DE102012216117A1 - Method for producing a self-tapping screw - Google Patents

Abstract

The invention relates to a method for producing a self-tapping screw in which a blank made of a martensitic hardenable steel is provided, from the blank a helical shape with a screw shank and a screw shaft arranged on the cutting thread is made, and the helical shape then at a temperature greater than 900 ° C is cured in a nitrogen-containing gas atmosphere. The invention further relates to a self-tapping screw.

Description

The invention relates to a method for producing a self-tapping screw, in particular for hard mineral substrates, for example for concrete, and such a self-tapping screw.

Although screws made of austenitic or ferritic steels or multiphase steels can be made comparatively corrosion-resistant, they are often not sufficiently hard and resistant to wear in order to be self-drilling and / or thread-cutting in hard, abrasive substrates such as concrete. It is therefore, for example, from the EP 2 204 244 A1 known to provide additional carbide cutting in the thread, which ensure the required edge retention even in hard surfaces.

While certain types of martensitic steels can achieve the hardness required to cut concrete, they are typically moderately resistant to corrosion. For example, from the WO 88/05991 A2 It is therefore known to combine different materials in one and the same screw, on the one hand to achieve sufficient corrosion resistance on the shaft and head and on the other hand, sufficient cutting strength in the drill bit. However, such multi-part screws require relatively complex and expensive joining operations, which under certain circumstances can also adversely affect the product properties.

The EP 0 652 300 A1 discloses a heat treatment process for forming an austenitic surface layer containing dissolved nitrogen in near-net stainless steel parts by embroidering at a temperature between 1000 and 1200 ° C in a nitrogen-containing gas atmosphere and subsequently cooling at a rate such that nitride precipitation is avoided.

The object of the invention is to provide a method for producing a self-tapping screw and a self-tapping screw, which ensure a particularly good corrosion resistance, a particularly high reliability, and in particular a particularly good setting and Lastabtragverhalten at low cost and ease of manufacture.

The object is achieved by a manufacturing method with the features of claim 1 and a screw with the features of claim 11. Preferred embodiments are given in the dependent claims.

In a method according to the invention for producing a self-tapping screw, it is provided that a blank of a martensitic hardenable steel is provided, from the blank a helical shape with a screw shank and a cutting thread arranged on the screw shank is produced, and then the helical shape at a temperature greater than 900 ° C, in particular greater than 1000 ° C, preferably between 1000 ° C and 1200 ° C, is cured in a nitrogen-containing gas atmosphere.

• – Im Gegensatz zum Niedrigtemperaturhärten, mit dem sich nur relativ dünne Randschichten erzeugen lassen, können mit dem erfindungsgemässen Hochtemperaturverfahren Randschichtdicken im Bereich einiger 100 μm ohne besonderen Zusatzaufwand realisiert werden. Es können also Schichtdicken erzeugt werden, die in der Grössenordnung der Querschnittsabmessungen typischer Schneidgewinde liegen, und daher kann ein ausreichendes Verschleissvolumen (typischerweise > 0,2 mm) realisiert werden. Unerwünschte Deformationen des Schneidgewindes beim selbstschneidenden Einschrauben können somit vermieden werden, so dass ein besonders gutes Setzverhalten gegeben ist. Der Grund für die vergleichsweise grossen Schichtdicken wird darin gesehen, dass es bei den erfindungsgemäss vorgesehenen Temperaturen an der Metalloberfläche zu einer Dissoziierung des molekularen Stickstoffs in Einzelatome kommt, welche durch die bei hohen Temperaturen rasch vonstattengehenden Diffusionsvorgänge vergleichsweise gut in den Stahl eindiffundieren können.
• – Mit dem erfindungsgemässen Hochtemperaturverfahren kann die Korrosionsbeständigkeit des Grundwerkstoffs erhöht werden. Dies erlaubt es, mässig korrosionsbeständige aber relativ harte Grundwerkstoffe einzusetzen, mit denen die Schneidhaltigkeit weiter verbessert werden kann. Der Grund für die vergleichsweise grosse erreichbare Korrosionsbeständigkeit wird darin gesehen, dass der Stickstoff nicht in Form von Nitriden, sondern im Eisengitter gelöst vorliegt.

