Crystallographic analysis of nano-structured bainitic steels

A 0.79C-1.5Si-1.98Mn-0.98Cr-0.24Mo-1.06Al-1.58Co (wt%) steel was isothermally heat treated at 200 and 350°C bainitic transformation temperatures. The microstructure was consisted of bainitic ferrite lath and retained austenite for both heat treatment conditions. The crystallographic analysis revealed that the bainitic ferrite laths are close to the Nishiyama-Wassermann orientation relationship with their parent austenite. The isothermal bainite transformation temperature has a significant effect on the retained austenite characteristics and the variant selection of the bainitic ferrite laths. In general, a decrease in the isothennal bainite transformation temperature refined the bainitic structure and weakened the variant selection.

A study of the strengthening mechanism in the thermomechanically processed TRIP/TWIP steel

The strengthening mechanism responsible for the unique combination of ultimate tensile strength and elongation in a multiphase Fe-0.2C-1.5Mn-1.2Si-0.3Mo-0.6Al-0.02Nb (wt%) steel was studied. The microstructures with different volume fraction of polygonal fenite, bainite and retained austenite were simulated by controlled thermomechanical processing. The interupted tensile test was used to study the bainitic ferrite, retained austenite and polygonal ferrite behavior as a function of plastic strain. X-ray analysis was used to characterize the volume fraction and carbon content of retained austenite. TEM and heat-tinting were utilized to analyze the effect of bainitic fenite morphology on the strain induced transformation of retained austenite and retained austenite twinning as a function of strain in the bulk material. The study has shown that the austenite twinning mechanism is more preferable than the transformation induced plasticity mechanism during the early stages of deformation for a microstructure containing I5% polygonal ferrite, while the transformation induced plasticity effect is the main mechanism in when there is 50% of polygonal ferrite in the microstructure. The baillitic fenite morphology affects the deformation mode of retained austenite during straining. The polygonal fenite behavior during straining depends on dislocation substructure tonned due to the deformation and the additional mobile dislocations caused by the TRIP effect. TRIP and TWIP effects depend not only on the chemical and mechanical stability of retained austenite, but also on the interaction of the phases during straining.

Crystallographic analysis of nanobainitic steels

Electron back-scattered diffraction in conjunction with transmission electron microscopy was employed to investigate the crystallographic nature of bainitic laths formed at relatively low transformation temperatures where a nanostructured bainite forms. It was revealed that the bainitic ferrite laths are close to the Nishiyama–Wasserman orientation relationship with the parent austenite. Furthermore, the temperature showed a significant effect on the retained austenite characteristics and the variant selection of the bainitic ferrite laths. A decrease in temperature generally refined the bainitic structure and weakened the variant selection.

Cyclic deformation of advanced high-strength steels : mechanical behavior and microstructural analysis

The fatigue properties of multiphase steels are an important consideration in the automotive industry. The different microstructural phases present in these steels can influence the strain life and cyclic stabilized strength of the material due to the way in which these phases accommodate the applied cyclic strain. Fully reversed strain-controlled low-cycle fatigue tests have been used to determine the mechanical fatigue performance of a dual-phase (DP) 590 and transformation-induced plasticity (TRIP) 780 steel, with transmission electron microscopy (TEM) used to examine the deformed microstructures. It is shown that the higher strain life and cyclic stabilized strength of the TRIP steel can be attributed to an increased yield strength. Despite the presence of significant levels of retained austenite in the TRIP steel, both steels exhibited similar cyclic softening behavior at a range of strain amplitudes due to comparable ferrite volume fractions and yielding characteristics. Both steels formed low-energy dislocation structures in the ferrite during cyclic straining.

Microstructure and mechanical properties of thermomechanically processed TRIP steel

The strengthening mechanism responsible for the unique combination of ultimate tensile strength and elongation in a multiphase Fe-0.2C-1.5Mn-1.2Si-0.3Mo-0.6Al-0.02Nb (wt%) steel was studied. The microstructures with different volume fractions of polygonal ferrite, bainite and retained austenite were simulated by controlled thermomechanical processing. The interrupted tensile test was used to study the bainitic ferrite, retained austenite and polygonal ferrite behaviour as a function of plastic strain. X-ray analysis was used to characterise the volume fraction and carbon content of retained austenite. Transmission electron microscopy was utilised to analyse the effect of bainitic ferrite morphology on the strain induced transformation of retained austenite and retained austenite twinning as a function of strain in the bulk material. The study has shown that the austenite twinning mechanism is more preferable than the transformation induced plasticity (TRIP) mechanism during the early stages of deformation for a microstructure containing 15% polygonal ferrite, while the transformation induced plasticity effect is the main mechanism when there is 50% of polygonal ferrite in the microstructure. The bainitic ferrite morphology affects the deformation mode of retained austenite during straining. The polygonal ferrite behaviour during straining depends on dislocation substructure formed due to the deformation and the additional mobile dislocations caused by the TRIP effect. Operation of TRIP or twinning mechanisms depends not only on the chemical and mechanical stability of retained austenite, but also on the interaction of the phases during straining.

