High cleanliness spring steel and high cleanliness spring excellent in fatigue properties

Abstract

High cleanliness spring steel useful in manufacturing a spring with SiO 2 -based inclusions being extremely controlled and excellent in fatigue properties is provided. High cleanliness spring steel which is steel containing; C: 1.2% (means mass %, hereafter the same with respect to the component) or below (not inclusive of 0%), Si: 1.2-4%, Mn: 0.1-2.0%, Al: 0.01% or below (not inclusive of 0%), and the balance comprising iron with inevitable impurities, wherein; the total of oxide-based inclusions of 4 or above of L (the large diameter of an inclusion)/D (the short diameter of an inclusion) and 25 μm or above of D and oxide-based inclusions of less than 4 L/D and 25 μm or above of L, in the oxide-based inclusions of 25 mass % or above of oxygen concentration and 70% (means mass %, hereafter the same with respect to inclusions) or above of SiO 2 content when Al 2 O 3 +MgO+CaO+SiO 2 +MnO=100% is presumed, out of inclusions in the steel, is 20 nos./500 g or below.

1 . A spring steel, comprising:
 an oxide-based inclusion with a width of 3 μm or above present in a surface layer side of from a surface of a wire rod to a depth of ¼ of the diameter, satisfies an inequality relationship (1) below, wherein the width is defined by a diameter of the inclusion in a direction perpendicular to an axial direction when observed by a section comprising an axial center line of the wire rod;   a concentration of MgO of 5 mass % or below (inclusive of 0 mass %) based on the total weight of the inclusion; and   a concentration of MnO: 10 mass % or below (inclusive of 0 mass %) based on the total weight of the inclusion,   wherein: [Cn], [An] and [Sn] of CaO, Al 2 O 3  and SiO 2  in the oxide-based inclusion are respectively defined by (2)-(4) below;   the concentrations of [Cn], [An] and [Sn] satisfy relationships of (5) and (6) below; and   a melting point of the oxide-based inclusion in a CaO—Al 2 O 3 —SiO 2  three component system phase diagram is 1,500 DEG C. or below, and   wherein: based on the total weight of the inclusion,
   CaO+Al 2 O 3 +SiO 2 >80(mass %)  (1) 
   [Cn](mass %)=[(CaO)/(CaO+Al 2 O 3 +SiO 2 )]×100  (2) 
   [An](mass %)=[(Al 2 O 3 )/(CaO+Al 2 O 3 +SiO 2 )]×100  (3) 
   [Sn](mass %)=[(SiO 2 )/(CaO+Al 2 O 3 +SiO 2 )]×100  (4) 
   [An]+4.29[Sn]≦221.9(mass %)  (5) 
   [An]≦30(mass %)  (6) 
   
     
     
         2 . The spring steel according to  claim 1 , comprising: based on the total weight of the steel,
 C, 1.2 mass % or below (not inclusive of 0%);   Si: 1.2-4 mass %;   Mn: 0.1-2.0 mass %; and   Al: 0.01 mass % or below (not inclusive of 0%).   
     
     
         3 . The spring steel according to  claim 2 , further comprising at least one element selected from the group consist of Cr, Ni, V, Nb, Mo, W, Cu, Ti, Li, Na, K and rare earth elements. 
     
     
         4 . The spring steel according to  claim 3 , comprising: based on the total weight of the steel,
 Cr: 0.5-3 mass %;   Ni: 0.5 mass % or below;   V: 0.5 mass % or below;   Nb: 0.1 mass % or below;   Mo: 0.5 mass % or below;   W: 0.5 mass % or below;   Cu: 0.1 mass % or below;   Ti: 0.1 mass % or below;   Li: 0.0005 mass % or below;   Na: 0.001 mass % or below;   K: 0.001 mass % or below; and   REM: 0.001 mass % or below.   
     
     
         5 . The spring steel according to  claim 1 , wherein the inclusion comprises CaO, Al 2 O 3 , SiO 2 , MnO and MgO. 
     
     
         6 . The spring steel according to  claim 1 , wherein the melting point of the oxide-based inclusion in a CaO—Al 2 O 3 —SiO 2  three component system phase diagram is 1,400 DEG C. or below. 
     
     
         7 . The spring steel according to  claim 1 , wherein the amount of MnO in the inclusion is less than 3.0 mass %. 
     
     
         8 . The spring steel according to  claim 1 , wherein the amount of MgO in the inclusion is 2.5 mass % or less. 
     
     
         9 . A spring comprising a high cleanliness spring steel according  claim 1 .