JP5505214B2 - High corrosion resistance titanium alloy having a large 0.2% proof stress in the rolling direction and its manufacturing method - Google Patents

Description

  The present invention is a titanium alloy with excellent corrosion resistance that is used in severe corrosive environments such as non-oxidizing acids and gaps, does not contain noble metal elements such as Pd and Ru, and has a sufficient rolling direction of 0.2%. The present invention relates to an alloy having yield strength and workability and a method for producing the same.

  Pure titanium exhibits high corrosion resistance in the environment of oxidizing acids such as nitric acid and chromic acid, seawater and chloride ion-containing solutions, but in non-oxidizing acids such as hydrochloric acid and sulfuric acid, so-called gaps are formed in the gaps. May cause corrosion. As an alloy improved in this respect, an alloy added with a noble metal element such as Pd or Ru (Ti-0.2% Pd; ASTM standard grades 7, 11, Ti-0.5% Ni-0.05% Ru; ASTM standards grades 13, 14, 15) are used. On the other hand, in Patent Document 1, although it is slightly inferior to a noble metal-added alloy, as an inexpensive alloy excellent in crevice corrosion resistance, an alloy in which Ni and Mo are added in combination (Ti-0.8% Ni-0.3% Mo). , ASTM standard grade 12). This alloy is used in various chemical plants such as a desalinator, a salt evaporator, and a chlorine electric field tank used in a hot brine environment. In such an application, since it is processed into a welded tube, workability (elongation) is required, and strength as a structure is also required.

  Patent Documents 2 to 4 describe a production method for suppressing the formation of a macroscopic heterogeneous structure of pure titanium. Patent Document 5 discloses a Ti-4.0-5.0% Al-2.5-3.5% V-1.5-2.5% Fe-1.5-2.5% Mo alloy. Describes a method for producing a high toughness titanium alloy. Patent Document 6 describes a method for producing a titanium alloy sheet having a surface roughness after acid treatment and surface waviness, which are below a reference value. Patent Document 7 discloses that Fe is 0.8 to 2.3%, N is 0.05% or less, O is an oxygen equivalent value Q = [O] +2.77 [N] +0.1 [Fe. ] Of 0.45 to 1.00 is described. Patent Document 8 describes that in a Ti-20V-4Al-1Sn alloy, anisotropy of mechanical properties is reduced by hot cross rolling at a temperature not higher than the β transformation point. Patent Document 9 describes a method of improving elongation and workability by hot cross rolling below the β transformation point of a Ti—Fe—O—N titanium alloy sheet. Patent Documents 10 and 11 describe the improvement of impact absorption characteristics of Ti—Fe—Ni—Cr alloys.

Japanese Patent Publication No.54-8529 Japanese Patent No. 4094292 Japanese Patent No. 4094244 Japanese Patent No. 3967897 Japanese Patent No. 4019668 Japanese Patent No. 3535027 JP-A-8-295969 JP 2010-82688 A Japanese Patent Laid-Open No. 11-61297 JP 2003-147462 A JP 2001-262257 A

  The Ti-0.8% Ni-0.3% Mo alloy described in Patent Document 1 has a small 0.2% proof stress in the rolling direction (hereinafter referred to as L direction), and is a direction orthogonal to the L direction and the rolling direction (hereinafter referred to as “lower direction”). There is a problem that the difference in 0.2% proof stress in the (T direction) is large. This alloy is used for piping in various chemical plants, such as pipes for wet heat exchangers containing brine containing aggregated chloride and sulfurous acid gas, so it must have sufficient proof stress and workability. Therefore, it is necessary to make the rolling direction anisotropy of 0.2% proof stress as small as possible.

  In Patent Document 1, only the component ranges of Ni, Mo, and Fe are defined, and there is no description about the manufacturing method. For the mechanical properties, only the ultimate tensile strength and the elongation are described. There is no description about 2% yield strength.

  In Patent Documents 2 to 4, it relates to a pure titanium drum electrode for copper foil production, and refers only to controlling the texture by cross rolling in order to prevent the appearance of a heterogeneous macrostructure on the surface, The effect on mechanical properties is not described.

Patent Document 5 discloses a high-strength α + β alloy having a 0.2% proof stress of 870 MPa or more, Ti-4.0 to 5.0% Al-2.5 to 3.5% V-1.5 to 2.5. % Fe-1.5-2.5% Mo alloy, which is greatly different from Ti-0.8% Ni-0.3% Mo alloy mainly composed of α phase and Ti ₂ Ni phase. .

