JP4595958B2 - Ingot manufacturing method by vacuum arc melting method - Google Patents

Description

  The present invention relates to a method for producing an ingot by a vacuum arc melting method, and particularly relates to a method for producing an ingot by a vacuum arc melting method suitable for casting a large-diameter ingot.

  The vacuum arc melting method (hereinafter sometimes referred to as “VAR method”) is one of the remelting and solidification methods. According to this VAR method, in general, compared with the ingot by the normal ingot-making method, Therefore, a sound ingot having excellent cleanliness and less component segregation can be obtained. In other words, in the VAR method, a directional solidification structure can be formed by laminating solidification, which is a major feature of the VAR method, and a high-quality ingot with reduced macrosegregation and semi-macrosegregation can be obtained. Therefore, the VAR method is a material that requires a high level of reliability, including space, aircraft parts, and turbine materials, although the process cost is higher than that of the normal ingot casting method and continuous casting method. Is often used in the manufacture of

  FIG. 1 is a longitudinal sectional view showing an outline of a VAR furnace used for manufacturing an ingot by the VAR method. As shown in the figure, in the VAR method, an arc discharge is generated between the consumable electrode 3 as a base material of the ingot 1 and the molten metal (molten pool 2) in the mold (water-cooled copper mold) 4 under high vacuum. Then, the consumable electrode 3 is sequentially melted from the lower end by an arc. The melted consumable electrode 3 is sequentially dropped and stored in the mold 4 and stacked and solidified in the mold 4. Thus, the ingot 1 having a directional solidified structure is obtained.

  In the VAR furnace shown in FIG. 1, the mold 4 is wrapped by a jacket 5. The jacket 5 is provided with a cooling water supply port 6 for feeding cooling water to the mold 4 and a cooling water drain port 7 for discharging the cooling water from the mold 4. A furnace body 8 is attached on the jacket 5. The furnace body 8 is provided with a vacuum exhaust port 9 through which air is exhausted to bring the inside of the furnace body 8 into a high vacuum. The consumable electrode 3 is connected to and supported by the stinger rod 10 in the furnace body 8 and is lifted and lowered as the stinger rod 10 is lifted and lowered by a lifting mechanism (not shown).

  FIG. 2 is a longitudinal sectional view schematically showing the state of dissolution and solidification during casting by the VAR method. As shown in the figure, in the ingot of the directional solidification structure, the entire surface is composed of a columnar crystal structure, and the columnar crystal is almost symmetrically upward from the bottom of the ingot to the central axis in the longitudinal direction of the ingot. It extends in the same direction. In order to obtain a directional solidified structure, when casting a small ingot having a small diameter D, it has been conventionally effective to keep the molten pool shallow by melting at a low speed.

  By the way, in the diversified material situation in recent years, even if it is VAR method, if the selection of dissolution conditions is wrong, there is a mixture of non-directional branched columnar crystals and equiaxed crystals in the solidified structure, macro segregation and This will cause semi-macro segregation and further porosity. For example, when the diameter of an ingot is increased for the purpose of improving production efficiency, if the melt is melted at high speed, the molten pool becomes deeper at the center of the ingot and the temperature gradient in the molten pool becomes smaller, so that the initial solidified shell Is formed from the molten metal and causes the formation of equiaxed crystals.

  Therefore, if the melt pool is made shallower by adjusting the dissolution rate of the consumable electrode, the temperature gradient at the solidification interface increases, so that a new solidification shell is generated at the solidification interface, and the solidification shell sequentially updates the solidification interface. Will grow like so. By maintaining such laminated solidification, an ingot having a directional solidified structure can be obtained.

  Here, when the solidified shell is formed and grows in the molten metal pool, the gap filled with the solidified shell becomes a closed space. When the residual molten metal sandwiched therebetween solidifies, the microsegregated molten metal in the surrounding solidified shell is sucked out by solidification shrinkage, and segregation defects such as macrosegregation and semi-macrosegregation appear in the center of the ingot. Furthermore, if the molten metal is insufficiently supplied due to solidification shrinkage, it causes porosity. The segregation defect includes a granular segregation defect generally called Freckle, in which specific components (C, Si, Mn, O, S, etc.) are locally enriched.

