JP3171053B2 - Hot rolling method of titanium slab - 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 for hot rolling a titanium material such as pure titanium or a titanium alloy, and more particularly, to a rolled titanium material having no surface flaws and excellent surface properties. The present invention relates to a method for hot rolling a titanium slab.

[0002] The rolled titanium material obtained by the method of the present invention is:
As it is hot-rolled or subjected to further processing such as cold-rolling, materials for the chemical industry such as plates, bars and tubes, general industrial materials such as condensers for nuclear power plants, and side plates for watches, It can be used for a wide range of uses such as consumer materials such as golf club members.

[0003]

2. Description of the Related Art Industrially used titanium includes industrial pure titanium, α-type, α + β-type, and β-type titanium alloys (hereinafter, these pure titanium and titanium alloy are collectively referred to as titanium). I will call it). The general production method of these titanium is that, first, in the case of pure titanium, a sponge-like metal is solidified as a consumable electrode, and in the case of a titanium alloy, a sponge-like metal in which an alloy material is wrapped is used. Titanium is solidified to form a consumable electrode and melted in an arc under vacuum to produce an ingot. Next, a material for hot rolling (hereinafter, referred to as a titanium slab) is manufactured by slab-rolling or forging the ingot. The surface of this titanium slab is machined and subjected to baking, surface treatment such as removal of surface flaws generated in the rolling step, and then hot rolling, thereby obtaining a hot-rolled material such as a plate-shaped or rod-shaped material. ing.

[0004] During hot rolling of titanium slabs, the surface of the rolled material tends to have flaws. Examples of the surface flaw include a flaw caused by scale generated during heating before rolling, a flaw generated due to low deformability, and a flaw generated by a mechanical impact (such as contact between rolled materials) during hot rolling. There is. Since these flaws need to be removed from the rolled material,
Problems such as a decrease in yield and an increase in manufacturing cost occur. Therefore, measures have been taken to minimize surface flaws generated during rolling.

[0005] Among the above-mentioned flaws, it is the most difficult problem to prevent flaws caused by scale generated during heating before rolling, and several measures have been taken. For example, Japanese Patent Publication No. 54-29458 discloses a method in which the heating temperature before rolling is set in a predetermined range, specifically, 700 ° C. to 950 ° C.
A method is disclosed in which a titanium slab is heated at a temperature in the range of ° C. and then rolled. The reason why the temperature is set to 700 ° C. or higher is to secure the deformability of the titanium slab during rolling.
The reason why the temperature is set to 50 ° C. or lower is that, in addition to suppressing the titanium material from absorbing hydrogen during heating, scale is prevented from being formed on the titanium slab surface during heating, and the surface by the scale during rolling is suppressed. This is to prevent generation of flaws.

In addition, as a method for suppressing the generation of scale during heating, there is known a method in which an antioxidant made of aluminum powder or the like is applied to the surface of a titanium slab and then heated (for example, Nippon Steel Technical Report, No. 352 (199
4), p. 50). This method is considered to be an effective means particularly when the purpose is to heat to a high temperature in the β-phase region and roll under low deformation resistance conditions.

In the case of a metal material other than titanium, a method of heating in a non-oxidizing atmosphere or an oxidizing atmosphere having a low oxygen potential is generally employed as a measure for preventing oxidation during heating. However, in the case of titanium, when heated in a non-oxidizing atmosphere, titanium causes hydrogen absorption. Hydrogen absorption is a fatal material defect for metallic materials, and titanium is no exception. Therefore, it is not possible to employ a method of heating the titanium material in a non-oxidizing atmosphere to prevent oxidation during heating of the titanium slab, that is, to take measures to suppress the generation of scale.

For this reason, as described above, in order to prevent the titanium slab from being oxidized during heating, it is necessary to employ means such as heating in a temperature range where oxidation is relatively unlikely to occur or application of an antioxidant.

[0009]

The crystal phase of titanium is an α phase having a dense hexagonal (HCP) structure near room temperature, and a β phase having a body-centered cubic (BCC) structure at high temperatures. During the heating process, the low-temperature α phase disappears and transforms into the β phase. This temperature is referred to as β transus (transus, Tβ)
It is called. This β transus is a temperature around 900 ° C. in the case of soft industrial titanium. The method of rolling a titanium slab by heating it to a temperature equal to or higher than β transus has the advantage that rolling is easy because the deformation resistance is small.However, since the titanium slab surface is severely oxidized, the surface of the material after rolling is markedly damaged. Remains. In the method of heating at a temperature lower than that of β transus, that is, in the α phase region, the oxidation of the titanium slab surface is relatively small, and the thickness of the scale layer is small. However, the adhesion between the scale layer and the metal layer is high,
It is difficult to completely remove the scale in the process from the removal of the titanium slab from the heating furnace to the rolling. Therefore, when this titanium slab is rolled, there is a problem that scale seizure occurs on the rolling roll, and the seizure leaves a flaw on the surface of the titanium material after rolling. Therefore, according to the method disclosed in Japanese Patent Publication No. 54-29458,
The generation of surface flaws during hot rolling of titanium slabs cannot be sufficiently prevented.

