TECHNICAL FIELD

The present invention relates to a method of manufacturing surface hardened forged steel components, and in particular it relates to manufacturing components having forged gear teeth, such as steering racks.

BACKGROUND

Surface hardening steel components by induction hardening is well known. The process provides a hard, wear resistant outer layer, whilst retaining a relatively soft, tough core, and is well suited to mass production. Induction hardening is typically applied to medium carbon steels, having between 0.3% and 0.8% carbon (0.3 - 0.8 %C) and more typically about 0.4%C. The theory and practice of induction hardening steel is well known. In brief, induction hardening uses an electrical conductor, usually in the form of a coil with at least one turn, placed close to the surface area to be hardened and energised by an electric current at a suitable frequency. This heats the surface layer of the steel component above the austenising temperature, which is then rapidly cooled to harden it, typically by quenching using a coolant such as water. Specially developed quenching systems may be used to minimize distortion. After hardening, the component may be tempered to improve its toughness.

Gear teeth, and in particular the teeth of steering racks for automotive rack and pinion steering, are commonly induction hardened. A problem with induction hardening gear teeth is that it is difficult to obtain even hardness depth between the tips and roots of the teeth. Often the tips of the teeth will be nearly through hardened in order to obtain sufficient hardness depth at the roots. Another problem is that heating and quenching the surface of an otherwise cold component may cause cracking or excessive distortion. It is known that pre-heating the component to a temperature below the austensing temperature prior to induction hardening reduces these problems. Also, the energy used to induction harden a pre-heated component is significantly lower than induction hardening a cold component because the increase in temperature required to reach the austenising temperature is lower. Also, there is less conduction away from the surface due to the reduced temperature differential between the surface and core of the component. One method of pre-heating uses two frequencies for induction heating. This method is described in US Patent 6,315,841 (Fisher et al), as applied to the teeth of forged bevel gears. The first lower frequency pre-heats the component before swapping to a higher frequency to further heat just the surface layer for hardening. The disadvantage of this method is the additional cost and complexity of the induction hardening equipment.

Conduction heating is an alternative to induction heating, although it is less commonly used. Two electrical contacts are made with the component, one at each end of the surface to be hardened, and a high frequency electrical current is made to pass through the component. An inductor placed close to the surface induces the current to flow near the surface of the component thus localising the heating to the surface layer, in a similar manner to induction hardening. After heating, the component is quenched to harden it. Since conduction hardening primarily only heats the surface layer, like induction hardening, it has the same problems as induction hardening and also benefits from pre-heating.

Warm forging of steel is well known. The actual temperature used for warm forging varies with the application from about 6000C up to 10000C. The advantages of warm forging over hot forging include minimisation of scale and increased forging precision. Warm forging is particularly applicable to forging net shape gear teeth. Net shape means that no finish machining of the teeth is required after forging. It is well known to warm forge the teeth of a steering rack to net shape from a round bar, and in this application the warm forging temperature is typically between 6500C and 85O0C. An apparatus for warm forging steering racks is disclosed in US Patent 5,862,701 (Bishop et al).

After a steering rack is warm forged, it is typically cooled to room temperature in a controlled manner and later induction hardened. The component is thus heated twice, once to reach the forging temperature and again for induction hardening, which wastes energy. Medium carbon steels that are commonly used for manufacturing forged and induction hardened components, and particularly steering racks, include SAE 1040 and DIN 37OS4. Medium carbon steels having a bainitic structure (bainite) may also be used. An advantage of steel having a bainitic structure is that it is stronger than a pearlitic structure whilst still maintaining good levels of ductility. Another advantage of a bainitic structure, as described in US Patent 5,667,605 (Bellus et al), is that it maintains its hardness after re-heating, providing it is not held at temperature for an excessive period of time. This is because a steel with a bainitic structure is slower to transform to austenite than other structures. A steel grade disclosed in US 5,667,605 having a composition of 0.35%C, 1.8%Mn, 0.12%V, and other elements, which is equivalent to DIN 35MnV7, is particularly suitable for producing a bainitic structure for forging applications.

A fine microstructure, such as bainite or a mixture of fine pearlite and ferrite, can be obtained in a plain iron-carbon steel, such as SAE 1040 or 37CrS4, by firstly rapidly cooling from an austenising temperature to a temperature above the temperature that transformation to martensite starts, and then holding this temperature until the structure transforms to bainite, or a mixture or fine pearlite and ferrite. The temperature at which transformation to martensite starts varies with steel grade and is typically between 230° and 35O0C. The actual type of fine microstructure that develops depends on the temperature that the steel is rapidly cooled to and the rate of cooling. Like bainite, a mixture of fine pearlite and ferrite improves strength whilst maintaining good levels of ductility.

