HIGH TENSILE STEEL WIRE

Abstract

An elongated steel element having a non-round cross-section and being in a work-hardened state, said elongated steel element having as steel composition: a carbon content ranging from 0.20 weight percent to 1.00 weight percent, a silicon content ranging from 0.05 weight percent to 2.0 weight percent, a manganese content ranging from 0.40 weight percent to 1.0 weight percent, a chromium content ranging from 0.0 weight percent to 1.0 weight percent, a sulfur and phosphor content being individually limited to 0.025 weight percent, contents of nickel, vanadium, aluminium, molybdenum or cobalt all being individually limited to 0.5 weight percent, the remainder being iron and unavoidable impurities, said steel having martensitic structure that comprises martensitic grains, wherein a fraction of at least 10 volume percent of martensitic grains is oriented.

Claims

1. An elongated steel element having a non-round cross-section and being in a work-hardened state, said elongated steel element having as steel composition: a carbon content ranging from 0.20 weight percent to 1.00 weight percent, a silicon content ranging from 0.05 weight percent to 2.0 weight percent, a manganese content ranging from 0.40 weight percent to 1.0 weight percent, a chromium content ranging from 0.0 weight percent to 1.0 weight percent, a sulfur and phosphor content being individually limited to 0.025 weight percent, contents of nickel, vanadium, aluminium, molybdenum or cobalt being individually limited to 0.5 weight percent, the remainder being iron and unavoidable impurities, said steel having martensitic structure that comprises martensitic grains, wherein a fraction of at least 10 volume percent of martensitic grains is oriented.

2. An elongated steel element according to claim 1, wherein a fraction of at least 20 volume percent of martensitic grains is oriented.

3. An elongated steel element according to claim 1, wherein a fraction of at least 40 volume percent of martensitic grains is oriented.

4. An elongated steel element according to claim 1, said elongated steel element having a yield strength Rp0.2 which is at least 80 percent of the tensile strength Rm.

5. An elongated steel element according to claim 1, said elongated steel element having a tensile strength Rm of at least 1200 MPa and an elongation at fracture At of at least 3 percent.

6. An elongated steel element according to claim 1, said elongated steel element having a tensile strength Rm of at least 1200 MPa for cross-section area below 300 mm.sup.2 and at least 1300 MPa for cross-section area below 100 mm.sup.2 and at least 1400 MPa for cross-section area below 5 mm.sup.2.

7. An elongated steel element according to claim 1, said elongated steel element being in a cold-rolled state.

8. An elongated steel element according to claim 1, said elongated steel element being in a warm-rolled state.

9. An elongated steel element according to claim 1, said elongated steel element is a flat shaped wire.

10. An elongated steel element according to claim 9, wherein said flat shaped wire has a “blacksmith cross” visible on its cross-section.

11. Use of an elongated steel element according to claim 1 as a spring wire or an element for producing a rope.

12. A process of manufacturing an elongated steel element, said elongated steel element having a non-round cross-section and being in a work-hardened state, said elongated steel element having as steel composition: a carbon content ranging from 0.20 weight percent to 1.00 weight percent, a silicon content ranging from 0.05 weight percent to 2.0 weight percent, a manganese content ranging from 0.40 weight percent to 1.0 weight percent, a chromium content ranging from 0.0 weight percent to 1.0 weight percent, a sulfur and phosphor content being individually limited to 0.025 weight percent, contents of nickel, vanadium, aluminium, molybdenum or cobalt being individually limited to 0.5 weight percent, the remainder being iron and unavoidable impurities, said steel having martensitic structure that comprises martensitic grains, wherein a fraction of at least 10 volume percent of martensitic grains is oriented, said process comprising the following steps in order: a) austenitizing a steel ingot, a steel wire rod or a steel (drawn or rolled) wire above Ac3 temperature during a period less than 120 seconds, b) quenching said austenitized steel ingot, steel wire rod or steel wire below 100° C. during a period less than 60 seconds, c) tempering said quenched steel ingot, steel wire rod or steel wire between 320° C. and 700° C. during a period ranging from 10 seconds to 600 seconds, d) work hardening said quenched and tempered steel ingot, steel wire rod or steel wire into an elongated steel element.

13. A process according to claim 12, said process further comprising the step of e) annealing said work hardened elongated steel element at a temperature between 350° C. and 700° C.

