Screw

Abstract

To provide a method for manufacturing a screw of which shaft has low proneness to hydrogen embrittlement, and such a screw.SOLUTION: A method for manufacturing a screw has the steps of: rolling a screw wire made of low-alloy carbon steel to manufacture a screw having a thread; heating the entire screw to an austenitizing temperature under a carbon atmosphere and / or nitrogen atmosphere and maintaining the temperature; and quenching the entire screw to a bainite temperature and maintaining the bainite temperature until the screw has a bainitic structure over its cross section. Thereafter, the screw is hardened locally at its tip 22, by the tip 22 being heated to the austenitizing temperature and the screw being subsequently quenched to a temperature below a martensite starting temperature (MS).SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a screw as defined in the preamble of claim 1 and to a screw for direct fastening as defined in the preamble of claim 5. [Background technology]

[0002] EP 3 276 189 A1 describes threads with a softer bainite structure in the edge regions along the thread shaft compared to the center of the thread. DE 10 2017 101 931 A1 describes threads with a bainite structure, which has a lower axial hardness at the tip of the thread than in the central region located towards the head of the thread.

[0003] DE 10 2010 055 210 A1 discloses a method for producing a screw with a double-hardened tip, the tip of which has a higher carbon content than the tempered martensitic shank region of the screw. As a result, the shank with the retaining region of the screw is less susceptible to hydrogen embrittlement than the hardened tip of the screw. For this reason, the tip of a low-alloy carbon screw is partially carburized, the entire screw is then tempered, and the tip is then locally re-hardened.

[0004] This method is complex and costly as it involves partially carburizing the threads. Summary of the Invention

[0005] The object of the present invention is to provide a faster and / or more efficient method for manufacturing a screw, which achieves a particularly high hardness at the tip of the screw and a lower hardness in the shaft with its head and holding area, so that the shaft is less susceptible to hydrogen embrittlement. The expression "tip" in the present invention means the front area of ​​the screw, which extends from the very front end of the screw towards the head of the screw. This is preferably the area designed for grooving the female thread, in particular in a female part made of high strength metallic material.

[0006] The screw according to the invention is produced by forming the screw from a low-alloy carbon steel wire, in particular with a content of alloying elements of less than 3%. The threads are rolled onto the screw during its production. The material used for the screw wire is preferably 23MnB4 or 38B2.

[0007] The screw is then heated to an austenitizing temperature, which is the temperature at which the respective wire used is in the austenitic phase region of its TTT diagram, specifically, the austenitizing temperature is higher than the A3 temperature of the wire.

[0008] After heating the thread to the austenitizing temperature, the thread is quenched to the bainitizing temperature, which is maintained until the thread has a bainite structure, specifically across the cross section of the thread shaft. The bainitizing temperature is the temperature at which the wire is in the bainite phase field. Specifically, the quenching time is selected such that both ferrite and pearlite are not formed during quenching. Specifically, quenching is performed by immersing the thread in a molten salt bath at the bainitizing temperature. A bainite-containing structure is present when the structure portion of interest has a significant and measurable bainite content, specifically above 25%. The structure portion is preferably quenched to a temperature of 0.05 mm. 2 It has a size of

[0009] In the screw according to the present invention, the total area of ​​the structure parts having a bainite content of more than 25% occupies a surface area percentage of more than 80% of the cross-sectional area of ​​the screw.

[0010] According to the invention, after the thread has been held at the bainitization temperature for a given time, the tip of the thread is locally heated again to the austenitization temperature and then cooled below the martensite start temperature, in particular to room temperature, after which at least the tip of the thread is quenched again below the martensite start temperature with a quenching time selected such that the formation of ferrite, pearlite and bainite is largely suppressed.

[0011] This means that the tip is re-hardened locally, especially in its edge zone, which provides an ultra-hard tip.

