RELATIONSHIP BETWEEN RETAINED AUSTENITE AND STRAIN FOR Si-Mn TRIP STEEL

The equation which reflects the relationship between the retained austenite and strain has been proposed and clear TRIP can be obtained while the S value (An index of retained austenite stability) is less than 6.5 for Silicon-Manganese TRIP steel

EFFECT OF CHEMICAL COMPOSITION ON RETAINED AUSTENITE IN TRIP STEEL

The systematic chemical compositions including common C, Si, Mn, Al, and micro-alloying elements of Ti and Nb were designed for high volume fraction of retained austenite as much as possible. The thermo-cycle experiments were conducted by using Gleeble 2000 thermo-dynamic test machine for finding the appropriate composition. The experimental results showed that chemical composition had a significant effect on retained austenite, and the appropriate compositions were determined for commercial production of TRIP steels.

Effects of Si on the stability of retained austenite and temper embrittlement of ultrahigh strength steels

Effects of silicon （Si） content on the stability of retained austenite and temper embrittlement of ultrahigh strength steels were investigated using X-ray diffraction （XRD）,transmission electron microscopy （TEM）,and other experimental methods.The results show that Si can suppress temper embrittlement,improve temper resistance,and hinder the decomposition of retained austenite.Reversed austenite appears gradually with the increase of Si content during tempering.Si has a significant effect on enhancing carbon （C） partitioning and improving the stability of retained austenite.Si and C atoms are mutually exclusive in lath bainite,while they attract each other in austenite.ε-carbides are found in 1.8wt% Si steel tempered at 250℃,and they get coarsened obviously when tempered at 400℃,leading to temper embrittlement.Not ε-carbides but acicular or lath carbides lead to temper embrittlement in 0.4wt% Si steel,which can be inferred as cementites and composite compounds.Temper embrittlement is closely related to the decomposition of retained austenite and the formation of reversed austenite.

Effect of lower bainite/martensite/retained austenite triplex microstructure on the mechanical properties of a low-carbon steel with quenching and partitioning process

We present a study concerning Fe-0. 176C-1.31Si-1.58Mn-0.26Al-0.3Cr （wt%） steel subjected to a quenching and partitioning （Q＆P） process. The results of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and tensile tests demon- strate that the microstructures primarily consist of lath martensite, retained austenite, lower bainite （LB）, and a small amount of tempered martensite; moreover, few twin austenite grains were observed. In the microstrucmre, three types of retained austenite with different sizes and morphologies were observed： blocky retained austenite （-300 nm in width）, film-like retained austenite （80-120 nm in width）, and ul- tra-fine film-like retained austenite （30-40 nm in width）. Because of the effect of the retained austenite/martensite/LB triplex microstructure, the specimens prepared using different quenching temperatures exhibit high ultimate tensile strength and yield strength. Furthermore, the strength effect of LB can partially counteract the decreasing strength effect of martensite. The formation of LB substantially reduces the amount of retained austenite. Analyses of the retained austenite and the amount of blocky retained austenite indicated that the carbon content is critical to the total elongation of Q＆P steel.

EFFECTS OF CARBON CONTENT AND ROLLING PROCESSING ON RETAINED AUSTENITE FOR HOT-ROLLED TRIP STEELS

The effects of finishing rolling temperature and coiling temperature on retained austen-ite were studied for hot-rolled transformation induced plasticity (TRIP) steels with different carbon content. The experimental results showed that an appropriate volume fraction of retained austenite from 6% to 11% could be obtained according to the different carbon content less than 0.20% by controlled finishing rolling and coiling for the hot-rolled TRIP steels. It can be concluded that carbon content has a significant effect on the fraction of retained austenite and coiling processing plays stronger role on retaining austenite than fishing rolling processing.

Characteristics of Retained Austenite in TRIP Steels with Bainitic Ferrite Matrix

Heat treatment process for producing cold rolled transformation induced plasticity-aided （TRIP-aided） steels with bainitic ferrite matrix was adopted. Characteristics of retained austenite （RA） in such TRIP steels were investigated. SEM and OM determination results showed that the stable austenite retained at room temperature were mainly located between laths and some of them inside the coarse ferrite. The grains were uniformly distributed in heat treated steel matrix and the regularly dispersed RA represented to be triangular morphology. XRD analysis indicated that RA content in matrix was not less than 10%, and TEM testified that RA inside the matrix were formed at the prior austenite boundaries and represented to be single or twin crystals. The ductile fracture originated from the boundaries of martensite islands from RA and ferrite. The cracks propagated along grain boundaries and some passed through the large ferrite grains and induced transgranular fracture.

