JP2017066431A - Ferritic stainless linear steel material for fastening component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless linear steel material for fastening component having excellent heat galling resistance and toughness.SOLUTION: There is provided a ferritic stainless linear steel material for fastening component having a chemical component of, by mass%, C:0.02% or less, Si:2.0% or less, Mn:1.0% or less, P:0.040% or less, S:0.005% or less, Cr:10.0 to 13.0%, Ni:0.01 to 1.0%, Mo:0.01 to 2.0%, Cu:0.01 to 2.0%, Al:0.3% or less, Ti: less than 0.05%, O:0.010% or less, N:0.02% or less, V:0 to 0.5%, Nb:0 to 1.0%, B:0 to 0.010%, Zr:0 to 1.0%, Ca:0 to 0.02%, Sn:0 to 0.5%, Mg:0 to 0.1%, Ga:0 to 0.05%, Sb:0 to 0.5%, W:0 to 0.5%, Ta:0 to 0.5%, Co:0 to 0.5%, REM:0 to 0.1% and the balance Fe with inevitable impurities and satisfying [0.5 Ni+0.5Cu-4Ti-4O≥0] and [40≤420C+470 N+23 Ni+9Cu+7Mn-11.5(Cr+Si)-12Mo-23 V-49Ti-52Al-47Nb+189<80].SELECTED DRAWING: None

Description

  The present invention relates to a ferritic stainless steel linear steel material for fastening parts used in a high temperature environment.

For measures and thermal efficiency of the emission control of the motor vehicle, O ₂ sensor housing and a cylindrical member for mounting the gas measurement and the temperature measurement (the boss member) are widely used in the exhaust system components. Since these sensor housings and boss materials are used in a high temperature environment such as 800 ° C., heat resistance and corrosion resistance are required. Therefore, stainless steel is used as these materials.

  These materials are produced by forming a linear steel material by cold working and then screwing in cold for component mounting, and then are attached to each part as a sensor housing or a boss material. Conventionally, austenitic stainless steel containing Ni has been used as a material for the sensor housing and the boss material from the viewpoint of heat resistance and corrosion resistance.

  However, since austenitic stainless steel wire has a high coefficient of thermal expansion and high elongation at high temperatures, it is difficult to meet the precision design associated with downsizing automobile parts, and the price is higher than that of Cr. Since a large amount of Ni with poor stability is contained, there is a problem that it is difficult to further reduce the cost of the automobile member. In order to solve these problems, it has been demanded to apply ferritic stainless steel with reduced Ni to the above materials.

  As described above, members such as the sensor housing and the boss material are used in a high temperature environment. For this reason, these members are subject to scale adhesion due to oxidation and deformation due to thermal expansion after long-term use, and seizure occurs between the sensor parts attached to the threaded parts, which prevents them from sticking out. Has occurred. Therefore, in addition to corrosion resistance and toughness, the stainless steel used as the material for the above member needs to have a characteristic that prevents seizure during use (heat resistance).

  The term “seizure” as used herein refers to “a friction surface that has been in a stable state until then due to the friction of the lubricating film and the like. It is a phenomenon that causes wearing and slipping.

  In the above-mentioned problem, with respect to improvement of corrosion resistance, in order to suppress Cr carbonitride causing corrosion resistance deterioration, a method of preventing sensitization by containing Ti and Nb fixing C and N as carbonitride is common. It is used for.

For example, Patent Document 1 proposes a low chromium-containing stainless steel that controls γp described by the formula (A) to 80% or more, increases austenite stability, and secures the toughness of the welded portion.
γp = 420C + 470N + 23Ni + 9Cu + 7Mn-11.5 (Cr + Si) -12Mo-23V-49Ti-52Al-47Nb + 189 (A)

  Furthermore, in Patent Documents 2 and 3, a technique is adopted in which Al and Ti are added to form nitrides, thereby making the crystal grains finer and preventing cracks during high temperature use.

