Research On Controlling Corner Crack Of A36-LB Slab

Boron steel is a kind of alloy structural steel that uses the trace boron as alloyelements to increase the hardenability to improve the properties of the steel and savealloying elements. However, the steel often has corner crack in the production process.The causes of cracks in the corner of Lianyuan Steel A36-LB are analyzed in this paper.Through the adjustment of cooling system of boron steel, the corner crack of A36-LB issolved in the production process. The results show that:（1）Based on the thermodynamic calculation during the solidification of boron steel,the precipitation laws of various precipitates within the scope of the internal controlcomponents are obtained. The TiN, TiC, AlN and BN in liquid steel is not likely toprecipitate. The temperature reduced to1512℃during solidification process, TiN（Ti0.025%, N0.006%） reachs to the thermodynamic conditions of precipitation. At theend of solidification, TiC (Ti0.025%、C0.20%) has a small amount of precipitation.During austenite process, the precipitation of TiC（Ti0.025%、C0.20%） begins at1112℃, there are a large number of precipitations during this process. Due to the high Ticontent, AlN and BN will not precipitate within the scope of Lianyuan Steel internalcontrol components, which reveals that the precipitation of BN is not the main reason ofthe corner cracking of A36-LB steel.（2）The thermoplastic of carbon steel containing boron is measured by usingGleeble-1500thermal simulation testing machine, the thermoplastic of A36-LB steel ispoor at700~900℃and over1250℃. The cause of two brittle zone is explained accordingto B segregation calculations and thermal simulation testing, and then the main reason ofproducing slab corner crack A36-LB steel is revealed, which is that A36-LB steel is in thescope of peritectic steels and the straightening zone temperature of actual control ofA36-LB is in the low temperature brittle zone.（3）Through analysis of oxygen and nitrogen of samples of A36-LB refining processand subsequent process, the nitrogen increase of ionization is serious in arc heating of LFrefining process. Nitrogen content is increased from19×10-6to about25×10-6, and thenitrogen content is increased from25×10-6to27×10-6after adding alloy,it is recommendedthat the pre-melted slag and aluminum deoxidizer, a small current, small voltage operation,homogenizing foaming slag to make the molten steel and air isolation and other measures should be used in the early process of LF refininig to reduce the nitrogen increase in liquidsteel and control nitride precipitation.（4）By solidification and heat transfer simulation, it is found that the mold coolingwater has reduced space in Lianyuan Steel, combined with the testing in production site,when mold cooling water is4000/380L/min, it has the lowest incidence of cracks. Theratio of heat on the narrow and wide surface is low, and it is recommended that the tapercan be adjusted to0.95%/m.The goals temperature of secondary cooling zone isreasonable, but the water yield of secondary cooling zone is unreasonable, and the wateryield is high in part of casting stage. Through the optimization of secondary cooling water,slab cracks have been controlled effectively.