KR101612474B1 - Ferritic stainless-steel wire with excellent cold forgeability and machinability - Google Patents

Abstract

An object of the present invention is to obtain a ferritic stainless steel wire having both cold forging, machinability and corrosion resistance.
With respect to corrosion resistance, Cr-based ferritic stainless steel has corrosion resistance, but in the welded portion, since Cr carbonitride is formed along the grain boundaries, a Cr-depleted layer is partially formed, and corrosion resistance is deteriorated. Therefore, Ti and Nb are added and carbonitride of Ti and Nb is precipitated preferentially, thereby restricting precipitation of Cr carbonitride and suppressing formation of a Cr-depleted layer.
It is preferable that a large number of inclusions are present in the cutting property, but in the cold forging property, it is preferable to have fewer inclusions. Accordingly, the present inventors have found that a ferrite-based material having both cold-cutting property, machinability, and corrosion resistance can be obtained by securing toughness by securing cutting property by relatively precipitating inclusions in the surface layer of the steel wire, securing toughness by relatively reducing inclusions at the center of the steel wire, A stainless steel wire could be obtained.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a ferritic stainless steel wire having excellent cold-

The present invention relates to a ferritic stainless steel wire having both cold forging, machinability and corrosion resistance.

In addition to strength characteristics and corrosion resistance, parts of electronic devices are required to have a high degree of flatness and dimensional precision and high workability. Therefore, if the cutting is performed only by cutting, the productivity is low and the cost is high as compared with the cold forging. On the other hand, cold forging has high productivity, but the surface roughness standard required for products is strict, and it is difficult to finish the surface by cold forging only. Therefore, the method of manufacturing the steel sheet as close to the finish shape as possible by cold forging and finally finishing the surface by cutting is the lowest cost by satisfying the quality, improving the productivity, and processing the product from the steel wire.

However, the conventional stainless steel wire is difficult to cut. Therefore, in order to improve machinability, an alloy element such as Pb or Bi, which is effective for improving machinability, is added to a ferritic stainless steel having a relatively high thermal conductivity, and inclusions and precipitates having a low melting point are dispersed. However, such a means lowers the amount of processing strain and tends to cause cracks. In addition, when the inclusions on the surface become larger or larger in number, rust tends to be generated from these as starting points, making it difficult to secure corrosion resistance. That is, it is very difficult to improve cold forging, machinability and corrosion resistance.

In Patent Document 1, S: 0.10% or less, Sb: 0.03 to 0.30%, Te: 0.01 to 0.10%, Pb: 0.03 to 0.30%, Bi: 0.03 to 0.30 %, And a ferrite-based cutting stainless steel excellent in toughness, cold workability and surface properties after machining at a Charpy transition temperature of 50 占 폚 or less and an area ratio occupied by crystal grains having a circle equivalent diameter of 200 占 퐉 or less of 60% . However, Patent Document 1 does not disclose the composition or distribution of inclusions.

Patent Document 2 discloses a ferritic stainless steel which comprises, as mass%, C: ≦ 0.15%, Si: ≦ 1.00%, Mn: ≦ 1.50%, Cr: 12 to 30%, S: ≧ 0.010% And 2.0 to 6.0% of Al, and the sintering property is improved without deteriorating toughness and ductility by setting the width of the sulfide to 3 m or less. However, the limitation of the size of the inclusions is disclosed, but the composition and distribution of the inclusions are not disclosed.

Patent Document 3, in mass%, Cr: 10 to in the ferritic stainless steel containing 20%, a circle equivalent diameter of more than 1㎛, TiO ₂ in weight percent: 5 to 30%, CaO: 5 to 30% , Al ₂ O _3: 24.1 to 49.7%, SiO _2: 6.2 to 26.8% _{containing, Al 2 O 3 -SiO 2 -CaO} -TiO 2 series of oxide-based inclusions of any cross section 1mm × 30 or more per 1mm A ferritic stainless steel excellent in machinability is disclosed. The composition and ratio of the inclusions are disclosed, but the distribution of the inclusions is not disclosed.

Patent Document 4 discloses a rotor hub that improves productivity after sintering to improve productivity and prevents contamination due to dropping of inclusions. The ferrite based free cutting stainless steel used in this rotor hub is composed of P: 0.05 to 0.15%, S: 0.10 to 0.30%, Mn: 0.15 to 0.30%, and Cu: 0.40 to 1.00% The size, the distribution density (the size in the thickness direction exceeds 50 탆, but the distribution density is less than 3 pieces / mm 2) of the A-type inclusions (inclusions having the shape elongated by rolling) appearing on the cutting plane along the axis A stipulated rotor hub is disclosed. However, as to the inclusion state of the bar material before the plastic working, it is described that the A-system inclusions having a length of 100 占 퐉 or more are distributed more than 1 / mm < 2 > or more on the cutting face. The inclusion distribution in the bar material is not disclosed.

