Steel wire material for spring and its producing method

Abstract

The steel wire material for a spring of the invention contains; C: 0.37-0.54%, Si: 1.7-2.30%, Mn: 0.1-1.30%, Cr: 0.15-1.1%, Cu: 0.15-0.6%, Ti: 0.010-0.1%, Al: 0.003-0.05%, and the balance including iron with inevitable impurities, wherein ferrite decarburized layer depth is 0.01 mm or less, whole decarburized layer depth is 0.20 mm or less, and fracture reduction of area is 25% or more. It alternately may contain; C: 0.38-0.47%, Si: 1.9-2.5%, Mn: 0.6-1.3%, Ti: 0.05-0.15%, Al: 0.003-0.1%, and the balance including iron with inevitable impurities, wherein ferrite decarburized layer depth is 0.01 mm or less, Ceq1 in the equation (1) below is 0.580 or more, Ceq2 in the equation (2) below is 0.49 or less, and Ceq3 in the equation (3) below is 0.570 or less. Ceq 1=[C]+0.11[Si]−0.07[Mn]−0.05[Ni]+0.02[Cr]  (1) Ceq 2=[C]+0.30[Cr]−0.15[Ni]−0.70[Cu]  (2) Ceq 3=[C]−0.04[Si]+0.24[Mn]+0.10[Ni]+0.20[Cr]−0.89[Ti]−1.92[Nb]  (3) (In the above equations, [ ] shows the content (mass %) of each element in steel.)

1 . A steel wire material for a spring comprising;
 C: 0.37-0.54% (in mass %, hereafter the same)   Si: 1.7-2.30%   Mn: 0.1-1.30%   Cr: 0.15-1.1%   Cu: 0.15-0.6%   Ti: 0.010-0.1%   Al: 0.003-0.05%, and   the balance composed of iron with inevitable impurities, wherein;   the depth of ferrite decarburized layer is 0.01 mm or less,   the depth of whole decarburized layer is 0.20 mm or less, and   fracture reduction of area is 25% or more.   
   
   
       2 . The steel wire material for a spring as set forth in  claim 1 , further comprising Ni: 0.15-0.7%. 
   
   
       3 . The steel wire material for a spring as set forth in  claim 1 , further comprising either one of V: 0.07-0.4% and Nb: 0.01-0.1%. 
   
   
       4 . The steel wire material for a spring as set forth in  claim 1 , further comprising Mo: 0.01-0.3%. 
   
   
       5 . The steel wire material for a spring as set forth in  claim 1 , wherein P is 0.020% or less, S is 0.020% or less, N is 0.0070% or less, and O is 0.0015% or less. 
   
   
       6 . A method for producing the steel wire material for a spring as set forth in  claim 1 , comprising the successive steps of hot rolling, coiling, and cooling on a cooling bed of steel, wherein;
 when A 1  transformation point, A 3  transformation point, and A 4  transformation point at the time C=0 wt % in the phase equilibrium diagram of the steel are designated respectively as A 1(c=0)  transformation point, A 3(c=0)  transformation point, A 4(c=0)  transformation point,   the heating temperature of steel before hot rolling is 900° C. or higher and A 4(c=0)  transformation point or lower,   the maximum reaching temperature of steel during finish rolling of hot rolling is A 3(c=0)  transformation point or higher and A 4(c=0)  transformation point or lower,   the placing temperature of the coil onto the cooling bed is A 1(c=0)  transformation point or higher and A 1(c=0)  transformation point +50° C. or lower, and   cooling is performed in the temperature range where ferrite precipitates on the continuous cooling curve corresponding to 8.0-11 crystal grain size number of austenite grains at the cooling speed of 1.0° C./s or faster at the close parts of the coil and 8° C./s or slower at the rough parts of the coil.   
   
