| The analysis and research was done about the surface defects of hot rolling plate and the evolution behavior of strand transverse corner crack during rolling. The strand transverse corner crack correlates well with the surface linear defects of hot rolling plate. Based on transverse corner crack phenomenon of microalloy steel during continuous casting, the characteristics of transverse corner crack, the microstructure of slab and the morphology and distribution of second phase precipitation in micro-alloyed slab corner surface were investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrum (EDS) and transmission electron microscopy (TEM). The causes of transverse corner cracks formation are concluded as follows: During continuous casting MnS precipitation along austenite grain boundary at a high temperature, then coarsen in the subsequent mild cooling condition, promoting the V(C,N) precipitation at low temperature. (MnS+V(C,N)) complex precipitate, as the effective nucleation sites for ferrite, promote the formation of film-like ferrite along austenite grain boundaries. Strain concentrated on the softer ferrite because the inhomogenous deformation in the austenite +ferrite two phase region originating from the strength difference of them when straightened during continuous casting.This causes voiding around the precipitates situated at the boundaries and these voids gradually link up to give crakes.To prevent of slab surface corner transverse cracking, slab surface microstructure control process was adopted. The microstructure of as-cast slab was duplicated through simulating the as-cast cooling condition by the remelting solidification cooling equipment. The strand surface temperature curve was obtained by thermocouples placed at the ingot corner and 5mm inside to the surface before testing. The heat transfer boundary condition of numerical simulation was modified by the surface temperature curve measured. The secondary cooling system was optimized by numerical simulation and remelting solidification cooling experiment was put forward. Finally the rational secondary cooling system was confirmed.Based on the above research results, the process of the slab surface microstructure control cooling was concluded as follows: The slab suface was rapidly cooled at 2.5℃/s to 720℃by intensive cooling in cooling zone I and II with slab section dimensions of 1000mm, 1160mm and 1250mm.as the cooling rate is fast enough and sufficient, ferrite not only precipitation at austenite grain boundaries but also inside the grain. In the meantime, precipitation of V(C,N) was be promoted due to the low interstitial solubility in ferrite compared to austenite. Then it was reheated up to 860℃by decreasing cooling water in cooling zone III andⅥ.During the reheating, all ferrite must transform to austenite. However, fine precipitates must be undissolved within the re-transformed austenite because of inadequate solubility product. At the following stage of cooling, the ferrite grain must originate from these fine precipitates which are dispersed in martrix, brings fine ferrite structure. The susceptibility to cracks was alleviated.The method is applied to commercial continuous caster, Pansteel 1# CC. Microstructure with no chain-like precipitated carbonitrides and film-like proeutectoid ferrite free structure was obtained. The fine precipitation is dispersed in martrix, and the microstructure is fine and uniform. As a result, transverse corner cracks of low alloy steel have been eliminated. |
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