JP2985679B2 - Forging method of titanium ingot - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of forging a cylindrical titanium ingot which can prevent roughening or indentation flaws occurring on the surface of a titanium slab after slab rolling.

[0002]

2. Description of the Related Art The crystal structure of titanium is an α phase having an HCP structure at a low temperature and a β phase having a BCC structure at a high temperature. The transformation temperature of the α phase and the β phase is called a β transformation point.
The β transformation point of pure titanium is 883 ° C.

[0003] A general method for producing wrought material such as a titanium plate material is to ingot an ingot by a consumable electrode type vacuum arc melting method or the like, forge it, form a slab by slab rolling, and then heat it. This is a method of manufacturing by performing cold rolling.

[0004] In this case, in the early stage of forging, forging is performed at a high temperature which is acceptable in terms of material in the β region. That is, the ingot of titanium is heated at once from the room temperature to the β temperature region, that is, the temperature equal to or higher than the β transformation point in the forging process.

A problem with the method of forging this ingot and producing a slab by slab rolling is that roughening or indentation flaws are generated on the slab surface, and it is necessary to cut and remove these flaws before hot rolling. In some cases, the product yield is reduced.

When a titanium slab is manufactured from a cylindrical ingot, the upper and lower peripheral edges of the cylindrical ingot are pressed into the slab surface, which becomes flaws. This indentation flaw is lower at the upper and lower peripheral edges of the cylindrical ingot during forging than in other parts, and the deformation resistance in this part increases, so the cylindrical edge part is on a flat plate. It is generated by being pushed into.

[0007] The rough surface is caused by the formation of coarse columnar crystals accompanying the unidirectional solidification on the surface of the ingot when the ingot solidifies.

As a countermeasure against this roughening of the skin, Japanese Patent Laid-Open No.
No. 456 discloses a method of applying a working strain to the surface of an ingot and then heating it to a temperature higher than a recrystallization temperature.

An attempt has been made to omit a part of the manufacturing process from the above conventional manufacturing method for the purpose of simplifying the process. For example, Japanese Unexamined Patent Publication (Kokai) No. 61-159562 discloses a method of manufacturing a slab by directly rolling an ingot without performing rough forging. In the ingot required to prevent surface flaws generated at the time of direct rolling, Are disclosed.

In this method, the ingot is heated to a temperature range of 930 to 1000 ° C.
The hot rolling of 20% or more is performed in the above-mentioned temperature range below the β transformation point. Rolling here is considered to be lumping or plate rolling.

For the same purpose, the ingot is heated to 900 ° C. or higher.
Japanese Patent Publication No. 4-46643 discloses that rough rolling is performed at a temperature of 00 ° C. or less, and further, adjustment rolling is performed in a β temperature range.

Japanese Patent Publication No. 1-156456 discloses a method for preventing surface flaws when performing hot working such as direct rolling or extrusion of an ingot. In this method, a work strain is applied to the surface of the ingot, which is heated above the recrystallization temperature to recrystallize the surface layer.

[0013]

When considering the reduction of titanium production cost, it is an important measure to omit a part of the above-mentioned conventional production steps. However, in the case of titanium, the yield is improved. Thinking about is also an important issue.

That is, titanium consumes a large amount of electric power in the refining process from ore to titanium sponge and in the melting process of melting titanium sponge to produce an ingot.
The cost of the ingot increases, and the cost of the ingot accounts for a large proportion of the production cost of the final product. In order to reduce the manufacturing cost of industrial products with such a cost structure, it is important to omit some of the manufacturing processes, but most importantly, to improve the yield in the manufacturing process. .

In the conventional method disclosed in Japanese Patent Application Laid-Open No. 1-156456, the method for preventing the occurrence of surface roughness during bulk-rolling is not specified, but the processing strain is limited to room temperature or a warming temperature below the recrystallization temperature. It is thought that it is necessary to be given by, when working strain is applied to the ingot in these temperature ranges,
There is a concern that this processing may cause cracks in the ingot.
In addition, this method cannot prevent indentation flaws on the slab surface.

The present invention is directed to a process of slab production in which an ingot is forged and then subjected to slab-rolling. An object of the present invention is to generate a slab in the ingot-rolling after forging of a cylindrical titanium ingot. An object of the present invention is to provide a forging method capable of preventing roughening and indentation flaws.

If this rough surface occurs on the slab, as in the case of the above-mentioned indentation flaw, it is necessary to remove it before hot rolling. For this reason, the occurrence of rough skin reduces the yield at the time of manufacturing, and causes a serious problem that the number of work steps associated with rough skin and removal of flaws increases.

[0018]

The inventors of the present invention have conducted intensive studies to achieve the above object, and have obtained the following findings.

That is, the rough surface generated on the slab surface after the ingot rolling is caused by rapid grain growth of the cast structure at the time of heating at the β transformation point or higher. Preliminary forging to break the crystal in the temperature range from 800 ° C to the β transformation point, and then forging in the temperature range above the normal β transformation point to prevent roughening of the surface. By producing a bloom by two steps of preliminary forging and normal forging, indentation flaws can also be prevented.

The present invention has been made based on the above findings, and the gist of the present invention is that “titanium cylindrical ingot is
Heat to a temperature range from 00 ° C to β transformation point,
% Of the titanium ingot, characterized in that the columnar crystal is broken by pre-forging in which the rolling is performed within a range of not less than 40% and not more than 40%, and then heated to a temperature of not less than the β transformation point and not more than 1100 ° C. is there.

