JP6432330B2 - Titanium plate and manufacturing method thereof - Google Patents

Description

  The present invention relates to a titanium plate excellent in the balance between strength and formability and a method for producing the same.

  In recent years, products have been made thinner in order to reduce the required amount of titanium material. Along with this, there is a growing need for higher strength titanium materials. In order to meet the need for higher strength, it is necessary to have excellent moldability at the same time. However, it is generally known that the formability deteriorates when the strength is high. For this reason, a titanium plate that satisfies both high strength and formability is desired.

  By the way, pure titanium is stipulated in JIS 1-4 by the content of Fe and O in the JIS standard. JIS type 1 with a low content of Fe and O has the lowest strength and excellent formability, and is therefore used as a plate material for plate heat exchangers that are pressed into complex shapes. In JIS 3-4 types, although the strength increases, the moldability is inferior. This is because solid solution strengthening by oxygen and stabilization of the β phase by Fe suppress grain growth at the α phase grain boundary, refine the structure, and make twin deformation difficult.

  As described above, it is difficult to satisfy the material characteristics in the trade-off relationship in the JIS 1-4 types defined in the JIS standard. Therefore, in order to respond to recent needs, studies have been made on titanium plates that achieve both high strength and excellent formability.

  For example, Patent Documents 1 to 4 disclose a manufacturing method that suppresses the content of Fe or Ni and enhances press moldability. However, although it is described that these contents are suppressed, the content of Fe exceeds 0.1%, or the content of Ni which is a β-phase stabilizing element similar to Fe is 0.5%. The chemical composition it contains is disclosed. If the content of Fe or Ni is large, the α-phase crystal grains may be small, so that twin deformation is difficult and sufficient formability cannot be obtained.

  Patent Document 5 discloses a chemical composition having a large content of Fe and O, and the α particle size does not reach 25 μm. For this reason, although the strength is high, twin deformation hardly occurs and sufficient formability cannot be obtained.

  Patent Documents 6 and 7 disclose examples of the invention in which the crystal grain size is less than 25 μm. If the crystal grain size is smaller than 25 μm, formability may be reduced.

  Patent Document 8 discloses a manufacturing method in which finish cold rolling with an elongation of 3.4 to 5.8% is performed after annealing to reduce the anisotropy of the r value. However, the moldability may deteriorate due to the large elongation rate.

  In Patent Document 9, the ratio between the Schmid factor of twin deformation during tension in the rolling direction and the Schmid factor of slip deformation during tension in the vertical direction of rolling is defined.

On the other hand, JIS 1 and 2 types of soft pure titanium materials are prone to mold galling during molding. Therefore, conventionally, a technique of forming a compound such as TiC, TiN or TiO ₂ on the surface has been adopted to increase the surface hardness in order to reduce the friction coefficient with the mold by press working and improve the formability. It was. However, the molded product may be wrinkled by nitrides or carbides. Thus, for example, Patent Document 10 discloses a technique for forming an oxide film exhibiting lubricity on the outermost surface of a titanium plate in order to suppress wrinkles while maintaining formability.

Japanese Patent No. 1696795 Japanese Patent No. 1593477 JP 2004-285457 A JP 2007-162070 A JP 2009-215601 A JP 2011-025269 A JP 2011-026649 A JP 9-216044 A JP 2012-214863 A Japanese Patent No. 5081570

  However, in molding such as press working, the direction of deformation differs depending on the mold. In the case of a thin plate, generally, plane deformation (deformation that projects only in one direction so as not to reduce the width) is most difficult to deform. For this reason, it is hard to say that the invention described in Patent Document 9 that uniquely defines the processing direction accurately reflects the molding such as realistic press processing.

  Further, in the titanium plate disclosed in Patent Document 10, since nitrides and carbides are located immediately below the oxide film, there is a problem in formability that may become a starting point of cracking during processing.

  As described above, the conventional titanium plate does not satisfy the recent severe needs of both high strength and formability by reducing the thickness of the product, and further improvement is necessary.

