非调质塑料模具钢热加工的模拟研究

采用Gleeble 3800热模拟试验机研究了非调质塑料模具钢(SWFT钢)的热变形工艺,试验结果表明,SWFT钢分别在1000、1050、1100和1150 ℃单道次变形50%后以0.083 ℃/s的速度冷却时,随热变形温度的升高,先共析铁素体组织减少直至消失,但晶粒较粗大.多道次热变形后在相同冷却速度时确保先共析铁素体组织不出现,同时细化因单道次变形温度较高出现的粗大晶粒,为SWFT钢应用于一定截面尺寸的非调质塑料模具钢模块提供了制订锻造工艺的依据.     

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%。     

等温压缩对高强度汽车钢WHT1300HF的马氏体相变及微观组织的影响

通过等温形变研究了形变参数(形变温度、形变速率、形变量)对高强度汽车钢WHT1300HF的微观组织转变和形貌的影响规律。研究结果表明:增加奥氏体等温形变量,有利于铁素体的缺陷形核,促进了形变奥氏体向铁素体转变;奥氏体的形变强化导致马氏体相变阻力增大,马氏体相变开始温度(Ms)下降,细小晶粒数量和小角度晶界数量增多;增加奥氏体等温形变(40%)速率能同时促进马氏体和铁素体相变,但马氏体体积分数和小角度晶界数量减少,细小晶粒数量略有提高;降低等温形变温度加剧奥氏体的形变强化,导致Ms温度下降,马氏体体积分数、小角度晶界比例减少,细小晶粒数量增多,铁素体含量明显增加。     

45钢等径弯曲通道变形及组织细化研究

研究了等径弯曲通道(ECAP)变形后45钢中先共析铁素体及珠光体组织的演变特征.结果表明,ECAP变形4道次后,片层状的珠光体组织演变成了超细的渗碳体颗粒均匀分布于亚微晶铁素体基体的组织.先共析铁素体由原始的平均晶粒尺寸约为30 μm演变为大角度晶界分离的、平均晶粒尺寸约为0.4μm的超细晶组织.ECAP变形后,先共析铁素体首先在其内部会形成具有薄片层界面(LBs)的板条位错胞甚至亚晶组织.进一步变形时位错胞或亚晶可继续细化.再进一步变形时通过晶界滑移和晶粒旋转的方式可以获得具有大角度晶界分离的、等轴的超细晶组织.     

形变参数对低碳钢形变诱导相变的影响

低碳钢在略高于Ar3温度大变形会发生形变诱导铁素体相变（DIFT）,通过热模拟实验方法考察了变形参数（变形前冷速、变形温度和应变速率）对形变诱导铁素体晶粒尺寸的影响规律。结果表明,大变形的条件下,形变诱导铁素体晶粒尺寸随着相变前冷速和应变速率的提高、变形温度的降低而减小。且通过变形参数对相变驱动力和晶粒长大的影响讨论了其对诱导铁素体晶粒尺寸产生影响的机制。     

热处理温度对345 MPa级耐火钢组织及性能的影响

研究了不同热处理温度对345 MPa级耐火钢组织和性能的影响.结果表明,经过热处理后试验耐火钢的组织特征是由铁素体基体、少量粒状贝氏体和M-A岛组成的混合组织.随着热处理温度的升高,铁素体晶粒尺寸长大,粒状贝氏体和M-A岛数量减少,耐火钢的抗拉强度、屈服强度和屈强比下降,延伸率升高.在加热温度为800℃时,铁素体晶粒形状规则,贝氏体分布均匀,综合力学性能最好.     

超快冷工艺对X70管线钢组织的影响

通过热模拟机研究超快冷工艺中冷却速率和终轧温度对X70管线钢组织细化及马氏体/奥氏体小岛的影响.随着冷却速率的增大,铁素体晶粒尺寸减小,M/A岛的体积分数先增大后降低,M/A岛的尺寸变化则相反.提高终轧温度,铁素体晶粒尺寸略微增大,M/A岛的体积分数增加;但在900~940℃范围内,随着终轧温度的升高,试样中M/A岛的体积分数略减小,尺寸增大.      

