JP2020012180A - Two-phase stainless steel bolt excellent in low temperature impact resistance, and manufacturing method therefor - Google Patents

Abstract

To provide a two-phases stainless steel bolt having largely improved low temperature impact resistance while using a two-phase stainless steel of SUS329J4L as a raw material, and a manufacturing method therefor.SOLUTION: A two-phase stainless steel bolt has a structure by cold rolling a wire of a two-phase stainless steel SUS329J4L with a chemical component of, by vol%, C≤0.030, Si≤1.00, Mn≤1.50, P≤0.040, S≤0.030, 5.50≤Ni≤7.50, 24.00≤Cr≤26.00, 2.50≤Mo≤3.50, 0.08≤N≤0.30, and the balance Fe, and conducting a solid solution heat treatment, and having a female thread by a cold rolling in a thread lower diameter with a bolt raw shape material having a bolt head part and the thread lower diameter.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a duplex stainless steel bolt excellent in low-temperature impact resistance and a method of manufacturing the same, and particularly to a bolt having significantly improved low-temperature impact resistance of a bolt manufactured in a cold process for stainless steel SUS329J4L excellent in corrosion resistance. It relates to the manufacturing method.

  Duplex stainless steel SUS329J4L defined by JIS B4303 B4308 (C: 0.030, Si <1.00, Mn <1.50, P <0.040, S <0. 030, 5.50 ≦ Ni ≦ 7.50, 24.00 ≦ Cr ≦ 26.00, 2.50 ≦ Mo ≦ 3.50, 0.08 ≦ N ≦ 0.30, balance Fe) has excellent corrosion resistance Due to its mechanical properties, it has begun to be widely used for structural materials such as plates and angles (Patent Documents 1 and 2). When constructing a structure using a duplex stainless steel plate or an angle, it is necessary to establish a manufacturing technology for fastening parts such as bolts, nuts, and washers.

  Conventionally, when austenitic stainless steel bolts are manufactured cold, they are manufactured in the following steps. The solution-treated heat-treated wire is lightly stretched by about 0.3 mm to improve the outer diameter accuracy to obtain a bolt steel wire. This steel wire is manufactured by forming the head and screw lower diameter part (or shaft part and screw lower diameter part) by cold heading, and forming the screw by cold rolling on the screw lower diameter part. There is a track record and a track record of use.

  On the other hand, SUS329J4L has a special metal structure in which an austenite phase and a ferrite phase are mixed at a predetermined area ratio, and nitrogen which exceeds 800 ppm is added. Due to such physical properties, the establishment of a technology for manufacturing bolts by cold heading of SUS329J4L duplex stainless steel has just begun.

JP 2005-213634 A Japanese Patent No. 5275639

When a SUS329J4L duplex stainless steel bolt is manufactured in the same cold process as a conventional austenitic stainless steel bolt, a high-strength bolt having an excellent tensile strength of 900 to 1100 N / mm ² can be obtained. When the V-notch Charpy test at the specified -40 ° C is performed, the value becomes extremely low. Since a bolt used for a structural member may receive an impact load in a low-temperature environment, it is required to commercialize a bolt with an improved low-temperature Charpy value at −40 ° C.

  In view of the above problems, the present invention is based on the use of SUS329J4L duplex stainless steel, and has an object to provide a duplex stainless steel bolt with significantly improved low-temperature impact resistance and a method for manufacturing the same.

  Therefore, the duplex stainless steel bolt excellent in low-temperature impact resistance according to the present invention is, by volume%, C ≦ 0.030, Si ≦ 1.00, Mn ≦ 1.50, P ≦ 0.040, S ≦ 0. 0.030, 5.50 ≦ Ni ≦ 7.50, 24.00 ≦ Cr ≦ 26.00, 2.50 ≦ Mo ≦ 3.50, 0.08 ≦ N ≦ 0.30, and the balance of Fe An internal thread is formed by cold rolling on the lower threaded portion of a bolt material having a bolt head and a lower threaded portion, which is cold forged using a stainless steel SUS329J4L wire as a material and subjected to solution heat treatment. It is characterized by being.

  Further, the method for producing a duplex stainless steel bolt excellent in low-temperature impact resistance according to the present invention is as follows: volume%, C ≦ 0.030, Si ≦ 1.00, Mn ≦ 1.50, P ≦ 0.040, S ≦ 0.030, 5.50 ≦ Ni ≦ 7.50, 24.00 ≦ Cr ≦ 26.00, 2.50 ≦ Mo ≦ 3.50, 0.08 ≦ N ≦ 0.30, balance Fe chemistry Using a rod material of duplex stainless steel SUS329J4L as a material, a bolt material having a bolt head and a screw lower diameter portion is manufactured by cold heading, and the bolt material is in a range of 950 ° C to 1100 ° C. The temperature is kept within 2 to 60 minutes (preferably selected according to the dimensions of the bolt material and the amount of solution treatment) and then rapidly cooled to 100 ° C. or less to perform a solution heat treatment. Cold-roll female threads in the lower diameter part of the heat-treated screw. Characterized in that it was.

