Keniti Amano

EFFECT OF ACCELERATED COOLING CONDITION ON FERRITIC AND SUBSEQUENT SECOND PHASE TRANSFORMATION FROM DEFORMED AUSTENITE

ABSTRACT This paper describes the result of a study concerning transformation behavior of an accelerated cooling after controlled rolling. In the conventional accelerated cooling, when a finish-cooling temperature is lowered below 450°C, the tensile strength (T.S.) is increased accompanying with decrease in the yield strength (Y.S.) due to the formation of martensite as a second phase. On the other hand, by applying the new cooling pattern, in which the cooling in ferritic transformation range is accelerated and subsequent cooling in bainitic transformation range is furthermore controlled, both T.S. and Y.S. are increased and the impact toughness is not deteriorated. The improved combination of T.S., Y.S., and toughness, which can not be realized by conventional accelerated cooling, is thus obtained. This behavior can be explained in terms of both the ferrite grain refinement and the formation of lower bainite-like transformed products, which are transformed from 500°C to 400°C as the second phase.

Study on Unstable Brittle Crack Arrest Toughness of Extremely-Low Carbon Bainitic Steel Plates

The extremely-low carbon bainitic steel (ELCB steel) is a high strength steel with about 0.02 mass% or less carbon. In this research, unstable brittle crack arrest toughness of ELCB steel plates was investigated by temperature-gradient ESSO tests, compared with that of conventional TMCP steel plates. Both of ELCB and TMCP steel plates without pre-straining had sufficient crack-arrest toughness at 0°C. After 10% prestraining, the TMCP steel plate had not sufficient crack-arrest toughness at 0 °C . The ELCB steel plates, however, maintained high crack arrest toughness at 0°C. even after 10% pre-straining. ELCB steel were also different from TMCP steels in the correlation between transition temperature of crack arrest toughness and fracture appearance transition temperature (vTrs) obtained by Charpy impact test. When the vTrs of an ELCB steel and that of a TMCP steel were the same value, crack arrest toughness of an ELCB steel was higher than that of a TMCP steel. In the cross section of the ESSO test piece of the ELCB steels, many sub-cracks and micro-crack branching were observed. However, in the cross section of the ESSO test piece of the conventional TMCP steels, there were few subcracks and branching. Initiation of sub-cracks and branching around the main crack tip reduces the stress intensity factor of the main crack. It was considered that the above features of the ELCB steel were caused by initiation of sub-cracks and branching at the tip of the main brittle crack.Copyright

APPLICATION OF ACCELERATED COOLING FOR PRODUCING 360MPa YIELD STRENGTH STEEL PLATES OF UP TO 150 MM IN THICKNESS WITH LOW CARBON EQUIVALENT

ABSTRACT An accelerated cooling (ACC) process including direct quenching (DQ), which commonly is called TMCP (Thermo-Mechanical Control Process), is a technique which can overcome problems for the production of heavy wall steel plate which meets required strength with a chemistry suitable for low temperature applications. The TMCP process enhances strength properties without sacrificing notch toughness through grain refinement under controlled cooling and introduction of second phase microstructures resulting in the decrease in Carbon equivalent and omission of pre-heating prior to commencement of welding. This TMCP process has been applied to produce heavy wall steel plates for ships, offshore structures and pressure vessels which require high toughness at low temperature and other superior fabrication characteristics. This paper describes the mechanism of TMCP ranging from controlled rolling to accelerated controlled cooling for producing 360MPa yield strength in steel plates of up to 150mm in thickness for diverse use.