  Patent Document 4 proposes a ferritic stainless steel wire that regulates Cr, Si, Mn, etc., improves cold workability and toughness, and can suppress surface flaws.

JP 2011-174122 A JP 2003-320476 A JP 2006-231404 A JP 2006-016665 A

Metallic Materials Technology Laboratory, Illustrated Metallic Materials Technical Glossary-Second Edition-January 30, 2000, published by Nikkan Kogyo Shimbun, p. 453

  However, since the steels described in Patent Documents 1 to 4 contain a considerable amount of Ti for preventing sensitization and refining crystal grains, there is a concern that a large amount of TiN that tends to become coarse inclusions is generated. In addition, when the Cr content is high, deterioration of cold workability due to an increase in the frequency of occurrence of surface defects and an improvement in material strength can be a problem.

  Furthermore, if the value of γp, which is an index for evaluating the stability of austenite, is controlled to be high, the amount of austenite generated increases at the time of use at a high temperature, Cr supply to the oxide scale becomes insufficient, and the oxidation resistance deteriorates. There is a case. Moreover, scale peeling tends to occur due to a difference in thermal expansion coefficient. Therefore, there is still room for improvement in any technique.

  An object of the present invention is to provide a ferritic stainless steel linear steel material for fastening parts having excellent heat resistance and toughness.

  As a result of various studies to solve the above problems, the present inventors have obtained the following knowledge.

  (A) Based on the low Cr-type stainless steel wire steel, Ti contained for preventing sensitization is a factor that lowers toughness in the present invention, and is regulated to be lower than a certain amount.

  (B) Regarding Ni, Cu, Ti and O, which are elements that affect toughness, the respective contents are regulated, and also in the relationship, 0.5Ni + 0.5Cu-4Ti-4O ≧ 0 is satisfied. Need to be adjusted.

  (C) By containing a small amount of Ni and Cu, the value of γp serving as an index for evaluating the stability of austenite is controlled within an appropriate range. By controlling the value of γp, even if the temperature environment at the time of use is relatively severe, even if the temperature environment is about 800 to 850 ° C., the amount of austenite to be generated is moderately suppressed. Since martensite can be generated, toughness is improved and cracking during use can be prevented.

  (D) With regard to heat resistance, the value of γp is appropriately controlled, so that abnormal oxidation due to austenite generation is prevented during use at high temperatures, and martensite is appropriately generated at lower temperatures. The expansion of the material is reduced, and seizure of the threaded portion due to abnormal oxidation and thermal deformation can be prevented.

  This invention is made | formed based on said knowledge, and makes a summary the following ferritic stainless steel linear steel materials for fastening components.

(1) The chemical composition is mass%,
C: 0.02% or less,
Si: 2.0% or less,
Mn: 1.0% or less,
P: 0.040% or less,
S: 0.005% or less,
Cr: 10.0-13.0%,
Ni: 0.01 to 1.0%,
Mo: 0.01 to 2.0%,
Cu: 0.01 to 2.0%,
Al: 0.3% or less,
Ti: less than 0.05%,
O: 0.010% or less,
N: 0.02% or less,
V: 0 to 0.5%
Nb: 0 to 1.0%,
B: 0 to 0.010%,
Zr: 0 to 1.0%,
Ca: 0 to 0.02%,
Sn: 0 to 0.5%,
Mg: 0 to 0.1%,
Ga: 0 to 0.05%,
Sb: 0 to 0.5%,
W: 0 to 0.5%
Ta: 0 to 0.5%
Co: 0 to 0.5%,
REM: 0-0.1%
The remainder: Fe and inevitable impurities
A ferritic stainless steel linear steel material for fastening parts that satisfies the following formula (i) and the value of γp represented by the following formula (ii) satisfies the following formula (iii).
0.5Ni + 0.5Cu-4Ti-4O ≧ 0 (i)
γp = 420C + 470N + 23Ni + 9Cu + 7Mn-11.5 (Cr + Si) -12Mo-23V-49Ti-52Al-47Nb + 189 (ii)
40 ≦ γp <80 (iii)
However, each element symbol in a formula represents content (mass%) of each element contained in steel materials, and is set to zero when not contained.