Patent Document 1: JP-A-2001-200345 Patent Document 2: JP-A No. 2006-131972 Patent Document 3: Japanese Patent Publication No. 4350323 Patent Document 4: JP-A-2009-106085

Non-patent reference: Shin-Nittsu Gibo No. 376, pages 57 to 62

Disclosed is a ferritic stainless steel wire having cold forging, machinability and corrosion resistance at the same time.

The present inventors studied the characteristics of ferritic stainless steels in addition to the alloy components necessary for exhibiting cold forging, machinability and corrosion resistance, and examined the size of inclusions and the distribution state in the steel wires, Based ferritic stainless steel wire which is very suitable as an electromagnetic base material such as a hub.

With respect to corrosion resistance, corrosion resistance as a Cr-based ferritic stainless steel is provided, but in the welded portion, Cr carbonitride is formed along grain boundaries, and corrosion resistance is deteriorated because a Cr-depleted layer is partially formed. Therefore, it has been found that Ti and Nb are added and carbonitride of Ti and Nb is precipitated preferentially so that precipitation of Cr carbonitride can be restricted and formation of a Cr-depleted layer can be suppressed.

The machinability and cold hardening are contradictory in terms of inclusions (also referred to as non-metallic inclusions). That is, it is preferable that a large number of inclusions exist in the cutting property, but in the cold forging property, the inclusions are preferably small. Accordingly, the present inventors have found that a relatively large amount of inclusions are precipitated in the surface layer of the steel wire to ensure machinability, and in the central portion of the steel wire, relatively few inclusions are secured to secure toughness and to secure cold-rolled steel.

The present invention has been made based on the above knowledge, and its gist is as follows.

[1]

C: not less than 0.001% and not more than 0.02%

Si: 0.1% or more and 1.0% or less,

Mn: not more than 1.0% (not including 0%),

Cr: not less than 10.5% and not more than 30.0%

N: not less than 0.001% and not more than 0.025%

And, further,

Nb: not less than 0.20% and not more than 0.80%

Ti: 0.10% or more and 0.50% or less

, One or two kinds of

Cu: not less than 0.05% and not more than 1.0%

Ni: not less than 0.05% and not more than 1.0%

Mo: not less than 0.02% and not more than 2.5%

Or a mixture of two or more thereof,

(Nb + Ti) / (C + N)? 10

And the remainder being Fe and inevitable impurities,

As an unavoidable impurity

P: 0.04% or less,

S: 0.03% or less,

Also

In a cross section passing through the center of the steel wire and parallel to the axial direction of the steel wire,

There is no inclusions having an area of more than 100 μm ² ,

0.5㎛ area of more than ² inclusions area average value is more than ² 1㎛ 5㎛ ² or less,

On the other hand, when the portion extending from the surface of the steel wire to the center of the steel wire to 1/6 of the outer diameter of the steel wire is defined as the center portion of the steel wire beyond the steel wire surface layer portion,

The density of inclusions having an area of 0.5 mu m < ² > or more in the surface layer portion of the steel wire is 3000 to 7000 pieces /

And the central portion of the steel wire is smaller than that of the steel wire surface layer portion with respect to the density of the inclusions having an area of 0.5 탆 ² or more.

[2] The ferritic stainless steel wire according to [1], wherein the density of the inclusions having an area of 0.5 탆 ² or more at the central portion of the steel wire is 3,000 pieces / mm ² or less.

[3] The ferritic stainless steel wire according to [1] or [2], wherein the difference in density of the inclusions having an area of 0.5 μm ² or more in the center portion of the steel wire and the surface layer portion of the steel wire is 1000 pieces / mm ² or more.

[4] In addition, in mass%

Al: 0.05% or less,

Ca: not more than 0.05%

V: 0.1% or less,

B: 0.01% or less,

Bi: not more than 0.5%

Mg: not more than 0.1%

Zr: 0.5% or less,

REM: Not more than 0.1%

The ferritic stainless steel wire according to any one of [1] to [3], wherein the ferritic stainless steel wire contains one or two or more of the following.

According to the present invention, a ferritic stainless steel wire having cold forging, machinability and corrosion resistance can be obtained.

1 is a view showing a section of a steel wire for irradiating an inclusion;
Fig. 2 is a view showing a sample simulating a hub shape. Fig.

Hereinafter, the present invention will be described in detail. First, the reason why the composition of the stainless steel wire is limited to the above range in the present invention will be described. In the present specification, unless otherwise specified,% refers to% by mass.

C: not less than 0.001% and not more than 0.02%

C improves the strength of the ferrite structure of the matrix and is also an element of austenite phase and carbide. The austenite phase forms a martensite structure after welding to improve the strength, and also contributes to the improvement of the strength of the fine carbide. Therefore, the austenite phase is contained in an amount of 0.001% or more. In order to ensure this effect, it is preferably 0.003% or more, and more preferably 0.004% or more. However, when the Cr content exceeds 0.02%, a Cr-depleted layer is formed around the Cr carbide due to precipitation of Cr carbide in the grain boundaries depending on the cooling rate after hot rolling. In the corrosive environment, so-called intergranular corrosion occurs because the Cr depletion layer along the grain boundary is dissolved. Therefore, the C content was limited to 0.02% or less. Further, in order to prevent grain boundary corrosion, the C content is preferably set to a range of 0.015% or less. In addition, it is preferably 0.010% or less.

