Application of mathematical modeling in two-stage rolling of hot rolled wire rods

The no-recrystallization temperature (T_nr) is an important parameter in the design of two-stage rolling schedule to obtain finer grain size. T_nr was obtained both by continuous cooling compression testing and tension-compression testing. However, due to the limitations of experimental installation, both compressing testing and tension-compression testing have a scaling down of practical pass strain and strain rate in rolling mill. The mathematical model that calculates mean flow stress (MFS) can eliminate these limitations and the pass strain and strain applied in mathematical model are approximately equal to the mean value of that in wire-rod rolling mill. Therefore, mathematical calculation is a new method to determine T_nr and the predicted T_nr is similar to experimental results. Due to the high strain rate and short interpass time at the finishing strain of wire rods mills, mathematical modeling is also an effective method to simulate microstructure-evolution in wire rods rolling. An expert system was established to study the microstructure evolution in two-stage rolling through the obtained dynamic recrystallization (DRX) model combined with metadynamic recrystallization (MRX) and static recrystallization (SRX) model in literature. In the present work, it is simplified that the complete metadynamic recrystallization (MRX) is achieved when strain for deformation exceeds critical strain ɛ_c. It was found that strain accumulation played an important role in finishing rolling. The recrystallization behavior during finishing rolling stage was repeated by static and dynamic model. The predicted austenite grain size and mean flow stress at each pass are expected to provide guidance for appropriate rolling schedule design.     

Assessment of austenite static recrystallization and grain size evolution during multipass hot rolling of a niobium-microalloyed steel

Double-deformation isothermal tests and multipass continuous-cooling hot torsion tests were used to study the evolution of austenite microstructures during isothermal and non-isothermal hot deformation of an Nb microalloyed steel. These tests, coupled with microstructural characterization, have verified that the no-recrystallization temperature (T _nr ) corresponds roughly to the temperature where recrystallization starts to be incomplete during rolling. An accurate method to estimate the recrystallized fraction during hot rolling based on stress-strain data, and which does not require metallographic studies, is proposed. The results of this method have been successfully compared to metallographic measurements, the values of non-isothermal fractional softening and the accumulated stress measured in the plots of mean flow stress (MFS) versus the inverse of temperature. A remarkable austenite grain refinement occurs in the first hot rolling passes after reheating. The correlation of isothermal and continuous cooling tests is better understood if the effect of grain size on recrystallization and precipitation is taken into account.     

高温、中应变速率下预测四道次连续轧制力的流变应力方程(英文)

为了计算在应变速率100～400s-1、温度900～1050℃条件下四道次连续线材轧制过程中的轧制力,提出了一个流变应力方程。基本概念是对Shida模型和Misaka模型进行改进。通常用这2种模型建立的流变应力本构方程来描述高温材料在不同应变率下的变形行为。将改进模型与有限元方法相结合来计算应变速率100～400s-1、温度900～1050℃条件下的四道次连续轧制过程中的轧制力。测量材料在每个道次的轧制力和表面温度,并与预测值进行比较。结果表明,在高温、中应变速率条件下,Misaka模型比Shida模型更好。在900℃时,采用Misaka模型的轧制力误差为-5.7%。在1050℃时,采用Misaka模型的轧制力误差为-15.2%,而采用改进的Misaka模型的轧制力误差降低到1.8%。由此可以得出,对于高温、中应变速率的线材轧制过程,改进的Misaka模型能用来预测高温材料的变形行为。     

基于Deform高线粗轧奥氏体晶粒细化研究

本文结合GCr15再结晶模型, 根据轧线实际孔型参数、轧线布置与轧制程序, 采用刚塑性有限元法, 利用模拟软件Deform对轴承钢线材GCr15粗轧进行了三维有限元模拟, 分析总结了粗轧过程中轧件温度场、等效应变和应变速率的变化规律, 得出粗轧过程动态、亚动态和静态再结晶的百分数和对应晶粒尺寸, 揭示了轧件在粗轧过程中再结晶规律及奥氏体晶粒细化规律, 并且证实了初始晶粒尺寸对粗轧过程奥氏体晶粒细化的影响规律。     

热变形参数对40CrNiMo钢组织的影响

利用Gleeble1500热/力学模拟实验机,对40CrNiMo钢进行双道次热模拟单向压缩试验。分析了40CrNiMo钢在变形温度为950、760℃,变形速率为0.5～30s-1,变形量为0.05～0.4,热变形后的奥氏体组织特征。结果表明,在950℃时,奥氏体组织能发生动态再结晶,最终奥氏体晶粒形状取决于变形速率和变形量;40CrNiMo钢在760℃时奥氏体组织发生动态回复,最终奥氏体晶粒呈扁平的"薄饼"状,奥氏体晶粒的变形程度取决于变形量的大小。     