  (2) The ferritic stainless steel linear steel for fastening parts according to (1) above, wherein the amount of Fe in the extraction residue obtained by electrolytic extraction is 0.20% or less.

(3) The chemical composition is mass%,
V: 0.01-0.5%
Nb: 0.05-1.0%
B: 0.0005 to 0.010%,
Zr: 0.05 to 1.0%,
Ca: 0.0005 to 0.02%,
Sn: 0.001 to 0.5%,
Mg: 0.0005 to 0.1%,
Ga: 0.0005 to 0.05%,
Sb: 0.001 to 0.5%,
W: 0.01-0.5%
Ta: 0.01 to 0.5%
Co: 0.01-0.5% and REM: 0.001-0.1%,
The ferritic stainless steel linear steel material for fastening parts according to the above (1) or (2), which contains one or more selected from the above.

  In the present invention, “linear steel material” means a steel material having a linear shape used as a ferritic stainless steel material for fastening parts used in a high-temperature environment, such as a steel bar, a wire material, and a steel wire. It is not limited to any of the above.

  According to the present invention, a ferritic stainless steel wire for fastening parts that has sufficient corrosion resistance and is excellent in heat resistance and toughness even in ferritic stainless steel with a relatively low Cr content. Can be obtained. The linear steel material according to the present invention has low thermal expansion when used at high temperatures and has high oxidation resistance, and therefore can prevent seizure during use at high temperatures. Therefore, it can be suitably used as a material for a member used in a high temperature environment.

  Hereinafter, each requirement of the present invention will be described in detail.

1. Chemical composition The reasons for limiting each element are as follows. In the following description, “%” for the content means “% by mass”.

C: 0.02% or less C is an element that has an effect of increasing the strength of the ferrite structure of the matrix and is an austenite-forming element. The austenite phase generates a martensite phase when the temperature is low, and plays a role of preventing coarsening of crystal grains. Therefore, C improves toughness and contributes to securing properties as a linear steel material. On the other hand, C combines with Ti and Nb to form carbides. However, if the C content exceeds 0.02%, Cr carbides are formed, and corrosion resistance and toughness are deteriorated. Therefore, the C content is 0.02% or less. The C content is preferably 0.015% or less. In addition, when obtaining said effect, it is preferable to make C content 0.001% or more.

Si: 2.0% or less Si not only acts as a deoxidizer but also suppresses abnormal oxidation by forming an oxide film of SiO ₂ , and thus is an element useful for improving oxidation resistance. However, if the content exceeds 2.0%, the toughness deteriorates. Therefore, the Si content is 2.0% or less. The Si content is preferably 1.0% or less, and more preferably 0.5% or less. In addition, when obtaining said effect, it is preferable to make Si content into 0.05% or more, and it is more preferable to set it as 0.1% or more.

Mn: 1.0% or less Mn, like C, is an element that has the effect of increasing the strength of the ferrite structure of the matrix and is an austenite-forming element. Therefore, Mn is an element effective for improving toughness. However, if its content exceeds 1.0%, corrosion resistance and toughness deteriorate. Therefore, the Mn content is 1.0% or less. The Mn content is preferably 0.3% or less. In addition, when acquiring said effect, it is preferable that Mn content shall be 0.05% or more, and it is more preferable to set it as 0.1% or more.

P: 0.040% or less P is an element that deteriorates mechanical properties such as toughness. In particular, when the P content exceeds 0.040%, the adverse effect becomes remarkable, so the P content is set to 0.040% or less. The P content is preferably 0.025% or less.

S: 0.005% or less S is an element for degrading toughness and corrosion resistance. In particular, when the S content exceeds 0.005%, the adverse effect becomes remarkable, so the S content is set to 0.005% or less. The S content is preferably 0.003% or less.

