US20190055633A1

US20190055633A1 - Methods and compositions for improved low alloy high nitrogen steels

Info

Publication number: US20190055633A1
Application number: US15/678,637
Authority: US
Inventors: John F. Chinella
Original assignee: US Army Research Laboratory; US Department of Army
Current assignee: US Army Research Laboratory; US Department of Army
Priority date: 2017-08-16
Filing date: 2017-08-16
Publication date: 2019-02-21

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.

Description

GOVERNMENT INTEREST

The embodiments herein may be manufactured, used, and/or licensed by or for the United States Government without the payment of royalties thereon.

BACKGROUND

Technical Field

The embodiments herein generally relate to low alloy high nitrogen steels and methods of making the same, and more particularly to a high strength low alloy high nitrogen martensitic steel and methods of making the same.

Description of the Related Art

Conventional low alloy carbon steels (CS), and high to ultra-high strength low alloy martensitic steels, for example, alloy 4340 and alloy D6A6, are alloys of iron (Fe) and carbon (C) typically with nickel (Ni) and manganese (Mn), and solutes chromium (Cr) and molybdenum (Mo). A desired strength level of the high strength steels may be obtained by processing to a martensite structure by austenitizing solution treatment followed by quench and tempering. The strength of these martensitic steels is obtained by an effect of interstitial carbon effecting a long range non-symmetrical distortion of the steel microstructure, microstructure refinement by martensite and dislocations, and following tempering, which toughens and which may add further strengthening by transforming the quenched martensite into ferrite and fine carbides. To achieve the highest strength levels, these carbon steels require a process temper at low temperature, for example 205° C. from which issues can arise, for example, low toughness and low resistance to stress corrosion cracking. To achieve beneficial toughness properties with carbon steel with increasing temper temperatures there is a disadvantage of a concomitant loss in strength and hardness, for example at 800° F. or 900° F. or 427° C. to 482° C. Furthermore, the temper process of carbon steels occurs, in part, with the rejection of carbon from martensite and the covalent interatomic bonding of C which strongly clusters C with solute elements; e.g., Fe, Cr, and Mo, in the form of carbide structures. Tempered martensite embrittlement may contribute to significant lessening of toughness following temper processes carried out at 200-500° C. The tempered martensite embrittlement follows from effects of carbide morphology from thin layers of retained austenite and segregation of minor elements to austenite grain boundaries, especially phosphorous (P). For example, with reheat or tempering, low alloy carbon steels have a tendency to cluster C with solutes to form cementite Fe₃C, elemental C as graphite, and higher carbides M₇C₃and M₂₃C₆, which coarsen in size with time at elevated temperatures. The Fe₃C carbide is especially deleterious as it forms with chemical bond enthalpy of low strength. It follows that the alloy may have low strength and toughness due to fracture that may readily initiate around large carbide particles under tension and shear loads by initiation of microcracks and microvoids and their growth and coalescence. In compression loading, elastic incompatibilities of the matrix and carbides localize deformation which again may lead to localized initiation of fracture. At combined elevated temperatures and pressure; e.g., 250° C. to 600° C., the carbides in carbon steel lose strength with respect to the matrix which may lead to fracture, initiated by softening of the alloy during plastic flow or dynamic loading.
One technical solution approach to the problem of weak deleterious carbides in steel and the loss of strength during tempering has been the development of the high alloy (HA) “secondary hardening” steels (SHS); e.g., alloy AF1410, such as alloy AF1410 steel with 14Co-10Ni-2Cr-1Mo-0.16C, and Aermet 100 alloy. During tempering around 500° C., the coarse dispersed cementite in alloy AF1410 and these SHS may be dissolved and replaced with a finer, more dispersed, strongly bonded M₂C carbide precipitate which can provide hardening and more resistance to decohesion, thereby maintaining toughness with strength. A disadvantage of the high alloy secondary hardening steels is the high level of alloying elements which increases the raw material costs for HA SHS.
High nitrogen steels now commercially available are made by cast ingot metallurgy, for example stainless steels Energietechnik alloys Cronidur 30, P900, Carpenter's 15-15HS, or Outokumpu 2507 and 2205 duplex steels, have high alloy contents greater than 8 weight percent of Cr, Mn, Ni, and Mo so as to dissolve nitrogen in liquid and retain during solidification. Disadvantages of the austenitic and duplex HNS with high levels of Cr, Ni, Mo, or Mn along with C and N are equilibrium phases of carbides and nitrides that form upon reheat and which are detrimental to mechanical and corrosion properties. Furthermore, there is higher cost of alloy elements for higher levels of alloy content. The duplex and austenitic grades of HNS have disadvantages of lower levels of strength than martensitic steel. The pressurized electroslag remelt (PESR) ingot cast high alloy martensitic Cronidur 30 HNS has disadvantages of a high Cr, Mo, C, and N contents which upon heating at extended periods have tendency to form deleterious weak M₂₃C₆and the sigma phase which may be detrimental to corrosion resistance and toughness.
The claims embodied herein are not meant as a panacea for poor steelmaking practice which may encompass issues of composition, impurity levels, and the making, shaping, and heat treatment of low alloy steel. Nevertheless, the claims provide methods and compositions which may mitigate issues and provide new methods, compositions and improvements not otherwise available for low alloy steel.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form any part of the prior art nor what the prior art may suggest to a person of ordinary skill in the art.

SUMMARY

In view of the foregoing, an embodiment herein provides 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. Cobalt (Co) substitutes for any part of nickel in the low alloy high nitrogen steel.
The steel in the embodiment may further include, 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. The steel in the embodiment may further include a microstructure of tempered martensite. The steel in the embodiment may further include, 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.
The steel in the embodiment may further include, 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. The steel in the embodiment may further include, 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.
The steel in the embodiment may, at gas pressure of 40 bar (40 MPa) or greater, upon transition from liquid to solid at high temperature, comprise, by weight: 0.008% or less nitrogen gas; and up to 14% delta ferrite.
Another embodiment provides a method of making a low alloy high nitrogen steel structure, the method comprising providing a steel composition under a first gas atmosphere of 1 bar to 40 bars pressure or more by casting liquid to solid either as an ingot or direct to shape, or as rapidly solidified powder or granules, or providing a composition by solid state processing, then for ingot or optional processing hot working or forming said steel composition to form to a shape, heating said steel composition to normalize or austenitize, quenching said steel composition at a rate to produce a substantially martensitic microstructure, and heating tempering said steel composition under a second gas atmosphere, wherein said second gas atmosphere comprises air, controlled atmosphere, or inert nitrogen or nitrogen and argon. The method of making may include additive manufacture by direct laser sinter or welding of powder or granules to form a shape. In the method, the steel composition comprises 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. Cobalt (Co) is substitutable for any part of the nickel.
In the method, the heating austenitizing may further include heating and holding the steel composition to a temperature in a range of about 890° C. to about 950° C. In the method, the quenching from the heating austenitizing may include 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 the steel composition, followed by secondary quenching to a lower temperature.
In the method, the heating tempering may include 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, including intermediate quenching to a temperature in a range from about 440° C. to about 550° C. and holding prior to the quenching; and the quenching proceeding after a thermal mechanical treatment at a temperature in a range from about 860° C. to about 1000° C. In the embodiment, the tempering may include at least one of a primary hardening at a temperature in a range from about 220° 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.
The method may further include hot working by rolling, forging or extrusion of the steel composition at a temperature in a range from about 1000° C. to about 1190° C. to a predetermined structure shape, wherein the hot working may include either increments or single steps of heating and reduction. The method may further include performing heat treatments of a softening anneal process following the hot working, and following any softening anneal with an optional heat treatment including normalizing the steel composition at a temperature in a range from about 870° C. to about 1020° C., followed by air cooling. The method may further include, prior to hot work, homogenizing the steel composition at a temperature in a heating range from about 870° C. to 1121° C.
In the method, after softening annealing, the method may further include performing at least one of mechanical cutting, flame cutting, plasma cutting, grinding, and sanding a surface of the low alloy high nitrogen steel structure.
In the method, the forming may include alloying via solid state comprising at either: mechanical alloying of powder materials under N gas, or N plus Ar gas, or N with ammonia, or alloying powders or thin sheet materials under N gas or a controlled atmosphere with ammonia to diffuse N gas into solid surfaces of the powders or thin sheet materials, wherein following mechanical alloying, or a gas-solid diffusion, N alloyed powder or thin sheets, or a surface treatment, is manufactured and following consolidation brought to a final structure at or greater than atmospheric pressure by any combinations of cleaning, surface finishing, cold isostatic pressing, hot isostatic pressing, sintering, additive manufacture laser sinter or welding of powder, hot work, austenitization, quench, or temper processing.
In the method, the forming may further include hot isostatic pressing the steel composition prior to hot working. The hot isostatic pressing may include packing and sealing a powder or thin sheets of the steel composition in a container under nitrogen gas or controlled atmosphere. The hot isostatic pressing may include performing at least one of (i) remotely pressurizing the 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 the container to diffuse at least a portion of the mass of N into the powder or thin sheets; or (ii) vacuum evacuating the container to remove gas, cold isostatic pressing to remove bulk. The hot isostatic pressing may include consolidating the steel composition via pressing at approximately 1000 to 1500 bars and at a temperature in a range of about 1090° C. to about 1250° C.
The method may further include consolidating by any of: cold isostatic pressing the 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 or controlled atmosphere at pressures of approximately 120 bars up to 250 bars, followed by optional consolidation and shaping by hot extrusion or hot rolling at a temperature in a range of approximately 1070° C. to 1300° C. The method may include: packing and sealing the 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. The method may further include additive manufacture laser sintering or welding of powder or thin sheets in nitrogen gas or controlled atmosphere.
These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments herein will be better understood from the following detailed description with reference to the drawings, in which:

FIG. 1A is a flow diagram showing a cast ingot and wrought hot work method of making a low alloy high nitrogen steel according to an embodiment herein;

FIG. 1B is a flow diagram showing a cast method of making a low alloy high nitrogen steel according to an embodiment herein;

FIG. 1C is a flow diagram showing solid state alloying plus hot isostatic press methods of making a low alloy high nitrogen steel according to embodiments herein;

FIG. 1D is a flow diagram showing a mechanical alloying plus cold isostatic press, pressure sintering, plus wrought hot work method of making a low alloy high nitrogen steel according to an embodiment herein;

FIG. 1E is a flow diagram showing additive manufacture laser sinter or weld consolidation of powder into solid form or shape, followed by optional sintering under pressure or wrought hot work, followed by austenitization, quench and temper method of making a low alloy high nitrogen steel according to embodiments herein;

FIG. 2A illustrates a 1 gram system schematic diagram of phase versus temperature at 40 bars or 4.0 MPa pressure for a first Example Composition 1 according to an embodiment herein;

