CA1217997A - High strength, low carbon, dual phase steel rods and wires and process for making same - Google Patents
High strength, low carbon, dual phase steel rods and wires and process for making sameInfo
- Publication number
- CA1217997A CA1217997A CA000442845A CA442845A CA1217997A CA 1217997 A CA1217997 A CA 1217997A CA 000442845 A CA000442845 A CA 000442845A CA 442845 A CA442845 A CA 442845A CA 1217997 A CA1217997 A CA 1217997A
- Authority
- CA
- Canada
- Prior art keywords
- steel
- ferrite
- low carbon
- martensite
- microstructure
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
- B21C1/00—Manufacture of metal sheets, metal wire, metal rods, metal tubes by drawing
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
-
- D—TEXTILES; PAPER
- D07—ROPES; CABLES OTHER THAN ELECTRIC
- D07B—ROPES OR CABLES IN GENERAL
- D07B1/00—Constructional features of ropes or cables
- D07B1/06—Ropes or cables built-up from metal wires, e.g. of section wires around a hemp core
- D07B1/0606—Reinforcing cords for rubber or plastic articles
- D07B1/066—Reinforcing cords for rubber or plastic articles the wires being made from special alloy or special steel composition
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Abstract
ABSTRACT OF THE DISCLOSURE
A high strength, high ductility, low carbon, dual phase steel wire, bar or rod and process for making the same is provided. The steel wire, bar or rod is produced by cold drawing to the desired diameter in a single multipass operation a low carbon steel composition characterized by a duplex microstructure consisting essentially of a strong second phase dispersed in a soft ferrite matrix with a microstructure and morphology having sufficient cold formability to allow reductions in cross-sectional area of up to about 99.9%. Tensile strengths of at least 827 mpa to over 2756 mpa may be obtained.
A high strength, high ductility, low carbon, dual phase steel wire, bar or rod and process for making the same is provided. The steel wire, bar or rod is produced by cold drawing to the desired diameter in a single multipass operation a low carbon steel composition characterized by a duplex microstructure consisting essentially of a strong second phase dispersed in a soft ferrite matrix with a microstructure and morphology having sufficient cold formability to allow reductions in cross-sectional area of up to about 99.9%. Tensile strengths of at least 827 mpa to over 2756 mpa may be obtained.
Description
~79~
HIGH STRENGTH, LOW CARBON, DUAL PHASE STEEL RODS AND
WIRES AND PROCESS FOR MAKING SAME
The present invention is directed to a process for making high-strength, high-ductility, low-carbon steel wires, bars and rods by cold drawing dual-phase steels.
Here, the term "dual-phass steels" refers to a class of steels which are processed by continuous annealing, batch annealing, or conventional hot rolling to obtain a ferrite matrix with a dispersed second phase such as marten site, Bennett and/or retained austenite. The second phase is controlled to be a strong, tough and deformable phase unlike the hard, non-deformable carbide phase found in pyrolytic rods and wires. It must be suitably dispersed and in sufficient volume fraction i.e. greater than 10%, to provide a substantial contra-button to the strength in the as-heat-treated condition and to increase the work hardening rate during wire drawing. Various heat treatment paths can be used to develop the dual-phase micro structure and the morphology depends on the particular heat treatment employed. A
preferred heat treatment is the intermediate quench method i.e. austenitize and quench to 100% marten site prior to annealing in the two phase my field and quenching to a ferrite marten site structure The invention it further directed to the high-strength, high ductility steel wires, bars and rods produced by the process of the present invention.
Steel wire has many known uses, such as for maXir.g cables, chains, and springs. It is also used to make steel belts and bead wire for tires, and steel strands are included in multi strand electrical wire to improve the tensile strength of the wire. In these applique-lions, the diameter requirements range from 0.127 mm to more than 6.35 mm with strength requirement ranging from 1722.5 ma to as much as 2756 ma in the smaller diameters In all of these applications, it is important to provide a steel wire having a high tensile strength and good ductility at the required diameter.
The oldest and most common method of producing high strength, high ductility wire is by patenting a near eutectoid composition pyrolytic steel. However, this process it complex and expensive. A further disadvan-tare of the patenting method is an inherent limitation in the maximum wire diameter that can be produced at a given strength level.
There is a need for steel wire and rods having higher tensile strength and higher ductility than steel wire and rods produced by the known methods, as well as a more economical method for producing high strength steel wire and rods. The present invention would replace the conventional method of patenting pyrolytic steel to produce wiry with a process whereby an alloy of rota-lively simple composition is cold drawn into wire or rods in a jingle multipas~ operation, i.e., without intermediate annealing or patenting heat treatments.
Elimination of the patenting heat treatments in the production of high strength steel wire should lower the cost of producing high strength steel wire, especially in light of the present fuel situation.
The cold drawing process requires a low alloy steel composition with a micro structure and morphology which provides high initial strength, high ductility, rapid work hardening, and good cold formability. The steel should be capable of being cold drawn, without inter-mediate anneals or patenting heat treatments, to the desired diameter, tensile strength, and ductility.
A specific group of steels with a chemical composition specifically developed to impart higher mechanical property values is known in the art as high-strength, low-alloy (HULA) steel. These steels contain carbon as a strengthening element in an amount reasonably consist tent with weld ability and ductility. Various levels and types of alloy carbide former are added to achieve the mechanical properties which characterize these steels.
However, the high tensile strength and high ductility needed in many applications for steel wire and rods do not seem to be attainable using HULA steels.
The factors governing the properties of low carbon steels are primarily its carbon content and micro-structure, and secondarily the residual alloy. Commonly, low carbon steels contain silicon, manganese, or a combination of silicon and manganese. In addition, carbide forming elements such as, vanadium, chromium, I niobium, molybdenum may be added.
