Kinetics of deformation processes in high-alloyed cast transformation-induced plasticity/twinning-induced plasticity steels determined by acoustic emission and scanning electron microscopy: Influence of austenite stability on deformation mechanisms

The austenite stability and the stacking fault energy of high-alloyed metastable transformation-induced plasticity/twinning-induced plasticity (TRIP/TWIP) steels, both depending on the chemical composition, have a strong influence on the deformation processes and stress/strain-induced martensitic phase transformation. Aiming at a better understanding of the kinetics of TRIP/TWIP-assisted plastic deformation, acoustic emission (AE) measurements were performed during room temperature tensile deformation of high-alloyed cast model steels with different austenite stability. The real-time AE investigations were complemented by detailed scanning electron microscopy investigations of deformed microstructures using electron backscattered diffraction to determine the martensitic phase transformation and electron channelling contrast to visualize dislocations and their arrangements. The quantitative AE analysis revealed different AE patterns at different plastic strains, which were correlated with underlying deformation mechanisms and microstructural transformations.     

A macroscopic model to simulate the mechanically induced martensitic transformation in metastable austenitic stainless steels

Mechanically induced martensitic transformation and the associated transformation plasticity phenomena in austenitic stainless steels are studied. The mechanisms responsible for the transformation are investigated and put into perspective based on experimental evidence. The stress and strain partitioning into the austenite and martensite phases are formulated using a mean-field homogenization approach. At this intermediate length-scale the average stress in the austenite phase is computed and utilized to compute the mechanical driving force resolved in the material. The amount of transformation and the transformation plasticity is derived as a function of the driving force. The mechanical response of the material is obtained by combining the homogenization and the transformation models. The model is verified by mechanical tests under biaxial loading conditions during which different transformation rates are observed. As a final verification of the model, a bending test is used which manifests the stress-state dependency of the transformation.     

低碳-硅-锰系TRIP钢的组织演变规律研究

采用彩色金相、SEM、TEM和X射线衍射技术研究了低碳-硅-锰TRJP钢在单向拉伸状态下的组织演变规律.结果表明,TRIP钢变形前的组织为F、B和残余奥氏体,经拉伸变形后部分残余奥氏体在应变作用下转变为孪晶结构的马氏体,提高了钢的强度;TRIP钢的断裂为韧性断裂,位于F晶界处的残余奥氏体发生相变从而松弛了应力,延缓了断裂的产生,使TRIP钢板获得高塑性.     

Thermodynamics-based alloy design criteria for austenite stabilization and transformation toughening in the Fe---Ni---Co system

Transformation toughening has been widely applied in metastable austenitic steels. Recently this toughening mechanism has been extended to ultrahigh strength secondary-hardening martensitic steels, bearing suitable austenitic dispersions. The resulting dispersed-phase transformation toughening depends on the stability of the austenitic dispersions. The stability of dispersed austenite depends on various factors including the chemical composition and size of austenite particles, the stress state and the yield strength of the matrix. A single-parameter characterization of the stability of the austenitic dispersion is provided by the M_s^σ temperature and a functional form relating that temperature with the above-mentioned factors is developed. The microstructural requirements for dispersed-phase transformation toughening are then derived in terms of the austenite particle size and chemical enrichment in stabilizing solutes. Compositional effects on austenite stability have been studied by performing thermodynamic calculations using the Thermo-Calc software. The free-energy change ΔG^ch = G^b.c.c. − G^f.c.c. for martensitic transformation (a measure of austenite stability) has been evaluated as a function of composition in the ternary Fe---Ni---Co system. This information, when superimposed on isothermal sections at the tempering temperatures of interest, provides a way for selecting alloy compositions that maximize the thermodynamic stability of dispersed austenite.     

Transformation potential predictions for the stress-induced austenite to martensite transformation in steel

The stress-induced transformation behavior of retained austenite is considered in this work. With the development of transformation-induced plasticity (TRIP) steels this deformation mode is of growing importance. Twinned martensite structures were calculated using the crystallographic theory of martensite. An available work criterion was used to predict the transformation potentials for 16 different in-plane stress states for sheet sample geometry. By rotating the twinned martensite structures over all crystallographic orientations using Euler angles, the magnitude of the transformation potential was plotted as an orientation distribution plot for comparison with typical texture components. From these data, the Brass and Copper orientation components that are typical in retained austenite such as in TRIP steels were found to have low transformation potential values. Grains aligned with these orientations would require higher stresses to transform than other orientations, and may therefore never transform. This correlates to experimental observations that heavily deformed TRIP steel contains residual retained austenite.     