Si: 0.1% or more and 1.0% or less

As a deoxidizing agent, Si is an element useful for high-temperature oxidation because it forms an oxide film of SiO ₂ and suppresses the progress of oxidation against the temperature rise of the surface during cutting. However, when the content exceeds 1.0%, it becomes hard and deteriorates the mechanical properties. Therefore, the amount of Si was 1.0% or less. In order to ensure this effect, it is preferably 0.9% or less, and more preferably 0.8% or less.

If the content is less than 0.1%, a deoxidizing effect can not be obtained. In order to ensure this effect, it is preferably 0.2% or more, more preferably 0.25% or more.

Mn: not more than 1.0% (not including 0%)

As in the case of Mn, the strength of the ferrite structure of the matrix is increased and the austenite phase-forming element is present. However, if the content of austenite Mn exceeds 1.0%, the amount of inclusions remaining in the steel increases to deteriorate the corrosion resistance. Therefore, the Mn content was set to be 1.0% or less. In order to ensure this effect, it is preferably 0.9% or less, and more preferably 0.8% or less. Mn may not necessarily be contained, but if it is less than 0.1%, the strength of the steel wire becomes low. Therefore, it is preferable that the Mn content is 0.1% or more. In addition, preferably 0.15% or more.

P: not more than 0.04%

P is an element that is inevitably incorporated from a raw material and deteriorates mechanical properties such as toughness and is also an element harmful to corrosion resistance. Therefore, P should be extremely reduced. Particularly, when the P content exceeds 0.04%, the adverse effect becomes remarkable, so that the P content is limited to 0.04% or less.

S: not more than 0.03%

S, like P, is an element that is inevitably incorporated from the raw material and combines with Mn to form MnS, which is a starting point of initial rust generation. S is also a harmful element which is segregated at crystal grain boundaries to promote grain boundary embrittlement. Therefore, S should be extremely reduced. In particular, when the S content exceeds 0.03%, the adverse effect becomes remarkable, so the S content is limited to 0.03% or less.

Cr: not less than 10.5% and not more than 30.0%

Cr is an important element as a corrosion-resistant expression component in the present invention. The steel wire to be used in the present invention requires at least 10.5% as Cr amount necessary for corrosion resistance. This is because when it is less than 10.5%, a strong passivation film or high temperature oxidation film is hardly produced. In order to ensure this effect, it is preferable to set it to 12.0% or more, and more preferably to 15.0% or more. On the other hand, if the Cr concentration exceeds 30.0%, the corrosion resistance is improved but the toughness is lowered or the cost is increased. Therefore, the Cr concentration was 30.0% or less. In order to ensure this effect, it is preferably 21.0% or less, more preferably 20.0% or less.

N: not less than 0.001% and not more than 0.025%

N is contained in an amount of 0.001% or more in order to increase the strength of the ferrite structure of the matrix and also to increase the strength of the austenite phase and the nitride and the element to be formed. In order to ensure this effect, it is preferably 0.005% or more, and more preferably 0.008% or more. On the other hand, if it is contained in excess, the upper limit is set to 0.025% because the corrosion resistance and the cold hardening are deteriorated. Preferably 0.020% or less, and more preferably 0.018% or less.

Nb: 0.20% or more and 0.80% or less, or Ti: 0.10% or more and 0.50% or less

Nb and Ti are both elements forming carbide or nitride and inhibit the formation of Cr carbide. As a result, generation of Cr-depleted layer is suppressed, and intergranular corrosion can be prevented. Therefore, at least one of Nb and Ti is contained.

In order to obtain the corrosion resistance improving effect with Nb, it is necessary to contain 0.20% or more. In order to ensure this effect, it is preferable to set 0.25% or more, more preferably 0.30% or more.

However, when the content exceeds 0.80%. Which results in deterioration of cold-startability and high-temperature oxidation resistance. In order to ensure this effect, it is preferable to set it to 0.75% or less, and more preferably to 0.70% or less.

In order for Ti to exhibit the above effect, it is preferable to add it by 0.10% or more. In order to ensure this effect, it is preferably 0.15% or more, more preferably 0.20% or more. On the other hand, if the content exceeds 0.50%, the cold-rolling and the high-temperature oxidation resistance deteriorate. Particularly, when Ti is contained in a large amount, the oxidation film is not densified and high-temperature oxidation tends to proceed easily. Therefore, it was 0.50% or less. In order to ensure this effect, it is preferably 0.45% or less, and more preferably 0.40% or less.

