METHOD FOR PRODUCING HOT-FORMED STEEL SPRINGS

Abstract

A process for producing a spring or torsion bar from a steel wire by hot forming may involve providing a steel wire; thermomechanically forming the steel wire; cooling the steel wire thermomechanically; cutting the steel wire to length to give rods; heating the rods; hot forming the rods; and tempering the rods to give a spring or torsion bar, comprising quenching the rods to give a spring or torsion bar to a first cooling temperature, reheating the spring or torsion bar to a first annealing temperature, and cooling the spring or rod to a second cooling temperature. Further, in some examples, the cooling of the steel wire may be cooled to a temperature below a minimum recrystallization temperature such that at least a partly ferritic-pearlitic structure is established in the steel wire.”

Claims

1.-15. (canceled)

16. A process for producing a spring or torsion bar from a steel wire by hot forming, the process comprising: providing a steel wire; thermomechanically forming the steel wire above a minimum recrystallization temperature of the steel wire, the steel wire having an at least partly austenitic structure; cooling the thermomechanically-formed steel wire such that at least a partly ferritic-pearlitic structure is formed in the steel wire; cutting the steel wire to length to form rods; heating the rods at least to a forming temperature, the forming temperature being above the minimum recrystallization temperature of the steel wire; hot forming the heated rods; tempering the hot-formed rods, wherein the tempering comprises: quenching the rods to a first cooling temperature, the first cooling temperature being a temperature below the minimum recrystallization temperature of the steel wire, wherein an at least partly martensitic structure is formed in the rods, reheating the quenched rods to a first annealing temperature that is less than an austenite start temperature, and cooling the reheated rods to a second cooling temperature, the second cooling temperature being less than the first annealing temperature.

17. The process of claim 16 wherein the forming temperature to which the rods are heated is equal to or greater than the austenite start temperature.

18. The process of claim 16 wherein the forming temperature to which the rods are heated is equal to or greater than an austenite end temperature.

19. The process of claim 16 wherein the cooling of the steel wire to the temperature below the minimum recrystallization temperature occurs before the steel wire is cut to length to form the rods.

20. The process of claim 16 wherein the steel wire is cut to length to form the rods from the steel wire before the cooling of the steel wire to the temperature below the minimum recrystallization temperature occurs.

21. The process of claim 16 wherein the steel wire is cut to length to form the rods from the steel wire before the rods are heated to the forming temperature.

22. The process of claim 16 wherein the steel wire is heated to the forming temperature before the steel wire is cut to length to form the rods.

23. The process of claim 16 wherein the thermomechanical forming occurs at a temperature equal to or greater than the austenite start temperature.

24. The process of claim 16 wherein the thermomechanical forming occurs at a temperature equal to or greater than an austenite end temperature.

25. The process of claim 16 wherein the thermomechanical forming occurs at a temperature in a range between an austenite end temperature and 50° C. above the austenite end temperature.

26. The process of claim 16 wherein the cooling of the thermomechanically-formed steel wire is effected to a temperature below the minimum recrystallization temperature.

27. The process of claim 16 wherein the cooling of the thermomechanically-formed steel wire is effected to a temperature below 200° C.

28. The process of claim 16 wherein the cooling of the thermomechanically-formed steel wire is effected to a temperature below 90° C.

29. The process of claim 16 wherein the quenching causes the rods to at least partially convert to martensite and to be exposed to at least a martensite start temperature, wherein the first cooling temperature is less than or equal to 200° C.

30. The process of claim 16 wherein the tempering establishes a hardness profile over a cross section of the rods.

31. The process of claim 16 wherein after the tempering the process further comprises edge heating and then re-cooling the rods, wherein a hardness of the rods increases from an edge to a core of the rods.

32. The process of claim 16 further comprising coiling the steel wire after cooling the thermomechanically-formed steel wire.

33. The process of claim 16 wherein after the cooling, the cutting, the heating, the hot forming, or the tempering, the process further comprises surface treating the steel wire or the rods to create the spring or torsion bar, wherein the surface treating comprises at least partly removing a surface of the steel wire or the rods.

Description

PREFERRED WORKING EXAMPLE OF THE INVENTION

[0101] Further measures which improve the invention will be discussed in detail hereinafter together with the description of a preferred working example of the invention and with reference to the figures. The figures show:

[0102] FIG. 1 a schematic diagram of the process of the invention in one embodiment of the invention,

[0103] FIG. 2 a temperature profile for the embodiment according to FIG. 1,

[0104] FIG. 3 a schematic diagram of a process according to the prior art.

