JP2015224358A - Ferritic stainless steel wire excellent in formability and corrosion resistance and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel wire excellent in formability, corrosion resistance, wear resistance and economical efficiency, for use in a muffler hanger of an automobile running in a salt damage district.SOLUTION: The ferritic stainless steel wire excellent in formability and corrosion resistance is provided which contains C:0.02% or less, Si:1.0% or less, Mn:1.0% or less, P:0.04% or less, S:0.03% or less, Cr:10.5% to 25.0%, N:0.025% or less, and further one or more kind of Cu:2.0% or less, Mo:3.0% or less, Ni:5.0% or less and one or more kind of Nb:0.8% or less and Ti:0.5% or less, and which is characterized in that: a crystal grain size number in an inner layer of a cross section vertical to a longer direction is 4.0 to 9.0; a crystal grain size number in a surface layer from a surface to a depth of 1/4×(diameter) is 5.0 to 10.0, namely, the crystal grain size number in the surface layer is larger than that in the inner layer; an average intergranular erosion depth is 0.5 to 1 times the average crystal grain diameter in the surface layer; and hardness of the cross section from the surface to the depth of 1/4×(diameter) is 140 or more.

Description

  The present invention relates to a ferritic stainless steel wire excellent in strength, formability and pitting corrosion resistance.

  Conventionally, carbon steel for mechanical structures such as STKM11A has been used as a stainless steel muffler hanger. However, in order to ensure corrosion resistance, metal plating or coating has been applied to the carbon steel. However, in automobiles traveling along the coast or in snowy areas where snow melting salt is sprayed in winter, a large amount of salt is attached to the muffler hangers, resulting in severe bruising and, in some cases, breakage due to the progress of pitting corrosion. The muffler may fall off.

  Therefore, the use of austenitic stainless steels such as SUS304 and SUSXM7 has been studied together with ferritic stainless steels having a low chromium content such as SUS410. Ferritic stainless steel has a small amount of alloy other than Cr and is low in cost, but is inferior in corrosion resistance. Although austenitic stainless steel is excellent in corrosion resistance and formability, it has a high Ni content and a high cost. From the economical point of view, ferritic stainless steel with a low Ni content is advantageous.

However, in order to commercialize ferritic stainless steel as a muffler hanger,
(1) improving the cold forgeability of the head,
(2) In order to prevent the occurrence of rust when a salt spray test is performed after molding, it is necessary to include a large amount of alloy, including Cr, and to reduce costs, the amount of alloy is reduced to improve corrosion resistance. Letting
(3) Stones and sand jump while driving, and they collide and wear continuously, improving wear resistance; and
(4) The challenge is to increase the resistance to corrosion generation / development from scratches caused by stone and sand collisions.

  Patent Documents 1 to 3 disclose techniques for individually solving these problems. Patent Document 1 discloses stainless steel that is excellent in cold forgeability, corrosion resistance, and electromagnetic properties, and is used as a material for electromagnetic components and the like. Patent Document 2 discloses a ferritic stainless steel excellent in wear resistance and secondary workability. Patent Document 3 discloses a ferritic stainless steel excellent in cold workability, toughness, and corrosion resistance and a method for producing the same. However, none of the documents discloses means for solving all of the above four problems.

Japanese Patent No. 2734035 Japanese Examined Patent Publication No. 05-081656 Japanese Patent No. 2906445

  The present invention maintains high formability, sufficient corrosion resistance, wear resistance, etc., and is economical for use in automobile muffler hangers that run in salt damage areas such as snowy areas where snow melting salt is sprayed in the winter or in winter. Another object of the present invention is to provide a ferritic stainless steel wire and a method for producing the same.

  In order to solve the above problems, the present inventors have investigated the influence of alloy elements on formability (cold forgeability), corrosion resistance, and wear resistance. As a result, in addition to the component composition of steel, it was found that the material and surface properties affect these properties.

