JP2018083232A - Titanium slab for surface melting treatment and titanium material for hot rolling using same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a titanium slab for surface melting treatment which is excellent in workability in cold rolling or cold molding after hot rolling.SOLUTION: A titanium slab for surface melting treatment is used in production of titanium material by forming a re-melted/solidified layer having depth of d1, through surface melting treatment, on a surface of a titanium slab in an as-cast state obtained by a DC slab casting method in a vacuum or in an inert gas atmosphere, and performing hot rolling using the surface as a rolling surface, where an increase amount C1 in average oxygen concentration in a first region (a region between the surface and a position at d1/2) is 0.20 mass% or less, and an increase amount C2 in average oxygen concentration in a second region (a region between the position at d1/2 and a position at d1) is 0.05 mass% or less, with respect to average oxygen concentration of base material for the titanium slab, in a thickness direction of the titanium slab, and Cd (= C1 - C2) is more than 0 and 0.15 mass% or less.SELECTED DRAWING: Figure 7

Description

  The present invention relates to a titanium slab for surface melting treatment made of industrial pure titanium and a titanium material for hot rolling in which the surface melting treatment is performed on the slab.

  Generally, pure titanium for industrial use is made of sponge titanium or titanium scrap obtained by the crawl method, and uses high-density thermal energy sources such as vacuum arc melting (VAR), electron beam melting (EBR), or plasma arc melting. It was usual to melt | dissolve by the melt | dissolution method which was made, and to make it a large slab (ingot). Here, the shape of the slab is limited to a cylindrical slab (billet) in the case of vacuum arc melting, while it is cast into a rectangular slab, that is, a slab in the case of electron beam melting or plasma arc melting. Can do.

  Furthermore, in manufacturing a titanium thin plate using such a large slab as a raw material, the surface of the large slab is subjected to surface cutting as necessary, and then subjected to hot rolling or forging, and then In general, it is processed into a hot rolling material having a shape and size suitable for hot rolling. The hot working process by these block rolling or forging is referred to herein as a breakdown process. And in order to further remove the oxide layer and oxygen-enriched layer formed in the surface layer of the material for hot rolling after breakdown, after cutting the surface layer to about several mm or more to nearly 10 mm by cutting, It was usual to use for hot rolling.

  However, in such a conventional general method, a large amount of time and cost is required for a breakdown process by split rolling or forging to process a large slab into a shape and size suitable for hot rolling, This has been a major bottleneck in improving the productivity and cost reduction of titanium sheet manufacturing.

  Recently, as a method for producing a relatively thin slab-like slab, that is, a slab having a shape and size that can be directly subjected to hot rolling, in place of large-sized ingot casting, a DC (direct cast) slab casting technique is used. Is being established. In the DC slab casting method, molten titanium melted in a hearth by electron beam melting or plasma arc melting is continuously injected into a water-cooled copper mold of a predetermined shape held in a vacuum or an inert gas atmosphere, and In this method, a solidified portion in a water-cooled copper mold is continuously drawn from the lower end side of the mold to obtain a slab-shaped slab having a predetermined length.

  If this DC slab casting method is applied, it becomes possible to omit the breakdown step that has been conventionally required, and as a result, it is possible to improve the productivity of titanium thin plate manufacturing and reduce the manufacturing cost. .

  However, the slab obtained by this DC slab casting method (hereinafter, also referred to as “DC slab”) usually has severe surface irregularities and many defects. If such a slab is subjected to hot rolling as it is, the surface properties of the hot rolled plate (hot rolled plate) will deteriorate. For this reason, before hot rolling, it is the actual condition that the surface must be cut by several mm to 20 mm. Therefore, the yield of the material is reduced, and the labor and cost of the cutting work are required, so there is still a strong demand for improvement.

  Furthermore, even when the DC slab is subjected to hot rolling after being subjected to surface cutting, there is a problem that the surface properties of the hot rolled sheet after hot rolling are not necessarily good. That is, there is a problem in that many large and small cover-like wrinkles extending from a few mm to a length of about 10 mm are generated on the surface of the hot rolled sheet. Such surface defects of the hot-rolled sheet are derived from the coarse cast structure of the cast slab. That is, the material for hot rolling that has not undergone the breakdown process, which is hot working, has a cast structure composed of coarse crystal grains as cast (“as cast”). Even if the cutting process is performed, a coarse structure exists in the surface layer after the cutting, and surface flaws are generated in the hot-rolled sheet due to such a coarse surface casting structure.

  By the way, in order to prevent the occurrence of surface flaws occurring on the surface of the hot rolled sheet after hot rolling, the surface layer of the titanium material for hot rolling obtained without undergoing the breakdown process is modified. A method of applying quality treatment has already been proposed.

  For example, in Patent Document 1, high energy is applied to the surface of the titanium slab, particularly the surface that becomes the rolling surface during hot rolling, by high frequency induction heating, arc heating, plasma arc heating, electron beam heating, or laser heating. A method has been proposed in which only the surface layer is melted over a depth of 1 mm or more and immediately cooled and re-solidified.

  According to such a method, the surface layer of the titanium slab is melted so that the surface is smoothed, and defects such as blow holes in the surface layer disappear, as well as heat removal from the base material side. As a result, the molten layer is rapidly cooled and solidified, and at the same time the lower heat-affected layer (HAZ layer) is rapidly cooled, so that the molten layer and the HAZ layer have a fine transformation structure. The surface layer thus refined is recrystallized at the time of subsequent slab heating before hot rolling, and becomes a granular structure (equiaxial grain structure) having a fine and irregular orientation. Therefore, it is possible to prevent the occurrence of dents due to the coarse structure, and it is possible to eliminate the surface flaws of the hot-rolled sheet after hot rolling.

  As described above, a method for modifying the surface layer of the titanium slab by applying high energy to the surface of the titanium slab to melt only the surface layer and immediately rapidly solidifying it is referred to as surface melting treatment in this specification. It is called. A layer obtained by melting the surface layer by the surface melting treatment and further resolidifying the molten layer is referred to as a remelted solidified layer in this specification. Further, the material for hot rolling (slab) after the surface melting treatment is referred to as a hot rolling material in this specification.

JP 2007-332420 A

  By the way, a hot rolled material obtained by subjecting a DC slab to surface melting treatment and then hot rolling is generally subjected to cold rolling to obtain a thickness corresponding to the product application. is there. In some cases, cold forming such as bending, drawing, and overhanging may be performed. Even when a hot-rolling material subjected to surface melting treatment using a DC slab as a starting material is used, cracks may occur on the surface in cold rolling or cold forming after hot rolling. As a result of various experiments and examinations by the present inventors regarding the cause of this problem, it has been found that it is caused by the following phenomenon.

  That is, first, it was confirmed that surface cracks in the cold rolling and cold forming processes as described above are likely to occur when oxygen is significantly concentrated in the hot rolled sheet surface layer. That is, there is usually a large difference in cold workability between a layer having a high oxygen concentration on the surface of the hot rolled sheet and a region having a low oxygen concentration inside the hot rolled sheet. It is considered that cracks are likely to occur in the surface layer having a high oxygen concentration during cold rolling or cold forming.

  Furthermore, when experiments and examinations were conducted on the cause of oxygen concentration in the hot rolled sheet surface layer, it was attributed to the DC slab casting process, and the subsequent surface melting treatment also affected the oxygen concentration distribution. Turned out to be giving.

  That is, the surface layer of the DC slab contains oxygen at a considerably high concentration (for example, an oxygen concentration in which the average oxygen concentration from the surface to a depth of 0.5 mm is about 0.3 to 0.5 mass% higher than the base material). It has been found that there may be a layer (hereinafter referred to as an oxygen-contaminated layer). There are several possible causes for the oxygen contamination layer on the surface of the slab. One of them is air that enters from the outside when the chamber is opened in order to remove the cast slab from the drawing chamber after casting. May be absorbed by the slab surface. Also, for some reason, the degree of vacuum in the chamber during casting, in particular, the degree of vacuum around the hot slab drawn from the mold is not sufficiently high, or residual oxygen-containing gas in an inert gas atmosphere The concentration may be high, and in that case, it is considered that oxygen may be absorbed by the slab surface from the atmosphere around the slab.

