Ultra-high strength thermo-mechanically processed steel

Abstract

The present invention disclosed an ultra-high strength steel for structural components, a process of making such steel that has a desirable microstructure in the thermo-mechanically processed and differently cooled conditions that delivers high fatigue performance in service, and a process of making forged components using such steel. The steel and the process of its manufacturing enables manufacture of components that exhibit bainitic microstructure that impart ultra-high strength ranges with very high fatigue performance. The invention enables saving in alloying additives compared to hardened and tempered alloy steels and in addition avoid expensive heat treatment operations to achieve the desired range of mechanical properties. The steel of the invention is a suitable replacement for micro alloyed steel or heat treated steel bars used for structural component development. The steel can be used for applied as the hot rolled and air cooled long products that can be directly used for applications or it can be directly hot forged in open or closed die forging followed by controlled cooling to achieve the desired microstructure and range of mechanical properties.

1 . An ultra-high strength steel of present invention characterised in that the said steel comprises in weight percentage carbon in the proportion of 0.1 to 0.25% manganese in the proportion of 1.2 to 2.5%, silicon in the proportion of 0.5 to 1.7%, chromium in the proportion of 0.8 to 1.4%, molybdenum in the proportion of 0.05 to 0.1, niobium in the proportion of 0.05 to 0.10 titanium in the proportion of 0.01 to 0.03%, and the residual elements less than 0.4%, nickel in the proportion of less than 0.4%, vanadium in the proportion of less than 0.1%. Sulphur in the proportion of 0.03%, phosphorus in the proportion of less than 0.02%, boron less than 30 ppm, and oxygen in an amount of less than 15 ppm. 
     
     
         2 . A process of making an ultra-high strength steel, the said process comprising the steps of:
 melting the steel of the chemical composition disclosed in  claim 1 , followed by secondary refining and vacuum degassing,   casting of said steel through ingot casting or continuous casting into a bloom, wherein said secondary refining comprises the step of killing with aluminium to bring the oxygen level in said steel to under 15 ppm,   thermo-mechanical processing said cast steel by hot charging the as-cast ingot or the continuously cast bloom, or alternatively, cold-charging them in a furnace for further deformation,   in the case of hot deformation, initially soaking said ingot or said bloom at a temperature range between 1280° C. and 1220° C.,   subjecting said soaked bloom to hot forging or hot rolling,   carrying out reduce rolling,   hot finishing the material at a temperature range 1000 to 900° C.,   control cooling the hot deformed steel in any of the cooling medium selected from a group comprising vermiculite, air, oil, polymer, or water.   
     
     
         3 . A process of making forged components with an ultra-high strength steel, characterised in that said process comprises the steps of:
 heating and soaking of rolled bar made using the process disclosed in  claim 2  at a temperature range between 1280° C. and 1220° C.,   hot forging of component,   finish forging the component at a temperature range 1000 to 900° C.,   cooling the finish forged component in any of the cooling medium selected from a group comprising controlled cooled, air, oil, or polymer.   
     
     
         4 . A process of making an ultra-high strength steel as claimed in  claim 3 , characterised in that melting in the step of casting is carried out using an induction furnace or an electric arc furnace or a basic oxygen furnace. 
     
     
         5 . A process of making an ultra-high strength steel as claimed  claim 4 , characterised in that the step of thermo-mechanical processing is carried out by a technique selected from a group comprising hot rolling, or closed die forging, or extrusion. 
     
     
         6 . A process of making forged components as claimed in  claim 3 , characterised in that the step of hot forging of component comprises the step of preform manufacturing for blocker forging and finisher forging, followed by cooling. 
     
     
         7 . A process of making forged components as claimed in  claim 6 , characterised in that the step of preform manufacturing comprises the steps of reduce rolling and bending.