  It is an object of the present invention to provide a titanium plate that is high in strength but excellent in formability and a method for producing the same.

As a result of intensive studies, the present inventors have obtained the following knowledge.
(1) By performing temper rolling to apply shear stress to the surface of the titanium plate under predetermined conditions, twin deformation occurs in the surface layer and the surface hardness increases, so that both high strength and formability can be achieved. become.

  (2) Twin deformation tends to occur as the crystal grains become larger. In order to enlarge the crystal grains, it is necessary to set the component range in which the content of Fe, O, etc. is small and to optimize the annealing conditions.

  (3) On the other hand, twin deformation is a beneficial deformation during processing as well as slip deformation in pure titanium having a hexagonal crystal structure. If twin deformation is excessively performed, the subsequent formability deteriorates.

  (4) Therefore, by strictly adjusting the amount of twin deformation on the surface of the titanium plate and not forming a large amount of twin deformation inside the titanium plate, both excellent moldability is achieved while suppressing mold galling during processing. can do.

  (5) Further, nitrides and carbides are formed due to the remaining carbon component of the rolling oil used in cold rolling. The present invention is directed to a titanium plate that is subjected to salt dipping and pickling on the titanium plate after annealing to remove nitrides and carbides on the surface and suppress deterioration of formability due to these.

Here, the present invention is as follows.
(1) by mass%, Fe: 0.08% or less, O: 0.05-0.15%, the balance has a chemical composition consisting of Ti and inevitable impurities,
The average crystal grain size of the α phase is 25 to 100 m, in the cross section perpendicular to the rolling direction, the number of twins M (present), all crystal grains of the individual number when a N (pieces) The twin density (M ÷ N) satisfies the formula (1),
A value obtained by dividing the number of twins existing in the region from the surface to 25% of the plate thickness by the N is a twin density X, and a value obtained by dividing the number of twins existing in the region other than the region by the N. When the twin density is Y, the twin density X is twice or more the twin density Y , and the twin density Y is 0.25 or less .
The tensile strength in the rolling width direction is 340 MPa or more,
A titanium plate characterized by having an overhang height of 18 mm or more in a ball head overhang test conducted using a ball head punch with a diameter of 40 mm .

M ÷ N ≧ 0.05 (1)
In the present invention, the “number of twins” represents the number of combinations of line segments when the twin plane is observed as a line segment by cross-sectional observation of the titanium plate. For example, as shown in FIG. 1, there are two line segments shown on the twin plane, but in the present invention, this is defined as one combination.

  (2) It is a manufacturing method of the titanium plate as described in said (1) which performs hot processing, annealing, cold processing, final annealing, pickling, and temper rolling in order, The said final annealing is a batch type annealing furnace. Is used, it is held at a temperature range of 600 to 750 ° C. for 200 to 600 minutes, and when a continuous annealing furnace is used, it is held at a temperature range of 750 to 820 ° C. for 1 to 10 minutes. Is 0.5 to 3%, the roll roughness of the work roll is # 600 or more, the roll diameter is 700 mm or more, and is performed without lubrication.

  According to the present invention, a titanium plate having high strength and excellent formability and a method for producing the same can be provided.

FIG. 1 (a) is an SEM photograph of the microstructure after twin deformation of the titanium plate according to the present invention, and FIG. 1 (b) is an enlarged photograph of the dotted line region of FIG. 1 (a). FIG. 2 is a schematic cross-sectional view of a titanium plate according to the present invention.

The present invention will be described in detail. Hereinafter, “mass%” is simply referred to as “%”.
1. Titanium plate (1) Chemical composition (1-1) Fe: 0.08% or less Fe is a β-phase stabilizing element in the titanium material. Although a part of Fe is dissolved in the α phase, many are known to be dissolved in the β phase. That is, as the Fe content increases, the amount of β phase increases, and accordingly, α phase grain growth is suppressed. For this reason, when performing the final annealing described later, it takes a long time until the crystal grains become coarse. Although the setting of the upper and lower limits of the average crystal grain size will be described later, twin deformation, which is important for the processing of a titanium plate, becomes active when the average crystal grain size of the α phase is 25 μm or more. Therefore, the reason why the Fe content is set to 0.08% or less in the titanium plate of the present embodiment is to suppress the amount of β phase and facilitate the growth of α phase grains. Preferably it is 0.07% or less, More preferably, it is 0.06% or less.

