Ultrahigh strength welded joint and ultrahigh strength welded steel pipe excellent in cold cracking resistance of weld metal, and methods for producing the same

Abstract

The present invention, in a welded joint of steel sheets and a steel pipe body having a tensile strength of 800 MPa or more (over X100 in API Standards), provides: the welded joint of the steel sheets and the steel pipe produced by forming a steel sheet into a cylindrical shape and welding both the ends thereof, both excellent in cold cracking resistance; and methods for producing them. The present invention includes an ultrahigh strength welded joint and an ultrahigh strength welded steel pipe excellent in the cold cracking resistance of a weld metal, characterized in that the amount of non-diffusible hydrogen in the inner side weld metal is 0.01 ppm or more. It is preferable that the ratio of the non-diffusible hydrogen amount to the total hydrogen amount in said inner side weld metal is 0.5% or more. Further, it is preferable that Mo carbide is contained by not less than 1 piece/μm 2 in said inner side weld metal. The present invention also includes a method for producing said welded joint and welded steel pipe, characterized by welding the butted portion from the inner side and thereafter welding it from the outer side so that the reheating temperature of the inner side weld metal may reach within the range from 500° C. to 700° C.

1 - 10 . (canceled) 
     
     
         11 . A welded joint for a steel pipe, comprising an inner side weld metal, an outer side weld metal, and a base metal, wherein weld metal is excellent in cold cracking resistance
 said welded joint being formed from a steel plate having a first longitudinal edge and a second longitudinal edge, said steel plate having a pipe shape with the first edge adjacent the second edge and said pipe shape steel plate having an inner side and an outer side, said pipe shape steel plate comprising said base metal;   said welded joint located between and joining said adjacent first longitudinal edge and said second longitudinal edge of said pipe shape steel plate;   wherein the inner side and outer side weld metals contain, in mass,
 C: 0.04 to 0.14%, 
 Si: 0.05 to 1.0%, 
 Mn: 1.2 to 2.2%, 
 P: 0.015% or less, 
 S: 0.010% or less, 
 Ni: 1.3 to 6.0%, 
 Mo: 0.5 to 2.0%, 
 Cr: 0.5 to 2.0%, 
 Nb: 0.001 to 0.1%, 
 V: 0.01 to 0.2%, 
 Ti: 0.003 to 0.05%, 
 Al: 0.02% or less, and 
 B: 0.005% or less, 
   with the balance being Fe and unavoidable impurities;   said inner side weld metal containing fine Mo carbides in an amount of not less than 1 piece/μm 2 , wherein said inner side weld metal is formed prior to said outer side weld metal, and wherein said fine Mo carbides is produced by reheating said inner side weld metal to a temperature within a range from 500° C. to 700° C. by the outer side welding;   wherein said inner side weld metal contains non-diffusible hydrogen in an amount of 0.01 ppm or higher and a ratio of non-diffusible hydrogen amount to total hydrogen amount in said inner side weld metal is 0.5% or higher.   
     
     
         12 . A welded joint excellent in the cold cracking resistance of a weld metal according to  claim 11 , characterized in that the base metal contains, in mass,
 C: 0.03 to 0.10%   Si: 0.6% or less,   Mn: 1.7 to 2.5%,   P: 0.015% or less,   S: 0.003% or less,   Ni: 0.1 to 1.0%,   Mo: 0.15 to 0.60%,   Nb: 0.001 to 0.10%, and   Ti: 0.005 to 0.030%,   and further one or more of   Cr: 0.8% or less,   V: 0.10% or less,   Al: 0.06% or less,   B: 0.005% or less,   N: 0.001 to 0.006%,   Cu: 1.0% or less,   Ca: 0.01% or less,   REM: 0.02% or less, and   Mg: 0.006% or less,   with the balance being Fe and unavoidable impurities.