JPH08120358A - Mold for melting high melting point active metal - Google Patents

Abstract

PURPOSE: To stably produce an ingot having fine crystal and excellent surface characteristic at the time of hot-working by forming the bottom part of a cylindrical mold for casting to a specific shape, at the time of casting into the ingot after melting a high m.p. active metal with a consumable electrode type vacuum arc melting method. CONSTITUTION: At the time of producing a slab by melting the high m.p. active metal of an industrial pure titanium or titanium alloy, etc., with the consumable electrode type vacuum arc method, casting into the ingot and rough-casting, blooming and hot-rolling, the inner surface of the mold used at the time of casting into the ingot is formed to the cylindrical shape. Further, related to the vertical cross section passing the vertical center line in the height direction of the mold at the time of setting a two-dimensional coordinate by using Z axis for the vertical center line and R axis for the horizontal center line in the diameter direction of the mold, a side becoming Z<=0 to the orgin (R, Z)=(0, 0) is made to be the bottom side of the mold. The shape of the bottom part of the mold is made to be the shape in the three-dimensional range forming by rotating the two-dimensional range surrounded with a parabola shown in the formulas I and II while centering around the Z axis.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold for producing a refractory active metal ingot having fine crystal grains and excellent surface properties during hot working by a consumable electrode vacuum arc melting method. Regarding

[0002]

2. Description of the Related Art Generally, high melting point active metal ingots such as industrial pure titanium and titanium alloys are manufactured by a consumable electrode type vacuum arc melting method.

In this melting method, an electrode itself of a melting raw material is melted by arc heat generated, and a molten pool is formed in a water-cooled copper mold and solidified from the bottom of the pool to manufacture an ingot.

The most commonly used mold is the one described in "Metallic Titanium and Its Applications" (published by Nikkan Kogyo Shimbun, page 20), in which the inside of the mold is cylindrical and the bottom of the mold is flat. It has become.

Subsequent to this melting step, in order to process the ingot into a product such as a plate material, for example, in the case of a pure titanium plate material, a rough forging and slab rolling process for converting a cylindrical ingot into a rectangular slab and a slab are performed. A hot rolling process for forming a coil will be performed.

When a hot rolled coil is processed from an ingot and a slab, the surface area of the coil becomes much larger than the surface areas of the ingot and the slab. Therefore, even if there are few "surface defects" generated in the slab, this causes heat It is important to improve the surface quality of the ingot and slab, as the surface quality will deteriorate over a large area of the rolled coil.

Therefore, the "surface defects" of the slab must be removed before the hot rolling process, and the "surface defects" generated in the slab should be reduced or easily removed. Is preferred.

The term "surface defect" of the slab as used herein means that when the slab is hot-rolled, the surface quality of the coil is deteriorated over a part or the whole surface of the hot-rolled coil, and the deteriorated part is removed from the hot-rolled coil. If it is not a product unless it is a product, it is a general term for what causes the yield to be greatly reduced, and is a general term for "surface flaws" and "surface roughness" of slabs.

Further, the occurrence of "surface defects" of the slab requires a "slab care process" for removing them.
Work man-hours increase and production efficiency decreases.

One of the causes of "surface defects" of the slab is "casting surface defects". "Casting surface defects" are irregularities on the surface of the ingot and voids in the surface layer, and when the slab is made through the rough forging and slabbing process without removing these, what becomes the "surface defect" of the slab. Say.

Further, both ends of the ingot are chamfered in order to prevent cracks at both ends of the slab when the ingot is roughly forged and slab-rolled.

Another cause of the "surface defect" of the slab is a problem inside the ingot. It occurs when coarse crystal grains of the ingot (hereinafter referred to as coarse grains) remain without being broken by rough forging and slabbing.

Therefore, in order to reduce the surface defects caused by the coarse grains, the coarse grains formed in the ingot at the time of melting are reduced in advance, and the coarse grains are destroyed in the rough forging and slabbing process. Then, a method of miniaturizing is considered.

