Linepipe steel

Abstract

A method of producing coil plate on a hot strip mill is disclosed. The method includes coiling hot rolled coil plate strip at a temperature that is selected (a) to minimise precipitation of Cr/Mo carbides or (b) so that any Cr/Mo carbides that form are sufficiently fine that they go into solution in any subsequent heat treatment of coil plate made from the strip.

1 . A linepipe steel includes the following composition, in wt %:
 C: up to 0.18;   Mn: 0.10 to 0.50;   Ti: at least 0.01;   Si: up to 0.35;   Nb: up to 0.10;   Al: up to 0.05;   Ca: up to 0.005;   S: up to 0.015;   P: up to 0.020;   Cr: up to 1.0;   Mo: up to 0.5;   B: up to 0.002;   Ni: up to 0.35;   Cu: up to 0.35;   V: up to 0.06;   Fe: balance; and   incidental impurities.   
   
   
       2 . The linepipe steel defined in  claim 1  includes less than 0.10 wt. % C. 
   
   
       3 . The linepipe steel defined in  claim 1  or  claim 2  includes at least 0.02 wt. % C. 
   
   
       4 . The linepipe steel defined in  claim 3  includes at least 0.03 wt. % C. 
   
   
       5 . The linepipe steel defined in  claim 4  includes at least 0.04 wt. % C. 
   
   
       6 . The linepipe steel defined in any one of the preceding claims includes less than 0.35 wt. % Mn. 
   
   
       7 . The linepipe steel defined in any one of the preceding claims includes at least 0.15 wt. % Mn. 
   
   
       8 . The linepipe steel defined in  claim 7  includes at least 0.20 wt. % Mn. 
   
   
       9 . The linepipe steel defined in  claim 8  includes at least 0.25 wt. % Mn. 
   
   
       10 . The linepipe steel defined in any one of the preceding claims includes less than 0.05 wt. % Ti. 
   
   
       11 . The linepipe steel defined in  claim 10  includes less than 0.03 wt. % Ti. 
   
   
       12 . The linepipe steel defined in any one of the preceding claims includes less than 0.25 wt. % Si. 
   
   
       13 . The linepipe steel defined in any one of the preceding claims includes at least 0.005 wt. % Si. 
   
   
       14 . The linepipe steel defined in any one of the preceding claims includes less than 0.08 wt. % Nb. 
   
   
       15 . The linepipe steel defined in any one of the preceding claims includes at least 0.001 wt. % Nb. 
   
   
       16 . The linepipe steel defined in  claim 15  includes at least 0.01 wt. % Nb. 
   
   
       17 . The linepipe steel defined in any one of the preceding claims includes at least 0.01 wt. % Al. 
   
   
       18 . The linepipe steel defined in any one of the preceding claims includes less than 0.001 wt. % Ca. 
   
   
       19 . The linepipe steel defined in any one of the preceding claims includes less than 0.012 wt. % S. 
   
   
       20 . The linepipe steel defined in  claim 19  includes less than 0.01 wt. % S. 
   
   
       21 . The linepipe steel defined in any one of the preceding claims includes at least 0.005 wt. % S. 
   
   
       22 . The linepipe steel defined in any one of the preceding claims includes less than 0.7 wt. % Cr. 
   
   
       23 . The linepipe steel defined in  claim 22  includes less than 0.5 wt. % Cr. 
   
   
       24 . The linepipe steel defined in any one of the preceding claims includes less than 0.3 wt. % Mo. 
   
   
       25 . A linepipe made from the linepipe steel defined in any one of the preceding claims. 
   
   
       26 . A method of producing a coiled strip of the linepipe steel defined in any one of the preceding claims that is suitable to be used as a feedstock for producing a linepipe, which method includes the steps of:
 (a) casting a slab of the linepipe steel defined in any one of the preceding claims;   (b) hot rolling the slab to form a strip having a required thickness, typically 5-10 mm; and   (c) coiling the strip.   
   
   
       27 . The method defined in  claim 26  wherein the microstructure of the linepipe steel in the coiled strip is predominantly fine grained polygonal ferrite. 
   
   
       28 . The method defined in  claim 26  or  claim 27  wherein the microstructure includes a small (up to 15%) volume fraction of pearlite in the case of medium strength linepipe grades, such as X42-X60. 
   
   
       29 . The method defined in  claims 26  or  claim 27  wherein the microstructure includes acicular ferrite and/or martensite/austenite in the case of high strength linepipe grades, such as X65 and X70. 
   
   
       30 . A method of producing a linepipe which includes electric resistance welding the linepipe steel strip defined in any one of  claims 26  to  29  and forming the linepipe.