Abstract
The combined effect of deformation temperature and strain value on the continuous cooling transformation (CCT) diagram of low-alloy steel with 0.23% C, 1.17% Mn, 0.79% Ni, 0.44% Cr, and 0.22% Mo was studied. The deformation temperature (identical to the austenitization temperature) was in the range suitable for the wire rolling mill. The applied compressive deformation corresponded to the true strain values in an unusually wide range. Based on the dilatometric tests and metallographic analyses, a total of five different CCT diagrams were constructed. Pre-deformation corresponding to the true strain of 0.35 or even 1.0 had no clear effect on the austenite decomposition kinetics at the austenitization temperature of 880 °C. During the long-lasting cooling, recrystallization and probably coarsening of the new austenitic grains occurred, which almost eliminated the influence of pre-deformation on the temperatures of the diffusion-controlled phase transformations. Decreasing the deformation temperature to 830 °C led to the significant acceleration of the austenite → ferrite and austenite → pearlite transformations due to the applied strain of 1.0 only in the region of the cooling rate between 3 and 35 °C·s−1. The kinetics of the bainitic or martensitic transformation remained practically unaffected by the pre-deformation. The acceleration of the diffusion-controlled phase transformations resulted from the formation of an austenitic microstructure with a mean grain size of about 4 µm. As the analysis of the stress–strain curves showed, the grain refinement was carried out by dynamic and metadynamic recrystallization. At low cooling rates, the effect of plastic deformation on the kinetics of phase transformations was indistinct.
Highlights
The individual phase transformations can be effectively controlled by the cooling rate of the rolled or forged steel products
The aim of the experimental work was to investigate the combined effect of the deformation temperature and strain value on the continuous cooling transformation diagram of steel low-alloyed with manganese, nickel, chromium, and molybdenum
Using dilatometric tests and metallographic analyses, a total of five different continuous cooling transformation diagrams were made for 23MnNiCrMo5-3 steel
Summary
The individual phase transformations can be effectively controlled by the cooling rate of the rolled or forged steel products. The major effect of the previous plastic deformation should be the displacement of the phase transformations to higher temperatures and shorter times This is caused by the increased diffusivity of elements in the deformed austenite and a higher density of the favored nucleation places for the diffusional transformation products. Varying the strain value from 5 to 25% had no influence on the phase transformations in manganese–nickel and manganese low-alloy steels but modified the microstructure after cooling due to the formation of coarser austenite grains under deformation of the “critical” strain value [52]. It depends considerably on the deformation temperature and on whether the already recrystallized austenitic or deformed grains enter the phase transformation This is evidenced by the comparison of the CCT and DCCT diagrams constructed for the 34CrMo4, 42CrMo4, 52CrMo4, 51CrV4, and 34NiMo6 steels at a strain value of 0, 30, and 60% [22]. Due to the pre-deformation, only slight changes in the curves corresponding to the start and finish of the phase transformations were observed
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