Showing papers on "TRIP steel published in 2019"

Liquid Metal Embrittlement of Resistance Spot Welded 1180 TRIP Steel: Effect of Electrode Force on Cracking Behavior

Abstract: Liquid metal embrittlement (LME) caused cracking of Galvanized transformation induced plasticity steels was investigated during resistance spot welding (RSW). Effect of electrode force (3–5 kN) for a weld time of 400, and 800 ms on crack resistance of TRIP steel was examined in relation to LME phenomenon. The microstructural characteristics of spot weld joints and LME cracking tendency were investigated using dye penetration tests, optical microscopy, scanning electron microscopy together with energy dispersive X-ray spectroscopy. It was found that as the electrode force increases, the crack size decreases due to a fast increase in crack tip temperature which rises further with more holding time up to 800 ms in high temperature zone. Least amount of crack size was observed at 5 kN electrode force and 400 ms of welding time. Finally, the experimental results have also been simulated by finite element modeling (FEM) to find suitable mechanism of crack formation, and a combination of 4 kN and 400 ms was suggested for the crack free and less thermal deformation in the spot welded TRIP steel.

A comparison of cross-tension properties and fracture behavior between similar and dissimilar resistance spot-weldments in medium-Mn TRIP steel

Abstract: The current study characterizes the degradation of cross-tension properties in medium-Mn transformation-induced plasticity (TRIP) steel weldments by comparison of fracture behaviors in both similar and dissimilar weldments with DP980 dual phase steel. The experimental results reveal that the cross-tension strengths of medium-Mn TRIP steel weldments is inferior to those of conventional TRIP steel having lower carbon equivalent. Although the failure modes of similar and dissimilar weldments were different from each other, the brittleness of the martensite was a common key factor governing crack propagation. In similar weldment, the decreased tensile toughness in the coarse grain heat-affected zone due to the insufficient self-tempering of martensite and the decreased retained austenite fraction. Meanwhile, understanding the changes in the carbon equivalent caused by dilution in the dissimilar welds and segregation in the fusion zone is necessary to characterize medium-Mn TRIP steel spot welds.

Effect of intermediate temperature annealing on the stability of retained austenite and mechanical properties of medium Mn-TRIP steel

Abstract: The medium Mn-TRIP steels usually exhibited excellent strength and elongation at 25 °C. However, the early plastic instability occurred when the deforming temperature increased to 100 °C due to the lack of work hardening in ultrafine grains and deformation induced martensitic transformation. In this study, an intermediate temperature annealing heat treatment was suggested to inhibit the plastic instability at 100 °C without markedly decreasing the mechanical properties. The results showed that two scenarios happened during the annealing process: i) austenite decomposed into M23C6 and ferrite, ii) C-Mn clusters formed in the austenite. It is revealed that such microstructural evolutions can increase the hardening effect by pinning the dislocations, decreasing the stability of austenite and promoting the TRIP effect during the tensile test.

Remarkable improvement in resistance spot weldability of medium-Mn TRIP steel by paint-baking heat treatment

Abstract: In this study, a remarkable improvement in resistance spot weldability of medium-Mn transformation-induced plasticity (MT) steel through a paint-baking heat treatment was addressed using dissimilar resistance spot weldments between MT and dual phase (DP) steels with various soaking temperatures. Experimental results showed that the cross-tension strength in the dissimilar MT/DP weldment increased up to 128% when the soaking temperature was more than 110 °C, and was at its highest at 210 °C. The improved cross-tension strength is attributed to the failure mode change from a pull-out to a partial pull-out failure. The brittle-to-ductile transition of martensitic matrix in coarse-grained heat-affected zone in MT steel was also considered to be a factor. Austenite reversion and carbide precipitation occurred in the martensitic matrix during the paint-baking heat treatment, whereas there was no significant change in alloying element segregation at the prior austenitic grain boundary. The driving force for the tempering was discussed based on the calculated carbon diffusion distance.

Influence of the Quenching and Partitioning Process on the Transformation Kinetics and Hardness in a Lean Medium Manganese TRIP Steel

