EP0460591A1

EP0460591A1 - High performance high strength low alloy steel

Info

Publication number: EP0460591A1
Application number: EP91109084A
Authority: EP
Inventors: Rao V. Bangaru
Original assignee: Ferrous Wheel Group Inc
Current assignee: Ferrous Wheel Group Inc
Priority date: 1990-06-05
Filing date: 1991-06-04
Publication date: 1991-12-11
Also published as: AU7812991A; BR9102318A; CA2043836A1; MX173753B; AU660928B2; JPH04231437A

Abstract

A high strength, low alloy, low to medium carbon wrought or cast steel is provided of the Fe/Cr/C type, said steel being characterized by the presence of a small but effective amount of each of Cu and Ni sufficient to enhance the mechanical stability of retained austenite formed following quenching of said steel from its austenitizing temperature, and also including small but effective amounts of Al, Ti and Nb sufficient to provide a fine grained microstructure. The steel consists essentially of about 0.05 to 0.5% C, about 0.1 to 0.5% Si, about 0.5 to 2% Mn, about 0.5 to 4% Cr, about 0.1 to 2% Cu, about 0.1 to 3% Ni, about 0.01 to 0.05% Al, up to about 0.02% Ti, about 0.005 to 0.04% Nb and the balance essentially iron together with minor amounts of impurities, such as P, S, and N in amounts not exceeding about 0.02% P, about 0.015% S, and about 150 ppm N. The steel has an average grain size of less than 100 microns.

Description

Background of the Invention

The present invention relates to a class of high performance, high strength, low alloy, low to medium carbon wrought and cast steels. High performance is defined as the ability to provide a superior property mix including high combinations of strength, hardness and toughness, together with environmental resistance, and including production flexibility and durability in field service.

State of the Art

High strength, low alloy, low to medium carbon steels are of particular interest because of their wide variety of uses. These steels are used extensively for components in aircraft landing gears, power transmission gears, chains, links, fasteners, shafting, armor plate, missile and rocket cases, and the like. Where such steels have high hardness accompanied by abrasion resistance, they find particular use in mining operations, such as buckets, milling equipment and other mineral processing operations.
Because of their relative high strength, such steels provide saving in weight for structural components used in bridges, ship building and automobile parts, as well as a wide variety of other uses.
Examples of commercial high strength structural steels include AISI 4340, such steels being referred to by the general classification of AISI 43XX. Other commercial steels also include AISI 4130, or more generally AISI 41XX.
Where these steels combine high hardness with high toughness, they find special application in military (ordnance) applications including armor castings (commander's work station castings, etc.), muzzle brake castings, lightweight retrofit armor and slotted retrofit armor; and also in mining and comminution industries, including track shoes, hoist drums, ball mill feed end heads, boom clevis castings, sprockets, drag chain and ring gears for ball mills. Other uses include automotive trucks, construction machinery, as well as structural applications including, for example, fifth wheels, suspension components, trailer hitches, axle housings, front dipper bucket components and teeth, hitch housing, yokes, etc., for road building equipment, bridge shoes and saddles, pile driving machinery (pile followers), and off-highway truck components (transmission housings, torque tubes). The steels also have use on railroads for lighter draft gear, sideframes, bolsters, motor truck frames, and draft gear for municipal light rail applications.
Where such steel castings combine moderate hardness/strength with excellent weldability, they can be used for nodes for offshore oil/gas drilling/production platforms. Where such castings combine high wear/abrasion resistance with excellent corrosion and impact strength, they can be used for sugar cutting knives and tool and die steel castings. Where such castings combine moderate strength with high corrosion resistance in sour environments, they find special application in sour service including valve and stem and stem castings in oil/gas production and transport.
The purpose of the present invention is to replace the foregoing types of high strength low alloy steels and to provide steels characterized by markedly improved cost/performance parameters compared to the commercial low alloy structural steels mentioned hereinabove, including SAE 8620 or, more generally 86XX.
Many of the commercial low to medium carbon (about 0.1 to 0.5 wt % C) high strength, low alloy structural steels available today with hardness in the range about Rockwell C 20 to about Rockwell C 50 provide good strength or toughness but rarely a combination of good strength and toughness. Furthermore, since most of these steels have been designed by trial and error without a sound scientific and technical basis, the history of development of these steels has been one of compromises -- that is, sacrificing one property to achieve gains in some other property. For example, silicon is present in most steels as a deoxidizer which aids in the steel making process. However, it's detrimental effects on mechanical behavior, especially on ductility and toughness are either ignored or not considered. Similarly, strong carbide formers such as Nb and Ti are added to achieve grain refining to benefit the strength-toughness combination. However, if the addition of these elements is not controlled judiciously, they may lead to precipitation hardening under certain tempering conditions which in turn could lead to a sudden and catastrophic loss of ductility and toughness of the steel. Virtually no commercial steel in existence today has optimized levels of grain refiners such as Nb and Ti with little or no penalty on other properties of the steel.
Structural steels in practice are not only subject to load bearing but also are exposed to various environments, often aggressive, and as such are required to possess good environmental resistance; and good load bearing capacity under the simultaneous action of load and environment for a variety of environmental conditions. Unfortunately, however, even those few structural high strength steels which have been designed based on a sound scientific basis have addressed either the mechanical properties or the environmental properties but were rarely designed to optimize both of these essential parameters for optimum engineering performance. Thus, many of the state-of-the-art steels which exhibit superior combination of strength and toughness are susceptible to stress corrosion cracking and hydrogen induced cracking.
From a practical point of view, structural steels must be designed to confer some flexibility for processing under a variety of steel mill or foundry conditions, for example, the ability to develop the desired microstructure and properties under a variety of rolling mill or foundry shop conditions. Also modern steels should be easily fabricable. For example, the steel should be weldable under a variety of welding conditions and it should have excellent weld heat-affected-zone (HAZ) toughness. These complex and varied requirements for a truly outstanding high strength low alloy steel for the modern world requires an integrated design approach based on a sound scientific and technical basis. The input for such an approach should include as many practical considerations and requirements as possible, such as weld HAZ toughness, tolerance for a wide variety of steel and rolling mill and foundry shop parameters, stress corrosion cracking resistance and resistance to hydrogen induced cracking. Thus, new steel grades are required which are designed to integrate superior mechanical properties with superior performance in other practical aspects as listed hereinabove.
The importance of designing structural steels having high strength and toughness is described in a paper entitled "Structure-Property Relations and the Design of Fe-4Cr-C Base Structural Steels for High Strength and Toughness" by Rao and Thomas which appears in Metallurgical Transactions A, Volume 11A, 1980, pp. 441-457.
In this paper some design guidelines are given for improving strength-toughness combinations in medium carbon structural steels of the Fe/Cr/C type by employing Mn and/or Ni additions. These additions were used to promote improvement in toughness properties due to the formation of retained austenite and due to the fact that the addition of Ni and/or Mn tended to improve the thermal stabilization of austenite.
U.S. Patent Nos. 4,170,497 and 4,170,499 issued to the aforementioned authors describe a structural steel with superior strength-toughness combinations. This steel is based on developing a composite microstructure of dislocated, auto-tempered lath martensite surrounded by films of untransformed austenite, that is, retained austenite. While these patents describe in broad terms the desirable microstructural features from a purely mechanical property point of view, the optimization of these microstructural features for the best combination of mechanical properties is not considered. Most importantly, other practical requirements including the environmental resistance aspects (such as stress corrosion cracking resistance (SCC), hydrogen induced cracking resistance) and processing and fabricability aspects are not considered. These patents describe high strength, tough alloy steels consisting essentially of from about 0.20 to about 0.35 weight % carbon, about 3.0 to 4.5 weight % chromium, and at least 1 weight % of at least one other substitutional alloying element selected from the group consisting of nickel, manganese, molybdenum, cobalt, silicon, aluminum and mixtures thereof, and the remainder iron. These patents also describe complex double heat-treatments to produce grain refining and the desired microstructure within these grains. In U.S. Patent No. 4,671,827 an alternate controlled rolling procedure is described which requires the use of certain rolling mill sophistication to induce fine grain structure.
It would be desirable to provide a low alloy, low to medium carbon steel of the Fe/Cr/C variety containing a novel combination of alloying constituents capable of optimizing the microstructural characteristics of the steel without requiring the use of complex heat treatments or complex working operations.

