Method for refining molten iron containing chromium

Abstract

A refining method and refining apparatus, able to shorten the time required for refining and reduce the refining costs in decarburization refining of a chromium-contained molten steel, which refining method for chromium-contained molten steel etc. performing decarburization refining by blowing a gas containing oxygen gas into a chromium-contained molten steel under a vacuum or atmospheric pressure and vacuum, said refining method for a chromium-contained molten steel etc. characterized by having a first step for blowing oxygen gas while making the inside of the vessel a pressure of a range of 400 Torr (53 kPa) to atmospheric pressure, a second step for blowing oxygen gas while evacuating the inside of the vessel to 250 to 400 Torr (33 to 53 kPa), and third step for blowing gas while evacuating the inside of the vessel to not more than 250 Torr (33 kPa). Further, a refining method and refining apparatus for an ultra-low carbon chrome melt characterized by performing a first vacuum refining until the third step, then restoring the pressure in the vessel to at least 400 Torr (53 kPa), then performing second vacuum refining while making the bottom blowing gas blow rate at least 0.4 Nm 3 /min per ton steel.

1 . A refining method refining by blowing a mixed gas including oxygen gas into a chromium-contained molten steel in a refining vessel, said refining method for a chromium-contained molten steel characterized by having a first step of blowing in said mixed gas while making the inside of the vessel a pressure of a range of 400 Torr (53 kPa) to atmospheric pressure, a second step of blowing said mixed gas while evacuating said vessel to 250 to 400 Torr (33 to 53 kPa), and a third step of blowing said mixed gas while further evacuating the inside of the vessel to not more than 250 Torr (33 kPa) and by refining step by step while switching from the first step to the second step at a concentration of carbon in the melt of 0.8 to 0.3% and switching from the second step to the third step at a concentration of carbon in the melt of 0.4 to 0.1%.  
     
     
         2 . A refining method for a chromium-contained molten steel as set forth in  claim 1 , characterized by refining while making the mixed gas blow rate at said second step at least 0.4 Nm 3 /min per ton melt.  
     
     
         3 . A refining method for a chromium-contained molten steel as set forth in  claim 1 , characterized by, in said first step, performing refining comprising refining the entire amount under atmospheric pressure, refining the entire amount under a vacuum, or refining first at atmospheric pressure, then under a vacuum.  
     
     
         4 . A refining method for a chromium-contained molten steel as set forth in  claim 1 , characterized by, when refining under atmospheric pressure of said first step, refining using both top blowing and bottom blowing as the blowing of said mixed gas.  
     
     
         5 . A refining method for a chromium-contained molten steel as set forth in  claim 1 , characterized by, when refining under atmospheric pressure of said first step, refining using only oxygen for the blowing of said mixed gas.  
     
     
         6 . A refining method for a chromium-contained molten steel as set forth in  claim 1 , characterized by, in said third step, refining by further evacuating step by step the inside of the vessel along with the decrease in concentration of carbon in the melt.  
     
     
         7 . A refining method for a chromium-contained molten steel as set forth in  claim 1 , characterized by, in said third step, refining by any means of supplying only inert gas for the blowing of said mixed gas, gradually reducing the ratio of supply of oxygen gas in said mixed gas along with the decrease in concentration of carbon in the melt, or supplying inert gas after the ratio of oxygen gas in said mixed gas decreases.  
     
     
         8 . A refining method for a chromium-contained molten steel as set forth in  claim 1 , characterized by starting evacuating the inside of said refining vessel, then blowing inert gas, nitrogen, or another non-oxidizing gas or a mixed gas of the same to reduce the concentration of oxygen in the exhaust gas to not more than 7 vol %, then blowing said mixed gas into said evacuated refining vessel and starting refining.  
     
     
         9 . A refining method for a chromium-contained molten steel as set forth in  claim 1 , characterized by, in said third step, reducing the concentration of carbon in the melt to not more than 0.08%, then restoring the pressure in the vessel to at least 400 Torr (53 kPa), then bottom blowing mixed gas and vacuum refining at a mixed gas blow rate of at least 0.4 Nm 3 /min per ton melt so as to reduce the carbon to an ultra-low level.  
     
     
         10 . A refining method for a chromium-contained molten steel as set forth in  claim 9 , characterized, after said third step, by restoring the pressure inside the vessel to at least 400 Torr (53 kPa), then bottom blowing mixed gas, reducing the ratio of the oxygen gas in the blown mixed gas to not more than 30%, reducing the pressure inside the vessel to not more than 100 Torr (13 kPa), and continuing refining.  
     
