JP3367392B2 - Manufacturing method of titanium slab - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a titanium slab for hot rolling into a fine and uniform structure by forging an industrial titanium ingot with a press. On how to do it. 2. Description of the Related Art In the production of hot and cold rolled sheets of industrial pure titanium and titanium containing a small amount of palladium (hereinafter simply referred to as titanium in the present invention), sponge titanium is generally produced by vacuuming. A circular ingot obtained by melting by arc melting or the like is used. Usually the diameter of this ingot is 600-
At about 1000 mm, it is hot crushed by a press or a large-diameter roll, formed into a slab, and subjected to hot rolling. [0003] In the method of sizing titanium, a large sizing roll is often used, but in some cases, the slab is formed by forging. However, in each case, the morphology of the structure depends on the final processing temperature of the lumping step. [0004] If the final processing temperature in the agglomeration step is a temperature below the transformation point of titanium, it becomes an equiaxed α phase, and if it is above the transformation point, it becomes a needle-like α phase affected by the β phase. This has been known for some time. From general metallurgical findings, the size of the slab crystal grains affects the deformability of the subsequent hot working.
In other words, the smaller the crystal grains of the slab, the easier the deformation of the crystal grains becomes, and as a result, the overall deformability increases.
The larger the deformability is, the less the flaw is generated on the surface of the hot pressing plate in the subsequent hot rolling. Therefore, the crystal grains of the slab are preferably fine. As described above, slabs subjected to hot rolling of titanium are often manufactured using large sizing rolls. The size of the crystal grains can be controlled to some extent by controlling the heating temperature of the ingot before the ingot and the temperature control at the end of the ingot, but the control of the crystal grains is limited due to the large size equipment. . When manufacturing slabs by forging, temperature control is relatively easy, but unlike lumping by rolls,
The lumping process tends to be uneven in the slab thickness direction. In particular, the crystal grains tend to be large in the vicinity of the surface of the slab, and the crystal grains become small in the central portion in the thickness direction, resulting in a slab in which the crystal grains are unevenly distributed. The reason is that it is difficult to forge at a portion directly below the forging die (hereinafter referred to as dead metal). FIG. 1 is a cross-sectional view for explaining a dead metal. This figure shows a state where a titanium ingot 1 is forged by an upper die 3 of a press, and 2 shows a forged material after forging. Fig. 1 just below the mold when pressed down by the upper mold 3
The processing rate of the triangular portion indicated by the oblique line becomes smaller, and this portion is called dead metal. [0008] Japanese Patent Application Laid-Open No. Hei 8-232601 discloses that in a forging process of a high-purity titanium material, primary forging combining forging and upsetting so that the forging ratio becomes 5 or more at a temperature equal to or higher than the transformation point. After processing at least once,
A forging method is disclosed in which a secondary forging process combining forging and upsetting is performed at least once so that a forging ratio at a temperature equal to or lower than the transformation point is 5 or more to refine crystal grains. [0009] However, in the range of production on an industrial scale, it is often difficult to easily increase the forging ratio due to dimensional restrictions in forming a slab from an ingot. In addition, the number of times of heating increases, and the production efficiency is not good. SUMMARY OF THE INVENTION The present invention provides a method for producing a titanium slab for hot rolling having fine and uniform crystal grains and excellent in hot deformability by press forging. As an issue. The inventors of the present invention have conducted experiments and studies in order to develop a method for making titanium crystal grains fine and uniform without increasing the forging ratio by press forging. The following knowledge was obtained. A) In order to refine the structure of the titanium slab after forging, relatively light forging is performed at a low temperature below the transformation point, strain is given to the ingot in advance, and then the temperature is increased to a temperature above the transformation point. It is preferable to start the forging at a temperature higher than the transformation point by heating, and to end the forging at a temperature lower than the transformation point. B) Prevention of non-uniformity of crystal grains due to dead metal, which is inevitable during forging, is achieved by limiting the contact length between the mold and the surface of the ingot in the direction of ingot transfer in forging below the transformation point. it can. C) The magnitude of the working ratio and the temperature during forging at a temperature below the transformation point for giving strain to the ingot in advance have little effect on the refinement of the crystal grains. The present invention has been made based on such knowledge, and the gist is as follows. A method for producing a slab by forging a titanium ingot with a press, wherein the contact length between the mold and the surface of the ingot in the ingot transfer direction is 300 mm at a temperature below the transformation point.