[0105] FIGS. 1 and 2 are described together hereinafter. A wound steel wire 1 is provided on a ring 10. This is at first heated to a first forming temperature T.sub.1 of about 800° C., which is above minimum recrystallization temperature of the steel wire 1s, especially above the austenitization temperature Ac.sub.3 of in the present case 785° C., 11. Then the steel wire 1 is subjected to thermomechanical forming 12. The heating 11 can be dispensed with when the TMF immediately follows the steel wire rolling process and the temperature of the steel wire 1s is still at the desired forming temperature T.sub.1.

[0106] The thermomechanical forming 12 can be effected by multistage caliber rolling. Subsequently, the steel wire 1 is cooled 13 at such a slow rate that a pearlitic-ferritic structure, i.e. a soft structure, is established in the steel wire 1. The cooling can be effected without any further intervention by simple storage at room temperature or ambient temperature, but the cooling is preferably effected in a controlled manner. The cooling can additionally or alternatively also be effected during or after the coiling 14 of the steel wire 1, which is readily possible because of the soft microstructural state. For cooling, a heat exchanger may be provided, such that the waste heat can be fed back to the process.

[0107] When the steel wire 1 is then coiled, it can be transported from one processing site to the next processing site and processed further there. In FIG. 2, this is illustrated by a gap in the temperature profile after the coiling 14. A spring manufacturer can then purchase the steel wire 1 pretreated by thermomechanical forming 12 from a steel wire manufacturer, and need not keep the equipment required for the TMF in house. This saves space and capital costs for the spring manufacturer.

[0108] After any desired period of storage and/or transport, the hot forming 18′, 18″ and tempering of the steel wire 1 commences, and consequently need not follow the TMF directly (or even in terms of location). After uncoiling 15, the steel wire 1 is cut 16 into individual rods 2. In connection with the cutting of the steel wire 1, it is possible for further processing steps to be conducted on the steel wire 1 or on the cut rod 2, for example cleaning, straightening, grinding and/or peeling.

[0109] Subsequently, for preparation for the hot forming 18′, 18″, the rod 2 is heated 17 to a second forming temperature T.sub.2 above the austenitization temperature Ac.sub.3. In the present case, the second forming temperature T.sub.2 is about 950° C. The heating is effected very quickly and is preferably conducted by inductive or conductive means. The heating is effected at a heating rate of at least 50 K/s, preferably at least 100 K/s. Subsequently, the bar 2 is subjected to hot winding 18′ to give the spiral spring 3′ or to hot bending 18″ to give the torsion bar 3″. Without any significant change in temperature (although the temperature of the spring 3 can quite possibly be slightly reduced during the hot forming 18′, 18″ and prior to the quenching), the quenching 19 is effected, for example, in an oil bath, which establishes a martensitic structure in the spring 3 produced. Subsequently, the spring 3 is annealed 20.

[0110] The process according to prior art is shown in schematic form in FIG. 3. In a departure from the inventive concept, the cutting to give the bars 2 precedes the performance of the TMF. The TMF 12 was conducted in an integral manufacturing line together with the hot forming 18, 18′, 18″ and the quenching 19. The steel wire 1 heated for the TMF was kept at a temperature above the austenitization temperature apart from the quenching.

[0111] In terms of its execution, the invention is not restricted to the preferred working example specified above. Instead, there is a number of conceivable variants which make use of the solution presented even in executions of a fundamentally different kind. All the features and/or advantages that are apparent from the claims, description or drawings, including details of construction or three-dimensional arrangements, may be essential to the invention either on their own or in a wide variety of different combinations.

INDUSTRIAL APPLICABILITY

[0112] Springs and/or torsion bars of the above-described type are used, for example, in the production of motor vehicles, especially of motor vehicle chassis.

LIST OF REFERENCE SIGNS

[0113] 1 steel wire

[0114] 2 rod

[0115] 3 spring

[0116] 3′ helical spring

[0117] 3″ torsion bar

[0118] 10 ring

[0119] 11 heating

[0120] 12 thermomechanical forming (TMF)

[0121] 13 cooling

[0122] 14 coiling

[0123] 15 uncoiling

[0124] 16 cutting to length

[0125] 17 heating

[0126] 18 hot forming

[0127] 18′ hot winding

[0128] 18″ hot bending

[0129] 19 quenching

[0130] 20 annealing