Studying On The Corner Crack Of New Refined Mill System In Medium-carbon Steel Containing Boron

Boron steel as a kind of special steel，has a wide range of uses, but the slab’s corner crackproblem is a major focus in the research and discussion at home and abroad due to the cracksensitivity, whereas the study on slab corner crack in medium carbon and boron steel is little, sothis paper mirror the influence factors of ordinary slab’s corner crack and the may cause of theaddition of boron to study the influence of medium carbon and boron steel’s angle crackproblems.The conclusions through the analysis were as following：(1) Carbon content in steel is not the main cause of corner crack; The unsteadiness ofnitrogen content cause thermoplastic instability; High levels of Al content can increased theincidence of slab corner crack; The unsteadiness of sulfur is bigger, and part of sulfur content ismore than the target, The Mn/S in steel which less than37.6reach61%, indicating the Mn/S islow; The actual control of w(Ca)/w(Als) in steel than is low. Superheating temperaturefluctuation is larger, part superheating temperature more than35℃.(2) The carbon content control of BOF is not stable, therefore it suggested that stable carboncontent in high level of the internal components control; In the whole process, the increase ofnitrogen content in LF is too much, and the analysis believe that the raw materials causing theincreased nitrogen is not main reason, while the electric arc heating process causing the increasenitrogen is more severe; the continuous casting process increaseing less nitrogen content, whichmeans that the protection of casting is great, but the volatile of end nitrogen content isfluctuations.(3) When water flow of the Mould reach4000/380l/min, the slab corner cracks areobviously improved and when water flow of the Mould decrease to3800/360L/min，even to3600/340L/min, the transverse corner cracks of the slab have different degrees of improvement.Stable water flow of the Mould near the target value is more important during the process ofproduction.(4) The negative slide time is0.21s before majorization, which closeing to the upper limit ofrecommended (0.1s~0.25s). By reduceing1mm stroke，the negative sliding time reduced byabout32%, vibration marks spacing reduced3.31mm~3.86mm.(5) The content of boron element oxidizing into casting powder is0.74%， viscosity of theslag change small during the process of pouring and the stability is good, which indicate that it isno need a special casting powder. And optimizing the casting powder composition by increasingalkalinity, melting、pointmelting speed, and reducing viscosity.(6) The precision of segment row control is poor, arc alignment precision and the roll gapprecision have a bigger deviation.(7) High temperature tensile experiments show that A36-LB steel slab in650℃~1250℃have two brittleness temperature area, namely the first brittleness temperature area higher than1250℃and between700℃and900℃, including thermoplastic bottom temperature in800℃orso.(8) Lateral gradient of slab surface temperature is large, the slab corner temperaturesignificantly less than the slab of1/4，and are in the thermoplastic bottom temperature ofthird brittleness temperature area, so the accordingly advice is reduceing slab edge water volume andtotal amount, so as to reduce the temperature gradient of the slab cross section and avoid thehappening of corner crack.Through the above analysis and research, corresponding measures were taked and thecorner crack got good control.