高性能低合金钢中残余奥氏体调控机理及性能研究

发布时间：2018-02-14 22:05

本文关键词： 低合金高强钢塑性低温韧性残余奥氏体多相组织　出处：《北京科技大学》2016年博士论文　论文类型：学位论文

【摘要】：为了满足日益增长的工程结构和装备轻量化、高服役性及节能减排的要求,发展具有高强度且兼具高塑性、高韧性以及良好的焊接性的第三代低合金钢,本文针对0.23C-1.35Si-1.85Mn低成本低合金钢及0.08C-2～3Mn-0.5Ni体系低碳低合金钢进行残余奥氏体调控以期开发高性能钢,重点研究了组织调控过程中残余奥氏体的稳定化机制及其对力学行为的影响。对0.23C-1.35Si-1.85Mn钢经两相区760℃临界退火加300～500℃中温一步等温处理,获得了临界铁素体、贝氏体以及残余奥氏体的多相组织。研究表明,残余奥氏体含量与等温过程中贝氏体相变量有关。在300℃等温时,奥氏体发生持续分解生成无碳化物贝氏体,最终获得的残余奥氏体含量较少；在400℃等温时,大部分奥氏体等温分解成贝氏体后,仍有8%左右的奥氏体未完全分解,此时有足够的贝氏体中过饱和C向奥氏体中富集,进而获得稳定的残余奥氏体;在500℃等温时,仅少量奥氏体分解成贝氏体,此时没有足够的C从贝氏体中向未分解的奥氏体中富集,最终无法获得稳定的残余奥氏体。实验钢先经淬火至300℃短时间等温,再进行400℃等温不同时间的两步热处理,也获得了6～9%的残余奥氏体。对0.08C-2～3Mn-0.5Ni体系低合金钢采用临界热处理进行了残余奥氏体调控及性能研究。研究表明,低C 3%Mn钢进行680℃临界退火30 min可以获得13%左右的残余奥氏体,而低C 2%Mn钢采用两步临界热处理也能获得8%稳定的残余奥氏体。通过对3%Mn和2%Mn钢分别采用一步和两步临界热处理,均能实现700 MPa级屈服强度,均匀延伸率大于10%,总延伸率大于25%和良好低温冲击韧性的优异性能。对0.08C-2～3Mn-0.5Ni体系低合金钢采用两步临界热处理获得的细残余奥氏体进行了细致表征。结果表明,在两步临界热处理过程中残余奥氏体获得受益于以下三个方面：首先,第一步逆转变获得的亚稳奥氏体由于合金元素的富集,从而使其在后续的冷却过程中形成合金元素富集的马氏体/贝氏体,这些富集了合金元素的马氏体/贝氏体在第二步的临界回火过程中由于其具有较低的Ac1温度而优先成为逆转奥氏体的形核位置：其次,在第二步临界回火过程中合金元素向再次逆转的亚稳奥氏体中高度富集(C达到0.4%以上,Mn达到6%以上,Ni大于0.6%),进一步提高了奥氏体的稳定性；第三,第二步临界回火形成的二次逆转奥氏体均匀弥散,大都呈薄膜状,且尺寸极其细小(平均尺寸约300 nm),奥氏体的纳米尺寸效应是稳定残余奥氏体的重要原因。通过不同拉伸应变量下的原位EBSD分析表明,低合金钢中残余奥氏体在不同应变阶段逐步发生TRIP效应,导致实验钢瞬时加工硬化指数持续上升形成三阶段加工硬化行为,推迟颈缩的发生,是实现高均匀延伸率的重要原因。此外,低合金钢中残余奥氏体含量对韧性影响也很大,且随着冲击测试温度的降低变得更加显著。残余奥氏体的存在有利于通过TRIP效应提高低温冲击过程中的塑性变形能力,延迟起裂,提高起裂功,从而获得优异的低温冲击韧性。利用残余奥氏体的增塑增韧机制,实现了500MPa级屈服强度,均匀延伸率大于20%,-100℃冲击功达到60 J左右的高强-高塑-高韧低合金钢。
[Abstract]:In order to meet the increasing engineering structure and equipment lightweight, high service and energy-saving emission reduction requirements, the development of high strength and high ductility, high toughness and good weldability of the third generation of low-alloy steel, this paper 0.23C-1.35Si-1.85Mn low cost low alloy steel and 0.08C-2 ~ 3Mn-0.5Ni system of low carbon low alloy steel the residual austenite regulation to the development of high performance steel, focuses on the stabilization mechanism of retained austenite in the process of regulation and its effect on the mechanical behavior of 0.23C-1.35Si-1.85Mn steel after intercritical. 760 C 300~500 C with critical annealing temperature step isothermal treatment, the critical ferrite, bainite and residual austenite multiphase structure. The results show that the content of residual austenite and bainite isothermal process. In the 300 phase variable C isothermal austenite, continuous decomposition of carbon free of From bainite, less residual austenite content obtained at 400 DEG C; isothermal, most isothermal decomposition of austenite into bainite, there is still about 8% of the austenitic incomplete decomposition of the bainite enough in supersaturated C to the austenite enrichment, and obtain stable residual austenite in 500; C the isothermal decomposition of austenite into, only a small amount of bainite, at this time there is not enough C from bainite to no decomposition of austenite enrichment, ultimately unable to obtain stable residual austenite after quenching. The steel to 300 DEG C short isothermal time, two step heat treatment and isothermal 400 degrees at different time. Won 6 