Effect of Martensite Fraction on Tensile Properties of Dual-Phase Steels

Abstract

Synopsis : Effects of martensite fraction on tensile properties and inhomogeneous deformation of microstructures in dual-phase steels have been investigated by TEM, SEM-EBSD and strain mapping using high-precision grid markers drawn by electron beam lithography. As is well known, when volume fraction of martensite is increased from 25% to 75%, yield and tensile strengths markedly increase while elongation decreases. It was found by EBSD that the average value of kernel average misorientation in ferrite increase with martensite fraction. Strain mapping using the marker method showed that both equivalent plastic strains of ferrite and martensite increase with martensite fraction, where it is to be noted that the strain increase of martensite is larger than that of ferrite, suggesting that the stress redistribution between ferrite and martensite is enhanced with increasing the martensite fraction.

Microscopic deformation and strain hardening analysis of ferrite–bainite dual-phase steels using micro-grid method

“…Many experimental and analytical investigations were carried out to improve strain hardenability of steels. It has been reported that microstructural characteristics such as volume fraction and morphology of second phase and strength difference between soft and hard phases affect the tensile properties of dual-phase steels to a significant degree [5][6][7][8][9]. In the case of ferrite-bainite steel, it was reported that ferrite with 30-40% of bainite shows highest strain hardenability [7].…”

Reason for high strength and good ductility in dual phase steels composed of soft ferrite and hard martensite

“…Since the microstructures of DP steels are micro-scale mixtures of ferrite and martensite, mechanical interactions between the soft and hard phases must be clarified for fundamentally understanding their good mechanical properties. There are various microstructural parameters that can influence the deformation behavior of DP steels, namely, the volume fraction, size, hardness ratio, and distribution of the two phases [2][3][4][5][6][7]. In the present study, two different types of DP microstructures composed of soft ferrite and hard martensite are fabricated by different heat treatment routs in a low carbon steel.…”

Crystallographic Analysis of Lath Martensite in Ferrite-Martensite Dual Phase Steel Sheet Annealed after Cold-Rolling

“…On the other hand, Hasegawa et al 6) recently investigated the relationship between the volume fraction of martensite and its mechanical properties in DP steels, and showed that the plastic strain of martensite increases with the increase in martensite fractions. Ueji et al 7) also showed that the groups of the laths in the martensite deformed nonuniformly under the plastic strain.…”