FIG. 2B illustrates the phase versus temperature diagram of FIG. 2A with an expanded scale;

FIG. 3A illustrates a 1 gram system schematic diagram of phase versus temperature at 40 bars or 4.0 MPa pressure for a second Example Composition 1 according to an embodiment herein;

FIG. 3B illustrates the phase versus temperature diagram of FIG. 3A with an expanded scale;

FIG. 4A illustrates a 1 gram system schematic diagram of phase versus temperature at 40 bars or 4.0 MPa pressure for a third Example Composition 1 according to an embodiment herein;

FIG. 4B illustrates the phase versus temperature diagram of FIG. 4A with an expanded scale;

FIG. 5A illustrates a 1 gram system schematic diagram of phase versus temperature at 40 bars or 4.0 MPa pressure for a fourth Example Composition 1 according to an embodiment herein;

FIG. 5B illustrates the phase versus temperature diagram of FIG. 5A with an expanded scale;

FIG. 6A illustrates a 1 gram system schematic diagram of phase versus temperature at 40 bars or 4.0 MPa pressure for a fifth Example Composition 1 according to an embodiment herein;

FIG. 6B illustrates the phase versus temperature diagram of FIG. 6A with an expanded scale;

FIG. 7A illustrates a 1 gram system schematic diagram of phase versus temperature at 40 bars or 4.0 MPa pressure for a first Example Composition 2 according to an embodiment herein;

FIG. 7B illustrates the phase versus temperature diagram of FIG. 7A with an expanded scale;

FIG. 8A illustrates a 1 gram system schematic diagram of phase versus temperature at 40 bars or 4.0 MPa pressure for a second Example Composition 2 according to an embodiment herein;

FIG. 8B illustrates the phase versus temperature diagram of FIG. 8A with an expanded scale;

FIG. 9A illustrates a 1 gram system schematic diagram of phase versus temperature at 40 bars or 4.0 MPa pressure for a third Example Composition 2 according to an embodiment herein;

FIG. 9B illustrates the phase versus temperature diagram of FIG. 9A with an expanded scale;

FIG. 10A illustrates at the same cast pressure of 40 bars (580 pounds/inch²) the range of phase stability versus temperature and the phase enthalpy of all phases of the same Example Composition 1 alloy shown in FIG. 5A and FIG. 5B according to an embodiment herein;

FIG. 10B illustrates at pressure of 13,790 bars (200,007 pounds/inch²), the range of phase stability versus temperature and the phase enthalpy of all phases of the same Example Composition 1 alloy shown in FIG. 5A and FIG. 5B according to an embodiment herein;

FIG. 11A illustrates a 1 gram system schematic diagram of phase versus temperature at 175 bars pressure for the solid state alloy first Example Composition 3 according to an embodiment herein; and

FIG. 11B illustrates the phase versus temperature diagram of FIG. 11A with an expanded scale of 0.00 to 0.10 gram.

DETAILED DESCRIPTION

The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
It will be understood that when an element or layer is referred to as being “on”, “connected to”, or “coupled to” another element or layer, it can be directly on, directly connected to, or directly coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element or layer is referred to as being “directly on”, “directly connected to”, or “directly coupled to” another element or layer, there are no intervening elements or layers present. It will be understood that for the purposes of this disclosure, “at least one of X, Y, and Z” can be construed as X only, Y only, Z only, or any combination of two or more items X, Y, and Z (e.g., XYZ, XYY, YZ, ZZ).
Referring now to the drawings, and more particularly to FIGS. 1A through 11B, where similar reference characters denote corresponding features consistently throughout the figures, there are shown exemplary embodiments.
An embodiment herein provides a low alloy high nitrogen steel. The low alloy high nitrogen steel includes 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; 0.60-1.50% molybdenum; not more than 0.05% tungsten; not more than 0.02% vanadium; not more than 0.60% silicon; not more than 0.10% copper; not more than 0.02% titanium; not more than 0.02% niobium; not more than 0.008% aluminum; and not more than 0.02% of any other element with not more than 0.10% total other elements. Cobalt may substitute for any part of nickel in the low alloy high nitrogen steel.
The alloy elements to the left of Fe in the periodic table; e.g., Cr, Mn, and Mo, can promote covalent bonding and enhance clustering, and elements to the right; e.g., Ni and
Co, can promote metallic bonding and short range order (SRO). Ab initio calculations and tests with electron spin resonance with interstitial alloy constituents, show that the concentration of free electrons can increase to an optimal level with alloying of N in CrMnCN steels which can enhance the ductile metallic character of atomic bonding. The N can promote SRO of N and C, Cr, or Mo solutes which can increase the solubility for alloy atoms due to the result of a more even distribution of solutes in the austenitic lattice. The addition of C alone can promote covalent bonding and clustering. The short range order of N and solutes, which can form a complex interstitial-solute (i-s) bond, can promote a non-cubic crystalline symmetry and an effect of lattice distortion, and solid solution strengthening greater than carbon. A stronger N-dislocation binding energy, enthalpy (H), than from carbon, can interact more strongly with dislocations, and provide a higher level of flow stress. The highest effect of SRO can be obtained with alloys of N+C.
The approach in the low alloy compositions of the embodiments herein assume that the short range order effect and more even distribution of alloy atoms of austenite can be inherited by the essentially near complete body centered cubic (BCC) phase structure upon the face centered cubic (FCC) to BCC transformation obtained by quenching of low alloy steel to martensitic steel, then tempering. As described later, calculations using Thermo-Calc software (available from Thermo-Calc Software, Inc.) and Schaeffler plots were used to calculate such a possibility. The resultant microstructure having the short range order effect and more even distribution of alloy atoms provides a more refined microstructure to resist initiation of fracture and the effect of N has improved ductility, level of flow stress and strain hardening. A high level of Mo alloying that is possible without deleterious carbides will improve the mechanical strength, toughness, and resistance to corrosion. Contrariwise, the first to solidify steel phase of delta ferrite has low solubility for N. In the pressure cast ingot manufacturing process for the compositions of the embodiments herein, in the specific low alloy levels of N, C, Mn, Cr, Mo, and Ni compositions of steel, the N solubility in high temperature austenite at elevated manufacture pressures may be maintained in part through enhancing austenite and limiting delta ferrite by optimal levels of composition and balanced levels of near equal C and N. The delta ferrite is either eliminated or limited to be under a specific amount; for example, 14%. Furthermore, with Thermo-Calc calculations cast ingot can be successfully manufactured in accordance with the embodiments herein. Wrought billet and wrought and finished plate structures can be manufactured and similar novel experimental low alloy Fe—Cr—Ni—Mn—V—C—N steels that contain W have been tested, for which N steel characteristics have been proven with martensitic structure, with or without retained austenite, high hardness and impact toughness, and excellent austenitize, quench and temper hardenability for both low 204° C. to 300° C. and high temper temperatures around 460° C. to 525° C.
The solid state alloy process is an alternative manufacturing approach from the pressure casting of ingot and may be either conducted by mechanical alloying of powders or gas-solid diffusion of powders or thin sheet with N or controlled atmosphere with ammonia gas at specific ranges of temperatures and pressures. The solid state alloy approach is an alloy manufacture process which enhances N solubility without the need to avoid delta ferrite or to enhance high temperature austenite over delta ferrite. With specific contents of manganese, nickel as low as 0.10%, high chromium up to 2.8% and molybdenum up to around 1.40%, the solid state alloy process allows minimal austenite content and provides alloys of highly enhanced 0.16% to 0.60% nitrogen and the choice of maintaining the transition temperature of the ferritic body centered cubic (BCC) lattice structure to the austenitic face centered cubic (FCC) lattice crystallographic structure at high temperatures. With no need for pressure remelt additive agents such as silicon nitride Si₃N₄, the level of silicon may be lessened to improve interstitial solubility of C and N constituents in the steel. With specific compositions of Fe—Cr—Mn—Mo—C and the higher N level possible, the mechanical alloy process or the gas-solid diffusion methods allow low alloy steel use of low Si, high Mo, and higher levels of alloying with Cr, up to around 2.8% without tendency for Cr—Cr clustering or to form carbides.
Hardness of martensitic steels can be dependent upon the level of interstitial C and N content. While still not wishing to be bound by theory, martensite which can be formed in C steels at high temperature near room temperature on cooling can be formed by dislocation movements and is called lath martensite. Lath martensite contains a high density of dislocations. During the quenching of steel following austenitization to form martensite, interstitial atoms can diffuse and segregate around dislocations. Hardening contributions of martensite structure of the low alloy high nitrogen steel alloys according to embodiments herein can be provided by the fine dislocation structure, pinning of the dislocated martensite structure by interstitial atoms C and N, and a lattice distortion which is non-cubic and which serves as an obstacle to dislocation movement.
When quenched martensite is saturated with interstitial C, reheat stage 1 tempering of conventional C-steel quenched martensite to less than 200° C., can result in precipitation of fine carbides which provides an additional contribution to strengthening; and the martensite which had expanded, shrinks. The second stage of tempering in conventional C-steel can involve transformation of retained austenite to bainite at around 200-350° C. for which the product of austenite is BCC ferrite and carbide. The third stage in tempering 200-700° C. in conventional C-steel can involve further decomposition of martensite into BCC ferrite and carbide structures, rapidly and beginning around 250° C. Around 400° C. in the conventional C-steel, the initially formed carbides can become replaced by Fe₃C precipitate, which can form preferentially along lath boundaries and former grain boundaries. Around 500° C. to 600° C., recovery of dislocations inherited from the conventional C-steel martensite can take place in the stage 3 ferrite to produce a low-dislocation-density acicular ferrite structure, and on further heating 600° C. to 700° C. the acicular ferrite grains can recrystallize to form an equiaxed ferrite structure.
Elements of conventional steels tend to cluster together; that is the Cr tends to cluster with itself, and the same with Mn, Mo, and C. The cluster effects promote formation of carbides, which deplete the matrix of solute elements, and the discontinuities of physical and mechanical properties of the carbides may initiate fracture under load. The compositions of embodiments with N and measured amounts of C, Cr, Mn, Mo, and N herein can better disperse solutes by short range ordering effects with C and N during austenitization, and upon quenching and tempering the Fe, Cr, Ni, Mn, Mo solutes are anticipated to largely resist diffusion more so than C and N, and the composition may thereby achieve a more refined quench and tempered microstructure than conventional C steel. Furthermore, unlike C steels, at high temper temperatures, the embodiments with N compositions which tend to form ordered phases may better contribute a secondary hardening effect. In this manner, the compositions of the embodiments herein can achieve an interstitial content of approximately 0.28 to 0.80 wt % to provide high strength and hardness combined with ductility from effects of a highly dislocated substructure of martensite, the pinning of martensite dislocations by interstitial N or SRO of N and solute, and the non-cubic long range distortion effect of the martensite. The alloys of these embodiments can resist formation of coarse carbides, the decomposition of martensite and provide hardening up to around 500° C. rather than the softening of C steel. The initial alloys are iteratively selected by Thermo-Calc software property diagram plots for utility of minimal effects from precipitation of carbides, most specifically cementite, and nitrides; therefore, it appears that there would be an enhanced effect of more finely distributed microstructure. The ordered structures of C and N and solute atoms within high temperature phases during austenization will largely be retained upon the quenching and tempering due to lower atomic mobility of the larger solute Fe, Cr, Mn, Mo atoms. The pinning of the dislocations of the quenched austenite martensite structure together with an enhancement of non-cubic distortion of the martensitic lattice may further provide a strengthening effect from dislocation movement and plastic flow. The property diagrams of the low alloys of the embodiments suggests that a high proportion of lath martensite with 1.5 wt % to 2.5 wt % austenite or more can result from a quench or equilibrium process.
N may readily dissolve in liquid steels with high Cr, Mn, and Mo contents. The amount of N dissolved in steel liquid under pressure of N may be estimated by Sieverts law and interaction parameters or by use of the Cr equivalent method of Feichtinger, Satir-Kolorz, and Xiao-Hong. During the first stage of solidification, specific compositions may form a portion of the delta ferrite phase which has low solubility of N. Ni, Mn, C, and N limit or eliminate delta ferrite. With appropriate low alloy Ni, Mn, C and N contents of the compositions of the embodiments, the amount of delta body-centered cubic (BCC) structure ferrite may be controlled or eliminated so that N dissolved in steel liquid may be more readily directly consolidated in solid steel of the face centered (FCC) phase. With near equal levels of C and N in the low alloy compositions of the embodiments, N is better retained following solidification in high temperature austenite to better resist degassing and gas porosity. Experimental trials reveal that W, and likely Mo, promotes consolidation of N under pressure from liquid into solid steel further beyond the solubility predicted for liquid steel, and that there may be a size effect of the solutes atoms. From composition, the class of steel; e.g., low-alloy or high alloy, may be determined by total alloy content, or by phase type with use of Cr and Ni equivalent coordinates on a Schaeffler diagram. A Schaeffler diagram closely distinguishes steel structures as either: ferritic-martensitic; martensitic; austenitic-martensitic; austenitic; martensitic-ferritic; ferritic; martensitic-austenitic-ferritic; and duplex austenitic-ferritic steels.
The activity, a_i, of any component in an ideal solution is equal to its mole fraction, and the activity coefficient of a component may be defined as the ratio of the activity of the component to its mole fraction. In the industry the activity coefficient; i.e., the interaction coefficient, f_N, of a component can be defined by the ratio of its activity and mass concentration. For multicomponent or non-ideal alloys of Fe, the solutes “i” affect their solubility and the influence is accounted for by interaction parameters.
An estimate of N solubility in liquid iron at 1600° C. and 1-atmosphere pressure was found to be 0.045 wt % N₂. The solubility of N in liquid iron may be determined by N wt %=(0.045*p_N2 ^1/2)/f_Nwith Sievert's law where the solubility of nitrogen is proportional to the square root of pressure.
For Fe or dilute solutions, the interaction coefficient f_N, for low alloy steels with less than 4% by mass percent of each constituent, with multicomponent constituents in Fe, f_Nmay be represented by a series shown in Equation 1 comprised of summed products of solute fractions in percent and their first order interaction parameters, e_N, which describe the influence of individual elements, and which may include the action of N on itself,
log f _N =e _N ^Cr*[% Cr]+e _N ^Ni*[% Ni]+e _N ^Mn*[% Mn]+e _N ^Mo*[% Mo]+E_N ^W*[W]+e _N ^Si*[% Si]+e _N ^C*[% C]+e _N ^N*[% N]. (1)
The calculations and experimental results indicate that interaction parameters cannot be calculated ab initio, that researchers obtain different values for their parameters, and that interaction parameters for calculation of N solubility in steels allows a semiquantitative result only. Table 1 lists a set of interaction parameters for solutions of N in steels. Elements with negative interaction parameters assist solution of N in liquid steel, while elements with positive interaction parameters inhibit solution of N in liquid steel. High levels of elements with more negative values of interaction parameters are not of interest in compositions of the embodiments; for example, the Ti and V constituents, beyond specific amounts may react with N strongly to form nitrides which remove N from solution of either austenitic FCC or BCC phases and do not provide a practical means to assist bringing N into solution in the matrix to provide the kinds of short range ordering and metallic electronic bonding whereby N may enhance solubility of atoms in HNS. Table 1 provides first order interaction parameters e_N ^Xfor solution of N in iron at 1600° C. (from Satir-Kolorz, 1990).

