Steel For Springs, Process Of Manufacture For Spring Using This Steel, And Spring Made From Such Steel

Abstract

A spring steel with high fatigue resistance in air and in corrosive conditions and with high resistance to cyclic sag, having the composition in weight percent: C=0.45-0.70% Si=1.65-2.50% Mn=0.20-0.75% Cr=0.60-2% Ni=0.15-1% Mo=traces-1% V=0.003-0.8% Cu=0.10-1% Ti=0.020-0.2% Nb=traces-0.2% AI=0.002-0.050% P=traces-0.015% S=traces-0.015% O=traces-0.0020% N=0.0020-0.0110% the balance being iron, and impurities resulting from the steel making process, where the carbon equivalent Ceq content calculated according to the formula: Ceq %=[C %]+0.12 [Si %]+0.17 [Mn %]−0.1 [Ni %]+0.13 [Cr %]−0.24 [V %] is between 0.80 and 1.00%, and whose hardness, after quenching and tempering, is greater than or equal to 55 HRC.

1 : A spring steel composition characterized by high fatigue resistance in air and in corrosive conditions and high resistance to cyclic sag, said composition comprising in weight percent:
 C=0.45-0.70%   Si=1.65-2.50%   Mn=0.20-0.75%   Cr=0.60-2%   Ni=0.15-1%   Mo=traces-1%   V=0.003-0.8%   Cu=0.10-1%   Ti=0.020-0.2%   Nb=traces-0.2%   Al=0.002-0.050%   P=traces-0.015%   S=traces-0.015%   O=traces-0.0020%   N=0.0020-0.0110%   the balance being iron, and impurities resulting from the steel making process, wherein the carbon equivalent (Ceq) content calculated according to the formula:
   Ceq %=[C %]+0.12 [Si %]+0.17 [Mn %]−0.1 [Ni %]+0.13 [Cr %]−0.24 [V %] 
   
       is between 0.80 and 1.00%, and whose hardness, after quenching and tempering, is greater than or equal to 55 HRC. 
     
     
         2 : The sing steel according to  claim 1 , characterized in that the maximum size of titanium nitrides or carbonitrides observed at 1.5±0.5 mm of the surface area of a bar, a wire rod, a slug or a spring over 100 mm 2  of the surface area of the section is less than or equal to 20 μm, said size being the square root of the surface area of the inclusions considered as squares. 
     
     
         3 : The spring steel according to  claim 1 , characterized in that its composition comprises:
 C=0.45-0.65%   Si=1.65-2.20%   Mn=0.20-0.65%   Cr=0.80-1.7%   Ni=0.15-0.80%   Mo=traces-0.80%   V=0.003-0.5%   Cu=0.10-0.90%   Ti=0.020-0.15%   Nb=traces-0.15%   AI=0.002-0.050%   P=traces-0.010%   S=traces-0.010%   O=traces-0.0020%   N=0.0020-0.0110%   the balance being iron and impurities resulting from the steel making process.   
     
     
         4 : A manufacturing process for a spring steel with high fatigue resistance in air and in corrosive conditions and high resistance to cyclic sag, according to which a liquid steel is made in a converter or an electric furnace, its composition is adjusted, it is cast into blooms or continuous flow billets or ingots that are left to cool to room temperature; that are rolled into bars, wire rods or slugs for transformation into springs, characterized in that:
 the steel is the type according to  claim 1 :   the blooms, billets or ingots after they become solid have a minimum mean cooling rate of 0.3° C./s between 1450-1300° C.;   said blooms, billets or ingots are rolled between 1200-800° C. in one or two reheating and rolling cycles; and   bars, wire rods or slugs, or springs made from these, are austenitized between 850-1000° C., followed by a water quench, a polymer quench or an oil quench, and by tempering at 300-550° C., so as to deliver steel with a hardness greater than or equal to 55 HRC.   
     
     
         5 : A spring, characterized in that it is made of a steel according to  claim 1 . 
     
     
         6 : A spring according to  claim 5 , characterized in that it is made of a steel comprising in weight percent:
 C=0.45-0.70%   Si=1.65-2.50%   Mn=0.20-0.75%   Cr=0.60-2%   Ni=0.15-1%   Mo=traces-1%   V=0.003-0.8%   Cu=0.10-1%   Ti=0.020-0.2%   Nb=traces-0.2%   Al=0.002-0.050%   P=traces-0.015%   S=traces-0.015%   O=traces-0.0020%   N=0.0020-0.0110%   the balance being iron, and impurities resulting from the steel making process, wherein the carbon equivalent (Ceq) content calculated according to the formula:
   Ceq %=[C %]+0.12 [Si %]+0.17 [Mn %]−0.1 [Ni %]+0.13 [Cr %]−0.24 [V %] 
   
       is between 0.80 and 1.00%, and whose hardness, after quenching and tempering, is greater than or equal to 55 HRC,
 wherein said steel is made in a converter or an electric furnace, its composition is adjusted and is cast into blooms or continuous flow billets or ingots that are left to cool to room temperature and are rolled into bars, wire rods or slugs for transformation into springs, characterized in that: 
 the blooms, billets or ingots after they become solid have a minimum means cooling rate of 0.3° C./s between 1450-1300° C.; 
 said blooms, billets or ingots are rolled between 1200-800° C. in one or two reheating and rolling cycles; and 
 bars, wire rods or slugs, or springs made from these, are austenitized between 850-1000° C., followed by a water quench, a polymer quench or an oil quench, and by tempering at 300-550° C., so as to deliver steel with a hardness greater than or equal to 55 HRC.