(1-2) O: 0.05 to 0.15%
O activates twin deformation during the processing of the titanium plate. If the O content exceeds 0.15%, even if the crystal grain size of the α phase is adjusted, twin deformation is less likely to be active and formability deteriorates. Preferably it is 0.12% or less, More preferably, it is 0.10% or less. If the O content is less than 0.05%, not only is there a concern of high costs associated with the use of high-purity titanium sponge, but the strength of the titanium plate is reduced, and the product manufactured using the titanium plate It may be difficult to impart sufficient strength. Preferably it is 0.06% or more, More preferably, it is 0.07% or more.

(1-3) The balance is Ti and unavoidable impurities The balance other than the above is Ti and unavoidable impurities. In the manufacture of titanium plates, scrap raw materials are sometimes used from the viewpoint of promoting recycling. Since various impurity elements are inevitably mixed in the titanium plate, it is difficult to uniquely determine the content of the impurity elements. Therefore, the inevitable impurities in the present invention mean an element contained in an amount that does not impair the effects of the present invention. Examples of such inevitable impurities include N: 0.01% or less, C: 0.01% or less, and H: 0.01% or less.

(2) Ti metal structure (2-1) α-phase average crystal grain size: 25 to 100 μm
In the present invention, by forming the titanium plate so that the average crystal grain size of the α phase is 25 to 100 μm, the twin deformation activity becomes active and the formability is improved. When the average crystal grain size of the α phase is less than 25 μm, even if the amount of oxygen is sufficiently small, the twin deformation activity is suppressed and the formability may be lowered. In the α phase, which is a hexagonal close-packed structure with few slip systems, the presence or absence of twin deformation affects the plastic deformation ability. Therefore, by setting the average crystal grain size of the α phase to 25 μm or more, stress concentration occurs in the α phase crystal grain boundary and twin deformation is likely to occur. Preferably it is 30 micrometers or more, More preferably, it is 40 micrometers or more. On the other hand, when the average crystal grain size of the α phase is larger than 100 μm, there is a concern that rough skin is caused by press working or the like. Preferably it is 90 micrometers or less, More preferably, it is 80 micrometers or less.

  In the present invention, the average crystal grain size of the α phase represents the average value of the equivalent circle diameter of the α phase. Moreover, in this invention, in order to improve a moldability and the smoothness of a processed surface, it is preferable that it is an equiaxed (alpha) phase.

(2-2) Twin density (M ÷ N): satisfying formula (1) The titanium plate of the present invention has the number of twins in the range of 400 μm × 400 μm in a cross section perpendicular to the rolling direction. When M (the number) and the number of crystal grains are N (the number), the expression (1) needs to be satisfied.

M ÷ N ≧ 0.05 (1)
In the present invention, twins are introduced into the titanium plate by temper rolling described later. Titanium plates are difficult to slip because there are few slip systems. On the other hand, if the twin density is 0.05 or more before pressing or the like, slip deformation is likely to occur due to the presence of crystal grains rotated by twin deformation that occurs in temper rolling. Good moldability is obtained. In the present invention, the twin density is a value obtained by dividing the number of twins by the number of all crystal grains.

