JP6247477B2 - Induction hardening tapping screw - Google Patents

Description

  The present invention relates to a tapping screw subjected to induction hardening.

  Conventionally, JIS (Japanese Industrial Standards) B1055 (Non-Patent Document 1) has recommended that steel tapping screws have a surface hardness of 450 HV or more and a core hardness of 200 HV to 400 HV, and heat treatment for that purpose. The method of carburizing and quenching a tapping screw made of a carbon steel wire is described.

  Moreover, the self-tapping screw shown in JP 2004-3548 A (Patent Document 1) is obtained by subjecting a self-tapping screw made of a low carbon steel wire having a carbon content of 0.3% or less to carbonitriding. The surface hardness is 480 HV or more and the core hardness is 320 HV or less.

  Further, the tap-formed bolt shown in Japanese Patent Application Laid-Open No. 2006-16648 (Patent Document 2) has a surface hardness of 350 HV or more and a core hardness by subjecting the tip of a bolt made of a low carbon steel wire to induction hardening. 350HV or less is realized.

  In addition, the heat resistant drill screw disclosed in Japanese Patent Application Laid-Open No. 2002-195918 (Patent Document 3) performs induction hardening on the tip of a drill screw made of a high carbon steel wire having a carbon content exceeding 0.3%. By applying, surface hardness of 600HV or more is realized.

JIS (Japanese Industrial Standards) B1055

JP 2004-3548 A JP 2006-16648 A JP 2002-195918 A

  However, those shown in Non-Patent Document 1 and Patent Documents 1 and 2 cannot be screwed into an austenitic stainless steel plate (SUS304) as an example of a hardness-fastened member. In order to screw the austenitic stainless steel plate without causing thread fracture, the surface hardness of the tapping screw is required to be 650 HV or more. However, since these conventional tapping screws have a low carbon concentration, the surface hardness is 650 HV. No more.

  On the other hand, what is shown in the said patent document 3 is what gave the high-carbon steel wire the induction hardening process, and the surface hardness is fully raised. However, with a high carbon steel wire, it becomes difficult to process into a tapping screw. Even if it can be processed into a tapping screw, the core part hardness is too high, so that the toughness is inferior, and there is a danger that the shaft part and the neck rounded part will break brittlely and break during screwing.

  As described above, hardness and toughness are contradictory properties, and there was no tapping screw having both of these characteristics.

  The present invention was created in view of the above problems, and has an object of providing an induction-hardened tapping screw that is excellent in surface hardness and toughness and can be self-tapped on a high-hardness fastened member. .

Forging a tapping screw having a head portion and a shaft portion on which a screw thread is formed from a low carbon steel wire having a carbon content of 0.06% or more and 0.22% or less, and diffusing carbon from the surface of the screw. By forming a carburized layer having a carbon concentration of 0.7% or more and 1.0% or less by quenching and hardening the carburized layer, and further subjecting the tip of the shaft portion to induction hardening, the surface hardness is increased to HV650 or more. On the other hand, the core hardness is suppressed to HV200 or more and HV400 or less, the shaft part is formed into a substantially polygonal shape in cross-sectional view, and induction hardening is applied to the screw thread formed on the top part from other screw threads. In the method of manufacturing an induction-quenched tapping screw, which is hardened to have a high hardness .

  The carburized layer preferably has nitrogen diffused in addition to carbon.

  Since the induction hardening tapping of the present invention is made of a low carbon steel wire for cold heading, the core hardness remains a flexible structure. In addition, since the carbon concentration of the carburized layer is set to 0.7% or more and 1.0% or less, the thickness of the round neck portion does not become a carburized layer, and the tip end of the shaft portion is induction-hardened. Has a surface hardness of HV650 or more. The induction-hardened tapping screw of the present invention has excellent surface hardness of the shaft portion and toughness of the neck rounded portion, and can be self-tapped to a high-hardness fastening member. Furthermore, since the wire is a low carbon steel for cold heading, it is excellent in workability and cost.

  In addition, it becomes easy to harden a carburized layer by diffusing nitrogen to a carburized layer.

  In addition, when the cross-sectional shape of an axial part is a polygon, when the induction hardening process is performed to the top part, it becomes the heat processing specialized for the said cross-sectional shape. This is because it is known that the cross-sectional shape of the shaft portion is superior in screwing performance when it is a polygonal shape (in particular, a triangular shape). In the case of such a shape, the top portion makes the strongest contact with the inner wall of the prepared hole formed in the fastened member when screwed. Therefore, the top is hardened most by induction hardening.

