低碳微量铌钢形变强化相变的组织演变

利用热模拟压缩变形实验研究了含Nb钢过冷奥氏体形变强化相变的组织演变规律,探讨了微量Nb对形变强化相变的影响,并对转变动力学和晶体取向进行了分析.结果表明,含Nb钢在A3-Ar3之间的形变过程中主要以形变强化相变为主.与低碳钢相比,含Nb钢形变强化相变的孕育期变长,完成相变所需的应变量也相应增加,使得其转变动力学曲线向高应变方向平移.含Nb钢的转变动力学曲线可划分成两个阶段,其中第一阶段固溶Nb阻碍铁素体相变,使孕育期推迟,而第二阶段形变过程中动态析出的Nb(C,N)既为铁素体相变提供了大量的形核位置,又钉扎铁索体晶界,阻碍铁索体晶粒的长大.在这种以形核为主的过程中出现了<111>和<001>两种织构的交替变化.     

Q235碳素钢应变强化相变的基本特点及影响因素

在热模拟单向压缩实验中,通过形变参数的变化考察了Q235碳素钢应变强化相变的基本规律及铁素体晶粒细化效果,结果表明,铁素体的超细化在热力学上是由于应变强化相变最大限度地提高了相变过冷度,在动力学上是由于形核集中在局部的高应变区,同时在转变过程中形变不断产生新的形核地点并抑制铁素体生长的结果,实现铁素体的超细化需要一最小变量及一定的应变速率,以使转变完毕并加抑制铁素伯的生长及形变成长条状,应变明显削弱了奥氏体晶粒尺寸的差异带来的铁素体尺寸的差异,应变造成的铁素体动态再结晶进一步细化了晶粒,这种特征是动态转变所特有的,此外,还比较了应变强化相变与无应变及传统近轧控冷铁素体形成时的差异。     

形变条件下过冷奥氏体组织转变特征

结合过冷奥氏体形变的真应力-真应变变化特征和显微组织观察,对Q235钢冷奥氏体的形变过程进行了深入分析,讨论了不同阶段的形变特点,并初步讨论了形变强化铁素体相变和铁素体动态再结晶在获得均匀、超细、等轴的铁素体组织方面所起的作用.     

变形温度对形变强化相变完成时临界应变量的影响

利用“形变强化相变”机制研究了低碳钢过冷奥氏体在740℃和780℃,10s-1变形时的变形温度对相变完成时临界应变量εc的影响。结果表明,变形温度对εc和组织演变的影响很大。在740℃和780℃变形时,εc分别为0.96和1.39,变形温度降低明显促进了相变。变形温度对εc的影响在组织演变上主要表现为铁素体形核地点的不同。740℃变形时,铁素体由奥氏体晶界形核过渡到以形变带形核为主,形核速率极高;780℃变形时,铁素体由奥氏体晶界形核过渡到在铁素体/奥氏体相界面前沿高畸变区快速形核。     

热变形对Fe-0.2C-2Mn合金γ→α转变动力学的影响及理论探讨

通过热处理实验和理论计算研究了热变形条件下Fe-0.2C-2Mn合金先共析铁素体转变动力学.金相观察表明,热变形细化铁索体组织,并使铁素体形貌趋于等轴状.在PLE/NPLE理论基础上,运用Pillbox模型和抛物线长大模型计算了变形前后铁素体的晶界形核率和长大系数,结果表明,过冷奥氏体变形促进NPLE模式下铁素体形核的主要原因是奥氏体晶界面积增加和元素扩散加快,而PLE模式下则是相变驱动力增大占主导.最后对比分析了热变形对形核和长大的影响程度,阐明了热变形细化不同温度区间转变的铁素体组织的机制.     

