RECENT DEVELOPMENTS IN ADVANCED HIGH STRENGTH SHEET STEELS FOR AUTOMOTIVE APPLICATIONS: AN OVERVIEW

 Abstract

In recent years there has been an increased emphasis on the development of new advanced high strength sheet steels (AHSS), particularly for automotive applications driven by needs for vehicle weight reduction to improve fuel economy and for materials which lead to enhanced crash performance and passenger safety. Steels of current interest involve novel alloying and processing combinations to produce unique microstructural combinations and have been referred to by a variety of identifiers including, among others, DP (dual-phase), TRIP (transformation induced plasticity), HSLA (high strength low alloy), CP (complex phase), TWIP (twinning induced plasticity), and martensitic steels. The properties of these multi-phase steels are derived from appropriate combinations of strengthening mechanisms, the basics of which have been well developed in the steel literature. Continued developments of AHSS steels, designed for specific applications, will require careful microstructure control to optimize the specific strengthening mechanisms responsible for the desirable final properties. In this paper recent AHSS developments are examined, and approaches to produce high strength sheet steels with unique strength/ductility combinations are discussed.

Key Words: AHSS, retained austenite, quenching and partitioning.

Introduction

Throughout the world there is increasing interest in the development of new Advanced High Strength Steels (AHSS) with enhanced combinations of strength and ductility to provide sheet materials for demanding applications in future vehicles. Correspondingly, research is ongoing at universities, research institutes, and companies to assess different processing routes that have been identified for the production of new sheet steel grades. Significant literature is evolving which describes the fundamentals associated with new steel developments and suggests processing routes to achieve desired properties. In this paper, excerpts from three recently-published papers from the ongoing work at the authors’ laboratories are presented to illustrate the status of current AHSS developments. 

Summary

This paper has highlighted recent developments leading to the production of the third generation of advanced high strength sheet steels (AHSS) which will be required in future light-weight, fuel-efficient vehicles. The basic principles for these developments were shown to evolve from early dual-phase and TRIP steel research in the late 1970’s and early 1980’s. The importance of controlling austenite stability and volume fraction in order to produce high-ductility TRIP steels was emphasized as a critical aspect in the growth of new third generation AHSS. Achievement of desired final microstructures in both coated and uncoated products requires control of a large number of variables including alloy content, starting microstructure, annealing temperature prior to cooling, time at temperature, heating and cooling rates, and incorporation of additional thermal cycles. It is important to note that alloy designs have been based primarily on low carbon steels due to welding considerations. Modifications to weld designs or joining processes may allow the use of higher carbon equivalent grades in the future, enabling new process/product concepts that are considered infeasible at present. With the extensive current research activities leading to new AHSS products, the challenges to researchers, steel producers, and designers are to identify and optimize promising alloys and processing routes to minimize the time required for economic implementation of new AHSS products in vehicles of the future.

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