Properties Of Dual Phase Steel Research Articles

Microstructure and Tensile Properties in Dual Phase and Trip Steels

Advanced high‐strength steels, like dual phase and TRIP steels, have gained much interest for automotive application. The complex microstructures in dual phase steels, and even more critical, the metastable microstructure in TRIP steels, do not follow the well‐established traditional microstructure‐property relationships for deep drawing steels. The volume fraction of the different phases, the phase distribution, and the stability of metastable phases influence significantly the forming potential.This paper discusses the correlation between different microstructural features and the mechanical properties. The tensile test properties of dual phase steels are governed by the martensite volume fraction, the martensite hardness and to a much smaller extent the martensite island diameter. Both in dual phase and more pronounced in TRIP steels the retained austenite content plays a vital role in determining the formability. The stability of the retained austenite can be described by different methods, it needs to be adjusted according to the forming temperature and the type and amount of strain. In general, multiphase steels require a very strict microstructure control in order to develop predictable forming behaviour.

Development of wear resistant medium carbon dual phase steels and their mechanical properties

Dual phase (DP) steels containing four different amounts of martensite ranging from 42 to 72 vol.-% have been developed from 0.42 wt-% carbon normalised steel by intercritical heat treatment at a fixed temperature of 740°C but varying holding times followed by water quenching. Mechanical properties of dual phase steels with increasing volume fraction of martensite have been investigated highlighting the effect of martensite content. The macrohardness has been found to increase with increasing martensite content in dual phase steel. The yield and tensile strengths have been found to increase with increasing amount of martensite whereas the percentage elongation and the percentage area reduction have been found to decrease. This has been attributed to the presence of hard and brittle martensite phase, which increases the strength at the expense of ductility. The mode of fracture has been found to change from purely ductile to mixed (ductile + brittle) as the martensite volume fraction increases from 42 to 72% in dual phase steels. Friction and wear properties under dry sliding conditions have also been found to improve with increasing martensite volume fraction in dual phase steels. The applications of medium carbon DP steels in the field of mineral processing and mining have been discussed.

Properties of Dual Phase Steels

Two different heat treatment (intercriticaly and intermediate heat treatments) have been applied to steels containing 0.16% C in order to obtain dual-phase steels (DPS) with different microstructures. One set of intermediate quenched samples were tempered. The specimens that have different microstructures were subjected to tensile testing and compared according to their heat treatment conditions. The strain-hardening exponent and strength coefficient of DPSs were determined by using Ludwick equation. It was observed that volume fraction of martensite and grain size increased as the annealing temperature and time increased in the intercriticaly annealed steels. The tensile strength and strength coefficient of DPSs decreased as the annealing temperatures and time increased but the strain and strain hardening coefficient of DPSs increased. In the case of intermediate quenching, the volume fraction of martensite increased with the increase in annealing temperature. Tempered DPS exhibited a lower strength than samples obtained with intermediate quenching. Consequently, it was determined that the tensile strength and the strength coefficient of DPS increased. However the total strain and strain hardening exponent decreased with the increment of the annealing temperature in the case of intermediate quenching. Deformation hardening in these steels has occurred in three different specific areas.

Physical modelling of microstructure and mechanical properties of dual-phase steel

The control of microstructure through physical models is of great interest in the development of new processes and products. Because microstructure results mainly from phase transformations occurring during the manufacturing process, modelling of this phenomenon is of great importance. Among the different hardening mechanisms of steels (solid solution, precipitation, grain size, second phase...) the dispersion of a hard second phase in the ferritic matrix is one of the best methods to optimise the strength/ ductility ratio. To achieve weight reductions and fuel saving in vehicles, materials designers have concentrated on Dual-Phase steels (DP) containing ferrite and martensite phases which can be obtained by relatively simple heat treatments. This paper deals with the combination of two types of models developed at IRSID, with the aim of prediction of final properties of hot rolled dual phase steels. The microstructure/mechanical properties relationships in DP steels are first presented. A physical approach for austenite decomposition into grain boundary ferrite for C-Mn steels is described. Finally, the combination of both models is used to predict the effect of process parameters on microstructure and on the mechanical properties.

Effect of Hot Rolling Conditions on the Microstructure and Mechanical Properties of Fe-C-Mn-Si Multiphase Steels

There is a growing need for the high strength steels with high formability for many applications. Using the concept of transformation induced plasticity (TRIP) observed in austenitic steels, one can obtain the combination of high strength and high formability. However, these austenitic steels require a fairly large amount of alloying elements which impose several practical problems for their widespread applications. On the other hand, it has been shown that the dual phase steels possess the relatively good combination of strength and formability due to their characteristic microstructure. The major microstructural constituents of dual phase steels are soft ferrite and hard martensite/bainite with small amount of retained austenite. In recent years, the beneficial effect of retained austenite on the mechanical properties of dual phase steels have been re-recognized and accordingly, there have been several investigations on developing the so-called TRIP-aided dual phase steels having increased amount of retained austenite in the microstructure. Most investigations on these TRIP-aided dual phase steels have been concentrated on the effects of heat treatment conditions and alloying elements such as Si, Mn, C and P on the microstructure and mechanical properties of cold rolled sheet steels. There have also been a few studies on developing suchmore » steels by hot rolling, and the emphases of these studies are placed mostly on increasing the amount of retained austenite. However, the microstructure of these hot rolled steels is far from optimum, consisting of large packets of hard second phase particles. The main objective of the present study is to modify the microstructure of this class of steels by varying the hot rolling conditions. The steel used in the present study is Fe-0.2C-2Si-1.5Mn.« less