(2-3) Density of twins existing in the region from the surface to 25% of the plate thickness: more than twice the density of twins existing in the region other than the region, and twins in the “region other than the region” Crystal density: 0.25 or less In the present invention, the density of twins existing in the region from the surface to 25% of the plate thickness is at least twice the density of twins existing in the region other than the region. is necessary. FIG. 2 is a schematic cross-sectional view of the titanium plate 1 according to the present invention. The “region from the surface to 25% of the plate thickness” means a region (plate thickness) obtained by combining the region A (1/4 of the plate thickness) and the region B (1/4 of the plate thickness) indicated by reference numeral 10 in FIG. 1/2). “Up to 25%” does not include 25%. “Area other than the above-mentioned area” represents an area from 25% to the center of the plate thickness (1/2 of the plate thickness), and is a region other than the region A and the region B indicated by reference numeral 20 in FIG. C. That is, in the present invention, it is necessary to satisfy (Twin density in region A) + (Twin density in region B) ≧ (Twin density in region C) × 2. As a result, the surface becomes harder than the vicinity of the center of the plate thickness. Therefore, when processing such as press molding is performed, lubrication is good and press cracks are less likely to occur. In addition, since the surface is likely to undergo twinning deformation, good stretch forming is possible. Preferably, it is 2.5 times or more, more preferably 3 times or more.

  Even in the case of having a tilted structure as described above, if the twin density in the “region other than the region” is high, twin deformation hardly occurs during processing such as press working. The twin density in the “region other than the region” needs to be 0.25 or less. Although the lower limit is not particularly limited, it is preferable that the lower limit is 0.001 or more because a slight shearing force is applied to the center by temper rolling, and twins are slightly generated in this region. The twin density in the “region from the surface to 25% of the plate thickness” is not particularly limited as long as it is at least twice the twin density in the “region other than the region”.

2. Manufacturing method (1) Hot working, annealing, cold working An ingot to be used for hot working in the present invention is produced by ordinary arc melting. The obtained ingot is processed into a shape that can be hot-worked by cutting or the like, and hot-worked at a rolling rate of 50% or more in a temperature range of about 800 to 850 ° C. to produce a hot-rolled sheet. Then, after annealing in a temperature range of about 700 to 800 ° C., the same pickling treatment as before may be performed to remove the scale. And cold-rolling of 50-90% of a rolling rate is performed, and a 0.2-2.0-mm cold-rolled sheet is manufactured. Examples of the hot working method and the cold working method include rolling and forging.

(2) Final annealing Final annealing is performed to increase the crystal grain size of the cold-rolled sheet. In the present invention, either a batch annealing furnace or a continuous annealing furnace may be used, but annealing in a continuous annealing furnace is desirable from the viewpoint of productivity. When the batch annealing furnace is used, the final annealing is held at a temperature range of 600 to 750 ° C. for 200 to 600 minutes. Preferably it is 240-600 minutes, More preferably, it is 240-360 minutes. When using a continuous annealing furnace, hold | maintain for 1 to 10 minutes in the temperature range of 750-820 degreeC. If the annealing temperature or annealing time is out of these ranges, the β phase increases and the growth of the α phase is inhibited, so that it is difficult to increase the crystal grain size.

Moreover, what is necessary is just to perform an annealing atmosphere in a vacuum, air | atmosphere, or inert gas atmosphere from a viewpoint of suppressing the oxidation of titanium. The same applies to the annealing described above. Moreover, although the temperature raising / lowering speed | rate is not specifically limited, Generally in the case of a batch type annealing furnace, it is 0.4-5 degreeC / min, and in the case of a continuous type annealing furnace, it is about 50-2000 degreeC / min. When annealing is performed in the atmosphere, the oxide scale TiO ₂ is generated on the surface in the above temperature range and processing time, but there is almost no solid solution in the internal metal titanium. For this reason, after the oxide scale TiO ₂ is removed by pickling to be described later, the surface hardness becomes equal to the center of the plate thickness. Therefore, twins can also be introduced into the surface layer of a titanium plate that has been annealed, salt-immersed and pickled in air.

