Study On Crack Forming During Hot-charging And Improving Of The Medium And Heavy Plate

In order to solve the problem of surface crack appeared on medium and heavy plate, spectrum analysis instrument, SEM, metallurgical analysis method and Thermo-simulation experiment were employed to examine the chemical composition of steel plate, microstructure of surface crack, type of inclusion in crack,rang of the third brittle zone, Gleeble-2000 Thermo-simulation testing machine were also used to simulate the whole cause of casting-heating-rolling of correlative steel, then used TEM to detect the precipitation and redissolve of carbonitride under different thermol history conditions, as well as EDX analysis. Through the above experiment result contrasting the actual processing condition, the primary forming reason of surface crack was found, simultaneity improved in practice.The following results were given through the study on suface crack and processing practice of energy steel:(1) The carbon content of the steel plate occurred surface crack was in the range of peritectic reaction, and most surface cracks were on low alloy steel, especially microalloying low alloy steel with Nb and Ti element. The surface cracks on steel plate were neither occurred by the over-content of Sn, Cu, As element in basal, nor occurred during the process of rolling, but forming from the original crack growth on continuous casting slab. Volumetric shrinkage induced by peritectic reaction, inclusion inside the steel and the precipitation of carbonitride during the process of hot charging were the main reasons inducing surface cracks.(2)Controlling liquid surface waving at the time LF refining, changing tundish and embedding nozzle, reducing the probability of slag-entrapment; controlling the temperature of straightening in order to avoid the third brittle zone;Controlling the hot charging temperature, heating temperature and heating time of casting slab, then avoiding the fastigium of carbonitride precipitating and transform, making carbonitride redissolved fully, through the above measures the surface cracks were reduced effectively.The reason forming surface cracks on energy steel plate in JiGang was analyzed in this paper, also the corresponding measures were proposed and applied in actual processing, obtained good effect.