Cr: 10.0-13.0%
Cr is an element effective for ensuring the corrosion resistance of the base material. However, if it is less than 10.0%, it is difficult to ensure sufficient corrosion resistance. On the other hand, since Cr is a ferrite-forming element, if its content exceeds 13.0%, the above-described value of γp is greatly reduced, an appropriate amount of martensite cannot be secured, and toughness is reduced. Therefore, the Cr content is set to 10.0 to 13.0%. The Cr content is preferably 10.5% or more, and preferably 12.0% or less.

Ni: 0.01 to 1.0%
Ni is an element effective for improving the corrosion resistance of the base material. Moreover, since Ni is an austenite forming element, it has the effect of promoting martensite formation and increasing toughness. Therefore, the Ni content is 0.01% or more. However, if the content exceeds 1.0%, the toughness decreases. Therefore, the Ni content is set to 0.01 to 1.0%. The Ni content is preferably 0.1% or more, and preferably 0.3% or less.

Mo: 0.01 to 2.0%
Mo is an element that has the effect of enhancing corrosion resistance and high-temperature strength and preventing seizure. Therefore, the Mo content is 0.01% or more. However, if the content exceeds 2.0%, a σ phase is likely to be generated, and since it is a more expensive element, it does not contribute to cost reduction. Therefore, the Mo content is set to 0.01 to 2.0%. The Mo content is preferably 0.05% or more.

Cu: 0.01 to 2.0%
Cu is an austenite-forming element next to C, N and Ni, and is also effective for controlling the value of γp represented by the above formula (ii). It is also an effective element for improving corrosion resistance. Therefore, the Cu content is 0.01% or more. However, if the content exceeds 2.0%, the toughness decreases. Therefore, the Cu content is set to 0.01 to 2.0%. The Cu content is preferably 0.1% or more, and preferably 0.5% or less.

Al: 0.3% or less Al is a deoxidizing element and an element that has the effect of forming fine nitrides, forming fine ferrite, and improving toughness. However, if the content exceeds 0.3%, the toughness is deteriorated. Therefore, the Al content is set to 0.3% or less. The Al content is preferably 0.05% or less, and more preferably 0.03% or less. In addition, when obtaining said effect, it is preferable to make Al content into 0.001% or more, and it is more preferable to set it as 0.005% or more.

Ti: Less than 0.05% Ti is an element having an effect of suppressing formation of Cr carbonitride by forming carbonitride and fixing C, and improving corrosion resistance and toughness. However, when the content is 0.05% or more, coarse TiN or oxide is formed, which causes a decrease in toughness. Therefore, the Ti content is less than 0.05%. The Ti content is preferably 0.04% or less.

O: 0.010% or less O is an element that affects the formation and desulfurization of inclusions. If the content exceeds 0.010%, toughness deteriorates. Therefore, the O content is 0.010% or less.

N: 0.02% or less N increases the strength of the ferrite structure of the matrix, further forms nitrides with Ti and Al, and contributes to the formation of a fine structure. However, when the content exceeds 0.02%, the corrosion resistance is deteriorated because Cr nitride is formed, and the toughness is further lowered. Therefore, the N content is 0.02% or less. The N content is preferably 0.015% or less. In addition, when obtaining said effect, it is preferable that N content shall be 0.005% or more.

  The stainless steel wire material of the present invention contains the above-mentioned elements from C to N, and the remainder has a chemical composition consisting of Fe and inevitable impurities.

  Here, “inevitable impurities” are components mixed in due to various factors of raw materials such as ores and scraps and manufacturing processes when steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. Means what will be done.

  In addition to the above elements, the stainless steel wire material according to the present invention includes V, Nb, B, Zr, Ca, Sn, Mg, Ga, Sb, W, Ta, Co and the following amounts as necessary. You may contain 1 or more types selected from REM.

