The market has higher and higher requirements for the quality of large and medium-sized steel castings. The castings requiring ultrasonic flaw detection (U) and magnetic particle flaw detection (MT) account for a large proportion. Without good molten steel quality, the qualified rate of flaw detection will be seriously affected. The quality of molten steel involves two levels: chemical composition and purity of molten steel.
(1) the chemical composition meets the scope specified in relevant standards, which is only a minimum requirement, and it is far from enough to meet the standards. S. P when the content of harmful elements is high, it will have adverse effects on ultrasonic flaw detection and magnetic particle flaw detection. For example, when the content of S and P is reduced from 0.04% to 0.02%, it is equivalent to reducing sulfide (FES, MNS) and phosphide (fe3p) by 1.834 kg per ton of casting [atomic weight of S: 32; atomic weight of P: 31; atomic weight of Fe: 56; atomic weight of Mn: 5. In which 0.55 kg of FES and MNS inclusions and 1.284 kg of fe3p inclusions are formed]. In other words, a 10 t casting can reduce 18.34 kg of inclusions, which is very important to improve the qualified rate of flaw detection.
Therefore, the content of S and P harmful elements must be reduced. It is very difficult to reduce s and P in acid furnace steelmaking, and the qualified rate of flaw detection of castings poured with molten steel in acid furnace is low. It is necessary to use alkaline furnace molten steel for casting.
(2) the purity of molten steel is often ignored. The main factors affecting the purity of molten steel are (n), (H), (0), deoxidation products (1a203) and foreign inclusions, which will also have a great impact on the quality of molten steel, affect the fluidity of molten steel and the flaw detection results of castings.
The speed of steelmaking has an important impact on the quality of molten steel. In the process of molten steel steelmaking, not only a certain amount of decarburization, but also a sufficient decarburization speed. Degassing and suction are carried out at the same time. Only when the decarburization speed is greater than the critical decarburization speed, the degassing speed can be greater than the suction speed, so as to effectively eliminate the gas in molten steel and improve the quality of molten steel The quality of molten steel. Otherwise, it will have an adverse impact on the quality of molten steel, and even seriously affect the flaw detection results of castings.
Large enterprises have significantly improved the quality of molten steel through large investment in transformation or increase of equipment, including “high-capacity and high-power alkaline electric furnace”, “ladle refining furnace”, “VD vacuum refining furnace” and “RH vacuum circulation degassing” It is impossible for some medium and small steel casting enterprises to invest heavily in these equipment, but the simplest means of out of furnace refining should be used, such as hydrogen blowing at the bottom of the ladle. The investment is small and the effect is obvious. If you haven’t adopted this method, you might as well try it.
