Process for obtaining a fine-grained martensitic structure component

Abstract

The disclosure relates to a process for the manufacture of a steel component comprising a fine-grained martensite structure component. The process comprises the steps of providing a steel component having an initial steel composition; introducing nitrogen into the steel component at a temperature T 1 above 950° C., thereby creating an at least partly austenitic nitrogen-containing steel component; bringing the at least partly austenitic nitrogen-containing steel component to a temperature T 2 , such that austenite is decomposed into a steel component comprising at least an amount of carbon- and/or nitrogen-containing precipitates; bringing the steel component comprising at least an amount of carbon- and/or nitrogen-containing precipitates to a temperature T 3 which is above T 2 , thereby creating an at least partly austenitic nitrogen-containing steel component optionally comprising at least an amount of carbon- and/or nitrogen-containing precipitates; and bringing the at least partly austenitic nitrogen-containing steel component to a temperature T 4 that is below a martensite start temperature of the at least partly austenitic nitrogen-containing steel component for initiating transformation of at least some of the austenite into fine-grained martensite, thereby producing a steel component comprising a fine-grained martensite structure component.

What is claimed is: 
     
         1 . A process for the manufacture of a steel component comprising a fine-grained martensite structure component, the process comprising the steps:
 vi) Providing a steel component having an initial steel composition;   vii) Introducing nitrogen into the steel component at a temperature T 1  above 950° C., thereby creating an at least partly austenitic nitrogen-containing steel component;   viii) Bringing the at least partly austenitic nitrogen-containing steel component to a temperature T 2 , wherein T 2  is below 800° C., such that austenite is decomposed into a steel component comprising at least an amount of carbon- and/or nitrogen-containing precipitates;   ix) Bringing the steel component comprising at least an amount of carbon- and/or nitrogen-containing precipitates to a temperature T 3  which is above T 2 , wherein T 3  is preferably above 900° C., thereby creating an at least partly austenitic nitrogen-containing steel component optionally comprising at least an amount of carbon- and/or nitrogen-containing precipitates; and   x) Bringing the at least partly austenitic nitrogen-containing steel component to a temperature T 4  that is below a martensite start temperature of the at least partly austenitic nitrogen-containing steel component for initiating transformation of at least some of the austenite into fine-grained martensite, thereby producing a steel component comprising a fine-grained martensite structure component.   
     
     
         2 . The process according to  any one of the preceding claims , wherein the initial steel composition is characterised by comprising at least 4.5 wt % Cr and 0.05 wt % C. 
     
     
         3 . The process according to  any one of the preceding claims , wherein the initial steel composition is characterised by comprising at least 10.5 wt % Cr and 0.05 wt % C. 
     
     
         4 . The process according to  any one of the preceding claims , wherein the initial steel composition is characterised by comprising 11.5 to 17.0 wt % Cr and 0.1 to 1.0 wt % C. 
     
     
         5 . The process according to  any one of the preceding claims , wherein the initial steel composition is characterised by comprising 0.01 to 0.1 wt % N. 
     
     
         6 . The process according to  any one of the preceding claims , wherein the at least partly austenitic nitrogen-containing steel component comprises nitrogen in the range of 0.03-3.0 wt % following the process in step ii). 
     
     
         7 . The process according to  any one of the preceding claims , wherein the steel component, such as a core of the steel component, produced in step iii) further comprises retained austenite in an amount equal to or less than 50%. 
     
     
         8 . The process according to  any one of the preceding claims , wherein the fine-grained martensite structure component is characterised by a composition comprising:
 a. 0.02 wt % to 3 wt % N;   b. 0.02 wt % to 3 wt % C; and   c. 4.5 wt % to 30 wt % Cr.   
     
     
         9 . The process according to  any one of the preceding claims , wherein the fine-grained martensite structure component is formed from austenite grains of step iv) having a grain size of less than 50 μm. 
     
     
         10 . The process according to  any one of the preceding claims , comprising an additional step, step vi) comprising tempering the steel component at least once in the temperature range of 60° ° C. to 700° C. 
     
     
         11 . The process according to  any one of the preceding claims , comprising a thermal treatment at least once at sub-zero Celsius temperature between step v) and step vi), such as at temperatures between −200° ° C. to 0° ° C., such as −150° C. to 0° ° C., such as −100° ° C. to 0° C. 
     
     
         12 . The process according to  any one of the preceding claims , wherein step ii) comprises either gas nitriding or plasma nitriding. 
     
     
         13 . The process according to  any one of the preceding claims , wherein introducing nitrogen is performed using a gas mixture comprising one or more of ammonia, nitrogen, hydrogen, cyanides and/or argon. 
     
     
         14 . The process according to  any one of the preceding claims , wherein step iii) is performed under vacuum. 
     
     
         15 . The process according to  any one of the preceding claims , wherein step iii) and/or step iv) is/are performed within an inert atmosphere comprising one or more of the followings:
 nitrogen, argon and/or helium.   
     
