Work Hardening in Ferritic Steel Containing Ultra-fine Carbides

Work hardening of ferritic steels containing fine carbides varied from 3 nm to 15 nm was investigated and compared to Ashby's model, which is well known as a work hardening theory of metals containing hard particles. A specific work hardening behavior was observed in the steels strengthened by the nano meter-sized carbides; work hardening proceeded in two stages within a few plastic strains. In the former step, the matrix deformed without the geometrically necessary dislocation since a misfit strain between the carbides and matrix is close to the Burgers vector. So the Ashby's model cannot explain this phenomenon. Yet in the later stage, the amount of work hardening was close to predicted value based on the Ashby's model. The plastic strain at which the later stage started decreased with the increase in the diameter of carbides since the geometrically necessary dislocation is easier to be generated by the larger carbides. A new model which can be applied to steels containing the nanometer-sized carbides by focusing generating dislocation into the matrix around carbides was established.