Showing papers on "TRIP steel published in 2017"

Martensite phase stress and the strengthening mechanism in TRIP steel by neutron diffraction.

Abstract: Two TRIP-aided multiphase steels with different carbon contents (0.2 and 0.4 mass%) were analyzed in situ during tensile deformation by time-of-flight neutron diffraction to clarify the deformation induced martensitic transformation behavior and its role on the strengthening mechanism. The difference in the carbon content affected mainly the difference in the phase fractions before deformation, where the higher carbon content increased the phase fraction of retained austenite (γ). However, the changes in the relative fraction of martensitic transformation with respect to the applied strain were found to be similar in both steels since the carbon concentrations in γ were similar regardless of different carbon contents. The phase stress of martensite was found much larger than that of γ or bainitic ferrite since the martensite was generated at the beginning of plastic deformation. Stress contributions to the flow stress were evaluated by multiplying the phase stresses and their phase fractions. The stress contribution from martensite was observed increasing during plastic deformation while that from bainitic ferrite hardly changing and that from γ decreasing.

Deformation behavior in cold-rolled medium-manganese TRIP steel and effect of pre-strain on the Lüders bands

Abstract: Deformation behavior was studied in cold-rolled 0.2C-1.6Al-6.1Mn-Fe transformation-induced plasticity (TRIP) steel subjected to intercritical annealing. The steel intercritically hardened at 650 ℃ exhibited excellent mechanical properties, and the excellent ductility was primarily associated with the discontinuous TRIP effect. Moreover at 650 ℃, the formation of Luders bands was associated with TRIP effect and cooperative dislocation glide. The length of Luders strain was gradually reduced with increasing pre-strain, and was eventually eliminated when the pre-strain was increased to 10%. The increased average stability of retained austenite and increased dislocation density in ferrite induced by pre-strain was responsible for decrease and ultimate elimination of Luders bands. While in steel intercritically annealed at 600 ℃, ferrite and austenite was predominantly deformed, which was responsible for poor work hardening rate and inferior tensile properties.

Tensile behaviors and deformation mechanism of a medium Mn-TRIP steel at different temperatures

Abstract: We investigated the effect of deformation temperature and gain size on the tensile behaviors of a new medium Mn-TRIP steel processed by batch and continuous annealing. It was found that the tensile strength decreased when the testing temperature increased from −20 to 300 °C. The total elongation firstly increased, reaching the maximum at 25–100 °C, and then decreased when the temperature was higher than 100 °C. The results can be explained by (i) the influence of temperature on chemical driving force for martensitic transformation and (ii) the grain size effect on the stability of austenite. To achieve optimal mechanical properties, the stability of austenite should be tailored so that the transformation-induced plasticity effect occurs under continuous deformation. Also, ferrite should have appropriate grain sizes so that work hardening of ferrite can coordinate the deformation of austenite.

Effect of two-step intercritical annealing on microstructure and mechanical properties of hot-rolled medium manganese TRIP steel containing δ-ferrite

Abstract: The microstructure-properties relationship, work-hardening behavior and retained austenite stability have been systematically investigated in a hot-rolled medium manganese transformation-induced-plasticity (TRIP) steel containing δ-ferrite subjected to one-step and two-step intercritical annealing. The steel exhibited tensile strength of 752 MPa and total elongation of 52.7% for one-step intercritical annealing at 740 °C, tensile strength of 954 MPa and total elongation of 39.2% in the case of intercritical quenching at 800 °C and annealing at 740 °C. The austenite obtained by two-step annealing mostly consists of refined lath structures and increased fraction of block-type particles existing at various kinds of sites, which is highly distinguished from those characterized by long lath morphology and small amounts of granular shape in one-step annealed samples. In spite of a higher C and Mn content in austenite and finer austenite laths, two-step annealing can lead to an active and continuous TRIP effect provided by a mixed blocky and lath-type austenitic structure with lower stability, contributing to a higher UTS. In contrast, one-step annealing gave rise to a less active but sustained TRIP effect given by the dominant lath-like austenite having higher stability, leading to a very high elongation. The further precipitation of vanadium carbides and the presence of both dislocation substructure and fine equiaxed grain in ferrite matrix facilitate the increase of yield strength after double annealing.

