JPH07251202A - Manufacture of hot rolled plate of pure titanium - Google Patents

Abstract

PURPOSE:To suppress evil influence due to surface flaw and seam flaw and to manufacture a high-quality titanium plate at the high yield of material and at low cost by directly hot rolling an ingot. CONSTITUTION:After a rectangular ingot of industrial pure titanium having the width/thickness of >=3.5 is heated to 900-1000 deg.C and reduction of 10 <= draft < 40% is applied at a surface temp. of >=880 deg.C at the time of starting rolling without executing forging or blooming rolling, successively, rolling such as the total draft is >=70% in the temp. region that the surface temp. is <880 deg.C and the surface temp. just after the completion of the final rolling is not below 650 deg.C is executed.

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 high-quality pure titanium hot-rolled sheet material by directly hot-rolling an ingot without performing forging and slabbing.

[0002]

2. Description of the Related Art Presently, in the case of producing an industrial pure titanium hot rolled sheet material (hereinafter referred to as "hot rolled sheet"),
The most common means is to forge a cylindrical ingot (ingot) manufactured by consumable electrode vacuum arc melting and forge-roll it into a rectangular slab, which is then rolled by a continuous hot rolling mill. Has been adopted. Then, in the hot rolling, the heating temperature of the slab before rolling is set to 700 to 950 ° C. in order to secure the soundness such as surface properties and mechanical properties of the obtained hot rolled sheet, and the heat at the end of finish rolling is set. The temperature of the rolled sheet (hereinafter referred to as "finishing temperature") is 650 to 800 ° C.
It is good to say

On the other hand, recently, a rectangular ingot (ingot) of titanium is directly hot-rolled as it is, thereby omitting the forging and slab-rolling steps and simplifying the hot-rolled sheet manufacturing process. Techniques for reducing costs have been studied (see, for example, JP-A-61-143528 and "Iron and Steel, vol.74 No. 6", pages 81 to 87).

However, there is a problem that the hot-rolled sheet obtained by the hot-rolling of titanium is apt to have flaws, whether it is a normal hot-rolling through the forging or slabbing process or the above direct hot-rolling. However, strict control of rolling conditions was necessary to ensure product quality.

However, the above-mentioned Japanese Patent Laid-Open No. 61-143528.
Although the technology related to Japanese Patent Publication No. 6-52, "Iron and Steel, vol.74 No. 6" also attempts to suppress the occurrence of the above defects by using a rectangular ingot, apart from this, the direct hot work From the viewpoint of more stably preventing surface defects by rolling, the titanium ingot is heated to 930 to 1000 ° C. for 30 minutes or more.
After soaking for 2 hours, rolling at least 1 pass with a rolling reduction of 10% or more is performed for 2 or more passes in a temperature range of 883 ° C or higher, and a total rolling reduction of 40% or more is secured, and the temperature is lowered to 883 ° C or lower. A method of finishing hot rolling with a rolling reduction of 20% or more has also been proposed (see Japanese Patent Laid-Open No. 61-159562). However, although these methods are effective in suppressing the surface flaws that occur in the hot rolled sheet, they cannot stably prevent even "seam flaws" that occur at the edges of the hot rolled sheet. After all, it was not completely satisfactory from the viewpoint of yield improvement.

Here, the defects that occur in the titanium hot-rolled sheet will be described. Defects generated on the hot-rolled titanium sheet include "surface defects" that occur on the entire rolled surface of the rolled plate and "seam defects" that occur on the edge of the rolled plate. "Surface flaws" is a general term for surface defects excluding seam flaws among the flaws that occur on the entire rolling surface, but it is the most problematic when rolling an ingot (ingot) among "surface flaws". No. is the "wrinkle defect" shown in FIG.

The mechanism of occurrence of this "wrinkle flaw" is considered as follows. That is, the rolled material is an ingot
Therefore, the crystal grains are coarse, but if the temperature before or during rolling is in the α phase (hexagonal) stable temperature range below the transformation point, the α phase has few slip systems, so during rolling Deformation of each grain differs in its grain direction (see Fig. 2). Therefore, when rolling is performed, the grain size may be large, and the grain part with a different slip direction may be "wrinkled."
Will remain (see Figure 3). This is a "wrinkle defect". Therefore, as a measure against "wrinkle flaws", it is effective to perform rolling in a β stable temperature range in which many slip systems are likely to be deformed.

On the other hand, "seam flaws" are continuous flaws parallel to the rolling direction which occur on the edge surface as shown in FIG. 4, and the mechanism of occurrence thereof is "wrinkle flaws". It is considered to be completely different from the case of No. 1 and no effective countermeasure has been found yet.