Phenomena and mechanism on superplasticity of duplex stainless steels

The superplasticity of Fe-24Cr-7Ni-3Mo-0.14N duplex stainless steel after being solution treated at 1350°C followed by 90% cold rolling was investigated at 850°C with a strain rate ranging from 10^-3-10^-1s^-1. The microstructure of duplex stainless steel consists of a matrix γ phase having low angle grain boundaries and a σ phase as second phase particles before the deformation at 850°C. It is well known that the constituent phases in duplex stainless steel is changed following α→α+γ→α+γ+σ→γ+σ through phase transformation during deformation at 850°C. The final microstructure of duplex stainless steel consisted of 70 vol.% of γ and 30 vol.% of the σ phase. A maximum elongation of 750% was obtained at 850°C with a strain rate of 3.16xl0^-3s^-1. The dislocation density within matrix γ grains was low and a significant strain-induced grain growth was observed during the deformation. The misorientation angles between the neighboring γ grains increased as the strain increased, thus the low angle grain boundaries were transformed into high angle grain boundaries suitable for sliding by dynamic recrystallization during the deformation at 850°C. The grain boundary sliding assisted by dynamic recrystallization is considered to be a controlling mechanism for superplastic deformation at 850°C.     

Recrystallization in as-cast polycrystalline intermetallic compound Ni3Al

The Ni₃Al-based alloys were hot formed and their dynamic as well as postdynamic recrystallization capability studied. In the as-cast material, the large original grain size and inhomogeneous strain distribution complicate the softening processes. It is very easy to provoke recrystallization in such a material. Strains ranging from 0.05 to 0.2 lead to the nucleation of dynamically recrystallized grains along the primary grain boundaries and twins. On the other hand, to achieve thorough recrystallization is often difficult. It depends chiefly on previous deformation, less on annealing time, temperature or microalloying. In some cases, annealing does not lead to any significant increase of recrystallization fraction but only to the growth of selected grains. The localization of strain plays a dominant role.     

Incoloy825合金热变形过程中的本构模型和特殊晶界演变

采用原始JC模型、修正JC模型和应变补偿Arrhenius方程，描述了Incoloy825合金在不同温度（950～1150 °C）和应变速率（1~10 s^-1）下经摩擦和温升修正后的应力-应变曲线。结果表明，修正后曲线具有明显的动态再结晶特征。与原始JC模型和修正的JC模型相比，Arrhenius应变补偿模型更适合于描述Incoloy825合金热变形过程中的应力应变行为。温度和应变速率对特殊晶界的演变有显著影响。特殊晶界长度分数与动态再结晶分数呈正相关。与冷轧后退火处理工艺相比，热变形工艺调控的特殊晶界长度分数较低，热变形工艺不适合用来调整特殊晶界分数，其原因是在热变形过程中动态再结晶的大量形核造成较小的晶粒团簇。     

Modeling the initiation of dynamic recrystallization using a dynamic recovery model

Dynamic recrystallization flow curve was studied in AISI 410 martensitic stainless steel by performing hot compression tests in a temperature range of 900-1150 °C and at strain rates of 0.001-1 s⁻¹. The Estrin and Mecking's equation for dynamic recovery was used to model the work hardening region of the flow curves. The critical strain and stress for the initiation of dynamic recrystallization were determined using the method developed by Poliak and Jonas. The critical dislocation density for starting dynamic recrystallization was estimated using the Estrin and Mecking's dynamic recovery model. A modified Arrhenius-type equation was used to relate the critical dislocation density to strain rate and temperature. The proposed model was also verified by the model proposed by Roberts and Ahlblom and developed to describe the variation of dislocation density and fractional softening due to dynamic recrystallization up to the peak of flow curve.     

Finite Element Analysis for Microstructure Evolution in Hot Finishing Rolling of Steel Strips

A computer model that describes the evolution of microstructures during the hot finishing rolling of SS400 steel has been proposed. It has been found that the microstructure strongly depends on processing of materials and on their parameters,which affected the history of the thermomechanical variables,such as temperature,strain,and strain rate. To investigate the microstructural evolutions during the hot finishing rolling process,the rigid-thermoviscoplastic finite element method(FEM) has been combined with dynamic recrystallization,static recrystallization,and grain growth models. The simulation results show a good agreement with those from the prediction software online.     

