Mechanism And Control Research On Billet Transverse Corner Crack Formation Of Boron Micro-alloyed Steel

Boron can significantly improve the steel hardenability,good mechanical properties can beobtained after heat treatment.But the transverse corner cracks on boron micro-alloyed steel is aworldwide problem. Based on the actual production of Tianjin Iron and Steel Group4#slabcaster,the project is aimed at solving this problem.The brittle mechanism of boron micro-alloyedsteel is researched in laboratory,and the rate of slab transverse corner cracks is controlled in lessthan1%after optimization.High temperature mechanical properties of boron micro-alloyed steel are researched byGleeble-3800thermal simulation testing machine. The third brittle temperature ranges of PQ235B-2B,PSS400-2B,45C-B are770℃~1020℃,720℃~920℃,600℃~1020℃respectively,when itcomes to800℃~850℃, the thermoplastic is worst,R.A%is lower than30%. Continuous castingstraightening should avoid this temperature region.The samples were incubated different times at aspecific temperature to promote the precipitation, and then subjected to a tensile test,and it showsthat the longer the samples were incubated the worse of the thermoplastic would be.That is tosay,precipitations could worsen the thermoplatis.Thermodynamic calculation results show the precipitation sequence for major second phaseparticles in boron micro-alloyed steels during the solidification process is MnS> BN> AlN.BN cansuppress the precipitation of AlN for its precipitation temperature is higher.When the temperaturedrops to Ac3, most of the element B combined with N, generated BN. Micro-organizations wereresearched by auger spectroscopy,it shows that the grain boundary segregation of B is more than300times, the atomic percent of boron in some areas can even reach38.65%.Samples were observed by SEM which were quenched at high temperatures.There are manyMnS precipitations and BN+MnS composite precipitations found in the steels,which are allspherical. The BN+MnS composite precipitations are distributed in the grain boundary and graininterior,its size is about2μm. A phenomenon be discovered that the nucleation and growth of BNis on MnS.The pull off samples of the thermal simulation experiment is also observed by SEM.Wecan find the BN+MnS composite precipitations deformed due to the tensile stress,BN is separatedfrom the MnS.The brittle mechanism of the boron micro-alloyed steel is obtained based on theabove research:(1) Lots of BN precipitations gathered at the grain boundaries due to its strongboron grain boundary segregation characteristics,making the grain boundaries Embrittled.(2)BNis easy to combine with MnS forming BN+MnS composite precipitations. As MnS precipitations are ductile, they are easily to be deformed and separated from the BN when subjected to tensilestress. Voids formed in this process, and cracks formed in the polymerization of these voids.Optimization experiment was organized based on the original BOF-LF-CC process.Thetitanium is added to steel after aluminum deoxidation controlled to150~250ppm, and theparameters of the vibration is optimized. After adding titanium,the rate of slab transverse cornercracks is significantly reduced, it’s a effective solution to the slab surface quality problems.Afteradding Ti to boron mico-alloyed steel,lots of small Ti(C,N) precipitations can be found bySEM,size of about100nm,no BN precipitations can bu found. So,adding titanium to the boronmicro-alloyed steel can indeed suppress precipitation of BN, thereby improving the quality of theslab.