Why yield point phenomenon is not shown in if steel?
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Very simple explanation of the effect you observe could be the following.
In low carbon steel carbon atoms are dissolved in the matrix as interstitials, which are known to expand lattice around them. On the other hand, dislocations create tensile stresses below the half-plane. Therefore, highly mobile carbon atoms readily occupy these positions near dislocation core. The binding energy of carbon with dislocation is about 0.5 eV. So pinning of dislocation on carbon atoms trapped in the dislocation core explains high yield stress, which rapidly drops down to Peierls-Nabarro stress when dislocation is released.
In high carbon steel carbon segregates and forms carbides. Therefore its concentration in the matrix reduces and no yield point phenomena is observed.
In low carbon steel carbon atoms are dissolved in the matrix as interstitials, which are known to expand lattice around them. On the other hand, dislocations create tensile stresses below the half-plane. Therefore, highly mobile carbon atoms readily occupy these positions near dislocation core. The binding energy of carbon with dislocation is about 0.5 eV. So pinning of dislocation on carbon atoms trapped in the dislocation core explains high yield stress, which rapidly drops down to Peierls-Nabarro stress when dislocation is released.
In high carbon steel carbon segregates and forms carbides. Therefore its concentration in the matrix reduces and no yield point phenomena is observed.
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