Permanent Strength of Interstitial-free Steel Processed by Severe Plastic Deformation and Subsequent Annealing

Abstract

The permanent strengths of interstitial-free (IF) steels with different grain sizes and dislocation densities processed by severe plastic deformation (SPD) and subsequent annealing are systematically investigated. Permanent strength, which is athermal and time-independent, corresponds to the fundamental capability to bear stresses caused by external forces. Sufficiently long-time (24 h) stress relaxation tests were carried out and experimental stress-relaxation time relationships were extrapolated to estimate the permanent strength that remained after an infinite time passed. The flow stresses observed in standard uniaxial tension tests increased with repeated SPD processes, and the fraction of permanent strength to the observed flow stress was mostly above 65%. The permanent strength also increased with repetition of SPD processes, and subsequent low-temperature annealing further augmented the permanent strength. During SPD processes, the dislocation-related strengthening was dominant, while the grain-size-related strengthening was minor, i.e., the Hall-Petch relation does not hold. On the other hand, after low-temperature annealing, the grain-sizerelated strengthening became dominant, quickly replacing the dislocation-related strengthening. In a coarse-grain region, the grain-size-related strength was consistent with the classical Hall-Petch relation. It was confirmed that the original Hall-Petch relation holds only in the coarse-grain region and it indicates "softening with grain coarsening due to annealing", not "strengthening by grain refinement due to SPD".

Predictive Modeling of Strip Temperature in Continuous Annealing Furnace: An Improved Optimization Algorithm

“…The vertical CAF is heated by the heat radiation of the radiant tube (RT) with a better surface quality, which is widely used in practical production. [4][5][6] Since the strip temperature characteristics grow increasingly complex as a result of the influence of parameters such as RT temperature, furnace temperature, strip width and thickness, production line speed, and materials. 7,8) It is worth pointing out that in the production of the iron and steel industry, the modeling and control of CAF are of great significance to improving product quality, reducing energy consumption, and safe production.…”