JP2017203190A - Ferritic free-cutting stainless steel and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic free cutting stainless steel that is excellent in machinability, corrosion resistance and surface fatigue strength.SOLUTION: Said ferritic stainless steel is obtained by containing, in mass%, 0.003≤C≤0.030, 0.05≤Si≤1.00, 1.00≤Mn≤2.00, P≤0.05, 0.25≤S≤0.40, Cu≤0.50, Ni≤0.50, 17.00≤Cr≤25.00, 0.50≤Mo≤3.00, 0.0020≤O≤0.0500, 0.012≤N≤0.050, 0.010≤V≤0.050, 0.001≤W≤0.020, 0.001≤Nb≤0.020 and 0.001≤Ti≤0.020 as well as satisfying the following equations (1) to (3): 0.006≤([V]+[W]+8[Nb]+15[Ti])/([Cr]+3[Mo])≤0.020 -- equation (1), 4.2≤Mn/S≤6.0 -- equation (2), and 10≤S/O≤30 -- equation (3), and further containing any 2 or more of 0.05≤Se≤0.30, 0.08≤Pb≤0.20, 0.08≤Bi≤0.20 and 0.005≤Te≤0.100, and having a composition of the balance Fe and inevitable impurities.SELECTED DRAWING: None

Description

  The present invention relates to a ferritic free-cutting stainless steel excellent in machinability, corrosion resistance and surface fatigue strength, and a method for producing the same.

While ferritic stainless steel has excellent corrosion resistance, it is difficult to cut because it is a high alloy steel. As a means of imparting free-cutting properties to ferritic stainless steel, for example, by adding S to steel, MnS is generated as inclusions in the steel, and machinability is enhanced by the effect of stress concentration on the inclusions during cutting. There is something.
Steel that produced MnS inclusions reduces the excellent corrosion resistance characteristic of stainless steel, but in the case of stainless steel, a part of Cr contained in the steel replaces part of Mn in sulfide. Therefore, both the machinability and the corrosion resistance are achieved by controlling the Cr concentration in the sulfide.

On the other hand, in applications such as bearings for precision equipment and ballpoint pen tips, erosion and erosion are caused by physical friction with fluids and different types of materials, especially in corrosive fluids such as water-soluble lubricants and water-soluble inks. There are problems such as corrosion, peeling due to friction, dropping off, and deformation, and there is a strong demand for improvement on these problems.
In the free-cutting stainless steel used for such applications, not only machinability and corrosion resistance but also excellent surface fatigue strength are required, but no ferritic free-cutting stainless steel satisfying these characteristics has been conventionally provided.

  The following Patent Document 1 discloses an invention relating to a free-cutting ferritic stainless steel that suppresses a decrease in corrosion resistance as much as possible and has improved strength, and the following Patent Document 2 has excellent machinability and corrosion resistance. An invention for ferritic free-cutting stainless steel is disclosed. However, these patent documents do not disclose composition control and form control of carbides and sulfide inclusions in the present invention, and are different from the present invention.

Japanese Patent Laid-Open No. 10-130794 JP-A-11-140597

  The present invention has been made for the purpose of providing a ferritic free-cutting stainless steel excellent in machinability, corrosion resistance and surface fatigue strength, and a method for producing the same, against the background as described above.

Thus, claim 1 relates to ferritic free-cutting stainless steel, 0.003 ≦ C ≦ 0.030, 0.05 ≦ Si ≦ 1.00, 1.00 ≦ Mn ≦ 2.00, P ≦ 0. .05, 0.25 ≦ S ≦ 0.40, Cu ≦ 0.50, Ni ≦ 0.50, 17.00 ≦ Cr ≦ 25.00, 0.50 ≦ Mo ≦ 3.00, 0.0020 ≦ O ≦ 0.0500, 0.012 ≦ N ≦ 0.050, 0.010 ≦ V ≦ 0.050, 0.001 ≦ W ≦ 0.020, 0.001 ≦ Nb ≦ 0.020, 0.001 ≦ Ti ≦ 0.020 and satisfy the following formulas (1) to (3),
0.006 ≦ ([V] + [W] +8 [Nb] +15 [Ti]) / ([Cr] +3 [Mo]) ≦ 0.020 Equation (1), 4.2 ≦ [Mn] /[S]≦6.0...Equation (2), 10 ≦ [S] / [O] ≦ 30 ... Equation (3), Equation (1), Equation (2), Equation (3) [] Indicates mass% of each element)
Furthermore, 0.05 ≦ Se ≦ 0.30, 0.08 ≦ Pb ≦ 0.20, 0.08 ≦ Bi ≦ 0.20, and 0.005 ≦ Te ≦ 0.100 are contained. , Remaining Fe and inevitable impurities.

