奥氏体变形及其对连续冷却相变的影响

利用THERMECMASTOR-Z型热模拟实验机，研究了低碳Mn-Nb-Ti微合金钢在高温下的奥氏体变形及其在连续冷却过程中的相变特征，建立了一定变形条件下的动态CCT曲线。实验结果表明，该钢种再结晶温度为950～980℃。当总的变形程度相同时，在相同的冷却速度下，再结晶温度以下累积变形程度增加将对变形后组织的开始转变温度有一定影响。随着冷却速度的提高，相变温度下降。     

Al-Zn-Mg-Sc合金连续冷却转变曲线的测定

测定了Al-Zn-Mg-Sc合金固溶处理后的连续冷却转变(CCT)曲线,通过动态电阻法测得冷却过程的电阻-温度曲线,根据曲线斜率的变化规律确定相变开始点、结束点以及临界冷却速度范围,绘制出该合金的CCT图,通过扫描电镜和透射电镜分析观察连续冷却过程的组织转变.结果表明,动态电阻法测得的CCT图是可信的;在470℃,保温1 h固溶处理后,抑制相变发生的临界冷速在2 168.0℃/min以下,但高于716.8℃/min,相变主要集中在150~420℃的温度区间发生;当冷却速度较慢时,平衡相η在晶内和晶界大量析出并逐渐长大和粗化,当冷却速度较快时,合金保持了较高的过饱和度,冷却到70℃以下仍有相变发生.     

稳恒磁场对低碳锰铌钢γ→α相变的影响

用X－Y记录仪测定了低碳锰铌钢在稳恒磁场下的温降曲线,分析标定了奥氏体向铁素体转变的起始相变点,通过对空冷至不同温度后淬火试样金相显微组织的分析,研究了稳恒磁场对γ→α相变的影响。研究结果表明：在奥氏体化后的空冷过程中加入稳恒磁场,可以提高奥氏体向铁素体相变的起始温度。随着磁通密度的增大,铁素体相变点也随着提高,而且铁素体晶料得到了显的细化;稳恒磁场下小幅度的冷却速度变化对铁素体相变点的影响不大,但对铁素体晶粒长大过程的影响较大。     

Q345E钢冷却过程中相变的研究

通过模拟试验,测定了Q345E钢的CCT图,确定了不同冷却条件下钢中相变的开始点和终了点,研究了变形量对转变开始点的影响,发现高于再结晶温度时转变开始温度随着变形量的增加而升高,这是变形诱导相变影响的结果。文中还用CCT图曲线对现场冷却过程的相变进行了模拟分析。     

冷却速度对ULCB钢相变和组织结构的影响

用工业性生产的ULCB钢进行热模拟实验,在非再结晶区变形后驰豫降温到740 ℃,再以不同的冷却速度冷却,研究了冷却速度对ULCB钢的贝氏体转变点和组织结构特征的影响.结果表明,在20 ℃/s以上的快速冷却时,贝氏体相变温度较低而温度区间较宽,发生大量贝氏体相变,得到的组织主要为板条贝氏体.随着冷却速度的降低,贝氏体相变点逐渐升高,相变温度区间变窄,得到板条贝氏体和粒状贝氏体的混合组织.冷速低于3 ℃/s时,相变点快速升高,开始点达660 ℃以上,组织中出现较多的多边形铁素体,此时的相变开始点已不是贝氏体相变点,而是铁素体相变点.     

高碳钢82B不同连续冷却条件下基体相变行为研究

采用Formastor-FII全自动相变仪对高碳钢82B盘条进行连续冷却试验,采用膨胀法测得钢的临界相变点。对不同冷却速度下的试样,利用光学显微镜来观察试样组织,并用显微维氏硬度仪测定维氏硬度,根据测定的相变点绘出试验钢82B静态CCT曲线。结果表明,最佳冷却速度不应超过5℃/s,该条件下可避免产生马氏体组织,得到大量珠光体和索氏体组织,有利于改善盘条的拉拔加工性能。     

20CrMnTiH连续冷却相变预测模型

采用DIL805型淬火变形膨胀仪测定了三种不同成分20CrMnTiH实验钢在不同冷却速度下的热膨胀曲线,对室温显微组织进行观察,并绘制连续冷却转变（CCT）曲线。实验结果表明：成分波动主要影响20CrMnTiH钢冷却转变过程中贝氏体与马氏体相变冷却区间,对临界相变温度影响较小。采用K-M方程拟合了三种实验钢的马氏体相变动力学参数。结合优化的Li经验模型及临界转变温度的回归关系式,建立了20CrMnTiH钢在连续冷却过程中的铁素体、珠光体与贝氏体的相变预测模型,成功预测了成分波动对实验钢CCT曲线的影响。进而,采用有限元分析方法建立了20CrMnTiH钢端淬仿真模型,较好地预测成分波动对实验钢淬透性的影响,此方法可为齿轮钢的成分优化设计与合理选材提供参考。     

