Mechanical properties of high strength Q690 steel at elevated temperature at low strain rates

The high temperature mechanical properties of high strength Q690 steel at different tensile strain rates were studied by tensile tests. And the effects of tensile strain rates on mechanical properties at high temperatures were quantified. According to the data from tensile tests, the stress-strain curves were drawn, and the mechanical properties of steel were obtained, including elastic modulus, yield strength, tensile strength and elongation after fracture. The test results show that necking generally occurs after high temperature tensile. With the increase of temperature, the shrinkage rate of the specimen section increases and the fracture section decreases, indicating that the plasticity of the material is significantly improved at high temperature. High tensile strain rate can slow down the decrease of elastic modulus, and the elastic modulus reduction factor of 0.200/min at 600 degrees C is about 36 % higher than that of 0.002/min. At 400 degrees C and below 400 degrees C, the strength and elastic modulus of Q690 steel remain at above 75 %, but the strain hardening effect is significantly weakened, and the high temperature softening begins to be obvious. When the temperature exceeds 400 degrees C, the reduction factor of mechanical properties decreases significantly. When the temperature is less than or equal to 400 degrees C, the higher tensile strain rate will generally lead to lower mechanical properties But when the temperature is greater than 400 degrees C, the higher tensile strain rate will produce greater mechanical properties. In addition, the experimental results were compared with the predicted values of the current specifications and the experimental results of the other two high strength steels at different tensile strain rates. The results show that GB51249, Eurocode3, AISC and AS4100 are generally unsafe in predicting the high temperature strength of Q690 steel (the predicted value is higher than the experimental value). Only AS4100 is conservative in predicting some strength indexes of Q690 steel at 400 degrees C. The strength reduction of Q690 steel at high temperature is significantly higher than that of Q460 and Q960, especially at low tensile strain rate. The elastic modulus of Q460 has the highest elevated temperature stability.