Treatment of steel

3,488,231

Patented Jan. 6, 1970

United States Patent Office

3,488,231

TREATMENT OF STEEL

Victor F. Zackay, Berkeley, and Earl R. Parker, Oakland, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission

No Drawing. Filed Nov. 22, 1966, Ser. No. 596,350 Int. Cl. C21d 7/13

U.S. Cl. 148—12 9 Claims

ABSTRACT OF THE DISCLOSURE

This invention is concerned with a process for imparting to steel a combination of high strength and high elongation, said process comprising subjecting sigle phase, austenitic steel which has both M_s and M_D below ambient temperature, a total carbon plus nitrogen content of from about 0.2% to about 0.5% and which contains at least 0.5% of at least one alloying element selected from the group consisting of molybdenum, chromium, manganese, vanadium, niobium, tantalum and tungsten, to deformation at a temperature above about 650° F., but below the recrystallization temperature of the steel.

The invention described herein was made in the course of, or under a contract with the United States Atomic Energy Commission.

The present invention is concerned with a process for the treatment of steel. More particularly it is concerned with a process for imparting to steel a combination of high strength and high elongation. It is generally recognized in steel making that high elongation has usually been achieved only at the sacrifices of high strength, and conversely, high strength has been achieved only at the sacrifice of elongation. It is an object of the present invention to provide a process whereby there is obtained steel simultaneously possessing both high strength and high elongation.

The starting material of the process of the present invention is a particular kind of steel having a composition differing from most commonly encountered steels. There are several requirements for steel to be suitable for use in the present process. The steel must have an M_s below ambient temperature. M_s is a standard metallurgical expression defined as the temperature at which the martensitic phase begins to form when the temperature is lowered. An M_s of ambient temperature or below is extremely low compared to the M_s of prior art ultra high strength steels. The M_s of a specimen may readily be measured by routine tests which are familiar to those skilled in the art, and are described in standard metallurgical publications.

The M_d of the starting material, like the M_s, must also be below ambient temperature. The M_D is a standard metallurgical term defined as the temperature above which the martensitic phase cannot form during working of the metal. The M_D may readily be determined by standardized testing familiar to those skilled in the art.

The steels of the present invention contain a large amount of alloying elements. This large amount of alloying element is necessary to achieve an M_s below ambient temperature. Typical alloying elements in these steels are: molybdenum, chromium, nickel, manganese, vanadium, niobium, tantalum, tungsten, silicon, nitrogen, and carbon. Typical compositions are: (a) 8% chromium, 8% nickel, 2% manganese, 2% silicon, 4% molybdenum, 0.30% carbon, balance iron; (b) 24% nickel, 4% molybdenum, 0.25% carbon, balance iron; (c) 21% nickel, 2% manganese, 2% silicon, 4% molybdenum, 24% carbon, balance iron.

It is also essential that the steel contain at least 0.5%, preferably about several percent, of at least one alloying element selected from the group consisting of molybdenum, chromium, manganese, vanadium, niobium, tantalum, and tungsten.

The steel used as the starting material in the present process must have a total carbon plus nitrogen content of from about 0.2% to about 0.5%. The combined carbon and nitrogen content of these steels is in sharp contrast to that of conventional stainless steel wherein the carbon and nitrogen content are generally kept as low as possible. The carbon content in conventional stainless steels is generally kept below 0.1%.

When a sample of steel is subjected to the process of the present invention and the combined carbon and nitrogen content of the steel is below about 0.2%, the steel does not have high strength; on the other hand, when the combined carbon and nitrogen content of the steel is above about 0.5%, the steel does not have good elongation.

To be suitable for use as a starting material in the operation of the present process, the steel must first be converted to a single phase, austenitic state. The usual way of doing this is by heating the steel above the critical temperature for austenite formation for a length of time sufficient so that the entire specimen has been heated throughout. Those skilled in the art are familiar with this operation. In general, for the steels of the present invention, the temperature will be from about 1800° F. to about 2200° F.

The actual operation of the present process involves deforming a steel specimen having the proper compositon as described above. This deformation must take place within a particular temperature range. The lower limit of the temperature range is about 650° F. The upper limit of the temperature range is the recrystallization range of the steel. Recrystallization temperature is a standard term in metallurgy and is customarily defined as that temperature above which a completely new set of strain-free grains are formed. The recrystallization temperature may be determined by routine tests familiar to those skilled in the art and described in the literature.

