Showing papers on "TRIP steel published in 2003"

Supra-Ductile and High-Strength Manganese-TRIP/TWIP Steels for High Energy Absorption Purposes

Abstract: The microstructural properties of advanced high strength and supra-ductile TRIP and TWIP steels with high-manganese concentrations (15 to 25 mass%) and additions of aluminum and silicon (2 to 4mass%) were investigated as a function of temperature (−196 to 400°C) and strain rate (10−4≤e≤103 s−1). Multiple martensitic γfcc (austenie)→ehcpMs (hcp-martensite)→αbccMs (bcc-martensite)-transformations occurred in the TRIP steel when deformed at higher strain rates and ambient temperatures. This mechanism leads to a pronounced strain hardening and high tensile strength (>1 000 MPa) with improved elongations to failure of >50%. The austenitic TWIP steel reveals extensive twin formation when deformed below 150°C at low and high strain rates. Under these conditions extremely high tensile ductility (>80%) and energy absorption is achieved and no brittle fracture transition temperature occurs. The governing microstructural parameter is the stacking fault energy Γfcc of the fcc austenite and the phase stability determined by the Gibbs free energy ΔGγ→e. These factors are strongly influenced by the manganese content and additions of aluminum and silicon.The stacking fault energy Γfcc and the Gibbs free energy G were calculated using the regular solution model. The results show that aluminum increases Γfcc and suppresses the γfcc→ehcpMs transformation, whereas silicon sustains the γfcc→ehcpMs transformation and decreases the stacking fault energy. At the critical value of Γfcc≈25 mJ/mol and for ΔGγ→e>0, the twinning mechanism is favored. At lower stacking fault energy of (Γfcc 0, martensitic phase transformation will be the governing deformation mechanism.The excellent ductility and the enhanced impact properties enable complex deep drawing or stretch forming operations of sheets and the fabrication of crash absorbing frame structures.

A physically based model for TRIP-aided carbon steels behaviour

Abstract: A physically based model for TRIP carbon steels is developed suitable to predict the macroscopic behaviour of multi-constituent aggregates. It includes the effects of phase composition and morphology on flow stress and strain hardening. In a first part, a detailed description of the stress-assisted and strain-induced martensitic transformation kinetics is given based on a generalised form of the Olson–Cohen model. The appearance of the much harder martensitic phase during plastic straining gives rise to a strong hardening of the retained austenite islands. The matrix behaviour is described using a model previously developed for ferritic–martensitic steels. A quite simple but accurate homogenisation approach is used to determine the TRIP steel behaviour. The predicted evolution of strain-induced martensite volume fraction, flow stress and incremental work hardening is in good agreement with experimental data and illustrates the critical importance of the retained austenite stability on the formability of TRIP steels.

Effect of deformation schedule on the microstructure and mechanical properties of a thermomechanically processed C-Mn-Si transformation-induced plasticity steel

Abstract: Thermomechanical processing simulations were performed using a hot-torsion machine, in order to develop a comprehensive understanding of the effect of severe deformation in the recrystallized and nonrecrystallized austenite regions on the microstructural evolution and mechanical properties of the 0.2 wt pct C-1.55 wt pct Mn-1.5 wt pct Si transformation-induced plasticity (TRIP) steel. The deformation schedule affected all constituents (polygonal ferrite, bainite in different morphologies, retained austenite, and martensite) of the multiphased TRIP steel microstructure. The complex relationships between the volume fraction of the retained austenite, the morphology and distribution of all phases present in the microstructure, and the mechanical properties of TRIP steel were revealed. The bainite morphology had a more pronounced effect on the mechanical behavior than the refinement of the microstructure. The improvement of the mechanical properties of TRIP steel was achieved by variation of the volume fraction of the retained austenite rather than the overall refinement of the microstructure.

In situ observations on the mechanical stability of austenite in TRIP-steel

Abstract: In-situ tensile deformation tests have been performed on a high Al TRIP steel (composition 0.26 wt. % Si, 1.5 wt. % Mn, and 1.8 wt. % Al) displaying the transformation-induced plasticity (TRIP) effect, while monitoring the phase transformation by means of X-ray microdiffraction in transmission geometry. Due to the small beam size (25 μm x 25 μm) every retained austenite grain appears as a discrete spot on the diffraction patterns. The diffraction patterns are treated like a powder pattern for different η-angles, with η representing the angle between the tensile direction and the normal direction of the diffracting {200}planes. The disappearance of austenite {200} reflections is analyzed during as a function of the imposed stress and orientation. Grains with η = 0 or 90° tend to transform to martensite more easily. A unique feature of this microdiffraction experiment is the possibility of detecting the average carbon concentration of the retained austenite as a function of stress. Direct proof has been obtained that austenite with a lower carbon content x c transforms at lower stress levels. The average X c increases from 1.0 to to 1.05 wt.%. This increase indicates a relatively narrow distribution of the carbon content.