The "surface flaws" generated on the hot rolled sheet are removed by grinding the surface, and the "seam flaws" are removed by cutting the edge portion outside the seam flaws. Quality is maintained. Therefore, the occurrence of these flaws greatly affects the material yield of the hot rolled sheet. In particular, when a pure titanium ingot (ingot) is directly hot-rolled, "seam flaws" are likely to enter from the edge portion to the inside, and the number of cut and removed portions of the hot-rolled sheet is particularly large. Therefore, the decrease in yield due to the removal of the seam flaw is a big problem, and the "seam" which leads to an increase in manufacturing cost.
It had to be said that the prevention of "scratch defects" is extremely important. In other words, in the direct hot rolling of pure titanium ingots, even if "scratch defects" could be suppressed, it would be possible to get inside from the edge part. There is no effective way to prevent seam flaws, and only "surface flaws" are used as the quality evaluation value of the product. The situation was becoming less satisfactory in terms of cost.

In view of the above, the object of the present invention is not only to suppress "surface flaws" but also to show almost no "seam flaws", or at least the seam flaw generation portion. It is to establish a means for stably producing a pure titanium hot-rolled sheet with a very small cutting margin.

[0011]

Means for Solving the Problems As a result of intensive studies to achieve the above-mentioned object, the inventors of the present invention have found that "when a pure titanium ingot is directly hot-rolled without forging or slabbing, In addition to using an ingot, the "width / thickness" ratio is regulated to 3.5 or less in order to suppress the "width expansion" after rolling. For rolling, a pass schedule with a small amount of reduction especially in the β stable temperature range. Is adopted,
It has been possible to obtain the knowledge that "a titanium hot-rolled sheet can be manufactured with a small amount of" surface flaws "and a very small cutting margin in the portion where" seam flaws "occur."

That is, the inventors of the present invention firstly examined in detail the mechanism of seam flaw generation. When a titanium rectangular ingot was hot-rolled, "wrinkles" were generated on the side surface of the edge portion by the rolling ( It was confirmed that the "wrinkles" subsequently rolled up on the surface as the rolling progressed to form "seam flaws" (see FIG. 6). The occurrence of "wrinkles" and the curling up are due to "the deformation of the side surface of the edge is in an open state" due to the rolling process. Therefore, the occurrence of "seam flaws" is completely eliminated. It is practically difficult to prevent. The occurrence of "wrinkles" on the side surface of the edge of the material to be rolled occurs from the initial stage of rolling, and "width expansion of the material to be rolled (roller material width expansion in a direction perpendicular to the rolling direction during rolling)" due to reduction. Since it moves to the position where it enters from the edge part to the inside, the "seam flaw" that is left behind is more likely to occur at the position where it enters further from the edge part as the "width spread" of the rolled material increases. Become. Therefore, it was also confirmed that it is important to suppress the "width expansion" of the material to be rolled as a measure against "seam flaws".

Therefore, this time, a basic study was conducted on the mechanism of "width expansion" of the material to be rolled, and the following was confirmed. Most of the deformation of the material in the rolling process is the deformation in the rolling direction, but some of it is also deformed in the direction perpendicular to the rolling direction to widen the plate width. This "width expansion" increases when the "width / thickness" ratio of the titanium ingot is small (form factor), and if the material being rolled is the β phase, it has a large slip system and is easily deformed. The "width spread" becomes large (temperature factor).

Therefore, making the shape of the ingot to be rolled into a rectangle having a large "width / thickness" ratio suppresses "width expansion", and the portion where "seam flaws" occur is located inside the edge portion as much as possible. This is a point to prevent it from entering, and by this, the cutting margin of the portion where the "seam flaw" occurs can be reduced. However, in the direct hot rolling of titanium ingot, it is necessary to carry out reduction at a high temperature (β-phase stable temperature range) at the initial stage of rolling in order to prevent “surface flaws”. Since there is a tendency that the “ingot” tends to become large, it is not a sufficient countermeasure against seam flaws if the shape of the ingot is a rectangle having a large “width / thickness” ratio. However, if the amount of reduction at high temperature (β-phase stable temperature range) at the initial stage of rolling is suppressed below a specific value together with ingenuity of the ingot shape, the material "widens".
Becomes noticeably smaller, and the portion where "seam flaws" occur is prevented from entering as much as possible from the edge to the inside, and the cutting margin is extremely reduced.