Low carbon, dual-phase micro structured steels kirk-terraced by a strong second phase dispersed in a soft ferrite matrix show potential for satisfying the tensile strength ductility, flexibility and diameter require-mints of high strength steel wire. Furthermore, thieve potential for achieving a level of cold formability which allows cold drawing without patenting or inter-mediate heating. In particular, a low carbon, duplexferrite-martensite steel, disclosed in US. Patent No.
4,067,756 issued January 10, 1978, is of interest in the present invention because it has high strength, high ductility characteristics and is composed of inexpensive elements. However, as generally fabricated, it has a tensile strength of about 827 ma which is much lower than the tensile strength required for most applications of high strength steel wire. The process of the present lo invention is directed to producing a high-strength steel wire having a tensile strength of at least about 827 ma A preferred tensile strength range it 827 ma to 2687 ma, but strengths above 2756 ma may be achieved.
It is therefore an object of the present invention to provide an improved process for making high strength, high ductility steel wires and rods and to produce steel wires or rods with increased tensile strength, ductile fly, and flexibility at the desired diameter.
Another object of the present invention is to provide a process for making high strength, high ductility steel wires or rods comprising the step of cold drawing a dual-phase steel composition to the required strength and ductility without intermediate anneals or patenting heat treatments, thereby providing complete flexibility in choosing the wire diameter.
It is another object of the present invention to provide a process for making high strength, high ductility steel wires or rods that eliminates the intermediate patenting step used in the present process for making pyrolytic steel wire, thereby reducing the complexity, cost, and energy consumption of the process for making high strength steel wires and rods.
to A further object of the present invention is to provide a process for making high strength steel wires and rods which is versatile, allowing for a wide range of dime-lens, strength, and ductility properties in the final steel wire or rod based on the choice of the initial duplex micro structure and manipulation of the micro-structure through appropriate thermal processing.
A further object of the present invention is to provide high strength, high ductility steel wires or rods which have a tensile strength at least about 827 ma.
Additional objects and advantages of the present invent lion will become evident from the following description taken in conjunction with the accompanying drawings.
In general, the present invention is directed to high strength, high ductility, low carbon steel wires or rods and the process for making the same. The process involves cold drawing a low carbon dual-phase steel to the desired diameter in a single multi pass operation.
The steel is characterized by a duplex micro structure consisting essentially of a strong second phase dispersed in a soft ferrite matrix and a micro~tructure and morphology having sufficient cold formability to allow reductions in cross-sectional area of up to about 39.9%.
One preferred embodiment of the invention is a high strength high ductility, low carbon steel rod or wire produced from a steel composition characterized by an appropriate duplex ferrite-martensite micro structure, for example as shown in FIG. 1, and the process for making the same The process involves cold drawing the duplex ferrite-martensite steel to the desired diameter ~LZ~9~7 in a single multi pass operation. In high strength steels with a duplex ferrite-martensite micro structure, the strong, deformable second phase consists predominately of marten site but may contain Bennett and retained austenite. the strong second phase is dispersed in a soft ductile ferrite matrix; the marten site provides the strength in the composite whereas the ferrite provides the ductility.
FIG. 1 is an optical micro graph showing a typical low carbon dual-phase ferrite-martensite micro structure prior to cold drawing.
FUGUE. 2 is a transmission electron micro graph of disk located lath marten site which comprises the strong second phase in a dual-phase steel according to the present invention.
FIG. 3 is a graph exemplifying a typical comparison between a cold drawing schedule for duplex microstruc-lure steel wire according to the present invention and a drawing schedule for pyrolytic steel wire according to a patenting method According to the present invention, the high strength, high ductility steel wires or rods are produced by a process whereby a low carbon steel composition, kirk-terraced by a duplex micro structure consisting essential lye of a strong second phase dispersed in a soft ferrite matrix, is cold drawn to the desired diameter in a single multi pass operation. The starting steel compost-lion prior to the cold drawing should possess duplex micro structure and a morphology which are sufficient to provide a level of cold formability allowing reductions I
in cross-sectional area of up to 99.9% during cold drawing.
The process of the invention provides an advantage over known processes in that it eliminates the intermediate heat treatments or patenting steps used in the known process for making pyrolytic steel wire, and thereby reduces the complexity, cost, and energy consumption of the process. Furthermore, a wider range of rod and wire diameter sizes can be produced by the process of the invention than in the patenting method. In the patent-in method, there is an inherent limitation in the maximum wire diameter that can be produced at a given strength level.
Referring to FIG. 3, the differences between the process of the present invention and the patenting process are shown. The solid line illustrates the cold drawing schedule of a low carbon duplex steel wire according to the present invention and the tensile strengths which can be achieved at different diameters. The broken lines indicate the drawing schedule of a pyrolytic steel wire according to the patenting method, including the intermediate heat treatments. In the drawing of the pyrolytic steel wire, the intermediate heat treatments are necessary in order to achieve the greater tensile strength that the process of the present invention can achieve at various diameters. These intermediate heat treatments increase the complexity and expense of the process for making high-strength steel wire, The process according to the present invention does not involve intermediate heat treatments and thus provides a significant improvement over the known process.
The process of the present invention can produce steel wires and rods with a wide range of tensile strength, ductility, and diameter. The final properties of the steel wire or rod at a given diameter are determined by a combination of the initial micro structure, the prop-reties of the starting steel and the amount of subset quint reduction in cross-sectional area during the cold drawing process. Since the micros~ructure of the steel is easily manipulated through appropriate thermal processing, the properties of the drawn wire can be tailored to match the required specifications of the desired application. The choice of alloying elements such as silicon, aluminum, manganese, and carbide forming elements, such as, molybdenum, niobium, vanadium and the like, is determined by the micro structure and properties desired. Thus, a wide range of alloys, including many simple and inexpensive alloys, can be used as long as they can be heat treated to the desired dual-phase micro structure.