Effect of Intercritical Deformation on Bainite Formation in Al-containing TRIP Steel

Abstract: The Influence of intercritical deformation, cooling rate and prior austenite grain size on bainite formation were investigated by dilatometry tests. Intercritical deformation (0‐40 %) performed in steels with a prior austenite grain size of 15‐28 mm leads to formation of more ferrite during the cooling and less bainite during the subsequent isothermal stage, and even almost no bainite is formed after 40 % strain. Fast cooling after deformation can suppress ferrite transformation. Relaxation following deformation can significantly, but not completely, reduce such effect of deformation due to the occurrence of recovery and recrystallization, particularly for the finer prior austenite grain size. When the prior austenite grain size was changed from 26.8 to 16.8 mm, bainite formation was suppressed. The mechanism for influence of deformation on bainite formation was discussed on the basis that deformation could refine the austenitic microstructure. Further, it is suggested that there is a critical size of austenite grains or subunits after deformation for the formation of bainite.

A steel composition for the production of cold rolled multiphase steel products

Abstract: The present invention is related to a steel composition intended to be used in a process comprising a cold rolling step, for the production of uncoated, electro-galvanised or hot dip galvanised TRIP steel products, said composition being characterised by a specific addition of phosphorus. The latter is added in order to reach the desired mechanical properties (high tensile strength in combination with high elongation) while keeping a good weldability by sufficiently reducing the carbon content. The invention is further related to a process for producing a steel product, and to said steel product obtained, said product having the composition of the invention.

Influence of silicon, aluminium, phosphorus and boron on hot ductility of TRansformation Induced Plasticity assisted steels

Abstract: The hot ductility of steels having high aluminium or phosphorus contents, which are currently being considered as possible replacements for the conventional high silicon TRansformation Induced Plasticity (TRIP) steel, has been examined. Tensile specimens were cast in situ and tested in the temperature range 750 - 1000 ° C at a strain rate of 3 × 10-3 s-1. The ductility trough for the conventional high silicon TRIP steel was controlled by the austenite - ferrite transformation, intergranular failure occurring when a thin band of the softer ferrite phase formed around the austenite grains. Void formation at the sulphides situated in the soft ferrite at the boundaries then occurred, and the strain concentrated locally there. The thin bands of ferrite were deformation induced and, as such, formed at temperatures above Ar 3 and could form at as high a temperature as Ae 3. Adding ferrite formers such as silicon, phosphorus and aluminium increased the Ae 3 temperature and thus widened the trough. The hig...

Microstructure and cleanliness of rapidly solidified steels

Abstract: In the present work the relations between microstructure, cleanliness and solidification parameters of the low alloyed and high alloyed steels in the case of rapid solidification have been investigated. Rapid solidification was investigated experimentally under laboratory conditions by immersion of a cold copper rod into a steel bath. Samples from the frozen steel shell were investigated under an optical microscope. As a structure-relevant characteristic, the secondary dendrite arm spacing (SDAS) and the nonmetallic inclusion diameter have been obtained as a function of the distance from the contact area and the solidification conditions.

Fundamental Study of Phase Transformations in Si-Al TRIP Steels

Abstract: TRIP steels are under development for automotive applications that require high strength and simultaneous high formability. This study was undertaken to examine the phase transformations occurring during the processing of a cold-rolled TRIP steel. The TRIP microstructure is comprised of ferrite, bainite and retained austenite. This microstructure is obtained by controlled cooling from the intercritical annealing temperature to the isothermal bainitic holding temperature. The effects of cooling rate from intercritical annealing temperature to isothermal transformation temperature, as well as isothermal transformation time, were studied via optical microscopy, SEM, TEM, XRD, magnetometry, dilatometry and mechanical testing. A CCT curve for one intercritically annealed TRIP steel was produced. Conclusions are drawn regarding the effects of processing parameters on phase balance (ferrite, bainite, austenite, martensite and pearlite) and mechanical properties. Suggestions for future work are also made.

