Steel for leaf springs of automobiles and a method of manufacturing of a leaf thereof

Abstract

A steel for leaf spring including of the following elements 0.4% ≦ C ≦ 0.7 %; 0.5% ≦ Mn ≦1.5 %;1% ≦ Si ≦ 2.5 %; 0.001% ≦ Al ≦ 0.1%; 0.1% ≦ Ni ≦ 1%;0.2% ≦ Cr ≦ 1.5 %; 0 ≦ P ≦ 0.09%; 0 ≦ S ≦ 0.09%; 0% ≦ N ≦ 0.09%; 0% ≦ Mo ≦ 0.5%; 0% ≦ V ≦ 0.2%; 0% ≦ Nb ≦ 0.1%; 0% ≦ Ti ≦ 0.1%; 0% ≦ Cu ≦ 1%; 0% ≦ B ≦ 0.008%; 0% ≦ Sn ≦ 0.1%; 0% ≦ Ce ≦ 0.1%; 0% ≦ Mg ≦ 0.10%; 0% ≦ Zr ≦ 0.10%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel including, by area percentage, 75% to 98% of Martensite, 2% to 20% of Residual Austenite, with a cumulative optional presence of bainite and ferrite between 0% to 5%.

What is claimed is: 
     
         1 - 21 . (canceled) 
     
     
         22 . A steel for leaf spring comprising a composition of the following elements, expressed in percentage by weight:
         0.4   %   ≦   C   ≦   0.7   %   ;                   0.5   %   ≦   Mn   ≦   1.5   %   ;                   1   %   ≦   Si   ≦   2.5   %   ;                   0.001   %   ≦   Al   ≦   0.1   %   ;                   0.1   %   ≦   Ni   ≦   1   %   ;                   0.2   %   ≦   Cr   ≦   1.5   %   ;                   0   ≦   P   ≦   0.09   %   ;                   0   ≦   S   ≦   0.09   %   ;                   0%   ≦   N   ≦   0.09   %   ;           and optionally one or more of the following elements           0   %   ≦   Mo   ≦   0.5   ;                   0   %   ≦   V   ≦   0.2   %   ;                   0   %   ≦   Nb   ≦   0.1   %   ;                   0   %   ≦   Ti   ≦   0.1   %   ;                   0   %   ≦   Cu   ≦   1   %   ;                   0   %   ≦   B   ≦   0.008   %   ;                   0   %   ≦   Sn   ≦   0.1   %   ;                   0   %   ≦   Ce   ≦   0.1   %   ;                   0   %   ≦   Mg   ≦   0   .10%;                   0   %   ≦   Zr   ≦   0.10   %   ;           a remainder of the composition being composed of iron and unavoidable impurities caused by processing,   a microstructure of the steel comprising, by area percentage, 75% to 98% of Martensite, 2% to 20% of Residual Austenite, with a cumulative optional presence of bainite and ferrite between 0% to 5%.   
     
     
         23 . The steel as recited in  claim 22  wherein the composition includes 1.2% to 2.4% of Silicon. 
     
     
         24 . The steel as recited in  claim 22  wherein the composition includes 0.45% to 0.6% of Carbon. 
     
     
         25 . The steel as recited in  claim 22  wherein the composition includes 0.001 % to 0.09% of Aluminum. 
     
     
         26 . The steel as recited in  claim 22  wherein the composition includes 0.6% to 1.4% of Manganese. 
     
     
         27 . The steel as recited in  claim 22  wherein the composition includes 0.3% to 1.4% of Chromium. 
     
     
         28 . The steel as recited in  claim 22  wherein the composition includes 0.1% to 0.9% of Nickel. 
     
     
         29 . The steel as recited in  claim 22  wherein the Martensite is between 80% and 97%. 
     
     
         30 . The steel as recited in  claim 22  wherein the Residual Austenite is between 3% and 18%. 
     
     
         31 . The steel as recited in  claim 22  wherein the cumulative optional presence of bainite and ferrite is between 0% and 4%. 
     
     
         32 . The steel as recited in  claim 22  wherein the presence of ferrite is between 0% and 1%. 
     
     
         33 . The steel as recited in  claim 22  wherein, the Ultimate tensile strength is greater than 1650 MPa. 
     
     
         34 . The steel as recited in  claim 22  wherein the steel has hardness of 480Hv or more. 
     
     
         35 . The steel as recited in  claim 22  wherein the steel has a fatigue endurance of at least 120000 cycles when tested at minimum stress of 1100 MPa. 
     
     
         36 . The steel as recited in  claim 22  wherein the steel has a striction more than 25%. 
     
     
         37 . A method of production a leaf of a leaf spring of steel comprising the following successive steps: 
 providing in the form of a semi-finished product a steel composition of the following elements, expressed in percentage by weight:
         0.4   %   ≦   c   ≦   0.7   %   ;         
         0.5   %   ≦   Mn   ≦   1.5   %   ;         
         1   %   ≦   Si   ≦   2.5   %   ;         
         0.001   %   ≦   Al   ≦   0.1   %   ;         
         0.1   %   ≦   Ni   ≦   1   %   ;         
         0.2   %   ≦   Cr   ≦   1.5   %   ;         
         0   ≦   P   ≦   0.09   %   ;         
         0   ≦   S   ≦   0.09   %   ;         
         0   %   ≦   N   ≦   0.09   %   ;         
 and optionally one or more of the following elements 
         0   %   ≦   Mo   ≦   0.5   %   ;         
         0   %   ≦   V   ≦   0.2   %   ;         
         0   %   ≦   Nb   ≦   0   .%;         
         0   %   ≦   Ti   ≦   0   .1%;         
         0   %   ≦   Cu   ≦   1   %   ;         
         0   %   ≦   B   ≦   0.008   %   ;         
         0   %   ≦   Sn   ≦   0.1   %   ;         
         0   %   ≦   Ce   ≦   0   .1%;         
         0   %   ≦   Mg   ≦   0   .10%;         
         0   %   ≦   Zr   ≦   0   .10%;         
 a remainder of the composition being composed of iron and unavoidable impurities caused by processing; 
   reheating the semi-finished product to a temperature between Ac3 and Ac3 +300° C.;   performing one or more mechanical operations on the semi-finished product in the austenitic range wherein the mechanical operation finishing temperature shall be between Ac3 and Ac3 +300° C. to obtain a hot leaf of a leaf spring;   cooling the hot leaf down to a temperature QT in a range from Ms-10° C. to 20° C. at a cooling rate less than 50° C./s; thereafter   heating the hot leaf at an average heating rate between 0.5° C./s and 150° C./s from QT to a temperature TT which is in a range from 250° C. to 500° C.; then   holding the hot leaf at a temperature TT during 10 seconds to10000 seconds; then   cooling the hot leaf at an average cooling rate below 5° C./s, from TT to room temperature to obtain the leaf of the leaf spring.   
     
     
         38 . The method as recited in  claim 37  wherein the reheating temperature of the semi-finished product is between Ac3+30° C. and Ac3 +300° C. 
     
     
         39 . The method as recited in  claim 37  wherein the temperature TT is from 300° C. to 475° C. 
     
     
         40 . The method as recited in  claim 37  wherein the temperature QT is from Ms-50° C. to 20° C. 
     
     
         41 . A method for the manufacture of structural or safety parts of a vehicle or an engine comprising employing the leaf manufactured according to the method as recited in  claim 37 . 
     
     
         42 . A vehicle comprising a part obtained according to the method as recited in  claim 41 . 
     
     
         43 . A method for the manufacture of structural or safety parts of a vehicle or an engine comprising employing the steel as recited in  claim 22 . 
     
     
         44 . A vehicle comprising a part obtained according to the method as recited in  claim 43 .