7　Secondary Refining

7.1　Process Design

7.1.1　For the design of secondary refining unit，in order to optimize the general steelmaking-casting production process and meet the requirement of variety and quality of product，one or some types of the following refining units shall be selected to form a complete production line together with melting furnace and caster considering investment cost，production cost and media supply condition:

1　Argon blowing of ladle；

2　LF；

3　CAS/CAS-OB；

4　AOD；

5　VD；

6　VOD；

7　RH；

8　RH-TB；

9　WF.

7.1.2　If secondary refining is adopted，the melting furnace shall be provided with slag-free or less-slag tapping technology and the tapping shall be accurately controlled.The temperature and composition of liquid steel from melting process shall be in conformity with the requirement for secondary refining.

7.1.3　The nominal volume of the refining unit shall be the average capacity and shall be in conformity with that of melting furnace.The actual capacity shall be adaptive to the rational fluctuation of tapping from melting furnace.

7.1.4　The production capacity of the refining unit shall be determined according to its refining cycle and operating rate and also according to the matching relation with melting furnace and caster.The operating rate of common refining unit shall be in accordance with the following requirement:

1　The operating rate of LF and VD should be 80%-90%.

2　The operating rate of VOD should be 50%-90%.

3　For RH and RH-TB，the operating rate of single-vacuum-vessel operation shall be 50%-60% and that of double-vacuum-vessel operation shall be 75%-80%.

7.1.5　The vacuum refining units like VD，VOD and RH shall be equipped with color industrial TV for monitoring liquid steel level in vacuum vessel（chamber）.

7.1.6　For the steam jet vacuum pump，the input temperature of cooling water shall not be higher than 35℃.

7.1.7　For the argon，nitrogen，oxygen，and compressed air for refining，the quality requirement and the operation parameters like pressure and flow rate at TOP shall be ensured.The quality of media shall be in accordance with the following provisions:

1　The purity of argon for liquid steel stirring of ladle and for final bath stirring of top and bottom blown converter shall not be lower than 99.99%.For the argon for liquid steel stirring of ladle，impulse gas flow shall be provided in addition to the operating gas flow and the pressure of impulse gas flow shall not be lower than 1.6MPa.

2　The purity of nitrogen as substitute of argon shall not be lower than 99.9%.

3　The purity of oxygen shall not be lower than 99.5%.

7.1.8　The rail foundation of transfer car for vacuum vessel of VD and VOD and ladle for refining units shall be provided with countermeasures to liquid steel leakage failure.The preventive measures shall be taken for RH unit with ladle or with lifting by ladle transfer car to prevent liquid steel leakage to the underground hydraulic equipment.

7.1.9　If VD，VOD and RH refining units are provided with steam jet vacuum pump，the measure to prevent escape of gas must be taken for the water-seal tank（or hot water tank）.The fume exhaust pipe for vacuum pump and water-seal tank shall be extended above roof of shop building and the extended height shall not be less than 2m.

7.1.10　For the vacuum decarbonization units with oxygen injection，such as VOD and RH-TB，nitrogen shall be used for dilution and vacuum breaking.In addition，the device for automatic pressure equilibrium to atmospheric pressure shall be provided.The valve and piping of oxygen supply system shall be made of stainless steel or copper.For the vacuum degassing unit like VD，air breaking of vacuum should be adopted.The gas filling point shall be located close to vacuum vessel and should be directly located on vacuum vessel cover.

7.1.11　High-efficient preheating device shall be equipped as ladle and RH vacuum vessel preheating device and the preheating and drying temperature shall not be lower than 1000℃，which should be online arranged on the tapping line for melting furnace.Before use of RH vacuum vessel，the preheating temperature should not be lower than 1200℃.Online or local preheating device should be used.

7.1.12　For the vacuum refining units in a clod area where ambient temperature in winter is below freezing point，antifreezing measures shall be taken for the return water pumps and water ring pumps of water-seal tank（or hot water tank）for the vacuum pump.

7.1.13　At the discharging pipe of the vacuum alloy hopper for RH vacuum refining unit，nitrogen purging shall be provided.