Method of fabricating metal parts of different ductilities

Abstract

Hardened metal articles such as slabs, plates or sheets or preformed metal articles which are to have regions of different ductility are subjected to heating to an austenization temperature and then to quenching of the region of greater ductility to form a start temperature lying above the gamma-alpha transformation temperature of the workpiece structure. The quenching is terminated at a stop temperature lying above the martensitic temperature and prior to any substantial transformation into ferritic or perlitic structures. The higher ductility region is then maintained prior to any substantial transformation into ferritic or perlitic structures. The higher ductility region is then maintained isothermically for condition of austenitite ferrite and/or perlite. The lower ductility structure is brought to a temperature sufficient for martensitic formation and the hardening is then carried out.

We claim:  
     
         1 . A method of producing a hardened metal article with at least two regions of different ductility including at least one first region with higher ductility and at least one second region of lower ductility, comprising the steps of: 
 (a) heating a metal slab or preformed metal part constituting a workpiece to an austenitization temperature;    (b) thereafter transporting said workpiece over a transport path and during transport of the workpiece along said path subjecting said first region to cooling by: 
 (b1) quenching said first region from a predetermined cooling start temperature ( Tstart ) lying above a γ-α transformation temperature of the workpiece,  
 (b2) terminating the quenching when a predetermined cooling stop temperature ( Tstop ) is reached which lies above the martensitic start temperature and prior to any transformation into ferritic or perlitic structures or prior to any but a slight transformation into ferritic or perlitic structures, and  
 (b3) then maintaining the workpiece approximately under an isothermal condition for conversion of austentite in the structure of the workpiece to at least one structure selected from the group consisting of ferrite and perlite;  
   (c) during step (b) maintaining said second region at a hardening temperature (T H ) at least sufficient for martensite formation in said second region; and    (d) then effecting a hardening process for said workpiece during transport thereof along said path.    
     
     
         2 . The method defined in  claim 1  wherein said second region is brought to said hardening temperature (T H ) during step (b3) which is less than the heating temperature in step (a).  
     
     
         3 . The method defined in  claim 1  wherein said second region is subjected to heating to maintain an austentite structure therein.  
     
     
         4 . The method defined in  claim 1  wherein heating radiated from said second region is captured by a reflecting mirror and reflected back to said second region.  
     
     
         5 . The method defined in  claim 1  wherein said first region is cooled in step (b1) by a nozzle directing a cooling medium onto said first region and matched to the geometry of the workpiece.  
     
     
         6 . The method defined in claims  5  wherein said cooling medium is air.  
     
     
         7 . The method according to  claim 1  wherein the hardening process is carried out in a cooled reshaping tool in the course of a hot forming of the workpiece.  
     
     
         8 . The method defined in  claim 1  wherein said first and second regions are subjected to processes separated by a partition from one another.  
     
     
         9 . The method defined in  claim 1  wherein the workpiece is composed of a steel alloy containing manganese and baron components.  
     
     
         10 . The method defined in  claim 9  wherein the workpiece is comprised of a steel alloy consisting, in weight percent, essentially of: 
 carbon (C) 0.18% to 0.3%  
 silicon (Si) 0.1% to 0.7%  
 manganese (Mn) 1.0% to 2.50%  
 phosphorus (P) maximum 0.025%  
 chromium (Cr) 0.1% to 0.8%  
 molybdenum (Mo) 0.1% to 0.5%  
 sulfur (S) maximum 0.01%  
 titanium (Ti) 0.02% to 0.05%  
 boron (B) 0.002% to 0.005%  
 aluminum (Al) 0.01% to 0.06%  
 balance iron and unavoidable smelting impurities.