The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) By mass%, C: 0.02% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.03% or less, Cr: 10 1% or more of 5% or more and 25.0% or less, N: 0.025% or less, Cu: 2.0% or less, Mo: 3.0% or less, Ni: 5.0% or less Nb: 0.8% or less, Ti: 0.5% or less, and the grain size number of the microstructure of the inner layer in the cross section perpendicular to the longitudinal direction is 4. 0 to 9.0, the grain size number of the surface layer from the surface to a depth of 1/4 × (diameter) is 5.0 to 10.0, the grain size number of the surface layer is larger than the inner layer, and the average grain boundary erosion depth Is 0.5 to 1 times the average crystal grain size of the surface layer, and the hardness Hv1 (JIS Z 2244) for the cross section from the surface to a depth of 1/4 × (diameter). There formability and pitting corrosion resistance superior ferritic stainless steel wire, characterized in that at more than 140.
(2) The ferritic stainless steel wire having excellent formability and pitting corrosion resistance according to (1) above, further containing Sn: 1.0% or less by mass%.
(3) Further, in mass%, W: 0.5% or less, Ta: 0.5% or less, Co: 0.5% or less, Al: 0.05% or less, Ca: 0.05% or less, V : 0.5% or less, B: 0.01% or less, Bi: 0.5% or less, Mg: 0.1% or less, Zr: 0.5% or less, REM: 0.1% or less, Ga: 0 0.05% or less, Sr: 0.05% or less, Se: 0.05% or less, Ru: 0.05% or less, Rh: 0.05% or less, Pd: 0.05% or less, Ag: 0.05 % Or less, Cd: 0.05% or less, In: 0.05% or less, Sb: 0.05% or less, Te: 0.05% or less, Hf: 0.05% or less, Re: 0.05% or less Os: 0.05% or less, Ir: 0.05% or less, or one or more of Os: 0.05% or less, excellent in moldability and pitting corrosion resistance as described in (1) or (2) above Ferritic stainless steel wire was.
(4) The ferritic stainless steel wire excellent in formability and pitting corrosion resistance according to any one of the above (1) to (3), which is used for a muffler hanger.
(5) A step of hot rolling the slab having the component composition according to any one of (1) to (3), a step of performing a shot blast treatment after the hot rolling, and 900 ° C. or more after the shot blast treatment It is excellent in formability and pitting corrosion resistance according to any one of the above (1) to (4), comprising a step of performing a heat treatment at a temperature of 1100 ° C. or less and a step of pickling after the heat treatment A method for manufacturing ferritic stainless steel wires.

  ADVANTAGE OF THE INVENTION According to this invention, the ferritic stainless steel wire used suitably for the automobile muffler hanger excellent in a moldability, pitting corrosion resistance, and abrasion resistance can be provided.

  Hereinafter, the ferritic stainless steel wire of the present invention will be described. Hereinafter, “%” means “% by mass” unless otherwise specified.

  First, the reasons for limiting the component composition of the stainless steel wire will be described.

C: 0.02% or less C is an element that increases the strength of the ferrite structure of the matrix and further generates an austenite phase and carbide. The austenite phase produces a martensite structure after welding to improve the strength, and fine carbides also contribute to improving the strength and improve the high-temperature strength as a steel wire.

  However, if it exceeds 0.02%, Cr carbide precipitates at the grain boundaries, and a Cr-depleted layer is generated around it. In a corrosive environment, so-called intergranular corrosion occurs because the Cr-depleted layer along the grain boundary dissolves. Therefore, it was limited to 0.02% or less. Furthermore, 0.015% or less is preferable to prevent intergranular corrosion. Moreover, if less than 0.001%, the amount of austenite phase and fine carbides produced in the steel wire is too small and the strength is insufficient, so 0.001% or more is preferable.

