Mingxing Zhou

An advanced bainite rail with high strength–toughness combination was produced in a steel mill and the effects of tempering on the microstructure and properties of the bainite rail steel were investigated by optical microscopy, transmission electron microscopy, electron back-scattering diffraction and X-ray diffraction. Results indicate that the tensile strength, elongation and impact toughness were about 1470 MPa, 14.5% and 83 J/cm², respectively, after tempering at 400 °C for 200 min. Therefore, a high-strength bainite rail steel with good toughness was developed. In addition, the amount of retained austenite (RA) decreased due to bainite transformation after low-temperature tempering (300 °C) and RA almost disappeared after high-temperature tempering (500 °C). Moreover, as the tempering temperature increased, the tensile strength of the rail head first decreased due to the decreased dislocation density and carbon content in bainite ferrite and the coarseness of bainite ferrite, and then increased because of carbide precipitation at high-temperature tempering. Furthermore, RA played a significant role in the toughness of bainite rail. The elongation and toughness of the rail obviously decreased after tempering at 500 °C for 200 min because of the disappearance of RA and appearance of carbides.

Two low carbon carbide-free bainitic steels (with and without Cr addition) were designed, and each steel was treated by two kinds of heat treatment procedure (austempering and continuous cooling). The effects of Cr addition on bainitic transformation, microstructure, and properties of low carbon bainitic steels were investigated by dilatometry, metallography, X-ray diffraction, and a tensile test. The results show that Cr addition hinders the isothermal bainitic transformation, and this effect is more significant at higher transformation temperatures. In addition, Cr addition increases the tensile strength and elongation simultaneously for austempering treatment at a lower temperature. However, when the austempering temperature is higher, the strength increases and the elongation obviously decreases by Cr addition, resulting in the decrease in the product of tensile strength and elongation. Meanwhile, the austempering temperature should be lower in Cr-added steel than that in Cr-free steel in order to obtain better comprehensive properties. Moreover, for the continuous cooling treatment in the present study, the product of tensile strength and elongation significantly decreases with Cr addition due to more amounts of martensite.

Three low‐carbon bainitic steels were designed to investigate the effects of Cr and Al addition on bainitic transformation, microstructures, and properties by metallographic method and dilatometry. The results show that compared with the base steel without Cr and Al addition, only Cr addition is effective for improving the strength of low‐carbon bainitic steel by increasing the amount of bainite. However, compared with the base steel, combined addition of Cr and Al has no significant effect on bainitic transformation and properties. In Cr‐bearing steel, Al addition accelerates initial bainitic transformation, but meanwhile reduces the final amount of bainitic transformation due to the formation of a high‐temperature transformation product such as ferrite. Consequently, the composite strengthening effect of Cr and Al addition is not effective compared with individual addition of Cr in low‐carbon bainitic steels. Therefore, in contrast to high‐carbon steels, bainitic transformation in Cr‐bearing low‐carbon bainitic steels can be finished in a short time, and Al should not be added because Al addition would result in lower mechanical properties.

In this study, two silicon-containing steels with different P contents were used, and reheating tests were conducted in an industrial furnace in a hot strip plant. The effect of P on the microstructure and melting temperature of Fe₂SiO₄ in silicon-containing steels was investigated using a backscattered electron (BSE) detector and energy-dispersive spectroscopy (EDS). The melting process of Fe₂SiO₄ was also observed in situ for the two steels with different P contents. The results show that the addition of P could lower the melting point of the eutectic compound Fe₂SiO₄/FeO, which is helpful for descaling the oxide scale. The melting point decreases with the increasing P content, and the melting point of Fe₂SiO₄/FeO can reduce up to 954.2 °C when the content of P reaches 0.115 wt %. Furthermore, P-compounds form in the dispersive particles located in the iron matrix near the interface between the matrix and inner oxide scale when the P content is relatively high. In addition, a method of in situ observation was proposed to study the effect of P on the melting point of Fe₂SiO₄/FeO in silicon-containing steel. The results are of more practical significance for the descaling of oxide scale in silicon-containing steel.

Red scale defect is known to be mainly caused by net-like Fe₂SiO₄–FeO. In the present study, the morphology of Fe₂SiO₄–FeO in a Si-containing steel was investigated by simultaneous thermal analysis, high-temperature laser scanning confocal microscopy, scanning electron microscopy, and energy dispersive spectroscopy. Only liquid Fe₂SiO₄–FeO can form a net-like morphology. Liquid Fe₂SiO₄–FeO is classified into two types in this work. Type-1 liquid Fe₂SiO₄–FeO is formed by melting pre-existing solid Fe₂SiO₄–FeO that already exists before the melting point of Fe₂SiO₄–FeO. Type-2 liquid Fe₂SiO₄–FeO is formed at a temperature higher than the melting point of Fe₂SiO₄–FeO. The results show that type-1 liquid Fe₂SiO₄–FeO is more likely to form a net-like morphology than is type-2 liquid Fe₂SiO₄–FeO. The penetration depth of type-1 liquid Fe₂SiO₄–FeO is also larger at the same oxidation degree. Therefore, type-1 liquid Fe₂SiO₄–FeO should be avoided in order to eliminate red scale defect. Net-like Fe₂SiO₄–FeO may be alleviated by two methods: decreasing the oxygen concentration in the heating furnace before the melting point of Fe₂SiO₄–FeO is reached and increasing the reheating rate before the melting point. In addition, FeO is distributed with a punctiform or lamellar morphology on Fe₂SiO₄.

In this work, three low-carbon bainitic steels, with different Mo contents, were designed to investigate the effects of Mo addition on microstructure and mechanical properties. Two-step cooling, i.e., initial accelerated cooling and subsequent slow cooling, was used to obtain the desired bainite microstructure. The results show that the product of strength and elongation first increases and then shows no significant change with increasing Mo. Compared with Mo-free steel, bainite in the Mo-containing steel tends to have a lath-like morphology due to a decrease in the bainitic transformation temperature. More martensite transformation occurs with the increasing Mo, resulting in greater hardness of the steel. Both the strength and elongation of the steel can be enhanced by Mo addition; however, the elongation may decrease with a further increase in Mo. From a practical viewpoint, the content of Mo could be ~0.14 wt. % for the composition design of low-carbon bainitic steels in the present work. To be noted, an optimal scheme may need to consider other situations such as the role of sheet thickness, toughness behavior and so on, which could require changes in the chemistry. Nevertheless, these results provide a reference for the composition design and processing method of low-carbon bainitic steels.