Xiuhua Gao

This study systematically investigated the mechanism of micro-segregation reduction in a high-Mn austenitic cryogenic steel continuous casting slab using thermodynamic calculations and homogenization experiments. The high-Mn austenitic cryogenic steel continuous casting slab exhibits obvious non-equilibrium solidification characteristics, with severe interdendritic segregation of C and Mn. The solidus temperatures of equilibrium, Scheil, and Scheil–Back solidification are 1324 °C, 953 °C, and 1272 °C, respectively. According to thermodynamic calculations, there is only a slight decrease in the highest segregation C content when the homogenization temperature is 900 °C. When the specimens were homogenized at 1000 °C, the segregation of C and Mn was significantly alleviated, and the segregation degree further decreased when the homogenization temperature was 1100 °C. Two feasible and industrially applicable strategies for alleviating the micro-segregation of a high-Mn steel continuous casting slab are proposed. First, reduce the cooling intensity of the secondary cooling stage during continuous casting, slow down the cooling rate around 1000 °C, and promote the limited diffusion of solute elements to reduce initial segregation. Second, introduce a holding stage at around 1000 °C during slab reheating prior to hot rolling, eliminating residual segregation and stabilizing the local solidus temperature above 1200 °C.

With the industrial production and application of a series of advanced high-strength steels containing austenite phases, quantifying austenite content has become vital for optimizing production processes and guaranteeing product quality. Traditional austenite measurement methods typically demand laborious, meticulous sample preparation and complex subsequent data processing, rendering them unsuitable for large-scale industrial standardization. The industry urgently requires a simple, stable technique for determining austenite content in bulk samples of these advanced steels. This article explored an innovative permeameter method to achieve the simple and stable quantitative determination of the austenite content in multiphase steels. This method measures the magnetization data within the near-saturation range, then utilizes the linear Frölich–Kennelly relationship to fit the data and indirectly derive the saturation magnetization, and subsequently determines the austenite content. Using medium manganese steels with varied austenite fractions as experimental materials, we conducted cross-comparisons with conventional methods—using a vibrating sample magnetometer and X-ray diffraction—to rigorously demonstrate the method’s feasibility, accuracy, and reliability. Extensive experimental results confirm that this method is both feasible and sensitive, with the measurement error meeting industrial requirements for quantitative austenite determination. It offers significant advantages in operational simplicity, analysis efficiency, measurement stability, and sensitivity, making it particularly well-suited for the quantification of austenite in medium manganese steels.

In this paper, the austenite grains growth behavior in the austenitizing process of Nb–Ti micro-alloyed medium manganese steel was studied through in situ observation by high temperature laser confocal microscope. The results show that the average austenite grain sizes change from about 3 μm at 1050 °C to over 50 μm at 1250 °C. When the grain boundary is a small-angle grain boundary, one grain boundary will split into several dislocations. With the extension of heating time, the lattice orientation difference further decreases, and the remaining dislocations may merge into new grain boundaries. The most suitable heating temperature for the medium manganese steel in this paper is from 1100 °C to 1150 °C, taking into account influences such as grain size, grain boundary damage, and deformation resistance.

In this paper, the austenite grains growth behavior in the austenitizing process of Nb-Ti micro-alloyed medium manganese steel was studied through in-situ observation by high temperature laser confocal microscope. The results show that the average austenite grain sizes change from about 3 μm at 1050℃ to over 50 μm at 1250℃. When the grain boundary is a small angle grain boundary, one grain boundary will split into several dislocations. With the extension of heating time, the lattice orientation difference further decreases, and the remaining dislocations may merge into new grain boundaries. The most suitable heating temperature for the medium manganese steel in this paper is from 1100℃ to 1150℃. This takes into account influences such as grain size, grain boundary damage, and deformation resistance.