One preferred duplex micro structure is the ferrite-marten site micro structure. Another preferred micro-structure is the duplex ferrite-bainite micro structure.
In both cases, the strong second phase, either marten-site or Bennett, is dispersed in a soft, ductile ferrite matrix.
In one preferred embodiment of the process of the present invention the starting steel composition con-sits essentially of iron, from about 0.05 to 0.15 weight % carbon, and from about 1.0 to 3 0 weight %
silicon. In another preferred embodiment the starting steel composition consists essentially of iron, from about 0.05 to 0.15 weight percent carbon, from about 1-3 weight percent silicon, and from byway 0.05 to 0.15 weight percent vanadium. In both preferred embodiments, the steel composition is thermally treated to form a duplex ferrite-martensite micro structure in a fibrous morphology. Briefly, the preferred process comprises the steps of austenitizing the steel composition, quenching the steel composition to transform the austenite to substantially 100% marten site, heating the resulting steel composition to an annealing temperature for a time sufficient to provide the desired ratio of austenite and ferrite, quick quenching the austenite ferrite composition to transform the austenite to marten site, and cold drawing the resulting dual-phase steel which is characterized by a duplex ferrite-marten site micro structure in a fibrous morphology to the desired diameter in a single multi pass operation.
More specifically, the starting steel composition is heated to a temperature, To, above the critical temperature at which austenite forms. The temperature range for I is from about 1050C to 1170C. The composition is held at that temperature for a period of time sufficient to substantially and completely Austin-tire the steel. The resulting composition is quenched in order to transform the austenite to substantially 100~ marten site. The composition is then reheated to an annealing temperature, To, in the two phase (I ye range. The air temperature range is from about 800C to 1000C. The composition it held at this temperature for a period of time sufficient to transform the martensitic steel composition to the desired volume ratio of ferrite and austenite. Upon final quenching, the austenite transforms to marten site, resulting in a strong second phase of marten site dispersed in a soft or ductile ferrite matrix.
The steel composition at this point is characterized by a unique micro structure which is a fine, isotropic, acicular marten site in a ductile ferrite matrix. The micro structure results due to the combination of the double heat treatment and the presence of silicon in the above-specified amount. The unique micro structure maximizes the potential ductility of the soft phase ferrite and also fully exploits the strong marten site phase as a load carrying constituent in the duplex micro structure. It is the micro structure as well as the morphology of the steel composition that enables the steel to be cold drawn to the desired wire or rod diameter in a single multi pass operation.
Any dual-phase steel may be used in the process of the present invention as long as a duplex micro structure and morphology can be produced having sufficient cold formability to allow reductions in cross-sectional area of up to about 99.9% when the composition is cold drawn.
In particular, dual-phase ferrite marten site steels have a greater continuous yielding behavior, higher ultimate tensile strength, and better ductility than commercial high strength low alloy steels, including MacWorld, fine-grain steels. Furthermore, the high tensile to yield ratio and high strain hardening rate in ferrite-US marten site dual-phase steel provides excellent cold formability.
The exact temperature, To, to which the steel compost-lion is heated in the first austenization step is not critical as long as it is above the temperature at which complete austenization occurs. The exact temperature, To, in the second heating step where the composition is transformed to the two phases of ferrite and as-twenty depends upon the desired volume ratio of ferrite I
and austenite, which in turn depends upon the desired volume ratio of ferrite-to-martensite. In general, the desired volume ratio of ferrite and marten site depends upon the ultimate properties desired for the steel wire or rod. Generally, 10-40 volume percent of marten site in the ferrite-martensite micro structure will allow the steel composition to be cold drawn to diameters repro-setting up to 99.9~ reductions in cross sectional area and will still result in steel wires and rods having a lo tensile strength at least about 827 ma. Usually, tensile strengths in the range of 827 ma to 2687 ma are obtained, but 2756 ma and above may also be obtained.
The following examples will illustrate the process of the invention more clearly, the resulting properties of the steel wires and rods produced by the process, as well as the flexibility of the process in allowing a choice of alloys, tensile strengths, ductility, and diameters.
A high strength, high ductility steel wire was made to satisfy the requirements for bead wire used in the manufacture of automobile tires. The bead wire requires a tensile strength of 1860 ma with I elongation, and a proportional limit of 1488 ma. The bead wire should be about 0.94 mm in diameter with sufficient ductility to pass a torsion test requiring 58 axial twists in a 203 mm length. A 5.6 mm diameter steel rod having a composition consisting essentially of iron, 0.1 weight percent carbon, 2 weight percent silicon, and 0.1 weight percent vanadium was austenitized and rapidly quenched to yield a substantially 100% martensitic composition. The rod was then reheated to a temperature of 950 C in the two phase I range and rapidly quenched to produce a duplex ferrite-martensite micro-structure of approximately 30 volume percent marten site and 70 volume percent ferrite. The needle-like, act-cuter character of the ferrite-martensite micro structure is shown in the optical micro graph in FIG. 1. The heat treated rod was then cold drawn through lubricated conical dies down to a diameter of 0.94 mm in 8 passes of approximately 36% reduction in area per pass.
After a short stress relief anneal at 425C similar to the current practice, an ultimate tensile strength of 1902 ma was achieved, thus satisfying the tensile strength requirement of bead wire. The ductility of the steel wire was sufficient to satisfy the twist test requirement.
Example 2 A steel rod consisting essentially of iron, 0.1 weight percent carbon, and 2.0 weight percent silicon was hot rolled to a diameter of 6.4 mm. The rod was then heated to a temperature of about 1150C for about 30 minutes to austenitize the composition. the steel was then quenched in iced brine to transform the austenite to substantially 100~ marten site. The rod was then rapidly reheated to a temperature of 950C in order to convert the structure to approximately 70% ferrite and 30% austenite~ The steel rod was then quenched in iced brine to convert the austenite to marten site. Finally, the rod was cold drawn to a diameter of 0.76 mm where its tensile strength was 2460 ma, and also drawn to a diameter of 0.61 mm where its tensile strength was 2480 ma. Continued cold drawing may achieve tensile strengths to 2756 ma or higher.
