Method for producing rust-proof high-strength self-tapping screw

Abstract

The invention provides a method for producing a rust-proof high-strength self-tapping screw. The self-tapping screw does not need to be subjected to a heat treatment process for improving material hardness downstream of a manufacturing process. Providing a rod-shaped blank with a wire part of a stainless steel material; forming a screw head at a first longitudinal end of the blank by means of cold forming; reducing the diameter of the end section of the blank by cold forming; preforming the drill tip at the second longitudinal end of the blank by means of a kneading movement transverse to the longitudinal axis; the drill tip is finally manufactured in the subsequent process step; during a subsequent thread rolling process, the material flag remaining protruding at the cutting edge of the drill tip is cut off. In the preforming, a contoured edge is formed in a subsequent cut edge region, with a residual material thickness between 0.3 mm and 1.0 mm. The final forming of the drill tip is performed by forming the contoured edge into a cut edge, with a residual material thickness between the cut edge and the material flag being between 0.05 mm and 0.2 mm and allowing local perforation (perforation).

Description

Field of the Invention

[0001] The present invention relates to a method for producing high-strength fasteners (especially self-tapping screws) made of stainless steel, and self-tapping screws produced in this way. Background Art

[0002] Steel is generally defined as an iron-carbon alloy with a carbon content of up to 2%. Carbon steel or non-alloy steel refers to those variants that have only a small amount of impurities or no specifically added alloying components (such as chromium, nickel, copper, manganese or silicon). Non-rusting steel (usually also called ferritic stainless steel) is characterized by a chromium alloy content of > 10% and a carbon content of less than 1.2%.

[0003] Due to the incorporation of carbon in the steel matrix, carbon steel is generally easier to harden than stainless steel. This hardening is usually achieved by hardening methods (heat treatment, surface hardening such as carbonitriding) or cold forming processes. However, this hardness of the steel (usually the desired hardness) comes at the cost of the basic drawback of being prone to corrosion. Therefore, certain end products need to be post-treated (coated, passivated) to make them tolerate environmental influences to a certain extent.

[0004] The most common stainless steels are those containing the alloying components chromium and nickel, such as steel types 1.4301 (V2A or A2) or 1.4401 (V4A or A4). There are standards for these steels with specified compositions, enabling these types with comparable properties to be obtained from various sources. These steels are also called austenitic steels because the alloying components Ni, C, Mn and N stabilize the austenite phase in the steel during production.

[0005] Duplex steel is a steel with a two-phase structure, consisting of a ferritic matrix and islands composed of austenite. Compared with pure austenitic steel, duplex steel has a lower nickel content, which means that the entire microstructure does not turn into austenite at room temperature. Here, for example, types 1.4462 and 1.4362.

[0006] Many products for daily needs (also, for example, in the construction industry) are made of steel, including fasteners, such as screws of all conceivable sizes suitable for many application fields. Here, it is often desirable to combine the hardness of carbon steel with the corrosion resistance of stainless steel. This is especially the case for drilling screws, usually by welding a carbon steel wire part to a stainless steel wire of the same diameter and processing this blank into a fastener in a known manner. Here, the rolled or formed tip (Spitze) made of carbon steel can be hardened by heat treatment, while the shaft (Schaft) made of stainless steel can usually remain untreated, so that the anti-corrosion properties of the steel can also be maintained.

[0007] The fasteners used in this document refer to mechanical components that can permanently connect two components (detachable and non-detachable) to each other. As a screw, it specifically refers to a fastener with a rod having a substantially longitudinal extension with a cylindrical or similar cylindrical cross-section. At one longitudinal end of the rod, there is a force-receiving portion, which can be designed as a head with a force-receiving surface. The tip of the screw is located at the opposite end of the rod. The rod is provided with threads at least in some sections, and the threads can be single-threaded or multi-threaded, and the pitch can be constant or variable. The tip can be designed as a drill tip with a cutting edge, a blunt conical non-threaded displacement tip, or a tapered tip with self-formed holes and threads. Depending on different applications, the threads can extend from the rod to the cone portion or the screw tip.