Si: 1.0% or less Si is an element useful for high-temperature oxidation resistance because it suppresses the progress of oxidation as a deoxidizer and also by forming an oxide film of SiO ₂ . However, if it exceeds 1.0%, it becomes hard and the mechanical properties deteriorate, so it was made 1.0% or less. Further, if it is less than 0.1%, a deoxidation effect cannot be obtained, so 0.1% or more is preferable.

Mn: 1.0% or less Mn, like C, is an element that increases the strength of the ferrite structure of the matrix and further generates an austenite phase. However, if it exceeds 1.0%, more inclusions remain in the steel and the corrosion resistance deteriorates, so the content was made 1.0% or less. Further, if it is less than 0.1%, the strength of the steel wire is low, and the strength is insufficient after welding, so 0.1% or more is preferable.

P: 0.04% or less P is an element not only deteriorating mechanical properties such as toughness but also harmful to corrosion resistance. If it exceeds 0.04%, the adverse effect becomes remarkable, so it was set to 0.04% or less. The lower limit is not particularly limited, but if it is reduced to 0.001% or less, the production cost is increased, so 0.001% is a practical lower limit on a practical steel wire.

S: 0.03% or less S is an element that combines with Mn to form MnS serving as a starting point of initial firing. S is also a harmful element that segregates at the crystal grain boundary and promotes embrittlement of the grain boundary, and is preferably reduced as much as possible. If it exceeds 0.03%, the adverse effect becomes remarkable, so 0.03% or less was set. The lower limit is not particularly limited, but if it is reduced to 0.0001% or less, the production cost is increased, so 0.0001% is a practical lower limit on a practical steel wire.

Cr: 10.5% to 25.0%,
Cr is an important element as a corrosion resistance developing component in the present invention. In the steel wire which is the subject of the present invention, at least 10.5% is necessary in order to ensure the required corrosion resistance. Preferably, it is 11.0% or more. This is because when the content is less than 10.5%, it is difficult to form a strong passive film.

  On the other hand, if it exceeds 25.0%, the corrosion resistance will be improved, but the amount of ferrite phase produced will increase, leading to insufficient toughness and increased costs. This tendency occurs at more than 20.0%, so 20% or less is preferable. From the above, the content was made 10.5% or more and 25.0% or less. Preferably, it is 11.0% or more and 20.0% or less.

N: 0.025% or less N is an element that increases the strength of the ferrite structure of the matrix and further generates an austenite phase and a nitride. However, if it exceeds 0.025%, corrosion resistance and cold forgeability deteriorate, so the content was made 0.025% or less. Further, if it is less than 0.001%, the production of steel wire nitride is small and the effect of improving the strength cannot be obtained, so 0.001% or more is preferable.

  Moreover, in order to improve each characteristic, the ferritic stainless steel wire of this invention contains 1 type (s) or 2 or more types of Cu, Mo, and Ni further.

Cu: 2.0% or less Cu is a useful element that improves corrosion resistance and strength. However, if it exceeds 2.0%, not only the toughness is deteriorated but also the cold forgeability is deteriorated. Further, if it is less than 0.2%, it is difficult to obtain the effect of improving corrosion resistance, so 0.2% or more is preferable.

Mo: 3.0% or less,
Mo is an element that improves corrosion resistance and wear resistance. However, if it exceeds 3.0%, the σ phase tends to precipitate and becomes brittle, and the cold forgeability deteriorates. If less than 0.01%, corrosion resistance is not sufficiently exhibited, so 0.01% or more is preferable. More preferably, it is 0.2% or more and 3.0% or less.

Ni: 5.0% or less Ni, like Cu, is an element effective in improving corrosion resistance, strength, and wear resistance. However, if it exceeds 5.0%, the cold forgeability deteriorates, so the content was made 5.0% or less. Further, if it is less than 0.01%, it is difficult to obtain an effect of improving corrosion resistance and strength, so 0.01% or more is preferable.

  Moreover, in order to suppress the production of Cr carbide and prevent intergranular corrosion, the present invention further contains one or two of Nb and Ti.

