Titanium alloy bar and method for manufacturing the same

Abstract

The invention relates to an α+β type titanium alloy bar consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, and having 10 to 90% of volume fraction of primary α phase, 10 μm or less of average grain size of the primary α phase, and 4 or less of aspect ratio of the grain of the primary α phase on the cross sectional plane parallel in the rolling direction of the bar. The α+β type titanium alloy bar has excellent ductility, fatigue characteristics and formability.

What is claimed:  
     
         1 . An α+β type titanium alloy bar consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, and having 10 to 90% of volume fraction of primary α phase, 10 μm or less of average grain size of the primary α phase, and 4 or less of aspect ratio of the grain of the primary α phase on the cross sectional plane parallel in the rolling direction of the bar.  
     
     
         2 . The α+β type titanium alloy bar of  claim 1 , wherein the volume fraction of primary α phase is 50 to 80%, and the average grain size of the primary α phase is 6 μm or less.  
     
     
         3 . A method for manufacturing an α+β type titanium alloy bar comprising the step of hot rolling an α+β type titanium alloy consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, while keeping the surface temperature thereof to β transus or below.  
     
     
         4 . The method for manufacturing an α+β type titanium alloy bar of  claim 3  comprising the steps of: heating an α+β type titanium alloy having a β transus of Tβ ° C. while keeping the surface temperature thereof between (Tβ−150) and Tβ ° C.; and hot rolling the heated α+β type titanium alloy while keeping the surface temperature thereof during hot rolling between (Tβ−300) and (Tβ−50) ° C. and keeping the finish surface temperature thereof, as the surface temperature immediately after the final rolling pass, between (Tβ−300) and (Tβ−100) ° C.  
     
     
         5 . The method for manufacturing an α+β type titanium alloy bar of  claim 4 , wherein the α+β type titanium alloy is hot rolled at a reduction rate of 40% or less per rolling pass.  
     
     
         6 . The method for manufacturing an α+β type titanium alloy bar of  claim 4 , wherein the rolling speed is selected to 6 m/sec or less when a reverse rolling mill is applied to hot rolling.  
     
     
         7 . The method for manufacturing an α+β type titanium alloy bar of  claim 4 , wherein the rolling speed is selected to 1.5 m/sec or less when tandem rolling mills are applied to hot rolling.  
     
     
         8 . The method for manufacturing an α+β type titanium alloy bar of  claim 4 , wherein when the α+β type titanium alloy having 3500 mm 2  or larger cross sectional area in normal to the rolling direction is hot rolled to the cross sectional area of S mm 2 , a waiting time before starting succeeding rolling is 0.167×S 1/2  or more sec.  
     
     
         9 . The method for manufacturing an α+β type titanium alloy bar of  claim 4 , wherein the α+β type titanium alloy is reheated during hot rolling.