Characterization of microstructure and mechanical properties in structural steels

Abstract

Steels have been widely used as structural materials due to an extensive range of mechanical properties from moderate strength levels with excellent ductility to very high strengths with adequate ductility. However, as the increasing demands for the fuel efficiency and safety in industrial fields, various researches for improving the mechanical properties of steels have conducted based on the microstructure control through the chemical composition and heat treatments. This means that the microstructure can be a strong parameter for improving the mechanical properties of steel. Therefore, it is important to define how the microstructure contributes to the mechanical properties of steels. In this study, the microstructural analyses by using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) carried out to understand the relationship between the microstructure and mechanical properties in conventional structural steels such as high-strength low alloy (HSLA) steel, high manganese (Mn) steel and high chromium (Cr) heat-resistant steel. Mechanical properties of these steels were examined through the tensile test, hardness test, and thermo-mechanical deformation system, and all tests were performed in accordance with the standard. In addition, a commercial thermodynamic calculation program (FactSageTM) was used to investigate the equilibrium phase according to the conditions of chemical compositions and heat treatments. In HSLA steel, the static recrystallization behavior with carbide forming elements, such as titanium (Ti), niobium (Nb) and vanadium (V) was investigated. As a result of TEM analysis, the static recrystallization was retarded by the MX types of precipitates formed during the hot deformation. The results of nano-indentation and tensile tests showed that the precipitates formed at elevated temperature decreased the strengthening effect by interphase precipitates (IP). Therefore, the hot rolling process should be conducted above the non-recrystallization temperature (Tnr) to maximize the IP strengthening effect in HSLA steel. Consequently, the high-strength steel sheet with a 980 MPa grade was produced at a pilot scale. In high Mn steel, the precipitation strengthening was obtained by the formation of the fine V(C, N) precipitates had a cube-cube orientation relationship with the austenite matrix. Meanwhile, the precipitation strengthening was not obtained in the steel containing titanium elements because the coarse Ti(C, N) precipitates with a size of several hundred nanometers or more were formed. Furthermore, ε-martensite transformation after plastic deformation was observed only in steel containing the coarse Ti(C, N) precipitates. As a result of TEM analysis, this phenomenon was due to the reduction of stacking fault energy by the consumption of soluble carbon elements. Consequently, in Fe-18Mn-0.4C alloy system, the precipitation strengthening by the fine V(C,N) precipitates without the ε-martensite transformation was obtained when the remaining carbon contents were 0.374 wt%. In high chromium (Cr) heat-resistant steel, Laves phase precipitates were mainly formed at prior austenite grain boundaries after long-term annealing treatment. Moreover, the growth rate of the Laves phase precipitates was faster than that of the other precipitates such as M23C6 and MX precipitates. As a result of TEM analysis, the values of Fe/Cr and W/Mo in Laves phase precipitates were found to steadily increase during the annealing treatments. In addition, these compositional changes resulted in an increase of strain energy at the interface between the Laves phase precipitates and ferrite matrix. Consequently, this promoted the attainment of the critical strain for the formation of cavities, even though the Laves phase precipitates had the same size.