Validation of a Model of Plastic Deformation of C‐Mn Steels in the Two‐Phase Temperature Region

The paper describes the validation of a thermal‐mechanical‐microstructural model of deformation of carbon‐manganese steels in the two‐phase temperature region. The model has been developed on the basis of dilatometric and plastometric tests performed for a wide range of temperatures and strain rates. Dilatometric tests were used to identify the phase transformation model and plastometric tests were used to identify the flow stress model. Inverse analysis was applied to find the parameters of the models which were further implemented into a finite element code. Numerical simulations of the deformation of steels in the two‐phase temperature range were performed. Multi‐stage plane strain compression tests were performed to validate the model. The samples were quenched after subsequent stages of the tests and metallographic analysis was performed. Predicted loads, grain size and volume fractions of the microstructural components were compared with measurements and the coefficients in the models were updated.     

Validation of a Model of Plastic Deformation of Niobium Microalloyed Steels in the Two‐Phase Temperature Region

This article describes a thermo‐mechanical‐microstructural model for deformation of niobium microalloyed steel in the two‐phase range of temperatures. Results of physical and numerical modeling are presented. The physical simulation experiments include plastometric and dilatometric tests, as well as industrial rolling trial. Plastometric tests were performed to describe the flow stress for the wide range of temperatures, including ferritic, two‐phase, and austenitic states. Two‐stage deformation tests were performed to identify the microstructure evolution model. Dilatometric tests were used to identify the model of phase transformation. A model of the kinetics of the precipitation was adapted from the literature. The coefficients in all the models were identified using inverse analysis. Developed models were implemented in the finite element code. In order to improve the accuracy of the flow stress predictions in the two‐phase phase temperature range, internal variable dislocation density model was included, as well. The proposed combination of models correctly predicted microstructure changes and mechanical properties in the two‐phase range, during the transformations of the thermo‐mechanical treatment. Industrial trials were performed for the final validation of the models.     

Hot‐Rolling Experiments on Deformation Behaviour of Oxide Scale

A new test technique developed by the authors allows to investigate the brittle‐ductile behaviour of oxide scale during the hot‐rolling process. Sandwich specimens were pre‐oxidised and then welded so as to be gas‐tight. A micro‐alloyed deep‐drawing quality steel (interstitial free steel) and a fine‐grained low‐pearlite structural steel served as test material. The experiments were performed at temperatures up to 1050 °C. Optical metallography was used to describe the changes of the scale layers. The method is quite good to describe the results qualitatively whereas it is rather difficult to derive quantitative results.     

IF钢铁素体区热变形后的静态软化行为

在Ｇｌｅｅｂｌｅ热模拟实验机上,采用双道次压缩法研究了ＩＦ钢铁素体区变形后的静态软化行为。结合静态软化过程中的组织变化和软化率曲线,总结了变形量与变形温度对静态软化率的影响。结合实验数据,求出ＩＦ钢的铁素体静态再结晶激活能Ｑｒｅｘ＝１７３ｋＪ／ｍｏｌ,并采用Ａｖｒａｍｉ方程得到ＩＦ钢的静态再结晶动力学方程     

Deformation Behaviour of Iron‐Rich Iron‐Aluminium Alloys with Ternary Transition Metal Additions

The hardness and yield stress at room temperature and the brittle‐to‐ductile transition temperature of Fe‐Al alloys with 16 at.% Al, which is in the range of the so‐called K‐state with possible short‐range ordering reactions, and ternary additions of 0.5 and 4 at.% of the transition metals Cr, Mo, Mn, V, Ti and Ni were studied with respect to possible hardening effects of the ternary additions. The addition of Cr, Mo and Mn to the Fe‐Al alloys produce solid‐solution hardening which corresponds to the hardening effect of Al. Only Ti, V and Ni produce extra hardening effects which cannot be related to solid‐solution hardening. This extra hardening is attributed to possible fine NiAl precipitates in the Fe‐Al‐Ni case and to possible enhanced short‐range ordering and/or fine carbide precipitates in the cases of Fe‐Al‐V and Fe‐Al‐Ti.     

