ES2386425T3 - Cold worked steels with packing net with martensite / austenite microstructure - Google Patents

Abstract

A process for manufacturing a high strength and high ductility carbon steel alloy, said process comprising: (a) forming a carbon steel alloy having a microstructure comprising martensite networks alternated with retained austenite films, and (b ) cold work said carbon steel alloy in a series of passes without intermediate heat treatment between passes to a sufficient reduction to achieve a tensile strength of at least 1034 MPa (150 ksi), in which step (a) it comprises: (i) forming a carbon steel alloy composition having a martensite starting temperature of at least 300 ° C, (ii) heating said carbon steel alloy composition to a temperature high enough to cause austenization of the same, to produce a homogeneous austenite phase with all alloy elements in solution, and (iii) cooling said homogeneous austenite phase through d and said martensite transition interval at a sufficiently rapid cooling rate to achieve said microstructure, preventing the formation of carbide at the interfaces between said martensite networks and said retained austenite films, wherein said composition of the steel alloy at Carbon contains 0.004% to 0.12% carbon, 0% to 11% chromium, 0% to 2.0% manganese and 0% to 2.0% silicon, all by weight, being the iron rest along with any inevitable impurity.

Description

Cold worked steels with packing net with martensite / austenite microstructure.

Background of the invention

1. Field of the invention

This invention relates to the technology of low and medium carbon steel alloys, particularly those of high strength and toughness, and cold formability of said alloys.

2. Description of the prior art

An important stage in the processing of high-performance steels is cold working, which typically consists of a series of compressions and / or expansions achieved by processes such as stretching, extrusion, cold-highlighting or rolling. Cold working causes the plastic deformation of the steel that produces the hardening of tensions while the steel is formed in the form in which it will finally be used. The cold worked, which in the case of the steel cable is analyzed by wire drawing, is typically performed in a succession of phases with intermediate heat treatments, which in the case of the steel cable are called "patented".

The high-strength steel cable is an example of a high-performance steel, and is useful in a variety of engineering applications including tire cord, wire cable and strands for prestressed concrete reinforcements. The most commonly used steel in high strength steel cable is medium or high carbon steel. In the typical procedure for forming the cable, hot rolled bars with perlitic microstructures are cold stretched in several phases, with intermediate patent treatments to soften the pearlite to continue cold drawing. For example, hot rolled bars of approximately 5.5 mm in diameter could be reduced in several phases to a diameter of approximately 3 mm. The patenting could then be performed at 800-900 ° C causing austenization of the steel, followed by transformation of the steel at 500-550 ° C into fine perlite lamellae. The steel could then be etched, in hydrochloric acid for example, to remove the scale formed during the patented. The pickling could be followed by various additional stretching phases to reduce the diameter to about 1 mm, then patented and further pickling. The final stretching would then be carried out in several phases up to the final desired diameter, which for example can be approximately 0.4 mm, to achieve the desired properties, in particular strength. This may be followed by additional processing, such as braiding, depending on the end use.

The purpose of the initial patented treatment is to produce a wire rod with a fine lamellar perlitic structure, which requires a low transformation temperature. To achieve the desired temperature control, the process is typically performed in a molten lead bath. In the successive stretching phases, the cable is stretched at true tensions (defined below) of 6-7 to obtain high strength levels of approximately 3,000 MPa. For conventional perlitic cables, these high voltages and resistances can be achieved simply by applying a series of patented treatments. Without these patented treatments, cold drawing would cause shear cracking of the perlitic lamellae. Due to the need for a molten lead bath the entire process is expensive and tends to create environmental concerns.

Cold working is also used in the production of expandable steel tubes, that is, tubes that expand at the site and, in some cases, below ground.

A recent development in steel alloys is the formation of microstructures that contain both phases martensite and austenite in an alternate configuration, in which martensite is present as networks that are separated by thin austenite films. Microstructures are condensed grains in which the individual grains contain several martensite networks separated by thin austenite films with, in some cases, an austenite coating surrounding each grain. These structures are called "dislocated martensite" structures or "martensite / austenite packing network" structures. The patents describing these microstructures are the following:

4,170,497 (Gareth Thomas and Bangaru V.N. Rao), issued on October 9, 1979 on an application filed on August 24, 1977

4,170,499 (Gareth Thomas and Bangaru V.N. Rao), issued on October 9, 1979 on an application filed September 14, 1978 and as a partial continuation of the previous application filed on August 24, 1977

4,671,827 (Gareth Thomas, Nack J. Kim, and Ramamoorthy Ramesh), issued on June 9, 1987, on an application filed October 11, 1985

6,273,968 B1 (Gareth Thomas), issued on August 14, 2001, on an application filed on March 28, 2000

Although these microstructures offer certain performance benefits, particularly high corrosion resistance, it has not been known until now that the processing steps typically used for steel alloys could be simplified or eliminated when these microstructures are present.

