DRAWING PROCESS AND WIRE OBTAINED BY DRAWING PROCESS

Abstract

A steel wire has a microstructure that is completely ferritic, a mixture of ferrite and cementite or a mixture of ferrite and pearlite and has a weight content of carbon C such that C<0.05% and a weight content of chromium Cr such that Cr<12%. The process for drawing the wire comprises: at least one first uninterrupted series of steps of drawing the wire from a diameter D to a diameter d, at least one second uninterrupted series of steps of drawing the wire of diameter d to a diameter d, and one or more intermediate steps between the first and second uninterrupted series of steps of drawing the wire, the wire having a temperature less than or equal to 300 C. during the or each intermediate step.

Claims

1.-29. (canceled)

30. A process for drawing a steel wire, the steel wire having a microstructure that is selected from the group consisting of completely ferritic, a mixture of ferrite and cementite, and a mixture of ferrite and pearlite, and the wire having a weight content of carbon C such that C<0.05% and a weight content of chromium Cr such that Cr<12%, the process comprising: at least one first uninterrupted series of steps of drawing the wire from a diameter D to a diameter d; at least one second uninterrupted series of steps of drawing the wire of diameter d to a diameter d; and one or more intermediate steps between the first and second uninterrupted series of steps of drawing the wire, the wire having a temperature less than or equal to 300 C. during the one or more intermediate steps.

31. The process according to claim 30, wherein d is greater than or equal to 0.5 mm.

32. The process according to claim 31, wherein d is greater than or equal to 1 mm.

33. The process according to claim 32, wherein d is greater than or equal to 1.3 mm.

34. The process according to claim 30, wherein d is less than or equal to 2.5 mm.

35. The process according to claim 34, wherein d is less than or equal to 2.2 mm.

36. The process according to claim 35, wherein d is less than or equal to 2 mm.

37. The process according to claim 30, wherein, with d and d being expressed in mm, the true strain =2.Math.ln (d/d) is such that >3.

38. The process according to claim 30, wherein, with d and d being expressed in mm, the true strain =2.Math.ln (d/d) is such that 5.

39. The process according to claim 37, wherein, with d and d being expressed in mm, the true strain =2.Math.ln (d/d) is such that 3.5.

40. The process according to claim 39, wherein, with d and d being expressed in mm, the true strain =2.Math.ln (d/d) is such that 3.7.

41. The process according to claim 40, wherein, with d and d being expressed in mm, the true strain =2.Math.ln (d/d) is such that 4.

42. The process according to claim 38, wherein, with d and d being expressed in mm, the true strain =2.Math.ln (d/d) is such that 4.7.

43. The process according to claim 42, wherein, with d and d being expressed in mm, the true strain =2.Math.ln (d/d) is such that 4.5.

44. The process according to claim 30, wherein, with D and d being expressed in mm, the true strain =2.Math.ln (D/d) is such that 2.

45. The process according to claim 30, wherein, with D and d being expressed in mm, the true strain =2.Math.ln (D/d) is such that 5.

46. The process according to claim 45, wherein, with D and d being expressed in mm, the true strain =2.Math.ln (D/d) is such that 4.

47. The process according to claim 46, wherein, with D and d being expressed in mm, the true strain =2.Math.ln (D/d) is such that 3.5.

48. The process according to claim 47, wherein, with D and d being expressed in mm, the true strain =2.Math.ln (D/d) is such that 3.

49. The process according to claim 44, wherein, with D and d being expressed in mm, the true strain =2.Math.ln (D/d) is such that 2.3.

50. The process according to claim 49, wherein, with D and d being expressed in mm, the true strain =2.Math.ln (D/d) is such that 2.5.

51. The process according to claim 50, wherein, with D and d being expressed in mm, the true strain =2.Math.ln (D/d) is such that 2.7.

52. The process according to claim 30, wherein, with D and d being expressed in mm, the true strain T=2.Math.ln (D/d) is such that T6.

