Conventional Transformation-induced Plasticity Research Articles

Metallurgical Factors Affecting the Tensile Properties of Weld Heat-Affected Zone in Medium Mn Transformation-Induced Plasticity Steel

The role of retained austenite in the mechanical property changes in the medium Mn transformation-induced plasticity (TRIP) steel heat-affected zone (HAZ) was elucidated throug a comparison with conventional TRIP steel. The two steel grades had similar microstructures consisting of ferritic phases with a high fraction of retained austenite in the as-received state; however, the difference in properties after welding was significant. The microstructures of both steels were dramatically changed to a martensitic matrix in the coarse-grained HAZ region. The medium Mn TRIP steel HAZ can have a higher retained austenite fraction than the conventional one; hence, an excellent combination of tensile strength and elongation can be obtained. The decisive difference between the two steels was determined in terms of austenite stability and the initial grain size.

Ductile–Brittle Variation Phenomenon and a Special Transformation-Induced Plasticity Effect in NbTi-NiTi Composite

In order to study the transformation-induced plasticity (TRIP) effect in a NiTi shape-memory alloy composite, an in situ NbTi-NiTi composite was prepared by vacuum arc melting, hot forging and wire drawing. An unusual ductile–brittle variation phenomenon was observed by means of a series of tensile tests. The composite was brittle in the absence of transformation at 200 °C, and the fracture strain was about 2%. With lowering the tensile temperature, the composite became ductile when stress-induced martensitic transformation occurred, and the elongation increased to about 18% (a ninefold increase) during tensile test at room temperature. This ductile–brittle variation phenomenon indicates that the stress-induced martensitic transformation of NiTi alloy helps to improve the ductility of the composite, which is just the exhibition of TRIP effect in the NbTi-NiTi composite. Different from conventional TRIP steels, the TRIP behavior in NiTi induces only an increase in the elongation, but not any increase in fracture strength.

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

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.

The effect of an austenitizing pretreatment on the morphology and distribution of the retained austenite and mechanical properties of TRIP590 steel

Transformation-induced plasticity (TRIP) 590 steel, with a chemical composition of 0.16C–0.25Si–1.46Mn–1.5Al (wt%), was treated with an austenitizing pretreatment followed by conventional TRIP heat treatments. Retained austenite (RA) is an important phase in TRIP steel. However, the physical characteristics of nanoscale RA grains cannot be systematically characterized by conventional electron backscatter diffraction and transmission electron microscopy analyses. In contrast, transmission Kikuchi diffraction (TKD) analysis can be employed to systematically characterize the grain size, morphology, and distribution of RA grains. The TKD analysis of TRIP590 steel samples subjected to austenitizing pretreatment revealed a large proportion of lamellar RA grains with a grain size range of 80 nm to 3 μm, and a more homogeneous distribution of RA grains was obtained around the ferrite grains than samples formed by conventional heat treatments alone. In addition, the volume fraction of the RA phase and its carbon content were greater than those obtained in samples formed by conventional heat treatments alone. Moreover, the austenitizing pretreatment significantly increases the ultimate tensile strength and elongation of the TRIP steel samples, and the product of the ultimate tensile strength and elongation increased by an average of 23%. The pretreated steel samples exhibit excellent mechanical properties, where the product of the strength and elongation reached 26448 MPa%, with a yield strength of 556 MPa, tensile strength of 696 MPa, and an elongation of 38%.

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

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 Ac3 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 ε-Cu particles in the Cu-TRIP steel led to a high strength of 1GPa 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.

Medium Mn transformation-induced plasticity steels: Recent progress and challenges

We highlight the recent advance in strong transformation-induced plasticity (TRIP) steels, which have higher alloying contents compared to the conventional TRIP steels for achieving tensile strength higher than 1GPa. Given that the extraordinary strength-elongation balance is led by the characteristic mixture of coarse and submicron-sized grains of ferrite and austenite, diverse strategies of materials design to obtain unprecedented properties are reviewed. Challenges to achieve the vital engineering parameters other than tensile properties are discussed as well, which should be resolved to secure competitiveness over other emerging structural alternatives.