It is a perspective view which shows the whole induction hardening tapping screw of this invention. It is a flow which shows the manufacturing process of an induction hardening tapping screw. It is a longitudinal cross-sectional view which shows the principal part of the induction hardening tapping screw of this invention. It is a graph which shows the hardness distribution of the induction hardening tapping screw of this invention. It is a figure which shows the quenching state of the front-end | tip part of the induction hardening tapping screw of this invention.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. In FIG. 1, reference numeral 1 denotes an induction-hardened tapping screw that is screwed into an austenitic stainless steel plate that is an example of a high-hardness fastened member. A pilot hole is formed in advance in the member to be fastened, and a female screw is formed into the pilot hole while being screwed. This induction hardening tapping screw 1 is made of a low carbon steel wire for cold heading having a carbon content of 0.06% or more and 0.22% or less. Similar to the processing of a general tapping screw, It is formed by a rolling process. The forging step uses a forging machine (commonly known as a header) to cut a low carbon steel wire for cold forging into a certain length, and with one end of the cut wire fixed, the opposite side is punched The head 2 and the cross hole 3 and the shaft portion 4 are formed by driving with a so-called mold. In the rolling process, a rolling machine (commonly called rolling) is used to roll while pressing a die called a rolling die against the shaft portion, whereby the thread 5 on the die surface is reflected on the shaft portion. 4 is formed with a thread 5.

  As shown in FIG. 2, carburizing and quenching is performed as the primary quenching on the tapping screw after the forging process. In carburizing and quenching, a carburized layer is formed on the surface of a tapping screw 1 made of a low carbon steel wire for cold heading, and this is hardened and hardened. The carbon concentration of the carburized layer is set to 0.7% or more and 1.0% or less. The primary quenching is not limited to carburizing and quenching, and may be vacuum carburizing and quenching or carbonitriding and quenching in which nitrogen is included in the carburized layer in addition to carbon.

  Further, as shown in FIG. 3, the neck rounded portion 6 is a connecting portion between the head portion 2 and the shaft portion 4. Further, the thickness t of the neck rounded portion 6 is defined as a position where the distance between the inner wall of the cross hole 3 and the neck rounded portion 6 is the shortest. The thickness t of the neck rounded portion 6 is the thinnest portion of the head 2, and if the entire portion is hardened as a carburized layer, the head 2 is broken from the shaft portion 4 due to brittle fracture. There is a risk that Therefore, as a configuration for avoiding such a danger, the depth of the carburized layer is set in a range in which the thickness t of the lower neck round portion 6 does not become a carburized layer.

  Further, the tapping screw 1 after carburizing and quenching is subjected to a tempering process for the purpose of lowering the hardness of the neck rounded portion 6 to give toughness. In this tempering process, the tempering temperature is set so that the hardness of the round neck portion 6 is 400 HV or less. If the set temperature is a high-temperature tempering treatment of 400 ° C. or more and less than 650 ° C., the hardness of the neck rounded portion 6 is surely 400 HV or less and has toughness. Depending on the properties of the wire and the setting of the thickness t of the necked round portion 6, even if it is a medium temperature tempering of 200 ° C. or more and less than 400 ° C. or a low temperature tempering of 180 ° C. or more and less than 200 ° C., the hardness is 400 HV or less. And may have a desired toughness.

According to Japanese Industrial Standard B1055 (tapping screw), the core portion 4 has a core hardness of 200 HV or more and 400 HV or less. The tapping screw 1 of the present invention realizes this core hardness. Therefore, a low carbon steel wire for cold heading having a carbon content of 0.06% or more and 0.22% or less is used. In the low carbon steel wire for cold heading having a carbon content of 0.06% or more, the core hardness becomes 200HV or more by carburizing and quenching, while the low carbon steel for cold heading having a carbon content of 0.22% or less. In the wire, the core hardness can be suppressed to 400 HV or less. The measurement value of the core hardness is set at a substantially middle point between the valley bottom of the cross section obtained by cutting a parallel thread portion sufficiently away from the tip of the screw at a right angle to the axis and the shaft center.
Yes.

