A process of producing a duplex stainless steel tube

A process of producing a duplex stainless steel tube, said duplex stainless steel having the following composition (in weight %),

• C
• 0-0.3;
• Cr
• 22-26;
• Cu
• 0-0.5;
• Mn
• 0-1.2;
• Mo
• 3.0-4.0,
• N
• 0-0.35;
• Ni
• 5.0-7.0;
• Si
• 0.2-0.8;
• balance Fe and unavoidable impurities,
• said process comprising the steps of
• a) producing an ingot or a continuous casted billet of said duplex stainless steel;
• b) hot extruding the ingot or the billet obtained from step a) into a tube;
• and
• c) cold rolling the tube obtained from step b) to a final dimension thereof;
• wherein the outer diameter D and the wall thickness t of the cold rolled tube is 50-250 mm respectively is 5-25 mm,
• wherein, for the cold rolling step, R and Q are set such that the following formula is satisfied:
• Rp0.2target=416.53+113.26·log Q+4.0479·R+2694.9·C %−82.750·(log Q)2−0.04279·R2−2.2601·log Q·R+16.9·Cr %+26.1·Mo %+83.6·N %±Z (1)
• wherein
• Rp0.2target is targeted yield strength and is 800-1100 MPa,
• Q=(W0−W1)×(OD0−W0)/W0((OD0−W0)−(OD1−W1)) (2)
• wherein W1 is tube wall thickness before cold rolling, W0 is tube wall thickness after cold rolling, OD1 is outer diameter of tube before cold rolling, and OD0 is outer diameter of tube after cold rolling,
• R is cold reduction and is defined as
• R
• =
• 1
• -
• A
• 
• 
• 1
• A
• 
• 
• 0
• (
• 3
• )
• wherein A1 is tube cross section area before cold rolling and A0 is tube cross section area after cold rolling
• Z=65,
• and wherein 0<Q<3.6.