Ultrafine Grained Dual Phase Steels Research Articles

Effect of Intercritical Annealing on Microstructure and Tensile Properties of an Ultrafine‐Grained Dual‐Phase Low Alloy Steel Containing Titanium

Ultrafine‐grained (UFG) dual‐phase (DP) AISI 5115 steel containing 0.12 wt% Ti is developed using thermomechanical treatment of cold rolling and intercritical annealing. The aim is to investigate the effect of intercritical annealing time on microstructural changes, mechanical properties, and fracture behavior. The results show that, with increasing intercritical annealing time up to 8 min, the martensite island size and the martensite volume fraction increase, whereas the ferrite grain size is not changed. Significant TiC precipitation is found inside the ferrite grains that retards grain growth during annealing. The yield strength (YS) and ultimate tensile strength (UTS) of the UFG DP steels are enhanced by increasing the martensite volume fraction, while slightly attenuated by enlarging the size of martensite islands. A suitable combination of strength and ductility is obtained in terms of the amount of energy absorbed for UFG DP steels (UTS × UE > 160 MPa%) compared with the coarse‐grained DP steel (≈100 MPa%). The fracture mechanism of the UFG DP steel is ductile in which increasing annealing time results in a smaller average diameter of the dimples.

Ultrafine grained dual-phase martensite/ferrite steel strengthened and toughened by lamella structure

Grain refinement is one of the most effective methods for strengthening steels, but grain refinement to 1 µm or less deteriorates the ductility considerably. To evade the strength-ductility trade-off, a lamellar-structured (LS) ultrafine-grained (UFG) dual-phase (DP) ferrite/martensite (F/M) steel was produced by warm rolling an intercritical annealed low-carbon martensite and subsequent air cooling in this paper. The microstructure is similar to a short fiber reinforced composite, and the highest strength (1432 ± 15 MPa) combined with a decent ductility (16.5 ± 0.5% elongation) is achieved in DP steel. Compared with conventional equiaxed coarse-grained (CG) and UFG DP counterparts, the LS martensite in UFG DP steel can produce extra strengthening effect and is also beneficial to ductility. And the short fiber theory can predict the strength very well.

High Strength-High Ductility Combination Ultrafine-Grained Dual-Phase Steels Through Introduction of High Degree of Strain at Room Temperature Followed by Ultrarapid Heating During Continuous Annealing of a Nb-Microalloyed Steel

Ultrafine-grained dual-phase (UFG-DP) steel consisting of ferrite (1.2 μm) and martensite (1 μm) was uniquely processed via combination of hot rolling, cold rolling and continuous annealing of a low-carbon Nb-microalloyed steel. Room temperature tensile properties were evaluated and fracture mechanisms studied and compared to the coarse-grained (CG) counterpart. In contrast to the CG-DP steel, UFG-DP had 12.7% higher ultimate tensile strength and 10.7% greater uniform elongation. This is partly attributed to the increase in the initial strain-hardening rate, decrease in nanohardness ratio of martensite and ferrite. Moreover, a decreasing number of ferrite grains with {001} orientation increased the cleavage fracture stress and increased the crack initiation threshold stress with consequent improvement in ductility UFG-DP steel.

Microstructure and Mechanical Properties of V-Nb Microalloyed Ultrafine-Grained Dual-Phase Steels Processed Through Severe Cold Rolling and Intercritical Annealing

Ultrafine-grained (UFG) dual-phase (DP) steel was produced by severe cold rolling (true strain of 2.4) and intercritical annealing of a low carbon V-Nb microalloyed steel in a temperature range of 1003 K to 1033 K (730 °C to 760 °C) for 2 minutes, and water quenching. The microstructure of UFG DP steels consisted of polygonal ferrite matrix with homogeneously distributed martensite islands (both of size <1 µm) and a small fraction of the inter lath films of retained austenite. The UFG DP steel produced through intercritical annealing at 1013 K (740 °C) has good combination of strength (1295 MPa) and ductility (uniform elongation, 13 pct). The nanoscale V- and Nb-based carbides/carbonitrides and spheroidized cementite particles have played a crucial role in achieving UFG DP microstructure and in improving the strength and work hardening. Analysis of work hardening behavior of the UFG DP steels through modified Crussard–Jaoul analysis showed a continuously varying work hardening rate response which could be approximated by 2 or 3 linear regimes. The transmission electron microscopy analysis on post tensile-tested samples indicated that these regimes are possibly related to the work hardening of ferrite, lath, and twin martensite, respectively.

