Search

The microstructure and mechanical properties of marine 10Ni5CrMoV steel are systematically investigated at tempering temperatures ranging from 630 to 670 °C. With increasing tempering temperature, several key parameters exhibit notable changes: the volume fraction of reversed austenite decreases from 17.36% to 13.39%. The equivalent grain size increases from 0.91 to 1.09 μm. The equivalent diameter of precipitates increases from 59 to 79 nm. The dislocation density decreases from 0.82 × 10−15 to 0.51 × 10−15 m−2. As the tempering temperature increases, the pinning effect of precipitates on dislocations and martensite laths decreases. This leads to reductions in both dislocation strengthening and precipitation strengthening mechanisms. Consequently, the yield strength of the specimen decreases from 876 to 683 MPa. The increase in the ductile‐brittle transition temperature of the specimens from −116 to −95 °C can be attributed to several factors. First, the increase in precipitate size reduces the critical nucleation stress of cleavage cracks and promotes the nucleation of such cracks. Additionally, the decrease in dislocation density mobility leads to a reduction in the dimple nucleation rate, thereby diminishing the role of dimples in hindering crack propagation. [ABSTRACT FROM AUTHOR]

High‐nitrogen stainless bearing steel (HNSBS) is the most representative third‐generation aviation bearing steel, and the cleanliness and microstructure characteristics of bearing steel are crucial to its high performance and long service life. In this work, the effect of cerium addition on the cleanliness and microstructure characteristics of as‐cast HNSBS was systematically studied by combining microstructural characterization and thermodynamic analysis methods. The results show that with increasing Ce content, the cleanliness of steel first improved and then decreased, and Ce2O2S, Ce2O3 and CeN inclusions were generated successively. The Ce content should be controlled to ≈0.012 wt% to ensure that all Al2O3, MgO · Al2O3, and MnS inclusions were modified, and minimize the formation of large‐size Ce‐bearing inclusions. Furthermore, cerium addition refined the dendrite structure due to more nucleation sites for γ‐Fe and the enrichment of Ce in the liquid phase at solidifying front. The finer and more dispersed distribution in precipitates was ascribed to the reduction of growth space and Cr segregation, as well as the increase in effective nucleation sites. These results can offer theoretical guidance for inhibiting the formation of harmful inclusions in high‐nitrogen alloy systems and refining the microstructure of alloy systems containing M23(C, N)6 and M2(C, N) precipitates. [ABSTRACT FROM AUTHOR]

The removal mechanism of vanadium addition on nitrogen pores in 30Cr15Mo1N ingot is systematically investigated through statistical analysis and theoretical calculation. The nitrogen solubility is obtained by the thermodynamic model, and the nitrogen content is obtained by combining the ProCAST software and the C–K model. Then, according to the formula of critical nucleation radius and growth rate of nitrogen bubbles, the variation of critical nucleation radius and growth rate of nitrogen bubbles with different vanadium contents is calculated. As the vanadium content increases, the number and mean area of nitrogen pores decrease. In comparison to the 0 V ingot, the number of nitrogen pores decreases in the 0.2 V ingot, but there is an increase in the number of large‐sized nitrogen pores (>3000 μm in diameter). When the vanadium content increases to 0.4, the nitrogen pores are completely removed. The results demonstrate that increasing vanadium content can remove nitrogen pores. Since the solidification mode remains unchanged with increasing vanadium content, increasing the vanadium content cannot remove nitrogen pores by changing the solidification mode of the 30Cr15Mo1N ingots. Therefore, increasing the vanadium content removes the nitrogen pores mainly by inhibiting the nucleation of nitrogen bubbles and promoting nitrogen bubble overflow in 30Cr15Mo1N ingots. [ABSTRACT FROM AUTHOR]

