Effect of Martensite Plasticity on the Deformation Behavior
of a Low-Carbon Dual-Phase Steel
M. MAZINANI and W.J. POOLE
An experimental study has been conducted to quantify the effects of martensite plasticity on the
mechanical properties of a commercial low-carbon (0.06 wt pct) dual-phase steel. The volume
fraction and morphology (banded and more equiaxed) of the martensite second phase were
systematically varied by control of the intercritical annealing temperature and the heating rate
to this temperature. It was observed that the yield and tensile strengths were dependent on the
volume fraction of martensite but not on the morphology. In contrast, the true uniform strain,
fracture strain, and fracture stress were found to have a significant dependence on martensite
morphology. These results were rationalized by considering an Eshelby-based model, which
allowed for the calculation of the stress in the martensite islands for different morphologies and
volume fractions. By comparing the stress in the martensite with an estimate of its yield stress, it
was possible to rationalize the conditions under which martensite plasticity occurs. The implications of martensite plasticity affect the work hardening of the steels but most importantly the
fracture properties. For conditions where martensite codeforms with the ferrite matrix, void
nucleation is suppressed and the final fracture properties are dramatically improved.
DOI: 10.1007/s11661-006-9023-3
The Minerals, Metals & Materials Society and ASM International 2007
I.
INTRODUCTION
DUAL-PHASE steels were developed in the late
1970s and early 1980s in response to the demand for highstrength, highly formable steels. Research from this
period is well documented in a series of conference
proceedings and journal papers, examples of which can be
found in References 1 through 7. In recent years, there has
been resurgence of interest in dual-phase steels because
the application of these materials has made significant
inroads, particularly in the automotive sector.
Dual-phase steels have a microstructure consisting of
a hard second phase (mostly martensite sometimes with
small amounts of bainite or retained austenite[1,8]).[9,10]
Due to their composite microstructure, dual-phase steels
exhibit interesting characteristic mechanical properties
such as continuous yielding, low yield stress to tensile
strength ratios, and relatively high formability, which
offer advantages compared with conventional highstrength low-alloy steels.[1–7,9–11]
Dual-phase steels can be produced industrially by two
processing routes, either intercritical annealing of coldrolled steels[12–14] (i.e., annealing in the austenite/ferrite
two phase region), often done in association with the
galvanizing process, or by hot rolling followed by step
cooling on the runout table.[15] The mechanical behavior
of dual-phase steels depends on the properties of the
constituent phases as well as the martensite volume
fraction[2,16] and its morphology.[17,18] The strength of
the ferrite phase is mainly controlled by the steel
M.MAZINANI, Doctoral Student and W.J.POOLE, Professor, are
with the Department of Materials Engineering, The University of
British Columbia, V6T 1Z4 Vancouver, BC, Canada. Contact e-mail:
warren.poole@ubc.ca
Manuscript Submitted: May 6, 2006.
328—VOLUME 38A, FEBRUARY 2007
chemistry, grain size,[9,10,19] and initial dislocation density, which may be affected by the compatibility stresses
and strains when martensite forms.[20–22] The strength of
martensite depends primarily on its carbon content[9,23]
and, to a lesser extent, its structure (lath vs plate).[23] In
order to predict the overall deformation behavior of
dual-phase steels, one needs to have knowledge of these
constituent properties and also the partitioning of stress
and strain between the two phases during deformation.
For example, if during straining the ferrite matrix
deforms plastically while the martensite phase remains
elastic, the highest possible strengthening effect of the
martensite phase will be achieved. However, there may
be a penalty in terms of ductility due to the intervention
of alternative processes such as martensite island cracking or interfacial decohesion. Conventionally, dualphase steels have had relatively high average carbon
concentrations (0.1 to 0.2 wt pct),[10] which lead to a
relatively high yield strength for the martensite islands
such that most experimental evidence shows that plastic
deformation of the martensite phase is absent except for
the regions very close to the fracture surface.[11,24–26]
However, there are a limited number of results for steels
toward the low end of the range of carbon concentrations (0.1 wt pct carbon), where there is evidence for
martensite plasticity in these steels, at least under certain
conditions.[26–29] Recently, there has been considerable
interest in using lower carbon concentrations (below
0.1 wt pct) to improve properties. In this case, it might
be expected that martensite plasticity would be favored
given its lower strength.
It is the main objective of this work to determine the
conditions under which martensite plasticity occurs and
to then consider the implications of this on the strength
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