【作者】 乔志霞；

【导师】 刘永长；

【作者基本信息】 天津大学 ， 材料学， 2010， 博士

【摘要】 目前钢铁工业发展所面临资源和环境的压力越来越大,为保护环境,节约能源和原材料,研究和发展超高强度钢、减轻钢制品重量,提高钢铁产品附加值迫在眉睫。低合金超高强度钢由于成本相对低廉,生产工艺比较简单,因而具有广阔的发展应用前景。30CrNi3MoV钢是在原Cr-Ni-Mo系低合金超高强钢的基础上进行V微合金化,并适当调整其它合金元素含量而发展起来的。为充分发挥V微合金化的强化优势,生产中须配以合理、先进的控轧控冷或热处理工艺,才能使钢的性能得到最大限度的发挥。然而,目前对此钢的实际应用仍采用传统的淬火、回火热处理工艺,这一方面不利用钢材潜能的最大发挥,而且还浪费资源和能源,增加生产成本,不利于其推广应用。钢材先进生产工艺的制定有赖于对钢本身固态相变过程、组织控制的深入研究,以期澄清其相变过程的一般规律。为此,本文采用高精度差分膨胀测量和微观组织分析方法系统研究了30CrNi3MoV钢在连续冷却条件下的组织转变规律,并在此基础上探讨粒状贝氏体的形成机制、贝氏体相变的不完全性、形变热处理对贝氏体相变的影响,以及30CrNi3MoV钢中马氏体相变特征等,取得如下研究成果:(1)系统研究了30CrNi3MoV钢连续冷却过程中的相变行为。采用高精度线膨胀仪并结合显微组织分析,澄清了30CrNi3MoV钢以1～2000℃/min连续冷却至室温所发生的组织转变,绘制了过冷奥氏体连续冷却转变曲线(CCT图),并探讨了连续冷却速度对相变产物显微硬度的影响。结果表明:从1℃/min到2000℃/min,30CrNi3MoV钢过冷奥氏体连续冷却转变产物发生“粒状贝氏体→下贝氏体和少量上贝氏体→板条与片状混合马氏体”的逐步过渡;30CrNi3MoV钢的CCT图中没有出现珠光体转变区,只存在贝氏体转变和马氏体转变以及极少的先铁素体析出区;30CrNi3MoV钢发生马氏体相变的临界冷却速度在20～25℃/min之间,该钢具有相当好的淬透性;由于受到马氏体自回火过程引起沉淀强化的影响,30CrNi3MoV钢相变产物的显微硬度并非随冷却速度增大而均匀增大。(2)研究了30CrNi3MoV钢中贝氏体相变规律。结果表明:30CrNi3MoV钢过冷奥氏体缓慢冷却至室温,相变产物为Bg1和Bg2两种不同形态的粒状贝氏体,其中Bg1中粒状物分布较稀疏,形状和分布都不规则,而另一种Bg2中的粒状物分布较密集,且沿某一方向平行分布,在透射电镜下形状为短棒状,两者具有不同的形成机制;在连续冷却情况下,30CrNi3MoV钢中贝氏体相变不完全性普遍存在,因相变不完全而残留的奥氏体具有很高的富碳程度,且随冷速增大,富碳程度有所降低;在极缓慢冷却条件下,贝氏体相变后的残余奥氏体不具备进一步相变的驱动力而被永久保留下来,而若冷速足够快,富碳残余奥氏体将在低温区转变为高碳孪晶马氏体,使贝氏体相变呈现停滞现象。(3)利用能施加载荷的高精度线膨胀仪,研究了奥氏体未再结晶区变形对30CrNi3MoV钢贝氏体相变特征的影响,结果表明:由于变形使母相奥氏体中储存了较高的形变能,相当于为相变提供了一个额外的机械驱动力,使贝氏体转变所需最小化学驱动力降低,因而经奥氏体形变处理的30CrNi3MoV钢贝氏体相变起始温度Bs显著升高;经奥氏体形变30CrNi3MoV钢在以20℃/min连续冷却至室温后得到的相变产物中新增了少量粒状贝氏体组织;奥氏体区变形使30CrNi3MoV钢中贝氏体条或片尺寸减小、分布位向增多,贝氏体条或片彼此相互交叉,很多还相互穿越,使过冷奥氏体在贝氏体相变过程中表现出更大的稳定性。(4)通过对微观亚结构和相变动力学的分析,研究了30CrNi3MoV钢中马氏体相变特征。研究发现:淬火冷却30CrNi3MoV钢的相变产物包括板条状和针状两种马氏体形态,其中针状高碳马氏体发生了一定程度的自回火,析出了多种弥散细小的合金碳化物;30CrNi3MoV钢的马氏体相变过程中发生了碳的重新分配,造成低碳板条马氏体与高碳针状马氏体的形成在相变动力学曲线中截然分开,板条马氏体形成速率远高于针状马氏体。(5)研究了奥氏体化过程对30CrNi3MoV钢马氏体相变的影响。结果表明:V微合金化使得30CrNi3MoV钢奥氏体晶粒粗化温度在约1000℃,因此对该钢进行热处理时奥氏体化加热温度不能超过1000℃;在奥氏体化加热过程中,奥氏体化温度会通过以下两方面影响30CrNi3MoV钢Ms点,一是晶粒尺寸和位错缺陷组态,它们决定马氏体相变前母相奥氏体强度,二是碳及合金元素溶入奥氏体的程度,这两方面综合作用使得Ms点随奥氏体化温度升高先上升,后又随之下降;奥氏体化保温时间对30CrNi3MoV钢奥氏体晶粒尺寸长大的影响远不如奥氏体化温度那样显著,在900℃的奥氏体化的温度下延长保温时间并不能造成晶粒尺寸的粗化;30CrNi3MoV钢Ms点随奥氏体化保温时间的延长而单调升高,但Ms点的变化受到晶粒尺寸和奥氏体中位错缺陷组态双方面影响,致使Ms点并非随奥氏体晶粒尺寸长大而均匀升高。更多还原