普通低碳钢奥氏体晶粒细化及其对变形强化相变铁素体晶粒尺寸的影响

研究了普通低碳钢奥氏体昌粒的细化及其对形变强化铁素体晶粒尺寸的影响。结果表明：通过在奥氏体区进行低温（850℃）较大变形，奥氏体晶粒尺寸可细化至约10μm。奥氏体晶粒尺寸对形变相变后的铁素体晶粒尺寸产生影响。奥氏体晶粒细小，则铁素体晶粒细小且均匀。但其影响随着变形温度的降低而逐渐减弱。     

超低碳钢等温时效过程中析出物的电镜观察

采用扫描电子显微镜研究了超低碳钢在650 ℃经100 s、1000s、100 h、300 h等温时效过程的组织变化及铜的析出颗粒,探讨了时效时间与铜原子扩散对富铜析出颗粒的影响.研究结果表明,等温时效后的组织为多边形铁素体晶粒,富铜析出颗粒优先在铁素体晶界处析出,随着时效时间的延长,在铁素体晶粒内部也出现了富铜析出颗粒.计算表明在晶粒内部的铜原子完全可以借助位错和晶界的高扩散率通道,逐渐富集、长大并粗化.     

超纯21%Cr铁素体不锈钢的热变形行为研究

采用热力模拟试验机对超纯21%Cr铁素体不锈钢进行了单道次压缩试验。研究了在650~1 000℃范围内铁素体不锈钢的热变形行为,确定了流变应力、变形温度和变形速率之间的关系及组织演变规律,并建立了变形抗力数学模型。结果表明,超纯铁素体不锈钢在试验温度范围内的热变形软化机制以动态回复为主。在800℃以下变形时,铁素体晶粒内部出现剪切变形带。随着温度的降低和压下量的增加试样中亚晶粒得到细化,位错密度增加。     

Dilatometric Investigation on Isothermal Transformation after Hot Deformation

Itiswellknownthattheferritetransformationisacceleratedandenhancedbyaustenitedeforma tion ,especiallyinnon recrystallizationregion ,andbysubsequentcoolingwithrelativehighrate[1] .Suchprocessobviouslyrefinestheferritegrainandotherphasetransformationproductsandbecomesaneffectivewaytoimprovethepropertiesofsteel ,basedonwhichthecontrolledrollingandcontrolledcoolingtechniquegoesforwardfurther .　　However ,someyearsago ,itwasfoundthatthedeformationofunder cooledaustenitecaninducefer ritetransformati…     

Ferrite Evolution During Isothermal Process in a High Deformability Pipeline Steel

 In order to ensure the safety of long-distance oil and natural gas transmission pipeline installed in seismic and/or permafrost region, high strength pipeline steel with excellent deformability has been developed. The ferrite and bainite dual phase pipeline steel is a very important kind of high deformability pipeline steel. Polygonal ferrite is a key microstructure in ferrite and bainite dual phase deformability pipeline steel. Ferrite evolution during isothermal process at 700 ℃ after 50% deformation at 800 ℃ was conducted by using a Gleeble-3800 thermal simulator, and microstructure was characterized by using an optical microscope, a scanning electron microscope and a transmission electron microscope. There are two types of ferrite, ferrite with high density dislocation and ferrite with a little dislocation. There is about 7% (volume percent) deformation induced ferrite (DIF) for compression of 50% at 800 ℃ and strain rate of 1 s-1. During the isothermal process at 700 ℃, with the holding time increasing, ferrite volume percent, ferrite grain number and average ferrite grain size increase. As the holding time is prolonged, dislocation recovery occurs in DIF. There are secondary phases in ferrite when the holding time is too long, and secondary phases and dislocation formation in dislocation pinning.     

The Effect of Pre-Deformation at Elevated Temperatures on the Microstructure and Hardness of Boron Steel 22MnB5

To investigate the effects of the pre-deformation and deformation temperature on the microstructure and hardness of boron steel 22MnB5,isothermal tensile tests were carried out on the Gleeble 3800 system when the temperatures are 700℃,750℃,800℃,respectively.Three specimens were employed to obtain different deformation levels at each temperature.The cooling rate before and after deformation was 30℃/s.Then the microstructures of the specimens were observed and the harness of each specimen was measured.The results show the martensitic fraction decreases and the ferrite fraction increases with decreasing the deformation temperature or increasing the deformation level.The specimens deformed at higher temperature have greater hardness.The hot plastic deformation of austenite shortens the incubation period of ferrite transformation and accelerates the transformation rate.In addition,the isothermally holding time is longer in the case of a larger deformation amount,more austenite will transform to ferrite.Especially,the specimen deformed at 700℃ and with large deformation has almost full massive ferrite microstructure with larger size.     