[0012] This ensures that screws manufactured according to the present invention are less susceptible to hydrogen embrittlement of the shaft and have an ultra-hard tip.

[0013] According to a preferred embodiment of the invention, the heating of the thread to the austenitizing temperature before quenching the thread to the bainitizing temperature may be carried out in a carbon atmosphere having a higher carbon content than the carbon content of the thread, resulting in the formation of a layer in the edge region of the thread having a higher carbon content than the central region (or core), the carbon content of the edge region of the thread being at least 0.2% higher than the central region of the thread. Alternatively, the so-called nitriding of the thread may be carried out in a similar manner. This is particularly effective for wire rods with a carbon content of less than 0.4%.

[0014] This process of introducing carbon or nitrogen into the threads at the austenitizing temperature, followed by rapid cooling to the bainitizing temperature, is referred to below as case-hardening bainitizing, and the structure thus produced is referred to as a case-hardened bainitic structure.

[0015] The screw thus produced may have a case-hardened bainite structure in its shaft and in its head region, especially in the edge region, and a superhard martensite structure in its tip, especially in the edge region of the tip, the edge region having in particular a carbon content between 0.6% and 1.5%.

[0016] In this way, compared to direct fastening, the screw according to the present invention achieves both high hardness at its tip and high ductility in its shaft, and furthermore, the shaft is less susceptible to hydraulic embrittlement.

[0017] According to a further embodiment of the invention, after case hardening bainitization and after tip hardening, the threads may be tempered.

[0018] Preferably, the tempering step may be carried out together with a coating step, in particular the coating may be a zinc flake coating.

[0019] In another of its aspects, the invention relates to a screw having a shaft comprising a head of the screw and a tip comprising an opposite end of the screw, the shaft having a substantially bainite structure over its cross section, and according to the invention the screw having a tip comprising a martensitic edge region.

[0020] In particular, the edge region has a higher carbon content than the center, the difference in concentration being at least 0.2%.

[0021] The shaft has a substantially tempered bainite structure in its center and a tempered structure in its edge regions, the latter structure having a higher carbon content than the center, and the tip may have a temper-hardened martensite structure at least in its edge regions.

[0022] The screw according to the invention is preferably manufactured in the manner described above. [Brief description of the drawings]

[0023] Further advantages, features and possible applications of the present invention will become apparent from the following description with reference to embodiments illustrated in the drawings.

[0024] [Figure 1] FIG. 1 is a schematic cross-sectional view of a low-alloy carbon steel screw manufactured by rolling threads onto a shaft; [Diagram 2] FIG. 2 is a schematic cross-sectional view of a screw after case hardening bainitization; [Diagram 3] FIG. 3 is a schematic cross-sectional view of a screw after its tip has been locally case-hardened (or surface-hardened); [Figure 4] FIG. 4 is a schematic temperature-time diagram of a method according to the present invention for manufacturing a screw and case hardening the tip. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] FIG. 1 is a schematic cross-sectional view of a rolled screw 10 made of a conventional screw steel after a process step. The screw has a shaft 20 including a screw head and a free threaded end, referred to here as tip 22, located axially opposite the head. The screw according to the invention is produced in a process step by forming the screw from a screw wire of low-alloy carbon steel, the content of alloying elements being less than 3%. In particular, the production of the screw comprises rolling a thread on the screw. Preferably, the material of the screw wire is 23MnB4. This type of steel can be processed well in the rolling process.

[0026] FIG. 2 is a schematic cross-sectional view of the screw 10.

[0027] To achieve the state shown in Figure 2, the screw 10 is heated to the austenitizing temperature in a carbon atmosphere having a higher carbon content than the screw 10 itself and exposed to this carbon atmosphere until the carbon content in the edge region 12 of the shaft and in the edge region 18 at the tip of the screw 10 is at least 0.2% higher than in the center of the screw, specifically between 0.6% and 1.5%. In this figure, the edge regions 12 and 18 are only shown diagrammatically and may vary in depth. Instead of or in addition to carburizing, nitriding may also be performed as well.