It is an object of the present invention to ameliorate at least some of the problems of the prior art. SUMMARY OF INVENTION

The present invention consists of a method of manufacturing a steel component comprising the steps of

a) heating at least a portion of a steel blank to a first temperature of at least 6000C; b) forging said portion to shape; c) cooling said portion in a controlled manner to a second temperature above 2000C; d) heating at least part of the surface of said portion to at least an austenising temperature; and then e) quenching said portion thereby hardening said surface.

Preferably said first temperature is less than 10000C, and more preferably said first temperature is between 75O0C and 8500C.

Preferably said second temperature is less than 5000C, and more preferably said second temperature is above 3000C.

Preferably said steel blank is made from a medium carbon steel suited to induction hardening.

In one preferred embodiment, said steel blank is made from steel having a bainitic structure and said second temperature is above 600°C, and more preferably said second temperature is between 650°C and 7000C.

In another preferred embodiment, step (c) comprises rapidly cooling said portion to said second temperature and holding said portion at said second temperature until said portion forms a fine microstructure, and more preferably said rapid cooling takes less than 20 seconds and said second temperature is between 4000C and 5500C. Preferably at step (d) said surface is heated by induction heating. Preferably the frequency of said induction heating is between 1kHz and 60OkHz. Preferably said surface is progressively locally heated and quenched by an induction coil and quench ring that traverses said surface together. Alternatively, at step (d) said surface is heated by conduction heating.

Preferably at step (b) said portion is forged to a shape comprising net shape gear teeth. Preferably said component is a steering rack and said blank is a round bar.

Preferably said method further comprises the step of tempering said portion after step (e).

In one preferred embodiment, the present invention consists of a method of manufacturing a steering rack from a round bar of steel comprising the steps of

a) heating at least a portion of said bar to a first temperature between 7500C and 8500C ; b) forging net shaped gear teeth on said portion; c) cooling said portion to a second temperature between 4000C and 5500C in less than 20 seconds; d) holding said portion at approximately said second temperature until said portion forms a fine microstructure; e) heating at least the surface of said teeth to at least an austenising temperature; f) quenching said portion thereby hardening said surface; and then g) tempering said portion. BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 schematically illustrates a method of manufacturing a surface hardened forged steel steering rack in accordance with the present invention.

Fig. 2 shows a steering rack manufactured in accordance with the present invention.

Fig. 3 is a sectional view of the steering rack of Fig. 2 along line III-III.

Fig. 4 is a sectional view of the teeth of the steering rack of Fig. 3 along line IV-IV.

Fig. 5 shows a method of induction hardening the toothed portion of the steering rack depicted in Fig. 2.

BEST MODE OF CARRYING OUT THE INVENTION

The invention will be described as applied to steering racks. However, the invention is equally applicable to other forged steel components requiring surface hardening, and in particular to other components that may be made with net shape forged gear teeth such as bevel gears, ring gears, crown wheels, hypoid gears, steering pinions or differential pinions.

Fig. 1 schematically illustrates a method of manufacturing a surface hardened forged steel steering rack in accordance with the present invention comprising steps 1 to 5.

Step 1 comprises heating a steel blank in the form of a round bar to a temperature above 6000C. Preferably the bar is heated to a temperature between 6000C and 10000C suitable for warm forging, and more preferably between 7500C and 85O0C. Preferably heating is done by induction and only the portion of the bar that will be forged is heated. The round bar is made from a medium carbon steel suitable for induction hardening, such as SAE 1040 or DIN 37CrS4. The medium carbon steel may have a bainitic structure, in which case it will preferably be grade DIN 35MnV7. Step 2 comprises forging a toothed portion from the heated portion of the round bar. Fig. 2 shows a steering rack 10 forged from the heated round bar in a die apparatus as described in US 5,862,701. Forged toothed portion 11 has a Υ' section as a result of being forged in the apparatus described in US Patent 5,862,701. However, the section of forged toothed portion 11 may have other shapes, such as a conventional 'D' shape, if other types of forging die are used. Forged teeth 12 are net shape and hence finish machining is not required.

