Process for producing patented steel wire

Abstract: This invention discloses a process for producing a high strength filament, said process comprising the steps of: (1) heating a steel wire to a temperature which is within the range of about 850°C to about 1100°C for a period of at least about 2 seconds; wherein said steel wire consists essentially of about 96.61 to about 98.905 weight percent iron, from about 0.72 to about 1.04 weight percent carbon, from about 0.3 to about 0.8 weight percent manganese, from about 0.05 to about 0.4 weight percent silicon, from about 0.02 to about 0.3 weight percent copper, and from about 0.005 to about 0.85 weight percent of at least one member selected from the group consisting of chromium, vanadium, nickel and boron, with the proviso that the total amount of silicon, manganese, chromium, vanadium, nickel and boron in the microalloyed high carbon steel is within the range of about 0.7 to 0.9 weight percent to produce a heated steel wire; (2) continuously cooling the heated steel wire at a cooling rate of less than about 60°C per second until a transformation from austenite to pearlite begins; (3) allowing the transformation from austenite to pearlite to proceed with an increase in the wire temperature resulting from recalescence to produce a patented steel wire; (4) cooling the patented steel wire to ambient temperature; (5) brass-plating the patented steel wire to produce a brass-plated wire; and (6) cold-drawing the brass-plated steel wire to a diameter which is within the range of about 0.10 mm to about 0.45 mm to produce a high strength filament.

Description: Background of the Invention It is frequently desirable to reinforce rubber articles, for example, tires, conveyor belts, power transmission belts, timing belts, hoses and the like products, by incorporating therein steel reinforcing elements. Pneumatic vehicle tires are often reinforced with cords prepared from brass-coated steel filaments. Such tire cords are frequently composed of high carbon steel or high carbon steel coated with a thin layer of brass. Such a tire cord can be a monofilament, but normally is prepared from several filaments which are stranded or bunched together. In some instances, depending upon the type of tire being reinforced, the strands of filaments are further cabled to form the tire cord. It is important for the steel alloy utilized in filaments for reinforcing elements to exhibit high strength and ductility as well as high fatigue resistance. Unfortunately, many alloys which possess this demanding combination of requisite properties cannot be processed in a practical commercial operation. The alloys which have proved to be commercially important have typically required a patenting procedure wherein they are subjected to an isothermal transformation from austenite to pearlite. US-A-5,167,727 describes such a process wherein steel filaments are manufactured utilizing a patenting step wherein the transformation from austenite to pearlite is carried out under isothermal conditions at a temperature which is within the range of 540°C to 620°C. Such isothermal transformations are normally carried out in a fluidized bed or in a molten lead medium to maintain a constant temperature for the duration of the transformation. However, the utilization of such an isothermal transformation step requires special equipment and adds to the cost of the patenting procedure. A fine lamellar spacing between carbide and ferrite platelets in the patented steel wire is required to develop high tensile strengths while maintaining the good ductility required for drawing the wire. To achieve this goal, small quantities of various alloying metals are sometimes added to the steel in order to improve the mechanical properties which can be attained by using isothermal patenting techniques. An alternative to isothermal patenting is continuous cooling or "air" patenting. In this process, high carbon steel wire is allowed to cool in air or other gas, such as cracked ammonia, which can be either still or forced in order to control the rate of cooling. This process typically produces a microstructure which has a lamellar structure which is somewhat coarser than that achieved with isothermal patenting. As a result, the tensile strength of the wire is significantly lower than that achieved by isothermal patenting and filaments drawing from the wire have lower tensile strengths. An additional drawback to the use of continuous cooling in patenting procedures is that as the diameter of the wire increases, the rate at which the wire cools is reduced and the microstructure becomes even coarser. As a result, it is more difficult to produce wires of a larger diameter with acceptable properties. EP-A- 0 620 284 discloses a process for producing a patented steel wire having a microstructure which is essentially pearlite with a very fine lamellar spacing between carbide and ferrite platelets which