Objects of the Invention

It is thus an object of the invention to provide a low alloy, low to medium carbon steel composition of the Fe/Cr/C type containing a novel combination of alloying constituents and characterized by optimum combination of mechanical properties.
Another object of the invention is to provide a low alloy, low to medium carbon structural steel of the Fe/Cr/C type containing a novel combination of alloying constituents sufficient to enhance the mechanical stability of retained austenite formed in said steel.
A further object of the invention is to provide as an article of manufacture a heat treated steel component of the Fe/Cr/C type characterized by a hardness of at least about 20 R_c, a fine grained microstructure consisting essentially of lath martensite enveloped by a thin film of retained austenite, said austenite being further characterized by enhanced mechanical stability.
A still further object of the invention is to provide a low alloy, low to medium carbon structural wrought steel composition of the Fe/Cr/C type containing controlled amounts of carbon, nickel, copper, niobium, titanium and aluminum, said steel composition characterized in the heat treated state by optimum hardness, optimum combination of mechanical properties, and thermally stable retained austenite and very fine grain size.
An additional object is to provide a method for producing a wrought low alloy, low to medium carbon structural steel of the Fe/Cr/C type characterized by an improved combination of mechanical properties, optimum hardness, impact toughness and plane strain fracture toughness (K_1c expressed as KSi-in½).
A still further object of the invention is to provide a low alloy, low to medium carbon cast steel composition of the Fe/Cr/C type containing about 0.1 to 0.5% Si, preferably, about 0.2 to 0.4% Si together with controlled amounts of carbon, nickel, copper, niobium, titanium and aluminum, said cast steel composition characterized in the heat treated state by optimum hardness, optimum combination of mechanical properties, and thermally stable retained austenite and fine grain size.
These and other objects, features and advantages will become more apparent when considered in conjunction with the accompanying disclosure, claims, and appended drawings.

The Drawings

FIGURE 1A is a reproduction of a photomicrograph taken at 200 times magnification of the steel of the invention showing a fine as - forged grain size of substantial uniformity;
FIGURE 1B is similar to Fig. 1A but differs in that it shows the steel at 200 times magnification in the as quenched condition following heating of the steel at the austenitizing temperature of 900°C for 45 minutes;
FIGURE 2A is a reproduction of a photomicrograph of the wrought steel of the invention taken by transmission electron microscope at 31,500 times magnification under bright-field conditions showing lath martensite obtained by heating the steel at 1000°C for 45 minutes, rapidly quenched in water, and tempered at 200°C for 60 minutes;
FIGURE 2B is the same as Fig. 2A, except that the photomicrograph was obtained under dark-field conditions to show retained austenite disposed between lath martensite;
FIGURE 3 is a reproduction of a photomicrograph of the steel of the invention taken by transmission electron microscope at 60,000 times magnification, the wrought steel having been heat treated by austenitization at a temperature of 1000°C for 45 minutes, rapidly quenched in water, and thereafter tempered at 200°C for 60 minutes to show intralath fine carbide particles;
FIGURE 4 is a graph showing the effect of the austenitizing temperature on the mechanical properties of the wrought steel of the invention; and
FIGURE 5 is a graph showing the effect of tempering temperature on the mechanical properties of the wrought steel of the invention.
Figure 6 is a plot illustrating critical flaw size versus service stress for various fracture toughness levels indicated on each curve based on simple linear elastic fracture mechanics; and
Figure 7 is a schematic of an idealized microstructure of the steel showing equiaxed grains at about 200 times magnification and the microcomposite microstructure within an equiaxed grain showing layers of rows of retained austenite disposed between lath martensite as viewed with an electron microscope at about 60,000 time magnification.