Relations of abrasion resistance and hardness of 16Cr-3C white irons with retained austenite content

The relationship between the retained austenite content of the matrix in16Cr-3C white irons and the abrasion resistance was investigated. The results show that: (1) theabrasion resistance can be improved by sub-critical heat treatment, which could be attributed to thedecrease of the retained austenite content; (2) both the abrasion resistance and hardness can beimproved by controlling the retained austenite content below 20 percent-30 percent and arrive at themaximum when the retained austenite content is reduced to about 10 percent; (3) the abrasionresistance decreases abruptly once the retained austenite content is lower than 10 percent, whichstems from both the in situ transformation of (Fe, Cr)_(23)C_6 to M_3C carbides and the formation ofpearlitic matrix.

Interaction of Hydrogen and Retained Austenite in Bainite/Martensite Dual-Phase High Strength Steel

The hydrogen trapping phenomena in two bainite/martensite dual-phase high strength steels(U20Si and U20DSi)were investigated by electrochemical permeation technique.The hydrogen diffusivity was calculated from data of permeation delay time,and the diffusion coefficient in U20 Si is far less than that in U20 DSi.Moreover,the hydrogen diffusivity decreases as the volume percent of retained austenite increases.The experiment results show that there are different hydrogen trappings and different volume percents of retained austenite in U20 Si and U20 DSi.The retained austenite is precipitated as films.The trap binding energy for the retained austenite and hydrogen is calculated to be 40.4kJ·mol-1.

RETAINED AUSTENITE IN LASER PROCESSED STEELS

This paper Presents experimental data on effect of carbon concentration and laser processing regimes on retained austenite quantity. The data on retained austenite decomposition during subsequent temperings at various temperatures as well as after holding at room temperature during 3 years are given.Correla- tion between structural broadening of the X - ray lines of retained gamma - phase and the amount of the latter has been discovered.Mechanisms responsible for the increased quantity of the retained austen- ite in carbon and low alloyed steels after laser hardening are described.

TRIP Steel Retained Austenite Transformation under Cyclic V-bending Deformation

To investigate the transformation behavior of TRIP steel retained austenite under cyclic load, cyclic V-bending deformation of low carbon Si-Mn TRIP600 was studied by experiment and finite element in this paper. The results showed that, under cyclic V-bending deformation, retained austenite in TRIP steel transformed into martensite gradually with the increasing of bending times, and for the symmetrical characteristic, upper surface and lower surface presented the same transformation tendency. From the first to the fourth V-bending deformatiort, retained austenite volume fraction decreased nearly linearly and then attained saturation step by step. Compressive stress state was helpful for martensite transformation than tension stress state with V-bending deformation, and strain magnitude was the determining factor for retaining anstenite martensitic transformation. With the increasing of bending times effective stress increased and the relationship between maximum effective stress and bending times was nearly linear. Effective stress and effective strain distribution were non-uniform, the maximum effective stress and effective strain were present in the center of the samples. The relationships between retained austenite and V-bending times, and retained austenite with effective strain were set up as Eqs.（1）-（5）. The relationship was typical quadric function, decreased linearly for the initial deformation and attained saturation finally.

EFFECTS OF RETAINED AUSTENITE ON DUPLEX MICROSTRUCTURE OF MARTENSITE AND LOWER BAINITE

The morphology and amount of the retained austenite in the duplex microstructure of martensite and tower bainite of a tow,alloy cold die steel(it is called GD steel for short) with high strength and high toughness have been investigated.The thermal and mechanical stabilities of the retained austenite were analyzed.Furthermore the effects of retained austenite on the mechanical properties of the steel were studied.The results show that the morphology and amount of retained austenite vary with silicon content in the steel.Retained austenite with high stability was responsible for the simultaneous increase in strength and toughness of the steel.The duplex microstructure of martensite and abnormal lower bainite of the steel with the maximum silicon content exhibits a relatively good strength-toughness combination.

