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Research On Evolution Of Microstructure In Low Carbon Steels For Fasteners

Posted on:2009-08-04Degree:MasterType:Thesis
Country:ChinaCandidate:K YangFull Text:PDF
GTID:2131360308479219Subject:Materials science
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Fasteners, including bolt, screw, nut and so on, are widely used in automobile, electronics and architectural industries and so on. Hot rolled wire rods are raw materials for producing fasteners. The fast development of fastener industry has caused an urgent demand on high quality hot-rolled wire rods. However, the domestic wire rods have serious problems such as mixed grain structure, which has large disadvantageous influence on the strength, plasticity, impact toughness and dimensional precision of materials. Therefore, analysis on formation mechanism of mixed grain structure has an important significance to improve the comprehensive properties of hot rolled wire rods.In this paper, the hot rolled 20 and 10B21 steel wire rods produced in a domestic iron and steel company were used as a research target. The tendency of grain growth in the two types of steel was examined through measuring the grain size of the two steel samples subjected to different heat treatments. The hot deformation behavior of two steels was investigated to determine the deformation temperature and strain rate at which dynamic recrystallization take place by using Gleeble 2000 thermo-simulator. The obtained main results are as follows.1. Austenitic grain size of hot rolled 20 steel and 10B21 steel wire rods were examined. The results showed that the austenitic microstructures of the two steels are severe mixed grain structure. The level of mixed grain structure is severer in the edge of wire rod than in the central part, and in 10B21 steel than in 20 steel.2. Study on growth tendency of austenitic grains of two steels showed that average grain size of 20 steel is larger than that of 10B21 steel after subjected to the same heat treatment. The grain growth rate of 20 steel decreases as the heating temperature increases, while it increases first and then decreases as the heating temperature increases in 10B21 steel.3. Different level of mixed grain structure was observed in 10B21 steel at whole heating temperatures of 900-1200℃, the degree of mixed grain is more serious at 950℃and 1000℃ and alleviates as the heating temperature increases. Mixed grain structure occurs only at 900℃and 950℃in 20 steel, and whose degree is less serious than in 10B21 steel.3. The hot deformation behavior of 10B21 and 20 steels at 800-1050℃and at strain rates of 0.01-10s-1 was studied through single-pass compression experiment. The experimental result showed that the onset temperarure of dynamic recrystallization increases when the steel specimens deformed as the strain rate increases. In 10B21 steel, the onset temperature is 875℃,950℃,1000℃and 1050℃when deformed at a strain rate of 0.01s-1,0.1s-1, 1s-1 and 10s-1, respectively. In 20 steel, the onset temperature is 825℃,900℃,1000℃and 1050℃when deformed at a strain rate of 0.01s-1,0.1s-1, 1s-1 and 10s-1, respectively. Under the deformation temperature and strain rate, the activation energy of hot deformation of 10B21 steel is determined to be 186kJ/mol, and the hot deformation equation can be described as The activation energy of hot deformation of 20 steel is determined to be 187kJ/mol, and the hot deformation equation can be described as...
Keywords/Search Tags:fastener, mixed grain structure, grain growth, thermal simulation, hot deformation
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