[0008] A self-tapping screw is a screw with a drill tip having a cutting edge. In this screw, the thread (rod) is roll-pressed and the drill tip is pinched. Pinching (Zwicken) here refers to a special cold-forming process in which the rod end of the blank is pressed into the shape of the desired drill tip by gradually tapering pinch jaws. Other cold-forming processes include rolling, upsetting, stretching, and impact extrusion. Basically, those skilled in the art consider cold forming to refer to the plastic forming of metals below the recrystallization temperature, and it is well known that the recrystallization temperature will cause work hardening of the formed material.

[0009] It is well known to produce fasteners or screws purely from stainless steel. However, the steel penetration ability of corresponding (self-)drilling screws or hole and thread forming screws is limited. Bimetallic screws are complex to manufacture and costly to implement. Therefore, there is a need for a fastener, especially a screw or a self-tapping screw, which can be completely made of corrosion-resistant steel types and can penetrate steel plates with a thickness greater than 1.5 mm without pre-drilling. Prior Art

[0010] The published text DE 29 29 179 describes a corrosion-resistant, self-drilling and thread-forming screw made of austenitic stainless steel material (according to the US standard series 300). The process steps in manufacturing include upsetting the head at the end of the wire part of the material, and then forming the drill tip at the opposite end through a pinching process, where the maximum closing speed of the pinch jaws is defined. Thereby, the austenitic structure of the drill tip is transformed into a martensitic structure. This text also suggests cooling the punched blank below 0°C, for example, by dry ice.

[0011] The text EP 2 080 572 describes a method for manufacturing high-strength fasteners from austenitic steel of the 300 series (according to the US standard). In the first working step, the diameter of the rod blank is reduced by 15% through cold forming. Then, the head and the tip are also manufactured through cold forming. The threads are produced on the rod part through a roll-pressing process. It is also suggested to improve the rust prevention ability of the cold-formed fasteners through post-treatment or coating.

[0012] The texts DE 2 103 053 and US 3 683 436 describe the manufacture of drilling screws with a knurled drill tip. By impact extrusion, the diameter of one end of the wire blank is reduced here, and then it is brought to its final shape by knurling.

[0013] The text EP 2 617 500 A1 describes a method for manufacturing an integral stainless steel drilling screw, in which the drill tip is manufactured in two forming steps. The first step is to flatten the end section of the blank, and then the flattened end section is formed into the final drill tip.

[0014] The object of the present invention is to improve the above methods, in particular to provide method steps for producing corrosion-resistant, hole-forming and thread-forming screws with drill tips, and the method steps make the use of bimetallic screws redundant as much as possible.

[0015] The features of the independent claims solve this object. The dependent claims give advantageous embodiments of the present invention. Summary of the Invention

[0016] Hereinafter, the manufacturing process of the self-tapping screw according to the present invention is described as a series of process steps. In terms of production technology, these process steps represent a series of consecutive working steps, usually carried out continuously in a short time. A multi-stage cold forming machine is generally used for production, and it processes the workpiece into the required shape in a synchronous manner and at a specified forming speed with the help of various tools. Intermediate steps in the process chain, such as quality control, transportation, cleaning, sorting and assembly, are not mentioned and have no impact on the feasibility of the present invention.

[0017] The self-tapping screw according to the present invention is made entirely of stainless steel material and can achieve its high strength even without a heat treatment process that specifically improves the material hardness after production steps A - F. Therefore, this process is explicitly excluded. The advantage lies in saving time and energy.

[0018] Step A:

[0019] This process step includes providing a wire section, as is known in the prior art, the wire section can be produced as a section of a wire coil or as a section of a corresponding bar of stainless steel material. Hereinafter, this is also referred to as a blank. According to well-known methods, the section can be obtained from the pre-material by shearing, sawing or other means. The length and diameter of the blank depend on the planned dimensions of the self-tapping screw to be produced and are designed according to the known rules of the prior art.

