Nano-scale analysis of nano-bainite formed in advanced high strength steels

The effect of composition and processing schedule on the microstructure of C-Mn-Si-Mo-(Al)-(Nb) steels containing nano-bainite was studied using transmission electron microscopy (TEM) and atom probe tomography (APT). The major phase formed in all steels was nano-bainite. However, the steels with lower carbon and alloying addition content subjected to TMP had better mechanical properties than high alloyed steel after isothermal treatment. The presence of ferrite in the microstructure can improve not only ductility but lead to the formation of retained austenite with optimum chemical stability.

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.

Understanding of the bainite transformation in a nano-structured bainitic steel

A 0.79C-1.5Si-1.98Mn-0.98Cr-0.24Mo-1.06Al-1.58Co (wt%) steel was isothermally heat treated at 200°C for 10 days to form a nano-scale bainitic microstructure consisting of nanobainitic ferrite laths with high dislocation density and retained austenite films. The crystallographic analysis using TEM and EBSD revealed that the bainitic ferrite laths are close to the Nishiyama-Wassermann orientation relationship with the parent austenite. There was only one type of packet identified in a given transformed austenite grain. Each packet consisted of two different blocks having variants with the same habit plane, but different crystallographic orientations. The presence of fine C-rich clusters and Fe-C carbides with a wide range of compositions in bainitic ferrite was revealed by Three-dimensional Atom Probe Tomography (APT). The high carbon content of bainitic ferrite compared to the para-equilibrium level of carbon in ferrite, absence of segregation of carbon to the austenite/bainitic ferrite interface and absence of partitioning of substitutional elements between the retained austenite and bainitic ferrite were also found using APT.

Effect of composition and processing parameters on the formation of nano-bainite in advanced high strength steels

The nano-bainitic microstructures were compared in a 0.79C-1.5Si-1.98Mn-0.24Mo-1.06Al (wt%) steel after isothermal heat-treatment and a Fe-0.2C-1.5Mn-1.2Si-0.3M0-0.6Al-0.02Nb (wt%) steel after controlled thermo-mechanical processing. The microstructure for both steels consisted of bainite. The microstructural characteristics of bainite, such as the morphology of the nano-bainite and thicknesses of bainitic ferrite and retained austenite layers, as a function of steel composition and processing was studied using transmission electron microscopy (TEM). It was found that the nano-bainitic structure can be formed in the low alloy steel through thermomechanical processing. Atom probe tomography (APT) was employed as a powerful technique to determine local composition distributions in three dimensions with atomic resolution. The important conclusions from the APT research were that the carbon content of bainitic ferrite is higher than expected from paraequilibrium level of carbon in ferrite for both steels and that Fe-C clusters and fine particles are formed in the bainitic ferrite in both steels despite the high level of Si.

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.

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.

On the low temperature strain aging of bainite in the TRIP steel

The aging behavior of a thermomechanically processed Mo-Al-Nb transformation-induced plasticity steel with ultrafine microstructure was investigated using transmission electron microscopy and atom probe tomography (APT). Strain aging at 73 K (200 °C) for 1800 seconds led to a significant bake-hardening response (up to 222 MPa). Moreover, aging for 1800 seconds at room temperature after 4 pct pre-strain also revealed a bake-hardening response (~60 MPa). The experimental results showed the formation of carbon Cottrell atmospheres around dislocations and the formation of carbon clusters/fine carbides in the bainitic ferrite during aging. It is proposed that this is associated with the high dislocation density of bainitic ferrite with formation of a complex dislocation substructure after pre-straining and its high average carbon content (~0.35 at. pct). The segregation of carbon and substitutional elements such as Mn and Mo to the retained austenite/bainitic ferrite interface during aging was observed by APT. This segregation is likely to be the preliminary stage for Mo-C particles’ formation. The aging after pre-straining also induced the decomposition of retained austenite with formation of ferrite and carbides.

Acceleration of the super bainite transformation through a coarse austenite grain size

The effect of austenite grain size on the kinetics of the isothermal bainitic transformation in a high-carbon super-bainitic steel was investigated. Experimental results showed that the transformation of super bainite was accelerated by a coarse austenite grain size. This is because while coarse austenite grains provide less nucleation sites, it is beneficial for bainite sheaf growth. Meanwhile, there is a critical austenite grain size below which there is a distinct grain size effect and above which it is not evident. © 2014 Elsevier B.V.

On low temperature bainite transformation characteristics using in-situ neutron diffraction and atom probe tomography

In-situ neutron diffraction was employed to monitor the evolution of nano-bainitic ferrite during low temperature isothermal heat treatment of austenite. The first 10 peaks (austenite, γ and ferrite, α) were monitored during austenization, homogenization, rapid cooling and isothermal holding at 573 K. Changes in the α-110 and γ-111 peaks were analysed to determine the volume fraction changes and hence the kinetics of the phase transformation. Asymmetry and broadening in the α-200 and γ-200 peaks were quantified to lattice parameter changes due to carbon redistribution as well as the effects of size and dislocation density. Atom Probe Tomography was used to confirm that, despite the presence of 1.5 mass % Si, carbide formation was evident. This carbide formation is the cause of poor ductility, which is lower than expected in such steels.

An experimental study of the crystallography of the austenite bainite reaction.

Mode of access: Internet.

In situ observation of bainite growth during isothermal holding

The mechanism of bainite growth has been investigated using in situ transmission electron microscopy observations. It was found that, in a number of alloys studied, a bainitic embryo is made of basic transformation units. These units are either a group of stacking faults or, in two dimensions, a series of parallelograms of different sizes. Thickening/widening of the bainite embryo takes place through shear along the stacking fault planes or twining planes. The bainite embryo is elongated by the formation of new transformation units at both tips of the bainite plate. The three-dimensional morphology of bainite is a convex tens-like lath. It is believed that the bainite embryo grows by shearing, which is controlled by the diffusion of solute atoms during the transformation. As the growth rate is much lower than that of martensite, it is therefore detectable. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.