Dimensional stability and residual stress relief of post-forming nitrided sheet steel

摘要

Ultra-low carbon sheet steel with excess stabilizing elements such as titanium, niobium and vanadium can be strengthened via internal nitriding in an ammonia/nitrogen atmosphere. Strength can be imparted to the sheet either in coil form in an open coil annealing (OCA) furnace or after forming in a batch-type component nitriding process. The latter strengthening method presents several advantages over conventional high strength steels including lower forming loads and increased part complexity (enhanced formability). To illustrate the effects of the nitriding thermal cycle on the dimensional stability and residual stress levels of formed parts, a post-forming nitrided steel was compared to a conventional commercially produced high-strength low- alloy (HSLA) steel in laboratory simulations. A slit-ring technique was used to measure residual stresses and dimensional stability. The residual stress relief as a function of nitriding time was measured for various nitriding temperatures. The results indicate that with post-forming nitrided steel, strength levels greater than or equal to the HSLA steel can be reached in the final part with the added benefits of better formability, lower forming loads, better dimensional stability and complete residual stress relief.