has good ductility and which can be drawn to develop high tensile strength, said process comprising the steps of: (1) heating a steel wire to a temperature which is within the range of approximately 850°C to 1050°C for a period of at least 2 seconds; wherein said steel wire is comprised of a microalloyed high carbon steel which consists essentially of 97.03 to 98.925 weight percent iron, from 0.72 to 0.92 weight percent carbon, from 0.3 to 0.8 weight percent manganese, from 0.05 to 0.4 weight percent silicon, and from 0.005 to 0.85 weight percent of at least one member selected from the group consisting of chromium, vanadium, nickel, and boron, with the proviso that the total amount of silicon, manganese, chromium, vanadium, nickel, and boron in the microalloyed high carbon steel is within the range of 0.7 to 0.9 weight percent; (2) continuously cooling the steel wire at a cooling rate of less than 100°C per second until a transformation from austenite to pearlite begins; (3) allowing the transformation from austenite to pearlite to proceed with an increase in the wire temperature resulting from recalescence; and (4) cooling the patented steel wire to ambient temperature. Summary of the Invention This invention discloses a process for producing a high strength filament for use in elastomeric reinforcements as recited in the appended claims. The patented steel wire has good ductility and can be drawn to develop high tensile strength using a steel alloy that contains a small amount of copper. Such patented steel wire is particularly suitable for utilization in manufacturing reinforcing wire for rubber products, such as tires. By utilizing this process, continuous cooling can be employed in the patenting procedure with the properties attained being more representative of those which are normally only attained under conditions of isothermal transformation. It has been unexpectedly found that certain copper containing microalloyed high carbon steel wires having good ductility and which can be drawn to develop high tensile strength can be prepared by a patenting procedure which utilizes a continuous cooling step for the transformation from austenite to pearlite. These plain carbon steels are comprised of 96.61 weight percent to 98.905 weight percent iron, from 0.72 weight percent to 1.04 weight percent carbon, from 0.3 weight percent to 0.8 weight percent manganese, from 0.05 weight percent to 0.4 weight percent silicon, from 0.02 weight percent to 0.3 weight percent copper, and from 0.005 weight percent to 0.85 weight percent of at least one member selected from the group consisting of chromium, vanadium, nickel and boron. The total amount of silicon, manganese, chromium, vanadium, nickel and boron in such copper containing microalloyed high carbon steel is within the range of 0.70 weight percent to 0.9 weight percent. By utilizing such alloys, the costly equipment required for isothermal transformation is eliminated. This, in turn, simplifies and reduces the cost of the patenting procedure. Detailed Description of the Invention Certain plain carbon steel microalloys that contain copper are utilized in the process of this invention. These microalloyed high carbon steels contain from 0.02 to 0.3 weight percent copper. This alloy is comprised of 96.61 weight percent to 98.905 weight percent iron, from 0.72 weight percent to 1.04 weight percent carbon, from 0.3 weight percent to 0.8 weight percent manganese, from 0.05 weight percent to 0.4 weight percent silicon, from 0.02 weight percent to 0.3 weight percent copper, and from 0.005 weight percent to 0.85 weight percent of at least one member selected from the group consisting of chromium, vanadium, nickel and boron, with the proviso that the total amount of silicon, manganese, chromium, vanadium, nickel and boron in the microalloyed high carbon steel is within the range of 0.7 to 0.9 weight percent. The copper containing steel alloys of this invention will preferably contain from 0.05 to 0.2 weight percent copper. Such copper containing steel alloys will more preferably contain from 0.10 to 0.15 weight percent copper. It is accordingly preferred for the microalloy to consist of from 97.54 weight percent to 98.59 weight percent iron, from 0.76 weight percent to 0.96 weight percent carbon, from 0.40 weight percent to 0.60 weight percent manganese, from 0.15 weight percent to 0.30 weight percent silicon, from 0.05 weight percent to 0.2 weight percent copper, and from 0.05 weight percent to 0.4 weight percent of at least one member selected from the group consisting of chromium, vanadium and nickel. In cases where boron is used in the copper containing microalloy, it is generally preferred for the microalloy to consist essentially of from 97.92 weight percent to 98.63 weight percent iron, from 0.76 weight percent to 0.88 weight percent carbon, from 0.40 weight percent to 0.60 weight percent manganese, from 0.15 weight percent to 0.30 