     
         11 . A refining apparatus for a chromium-contained molten steel, said refining apparatus for a chromium-contained molten steel characterized by comprising a vacuum refining vessel, an alloy and sub-material addition unit provided above the vacuum refining vessel, exhaust gas cooler, vacuum valve, one-stage or multiple-stage ejector type vacuum exhaust unit, and water-sealed type vacuum pump arranged successively and by having a pressure control valve under vacuum for returning part of the exhaust gas from down stream side of said water-sealed type vacuum pump to the upstream side of said water-sealed type vacuum pump.  
     
     
         12 . A refining apparatus for a chromium-contained molten steel as set forth in  claim 11 , characterized by being provided with a means for adjusting the opening degree of said vacuum control use pressure adjusting valve to control the degree of vacuum inside said vacuum refining vessel to return part of the exhaust gas exhausted from said water-sealed type vacuum pump to the upstream side of the exhaust gas passage of said water-sealed type vacuum pump.  
     
     
         13 . A refining apparatus for a chromium-contained molten steel as set forth in  claim 11 , characterized by providing a means arranging a vacuum valve between an exhaust side of said one-stage or multiple-stage ejector type vacuum exhaust unit and said water-sealed type vacuum pump and said vacuum refining vessel side of said exhaust gas cooler, closing said vacuum valve before the start of vacuum refining to place said ejector type vacuum exhaust unit and said water-sealed type vacuum pump in a vacuum state in advance, and opening said vacuum valve simultaneously with the start of vacuum refining to raise the degree of vacuum of the vacuum refining vessel.  
     
     
         14 . A refining apparatus for a chromium-contained molten steel as set forth in  claim 11 , characterized by providing a means for adjusting the opening degree of said vacuum control use pressure control valve under a vacuum in advance to restore up to 10% of the flow of exhaust gas to the upstream side of said water-sealed type vacuum pump and then immediately adjusting the degree of vacuum in said vacuum refining vessel when adding alloy and sub-material during refining under a vacuum in the vacuum refining vessel.  
     
     
         15 . A refining apparatus for a chromium-contained molten steel as set forth in  claim 11 , characterized by providing a seal unit having a seal valve for sealing an addition port at the bottom of said alloy and secondary material adding unit and setting a dummy lance integrally with said seal unit at the bottom of said seal valve or setting it elevatably linked with said seal unit.  
     
     
         16 . A refining apparatus for a chromium-contained molten steel as set forth in  claim 15 , characterized by providing a seal port for blowing seal gas to a clearance between inside walls of the addition port of said alloy and secondary material unit and said dummy lance.  
     
     
         17 . A refining apparatus for a chromium-contained molten steel as set forth in  claim 11 , characterized by providing a center cover having a cooling function at the bottom of said alloy and secondary material addition unit.  
     
     
         18 . A refining apparatus for a chromium-contained molten steel as set forth in  claim 11 , characterized by providing at the back of said exhaust gas cooler inside the refining apparatus system a water leakage detection unit able to detect water leakage by measuring at least one of a steam temperature or steam pressure in the exhaust gas.  
     
     
         19 . A refining apparatus for a chromium-contained molten steel as set forth in  claim 11 , characterized by arranging at the back of said one-stage or multiple-stage ejector type vacuum exhaust unit and said water-sealed type vacuum pump a return water storage tank linked with these and attached to a gas ventilation unit.  
     
     
         20 . A refining apparatus for a chromium-contained molten steel as set forth in  claim 19 , characterized by providing a water-sealed cover having a partition cover provided, without being fixed, at the top of said return water storage tank.  
     
     
         21 . A refining apparatus for a chromium-contained molten steel as set forth in  claim 20 , characterized in that the weight of said water-sealed cover satisfies the following formula (1): 
       ( W 1+ W 2)×9.8> P×S   (1) where,    W 1 : weight of partition cover (kg)    W 2 : weight of weight placed on partition cover (kg)    P: maximum gas pressure acting inside return water storage tank (Pa)    S: maximum area of projection of inside surface of movable partition cover on horizontal plane (m 2 ).    
     
     
         22 . A refining apparatus for a chromium-contained molten steel as set forth in  claim 20 , characterized in that the water-sealing height of said water-sealed cover satisfies the following formula: 
         H−L> 9.8×10 3   ×P   (2) where,    H: height of outside outer tube of partition cover side walls of water-sealed cover (m)    P: maximum gas pressure acting at inside of return water storage tank (Pa)    L: height of sealing water passage between inner tube and outer tube in water-sealed cover (m).