Damage Mechanisms and Mechanical Properties of High-Strength Multiphase Steels

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.

Microstructure and Mechanical Properties of Nb and V Microalloyed TRIP-Assisted Steels

“…In addition to the formability evaluation, the difference of elongation between the steel types of local and global formability is typically larger than the values measured in this investigation. Heibel et al reported that there was a difference of 0.04 in ε 3f and 0.10 in true uniform elongation between high-formability dual-phase 800 MPa strength grade and 1000 MPa grade CP steels [35]. As it was observed from the microstructures, all the steels in the present study were more or less dual-phase type steel with varying amounts of retained austenite and hard phases in the ferritic steel matrix.…”

“…The effect of IBT temperature on the local and global formability was evaluated using true thickness strain at fracture diagram, which was earlier reported to be suitable classification type for high-strength multiphase steels [35]. Figure 11 presents an evaluation of the formability types of the investigated steels, where Steel A and Steel C IBT 450 demonstrated the best balanced local/global formability.…”

“…Even though Steels B and C did not surpass the properties of Steel A, their balanced local/global formability was near to the lower strength reference steel. It was concluded that the effective phenomenon was grain refinement which increases local formability with the simultaneous strengthening [35].…”

“…In the case where only a limited amount of test material is available, the elongation test data and test pieces can be investigated using advanced methods. Heibel et al proposed that the measurement of true thickness strain at fracture and true fracture strain allows the estimation of damage tolerance, edge-crack sensitivity, fracture toughness, and bendability of high-strength multiphase steels [35]. True thickness strain at fracture is suitable measure for local formability, whereas true uniform strain depicts the global formability of the material.…”

Microstructure, Crystallographic Texture, and Stretch‐Flangeability of Hot‐Rolled Multiphase Steel

“…The hole expandability of AHSSs has been investigated . However, a detailed characterization of the microstructure of the sheet and the changes at the hole edge‐punched has not been deeply realized.…”