Strain hardening of steels at large strain deformation. Part I: Relationship between strain hardening and microstructures of b.c.c. steels

In the present work, the relation between strain hardening behaviour and microstructure of steels, especially for large deformations, was investigated with the help of the Kocks model. First, the different origins of flow stresses and the development of the dislocation density are discussed because of their importance to the Kocks model. For the measurement of stress strain curves at large deformations, a new method of tensile testing was developed. Steels with different but defined microstructures were produced. It was found that the strain hardening behaviour of Ti micro-alloyed steels and ferritic pearlitic steels depends significantly on the microstructure. Small precipitates or increasing volume fraction of hard second phase increase the strain hardening rate. A quantitative characterization of the microstructure of well-annealed steels can be given by the yield stress and the relation between strain hardening and microstructure can be replaced by the relation between strain hardening and yield stress.