TECHNICAL FIELD This invention relates to a turbocharger component made of cast austenitic stainless steel. BACKGROUND ART A turbocharger is used as means for improving the power output and the performance of an automobile engine. In a turbocharger, a turbine wheel is driven by the exhaust energy of the engine to rotate a compressor with the power of the engine, whereby the engine is supercharged to have more air fed into it than fed into it by natural suction. To improve operating efficiency, a variable nozzle turbocharger has been proposed. Such a turbocharger has a variable nozzle device for controlling the angle at which the exhaust has impacts the blades of the turbine wheel. A turbocharger having a variable nozzle device is disclosed in U.S. Patent No. 4,804,316. Fig. 1 shows a partial plan view of the variable nozzle device, Fig. 2 a partial sectional view taken along line 2-2 of Fig. 1, and Fig. 3 a partial sectional view taken along line 3-3 of Fig. 1. As shown best in Fig. 1, there is provided a turbine housing 10 which forms a generally scroll-shaped volute 12. The volute 12 accepts the exhaust gas from the engine and directs it onto the turbine wheel blades through an annular nozzle passage 14. Thereafter, the exhaust gas flows axially through a turbine shroud 16 and exits the turbocharger through an outlet 18. Located within the nozzle passage 14 are a plurality of pivotable vanes 20 which operate to vary the geometry of the nozzle passage 14 to control the angle at which the exhaust gas impacts the turbine wheel and ultimately the amount of air supplied to the engine. - 2 - WO 42143 July 28 , 2004

As shown in Fig. 3, the plurality of vanes 20 is mounted to a nozzle ring 30 having a plurality of radially spaced bores therethrough to accommodate a vane pin 22 associated with each vane 20. Attached to the other end of the vane pin 22 is a vane arm 24 such that the nozzle ring 30 is located between the vane 20 and the vane arm 24. As best shown in Fig. 2, the nozzle ring 30 is bolted directly to the turbine housing 10 so as to form the nozzle passage 14 therebetween. Associated with each bolt 26 is a spacer 28 which has an axial length slightly longer than the axial length of the vanes 20. The spacers 28 prevent sticking or binding of the vanes 20. Fig. 1 shows that there is provided an unison ring 40 which is an annular ring with a plurality of slots 42 on its inner radial surface. Each slot 42 receives a vane arm 24. Located at the internal periphery of the unison ring 40 are at least three spaced rollers 44. The rollers 44 are rotatably mounted on dowels 46 spaced radially inwardly of the unison ring 40 and secured between the nozzle ring 30 and the center housing of the turbocharger; each having bores therein for acceptance and location of the dowels 46. The rollers 44 include an annular groove there around for acceptance and guidance of the inner periphery of the unison ring 40. The rollers 44 and dowels 46 ensure the concentricity between the unison ring 40 and the nozzle ring 30. The above variable nozzle turbocharger has the turbine housing 10 coupled to the nozzle ring 30, so that mechanical and thermal load is directly transmitted from the turbine housing 10 to the components of the variable nozzle device. To avoid transmitting mechanical and thermal load from the turbine housing to the variable nozzle device, - 3 - WO 42143 July 28, 2004

WO 2004/022926 Al proposes a variable nozzle turbocharger having the components of the variable nozzle device mounted to the center housing of the turbocharger. Fig. 4 shows a partial sectional view of the turbine end of this turbocharger. As shown in Fig. 4, the turbocharger has a center housing 50 carrying a shaft 52 with a turbine wheel 54 attached at one end thereof, and a turbine housing 60 which defines a generally scroll-shaped volute 62 for accepting the exhaust gas from the engine and an outlet 68 for discharging the exhaust gas to a suitable pollution-control device or the atmosphere. The turbine housing 60 is mounted to the center housing 50 by means of V-bands 56. In the inner space defined by both housings 50, 60 there is provided an insert 70 that defines an annular nozzle passage 64 and a turbine shroud 66 for directing the exhaust gas from the volute 62 to the outlet 68. The insert 70 is bolted to the central housing 50 and carries all movable components of the variable nozzle device, including a nozzle ring 72 provided with a plurality of adjustable vanes 74 and a unison ring 76 for actuating the vanes 74. By means of the insert 70 and the bolts the variable nozzle device is decoupled from the turbine housing 60. The exhaust gas temperature of modern diesel engines is increasing above 8300C, so that the turbocharger components exposed to the exhaust gas require improved heat resistance and corrosion resistance. In particular, it is necessary to improve the heat and corrosion resistance of the variable nozzle device to prevent sticking and binding of the vanes in the nozzle passage which direct the exhaust gas onto the turbine wheel blades. - 4 - WO 42143 July 28, 2004