(3) Pickling After the final annealing, pickling is performed after the immersion in the salt to remove nitrides and carbides resulting from the rolling oil during cold working and oxidation scales by annealing in the air. The salt dipping is performed, for example, by dipping in an alkali molten salt containing NaOH at 450 to 550 ° C. as a main component. The pickling is performed under the same conditions as in the prior art using nitric acid. The salt dipping and pickling are not limited to these, and any conditions may be used as long as nitrides and carbides generated after the final annealing can be removed.

(4) Temper rolling In order to distribute more twins on the surface of the titanium plate after pickling, temper rolling is performed under the following conditions.

(4-1) Elongation rate: 0.5 to 3%
By setting the elongation to 0.5 to 3%, an excellent balance between strength and formability can be obtained. If it is less than 0.5%, such an effect is not exhibited. If it exceeds 3%, the twin density increases even in the range exceeding 25% of the plate thickness from the surface, and twin deformation is less likely to occur during molding such as press working. Preferably it is 0.8 to 2.0%.

(4-2) Roll roughness of the work roll: # 600 or more, roll diameter: 700 mm or more, unlubricated As in the titanium plate of the present invention, in order to distribute many twins in the vicinity of the surface, the conventional tempering By increasing the friction coefficient between the surface of the titanium plate and the rolling roll rather than rolling, it is necessary to apply a large shear force near the surface. In the present invention, the roughness of the work roll is finely polished to # 600 or more. By making the roll surface finer, the contact area with the surface of the titanium plate becomes as large as possible, and a greater shearing force can be applied to the titanium plate. If it is less than # 600, the twin density on the surface of the titanium plate does not increase. Preferably it is # 700 or more, more preferably # 1000 or more.

  In the present invention, in order to apply a large shear force to the titanium plate, the roll diameter is set to 700 mm or more. If the thickness is less than 700 mm, the twin density on the surface of the titanium plate does not increase. Preferably it is 800 mm or more, More preferably, it is 900 mm or more.

  In the present invention, it is necessary to perform temper rolling without lubrication. By performing temper rolling without lubrication, the friction coefficient increases, the shear force near the surface increases, and more twins are generated near the surface. If lubricating oil is used during temper rolling, the friction coefficient between the roll and the titanium plate decreases, and the twin density does not increase.

  A titanium ingot having a chemical composition in which the Fe content and the O content are the contents shown in Table 1 and the balance is composed of Ti and inevitable impurities was manufactured by vacuum arc melting. This titanium ingot is forged, hot rolled at a rolling rate of 50% or more at 850 ° C., annealed at 725 ° C., surface-cut, and cold rolled at a rolling rate of 89% to produce a titanium plate having a thickness of 0.5 mm. did. Then, final annealing was performed in vacuum or in the air at the annealing temperature and annealing time shown in Table 1. Then, in order to remove oxide scales, carbides and nitrides, the plate that has been continuously annealed in the atmosphere is immersed in an alkali molten salt mainly composed of NaOH at 450 to 550 ° C., and pickled with nitric acid. It was. Finally, temper rolling was performed under the temper rolling conditions shown in Table 1. In addition, No. 16-18, 26-30, 44-46, and 61-64 performed final annealing in a batch type annealing furnace. Otherwise, final annealing was performed in a continuous annealing furnace.

The evaluation was performed under the following conditions.
-Fe content and O content Measured according to JIS H 1614 (2010).

-Average crystal grain diameter In the structure | tissue photograph image | photographed with the optical microscope, the number N of crystal grains in the area of 400 micrometers x 400 micrometers was calculated | required, and the average area of the crystal grain of (alpha) phase was computed. Thus, the crystal grains were equivalent to a circle, and the diameter was calculated as the average crystal grain size of the α phase.

Twin density The twin density M / N was calculated by measuring the number M of twins within the same field of view as the average crystal grain size and dividing by the number N of crystal grains.