Nb: 0.8% or less Solid solution Nb increases the strength and forms carbonitrides, so it suppresses the formation of Cr carbides and, as a result, suppresses the formation of Cr-deficient layers. It is an element that contributes to prevention. That is, Nb is an element useful for improving corrosion resistance. However, if it exceeds 0.8%, the cold forgeability deteriorates, so the content was made 0.8% or less. Further, if it is less than 0.05%, it is difficult to obtain the effect of improving the corrosion resistance, so 0.05% or more is preferable.

Ti: 0.5% or less Ti, like Nb, forms carbides and nitrides, so suppresses the formation of Cr carbides, and as a result, suppresses the formation of Cr-deficient layers and contributes to the prevention of intergranular corrosion. Element. That is, Ti is an element useful for improving corrosion resistance. However, if it exceeds 0.5%, the cold forgeability deteriorates, so the content is made 0.5% or less. Moreover, since the corrosion resistance improvement effect of Ti cannot be expressed as it is less than 0.05%, 0.05% or more is preferable.

  The above is an element required by this invention. Furthermore, for the purpose of improving each characteristic, the following elements can be optionally contained as required.

Sn: 1.0% or less Sn is an element useful for improving strength by suppressing active dissolution in acid. However, if it exceeds 1.0%, the hot workability deteriorates, so the content was made 1.0% or less. Moreover, since the effect of Sn cannot be expressed as it is less than 0.001%, 0.001% or more is preferable.

  In the present invention, W, Ta, Co, Al, Ca, V, B, Bi, Mg, Zr, and REM (Sc, Y and elements having atomic numbers 57 to 71) are further used according to various purposes. One, or two or more selected from Ga, Sr, Se, Ru, Rh, Pd, Ag, Cd, In, Sb, Te, Hf, Re, Os, and Ir can be added.

W: 0.5% or less W is an element useful for improving wear resistance and corrosion resistance. If it exceeds 0.5%, the toughness deteriorates, so the content was made 0.5% or less. If it is less than 0.01%, wear resistance and corrosion resistance cannot be expressed, so 0.01% or more is preferable.

Ta: 0.5% or less,
Ta is a useful element that improves wear resistance and corrosion resistance. If it exceeds 0.5%, the toughness deteriorates, so the content was made 0.5% or less. If it is less than 0.01%, wear resistance and corrosion resistance cannot be expressed, so 0.01% or more is preferable.

Co: 0.5% or less Co is an element useful for improving wear resistance. If it exceeds 0.5%, the toughness deteriorates, so the content was made 0.5% or less. If it is less than 0.01%, wear resistance cannot be exhibited, so 0.01% or more is preferable.

Al: 0.05% or less Al is an element useful as a deoxidizer. If it exceeds 0.05%, the toughness deteriorates, so it was made 0.05% or less. In order to exhibit the effect as a deoxidizer, 0.001% or more is preferable.

Ca: 0.05% or less Ca is an element useful for improving hot workability. If it exceeds 0.05%, the toughness deteriorates, so it was made 0.05% or less. In order to exhibit the effect of improving hot workability, 0.001% or more is preferable.

V: 0.5% or less V is an element useful for improving corrosion resistance. If it exceeds 0.5%, the toughness deteriorates, so the content was made 0.5% or less. In order to exhibit the effect of improving corrosion resistance, 0.03% or more is preferable.

B: 0.01% or less B is an element useful for improving hot workability. If it exceeds 0.01%, the toughness deteriorates, so the content was made 0.01% or less. In order to exhibit the effect of improving hot workability, 0.0003% or more is preferable.

Bi: 0.5% or less Bi is an element useful for improving corrosion resistance and cutting workability. If it exceeds 0.5%, the toughness deteriorates, so the content was made 0.5% or less. In order to exhibit the effect of improving corrosion resistance and cutting workability, 0.005% or more is preferable.

