【作者】 颜飞；

【导师】 徐洲； 史海生；

【作者基本信息】 上海交通大学 ， 材料学， 2007， 博士

【摘要】 本论文工作结合宝钢集团重大科研项目“喷射成形先进冶金加工技术的应用开发”而完成。模具钢大多数采用常规铸造方法,再经大变形量锻造及复杂的热处理来改善钢中碳化物尺寸和分布,以获得良好的力学性能。但常规方法难以制备高品质高合金模具钢,而只能采用工艺复杂、成本高的粉末冶金方法生产。本论文选取瑞典UDDEHOLM公司生产的粉末冶金Vanadis4(V4)钢作为对比材料,采用喷射成形这一先进的冶金加工技术制备了V4钢,有效细化了晶粒,减小了偏析,极大地改善了钢中碳化物形貌和分布。在此基础上分析了喷射成形V4钢微观组织及形成特点,揭示了该钢的细化机理及区别于其它喷射成形钢材的特点;系统的研究了喷射成形V4钢热加工过程微观组织演变行为,给出了制备V4钢最优化的控制参数;对比研究了喷射成形V4钢与粉末冶金V4钢的淬回火后的力学性能,分析了喷射成形V4钢回火过程二次碳化物的析出行为。自主创新的工艺路线相对粉末冶金工艺而言工艺简单成本更低,有力地证明了喷射成形取代粉末冶金制备高合金V4钢的可行性。喷射成形工艺得到的高合金V4钢为等轴晶,晶粒大小在8-10μm左右分布均匀,无宏观偏析及粗大网状碳化物,且组织中未见类似常规铸态材料中的共晶组织。其组成相包括马氏体、残余奥氏体、MC及M7C3型碳化物,其中,残余奥氏体含量高达33%,VC颗粒大部分尺寸在0.5-2μm均匀分布在晶界,而大部分M7C3颗粒尺寸在180nm左右均匀分布在晶内。喷射成形所具有的快速凝固特征是V4钢组织细化的最关键因素;此外,喷射成形雾化过程先凝固粒子的异质形核作用,以及凝固过程中在较高温度析出的VC限制了γ枝晶生长的时间和空间,从而促进γ枝晶细化也是其能获得细小晶粒的重要原因。V4钢加热过程的相转变点为845℃(A1)和890℃(A3),加热过程中由于基体组织分解及大量碳化物析出而导致热膨胀曲线出现明显的弯曲。等温压缩热模拟实验结果表明,V4钢在850-1150℃温度范围内的真应力-真应变曲线均为典型的再结晶曲线,当变形温度一定,变形速率越高,流变应力越大;当变形速率一定,变形温度越高,流变应力越小。热轧实验研究结果表明,V4钢在850-950℃温度轧制后基体中析出了大量M7C3及M3C碳化物,而一次碳化物则会发生不均匀长大,因此轧后组织很不均匀;1050℃轧后组织中的碳化物最为均匀细小;当轧制温度升高至1100℃以上时,回溶至基体的合金元素在轧后冷却过程中将沿晶界不均匀析出。因此,热变形温度是决定变形后组织中碳化物形貌、尺寸和分布的最关键热力学条件。退火实验结果表明,退火温度显著影响轧后组织中碳化物球化过程。850℃退火时元素扩散速率较低,部分条状碳化物还未熔断球化;900℃是最理想的退火温度;温度升高至950℃,元素扩散大大能力增加,碳化物将发生较为明显的粗化。通过本文新工艺得到的V4钢组织相对粉末冶金V4钢而言碳化物更加均匀细小,且所采用的轧制或锻造工艺相对简单,工业上也易于实现,这是喷射成形高合金模具钢的独特优点。本研究成果能填补国内采用喷射成形取代粉末冶金制备高合金模具钢的空白。经过相同工艺淬回火后的喷射成形V4钢硬度值高于粉末冶金V4钢,两者冲击功相当。V4钢回火过程存在明显的二次硬化,在500℃回火后,极其细小且弥散析出的纳米级VC是材料达到二次硬化峰的主要原因,VC热稳定性好,经过长时间时效也很难长大;当回火温度进一步升高至550℃后,析出的碳化物为尺寸相对较大的M3C,大量C的析出使基体强度降低,且M3C的弥散强化效果低于MC,因此V4钢硬度值有较为明显下降;而700℃过时效处理5min后基体便迅速分解,位错密度显著降低,首先发现析出的碳化物为M7C3且多沿马氏体板条边界析出,随回火时间延长析出的碳化物还包括M23C6、M6C、MC。基体在回复过程中,胞壁的位错重新排列和对消逐渐变锋锐,胞壁完全锋锐了的胞块转化为亚晶,亚晶逐渐合并,形成多边形铁素体。基体中析出的大量碳化物能有效钉扎亚晶界,阻止亚晶合并和再结晶的进行。喷射成形V4钢与粉末冶金V4钢的耐磨性对比研究结果表明,两种钢的磨损过程均包括跑合与稳定磨损两个阶段,在二次硬化峰值处两种材料具有最好的耐磨性。磨损表面的扫描电镜形貌观察表明两种钢的主要磨损机制为磨粒磨损,但粉末冶金V4钢磨损表面还存在粘着磨损形貌,而该特征在喷射成形V4钢磨损表面并未发现,这是两种钢磨损过程中的摩擦系数和表面粗糙度差别产生的主要原因。相同实验条件下喷射成形V4钢摩擦系数小于粉末冶金V4钢摩擦系数;基于激光扫描共焦显微镜的两材料磨损表面三维粗糙度表征结果表明喷射成形V4钢的表面粗糙度低于粉末冶金V4钢表面粗糙度。粘着磨损的产生与两种材料中碳化物总量、分布、碳化物间距相关,喷射成形V4钢中碳化物更加均匀细小,能有效避免对磨材料基体间的直接接触而避免粘着磨损。相同淬回火条件下的喷射成形V4钢的耐磨性优于粉末冶金V4钢。更多还原

【Abstract】 The work of this dissertation was done by combined with the key research project‘Application and Exploitation of the Advanced Metallurgical Technology– Spray Forming’of Baosteel Group Cooperation. In order to obtain high mechanical properties, most of the cold work die steels (CWDS) which were produced by conventional cast technique must be treated by large strain deformation and complex heat treatment in order to refine the size and distribution of the carbides. However, high grade high alloyed CWDS are hard to be produced by conventional cast technology; only the complex and high cost powder metallurgical technology can be used. In this dissertation, a powder metallurgical Vanadis4 (V4) CWDS produced by UDDEHOLM, Sweden, was chosen as the material for comparison, and spray forming technique was firstly proposed to produce the V4 steel. It was found that macro-segregation was reduced, and the grain size, the morphology and distribution of the carbides were refined greatly. In addition, microstructure and forming characteristics of the as-sprayed V4 steel were