Experimental Research On Mechanical Properties Of 600MPa Steel Bars Under High Temperature

The security requirement toward reinforced concrete structure has been improved constantly, which prompts higher levels of steel is applied to the structure field gradually. The 600 MPa steel bars is a kind of high strength steel that is a new development achievement of the technology of micro alloy and reinforcement. The 600 MPa steel bars has been included in Steel for the reinforcement of concrete-Part 2: Hot rolled ribbed bars(GB1499.2-2013)(draft for deliberation) and its index of mechanical properties in normal temperature has been stipulated. The 600 MPa steel bars has gradually become the new target of engineering application, but lacking the research date of mechanical materials in high temperature function when structure fire resistance design is done. Therefore, in this paper, the strength and deformation rule of 600 MPa steel bars under high temperature is studied, which provides theoretical basis and experimental for its popularization and application.The main research contents and results are as follows:1. The strength experiment in the way of temperature-stress is done.On the one hand,the temperature is settled and the load is increasing. On the other hand, the load is constant and the temperature is increasing. In the process, damage phenomenon should be observed. Through the experiment, the ultimate strength and yield strength between 20℃ and 800 ℃ can be got, then the fitting formula about strength can be got, and the two kinds of strength change rule need to be compared. The experiment results show that:(1) With the rise of temperature, the ultimate strength and yield strength decrease, especially in the range from 400℃ to 600℃, they decrease most fastly, but the declining rate of yield strength is faster than ultimate strength;(2) In the range from 300℃ and 700℃, the strength in the second way is slightly higher than that in the first way. Beyond the range, the strength in the two ways has a little difference;(3) Yield-to-ultimate strength ratio under high temperature is different because of the diversity of steel strength.(4) Strength test data are in good agreement with that in the fitting formula.2. The deformation test is done under high temperature, then the stress-strain relationship, elastic modulus, free expansion deformation in linear way, temperature deformation under constant stress and high temperature creep within two hours between 20℃ and 800℃ can be got. At the same time, the corresponding calculation model also can be gained and the thermal-elastic-plastic integral scheme of reinforcements is presented briefly. The experiment results show that:(1) the stress-strain curve under high temperature is very different with it in normal temperature obviously. With the rise of temperature, the yield step will disappear and the strengthening segment shortened gradually. When the temperature is 400℃, the ultimate elongation decreases to the minimum. In the range from 500℃ to 800℃, it is composed of the rising slope with decreasing gradient gradually and gentle decline after peak point;(2) With the rise of temperature, the elastic modulus decreases and its change trend is the same as the strength basically. But its reduction is greater;(3) Under different stress levels, the temperature deformation between 20℃ and 400℃ is relatively small and has a linear growth approximately. Within the range between 400 ℃ and 800℃, it is akin to exponential growth; When the temperature is same, the temperature deformation has a linear growth trend with the increase of stress level;(4) Free expansion deformation has a growth trend like the power function with the increase of temperature. The difference of steel bar intensity has a little influence on it;(5) When the temperature is lower and stress level is higher, the creep is growing slowly over time and its total value is smaller; When the stress level is lower and the temperature is higher, the creep has a linear trend; When the stress level is same, the creep grows rapidly with the rise of temperatures and its total value in two hours increases greatly; When the temperature is same, the total value of creep also increases greatly with the increase of stress level.