微合金元素Nb对高碳合金钢动态再结晶行为的影响

为研究微合金元素Nb对高碳合金钢动态再结晶行为的影响,利用Gleeble-3500热模拟试验机进行单道次压缩试验,测定了高碳合金钢在变形温度为950~1150 ℃、应变速率为0.01~5 s^-1的流变应力曲线,利用Zeiss光学显微镜观察了奥氏体动态再结晶晶粒形态,通过回归计算获得了相应的再结晶激活能,建立了热变形方程。结果表明:较高的变形温度和较低的应变速率有利于含铌高碳合金钢发生动态再结晶;含铌高碳合金钢的动态再结晶晶粒尺寸随着变形温度的升高而增大,当变形温度为1050 ℃时,含铌高碳合金钢已大量出现动态再结晶晶粒;0.040%铌加入到高碳合金钢中,在应变速率为0.1 s^-1,变形温度为1150 ℃时推迟了钢的动态再结晶开始时间约2.23 s,动态再结晶形变激活能增加了52.26 kJ/mol。     

AZ31镁合金铸轧和常规轧制板的变形组织及形变特征

在变形温度为150～400 ℃、应变速率为0.3～0.000 3 s~(-1)条件下,在Gleeble1500热模拟机上采用等温拉伸试验对AZ31镁合金铸轧和常规轧制板的高温塑性及组织演变进行研究.结果表明:两种AZ31镁合金板的峰值应力和峰值应变均随着变形温度的降低和应变速率的增加而逐渐增大.铸轧板的应变硬化指数和应变速率敏感系数均大于常规轧制板的.在高温低应变速率变形条件下,铸轧板的晶界滑移引起的空洞尺寸、体积分数和密度均大于常规轧制板的.低应变速率下拉伸变形后的动态再结晶晶粒尺寸随温度的升高逐渐增加;不同变形条件下铸轧板的晶粒尺寸均小于常规轧制板的;再结晶晶粒尺寸和Z参数呈幂律关系.     

Nb-Ti连铸坯热芯大压下轧制动态再结晶行为研究

采用热模拟单道次压缩实验,研究了Nb-Ti连铸坯热芯大压下轧制中动态再结晶行为及奥氏体晶粒转变规律。结果表明,变形温度越高,应变速率越低,发生动态再结晶的临界应变值越小,动态再结晶越充分。在变形温度1 350 ℃,继续增加应变至0.8和增加应变速率至10 s^-1,奥氏体晶粒尺寸并未得到进一步细化,反而较应变0.5和应变速率5 s^-1下的奥氏体晶粒更加粗大。这是因为高温粘塑性区的金属晶间粘性流动增加,位错增殖速度增大,在动态再结晶过程中会重新形成新的无畸变再结晶晶粒,这些新的无畸变晶粒的亚动态再结晶动力学极快,在较大驱动力下使奥氏体晶界快速迁移,从而使奥氏体发生一定程度的粗化。     

Nb-Ti连铸坯热芯大压下轧制动态再结晶行为研究

采用热模拟单道次压缩实验，研究了Nb-Ti连铸坯热芯大压下轧制中动态再结晶行为及奥氏体晶粒转变规律。结果表明，变形温度越高，应变速率越低，发生动态再结晶的临界应变值越小，动态再结晶越充分。在变形温度1 350 ℃，继续增加应变至0.8和增加应变速率至10 s^-1，奥氏体晶粒尺寸并未得到进一步细化，反而较应变0.5和应变速率5 s^-1下的奥氏体晶粒更加粗大。这是因为高温粘塑性区的金属晶间粘性流动增加，位错增殖速度增大，在动态再结晶过程中会重新形成新的无畸变再结晶晶粒，这些新的无畸变晶粒的亚动态再结晶动力学极快，在较大驱动力下使奥氏体晶界快速迁移，从而使奥氏体发生一定程度的粗化。     

Static recrystallization behavior of a martensitic heat-resistant stainless steel 403Nb

A static recrystallization behavior between the rolling passes of a martensitic heat-resistant stainless steel 403Nb has been studied by OM,TEM and double-hit thermo-mechanical simulator to explore the effects of deformation temperature,strain rate,strain and the prior austenite grain size. The results show that increases of deforma-tion temperature and strain rate and strain can promote the static recrystallization of 403Nb steel. Static recrystallization also proceeds faster when the prior austenite grain...     

