Hot-working tool steel having excellent toughness and high-temperature strength and method for production thereof

Abstract

Disclosed is a hot-working tool steel having improved toughness and high-temperature strength. Also disclosed is a method for producing the hot-working tool steel. The hot-working tool steel comprises the following components (by mass): C: 0.34-0.40%, Si: 0.3-0.5%, Mn: 0.45-0.75%, Ni: 0-0.5% (exclusive), Cr: 4.9-5.5%, (Mo+1/2W): 2.5-2.9% (provided that Mo and W are contained singly or in combination), and V: 0.5-0.7%, with the remainder being Fe and unavoidable impurities. Preferably, the cross-sectional structure of the hot-working tool steel upon quenching contains a granular structure and an acicular structure, wherein the granular structure (A %) accounts for 45 area % or less, the acicular structure (B %) accounts for 40 area % or less, and the remaining austenite (C %) accounts for 5 to 20 volume %.; Also disclosed is a method for producing a hot-working tool steel, which comprises tempering the above-mentioned hot-working tool steel so that a value X determined by the following relational expression between a tempered hardness (HRC) and the percentages of the tissues becomes 40 or greater. X =[−0.36(HRC)−1.47( A %)−1.67×( B %)+6.55×( C %)+72.91]

1 . A hot-working tool steel having excellent toughness and high-temperature strength, consisting essentially of, by mass percent,
 0.34 to 0.40% of carbon,   0.3 to 0.5% of Si,   0.45 to 0.75% of Mn,   0 to less than 0.5% of Ni,   4.9 to 5.5% of Cr,   2.5 to 2.9% of a total amount of Mo and a half of W, Mo and W being contained singly or in combination,   0.5 to 0.7% of V, and   the balance being Fe and unavoidable impurities.   
   
   
       2 . The hot-working tool steel according to  claim 1 , containing 0.35 to 0.39% of carbon, by mass percent. 
   
   
       3 . The hot-working tool steel according to  claim 1 , containing 0.35 to 0.45% of Si, by mass percent. 
   
   
       4 . The hot-working tool steel according to  claim 1 , containing 0.5 to 0.7% of Mn, by mass percent. 
   
   
       5 . The hot-working tool steel according to  claim 1 , containing 0.01 to 0.3% of Ni, by mass percent. 
   
   
       6 . The hot-working tool steel according to  claim 1 , containing 5.0 to 5.4% of Cr, by mass percent. 
   
   
       7 . The hot-working tool steel according to  claim 1 , containing 2.6 to 2.8% of a total amount of Mo and a half of W, by mass percent, Mo and W being contained singly or in combination. 
   
   
       8 . The hot-working tool steel according to  claim 1 , containing 0.55 to 0.65% of V, by mass percent. 
   
   
       9 . The hot-working tool steel according to  claim 1 , having a hardness of not lower than 40HRC. 
   
   
       10 . The hot-working tool steel according to  claim 1 , having a hardness of not lower than 43HRC. 
   
   
       11 . The hot-working tool steel according to  claim 1 , having a hardness of not lower than 45HRC. 
   
   
       12 . The hot-working tool steel according to  claim 9 , having a hardness of not higher than 49HRC. 
   
   
       13 . The hot-working tool steel according to  claim 1 , wherein the steel has a cross-sectional structure, after quenching, including a granular structure and an acicular structure, an area percentage of the granular structure (A %) being not more than 45%, an area percentage of the acicular structure (B %) being not more than 40%, and a volume percentage of retained austenite (C %) being 5 to 20%. 
   
   
       14 . A method of producing a hot-working tool steel having excellent toughness and high-temperature strength, wherein the steel according to  claim 13  is tempered so that a value X determined by a following relational expression between a tempered hardness (HRC) and percentages of the structures is not smaller than 40:
     X=[− 0.36×(HRC)−1.47×( A  %)−1.67×( B  %)+6.55×( C  %)+72.91]   
   
   
       15 . The method according to  claim 14 , wherein the steel is tempered to 40 to 49HRC. 
   
   
       16 . The method according to  claim 14 , wherein the steel is tempered to 43 to 49HRC. 
   
   
       17 . The method according to  claim 14 , wherein the steel is tempered to 45 to 49HRC. 
   
   
       18 . The hot-working tool steel according to  claim 10 , having a hardness of not higher than 49HRC. 
   
   
       19 . The hot-working tool steel according to  claim 11 , having a hardness of not higher than 49HRC.