The recrystallization and precipitates of deformed austenite for three Ti-microalloyed low carbon steels with different amount of Zr and Ti in the temperature range of 950°C to 1050°C were investigated by means of multi-pass compression test to simulate the actual rolling situation, in order to acquire the optimum deformation temperature for the alloys. The results show that the increase of Ti content will retard the occurrence of the recrystallization of deformed austenite and the recrystallization grain growth in Ti-microalloyed steels, while the addition of Zr will also retard the occurrence of recrystallization of deformed austenite in Ti-microalloy steel, and inhibit the growth of recrystallization grains. Besides, the addition of Zr increases the number of precipitates in Ti-microalloyed steel, and improves the size uniformity of precipitated phase, so that the Ti-Zr microalloyed steel consists of a relatively uniform austenite microstructure. When the deformation temperature is 1000℃, the Ti-Zr microalloyed steel has the finest uniform austenite microstructure.

通过多道次模拟压缩实验，研究不同Zr和Ti含量的三种Ti微合金化低碳钢在950℃~1050℃形变奥氏体再结晶和析出相的变化和最佳变形温度。结果表明，Ti含量的提高和Zr的加入使Ti微合金钢形变奥氏体的再结晶和晶粒长大延迟。Zr的加入还能增加Ti微合金钢中析出相的数量、改善析出相尺寸分布的均匀性进而得到相对均匀的奥氏体组织。变形温度为1000℃时的Ti-Zr微合金钢奥氏体组织最细小均匀。

A novel secondary hardened steel 25CrMo3NiTiVNbZr was developed via optimization the chemical composition of the 25Cr3Mo3NiNbZr steel bymeans of the so called JMatpro and Thermo-Calc software. Then the microstructure, phase composition, morphology of precipitates and mechanical performance at 700℃ of the 25CrMo3NiTiVNbZr steel were characterized by means of transmission electron microscopy (TEM), electron backscattering diffraction (EBSD), while its the high-temperature streng-thening mechanism was also elucidated. The results show that the tensile strength and yield strength of 25CrMo3NiTiVNbZr steel at 700oC are 543 MPa and 409 MPa, respectively, which are 139 MPa and 123 MPa higher than that of 25Cr3Mo3NiNbZr steel. The high-temperature strengthening of the 25CrMo3NiTiVNbZr steel may mainly ascribed to themechanism, precipitation strengthening. In the process of being stretched at 700oC, the precipitation strengthening increment derived from the precipitation of the strengthening phase for the 25CrMo3NiTiVNbZr steel reached 367 MPa, which was 147 MPa higher than that of the 25Cr3Mo3NiNbZr steel. This is mainly due to the higher thermal stability and smaller size (average size at 7.93 nm) of the precipitated strengthening phases in the 25CrMo3NiTiVNbZr steel. After high-temperature stretching, the average size of precipitates remained at 8.14 nm.

使用JMatpro和Thermo-Calc软件优化25Cr3Mo3NiNbZr钢的成分，使用透射电镜(TEM)、电子背散射衍射(EBSD)和相分析等手段表征在700℃高温拉伸前后试样的显微组织和析出相以研究其高温强化机理。结果表明：25CrMo3NiTiVNbZr钢700℃的高温抗拉强度和屈服强度分别为543 MPa、409 MPa，比25Cr3Mo3NiNbZr钢提高了139 MPa和123 MPa。根据对这种钢高温强化机理的分析，析出强化是主要强化方式。在700℃高温拉伸25CrMo3NiTiVNbZr钢其析出强化增量为367 MPa，比25Cr3Mo3NiNbZr钢提高147 MPa，其主要原因是在25CrMo3NiTiVNbZr中析出了热稳定性更高、尺寸更小(平均尺寸为7.93 nm)的析出相。