The two United States Patents describing cold-worked steel bars and cables without patents are of great relevance for this invention. These patents are:

4,613,385 (Gareth Thomas and Alvin H. Nakagawa), issued on September 23, 1986, on an application filed on December 9, 1982

4,619,714 (Gareth Thomas, Jae-Hwan Ahn, and Nack-Joon Kim), issued on October 28, 1986 on an application filed on November 29, 1984 as a partial continuation of the previous application filed on August 6, 1984.

The microstructures of the steels in these patents are considerably different from those of the first four patents shown above.

Summary of the invention

The invention provides a process for manufacturing a high strength and high ductility carbon steel alloy according to claim 1 or claim 2 of the appended claims thereto.

It has now been discovered that the microstructure of martensite / austenite packing network is unique in its crystallographic characteristics and how these characteristics cause it to respond to cold work. Due to the high displacement density of this microstructure and the ease with which tensions in the structure can move between the martensite and austenite phases, cold-working provides the microstructure with the unique mechanical properties that include high tensile strength. As a result, these alloys can be cold worked without intermediate heat treatments, while still achieving tensile strengths comparable to the tensile strengths of conventional steel alloys that have been processed by cold treatment with intermediate heat treatments. In the case of a steel cable having the microstructure of martensite / austenite packing network, this invention lies in the discovery that cold drawing can be performed without intermediate patenting treatments. According to the present invention, therefore, carbon steel alloys having the martensite / austenite packing network microstructure, that is, those whose microstructure includes alternate martensite nets with retained austenite thin films, are formed cold, preferably without intermediate heat treatments, to a sufficient reduction to achieve a tensile strength of approximately 150 ksi or greater ("ksi" denotes kilo pounds force per square inch), equivalent to approximately 1,085 MPa or greater ("MPa" denotes megapascals, that is, newtons per square millimeter). Cold work at tensile strengths of 2,000 MPa (290 ksi) or greater is of particular interest and, in fact, tensile strengths of 3,000 MPa (435 ksi) and as high as 4,000 MPa (580 ksi) can be achieved by The practice of this invention. These values are approximate; The conversion factor to the nearest thousand is 6,895 MPa equal to 1 ksi.

The benefits of this invention extend to simple microstructures of martensite / austenite packing net that do not contain ferrite or contain insignificant amounts of ferrite, and also to microstructures that include a packing network of condensed grains with ferrite grains, and variants of these structures, including those whose grains in the packing network are covered by austenite coatings, those that are free of carbide precipitates at the interfaces and those in which the austenite films are of a uniform orientation. The discovery of the ability of the martensite / austenite packing network microstructures to respond to cold work in this way is surprising with respect to the descriptions in patents No. 4,613,385 and 4,619,714 referred to above. , since the ferrite in the microstructure of these patents has a lower elastic limit than martensite. As a result, the ferrite will preferably absorb the tension introduced by the cold worker, while the martensite will not respond to the cold worked until the ferrite phase is worked by hardening at a level above the elastic limit of the martensite. In the microstructures addressed by the present invention, the level of ferrite was relatively low or its absence, when no ferrite is present, will cause the martensite to absorb tension at an earlier stage of the cold working process. Martensite and ferrite are distinctly different from each other in crystalline structure and hardening behavior.

These and other features, objects, advantages and embodiments of the invention will be better understood from the following description.

Brief description of the figures

Figure 1 is a representation of the tensile strength against true total tension for two alloys of dual phase microstructure steel of martensite / austenite packing net, after cold working according to this invention in the absence of treatments intermediate thermal.

Figure 2 is a representation of the tensile strength against total stress for three steel alloys of a triple phase martensite / austenite packing network microstructure and a martensite / austenite packing network microstructure steel alloy double phase, after cold working according to this invention in the absence of intermediate heat treatments.

Detailed description of the invention

and preferred embodiments

The cold worked in the practice of this invention can be carried out by using the techniques and equipment that has been used for cold working in the prior art in other steel alloys and microstructures for alloys in the form of square slabs, billets , bars, plates or sheets, cold working can consist of rolling the steel between the rollers or other compression means to reduce excess, and lengthen the steel. When cold working is done by rolling, multiple reductions are achieved by multiple passes through the rolling train. For workpieces in the form of a bar or a cable, the cold work may consist of cold drawing or extrusion through a die. For multiple reductions, the workpiece is extruded through a series of successively smaller dies. The tubes are achieved by stretching the steel through a ring-shaped die with a mandrel inside the die. For multiple passes, the tube that has already been stretched is further stretched through a smaller ring-shaped die with a mandrel located inside the tube.

