Effects of Austenitization Temperature and Pre-Deformation on CCT Diagrams of 23MnNiCrMo5-3 Steel

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.

Effect of austenitizing temperature on the phase transformations of Mn-Cr-Mo pipe steel

“…After austenitizing and deformation at a temperature of 860 °C, the mean grain size was 9.7 ± 0.9 µm, but after deformation at a temperature of 1,000 °C it was roughly twice (21.7 ± 2.0 µm; the data scatter was characterized by the confidence interval with a 95% confidence level). Thus, it was confirmed that a coarser austenitic grain is associated with a slowing down of the ferritic and pearlitic transformation, the grains or formations of which are originated by a diffusion mechanism [11,12]. The reason is the formation of a smaller amount of suitable nucleation sites.…”

Determination of CCT Diagram by Dilatometry Analysis of High-Strength Low-Alloy S960MC Steel

“…The sample's average hardness value for a 100 • C/s cooling rate corresponded with the hardness of the base material. The base material's microstructure is predominantly martensitic and was the same for the 100 • C/s cooling rate, the typical cooling rate for hardening HSLA steels [36,43,45,47,48]. The first significant hardness drop was recorded at a 10 • C/s cooling rate with a 23% decrease compared with the base material.…”

“…This process occurred with the shear transformation of austenite [44]. Hardness measurements for all samples were performed to compare with the microstructure while considering hardness the ferrite-perlite steel structure, next bainite, martensite and their mixture [4,6,36,47]. The cooling rate's effect on the values of the first and last austenite decomposition transformation temperature for both methods of determination is shown in Figure 9.…”

Hot Deformation and Microstructure Evolution of Metallic Materials

“…By lowering the rolling temperature, it was possible to reduce the grain size by up to 56 μm. Schindler et al [ 13 ] studied the combined effect of austenitization temperature and pre-deformation on continuous cooling transformation (CCT) diagrams of 23MnNiCrMo5-3 steel. Based on the dilatometric tests and metallographic analyses, a total of five different diagrams were constructed and compared.…”