Effect of Island-Dispersed Martensite and Austenite Constituents on Plastic Deformation Behavior of Low Carbon Bainite Steels

Uniaxial tensile tests were conducted for measuring the plastic deformation behavior of low carbon bainite steels before and after aging. Small pieces were cut from the inside of the tensile specimens subjected to several plastic strains, and microstructural observation and X-ray diffraction were conducted. The bainite steel before aging has island-dispersed martensite and austenite (MA) constituents. The proof strength of the bainite steel before aging was found to be 600 MPa. The steel before aging began to deform plastically at 83 % of 700 MPa, which is the proof strength of the bainite steel after aging. The yield ratio before aging was 0.64. The bainite steel having MA-constituents is easy to deform plastically. The bainite steel consists of 89 vol% pro-eutectoid ferrite grains and 11 vol% MA-constituents. The ratio of volume fractions is constant up to 2 % plastic strain. The volume fraction of martensite phase in the MA- constituents is also constant up to fracture. On the other hand, it was newly found that the austenite phase disappears and the volume fraction of ferrite phase increases in the necking portion. Most of the carbon content in the bainite steel is solidly soluble in MA-constituents, and there is no cementite in the bainitic steel before aging. The MA-constituents disappears with aging treatment. The bainite steel after aging consists of 98 vol% ferrite grains and 2 vol% cementite. The ratio of volume fractions remains constant until fracture. And then, the low-carbon bainite steel having the original microstructure induced by aging treatment are returning to reinforce the proof strength of 700 MPa and the yield ratio of 0.76.