analyzed; the refine mechanisms and characteristics which different to other materials were revealed. Microstructural evolution during hot working and heat treatment were studied, and optimum parameters for the production of V4 steel were provided. Mechanical properties of the spray formed and powder metallurgical V4 steels after quenching and tempering were compared, and the secondary carbides precipitated during tempering were studied also. Compared with powder metallurgical technology, this self-developed route is more simple and cheaper. It strongly proves the possibility that spray forming can replace powder metallurgy to produce high alloy CWDS.Microstructural study of the as-sprayed V4 steel shows that it was composed of martensite, retained austenite (amount to 33%), MC and M7C3 carbides. Fine, homogeneous and fully spheroidal grains ranging from 8 to 10μm, which was substantially finer than the conventionally cast equivalent, are found. No coarse net-work carbides and eutectic structures were found in the matrix. Spheroidal VC carbides ringing from 0.5-2μm were uniformly distributed along grain boundaries; however, most of the M7C3 carbides with the size of about 180 nm were distributed in the grains. Rapid solidification inherent in the spray forming is the key refinement factor of the microstructure, in addition, the facts that the pre-solidified particles acted as heterogeneous nucleate sites, and that the VC carbides which were precipitated at high temperature during solidification and then confined the time and space of the growth of the dendrites, are other two important factors.The austenitizing temperatures of the as-sprayed V4 steel are 845℃(A1) and 890℃(A3). An obvious flexure appears in the measured dilatation curves due to the matrix decomposition and the precipitation of a large number of carbides. Isothermal compression test was carried out on the as-sprayed V4 steel within the range of temperatures between 850 and 1150℃. The obtained true stress-strain curves showed that the true stress increase with the decrease of temperature at a given strain rate and the increase in strain rate at a given temperature. Results of the hot rolling test showed that, when the steels were rolled within the ranges of temperatures between 850 and 950℃, a large number of M7C3 and M3C carbides were precipitated, and the primary carbides would grow also, therefore, uneven microstructures were obtained after rolling. Attractive microstructure could be obtained when rolled at 1050℃, however, when the rolling temperature was elevated to equal or above 1100℃, irregular carbides would precipitated along the grain boundaries after rolling. Thus, the hot rolling temperature is the key factor in controlling the evolution of type, morphology and distribution of carbides. During annealing, spheroidization of the carbide was greatly influenced by the temperature. The diffusion ability of the elements was low at 850℃, and many undissolved carbide stringers were found; 900℃was proved to be an ideal annealing temperature; with further increasing annealing temperature, the diffusion ability of the elements increased greatly and therefore, obvious carbide