14. A process according to claim 12, wherein said work hardening is cold rolling.

15. A process according to claim 12, wherein said work hardening is warm rolling occurring between 400° C. and 700° C.

16. A process according to claim 13, wherein said work hardening is cold rolling.

17. A process according to claim 13, wherein said work hardening is warm rolling occurring between 400° C. and 700° C.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

[0040] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0041] FIG. 1 schematically shows grain orientation in poly-crystallographical materials.

[0042] FIG. 2 illustrates a thermo-mechanical process for steel wires according to the prior art.

[0043] FIG. 3 illustrates the thermo-mechanical process for steel wires according to the present invention.

[0044] FIG. 4 illustrates a temperature versus time curve for a thermal process according to the present invention.

[0045] FIG. 5 shows the tensile/yield strength, and elongation as a function of thickness reduction according to the second embodiment of the present invention.

[0046] FIG. 6 is a schematic view of “blacksmith-cross” on the cross-section of flat shaped elongated steel elements produced according the present invention.

[0047] FIG. 7 (a) shows the scanning electron microstructure (SEM) near the center of the “blacksmith-cross” of flat shaped steel wire.

[0048] FIG. 7 (b) shows the scanning electron microstructure at the short edge of the cross-section of the flat shaped steel wire.

[0049] FIG. 7 (c) shows the scanning electron microstructure at the long edge of the cross-section of the flat shaped steel wire.

[0050] FIG. 8 is a schematic view of the cross-section of a wire rod after a same thermal treatment according to the present invention.

[0051] FIG. 9 (a) shows the scanning electron microstructure near the center of the wire rod.

[0052] FIG. 9 (b) shows the scanning electron microstructure at the edge of the wire rod.

[0053] FIG. 10 shows the development of tensile/yield strength, and elongation of the steel wire according to the present invention as a function of annealing temperature.

MODE(S) FOR CARRYING OUT THE INVENTION

[0054] FIG. 4 illustrates a suitable temperature versus time curve applied to a steel wire or wire rod with a diameter of 6.5 mm and with following steel composition: [0055] % wt C=0.55 [0056] % wt Mn=0.65 [0057] % wt Si=1.4 [0058] % wt Cr=0.6
the balance being iron and unavoidable impurities.

[0059] The starting temperature of martensite transformation Ms of this steel is about 280° C. and the temperature Mf, at which martensite formation ends is about 100° C.

[0060] The various steps of the process are as follows: [0061] a first austenitizing step (10) during which the steel wire stays in a furnace at about 950° C. during 120 seconds, [0062] a second quenching step (12) for martensite transformation in oil at a temperature below 100° C. during at least 20 seconds; [0063] a third tempering step (14) for increase the toughness at a temperature about 450° C. during less than 60 seconds; and [0064] a fourth cooling step (16) at room temperature during 20 or more seconds.

[0065] Curve 18 is the temperature curve in the various equipment parts (furnace, bath . . . ) and curve 19 is the temperature of the steel wire or wire rod.

[0066] The steel wire or wire rod after above thermal treatment has a tempered martensitic microstructure.

[0067] The formed martensitic steel wire or wire rod is continued with cold rolling, i.e. below 400° C., to flat shape. The steel element is cold rolled to final dimension through several rolling stands. The more rolling stands the steel wire pass, the more thickness reduction. The tension of the steel wire may be measured and controlled. It is important to minimize or eliminate the tension in the steel wire moving between stands. Tension can result in a substantial narrowing of the steel. A precision speed regulation system can be used to control the speed at which the rollers are driven to minimize tension. As an example, an edge rolling is inserted between two thicknesses rolling.

[0068] The yield (R.sub.p0.2) and tensile (Rm) strength at different level of thickness reduction together with the elongation at fracture At are shown in FIG. 5. As shown in FIG. 5, both the tensile and yield strength increase with the thickness reduction. The yield to tensile ratio is between 80 and 96. Having a thickness reduction of 60%, the tensile strength of the flat shaped steel wire can go up to 2200 MPa without failure or breaking. Such a flat shaped steel wire has an elongation at fracture At about 2%, which is acceptable for further processing or operations such as bending.

[0069] This very high tensile strength is a consequence of oriented martensitic grains in the steel wire after rolling. The orientation was analyzed by image analysis and it appears a fraction of at least 10 volume percent of martensitic grains is oriented.