In addition, by containing both Ti and Nb, the effect of increasing the denseness of the oxide film is exhibited and the corrosion resistance is improved.

(Nb + Ti) / (C + N)? 10

It is necessary that the above-mentioned Nb and Ti are contained so that (Nb + Ti) / (C + N) > = 10. In the formula, Nb, Ti, C and N mean the content (mass%) of each element.

In the ferritic stainless steel, Cr carbonitride is formed along the grain boundaries at the time of welding, and there is a Cr-depleted layer around the grain boundary, which is deteriorated in corrosion resistance. It is difficult to suppress the formation of the Cr-depleted layer even if quenched. Therefore, Ti and Nb are added and Ti and Nb carbonitrides are precipitated preferentially, thereby suppressing Cr carbonitride deposition and suppressing formation of a Cr-depleted layer. As a test method for detecting the intergranular corrosion susceptibility of welds, a sulfuric acid-copper sulfate corrosion test was carried out in which the sulfuric acid concentration of JIS G 0575 was changed. The test conditions were continuously immersed for 16 hours (h) in a boiling solution (sulfuric acid concentration: 0.5%, copper sulfate: 5%) while the test piece was in contact with the billet. After the test, the cross section of the test piece was cut out, and the grain boundary corrosion state of the weld metal part was observed with an optical microscope. As a result of investigation of the presence or absence of intergranular corrosion in the sulfuric acid-copper sulfate corrosion test and the relationship between the precipitated metal components,

 (Nb + Ti) / (C + N)? 10

The occurrence of intergranular corrosion is suppressed. In order to ensure this effect, (Nb + Ti) / (C + N)? 15 is preferable, and (Nb + Ti) / (C + N)? 20 is more preferable.

The inclusion of Cu, Ni, and Mo improves corrosion resistance and strength, so that at least one of them is contained.

Cu: not less than 0.05% and not more than 1.0%

Cu is a useful element for improving corrosion resistance and strength. In order to obtain this effect, it is preferable to contain 0.05% or more. In order to ensure this effect, it is preferable to set it to not less than 0.1%, more preferably to not less than 0.2%. On the other hand, when the content exceeds 1.0%, not only the toughness deteriorates but also the cold hardening is deteriorated. Therefore, it was 1.0% or less. In order to ensure this effect, the content is preferably 0.9% or less, more preferably 0.8% or less.

Ni: not less than 0.05% and not more than 1.0%

Ni is an effective element for improving corrosion resistance and strength as well as Cu. In order to obtain this effect, it is preferable to contain 0.05% or more. In order to ensure this effect, it is preferable to set it to 0.1% or more, more preferably 0.2% or more. On the other hand, when the content exceeds 1.0%, the cold-rolled steel composition deteriorates. Therefore, it was 1.0% or less. In order to ensure this effect, the content is preferably 0.9% or less, more preferably 0.8% or less.

Mo: not less than 0.02% and not more than 2.5%

Mo improves corrosion resistance and high-temperature strength, and is a ferrite-forming element, and the effect can be obtained by containing 0.02% or more. In order to ensure this effect, it is preferable to set it to not less than 0.1%, more preferably to not less than 0.3%. On the other hand, if it is more than 2.5%, the α-phase tends to precipitate and becomes brittle and deteriorates the cold-phase composition. In order to ensure this effect, the content is preferably 2.3% or less, more preferably 2.0% or less.

In the present invention, at least one or more species selected from Al, Ca, V, B, Bi, Mg, Zr and REM (elements of Sc, Y and atomic numbers 57 to 71) May optionally be added.

Al: not more than 0.05%

Al is an element useful as a deoxidizing agent, and may be added in an amount of 0.001% or more, if necessary. However, if the content exceeds 0.05%, the toughness deteriorates. Therefore, it was made 0.05% or less.

Ca: not more than 0.05%

Ca is a useful element for improving hot workability, and may be added in an amount of 0.001% or more, if necessary. On the other hand, when the content exceeds 0.05%, the toughness deteriorates. Therefore, it was made 0.05% or less.

V: not more than 0.1%

V is a useful element for improving the corrosion resistance, and may be added in an amount of 0.03% or more if necessary. On the other hand, when the content exceeds 0.1%, the toughness deteriorates. Therefore, it was set to 0.1% or less.

B: 0.01% or less

B is a useful element for improving hot workability, and may be added in an amount of 0.0003% or more, if necessary. On the other hand, when the content exceeds 0.01%, the toughness deteriorates. Therefore, it is 0.01% or less.

Bi: not more than 0.5%

Bi is a useful element for improving the corrosion resistance and cutting workability, and may be added in an amount of 0.005% or more if necessary. On the other hand, if the content exceeds 0.5%, the toughness deteriorates. Therefore, it was 0.5% or less.

Mg: not more than 0.1%

Mg is a useful element for improving corrosion resistance, and may be added in an amount of 0.0002% or more, if necessary. On the other hand, when the content exceeds 0.1%, the toughness deteriorates. Therefore, it was set to 0.1% or less.

