Forged or Stamped Average or Small Size Mechanical Part

Abstract

The inventive average or small size mechanical steel part is produced by hot forging or cold stamping i.e. by plastic processing of a long ferrous semi-product which is obtainable by continuous casting and hot rolling in the austenitic phase and, afterwards is shaped by plastic deformation and heat treated in order to obtain a metallographic structure substentially containing an acicular ferrite at least in mechanical toughness and fatigue stress areas. The composition of said steel, apart from iron and inevitable residual impurities resulting from steel production, corresponds at least to the following analysis: 0.2-0.5% C, 0.5-2.0% Mn, 0.05-0.5% V, 0.6 1.5% Si, 0.05 1.0% Cr, 0.01-0.5% Mo, 0.02-0.10 S, preferably from 0.01 to 0.02% Ti and/or up to 0.20% Al, and from 5 to 30 ppm of Ca.

1 . A mechanical part made of steel derived from the hot forging or the cold pressing thereof, of medium or small size, and resulting from plastic transformation of a long siderurgical semiproduct, which the steel of which it is composed has a composition that, besides iron and the inevitable residual impurities resulting from processing of the steel, corresponds at least to the following analysis, given in weight percentages: 
 0.2≦C≦0.5,    0.5≦Mn≦2.0,    0.05≦V≦0.5,    0.6≦Si≦1.5,    0.05≦Cr≦1.0,    0.01≦Mo≦0.5, and    0.02≦S≦0.10,    and optionally up to 50 ppm of boron, wherein the said part is obtained from a long semiproduct derived from continuous casting and hot-rolling in the austenitic area, then formed by plastic deformation and treated thermally in order to obtain a metallographic structure containing essentially acicular ferrite at least in the zones of mechanical stressing in tenacity and fatigue.    
   
   
       2 . The mechanical part according to  claim 1 , wherein the steel further comprises from 0.01 to 0.02% titanium and/or up to 0.20% aluminum.  
   
   
       3 . The mechanical part according to  claim 1 , wherein the steel further comprises between 5 and 30 ppm of calcium.  
   
   
       4 . A steel for the manufacture of a mechanical part by plastic deformation, wherein, besides the inevitable residual impurities resulting from processing of the steel, its chemical composition comprises at least, expressed in weight content: 
 0.2≦C≦0.5,    0.5≦Mn≦2.0,    0.05≦V≦0.5,    0.6≦Si≦1.5,    0.05≦Cr≦10,    0.01≦Mo≦0.5, and    0.02≦S≦0.10,    and optionally up to 50 ppm of B, wherein the metallographic microstructure that the steel will have, once the part is implemented, is essentially composed of acicular ferrite at least in the zones of the part subjected to mechanical stressing in tenacity and fatigue.    
   
   
       5 . The steel according to  claim 4 , wherein, in order to protect the vanadium, the steel further comprises from 0.01 to 0.02% titanium and/or up to 0.20% aluminum.  
   
   
       6 . The steel according to  claim 4 , further comprising between 5 and 30 ppm of calcium.  
   
   
       7 . A process for the manufacture of a mechanical part made of steel, wherein, for the purpose of obtaining acicular ferrite at least locally on the part, the process comprises the following stages: 
 providing a continuous casting billet made of steel with a composition according to  claim 4 , which is hot-rolled at a temperature in excess of 1000° C. into a bar or wire before being cooled to room temperature after rolling;    subjecting the wire to a controlled cooling prior to formation into rings to obtain a metallographic structure composed essentially of acicular ferrite, which wire then is cut into pieces and cold-pressed into a finished part ready for use; and    cooling the bar naturally in the rolling heat prior to cutting the bar into pieces which then are hot-forged into a rough shape of a part that is cooled by controlled cooling to obtain a structure essentially composed of acicular ferrite at least in the stressed zones of the part, which rough shape then is machined, as need be, to the desired dimensions to make it into a finished part ready for use.    
   
   
       8 . The process according to  claim 7 , wherein the controlled cooling is a natural cooling to room temperature.  
   
   
       9 . The process according to  claim 7 , wherein the controlled cooling is a forced cooling ensuring a surface cooling speed of approximately 0.5 to 15° C./s.  
   
   
       10 . A long, medium carbon siderurgical semiproduct, intended to be transformed by forge or by press into a mechanical part with high characteristics, of small size or of medium size, wherein, in order that the part may have a metallographic microstructure essentially composed of acicular ferrite at least in the zones of the part subjected to mechanical stressing in tenacity and fatigue, the steel that constitutes the part corresponds at least to the following analysis, given in weight percentages: 
 0.2≦C≦0.5,    0.5≦Mn≦2.0,    0.05≦V≦0.5,    0.6≦Si≦1.5,    0.05≦Cr≦1.0,    0.01≦Mo≦0.5. and    0.02≦S≦0.10,    and optionally up to 50 ppm of boron, wherein the metallographic microstructure that it will have after transformation will be essentially composed of acicular ferrite at least in the zones of the part subjected to mechanical stressing in tenacity and fatigue.