Based on Thermo-Calc calculation, a nanostructured bainite steel, which is suitable for making steel wire, with composition of Fe-0.8C-2Mn-1.5Si-1.5Cr-0.25Mo-0.25Ni-1Al-0.25Co-0.1V was designed. The influence of austempering treatment on microstructure and mechanical properties of the steel was investigated by means of scanning electron microscope (SEM), X-ray diffractometer (XRD), transmission electron microscope (TEM), thermal dilatometer, and tensile tester in terms of microstructures, thermal and mechanical properties etc. Results show that after austempering at lower temperature, the microstructure of the nanostructured bainitic steel is composed of nanostructured bainite ferrite lath, retained austenite and a little proportion of martensite. With the increase of austempering temperature, the transformation rate and the volume fraction of bainite ferrite increase. As the austempering time increases, the volume fraction of bainite ferrite increases, while the amount of undercooled austenite decreases, and the size and volume fraction of the massive M/A islands obtained at room temperature decrease. Due to the carbon partitioning sufficiently between the bainite ferrite and austenite, the stability of undercooled austenite is improved, and the proportion of brittle martensite in M/A islands is reduced significantly. This induces the transition of tensile fracture from mixed fracture to quasi-cleavage fracture. After being heated at 230oC for 48 hours, the tensile strength and yield strength of the experimental steel were 1625 and 1505 MPa, respectively, with an elongation of 34.5%, indicating the best match for strength and toughness.

依据Thermo-Calc计算设计了一种成分为Fe-0.8C-2Mn-1.5Si-1.5Cr-0.25Mo-0.25Ni-1Al-0.25Co-0.1V可用于制造钢丝的纳米贝氏体钢，使用热膨胀相变仪、扫描电镜(SEM)、X射线衍射(XRD)、透射电镜(TEM)和拉伸实验等手段研究了等温淬火温度和时间对其组织和力学性能影响。结果表明：这种纳米贝氏体钢低温等温淬火后的组织，由纳米结构的贝氏体铁素体板条、残余奥氏体和少量的马氏体组成。随着等温淬火温度的提高相变速率随之提高，贝氏体铁素体的体积分数增大。随着等温淬火时间的延长，贝氏体铁素体的体积分数增大而过冷奥氏体的量减少，在室温下生成的块状M/A岛的尺寸减小和体积分数降低，碳的配分使过冷奥氏体的稳定性提高，M/A岛中的脆性马氏体比例大幅度降低，拉伸断口由混合型断裂向准解理断裂转变。将这种钢在230℃保温48 h后强塑性匹配最佳，其抗拉强度和屈服强度分别达到1625和1505 MPa，延伸率达到34.5%。

Nanoscale co-precipitation strengthening in steels has attracted increasing attention in recent years and has become a new cornerstone for the development of advanced high performance steels with superior combination of strength and ductility. Rolling process, finishing temperature, cooling rate and coiling temperature are the main factors which affect the mechanical properties of microalloyed steels by changing the volume fraction and particle size of precipitates. Nevertheless, the influence of cooling rate on microstructure evolution, precipitates and mechanical properties of complex microalloyed ferritic Ti-V-Mo steel has been rarely reported. In this work, the precipitation law of (Ti, V, Mo)C carbides at different cooling rates and its effect on microstructue evolution and mechanical properties of Ti-V-Mo complex miroalloyed steel were studied by OM, EBSD, HRTEM and Vickers-hardness test. The results indicated that the hardness first increased quickly and then increased slowly as the cooling rate increased (lower than 20 ℃/s); the mean size of (Ti, V, Mo)C particles decreased from 13.2 nm to 6.9 nm and the average size of ferrite grain reduced from 5.06 μm to 2.97 μm; the hardness of Ti-V-Mo steel was improved by the means of grain refinement hardening and precipitation hardening. However, when the cooling rate increased from 20 ℃/s to 30 ℃/s, its effects on grain refinement hardening and precipitation hardening has become saturated, so the hardness was kept flat and achieved a maximum vlaue of 410 HV, and the yield strength reached as high as 1090 MPa. The hardness y of Ti-V-Mo microalloyed steel and cooling rates x accord with a exponential decay relationship: y=-229exp(-x/5)+412.

