超声振动影响低碳钢热压缩过程的分析和试验

为研究超声振动对低碳钢热变形中奥氏体再结晶过程的影响,基于Gleeble-1500热模拟试验和对变形后淬火试样原始奥氏体晶界观测,构建Q235钢热压缩变形抗力和奥氏体平均晶粒尺寸数学模型,对超声振动和常规压缩变形条件下低碳钢圆柱试样的热压缩过程进行仿真分析。结果表明,超声振动导致试样一端变形如倒置的伞状,发生奥氏体动态再结晶所需临界变形程度明显降低;奥氏体动态再结晶首先从试样超振激励端开始,变形后试样内最小奥氏体晶粒尺寸约为20μm;超声振动变形试验表明,试样表层组织纳米化,内部晶粒尺寸相比常规压缩明显细化。     

CSP热轧TRIP钢动态再结晶行为研究

摘要：采用Gleeble-3500热模拟试验机,在温度为950~1150℃、应变速率为0.1~10s-1和变形量为65%的条件下研究了CSP热轧TRIP钢的动态再结晶行为,探讨了初始奥氏体晶粒尺寸对TRIP钢动态再结晶行为的影响。研究结果表明,初始奥氏体晶粒尺寸越小,变形温度越高,应变速率越慢时,TRIP钢中奥氏体越易发生动态再结晶。其中,粗晶试样（初始奥氏体晶粒尺寸为767.54μm）在1050~1150℃内变形时,将发生动态再结晶。其热变形激活能为361539.17J/mol,确定了Zener-Holloman参数与应变速率和温度的关系式,建立了动态再结晶临界应变模型、高温奥氏体流动应力模型和动态再结晶晶粒尺寸模型,理论模拟结果与试验结果吻合较好。     

含铌低碳贝氏体钢再结晶与应变诱导析出研究

利用热模拟技术研究了不同热变形温度条件下双道次变形及不同道次间隙时间等工艺参数对一种铌微合金化低碳贝氏体钢再结晶规律的影响。根据静态再结晶软化率曲线图和双道次热变形后奥氏体晶粒随着间隙时间的晶粒长大趋势,研究了应变诱导析出Nb（C,N）与热变形中奥氏体再结晶的相互作用规律。结果表明,1000—900℃的变形温度内,由于发生应变诱导析出,再结晶过程被延迟,在本试验的道次间隙时间内都为部分再结晶,同时随着变形温度的降低,经过双道次累计应变后,发生完全再结晶,奥氏体晶粒明显细化。     

FGH96合金在等温变形条件下的变形及微观组织演化

通过热模拟试验,深入分析了FGH96合金在高温变形条件下的动态再结晶行为,研究其发生动态再结晶的临界条件及动态再结晶晶粒尺寸的影响因素.深入研究了加工硬化率-应力曲线、加工硬化率-应变曲线和动态再结晶的RTT曲线,确定了FGH96合金热变形时动态再结晶的特征应变、峰值应变、临界应变以及最大软化率应变,同时,采用该方法,还准确地判断了动态再结晶的类型.采用定量金相分析方法,测定了再结晶晶粒尺寸,利用数学回归的方法建立了FGH96合金热变形时的动态再结晶数学模型.     

热变形对奥氏体晶粒尺寸的影响

本文研究了热变形对12CrNi3A钢和18Ni马氏体时效钢再结晶行为和奥氏体组织状态的影响,并着重讨论了不同状态奥氏体的晶粒大小与热变形参数间的关系。文中还探讨了热变形对细化奥氏体晶粒的极限值,续动态再结晶的晶粒长大和形变量对动态再结晶晶粒尺寸的影响等问题。     

34CrMoV结构钢动态再结晶行为研究

应用Gleeble-1500热模拟实验机进行等温压缩试验,研究了变形工艺参数对34CrMoV结构钢动态再结晶行为的影响.结果表明,其流变应力随变形温度升高而降低,随应变速率提高而增大;随着应变速率的降低,应变量的增加和变形温度的升高,动态再结晶速度加快且奥氏体晶粒得到细化.     

