クロム添加による極低炭素冷延鋼板の降伏強度変化

概要

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The relationship between ferrite grain diameter and upper yield strength was investigated for cold-rolled and annealed ultra low carbon steel sheets containing various chromium contents from 0 to 23%. The upper yield strengths were dropped with annealing temperature and the tangent of the change in the yield strength was decreased with increasing in chromium content. The coefficient in Hall–Petch equation decreased linearly with chromium content. This is attributed to lowering solute carbon content at grain boundaries with precipitation of chromium carbide since the decrease was not due to ferrite region hot-rolling nor to α′ phase precipitation in high chromium steel. While friction stress (σ0) in Hall–Petch equation of air-cooled sheets after annealing showed the minimum at chromium content of 5%, σ0 of the lower chromium content steels increased and became to show linear relationship for chromium content after 150°C aging. This result indicates that σ0 increases with chromium content by solid solution strengthening. The reason why the minimum σ0 is exhibited in the 5% chromium steel air-cooled after annealing is that the amount of decrease in σ0 by retardation of aging during air-cooled process is larger than that of solid-solution strengthening by chromium.