Zr: not more than 0.5%

Zr is a useful element for improving the corrosion resistance and hot workability, and may be added in an amount of 0.03% or more, if necessary. On the other hand, if the content exceeds 0.5%, the effect becomes saturated. Therefore, it was 0.5% or less.

REM (elements of Sc, Y and atomic numbers 57 to 71): not more than 0.1%

REM is a useful element for improving hot workability, and may be added in an amount of 0.03% or more as needed. On the other hand, when the content exceeds 0.1%, the toughness deteriorates. Therefore, it was set to 0.1% or less.

In the present invention, reasons for defining inclusions in the steel wire will be explained.

As described above, the inventors of the present invention have studied in detail the size and density of inclusions in order to find a condition that both cold forging and free cutting properties are compatible.

The inclusions were measured as follows. That is, it was measured by microscopic observation and image processing according to the method of Annex 1 of JIS G 0555 (2003). As shown in Fig. 1, the surface to be inspected (the surface to be inspected) 2 passes from the center of the steel wire to the end of the test piece 1 (the surface layer of the steel wire) at the cut surface parallel to the steel wire axis direction Respectively. Further, in the surface 2 to be tested, the length from the surface of the steel wire to the center of the steel wire is 1/6 of the outer diameter of the steel wire (the outer diameter of the steel wire is t and the length corresponding to 1/6 of the steel wire diameter is expressed as 1/6 t). Hereinafter the same) is referred to as a steel wire surface layer 3. A region extending from the surface of the steel wire to the center of the steel wire in a length exceeding 1/6 of the outer diameter of the steel wire and half the length of the steel wire outer diameter (i.e., the center of the steel wire) is referred to as a steel wire center portion 4. The observation region in the surface 2 to be inspected was 300 mm 2 at the surface layer portion of the steel wire and 600 mm 2 at the center portion of the steel wire. Then, a measurement field of 125 mu m in length x 150 mu m in width was extracted from each observation area by 200-field and 400-field, and the area and density of the inclusions were measured. The magnification of the microscope was set at 400 times, and in the case where the entirety of the inclusions could not be observed on the basis of the fact that a part of the inclusions protruded out of the visual field at the end of each measurement field, In addition, the inclusion area of less than 0.5 mu m < ² > in each measurement field is not subject to measurement because the effect on cold-drawing and free-cutting is very small. In this measurement method, the relationship between the size and density of the inclusions, the cold hardening, and the free cutting performance was investigated for various steel wires.

Incidentally, the method of inclusion is not particularly limited to the above-mentioned method. The size of the observation area and the measurement visual field may be appropriately selected.

As a result, in the bigeom surface, when the largest inclusion in excess (e.g. greater than ² inclusions 100㎛ inclusions in the area) is present, it was found that a crack occurs in the cold forging. In this case, the inclusion area refers to the area of the inclusions measured in the measurement visual field. Therefore, the maximum value of the area of the inclusions is set to 100 μm ² or less. In addition, when the average value of the inclusion area is less than 1 占 퐉 ^{2, the} cutting workability is deteriorated. Therefore, the average value of the inclusion area is 1 占 퐉 ² or more. If the average value of the inclusion area is larger than 5 占 퐉 ² , cracks will be generated in the cold forging, so that the average value of the inclusion area is 5 占 퐉 ² or less.

Further, it has been found that the inclusions in the surface layer of the steel wire are large, and as the density is large, the cutting chips are small and easy to separate, so that cutting workability is improved. On the other hand, in the cold forging, it was found that the smaller the inclusions and the smaller the density, the more advantageous the work cracks starting from the inclusions are less likely to occur. It was also found that the smaller the inclusions and the smaller the density, the less the starting points of rust generation and the higher the corrosion resistance. For this reason, it has been found that it is important not only to control the maximum value or the average value of the inclusion area of the inclusions, but also to control the density of inclusions in each of the steel wire surface portion and the steel wire center portion.

First, the control of the inclusions in the surface layer of the steel wire contributing to the improvement of cutting workability will be described.

When the density of inclusions having an inclusion area of 0.5 mu m ² or more in the surface layer portion of the steel wire is less than 3000 pieces / mm < ² >, the cutting workability deteriorates, and when the number of inclusions exceeds 7000 pieces / mm & Therefore, the density of the inclusions may be 3,000 / mm 2 or more and 7,000 / mm 2 or less.

The inclusion of the surface layer of the steel wire is subject to the following reasons. That is, the lower the degree of cutting that requires cutting after cold forging, the lower the cost. However, if the cutting property of the steel wire surface portion is good, the machining cost can be suppressed to be low and the flatness and surface roughness of the product can be ensured . The outer diameter of the steel wire is not particularly limited, but is preferably from 1 mm to 20 mm.

From the viewpoint of corrosion resistance, it has also been found that the generation of rust is suppressed by the provision of alloying elements of the mother earth in the above composition range.

