Prediction of steel flow stresses at high temperatures and strain rates

The flow behavior of steels during deformation in the roll gap was simulated by means of single hit compression tests performed in the temperature range 800 °C to 1200 °C. Strain rates of 0.2 to 50 s⁻¹ were employed on selected low-carbon steels containing various combinations of niobium, boron, and copper. The stress/strain curves determined at the higher strain rates were corrected for deformation heating so that constitutive equations pertaining to idealized isothermal conditions could be formulated. When dynamic recovery is the only softening mechanism, these involve a rate equation, consisting of a hyperbolic sine law, and an evolution equation with one internal variable, the latter being the dislocation density. When dynamic recrystallization takes place, the incorporation of the fractional softening by dynamic recrystallization in the evolution equation makes it possible to predict the flow stress after the peak. These expressions can be employed in computer models for on-line gage control during hot-rolling.     

Hot Compression Behavior of AsCast Precipitation Hardening Stainless Steel

 High temperature deformation characteristics of a semiaustenitic grade of precipitation hardening stainless steels were investigated by conducting hot compression tests at temperatures of 900-1 100 ℃ and strain rates of 0001-1 s-1. Flow behavior of this alloy was investigated and it was realized that dynamic recrystallization (DRX) was responsible for flow softening. The correlation between critical strain for initiation of DRX and deformation parameters including temperature and strain rate, and therefore, Zener Hollomon parameter (Z) was studied. Metallographic observation was performed to determine the as deformed microstructure. Microstructural observation shows that recrystallized grain size increases with increasing the temperature and decreasing the strain rate. The activation energy required for DRX of the investigated steel was determined using correlations of flow stress versus temperature and strain rate. The calculated value of activation energy, 460 kJ/mol, is in accordance with other studies on stainless steels. The relationship between peak strain and Z parameter is proposed.     

High Temperature Deformation Behavior of 4340 Steel: Activation Energy Calculation and Modeling of Flow Response

The 4340 steel is extensively utilized in several industries including automotive and aerospace for manufacturing a large number of structural components. Due to the importance of thermo-mechanical processing in the production of steels, the dynamic recrystallization (DRX) characteristics of 4340 steel were investigated. Namely, hot compression tests on 4340 steel have been performed in a temperature range of 900–1200 °C and a strain rate range of 0. 01–1 s^?1 and the strain of up to 0. 9. The resulting flow stress curves show the occurrence of dynamic recrystallization. The flow stress values decrease with the increase of deformation temperature and the decrease of strain rate. The microstructure of 4340 steel after deformation has been studied and it is suggested that the evolution of DRX grain structures can be accompanied by considerable migration of grain boundaries. The constitutive equations were developed to model the hot deformation behavior. Finally based on the classical stress-dislocation relations and the kinematics of the dynamic recrystallization; the flow stress constitutive equations for the dynamic recovery period and dynamic recrystallization period were derived for 4340 steel, respectively. The validity of the model was demonstrated by demonstrating the experimental data with the numerical results with reasonable agreement.     

铸态超级双相不锈钢S32750热加工性能

 为研究热变形参数对铸态超级双相不锈钢S32750热变形行为和显微组织的影响,运用Gleeble-3800热模拟试验机对S32750进行不同温度和应变速率下的高温拉伸和压缩试验。结果表明,S32750在1000~1200℃范围内具有较好的热塑性。在变形温度较低、应变速率较低时,流变曲线表现出不同于单相不锈钢的“类屈服平台”特征;当应变速率较高或变形温度较高、应变速率较低时,流变曲线为典型的动态再结晶特征。微观组织演变显示,铁素体和奥氏体两相都发生动态再结晶,且铁素体的再结晶先于奥氏体。降低应变速率,提高变形温度,可促进动态再结晶发生。基于热变形动力学模型建立了本构方程,表观应力指数为3.99,热变形激活能为393.75kJ/mol。S32750的高温软化机制与Zener-Hollomon(Z)参数有关,随Z参数增加,热变形峰值应力增加。     

Analysis of the Effect of Al on the Static Softening Kinetics of C-Mn Steels Using a Physically Based Model

The effect of Al addition on the static softening behavior of C-Mn steels was investigated. Double-hit torsion tests were performed at different deformation temperatures ranging from 1198 K to 1338 K (925 °C to 1065 °C) with pass strains of ε = 0.2 and 0.35. It was found that solute Al produced a significant delay on the static softening kinetics. Additionally, at the lowest temperatures [1198 K to 1238 K (925 °C to 965 °C)] and highest Al level (2 wt pct), austenite to ferrite phase transformation was found to be concurrent with softening, leading this to higher softening retardation. The softening kinetics of the steels investigated were analyzed using a physically based model which couples recovery and recrystallization mechanisms. The main parameters of the model were identified for the present alloys. An expression for the grain boundary mobility of the base C-Mn steel was derived and the retarding effect of Al in solid solution on the static recrystallization kinetics was introduced in the model. Reasonable agreement was obtained between model and experimental results for a variety of deformation conditions.     

