This kind of crack can only be found after pickling on the surface of the slab, and the depth can reach 5mm. Causes:
(1) The surface of the high-temperature casting billet absorbs the copper of the mold, and the copper becomes liquid and then penetrates along the austenite grain boundaries.
(2) Selective oxidation of iron on the surface of the slab, so that residual elements (such as Cu, Sn, etc.) in the steel remain on the surface and penetrate along the grain boundaries to form cracks. Studies have shown that the crack area is enriched with elements such as Cu, Sn, and Sb. The content of Cu in the steel is greater than 0.1%, and the crack is aggravated; the content of Al in the steel increases, and the network crack is aggravated.
1) The surface of the mold is plated with Cr or Ni to increase the hardness.
2) Appropriate amount of secondary cooling water.
3) Control residual elements in steel such as Cu<0.2%.
4) Control Mn/S>40.