Generally speaking, commercial high/ultra-high strength steels offer good hardness/strength or toughness but rarely a combination of both. This is especially true for steels in the hardness range HRc 42-50 (BHN 400-500) wherein the toughness properties, both the sharp notch plane strain fracture toughness, KIc and the blunt notch Charpy impact toughness, are generally very poor. This is an especially serious limitation for cast grades. For example, the widely used ultra-high strength wrought steel, AISI/SAE 4340 is characterized at ambient by only 10 to 15 ft-lbs Charpy-V-Notch (CVN) impact toughness and only about 40 to 50 ksi-in½ plane strain fracture toughness at ambient when the steel is heat-treated in the range 440-500 BHN. For cast grade, these properties are even lower and the wrought properties can be considered as upper limit for such castings. These toughness properties are much below those needed to fully exploit the steel's available strength for many structural applications wherein fracture mechanics based design is used.
This is particularly illustrated in Figure 6 which shows a plot of critical flaw size versus service stress for various fracture toughness levels as indicated on each curve based on simple linear elastic fracture mechanics. Experimental limitations make the detection of tiny cracks extremely difficult and about 0.1 inch is generally considered to be the limit for detection by conventional methods. Ultra-high strength steels, due to their low toughness to strength ratio, have extremely small critical flaw sizes for catastrophic failure. Quite simply, Figure 6 defines the acceptable service stresses for assumed flaw size detectability limits.
For example, for a flaw size detectability limit of 0.1" and a safety factor of one, to prevent catastrophic fracture in service for AISI 4340 steel, service stress has to be limited to less than about 50 ksi which is less than about 25% of the yield strength of the steel. The allowable service stress quickly increases to about 130 ksi with doubling of fractures toughness to 80 ksi-in½. Even at this level, the full strength of 4340 (yield strength in the range 200-230 ksi) will not be utilized. In order to fully utilize the available strength for design, steels in the strength range under discussion should have fracture toughnesses in excess of 100 ksi-in½. The present wrought and cast steels have been developed to fulfill this need.
There are important differences between wrought and cast grades of steels and the alloy design to produce the desired superior properties and these differences have to be addressed. First, articles of use are formed or fabricated from the wrought grades. Thus, the wrought steel is mechanically worked in this process starting from an initially large ingot. However, in the case of castings, the object is cast near net shape and is not subject to further mechanical work as a means to shape the article, except for some minor machining. Thus, the desired microstructure has to be established in a casting without depending on the mechanical part of the thermomechanical processing, in other words, the desired microstructure in a casting can only be established through thermal (heat-treatment) processing once the chemistry is chosen. Furthermore, since castings are not subject to further mechanical working as is the case with wrought products, it is important that the castings have good quality (soundness) and free from defects and shrinkage, both macro and micro, in order to ensure that the casting soundness will not be limiting to its performance. For these reasons, modifications to the chemistry and heat-processing have been implemented as compared to the wrought counterpart to obtain superior casting steels in the present invention.

Statement of the Invention

One embodiment of the invention resides in a method for enhancing the mechanical stability of retained austenite of high strength, low alloy, low to medium carbon steel of the Fe/Cr/C type, including steels containing about 0.1 to 0.5% Si, said method comprising adding a small but effective amount of both copper and nickel to said steel composition, and when Si is present the amount of nickel being at least sufficient to counteract the destabilizing effect of Si on austenite.
Another embodiment of the invention is directed to a high strength, low alloy, low to medium carbon steel of the aforementioned Fe/Cr/C low alloy steel containing said copper and nickel.
Such wrought steels include the composition by weight of about 0.5 to 4% Cr, about 0.05-0.5% C, said steel also containing small but effective amounts of about 0.1 to 2% Cu and of about 0.1 to 3% Ni at least sufficient to enhance the mechanical stability of retained austenite.
Such cast steels include the composition by weight of about 0.5 to 4% Cr, about 0.05-0.5% C, and about 0.1 to 0.5% Si, said steel also containing small but effective amounts of about 0.1 to 2% Cu and of about 0.1 to 3% Ni at least sufficient to enhance the mechanical stability of retained austenite.
A further embodiment of the invention resides in a method of producing fine grained low alloy, low to medium carbon steel consisting essentially of an Fe/Cr/C/Cu/Ni steel to which small but effective amounts of Al, Ti and Nb are added sufficient to provide fine grained steel following rapid cooling from the austenitizing temperature.
A still further embodiment of the invention is directed to a fine grained high strength low alloy, low to medium carbon steel consisting essentially of Fe/Cr/C/Mn/Cu/Ni/Al/Ti/Hb and containing about 0.1 to 0.5% Si for cast steels.