Microstructures and mechanical properties of TRIP steel produced by strip casting simulated in the laboratory

Conventional transformation induced plasticity (TRIP) steel (0.17C-1.52Si-1.61Mn-0.03Al, wt%) was produced via strip casting technology simulated in the laboratory. Effects of holding temperature, holding time and cooling rate on ferrite formation were studied via analysis of the continuous cooling transformation diagram obtained here. A typical microstructure for conventional TRIP steels consisting of ~ 0.55 fraction of polygonal ferrite with bainite, retained austenite and martensite was obtained. However, coarse prior austenite grain size of ~80µm led to large polygonal ferrite grain size of ~17µm, coarse second phase regions of ~21µm size, small amount of retained austenite (0.02–0.045) and the presence of Widmanstätten ferrite. Optimisation of the microstructure-property relationship was reached via a variation in the isothermal bainite transformation temperature. The highest retained austenite fraction of 0.045±0.003 with medium carbon content of 1.23±0.01wt% was obtained after holding at 400°C, resulting in the highest ultimate tensile strength of 590±35MPa and largest total elongation of 0.27±0.05. The presence of TRIP effect in the studied steel was revealed through the analysis of strain hardening exponent and modified Crussard–Jaoul model. Effect of processing parameters on retained austenite retention and stress-strain behaviour was discussed.

Multi-phase microstructure design of a novel high strength TRIP steel through experimental methodology

The multi-phase structure of a novel low-alloy transformation induced plasticity (TRIP) steel was designed through experimental analysis. The evolutions of both microstructure and mechanical properties during the two-stage heat treatment were analyzed. The phase transformations during the intercritical annealing and the isothermal bainitic transformation were investigated by means of dilatometry. It was shown that two types of C diffusion were detected during intercritical annealing and a complex microstructure was formed after heat treatment. The processing parameters were selected in such a way to obtain microstructures with systematically different volume fractions of ferrite, bainite and retained austenite. The volume fractions of ferrite and retained austenite were found to be two main factors controlling the ductility. Furthermore, a high volume fraction of C-rich retained austenite, which was stabilized at room temperature, was the origin of a TRIP effect. The resulting material demonstrates a significant improvement in the ultimate tensile strength (1077MPa) with good uniform elongation (22.5%), as compared to conventional TRIP steels.

Effect of isothermal bainitic processing on microstructures and mechanical properties of novel Mo and Nb microalloyed TRIP steel

A transformation induced plasticity (TRIP) assisted steel with a new composition of C–Mn–Si–Al–Cu–Mo–Nb was designed in order to achieve significant improvements in both strength and ductility. In this study, computational thermodynamics calculations and a dilatometric method were used to develop a predictive methodology to determine the processing parameters in order to reach the desired amount of ferrite and retained austenite (RA) fractions during a conventional two-stage heat treatment. The heat treatment at the bainitic temperature affects RA content and carbon concentration in the γ phase. The effect of isothermal temperature on the microstructures and the mechanical properties of the investigated steel were investigated, and the mechanical stability of RA was evaluated using the Ludwigson and Burger relation. Direct experimental evidence by electron probe microanalysis confirms the distribution of alloying elements in different phases, especially C. Most of the C existed in the RA and small M/A islands. Observation by TEM reveals that the multiple additions of Mo and Nb not only refine the final microstructure but also make fully fine and dispersive (Nb,Mo)C carbide precipitate in ferrite grains. The samples annealed at 830°C and isothermally treated at 440°C demonstrated a significant improvement in the ultimate tensile strength (1080 MPa) with good uniform elongation (23%), as compared to conventional TRIP steels.