	TABLE 1

	Constituent,	Interaction
	X	parameter

	Ti	−0.930
	V	−0.098
	Nb	−0.050
	Cr	−0.048
	Mn	−0.021
	Mo	−0.013
	W	−0.002
	Cu	0.006
	Co	0.010
	Ni	0.011
	Al	0.040
	Si	0.043
	B	0.083
	C	0.118
	N	0.130

From the above, a Sievert's law estimate for N solution in multicomponent steel alloys at 1600° C. is: N %=(0.045*p_N2 ^1/2)/f_N.
Compositions 1 and 2 that are adaptable to cast ingot processes according to exemplary embodiments herein are shown in Table 2, Table 3, and Tables 6 through 9. These Mo alloy compositions may trend to the lower levels of Cr and N. Composition 3 that is adaptable to the solid state alloying methods of mechanical alloying or the gas-solid diffusion alloying methods of powder or thin sheet materials according to exemplary embodiments herein are shown in Table 2, Table 3, and Tables 10 through 11.

	TABLE 2

	Alloy	Composition

	1	Fe-0.30-1.00Ni, 1.60-2.00Mn, 1.30-1.45Cr, 1.10-1.50Mo,
		0.15-0.20C, 0.16-0.21N, 0.02V
	2	Fe-1.00-2.00Ni, 1.40-2.00Mn, 1.30-1.45Cr, 0.60-1.50Mo,
		0.15-0.20C, 0.16-0.21N, 0.02V
	3	Fe-0.10-1.40Ni, 0.60-1.70Mn, 1.30-2.80Cr, 0.60-1.50Mo,
		0.03-0.20C, 0.20-0.60N, 0.02V

According to the embodiments herein, methods of making low alloy high nitrogen steels fall within the steels described as low-alloy, with constituents less than 8 wt % alloy elements. Iterative property diagram plots of Thermo-Calc v4.0 2015a and 2016a software and successful experimental PESR trials and tests of similar Fe—Cr—Ni—W—Mn—N—C alloys were used for the selection of compositions. The compositions were referenced to a Schaeffler Ni equivalent versus Cr equivalent phase diagram to verify approximate compositions of the low alloy martensitic steels. Compositions were given upper and lower limits following iterative runs of Thermo-Calc software and included a Sievert's law estimate for N solution in multicomponent steel. Example objectives for pressure cast or pressurized electroslag remelt cast Compositions 1 and 2 alloys were to obtain: (1) alloy activity coefficients to ensure N solubility in the liquid steels with low alloy compositions; (2) to limit, minimize, or eliminate delta ferrite during initial solidification to minimize any rejection of N during solidification of high temperature delta ferrite; (3) to eliminate or minimize de-gassing and porosity of austenite solid following solidification; and (4) to obtain at specific temperatures of hot working, solution treatment, and tempering, phase structures with absence or near absence of solute and C-clustering formation of graphite, cementite, or complex M₂₃C₆or M₇C₃carbides, rather to obtain more homogeneous short range ordered phases and carbides in form of a fine dispersion of alloy constituents in either austenite, body centered cubic (BCC), or martensitic matrices within respective temperature ranges of FCC austenite and BCC phases. Furthermore, the exemplary embodiment compositions were selected with respect to the level of C, Cr and ratios of Cr/C, N/C, and Cr/N to minimize clustering effect of excess Cr, Mo, or C, as rather not to achieve high levels of Cr for corrosion resistance or to achieve an austenitic structure with high levels of Ni, Mn or N. The specific levels of Cr, Mn, and Mo constituents that are within low amounts serve to enhance solubility of N and C in the steel without enhancing the formation of complex carbides. Specific amounts of nitrogen and carbon minimize clustering effects of Cr—Cr or Mn—Mn by i-s interaction with these solutes to form more metallic-like electron bonds. The compositions of the exemplary embodiments can be specifically limited in ranges of solutes and interstitials. For the compositions of the exemplary embodiments, carbon levels more than the composition limits promote cementite and graphite below 700° C.; while a level too low of carbon may promote complex carbides at low temperatures below 400° C. In some embodiments, cast alloys with high N/C ratios may be possible with some porosity during solidification which may be healed at lower at temperature during wrought processing, or pressure over 40 bars may be used during ingot casting.
At 1 bar or more pressure around approximately 600° C. to 800° C., below the lower or upper transformation temperatures of the ferritic BCC to austenitic FCC crystallographic lattice structure, the embodiments of solid state mechanical processed Composition 3 alloys are designed to maximize N content dispersed in ordered phases with C, vacancies (VA), or alloy solutes. At low temperatures less than around 800° C., the alloys of the embodiments retain N without outgassing. At 1 bar pressure and at elevated temperatures; for example greater than 800° C., some gas may form from surfaces of the Composition 3 alloys with the lowest levels of Cr, and Mn constituents. At more elevated pressures of N, for example 120-250 bars or 12-25 MPa any outgas effect at the high range of temperature over 650-800° C. may be suppressed. Therefore, in the solid state alloy approach, the embodiments of solid state mechanical processed Composition 3 alloys may be processed to end composition beginning with powders or thin sheet from the semi-finished composition. The finish amount of N may be limited to before onset of formation of carbides, for example M₇C₃, and M₆C, or graphite and N gas. While meeting design objectives in these high nitrogen low alloy steels, the solid state alloy approach allows use of low levels of Ni, Mn, C, and Si, and higher levels of N, Mo, and Cr, than that of pressure cast or pressurized electroslag remelt ingot metallurgy. With low nickel content possible, the solid state process therefore provides the choice of maintaining the transition temperature of the ferritic body centered cubic BCC lattice structure to the austenitic face centered cubic FCC lattice crystallographic structure at high temperatures, and allows minimal austenite content.
Moreover, low alloy high nitrogen steel HNS martensitic compositions with Mo can have properties and structures, which were verified with iterative runs of the Thermo-Calc software with a range of solutes and interstitial C and N, which retain or reform strong bonds of hardening and strengthening phases relative to the matrix under conditions of high pressure. These Mo-modified alloys can withstand service conditions of elevated temperature and pressure and may better resist softening than the non-molybdenum-modified low alloy carbon steels.
The embodiments of steel compositions and processing can provide unique microstructures and properties from secondary hardening with temper temperatures of approximately 440-550° C., with a preferred temper temperature ranges of about 200-300° C. and 460-600° C.
The constituents in compositions of the embodiments can provide specific contribution to microstructure and properties depending on the specific ratios and amounts added. Table 3 shows the role of the solutes and interstitials, N and C, in hardening and strengthening of the Fe HNS alloys.