[0020] Step B:

[0021] The screw head is upset at the first longitudinal end of the blank by cold forming. The upsetting of the screw head can also be carried out in one or more intermediate steps to control the formability of each upsetting process. Step B includes manufacturing a stress-bearing part (external hexagon, internal hexagon, internal hexagon...) at or within the screw head.

[0022] Step C:

[0023] The diameter of the (longitudinal) section or rod section at the second longitudinal end of the blank is reduced by cold forming. The diameter reduction is carried out at the second longitudinal end opposite to the screw head. This can cause an increase in surface hardness and an extension of the relevant section. This diameter reduction can also be carried out in several sub-steps; this variant is also included in step C.

[0024] The length of the rod section at least includes the (longitudinal) section provided in the subsequent steps D and E for producing the drill tip. However, if necessary, it can also additionally include the rod section for providing a thread on the rod. If it is useful or reasonable from a production technology perspective, the diameter reduction can also include the entire rod length down to below the head.

[0025] Step D:

[0026] A drill tip is preformed at the second longitudinal end of the blank by a squeezing movement transverse to the longitudinal axis between two opposing tool jaws. The second longitudinal end of the blank with a gradually tapered diameter in step C is preformed. In this case, preforming means that two so-called squeezing jaws (i.e., the tool halves of the squeezing device) make the longitudinal end of the blank into a preform that can be recognized as a drill tip but does not correspond to the final dimensions.

[0027] During the pressing process, the produced preform includes a produced material flag (Materialfahne) formed by the excess material extruded outwards. Therefore, the preformed drill tip is surrounded by a flat, thin, irregularly formed material disk. The plane of the material disk corresponds to the closing plane or closing surface defined by the two gradually tapering tool jaws.

[0028] The preforming of the drill tip also means that the main cutting part, cross-cutting part, chip groove, and free surface of the future drill tip provided according to the selected drill tip layout can be recognized in terms of their shape and position, but do not yet correspond to the final dimensions in terms of size. The tool jaws are usually designed such that the future cutting edge is at least partially located in the material disk and has been marked out by the embossed contour edge in the preforming.

[0029] Step E:

[0030] Final shaping of the drill tip. Through a further cold forming step, the shape and position of the main cutting part, cross cutting part, chip flutes and clearance face of the drill tip setting are finally shaped according to the specific design, so as to achieve the final dimensions of the setting. The profile edges are further shaped here and become cutting edges as required. In this step, the above-mentioned material disc is further thinned and can be partially perforated or present along the profile edges or the finally shaped cutting edges.

[0031] Step F:

[0032] During the subsequent thread rolling process, the protruding material flags remaining at the cutting edges or profile edges of the drill tip are trimmed off. Depending on the design, the thread on the shank can be used over the entire length between the screw head and the drill tip or in a partial area.

[0033] Here, the features of the present invention in steps D and E are:

[0034] - During the preforming process of step D, profile edges are formed in the subsequent cutting edge area, where the remaining material thickness between the profile edges and the material flags is between 0.3 mm (inclusive) and 1.0 mm (exclusive). These profile edges can be recognized as notches in the blank between the provided drill tip and the material flags.

[0035] - After the final shaping in step E, the profile edges are shaped into cutting edges, and the remaining material thickness between the cutting edges and the material flags is between 0.05 mm (inclusive) and 0.3 mm (exclusive), where local punching (perforation) may also occur. This means that in individual areas, the cutting edges have been separated from the material flags.

[0036] The final shaping of the drill tip in step E is preferably carried out analogously to step D by a squeezing movement of two opposed tool jaws transverse to the longitudinal axis of the blank. In order to achieve the effects described according to steps D and E and to simplify the manufacture of the tool, the tool jaws used in steps D and E are shaped similarly but not identically designed. Thus, compared with the preforming known in the prior art for a similar shape, it can be seen that the preforming only provides a flattened, pressed or purely cylindrical blank shape. While the tool used in step D achieves an effect already similar to that of a drill tip, it still cannot be used as a drill tip.

