Design and Development of Complex Phase Steels with Improved Combination of Strength and Stretch-Flangeability

Alexis Graux,David San-Martín,David De Castro,Jose Maria Cabrera,Sílvia Molas,Damien Fabrègue,Michel Perez,Djordje Mirković,Carlos Capdevila,Frédéric Danoix,Sebastian Schreiber,Sophie Cazottes

doi:10.3390/met10060824

Abstract

This study presents the design and development of a hot-rolled bainitic steel, presenting a good combination of strength and stretch-flangeability, for automotive applications. Ti, Nb, and Mo were added in the steel composition in order to control austenite grain sizes, enhance precipitation hardening, and promote the formation of bainite. This study focuses on the effect of process parameters on final microstructures and mechanical properties. These parameters are the finishing rolling temperature, which conditions the austenite microstructure before its decomposition, and the coiling temperature, which conditions the nature and morphology of the ferritic phases transformed. A preliminary study allowed to determine the austenite grain growth behavior during reheating, the recrystallization kinetics, and the continuous cooling transformation curves of the studied steel. Then, a first set of parameters was tested at a semi-industrial scale, which confirmed that the best elongation properties were obtained for homogeneous bainitic lath/granular microstructures, that can be produced by choosing a coiling temperature of 500 ∘ C . When choosing those parameters for the final industrial trial, the microstructure obtained consisted of a homogeneous lath/granular bainite mixture that presented a Ultimate Tensile Strength of 830 MPa and a Hole Expansion Ratio exceeding 70%.

Highlights

Over the last 20 years, car designers have been facing increasing challenges concerning passenger safety, vehicle performance, and fuel economy
The industrial production trial demonstrates the industrial feasibility of the steel design concept, even though the use of an industrial process induces significant changes in the final microstructure compared to pilot plant trials
The design and development of a bainitic steel grade presenting a combination of strength and stretch-flangeability for automotive application is presented

Summary

Introduction

Over the last 20 years, car designers have been facing increasing challenges concerning passenger safety, vehicle performance, and fuel economy. Increasing steel strength allows to reduce the total weight of the car, contributing to reduce CO2 emissions. For safety issues, increasing both yield and ultimate tensile strengths leads to enhanced crash performances. Numerous chassis parts of the cars present complex shapes, and are manufactured by cold forming operations from steel plates, which requires good stretch-flangeability. It is related to the local ductility of the material, which can be quite different from the “global” ductility measured from a conventional tensile test. To improve the local ductility, it is important to limit the differences in hardness between phases present in the material, or to favor single phase microstructures

Methods

Discussion

Conclusion