The reason why the density of the inclusions in the central portion of the steel wire is defined to be smaller than the density of the steel wire surface layer portion will be described. In the central portion of the steel wire, it is more important to ensure cold-cutting and corrosion resistance than machinability. For this purpose, it is effective to reduce the density of the inclusions. Therefore, the density of the inclusions from the surface layer portion of the steel wire may be reduced. The difference in density of inclusions between the surface layer portion of the steel wire and the center portion of the steel wire may be 1000 pieces / mm2 or more.

It is preferable that the density of inclusions having an inclusion area of 0.5 mu m < ^{2 >} or more is 1000 pieces / mm < ² > When the number is 3,000 / mm 2 or less, it is possible to sufficiently secure the cold forging and the corrosion resistance. If the number is less than 1000 pieces / mm 2, labor and time are required for the refining process and the casting process, and the productivity is lowered and the manufacturing cost is greatly increased.

A method of manufacturing the stainless steel wire of the present invention will be described.

The steel wire is manufactured by sequentially machining from the billet as the material, and then the part is manufactured from the steel wire. Typical manufacturing steps are as follows.

[1] Billets - [2] Rolling wire - [3] Annealing - [4] Freshness - [5] Annealing - [6] Skin pass drawing - [7] Cold forging - [8] Cutting - [9] use.

The billet is also manufactured by a process of melting an alloy raw material-b refining-c continuous casting.

The size and density of the inclusions described above can be controlled by the process conditions of b refining-c continuous casting. The relationship between the various conditions of smelting and casting and the inclusion properties is illustrated. For example, large inclusions exceeding a cross-sectional area of 100 mu m < ² > can float in the refining process or casting process and can be separated from molten steel as slag. Therefore, if the floating time is prolonged, large inclusions are liable to float. In order to increase the lifting time in the casting process, the size (cross-sectional area) of the largest inclusion can be reduced as the drawing speed of the billet at the time of continuous casting is slowed down. In addition, the average inclusion area of the inclusions having an inclusion area of 0.5 mu m < ² > Therefore, by appropriately setting the drawing speed of the billet, there can be no inclusions having an inclusion area of more than 100 탆 ² , and an average inclusion area of inclusions having an inclusive area of 0.5 탆 ² or more can be made 5 탆 ² or less. However, if the drawing speed is too slow, the average inclusion area of the inclusions having an inclusion area of 0.5 탆 ² or more becomes less than 1 탆 ² , so that the drawing speed is not lowered too much. Specifically, in the case of the continuous casting apparatus used in the later-described embodiment, the drawing speed of the billet may be set to 0.1 to 1.0 m / min. If the drawing speed of the billet is less than 0.1 m / min, the floating time becomes excessively long, so only minute inclusions remain in the steel, and the required inclusion area can not be obtained. Further, since it is excessively cleaned, the inclusion density of the surface layer portion necessary for the free-cutting property can not be obtained. On the other hand, in the case of exceeding 1.0 m / min, since the inclusion floating time is too short, large inclusions remain and the inclusion density becomes too large because the purification does not proceed, resulting in poor cold forging.

It is also possible to control the intracellular inclusions by electromagnetic stirring in the mold. When the electromagnetic stirring in the mold is performed, a flow occurs in the molten steel, and the inclusions repeatedly collide with each other. Thereby, some inclusions aggregate and become large inclusions which are liable to float. By this action, it is possible to purify the steel by appropriately setting the conditions of the electron stirring in the mold. Electron agitation also affects the inclusion density distribution in the surface layer portion and the center portion. That is, such large inclusions are liable to float, and the velocity difference in the same inclusions is remarkable. That is, in the case of performing electromagnetic stirring, the molten steel flow velocity is fast on the outer peripheral side of the billet, and the molten steel flow velocity is slowed in the central portion. When the inclusions are large, the large inclusions move toward the center portion because the velocity of the outer periphery of the inclusions is high and the velocity of the large inclusions is low at the center portion side. In addition, the large inclusions move toward the center portion and float, so that the inclusions near the center portion float together while being mixed with the large inclusions. As a result, the density of the inclusions becomes smaller toward the center portion than the surface layer portion. Therefore, by adjusting the flow rate of the molten steel by electromagnetic stirring, (1) the inclusion area 0.5㎛ wire density is present in ^two or more inclusions 3000 to 7000 in the surface layer portion dog / ㎟, (2) the density of the inclusions in the area of ^two or more inclusions 0.5㎛ The central portion of the steel wire can be made smaller than the steel wire surface layer portion. In the case of the continuous round billet continuous casting apparatus used in the following examples, the conditions of the electromagnetic stirring were set such that the flow rate was 10 to 20 cm / sec at 5/6 of the mold radius from the center of the mold. When the flow rate by the electron stirring is less than 10 cm / sec, the generation of large inclusions is small and the inclusion density becomes too large. On the other hand, when it exceeds 20 cm / sec, large inclusions are generated in a large amount, and the inclusion of the inclusions becomes excessive. As a result, only minute inclusions remain in the steel, and the necessary inclusion average area can not be obtained. Or excessively cleaned, so that the density of the surface layer inclusions required for the free-cutting property can not be obtained.

