A type of ultra-high strength steel that can be bent at small angles and its manufacturing method

Abstract

This invention belongs to the field of steel materials, specifically relating to an ultra-high-strength steel that can be bent at small angles and its manufacturing method. Its chemical composition, by mass percentage, includes: carbon (C) 0.18-0.22%, manganese (Mn) 2.0-2.5%, silicon (Si) 0.8-1.2%, aluminum (Al) 0.03-0.06%, phosphorus (P) ≤0.012%, sulfur (S) ≤0.003%, rare earth elements (RE) 0.001-0.005%, with the balance being iron (Fe) and unavoidable impurities. This method eliminates the expensive laser local processing process and avoids the strength loss caused by pre-deformation. Through parameter reconstruction of conventional metallurgical and heat treatment equipment, a qualitative leap in material properties is achieved. This has a profound impact on promoting the application of ultra-high-strength steel in the automotive industry, engineering machinery, and high-end equipment manufacturing.

Description

Technical Field

[0001] This invention belongs to the field of steel materials, specifically relating to an ultra-high strength steel that can be bent at a small angle and its manufacturing method. Background Technology

[0002] With the accelerated pace of lightweighting in the global automotive industry and increasingly stringent collision safety regulations, the application of ultra-high strength steel (UHSS) with a strength of 1000 MPa and above in key structural components such as vehicle body longitudinal beams, A-pillar / B-pillar reinforcements, and anti-collision beams continues to rise. In the current field of materials science, dual-phase steel (DP steel) and martensitic steel (MS steel), as typical representatives of UHSS, provide core support for achieving vehicle weight reduction and improving passive structural safety due to their superior tensile and yield strength. Traditional technical solutions typically rely on precisely controlling the transformation process from austenite to martensite or bainite to obtain a dispersed distribution of high-hardness reinforcing phases, thereby macroscopically endowing the material with extremely high load-bearing capacity.

[0003] However, as automotive manufacturing processes become increasingly integrated and complex, the design structures of components exhibit sharper geometric features. This poses unprecedented and stringent challenges to the cold forming performance of ultra-high-strength steel, especially its performance in small-angle bending. In actual production conditions, traditional 1000MPa-grade ultra-high-strength steel often experiences cracking failures exceeding 80% when faced with extreme forming requirements where the bending radius to thickness ratio (R / t) is less than or equal to 2. This significantly limits the application of high-strength steel in complex structural components. The underlying technical reasons for this lie in several inherent contradictions in the existing material systems and manufacturing processes. First, controlling the uniformity of the microstructure becomes extremely difficult at the ultra-high strength scale. Martensite laths formed under traditional processes are generally large, with an average size often exceeding 5μm, accompanied by a continuous network of bainite. This heterogeneous structure, when subjected to severe plastic deformation caused by bending, easily generates significant dislocation pile-up and stress concentration at phase boundaries, thereby inducing the initiation of microcracks.

[0004] Secondly, the imbalance in the distribution of grain boundary characteristics is another core contributing factor to increased material brittleness. In conventional heat treatment cycles, the proportion of large-angle grain boundaries (orientation difference greater than 15°) within the material is often less than 65%. This means the material lacks sufficient effective barriers to absorb the energy of crack propagation, causing cracks to rapidly penetrate the entire matrix along continuous low-angle grain boundaries or brittle phase boundaries once they initiate on the surface. Simultaneously, the deterioration of the surface stress state becomes the "trigger" for bending cracking. Due to the limitations of existing hot rolling and cold rolling annealing processes, ultra-high-strength steel plates often retain significant tensile stress on their surface in the delivered state. During bending, the tensile strain and residual tensile stress on the outer surface are linearly superimposed, causing the peak surface stress to often exceed 1200 MPa, far exceeding the material's cleavage fracture strength, leading to macroscopic cracking on the outer curved surface in the early stages of deformation.

[0005] To address the aforementioned bottlenecks, existing technologies have attempted to remedy the situation through various approaches, but often fall into a dilemma of "paying for one thing while losing another." For example, while laser local annealing technology (such as JP2022156780A) can locally improve the plasticity of the bending area, its high equipment investment and extremely low production efficiency significantly increase the cost per ton. Utilizing the pre-deformation induced phase transformation-induced plasticity (TRIP) effect (such as CN113755754A) improves formability, but the pre-deformation process often leads to a significant decrease in matrix strength, making it difficult to maintain a stable strength level of 1000MPa. While single microalloying methods (such as US20230087621) can refine grains to some extent, they cannot fundamentally reverse the trend of surface tensile stress concentration and rapid crack propagation. Therefore, how to construct a new type of ultra-high strength steel that can balance high strength of 1000MPa and small-angle bending capability of R≤2t, and its systematic manufacturing solution, from multiple dimensions such as composition design, microstructure refinement, grain boundary engineering and surface stress field reconstruction, has become a key challenge and core problem that urgently needs to be solved in the current research and development of high-performance metal materials. Summary of the Invention

[0006] This invention aims to solve the technical problems of existing 1000MPa grade ultra-high strength steel, such as easy cracking, poor microstructure uniformity, grain boundary embrittlement, and severe surface stress concentration under small-angle bending conditions (bending radius R≤2t, where t is the steel plate thickness). It provides an ultra-high strength steel that can be bent at small angles and its manufacturing method. This invention, through precise control of chemical composition and combined with specific heat treatment processes and surface strengthening techniques, constructs a multiphase synergistic microstructure composed of fine-grained martensite, granular bainite, and retained austenite. A nanocrystalline layer with controlled thickness and a high residual compressive stress field are formed on the steel plate surface, thereby significantly improving the bending performance and overall mechanical properties of the material.

[0007] To achieve the above-mentioned objectives, this invention provides an ultra-high strength steel that can be bent at a small angle, the chemical composition of which, by mass percentage, includes: carbon (C) 0.18-0.22%, manganese (Mn) 2.0-2.5%, silicon (Si) 0.8-1.2%, aluminum (Al) 0.03-0.06%, phosphorus (P) ≤0.012%, sulfur (S) ≤0.003%, rare earth elements (RE) 0.001-0.005%, with the balance being iron (Fe) and unavoidable impurities. In the composition design, the manganese content is controlled at 2.0-2.5% to ensure the hardenability of the matrix and stabilize the austenite phase; the silicon content is in the range of 0.8-1.2%, its main function is to inhibit the precipitation of carbides during the isothermal transformation process and promote the enrichment of carbon into the untransformed austenite, thereby obtaining stable retained austenite; the rare earth element RE is added at 0.001-0.005%, and the RE is specifically a mixture of cerium (Ce) and lanthanum (La), wherein the mass ratio of Ce to La is set at 7:3. This specific ratio of rare earth elements can effectively purify grain boundaries, modify sulfide inclusions, transform them from elongated shapes to spherical shapes, and reduce stress concentration sources during bending.

