The aim of the present work is to study the microstructure evolution of a cold‐rolled low carbon steel alloyed with Cr, Mn, Mo, and Nb during continuous heating. The formation of austenite and its further transformation is examined by means of peak annealing experiments at three different heating rates, followed by quenching. The effect of carbide‐forming alloying elements in the kinetics of austenite formation and decomposition is discussed with the aim of DICTRA calculations. Electron Probe Micro Analysis allows the detection of small compositional fluctuations within the microstructure, which are responsible for local changes in the mechanism of austenite formation. It is experimentally demonstrated that the temperature dependence of the austenite fraction is relatively insensitive to the heating rate. It is suggested that carbide‐forming alloying elements slow down the kinetics of austenite formation.@en

Atomic-scale investigation was performed on 51CrV4 steel, isothermally held at different temperatures within the bainitic temperature range. Transmission electron microscopy (TEM) analysis revealed three different morphologies: lower, upper, and inverse bainite. Atom Probe Tomography (APT) analysis of lower bainite revealed cementite particles, which showed no evidence of partitioning of substitutional elements; only carbon partitioned into cementite to the equilibrium value. Carbon in the bainitic ferrite was found to segregate at dislocations and to form Cottrell atmospheres. The concentration of carbon remaining in solution measured by APT was more than expected at the equilibrium. Upper bainite contained cementite as well. Chromium and manganese were found to redistribute at the cementite-austenite interface and the concentration of carbon in the ferritic matrix was found to be lower than the one measured in the case of lower bainite. After isothermal treatments close to the bainite start temperature, another austenite decomposition product was found at locations with high concentration of Mn and Cr, resembling inverse bainite. Site-specific APT analysis of the inverse bainite reveals significant partitioning of manganese and chromium at the carbides and at the ferrite/martensite interfaces, unlike what is found at isothermal transformation products at lower temperatures.

@en

The aim of the present investigation is to study the microstructure and texture evolution in a cold-rolled low carbon steel subjected to continuous heating and subsequent quenching. Peak-annealing and quenching experiments are carried out at different temperatures at three heating rates (10, 400, and 1000 °C s−1). The texture evolution in ferrite and martensite is analyzed separately via Electron Backscattered Diffraction (EBSD) measurements. It is observed that the recrystallization is virtually completed at 771 °C in samples heated at 10 °C s−1. Conversely, the recrystallization is suppressed in samples heated at rates ≥400 °C s−1. In samples heated at 10 °C s−1, the ferrite texture evolves according to the well-known recrystallization behavior, showing a maximum in {113}<110> components, whereas in samples heated at rates ≥400 °C s−1, the texture is similar to the one of the cold-rolled material. It is concluded that the texture evolution in martensite is strongly dependent on the evolution of recrystallization textures in ferrite. A particular grain substructure product of the interaction between the recrystallization of ferrite and massive austenite formation is observed after heating at rates ≥400 °C s−1. However, this substructure seems to have effect neither on the textures of ferrite nor on the work hardening behavior.@en

A comparative study on the microstructural changes after conventional (20 °C s⁻¹) and ultrafast (300 °C s⁻¹) heating is performed on a medium carbon steel in the soft annealed condition. Continuous-heating dilatometry experiments are carried out. The phase transformations and the volume phase fraction of austenite are determined among other microstructural changes. The microstructure is first observed using Optical Microscopy (OM), further characterized by Scanning (SEM), and detailed analyzed by Transmission Electron Microscopy (TEM). The effect of heating rate on the kinetics of cementite dissolution and austenite formation is rationalized. The experimental results are compared with Dictra calculations, and the possible effects on the kinetics of diffusion-controlled austenite formation are rationalized as well. Metallographic observations indirectly suggest the enhanced nucleation of austenite above A_m.

@en