Transformation during the isothermal deformation of low-carbon Nb-B steels

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Vol. 29; no. 5; pp. 1383 - 1394
• Main Authors: BAI, D. Q, YUE, S, MACCAGNO, T. M, JONAS, J. J
• Format: Journal Article
• Language: English
• Published: New York, NY Springer 01.05.1998; Springer Nature B.V
• Subjects: Applied sciences; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Martensitic transformations; Materials science; Metals. Metallurgy; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Isothermal condition; Plastic flow; Phase transformations; Experimental study; Stress-strain relations; Microalloyed steel; Temperature effects; Steels; Microstructure; True stress true strain curve; Flow curve; Flow stress; Bainitic transformation; Strain rate
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-998-0353-1

The transformation behavior during isothermal deformation of four steels containing different microalloying additions was investigated by means of the "strain-rate change" technique. The flow curves obtained at temperatures ranging from 620-850 deg C, and the associated microstructures, indicate that the transformation in the Mo-Nb-B and Mo-B steels is of the austenite-to-bainite type. Here, dramatic increases in flow stress are observed at lower temperatures. By contrast, the transformation in the Nb-15B and Nb-64B steels is basically of the austenite-to-ferrite type; in these two grades, the flow stress increases observed are attributable to strengthening by NbC precipitation. Large intergranular and intragranular Fe sub 23 (C,B) sub 6 particles were found in the Nb-64B steel samples deformed to epsilon =0.1 after holding for 60 s at 800 deg C. These large precipitates are considered to be responsible for accelerating the transformation in the Nb-64B steel by reducing the concentration of boron atoms available for boundary segregation and by acting as nucleation sites for the formation of polygonal ferrite. The flow curves of the Mo-Nb-B steel exhibit distinct serrations, indicating that a displacive mechanisms is involved in the gamma -to-B transformation.