Showing papers on "TRIP steel published in 2010"

Influence of Al on the Microstructural Evolution and Mechanical Behavior of Low-Carbon, Manganese Transformation-Induced-Plasticity Steel

Abstract: Microstructural design with an Al addition is suggested for low-carbon, manganese transformation-induced-plasticity (Mn TRIP) steel for application in the continuous-annealing process. With an Al content of 1 mass pct, the competition between the recrystallization of the cold-rolled microstructure and the austenite formation cannot be avoided during intercritical annealing, and the recrystallization of the deformed matrix does not proceed effectively. The addition of 3 mass pct Al, however, allows nearly complete recrystallization of the deformed microstructure by providing a dual-phase cold-rolled structure consisting of ferrite and martensite and by suppressing excessive austenite formation at a higher annealing temperature. An optimized annealing condition results in the room-temperature stability of the intercritical austenite in Mn TRIP steel containing 3 mass pct Al, permitting persistent transformation to martensite during tensile deformation. The alloy presents an excellent strength-ductility balance combining a tensile strength of approximately 1 GPa with a total elongation over 25 pct, which is comparable to that of Mn TRIP steel subjected to batch-type annealing.

Microstructure and mechanical properties of laser-welded joints of TWIP and TRIP steels

Abstract: With the aim of investigating a laser-welded dissimilar joint of TWIP and TRIP steel sheets, the microstructure was characterized by means of OM, SEM, and EBSD to differentiate the fusion zone, heat-affected zone, and the base material. OIM was used to differentiate between ferritic, bainitic, and martensitic structures. Compositions were measured by means of optical emission spectrometry and EDX to evaluate the effect of manganese segregation. Microhardness measurements and tensile tests were performed to evaluate the mechanical properties of the joint. Residual stresses and XRD phase quantification were used to characterize the weld. Grain coarsening and martensitic areas were found in the fusion zone, and they had significant effects on the mechanical properties of the weld. The heat-affected zone of the TRIP steel and the corresponding base material showed considerable differences in the microstructure and properties.

Deformation behaviour of spot-welded high strength steels for automotive applications

Abstract: Numerical simulation of component and assembly behaviour under different loading conditions is a main tool for safety design in automobile body shell mass production. Knowledge of local material behaviour is fundamental to such simulation tests. As a contribution to the verification of simulation results, the local deformation properties of spot-welded similar and dissimilar material joints in shear tension tests were investigated in this study for a TRIP steel (HCT690T) and a micro-alloyed steel (HX340LAD). For this reason, the local strain distribution was calculated by the digital image correlation technique (DIC). On the basis of the hardness values and microstructure of the spot welds, the differences in local strain between the selected material combinations are discussed. Additionally, the retained austenite content in the TRIP steel was analysed to explain the local strain values. Results obtained in this study regarding similar material welds suggest significant lower local strain values of the TRIP steel HCT690T compared to HX340LAD. One reason could be the decrease of retained austenite in the welded area. Furthermore, it has been ascertained that the local strain in dissimilar material welds decreases for each component compared with the corresponding similar material weld.

Effect of Continuous Galvanizing Heat Treatments on the Microstructure and Mechanical Properties of High Al-Low Si Transformation Induced Plasticity Steels

