The Study On High-strength Steel By Precipitates Of Nano-size

In recent years, with the rapid development of the automobile industry, the demands for the performance of steel plates using in automobile are being increased higher, and more and more specialized. The demands of safety, durability, environmental protection and the requirements of sustainable development making the lightweight direction of the automobile production, which drive the development and application of high-strength low-alloy steel plate.The steel using in automobile chassis needs high strength of 780 MPa and excellent stretch flange formability, which is also the object of this paper. Chemical composition of steel which meet the performance was brought forward, and smelting and rolling were carried out in the key laboratory in the ENU, on the basis of the analysis of micro-alloy steel. The results of simulation experiments showed the possibility that there are precipitations with nano-size distributed in ferrite matrix, which provide the theory and basis of practice in the exploiting of the new steel.1. The CCT curve of the experimental steel was determined, which provide a theoretical basis for the establishment of rolling process. Experiments show that to obtain the ferrite matrix, the cooling rate must be controlled not more than 5°C/s.2. The processing simulation of actual production was carried out on the MS-200 thermo-mechanical simulator. The optical and TEM analysis of the compressed sample show that there are precipitations between 5～50nm distributed in ferrite matrix and the dislocation became bendt suffocated by fine precipitates inside grain, which different from the traditional precipitation behavior.3. The hot rolling process was carried out :the roughing temperature is 1100°C, the finishing deformation temperature is 800～900°C, the coiling temperature is 580～620°C and the thickness of product is 3mm. Mechanical testing showed that the yield strength and the tensile strength could reach 574 MPa and 631 MPa respectively. Calculating analysis showed that the increase of yield strength was 257MPa because of the precipitates in such Micro-scale.