TABLE 3

Con-
stit-
uent	Process-Specific Design Effect or Role of Constituent

Fe	Base alloy, the matrix
Ni	Matrix solute, Fe-Ni short range order (SRO), toughens, Ni +
	Mn toughens
Ni	Minimizes, eliminates, delta ferrite during pressurized electroslag
	remelt (PESR) solidification to improve N level in solid
Cr	Assist solution of N and C, solid solution hardening (SSH),
	strong enthalpy (H) bond¹, improve resistance to corrosion,
	effective through-thickness hardenability
Mo	Assist solution of N and C, SSH and SRO strengthening¹, resist
	corrosion
Co	Similar to Ni, but raises the BCC to FCC transition temperature,
	enhances amount of delta ferrite
Mn	Shape control of S, eliminate FeS. Assist solution of N and C,
	SSH, minimize solidification defects, toughens
Mn	Minimize & eliminate delta ferrite during solidification to
	improve N level in solid, highly effective through-thickness
	hardenability for martensite processing
Si	Residual of Si₃N₄→ 3Si + 4N PESR additive and an impurity of
	slag or refractory, improves through-thickness hardenability
N	Homogenize solutes, improve hardness, flow strength, corrosion
	resistance, shock resistance, help limit amount of delta ferrite
	with Mn + Ni
N	Dissolve deleterious carbide, and (FeCuNiCr)3P intermetallics
	during temper treatment
C + N	Form SRO structures with Cr, Mo, W in matrix, improve
	mechanical & corrosion properties, minimizes delta ferrite in iron
C	Strengthens, minimize degassing of N in of low alloy HNS when
	N/C ~0.85-1.3

¹Strong bonding energy relative to the matrix at both low 1-40 bars pressure and high pressures to 17-kbars and 350° C. to ~650-700° C. throughout the martensitic or BCC, and much of the FCC austenite phase regions, to provide homogenization of solutes and strengthening.