Effect of Nb on Microstructures and Mechanical Properties of an Ultrafine-Grained Dual Phase Steel

The effect of Nb addition from 0.06 to 0.18 wt.% on microstructural evolutions, mechanical properties, strain-hardening behavior, and fracture mechanism of an ultrafine-grained dual phase (UFG-DP) steel was investigated. The DP steels were cold rolled up to 80% thickness reduction followed by intercritical annealing at 770 °C for 6 min to form the UFG structures. Results showed that increasing Nb content up to 0.12 wt.% increased the volume fraction of martensite and decreased the average grain size of ferrite; however, lower martensite content with no further grain refinement was detected in the steel containing 0.18 wt.% Nb. The variations of strength, elongation, strain-hardening exponent, and fracture behavior of the Nb-bearing UFG-DP specimens were explained in terms of the microstructural features. The UFG-DP steel containing 0.12 wt.% Nb showed a superior strength-elongation balance (UTS × UE ≈ 116 MPa) compared to both the as-received ferritic-pearlitic and the commercially used DP980 steels.

Microstructure refinement of dual-phase steels with 3.5 wt% Mn: Influence on plastic and fracture behavior

The plastic and fracture behavior of ultrafine-grained dual-phase (UFG-DP) steels (Fe-0.1wt%C-3.5wt%Mn) were analysed from the comparison with the coarser grained DP microstructures taken as references. The UFG-DP steel exhibits a superior balance between strength, uniform elongation and true fracture strain. The higher mechanical performance is explained by both a size effect and a relatively high Mn content in austenite which lead to a Transformation-Induced-Plasticity effect. Furthermore, the analysis shows that microstructure refinement significantly retards damage accumulation and modifies the dominant damage mechanism and fracture behavior. The analysis confirms that the present grade of ultrafine-grained DP steels is a serious candidate for automotive applications.

Strengthening Mechanisms of Ultrafine Grained Dual Phase Steels Developed by New Thermomechanical Processing

Dual phase (DP) steels have been investigated using a new approach utilizing simple cold-rolling and subsequent intercritical annealing of a martensite-ferrite duplex starting structure. The ferrite grain size and volume fraction of martensite were varied by changing the rolling reduction and intercritical annealing time. Ultrafine grained DP (UFG-DP) steel with an average grain size of about 2 μm was achieved by short intercritical annealing of the 80% cold–rolled duplex microstructure. Tensile testing revealed superior mechanical properties (the ultimate tensile strength of 1100 MPa and elongation of 13%) for the new DP steel in comparison with the commercially used high strength DP980 steel. The variations in hardness, strength and elongation of the specimens with rolling reduction and intercritical holding time were correlated to microstructural characterizations. The inherent mechanism for strengthening of the DP steel was discussed and the contribution of each strengthening factor was quantitatively calculated. The results showed that the calculated yield strength (430.6 MPa) is very close to the measured value (422 MPa).

Effects of initial microstructure and thermomechanical processing parameters on microstructures and mechanical properties of ultrafine grained dual phase steels

Dual phase (DP) steels was investigated using thermomechanical processes designed to obtain ultrafine/nanoferrite-carbide aggregates and martensite–ferrite duplex starting structures prior to intercritical annealing. The effects of processing parameters such as intercritical annealing temperature and time on the microstructural evaluations, mechanical properties, strain hardening behaviors and fracture mechanisms have been studied. The ferrite grain size and martensite volume fraction were depended on the initial microstructure and thermomechanical processing parameters. Ultrafine grained DP (UFG-DP) steel with an average grain size of about 2μm was achieved by short intercritical annealing of the 80% cold-rolled duplex microstructure. Tensile testing revealed an excellent strength–elongation balance (UTS×UE>110Jcm−3) in the DP steels. The new UFG-DP steels showed superior mechanical properties in comparison with the commercially used high strength steels. The variations of strength, elongation, strain hardening behavior and fracture mechanism of the specimens with thermomechanical parameters were correlated to microstructural features.