・ Tensile strength Tensile test pieces with a parallel part of 6.25 × 32 mm, a gauge interval of 25 mm, a chuck part of 15 mm width, and a total length of 100 mm are prepared so that the tensile axis is in the rolling direction or the rolling width direction. Until the measurement, a tensile test was performed at a tensile rate of 0.5% / min between the gauge points and 20% / min after the proof stress measurement. Here, 340 MPa or more in which the tensile strength in the rolling width direction (T direction) exceeds that of JIS class 2 pure titanium was determined as acceptable. In the tensile test, an apparatus manufactured by Shimadzu Corporation: model number AG20kNG was used.

-Formability: Made by Tokyo Test Machine Co., Ltd .: Using a spherical head punch with a diameter of 40 mm in a deep drawing tester of model number SAS-350D, a titanium plate is processed into a 70 mm x 100 mm shape so as to have a plane strain deformation. A head overhang test was performed. For stretch forming, high viscosity oil (# 660) manufactured by Nippon Tool Oil Co., Ltd. is applied, and a poly sheet is placed on top of this to prevent direct contact between the punch and the titanium plate, and the punch raising speed is 8 mm / min. It was. The overhang height of the test piece at this time was comparatively evaluated. An overhang height of 18 mm in the ball head overhang test was determined to be acceptable.

  Invention example No. satisfying the conditions of the present invention. 11-14, 19-22, 26-29, 31-37, 39, 43-46, 49, 50, 52, 53 all had a tensile strength of 340 MPa or more and an overhang height of 18 mm or more. .

Comparative Example No. Nos. 1 to 9 were inferior in strength because of low O content.
Comparative Example No. No. 10, since the elongation of temper rolling was 0%, twins were not generated and the strength was inferior.

  In Comparative Examples 15, 23 and 30, the elongation of temper rolling was as high as 4%, so the Y value was high and the overhang height was inferior.

  Comparative Example No. Nos. 16 to 18 were short in annealing time, so the crystal grain size was small and the overhang height was inferior.

  Comparative Example No. Since 24, 40-42, and 51 had a small roll diameter, the twin density did not satisfy X ≧ 2Y, and the overhang height was inferior.

  Comparative Example No. Nos. 25 and 54 were subjected to temper rolling using rolling oil, so the twin density did not satisfy X ≧ 2Y, and the overhang height was inferior.

  Comparative Example No. Since 38, 47 and 48 had low roll roughness, the twin density did not satisfy X ≧ 2Y, and the overhang height was inferior.

  Comparative Example No. Nos. 55 and 56 had a small crystal grain size and an inferior overhanging height because of a large Fe content.

  Comparative Example No. Since 57-64 had much O content, the overhang height was inferior.

Claims (2)

In mass%, Fe: 0.08% or less, O: 0.05 to 0.15%, the balance has a chemical composition consisting of Ti and inevitable impurities,
The average crystal grain size of the α phase is 25 to 100 m, in the cross section perpendicular to the rolling direction, the number of twins M (present), all crystal grains of the individual number when a N (pieces) The twin density (M ÷ N) satisfies the formula (1),
A value obtained by dividing the number of twins existing in the region from the surface to 25% of the plate thickness by the N is a twin density X, and a value obtained by dividing the number of twins existing in the region other than the region by the N. When the twin density is Y, the twin density X is twice or more the twin density Y , and the twin density Y is 0.25 or less .
The tensile strength in the rolling width direction is 340 MPa or more,
A titanium plate characterized by having an overhang height of 18 mm or more in a ball head overhang test conducted using a ball head punch with a diameter of 40 mm .
M ÷ N ≧ 0.05 (1) It is a manufacturing method of the titanium plate according to claim 1 which performs hot processing, annealing, cold processing, final annealing, pickling, temper rolling in order,
When the batch annealing furnace is used, the final annealing is held at a temperature range of 600 to 750 ° C. for 200 to 600 minutes, and when the continuous annealing furnace is used, it is held at a temperature range of 750 to 820 ° C. for 1 to 10 minutes. ,
The temper rolling is performed with no lubrication, the elongation is 0.5 to 3%, the roll roughness of the work roll is # 600 or more, the roll diameter is 700 mm or more, and is unlubricated. Manufacturing method.