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Austenite formation during intercritical annealing was studied in a cold-rolled dual-phase (DP) steel based on a low-carbon DP780 composition processed in the mill. Two heating rates, 10 and 50 K/s, and a range of annealing temperatures from 1053 K to 1133 K (780°C to 860°C) were applied to study their effects on the progress of austenitization. The effect of these process parameters on the final microstructures and mechanical properties was also investigated using a fixed cooling rate of 10 K/s after corresponding annealing treatments. It was found that the heating rate affects the austenite formation not only during continuous heating, but also during isothermal holding, and the effect is more pronounced at lower annealing temperatures. Faster heating delays the recrystallization kinetics of the investigated steel. The rate of austenite formation and its distribution are strongly influenced by the extent of overlapping of the processes of recrystallization and austenitization. It appeared that the heating rate and temperature of intercritical annealing have a stronger effect on the final tensile strength (TS) of the DP steel than holding time. Both higher annealing temperatures and long holding times minimize the strength difference caused by a difference in heating rate.
An experimental investigation was conducted using laboratory-processed, low carbon 0.08C-2.0Mn-0.2Cr-0.15Mo steels with different Si contents to evaluate the influence of Si additions on the mechanical properties and microstructure of dual-phase steels. The heat treatment was carried out in a salt bath furnace to heat the samples between 720 and 860°C; samples were held isothermally for 60 s, followed by air cooling or water quenching. This was accomplished by evaluating the formation of austenite at various intercritical temperatures during annealing and its decomposition during subsequent cooling. It was found that Si addition accelerates the recrystallization of ferrite during heating in the intercritical temperature range, which in turn promotes the formation of austenite through the nucleation process, followed by grain growth. Addition of Si favors the formation of a homogeneous austenite of higher hardenability resulting in a higher volume of martensite in the final structure. Thus, a silicon-bearing steel has been demonstrated to possess a higher strength in comparison with Si-free steel.
The refinement of microstructure and its homogeneity during controlled hot strip rolling is primarily achieved by controlling the austenite recrystallization before its transformation during accelerated cooling. The paper describes the methodology to determine the deformation conditions favorable for dynamic recrystallization (DRX). Using this methodology it becomes possible to delineate the conditions for postdeformation static and metadynamic recrystallization as well. The work is based on viscoplastic power law formalism applied to steady state flow within wide range of deformation temperatures and strain rates. Two equations of the same form but with different coefficients can be used depending on whether the steady state flow is controlled by dynamic recovery (DRV) or DRX. The transition from DRV-to DRX at the corresponding value of Zener-Hollomon parameter Zt can be viewed as the demarcation between static and metadynamic recrystallization occurring after deformation. The approach is illustrated using low carbon Mn-V steel. Alloying with Cr and especially with Mo suppresses DRX and MDRX as manifested by increasing Zt.
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