Research Progress and Prospect of Ultra-High Strength Steel
Abstract: Ultra-high strength steel is the core material for key load-bearing components of major equipment such as aerospace and construction machinery. This article reviews the current technological status of ultra-high strength steel in three development stages from theoretical breakthroughs to engineering applications: In the laboratory stage, the strength limit of 2,600 to 3,000 MPa (for bulk materials) has been broken through; The stable preparation of 2,000-2,500 MPa grade steel was achieved in the pilot stage. A mature production system for steel grades ranging from 1,500 to 2,000 MPa was established during the industrialization stage. However, with the increasingly harsh extreme service environment, the existing mature ultra-high strength steel is no longer able to meet the engineering and technical requirements. The fundamental constraint lies in the intrinsically inverted relationship between strength and toughness. This article clarifies that the key to breaking the inversion of strength and toughness lies in the systematic optimization of the entire material preparation process, and condenses three major materials science elements that determine performance breakthroughs: purity - controlling impurity elements and inclusions to eliminate crack sources; Uniformity - Eliminate component segregation and tissue gradient to ensure overall reliability; Organizational degree - Regulating multi-scale microstructure to achieve the synergy of strengthening and resilience. Based on this theoretical framework, this paper introduces the technical system of the 1,700-2,700 MPa series of ultra-high strength steel developed by the author's team. Looking ahead, the development of ultra-high strength steel will focus on directions such as ultra-pure smelting, high-uniformity preparation, intelligent material design, and exploration of the 3000 MPa grade limit. Key words: Ultra-high strength Purity Uniformity Degree of organization
Ultra-high strength steel, as a strategic structural material, serves as the core material foundation in key fields such as aerospace, construction machinery, transportation, and the energy industry. It plays an irreplaceable role in applications such as landing gear, armor protection, lightweight structures and deep-sea engineering.
Facing increasingly harsh extreme service environments, the development of traditional ultra-high strength steel is confronted with major bottlenecks, and there is an inherent inverted relationship between strength and toughness. This contradiction stems from the essential defect of the traditional strengthening mechanism - while introducing microscopic obstacles to enhance strength, it suppresses the uniform plastic deformation capacity, leading to an increased risk of stress concentration and brittle fracture. The fundamental problem lies in that strengthening the microstructure increases the driving force for crack propagation but fails to simultaneously enhance the resistance to crack propagation.
To achieve a coordinated improvement in higher strength and toughness, systematic work needs to be carried out in the three dimensions of material purity, uniformity and microstructure: purity control requires minimizing harmful impurities and inclusions to clear obstacles for performance improvement; Uniformity control requires achieving precise and uniform distribution of components and tissues to avoid weak performance links. The control of organizational degree requires ingenious design to achieve the orderly arrangement and collaborative effect of multi-scale structures.
To break through the existing bottlenecks, it is necessary to transcend the limitations of traditional alloying and process optimization, and achieve the synergy of interface strengthening and plastic deformation through ingenious microstructure design, creating an effective micro-path for energy dissipation. At the same time, engineering challenges such as performance consistency control in industrial production and maintaining organizational stability during long-term service also require a systematic perspective to reconstruct material design concepts, laying a theoretical and technical foundation for the development of the next generation of ultra-high strength steel.
The improvement of the strength of steel materials has always been an important topic in the field of materials science. The differences in the macroscopic properties of materials stem from their microstructure. Specifically, the strength of a material is strongly correlated with the specific configuration of its internal structure. Microstructures of different forms function through distinct strengthening mechanisms, resulting in significant differences in their intensity values, as shown in Figure 1. The strength of pure iron single crystals is only about 80 MPa, that of ferritic steel can reach 900 MPa, that of ferritic-pearlite steel can reach 1,000 MPa, that of hot-rolled eutectoid steel exceeds 1,000 MPa, that of martensitic steel can reach 3 GPa, and that of cold-drawn pearlite steel wire exceeds 7 GPa, while the ideal strength is approximately 13 GPa.