[0037] This design enables the last step of giving the drill tip its final shape to remain a forming with a low degree of forming and thus generate less forming heat. The work hardening achieved remains in the cutting edges.

[0038] The forming heat generated during preforming can better flow into the surrounding material disk or material flag through only the imprinted contour edges and reduce the recrystallization tendency of the steel structure.

[0039] As mentioned at the beginning, the methods presented here allow for favorable applications of many stainless steel materials. These stainless steels include standard 1.4301, 1.4551, or 1.4307 (V2A), or 1.4401, 1.4571, or 1.4404 (V4A), or 1.4462, 1.4410, or 1.4501 (duplex). Similar steel types from other standardization bodies are also included.

[0040] It is particularly advantageous to use stainless steel materials as the starting material, which already have a surface strength between 200 Hv and 350 Hv before being processed according to step A.

[0041] The diameter reduction described in step C is advantageously at least 20% and at most 40%. Within the scope of the present invention, forming also includes multiple sub-steps.

[0042] After the above cold forming steps A to F, coating is advantageously carried out to further improve the usability of the screw. The electroplated Zn-Ni coating has proven to be particularly effective. This process step is outlined in step G. The Zn-Ni coating in step G preferably has 12 - 15% nickel.

[0043] In a further process step H, a single-layer or multi-layer sliding coating made of wax, plastic, or a mixture thereof can be applied on the Zn-Ni coating according to step G. These layers have the effect of reducing friction, especially on the drill tip and its cutting edge.

[0044] To ensure the high strength of the self-tapping screw described here, a particularly effective additional step is to carry out a cooling process on the blank between the preforming (step D) and final forming (step E) of the drill tip.

[0045] It has been shown that both actively cooling the blank with a cooling fluid and passively cooling the blank in a temperature-controlled environment are effective. The cooling fluid can be a liquid medium that is sprayed, injected, or pumped onto the blank. This can also be an immersion process, a bath, or an equivalent cooling method. Gaseous media, cooling air, and ambient air cooled with dry ice are also effective.

[0046] Passive cooling is understood as releasing the heat of the blank to the environment without targeted cooling measures before reaching the desired end temperature. It has been shown that the consumption required to cool to normal ambient temperature is the least. In any case, it should be cooled to below 100 degrees Celsius, preferably below 50 degrees Celsius.

[0047] Facts have shown that, by the method of the present invention, after step C, the surface hardness of the blank is substantially 300 Hv to 380 Hv in the region formed thereby. If the cooling process as described above is carried out between steps D and E, the surface hardness of the blank can be increased after the cooling process (measured in the region formed in this way after step E, substantially reaching 450 Hv to 550 Hv).

[0048] Thus, high-strength self-tapping screws can be integrally made of stainless steel by the above process. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] FIG. 1 shows the sequential order of production stages A to F similar to the process steps described above. DETAILED DESCRIPTION OF THE INVENTION

[0050] FIG. 1 shows the sequential order of the method steps A - F of the core material of the present invention.

[0051] A shows a wire part 105 having a first longitudinal end 120 and a second longitudinal end 130. In step B, by cold forming, this blank 110 obtains a screw head 140. The type of the shown screw head 140 represents all technically significant screw head types. C shows the (end) section 135 of the blank 110 that is tapered by cold forming.

[0052] D shows the state of the blank 110 after pre-forming a drill tip 150. The profile edge 190 is shown as a thick line. The shown profile edges correspond to the possibilities of constructing the drill tip, and a large number of such methods are known in the prior art. Where the profile edge 190 and the material flag 170 intersect each other, the requirement standard of "residual material thickness" applies. In addition, any edge of the formed drill tip design is also regarded as a profile edge, even if it is not located in the material plate or the plane of the material flag. Logically, the requirement standard of "residual material thickness" does not apply to these profile edges.

[0053] In the E region of FIG. 1, for the cutting edge 180 emerging from the profile edge 190, the state after final forming is shown by a thin line. The above requirement standard of "residual material thickness" applies to the cutting edge 180 adjacent to the material flag 170. As shown, the material flag 170 is enlarged by a second forming. The allowed perforations are not shown here.

