Method for producing line pipe

Abstract

A method for producing steel line pipe having low yield strength to tensile strength ratio in order to improve the capability of steel line pipe to undergo reeling into coil form and unreeling therefrom. The method includes (a) providing a steel pipe having a composition consisting essentially of in weight percent: C 0.01 to 0.40, Mn 0.25 to 2.0, P residual to less than 0.5, S residual to less than 0.020, Si residual to 2.0, Cu residual to 1.0, Ni residual to 1.0, Cr residual to 2.0, Mo residual to 1.0, Al 0.010 minimum to less than 1.0, N residual to 0.030, V residual to less than 0.5, B residual to less than 0.02, Ti residual to less than 0.3, and Nb residual to less than 0.3, balance iron and incidental impurities. The pipe is heated to a temperature within the intercritical A c1 to A c3 temperature range, cooled to a temperature below the M s (martensite start) temperature in order to obtain martensite, reheated to a temperature below the A c1 temperature for a time sufficient to obtain the desired yield strength, tensile strength and yield strength to tensile strength ratio, and then air cooled.

1 . A method for the production of steel line pipe, comprising: 
 (a) providing a steel pipe having a composition consisting essentially of in weight percent: C 0.01 to 0.40, Mn 0.25 to 2.0, P residual to less than 0.5, S residual to less than 0.020, Si residual to 2.0, Cu residual to 1.0, Ni residual to 1.0, Cr residual to 2.0, Mo residual to 1.0, Al 0.010 minimum to less than 1.0, N residual to 0.030, V residual to less than 0.5, B residual to less than 0.02, Ti residual to less than 0.3, and Nb residual to less than 0.3, balance iron and incidental impurities;    (b) heating the pipe to a temperature within the intercritical A c1  to A c3  temperature range to obtain ferrite and austenite in the microstructure;    (c) cooling the heated pipe to a temperature below the M s  (martensite start) temperature at a cooling rate sufficient to cause the austenite present to transform to martensite;    (d) after cooling from the intercritical annealing temperature, reheating the pipe to a temperature below the A c1  temperature for a time sufficient to obtain the desired yield strength, tensile strength and yield strength to tensile strength ratio; and    (e) then cooling the pipe to room temperature.    
     
     
         2 . The method of  claim 1 , wherein the heating step comprises heating the pipe to a temperature within the intercritical range that is sufficient to obtain between 5 to 90 percent austenite in the microstructure.  
     
     
         3 . The method of  claim 1 , wherein said heating step comprises heating the pipe to a temperature within the range of 1346 to 1562° F. (730 to 850° C.) for a time at temperature within the range of 5 to 120 minutes.  
     
     
         4 . The method of  claim 1 , wherein said cooling step (c) comprises cooling the pipe at a rate within the range of 1° C./second to 200° C./second.  
     
     
         5 . The method of  claim 4 , wherein said cooling step comprises water quenching the pipe.  
     
     
         6 . The method of  claim 1 , wherein said reheating step (c) comprises reheating the pipe to a temperature within the range of 572 to 1292° F. (300 to 700° C.).  
     
     
         7 . The method of  claim 1 , wherein said cooling step (d) comprises air cooling the pipe to room temperature.  
     
     
         8 . The method of  claim 1 , wherein said pipe has a yield/tensile ratio of 0.40 to 0.85 after cooling step (d).  
     
     
         9 . The method of  claim 1 , wherein steps (b), (c) and (d) are performed in-line on a seamless pipe mill.  
     
     
         10 . The method of  claim 1 , wherein steps (b), (c) and (d) are not performed in-line on a pipe mill.  
     
     
         11 . The method of  claim 1  which further comprises reeling said steel line pipe into coil form.  
     
     
         12 . The method of  claim 1  wherein said steel line pipe is in coil form, said method further comprising unreeling said steel line pipe.  
     
     
         13 . A method for the production of steel line pipe, comprising: 
 (a) providing a steel pipe having a composition consisting essentially of in weight percent: C 0.01 to 0.40, Mn 0.25 to 2.0, P residual to less than 0.5, S residual to less than 0.020, Si residual to 2.0, Cu residual to 1.0, Ni residual to 1.0, Cr residual to 2.0, Mo residual to 1.0, Al 0.010 minimum to less than 1.0, N residual to 0.030, V residual to less than 0.5, B residual to less than 0.02, Ti residual to less than 0.3, and Nb residual to less than 0.3, balance iron and incidental impurities;    (b) heating the pipe to a temperature within the temperature range of 1346 to 1562° F. (730 to 850° C.) for a time at temperature within the range of 5 to 120 minutes in order to obtain ferrite and austenite in the microstructure;    (c) cooling the heated pipe to a temperature below the M s  (martensite start) temperature at a cooling rate sufficient to cause the austenite present to transform to martensite;    (d) after cooling from the intercritical annealing temperature, reheating the pipe to a temperature reheating the pipe to a temperature within the range of 572 to 1292° F. (300 to 700° C.) in order to obtain the desired yield strength, tensile strength and yield strength to tensile strength ratio; and    (e) then air cooling the pipe to room temperature.