A ferritic steel precipitation-strengthened by nanometer-sized carbides was developed to obtain a high strength hot-rolled sheet steel having tensile strength of 780 MPa grade with excellent stretch flange formability.Manganese in a content of 1.5 % and molybdenum in a content of 0.2 % were added to 0.04 % carbon Tibearing steel in order to lower austenite-ferrite transformation temperature for fine carbides and to retard generating of pearlite and large cementites, respectively. Tensile strength of hot-rolled sheet steel increased with titanium content and it was achieved to 800 MPa in a 0.09 % Ti steel. Microstructure of the 0.09 %Ti steel was ferrite without pearlite and large cementites. Fine carbides of 3 nm in diameter were observed in rows in the ferrite matrix of the 0.09 % Ti steel with transmission electron microscope. The characteristic arrangement of the nanometer-sized carbides indicates that the carbides were formed at austenite-ferrite interfaces during transformation. By energy dispersive X-ray spectroscopy, the carbides were found to contain molybdenum in the same atomic concentration as titanium. Crystal structure of the nanometer-sized carbides was determined to be NaCl-type by X-ray diffractometry. The calculated amount of precipitationstrengthening by the carbides was approximately 300 MPa. This is two or three times higher than that of conventional Ti-bearing high strength hot-rolled sheet steels.Based on the results obtained in the laboratory investigation, mill trial was carried out. The developed hotrolled high strength sheet steel exhibited excellent stretch flange formability.
Tensile behavior and structure-property relationship of ferritic steels with nano-sized carbide dispersion were invesigated using Ti-added steel and Ti,Mo-added low carbon steels. By austenitizing followed by isothermal heat treatment at 700°C, polygonal ferrites containing very fine carbides of TiC and (Ti,Mo)C were obtained in the Ti-added and the Ti,Mo-added steels, respectively. The size of such carbides was finer in the Ti,Mo-added steel than in the Ti-added steel at the same isothermal holding. The results of tensile tests for these samples showed that the strength is higher as the carbide size is smaller. The structure-based strength calculation led to a good agreement with the experiments, when it was assumed that the Ashby-Orowan mechanism is dominant for precipitation strengthening of nano-sized alloy carbides. It was also suggested that a relatively large tensile ductility is related to enhanced recovery during the tensile deformation, accompanied with promotion of secondary slips or cross slips in a finer scale due to the nano-sized particles.
The hydrogen embrittlement behavior of an ultra-high strength (1180 MPa grade) dual phase steel sheet composed of ferrite and tempered martensite, as compared with that of a single phase steel sheet composed of tempered martensite, has been investigated by a sustained tensile-loading test. No fracture of the dual phase steel occurs under the low hydrogen-charging current density of 5 A/m 2 except under high applied stress substantially larger than the yield stress. With the high current density of 50 A/m 2 , the time to fracture of the dual phase steel varies widely, but is almost the same as that of the single phase steel. The critical applied stress for fracture of the dual phase steel is higher than that of the single phase steel. Under the high applied stress, however, the time to fracture of the dual phase steel is shorter than that of the single phase steel, and a unique intergranular-like morphology is observed at the crack initiation area on the fracture surface. Upon plastic deformation before the sustained tensile-loading test under the high applied stress, the time to fracture of the dual phase steel increases and the initiation area on the fracture surface exhibits typical quasi-cleavage features. The results of the present study indicate that the hydrogen embrittlement of the dual phase steel displays some anomalous behavior.KEY WORDS: hydrogen embrittlement; delayed fracture; high strength steel; dual phase steel; martensite; ferrite.
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