The present invention relates to a screw consisting in the main of stainless steel with a shank with a point and at least one threaded portion. The invention aims especially at two types of screw, namely self- drilling screws intended for fastening sheet metal, in particular stainless sheet metal, on roofs and house facades, and self-tapping screws. The invention also relates to a method for manufacturing and treating a self-tapping or self- drilling screw and also use of a specific steel material for this manufacturing.

A self-drilling screw intended as a fastening element for stainless sheet metal on roofs and house fa ades has to satisfy a number of requirements, some of which are difBcult to combine: the material must have cold-workability such that it can be shaped in the cold state into a screw with a point which can drill the screw through a stainless metal sheet, the screw, or at least an outer layer, and in particular the surface in the screw point, must have very great hardness, the screw must have very good inherent corrosion-resistance and no stress corrosion must arise as a result of galvanic reactions between the screw and the stainless metal sheet, at least the screw head must have the same colour as the stainless metal sheet for aesthetic reasons, the screw must not be too expensive.

Many different alloys, in particular different steel alloys, have been proposed for self- drilling screws, inter aha martensitic, stainless chrome steel, but stainless austenitic steel also has been proposed for this purpose. EP- 0 523 298 proposes using a conventional austenitic stainless steel of the 316 type which is nitrided in order to produce a hard outer layer, after which the nitrided outer layer is covered by a metal layer or by a plastic layer in order to prevent rust formation on the nitrided layer. As far as the applicants know, however, none of the previously proposed materials has satisfied all the requirements indicated above. One aim of the invention, therefore, is to offer a screw made of such a material and treated in such a manner that it satisfies all the requirements made of self-drilling screws intended to be used as fastening elements for stainless roof or facade covering.

As far as self-tapping screws are concerned, it is of primary importance that the screw has very high hardness, at least in an outer layer, in order to be able to function as "its own screw tap" in order to bring about threads in a bore hole. In the event of the screw being used as a fastening element for stainless sheet metal or other stainless elements, the same requirements with regard to corrosion resistance exist in the case of self-drilling screws also.

In order to achieve the abovementioned and other aims, the screw is characterized, according to a first aspect of the invention, in that at least the point and a length of said at least one threaded portion has a surface-hardening layer produced by ion nitriding which has a thickness of 0.01-0.2 mm and a hardness of at least 900 HV, preferably 1000-1300 HV. Preferably only the screw point and at least a front portion of the threaded screw shank have said surface-hardening layer produced by ion nitriding, while at least the screw head lacks such surface-hardening layer. The ion nitriding according to the method according to the invention amounts to the screw being exposed to electric high voltage in a furnace which is filled with a mixture of nitrogen gas and hydrogen gas so that the gas is ionized and at least the point of the screw and at least a portion of the threaded part of the screw shank are supplied with nitrogen by ion bombardment for a time of 5-50 h and at a temperature of 450-600°C, as a result of which said screw surfaces are ion nitrided to create said surface-hardening layer which has a thickness of 0.01-0.2 mm and a hardness of at least 900 HV. In order that only the screw point and at least a front portion of the threaded screw shank are surface-hardened by ion nitriding, the screw can be placed in a hole in a fixture so that the point and said portion of the threaded shank project from the fixture and are exposed to the ion bombardment in the furnace, while the rest of the screw is shielded by a protection which covers the screw head.

According to another aspect of the invention, the stainless steel contains a maximum of 0.1% carbon, preferably a maximum of 0.08% carbon, and, as a stabilizing element, at least one of the elements niobium, tantalum and titanium in a total content amounting to at least five times the carbon content, but a maximum of 1%. By these means, it is possible to counteract local depletion of chromium as a result of the formation of chromium carbides at the grain boundaries, which could otherwise take place during ion nitriding treatment, which in turn could entail a risk of grain boundary corrosion.

For self-drilling screws with small or modest diameters, e.g. diameters up to approx. 4.8 mm stabilized austenitic stainless steel, e.g. stabilized stainless steel of 18/9 type, can

have not only adequate corrosion resistance but also adequately high hardness, normally a maximum of 250 HV. For self-drilling screws of larger dimensions, however, the risk can arise that the hard outer layer is peeled off from the underlying steel during the drilling operation. At least for such applications, it may be more expedient to select as starting material a stabilized stainless steel which is precipitation hardening by ageing treatment. After ageing treatment, it is possible to obtain with such a steel a hardness in the steel mass of 300-500 HV.

Further aspects and characteristics of the invention emerge from the following description of exemplary embodiments and from the subsequent patent claims.

In the following description, reference will be made to attached drawing figures, of which

Fig. 1 shows a self-drilling screw of a design known per se, in the case of which the invention can be applied, Fig. 2 shows diagrammatically and on very large scale the construction of an outer layer on the screw within a ringed area A, Fig. 1, according to a first embodiment, Fig. 3 shows diagrammatically and on very large scale the construction of an outer layer within the ringed area of the screw according to another preferred embodiment, and Fig. 4 illustrates how the screws can be arranged in a fixture during ion nitriding.

The starting material for the screw, like the basic mass of the finished screw, i.e. the entire screw with the exception of the outer layer, consists of carbon-stabilized stainless steel. Examples of types of stabilized stainless steel are stabilized austenitic stainless steel and stabilized precipitation-hardening stainless steel. Examples of specific steel alloys which can be used for the screw according to the invention are given in the table below, in which steels no. 1-3 are constituted by stabilized stainless steel and steels no. 4 and 5 represent stabilized precipitation-hardening stainless steel.