[0070] In particular, the martensitic grains are well oriented near the so called “blacksmith cross” (as shown in FIG. 6) characterized by a maximal strain area created due to rolling. In some instance, it is also called “lamination cross” since it is a formation of macroscopic shear bands. In terms of stresses, rolling has a heterogeneous repartition of stress components between the center, the long edge and the short edge of the flat shaped wire. The highest strains or strongest deformation takes place at a cross area as schematically shown in FIG. 6. The strain distribution determines the orientation of lenticular shaped martensitic grains such that the martensites are much better compressed and consequently oriented near this cross area (e.g. position indicated by (a) in FIG. 6) in comparison with the orientation near the short and long edges (positions indicated respectively by (b) and (c) in the cross-section view of FIG. 6). FIG. 7(a) and FIGS. 7(b)&(c) shows respectively the microstructures of the cross-section near the center (indicated by (a) in FIG. 6) and near the short and long edges of the flat shaped wire (indicated respectively by (b) and (c) in FIG. 6) cold-rolled to 11.9 mm in width and 3.5 mm in thickness. As shown in FIG. 7(a), the lenticular shaped martensitic grains appear needlelike shape microstructure and are well oriented. It has been found in particular near the center of the cross-section the axes of lenticular (lens-shaped) martensitic crystal grains are oriented substantially normal to the long edge of the flat shaped wire. The degree of orientation of martensitic grains at the edges as shown in FIGS. 7(b) &(c) are not as high as that shown in FIG. 7(a) which is near the center.

[0071] As a comparison, the microstructure at the edge (indicated by position (b) in FIG. 8) and near the center (indicated by position (a) in FIG. 8) of a wire rod with a round cross-section (FIG. 8) is also observed and shown in FIG. 9. The wire rod went through a same thermal treatment as the flat shaped wire of the invention, and there is no cold deformation applied to this wire rod during or after the thermal treatment. Without cold deformation, the wire rod appears a homogeneous microstructure. The martensitic grains are randomly oriented either near the center (FIG. 9(a)) or at the edge (FIG. 9(b)) of the wire rod.

[0072] As an additional and optional step, an anneal treatment may be used after rolling to remove stresses. The initial cold-rolled flat shaped wire has a tensile strength of about 2020 MPa, yield strength of about 1750 MPa and an elongation at fracture of about 4.2%. The work hardened steel wires continuously pass at a speed of 15 m/min through an annealing furnace or oven at a temperature between 350° C. and 750° C. The development of tensile strength (Rm-R), yield strength (Rp0.2-R) and elongation at fracture (At-R) of the steel wire as a function of the annealing temperature (AT) are shown in FIG. 10. When the wire was annealed at low temperature, i.e. about 400° C. or 450° C., the elongation was not improved and even slightly decreased. However, when annealed at a temperature above 500° C., the elongation at fracture (At-RTA) of work hardened steel wire increases with annealing temperature as shown in FIG. 10. When the steel wire was annealed at 700° C., the elongation at fracture (At-RTA) of steel wire can go up to about 9.5%. Both the tensile strength (Rm-RTA) and the yield strength (Rp0.2-RTA) decrease with the annealing temperature of the steel wire.

[0073] As an example, the work hardened steel wire is annealed so as to reduce its tensile strength Rm from about 2020 MPa to a value comprised between 1000 MPa and 1500 MPa, preferably comprised between 1200 MPa and 1500 MPa. As another example, the work hardened steel wire is annealed so as to reduce its tensile strength Rm from about 2020 MPa to a value comprised between 1500 MPa and 1900 MPa, preferably comprised between 1600 MPa and 1800 MPa. The annealing treatment on the one hand significantly influences the strength and the elongation of the wire, and on the other hand can also be controlled to improve fatigue resistance, corrosion resistance and resistance to hydrogen embrittlement.

[0074] According to the present invention, alternatively, warm rolling is used to flatten or reduce the thickness of the steel wire. The quenched and tempered round or flat wire is first warmed up to a temperature between 400° C. and 700° C. in a furnace or oven before the warm rolling, preferably in a median frequency induction heating furnace. Here, median frequency means a frequency in the range of 10 to 200 kHz. Preferably, a trimming unit is used during warm rolling that adjusts the temperature of the steel to compensate for heat loss that may occur during the rolling step.