JP2016128171A - Titanium hot rolling slab being hard to cause surface flaw and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a titanium slab in which surface properties of a strip-like coil after the hot rolling are superior, even if omitting a breakdown process such as slabbing or forging, in the titanium slab casted by using an electron beam melting method and a plasma arc melting method.SOLUTION: A titanium slab is a commercially pure titanium cast-remaining slab manufactured by an electron beam melting method and a plasma arc melting method. At least a slab surface layer equivalent to a rolled surface is melted together with the material containing Fe, Cr, Ni being β stabilization element, and thereby a molten re-solidification structure comprising a fine acicular structure of 1 mm or more in depth is formed. The Fe concentration to this surface layer portion of 1 mm or the total content of each element obtained by substituting a portion or all of the Fe by one or two kinds of Cr and Ni is 0.10 mass% or more and 1.50 mass% or less. Powder, chip, wire and foil are used as the material containing the β stabilization element. Also, the electron beam heating and the plasma arc heating are used as means for melting the surface layer.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing industrially pure titanium hot rolling slabs and hot rolling slabs, and in particular, hot rolling of titanium slabs in which breakdown processes such as split rolling and forging are omitted. The present invention relates to a hot-rolling slab capable of maintaining good coil surface properties when a strip-like coil is formed, and a method for manufacturing the same.

  Pure titanium and titanium alloys for industrial use are generally made from titanium sponge and titanium scrap as raw materials, and are melted by a non-consumable electrode type arc melting method, an electron beam melting method, a plasma arc melting method, or the like to form a titanium ingot. In the non-consumable arc melting method, an ingot is obtained by using a briquette formed by pressurizing sponge titanium as an electrode, causing arc discharge between the electrode and the mold, melting the electrode itself, and casting in the mold. Therefore, since it is necessary to discharge the mold and the electrode uniformly, the mold shape is limited to a cylindrical shape. On the other hand, the electron beam melting method and the plasma arc melting method use an electron beam and a plasma arc, respectively. The melting method is different, but the molten titanium melted on the hearth is poured into the mold at the time of melting. It is possible to manufacture a rectangular ingot.

  In the current titanium slab manufacturing process, hot rolling is performed after a hot working process such as ingot rolling, forging, etc., called the ingot breakdown process, and the breakdown process is an essential process. It has become. However, it is considered that a rectangular ingot can omit the breakdown step because of its shape, and a technique of performing hot rolling without the breakdown step is being studied. Once this technology is established, cost reductions can be expected by omitting the process and improving yield.

  However, titanium ingots manufactured by using an electron beam melting method or a plasma arc melting method are cast as they are, so that there are coarse grains of several tens of millimeters. For such a titanium ingot, when the hot rolling is performed by omitting the breakdown step, unevenness occurs on the surface due to the influence of deformation anisotropy within the grains and between each crystal grain due to the coarse grains, This becomes a surface defect. In order to remove the surface flaws generated by hot rolling, it is necessary to increase the amount of hot-rolled sheet on the surface of the hot-rolled sheet in the next pickling process, resulting in a decrease in yield and an increase in cost. Concerned.

  Therefore, titanium ingots manufactured by the electron beam melting method or the plasma arc melting method are expected to improve costs by omitting breakdown processes such as ingot rolling and forging, but there is a concern about an increase in costs due to an increase in surface defects. This has hindered the practical application of hot rolling slabs that omit the breakdown step.

  In Patent Document 1, in a cross-sectional structure of a titanium slab melted in an electron beam melting furnace and directly drawn from the mold, an angle θ formed by a solidification direction from the surface layer to the inside and a casting direction of the slab is 45 ° to 90 °, Alternatively, in the crystal orientation distribution of the surface layer, when the angle between the hcp c-axis and the normal line of the slab surface layer is 35 ° to 90 °, the casting surface is good and the ingot breakdown process is omitted. However, a method is disclosed that can improve the surface defects after hot rolling. That is, the generation of wrinkles due to such coarse crystal grains can be suppressed by controlling the shape and crystal orientation of the surface crystal grains. However, in Patent Document 1, due to variations in operating conditions, the solidification direction and crystal orientation distribution of crystal grains near the surface layer rarely change, and surface defects may occur.

