ALPHA + BETA TITANIUM ALLOY WELDED PIPE EXCELLENT IN STRENGTH AND RIGIDITY IN PIPE LONGITUDINAL DIRECTION AND METHOD FOR PRODUCING THE SAME

1 . An α+β titanium alloy welded pipe produced by processing an α+β titanium alloy cold-rolled and annealed sheet consisting of, in mass %,

Fe: 0.8% to 1.5%,

N: 0.02% or less, and

the balance: Ti and impurities, and

satisfying Q shown in Formula (1) below being 0.34 to 0.55,

wherein a tensile strength in a pipe longitudinal direction is more than 900 MPa and a Young's modulus in the pipe longitudinal direction is more than 130 GPa,

Q =[O]+2.77×[N]+0.1×[Fe]  (1)

where [Fe], [O], and [N] represent the amounts of the respective elements contained [mass %].

2 . A method for producing an α+β titanium alloy welded pipe, comprising:

producing a welded pipe by processing an α+β titanium alloy cold-rolled and annealed sheet consisting of, in mass %,

Fe: 0.8% to 1.5%,

N: 0.02% or less, and

the balance: Ti and impurities, and

satisfying Q shown in Formula (1) below being 0.34 to 0.55,

wherein, in a texture of the α+β titanium alloy cold-rolled and annealed sheet, assuming that a normal-to-rolling-plane direction is denoted by ND, a sheet longitudinal direction is denoted by RD, a sheet width direction is denoted by TD, a direction normal to a (0001) plane of an α-phase is taken as a c-axis direction, an angle between the c-axis direction and ND is denoted by θ, an angle between a line of projection of the c-axis direction onto a sheet plane and the sheet width direction (TD) is denoted by φ, a strongest intensity out of (0002)-reflection relative intensities of X-rays caused by crystal grains falling within a range of angle θ of not less than 0 degrees and not more than 30 degrees and angle φ of −180 degrees to 180 degrees is denoted by XND, and a strongest intensity out of (0002)-reflection relative intensities of X-rays caused by crystal grains falling within a range of angle θ of not less than 80 degrees and less than 100 degrees and angle φ of ±10 degrees is denoted by XTD, a ratio XTD/XND is 5.0 or more, and

when the α+β titanium alloy cold-rolled and annealed sheet is processed into a pipe shape, the sheet width direction of the α+β titanium alloy cold-rolled and annealed sheet is set as a longitudinal direction of the α+β titanium alloy welded pipe and the sheet longitudinal direction of the α+β titanium alloy cold-rolled and annealed sheet is set as a round direction of the α+β titanium alloy welded pipe,

Q =[O]+2.77×[N]+0.1×[Fe]  (1)

where [Fe], [O], and [N] represent the amounts of the respective elements contained [mass %].

3 . The method for producing an α+β titanium alloy welded pipe according to claim 2 , wherein

the α+β titanium alloy cold-rolled and annealed sheet is produced by using a unidirectionally hot-rolled sheet as a material and performing unidirectional cold rolling in the same direction as a direction of hot rolling and annealing, and

annealing for a holding time of not less than t of Formula (2) below is performed at not less than 500° C. and less than 800° C. in a case where a cold rolling rate of the unidirectional cold rolling is less than 25% and annealing for a holding time of not less than t of Formula (2) below is performed at not less than 500° C. and less than 620° C. in a case where the cold rolling rate is 25% or more,

t =exp(19180/ T− 15.6)   (2)

where t: holding time (s), and T: holding temperature (K).

4 . An α+β titanium alloy welded pipe produced by processing an α+β titanium alloy cold-rolled and annealed sheet comprising, in mass %,

Fe: 0.8% to 1.5%,

N: 0.02% or less, and

the balance: Ti and impurities, and

satisfying Q shown in Formula (1) below being 0.34 to 0.55,

wherein a tensile strength in a pipe longitudinal direction is more than 900 MPa and a Young's modulus in the pipe longitudinal direction is more than 130 GPa,

Q =[O]+2.77×[N]+0.1×[Fe]  (1)

where [Fe], [O], and [N] represent the amounts of the respective elements contained [mass %].

5 . A method for producing an α+β titanium alloy welded pipe, comprising:

producing a welded pipe by processing an α+β titanium alloy cold-rolled and annealed sheet comprising, in mass %,

Fe: 0.8% to 1.5%,

N: 0.02% or less, and

the balance: Ti and impurities, and

satisfying Q shown in Formula (1) below being 0.34 to 0.55,

wherein, in a texture of the α+β titanium alloy cold-rolled and annealed sheet, assuming that a normal-to-rolling-plane direction is denoted by ND, a sheet longitudinal direction is denoted by RD, a sheet width direction is denoted by TD, a direction normal to a (0001) plane of an α-phase is taken as a c-axis direction, an angle between the c-axis direction and ND is denoted by θ, an angle between a line of projection of the c-axis direction onto a sheet plane and the sheet width direction (TD) is denoted by φ, a strongest intensity out of (0002)-reflection relative intensities of X-rays caused by crystal grains falling within a range of angle θ of not less than 0 degrees and not more than 30 degrees and angle φ of −180 degrees to 180 degrees is denoted by XND, and a strongest intensity out of (0002)-reflection relative intensities of X-rays caused by crystal grains falling within a range of angle θ of not less than 80 degrees and less than 100 degrees and angle φ of ±10 degrees is denoted by XTD, a ratio XTD/XND is 5.0 or more, and

when the α+β titanium alloy cold-rolled and annealed sheet is processed into a pipe shape, the sheet width direction of the α+β titanium alloy cold-rolled and annealed sheet is set as a longitudinal direction of the α+β titanium alloy welded pipe and the sheet longitudinal direction of the α+β titanium alloy cold-rolled and annealed sheet is set as a round direction of the α+β titanium alloy welded pipe,

Q =[O]+2.77×[N]+0.1×[Fe]  (1)

where [Fe], [O], and [N] represent the amounts of the respective elements contained [mass %].

6 . The method for producing an α+β titanium alloy welded pipe according to claim 5 , wherein

the α+β titanium alloy cold-rolled and annealed sheet is produced by using a unidirectionally hot-rolled sheet as a material and performing unidirectional cold rolling in the same direction as a direction of hot rolling and annealing, and

annealing for a holding time of not less than t of Formula (2) below is performed at not less than 500° C. and less than 800° C. in a case where a cold rolling rate of the unidirectional cold rolling is less than 25% and annealing for a holding time of not less than t of Formula (2) below is performed at not less than 500° C. and less than 620° C. in a case where the cold rolling rate is 25% or more,

t =exp(19180/ T− 15.6)   (2)

where t: holding time (s), and T: holding temperature (K).