Process of producing a duplex stainless steel tube
A process of producing a duplex stainless steel tube comprises 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. The outer diameter D and the wall thickness t of the cold rolled tube is 50-250 mm respectively is 5-25 mm, and, for the cold rolling step, R and Q are set such that the following formula is satisfied: Rp 0.2target=416.53+113.26·log Q +4.0479·R+2694.9·C %−82.750·(log Q ) 2 −0.04279·R 2 −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 and 0<Q<3.6.
1. A process of producing a duplex stainless steel tube, the process comprising:
producing an ingot or a continuous casted billet of the duplex stainless steel;
hot extruding the produced ingot or the produced billet into a tube;
selecting a cold reduction (R) and a Q-value (Q) to be used in cold rolling so as to obtain a value for targeted yield strength (Rp0.2target) that is 1000-1100 MPa and a value for Q-value (Q) is 0<Q<3.6; and
cold rolling the hot extruded tube to a final dimension using the selected cold reduction (R) and Q-value (Q),
wherein the cold reduction (R) is defined as:
R
=
1
-
A
1
A
0
where A1 is tube cross section area before cold rolling and A0 is tube cross section area after cold rolling,
wherein the Q-value (Q) is calculated using the following formula:
Q =( W 0− W 1)×( OD 0− W 0)/ W 0(( OD 0− W 0)−( OD 1− W 1))
where
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,
wherein the value for targeted yield strength (Rp0.2target) is calculated using the following formula:
Rp
0.2
target
=
416.53
+
113.26
·
log
Q
+
4.0479
·
R
+
2694.9
·
C
%
-
82.750
·
(
log
Q
)
2
-
0.04279
·
R
2
-
2.2601
·
log
Q
·
R
+
16.9
·
Cr
%
+
26.1
·
Mo
%
+
83.6
·
N
%
±
Z
,
wherein Z=65, 50, 20 or 0, and
wherein the duplex stainless steel has a composition including (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,
and
balance Fe and unavoidable impurities.
2. The process according to claim 1 , wherein the outer diameter of the tube after cold rolling is 50-250 mm, and wherein the tube wall thickness after cold rolling is 5-25 mm.
3. The process according to claim 2 , wherein, if 0<Q<1, then 25*Q<R<40*Q+20.
4. The process according to claim 2 , wherein, if 1≤≤2, then 25*Q≤R≤60.
5. The process according to claim 2 , wherein, if 2<Q<3.6, then 50<R<60.
6. The process according to claim 2 , wherein the duplex stainless steel contains 30-70 vol. % austenite and 30-70 vol. % ferrite.
7. The process according to claim 2 , said duplex stainless steel having the following composition (in weight %),
C
0.008-0.03;
Cr
22-26;
Cu
0.1-0.2;
Mn
0.35-1.0;
Mo
3.0-4.0
N
0.1-0.35;
Ni
5.0-7.0;
Si
0.2-0.7;
and
balance Fe and unavoidable impurities.
8. The process according to claim 2 , wherein Z=50.
9. The process according to claim 2 , wherein Z=20.
10. The process according to claim 2 , wherein Z=0.