  Then, if the slab in which the oxygen-contaminated layer is generated on the surface is subjected to the surface melting treatment without performing surface cutting, the state in which the oxygen concentration is high becomes the depth of the remelted solidified layer by the surface melting treatment. If it is deeply spread in the whole direction and further heated as a raw material for hot rolling, the oxygen concentration in the remelted solidified layer may be increased, but it will not be lowered. A deep oxygen-contaminated layer (a remelted and solidified layer having a high oxygen concentration) remains on the plate. Such a deep oxygen-contaminated layer cannot be sufficiently removed even by pickling before cold rolling, and remains in a region near the surface (remelted solidified layer if it is subjected to cold rolling or cold forming). ) And the internal region, it is considered that cracking is likely to occur due to the difference in cold workability derived from the difference in oxygen concentration.

  Here, in the state where the DC slab is cast, it is normal that the oxygen concentration of the surface layer (oxygen-contaminated layer) has a gradient (oxygen concentration gradient) that increases from the inside toward the surface. However, if the surface layer is melted and re-solidified by the surface melting process, the oxygen concentration in the re-melted and solidified layer (oxygen-contaminated layer) is averaged and the concentration gradient disappears. It is observed that the oxygen concentration becomes almost constant. However, the average oxygen concentration in this state is the oxygen in the non-contaminated area (the area of the original slab that has not been melted and resolidified) inside the remelted and solidified layer (oxygen-contaminated layer). It is definitely higher than the concentration. This means that the oxygen concentration is stepped in the surface layer (remelted solidified layer; oxygen-contaminated layer) in the slab (material for hot rolling) after the surface melting treatment and in the vicinity of the inner region and the boundary. Means a sudden change.

  In such a material for hot rolling after surface melting treatment (melting-resolidification treatment), a rapid change in the oxygen concentration near the boundary between the surface layer (remelting solidification layer) and the internal region is caused by hot rolling. It is not solved by the previous slab heating, and it is inherited even after hot rolling. Further, as described above, the oxygen-contaminated layer is deepened by the surface melting treatment, and has a depth that cannot be removed by pickling hot-rolled sheets. Therefore, in cold rolling and cold forming after pickling, cracks start near the boundary due to a significant difference in cold workability near the boundary between the surface layer (remelted solidified layer) and the internal region. It is thought that it becomes easy to generate. That is, surface melting treatment for hot rolling material (slab) is extremely effective for smoothing the surface and eliminating surface layer defects (such as blowholes), but the surface layer is contaminated with oxygen. If there is, the oxygen concentration in the contaminated layer is averaged (the oxygen concentration is uniform from the inside to the surface) and the oxygen contaminated layer is deepened (the region with a high oxygen concentration expands in the depth direction). It is thought that this causes the problem of cracking during cold rolling or cold forming as described above.

  By the way, even if it is DC slab, the surface layer of the as-cast slab is usually a layer with severe irregularities and many defects. It has been conventionally considered that the surface layer of the slab is removed by cutting over a depth of about several mm and then the surface melting treatment as described above is performed. The cutting in this case is mainly aimed at removing surface irregularities and defects, but following the technique, it is also possible to remove the oxygen-contaminated layer on the surface of the DC slab by cutting and then subject it to surface melting treatment. It is done. If the surface layer (oxygen-contaminated layer) is removed prior to the surface melting process by cutting in this way, cracking occurs during cold working or cold forming due to oxygen contamination during DC casting as described above. It is possible to avoid this problem.

  However, when the oxygen concentration of the oxygen-contaminated layer in the DC slab is high and the depth of the oxygen-contaminated layer is large, the cutting depth must be increased, and in this case, a great deal of labor and time are required for surface cutting. The yield is also greatly reduced. Therefore, if such cutting before the surface melting treatment is omitted or at least the cutting depth is reduced, cracks will occur during cold rolling and cold forming due to oxygen contamination during DC casting as described above. Therefore, it is possible to manufacture a titanium thin plate excellent in cold workability at a low cost with high productivity.

  Therefore, the present invention does not perform the cutting process before the surface melting process, or at least when the cutting depth of the cutting process before the surface melting process is reduced, In the hot forming, providing a titanium slab for surface melting treatment that can reliably and stably prevent cracks on the surface, and a titanium material for hot rolling that has been subjected to surface melting treatment using the slab, As a result, it is an object to improve the productivity of titanium hot-rolled sheet manufacturing and to reduce costs.

  In order to solve the above-mentioned problems, the present inventors have conducted extensive experimental studies, and as a result, at the stage of the titanium slab before the surface melting treatment, the oxygen concentration gradient in the slab thickness direction of the surface layer is appropriately reduced. As a result, the inventors have found that the above-described problems can be solved, and have reached the present invention.

Accordingly, in the titanium slab for surface melting treatment of the basic aspect (first aspect) of the present invention, a remelted solidified layer having a depth d1 is formed on the surface of the titanium slab by surface melting treatment, and the surface is rolled. A titanium slab for surface melting treatment used in producing a titanium material by hot rolling,
The titanium slab is an as-cast titanium slab obtained by a DC slab casting method in a vacuum or an inert gas atmosphere,
In the thickness direction of the titanium slab,
A region from the surface to a position of d1 / 2 is a first region,
A region from the position of d1 / 2 to the position of d1 is a second region,
For the average oxygen concentration of the base material of the titanium slab,
Increment of average oxygen concentration in the first region is C1,
The increment of the average oxygen concentration in the second region is C2,
When the difference C1-C2 between C1 and C2 is Cd,
C1: 0.20 mass% or less,
C2: 0.05 mass% or less, and Cd: more than 0 and 0.15 mass% or less,
It is a titanium slab for surface melting treatment.

Furthermore, the titanium slab for surface melting treatment of the second aspect of the present invention is the titanium slab for surface melting treatment of the first aspect,
Said Cd is 0.10 mass% or less.

Furthermore, the titanium slab for surface melting treatment of the third aspect of the present invention is the titanium slab for surface melting treatment of the first or second aspect,
Said d1 exists in the range of 3.0-10.0 mm.

Moreover, the titanium slab for surface melting treatment of the fourth aspect of the present invention is the titanium slab for surface melting treatment of any one of the first to third aspects,
The surface is cut and removed with a thickness of 3.0 mm or less.

  Further, the following fifth to seventh aspects are for hot rolling, wherein the surface of the titanium slab for surface melting treatment as described above is subjected to melting-rapid resolidification processing (surface melting processing) by high-density energy. This is specified for titanium materials.

  That is, the titanium material for hot rolling according to the fifth aspect of the present invention forms a remelted solidified layer having a depth d1 by surface melting treatment on the surface of the titanium slab according to any one of the first to fourth aspects. It is a titanium material for hot rolling used when producing a titanium material by hot rolling with the surface as a rolling surface.

  And the titanium raw material for hot rolling according to the sixth aspect of the present invention is the titanium raw material for hot rolling according to the fifth aspect, wherein the oxygen concentration distribution in the thickness direction is a boundary between the remelted solidified layer and the base material. In the position, it increases in steps from the base material toward the surface.

The titanium material for hot rolling according to the seventh aspect of the present invention is the titanium material for hot rolling according to any of the fifth and sixth aspects,
The increment of the average oxygen concentration of the remelted solidified layer with respect to the average oxygen concentration of the base material is 0.1 mass% or less.

  According to the present invention, a remelted solidified layer is formed on the surface of the titanium slab by surface melting treatment, and hot rolling is performed by hot rolling using the surface as a rolling surface. When a cold working such as cold rolling is performed after descaling, or when cold forming is performed after cold rolling and annealing, it is possible to prevent processing defects such as cracks from occurring. . Moreover, according to the present invention, even if the slab surface is not subjected to cutting before the surface melting treatment or the amount of cutting is reduced, processing such as cracking when the cold working as described above is performed. Since the occurrence of defects can be prevented, cutting can be omitted or the amount of cutting can be reduced, so that productivity can be improved and manufacturing costs can be reduced.

