Alpha + beta titanium alloy shape and method for manufacturing the same

Abstract

An α+β titanium alloy shape includes an acicular microstructure, wherein a 0.2% proof stress is 830 MPa or more, an elongation is 10% or more, a fatigue strength is 450 MPa or more, an area fraction of voids is 1.0×10 −5 % or less, a twist angle from one end to the other end is within ±3.0°, and a warpage height (mm)/total length (m) is within ±2.17.

1 . An α+β titanium alloy shape comprising an acicular microstructure, wherein
 a 0.2% proof stress is 830 MPa or more, 
 an elongation is 10% or more, 
 a fatigue strength is 450 MPa or more, 
 an area fraction of voids is 1.0×10 −5  % or less, 
 a twist angle from one end to the other end is within ±3.0°, and 
 a warpage height (mm)/total length (m) is within ±2.17. 
 
     
     
         2 . The α+β titanium alloy shape according to  claim 1 , wherein the 0.2% proof stress is 850 MPa or more. 
     
     
         3 . The α+β titanium alloy shape according to  claim 1 , wherein the area fraction of voids is 1.0×10 −6  % or less. 
     
     
         4 . The α+β titanium alloy shape according to  claim 1 , wherein the average prior β grain size is 500 μm or less. 
     
     
         5 . The α+β titanium alloy shape according to  claim 1 , wherein the maximum residual stress in the cross section is +400 MPa or less. 
     
     
         6 . The α+β titanium alloy shape according to  claim 1 , wherein the α+β titanium alloy shape is an extruded shape. 
     
     
         7 . The α+β titanium alloy shape according to  claim 1 , comprising in mass %,
 Al: 4.4 to 6.5%, 
 Fe: 0.5 to 2.9%, 
 Si: 0 to 0.50%, 
 O: 0 to 0.25%, 
 C: 0 to 0.08%, 
 N: 0 to 0.05%, 
 Ni: 0 to 0.15%, 
 Cr: 0 to 0.25%, and 
 Mn: 0 to 0.25%, 
 with the remainder being Ti and impurities, 
 wherein the contents of Fe, Ni, Cr, and Mn, % Fe, % Ni, % Cr, and % Mn, expressed in mass %, satisfy 0.5%≤% Fe+% Ni+% Cr+% Mn≤2.9%. 
 
     
     
         8 . The α+β titanium alloy shape according to  claim 1 , comprising in % by mass,
 Al: 4.4 to 5.5%, 
 Fe: 1.4 to 2.3%, 
 Mo: 1.5 to 5.5%, 
 O: 0 to 0.20%, 
 C: 0 to 0.08%, 
 N: 0 to 0.05%, 
 Si: 0 to 0.10%, 
 Ni: 0 to 0.15%, 
 Cr: 0 to 0.25%, and 
 Mn: 0 to 0.25%, 
 with the remainder being Ti and impurities, 
 wherein the contents of Fe, Ni, Cr, and Mn, % Fe, % Ni, % Cr, and % Mn, expressed in mass %, satisfy 1.4%≤% Fe+% Ni+% Cr+% Mn≤2.3%. 
 
     
     
         9 . A method for manufacturing an α+β titanium alloy shape comprising an acicular microstructure, wherein
 a 0.2% proof stress is 830 MPa or more, 
 an elongation is 10% or more, 
 a fatigue strength is 450 MPa or more, 
 an area fraction of voids is 1.0×10 −5  % or less 
 a twist angle from one end to the other end is within ±3.0°, and a warpage height (mm)/total length (m) is within ±2.17, the method comprising: 
 hot working an α+β titanium alloy to obtain a shape; 
 heating the shape to a straightening temperature of equal to or higher than β transus temperature −400° C. and equal to or lower than β transus temperature −200° C., applying a strain of 0.1% or more and 8% or less in a longitudinal direction at the straightening temperature, and further applying a torque that makes a twist in the longitudinal direction of the shape within ±3.0%; and 
 cooling the shape to 500° C. or lower while applying tensile stress and the torque to the shape. 
 
     
     
         10 . The method for manufacturing an α+β titanium alloy shape according to  claim 9 , wherein the tensile stress applied during cooling of the shape is 20% or less of 0.2% proof stress at room temperature. 
     
     
         11 . The method for manufacturing an α+β titanium alloy shape according to  claim 9 , further comprising holding the shape at 500 to 650° C. during cooling of the shape. 
     
     
         12 . The method for manufacturing an α+β titanium alloy shape according to  claim 9 , wherein an average cooling rate of the shape from the straightening temperature to 500° C. is 10° C./s or less.