coarsening behavior was found. The average carbide size in the V4 steel obtained by the new method is more finer than the equivalent in the powder metallurgical V4 steel, however, the hot rolling and forging are much simple and can be easily used in mass production, and this is the unique merit of spray formed high alloyed cold work steels.The hardness of the spray formed V4 steel is slightly higher than that of the powder metallurgical V4 steel when quenched and tempered by the same technology, and they have equal impact energies. Obvious secondary hardening was found during tempering of the V4 steel. It was confirmed by TEM that the very fine and dense nature of secondary VC precipitates are responsible for the secondary hardening peak at 500℃. The nano-sized VC particles are thermodynamically very stable and show little tendency to coarsen when aged for prolonged times. The decrease of the hardness when the steel was tempered at 550℃can be attributed to the precipitation of M3C, which had lower dispersion strengthen effect as that of MC. The matrix decomposed quickly, and the dislocation density greatly decreased when the steel aged at 700℃for 5 min. The decomposition initiated with the nucleation of fine M7C3 carbides preferentially at the martensite lath boundaries; M23C6, M6C, and MC carbides were also found when aged for prolonged times. During the restoring process of the matrix, the dislocations in the cell boundaries rearranged and neutralized, and then gradually became sharp and the cell translated into sub-grains. The sub-grains coalesce and then the polygonal ferrite came into being. The precipitates, especially a high density of fine precipitates, retard the coalescence of sub-grains and recrystallization of ferrite.Sliding wear tests showed that the wear process of the spray formed and powder metallurgical V4 steels can all be divided into running-in and steady state regimes. The V4 steel has the best wear resistance when tempered at 500℃. SEM morphologies of the worn surface indicate that the main wear mechanism of the V4 steel was abrasive wear. Adhesive wear morphologies were also found on the surface of the powder metallurgical V4 steel, however, this characteristic was not found on the surface of the spray formed V4 steel. This is the main factor which caused the difference of the friction coefficient and surface roughness between the two kinds of steels. The results showed that the friction coefficients of the spray formed V4 steel were smaller than those of the powder metallurgical V4 steel; and the worn surface roughness of the spray formed V4 steel were lower too, which was obtained by laser scanning confocal microscope. Formation of the adhesive wear correlated with the total amount, distribution, and space between the carbides. The carbides in the spray formed V4 steel are finer and more uniformly distributed than those of the powder metallurgical V4 steel, which prevents the directly contact of the matrix, and therefore, adhesive wear was being prevented. The wear resistance of the spray formed V4 steel is finer than that of the powder metallurgical V4 steel when treated by the same quenching and tempering technique.更多还原