JPH0551643B2 - - Google Patents

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a stainless steel T-head bolt.

(Prior art) A T-head bolt is a bolt that is installed on a shaft so that the head and the shaft form a T-shape, and is used for tightening pipe joints such as water pipes. has been done.

As conventional T-head bolts, those made of spheroidal graphite cast iron and those made of stainless steel are known. T-head bolts made of spheroidal graphite cast iron have a high tensile strength.
One type of JISG5502 achieves 40Kgf/mm ² or more, and two types achieve 45Kgf/mm ² or more. On the other hand, there are stainless steel T-head bolts made of martensitic material such as SUS403, which has a tensile strength of 60 kgf/mm ² or more.

The above-mentioned stainless steel T-head bolt is obtained by hot forging from a stainless steel bar, and the general processing steps thereof are as follows.

Annealing of steel bar → Cutting to required length → Heating (approximately 1200℃) → Forging → Quenching and tempering → Passivation treatment → Thread rolling In this case, the above quenching is performed on the bolt head and shaft after hot forging. Tempering is necessary because the martensitic structure created by quenching significantly reduces toughness and increases residual stress. It is processing. Therefore, this stainless steel T-head bolt has a uniform tempered structure throughout (see Fig. 16).

(Problems to be solved by the invention) Conventional stainless steel T-head bolts have high tensile strength, but they are expensive because they are hot forged, and they are also expensive because they are further quenched and tempered. The problem is that they are becoming increasingly expensive. In addition, in the case where quenching and tempering are omitted, only the bolt head is hardened by heating and air cooling, and there is a problem that the strength becomes weak due to the difference in structure from the shaft part.

(Means and effects for solving the problems) The present invention, as means for solving the above problems,
T-head bolts are made by cold or warm forging stainless steel with reduced carbon and nitrogen content to improve workability, forming a continuous streamlined metal flow from the head to the shaft. This is what we provide.

That is, the specific means is that the head is provided with respect to the shaft so that it forms a T-shape together with the shaft, and the length of the head is set to 2.60 to 2.90 times the nominal diameter. A T-head bolt made of cold-formed stainless steel, with C: 0.07% or less, N: 0.02% or less, and Cr: 10.0.
% or more, and the entire bolt has a completely annealed structure with a small amount of CrC compound interspersed with ferrite, and it curves and spreads from the center of the top surface of the head to the side of the head. In other words, a continuous streamlined metal flow is formed in the shaft portion.

(Function) The T-head bolt above has a head length of 2.60 mm of the nominal diameter.
This is a special bolt with ~2.90 times, but due to cold forging, the part from the bolt head to the shaft is curved from the center of the top surface of the head to the side of the head and converged at the shaft. Because a continuous streamlined metal flow is formed, the strength of the bolt head and the boundary between this head and the shaft is high, and heat treatment such as quenching and tempering is not necessary to improve strength. , does not result in high costs.

Then, the amount of C is 0.07% or less, and the amount of N is 0.02%.
As shown below, while improving corrosion resistance,
It is possible to suppress the tensile strength from increasing in the annealed state, and the continuous streamlined metal flow described above can be formed by cold forging.

Furthermore, since the Cr content is 10.0% or more, it is possible to obtain the basic corrosion resistance of stainless steel.

(Effects of the Invention) Therefore, according to the present invention, in a T-head bolt whose head length is set to 2.60 to 2.90 times the nominal diameter, the C content is 0.07% or less, the N content is 0.02% or less, The Cr content is set to 10.0% or more, and the entire bolt is made into a completely annealed structure with a small amount of CrC compound interspersed with ferrite by cold forging, and from the center of the top surface of the head to the side of the head. Since it curves and spreads to form a continuous streamlined metal flow that converges at the shaft, it has excellent mechanical properties and corrosion resistance without the risk of head breakage during use, even without heat treatment such as quenching and tempering. Head bolts can be obtained cheaply.

(Example) Hereinafter, an example of the present invention will be described based on the drawings.

In the T-head bolt 1 shown in FIG. 1, 2 is a shaft portion having a threaded portion 2a at its lower part, and a head 3 is attached to the upper end of this shaft portion 2 so that it joins with the shaft portion 2 to form a T-shape. It's connected. The nominal diameter of the T-head bolt 1 in this embodiment is 20 mm, and the length of the head 3 is 55 mm. In other words, this ratio of (head length/nominal diameter) is
2.75, and this ratio can also be, for example, between 2.60 and 2.90. FIG. 2 is a plan view of the T-head bolt 1, and FIG. 3 is a bottom view thereof, in which an I-shaped positioning recess 4 is engraved on the end surface of the shaft portion 2.