[0008] Furthermore, the ultra-high strength steel capable of small-angle bending provided by the present invention has a microstructure with a clearly defined volume fraction: the microstructure consists of 40-50% granular bainite, 12-15% retained austenite, and the remainder martensite. Regarding martensite morphology control, the present invention requires that the average width of the martensite laths not exceed 0.5 μm. The granular bainite is dispersed in the matrix, and the bainitic ferrite matrix within it forms a discontinuous interface with the discrete carbon-rich islands (M / A islands). This structure can induce crack deflection during bending deformation. According to experimental observations, when the crack propagates to the granular bainite region, due to the hindering effect of the phase interface, the actual crack propagation path length increases by more than 30% compared to a straight path, thus significantly consuming crack propagation energy.

[0009] Furthermore, the ultra-high strength steel capable of being bent at small angles according to the present invention exhibits significant characteristics in its grain boundary distribution. Within the microstructure, large-angle grain boundaries (i.e., grain boundaries with a crystal orientation difference greater than 15°) account for no less than 80%. This high proportion of large-angle grain boundaries significantly increases the resistance to dislocation slip and plays a significant role in inhibiting crack initiation, preventing rapid crack propagation along a single direction.

[0010] Furthermore, the ultra-high-strength steel capable of small-angle bending described in this invention features a specific functionalized surface layer structure. The steel plate's surface layer has a nanocrystalline layer with a thickness of not less than 40 μm. Within this nanocrystalline layer, the grain size exhibits a gradient distribution, with the outermost surface grain size ranging from 20 nm to 100 nm. Simultaneously, the surface layer of the steel plate contains a deep-distributed residual compressive stress, with a surface residual compressive stress value not less than -800 MPa. This high residual compressive stress field effectively counteracts the tensile stress on the outer arc surface during bending, significantly improving the crack initiation energy. Under extreme deformation conditions with a bending radius R = 2t, this surface structure increases the microcrack initiation energy by more than 50%, thereby ensuring surface integrity.

[0011] This invention also provides a method for manufacturing the aforementioned ultra-high-strength steel that can be bent at small angles. This method achieves deep coupling between microstructure and properties by synergistically controlling precipitation kinetics, phase transformation characteristics, and surface plastic deformation. The specific manufacturing steps are as follows: During the smelting and casting stages, a vacuum induction furnace or converter is used for smelting, and the content of impurity elements is strictly controlled. After smelting, continuous casting is carried out to obtain a continuously cast billet. The continuously cast billet is then heated to 1200±20℃ and held for 2-3 hours to ensure sufficient solid solution of alloying elements and homogenization of initial austenite grains.

[0012] During the hot rolling stage, the final rolling temperature is controlled at 900±20℃. The total hot rolling compression ratio is not less than 80%, and the initially coarse austenite grains are broken down through large deformation. After hot rolling, ultra-fast cooling technology is used to cool the steel plate to room temperature.

[0013] During the relaxation precipitation stage, the hot-rolled steel plate is reheated to 650±20℃ and held for 200-400 seconds. Within this temperature range, rare earth elements (RE) interact with trace impurity elements in the matrix, resulting in micro-relaxation phenomena at the grain boundaries. The core purpose of this step is to induce the dispersed precipitation of fine inclusions and utilize the oxygen- and sulfur-loving properties of rare earth elements to purify the austenite grain boundaries, laying the foundation for subsequent grain refinement.

[0014] During the critical zone quenching stage, the steel plate is heated to 820±10℃ for austenitization. This temperature is located in the upper part of the ferrite-austenite dual-phase region, aiming to achieve partial austenitization and form fine austenite grains. Subsequently, the steel plate is rapidly cooled to 400±10℃ at a cooling rate greater than 50℃ / s.

[0015] During the isothermal transformation stage, the steel plate is held in an isothermal salt bath or induction heating device at 400±10℃ for 100-150s. During this process, a portion of the austenite transforms into granular bainite. Due to the hindering effect of silicon, carbon atoms are repelled from the transformed bainitic ferrite into the remaining austenite, giving the untransformed austenite extremely high thermodynamic stability. Therefore, after subsequent cooling to room temperature, it can exist in the matrix as retained austenite.

[0016] In the final cooling stage, the isothermal treated steel sheet is directly water-quenched or oil-quenched to room temperature. At this point, the remaining unstable austenite portion transforms into fine lath martensite. Due to the precise control of grain boundaries and solute distribution in the preceding processes, the width of the generated martensite laths is strictly limited to below 0.5 μm.

[0017] In the online warm shot peening stage, this is a crucial process for achieving small-angle bending performance. The cooled steel plate is reheated to 200-300℃, and online shot peening is performed within this temperature range. The shot peening intensity is controlled at 0.35-0.45 mmN (measured using standard test pieces). The shot peening medium uses cast steel shot or ceramic shot with a hardness of not less than 650 HV and a diameter of 0.3-0.5 mm, with a coverage of not less than 200%. At 200-300℃, the yield strength of the material decreases while its plasticity increases, making it easier for the impact energy generated by shot peening to induce severe plastic deformation of the surface metal. This warm shot peening process not only forms a nanocrystalline layer with a depth greater than 40 μm on the surface, but more importantly, under the combined effect of the temperature rise and plastic deformation, the residual compressive stress distribution is more profound and stable, with the peak value of the surface residual compressive stress stabilizing between -800 MPa and -1000 MPa.

[0018] The ultra-high strength steel capable of being bent at small angles and its manufacturing method described in this invention are characterized by the following technical principles and advanced features: First, the non-uniform crack resistance mechanism at the microstructure level. This invention does not pursue simple toughening, but rather constructs a "soft and hard alternating" microstructure through a composite of 40-50% granular bainite and martensite. The M / A islands in the granular bainite, acting as dispersed hard particles, can induce frequent microscopic deflections of the main crack under bending and tensile forces. This deflection not only increases the total crack propagation area but also consumes a large amount of fracture energy during each deflection process, making it difficult for the crack to form a penetrating macroscopic crack.

[0019] Second, grain boundary engineering suppresses brittle fracture. Traditional ultra-high strength steels have a low proportion of large-angle grain boundaries, making them prone to rapid, linear crack penetration along grain boundaries with small orientation differences. This invention, through a precise combination of rare-earth microalloying and critical zone quenching processes, increases the proportion of large-angle grain boundaries to over 80%. Large-angle grain boundaries possess higher interfacial energy and stronger dislocation blocking capabilities. When a microcrack attempts to cross a large-angle grain boundary, it must overcome significant dislocation recombination resistance, thus locking the crack origin at the microscale.

[0020] Third, the reconstruction and strengthening of the surface stress field. Bending cracking is essentially caused by the tensile stress on the outer surface exceeding the material's critical fracture strength. This invention introduces a high value of residual compressive stress into the steel plate surface through online temperature shot peening. This compressive stress acts as a "preload" during the bending process; only after the externally applied bending tensile stress first offsets this compressive stress will the surface metal enter a tensile state. Experimental data shows that a residual compressive stress of -800 MPa reduces the actual net tensile stress on the outer curved surface of the bend by more than 60%, significantly delaying crack initiation.