V: 0 to 0.5%
V generates fine carbonitrides, forms fine ferrite grains, and contributes to improvement of toughness. Therefore, V may be contained as necessary. However, if its content exceeds 0.5%, coarse V-based carbonitrides are produced, and on the contrary, the toughness deteriorates. Therefore, the V content is 0.5% or less. The V content is preferably 0.3% or less. In addition, when obtaining said effect, it is preferable to make V content into 0.01% or more, and it is more preferable to set it as 0.1% or more.

Nb: 0 to 1.0%
Nb, like Ti, forms carbonitride and fixes C, thereby suppressing the formation of Cr carbonitride and contributing to improvement of corrosion resistance and toughness. Therefore, Nb may be contained as necessary. However, if its content exceeds 1.0%, coarse carbonitrides are formed, and on the contrary, the toughness is lowered. Therefore, the Nb content is 1.0% or less. The Nb content is preferably 0.8% or less, and more preferably 0.6% or less. In addition, when obtaining said effect, it is preferable that Nb content shall be 0.05% or more, it is more preferable to set it as 0.1% or more, and it is further more preferable to set it as 0.3% or more.

B: 0 to 0.010%
B may be contained as necessary in order to suppress P segregation at grain boundaries and further improve toughness. However, if the content exceeds 0.010%, coarse boride is formed, and on the contrary, the toughness is lowered. Therefore, the B content is 0.010% or less. The B content is preferably 0.0050% or less. In addition, when obtaining said effect, it is preferable to make B content into 0.0005% or more, and it is more preferable to set it as 0.0020% or more.

Zr: 0 to 1.0%
Zr may be contained as necessary in order to suppress the formation of Cr carbonitride. However, when the content exceeds 1.0%, corrosion resistance and toughness are deteriorated due to coarse carbonitride. Therefore, the Zr content is 1.0% or less. In addition, when obtaining said effect, it is preferable that Zr content shall be 0.05% or more.

Ca: 0 to 0.02%
Since Ca is a useful element that improves hot workability, it may be contained as necessary. However, when the content exceeds 0.02%, the toughness is deteriorated. Therefore, the Ca content is 0.02% or less. In addition, when obtaining said effect, it is preferable to make Ca content 0.0005% or more.

Sn: 0 to 0.5%
Sn is a useful element that improves the corrosion resistance, so it may be contained if necessary. However, if its content exceeds 0.5%, it leads to a decrease in toughness, and the effect of improving corrosion resistance is saturated. Therefore, the Sn content is 0.5% or less. In addition, when obtaining said effect, it is preferable to make Sn content 0.001% or more.

Mg: 0 to 0.1%
Since Mg is an element useful for strengthening deoxidation, it may be contained if necessary. However, if its content exceeds 0.1%, the toughness deteriorates. Therefore, the Mg content is 0.1% or less. The Mg content is preferably 0.08% or less. In addition, when obtaining said effect, it is preferable to make Mg content 0.0005% or more.

Ga: 0 to 0.05%
Since Ga is an element useful for improving workability in cold forging or bending, it may be contained as necessary. However, if the content exceeds 0.05%, the toughness deteriorates. Therefore, the Ga content is 0.05% or less. In addition, when obtaining said effect, it is preferable to make Ga content into 0.0005% or more, and it is preferable to set it as 0.0010% or more.

Sb: 0 to 0.5%
Since Sb is an element useful for improving the corrosion resistance, it may be contained as necessary. However, if its content exceeds 0.5%, the corrosion resistance deteriorates due to segregation. Therefore, the Sb content is 0.5% or less. In addition, when obtaining said effect, it is preferable to make Sb content into 0.0010% or more.

W: 0 to 0.5%
Since W is an element useful for improving wear resistance and corrosion resistance, it may be contained if necessary. However, when the content exceeds 0.5%, the toughness deteriorates. Therefore, the W content is 0.5% or less. In addition, when obtaining said effect, it is preferable to make W content 0.01% or more.