The method disclosed in Japanese Unexamined Patent Publication No. 4-46643 is the latter case described above, in which a slab for continuous hot sheet rolling is formed from a cylindrical pure titanium ingot only by slab rolling without forging. It is a manufacturing method, and the temperature is controlled until the thickness becomes 40% or less of the ingot diameter.
This is a method for producing a slab with few wrinkles (surface defects) by setting the temperature to 00 ° C. or higher and 1100 ° C. or lower, and then rolling in the width direction using a hole roll at 883 ° C. or higher and 1100 ° C. or lower.

This method is a method for manufacturing a slab by only rolling process without forging the ingot, and since the coarse-grained structure of the ingot is destroyed by forging, wrinkle marks are not formed on the surface of the slab after the forging process. However, if you want to manufacture slabs only by rolling without forging,
Coarse grains are not sufficiently destroyed and become wrinkle flaws. Therefore, rolling conditions are specified to prevent such wrinkle flaws.

However, since this method is limited to pure titanium, it cannot deal with high melting point active metals other than pure titanium such as zirconium and zirconium alloys.

As a result of various investigations by the present inventors, as will be described later, coarse grains are concentrated at the bottom of the ingot, which corresponds to the initial stage of melting, so the above method cannot be adequately applied, and a forging step is introduced. However, the coarse grains were not sufficiently destroyed, and the surface defects of the slab generated at the portion corresponding to the bottom of the cylindrical ingot could not be suppressed.

[0018]

The present invention has been made in order to prevent slab surface defects due to coarse crystal grains of an ingot, and provides a mold capable of producing an ingot having a small number of coarse grains. The purpose is to

[0019]

In order to achieve the above object, the inventors of the present invention have conducted extensive studies on the cause of generation of coarse crystal grains of a high melting point active metal ingot manufactured by consumable electrode type vacuum arc melting. The following findings have been obtained.

(A) Coarse grains are always present immediately below the surface defects of the slab, and no coarse grains are present in the sound part.

(B) The bottom portion of the ingot corresponding to the initial stage of melting has a longer and larger cast structure, that is, columnar crystals, than the top portion corresponding to the final stage of melting, and the number and scale of surface defects of the slab corresponding to the bottom portion of the ingot are large. Both must be larger than the rest.

(C) The bottom of the ingot has longer columnar crystals than the top, and the reason why the columnar crystals grow larger is that the bottom of the mold is flat.

(D) In order to suppress the growth of columnar crystals at the bottom of the ingot, it is effective to make the mold bottom have a concave curvature.

The present invention has been completed based on the above findings, and its gist is "a mold for melting a high melting point active metal used for vacuum arc melting of a consumable electrode, in which the mold has a cylindrical shape and a high mold height. The vertical center line in the mold height direction is Z
When the axis and the horizontal center line in the mold radial direction are two-dimensional coordinates with the R axis as the origin (R, Z) = (0, 0),
The side where Z ≦ 0 is the mold bottom side, and the shape of the mold bottom is the range of the three-dimensional area formed by rotating the two-dimensional area surrounded by the parabola of the following equations (1) and (2) around the Z axis. A mold for melting a high melting point active metal, which is characterized by:

Z ≦ −1.2 / d · (R−d / 2) · (R + d / 2) ··· (1) Z ≧ −4.0 / d · (R−d / 2) · ( R + d / 2) ... (2) where d is the mold inner diameter ”.

[0026]

Next, the reasons and functions for limiting the shape of the mold of the present invention will be described below.

As a result of detailed investigation of the surface defects of the slab, the following phenomenon was confirmed.

First, the structures of the cross-sections of the surface layer portion of the slab containing the surface defects and the surface layer portion of the slab in the sound part were observed, and it was confirmed that coarse grains were always present immediately below the surface defects. did. From this, it was revealed that the surface defects of the slab were the result of non-uniform deformation in which the coarse grains of the ingot were not destroyed in the rough forging / bulk rolling process.

