DE102013108032A1 - Concrete screw and method for its production - Google Patents

Abstract

The invention relates to a concrete screw (1) and their hardening of a stainless steel. In order to provide such a stainless steel screw which does not require cutting elements of another material, the invention proposes that a ferritic stainless steel is used whose edge layer is embroidered and quenched at 1000 to 1200 degrees. The nitriding significantly increases the hardness and at the same time improves the corrosion resistance.

Description

The invention relates to a concrete screw with the features of the preamble of claim 1 and a method for producing a concrete screw having the features of the preamble of claim 14.

Concrete screws are screws that can be screwed into a pre-drilled hole in concrete. They cut their threads themselves. Regarding the corrosion resistance, a distinction is made between so-called "normal steel concrete screws" and "stainless steel concrete screws". By the latter are meant concrete screws made of stainless steel and thus suitable for outdoor or wet room applications. The problem with these screws is that stainless steel is not hard enough to cut sufficiently into the concrete.

In the past, therefore, stainless steel concrete screws have been developed that have one or more cutting elements of a different material. For example, the document shows DE 198 41 135 A1 a concrete screw, wherein the front end in the insertion direction has a cutting sleeve as a cutting element made of a hardened, corrosion-sensitive material. The cutting sleeve forms the front part of the thread and is pushed onto a stump-like extension of the base body made of stainless steel and bonded thereto. The cutting sleeve thus forms with its thread a cutting element, since in this region of the thread, the external thread is cut in the concrete. A similar solution suggests the document EP 0 861 379 B1 before, except that here instead of a cutting sleeve, a cutting tip is used in the form of a solid cylinder with external thread, which is welded to the base body made of stainless steel. Instead of these larger cutting elements and cutting inserts can be used as cutting elements, as for example in the document DE 198 52 338 A1 is proposed. Here only cylindrical pins made of a hard material in the front region of the thread are arranged. The pins protrude from radial receiving holes in the thread flank. Otherwise, the screw is again made of a stainless steel. Similarly, the document proposes DE 10 2007 000 605 A1 Cutting inserts made of hard material, wherein the cutting inserts along the cutting edge are arranged in a recess and connected to the remaining thread of stainless steel by resistance welding. All these concrete screws are very expensive to manufacture.

The object of the invention is to provide a simple to produce concrete screw, which is resistant to corrosion.

This object is achieved by a concrete screw with the features of claim 1 and a method for producing a concrete screw with the features of claim 14. Under a concrete screw is understood here a screw without drill bit, which is in a pre-drilled hole in mineral substrates, especially concrete, screwed. A concrete screw thereby cuts its thread itself, to which this at least locally has a hardness of over 50 HRC, in particular above 55 HRC and preferably above 58 HRC. The concrete screw forms a fastening arrangement together with the substrate. Preferably, the concrete screw in a mineral substrate, in particular made of concrete, anchored.

The concrete screw according to the invention consists of a stainless steel, according to which EN 10 088 Steels with a chromium content of at least 10.5 percent and a maximum carbon content of 1.2 percent. The percentages are always based on the weight here. In order to achieve the hardness required for cutting into concrete, the invention proposes that the concrete screw, at least in the region of the thread, has an edge layer with a thickness of at least 0.05 mm, in particular of at least 0.2 mm and in particular not more than 2.5 mm , having. In this boundary layer there is an increased content of dissolved nitrogen compared with the rest of the structure, whereby the surface layer is hard, preferably with a hardness of at least 57 HRC, in particular of at least 60 HRC. In particular, the surface layer is not produced by nitriding, which means "nitriding" here, a diffusion at temperatures below 1000 degrees Celsius, especially below 900 degrees Celsius. Due to the increasing susceptibility to corrosion at elevated temperatures, nitriding is typically conducted at only about 500 degrees Celsius and results in layer thicknesses of only 10 to 30 microns.