[0021] Fourth, the crack shielding effect of the nanocrystalline layer. The nanocrystalline layer formed on the surface in this invention exhibits extremely fine grains, resulting in a geometrically increased number of grain boundaries per unit volume. This high-density grain boundary network not only significantly improves the surface hardness and strength, but also, due to the good synergistic deformation capability of nanocrystals, maintains the continuity of the microstructure under extremely large strain gradients, effectively shielding the path for microcracks to penetrate into the matrix.

[0022] In a preferred embodiment of the present invention, the rare earth element RE is added to the chemical composition by means of a rare earth alloy wire during the final stage of refining via a wire feeding method. The Ce and La in the rare earth alloy wire are encased in a low-carbon steel sheet to prevent oxidation of the rare earth elements before they enter the molten steel.

[0023] In a preferred embodiment of the present invention, in the manufacturing method, the cooling rate after the online shot peening step is controlled at 5-10℃ / s to ensure that the residual compressive stress is fully relaxed, recovered and stabilized on the surface, and to avoid the stress gradient being too steep due to excessive cooling.

[0024] Furthermore, the ultra-high-strength steel capable of small-angle bending described in this invention exhibits a tensile strength stable at 1000-1100 MPa, a yield strength of 850-950 MPa, and an elongation (A50) of not less than 12%. During a 90-degree bending test with a radius of 2t, no visible cracks were observed on the outer surface of the sample, and under a 100x metallographic microscope, the depth of microcracks was less than 10 μm. Comparative experiments show that steel plates with the same composition but without relaxation precipitation and warm shot peening treatment all exhibited a cracking rate greater than 80% during bending with a radius of 2t, demonstrating the significant technical effectiveness of the combined process of this invention.

[0025] In a preferred embodiment of the present invention, the austenitizing holding time in the critical zone quenching step is calculated as 0.5-1.0 minutes per millimeter of plate thickness to ensure the synchronization of the microstructure transformation between the core and the surface of the plate, thereby obtaining a high degree of consistency in longitudinal properties.

[0026] Furthermore, in the warm shot peening process, the shot peening angle is set to 75° to 90° with the steel plate surface. By adjusting the shot peening pressure and flow rate, the surface roughness Ra of the steel plate is controlled between 1.6 and 3.2 μm. This roughness range not only benefits subsequent coating but also avoids the micro-notch effect caused by excessive surface roughness, further improving the fatigue strength of the material.

[0027] As a specific implementation step of the present invention, the actual yield of rare earth element RE in molten steel is achieved by precisely controlling the slag basicity, requiring the slag basicity R to be stable between 3.0 and 3.5. This operation minimizes side reactions between rare earth elements and oxides in the slag, ensuring that the segregation concentration of Ce and La at grain boundaries reaches the design expectation, thereby achieving effective control over the proportion of large-angle grain boundaries.

[0028] As an improvement of the present invention, the heating method for the relaxation precipitation stage is induction heating, with the heating rate controlled at 30-50℃ / s. Induction heating can utilize the skin effect generated by electromagnetic induction to allow the surface of the steel plate to reach the set temperature preferentially. This is beneficial for the formation of finer initial grains on the surface, providing good initial microstructure conditions for the subsequent construction of nanocrystalline layers.

[0029] Furthermore, in the method provided by this invention, the medium for the isothermal transformation stage is a nitrate bath. The nitrate bath is composed of a mixture of 50% sodium nitrate and 50% potassium nitrate. This isothermal medium has extremely high heat capacity and thermal stability, ensuring that the temperature fluctuation of the steel plate during the isothermal process is controlled within ±2℃, thereby precisely controlling the phase transformation ratio of granular bainite and retained austenite, and ensuring the accuracy of the microstructure composition.

[0030] In a preferred embodiment of the present invention, the width distribution of the martensitic laths is calibrated using transmission electron microscopy (TEM). In the remaining martensitic structure, lath martensite with high dislocation density provides the primary strength support. Due to the spatial segmentation effect of the preceding granular bainite during the formation of the martensitic laths, their growth is mechanically hindered, thereby achieving a lath width ≤0.5 μm. These fine martensitic units significantly reduce stress concentration and enhance the local plastic deformation capacity of the matrix.

[0031] Furthermore, a transition layer with a thickness of 20-30 μm exists between the nanocrystalline layer and the substrate. In this transition layer, the microstructure consists of fine-grained ferrite and deformed martensite, with the grain size smoothly transitioning from the nanoscale to the microscale. This gradient distribution structural design avoids the risk of interfacial delamination caused by modulus mismatch and enhances the bonding strength between the surface reinforcement layer and the substrate.

[0032] In one embodiment of the present invention, the final surface hardness of the steel plate before bending is controlled at 360-400 HV. This hardness matching, combined with high residual compressive stress, allows the material to dissipate energy through localized lattice distortion and the release of residual compressive stress when subjected to enormous shear stress on the outer curved surface of the bend, without resulting in cleavage fracture.

[0033] This invention provides an ultra-high-strength steel capable of being bent at small angles and its manufacturing method. Through the precise coordination of the aforementioned process parameters, it systematically resolves the inherent contradiction between the strength and formability of ultra-high-strength steel. Experimental tests show that the steel plate prepared by this invention maintains surface integrity without any cracking when subjected to an extreme bending test of R=1.5t. This performance is significantly superior to similar products obtained by simply adding alloying elements or through simple heat treatment in the prior art. Its core advantage lies in not only achieving a balanced strengthening and toughening of the matrix structure at the microscale, but also providing a significant safety margin for the forming process at the macroscopic stress field level through surface engineering techniques.

[0034] Furthermore, the chemical composition and manufacturing process of this invention are applicable not only to cold-rolled steel sheets but also to hot-rolled pickled sheets. For steel sheets of varying thicknesses (1.2mm to 3.5mm), consistent small-angle bending performance can be achieved by finely adjusting the shot peening intensity and isothermal time. The versatility and stability of this process lay a solid engineering foundation for the large-scale application of this type of ultra-high-strength steel in the automotive parts manufacturing field.

[0035] In another preferred embodiment of the present invention, during the smelting process, the oxygen content in the steel is controlled to below 10 ppm through a vacuum carbon deoxidation process. This extremely low oxygen content ensures that rare earth elements (REs) exist primarily in solid solution or as fine sulfides, avoiding the formation of large rare earth oxide inclusions. This precise control over the size and distribution of inclusions is one of the key prerequisites for achieving a high-angle grain boundary ratio of over 80%, and is also an inherent guarantee against brittle fracture during bending.

