Methods and compositions for improved low alloy high nitrogen steels

Abstract

A low alloy high nitrogen steel and method of making the same are provided. The method can include forming a steel composition using nitrogen additive constituents and under a first gas atmosphere that can include any of inert argon, nitrogen, or a controlled atmosphere conveying nitrogen. The method can include hot working the steel composition, heating the steel composition to normalize or austenitize, quenching the steel composition at a rate to produce substantially a martensitic microstructure, and heating tempering the steel composition under vacuum or a second gas atmosphere. The second gas atmosphere can include air, controlled atmosphere with ammonia, inert nitrogen, or nitrogen and argon. The steel composition can include iron and, by weight: 0.16-0.60% nitrogen, 0.03-0.20% carbon, 0.10-2.00% nickel, 0.60-2.00% manganese, 1.30-2.80% chromium, and 0.60-1.50% molybdenum. Cobalt can be substituted for any part of the nickel.

What is claimed is: 
     
         1 . A low alloy high nitrogen steel comprising iron and, by weight:
 0.16-0.60% nitrogen (N);   0.03-0.20% carbon (C);   0.10-2.00% nickel (Ni);   0.60-2.00% manganese (Mn);   1.30-2.80% chromium (Cr);   0.60-1.50% molybdenum (Mo);   not more than 0.05% tungsten (W);   not more than 0.02% vanadium (V);   not more than 0.60% silicon (Si);   not more than 0.10% copper (Cu);   not more than 0.02% titanium (Ti);   not more than 0.02% niobium (Nb);   not more than 0.008% aluminum (Al); and   not more than 0.02% of any other element with not more than 0.10% total other elements,   wherein cobalt (Co) is substitutable for any part of the nickel.   
     
     
         2 . The steel of  claim 1 , further comprising, by weight:
 not more than 0.008% sulfur;   not more than 0.015% phosphorus;   not more than 40 ppm oxygen;   not more than 4 ppm hydrogen;   not more than 0.005% antimony;   not more than 0.005% tin; and   not more than 0.005% arsenic.   
     
     
         3 . The steel of  claim 1 , further comprising a microstructure comprising tempered martensite. 
     
     
         4 . The steel of  claim 1 , further comprising, by weight:
 0.16-0.21% nitrogen;   0.15-0.20% carbon;   0.30-1.00% nickel;   1.60-2.00% manganese;   1.30-1.45% chromium; and   1.10-1.50% molybdenum.   
     
     
         5 . The steel of  claim 1 , further comprising, by weight:
 0.16-0.21% nitrogen;   0.15-0.20% carbon;   1.00-2.00% nickel;   1.40-2.00% manganese;   1.30-1.45% chromium; and   0.60-1.50% molybdenum.   
     
     
         6 . The steel of  claim 1 , further comprising, by weight:
 0.20-0.60% nitrogen;   0.03-0.20% carbon;   0.10-1.40% nickel;   0.60-1.70% manganese;   1.30-2.80% chromium; and   0.60-1.50% molybdenum.   
     
     
         7 . The steel of  claim 1 , wherein said steel at gas pressure of 40 bar (40 MPa) or greater, upon transition from liquid to solid at high temperature, comprises, by weight:
 0.008% or less nitrogen gas; and   up to 14% delta ferrite.   
     
     
         8 . A method of making a low alloy high nitrogen steel structure, the method comprising:
 forming a steel composition using nitrogen additive constituents and under a first gas atmosphere of 1 bar to 40 bars pressure or more, wherein said first gas atmosphere comprises any of inert argon, nitrogen, or a controlled atmosphere conveying nitrogen, and wherein said steel composition comprising iron and, by weight:
 0.16-0.60% nitrogen (N), 
 0.03-0.20% carbon (C), 
 0.10-2.00% nickel (Ni), 
 0.60-2.00% manganese (Mn), 
 1.30-2.80% chromium (Cr), 
 0.60-1.50% molybdenum (Mo), 
 not more than 0.05% tungsten (W), 
 not more than 0.02% vanadium (V), 
 not more than 0.60% silicon (Si), 
 not more than 0.10% copper (Cu), 
 not more than 0.02% titanium (Ti), 
 not more than 0.02% niobium (Nb), 
 not more than 0.008% aluminum (Al), and 
 not more than 0.02% of any other element with not more than 0.10% total other elements, 
 wherein cobalt (Co) is substitutable for any part of the nickel; 
   casting liquid to solid or solid state processing under a first atmosphere to said steel composition or shape;   hot working or forming said steel composition to a shape;   heating said steel composition to first normalize and cool, then to austenitize;   quenching said steel composition at a rate to produce substantially a martensitic microstructure; and   heating tempering said steel composition under vacuum or a second gas atmosphere, wherein said second gas atmosphere comprises air, controlled atmosphere with any of ammonia, hydrogen, inert nitrogen, or nitrogen and argon.   
     