Structure and Properties of High-Manganese TWIP, TRIP and TRIPLEX Steels

The aim of this paper is to determine the high-manganese austenite propensity to twinning induced by the cold working and its effect on structure and mechanical properties, and especially the strain energy per unit volume of newly-developed high-manganese Fe-Mn-(Al, Si) investigated steels. Newly-developed steels achieve profitable connection of mechanical properties. The newly-developed high-manganese steels provide an extensive potential for automotive industries through exhibiting the twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) mechanisms. TWIP steels not only show excellent strength, but also have excellent formability due to twinning, thereby leading to excellent combination of strength, ductility, and formability over conventional dual phase steels or TRIP steels. Conditions applied to high-manganese TWIP/TRIP/TRIPLEX steels sheets during deep drawing are different from those applied during tensile testing; the formability cannot be evaluated by mechanical properties obtained from the tensile test. The microstructure evolution in successive stages of deformation was determined in metallographic investigations using light, scanning and electron microscopies as well as x-ray diffractometry. Results obtained for newly-developed high-manganese austenitic steels with the properly formed structure and properties in the heat treatment- or thermo-mechanical processes indicate the possibility and purposefulness of their employment for constructional elements of vehicles, especially of the passenger cars to take advantage of the significant growth of their strain energy per unit volume.

Multi-phase microstructure design of a low-alloy TRIP-assisted steel through a combined computational and experimental methodology

The multiphase constitution of a transformation-induced plasticity (TRIP)-assisted steel with a nominal composition of Fe–1.5Mn–1.5Si–0.3C (wt.%) was designed, utilizing a combination of computational methods and experimental validation, in order to achieve significant improvements in both strength and ductility. In this study, it was hypothesized that a microstructure with maximized ferrite and retained austenite volume fractions would optimize the strain hardening and ductility of multiphase TRIP-assisted steels. Computational thermodynamics and kinetics calculations were used to develop a predictive methodology to determine the processing parameters in order to reach maximum possible ferrite and retained austenite fractions during conventional two-stage heat treatment, i.e. intercritical annealing followed by bainitic isothermal transformation. Experiments were utilized to validate and refine the design methodology. Equal channel angular pressing was employed at a high temperature (950°C) on the as-cast ingots as the initial processing step in order to form a homogenized microstructure with uniform grain/phase size. Using the predicted heat treatment parameters, a multiphase microstructure including ferrite, bainite, martensite and retained austenite was successfully obtained. The resulting material demonstrated a significant improvement in the true ultimate tensile strength (∼1300MPa) with good uniform elongation (∼23%), as compared to conventional TRIP steels. This provided a mechanical property combination that has not been exhibited before by low-alloy first-generation high-strength steels. The developed computational framework for the selection of heat treatment parameters can also be utilized for other TRIP-assisted steels and help design new microstructures for advanced high-strength steels, minimizing the need for cumbersome experimental optimization.

Role of microstructure in the low cycle fatigue of multi-phase steels

The low cycle fatigue (LCF) behaviour of several commercially-produced multiphase steels was studied; including dual-phase (DP) and transformation induced plasticity (TRIP). In addition, a novel TRIP980 hybrid microstructure was examined that consisted of coarse ferrite grains along with low temperature bainite regions interspersed with retained austenite. Fully reversed strain controlled fatigue tests were conducted on the different steels to determine the cyclic stress response and strain to failure. The effects of the cyclic deformation on the microstructures were analysed using electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). Results showed that the initial cyclic hardening behaviour and low cyclic softening ratio observed in the TRIP steels was not necessarily due to austenite to martensite transformation. Differences between the austenite transformation behaviour of the conventional and novel hybrid TRIP microstructures was related to the different surrounding phases and the size of the retained austenite.

INFLUENCE OF MARTENSITE ON MECHANICAL PROPERTIES OF HOT--DIP GALVANIZED TRIP STEEL

The hot-dip galvanized transformation induced plasticity(TRIP) steel is an advanced automobile steel with excellent mechanical proprieties,which meets the requirements of energy preservation and environment protection.The microstructure and mechanical properties of hot-dip galvanized TRIP steel are different from conventional TRIP steel treated by continuous annealing processing.Three hot-dip galvanizing processes(main difference is isothermal bainitic transformation(IBT) time) arc designed according to the characteristics of hot dip galvanized TRIP steel manufacturing process to study the influence of microstructure on mechanical properties.By means of SEM,TEM,XRD,dilatometry and tensile test,the microstructure characteristic,phase transformation and mechanical properties were investigated.Combined these experimental results and the carbon diffusion balance theory,the content of martensite and its effect on mechanical properties were analyzed.The results show that the IBT time has a significant impact on the content of martensite.The volume fractions of martensite are 7.08%,4.17%and 1.18%.when IBT times are 20,30 and 60 s,respectively.Both the transformation induced plasticity of retained austenite and the martensite hardening are effective but the latter plays a dominant role,when the content of martensite is 7.08%the strength is similar as dual-phase steel,but a better elongation.When the volume fraction of martensite decrease to 1.18%,the mechanical properties are similar to the conventional TRIP steel,as so small amount of martensite has no obvious effect on the mechanical properties.By changing the hot dip galvanizing process,the amount of martensite and retained austenite can be controlled,and hot-dip galvanized TRIP steel with different mechanical properties can be obtained.