904L不锈钢动态再结晶行为

采用单道次热压缩试验,研究了904L钢在不同变形温度、不同应变速率下的真应力-应变曲线以及组织形貌,阐明了热加工过程中热变形参数对其在变形过程中发生的动态再结晶行为及微观组织演变规律的影响,揭示了其相应的软化机制。结果表明:变形温度越高,流变应力越小,动态再结晶体积分数越高,晶粒尺寸越大;同温度下,变形速率越小,应力峰值越小,晶粒尺寸越大且晶界越平直化;904L钢的动态再结晶行为随着变形温度的升高,应变速率的减小,应变量的增大而进行得越充分且较高的变形温度有利于动态再结晶的进行。     

RAFM钢应变补偿本构关系及热加工图

低活化铁素体/马氏体(RAFM)钢具有较低的辐照肿胀率和优异的力学性能,被认为是聚变堆首选的结构材料。然而,低活化钢强度高、冷塑性变形抗力大的特点,使其难以通过冷加工或低温加工实现大规模生产。使用MMS-200型热模拟试验机,在变形温度为950～1 200℃、应变速率为0.1～5 s^-1和最大变形量为50%条件下,进行了低活化铁素体/马氏体钢(0.11C-9.4Cr-1.35W-0.22V-0.05Si-0.11Ta-0.50Mn)单道次热压缩试验,研究其热变形行为。基于动态材料模型构建了不同应变量下的低活化钢变形本构方程和热加工图,确定了最优热加工参数,结合金相结果分析了材料变形过程中微观组织演化规律,为低活化钢的热加工成形工艺及组织优化提供理论参考。结果表明,在相同应变速率下,随着变形温度升高,流变应力逐渐降低,在一定变形温度下,流变应力随应变速率增大而增大;温度和应变速率对组织的影响主要取决于变形过程中材料内部发生的动态回复和再结晶等机制的交互作用。使用六阶多项式拟合进行应变补偿建立的低活化钢变形本构方程具有较高的预测精度,平方相关系数为0.972。显微组织...     

Prediction of Rolling Load, Recrystallization Kinetics, and Microstructure During Hot Strip Rolling

 The recrystallization kinetics and grain size models were developed for the C Mn and niobium containing steels to describe the metallurgical phenomenon such as softening, grain growth, and strain accumulation. Based on the recrystallization kinetics equations, the mean flow stress and the rolling load of each pass were predicted and the optimum rolling schedule was proposed for hot strip rolling. The austenite grain refinement is associated with the addition of niobium, the decrease of starting temperature of finish rolling, and the reduction of finished thickness. The mean flow stress curve with a continuous rising characteristic can be usually observed in the finish rolling of niobium containing steel, which is formed as a result of the heavy incomplete softening and strain accumulation. The predicted rolling loads are in good agreement with the measured ones.     

Influence of Strain Rate and Temperature on Tensile Deformation and Fracture Behavior of Type 316L(N) Austenitic Stainless Steel

Tensile tests were performed at strain rates ranging from 3.16 × 10^?5 to 3.16 × 10^?3 s^?1 over the temperatures ranging from 300 K to 1123 K (27 °C to 850 °C) to examine the effects of temperature and strain rate on tensile deformation and fracture behavior of nitrogen-alloyed low carbon grade type 316L(N) austenitic stainless steel. The variations of flow stress/strength values, work hardening rate, and tensile ductility with respect to temperature exhibited distinct three temperature regimes. The steel exhibited distinct low- and high-temperature serrated flow regimes and anomalous variations in terms of plateaus/peaks in flow stress/strength values and work hardening rate, negative strain rate sensitivity, and ductility minima at intermediate temperatures. The fracture mode remained transgranular. At high temperatures, the dominance of dynamic recovery is reflected in the rapid decrease in flow stress/strength values, work hardening rate, and increase in ductility with the increasing temperature and the decreasing strain rate.     