Cold work is performed at a temperature below the lowest temperature at which recrystallization occurs. Suitable temperatures are, therefore, those that do not induce any phase change in the steel. For carbon steels, recrystallization typically occurs at approximately 1,000 ° C (1,832 ° F), and consequently cold working according to this invention is performed well below this temperature. Preferably, cold working is performed at temperatures of about 500 ° C (932 ° F) or less, more preferably about 100 ° C (212 ° F) or less and, most preferably, at a temperature that is within about 25 ° C of ambient temperature

Cold work can be done in a single pass or in a succession of passes. In any case, intermediate heat treatments (which, in the case of the steel cable are called "patented") can be performed for the further improvement of the properties, although the properties resulting from cold working alone are sufficiently high so that the treatments Thermal intermediates are not required and, preferably, are not performed. The degree of reduction per pass is not critical to the invention and can vary widely, although the reductions should be large enough to prevent hardening of the steel so much that the steel becomes susceptible to breakage after a small total reduction. In most cases, the preferred reductions are at least about 20% per pass, more preferably at least about 25% per pass and, most preferably, from about 25% to about 50% per pass. The reduction by pass is governed at least partially by factors such as the angle of the die and the coefficient of effectiveness of the stretching. The greater the angle of the die, the greater the minimum reduction that is required to prevent cracking by central discharge. The lower the stretching efficiency coefficient, however, the lower the maximum reduction for a steel with a given strain hardening exponent. A compromise is typically sought between these two considerations in competition. In terms of the tensile strength of the final product, cold working will preferably be performed at a tensile strength within the range of about 1034 MPa (150 ksi) to about 3450 MPa (500 ksi).

The process of this invention is applicable to carbon steel alloys having martensite / austenite packing network microstructures, such as those described in the aforementioned patents, as well as those described in United States Patent Applications pending together with the present Nº 10 / 017.847, presented on December 15, 2001 (entitled "Triple-Phase Nano-Composite Steels", inventors Kusinski, GJ, Pollack, D., and Thomas, G.), and 10 / 017.879, presented on December 14, 2001 (entitled "Nano-Composite Martensitic Steels", inventors Kusinski, GJ, Pollack, D., and Thomas, G.). To allow the formation of the martensite / austenite packing network microstructure, the alloy composition will typically have a martensite starting temperature M of approximately 300 ° C or greater, and preferably 350 ° C or greater. Although the alloy elements generally affect M, the alloy element that has the greatest influence on M5 is carbon, and achieving an alloy with M6 above 300 ° C can be achieved by limiting the carbon content of the alloy to a maximum 0.35% by weight. In a reference example, the carbon content is within the range of about 0.03% to about 0.35% and, in a reference example, the range is about 0.05% to about 0 , 33%, all by weight. Additional alloy elements, such as molybdenum, titanium, niobium and aluminum, may also be present in amounts sufficient to serve as nucleation sites for fine grain formation, although at a concentration sufficiently low to avoid affecting the properties of the alloy. finished by his presence. The concentration should also be low enough to prevent the formation of inclusions and other large precipitates, which can make steel susceptible to early fracture. In certain reference examples, it would be advantageous to include one or more austenite stabilization elements, examples of which are nitrogen, manganese, nickel, copper and zinc. Particularly preferred among these are manganese and nickel. When nickel is present, the nickel concentration is preferably in the range of about 0.25% to about 5% and when manganese is present, the manganese concentration is preferably in the range of about 0.25% at approximately 6%. Chromium is also included in many embodiments of the invention. All concentrations in this document are by weight.

Certain embodiments of the invention involve alloys that include a ferrite phase in addition to the martensite / austenite packing net grains (triple phase alloys) while others only contain the martensite / austenite packing net grains and do not include a phase. ferrite (double phase alloys). In general, the presence or absence of the ferrite phase is determined by the type of heat treatment in the initial austenization phase. By appropriate temperature selection, the steel can be transformed into a single phase of austenite or a biphasic structure that contains both austenite and ferrite. In addition, the alloy composition can be selected or adjusted to cause the formation of ferrite during the initial cooling of the austenite phase alloy or prevent the formation of ferrite during cooling, that is, to prevent the formation of ferrite grains before additional cooling of the austenite to form the microstructure of packing network.

As indicated above, in certain cases it would be beneficial to use alloys with microstructure of martensite / austenite packing net in which the austenite films on a single grain of the packing net are all of approximately the same orientation, although the orientation Crystallography may vary, such as those in which austenite films on a single grain of packing net are all of the same orientation of the crystalline plane. The latter can be achieved by limiting the grain size to 10 smaller. Preferably, the grain size in these cases is within the range of about 1 m (micrometer) to about 10 m (micrometers) and more preferably from about 5 m (micrometers) to about 9 m (micrometers).

