Effects on the Final Mechanical Properties by Soft Annealing 22MnB5 Steel Sheets Prior to Austenitization

Abstract Using two different versions of a cold rolled steel, the effect of the initial microstructure on the final mechanical properties were studied. Prior to austenitisation and cooling, half of the cold rolled material underwent a soft annealing treatment to modify the initial microstructure. From these two initial materials, tensile samples were produced to assess the mechanical properties, and the variations in these properties caused by the altered initial microstructure. The samples were isothermally austenitised at 930 °C using a Gleeble 3800 Thermo-mechanical Simulator, followed by cooling at either 5 or 15 °C/s and finally a bake hardening treatment at 170 °C for 20 min. The tensile testing was performed at room temperature using notched tensile samples, where the deformation was measured using both an extensometer and digital image correlation. The tensile testing revealed that the cold rolled samples, without any initial heat treatments, were stronger and less ductile than the soft annealed equivalent. The difference in strength was most pronounced for the higher cooling rate, and the difference in ductility was most apparent for the samples cooled at a lower rate. In the samples cooled at 5 °C/s, the final microstructure was composed of mainly ferrite and pearlite. The morphology differed between the two different materials, with the soft annealed samples having smaller regions of pearlite, and the ferrite grains containing carbides. The faster cooling rate, 15 °C/s, produced a microstructure containing mainly bainite and martensite, with smaller regions of ferrite. Measurements of the parent austenite grain size revealed a larger average parent austenite grain size for the samples which were soft annealed prior to austenitisation, both at the beginning and at the end of the isothermal austenitisation. This is one of the parameters which can affect the phase transformations kinetics, as the grain boundaries act as nucleation sites during austenite decomposition. Thus, a smaller parent austenite grain size could be expected to lead to a softer microstructure formed earlier during the cooling. However, this was not true for the tensile testing performed as part of this study. The initial microstructure affects the final microstructure and mechanical properties in a manner not captured by simplified models using the average austenite grain size and chemical composition.