FIG. 1 is a view showing the structure of a slab observed by a surface defect microscope.

Further, the structure of the vertical cross section of the cylindrical ingot melted by the consumable electrode type vacuum arc melting method was examined, and as shown in FIG. 2, coarse grains grown in a columnar shape in the height direction at the bottom of the ingot. Had formed a triangular pyramid.

This is because in the initial stage of melting, the bottom of the molten pool, which is the solidification interface, is close to the bottom of the mold, so heat removal to the bottom of the mold, that is, the heat flux in the height direction is dominant.

As the melting progresses and the height of the ingot increases, the bottom of the molten pool, which is the solidification interface, also separates from the bottom of the mold, so that heat removal to the side of the mold, that is, the heat flux in the radial direction becomes dominant, The growth of crystal grains in the height direction stops.

This phenomenon in the initial stage of melting cannot be avoided as a fundamental problem of the vacuum arc melting method of the consumable electrode type, but it becomes more remarkable in the conventional method using a mold having a flat bottom surface.

Because the height direction of the ingot is perpendicular to the bottom surface of the mold, the heat flux becomes perpendicular to the bottom surface of the mold, that is, parallel to the height direction, which is a disadvantageous condition for stopping the growth of crystal grains. Because.

Considering these phenomena as the shape of the bottom of the molten pool, the flatter the bottom of the molten pool is, the easier the crystal grains are to grow. That is, the bottom of the mold is flat. To do.

From the above findings on the growth of coarse grains of the ingot, in order to suppress the coarse grains, it is necessary to correct the condition that the heat flux is biased in the height direction, that is, the condition that the bottom surface of the mold is a horizontal surface.

Specifically, by making the horizontal surface portion of the mold bottom smaller and gradually increasing the inner diameter of the mold bottom upward, the growth direction of the crystal grains in the height direction is changed to the mold diameter. It was possible to reduce the coarse grains by stopping the growth by changing the direction and causing the columnar crystal grains to collide with each other at an early stage.

FIG. 4 is a view showing the shape of the bottom of the mold defined by the present invention, and the shaded area in the figure is the range defined by the present invention. In the figure, the Z axis is the vertical center line in the mold height direction,
R is a horizontal center axis in the radial direction of the mold. The side where Z ≦ 0 with respect to the origin of the Z and R axes = (0, 0) is the mold bottom side.

FIG. 3 shows the results of examining the macrostructure by longitudinally cutting an ingot melted using the mold of the present invention. As is clear from this figure, the conical coarse grains formed at the bottom of the vertical cross section of the conventional ingot shown in FIG. 2 are eliminated.

The reason for limiting the shape of the mold will be described below.

Z ≦ −1.2 / d · (R−d / 2) · (R + d / 2): Z> −1.2 / d · (R−d / 2) · (R + d / 2)
When the shape becomes a three-dimensional shape corresponding to a region where the horizontal axis (R axis) is large, or when there are many horizontal planes, columnar crystal grains grow in the axial direction and coarse grains are formed at the bottom of the ingot. Since the area is formed and causes the surface defect of the slab, the lower limit of the depth of the concave shape of the mold bottom is set to Z ≦
It was set to -1.2 / d * (R-d / 2) * (R + d / 2).

Z ≧ −4.0 / d · (R−d / 2) · (R + d / 2): Z = −4.0 / d · (R−d / 2) · (R + d / 2) In the case of a parabolic three-dimensional shape, the growth suppression of coarse particles is saturated. Z <-4.0 / d · (R−d / 2) · (R +
In the case of a three-dimensional shape corresponding to the area of d / 2), the length of the portion having a smaller outer diameter becomes larger than that of the main portion of the ingot, although the growth of coarse grains has already been suppressed. Therefore, for the batch type consumable electrode vacuum arc melting method in which the total height of the ingot is limited, the ingot volume that can be melted in one melting, that is, the ingot weight is reduced, so that the manufacturing efficiency is significantly reduced. The upper limit of the concave depth of the mold bottom is Z ≧ −4.
It was set to 0 / d * (R-d / 2) * (R + d / 2).