Optimization of process parameters for the manufacturing of rocket casings: A study using processing maps

Maraging steels possess ultrahigh strength combined with ductility and toughness and could be easily fabricated and heat-treated. Bulk metalworking of maraging steels is an important step in the component manufacture. To optimize the hot-working parameters (temperature and strain rate) for the ring rolling process of maraging steel used for the manufacture of rocket casings, a systematic study was conducted to characterize the hot working behavior by developing processing maps for γ-iron and an indigenous 250 grade maraging steel. The hot deformation behavior of binary alloys of iron with Ni, Co, and Mo, which are major constituents of maraging steel, is also studied. Results from the investigation suggest that all the materials tested exhibit a domain of dynamic recrystallization (DRX). From the instability maps, it was revealed that strain rates above 10 s⁻¹ are not suitable for hot working of these materials. An important result from the stress-strain behavior is that while Co strengthens γ-iron, Ni and Mo cause flow softening. Temperatures around 1125 °C and strain rate range between 0.001 and 0.1 s⁻¹ are suitable for the hot working of maraging steel in the DRX domain. Also, higher strain rates may be used in the meta-dynamic recrystallization domain above 1075 °C for high strain rate applications such as ring rolling. The microstructural mechanisms identified from the processing maps along with grain size analyses and hot ductility measurements could be used to design hot-working schedules for maraging steel.     

Static Recrystallization of 30Cr2Ni4MoV Ultra-Super-Critical Rotor Steel

In order to investigate the static recrystallization (SRX) behavior of 30Cr2Ni4MoV ultra-super-critical rotor steel, the double-hit hot compression tests were conducted in the deformation temperature range of (970-1250) °C, the strain rate range of (0.001-0.1) s^?1, and the inter-pass time range of (1-100) s. It is found that the effects of forming parameters (forming temperature and strain rate) on the microstructural evolution during SRX are not significant, while those of the initial austenitic grain size are obvious only when the initial austenitic grain size is less than 225.4 µm. Based on the experimental results, the kinetic equations have also been developed to predict the SRX behaviors of hot-deformed 30Cr2Ni4MoV steel. The proposed kinetic equation correlates well with the experimental results confirming that the proposed kinetic equations can give an accurate estimate of the static softening behaviors for 30Cr2Ni4MoV steel.     

Fe—16Al基合金与Fe—28Al基合金的形变与再结晶织构

研究了Ｆｅ１６Ａｌ（摩尔分数,％,下同）基合金与Ｆｅ２８Ａｌ基合金的温轧形变与再结晶织构。结果表明,ＦｅＡｌ合金系中,不同Ａｌ含量对塑性变形及再结晶机制的影响是与由Ａｌ含量不同引起的结构变化密切相联系的。Ｆｅ１６基合金形变与再结晶行为与纯ＢＣＣ金属类似。Ｆｅ２８Ａｌ基合金变形时外力对织构形成有敏感影响,变形时晶界的应变协调能力较弱。且再结晶后晶粒取向分布有明显的随机化倾向。     

Relating Initial Texture to Deformation Behavior During Cold Rolling and Static Recrystallization Upon Subsequent Annealing of an Extruded WE43 Alloy

Deformation behaviors during cold rolling and static recrystallization behaviors upon subsequent annealing of an extruded WE43 alloys with different initial textures were investigated in this study. Three types of differently textured WE43 initial alloys were labeled as samples Ⅰ, Ⅱ and Ⅲ. The results showed that multiple twinning modes and basal slip dominated the deformation of samples during cold rolling. Cold-rolled sample Ⅰ activated the larger number of double twins with high strain energy...     

30CrNi3MoV钢动态再结晶行为及其模型的构建

根据Gleeble-3500热模拟试验机测量30CrNi3MoV钢的真应力-真应变曲线,系统研究了应变速率为0.01、0.1 s^-1时钢材的动态再结晶行为,并构建了其动态再结晶模型。结果表明:30CrNi3MoV钢在高温小应变速率下更容易发生动态再结晶,其热变形激活能为328.2 kJ/mol;通过加工硬化率随流变应力变化曲线(θ-σ)的拐点确定临界应变,可得动态再结晶临界应变方程为ε_c=0.001 22Z^0.175;构建的动态再结晶体积分数及其平均晶粒尺寸模型能够较好地预测试验钢的动态再结晶体积分数及其晶粒尺寸;当应变速率为0.1 s^-1、变形温度为1050 ℃时,试验钢的晶粒最细小、均匀,平均晶粒尺寸约为19.9 μm。