Separations Occurring at the Fracture Surfaces of Controlled Rolled Hot-Strips

Abstract

Using a laboratory simulation method for producing coiled-strips and plates, a study has been made on the separations occurring at the fracture surfaces of tensile and Charpy specimens of high strength low alloy steel containing 0.04%Nb and 0.09%V. Main results are summarized as follows:
(1) Even by the same rolling process with a finishing temperature of 780°C, the separations of coiledstrips on Charpy specimens are more remarkable than those of plates. Also, separations occur at the fracture surfaces of tensile specimens for coiled-strips, although they can hardly be observed in case of aircooled plates.
(2) The plates which are air-cooled to room temperature and then tempered at the temperature similar to that of coiling, show the separations on tensile specimens.
(3) The separations in coiled-strips are accelerated by temper-embrittling elements such as phosphorus or manganese, and are reduced by molybdenum which is known to be the suppressing element for temperbrittleness.
(4) For coiled-strips, the separations are found to closely correlate with the layer of segregation of phosphorus, and the cracks are mainly through the grain boundary of the ferrite.
(5) From these results, it is concluded that the separations of coiled-strips are caused by temper-brittleness during the slow cooling after coiling i. e., phosphorus in a segregated layer moves to ferrite grain and deteriorates the toughness of the boundaries.