Ultra-high strength stainless steels often used as load-bearing structural component in aviation, aerospace, marine and other fields, due to its ultra-high strength and good corrosion resistance. For the ultra-high strength stainless steel, simply improving its strength is not enough for the practical application. The future development of ultra-high strength stainless steel should take into account the plastic, toughness and good corrosion properties of steel in addition to ultra-high strength. In order to achieve the optimal combination of strength and toughness in 2.1GPa stainless steel, this study employs the concept of composite precipitation strengthening with various types of second phase combined with reverse transformation of austenite to enhance toughness, aiming to surpass the limitations of strong-toughness matching in 2.1GPa ultra-high-strength stainless steel. The effects of secondary aging temperature on the microstructure and mechanical properties of 2.1GPa stainless steel are studied by means of SEM, XRD, TEM and mechanical properties testing. The results show that a large number of nanoscale second phases are dispersed on martensitic matrix with high density dislocation after the steel is treated at different secondary aging temperatures. These second phases are mainly composed of Laves phase and M2C phase. At the same time, thin film austenite is found in the boundary of martensitic lath. During the aging process, the precipitation, growth, coarsening of M2C and Laves phases, as well as the increase in austenite content, result in an initial increase followed by a decrease in the strength and toughness of the steel. When the aging temperature is 520°C, the strength and toughness of steel reach the best match.

High-strength ferrite-martensite dual-phase steels usually suffer from low elongation and fracture strain. To solve this problem, a Ti-bearing low carbon DP steel was designed in this study to prepare a 980 MPa grade DP steel with excellent elongation and fracture strain by narrowing the strength difference between ferrite and martensite as well as refining the grains. It was demonstrated that the ferrite volume fraction decreased from 57% to 32% and the ferrite grain size from 2.1 μm to 1.6 μm for the experimental steel as the annealing temperature was increased from 760 °C to 800 °C. Regarding the mechanical properties, the ultimate tensile strength increased from 983 MPa to 1081 MPa, the total elongation decreased from 17.6% to 13.9%, and the fracture strain increased from 0.41 to 0.62 for the experimental steels as the annealing temperature was increased from 760 °C to 800 °C. The increasing annealing temperature increased the martensite volume fraction and therefore reduced the martensite carbon content, which in turn reduced the ferrite-martensite strength difference, promoted the coordinated deformation ability between the two phases, and ultimately inhibited the ferrite-martensite interface decohesion. Moreover, the reduced martensite carbon content also improves the martensite toughness, thereby inhibiting martensite fracture. The combined effect of these two factors is the main reason for the increase in fracture strain of the experimental steel with increasing annealing temperature. In addition, compared with the reported DP steels, the 800 °C annealed experimental steels exhibit higher fracture strains at identical ultimate tensile strength. This can be attributed to three factors. Firstly, TiC particles reduce the strength difference between ferrite and martensite, promote the coordinated deformation ability between the two phases, and thereby suppress the ferrite-martensite interface decohesion. Secondly, the combination of low carbon content and high martensite volume fraction reduces the martensite carbon content, which improves the martensite toughness and thus suppresses the martensite cracking. Finally, TiC particles refine the grains and further improve the matrix toughness.

As a widely used type of steel, gear steel needs to be processed into complex shapes and withstand high friction during operation. Therefore, it is required to have excellent cutting performance and wear resistance. Sulfide inclusions in steel are a key factor in the coordinated control of the two, and the addition of tellurium can act as a modifier for sulfides in steel. Therefore, this article explores the effect of tellurium addition on the wear resistance and inclusion properties of 20MnCrS5 gear steel through scanning electron microscopy, automatic analysis of inclusions, nanoindentation of inclusions, and dry friction experiments. The results indicate that the addition of tellurium reduces the number of elongated and chain like inclusions in 20MnCrS5 steel, and the morphology of sulfide inclusions tends to spheroidize. When the mass fraction of Te in the steel is 0.053%, the aspect ratio of inclusions decreases from 2.302 without Te to 1.720. Combined with thermodynamic calculations, it is shown that it is mainly due to the transformation of sulfide precipitation characteristics caused by Te addition. The nanoindentation experiment on the inclusions themselves showed that adding tellurium element to steel can improve the hardness of sulfide inclusions. After Te modification, the hardness of MnS inclusions increased from 7.82 GPa without Te to 9.42 GPa. And the statistical results of the deformation amount of sulfide inclusions during the rolling deformation process show that their deformation rate can be reduced from 16.6% to up to 3.8%. Meanwhile, dry friction experiments showed that the addition of Te reduced the wear and friction coefficient of 20MnCrS5 gear steel by spheroidizing inclusions in the steel and increasing the hardness of sulfides, resulting in enhanced wear resistance. This article explains the influence of inclusions steel matrix on deformation and friction processes, providing corresponding references for the further development and optimization of Te containing special steels in the future.