核电用奥氏体不锈钢的动态再结晶行为

利用Gleeble-1500D热模拟试验机对316LN奥氏体不锈钢进行单道次热压缩试验,分别设置变形温度为900~1200℃、应变速率为0.001~10 s-1、真应变为0.1~0.9及试样的初始晶粒度为122~297μm之间,以研究热变形条件及初始晶粒度对316LN钢动态再结晶行为的影响.对试验数据进行处理,得到临界应变与峰值应变以及临界应力与峰值应力的比值分别为0.38和0.89,建立了动态再结晶动力学方程和晶粒尺寸演变方程.对建立的动态再结晶模型进行修正,将修正后的模型嵌入DEFORM-3D有限元模拟软件中进行计算,发现修正模型的模拟值和试验值符合较好,证明修正模型的准确性.      

L360管线钢热轧过程静态再结晶行为的研究

奥氏体再结晶行为是影响热轧钢带组织和力学性能的1个主要因素。在热模拟试验的基础上,采用应力松弛法和显微组织观察法,对含Nb和Ti的L360管线钢热轧过程奥氏体静态再结晶行为进行了研究,分析了应变、变形温度、应变速率、原始奥氏体晶粒尺寸对奥氏体静态再结晶的影响。利用线性回归分析,计算出试验钢静态再结晶激活能为203kJ;同时通过回归,得出试验钢热轧过程静态再结晶动力学方程。     

低碳钢Q235形变奥氏体的静态再结晶

在Thermecmastor-Z热模拟试验机上进行了低碳钢Q235的双道次压缩试验,确定了该钢静态再结晶的动力学方程.结果表明,在大变形量下,即使变形温度较低(850～800℃),应变速率较高时,静态再结晶的速度仍然很快.通过静态再结晶可以将奥氏体晶粒尺寸细化至10～20μm.     

ULCB钢的奥氏体形变组织研究

采用单道次压缩实验方法,用THERMECHMASTOR-Z热模拟试验机在1 100～850℃、变形速率2 s-1和变形量10%～50%的应变条件下,对900 Mpa级ULCB钢进行应力-应变曲线和奥氏体形变组织的试验.结果表明,在950℃以下的低温变形中不发牛形变再结晶,随着变形量增大,先出现晶内形变带直到晶粒拉长变形.在1 000℃以上的高温变形中,当变形量大于临界变形量时发生形变再结晶.随着变形量增大,奥氏体再结晶晶粒面积百分数依次增加,形变组织为部分或完全再结晶奥氏体,奥氏体晶粒平均截距的大小取决于形变再结晶奥氏体的晶粒尺寸和面积百分数.     

X70管线钢热连轧过程奥氏体再结晶、晶粒尺寸和平均流变应力的预测

在考虑动态、亚动态再结晶及静态再结晶的基础上,建立了X70管线钢的物理冶金模型,并应用于板带钢热连轧过程奥氏体再结晶、晶粒尺寸和流变应力的预测.结果表明,在合理的温度和压下条件下,应变累积可导致在精轧过程出现动态 亚动态再结晶行为,促进奥氏体晶粒的进一步细化.终轧温度的降低可引起奥氏体晶粒的粗化和残余应变的显著提高.建立了考虑晶粒尺寸和残余应变影响的平均流变应力(MFS)的人工神经网络预测模型,大大提高了热连轧过程MFS预测精度.     