In addition, the method of applying an antioxidant to a titanium slab before heating not only has a problem such as a decrease in productivity and an increase in running cost, but also completely removes the antioxidant from the titanium slab during rolling. Since it is difficult, there is a problem that the antioxidant remaining on the surface of the slab bites into the rolls and indentation flaws occur on the surface of the rolled material.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and can produce a rolled material having no surface flaws and excellent in surface properties by a simple process with good productivity. It is an object of the present invention to provide a method for hot rolling a titanium slab.

[0012]

Means for Solving the Problems The present inventors have obtained the following findings in the course of continuing research on a method for solving the above-mentioned problems. That is, if the titanium slab is heated in a temperature range equal to or higher than β transus for a time sufficient to equalize the temperature of the titanium slab, the slab surface is oxidized at a very high rate. At the same time that the scale layer is formed on the entire surface, the grain boundaries of β crystal grains at the interface between the scale layer and the titanium metal layer are selectively and strongly oxidized. For this reason, a deep recess is formed on the surface of the titanium metal layer, and the unevenness of the surface of the titanium slab caused by the recess causes a surface flaw of the rolled material.

If the heating in the temperature range above β transus is short, a thin scale layer is formed on the surface of the titanium slab, but selective oxidation of the grain boundaries of β crystal grains does not occur.

A scale layer of a titanium slab formed by heating in a temperature range of β transus or higher has a property of easily peeling. That is, the scale can be easily removed in the descaling step from the heating furnace to the rolling.

The titanium slab can be sufficiently rolled by heating and soaking in a temperature range of 800 ° C. or more and less than β transus. However, the scale layer formed on the titanium slab surface in this temperature range has poor peelability. On the other hand, the metal layer below the scale layer has a smooth surface and good properties.

The present inventors have recognized that the following solution is presumed to be effective based on the above findings.

"Titanium slab is heated and soaked at a temperature of 800 ° C. or more and less than β transus, and then heated to a temperature of β transus or more for a short time to roll the slab. This method is the object of the present invention. It is possible to produce a rolled titanium material having good surface properties without surface flaws with good yield. "The present invention was completed as a result of research on conditions that can realize the above basic idea. The gist is that the slab heating conditions and steps include the following.

A first heating step of heating the titanium slab at a temperature of 800 ° C. or more and less than the β transus temperature (Tβ), soaking the titanium slab in a hot-rollable state; A second heating step of heating to a temperature (T ₂ ) satisfying the equation (1) for a time (t) specified by the following equation (2):, a titanium slab heated by the first and second heating steps of T ₂ (° C.) ≧ Tβ (° C.) + 10 Equation (1) 5 ≦ t (minute) ≦ 35− (ΔT / 2) Equation (2) where ΔT (° C.) = T ₂ −Tβ

[0019]

According to the method of the present invention, in the first heating step, the titanium slab is soaked at a temperature lower than the β transus temperature (Tβ) for a sufficient heating time to give the titanium slab the deformability required for rolling. . Tβ is, for example, 890 ° C. for industrial pure titanium and 1000 ° C. for Ti-6Al-4V alloy. The lower limit of the heating temperature may be a hot rollable temperature, but the deformation resistance during rolling of the titanium slab, from the viewpoint of flaw generation of slabs, and 800 ° C.. In addition, Ti-6Al
In the case of a -4V alloy, the temperature is desirably about 900 ° C.
The heating is usually performed in a heating furnace with a burner combustion flame as a heat source and a weakly oxidizing atmosphere or an atmosphere in the atmosphere by electric resistance heating. This heating forms a thin scale layer that is difficult to peel off on the slab surface. The thickness of the scale layer formed at this stage varies depending on the heating conditions, but is at most about 50 μm. The time required for soaking the titanium slab varies depending on conditions such as the size and shape of the slab, the heating method, and the material of titanium.
It is about an hour.

