Nb添加非調質高張力鋼板に関する研究(第1報)

Abstract

At first the effect of niobium addition on the mechanical and welding properties of low carbon steel small plate as rolled was investigated in laboratory as compared with vanadium addition. Then the properties of large plate as rolled in works of killed and semikilled steel containing niobium, especially the toughness of the plate were investigated.1) To increase the tensile strength of low carbon steel plate as rolled by 5-6 Kg/mm2 the addition of about 0.05% vanadium is a little more effective than the one of niobium equivalent to 0.05% vanadium with respect the impact properties of the plate. But niobium increases the tensile strength of the plate more than vanadium in the range up to about 0.05% individually.2) It may be said that we have had for the first time nonheat-treated weldable high tension steel plate with both tensile strength over 60 kg/mm2 and structure consisting of ferrite and pearlite. The addition of both niobium and vanadium to the low alloy steel containing lower content of carbon further improves the impact properties of the plate as rolled.3) The high tension steel plate containing niobium has a higher yield ratio as well known and can be welded easily, but the absorbed energy in Charpy impact test of the plate is comparativelly small and this phenomenon is especially controversial in the transverse direction to rolling one. The absorbed energy in Charpy test being small is probably due to the higher tensile strength for the structure consisting of ferrite and pearlite. But the transition temperature of fracture surface in Charpy test is comparativelly good and not worse than the one of low tension steel plate containing no niobium.4) The new evaluation criteria for brittle fracture characteristic of structural steels for low temperature application (recently proposed by Japan Welding Engineering Society) adopt the evaluation method by transition temperature of fracture surface in Charpy test, rejecting the one by the absorbed energy. The transition temperature of fracture surface of high tension steel plate containing niobium being good, the small absorbed energy of the plate seems not to be a large defect.5) The fracture tests on a larger scale such as press-notch crack starter test (wedge impact test) and double tension test of the plate were also carried out. The results of these fracture tests matched the transition temperature of fracture surface in Charpy test and supported the evaluation results by the above new criteria.6) The tensile strength of carbon steel plate containing no niobium coiled immediately after hot-rolling is lowered in general due to the pearlite structure being globularized during slow cooling from coiling temperature, but the one of coiling plate containing niobium is little lowered by precipitation hardening of niobium carbo-nitride. The method to apply globularizing annealing to uncoiled flat plate after hot-rolling may be suitable to improve the impact properties of large plate unable to be coiled.