高锰钢高速冲击时剪切区TRIP行为的准原位分析

利用背散射电子衍射技术对高速冲击前后高锰钢样品强制剪切区域的晶粒进行准原位观察,分析了剪切区域不同位置晶粒的相变情况,并借助有限元模拟及受力计算对不同晶粒相变程度差异的原因做了进一步分析.结果表明,在高速变形下,应力应变水平、奥氏体取向及晶粒间的相互作用共同影响TRIP行为:应力应变水平越高,相变程度越大;由于帽型样中剪切应力的存在,相比于近〈111〉取向奥氏体,近〈100〉和近〈110〉取向奥氏体相变程度更大,近〈110〉取向相变程度最大.具有有利取向的奥氏体,晶粒尺寸越大,其相变行为受周围晶粒影响越小,越容易充分相变;具有有利取向的长条状奥氏体晶粒,若其两侧晶粒难相变,则该晶粒相变将受到束缚;带有尖角的晶粒,变形时应力集中难以释放,易发生相变;当晶粒的孪生分力大于滑移,但其最大和次大的孪生分力相差不大,可能导致在这两个方向孪生互相竞争,反而不易相变.高速变形时体心马氏体多在晶界应力集中处产生,很少在晶粒内部大量产生,形态多为细片状,变体选择强.      

Effect of initial grain size of austenite on hot-deformed structure of Ni-30Fe alloy

The microstructural evolutions of Ni-30Fe alloys during hot deformation are investigated. Hot-deformed structures of Ni-30Fe alloys with initial austenite grain sizes of 20 and 140 μm are examined under various compressive strains and deformation temperatures. As the initial austenite grain size decreases, dynamic recrystallization (DRX) occurs at lower compressive strain and lower deformation temperature. At deformation temperatures where dynamic recovery occurs instead of the DRX, hot-deformed structures consist of recovered elongated grains until fine-equiaxed grains are evolved by geometric DRX. Critical compressive strain for the geometric DRX decreases with the decrease of initial austenite grain size. Geometric DRX is evolved by the impingement of serrated grain boundaries. The decrease of initial grain size is considered to reduce the critical compressive strain needed for the impingement of serrated grain boundaries. The changes in the effective thickness of austenite grain according to the compressive deformation are examined and the effects of the restoration processes on the effective thickness of austenite grain are discussed.     

Recrystallization behavior of twin roll cast low carbon steel strip

The dynamic and static recrystallization behaviors of twin roll cast low carbon steel strip were investigated with an attempt to provide guiding deformation parameters for the on line hot rolling.In order to investigate dynamic recrystallization behavior,as cast strip was reheated and soaked with austenite grain size similar to the width level of the as cast columnar structure.Tensile test was used and the deformation temperature is in the range of 900℃to 1 100℃and strain rates are 0.01 s^-1,0.1 s^-1,1 s^-1.The activation energy and stress exponent were determined as 306kJ/mol and 4.69 respectively.The ratio of critical strain to the peak strain is 0.65,and that of critical stress to the peak stress is 0.92.The dependence of the peak strain on the initial grain size and Zener - Hollomon parameters Z isε_p =9.1×10^-4×D₀^0.48Z^0.13.The kinetics of the dynamic recrystallization and recrystallized grain size was predicted using models published.The as cast coarse austenite were dramatically refined after complete dynamic recrystallization.For static recrystallization,the tensile test was carried out on Gleeble -3500 thermo - mechanical simulator.The deformation temperature is in the range of 800℃to 1 200℃with strain rate 0.01 s^-1 to 1s^-1.The pre strain is fixed at 0.04 to 0.12 and the inter-hit delay time varies from 1 s to 3 000 s.The activation energy and Avrami exponent of static recrystallization were determined as 241 kJ/mol and 0.54 respectively.A kinetics model was proposed to describe the static recrystallization kinetics.The predicted results were in good agreement with the experimental results.     

Microstructure Refinement and Property Improvement of Metastable Austenitic Manganese Steel Induced by Electropulsing

Grain refinement efficiency of electropulsing treatment(EPT)for metastable austenitic manganese steel was investigated.The mean grain size of original austenite is 300μm.However,after EPT,the microstructure exhibits a bimodal grain size distribution,and nearly 70vol.%grains are less than 60μm.The refined austenite results in ultrafine martensitic microstructure.The tensile strengths of refined austenitic and martensitic microstructures were improved from 495to 670,and 794to 900MPa respectively.The fine grained materials possess better fracture toughness.The work-hardening capacity and wear resistance of the refined austenitic microstructure are improved.The reasonable mechanism of grain refinement is the combination of accelerating new phase nucleation and restraining the growth of neonatal austenitic grain during reverse transformation and rapid recrystallization induced by electropulsing.     