The preparation of phase microstructures of martensite / austenite packing network that does not contain ferrite (ie "double phase" microstructures) begins with the selection of the alloy components and the combination of these components in the appropriate portions, as indicated above. The combined components are then homogenized ("soaked") for a sufficient period of time and at a temperature sufficient to achieve a uniform austenitic structure with all the elements and components in solid solution. The temperature will be above the austenitic recrystallization temperature although preferably at a level that will cause very fine grains to form. The austenitic recrystallization temperature typically varies with the composition of the alloy, although in general it will be readily apparent to those skilled in the art. In most cases, better results will be achieved by soaking at a temperature within the range of 800 ° C to 1,150 ° C. Laminating, forging or both are optionally made on the alloy at this temperature.

Once the homogenization has been completed, the alloy is subjected to a combination of cooling and refinement of grain to the desired grain size which, as indicated above, may vary. Grain refinement can be carried out in phases, although final grain refinement is generally achieved at an intermediate temperature that is above even the austenitic recrystallization temperature. The alloy can be laminated, first, at the homogenization temperature to achieve dynamic crystallization, then cooled to an intermediate temperature and laminated again for further dynamic recrystallization. The intermediate temperature is between the austenitic recrystallization temperature and a temperature that is approximately 50 ° C above the austenitic recrystallization temperature. For alloy compositions whose austenitic recrystallization temperature is about 900 ° C and the intermediate temperature at which the alloy cools is preferably between about 900 ° C and about 950 ° C, and more preferably between about 900 ° C and about 925 ° C. For alloy compositions whose austenitic recrystallization temperature is approximately 820 ° C, the preferred intermediate temperature is approximately 850 ° C. Dynamic recrystallization can also be achieved by forging or by other means known to those skilled in the art. Dynamic recrystallization results in a grain size reduction of 10% or greater and, in many cases, a grain size reduction of about 30% to about 90%.

Once the desired grain size has been achieved, the alloy is quenched by cooling from a temperature above the austenitic recrystallization temperature to the starting temperature of martensite Ms, then through the martensite transition interval to convert the crystals of austenite in the microstructure of martensite / austenite packing network. When ferrite crystals are present between austenite crystals, conversion only occurs in austenite crystals. The optimum cooling rate varies with the chemical composition and, thus, the hardening capacity of the alloy. The resulting packages are approximately the same small size as the austenite grains produced during the rolling phases, but only the austenite that remains in these grains is in the thin films and in some cases in the lining that surrounds each network grain. packing When thin austenite films have to be a single variant in the crystalline orientation, this is achieved by controlling the process to achieve a grain size of less than 50 m (micrometers). As an alternative to dynamic recrystallization, grain refinement to the desired grain size can be achieved by heat treatment alone. To use this method, the alloy is inactivated as described in the previous paragraph, then reheated to a temperature that is approximately equal to the austenitic or slightly lower recrystallization temperature, then inactivated once more to achieve or return the microstructure martensite / austenite packing net. The reheating temperature is preferably within about 50 degrees Celsius of the austenitic recrystallization temperature, for example about 870 ° C.

The processing steps such as heating the alloy composition to the austenite phase, cooling the alloy with controlled rolling or forging to achieve the desired reduction and grain size, and inactivating the austenite grains throughout the region of Martensite transition to achieve the packing network structure are performed by methods known in the art. These methods include casting, heat treatment and hot working of the alloy, such as by forging or rolling, followed by temperature controlled finishing for optimum grain refinement. The controlled laminate serves several functions, including assisting in the diffusion of the alloy elements to form a homogeneous crystalline phase of austenite and in the storage of the tension energy in the grains. In the process inactivation phases, the controlled laminate guides the newly formed martensite phase towards a packing network arrangement of martensite networks separated by thin films of retained austenite, the degree of lamination reduction can vary and will be readily apparent to the subject matter experts. The inactivation is preferably carried out quickly enough to prevent the formation of harmful microstructures including perlite, vainite and particles or precipitates, particularly interface precipitation and particle formation, including the formation of undesirable carbides and carbonitrides. In the grains of the martensite / austenite packaging network, the retained austenite films will constitute from about 0.5% to about 15% by volume of the microstructure, preferably from about 3% to about 10% and, more preferably, a maximum of about 5%.