It is an approximate solution figure showing typically an example of a method of manufacturing a titanium slab by DC casting under vacuum. It is a typical perspective view which shows an example of the condition which surface-melts with respect to a titanium slab. It is a schematic diagram which shows an example of the condition of the titanium slab surface layer when performing the surface melting process with respect to a titanium slab in the cross-sectional position. It is a schematic diagram which shows an example of the oxygen concentration distribution of the thickness direction in the cross-sectional position in the as-cast titanium slab corresponding to a slab cross section. FIG. 5 is an enlarged schematic view showing an IV portion of FIG. 4 as an example of an oxygen concentration distribution in a thickness direction at a cross-sectional position in an as-cast titanium slab. It is a schematic diagram which shows an example of the oxygen concentration distribution of the thickness direction in the cross-sectional position in the stage after performing a surface melting process with respect to a titanium slab, ie, the stage used as the raw material for hot rolling. It is a schematic diagram which shows an example of the oxygen concentration distribution of the thickness direction in the cross-sectional position in the stage after performing surface melting process and hot rolling with respect to a titanium slab. It is a schematic diagram which shows an example of the oxygen concentration distribution of the thickness direction in the cross-sectional position in the stage after performing surface melting process, hot rolling, and further pickling with respect to a titanium slab. It is a schematic diagram which shows the comparative example of the oxygen concentration distribution of the thickness direction in the cross-sectional position in the as-cast titanium slab on the same scale as FIG. 5A. It is a schematic diagram which shows the comparative example of oxygen concentration distribution after performing the surface melting process with respect to a titanium slab on the same scale as FIG. 6A. An example of the oxygen concentration distribution in the thickness direction at the cross-sectional position in the titanium slab according to the present invention and the concentration distribution after the surface melting treatment are as follows: It is a schematic diagram shown in contrast.

  Embodiments of the present invention will be described below in detail with reference to the drawings.

  First, before describing the titanium slab for surface melting treatment of the present invention, the method for producing the titanium slab for surface melting treatment, that is, the titanium melting raw material is vacuum or non-reacted using a high-density energy heat source such as an electron beam or plasma. For DC slab casting method (direct cast method) that melts in an active gas atmosphere and continuously casts over a predetermined length on a slab having a rectangular cross section (slab), A description will be given with reference to FIG.

  In FIG. 1, when casting a titanium slab 10, a water-cooled copper hearth 2 disposed in a melting chamber 1 is mixed with an industrial pure titanium melting raw material, for example, a titanium sponge obtained by a crawl method, or pure titanium scrap. , And the electron beam 3 is irradiated by the electron beam irradiation gun 12 to melt the melting raw material in the hearth 2. Then, the obtained titanium melt 4 is made into a water-cooled copper mold 6 for DC slab casting disposed in the upper part of the casting-drawing chamber 5, that is, the upper and lower sides are open and the horizontal section is rectangular (chamber at the corner). The water is continuously poured into the water-cooled copper mold 6. At this time, in order to keep the surface of the molten titanium 4 in the mold 6 warm, the electron beam 7 is generally irradiated to the surface of the molten titanium 4 in the mold 6 separately from the melting electron beam 3. is there. The hearth 2 for melting the melting raw material may be plural or multistage.

  Titanium solidified in the mold 6 is continuously drawn downward by lowering a drawing member (liftable receiving member) 8 disposed below in the casting-drawing chamber 5. A titanium slab 10 having a rectangular shape (including a case where chamfers are formed at corners) and having a predetermined length is obtained in an extraction chamber 5B described below.

  Here, the casting-drawing chamber 5 has a structure in which a casting chamber 5A that surrounds the mold 6 and a drawing chamber 5B below the mold 5 are vertically connected, and the lower drawing chamber 5B has a predetermined length of casting. After the completion, there is a case in which it is configured to move away from the upper casting chamber 5A and move to one side (for example, the left side in FIG. 1) from the original position (position at the time of casting) together with the drawing member 6. In this case, a variable partition plate (valve plate) (not shown) is inserted between the casting chamber 5A and the drawing chamber 5B in order to maintain the vacuum state of the casting chamber 5A, or a variable partition plate (valve) It is common to install a short gate chamber containing a plate. In addition, on the other side (for example, the right side of FIG. 1) of the position (original position) of the extraction chamber 5B at the time of casting, another extraction chamber (a chamber provided with another extraction member) is disposed. After the casting of the titanium slab 10 having a predetermined length is completed, the other drawing chamber moves below the casting chamber 5A as the original drawing chamber 5B moves to one side (for example, the left side in FIG. 1). Composed to come to the side.

  Here, when the variable partition plate (valve plate) is open, the space in the melting chamber 1 and the space in the casting-drawing chamber 5 communicate with each other by a gap around the mold 6. For example, a vacuum pump 9 is connected to the upper part of the melting chamber 1 through an exhaust pipe 9a, and the space in the melting chamber 1 and the space in the casting-drawing chamber 5 are evacuated. . Therefore, melting and casting of titanium is basically performed under vacuum exhaust. However, in practice, a very small amount of diluted inert gas may be introduced.

  As described above, after the casting of the titanium slab 10 having a predetermined length in the casting-drawing chamber 5 is finished and the titanium slab 10 is cooled to some extent, the variable partition plate is inserted. The lower drawing chamber 5B moves away from the upper casting chamber 5A together with the titanium slab 10 and the drawing member 6, for example, to the left in FIG. 1, and one lot of melting / casting is completed. Thus, when the lower drawing chamber 5B moves away from the upper casting chamber 5A, the evacuation in the drawing chamber 5B is interrupted, and further lowered to an appropriate temperature and then released to atmospheric pressure. It will be.

  At the same time or after that, another drawing chamber (for example, provided with another drawing member and pre-evacuated) located on the right side moves to the lower position of the casting chamber 5A, and the drawing chamber Is connected to the casting chamber 5A, the variable partition plate is opened again, the melting chamber 1 and the casting-drawing chamber 5 are connected to the melting chamber 1, and are evacuated by the vacuum pump 9, so that the next one lot of melting is performed. We will prepare for casting.

  Next, an example of a method for subjecting the titanium slab 10 obtained by melting and casting as described above to surface melting treatment will be described with reference to FIGS.

  For example, as shown in FIG. 2, in the titanium slab 10 having the chamfer 11, the wide 2 of the four surfaces 10 </ b> A to 10 </ b> D along the length direction LD (the slab drawing direction in the DC slab casting) LD. The surfaces 10A and 10B (surfaces including the chamfer 11) serve as rolling surfaces during hot rolling. Therefore, surface melting treatment is performed on the wide two surfaces 10A and 10B including at least the chamfer 11. Of course, not only the wide two surfaces 10A and 10B but also the side surfaces 10C and 10D can be subjected to surface melting treatment, and thereby benefits such as omission of cutting can be enjoyed. Here, the wide two surfaces 10A The explanation will be made on the assumption that surface melting treatment is performed for 10B.

  Specifically, first, an electron beam irradiation gun 13 irradiates the surface of one wide surface 10A among the outer surfaces of the titanium slab 10 with an electron beam irradiation gun 13 to rapidly melt only the surface layer on the surface 10A. At this time, the electron beam irradiation gun 13 is continuously moved or the rectangular cast piece 10 is continuously moved along the longitudinal direction LD (or short direction) of the titanium slab 10. The melting position is moved to. The molten layer on the surface of the titanium slab 10 at that time is denoted by reference numeral 16a in FIG.

  When the electron beam irradiation gun 13 is irradiated with the electron beam while being continuously moved relative to the titanium slab 10, the molten layer 16a in the irradiated portion has a base material (titanium slab as shown in FIG. 3). 10 is cooled by heat removal from the inside), and when it reaches a solidification temperature or lower, it solidifies and becomes a remelted solidified layer 20. In addition, due to the heat effect of the surface melting treatment, a heat affected zone (HAZ) 18 heated to a temperature equal to or higher than the β transformation point temperature (about 900 ° C.) is generated. Thereafter, the heat affected layer 22 is formed by being cooled by heat removal from the base material (inside the titanium slab 10) and reaching the β transformation point temperature or lower.

  In this specification, the remelted solidified layer 20 and the heat-affected layer 22 are collectively referred to as a surface melt-treated layer 21.

  When the melting-re-solidification process (surface melting process) for the entire area of the one surface 10A of the titanium slab 10 (or the area that needs to be processed) is completed, the above-described process is performed on the other surface 10B of the titanium slab 10. The same processing is performed. Further, if necessary, the same processing is performed on the other surfaces 10C and 10D of the titanium slab 10.