A basic idea of the invention can be seen in the use of a martensitic hardenable steel in the production of self-tapping screws, which is hardened in a high-temperature curing process under a nitrogen atmosphere. This thermochemical process leads to a nitridation of the edge zone and, associated therewith, to a hardness which is higher than that of the base material. The method according to the invention has, in connection with self-tapping screws, two important advantages, among others:

• In contrast to low-temperature curing, with which only relatively thin edge layers can be produced, edge layer thicknesses in the range of a few 100 .mu.m can be realized without any special additional expenditure with the high-temperature process according to the invention. It is thus possible to produce layer thicknesses which are of the order of magnitude of the cross-sectional dimensions of typical cutting threads and therefore a sufficient wear volume (typically> 0.2 mm) can be realized. Unwanted deformations of the cutting thread during self-tapping screwing can thus be avoided, so that a particularly good setting behavior is given. The reason for the comparatively large layer thicknesses is seen in the fact that at the temperatures provided according to the invention at the metal surface a dissociation of the molecular nitrogen into single atoms occurs, which can diffuse comparatively well into the steel due to the diffusion processes which take place rapidly at high temperatures.
• - With the inventive high-temperature process, the corrosion resistance of the base material can be increased. This makes it possible to use moderately corrosion-resistant but relatively hard base materials, with which the cutting strength can be further improved. The reason for the comparatively large achievable corrosion resistance is seen in the fact that the nitrogen is present not dissolved in the form of nitrides, but in the iron grid.

In particular, the invention may enable one-piece corrosion-resistant and self-tapping screws to be made for use in hard and / or abrasive substrates. The corrosion resistance is sufficiently high for use outdoors or in wet indoor areas and the cutting edge is sufficient for cutting a thread in mineral substrates (eg concrete, solid brick, ...) or in steel substrates. According to the invention, such screws can also be produced in one piece without joining operation, ie as "monolithic screws".

It is particularly preferred that the steel of the blank and thus preferably also the finished screw has a carbon content of less than 0.07%. By using such Weichmartensite a particularly good resistance to hydrogen embrittlement and a particularly high Kernduktilität can be achieved. This in turn allows a particularly good load transfer. Since, according to the invention, the edge hardness is increased by embroidering, the relatively low core hardness (core tensile strength of the thread, for example, less than 1100 MPa), which results at low carbon contents, can be realized without significant impairment of the cutting edge strength.

Furthermore, it is advantageous that the steel of the blank and thus preferably also the finished screw has a carbon content greater than 0.02%, in particular greater than or equal to 0.03%. The associated core hardness can ensure that the relatively large torques occurring when screwing in the screw in concrete can be reliably transmitted.

A development of the invention consists in that the steel of the blank has a chromium content greater than 12% and / or a nickel content greater than 2.5%. Hereby, even with the mentioned relatively low carbon contents, a particularly comprehensive martensitic microstructure can be realized. In addition, the corrosion resistance and toughness is further improved.

Furthermore, it is expedient that the steel of the blank and thus preferably also the finished screw has a molybdenum content greater than 0.3%. The addition of molybdenum can further increase the resistance in a chloride-containing environment.

Insofar as percentages of quantities are contained in the present application, these may in particular be mass percentages in the usual way.

For example, the steel of the blank, and thus preferably also of the finished screw, may be X3CrNiMo13-4 (1.4313). This combines good low-temperature toughness with high strength. In particular, a low temperature toughness of greater than 27J (ISO-V) at -20 ° C may be provided.

Furthermore, it is preferred that the helical shape has a rotary engagement means for, preferably form-fitting, transmitting a torque to the screw shaft. Preferably, the helical shape is cured together with the rotary control means, in particular at the temperature according to the invention and in the nitrogen-containing gas atmosphere according to the invention. According to this embodiment, the rotary engagement means of the screw is also made of the martensitic hardenable steel and then high temperature hardened. The screw is therefore preferably formed in one piece, in particular including the rotary engagement means. The rotary grip means may in particular comprise an external polygon or an internal polygon. For example, the rotary engagement means may be a screw head with an external polygon or a polygon socket.

Suitably, the helical shape is quenched so rapidly at the end of the hardening process that nitride precipitation is avoided. As a result, the strength and the corrosion resistance can be further improved. During quenching, the martensitic structure is conserved metastable.

Furthermore, it is according to the invention that the helical shape is tempered after curing and optionally quenching, preferably in a temperature range between 150 ° C and 450 ° C. Tempering can reduce the residual austenite content in the martensite, which also has an advantageous effect on the strength. In addition, the ductility of the screw increases.

For example, in view of the production cost, it is advantageous that the helical shape is formed from the blank by cold forming. The cutting thread may preferably be formed by rolling. The load application means and / or the screw head can be formed by upsetting. The blank may in particular be a wire section.

It is further preferred that, prior to curing in the nitrogen-containing gas atmosphere, at least a portion of the helical shape is covered with a diffusion-inhibiting barrier. By locally covering screw areas with diffusion-inhibiting barriers (eg layers), the embroidering effect can be targeted to specific areas be limited, whereby the product can be further optimized in terms of ductility and / or hardness. For example, the rotary engagement means and preferably also the shaft region immediately below the rotary engagement means can be covered with such a diffusion-inhibiting barrier.

Surface hardnesses of greater than 600 HV, which corresponds approximately to a tensile strength of greater than 1800 MPa, can be achieved by means of the process according to the invention. Following the inventive curing in the nitrogen-containing gas atmosphere, but also before the inventive curing, additional thermal treatment process on the blank or on the helical shape can be provided.