The triple phase alloys have a microstructure consisting of two types of grains, ferrite grains and martensite / austenite packing net grains, condensed together as a continuous mass. As with double phase alloys, individual grain size is not critical and can vary widely. For best results, grain sizes will generally have diameters (or other appropriate characteristic dimensions) that are within the range of about 2 micrometers = to about 100 micrometers, or preferably within the range of about 5 micrometers to about 30 micrometers. The amount of ferrite phase with respect to the martensite / austenite phase may vary. In most cases, however, the best results will be obtained when martensite / austenite grains constitute from about 5% to about 95% of the triple phase structure, preferably from about 15% to about 60 % and, more preferably, from about 20% to about 40%, all by weight.

The triple phase alloys can be prepared by first combining the appropriate components necessary to form an alloy of the desired composition, then soaking to achieve a uniform aphenic structure with all the elements and components in solid solution, as in the preparation of the alloys of double phase described above. A preferred soaking temperature range is from about 900 ° C to about 1,170 ° C. Once the austenite phase is formed, the alloy composition is cooled to a temperature in the intercritical region, which is defined as the region in which the austenite and ferrite phases coexist in equilibrium. The cooling causes, therefore, that a portion of the austenite is transformed into ferrite grains, leaving the rest as austenite. The relative amounts of each of the two equilibrium phases vary with the temperature at which the composition cools in this phase and also with the levels of the alloy elements. The distribution of coal between the two phases (again in equilibrium) also varies with temperature. The relative amounts of the two phases are not critical to the invention and may vary. The temperature at which the composition is cooled to achieve the double phase ferrite-austenite structure is preferably in the range of about 800 ° C to about 1,000 ° C.

Once the ferrite and austenite crystals have formed (i.e., once the equilibrium has been achieved at the selected temperature in the intercritical phase), the alloy is rapidly inactivated by cooling through the transition range of martensite to convert the austenite crystals in the microstructure martensite / austenite packing network. The cooling rate used during this transition is large enough to substantially avoid any change in the ferrite phase and prevent undesirable decomposition of austenite. Depending on the composition of the alloy and its hardening capacity, water cooling may be required to achieve the desired cooling rate, although for certain alloys air cooling will be sufficient. In some alloys, specifically triple phase containing 6% Cr, the desired cooling rate is sufficiently low so that air cooling can be used. The considerations indicated above in relation to double phase alloys apply here as well.

The double phase alloy compositions are those containing from about 0.04% to about 0.12% carbon, from 0 to about 11.0% chromium, from 0 to about 2.0% of manganese, and from 0 to approximately 2.0% silicon, all by weight, the rest being iron. The triple phase alloy compositions are those containing 0.02% to about 0.14% carbon, 0 to about 3.0% silicon, 0 to about 1.5% manganese and 0 to approximately 1.5% aluminum, all by weight, the rest being iron.

The formation of precipitates or other small particles within the microstructure after cooling is collectively referred to as "self-tempering." In certain applications of this invention, whether double phase or triple phase alloys, self-tempering will be purposely avoided using a relatively rapid cooling rate. The minimum cooling rates that will prevent self-tempering are evident from the temperature / time transformation diagram for the alloy. In the typical diagram, the vertical axis represents temperature and the horizontal axis represents time, while the curves in the diagram indicate the regions where each phase exists by itself or in combination with other phases. One of these typical diagrams is shown in Thomas, US Patent No. 6,273,968 B1, referred to above. In these diagrams, the minimum cooling rate is a descending temperature line with the time that rests on the left side of a C-shaped curve. The region to the right of the curve represents the presence of carbides and the velocities of Cooling that prevents the formation of carbides are, therefore, those represented by the lines that remain to the left of the curve. The line that is tangential to the curve has the lowest slope and, therefore, is the slowest speed that can be used while carbide formation is still avoided.

The terms "interface precipitation" and "interface precipitates" are used herein to denote the formation of small alloy particles at locations between the martensite and austenite phases, that is, between the networks and the thin films that separate the networks. . "Interface precipitates" does not refer to austenite films themselves. Interphase precipitates should be distinguished from "intraphase precipitates," which are precipitates located within martensite networks rather than along the interfaces between demartensite networks and austenite films. Intraphase precipitates that are approximately 500 Å or smaller in diameter are not detrimental to thickness and, in fact, can improve toughness. In this way, the self-return is not necessarily detrimental with the condition that the self-tempering is limited to the intraphase precipitation and not as a result the interface precipitation. The term "substantially carbide free" is used herein to indicate that if any carbide is present, its distribution and quantity is such that it has an insignificant effect on the performance characteristics and particularly the corrosion characteristics of the finished alloy.

Depending on the composition of the alloy, a cooling rate that is high enough to prevent carbide formation or self-tempering in general may be one that can be achieved by air cooling or one that requires water cooling. In alloy compositions in which self-tempering can be avoided by air cooling, air cooling can still be performed when the levels of certain alloy elements are reduced, provided that the levels of other alloy elements are reduced. raise. For example, a reduction in the amount of carbon, chromium or silicon can be compensated by increasing the level of manganese.

