The influence of the heating rates from 10 to 1000 °C/s and annealing temperatures on the microstructure and mechanical properties of two 0.2%C, 1.9%Mn, 1.4%Si cold-rolled steels with and without the addition of carbide-forming elements (Mo, Nb, and Ti) have been investigated. Results show that the increase of the heating rate above 100 °C/s refines the parent austenitic grains in both alloys. The increment of the heating rate led to carbon heterogeneities in the austenite, which after subsequent cooling promoted the formation of a complex mixture of fine-grained constituents. As expected, at the lower heating rates the presence of Nb and Ti-rich carbides and carbonitrides controls the austenite grain growth during the annealing treatment. The tensile test results reveal that high heating rates do not have a significant influence on the tensile strength of the alloy with carbide-forming elements. On the other hand, both the ultimate tensile strength (UTS) and total elongation of the alloy without carbide-forming elements decrease, due to the formation of bands of ferrite and high carbon martensite. However, samples treated at heating rates above 100 °C/s show a combination of UTS in the range of 1400–1600 MPa, and 12–18% of total elongation. The results suggest that the microstructure heterogeneity obtained after high heating rates, especially the ferrite content, has the major effect on the mechanical behavior of the studied steels.

Single and two-step heat treatments were performed on hot-rolled 0.2C-2Mn-1.4Si-0.3Mo steel. The microstructure was characterized by means of optical and scanning electron microscopy and X-ray diffraction measurements. The results showed that the main microstructural components are tempered martensite, bainite, and retained austenite. The thermodynamic analysis showed that the constrained carbon equilibrium in the presence of cementite CCEθ model fits the experimental data for the isothermal heat treatment at a temperature well below the M_S. Neither of the existing models for predicting the equilibrium between martensite and austenite (i.e., constrained carbon equilibrium CCE and CCEθ) seems to predict the measured austenitic carbon content nearby the M_S, which might indicate that local equilibrium (LE) conditions during bainitic transformation govern the final stage of both single and two-phase heat treatments.

In this study an Fe-0.28C-1.91Mn-1.44Si cold-rolled steel was subjected to conventional (10 °C/s) and ultrafast (100 °C/s - 700 °C/s) heating peak annealing treatments, followed by quenching and partitioning (Q&P). The microstructural characterization results showed that grain refinement of the parent austenite and its transformation products occurred with the increment of the heating rate from 10 °C/s to 100 °C/s, without further refining at 700 °C/s. The formation of complex microstructures after the end of the thermal treatment, accompanied by the reduction in the retained austenite carbon content, suggested that local chemical heterogeneities in austenite appear upon ultrafast heating. Regardless of the prior heating rate, similar mechanical properties and strain hardening were measured, revealing that both, the microstructure development and the extent of austenite stabilization during quenching and partitioning stage play a fundamental role on the mechanical behavior of the peak annealed Q&P steels.