HIGH STRENGTH, LOW CARBON, DUAL PHASE STEEL RODS AND
WIRES AND PROCESS FOR MAKING SAME
The present invention is directed to a process for making high-strength, high-ductility, low-carbon steel wires, bars and rods by cold drawing dual-phase steels.
Here, the term "dual-phass steels" refers to a class of steels which are processed by continuous annealing, batch annealing, or conventional hot rolling to obtain a ferrite matrix with a dispersed second phase such as marten site, Bennett and/or retained austenite. The second phase is controlled to be a strong, tough and deformable phase unlike the hard, non-deformable carbide phase found in pyrolytic rods and wires. It must be suitably dispersed and in sufficient volume fraction i.e. greater than 10%, to provide a substantial contra-button to the strength in the as-heat-treated condition and to increase the work hardening rate during wire drawing. Various heat treatment paths can be used to develop the dual-phase micro structure and the morphology depends on the particular heat treatment employed. A
preferred heat treatment is the intermediate quench method i.e. austenitize and quench to 100% marten site prior to annealing in the two phase my field and quenching to a ferrite marten site structure The invention it further directed to the high-strength, high ductility steel wires, bars and rods produced by the process of the present invention.
Steel wire has many known uses, such as for maXir.g cables, chains, and springs. It is also used to make steel belts and bead wire for tires, and steel strands are included in multi strand electrical wire to improve the tensile strength of the wire. In these applique-lions, the diameter requirements range from 0.127 mm to more than 6.35 mm with strength requirement ranging from 1722.5 ma to as much as 2756 ma in the smaller diameters In all of these applications, it is important to provide a steel wire having a high tensile strength and good ductility at the required diameter.
The oldest and most common method of producing high strength, high ductility wire is by patenting a near eutectoid composition pyrolytic steel. However, this process it complex and expensive. A further disadvan-tare of the patenting method is an inherent limitation in the maximum wire diameter that can be produced at a given strength level.
There is a need for steel wire and rods having higher tensile strength and higher ductility than steel wire and rods produced by the known methods, as well as a more economical method for producing high strength steel wire and rods. The present invention would replace the conventional method of patenting pyrolytic steel to produce wiry with a process whereby an alloy of rota-lively simple composition is cold drawn into wire or rods in a jingle multipas~ operation, i.e., without intermediate annealing or patenting heat treatments.
Elimination of the patenting heat treatments in the production of high strength steel wire should lower the cost of producing high strength steel wire, especially in light of the present fuel situation.
The cold drawing process requires a low alloy steel composition with a micro structure and morphology which provides high initial strength, high ductility, rapid work hardening, and good cold formability. The steel should be capable of being cold drawn, without inter-mediate anneals or patenting heat treatments, to the desired diameter, tensile strength, and ductility.
A specific group of steels with a chemical composition specifically developed to impart higher mechanical property values is known in the art as high-strength, low-alloy (HULA) steel. These steels contain carbon as a strengthening element in an amount reasonably consist tent with weld ability and ductility. Various levels and types of alloy carbide former are added to achieve the mechanical properties which characterize these steels.
However, the high tensile strength and high ductility needed in many applications for steel wire and rods do not seem to be attainable using HULA steels.
The factors governing the properties of low carbon steels are primarily its carbon content and micro-structure, and secondarily the residual alloy. Commonly, low carbon steels contain silicon, manganese, or a combination of silicon and manganese. In addition, carbide forming elements such as, vanadium, chromium, I niobium, molybdenum may be added.
Low carbon, dual-phase micro structured steels kirk-terraced by a strong second phase dispersed in a soft ferrite matrix show potential for satisfying the tensile strength ductility, flexibility and diameter require-mints of high strength steel wire. Furthermore, thieve potential for achieving a level of cold formability which allows cold drawing without patenting or inter-mediate heating. In particular, a low carbon, duplexferrite-martensite steel, disclosed in US. Patent No.
4,067,756 issued January 10, 1978, is of interest in the present invention because it has high strength, high ductility characteristics and is composed of inexpensive elements. However, as generally fabricated, it has a tensile strength of about 827 ma which is much lower than the tensile strength required for most applications of high strength steel wire. The process of the present lo invention is directed to producing a high-strength steel wire having a tensile strength of at least about 827 ma A preferred tensile strength range it 827 ma to 2687 ma, but strengths above 2756 ma may be achieved.
It is therefore an object of the present invention to provide an improved process for making high strength, high ductility steel wires and rods and to produce steel wires or rods with increased tensile strength, ductile fly, and flexibility at the desired diameter.
Another object of the present invention is to provide a process for making high strength, high ductility steel wires or rods comprising the step of cold drawing a dual-phase steel composition to the required strength and ductility without intermediate anneals or patenting heat treatments, thereby providing complete flexibility in choosing the wire diameter.
It is another object of the present invention to provide a process for making high strength, high ductility steel wires or rods that eliminates the intermediate patenting step used in the present process for making pyrolytic steel wire, thereby reducing the complexity, cost, and energy consumption of the process for making high strength steel wires and rods.
to A further object of the present invention is to provide a process for making high strength steel wires and rods which is versatile, allowing for a wide range of dime-lens, strength, and ductility properties in the final steel wire or rod based on the choice of the initial duplex micro structure and manipulation of the micro-structure through appropriate thermal processing.
A further object of the present invention is to provide high strength, high ductility steel wires or rods which have a tensile strength at least about 827 ma.