Effect of bake-hardening on the structure-property relationship of multiphase steels for the automotive industry

The effect of a bake-hardening (BH) treatment on the microstructure and mechanical properties has been studied in C-Mn-Si TRansformation Induced Plasticity (TRIP) and Dual Phase (DP) steels after: (i) thermomechanical processing (TMP) and (ii) intercritical annealing (IA). The steels were characterized using X-ray diffraction, transmission electron microscopy (TEM) and three-dimensional atom probe tomography (APT). All steels showed high BH response. however, the DP and trip steels after IA/BH showed the appearance of upper and lower yield points, while the stress-strain behavior of the trip steel after TMP/BH was still continuous. This was due to the higher volume fraction of bainite and more stable retained austenite in the TMP/BH steel, the formation of plastic deformation zones with high dislocation density around the "as-quenched” martensite and “TRIP” martensite in the IA/BH DP steel and IA/BH TRIP steel, respectively.

Effect of pre-straining and bake hardening on the microstructure and mechanical properties of CMnSi trip steels

The effects of pre-straining and bake hardening on the mechanical behaviour and microstructural changes were studied in two CMnSi TRansformation-Induced Plasticity (TRIP) steels with different microstructures after intercritical annealing. The TRIP steels before and after pre-straining and bake hardening were characterised by X-ray diffraction, optical microscopy, transmission electron microscopy, three dimensional atom probe and tensile tests. Both steels exhibited discontinuous yielding behaviour and a significant strength increase with some reduction in ductility after pre-straining and bake hardening treatment. The following main microstructural changes are responsible for the observed mechanical behaviours: a decrease in the volume fraction of retained austenite, an increase in the dislocation density and the formation of cell substructure in the polygonal ferrite, higher localized dislocation density in the polygonal ferrite regions adjacent to martensite or retained austenite, and the precipitation of fine iron carbides in bainite and martensite. The mechanism for the observed yield point phenomenon in both steels after treatment was analysed.

Numerical simulations on warm forming of stainless steel with trip-effect

In steels with TRIP-effect, a phase transformation from the retained-austenite to martensite occurs during forming, and it significantly affects hardening behaviours. Such an effect is sensitive to the amount of strain as well as the temperature variation. For materials with a strong TRIP-effect, new forming techniques are needed to develop that can lead to lighter and stronger components in automotive industry. This paper presents a coupled thermo-mechanical finite element modelling and simulation of a warm deep drawing of austenitic stainless steel (including a TRIP-effect) using LS-DYNA and temperature effect on forming process of such materials is investigated.

Characterization of the bake-hardening behavior of transformation induced plasticity and dual-phase steels using advanced analytical techniques

The bake-hardening (BH) behavior of TRansformation Induced Plasticity (TRIP) and Dual-Phase (DP) steels after intercritical annealing (IA) has been studied using transmission electron microscopy, X-ray diffraction and three dimensional atom probe tomography. It was found for the DP steel that carbon can segregate to dislocations in the ferrite plastic deformation zones where there is a high dislocation density around the "asquenched" martensite. The carbon pinning of these dislocations, in turn, increases the yield strength after aging. It was shown that bake-hardening also leads to rearrangement of carbon in the martensite leading to the formation of rod-like low temperature carbides in the DP steel. Segregation of carbon to microtwins in retained austenite of the TRIP steel was also evident. These factors, in combination with the dislocation rearrangement in ferrite through the formation of cells and microbands in the TRIP steel after pre-straining, lead to the different bake-hardening responses of the two steels.

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.

Understanding the behavior of advanced high-strength steels using atom probe tomography

The key evidence for understanding the mechanical behavior of advanced high strength steels was provided by atom probe tomography (APT). Chemical overstabilization of retained austenite (RA) leading to the limited transformation-induced plasticity (TRIP) effect was deemed to be the main factor responsible for the low ductility of nanostructured bainitic steel. Appearance of the yield point on the stress-strain curve of prestrained and bake-hardened transformationinduced plasticity steel is due to the unlocking from weak carbon atmospheres of newly formed during prestraining dislocations.

Role of microstructure in the low cycle fatigue of multi-phase steels

The low cycle fatigue (LCF) behaviour of several commercially-produced multiphase steels was studied; including dual-phase (DP) and transformation induced plasticity (TRIP). In addition, a novel TRIP980 hybrid microstructure was examined that consisted of coarse ferrite grains along with low temperature bainite regions interspersed with retained austenite. Fully reversed strain controlled fatigue tests were conducted on the different steels to determine the cyclic stress response and strain to failure. The effects of the cyclic deformation on the microstructures were analysed using electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). Results showed that the initial cyclic hardening behaviour and low cyclic softening ratio observed in the TRIP steels was not necessarily due to austenite to martensite transformation. Differences between the austenite transformation behaviour of the conventional and novel hybrid TRIP microstructures was related to the different surrounding phases and the size of the retained austenite.

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.