[0036] Furthermore, the manufacturing method also includes a leveling process after warm shot peening. The leveling elongation is controlled between 0.3% and 0.5%. This process aims to eliminate the extremely fine surface waviness that may be caused by warm shot peening, improve the flatness of the steel plate, and at the same time, the slight plastic deformation can further stabilize the residual austenite and prevent its premature decomposition during subsequent storage.

[0037] The ultra-high strength steel that can be bent at small angles as described in this invention also exhibits significantly enhanced service stability under complex environments. Due to the purifying effect of rare earth elements on grain boundaries, the material's delayed crack resistance is increased by more than 30%, enabling this steel to maintain long-term mechanical property stability in formed structural components, thus meeting the high reliability requirements of the automotive industry for safety components.

[0038] In summary, this invention, through a comprehensive four-dimensional design considering composition, microstructure, grain boundaries, and surface stress field, constructs a complete and highly feasible technical solution. It not only theoretically elucidates the mechanism of synergistic crack resistance between granular bainite and nanocrystalline layers, but also, in engineering practice, achieves the industrialization goal of crack-free bending of 1000MPa-grade ultra-high-strength steel with a radius of R≤2t through precise control of temperature, time, and mechanical parameters, demonstrating significant inventiveness and practical value.

[0039] Furthermore, the manufacturing method for ultra-high-strength steel capable of being bent at small angles provided by this invention requires extremely high precision in temperature control. During the isothermal transformation stage, a segmented temperature control device is used to control the transverse temperature difference of the steel plate within ±3℃. This uniform temperature field distribution ensures that the volume fraction fluctuation of 40-50% granular bainite across the entire width of the steel plate is less than 2%, thereby guaranteeing that the bending performance of each steel plate and each part meets strict process specifications during mass continuous production.

[0040] Furthermore, the retained austenite (12-15%) described in this invention exhibits a significant transformation-induced plasticity (TRIP) effect during bending deformation. As the bending strain increases, the retained austenite gradually transforms into martensite. Because this transformation is accompanied by volume expansion, it not only produces significant work hardening and homogenizes plastic deformation, but also generates a local compressive stress field at the microscale, further counteracting the destructive effect of external tensile stress. This dynamic process, together with the residual compressive stress field on the surface, forms a spatial-scale linkage, jointly constructing a multi-layered crack-resistant barrier.

[0041] The control of martensitic lath width described in this invention is achieved through the sequential application of multi-stage cooling rates. During the cooling process from the critical austenitizing temperature to the isothermal temperature, the high cooling rate (>50℃ / s) suppresses the formation of a large amount of ferrite, maintaining the uniformity of austenite composition. In the final quenching stage, martensite nucleates and grows within the small residual spaces already divided by granular bainite. Its growth space is geometrically limited, thus forcing the lath width to remain at the nanometer to submicrometer level (≤0.5μm). This level of microstructural refinement is difficult to achieve with conventional heat treatment processes and is the core microscopic basis for enabling extremely small radius bending in this invention.

[0042] In the use of rare earth elements (RE), this invention particularly emphasizes the specific ratio (7:3) of Ce to La. This ratio was determined based on in-depth research into the segregation kinetics of rare earth elements in molten steel. Ce has strong deoxidation and desulfurization capabilities, while La exhibits higher stability in grain boundary segregation. The combination of the two in a 7:3 ratio maximizes the improvement of grain boundary properties, ensuring the purity of the molten steel and effectively reducing the segregation energy of harmful elements such as phosphorus and sulfur at the grain boundaries through stable arrangement at the grain boundaries, thereby minimizing the risk of grain boundary embrittlement.

[0043] For the detection of the nanocrystalline layer (thickness ≥ 40 μm), this invention uses cross-sectional electron microscopy and nanohardness indentation to verify its properties. Within a depth range of 40 μm from the surface, its hardness curve exhibits a continuous gradient change without obvious brittle abrupt change points. This demonstrates that the plastic deformation layer induced by warm shot peening possesses extremely high microstructural stability and excellent strength-plasticity matching. During actual bending, this nanocrystalline layer can wrap around the substrate like a "metal skin," and even under extremely high outer edge tensile strain, it can accommodate deformation through grain boundary slip and rotation, without generating microcrack initiation centers.

[0044] As a supplement to the manufacturing method of this invention, the preheating method in the online temperature shot peening process is preferably rapid infrared heating to avoid surface oxidation and decarburization caused by prolonged heating. The depth of the surface decarburized layer is strictly controlled within 10 μm, thereby ensuring that the shot peening strengthening effect can directly act on the high carbon concentration strengthened structure and maximize the excitation of the residual compressive stress.

[0045] Furthermore, the silicon-aluminum composite deoxidation process in the composition of the ultra-high strength steel capable of small-angle bending described in this invention is also crucial. The combined use of Si and Al not only ensures deep deoxidation of the molten steel but also alters the morphology of alumina inclusions, causing them to form complex composite inclusions with rare earth elements. These composite inclusions do not break or elongate during rolling, maintaining a spherical state, thereby avoiding stress concentration caused by elongated inclusions at bending points and significantly improving the isotropic formability of the steel plate.

[0046] In the manufacturing process of this invention, the parameter settings for each step have undergone rigorous physical simulation and engineering verification. For example, the relaxation precipitation temperature of 650±20℃ is selected within the range where the diffusion energy barrier of alloying elements and the precipitation driving force reach the optimal balance. If the temperature is too high, the precipitated phase is prone to coarsening and loses its function of pinning grain boundaries; if the temperature is too low, the diffusion rate of rare earth elements is insufficient, making it difficult to effectively purify the grain boundaries. This invention, through this precise temperature window, achieves dynamic repair and strengthening of the original austenite grain boundaries, providing a microstructure genetic basis for subsequently obtaining a large-angle grain boundary ratio of over 80%.

[0047] Similarly, the critical heating temperature of 820±10℃ has a tolerance range of only ±10℃. This is because the proportion of austenite is extremely sensitive to temperature changes within this temperature range. To obtain the designed 12-15% retained austenite and 40-50% granular bainite, the heating state of the critical region must be precisely locked at a specific phase equilibrium point. This precise control ensures that the contribution of strength (martensite) and the contribution of plasticity (granular bainite and retained austenite) reach the optimal ratio.

[0048] Through the aforementioned systematic technical means, this invention not only successfully manufactured a high-performance ultra-high-strength steel, but also proposed a new manufacturing paradigm based on a microstructure-property coupling mechanism. This approach eliminates the need for expensive laser local processing and avoids strength loss caused by pre-deformation. By reconstructing the parameters of conventional metallurgical and heat treatment equipment, a qualitative leap in material properties is achieved. This has a profound impact on promoting the application of ultra-high-strength steel in the automotive industry, engineering machinery, and high-end equipment manufacturing.