  Here, in the above surface melting treatment, if the surface of the titanium slab is irradiated with an electron beam and the surface is heated to a temperature equal to or higher than the melting point of industrial titanium (usually about 1700 ° C.), the plate of the titanium slab 10 The surface layer of the surface 10A is melted by a depth corresponding to the amount of heat input. That is, the region from the surface to the position of the depth d1 in the thickness direction is melted, and the remelted solidified layer 20 having the depth d1 in the thickness direction is formed by subsequent resolidification. Further, the heat-affected layer 22 having a depth d2 is formed. Therefore, the depth of the entire region (surface melt treatment layer 21) in which the coarse cast structure is converted into a fine transformation structure by such surface melting treatment is d (= d1 + d2).

  The melting depth by the surface melting treatment as described above, and therefore the depth d1 of the remelted solidified layer 20 is usually in the range of 3 mm to 10 mm.

  Since the amount of heat input is mainly related to the melting depth by electron beam irradiation, the electron beam irradiation conditions are selected so that the amount of heat input is such that the melting depth d1 is obtained. Actually, the amount of heat input required varies depending on the slab thickness (heat capacity), slab base material temperature, cooling conditions on the slab base material side, etc. Although not determined, normally, the heat input per unit area (per 1 cm 2) is about 80 to 300 J. Here, the electron beam irradiation conditions that affect the amount of heat input per unit area include the output and beam diameter of the irradiation gun, and the gun movement speed when irradiating while moving the irradiation gun continuously as described above ( (Irradiation position moving speed) and the like, and these are appropriately set to secure the above heat input.

  The titanium slab subjected to the surface melting treatment as described above is heated as a hot rolling material to a temperature higher than the hot rolling start temperature by an appropriate heating furnace, and then the hot rolling material is hot rolled. Thus, a hot-rolled sheet having a required thickness is obtained. Then, the hot-rolled sheet is subjected to descaling treatment such as pickling, and then cold-rolled to reduce the thickness to the product sheet thickness and then annealed. In addition, it is subjected to cold forming as necessary and used for various purposes.

  In the above, the titanium slab (as cast DC slab) 10 obtained by melting and casting has a cross section in the thickness direction as shown in FIG. There is an oxygen-contaminated layer 10P containing oxygen at a high concentration, for example, about 0.1 mass% or more higher than the oxygen concentration in the inner region), and in some cases at a high concentration about 0.3-0.5 mass% or more. There are things to do. The oxygen in the oxygen-contaminated layer 10P is mainly due to oxygen from the atmosphere outside the slab, and is caused by absorption and inward diffusion of the oxygen-containing gas. It has a gradient (oxygen concentration gradient) that decreases from the slab surface toward the inside. In this specification, the oxygen-contaminated layer 10P is a region where the oxygen concentration is 0.05 mass% or more from the base material, for example, an oxygen gradient in the slab thickness direction in FIG. The region from the position Pq of the oxygen concentration Cq (= C0 + 0.05 mass%) higher than 05 mass% to the slab surface is meant.

  Here, in the electron beam melting / casting method as described above, the space in the melting chamber 1 and the casting-drawing chamber 5 is originally maintained in vacuum during the melting / casting and the subsequent cooling period. In addition, oxygen contamination of the titanium slab should not occur. However, in practice, the oxygen-contaminated layer 10P is often generated.

  There are several possible causes for the oxygen contamination layer on the surface of the slab. As already mentioned, one of the reasons is that when one lot of titanium slab is cast and the casting-drawing chamber is opened, the external surface is exposed. It is considered that oxygen in the air entering from the slab may be absorbed by the slab surface. In particular, when the temperature of the titanium slab when the casting-drawing chamber is opened to the atmosphere is still high, oxygen entering from the outside is easily absorbed by the titanium slab, and oxygen contamination is considered to be severe.

  Also, if for some reason the degree of vacuum in the chamber during casting, especially around the hot slab drawn from the mold, is not sufficiently high, oxygen is absorbed from the atmosphere around the slab to the slab surface. It is thought that it may end up. In particular, in a conventional general chamber, as shown in FIG. 1, a vacuum pump 9 for exhausting the inside of the chamber is generally provided on the side of the melting chamber 1 to prevent oxygen absorption during melting of the melting raw material. In this case, since the exhaust location is separated from the casting-drawing chamber 5, the degree of vacuum in the space in the casting-drawing chamber 5, especially the space in the drawing chamber 5B may not be sufficiently high. Therefore, it is considered that the oxygen-containing residual gas may be absorbed by the high-temperature titanium slab 10 in the extraction chamber 5B.

  Further, when oxygen or oxygen-containing gas that has entered from the outside when the chamber is opened is adsorbed or adhered to the inner wall of the casting-drawing chamber 5 or the outer surface of the mold 6 and is not sufficiently exhausted or evacuated. Absorption on the surface of the hot slab is considered to be one cause. In any case, it is considered that the concentration of oxygen or oxygen-containing gas in the atmosphere in the chamber becomes unexpectedly high, and an oxygen-contaminated layer is generated on the slab surface.

  The slab in which the oxygen-contaminated layer is formed on the surface in this way is subjected to the surface melting treatment without performing surface cutting (and thus without removing the oxygen-contaminated layer), and the slab after the surface melting treatment (hot rolling) Material) is heated and hot-rolled into a hot-rolled sheet, and further cold-rolled into a cold-rolled plate or cold-rolled annealed sheet, as described above, during cold rolling or cold forming The problem of cracking will occur. That is, the state in which the oxygen concentration of the surface layer in the slab is high (the state in which the oxygen-contaminated layer exists) remains after the surface melting treatment, and further heating it as a hot rolling material increases the oxygen concentration of the surface layer. Even if it is, it will not be lowered, it will be inherited by the hot rolled sheet after hot rolling, and the surface layer at the time of cold rolling or cold forming of cold rolled sheet or cold rolled annealed sheet It is considered that cracking is likely to occur due to the difference in workability derived from the difference in oxygen concentration from the internal region.

  The change behavior of the oxygen concentration distribution of the oxygen-contaminated layer from such slab melting casting to surface melting treatment, further hot rolling / pickling will be specifically described with reference to FIGS. 5A to 5D.

  FIG. 5A shows a part of the oxygen concentration distribution in the slab thickness direction in the as-cast titanium slab (original material) 10 shown in FIG. 4, that is, the oxygen concentration in the vicinity of the surface 10A (circled portion IV in FIG. 4). The distribution is shown enlarged. FIG. 5B shows the oxygen concentration distribution of the slab after the surface melting treatment is performed on the as-cast titanium slab having the oxygen concentration distribution shown in FIG. 5A by a solid line. The broken line in FIG. 5B indicates the oxygen concentration distribution of the as-cast titanium slab shown by the solid line in FIG. 5A, that is, the oxygen concentration distribution before the surface melting treatment. 5C shows the oxygen concentration distribution of the hot-rolled sheet after hot rolling the slab after the surface melting treatment having the oxygen concentration distribution shown by the solid line in FIG. 5B. FIG. 5D shows the oxygen concentration distribution after the hot-rolled sheet having the oxygen concentration distribution shown in FIG. 5C is pickled.

  As shown in FIG. 5A, the oxygen concentration of the as-cast slab (base material) increases from the inside (base material side inside the base material) toward the slab surface in the surface layer. The maximum value Cmax1 of the oxygen concentration on the surface may reach 0.5% or more.

  FIG. 5B shows a state after subjecting such an as-cast titanium slab to surface melting treatment. The oxygen concentration distribution changes significantly when the surface melting treatment is performed (FIG. 5B) from the as-cast state (FIG. 5A). That is, in the surface melting treatment, the surface layer is melted by irradiation with an electron beam having a high density energy, and the molten metal in the molten pool is forcibly stirred by the energy, so that oxygen is stirred and flowed in the molten pool, As a result, the oxygen concentration in the molten layer is averaged. As a result, the oxygen concentration gradient before the surface melting treatment substantially disappears, and in the remelted solidified layer (region from the surface to the depth d1) 20 after melting, The density is almost uniform. Therefore, the oxygen concentration of the remelted solidified layer 20 increases stepwise with respect to the substantially uniform oxygen concentration C0 (usually about 0.04 to 0.2 mass%) in the inner region (base material) free from oxygen contamination. State (oxygen concentration Cm). That is, in the slab (hot rolling material) after the surface melting treatment, the vicinity of the boundary between the surface layer (remelted solidified layer 20) and the inner side region (strictly speaking, in the surface melting treatment layer) In the vicinity of the boundary between the remelted solidified layer 20 and the heat-affected layer 22), the oxygen concentration changes steeply in steps.