  Patent Documents 6 and 7 reduce the in-plane anisotropy of workability such as bending characteristics when a plurality of titanium alloy materials are stacked and rolled by rolling to produce a sheet material. .8% Ni-0.3% Mo alloy is greatly different from the present invention in which improvement of 0.2% yield strength in the L direction is a problem.

  In the present invention, among the directions parallel to the plate surface, the direction with the minimum 0.2% yield strength is defined as the minimum yield strength direction, and the direction perpendicular to the minimum yield strength direction is defined as the orthogonal yield strength direction. Ti-0.8% Ni-0.3% Mo, characterized in that the yield strength is 363 MPa or more and 400 MPa or less, and the difference in 0.2% yield strength between the minimum yield strength direction and the orthogonal yield strength direction is less than 180 MPa. It is an object to provide an alloy (ASTM standard grade 12) and a method for producing the same. 363 MPa is a large value with a margin with respect to the standard value of 0.2% proof stress of ASTM standard grade 12. If the difference in 0.2% proof stress between the minimum proof strength direction and the orthogonal proof strength direction is less than 180 MPa, there is no problem even if the material is collected from the base material without worrying about the in-plane anisotropy of the proof stress.

Patent Document 8 is for reducing the ductility anisotropy of β-type titanium alloys such as Ti-20% V-4% Al-1% Sn, and Patent Document 9 further describes Ti-Fe-O. This relates to an N-based alloy. In these inventions, in the 0.2% proof stress and the rolling conditions, Ti-0.8, which is the object of the present invention, is excellent in crevice corrosion resistance, and has an α phase and a Ti ₂ Ni phase as main constituent phases. It is very different from the% Ni-0.3% Mo alloy. Further, Patent Documents 10 and 11 relate to shock absorption, and are greatly different from the object of the present invention.

  The inventors have increased the 0.2% proof stress in the L direction of Ti-0.8% Ni-0.3% Mo alloy, have sufficient ductility, and are anisotropic in the rolling direction with 0.2% proof stress. As a result of earnest research, we considered that the oxygen concentration, oxygen equivalent, hot rolling method, especially the heating temperature of hot rolling, reduction ratio and cross rolling ratio are important in order to reduce the properties. (Ni, Mo, O, Fe, N) is appropriately selected, the heating temperature during hot rolling is performed within a certain range, the reduction ratio is set to a certain value or more, and cross rolling is performed during hot rolling to obtain a cross ratio. It has been found that the above-mentioned target can be solved by setting the annealing temperature to a certain temperature range when the annealing temperature is less than a certain value and annealing is performed after hot rolling. However, in the present invention, the reduction ratio is a numerical value obtained by dividing the rolling start sheet thickness by the rolling end sheet thickness, and the cross ratio is the reduction ratio in one rolling direction of the cross rolling and the other orthogonal to the one rolling direction. It is defined as the ratio to the rolling ratio in the rolling direction (however, the larger of the two rolling ratios is the numerator).

The present invention is based on such knowledge, and the gist thereof is as follows.
(1) In mass%, Ni: 0.7% or more, 0.9% or less, Mo: 0.20% or more, 0.40% or less, O: 0.10% or more, 0.20% or less, Fe : 0.02% or more, 0.10% or less, N: 0.001% or more and 0.010% or less, and defined by the following formula (1) Q: 0.20% or more, 0.30% It consists of the remainder titanium and inevitable impurities,
Of the directions parallel to the plate surface, the direction with the minimum 0.2% proof stress is the minimum proof stress direction, the direction orthogonal to the minimum proof stress direction is the orthogonal proof stress direction, and the 0.2% proof stress in the minimum proof stress direction is 363 MPa or more, the minimum A highly corrosion-resistant titanium plate, characterized in that the elongation in the proof stress direction and the orthogonal proof stress direction is 23% or more, and the difference in 0.2% proof stress between the minimum proof stress direction and the orthogonal proof stress direction is less than 180 MPa.
Q = [O] +2.77 [N] +0.1 ([Fe] + [Ni]) (1)
However, [O], [N], [Fe], and [Ni] mean the content (mass%) of each element.
(2) A hot slab having a component content specified in (1) above is heated and rolled at a temperature of 800 ° C. or higher and 840 ° C. or lower and then hot-rolled at a reduction ratio of 4.0 or higher and 25.0 or lower. And the cross rolling is performed during the same hot rolling, and the ratio between the rolling ratio in one rolling direction and the rolling ratio in the other rolling direction orthogonal to the one rolling direction is defined as a cross ratio, and the cross ratio is The denominator of the cross ratio is determined to be 1 or more, and hot rolling is performed so that the cross ratio is 1.0 or more and less than 10.0, and the high corrosion resistance according to (1) above A method for producing a titanium plate.
(3) The method for producing a highly corrosion-resistant titanium plate according to (2) above, wherein annealing is performed in a temperature range of 550 ° C. or more and less than 750 ° C. following the hot rolling.

