Advanced High Strength Steels: Improved Properties by Design of Textures and Microstructures

Leo A.I Kestens,Dorien De Knijf,Roumen H Petrov,Felipe Castro Cerda

doi:10.1088/1757-899x/219/1/012004

Abstract

Texture and microstructure formation during annealing of a 75% cold rolled AHS steel, reheated at a rate of 10°C/s to ∼3000°C/s were studied with the purpose to investigate the interaction between the recrystallization and the austenite formation as well as the possibility of grain refinement. It was found that at ultra-high heating rates, the bcc-fcc phase transformation starts before the completion of recrystallization and the deformation texture is partially retained in the samples after annealing. The crystallographic texture observed in the martensitic phase, which is the product of intercritical austenite transformation in as-quenched samples, i.e., after the double α-γ-α’ transformation, is similar to the initial cold rolling texture, which is an indication for the texture memory effect. After ultra-fast reheating with average reheating rates of 1000°C/s and higher, a significant grain refinement was observed with an average calculated ferrite grain diameter of ∼2 μm. The obtained final ferrite grain size depends significantly on both the reheating temperature and the heating rate. The same effect is observed in 75% cold rolled quenching and partitioning steels for heating rates of 400°C/s, 800°C/s and 1000°C/s. In both cases the combination of fine grains, and weak RD/ND-fiber textures produce an increase of strength and ductility compared to material annealed using a conventional heating rate of ∼10°C/s.

Highlights

The development of advanced high strength steels (AHSSs) was a reaction of the steel industry to societal requests to decrease the CO2 emissions of the cars without sacrificing passenger safety [1]
An increase of the temperature with heating rates of 140°C/s triggers initially recovery and recrystallization in the cold rolled steel, which is associated with a decrease in the hardness
The results in this study show that the microstructure and mechanical properties of existing AHSS compositions can be improved without addition of expensive alloying elements if fast heating without isothermal soaking is used

Summary

Introduction

The development of advanced high strength steels (AHSSs) was a reaction of the steel industry to societal requests to decrease the CO2 emissions of the cars without sacrificing passenger safety [1]. The development of AHSS grades is one of the enduring priorities of the steel manufacturing industry today and requires often non-conventional approaches in development of stronger and tougher steel grades. The use of high strength steels in the automotive industry appears to be an effective way to decrease the weight of vehicles, to increase the safety of the passengers and to decrease the fuel consumption.

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