Abstract: Heat treatments were performed using an isothermal bainitic transformation (IBT) temperature compatible with continuous hot-dip galvanizing on two high Al–low Si transformation induced plasticity (TRIP)-assisted steels. Both steels had 0.2 wt pct C and 1.5 wt pct Mn; one had 1.5 wt pct Al and the other had 1 wt pct Al and 0.5 wt pct Si. Two different intercritical annealing (IA) temperatures were used, resulting in intercritical microstructures of 50 pct ferrite (α)-50 pct austenite (γ) and 65 pct α-35 pct γ. Using the IBT temperature of 465 °C, five IBT times were tested: 4, 30, 60, 90, and 120 seconds. Increasing the IBT time resulted in a decrease in the ultimate tensile strength (UTS) and an increase in the uniform elongation, yield strength, and yield point elongation. The uniform elongation was higher when using the 50 pct α-50 pct γ IA temperature when compared to the 65 pct α-35 pct γ IA temperature. The best combinations of strength and ductility and their corresponding heat treatments were as follows: a tensile strength of 895 MPa and uniform elongation of 0.26 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 90-second IBT time; a tensile strength of 880 MPa and uniform elongation of 0.27 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 120-second IBT time; and a tensile strength of 1009 MPa and uniform elongation of 0.22 for the 1 pct Al-0.5 pct Si TRIP steel at the 50 pct γ IA temperature and 120-second IBT time.

Non-equilibrium solidification and ferrite in δ-TRIP steel

Abstract: Microscopy and microanalysis experiments on two cast alloys, designed on the basis of equilibrium to contain substantial amounts of δ-ferrite, reveal zero or much reduced fractions of this phase in the solidified condition. It appears that the solid state transformation of δ-ferrite into austenite occurs without the required partitioning of solutes and that this is responsible for the development of non-equilibrium microstructures. This conclusion is supported by microanalytical data and through calculations of limiting phase diagrams based on paraequilibrium rather than equilibrium. Kinetic simulations confirm that this interpretation is consistent with the majority of austenite growing in the solid state without the partitioning of the substitutional solutes.

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

Abstract: 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.

Novel TRIP-Steel/Mg-PSZ Composite-Open Cell Foam Structures for Energy Absorption

Abstract: Porous materials have received extensive attention for energy absorption in the last years. In terms of this study austenitic TRIP-steel/Mg-PSZ composite–open cell foam structures are formed based on replicas using open-celled polyurethane foam as a skeleton with and without a supporting dense face (jacket) coating. Their compression strength as well as their specific energy absorption SEA has been registered as a function of the compressive strain. The zirconia addition has reinforced the composite material with the face coating up to a compressive strain of 50%. The stress-induced martensitic transformation of partially stabilized zirconia phases has been investigated as a function of the compressive strain by EBSD. The zirconia phase transformation is triggered already at low compressive strains below 2%.

High-strength cold-rolled plate with favorable forming property and preparation method thereof

Abstract: The invention provides a high-strength cold-rolled plate with favorable forming property and a preparation method thereof. The high-strength cold-rolled plate comprises the following chemical components: 0.05-0.6% of C, being more than or equal to 0.3% and being less than 0.8% of Si, 0.5-4.0% of Mn, being more than ore equal to 0.2% of P, being more than or equal to 0.002% of S, being more than or equal to 0.1% of Al, being more than or equal to 0.01% of N, and the balance of Fe and inevitable impurities. The preparation method comprises the following steps: smelting, continuous-casting, hot-rolling, cold-rolling and continuous-annealing, wherein annealing temperature is higher than that of Ac3, holding time is 3-30 minutes; quickly cooling to 100-600 DEG C at the rate of 30-100 DEG C/s, keeping the temperature for 3-30 minutes, and finally cooling to room temperature at the rate of 5-30 DEG C/s. The invention adopts low-silicon low-aluminum component design and heat treatment technique; the strength level of the produced bainite TRIP steel is greater than 780 MPa; and the produced bainite TRIP steel has favorable strength, plasticity, extensibility, chambering property and coating property.