Additional objects and advantages of the present invent lion will become evident from the following description taken in conjunction with the accompanying drawings.
In general, the present invention is directed to high strength, high ductility, low carbon steel wires or rods and the process for making the same. The process involves cold drawing a low carbon dual-phase steel to the desired diameter in a single multi pass operation.
The steel is characterized by a duplex micro structure consisting essentially of a strong second phase dispersed in a soft ferrite matrix and a micro~tructure and morphology having sufficient cold formability to allow reductions in cross-sectional area of up to about 39.9%.
One preferred embodiment of the invention is a high strength high ductility, low carbon steel rod or wire produced from a steel composition characterized by an appropriate duplex ferrite-martensite micro structure, for example as shown in FIG. 1, and the process for making the same The process involves cold drawing the duplex ferrite-martensite steel to the desired diameter ~LZ~9~7 in a single multi pass operation. In high strength steels with a duplex ferrite-martensite micro structure, the strong, deformable second phase consists predominately of marten site but may contain Bennett and retained austenite. the strong second phase is dispersed in a soft ductile ferrite matrix; the marten site provides the strength in the composite whereas the ferrite provides the ductility.
FIG. 1 is an optical micro graph showing a typical low carbon dual-phase ferrite-martensite micro structure prior to cold drawing.
FUGUE. 2 is a transmission electron micro graph of disk located lath marten site which comprises the strong second phase in a dual-phase steel according to the present invention.
FIG. 3 is a graph exemplifying a typical comparison between a cold drawing schedule for duplex microstruc-lure steel wire according to the present invention and a drawing schedule for pyrolytic steel wire according to a patenting method According to the present invention, the high strength, high ductility steel wires or rods are produced by a process whereby a low carbon steel composition, kirk-terraced by a duplex micro structure consisting essential lye of a strong second phase dispersed in a soft ferrite matrix, is cold drawn to the desired diameter in a single multi pass operation. The starting steel compost-lion prior to the cold drawing should possess duplex micro structure and a morphology which are sufficient to provide a level of cold formability allowing reductions I
in cross-sectional area of up to 99.9% during cold drawing.
The process of the invention provides an advantage over known processes in that it eliminates the intermediate heat treatments or patenting steps used in the known process for making pyrolytic steel wire, and thereby reduces the complexity, cost, and energy consumption of the process. Furthermore, a wider range of rod and wire diameter sizes can be produced by the process of the invention than in the patenting method. In the patent-in method, there is an inherent limitation in the maximum wire diameter that can be produced at a given strength level.
Referring to FIG. 3, the differences between the process of the present invention and the patenting process are shown. The solid line illustrates the cold drawing schedule of a low carbon duplex steel wire according to the present invention and the tensile strengths which can be achieved at different diameters. The broken lines indicate the drawing schedule of a pyrolytic steel wire according to the patenting method, including the intermediate heat treatments. In the drawing of the pyrolytic steel wire, the intermediate heat treatments are necessary in order to achieve the greater tensile strength that the process of the present invention can achieve at various diameters. These intermediate heat treatments increase the complexity and expense of the process for making high-strength steel wire, The process according to the present invention does not involve intermediate heat treatments and thus provides a significant improvement over the known process.
The process of the present invention can produce steel wires and rods with a wide range of tensile strength, ductility, and diameter. The final properties of the steel wire or rod at a given diameter are determined by a combination of the initial micro structure, the prop-reties of the starting steel and the amount of subset quint reduction in cross-sectional area during the cold drawing process. Since the micros~ructure of the steel is easily manipulated through appropriate thermal processing, the properties of the drawn wire can be tailored to match the required specifications of the desired application. The choice of alloying elements such as silicon, aluminum, manganese, and carbide forming elements, such as, molybdenum, niobium, vanadium and the like, is determined by the micro structure and properties desired. Thus, a wide range of alloys, including many simple and inexpensive alloys, can be used as long as they can be heat treated to the desired dual-phase micro structure.
One preferred duplex micro structure is the ferrite-marten site micro structure. Another preferred micro-structure is the duplex ferrite-bainite micro structure.
In both cases, the strong second phase, either marten-site or Bennett, is dispersed in a soft, ductile ferrite matrix.
In one preferred embodiment of the process of the present invention the starting steel composition con-sits essentially of iron, from about 0.05 to 0.15 weight % carbon, and from about 1.0 to 3 0 weight %
silicon. In another preferred embodiment the starting steel composition consists essentially of iron, from about 0.05 to 0.15 weight percent carbon, from about 1-3 weight percent silicon, and from byway 0.05 to 0.15 weight percent vanadium. In both preferred embodiments, the steel composition is thermally treated to form a duplex ferrite-martensite micro structure in a fibrous morphology. Briefly, the preferred process comprises the steps of austenitizing the steel composition, quenching the steel composition to transform the austenite to substantially 100% marten site, heating the resulting steel composition to an annealing temperature for a time sufficient to provide the desired ratio of austenite and ferrite, quick quenching the austenite ferrite composition to transform the austenite to marten site, and cold drawing the resulting dual-phase steel which is characterized by a duplex ferrite-marten site micro structure in a fibrous morphology to the desired diameter in a single multi pass operation.
More specifically, the starting steel composition is heated to a temperature, To, above the critical temperature at which austenite forms. The temperature range for I is from about 1050C to 1170C. The composition is held at that temperature for a period of time sufficient to substantially and completely Austin-tire the steel. The resulting composition is quenched in order to transform the austenite to substantially 100~ marten site. The composition is then reheated to an annealing temperature, To, in the two phase (I ye range. The air temperature range is from about 800C to 1000C. The composition it held at this temperature for a period of time sufficient to transform the martensitic steel composition to the desired volume ratio of ferrite and austenite. Upon final quenching, the austenite transforms to marten site, resulting in a strong second phase of marten site dispersed in a soft or ductile ferrite matrix.
