JPH08104961A - Production of hot rolled sheet of pure titanium for industry - Google Patents

Abstract

PURPOSE: To produce a hot rolled sheet free from cracks and flaws and having a completely recrystallized uniform structure by subjecting the cast slab of pure titanium for industry to specified hot rolling at the specified temp. range according to component composition and recrystallizing and annealing. CONSTITUTION: The cast slab of pure titanium for industry is rolled at >=20% draft in the temp. range of >=T deg.C shown in the formula of T=886+147.7[O]+161.8[C]+294.3[N]-19.8[Fe] ([O], [C], [N] and [Fe] respectively denote the weight% of O, C, N and Fe as impurities. The titanium material is successively rolled at >=80% draft in the temp. range of 600 deg.C to T-30 deg.C. After that, the rolled stock is subjected to recrystallization annealing preferably at 600 to 750 deg.C. Thus, the hot rolled sheet of pure titanium for industry free from the residual structure of unrecrystallized ones and good in workability can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled sheet of industrial pure titanium (hereinafter referred to as CP-Ti) which is produced by directly hot rolling a cast slab.

[0002]

2. Description of the Related Art Conventionally, a CP-Ti hot-rolled sheet is made of sponge titanium as a raw material, is melted into a cylindrical ingot by a vacuum arc melting (hereinafter referred to as VAR) method, and is slab-shaped by slab forging. It is manufactured by a method of forming, hot rolling and then recrystallization annealing. The CP-Ti hot-rolled sheet thus produced has a structure composed of uniform recrystallized grains and exhibits excellent workability.

In the case of the VAR method, at present, the shape of the ingot has to be cylindrical due to the problem of the melting method, and therefore direct hot rolling is difficult, and the ingot is formed into a slab shape capable of being rolled as described above. A slab forging process is required. On the other hand, electron beam melting (hereinafter,
It is called EBR. ) Method and vacuum plasma melting (hereinafter VPR
Called. ) Method can be used to cast into a slab-shaped ingot, so if the cast slab can be directly hot-rolled to produce a CP-Ti hot-rolled sheet, the slab forging process is unnecessary and the production cost is greatly reduced. There is a possibility.
In addition, when the EBR method or the VPR method is used, it is possible to use a large amount of scrap, and there is a possibility that the manufacturing cost can be further reduced.

However, in general, a cast slab is significantly inferior in workability to a forged slab because of its coarse solidified structure, and cracks and flaws are likely to occur when rolled under normal rolling conditions. Therefore, careful maintenance of the surface of the hot-rolled sheet, which is a product, is required, resulting in a large increase in manufacturing cost and a decrease in yield. Therefore, in order to solve these problems, Japanese Patent Laid-Open No. 1-156456 discloses that the surface layer has a texture obtained by applying working strain to the surface layer of pure titanium or titanium alloy ingot and then heating it to a temperature higher than the recrystallization temperature. A method of hot-rolling after recrystallization, and in JP-A-61-159562, in producing a titanium plate or a titanium slab from an ingot by hot rolling, a temperature range of 930 to 1000 ° C. is used. The soaking is performed for 2 minutes to 2 hours, and at least 1 pass is 10% or more and 2 passes or more in the β-phase temperature range of 883 ° C. or more.
And rolled at 40% or more at the total reduction rate, and then 88
20 in the temperature range of (α + β) phase or α single phase below 3 ℃
A method of hot rolling at a rolling reduction of not less than% has been proposed.

[0005]

However, when the methods described in JP-A-1-156456 and JP-A-61-159562 are additionally tested, the occurrence of cracks and flaws during rolling can be prevented. However, for the hot rolled sheet, the current VA
Even if the same recrystallization annealing as that performed on the hot rolled sheet manufactured using the R melting material is applied, an unrecrystallized structure remains,
The thing having sufficient workability was not obtained.

The present invention has been made in order to solve the above problems, and a cast slab is directly hot-rolled and has no cracks or flaws, and is completely recrystallized by recrystallization annealing after hot rolling. An object of the present invention is to provide a method for producing a CP-Ti hot-rolled sheet having a texture.

[0007]

[Means for Solving the Problems] The above-mentioned problems are obtained by using a casting slab manufactured by the EBR method, the VPR method, or the like by the formula (1).
CP-T for recrystallization annealing after rolling at a rolling reduction rate of 20% or more in a temperature range of T ° C or higher and subsequently rolling at a rolling reduction rate of 80% or more in a temperature range of 600 ° C to T-30 ° C.
It is solved by the manufacturing method of the hot rolled sheet of i.

T = 886 + 147.7 [O] +161.8 [C] +294.3 [N] -19. 8 [Fe] (1) Here, [O], [C], [N] and [Fe] are respectively O, C, N, contained as impurities in the cast slab of pure titanium. It is a value in which the amount of Fe is expressed in% by weight.

Further, recrystallization annealing after hot rolling is performed at 600 ° C.
When it is performed in the temperature range of 750 ° C or lower, a structure more preferable for workability can be obtained.

[0010]

The above temperature T is 100% of the cast structure of the slab.
O, C, which are the main impurities contained in CP-Ti, at the temperature at which the crystal structure has a BCC structure with good workability.
It changes depending on the amounts of N and Fe. As a result of multiple regression analysis of the relationship between T and the contents of O, C, N and Fe using various CP-Ti, the formula (1) was obtained. This temperature T is
As shown below, it plays an important role in microstructure control in hot rolling.

CP-Ti having a composition with a temperature T of 899 ° C.
Using a cast slab that was melted by the EBR method, the hot rolling conditions were examined to obtain a uniform structure that was completely recrystallized by recrystallization annealing (here, 700 ° C) without cracks or flaws. It became clear that rolling needs to be performed in two steps in a temperature range of T ° C or higher and a temperature range of T-30 ° C or lower. The details will be described below.

