optimization scheme for the finishing rolling process parameters in step (3) of the above production method, specifically including: the finishing rolling adopts 7-stand continuous rolling, the inlet descaling pressure is ≥18MPa, the initial rolling temperature is 1000~1080℃, the final pass reduction rate is ≥10%, and the final cooling temperature is 865~895℃.

[0053] Specifically, the finishing rolling process employs a 7-stand continuous rolling mill, meaning that the steel undergoes plastic deformation through seven consecutive rolling stands during the finishing stage. This multi-stand continuous rolling method ensures that the steel achieves sufficient and uniform deformation during the rolling process, effectively refining the grains and enabling precise control of the steel thickness, thus providing a foundation for obtaining ultra-thin steel products. The inlet descaling pressure is ≥18MPa, meaning that before the steel enters the finishing mill stand, the surface of the steel is descaled using high-pressure water at a pressure of not less than 18 MPa. High-pressure descaling efficiently and thoroughly removes the iron oxide scale formed during heating and rough rolling, preventing it from being pressed into the steel surface during finishing rolling, thereby effectively avoiding surface defects and ensuring the surface quality of the final product. The initial rolling temperature is 1000~1080℃, meaning that the temperature of the steel entering the first stand of the finishing mill is controlled between 1000℃ and 1080℃. This temperature range ensures that the steel exhibits good plasticity during finishing rolling, making it easy to deform. It also promotes dynamic recrystallization, effectively refining the grain structure and laying the foundation for excellent mechanical properties. A final pass reduction of ≥10% means that the reduction in steel thickness in the last finishing pass is no less than 10% of the thickness before entering that pass. A larger final pass reduction applies sufficient deformation to the steel, further promoting grain refinement and homogenization, contributing to the formation of an ideal microstructure, thereby improving the steel's strength and toughness. A final cooling temperature of 865–895℃ refers to the temperature of the steel leaving the last rolling stand after completing all finishing passes being controlled between 865℃ and 895℃. This temperature range ensures that the steel is in an austenitic state with a moderate grain size before entering the subsequent ultra-fast cooling and layer cooling combined cooling stages. This creates favorable conditions for precise control of phase transformation during subsequent cooling, thereby optimizing the final microstructure and mechanical properties.

[0054] Through the above technical solutions, the use of 7-stand continuous rolling mills enables precise control of steel thickness and uniform deformation, laying the foundation for obtaining ultra-thin products. High inlet descaling pressure effectively removes iron oxide scale, preventing surface defects and ensuring subsequent rolling quality. Precise control of the initial rolling temperature between 1000 and 1080℃ ensures good plasticity of the steel during finishing rolling, promotes dynamic recrystallization, and refines the grains. A final pass reduction rate of ≥10% further enhances the grain refinement effect and helps form a uniform microstructure. Final cooling temperature control between 865 and 895℃ ensures the steel is in its optimal microstructure before entering the cooling stage, creating favorable conditions for subsequent ultra-fast cooling and layer cooling combinations. This effectively controls the phase transformation process, ultimately yielding ultra-thin 960MPa grade non-quenched and tempered high-strength steel with high strength, high toughness, excellent formability, and good surface quality.

[0055] In some embodiments described above in this application, a method for producing high-strength, ultra-thin 960MPa grade non-quenched and tempered steel is proposed, wherein a combination of ultra-fast cooling and layer cooling is used for cooling after the rolling step (3). However, if the cooling parameters are not properly controlled, the internal structure of the steel may be uneven and the grains may be coarse, thereby affecting the mechanical properties of the final product. In particular, key indicators such as yield strength, tensile strength, and elongation after fracture may not meet the expected high strength and high toughness requirements, and it may be difficult to effectively control the phase transformation process to obtain the ideal microstructure.

[0056] In this regard, this application further proposes the following specific cooling parameters in step (3): cooling rate of 80-200℃ / s in the ultra-fast cooling stage, cooling rate of ≥15℃ / s in the layer cooling stage, and coiling of strip steel after cooling to 550-580℃.