Development of a new ultrafine grained dual phase steel and examination of the effect of grain size on tensile deformation behavior

Ultrafine grained dual phase (DP) steels are among the newest grades of DP steels that incorporate the uniform distribution of fine martensite particles (in the order of 1–2μm) within a ferrite matrix. These new grades of steels have been developed in response to the world׳s demand for decreasing the fuel consumption in automobiles by increasing the strength to weight ratio. In the present research, a new kind of ultrafine grained DP (UFG-DP) steel with an average grain size of about 2μm as well as a coarse grained DP (CG-DP) steel with an average grain size of about 5.4μm was produced by consecutive intercritical annealing and cold rolling of low carbon AISI 8620 steel. The martensite volume fraction for both microstructures was the same and about 50 percent. Scanning electron microscopy (SEM) microstructural examination and room temperature tensile deformation analyses were performed on both UFG-DP and CG-DP steels and their deformation behavior in terms of strength, elongation and strain hardening was studied and compared. Room-temperature uniaxial tensile tests revealed that for a given martensite volume fraction, yield and tensile strengths were not very sensitive to martensite morphology. However, uniform and total elongation values were noticeably affected by refining martensite particles. The higher plasticity of fine martensite particles as well as the more uniform strain distribution within the UFG-DP microstructure resulted in higher strain hardenability and, finally, the higher ductility of the UFG-DP steel.

경정립 미세화에 따른 이상조직 탄소강의 하중에 따른 마멸기구

Dry sliding wear behavior of ultra-fine grained(UFG) plain low carbon dual phase steel, of which microstructure consists of hard martensite in a ductile ferrite matrix, has been investigated. The wear characteristics of the UFG dual phase steel was compared with that of a coarse grained dual phase steel under various applied load conditions. Dry sliding wear test were carried out using a pin-on-disk type tester at various loads of 1N to 100N under a constant sliding speed condition of 0.20m/s against an AISI 52100 bearing steel ball at room temperature. The sliding distance was fixed as 1000m for all wear tests. The wear rate was calculated by dividing the weight loss, measured to the accuracy of 10-5g by the specific gravity and sliding distance. The worn surfaces and wear debris were analyzed by SEM, EDS and profilometer. Micro-vickers hardness of the cross section of worn surfaces were conducted to analyze strain hardening underneath the contact surfaces. The wear mechanism of the UFG dual phase steel was investigated with emphasis on the unstable nature of the grain boundaries of the UFG microstructure.

Ultrafine Grained Dual Phase Steels Fabricated by Equal Channel Angular Pressing

Ultrafine grained (UFG) ferrite-martensite dual phase steels were fabricated by equal channel angular pressing and subsequent intercritical annealing. Their room temperature tensile properties were examined and compared to those of coarse grained counterpart. The formation of UFG martensite islands of ~ 1 μm was not confined to the former pearlite colonies but they were uniformly distributed throughout UFG matrix. The strength of UFG dual phase steels was much higher than that of coarse grained counterpart but uniform and total elongation were not degraded. More importantly, unlike most UFG metals showing negligible strain hardening, the present UFG dual phase steels exhibited extensive rapid strain hardening.

Ultrafine grained ferrite–martensite dual phase steels fabricated via equal channel angular pressing: Microstructure and tensile properties

Ultrafine grained (UFG) ferrite–martensite dual phase steels containing different amounts of vanadium were fabricated by equal channel angular pressing (ECAP) and subsequent intercritical annealing. Their room temperature tensile properties were examined and compared to those of a coarse grained counterpart. The formation of UFG martensite islands of ∼1 μm was not confined to the former pearlite colonies but they were uniformly distributed throughout UFG ferrite matrix. A diffusion analysis showed that this specific microstructure may result from dissolution of carbon atoms from pearlitic cementite and their concurrent diffusion into UFG ferrite during ECAP, making the average carbon content reach the equilibrium content to form austenite during subsequent intercritical annealing. The strength of UFG dual phase steels was much higher than that of the coarse grained counterpart, but uniform and total elongations were not degraded. More importantly, the present UFG dual phase steels exhibited extensive rapid strain hardening unlike most UFG materials. The addition of vanadium slightly increased the strength and elongation of the present UFG dual phase steels, but it was found that excessive vanadium addition did not lead to further improvement of their mechanical properties. An excellent combination of strength, elongation and strain hardening of the present UFG dual phase steels was explained in terms of their specific microstructural features.