Rolled material for high strength spring, and wire for high strength spring using the same

Abstract

An object of the present invention is to provide a rolled material for high strength spring, which has excellent wire drawability even when suppressing the addition amount of an alloying element, and which can exhibit corrosion fatigue properties after quenching and tempering. The present invention provides a rolled material for high strength spring, including C, Si, Mn, P, S, Al, Cu and Ni, wherein an amount of nondiffusible hydrogen is 0.40 ppm by mass or less, and an area ratio of ferrite expressed as a percentage satisfies an inequality expression (1) below, and a total area ratio of bainite and martensite is 2% or less: Ferrite area ratio<{(0.77−[C])/0.77−[C]/3+0.08}×100  (1) where [name of element] in the above inequality expression (1) means a content expressed in % by mass of each element.

1 .- 5 . (canceled) 
     
     
         6 . A rolled material suitable for a high strength spring, comprising, in % by mass:
 C: 0.39 to 0.65%,   Si: 1.5 to 2.5%,   Mn: 0.15 to 1.2%,   P: exceeding 0% and 0.015% or less,   S: exceeding 0% and 0.015% or less,   Al: 0.001 to 0.1%,   Cu: 0.1 to 0.80%, and   Ni: 0.1 to 0.80%, with the balance being iron and inevitable impurities,   wherein:   an amount of nondiffusible hydrogen is 0.40 ppm by mass or less, and   an area ratio of ferrite expressed as a percentage satisfies an inequality expression (1) below, and a total area ratio of bainite and martensite is 2% or less:
   ferrite area ratio<{(0.77−[C])/0.77−[C]/3+0.08}×100  (1),
 
   where [name of element] in the above inequality expression (1) means a content expressed in % by mass of each element.   
     
     
         7 . The rolled material suitable for a high strength spring according to  claim 6 , further comprising, in % by mass, at least one belonging to any one of the following (a), (b), (c) and (d):
 (a) Cr: exceeding 0% and 1.2% or less,   (b) Ti: exceeding 0% and 0.13% or less,   (c) B: exceeding 0% and 0.01% or less, and   (d) at least one selected from the group consisting of Nb: exceeding 0% and 0.1% or less, Mo: exceeding 0% and 0.5% or less, and V: exceeding 0% and 0.4% or less.   
     
     
         8 . The rolled material suitable for a high strength spring according to  claim 6 , wherein an ideal critical diameter D i , which is calculated using an equation (2) below when B is not included, is in a range of 65 to 140 mm:
     D   i =25.4×(0.171+0.001×[C]+0.265×[C] 2 )×(3.3333×[Mn]+1)×(1+0.7×[Si])×(1+0.363×[Ni])×(1+2.16×[Cr])×(1+0.365×[Cu])×(1+1.73×[V])×(1+3×[Mo])  (2)
   where [name of element] in the above equations (2) means a content expressed in % by mass of each element.   
     
     
         9 . A wire suitable for a high strength spring, having a tensile strength of 1,900 MPa or more, obtained by wire-drawing the rolled material for high strength spring according to  claim 6 , followed by a quenching and tempering treatment. 
     
     
         10 . A wire suitable for a high strength spring, having a tensile strength of 1,900 MPa or more, obtained by wire-drawing the rolled material for high strength spring according to  claim 8 , followed by a quenching and tempering treatment. 
     
     
         11 . The rolled material suitable for a high strength spring according to  claim 7 , wherein an ideal critical diameter D i , which is calculated using an equation (2) below when B is not included or using an equation (3) below when B is included, is in a range of 65 to 140 mm:
     D   i =25.4×(0.171+0.001×[C]+0.265×[C] 2 )×(3.3333×[Mn]+1)×(1+0.7×[Si])×(1+0.363×[Ni])×(1+2.16×[Cr])×(1+0.365×[Cu])×(1+1.73×[V])×(1+3×[Mo])  (2)
       D   i =25.4×(0.171+0.001×[C]+0.265×[C] 2 )×(3.3333×[Mn]+1)×(1+0.7×[Si])×(1+0.363×[Ni])×(1+2.16×[Cr])×(1+0.365×[Cu])×(1+1.73×[V])×(1+3×[Mo])×(6.849017−46.78647×[C]+196.6635×[C] 2 −471.3978×[C] 3 +587.8504×[C] 4 −295.0410×[C] 5 )  (3)
   
       where [name of element] in the above equations (2) and (3) means a content expressed in % by mass of each element. 
     
     
         12 . A wire suitable for a high strength spring, having a tensile strength of 1,900 MPa or more, obtained by wire-drawing the rolled material for high strength spring according to  claim 7 , followed by a quenching and tempering treatment. 
     
     
         13 . A high strength spring comprising rolled material according to  claim 6 . 
     
     
         14 . A material comprising in % by mass:
 C: 0.39 to 0.65%,   Si: 1.5 to 2.5%,   Mn: 0.15 to 1.2%,   P: exceeding 0% and 0.015% or less,   S: exceeding 0% and 0.015% or less,   Al: 0.001 to 0.1%,   Cu: 0.1 to 0.80%,   Ni: 0.1 to 0.80%, with the balance being iron and inevitable impurities, and   nondiffusible hydrogen: 0.40 ppm by mass or less.   
     
     
         15 . The material according to  claim 14 ,
 wherein an area ratio of ferrite expressed as a percentage satisfies an inequality expression (1): ferrite area ratio<{(0.77−[C])/0.77−[C]/3+0.08}×100, and   wherein a total area ratio of bainite and martensite is 2% or less.