The structure of the above T-head bolt 1 is shown in a micrograph (500x) in FIG. 4, and the entire bolt has a completely annealed structure with small amounts of CrC compound scattered on the ferrite base. In addition, in the part extending from the bolt head 3 to the shaft part 2, as shown in the photograph in FIG. A metal flow is formed.

The T-head bolt 1 is obtained by the process shown in FIG. 6 including a forging process, and the test material is a martensitic stainless steel wire having the following components (% by weight).

C 0.07 or less, Si 0.40 or less, Mn 0.10-1.0,
Cr 10.0 to 13.0, N 0.02 or less, balance Fe and impurity elements In the treatment process, first, the above material is completely annealed. This complete annealing is to improve the cutting and plastic workability of the material.
This is done by holding the material at a temperature above its transformation point and slowly cooling it to complete the transformation. Material (12Cr)
The relationship between the amount of C and the tensile strength in the annealed state is shown in Figure 7. When the amount of C is 0.07% by weight or less, the tensile strength is 47 kgf/mm ² or less, and the forgeability is good. I understand. In other words, as shown in Figure 8, which shows the relationship between the tensile strength of the material and the life of the forging die (the number of pieces of material that can be forged before cracking occurs), when the tensile strength becomes 47Kgf/ ^mm2 or less, the die average lifespan
There are over 2000 pieces. Moreover, corrosion resistance can also be improved by reducing the amount of C.

Next, using the bolt former 5 shown in FIG.
The material is cut to the required length and undergoes a series of cold forging steps including upsetting, head forming, shaft drawing, and trimming.

Although the illustration of the material cutting part is omitted in this bolt former 5, each process surrounded by a two-dot chain line in the process diagram shown in FIG.
It will be carried out by. Furthermore, when using a material that has been cut to a required length in advance, it is also possible to shift from the complete annealing process to the upsetting process. The bolt homer 5 will be explained below.

That is, in FIG. 9, 6 is an upper mold, and 7 is a lower mold, and from the right side of the drawing, a (preliminary) upsetting part 8, a head forming part 9, a shaft drawing part 10, and a trimming part 11 are provided, respectively. Upsetting punch 12 and die 1
3. A punch 14 and die 15 for head shaping, a punch 16 and die 17 for shaft drawing, and a punch 18 and receiver 19 for trimming are provided, and the knockout pins 20 to 24 are used to remove the workpiece W. ₁ to _W3 are transported by a push-out chuck, and workpiece _W4 is pushed out and dropped into a product chute.

The machining conditions in the upsetting section 8 are as follows using the dimensional symbols shown in FIGS. 10 to 12.

Upsetting angle θ = 45 degrees, free upsetting ratio b/d = 1.5, upsetting height ratio c/a = 0.6, f/d = 2.0, g/d = 1.0 Due to this upsetting, the bolt head 3 is determined, and the upsetting angle is preferably 30 to 60 degrees. That is, when the swaging angle is less than 30 degrees, a T-head bolt a is obtained in which the metal flow a ₁ is irregularly distorted at the head due to head shaping, as shown in Fig. 13, and the swaging angle is If the angle exceeds 60 degrees, a T-head bolt b is obtained in which the metal flow b ₁ breaks near both sides of the bolt top, as shown in FIG. 14, and in any case, strength cannot be improved using the metal flow. The most preferable upsetting angle is about 45 degrees, and the streamlined metal flow shown in FIG. 5 can be obtained.

In the cold forging using the bolt former mentioned above, the processed shape is non-axisymmetric, that is, the processed shape during upsetting is such that the overhangs W ₁₁ and W ₁₁ are formed symmetrically following the product shape, as shown in Fig. 12. There is no need for cutting at the trimming section 11.
In addition, in this case, each processed portion 8 to 8 of the bolt former 5
The positioning of the workpieces W ₁ to _{W 4} in 11 is performed using the positioning recess (I-shaped) 4 at the bottom of the shaft, and there is no rotation of the workpieces W ₁ to W ₄ due to movement, and the workpieces W ₁ The above positioning prevents dents or damage to the mold due to W ₄ runout.

After the above-mentioned cold forging, the forged product is subjected to thread rolling and further subjected to passivation treatment. The passivation treatment is performed, for example, by immersing the processed material in an aqueous HNO ₃ solution at 30° C. for a predetermined period of time. In this case, since the passivation treatment is performed after thread rolling, the corrosion resistance of the threaded portion 2a can be improved.