[0057] Specifically, ultra-rapid cooling refers to the rapid cooling of strip steel immediately after hot rolling. Its purpose is to suppress high-temperature phase transformations, such as the formation of ferrite and pearlite, promote austenite grain refinement, and create favorable conditions for subsequent phase transformations, thereby obtaining a finer, harder microstructure. Controlling the cooling rate within the range of 80–200°C / s effectively avoids unwanted phase transformations, laying the foundation for subsequent microstructure formation. This cooling is typically achieved through high-pressure water jet or atomized cooling systems, with the cooling rate adjusted by precisely controlling water flow, pressure, nozzle design, and strip running speed.

[0058] Laminar cooling is a relatively gentle cooling stage that follows ultra-rapid cooling. Its purpose is to further precisely control the phase transformation kinetics, promoting the formation of specific microstructures, such as fine bainite or tempered martensite, while avoiding excessive thermal stress or cracking. The cooling rate is no less than 15°C / s, ensuring continuous control of the phase transformation process and contributing to the formation of a uniform and high-performance microstructure. This stage is typically achieved through a series of water curtains or spray systems, with the cooling rate controlled by adjusting the length of the cooling section, water flow rate, and strip speed.

[0059] Coiling temperature refers to the temperature at which strip steel is coiled. Cooling the strip steel to 550–580°C before coiling is crucial for the formation of the final microstructure and mechanical properties. Within this temperature range, the strip steel undergoes a degree of self-tempering or precipitation strengthening during slow cooling within the coil, thus optimizing the strength-toughness balance of the steel without additional tempering treatment. Simultaneously, this temperature range also helps avoid excessive hardening or softening and reduces residual stress. Achieving this temperature relies on precise control of the ultra-rapid cooling and laminar cooling processes, and real-time monitoring of the strip steel temperature using temperature sensors such as pyrometers.

[0060] By precisely controlling the cooling rate of the ultra-rapid cooling stage to 80–200℃ / s, the high-temperature phase transformation can be effectively suppressed, and austenite grains can be refined, laying the foundation for the subsequent formation of a high-strength microstructure. Subsequently, by controlling the cooling rate of the layer cooling stage to be no less than 15℃ / s, the phase transformation process is further guided, promoting the formation of fine and uniform bainite or tempered martensite microstructures, thereby significantly improving the toughness and plasticity of the steel. Finally, the strip steel is cooled to 550–580℃ before being coiled, allowing the steel to undergo sufficient self-tempering and precipitation strengthening within the coil, optimizing the match between strength and toughness, and effectively avoiding problems such as uneven microstructure and coarse grains caused by improper cooling. Overall, this precise control of cooling parameters ensures that the high-strength, ultra-thin 960MPa grade non-quenched and tempered steel can obtain an ideal microstructure, thereby enabling its mechanical properties such as yield strength, tensile strength and elongation after fracture to meet or exceed the expected targets. In particular, under the requirements of ultra-thin specifications and high formability, it can stably achieve a good combination of high strength and high toughness.

[0061] In the production of high-strength, ultra-thin 960MPa grade non-quenched and tempered steel, the removal of oxides from the strip surface is a critical step. Improper control of pickling process parameters can lead to incomplete oxide removal, affecting subsequent rolling and product surface quality; or it can cause over-corrosion, damaging the strip matrix and reducing material properties. Therefore, precise control of pickling and post-treatment parameters is essential to ensuring product quality.

[0062] In this regard, this application further proposes specific limitations on the pickling and post-treatment parameters in step (4) of the production method, including acid concentration of 70-240 g / L, pickling temperature of 75-85℃, pickling speed of ≤40 m / min; rinsing temperature of ≥50℃, conductivity of pickling tank of ≤650 μs / cm, and drying hot air temperature of ≥110℃.