Low alloy high to ultra-high strength martensitic steel articles of specific compositions compatible with specific heat treatment processes are provided according to exemplary embodiments. Specific compositions, levels and ranges of Cr, Mo, Mn, C, and Ni are provided to solutionize N in liquid steel, to allow solidification with little or no delta ferrite and degassing of high temperature austenite, and to obtain structures of alloys with homogeneous distribution of constituent elements in phases according to exemplary embodiments. Along with Cr, Mo, Mn, and C, the HNS alloys are provided with specific levels of the austenite forming Ni, Mn, N, and C and by use of specific ratios of Cr/C, Cr/N, and N/C to provide: (1) homogeneous phase structures that can be free of complex carbides with elements dispersed in the structures over a wide range of temperatures and/or wide range of temperature and pressure; (2) alloys that can be manufactured to achieve composition ranges compatible with the variability found in production; e.g., ±0.02-0.03C or N and ±0.05Cr; (3); pressure cast alloys that can achieve specific average range levels of C from 0.15 to 0.20 wt % with N from 0.16 to 0.21 wt % as for design of low-alloy high to ultra-high strength martensitic steels; and (4) a martensitic structure that can be obtained from austenitize-quench treatment according to exemplary embodiments. Specific compositions of the embodiments together with solid state processing either at low temperatures for example around 440-640° C., or high temperatures, for example 880-1100° C. gain utility of high N contents to 0.60 wt % with the absence of cementite or M₇C₃, or M₂₃C₆carbides.
Methods and Compositions
According to an exemplary embodiment herein, the manufacture of low alloy high nitrogen steel compositions of the embodiments may include forming direct to shape by either additive manufacture laser sintering of powder, solid state forming of powders, or squeeze or pressure casting of liquid to solid followed by austenitization and quench-temper heat treatment. According to another exemplary embodiment, the manufacture of low alloy high nitrogen steel compositions of the embodiments may include wrought methods beginning either with material sourced from mechanical alloying or controlled atmosphere gas-solid alloying, rapid solidification of powders or, pressurized electroslag remelt casting of ingots with the use of a slag under pressure of N or N plus Ar followed by hot working to billets, and shapes followed by heat treatments. In these embodiments, gas pressure can be maintained at a level above any of cooling water pressure level, and remelting methods can be a preferred method to minimize volume of liquid metal under pressure.
According to another embodiment the manufacture of the low alloy high nitrogen steel compositions may occur in the solid state at low temperature less than 800° C. by either mechanical alloying of powders or the controlled atmosphere gas-solid diffusion of N or of ammonia to assist N diffusion into powders or thin sheet, followed by consolidation processes of hot isostatic pressing (HIP). For the gas-solid diffusion alloy approach, the powder may be infused with N at low temperature around 450° C. to 650° C. and at atmospheric pressure. Another gas-solid diffusion alloy approach may be by using the method of elevated temperature greater than 800° C. and at elevated pressure similar to Feichtinger's nitriding method (as described in Feichtinger, H., “Alternative Routes to the Production of High-Nitrogen Steels,” Stein, G; Witulski, H., editors, High Nitrogen Steels FINS 90; Dusseldorf, Germany: Verlag Stahleisen GmbH; 1990, pp. 298-302); for example at 175 bars or 17.5 MPa during a low pressure hot isostatic press process in which an encapsulated unconsolidated can of porous powder is pressurized by a remote source of N to sufficient pressure to convey the necessary mass of N to meet the finish composition; and coincident the press pressure is maintained equal to can pressure along with outside heating of the can. Another solid state powder consolidation approach may include cold isostatic pressing (CIP) extrusion of powder, for example into tubing or a shape, followed by sintering under pressure of N followed by hot work extrusion, forging, or rolling. Both the HIP and CIP consolidation approaches of powders may end with austenitization, quench, and temper treatments. Furthermore, consolidation may occur by the additive manufacture of powder and a laser method under controlled atmosphere and elevated levels of pressure.
The low alloy compositions of the embodiments with moderate to high Mn and Ni constituents and melting and casting under N or N—Ar pressure can provide capability for casting to shape by squeeze casting or pressure casting methods with qualities of good fluidity. During solidification and cooling of solid metal, the compositions can provide high resistance or immunity to both gas evolution and porosity. The low alloy content balanced in levels of Cr, Mo, Mn, C, and N can prevent clustering and precipitation of massive carbides and nitrides that can be common in high alloy or higher Cr level compositions. Cast methods may use a pressure chamber with N or N plus Ar atmosphere to remelt a master alloy(s) with or without final addition of N and C. Pressure or squeeze cast melting may have material re-melt stock pre-prepared by vacuum induction melting, vacuum arc remelting, pressurized electroslag remelting, electric induction, or arc spray castings. The remelt method can include a method of stirring to initially homogenize alloy content. Ladle or remelt refinements may include injection of calcium, oxygen, nitrogen, and argon to adjust S, C, and N levels. Pouring into molds of specific shape may occur by differential pressure with feed from the bottom of a ladle or crucible. Molds can include risers, gates, and differential cooling to allow directional cooling and solidification and adequate feeding of liquid into the solidifying metal. Solidification can be completed under pressure, at the pressure of melting or at a pressure level greater than pressure at the melt temperature. Following solidification and cooling the cast shapes may be trimmed and rough finished, then heat treated in either air or in Ar—N inert atmosphere, for stress relief, softening, or austenitization. Austenitization temperatures at approximately 890-920° C. can prevent any significant decarburization or denitriding of the casting surface. For heat treatment at temperatures greater than approximately 860° C., a pressure or atmospheric retort with inert N or N+Ar atmosphere may be used to prevent decarburization and denitriding of workpiece surfaces. Following any austenitization treatment, the casting can be quenched then promptly tempered for hardening and toughening; for example, tempering at temperatures of around 200-300° C. or 460-600° C. for maximum combined strength and toughness. A controlled atmosphere or atmosphere of N and Ar+N during temper heat treatment may be used to minimize decarburization and improve surface quality resistance to corrosion. Thermal cycling of normalizing, or austenitization followed by quenching and tempering may be used to refine grain size of the casting.
The manufacture of the low alloy high nitrogen steel alloy in wrought form may occur by the pressurized electroslag remelt method and can begin with melting and casting of a consumable electrode (CE) by either one or more steps of air melt (AM), vacuum induction melt (VIM), argon oxygen decarburization (AOD) with or without calcium injection, or vacuum arc remelt (VAR) and the like or combinations thereof, to a composition near the final composition of steel with the exception of N and Mn, according to exemplary embodiments. Master alloys may be used to ensure solution in the melt of high temperature melting point alloy constituents during the cast of the consumable electrode. The consumable electrode may have a composition of up to or greater than approximate 0.04% or more N for final alloying by PESR to an ingot or other methods. Purity of the CE composition can be selected by first use of including a VIM, AOD melt with or without calcium injection to remove sulfur (S) or by the VAR method. Purity may be maintained by controlling levels of impurities in the melt stock, most specific strong carbide forming constituents Ti, Zr, V, and Nb, and the impurities S, P, As, Sn, O, and H. V may be added in small amounts; for example, up to 0.02 wt %, to assist control of grain size during heat treatment and hot work or forming. For any composition, it is preferred that the alloy range of more variable interstitials be matched to a corresponding range level of solutes; that is, high solute matched with a mid-high level interstitials. The PESR process may use high pressures of N; e.g., 40 bars pressure or greater, to remelt under pressure and bring N into solution of the alloy, and to solidify under pressure. The PESR process can ensure temperature and melt rate of the electrode for a progression of ingot solidification in which liquid material is fed gradually to the solidification front. Sievert's law may be used as an estimate for the necessary alloy compositions to bring N into solution. Thermo-Calc software may be used with specific compositions, temperature, and pressure for design and estimates of phase structure during solidification and heat treatments.
The consumable electrode can be remelted with a PESR plant which immerses the electrode end under a slag and under pressure with N. The slag may be preheated to ensure dryness at the start of the PESR remelt process. The PESR ingot can be built up in a pressurized, water cooled mold. During PESR remelt processing alternating electric current resistance heats the slag and electrode tip, metal droplets fall through the molten slag and chemical reactions can reduce sulfur and nonmetallic inclusions. N can be added under pressure as necessary to meet the composition requirement by addition of silicon nitride (Si₃N₄), or intermetallic compounds of Cr and N, or N gas. Dry N may be added to the furnace to assist solution of N. Low alloy steels of the embodiments can be subjected to about 40 bars (580 psia, 4.0 MPa) N pressure, and up to about 45 bars (653 psia, 4.5 MPa) to solutionize N contents. Excess Si₃N₄can result in porosity of the PESR casting or ingot. PESR processing can be performed under optimal conditions of power and melt rate so that solidification structure is directional from bottom to top, fed by liquid, and completed in a manner that results in a solidification structure that is high in density and homogeneity with minimal or no porosity and with minimal segregation in the ingot. Deoxidizers may be added as necessary to the slag. A proper melt rate and power setting can be used to provide an absence of segregation and shrinkage.
Following casting, cast ingots can be homogenized at elevated temperatures of about 870° C. to about 1120° C. for a sufficient time to solutionize elemental constituents, lessen segregation, and to bring the ingot to a uniform high temperature to allow plastic deformation of the steel ingot. Homogenization of the alloy ingot to lessen segregation and for solution treatment can be performed followed by hot work to billets and plate or structure shapes with hot work performed with increments of reduction and reheating as needed. Following homogenization or as part of homogenization, the ingots can be mechanically hot worked by rolling, forging, hammering, or squeezing to improve the steel by closing any small cavities or voids, breaking up and dispersing solutes and any small impurities, and recrystallizing and refining the grain structure to a more homogeneous product which may be a bloom or billet. To make billets the steel can be shaped into blooms then further incrementally reduced in a mill. Each time the ingot is forced through rolls, it can be reduced in one dimension. After one or two passes the steel may be turned to bring the side surfaces under the rolls for a more uniform material. After the steel is rolled the uneven ends can be sheared off and the length cut to shorter lengths. Mill scale and surface defects can be removed from the surfaces. The product billet may be stress relief annealed at low temperature to minimize residual stress and to soften the steel for handling, cutting, and shipping.
By mechanical hot working, for example, by heating uniformly then rolling beginning with incremental reductions at temperatures of around 1050° C. to 1120° C., billet, blooms, or slabs may be used to manufacture end products which may be plate shapes. To refine grain size, more evenly distribute alloy constituents, and improve toughness of wrought products the hot work shall ensure adequate reductions and have multipass shape reduction at about 1050° C. to about 1080° C. with finish roll passes at about 1000° C. to about 1040° C. Hot work and homogenization may include slow heating with low temperature holds around 580° C. and 880° C. before heating to higher temperatures. Finish hot rolling of final passes through rolls may be used at the lower temperatures of about 1000° C. to about 1040° C. to improve impact toughness and refine grain size. Slabs can be used for rolling of large plate shapes.
The roll process may include increments of rolling down to rough thickness, followed by rough flattening around 850° C. to about 930° C. and stress relief anneal with cooling slowly and to a hold at about 580° C. to about 610° C., a softening anneal at about 580° C. to about 610° C., followed by finish flattening and air cooling, then mechanical surface finishing. The alloys that contain cobalt (Co) may require annealing temperatures adjusted upwards by 30° C. to 40° C. or greater. Mechanical finishing during and following billet and plate manufacture may include removal of mill scale by jets of water, grit blast, brushing, followed by surface grinding, and sanding to remove surface defects and oxide scale and to bring dimension to the shape.
Trimming of ingots or hot worked shapes by mechanical or flame or plasma cutting may be performed. Grinding or sanding may be used to finish surfaces or to remove any defects. The specific method may be chosen based, for example, on the specific stage of processing from ingot to product.
The forming into shape of the solid state mechanical alloyed or controlled atmosphere-solid alloyed, powders by the sintering process may occur first with handling under N atmosphere sealing inside a can or envelope followed by cold isostatic pressing compaction to reduce bulk, then sintering under controlled atmosphere with any of ammonia, N, and Ar at pressures of 120 to 250 bars and temperatures of 1150° C. to 1400° C. The higher 1315° C. to 1400° C. sinter temperatures provide greater near theoretical density, the lower temperatures provide finer grain size. The powder may be formed and consolidated into shape by the additive manufacture process by laser sintering under temperatures greater than 900° C. and elevated pressures of 120 to 250 bars. The sintered product may be further consolidated to full density by hot extrusion, forging, or rolling.
Alternatively, for consolidation forming by hot isostatic pressing, the finish composition mechanically alloyed or controlled atmosphere-solid alloyed powders or thin sheets may be can packaged under N atmosphere, vacuum evacuated, consolidated or not by cold isostatic pressing, then densified by hot isostatic pressing in atmosphere of N or Ar at temperatures of 1150° C. to 1250° C. and pressures of 1000 bars to 1500 bars or 100 MPa to 150 MPa.
Alternatively, by Feichtinger's nitriding method (German Patent No. DE 3624622 C2, the complete disclosure of which is incorporated by reference herein), the powder or thin sheets with sufficient open space in the form of a semi-finished composition may be infused with N to average finish composition by remote charging of the can packaged powders or sheet with sufficient mass level of N by pressure, and with equal pressure of the applied hot isostatic press, heated by outside energy source inside the hot isostatic press then held at temperatures of 450-650° C. or ideally at more elevated temperature of over 820° C.
Both the sintering and HIP methods may subsequently use hot work by extrusion or forging or rolling followed by heat treatment austenitization, quench and tempering. Alternatively, the solid state alloyed powder may be consolidated by packaging, evacuation under vacuum, cold isostatic pressing to remove bulk, then hot extrusion, followed by austenitization, quench, and temper heat treatments.
According to exemplary embodiments, specific heat treatments following full consolidation of solid state alloyed powder material or cast ingot hot rolling, and the stress relief, soften annealing, finish flattening, air cooling, and surface grinding and sanding may include normalization heat treatment at temperatures around 870° C. to about 1050° C., by heating to and holding at temperature, followed by air cool of the product. The normalize treatment can refine grain size, and bring into solution any segregated or gross precipitates of constituents of the alloy to homogenize solutes, and avoid residual stress during cooling.
According to exemplary embodiments, the low alloy high nitrogen steel composition, with or without prior normalizing heat treatment, can undergo an austenitization process heat and/or thermomechanical treatment followed by quenching and tempering. The austenitization can include the heating of the composition according to the embodiments herein to a temperature at which changes crystal structure from BCC ferrite to FCC austenite, and brings into solution the major hardening and strengthening constituents, for example, Cr, Mo, C, and N, followed by a quench or air cooling specifically to harden the steel. An austenitization treatment can be performed near 890° C. to about 940° C., or at a sufficient temperature and time to uniformly heat and solutionize many of the N and alloy constituents in FCC austenitic phases, but with a low enough temperature to avoid excessive grain growth or to avoid formation of extreme surface scale or porosity. Austenitize temperature excessively high may lead to microcracks during a subsequent quench. The exact process time-temperature schedule can be verified by microstructure and mechanical property tests for each alloy so as to obtain the required grain size, hardness, strength levels, impact toughness, and fracture toughness. At high temperatures of normalizing or austenitizing an inert atmosphere mainly of argon gas (Ar) with nitrogen may be used to minimize surface decarburization, denitriding, or oxidation. The heating rate may be stepped first to around 580° C. to 610° C., held briefly, and then heated to the austenitization temperature.
Quenching from the austenitization temperature can proceed directly without delay, or may include an intermediate temperature hold, or a final roll thermomechanical treatment. The quench media may include forced or still air, water spray, warmed agitated polymer-water solution or oil, liquid salt, and the like, or specific combinations thereof.
Quenches may either proceed immediately directly from austenitization temperatures, from thermomechanical treatment (TMT) final hot roll reduction process, or from intermediate hold temperatures. For example, a quench may proceed by quenching the alloy of the embodiment directly into oil held at about 38° C. to about 177° C., or polymer plus water solution at about 27° C. to 66° C., followed, first by a temper treatment, or by freezing to less than about −20° C. down to about −78.5° C., then a final temper treatment as desired upon end application and alloy and required resultant product hardness and toughness by control of the level of any retained austenite and to more further complete transformation to a martensitic structure. As another example, an intermediate quench may proceed by quenching the alloy of the embodiment into media at about 460° C. to about 550° C. and held at temperature, referred to as austemper, consistent with secondary hardening at, for example, about 460° C. to about 550° C., followed by a quench to lower temperature as described.
Temper methods can immediately follow austenitization and quench and may use a single temper step or multiple temper steps. The multiple temper steps can proceed with or without an intermediate chill, for example, cryogenic or about −20° C. to about −78.5° C., including a hold step at chill temperature. A single temper step can include one temper at elevated temperature followed by a quench to ambient temperature with or without an immediate chill for example, cryogenic or about −20° C. to about −78.5° C., including hold step at chill temperature. The temper step can include a thermomechanical treatment (TMT) prior to quenching. The temper step can include an austemper step; e.g., a quench to and hold at about a 440° C. to about 550° C. temperature range followed by a lower temperature quench and an optional second temper. An autotemper step may include a soft quench; e.g., by air convection or forced air quench entirely or as a portion of a temper process. Normalizing may include air cooling to an ambient temperature. A steel alloy of the embodiments herein tempered at less than 440° C. can be referred to as a primary hardened alloy. Whereas a steel alloy of the embodiments tempered at greater than 440° C. can be referred to as a secondary hardened alloy.
FIG. 1A is a flow diagram showing a method of making a wrought low alloy high nitrogen steel according to an embodiment herein. Operation 110 represents providing remelt casting ingots under N; e.g., by PESR, of the low alloy high nitrogen steel composition according to the embodiments herein. In operation 120 of the method, the low alloy high nitrogen steel composition can undergo homogenization as described. In operation 130 of the method, low alloy high nitrogen steel composition can undergo hot work. In operation 140 the low alloy high nitrogen steel composition can undergo a stress relief anneal process. In operation 150, the low alloy high nitrogen steel composition can be subjected to a softening anneal process, and in operation 160 the composition can be subjected to a normalization process. In operation 170 of the method, the composition can undergo austenitization. In operation 180, during quenching, the method can have an optional intermediate hold. Operation 190 represents a quench directly from austenitization in operation 170, or directly from the optional intermediate hold in operation 180. The method can further include tempering of the low alloy high nitrogen steel composition in operation 200. To refine grain size as described, in operation 210, the method can include repeating normalizing (160), or austenitizing (170) and quenching (190), with or without the optional intermediate hold (180) and tempering (200) steps may be performed. To lower amounts of residual austenite following austenitization and quench, a chill or multiple steps (200) of tempering may be performed.
FIG. 1B is a flow diagram showing another method of making a low alloy high nitrogen steel according to an embodiment herein. In the illustrated method of FIG. 1B, squeeze or pressure casting direct to shape; e.g., by PESR, of the low alloy high nitrogen composition can be performed in operation 310. In operation 320, the method can include stress relief annealing the low alloy high nitrogen steel composition. As illustrated by operation 330, the low alloy high nitrogen steel composition can be subjected to a normalizing anneal process, and in operation 340 of the method, the composition can undergo austenitization. In operation 350, during quenching, the method can have an optional intermediate hold. Operation 360 represents a quench directly from austenitization in operation 340, or directly from the optional intermediate hold in operation 350. The method can further include tempering of the low alloy high nitrogen steel composition in operation 370. In operation 380, the method can include repeating normalizing (330) or austenitizing (340), quenching (360), with or without the optional intermediate hold (350), and tempering (370) to refine grain size as described.
FIG. 1C is a flow diagram showing another method of making a low alloy high nitrogen steel according to an embodiment herein. In operation 410 of the illustrated method of FIG. 1C, solid state alloying can be performed and may begin with a powder near the low alloy high nitrogen finish composition for example by N gas-solid mechanical alloying to the finished composition, or for powder or thin sheet by controlled atmosphere gas-solid diffusion alloying such as with N or an ammonia stream for gas-solid alloying. The diffusion of N into powders or thin sheet may occur both within a hot isostatic press and the encapsulating can which is connected to a source of variable N pressure sufficient to convey the required mass of N for alloying and equal to the isostatic pressure; and while equal pressure is maintained the furnace may be brought to N diffusion temperature. In operation 420, the method can include canning under N atmosphere then vacuum encapsulation the powders or sheets of the low alloy high nitrogen steel composition. As illustrated by operation 430, the vacuum encapsulated low alloy high nitrogen steel composition may be subjected to a hot isostatic process, and in operation 440 of the method, the composition may undergo optional hot work to shape. In operation 450 the method may involve an optional normalizing anneal. Operation 460 represents an austenitization process followed by an optional intermediate hold in operation 470, and quench in operation 480. The method may further include tempering of the low alloy high nitrogen steel composition in operation 490. In operation 495, the method may include repeating tempering (490) and quenching (480), to lessen any amount of residual austenite.
FIG. 1D is a flow diagram showing another method of making a low alloy high nitrogen steel according to an embodiment herein. In the illustrated method of FIG. 1D, the solid state alloying of powder or thin sheet material of the low alloy high nitrogen composition may be performed in operation 510. In operation 520, consolidation may be begin with optional enclosure within an envelope and cold isostatic compaction or pressing under N atmosphere followed by either operation 530 hot sintering under N or ammonia controlled atmosphere with pressure, for example of 120-250 bars, or operation 540 hot work forging or extrusion. An optional normalize treatment with soft quench may be performed in operation 550. Hardening and toughening of material may be performed by an austenitization operation 560, followed by optional operation 570 involving an intermediate hold, followed by a quench 580 operation and a tempering operation 590.
FIG. 1E is a flow diagram showing additive manufacture laser sinter consolidation of powder into solid form or shape, followed by optional sintering under pressure or wrought hot work, followed by austenitization, quench and temper method of making a low alloy high nitrogen steel according to embodiments herein. In operation 610 of the illustrated method of FIG. 1E additive manufacture by laser sinter welding of powder into shape may be performed. In operation 620, the method may include hot isostatic pressing, sintering, hot work forging or extrusion. An optional normalize treatment with soft quench may be performed in operation 630. Hardening and toughening of material may then be performed by an austenitization operation 640, followed by optional operation 650 involving an intermediate hold, followed by a quench operation 660, and a tempering operation 670. The various operations may be performed in various steps depending on whether the optional operations are performed, and as denoted by the dashed lines in FIG. 1E.
Table 4 provides ranges of approximate process temperatures according to exemplary embodiments for wrought hot work and heat treat operations shown in FIGS. 1A, 1B, 1C, 1D, and 1E.

