In the case of heating, the mechanical properties of steel with increasing temperature change. The general performance of the elastic modulus, yield strength, ultimate strength decreases with increasing temperature, plastic deformation and creep increases with increasing temperature. So-called "blue brittle" phenomenon at 200 ℃ ~ 350 ℃ hot rolling, then increase the ultimate strength of steel and plastic lower compared with other temperature ranges change "brittle." At 500 ℃, ultimate strength and yield strength of the steel is greatly reduced, increasing plasticity. At 450 ℃ ~ 600 ℃, the graphite and carbides tend ball. Graphitization product is due to the decomposition of iron carbide, graphite particles produce results free. If the heating temperature is higher, the longer, the higher the carbon content of steel, carbide ball is the more intense. The presence of graphite and ball, show steel at high temperatures weakened, reducing mechanical properties. Changes in the temperature of the alloy material is generally added to make the steel needs to improve. The results showed that: within 200 ℃ intensity did not change significantly, a slight decrease in yield strength, ultimate strength and unchanged. The yield strength with increasing temperature and decreasing the rate began to accelerate after 200 ℃. Ultimate strength due to "blue brittle" and slightly higher than the ambient temperature at 200 ℃ ~ 300 ℃, ultimate strength after 300 ℃ decreased with increasing temperature. At 600 ℃, low carbon steel yield strength and ultimate strength are only 35% to 40% of normal temperature, and lower the strength of carbon steel wire. As the temperature rises further, the strength of steel at 800 ℃ disappeared. While steel elongation and reduction of area surface increases with increasing temperature, indicating that the plastic properties of steel at high temperatures increases, easily deformed. In addition, the steel at a certain temperature and stress, over time will occur slowly deform, that creep, creep relaxation will result in material.
Steel at high temperatures reduce the yield point is an important factor in determining the fire resistance of steel structures, such as a steel member that is at room temperature and load application is half the value of the yield point, but since the fire steel fire at high temperature yield strength decreases, when the actual value reaches the stress reduces the yield strength showed an yield phenomenon and damage and the sharp decline in the carrying capacity of the structure, resulting in partial or total collapse of steel buildings destroyed.
