Ultrafine-grained Dual-phase Research Articles

A novel approach to producing architectured ultra-high strength dual phase steels

An ultrafine-grained dual phase (DP) microstructure has been produced in a steel that contains a combination of composition and deformation gradients. A carbon gradient was created by controlled decarburization of the steel. The compositionally graded steel was cold rolled to produce a large number of shear bands prior to recrystallization annealing at 650 °C for 5 h followed by intercritical annealing at 740 °C for 10 min. Microstructural characterization reveals a very fine heterogeneous dual phase microstructure with submicron grains appearing along the traces of the prior shear bands that had developed during rolling. Martensite volume fraction changes from the surface to the center and also within these shear band traces. Tensile tests of the new dual phase structure reveal that the work hardening rate changes spatially with martensite volume fraction. Detailed fractography using ex-situ X-ray computed tomography (XCT) reveals a unique form of multiple cup and cone fracture features that can be related to the evolution of damage along the shear band traces with a gradient of void density from the surface to the center. This strategy for fabricating dual phase steels enables the development of architectured ultra-high strength DP steels with different properties that can be tailored by controlling the composition and deformation gradients and subsequent annealing treatments.

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.

Correlation of microstructure and strain hardening behavior in the ultrafine-grained Nb-bearing dual phase steels

Ultrafine-grained dual phase (DP) steels with different Nb contents (0.00, 0.06 and 0.12wt%) were produced by cold-rolling followed by intercritical annealing of ferrite/martensite starting microstructure at 770°C for different holding times. Scanning electron microscopy, equipped with electron backscattered diffraction (EBSD) detector, nanoindentation and tensile testing were used to characterize microstructural evolutions and their correlations to the strain hardening and fracture behavior. EBSD results confirmed the retardation effect of Nb on recrystallization. It was found that the strains stored in the grains and density of geometrically necessary dislocations (GNDs) were increased with the addition of Nb. Strain hardening analysis showed that plastic deformation of the DP steels occurred in three distinct stages, which based on the EBSD results, nanoindentation and fracture analysis, were controlled by microstructural features such martensite volume fraction and size, density of GNDs and individual ferrite and martensite tensile properties.

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.

Thermal stability of an ultrafine-grained dual phase TWIP steel

This work investigates the thermal stability of an ultrafine-grained dual phase twinning-induced plasticity (TWIP) steel produced by friction stir processing (FSP). In exploring this, a set of annealing treatments were conducted on the friction stir processed steel in the temperature range of 500–1000°C for 1h. The results indicate that the microstructure of the ultrafine-grained steel possesses high thermal stability up to 700°C, where the grain size remained in submicron level. At higher temperatures, however, the grains experienced a significant growth. Moreover, the apparent activation energy for grain growth was calculated through isothermal annealing at 1000°C for different durations. The micro-texture analysis shows that the annealing has been accompanied by a substantial shear-type texture weakening. Concurrently, shear punch mechanical testing and microhardness measurements were employed to study the mechanical behavior of the annealed materials. The results were in line with the microstructural evolution i.e., a high stability of strength was recorded by 700°C, while material׳s strength significantly dropped at higher temperatures. The evolution of microstructure and mechanical properties were elaborately discussed considering the nature of FSP induced microstructure, restoration processes taking place during annealing and the variations of ferrite volume fraction with annealing temperature.

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).

Processing of Bimodal Grain-Sized Ultrafine-Grained Dual Phase Microalloyed V-Nb Steel with 1370 MPa Strength and 16 pct Uniform Elongation Through Warm Rolling and Intercritical Annealing

Ultrafine-grained dual phase microalloyed V-Nb steel with ultimate tensile strength of 1371 MPa and uniform elongation of 16 pct characterized by bimodal ferrite grain structure was obtained through warm rolling and subsequent intercritical annealing. The bimodal ferrite grain structure with uniform dispersion of Nb/V carbides and strong γ-fiber texture promoted high strain hardening rate and high uniform elongation and high strength is attributed to ultrafine-grained ferrite and martensite.

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.

Influence of nanoparticle reinforcements on the strengthening mechanisms of an ultrafine-grained dual phase steel containing titanium

To reduce cost, optimise mechanical properties and improve process tolerance, a series of 1000MPa grade ultrafine-grained dual phase (DP) steels with nanosized precipitates have been developed based on the C–Si–Mn–Ti alloy system. The grain size of ferrite in ultra-high strength DP steels ranges from 1.1 to 1.7μm. The amount of precipitations in the annealed sheet is much more than that in the hot-rolled plates with the highest distribution frequency being between 5 and 10nm. The grain refinement and precipitation strengthening interact significantly and have a considerable effect on yield strength. Therefore, the strengthening effects cannot be expressed as a simple linear relationship. A modified root-mean-square (RMS) relationship has been proposed to express the yield strength for dual phase steel with obvious grain refinement and precipitation strengthening.

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.