Metallographic observation is an important method to analyze the microstructure of materials, and the collection of high-quality metallographic photos is particularly important for material analysis. However, the acquisition of high-quality metallographic photographs is a time-consuming and labor-intensive process. Especially for ultra-low carbon steel, due to its low deformation resistance, scratches are easy to appear when grinding samples, in addition, it is difficult to obtain clear grain boundaries by corrosion means, which brings great difficulties to microstructure analysis. In this paper, a grain boundary strengthening method based on CycleGan-SA was developed based on the attention mechanism and the recurrent regression neural network algorithm to realize the grain boundary strengthening of ultra-low carbon steel microstructure photos. On this basis, the watershed segmentation algorithm is used to achieve fine grain size statistics for the microstructure images after grain boundary enhancement. The results show that: Compared with the traditional segmentation method, the CycleGan-SA model can realize the enhancement of microstructure characteristic information. The grain size distribution measured by the refined analysis model is basically the same as that measured by Image J, and the grain boundary ratio is only 0.008% different from that measured by Image Pro Plus.

Fe-based amorphous alloy has the structural characteristics of long-range disorder, uniform composition and no defects such as grain boundaries and dislocations. The wear and corrosion resistance of amorphous alloy is much better than that of traditional stainless steel. However, the preparation of corrosion-resistant amorphous alloys usually requires the addition of high content of chromium and molybdenum for remelting and alloying, leading to the high cost of raw materials and the difficulties in melting. In this study, a novel strategy for preparing corrosion-resistant amorphous alloy by using S31254 stainless steel was proposed. Combined with the analysis of the composition characteristics of S31254 stainless steel and the mixing enthalpy as well as the atomic size difference, a new corrosion-resistant amorphous alloy with the composition of S31254+15C+6B (at.%) was designed. The low melting point region and slag-metal equilibrium of refining slag were calculated in detail by using FactSage. As a result, a low melting point refining slag of 10%SiO2-40%CaO-25%Al2O3-25%B2O3 suitable for S31254+15C+6B amorphous alloy was determined. After alloying and refining, alloy ribbons were successfully prepared by using the single-roller melt-spinning technique. The amorphous forming ability and corrosion resistance of the ribbon samples were evaluated via XRD and electrochemical workstation. The results show that the manufacturability of ribbons is significantly improved after the alloying of B, C elements, and the ribbons with good surface quality can be readily obtained. After refining, the content of O and S in the alloy is significantly reduced, and the amorphous forming ability is significantly improved. Amorphous ribbons of 25-42 μm can be prepared at different melt-spinning rates of 20-35 m/s. Moreover, the S31254+15C+6B amorphous alloy exhibits a wide passivation interval and a large impedance radius, demonstrating the excellent corrosion resistance of the S31254+15C+6B amorphous alloy, which is much better than that of S31254 stainless steel. The above results and analyses prove the effectiveness of the composition design and refining slag design in this study. It is feasible to prepare corrosion-resistant amorphous alloys by using S31254 stainless steel. On one hand, the proposed strategy can make full use of Fe, Ni, Cr, Mo elements in S31254 stainless steel, which will then reduce the cost of raw materials and avoid the refractory problems of chromium and molybdenum. On the other hand, the resultant amorphous alloy possesses significantly enhanced corrosion resistance than S31254 stainless steel. This study is of great significance to the high-value utilization of scrap steel resources and the large-scale production and application of high-performance corrosion-resistant amorphous alloys.

It is common that microalloying elements have not been completely precipitated in hot rolled strip steel, and tempering treatment can make the solid-solved microalloying elements in the steel precipitate in the matrix, thus enhancing its mechanical properties. In this paper, the effect of tempering temperature on precipitation behavior of microalloyed second phase of hot-rolled Ti-Mo-V microalloyed ultra-high-strength steel was studied by scanning electron microscope, transmission electron microscope and Vickers hardness tester. The results show that the ferrite grain changes in the tested steel are not obvious in the range of 550℃-700℃. The microalloyed elements that are not completely precipitated in the hot rolling stage are gradually precipitated during the tempering process, and the precipitated phase is V-rich (Ti, Mo, V)C particles. At the same time, the volume fraction and the average particle size of (Ti, Mo, V)C particles in the tested steel gradually increase with the increase of tempering temperature, and the proportion of V atoms in (Ti, Mo, V)C particles increases and the lattice constant decreases. When tempering temperature is 700 ℃, the precipitation volume fraction of (Ti, Mo, V)C particles in the tested steel reaches 0.603%, the average particle size is about 8.39 nm, the proportion of V atoms is 51.9%, and the lattice constant is 0.436 nm. In addition, precipitation strengthening increment provided by the microalloyed second phase of the tested steel exhibits a tendency of increasing and then decreasing with the increase of tempering temperature. When the tempering temperature increases from 550 °C to 650 °C, the precipitation strengthening increment in the tested steel increased from 280.33 MPa to 314.81 MPa, and the microhardness reached a peak of about 282.07 HV. When the tempering temperature was further increased to 700 °C, the (Ti, Mo, V)C particles were significantly coarsened, and the precipitation strengthening increment produced by them decreased to 286.90 MPa, and the softening of the matrix microstructure further intensified, the microhardness of the tested steel decreased to 261.13 HV.