The effect of intercritical annealing heat treatment on mechanical and corrosion properties of fer-rite/martensite dual-phase steels with and without deformation by rolling has been investigated in this study. The X70 steel was rolled at intercritical temperatures (760 and 800 °C) with 30% amount of reductions followed by water quenching in order to produce dual-phase microstructure. The experimental results show that microstructures, mechanical and corrosion properties and were strongly affected by intercritical rolling conditions. It is established that the intercritical rolling provides a fibrous structure, and improves the mechanical and corrosion properties of X70 dual phase steel. It was found that after intercritical rolling (DQR), grain refinement and hardness improved significantly compared to intercritical heat-treated steel without rolling (DQ). A carefully conducted comparison of resistance corrosion of the representative specimens obtained after (DQ) and (DQR) treatments has been studied. (DQR) treatment shows better corrosion resistance than (DQ) treatment. The microstructure, mechanical and corrosion properties of the rolled X70 dual phase steel were investigated by optical microscopy, scanning electron microscopy, hardness tests and electrochemical measurements.
In the present investigation, some electrochemical properties of dual phase X70 steels with different martensite morphologies which have undergone boriding were studied. To obtain a variety of martensite morphologies, Direct Quenching (DQ), Intermediate Quenching (IQ) and Step Quenching (SQ) heat treatments were applied at an intercritical annealing temperature (IAT) of 760℃. The treatment (DQ) allowed the formation of fine martensite evenly distributed in the ferrite matrix. (IQ) treatment showed the formation of martensite along the ferrite / ferrite grain boundaries. In contrast, treatment (SQ) induced the formation of a banded morphology of martensite and ferrite. The realization of borides on X70 (DP) steel was carried out in a powder mixture containing 5% of B4C as source of boron, 5% of NaBF4 as activator and 90% of SiC as diluent at 950℃ for a period of time from 4 h. The corrosion behavior of X70 (DP) steel has been explored by the Tafel extrapolation method in a 5 wt. % H2SO4 solution. The corrosion resistance of steel which has undergone boriding (BDP) is higher than that of steel which has not undergone it (DP).
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