  In Patent Document 2, as a method of directly performing hot rolling while omitting the breakdown process of the titanium material ingot, the surface layer corresponding to the rolling surface is subjected to high frequency induction heating, arc heating, plasma heating, electron beam heating, laser heating, etc. By remelting and re-solidifying, a fine particle having a depth of 1 mm or more is performed from the surface layer. Generation of surface defects is prevented by providing a fine and irregular crystal orientation distribution by rapid solidification of the slab surface layer. However, in Patent Document 2, if the slab temperature is not sufficiently low at the time of the melt resolidification process, the slab temperature is increased if the melt resolidification process is continuously performed on the slab rolling surface without fine graining. Fine graining may not be achieved. Further, the crystal grains may be coarsened due to variations in hot rolling heating temperature and heating time. As described above, surface flaws may occur if fine graining is not achieved sufficiently.

International Publication No. 2010/090353 JP 2007-332420 A

  As mentioned above, the rectangular titanium ingot manufactured by melting method using hearth such as electron beam melting method and plasma arc melting method omits the breakdown process such as block rolling and forging, which were conventionally essential processes. When hot rolling is performed, irregularities are generated on the surface due to the deformation anisotropy within the grains and between each crystal grain due to the coarse grains on the surface of the ingot, and surface defects are generated. Therefore, it is necessary to remove this surface flaw in the next pickling step, and there is a concern that the yield will be reduced accordingly.

  Therefore, the present invention eliminates the breakdown step conventionally required in an as-cast titanium slab manufactured by an electron beam melting method or a plasma arc melting method, and even after hot rolling, An object of the present invention is to provide a titanium slab for hot rolling having a good surface property and a method for producing the same.

  As a result of diligent investigations to achieve the above-mentioned problems, the inventors of the present invention have conventionally required an as-cast titanium slab manufactured using an electron beam melting method or a plasma arc melting method as a melting method for industrial pure titanium. When the hot rolling is performed by omitting the breakdown process, the raw material (powder containing Fe, Ni, Cr β-stabilizing element of the rolled surface of the titanium slab as cast as a pre-process of hot rolling. , Wire, foil) can be spread or sprayed, and the slab surface layer can be melted together with the raw material to form a β-stabilizing element rich layer on the slab surface layer, so that the surface properties after hot rolling can be kept good. I found.

That is,
(1) An industrially pure titanium cast slab having a depth of at least 1 mm melted and re-solidified from the surface layer corresponding to the rolling surface, and from the surface layer in the melt-resolidified layer to a depth of 1 mm, β A titanium slab for hot rolling, characterized in that the concentration of Fe as a stabilizing element is 0.10 mass% or more and 1.50 mass% or less.
(2) The titanium slab for hot rolling according to (1), wherein a part or all of the Fe is replaced with one or two of Ni and Cr which are β-stabilizing elements.
(3) The surface layer of at least the rolling surface of the slab as cast as pure industrial titanium is melted and re-solidified with a material containing Fe, Cr, Ni as β-stabilizing elements, (1 ) Or the manufacturing method of the titanium slab for hot rolling as described in (2).
(4) The titanium slab for hot rolling according to (3), wherein the material containing Fe, Cr, Ni as β-stabilizing elements has a shape of powder, chip, wire, or foil. Production method.
(5) Electron beam heating or plasma heating is used as a means for melting a material containing Fe, Cr, Ni as β-stabilizing elements and a surface layer corresponding to a slab rolled surface (3) or (4) The manufacturing method of the titanium slab for hot rolling as described in any one of.

  The present invention is equivalent to conventional materials even when hot rolling is performed by omitting breakdown processes such as ingot rolling and forging, which were indispensable in the past when producing titanium coils from cast titanium slabs. The present invention relates to a hot-rolling titanium slab capable of producing a strip-like coil having a surface property and a method for producing the same, and includes a reduction in heating time by omitting a breakdown process and a cutting care associated with smoothing the slab surface by melting the surface. Reduction of the amount of cutting during pickling due to reduction, improvement of the surface properties of the strip coil, etc. can improve the yield, thereby reducing the manufacturing cost and immeasurable industrial effects.

  The present invention will be described in detail below.

  Normally, industrial pure titanium is hot-rolled in the α single phase region in the temperature range below the β transformation point. If it is pure titanium with high purity, all below the β transformation point will be in the α single phase region, but industrial pure titanium is slightly added with a β-stabilizing element such as Fe as an alloy element, There is an α + β two-phase region. Fe, which is contained in a large amount in industrial pure titanium as compared with other β-stabilizing elements, is contained in 0.020 mass% in JIS type 1 and 0.500 mass% in JIS type 4.