Table 1

Steel C Si Mn Cr Ni Other Rest no.

1 < 08 ≤l.O <2.0 17-20 9-13 lOxC≤Nb+Ta <1.0 Fe an unavoidable impurities

2 ≤.08 ≤l.O <2.0 17-20 9-13 5xC≤Ti<0.5

3 .01 0.5 0.6 18 9.5 3.5Cu, 5xC≤ Ti<0.5

4 < 07 ≤l.O ≤l.O 15-17.5 3-5 ≤0.5Mo, 3-5 Cu, 5xC≤Nb+ Ta<0.5

5 < 07 ≤l.O ≤l.O 15-17.5 6-8 1A1, 5xC≤ Ti< 0.5

In Fig. 1, a screw has been generally designated 1. In order that, for a self- drilling screw, it has adequate surface hardness in the drilling point 2, the screw has been surface- hardened by so-called ion nitriding, also called plasma nitriding. This is a method which has obtained its name from the fact that a plasma or in other words an ionized gas is utilized as heating and nitriding medium in the process. During this treatment, the screws 1 are placed in a fixture 10, Fig. 4. This consists of a flat box 11 with a plane bottom 12 which is provided with small openings 13 for the screws 1 which are to be ion nitrided. In the box, there is also a plate 14, the thickness of which determines the height of the screw heads 6 above the bottom 12. The plate 14 has through-holes 16 in front of the holes 13 in the bottom 12. The screws which are to be ion nitrided are mounted in the holes 16, 13, whereupon the box 11 is covered by a cover 17. Both the box 11 and the cover 17 consist of metal and form a shield against the ionization for the screw heads 6 and for the parts 9 of the shank 7, Fig. 1, which are situated inside the fixture 10, i.e. within the area of the holes 13 and 16. The only part of the screws 1 which is exposed to the plasma nitriding is thus the drilling screw point 1 and the front portion 8 of the shank 7.

The fixture 10 with the screws 1 is placed in a furnace which is filled with a mixture of nitrogen gas and hydrogen gas. A voltage of 1000 V is connected between the screws and the furnace wall. By these means, the gas is ionized, the ions striking with great kinetic energy the screw surfaces which are exposed outside the fixture 10, i.e. the point

2 and the front portion 8 of the shank 7. At the same time, the components are heated to the desired nitriding temperature so that no separate outer furnace heating is required. By ion bombardment, nitrogen is supplied to the screw surfaces, which produces the desired nitriding effect. By virtue of the fact that the steels are stabilized, the temperature can be allowed to rise to 550-600°C. Normally, the temperature lies between 450 and 600°C, preferably between 500 and 600°C. The treatment is carried out for a time of 5- 50 h, preferably for a maximum time of 30 h, which is possible if the temperature lies within the higher range. The treatment should preferably be carried out for 10-30 h.

As a result, a surface-hardened layer 3 is obtained. One problem with ion nitriding, however, is that the passivation layer of the stainless steel is partially destroyed by the ion bombardment, which means that the corrosion resistance is reduced. In order inter alia to restore the corrosion resistance, the ion nitrided screws are covered - according to one embodiment, Fig. 2 - with a thin zinc layer by gah anotechnical treatment. This zinc layer 4 has a thickness of at least 5 μm, preferably at least 8 μm, but does not exceed 25 μm thickness. A further effect of the zinc layer is that it gives the screw lubricating properties which are advantageous when the screw is to be used as a self-drilling screw. The zinc layer also gives the screw an aesthetically attractive surface and colour. Galvanization is carried out expediently by immersion in an acid zinc bath after pickling in an acid bath to remove oxides on the surface of the screw.

However, the zinc layer can also have defects. In order further to improve the corrosion resistance, the zinc layer can in a further operation be coated with a very thin chromium layer 5. A typical thickness of this is 1-5 μm or approximately 2 μm. The chromating can be carried out by depositing trivalent chromium - so-called blue chromate - by means of immersion for approximately 1 minute.

Finally, the screw heads can be covered with a lacquer layer, expediently a polyester lacquer, which is sprayed onto the heads in powder form, whereupon the lacquer is hardened in a manner known per se. This lacquer layer gives a further improvement of the corrosion protection on the part which remains exposed, at the same time as, by selecting a suitable colour of the lacquer, it is possible to obtain a screw can which completely matches the metal sheet in which the screw is to be used.

According to an alternative embodiment, Fig. 3, the screw can, after ion nitriding and pickling, be electrolytically polished, i.e. treated in an electrolytic bath according to principles known per se, so that a very thin layer of the raw surface which is produced by

ion nitriding is worked off In particular peaks on the surface are worked off so that a smooth surface finish is obtained. While the layer surface-hardened by ion nitriding has a thickness of 0.01-0.2 mm, with an average depth of approximately 0.05 mm, the surface layer worked off by electrolytic polishing is only 1-5 μm, normally 2 to 3 μm, in the ion nitrided areas 2, 8 of the screw and in the remaining areas 10-40 μm, normally of the order of 20 μm By means of this treatment, the abovementioned galvanization can be eliminated for certain applications. The electrolytically polished screw also can, if desired, of course be provided with a suitable lacquer colour on the screw head 6.