Statistical Reliability of Phase Fraction Determination Based on Electron Backscatter Diffraction (EBSD) Investigations on the Example of an Al-TRIP Steel

Abstract: The aim of this article is to discuss the representativeness of electron backscatter diffraction (EBSD) mapping data for phase fraction determination in multiphase materials. Particular attention is paid to the effect of step size and scanned area. The experimental investigations were carried out on a low-alloyed steel with transformation induced plasticity (TRIP) that shows a relatively heterogeneous distribution of residual austenite in a ferrite matrix. EBSD scans of various area sizes and step sizes were carried out and analyzed with respect to the determined austenite phase fraction. The step size has only an indirect influence on the results, as it determines the size of the investigated area if the number of measurement points is kept constant. Based on the experimental results, the optimum sampling conditions in terms of analyzed area size and the number of measurement points were determined. These values were compared with values obtained from Cochran’s formula, which allows calculation of sampling sizes for predefined levels of precision and confidence. A significant deviation of experimental from theoretical optimum sample sizes was found. This deviation is, for the most part, a result of the heterogeneous distribution of the austenite phase. Depending on grain size and volume fraction of the second phase, the false assignment of phases at grain boundaries also may introduce a significant error. A general formula is introduced that allows estimation of the error caused by these parameters. Finally, a new measurement scheme is proposed that allows improvement of reliability and representativeness of EBSD-based phase determination without large sacrifices in measurement time or data set sizes.

Screening of the Interactions Between Mg‐PSZ and TRIP‐Steel and Its Alloys During Sintering

Abstract: Ceramic-steel compound materials are used in a wide range of applications up to date. Major advantages are the mechanical properties due to the combination of brittle ceramic with tough steel. This study deals with effects of the sintering process on austenitic TRIP-steel/Mg-PSZ composite materials for mechanical load applications. Both, the Fe-Cr-Ni-steel and partially stabilized zirconia offer their special mechanical behavior only in a metastable state. The ability of phase transformation depends mainly on the chemical composition. Mutual interactions of the alloying metals (Cr, Ni, Mn, and Fe) and the ceramic stabilizer (MgO) during sintering may prevent the martensitic phase transformation. This may cause disadvantageous mechanical behavior on mechanical load in use.

Ultra Fine-grained 6wt% Manganese TRIP Steel

Abstract: Ultra-fine grained TRIP steels (UFG-TRIP) containing 6wt%Mn were produced by intercritical annealing. An ultra-fine grained microstructure with a grain size less than 1μm was obtained. The formation mechanism of the high volume fraction of retained austenite was investigated by dilatometry, XRD and magnetic saturation. The fraction of retained austenite was strongly dependent on the annealing temperature. The tensile properties were also found to be strongly influenced by the annealing temperature with poorer mechanical properties being observed at higher annealing temperatures. It was found that the stabilization of the retained austenite was both a composition and size-effect, made possible by the grain refinement due to the reversely transformed martensite.

The Role of New Ferrite on Retained Austenite Stabilization in Al-TRIP Steels

Abstract: Microstructure and mechanical properties of two high Al, low-Si TRIP steels with different Cr and Mo contents were studied using continuous galvanizing line (CGL) laboratory simulation. Combined use of specific etching methods, optical and electron microscopy observations and EBSD characterization led to verify the epitaxial growth of ferrite during cooling at a moderate rate from the intercritical annealing to the isothermal holding temperature. The amounts of “new” ferrite formed during cooling and retained austenite obtained after processing are much higher in the steel with lower content of hardenability-promoting elements. Measured tensile properties and mechanical behavior of the steel strongly depend on the amount of new ferrite and retained austenite. It is found that the formation of new epitaxial ferrite from intercritical austenite can effectively contribute to the chemical and particle size stabilization of untransformed austenite as well as to obtain the desired TRIP effect under processing conditions highly compatible with industrial practice, i.e. cooling rates near 15°C/s and isothermal holding times at 460°C shorter than 60 s.

Spot weldability of δ-TRIP steel containing 0·4 wt-%C

Abstract: Strong steels are usually difficult to resistance spot weld because of the tendency to form hard phases. This applies particularly to the transformation induced plasticity (TRIP) assisted steels with relatively high carbon equivalents. A new development in this context is the δ-TRIP steel, designed to retain δ-ferrite as a stable phase at all temperatures below melting. Fully martensitic regions are therefore avoided, making it possible to weld in spite of the high carbon concentration. The authors present here the first spot welding tests on the novel alloy system.