[0063] Specifically, acid concentration is a key factor affecting pickling efficiency and corrosion severity. Within the range of 70–240 g / L, sufficient dissolution capacity for oxides on the strip surface is ensured, while avoiding excessive corrosion or hydrogen embrittlement risks caused by excessive acid concentration, thus guaranteeing the integrity of the base metal. Pickling temperature directly affects the chemical reactivity of the acid solution. Controlling the temperature between 75 and 85°C significantly increases the pickling reaction rate, accelerates oxide dissolution, and shortens pickling time, thereby improving production efficiency. Simultaneously, this temperature range also helps maintain acid stability, reduces acid mist generation, and minimizes equipment corrosion. Pickling speed determines the residence time of the strip in the pickling solution. Controlling the pickling speed to no more than 40 m / min ensures sufficient immersion time for the strip, allowing the acid to fully act on the oxide layer and achieve thorough oxide removal. Excessively fast speeds may lead to incomplete pickling, while excessively slow speeds will reduce production efficiency and may cause over-corrosion.

[0064] In addition, rinsing is a crucial step in removing residual acid and dissolved products from the strip surface after pickling. Maintaining the rinsing temperature at no less than 50°C effectively reduces the surface tension of water, enhances the rinsing effect, ensures the cleanliness of the strip surface, and avoids adverse effects of residual acid on subsequent processes and product performance. Higher temperatures also help accelerate the evaporation of moisture from the strip surface, creating favorable conditions for subsequent drying. The conductivity of the pickling tank is an important indicator of the purity of the rinsing water. Controlling the conductivity of the pickling tank to no higher than 650 μS / cm indicates a low content of impurity ions in the rinsing water, effectively preventing secondary contamination by the pickling solution and ensuring a high level of cleanliness on the strip surface after rinsing, thereby avoiding surface defects or corrosion caused by ion residue. Drying is a crucial step in removing moisture from the strip surface and preventing secondary oxidation. Controlling the drying hot air temperature to no less than 110°C rapidly evaporates moisture from the strip surface, ensuring the strip is completely dry before entering the next process, effectively preventing defects such as water stains and rust, and providing good surface conditions for subsequent single-stand rolling.

[0065] By precisely controlling the pickling and post-treatment parameters described above, this application effectively solves the problems of incomplete oxide removal or excessive corrosion of the strip steel that may occur during traditional pickling processes. After steelmaking, continuous casting, heating, and rolling, oxides form on the surface of the strip steel. By controlling the acid concentration, pickling temperature, and pickling speed within specific ranges, the acid solution can efficiently and uniformly dissolve the oxide layer, ensuring thorough removal of the oxides while avoiding damage to the base metal. Subsequently, by controlling the rinsing temperature and the conductivity of the pickling tank, residual acid and dissolution products on the strip steel surface can be thoroughly removed, preventing secondary contamination and surface defects. Finally, by setting the drying hot air temperature, moisture on the strip steel surface can be quickly and effectively removed, preventing water stains and rust, and providing ideal surface conditions for subsequent single-stand rolling. This refined pickling and post-treatment process significantly improves the surface quality of high-strength, ultra-thin 960MPa grade non-quenched and tempered high-strength steel, laying a solid foundation for its excellent mechanical and forming properties.

[0066] In some embodiments described above in this application, a method for producing high-strength, ultra-thin 960MPa grade non-quenched and tempered high-strength steel with high formability is proposed, which includes a single-stand rolling step to achieve ultra-thin specifications. However, during the single-stand rolling process, if there is a lack of precise control over the raw material thickness, reduction rate, and tension, it is very easy to cause problems such as uneven strip thickness, poor strip shape, surface defects, and even strip breakage, which seriously affects the mechanical properties and forming quality of the final product, making it difficult to stably produce ultra-thin high-strength steel that meets high standards.

[0067] In this regard, this application further proposes that in the above production method, the thickness of the raw material rolled by a single stand in step (5) is 2.5 to 3 mm, the reduction rate per pass is 10 to 20%, the front tension is 4 to 12 KN / mm, and the back tension is 10 to 20 KN / mm.