Correlation between deformation behavior and austenite characteristics in a Mn-Al type TRIP steel

Abstract: We investigate here the correlation between deformation behavior and retained austenite characteristics in a medium-Mn transformation-induced plasticity (TRIP) steel. The sample was characterized by a dual-phase microstructure consisting of ultra-fine grained ferrite and retained austenite with relatively high mechanical stability after annealing at 700 °C for 5 h. Both lath-like and blocky (granular) retained austenite with volume fraction of 38.7% and relatively inhomogeneous grain size was obtained. The tensile specimen exhibited outstanding mechanical properties with yield strength of 745 MPa, tensile strength of 1005 MPa and total elongation of 46%, as well as a distinctive work hardening behavior. The in-depth investigation on deformation behavior demonstrated that the transformation mechanism of retained austenite during deformation was strain-induced and the yielding behavior was controlled mainly by the deformation of soft ferrite phase. As to the multi-peak work hardening behavior, it is believed to be attributed to the inhomogeneous and discontinuous occurrence of TRIP effect, which resulted from the inhomogeneous stability of retained austenite. Moreover, the orientation of retained austenite (Schmid factors) was proved an important factor in determining the mechanical stability of retained austenite upon deformation, in addition to the heterogeneity of grain size. These two factors together resulted in the inhomogeneous stability of retained austenite.

Selective laser melting of ultra-high-strength TRIP steel: processing, microstructure, and properties

Abstract: This paper presents results about the influence of the selective laser melting (SLM) process parameters on a FeCr4Mo1V1W8C1 (wt%) alloy regarding microstructure and mechanical behavior. Tailored parameter variation studies were performed to obtain crack-free and highly dense SLM parts. The microstructure was studied using scanning electron microscopy, X-ray diffraction, Auger electron spectroscopy, and scanning transmission electron microscopy. Additionally, the mechanical properties were investigated by compression and tensile tests. The obtained microstructure is composed of complex nanoscale carbides, retained austenite, and martensite. Caused by the fast directional cooling during SLM, a completely dendritic solidification aligned in building direction occurs. Non-equilibrium segregation leads to an orderly phase arrangement of complex carbides at the boundary of the dendrites surrounded by retained austenite and martensite in the center of the dendrites. A strong work hardening behavior was observed, based on an austenite-to-martensite phase transformation (TRIP effect). This effect accounts for the outstanding mechanical properties such as compression strength of 6000 MPa, a 0.2% tensile yield strength of 560 MPa, and an ultimate tensile strength of over 1000 MPa. These findings reveal that SLM is advantageous for the processing of ultra-high-strength FeCrMoVWC tool steel.

The Influence of Laser Welding on the Mechanical Properties of Dual Phase and Trip Steels

Abstract: Nowadays, a wide range of materials is used for car body structures in order to improve both the passengers’ safety and fuel consumption. These are joined by laser welding and solid state fiber lasers being used more and more in present. The article is focused on the research of laser welding influence on the mechanical and deformation properties, microstructure and microhardness of advanced high-strength steels: high-strength low-alloyed steel HC340LA, dual phase steel HCT600X and multi-phase residual austenite steel RAK40/70. The proper welding parameters have been found based on weld quality evaluation. The specimens for tensile test with longitudinal laser weld were used to measure mechanical and deformation properties. Microstructure and microhardness of laser welds were evaluated in the base metal, heat affected zone and fusion zone. The higher values of strength and lower ones for deformation properties of laser-welded materials have been found for dual and multi-phase steel. The microhardness strongly depends on the carbon equivalent of steel. Deformation properties are more sensitive than strength properties to the change of microstructure in the fusion zone and heat affected zone.

Microstructural evolution and mechanical properties of duplex TRIP steel produced by strip casting

Abstract: In the present study, a duplex TRIP steel (Fe-0.232C-2.0Si-2.39Mn-2.01Al wt%) was successfully produced by a novel processing route involving twin-roll strip casting (TRSC), heat treatment, cold rolling and annealing for the first time. The evolution of microstructure during the entire processing route was extensively studied that included determination of mechanical properties and fractography of tensile specimens of annealed sheets. It was observed that the as-cast strip microstructure consisted of ferrite and martensite. The morphology at the surface and the center was dendritic and equiaxed, respectively. On using an appropriate heat treatment schedule (heat treatment at 780 °C for 10 min followed by direct quenching), a duplex structure of ferrite and retained austenite (RA) in the heat-treated strip was obtained that was suitable for cold rolling. Annealing temperature was very critical for microstructure and mechanical properties of the annealed sheets. Annealing at 780 °C for 5 min followed by direct quenching yielded a duplex structure of ferrite and RA (volume fraction of ~22%), which exhibited an excellent combination of tensile strength and total elongation of 945 MPa and 28.56%, respectively.

MIG-welding of dissimilar advanced high strength steel sheets

Abstract: The need for steel materials with increasing strength is constantly growing. The main application of such advanced high strength steels (AHSS) is the automobile industry, therefore the welding process of different types of AHSSs in dissimilar welding joint was investigated. To simulate the mass production of thin steel sheet constructions (such as car bodies) automated metal inert gas (MIG) welding process was used to weld the TWIP (twinning induced plasticity) and TRIP (transformation induced plasticity) steel sheets together. The welding parameters were successfully optimized for butt welded joints. The joints were investigated by visual examination, tensile testing, quantitative metallography and hardness measurements. The TRIP steel side of the joints showed increased microhardness up to (450-500 HV0.1) through increased fraction of bainite and martensite. Macroscopically the tensile specimen showed ductile behaviour, they broke in the austenitic weld material.