53. The process according to claim 52, wherein, with D and d being expressed in mm, the true strain T=2.Math.ln (D/d) is such that T6.5.

54. The process according to claim 53, wherein, with D and d being expressed in mm, the true strain T=2.Math.ln (D/d) is such that T7.

55. The process according to claim 52, wherein, with D and d being expressed in mm, the true strain T=2.Math.ln (D/d) is such that T8.

56. The process according to claim 30, wherein D is greater than or equal to 4 mm.

57. The process according to claim 56, wherein D is greater than or equal to 5 mm.

58. The process according to claim 30, wherein the one or more intermediate steps do not comprise a step of heating the steel beyond an austenitizing temperature of the steel.

59. The process according to claim 30, wherein the one or more intermediate steps comprise a step of coating the wire of diameter d with at least one metal layer.

60. The process according to claim 59, wherein the step of coating the wire of diameter d is selected from the group consisting of a step of depositing a layer of an alloy of substantially pure metals, a step of depositing a first layer of a first substantially pure metal followed by a step of depositing a second layer of a second substantially pure metal, and a step of depositing a layer of a substantially pure metal.

61. The process according to claim 59, wherein the step of coating the wire of diameter d comprises a step of depositing a first layer of a first substantially pure metal followed by a step of depositing a second layer of a second substantially pure metal, and the one or more intermediate steps do not comprise a step of thermal diffusion of each of the first and second substantially pure metals respectively into the second and first layers.

62. A steel wire of diameter d, expressed in mm, and having a microstructure that is selected from the group consisting of completely ferritic, a mixture of ferrite and cementite and a mixture of ferrite and pearlite, wherein the wire has a weight content of carbon C such that C<0.05%, a weight content of chromium Cr such that Cr<12% and a maximum tensile strength R, expressed in MPa, such that RA+910.C600.In(d) with A=200.

63. A cord comprising at least two wires according to claim 62.

64. A semi-finished element comprising a rubber matrix in which at least one wire according to claim 62 is embedded.

65. A tire comprising at least one wire according to claim 62.

Description

[0090] The invention will be better understood on reading the following description, given solely by way of nonlimiting example and with reference to the drawings in which:

[0091] FIG. 1 is a cross-sectional view perpendicular to the circumferential direction of a tyre according to the invention;

[0092] FIG. 2 is a diagram illustrating steps of a drawing process according to the invention;

[0093] FIG. 3 is an optical microscope view of a ferritic-pearlitic microstructure;

[0094] FIG. 4 is a scanning electron microscope view of a ferritic-pearlitic microstructure; and

[0095] FIG. 5 is an optical microscope view of an acicular ferritic (Widmansttten) microstructure.

EXAMPLE OF A TYRE ACCORDING TO THE INVENTION

[0096] FIG. 1 depicts a tyre comprising wires obtained by the process according to the invention and that is denoted by the general reference 10.

[0097] The tyre 10 has a crown 12 reinforced by a crown reinforcement 14, two sidewalls 16 and two beads 18, each of these beads 18 being reinforced with a bead wire 20. The crown 12 is surmounted by a tread, not shown in this schematic figure. A carcass reinforcement 22 is wound around the two bead wires 20 in each bead 18 and comprises a turn-up 24 positioned towards the outside of the tyre 10, which is shown fitted onto a wheel rim 26 here.

[0098] The carcass reinforcement 22, in a manner known per se, consists of at least one ply reinforced by wires or cords. These wires or cords of the carcass reinforcement are referred to as radial wires or cords, that is to say that these wires or cords are positioned virtually parallel to one another and extend from one bead to the other so as to form an angle of between 80 and 90 with the median circumferential plane (plane perpendicular to the axis of rotation of the tyre which is situated midway between the two beads 18 and passes through the middle of the crown reinforcement 14).