The processes and conditions set forth in United States patents referred to above, particularly heat treatments, grain refinements, forged in line and the use of rolling mills for round, flat and other shapes, can be used in the practice of the present invention for the heating of the alloy composition to the austenite phase, the cooling of the alloy from the austenite phase to the intercritical phase, in the case of triple phase alloys, and then the cooling through the region of transition of martensite. The laminate is carried out in a controlled manner in one or more phases during austenization procedures and the first cooling phase, for example to aid in the diffusion of the alloy elements to form a homogeneous crystalline phase of austenite and then deform the crystalline grains. and storing the tension energy in the grains, while in the second cooling phase, the laminate can serve to guide the newly formed martensite phase to the martensite net packing network arrangement separated by thin films of retained austenite. The degree of rolling reductions may vary and will be readily apparent to those skilled in the art. In crystals of the martensite / austenite packaging network, the retained austenite films will constitute from about 0.5% to about 15% by volume of the microstructure, preferably from about 3% to about 10% and, more preferably, a maximum of about 5%. The proportion of austenite with respect to the entire triple phase microstructure will be a maximum of approximately 5%. The actual width of an individual retained austenite film is preferably in the range of about 50 Å to about 250 Å and preferably is about 100 Å. The proportion of austenite with respect to the total triple phase microstructure will generally be, at most, approximately 5%. The laminate analyzed in this paragraph must be distinguished from the cold-work performed in accordance with this invention after the martensite / austenite packing network microstructures, are double phase or part of a triple phase structure, have been formed .

The following examples are offered by way of illustration only.

EXAMPLE 1

This example illustrates the deformation of a carbon steel rod with a microstructure of martensite / austenite packing net by a cold rolling process according to the present invention up to a 99% area reduction.

5 The experiment shown in this example was carried out on a steel bar measuring 6 mm in diameter and having an alloy composition of 0.1% carbon, 2.0% silicon and 0.5% chromium, 0, 5% manganese, all by weight, and the rest iron, with a microstructure consisting of grains measuring approximately 50 (micrometers), each grain consisting of martensite nets measuring approximately 100 nm thick alternating with thin austenite films that They measured approximately 10 nm thick, without ferrite phases, and each

The grain was surrounded by an austenite coating that measured approximately 10 nm thick.

The bar was prepared by the method described in the United States Patent Application being processed together with the present with Serial No. 10 / 017,879, filed on December 14, 2001, referred to above.

The uncoated steel bar was superficially cleaned and lubricated, then cold stretched through

15 dies lubricated in 15 passes at a temperature of 25 ° C to a diameter of 0.0095 inches (0.024 cm). At a final cable diameter of 0.0105 inches (0.027 cm), which represented a 99% total area reduction, the cable had a tensile strength of 390 ksi (2690 MPa).

EXAMPLE 2

This example is another illustration of the cold work of carbon steel bars with network microstructures of

20 martensite / austenite packaging according to the present invention. In this example, two different alloys, Fe / 8Cr / 0.05C and Fe / 2Si / 0.1C, were used with a microstructure consisting of grains measuring approximately 50 m (micrometers) in diameter, each grain consisting of martensite networks measuring approximately 150 nm thick alternating with thin austenite films measuring approximately 10 nm thick, without significant ferrite phases, each grain being surrounded by a coating of austenite that

25 measured approximately 10 nm thick.

The iron bars were 6 mm in diameter and their surface was cleaned and lubricated, then they were cold drawn through lubricated dies in a series of passes at a temperature of 25 ° C. The drawing scheme shown in Table 1 was used for the Fe / 8Cr / 0.05C alloy, and a similar styling scheme was used for the Fe / 2Si / 0.1C alloy. In this table, Ao represents the initial bar diameter and A is the bar diameter after

30 of a particular pass.

TABLE I

Stretching Scheme for Fe / 8Cr / 0.05C with a Substantially Free Ferrite Packing Network Microstructure

No. of pass

Diameter (mm) Total Total Voltage (In (A / Ao)) Reduction of Area in a Single Pass (%) Total Area Reduction (%)

(initial)

6,000 0.0 0.0 0.0

one

4.3 0.7 48.2 48.2

2

3.4 1.1 37.0 67.3

3

2.7 1.6 37.1 79.4

4

2.2 2.0 34.0 86.4

5

1.8 2.5 36.6 91.4

6

1.4 2.9 38.5 94.7

7

1.0 3.5 45.4 97.1

Tensile strengths were measured at the initial bar and after each pass, and the results are plotted against the true total tension in Figure 1, in which the squares represent the alloy of Fe / 8Cr / 0.05C and 35 the diamonds represent the alloy Fe / 2Si / 0,1C. The Figure shows that the tensile strengths of both alloys reach 2,000 MPa at the end of the entire stretching sequence at a total area reduction of 97%.

