Abstract
The influence of microstructural parameters on the true stress-true strain diagrams between 0.02 strain and the necking strain was investigated in the steels whose microstructure was constituted of tempered martensite patches and free ferrite.
The results obtained are as follows:
(1) Under some conditions, the logarithmic true stress-logarithmic true strain diagram does not make a straight line but it consists of two straight lines having different slopes.
(2) The appearance of the knee, which is the intersecting point of two straight lines, is controlled by the value of the volume fraction of tempered martensite f, the mean free path between tempered martensite patches α2, the tempered martensite patch size β and tempering temperature.
(3) The stress at the knee is closely related to the yield strength of tempered martensite structure. It is supposed from this that, at the knee, the macroscopic yieldings of tempered martensite patches start.
(4) The lowering of α2, β and tempering temperature and the rising of f introduce the increasing flow stress at a constant strain.
(5) To explain the above experimental results, a model representing the microstructure of steel used in the present work was proposed. In the model, all the tempered martensite and some portion of free ferrite were connected in parallel and furthermore this unit and remaining portion of free ferrite were connected in series as shown in Fig. 6(c).