to 9%. The residual austenite of 0.08C-2 ~ 3Mn-0.5Ni system of low alloy steel with critical heat treatment was studied and the performance of regulation of residual austenite. The results show that the low C 3%Mn steel was 680 DEG C critical annealing can obtain 30 min 13% left Right of residual austenite, and low C 2%Mn steel using two step critical heat treatment can also obtain 8% residual austenite stability. Based on the 3%Mn and 2%Mn steel respectively by one step and two step critical heat treatment, can achieve 700 MPa yield strength, uniform elongation is greater than 10%, the total extension rate of more than 25% excellent performance and good low temperature impact toughness. The 0.08C-2 - 3Mn-0.5Ni system of low alloy steel by two step heat treatment to obtain the critical fine austenite was investigated. The results show that the residual austenite is obtained in the two critical step in the process of heat treatment to benefit from the following three aspects: first, the first step is obtained due to the metastable austenite reverse transformation the enrichment of alloy elements, so that the formation of martensite alloy element concentration of bainite in the subsequent cooling process, the enrichment of the martensite / bainite alloy elements in the critical second step of tempering In the process because of its low temperature Ac1 and become a priority of reversed austenite nucleation sites. Secondly, in the second step the critical process of tempering alloy elements to the metastable austenite reverse again in high abundance (C reached more than 0.4%, Mn reached more than 6%, Ni greater than 0.6%), to further improve the stability of austenite third, second; the two step critical tempering formation of reversed austenite uniform, mostly is film shaped, and the size is extremely small (the average size is about 300 nm), nanometer size effect of austenite is an important reason for stable residual austenite. Through different tensile stress in situ EBSD variables analysis showed that residual austenite in low alloy steel in different strain stages gradually occur TRIP effect, the steel instantaneous hardening index continued to rise the formation hardening behavior of three phase processing, delayed necking occurs, is to achieve high uniform elongation The important reason. In addition, effects of the content of residual austenite in low alloy steel on toughness is also great, and with the decrease of impact test temperature becomes more significant. The existence of residual austenite is helpful to improve the ability of plastic deformation in the process of low temperature impact by TRIP effect, delayed initiation, initiation and improve work, so as to obtain the low temperature impact toughness excellent. The residual austenite plasticized toughening mechanism to achieve the level of 500MPa, yield strength, uniform elongation is greater than 20%, -100 degrees of impact energy is about 60 J with high strength, high plasticity and high toughness - low alloy steel.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG142.33

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