It is practical to carry out the second heating step following the first heating step in the same heating furnace by a method of raising the temperature in the furnace. In the second heating step, the first heating step
It is sufficient to heat only the surface layer portion in order to convert the scale layer on the slab surface generated in the heating step into a scale layer having good releasability. Therefore, in this step, as described above, Tβ +
At a temperature (T ₂ ) of 10 ° C. or more, 5 minutes to Δ35− (ΔT /
2) Heat for｝ minutes.

The reason why the lower limit of the heating temperature is Tβ + 10 ° C. and the lower limit of the heating time is 5 minutes is that the scale layer on the slab surface generated in the first heating step is formed at a temperature lower than Tβ + 10 ° C. and a time shorter than 5 minutes. This is because it is difficult to change to a scale layer having good releasability. In addition, the upper limit of the heating time is 35- (ΔT
The reason why the heating time is not more than / 2) minutes is that the grain boundary of β crystal grains on the surface of the titanium slab is selectively oxidized to form a concave portion when the heating time is longer than that, and a surface flaw is easily generated in the rolled material. . The reason why the upper limit of the heating time is shortened as ΔT (= T ₂ −Tβ) is larger is that the oxidation speed of the titanium slab surface is higher as the temperature is higher. is there. The upper limit of the heating temperature in the second heating step is a case where the heating time is 5 minutes in the equation (2), and the temperature is ΔT of 60 ° C., that is, T ₂ is Tβ + 60 ° C.
Above this temperature, oxidation of the slab surface becomes excessive, which is not preferable.

The scale of the slab surface formed by heating in the second heating step, that is, the temperature of the β phase region of titanium, is easily peeled off by a small mechanical / thermal impact, and easily falls off the slab surface. There is nature. Therefore, in the process of removing the slab from the heating furnace and transferring the slab to the rolling step, the slab often peels off naturally due to vibration during transportation, cooling of the slab surface, and the like. Further, when thorough descaling is performed, it is preferable to perform a treatment such as injection of high-pressure water.

The titanium slab heated in the first and second heating steps is subsequently rolled to obtain a rolled titanium material. As the rolling method, in the case of titanium, a roll rolling method using a flat type or a hole type roll is frequently used. The temperature of the slab at the time of rolling is first and second at the start of rolling.
Approximately the temperature heated in the step, but gradually decreases with the progress of rolling. Since the deformation resistance of the titanium slab increases as the temperature decreases, the temperature at the end of rolling is, for example, 550 ° C. in the case of industrial pure Ti and Ti-6Al-
In the case of 4V, it is desirable to be about 700 ° C.

[0024]

Example 1 The titanium material targeted in this example is pure titanium for industrial use, and its chemical composition and Tβ are shown in Table 1 as Test Material 1. The slab was prepared by vacuum arc melting titanium sponge as a consumable electrode to produce an ingot having a diameter of 1000 mm and a thickness of 150 mm in a temperature range of β phase and α phase.
mm. In the present embodiment, a width of 60 mm and a length of 150
A rolled material having a thickness of 25 mm and a thickness of 25 mm was prepared by machining.

[0025]

[Table 1]

The rolled material was subjected to first and second two-stage heating under the conditions shown in Table 2. The heat pattern is as shown in FIG. As for the heating method, the heating furnace is an electric furnace, and the heating atmosphere is in the air. The heating time was 150 minutes for the first heating and the time shown in Table 2 for the second heating. After the heating was completed, the rolled material was lightly hit to remove the scale formed on the surface.

[0027]

[Table 2]

The rolling method is a rolling method using a flat roll, and the rolled material having a thickness of 25 mm was rolled to 5 mm by performing 4 passes under a condition of 5 mm / pass. In addition, this four-pass rolling was performed by the above-mentioned heating once. The temperature of the rolled material at the start of rolling was a temperature slightly lower than the first heating temperature in the inside and a temperature lower than that in the surface layer portion and about 50 ° C. lower than the first heating temperature. The temperature at the end of rolling was about 600 ° C.

The obtained rolled material was evaluated for surface flaws. First, after a shot blast was applied to the rolled material, it was pickled with nitric hydrofluoric acid to remove the scale layer generated during the rolling and to clean the surface. Next, the flaws were easily observed by a penetrant flaw detection method (die check), and the presence or absence of surface flaws was visually inspected on the front and back surfaces of the rolled material. In Table 2,
When no flaw was detected, the result was indicated by ○, and when a flaw was detected, the result was indicated by ×.

