Abstract
The hot deformation behavior of a high carbon carbide-free bainitic steel was studied through isothermal compression tests that were performed on a Gleeble-1500D thermal mechanical simulator at temperatures of 1223–1423 K and strain rates of 0.01–5 s−1. The flow behavior, constitutive equations, dynamic recrystallization (DRX) characteristics, and processing map were respectively analyzed in detail. It is found that the flow stress increases with increasing the strain rate and decreases with increasing the temperature, and the single-peak DRX can be easily observed at high temperatures and/or low strain rates. The internal relationship between the flow stress and processing parameters was built by the constitutive equations embracing a parameter of Z/A, where the activation energy for hot deformation is 351.539 kJ/mol and the stress exponent is 4.233. In addition, the DRX evolution and the critical conditions for starting DRX were discussed. Then the model of the DRX volume fraction was developed with satisfied predictability. Finally, the processing maps at different strains were constructed according to the dynamic material model. The safety domains and flow instability regions were identified. The best processing parameters of this steel are within the temperature range of 1323–1423 K and strain rate range of 0.06–1 s−1.
Highlights
IntroductionCarbide-free bainitic steels have attracted considerable interest because of their unusual combination of roughly 2.5 GPa strength, 700 HV hardness, and 130 MPa·m1/2 toughness [1,2,3]
In recent years, carbide-free bainitic steels have attracted considerable interest because of their unusual combination of roughly 2.5 GPa strength, 700 HV hardness, and 130 MPa·m1/2 toughness [1,2,3].The excellent mechanical properties depend on an ultra-fine alternative microstructure consisting of bainitic ferrite (BF) and carbon-enriched retained austenite (CERA)
The following in the temperature range from 1223–1423 K and strain rate range from 0.01–5 s −1 in the temperature range from 1223–1423 K and strain rate range from 0.01–5 s
Summary
Carbide-free bainitic steels have attracted considerable interest because of their unusual combination of roughly 2.5 GPa strength, 700 HV hardness, and 130 MPa·m1/2 toughness [1,2,3]. As we know, the bainitic reaction in the medium-to-high carbon carbide-free bainitic steels is a rather time-consuming process It may take several days or months to achieve the complete transformation, though the super bainitic microstructure can be obtained at low isothermal temperatures [3]. In order to effectively control the grain size of prior austenite in carbide-free bainitic steels, it is essential to clarify the hot deformation behavior and microstructural evolution, especially regarding DRX features. After the suitable heat treatment, the carbide-free bainitic microstructure can be obtained, which is supported by the reference [31] In this steel, the content of Cr is decreased, and the strength and hardenability reductions resulting from the lower Cr content are compensated by suitably increasing the quantity of Mn. The chemical composition modification will affect the deformation behavior at elevated temperatures. The in-depth research on the effects of variations in the parameters on the processing map and DRX evolution will be a great help for determining the optimal processing parameters in actual production
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