Transformation Induced Plasticity Assisted Steels Research Articles

TRIP assisted press hardened steel by the anisothermal bainitic ferrite transformation

Abstract A new steel chemical composition is combined with a new press hardening process, in which die-quenching is interrupted by opening the forming tool to permit slow cooling of the hot formed part through the anisothermal bainitic ferrite transformation. This promotes carbon partitioning to austenite before the forming tool is re-closed and die-quenching is resumed to near-ambient temperature. The final microstructure is predominantly bainitic ferrite with dispersions of martensite and up to 11 % retained austenite. Retained austenite can undergo stress induced transformation to martensite in an automobile crash event. The steel exhibits up to 25 % elongation and 930 MPa tensile strength. In contrast to traditional cold formable Transformation Induced Plasticity assisted steels, where retained austenite is consumed during work hardening of cold forming, here, the desired microstructure is achieved after hot forming meaning the retained austenite is more uniformly distributed within the formed part, which enhances energy absorption. The new steel chemical composition is carefully designed to provide optimal microstructural evolution within the constraints of the new press hardening process, yet relatively lean and manufacturer friendly. The new press hardening process is energy efficient as secondary heating is not required since retarded cooling through the bainitic ferrite transformation is provided by residual heat accumulation of the newly developed titanium alloy forming tool. Development of the new technology is demonstrated by press hardening experiments, tensile testing, microstructural analysis, transversal & axial crush testing of formed parts and numerical simulation of crush testing, including a new modelling technique that more accurately simulates deformation of hot versus cold formed parts. Results show a 22 % increase to energy absorption under axial crushing compared to traditional cold formed Transformation Induced Plasticity assisted steels owing to greater work hardening capacity in formed radii of the part, which are shown to be exposed to the highest stresses during crushing.

A comparison of the internal stresses in a transformation-induced plasticity-assisted steel and a twinning-induced plasticity steel

ABSTRACTAn analysis of the loading–unloading behaviour of tensile tests was performed in a transformation-induced plasticity-assisted (TRIP-assisted) steel, duplex high Mn steel (22% Mn–3% Al–3% Si) for a wide range of grain sizes, D. The Bauschinger effect observed during unloading was so strong that plastic deformation was detected before complete unloading was reached. In addition, the yield stress, , or the flow stress, , are related to the grain size according to the Hall–Petch relationship. This behaviour was compared with a twinning-induced plasticity (TWIP), steel (22% Mn–0.6% C). The high levels of internal stresses obtained in both steels can be rationalised on account of the reinforcement of the austenitic matrix by the increasing volume fraction of ϵ-martensite or deformation twins of nanometric thickness.

Crystal Plasticity Based Transformation Induced Plasticity Formulation for Predictions of Forming Limit Diagrams

Significant effort has been invested into the development and research of Advanced High Strength Steel. In particular, transformation induced plasticity (TRIP) steel has shown promise as a candidate for lightweighting of vehicles because of their enhanced formability. However, predictive modeling of TRIP steel in formability is difficult due the combination of dislocation and transformation mechanisms occurring during deformation for various strain paths. This work presents a crystal plasticity constitutive model to simulate the behaviour of deformation induced γ → α’ transformation. A physics based transformation model is implemented into the crystal plasticity framework to fully capture the behavior of TRIP assisted steels. Simulations are calibrated and compared to experimental measurements of Duplex Stainless Steel. The calibrated model is then used to predict the forming limit diagram using the Marciniak-Kuczynski approach. The results highlight the importance of a physics based used in simulation to fully utilize TRIP assisted steels.

Crack propagation in TRIP assisted steels modeled by crystal plasticity and cohesive zone method

The influence of transformation induced plasticity (TRIP) on materials mechanical behaviours, as well as failure phenomena including crack propagation and phase boundary debonding in multiphase steels (e.g. dual phase steels, TRIP steels) are studied by using an advanced crystal plasticity finite element method. We have coupled the crystal plasticity model Ma and Hartmaier (2015), which explicitly considers elastic-plastic deformation of ferrite and austenite, austenite-martensite phase, with a cohesive zone model designed for crack propagation, to study the deformations of several representative microstructural volume elements (RVE). Results shows that, the transformation induced plasticity enhances materials strength and ductility, hinders crack propagation and influences interface debonding. Furthermore, the martensitic transformation kinetics in TRIP steels was found depending on the crystallographic orientation and the stress state of a retained austenite grain. The current simulation results helps to investigate and design multiphase steels with improved mechanical properties.

Effect of Nb-Mo additions on precipitation behaviour in V microalloyed TRIP-assisted steels

The microstructural evolution and precipitation behaviour of Nb-V-Mo and single V containing transformation-induced plasticity-assisted steels with an acicular/bainitic ferrite matrix were investigated by a heat treatment up to the austenite formation range. It was found that during the heating stage the acicular/bainitic ferrite matrix resisted recrystallisation, while cementite and martensite were decomposed and austenite was formed in the acicular/bainitic ferrite. Both Nb-V-Mo and V containing steels after the heat treatment showed a microstructure consisting of a polygonal ferrite matrix with small islands of pearlite. During these transformations, the microscopy observations showed that 0.04 wt% Nb and 0.08 wt% Mo additions to the 0.16 wt% V microalloyed steel considerably reduced the growth-coarsening of microalloy precipitates.