  In addition, as a result of averaging the oxygen concentration of the surface layer before the treatment in the surface melting treatment, the value Cm of the oxygen concentration of the remelted solidified layer 20 after the surface melting is almost the entire region in the thickness direction before the treatment. Although it is smaller than the maximum value Cmax1 on the surface, it is surely higher than the average oxygen concentration C0 in the inner region without oxygen contamination. As can be understood from FIG. 5A, a region having a higher oxygen concentration (region having an oxygen concentration Cm) than the average oxygen concentration C0 in the inner region free from oxygen contamination is greatly increased on the inner side of the plate. It means expanding. Note that the increment of the oxygen concentration Cm of the remelted solidified layer 20 with respect to the average oxygen concentration C0 of the inner region (base material portion) is ΔCm (= Cm−C0).

  The slab after the surface melting treatment is heated and subjected to hot rolling as a material for hot rolling. The oxygen concentration distribution after hot rolling is shown by the solid line in FIG. 5C. In addition, since the whole thickness reduces by hot rolling, the part which was d, d1, d2 in the slab before hot rolling is reduced to d ', d1', d2 'by hot rolling, respectively. . Further, in the slab before hot rolling, the position represented by Pq, that is, the position corresponding to the oxygen concentration Cq (= C0 + 0.05 mass%) 0.05 mass% higher than the oxygen concentration C0 of the base material, In the hot-rolled sheet, it is written as Pq ′.

  Since heating before hot rolling and hot rolling are usually performed in the atmosphere, the surfaces of the slab and hot rolled sheet are oxidized, and the sheet after hot rolling is indicated by a solid line in FIG. 5C. As shown in the figure, the oxygen concentration at the very thin layer portion on the surface of the hot-rolled plate rapidly increases and becomes Cmax2 (for example, an oxide having an oxygen concentration of about 35% or more) at the surface position. After hot rolling, pickling is generally performed to dissolve and remove a very thin layer having a thickness d3 (generally a layer having a thickness d3 of about 0.03 to 0.1 mm). Thus, the surface oxygen-enriched layer generated by heating-hot rolling is usually removed. FIG. 5D shows the oxygen concentration distribution after the pickling.

  Here, since pickling treatment after hot rolling only removes a very thin layer on the surface, as shown in FIG. 5D, surface pickling treatment before hot rolling heating is performed even after pickling. The tendency of the generated oxygen concentration distribution (distribution in the thickness direction) remains almost as it is. That is, even in the state of the hot-rolled sheet after pickling, the oxygen concentration is near the boundary position P1 between the region of the remelted solidified layer 20 in the surface melt-treated layer subjected to the surface melting treatment and the region inside the plate. Is changing rapidly. And if such a difference in oxygen concentration is large, there will be a large difference in cold workability between the surface layer and the internal region, and cracking and peeling will occur in cold rolling and cold forming as described above. it is conceivable that.

  In the present invention, as shown in FIGS. 5A to 5D, the depth d of the surface treatment layer 21 is the depth of the oxygen-contaminated layer 10P, that is, the oxygen concentration from the slab surface is Cq (= C0 + 0. 05 mass%) is determined to be larger than the depth to the position Pq.

  The depth d1 of the remelted solidified layer 20 in the surface melt-treated layer 21 is the depth of the oxygen-contaminated layer 10P, that is, from the slab surface to the position Pq where the oxygen concentration is Cq (= C0 + 0.05 mass%). Although it may be larger or smaller than the depth, in practice, it is desirable to make it larger than the depth from the slab surface to the position Pq where the oxygen concentration is Cq.

  In summary, the surface melting treatment as described above is derived from the existence of the oxygen contamination layer on the surface of the titanium slab and the change in the oxygen concentration distribution of the surface layer when the surface melting treatment is applied to the slab. In addition, as a hot-rolled sheet after hot rolling, cracks are considered to occur during cold rolling or cold forming after cold-rolled sheet annealing.

  However, as a result of repeated experiments and examinations by the present inventors, a remelted solidified layer having a depth d1 is formed on the surface of the titanium slab by surface melting treatment, and titanium is obtained by hot rolling using the surface as a rolling surface. In the titanium slab for surface melting treatment used for manufacturing a material, a region from the surface to a position of d1 / 2 in the thickness direction of the titanium slab is defined as a first region, and the position of d1 from the position of d1 / 2 The region up to the position is the second region, the average oxygen concentration increment in the first region is C1 with respect to the average oxygen concentration of the base material of the titanium slab, and the average oxygen concentration increment in the second region is C2 When Cd is the difference C1-C2 between C1 and C2, C1: 0.20 mass% or less, C2: 0.05 mass% or less, and Cd: more than 0 and 0.15 mass If it is below, the difference in oxygen concentration between the remelted solidified layer after the surface melting treatment and the region inside it will be small enough not to adversely affect the cold workability, and the surface melting treatment as described above And as a hot-rolled sheet after hot rolling, it can effectively prevent cracks from occurring in cold rolling, and cracks can occur when forming cold-rolled annealed sheets cold. It can be effectively prevented.

  Here, the oxygen contamination layer having a high oxygen concentration is generated, the oxygen concentration gradient of the surface layer is also increased, and the slab (comparative slab) when the above conditions are not satisfied is the thickness before the surface melting treatment. The directional oxygen concentration distribution is shown in FIG. 6A on the same scale as the thickness direction oxygen concentration distribution in the stage before the surface melting treatment in the slab of the present invention shown in FIG. 5A. Moreover, about the comparative slab, the thickness direction oxygen concentration distribution after performing the surface melting treatment is the same as the thickness direction oxygen concentration distribution in the stage before the surface melting treatment in the slab of the present invention shown in FIG. Shown in

  Note that the average oxygen concentration in the region inside the oxygen-contaminated layer (non-oxygen-contaminated layer) is not substantially affected by oxygen absorption from the surface. Accordingly, the oxygen amount in the region inside the slab is not different between the comparison slab and the slab of the present invention, and can be regarded as the same. Therefore, in FIGS. 6A and 6B for the comparison slab, FIG. As in FIG. 5B, the internal average oxygen concentration is shown as the same value C0.

  Here, a slab (comparative slab; FIGS. 6A and 6B) when the oxygen contamination layer having a high oxygen concentration is generated as described above, the oxygen concentration gradient of the surface layer is increased, and the above condition is not satisfied. In the slab that satisfies the above conditions (the slab of the present invention; FIGS. 5A and 5B), the oxygen concentration distribution in the thickness direction before the surface melting treatment, and the slab (the material for hot rolling) after the surface melting treatment The oxygen concentration distribution in the thickness direction is shown in the same FIG.

  As shown in FIG. 6A, the comparative slab has a region (first region) R1 from the slab surface to the position of ½ (ie, d1 / 2) of the depth d1 of the remelted solidified layer 20 in the slab thickness direction. Region C2 'from the base material average oxygen concentration C0, and a region (second region) R2 from the position d1 / 2 in the slab thickness direction to the position corresponding to the depth d1 of the remelted solidified layer The difference Cd ′ (= C1′−C2 ′) between the average oxygen concentration and the increment C2 ′ from the base material average oxygen concentration C0 exceeds 0.15 mass%. In this comparative slab, the oxygen concentration distribution in the thickness direction before the surface melting treatment is indicated by a two-dot chain line in FIG. In this case, the maximum oxygen concentration (oxygen concentration at the surface position) of the surface layer (oxygen-contaminated layer) is indicated by Cmax1 ′.

  Further, for the comparative slab, the oxygen concentration distribution in the slab thickness direction after the surface melting treatment is performed from the surface at a depth d (the depth of the remelted solidified layer is d1) is shown by a dotted line in FIG. As already described, in the remelted solidified layer having the depth d1, the oxygen concentration in the thickness direction is averaged, and the oxygen concentration of Cm ′ is almost uniform. That is, the oxygen concentration in the thickness direction has a distribution that increases stepwise at the boundary position between the remelted solidified layer and the base material. In the vicinity of the position of the depth d1 (the boundary position between the remelted solidified layer 20 and the base material), the oxygen concentration changes rapidly and greatly from C0 to Cm ′ in the thickness direction.

