Abstract
In the past decade, efforts have been focused on developing very fine, medium-carbon bainitic steels via the low-temperature (typically 300–400 °C) ausforming process, which not only enables shorter isothermal holding times for bainitic transformation at low temperatures, but also offers significantly improved strength. This paper describes static recrystallization (SRX) characteristics of austenite in four medium-carbon 2%Mn-1.3%Si-0.7%Cr steels with and without microalloying intended for the development of these steels. The stress-relaxation method on a Gleeble simulator resulted in recrystallization times over a wide range of temperatures, strains and strain rates. Also, the occurrence of precipitation was revealed. Powers of strain (−1.7 to −2.7) and strain rate (−0.21 to −0.28) as well as the apparent activation energies (225–269 kJ/mol) were in the ranges reported in the literature for C-Mn and microalloyed steels with lower Mn and Si contents. The new regression equations established for estimating times for 50% SRX revealed the retardation effects of microalloying and Mo addition showing reasonable fits with the experimental data, whereas the previous model suggested for ordinary microalloyed steels tended to predict clearly shorter times on average than the experimental values for the present coarse-grained steels. The Boratto equation to estimate the non-recrystallization temperature was successfully modified to include the effect of Mo alloying and high silicon concentrations.
Highlights
Nanostructured bainite, called superbainite, is a novel type of microstructure for creating modern tool and construction steels exhibiting exceptional durability [1]
As far as the flow stress levels of the steels are concerned, it seems that there were no significant differences among the four steels tested, except the Mo-Nb steel (Figure 3b), which showed somewhat higher flow stresses especially at the two lowest deformation temperatures
◦ C/0.17/0.15 s−1, 1025 ◦ C/0.24/0.2 s−1 ) and using the deformation parameters, (1075 recrystallization (SRX) kinetics of coarse-grained hot-deformed austenite in four mediumcarbon Fe-2Mn-1.3Si-0.7Cr steels with different microalloying over a wide range of temperatures (900–1200 °C), strains (0.125–0.4) and strain rates (0.01–5 s−1)
Summary
Nanostructured bainite, called superbainite, is a novel type of microstructure for creating modern tool and construction steels exhibiting exceptional durability [1]. Efforts have been focused on developing very fine, medium-carbon bainitic steels by introducing a low-temperature (typically 300–400 ◦ C) ausforming process, facilitating a very fine nanostructured bainitic structure in the steels [2,3,4,5]. A careful alloy design is, necessary as a low martensite start temperature MS must be obtained. This is possible typically by the addition of austenite stabilizing elements such as Mn. Additions of Mo and microalloying elements V and Nb are quite usual in order to utilize their grain refinement and strengthening effects on the final bainitic microstructure
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