FIG. 1 shows the relationship between the first stage rolling temperature and the rolling reduction and the presence or absence of cracks generated on the surface of the final product. T
It can be seen that the occurrence of cracks can be prevented by rolling at a rolling reduction of 20% or more in the temperature range of ℃ or more. By rolling in a temperature range of T ° C or higher, which has relatively good workability, it is possible to break the cast structure and reduce the grain size while suppressing the occurrence of cracks and flaws during rolling. It is thought that the occurrence of scratches and flaws can be prevented.

FIG. 2 shows the relationship between the second stage rolling temperature and rolling reduction and the structure after recrystallization annealing. It can be seen that a completely recrystallized structure can be obtained by rolling at a rolling reduction of 80% or higher in a temperature range of T-30 ° C or lower. Further, all the completely recrystallized structures were composed of uniform crystal grains.
It is considered that by applying a reduction of 80% or more after the first rolling, the strain energy necessary for recrystallization during the recrystallization annealing is sufficiently accumulated, so that a completely recrystallized uniform structure can be obtained.

As the rolling temperature is lowered, the deformation resistance of the rolled material increases and it becomes difficult to roll at a rolling reduction of 80% or more. Therefore, the lower limit of the rolling temperature is set to 600 ° C.

Although not specified in the present invention, in the case of production using the current rolling equipment, considering the dimensions of the cast slab and the final product and the second stage rolling reduction of 80% or more,
The reduction rate of the first step cannot be increased as much as possible.
The upper limit is about 0%. There is no problem if the rolling temperature at the first stage is T ° C. or higher, but if it is too high, scale loss due to surface oxidation becomes large, so about 1100 ° C. is the upper limit.

The hot rolled sheet produced under the hot rolling conditions of the present invention described above was annealed in the temperature range of 500 to 1000 ° C., and the recrystallization annealing temperature was examined. FIG. 3 shows the relationship between the annealing temperature and the recrystallized grain size. When annealed in a temperature range of 600 ° C. or higher and 750 ° C. or lower, a structure composed of fine recrystallized grains smaller than 100 μm is obtained, and a hot-rolled sheet excellent in workability having a good strength-ductility balance can be manufactured. If the temperature is lower than 600 ° C, unrecrystallized portions remain, and if the temperature exceeds 750 ° C, coarse grains or a needle-like structure is formed, resulting in low ductility, which cannot be applied to a hot-rolled sheet that requires excellent workability.

[0017]

EXAMPLE Fe, O, C, H and N in the amounts shown in Table 1 were contained as impurities, and the balance substantially consisted of other unavoidable impurities and titanium. The temperatures T were 899 ° C. and 9 °, respectively.
Samples A and B at 18 ° C. were melted by the EBR method, cast into a slab shape, rolled under the hot rolling conditions shown in Table 2, and then 700
A CP-Ti hot rolled sheet was manufactured by performing recrystallization annealing at ℃. Note that the recrystallization annealing temperature was changed for some of the samples rolled under the hot rolling conditions within the scope of the present invention. With respect to the hot-rolled sheet thus manufactured, the presence or absence of cracks generated on the surface was visually inspected, the microstructure was examined by an optical microscope, and the workability was examined by a tensile test. The workability was evaluated by the total elongation of a hot-rolled sheet of the same plate thickness (hereinafter referred to as the current hot-rolled sheet) manufactured using the current VAR melting material. That is, if the same elongation as that of the current hot-rolled sheet can be obtained, ○ (within the invention range), the elongation of the current hot-rolled sheet is 80%.
% (Within the invention range) when the elongation was not less than 80%, and x (outside the invention range) when the elongation was less than 80% of the elongation of the current hot-rolled sheet.

The results are shown in Table 2. In the sample that was rolled under hot rolling conditions within the scope of the present invention and recrystallized and annealed, the occurrence of cracks was not recognized on the surface, the structure was completely recrystallized, and the elongation of the current hot-rolled sheet was Elongation of 80% or more can be obtained. Further, if the annealing temperature is 700 ° C., a uniform fine grain structure is obtained, elongation equivalent to that of the existing hot rolled sheet is obtained, and excellent workability is exhibited.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

Since the present invention is constituted as described above, the cast slab is directly hot-rolled, has no cracks or flaws, and has a uniform structure which is completely recrystallized by recrystallization annealing after hot rolling. A method for producing the obtained CP-Ti hot-rolled sheet can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a rolling temperature and a rolling reduction in the first stage and the presence or absence of cracks generated on the surface of a final product.

FIG. 2 is a diagram showing the relationship between the second stage rolling temperature and rolling reduction and the structure after recrystallization annealing.

FIG. 3 is a diagram showing a relationship between an annealing temperature and a recrystallized grain size.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masahiro Katahira 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. A casting slab is subjected to a temperature T ° C. shown in the formula (1).
Rolling at a rolling reduction of 20% or more in the above temperature range, and subsequently rolling at a rolling reduction of 80% or more in a temperature range of 600 ° C to T-30 ° C, followed by recrystallization annealing. A method for manufacturing a titanium hot-rolled sheet. T = 886 + 147.7 [O] +161.8 [C] +294.3 [N] -19. 8 [Fe] (1) Here, [O], [C], [N] and [Fe] are respectively O, C, N, contained as impurities in the cast slab of pure titanium. It is a value in which the amount of Fe is expressed in% by weight. 2. The method for producing a hot-rolled sheet of industrial pure titanium according to claim 1, wherein the recrystallization annealing is performed in a temperature range of 600 ° C. or higher and 750 ° C. or lower.