In addition, such setting conditions may be different depending on equipment used and various operating conditions, and the relationship between various conditions of the actual operation and the size and density of inclusions may be examined, and the inclusions may be appropriately distributed You can set it.

Example

b Alloy melting - b refining - c Billets are made by continuous casting process, and billet - [2] Rolled wire - [3] Annealed - [4] Fresh - [5] Annealed - [6] [7] A wire coil was prepared in the process of skin pass drawing.

The continuous casting was carried out by a round billet continuous casting machine having a diameter of 178 mm. Electromagnetic stirring was applied in a continuous casting mold. Electron agitation was performed to form a swirling flow in the molten steel in the mold. And the molten steel flow rate in the mold was measured. On the surface of the molten steel in the mold, the molten steel flow velocity at the position 5/6 of the mold radius from the center of the mold was measured by a molten steel flow velocity measuring sensor as described in Non-Patent Document 1.

[4] After the drawing, [5] annealing (900 to 950 ° × 1 to 3 h, water cooling, etc.) was performed. [7] The inclusion size and inclusion distribution after the skin pass drawing were measured (as shown in Table 1). After the skin pass drawing, the sample was cut out and subjected to cold forging to check for cracks. The cold forging conditions were such that a 16 mm steel wire was cut vertically in the longitudinal direction to cut the sample, and the sample was upset forged. The height reduction rate (%) was set at 70% below.

Height reduction rate (%) = ((H-h) / H) 100

However, the length of the sample before forging is H, and the length of the sample after forging is h

After the forging, the presence or absence of cracks on the surface was observed, and the absence of cracks at 70% of the height was evaluated as good (excellent), the number of cracks at the height reduction rate of 50% Respectively.

For evaluating the machinability of the steel wire, the cross section of the steel wire with a diameter of 16 mm was cut with a summation tool, and the surface condition after cutting at 100 m was evaluated as Ra. A peripheral speed of 150 m / min, a cutting depth of 0.04 mm, a feed of 0.03 mm / rev, and a water-soluble cutting oil as a cooling agent.

Ra: 0.5 占 퐉 or less and a small cutting column shortness was evaluated as? (Good)

Ra: Ra of more than 0.5 [mu]

Respectively.

After cold forging, the surface layer was subjected to cutting, and samples having a shape simulating a hub having a diameter of 20 mm and a thickness of 1.0 mm shown in Fig. 2 were produced.

Thereafter, a sample simulating the herb was subjected to a 35 ° C 5% salt spray test (168h) according to JIS Z 2371, and the occurrence of rust was observed. Rust occurrence No rust ◎ (excellent), little rust occurred (rating number 6 or more) ○ (good) and rust occurred (less than rating number 6).

Tables 1 and 2 show evaluation results and comparative examples of the chemical components of the stainless steel wire for the hub of the present invention, cold forging, cutting, and salt spray test. The " intrinsic corrosivity index " in the table means (Ti + Nb) / (C + N).

The electromagnetic stirring conditions in Nos. 1 to 29 listed in Table 1 are 10 to 20 cm / sec, and the billet drawing speed is 0.1 to 1.0 m / min.

Nos. 1 to 20 represent embodiments of the present invention. No deterioration of cold forging, machinability and corrosion resistance was detected.

On the other hand, in No. 21, the C content exceeded 0.02% and the corrosion resistance deteriorated. In No. 22, the Si content exceeded 1.0% and the cold-rolled steel composition deteriorated. In No. 23, the Mn content exceeded 1.0% and the corrosion resistance deteriorated.

In No. 24, the Cr amount was less than 10.5%, and the corrosion resistance was deteriorated. In No. 25, the Cr content exceeded 30.0% and the cold-rolled steel composition deteriorated. In No. 26, the Nb content was less than 0.20% and the corrosion resistance was deteriorated.

In No. 27, the Nb content exceeded 0.80% and the cold-rolled steel composition deteriorated. In No. 28, the N content exceeded 0.025% and the cold-rolled steel composition deteriorated

In No. 29, corrosion resistance was deteriorated because the intergranular corrosion index (Ti + Nb) / (C + N) was less than 10.

