ABSTRACT

The effect of calcium treatment on the resulting nonmetallic inclusions and the mechanical properties of 3 plate steels, A5l6-70, A633C, and A5l4F is reported. Both conventional and calcium treated quality of the 3 plate steel grades are investigated. The inclusion types in the conventional quality of these aluminum-killed steels were found to be Type II manganese sulfides and alumina inclusion galaxies. Calcium treatment has been observed to prevent the formation of both of these detrimental inclusion types. The through-thickness tensile percent reduction area in all 3 plate steels was significantly improved by calcium treatment. The upper shelf Charpy-V-notch, dynamic tear and J-Integral fracture toughness were substantially improved in all testing orientations for all 3 steels by calcium treatment. Although the fatigue endurance limit and fatigue crack propagation properties were only slightly enhanced in A5l6-70 by calcium treatment, the fatigue crack propagation properties were significantly improved in A633C and A5l4F.

INTRODUCTION

Offshore structures and in particular offshore oil drilling platforms have to perform under severely stressed situations and in very harsh environments. For these reasons many of the steels used in platform construction have been newly developed or improved to meet this more stringent service application. ne improvement has been the development of steels with low sulfur levels and inclusion shape control through using Calcium Treatment, Lukens "Fineline"R. These steels resist "lamellar tearing" on welding during fabrication and also have enhanced toughness and fatigue properties to improve service performance. Calcium Treatment (CaT) quality plate steels have already been applied in various offshore structures, for example in node areas where lamellar tearing is a concern. Also CaT steels have been employed in cases where low temperature toughness is required or in deep water applications where high reliability is demanded.

Steels produced by CaT have significantly lower levels of indigenous nonmetallic inclusions than steels produced using Conventional (CON) techniques. These inclusions precipitate as discrete phases during the solidification of molten steel, i.e. sulfides and oxides. A review of the CaT process in comparison to CON techniques, as well as the kinds of inclusions found will be given in this paper.

CaT has been found to improve levels of tensile ductility, particularly in the through-thickness testing orientation, impact and fracture toughness and fatigue properties in a number of aluminum killed steels (1-6). In many instances the properties have approached, although not equalled, the quality of electroslag remelted (ESR) steels, which have previously been shown to supply a superior level of properties in plate steels.

The steels used in offshore structures can have a wide range of strength levels. For this reason the properties of 3 different plate steel grades will be reported here namely, A5l6-70, A633C and A5l4F. These steels have minimum 0.2% offset yield strength minimums of 38 ksi (263 MPa), 50 ksi (345 MPa) and 100 ksi (690 MPa), respectively. Both CaT and CON quality steels in each grade will be examined. But first a review of the steelmaking practices involved and the nonmetallic inclusions found in these steels will be presented.

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