Detailed Description of the Invention

In accordance with a preferred embodiment of the invention, a class of high strength, high toughness low alloy steels of specified composition, cleanliness and microstructure are produced to integrate their mechanical property superiority with processing and fabrication advantages, the steels being characterized, in addition, with a set of unique engineering property and practical performance advantages. The preferred compositions of the steels consist of principal alloying elements, microalloying, grain refining/weld HAZ toughness improvement additives and are fabricated to certain cleanliness standards by controlling the amount of residuals. The principal alloying elements include about 0.05 to 0.5 weight % carbon, about 0.5 to 4 weight percent chromium, and about 0.5 to 2 weight % manganese and for cast steels about 0.1 to 0.5% silicon. The preferred microalloying ingredients include copper and, more preferably, combined additions of copper and nickel for enhancing the stability of retained austenite. The preferred ranges for copper and nickel are about 0.1 to 2.0 weight % and about 0.1 to 3.0 weight %, respectively. For cast steels having 0.1 to 0.5% Si, the amount of nickel being also at least sufficient to counteract the destabilizing effect of Si on austenite. The grain refining/weld HAZ toughness improvement additions include at least two and preferably all three combined additions of the following elements: niobium, titanium and aluminum.
The preferred ranges for these elements are as follows: niobium, about 0.005 to 0.04 weight %; titanium, up to about 0.02 weight % and aluminum, about 0.01 to 0.05 weight %. In addition to these preferred ranges, the steels of the present invention require strict control as to cleanliness, level of residuals, and other undesirable alloying additions that are common in steel melting practice. For example, the steels of the present invention require that maximum limits be placed on the following more common residual elements in order that these steels develop the desirable microstructure and properties: sulfur levels not to exceed about 0.015 weight %, phosphorus levels not to exceed about 0.02 weight %, soluble nitrogen not exceeding 150 weight parts per million (ppm), but more preferably not exceeding 75 weight ppm, and for wrought steels silicon levels be maintained as low as possible but not to exceed about 0.1 weight %.
Examples of preferred ranges of composition for the steels of the present invention are tabulated in weight % in Tables I and III. Within these ranges, specific steels can be designed to obtain certain combination of mechanical properties or other engineering and technological properties. Specifically, for a steel designed to have superior strength and toughness combinations compared to those of AISI 4340 and 4130, the preferred embodiments of steel chemistry are tabulated in weight % in Tables II and IV.
Another preferred wrought steel composition is one that ranges from about 0.2 to 0.25% C, about 1.4 to 1.6% Mn, about 1.8 to 2.4% Cr, about 0.25 to 0.5% Cu, about 0.2 to 0.5% Ni, about 0.01 to 0. 05% Al, about 0.005 to 0.02% Ti, about 0.01 to 0.03% Nb, less than about 0.005 to 0.015% P, less than about 0.001 to 0.01% S, less than about 0.1% Si, less than about 150 ppm N and the balance essentially iron.
In achieving the desired hardness and toughness in the wrought steel of the invention, the steel is quenched from an austenitizing temperature ranging from about 870°C to 1150°C, preferably about 900°C to 1100°C. Following the quench, the steel may be tempered at a temperature ranging from about 170°C to 250°C, preferably from about 190°C to 230°C in accordance with known procedure.
In achieving the desired hardness and toughness in the cast steel of the invention, the steel is first homogenized and thereafter quenched from an austenitizing temperature ranging from about 870°C to 1150°C, preferably about 900°C to 1100°C. Following the quench, the steel may be tempered at a temperature ranging from about 170°C to 250°C, preferably from about 190°C to 230°C in accordance with known procedure.
In carrying out the heat treatment, the cast steel is first homogenized by heating it to a temperature of about 870°C to 1150°C for a time sufficient to substantially relieve the steel of segregation formed during the solidification of the steel. This is followed by cooling to room temperature, and the homogenized steel thereafter subjected to an austenitizing treatment by heating to a temperature of about 870°C to 1100°C and rapidly quenched to room temperature.
Another method is to homogenize the cast steel at a temperature of about 900°C to 1150°C for a time sufficient to substantially relieve the steel of segregation formed during solidification followed by furnace cooling to the lower temperature range of 870°C to 1100°C to austenitize the steel and then followed by rapid quenching.
Preferably, the homogenizing temperature may range from about 950°C to 1100°C and the austenitizing temperature range from about 870°C to 1000°C.
A more preferred treatment is to homogenize the cast steel at approximately 1065°C, followed by austenitizing at approximately 950°C.
A major feature of the invention is the use of a four pronged approach to impart unique microstructure and cleanliness to the steel: first establish a framework of fine prior austenite grain structure, with average grain diameter below about 20 microns for cast steels and below about 50 microns for structural steels, preferably below about 50 microns and 25 microns, respectively, or ASTM grain size number in the range 8 to 11. Having achieved the fine grain size, the second part of the four pronged approach is to install a microcomposite microstructure within these grains consisting of the major phase comprising dislocated lath martensite enveloped by a minor phase of retained austenite of optimized mechanical stability. The third part is concerned with the judicious control of unwanted tramp elements in the steel and the overall cleanliness of the steel in terms of the inclusion control. A fourth distinguishing feature of the current invention is the minor alloying additions to impart some special processing and engineering properties to the steel while not adversely affecting the other three aspects discussed above. The four aspects mentioned above are dramatic and significant and provide a total integrated concept which results in a unique class of high strength and tough steels.
Before discussing the details of the four aspects, it would be helpful at this point to explain certain terms applicable to austenite and its transformation characteristics. Those familiar with the field have used a variety of technical terms to describe the transformation characteristics of austenite. Insofar as the present invention is concerned, the following technical terms will be used.
Stabilization of austenite refers to the processes and mechanisms responsible for retaining the high temperature austenite phase in the metastable condition at ambient. Stability of austenite is that property of retained austenite to transform when subjected to thermal ageing and/or mechanical deformation.
In the context of the above terminology, thermal stabilization refers to thermal processes, seen as carbon and nitrogen diffusion and precipitation effects, which lead to the retention of austenite when quenched from a high temperature.
Mechanical Stabilization refers to the retention of austenite during quenching from a high temperature to accommodate volume expansion which occurs when a major portion of austenite transforms to martensite.
Thermal Stability refers to the stability of retained austenite to transformation when subjected to thermal ageing. Mechanical Stability refers to the stability of retained austenite to transformation when subjected to mechanical deformation.
The four aspects of the present invention will now be discussed.

