【作者】 边红霞；

【导师】 朱亮；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2014， 博士

【摘要】 奥氏体不锈钢在热变形过程中会发生不均匀应变,这种应变集中在某一个区域会引起应变局部化,造成工件表面的局部几何失稳,甚至开裂。本文以Cr17Mn6Ni4Cu2N、Cr15Mn9Cu2Ni1N和304奥氏体不锈钢为研究对象,首先利用高温拉伸实验和高温拉伸卸载实验,结合用户自定义奥氏体不锈钢热变形材料模型的有限元分析方法,研究了高温变形条件下,奥氏体不锈钢的拉伸失稳特征,以及表面应变局部化的发展、演变机制；然后,利用改变变形温度、应变速率和应力状态的热变形实验,研究变形条件对几何失稳以及应变局部化演变的影响规律。认识了高温拉伸奥氏体不锈钢的几何失稳过程及机制；获得了不同应力状态下奥氏体不锈钢的几何失稳轨迹；提出了高温变形应变局部化的演变模型；确定了与变形条件相关的材料参数与几何失稳应变的关系,给出了降低热变形应变局部化的加工条件。高温变形条件下,奥氏体不锈钢的应变速率敏感性增大,使其在单轴拉伸变形过程中表现出了载荷失稳与几何失稳不同时发生的现象。同样,也因为应变速率敏感性的作用,载荷失稳后,试样上形成的应变速率增大,变形抗力增加,抑制了潜在几何失稳区的进一步发展,变形转移向其它位置,维持了试样的均匀变形,延迟了几何失稳的发生。几何失稳的发生与应变局部化区的最大长度Lmax和最大深度dmax有关。950℃拉伸试样几何失稳时的Lmax和dmaz分别达到9.8mmm和0.56mmm;1150℃拉伸试样几何失稳的Lmaz和dmaz分别达到11.1mm和0.82mm。受变形量和变形温度的影响,高温拉伸变形奥氏体不锈钢表面应变局部化区的数目、最大长度以及最大深度都在不断地变化。变形量增加,其数目先增大后减小,最大长度Lmax和最大深度dmax都增大；变形温度降低,其数目减少,但最大长度Lmax增大,最大深度dmax减小高温拉伸变形奥氏体不锈钢表面存在“活跃”的和“休眠”的两种应变局部化区。应变局部化的演变,实质上是就是“活跃”区分裂、合并邻近“休眠”区的过程。温度越高,这种分裂、合并的能力增强。以两个“活跃”区同时竞争合并“休眠”区建立的应变局部化演变模型,表明在应变速率敏感性的作用下,哪一个“活跃”区能对周围的“休眠”区进行合并,取决于这两个区域变形抗力的大小,变形抗力越大其合并能力越强。同时,变形抗力的大小由应变局部化区的横截面积和在同一时间内截面积的减小量决定。在950-1200℃温度范围内,温度升高奥氏体不锈钢的几何失稳应变增大,在0.01-10S1的应变速率范围内,应变速率降低,奥氏体不锈钢的几何失稳应变增大。分析材料参数与失稳应变的关系,发现加工硬化指数n控制了载荷失稳应变,应变速率敏感性系数m控制了载荷失稳到几何失稳阶段的应变。载荷失稳到几何失稳阶段的应变是几何失稳应变的主要构成,且随m值增大而增大。几何失稳应变与材料参数n、 m的关系为ε,=an+bnm,其中,6n=4.167+5.389n。n、m值与载荷失稳应变和载荷失稳到几何失稳阶段应变的关系分别为应变速率改变了几何失稳的形式,在0.1-2.5s-1的应变速率范围内,几何失稳是载荷失稳和几何失稳相分离的形式,而在10s-1时变为几何失稳和载荷失稳同时发生。应变速率也改变了奥氏体不锈钢应变局部化的演变过程机制,随着应变速率的增大,由同时存在几个“活跃”区域竞争对“休眠”局部化区域合并,转变为只存在唯一的“活跃”应变局部化区域合并“休眠”应变局部化区域。分析应变速率影响的材料性能发现,奥氏体不锈钢在低应变速率下既存在应变强化又存在应变速率强化,而在较高应变速率下,只存在应变强化。不同应力三轴度η和Lode参数μσ下,奥氏体不锈钢的几何失稳轨迹表明,几何失稳应变随η值和μσ心值的增大,先增大后减小,最大的几何失稳应变并不出现在单轴拉伸条件下,与材料所处的应力状态有关。几何失稳应变随η和μσ值的增大而减小是试样缺口中心和边部受力和变形的差异性加剧的结果。应力三轴度η值和Lode参数μ。的增大,使奥氏体不锈钢的几何失稳形式由载荷失稳与几何失稳分离转变为载荷失稳和几何失稳同时发生；也使缺口作为应变局部化区表现出的活跃性逐渐降低,由能够作为“活跃”应变局部化区域不断合并缺口以外其它应变局部化区域的状态转变为固定在缺口内部的“活跃”应变局部化区或者几何失稳区。本文对奥氏体不锈钢在高温变形条件下的几何失稳过程和应变局部化演变机制进行了全面的研究与分析,为认识材料的高温塑性失稳提供了实验和理论依据；提出在1000-1150℃温度范围和01-2.5s-1的应变速率范围对奥氏体不锈钢进行热加工能获得较大的延伸量和较好的表面质量,期望本文的研究结果能为实际生产中改进生产工艺提供帮助。更多还原

