PSI - Issue 13

Kejin Zhang et al. / Procedia Structural Integrity 13 (2018) 1047–1052 Kejin ZHANG / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

With increasing demand for weight reduction in the automobile industry, the use of high-strength steel is increasing. Punching is used to make holes in steel plates, and the punched plate has an area, called the shear-affected-zone (SAZ) [Levy et al. (2008)], where the punching-induced plastic deformation has locally changed the properties of the steel. These changes comprise reductions in tensile properties [Dalloz et al. (2009)] and fatigue properties [Korsgren et al. (1989)], leading to certain problems. The vicinity of the punched hole experiences a large plastic strain. Uniform plastic strain in an unpunched specimen causes an increase in the yield strength and tensile strength due to dislocation hardening, grain refinement, etc., and a decrease in toughness due to void initiation and work hardening. However, punched steel plates differ in characteristics because, in the SAZ, there are microstructural changes with a plastic strain gradient from the punched surface, where the strain reaches a maximum value, decreasing in the inward direction [Dalloz et al. (2009)]. In the inner surface of a punched hole, there are surface roughness and micro-cracks [Sánchez et al. (2004)] and tensile residual stress is generated [Korsgren et al. (1989)]. The consequent expression of the change in properties varies. Therefore, it is difficult to universally evaluate their influence on the steel’s strength characteristics. In this study, as a method to evaluate the influence of SAZ under tensile loading while focusing on crack initiation and propagation, our objectives were: (1) Determine whether any special events occur under tensile loading in the fracture mechanism of holed specimens with a SAZ vs. holed specimens without a SAZ. (2) Find the reasons why the special events occur. We carried out the following to solve these problems. We generated a triaxial stress field in holed specimens due to stress concentration under tension. We used two types of specimens with the same hole diameter. The type called “punched specimen s ” had a hole formed by punching, and the type called “honed specimen s ” had a hole processed by drilling and polishing, and thus have no SAZ. The steel being studied is TiC precipitation-hardened steel (PH steel) used as a high strength steel sheet for automobiles. We prepared two kinds of PH steels with different strength levels. After performing tensile tests on these specimens, we examined the load-stroke relationships and observed the change in the surface of the specimen with increasing load. Furthermore, we observed the fracture surface to investigate the failure mechanisms. 2. Experimental Procedure We prepared TiC precipitation-hardened steel specimens, with strength grades of both 590 and 780 MPa classes, hereafter referred to as PH6 and PH8, respectively. The PH steels have ferritic microstructures with different amounts of TiC precipitates. Fig. 1 shows the schematic diagram of the specimen. The stress concentration factor in the hole edge was calculated to be approximately 2.4 using ANSYS software. Fig. 2 (a) shows the schematics of the punching process used to make the punched specimens. The clearance was 12.5%, and the punching speed was 200 mm/min. As a consequence, the depth of the SAZ from the hole inner surface was measured at about 1 mm via hardness measurement [Hamada et al. (2018a)]. For the honed specimens, we drilled the hole and removed the process-affected zone by polishing. The tensile speed of all specimens was 10 mm/min, and video was recorded to observe the behavior of the specimen under test. Fracture surface observations were conducted as shown in Fig. 2 (b) using a scanning electron microscope (SEM). 3. Experimental Results 3.1. Tensile test Fig. 3 shows the engineering stress-stroke curves. The nominal stress was defined as the stress at the minimum specimen section. The PH6 punched specimens showed no obvious difference between the punched and honed specimens, but the PH8 punched specimens showed brittle fracture behavior with extremely low elongation, while the PH8 honed specimen showed ductile failure behavior.