Transformation of Low-Carbon Austenite after Small Plastic Strains

The effects have been measured of tensile strains of < 0.21 at 950° C on the transformation of three low-carbon steel austenites. Hot deformation of 0.1% C and of 0.1 C, 1.5 Mn, 0.04% Nb steels austenitized at 950° C hastened their transformation to ferrite and softened the transformation product. In the niobium steel austenitized at a high temperature to dissolve NbC and coarsen the grain size before testing at 950° C, deformation also accelerated transformation to ferrite, but without affecting the strength. In these steels transformation to ferrite occurred before the deformed austenites recrystallized. In 0.1 C, 1.5% Mn steel transformation was slow enough for austenite deformed to a strain of 0.21 to recrystallize before transformation, which retarded transformation to ferrite; only at a smaller strain of 0.105 was transformation of this austenite accelerated. These direct effects of hot deformation do not account for the improved strength and toughness of control-rolled low-carbon steels.