[0068] Specifically, this scheme limits the initial thickness of the strip entering the single-stand rolling process to 2.5–3 mm. Choosing an appropriate initial thickness is crucial for producing ultra-thin high-strength steel. If the raw material is too thick, more rolling passes or larger single-pass reductions are required, which not only increases energy consumption and roll wear but may also lead to material performance degradation or defects due to excessive cumulative deformation. Conversely, if the raw material is too thin, it may be difficult to effectively control the strip shape and thickness accuracy, and even tearing may occur during rolling. Controlling the raw material thickness within the range of 2.5–3 mm provides a stable and suitable starting condition for subsequent fine rolling, ensuring efficient rolling of the strip to the target ultra-thin thickness within a limited number of passes while maintaining good strip shape and surface quality.

[0069] Meanwhile, the scheme specifies that during single-stand rolling, the reduction in strip thickness per rolling pass should be 10-20%. For high-strength steel, which has greater resistance to deformation, excessive reduction can easily lead to severe work hardening, a sharp increase in rolling force, edge cracking, or strip breakage. Conversely, excessive reduction will increase the number of rolling passes and reduce production efficiency. Controlling the reduction rate per pass within the range of 10-20% can effectively control the amount of strip deformation and the degree of work hardening while ensuring rolling efficiency, avoiding defects caused by excessive deformation, and ensuring that the strip maintains good plasticity and toughness during rolling, laying the foundation for the high formability of the final product.

[0070] Furthermore, this scheme limits the tension during the rolling process: the initial tension is 4–12 kN / mm, and the subsequent tension is 10–20 kN / mm. Tension is a key process parameter for controlling strip shape, thickness accuracy, and preventing defects during strip rolling. The initial tension mainly acts before the strip enters the rolls, helping to smoothly introduce the strip into the roll gap and influencing the rolling force. The subsequent tension acts after the strip leaves the rolls, its main function being to keep the strip straight, eliminate shape defects (such as waviness and warping), and help improve rolling stability. By precisely controlling the initial tension within the range of 4–12 kN / mm and the subsequent tension within the range of 10–20 kN / mm, the transverse flow and longitudinal uneven deformation of the strip during rolling can be effectively suppressed, significantly improving the strip shape and thickness uniformity, while reducing the risk of surface defects such as folds and scratches during rolling, ensuring the dimensional accuracy and surface quality of ultra-thin high-strength steel.

[0071] By employing the aforementioned technical solutions, precise control of key parameters in the single-stand rolling process significantly improves the production stability and product quality of ultra-thin 960MPa grade non-quenched and tempered high-strength steel. Specifically, limiting the raw material thickness for single-stand rolling to 2.5–3 mm provides optimal starting conditions for subsequent ultra-thin rolling, avoiding rolling difficulties and defect risks caused by excessively thick or thin raw materials. Simultaneously, by precisely controlling the reduction rate per pass to 10–20%, rolling efficiency and material deformation control are effectively balanced, preventing cracking or strip breakage due to excessive work hardening during high-strength steel rolling, ensuring good material plasticity. Furthermore, by setting reasonable pre-tension (4–12 KN / mm) and post-tension (10–20 KN / mm), the strip's operating state during rolling is effectively stabilized, significantly improving strip shape and thickness uniformity, and minimizing surface defects. Therefore, this technical solution can ensure the production of high-strength, non-quenched and tempered 960MPa grade ultra-thin steel with excellent dimensional accuracy, plate shape quality and mechanical properties, effectively solving the quality control problems that are easy to occur in the ultra-thin rolling process, thereby meeting the market demand for high-performance ultra-thin high-strength steel.

[0072] In some embodiments described above in this application, the production method of high-formability, ultra-thin 960MPa grade non-quenched and tempered high-strength steel includes a heat treatment step, namely, tempering of the single sheet followed by air cooling to room temperature. However, in actual production, if the tempering temperature and time parameters are not properly selected, the microstructure of the steel may not be fully optimized, thereby affecting its final mechanical properties, especially the balance of yield strength, tensile strength, and elongation after fracture, making it difficult to consistently meet the requirements of high formability and high strength.

[0073] In this regard, this application further proposes that in the above-mentioned production method, the tempering temperature in step (6) is 600-650℃ and the tempering time is 25-35min, and the tempering temperature and time satisfy: when the tempering temperature is 600-620℃, the tempering time is 32-35min; when the tempering temperature is 630-650℃, the tempering time is 25-30min.

