0.20C-0.28Si-1.25Mn-25 ppm B Steel

Steel name: 0.20C-0.28Si-1.25Mn-25 ppm B
Diagram No.: 5144
Type of diagram: CCT
Chemical composition in weight %: 0.20% C, 0.28% Si, 1.25% Mn, 0.015% P, 0.008% S, 0.15% Cr, 0.044% Al, 25 ppm B
Steel group: Boron steels, low alloyed steels
Note: Boron exerts a large influence on mechanical properties of steel through microstructural control. It increases the hardenability of steel by retarding the heterogeneous nucleation of ferrite at the austenite grain surfaces and the decomposition kinetics of austenite to ferrite transformation are governed by its location and chemical state. To understand the effect of boron (~ 25 ppm) on microstructural evolution and change in continuous cooling transformation (CCT) diagram , a systematic study has been carried out using Gleeble thermo-mechanical simulator in unalloyed (C: 0.05 wt%; Mn: 0.2 wt%) and low alloyed (C: 0.2 wt%; Mn: 1.2 wt%: Cr: 0.15 wt%) steels. CCT diagrams, plotted for the onset and end of pearlitic, bainitic and martensitic reactions, consisted essentially of two C-curves and a remarkable difference was observed on comparing the results for both steels. It is interesting to note that bainite and martensite are completely absent even at higher cooling rate of 70C/sec in the unalloyed steel with boron addition. In contrast, the addition of boron was observed to promote significant bainite formation even at a slower cooling rate of only 20C/sec in the low alloyed steel. These contradicting effects on hardenability can be explained by the effect of boron in shifting only the upper C curve to the right for reconstructive transformation.
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