  On the other hand, in the directional solidified structure, since there is no such factor, segregation defects such as macrosegregation and semi-macrosegregation do not occur, and only minute microsegregation formed between dendrite trees appears.

  As a technique for suppressing segregation defects in a large-diameter ingot, Patent Document 1 proposes a technique in which molten metal is injected into the gap between the mold and the ingot or helium gas is allowed to flow through the gap. . According to this method, heat transfer between the mold and the ingot is improved by the presence of molten metal or helium gas, and the cooling rate of the ingot can be increased. Thereby, a molten metal pool becomes shallow and the large diameter ingot which has a directional solidification structure with few segregation defects is obtained.

JP-A-9-70656

  However, although the above-described method can obtain a large-diameter ingot having a directional solidification structure with few segregation defects, it is technically introduced in the VAR method that requires melting under a high degree of vacuum. It is difficult to implement, and even if it is introduced, it is not realistic because it involves major equipment modifications.

  Further, segregation defects are likely to occur as the dissolution rate increases and the diameter of the ingot increases, and the critical condition of the dissolution rate that becomes a directional solidified structure varies depending on the diameter of the ingot. As a result, there is no problem in quality even if the dissolution rate for casting a small-diameter ingot is applied as it is as the dissolution rate for casting a large-diameter ingot, but in this case, the operation time becomes long. However, it cannot be said that the production efficiency can be maximized.

  Therefore, in the past, when casting a large-diameter ingot in earnest, the internal quality of the ingot obtained individually was tested in advance by variously changing the dissolution rate in the casting of the large-diameter ingot. By verifying the above, it was found that the dissolution rate was suitable for full-scale operation in which a directional solidified structure free from segregation defects was obtained. The determination of an appropriate melting rate for casting such a large-diameter ingot requires a large amount of time before the full-scale operation, and therefore there is a problem that the full-scale operation cannot be efficiently performed. .

  Therefore, the present invention has been made in view of the above problems, and can be efficiently transferred to full-scale operation, and can produce a large-diameter ingot having a directional solidification structure free from segregation defects with high production efficiency. It aims at providing the manufacturing method of the ingot by.

  In order to achieve the above object, the present inventors have conducted extensive studies on a method for producing an ingot by the VAR method on the premise that the melting conditions for casting a small-diameter ingot are cast. Knowledge was obtained and the present invention was completed.

That is, when casting a small-diameter ingot having a diameter D ₀ by the VAR method, the entire longitudinal section of the ingot after casting is composed of a columnar crystal structure extending symmetrically upward from the bottom in the longitudinal central axis. When the upper limit dissolution rate at which a directional solidified structure can be obtained and the occurrence of freckle is M _0max , when casting a large-diameter ingot having a diameter D ₁ larger than the small-diameter ingot by the VAR method, adjust the dissolution rate M ₁ so as to satisfy the relationship of formula (a) below, to obtain the directional solidification structure in the ingot of the large diameter.

M ₁ ≦ (M _0max / D ₀ ) × D ₁ Formula (a)
Thus, when casting a large-diameter ingot by the VAR method, the dissolution rate when casting a small-diameter ingot with excellent internal quality, the diameter of the small-diameter ingot, and the large-diameter From the diameter of the ingot, it is possible to calculate and set an appropriate dissolution rate for obtaining a large-diameter ingot having a directional solidified structure without segregation defects.

  According to the method for producing an ingot by the VAR method of the present invention, it is possible to efficiently shift to a full-scale operation of a large-diameter ingot by applying a melting rate calculated by utilizing the melting condition of the small-diameter ingot. Moreover, it is possible to obtain a sound large-diameter ingot having a directional solidification structure with high production efficiency and no segregation defects.

  Hereinafter, embodiments of the present invention will be described in detail.

  In general, the critical condition between the formation of columnar crystals and the formation of equiaxed crystals is determined by the magnitude of the value of V / G using the local solidification rate V and the temperature gradient G of the solid-liquid phase as parameters. Conceivable. Therefore, the critical constant α is defined by the following equation (1).