Forging in the following manner, then heating to a temperature above the transformation point, starting forging at a temperature above the transformation point, and terminating the forging at a temperature below the transformation point. Is a small amount of P in addition to pure titanium for industrial use.
It also includes titanium containing a corrosion resistance improving element such as d, Co, and Ta. For example, Pd is 0.04 to 0.25
%, The corrosion resistance is improved. The ingot surface refers to a surface in an as-cast state that has not been forged. FIG. 2 is a view showing a heat pattern of a titanium ingot in the production method of the present invention. As shown in the figure, the manufacturing method of the present invention has a transformation point (880-9).
00 ° C) or less in a dense hexagonal α-phase temperature range (zigzag part in Fig. 2), and then heated to a body-centered cubic β-phase temperature range above the transformation point and forged in a β-phase temperature range. In the forging in the α phase temperature range below the first transformation point, the contact length between the mold and the ingot surface in the direction of the ingot is transferred by the method of starting forging in the temperature range below the transformation point. Make it 300 mm or less. FIG. 3 is a longitudinal sectional view of the vicinity of the forged portion for explaining the contact length between the mold and the surface of the ingot in the ingot transfer direction. This figure shows that the titanium ingot 1 before forging is
And a state in which the forged material 2 is forged with a lower die (not shown). It shows a state where forging of the D portion of the ingot 1 is started. The length D of contact between the mold 3 and the ingot surface 1S in the ingot transfer direction is the contact length. The contact length in the width direction (the direction perpendicular to D) is not particularly defined because it has a small effect on grain refinement, but is usually about 300 mm because the ingot is usually circular in cross section.
Hereinafter, the reasons for limiting the production conditions of the present invention will be described. The titanium ingot is a generally used ingot having a circular cross section and a diameter of about 600 to 1000 mm. This ingot is forged at a temperature below the transformation point, the contact length between the mold and the surface of the ingot in the direction of ingot transfer is 300 mm or less, and then heated above the transformation point for the following reasons. . The reason for forging at a temperature lower than the transformation point is to give strain to the ingot. In addition, heating to a temperature above the transformation point after strain is applied is to heat the α phase incorporating the processing strain to a temperature above the transformation point, and to recrystallize it into a β phase with finer β grains. To do that. The forging temperature below the transformation point may be any temperature as long as it is below the transformation point, but it is preferable to heat to 880 to 700 ° C. and forge in the range of 700 to 600 ° C. for easy forging. Further, the working ratio in forging below the transformation point is not particularly limited because it does not significantly affect the refinement of crystal grains. The following tests were conducted to investigate the effects of the forging rate and the forging temperature on the crystal grain refinement at temperatures below the transformation point. From an ingot of 20 kg of pure titanium corresponding to one of JIS H-4600 having a diameter of 140 mm, a thickness of 30 m
m, cut out a test piece of width 100mm, length 150mm,
5% to 70% reduction at 750 ° C and 850 ° C
After being rolled, and then heated to 950 ° C. and cooled, the crystal grain size was determined by a comparative method using an optical microscope to determine the average area of the crystal grains. The length of contact between the mold and the surface of the ingot in the ingot transfer direction was all 150 mm. FIG. 4 is a diagram showing the relationship between the processing rate and the crystal grain area obtained by the above test. As is apparent from this figure, the average area of the crystal grains was 0.38 under all conditions.
A ～0.48Mm ^2, crystal grains are fine, and the size of the high and low and processing rate of processing temperature is seen to have little effect on the grain refinement. Next, the reason why the contact length between the mold and the surface of the ingot in the ingot transfer direction is set to 300 mm or less is 300 m
If it exceeds m, the processing rate of the dead metal portion becomes extremely small, strain is not uniformly applied to the entire ingot, and the size of crystal grains becomes non-uniform. By setting the diameter to be less than 300 mm, it is possible to reduce the size of the dead metal portion. From the viewpoint of forging efficiency and imparting distortion to a dead metal portion, the thickness is preferably 100 to 300 mm. This 300 mm was determined by the following experiment. Using a pure titanium ingot corresponding to one type of JIS H-4600 having a diameter of 760 mm, a temperature of 8
At 50 ° C., the working ratio is set to 20% and 40%, and the contact length between the mold and the ingot surface in the ingot transfer direction is 100 to 100%.
