Effect of decreased hot-rolling reduction treatment on fracture toughness of low-alloy structural steels
- PDF / 2,613,337 Bytes
- 9 Pages / 594 x 774 pts Page_size
- 49 Downloads / 225 Views
I.
INTRODUCTION
COMMERCIAL low-alloy structural steels have been used in many engineering components in quenched and highly tempered conditions. Recently, increased use has been made in critical structural applications under severely stressed situations. As a result, the steels used for these purposes require better ductility and toughness as well as higher strength. The isotropy regarding these properties is an especially necessary requirement. In order to meet such requirements, many improvements have been made in the alloying composition and heat treatment of steel. One of the effective approaches for solving this problem has been the development of the mechanical properties through sulfide-inclusion shape control, which is worth noting for commercial use. In such a case, the better approach taken to control the shape of the sulfide inclusions is either to decrease aspect ratios of the inclusions by mechanical treatment, e.g., hot rolling, or to modify the morphology of the inclusions by adding cbemical elements, i.e., rare earth metals, tl] Although recently the rare earth treatment has markedly advanced due to newer and more effective adding techniques, t2,3,4] it is not necessarily practicable, because some difficulty may be encountered in commercial steelmaking practice: (1) rare earth elements are liable to react with activated oxygen [O] in the melt, refractory materials, and atmosphere; (2)rare earth oxides and oxysulfides accumulate at the bottom of the ingot and reduce ductility and toughness; and (3) the process of rare earth injection requires comparatively detailed techniques and high operating costs. Recent investigations t5,6,7] have shown that decreasing aspect ratios of the sulfide inclusions (MnS) improves both long-transverse and short-transverse ductility of commercial steels while not influencing the Ion-
YOSHIYUKI TOMITA, Associate Professor, is with the Deparlment of Metallurgical Engineering, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan. Manuscript submitted September 6, 1989. METALLURGICALTRANSACTIONSA
gitudinal ductility. Thus, decreasing the aspect ratios of the sulfide inclusions by mechanical treatments has found more favor for improved mechanical properties. In such situations, a potential approach has been suggested by the author whereby aspect ratios of the MnS inclusions decreased and, consequently, the mechanical properties were significantly improved: decreasing hot-rolling reduction from 98 to 80 pct modified the shape of the MnS inclusions of the steel from a stringer to an ellipse, which greatly improved the plane-strain fracture toughness (Klc) of ultrahigh-strength 4340 steel, independent of testing orientation. [sl Improved mechanical properties through decreased hot-rolling reduction treatment (DHRRT) are attributed to the following facts: (1) in the longitudinal orientation, modified sulfide inclusions act to blunt and arrest cracks propagating across the specimen which would normally cause failure; and (2) in the transverse orienta
Data Loading...
