Evaluation of the fracture toughness of hot-rolled low-alloy Ti-V plate steel
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I.
INTRODUCTION
IT is well established that thermomechanical processing of steels produces optimum mechanical properties.~ In hot rolling, the finish rolling temperature (FRT) and the cooling rate after rolling determine the strength and toughness of a product. As the FRT decreases, the microstructure becomes more refined, although banding occurs at lower temperatures. High cooling rates after rolling introduce hard brittle phases that increase strength but decrease toughness. Ferritic steels exhibit transition from low energy brittle fracture at low temperatures to high energy ductile tearing at higher temperatures. A good indication of the onset of brittleness is obtained from the ductile-brittle transition temperature (DBTT), at which a specific toughness value is achieved. For design purposes, fracture toughness can be characterized by the critical value of one fracture mechanics parameter, such as the stress-intensity factor, Jintegral, or crack-tip-opening displacement (CTOD). These parameters are related because they are all determined at the onset of crack growth of sharp fatigue-precracked specimens at low deformation rates. Toughness is more commonly measured-by the energy needed to break a Charpy V-notch specimen under impact. The energy absorbed in a Charpy test is required to initiate and propagate a crack and is not exclusively related to the onset of crack growth. Charpy test conditions are also different from those used in fracture mechanics tests. Nevertheless, correlations have been sought between Charpy energies and fracture toughness, 2 because of the convenience of the Charpy test. Therefore, it is not surprising that sometimes these correlations are not valid 3 and that when they are valid it is only under restricted conditions. The aim of the work reported here is to investigate the relationship between several measures of toughness and to evaluate the effect of thermomechanical processing parameters on the fracture toughness properties of a microalloyed Ti-V plate steel.
B. FAUCHER is Research Scientist, Physical Metallurgy Research Laboratories, CANMET, Energy, Mines and Resources, 568 Booth Street, Ottawa, ON, Canada K1A 0G1. B. DOGAN is Research Scientist, GKSS-Forschungszentrum Geesthacht GmbH, Federal Republic of Germany. Manuscript submitted November 24, 1986. METALLURGICALTRANSACTIONS A
II.
MATERIAL
An experimental heat of low C, micro-alloyed Ti-V steel was produced at the Physical Metallurgy Research Laboratories (PMRL) in the form of a 133 mm thick ingot. The composition of the steel, given in Table I, is typical of plate steels for offshore applications. In the as-cast condition, the area fraction of inclusions was about 0.2 pct. To produce plate, ingots were preheated at 1120 ~ then rolled to 25 mm thickness. Three schedules were employed with different finish rolling temperatures, 9, 12, and 17 rolling passes being used for FRT's of 950, 830, and 700 ~ respectively. After rolling, the plates were either air-cooled (AC) or water-quenched (WQ). Six different microstructures,
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