[0099] The crown reinforcement 14 comprises at least one ply reinforced by wires or cords in accordance with the invention. In this crown reinforcement 14 that is depicted in a very simple manner in FIG. 1, it will be understood that the wires or cords of the invention may for example reinforce all or some of the working crown plies or triangulation crown plies (or half plies) and/or protective crown plies, when such triangulation or protective crown plies are used. Besides the working plies, and the triangulation and/or protective plies, the crown reinforcement 14 of the tyre of the invention may of course comprise other crown plies, for example one or more hooping crown plies.

[0100] Of course, the tyre 10 additionally comprises, in a known manner, an inner rubber or elastomer layer (commonly referred to as inner liner) which defines the radially inner face of the tyre and which is intended to protect the carcass reinforcement from the diffusion of air originating from the space inside the tyre. Advantageously, in particular in the case of a tyre for a heavy-duty vehicle, it may also comprise an intermediate reinforcing elastomer layer which is located between the carcass reinforcement and the inner layer, intended to reinforce the inner layer and, consequently, the carcass reinforcement, and also intended to partially delocalize the stresses to which the carcass reinforcement is subjected.

[0101] The tyre is manufactured by assembling the various elements described above present in the form of semi-finished elements comprising a rubber matrix in which the wires or cords are embedded.

[0102] Example of a Cord According to the Invention

[0103] In the case where the crown and/or carcass reinforcement is reinforced by cords, these are manufactured by assembling several steel wires in accordance with the invention, either by cabling or by twisting.

[0104] In the case of a tyre for industrial vehicles selected from vans, heavy vehicles such as heavy-duty vehicles (i.e. underground trains, buses, heavy road transport vehicles (lorries, tractors, trailers) and off-road vehicles), agricultural or civil engineering machinery, aircraft, and other transport or handling vehicles, the crown and/or carcass reinforcement is reinforced by cords in accordance with the invention in particular selected from layered cords of 1+3+8, 1+6+11, 1+6+12, 2+7, 3+8, 3+9 and 3+9+15 structure and stranded cords of 3(1+5), (1+6)(3+8), (1+6)(3+9+15) and (1+6)(4+10+16) structure. Other cords that can reinforce the crown and/or carcass reinforcement are also described in document WO 2010/139583.

[0105] In the case of a tyre for passenger vehicles, the crown and/or carcass reinforcement is reinforced by cords in accordance with the invention and in particular selected from the cords of 2+1, 2+2, 2+4 and 43 structure.

[0106] The cords in accordance with the invention may be rubberized in situ, as is described, inter alia, in document WO 2010/139583.

[0107] The crown and/or carcass reinforcement may also be reinforced by one or more individual wires in accordance with the invention but that are not assembled.

[0108] Example of a Wire According to the Invention

[0109] The wire is made of steel. Preferably, the steel is an unalloyed steel as defined in the standard NF EN10020. Wires made of alloy steel or stainless steel as defined in the standard NF EN10020 can also be envisaged.

[0110] The steel used may therefore preferably comprise known alloying elements such as for example Mn, Si, P, S, N, V, Cr, Mo, Ni, B and Co (see, for example, Research Disclosure 34984Micro-alloyed steel cord constructions for tyresMay 1993; Research Disclosure 34054High tensile strength steel cord constructions for tyresAugust 1992) that make it possible to adapt the steel.

[0111] The preferred unalloyed steel in accordance with the standard NF EN10020 comprises at most 1.65%, preferably at most 1.30% and more preferably still at most 1% and very preferably at most 0.5% by weight, here 0.273% by weight of Mn.

[0112] The preferred unalloyed steel in accordance with the standard NF EN10020 comprises at most 0.60% by weight, here 0.039% by weight of Si.

[0113] The preferred unalloyed steel in accordance with the standard NF EN10020 comprises at most 0.10% by weight, here 0.011% by weight of P.

[0114] The preferred unalloyed steel in accordance with the standard NF EN10020 comprises at most 0.10% by weight, here 0.011% by weight of S.