The present invention has been made on the basis of the above-mentioned findings and the like. For example, an industrial pure titanium rectangular ingot of "" width / thickness ≥3.5 "is heated to a temperature of 900 to 1000 ° C and forged or mined. At the beginning of rolling without rolling, a reduction of 10% or more and less than 40% was applied at a surface temperature of 880 ° C. or higher, and subsequently, a surface temperature of 650 ° C. or less and a surface temperature immediately after the final rolling was 650 ° C. By rolling at a total reduction of 70% or more in a temperature range not lower than ℃, it is possible to stabilize a pure titanium hot-rolled sheet with very few "surface defects" and "cutting margins of seam defects". It has a major feature in that it can be manufactured by manufacturing.

As described above, according to the present invention, the titanium ingot used for hot rolling is a rectangular ingot having a large "width / thickness" ratio, and the amount of reduction in the β-phase stable temperature range is regulated so that the titanium alloy ingot becomes -The cutting allowance of the hot-rolled sheet due to "defects" is also reduced, the material yield is increased, and the production cost of the titanium hot-rolled sheet is further improved. By the way, in the technique described in the above-mentioned JP-A-61-159562,
Even if the control of "surface flaws" is realized, the cutting margin due to the occurrence of "seam flaws" on the inside of the plate increases and the yield decreases. It is thought that this is because the "width expansion" due to the temperature factor becomes large because the rolling rate is required.

Next, the reason why the manufacturing conditions of the pure titanium hot-rolled sheet are specified as described above in the present invention will be explained more specifically together with the action thereof.

[Action]

A) Conditions relating to ingot In the present invention, a rectangular ingot of industrial pure titanium is used as a material to be subjected to hot rolling, and as a method of manufacturing this rectangular ingot, "a consumable electrode type vacuum arc melting using a rectangular mold is used. Method "or" cold heart melting method using electron beam, plasma arc, plasma beam, etc. as a heat source ". However, the electron beam melting method is recommended in the sense that a rectangular ingot can be most easily manufactured and inclusions inside the ingot can be reduced.

Regarding the rectangular ingot to be used, the limitation of "width / thickness ≥3.5" is set. When the dimension is "width / thickness <3.5", the "width expansion" becomes large. This is because the "seam flaw" generation position largely enters from the edge portion of the hot-rolled sheet to the inner side, so that the material yield is remarkably reduced. When the structure is made fine by forging and slabbing, it can be rolled in the α-phase stable temperature range from the beginning,
Even if the "width / thickness" ratio is small, the "width expansion" can be suppressed by the temperature factor, and thus this condition for suppressing the "width expansion" is not necessary. That is, the above "width / thickness ≥ 3.5"
Regulation is a necessary condition under the assumption that it is a direct rolling of ingot which requires some reduction in the β-phase stable temperature range in order to suppress the occurrence of “surface defects”. . By the way, in the method of the present invention, an elliptic cylinder ingot can be used, but in that case, the condition of "major axis / minor axis ≧ 3.5" is satisfied.

As mentioned above, in the present invention, the rectangular ingot is subjected to hot rolling without forging and slabbing. Because if forging and slabbing are carried out in the sense of forming to pass through a hot rolling mill, the rectangular ingot already has a shape that allows it to pass through a hot rolling mill. This is because the steps are not always necessary. Further, even if the forging and the slabbing are performed in the sense that the casting structure is destroyed, the present invention aims to obtain a high quality hot-rolled sheet while reducing the cost by omitting these steps. Therefore, these steps are not necessary.

B) Heating conditions The heating temperature is for hot rolling. To reduce "surface defects", it is desirable to heat the ingot surface layer to a recrystallized microstructure at the initial stage of rolling, so that the temperature should be as high as possible. good. In this case, if the heating temperature is less than 900 ° C., the recrystallization refinement during rolling becomes insufficient and the number and depth of “surface defects” extremely increase, which causes “seam defects”.
The effect of the present invention, which suppresses the occurrence of the inside of the plate away from the edge portion and improves the yield, is offset. On the other hand, when the heating temperature exceeds 1000 ° C, oxidation progresses and "surface defects" due to the inclusion of the oxide scale.
Occurs. Therefore, the heating temperature during hot rolling is limited to 900 to 1000 ° C. The heating at the time of hot rolling may be performed using any of a gas furnace, a heavy oil furnace, and an electric furnace, and the heating atmosphere may be in the atmosphere or an inert gas. However, since it is a technique applicable to rolling using a continuous rolling mill, the number of times of heating should be once.