  According to the present invention, Ti-0.8% Ni has a sufficiently high 0.2% proof stress in the minimum proof stress direction, sufficient ductility, and a small difference between the 0.2% proof stress in the minimum proof stress direction and the orthogonal proof stress direction. -It became possible to provide 0.3% Mo alloy, and it became possible to manufacture pipes for heat exchangers in humid environments containing brine and sulfite gas containing aggregated chlorides with excellent durability. The spread to the environment is greatly advanced, and the contribution to the environment and industry is extremely remarkable.

  In the ASTM standard grade 12 component range, the present invention uses Ni, Mo, O, Fe, and N as appropriate components shown below, and is intended to achieve a predetermined purpose under the manufacturing conditions specified below. It is.

  In the present invention according to claim 1, in mass%, Ni: 0.7% or more, 0.9% or less, Mo: 0.20% or more, 0.40% or less, O: 0.10% or more, 0.20% or less, Fe: 0.02% or more, 0.10% or less, N: 0.001% or more, 0.010% or less, Q defined by the above formula (1): 0.20 %, 0.30% or less, consisting of the remaining titanium and inevitable impurities, and the direction parallel to the plate surface, the direction with the smallest 0.2% proof stress is the minimum proof stress direction, and the direction perpendicular to the minimum proof stress direction In the orthogonal proof direction, the 0.2% proof stress in the minimum proof stress direction is 363 MPa or more, the elongation in the minimum proof stress direction and the orthogonal proof stress direction is 23% or more, and the difference in 0.2% proof stress between the minimum proof stress direction and the orthogonal proof stress direction Was a high corrosion resistance titanium plate characterized by being less than 180 MPa.

  The reasons for limiting the amounts of Ni, Mo, O, Fe, and N are as follows.

Ni contains crevice corrosion resistance in a non-oxidizing acid by containing 0.7% or more and 0.9% or less by mass%. This is due to the precipitation of Ti ₂ Ni which controls crevice corrosion against non-oxidizing acids. By containing 0.7 to 0.9% of Ni, an appropriate amount of Ti ₂ Ni is precipitated, and this strengthens the oxide film on the surface of the titanium alloy, so that excellent crevice corrosion resistance is obtained. If the Ni content is less than 0.7%, sufficient crevice corrosion resistance cannot be obtained, and if it exceeds 0.9%, the elongation becomes less than 23%, which causes troubles in processing such as welded pipe manufacturing. .

  Mo was 0.20% to 0.40% by mass. Mo has the effect of reducing the corrosion rate, and the combined addition with Ni makes it possible to raise the crevice corrosion resistance to a level close to that of a noble metal-added alloy. If it is less than 0.20%, sufficient crevice corrosion resistance cannot be obtained, and if it exceeds 0.40%, workability is affected, so 0.20 to 0.40%.

  The crevice corrosion resistance was investigated by pressing a test piece having a plate thickness × 25 mm × 25 mm and a hole having a diameter of 10 mm in the center with a gap forming material made of ethylene tetrafluoride and immersing it in a boiling NaCl solution for 168 hours. Visually evaluate the presence or absence of corrosion products.

  O was 0.10% or more and 0.20% or less. O is important as an element for controlling mechanical properties. The 0.2% proof stress in the minimum proof stress direction does not reach 363 MPa when O is less than 0.10%, and when it exceeds 0.20%, the elongation becomes less than 23%, which causes troubles in processing such as welded pipe manufacturing. The

  In this alloy, Fe and N are also elements for controlling the mechanical strength. As a single content, in order to make Fe less than 0.02%, a high-purity raw material must be used, which leads to an increase in cost, so 0.02% or more, and exceeding 0.10% In order to adversely affect the room temperature ductility, the content was made 0.10% or less. The reason why N is also less than 0.001% is that a high-purity raw material must be used, and if it exceeds 0.010%, the room temperature ductility is adversely affected.