Limit of ferrite grain refinement by severe plastic deformation of austenite

High-angle grain-boundary spacing in deformed austenite is analyzed using Ni-30Fe alloy to explain the change of ferrite grain size by severe plastic deformation (SPD) of austenite in low carbon steel. It is suggested that constant high-angle grain-boundary spacing in deformed austenite resulting from dynamic recrystallization (DRX) or geometric DRX is responsible for the limit of ferrite grain refinement over a certain level of plastic deformation of austenite.     

Study on dynamic recrystallization behavior of hot rolled TRIP steel produced by CSP process

The dynamic recrystallization behavior of hot rolled TRIP steel produced by CSP process was studied by means of Gleeble-3500 thermal simulation testing machine in the temperature range of 950-1150℃ with the strain rate of 0.1-10s-1 and the strain of 65%. And the effect of initial austenite grain size on the dynamic recrystallization behavior of TRIP steel was explored. The results show that the finer initial austenite grain size, the higher deformation temperature and the lower strain rate, the more positive austenite dynamic recrystallization of TRIP steel. Moreover, it is found that when the coarse grained samples (initial austenite grain size is 767.54μm) deform in the range of 1050℃ to 1150℃, the austenite dynamic recrystallization will take place, and the dynamic recrystallization activation energy of TRIP steel is deduced as 361539.17J/mol. The Zener-Hollomon parameter equation as a function of strain rate and temperature is determined. And the model of critical strain for dynamic recrystallization, the flow stress model of austenite at high temperature and the grain size model for dynamic recrystallization are also established. The calculation results are coincided well with the experimental results.     

热变形对轴承钢G80T晶粒细化行为的影响

利用Gleeble-3800热模拟变形试验机,对高温轴承钢G80T的动态再结晶行为及相关力学性能进行了研究。通过对该钢在1 050℃下以10s-1的变形速率进行0%~70%的不同形变量的单道次压缩,研究了不同形变量下热变形钢的微观组织结构及硬度。结果表明:当形变量在20%时,无动态再结晶发生;当形变量达到40%时,热变形组织出现了部分的再结晶晶粒;随着形变量的进一步加大,再结晶晶粒数目增多,形变量达到60%后,形变组织形成了平均晶粒尺寸为2.8μm的完全再结晶组织。同时由于形变和晶粒尺寸大幅度细化,钢中的碳化物也随着形变量的增加而逐步减少。通过对压缩应力-应变曲线软化行为的分析,认为该钢的再结晶属于动态再结晶;在1 050℃进行60%的形变可以实现该钢的完全动态再结晶,将平均晶粒尺寸从原始的22μm细化到2.8μm,同时将钢的硬度从820 HV提高到895 HV。研究结果表明,动态再结晶是细化高温轴承钢G80T晶粒尺寸和提高性能的一种有效措施。     

Microstructural Evolution During Friction Stir Welding of Mild Steel and Ni-Based Alloy 625

Microstructure evolution during friction stir welding (FSW) of mild steel and Ni-based alloy 625 was studied. Regarding the Ni-based alloy, the welding process led to grain refinement caused by discontinuous and continuous dynamic recrystallization, where bulging of the pre-existing grains and subgrain rotation were the primary mechanisms of recrystallization. In the steel, discontinuous dynamic recrystallization was identified as the recovery process experienced by the austenite. Simple shear textures were observed in the regions affected by the deformation of both materials. Although the allotropic transformation obscured the deformation history, the thermo-mechanically affected zone was identified in the steel by simple shear texture components. A new methodology for the study of texture evolution based on rotations of the slip systems using pole figures is presented as an approximation to describe the texture evolution in FSW.