Research On The Surface Cracks Of Ccsd36 Ship Plate In Tangshan Medium Plate Co.

After committing 2,500,000 t/a Slab casting project in Tanggang Iron and Steel Co, The slabs are disqualified by ultrasonic inspection,transverse cracks in slabs occurred frequently for a period of time, especially detrimental to the quality of micro-alloyed steel grades.These quality defects immensely constrained the production capacity, and did harm to the quality of rolled products afterwards as well. The reason of cracks was studied by analysis to the surface of cracked slabs and experimental research in this dissertation.Surface microcracks on steel plate is slim and curving, but is long and remarkable. It originates in mold and extends in secondary cooling zone and rolling. Oxidation layer along the cracks formed in high temperature makes cracks hard to be welded. A large amount of inclusions with high melting point and low plasticity such as silicates, the content is deviated from the normal state seriously, brittle particles is formed,when they are captured by exterior and oxidation layer, the surface will breaks when it is being rolled, so the cracks extend or newly formed.By ultrasonic inspection, macro inclusions are considered as cracks.Through researching the inclusions of slab, the inclusion volume is 0.16per centum,with the number of the micro inclusions 36/mm2 of CCSD36; the micro inclusions'diameter is small. Inclusions diameter is less than 5μm account for about 90%, inclusions diameter is between 5μm and 10μm account for 9%, and inclusions diameter is more than 10μm less than 1%. The content of inclusion is high in inner arc of the slab, the inclusions aggregated in the area of apart from 1/5 to 3/5 of inner arcs. This trait of distribution is mainly caused by inclusions which diameter is between 5μm and 10μm. Micro-inclusions is mainly divided into four categories, micro inclusions in the steel appear to be silicate inclusions and block and cluster of Al2O3 before LF treament,after Ca treament in LF,CaO- Al2O3 and CaO- Al2O3–CaS become the main inclusions in the steel of slab and in tundish;It is demonstrated by analyzing the tracer that the micro inclusions in slab mainly come from the the turdish cover material, secondly mold flux and molten steel container cinders.Macro-inclusions are mainly CaO-Al2O3 and SiO2-Al2O3, and the content is 15.21mg/10kg. It is demonstrated by analyzing the tracer that the macro inclusions in slab mainly come from the mold flux, secondly the turdish cover material and molten steel container cinders.The inclusions of slab is important to the production and expansion of the surface cracks.