Low temperature superplasticity and thermal stability of a nanostructured low-carbon microalloyed steel

J Hu,L.-X Du,H Xie,R.D.K Misra,G.-S Sun

doi:10.1038/srep18656

Abstract

We describe here for the first time the low temperature superplasticity of nanostructured low carbon steel (microalloyed with V, N, Mn, Al, Si, and Ni). Low carbon nanograined/ultrafine-grained (NG/UFG) bulk steel was processed using a combination of cold-rolling and annealing of martensite. The complex microstructure of NG/UFG ferrite and 50–80 nm cementite exhibited high thermal stability at 500 °C with low temperature elongation exceeding 100% (at less than 0.5 of the absolute melting point) as compared to the conventional fine-grained (FG) counterpart. The low temperature superplasticity is adequate to form complex components. Moreover, the low strength during hot processing is favorable for decreasing the spring back and minimize die loss.

Highlights

The effect of isothermal holding time at 500 °C on thermal stability of NG/UFG steel was studied because this is the temperature at which tensile tests was carried out after annealing
The undamaged lath martensitic structure was rarely observed and dislocation density was one order of magnitude higher in specimens subjected to severe cold reduction greater than 80%, leading to increase in nucleation rate of recrystallized ferrite grains
The deformation-induced damaged martensitic structure transformed into NG/UFG ferritic grains via recovery process involving escape of interstitial C atoms and annihilation of dislocations

Summary

Introduction

When cold-rolled plate of 0.9 mm thickness was annealed at 550 °C for 5 min, recrystallization occurred, and the microstructure consisted of UFG (100–300 nm) and few NG (< 100 nm) ferrite grains, and 50–80 nm precipitates distributed along the grain boundaries. The effect of isothermal holding time at 500 °C on thermal stability of NG/UFG steel was studied because this is the temperature at which tensile tests was carried out after annealing.

Results

Conclusion