[0049] Finally, the ultra-high-strength steel capable of being bent at small angles and its manufacturing method described in this invention have demonstrated excellent process robustness in practical industrial applications. Its composition system does not rely on expensive precious metals such as nickel and molybdenum; instead, through a combination of manganese, silicon, and trace amounts of rare earth elements, coupled with a precise heat treatment process, it achieves ultra-high performance while ensuring low cost. This not only aligns with the current development trend of green, low-carbon, cost-reduction, and efficiency-enhancing manufacturing, but also provides crucial technical support for the independent and controllable production of high-grade ultra-high-strength steel. Detailed Implementation

[0050] This invention provides an ultra-high-strength steel capable of being bent at small angles and its manufacturing method. The core of this invention lies in its systematic solution to the engineering challenge of easy cracking of ultra-high-strength steel under small-angle bending conditions through precise control of composition design, synergistic coupling of multiphase structures, optimization of grain boundary characteristic distribution, and reconstruction of the surface stress field. To enable those skilled in the art to more clearly understand the technical solution and implementation details of this invention, a detailed description is provided below, combining specific chemical compositions, process parameters, microstructure characteristics, and performance experimental data.

[0051] In the chemical composition design of the ultra-high strength steel capable of small-angle bending provided by this invention, carbon, as a key element for strengthening the matrix and stabilizing austenite, is strictly controlled within a mass percentage range of 0.18% to 0.22%. This range is chosen based on a deep consideration of the balance between material strength and toughness. If the carbon content is below 0.18%, it is difficult to guarantee that the material will achieve a tensile strength of 1000 MPa after quenching; while if the carbon content is above 0.22%, it will lead to a decrease in the martensitic transformation temperature, and the generated lath martensite will have excessively high hardness and increased brittleness, greatly impairing the material's local deformation capacity during bending. Manganese, as the main alloying element, is controlled within a content of 2.0% to 2.5%. Its primary role is to improve the hardenability of the molten steel, ensuring a uniform quenched structure even with large cross-sectional dimensions. At the same time, manganese enriches in austenite during critical heating, significantly improving the chemical stability of austenite and laying the foundation for obtaining a residual austenite of 12% to 15% by volume. Silicon accounts for 0.8% to 1.2% of the composition system. Its function is to effectively suppress the precipitation of brittle phases such as cementite during the isothermal transformation process by utilizing its extremely low solubility in carbides. This promotes the continuous migration of carbon atoms into the untransformed austenite. This carbon enrichment effect gives the retained austenite extremely high thermodynamic stability at room temperature.

[0052] To control steel purity and inclusions, this invention incorporates 0.001% to 0.005% of rare earth element (RE). Specifically, this rare earth element is composed of cerium and lanthanum in a 7:3 mass ratio. The addition of rare earth elements not only provides deoxidation and desulfurization, but more importantly, its micro-segregation at grain boundaries effectively reduces the segregation energy of low-melting-point impurities such as sulfur and phosphorus, thus preventing grain boundary weakening. The high chemical reactivity of cerium allows it to form fine, spherical composite inclusions with oxygen and sulfur in the steel, while lanthanum tends to form stable segregated layers at the original austenite grain boundaries. This specific combination optimizes the interfacial energy of the grain boundaries, significantly increasing the proportion of large-angle grain boundaries. The aluminum content is controlled at 0.03% to 0.06%, forming a composite deoxidation system with silicon, ensuring that the oxygen content in the steel is compressed to below 10 ppm, thus preventing coarse alumina particles from acting as crack initiation sources. Phosphorus and sulfur, as harmful impurities, are strictly limited to below 0.012% and 0.003%, respectively, in order to eliminate the negative impact of low-melting-point eutectic on hot working and cold forming properties.

[0053] At the microstructure level, this invention constructs a multiphase microstructure consisting of 40% to 50% granular bainite, 12% to 15% retained austenite, and the remainder fine-grained lath martensite. Furthermore, the average width of the martensite laths is precisely controlled to below 0.5 μm. The advantage of this microstructure lies in the fact that the ferrite matrix within the granular bainite provides excellent matrix plasticity, while the dispersed M / A islands act as hard barriers at the microscale, inducing a local redistribution of the stress field. When the material is bent at a small angle with a bending radius R ≤ 2t, the extremely high tensile stress on the surface can lead to the initiation of microcracks. However, in the microstructure of this invention, cracks must frequently cross the phase interface between the granular bainite and martensite as they propagate inward. Furthermore, since large-angle grain boundaries account for no less than 80%, each time a crack crosses a grain boundary, it requires extensive dislocation reorganization and orientation adjustment. Experimental calibration shows that the presence of this discontinuous interface makes the crack propagation path serrated, and the actual propagation work is several times higher than that of the traditional single-phase structure, thus exhibiting excellent resistance to bending cracks on a macroscopic scale.

[0054] As a key technical indicator of this invention, the nanocrystalline layer with a thickness of not less than 40 μm on the surface of the steel plate is achieved through a subsequent warm shot peening process. Within this region, the grains undergo multi-level refinement due to intense plastic shear forces, with the grain size on the outermost surface limited to 20 nm to 100 nm. This gradient microstructure not only exhibits a smooth transition in hardness from the surface inwards, effectively avoiding interfacial delamination caused by modulus mismatch, but more importantly, the nanocrystals possess extremely high grain boundary density, enabling coordinated deformation at extremely high strain rates through mechanisms such as grain boundary slip. Simultaneously, a residual compressive stress field with a value of not less than -800 MPa exists on the surface of the steel plate. This deeply distributed compressive stress layer can pre-compensate for the tensile stress generated by the outer curved surface during bending. Specifically, the surface metal only truly enters a tensile stress state when the applied external bending tensile stress exceeds the absolute value of the residual compressive stress. This "stress cancellation" effect significantly increases the crack initiation threshold of the material.

[0055] This invention provides a method for manufacturing ultra-high-strength steel that can be bent at small angles, with the production process strictly following the synergistic control laws of thermodynamics and kinetics. First, in the smelting and continuous casting stages, vacuum induction or converter smelting is employed, and the oxygen content is reduced to an extremely low level through a vacuum carbon deoxidation process. At the end of refining, Ce-La alloy wire encased in a low-carbon steel sheet is fed into the molten steel using a wire feeding method. To ensure a high rare earth yield, the slag basicity must be strictly controlled between 3.0 and 3.5, which effectively suppresses side reactions between rare earth elements and slag components. After obtaining the continuously cast billet, it is heated to 1200±20℃ and held for 2 to 3 hours. This step is not only for the complete solid solution of alloying elements but also to eliminate component segregation during continuous casting, ensuring that the initial austenite grains reach a uniform state before subsequent rolling.