Conversional model of transformation strain to phase fraction in low alloy steels

Computational models of austenite decomposition are widely used to predict external shape and internal microstructural changes as functions of time and temperature. One modeling approach involves converting the measured transformation strain to phase fractions using the lattice parameters of parent and product phases, because the transformation strain is caused by crystal structural change and an enrichment/depletion of the solute atoms in solid solution. A conversional model of transformation strains occurring during austenite decomposition to phase fractions was developed using an optimal set of lattice parameters and thermal expansion coefficients of each phase associated with low alloy steels. The model is composed of four different transformation strain formulae for ferrite, pearlite, bainite and martensite transformations, respectively, and can be applied to continuous cooling transformations as well as isothermal ones. The conversional model demonstrated a good match between calculated and measured phase fractions of industrial low alloy steels cooled at the different rates after austenitization.     

The Structure Stability of Carbide-Free Bainite Wheel Steel

Metallographic structures of carbide-free bainite steel wheel rim are mainly composed of supersaturated lath ferrite and retained austenite film among bainitic ferrite laths. It is suspected that supersaturated ferrite and retained austenite are likely to decompose under the influence of temperature change and mechanical stress. Stability of wheel rim structure is studied by means of x-ray diffraction, dye microscopy, and micro-hardness test. When the samples are tempered in the range of 150-350 °C, the retained austenite films are at the state of relative stability. Fifty percent of retained austenite is decomposed when the sample is tempered at 400 °C. Microhardness increases when the sample is tempered at 150 °C. The decrease in hardness is mild when the samples are tempered from 200 to 500 °C. The mechanical stability of retained austenite film is studied with tensile sample under the effect of tensile stress. The retained austenite appears to be stable in low and middle degree of deformation, and decomposition occurs at great amount of deformation. Diffraction peak of carbide is not found in all above experiments. The steel enriched silicon prevents the carbide precipitation during the transformation. It indicates the carbide-free bainite wheel steels have an excellent thermal and mechanical stability.     

Nb对中锰TRIP钢临界退火过程中组织演变的影响

采用临界退火热处理工艺,利用场发射扫描电镜(FE-SEM)观察含铌和不含铌的两种热轧中锰TRIP钢在不同退火制度下的碳化物演变行为及铌对中锰TRIP钢微观组织、残留奥氏体体积分数与稳定性的影响。结果表明:试验钢经临界退火处理后获得超细晶铁素体与残留奥氏体复相组织。随着退火温度的提高,残留奥氏体体积分数出现先升高后降低的趋势;随着退火时间的延长,碳化物逐渐溶解,残留奥氏体体积分数逐渐增加,达到平衡后保持不变。Nb元素的加入可细化奥氏体晶粒,延缓碳化物溶解,推迟奥氏体转变,增加膜状奥氏体,提高奥氏体稳定性。     

High-energy X-ray diffraction study on the temperature-dependent mechanical stability of retained austenite in low-alloyed TRIP steels

The stability of the retained austenite has been studied in situ in low-alloyed transformation-induced-plasticity (TRIP) steels using high-energy X-ray diffraction during tensile tests at variable temperatures down to 153 K. A detailed powder diffraction analysis has been performed to probe the austenite-to-martensite transformation by characterizing the evolution of the phase fraction, load partitioning and texture of the constituent phases simultaneously. Our results show that at lower temperatures the mechanically induced austenite transformation is significantly enhanced and extends over a wider deformation range, resulting in a higher elongation at fracture. Low carbon content grains transform first, leading to an initial increase in average carbon concentration of the remaining austenite. Later the carbon content saturates while the austenite still continues to transform. In the elastic regime the probed {h k l} planes develop different strains reflecting the elastic anisotropy of the constituent phases. The observed texture evolution indicates that the austenite grains oriented with the {2 0 0} plane along the loading direction are transformed preferentially as they show the highest resolved shear stress. For increasing degrees of plastic deformation the combined preferential transformation and grain rotation results in the standard deformation texture for austenite with the {1 1 1} component along the loading direction. The mechanical stability of retained austenite in TRIP steel is found to be a complex interplay between carbon concentration in the austenite, grain orientation, load partitioning and temperature.     