EXAMPLE 3

This example illustrates cold working according to the present invention, using carbon steel bars with microstructures of martensite / austenite packing net containing ferrite crystals and a third phase (in addition to martensite nets and thin films of austenite, that is, a microstructure

5 three phase).

In this example, the alloy was Fe / 2Si / 0.1C, with a microstructure consisting of condensed ferrite with grains of the packing net similar to those described above in Examples 1 and 2, which contained martensite networks alternated with films thin austenite and coated in an austenite coating. The bars were prepared by the method described in US Pat. No.

10 Application 10 / 017,847, filed on December 14, 2001, referred to above, using a reheating temperature of 950 ° C to achieve a ferrite content of 70% by volume of the microstructure. The initial bar diameter was 0.220 inches (5.59 mm), and cold work consisted of stretching the bars through lubricated conical dies at a temperature of 25 ° C in 15 passes with approximately 36% reduction per pass, up to a final diameter of 0.037 inches (0.94 mm).

The drawing scheme is shown in Table II, in which Ao represents the initial bar diameter and A is the bar diameter after a particular pass.

Table II

Drawing scheme for Fe / 2Cr / 0.1C with Triple Phase Microstructure

No. of Pass

Diameter (mm) Total Total Voltage (ln (Y / Y) Reduction of Area in a Single Pass (%) Total Area Reduction (%)

(initial)

6,050 0.00 0.00 0.00

one

4,580 0.56 42.69 42.69

2

3,650 1.01 36.49 63.60

3

2,910 1.46 36.44 76.86

4

2,320 1.92 36.44 85.29

5

1,870 2.35 35.03 90.45

6

1,660 2.59 21.20 92.47

7

1,320 3.04 36.77 95.24

8

1,090 3.43 31.81 96.75

9

0.910 3.79 30.30 97.74

10

0.756 4.16 30.98 98.44

eleven

0.624 4.54 31.87 98.94

12

0.526 4.89 28.94 99.24

13

0.437 5.26 30.98 99.48

14

0.390 5.48 20.35 99.58

fifteen

0.359 5.65 15.27 99.65

The tensile strength of the final cable was 2760 MPa (400 ksi).

20 EXAMPLE 4

This example is a further illustration of the cold work of carbon steel bars whose microstructure consisted of a martensite / austenite and ferrite crystals packing network, in accordance with the present invention.

In this example, the alloy was Fe / 2Si / 0.1C as in Example 3, with a microstructure consisting of condensed ferrite with packing net grains similar to those described above in Examples 1 and 2, which contained networks of Alternate martensite with thin austenite films and surrounded by an austenite coating. A bar of this composition was prepared by the general method described in US Pat. No. Application 10 / 017,847, filed on December 14, 2001, referred to above. In this case, the bar was initially hot rolled to a diameter of 0.25 inches (6.35 mm), then heated at 1,150 ° C for approximately 30 minutes to austenize the composition, then quenched in ice-cold brine to transform the austenite into substantially 100% martensite, then quickly reheated to convert the structure to approximately 70% ferrite and 30% austenite. The bar was then quenched in ice-cold brine to convert the austenite into a martensite / austenite packing net structure. The bar was cold stretched after 7 passes at a 35% reduction per pass, to a final diameter of 0.055 inches (1.40 mm), resulting in a tensile strength of 1,875 MPa (272 ksi). In a parallel experiment, a bar of the same composition and treated identically was cold drawn in 13 passes at a 35% reduction per pass, to a final diameter of 0.015 inches (0.37 mm), resulting in a resistance of 2,480 MPa (360 ksi).

EXAMPLE 5

This example is a further illustration of the cold working of carbon steel bars whose microstructure consisted of a martensite / austenite and ferrite crystals packing network, in accordance with the present invention, demonstrating the effect of varying relative quantities. of the martensite / austenite and ferrite packing network.

The steel alloy was Fe / 2Si / 0.1C as Examples 3 and 4, and the bars were prepared as described in Example 4, using different reheating temperatures to achieve ferrite contents of 0%, 56%, 66% and 75%, corresponding to contents of martensite / austenite packing network of 100%, 44%, 35% and 25% respectively, all in volume. Stretching schemes similar to those shown in Table II were used for the four microstructures and the resulting tensile strengths are drawn against the true total tension in Figure 2, in which the squares represent 100% packing net alloy, the triangles represent 44% packing net alloy, the circles represent 34% packing net alloy, and the rhombuses represent 25% packing net alloy. The drawing shows that the four microstructures achieved a tensile strength well above 2,000 MPa, and those in which the portions of martensite / austenite packing net exceeded 25% produced greater tensile strengths than the microstructure in which the portion of packing net was 25%.

