High strength forged steel and large forged component

Abstract

A high strength forged steel comprises composition which comprises predetermined amounts of predetermined chemical elements and a balance being Fe and inevitable purities. A major constituent of a metallographic structure of the steel comprises bainite, martensite or a combination thereof. An average crystal grain size in steel microstructures between a near-surface level below a surface of the steel at a distance of 5 mm and a mid level below the surface at a distance of one quarter of the thickness of the steel is less than or equal to 50 μm. A difference between peak values of crystal grain size distributions at the near-surface level and the mid level is greater than or equal to 10 μm and less than or equal to 30 μm. The total index as determined in accordance with DIN 50602-1985, method K0 for evaluating non-metallic inclusions is preferably less than or equal to 10.

1 . A high strength forged steel, comprising Fe and:
 C in an amount greater than or equal to 0.40% by mass and less than or equal to 0.60% by mass;   Si in an amount greater than or equal to 0.05% by mass and less than or equal to 0.40% by mass;   Mn in an amount greater than or equal to 0.70% by mass and less than or equal to 1.20% by mass;   Cu in an amount greater than or equal to 0% by mass and less than or equal to 0.50% by mass;   Ni in an amount greater than or equal to 0.40% by mass and less than or equal to 1.00% by mass;   Cr in an amount greater than or equal to 1.00% by mass and less than or equal to 2.00% by mass;   Mo in an amount greater than or equal to 0.20% by mass and less than or equal to 0.50% by mass;   V in an amount greater than or equal to 0.05% by mass and less than or equal to 0.30% by mass;   Al in an amount greater than or equal to 0.020% by mass and less than or equal to 0.060% by mass;   N in an amount greater than or equal to 0.0040% by mass and less than or equal to 0.0080% by mass;   O in an amount greater than 0 ppm and less than or equal to 30 ppm; and   wherein a major constituent of a metallographic structure of the high strength forged steel comprises at least one selected from the group consisting of bainite and martensite;   wherein an average crystal grain size in steel microstructures between a near-surface level of the high strength forged steel and a mid level of the high strength forged steel is less than or equal to 50 μm;   wherein the near-surface level is 5 mm below a surface of the high strength forged steel and the mid level is below the surface at a distance of one quarter of a total thickness of the high strength forged steel; and   wherein a difference between a peak value of a crystal grain size distribution at the near-surface level and a peak value of a crystal grain size distribution at the mid level is greater than or equal to 10 μm and less than or equal to 30 μm.   
     
     
         2 . The high strength forged steel of  claim 1 , wherein a total index, as determined in accordance with method K0 of DIN 50602-1985, is less than or equal to 10. 
     
     
         3 . A large forged component comprising the high strength forged steel of  claim 1 . 
     
     
         4 . A large forged component comprising the high strength forged steel of  claim 2 . 
     
     
         5 . The high strength forged steel of  claim 1 , wherein a lower limit of an area fraction of the major constituent of the metallographic structure is 0.95. 
     
     
         6 . The high strength forged steel of  claim 1 , wherein a lower limit of an area fraction of the major constituent of the metallographic structure is 0.98. 
     
     
         7 . The high strength forged steel of  claim 1 , wherein an area fraction of the major constituent of the metallographic structure is 1.00. 
     
     
         8 . The high strength forged steel of  claim 1 , comprising C in an amount greater than or equal to 0.45% by mass and less than or equal to 0.55% by mass. 
     
     
         9 . The high strength forged steel of  claim 1 , comprising Si in an amount greater than or equal to 0.05% by mass and less than or equal to 0.20% by mass. 
     
     
         10 . The high strength forged steel of  claim 1 , comprising Mn in an amount greater than or equal to 0.70% by mass and less than or equal to 1.00% by mass. 
     
     
         11 . The high strength forged steel of  claim 1 , comprising Cu in an amount greater than or equal to 0.10% by mass and less than or equal to 0.50% by mass. 
     
     
         12 . The high strength forged steel of  claim 1 , comprising Ni in an amount greater than or equal to 0.55% by mass and less than or equal to 0.80% by mass. 
     
     
         13 . The high strength forged steel of  claim 1 , comprising Cr in an amount greater than or equal to 1.50% by mass and less than or equal to 1.80% by mass. 
     
     
         14 . The high strength forged steel of  claim 1 , comprising Mo in an amount greater than or equal to 0.30% by mass and less than or equal to 0.40% by mass. 
     
     
         15 . The high strength forged steel of  claim 1 , comprising V in an amount greater than or equal to 0.05% by mass and less than or equal to 0.15% by mass. 
     
     
         16 . The high strength forged steel of  claim 1 , comprising N in an amount greater than or equal to 0.0040% by mass and less than or equal to 0.0060% by mass. 
     
     
         17 . The high strength forged steel of  claim 1 , comprising O in an amount greater than or equal to 0 ppm and less than or equal to 10 ppm. 
     
     
         18 . The high strength forged steel of  claim 1 , wherein a difference between a peak value of a crystal grain size distribution at the near-surface level and a peak value of a crystal grain size distribution at the mid level is greater than or equal to 18 μm and less than or equal to 25 μm. 
     
     
         19 . The high strength forged steel of  claim 1 , wherein the major constituent of the metallographic structure is present in an amount greater than 50% by mass.