Hot Deformation and Dynamic Recrystallization Behaviour of Medium Carbon Steel in Austenite Region

The hot deformation behaviour of a 0.47%C (JIS‐S45C) steel in the stable austenite region was systematically investigated under various deformation conditions to collect fundamental data on its high‐temperature deformation and microstructure evolution. The medium carbon steel showed dynamic recrystallization in a wide range of temperatures (850°C～1150°C) and strain rates (10^‐3 s^‐1～100 s^‐1) in the stable austenite region. The dynamically recrystallized grain size was monotonically decreasing with increasing steady state stress. The minimum grain size obtained through dynamic recrystallization was 8.3 μm when the S45C specimen was deformed at 850°C and 1 s^‐1. The stress‐strain relationships were formularized based on a phenomenological model. The stress‐strain curves estimated by the obtained equation were in good agreement with the experimental results.     

Effect of Nitrogen on the Deformation Behaviour of High‐Nitrogen Austenitic Stainless Steels

The deformation behaviour of high‐nitrogen austenitic steels with the base composition of Fe‐18Cr‐10Mn containing various contents of nitrogen was investigated. Two deformation modes including deformation‐induced martensitic transformation (DIMT) and deformation twinning (DT) were observed depending on the nitrogen content. In the alloys with lower nitrogen contents, γ→?→α' martensitic transformation sequentially occurred, whereas DT acted as a main deformation mode and DIMT was suppressed in the alloys with increasing nitrogen content. Both DIMT and DT showed strong crystallographic orientation dependence. The competing mechanism between them was discussed in terms of the variation of stacking fault energy with nitrogen content.     

Microstructure and Deformation Behaviour of Iron‐Rich Iron‐Aluminium Alloys with Ternary Carbon and Silicon Additions

The microstructure, hardness, yield stress, fracture strain, and brittle‐to‐ductile transition temperature of Fe‐Al alloys with Al contents of 12‐18 at.% Al, which are in the range of the so‐called K‐state with possible short‐range ordering reactions, and with ternary additions of carbon and silicon were studied with respect to the effects of possible impurities on the hardening of Fe‐Al alloys. It was found that perovskite‐type Fe₃AlC carbide particles precipitate even in alloys with low C and Si contents; they are controlled by prior heat treatments and strongly affect the deformation behaviour.     

Phase‐Field Simulation of Solidification and Solid‐State Transformations in Multicomponent Steels

Calculating microstructures for technical materials is an ambitious task which not only implies different length scales but also the complex thermodynamic properties of multicomponent and multiphase alloys. We report some of the recent progress in simulating microstructure evolution in multicomponent steels using the multiphase‐field software MICRESS®. Several applications are reviewed in order to demonstrate the current status of applied phase‐field techniques.     

Thermo‐elastic Homogenization of 3‐D Steel Microstructure Simulated by the Phase‐field Method

A multi‐scale approach based on the asymptotic homogenization method of periodic material structures is applied here to determine the effective thermo‐elastic properties of 3D steel microstructures, which have been calculated by phase‐field simulations. A multiphase‐field model, coupled to thermodynamic databases, is used to evaluate the microstructure evolution during the austenite to ferrite phase transformation of low carbon Fe‐C‐Mn steel. In order to derive effective mechanical properties, geometrical information about the grains, their phase properties and crystallographic orientations are transferred to the homogenization tool. Effective cubic Young and shear modules and Poisson coefficients are predicted for different ferrite volume fractions. Moreover, the volume change is derived as function of the phase fractions, leading to a calculated dilatometer curve. The effects of the thermal shrinkage and the volume expansion caused by the phase transformation are taken into account.     

Relationship between Work Hardening Behaviour and Deformation Structure in Ni‐free High Nitrogen Austenitic Stainless Steels

The work hardening behaviour of high nitrogen austenitic steel (HNS) depends not only on the nitrogen content but also on the addition of substitutional alloying elements such as Mn and Ni, although the effect of nitrogen content has been considered to be a main factor controlling the work hardening rate in HNS. In this study, two kinds of high nitrogen austenitic steels containing nearly 1 mass‐% of nitrogen with and without Mn (Fe‐25%Cr‐1.1%N and Fe‐21%Cr‐0.9%N‐23%Mn alloys) were tensile‐tested and their work hardening behaviour was investigated for the purpose of clarifying the effect of Mn on the work hardening behaviour. Then the results were related to the change in deformation substructure. In the Fe‐25Cr‐1.1N alloy, the work hardening rate kept high until fracture occurred, while in the Fe‐21Cr‐0.9N‐23Mn alloy it tended to decrease gradually with tensile deformation in the high strain region. It was concluded that the difference in work hardening behaviour between both alloys is attributed to the change in dislocation substructure from planar dislocation array to dislocation cell by the addition of Mn.     