However, these are not sufficiently robust for a concrete screw. In contrast, it has been shown that the edge layer hardened according to the invention is sufficient to cut in concrete. Preferably, the concrete screw on a softer core, so is not through-hardened over the entire cross-section, so as not to be too brittle. The concrete screw according to the invention, for example, by simply cold forming the starting material, in particular rolling and upsetting a rod material made of stainless steel, and then hardening by diffusion of nitrogen, hereinafter called Aufsticken, getting produced. The embroidering is carried out by diffusing and dissolving nitrogen in the structure of the edge zone at temperatures of 1000 to 1200 degrees Celsius, in particular from 1050 to 1150 degrees Celsius, and a pressure of 0.1 to 5 bar, in particular at most 3 bar, and subsequent rapid quenching , The dissolution of nitrogen causes a particularly hard, high-strength surface layer. The concrete screw can thus be completed in two simple steps, without any reworking for attaching cutting elements is necessary. In addition, it has been found that the corrosion resistance is significantly increased by the nitrogen. So if a certain level of corrosion resistance can be achieved, it can be assumed that a stainless steel, which has a significantly lower corrosion resistance. This extends the choice of steels, which makes it possible to select particularly strong, easy-to-process and / or cost-effective steels. The nitrogen content in the edge zone is between 0.1 and 0.7 percent, in particular 0.3 to 0.5 percent.

For a particularly high hardness, it is necessary that the nitrogen does not precipitate out as chromium nitride during quenching. Therefore, the quenching process must be done very quickly. The core of the concrete screw can be disturbing because it stores the heat as a reservoir. To counteract this effect, the invention proposes that the concrete screw is heated inductively during curing and then quenched. In inductive hardening an eddy current is generated in the concrete screw with an induction coil arranged around the longitudinal axis, which leads to the heating due to the internal resistance. Due to the skin effect, the eddy currents flow mainly on the outer circumference, so that it comes mainly in the edge zone for heating. This is favored by a particularly high-frequency excitation voltage. The core of the screw thus does not act as a heat reservoir. This is further facilitated by cooling the core, for example by a heat sink. The inductive hardening takes place in particular in a second step of the curing, after in a first step under the above-mentioned process conditions, the nitrogen was first dissolved in the edge zone. If necessary, nitrogen dissolved in chromium nitrides during the first quenching process can be released again and the concrete screw then quenched even faster.

In contrast to the prior art, the concrete screw according to the invention preferably does not have a cutting element made of a different material, wherein a cutting element is meant by a joining with a body of the screw part that is suitable for shearing a thread in concrete, ie in particular a cutting sleeve , a cutting tip or a cutting insert.

Preferably, the concrete screw in the boundary layer has a martensitic structure and otherwise a ferritic structure. Due to the body-centered lattice, ferritic steels are regarded as poorly formable and their notched impact strength is low (cf. Bargel / Schulze "Material Science", 5th edition, page 216 ). Thus, it seems at first absurd to use such steels for concrete screws, since the concrete screw is preferably to be made reshaping of a cylindrical wire and thereby considerable degrees of deformation can be achieved. In addition, the notched impact strength initially speaks against the use of concrete screws in comparison with austenitic chromium-nickel steels, as these are usually screwed into the concrete with a tangenzial impact wrench and also strike hard stones over and over again. The concrete screw is thus stressed dynamically. However, if the ferritic microstructure is heated in the abovementioned process, it transforms into an austenitic microstructure with increasing carbon and nitrogen content even at relatively high chromium contents. Just by embroidering so the Austenitbildung is favored. If the steel is quenched after the nitrogen has diffused in, a martensitic structure can be formed at a sufficient quenching rate. Chromium-nickel steels, on the other hand, have an austenitic structure at room temperature. This is even stabilized by the diffusing nitrogen, so that it forms no martensite during quenching. However, a martensitic structure is much harder than an austenitic structure. At the same time, the ferritic steels have proved to be sufficiently deformable and tough for a concrete screw.

Preferably, the steel is substantially free of nickel. By this is meant that the steel contains less than 2.5 percent, in particular not more than 1.0 percent and preferably not more than 0.75 percent nickel. Since nickel threatens to bind the nitrogen in the form of nitrides during embroidering, a low nickel content ensures good solubility for nitrogen in the iron grid. This leads to a hard and firm surface layer as described above. An important further effect results from the microstructure: Nickel-free chromium steels have a ferritic microstructure at room temperature, with the advantages mentioned above. The special feature of the nitriding in a nickel-free, stainless steel is thus a double hardening effect, namely on the one hand by the release of the nitrogen atoms in the structure and on the other hand by a martensite during quenching.

To achieve good corrosion resistance, the steel preferably has one Carbon content of less than 0.1 percent, in particular a maximum of 0.06 percent. Carbon can bind chromium in the form of carbides, thus leading to a so-called chromium depletion. A low carbon content thus promotes corrosion resistance.