After forging with various changes of 400 mm, heating to 950 ° C. and forging and cooling, a specimen for examining crystal grains with a microscope was sampled from the surface layer of the slab, and the average area of the crystal grains was determined by the same method as described above. FIG. 5 is a diagram showing the relationship between the amount of feed during forging and the area of crystal grains, obtained by this test. As shown in this figure, when the contact length between the mold and the surface of the ingot exceeds 300 mm, the crystal grains are not refined. The forging with a contact length of 300 mm or less is, of course, performed over the entire length of the ingot. However, when a portion forged once at 300 mm or less is subjected to a second forging, the feed amount is reduced. May exceed 300 mm, and there is no significant effect on grain refinement. Next, the forging is started at a temperature higher than the transformation point by heating to a temperature higher than the transformation point, and the forging is terminated at a temperature lower than the transformation point for the following reasons. Forging in a body-centered cubic β-phase temperature region higher than the transformation point is easy to work, can be efficiently forged to a target size, and can make crystal grains finer. Because. Further, the reason why the forging end temperature is set to the transformation point or lower is to prevent the crystal grains of the slab from being affected by the β phase and having a needle-like structure which is not preferable as a material for hot rolling. The effects of the present invention will be described below with reference to examples. JIS-H-4 melted twice by vacuum melting
Press forging was performed under the conditions shown in Table 1 using a 6-ton ingot having a diameter of 760 mm corresponding to one of the 600 types. That is, the forging temperature below the transformation point is 75
There were three types, 0 ° C. and 850 ° C., and the processing rates at each temperature were 10%, 30% and 50%. The heating temperature above the transformation point is 950 ° C.
The forging was started at the transformation point or higher, and the work was performed while adjusting the temperature at the end of the forging to be equal to or lower than the transformation point. As the temperature after the end of the forging, the temperature of the slab was actually measured. As a comparative example, conditions for forging at a temperature above the transformation point without forging at a temperature below the transformation point are as follows:
Forging at a temperature below the transformation point, heating to a temperature above the transformation point,
Forging was performed under the condition that the forging end temperature was equal to or higher than the transformation point. The processing rates at temperatures above the transformation point were all about 70%. In addition, all ingots are fed 150mm
And [Table 1] An air atmosphere heating furnace was used for heating the ingot, and a 2500-t free forging machine having a 400 mm square cross section was used for forging. After the forging, a sample for microstructure observation was cut out from the slab, and the area of crystal grains was determined. Samples were taken from the surface layer and the center of the slab in the slab thickness direction in order to check the uniformity of the crystal grains of the slab. The microstructure was observed in a vertical section of the slab, and the average area of the crystal grains at that time was determined. The results are shown in Table 1. As is clear from Table 1, the crystal grains of the slab forged at a temperature lower than the transformation point are finer than those of the slab not subjected to the transformation. In addition, the slab which has not been processed below the transformation point has a large difference in the area of the crystal grains between the surface and the center, but the slab forged at a temperature below the transformation point has a uniform crystal grain size. . In Comparative Examples Nos. 4, 8, and 11, the forging end temperature was 900 ° C., so that an undesired needle-like structure remained in the hot rolling in the subsequent step. According to the slab manufacturing method of the present invention, a slab in which crystal grains are finely and uniformly distributed can be obtained.
By subjecting it to subsequent hot rolling, a flawless titanium hot rolled sheet is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view for explaining dead metal. FIG. 2 is a view showing a heat pattern of a titanium ingot in the production method of the present invention. FIG. 3 is a diagram for explaining a feed amount. FIG. 4 is a diagram showing a relationship between a processing rate and a crystal grain area. FIG. 5 is a diagram showing a relationship between a feed amount of an ingot and an average area of crystal grains. [Description of Signs] 1 Titanium ingot 2 Forged material 3 Upper mold 1S Titanium ingot surface

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-8-2322061 (JP, A) JP-A-8-81747 (JP, A) JP-A-59-104233 (JP, A) JP-A-53-1983 1617 (JP, A) JP-A-2-213453 (JP, A) JP-A-64-28347 (JP, A) JP-A-62-286639 (JP, A) JP-A-1-156456 (JP, A) JP-B4-46643 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) B21J 1/04 B21J 5/00

Claims (1)

(1) A method for producing a slab by forging a titanium ingot by a press, wherein the surface of the mold and the surface of the ingot in the ingot transfer direction are formed at a temperature not higher than a transformation point. Forging with a contact length of 300 mm or less, and then heating above the transformation point, starting forging at a temperature above the transformation point, and terminating forging at a temperature below the transformation point. Production method.