双相不锈钢2205热变形行为的实验

利用扫描电镜的EBSD技术,通过对单道次压缩热模拟实验后淬火试样组织的深入系统分析,研究了2205双相不锈钢热变形过程中的软化行为。研究结果表明,2205双相不锈钢的主要软化机制为铁素体的连续动态再结晶和奥氏体与铁素体之间的相转变,变形速率为主要影响参数,并通过影响应变在两相之间的分配控制组织在变形过程中的软化进程。变形速率很小时,铁素体的动态再结晶和铁素体向奥氏体的相转变是互为竞争的两个软化机制;随着变形速率的增加,主导软化机制转变为铁素体的动态再结晶和奥氏体相向铁素体相的转变。     

Dynamic Ferrite Transformation Behaviors in 6Ni-0.1C Steel

Phase transformation from austenite to ferrite is an important process to control the microstructures of steels. To obtain finer ferrite grains for enhancing its mechanical property, various thermomechanical processes followed by static ferrite transformation have been carried out for austenite phase. This article reviews the dynamic transformation (DT), in which ferrite transforms during deformation of austenite, in a 6Ni-0.1C steel recently studied by the authors. Softening of flow stress was caused by DT, and it was interpreted through a true stress–true strain curve analysis. This analysis predicted the formation of ferrite grains even above the Ae₃ temperature (ortho-equilibrium transformation temperature between austenite and ferrite), where austenite is stable thermodynamically, under some deformation conditions, and the occurrence of DT above Ae₃ was experimentally confirmed. Moreover, the change in ferrite grain size in DT was determined by deformation condition, i.e., deformation temperature and strain rate at a certain strain, and ultrafine ferrite grains with a mean grain size of 1 μm were obtained through DT with subsequent dynamic recrystallization of ferrite.     

低碳钢低温变形γ→α相变行为的预测模型

从变形使孕育期缩短的角度，讨论了形变诱导相变的发生条件，计算了不同变形条件下几种成分的低碳钢变形过程析出铁素体的开始温度Ar3d，发现同样变形条件下，碳含量越低的钢种，其Ar3d越高；同一成分钢种，随着变形量增加或变形速率减小，Ar3d提高，基于相变动力学理论，在形核速率计算中充分考虑变形和过冷的双重作用，探索了低温变形诱导铁素体相变的动力学模型，用该模型进行的计算机模拟结果和实验结果吻合良好，表明这种理论处理方法可用来模拟这种相变过程。     

INFLUENCE OF HOT DEFORMATION ON CONTINUOUS COOLING BAINITIC TRANSFORMATION IN A LOW CARBON STEEL

1.IntroductionRecently,greatprogresshasbeenmadeintheresearchofthebainiticmicrostructuralcharacteristicsandtransformationbehaviorfor(ultra)lowcarbonsteel,especiallyoncontinuouslycoolingtransformatio.[ll.Itisknownthatthemorphologyofbainiticstructurefromcontinuouslycoolingtransformationisverydifferentfromthatfromisothermalheatt...t...t[2--v].Fromtheviewpointofindustrialapplicationthattheultralowcarbonbainiticsteelareproduc,gdbythermomechanicalcontrolledprocess(TMCP),thestudyofcontinuouscoolingt…     

形变诱导铁素体的形成机制

在Ｇｌｅｅｂｌｅ２０００热模拟实验机上采用变形开始的同时喷水淬水和产减少变量的方法,对形变诱导铁素体的形成机制进行了一系列实验研究,实验研究表明,形变导致了铁素体的块型相变和针状铁素体的相变析出,同进在奥氏体温区形变诱导铁素体在变形后具有不稳定、自发逆相变的特点。     

Numerical simulation of dynamic strain-induced austenite–ferrite transformation in a low carbon steel

A modified cellular automaton modeling has been performed to investigate the dynamic strain-induced transformation (DSIT) from austenite (γ) to ferrite (α) in a low carbon steel. In this modeling, the γ–α transformation, ferrite dynamic recrystallization and the hot deformation were simulated simultaneously. The simulation provides an insight into the mechanism of the ferrite refinement during the DSIT. It is found that the refinement of ferrite grains derived from DSIT was the result of the increasing ferrite nuclei density by the “unsaturated” nucleation, the limited ferrite growth and the ferrite dynamic recrystallization. The effects of prior austenite grain size and strain rate on the microstructural evolution of the DSIT ferrite and the characteristics of the resultant microstructure are also discussed.     