The steel composition at this point is characterized by a unique micro structure which is a fine, isotropic, acicular marten site in a ductile ferrite matrix. The micro structure results due to the combination of the double heat treatment and the presence of silicon in the above-specified amount. The unique micro structure maximizes the potential ductility of the soft phase ferrite and also fully exploits the strong marten site phase as a load carrying constituent in the duplex micro structure. It is the micro structure as well as the morphology of the steel composition that enables the steel to be cold drawn to the desired wire or rod diameter in a single multi pass operation.
Any dual-phase steel may be used in the process of the present invention as long as a duplex micro structure and morphology can be produced having sufficient cold formability to allow reductions in cross-sectional area of up to about 99.9% when the composition is cold drawn.
In particular, dual-phase ferrite marten site steels have a greater continuous yielding behavior, higher ultimate tensile strength, and better ductility than commercial high strength low alloy steels, including MacWorld, fine-grain steels. Furthermore, the high tensile to yield ratio and high strain hardening rate in ferrite-US marten site dual-phase steel provides excellent cold formability.
The exact temperature, To, to which the steel compost-lion is heated in the first austenization step is not critical as long as it is above the temperature at which complete austenization occurs. The exact temperature, To, in the second heating step where the composition is transformed to the two phases of ferrite and as-twenty depends upon the desired volume ratio of ferrite I
and austenite, which in turn depends upon the desired volume ratio of ferrite-to-martensite. In general, the desired volume ratio of ferrite and marten site depends upon the ultimate properties desired for the steel wire or rod. Generally, 10-40 volume percent of marten site in the ferrite-martensite micro structure will allow the steel composition to be cold drawn to diameters repro-setting up to 99.9~ reductions in cross sectional area and will still result in steel wires and rods having a lo tensile strength at least about 827 ma. Usually, tensile strengths in the range of 827 ma to 2687 ma are obtained, but 2756 ma and above may also be obtained.
The following examples will illustrate the process of the invention more clearly, the resulting properties of the steel wires and rods produced by the process, as well as the flexibility of the process in allowing a choice of alloys, tensile strengths, ductility, and diameters.
A high strength, high ductility steel wire was made to satisfy the requirements for bead wire used in the manufacture of automobile tires. The bead wire requires a tensile strength of 1860 ma with I elongation, and a proportional limit of 1488 ma. The bead wire should be about 0.94 mm in diameter with sufficient ductility to pass a torsion test requiring 58 axial twists in a 203 mm length. A 5.6 mm diameter steel rod having a composition consisting essentially of iron, 0.1 weight percent carbon, 2 weight percent silicon, and 0.1 weight percent vanadium was austenitized and rapidly quenched to yield a substantially 100% martensitic composition. The rod was then reheated to a temperature of 950 C in the two phase I range and rapidly quenched to produce a duplex ferrite-martensite micro-structure of approximately 30 volume percent marten site and 70 volume percent ferrite. The needle-like, act-cuter character of the ferrite-martensite micro structure is shown in the optical micro graph in FIG. 1. The heat treated rod was then cold drawn through lubricated conical dies down to a diameter of 0.94 mm in 8 passes of approximately 36% reduction in area per pass.
After a short stress relief anneal at 425C similar to the current practice, an ultimate tensile strength of 1902 ma was achieved, thus satisfying the tensile strength requirement of bead wire. The ductility of the steel wire was sufficient to satisfy the twist test requirement.
Example 2 A steel rod consisting essentially of iron, 0.1 weight percent carbon, and 2.0 weight percent silicon was hot rolled to a diameter of 6.4 mm. The rod was then heated to a temperature of about 1150C for about 30 minutes to austenitize the composition. the steel was then quenched in iced brine to transform the austenite to substantially 100~ marten site. The rod was then rapidly reheated to a temperature of 950C in order to convert the structure to approximately 70% ferrite and 30% austenite~ The steel rod was then quenched in iced brine to convert the austenite to marten site. Finally, the rod was cold drawn to a diameter of 0.76 mm where its tensile strength was 2460 ma, and also drawn to a diameter of 0.61 mm where its tensile strength was 2480 ma. Continued cold drawing may achieve tensile strengths to 2756 ma or higher.
Claims (28)
1. A process for making high strength, high ductility steel wire and rods comprising the step of cold drawing to the desired diameter in a single multipass operation a low carbon dual-phase steel composition characterized by a microstructure consisting essentially of a strong second phase dispersed in a soft ferrite matrix and a microstructure and morphology having sufficient cold formability to allow reductions in cross- sectional area of up to about 99.9%.
2. A process according to Claim 1 wherein said steel composition is characterized by a duplex ferrite-marten-site microstructure, said martensite comprising the strong second phase dispersed in said soft ferrite matrix.
3. A process according to Claim 1 wherein said steel composition is characterized by a duplex ferrite-bainite microstructure, said bainite comprising the strong second phase dispersed in said soft ferrite matrix.
4. A process for making high strength, high ductility steel wire and rods comprising the steps of:
heating a steel composition consisting essentially of iron, from about 0.05 to 0.15 wt. % carbon, and from about 1.0 to 3.0 wt. % silicon to a temperature, T1, for a period of time sufficient to substantially completely austenitize said steel;
quenching the resulting austenitized steel compo-sition to transform said austenite to 100% martensite;
heating the resulting martensitic steel composition to a temperature, T2, in the two phase .alpha. + .gamma. range for a period of time sufficient to transform said marten-sitic steel composition to a desired volume ratio of ferrite and austenite;
quenching the resulting ferrite-austenite steel composition to transform the austenite to martensite;
and cold drawing the resulting steel composition characterized by a duplex ferrite-martensite microstruc-ture to the desired diameter.
heating a steel composition consisting essentially of iron, from about 0.05 to 0.15 wt. % carbon, and from about 1.0 to 3.0 wt. % silicon to a temperature, T1, for a period of time sufficient to substantially completely austenitize said steel;
quenching the resulting austenitized steel compo-sition to transform said austenite to 100% martensite;
heating the resulting martensitic steel composition to a temperature, T2, in the two phase .alpha. + .gamma. range for a period of time sufficient to transform said marten-sitic steel composition to a desired volume ratio of ferrite and austenite;
quenching the resulting ferrite-austenite steel composition to transform the austenite to martensite;
and cold drawing the resulting steel composition characterized by a duplex ferrite-martensite microstruc-ture to the desired diameter.