C) Rolling Conditions [Conditions for Primary Rolling (Reduction at the Start of Rolling)] Rolling is performed at a high temperature and with a sufficient reduction ratio in order to suppress the occurrence of "surface defects" on the hot-rolled titanium sheet. Is good. But,
It is necessary to avoid "width expansion" of the rolled material and prevent "seam flaws" from occurring inside the plate away from the edge. And in order to meet these aims,
10% or more and 40% at surface temperature of 880 ° C or more at the start of rolling
It is necessary to apply a reduction of less than. That is, if the reduction at the start of rolling (primary rolling) is performed at a surface temperature of less than 880 ° C, or if the reduction rate is less than 10%,
Recrystallization of the surface layer of the ingot becomes insufficient, and the number and depth of "surface defects" increase. On the other hand, at this time, the rolling reduction is 40
When a rolling reduction of more than 0.1% is applied, the "width expansion" in the rolling T direction (direction perpendicular to the rolling direction) becomes large, and "seam flaws" appear at the position that enters from the edge of the hot-rolled sheet to the inside. If they occur, the material yield will be significantly reduced.

[Conditions for Secondary Rolling] As described above, it is necessary to suppress "width expansion" in order to suppress the position where "seam flaws" occur outside the edge of the hot-rolled sheet. . In order to do so, reduction in the β-phase stable temperature range should be minimized, and the amount of reduction necessary to secure the desired characteristics should be ensured in the α-phase stable temperature range, and rolling with minimal deformation should be performed. I won't. Therefore, after applying a reduction of 10% or more and less than 40% at a surface temperature of 880 ° C. or higher at the start of rolling, the total reduction ratio of 70% in a temperature range of the surface temperature of less than 880 ° C.
It is necessary to carry out rolling (secondary rolling) to secure the above percentage. That is, the reduction of this secondary rolling is 8 at the surface temperature.
When it is carried out in a temperature range of 80 ° C. or higher, the material is in the β phase in the temperature range, so that it is easily deformed and the “width spread” becomes large, so that the intended purpose cannot be achieved. Further, if the total reduction ratio after reduction in the temperature range of the surface temperature less than 880 ° C. is less than 70%, the cast structure remains and the mechanical properties and cold rollability of the product deteriorate.

The secondary rolling must be carried out in a temperature range where the surface temperature immediately after the final rolling is not lower than 650 ° C. This is because if the material surface temperature is below 650 ° C. immediately after the final rolling, the deformability is insufficient in the final rolling of the hot rolling, and “surface defects” frequently occur. Here, the hot rolling using a rectangular ingot as a rolling material directly,
It goes without saying that it does not matter whether it is "unidirectional rolling" or "bidirectional rolling".

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

Example A pure titanium rectangular ingot of 120 mm x 1150 mm x 4250 mm (JIS class 2 equivalent material) was manufactured by using sponge titanium as a main raw material and by melting an electron beam. The analysis results of the main components were as follows. Fe: 0.07wt%, O: 0.13wt%, C: 0.01wt%,
H: 0.0009 wt%, N: 0.002 wt%, Ti and impurities: balance.

Then, various size rolling test pieces were cut out from the ingot, heated to a predetermined temperature in an electric furnace in the atmosphere and held for 2 hours, and then hot rolled (roll diameter: 480 mmφ, roll diameter). -Rule speed: 17.5 rpm, rolling method: unidirectional rolling, reduction ratio per pass (excluding some parts): 12.5 to 17.5
%) Was applied to obtain a plate material. Table 1 shows the rolling conditions at this time. Note that, here, “rolling corresponding to reduction at a surface temperature of 880 ° C. or higher” is called “primary rolling”, and rolling corresponding to reduction at a surface temperature of less than 880 ° C. performed thereafter is called “secondary rolling”. . In addition, temperature measurement and temperature control were performed with a radiation thermometer.

[0026]

[Table 1]

Then, the sheet width of the hot-rolled titanium sheet thus obtained was measured, and the "surface flaw" and "seam flaw" were examined, and the mechanical properties were measured by a tensile test. Also carried out. Here, the depths of "surface flaws" and "seam flaws" were measured by cutting out the portion which seems to be the deepest by visual observation and observing the cross section with an optical microscope. The results of these investigations are shown in Table 2. In addition, the "width expansion ratio" mentioned in Table 2 means "the width of the test piece before rolling".
To the "width of the test piece after rolling".

