摘要
高碳淬火钢难以在室温下发生塑性变形,人们曾将此现象归因于成分(碳含量过高)和组织(马氏体塑性差),未考虑几何尺寸的影响.为此,本研究在室温下对厚度为400?m的9Si Cr淬火马氏体钢进行微轧制实验,发现微轧制下实验钢的塑性良好,延展率达200%以上,且硬度比淬火态提高了39%.本文对此新发现进行了研究.首先,负辊缝轧制使横向压应力增大,同时由于单位体积的接触表面积变大,而引起纵向压应力和垂直方向压应力同时增大,因而静水压力增加.这些因素有利于抑制轧件边裂、头部劈裂以及轧件内部缺陷生成和扩展,致使塑性改善.这一现象可以用传统理论进行解释,属于第一类尺度效应.其次,由于位错和孔洞一旦到达试样表面就会释放和消失,所以轧件的比表面积越大对提高塑性越有利.因此,比表面积充分大的薄轧件已不能视为均匀连续、各向同性的理想材料,其表面效应属于第二类尺度效应.实验结果表明:长期以来认为高碳马氏体组织塑性差的认识在薄轧件轧制中受到挑战,需要同时考虑两种尺度效应,才能深入认识其力学表现和物理本质.
It is difficult for high-carbon quenched steel to deform plastically at room temperature; this has been attributed to the composition（high carbon content） and microstructure（martensite without plasticity）, but the geometric scale of the sample has not previously been considered. In the present study, a quenched 9Si Cr steel with an initial thickness of 400 ？m was micro-rolled at room temperature in order to understand the size effects. It was found that the extensibility of the test steel was over 200%, and the hardness was 39% higher than that of the as-quenched steel. Additional research was performed to explore these novel findings. We found that the transverse compressive stress was increased by negative-roll-gap rolling, while the longitudinal and vertical compressive stresses were also increased because of the increase in contact surface area per unit volume; consequently, the hydrostatic stress also increased. These above factors are helpful to prevent edge-cracking and the formation of split ends in rolled products, and they also inhibit the generation and expansion of internal defects; hence the plasticity is improved. We refer to the above effects, which can be explained by conventional theories, as first-order size effects. However, once dislocations and voids arrive on the surfaces of the sample, they can be released and disappear, causing the plasticity to improve even more than usual when the specific surface area becomes very large. A ultrathin rolled product with a large specific surface area cannot be regarded as an ideal homogeneous, continuous, isotropic material. The surface effects of the thin-rolled product are referred to as second-order size effects. These results indicate that the traditional assumption that high-carbon martensite must have extremely low plasticity is no longer valid in the case of ultrathin rolled products, whose mechanical characteristics and physical essence can be understood only by considering two distinct orders of size effects.
出处
《中国科学：技术科学》
EI
CSCD
北大核心
2015年第11期1187-1194,共8页
Scientia Sinica(Technologica)
基金
国家自然科学基金(批准号:51374069)
江苏省自然科学基金(批准号:BK20131123)
江苏省"六大人才高峰"项目(批准号:2012-ZBZZ-007)
住房与城乡建设部研究开发项目(批准号:2013-k4-28
2014-k4-005)资助
关键词
高碳马氏体钢
超延展性
尺度效应
静水压力
微轧制
high-carbon martensitic steel
extreme extensibility
size effect
hydrostatic stress
micro-rolling
