Alpha + beta type titanium alloy, process for producing titanium alloy, process for coil rolling, and process for producing cold-rolled coil of titanium alloy

Abstract

A high strength and ductility α+β type titanium alloy, comprising at least one is isomorphous β stabilizing element in a Mo equivalence of 2.0-4.5 mass %, at least one eutectic βstabilizing element in an Fe equivalence of 0.3-2.0 mass %, Si in an amount of 0.1-1.5 mass %, and C in an amount of 0.01-0.15% mass, and has a β transformation temperature no lower than 940° C.

What is claimed is:  
     
         1 . An α+β titanium alloy comprising at least one isomorphous β-stabilizing element in a Mo equivalence of 2.0-4.5 mass %, at least one eutectic β-stabilizing element in an Fe equivalence of 0.3-2.0 mass %, Si in an amount of 0.1-1.5 mass %, and C in an amount of 0.01-0.15 mass %, and has a P transformation temperature no lower than 940° C.  
     
     
         2 . The α+β titanium alloy according    claim 1   , wherein an Al equivalence is more than 3 mass % and less than 6.5 mass %.  
     
     
         3 . The α+β titanium alloy according    claim 2   , wherein those elements of Al equivalence are entirely Al.  
     
     
         4 . The α+β titanium alloy according to    claim 1   , which substantially contains Mo in an amount of 1.0-3.0 mass %, V in an amount of 1.0-2.0 mass %, Fe in an amount of 0.3-1.0 mass %, Al in an amount of 3.5-5.5 mass %, Si in an amount of 0.2-0.5 mass %, and C in an amount of 0.02-0.15 mass %, with the remainder being Ti and inevitable impurities.  
     
     
         5 . The α+β titanium alloy according to    claim 1   , which contains O as an additional element such that the amount of Mo-equivalenve, the amount of Fe-equivalence, and the content of 0 satisfy the following inequality [1].  
       7.0 mass %≦(Mo-equivalence+2.5×Fe-equivalence+40×O mass %)≦19 mass %  [1] 
     
     
         6 . The α+β titanium alloy according to    claim 1   , which further contains a platinum group element in an amount of 0.03-0.2 mass %.  
     
     
         7 . A process for hot-rolling the titanium alloy of any of    claims 1    to    6   , said process comprising heating the titanium alloy at a temperature (T1) which satisfies the following inequatity [2] and then rolling it.  
       [β-transus−20° C.−(770× C  mass %)° C.]≦ T 1<β-transus  [2] 
     
     
         8 . A process for rolling the titanium alloy of any of    claims 1    to    6   , said process comprising annealing the titanium alloy at a temperature (T2) which satisfies the following inequality [3] and then rolling it, thereby producing a coil of titanium alloy.  
       [β-transus−270° C.]≦ T 2≦(β-transus−50° C.)  [3] 
     
     
         9 . The process for rolling to produce a coil according to    claim 8   , wherein rolling is carried out under a tension of 49-392 MPa such that the draft is no lower than 20%.  
     
     
         10 . The process for rolling to produce a coil according to    claim 8   , wherein rolling is repeated more than once, with annealing in the cc+p region intervening between consecutive rolling steps.  
     
     
         11 . A process for annealing a cold-rolled coil of the titanium alloy of any of    claims 1    to    6   , characterized in that the heating temperature for annealing is higher than the temperature at which work hardening due to cold-rolling is relieved and lower than the β transus but excludes the temperature range in which a alloy of brittle hexagonal crystals emerges, thereby improving the elongation in the transverse direction of the rolled strip of the titanium alloy.  
     
     
         12 . A process of annealing a coil cold-rolled strip of the titanium alloy of any of    claims 1    to    6   , wherein annealing is carried out at the temperature (T3) which satisfies the inequality [4] below so as to give a coil rolled titanium alloy strip superior in bending properties.  
       (β-transus−130° C.)≦T3≦(β-transus−15° C.)  [4] 
     
     
         13 . A process of annealing a coil cold-rolled strip of the titanium alloy of    claim 4   , wherein annealing is carried out at a temperature no lower than 850° C. and no higher than 963° C. so as to give a coil rolled titanium alloy strip superior in bending properties.