[0056] In the hot rolling process, the final rolling temperature is precisely locked at 900±20℃, and the total hot rolling compression ratio must be no less than 80%. The introduction of large deformation generates a high-density dislocation pile-up, providing a huge driving force for subsequent dynamic recrystallization, thereby achieving significant refinement of the initial grains. Ultra-rapid cooling is used after rolling to suppress carbide coarsening. Subsequently, the steel plate enters the critical relaxation precipitation stage, heated to 650±20℃ and held for 200 to 400 seconds. At this time, the skin effect of induction heating is used to rapidly bring the surface of the steel plate to the set temperature. In this temperature range, rare earth elements begin to segregate towards the grain boundaries and induce the dispersion and precipitation of fine inclusions at the grain boundaries. This synergistic effect of "grain boundary purification and pinning" directly determines the proportion of large-angle grain boundaries formed in subsequent processes, which is the microstructure and genetic basis for achieving high crack resistance of the material.

[0057] Next, the steel plate is heated to 820±10℃ to enter the critical zone quenching stage. At this temperature, the microstructure is in the two-phase region of ferrite and austenite. By controlling the holding time (0.5-1.0 minutes per millimeter of plate thickness), a precise austenitization ratio is achieved. Subsequently, it is rapidly cooled to 400±10℃ at a cooling rate greater than 50℃ / s. After entering the isothermal transformation stage, the steel plate is held in a nitrate bath composed of 50% sodium nitrate and 50% potassium nitrate for 100 to 150 seconds. This high heat capacity medium ensures that the temperature difference during the isothermal process is less than ±2℃. During this stage, some austenite transforms into granular bainite, while carbon elements are strongly enriched in the untransformed austenite due to the repulsion of silicon atoms, ensuring that it exists stably as 12% to 15% retained austenite after subsequent quenching to room temperature. During the final quenching process, since the granular bainite in the early stage has geometrically divided the original space, the newly generated martensite is confined in the narrow remaining space and is forced to grow in the form of extremely fine laths (≤0.5μm).

[0058] The key to this invention's manufacturing method lies in the online warm shot peening strengthening process. The cooled steel plate is reheated to 200-300°C and shot peening is performed within this plasticity window. Cast steel shot with a diameter of 0.3-0.5 mm and a hardness of not less than 650 HV is used for full-coverage shot peening with an intensity of 0.35-0.45 mmN. Warm shot peening utilizes the superposition effect of thermal activation energy and mechanical energy, causing the surface layer to undergo severe plastic deformation to form a nanocrystalline layer (thickness ≥ 40 μm), while simultaneously introducing deep and stable residual compressive stress (-800 MPa to -1000 MPa). After shot peening, the cooling rate must be controlled at 5-10°C / s to prevent excessively steep strain-induced stress gradients that could lead to surface brittleness. Finally, a 0.3% to 0.5% leveling process optimizes the plate shape and further stabilizes the microstructure.

[0059] To further demonstrate the superiority of the present invention, our laboratory prepared several examples and comparative examples. In the experimental data below, the thickness of all samples was 2.0 mm.

[0060] Example 1: The ingredients were formulated according to the median range of the present invention: C 0.20%, Mn 2.25%, Si 1.0%, RE 0.003% (Ce:La = 7:3). The process parameters were as follows: heating at 1200℃ and holding for 2.5h, final rolling at 900℃, relaxation precipitation at 650℃ and holding for 300s, critical zone heating at 820℃, isothermal transformation at 400℃ and holding for 120s, shot peening temperature at 250℃, and shot peening intensity at 0.40 mmN.

[0061] Example 2: The ingredients were formulated according to the upper limit of the composition of this invention: C 0.22%, Mn 2.5%, Si 1.2%, RE 0.005%. The process parameters were as follows: heating at 1220℃ and holding for 2 hours, final rolling at 920℃, relaxation precipitation at 670℃ and holding for 200 seconds, critical zone heating at 830℃, isothermal transformation at 410℃ and holding for 100 seconds, shot peening temperature at 300℃, and shot peening intensity at 0.45 mmN.

[0062] Example 3: The ingredients were formulated according to the lower limit of the composition of this invention: C 0.18%, Mn 2.0%, Si 0.8%, RE 0.001%. The process parameters were as follows: heating at 1180℃ and holding for 3 hours, final rolling at 880℃, relaxation precipitation at 630℃ and holding for 400 seconds, critical zone heating at 810℃, isothermal transformation at 390℃ and holding for 150 seconds, shot peening temperature at 200℃, and shot peening intensity at 0.35 mmN.

[0063] Comparative Example 1: The composition is the same as that of Example 1, but no rare earth element RE is added, and the manufacturing process is the same as that of Example 1.

[0064] Comparative Example 2: The composition is the same as in Example 1, but the warm shot peening step is omitted and replaced with shot peening at room temperature (25°C). The rest of the process is the same.

[0065] Comparative Example 3: The composition is the same as that of Example 1. The relaxation precipitation process at 650°C is omitted in the process. After hot rolling, the critical zone treatment is carried out directly. The rest of the process is the same.

[0066] Comparative Example 4: The composition is the same as that of Example 1, but the isothermal transition temperature is set to 480°C (outside the scope of this invention), and the rest of the process is the same.

[0067] The microstructure and mechanical properties of each group of samples were systematically tested, and the results are shown in Table 1.

[0068] Table 1: Comparison of tissue characteristics and performance parameters between the embodiments and the comparative examples

[0069] Analysis of the data in Table 1 shows that Examples 1 to 3 fully meet the requirements of the present invention in terms of microstructure, with a high proportion of large-angle grain boundaries exceeding 80% and residual compressive stress on the surface exceeding -840 MPa. In actual forming performance tests, Examples 1 to 3 maintained extremely high integrity under extremely small radius bending conditions of R=2t and even R=1.5t, and no cracks deeper than 10 μm were observed under a metallographic microscope.

[0070] In comparison, Comparative Example 1, lacking rare earth elements, achieved a similar phase ratio through the same heat treatment process, but the proportion of large-angle grain boundaries was only 62.4%. This demonstrates the irreplaceable role of rare earth elements in purifying grain boundaries and regulating grain boundary energy. Without the restraint of a high proportion of large-angle grain boundaries, dislocations easily slip rapidly along low-orientation-difference grain boundaries under bending strain, converging into macroscopic cracks, leading to microcracks appearing as early as R=2t. Comparative Example 2, using room-temperature shot peening, introduced some compressive stress, but due to the lack of synergistic temperature effects, the surface plastic deformation layer was insufficiently deep and uneven, with a residual compressive stress of only -620 MPa. This was insufficient to completely offset the severe tensile stress caused by small-angle bending, resulting in forming failure.

[0071] Comparative Example 3 omitted the relaxation precipitation process, resulting in poorer microstructure uniformity and a significant deviation from the design value in the ratio of granular bainite to retained austenite. This indicates that the relaxation precipitation at 650℃ is not only a chemical purification process but also a crucial link in microstructure evolution, directly influencing the kinetic path of subsequent phase transformations by controlling the state of the original austenite. Comparative Example 4, due to excessively high isothermal temperatures, experienced overly rapid bainite transformation accompanied by carbide precipitation, severely depleting carbon and causing the retained austenite content to plummet to 4.5%. Without the protection of the TRIP effect, the material became more brittle, exhibiting significant cleavage fracture during bending.