工程机械用超高强钢的相变行为研究

借助MMS-200热模拟试验机研究了Cr-Mo-Ni-B系工程机械用超高强钢连续冷却条件下的相变行为,通过热膨胀曲线、光学显微镜(OM)和扫描电镜(SEM)分析了冷却速度(0.5～40℃/s)对其相变温度和微观组织的影响。结果表明,随着冷却速度增大,钢相变温度B_s、B_f、M_s、M_f均降低,中低温相变加强。冷却速度在2℃/s以下时,发生珠光体相变和贝氏体相变;冷却速度在2～5℃/s时,出现粒状贝氏体和板条马氏体的混合组织;冷却速度在5℃/s以上时,粒状贝氏体消失,微观组织为单一的板条马氏体。在中低温相变组织形成温度范围内,冷却速度对M-A岛的形貌、尺寸、数量以及马氏体板条宽度有显著的影响。随着冷速的增大,M-A岛的形貌由块状向颗粒状变化,其尺寸减小,数量增多;马氏体板条的平均宽度减小。     

Nb-Ti微合金钢连续冷却相变的研究

利用Gleeble-2000热模拟实验机,测定了Nb-Ti微合金钢变形奥氏体的CCT曲线,研究了冷却速度对连续冷却相变及显微组织的影响,利用碳萃取复型法对第二相析出物进行了分析。结果表明,随冷却速度提高,Nb-Ti微合金钢相变温度降低,组织细化,铁素体的形貌从多边形逐渐向针状转变;随冷却速度的增加,第二相析出物增多且变得细小。     

热变形参数对含铌钢奥氏体未再结晶区变形相变的影响

利用Thermecomastor-Z热模拟试验机,研究了铌质量分数为0.128%的微合金低碳钢在奥氏体未再结晶区变形及连续冷却过程的相变,分析了变形温度、变形速率、变形量等热变形参数对相变的影响规律.研究表明,在连续冷却条件下,随着冷却速度的增加,相转变开始点降低,当冷却速率大于5℃/s时,90%以上的组织为粒状贝氏体.变形温度的升高、变形量及变形速率的增加对相变有促进作用.     

氧化铈掺杂氧化锆的等温和非等温马氏体相变

Martensitic transformation behavior was studied for zirconia containing 4%～10% CeO2 (in mole fraction) by using a dilatometric method. The Ms (Martensite start temperature) decreased near linearly with increasing CeO2. Different transformation modes were observed depending on the composition and cooling rate. ZrO2 containing 6% CeO2 showed isothermal transformation behavior, whereas ZrO2 containing 9% and 10% CeO2 showed athermal transformation behavior. However, ZrO2 containing 8% CeO2 showed either isothermal or athermal transformations behavior depending on the cooling rate. A TTT (Time-Temperature-Transformation) diagram was proposed for ZrO2 containing 8% CeO2.     

钼、铬对低碳铌钛微合金钢连续冷却转变行为的影响

 为研究合金元素钼、铬对低碳铌钛微合金钢连续冷却转变行为的影响,采用Gleeble-3800热模拟试验机和热膨胀试验方法,测定了钼、铬含量不同的3种低碳铌钛成分微合金钢在不同冷却速度下的相变点,采用光学显微镜及扫描电子显微镜观察了其转变产物的微观组织,同时结合维氏硬度测试,绘制了动态CCT曲线。结果表明,钼和铬均降低奥氏体向针状铁素体转变的相变温度,并且在冷速大于1℃/s时,钼比铬的作用效果更加明显。钼、铬均能抑制先共析铁素体和珠光体的转变,扩大针状铁素体形成冷速范围,并能够显著细化组织。     

