LOW YIELD RATIO STEEL PLATE FOR LOW TEMPERATURE USE, EXCELLENT IN TOUGHNESS IN WELD HEAT-AFFECTED ZONE AT LARGE HEAT INPUT WELDING AS WELL AS IN AMMONIA STRESS CORROSION CRACKING RESISTANCE

摘要

PROBLEM TO BE SOLVED: To provide, as the first priority, a steel plate for low temp. use, having ≥400 N/mm² class tensile strength and excellent toughness at low temp. in the vicinity of -60°C and free from ammonia stress corrosion cracking, to provide, as the second priority, a steel plate for low temp. use, having excellent toughness in HAZ even if subjected to large heat input welding of ≥50 kJ/cm, and to provide, as a third problem, a steel plate for low temp. use, having yield ratio as low as ≤88%. ;SOLUTION: The steel plate whose first priority has been solved has ≤0.06% C content. The steel plate whose second priority has been solved has ≤0.06% C content and also contains 0.005-0.020% Ti and 0.002-0.005% N. Further, the steel plate whose third priority has been is a steel plate for low temp. application, having a lamellar pearlitic structure which is composed essentially of ferrite and in which pearlite existing at ferrite grain boundaries is dispersed in layers. Moreover, the average grain size of ferrite and the interlamellar spacing of lamellar pearlite are regulated to 5-15 μm and ≤30 μm, respectively.;COPYRIGHT: (C)1999,JPO