It is a figure which shows typically the manufacturing method of the duplex stainless steel bolt excellent in the low temperature impact property which concerns on this invention.

  Hereinafter, the present invention will be described in detail based on specific examples shown in the drawings. FIG. 1 schematically shows an embodiment of a method for producing a duplex stainless steel bolt excellent in low-temperature impact resistance according to the present invention.

(Sample)
A bolt wire 10 of duplex stainless steel SUS329J4L M16 × 55 mm under the neck length was used as a test material.

(Step of the present invention)
1. The bolt wire 10 is lightly stretched from φ16 mm to φ15.7 mm with a reduction in area of 3.7% (FIG. 1A).
2. The head 11 is set up in the forging machine (FIG. 1 (b)), trimmed to form a bolt head 13, and the front end half of the body 12 is forged to a φ14.57 mm area reduction rate of 17%. The screw lower diameter portion 14 is formed, and a bolt material is manufactured (FIG. 1C).
3. Thereafter, the bolt material is held at a temperature in the range of 950 ° C. to 1100 ° C. for 2 minutes to 10 minutes, and then rapidly cooled to 100 ° C. or less to perform a solution heat treatment (FIGS. 1D and 1E). ).
4. An external thread is cold-rolled on the lower thread portion 14 to form an external thread portion 15 (FIG. 1 (f)).

[Conventional process]
1. Lightly stretched from φ16mm to φ15.7mm with a reduction of 3.7%.
2. The head is upset in a forging machine, trimmed, and forged to a screw lower diameter of φ14.57 mm and a reduction rate of 17%.
3. Thereafter, a male screw is cold rolled at the lower portion of the screw.

  The physical properties of the bolts manufactured by the process of the present invention and the conventional process were compared and confirmed. First, the tensile strength (TS), proof stress (YP), and elongation (EL) were determined, and the cut site was examined. Table 1 shows the results.

From Table 1, the tensile strength (TS) of the bolt manufactured by the process of the present invention (hereinafter referred to as “product of this process”) is 882 N / mm ² and 884 N / mm ² , and the bolt manufactured by the conventional process (hereinafter referred to as “the product”). , reduced to 84% of the tensile strength of the "conventional process product" ^{hereinafter) (1067N / mm 2, 1068N} / mm 2), yield strength (YP) yield strength of the conventional process products ^{(965N / mm 2, 936N /} mm 2 ) To 722 N / mm ² .
Furthermore, the elongation (EL) of this process product is 11.1 mm and 11.3 mm, which is smaller than the elongation of the conventional process product (16.8 mm and 20 mm). On the other hand, the conventional process product was the body.

  Next, the hardness (HRC) was measured. Table 2 shows the measurement results.

  From Table 2, in this process product, the hardness of the body portion 1 / 4D is HRC22.8 and HRC22.5, the hardness of the body core is HRC23.4 and HRC24.6, and the hardness of the screw portion 1 / 4D is HRC23.3 and HRC23. 0.5, the hardness of the thread core is HRC23.9 and HRC23.8, whereas the hardness of the body portion 1 / 4D is HRC28 and HRC27.9, and the hardness of the body core is HRC25.6 and HRC25 in the conventional process product. .4, the hardness of the screw portion 1 / 4D was HRC34.8 and HRC33.6, and the hardness of the screw portion core was HRC29.6 and HRC31.5.

  Next, a V-notch low-temperature (−40 ° C.) Charpy test was performed on the present process product and the conventional process product, and the fracture surface was observed. Table 3 shows the results.

From Table 3, the low temperature Charpy values of the body of this process are 288 J / cm ² and 280 J / cm ² , and the low Charpy values of the thread are 302 J / cm ² and 332 J / cm ^2. The values were 29 J / cm ² and 28 J / cm ² , and the low temperature Charpy value of the thread part was improved as compared with 32 J / cm ² and 32 J / cm ² .
Also, in this process product, the brittle fracture rate of the fracture surface is 10% and 0% in the body portion, and 10% and 5% in the threaded portion. % And 70%, and 75% and 70% in the thread part.

  From the above, it was confirmed that the toughness of the present process product was improved by about 9 times compared with the conventional process product.