Q, which is defined by the following formula (1), is used as an index indicating the strengthening ability of elements including elements other than O that affect the mechanical properties. Q is an oxygen equivalent value. Q is an equation in which the strengthening ability of O, N, Fe, and Ni is linearly combined with O being 1 and Ni has the strengthening ability equivalent to Fe, so the coefficient is the same as that of Fe. In the present invention, the Q range is set to 0.20% or more and 0.30% or less. If Q is less than 0.20, sufficient 0.2% proof stress in the minimum proof stress direction cannot be obtained, and if it exceeds 0.30%, the elongation is insufficient and the workability is affected.
Q = [O] +2.77 [N] +0.1 ([Fe] + [Ni]) (1)
However, [O], [N], [Fe], and [Ni] mean the content (mass%) of each element.

  The balance is titanium and inevitable impurities. Typically, H and C are inevitable impurities, and both are 0.02% or less.

  In ASTM standard grade 12, the 0.2% yield strength is 345 MPa or more, but exceeds this value with a margin of 5% or more, so the 0.2% yield strength in the minimum yield direction was set to 363 MPa or more. This is sufficient as the strength required for a structure such as chemical plant piping. The elongation in the minimum proof stress direction and the orthogonal proof stress direction was set to 23% or more in order to secure the ductility necessary for rounding a plate having a thickness of 5 to 10 mm to process it as a welded pipe. Further, in order to reduce the in-plane anisotropy of the proof stress, the difference in 0.2% proof stress between the minimum proof stress direction and the orthogonal proof stress direction was set to less than 180 MPa.

  Next, the manufacturing method of the high corrosion resistance titanium plate of the present invention will be described.

  The highly corrosion-resistant titanium plate of the present invention is usually produced through steps of melting, slabbing, hot rolling and, if necessary, annealing. In the hot rolling step in the manufacturing process, the slab is heated and held at 800 ° C. or higher and 840 ° C. or lower, and then rolled at a reduction ratio of 4.0 or higher, and cross-rolled. It was set as the manufacturing method characterized by rolling so that it might become 0 or more and less than 10.0. Here, the ratio of the rolling ratio in one rolling direction in cross rolling and the rolling ratio in the other rolling direction orthogonal to the one rolling direction is defined as a cross ratio, and the cross ratio is set so that the cross ratio is 1 or more. Define the denominator numerator. That is, of the two rolling reduction ratios, the larger rolling reduction ratio is defined as a numerator.

  These rules for rolling temperature, reduction ratio, and cross ratio are all for obtaining a sufficiently large minimum proof stress direction 0.2% proof stress. The Ti-0.8 mass% Ni-0.3 mass% Mo alloy has a β-phase ratio of about 50% or more at a high temperature exceeding 850 ° C. By carrying out hot rolling at 840 ° C. or lower, a sufficient work structure is introduced in a temperature range where the proportion of α phase is high even when processing heat is generated, so avoiding a decrease in 0.2% proof stress in the rolling direction, and The difference between the 0.2% proof stress in the direction orthogonal to the rolling direction can be reduced. On the other hand, when the temperature is lower than 800 ° C., a lot of soot is introduced during hot rolling, and a great deal of labor is required to remove it. As the hot rolling temperature, a temperature range of 820 ± 10 ° C. is desirable.

  The reason why the reduction ratio is 4.0 or more is that when the reduction ratio is less than 4.0, a sufficient work structure is not introduced, and the rolling direction proof stress is low. Although there is no upper limit in particular, it was made 25.0 or less as a level which can be rolled.

  In the titanium plate manufactured by performing cross rolling by hot rolling as in the present invention, the rolling direction (hereinafter also referred to as “L direction”) having the larger rolling reduction ratio matches the minimum proof stress direction. The rolling direction with the smaller rolling reduction (hereinafter also referred to as “T direction”) coincides with the orthogonal proof stress direction. Therefore, in defining the highly corrosion resistant titanium plate of the present invention, the minimum proof stress direction can be read as the L direction and the orthogonal proof stress direction can be read as the T direction.