Abstract: The quenching and partitioning (Q&P) process of lean medium Mn steels is a novel approach for producing ultra-high strength and good formable steels. First, the steel is fully austenitized, followed by quenching to a specific quenching temperature (TQ) in order to adjust an appropriate amount of initial martensite (α’initial). Subsequently, the steel is reheated to a partitioning temperature (TP) in order to ensure C-partitioning from α’initial to remaining austenite (γremain) and thus retained austenite (RA) stabilization. After isothermal holding, the steel is quenched to room temperature (RT), in order to achieve a martensitic-austenitic microstructure, where the meta-stable RA undergoes the strain-induced martensitic transformation by the so-called transformation induced plasticity (TRIP) effect. This paper systematically investigates the influence of the Q&P process on the isothermal bainitic transformation (IBT) kinetics in a 0.2C-4.5Mn-1.3Al lean medium Mn steel by means of dilatometry. Therefore, the Q&P annealing approach was precisely compared to the TRIP-aided bainitic ferrite (TBF) process, where the samples were directly quenched to the temperature of the IBT after full austenitization. The results indicated an accelerated IBT for the Q&P samples, caused by the formation of α’initial during quenching below the martensite start (MS) temperature. Furthermore, a significant influence of the annealing parameters, such as TQ and TP, was observed with regard to the transformation behavior. For further characterization, light optical microscopy (LOM) and scanning electron microscopy (SEM) were applied, showing a microstructure consisting of a martensitic-bainitic matrix with finely distributed RA islands. Saturation magnetization method (SMM) was used to determine the amount of RA, which was primarily depending on TQ. Furthermore, the hardness according to Vickers revealed a remarkable impact of the annealing parameters, such as TQ and TP, on the predicted mechanical properties.

Suppression of liquid metal embrittlement in resistance spot welding of TRIP steel

Abstract: Third-generation advanced high strength steels are typically given a zinc coating that provides excellent resistance to corrosion. During the resistance spot welding process, the melted zinc coating enables liquid metal embrittlement (LME) that causes cracking in the weld indent. In this study, LME in TRIP 1100 and TRIP 1200 steels was suppressed by placing aluminium interlayers added between the electrode and steel contact surface. Compared to welds exhibiting LME, TRIP 1100 with aluminium interlayers showed complete strength recovery while TRIP 1200 with aluminium interlayers resulted in a recovery of strength by 90%. Aluminium interlayers suppress LME by the formation of iron aluminides that hinder liquid zinc from coming in contact with the steel substrate, thus preventing LME.

Mechanical properties and microstructural characterization of simulated heat-affected zones in 10 wt pct Ni steel

Abstract: The effects of welding thermal cycles on the mechanical properties of a high strength, high ballistic resistance 10 wt pct Ni steel were studied. Welding simulations were performed using a Gleeble 3500 thermal-mechanical simulator to replicate microstructures observed in the heat-affected zone of this steel, which is a transformation induced plasticity or TRIP steel. The microstructural influences on austenite content, strength, and toughness were determined using a variety of characterization techniques including XRD, SEM, EBSD, and STEM/EDS. The results demonstrate that with increasing peak temperature of the welding thermal cycle, the amount of austenite present in the microstructure decreases. However, the toughness results do not directly correlate with the austenite contents. Poor toughness is observed in the intercritical heat-affected zone as a result of brittle high carbon martensite in this region. This region represents the greatest challenge in terms of maintaining high ballistic resistance of welds of 10 wt pct Ni steel. These results are significant in that the toughness of this steel is not solely based on the austenite content, which would be expected given it is a TRIP steel, but instead is a function of other microstructural influences.

Formation of Surface Depression during Continuous Casting of High-Al TRIP Steel

Abstract: High aluminum transformation-induced plasticity (TRIP) steels offer a unique combination of high tensile strength and ductility, high impact energy absorption and good formability. The surface of the slab is prone to depressions and longitudinal cracks during continuous casting due to the high Al content in steels. Surface depressions of the 1.35 wt.% Al-TRIP steel slab in a steel works were investigated by scanning electronic microscopy (SEM) and mold fluxes with different Al2O3/SiO2 ratios were researched by thermodynamic calculations and high-temperature static balance experiments. The results show that some micro-cracks were distributed along the grain boundary in the surface depression of the slab. Inclusions containing K and Na, which were probably from mold flux, were found in the depression samples. Meanwhile, the components of reactive mold flux showed significant variation in their chemical composition during the continuous casting process of the Al-TRIP steel. A large number of depressions and irregular oscillation marks on the Al-TRIP steel slab surface were generated due to serious deterioration in the physical properties of the mold flux. Since the TRIP steel is a typical hypo-peritectic steel, the overly large thermal contraction and volume contraction during initial solidification is the intrinsic cause of surface depression. The change of mold flux properties during casting aggravates the formation of depressions.