Mg: 0.1% or less Mg is an element useful for improving corrosion resistance. If it exceeds 0.1%, the toughness deteriorates, so the content was made 0.1% or less. In order to exhibit the effect of improving corrosion resistance, 0.0002% or more is preferable.

Zr: 0.5% or less Zr is an element useful for improving corrosion resistance and hot workability. If it exceeds 0.5%, the effect of improving the corrosion resistance and hot workability is saturated, so the content is made 0.5% or less. In order to exhibit the improvement effect of corrosion resistance and hot workability, 0.03% or more is preferable.

REM: 0.1% or less REM is an element useful for improving hot workability. If it exceeds 0.1%, the toughness deteriorates, so the content was made 0.1% or less. In order to exhibit the effect of improving hot workability, 0.03% or more is preferable.

Ga: 0.05% or less Ga is a useful element that improves workability in cold forging and bending. In order to surely obtain the effect of addition, 0.0010% or more is preferable. On the other hand, if it exceeds 0.05%, the toughness deteriorates, so it was made 0.05% or less.

Sr: 0.05% or less Sr, like Ca, is an element useful for improving hot workability. In order to reliably obtain the effect of improving hot workability, 0.001% or more is preferable. On the other hand, if it exceeds 0.05%, the toughness deteriorates, so it was made 0.05% or less.

Se, Ru, Rh, Pd, Ag, Cd, In, Sb, Te, Hf, Re, Os, Ir: 0.05% or less Se, Ru, Rh, Pd, Ag, Cd, In, Sb, Te, Hf , Re, Os and Ir are elements useful for improving corrosion resistance. In order to obtain the effect of improving the corrosion resistance, 0.0010% or more is preferable respectively. On the other hand, if it exceeds 0.05%, the corrosion resistance deteriorates due to segregation.

  Next, the grain size number of the microstructure of the inner layer in the vertical section with respect to the longitudinal direction of the steel wire is 4.0 to 9.0, and the grain size of the surface layer from the surface to a depth of 1/4 × (diameter) The reason why the number is 5.0 to 10.0 and the surface grain size number is larger than the inner layer will be described.

  First, a method for measuring the average grain size number will be described.

  The number of intersections of crystal grain boundaries per unit length was measured by the method specified in Appendix C of JIS G 0551 (2013) (steel-grain size microscopic test method), and JIS G 0551 (2013) Table B. Determine the grain size according to 1 or Annex JB.

  For the cross section perpendicular to the center line of the steel wire, the grain size number for the cross section from the surface to a depth of 1/4 × (diameter) is the average grain size number of the surface layer, and 1/4 × (diameter) from the surface The grain size number for the cross section from the depth to the center is the grain size number of the inner layer.

  Next, the reason for limiting the average grain size number will be described.

  When the crystal grain size number is 4.0 or more, the bending process becomes easy and the muffler hanger shape process can be achieved. Preferably, it is 5.0 or more. If it is less than 4.0, the bending workability is low, so that the molding itself becomes difficult, for example, cracking occurs during the molding. In the case of more than 10.0, the bending load is too large and it becomes difficult to process, so it was set to 10.0 or less. Preferably, it is 9.0 or less.

  Since the surface layer is required to have wear resistance, increasing the crystal grain size number increases strength and increases elongation, thereby improving workability. Therefore, the crystal grain size number of the surface layer was made 1 or more larger than that of the inner layer. The crystal grain size number increases as the surface layer increases and decreases toward the inner layer. It has been found that if the difference between the crystal grain number of the surface layer and the crystal grain number of the inner layer is clear, the cold forgeability, wear resistance and corrosion resistance are satisfied.

  Therefore, with respect to the longitudinal direction of the steel wire, the grain size number of the inner layer in the vertical section is 4.0 to 9.0, the grain size number of the surface layer is 5.0 to 10.0, and the grain size of the surface layer from the inner layer Increased the number.

  Next, the reason why the average grain boundary erosion depth is 0.5 to 1 times the average crystal grain size of the surface layer will be described.

