A Unified Model for Plasticity in Ferritic, Martensitic and Dual-Phase Steels

Enrique I. Galindo-Nava,Shuntaro Matsuyama

doi:10.3390/met10060764

Abstract

Unified equations for the relationships among dislocation density, carbon content and grain size in ferritic, martensitic and dual-phase steels are presented. Advanced high-strength steels have been developed to meet targets of improved strength and formability in the automotive industry, where combined properties are achieved by tailoring complex microstructures. Specifically, in dual-phase (DP) steels, martensite with high strength and poor ductility reinforces steel, whereas ferrite with high ductility and low strength maintains steel’s formability. To further optimise DP steel’s performance, detailed understanding is required of how carbon content and initial microstructure affect deformation and damage in multi-phase alloys. Therefore, we derive modified versions of the Kocks–Mecking model describing the evolution of the dislocation density. The coefficient controlling dislocation generation is obtained by estimating the strain increments produced by dislocations pinning at other dislocations, solute atoms and grain boundaries; such increments are obtained by comparing the energy required to form dislocation dipoles, Cottrell atmospheres and pile-ups at grain boundaries, respectively, against the energy required for a dislocation to form and glide. Further analysis is made on how thermal activation affects the efficiency of different obstacles to pin dislocations to obtain the dislocation recovery rate. The results are validated against ferritic, martensitic and dual-phase steels showing good accuracy. The outputs are then employed to suggest optimal carbon and grain size combinations in ferrite and martensite to achieve highest uniform elongation in single- and dual-phase steels. The models are also combined with finite-element simulations to understand the effect of microstructure and composition on plastic localisation at the ferrite/martensite interface to design microstructures in dual-phase steels for improved ductility.

Highlights

Further improvement of advanced high strength steels (AHSSs) is required to reduce weight in electric vehicles (EVs), which controls their range of operation, the most crucial factor for their practical use
The results indicate that the previously used parameters are just fitting parameters, which prevents the application of the models to a variety of steels and compositions
For Mart6 and Mart7, the model tends to predict larger K than experiments. These steels contain the highest amount of carbon amongst experimental data (Table 2), which could indicate that some fraction of carbon atoms precipitate as cementite instead of forming Cottrell atmospheres [83]

Summary

Introduction

Further improvement of advanced high strength steels (AHSSs) is required to reduce weight in electric vehicles (EVs), which controls their range of operation, the most crucial factor for their practical use. High strength steels (HSSs) have contributed to the reduction of the weight of vehicles, their use is limited to parts of simple geometries, because of their poor formability. AHSSs have been widely used for car body parts and reduce much more weight of vehicles than HSSs [1]. That is the reason AHSSs are expected to realise further weight reduction required for EVs. its complex microstructure makes it easy to promote voids between different kinds of microstructures. Voids promote ductile fracture and prevent AHSSs’ improvement of formability [2]

Objectives

Results

Discussion

Conclusion