Damage Mechanisms and Mechanical Properties of High-Strength Multiphase Steels.

Sebastian Heibel,A Tekkaya,Winfried Nester,Thomas Dettinger,Till Clausmeyer

doi:10.3390/ma11050761

Sebastian Heibel, A Tekkaya + Show 3 more

Open Access

https://doi.org/10.3390/ma11050761

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Abstract

The usage of high-strength steels for structural components and reinforcement parts is inevitable for modern car-body manufacture in reaching lightweight design as well as increasing passive safety. Depending on their microstructure these steels show differing damage mechanisms and various mechanical properties which cannot be classified comprehensively via classical uniaxial tensile testing. In this research, damage initiation, evolution and final material failure are characterized for commercially produced complex-phase (CP) and dual-phase (DP) steels in a strength range between 600 and 1000 MPa. Based on these investigations CP steels with their homogeneous microstructure are characterized as damage tolerant and hence less edge-crack sensitive than DP steels. As final fracture occurs after a combination of ductile damage evolution and local shear band localization in ferrite grains at a characteristic thickness strain, this strain measure is introduced as a new parameter for local formability. In terms of global formability DP steels display advantages because of their microstructural composition of soft ferrite matrix including hard martensite particles. Combining true uniform elongation as a measure for global formability with the true thickness strain at fracture for local formability the mechanical material response can be assessed on basis of uniaxial tensile testing incorporating all microstructural characteristics on a macroscopic scale. Based on these findings a new classification scheme for the recently developed high-strength multiphase steels with significantly better formability resulting of complex underlying microstructures is introduced. The scheme overcomes the steel designations using microstructural concepts, which provide no information about design and production properties.

Highlights

Body-in-white development is strongly driven by both targets reducing CO2 emissions and increasing crash safety
The investigated materials show all common damage mechanisms. These damage mechanisms and whole damage evolution are strongly dependent on the local microstructural morphology as stated before by [1,10]
The DP600 is much more damage tolerant than the DP1000 which fits well with the results presented here

Summary

Introduction

Body-in-white development is strongly driven by both targets reducing CO2 emissions and increasing crash safety. One way to reduce CO2 emissions via lightweight design is the reduction of component thickness. This is only possible with increasing materials strength. A wide range of high-strength multiphase steels for various applications has been developed in the last decades Their respective mechanical properties are directly related to the underlying thermo-mechanical processing and composition dependent microstructure. Characteristics of this microstructure are: phase distribution, phase morphology, phase hardness and hardness difference between phases, grain and particle size, texture and density of geometrically necessary dislocations [1].

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