The research on the strengthening mechanism of steel materials mainly follows two completely different technical paths: the theoretical strength approach based on perfect crystal structure and the strengthening mechanism based on microscopic defects. There is an order of magnitude gap between the theoretical strength of perfect crystals and the actual strength of steel, mainly due to the existence of microscopic defects. As early as 1956, Brenner first achieved a tensile strength of 13 GPa close to the theoretical strength through iron whiskler experiments [1].
On the contrary, the strengthening effect of microscopic defects is positively correlated with the defect density. By precisely controlling the density of microscopic defects such as dislocations, grain boundaries, and precipitates, the strength of materials can be significantly enhanced. By introducing high error density (1015 m-2) as the core strengthening mechanism, the material can achieve extremely high strength. For instance, a yield strength of 2.2 GPa was achieved in medium manganese steel through this mechanism [2]. Based on a similar dislocation strengthening principle, by introducing high-density dislocations and the synergistic effect of nanoscale obstacles (such as carbides) in martensitic steel, its tensile strength can also reach 2.2 GPa [3]. This strategy can even be extended to pure metals. Research shows that pure iron deformed under high pressure can achieve a uniaxial compressive ultimate strength of 3.7 GPa due to the high-density dislocations generated inside it [4]. The introduction of high-density microscopic defects has enabled the strength of steel materials to reach the 7 GPa level [5].
The comparison of these two strengthening paths reveals the essential mechanism of material strengthening: the perfect crystal path approaches the theoretical limit by eliminating defects, while the defect strengthening path optimizes the strength-toughness match by precisely regulating the type, density and spatial distribution of defects. This understanding provides important theoretical guidance for the structural design and performance optimization of modern high-performance steel materials.
At present, the widely used ultra-high strength materials are mainly based on defect strengthening mechanisms. Ultra-high strength steels mainly include low alloy ultra-high strength steels, martensitic age-hardening steels, high Co-Ni secondary hardening steels, martensitic stainless steels and medium/high manganese steels, etc. In recent years, significant progress has been made in the research of medium and high entropy alloys, and ultra-high strengthening effects have also been achieved. Figure 2 shows the strength-plastic-toughness of typical ultra-high strength steels. For comparison, the data of medium and high entropy alloys have been added. Figure 2 shows a typical inverted relationship between strength and plasticity and toughness. The maximum strength of traditional ultra-high strength steel (low alloy ultra-high strength steel, martensitic aging steel, high Co-Ni secondary hardening steel, martensitic stainless steel) can reach 3 GPa. Medium/high manganese steels and medium/high entropy alloys, with their complex multi-scale microstructures (such as transformation-induced plasticity, severe lattice distortion, etc.), demonstrate great potential to surpass traditional ultra-high strength steels in terms of strength-plasticity synergy. However, due to the current complex preparation processes and controllability challenges, most of the research on these two types of high-performance materials is still at the laboratory stage, and their industrial application still needs to be broken through.
The research and development process of ultra-high strength steel usually goes through three stages: laboratory research, pilot-scale amplification and industrial mass production. In the laboratory stage, principle verification is completed; in the pilot-scale stage, engineering amplification is achieved; and finally, in the industrial stage, large-scale stable application is realized. Its technological maturity and application scope have been gradually enhanced step by step.
During the industrialization stage, large-scale production of steel grades such as 4340, 300M, AerMet100, and PH 13-8Mo has been achieved, with strength grades ranging from 1,500 to 2,000 MPa and fracture toughness ranging from 60 to 120 MPa·m1/2. Among them, the tensile strength of 300M steel can reach over 1,860 MPa and is widely used in aircraft landing gear. The tensile strength of AerMet100 is 1,930 MPa, and its fracture toughness is as high as 110 MPa·m1/2 [6]. The strengthening of these steel grades mainly achieves ultra-high strength through martensitic transformation and fine and dispersed precipitated phase strengthening. At present, the tensile strength of AerMet360 steel has reached 2,580 MPa, but its elongation is only 5.2% and its fracture toughness is only 22.6 MPa·m1/2. The relatively low plasticity and toughness limit its wide application.
