Bei He

New insights into the effects of silicon content on the oxidation process in silicon-containing steels

Simultaneous thermal analysis (STA) was used to investigate the effects of silicon content on the oxidation kinetics of silicon- containing steels under an atmosphere and heating procedures similar to those used in industrial reheating furnaces for the production of hot-rolled strips. Our results show that when the heating temperature was greater than the melting point of Fe2SiO4, the oxidation rates of steels with different silicon contents were the same; the total mass gain decreased with increasing silicon content, whereas it increased with increasing oxygen content. The oxidation rates for steels with different silicon contents were constant with respect to time under isothermal conditions. In addition, the starting oxidation temperature, the intense oxidation temperature, and the finishing oxidation temperature increased with increasing silicon content; the intense oxidation temperature had no correlation with the melting of Fe2SiO4. Moreover, the silicon distributed in two forms: as Fe2SiO4 at the interface between the innermost layer of oxide scale and the iron matrix, and as particles containing silicon in grains and grain boundaries in the iron matrix.

Effects of Austenitization Temperature and Compressive Stress During Bainitic Transformation on the Stability of Retained Austenite

The effects of stress and austenitization temperature on the carbon content and morphology of retained austenite (RA) have been investigated. The stability of RA has been analyzed. An interesting finding is that the increase in the amount of bainite is not accompanied by the increase in the carbon content in RA under the effect of stress. This does not match with what is expected from the bainitic transformation theory. The amount of bainitic transformation and stress both affect the carbon content in RA. In addition, the stress during bainitic transformation helps to increase the stability of RA by decreasing the amount of blocky RA, whereas it has little effect on the carbon content in RA. Moreover, the increase in austenitization temperature is beneficial to increasing the stability of RA, whereas it has no significant effect on the morphology of RA.

A Method to Reduce the Oxide Scale of Silicon-Containing Steels by Adjusting the Heating Route

A method to reduce the scale of silicon-containing steels by adjusting the heating route has been proposed in the present study. The influence of heating rate on the oxidizing behavior of steels containing Si was studied by simultaneous thermal analyzer with various heating rates under the condition of the fixed heating time and same heating temperature. The heating process was divided into two sections, i.e., fast heating stage and slow-heating stage. Three kinds of heating route with different end temperatures at fast heating stage and different heating rates at slow-heating stage were designed. Additionally, the solidification process of Fe2SiO4 at different cooling rates was observed by in situ observation. The results showed that the amount of Fe2SiO4 and oxidation mass gain both decreased with the decrease of end temperature at fast-heating stage. Likewise, the net-like distribution of Fe2SiO4 became less apparent with the decrease of end temperature at fast-heating stage and the increase of the heating rate at slow-heating stage. Therefore, it would be beneficial for descaling with a heating route of a lower end heating temperature at the fast-heating stage and a higher heating rate at slow-heating stage. Besides, the solidifying point of Fe2SiO4 decreased apparently at a higher cooling rate.

New insight to the oxidation kinetics of silicon-containing steel at high temperature

Abstract This study investigated the oxidation kinetics of silicon-containing steel at 1150–1300 °C using a Simultaneous Thermal Analyzer under atmospheric conditions similar to that of an industry reheating furnace. There is a critical time point for the oxidation kinetics at an oxygen concentration of 4·0 vol.-%., following which the oxidation rate constant increases with the increasing oxidation temperature. The model coefficient A in the kinetic oxidation equation was found to be constant. However, before the critical time point, the oxidation rate constant remained unchanged; the model coefficient A decreased with the increasing temperature. Therefore, the kinetic model of silicon-containing steel for isothermal oxidation was observed to be a modified one on the basis of the experimental data. In addition, the critical time point was prolonged with the increasing isothermal oxidation temperature. Moreover, the oxidation activation energy of the tested silicon-containing steel was 366·16 kJ mol−1.

Effects of oxygen content on the oxidation process of Si-containing steel during anisothermal heating

The oxidizing behavior of Si-containing steel was investigated in an O2 and N2 binary-component gas with oxygen contents ranging between 0.5vol% and 4.0vol% under anisothermal-oxidation conditions. A simultaneous thermal analyzer was employed to simulate the heating process of Si-containing steel in industrial reheating furnaces. The oxidation gas mixtures were introduced from the commencement of heating. The results show that the oxidizing rate remains constant in the isothermal holding process at high temperatures; therefore, the mass change versus time presents a linear law. A linear relation also exists between the oxidizing rate and the oxygen content. Using the linear regression equation, the oxidation rate at different oxygen contents can be predicted. In addition, the relationship between the total mass gain and the oxygen content is linear; thus, the total mass gain at oxygen contents between 0.5vol%–4.0vol% can be determined. These results enrich the theoretical studies of the oxidation process in Si-containing steels.

A Study of Growth of Austenite Grains in a Steel Microalloyed with Ti and Nb

The growth of austenite grains and the dissolution of carbonitrides under heating of a structural steel microalloyed with niobium and titanium are studied. The microstructure of the steel is investigated with the help of light and transmission electron microscopes. The content of elements in precipitated particles is determined using an energy dispersive spectrometer. The kinetic equation of grain growth is derived and the critical temperature above which the carbonitrides stop to hinder grain growth is determined. Recommendations are given on the temperature modes of hot rolling.