  In the present invention, by heating only the surface layer portion of the slab as cast and melting a depth of 1 mm or more, the cross-sectional structure of the melted and re-solidified layer when cooled to room temperature after melting is fine needles. It becomes a state organization. And by melt | dissolving simultaneously with Fe at the time of surface layer melt | dissolution, Fe will be contained in a melt re-solidification layer, and a fusion | melting re-solidification layer can be made into a finer structure | tissue by the hardenability improvement by Fe. Also, by melting together with Fe, Fe is concentrated in the melt-resolidified layer, and this portion becomes an α + β two-phase region during hot rolling heating, and the growth of α grains is suppressed in this portion. It has been found that a strip-like coil having excellent surface properties can be produced without generating surface defects due to the fact that the crystal grains at the time of extension are kept fine.

  By melting and re-solidifying a slab surface layer depth of 1 mm or more as described above, a fine acicular structure having a depth of 1 mm or more from the surface layer is melted and re-solidified. Become an organization. At least the surface layer corresponding to the rolling surface of the slab is melted and resolidified together with the material containing Fe, so that the Fe concentration from the surface layer in the melted and resolidified layer to 1 mm depth is 0.10 mass% or more and 1.50 mass%. The following should be done. If the Fe concentration in this part is less than 0.10 mass%, the effect of improving the hardenability and the effect of suppressing the growth of crystal grains due to the addition of Fe cannot be sufficiently obtained, and surface flaws are generated in the strip-like coil after hot rolling. . Further, if the Fe concentration in this region is within the above range, it is concentrated in the melted and re-solidified layer by hot-rolling and subsequent processes such as surface layer cutting by the shot pickling process and diffusion of Fe by the annealing process. Fe is rendered harmless. However, if the Fe concentration is higher than 1.50 mass%, the ratio of the β phase of the slab surface layer increases during hot rolling, and the slab surface layer portion is oxidised vigorously, resulting in a significant decrease in yield. In addition, since it becomes difficult to render the Fe concentrated layer harmless in the subsequent process, the Fe concentration from the surface layer to a depth of 1 mm is set to 1.50 mass% or less. When a melt resolidified layer having a depth of 1 mm or more is formed from the surface layer, the component composition from the surface layer to the depth of 1 mm is almost uniform. In addition, although the melt depth is set to 1 mm or more, there is a concern that a concentrated layer of Fe may remain after the shot pickling process or the annealing process if the melt depth becomes too deep, so the melt depth is about 5 mm. Is desirable.

  It has been found that the material that melts together with the surface layer of the slab as cast is not limited to Fe, and the same effect can be obtained if it is a material containing a β-stabilizing element. Therefore, a part or all of Fe can be replaced with a β-stabilizing element, and Mo or the like may be used as the β-stabilizing element. However, since Mo is expensive, Fe, Ni which is a β-stabilizing element and is relatively inexpensive. It is preferable to use Cr. As described above, since the β-stabilizing element includes Ni and Cr, it is effective to use a stainless steel material such as stainless steel powder. At this time, instead of the Fe concentration from the surface layer to a depth of 1 mm, the total content of each element ([mass% Fe] + [mass% Cr] + [mass% Ni]) is 0.10 mass% or more, and It may be 1.50 mass% or less. In addition, although it concentrates on industrially practical three elements of Fe, Ni, and Cr, it is effective even if it uses other (beta) stabilization elements. Even when a part of Fe is replaced with one or two of Ni and Cr, by making the total content of each element not less than 0.10 mass% and not more than 1.50 mass%, good surface properties can be obtained. A strip coil could be obtained.

  The material used for adding the β-stabilizing element Fe, Cr, Ni to the slab surface layer may be any of powder, chip, wire, and foil. It is effective to use a material with a particle size of 1 μm to 0.5 mm, a chip with a size of 2 mm square to 5 mm square, a wire with φ0.5 mm to φ5 mm, and a foil with a film thickness of 1 μm to 0.1 mm. It is. These materials can be uniformly added to the surface of the slab by placing them uniformly on the surface of the slab when they are placed or dispersed on the surface of the slab, and a strip coil having a better surface property can be obtained.