α = V / G Formula (1)
The unit of the solidification rate V is, for example, [mm / min], and the unit of the temperature gradient G is, for example, [° C./mm].

  Next, in considering the solidification structure transition of the ingot by the VAR method, the temperature gradient is the smallest and the value is represented by the value of the center portion of the ingot where segregation defects are likely to occur. That is, the following solidification parameters are values at the center of the ingot.

  When the ingot rising speed by the VAR method is U, and the shape of the molten pool reaches a steady state, the rising speed U becomes equal to the solidification speed V (V = U). In the solidification process, there is a relationship of G = R / V among the ingot cooling rate R, the solidification rate V, and the temperature gradient G. By substituting these relational expressions into the above expression (1), the following expression (2) is derived.

α = U ² / R (2)
The unit of the hot water rising speed U is, for example, [mm / min], similarly to the unit of the solidification speed V, and the unit of the cooling speed R is, for example, [° C./min].

Next, assuming that the melting rate of the consumable electrode by the VAR method is M, the diameter of the ingot is D, and the material density is ρ, the relationship M = πρD ² U / 4 holds. By substituting this relational expression into the above expression (2), the following expression (3) is derived.

α = (4M / πρD ² ) ² / R (3)
The unit of the dissolution rate M is, for example, [kg / min], the unit of the ingot diameter D is, for example, [mm], and the unit of the material density ρ is, for example, [kg / mm ³ ].

Here, for a small-diameter ingot, the diameter is D ₀ , the melting rate is M ₀ , and the cooling rate is R _{0. For} the large-diameter ingot, the diameter is D ₁ , the melting rate is M ₁ , and the cooling rate is Let R ₁ . When the critical constant α for the small diameter ingot and the large diameter ingot is the same, the following formula (4) is derived from the above formula (3).

(4M ₁ / πρD ₁ ² ) ² / R ₁ = (4M ₀ / πρD ₀ ² ) ² / R ₀ Formula (4)
The units of the dissolution rates M ₁ and M ₀ are, for example, [kg / min], the units of the ingot diameters D ₁ and D ₀ are, for example, [mm], and the units of the cooling rates R ₁ and R ₀ . Is, for example, [° C./min].

From the above equation (4), the dissolution rate M ₁ of the large-diameter ingot by the VAR method is expressed by the following equation (5).

M ₁ = M ₀ (D ₁ / D ₀ ) ² × (R ₁ / R ₀ ) ^1/2 Formula (5)
Since the cooling rate R is inversely proportional to the square of the diameter D of the ingot, the relationship R ₁ / R ₀ = (D ₀ / D ₁ ) ² is established. By substituting this relational expression into the above expression (5), the following expression (6) is derived.

M ₁ = (M ₀ / D ₀ ) × D ₁ Formula (6)
If it is confirmed that a directional solidified structure of columnar crystals without segregation defects can be obtained with a melting rate M ₀ applied in the full-scale operation of a small-diameter ingot, the small-diameter ingot When casting a large ingot having a diameter D ₁ larger than the diameter D _0, the dissolution rate M ₁ calculated based on the above equation (6) is set. Thereby, the internal quality of the obtained large-diameter ingot is equal to that of the small-diameter ingot, and a directional solidified structure free from segregation defects is obtained.

Furthermore, in the casting of a small-diameter ingot, if the upper limit dissolution rate M _0max is obtained to obtain a directional solidified structure of columnar crystals without segregation defects, the diameter D ₁ is larger than the diameter D _{0 of the} small-diameter ingot. When casting the large-diameter ingot, the dissolution rate M ₁ calculated based on the following equation (7) derived from the above equation (6) (corresponding to the above equation (a)) is set.

M ₁ ≦ (M _0max / D ₀ ) × D ₁ (7)
By setting the dissolution rate M ₁ to satisfy the above formula (7), the internal quality of the obtained large-diameter ingot is at least equivalent to that of the small-diameter ingot, and a directional solidified structure free from segregation defects is obtained. In particular, if the dissolution rate M ₁ when casting a large-diameter ingot is the upper limit (M _0max / D ₀ ) × D ₁ , as a result, the operation time is greatly shortened. Large diameter ingots can be manufactured with maximum improvement.