Springback characteristics of steel sheets for warm U-draw bending

Abstract: The purpose of this study is to investigate the characteristics of springback for various process conditions of the U-draw bending operation. The process variables are the forming temperature and the tool geometry, including punch profile radius (Rp) and die profile radius (Rd). In order to control springback, the use of a warm forming method is applied. For the warm draw-bending, five steps of temperature ranges, from room temperature to 200°C, were adopted. Two kinds of steel sheets, cold rolled carbon steel (SCP1, for general purposes in the automobile industries) and TRIP (transformation-induced plasticity) steel, were adopted. TRIP steel was the newly developed high-strength steel sheet. The results indicated that elevated temperature and the geometry of tools in the two kinds of steel sheet affected the springback.

Effects of load mode on mechanical properties of ZrO2 （2Y）／TRIP steel composites

Abstract: The ZrO2 (2Y)/TRIP steel composites were prepared by vacuum hot-pressing sintering. The room temperature static tensile and dynamic yield strength were tested using the static tensile and Split Hopkinson Pressure Bar methods, respectively. The effects of load mode on the static and dynamic mechanical behaviors were studied. The results show that the static tensile strengths of the composites decrease with the increase of ZrO2 content, for the weak bonding of ZrO2/ZrO2. Under the dynamic load, the matrix TRIP steel produces the martinsitic phase transformation, which improves the dynamic strength and deformation ability of the composites. When the volume fraction of ZrO2 exceeds 20%, the strain-hardening coefficient and the dynamic deformation ability of the composites decrease.

Microstructure and Performance of 2Y-PSZ／TRIP Steel Composites

Abstract: 2Y-PSZ/TRIP steel composites have been sintered by hot-pressing method. Their microstructure and mechanical properties were investigated by means of SEM, TEM, XRD and static tension, split Hopkinson pressure bar method.The results showed that the strength and elastic modulus of 2Y-PSZ/TRIP steel composites at room temperature decreased with the increase of 2Y-PSZ content. The main reason was that the combining strength was quite weak between the grains of ZrO2. Distortion induced martensite transformation and plasticity during the dynamic loading increased the strength and distortion capability of the composites. The transformation was carried out mainly through twins formation. The shape of martensite induced by distortion was lamellate with substructures of twins. The habit plane was near {259}γ with no mid-ridge and no explosion phenomena. The interface was straight between the austenite and martensite induced by distortion. The increase of 2Y-PSZ content, on the one hand, made the composite dynamic flow stress improved. Thereby, the fracture strength was improved. On the other hand, it depressed both the distortion capability and the martensite transformation induced by distortion. This resulted in the decrease of dynamic fracture strength.

Effects of load mode on mechanical properties of ZrO_2(2Y)/TRIP steel composites

Abstract: The ZrO 2 (2Y)/TRIP steel composites were prepared by vacuum hot pressing sintering. The room temperature static tensile and dynamic yield strength were tested using the static tensile and Split Hopkinson Pressure Bar methods, respectively. The effects of load mode on the static and dynamic mechanical behaviors were studied. The results show that the static tensile strengths of the composites decrease with the increase of ZrO 2 content, for the weak bonding of ZrO 2/ZrO 2. Under the dynamic load, the matrix TRIP steel produces the martinsitic phase transformation, which improves the dynamic strength and deformation ability of the composites. When the volume fraction of ZrO 2 exceeds 20%, the strain hardening coefficient and the dynamic deformation ability of the composites decrease.

Corrosion-resistant austenitic trip-steel for cold plastic deformation and article produced from such steel

Abstract: FIELD: metallurgy, mechanical engineering. SUBSTANCE: corrosion-resistant austenitic trip-steel comprises, wt%: hydrogen 0.20-0.25; silicon 0.25-0.50; manganese 0.70-0.85; chromium 14.5-16.0; nickel 4.8-5.8; molybdenum 2.7-3.0; nitrogen 0.10-0.13; titanium 0.012-0.020; aluminum 0.05-0.05; unavoidable admixtures including sulfur about 0.015; phosphor about 0.015; oxygen about 0.003; iron the balance. Chromium, hydrogen, nickel and molybdenum content makes the following dependence: wt% Cr=30-(16-17)%C-(1.4-1.5)%Ni-(1.2-1.3)%Mo, titanium to hydrogen ratio is (0,5/0,8).10-2, and grain fineness number is at least 7 according to State Standard GOST 5639. Corrosion-resistant austentic trip-steel additionally comprises 0.03-0.10 wt% of cerium provided the ratio be kept. Amount of alpha-phase after melting is no more than 10 wt%. Steel of such composition may be also used in aviation for manufacture of steel strips, for example, aircraft parts of essential importance, in particular, torsion bars for 0.3 mm thick helicopter rotors, in manufacture of compressor valve plates with the thickness of up to 1 mm and other parts to be heavily loaded during operation under dynamic force conditions. Steel and articles produced from such steel have good corrosion resistance and cold plastic deformation capacity. EFFECT: increased strength, ductility and corrosion fatigue life of steel and parts fabricated from it. 4 cl, 3 ex