  On the other hand, in the slab of the present invention, the difference between the average oxygen concentration in the first region R1 with respect to the base material average oxygen concentration C0 and the difference between the average oxygen concentration in the second region R2 and the increment C2 with respect to the base material average oxygen concentration C0. Cd is 0.15 mass% or less. In the present slab, the oxygen concentration distribution in the thickness direction before the surface melting treatment is shown by broken lines in FIG. In this case, the maximum oxygen concentration (oxygen concentration at the surface position) of the surface layer (oxygen-contaminated layer) is indicated by Cmax1.

  Here, in the case of the slab of the present invention that satisfies the condition that the difference Cd of the average oxygen concentration of the first region R1 and the second region R2 from the base material average oxygen concentration C0 is 0.15 mass% or less (that is, Cd ≦ 0.15 mass%) in the case of a comparative slab that does not satisfy the condition that the difference Cd of the average oxygen concentration of the first region R1 and the second region R2 from the base material oxygen concentration C0 is 0.15 mass% or less ( That is, in the comparative slab, the oxygen concentration gradient (slope) in the thickness direction is smaller than Cd ′> 0.15 mass%. As described above, if the average oxygen concentration in the non-oxygen-contaminated region inside the slab is the same C0, the smaller the oxygen concentration gradient (slope) in the thickness direction, the smaller the oxygen concentration (maximum oxygen concentration) at the surface position. . Therefore, as is apparent from comparing the two-dot chain line (comparison slab; before surface melting treatment) and the broken line (present slab; before surface melting treatment) in FIG. 7, the oxygen concentration (at the surface position of the slab of the present invention ( The maximum oxygen concentration (Cmax1) is lower than the oxygen concentration (maximum oxygen concentration) Cmax1 ′ at the surface position in the comparative slab. Furthermore, the total amount of oxygen contained from the surface to the position of the depth d1 is also smaller in the slab of the present invention than in the comparative slab.

  Further, the solid line in FIG. 7 shows the slab of the present invention (slab that satisfies the above conditions) having the oxygen concentration distribution shown by the chain line after the surface melting treatment is performed at a depth of d (= d1 + d2) from the surface. The oxygen concentration distribution in the slab thickness direction is shown. Also in the case of the slab of the present invention, the tendency of the oxygen concentration distribution in the thickness direction after the surface melting treatment is similar to the oxygen concentration distribution in the thickness direction after the surface melting treatment of the comparative slab, and the remelted solidified layer having the depth d1. 20, the oxygen concentration in the thickness direction is averaged to obtain a substantially constant oxygen concentration Cm, and the increment ΔCm from the base material oxygen concentration C0 is also substantially constant in the thickness direction. That is, the oxygen concentration distribution in the thickness direction is a step-like distribution. However, the substantially constant oxygen concentration Cm from the vicinity of the surface to the position of the depth d1 in the slab of the present invention is from the vicinity of the surface to the position of the depth d1 in the comparative slab after the surface melting treatment (broken line in FIG. 7). It becomes smaller than the substantially constant oxygen concentration Cm ′. This is because, as described above, the total amount of oxygen contained from the surface to the position of the depth d1 is smaller in the slab of the present invention than in the comparative slab. And in the vicinity of the position of the depth d1 (the boundary position between the remelted solidified layer and the inner base material region), the oxygen concentration changes from C0 to Cm stepwise in the thickness direction. It can be seen that the degree of change, that is, the oxygen concentration increment ΔCm (= Cm−C0) is smaller than the degree of change from C0 to Cm ′ (oxygen concentration increment ΔCm ′ = Cm′−C0) in the comparison slab. That is, in the slab of the present invention, the rapid change in the oxygen amount in the thickness direction near the boundary position between the remelted solidified layer and the inner region is alleviated.

  As described above, by reducing the oxygen concentration gradient in the plate thickness direction in the surface layer, changes in the oxygen concentration in the vicinity of the boundary between the remelted solidified layer by the surface melting treatment and the inner region are alleviated. This tendency is inherited even after the pre-hot rolling and further hot rolling, and the increment of the oxygen concentration of the remelted solidified layer from the oxygen concentration of the base metal is suppressed to a small value. That is, even in the state of a hot-rolled sheet, a rapid change in the amount of oxygen in the vicinity of the boundary between the remelted solidified layer and its inner region is alleviated. As a result, there is less risk of cracking in cold rolling due to the difference in cold workability between the surface layer portion (remelted solidified layer) and the region inside it, and after cold rolling annealing There is less risk of cracking when cold forming.

  As described above, in the present invention, as an index representing the degree (gradient) of the oxygen concentration gradient in the thickness direction in the surface layer of the slab, the increment C1 from the base material average oxygen concentration C0 for the first region R1 and the first The difference Cd with respect to the increment C2 from the base material average oxygen concentration C0 for the two regions R2 is used, and the difference Cd (= C1-C2) is regulated to 0.15 mass% or less.

  Here, if there is a large concentration gradient in the surface layer of the slab such that the oxygen concentration difference Cd exceeds 0.15 mass%, the vicinity of the boundary between the remelted solidified layer after the surface melting treatment and the region inside it (For example, the increment ΔCm of the oxygen concentration Cm of the remelted solidified layer from the base material average oxygen concentration C0 exceeds 0.1% in the state after the surface melting treatment). As a result, it becomes difficult to prevent the occurrence of cracking during cold rolling of the hot-rolled sheet as described above and crack peeling during cold forming of the cold-rolled annealed sheet.

  Further, when the increment C1 from the base material oxygen concentration C0 for the first region R1 exceeds 0.2 mass%, or the increment C2 from the base material oxygen concentration C0 for the second region R2 exceeds 0.05%. Even in this case, the change in the oxygen concentration in the vicinity of the boundary between the remelted solidified layer and the inner region after the surface melting treatment becomes large (for example, the reconstitution from the base material oxygen concentration C0 in the state after the surface melting treatment). The increment ΔCm of the oxygen concentration Cm of the melt-solidified layer exceeds 0.1%). As a result, it becomes difficult to prevent the occurrence of cracking during cold rolling of the hot-rolled sheet as described above and crack peeling during cold forming of the cold-rolled annealed sheet.

  Note that the effect of the present invention is further improved if the difference Cd in increments of oxygen concentration before the surface melting treatment is suppressed to 0.10 mass% or less.

  In addition, as a concrete numerical value of the depth (depth from the surface position of the base plate before the treatment) d1 of the remelted solidified layer in the surface melting treatment applied to the slab, within a range of 3.0 to 10.0 mm It is preferable that If the depth of the remelted solidified layer is less than 3.0 mm, the effect of smoothing the surface layer and removing defects in the surface layer by performing the surface melting treatment cannot be obtained sufficiently. On the other hand, even if the thickness exceeds 10.0 mm, the effect of the surface melting treatment does not increase any more, and unnecessarily increases the energy cost and decreases the productivity.

  In addition, the depth of the first region and the second region as a reference for the thickness direction concentration gradient of the surface layer of the slab is determined to be ½ of the depth d1 of the surface melting treatment to be performed thereafter. Therefore, the specific depth (d1 / 2) of each region is about 0.15 to 5 mm.

  Also, as described above, if the oxygen concentration gradient in the thickness direction of the slab surface layer is regulated, the maximum oxygen concentration at the surface position is usually reduced as described above. Even without cutting, it is possible to obtain the actions and effects as described above. However, in some cases, it is allowed to perform surface melting treatment after removing a very thin layer on the surface by cutting. However, the cutting depth in that case is a depth smaller than 3.0 mm.

  Here, even if the high oxygen concentration portion of the surface layer portion is removed by cutting, the concentration gradient in the depth direction of oxygen does not change, but since the high oxygen concentration portion has been removed by cutting, the total of the remelted solidified layer The oxygen amount of the first region R1 and the second region R2 is reduced, and the difference Cd between the increments C1 and C2 from the base material oxygen concentration C0 in each region is as cast. Even if it is not 0.15 mass% or less in the state before cutting, a slab having the difference Cd of 0.15 mass% or less can be obtained by cutting. However, if the cutting depth of the surface layer exceeds 3.0 mm, the burden of cutting processing increases, and the productivity may be hindered. Therefore, when cutting, the depth is set to 3.0 mm or less.

  In manufacturing (melting / casting) the surface molten titanium slab of the present invention, a specific method for regulating the oxygen concentration of the surface layer of the slab as described above is not particularly limited. Various methods as shown in A to C may be applied. Countermeasure A is a technique for mainly preventing oxygen contamination when the atmosphere is released after completion of melting and casting. Countermeasure B and Countermeasure C are methods mainly for preventing oxygen contamination during melting and casting. In practice, it is preferable to apply a combination of two or more of these.