To comprehensively understand the effect of oscillation on cooling capacity of steel strip, a mathematical model is proposed to investigate the distribution of temperature fields in continuous cast slabs, taking into account the oscillatory feeding process. Strip oscillation results in a periodic variation of nearby molten steel flow. It intensifies the turbulence of surrounding molten steel and enhances the interfacial heat transfer. During the feeding process, the morphology of strip undergoes three stages: upheaval, fluctuation, and disappearance. As the strip oscillation frequency increases, the maximum cross‐sectional area of the strip gradually decreases during the upheaval stage, the fluctuation stage gradually disappears, and the strip disappearance speed gradually increases during the disappearance stage. These observations suggest that an increase in the oscillation frequency accelerates the melting of the strip, which is caused by the enhanced heat transfer between the strip and molten steel. This results in a gradual growth of strip's cooling capacity and a decrement in the liquid fraction of molten steel surrounding the strip. In addition, feeding strip is advantageous in preventing the jet from impinging onto narrow surfaces of slab. The blocking effect initially increases and then remains almost unchanged as the strip oscillation frequency increases. [ABSTRACT FROM AUTHOR]

In this study, the microstructural evolution of H13 hot‐work die steel during isothermal tempering at 500 ≈ 650 °C is evaluated, and kinetic equations and a neural network are constructed to describe the softening behavior. The results show that the softening behavior appears as a loss of hardness with increasing isothermal tempering temperature and time. The softening mechanism is ascribed to the decomposition of martensitic laths, annihilation and rearrangement of dislocations, formation of sub‐grains, and precipitation and coarsening of secondary carbides. The evolution of secondary carbides includes the coarsening of V‐rich MC nanoprecipitates, and the precipitation and coarsening of secondary phases (Cr‐rich M23C6 and Mo‐rich M6C carbides) during long‐term isothermal tempering. Regarding the applicability of the model in precipitation evolution and hardness prediction, the Johnson–Mehl–Avrami model is considered to be more suitable than the Hollomon–Jaffe, Lifshitz–Slyozov–Wagner, and back‐propagation neural network models. [ABSTRACT FROM AUTHOR]

By comparing the advantages and disadvantages of the existing single‐step pressurized metallurgical manufacturing technologies of high nitrogen stainless steel (HNS), a promising pressurized duplex manufacturing route, i.e., pressurized induction melting (PIM) and pressurized electroslag remelting (PESR), is proposed. The PIM stage mainly carries on the nitrogen alloying, deoxidation and desulfurization, and the PESR process mainly takes responsibility for further desulfurization, removal of large‐size inclusion, and elevating solidification quality. During PIM, the deoxidation is achieved by vacuum carbon deoxidation combined with magnesium and rare‐earth compound treatment. Subsequently, the PESR is conducted under high nitrogen pressure without adding nitrided ferroalloys. To precisely control the nitrogen content and avoid segregation and escape of nitrogen, the three‐stage nitrogen pressure collocation is developed. The high nitrogen stainless bearing steel 30Cr15Mo1N (DIN 1.4108) is manufactured as an example by the duplex route, and a PIM–PESR ingot with high nitrogen content, high cleanliness, fine and dispersive inclusions, and compact solidification structure is obtained. Moreover, the steel possesses a homogeneous microstructure, slightly better mechanical properties, and corrosion resistance compared with that manufactured by the single‐step PESR process. Finally, a pressurized ladle refining and PESR duplex route are proposed for large‐scale industrial production of HNSs. [ABSTRACT FROM AUTHOR]

The tempering process is applied in marine 10Ni5CrMoV steel to study microstructure, and mechanical properties by multi‐scale characterizations, strain hardening behavior, and cryogenic toughening mechanism are further investigated. As the tempering temperature increases from 590 to 630 °C, the dislocation density decreases by up to 25% and the austenite volume fraction decreases by up to 35%. Additionally, the morphology of austenite changes from strip to bulk, which weakened the pinning effect on the laths, leading to a coarsened martensite and an increase in the equivalent grain size. Based on the modified Crussard–Jaoul analysis, the specimens at different tempering temperatures exhibit a single‐stage strain hardening behavior during plastic deformation. The increase in strength is attributed to the continuous transformation‐induced plasticity effect and the increasing dislocation density. Furthermore, high austenite volume fraction and the proportion of grain boundary misorientation above 45° promote the release of stress concentration and hinder the propagation of cracks. This results in an increase in the ductile–brittle transition temperature (DBTT) of the specimens from −105 to −135 °C. At a tempering temperature of 610 °C, the specimen demonstrates an outstanding balance between yield strength (868 MPa) and cryogenic toughness (DBTT of −135 °C). [ABSTRACT FROM AUTHOR]