Strain hardening behaviour and its relationship to tensile mechanical properties of dual phase steel

Using the modified Crussard–Jaoul analysis, the strain hardening behaviour of a predeformed 1020 dual phase steel with quenching and quenching plus tempering treatments was examined, showing that this dual phase steel deforms in two stages. The relationships between the maximum uniform strain εu, the stress ratio (true initial stress σ0 true maximum stress σu) and the strain hardening exponents in the first stage and the second stage (m1 and m2) were derived based on the Swift equation and the Considere instability criterion, which showed good agreement with the experimental results for the present 1020 dual phase steel. The dependence of strength σu and ductility ɛu of dual phase steel on the exponents m1 and m2 was discussed using the two derived relationships.MST/1631

Strain hardening behaviour and its relationship to tensile mechanical properties of dual phase steel

Using the modified Crussard–Jaoul analysis, the strain hardening behaviour of a predeformed 1020 dual phase steel with quenching and quenching plus tempering treatments was examined, showing that this dual phase steel deforms in two stages. The relationships between the maximum uniform strain εu, the stress ratio (true initial stress σ0 true maximum stress σu) and the strain hardening exponents in the first stage and the second stage (m1 and m2) were derived based on the Swift equation and the Considere instability criterion, which showed good agreement with the experimental results for the present 1020 dual phase steel. The dependence of strength σu and ductility ɛu of dual phase steel on the exponents m1 and m2 was discussed using the two derived relationships.MST/1631

Effect of morphology of second-phase martensite on tensile properties of Fe-0.1C dual phase steels

Fe-0.1 C steel has been studied to determine the effect of morphology (shape, size and distribution) of the second-phase martensite on the tensile properties of dual phase steels. Retention at an intercritical temperature of 1023 to 1073 K followed by ice-brine quenching (intercritical quenching treatments), whereby martensite appears to surround the ferrite grain, increased strength, but produced an increased yield ratio and decreased ductility. Incorporation of 1223 K direct ice-brine quenching prior to the intercritical quenching treatment at 1053 K gave rise to the distribution of fine spheroidized martensite in a refined ferrite matrix. The heat treatment significantly improved strength and ductility, but produced a somewhat increased yield ratio. Austenitization at a temperature of 1223 K followed by step quenching to 1023 K prior to ice-brine quenching, whereby martensite was randomly scattered in massive form in the ferrite matrix, gave a better combination of strength and ductility and produced a decreased yield ratio. These results are briefly discussed in terms of stress-strain analysis and fractography.

Effect of bainite on the mechanical properties of dual-phase steels

Effect of bainite on the mechanical properties of dual-phase steels

The effect of ferrite grain size and martensite volume fraction on the tensile properties of dual phase steel

The yield strength and tensile strength of dual phase steel are found to follow a Hall-Petch type dependence on the mean free path in ferrite d ƒ , i.e. σ = σ 0 + kd ƒ − 1 2 . The Hall-Petch slope k increases with increasing volume fraction of martensite, V m , in the dual phase in both cases, and σ 0 is independent of V m for yield strength but increases with V m for tensile strength. The effect of ferrite grain size and the martensite volume fraction on the yield strength of a dual phase steel can be explained by extending the dislocation pile-up model to dual phase steels. Also, it is found that the effect of ferrite grain size on yield strength is much stronger than on tensile strength. This naturally leads to a decrease of the yield to tensile ratio with increasing ferrite grain size. The yield ratio approaches a constant value at large ferrite grain size and this limiting value decreases with V m . It is demonstrated that these two observations are predictable from the Hall-Petch equations for the yield strength and tensile strength of the dual phase steel.

ラスマルテンサイトを二相域焼鈍して得た複合組織鋼の組織と機械的諸性質

ラスマルテンサイトを二相域焼鈍して得た複合組織鋼の組織と機械的諸性質

Influence of martensite composition and content on the properties of dual phase steels

A study has been made of the mechanical properties of dual phase (martensite plus ferrite) structures produced when Fe-Mn-C alloys are quenched from the austenite plus ferrite phase field, so as to give a series of alloys with constant ferrite and martensite compositions but varying percent martensites. It is found that the strength of a dual phase structure is dependent on the ferrite grain size and the volume fraction of martensite, and is independent of the composition and strength of the martensite. In agreement with previous work the ductility of these steels is superior to that for standard HSLA steels at the same tensile strength. As shown in a previous paper the strength and ductility as a function of percent martensite are in agreement with Mileiko’s theory of composites of two ductile phases. This theory and the results indicate that the superior ductility of dual phase steels is largely a consequence of the high strength (fine grained), highly ductile (low interstitial content) ferrite matrix.