Bainitic steels with boron

Abstract

Steel compositions contain micro-alloying additions of boron and titanium, with yield strength of at least 100 ksi (690 MPa), excellent toughness and good weldability. Boron additions are used to increase hardenability. Strong nitride formers, such as titanium, may be added to the steel composition in order to prevent boron nitrides from forming. These compositions may be cooled from hot rolling in air or using accelerated cooling. After air cooling, the composition may be quenched or quenched and tempered. The compositions are suitable for high strength line pipes (for example, X100 in API 5L standard) and other applications.

1 . A method of making a steel pipe, comprising:
 providing a steel composition comprising:
 about 0.04-0.12 wt. % carbon (C); 
 about 0.01 to 0.03 wt. % titanium (Ti); 
 about 0.0005 to 0.003 wt. % boron (B); and 
 less than or equal to about 0.008 wt. % nitrogen (N); 
 the concentration of each element being based upon the total weight of the steel composition; 
 wherein about 0.0005 to 0.002 wt. % boron is in solid solution for improving hardenability; 
 wherein substantially all of the nitrogen is present in the form TiN particles to avoid the formation of boron nitrides and achieve said boron content in solid solution, and 
   cooling a bar cast from the steel composition, wherein the cooling rate at about the center of the bar is selected such that TiN particles formed in the bar exhibit a mean diameter less than about 50 nm; and   forming a pipe from the bar;   wherein the yield strength of the steel pipe, measured according to ASTM E8, is greater than about 100 ksi (690 MPa).   
     
     
         2 . The method of  claim 1 , wherein the steel bar is cooled from casting at a rate greater than about 10° C./min at about the center of the bar. 
     
     
         3 . The method of  claim 2 , wherein the steel bar is cooled from casting at a rate greater than about 30° C./min at about the center of the bar. 
     
     
         4 . The method of  claim 1 , wherein the steel composition further comprises:
 about 0.6 to 1.6 wt. % manganese (Mn);   about 0.05 to 0.3 wt. % silicon (Si);   less than or equal to about 0.5 wt. % nickel (Ni);   less than or equal to about 0.5 wt. % chromium (Cr);   less than or equal to about 0.5 wt. % molybdenum (Mo);   less than or equal to about 0.15 wt. % vanadium (V); and   less than or equal to about 0.05 wt. % niobium (Nb).   
     
     
         5 . The method of  claim 4 , wherein the elements of the steel composition are selected in concentrations such that the carbon equivalency (CE Pcm ) of the composition is less than about 0.22, where CE Pcm  is calculated according to: 
       
         
           
             
               CE 
               = 
               
                 C 
                 + 
                 
                   Si 
                   30 
                 
                 + 
                 
                   
                     Mn 
                     + 
                     Cu 
                     + 
                     Cr 
                   
                   30 
                 
                 + 
                 
                   Ni 
                   60 
                 
                 + 
                 
                   Mo 
                   15 
                 
                 + 
                 
                   V 
                   10 
                 
                 + 
                 
                   5 
                    
                   B 
                 
               
             
           
         
       
       wherein the concentration of each element is provided in wt. %. 
     
     
         6 . The method of  claim 1 , further comprising:
 hot rolling the steel pipe and cooling the steel pipe in air from hot rolling at a rate less than about 1° C./sec; and   austenizing and quenching the hot rolled steel pipe.   
     
     
         7 . The method of  claim 6 , further comprising tempering the quenched steel pipe at a temperature ranging between about 400 to 700° C. for between about 10 to 60 minutes. 
     
     
         8 . The method of  claim 1 , further comprising cooling the steel pipe from hot rolling, without an intermediate cooling step, at a rate between about 5 to 50° C./sec. 
     
     
         9 . A method of making a steel composition, comprising:
 providing a steel composition comprising:
 about 0.04 to 0.12 wt. % carbon (C); 
 about 0.8 to 1.6 wt. % manganese (Mn); 
 about 0.05 to 0.3 wt. % silicon (Si); 
 less than or equal to about 0.5 wt. % nickel (Ni); 
 less than or equal to about 0.5 wt. % chromium (Cr); 
 less than or equal to about 0.5 wt. % molybdenum (Mo); 
 less than or equal to about 0.15 wt. % vanadium (V); 
 less than or equal to about 0.05 wt. % niobium (Nb); 
 about 0.01 to 0.03 wt. % titanium (Ti); 
 about 0.0005 to 0.0030 wt. % boron (B); and 
 less than or equal to 0.008 wt. % nitrogen (N); 
 wherein the concentration of each element is based upon the total weight of the steel composition and wherein about 0.0005 to 0.002 wt. % boron is kept in solid solution for improving hardenability; 
   casting the steel composition, wherein substantially all of the nitrogen in the cast steel composition is present in the form of TiN particles having a size less than about 50 nm to avoid the formation of boron nitrides and achieve said boron content in solid solution;   hot rolling the cast steel composition; and   cooling the cast steel composition directly after hot rolling at a rate between about 5 to 50° C./sec.   
     
     
         10 . The method of  claim 9 , further comprising:
 reheating the cast steel composition to about 1200 to 1300° C.;   piercing the cast steel composition at temperatures ranging between about 1100 to 1200° C.; and   hot rolling the cast steel composition at temperatures ranging between about 900-1100° C.   
     