(I) Grain Size Control

A key aspect of the steels of the current invention is the ability of the steel to develop and maintain fine and ultrafine austenite grain sizes for a variety of rolling mill and forging mill on-line and off-line processing conditions. A major feature of the invention is the realization that a well controlled addition of mixtures of niobium and titanium microalloyed together with control of aluminum and nitrogen enable the accomplishment of this goal. The steels of the present invention are designed to fully exploit the benefits of minor co-additions of niobium and titanium to make the steels much more forgiving for process variability than comparable steels in the same class, including the single niobium added steels, and yet have the ability to develop ultrafine grain sizes. A key finding of the invention is that the amount of niobium-titanium co-additions is controlled, so that the total addition preferably does not exceed 0.05 weight %. More importantly, the individual amounts of these elements should also be controlled; that is, titanium addition should not exceed about 0.02 weight % and the niobium addition should not exceed about 0.04 weight %.
It has been found that with these upper limits on niobium/titanium, their primary purpose of achieving the grain refining and other processing flexibility as described below is attained while avoiding the drawbacks of such additions on precipitation hardening and the consequent severe loss of matrix toughness. Moreover, at these levels, it has been discovered that the grain coarsening in the weld heat-affected-zone (HAZ) is substantially reduced which has a very positive effect on weld HAZ toughness, especially for high heat input welding. At the same time, the amount of detrimental formation of the high carbon (greater than about 0.6% by weight) martensite-austenite constituent, which severely degrades the toughness of local regions of the weld HAZ, is avoided in the intercritical/subcritical and intercritically reheated regions of the weld HAZ; these being the two most sensitive regions for the formation of this constituent.
This is but one example of the unique approach used in the alloy design of the current class of structural steels, that is, taking advantage of the beneficial attributes of the addition of niobium and titanium, while at the same time, minimizing or eliminating the detrimental side effects of these additions by employing an integrated design philosophy which is dependent upon judiciously controlling these additions using a firm scientific and technical approach. The co-additions of niobium and titanium in the specified amounts also lead to substantial processing flexibility for the following reasons:
The niobium-titanium treated steels have minimal tendency for grain coarsening even at very high temperatures, as high as 1250 degrees Celsius. This allows flexibility to use relatively high ingot and billet soaking and rolling/forging temperatures which in turn allow for the use of large rolling/forging pass reductions per pass even with old rolling/forging mill facilities having limited rolling force and torque capabilities. The use of large rolling/forging pass reduction is extremely desirable for two main reasons: (1) it results in markedly enhanced nucleation rate for recrystallized austenite grains which leads to much finer and more uniform austenite grain structure in the as rolled/forged products; and (2) the use of large per pass reductions is also beneficial in promoting uniform recrystallized grain structure for thick sections. Moreover, niobium-titanium additions help to stabilize the recrystallized grain structure during the hold period, such that after the finish rolling/forging operation, significant grain coarsening is avoided, thereby adding to processing flexibility.
One of the key advantages of the current invention is that inducement of fine grain size does not require the use of sophisticated controlled rolling/forging practices. In fact, austenite grain sizes in the range 10 to 15 micrometers diameter (ASTM grain size numbers in the range 8 to 11) may easily be obtained by a simple austenitizing treatment in the temperature range of about 870 to 1000°C. This simple treatment can be performed off-line thereby providing significant processing flexibility to induce fine grain structure in situations where a particular rolling/forging mill does not have facilities to do on-line heat-treatment (quenching). Furthermore, in many instances, it may be desirable to provide steel in a softened as-rolled/forged condition which would require that the steel not be quenched on-line. This is because in many applications, the steel is worked in its softer condition to fabricate different engineering shapes or components. Thereafter, the steel's hardness and microstructure can be restored by a final off-line heat-treatment.
The steels of present invention are capable of developing fine austenite grain structures for the foregoing specific situation. Those steels that depend on sophisticated on-line controlled rolling/forging practices are not able to provide fine grain structures during an off-line final heat-treatment after fabrication into shapes. Examples of fine grain structures in two conditions, are shown, for example, in the forged condition (Fig. 1A) and in the off-line heat-treated condition illustrated in Fig. 1B. The steel contains by weight 0.21 % C, 1.55 % Mn, 1.93 % Cr, 0.26 % Ni, 0.32 % Cu, 0.027 % Al, 0.012 % Ti, 0.02 % Nb, 0.012% P, 0.004 % S, 0.10 % Si, about 125 ppm N and the balance essentially iron and has a grain size in the range ASTM 9 to 11.
Grain refining in the presence of combined titanium-niobium additions is primarily achieved by the precipitation of grain pinning, thermally highly stable titanium, niobium carbo-nitride Ti, Nb (C,N) particles. These particles are most effective if they are uniformly precipitated in sizes preferably less than a micrometer or micron in their longest dimension. This takes place during solidification of the ingot from the melt and\or the subsequent cooling. The secondary grain refining particles in the steels of the present invention are aluminum nitrides and oxides.

