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NANO-HITEN - Development of High Strength Hot-rolled Sheet Steel Consisting of Ferrite and Nanometer-sized Carbides

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NANO-HITEN - Development of High Strength Hot-rolled Sheet Steel Consisting of Ferrite and Nanometer-sized Carbides

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 Ti- bearing 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 con- tain 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 precipitation- strengthening 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 hot- rolled high strength sheet steel exhibited excellent stretch flange formability. [more]

 
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