Effect of Deformation in Controlled Rolling on Ferrite Nucleation

Abstract

Nucleation of ferrite either at austenite grain boundaries or within them on deformation defects has been examined experimentally and analyzed on the basis of the theory of heterogeneous nucleation. Low-carbon low-alloy steels were deformed by controlled rolling schedules to a total reduction of 50 or 68% in finish rolling at 730-800 °C. Deformation accelerates the kinetics of the γ→∞ transformation and strongly activates ferrite nucleation at grain boundaries. Both experimental and theoretical estimations showed that the rate of intragranular nucleation is much less compared to nucleation at grain boundaries. Intragranular nuclei develop notably only in the final stages of transformation in deformed austenite and affect the formation of structure only in the small separated areas.

On a examiné expérimentalement et analysé la nucléation de la ferrite, soit aux joints de grain de l'faustenite, ou à l'finterieur de ceux-ci, aux défauts de déformation, en se basant sur la théorie de la nucléation hétérogéne. On a déformé des aciers peu alliés à faible carbone, en suivant des plans de laminage contrôlé, jusqu'fa une réduction totale de 50 ou 68% pour le laminage final a 800-730 °C. La déformation accélère la cinétique de transformation de γ→α et active fortement la nucleation de la ferrite aux joints de grains. Tant les évaluations expérimentales que theoriques ont montré que le taux de nucléation intragranulaire était beaucoup moins élevé en comparaison avec la nucleation aux joints de grain. Les noyaux intragranulaires se développent particuliérement seulement lors des étapes finales de transformation de l'faustenite déformée et affectent la formation de la structure seulement dans de petites zones séparées.     

Phase‐Field Simulation of Solidifying Microstructure in the Presence of a Convective Field

Here we present a contribution to the microstructure based design of new steel alloys by a careful investigation of multiphase microstructure solidification in a convective field. To this end we present an extension of the quantitative phase‐field model proposed in [1] to investigate the influence of hydrodynamic convection on the growth of eutectic/peritectic alloys. We study directional solidification of a eutectic alloy under the influence of a shear flow ahead of the solidifying front. We mainly investigate the growth of a eutectic lamellar structure. We show that the imposed flow tilts the whole lamellar structure away from the flow direction. Moreover, we show that forced flow alters the growth morphology of Fe‐Ni peritectic alloys, having a strong influence on the nucleation of the peritectic phase. Based on these investigations, a scale relation between the strength of flow and the solid volume fraction is derived. Further we propose an application of these models as an alternative approach to study heterogeneous nucleation kinetics in the solidification of peritectic materials systems.     

Modeling Kinetics of Deformation Induced Ferrite Transformation in Fe-C-Mn Alloy

Thedeformation inducedferritetransformation (DIFT)inthelowtemperatureregioncanbeusedto effectivelyproduceultra finegrainedsteels[1-7]. Thedeformationleadstotheincreaseofstoredener gyandshortenstheincubationperiod.Abovethe pointAr3,ferritetransformationwi…     

形变及冷却条件对35钢铁素体组织的影响

利用热模拟单道次压缩形变试验研究了35钢在临界奥氏体区(Ae3～Ar3)形变后铁素体组织的变化规律。结果表明,形变后水淬,小形变时铁素体首先在原奥氏体晶粒的界隅和晶界上形核,随着形变温度的降低和应变量的增大,原奥氏体内部出现大量细小、等轴的铁素体。在此温度区间形变后缓慢冷却,能够得到超平衡数量(共析转变温度的铁素体转变量)的铁素体,并且随形变温度的降低和形变量的增大,铁素体晶粒明显细化和等轴化。当形变温度降到690℃(Ar3附近),真应变0.92时,铁素体晶粒细化到2～4μm,其体积分数达到76.86%。     

低碳钢铁素体相区变形特性

采用平面应变压缩试验研究了Q235级别低碳钢铁素体相区在550~720℃,应变速率在5×10^-4~10S^-1范围的热变形特性.结果表明,在铁素体相区范围,所有流变曲线都观察到了峰值应力的出现及随后\