  Thus, when casting a large-diameter ingot by the VAR method, the dissolution rate when casting a small-diameter ingot that has already been determined to have excellent internal quality, the diameter of the small-diameter ingot, and the An appropriate dissolution rate can be calculated and set from the diameter of the large ingot. Therefore, it is not necessary to verify the internal quality of the ingot at each melting rate by a preliminary test, and it is possible to efficiently shift to full-scale operation of a large-diameter ingot by applying the calculated melting rate, and high production It is possible to obtain a sound large-diameter ingot having a directional solidification structure that is efficient and free from segregation defects.

  About the manufacturing method of the ingot by the above VAR method, the effectiveness was clarified from the following examples.

A small diameter ingot having a diameter D ₀ of φ500 mm and a large ingot having a diameter D ₁ of φ775 mm were tested using a VAR furnace. The diameters D ₀ and D ₁ of the ingot correspond to the inner diameter of the mold (water-cooled copper mold).

  A low alloy steel of C: 0.5 wt% -Si: 0.3 wt% -Mn: 0.9 wt% -Ni: 1.8 wt% -Cr: 0.8 wt% is prepared as a test base material to be a consumable electrode. did. The diameter of the consumable electrode for the small diameter ingot (φ500 mm) was φ400 mm, and the diameter of the consumable electrode for the large diameter ingot (φ775 mm) was φ690 mm.

Regarding the melting voltage value, both the small diameter ingot (φ500 mm) and the large diameter ingot (φ775 mm) are changed within the range of 25 to 27 V, and the melting current value is within the range of 7 to 16 kA for the small diameter ingot (φ500 mm). In the case of a large-diameter ingot (φ775 mm), the input power value was changed by changing it within a range of 13 to 20 kA, and ingots with different melting rates M ₀ and M ₁ were manufactured in stages. The casting length of the ingot was 3 m for the small-diameter ingot (φ500 mm) and 3.5 m for the large-diameter ingot (φ775 mm).

  And in each ingot, the macro sample of the center longitudinal section was taken from the position of the steady melting period at the center of the ingot height, and the internal quality was confirmed from the presence of solidification structure and segregation defects at the center of the ingot. .

  Table 1 shows examples of the present invention included in the scope of the present invention and comparative examples outside the scope of the present invention.

First, a preliminary test by the VAR method was performed in the order of Invention Examples 1 to 4 and Comparative Examples 1 and 2, using a mold for a small diameter ingot (φ500 mm). When the melting rate M ₀ in the stationary phase excluding the hot start phase at the initial stage of the start and at the end of melting was increased stepwise, the melting rate M _{0 up} to 6 kg / min (Invention Examples 1 to 4) was also in the center of the ingot. Columnar crystals were formed. As a result, no segregation defects such as macrosegregation and semi-macrosegregation occurred.

However, at 6.5 kg / min (Comparative Example 1) and 7 kg / min (Comparative Example 2) in which the dissolution rate M ₀ exceeds 6 kg / min, an equiaxed crystal is formed in the structure of the center of the ingot. Thus, granular segregation (maximum diameter 3 mm in comparative example 1, maximum diameter 5 mm in comparative example 2) appeared.

Based on the preliminary test of this small diameter ingot (φ500 mm), the critical M _0max / D ₀ value for obtaining a directional solidification structure of columnar crystals without segregation defects is 0.012 kg / min · mm (= 6/500). It became.

Next, tests by the VAR method were performed in the order of Invention Examples 5 to 8 and Comparative Examples 3 and 4 using a mold for a large diameter ingot (φ775 mm). At that time, the dissolution rate M ₁ based on the above formula (7) under the condition that the critical M _0max / D ₀ value (0.012) obtained from the test result in the previous small-diameter ingot (φ500 mm) is included. Was set in stages. The dissolution rate M ₁ in Invention Example 8 is 9 kg / min corresponding to the M _0max / D ₀ value (0.012), and the dissolution rate M ₁ in Invention Examples 5 to 7 is M _0max / D. Corresponds to a value not exceeding ₀ (0.012). The dissolution rates M ₁ in Comparative Examples 3 and 4 correspond to those exceeding the M _0max / D ₀ value (0.012).