As is clear from Table 2, a rolled material heated and rolled under the conditions of the present invention (Example of the present invention: treatment Nos. 1 to 9)
In the case of No. 10, no surface flaw was detected, but the rolled material out of the conditions of the present invention (Comparative Example: Treatment No. 10)
Regarding Examples 17 to 17), surface flaws occurred.
No. 2 in which the second heating was not performed. 10 and 17, the heating time is shorter than the condition of the present invention defined by the formula (2).
o. In Nos. 11, 13, and 15, surface flaws, which were determined to be bitten by the scale generated during the first heating, were observed in the rolled material. The cause is presumed to be that the second heating does not satisfy the conditions of the present invention, and the scale generated by the first heating during the second heating did not change to a scale that easily peeled off. In addition, in the case of No. 3 in which the heating time is longer than the condition of the present invention obtained from the equation (2). 12, 14 and 1
As for No. 6, it was observed that minute surface flaws were generated on the entire surface of the rolled material. Judging from the form of surface flaws,
It was considered that the crystal grain boundary of the β phase was selectively oxidized during the heating to form a concave portion to form an uneven surface, and the unevenness was rolled to become a flaw.

[0031]

Embodiment 2 The titanium material targeted in this embodiment is Ti
-4V-6Al alloy, and its chemical composition and Tβ are shown in Table 1.
Specimen 2 is shown in FIG.

The heating conditions are as shown in Table 3. Other conditions were the same as in Example 1. The temperature of the rolled material at the start of rolling was slightly lower than the first heating temperature in the inside, and lower than that in the surface layer by about 50 ° C. than the first heating temperature. The temperature at the end of rolling was about 750 ° C. Table 3 shows the inspection results of the surface flaws of the rolled material.

[0033]

[Table 3]

As is clear from Table 3, as in the case of Example 1, the rolled material heated and rolled under the conditions of the present invention (Example of the present invention: treatment Nos. 1 to 9) had no surface flaws. On the other hand, no surface defects were found in any of the rolled materials (comparative examples: treatment Nos. 10 to 17) out of the conditions of the present invention, though not detected. N without second heating
o. In Nos. 10 and 17, the heating time was shorter than the condition of the present invention of the formula (2). In Nos. 11, 13, and 15, surface flaws, which were determined to be bitten by the scale generated during the first heating, were observed in the rolled material. In addition, in the case of No. 3 in which the heating time is longer than the condition of the present invention obtained from the equation (2) With respect to 12, 14, and 16, it was observed that minute surface flaws occurred on the entire surface of the rolled material. The causes of these surface flaws are:
It is considered to be the same as that described in the first embodiment.

As described above, according to the results of Example 1 and Example 2, according to the method of hot rolling a titanium slab according to the conditions of the present invention, the surface of the rolled material has no flaws and excellent surface properties. It was found that a rolled titanium material was obtained. Further, the method of the present invention can utilize the conventional heating and rolling equipment as it is, and it is only necessary to change the heat pattern at the time of heating the titanium slab. Therefore, the method is simple without adding a new step. Furthermore, since there is no need to apply a scale generation inhibitor before heating, the productivity is excellent. In addition, the method of the present invention has other excellent features such as a small amount of scale generation and a high rolling yield due to a short heating time in the β phase temperature range.

[0036]

According to the hot rolling method for a titanium slab of the present invention, a scale having good releasability can be formed when the titanium slab is heated. Absent. In addition, since heating of titanium in the β phase temperature range is limited, there is no generation of surface flaws due to unevenness of the titanium slab surface caused by selective oxidation of β grain boundaries. In addition, conventional equipment can be used as the heating equipment, and the process is simpler than changing the heat pattern at the time of heating compared to the conventional method.
Excellent productivity. In addition, the rolling yield is high because the amount of generated scale is small.

As described above, the hot rolling method for a titanium slab of the present invention has an excellent effect that a rolled titanium material having good surface properties can be economically produced with high productivity.

[Brief description of the drawings]

FIG. 1 is a view showing a heat pattern at the time of heating a titanium slab including a first heating step and a second heating step.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22F 1/00-3/02 B21B 3/00

Claims (1)

(57) [Claims] 1. A first heating step of heating a titanium slab at a temperature of 800 ° C. or more and less than a β transus temperature (Tβ) and soaking in a state where hot rolling is possible, and following the first heating step, A second heating step of heating to a temperature (T ₂ ) satisfying 1) for a time (t) specified by the following equation (2), and the titanium slab heated by the first and second heating steps: A hot rolling method for a titanium slab, comprising a step of rolling. T ₂ (° C.) ≧ Tβ (° C.) + 10 Equation (1) 5 ≦ t (minute) ≦ 35− (ΔT / 2) Equation (2) where ΔT (° C.) = T ₂ −Tβ