  First, a method for measuring the average grain boundary erosion depth is described.

  Grain boundaries are clarified by a method such as oxalic acid electroetching, and the surface grain boundary erosion depth (distance between the surface and the deepest point of grain boundary erosion) is measured using an optical microscope for a field of view of 200 μm × 200 μm to 300 μm × 300 μm. Is measured in μm and the sum (ΣD) is calculated. Next, the number (N) of crystal grains in contact with the measured grain boundary is measured. Finally, the sum of the grain boundary erosion depth (ΣD) is divided by the number of crystal grains (N) to obtain a quasi-average grain boundary erosion depth (= ΣD / N). The quasi-average grain boundary erosion depth is measured for five visual fields, and the average is defined as the average grain boundary erosion depth.

  For the crystal grain size, for one field of view of 500 μm × 500 μm, the diameter of the crystal grain regarded as a circle is calculated from the cross-sectional area of the crystal grain by image analysis, and the average diameter of all crystal grains in one field of view is calculated. It is measured for five arbitrary visual fields, and the average diameter of the five visual fields is defined as the average crystal grain size.

  Next, the reason for limiting the average grain boundary erosion depth will be described.

  In the steel wire, after heat treatment, a Cr-deficient layer generated near the grain boundary of the surface layer is preferentially welded by pickling to form a small groove (micro groove) along the grain boundary. When the microgroove is drawn, the microgroove is an effective part for holding the lubricant on the surface. The depth of this microgroove affects the surface beauty and cold workability due to lubricity, gloss, etc., and the depth is organized in relation to the average crystal grain size regardless of whether or not shot blasting is performed. I found out that I can do it.

  If the microgroove depth (grain boundary erosion depth) is less than 0.5 times the average crystal grain size of the surface layer, the surface lubricant is too small to be drawn or the surface is seized. Or If the microgroove depth exceeds 1 times the average crystal grain size, the grain boundary erosion remains on the surface after wire drawing, not only deteriorating the surface beauty, but also cracking during cold forging and bending. Is the starting point. From the above, the average grain boundary erosion depth was set to 0.5 to 1 times the average crystal grain size of the surface layer.

  Next, the reason why the hardness Hv1 of the cross section from the surface to a depth of ¼ × (diameter) is 140 or more will be described.

  The wear resistance of the steel wire is controlled by the surface hardness because the higher the surface hardness, the better the wear resistance. If the hardness of the surface layer is less than 140 in Hv1 (JIS Z 2244), when used as a muffler hanger, stones and sand will bounce during running and wear out due to continuous collision. The thickness was 140 or more. Preferably, it is 160 or more. The upper limit is not particularly limited, but about 250 is the upper limit in terms of workability.

  As described above, the ferritic stainless steel wire of the present invention that has been described satisfies all the conditions necessary for the muffler hanger described above, and therefore can be particularly suitably used for the muffler hanger.

  Next, the manufacturing method of the ferritic stainless steel wire of this invention is demonstrated.

  The ferritic stainless steel wire of the present invention is manufactured by hot-rolling the slab adjusted to the above component composition, then performing shot blasting, performing heat treatment, pickling treatment, and drawing, It is characterized in that after hot rolling, shot blasting is performed, and then further heat treatment is performed.

Hot rolling process, shot blasting process In hot rolling (wire rolling), the area reduction ratio ((cross-sectional area of billet) − (cross-sectional area after hot rolling)) / (cross-sectional area of billet) is about 98. It becomes as large as ˜99%, and strain is introduced over the entire cross section. Subsequent to hot rolling, by performing shot blasting, finer strain is introduced into the surface layer. The shot blast treatment is preferably 100 kg / m ² or more and 300 kg / m ² or less. If it is less than 100 kg / m ² , there is no effect of adding strain, and if it exceeds 300 kg / m ² , the surface layer is greatly deformed and the surface is not beautiful.