[0115] The preferred unalloyed steel in accordance with the standard NF EN10020 comprises at most 0.10% by weight of nitrogen, at most 0.10% by weight of vanadium, at most 0.30% by weight of chromium, at most 0.08%, limit included, by weight of molybdenum, at most 0.3%, limit included, by weight of nickel, at most 0.0008%, limit included, by weight of boron and at most 0.3%, limit included, by weight of cobalt. Here, unalloyed steel comprises negligible amounts of nitrogen, vanadium, chromium, molybdenum, nickel, boron and cobalt present in the form of impurities

[0116] In the case of an alloy steel in accordance with the standard NF EN10020, the steel used comprises a weight content of chromium Cr such that Cr<10.5%, preferably such that Cr5%, more preferably such that Cr1%, and more preferably still such that Cr0.2%.

[0117] The values of the weight contents of the elements described above may be measured according to the standard FD CEN/TR 10261 entitled Iron and steelEuropean standards for the determination of chemical composition.

[0118] The microstructure of the steel is selected from ferrite, a mixture of ferrite and cementite or a mixture of ferrite and pearlite. The wire is preferably made of ferritic steel, illustrated in FIGS. 3 and 4. The steel used comprises a content of carbon C, expressed in %, by weight of steel such that C<0.05%. Preferably, 0.01%C<0.05%, preferably 0.01%C0.045% and more preferably still 0.01%C0.045%.

[0119] The wire may be coated with a metal layer that improves, for example, the processing properties of the wire, or the usage properties of the wire, of the cord and/or of the tyre themselves, such as the adhesion, corrosion resistance or else ageing resistance properties. The wire is coated with a coating selected from a layer of an alloy of substantially pure metals, for example of brass or bronze, a first layer of a first substantially pure metal, for example copper, itself coated with a second layer of a second substantially pure metal, for example zinc, and a layer of a substantially pure metal, for example zinc. As a variant, the wire may have no metal coating.

[0120] Given in Table 1 below are wires EDT1, EDT2 according to the prior art and F1 according to the invention.

[0121] The wires of the examples from Table 1 have a diameter d of greater than or equal to 0.10 mm and preferably greater than or equal to 0.12 mm. Moreover, the wires of the examples from Table 1 have a diameter d of less than or equal to 0.40 mm, preferably less than or equal to 0.25 mm, more preferably less than or equal to 0.23 mm and more preferably still less than or equal to 0.20 mm.

TABLE-US-00001 TABLE 1 EDT1 EDT2 F1 F2 C (%) 0.71 0.585 0.03 0.03 d (mm) 0.18 0.18 0.15 0.18 R (MPa) 2820 2903 1901 1907

[0122] The wire F1 is such that the maximum tensile strength R of the wire, expressed in MPa, is such that RA+910.C600.In(d) with A=200 and d expressed in mm.

[0123] It will be noted that the wire F1 is such that A=400, preferably A=600 and more preferably A=700.

[0124] It will be noted that the wires F1 and F2 are such that R1200 MPa, preferably R1600 MPa and more preferably R1800 MPa.

[0125] Example of Process for Drawing the Wire According to the Invention

[0126] Represented in FIG. 2 is a diagram of a process that makes it possible to draw the wires as described above.

[0127] In an uncoiling step 100, a steel wire of initial diameter preferably here equal to 5.5 mm and having a maximum tensile strength of between 300 MPa and 700 MPa, is uncoiled. The wire, referred to as wire stock, is stored in the form of a coil on a pay-off reel from which it is uncoiled using automated uncoiling means, for example an uncoiler. The steel microstructure is then ferritic-pearlitic.

[0128] In a step 200 of descaling the wire stock, the wire stock is passed into several successive pulleys and into two straighteners each formed by several pulleys, the pulleys of each straightener being rotatably mounted about an axis perpendicular to the axis of rotation of the pulleys of the other straightener. A layer of iron oxides, referred to as scale, present at the surface of the wire stock is thus removed.

[0129] In a step 300, the wire stock is coated with a layer of an adhesion promoter for a drawing lubricant.

