In-situ observations of Widmanstätten ferrite formation in a low carbon steel

An in situ laser-scanning confocal microscopy study has been undertaken into Widmanstätten ferrite formation in an Fe–C alloy, in combination with electron backscattered diffraction. It has been found for the first time that the sympathetic nucleation of Widmanstätten ferrite on grain boundary allotriomorphs can exhibit a step wise change in orientation and growth direction until the most favourable growth conditions are achieved.

Effect of transformation mechanism (static or dynamic) on final ferrite grain size

A simple series of test was developed to highlight and compare the difference between the static strain induced transformation (SSIT) and the dynamic strain induced transformation (DSIT) mechanism in grain refinement and also to investigate the origin of the difference between the two mechanisms. The results showed that while the SSIT sets up a two-dimensional impingement among the ferrite grains, it cannot avoid their coarsening (normal growth). However, the DSIT forms a group of grains with a three-dimensional impingement which does not coarsen and maintains their fine size throughout the transformation, thereby, reduces the final average grain size.

Ultrafine ferrite formation in steels through thermomechanical processing

The main aim of this study was to investigate the critical conditions for the formation of ultrafine grain structures using hot torsion and wedge rolling techniques. In addition, the effect of thermomechanical parameters and steel composition on the critical conditions for ultrafine grain structure formation has been systematically evaluated.

The formation of ultrafine ferrite and low temperature bainite through thermomechanical processing

In the current study, a novel approach was employed to produce a unique combination of ultrafine ferrite grains and low temperature bainite in a low carbon steel with a high hardenability. The thermomechanical route included warm deformation of supercooled austenite followed by reheating in the ferrite region and then cooling to bainitic transformation regime (i.e. 400-250°C). The resultant microstructure was ultrafine ferrite grains (i.e. <4μm) and very fine bainite consisting of bainitic ferrite laths with high dislocation density and retained austenite films. This microstructure offers a unique combination of ultimate tensile strength and elongation due to the presence of ductile fine ferrite grains and hard low temperature bainitic ferrite laths with retained austenite films. The microstructural characteristics of bainite were studied using optical microscopy in conjunction with scanning and transmission electron microscopy techniques.

Deformation and fracture characteristics of ferrite/bainite dual-phase steels

The deformation and fracture characteristics of a low carbon Si–Mn steel with ferrite/bainite dual–phase structure were investigated by thermo–mechanical controlled process (TMCP). The results showed that the curves of the instantaneous work–hardening factor n* value versus true strain ε are made up with three stages during uniform plastic deformation: n* value is relatively higher at stage I, decreases slowly with ε in stage II, and then decreases quickly with ε in stage III. Compared tothe equiaxed ferrite/bainite dual–phase steel, the quasi–polygonal ferrite/bainite dual–phase steel shows higher tensile strength and n*value in the low strain region. The voids or micro–cracks formed not only at ferrite–bainite interfaces but also within ferrite grains in the necked region, which can improve the property of resistance to crack propagation by reducing local stress concentration of the crack tips.

Microstructures and properties of ferrite/bainite dual-phase steels with high hole-expanding ratio

The effects of Si and Mn contents on microstructure, mechanical properties and formability of low carbon Si-Mn steels were studied, and the crack propagation of ferrite/bainite dual-phase steel was also investigated. The results showed that the increase in Si content increases the volume fraction of equiaxed ferrite. However, the increase in Mn content increases both strength and ductility, but decreases elongation and hole-expanding ratio. The crack of ferrite/bainite dual-phase steel is formed by the mode of microvoid coalescence. When a microcrack meets the bainite, it mostly propagates along the phase interface between ferrite and bainite and by cutting off ferrite grains. The hot-rolled ferrite/bainite dual-phase steel, which has a hole-expanding ratio of 95% and good property combination, could be produced by designing proper contents of Si and Mn as well as parameters of TMCP.

Dynamic strain-induced ferrite transformation during hot compression of low carbon Si-Mn steels

The dynamic strain-induced transformation (DSIT) of austenite to ferrite was investigated under different undercooling conditions using three low carbon Si-Mn steels. The undercooling of austenite (ΔT) was controlled by varying the cooling rate between austenitization and deformation temperatures. Uniform DSIT ferrite grains (∼2.3 μm) were produced at a relatively high deformation temperature above 840°C using a low carbon high Si steel (0.077C-0.97Mn-1.35Si, mass%) in connection with a larger ΔT. The critical conditions for DSIT were determined based on the flow stress-strain curves measured during hot compression tests. Influence of deformation temperature on DSIT of low carbon Si-added steel was also discussed.

Effects of Si on microstructural evolution and mechanical properties of hot-rolled ferrite and bainite dual-phase steels

Based on the thermo-mechanical controlled process, the effects of Si on microstructural evolution, tensile properties, impact toughness, and stretch-flangeability of ferrite and bainite dual-phase (FBDP) steels were systematically investigated. The addition of Si from 0 to 0.95% promoted the formation of fine and equiaxed ferrite grains, and high Si (0.95%) also resulted in the formation of blocky martensite islands and retained austenite. Yield and tensile strengths, and uniform and total elongations all increased with increasing Si content. Therefore, the tensile strength and ductility balance was improved by Si addition due to the increasing strain-hardening rate. The fractured morphologies after hole-expansion showed that the excellent stretch-flangeability of FBDP steels was associated with the micro-cracks propagating through in ferrite phase as well as the elongated ferrite grains along the direction perpendicular to the crack. 0.95% Si steel had a similar high combination of tensile strength and impact toughness to 0.55% Si steel, and especially 0.95% Si steel exhibited an excellent combination of tensile strength and stretch-flangeability.

Fabrication, microstructure and mechanical properties of high performance ferrite-bainite steels

Low cost ferrite and bainite(FB) steels offer the prospect of high ultimate tensile strength combined with high hole expansion ratio. The enhanced strain hardening and formabilityof FB steels were primarily associated with the fine ferrite matrix, the low residual stresses and dislocation densityand compatible deformation between both phases.This overview describes the various techniques to produce FB steels, and comparestheresulting microstructure, tensile propertiesand tretchflangeabilitywith conventional HSLA and DP steels.A new generation of ultrafine ferrite and nano-scalebainiteautomotive steelsisunder development forthe futuredemands of extremely high strength and ductilitythroughthe fabricationtechnologiesinvolvingphase transformationsandplastic deformation.

Biocompatibility of transition metal-substituted cobalt ferrite nanoparticles

Transition metals of copper, zinc, manganese, and nickel were substituted into cobalt ferrite nanoparticles via a sol-gel route using citric acid as a chelating agent. The microstructure and elemental compositions of the nanoparticles were characterized using scanning electron microscopy combined with energy dispersive X-ray spectroscopy. The particle size of the nanoparticles was investigated using particle size analyzer, and the zeta potentials were measured using zeta potential analyzer. The phase components of the synthesized transition metal-substituted cobalt ferrite nanoparticles were studied using Raman spectroscopy. The biocompatibility of the nanoparticles was assessed using osteoblast-like cells. Results indicated that the substitution of transition metals strongly influences the physical, chemical properties, and biocompatibility of the cobalt ferrite nanoparticles. © 2014 Springer Science+Business Media.