Microstructural Evolution of Nb–V–Mo and V Containing TRIP-assisted Steels during Thermomechanical Processing

The microstructural evolution and precipitation behaviour of Nb–V–Mo and single V containing transformation induced plasticity assisted steels were investigated during thermomechanical processing. A plane strain compression testing machine was used to simulate the thermomechanical processing. Microstructures were characterised by optical microscopy, scanning-transmission electron microscopy and microanalysis, and X-ray diffraction analysis, and Vickers hardness was obtained from the deformed specimens. The resulting microstructure of both Nb–V–Mo and V steels at room temperature primarily consisted of an acicular/bainitic ferrite, retained austenite and martensite surrounded by allotriomorphic ferrite. The TEM analysis showed that a significant number of Nb(V,Mo)(C,N) precipitates were formed in the microstructure down to the finishing stage in Nb–V–Mo steel (i.e. 830 °C). It was also found that the V(C,N) precipitation primarily occurred in both ferrite and deformed austenite below the finishing stage. The results suggested that Nb–Mo additions considerably increased the temperature stability of microalloy precipitates and controlled the microstructural evolution of austenite. However, the microalloy precipitation did not cause a significant precipitation strengthening in both Nb–V–Mo and V steels at room temperature.

Microstructural evolution during bainite transformation in a vanadium microalloyed TRIP-assisted steel

The microstructural evolution and subsequent properties of a cold rolled 0.12wt% V microalloyed transformation induced plasticity assisted steel were studied after intercritical annealing, focusing on isothermal bainite transformation at 460°C. The heat treated steel was examined by optical microscopy, scanning-transmission electron microscopy and microanalysis, X-ray diffraction analysis and tensile testing. The tensile testing showed considerable improvements in yield strength and total elongation with longer isothermal bainite holding times (i.e. 180s). According to microscopy observations, these effects were attributed to the increase in the population of film-type retained austenite and newly formed dislocations as a result of bainite transformation. No significant V(C,N) precipitation and growth-coarsening were observed during isothermal bainite transformation, which is consistent with conventional vanadium microalloyed steels. However, the V(C,N) precipitates in dislocation tangles suggested the possible contribution of precipitates on the tensile properties of steel.

Effect of B2O3 addition on viscosity of mould slag containing low silica content

The viscosity of SiO2–30Al2O3–B2O3–12Na2O–CaO slag system (CaO/SiO2 = 2·68–7·35, B2O3 = 0–9 wt-%) was investigated in the present study. The objective is to study the influence of B2O3 on the viscosity and structure of mould slags containing low silica content. The research of the low silica slag system is meaningful because of it is less reactive with Al during continuous casting of steels containing high amounts of dissolved aluminium, such as transformation induced plasticity assisted steels. The viscosities of these quinternary slags decreased slightly with an increase in the content of additive B2O3 above the break temperature. The apparent activation energy of viscous flow was found to decrease with increasing B2O3 content. The break temperature also decreased with increasing B2O3 content. Based on the experimental data, the decrease in break temperature per mol.-% B2O3 ΔTbr is 15·23 K. An empirical model was then established to predict the break temperature of mould slags containing B2O3, and the deviation is between −25 and +25 K. Based on the viscosity and Fourier transform infrared analysis, it is proposed that the three-coordinated boron is dominated in these slags over the entire compositional range, and the borosilicate networks are loosened with increasing B2O3 content in the present study. The Riboud model was modified empirically based on the present experimental data, and the mean deviation is ∼13%.

Spot weldability of TRIP assisted steels with high carbon and aluminium contents

We examine here the spot welding characteristics of transformation induced plasticity assisted steels, which contain δ-ferrite as a consequence of their aluminium concentrations of 3·5 or 5·6 wt-% and which also have high carbon contents of 0·3 or 0·4 wt-% when compared with conventional automotive steels. The resistance spot welds are tested in both shear and cross-tensile tests in order to determine the so called ductility ratio, which is a parameter associated with the fitness of such welds for automotive applications. With an increase in the δ-ferrite fraction from 0·19 to 0·5, the hardness variation across the weld and heat affected zone is decreased approximately from 400 to 150 HV. It seems that the presence of stable δ-ferrite is helpful in reducing hardness variations and in achieving a significant ductility ratio of 0·39.

Transformation induced plasticity assisted steels: stress or strain affected martensitic transformation?

Transformation induced plasticity (TRIP) assisted steels contain a small quantity of carbon enriched retained austenite, which transforms into martensite during the course of plastic deformation. Transformation of this kind can be induced by both stress and plastic strain. The detailed mechanism by which the martensite is induced is different for these two scenarios. An attempt is made here to discover the relative importance of these mechanisms and it is found that stress affected transformation can explain much of the variation in retained austenite content as a function of plastic strain.

New thermomechanical strategy for production of high strength low alloyed multiphase steel showing transformation induced plasticity effect

In the past few years a lot of research was carried out on the development of transformation induced plasticity (TRIP) assisted multiphase steels. Two principal ways were proposed: (i) controlled cooling during the hot rolling process to obtain hot rolled TRIP assisted multiphase steels and (ii) the combination of intercritical annealing and isothermal holding at bainite formation temperatures during continuous annealing resulting in cold rolled TRIP assisted steel products. Unfortunately, both thermomechanical methods proposed require a high silicon level to inhibit cementite precipitation to avoid a loss of stability for the metastable retained austenite. In addition, due to high silicon levels, red scale surface defects appear and only moderate hot dip galvanisability is possible. In this paper a new thermomechanical strategy for the production of high strength low alloyed TRIP assisted multiphase steels with good hot dip galvanisability and without red scale defects will be presented.