5. A process according to Claim 4 wherein the cold drawing step comprises a single multipass cold drawing step.
6. A process according to Claim 4 wherein said steel composition consists essentially of iron, about 0.1 wt.
% carbon, and about 2 wt. % silicon.
% carbon, and about 2 wt. % silicon.
7. A process according to Claim 4 wherein said steel composition contains from about 0.05 to 0.15 wt. %
vanadium.
vanadium.
8. A process according to Claim 7 wherein said vana-dium content is about 0.1 wt. percent.
9. A process according to Claim 4 or 7 wherein T1 is in the range from about 1050°C to 1170°C and T2 is in the range from about 800°C to 1000°C.
10. A process according to Claim 4 or 7 wherein T1 is about 1150°C.
11. A process according to claim 2, 4 or 7 wherein said steel composition is heated at T2 for a period of time sufficient to achieve a ratio of ferrite and austenite such that subsequent quenching results in a microstructure containing 10-40 volume percent martensite.
12. A process according to claim 2, 4 or 7 wherein said steel composition is heated at about 950°C for a period of time sufficient to achieve a ratio of ferrite and austenite such that the subsequent quenching results in a microstructure containing about 30 volume per cent martensite.
13. A high strength, high ductility, low carbon steel product having a tensile strength of at least 827 mpa and having been cold drawn from a steel composition characterized by a micro-structure consisting essentially of a strong second phase dispersed in a soft ferrite matrix.
14. A product according to claim 13 wherein said tensile strength is in the range 827 mpa to 2687 mpa.
15. A steel product according to claim 14 wherein said product is steel wire.
16. A steel product according to claim 14 wherein said product is steel bar or rod.
17. A high strength, high ductility, low carbon steel product according to claim 15 or 16 wherein said microstructure is a cold-drawn duplex ferrite-martensite microstructure.
18. A high strength, high ductility, low carbon steel product according to claim 15 or 16 wherein said microstructure is a cold-drawn duplex ferrite-bainite microstructure.
19. A high strength, high ductility, low carbon steel product having a steel composition consisting essentially of iron, from about 0.05 to .15 wt. % carbon, and from about 1 to 3 wt. % silicon, having a tensile strength of at least 827 mpa and having been cold drawn from a steel composition characterized by a duplex ferrite-martensite microstructure.
20. A product according to claim 19 wherein said tensile strength is in the range of 827 mpa to 2687 mpa.
21. A steel product according to claim 20 wherein said product is steel wire.
22. A steel product according to claim 20 wherein said product is steel bar or rod.
23. A high strength, high ductility, low carbon steel product according to claim 21 or 22 wherein said carbon content is about .1 wt. %, said silicon content is about 2 wt. %, said duplex ferrite-martensite microstructure comprises about 30 volume per cent martensite, and said tensile strength is from about 2460 mpa to about 2480 mpa after about 99.9% reduction in cross-sectional area.
24. A high strength, high ductility, low carbon steel product according to claim 21 or 22 wherein the steel composition contains from about 0.05 to 0.15 wt. % vanadium.
25. A high strength, high ductility, low carbon steel product according to claim 21 or 22 wherein said carbon content is about 0.1 wt. %, said silicon content is about 2.0 wt. %, said vanadium content is about 0.1 wt. %, said duplex ferrite-martensite microstructure comprises 30 volume per cent martensite, and said tensile strength is about 1902 mpa after about 97% reduc-tion in cross-sectional area.