  As described above, by taking a rolling temperature and a reduction ratio within a certain range and rolling the cross rolling so that the cross ratio is 1.0% or more and less than 10.0, 0.2% proof stress can be obtained. In addition, the difference in 0.2% proof stress between the minimum proof stress direction and the orthogonal proof stress direction can be made less than 180 MPa. When a Ti-0.8 mass% Ni-0.3 mass% Mo alloy is rolled in one direction, a texture is formed that is easily deformed in the rolling direction and is unlikely to deform in the direction perpendicular to the rolling direction. The deformation in the rolling direction becomes easy, and the 0.2% proof stress in the minimum proof stress direction is lowered. On the other hand, if cross rolling is performed in the direction perpendicular to the rolling direction, the texture is easily deformed in the rolling direction and difficult to deform in the direction perpendicular to the rolling direction, so the 0.2% yield strength in the minimum yield strength direction is improved. And the difference in 0.2% proof stress between the minimum proof stress direction and the orthogonal proof stress direction can be reduced. It should be noted that the 0.2% yield strength is sufficiently improved only when the rolling temperature of 800 to 840 ° C., the reduction ratio of 4.0 to 10.0, the cross ratio of 1.0 or more and less than 10.0 are satisfied, and Reduction of 0.2% proof stress difference between minimum proof stress direction and orthogonal proof stress direction is obtained, 0.2% proof stress in minimum proof stress direction is 363 MPa or more, and 0.2% proof stress difference between minimum proof stress direction and orthogonal proof stress direction Is less than 180 MPa. By the way, the cross ratio 1.0 is the minimum value of the cross ratio, and is the most effective cross ratio for improving 0.2% proof stress in the minimum proof stress direction. Is preferably 1.5 or more, more preferably 2.0 or more.

Although the high corrosion resistance titanium plate of the present invention can be used as it is in hot rolling, it is desirable to perform annealing in a temperature range of 550 ° C. or more and less than 750 ° C. following hot rolling. By performing annealing at a temperature range of 550 ° C. or more and less than 750 ° C., an appropriate amount of Ti ₂ Ni that controls crevice corrosion resistance of the Ti-0.8% Ni-0.3% Mo alloy can be precipitated. The reason why the temperature is set to 550 ° C. or higher is that sufficient recrystallization is not performed at a temperature lower than this temperature and the room temperature ductility is small, and the temperature lower than 750 ° C. is that the precipitation amount of Ti ₂ Ni decreases at a temperature higher than this. This is because the crevice corrosion resistance decreases. From the viewpoint of recrystallization and sufficient Ti ₂ Ni precipitation, annealing at 650 to 700 ° C. is desirable. The annealing time may be 30 minutes to 1 hour when the entire material reaches a predetermined temperature.

  Hereinafter, an example is given and the composition and operation effect of the present invention are explained more concretely.

  A titanium material having the composition shown in Table 1 is melted by vacuum arc melting or electron beam melting, and this is made into a slab by hot forging, held at the heating temperature shown in Table 1, and then hot-rolled. A 30 mm thick plate was used. Then, when annealing, it annealed for 30 minutes at the temperature shown in Table 1, shot blasting, and pickling, and it was set as the test material. The example which put "-" in the annealing temperature column does not anneal.

  Test pieces of JIS No. 14 and No. 15 were cut out from these test materials, and it was confirmed that the rolling direction (L direction) with the larger rolling reduction of the cross rolling coincided with the minimum proof stress direction. Thus, the L direction was selected as the minimum proof stress direction, and the room temperature tensile test in the minimum proof stress direction and the orthogonal proof stress direction was performed with the T direction orthogonal to the L direction as the orthogonal proof stress direction. Also, a corrosion test piece having a plate thickness × 25 mm × 25 mm and a hole with a diameter of 10 mm in the center was cut out, pressed with a gap forming material made of tetrafluoroethylene, immersed in boiling NaCl solution for 168 h, and presence or absence of corrosion products. The crevice corrosion resistance was evaluated. Both the tensile test and the crevice corrosion resistance test were performed at n = 3. The mechanical test values shown in Table 1 are average values.

  The measurement results are summarized in Table 1.