weight percent silicon, from about weight percent to 0.2 weight percent copper, and from 0.01 weight percent to 0.1 weight percent of boron. It is normally more preferred for the copper containing high carbon steel microalloy to consist of from 97.85 weight percent to 98.3 weight percent iron, from 0.9 weight percent to 0.95 weight percent carbon, from 0.40 weight percent to 0.50 weight percent manganese, from 0.2 weight percent to 0.25 weight percent silicon, from 0.10 weight percent to 0.15 weight percent copper, and from 0.1 weight percent to 0.3 weight percent of at least one element selected from the group consisting of chromium, vanadium and nickel. In cases where boron is included in the microalloy, it is normally more preferred for the high carbon steel microalloy to consist essentially of from 98.15 weight percent to 98.44 weight percent iron, from 0.8 weight percent to 0.85 weight percent carbon, from 0.45 weight percent to 0.55 weight percent manganese, from 0.2 weight percent to 0.25 weight percent silicon, from 0.10 weight percent to 0.15 weight percent copper, and from 0.01 weight percent to 0.05 weight percent boron. It is generally most preferred for such microalloys to contain a total of 0.75 weight percent to 0.85 weight percent of silicon, manganese, chromium, vanadium, nickel and boron. Rods having a diameter of 5 mm to 6 mm which are comprised of the steel alloys of this invention can be manufactured into steel filaments which can be used in reinforcing elements for rubber products. Such steel rods are typically cold-drawn to a diameter which is within the range of 1.2 mm to 2.4 mm and which is preferably within the range of 1.6 mm to 2.0 mm. For instance, a rod having a diameter of 5.5 mm can be cold-drawn to a wire having a diameter of 1.8 mm. This cold drawing procedure increases the strength and hardness of the metal. The cold-drawn wire is then patented by heating the wire to a temperature which is within the range of 850°C to 1100°C and allowing the wire to continuously cool to ambient temperature. In cases where the wire is heated by electrical resistance by passing an electrical current through it, the heating time is typically between 2 seconds and 10 seconds. In cases where electrical resistance heating is used, the heating period is more typically within the range of 4 to 7 seconds and the heating temperature is typically within the range of 950°C to 1050°C. It is, of course, also possible to heat the wire in a fluidized bed oven. In such cases, the wire is heated in a fluidized bed of sand having a small grain size. In fluidized bed heating techniques, the heating period will generally be within the range of 5 seconds to 30 seconds. It is more typical for the heating period in a fluidized bed oven to be within the range of 10 seconds to 20 seconds. It is also possible to heat the wire in a convection oven or in a furnace. In this case, the heating time will be in the range of 25 seconds to 50 seconds. The exact duration of the heating period is not critical. However, it is important for the temperature to be maintained for a period which is sufficient for the alloy to be austenitized. The alloy is considered to be austenitized after the microstructure has been completely transformed to a homogeneous face centered cubic crystal structure. In the next step of the patenting procedure, the austenite wire is continuously cooled at a cooling rate of less than 60°C per second. In most cases, the cooling rate employed will be between 15°C per second and 60°C per second. It is normally preferred to utilize a cooling rate which is within the range of 20°C per second to 60°C per second. This continuous cooling step can be brought about by simply allowing the wire to cool in air or another suitable gas, such as cracked ammonia. The gas can be still or circulated to control the rate of cooling. The continuous cooling is carried out until a transformation from austenite to pearlite begins. This transformation will typically begin at a temperature which is within the range of 500°C to 650°C. The transformation from austenite to pearlite will more typically begin at a temperature which is within the range of 540°C to 600°C. The transformation will more typically begin at a temperature which is within the range of 550°C to 580°C. After the transformation from austenite to pearlite begins, the temperature of the wire will increase from recalescence. At this point in the process, the transformation is simply allowed to proceed with the temperature of the wire increasing solely by virtue of the heat given off by the transformation. A temperature increase which is within the range of 20°C to 80°C will normally be experienced with temperature increases within the range of 20°C to 70°C being typical. A temperature increase which is within the range of 30°C to 60°C is more typically experienced. It is most typical for the temperature of the wire to increase by 