With the increasing cleanliness of steels in recent years, it has made possible for making micro-alloyed steels with rare earth elements. It is found that the addition of rare earths has a significant effect on the solid-state phase transformation behavior of steels, especially for low-carbon low-alloy steels. The effectiveness in modifying the inclusions and inducing nucleation by RE addition has been studied intensively and approved substantially. Whereas, the micro-alloying effect of RE on the ferrite phase transformation of steel is still unclear. The effect of rare earth elements (RE) on austenite-ferrite transformation temperature during continuous cooling, and the isothermal transformation kinetics of Fe-C alloys and Fe-C-Si-Mn low carbon steels has been investigated in this article. It is found that a tiny amount of RE addition can reduce the starting point temperature of proeutectoid ferrite during continuous cooling. Additionally, the addition of RE also changes the ferrite transformation kinetics in the isothermal process: for Fe-C-(RE) alloys, the addition of RE slows down the transformation rate during the whole transformation process due to the pinning carbon diffusion effect; For Fe-C-Si-Mn alloys, RE elements can play a double role of pinning carbon diffusion and changing grain boundary energy, and then prolong the incubation period and decreases the rate of phase transformation during the initial stage, while increase the phase transformation rate during the middle and late of phase transformation.

Aluminum (Al) alloys, a typical lightweight material, are limited to applications at temperatures below about 200 oC. The high-temperature range of 300-400 oC has been a longstanding bottleneck for traditional Al alloys. In this study, the underlying mechanisms of this service bottleneck are first discussed, and key scientific solutions aimed at overcoming the bottleneck are proposed. A new microstructure designing strategy is proposed to develop advanced heat-resistant Al alloys through phase transformation that couples rapidly diffusing solute atoms with slowly diffusing ones. This strategy leads to three design approaches for thermal stability: (1) interfacial solute segregation at the nanoprecipitate/matrix interfaces, (2) interstitial solute ordering within the coherent nanoprecipitates, and (3) multiple interfacial coherency coupling with multiscale microstructural features. By manipulating the microalloying effect at the atomic length scale, a series of 300-400 oC heat-resistant Al alloys were developed. Furthermore, the potential development directions of the heat-resistant Al alloys are also explored as possible references for future work.

利用透射电镜对含Ti微合金钢热轧及回火后的析出相分布、形貌和尺寸进行了观察和分析，结合拉伸试验结果研究了回火过程中纳米析出颗粒的变化特征及其对试验钢强度变化的影响。结果表明,600℃回火2h试验钢的屈服强度达到最大值，这是由于除了热轧后空冷形成的高体积分数相间析出以外，回火过程中又形成了大量的过饱和析出，此时纳米析出相体积分数较其他温度增加约3%～5%，颗粒平均尺寸为5～6nm，分布最均匀。随着回火温度的升高和回火时间的延长，2类析出颗粒均发生了不同程度的粗化，相间析出颗粒尺寸均在8nm以下，较过饱和析出颗粒具有较好的热稳定性，析出强化作用更明显。

Among various hardening factors of steels, precipitation hardening has the least embrittlement vector value except grain refinement hardening. Giving full play to the precipitation hardening of microalloyed carbonitrides is an important aspect in the development of microalloyed high strength steels. Recently, the research on behaviors of precipitation and development of microalloyed high strength steels is mainly focused on these relatively simple microalloyed steels including single V, single Ti, Ti-V and Ti-Mo microalloyed steels, while paid less attention on complex microalloyed steels such as Ti-V-Mo steels. Therefore, it is expected to provide a theoretical basis and a practical significance for the development of Ti-V-Mo microalloyed high strength steel. Various hardening increments at different coiling temperatures were calculated. Meanwhile, the effect of coiling temperatures on yield strength and the influence of MC particles on uniform elongation were discussed by means of OM, EBSD, TEM, XRD and physical-chemical phase analysis. The results show that Ti-V-Mo steel has the best mechanical properties with ultimate tensile strength of 1134 MPa, yield strength of 1080 MPa, elongation of 13.2% and uniform elongation of 6.8% at coiling temperature of 600 ℃. The precipitation hardening increment was high to about 444~487 MPa due to the mass fraction of about 72.6% of total precipitates with a size of ?10 nm. In addition, precipitation hardening and grain refinement hardening are the main mechanisms to improve the strength of Ti-V-Mo steel, while the variation in precipitation hardening increment causes a significent difference in yield strength. With the coiling temperature increases from 500 ℃ to 600 ℃, the ultimate tensile strength and yield strength increase continuously, but the uniform elongation increases slowly instead of decreasing, which is mainly attributed to an increase of precipitation hardening increment.