     
         11 . The method of  claim 9 , wherein the austenitic grain size of the steel composition, prior to cooling from hot rolling, ranges between about 20 to 50 μm. 
     
     
         12 . The method of  claim 11 , wherein the composition is cooled directly from hot rolling at a rate between about 10 to 50° C./sec. 
     
     
         13 . The method of  claim 11 , wherein the composition is cooled directly from hot rolling at a rate between about 10 to 20° C./sec. 
     
     
         14 . The method of  claim 12 , wherein the yield strength of the cast steel composition after hot rolling and cooling, measured according to ASTM E8, is at least about 100 ksi (690 MPa). 
     
     
         15 . The method of  claim 12 , wherein the Charpy V-notch impact energy of the composition, strength of the cast steel composition after hot rolling and cooling, measured according to ASTM E23 in full size samples is greater than about 220 J at temperatures greater than or equal to −20° C. 
     
     
         16 . The method of  claim 9 , wherein the composition comprises:
 about 0.05-0.10 wt. % carbon (C);   about 0.8 to 1.6 wt. % manganese (Mn);   about 0.05 to 0.30 wt. % silicon (Si);   up to about 0.4 wt. % nickel (Ni);   up to about 0.3 wt. % chromium (Cr);   up to about 0.3 wt. % molybdenum (Mo);   up to about 0.1 wt. % vanadium (V);   up to about 0.04 wt. % niobium (Nb);   about 0.015 to 0.025 wt. % titanium (Ti);   about 0.0005-0.015 wt. % boron (B); and   less than or equal to 0.007 wt. % nitrogen (N);   
     
     
         17 . A pipe formed according to the method of  claim 9 . 
     
     
         18 . A method of making a steel composition, comprising:
 providing a steel composition comprising:
 about 0.04-0.12 wt. % carbon (C); 
 about 0.8 to 1.6 wt. % manganese (Mn); 
 about 0.05 to 0.3 wt. % silicon (Si); 
 less than or equal to about 0.5 wt. % nickel (Ni); 
 less than or equal to about 0.5 wt. % chromium (Cr); 
 less than or equal to about 0.5 wt. % molybdenum (Mo); 
 less than or equal to about 0.15 wt. % vanadium (V); 
 less than or equal to about 0.05 wt. % niobium (Nb); 
 about 0.01 to 0.03 wt. % titanium (Ti); 
 about 0.0005-0.0030 wt. % boron (B); and 
 less than or equal to 0.008 wt. % nitrogen (N); 
   wherein the concentration of each element is based upon the total weight of the steel composition;   wherein about 0.0005 to 0.002 wt. % boron is kept in solid solution for improving hardenability;   casting the steel composition, wherein substantially all of the nitrogen in the cast steel composition is present in the form of TiN particles having a size less than about 50 nm to avoid the formation of boron nitrides and achieve said boron content in solid solution;   hot rolling the cast steel composition;   air cooling the formed steel composition directly after hot rolling at a rate less than about 1° C./sec; and   austenizing and quenching the composition.   
     
     
         19 . The method of  claim 18 , further comprising:
 reheating the cast steel composition up to about 1200 to 1300° C.;   piercing the cast steel composition at temperatures ranging between about 1100 to 1200° C.; and   hot rolling the cast steel composition at temperatures ranging between about 900-1100° C.   
     
     
         20 . The method of  claim 19 , wherein the steel composition comprises:
 about 0.07 to 0.10 wt. % carbon (C);   about 1.0 to 1.4 wt. % manganese (Mn);   about 0.05 to 0.15 wt. % silicon (Si);   up to about 0.4 wt. % nickel (Ni);   up to about 0.35 wt. % chromium (Cr);   up to about 0.3 wt. % molybdenum (Mo);   up to about 0.1 wt. % vanadium (V);   up to about 0.04 wt. % niobium (Nb);   about 0.02 to 0.03 wt. % titanium (Ti); and   about 0.001 to 0.002 wt. % boron (B).   
     
     
         21 . The method of  claim 20 , wherein the quenched steel is tempered at a temperature between about 400 to 600° C. 
     
     
         22 . The method of  claim 21 , wherein after hot rolling, cooling, austenizing and quenching, the yield strength of the composition, measured according to ASTM E8, is greater than about 100 ksi, and the Charpy V-notch impact energy of the composition, measured according to ASTM E23 in full size samples, is greater than about 170 J at temperatures equal to or greater than about −40° C. 
     
     
         23 . The method of  claim 19 , wherein the steel composition comprises:
 about 0.04 to 0.08 wt. % carbon;   about 1.0 to 1.4 wt. % manganese;   about 0.05 to 0.15 wt. % silicon;   up to about 0.35 wt. % chromium;   about 0.2 to 0.3 wt. % molybdenum;   about 0.03 to 0.04 wt. % niobium;   about 0.02 to 0.03 wt. % titanium; and   about 0.001 to 0.002 wt. % boron.   
     
     
         24 . The method of  claim 23 , wherein the steel is reheated in the austenitic region and quenched without subsequent tempering. 
     
     
         25 . The method of  claim 24 , wherein after hot rolling, cooling, austenizing and quenching, the yield strength of the composition, measured according to ASTM E8, is greater than about 100 ksi and the Charpy V-notch impact energy, measured according to ASTM E23 in full size samples, is greater than about 90 J at temperatures equal to or greater than about −40° C. 
     
     
         26 . A pipe formed according to the method of  claim 18 .