热轧590MPa级车轮钢的奥氏体相变行为

 利用热力模拟试验技术,研究一种Nb-V-Ti复合微合金化C-Mn钢的奥氏体连续冷却相变行为,为低成本高性能热轧590MPa级车轮钢的控制轧制和控制冷却工艺制定提供必要的理论依据。研究表明：无形变条件下,铁素体转变存在的冷却速率范围为0. 5~5℃/s,珠光体转变存在的冷却速率范围为0. 5~2℃/s;形变条件下,铁素体转变存在的冷却速率范围为0. 5~25℃/s,珠光体转变存在的冷却速率范围为0. 5~10℃/s;不论是否存在形变,贝氏体转变存在于整个冷却速率范围(0. 5~30℃/s);奥氏体区形变增加了奥氏体内部的缺陷密度,促进了非均匀形核的发生,故形变促进了铁素体转变;由于试验钢的碳的质量分数较低(＜0. 10%),形变通过促进铁素体相变而间接促进珠光体相变;当贝氏体相变前无铁素体相变时,形变对贝氏体相变有促进作用;试验钢在实际热轧试验中冷却速率宜控制在20℃/s左右,卷取温度控制在550~650℃。     

不同硼含量微合金钢的连续冷却相变行为

 利用热模拟试验技术对实验室制备的含硼微合金钢连续冷却转变形为进行了试验研究,利用光学显微镜研究冷却速度、变形对试验钢显微组织的影响,探讨了硼对转变行为的影响规律。结果表明：适量硼延缓多边形铁素体生成,有利于获得贝氏体组织;无硼及wB=00020%时,分别在1~25及05~25℃/s的冷速都能得到贝氏体组织;wB=00030%时,冷速在2℃/s 以上能得到贝氏体组织;与未变形相比,变形导致试验钢贝氏体冷速区间变窄。在同一冷速下,随硼含量增加贝氏体开始转变温度先降低再升高,显微硬度随硼含量增加先增加而后降低。     

Study on transformation behavior of hot- rolled microalloyed TRIP steel

Effects of deformation mode, deformation temperature, deformation rate, cooling rate and slow- cooling stop temperature on the transformation behavior of hot- rolled microalloyed TRIP steel were studied by means of MMS- 300 thermomechanical simulator. The results show that for the samples subjected to the single or double pass deformation, ferrite transformation area is expanded, pearlite transformation area appears, and martensite transformation area disappears in the continuous cooling transformation diagrams. Transformation temperatures of Ar3, Bs and Bf decrease, diffusional transformation is prevented and intermediate temperature transformation is promoted with the increase of deformation temperature or cooling rate. When deformation temperature is 850??, transformation temperatures of Ar3, Bs and Bf increase, the amount of ferrite also increases, and the amount of bainite decreases in the microstructure with the increase of deformation rate. With the decrease of slow- cooling stop temperature, ferrite amount increases, ferrite grains grow and retained austenite amount first increases and then decreases.     

热处理对AISI4340钢临界点及CCT曲线的影响

 采用热膨胀法测定了AISI4340钢的固态相变点,观察了组织和硬度,研究了不同热处理状态对临界点和奥氏体连续冷却转变曲线（CCT曲线）的影响。结果表明,试验钢具有较好的淬透性,冷速为0.03 ℃/s 时得到单一贝氏体组织,冷速为0.03～0.78 ℃/s之间得到马氏体和贝氏体混合组织,冷速大于0.78 ℃/s时相变组织都为马氏体。不同热处理状态对ATSI4340钢的[Ac1、][Ac3]有影响,而CCT曲线没有明显差别。弥散分布、介稳定的组织具有较低的[Ac1,]原始组织对[Ac1]的影响比[Ac3]大。     

过共析钢线材组织与性能的控制

采用热／力物理模拟和微观分析相结合的方法研究了过共析钢在连续冷却过程中的固态相变和热加工工艺对钢的组织性能影响,结果表明,变形温度降低120℃,引起相变温度升高22℃;冷却速度提高到15℃／s,使得相变温度下降了90℃;提高变形温度有利于增加抗拉强度,但断面收缩率呈现下降趋势。试验结果用于生产过程优化和控制,使线材的合格率达到95%以上。     

不同冷却速率下X70管线钢铁素体相变的模拟

为更精确地控制及优化X70管线钢的目标组织,以经典相变理论模型为基础,建立了先共析铁素体周围的临界碳浓度与原奥氏体的碳浓度之间的数学模型,并采用逆向回归法确定了铁素体相变分数的关键性参数,经试验验证,模型具有良好的精度。结果表明：临界碳浓度满足C^k=1．8,关系;铁素体相变分数的关键性参数m=1．3,b1=0．026...     