TABLE 4

a. Homogenization: casting, ingot, bloom, billet, shape	870-1121°	C.
b. Hot work; e.g. roll billet and plate	1050-1121°	C.
c. Hot work, finish roll plate	1000-1040°	C.
d. Normalizing: casting, ingot, bloom, billet, shape	870-1050°	C.
e. Austenitization: casting, ingot, bloom, billet, shape	890-940°	C.
f. Stress relief, slow cool to/hold: casting, bloom,	580-610°	C.
billet, shape
g. Soften Anneal, hold/air cool: casting, ingot, bloom,	580-610°	C.
billet, shape
h. TMT low	860-910°	C.
i. TMT high	910-1000°	C.
j. Temper (primary, hardening)	220-440°	C.
k. Temper (secondary, hardening)	440-600°	C.

Table 5 presents composition ranges of the low alloy high nitrogen steel according to exemplary embodiments. Single values are maximums.

TABLE 5

Chemical Composition Ranges

	Element	Weight (mass) percent

	Nitrogen (N)	0.16-0.60
	Carbon (C)	0.03-0.20
	Nickel (Ni)	0.10-2.00
	Manganese (Mn)	0.60-2.00
	Chromium (Cr)	1.30-2.80
	Molybdenum (Mo)	0.60-1.50
	Vanadium (V)	0.02
	Silicon (Si)	0.60
	Copper (Cu)	0.10
	Titanium (Ti)	0.02
	Niobium (Nb)	0.02
	Aluminum (Al)	0.008
	Sulfur (S)	0.008*
	Phosphorous (P)	0.015*
	Oxygen (O)	10-40*
	Hydrogen (H)	4*
	Antimony (Sb)	0.005*
	Tin (Sn)	0.005*
	Arsenic (As)	0.005*
	Others, Each	0.02
	Others Total	0.10

	Single values = maximums as objectives;
	Co may substitute for any part of Ni;
	*Impurities of S, P, H, O, Sb, Sn, As are preferred to be held to as low as possible levels based on electrode and PESR methods.

Example compositions of alloys were developed and are presented here as illustrative embodiments and not for the purpose of limitation. Examples of the phase constituents of the alloys which provide an ordered or a more uniform distribution of elemental constituents are as follows: FCC_A1 (Fe,Mn,Ni,Cr,Mo,V,Si)1(VA,C,N)1 where VA is a lattice vacancy; FCC_A1#2 (Cr,Fe,Mn,Mo,Ni,S,Si,V)1(C,N,VA,)1; HCP_A3#2 (Cr,Fe,Mn,Mo,Ni,Si,V)(VA,C,N)0.5, and BCC_A2 (Fe,Mn,Ni,Cr,Mo,V,Si)1(VA,C,N)3. Table 6 provides Example Composition 1 low alloy (LA) with 0.15 to 0.20 C and 0.16-0.21 N objective.

TABLE 6

Composition 1 - Chemical Composition Limit Ranges

	Element	Weight (mass) percent

	Nitrogen (N)	0.16-0.21
	Carbon (C)	0.15-0.20
	Nickel (Ni)	0.30-1.00
	Manganese (Mn)	1.60-2.00
	Chromium (Cr)	1.30-1.45
	Molybdenum (Mo)	1.10-1.50

Proposed alloys of Example Composition 1 with 0-0.008 wt % gas and 0-14 wt % delta ferrite at 40 bar pressure and initial temperatures of solidification and cooling are presented in Table 7.

TABLE 7

Cr	Ni	Mn	Mo	Si	V	N	C

1.30	0.70	1.70	1.40	0.50	0.01	0.19	0.19
1.40	0.80	1.90	1.30	0.50	0.01	0.19	0.18
1.30	0.80	1.70	1.50	0.50	0.01	0.19	0.18
1.40	0.60	1.90	1.30	0.50	0.01	0.19	0.18
1.40	0.80	1.60	1.30	0.50	0.01	0.19	0.18
1.30	0.80	1.80	1.30	0.50	0.01	0.19	0.17
1.30	0.70	1.80	1.30	0.50	0.01	0.19	0.16
1.40	0.90	1.80	1.10	0.50	0.01	0.19	0.18
1.30	0.90	1.80	1.50	0.50	0.01	0.19	0.18

Table 8 provides Example Composition 2 low alloy (LA) with 0.15 to 0.20 C and 0.16-0.21 N objective.

TABLE 8

Composition 2 - Chemical Composition Limit Ranges

	Element	Weight (mass) percent

	Nitrogen (N)	0.16-0.21
	Carbon (C)	0.15-0.20
	Nickel (Ni)	1.00-2.00
	Manganese (Mn)	1.40-2.00
	Chromium (Cr)	1.30-1.45
	Molybdenum (Mo)	0.60-1.50

Proposed alloys of Example Composition 2 with 0.003 wt %-0.008 wt % gas and 0 wt % to 1 wt % delta ferrite at 40 bar pressure and initial temperatures of solidification and cooling are presented in Table 9.

TABLE 9

Cr	Ni	Mn	Mo	Si	V	N	C

1.40	1.00	1.80	1.30	0.50	0.01	0.19	0.18
1.40	1.20	1.80	1.30	0.50	0.01	0.19	0.18
1.30	1.40	1.60	1.00	0.50	0.01	0.18	0.16
1.30	1.40	1.60	0.80	0.50	0.01	0.18	0.16
1.40	1.50	1.80	0.70	0.50	0.01	0.18	0.16
1.30	1.40	1.80	1.00	0.50	0.01	0.18	0.16
1.40	1.50	1.90	1.30	0.50	0.01	0.19	0.18
1.40	1.30	1.90	1.30	0.50	0.01	0.19	0.18
1.40	1.20	1.90	1.30	0.50	0.01	0.19	0.18
1.40	1.00	1.90	1.30	0.50	0.01	0.19	0.18

Table 10 provides Example Composition 3 low alloy (LA) steel with 0.12 to 0.20 C and 0.16-0.60 N objective which is capable of solid state alloying to composition with absence of gas phase constituents specifically either at 1 bar pressure or more and below the upper transformation temperature or below 800° C., or above 800° C. at moderate pressures around 120 to 250 bars or 12 to 25 MPa. Composition 3 alloys may be made free of internal gas, carbide or graphite constituents following the embodiments of consolidation, austenitization, and quench and temper processing.