Metadynamic Recrystallization Behavior of As-cast 904L Superaustenitic Stainless Steel

The metadynamic recrystallization(MDRX)behavior of as-cast 904 Lsuperaustenitic stainless steel was investigated by double pass isothermal compression tests at temperatures of 950-1 150℃,strain rates of 0.05-5s~(-1)and interval of 1-100 s.The effects of working parameters(deformation temperature,strain rate,pre-strain and interval time)on the flow curves and microstructural evolution were discussed.The MDRX fraction increased obviously with the increase of deformation temperature,strain rate and interval time.The MDRX softening was controlled by the migration of grain boundary,annihilation of dislocation and dynamic recrystallization.Moreover,the kinetic model was established for the prediction of MDRX behavior of as-cast 904 Lsuperaustenitic stainless steel based on the experimental data.A good agreement between the predicted and the experimental values was achieved(correlation coefficient R~2=0.98),indicating a satisfactory accuracy.     

Flow stress and ductility of duplex stainless steel during high-temperature torsion deformation

Hot torsion tests were performed on a duplex stainless steel (DSS)-type EN1.4462 steel. The temperature was varied in the range from 950 °C to 1200 °C, while the strain rate was varied from 0.01/s to 2/s. The mean flow stress (MFS) was fitted to the hyperbolic sine function proposed by Sellars and Tegart. An activation energy for plastic deformation of Q _HW = 425 kJ/mol was obtained. The high value was explained by the fact that, in addition to the softening of ferrite (α) and austenite (γ), there was a decrease in the volume fraction of the high-strength austenite with increasing temperature. For higher values of the Zener-Holomon parameter (Z), the MFS showed a linear dependence on ln (Z), which was related to the gradual disappearance of a yield-point-elongation-like effect. This yield-point-elongation-like effect was characterized by a nonstrengthening plateau during the initial stages of plastic deformation. The strain to rupture and the dynamic softening were both found to decrease for higher values of ln (Z). Therefore, the ductility was directly related to the amount of dynamic softening, i.e., dynamic recrystallization (DRX) of the austenitic phase. At higher strain rates, significant dynamic softening was only observed for temperatures above 1100 °C. The strain-rate sensitivity (m) was found to vary from 0.13 at 950 °C to 0.22 at 1200 °C.     

Modeling of Mechanical Characteristics in Hot Deformation of 4130 Steel

In this investigation, hot compression tests were performed at 900 °C ? 1100 °C and strain rate of 0.001 ? 0.1 s^?1 to study hot deformation behavior and flow stress model of 4130 steel. Based on the classical stress–dislocation relations and the kinematics of the dynamic recrystallization, the flow stress constitutive equations of the work hardening‐dynamical recovery period and dynamical recrystallization period were established for 4130 steel, respectively. The validity of the model was demonstrated by comparing the experimental data with the numerical results. The agreement of this comparison is quite reasonable.     

两种高等级管线钢的奥氏体热变形行为

利用MMS300热模拟试验机在温度为850~1 150℃,应变速率为5 s-1,道次间隔时间为1~400 s的条件下进行了X80及X100管线钢双道次压缩变形试验,测定了不同间隔时间、不同变形温度条件下的软化率。研究结果表明：当道次间隔时间相同时,随变形温度升高,X80及X100管线钢静态再结晶分数均增加,再结晶进程加...     

Effect of deformation parameters on the

The recrystallization behavior of three Nb-bearing high-strength low-alloy (HSLA) steels was investigated during multipass deformation under continuous cooling conditions. The niobium concentrations of these steels varied from 0.05 to 0.09 wt pct. The specimens were tested on a computerized torsion machine using a simulation schedule of 17 passes. Deformation tem-peratures of 1180 °C to 700 °C were employed, together with pass strains of 0.1 to 0.7, strain rates of 0.2 to 10 s^-1, and interpass times of 5 to 200 seconds. By means of mean flow stressvs 1000/T diagrams, the effect of the deformation conditions on the no-recrystallization tem-perature (T _nr ), the temperature at which recrystallization is no longer complete, was determined. It decreases with increasing strain and also decreases slightly with increasing strain rate. There is aT _r minimum at times of about 12∼15 seconds, and both increases and decreases from this value raise this characteristic temperature. When the interpass times are short, solute atoms control the rate of recrystallization, the extent of which decreases as the time is decreased. When the interpass times are long, precipitation takes place and retards recrystallization, so that the extent of softening decreases. Formerly Research Associate, McGill University, Department of Metallurgical Engineering.     

