Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： As an important carbide forming element, Nb plays an important role in steel. Precipitated Nb can restrain the austenite grain growth during soaking process and provide precipitation strengthening after g /a phase transformation. Precipitated or dissolved Nb can inhibit recrystallizaton of deformed austenite. Recently, both Nb and Mo are added in steel to enhance the role of Nb. However, these kinds of researches mostly focused on continual cooling process of g /a transformation or isothermal process during tempering, and precipitation behavior of MCtype carbide in steel containing Nb and Mo during reheating process and the effect of Mo on precipitation of NbC in ferrite were rarely reported. Therefore, in this work, precipitation behaviors of MC-type carbide and the synergistic effect of Nb and Mo in steel containing Nb or Nb-Mo during reheating process at the heat rate 20 ℃/min were investigated by means of Vickers hardness test, SEM, HRTEM and DSC. The results show that both Nb and Nb-Mo steels have hardness peaks at 300 and 700 ℃, which are attributed to the precipitation of e-carbide and MCtype carbide, respectively. The MC-type carbide precipitates at about 650 ℃ during reheating process, which is in a good agreement with the nose temperature of MC-type carbide calculated by Avrami equation. (Nb, Mo)C particle forming in Nb-Mo steel during precipitation has a small mismatch with ferrite matrix compared with NbC, leading to the decrease of interfacial energy. Thus, the precipitation kinetic of MC-type carbide in Nb-Mo steel is faster than that in Nb steel, which results in the denser and finer MC-type carbide and higher precipitation strengthening effect.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： With the development of Ti microalloying technology, the application and theory research of Ti in microalloyed steels are becoming more deeply and widely. However, the effect of tempering time on the microstructure and mechanical properties of high Ti microalloyed quenched martensitic steel has been rarely touched upon, meanwhile, it has long been inconclusive whether precipitated phases coarsening or the recovery and softening of martensitic matrix is the dominant role resulting in the decrease of hardness along with long time tempering of microalloyed steel. In this work, the effect of tempering time on the microstructure and mechanical properties of high Ti microalloyed quenched steel was systemactically investigated by TEM, XRD and Vickers-hardness test, and the interaction between precipitation hardening and microstructural softening of the high Ti microalloyed steel was also studied. The results indicate that the hardness increases for Ti microalloyed steel with tempering time 10~300 s, which is attributed to the fact that the precipitation hardening by nano-sized TiC particles is greater than the recovery and softening of matrix. With the tempering time from 300 s to 10 h, nano-sized TiC particles precipitate more and more and the mass fraction of TiC with the size less than 5 nm increases, owning to the precipitation hardening produced by tiny TiC which offsets the hardness decrease due to the gradual softening with recovery of matrix, and therefore, the hardness can keep a long platform; in addition, with the tempering time 10~20 h, the hardness decreases significantly and the deacreasing rate of hardening for steel with Ti microalloying is higher than that for steel without Ti microalloying. The average particle size of TiC increases from 2.76 nm at 10 h to 3.15 nm at 20 h. Calculation results show that the decrease of hardness caused by coarsening of TiC is 11.94 HV, while caused by recovery of matrix is 24.56 HV. It is shown that the recovery of matrix is the dominating factor for reduction in hardness, but coarsening of tiny TiC speeds the decrease of hardness and is also an important factor resulting in the decrease of hardness.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-18 Cooperative journals: 《材料研究学报》
Abstract： The volume fraction, morphology and size of reversed austenite in 5.5Ni steel tempered at different temperatures were characterized by X-ray diffraction, scanning electron microscope and transmission electron microscope, and the influence of tempering temperature on mechanical properties of 5.5Ni steel was investigated. The results show that there was no significant change in either the tensile strength or yield strength for the steel tempered in the range from 580℃ to 600℃. There was a slight increment in tensile strength but a great decrement in yield strength, besides, a maximum elongation was obtained for the steel tempered at 620℃. As the tempering temperature increased from 580℃ to 620℃, the volume fraction of reversed austenite in 5.5Ni steel increased gradually but impact energy decreased. Stable, homogeneous, dispersive and fine lamella-like reversed austenite is the main reason of the high impact energy of 148 J when the steel tempered at 580℃. Two types of reversed austenite including lamella ones and block ones were detected in this steel. The former had different length with a width of about 20 nm which could improve the low temperature toughness of the steel. The latter had a size of about 200 nm and tended to gathering together as clusters which were detrimental to the low temperature toughness of the steel.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-18 Cooperative journals: 《材料研究学报》
Abstract： Fe-C-Mo-M steels (where M is Nb, V or Ti, ~0.1%, and Mo ≤0.2% ) were produced by thermal mechanical control processing (TMCP), and then their performance was characterized in terms of failure temperature by means of constant load tensile test while heating from ambient temperature up to 800oC with a heating rate 28 oC/min. The boundary misorientation of the steels after TMCP was examined by electron back scattered diffraction (EBSD), and the precipitates of MC type carbides were characterized by transmission electron microscopy (TEM). The results show that the addition of 0.2% Mo in Fe-C-Nab/V steels increases the failure temperature of steels by 40℃. It is believed that the low-angle grain boundary provided the favorable nucleation site for MC type carbides, which in turn will accelerate the kinetics of precipitation process. The fine and dispersed precipitates of MC type carbides induce significant precipitation strengthening for the steels during the constant load tensile process, thus resulting in higher failure temperature. Among the tested steels, the failure temperature of Ti-Mo steel is the highest due to its highest low-angle grain boundary density which results in the fast precipitation of MC type carbides. The failure temperature of Nb-Mo steel comes the second and that of the V-Mo steels is the lowest because of its lowest low angle grain boundary density leading to the lowest density of precipitated MC type carbides.