(II) Microcomposite Microstructure

The steels of the present invention are designed to develop, upon quenching (or fast cooling) from a suitably high austenitizing temperature, a microcomposite microstructure consisting of soft and tough retained austenite films surrounding strong lath martensite. U.S. Patent No. 4,170,497 and No. 4,170,499 disclose such structures to be very desirable to provide high strength and toughness. U.S. Patent No. 4,170,499 discloses a method of making a high strength, tough alloy steel having a fine grained structure with the duplex microstructure as described in U.S. Patent No. 4,170,497. The method of making high strength, tough alloy steel as disclosed in these patents requires the use of complex thermal treatments which are inefficient and costly.
U.S. Patent No. 4,671,827 discloses a process of controlled rolling and controlled cooling to produce fine grain structure during on-line processing. However, most existing mills are not capable of implementing the stringent rolling pass reductions and cooling disclosed in this patent. Furthermore, as already mentioned, it is desirable in many instances not to produce the desired grain structure-microstructure during on-line processing but rather during a subsequent heat-treatment after fabrication of the intermediate and final engineering components. Thus, the methods of prior art have serious limitations in their practical applicability. What is truly required is a steel that is forgiving and has the flexibility to respond to a variety of on-line and off-line heat-treatment conditions to give the desired grain structure-microstructure combinations. The steels of the present invention provide this flexibility.
As previously mentioned, the steels of the present invention are capable of producing a fine austenite grain structure with the desired microstructure with a simple heat-treatment in the 870 to 1150°C range for about 45 minutes to 60 minutes followed by rapid quenching to ambient, for example, in water. This heat-treatment can be performed at the final stage after fabrication of an engineering component. Thus, the raw material steel can be supplied in a softer, hot forged or as rolled condition as it comes off the rolling/forging line for ease of workability and fabrication into complex engineering shapes.
Alternately, the steels of the present invention are particularly capable of developing extremely fine grain structures during controlled rolling/forging operations. What is remarkable about the steels of the present invention is the ability of the steel to develop fine grain structure even with sophisticated controlled rolling/forging or semi-sophisticated controlled rolling/forging, and including a variety of finish rolling/forging temperatures with finishing temperatures as high as 1100°C. Furthermore, the steels of the present invention can be quenched on-line to induce the formation of the microcomposite microstructure within the grains. Alternatively, the steels can be reheated on a separate line in the high temperature austenite region in the temperature range of about 870 to 1150°C and then quenched rapidly to ambient to produce the desired grain structure-microstructure combinations. This processing flexibility is quite important as many rolling/forging mills across the world do not have sophisticated rolling/forging lines or facilities for on-line quenching. The installation of these facilities involves significant capital outlays and many steel mills are reluctant to install such facilities. The results achieved with the present invention show that the control of the individual and combined amounts of titanium, niobium and aluminum together with the other alloying elements set forth in Tables I and II is necessary to provide the processing flexibility discussed hereinabove.
In the course of the present discovery, it has been found that the properties of the microcomposite microstructure comprising the retained austenite films at the dislocated lath martensite boundaries are particularly dependent on and are optimized by suitable control of the amount and stability of the retained austenite and the amount of transformation thereof by mechanical stress or strain. Austenite in these steels is a metastable phase and is retained at ambient conditions due to the interplay between complex stabilization mechanisms, including chemical, thermal and mechanical stabilization factors. The amount of austenite retained during on-line or off-line heat-treatments is very sensitive to the chemistry of steel and the heat-treatment itself. Once retained at ambient, the austenite is metastable and can decompose when exposed to mechanical stress/strain or when subject to thermal exposure of temperatures above about 250°C. The nature of the decomposition of this retained austenite profoundly affects the mechanical and environmental properties of the steels.
It has been found that excessive amounts of retained austenite are detrimental to yield strength of the steel and may lower the overall strength-toughness combinations. Thus, about 1 to 10 volume percent and, more preferably, between about 2 to 4 volume percent of retained austenite, is ideal for excellent combinations of strength and toughness. Chromium in the preferred range of about 1.5 to 2.5 weight % and manganese in the range of about 0.8 to 1.8 weight % are shown to establish the desired volume fractions of retained austenite without affecting the desirable dislocated lath martensitic structure which forms the major phase of the microstructure and acts as the load bearing constituent and provides excellent strength. However, the austenite so generated in the steel with controlled chromium and manganese has been found to have inadequate stability when exposed to mechanical stress/strain leading to premature decomposition in the stress/strain field, for example, in the plastic zone ahead of a running crack. When the austenite transforms prematurely, it's crack blunting ability is lost and the toughness benefit due to the existence of this phase in the microstructure is not optimized. However, if the austenite is overly stable, then again the benefits associated with Transformation Induced Plasticity (TRIP) are not achieved resulting in an overall reduction in the achievable toughness of the steel. These aspects are important not only for the case of straight mechanical stress/strain but also for the combined action of mechanical stress/strain and environment. In the prior art, the full implications of an unoptimized austenite phase in the microstructure on the mechanical properties of the steel in both the presence and absence of an environment are not recognized and addressed in low alloy steels (especially in steels where the total weight percent of alloying elements does not exceed about 6% and preferably not exceed about 5 weight %). Attempts have been made to modify the stability of the austenite phase by excessive additions of expensive nickel in amounts greater than about 1 weight % and in a preferred embodiment about 3 to 5 weight %. While this amount of nickel is conducive to the retention of austenite, it also tends to overstabilize the austenite to mechanical stress/strain, thereby leading to less than optimum combinations of strength and toughness.
Thus, in carrying out the invention, it has been found that controlled additions in the range of about 0.1 to 2.0 weight % and more preferably in the range of about 0.25 to 0.6 weight % copper produce the desired optimum stability of retained austenite during mechanical stressing/straining both in the presence of as well as in the absence of the environment. This small but essential addition of copper to steel has been found to postpone the stress/strain induced transformation of retained austenite to greater strains/stresses than is the case in steels without essential amounts of copper. For example, in an uniaxial tensile test, the onset of transformation of the austenite is observed at or beyond the start of plastic instability (necking) in copper bearing steels whereas in steels without copper, such transformation takes place prematurely at the onset of yielding. This effect in turn leads to maximization of the beneficial effect of retained austenite on the toughness properties of the steel.
What is also important is the observation that judicious copper additions are found to accomplish these desired effects on the minor phase, vis-a-vis, retained austenite, without any adverse effects on the major phase of the microstructure, vis-a-vis, dislocated lath martensite. Copper additions at the specified limits have been found to maintain the dislocated substructure of the base martensite without any indication of the presence of detrimental substructural twinning. Figs. 2A and 2B illustrate the ideal microstructure obtained in the steels of the present invention containing copper.
It is well known by those skilled in the art that copper, even in small amounts, leads to "hot shortness", a phenomenon associated with formation of low melting phases in copper bearing steels. For this reason, a small amount of nickel is added with copper to overcome this adverse phenomenon. Thus, the steels of the present invention are modified with judicious amounts of nickel. The amount of nickel is controlled and optimized so that the steels do not suffer the overstabilization of austenite as has been the case with some of the steels of prior art. The amount of nickel for one of the preferred embodiments is shown in Table II. At these low levels of nickel, it has been found that the deleterious effect of copper on hot shortness of steel is obviated, and also nickel contributes to the low temperature toughness of the steel, that is, particularly by lowering the ductile to brittle transition temperature, and that nickel counteracts the destabilizing effect of Si on austenite.