TRANSFORMATION OF RETAINED AUSTENITE IN CARBURIZED CASE DURING FATIGUE CRACK GROWTH

The morphology of the retained austenite in the carburized case of 20CrNiMo steel and its transformation during fatigue crack propagation through the case were investigated by using X-ray and TEM analysis.In the carburized case both film and block shaped retained austenite were found.Due to the crystallographic orientation relationship at the interface,the fatigue crack is inclined to pass through the block shaped retained austenite and thereby stim- ulates its strain-induced martensitie transformation.During the process of the fatigue frac- ture,most of the retained austenite structures on the crack path are transformed into the martensite,and the untranaformed parts on the fracture surface remain less than 6%.The transformation of the retained austenite,which is restrieted mainly within the plastic zone,oc- curs only during the proeess of fracture,and is independent of the magnitudes of the external stress,stress ratio and cyclic number.The volume expansion accompanying the transforma- tion creates an additional residual displacement of about 0.44μm on fracture surfaces,which is equivalent to the magnitude of the plasticity-induced residual displacement.The phase transformation induced fatigue crack closure is believed to be an important factor affecting the fatigue crack behaviors in the high carbon laver of the carburized case.

Quantitative Mossbauer Study on Retained Austenite and Carbide in High Speed Steel

The retained austenite in high speed steel of two different heat treatment regimes was quantitatively,determined by transmission Mssbauer spectroscopy (TMS),backscattering X-ray geometry,Mossbauer spectroscopy (BXMS) and X-ray diffraction technique.The results meas- ured by these methods were in agreement with each other.It was demonstrated that the concentration of carbides in high speed steel could be measured by transmission Mossbauer spectroscopy or Mossbau- er spectroscopy in backscattering X-ray geometry accurately.

A novel design to enhance the stability of local austenite and the volume fraction of retained austenite in a low-carbon Si Mn Q-P steel

Pre-quenching prior to intercritical annealing quenching and partitioning(Q-P)process was proposed to enhance the volume fraction of retained austenite and the mechanical properties of a low-carbon Si Mn steel.The intercritical austenite exhibited a lath morphology due to the martensitic microstructure maintained prior to intercritical annealing.Consequently,the alloy element enrichment of intercritical austenite,in which the alloy element was aggregated at the austenitic boundaries and further diffused inside,improved the stability of intercritical austenite and decreased the M_(s) of it.As a result,the fraction of retained austenite in steel was increased,which improved the mechanical properties of the experimental Q-P steel.

Review on the plastic instability of medium -Mn steels for identifying the formation mechanisms of Lüders and Portevin -Le Chatelier bands

Plastic instability,including both the discontinuous yielding and stress serrations,has been frequently observed during the tensile deformation of medium-Mn steels(MMnS)and has been intensively studied in recent years.Unfortunately,research results are controversial,and no consensus has been achieved regarding the topic.Here,we first summarize all the possible factors that affect the yielding and flow stress serrations in MMnS,including the morphology and stability of austenite,the feature of the phase interface,and the deformation parameters.Then,we propose a universal mechanism to explain the conflicting experimental results.We conclude that the discontinuous yielding can be attributed to the lack of mobile dislocation before deformation and the rapid dislocation multiplication at the beginning of plastic deformation.Meanwhile,the results show that the stress serrations are formed due to the pinning and depinning between dislocations and interstitial atoms in austenite.Strain-induced martensitic transformation,influenced by the mechanical stability of austenite grain and deformation parameters,should not be the intrinsic cause of plastic instability.However,it can intensify or weaken the discontinuous yielding and the stress serrations by affecting the mobility and density of dislocations,as well as the interaction between the interstitial atoms and dislocations in austenite grains.

Role of retained austenite in advanced high-strength steel:ductility and toughness

Enhancing the ductility and toughness of advanced high-strength steels is essential for the wide range of promising applications.The retained austenite(RA)is a key phase due to the austenite-to-martensite transformation and its transformation-induced plasticity effect.It is commonly accepted that slow RA-to-martensite transformation is beneficial to ductility;therefore,the RA fraction and stability should be carefully controlled.The RA stability is related to its morphology,size,carbon content,neighboring phase and orientation.Importantly,these factors are cross-influenced.It is noteworthy that the influence of RA on ductility and fracture toughness is not consistent because of their difference in stress state.There is no clear relationship between fracture toughness and tensile properties.Thus,it is important to understand the role of RA in toughness.The toughness is enhanced during the RA-to-martensite transformation,while the fracture toughness is decreased due to the formation of fresh and brittle martensite.As a result,the findings regarding to the effect of RA on fracture toughness are conflicting.Further investigations should be conducted in order to fully understand the effects of RA on ductility and fracture toughness,which can optimize the combination of ductility and toughness in AHSSs.