TABLE 10

Composition 3 - Chemical Composition Limit Ranges

	Element	Weight (mass) percent

	Nitrogen (N)	0.16-0.60
	Carbon (C)	0.03-0.20
	Nickel (Ni)	0.10-1.40
	Manganese (Mn)	0.60-1.70
	Chromium (Cr)	1.30-2.80
	Molybdenum (Mo)	0.60-1.50

Proposed alloys of Example Composition 3 with enhanced levels of Cr and N, low levels of Si, low to moderate levels of Ni, and high or low Mn and C are presented in Table 11. The specific Example Composition 3 alloys shown in Table 11 reveal over wide ranges of composition, the manner in which N, C, Cr, and Mo may be both balanced and enhanced fully within the embodiments. Thermo-Calc models reveal specific Example Composition 3 alloys have minimal or no M₂₃C₆, M₇C₃, or M₆C carbides over a wider range of temperature. The lower temperature limit below which no equilibrium state gas forms from free surfaces in these specific Example Composition 3 alloys is 650-825° C. These Composition 3 alloys can have significantly less propensity for carbides than the Example Composition 1 and 2 alloys, which must meet issues of rejection of gas during solidification of any delta ferrite at high temperatures in austenite.

TABLE 11

Cr	Ni	Mn	Mo	Si	V	N	C

1.40	0.40	0.80	0.90	0.15	0.01	0.45	0.06
1.40	0.40	0.80	0.90	0.15	0.01	0.42	0.06
1.40	0.40	1.00	0.80	0.15	0.01	0.40	0.06
1.30	0.40	0.60	0.70	0.15	0.01	0.42	0.03
1.40	0.40	0.60	0.80	0.15	0.01	0.38	0.07
1.35	0.40	0.80	0.80	0.15	0.01	0.40	0.05
1.40	0.40	0.60	0.60	0.15	0.01	0.38	0.09
1.30	1.40	1.60	1.00	0.15	0.01	0.26	0.16
1.50	0.15	1.60	1.00	0.15	0.01	0.30	0.15
1.50	0.15	1.70	1.00	0.15	0.01	0.30	0.15
1.70	0.15	1.60	1.25	0.15	0.01	0.36	0.17
1.80	0.35	1.60	1.30	0.15	0.01	0.38	0.18
1.80	0.35	1.60	1.35	0.15	0.01	0.38	0.18
1.80	0.15	1.60	1.35	0.15	0.01	0.40	0.18
1.90	0.15	1.60	1.35	0.15	0.01	0.42	0.18
2.00	0.15	1.60	1.30	0.15	0.01	0.44	0.18
2.10	0.15	1.60	1.20	0.15	0.01	0.46	0.18
2.40	0.15	1.60	1.00	0.15	0.01	0.52	0.18
2.60	0.15	1.60	0.80	0.15	0.01	0.56	0.18
2.80	0.15	1.60	0.75	0.15	0.01	0.60	0.18
2.80	0.15	1.50	0.75	0.15	0.01	0.60	0.18

