According to 30CrMnSiA of the fracture defects, scanning electron microscope（SEM）and optical micro- scope（OM） were used to observe and find large former crack in the fracture, crack deepness contains half of the tube, crack was originated from the former root crack, which is the former crack propagation. Because of the big grain in the root crack, alumina slag and decarburization nearby the crack, the crack is concerned with the pierced tube. Pierced tube crack, alumina slag and big grain cause improper breakage. And the rising temperature leads to the big grain and fracture.

The normalizing and fully annealing tests were performed on the two samples of carburized bearing steel G20CrNi2MoA,which has two kinds of original structures;the isothermal annealing test has been done on the sample with normal structure,it was observed and shown that band structure can be eliminated by normalizing,while fully annealing makes the band structure worse,and the higher the isothermal annealing temperature is,the worse the banded structure is,which happens more obviously when the temperature is above 650 ℃.

New LZ45CrV steel blooms(/%:0.43C,0.27Si,0.77Mn,0.007P,0.005S,0.55Cr,0.10V, 0.lOMo) of 260 mm×260 mm and 280 mm×280 mm for heavy load railway axle at daye special steel are pilot-produced by the flow sheet 60 t UHP EBT EAF-LF-VD-6.5 t ingot-850 primary mill rolling process.With process measures including EAF charging 45%hot metal and quality scrap,controlling end[C]≥0.15%and[P]≥0.005%,feeding aluminium wire in LF refining to control 0.035%～0.045%Al and VD vacuum treatment≥20 min,the oxygen content in LZ45CrV steel bloom is 8.5×10~(-6),rating of nonmetallic inclusions≤1.0,rating of grain size 7～8,and the mechanical properties of steel bloom meet the requirement of standard and design.

Large inclusions have a significant impact on the fatigue life of bearing steel.The evaluation methods for inclusions in bearing steel were compared and analyzed.Bearing steel were tested by high frequence ultrasonic C-scan,and using metallographic method to validate the results.The results showed that the testing method of high frequence ultrasonic C-scan was effective for the testing of large inclusions in bearing steel.

Surface cracks were found on the surface of normal rolling steel.The morphology of the crack was observed by scanning electron microscopy(SEM) and optical microscopy(OM)and high-temperature oxidation products were found near the exist,with serious decarburization near cracks after corrosion.The result shows that high temperature oxidation would arise after heating the cracks of continuous casting billet,which can not be welded by normal hot rolled technology,and defects of the rolling steel will appear finally.