[0028] After reaching the desired carbon saturation in edge regions 12 and 18, the screw 10 is quenched, specifically in a molten salt bath, to a bainitization temperature, which is approximately equal to the martensite start temperature M SThe screw is kept at the bainitic temperature until the shaft has a bainitic structure 14 substantially over its cross-sectional area. The screw 10 according to FIG. 2 has a higher carbon content in the edge layers 12 and 18 than in the center. A substantially bainitic structure is defined as at least 80% of the cross-sectional area having a bainitic structure. In some cases, other structures may be present.

[0029] FIG. 3 is a schematic cross-sectional view of a screw 10 according to the invention, the tip 22 of which is again locally heated to the austenitizing temperature and then to the martensite start temperature M S Upon cooling to a temperature below 100° C., martensite is formed and a hardened martensitic microstructure 16 having a carbon content of about 1% is present in the tip 22, particularly in the edge layer 18. This produces a carbide tip.

[0030] 4 is a schematic temperature-time diagram of the manufacturing method according to the invention. First, the thread is heated to an austenitizing temperature higher than the A3 temperature. If the wire does not have a sufficient carbon content between 0.6% and 1.5%, such heating may be performed in a carbon-enriched atmosphere, which has a higher carbon concentration than the wire, so that carbon diffuses from the carbon atmosphere to the edge region of the thread during heating.

[0031] The thread is then quenched to the bainitization temperature, which is the temperature at which the wire is in the bainite phase region of its time-temperature diagram. The quench time is selected so that both ferrite and pearlite are not formed during quenching. The thread is held at the bainitization temperature until a significant portion of the thread cross section is bainitic. It is then cooled to room temperature.

[0032] After the screw thus produced has been cooled to room temperature RT, its tip is locally reheated to the austenitizing temperature and then heated to the martensite start temperature M S Rapid cooling again to below 0.5 creates a martensitic structure at least in the edge region of the tip.

Claims

1. a. rolling a low alloy carbon steel screw wire to produce a threaded screw (10); b. heating the entire screw (10) to an austenitizing temperature under a carbon and / or nitrogen atmosphere and maintaining said temperature; c. quenching the entire screw (10) to the bainitization temperature and maintaining the bainitization temperature until the screw has a bainitic structure throughout its cross section; A method for manufacturing a screw (10) having the following features: d. The screw (10) is then heated to the martensite start temperature (M Ｓ 2. A method for manufacturing a screw, comprising the steps of: (a) locally hardening the screw (10) at its tip (22) by quenching it to a temperature below 1000° C.

2. 2. The method of claim 1, wherein the carbon atmosphere has a higher carbon content than the screw and / or the nitrogen atmosphere has a higher nitrogen content than the screw.

3. 3. A method according to claim 1 or 2, characterized in that the thread is kept at the austenitizing temperature in a carbon atmosphere until the edge regions (12, 18) of the thread have a carbon content at least 0.2% higher than the carbon content of the core.

4. 4. The method according to claim 1, wherein the screw (10) is tempered.

5. A screw (10) comprising a threaded shaft (20) and a tip (22), the shaft (20) having a substantially bainite structure in its central portion, A screw characterized in that the tip (22), at least in its edge region (18), consists essentially of a hardened martensitic structure.

6. 6. A screw (10) according to claim 5, characterized in that the screw (10) has a higher carbon content in its edge regions (12, 18) than in its central portion.

7. The shaft has a substantially tempered bainite structure in its center and a tempered structure in its edge regions (12) having a higher carbon content than the center; 7. A screw according to claim 5 or 6, characterized in that the tip (22) has a temper-hardened martensitic structure at least in its edge region (18).

8. The screw (10) according to any one of claims 5 to 7, characterized in that the screw (10) is manufactured using a method according to any one of claims 1 to 4.

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