     
         16 . The process according to  claim 15 , wherein the pressure is at least 0.15 bar, preferably between 0.15 and 15 bars, more preferably between 0.25 and 15 bars. 
     
     
         17 . The process according to  any one of the preceding claims , wherein prior to step iii) and/or step iv) the steel component is taken to a temperature which is below 600° C., such as below 400° C., such as below 200° ° C., such as below 100° C., to produce phases and/or microstructures comprising carbides and/or nitrides. 
     
     
         18 . The process according to  any one of the previous claims , wherein the temperature T 1  is above 1000° C., such as equal to or above 1100° C., such as equal to or above 1150° C., such as equal to or above 1200° C., with the prerequisite that the T 1  is still below the melting point of the steel component. 
     
     
         19 . The process according to  any one of the previous claims , wherein the temperature T 2  is below 850° C., such as equal to or below 800° C., such as equal to or below 750° C., such as equal to or below 650° ° C. 
     
     
         20 . The process according to  any one of the previous claims , wherein the temperature T 2  is in the range of 500-800° C., preferably between 600 and 750° C. 
     
     
         21 . The process according to  any one of the previous claims , wherein the temperature T 3  in step iv) is at a temperature below T 1 , such as at a temperature that does not exceed the melting point of the steel component. 
     
     
         22 . The process according to  any one of the previous claims , wherein the temperature T 3  is at least 900° C., preferably between 980° C. and 1150° C., more preferably between 1000° C. and 1100° C. 
     
     
         23 . The process according to  any one of the previous claims , wherein steps iv) and v) are performed at a pressure between 0.15-15 bar, preferably between 0.25 and 15 bar, more preferably between 0.25 and 10 bar, more preferably between 0.36 and 4 bar. 
     
     
         24 . The process according to  any one of the previous claims , further comprising the step of applying a carburizing step or a carbo-nitriding step prior to the step ii). 
     
     
         25 . The process according to  any one of the previous claims , wherein the fine-grained martensite structure component is in the form of a case surrounding a core having a second structure component, which is different from the fine-grained martensite structure component. 
     
     
         26 . The process according to  any one of the previous claims , wherein the fine-grained martensite structure component is in the form of a case, wherein the case has a depth in the range of 0.05-30 mm, such as 0.05-20 mm, such as 0.05-10 mm. 
     
     
         27 . A steel component comprising a fine-grained martensite structure component obtained by the process according to  any one of the preceding claims 1-26 , wherein the fine-grained martensite structure component is characterised by a composition comprising:
 a. 0.02 wt % to 3 wt % N;   b. 0.02 wt % to 3 wt % C; and   c. 4.5 wt % to 30 wt % Cr,   wherein the fine-grained martensite structure component is in the form of a case surrounding a core having a second structure component which is different from the fine-grained martensite structure component.   
     
     
         28 . A steel component comprising a fine-grained martensite structure component, wherein the fine-grained martensite structure component is in the form of a case surrounding a core having a second structure component, wherein the core and the case are different, and further wherein the fine-grained martensite structure component is characterised by a composition comprising:
 a. 0.02 wt % to 3 wt % N;   b. 0.02 wt % to 3 wt % C; and   c. 4.5 wt % to 30 wt % Cr.   
     
     
         29 . The steel component according to  claim 28 , wherein the steel component comprises at least an amount of carbon- and/or nitrogen-containing precipitates. 
     
     
         30 . The steel component according to  claim 29 , wherein the nitrogen level in the core is lower than the nitrogen level in the case. 
     
     
         31 . The steel component according  claims 28-30 , wherein the fine-grained martensite structure component has a hardness in the range of 300-1400 HV1. 
     
     
         32 . The steel component according to  claims 28-31 , wherein the fine-grained martensite structure component is formed from austenite grains, wherein the austenite grains, prior to transformation into the fine-grained martensite structure component, have a grain size of less than 50 μm. 
     
     
         33 . The steel component according to  claims 28-32 , wherein the case has a depth in the range of 0.05-30 mm, such as 0.05-20 mm, such as 0.05 to 10 mm. 
     
     
         34 . The steel component according  claims 28-33 , wherein the fine-grained martensite structure component and/or the second structure component is/are not identical to SAE AMS 5898. 
     
     
         35 . The steel component according to  claims 28-34 , further comprising at least one other alloying elements. 
     
     
         36 . The steel component according to  claims 28-35 , wherein the at least one alloying elements are one or more selected from Si, Mo, Mn, Nb, V, Ta, Ti, Ni, Cu and Co. 
     
     
         37 . A steel component according to any one of  claims 27-36  for use in bearings, gears, crankshafts, screws, tools, weapons, armours, knives, medical and/or dental utensils. 
     
     
         38 . A steel component according to any one of  claims 27-36  for use in wind turbine components. 
     
     
         39 . A steel component according to any one of  claims 27-36  for use preventing white etching cracks in bearings. 
     
     
         40 . A steel component according to any one of  claims 27-36  for use in and/or for prevention of rolling and/or sliding contact fatigue.