Micromechanical modelling of TRIP steels

Abstract: The combined thermodynamic-micromechanical model of Fischer [1] is applied to a low-alloyed TRIP steel with a volume fraction of 16% of retained austenite. The model is implemented in a finite element code. The mesh consists of 9 by 9 by 9 cubical elements, each representing a single grain with a random crystallographic orientation. The retained austenite grains are randomly dispersed through the entire mesh. The extent of the martensitic transformation in all austenite grains is calculated. A large spread in transformation rate is observed. The most favourably oriented grains reach a full martensitic structure, while the martensite volume fraction of less favourably oriented grains is less than 50%. When the chemical driving force is more negative, the onset of the transformation is delayed and the increase of the martensite volume fraction is slower. The calculated results are compared with experimentally obtained values. Although in general a reasonable agreement is found, Fischer's approach leads to some discrepancies with experimental observations.

Effect of Microstructure in TRIP Steel on Its Tensile Behavior at High Strain Rate

Abstract: The relationships between microstructure of 0.195C-1.6Si-1.58Mn TRIP steel and its dynamic mechanical properties at high strain rate were investigated. The effect of microstructures on dynamic properties was discussed and the comparison with its static mechanical properties was also presented. The specimens of TRIP steel via three heat treatment techniques exhibit different morphological structures, responsible for their dynamic mechanical performances. The dynamic tensile testing was performed on self-made pneumatic tensile impact tester. The results showed that the size, volume fraction, morphology and distribution of retained austenite all affect the final mechanical properties at high strain rate. Among them, the second phase (retained austenite+bainite) with net structure severely decreases the elongation of TRIP steel in spite of the fact that it enhances strength because it restrains ferrite deformation. In order to obtain the excellent combination of strength and elongation, rational matching of morphology, size and volume fraction of several phases in TRIP steel can be obtained via proper heat treatment techniques.

Finite element modelling of smart TRIP steel sensors and systems.

Abstract: Transformation Induced Plasticity (TRIP) steels undergo a phase transformation when subjected to high levels of mechanical strain. This transformation from a paramagnetic austenitic parent phase to a ferromagnetic martensitic phase is irreversible and the resultant magnetic properties may therefore be used as a measure of strain history. The transformat.ion behaviour of TRIP steels has been recognised as a potential smart characteristic and various proposals have appeared aimed at producing a structure that performs its primary structural function as well a strain sensing function simultaneously. However the strain induced nature of the transformation implies that transformation will occur in areas of high stress concentration and therefore engineered stress concentration features will be required to provide a consistent measure of the changes in the magnetic properties of the material as a function of applied load. In order to predict the performance of smart TRIP steel sensors, an analysis method capable of quantifying the effectiveness of a component in its dual role as structure and sensor is needed. The thesis addresses the development of a methodology for correlating the changing magnetic permeability of TRIP steel sensors and structures with martensitic transformation behaviour. The prediction of the deformation behaviour including transformation is implemented by considering a mechanical analysis based on the finite element method and a constitutive model incorporating strain-induced martensitic transformation kinetics. .Extensions to the model which allow for a wide range of deformation rates and temperatures are also discussed. In order to demonstrate the application of the methodology, an analysis of a simple tensile element used in strain measurement applications is presented. The analysis also includes the effect of temperature on the performance of the sensor. An analysis of a design proposal for a smart aircraft bolt is also included to investigate the effects of geometry, particularly engineered stress concentrations, and sensor placement.

Effect of the thermomechanical processing parameters on the microstructure-property relationships in C-Mn-Si TRIP steel

Abstract: In this work a wide range of roughing (deformation in the austenite recrystallised region) and finishing (deformation in the non-recrystallised region) strains and isothermal holding times were used to clarify the effect of processing parameters on the transformation kinetics and mechanical properties of 0.2C-1.55Mn-1.55Si (wt%) TRIP steel. The results have highlighted the complex relationships between multi-phase microstructure and mechanical properties of TRIP steel. The presence of the triclinic carbides, formed during isothermal holding, deteriorated the mechanical properties of steel studied.