The recrystallization temperatures of the steels of the present invention are generally in the range of about 1500° F. to about 1800° F. It is thus seen that the temperature at which deformation occurs is generally in the range of 650° F. to 1800° F. This is a rather unusual working range. Ordinarily it is desired to deform a metal at a temperature well above recrystallization temperature because the metal is soft at these temperatures. On the other hand when it is desired to strengthen a metal by thermo-mechanical processes, a temperature below 650° F. is usually employed, because at higher temperatures the metal loses the strength imparted to it by cold working.

It should be noted that the present invention employs unusual compositions for the starting materials and also unusual temperatures for the deforming operation. It has now been discovered that this novel combination of operating parameters results in the production of steels having markedly and unexpectedly superior properties.

When the expression deforming is used in the present application it is intended to mean subjecting the specimen to a stress beyond its elastic limit and thereby causing a change in the shape of the entire specimen. Deformation may be carried out by any of the standard metal working techniques such as rolling, swaging, wire drawing, forging, shear forming, etc. Any application of mechanical force sufficient to cause a change in shape is effective provided the change in shape extends throughout the entire specimen. In general it is preferred that the amount of deformation be at least about 10% at temperatures in the middle of the operating range. At the lower end of the tempera3,488,231 ture range a greater amount of deformation may be required to induce the desired chemical and structural changes, while at the upper end of the temperature range a greater amount of deformation may be required because the higher temperatures may serve to modify the structure in an undesired way.

The benefits of the present process are achieved once the steel of the proper composition has been deformed at the proper temperature. From then on the specimen may be treated in a variety of ways depending on the ultimate use. For example, the steel may be allowed to cool slowly to room temperature from the deforming temperature. Alternatively, it may be quenched rapidly to room temperature in an appropriate quenching medium, and, if desired, even subsequently cooled to a temperature below room temperature such as the temperature of liquid nitrogen. . .

In one preferred variation of the present invention, the samples are subjected to a second deformation process. The elongation of the steels of this invention is such that the steels may at any desired temperature be subjected to straining and tempering and by this straining acquire greater strength.

In summary, the important step is the deforming within the specified temperature range, and once that step has been accomplished, the speciment will retain its desirable properties through any of a wide variety of subsequent steps.

The results obtained by the present invention are totally unexpected and are not fully understood. Without in any way wishing to be bound thereby, we have formulated the following theory which may account for the observed results.

The ductility of conventionally heat treated steel is limited by the onset of necking. Necking refers to a local reduction in area which usually proceeds fracture when a sample is subjected to tensile stresses. In conventionally processed steels or other alloys there is little or nothing that can be done to delay or eliminate the onset of necking or early failure. However, in steels processed by the present invention the early onset of necking is prevented by an increased rate of work hardening whenever a neck starts to form. This increased rate of work hardening increases the strength of the steel at the incipient neck area and forces the metal to flow plastically in an area adjacent to the neck. The increased rate of work hardening is believed to be caused by a phase transformation; in other words the strain induces transformation of the austenite to martensite. This transformation is readily seen as a discontinuity in the autographic record of the stress-strain curve of a steel processed according to this invention. The high elongation of these steels is thus attributed to the prevention of early necking by the production of martensite from austenite during straining.

Since the increased elongation results directly from the control of the austenite to martensite transformation, we have labeled the phenomenon “TRIP,” which is abbreviated from the words “Transformation Induced Plasticity.” Thus during the process of the present invention the M_Dis caused to be raised to above ambient temperature, thereby assuring the occurrence of the of the austenite to martensite transformation during straining over a very wide range of strain and thereby resulting in a neck resistant steel of high strength and high elongation.

The following examples show the improvement in mechanical properties over those obtained by conventional processing and describe in detail our process and some of its modifications.

EXAMPLE I

A typical steel suitable for our process has the following composition: 8% chromium, 8% nickel, 4% molybdenum, 2% manganese, 2% silicon, and 0.30 carbon, balance iron. To produce maximum ductility in this steel the conventional heat treatment of preference would consist of the following; Heat to 2080° F. and hold for one hour at this temperature and then water quench to room temperature. A treatment such as this conventional one results in the following properties: Yield strength: 54,700 p.s.i.; ultimate tensile strength: 80,000 p.s.i.; and elongation: 27%.