[0028]

[Table 2]

In Tables 1 and 2 above, test numbers 1 to 1
4 is an example of the present invention (example of the present invention), in which the ratio of "width / thickness" was changed within the range defined by the present invention. Test Nos. 5 to 8 are examples in which the heating temperature is changed, and the temperature before the start of the primary rolling is also changed accordingly. Of these, test numbers 6 and 7 are examples of the present invention, test number 5 is an example in which the heating temperature is deviated downward, and test number 8 is an example in which the heating temperature is deviated upward. Test Nos. 9 to 12 are examples in which the rolling reduction of the primary rolling was changed. Among them, Test Nos. 10 and 11 are examples of the present invention, and Test No. 9 shows that the rolling reduction of the primary rolling deviates downward. Test number 12 is an example in which the rolling reduction of the primary rolling is deviated upward. Test No. 13 is an example in which reheating is performed before the secondary rolling and the temperature before the start of the secondary rolling is out of the range defined by the present invention, and Test No. 14 is the end of the secondary rolling. This is an example in which the immediate temperature is below the range specified by the present invention. Test Nos. 15 to 17 are examples in which the total rolling reduction is changed. Among them, Test Nos. 16 and 17 are examples of the present invention, and Test No. 15 is an example in which the total rolling reduction deviates downward. And, test Nos. 18 to 21 are examples (conventional examples) rolled within the condition range adopted for the conventional forged and slab-rolled slabs, and test Nos. 22 to 25 are described in the above-mentioned JP-A-61-161. This is an example (conventional example) of rolling within the condition range in the invention described in Japanese Patent Publication No. 159562.

As is clear from the results shown in Table 2, test numbers 1 to 4, 6, 7, 10 to 11 and 16 to
In the present invention example according to 17, the width expansion ratio in all of them
It is less than 1.19 and the "width expansion" is small. Therefore, the "seam flaw" is limited to the edge portion, and the cutting margin is small, so that the yield is improved. Further, it can be seen that the "surface flaw" is also shallow, and a preferable hot rolled sheet can be obtained.

On the other hand, in the comparative examples of Test Nos. 5, 8-9, 14 and 18-21, "surface flaws" frequently occur in all of the obtained hot-rolled sheets and have a cast structure. It turns out that this is a disadvantageous condition for the direct rolling of the ingot. Further, in the comparative examples of test numbers 12 to 13 and 22 to 25, "width expansion" was large in all of them (width expansion ratio: 1.
38 ~ 1.59), "Seam flaws" are generated on the inner side of the plate, so a lot of cutting margin was required to remove them.

Further, a plate-shaped tensile test piece having a parallel portion having a width of 6 mm, a thickness of 2 mm and a gauge length of 25 mm was cut out from a part of the hot rolled sheet along the direction perpendicular to the rolling direction, and vacuum cut at 725 ° C. for 1 hour. In a room temperature tensile test carried out after annealing, it was confirmed that the test piece of the comparative example of test number 15 showed a larger decrease in elongation than the test piece of the present invention. Therefore, it was found that the hot-rolled sheet obtained in Test No. 15 was inferior in performance as a hot-rolled sheet and was difficult to cold-roll.

[0033]

[Summary of Effects] As described above, according to the present invention, a "surface defect" or "surface flaw" which is a problem when directly hot rolling a pure titanium ingot as a raw material without performing forging or slabbing. It suppresses seam flaws, drastically reduces the equipment cost and the number of working steps, and also can reduce the amount of surface grinding and the cutting margin of the edge part and stabilizes the high quality pure titanium hot-rolled sheet with good material yield. Industrially useful effects such as the ability to manufacture are brought about.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of “wrinkle flaws (surface flaws)” generated in a pure titanium hot-rolled sheet.

FIG. 2 is an explanatory diagram of a mechanism of occurrence of “wrinkle defects”.

FIG. 3 is an explanatory diagram of a mechanism of occurrence of “wrinkle defects”.

FIG. 4 is an explanatory view of “seam flaws” generated in a pure titanium hot-rolled sheet.

FIG. 5 is an explanatory diagram of a mechanism of occurrence of “seam flaw”.

FIG. 6 is an explanatory diagram of a mechanism of occurrence of “seam flaw”.

Claims (1)

[Claims] 1. An industrial pure titanium rectangular ingot of "width / thickness ≥3.5" is heated to a temperature of 900 to 1000 ° C. and rolled at a surface temperature of 880 ° C. or higher at the start of rolling without forging or slabbing. After applying a reduction of 10% or more and less than 40%, the total reduction rate is 70% or more in a temperature range where the surface temperature is less than 880 ° C and the surface temperature immediately after the final rolling does not fall below 650 ° C. The method for producing a pure titanium hot-rolled sheet material, comprising: