Leaf spring material and manufacturing method thereof

Abstract

The present invention provides a leaf spring material superior in mechanical characteristics and a manufacturing method of the leaf spring material capable of reliably achieving the same, utilizing induction hardening. The manufacturing method of the leaf spring material comprises the steps of imparting tensile stress on a first surface along the longitudinal direction of the first surface and compressive stress on a second surface along the longitudinal direction of the second surface of a substantially strip-shaped steel plate, and subjecting the first surface to induction hardening. With this induction hardening, an induction-hardened structure having a higher average hardness than that of a parent material structure in the vicinity of the second surface and comprising martensite and finely and evenly dispersed austenite is imparted on a surface layer in the vicinity of the first surface.

1 . A manufacturing method of a leaf spring material that comprises the steps of: imparting tensile stress on a first surface along the longitudinal direction of said first surface and compressive stress on a second surface along the longitudinal direction of said second surface of a substantially strip-shaped steel plate; and subjecting said first surface to induction hardening, wherein:
 said induction hardening imparts in the vicinity of said first surface an induction-hardened structure having a higher average hardness than a parent material structure in the vicinity of said second surface and comprising martensite and finely dispersed austenite.   
   
   
       2 . The manufacturing method of a leaf spring material according to  claim 1 , wherein said parent material structure comprises one or more of martensite, tempered martensite structure, troostite, sorbite, bainite, ferrite, and pearlite. 
   
   
       3 . The manufacturing method of a leaf spring material according to  claim 2 , wherein said induction-hardened structure has a grain size smaller than that of said parent material structure. 
   
   
       4 . The manufacturing method of a leaf spring material according to  claim 3 , wherein the grain size number of said parent material structure is 8 to 10, and the grain size number of said induction-hardened structure is 11 to 13. 
   
   
       5 . The manufacturing method of a leaf spring material according to  claim 1 , comprising the step of:
 releasing the load of said tensile stress after said induction hardening, wherein said tensile stress is 1000 MPa or higher at ambient temperature.   
   
   
       6 . The manufacturing method of a leaf spring material according to  claim 5 , comprising the step of low-temperature tempering after the step of releasing the load of said tensile stress. 
   
   
       7 . The manufacturing method of a leaf spring material according to  claim 6 , wherein said the step of low-temperature tempering is processed at the temperature of 80° C. to 160° C. for a duration shorter than 120 minutes. 
   
   
       8 . The manufacturing method of a leaf spring material according to  claim 1 , wherein said induction-hardened structure has a hardness of 650 HV or higher. 
   
   
       9 . The manufacturing method of a leaf spring material according to  claim 8 , wherein the volumetric content of said austenite of said induction-hardened structure is 15% or higher. 
   
   
       10 . The manufacturing method of a leaf spring material according to  claim 1 , wherein said austenite of said induction-hardened structure has an average grain size of 500 mm or less. 
   
   
       11 . The manufacturing method of a leaf spring material according to  claim 1 , wherein said induction-hardened structure is distributed at a thickness of 5 to 10% of the thickness of said steel plate. 
   
   
       12 . The manufacturing method of a leaf spring material according to  claim 1 , wherein said induction-hardened structure has a compressive residual stress of 650 MPa or higher. 
   
   
       13 . The manufacturing method of a leaf spring material according to  claim 1 , wherein a half value breadth of an Fe (200) α  diffraction peak of said induction-hardened structure based on a CrKα beam diffractometer is 8° or higher. 
   
   
       14 . A leaf spring material that comprises a substantially strip-shaped steel plate having a first surface and a second surface, comprising:
 in the vicinity of said first surface an induction-hardened structure having a higher average hardness than that of a parent material structure in the vicinity of said second surface and comprising martensite and finely dispersed austenite.   
   
   
       15 . The leaf spring material according to  claim 14 , wherein said parent material structure comprises one or more of martensite, tempered martensite structure, troostite, sorbite, bainite, ferrite, and pearlite. 
   
   
       16 . The leaf spring material according to  claim 15 , wherein said induction-hardened structure has a grain size smaller than that of said parent material structure. 
   
   
       17 . The leaf spring material according to  claim 16 , wherein the grain size number of said parent material structure is 8 to 10, and the grain size number of said induction-hardened structure is 11 to 13. 
   
   
       18 . The leaf spring material according to  claim 14 , wherein said induction-hardened stricture has a hardness of 650 HV or higher. 
   
   
       19 . The leaf spring material according to  claim 18 , wherein the volumetric content of said austenite of said induction-hardened structure is 15% or higher. 
   
   
       20 . The leaf spring material according to  claim 14 , wherein said austenite has an average grain size of 500 nm or less. 
   
   
       21 . The leaf spring material according to  claim 14 , wherein said induction-hardened structure is distributed at a thickness of 5 to 10% of the thickness of said steel plate. 
   
   
       22 . The leaf spring material according to  claim 14 , wherein said induction-hardened structure has a compressive residual stress of 650 MPa or higher. 
   
   
       23 . The leaf spring material according to  claim 14 , wherein a half value breadth of an Fe (200) α  diffraction peak of said induction-hardened structure based on a CrKα beam diffractometer is 8° or higher.