A comparable, negative effect by binding of dissolved nitrogen in ionic compounds is found in titanium, niobium and sulfur. The invention therefore proposes that the steel be substantially free of both titanium, niobium and sulfur. "Free from" means less than 0.01 percent for titanium and niobium and less than 0.15 percent for sulfur, in particular less than 0.1 percent and in particular not more than 0.03 percent.

Preferably, moreover, the sum is off Proportion of chromium [%] + 3.3 · proportion of molybdenum [%] more than 18, in particular more than 20, and preferably more than 22. In this case, the proportion of chromium in percent with "proportion of chromium [%]" and the "content of 3.3" of molybdenum [%] is 3.3. means the percentage of molybdenum content. Selected alloying components ensure that the concrete screw has a high corrosion resistance.

Preferably, the chromium content of the steel is at least 16 percent, but most preferably at most 18 percent, whereby the solubility for nitrogen is best. The molybdenum content is preferably at least 0.9 percent. Molybdenum increases the solubility of nitrogen. Both thus ensure high hardness and strength values.

Particularly suitable for the concrete screw according to the invention has the steel of the number 1.4113 after EN 10 088 exposed. This steel contains 16-18 percent chromium, 0.9-1.4 percent molybdenum, 0.08 percent maximum carbon and is free of nickel, titanium, niobium and sulfur. As a ferritic, stainless steel can be achieved by the Aufsticken described a double hardness effect by dissolving nitrogen atoms and martensite. At the same time, the solubility of nitrogen is promoted by the molybdenum and the steel achieves a very good corrosion resistance by dissolving nitrogen. On the other hand, this ferritic steel is still sufficiently deformable for producing the geometry of a concrete screw from a round material and sufficiently tough for self-tapping screwing such a concrete screw in a borehole.

In order to achieve an even greater hardness, the invention proposes that an additional hard material layer, in particular a chromium layer, is arranged on the edge layer. A chrome layer can be applied by so-called "hard chrome plating". As an alternative to a chrome layer, the concrete screw can also be bored or nickel-plated. Further alternatives result from the application of a coating in PVD (Physical Vapor Deposition) based on titanium nitride (TiN), titanium carbonitride (TiCN), titanium aluminum nitride (TiAlN), chromium nitride (CrN), chromium vanadium nitride (CrVN) or chromium aluminum nitride (CrAlN). or in the CVD process (Chemical Vapor Deposition). The list is not exhaustive. While such hard material layers often detach rapidly due to the so-called "eggshell effect", that is to say due to the collapse of the hard material layer due to an underlying, too soft substrate, this effect can be avoided by the combination according to the invention with the stable boundary layer. The hard material layer is thus supported by the surface layer and can thus bring their hardness to advantage.

The concrete screw according to the invention has a ratio of thread pitch to outer diameter of the thread greater than 0.5, in particular greater than 0.6 and preferably greater than 0.7, on. As a result, a good ratio of applied torque, propulsion and holding values of the concrete screw is achieved.

Preferably, the concrete screw also has a ratio of outer diameter to core diameter of less than 1.5, in particular less than 1.4 in the region of the thread. This also favors the ratio of applied torque, propulsion and holding values of the concrete screw.

The invention will be explained below with reference to an embodiment. The single figure shows the concrete screw according to the invention. The drawing is not to scale.

The thread-cutting concrete screw according to the invention shown in the drawing 1 is for screwing into a pre-drilled hole 2 in a subsoil 3 concrete, masonry or other mineral material. The concrete screw 1 is placed in the well without the use of a dowel or other means 2 screwed in, cuts itself an internal thread into the ground 3 and serves to fasten an object 4 in the form of an angular steel console. To do this, the concrete screw 1 before screwing into the ground 3 through a through hole 5 of the object 4 through put. The drawing shows the fully anchored condition.

The concrete screw 1 has a frusto-conical taper 6 at a front in the direction of insertion E screw end 7 and a screw head 8th at a rear end of the screw 9 on. In the illustrated embodiment, the screw head 8th a hexagonal head, this head shape is not mandatory.

