【作者】 程眉；

【导师】 张治民；

【作者基本信息】 中北大学 ， 火炮、自动武器与弹药工程， 2013， 博士

【摘要】 304不锈钢以其优异的化学稳定性和良好的加工性,广泛应用于军工、核电、石油化工等领域的重要构件,尤其在耐腐蚀性要求高的舰船、核反应堆等设备中的储存、冷却传输系统中应用广泛。该材料为奥氏体组织,铸态强度低且不能通过热处理强化,无法满足高温高压系统构件的使用需求,严重制约了其应用范围。塑性加工能提高强度,但变形的方式和变形程度对304合金金相组织和力学性能影响异常剧烈。本文研究了三通阀体构件多向成形过程金属的变形规律,通过对304合金变形温度、变形方向、变形速度及变形均匀程度的控制,利用多向柔性加载技术控制零件各部位交替变形,得到综合性能优良的变形组织,从形变强化和细晶强化两方面提高了材料的力学性能。研究304合金流变应力随变形温度及变形速率的变化规律,建立温变形状态下流变应力计算模型；从相变热力学角度讨论塑性变形能和弹性畸变能对形变诱发马氏体形核功的影响。研究表明304不锈钢在200℃左右挤压变形时真应变量可达到1.2以上而不产生变形缺陷,变形过程中不产生马氏体相变,也没有第二相析出,变形组织为单一奥氏体组织,且变形抗力比室温变形有较大程度的减小。针对三通阀体多向主动加载条件下的金属变形过程进行实验研究和数值模拟,分析了不同变形区域的流动特性及应变状态。采用坯料分块挤压试验测定金属流动趋向及缺陷形成条件,分析了缺陷形成的力学原因,揭示了多向变形过程中缺陷形成机制：金属在多向受力条件下由于主动力交叉而引起流线交叉,在流线交叉部位,多向拉应变易形成拉伸缺口,多向压应变易形成流线折叠。研究多向挤压多区域流动状态下金属的变形规律,发现了应变强化型合金稳定变形阶段不同变形区域顺序交替变形特征；并根据流变学原理,建立多向加载条件下塑性变形的流函数速度场,推导了基于能量理论的多变形区域的交替塑性变形判别式,确定了流量方程。讨论了在变形区域内变形体随主动模拖曳流动对减少局部剧烈不均匀变形的贡献；通过多主动模加载形成多个局部主动变形区,有利于实现变形体整体变形的均匀性。根据以上研究成果,对304不锈钢三通阀体进行了多向温挤压成形的试验研究,采用200℃下两工步多向挤压成形技术实现外形和内孔的挤压成形；研究表明,两工步多主动模成形可减轻工件剧烈变形的程度,使工件均匀变形程度提高；变形过程不改变材料的微观组织,而产生形变强化和细晶强化,因此在不降低其耐腐蚀性条件下,使材料的强度指标提高3-4倍,较大程度提高了材料的综合力学性能。更多还原

【Abstract】 304stainless steel products are widely applied in the field of military, nuclear power, petrochemical and other important fields for its excellent chemical stability and good processability, especially in the high corrosion resistance requirements such as ships and nuclear reactors of storage space, cooling system, transport system. As austenitic steel, the casting intensity of304is low, can’t be strengthened with heat treatment at the same time, and can’t meet the demand of the use of high temperature and high pressure system components, severely restricting its application scope. The strength can be improved by plastic processing, but the deformation mode and degree produce a great impact on the microstructure and mechanical properties. In this paper, homogeneous austenitic stainless steel triple valve body was obtained by multidirectional extrusion under200℃, through research on the microstructure of304alloy in cold deformation state and the active load to realize flexible control. The deformation process improve the overall performance of the material from two aspects of deformation strengthening and refined crystalline strengthening at the same time.The transformation law of was studied304alloy rheological stress changing with the deformation temperature and deformation rate, temperature deformation condition rheological stress calculation model was established. The influence of plastic deformation energy and elastic distortion energy was discussed form phase transition thermodynamics. Research shows that the material can’t produce deformation defects while compression strain reach more than1.2at200℃, and do not produce martensite phase transformation and the second phase precipitation in the process of deformation. Deformation texture was single austenite, and the deformation resistance was decrease with room temperature deformation. The experimental study and numerical simulation was done for the triple valve body deformation process by multidirectional active load, and flow characteristics and strain state of different deformation zone was analyzed. Metal flow trend and defect forming conditions was studied through divided the workpiece into several parts, the mechanics reasons for defect formation was analysesed, defect formation mechanisms was revealsed in the process of multidirectional deformation.The workpiece produce streamline crossover caused by active load cross under the condition of multidirectional stress, the partes with multidirectional tensile strain produced tends to stretch gap, and multidirectional compressive strain to streamline folding.The deformation law of multidirectional active loades and multidirectional extrusion area was studied, alternating deformation characteristics of strain-hardening alloy with different deformation area was founded in stable deformation stage; the velocity field of flow function was obtained with multidirectional loades, alternating deformation discriminant in multiple deformation area was deduced based on energy theory, the flow equation was determined.The contribution to reduce the local the non-uniform deformation degree was discussed for materials couette flow with moving die. More active moving dies can get multiple couette flow deformation zone, the deformation parts of workpiece evenly deformed.The research shows that the multidirectional warm extrusion experiment of304stainless steel triple valve body was studied, the profile and inner hole was formed will two step multidirectional extrusion forming in200℃. Studies have shown that two step more moving dies active load can reduce deformation extent, increase deformation uniform. Deformation process does not change the material microstructure, and the deformation strengthening and fine-grain strengthening was obtained, therefore, the intensity of material increase3-4times under the condition of not reducing its corrosion resistance,, largely improve the comprehensive mechanical properties of the material.更多还原