In Situ Observation of Bainite Transformation and Simultaneous Carbon Enrichment in Austenite in Low-Alloyed TRIP Steel Using Time-of-Flight Neutron Diffraction Techniques

Abstract: An in situ neutron diffraction experiment during austempering of low-alloyed transformation-induced plasticity steel, Fe-1.48Si-1.52Mn-0.15C, in wt pct was conducted. In this study, time-of-flight neutron diffractometer with a large detector coverage, iMATERIA at J-PARC MLF, was employed. The phase fraction and carbon concentration in austenite could be quantitatively determined with a time resolution 1 minute although considerable textures existed for both ferrite and austenite. The carbon concentration in austenite during austempering was found to be inhomogeneous, resulting in a bimodal concentration distribution. The low-carbon region was consumed by bainite transformation whereas the high-carbon austenite slightly increased and even survived the final cooling to room temperature, forming a retained austenite. The rate of bainite transformation was affected by the state prior to the start of austempering. Consequently, different morphological features of the retained austenite were formed. More block-shaped austenite was observed in the case of slower bainite transformation, and it was determined that film-shaped austenite could also exist. The average carbon concentration was similar to that of high-carbon austenite during austempering. Hence, the film and block shapes of the retained austenite do not necessarily reflect the difference in carbon concentration.

Hot Deformation Behavior and Processing Maps of a Medium Manganese TRIP Steel

Abstract: The hot deformation behavior of a medium-Mn steel was studied in terms of hot compression flow curves in the temperature range of 850–1050 °C and strain rates of 0.05–10 s−1. The thermo-mechanical analysis was carried out and suggested that the microstructure during deformation was completely austenite which had high tendency for dynamic recrystallization (DRX). The flow behavior was characterized by significant flow softening at deformation temperatures of 950–1050 °C and lower strain rates of 0.05–5 s−1, which was attributed to heating during deformation, DRX and flow instability. A step-by-step calculating procedure for constitutive equations is proposed. The verification of the modified equations indicated that the developed constitutive models could accurately describe the flow softening behavior of studied steel. Additionally, according to the processing maps and microstructure analysis, it suggested that hot working of medium-Mn steel should be carried out at 1050 °C, and the strain rate of 0.05–10 s−1 resulted in significantly recrystallized microstructures in the in steel. The flow localization is mainly flow instability mechanism for experimental steel.

Constitutive Modeling of Asymmetric Hardening Behavior of Transformation-Induced Plasticity Steels

Abstract: Transformation-induced plasticity (TRIP) steels are part of advanced high strength steels capable of phase transformation, having good strength and ductility. The transformation rate is known to be dependent on the stress state, which may lead to asymmetric hardening behaviour for TRIP steels with compressive flow stresses larger than tensile ones. Sheet stamping products of TRIP steels show complex springback because of the asymmetry in addition to the large strength, which will complicate the analysis of sheet metal forming processes. In this work, the asymmetric hardening behaviour of a TRIP steel with a tensile strength of 1180 MPa was measured using the sheet tension-compression tester. An asymmetric hardening model was developed by introducing an off-centred bounding surface for the kinematic back-stress evolution, to depict the asymmetric hardening behaviour. The model parameters of the proposed constitutive equations were obtained from the stressstrain curves under tension followed by compression. The stress-strain curves were well captured by the developed constitutive model, whereas the conventional symmetric model fails to describe the asymmetric hardening behaviour of the TRIP steel. For validation, load-displacement curve and springback angles of three-point bending test were compared with the predictions by the proposed model.

Effect of Microstructure on Tensile Behavior and Mechanical Stability of Retained Austenite in a Cold-Rolled Al-Containing TRIP Steel

Abstract: In the present study, a quenching treatment prior to two-stage heat treatment was conducted on a Fe–0.28C–1.55Mn–2.06Al transformation-induced plasticity steel to tailor the final microstructure. Compared with the microstructure of the ferrite, bainite and blocky retained austenite obtained by conventional two-stage heat treatment, the microstructure subjected to quenching plus two-stage heat treatment was composed of the ferrite, lath bainite and film-like retained austenite. The corresponding tensile behavior and mechanical stability of retained austenite were investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The results show that the mechanical stability of blocky retained austenite grains is lower and most of them transform to martensite during the tensile deformation, which leads to higher ultimate tensile strength and instantaneous work hardening exponent. Film-like retained austenite has relatively higher stability, which could cause sustained work hardening and high ductility as well as product of strength and elongation.

Use of multi-phase trip steel for press-hardening technology

Abstract: Development of high strength or even ultra-high strength steels is mainly driven by the automotive industry which strives to reduce the weight of individual parts, fuel consumption, and CO2 emissions. Another important factor is to improve the passenger safety. In order to achieve the required mechanical properties, it is necessary to use suitable heat treatment in addition to an appropriate alloying strategy. The main problem of these types of treatments is the isothermal holding step. For TRIP steels, the holding temperature lies in the field of bainitic transformation. New possibilities are sought to integrate these methods directly into the production process. One way to produce high-strength sheet is the press-hardening technology. Physical simulation based on data from a real-world press-hardening process was tested on CMnSi TRIP steel. Mixed martensitic-bainitic structures with ferrite and retained austenite (RA) were obtained, having tensile strengths in excess of 1000 MPa.