Prediction of post-dynamic austenite-to-ferrite transformation and reverse transformation in a low-carbon steel by cellular automaton modeling

The post-dynamic transformation that takes place during the subsequent isothermal holding for the case when dynamic strain-induced transformation (DSIT) from austenite to ferrite occurs during hot deformation is investigated by cellular automaton modeling. The simulation provides a better understanding of carbon diffusion in retained austenite and the resulting microstructure evolution during the post-dynamic transformation. The predictions reveal that continuing transformation from retained austenite to ferrite and the reverse transformation can occur simultaneously in the same microstructure during post-deformation isothermal holding owing to the locally acting chemical equilibrium conditions. Competition between forward and reverse transformation exists during the early stage of post-dynamic heat treatment. It is also revealed that increasing the final strain of DSIT might promote the reverse transformation, whereas the continuous austenite-to-ferrite transformation yields a diminishing effect. The influence of the DSIT final strain on the grain size of ferrite and the characteristics of the resultant microstructure is also discussed.     

低碳钢形变诱导铁素体相变过程中碳的扩散行为

在Gleeble 1500热模拟机上对低碳钢Q235进行了热压缩实验，用电子探针分析了热变形试样微观组织中的碳浓度分布．结果表明，形变诱导铁素体晶粒中的碳含量明显过饱和．这表明在形变诱导铁素体相变过程中，碳没有发生明显的从铁素体向奥氏体扩散．从热力学的角度分析，在高于奥氏体-铁素体平衡转变温度Ae3变形，变形存储能的作用最终降低了体系相变后的自由能，使得在形变诱导铁素体相变过程中，碳无需发生从铁素体向奥氏体的扩散。     

Prediction of Carbon Concentration and Ferrite Volume Fraction of Hot-Rolled Steel Strip During Laminar Cooling

A phase transformation model was presented for predicting the phase fraction transformed and the carbon concentration in austenite for austenite to ferrite transformation during laminar cooling on run-out table in hot rolling strip mill. In this model, the parameter k in Avrami equation was developed for carbon steels. The wide range of chemical composition, the primary austenite grain size, and the retained strain were taken into account. It can be used to predict the ferrite volume fraction and the carbon concentration in austenite of hot-rolled steel strip during laminar cooling on run-out table. The coiling temperature controlling model was also presented to calculate the temperature of steel strip. The transformation kinetics of austenite to ferrite and the evolution of carbon concentration in austenite at different temperatures during cooling were investigated in the hot rolled Q235B strip for thickness of 9.35, 6.4, and 3.2mm. The ferrite volume fraction along the length of the strip was also calculated. The calculated ferrite volume fraction was compared with the log data from hot strip mill and the calculated results were in agreement with the experimental ones. The present study is a part of the prediction of the mechanical properties of hot-rolled steel strip, and it has already been used on-line and off-line in the hot strip mill.     

热变形条件下C-Mn钢奥氏体→铁素体相变模拟

为了定量考虑热变形对普碳钢奥氏体向铁素体转变的影响，计算了热变形的C－Mn钢中的位错密度和变形储存能。在计算变形奥氏体向铁素体的平衡转变温度时，将计算所得变形储存能加在母相γ的能量项中，从而使变形奥氏体向铁素体转变的平衡转变温度Ae3提高，在本工作的热变形条件下，变形储存能为10－20J/mol，使平衡转变温度Ae3提高10K左右，因而相同的冷却条件下奥氏体向铁素体转变的实际温度Ar3也会提高。从γ/α界面移动速度控制铁素体生长速度角度的计算表明，奥氏体中储存能ΔGdef使相变驱动力ΔG^γ→α增加，使铁素体的长大速度增加，加速奥氏体向铁素体的转变过程，但长大速度并未发生数量级的变化。在连续冷却相变模拟的过程中，利用超组元模型计算相变的平衡参数，计算结果与文献实验结果吻合良好。     

Deformation behavior of ferrite-austenite duplex high nitrogen steel

The tensile deformation behavior of ferrite-austenite duplex high nitrogen steel has been investigated by interrupted tensile tests and compared with that of fully austenitic high nitrogen steel. It shows that ferrite is softer than austenite and most of the strain in early and later stages of deformation is accommodated by ferrite, while austenite undergoes a deformation-induced martensitic transformation. Such accommodation of a large amount of strain in ferrite is responsible for rapidly increasing work hardening rate and the resultant higher ultimate tensile strength of duplex high nitrogen steel as compared to those of fully austenitic high nitrogen steel, although duplex steel contains a smaller amount of N than austenitic steel.     