  Measure A: Oxygen absorption of the slab due to the intrusion of the atmosphere into the chamber when the atmosphere is open is likely to occur when the slab is still at a high temperature. Prone to occur in some cases. Therefore, after melting and casting a slab having a predetermined length, the air is released to the atmosphere after the surface temperature of the slab becomes about 900 ° C. or less. At this time, it may be left in the chamber, or accelerated cooling may be performed to improve productivity. For example, a cooling means for cooling the slab may be provided in the drawing chamber, and the air may be opened to the atmosphere in a state where the surface temperature is about 900 ° C. or lower. As the cooling means, for example, it is possible to apply a cooling plate whose inside is cooled with water in a state of being close to the slab. In addition, when the atmosphere in the chamber is replaced with a low-temperature gas such as an inert gas before the atmosphere is released, the surface temperature is quickly reduced to about 900 ° C. or lower, and the slab surface becomes a low temperature of 900 ° C. or lower. It is good also as opening in the atmosphere.

  Countermeasure B: As already described, the vacuum pump 9 (see FIG. 1) for evacuating the chamber is generally evacuated at a location away from the casting-drawing chamber 5 mainly for the purpose of keeping the melting chamber 1 in a vacuum. Are often connected in such a way. In that case, a second vacuum pump 91 is provided separately from the vacuum pump 9 for evacuation of the melting chamber, for example, as shown by a chain line in FIG. The exhaust pipe 91a of the second vacuum pump 91 is connected to the side of the drawing chamber 5B that is opened to increase the degree of vacuum in the casting / drawing chamber 5 during melting / casting.

  Countermeasure C: In order to lower the partial pressure of oxygen and oxygen-containing gas in the atmosphere during melting and casting, for example, as shown by the chain line in FIG. A member 92 made of a high target material such as titanium or zirconium is provided. In addition, in order to increase the degree of vacuum in the chamber during casting and melting, a vacuum pump having a larger exhaust capacity may be used. Moreover, you may use together means, such as improving the airtightness in the chamber at the time of casting and melt | dissolution.

  Here, the industrial pure titanium constituting the titanium slab of the present invention includes 1 to 4 types of JIS standards, Grades 1 to 4 of ASTM standards corresponding thereto, and 3 · 7025, 3 · 7025, 3 · of DIN standards. Industrial pure titanium specified in 7025 shall be included. That is, the industrial pure titanium targeted in the present invention is, in mass%, C: 0.1% or less, H: 0.015% or less, O: 0.4% or less, N: 0.07% or less, It can be said that Fe: 0.5% or less and the balance Ti. Further, a small amount of platinum group element is added to these, and a high corrosion resistance alloy (ASTM Grade 7, 11, 16, 26, 13, 30, 33) called modified (improved) pure titanium or JIS corresponding thereto. In the present invention, the seed is also treated as being contained in industrial pure titanium.

  Moreover, the dimension of the titanium slab 10 for surface treatment of the present invention is not particularly limited as long as it can be directly subjected to hot rolling, but coil rolling is applied as hot rolling, and a hot rolled coil having a thickness of about 3 mm to 8 mm. When manufacturing a thin plate, the titanium slab may be about 150 mm to 280 mm in thickness, about 3 m to 10 m in length, and about 600 mm to 1500 mm in width.

  In actually using the titanium material for hot rolling according to the present invention, hot rolling is performed to obtain a hot rolled sheet having a desired thickness. The hot rolling method is not particularly limited, but in the case of a thin hot-rolled sheet product, coil rolling is usually applied. In this case, the hot rolled plate thickness is not particularly limited, but is usually about 3.0 mm to 8.0 mm. Although the hot rolling conditions are not particularly limited, like normal titanium hot rolling, heating is performed from 720 ° C. to 920 ° C. for about 60 minutes to 420 minutes, and hot rolling is started at a temperature within the range, What is necessary is just to complete | finish hot rolling at the temperature more than room temperature according to the capability of a rolling mill.

  Examples of the present invention will be described below together with comparative examples.

[Test Example 1]
Using JIS type 1 pure titanium as a melting raw material, DC casting was performed by electron beam melting using equipment as shown in FIG. 1 to obtain a titanium slab having a cross section of about 1300 mm wide × about 400 mm thick × about 7500 mm long. The casting speed was 2 ton / h.

  In melting and casting, any one or more of the measures A to C was applied to suppress the oxygen concentration in the surface layer of some slabs (Nos. 1 to 6 in Table 1). For some slabs (No. 7 in Table 1), no measures were taken to suppress the oxygen concentration in the surface layer.

  No. The DC slab manufactured under the same conditions as in No. 7 was subjected to surface cutting (cutting depth of 0.5 to 2.5 mm) before the next surface melting treatment. 8-No. There were 12 slabs.

  Thereafter, surface melting treatment by electron beam irradiation was performed while continuously moving the slab on two wide surfaces of the slab to obtain a titanium material for hot rolling. In the surface melting treatment, an electron beam adjusted to have a rectangular electron beam size of 2.5 cm is used, and other electron beam irradiation conditions (electron beam output, slab moving speed during irradiation, heat input per cm, etc.) The melting depth from the surface position of the slab (the depth of the remelted solidified layer) d1 was changed.

  In the above process, the oxygen concentration and its distribution of the surface layer of the wide surface of each titanium slab before the surface melting treatment were quantitatively examined by EPMA analysis (X-ray microanalyzer) on the cross section. That is, the surface oxygen concentration Cmax and the average oxygen concentration C0 of the base material portion are examined, and the depth of the target (scheduled) remelted solidified layer by the surface melting process performed thereafter is defined as d1 up to the depth d1 / 2. The increment C1 of the oxygen concentration of the region (first region R1) with respect to the average oxygen concentration C0 of the base material portion and the oxygen concentration of the region (second region R2) from the depth d1 / 2 to d1 of the base material portion The increment C2 with respect to the average oxygen concentration C0 was examined.

  In addition, about the example (No. 8-12) which performed the surface cutting before performing a surface melting process, each oxygen concentration after cutting was investigated.

  Each slab No. before these surface melting treatments. Tables 1 and 2 show the results of examining the oxygen concentration of the surface layer and the distribution thereof for 1 to 12.

  On the other hand, for the wide surface of each titanium slab after surface melting treatment (titanium material for hot rolling; if surface cutting is performed, the material after cutting), the surface oxygen concentration Cmax, the average oxygen concentration of the base material portion C0 was examined, and an increment ΔCm of the oxygen concentration from the surface to the position d1 (that is, from the surface to the bottom of the remelted solidified layer) with respect to the average oxygen concentration C0 of the base material portion was examined. The results are shown in Table 3.

  Further, the surface-melted titanium material for hot rolling obtained as described above was inserted into a furnace at 800 ° C., heated for about 240 minutes, and then hot rolled to a thickness of 5 mm by a continuous hot rolling strip mill. A plate coil was manufactured, passed through a continuous pickling line made of nitric hydrofluoric acid, and cut by about 40 μm per side.

  After that, it is cold-rolled to 0.75 mm, and the surface is observed centering on the end portion to examine the occurrence of cracks, and the degree of cracks is determined as follows: no cracks (A judgment), even if there are cracks However, the evaluation was made in four stages: a situation where there is no practical problem (B determination to C determination) and a severe cracking situation (D determination).

  Further, after annealing in an argon gas atmosphere at 650 ° C. for 5 hours, the cold formability was examined. The cold formability test was performed by the Eriksen test (based on JIS Z 2247).

  These evaluation results are also shown in Table 3.

  As shown in Tables 1 to 3, the titanium slab before the surface melting treatment is any of the present invention examples (No. 1 to 6, No. 8 to 12) and the comparative example (No. 7). It was also confirmed that C1> C2 and an oxygen concentration gradient in which the amount of oxygen decreases from the surface toward the inside.

  In each example of the present invention, Cd (= C1-C2) was 0.15 mass% or less. Further, with respect to the titanium slab after the surface melting treatment (titanium material for hot rolling), in each example of the present invention, the oxygen concentration from the surface to the position of d1 (that is, from the surface to the bottom of the remelted solidified layer) is almost constant. The increment ΔCm of the base material portion with respect to the oxygen concentration C0 was 0.1 mass% or less.