  In Table 1, no. 1 to No. 12 is an embodiment of the present invention. In any case, the 0.2% proof stress in the minimum proof stress direction is 363 MPa or more, the difference between the 0.2% proof stress in the minimum proof stress direction and the orthogonal proof stress direction is less than 180 MPa, and the elongation in the minimum proof stress direction and the orthogonal proof stress direction is 23%. That was all. In addition, no crevice corrosion was observed with respect to crevice corrosion resistance. On the other hand, No. whose hot rolling temperature is higher than 840 ° C. In No. 13, the 0.2% proof stress in the minimum proof stress direction is as small as 349 MPa, and the difference in 0.2% proof stress between the minimum proof stress direction and the orthogonal proof stress direction is also 195 MPa, and the anisotropy in the rolling direction is large. No. whose hot rolling temperature is lower than 800 ° C. In 14, the minimum elongation in the orthogonal proof stress direction is small. No. with a cross ratio greater than 10.0. 15, the 0.2% proof stress in the minimum proof stress direction is smaller than 363 MPa, the difference between the 0.2% proof stress in the minimum proof stress direction and the orthogonal proof stress direction is also larger than 180 MPa, and the rolling direction anisotropy is remarkable. The No. reduction ratio is smaller than 4.0. 16, the 0.2% proof stress in the minimum proof stress direction is smaller than 363 MPa. No. with annealing temperature exceeding 750 ° C. In No. 17, in the crevice corrosion resistance test, the progress of corrosion was observed in one test piece among n = 3. Further, the contents of Ni, Mo, O and the Q value each exceed the upper limit of the present invention. In 18, the elongation does not reach 23% in both the minimum and orthogonal strength directions. In addition, the contents of Ni, Mo, O and the Q value are respectively lower than the lower limit of the present invention. In No. 19, the 0.2% proof stress in the minimum proof stress direction was smaller than 363 MPa, and in the crevice corrosion resistance test, the progress of corrosion was observed in one test piece of n = 3. The content of Fe and N exceeded 0.10% and 0.010%, respectively, and Q value exceeded 0.30%. At 20, the minimum elongation in the orthogonal proof stress direction does not reach 23%.

  According to the present invention, a titanium alloy having a sufficient 0.2% yield strength, a 0.2% yield strength with small rolling direction anisotropy, excellent crevice corrosion resistance, and good ductility at room temperature is obtained, It becomes possible to manufacture heat exchanger pipes in a humid environment containing brine and sulfite gas containing aggregated chloride having excellent durability, and the use of pipes in various chemical plants is promoted.

Claims (3)

% By mass
Ni: 0.7% or more, 0.9% or less,
Mo: 0.20% or more, 0.40% or less,
O: 0.10% or more, 0.20% or less,
Fe: 0.02 or more and 0.10% or less,
N: 0.001% or more and 0.010% or less, Q defined by the following formula (1): 0.20% or more and 0.30% or less, comprising the balance titanium and inevitable impurities,
Of the directions parallel to the plate surface, the direction with the minimum 0.2% proof stress is the minimum proof stress direction, the direction orthogonal to the minimum proof stress direction is the orthogonal proof stress direction, and the 0.2% proof stress in the minimum proof stress direction is 363 MPa or more, the minimum A highly corrosion-resistant titanium plate, characterized in that the elongation in the proof stress direction and the orthogonal proof stress direction is 23% or more, and the difference in 0.2% proof stress between the minimum proof stress direction and the orthogonal proof stress direction is less than 180 MPa.
Q = [O] +2.77 [N] +0.1 ([Fe] + [Ni]) (1)
However, [O], [N], [Fe], and [Ni] mean the content (mass%) of each element.   The hot slab having a component content defined in claim 1 is heated and held at a temperature of 800 ° C. or higher and 840 ° C. or lower, and then hot-rolled at a reduction ratio of 4.0 or higher and 25.0 or lower, and Cross rolling is performed at the time of hot rolling, and the ratio between the rolling ratio in one rolling direction and the rolling ratio in the other rolling direction orthogonal to the one rolling direction is defined as a cross ratio, and the cross ratio becomes 1 or more. The method for producing a highly corrosion-resistant titanium plate according to claim 1, wherein the denominator of the cross ratio is determined as described above, and hot rolling is performed so that the cross ratio is 1.0 or more and less than 10.0. .   The method for producing a highly corrosion-resistant titanium plate according to claim 2, wherein annealing is performed in a temperature range of 550 ° C or higher and lower than 750 ° C following the hot rolling.