Deformation and fracture mechanisms in fine- and ultrafine-grained ferrite/martensite dual-phase steels and the effect of aging

Dual-phase (DP) steel is the flagship of advanced high-strength steels, which were the first among various candidate alloy systems to find application in weight-reduced automotive components. On the one hand, this is a metallurgical success story: Lean alloying and simple thermomechanical treatment enable use of less material to accomplish more performance while complying with demanding environmental and economic constraints. On the other hand, the enormous literature on DP steels demonstrates the immense complexity of microstructure physics in multiphase alloys: Roughly 50 years after the first reports on ferrite-martensite steels, there are still various open scientific questions. Fortunately, the last decades witnessed enormous advances in the development of enabling experimental and simulation techniques, significantly improving the understanding of DP steels. This review provides a detailed account of these improvements, focusing specifically on (a) microstructure evolution during processing, (b) exp...

An Overview of Dual-Phase Steels: Advances in Microstructure-Oriented Processing and Micromechanically Guided Design

Experimental and numerical study on geometrically necessary dislocations and non-homogeneous mechanical properties of the ferrite phase in dual phase steels

Large strain warm deformation at different temperatures and subsequent intercritical annealing has been applied to obtain fine grained (2.4m) and ultrafine grained (1.2m) ferrite/martensite dual-phase (DP) steels. Their mechanical properties were tested under tensile and impact conditions and compared to a hot deformed coarse grained (12.4m) reference material. Both yield strength and tensile strength follow a Hall–Petch type linear relationship, whereas uniform elongation and total elongation are hardly affected by grain refinement. The initial strain hardening rate as well as the post-uniform elongation increase with decreasing grain size. Ductile fracture mechanisms are considerably promoted due to grain refinement. Grain refinement further lowers the ductile-to-brittle transition temperature and leads to higher absorbed impact energies. Besides the common correlations with the ferrite grain size, these phenomena are explained in terms of the martensite particle size, shape and distribution and the more homogeneous dislocation distribution in ultrafine ferrite grains.

Effect of grain refinement to 1 μm on strength and toughness of dual-phase steels

The major scientific and technological advances and breakthroughs of advanced high strength steels (AHSS) were achieved due to the strong demands of automotive industry. The development of AHSS began in the early 1980s with the aim of improving passenger safety and weight-saving. The present paper presents the driving forces and logic of development of various AHSS for automotive applications since 1980s. The importance of crash performance, weight-saving, formability, and rigidity are critically reviewed for the development of new steel grades for automotive applications. The logical sequences of the development of dual phase (DP) steel, transformation induced plasticity (TRIP) steels, tempered DP steels, complex phases (CP) steels, Ferrite-Bainite steels, hot-stamping technology, twinning induced plasticity (TWIP) steels, Quench and Partitioning (Q&P) steels, Medium Mn steels, and steels–polymer composites are presented.

Driving Force and Logic of Development of Advanced High Strength Steels for Automotive Applications

Structure–property optimization of ultrafine-grained dual-phase steels using a microstructure-based strain hardening model

Separation of size-dependent strengthening contributions in fine-grained Dual Phase steels by nanoindentation

Elastic anisotropy of a transversely isotropic random network of interconnected fibres: non-triangulated network model

Different Dual Phase steels were processed with ferrite grain size between 0.7 μm and 4 μm and martensite islands dispersed along the grain boundaries of the ferrite phase. Improvement of both strength and elongation is obtained when non radial large strain paths are imposed as cold deformation prior to the thermal annealing. In the present work, specimens were deformed by cold swaging, which is an unusual but very efficient method to apply non radial compressive loading path at room temperature. Local probing of the mechanical properties of the ferrite phase was conducted by nanoindentation. By changing the indentation depth with respect to the size of the microstructure, separation is made between the various contributions to the strengthening : nanoindentation size effect, natural hardening of ferrite, grain size effect and reinforcement by the martensite. A simple model which relates the strengthening mechanisms to their characteristic sizes supports the interpretation of the experimental results and highlights the relationship existing between the different length scales.

Marc Delincé

Papers

Structure–property optimization of ultrafine-grained dual-phase steels using a microstructure-based strain hardening model

Separation of size-dependent strengthening contributions in fine-grained Dual Phase steels by nanoindentation

Elastic anisotropy of a transversely isotropic random network of interconnected fibres: non-triangulated network model

Separation of the different strengthening contributions in fine-grained dual phase steels