No abstract

Mg treatment is conducted in the production of peritectic steel, which greatly reduces the average value of incidence of transverse corner crack in continuous casting slab from 3.10% to 1.06% by 65.8%. In the industrial experiment with the stages of Ar station, before magnesium treatment, after magnesium treatment, and in the slab, the maximum Mg content is 30 ppm after Mg treatment. A large number of Mg containing complex inclusions are formed, and most of Al2O3 are transformed into MgO·Al2O3. In the inclusions, the content of MgO significantly increases from 3.58% to 27.2%. The number density of Mg containing inclusions increases from 45.2 to 168 mm−2. The oxide inclusions smaller than 2 μm account for more than 90%, indicating that Mg treatment can promote the refinement of inclusions. In the slab, most oxide inclusions are transformed into the complex inclusions with Mg containing oxides as the core covered by TiN and sulfide precipitates. These complex inclusions with a size above 1 μm can effectively promote the nucleation and growth of intragranular ferrite to prevent the generation of transverse corner cracks.

In this paper, in order to clarify the mechanism of improving the transverse corner crack of peritectic steel slab with Mg treatment, the inclusions and microstructure characteristics at the slab corner of Al deoxidized (S-Al), (Al+Ca) deoxidized (S-Ca) and (Al+Mg) deoxidized (S-Mg) peritectic steels were studied. The inclusions in S-Al sample are mainly Al2O3, Al2O3 · MnS and MnS inclusions. The S-Ca sample mainly contains Al2O3 inclusions and their complex inclusions. In the S-Mg sample, the number of Mg containing inclusions increase sharply. The S-Mg sample has the smallest average size of inclusions and the largest number of inclusions with the size smaller than 1 µm. The brittle structures are dominant in S-Al sample, which basically disappear in S-Ca sample and S-Mg sample. Mg treatment refines the size of grains at the slab corner, reduces the size of the proeutectoid ferrite films, and restrains the crack propagation along the grain boundary. More IAF microstructures are formed in S-Mg sample. IAFs take MgO · Al2O3 · MnS, MgO · Al2O3 · TiN · MnS and other complex inclusions as the nucleation cores, which diverge and grow outward from the inclusions. Thus, IAFs form a fine crack resistant microstructure, improving the microstructure at the slab corner to prevent the occurrence of transverse corner cracks.