The effect of copper atoms in solid solution (solute Cu) on yield strength was investigated in dislocation-strengthened steels such as a martensitic steel and a work-hardened steel, which have high dislocation density. The yield strength of the martensitic steel increases with increasing content of the solute Cu. However, the increment of the yield strength by the solute Cu is smaller in the martensitic steel than in the ferritic steel. Dislocation density of the martensitic steel increases with increasing Cu content and the yield strength is also enhanced depending on the dislocation density. The increment of the yield strength can be reasonably explained by the dislocation strengthening mechanism based on the Bailey-Hirsch relationship. In the work-hardened ferritic steel, the solid solution strengthening by Cu is significant when the dislocation density is low, but it tends to disappear with increasing the dislocation density through cold-rolling. These are indicative of the facts that the yield strength of the dislocation-strengthened steel is determined by the dislocation strengthening and the contribution of the solute Cu on the yield strength disappears owing to high dislocation density.
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