Cold Rolled Sheet Surface Linear Defect Analysis

14 March 18

The surface black sliver defects of cold-rolled DC03 steel sheet are found in certain block in a China steel-making plant, the surface and section of the defects are analyzed by SEM and EDS,it shows that there are many different size grains of Fe on the surface of the defects and there also many grains on the section of the defects which are found FeO in the center but Fe in the surface layer.

So we can conclude that The surface black sliver defects are formed by pressed FeO which entered in former working procedure, the surface layer FeO of grains is deoxidized by H in the atmosphere during anneal,and the center FeO of grains is immovable. Technics control should be enhanced to prevent the surface black sliver defects.

Linear defects is one of the most common defects of cold-rolled sheet. Due to the long process from steelmaking to finished product rolling, many factors can cause linear defects in cold-rolled sheets. Recently made a number of cold-rolled DC03 thin black surface defects appear on the surface analysis, inspection defect length of about 210 mm, a width of 0.5 ~ 3.0 mm, was intermittent, feel obvious, the specimen thickness 0.7 mm. The defects were analyzed by Quanta 400 scanning electron microscope and EDAX spectrometer.

DC03 is a high-quality carbon structural steel, the general thickness of 0.5 ~ 3.0 mm, mainly used in automobile and tractor manufacturing.

Surface analysis interception of the sample, after ultrasonic cleaning of alcohol, scanning electron microscopy linear defects under the microscope morphology shown in Figure 1, showed a bright band.

At higher magnification, we can see the characteristics of the bearing edge of the defect edge, see Fig. 2.

Defective parts of the skins off at the local magnification shows a large number of lumps and particles, the shape of a brittle characteristic, shown in Figure 3.

The brittle material is difficult to deform during the rolling process and is thus broken into lumps or particles of different sizes and distributed linearly in the rolling direction. EDS analysis showed that there were no other kinds of heterogeneous elements besides massive Fe and a small amount of Mn.

It can be seen that there are loose lumps in the area. The lumps are separated from the substrate. They are also separated into small pieces. The outer surface of each piece is lighter in color, consistent with the color of the substrate and darker in color, indicating that clumps material surface and the inner layer has obvious composition difference. EDS analysis showed that the darker Fe and O elements in the inner color were obvious, and the atomic ratio was close to 1: 1, which should be FeO. The lighter color of the surface layer is pure iron and elemental line scan also showed that the surface of the lumpy substance Fe content is higher, the internal Fe content decreased, O content increased.

The formation of the linear defects is caused by the brittle bulk FeO evolved after being pressed in the previous step. Iron oxide stamping is one of the important reasons for the decrease of surface quality of cold-rolled products. When the hot-rolled oxide scale is rolled off the surface of the strip, the pickling before cold rolling does not remove the scale, or the strip does not use protective measures during the heat treatment, so that the surface of the strip is formed with scale, Direct pickling cold-rolled, and slab surface cracks caused by low Mn / S, etc., will lead to the formation of linear defects on the surface of the finished product.

The formation and removal of scale involves the heating system, the furnace atmosphere configuration, high pressure descaling and finishing temperature and coiling temperature and many other factors. Due to the thinness of the cold-rolled sheet, the final fragile FeO is exposed linearly to the sample surface after it breaks. However, in the recrystallization annealing process of the cold-rolled sheet, the sample is in an atmosphere of reducing hydrogen, and the surface FeO is reduced to Fe. Due to the limitation of the annealing time, the central part retains the original FeO. Therefore, in order to prevent the occurrence of such linear defects, the process control should be strengthened to prevent the FeO from being pushed in by various factors such as the acid pickling and rust removal.

Steel mills need to strengthen the pickling management and heat treatment protection before cold rolling, reasonably control the Mn / S in the steelmaking, prevent surface cracks on the slab, and prevent any iron oxide on the strip surface from being brought into the cold rolling process. Through the above measures, the problem of linear defects on the surface of the cold-rolled sheet has been effectively solved.

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