Heat treatment step After the shot blast treatment, heat treatment is performed at a temperature of 900 ° C. or higher and 1100 ° C. or lower. As a result, the strain introduced by hot rolling and shot blasting is recrystallized from the surface layer to the entire center before recovering with residual heat, and the surface layer is fine and the center has a large grain distribution. It is formed.

  Moreover, the grain size number of the microstructure of the inner layer in the vertical cross section with respect to the longitudinal direction of the steel wire is 4.0 to 9.0, the grain size number of the surface layer is 5.0 to 10.0, and the crystal of the surface layer The particle size number can be larger than that of the inner layer. In addition, since the surface layer has a small crystal grain size, the surface roughness of the wire is small and a beautiful surface can be obtained, and wrinkles and cracks due to cold forging and bending are difficult to occur.

  Further, if the temperature of the heat treatment is less than 900 ° C., recrystallization is insufficient, and if it exceeds 1100 ° C., the crystal becomes coarse grains, and the crystal grain size number of the surface layer becomes smaller than 5.0. The heat treatment time is preferably 4 to 5 minutes.

Pickling process Pickling is performed after the heat treatment. By performing the pickling treatment, an appropriate grain boundary erosion depth can be obtained together with descaling. In the pickling treatment, steel is immersed in an acid that preferentially dissolves a Cr-deficient layer such as nitric hydrofluoric acid at a constant temperature for a predetermined time.

  The average intergranular corrosion depth can be controlled by the temperature of the pickling treatment and the immersion time. For example, when the temperature is high, the immersion time is shortened, and when the temperature is low, the immersion time is increased. When the immersion time with respect to temperature is shorter than the optimum range, the grain boundary erosion depth becomes smaller than 0.5 times the average crystal grain size of the surface layer. When the immersion time with respect to temperature is longer than the optimum range, the surface roughness becomes large, and a portion where the grain boundary erosion depth exceeds 1 times the average crystal grain size of the surface layer appears. Therefore, by adjusting the temperature of the pickling treatment and the dipping time, a steel wire having an average grain boundary erosion depth of 0.5 to 1 times the average crystal grain size of the surface layer can be obtained.

A muffler hanger was manufactured in the following steps (1) to (7).
(1) Billet (diameter 178mm round cross section)
(2) Wire rod rolling (11.5 mm diameter after rolling)
(3) Shot blasting (4) Heat treatment (900 ° C. or higher and 1100 ° C. or lower)
(5) Pickling (6) Wire drawing (10.5 mm diameter after wire drawing)
(7) Cold forging

Table 1 shows chemical components of the stainless steel wires of the present invention and comparative steel. The pickling of (5) was immersed in hydrochloric acid and then immersed in 30 ° C. nitric hydrofluoric acid (1% HF and 10% HNO ₃ ) for 5 minutes.

  Table 2 shows the evaluation result of the muffler hanger simulated product obtained by subjecting the steel wire to the cold forging of (7). The crystal grain size number was measured by the method described above.

The cold forgeability is upset forging a sample of φ10.5mm and length 150mm. After the head is machined, 90 ° bending is performed on the 1/2 length part to check for cracks on the surface after forging. Observed. An upsetting rate of 70% indicates no cracking, an upsetting rate of 50% indicates no cracking, and an upsetting rate of 50% indicates that there is no cracking. The upsetting rate (%) was obtained as follows.
Upsetting rate (%) = ((H−h) / H) × 100
H: Length of sample before forging h: Length of sample after forging

  As for the wear resistance, the higher the surface hardness, the better the wear resistance. The hardness Hv1 (JIS Z 2244) for a cross section having a depth of ¼ × (diameter) from the surface was evaluated based on an average of 5 points, and 140 or more was regarded as acceptable.

  For corrosion resistance, a sample simulating a muffler hanger was subjected to a 35 ° C., 5% salt spray test (168 h) in accordance with JIS Z 2371, and the state of rusting was observed. “No” indicates ◎, “Slightly” (with a rating number of 6 or more) indicates “◯”, and “No” (with a rating number of less than 6) indicates ×.