[0054] Figure 1 Region F shows the completed cold-formed self-tapping screw 100, which has a thread 200 and a drill tip 160 released from the material flag 170.

[0055] In this section, the term "blank (Rohling)" is a collective term for all manifestations of the self-tapping screw in steps A to E, inclusive of step E, even if the appearance of the screw changes in the different steps.

Claims

1. A method for producing a high-strength self-drilling screw (100) made entirely of stainless steel material, wherein the self-drilling screw (100) does not have to undergo a heat treatment process downstream of the manufacturing process to improve the hardness of the material, The method comprises the following steps: A. providing a rod-shaped blank (110) with a wire portion (105) made of a stainless steel material; B. upsetting a screw head (140) at a first longitudinal end (120) of said blank (110) by means of cold forming; C. reducing the diameter of a section (135) of the blank (110) at the second longitudinal end (130) by cold forming; D. preforming the drill tip (150) at the second longitudinal end (130) of the blank (110) by a pinching movement between two opposing tool jaws transversely to the longitudinal axis; E. Final drill tip (160); F. shearing off the protruding material flag (170) remaining at the cutting edge (180) of the drill tip (160) during the subsequent thread rolling process, It is characterized in that - during the preforming process of step D, the area of ​​the subsequent cutting edge (180) forms a contour edge (190), wherein the residual material thickness between the contour edge (190) and the material flag (170) is between 0.3 mm and 1.0 mm; and After final shaping in step E, the contour edge ( 190 ) is formed into a cutting edge ( 180 ), and the residual material thickness between the cutting edge ( 180 ) and the material flag ( 170 ) is between 0.05 mm and 0.3 mm, wherein local breakthroughs are also permitted.

2. The method according to claim 1, characterized in that: The stainless steel material is a stainless steel selected from the standard 1.4301, 1.4551 or 1.4307 (V2A), or 1.4401, 1.4571 or 1.4404 (V4A), or 1.4462, 1.4362, 1.4410 or 1.4501 (duplex).

3. The method according to claims 1 to 2, characterized in that Prior to processing according to step A, the starting stainless steel material has a surface strength between 200 Hv and 350 Hv.

4. The method according to claims 1 to 3, characterized in that The reduction in diameter in step C is at least 20% and at most 40%.

5. The method according to claims 1 to 4, characterized in that The final shaping of the drill tip in step E is brought about by a kneading movement of the two opposing tool jaws transversely to the longitudinal axis of the blank.

6. The method according to claims 1 to 5, characterized in that The tool jaws used in step D and step E are similar in shape, but not identical.

7. The method according to claims 1 to 6, characterized in that After the cold forming steps A to F, a coating step G is performed, wherein the coating is an electroplated Zn—Ni coating.

8. The method according to claim 7, characterized in that The Zn-Ni coating of step G has 12-15% nickel.

9. The method according to claims 7 to 8, characterized in that In step H, a single-layer or multi-layer sliding coating consisting of wax, plastic or a mixture thereof is applied to the Zn—Ni coating according to step G.

10. The method according to claims 1 to 9, characterized in that Between the preforming (step D) and the final forming (step E) of the drill tip, the blank is subjected to a cooling process.

11. The method according to claim 10, characterized in that The cooling process is achieved by actively cooling the blank by means of a cooling fluid.

12. The method according to claims 10 to 11, characterized in that The cooling process is achieved by passively cooling the blank in a temperature controlled environment.

13. The method according to claims 1 to 12, characterized in that The surface hardness of the blank after step C in the region formed thereby is substantially 300 Hv to 380 Hv.

14. The method according to claims 10 to 13, characterized in that The surface hardness of the blank after the cooling process and after step E in the region formed thereby is substantially 450 Hv to 500 Hv.

15. A high-strength self-drilling screw manufactured in one piece from rust-proof stainless steel according to the method of claims 1 to 14.

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