A significant breakthrough was achieved in the pilot stage. Wang et al. [7-8] fabricated 2200 and 2,500 MPa grade NiAl and M2C dual-precipitation ultra-high strength steels on the pilot line, while maintaining an elongation rate of 8-10%. The strengthening and toughening mechanism at this stage is mainly based on composite precipitation strengthening. Traditional martensitic age-hardening steels generally strengthen the matrix through the dispersion and precipitation of Mo and Ti intermetallic compounds, while the new dual-precipitation steel achieves performance breakthroughs through the NALI-M2C composite strengthening mechanism.
At the laboratory stage, ultra-high strength steel with tensile strength ranging from 2,600 to 3,000 MPa has been prepared. Liu et Al. [9] based on the addition of 1% Al to the composition of M54 steel and through the double precipitation strengthening of NiAl and M2C carbides, its tensile strength can reach 2700 MPa. Medium carbon low manganese steel can achieve an ultra-high tensile strength of 2,800 MPa and an elongation of 18%. Its outstanding performance stems from a multi-level structure. The microstructure of this steel is composed of martensite with volume fractions of 67%, retained austenite with 10%, ferrite with 23%, and nano-scale precipitates [10]. Based on the optimization of C350 aging steel, by adjusting the contents of Mo and Ti, the tensile strength can reach 3,002 MPa after 1.8% pre-deformation stretching [11].
The realization of the coordinated improvement of the strength and plasticity and toughness of ultra-high strength steel is fundamentally constrained by three key material science elements: purity, uniformity and microstructure. High purity is the prerequisite for eliminating the source of cracks and delaying fracture. High uniformity is the foundation for ensuring consistent performance and avoiding local weakening. High organizational degree is the fundamental guarantee for precisely regulating the strengthening phase and the toughening phase to achieve a match between strength and toughness. The three together form the intrinsic cornerstone of the outstanding mechanical properties of ultra-high strength steel.
2.1 High Purity Purity is the prerequisite for the performance of ultra-high strength steel. Harmful elements, impurity elements and non-metallic inclusions act as stress concentration sources and crack initiation points, seriously deteriorating the strength, toughness and fatigue properties of materials.
Davis et al. [12] found through comparative research that the vacuum arc remelting process can significantly reduce the gas content (mass fraction) in 4340 steel compared with atmospheric smelting: H, O and N were respectively reduced from 1.4×10-6, 25×10-6 and 100×10-6 to 0.9×10-6, 4×10-6 and 53×10-6. The uniformity of the material was improved, and the fracture toughness increased from 44.5 MPa·m1/2 to 60.4 MPa·m1/2. In the ultra-high strength steel system of 0.15C-0.3Si-1.0Mn-0.4Cr-0.1Mo-0.015Ti-0.0015B, the research shows that when the total content of impurities O, N and S increases from 55×10-6 to 91×10-6, The number density of coarse inclusions increased from 0.7 per mm ² to 1.2 per mm ² accordingly, resulting in a decrease of 15 J and 11 J in the transverse and longitudinal impact energies at -40℃, respectively [13]. The total mass fraction content of the five harmful elements (As, Sn, Sb, Bi, Pb) in 300M steel prepared from 4N grade high-purity iron can be as low as 2.7×10-6 [14].
Non-metallic inclusions affect the properties of steel through stress concentration effect, interfacial debonding mechanism and hydrogen-induced cracking [15-17]. The fatigue failure mechanism changes with the strength level: high-cycle fatigue is controlled by surface defects, and ultra-high-cycle fatigue is controlled by internal inclusions [18]. When the yield strength exceeds 1,500 MPa, internal inclusions become the main controlling factor of fatigue life [19]. The rolling contact fatigue life of M50 steel made from 4N grade high-purity iron has been significantly enhanced [14].