【Abstract】 When the nonuniform strain which occurs in hot deformation of Austenitic stainless steel is concentrated in some region, it can cause strain localization and lead to local geometry instability on the surface of workpiece, and even cracking. In order to study the phenomenon, this paper, the Cr17Mn6Ni4Cu2N, Cr15Mn9Cu2Ni1N and304austenitic stainless steel are taken as the main research materials. The tensile instability characteristics of austenitic stainless steel and the evolution mechanism of surface strain localization have been investigated by the high temperature tenile test and high temperature tensile unloading test and the finite element analysis method which material model is defined by user. The influence of deformation condition on geometry instability and strain localization evolution rule have been investigationg by the hot deformation tests which deformation temperature, strain rate and stress state were changed. Have cognized the process and mechanism of geometry instability in high temperature tensile, obtained the geometry instability trajectory austenitic stainless steel in various stress states, put forward the model of strain localization evolution in high temperature, determined the relation between material parameters which have relationship with deformation conditions and geometry instability strain and given the processing conditions to reduce strain localization in high temperature deformation.In the condition of high temperature deformation, it make the load instability and geometric instability do not occur at the same time in uniaxial tensile that the strain rate sensitivity of Austenitic stainless steel increase. As well, because of the reason strain rate sensitivity increase, it restrain the deformation of geometry instability region that the deformation resistance increase with the strain rate in potential geometry instability region which is formed in the specimen after the load instability and make it transfer to other positions. This maintains the sample homogeneous deformation and delay the geometric instability occurence.The occurrence of geometry instability relates with the maximum length Lmax and the maximum depth dmax of strain localization region. In950℃, the Lmax and dmax respectively is9.8mm and0.56mm when the geometry instability occur and in1150℃, the Lmax and dmax respectively is11.1mm and0.82mm when the geometry instability occur. Under the influence of elongation and deformation temperature, the number, the maximum length and the maximum depth of strain localization zone of austenitic stainless steel in high temperature tensile. With the increase of elongation, the number is first increases then decreases and the Lmax and dmax is increases. With the decrease of deformation temperature, the number is decreases and the Lmax is increases and dmax is increase.There are "active" and "sleep" two kinds of strain localization region in austenitic stainless steel surface in deformation tensile. In fact, the evolution of strain localization is the process that the "sleep" strain localization region is divided and merged by "active" strain localization region. With the temperature rise, the process is more obvious. The model that is built by the mechanism that two "active" strain localization regions merger "sleep" strain localization regions at the same time by competition showed it is determined by deformation resistance which one "active" strain localization region is able to merge the "sleep" strain localization around the region in the effect of strain rate sensitivity and the deformation resistance is greater and its ability stronger to merger. The deformation resistance depends on the size of the cross-sectional area of strain localization region and the amount of cross-sectional area to reduce.In950-1200℃temperature range, geometry instability strain of austenitic stainless steel is increase with temperature rise and in0.01to10s-1strain rate range, geometry instability strain of austenitic stainless steel is increase with strain rate reduce. Through the analysis of the relationship between material parameters and instability strain, found that strain hardening parameter index n control the load instability strain and strain rate sensitivity coefficient m control the strain from load instability to the geometric instability. This strain from load instability to the geometric instability is the main part of geometric instability strain, and is increase with the increase of m value. The relationship between the geometry instability strain and and material parameters isε1=an+bnm, and among themα,,=-0.153+1.268n, bn=4.167+5.389n. The relationship between、m valuesand load instability strain and the strain from load instability to geometric instability respectively isεn=en-1, ε1-εn=an+bnm-en-1.In0.1-2.5s-1strain rate range, load instability and geometrical instability is separation in the process of geometry instability, and geometric instability and the load instability occurs at the same time in10s-1.The evolution mechanism of strain localization has also changed with the strain rate change. At low strain rate, there are a few "active" strain localization regions in specimen, and they merger "sleep" localized region by competition, however, at high strain rate, there are only one "active" strain localization region in specimen to merger "sleep" localized region. To analyse the material’s parameter in various strain rate, found there are strain strengthening and strain rate strengthening at low strain rate and there are only strain strengthening at higher strain rate in austenitic stainless steel.The trajectorys of geometry instability in various triaxial stess degree7and Lode parameter μ6show that the geometry instability strain is first increases then decreases with η and μ6value increase and the maximum geometry instability strain is not in where η value equals to0.333and μ6value equals to-1. The increase of nonuniform of stress and deformation in specimen is the main reason that geometry instability strain decrease with with77and μ6value increase. With η and μ6value increase, the form of geometry instability change by load instability and geometrical instability separation to geometric instability and the load instability occurs at the same time. With77and μ6value increase, the gap that was taken as strain localization region change from "active" strain localization regions to the "active" strain localization region that is fixed inside the gap or direct geometric instability region.In this paper, the geometry instability process and evolvement mechanism of strain localization of austenitic stainless steel has benn carried on the comprehensive research and analysis in high temperature. The work provides experimen and theoretical foundation to recognize the plastic instability of matrial in high temperature, and expect the results of the work can help to improve the manufacturing technique in thermal deformation.更多还原