Subjects: Materials Science >> Materials Science (General) submitted time 2017-11-21 Cooperative journals: 《金属学报》
Abstract：利用OM、EBSD、HRTEM和Vickers硬度计等手段研究了冷却速率对Ti-V-Mo复合微合金钢组织转变、析出相及硬度的影响，阐明了(Ti, V, Mo)C在不同冷却速率下的析出规律及其对显微组织、硬度的作用机理。结果表明，当冷却速率低于20 ℃/s时，随着冷却速率的增加，析出相平均尺寸由13.2 nm逐渐减小至6.9 nm，铁素体平均晶粒尺寸由5.06 μm逐渐细化至2.97 μm，硬度呈先快速增大而后缓慢增大的趋势，铁素体的细晶强化和(Ti, V, Mo)C的沉淀强化是硬度升高的主要因素；冷却速率为20~30 ℃/s，其对晶粒细化和沉淀强化的影响效果已趋于饱和，硬度基本保持不变，此时Ti-V-Mo复合微合金钢的硬度具有最大值410 HV，屈服强度高达1090 MPa。Ti-V-Mo复合微合金钢的硬度y与冷却速率x符合指数衰减关系：y=-229exp(-x/5)+412。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：利用OM, EBSD, XRD及物理化学相分析法，对不同卷取温度下Ti-V-Mo复合微合金化热轧高强钢的强化增量进行了估算和分析，分别讨论了卷取温度对屈服强度和MC相粒子对均匀塑性的影响规律。结果表明，在600 ℃卷取时，实验钢具有最佳的综合力学性能：抗拉强度UTS为1134 MPa，屈服强度YS为1080 MPa, 延伸率A为13.2％, 均匀延伸率Au为6.8％，其析出强化增量σp高达444～480 MPa左右，主要是由质量分数高达72.6 wt％的10 nm以下的(Ti, V, Mo)C粒子提供的。析出强化和细晶强化是实验钢主要的强化方式，σp的改变是导致实验钢不同卷取温度下YS变化的主要因素。随着卷取温度由500 ℃升高至600 ℃，实验钢的UTS和YS不断增加，Au不但没有降低，反而呈线性缓慢增加。其主要原因是σp对屈服强度的贡献量不断提高，在提高强度的同时改善了均匀塑性。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：利用Gleeble热模拟实验机、显微硬度计、扫描电镜（SEM）、高分辨透射电镜（HRTEM）及差热分析仪（DSC）研究了淬火态Nb和Nb-Mo钢在升温过程中碳化物的析出行为, 并利用经典形核长大理论及Avami方程对淬火态钢中MC型碳化物的析出动力学进行了计算.结果表明：含Nb和Nb-Mo微合金钢以20℃/min的速率加热至不同温度再淬火后分别在300 ℃和700 ℃时由于渗碳体和MC型碳化物析出而出现了硬度峰值.MC型碳化物在650 ℃左右析出,导致析出强化硬度上升,与理论计算得到的MC型碳化物的析出鼻子点温度约650℃的结果相一致. 分析认为Mo进入NbC中降低了NbC与铁素体基体的错配度,从而减小了析出相与铁素体基体间的界面能,同时也使NbC的析出自由能降低,但界面能的降低占主导作用,最终使得(Nb,Mo)C析出动力学加快,所以Nb-Mo钢中析出相粒子分布更为密集,尺寸更为细小,具有较高的析出强化作用.
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