热变形制度对09CuPTiRE钢热变形组织行为的影响

通过热模拟试验和金相组织观察，具体研究了变形温度、平均应变速率和道次间隔时间对09CuPTiRE钢的热变形组织行为及其常温组织状态的影响。结果表明：变形温度的影响最大，动态再结晶的发生程度随其降低而减小；平均应变速率和道次间隔时间的影响相对较小，随着平均应变速率增加，动态再结晶的发生程度减小，静态再结晶的软化效果减弱；随着道次间隔时间延长，静态再结晶发生更加充分。     

17Cr2Ni2Mo齿轮用钢热变形动态再结晶行为

 为了探索材料热塑性变形工艺理论,针对热轧态17Cr2Ni2Mo齿轮用钢进行热力模拟试验,研究材料在应变速率=0.01～10 s-1、热变形温度t=1 050～1 150 ℃条件下的动态再结晶行为。结果表明,在较高应变速率下,应力在峰值后,出现动态回复或持续性动态再结晶软化,在较低应变速率下,应力呈现波浪多峰值状,出现多次动态再结晶软化。通过加工硬化率随应变变化曲线(θ ε),确定了动态再结晶临界特征应变量εc,结合峰值应变量εp统计得到εc/εp比值为0.629～0.854,并可知当应变速率一定时,εc随着温度升高而减小,当温度一定时,εc随应变速率的增大而增大。同时建立了流变应力本构方程,数据验证平均相对误差为1.705%。最后建立了动态再结晶动力学模型。     

中温中压容器用钢13MnNiMoR的热变形行为研究

为制定中温中压容器用钢13MnNiMoR的热加工工艺提供理论依据并实现其工业化生产,利用单道次热压缩模拟实验研究了变形温度(900~1150℃)和应变速率(0.01~1s~(-1))对其热变形行为的影响.结果表明:当应变速率低于0.1s~(-1)时,新晶粒有足够的时间进行形核和长大,奥氏体容易发生动态再结晶;当变形温度降低或应变速率增加时,实验钢在变形过程中主要发生动态回复,流变应力也随之提高.基于测定的流变应力曲线,通过拟合得到实验钢在热变形时的应力指数为4.29,动态再结晶激活能为319kJ/mol,据此建立了13MnNiMoR钢在高温变形时的热加工方程.     

Ti-B25合金高温变形时的塑性流动软化行为

通过热模拟压缩实验,研究了变形温度、应变速率对Ti-B25合金高温变形时流动应力和峰值流动应力的影响,并结合组织演变规律揭示了其高温塑性流动软化机理.结果表明:流动应力和峰值流动应力均随变形温度的下降以及应变速率的增大而增大.应变速率为10.0 s-1时,随着变形温度的升高,流动软化程度减小,并且α+β两相区的软化程度...     

Effects of Temperature and Strain Rate on Flow Behavior and Microstructural Evolution of Super Duplex Stainless Steel under Hot Deformation

Hot compression tests were carried out in the temperature range of 1 223-1 473 Kand strain rate range of0.01-30s-1 to investigate the flow behavior and microstructural evolution of super duplex stainless steel 2507(SDSS2507).It is found that most of the flow curves exhibit a characteristic of dynamic recrystallization(DRX)and the flow stress increases with the decrease of temperature and the increase of strain rate.The apparent activation energy Qof SDSS2507 with varying true strain and strain rate is determined.As the strain increases,the value of Qdeclines in different ways with varying strain rate.The microstructural evolution characteristics and the strain partition between the two constituent phases are significantly affected by the Zener-Hollomon parameter(Z).At a lower lnZ,dynamic recovery(DRV)and continuous dynamic recrystallization(CDRX)of the ferrite dominate the softening mechanism during the compression.At this time,steady state deformation takes place at the last stage of deformation.In contrast,a higher lnZ will facilitate the plastic deformation of the austenite and then activate the discontinuous dynamic recrystallization(DDRX)of the austenite,which leads to a continuous decline of the flow stress even at the last deformation stage together with CDRX of the ferrite.     

Hot workability of M2 type high-speed steel

The hot workability of an M2 type HSS in cast, forged and rolled condition has been studied by means of a torsion test and metallography. Continuous tests were used to determine the temperature and strain-rate dependence of the flow stress and temperature dependence of the strain to fracture, while multistage tests were used to determine the extent of interpass softening. Temperature and strain dependence of the flow stress is described by a relation of the form: in which the activation energy for hot working (Q_HW) is found to be temperature dependent. The hot ductility of the cast steel is not only lower but is also little affected by temperature in comparison to worked steels, in spite of the fact that critical strain for dynamic recrystallization is smaller, and the extent of metadynamic and static recrystallization during interpass interval is more extensive in the former. This is related to a large volume fraction of carbides, which give rise to a high stored energy and enhanced recrystallization on the one hand, and to the suppression of recrystallization within the continuous network of carbides on the other hand. Carbides give rise to an easy nucleation, growth and coalescence of cracks.