Thomas reported a shift of 1 to 3 ☌ of the A C3 temperature by increasing the heating rate from 100 ☌/s to 1000 ☌/s in 1020, 1019M and 15B25 cold-rolled steels. Previous evaluations of the A C3 evolution with the heating rate in cold-rolled low alloy steels indicated that the A C3 temperature shifts slightly when high heating rates are applied. Nevertheless, this value was obtained in samples heated at 500 ☌/s up to the A C1 temperature (767 ☌), then the heating rate declined to ≈380 ☌/s due to the decrease in efficiency of the longitudinal flux induction heating in dilatometer above the curie point and by the formation of paramagnetic austenite. The A C3 temperature for the samples heated at 500 ☌/s in dilatometer was estimated as 892 ☌. The A C3 for the last annealing step (step 4) was estimated as 855 ☌. The parameters of each cycle are a constant heating rate of 30 ☌/s to 885 ☌, soaking time of ≈2 s, and cooling at 160 ☌/s to room temperature. For TC, the first annealing step follows the same parameters as CA, and then three subsequent heating and cooling steps (cycles) were applied. Samples treated according to the CA treatment were heated at 10 ☌/s up to 885 ☌, i.e., ≈30 ☌ above the A C3 (≈852 ☌), and then soaked for 180 s followed by fast cooling at 160 ☌/s. The A C3 temperature in each annealing treatment was estimated via dilatometric analysis employing the methodology presented in. Moreover, current research on this topic has confirmed that solute heterogeneities in austenite, produced due to the lack of time for homogenization during the annealing step, are responsible for the formation of a complex mixture of constituents upon cooling. The microstructural grain refinement reached in ultrafast heating experiments is related to several factors including (i) preferential nucleation of austenite and interaction between ferrite recrystallization and austenite phase transformation (ii) pinning effect by undissolved cementite carbides (iii) restricted austenitic grain growth by the high heating rate employed. The enhanced combination of mechanical properties in lean alloyed UFH steels is developed through the formation of fine-grained heterogeneous microstructures. Pilot-scale installations for ultrafast heating applications are reported elsewhere. Thanks to the development of longitudinal and transverse flux induction heating technologies, the ultrafast heating of steel strips is feasible at small and large scales. This strategy represents an optimization of the heat treatment process by employing heating rates ≥100 ☌/s, reducing the annealing time from several minutes to a window of 1 to 10 s. The microstructural formation after UFH is discussed in terms of chemical heterogeneities in the parent austenite.Īnother promising annealing route towards the new generation of steels is the ultrafast heating (UFH). The outstanding mechanical response exhibited by the UFH steel is related to the formation of heterogeneous distribution of ferrite, bainite and retained austenite in proportions 0.09–0.78–0.14. On the other hand, the steel grade produced via a combination of ultrafast heating annealing and austempering exhibits enhanced ductility without decreasing the strength level with respect to TC and CA, giving the best strength–ductility balance among the studied steels. Nevertheless, the obtained microstructure via TC has not led to an improvement in the mechanical properties in comparison with the CA steel. It was found that TC and UFH strategies produce an equivalent level of microstructural refinement. After the annealing path, steel samples were fast cooled and isothermally treated at 400 ☌ employing the same parameters. Multistep thermo-cycling (TC) and ultrafast heating (UFH) annealing were carried out and compared with the outcome obtained from a conventionally annealed (CA) 0.3C-2Mn-1.5Si steel. This study focuses on the effect of non-conventional annealing strategies on the microstructure and related mechanical properties of austempered steels.
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