Analysis On The Causes Of Planking Roughness And Dotted Corrosion And The Corresponding Measures

In recent years, with the quick development of ship-building industry, the requirements for further improving ship plate surface quality have been put forward by domestic ship companies. Any roughness and surface corrosion phenomenon are not allowed. The production of with rough surface after shot peening, which is more concentrated in the thin specifications planking by Shoudu Iron & Steel Company. The rough depth is 0.03-0.06 mm. For this defect, Samples taken from the ship plate producted by Shoudu Iron & Steel Company were determined by using electron microscope, energy spectrum, XRD, etc. The reason for rough surface was firstly analyzed and the corresponding measures were put forward.The test results show that, the steel plate surface roughness and color difference after blasting are caused by secondary oxidation iron sheet during finishing stage. The secondary oxidation iron sheet is FeO after descaling which is residual in descaling substrate, then it is further oxidized to Fe3O4 and Fe2O3 during finishing stage. The residence time of the billet in the furnace temperature above 1178 ? is too long, especially in the soaking period, which can cause more molten binary eutectic Fe2SiO4 + FeO in the surface. So the descaling is difficult and some are residual in the slab surface. The effect for first descaling, roughing rolling descaling and finish rolling descaling by the high pressure water impact are not achieved, which is related to hydraulic pressure water shape, nozzle jet flow and the steel plate distance.By adopting the following measures, the rough on ship plate surface can be eliminated. Firstly, to reduce the furnace temperature of billet should be decreased. The soaking zone is controlled under 1178 ?. The slab in the soaking period of time should be reduced, and the total time of slab in the furnace should be reduced too. Secondly, the water nozzle height for first descaling should be decreased to 90 mm. The overlap area of jet stream should be reduced too, to avoid the slab of heart too cold. The roughing descaling, finishing descaling high pressure water capacity should be improved from 18.5 MPa to 20 MPa. The jet flow, nozzle shape and plate distance should be adjusted to ensure appropriate amount of overlap jet stream. In addition, after stable descaling, the ship plate under 18 mm and finishing temperature above 900? are descaled at finishing stage 1, 2. However, that above 18 mm and the finishing temperature 800-900?, are descaled with the decreasing finishing temperature.Dotted corrosion appears on the surface of ship plate with small dot distribution or aggregation distribution, and its shape is irregular shape. The depth of the dotted corrosion is mainly distributed between 0.1 mm to 0.1 mm, and the largest depth is more than 1 mm. The cause of the dotted corrosion is mainly steel plate in water for a long time between stacking storage, caused by crevice corrosion occurs, and the Cl- promote dotted corrosion. The dotted corrosion resistance of the ship plate with blue steel technology is higher than that with traditional one.