Modeling of structure of double-phase low-carbon chromium steels

A physical model for determining the relative amount of phase components and the size of ferrite grains after decomposition of austenite in the process of cooling of double-phase steels is suggested. The main products of austenite transformation, i.e., polygonal ferrite, pearlite, bainite, and martensite, are considered. The driving forces of the transformation and the concentration of carbon on the phase surface are determined with the use of methods of computational thermodynamics. The model is based on equations of the classical theory of nucleation and growth. It allows for the structural features of the occurrence of γ → α transformation and contain some empirical parameters. The latter are determined using data of dilatometric measurements of the kinetics of austenite transformation and metallographic measurements of the size of ferrite grains. The model is used for predicting the kinetics of the transformation under the complex cooling conditions implemented by the VOEST-ALPINE STAHL LINZ GmbH rolling mill within the computer system for control of mechanical properties of hot-rolled strip.     

Phase transformation theory: A powerful tool for the design of advanced steels

An innovative design procedure based on phase transformation theory alone has been successfully applied to design steels with a microstructure consisting of a mixture of bainitic ferrite, retained austenite, and some martensite. An increase in the amount of bainitic ferrite is needed in order to avoid the presence of large regions of untransformed austenite, which under stress decompose to brittle martensite. The design procedure addresses this diffi culty by adjusting the T′_o curve to greater carbon concentrations with the use of substitutional solutes such as manganese and chromium. The concepts of bainite transformation theory can be exploited even further to design steels with strength in excess of 2.5 GPa and considerable toughness.     

变形速度对TRIP钢冲压成形性的影响

在TRIP钢相变动力学模型的基础上,推导了残余奥氏体体积比变化速率与变形速度的关系,详细分析了变形模式和变形速度对残余奥氏体转变的影响规律,讨论了相变诱发塑性效应对TRIP钢冲压成形性的作用机理,并通过圆筒件拉深成形试验,验证了不同变形速度下TRIP钢的成形性。结果表明,残余奥氏体体积比变化速率随变形速度的增加而增加;变形速度和变形模式对相变诱发塑性效应的发挥影响较大,变形过快导致后续变形所需塑性补充不足;TRIP钢良好塑性得以发挥,必须具有合适的变形条件。     

Dynamic Ferrite Transformation Behaviors in 6Ni-0.1C Steel

Phase transformation from austenite to ferrite is an important process to control the microstructures of steels. To obtain finer ferrite grains for enhancing its mechanical property, various thermomechanical processes followed by static ferrite transformation have been carried out for austenite phase. This article reviews the dynamic transformation (DT), in which ferrite transforms during deformation of austenite, in a 6Ni-0.1C steel recently studied by the authors. Softening of flow stress was caused by DT, and it was interpreted through a true stress–true strain curve analysis. This analysis predicted the formation of ferrite grains even above the Ae₃ temperature (ortho-equilibrium transformation temperature between austenite and ferrite), where austenite is stable thermodynamically, under some deformation conditions, and the occurrence of DT above Ae₃ was experimentally confirmed. Moreover, the change in ferrite grain size in DT was determined by deformation condition, i.e., deformation temperature and strain rate at a certain strain, and ultrafine ferrite grains with a mean grain size of 1 μm were obtained through DT with subsequent dynamic recrystallization of ferrite.     

Mechanical Properties and Retained Austenite Stability of High Aluminum TRIP-aided Cold-rolled Steel Sheets

The mechanical properties, microstructure and retained austenite stability of CMnAlSi-TRIP steels were investigated in this paper. The steel sheets were hot-rolled, cold-rolled and heat treated by intercritical annealing and isothermal heat treatment. The microstructure, volume fraction of retained austenite and its carbon concentration were observed by Optical microscopy and X-ray diffraction. The mechanical properties were obtained through uniaxial tensile test. The results show that the CMnAlSi cold-rolled TRIP-aided steels have good combination of strength and ductility with proper isothermal heat treatment, the retained austenite stability determines incremental strain hardening exponent during strain-induced martensitic transformation, and affected by its volume fraction and carbon content. The retained austenite stability has a good correlation with the combination of strength and ductility.     