In the examples shown in Table 2, the molten steel flow rate (cm / sec) by electron stirring and the drawing rate (m / min) of the billet were varied. In the continuous casting apparatus to which the present embodiment is applied, if the drawing speed of the billet is less than 0.1 m / minute, the floating time becomes excessively long, and as a result, only minute inclusions remain in the steel and the necessary inclusion average area can not be obtained . On the other hand, if it exceeds 1.0 m / min, the inclusion lifting time is too short. Therefore, the maximum inclusion size exceeds the range of the present invention, or the average inclusion area exceeds the upper limit of the present invention. If the flow rate by the electron stirring is less than 10 cm / sec, the inclusion density at the center of the steel wire in the radial direction of the steel wire can not be reduced, and the distribution of inclusions is not appropriate. On the other hand, when it exceeds 20 cm / sec, large inclusions are generated in a large amount, and the inclusion of the inclusions becomes excessive. As a result, only minute inclusions remain in the steel, and the necessary inclusion average area can not be obtained. In addition, since the steel is excessively cleaned, the density of the surface layer inclusions required for the free-cutting property can not be obtained.

Nos. 30 to 34 of Table 2 represent examples of the present invention. The components are within the scope of the present invention and the molten steel flow rate by electron stirring and the drawing speed of the billet are set to favorable conditions. As a result, the size, density and distribution of the inclusions in the steel wire are within the scope of the present invention and all have cold- Was not detected.

In No. 35 of the comparative example, the billet pulling speed exceeded 1.0 m / min, the mean area of the inclusions exceeded 5 탆 ² , the inclusion density of the steel wire surface layer exceeded the upper limit, and the cold forging and corrosion resistance deteriorated. In No. 36, the billet drawing speed exceeded 1.0 m / min, the inclusion size exceeded 100 탆 ² , and the average inclusion area exceeded 5 탆 ² , resulting in deterioration of cold-forging and corrosion resistance. In No. 37, the billet drawing speed was less than 0.1 m / minute, the average area of inclusions was less than 1 탆 ² , and the inclusion density of the surface layer of the steel wire was less than the lower limit.

No.38 had an electron stirring speed exceeding 20 cm / sec, an inclusion average area of less than 1 占 퐉 ² and an inclusion density of the surface layer of the steel wire of less than 3000 pieces / mm < ² > In No. 39, the electron stirring speed was less than 10 cm / sec, and the inclusion density of the surface layer of the steel wire exceeded 7000 pieces / mm 2, resulting in deterioration of cold forging and corrosion resistance.

Industrial availability

The ferritic stainless steel according to the present invention is excellent in cold forging, machinability and corrosion resistance, and therefore can be used for electronic parts (hard disk, hub, etc.).

1 specimen
2 Inspection surface
3 strand surface layer
4 liner center

Claims (5)

In terms of% by mass,
C: not less than 0.001% and not more than 0.02%
Si: 0.1% or more and 1.0% or less,
Mn: not more than 1.0% (not including 0%),
Cr: not less than 10.5% and not more than 30.0%
N: not less than 0.001% and not more than 0.025%
And further,
Nb: not less than 0.20% and not more than 0.80%
Ti: 0.10% or more and 0.50% or less
And one or more of
Cu: not less than 0.05% and not more than 1.0%
Ni: not less than 0.05% and not more than 1.0%
Mo: not less than 0.02% and not more than 2.5%
Or a mixture of two or more thereof,
(Nb + Ti) / (C + N)? 10
And the balance being Fe and inevitable impurities,
As an unavoidable impurity
P: 0.04% or less,
S: 0.03% or less,
Also,
In a cross section passing through the center of the steel wire and parallel to the axial direction of the steel wire,
Without the inclusion of an area not greater than ² 100㎛ present, the average value of areas of the area over 0.5㎛ ² is at least ² inclusions 1㎛ 5㎛ ² or less,
Further, when the portion from the surface of the steel wire to the center of the steel wire to 1/6 of the outer diameter of the steel wire is defined as the center portion of the steel wire beyond the 1/6 of the steel wire outline,
In the surface layer portion of steel wire, an area 0.5㎛ a density of at least ² inclusions, 3000 to 7000 / ㎟, also
Wherein the density of the central portion of the steel wire is smaller than the density of the surface layer portion of the steel wire with respect to the density of the inclusions having an area of 0.5 탆 ² or more. The ferritic stainless steel wire according to claim 1, wherein the density of the inclusions having an area of 0.5 mu m < ^{2 >} or more at the central portion of the steel wire is 3,000 pieces / mm & The ferritic stainless steel wire according to Claim 1 or 2, wherein the difference in density of inclusions having an area of 0.5 mu m < ^{2 >} or more in the steel wire center portion and the steel wire surface layer portion is 1000 pieces / mm & 3. The method according to claim 1 or 2,
In addition, in mass%
Al: 0.05% or less,
Ca: not more than 0.05%
V: 0.1% or less,
B: 0.01% or less,
Bi: not more than 0.5%
Mg: not more than 0.1%
Zr: 0.5% or less,
REM: Not more than 0.1%
And a ferrite-based stainless steel wire. The method of claim 3,
In addition, in mass%
Al: 0.05% or less,
Ca: not more than 0.05%
V: 0.1% or less,
B: 0.01% or less,
Bi: not more than 0.5%
Mg: not more than 0.1%
Zr: 0.5% or less,
REM: Not more than 0.1%
And a ferrite-based stainless steel wire.

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