  Test No. which is an invention example. 1-12 are favorable in evaluation of cold forgeability, hardness, and corrosion resistance. Test No. which is a comparative example out of the component range. 13-23 are inferior in one or more evaluations of cold forgeability, hardness, and corrosion resistance. Test No. 1 is a comparative example in which the pickling method was changed to deviate from the range of average grain boundary erosion depth / surface average grain size. 24 and 25 are inferior in the evaluation of cold forgeability.

  In Table 3, steel No. 1, 5, and 8 show the evaluation results obtained by changing the conditions of (3) shot blast treatment and (4) heat treatment.

  Test No. which is an invention example. 26-No. No. 34 is good in the evaluation of cold forgeability, hardness, and corrosion resistance. Test No. which is a comparative example. 35-No. No. 43 is inferior in one or more evaluations of cold forgeability, hardness, and corrosion resistance.

  As described above, according to the present invention, it is possible to provide a ferritic stainless steel wire that is suitably used for an automobile muffler hanger excellent in formability, pitting corrosion resistance, and wear resistance. Therefore, the present invention has great industrial applicability.

Claims (5)

% By mass
C: 0.02% or less,
Si: 1.0% or less,
Mn: 1.0% or less,
P: 0.04% or less,
S: 0.03% or less,
Cr: 10.5% to 25.0%,
N: 0.025% or less,
Cu: 2.0% or less,
Mo: 3.0% or less,
Ni: containing 1 type or 2 types or less of 5.0% or less,
Nb: 0.8% or less,
Ti: 1 or 2% of 0.5% or less, the grain size number of the microstructure of the inner layer in the cross section perpendicular to the longitudinal direction is 4.0 to 9.0, and 1/4 × (( The grain size number of the surface layer up to the depth of (diameter) is 5.0 to 10.0, the grain size number of the surface layer is larger than the inner layer, and the average grain boundary erosion depth is 0.5 times the average grain size of the surface layer 1x
A ferritic stainless steel wire excellent in formability and pitting corrosion resistance, characterized in that the hardness Hv1 (JIS Z 2244) for a cross section from the surface to a depth of 1/4 × (diameter) is 140 or more. Furthermore, in mass%,
The ferritic stainless steel wire excellent in formability and pitting corrosion resistance according to claim 1, characterized by containing Sn: 1.0% or less. Furthermore, in mass%,
W: 0.5% or less,
Ta: 0.5% or less,
Co: 0.5% or less,
Al: 0.05% or less,
Ca: 0.05% or less,
V: 0.5% or less,
B: 0.01% or less,
Bi: 0.5% or less,
Mg: 0.1% or less,
Zr: 0.5% or less,
REM: 0.1% or less,
Ga: 0.05% or less,
Sr: 0.05% or less,
Se: 0.05% or less,
Ru: 0.05% or less,
Rh: 0.05% or less,
Pd: 0.05% or less,
Ag: 0.05% or less,
Cd: 0.05% or less,
In: 0.05% or less,
Sb: 0.05% or less,
Te: 0.05% or less,
Hf: 0.05% or less,
Re: 0.05% or less,
Os: 0.05% or less,
The ferritic stainless steel wire excellent in formability and pitting corrosion resistance according to claim 1 or 2, characterized by containing Ir: 0.05% or less.   The ferritic stainless steel wire excellent in formability and pitting corrosion resistance according to any one of claims 1 to 3, which is used for a muffler hanger.   A step of hot rolling the slab having the component composition according to any one of claims 1 to 3, a step of performing a shot blast treatment after the hot rolling, and a temperature of 900 ° C or higher and 1100 ° C or lower after the shot blast treatment. The ferritic stainless steel excellent in formability and pitting corrosion resistance according to any one of claims 1 to 4, comprising a step of performing a heat treatment at a temperature, and a step of pickling after the heat treatment. Wire manufacturing method.