The invention also relates to a screw which is produced by means of the method according to the invention.

The ratio of the outer diameter of the cutting thread to the thread pitch of the cutting thread can be in the range of 1 to 2, in particular in the range of 1.2 to 1.45. These are typical thread sizes for screws suitable for self-tapping into mineral substrates such as e.g. Concrete are provided. Under the slope can be understood in particular the axial distance of successive turns of a thread.

According to the invention, a concrete substrate may be provided with a bore into which a screw according to the invention is screwed, wherein in the concrete substrate, a negative mold to the cutting thread of the screw is formed. Accordingly, the screw is self-tapping screwed into the bore in the concrete substrate to form a counter-thread.

The invention will be explained in more detail below with reference to preferred embodiments, which are shown schematically in the accompanying figures. In the figures show schematically:

1 a schematic flow diagram of a production method according to the invention; and

2 : A schematic representation of a screw according to the invention, produced in a production method according to the invention.

1 schematically shows the sequence of steps of a manufacturing method according to the invention. First, in step 1 a blank made of a martensitic hardenable steel provided. Then in step 2 from the blank a helical shape with a screw shank 20 , one on the screw shaft 20 arranged cutting thread 21 and one on the screw shaft 20 arranged rotary control means 15 manufactured. The helical shape is then in step 3 at a temperature greater than 900 ° C, in particular greater than 1050 ° C, cured in a nitrogen-containing gas atmosphere and then in step 4 quenched so quickly that nitride excretion is avoided. Finally, the screw shape in step 5 tempered, preferably in a temperature range between 150 ° C and 400 ° C.

An embodiment of a screw according to the invention, which is designed as a concrete screw, is in 2 shown.

The screw 10 has a cylindrical screw shaft 20 on, at the end of a hex screw head is provided, which is a rotary handle 15 forms. Along the screw shaft 20 extends a cutting thread 21 with an outer diameter d and a pitch p. Optional can be on the screw shaft 20 also a smaller diameter support thread 28 be provided.

The screw shaft 20 the screw is in a hole in a mineral substrate 50 , in particular in a concrete substrate, screwed in, wherein the cutting thread 21 when screwing a corresponding thread in the substrate 50 has cut free. The screw shaft 20 is through a hole in a load 55 performed by the screw head 15 on the substrate 50 is secured.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 2204244 A1 [0002]
• WO 88/05991 A2 [0003]
• EP 0652300 A1 [0004]

Claims (13)

Method for producing a self-tapping screw ( 10 ), in which - a blank prepared from a martensitic hardenable steel ( 1 ), - from the blank a helical shape with a screw shank ( 20 ) and one on the screw shaft ( 20 ) arranged cutting thread ( 21 ) ( 2 ), and - then the helical shape is cured at a temperature greater than 900 ° C. in a nitrogen-containing gas atmosphere ( 3 ) becomes. A method according to claim 1, characterized in that the steel of the blank has a carbon content of less than 0.07%. Method according to one of the preceding claims, characterized in that the steel of the blank has a carbon content greater than or equal to 0.03%. Method according to one of the preceding claims, characterized in that the steel of the blank has a chromium content greater than 13% and a nickel content greater than 2.5%. Method according to one of the preceding claims, characterized in that the steel of the blank has a molybdenum content greater than 0.3%. Method according to one of the preceding claims, characterized in that the steel of the blank is X3CrNiMo13-4. Method according to one of the preceding claims, characterized in that the helical shape is a turning handle ( 15 ) for transmitting a torque to the screw shank ( 20 ), and that the helical shape together with the rotary handle ( 15 ) hardened ( 3 ) becomes. Method according to one of the preceding claims, characterized in that the helical shape quenched so quickly at the end of the hardening process ( 4 ) that nitride precipitation is avoided, and that the helical shape is tempered after hardening ( 5 ), preferably in a temperature range between 150 ° C and 400 ° C. Method according to one of the preceding claims, characterized in that the helical shape is formed from the blank by cold forming. Method according to one of the preceding claims, characterized in that before hardening ( 3 ) is covered in the nitrogen-containing gas atmosphere, at least a portion of the helical shape with a diffusion-inhibiting barrier. Self-tapping screw ( 10 ) prepared by a method according to any one of claims 1 to 10. Screw according to claim 11, characterized in that the ratio of the outer diameter (d) of the cutting thread ( 21 ) to the thread pitch (p) of the cutting thread ( 21 ) is in the range of 1 to 2, in particular in the range of 1.2 to 1.45. Concrete substrate ( 50 ) with a hole into which a screw ( 10 ) is screwed in according to claim 11 or 12, wherein in the concrete substrate ( 50 ) a negative mold to the cutting thread ( 21 ) of the screw ( 10 ) is formed.

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