Effect of Niobium on Isothermal Transformation of Austenite to Ferrite in HSLA Low-Carbon Steel

Using thermomechanical simulation experiment,the kinetics of the isothermal transformation of austenite to ferrite in two HSLA low-carbon steels containing different amounts of niobium was investigated under the conditions of both deformation and undeformation.The results of optical microstructure observation and quantitative metallography analysis showed that the kinetics of the isothermal transformation of austenite to ferrite in lower niobium steel with and without deformation suggests a stage mechanism,wherein there exists a linear relationship between the logarithms of holding time and ferrite volume fraction according to Avrami equation,whereas the isothermal transformation of austenite to ferrite in high niobium steel proceeds via a two stage mechanism according to micrographs,wherein,the nucleation rate of ferrite in the initial stage of transformation is low,and in the second stage,the rate of transformation is high and the transformation of residual austenite to ferrite is rapidly complete.Using carbon extraction replica TEM,niobium carbide precipitation for different holding time was investigated and the results suggested that NbC precipitation and the presence of solute niobium would influence the transformation of austenite to ferrite.The mechanism of the effect of niobium on the isothermal transformation was discussed.     

Effect of Dissolved and Precipitated Niobium in Microalloyed Steel on Deformation Induced Ferrite Transformation （DIFT）

 Hot deformation processing was designed to study the effects of niobium (Nb) on DIFT. A prestrain of 051 at 880 ℃ for different isothermal time was used for adjusting the deformed austenite constitution and Nb existing state, followed by a secondary heavy deformation at 780 ℃ for inducing the ferrite transformation. The volume fraction and grain size of deformation induced ferrite (DIF) obtained at different isothermal time between double hits were investigated. It was found that Nb dissolved in austenite is adverse to DIFT; however, the precipitation of Nb is beneficial to DIFT. As Nb plays the role in the conventional TMCP, Nb retards the recrystallization of deformed austenite and enhances the deformation stored energy in the multipass deformation, and in result, Nb promotes DIFT.     

Nb的析出对变形诱导铁素体相变的影响

通过Gleeble2000热模拟实验机，研究了X65管线钢中Nb在变形奥氏体中的析出状态对变形诱导铁素体相变（DIFT）的影响。试验结果表明，在奥氏体临界区变形时，第一道次变形后，随变形后等温时间延长，诱导铁素体量变化不大。等温时间达120S时，变形奥氏体仍未发生再结晶。在道次间随时间延长，Nb的析出量增加，第二道次变形后诱导的铁素体也显著增加。微合金元素Nb通过碳氮化物的析出作用促进变形诱导铁素体相变。     

2205双相不锈钢连铸坯加热过程组织转变

对2205双相不锈钢连铸坯试样在1 220、1 240、1 260、1 280℃保温10、20、30和40 min进行加热处理,通过光学显微镜和铁素体仪试验分析2205双相不锈钢的组织和铁素体含量随保温时间和加热温度的变化情况.结果表明,2205双相不锈钢连铸坯试样在相同的保温时间下,随着加热温度的升高,铁素体含量逐渐增加,1 260℃时奥氏体晶粒明显变得粗大;在相同的加热温度下,随着保温时间的延长,铁素体的含量逐渐减少.     

Effect of ausforming on isothermal bainitic transformation and microstructure

Deformation affects the microstructure and morphology of the parent austenite, which affects the subsequent bainite transformation. The effects of ausforming on bainite transformation and microstructure were investigated by means of thermal simulation experiment, TEM and SEM etc. Different deformation temperatures and deformation strains were designed. The amount of bainitic transformation during isothermal holding and the volume fraction of retained austenite at room temperature were analyzed. The results show that the isothermal bainitic transformation is promoted by the deformation at 300?? and 400??. Moreover, the lower deformation temperature leads to larger amount of bainite. In addition, the volume fraction of retained austenite increases with the increase of the deformation strain, and more retained austenite can be obtained by decrease the deformation temperature. It indicates that deformation at lower temperature contributes to the mechanical stabilization of austenite.     

The isothermal decomposition of austenite in hot-rolled microalloyed steels

The isothermal decomposition of austenite has been examined in a set of 0.1 C, 1.4 Mn steels containing small amounts of Ti, V, or Nb. The volume fraction of ferrite was measured as a function of transformation temperature and holding time, after hot rolling. Precipitation of carbonitrides, in both the austenite and the ferrite, was examined by electron microscopy of extraction replicas. The decomposition is slowest in the Nb-alloyed steel, in which the start of transformation is delayed and ferrite growth rates are much lower than in the other steels. In the V-alloyed steels, ferrite growth rates are lower than in the plain carbon or Ti alloyed steels. These results are discussed in terms of the effects of carbonitride precipitation in the austenite during high temperature deformation and in the ferrite during transformation. The roles of V and Nb in solution are also considered.