| 摘要 |
PURPOSE: A composite organized steel for bolt and a preparation method thereof are provided, which provide the bolt having high strength, high percentage of elongation and good delayed fracture resistance. CONSTITUTION: The composite composition steel for bolt comprises: (i) 0.4-0.60 wt% of carbon, 2.0-4.0 wt% of silicone, 0.2-0.8 wt% of manganese; 0.25-0.8 wt% of chromium, less than 0.01 wt% of sulfur, less than 0.01 wt% of phosphorous, 0.005-0.01 wt% of nitrogen and less than 0.005 wt% of oxygen; (ii) at least one selected from 0.05-0.2 wt% of vanadium, 0.05-0.2 wt% of niobium, 0.3-2.0 wt% of nickel, 0.001-0.003 wt% of boron, 0.01-0.5 wt% of molybdenum, 0.01-0.2 wt% of titanium, 0.01-0.5 wt% of tungsten, 0.01-0.2 wt% of copper and 0.01-0.5 wt% of cobalt; and (iii) the remained of Fe and inevitable impurities, wherein the fine composition has the composite composition of ferrite and tempered marten site and the phase proportion of the ferrite is 8-20%. The method comprises steps of: (i) controlling the phase proportion of the ferrite at 8-20% and the phase proportion of the austenite at 80-92% by heating the composite composition steel within the range of from Ac3-(Ac3-Ac1)/1.3 to Ac3-(Ac3-Ac1)/5.5 for more than 20 minutes; (ii) tempering the step (i) by quickly cooling it in a cooling rate of 70°C/second at a temperature of 300-600°C for more than 20 minutes.
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