Step 3 comprises cooling forged toothed portion 11 in a controlled manner to a temperature above 2000C, and the preferred temperature range for conventional medium carbon steels, such as SAE 1040 or DIN 37CrS4, is between 3000C and 5000C. However, if rack 10 is forged from a steel having a bainitic structure, such DIN 35MnV7, then it is preferable to only cool down to a temperature above 6000C, and more preferably between 65O0C and 7000C. This is because the bainitic structure allows the steel to be held at a high temperature longer than a conventional steel without transforming its structure. A suitable cooling method is to blow air over forged toothed portion 11 whilst rack 10 is held in a fixture. The cooling is controlled in such a manner to minimise distortion and bending of rack 10.

An alternative cooling method may be employed if rack 10 is forged from a plain iron- carbon steel, such as SAE 1040, and it is desired form a fine microstructure, such as bainite or a mixture of fine pearlite and ferrite. In this case, forged toothed portion 11 is first rapidly cooled to a temperature above the temperature that transformation to martensite starts, as discussed in the background. Cooling may be done by water mist, fluidised bed or an air pulsed water mist spray. Then, forged portion 11 is held at this temperature until the structure transforms to a fine microstructure. Preferably, to form a mixture of fine pearlite and ferrite in SAE 1040, forged portion 11 is firstly cooled from forging temperature to between 4000C and 5500C in less than 20 seconds. Then forged portion 11 is held at this temperature until the microstructure transforms to bainite or a mixture of fine pearlite and ferrite, which takes approximately 60 seconds. Step 4 starts with forged portion 11 at the temperature cooled to in step 3. The subsequent surface hardening process described below thus has the advantages of pre-heating as discussed in the background. Step 4 comprises heating the surface of forged toothed portion 11 , including the surface of forged teeth 12, to a temperature above the austenising temperature. The surface is heated quickly so that the core of forged toothed portion 11 remains at approximately the temperature cooled to in step 3. A steering rack requires the whole surface of forged toothed portion 11 , rather than just forged teeth 12, to be hardened because forged toothed portion 11 slides in a rack pad when assembled into a steering gear. Surface heating is preferably done by induction. This may be done by an induction coil surrounding forged toothed portion 11 and extending over its length. The frequency used for induction heating will depend on the exact application. For steering racks, suitable frequencies range from 1 kHz to 60OkHz. Alternatively, heating may be done by conduction as discussed in the background. Since forged toothed portion 11 has effectively been pre-heated, the time and energy required to heat the surface layer to sufficient depth is significantly reduced compared with surface heating a cold component.

Step 5 comprises quenching the surface of forged toothed portion 11 immediately after it has been heated above the austenising temperature. This hardens the surface by forming martensite. Preferably, quenching is done in such a manner that it controls the amount of distortion of the rack. After hardening, forged toothed portion 11 may be tempered to increase the toughness of the hardened surface layer. If rack 10 is forged from a steel having a bainitic structure, such DIN 35MnV7, and it was only cooled down to a temperature above 6000C at step 3, then it may be necessary to press quench forged toothed portion 11 to minimise distortion. Press quenching involves pressing toothed portion 11 against a correspondingly shaped fixture during quenching.

Fig. 3 is a sectional view of forged toothed portion 11 along line III-III of Fig. 2 after completion of steps 1 to 5. The hardened surface layer is indicated by reference numeral 13. Fig. 4 is a sectional view of forged teeth 12 along line TV-IV of Fig. 3. As shown, the roots 14 of teeth 12 have sufficient hardness depth 16 to resist fatigue failure, whilst the teeth 12 themselves are not through hardened. This desirable balance between sufficient hardness depth in the roots of the teeth and hardness depth not being excessive in the teeth themselves is partially due to the heat remaining after cooling step 3.

Step 4, surface heating, and step 5, quenching, may alternatively be performed progressively along forged toothed portion 11 as shown in Fig. 5. Induction heating coil 17 is narrow relative to the length of forged toothed portion 11 , and adjacent to it is quench ring 18 (both shown sectioned). Coil 17 and quench ring 18 surround forged toothed portion 11 and traverse along it together, as indicated by arrow 21. Coil 17 locally heats forged toothed portion 11 in region 19, and coolant 20 sprayed from quench ring 18, following behind coil 17, immediately quenches and hardens locally heated region 19. Thus, forged toothed portion 11 is progressively hardened along its length. Quench ring 18 may be segmented in a manner where spray holes are at varying axial locations to control the straightness of toothed portion 11 during quenching. For example, the spray holes that direct coolant to the teeth may be closer to coil 17 than the spray holes that are directed at the back of the rack.