(III) Impurity and Inclusion Control

While the mechanical and environmental properties of steel are dependent on its grain size and microstructure, impurities and inclusions introduced during the steel making process also can degrade its properties. With this in mind, steels of the present invention are designed with acceptable upper limits of tramp impurities which may dissolve in the steel or may precipitate in the form of damaging inclusions. Silicon, although a common alloying element in most commercial steels, is an unwanted impurity for the structural steels of the present invention. Silicon is present in most commercial steels as a deoxidizer and an economical strengthener of steel. However, silicon can produce adverse effects from the point of view of the desired microcomposite microstructure of the present steels. In particular, it has been found that small amounts of silicon adversely affect and decrease the stability of retained austenite to mechanical stress/strain. This leads to premature decomposition of the austenite and consequently the degradation of the achievable property limits in the steel. Furthermore, silicon while strengthening the major martensite phase actually results in rather substantial decrease in the flow, ductility and toughness of this phase with the net result that the increased increment in strength is obtained at a rather steep price in toughness. An interesting aspect of the present invention is that silicon is not only harmful to the toughness of the base steel but also is even more detrimental to the toughness of the weld heat-affected-zone (HAZ) where it promotes the formation of embrittling microstructures resulting from the complex thermal cycles of the multipass welding. This subject is discussed in more detail later. Due to the above observations, the structural steels of the present invention are fabricated with as little silicon as practicable but preferably with an upper limit of 0.1 weight %. This requirement is particularly applicable to wrought products in which silicon is not necessary.
However, since Si is known to enhance the fluidity of the steels, a prime requirement to the casting soundness, for the cast grade, this restriction is relaxed to an upper limit of about 0.5 weight %. In the present cast steels it has been found that high Si can also have adverse HAZ toughness. Thus, it is preferable that even for cast grade that Si be as low as possible, preferably ≦0.3 weight %.
The other important tramp elements in steel are sulfur and phosphorus. Generally, these elements are precipitated out during steel solidification and ingot casting in the form of inclusions. These inclusions can lower the impact upper-shelf energy and increase the ductile-to-brittle transition temperature. Both of these effects are detrimental to steel toughness. Moreover, sulfide inclusions can lead to lamellar tearing during welding and hence can create some serious practical problems. Also, the formation of sulfide inclusions in these steels ties up valuable manganese therewithin which could have otherwise been available for the development of the desired microcomposite microstructure. Phosphorus that is not precipitated out and present in the dissolved state in steel has the same detrimental effects as described above with silicon. Because of the negative effects of these elements, they should be maintained as low as possible in the steel and be restricted to a maximum of 0.015 weight % for sulfur and 0.02 weight % for phosphorus. All the other residual tramp elements including antimony, arsenic, lead, etc. should be as low as is practically feasible.
Gases such as nitrogen, oxygen and hydrogen either dissolved or precipitated in the steel, tend to degrade the steel's mechanical properties. In this regard, some nitrogen can actually be desirable if precipitated out in the form of stable carbonitrides for grain refining as stated previously. However, unstabilized or free nitrogen dissolved in steel has been found to be detrimental to the toughness both in the base steel as well as in the weld HAZ. For this reason, an upper limit of about 150 weight parts per million (wppm) is specified for soluble nitrogen for the steels of the present invention. The oxygen and hydrogen levels should also be as low as is practical.

(IV) Control of Minor Alloying for Processing and Engineering Property Advantage

As already indicated, the steels of the present invention are based on an integrated design approach to optimize the base mechanical and environmental properties while at the same time controlling the minor alloying ingredients so as to impart some unique processing and performance advantages compared to the state-of-the-art low alloy steels so that they exceed the competition and set new standards for performance in several practical requirements. These improvements are achieved without compromising the base or core properties. Some of these unique aspects of the steels of the present invention are described below.
The titanium-niobium co-additions, besides inducing the formation of a desirable fine grain structure in the steel, are also advantageous in restraining grain coarsening during the intense heat that prevails during the welding thermal cycle in the HAZ. However, if the total amount of these additions are not judiciously controlled, they can actually degrade the HAZ toughness in two important aspects: (1) formation of dislocation nucleated needle type precipitates, and (2) promotion of high carbon twinned martensite or martensite-austenite constituent in certain regions of HAZ. It has been found that the total addition of these elements should be restricted to less than about 0.05 weight % to minimize their deleterious effects on HAZ toughness. At these restricted amounts, the steels derive their beneficial effects on grain refining and stabilization of free nitrogen without the detrimental side effects on HAZ toughness.
The copper-nickel microalloying is found to be by far the most desirable alloying from the point of view of minimizing the adverse effects of any alloying in lowering the HAZ toughness brought about by its effect on increasing the hardenability of the steel.
The required copper additions of the steels of the present invention have been found to have some very interesting and desirable environmental resistance and practical fabrication benefits. The copper in steels of the present invention is found to increase the resistance of these steels to atmospheric corrosion and corrosion in sour environments, a property that is not readily available in the steels of the prior art. The increased corrosion resistance decreases the generation and absorption in the steel of atomic hydrogen, an extremely damaging by-product of the corrosion process. The decreased hydrogen is hypothesized to be responsible for the steels resistance to hydrogen-induced-cracking (HIC) in sour service. This special property is also desirable from the point of view of reducing the cold cracking susceptibility during welding. The beneficial side effect of copper on corrosion, together with its primary beneficial effect on the optimized mechanical stability of retained austenite in the microstructure, are hypothesized to confer on these steels some unique stress corrosion cracking resistance (SCC). In SCC, both the generation and transport of hydrogen are important as most mechanisms of SCC are based on hydrogen assisted cracking (HAC). Copper, by reducing the generation of hydrogen as the by-product of corrosion and by stabilizing the retained austenite against premature transformation, provides an ideal microstructure to resist hydrogen transport to regions where it can damage the material.
The restrictions on the silicon level in the steel are conducive to avoiding the formation of brittle, high carbon twinned martensite and/or martensite-austenite constituents in certain regions of HAZ. This allows the use of high welding heat inputs for welding, a production and fabrication flexibility, without undue penalty on HAZ toughness.
Several tests have been conducted which demonstrate the markedly improved combination of properties obtainable using the novel inventive concepts set forth hereinbefore.

EXAMPLE 1

A 25 kg melt was prepared for wrought product by vacuum melting having the following composition:
Ingots were cast, homogenized in the conventional manner followed by forging into slabs. Test samples were cut from the forged slabs.
One group of test samples was subjected to Heat Treatment A which comprised austenitizing the steel at 900°C for 45 minutes and rapidly quenched in water and the following mechanical properties obtained at room temperature.

Heat Treatment A

Specimens were also subjected to Heat Treatment B. The specimens were austenitized at 900°C for 45 minutes, rapidly quenched in water and then tempered for one hour at 225°C and the following mechanical properties obtained at room temperature.

Heat Treatment B

Mechanical Properties (at -40°C)

Heat Treatment E

Austenitize at 900°C for 45 minutes and rapidly quenched in water
Temper for one hour at 200°C and quenched in water

Mechanical Properties at Room Temperature

Mechanical Properties (at -40°C)

As illustrative of the improved results obtained with production sized heats, the following example is given.

EXAMPLE 2

The production sized heat for wrought grade had the following composition:

Heat Treatment A

Mechanical Properties at Room Temperature

Mechanical Properties (at -40°C)

Heat Treatment B

Mechanical Properties at Room Temperature

Heat Treatment F

The specimen was austenitized at 1000°C (45 minutes) and rapidly quenched in water.

Heat Treatment G

This specimen was austenitized at 1000°C (45 minutes) and rapidly quenched in water and tempered for one hour at 225°C.

Mechanical Properties at Room Temperature

Heat Treatment H

The specimen was austenitized at 1000°C for 45 minutes and rapidly quenched in water and tempered for one hour at 200°C.