40°C to 50°C during the transformation. The transformation from austenite to pearlite typically takes from 0.5 seconds to 4 seconds to complete. The transformation from austenite to pearlite will more typically take place over a time period within the range of 1 second to 3 seconds. The transformation is considered to begin at the point where a temperature increase due to recalescence is observed. As the transformation proceeds, the microstructure is transformed from a face centered cubic microstructure of the austenite to pearlite. The patenting procedure is considered to be completed after the transformation to pearlite has been attained wherein the pearlite is a lamellar structure consisting of an iron phase having a body centered cubic crystal structure and a carbide phase. After the patenting has been completed, the steel wire can be simply cooled to ambient temperature. In some instances, it may not be possible to draw the wire directly from wire rod to a diameter suitable for final patenting. In these cases, the wire may be initially cold-drawn to reduce its diameter between 40 percent to 80 percent to a diameter in the range of approximately 3.8 mm to 2.5 mm. After this initial drawing, the wire is then patented in a process referred to as intermediate patenting, by using a similar process to the one used in the first patenting step with the exception that the heating times are generally longer. After intermediate patenting, the wire is cold-drawn to a final diameter suitable for the final patenting step described above. After final patenting, the steel wire is then typically brass-plated. For instance, alloy plating can be used to plate the steel wire with a brass coating. Such alloy-plating procedures involve the electrodeposition of copper and zinc onto the wire simultaneously to form a homogeneous brass alloy in situ from a plating solution containing chemically complexing species. This codeposition occurs because the complexing electrolyte provides a cathode film in which the individual copper and zinc deposition potentials are virtually identical. Alloy-plating is typically used to apply alpha-brass coatings containing 70 percent copper and 30 percent zinc. Such coatings provide excellent drawing performance and good initial adhesion. Sequential plating is also a practical technique for applying brass alloys to steel wires. In such procedures, a copper layer and a zinc layer are sequentially plated onto the steel wire by electrodeposition followed by a thermal diffusion step. Such a sequential plating process is described in US-A- 5,100,517. In the standard procedure for plating brass onto steel wire, the steel wire is first optionally rinsed in hot water at a temperature of greater than 60°C. The steel wire is then acid-pickled in sulfuric acid or hydrochloric acid to remove oxide from the surface. After a water rinse, the wire is coated with copper in a copper pyrophosphate plating solution. The wire is given a negative charge so as to act as a cathode in the plating cell. Copper plates are utilized as the anode. Oxidation of the soluble copper anodes replenishes the electrolyte with copper ions. The copper ions are, of course, reduced at the surface of the steel wire cathode to the metallic state. The copper-plated steel wire is then rinsed and plated with zinc in a zinc-plating cell. The copper-plated wire is given a negative charge to act as the cathode in the zinc-plating cell. A solution of acid zinc sulfate is in the plating cell which is equipped with a soluble zinc anode. During the zinc plating operation, the soluble zinc anode is oxidized to replenish the electrolyte with zinc ions. The zinc ions are reduced at the surface of the copper-coated steel wire which acts as a cathode with a layer of zinc being deposited thereon. The acid zinc sulfate bath can also utilize insoluble anodes when accompanied with a suitable zinc ion replenishment system. The copper/zinc-plated wire is then rinsed and heated to a temperature of greater than 450°C and preferably within the range of 500°C to 550°C to permit the copper and zinc layers to diffuse thereby forming a brass coating. This is generally accomplished by induction or resistance heating. The filament is then cooled and washed in a dilute phosphoric acid bath at room temperature to remove oxide. The brass-coated wire is then rinsed and air-dried at a temperature of 75°C to 150°C. In some cases, it may be desirable to coat the steel alloy with an iron-brass coating. Such a procedure for coating steel reinforcing elements with a ternary iron-brass alloy is described in US-A-4,446,198. After brass plating, the wire is again cold-drawn while submerged in a bath of liquid lubricant. In this step, the cross section of the wire is reduced by 80 percent to 99 percent to produce the high strength filaments used for elastomeric reinforcements. It is more typical for the wire to be reduced by 96 percent to 98 percent. The