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. Full article

A linear piezoelectric actuator based on the stick-slip principle is presented and tested in this paper. With the help of changeable vertical preload force flexure hinge, the designed linear actuator can achieve both large travel stick-slip motion and high-resolution stepping displacement. The developed actuator mainly consists of a bridge-type flexure hinge mechanism, a compound parallelogram flexure hinge mechanism, and two piezoelectric stacks. The mechanical structure and motion principle of the linear actuator were illustrated, and the finite element method (FEM) is adopted. An optimal parametric study of the flexure hinge is performed by a finite element analysis-based response surface methodology. In order to investigate the actuator’s working performance, a prototype was manufactured and a series of experiments were carried out. The results indicate that the maximum motion speed is about 3.27 mm/s and the minimum stepping displacement is 0.29 μm. Finally, a vibration test was carried out to obtain the first natural frequency of the actuator, and an in situ observation was conducted to investigate actuator’s stick-slip working condition. The experimental results confirm the feasibility of the proposed actuator, and the motion speed and displacement are both improved compared with the traditional stick-slip motion actuator. Full article

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. Full article

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. Full article

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. Full article

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. Full article

The oxidation behavior of silicon-containing steel was studied by applying segmented heating routes similar to the atmosphere and heating process in an industrial reheating furnace. The oxidation tests were carried out on a simultaneous thermal analyzer at heating temperatures of 1150 °C–1300 °C. The morphologies of Fe₂SiO₄ were observed by SEM, and the penetration depths of the Fe₂SiO₄ layer at different oxidation temperatures were determined by using the Image-Pro Plus 6.0 software. The results show that at heating temperatures ≥1235 °C, the oxidation rate and total oxidation mass gain have no relation with the heating temperature; the mass gain versus time follows a linear law after about 1164 °C (lower than the eutectic temperature of fayalite). In addition, the oxidation rate first decreases slowly and then drops from 1190 °C to 1210 °C during the isothermal holding stage. With the increase in temperature, the oxidation rate and mass gain also increase gradually; the relationship between the mass gain and time is close to a parabolic law. Moreover, at a heating temperature of 1150 °C, the oxidation rate decreases rapidly during the isothermal holding stage, and the mass gain versus time follows a parabolic law. Full article

In this study, thermal simulation experiments under different austenitization temperatures and different stress states were conducted. High-temperature laser scanning confocal microscopy (LSCM), thermal dilatometry, and scanning electron microscope (SEM) were used to quantitatively investigate the effects of the uniaxial compressive stress on bainitic transformation at 330 °C following different austenitization temperatures. The transformation plasticity was also analyzed. It was found that the promotion degree of stress on bainitic transformation increases with the austenitization temperature due to larger prior austenite grain size as well as stronger promoting effect of mechanical driving force on selected variant growth at higher austenitization temperatures. The grain size and the yield strength of prior austenite are other important factors which influence the promotion degree of stress on bainitic transformation, besides the mechanical driving force provided by the stress. Moreover, the transformation plasticity increases with the austenitization temperature. Full article

In order to study the effect of the Si content on the morphology, amount, and distribution of fayalite (Fe₂SiO₄), three low-carbon steels with different Si contents were selected, and reheating tests were conducted in an industrial furnace in a hot strip plant. The results show that Si distributes in two forms—first, Fe₂SiO₄, in the innermost layer of the oxide scale, and, second, granular SiO₂, dispersively distributed in the matrix near the scale. In addition, Fe₂SiO₄ appears in a net-like form in the innermost layer of the oxide scale close to the iron matrix when the Si content is 1.21 wt. %. However, no obvious net-like Fe₂SiO₄ is observed when the Si content is less than 0.25 wt. %. Moreover, the inhibition effect of the solid Fe₂SiO₄ on the oxidation reaction plays a more important role than the promotion effect of the liquid Fe₂SiO₄ during the entire oxidation reaction. Therefore, the total thickness of the scale decreases with the increase in Si content. Full article