  In Examples 5 to 8 of the present invention, columnar crystals were formed in the structure at the center of the ingot, and the occurrence of segregation defects was not recognized. On the other hand, in Comparative Examples 3 and 4, the structure at the center of the ingot was equiaxed, and relatively large grain segregation (maximum diameter 4 mm in Comparative Example 3 and maximum diameter 7 mm in Comparative Example 4) occurred.

Thus, when casting an ingot (Fai775mm) of large diameter, the rate of dissolution M _1, based on the equation (7), the criticality of the time of casting the diameter of the ingot (φ500mm) M _0max / It has been clarified that a healthy ingot having a directional solidification structure free from segregation defects can be obtained by adjusting the D ₀ value (0.012) or less.

Here, the critical M _0max / D ₀ value when casting a small-diameter ingot is the range of the solidus temperature and liquidus temperature of the material to be cast, and the roughness of the dendrite structure unique to the material. It is thought that the properties related to dissolution and solidification such as Therefore, if the steel type of the large-diameter ingot is the same as the steel type of the standard small-diameter ingot, the above formula (7) can be used regardless of the steel type (carbon steel, low alloy steel, high alloy steel, etc.). A sound large-diameter ingot can be produced with the melting rate M ₁ set on the basis.

However, even if the steel types of the large-diameter ingot and the small-diameter ingot are strictly different from each other, for example, within the range of carbon steel having a C concentration of about 0.1 to 1 wt%, and further within the range of low alloy steel Since the characteristics relating to dissolution and solidification generally coincide with each other, the dissolution rate M ₁ of the large-diameter ingot may be set based on the above formula (7). The criticality of M _0max / D ₀ value at that time, it is possible to adopt a 0.012 kg / min · mm which can be verified in the above invention example.

In the case where the production of small diameter ingot as a reference has not been made yet, first the assess the M _0max / D ₀ value of the critical performing preliminary test by VAR method at a small-diameter ingot, its M _0max / The melting rate M ₁ of the large-diameter ingot may be set based on the above formula (7) from the D ₀ value. The determination of the critical M _0max / D ₀ value here is performed with a small-diameter ingot, so that it can be performed much more easily than with a conventional large-diameter ingot.

  According to the method for producing an ingot by the VAR method of the present invention, it is possible to efficiently shift to full-scale operation of a large-diameter ingot, and to obtain a large-diameter ingot excellent in internal quality with high production efficiency. It becomes possible. Therefore, the present invention is extremely useful as a method for producing a sound large-diameter ingot having a directional solidified structure by the VAR method.

It is a longitudinal cross-sectional view which shows the outline of the VAR furnace used for manufacture of the ingot by a VAR method. It is a longitudinal cross-sectional view which shows typically the melt | dissolution solidification state at the time of the casting by VAR method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Ingot 2 Molten metal pool 3 Consumable electrode 4 Mold 5 Jacket 6 Cooling water supply port 7 Cooling water drain port 8 Furnace body 9 Vacuum exhaust port 10 Stinger rod

Claims (1)

When casting a small-diameter ingot having a diameter D ₀ by the vacuum arc melting method, it is composed of a columnar crystal structure extending symmetrically upward from the bottom in the longitudinal direction on the entire longitudinal section of the ingot after casting. When the upper limit melting rate at which a directional solidified structure is obtained is M _0max , when casting a large-diameter ingot having a diameter D ₁ larger than the small-diameter ingot by the vacuum arc melting method, the following formula (a ), The melting rate M ₁ is adjusted so as to satisfy the relationship, and the directional solidified structure is obtained in the large-diameter ingot .
M ₁ ≦ (M _0max / D ₀ ) × D ₁ Formula (a)

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