  And in these examples of the present invention, cracks do not occur in hot rolling and cold rolling of pickled coil plates (A judgment), or even if cracks occur (B or C judgment), It was confirmed that there was practically no problem. Also, the cold formability after cold rolling-annealing was confirmed to be good, with an Erichsen value of 11.5 mm or more being obtained. In particular, in the case of a No. 1 Cd (= C1-C2) value specified in claim 2 of 0.1 mass% or less. In 3, 6, 11, and 12, cracks during cold rolling did not occur (A judgment), and the Erichsen value also showed a high value of 12 mm or more.

  On the other hand, No. which is a comparative example. 7, Cd (= C1-C2) exceeded 0.15 mass%. Further, with respect to the titanium slab after the surface melting treatment (titanium material for hot rolling), in each comparative example, the oxygen concentration from the surface to the position of d1 (that is, from the surface to the bottom of the remelted solidified layer) Although it was almost constant as in the example, the increment ΔCm with respect to the average oxygen concentration C0 of the base material portion exceeded 0.1 mass%.

  And in this comparative example, although the crack did not arise in cold rolling, it was confirmed that the cold formability after cold rolling-annealing is inferior (Erichsen value is 11 mm or less).

[Test Example 2]
Using ASTM grade 2 pure titanium as a melting raw material, DC casting was performed by electron beam melting using equipment as shown in FIG. 1 to obtain a titanium slab having a cross section of about 1100 mm wide × about 220 mm thick × about 7000 mm long. The average casting speed was 1.9 ton / h.

  In melting and casting, any one or more of the measures A to C was applied in order to suppress the oxygen concentration in the surface layer of some slabs (Nos. 13 to 18 in Table 4). For some slabs (No. 19 in Table 4), no measures were taken to suppress the oxygen concentration in the surface layer.

  No. The DC slab manufactured under the same conditions as the slab No. 19 was subjected to surface cutting (cutting depth of 0.5 to 2.5 mm) before the next surface melting treatment. The slab was 20-24.

  Thereafter, surface melting treatment by electron beam irradiation was performed while continuously moving the slab on two wide surfaces of the slab to obtain a titanium material for hot rolling. In the surface melting treatment, an electron beam adjusted to have a rectangular electron beam size of 2.5 cm is used, and other electron beam irradiation conditions (electron beam output, slab moving speed during irradiation, heat input per cm, etc.) The melting depth from the surface position of the slab (the depth of the remelted solidified layer) d1 was changed.

  In the above process, the oxygen concentration and distribution of the surface layer on the wide surface of each titanium slab before the surface melting treatment are quantitatively examined by EPMA analysis (X-ray microanalyzer) on the cross section, as in Test Example 1. It was.

  In addition, about the example (No. 21-24) which performed surface cutting before performing a surface melting process, each oxygen concentration after cutting was investigated.

  Each slab No. before these surface melting treatments. Tables 4 and 5 show the results of examining the oxygen concentration and the distribution of the surface layer for 13 to 24.

  On the other hand, for the wide surface of each titanium slab after surface melting treatment (titanium material for hot rolling; if surface cutting is performed, the material after cutting), the surface oxygen concentration Cmax, the average oxygen concentration of the base material portion C0 was examined, and an increment ΔCm of the oxygen concentration from the surface to the position d1 (that is, from the surface to the bottom of the remelted solidified layer) with respect to the average oxygen concentration C0 of the base material portion was examined. The results are shown in Table 6.

  Further, the surface-melted titanium material for hot rolling obtained as described above was inserted into a furnace at 800 ° C., heated for about 240 minutes, and then hot rolled to a thickness of 5 mm by a continuous hot rolling strip mill. A plate coil was manufactured, passed through a continuous pickling line made of nitric hydrofluoric acid, and cut by about 40 μm per side.

  Then, it cold-rolled to 1.2 mm, surface observation was performed centering on the edge part, and the occurrence condition of a crack was investigated. The evaluation of crack evaluation was performed in four stages (A determination to D determination) in the same manner as in Test Example 1. The evaluation results are shown in Table 6.

  As shown to Tables 4-6, about the titanium slab before a surface melting process, each invention example (No. 14, 15, 17, 18, 21-24), each comparative example (No. 13, 16) 19 and 20), it was confirmed that all have an oxygen concentration gradient in which the amount of oxygen decreases from the surface toward the inside (C1> C2).

  In each example of the present invention, Cd (= C1-C2) was 0.15 mass% or less. In addition, with respect to the titanium slab after the surface melting treatment (titanium material for hot rolling), in each example of the present invention, the oxygen concentration from the surface to the position of d1 (that is, from the surface to the bottom of the remelted solidified layer) is almost constant. The increment ΔCm of the base material portion with respect to the average oxygen concentration C0 was 0.10 mass% or less.

  And in these examples of the present invention, cracks do not occur in hot rolling and cold rolling of pickled coil plates (A judgment), or even if cracks occur (B or C judgment), It was confirmed that there was practically no problem. In particular, as defined in claim 2, the Cd (= C1-C2) value is 0.10 mass% or less. Nos. 18, 22, 23, and 24 did not cause cracks during cold rolling (determination A), and had extremely high cold rolling properties.

  On the other hand, No. which is a comparative example. 19 and 20, Cd (= C1-C2) exceeded 0.15 mass%. Moreover, No. which is a comparative example. Nos. 13 and 19 have C1 of more than 0.2% by mass and are comparative examples No. 13, 16, and 19 had C2 exceeding 0.05 mass%.

  In each of these comparative examples, for the titanium slab (titanium material for hot rolling) after the surface melting treatment, the oxygen concentration from the surface to the position of d1 (that is, from the surface to the bottom of the remelted solidified layer) However, the increment ΔCm of the base material portion with respect to the average oxygen concentration C0 exceeded 0.1 mass%.

  And in these comparative examples, the remarkable crack has arisen in cold rolling (D determination), and it was confirmed that it is inferior to cold workability.

  The preferred embodiments and examples of the present invention have been described above. However, the embodiments and examples are merely examples within the scope of the present invention, and the configuration of the present invention is not limited to the scope of the present invention. Additions, omissions, substitutions, and other changes are possible. That is, the present invention is not limited by the above description, is limited only by the appended claims, and can be appropriately changed within the scope.

DESCRIPTION OF SYMBOLS 10 Titanium slab 12 Electron beam irradiation gun 16 Surface melting process layer d1 Surface melting process layer depth R1 1st area | region R1 2nd area | region

Claims (7)

This is a titanium slab for surface melting treatment that is used when a remelted solidified layer having a depth d1 is formed on the surface of the titanium slab by surface melting treatment and a titanium material is produced by hot rolling with the surface as a rolling surface. And
The titanium slab is an as-cast titanium slab obtained by a DC slab casting method in a vacuum or an inert gas atmosphere,
In the thickness direction of the titanium slab,
A region from the surface to a position of d1 / 2 is a first region,
A region from the position of d1 / 2 to the position of d1 is a second region,
For the average oxygen concentration of the base material of the titanium slab,
Increment of average oxygen concentration in the first region is C1,
The increment of the average oxygen concentration in the second region is C2,
When the difference C1-C2 between C1 and C2 is Cd,
C1: 0.20 mass% or less,
C2: 0.05 mass% or less, and Cd: more than 0 and 0.15 mass% or less,
Titanium slab for surface melting treatment. The Cd is 0.10 mass% or less,
The titanium slab for surface melting treatment according to claim 1. The d1 is in the range of 3.0 to 10.0 mm.
The titanium slab for surface melting treatment according to claim 1 or 2. The surface was removed by cutting with a thickness of 3.0 mm or less,
The titanium slab for surface melting treatment according to any one of claims 1 to 3.   A remelted solidified layer having a depth of d1 is formed by surface melting treatment on the surface of the titanium slab according to any one of claims 1 to 4, and the titanium material is hot-rolled with the surface as a rolling surface. Titanium material for hot rolling used in manufacturing. The oxygen concentration distribution in the thickness direction increases stepwise from the base material toward the surface at the boundary position between the remelted solidified layer and the base material.
The titanium material for hot rolling according to claim 5. The increment of the average oxygen concentration of the remelted solidified layer with respect to the average oxygen concentration of the base material is 0.1 mass% or less.
The titanium material for hot rolling according to claim 6.

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