Microstructure and mechanical properties of hot-rolled and heat-treated TRIP steel with direct quenching process

Abstract: In the present study, a hot-rolled and heat-treated TRIP steel (Fe-0.25C-1.23Si-2.09Mn-2.92Al wt%) was successfully produced by a simple process that involved intercritical heat treatment (IHT) followed by direct quenching without bainite reaction step. The microstructure of the as-cast ingot, as-hot-rolled sheet and heat-treated sheet was studied and related to mechanical properties of the heat-treated sheet. It was observed that δ-ferrite was retained from the solidification stage and remained as a stable phase during different stages of processing because of the alloy design that included ~ 3 wt% Al. Banded structure was obtained in both as-hot-rolled sheet and heat-treated sheet. The microstructure of 770–820 °C heat-treated sheet consisted of δ-ferrite, retained austenite (RA), α-ferrite and martensite. Based on the alloy design and direct quenching process, ~ 20–30% volume fraction of RA was obtained in the heat-treated sheet. The 780 °C heat-treated sheet exhibited an excellent combination of tensile strength and total elongation of 880 MPa and 28%, respectively.

The Effect of Two-Step Heat Treatment Parameters on Microstructure and Mechanical Properties of 42SiMn Steel

Abstract: Medium-carbon steel 42SiMn (0.4C-0.6Mn-2Si-0.03Nb) was used for a two-step heat treatment consisting of a soaking hold and an annealing hold at bainite transformation temperature. Various heating temperatures, cooling rates, and bainitic hold temperatures were applied to the steel to obtain microstructures typical for TRIP (Transformation Induced Plasticity) steels. TRIP steels utilize the positive effects of a multiphase microstructure with retained austenite, creating a good combination of strength and total elongation and an excellent deep-drawing ability. Typical microstructures consist of ferrite, bainite, and 10–15% of retained austenite. In this work, tensile strengths in the region of 887–1063 MPa were achieved with total elongation A5mm of 26–47%, and the final microstructures contained 4–16% of retained austenite. The most suitable microstructure and the best combination of high strength and total elongation were achieved for the processing with intercritical heating temperature of 850 °C and cooling at 30 °C/s to the bainitic hold of 400 °C. Very fine pearlite persisted in the microstructures, even after applying a cooling rate of 50 °C/s, however these small areas with extremely fine laths did not prevent the retention of up to 16% of retained austenite, and high total elongation A5mm above 40% was still reached for these microstructures.

Numerical and experimental study on the effects of welding environment and input heat on properties of FSSWed TRIP steel

Abstract: In this paper, the effect of input heat and welding environment on microstructure and mechanical properties of friction stir spot welded TRIP steel sheets were investigated. Six types of joints produced in both air and water environments and under rotational speeds of 900, 1350, and 1800 rpm. Then, the microstructure and mechanical properties of them were studied. The thermal histories, strain, and strain rate distributions of cases obtained by finite element modeling. According to the temperature and strain distribution and microstructural observations, four different zones determined in welding region: stir zone, thermomechanically affected zone, and high and low temperature heat-affected zones. It is obtained at in-air welds by increasing the rotational speed, the strength of joints increase to a maximum value because of higher strain rate and more recrystallization of prior austenite grains. The strength then decreases due to high amount of heat input and growth of recrystallized grains. Thermal history of underwater welds showed lower peak temperature and rapid cooling rate. Also, by increasing rotational speed in underwater joints, the strength and hardness increased because of microstructure refinement. The fracture surfaces of joints showed a dimple pattern ductile fracture in all cases except 1800 rpm in-air joint that the fracture was less ductile which agrees with lower tensile elongation of it.

Changing oxide layer structures with respect to the dew point prior to hot-dip galvanizing of δ-TRIP steel

Abstract: Low-carbon, low-alloy δ-TRIP steel is a research topic of interest because it meets the lightweight requirements of automotive steel. To bolster its use as automotive steel, the present study focused on the galvanizing process of δ-TRIP steel. Samples were submitted to simulated annealing prior to hot-dip galvanization. The surface morphology of five groups of samples marked with different dew points were detected using scanning electron microscopy (SEM). The element distribution in the oxide layers of four groups of samples were detected using glow discharge optical emission spectroscopy (GDOES), and oxide layer structures of two representative groups of samples were analyzed using transmission electron microscopy (TEM) techniques. The results showed that the dew point affected the oxide layer structure of δ-TRIP steel. Increasing the dew point led to an extraordinary oxide layer of pure overflowing Fe platelets in the δ-TRIP steel. The present study is expected to prompt further developments in galvanized δ-TRIP steels.

Observation of deformation twinning and martensitic transformation during nanoindentation of a transformation-induced plasticity steel.

Abstract: For the first time, deformation twinning and martensitic transformation were observed in retained austenite in a low-alloyed transformation-induced plasticity steel using nanoindentation in conjunction with electron backscattering diffraction and transmission electron microscopy. Dislocation glide, martensite formation and deformation twinning were correlated to pop-ins and deviation from linearity in the load-displacement curve. Deformation twinning was found to enhance the stability of retained austenite. This observation furthers our understanding of RA stability during straining of low-alloyed multiphase TRIP steel.

Towards ultra-high ductility TRIP-assisted multiphase steels controlled by strain gradient plasticity effects

Abstract: TRansformation Induced Plasticity (TRIP) is a very effective mechanism to increase the strain hardening capacity of multiphase steels containing a fraction of metastable austenite, leading to both high strength and large uniform elongation. Excellent performances have been reached in the past 20 years, with recent renewed interest through the development of the 3rd generation of high strength steels often involving a TRIP effect. The microstructure and composition optimization is complex due to the interplay of coupled effects on the transformation kinetics and work hardening such as phase stability, size of retained austenite grains, temperature and loading path. In particular, recent studies have shown that the TRIP effect can only be quantitatively captured for realistic microstructures if strain gradient plasticity effects are taken into account, although direct experimental validation of this claim is missing. Here, an original computational averaging scheme is developed for predicting the elastoplastic response of TRIP aided multiphase steels based on a strain gradient plasticity model. The microstructure is represented by an aggregate of many elementary unit cells involving each a fraction of retained austenite with a specified stability. The model parameters, involving the transformation kinetics, are identified based on experimental tensile tests performed at different temperatures. The model is further assessed towards original experiments, involving temperature changes during deformation. A classical size independent plasticity model is shown unable to capture the TRIP effect on the mechanical response. Conversely, the strain gradient formulation properly predicts substantial variations of the strain hardening with deformation and temperature, hence of the uniform elongation in good agreement with the experiments. A parametric study is performed to get more insight on the effect of the material length scale as well as to determine optimum transformation kinetics to reach the highest possible strength-ductility balance. It is shown that the uniform elongation can potentially be increased by 50% or more, paving the way towards future microstructure engineering efforts.

Extraordinary strength and ductility obtained in transformation-induced plasticity steel by slightly modifying its chemical composition

Abstract: Extraordinary mechanical properties were obtained in a low-carbon transformation-induced plasticity (TRIP) steel by modifying its chemical composition and employing thermo-mechanical processing. It was found that Cu addition to the conventional CMnSiAl TRIP steel could improve the thermal stability of high temperature austenite by decreasing the A c3 temperature and retarding the bainitic transformation, which led to the formation of retained austenite with higher volume fraction but lower carbon content. The combined effects of high strain hardening due to the TRIP effect, uniform strain distribution, higher volume fraction of thermal martensite, and nano-scale precipitation of e-Cu particles in the Cu-TRIP steel led to a high strength of 1 GPa and total elongation of approximately 50%. Moreover, a detailed scanning electron microscopy analysis showed that the copper had a noticeable effect on the morphology of the martensite phase created within the microstructure.

Behind the Development of Advanced High Strength Steel (AHSS) Including Stainless Steel for Automotive and Structural Applications - An Overview

Abstract: Safety is paramount importance along with enhancing fuel efficiency of the transport car over the last three decades. Advanced high strength steels play a pivotal role towards achieving the desired structural characteristics of the motor vehicles. Many structural components have been replaced by advanced high strength steels like IF steel, Bake hardening steel, HSLA steel, Micro alloyed steel, Dual Phase steel, Ferrite Bainite steel, Martensitic steel, Hot formed steel, TRIP steel, TWIP steel etc. along with austenitic and ferrite grade stainless steels due to its superior strength and ductility. In the current context it has been attempted to see the causes behind the development of those mentioned steels from conventional to third generation as well the strengthening mechanisms employed towards the development of advanced high strength structural steels. It has been observed from literature study that substantial development have been progressed from metallurgical point of view in this matter over the last decade.

Acoustic emission analysis of the stages of deformation of TRIP steel

Abstract: The kinetics of damage accumulation and deformation martensite formation in cold-rolled austenitic–martensitic VNS9-Sh TRIP steel during static tension at room temperature is studied at various stages of plastic deformation and fracture by acoustic emission and X-ray diffraction. The threshold stresses that correspond to the beginning of dislocation motion and intense dislocation generation predominantly in surface layers are determined. Deformation martensite is shown to form after a yield plateau and its formation is most intensely at the stage of strain hardening.