Impact fracture behavior at low temperatures was investigated in medium manganese steel with bcc-fcc duplex microstructures. The impact energy was above 150 J (−80~20 °C) and the fractography showed dimples for inter-critical annealing at 630 °C (QHA) because of the high retained austenite stability and low martensite dislocation density. The impact energy was from 180 J (20 °C) to 60 J (−80 °C) and the fractography was intergranular for inter-critical annealing at 610 °C (QLA) because of the low stability of RA and carbides precipitated at the prior austenite grain boundaries. The impact energy was below 60 J (−80~20 °C) and the fractography showed cleavage for direct quenching (DQ) because of the high dislocation density of martensite.

Retained austenite (RA) transformation and its role in the mechanical properties of three low-carbon micro-alloyed quenching and partitioning (Q&P) steels was investigated utilizing in situ tensile tests and electron microscopy. Meanwhile, RA’s strain-induced martensite transformation (SIMT) was analyzed and discussed in terms of the strengthening mechanism. The results show that the ductility of the Q&P steels relies on the size and morphology of RA. In addition, both affect RA’s mechanical or thermostability. Dislocation density and carbon trapping should be considered in estimating the yield strength in the two-step Q&P process. V and Nb-Ti elements promote the formation of blocky RA. Ti accelerates the formation of film-like RA. For experimental Q&P steels with different processes and compositions, the true stress always keeps a linear relationship with the amount of transformed martensite, i.e., 30.38~46.37 MPa per vol. 1% transformed martensite, during the in situ tensile deformation.

Currently, the fish-scaling resistance of most hot-rolled enamel steels is improved by adding Ti to form fine TiC carbides as hydrogen traps. Given that the hydrogen capture capacity of NbC is higher than that of TiC, the manufacture of hot-rolled enamel steels via Ti-Nb microalloying has a promising future. In the present study, a Ti-Nb microalloyed hot-rolled enamel steel was developed, and its microstructure, mechanical properties, and fish-scaling resistance were studied by optical microscopy, transmission electron microscopy, tensile test, and hydrogen permeation test. The results show that the microstructure of hot-rolled experimental steel is composed of ferrite and fine carbides, with a large number of fine precipitates uniformly distributed in the ferrite grains. After the first and second enamel firings, the average sizes of ferrite grain and precipitates gradually increase, the yield strength decreases from 711 ± 9 MPa to 471 ± 17 MPa and 409 ± 8 MPa, the tensile strength decreases from 761 ± 7 MPa to 524 ± 15 MPa and 490 ± 12 MPa, and the elongation increases from 21.0 ± 2.8% to 27.8 ± 1.8% and 33.9 ± 1.1%. The hydrogen permeation value (TH value) decreases from 35.9 min/mm² to 6.8 min/mm² and 3.9 min/mm² after the first and second enamel firings. That is, the fish-scaling resistance of hot-rolled experimental steel is significantly reduced after enamel firing, which is caused by the coarsening of precipitates, resulting in a significant reduction in the density of irreversible hydrogen traps (from 1.21 × 10²⁵ cm⁻³ to 6.50 × 10²³ cm⁻³ and 4.27 × 10²³ cm⁻³). A large amount of semi-coherent precipitates is the key to obtaining the good fish-scaling resistance of hot-rolled enamel steel.

Compression deformation experiments are conducted to investigate the recrystallization behavior during the dynamic recrystallization (DRX) and static recrystallization (SRX) of high‐manganese steel in terms of flow stress and microstructure morphology. The experimental results indicate that strain rate plays a key role on DRX style. A low strain rate promotes discontinuous dynamic recrystallization (DDRX), whereas a high strain rate encourages continuous dynamic recrystallization (CDRX). The steady‐state stress of DRX behavior decreases with increasing deformation temperature regardless of the true strain and strain rate. Deformation temperature is a vital factor in determining the softening fraction of SRX behavior. The effect of interpass time on austenite grain size is weak for SRX behavior. The SRX kinetics equation for the designed high‐manganese steel is proposed according to experimental results.