Ta: 0 to 0.5%
Ta is a useful element that improves wear resistance and corrosion resistance, and may be contained as necessary. However, when the content exceeds 0.5%, the toughness deteriorates. Therefore, the Ta content is 0.5% or less. In addition, when obtaining said effect, it is preferable to make Ta content 0.01% or more.

Co: 0 to 0.5%
Co is an element useful for improving wear resistance, and therefore may be contained as necessary. However, when the content exceeds 0.5%, the toughness deteriorates. Therefore, the Co content is 0.5% or less. In addition, when obtaining said effect, it is preferable to make Co content 0.01% or more.

REM: 0 to 0.1%
Since REM is an element that improves hot workability, it may be included as necessary. However, if its content exceeds 0.1%, the toughness deteriorates. Therefore, the REM content is 0.1% or less. The REM content is preferably 0.08% or less. In addition, when obtaining said effect, it is preferable to make REM content 0.001% or more.

  Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to the total content of these elements.

0.5Ni + 0.5Cu-4Ti-4O ≧ 0 (i)
However, each element symbol in a formula represents content (mass%) of each element contained in steel materials, and is set to zero when not contained.
As described above, Ni and Cu are austenite forming elements, while Ti and O are elements that form carbonitrides or oxides. These elements have a great influence on toughness. Therefore, in order to prevent a decrease in toughness, it is necessary that the content of these elements is in the range specified above and that the above formula (i) is satisfied.

γp = 420C + 470N + 23Ni + 9Cu + 7Mn-11.5 (Cr + Si) -12Mo-23V-49Ti-52Al-47Nb + 189 (ii)
40 ≦ γp <80 (iii)
However, each element symbol in a formula represents content (mass%) of each element contained in steel materials, and is set to zero when not contained.
As described above, γp represented by the formula (ii) is an index for evaluating the stability of austenite. By controlling the value of γp to an appropriate range, it is possible to ensure toughness during use and prevent deterioration of oxidation resistance and thermal deformation due to excessive austenite formation at high temperatures. It becomes possible to improve.

  Therefore, it is necessary that the content of each element in the formula (ii) is in the range specified above, and that the formula (iii) is satisfied. The value of γp is preferably 50 or more, and preferably 65 or less.

  By satisfying the above formula (iii), an austenite phase in an amount of 1 to 30% by volume is formed in the metal structure when heated at a high temperature of about 850 ° C., for example. And the coarsening of a ferrite phase can be avoided by forming an appropriate amount of austenite layer. As a result, a martensite phase is obtained after cooling, and strength and toughness can be ensured.

  If γp is less than 40, a sufficient austenite phase cannot be formed during high-temperature heating, so the ferrite phase becomes coarse. And since a martensite phase cannot be secured after cooling, strength and toughness are lowered. On the other hand, when γp is 80 or more, an excessive martensite phase is obtained, and the toughness is greatly reduced. Therefore, in the linear steel material according to the present invention, the amount of martensite in the metal structure becomes 1 to 30% by volume after being exposed to a high temperature environment of about 850 ° C. In order to ensure better strength and toughness, the martensite content is preferably 10 to 20%.

2. As described above, in the present invention, Nb is contained as necessary in order to prevent intergranular corrosion. Along with this, the Lafes phase, which is a precipitate containing Nb and Fe, is precipitated. If the precipitation of the Lafes phase is suppressed, the toughness is further improved. Regarding the evaluation of the amount of precipitation of the Rafes phase, it can be carried out by the following method.

  The steel wire is polished and finished using an abrasive of grain size # 500, and the sample is used as an anode and dissolved using a 10% AA-based electrolytic solution (10% acetylacetone-1% tetramethyl-methanol). Thereafter, the extraction residue obtained by filtering the electrolytic solution with a filter paper or the like is dissolved using 0.5% bromine-methanol, and then the amount of Fe in the extraction residue is measured by chemical analysis such as atomic absorption method. In order to obtain a further improvement effect of toughness, the amount of Fe in the extraction residue is preferably 0.2% or less.