The concrete screw 1 has a thread 10 on. An outer diameter d _{a of} the thread 10 is so big that it is when screwing in the concrete screw 1 in the borehole 2 in a borehole wall 11 cuts. The ratio of the outer diameter d _a to a core diameter d _{K of} the thread 10 is 1.45 and the ratio of thread pitch p to outer diameter d _{a of} the thread 10 is 0.8. The concrete screw 1 forms together with the underground 3 a fastening arrangement.

The concrete screw 1 is made of a steel number 1.4113 EN 10 088 produced. Starting from a round material in wire form, the geometry is first produced by reshaping in a known manner. After that, the concrete screw 1 initially 110 minutes at 1100 degrees Celsius and 0.46 bar embroidered and then quenched to room temperature. Before starting at 150 degrees Celsius, the screw is cooled to minus 80 degrees. In a possible second step of the curing, the grain is refinned for another 80 minutes at 1100 degrees Celsius and 0.2 bar and quenched again. Instead of the second step, an inductive heating to 1100 degrees Celsius followed by quenching can be done to increase the hardness, especially in concrete screws with large diameter.

In addition, the concrete screw 1 be provided with a hard chrome layer.

LIST OF REFERENCE NUMBERS

1

concrete screw

2

well

3

underground

4

object

5

Through hole of the object 4

6

rejuvenation

7

front end of screw

8th

screw head

9

rear end of screw

10

thread

11

borehole wall

d _a

Outer diameter of the thread 10

d _K

Core diameter of the thread 10

e

of introduction

p

Thread

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

• DE 19841135 A1 [0003]
• EP 0861379 B1 [0003]
• DE 19852338 A1 [0003]
• DE 102007000605 A1 [0003]

Cited non-patent literature

• EN 10 088 [0006]
• Bargel / Schulze "Werkstoffkunde", 5th edition, page 216 [0010]
• EN 10 088 [0016]
• EN 10 088 [0024]

Claims (15)

Concrete screw ( 1 ) of a stainless steel, characterized in that the concrete screw ( 1 ) at least in the area of the thread ( 10 ) has an edge layer with a thickness of at least 0.05 mm, in particular of at least 0.2 mm, with a content of dissolved nitrogen which is increased compared to the rest of the structure. Concrete screw according to claim 1, characterized in that the concrete screw ( 1 ) has no cutting element made of a different material Concrete screw according to one of the preceding claims, characterized in that the concrete screw ( 1 ) has a martensitic structure in the surface layer and otherwise a ferritic structure. Concrete screw according to one of the preceding claims, characterized in that the steel is substantially free of nickel. Concrete screw according to one of the preceding claims, characterized in that the carbon content of the steel is less than 0.1%, in particular not more than 0.06%. Concrete screw according to one of the preceding claims, characterized in that the steel is substantially free of titanium, niobium and / or sulfur. Concrete screw according to one of the preceding claims, characterized in that the sum of Proportion of chromium [%] + 3.3 · proportion of molybdenum [%] is more than 18, in particular more than 20, and preferably more than 22. Concrete screw according to one of the preceding claims, characterized in that the chromium content of the steel is at least 16% and the molybdenum content is at least 0.9%. Concrete screw according to one of the preceding claims, characterized in that the steel corresponds to the number 1.4113 according to EN 10 088. Concrete screw according to one of the preceding claims, characterized in that on the edge layer, an additional hard material layer, in particular a chromium layer, is arranged. Concrete screw according to one of the preceding claims, characterized in that the ratio of thread pitch p to outer diameter d _{a of} the thread ( 10 ) is greater than 0.5, in particular greater than 0.6 and preferably greater than 0.7. Concrete screw according to one of the preceding claims, characterized in that in the region of the thread ( 10 ) the ratio of outer diameter d _a to core diameter d _{K is} less than 1.5, in particular less than 1.4. Concrete screw according to one of the preceding claims, characterized in that the concrete screw ( 1 ) in a mineral subsoil ( 3 ), in particular of concrete, is anchored. Method for producing a concrete screw, in particular according to one of the preceding claims, characterized in that the concrete screw ( 1 ) is cured, wherein at a temperature of 1000 to 1200 degrees Celsius, in particular from 1050 to 1150 degrees Celsius, and a pressure of 0.1 to 5 bar, in particular at most 3 bar, nitrogen in a boundary layer of the concrete screw ( 1 ) and the concrete screw ( 1 ) is then quenched. Method for producing a concrete screw according to claim 14, characterized in that the concrete screw ( 1 ) is inductively heated during curing and then quenched.

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