クロム添加による極低炭素冷延鋼板の降伏強度変化

摘要

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(σ_0) in Hall-Petch equation of air-cooled sheets after annealing showed the minimum at chromium content of 5%, σ_0 of the lower chromium content steels increased and became to show linear relationship for chromium content after 150℃ aging. This result indicates that σ_0 increases with chromium content by solid solution strengthening. The reason why the minimum σ_0 is exhibited in the 5% chromium steel air-cooled after annealing is that the amount of decrease in σ_0 by retardation of aging during air-cooled process is larger than that of solid-solution strengthening by chromium.%薄鋼板はプレス成形で三次元形状に加工された後，構造rn体の一部に使用される。構造用材料として求められる強度rnは主としてC量により制御されるが，さらに機能を付与すrnるために合金元素を加えることがある。Crは薄鋼板に熱rn処理材の機能である焼入れ性や環境材料の機能である耐食rn性を付与することから，1％未満の微量から数十％まで構rn造体の使用目的に応じて涼加量が選択されている。このrnCr添加薄鋼板のプレス成形時および成形後の構造休の強rn度を目的のものとするためには，Cr含有量が機械的性質rnに及ぼす影響を正確に把握していなければならない。