Formation of aligned two-phase microstructures by applying a magnetic field during the austenite to ferrite transformation in steels

Application of a magnetic field during the ferrite to austenite transformation in Fe–C alloys was found to yield a two-phase microstructure with the paramagnetic austenite grains aligned as chains or columns along the direction of the field in the matrix of ferromagnetic ferrite phase. The underlying mechanism of dipolar interactions suggests that similar alignment of microstructures should take place during the austenite to ferrite transformation under a magnetic field. In the present investigation, an experimental setup has been designed to study the magnetic alignment. Its concept is characterized by deforming steels prior to the austenite to ferrite transformation to introduce ample nucleation sites in addition to applying magnetic fields up to 12 T. Experiments have revealed successful conditions for aligned two-phase microstructures in carbon steels. The formation mechanism of the aligned structures is discussed from the viewpoint of the nucleation and growth of ferrite grains in austenite phase under a magnetic field. Furthermore, it is shown that the shape of the aligned ferrite grains is determined by a balance of the magnetostatic and the interfacial energies.     

Thermal stability of retained austenite in TRIP steels studied by synchrotron X-ray diffraction during cooling

We have performed in situ X-ray diffraction measurements at a synchrotron source in order to study the thermal stability of the retained austenite phase in transformation induced plasticity steels during cooling from room temperature to 100 K. A powder analysis of the diffraction data reveals a martensitic transformation of part of the retained austenite during cooling. The fraction of austenite that transforms during cooling is found to depend strongly on the bainitic holding time and the composition of the steel. It is shown that that austenite grains with a lower average carbon concentration have a lower stability during cooling.     

THEORETICAL PREDICTION OF THE KINETICS CURVES OF PEARLITIC TRANSFORMATION IN HYPO-PROEUTECTOID STRUCTURAL STEELS

1.IntroductionDuringthepastseveraldecades,researchonpeaxlitictransformationhasbeenfocusedonpearlitegrowthmechanism.In1938,Mehl[IJsuggestedthatthegrowthrateofpearlitedependsoncarboncontentgradientandcarbondiffusionrate.In1946,basedonthegeneraltheoryofdiffusion,Ze.e.[2]proposedanequationtodescribethepearlitegrowthrate.Soonafterwards,Zenner,BrandtandWagner[3]wereawareofthefactthattheGibbsThomsoncapillarityaffectsboththe7--aand7-Fe3Cboundaxiesandrevisedthekineticsequation.Lateronsin1957,based…     

In situ neutron diffraction investigation of the collaborative deformation–transformation mechanism in TRIP-assisted steels at room and elevated temperatures

The deformation behaviour of two transformation induced plasticity (TRIP)-assisted steels with slightly different microstructures due to different thermo-mechanically controlled processing (TMCP) was investigated by the in situ neutron diffraction technique during tensile straining at room temperature and two elevated (50 and 100 °C) temperatures. The essential feature of the TRIP deformation mechanism was found to be significant stress redistribution at the yield point. The applied tensile load is redistributed within the complex TRIP-steel microstructure in such a way that the retained austenite bears a significantly larger load than the ferrite–bainite α-matrix. The macroscopic yielding of the steel then takes place through the simultaneous cooperative activity of the austenite-to-martensite transformation in the austenite phase and plastic deformation in the α-matrix. It is concluded that, although its volume fraction is small, the martensitically transforming retained austenite phase dispersed within the α-matrix governs the plastic deformation of TRIP-assisted steels.     

双相钢中残余奥氏体对性能的影响及其热稳定化

用声发射法研究了一种低合金钢于双相区热处理所得残余奥氏体的马氏体相变。发现残余奥氏体的热稳定化程度和稳定性均与奥氏体的颗粒尺寸有关。颗粒愈小，热稳定化程度愈高，且愈稳定。不存在马氏体核胚的极小奥氏体颗粒不能仅靠过冷来使其转变。形变能诱发试验钢中残余奥氏体转变，且增加钢的塑性。但只有奥氏体颗粒尺寸有合适的分布，其中小部分稳定性很高，才能使马氏体相变随应变增加而逐渐发生并延伸到大的应变，使延伸率明显增加。     

Increasing the stability of austenite by programmed loading

Conclusion The stability of austenite under strain can be increased by programmed loading in the temperature range where the stability of heated austenite is highest and the diffusion mobility of lattice defects is adequate. For some parts of austenitic steels (of the Kh18NI0T type) operating at low temperatures under stress, programmed loading can be used as the final treatment.Physicotechnical Institute of the Academy of Sciences, Ukrainian SSR. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 71–72, June, 1970.