The Void Behavior Of The X80 Steel In The Process Of Rupture

The paper studied the law of the inclusion's deformation and the initiation of void in the tensile and impact test of the steel X80. The size, shape, and deformation regulation of the inclusion was observed in the X80 by SEM. Then, it obtained the initial strain and the rupture strain of the initiation of the void in the tensile test of X80 steel. By a series of analysis, such conclusions can be drawn as follows:(1)According to the void theory of the critical divergence ratio and the model of R-T ,the critical macro-parameter of void divergence ratio computed (V_GC) of X80 steel in tension test was approaching to documentation[41]'s. This explained that the critical macro-parameter of void divergence ratio (V_GC) in the tension test was still effective as the criterion to judge the critical situation of rupture, V_GC=ε_pexp(3/2T)=3.47±0.17, but that was rather conservative for fully using the X80 steel, so it was essential to revise the critical divergence ratio theory of void properly .(2)It was different obviously between the critical macro-parameter of void divergence ratio VGC in the tensile abruption sample and those in the impact abruption sample (diversity ratio up to the scale from 39% to 200% ). Such result showed that the theory of critical divergence ratio had been impractical during the final rupture course, probably because a large number of voids initiated in the final rupture course. Therefore, it was not appropriate to judge the critical situation of rupture by the critical macro-parameter of void divergence ratio under the situation of impact. More over, it was necessary to find the new suitable rupture criterion under all stress condition for the steel of high strength and tenacity such as X80 by further research. By the observation of SEM, there were more than 2,000 voids in the tensile fracture and 500 voids in the impact rupture of X80.(3)By analysis of the deformation of various tensile samples, it presented anisotropic properties; By observation of the longitudinal section of various tensile samples deformed differently, it could be obtained that the voids initiated at the tensile strain of 14% and the sample cracked at the tensile strain of 24%. In the final rupture course, the nucleation and growth of voids were so fast increasingly. From the observation results of the SEM, the shape of voids appears elliptical approximately. Comparing with the circle voids in the models of Gurson and the void theory of the critical divergence ratio, they are very different essentially.(4)The main inclusion of the X80 steel was the series of CaO-MgO-Al₂O.The deformation of inclusions was related to the degree of tension. Bigger the degree of tension, larger the axial ratios of inclusions were (the deformation of inclusion is larger), and the included angle between the long axis and the main arbor of tension would be smaller. Reversely, Smaller the degree of tension, smaller the axial ratios were, approaching to 1(the shape appeared circle),and the included angle would be larger.