Effect of strain rate on bake hardening response of BH220 steel

Anindya Das,S Sivaprasad,Debalay Chakrabarti,Soumitro Tarafder,E Cadoni

doi:10.1051/epjconf/20159401037

Anindya Das, S Sivaprasad + Show 3 more

Open Access

https://doi.org/10.1051/epjconf/20159401037

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Abstract

This study aims at understanding the bake hardening ability of ultra low carbon BH220 steel at different strain rates. The as-received material has been pre-strained to four different levels and then deformed in tension under (a) as pre-strained state and (b) after baking at 170 ∘ C for 20 minutes, at three different strain rates of 0.001, 0.1 and 100/s. In both the conditions, yield stress increased with pre-strain and strain rate, but bake hardening ability was found to decrease when strain rate was increased. The strain rate sensitivity of the material was also found to decrease with bake hardening. Generation of dislocation forests and their subsequent immobility during baking treatment enables them to act as long range obstacles during further deformation. At higher strain rates, less amount of dislocations are produced which can interact with themselves and produce hardening, because of which bake hardening ability and the strain rate drops. A dislocation based strengthening model, as proposed by Larour et al. 2011 [7], was used to predict the yield stress values obtained at different conditions. The equation produced excellent co-relation with the experimental data.

Highlights

Bake hardenable steels are popular grades in automobile industries due to their high strength to weight ratio and excellent formability
Cold deformation is essentially a pre-straining process during which fresh dislocations are produced. During baking process these dislocations are pinned down by the interstitial atoms by forming Cottrell atmosphere, providing an enhancement in strength during subsequent deformation through strain ageing phenomena [2]. This process of bake hardening is greatly influenced by the concentration of solute atoms, pre-strain levels, time and temperature of ageing, grain size of the material etc
The engineering stress-strain curves corresponding to PS and PS+BAK conditions, at three different strain rates

Summary

Introduction

Bake hardenable steels are popular grades in automobile industries due to their high strength to weight ratio and excellent formability These steels are used in the outer body panels of the automobiles. Cold deformation is essentially a pre-straining process during which fresh dislocations are produced During baking process these dislocations are pinned down by the interstitial atoms by forming Cottrell atmosphere, providing an enhancement in strength during subsequent deformation through strain ageing phenomena [2]. This process of bake hardening is greatly influenced by the concentration of solute atoms, pre-strain levels, time and temperature of ageing, grain size of the material etc. On further increasing the solute concentration or ageing time and temperature, carbides starts precipitating which again increase the bake hardenability [4]

Experimental design

Tensile and bake hardening properties

Strain rate sensitivity after bake hardening

Conclusion