High-temperature stability of retained austenite and plastic deformation mechanism of ultra-fine bainitic steel isothermally treated below Ms

The mechanical properties of the sample and the stability of retained austenite were studied by designing two kinds of ultra-fine bainitic steel with different heat treatment methods austempering above and below Ms(martensite start tem-perature),which were subjected to tensile tests at 20 and 450℃,respectively.The results show that compared to room temperature(20℃)tensile properties,the uniform elongation of the sample at high temperature(450℃)significantly decreased.Specifically,the uniform elongation of the sample austempered above Ms decreased from 8.0%to 3.5%,and the sample austempered below Ms decreased from 10.9%to 3.1%.Additionally,the tensile strength of the sample austempered above Ms significantly decreased(from 1281 to 912 MPa),and the sample austempered below Ms slightly decreased(from 1010 to 974 MPa).This was due to the high carbon content(1.60 wt.%),high mechanical stability,low thermal stability for the retained austenite of the sample austempered below Ms.Besides,the retained austenite decomposed at high temper-atures,the carbon content and transformation driving force were significantly reduced,the transformation rate increased,and the phase transformation content reduced.

Machine learning for prediction of retained austenite fraction and optimization of processing in quenched and partitioned steels

The metastable retained austenite(RA)plays a significant role in the excellent mechanical performance of quenching and partitioning(Q&P)steels,while the volume fraction of RA(V_(RA))is challengeable to directly predict due to the complicated relationships between the chemical composition and process(like quenching temperature(Qr)).A Gaussian process regression model in machine learning was developed to predict V_(RA),and the model accuracy was further improved by introducing a metallurgical parameter of martensite fraction(fo)to accurately predict V_(RA) in Q&P steels.The developed machine learning model combined with Bayesian global optimization can serve as another selection strategy for the quenching temperature,and this strategy is very effcient as it found the"optimum"Qr with the maximum V_(RA) using only seven consecutive iterations.The benchmark experiment also reveals that the developed machine learning model predicts V_(RA) more accurately than the popular constrained carbon equilibrium thermodynamic model,even better than a thermo-kinetic quenching-partitioning-tempering-local equilibrium model.

Development and microstructure characterization of BG110E high-strength expansion pipe

The BG110E high-strength expansion pipe was developed using medium manganese steel and subjected to a two-phase zone heat treatment process.Mechanical properties and microstructure analysis results have proven that the BG110E expansion pipe exhibits uniform elongation of more than 19%.Moreover,after undergoing expan-sion deformation,its strength,toughness,and plasticity are found to meet the stringent requirements of the P110 pipe.The microstructure of this high-strength expansion pipe,which has a strength of 110 ksi(1 ksi=6.895 MPa),consists of tempered martensite,ferrite,retained austenite,and granular bainite.The propotion of retained austenite reaches up to 12%,ensuring high plasticity and the occurrence of the transformation-induced plasticity effect during the deformation process.Consequently,it enhances the coordinated deformation ability between different phases,which significantly improves the internal yield pressure of the BG110E high-strength expansion pipe in turn.

Effect of heat treatment on microstructure and mechanical properties of Ti-containing low alloy martensitic wear-resistant steel

Effects of quenching temperature and cooling conditions(water cooling and 10%NaCl cooling)on microstructure and mechanical properties of a 0.2%Ti low alloy martensitic wear-resistant steel used for die casting ejector plate were investigated.The results show that lath martensite can be obtained after austenitizing in the range of 860-980℃and then water cooling.With an increase in austenitizing temperature,the precipitate content gradually decreases.The precipitates are mainly composed of TiC and Ti4C2S2,and their total content is between 1.15wt.%and 1.64wt.%.The precipitate phase concentration by water-cooling is higher than that by10%NaCl cooling due to the lower cooling rate of water cooling.As the austeniting temperature increases,the hardness and tensile strength of both water cooled and 10%NaCl cooled steels firstly increase and then decrease.The experimental steel exhibits the best comprehensive mechanical properties after being austenitized at 900℃,cooled by 10%NaCl,and then tempered at 200℃.Its hardness,ultimate tensile strength,and wear rate reach551.4 HBW,1,438.2 MPa,and 0.48×10^(-2)mg·m^(-1),respectively.