形变温度对Q235碳素钢应变诱导相变的影响

研究了热模拟单向压缩条件下形变温度对Q235碳素钢应变诱导相变过程组织变化的影响,结果表明,铁素体可在A3以上较高的温度下因形变而存在,大应变下该钢不存在单纯的形变奥氏体状态,随形变温度的降低,组织变化的规律为由奥氏体的动态再结晶为主,过渡到奥氏体动态再结晶与铁素体的诱导析出同时进行,再过渡为铁素体的析出与铁素体的动态相继进行的过程,碳素钢热加工过程在微观上实质是动态复合转变过程,奥氏体的动态再结晶影响了铁素体的形态、分布与细化效果,高温形变后的保温导致铁素体向奥氏体的逆转变。     

变形条件对Mn-Cr齿轮钢连续冷却相变的影响

使用Gleeble 1500热模拟实验机研究了Mn—Cr齿轮钢在奥氏体再结晶区不同温度变形后的连续冷却相变行为及相变组织．实验结果表明,变形温度降低,促进了多边形铁索体及珠光体相变,获得多边形铁索体加珠光体混合组织的临界冷速增大．贝氏体与针状铁索体之间存在相互竞争机制,随着变形温度及冷速的降低,大量的晶界仿晶型铁索体占据了奥氏体晶界,中温相变产物由贝氏体向针状铁索体转变．降低变形温度,奥氏体在中温相变区稳定性增加,相变结束温度下降,室温组织中马氏体／奥氏体岛的数量增多．     

Dynamic Phase Transformation and Spheroidization of Cementite of Hypereutectoid Steel Containing Aluminum during Deformation

利用Gleeble 1500热模拟试验机进行单轴热压缩实验, 研究了合金元素Al对过共析钢缓冷相变和 过冷奥氏体动态相变组织的影响. 结果表明: 在缓冷相变时, Al的加入抑制网状渗碳体形成, 细化珠光体 片层间距; 在过冷奥氏体形变过程中, 动态转变经历动态相变和相变所得珠光体中渗碳体球化及铁素体动 态再结晶等过程. 在动态相变过程中, 没有形成晶界网状渗碳体, 而直接产生珠光体. Al的加入使动态相变过程中奥氏体的稳定性提高、珠光体转变推迟, 进一步细化了珠光体片层间距. 在相变所得珠光体中渗碳体球化及铁素体动态再结晶的过程中, Al阻碍渗碳体粗化, 使渗碳体颗粒和铁素体晶粒尺寸细化.     

18.7Cr-1.0Ni-5.8Mn-0.2N节Ni型双相不锈钢的大变形热压缩软化行为

在1123~1423 K、0.1~10 s^-1条件下对18.7Cr-1.0Ni-5.8Mn-0.2N节Ni型双相不锈钢进行70%大变形量热压缩研究。利用OM、SEM和EBSD分析热变形组织。结果表明,铁素体动态再结晶(DRX)主要发生在1123 K较低变形温度,随应变速率增大,晶粒细化程度增加,晶粒不均匀程度减小。应变速率对铁素体DRX影响较大,而奥氏体DRX对变形温度更加敏感。在1223 K、10 s^-1条件下,铁素体相发生了以小角度晶界(LAGB)向大角度晶界(HAGB)转变的连续动态再结晶(CDRX),而在1323 K、0.1 s^-1条件下,奥氏体相以不连续动态再结晶(DDRX)为主。低应变速率条件下升高温度易诱发DDRX,而在高应变速率条件下易发生CDRX。在高温低应变条件下,奥氏体相晶粒取向主要为(001)和(111)再结晶织构,而铁素体相在(001)和(111)织构之间存在竞争关系。拟合获得临界应力(应变)并确定了其与峰值应力(应变)的关系。随着应变增加,热加工失稳区缩小,且稳定区逐渐向高温高应变速率方向移动,1323~1423 K、0.01~6.05 s^-1的热参数条件最适合热加工。