The Influence of Strain Rate and Strain Intensity on Retained Austenite Content in Structure of Steel with TRIP Effect

Abstract: TRIP (transformation induced plasticity) steels are low and medium-carbon steels containing soft ferritic groundmass responsible for low yield point and phases of hard particles such as martensite and/or bainite, which ensure high values of tensile strength. The most important content in structure of TRIP steel is occupied by a non-transformation retained austenite. The advantageous properties of these steels are obtained as a result of martensite transformation generated by plastic deformation process. The retained austenite induces increase of steel plasticity till the moment when by the impact of plastic deformation will undergo deformation in martensite, which results in the increase of steel mechanical properties. The speed of transformation of retained austenite in martensite is highly dependent on strain magnitude and strain rate magnitude. The paper presents the research of strain rate and intensity on retained austenite content ensuring TRIP effect in structure of low carbon steel (0.29%C). Finite element analysis for different strain rate and strain degree values in upsetting test was performed by means of software FORGE 3D. The practical analysis obtained from simulation results was realized by using metallurgical processes simulator Gleeble 3800.

Quantitative Analysis of Microstructural Constituents in Welded Transformation-Induced-Plasticity Steels

Abstract: A quantitative analysis of retained austenite and nonmetallic inclusions in gas tungsten arc (GTA)–welded aluminum-containing transformation-induced-plasticity (TRIP) steels is presented. The amount of retained austenite in the heat-affected and fusion zones of welded aluminum-containing TRIP steel with different base metal austenite fractions has been measured by magnetic saturation measurements, to study the effect of weld thermal cycles on the stabilization of austenite. It is found that for base metals containing 3 to 14 pct of austenite, 4 to 13 pct of austenite is found in the heat-affected zones and 6 to 10 pct in the fusion zones. The decomposition kinetics of retained austenite in the base metal and welded samples was also studied by thermomagnetic measurements. The decomposition kinetics of the austenite in the fusion zone is found to be slower compared to that in the base metal. Thermomagnetic measurements indicated the formation of ferromagnetic e carbides above 290 °C and paramagnetic η(e′) transient iron carbides at approximately 400 °C due to the decomposition of austenite during heating.

Yielding Behavior of Nb Micro-alloyed C–Mn–Si TRIP Steel Studied by In-situ Synchrotron X-ray Diffraction

Abstract: The stress and strain partitioning between the different micro-structural constituents, and the initiation of the martensitic transformation during the yielding of multi-phase transformation-induced plasticity steel were studied by in-situ synchrotron X-ray diffraction experiments under tensile tests. Position, intensity and width of retained austenite and ferrite diffraction peaks were used to determine lattice strain and phase fractions. At low tensile stress, small elastic lattice deformations were observed. Whereas the Poisson strains in the ferrite were found to be reduced at the macroscopic yield stress, the strain in the austenite increased. The results clearly show that at the elasto–plastic transition the transformation of some of the retained austenite is stress induced.

Cellular Energy Absorbing TRIP-Steel/Mg-PSZ Composite: Honeycomb Structures Fabricated by a New Extrusion Powder Technology

Abstract: Lightweight linear cellular composite materials on basis of austenite stainless TRIP- (TRansformation Induced Plasticity-) steel as matrix with reinforcements of MgO partially stabilized zirconia (Mg-PSZ) are described. Two-dimensional cellular materials for structural applications are conventionally produced by sheet expansion or corrugation processes. The presented composites are fabricated by a modified ceramic extrusion powder technology. Characterization of the microstructure in as-received and deformed conditions was carried out by optical and scanning electron microscopy. Magnetic balance measurements and electron backscatter diffraction (EBSD) were used to identify the deformation-induced martensite evolution in the cell wall material. The honeycomb composite samples exhibit an increased strain hardening up to a certain engineering compressive strain and an extraordinary high specific energy absorption per unit mass and unit volume, respectively. Based on improved property-to-weight ratio such linear cellular structures will be of interest as crash absorbers or stiffened core materials for aerospace, railway, or automotive applications.

