【作者】 姚圣杰；

【导师】 王国栋； 杜林秀；

【作者基本信息】 东北大学 ， 材料加工工程， 2009， 博士

【摘要】 现代钢铁工业的发展不再强调数量上的增长,而更多侧重技术层面上的研发。在新一代钢铁材料的开发中注重减量化的思想,即在不增加或者少增加成本的基础上提升产品的综合性能,而其中细晶强化无疑是一种被多数研究所证实的行之有效的方法。本文以此为着眼点,在前期863计划课题（2001AA332020）“500MPa碳素钢先进工业化制造技术”的研究基础上,通过奥氏体晶粒的超细化及对其相变行为的有效控制获得一种实现低碳结构钢组织超细化控制的新思路。在轧制技术及连轧自动化国家重点实验室自主课题经费的支持下,本文主要围绕含Nb钢和Nb-V-Ti复合微合金钢的奥氏体晶粒超细化方法、超细晶奥氏体的长大动力学、超细晶奥氏体的高温变形及其γ→α相变行为等方面进行研究,并在实验室现有条件下做了一些尝试性轧制试验。研究取得如下成果:（1）在不同原始组织条件下,利用循环加热-淬火工艺细化奥氏体晶粒,将奥氏体晶粒细化至1～3μm。在所采用的原始组织中,以温轧铁素体+珠光体所获得的细化奥氏体晶粒的效果最好;另外,Nb-V-Ti复合微合金钢要比单纯含Nb钢更有利于细化奥氏体晶粒。（2）利用形变热处理细化奥氏体晶粒是一个多种相关因素（如变形温度,变形量,应变速率以及变形方向等）耦合的过程,需要对每一个因素进行合理的关联控制才能实现奥氏体晶粒最大程度的细化。（3）在温轧铁素体+珠光体原始组织下,可以通过升温过程中的铁素体动态再结晶和变形促进奥氏体相变机制的综合作用实现奥氏体晶粒的亚微米化。（4）以Nb-V-Ti复合微合金钢为对象,讨论了原始晶粒尺寸1～3μm的超细晶奥氏体的等温长大行为,并建立了相应的长大动力学模型。分析不同尺寸奥氏体热变形的真应力-真应变曲线发现,随着奥氏体晶粒超细化以后,晶界行为在奥氏体的热变形过程中协调作用更加显著。奥氏体晶粒超细化使得晶粒（或晶粒簇）间对碳浓度的分布变得敏感,造成了单相奥氏体中“类两相”（硬相和软相）特征组织的存在,从而对超细晶奥氏体的热变形行为以及后续变形过程中铁素体相变均产生显著影响。（5）超细晶奥氏体在接近A₃点的A_d3～A_r3温度区间变形初期,除了有变形诱导铁素体相变发生外,同时伴随有超细晶奥氏体的长大行为。（6）奥氏体晶粒超细化对相变后铁素体的细化是显著的,应变速率的增加和相变温度的降低均有利于终态组织的细化,在两相区的较低温度变形时可以在高应变速率下将铁素体晶粒细化至100～300nm;而同样的应变量,低应变速率(0.1s^-1)时所获得的铁素体晶粒尺寸相对较大,约在500nm。（7）在实验条件下,分别制备了晶粒尺寸1μm左右的超细晶C-Mn钢样品和铁素体晶粒为100～300nm的Nb-V-Ti复合微合金钢样品,其室温拉伸曲线显示了超细晶钢所普遍具有的低应变硬化能力,且结合超细晶C-Mn钢样品的EBSD分析认为超细晶钢的低应变硬化能力除了与位错累积能力差有关之外,晶界的滑动也是原因之一。但是通过合理控制终态组织可以有效提高超细晶材料的应变硬化能力,如超细晶铁素体+珠光体（渗碳体）+马氏体复合组织所具有的加工硬化能力要明显优于超细晶铁素体+珠光体（渗碳体）。更多还原

【Abstract】 More emphasis has being laid on the improvement of technical content instead of quantitative increase in the modern steel industry. As for new generation steel, reduced rolling technology has already successfully applied to its research and development, where the most effective method utilized is fine-grained strengthening. By taking this strengthening method into account, basing on the advance researches in the 863 project of The Advanced Industrial Manufacture Technology of 500MPa Carbon Steel, a new way was explored to better control of ultra-fine microstructures in combination with ultra-refinement of austenite grains and reasonable control of transformation behaviors during the subsequent cooling stage.Being sponsored by autonomic-project fund of State Key Lab. of Rolling & Automation, researches about the processes for ultrafining austenite grains in Nb-containing steel and Nb-V-Ti steel, growth kinetics of ultra-fine austenite grains, hot-deformation of ultra-fine grained austenite and subsequentγ→αtransformation behaviors etc. were systematically undertaken in the current paper, including some tentative rolling experiments in laboratory. The main works involved as follows:（1） Ultra-fine austenite grains in size of 1～3μm can be prepared with different initial microstructures in the process of repetitive reheating and quenching, while the most effective initial microstructure used is warm-rolled ferrite + pearlite. Moreover, ultra-fine austenite grains obtained in Nb-V-Ti steel are much smaller than those in Nb-containing steel.（2） All the influencing factors are related to each other and should be controlled well in the thermomechanial treatment which is a coupling process, in order to minimize the obtained austenite grain size.（3） Submicron crystallization of austenite grains can be obtained through recrystallization of ferrite and deformation promoted austenite transformation during the reheating stage.（4） Isothermal growth behavior of ultra-fine austenite grains with average grain size 1～3μm in Nb-V-Ti steel was systematically discussed and corresponding model of growth kinetics was also established. True stress-true strain curve is inclined to softening type as austenite grain size decreases at 900～950℃, which means grain boundary behaviors gradually participate in coordinating the deformation process. Austenite grains （or grain clusters） become more sensitive to the distribution of carbon concentration, and then, such ’quasi two phase’ （hardening and softening phases） in austenite is formed which further influence the hot-deformation behaviors of ultra-fine grained austenite and subsequent dynamic ferrite transformation significantly.（5） Deformation-induced ferrite transformation and dynamic austenite grain growth simultaneously exist in the early stage of deformation at temperature range of A_d3～A_r3 around A₃.（6） Ferrite grain size remarkably decreases through refinement of austenite grain size. Also, lowering the deformation temperature and increasing the strain rate are both favorable for the refinement of final microstructures. Ferrite grains in size of 100～300nm can be obtained when samples are deformed at relatively low temperatures in two-phase region with high strain rate (10s^-1), while ferrite grains become a little larger （～500nm） if the strain rate is slowed down to 0.1s^-1.（7） Ultra-fine grained C-Mn steel and Nb-V-Ti steel with ferrite grains size about 1μm and 100～300nm respectively were successfully prepared under laboratory conditions, and low strain hardening capacities were entirely found in those ambient tensile tests, which are partially attributed to initial grain boundary sliding/grain rotation besides the weak piling up of dislocations in ultra-fine grains.更多还原