  In addition, there are methods such as electron beam heating, arc heating, laser heating, and induction heating for melting the surface layer together with β-stabilizing elements Fe, Cr, and Ni. Titanium is an active metal and is used in the atmosphere. When the surface layer is melted, the melted part is significantly oxidized. Therefore, an inert gas such as electron beam heating or arc heating (especially plasma arc heating or TIG (Tungsten Inert Gas) welding) that can be processed in a vacuum atmosphere or an inert gas atmosphere is used. The heating method used), laser heating, and the like are suitable, and the above-described treatment can be performed by any method. Among them, electron beam heating or plasma arc heating capable of imparting high energy at a time is industrially suitable, and these methods are preferably used.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  In the examples and comparative examples shown in Table 1, the titanium slab was manufactured using a rectangular mold by electron beam melting. A hot-rolled sheet having a thickness of 4 mm was manufactured from an ingot having a thickness of 200 mm, a width of 1000 mm, and a length of 4500 mm by hot rolling. As the slab variety, industrially pure titanium JIS type 1 was used. In addition, as a material containing a β-stabilizing element, for Fe, one of powder (particle size 100 μm), chip (2 mm square, 1 mm thickness), wire (φ1 mm), foil (20 μm) is used, Cr , Ni were austenitic stainless steel SUS304 and ferritic stainless steel SUS430 stainless powder (particle size 100 μm). In either case, a raw material containing Fe, Cr, Ni, which is a β-stabilizing element, is placed or sprayed on the as-cast surface of the slab as cast, and the surface of the slab is heated from above. By scanning the heated portion with a beam and a plasma arc, the entire surface of the slab rolled surface was processed so that the material containing Fe, Cr, Ni and the unmelted portion of the slab rolled surface did not remain. In addition, as-cast slabs have a relatively good casting surface, so that no unmelted residue due to the casting surface occurs when the surface layer melts. Moreover, the raw material containing Fe, Ni, and Cr was uniformly dispersed on the slab rolling surface so that Fe, Cr, and Ni were uniformly added to the inside of the slab. The method of measuring the depth of the melt-resolidified layer is to measure the thickness of the layer that has become a fine needle-like structure by observing with an optical microscope the part of the slab that has been melt-resolidified and cut and polished and etched. did. At this time, an analytical sample is taken from within 1 mm of the surface layer of any 10 places on the rolling surface of the slab, ICP emission spectroscopic analysis is performed, and the average value of 10 places is taken to obtain the concentration of Fe, Cr, Ni. investigated. In addition, the occurrence of surface defects was evaluated by visually observing the coil surface after hot rolling and pickling the hot-rolled sheet. In the comparative example in which the surface layer was not melted, the analysis sample was collected from within 1 mm of the surface layer, and in the comparative example having a melt resolidification layer thickness of less than 1 mm, the analysis sample was collected from within the melt resolidification layer.

  First, it describes about the result about the slab which fuse | melted the surface layer with the raw material containing Fe.

  No. 1, no. In the reference example 2 and the comparative example, hot rolling is performed without performing the melting treatment of the surface layer. Since the melting treatment is not performed, the Fe concentration up to 1 mm of the surface layer is equal to the base material Fe concentration. No. The reference example 1 is a case in which the ingot rolling is performed in the same manner as a normal titanium ingot. Partial rolling was performed from a thickness of 200 mm to 100 mm, then reheating and hot rolling to 4 mm. Since the batch rolling was performed, there was no abnormality in the surface properties after hot rolling. No. The comparative example of 2 is a case where no lump rolling was performed. Since the bulk rolling was not performed, coarse surface defects were generated on the hot-rolled sheet after pickling.

  No. 3, no. The comparative example 4 is a case where powder is used as a material containing Fe and the surface layer of the rolled surface is melted by electron beam heating. No. The comparative example 3 is a case where the Fe concentration up to 1 mm of the surface layer is 0.09 mass% and the depth of the molten resolidified layer is 2 mm. Since the Fe concentration was lower than 0.10 mass%, partially coarse wrinkles were generated on the surface of the hot-rolled sheet after pickling. No. The comparative example 4 is a case where the Fe concentration in the molten resolidified layer is 0.24 mass% and the depth of the molten resolidified layer is 0.5 mm. Since the depth of the melt resolidified layer was shallower than 1 mm, partially coarse wrinkles were generated on the surface of the hot-rolled sheet after pickling. No. 3, no. The comparative example of No. 4 Compared with the comparative example shown in 2, the surface properties of the hot-rolled sheet were improved, but somewhat large surface defects were generated and the quality was insufficient.

  No. 5 to No. In the thirteenth embodiment, electron beam heating is used as a melting method of the slab surface layer, and the hot rolling test is performed by changing the shape of the material containing the β stabilizing element.

  No. 5 to No. In Example 7, powder is used as a material containing Fe.