Claims (15)

1. A process for manufacturing a high strength and high ductility carbon steel alloy, said process comprising:

(to)

 forming a carbon steel alloy having a microstructure comprising martensite networks alternated with retained austenite films, and

(b)

 cold work said carbon steel alloy in a series of passes without intermediate heat treatment between passes to a sufficient reduction to achieve a tensile strength of at least 1034 MPa (150 ksi),

in which step (a) comprises:

(i)

 forming a carbon steel alloy composition having a martensite starting temperature of at least 300 ° C,

(ii)

heating said carbon steel alloy composition to a temperature high enough to cause austenization thereof, to produce a homogeneous austenite phase with all alloy elements in solution, and

(iii) cooling said homogeneous austenite phase through said martensite transition interval at a sufficiently rapid cooling rate to achieve said microstructure, avoiding carbide formation at the interfaces between said martensite networks and said retained austenite films, wherein said carbon steel alloy composition contains from 0.004% to 0.12% carbon, 0% to 11% chromium, 0% to 2.0% manganese and 0% to 2, 0% silicon, all by weight, the rest being iron along with any inevitable impurity. 2. A process for manufacturing a high strength and high ductility carbon steel alloy, said process comprising:

(to)

 forming the carbon steel alloy having a microstructure comprising alternate martensite nets with retained austenite films, and

(b)

 cold work said carbon steel alloy in a series of passes without intermediate heat treatment between passes at a sufficient reduction to achieve a tensile strength of at least

 1,034 MPa (150 ksi), in which step (a) comprises:

(i)

 forming a carbon steel alloy composition having a martensite starting temperature of at least 300 ° C,

(ii)

heating said carbon steel alloy composition to a temperature high enough to cause austenization thereof, to produce a homogeneous austenite phase with all alloy elements in solution,

(iii) cooling said homogeneous austenite phase to transform a portion of said austenite phase into ferrite crystals, thereby forming a biphasic microstructure comprising ferrite crystals fused with austenite crystals, and (iv) cooling said biphasic microstructure through the transition interval of martensite under conditions that cause the conversion of said austenite crystals to a microstructure containing alternate martensite networks with retained austenite films, wherein said carbon steel alloy composition contains 0.02% to 0.14% carbon, 0% to 3.0% silicon, 0% to 1.5% manganese and 0% 1.5% aluminum, all by weight, the rest being iron along with any inevitable impurity.

3.

 A process according to claim 1 or claim 2 wherein step (b) comprises cold working said carbon steel alloy to a sufficient reduction to achieve a tensile strength of 1034 MPa (150 ksi) at 3447 MPa (500 ksi).

Four.

 A process according to claim 1 or claim 2 wherein step (b) comprises working said carbon steel alloy cold to a reduction of the cross-sectional area of at least 20% per pass.

5.

 A process according to claim 1 or claim 2 wherein step (b) comprises working in

cold said steel alloy to a reduction of the cross-sectional area of at least 25% per pass.

6.

 A process according to claim 1 or claim 2 wherein step (b) comprises working said carbon steel alloy cold to a reduction of the cross-sectional area from 25% to 50% per pass.

7.

 A process according to claim 1 or claim 2 wherein step (b) is performed at a temperature of 100 ° C or less.

8.

 A process according to claim 1 or claim 2 wherein step (b) is performed at an ambient temperature of 25 ° C.

9.

 A process according to claim 1 or claim 2 wherein said carbon steel alloy is in the form of a bar or cable, and step (b) comprises stretching said carbon steel alloy through a die.

10.

A process according to claim 1 or claim 2 wherein said carbon steel alloy is in the form of a sheet, and step (b) comprises laminating said carbon steel alloy.

eleven.

A process according to claim 1 wherein said carbon steel alloy composition has a martensite starting temperature of at least 350 ° C.

12.

 A process according to claim 1, wherein said retained austenite films are of a uniform orientation.

13.

 A process according to claim 1 wherein said temperature of step (ii) is 800 ° C to 1150 ° C.

14.

 A process according to claim 2 wherein step (iii) comprises cooling said homogeneous austenite phase to a temperature of 800 ° C to 1,000 ° C.

fifteen.

 A process according to claim 2 wherein step (ii) comprises heating said carbon steel alloy composition to a temperature of 1,050 ° C to 1,170 ° C, and epata (iii) comprises cooling said homogeneous austenite phase to a temperature of 800 ° C to 1,000 ° C.