[0130] The objective of steps 400.sub.1 to 400.sub.n is to reduce the diameter of the wire from the initial diameter D to an intermediate diameter d, for example greater than or equal to 0.5 mm, preferably greater than or equal to 1 mm and more preferably greater than or equal to 1.3 mm and for example less than or equal to 2.5 mm, preferably less than or equal to 2.2 mm and more preferably less than or equal to 2 mm.

[0131] Steps 400.sub.1 to 400.sub.n (n varying from 6 to 12) form a first uninterrupted series of steps of dry drawing the wire from the initial diameter D to the intermediate diameter d. Each step 400.sub.1 to 400.sub.n is a dry drawing step in which the wire is passed into a die having a diameter smaller than the diameter of the wire upstream of the die. Thus, the wire has a diameter downstream of the die that is smaller than the diameter upstream of the die. The diameter of each die is smaller than the diameter of the die located upstream. For the first uninterrupted series of steps of dry drawing the wire from the initial diameter D to the intermediate diameter d, the true strain is defined as =2.Math.ln(D/d).

[0132] Means for pulling the wire that are positioned downstream of each die, here capstans, make it possible to exert a pulling force sufficient to draw the wire through each die. A drawing lubricant in pulverulent form is used.

[0133] In a step 600, the wire of intermediate diameter d is coated with at least one metal layer. The step 600 of coating the wire of intermediate diameter d is selected from a step of depositing a layer of an alloy of substantially pure metals, a step of depositing a first layer of a first substantially pure metal followed by a step of depositing a second layer of a second substantially pure metal, and a step of depositing a layer of a substantially pure metal. Here, the step 600 of coating the wire of intermediate diameter d is a step of depositing a step of depositing a first layer of a first substantially pure metal followed by a step of depositing a second layer of a second substantially pure metal, here a layer of copper then a layer of zinc.

[0134] The objective of steps 700.sub.1 to 700.sub.m (m varying for example from 8 to 23) is to reduce the diameter of the wire from the intermediate diameter d to the final diameter d and to increase the maximum tensile strength of the wire.

[0135] Steps 700.sub.1 to 700.sub.m form a second uninterrupted series of steps of wet drawing the wire from the intermediate diameter d to the final diameter d. Each step 700.sub.1 to 700.sub.m is a wet drawing step in which the wire is passed into a die having a diameter smaller than the diameter of the wire upstream of the die. Thus, the wire has a diameter downstream of the die that is smaller than the diameter upstream of the die. The diameter of each die is smaller than the diameter of the die located upstream. For the second uninterrupted series of steps of wet drawing the wire from the intermediate diameter d to the final diameter d, the true strain is defined as =2.Math.ln(d/d).

[0136] As a variant, steps 700.sub.1 to 700.sub.m will be carried out in a dry environment.

[0137] Means for pulling the wire that are positioned downstream of each die, here stepped capstans, make it possible to exert a pulling force sufficient to draw the wire through each die. The pulling means and the dies are immersed in a liquid bath of drawing lubricant, for example as described in document WO 2008/113481.

[0138] The drawing process thus comprises N uninterrupted series of drawing steps, for example one in a dry environment and one in a wet environment. Here N=2. Thus, it is possible to define the total true strain for the drawing process as T=2.Math.ln(D/d).

[0139] Thus, the process comprises one or more intermediate steps between the first and second uninterrupted series of wire drawing steps 400.sub.1-400.sub.m, 700.sub.1-700.sub.m. During the or each of these intermediate steps, the wire has a temperature less than or equal to 300 C. Here, the process comprises a single intermediate step, here the coating step 600 during which the wire has a temperature of between 15 C. and 300 C. and preferably between 15 C. and 200 C.

[0140] The intermediate step or steps do not comprise a step of thermal diffusion of each first and second metal respectively into the second and first layer, here copper in the second layer and zinc in the first layer.

