The Effect Of Alloying And Hot-rolling Process On The Microstructures And Mechanical Properties Of Low-carbon Bainitic Weathering Steels

Due to the corrosion of ocean engineering structural steel in the seawater and marine atmosphere, the service life of this kind of steel suffers great damage. Based on the studies that the corrosion resistance of steel in the atmosphere and seawater can be significantly improved by copper element and phosphorus element, the low-carbon bainitic weathering steels added copper element and phosphorus element were studied in this paper. The effect of steel composition, controlling the final rolling temperature and the annealing process at550℃on the microstructure and properties of the low-carbon bainitic weathering steels were studied. Furthermore, the strengthening mechanisms and toughening mechanisms of the low-carbon bainitic steels were discussed in the present study. The specific research includes the following content:firstly, the effect of experimental steel groups alloying by copper-phosphorus and alloying by molybdenum treated respectively with high-temperature final rolling and low-temperature final rolling on the microstructure of low-carbon bainitic steels were studied; secondly, the experimental steel groups alloying by molybdenum were annealed at550℃. Now there are the following results:The typical micro structures mainly made up of granular bainite in the experimental steel groups are obtained and the tensile strength and impact resistance are both high, which can meet the requirements of mechanical properties of weathering steel in500MPa level. The strength of the experimental steel is significantly improved by copper and phosphorus alloying. The impact toughness slightly decreases, but the low-temperature toughness increases with the decreasing of the final rolling temperature.With the decreasing of the final rolling temperature, the microstructures of the experimental steels alloying by copper-phosphorus are refined, and the size and the volume fraction of M/A islands in the microstructure gradually become small. Thus, the crack propagation ability decreases and the toughness of the steel can be improved. Besides, the M/A islands have the excellent deformation ability corresponded with the matrix in the plastic deformation. Meanwhile, the stress-induced transformation of austenitic to martensite contributes to the work hardening effect, thus the plasticity and the toughness of the steel are improved.The stress-induced particles Nb(CN) precipitating in the dislocation contributes to stabilizing the dislocation structure, delaying the reflex process and the dispersion strengthening. In addition, the decreasing of the final rolling temperature can enhance the pinning effect of the precipitation, decrease the formation of the cell structure, thus promote the formation of the fine lath bainite and improve the strength and the toughness.The strength of the experimental steel groups alloying by molybdenum are significantly improved. The same results are obtained with experimental steel groups alloying by copper and phosphorus. The experimental steels treated with final hot-rolling at850℃result in more excellent mechanical properties.The plasticity of Mo alloying steel is significantly improved, while the tensile strength are slightly reduced through annealing treatment at550℃. The decreasing extent of tensile strength of Mo added steels is far less than the comparative steel, which indicates that the molybdenum element contributes to improve the stability of microstructures in experimental steels. The microstructures also show that the M/A island in the experimental steels decompose after annealing treatment, being accompanied with carbide precipitation and formation of quasi-polygonal ferrites.