Galvannealing of (High‐)Manganese‐Alloyed TRIP‐ and X‐IP®‐Steel

Abstract: In this study the influence of Mn on galvannealed coatings of 1.7% Mn-1.5% Al TRIP- and 23% Mn X-IP®-steels was investigated. It is shown that the external selective oxides like Mn, Al and Si of the TRIP steel which occur after annealing at 800 °C for 60 s at a dew point (DP) of -25 °C (5% H2) hamper the Fe/Zn-reaction during subsequent galvannealing. Preoxidation was beneficially utilized to increase the surface-reactivity of the TRIP steel under the same dew point conditions. The influence of Mn on the steel alloy was investigated by using a 23% Mn containing X-IP®-steel which was bright annealed at 1100 °C for 60 s at DP -50 °C (5% H2) to obtain a mainly oxide free surface prior to hot dip galvanizing (hdg) and subsequent galvannealing. As well known from the literature Mn alloyed to the liquid zinc melt stabilizes δ-phase at lower temperatures by participating in the Fe-Zn-phase reactions, it was expected that the metallic Mn of the X-IP®-steel increases the Fe/Zn-reactivity in the same manner. The approximation of the effective diffusion coefficient (Deff(Fe)) during galvannealing was found to be higher than compared to a low alloyed steel reference. Contrary to the expectation no increased Fe/Zn-reaction was found by microscopic investigations. Residual η- and ζ-phase fractions prove a hampered Fe/Zn-reaction. As explanation for the observed hampered Fe/Zn-reaction the lower Fe-content of the high-Mn-alloyed X-IP®-steel was suggested as the dominating factor for galvannealing.

Advanced Bainitic and Martensitic Steels with Carbide-Free Microstructures Containing Retained Austenite

Abstract: Recent decades have witnessed some remarkable advances in engineering steels driven by the need to respond to challenges posed, for example, by recovery and transmission of oil and gas, or enhanced vehicle safety and fuel economy. Foremost amongst these must surely be the extended application of carbon steels, achieved principally through ferrite grain refinement by the practice of microalloying coupled with controlled thermomechanical processing. Limitations to strengthening ferrite/pearlite structures further by grain refinement or precipitation, however, has focused attention back to acicular forms of microstructure. One of the most interesting advances in this area has been the development of bainitic steels, which have been almost dormant since the mid-20th century. This resurgence may partly be attributed to a better appreciation of the bainite transformation mechanism, and the experimental work for this which unexpectedly spawned some interesting bainitic microstructures which have seen further development and application. These are the so-called ‘carbide-free’ bainites, which employ alloying to replace carbides, principally cementite, with carbon-stabilized retained austenite. Particularly noteworthy has been the emergence of the transformation induced plasticity (TRIP) sheet steels with enhanced properties principally targeted for automotive use. It is worth mentioning also that a parallel development has produced similar microstructure in austempered ductile irons (ADI), another important ferrous alloy which has seen recent expanding interest in its application. Even more recently, as we proceed into the 21st century, the concept of employing steel microstructures containing carbon-enriched retained austenite, has been developed further by combining both alloying and novel heat treatment procedures to exchange ‘bainitic’ ferrite with ‘martensitic’ ferrite. Interestingly, this non-equilibrium ‘quenching and partitioning’ process route also offers the possibility to increase the retained austenite carbon concentration to very high levels, potentially revealing new and previously unobtainable properties.