With the continuous advancement of smelting technology, the conventional EAF+LF+VD/VOD process has now been adopted to replace the expensive VIM+VAR double vacuum process in the production of some ultra-high strength steel varieties, significantly reducing production costs. Han Shun et al. [20] prepared 300M steel by optimizing the single vacuum melting process. They adopted key technical measures such as electric arc furnace oxidation refining for phosphorus removal, LF furnace white slag sulfur control, and VD/VHD strong stirring degassing, achieving an effective balance between cost control and performance guarantee. Compared with electroslag remelting, the oxygen content in steel after vacuum consumable smelting is lower. The oxygen content in steel decreases from 9×10-6 to 5×10-6, and the inclusions are also finer. The size of inclusions in vacuum consumable smelting is mainly distributed in the range of 2 to 5μm, while in electroslag remelting, it is mainly distributed in the range of 5 to 10μm. After vacuum consumable smelting, the crack induction drive around the inclusions is reduced The ultra-high cycle fatigue strength is higher. Under the condition of 109 cycle cycles, the fatigue strength is 21 MPa higher than that of electroslag remelting steelmaking [21].
2.2 High Uniformity Uniformity is the foundation for ensuring consistent performance. Component segregation and uneven structure can lead to significant differences in mechanical properties among various parts, creating a "barrel effect" and confining the overall performance to the weakest link. Achieving multi-scale organizational uniformity is the key to ensuring performance consistency and service reliability. The main factors affecting the uniformity of materials include segregation, banded formation, etc.
Component segregation damages the strength and toughness of materials. Segregation leads to uneven element concentration, forming a hard and brittle phase. It may cause solute enrichment and strengthening in local areas, but it is more likely to become a crack source, weakening the overall strength and triggering early brittle fracture, thereby causing premature neck shrinkage and failure of the material [22-23]. The performance difference between the segregation zone and the matrix leads to uncoordinated deformation and stress concentration. Guo et al. [24] demonstrated that in ultra-high strength steel at 1,500 MPa, the average impact energy at room temperature in the segregation zone decreased from approximately 47 J to about 32 J. The microstructure in the non-elemental segregation zone was uniformly dispersed, with more large-angle grain boundaries and a lower KAM value. It can effectively prevent crack propagation [25]. Ritchie et al. [26] found that the co-segregation of alloying elements Ni and Mn and impurity elements P and Si at the original austenite grain boundaries caused the embrittlement of 300M steel. The reduction of alloying elements such as Cr, Mo and Ni caused by macroscopic segregation will lower the corrosion resistance of 300M steel [27].
Banded structure, as the main defect in ultra-high strength steel, not only leads to significant anisotropy in mechanical properties, but also deteriorates the material's toughness, ductility and formability, and is more prone to stress concentration and crack initiation points under dynamic loads. The strain gradient between the matrix and the segregation band promotes crack initiation, especially generating stress concentration at the interface between the matrix and the segregation band [28]. In martensitic aging steel, the segregation of Ti and Mo is the main cause of the formation of banded structures. When the concentration of these elements exceeds the critical level, banded structures composed of different grain sizes will form, and inclusions preferentially exist at the interfaces of banded structures [29]. After smelting and forging, high alloy steel usually forms banded structures [30]. The presence of banded structures makes the steel structure uneven, prone to anisotropy, and reduces the plasticity and toughness of the steel. The main reason for the formation of banded structure is the selective resolution of molten steel during the ingot crystallization process, the different solubilities of various solute atoms, and the density differences of alloy compositions, which form dendrite structure with uneven chemical composition. During forging deformation, most of the coarse dendrites will be elongated along the deformation direction during forging and gradually align with the deformation direction And to a certain extent, dendrite segregation is still retained, thus forming depletion zones of carbon and alloying elements that alternate and stack with each other.
2.3 Degree of Organization Degree reflects the ingenuity of organizational design. By precisely controlling grain size, distribution of precipitated phases, interface structure, austenite distribution, etc., a multi-scale microstructure is constructed to achieve the synergistic effect of strengthening mechanisms, providing an effective path for plastic deformation while ensuring high strength.
Microstructure, as the core concept of the microstructure design of ultra-high strength steel, is achieved by precisely controlling multi-scale microstructure parameters such as grain size, distribution of precipitated phases, interface structure [31], and distribution of metastable austenite [32]
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