Dissertation
Dissertation > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metallic materials > Steel > Variety of steel products : chemical composition distinguish > Alloy steel

New ultra-high-strength steel alloy to optimize its organizational performance research

Author LuZuo
Tutor XieGang;SuJie
School Kunming University of Science and Technology
Course Iron and steel metallurgy
Keywords Eglin steel thermodynamics calculation undissolved carbides new type ultrahigh strength steel
CLC TG142.33
Type PhD thesis
Year 2011
Downloads 109
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Based on the analysis of strengthening and toughening mechanism of low-alloy ultra-high strength steels,30Cr3Si2Mn2NiWMoVNb steel and 30Cr3Si2Mn2NiMoVNb steel are designed. The machaical properties and microstructure of newly designed steels and Eglin steel are systematic investigated by means of mechanics performance testing, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Furthermore, the relations between undissolved carbides and austenited temperature are also studied. The main research and conclusions are obtained as follows:The toughness of G50 steel can be improved by adding Si and Nb in G50 steel. Based on the method, the toughness of Eglin steel can be improved by adding 2% Mn in Eglin steel. However the strength must be reduced. So the content of Mo can be increased to improve the strength of Eglin steel. So based on Eglin steel,30Cr3Si2Mn2NiWMoVNb steel, a new low cost ultra-high strength steel, can be by thermodynamic calculation by application of Thermo-Calc software. The results show that austenizing at 950℃for 1 hour, oil quenched and then tempered at 260℃for 2 hours followed by air cooling is the optimal heat treatment for 30Cr3Si2Mn2NiWMoVNb steel. Under the conditions, tensile strength of 187OMPa, yield strength of 1460MPa, Charpy U-notch impact toughness of 66J and ductility of 12% were obtained. The tensile strength of new steel is increased about 200Mpa with almost the same toughness against Eglin steel. The successful development of 30Cr3Si2Mn2NiWMoVNb steel can offer a method to develop a low cost ultrahigh strength steels.In order to reduce the cost, based on the G50 steel, the content of Mn increases by 1% to improve the toughness, the content Cr increases by 2% to improve hardenability and V is added to improve the strength.30Cr3Si2Mn2NiMoVNb steel, a new low cost ultra-high strength steel has been designed by thermodynamic calculation by assistant of Thermo-Calc software. The results show that austenizing at 950℃for 1 hour, oil quenched and then tempered at 260℃for 2 hours followed by air c(?)oling is the optimal heat treatment of 30Cr3Si2Mn2NiMoVNb steel. Under the conditions, tensile strength of 1750MPa, yield strength of 133OMPa, Charpy U-notch impact toughness of 74J and ductility of 13% were obtained. Tensile strength of 30Cr3Si2Mn2NiMoVNb steel is increased about 100MPa with almost the same toughness against Eglin steel and insensitive to austenitizing temperature. 30Cr3Si2Mn2NiMoVNb steel has almost the same mechanical properties as G50 steel, but is much cheaper.The microstructure of newly designed steels are characterized through OM, SEM, EDAX, XRD and TEM. The results show that matrix of new designed steels is lath-martensite with high dislocation density, interlath retained austenite and some undissolved carbides.30Cr3Si2Mn2NiWMoVNb steel has finer and more undissolved carbides of W, Mo-rich M6C and Nb, V-rich MC compared with Eglin steel. The undissolved carbides are lead to a finer microstructure, which ensured toughness and increased strength significantly. The undissolved carbides of 30Cr3Si2Mn2NiMoVNb steel are Mo-rich M6C and Nb, V-rich MC. Size and quantity of the undissolved carbides in 30Cr3Si2Mn2NiMoVNb steel are the same as Eglin steel. However, finer grain size according to Nb, V-rich MC lead to insensitive strength of 30Cr3Si2Mn2NiMoVNb steel.Variation of high temperature phases during heating in newly designed steels are calculated by Thermo-Calc software and verification test is carried out. Calculation results show that the W, Mo-rich M6C phases of 30Cr3Si2Mn2NiWMoVNb steel are dissolved completely at about 970℃. And the Mo-rich M6C phases of 30Cr3Si2Mn2NiMoVNb steel are dissolved completely at about 920℃. Experimental results showed that undissolved W, Mo-rich M6C carbides of 30Cr3Si2Mn2NiWMoVNb steel are dissolved completely in the range of 950℃-980℃; and undissolved Mo-rich M6C carbides of 30Cr3Si2Mn2NiMoVNb steel are dissolved completely in the range of 890℃-920℃. The calculation results were in accordance with test results.

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