A Study of As-rolled Low-carbon, High Notch-Toughness Steels

Abstract

Studies have been made in order to develop a new steel which is usable in as-rolled condition for low temperature service. Fine grained steels with low carbon contents of below 0.10% finish -rolled from relatively low reheating temperature, are expected to have ultrafine grains. The effects of chemical composition and rolling variables on the microstructures and notch toughness of low carbon fine grained steels are examined in relation to metallurgical factors.
The reasults are as follows:
(1) When the slabs are reheated at 900-950°C and finish-rolled with 50% total reduction, the ferrite grain size decreases progressively with the decrement of finish rolling temperature down to below 770°C until a limiting value.
(2) The addition of grain refiners such as vanadium, niobium, aluminium, titanium and manganese markedly refines the ferrite grain size of steels rolled from relatively low soaking temperature and yields a high notch toughness in as-rolled condition.
(3) As for the finish rolling variables, the soaking temperature of slab must be so selected that there are large amounts of precipitates remained insoluble and that they still act as grain refiners. The lower reheating tenperature yields a good notch toughness even after a small total deformation.
(4) The good notch toughness of steel plates rolled from low reheating temperature is mainly due to fine ferrite grain size, but partly due to mechanical fibering of ferrite and pearlite which may induce microfissuring in the plane of the plate at the notch root, with the effect that stress triaxiality is relieved and transition temperature depressed.
(5) A combioation of niobium and titanium as the grain refiners improves notch toughness, especially at the grain coarsed region of HAZ.
Based on the above results, a new weldable structural steel has been developed which contains small amount of nickel, but provides transition temperature as low as -100°C