  No. In Example 5, the Fe concentration up to 1 mm on the surface layer is 0.10 mass%, and the depth of the molten resolidified layer is 3 mm. In the hot-rolled sheet after pickling, a slightly coarse surface flaw occurred, but this was an acceptable level. 3, no. Compared with Comparative Example 4, the surface property was very good.

  No. Example 6 is a case where the Fe concentration up to 1 mm of the surface layer is 0.89 mass% and the depth of the molten resolidified layer is 1 mm. No. In Example 7, the Fe concentration up to 1 mm of the surface layer is 1.50 mass%, and the depth of the molten resolidified layer is 5 mm. No. 6, no. In Example 7, the surface wrinkle after pickling was slight, and a very good surface property was obtained.

  No. No. 8 to no. In 13 examples, the surface layer was heated so that the depth of the melt-resolidified layer of the slab surface layer was 3 mm. No. 8, no. In the embodiment of No. 9, a chip is used as the material containing Fe, 10, no. In the 11th embodiment, the wire is 12, no. In the thirteenth embodiment, foil is used. No. No. 8 to no. In Example 13, the Fe concentration up to 1 mm on the surface layer was 0.10% or more, and the surface defects of the hot-rolled sheet were of an acceptable level and slight.

  From the above results, the surface property of the hot-rolled sheet was good even when any of powder, chip, wire, and foil was used as the shape of the material containing the β-stabilizing element.

  No. 14, no. In the 15th Example, the powder is used as a raw material containing (beta) stabilization element, The hot rolling test is done by changing the melting method of a slab surface layer. No. 14, no. In 15 examples, plasma arc heating was used as the melting method of the slab surface layer, and the depth of the melted and resolidified layer was 4 mm. No. 14, no. In 15 examples, the surface defects of the hot-rolled sheet after pickling were slight and very good. From the above results, good results were obtained for the surface properties of the hot-rolled sheet, regardless of whether electron beam heating or plasma arc heating was used as the melting method of the slab surface layer.

  Next, the result in the case of using stainless steel containing Cr and Ni in addition to Fe will be described.

  No. 16 to No. In 19 comparative examples and examples, SUS304 powder was used as stainless steel, and the surface layer was melted by electron beam heating, so that the depth of the molten resolidified layer was 2 mm. No. In Comparative Example 16, the total content of Fe, Cr and Ni up to 1 mm on the surface layer was less than 0.10 mass%, and the surface defects of the hot-rolled sheet were coarse. No. 17 to No. In 19 examples, the total content of Fe, Cr, and Ni up to 1 mm on the surface layer is 0.10 mass% or more, and the surface defects of the hot-rolled sheet are of an acceptable level and minor. It was.

  No. 20 to No. In Comparative Example 23 and Example 23, SUS430 powder was used as stainless steel, and the depth of the melted and resolidified layer was 2 mm by melting the surface layer by electron beam heating. No. In 20 comparative examples, the total content of Fe, Cr, and Ni up to 1 mm on the surface layer was less than 0.10 mass%, and the surface defects of the hot-rolled sheet were coarse. No. 21 to No. In the example of 23, the total content of Fe, Cr, Ni up to 1 mm of the surface layer is 0.10 mass% or more, and the surface defects of the hot-rolled sheet are of an acceptable level and a slight one. It was.

Claims (5)

  It is a cast slab of industrial pure titanium, and at least 1 mm deep from the surface layer corresponding to the rolling surface is melted and re-solidified, and the β-stabilizing element from the surface layer in the molten re-solidified layer to 1 mm depth A titanium slab for hot rolling, wherein the concentration of Fe is 0.10 mass% or more and 1.50 mass% or less.   The titanium slab for hot rolling according to claim 1, wherein a part or all of the Fe is replaced with one or two of Ni and Cr as β-stabilizing elements.   The surface layer of at least the rolling surface of the slab as cast as pure titanium for industrial use is melted and re-solidified together with a material containing Fe, Cr, Ni as β-stabilizing elements. Item 3. A method for producing a titanium slab for hot rolling according to Item 2.   The method for producing a titanium slab for hot rolling according to claim 3, wherein the material containing Fe, Cr, Ni as β-stabilizing elements has a shape of powder, chip, wire, or foil.   5. The electron beam heating or plasma arc heating is used as means for melting a material containing Fe, Cr, Ni as β-stabilizing elements and a surface layer corresponding to a slab rolling surface. Of manufacturing a titanium slab for hot rolling.