Advanced materials, which provide an improved heat and corrosion resistance and are suitable for the manufacturing of turbocharger components, are known, for example, from US 2002/01339448 Al, EP 1396620 Al and WO 2004/024970 Al. DISCLOSURE OF INVENTION The object of the present invention as to provide a turbocharger component made of an improved material which exhibits an optimum combination of heat resistance, corrosion resistance, wear resistance and machinability. According to the present invention, there is provided a turbocharger component as defined in claim 1, said turbocharger component being made of cast austenitic stainless steel consisting of C: 0.40 to 0.70 wt%, Si: 1.80 to 2.20 wt%, Mn: 1.50 wt% or less, S: 0.20 to 0.40 wt%, P: 0.06 wt% or less, Cr: 18.00 to 21.00 wt%, Ni: 12.00 to 14.00 wt%, balance Fe and inevitable impurities. The microstructure of the cast steel contains a network of eutectic chromium carbides in an austenitic matrix, thereby providing hot hardness and stability and improved creep resistance. The cast steel has, compared to a ferritic alloy, improved corrosion resistance at high temperature and, compared with other austenitic alloys, excellent high temperature oxidation resistance due to the high nickel content. Further, the cast steel has improved wear characteristics due to the enhanced carbon content, and also good machinability due to the presence of solid sulfur lubricant. The austenitic matrix may contain globular manganese sulfides as the solid sulfur lubricant. The hardness of the cast steel is preferably between 170 and 250 HBW 2.5/187.5 according to the European Norm - 5 - WO 42143 July 28, 2004

10003-1 or between 180 and 260 HVi-0 according to the European Norm ISO 6507-1. Due to optimum combination of heat resistance, corrosion resistance, wear resistance and machinability, the cast steel is particularly suitable for manufacturing a turbocharger component which is part of a variable nozzle device in a variable nozzle turbocharger. As explained in the discussion of the background art, the variable nozzle device comprises a nozzle passage for directing exhaust gas onto a turbine wheel, and a plurality of adjustable vanes placed located within the nozzle passage, for controlling the angle at which the exhaust gas impacts on the turbine wheel. Preferably, the turbocharger component is a nozzle ring carrying the plurality of adjustable vanes and forming part of the nozzle passage. In this case the properties of the cast steel help keeping the flatness of the nozzle ring even at high temperatures above 8300C and avoiding microwelding and wear between the vane pins extending through the nozzle ring and the corresponding bore under vibrating and aero force conditions. It is preferable that the other part of the nozzle passage is made of an austenitic alloy (i.e. iron or steel) to avoid a mismatch of the coefficients of thermal expansion. If the other part of the nozzle passage had a coefficient of thermal expansion substantially different from the nozzle ring, there would be the risk of sticking or binding of the vanes. The nozzle ring may be mounted to either the turbine housing or the central housing of the turbocharger by, for example, bolting. In the following, further details are given of the turbocharger component made of the cast austenitic stainless steel. - 6 - WO 42143 July 28, 2004

(Microstructure) The particular microstructure of the cast steel is illustrated in Figs. 5 and 6 which have different magnifications. While Fig. 5 shows the overall microstructure of the as-cast austenitic stainless steel, Fig. 2 illustrates the network of chromium carbides dispersed in the austenitic matrix. In the austenitic matrix, there are also dispersed some globular manganese sulfides which serve as a solid lubricant. (Hardness) The hardness range of the turbocharger component shall be 170-250 HBW 2.5/187.5 according to European Norm 10003-1. If the size or geometry of the turbocharger component will not allow a Brinell hardness test, an average of at least four Vickers measurements may be used according to European Norm ISO 6507-1. In this case the hardness range shall be 180-260 HVi.o« (Manufacture) The austenitic stainless steel may be supplied as- cast, and the turbocharger component may be machined from the as-cast material. In case of a nozzle ring, the austenitic stainless steel may be cast into the shape of a pipe. Then, the pipe is cut into slices to obtain a nozzle body. Finally, bores are drilled into the nozzle body for accommodating the vane pins of the vanes, and the bolts for attaching the nozzle ring to the turbine housing or central housing of the turbocharger. (Use of the turbocharger component) The claimed turbocharger component may be used in the conventional turbochargers illustrated in Figs. 1 to 4, but is not limited thereto. - 7 - WO 42143 July 28, 2004

As discussed above, the cast austenitic stainless steel exhibits an optimum combination of heat resistance, corrosion resistance, wear resistance and machinability. Therefore, the cast steel is suitable for all components of the turbocharger which are exposed to the hot and corrosive atmosphere of the exhaust gas, and especially for all components requiring good machinability. Although the cast steel may be used as a material for manufacturing the turbine housing or an insert that forms part of the nozzle passage where the vanes are located, the cast steel is particularly suitable for manufacturing the vanes and vane pins of the variable nozzle device and above all the nozzle ring. If the cast steel is used for manufacturing the nozzle ring, it is preferable that the thermal expansion characteristic of the turbine housing or insert forming the other part of the nozzle passage is adjusted to the austenitic behavior of the nozzle ring to prevent sticking or binding of the adjustable vanes. The thermal expansion characteristic may be adjusted by manufacturing the turbine housing or insert from austenitic alloy (i.e. iron or steel) . A suitable austenitic alloy for the turbine housing or insert is D5S Ni-resist, which consists of C: 2.0 wt% or less, Si: 4.8 to 5.3 wt%, Ni: 34.0 to 38.0 wt%, Cr: 1.6 to 2.2 wt%, Mn: 0.7 wt% or less, Mg: 0.035 to 0.9 wt%, P: 0.07 wt% or less, rare earth element: 0.01 wt% or less, balance Fe and inevitable impurities. D5S Ni-resist has a coefficient of thermal expansion of about 17xlO"6/°C between 800 and 9000C. It goes without saying that, if all the components forming the nozzle passage are made of austenitic alloy, the heat and corrosion resistance are improved as compared with a nozzle passage a part of which is made of - 8 - WO 42143 July 28 , 2004

ferritic alloy. Therefore, if improved wear resistance and machinability are no issue, it is acceptable if the components forming the nozzle passage are made of austenitic alloy other than the cast steel defined in the attached claims.