Since the effect of the present invention is not influenced by the material of the mold, it is not particularly limited, but it is preferable to use copper.

The high melting point active metal has a melting point of 1500 ° C.
As described above, it refers to a metal that has a strong affinity with oxygen and cannot be dissolved in the atmosphere, and the main metals are titanium, titanium alloy, zirconium, and zirconium alloy.

When an ingot is produced by using the mold of the present invention, if a conventional columnar electrode is used, the cross-sectional area of the electrode is too large with respect to the cross-sectional area of the mold at the initial stage of melting. The arc and solidification may become unstable, which may lead to the generation of casting surface defects at the bottom of the ingot, and if this portion is removed, the yield will decrease. In addition, the mold may be damaged because the distance between the electrode and the mold in the initial stage of melting is not constant. Damage to the mold causes a serious accident, and repair or replacement of the mold is required even if the accident does not occur, which increases the manufacturing cost.

As a method of avoiding this problem, it is effective to make the tip of the electrode equal to the shape of the bottom of the mold. Considering the shape of the above mold, the area of the cross section of the mold and ingot increases in the initial stage of melting, so by changing the cross section of the electrode according to this cross section, It is possible to maintain an appropriate distance between the electrodes and a stable current density.

[0047]

EXAMPLES Next, the effects of the present invention will be described more specifically by way of examples.

As the primary dissolution, about 20 k of titanium sponge
g is a compact electrode with an outer diameter of 55 mm, and this is the inner diameter 1
Using a water-cooled copper mold of 00 mm, melting current 2 kA,
It melt | dissolved in 25V of dissolution voltage.

The bottom portion was cut by leaving one melted primary melting ingot to prepare secondary melting electrodes having various tip shapes.

FIG. 5 is a side view of the tip of the cut electrode (bottom portion of the primary melting ingot). Using these electrodes, a water-cooled copper mold having an inner diameter of a main portion of 145 mm and various bottom shapes was used and melted at a melting current of 3 kA and a melting voltage of 28 V.

FIG. 6 is a side view of the bottom of the mold used for this melting.

The total length of this secondary molten ingot was measured. If the total weight and the outer diameter of the main portion are the same, the conventional method ingot having a cylindrical bottom portion has the smallest overall height, which is advantageous in terms of smelting efficiency. Therefore, the conventional method was evaluated as ⊚ (good), those having a total height of 1.5 times or less as those of the conventional method were evaluated as ∘ (fair), and those exceeding 1.5 times were evaluated as x (not good).

After measuring the total length, the secondary melting ingot was subjected to removal of casting surface defects and chamfered at the end of test No. 8 in Table 1.

Regarding the chamfering of the end portion, the size of R after chamfering was set to 15 mm.

The yield was calculated from the weight change before and after this step, and 99% or more was excellent (good), and less than 99% was 97% or more.
(Fair) and less than 97% were evaluated as x (Fail).

Next, the casting surface defect was removed and the chamfering process was performed on the ingot by forging at a heating temperature of 950 ° C. to a thickness of 80 mm, followed by rolling at a roll diameter of 480 mm and a roll speed of 17.5 rpm to a thickness of 40 mm. It was a slab.

Surface defects of this slab were removed by cutting.

The yield was calculated from the weight change before and after this process, and 100%, that is, "no surface defect" was evaluated as ⊚ (good), 1
Less than 00% and 98% or more were evaluated as O (OK), and less than 98% were evaluated as X (NO).

Table 1 shows the shapes of the electrode tip and the mold bottom, which are the conditions for the secondary melting, and the yield investigation results of the ingot total length, ingot care and slab care.

