| 摘要 |
The invention concerns a cold-rolled and annealed steel sheet, the chemical composition of which comprises, the contents being expressed by weight: 0.09 ≤ C ≤ 0.11%, 2.6 ≤ Mn < 2.8%, 0.20 < Si < 0.55%, 0.25 < Cr < 0.5%, 0.025 ≤ Ti < 0.040%, 0.0015 < B ≤ 0.0025%, 0.005 ≤ Al < 0.18%, 0.08 < Mo < 0.15%, 0.020 ≤ Nb < 0.040%, 0.002 ≤ N ≤ 0.007%, 0.0005 < S ≤ 0.005%, 0.001 ≤ P ≤ 0.020%, Ca < 0.003%, the remainder consisting of iron and inevitable impurities resulting from production, the sheet having a microstructure consisting of martensite and/or lower bainite, said martensite comprising fresh martensite and/or self-tempered martensite, the sum of the area ratios of martensite and lower bainite being between 40 and 70%, 15 to 45% by area ratio of bainite with a small amount of carbides, 5 to 20% by area ratio of ferrite, the fraction of non-recrystallised ferrite relative to the total ferrite fraction being less than 15%, and less than 5% by area ratio of residual austenite in the form of islands. |