Mechanical Properties at Room Temperature

As will be noted, the lower tempering temperature of 200°C improves the toughness of the steel with a slight rise in hardness.

Heat Treatment I

The specimen was austenitized at 1000°C for 45 minutes and rapidly quenched in water and tempered for one hour at 190°C.

Mechanical Properties at Room Temperature

Lowering the tempering temperature to 190°C further improved the toughness.

EXAMPLE 3

Another laboratory heat was produced for wrought product by vacuum melting having the following composition:

Heat Treatment A

Mechanical Properties at Room Temperature

Heat Treatment B

Mechanical Properties at Room Temperature

EXAMPLE 4

Casting test blocks were produced by air induction melting followed by refining in an Argon-Oxygen-Decarburizer. The cast test blocks were subjected to two types of heat-treatment: the standard heat-treatment consisted of 1950°F (1065°C) homogenization for 2 hours followed by fan cooling to ambient and austenitizing at 1650°F (900°C) for 1 hour followed immediately by a water quench. The modified heat-treatment involved 1950°F (1065°C) homogenization as above but followed by furnace cooling to 1750°F (955°C) and water quenching. For the standard and modified heat-treatments both as quenched and quenched and tempered conditions were characterized. The tempering treatment consisted of 400°F (205°C) holding for 2 hours followed by cooling to ambient. Other types of initial homogenization treatments including 1750°F (955°C) have been studied and can be used depending on the limitations of particular foundry to heat-treat at a higher temperature. Similarly tempering temperatures of up to about 482°F (250°C) can be used depending on the strength-toughness requirements of a particular casting.
The types of physical properties obtained with cast steels of the invention is given in Table III as follows:
The results obtained on Heats 1 and 2 with respect to Charpy-V-Notch impact toughness is given in Table VII below:

Individual Charpy Values in ft-lbs in brackets, average value on first row in ft.lbs/joules

HEAT-TREATMENT KEY

955° Hom. Std.: 955°C/2 hrs.; Fan Air Cool, 900°C/2 hrs.
Water Quench; 205°C/2 hrs. temper
955°C As-Quenched: Same as above but no temper
1065°C/Std. Hom.: 1065°C/2 hrs.; Fan Air Cool, 900°C/1 hr.
Water Quench; 205°C/2 hrs. temper
1065°C As-Quenched: Same as above but no temper
1065°C Interr. Q&T: 1065°C/2 hrs. furnace cool to 955°C,
Water Quench, 205°C/2 hrs. temper
In summary, the invention provides a high strength, low alloy, low to medium carbon cast steel consisting essentially of about 0.5 to 4% Cr, about 0.05 to 0.5% C, about 0.5 to 2% Mn, about 0.1 to 2% Cu, 0.1 to 3% Ni, about 0.01 to 0.05% Al, up to 0.02% Ti, about 0.005 to 0.04% Nb, about 0.1 to 0.5% Si and the balance essentially iron, said steel in the hardened condition being characterized by an amount of retained austenite ranging from about 1 to 10 volume % and generally from about 1 to 5 volume %.
The invention also provides a high strength, low alloy, low to medium carbon wrought steel consisting essentially of about 0.5 to 4% Cr, about 0.05 to 0.5% C, about 0.5 to 2% Mn, about 0.1 to 2% Cu, 0.1 to 3% Ni, about 0.01 to 0.05% Al, about 0.005 to 0.02% Ti, about 0.005 to 0.04% Nb and the balance essentially iron, said steel in the hardened condition being characterized by an amount of retained austenite ranging from about 1 to 10 volume % and generally from about 1 to 5 volume %.
The steel in the hardened state is characterized by a microstructure of lath martensite surrounded by a layer of retained austenite.
By controlling the composition of the steel within the range stated hereinabove, the austenite is characterized by mechanical stability when subjected to deformation working.
One of the advantages of the present invention is the production of a microcomposite microstructure consisting of soft and tough retained austenite (minor phase) surrounding strong dislocated lath martensite (major phase). This is shown in the schematic of Fig. 7 which depicts a cross section of a steel bar/casting 10 from which a sample 10A is removed and examined metallographically at about 200 times magnification to show equiaxed grains 11, which reveal packets of lath martensite 12 shown more clearly in the idealized microcomposite microstructure indicated generally by the number 13, the microstructure at about 60,000 times magnification comprising packets 14 and 15 of the lath martenite/austenite structure.
The packets are made up of films of retained austenite 16 sandwiching therebetween dislocated lath martensite 12A having dispersed therethrough fine carbide particles 17. As shown in Fig. 7, the films of retained austenite are about 200 Angstroms thick (A) and are separated from each other by a distance of about 0.5 micron. This idealized microstructure accounts for the high strength and toughness of thewrought and cast steel of the invention.
Although the present invention has been described in conjunction with the preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

Claims

A fine grained, high strength tough, low alloy, low to medium carbon steel, said steel consisting essentially by weight of about 0.5 to 4% Cr, about 0.05 to 0.5% C, about 0.5 to 2% Mn, about 0.1 to 2% Cu, about 0.1 to 3% Ni, about 0.01 to 0.05% Al, up to about 0.02% Ti, about 0.005 to 0.04% Nb and the balance essentially iron including minor amounts of tramp elements, said steel being characterized by an average grain size of less than about 100 microns.
The fine grained steel as in claim 1, further consisting essentially of 0.1 to 0.5% Si by weight.
The fine grained steel as in claim 2, wherein said average grain size is less than about 50 microns.
The fine grained steel as in claim 3, wherein said tramp elements include P, S, and N in amounts not exceeding about 0.02% P, about 0.015% S, and about 150 ppm N.
As an article of manufacture, a steel component having the composition of claim 1.
As an article of manufacture, a steel cast component having the composition of claim 2.
A method of producing a fine grained steel according to claim 1 which comprises, co-adding to said steel a small but effective amount of each of Al, Ti, and Nb sufficient to provide a fine grained steel following quenching of said steel from its austenitizing temperature.
A method of producing a fine grained cast steel according to claim 2 which comprises, co-adding to said steel a small but effective amount of each of Al, Ti, and Nb sufficient to provide a fine grained cast steel following quenching of said steel from its austenitizing temperature.