【Abstract】 The 30CrNi3MoV steel is a microalloyed Cr-Ni-Mo ultra-high strength steel by vanadium addition. The phase transformation behavior from the undercooled austenite in continuously-cooled 30CrNi3MoV steel was systematically studied by means of high-resolution dilatometric measurements and microstructural analysis. The formation mechanism of granular bainite, incompleteness phenomenon of bainitic transformation, effect of thermo-mechanical treatment on bainite transformation and the characteristics of martensitic transformation in 30CrNi3MoV steel were investigated, the conclusions were as follows:(1) The transformation behavior of 30CrNi3MoV steel in the process of continuous cooling from the austenite was systematically investigated. All the possible transformations in the experimental steel cooled at 1 ~ 2000℃/min were clarified, and the corresponding CCT diagram was constructed. It shows that the transformation products from the austenite of 30CrNi3MoV steel evolves as“granular bainite→low bainite plus a small amount of upper bainite→lath martensite and acicular martensite”with the increase of the applied cooling rate from 1 to 2000℃/min. In the CCT diagram, no pearlite transformation was detected in the explored 30CrNi3MoV steel except for forming bainite, martensite and a little pre-eutectoid ferrite. The critical cooling rate for the martensite transformation is in the range of 20～25℃/min.(2) Two kinds of granular bainites (named as Bg1 and Bg2, respectively) were formed in the investigated 30CrNi3MoV steel with slow cooling from the high-temperature austenite field at rates from 1 to 5 oC/min. All secondary particles existing in the Bg1 tend to be irregular, which results from the formation and growth of ferrite in an equiaxed way from carbon-poor austenite areas. Granules in the Bg2 are parallel to each other at some preferred orientations, and the massive matrix form by merging of the ferritic laths. Incomplete bainite transformation phenomenon occurs generally in the continuously-cooled 30CrNi3MoV steel, leading to the formation of small carbon-rich retained austenite region after the completion of bainite transformation.(3) The deformation of austenite in the un-recrystallizing stage has remarkable influence on the characteristics of bainitic transformation in the explored 30CrNi3MoV steel. The Bs temperature of the 30CrNi3MoV steel with deformation in austenite increased greatly due to the additional transformation driving force from the high storaged energy in deformed austenite. The size of bainite laths in the 30CrNi3MoV steel decreased remarkably with the deformation in austenite, and their distribution exhibit more orientations.(4) The martensitic transformation in the explored 30CrNi3MoV steel was investigated through kinetic analysis and the microstructure observation in this part. There are two kinds of martensite with different morphology in the quenched 30CrNi3MoV steel, that is, lath martensite and plate martensite. Redistribution of carbon atoms occurred in the process of martensite transformation in the 30CrNi3MoV steel, which results in the detachment between the formation of lath martensite and plate martensite on the kinetic curve of martensite transformation.(5) The size of the austenite grains begin to coarsen when the austenization temperature exceeding 1000℃. The Ms temperature of the 30CrNi3MoV steel increases with the increasing austenization temperature when lower than 950℃, but begins to decrease below that. The great increase of austenization holding time at 900℃will not lead to the obvious growth of the austenite grains. The Ms temperature of the 30CrNi3MoV steel increases remarkably with the extension of holding time at austenzation temperature, but do not increase homogeneously with the increasing grain size of the austenite obtained by holding different time.更多还原