  In addition, when the physical properties of the bolts manufactured by the process of the present invention and the conventional process were compared and confirmed for each of the bolts of φ12 Mmm, φ10 mm, and φ8 mm, the same improvement in low-temperature impact resistance as the bolt of φ16 mm was observed.

DESCRIPTION OF SYMBOLS 10 Wire rod 11 Head 12 Body 13 Bolt head 14 Screw lower diameter part 15 Male screw

JIS B 4303 two-phase system is defined by the B4308 stainless steel SUS329J4L (chemical composition: by mass%, C ≦ 0.030, Si ≦ 1.00, Mn ≦ 1.50, P ≦ 0.040, S ≦ 0. 030, 5.50 ≦ Ni ≦ 7.50, 24.00 ≦ Cr ≦ 26.00, 2.50 ≦ Mo ≦ 3.50, 0.08 ≦ N ≦ 0.30, balance Fe) has excellent corrosion resistance. Due to its mechanical properties, it has begun to be widely used for structural materials such as plates and angles (Patent Documents 1 and 2). When constructing a structure using a duplex stainless steel plate or angle, it is necessary to establish a manufacturing technology for fastening parts such as bolts, nuts, and washers.

Therefore, the duplex stainless steel bolt excellent in low-temperature impact resistance according to the present invention is, in mass% , C ≦ 0.030, Si ≦ 1.00, Mn ≦ 1.50, P ≦ 0.040, S ≦ 0. 0.030, 5.50 ≦ Ni ≦ 7.50, 24.00 ≦ Cr ≦ 26.00, 2.50 ≦ Mo ≦ 3.50, 0.08 ≦ N ≦ 0.30, and the balance of the chemical composition of Fe An internal thread is formed by cold rolling at the lower diameter portion of the above-described bolt material having a bolt head and a lower diameter portion of a bolt, which is cold forged using a stainless steel SUS329J4L wire as a raw material and subjected to solution treatment. It is characterized by being.

Further, the method for producing a duplex stainless steel bolt excellent in low-temperature impact resistance according to the present invention is, in mass% , C ≦ 0.030, Si ≦ 1.00, Mn ≦ 1.50, P ≦ 0.040, S ≦ 0.030, 5.50 ≦ Ni ≦ 7.50, 24.00 ≦ Cr ≦ 26.00, 2.50 ≦ Mo ≦ 3.50, 0.08 ≦ N ≦ 0.30, balance Fe chemistry Using a wire of duplex stainless steel SUS329J4L as a material, a bolt material having a bolt head and a screw lower diameter portion is manufactured by cold heading, and the bolt material is in a range of 950 ° C to 1100 ° C. After holding at a temperature within the range of 2 to 60 minutes (preferably selected according to the dimensions of the bolt material and the amount of solution treatment), the solution is rapidly cooled to 100 ° C. or lower to perform a solution heat treatment. Cold-roll female threads in the lower diameter part of the heat-treated screw. Characterized in that it was.

Claims (2)

By volume%, C ≦ 0.030, Si ≦ 1.00, Mn ≦ 1.50, P ≦ 0.040, S ≦ 0.030, 5.50 ≦ Ni ≦ 7.50, 24.00 ≦ Cr ≦ 26.00, 2.50 ≦ Mo ≦ 3.50, 0.08 ≦ N ≦ 0.30, microstructure of a duplex stainless steel SUS329J4L having the chemical composition of Fe as the balance, which was cold forged and subjected to solution treatment. A two-phase with excellent low-temperature impact resistance, characterized in that a female screw is formed by cold rolling at the above-mentioned lower thread portion of a bolt forming material having a bolt head and a lower thread diameter portion and having a bolt head Stainless steel bolts.
By volume%, C ≦ 0.030, Si ≦ 1.00, Mn ≦ 1.50, P ≦ 0.040, S ≦ 0.030, 5.50 ≦ Ni ≦ 7.50, 24.00 ≦ Cr ≦ 26.00, 2.50 ≦ Mo ≦ 3.50, 0.08 ≦ N ≦ 0.30, the balance being Fe duplex stainless steel SUS329J4L, and the bolt head and screw lower diameter part Is manufactured by cold heading, holding the bolt shaped material at a temperature in the range of 950 ° C. to 1100 ° C. for 2 minutes to 60 minutes, and then quenching to 100 ° C. or less. A method for producing a duplex stainless steel bolt excellent in low-temperature impact resistance, wherein a solution heat treatment is performed, and a female screw is cold-rolled at a lower diameter portion of the solution heat-treated screw.

Images