General Method of Phase Transformation Modeling in Advanced High Strength Steels

Abstract: In the present paper, a new modeling approach is proposed for the austenite to ferrite and bainite transformation kinetics in transformation induced plasticity (TRIP) and complex phase (CP) steels. Based on experimental data obtained by dilatometry during continuous cooling, Rios' method has been successfully applied assuming additivity to calculate the parameters for the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, i.e. exponent n and the rate parameter k. Limitations of the Rios' method have been identified when k is a function of both temperature T and fraction transformed X. For these cases that are in particular relevant for the bainite transformation, a new modeling method has been developed to investigate the exact relationship of k with T and X. The new method has been used to describe the transformation kinetics in a TRIP and a CP steel. Good agreement has been obtained between the calculated and measured transformation data. The proposed new modeling method provides a general modeling approach that promises to be useful in predicting the complex phase transformation kinetics during industrial processing of advanced high strength steels (AHSS).

Control of Structures and Properties of Cold-Rolled Sheet Steels

Abstract: Precise microstructural control is essential to obtain the required properties of sheet steels. Since excellent formability is required for mild steels, the control of grain structure and texture are of utmost importance. First, the roles of initial grain boundaries prior to cold rolling, shear bands, and the interaction between C and Mn during recovery are critically reviewed as the essential factors controlling recrystallization texture. A model for the grain size of hot bands in interstitial free (IF) steels is presented in conjunction with texture control. Moreover, the grain growth during annealing, which leads to the development of ND//〈111〉texture, is discussed from the viewpoint of the pinning effect due to fine precipitates. The texture memory effect, typically found in Mn-IF steels, is also discussed. Concerning high strength steel sheets, the history of the development is firstly introduced. The discussion is focused on transformation induced plasticity (TRIP) steel by placing an emphasis on th...

Effect of Thermomechanical Control Processing on Microstructure and Mechanical Properties of Fe-0.2C-1.44Si-1.32Mn Hot Rolled TRIP Steel

Abstract: The effect of thermomechanical control processing (TMCP) on microstructure and mechanical properties of Fe-0. 2C-1. 44Si-1. 32Mn hot rolled TRIP steel was investigated through experiments. Strain-induced transformation and transformation-induced plasticity behavior of retained austenite were analyzed. The results show that with multipass deformation, reduction per pass of more than critical deformation in austenite recrystallization region and total reduction of more than 58% in non-recrystallization region and high temperature section of two-phase region, austenite can be refined before γ→α transformation. It is beneficial to obtain refined ferrite grain in final microstructure. To obtain fine and uniform microstructure and excellent strength-ductility balance, a three-stage cooling process (laminar cooling-air cooling-ultra-fast cooling) after hot rolling was conducted. The ultimate tensile strength and elongation of the investigated steel can reach 663 MPa and 41%, respectively.

Effect of Baking Process on Microstructures and Mechanical Properties of Low Silicon TRIP Steel Sheet With Niobium

Abstract: After 2% predeformation, the baking treatment with different schedule was carried out for low silicon TRIP steel sheet with niobium. The effects of baking temperature and time on microstructures and mechanical properties were investigated. The results showed that with increasing the baking temperature and time, the volume fraction of retained austenite decreases, and the volume fraction of tempered martensite increases; as baking temperature ranges from 80°C to 170°C, the bake-hardening (BH) value increases obviously, while from 170°C to 230°C, the variation of BH value is very slight; as baking time ranges from 2 min to 20 min, the BH value increases significantly, while the BH value decreases when baking time exceeds 20 min. So that when the baking temperature is 170°C and the baking time is 20 min, the low silicon TRIP steel sheet exhibits good bake-hardening behavior, and the highest BH value is above 70 MPa.

Influence of Al on Microstructure and Mechanical Behavior of Cr-Containing Transformation-Induced Plasticity Steel

Abstract: Chromium in transformation-induced plasticity (TRIP) steel is known to have a detrimental effect on the mechanical properties by increasing the hardenability of austenite introduced during intercritical heat treatment. In this study, it is suggested that an Al addition can counterbalance the effect of Cr by encouraging ferrite formation during fast cooling and austempering. This contributes to securing the thermal stability of austenite and to retrieving the excellent mechanical properties of TRIP steel even with the addition of Cr.