[0074] Tempering is a crucial step in the heat treatment of steel. Its main function is to eliminate internal stresses generated during rolling or cooling, stabilize the microstructure of the steel, and adjust its mechanical properties, particularly improving plasticity and toughness, while maintaining or optimizing strength. For non-quenched and tempered high-strength steels, precise control of tempering parameters has a decisive impact on the final grain size, morphology and distribution of precipitates, and residual stress state, thus directly affecting the comprehensive mechanical properties of the steel, especially the balance between high strength and high formability. Tempering is typically carried out in a specialized tempering furnace, achieved through precise control of the furnace temperature and the residence time of the steel within the furnace. Temperature control can be achieved through a thermocouple feedback system linked to the heating elements, while time control is accomplished through conveyor belt speed or batch processing timers.

[0075] The segmented matching control of tempering temperature and time proposed in this application aims to match corresponding tempering times according to different tempering temperature ranges, thereby achieving refined control of the microstructure of steel. Specifically, when the tempering temperature is in the lower range (600–620℃), due to the relatively slow atomic diffusion rate, a relatively long tempering time (32–35 min) is required to ensure sufficient microstructure transformation, effectively eliminate internal stress, and promote the uniform precipitation of fine carbides, thus significantly improving the plasticity and toughness of the steel while maintaining high strength. Conversely, when the tempering temperature is in the higher range (630–650℃), due to the accelerated atomic diffusion rate and increased microstructure transformation efficiency, the tempering time can be appropriately shortened (25–30 min) to avoid excessive grain growth or coarsening of precipitates, effectively suppressing strength loss while ensuring good formability. This synergistic control helps optimize the formability and elongation after fracture of the steel while ensuring high strength. In actual production, the tempering temperature and time can be precisely matched through an automated control system. For example, when the tempering furnace temperature is detected to be within a specific range, the system automatically adjusts the residence time of the steel in the furnace to meet the preset tempering process requirements.

[0076] Through the above technical solution, the tempering temperature and tempering time in step (6) of the heat treatment of high-strength steel with ultra-thin profile and non-quenched and tempered strength (960MPa grade) were precisely matched and controlled. Specifically, within the tempering temperature range of 600-650℃, different tempering time intervals were set according to the temperature. When the tempering temperature was low, the tempering time was extended to ensure sufficient microstructure transformation, effectively eliminate internal stress, and promote the uniform precipitation of fine carbides, thereby significantly improving the plasticity and toughness of the steel while maintaining high strength. When the tempering temperature was high, the tempering time was shortened to avoid excessive grain growth and coarsening of precipitates, effectively suppressing strength loss while ensuring good formability. This precise coordinated control of tempering parameters optimized the microstructure of the steel, ultimately achieving a good balance between yield strength, tensile strength, and elongation after fracture, ensuring that the high-strength steel can still meet the stringent requirements of high formability in ultra-thin profiles, and significantly improving the stability and reliability of the product.

[0077] In some embodiments described above in this application, a high-strength, ultra-thin 960MPa grade non-quenched and tempered steel is proposed, which aims to achieve high strength and good formability through a specific chemical composition ratio. However, in actual production, if the alloy element ratio is inappropriate, especially in ensuring high strength while further optimizing the steel's toughness and bending properties to meet the stringent forming requirements of ultra-thin steel, it remains a key issue requiring precise control. An unreasonable alloy element ratio may lead to cracking or performance instability in the steel during forming, thus limiting its widespread application in high-formability applications.