diameters of the high strength filaments produced by this process are normally within the range of 0.10 mm to 0.45 mm. The diameters of the high strength filaments produced by this process are typically within the range of 0.15 mm to 0.40 mm. More typically, the high strength filaments produced have a diameter which is within the range of 0.25 mm to 0.35 mm. In many cases, it will be desirable to twist two or more filaments into cable for utilization as reinforcements for rubber products. For instance, it is typical to twist two such filaments into cable for utilization in passenger tires. It is, of course, also possible to twist a larger number of such filaments into cable for utilization in other applications. For instance, it is typical to twist 50 filaments into cables which are ultimately employed in earthmover tires. The present invention will be described in more detail in the following examples. These examples are merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it may be practiced. Unless specifically indicated otherwise, all parts and percentages are given by weight. Comparative example 1 In this experiment, a chromium containing high carbon steel microalloy wire was patented utilizing a technique which included a continuous cooling step. The microalloy utilized in this experiment contains approximately 98.43 percent iron, 0.85 percent carbon, 0.31 percent manganese, 0.20 percent silicon and 0.21 percent chromium. In the process used, the chromium containing microalloy wire was very quickly heated by electrical resistance over a period of 5 seconds to a peak temperature of 950°C. This heating cycle was sufficient to austenitize the wire which was then allowed to continuously cool in air at a cooling rate of 40°C per second. After the wire had cooled to a temperature of 580°C, a transformation from austenite to pearlite began. This transformation caused the temperature of the wire to increase to 625°C over a period of 1 second after which the wire again began to continuously cool. The patented wire produced had a diameter of 1.75 mm and was determined to have a tensile strength of 1260 MPa (megapascals). The patented wire was also determined to have an elongation at break of 10.5 percent and a reduction of area at break of 47 percent. The patented wire was subsequently cold-drawn into a filament having a diameter of 0.301 mm. The filament made was determined to have a tensile strength of 3349 MPa and had an elongation at break of 2.61 percent. The tensile strength of the filaments made in this experiment utilizing the chromium containing high carbon steel microalloy compare very favorably to those which can be realized utilizing isothermal patenting techniques which employ standard 1080 carbon steel . More importantly, this experiment shows that very outstanding filament tensile strength can be realized utilizing a patenting procedure wherein a continuous cooling step is employed. Comparative Example 2 This experiment was carried out utilizing the same procedure as is described in Example 1 except for the fact that a 1080 carbon steel which contained 98.47 percent iron, 0.83 percent carbon, 0.48 percent manganese and 0.20 percent silicon was substituted for the chromium containing microalloy utilized in Example 1. The patented 1080 carbon steel wire made had a tensile strength of 1210 MPa with the drawn filament produced having a tensile strength of only 3171 MPa. The filament made was also determined to have an elongation at break of 2.52 percent. This example shows that the utilization of the chromium containing microalloy described in Example 1 resulted in a filament tensile strength increase of 178 MPa. Comparative example 3 This experiment was also carried out utilizing the general procedure described in Example 1 except that a vanadium containing plain carbon steel microalloy was utilized. The patented wire produced in this experiment was determined to have a tensile strength of 1311 MPa, an elongation at break of 10 percent and a reduction of area at break of 48 percent. The filament made in this experiment was determined to have a tensile strength of 3373 MPa and an elongation at break of 2.57 percent. This example shows that the tensile strength of the filaments was further improved by utilizing the vanadium containing microalloy. Inventive example 4 This experiment was carried out utilizing the general procedure described in Example 1 except that a copper containing steel microalloy was utilized. Also, the patented wire was cold-drawn into a filament having a diameter of 0.2 mm. The filament made in this experiment was determined to have a tensile strength of 3650 MPa and an elongation at break of 2.6 percent. This example shows that the tensile strength of the filaments was further improved by utilizing the copper containing microalloy. The inclusion of copper in the alloy provided a higher work hardening rate and also improved ductality.