This same steel, processed according to the teachings of our invention, has entirely different and superior properties. A typical process is the following: Heat to 2080 ⁰F. and hold at this temperature for one hour; cool to 840° F. and deform 20% by rolling at this temperature; finally, water quench to room temperature. The above processing results in the following properties: Yield strength: 122,000 p.s.i.; ultimate tensile strength: 168,300 p.s.i.; and elongation, 80%. Thus, application of our process has more than doubled the elongation. In striking contrast to conventional heat treatments, both the elongation and the strength (yield and ultimate tensile) have been increased. In virtually all conventional processes the strength is sacrificed for a gain in elongation, or conversely, the elongation is sacrificed for a gain in strength.

EXAMPLE II

To produce a combination of high strength and good ductility in the steel having the composition shown in the example above, another typical treatment by our process is as follows: Heat to 2080° F. and hold for one hour at this temperature; water quench to room temperature; reheat to 840° F. and deform 80% by rolling at this temperature. Alternately, the specimen can be cooled from 2080° F. to 840° F. rather than be cooled to room temperature and reheated. Either of these alternate processes results in the following mechanical properties: Yield strength: 222,500 p.s.i.; ultimate tensile strength: 254,000 p.s.i.; and elongation, 40%. The elongation of a conventionally treated steel at this strength level would typically be less than 12%.

EXAMPLE III

Even greater strengths, accompanied by good elongation, can be obtained with minor modifications of our process. A typical modification of the process is as follows: Heat a steel specimen having the composition given in the examples above to 2080° F. and hold for one hour at this temperature; water quench to room temperature; heat to 840° F. and deform 80% by rolling at this temperature; cool to room temperature; strain 15% at room temperature, and, finally, heat to 840° F. and hold at this temperature for thirty minutes. The above processing results in the following properties: Yield and ultimate tensile strength: 273,000 p.s.i.; and elongation, 25%. Virtually all conventionally processed steels would have considerably less elongation at this strength level.

A type of processing seemingly similar to our invention is that embodied in the “Precipitation Hardening” or “PH-Series” of stainless steels. However, an examination of the conventional processing of this type of steel reveals fundamental differences. High strength and good elongation in the PH-Series of stainless steels are usually obtained by a series of heat treatment. Normally these steels are not given thermo-mechanical treatments. In contrast to the teachings of the present invention, the deformation in the processing of the PH-Series of stainless steels is done below the M_D. In our invention, the deformation is deliberately done at a temperature above M_D with the said deformation resulting in a much superior balance of strength and elongation in the finished product.

Another prior art process involving deformation at intermediate temperatures is that of ausforming. In this process the steel is heated to a high temperature, usually about 1800° F. cooled to a temperature between about 800° and 1200° F., deformed at least 25%, cooled to room temperature, and reheated to a temperature less than 1000° F. for periods of about one hour. In the ausform process the composition of the steel must be such that its M_s is well above room temperature; in addition the de3,488,231 formation is necessarily restricted to a relatively narrow range of temperatures, unlike the teachings of the present invention. Further, the process is such that the deformation must be done in one temperature cycle as opposed to that of the present invention wherein the deformation can be done in steps, the specimen being heated to and cooled from the deformation temperature innumerable times without harmful effects. This latter feature of our process has considerable practical advantage over the ausform (and its modifications) process in that severe shaping and forming operations can be done in small steps. In this way, smaller, more conventional forming and shaping equipment can be utilized, thereby overcoming a major factor in the limited use of the ausform process.

Evidence can be found in the technical literature for improving the strength and ductility of steels by techniques superficially similar to those described in this invention. An example is the work of Bressanelli and Moskowitz, Transactions Quarterly of the American Society for Metals, vol. 59, 1966, p. 223. These investigators found that the strength and ductility of commercial stainless steels could be increased by the control of the composition and the temperature and strain rate of testing. The present invention teaches that prior to actual testing, deformation be done at a temperature above M_Dand below the recrystallization temperature. This critical step controls the amount of the second phase which is produced during straining in the final testing. Also, it is the control of this step of the processing of this invention that permits the attainment of such a wide range of combination of strength and elongation as described above. In the process of the present invention, the shaping or forming operation is done at a relatively high temperature, i.e., between 650° F. and the temperature of recrystallization. The only straining at or near room temperature is that encountered in actual service.