Specific Phases which May be Present are as Follows:
FIG. 2A illustrates a schematic diagram of phase versus temperature for an first Example Composition 1 of Fe-0.70Ni-1.70Mn-1.30Cr-1.40Mo-0.50Si-0.19N-0.19C, using a Thermo-Calc model, with 0.008 wt % gas and 8.1 wt % BCC delta ferrite at a cast pressure of 40 bars, for a system of 1 gram and where the phases are in gram scales of 0 to 1.00 according to an embodiment herein. FIG. 2B illustrates the phase versus temperature diagram of FIG. 2A with an expanded gram scale of 0 to 0.10.
FIG. 3A illustrates a schematic diagram of phase versus temperature for second Example Composition 1 of Fe-0.80Ni-1.90Mn-1.40Cr-1.30Mo-0.50Si-0.19N-0.18C, using a Thermo-Calc model, with 0.004 wt % gas and 3.6 wt % BCC delta ferrite, at a cast pressure of 40 bars, for a system of 1 gram and where the phases are in gram scales of 0 to 1.00 according to an embodiment herein. FIG. 3B illustrates the phase versus temperature diagram of FIG. 3A with an expanded gram scale of 0 to 0.10.
FIG. 4A illustrates a schematic diagram of phase versus temperature for third Example Composition 1 of Fe-0.80Ni-1.70Mn-1.30Cr-1.50Mo-0.50Si-0.19N-0.18C, using a Thermo-Calc model, with 0.008 wt % gas and 10.0 wt % BCC delta ferrite, at a cast pressure of 40 bars, for a system of 1 gram and where the phases are in gram scales of 0 to 1.00 according to an embodiment herein. FIG. 4B illustrates the phase versus temperature diagram of FIG. 4A with an expanded gram scale of 0 to 0.10.
FIG. 5A illustrates a schematic diagram of phase versus temperature for a fourth Composition 1 of Fe-0.80Ni-1.80Mn-1.30Cr-1.30Mo-0.50Si-0.19N-0.17C, using a Thermo-Calc model, with 0.008 wt % gas and 8.2 wt % BCC delta ferrite, at a cast pressure of 40 bars, for a system of 1 gram and where the phases are in gram scales of 0 to 1.00 according to an embodiment herein. FIG. 5B illustrates the phase versus temperature diagram of FIG. 5A with an expanded gram scale of 0 to 0.10.
FIG. 6A illustrates a schematic diagram of phase versus temperature for a first Example Composition 2 of Fe-1.00Ni-1.80Mn-1.40Cr-1.30Mo-0.50Si-0.19N-0.18C, using a Thermo-Calc model, with 0.006 wt % gas and 0.70 wt % BCC delta ferrite, at a cast pressure of 40 bars, for a system of 1 gram and where the phases are in gram scales of 0 to 1.00 according to an embodiment herein. FIG. 6B illustrates the phase versus temperature diagram of FIG. 6A with an expanded gram scale of 0 to 0.10.
FIG. 7A illustrates a schematic diagram of phase versus temperature for a second Example Composition 2 of Fe-1.20Ni-1.80Mn-1.40Cr-1.30Mo-0.50Si-0.19N-0.18C, using a Thermo-Calc model, with 0.008% gas and 0.0% BCC delta ferrite at a cast pressure of 40 bars, for a system of 1 gram and at 40 bars pressure, where the phases are in gram scales of 0 to 1.00 according to an embodiment herein. FIG. 7B illustrates the phase versus temperature diagram of FIG. 7A with an expanded gram scale of 0 to 0.10.
FIG. 8A illustrates a schematic diagram of phase versus temperature for a third Example Composition 2 of Fe-1.40Ni-1.60Mn-1.30Cr-1.00Mo-0.50Si-0.18N-0.16C, using a Thermo-Calc model, with 0.006 wt % gas and 0.0 wt % BCC delta ferrite at a cast pressure of 40 bars, for a system of 1 gram, where the phases are in gram scales of 0 to 1.00 according to an embodiment herein. FIG. 8B illustrates the phase versus temperature diagram of FIG. 8A with an expanded gram scale of 0 to 0.10.
FIG. 9A illustrates a schematic diagram of phase versus temperature for a fourth Example Composition 2 of Fe-1.40Ni-1.60Mn-1.30Cr-0.80Mo-0.50Si-0.18N-0.16C, using a Thermo-Calc model, with 0.007 wt % gas and 0.0% BCC delta ferrite at a cast pressure of 40 bars, for a system of 1 gram, where the phases are in gram scales of 0 to 1.00 according to an embodiment herein. FIG. 9B illustrates the phase versus temperature diagram of FIG. 9A with an expanded gram scale of 0 to 0.10.
Results of the Thermo-Calc model property diagrams of FIGS. 2A through 9B demonstrate that a more uniform distribution of the alloy constituents and ordered phases in the steels may be obtained by austenitization, quenching, and secondary hardening; e.g., tempering in a temperature range from about 460° C. to about 600° C. Furthermore, consecutive tempers and quenches may be used to lower the level of retained austenite to increase hardness.
Embodiments herein of the low alloy high nitrogen steel can provide characteristics in compositions and processes specific for combined high strength and toughness as a low cost low alloy steel. These embodiments can be obtained by adequate levels and ratios of C plus N, balanced N, C, Cr and Mo with Mn, and Cr, and Mo to achieve high levels of strength, beneficial microstructure, and durability with toughness and resistance to stress corrosion at high levels of strength where typical high strength carbon steels are susceptible to stress corrosion or low toughness.
Other embodiments can include compositions with Mo, high Mn with low-moderate Ni, and balanced C plus N, to provide increased near ideal phases, for example, either no or minimal delta ferrite during solidification for minimal or no N de-gassing during and immediately following solidification. Furthermore, the inheritance of the C plus N ordered austenitized Fe—Cr—Ni—Mn—Mo solute structure in ferritic BCC form may resist diffusional transformation during quenching then tempering at process temperatures up to about 460° C. to about 520° C., to provide more refined homogeneous phases and to preclude deleterious weak brittle carbides or massive nitrides in these embodiments.
In some embodiments including compositions with 1.40 to 2.00 Mn, N solubility can be enhanced in liquid and with relatively low levels of Ni. This can enable fewer tendencies for N to de-gas immediately following solidification and at high temperatures in the austenitic region.
The high Mn+N compositions of some embodiments provides an effect to substitute for Ni and enhances N solubility in liquid to solid transitions for alloys by inhibiting delta ferrite and thereby with high N assisting an effect to inhibit carbide formation at low temperatures such as with Cr. That is, carbides for example M₂₃C₆, are inhibited from forming at low temperatures according to these embodiments.
Embodiments herein including a Mo constituent in a 0.60 to 1.5 percent range can provide a composition and a process in service to retain strength and toughness by resisting and precluding the weakening and failure processes typical of carbon austenite FCC and carbon ferritic BCC precipitates and matrices at ambient and at moderately high temperatures and pressures as shown by the Thermo-Calc software up to 13.79-15.513 kbar (200-225 ksi) pressure by ordered phase; e.g., HCP_A3#2 enthalpy more negative than the matrix. Ordinarily, in carbon steel, Mo and W solutes are difficult to solutionize throughout process temperatures in steel, in contrast, the steel alloys of the embodiments herein having C—N compositions as described, enhance solubility of Mo and the alloy constituents Cr, Mn, N, and V.
These embodiments provide a unique Mn and balanced C and N constituents in low alloy high nitrogen steel, which can allow a greater window of Cr composition and process than Ni to provide fewer tendencies for N to de-gas immediately following solidification and at high temperatures in the austenitic region.
These embodiments provide low Ni by substitution of Mn combined with N for low cost, and a more stable alloy under high pressure with minimal reversion to austenite, and for more strengthening by conversion to martensite during quench hardening, than, for example, the known low alloy carbon steels or the high alloy secondary hardening steels.
The compositions and processes provided by the embodiments herein preclude deleterious weak, brittle cementite, or higher order carbides, for example, Fe₃C, M₃CN, M₇C₃, M₂₃C₆, M₆C, and the like, that deplete the matrix of corrosion protective Cr and Mo, C, and N.
According to the embodiments herein, the low alloy compositions with Mn and Ni constituents and melting/casting under N or N—Ar pressure promote capability for casting to shape by squeeze casting or pressure casting methods with qualities of good fluidity. For example, during solidification and cooling of solid metal, the compositions of 0% delta ferrite and 0%-0.008% gas at 40 bars pressure can promote high resistance or immunity to both gas evolution and porosity. The low alloy content balanced in levels of Cr, Mo, Mn, C, and N can prevent clustering and precipitation of massive carbides and nitrides common in high alloy or higher Cr level compositions. Furthermore, pressure of the casting during solidification may be raised to lower hazards or to enhance the level of N.
For wrought products, according to the embodiments herein, the PESR method can provide characteristics over alternative melt processes and simple N-pressure casting, by obtaining a high level of N solution into liquid and solid, high ingot quality through removal of S, dispersion of any oxides into small dispersed round shapes, and a refined dendrite structure with minimal segregation. Large production volumes may be obtained as ingots according to these embodiments. Inclusion of clean metal practices, for example, VIM/VAR, nitrogen/argon refining, to produce electrodes for the PESR process or squeeze casting can provide additional characteristics in toughness and resistance to stress corrosion.
FIG. 10A illustrates at the same cast pressure of 40 bars (580 pounds/inch²) the range of phase stability versus temperature and the phase enthalpy of all phases in joules/mole (J/mol) versus temperature for a 1 gram system of the same Example Composition 1 alloy Fe-1.30Cr-0.80Ni-1.30Mo-1.80Mn-0.50Si-0.01V-0.17C-0.19N shown before in FIG. 5A and FIG. 5B according to an embodiment herein.
FIG. 10B illustrates at pressure of 13,790 bars (200,007 pounds/inch²), the range of phase stability versus temperature and the phase enthalpy of all phases in joules/mole (J/mol) versus temperature for the same Example Composition 1 of Composition I alloy Fe-1.30Cr-0.80Ni-1.30Mo-1.80Mn-0.50Si-0.01V-0.17C-0.19N shown before in FIG. 5A and FIG. 5B alloy according to an embodiment herein.
FIG. 11A illustrates a 1 gram system schematic diagram of phase versus temperature at 175 bars pressure for Fe-1.90Cr-0.15Ni-1.35Mo-1.60Mn-0.15Si-0.01V-0.006S-0.18C-0.42N solid state alloy first Example Composition 3 according to an embodiment herein. FIG. 11B illustrates the phase versus temperature diagram of FIG. 11A with an expanded scale of 0.00 to 0.10 gram.
FIG. 10A and FIG. 10B reveal the phase stability over temperature and phase strength both at 1-40 bar ambient, and dynamic load level of pressure of 13,790 bars (200,007 lbs./inch) according to the same FIG. 5 Example Composition 1 Fe-1.30Cr-0.80Ni-1.30Mo-1.80Mn-0.50Si-0.01V-0.17C-0.19N. At near ambient process, and dynamic load levels of pressure, the ordered phases which strengthens the matrix; HCP_A3#2 (Cr,Fe,Mn,Mo,Ni,Si,V)1 (C,N,VA)0.5 remains largely intact and is maintained with a greater negative value of enthalpy than either the BCC or FCC matrix constituents.
FIG. 11A and FIG. 11B reveal that at 175 bars pressure and for all temperatures both N gas is absent, and that cementite or carbides are absent within a 440° C. to 600° C. temperature range of interest and further into the austenitic region. The results show that alloying by either gas-solid diffusion, or the solid state mechanical method is possible to attain high levels of Mo and enhanced levels of N and Cr over cast material while meeting the material design objectives. FIG. 11A and FIG. 11B further reveal that alloying of powders or thin sheet may proceed such as with the solid state method, and that material may be hot sinter consolidated at relatively low pressures, or may be HIP or hot worked processed to consolidation.
According to an embodiment herein, alloying nitrogen with a solid state metallurgy process approach either by mechanical alloying and gas-solid adsorption, or by surface gas-solid state N diffusion eliminates concerns of deleterious effects of N solubility from solidification phases at high temperature and allows greater levels of N, Cr, and Mo content limited only by the onset of cementite formation. The higher level of N, Cr, and Mo may provide greater strength, toughness, and corrosion resistance. Over the temperature range of up to around 800° C., the solid state manufacture of compositions provided by the embodiments herein allow a maximum of greater than 0.60 wt % N before onset of cementite or N gas. The typical limit of N will be limited by need for a specific level of ductility and toughness. Temperatures around 840° C. to 900° C. with moderate levels of pressure, for example 180 bars or 18 MPa, are required for solid state alloying of N at a rapid rate.
The compositions and processes provided by the embodiments herein have characteristics to optimize beneficial metallic, mechanical, and thermodynamic properties and effects of N in low alloy steel to help to resist fracture and fracture instability, improve plastic flow, strength, ductility, toughness, and resistance to corrosion and stress corrosion cracking (SCC). The specific low alloy high nitrogen steel embodiments cover a range of compositions in classes of high and ultra-high strength steels, and can minimize or eliminate in each class, over a wide and useful range of temperatures and time the deleterious precipitation or partitioning of C and solutes into carbide phases and minimize matrix depletion of solute Cr and Mo. Furthermore, the embodiments can enhance beneficial, more ideally metallic-like, short range ordered, yet a high entropy distribution of alloy solutes and low enthalpy strong N and N—C phases which can be ideally compatible for hardening and strengthening by methods of austenitizing, quenching, hardening, and secondary temper hardening near 460° C. to 600° C.
The embodiments have compositions of both high and enhanced N levels and high and enhanced Cr levels, and high Mo levels for the greatest levels of strength and resistance to corrosion, while meeting design objectives of avoidance of the deleterious carbides M₂₃C₆, Fe₃C cementite, or M₇C₃, or graphite.
Alloys of the embodiments herein can enable characteristics of a quench and temper low alloy martensitic steel strengthened by austenitizing and quench, then low temperature tempering or elevated temperature secondary hardening. The steels of the embodiments can have specific compositions and ratios of alloy elements directed over a broad range of compositions and strength specifically to optimize the characteristics of N, and N with C in steel, and to minimize or exclude deleterious cluster effects of C with solute atoms Cr, Mo, and Mn in steel. The applications of these low alloy steels may be in fields of aerospace, defense, industry, petrochemical, structure shapes, vehicles, machinery, and protection structures.
The characteristics of these embodiments can include a combination of features described herein. Foremost, the embodiments provide high levels of strength following tempering at low and high ranges of temperature. Furthermore, the low alloy steel compositions and processes of a wide range of compositions can minimize or eliminate, over useful and wide ranges of temperatures and time, deleterious precipitation or partitioning of carbide phases and matrix depletion of solute C, Cr, and Mo, whether during manufacture, processing, fabrication, or joining. Also, the embodiments provide quench and tempered martensitic steel with low sensitivity to quench rate from absence of cementite or carbides. The embodiments provide low alloy compositions and processes which can enhance beneficial, more homogeneous and ideally metallic-like distribution of alloy solutes and N and C and their associated phases and which are ideally capable for processing by methods of austenitizing, quenching, hardening, and most specifically, secondary hardening.
Additional characteristics of embodiments include compositions amendable to casting that may attain high levels of N, and variable levels of C, for example, each 0.16 to 0.21 N weight percent, 0.15 to 0.20 C weight percent, with N/C weight ratio of 0.85 to 1.3, for cast alloys a Cr/C ratio of 6.5 to 9.7. The embodiments include Mn as necessary for conversion of FeS to MnS intermetallic, and to enhance the high temperature FCC phase over BCC delta ferrite to minimize outgas from high temperature austenite or any BCC delta ferrite, and to achieve the greatest hardenability through thickness and hardness. The embodiments include sufficient levels, but not excessive, levels of Cr, Mn, and Mo which directly or indirectly assist strengthening and ordering with N and C, to enhance solubility of C and N in solid state solution, and the solubility of N in liquid steel under pressure to levels upward of 0.16-0.21 (wt %) N. The specific levels are as predicted by Thermo-Calc software and Sievert's law with interaction parameters of elements or Cr level equivalents with N in Fe. The compositions of the embodiments herein, following austenitization and quench, may provide a primarily martensitic structure, and not primarily austenite or ferrite from prediction of a Schaeffler diagram through Ni-equivalent contributions of Ni, Mn, Co, N, and C, and the Cr-equivalent parameters of Cr, Mo, and Si.
Characteristics of embodiments achieved by the alloy solid state solution are N solubility levels upwards to 0.60 wt % before onset of cementite or at 1 bar, N gas. The C, N, and substitutional solutes assist strengthening with ordered phases that do not deplete the matrix from formation of complex carbides of Fe, Cr, and Mo. The compositions of the embodiments herein provide primarily all martensitic structure, and not austenite or ferrite from prediction of Thermo-Calc software, following austenitizing and quench. The solid state alloys may have enhanced Cr, Mo, and N and C and lower levels of Si, Ni, and Mn. The lower levels of Mn and Ni, high N, and low to moderate C allow maintaining a high BCC to FCC transition temperature.
The compositions provided by the embodiments herein achieve microstructure and property characteristics, through modeling and a design method of computational materials engineering realized by property diagrams of phase amounts versus temperature, and thermodynamic parameters of enthalpy, entropy, and Gibbs energy per formula unit versus temperature, that are consistently directed to optimize N benefits and achieve characteristics over conventional C-steels, high alloy steels, or the high nitrogen steels (HNS's) that may not be of optimal compositions.
Aspects of the embodiments herein include steel alloys that can eliminate during processing, the detrimental phase transformation or precipitation, or partitioning of carbides to grain boundaries, or upon transformation of constituent phases, the coarsening and excessive segregation of alloy elements to carbide precipitates. This can lead to alloying compositions with greater levels of potentially improved strengthening and corrosion resisting elements; e.g., N, C, Cr, and Mo, a wide range of temper processing, and austenitizing at lower temperatures than may occur with high stability nitride or large and coarse carbides. Strengthening methods of secondary hardening of these alloys yield improved microstructure and properties of strength, ductility, and toughness, and resistance to corrosion and stress corrosion, all without reliance on high levels of relatively expensive nickel and cobalt such as used in aerospace alloys AF1410 and Aermet 100.
Aspects of the embodiments herein also include steel alloys which can be predicted to have under conditions of ballistic impact, for example, 10-50 kbar pressure, improved resistance to thermal softening or microcracking through equilibrium phases with high strength (enthalpy) and thermodynamic stability (Gibbs energy) from the enhanced contributions of enthalpy and entropy over a wide range of temperature; and absence of carbides which may lose strength under pressure and temperature, or which may form voids or initiate and localize deformation or microcracking under pressure or during plastic flow.
The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.

Claims

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

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.