     
         9 . The method of  claim 8 , wherein the heating austenitizing further comprises heating and holding said steel composition to a temperature in a range of about 890° C. to about 950° C. 
     
     
         10 . The method of  claim 9 , wherein quenching from said heating austenitizing comprises at least one of:
 quenching into oil held at a temperature in a range of about 38° C. to about 177° C.;   quenching into a solution of polymer and water held at a temperature in a range of about 27° C. to about 66° C.;   quenching into a controlled stream of air or inert gas;   applying a cryogenic treatment to a temperature in a range from about −78.5° C. to about −20° C.; and   quenching into media at an intermediate temperature in a range from about 460° C. to about 550° C., holding for a predetermined time to harden said steel composition, followed by secondary quenching to a lower temperature.   
     
     
         11 . The method of  claim 8 , wherein said heating tempering comprises at least one of:
 a single tempering step;   multiple tempering steps comprising at least one chilling between tempering steps;   multiple tempering steps without chilling between tempering steps;   austempering, comprising intermediate quenching to a temperature in a range from about 440° C. to about 550° C. and holding prior to said quenching; and   said quenching proceeding after a thermal mechanical treatment at a temperature in a range from about 860° C. to about 1000°,   wherein said tempering comprises at least one of a primary hardening at a temperature in a range from about 200° C. to about 440° C., or a secondary hardening at a temperature in a range from about 440° C. to about 550° C. or more.   
     
     
         12 . The method of  claim 8 , further comprising hot working by rolling, forging or extrusion of said steel composition at a temperature in a range from about 1000° C. to about 1190° C. to a predetermined structure shape, said hot working comprising either increments or single steps of heating and reduction. 
     
     
         13 . The method of  claim 12 , further comprising performing a softening anneal process following said hot working. 
     
     
         14 . The method of  claim 8 , prior to austenitizing, further comprising normalizing said steel composition at a temperature in a range from about 870° C. to about 1050° C. 
     
     
         15 . The method of  claim 12 , further comprising, prior to hot work, homogenizing said steel composition at a temperature in a heating range from about 870° C. to 1121° C. 
     
     
         16 . The method of  claim 13 , wherein after softening annealing, said method further comprising performing at least one of mechanical cutting, flame cutting, plasma cutting, grinding, and sanding a surface of said low alloy high nitrogen steel structure. 
     
     
         17 . The method of  claim 8 , wherein said forming comprises:
 alloying via solid state comprising at least one of:
 mechanical alloying of powder materials under controlled atmosphere, N gas, or N plus Ar gas; or 
 alloying powders or thin sheet materials under N gas or a controlled atmosphere or with ammonia to diffuse N gas into solid surfaces of said powders or thin sheet materials, 
 wherein said forming further comprises any treatments of cleaning, surface finishing, cold isostatic pressing, hot isostatic pressing, sintering, hot work, austenitization, quench, and temper processing at or greater than atmospheric pressure. 
   
     
     
         18 . The method of  claim 17 , wherein said forming further comprises hot isostatic pressing said steel composition prior to hot working, and wherein said hot isostatic pressing comprises:
 packing and sealing a powder or thin sheets of said steel composition in a container under nitrogen gas;   performing one of:
 (i) remotely pressurizing said container to provide predetermined N pressure and mass so as to substantially equal the argon pressure level of the surrounding hot isostatic press, and heating said container to diffuse at least a portion of said mass of N into said powder or thin sheets; or 
 (ii) vacuum evacuating said container to remove gas, cold isostatic pressing to remove bulk; and 
   consolidating said steel composition via hot isostatic pressing at approximately 1000 to 1500 bars and at a temperature in a range of about 1090° C. to about 1250° C.   
     
     
         19 . The method of  claim 17 , further comprising consolidating by any of:
 cold isostatic pressing said powder or thin sheets in nitrogen gas or controlled atmosphere in any of a sealed container or a shaped mold and then hot sintering or pressing at temperatures of approximately 1150° C. to 1400° C. under N at pressures of approximately 120 bars up to 250 bars; and   additive manufacture laser sintering of powder or thin sheets in nitrogen gas or controlled atmosphere at pressures of approximately 1 bar up to 250 bars.   
     
     
         20 . The method of  claim 17 , further comprising packing and sealing said powder or thin sheets in a container under nitrogen gas or controlled atmosphere, evacuating to remove gas, cold isostatic pressing to remove bulk, and fully consolidating by hot extrusion or hot rolling at a temperature in a range of approximately 1070° C. to 1300° C.