   
       7 . A method for producing the steel wire material for a spring as set forth in  claim 1 , comprising the successive steps of hot rolling, coiling, and cooling on a cooling bed of steel, wherein;
 the heating temperature of the steel before hot rolling is 900° C. or higher and 1,250° C. or lower,   the maximum reaching temperature of the steel during finish rolling of hot rolling is 1,050° C. or higher and 1,200° C. or lower,   the placing temperature of the coil onto the cooling bed is 900° C. or higher and 980° C. or lower, and   cooling is performed in the temperature range of the temperature 750° C.-600° C. at the cooling speed of 1.0° C./s or faster at the close parts of the coil and 8° C./s or slower at the rough parts of the coil.   
   
   
       8 . The method for producing the steel wire material for a spring as set forth in  claim 6 , wherein the maximum reaching temperature of the steel during finish rolling is controlled into the range by working heat generation of the steel in hot rolling without performing water cooling of the steel before finish rolling. 
   
   
       9 . The method for producing the steel wire material for a spring as set forth in  claim 6 , wherein the ideal critical diameter DCI of the steel as exhibited in the equation (1) below is 75-135 mm.
     DCI (mm)=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])  (1)   (In the above equation, [ ] shows the content (mass %) of each element in steel.)   
   
   
       10 . A steel wire material for a spring comprising;
 C: 0.38-0.47%   Si: 1.9-2.5%   Mn: 0.6-1.3%   Ti: 0.05-0.15%   Al: 0.003-0.1%, and   the balance composed of iron with inevitable impurities, wherein;
 the depth of ferrite decarburized layer is 0.01 mm or less, 
 Ceq1 as exhibited in the equation (1) below is 0.580 or more, 
 Ceq2 as exhibited in the equation (2) below is 0.49 or less, and 
 Ceq3 as exhibited in the equation (3) below is 0.570 or less.
     Ceq 1=[C]+0.11[Si]−0.07[Mn]−0.05[Ni]+0.02[Cr]  (1) 
     Ceq 2=[C]+0.30[Cr]−0.15[Ni]−0.70[Cu]  (2) 
     Ceq 3=[C]−0.04[Si]+0.24[Mn]+0.10[Ni]+0.20[Cr]−0.89[Ti]−1.92[Nb]  (3) 
 
   
     (In the above equation, [ ] shows the content (mass %) of each element in steel.) 
   
   
       11 . The steel wire material for a spring as set forth in  claim 10 , further comprising Cr: 0.1-0.4%. 
   
   
       12 . The steel wire material for a spring as set forth in  claim 10 , further comprising Cu: 0.1-0.7%. 
   
   
       13 . The steel wire material for a spring as set forth in  claim 10 , further comprising Ni: 0.1-0.7% 
   
   
       14 . The steel wire material for a spring as set forth in  claim 10 , further comprising Nb: 0.01-0.1%. 
   
   
       15 . The steel wire material for a spring as set forth in  claim 10 , wherein; P is 0.02% or less, S is 0.02% or less, N is 0.007% or less, and O is 0.0015% or less. 
   
   
       16 . The steel wire material for a spring as set forth in  claim 10 , further characterized in that; after performing the corrosion test described below, out of corrosion pits observed on the surface of the test piece, five or more corrosion pits are selected starting from one with a greater amount of depth, and the average value of aspect ratios as exhibited in the equation (4) below of those corrosion pits is 0.9 or less.
   Aspect ratio=(corrosion pit depth×2)/(corrosion pit width)  (4)   Corrosion test:
 after the steel wire material for a spring is heated at a temperature of 925° C. for 10 minutes, it is cooled and oil quenched by the oil of a temperature of 70° C., then, after tempering by heating at 400° C. for 60 minutes, the test piece for a corrosion test is fabricated with the surface being polished with #800 emery paper; 
 5 wt % NaCl aqueous solution is sprayed to this test piece at 35° C. for 8 hours in accordance with JIS Z 2371, then, letting the treatment of the test piece being kept in the wet environment of 60% humidity and a temperature of 35° C. for 16 hours be one cycle, 14 cycles total are carried out; and 
 the rust is removed and then the corrosion pits on the surface of the test piece are observed by a laser microscope. 
   
   
   
       17 . The steel wire material for a spring as set forth in  claim 1  or  claim 10 , further comprising B: 0.0003-0.005%.