[0078] In this regard, this application further proposes that the ratio of Mo to Cr content in the chemical composition of the aforementioned high-strength, ultra-thin 960MPa grade non-quenched and tempered steel is 1.0–1.36. Molybdenum (Mo), as a strong carbide-forming element, can effectively refine grains and improve the hardenability, strength, and toughness of the steel. In non-quenched and tempered steel, Mo helps form dispersed carbides, playing a precipitation strengthening role, and inhibiting the ferrite-pearlite phase transformation while promoting bainite or martensite phase transformation, thereby significantly improving the strength of the steel. Chromium (Cr), also an important carbide-forming element, can significantly improve the hardenability and strength of the steel, and works synergistically with Mo to further enhance the comprehensive mechanical properties of the steel. By precisely controlling the ratio of Mo to Cr content within the range of 1.0–1.36, the optimal synergistic effect of Mo and Cr elements in the steel can be achieved. The setting of this ratio range aims to optimize the precipitation behavior and distribution of carbides in the steel matrix, ensuring the formation of fine, uniform, and dispersed carbides, and having a favorable influence on phase transformation kinetics. This results in an ideal microstructure, such as fine-grained bainite or martensite, even in a non-tempered state. This precise ratio avoids problems such as microstructure coarsening, decreased toughness, or hardenability imbalance caused by an excess or deficiency of a single element, providing crucial support for achieving a balance between high strength and high formability.

[0079] By precisely controlling the ratio of Mo to Cr content in the chemical composition within the range of 1.0 to 1.36 using the above technical solution, this application can fully leverage the synergistic strengthening effect of Mo and Cr elements in steel. This precise alloy ratio optimizes the precipitation behavior and distribution of carbides in the matrix, promoting the formation of fine and uniform microstructures, such as fine-grained bainite or martensite. This not only significantly improves the yield strength and tensile strength of high-strength steel, making it stably reach the 960 MPa level, but more importantly, it effectively improves the plasticity and toughness of the steel without tempering. Specifically, this ratio helps to improve the elongation after fracture and bending performance of the steel, enabling ultra-thin steel to pass the 180° bending test without cracking even under the harsh condition of D=2a. This greatly enhances the formability and reliability of high-strength steel, effectively avoiding cracking problems caused by insufficient performance during forming, and solving the technical challenge of how to further optimize the toughness and bending performance of steel while ensuring high strength.

[0080] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The chemical composition of the steel plate of the present invention reduces the C content and increases the content of precious metals such as Nb, Mn and V to improve its welding performance and toughness, so as to ensure its forming performance.

[0081] (2) This invention employs a production process of "controlled rolling + pickling + single-stand rolling + tempering heat treatment," producing 960MPa grade high-strength steel with excellent performance and good uniformity. The pickling process ensures the surface quality of the steel plate; the single-stand process promotes the generation of numerous dislocations in the strip, increasing the internal grain dislocation density, while simultaneously elongating or fragmenting the grains to form a fibrous structure, further strengthening the material. It also disperses the second-phase particles, hindering dislocation movement and increasing the strip strength. Furthermore, the single-stand process improves the strip shape, reducing strip unevenness. Finally, tempering heat treatment improves the internal structure of the steel plate, softening it and obtaining a fine, uniformly distributed structure, optimizing the steel plate's performance and increasing its toughness. This meets the increasingly stringent demands of the engineering machinery steel market. The technical solution of this invention has strong adaptability and promotional value, while simultaneously meeting the "high-precision" and lightweight requirements of the engineering machinery field.

[0082] (3) The 960MPa grade non-quenched and tempered high-strength steel produced by the method of the present invention has a low carbon equivalent, excellent steel plate welding performance, steel plate thickness ≤2mm, excellent mechanical properties, and forming performance far superior to the existing standard 180° bending test D=4a, yield strength ≥960MPa, tensile strength ≥1000MPa, elongation after fracture (A80) ≥ 5%, 180° bending test D=2a (D=bending head diameter, a=steel thickness).

[0083] The chemical composition (wt%) of 960MPa grade high-strength steel with a thickness of 2.0 mm tested according to the above method is shown in Table 1, and the mechanical property test results are shown in Table 2.

[0084] Table 1. Chemical composition of each embodiment (wt%, balance Fe) Table 2 Mechanical properties of each embodiment As can be seen from the above, the 960MPa grade quenched and tempered high-strength steel produced by the method of the present invention has a low carbon equivalent, excellent weldability, a steel plate thickness of ≤2mm, excellent mechanical properties, and formability far superior to the existing standard 180° bending test D=4a, yield strength ≥960MPa, tensile strength ≥1000MPa, elongation after fracture (A80) ≥ 5%, and 180° bending test D=2a (D=bending head diameter, a=steel thickness).