The T-head bolt obtained in the above manner has a tensile strength of
It is 50Kgf/mm ² or more, and the guaranteed load is 6100Kgf without causing permanent deformation. The reason why it has such excellent mechanical strength is that the metal flow shown in FIG. 5 is formed in the completely annealed structure.

Therefore, the T-head bolt can be used as is for tightening pipe joints such as water pipes.

Further, according to the T-head bolt manufacturing method of the embodiment,
Since wire rods are sequentially cut to the required length and then cold forged, compared to hot forging using steel bars, the processing speed is higher due to the difference in forging method, reducing costs. The production of residual material and oxidation loss (due to the peeling off of the oxide film produced by heating the material during hot forging) that occur when cutting the material are virtually eliminated, improving material yield and increasing productivity. It gets expensive. Further, since quenching and tempering treatments are not required, further cost reduction can be achieved.

Of course, the T-head bolt in this example is made of Cr.
The equivalent weight is 9.0 to 10.5, and it is possible to improve its strength by quenching and tempering. In other words, the quenching in that case is 950 ~
Tempering can be carried out by oil cooling from 1000℃ (e.g. 970℃), and tempering can be performed from 700 to 750℃ (e.g. 700℃).
This can be carried out by water cooling from Through such heat treatment, the T-head bolt becomes completely martensitic, and its tensile strength becomes 60 Kgf/mm ² or more.

In addition, in the above embodiment, by using the positioning recess (I-shaped) 4, the workpiece can be easily removed during cold forging.
W ₁ to W ₄ can be conveyed accurately without wobbling, and products with good head dimensional accuracy can be obtained. Furthermore, since no heat treatment is required, the number of processing steps is reduced.

Furthermore, although the T-head bolt is obtained by cold forging in the above embodiment, a T-head bolt similar to that of the above embodiment can be obtained by warm forging. In other words, as shown in Figure 15, which shows the relationship between the deformation resistance and temperature of stainless steel (SUS403), the deformation resistance decreases significantly above 150°C, and from 400°C to 600°C, the deformation resistance decreases due to blue brittleness. Although the decrease in
The deformation resistance also decreases at temperatures above 600℃, and this is true not only for the materials used in the previous example but also for general stainless steel materials in the warm region (below the transformation point, especially in the region where no oxide film is formed by heating the material). By performing forging, a T-head bolt similar to the previous example can be obtained.

[Brief explanation of the drawing]

Figures 1 to 5 relate to the T-head bolt of the example. Figure 1 is a front view of the T-head bolt, Figure 2 is a top view of the same, Figure 3 is a bottom view of the same, and Figure 4 is a T-head bolt. 500x micrograph showing the metallographic structure (crystal structure) of
Figure 5 shows the metal structure (metal flow) of a T-head bolt.
Figure 6 is a process diagram of the example, Figure 7 is a photograph showing the
The figure is a characteristic diagram showing the relationship between the C content of the material and the tensile strength in an annealed state. Figure 8 is a characteristic diagram showing the relationship between the tensile strength of the material and the life of the forging die. Figure 9 is a characteristic diagram showing the relationship between the tensile strength of the material and the life of the forging die. A vertical cross-sectional view of the example bolt former, FIG. 10 is a front view showing the upper part of the material, FIG. 11 is a front view with the material upturned, FIG. 12 is a plan view of the same, and FIGS. 13 and 14 are Figure 15 is a front view showing the state of defective metal flow of a T-head bolt, Figure 15 is a characteristic diagram showing the relationship between deformation resistance and temperature of stainless steel, and Figure 1 is a characteristic diagram showing the relationship between deformation resistance and temperature of stainless steel.
Figure 6 is a 500x micrograph showing the metal structure of a conventional T-head bolt. 1...T-head bolt, 2...Shaft portion, 2a...Threaded portion, 3...Head, 5...Bolt former.

Claims (1)

[Scope of Claims] 1. A cold temperature control device in which a head is provided with respect to the shaft so as to form a T-shape together with the shaft, and the length of the head is set to 2.60 to 2.90 times the nominal diameter. Interformed stainless steel T-head bolt with C: 0.07% or less, N: 0.02% or less, and Cr: 10.0
% or more, and the entire bolt has a completely annealed structure with a small amount of CrC compound interspersed with ferrite, and it curves and spreads from the center of the top surface of the head to the side of the head. A T-head bolt characterized in that a continuous streamlined metal flow is formed in the shaft portion.