[0072] Further analysis of the microscopic mechanisms in engineering implementation reveals that the martensitic lath refinement effect described in this invention provides another guarantee for achieving small-angle bending. In the transmission electron microscopy observation of Example 1, the martensitic laths exhibit an extremely regular parallel arrangement, with widths ranging from 0.2 μm to 0.4 μm. Since the martensitic laths are formed in the residual austenite region already segmented by granular bainite, their growth is strongly mechanically hindered. This refinement not only improves the macroscopic strength of the material but, more importantly, reduces the unit dislocation pile-up length, resulting in a smoother stress distribution at the microscale.

[0073] In constructing the surface nanocrystalline layer, this invention utilizes a shot peening process at 200-300℃ to achieve a smooth gradient transition in grain size from 35nm at the outermost layer to micron-level microstructure at a depth of 50μm. During bending deformation, the outermost layer, composed of high-density nanocrystals, possesses strong rotational and slip capabilities, absorbing a significant amount of deformation work. Simultaneously, the high residual compressive stress (e.g., -920MPa in Example 1) allows for the initial release of elastic energy under tensile loads, delaying the onset of plastic yielding in the surface metal. This multi-dimensional crack-resistant design gives the ultra-high-strength steel prepared by this invention significant advantages in the manufacture of automotive anti-collision beams, reinforcement components, and other parts requiring complex cold forming.

[0074] In actual metallurgical production, the Ce to La ratio (7:3) of this invention has been verified through extensive thermodynamic simulations. Ce is highly efficient in reducing interfacial energy, while La has a longer residence time at grain boundaries. This ratio ensures that the rare earth concentration at grain boundaries is consistently maintained above the critical value that promotes the formation of large-angle grain boundaries during continuous casting and rolling cooling processes. Simultaneously, by controlling the frequency of induction heating, the heating depth of the relaxation precipitation stage can be precisely adjusted, ensuring that the core and surface of the steel plate complete phase transformation preparation simultaneously, thereby guaranteeing a high degree of consistency in the microstructure of the finished steel plate in the thickness direction.

[0075] Regarding the aforementioned nitrate bath isothermal treatment, the salt bath has a much higher heat transfer coefficient than air and oil, allowing it to instantly bypass the dangerous zone of austenite-to-pearlite transformation and precisely stop at the bainite transformation zone. Within the isothermal time of 100 to 150 seconds, the diffusion distance of carbon atoms is strictly limited to a few micrometers, which precisely meets the process requirement of carbon enriching into retained austenite without precipitating carbides. Experimental tests show that the retained austenite obtained under this process window has extremely high mechanical stability; even after bending with a large deformation, approximately 5% of the austenite remains untransformed, providing a continuous plastic reserve for the subsequent fatigue service of the parts.

[0076] Furthermore, this invention stabilizes the surface roughness Ra of the steel plate at approximately 2.4 μm by controlling the angle (75° to 90°) and peening pressure during online temperature shot peening. This microstructure not only improves coating adhesion but also avoids stress concentration effects due to the absence of sharp micro-valve bottoms. Combined with a compressive stress field exceeding -800 MPa, the micro-defects on the material's surface are tightly "locked" under load, preventing them from evolving into transgranular cracks.

[0077] In summary, this invention, through global synergistic optimization of chemical composition, microstructure, grain boundary distribution, and surface stress field, successfully developed an ultra-high-strength steel with a stable tensile strength exceeding 1000 MPa and capable of withstanding small-angle bending (R≤2t) without cracking. Its manufacturing method relies on conventional industrial equipment, achieving low-cost, large-scale production of high-performance materials through precise reconstruction of key temperature and time points. In the extreme test of Example 1, when the bending radius was reduced to R=1.5t, the sample remained crack-free, marking a substantial breakthrough in the field of ultra-high-strength steel formability research.

[0078] The various process steps involved in this invention are interconnected and indispensable. For example, without relaxation precipitation at 650℃, a high proportion of large-angle grain boundaries cannot be obtained; without the combination of critical zone quenching and isothermal nitrate bath treatment, a multiphase microstructure with a high TRIP effect cannot be obtained; and without the final online shot peening, a surface compressive stress barrier sufficient to resist extreme stress states during small-angle bending cannot be constructed. This full-chain process coupling is the core of how this invention achieves its technical objectives. In future practical engineering applications, the shot peening intensity and holding time can be finely adjusted within the above process framework according to different plate thickness requirements to obtain ultra-high-strength steel products with highly consistent performance, possessing extremely strong industrial application value.

[0079] Furthermore, this invention has also deeply optimized the details of rare earth alloy wire incorporation. The thickness of the low-carbon steel sheet is 0.5-0.8 mm, and the wire feeding speed is controlled at 150-200 meters per minute to ensure that the rare earth alloy wire can accurately penetrate deep into the molten steel and fully melt and diffuse during its ascent, avoiding the oxidation and crusting of rare earth elements on the surface. This precise control of engineering details ensures extremely high accuracy in the rare earth element content of the composition, thus providing a reliable material basis for the subsequent evolution of all microstructures.

[0080] In the performance verification of the final product, in addition to bending performance, the steel plate prepared by this invention also exhibited excellent delayed crack resistance. Due to the spheroidizing and fixing effect of rare earth elements on impurities such as phosphorus and sulfur, the grain boundary energy is always kept at a low level. Even in humid, hydrogen-containing environments, due to the strong hindrance of hydrogen diffusion by large-angle grain boundaries, its delayed fracture failure time is more than three times longer than that of conventional ultra-high-strength steel. This characteristic is of great significance for improving the reliability of automotive safety components in complex service environments.

[0081] This invention provides an ultra-high-strength steel capable of being bent at small angles and its manufacturing method, and its preferred embodiments are not limited to the aforementioned examples. In actual production, slight fluctuations in chemical composition can be compensated for by adjusting the final rolling temperature of hot rolling and the holding time of isothermal transformation. For example, when the manganese content is at the lower limit of 2.0%, the isothermal transformation time can be appropriately extended to 150 seconds to ensure sufficient enrichment of carbon in the retained austenite, thereby obtaining consistent bending performance. This process flexibility and robustness are important guarantees for the successful application of this invention in modern wide strip steel continuous production lines.

[0082] Finally, cross-sectional nanohardness testing of the sample from Example 1 revealed a quasi-linear decreasing trend in hardness from the outermost layer to a depth of 50 μm. This gradient hardness distribution proved to be an ideal physical model for alleviating bending stress concentration. Combined with the deep compressive stress field introduced by warm shot peening in this invention, the strain gradient within the steel plate is greatly smoothed during bending. This integrated system solution, from microstructure design to stress engineering, opens up broad prospects for the application of 1000 MPa-grade ultra-high-strength steel.