26. A high strength, high ductility, low carbon steel product produced by the process of claim 1.
27. A high strength, high ductility, low carbon steel product produced by the process of claim 4.
28. A high strength, high ductility, low carbon steel product with a steel composition consisting essentially of iron, from about 0.05 to .15 wt. % carbon, from about 1 to 3 wt. % silicon, and from about 0.05 to .15 wt. % vanadium, produced by the process of claim 4.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| PCT/US1982/001722 WO1984002354A1 (en) | 1982-12-09 | 1982-12-09 | High strength, low carbon, dual phase steel rods and wires and process for making same |
| US82/01722 | 1982-12-09 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1217997A true CA1217997A (en) | 1987-02-17 |
Family
ID=22168441
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000442845A Expired CA1217997A (en) | 1982-12-09 | 1983-12-08 | High strength, low carbon, dual phase steel rods and wires and process for making same |
Country Status (16)
| Country | Link |
|---|---|
| EP (1) | EP0128139A4 (en) |
| JP (1) | JPS60500177A (en) |
| KR (2) | KR890003401B1 (en) |
| AU (1) | AU561976B2 (en) |
| BR (1) | BR8208108A (en) |
| CA (1) | CA1217997A (en) |
| DK (1) | DK359084D0 (en) |
| ES (1) | ES8504946A1 (en) |
| FI (1) | FI78929C (en) |
| IN (1) | IN157840B (en) |
| IT (1) | IT1194512B (en) |
| NO (1) | NO843184L (en) |
| NZ (1) | NZ206472A (en) |
| PT (1) | PT77796B (en) |
| WO (1) | WO1984002354A1 (en) |
| ZA (1) | ZA83757B (en) |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4578124A (en) * | 1984-01-20 | 1986-03-25 | Kabushiki Kaisha Kobe Seiko Sho | High strength low carbon steels, steel articles thereof and method for manufacturing the steels |
| CA1332210C (en) * | 1985-08-29 | 1994-10-04 | Masaaki Katsumata | High strength low carbon steel wire rods and method of producing them |
| FR2672827A1 (en) * | 1991-02-14 | 1992-08-21 | Michelin & Cie | METALLIC WIRE COMPRISING A STEEL SUBSTRATE HAVING A WRINKLE - TYPE RECTANGULAR STRUCTURE AND A COATING; METHOD FOR OBTAINING THIS WIRE. |
| FR2743573A1 (en) * | 1996-01-16 | 1997-07-18 | Michelin & Cie | METAL WIRE READY FOR USE AND METHOD FOR OBTAINING THREAD |
| SG138444A1 (en) * | 2002-12-04 | 2008-01-28 | Inventio Ag | Reinforced synthetic cable for lifts |
| EP1428927B1 (en) * | 2002-12-04 | 2008-02-27 | Inventio Ag | Reinforced synthetic cable for lifts |
| BRPI0908575A2 (en) * | 2008-03-04 | 2015-09-22 | Bekaert Sa Nv | cold drawn low carbon steel filament and method of manufacturing said filament |
| WO2014083535A2 (en) | 2012-11-30 | 2014-06-05 | Pirelli Tyre S.P.A. | Reinforcement cord and tyre for vehicle wheels comprising such a reinforcement cord |
Family Cites Families (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3423252A (en) * | 1965-04-01 | 1969-01-21 | United States Steel Corp | Thermomechanical treatment of steel |
| US3502514A (en) * | 1968-01-30 | 1970-03-24 | United States Steel Corp | Method of processing steel |
| SE335547B (en) * | 1970-02-11 | 1971-06-01 | Fagersta Bruks Ab | |
| FR2238768A1 (en) * | 1973-07-23 | 1975-02-21 | Sgtm | Thermo-mechanical treatment of austenitic steel - followed by controlled quenching giving mech props similar to expensive alloys |
| JPS51144329A (en) * | 1975-06-09 | 1976-12-11 | Kobe Steel Ltd | Process for producing high tensile stregth wire rod of coiled figure |
| US4088511A (en) * | 1976-07-29 | 1978-05-09 | Lasalle Steel Company | Steels combining toughness and machinability |
| US4067756A (en) * | 1976-11-02 | 1978-01-10 | The United States Of America As Represented By The United States Department Of Energy | High strength, high ductility low carbon steel |
| JPS57126913A (en) * | 1981-01-27 | 1982-08-06 | Kobe Steel Ltd | Production of high-toughness high-strength wire or rod steel |
| JPH05235118A (en) * | 1992-02-24 | 1993-09-10 | Nec Yamagata Ltd | Semiconductor element |
-
1982
- 1982-12-09 EP EP19830900309 patent/EP0128139A4/en not_active Withdrawn
- 1982-12-09 JP JP83500427A patent/JPS60500177A/en active Pending
- 1982-12-09 AU AU11087/83A patent/AU561976B2/en not_active Ceased
- 1982-12-09 WO PCT/US1982/001722 patent/WO1984002354A1/en not_active Application Discontinuation
- 1982-12-09 BR BR8208108A patent/BR8208108A/en unknown
-
1983
- 1983-02-01 IN IN117/CAL/83A patent/IN157840B/en unknown
- 1983-02-04 ZA ZA83757A patent/ZA83757B/en unknown
- 1983-12-02 NZ NZ206472A patent/NZ206472A/en unknown
- 1983-12-08 CA CA000442845A patent/CA1217997A/en not_active Expired
- 1983-12-09 KR KR1019830005825A patent/KR890003401B1/en not_active IP Right Cessation
- 1983-12-09 ES ES528241A patent/ES8504946A1/en not_active Expired
- 1983-12-09 PT PT77796A patent/PT77796B/en not_active IP Right Cessation
- 1983-12-09 IT IT24103/83A patent/IT1194512B/en active
-
1984
- 1984-07-20 FI FI842931A patent/FI78929C/en not_active IP Right Cessation
- 1984-07-20 DK DK359084A patent/DK359084D0/en not_active Application Discontinuation
- 1984-08-08 NO NO843184A patent/NO843184L/en unknown
-
1989
- 1989-06-28 KR KR1019890009331A patent/KR890003402B1/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| IT8324103A0 (en) | 1983-12-09 |
| NZ206472A (en) | 1986-03-14 |
| FI78929C (en) | 1989-10-10 |
| EP0128139A1 (en) | 1984-12-19 |
| ES528241A0 (en) | 1985-04-16 |
| IT1194512B (en) | 1988-09-22 |
| DK359084A (en) | 1984-07-20 |
| FI842931A0 (en) | 1984-07-20 |
| ZA83757B (en) | 1984-03-28 |
| PT77796A (en) | 1984-01-01 |
| BR8208108A (en) | 1984-12-11 |
| AU1108783A (en) | 1984-07-05 |
| KR890003402B1 (en) | 1989-09-20 |
| FI842931A (en) | 1984-07-20 |
| JPS60500177A (en) | 1985-02-07 |
| KR840006920A (en) | 1984-12-04 |
| KR890003401B1 (en) | 1989-09-20 |
| EP0128139A4 (en) | 1985-09-16 |
| PT77796B (en) | 1986-03-27 |
| NO843184L (en) | 1984-08-08 |
| IN157840B (en) | 1986-07-05 |
| ES8504946A1 (en) | 1985-04-16 |
| FI78929B (en) | 1989-06-30 |
| AU561976B2 (en) | 1987-05-21 |
| WO1984002354A1 (en) | 1984-06-21 |
| DK359084D0 (en) | 1984-07-20 |
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