[0060]

[Table 1]

Test Nos. 1-5 are examples of the present invention, in which the shapes of the electrode tip and the mold bottom for secondary melting were changed within the range specified by the present invention. Specifically, as shown in FIG. 6, in the example of the mold diameter of 145 mm, the depth (Z axis) of the mold bottom at the position where the horizontal center line R is 0 is 25 to 145 mm, which is within the specified range of the present invention. Is.

Test Nos. 6-7 are examples outside the scope of the claims of the present invention. Test number 8 is an example to which the conventional technique is applied.

In all of the examples of the present invention of Test Nos. 1-5, the yield evaluation of ingot care was ◎ or ○.
Has become. This is due to the fact that the shape of the bottom of the mold is made proper and the maintenance of the bottom of the ingot is reduced. Furthermore, all the examples of Test Nos. 1-3 in which the shape of the electrode tip portion is similar to the shape of the mold bottom portion are evaluated as ⊚. This is because the arc becomes stable at the initial stage of melting and the casting surface defects at the bottom of the ingot are reduced.

Further, in all of the invention examples of test numbers 1-5, the slab maintenance yield evaluation is ⊚ or ∘. This is because the shape of the bottom of the mold is proper and coarse grains of the ingot are reduced.

Particularly, in the present invention example of test number 2, it is understood from the comprehensive evaluation of the total length of the ingot, the care of the ingot, and the care of the slab that the conditions are advantageous among the examples of the present invention.

On the other hand, in the comparative example of test No. 6 and the conventional method of test No. 8, the yield evaluation of ingot care and slab care is x. Such a decrease in yield is a result of the need for chamfering of the bottom in the ingot care, and in the slab care process,
This is a result of removing surface defects due to coarse grains at the bottom.

In the comparative example of Test No. 7, the evaluation for the decrease in yield is ⊚, but the overall evaluation is × because the total height of the ingot is too large.

[0068]

EFFECTS OF THE INVENTION The ingot melted by using the mold of the present invention eliminates the triangular pyramidal region due to the coarse grains formed on the bottom of the conventional ingot, and the bottom of the ingot is not located at a position having a special casting structure. As a result, the surface defects of the slab that frequently occurred at the position corresponding to the bottom of the ingot in the rough forging / lumping rolling process could be made equal to the positions corresponding to the other parts of the ingot.

Further, the chamfering of the bottom portion was required for the conventional ingot, but since the bottom portion already has a curvature and is smooth, the yield due to the chamfering processing and the number of working steps are reduced. There are secondary effects.

[Brief description of drawings]

FIG. 1 is a cross-sectional microscope observation view of a slab surface defect portion and a sound portion.

FIG. 2 is a diagram showing a textured state of a vertical section of a conventional ingot.

FIG. 3 is a diagram showing a texture state of a longitudinal section of an ingot melted by the mold of the present invention.

FIG. 4 is a view showing a shape of a mold bottom portion of the present invention.

FIG. 5 is a diagram showing a shape of an electrode tip portion used in Examples.

FIG. 6 is a diagram showing a shape of a mold bottom portion used in Examples.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location F27D 11/08 B 8926-4K

Claims (1)

[Claims] 1. A mold for melting a high melting point active metal used for consumable electrode type vacuum arc melting, wherein the mold has a cylindrical shape and a vertical cross section passing through a vertical center line in the mold height direction is a mold height direction. Where the vertical center line of Z is the Z axis and the horizontal center line of the mold radial direction is the R axis, and the side that satisfies Z ≦ 0 with respect to the origin (R, Z) = (0, 0) On the mold bottom side, the shape of the mold bottom is a range of a three-dimensional region formed by rotating a two-dimensional region surrounded by parabolas of the following formulas (1) and (2) around the Z axis. Mold for melting high melting point active metals. Z ≦ −1.2 / d · (R−d / 2) · (R + d / 2) ··· (1) Z ≧ −4.0 / d · (R−d / 2) · (R + d / 2 ) (2) where d is the mold inner diameter.