  Subsequently, the tapping screw 1 after the primary quenching is subjected to secondary quenching, and the tip end portion of the shaft portion 4 is subjected to induction hardening. Specifically, induction hardening is performed on the first to third threads of the thread that is counted from the tip of the shaft portion 4 and becomes a complete thread. In the induction hardening process, only the tip portion of the shaft portion 4 of the tapping screw is inserted into the high frequency heating coil 7 such as a donut type coil or a hairpin type coil. When an alternating current is passed through the coil 7 in this state, a magnetic force due to electromagnetic induction is generated inside the coil 7 and at the same time, an eddy current is generated at the tip of the shaft portion 4. Since this eddy current is collected only on the surface of the tip portion of the shaft portion 4, the current flows through the surface. When an electric current is generated, Joule heat is generated by the electric resistance of the shaft portion by induction heating. Since the surface of the shaft portion 4 has become a carburized layer by the carburizing and quenching process, it undergoes austenite transformation by the Joule heat. Subsequently, the toughness is increased by cooling and martensitic transformation. Thus, by subjecting only the tip end portion of the shaft portion 4 to the induction hardening process, only the surface hardness of the tip portion of the shaft portion 4 is increased without reducing the toughness of the round neck portion 6. In particular, it is preferable that the first crest of the screw thread 5 that is a complete screw thread counted from the tip of the shaft part 4 is induction hardened to be hardened most.

  According to the induction hardening tapping screw 1 of the present invention, the carbon concentration of the carburized layer is set to 0.7% or more in carburizing and quenching. This critical significance is a carbon concentration that can achieve 650 HV or higher by induction hardening. Furthermore, the carbon concentration of the carburized layer is set to 1.0% or less. This critical significance is the carbon concentration for preventing the thickness t of the lower neck rounded portion 6 from becoming a carburized layer. FIG. 4 shows the hardness distribution when induction hardening is performed on the shaft portion 2 on which the carburized layer set in this way is formed. The distance from the surface shown on the horizontal axis in FIG. 4 is a vertical line extending from the top of the thread 5 toward the center line of the shaft 2 in the longitudinal section of the shaft 2. It is measured. According to this, the hardness is gradually lowered from the surface of the screw toward the core, and therefore, the induction-hardened tapping screw 1 of the present invention is such that the screw thread 5 is crushed against the austenitic stainless steel plate. It can be screwed without. On the other hand, in the induction-hardened tapping pin 1 screw in which the hardness of the carburized layer becomes a level distribution from the surface to the core, even if the surface hardness is sufficiently increased, the carburized layer and other portions Since the hardness difference is large, the screw thread is crushed. That is, since not all of the high surface hardness can be screwed into the austenitic stainless steel sheet, it is preferable that the hardness distribution is as in the induction-hardened tapping 1 of the present invention.

  In particular, as shown in FIG. 5, when the cross-sectional shape of the shaft portion 4 is a polygon, when the induction hardening process is applied to the screw thread 5 at the top (hatched portion), the cross-sectional shape is specialized. Heat treatment. This is because it is known that the cross-sectional shape of the shaft portion 4 is superior in screwing performance when it is a polygonal shape (in particular, a triangular shape). In the case of such a shape, the top portion makes the strongest contact with the inner wall of the prepared hole formed in the fastened member when screwed. Therefore, induction hardening is performed only on the top portion and cured. Such a local hardening process can be realized by a laser hardening process instead of the induction hardening process.

DESCRIPTION OF SYMBOLS 1 Induction hardening tapping screw 2 Head 3 Cross hole 4 Shaft part 5 Screw thread 6 Neck lower round part 7 High frequency heating coil

Claims (2)

Forging a tapping screw having a head portion and a shaft portion on which a thread is formed from a low carbon steel wire having a carbon content of 0.06% or more and 0.22% or less,
By diffusing carbon from the surface of the screw, a carburized layer having a carbon concentration of 0.7% or more and 1.0% or less is formed and quenched and hardened.
Furthermore, by subjecting the tip of the shaft portion to induction hardening, the surface hardness is cured to HV650 or more, while the core hardness is suppressed to HV200 or more and HV400 or less,
The shaft portion is formed into a substantially polygonal shape in cross-sectional view, and is hardened to have a higher hardness than other screw threads by subjecting the screw thread formed on the top to induction hardening.
A method of manufacturing an induction-hardened tapping screw characterized by   2. The method of manufacturing an induction hardening tapping screw according to claim 1, wherein nitrogen is diffused in addition to carbon in the carburized layer.

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