[0021]

Next, the reason why the forging method of the present invention is limited as described above and the operation thereof will be described.

A) Pre-forging temperature for fracture of columnar crystals The temperature for destroying coarse columnar crystals in the ingot is 800 ° C.
The reason specified in the temperature range of the β transformation point is as follows.

If the heating temperature exceeds the β transformation point, the ingot structure undergoes rapid grain growth in the β phase temperature range, so that it is not possible to achieve the destruction of the coarse structure. The heating temperature is 800
If the temperature is lower than ℃, the workability of the ingot is insufficient, and cracks or coarse flaws are generated during the processing.

B) Reduction amount of pre-forging Next, if the degree of work is less than 5% of the diameter, the pre-strain in the α phase region is small, and when the material is heated to the β transformation point or more after working, the crystal grains may be reduced. Abnormal growth occurs, and it is not possible to achieve finer crystal grains. On the other hand, if the processing exceeds 40%, cracks are generated on the side surface of the ingot during this process, and the amount of care after forging is increased. Therefore, the rolling reduction for columnar crystal breakage was set to 5 to 40%.

When the rolling direction is reduced from two directions orthogonal to each other in the cross section of the ingot, columnar crystals can be efficiently destroyed.

FIG. 1 is a diagram showing the rolling direction and the cross-sectional shape after rolling in the cross section of the ingot. The rolling direction is, for example, as shown in FIG. 1 (a), a vertical direction (Y direction) and a horizontal direction. Direction (X direction). Therefore, the cross-sectional shape after reduction is shown in FIG.
(b). Although the columnar crystal can be broken even when the rolling is performed from only one direction, it is preferable to perform the rolling in two directions.

C) Forging temperature If the forging temperature is lower than the β transformation point, the workability of the ingot structure is insufficient, cracks are generated during forging, and if this temperature exceeds 1100 ° C., the forging temperature increases. Since the lump surface significantly absorbs gas during heating and causes an increase in the amount of care after processing, the β transformation point is set to １００1100 ° C.

The fact that the indentation flaw can be reduced by dividing the preforging for columnar crystal destruction into two steps, ie, the forging and the main forging, is that the processing of the upper and lower edges of the ingot is performed in two steps. Therefore, the temperature drop of the peripheral portion during processing in one processing is small, and the peripheral portion is easily deformed, so that the peripheral portion is hard to be pushed. On the other hand, in the conventional process, since the cylindrical ingot is processed into a flat bloom by a single forging process, the temperature drop at the peripheral edge portion is large, and this temperature drop portion has become a noticeable indentation flaw. .

[0029]

[Example] JI containing 0.03% of Fe and 0.04% of O by weight%
An ingot of pure titanium of the S1 type was produced by a consumable electrode type vacuum arc melting method. Ingot dimensions are 200mm in diameter and 600mm in length
And its β transformation point was 883 ° C.

The ingot was pre-forged under the conditions shown in Table 1. After the preliminary forging, it was reheated to each temperature shown in Table 1 and forged to give a bloom. Thereafter, the slab was heated to 950 ° C. and slab-rolled to produce a slab having a width of 180 mm and a thickness of 50 mm.
With respect to this slab, the side face was removed in 1 mm units by cutting, and the residual flaw was examined by a penetrant inspection method. Machine processing of the side surface was continued until it was confirmed by this penetrant inspection that there was no skin roughness, and the depth of the surface roughness was investigated.

Also, the indentation flaws at the edges are sampled from the center in the width direction on the upper surface of the product after forging, and a sample for a speculum embedded in resin is prepared. From the surface to the tip of the indentation flaw by an x50 optical microscope. Was measured and the depth of the indentation flaw was investigated.

The results of the survey are shown in Table 1. In this table, based on the results of the conventional process (No 16), the surface roughness was evaluated as good when the side surface roughness and the depth of each of the indentation flaws on the surface were shallower than the conventional process. ○, one or both of these were evaluated. If it was deeper than the conventional process, it was evaluated as defective.
And

[0033]

[Table 1]

From these results, the ingot forged under the conditions of the present invention has rough side surfaces and shallow indentation flaws on the upper surface, and the object of the present invention has been achieved.

The slab manufactured by the method of the present invention is
It can be seen that the roughened surface and the occurrence of indentation flaws are reduced as compared with the material omitted in the conventional rough forging process (No. 17).

[0036]

EFFECT OF THE INVENTION In the production of wrought materials such as titanium plate materials, the prevention of rough skin and indentation flaws, which are problems when producing plate-like slabs, which are intermediate products of wrought materials, from cylindrical ingots. At the same time, making it possible to manufacture products with good yield.

[Brief description of the drawings]

FIG. 1 is a diagram showing a rolling direction and a cross-sectional shape after rolling in a cross section of an ingot.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C22F 1/18 B21J 1/00 B21J 5/00

Claims (1)

(57) [Claims] 1. A columnar crystal is broken by pre-forging, in which a columnar titanium ingot is heated to a temperature range of 800 ° C. or more and a β transformation point or less and a rolling is performed within a range of 5% to 40% of the ingot diameter. A forging method for a titanium ingot, wherein the forging is performed by heating to a temperature from the β transformation point to 1100 ° C. or lower.