Isothermal and athermal type martensitic transformations in yttria doped zirconia

The phase transformation from the high temperature tetragonal phase to the low temperature monoclinic phase of zirconia had been long considered to be a typical athermal martensitic transformation until it was recently identified to be a fast isothermal transformation. The isothermal nature becomes more apparent when a stabilizing oxide, such as yttria, is doped, by which the transformation temperature is reduced and accordingly the transformation rate becomes low.Thus it becomes easy to experimentally establish a C-curve nature in a TTT (Time-Temperature-Transformation) diagram. The C-curve approaches that of well known isothermal transformation of Y-TZP (Yttria Doped Tetragonal Zirconia Polycrystals), which typically contains 3mol% of Y2O3. In principle, an isothermal transformation can be suppressed by a rapid cooling so that the cooling curve avoids intersecting the C-curve in TTT diagram. Y-TZP is the case, where the stability of the metastable tetragonal phase is relatively high and thus the tetragonal phase persists even at the liquid nitrogen temperature. On the other hand, the high temperature tetragonal phase of pure zirconia can never be quenched-in at room temperature by a rapid cooling; instead it always turns into monoclinic phase at room temperature. This suggests the occurrence of an athermal transformation after escaping the isothermal transformation, provided the cooling rate was fast enough to suppress the isothermal transformation. Thus, with an intermediate yttria composition, it would be possible to obtain the tetragonal phase which is not only metastable at room temperature but athermally transforms into the monoclinic phase by subzero cooling. The objective of the present work is to show that, with a certain range of yttria content, the tetragonal phase can be quenched in at room temperature and undergoes isothermal transformation and athermal transformation depending on being heated at a moderate temperature or under-cooied below room temperature. Because both of the product phases are essentially the same monoclinic phase, both transformations are regarded as martensitic transformation, i. e. isothermal and athermal martensite. In some steels such as Fe-Mn-Ni and Fe-Ni-C, the occurrence of both isothermal and alhermal martensitic transformations has been reported. However, in these cases, the isothermal transformation occurs at temperatures slightly above the Ms (Martensite start) temperatures, and thus these transformations are considered to conform the same C-curve. On the other hand, the Ms temperature of the present material is well below the C-curve, which suggests that completely different mechanisms are controlling the kinetics of these two modes of transformations. Other aspects on these transformations are also to be reported..     

添加Y2O3的ZrO2的等温和变温马氏体相变

The phase transformation from the high temperature tetragonal phase to the low temperature monoclinic phase of zirconia had been long considered to be a typical athermal martensitic transformation until it was recently identified to be a fast isothermal transformation. The isothermal nature becomes more apparent when a stabilizing oxide, such as yttria, is doped, by which the transformation temperature is reduced and accordingly the transformation rate becomes low.Thus it becomes easy to experimentally establish a C-curve nature in a TTT (Time-Temperature-Transformation) diagram. The C-curve approaches that of well known isothermal transformation of Y-TZP (Yttria Doped Tetragonal Zirconia Polycrystals), which typically contains 3mol% of Y2O3.In principle, an isothermal transformation can be suppressed by a rapid cooling so that the cooling curve avoids intersecting the C-curve in TTT diagram. Y-TZP is the case, where the stability of the metastable tetragonal phase is relatively high and thus the tetragonal phase persists even at the liquid nitrogen temperature. On the other hand, the high temperature tetragonal phase of pure zirconia can never be quenched-in at room temperature by a rapid cooling; instead it always turns into monoclinic phase at room temperature. This suggests the occurrence of an athermal transformation after escaping the isothermal transformation, provided the cooling rate was fast enough to suppress the isothermal transformation. Thus, with an intermediate yttria composition, it would be possible to obtain the tetragonal phase which is not only metastable at room temperature but athermally transforms into the monoclinic phase by subzero cooling.The objective of the present work is to show that, with a certain range of yttria content, the tetragonal phase can be quenched in at room temperature and undergoes isothermal transformation and athermal transformation depending on being heated at a moderate temperature or under-cooled below room temperature. Because both of the product phases are essentially the same monoclinic phase, both transformations are regarded as martensitic transformation, i. e. isothermal and athermal martensite. In some steels such as Fe-Mn-Ni and Fe-Ni-C, the occurrence of both isothermal and athermal martensitic transformations has been reported. However, in these cases, the isothermal transformation occurs at temperatures slightly above the Ms (Martensite start) temperatures, and thus these transformations are considered to conform the same C-curve. On the other hand, the Ms temperature of the present material is well below the C-curve, which suggests that completely different mechanisms are controlling the kinetics of these two modes of transformations. Other aspects on these transformations are also to be reported..