Claims: A process for producing a high strength filament for use in elastomeric reinforcements by: (1) heating a steel wire to a temperature which is within the range of 850°C to 1100°C for a period of at least 2 seconds; (2) continuously cooling the heated steel wire at a cooling rate of less than 60°C per second until a transformation from austenite to pearlite begins; (3) allowing the transformation from austenite to pearlite to proceed with an increase in the wire temperature resulting from recalescence to produce a patented steel wire, wherein the increase in wire temperature resulting from recalescence is an increase in temperature which is within the range of 20°C to 80°C; (4) cooling the patented steel wire to ambient temperature; (5) brass-plating the patented steel wire to produce a brass-plated wire; and (6) cold-drawing the brass-plated steel wire to a diameter which is within the range of 0.10 mm to 0.45 mm to produce a high strength filament; wherein said steel wire is comprised of a microalloyed high carbon steel which consists of 96.61 weight percent to 98.905 weight percent iron, from 0.72 weight percent to 1.04 weight percent carbon, from 0.3 weight percent to 0.8 weight percent manganese, from 0.05 weight percent to 0.4 weight percent silicon, from 0.02 weight percent to 0.3 weight percent copper, and from 0.005 weight percent to 0.85 weight percent of at least one member selected from the group consisting of chromium, vanadium, nickel and boron, with the proviso that the total amount of silicon, manganese, chromium, vanadium, nickel and boron in the microalloyed high carbon steel is within the range of 0.7 weight percent to 0.9 weight percent to produce a heated steel wire. A process as specified in claim 1 characterized in that the microalloyed high carbon steel consists of iron, carbon, manganese, silicon, chromium and copper. A process as specified in claim 2 characterized in that the carbon steel microalloy consists of 97.54 weight percent to 98.59 weight percent iron, from 0.76 weight percent to 0.96 weight percent carbon, from 0.4 weight percent to 0.6 weight percent manganese, from 0.15 weight percent to 0.3 weight percent silicon, from 0.05 weight percent to 0.2 weight percent copper, and from 0.05 weight percent to 0.4 weight percent chromium. A process as specified in claim 2 characterized in that the carbon steel microalloy consists of 97.85 weight percent to 98.3 weight percent iron, from 0.9 weight percent to 0.95 weight percent carbon, from 0.40 weight percent to 0.50 weight percent manganese, from 0.2 weight percent to 0.25 weight percent silicon, from 0.10 weight percent to 0.15 weight percent copper, and from 0.1 weight percent to 0.3 weight percent chromium. A process as specified in any of the preceding claims characterized in that the cooling rate of step (2) is within the range of 15°C per second to 60°C per second. A process as specified in any of the preceding claims characterized in that the cooling rate of step (2) is within the range of 20°C per second to 60°C per second. A process as specified in any of the preceding claims characterized in that the transformation from austenite to pearlite begins at a temperature which is within the range of 500°C to 600°C; and in that the continuous cooling of step (2) is carried out in air or cracked ammonia. A process as specified in any of the preceding claims characterized in that the microalloyed high carbon steel contains from 0.05 weight percent to 0.2 weight percent copper. A process as specified in any of the preceding claims characterized in that the brass-plated steel wire is cold-drawn in step (6) to a diameter which is within the range of 0.15 mm to 0.40 mm. A process as specified in any of the preceding claims characterized in that the microalloy contains a total of 0.75 weight percent to 0.85 weight percent silicon, manganese, chromium, vanadium, nickel and boron.

Inventors: BHAGWAT ANAND WAMAN, PALMER KENNETH JOSEPH, TODD RODGER NMN, TONTELING CHARLES NMN, BHAGWAT, ANAND WAMAN, PALMER, KENNETH JOSEPH, TODD, RODGER, (NMN), TONTELING, CHARLES, (NMN)
Applicants: GOODYEAR TIRE & RUBBER, THE GOODYEAR TIRE & RUBBER COMPANY
Application year: 1997
Main classification: C21D8
IPC codes: B21C1, C21D9, D07B1, C22C38, C23C30, C21D8