The invention concerns a self-screwthread-forming screw of corrosion-resistant steel.
In general screws have a shank which is provided with a male screwthread and which is delimited by a screw head at one end. As their name suggests, self-screwthread-forming screws automatically form their screwthread in a bore hole when they are screwed in. For that purpose a shank portion at the end of the shank, that is remote from the head, is in the form of a thread-forming region.
Self-screwthread-forming screws can be in the form of drilling screws. Drilling screws have a drilling tip which makes it unnecessary to pre-drill a hole for the screw. The hole for the screw on the contrary is cut by the drilling tip when the drilling screw is screwed in. In a second stage in the same working operation, the screw then forms a screwthread in the bore which has just been cut, and that screwthread affords the screw the desired hole in the bore. A drilling screw therefore avoids the need for the separate working operations which are otherwise necessary, of pre-drilling a hole and cutting a screwthread in the hole. It is therefore desirable to use self-screwthread-forming screws and in particular drilling screws, in place of conventional screws. An alternative to drilling screws is flow-hole-forming screws which also themselves produce the hole required for screwing engagement. Unlike a drilling screw with a drilling tip however the hole is not produced by a cutting procedure but by changing the shape of the material into which the screw is screwed. In that situation, a bead or ridge of material is produced around the hole, and that bead or ridge, also provided with a female screwthread, contributes to the strength of the screw connection in terms of the screw being torn out.
However a conflict occurs whenever a self-screwthread-forming screw as such or in the form of a drilling screw or flow-hole-forming screw is also to be corrosion-resistant. For the screw to form the screwthread and even more for the screw to drill a hole, the screw must have a quite particularly hard shaping region (in this case the cutting region of a drilling screw, formed by the drilling tip, as well as flow-hole-forming shank portions, are also deemed to constitute a shaping region in that respect). Good corrosion resistance and a high level of hardness however are considered to be properties which are generally mutually exclusive.
All previously known proposals for resolving this conflict of aims are unsatisfactory from one point of view or another. It is known for example to produce two-part drilling screws which comprise a hard but corrosion-susceptible material in the region of the drilling tip and the shaping region while in the rest of the shank region they comprise a material which is less hard but in return corrosion-resistant. It will be appreciated that the manufacture of two-part connecting elements of that kind is expensive. Two-part drilling screws are also mentioned in German Utility Model specifications Nos 297 09 932 and 297 06 372. A two-part nail is described in European patent specification No 0 545 852.
Above-mentioned Utility Model specifications Nos 297 09 932 and 297 06 372 also put forward proposals for resolving the above-indicated problem: DE U1 ""932 describes a one-part screw of austenitic high-quality steel with partially hard surface. In regard to that partially hard surface, the Utility Model specification only contains the indication that the screw has a hard coating at the drilling tip. The nature of the coating or how the coating is applied is not specified in the Utility Model specification. DE U1 ""372 proposes a high-quality steel drilling screw comprising a part of precipitation-hardenable or age-hardenable high-quality steel. It will be appreciated that, as a drilling screw, it has a drilling tip and it is corrosion-resistant as it consists of high-quality steel. What is still not clear in terms of the information set out in the Utility Model specification is how that high-quality steel drilling screw acquires its hardness. The specification merely states that the screw is subjected to through-hardening as a whole. In accordance with the general understanding on the part of the men skilled in the art, a screw is through-hardened when it involves the same level of hardness in the core region and in the edge zones. Conventional through-hardening is effected for example by heating a screw to the austenitisation temperature and quenching it. Heat-treated screws are then tempered.
Screws comprising austenitic stainless, that is to say high-quality steel, are moreover already known from German patent specification No 29 29 179 (column 2, lines 35-38). Those screws acquire their strength in the screwthread flank region by cold work-hardening when shaping the screwthread. In addition to cold work-hardening, transformation martensite formation occurs in the operation of working the metastable austenitic high-quality steel. That results in a higher level of hardness in the worked region.
German laid-open application (DE-OS) No 32 35 447 also discloses a drilling screw of stainless austenitic steel whose drilling and tapping portion acquires the required strength by virtue of case-hardening. This partial case-hardening operation is however expensive and reduces the level of corrosion-resistance. The same applies in regard to the proposal disclosed in German laid-open application (DE-OS) No 30 00 165 whereby the screwthread of a self-tapping screw is cold-worked at xe2x88x9240xc2x0 C. In that case the increase in hardness is achieved by cold work-hardening and by the formation of transformation martensite. It will be noted however that the method of manufacture is impracticable for mass-scale production at low cost levels, by virtue of the method parameters required.
The varied state of the art shows that the underlying object of the present invention, namely the provision of a self-screwthread-forming screw which at the same time is suitable for screwing into steel of great hardness and which is corrosion-resistant and which can be economically manufactured has hitherto not been satisfactorily attained.
In accordance with the invention that object is attained in a self-screwthread-forming screw of corrosion-resistant steel which is partially precipitation-hardened, more specifically preferably exclusively in the shank portion which is most highly loaded, the shaping region. In this respect, the shaping region includes the screwthread-forming region and also a hole-forming tip such as for example a drilling tip or a flow-hole-forming tip or also a drilling tip with a flow-hole-forming shank portion adjoining same.
The invention is based on the realisation that pure cold work-hardening of the steel on its own or in combination with transformation martensite formation in the cold work-hardening step in the area of the shaping region and possibly the drilling tip is not sufficient to give sufficient hardness and strength to a screw in the area of the shaping region and drilling tip, as is suggested by the state of the art. A desired level of hardness and strength of the screwthread flanks in the shaping region and possibly the drilling tip is surprisingly afforded however if the screw is precipitation-hardened in particular in those regions. Partial precipitation hardening of the screw also has a second advantage: in the precipitation-hardened parts, the resistance to corrosion can admittedly fall slightly depending on the respective alloying composition and heat treatment involved. As however the precipitation-hardened parts make up only a small part of the screwxe2x80x94in the preferred embodiment they even concern only that part of the screw which is only required for drilling the hole and forming the screwthread but not for load-bearing purposesxe2x80x94the screw in other respects retains its original resistance to corrosion without limitations.
A suitable production process for such a screw is distinguished in that the screw is firstly shaped in conventional manner by pressing and rolling and is then preferably partially precipitation hardened by inductive heating. The precipitation hardening Is in that respect preferably limited to the shaping region and possibly the drilling tip of the screw.
The screw is preferably produced from a material which has a high content of interstitially dissolved nitrogen (N). The following range of the chemical composition of the material is preferred:
Nitrides and/or to a slight extent carbides are precipitated due to the precipitation treatment. That causes triggering of the desired increase in hardnessxe2x80x94predominantly in the structure regions which are already strengthened and converted by the mechanical transformation operation.
By limiting the carbon content, chromium carbide formation which preferably involves the grain boundaries is deliberately kept low for chromium carbide formation would increase susceptibility in relation to intercrystalline corrosion.
Alternatively, materials which for example contain up to 1.5% of boron present themselves for the production of self-screwthread-forming screws with subsequent partial precipitation hardening. In that respect, borides are precipitated when using a boron-bearing material during the heat treatment. Vanadium-, niobium- or titanium-alloyed materials could possibly also be processed. Screws made of those alternative materials achieve an increase in hardness by virtue of the heat treatment due to the formation of vanadium, niobium or titanium carbide phases.
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