[0085] The above description is merely an embodiment of this application and is not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A high-strength, non-quenched and tempered steel with ultra-thin, high-formability profile and a strength of 960MPa, characterized in that... The chemical composition of the high-strength steel, by weight percentage, is as follows: C: 0.085–0.100%, Si: 0.15–0.25%, Mn: 1.75–1.85%, P≤0.018%, S≤0.0015%, Mo: 0.22–0.30%, V: 0.01–0.02%, Ti: 0.15–0.17%, Cr: 0.22–0.30%, Nb: 0.052–0.060%, N≤0.005%, Als: 0.01–0.06%, with the balance being Fe and unavoidable impurities. The mechanical properties of the high-strength steel meet the following requirements: yield strength ≥960 MPa, tensile strength ≥1000 MPa, elongation after fracture A80 ≥5%, and 180° bending test D=2a, where D is the diameter of the bending indenter and a is the thickness of the steel without cracks.

2. The production method of high-strength, ultra-thin 960MPa grade non-quenched and tempered steel as described in claim 1, characterized in that, Includes the following steps: (1) Steelmaking and continuous casting: smelting molten steel according to the chemical composition described in claim 1, and continuously casting it into slabs with a thickness of 230-240 mm; (2) Heating: After the slab is cooled for ≥24 hours, it is sent into the heating furnace. The holding temperature is 1260~1290℃, the holding time is 20~100min, the total time in the furnace is 180~400min, and the furnace exit temperature is 1260~1290℃. (3) Rolling: Rough rolling and finish rolling are performed in sequence. After rough rolling, a 36-59 mm thick intermediate billet is obtained. After finish rolling, a combination of ultra-fast cooling and layer cooling is used for cooling. (4) Pickling: Remove oxides from the surface of the strip steel, followed by rinsing and drying; (5) Single-stand rolling: Rolled in 3 passes to a thickness ≤2mm; (6) Heat treatment: After tempering, the single board is air-cooled to room temperature.

3. The high-strength, ultra-thin, 960MPa grade non-quenched and tempered steel according to claim 1, characterized in that, The Ti and N contents in the chemical composition satisfy Ti / N≥30.

4. The production method according to claim 2, characterized in that, In step (3), the roughing process is carried out in 5 to 7 passes, with an initial rolling temperature of 1260 to 1290°C and a final cooling temperature of 1060 to 1100°C.

5. The production method according to claim 2, characterized in that, In step (3), the finishing rolling adopts a 7-stand continuous rolling mill, with an inlet descaling pressure ≥18MPa, an initial rolling temperature of 1000~1080℃, a final pass reduction rate ≥10%, and a final cooling temperature of 865~895℃.

6. The production method according to claim 2, characterized in that, The specific cooling parameters in step (3) are: cooling rate of 80-200℃ / s in the ultra-fast cooling stage, cooling rate of ≥15℃ / s in the layer cooling stage, and coiling of the strip after cooling to 550-580℃.

7. The production method according to claim 2, characterized in that, The pickling and post-treatment parameters in step (4) are: acid concentration 70~240g / L, pickling temperature 75~85℃, pickling speed ≤40m / min; rinsing temperature ≥50℃, pickling tank conductivity ≤650μs / cm, and drying hot air temperature ≥110℃.

8. The production method according to claim 2, characterized in that, In step (5), the thickness of the raw material rolled by a single stand is 2.5-3 mm, the reduction rate per pass is 10-20%, the front tension is 4-12 KN / mm, and the back tension is 10-20 KN / mm.

9. The production method according to claim 2, characterized in that, In step (6), the tempering temperature is 600-650℃ and the tempering time is 25-35min. The tempering temperature and time satisfy the following conditions: when the tempering temperature is 600-620℃, the tempering time is 32-35min; when the tempering temperature is 630-650℃, the tempering time is 25-30min.

10. The high-strength, ultra-thin, 960MPa grade non-quenched and tempered steel according to claim 1, characterized in that, The ratio of Mo content to Cr content in the chemical composition is 1.0 to 1.36.

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