3. Dimensions There are no particular restrictions on the dimensions of the ferritic stainless steel wire for fastening parts according to the present invention. However, as a linear steel material used in a fastening portion in a high-temperature environment that has excellent toughness and heat resistance, the diameter of the linear steel material is preferably 8 mm or more, and more preferably 10 to 20 mm.

4). Manufacturing Method The manufacturing method of the ferritic stainless steel linear steel material for fastening parts according to the present invention is not particularly limited and can be manufactured by a general process. For example, a linear steel material can be manufactured through a casting process, a hot wire rolling process, an annealing process, and a pickling process.

  For the purpose of improving toughness, it is preferable to control the cooling rate in the annealing step in order to suppress the precipitation of the Lafes phase. In the annealing process, if the cooling rate after holding is less than 50 ° C./h, a considerable amount of the Raffes phase is precipitated, which may not contribute to the improvement of toughness. Therefore, the cooling rate after holding in the annealing step is preferably 50 ° C./h or more.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steels having the chemical compositions shown in Tables 1 to 4 were melted in a 100 kg vacuum melting furnace and cast into slabs having a diameter of 180 mm. The slab was subjected to hot wire rolling to a diameter of 15 mm. The hot rolling end temperature at this time was 1000 ° C. Subsequently, batch annealing was performed by heating at 800 ° C. for 2 hours. Thereafter, pickling was performed to obtain a wire (wire No. 1 to 88).

Furthermore, an impact test was performed at room temperature using a V-notch test piece having a square cross section of 55 mm in length and 10 mm on one side in accordance with the JIS standard from the wire. In the present invention, impact value 100 J / cm ² or more of the Yu (◎), 50J / cm ² or more the good ones (○), was evaluated of less than 50 J / cm ² as impossible (×).

  Further, a sample was cut out from the above-mentioned wire rod, subjected to internal thread processing after cold processing to form a boss shape, an external thread was attached to the internal female thread portion, and held in the atmosphere at 850 ° C. for 6 hours. Thereafter, the presence or absence of seizure was investigated when the male screw was removed, and the amount of martensite and the amount of increase in oxidation were also investigated.

The increase in oxidation was determined by measuring the weight of the sample before and after the test. Those having an oxidation increase of 5 mg / cm ² or less by the oxide scale generated in the test were evaluated as good (◯), and those exceeding 5 mg / cm ² were determined as unacceptable (x).

  As for the presence or absence of seizure, when the male screw was removed after the test, the one that was removed was judged as good (O), and the one that caused abnormal oxidation and could not be removed due to galling was judged as unsatisfactory (x).

  For the amount of martensite, image analysis using an optical microscope was performed at a measurement magnification of 50 times and 10 fields of view, and the area ratio of martensite was obtained in each field of view, and the average value of 10 fields of view was calculated. And the average area ratio of said martensite was made into the amount of martensite. Note that the area ratio obtained from the microstructure is the same as the volume ratio.

  The above results are summarized in Tables 5 and 6.

  As can be seen from Tables 1-6, Test No. which is an example of the present invention whose chemical composition satisfies the provisions of the present invention. Nos. 1 to 52 were all good without deterioration of toughness, excellent oxidation resistance, and without seizure.

  On the other hand, test no. 53-88 was inferior in at least any of oxidation resistance, toughness, and heat resistance.

  Next, a wire No. whose chemical composition satisfies the provisions of the present invention. 2 was used to change the annealing temperature and the cooling rate, and to evaluate the influence of the Raffes phase on toughness. Specifically, using the wire, the annealing temperature is either 800 ° C. or 900 ° C., and the cooling rate from that temperature is water cooling, 100 ° C./h, 60 ° C./h, 20 ° C./h, 10 Cooled as either ° C / h. In addition, the sample which cooled on conditions other than water cooling was de-furnaceed at 600 degreeC.