Influence of high-energy ball milling on Mg-PSZ-reinforced TRIP steel-matrix composites synthesized by FAST / SPS

Abstract: A composite with a high-alloy steel matrix was reinforced with varying amounts of Mg-PSZ and synthesized using Spark Plasma Sintering (SPS) technology. To prepare the powders for sintering, they were mixed in a planetary ball mill at two different rates of 100 rpm and 250 rpm. The influence of the rotation speed on the microstructures and compressive strengths of the sintered composites was investigated. Due to the high degree of deformation of the steel powder particles during milling at 250 rpm, the steel phase underwent grain refinement during SPS. Furthermore, the Mg-PSZ was more homogeneously distributed within the steel matrix and mechanically interlocked with the steel matrix. Thus, the compressive strength of the composite increased with increasing Mg-PSZ content and increasing rotation speed during milling of the powder due to grain refinement and the improved strength of the steel/ceramic interface. Furthermore, a larger amount of Mg-PSZ underwent stress-induced transformation under compressive loading.

Fracture toughness of high-alloy austenitic-martensitic TRIP steels after Q&P processing

Abstract: The recently developed austenitic-martensitic TRIP cast steel Fe–14Cr–3Ni–3Mn–0.4Si–0.11N–0.15C was subjected to different Quenching & Partitioning (Q&P) treatments in order to achieve a variation of the microstructural and mechanical properties. Subsequently, the fracture properties of three material variants were studied by means of tensile tests and fracture mechanical 3-point-bending tests to determine J– $$\Delta a$$ fracture resistance curves. Due to Q&P treatment, the steel achieved considerable strength and ductility values (UTS of about 1500 MPa with a total elongation of almost 30%) which qualify it for the 3rd generation of AHSS. The fracture toughness behavior was significantly influenced by the initial $$\upalpha ^\prime $$ -martensite content as well as by the austenite stability, which could be adjusted by varying the Q&P parameters. If the austenite stability was low, the formation of deformation-induced $$\upalpha ^\prime $$ -martensite became possible. This TRIP effect is known to be beneficial for fracture toughness of austenitic steels. However, the experimental results suggest that there was a contrary effect of embrittlement due to metastable austenite which undergoes martensitic transformation already in the early stages of deformation. Therefore, the Q&P parameters have to be carefully chosen in order to achieve a remarkable combination of strength, ductility and fracture toughness of the investigated high-alloy austenitic-martensitic TRIP steel.

Microstructure Evolution during the Production of Dual Phase and Transformation Induced Plasticity Steels Using Modified Strip Casting Simulated in the Laboratory

Abstract: Instead of conventional steel making and continuous casting followed by hot and cold rolling, strip casting technology modified with the addition of a continuous annealing stage (namely, modified strip casting) is a promising short-route for producing ferrite-martensite dual-phase (DP) and multi-phase transformation-induced plasticity (TRIP) steels. However, at present, the multi-phase steels are not manufactured by the modified strip casting, due to insufficient knowledge about phase transformations occurring during in-line heat treatment. This study analysed the phase transformations, particularly the formation of ferrite, bainite and martensite and the retention of austenite, in one 0.17C-1.52Si-1.61Mn-0.195Cr (wt. %) steel subjected to the modified strip casting simulated in the laboratory. Through the adjustment of temperature and holding time, the characteristic microstructures for DP and TRIP steels have been obtained. The DP steel showed comparable tensile properties with industrial DP 590 and the TRIP steel had a lower strength but a higher ductility than those industrially produced TRIP steels. The strength could be further enhanced by the application of deformation and/or the addition of alloying elements. This study indicates that the modified strip casting technology is a promising new route to produce steels with multi-phase microstructures in the future.

Austenite stabilisation by two step partitioning of manganese and carbon in a Mn-TRIP steel

Abstract: In addition to manganese, carbon partitioning has been proposed in a new medium Mn-TRIP steel by two-step partitioning during the first batching annealing and the final continuous annealing...

Laminated TRIP/TWIP Steel Composites Produced by Roll Bonding

Abstract: In order to investigate the roll bonding of high-alloy transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) steel, roll-bonded sheets of the TRIP and TWIP steel were manufactured starting from hot rolling, followed by brushing and cold rolling. Both, the microstructure and mechanical properties of the roll-bonded sheets were characterized by metallographic investigations, and tensile and T-peel tests. Preliminary results, such as an occurrence of an adhesive bonding between two TWIP steel sheets and between TRIP and TWIP steel sheet after a thickness reduction of approximately 50% were obtained. Moreover, the formation of deformation-induced martensite leads to outstanding mechanical properties of the roll-bonded composite sheet. An ultra-fine grained microstructure was observed in the bonding zone after only one roll-bonding process. The obtained promising results demonstrate the possibility of the development of an accumulative roll-bonding process for TRIP/TWIP steel composites.