hot-rolled plate is rolled, there are scar defects on the surface, and local scars are peeled off. The macroscopic morphology of the defect is shown in the figure.
Macro morphology
The hot-rolled plate production process is: (hot metal desulfurization) → converter smelting → argon blowing (vacuum treatment) → continuous casting → slab inspection → heating → dephosphorization → rough rolling → fine Rolling → laminar cooling → coiling → (finishing) → inspection → packaging.
Prepare a cross-sectional sample at the defect site, and observe it under electron microscope after grinding and polishing. The defect site can see cracks that are almost parallel to the board surface. Some parts of the cracks can see the oxide embedding of Fe :
A large amount of slag inclusions can be seen in other parts, as shown in the following figure:
There are cracks in the defect section, and there are more slag inclusions and a large number of FeO in the cracks. It can be inferred that the scars are caused by slag inclusions. cause. slag has a lower melting point than steel. When the slag is involved, during the cooling process of the cast slab, the steel matrix solidifies first, and the slag solidifies afterwards. Under the action of thermal stress, thermal cracks will occur during the solidification process. If the slag is very shallow from the surface of the steel, the crack is in air communication with the surface of the billet, and the oxygen in the air at high temperatures will oxidize the Fe on the surface of the crack.
This is the reason why slag inclusions and Fe oxides can be observed at the same time in the cracks.
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