[0141] Neither do the intermediate step or steps comprise a step of heating the steel beyond its austenitizing temperature. Such austenitizing steps are described in particular in Prcis de mtallurgie [Precis on metallurgy], ISBN 2-12-260121-6 and also in Atlas des courbes de transformation des aciers de fabrication francaise [Atlas of transformation curves of steels of French manufacture], IRDIS, 1974.

[0142] Such a heating step is well known to those skilled in the art during heat treatments as described in particular in Les principes de base du traitement thermique des aciers [The basic principles of heat treatment of steels], Andr Constant and Guy Henry, ISBN 2-85330-083-8.

[0143] The absence of heat treatment comprising generally a step of heating beyond the austenitizing temperature of the steel and then a cooling step makes it possible to avoid the associated problems. Thus, if the austenitizing is not sufficient, non-recrystallized bands remain and the austenite obtained is not homogeneous which is detrimental to the subsequent drawing. If the austenitizing is too great, the microstructure obtained during subsequent cooling is an acicular (Widmansttten) ferrite, illustrated in FIG. 5, and not a ferritic-pearlitic structure.

[0144] Given in Table 2 are various values of the characteristics of the wires according to the invention and wires from the prior art.

TABLE-US-00002 TABLE 2 EDT1 EDT2 F1 F2 C (%) 0.71 0.585 0.03 0.03 d (mm) 1 1.3 1.3 1.3 d (mm) 0.18 0.18 0.15 0.18 2.6 2.8 2.9 2.9 3.6 4 4.3 4.0 T 6.2 6.8 7.2 6.9

[0145] It will be noted that the true strain =2.Math.ln (d/d) is such that 3<5 for the wires F1 and F2. Preferably, 3.5, more preferably 3.7 and more preferably still 4 for the wires F1 and F2. It will also be noted that 4.7 and preferably 4.5 for the wires F1 and F2.

[0146] It will be noted that the true strain =2.Math.ln (D/d) is such that 25 for the wires F1 and F2.

[0147] It will also be noted that, for the wires F1 and F2, when 3.5, preferably 3.7 and more preferably 4, the true strain =2.Math.In (D/d) is such that 4, preferably 3.5 and more preferably 3.

[0148] It will also be noted that, for the wires F1 and F2, when 4.7, preferably 4.5, the true strain =2.Math.In D/d) is such that 2.3, preferably 2.5 and more preferably 2.7.

[0149] It will also be noted that, for the wire F1, T6.5, preferably T6.75. It will also be noted that T8 for the wires F1 and F2.

[0150] Comparative Tests and Trials

[0151] The wires from the prior art and the wires F1 and F2 were compared during a rotating bending test carried out in a wet atmosphere (at least 60% relative humidity).

[0152] This test makes it possible to measure the maximum rotating bending endurance stress in a wet environment .sub.F* of each wire tested. During this test, the wire tested is subjected to 10.sup.5 cycles about its axis of revolution at a predetermined stress. If the wire breaks, the test is restarted with a lower stress and if the wire doesn't break, the test is restarted with a higher stress. The value of .sub.F* is thus determined step-by-step, for example by the staircase method. The results of this test are given in Table 3 below:

TABLE-US-00003 TABLE 3 EDT1 EDT2 F1 F2 C (%) 0.71 0.585 0.03 0.03 d (mm) 0.18 0.18 0.15 0.18 R (MPa) 2820 2903 1901 1907 .sub.F* (MPa) <500 <500 1004 995

[0153] In a wet environment, the wires F1 and F2 according to the invention breaks at significantly higher stresses than those of the prior art, thus illustrating one of the advantages of the invention. Thus, even if the initial tensile strength of the wires F1 and F2 according to the invention is lower than that of the wires EDT1 and EDT2, the fatigue-corrosion endurance of the wires F1 and F2 is significantly greater than that of the wires EDT1 and EDT2.

[0154] The invention is not limited to the embodiments described above.

[0155] Indeed, the descaling step 200 may be carried out by the action of a chemical agent, for example acid.