循环加载下超高强钢力学性能表征研究

发布时间：2019-02-22 17:44

【摘要】：汽车工业的持续迅猛发展给能源和生态环境带来了巨大的负担,高强钢的研究与应用显得尤为重要。淬火配分钢(QP钢)作为第三代汽车先进高强度用钢,由于具备良好的强度和塑性,在汽车制造行业有着广阔的发展前景。本文主要研究循环加载方式对QP980超高强钢力学性能的影响,并从微观角度对其进行机理分析。主要内容包括:首先,对QP980超高强钢进行单向拉伸试验,获取材料的力学性能参数,并对材料的各向异性进行研究;其次,进行拉伸-卸载-拉伸循环加载试验,通过与单向拉伸试验对比分析QP980超高强钢的力学特性,包括材料弹性模量、屈服强度、抗拉强度、延伸率等方面,来获悉循环加载方式对材料力学性能的影响;最后,对不同应变量下的试样进行OM、SEM、TEM、XRD等微观检测试验,解释超高强钢在循环加载条件下的变形机理。通过单向拉伸试验可以得知QP980超高强钢拥有很高的强度和良好的塑性,具备优良的综合力学性能;循环加载方式会对材料产生增强增塑的效果;除此以外,循环加载中的弹性模量不是固定不变的值,而是随着应变量的变化,呈现先增大后逐渐减小的趋势;并且卸载与重新加载路径体现了非线性特性,即随着应力的增加,弹性模量发生变化,卸载与重新加载曲线的起点和终止点重合,形成一个封闭区域。微观检测试验得出:QP980超高强钢在应力作用下发生马氏体相变;马氏体含量的不断增加导致了材料强度的升高;并且由于马氏体含量增加使晶粒得到细化,增强了材料的塑性;材料内部残余应力、马氏体相变以及位错运动产生的微小塑性应变导致弹性模量的变化及卸载-重新加载曲线的非线性;循环加载过程,由于在卸载时位错发生反向滑移,为马氏体的继续增长提供了条件,导致相同的应变条件下,循环加载比单向拉伸试样的马氏体含量高,说明循环加载方式对马氏体相变起到一定的促进作用。
[Abstract]:The continuous rapid development of automobile industry has brought a huge burden to energy and ecological environment, and the research and application of high strength steel is particularly important. As the third generation advanced high strength steel for automobile, quenched steel (QP) has a broad prospect in automobile manufacturing industry because of its good strength and plasticity. In this paper, the effect of cyclic loading on the mechanical properties of QP980 ultra-high strength steel is studied, and the mechanism is analyzed from the microscopic point of view. The main contents are as follows: firstly, the uniaxial tensile test of QP980 ultra-high strength steel is carried out to obtain the mechanical properties of the material, and the anisotropy of the material is studied. Secondly, the mechanical properties of QP980 ultra-high strength steel, including elastic modulus, yield strength, tensile strength, elongation and so on, are compared with uniaxial tensile test. To learn the effect of cyclic loading on the mechanical properties of materials; Finally, the microcosmic tests such as OM,SEM,TEM,XRD were carried out to explain the deformation mechanism of ultra-high strength steel under cyclic loading. The uniaxial tensile test shows that QP980 ultra high strength steel has high strength, good plasticity and excellent comprehensive mechanical properties. In addition, the modulus of elasticity in cyclic loading is not a fixed value, but with the change of strain, it increases first and then decreases gradually. The unloading and reloading paths reflect the nonlinear characteristics, that is, the elastic modulus changes with the increase of the stress, and the starting point and the ending point of the unloading and reloading curves coincide to form a closed region. The microcosmic test results show that martensite transformation occurs in QP980 ultra-high strength steel under stress, the increasing of martensite content leads to the increase of the strength of the material, and the grain size is refined because of the increase of martensite content, and the plasticity of the material is enhanced. The internal residual stress, martensitic transformation and tiny plastic strain caused by dislocation movement lead to the change of elastic modulus and the nonlinearity of unloading-reloading curve. During cyclic loading, the reverse slip of dislocation occurs during unloading, which provides the condition for the further growth of martensite, resulting in higher martensite content in cyclic loading than in uniaxial tensile specimen under the same strain condition. The results show that cyclic loading plays an important role in promoting martensite transformation.
【学位授予单位】：北方工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG142.1

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