Thereafter, each sample was polished and finished using an abrasive having a particle size of # 500, and the sample was used as an anode, and a 10% AA-based electrolyte (10% acetylacetone-1% tetramethyl-methanol) was used, and -100 mV vs SCE. Then, an electric current of about 1200 coulomb / cm ² was applied to dissolve about 0.5 g. Then, the electrolytic solution was filtered using a polycarbonate filter paper having a mesh size of 0.2 μm to obtain an extraction residue. After the obtained extraction residue was dissolved using 0.5% bromine-methanol, the amount of Fe in the extraction residue was measured by chemical analysis by atomic absorption method.

Then, for each sample, subjected to an impact test in the same manner as in Example 1, impact value 100 J / cm ² or more of the Yu (◎), was evaluated as 50 J / cm ² or more the good ones (○). The results are shown in Table 7.

  As can be seen from Table 7, the cooling rate after annealing was 50 ° C./h or more. For 89 to 91 and 94 to 96, the amount of Fe extraction residue was 0.20% or less, and the toughness was further improved. On the other hand, Sample No. whose cooling rate after annealing is less than 50 ° C./h. For 92, 93, 97 and 98, the Fe extraction residue amount exceeded 0.20%, and as a result, no improvement in toughness was observed.

  According to the present invention, a ferritic stainless steel wire for fastening parts that has sufficient corrosion resistance and is excellent in heat resistance and toughness even in ferritic stainless steel with a relatively low Cr content. Can be obtained. The linear steel material according to the present invention has low thermal expansion when used at high temperatures and has high oxidation resistance, and therefore can prevent seizure during use at high temperatures. Therefore, it can be suitably used as a material for a member used in a high temperature environment.

Claims (3)

Chemical composition is mass%,
C: 0.02% or less,
Si: 2.0% or less,
Mn: 1.0% or less,
P: 0.040% or less,
S: 0.005% or less,
Cr: 10.0-13.0%,
Ni: 0.01 to 1.0%,
Mo: 0.01 to 2.0%,
Cu: 0.01 to 2.0%,
Al: 0.3% or less,
Ti: less than 0.05%,
O: 0.010% or less,
N: 0.02% or less,
V: 0 to 0.5%
Nb: 0 to 1.0%,
B: 0 to 0.010%,
Zr: 0 to 1.0%,
Ca: 0 to 0.02%,
Sn: 0 to 0.5%,
Mg: 0 to 0.1%,
Ga: 0 to 0.05%,
Sb: 0 to 0.5%,
W: 0 to 0.5%
Ta: 0 to 0.5%
Co: 0 to 0.5%,
REM: 0-0.1%
The remainder: Fe and inevitable impurities
A ferritic stainless steel linear steel material for fastening parts that satisfies the following formula (i) and the value of γp represented by the following formula (ii) satisfies the following formula (iii).
0.5Ni + 0.5Cu-4Ti-4O ≧ 0 (i)
γp = 420C + 470N + 23Ni + 9Cu + 7Mn-11.5 (Cr + Si) -12Mo-23V-49Ti-52Al-47Nb + 189 (ii)
40 ≦ γp <80 (iii)
However, each element symbol in a formula represents content (mass%) of each element contained in steel materials, and is set to zero when not contained.   The ferritic stainless steel linear steel material for fastening parts according to claim 1, wherein the amount of Fe in the extraction residue obtained by electrolytic extraction is 0.20% or less. The chemical composition is mass%,
V: 0.01-0.5%
Nb: 0.05-1.0%
B: 0.0005 to 0.010%,
Zr: 0.05 to 1.0%,
Ca: 0.0005 to 0.02%,
Sn: 0.001 to 0.5%,
Mg: 0.0005 to 0.1%,
Ga: 0.0005 to 0.05%,
Sb: 0.001 to 0.5%,
W: 0.01-0.5%
Ta: 0.01 to 0.5%
Co: 0.01-0.5% and REM: 0.001-0.1%,
The ferritic stainless steel linear steel material for fastening parts according to claim 1 or 2, comprising at least one selected from the group consisting of: