Heavy wall steel material having superior weldability and method for producing the same

Abstract

A tough high strength heavy wall steel material having superior weldability is provided, said steel material has a diameter or a side 5 mm or more in length, and comprises oxides 1 μm or less in particle diameter homogeneously dispersed at a dispersion density in a range of from 10,000 to 100,000 particles/mm 2 and uniform ferrite grains 2 μm or less in grain diameter formed over the entire plane making a right angle with respect to the rolling direction.

1 . A heavy wall steel material having superior weldability, which is a steel material having a diameter or a side 5 mm or more in length and which comprises oxides 1 μm or less in particle diameter homogeneously dispersed at a dispersion density in a range of from 10,000 to 100,000 particles/mm 2  and uniform ferrite grains 2 μm or less in grain diameter formed over the entire plane making a right angle with respect to the rolling direction.  
   
   
       2 . A method for producing a heavy wall steel material having superior weldability, which comprises allowing oxide crystals 1 μm or less in particle diameter to form in the texture and uniformly dispersing them at a dispersion density of from 10,000 to 100,000 particles/mm 2 , rolling the resulting steel material through a hole profile in the temperature range of 400° C. or higher but not higher than the Ac3 transformation point, and subjecting it to a recrystallization treatment to form uniform ferrite grains 2 μm or less in grain diameter over the plane making a right angle with respect to the rolling direction, thereby obtaining a heavy wall steel material having superior weldability and having a diameter or a side 5 mm or more in length.  
   
   
       3 . A method for producing a heavy wall steel material having superior weldability as claimed in  claim 2 , wherein the method comprises undercooling a molten steel by placing it inside an oxide slag, thereby allowing oxide crystals 1 μm or less in particle diameter to form in the texture and uniformly dispersing them at a dispersion density of from 10,000 to 100,000 particles/mm 2 .  
   
   
       4 . A method for producing a heavy wall steel material having superior weldability as claimed in  claim 3 , wherein the molten steel contains: 
 carbon (C) at an amount corresponding to a quantity as such that carbides account for 20% by volume or less in the resulting material;    silicon (Si) accounting for 0.8% by weight or less;    manganese (Mn) accounting from 0.05 to 3.0% by weight;    at least one element which forms oxides, which is selected from the group consisting of Ti, Mg, and Al, said elements accounting for 0.3% by weight either alone or in the form of a mixture thereof; and    balance iron (Fe) together with unavoidable impurities.