Comparison of Final Properties between Conventional Trip and Tempered Martensite Assisted Steels (TMAS)

Recent research in the field of advanced steels has shed some light on new grades of advanced automotive steel such as Tempered Martensite Assisted Steels (TMAS). Processing and analysis of the final properties of TMAS is the scope of this current work. This paper focuses on the influence of tempered martensite volume fraction on the final properties of cold rolled and subcritically annealed Al-containing TRansformation Induced Plasticity (TRIP) steel, otherwise called TMAS. In this work, an X-ray diffraction technique was used to measure retained austenite volume fractions of all samples after TRIP annealing. The steel samples were subsequently cold rolled to obtain strain-induced martensite, deformed ferrite and bainite in the microstructure. The cold rolled TRIP steel samples were subsequently subcritically annealed at 500 °C for 1 hour to obtain TMAS containing tempered martensite, fine ferrite and bainite in the microstructure. Shear Punch testing was used to evaluate the mechanical properties. The properties of the conventional TRIP steel and TMAS are compared and the results are discussed.

Microstructures and stability of retained austenite in TRIP steels

Transformation induced plasticity (TRIP) steels exhibit a combination of high strength and ductility due to their multi-phase microstructure, including ferrite, bainite and retained austenite which transforms to martensite under the external stress. The characterization of microstructures is necessary for understanding the relationship between microstructure and property. In the present work, an effort to determine the volume fraction of various phases was made for the conventional TRIP steel containing silicon. The microstructures in the TRIP steel were characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy, especially, an effective method was developed to identify multi-phase microstructures by atomic force microscopy based on the height difference. Furthermore, the stability of retained austenite determining TRIP effect was evaluated by electrical resistance tests and tensile tests. The results show that retained austenite does not generate martensitic transformation at −80 °C and exhibits a good thermodynamic stability, and the transition temperature from stress-induced martensitic transformation to strain-induced martensitic transformation is determined as about −5 °C, and thus strain-induced martensitic transformation over −5 °C (somewhat lower than room temperature) is favorable for the application of TRIP steels in the automobile industry.

Galvanisability of silicon free CMnAl TRIP steels

Transformation induced plasticity (TRIP) steels are a promising solution for the production of cars with low body mass because of the combination of high strength and large uniform elongation that they offer. However, conventional CMnSi TRIP steels with more than 1 wt-%Si have the drawback of poor Zn coating quality after continuous galvanising. This problem is due to the presence of complex Si–Mn oxides on the strip surface. The present research work therefore focused on the full substitution of Si by Al in TRIP steels and the detailed analysis of the galvanising behaviours of these Si free CMnAl TRIP steels. If the hot dipping is done after a combination of intercritical annealing and isothermal bainitic transformation in a furnace atmosphere with a high dewpoint, the wetting of the strip by the liquid Zn is improved significantly. However, the improvement is limited and not enough to avoid bare spots and coating defects cannot be avoided on conventional CMnSi TRIP steel. In contrast, the Si free CMnAl TRIP steel has a much better wettability when annealed at a low dewpoint. The surface segregation of the elements, which have a high affinity for oxygen, i.e. Si, Al, and Mn, was studied in detail and this revealed that Si was much more readily enriched on the surface than Al during the annealing in the low dewpoint atmosphere. The difference in the surface segregation between Si and Al resulted in a clear difference in the galvanisability. The limited presence of Al on the strip surface is due to the fact that Al can be oxidised internally during hot rolling. As a result, an Al depleted surface region is formed owing to selective internal oxidation of Al before the continuous galvanising.