Research On Microscopic Structure Of X70 Pipeline Steel And Inclusions Controlment In Casting Slab

Recently, with increasing demand of high-grade pipeline steel owing to contry economic development, some large scale iron and steel corporations were all motivated to explore higher-grade pipeline steel product. As the largest modern enterprise in Hebei Province, Tang Steel could urgently develop X70 and higher level pipeline steel to make up the configuration of alloy steel in Hebei Province.The quantity, dimension, morphologies and composition of non-metallic inclusions in X70 pipeline steel cast slab sampled from 1700 cast slab production line were investigated. Microstructure and mechanic performance of X70 plates were analysed. Simultaneously, influence of production technology on controllment of microscopic constitution and inclusions in X70 pipeline steel was accessed. Through numerical simulation of Fluent software, statement of liquid steel flow and movement track of inclusions were discussed, and the flow-control device in a 1700 slab tundish was evaluated and optimized. This paper could provide some useful information for production of X70 pipeline steel in Tang Steel.The main conclusions were as follows:Microscopic constitution of the high niobium X70 pipeline steel was the perfcet acicular ferrite. The precipitates on crystal interface were nitrides and carbonitrides of niobium and titanium. Construction and mechanical performance of the test steel samples were satisfied for the technology standard.Lots of micro-inclusions in X70 pipeline steel casting slab were spherical,scattering distributed and with less diameter. The average content of T[O] was more than that in similar product. The content of macro-inclusions reaches to 4.37mg/10kg. Most of macro-inclusions with larger diameter were silicon aluminates and calcium aluminates inclusions.Once the flow control device was installed in 1700 slab tundish, the state of liquid steel flow and temperature distribution could be improved, and the efficiency of inclusions removal could be increased. On the other hand, casting speed has little influence on liquid steel flow under the applied operation condition.Taking liquid steel flow field, temperature field and inclusion removal efficiency into account, the tundish integrated current regulator with 555mm dam was most optimized construction. In this tundish, the state of steel flow and temperature distribution could be improved, and the percentage of inclusions with equivalent diameter of 10~150μm could be increased to 6.5%~12.5%.