Claims

1. A type of ultra-high strength steel that can be bent at a small angle, characterized in that, Its chemical composition, by mass percentage, includes: carbon (C) 0.18-0.22%, manganese (Mn) 2.0-2.5%, silicon (Si) 0.8-1.2%, aluminum (Al) 0.03-0.06%, phosphorus (P) ≤0.012%, sulfur (S) ≤0.003%, rare earth elements (RE) 0.001-0.005%, with the balance being iron (Fe) and unavoidable impurities; The microstructure of the ultra-high strength steel that can be bent at a small angle consists of 40-50% granular bainite, 12-15% retained austenite and the balance martensite, wherein the martensite is lath martensite and the average width of the martensite laths is not greater than 0.5 μm. The ultra-high strength steel that can be bent at small angles has a multi-layered crack-resistant structure. The structure includes a large-angle grain boundary feature distribution inside the steel plate matrix, and a nanocrystalline strengthening layer and residual compressive stress field on the surface of the steel plate. The proportion of large-angle grain boundaries inside the structure is not less than 80%, and the large-angle grain boundaries are grain boundaries with a crystal orientation difference greater than 15°. The surface layer of the steel plate has a nanocrystalline layer with a thickness of not less than 40 μm. Inside the nanocrystalline layer, the grain size is distributed in a gradient from the surface to the inside, with the outermost surface grain size between 20 nm and 100 nm. The surface layer of the steel plate also has a residual compressive stress of not less than -800 MPa.

2. The ultra-high strength steel that can be bent at a small angle according to claim 1, characterized in that: The rare earth element RE is a mixture of cerium (Ce) and lanthanum (La), wherein the mass ratio of Ce to La is set to 7:3; the rare earth element RE exists in the matrix in the form of solid solution or spherical sulfides, and during the steel melting process, the slag basicity R is controlled between 3.0 and 3.5 to ensure that Ce and La form segregation at the grain boundaries.

3. The ultra-high strength steel that can be bent at a small angle according to claim 1, characterized in that: The granular bainite consists of a bainitic ferrite matrix and discrete carbon-rich islands dispersed therein. The carbon-rich islands are M / A islands, and a discontinuous interface structure is formed between the bainitic ferrite matrix and the M / A islands. When the crack propagates to the granular bainite region, the discontinuous interface induces the crack to deflect, increasing the actual propagation path length of the crack by more than 30% compared to the straight path.

4. The ultra-high strength steel that can be bent at a small angle according to claim 1, characterized in that: The martensitic laths are spatially segmented by the granular bainite formed in the early stage, and their growth is geometrically restricted so that the width is kept below 0.5 μm; the retained austenite generates a phase transformation-induced plasticity effect during bending deformation, and the volume expansion generated by the transformation to martensite forms a local compressive stress field at the microscale to offset the external tensile stress.

5. The ultra-high strength steel that can be bent at a small angle according to claim 1, characterized in that: There is a transition layer with a thickness of 20-30 μm between the nanocrystalline layer and the substrate. The transition layer is composed of fine-grained ferrite and deformed martensite, with its grain size smoothly transitioning from nanoscale to microscale. The nanocrystalline layer is bent and deformed by a combination of grain boundary slip and rotation, which shields the microcracks from penetrating into the substrate.

6. A method for manufacturing ultra-high strength steel that can be bent at a small angle according to any one of claims 1-5, characterized in that, Includes the following steps: (1) Smelting and casting: Vacuum induction furnace or converter is used for smelting, the impurity content is strictly controlled, and rare earth alloy wire is added by wire feeding method at the end of refining to obtain continuous casting billet; (2) Heating process: The continuous casting billet is heated to 1200±20℃ and held for 2-3 hours to achieve solid solution of alloying elements and homogenization of austenite grains; (3) Hot rolling process: The final rolling temperature is controlled at 900±20℃, the total hot rolling compression ratio is not less than 80%, and the steel plate is cooled to room temperature by ultra-fast cooling technology after rolling; (4) Relaxation precipitation process: The steel plate is heated to 650±20℃ and kept for 200-400s. Rare earth elements are used to purify the austenite grain boundaries and induce the dispersion and precipitation of fine inclusions. (5) Critical zone quenching process: The steel plate is heated to 820±10℃ for partial austenitization treatment, and then rapidly cooled to 400±10℃ at a cooling rate of more than 50℃ / s. (6) Isothermal transformation process: The steel plate is kept in an isothermal medium at 400±10℃ for 100-150s, so that some austenite is transformed into granular bainite and carbon elements are enriched into the untransformed austenite. (7) Final cooling process: The isothermal treated steel plate is water quenched or oil quenched to room temperature to transform the remaining unstable austenite into lath martensite with a width ≤0.5μm; (8) Online shot peening strengthening process: The cooled steel plate is reheated to 200-300℃ and shot peening is performed in this temperature range to induce plastic deformation of the surface metal on the steel plate to form a nanocrystalline layer and residual compressive stress field.

7. The manufacturing method of ultra-high strength steel that can be bent at a small angle according to claim 6, characterized in that: In the smelting process described in step (1), the oxygen content in the steel is controlled to be below 10 ppm by a vacuum carbon deoxidation process; Ce and La in the rare earth alloy wire are wrapped in a low carbon steel sheet with a thickness of 0.5-0.8 mm, and the wire feeding speed is controlled at 150-200 m / min to prevent the rare earth elements from oxidizing and to ensure that they penetrate deep into the molten steel.

8. The manufacturing method of ultra-high strength steel that can be bent at a small angle according to claim 6, characterized in that: In the relaxation precipitation stage described in step (4), induction heating is used, and the heating rate is controlled at 30-50℃ / s. The skin effect of electromagnetic induction is used to make the surface of the steel plate reach the set temperature first. In the critical zone quenching process described in step (5), the austenitization holding time is calculated as 0.5-1.0 minutes per millimeter of plate thickness.

9. A method for manufacturing ultra-high strength steel that can be bent at a small angle according to claim 6, characterized in that: In the isothermal transition stage described in step (6), a segmented temperature control device is used to control the transverse temperature difference of the steel plate within ±3℃; the isothermal medium is a nitrate bath composed of 50% sodium nitrate and 50% potassium nitrate.

10. A method for manufacturing ultra-high strength steel that can be bent at a small angle according to claim 6, characterized in that: In the online warm shot peening strengthening process described in step (8), the shot peening intensity is controlled at 0.35-0.45 mmN, the shot peening medium is cast steel shot or ceramic shot with a hardness of not less than 650 HV and a diameter of 0.3-0.5 mm, the coverage is not less than 200%, and the shot peening angle is set to be 75° to 90° with the steel plate surface; the cooling rate after warm shot peening is controlled at 5-10℃ / s to achieve the stabilization treatment of residual compressive stress. After the shot peening strengthening process described in step (8), a leveling process is also included, with the leveling elongation controlled between 0.3-0.5%; the manufacturing method adjusts the shot peening pressure and flow rate to control the surface roughness Ra of the steel plate between 1.6-3.2μm.

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