Method for producing alloy ingot

1. A method for producing alloy ingot, comprising:

a step of charging alloy starting material into a cold crucible in a cold-crucible induction melter, and forming a melt pool of the alloy starting material by induction heating in inert gas atmosphere;

a step of continuing the induction heating and adding first refining agent to the melt pool, and then reducing the content of at least phosphorus from among impurity elements present in the melt pool; and

a step of forming alloy ingot by solidifying the melt, the phosphorus content of which has been reduced, wherein

the first refining agent is a mixture of metallic calcium and flux containing calcium halide;

the flux contains calcium fluoride and at least one of calcium oxide and calcium chloride, with a proportion of a total weight of the calcium oxide and the calcium chloride with respect to a weight of the calcium fluoride ranging from 5 to 30 wt %; and a proportion of a weight of the metallic calcium in the first refining agent with respect to a weight of the melt pool before adding the first refining agent is 0.4 wt % or greater.

2. The method for producing alloy ingot according to claim 1 , wherein

the proportion of a weight of the flux in the first refining agent with respect to the weight of the melt pool before adding the first refining agent is equal to or greater than the proportion of a weight of the metallic calcium in the first refining agent with respect to the weight of the melt pool.

3. The method for producing alloy ingot according to claim 1 , wherein

the melt pool is held by continuing the induction heating from a point in time at which the first refining agent is added to the melt pool up to a predetermined point in time between t1 to t2, in the step of reducing the content of phosphorus,

where the t1 denotes a point in time by which there has elapsed half the time until fusion of all the first refining agent from the point in time at which the first refining agent is added to the melt pool, and the t2 denotes the point in time by which there has elapsed a time over which half the metallic calcium in the first refining agent evaporates, from the point in time at which the first refining agent is added to the melt pool.

4. The method for producing alloy ingot according to claim 1 , wherein

a process from the step of forming the melt pool up to the step of forming alloy ingot is performed at least once after the step of forming alloy ingot, by using the alloy ingot as alloy starting material.

5. The method for producing alloy ingot according to claim 1 , further comprising:

a step of charging into the cold crucible the alloy ingot formed by solidification, as primary ingot, and forming melt pool of the primary ingot out of the primary ingot by induction heating in inert gas atmosphere;

a step of continuing the induction heating and adding second refining agent to the melt pool of the primary ingot, and then reducing the content of at least carbon and calcium from among impurity elements present in the melt pool; and

a step of forming alloy ingot by solidifying the melt, the carbon and calcium content of which has been reduced, wherein

the second refining agent is mixture of the flux and first oxide containing one, two or more types of oxides of main component element in the primary ingot;

a weight of the first oxide in the second refining agent ranges from 0.2 to 4 times a calculated weight that is calculated in order to oxidize totally at least carbon and calcium from among impurity elements present in the melt pool of the primary ingot; and

a proportion of the weight of the flux in the second refining agent with respect to a weight of the melt pool of the primary ingot before adding the second refining agent ranges from 0.5 to 5 wt %.

6. The method for producing alloy ingot according to claim 5 , wherein

the second refining agent is added to the melt pool of the primary ingot, and thereafter, the melt pool of the primary ingot is held by continuing the induction heating for 15 minutes or longer in evacuated state resulting from evacuating inert gas to the exterior, in the step of reducing the content of carbon and calcium.

7. The method for producing alloy ingot according to claim 6 , further comprising:

a step of charging into the cold crucible, as secondary ingot, the alloy ingot formed by solidification of the melt, the carbon and calcium content of which has been reduced, and forming melt pool of the secondary ingot out of the secondary ingot by induction heating in inert gas atmosphere;

a step of continuing the induction heating and adding deoxidizing element-based alloy component into the melt pool of the secondary ingot thereby to form alloy; and a step of forming alloy ingot by solidifying the melt that forms the alloy.

8. The method for producing alloy ingot according to claim 1 , further comprising:

a step of supplying into a cold hearth-type electron beam melter the alloy ingot formed by solidification, as starting material electrode, and irradiating electron beam onto the electrode of the starting material under atmospheric pressure lower than 5×10 −4 mbar, and then forming melt pool of the starting material electrode in a cold hearth in the cold hearth-type electron beam melter;

a step of adding third refining agent to the melt pool of the starting material electrode, and then reducing the content of carbon as impurity element present in the melt pool; and

a step of forming alloy ingot by solidifying the melt, the carbon content of which has been reduced, wherein

the third refining agent is second oxide containing one, two or more types from among oxides of main component element in the starting material electrode; and a weight of the third refining agent is 1 to 4 times a calculated weight that is calculated in order to oxidize totally carbon from among the impurity elements present in the melt pool of the starting material electrode.