  According to a second aspect of the present invention, in the first aspect, any one of 0.0005 ≦ B ≦ 0.0500, 0.0005 ≦ Ca ≦ 0.0100, 0.0005 ≦ Mg ≦ 0.0100 in mass%. It further contains a seed or two or more kinds.

  In claim 3, the composition of the sulfide inclusions in any one of claims 1 and 2 is mass%, Mn: 45 to 60%, Cr: 5% or more, Mo: 0.6% or more And the sulfide inclusions are distributed in an area ratio of 0.2 to 2.5%.

  Claim 4 relates to a method for producing a ferritic free-cutting stainless steel, and in the ferritic free-cutting stainless steel according to any one of claims 1 to 3, after hot rolling in the range of 900 to 1200 ° C. , 550 to 780 ° C., and an annealing treatment for 10 hours or less is performed.

  A fifth aspect of the present invention is characterized in that, in the fourth aspect, the shape of the sulfide inclusions after the annealing treatment is an equivalent circle diameter of 2 to 50 μm and an aspect ratio of 2 to 10. Here, the aspect ratio is the value of the major axis of sulfide inclusions / the minor axis of sulfide inclusions.

In the present invention as described above, the content of carbide forming elements is balanced so as to disperse and precipitate an appropriate amount of fine carbides mainly composed of V, W, Nb, and Ti instead of Cr and Mo. It is characterized by.
In ferritic stainless steel, it is effective to precipitate a carbide having a predetermined hardness in order to obtain a predetermined surface fatigue strength. However, since the main composition of the carbide of ferritic stainless steel is Cr and Mo, when the carbide is precipitated, a Cr and Mo deficient layer is formed in the vicinity thereof, which becomes a starting point of corrosion and deteriorates the corrosion resistance.
In the present invention, V, W, Nb, and Ti, which have a higher tendency to form carbides than Cr and Mo, are added, and each element of V, W, Nb, Ti and Cr, Mo is satisfied so as to satisfy the above formula (1). By balancing the content of V, W, Nb, Ti-based carbides are precipitated to ensure surface fatigue strength, while Cr and Mo are maintained in a solid solution state to prevent deterioration of corrosion resistance.

Furthermore, in the present invention, another feature is that the contents of each element of Mn, S, and O are balanced so as to satisfy the expressions (2) and (3).
In Formula (2), the ratio of the amount of Mn and the amount of S is prescribed | regulated. Since there are MnS excellent in machinability and CrS excellent in corrosion resistance in the sulfide inclusions, the ratio between the Mn amount and the S amount according to the formula (2) depends on the machinability and sulfide inclusions. Affects the corrosion resistance of objects.
On the other hand, in the formula (3), the ratio between the S amount and the O amount is defined. Since sulfide inclusions grow with oxides in steel as nuclei, the ratio between the amount of S and the amount of O affects the dispersibility and size of the sulfide inclusions.
In the present invention, the composition and form of the sulfide inclusions are controlled by balancing the content of each element of Mn, S, and O so as to satisfy the expressions (2) and (3), thereby improving the corrosion resistance. The machinability can be effectively enhanced while maintaining.

  In the ferritic free-cutting stainless steel based on the composition of any one of claims 1 and 2, the composition of sulfide inclusions is Mn: 45-60%, Cr: 5% or more, Mo: 0.00. Excellent corrosion resistance and machinability can be imparted by adjusting the sulfide inclusions to an area ratio of 0.2 to 2.5% by setting the content to 6% or more.

  The ferritic free-cutting stainless steel of the present invention contains any two or more of elements Se, Pb, Bi, and Te that are excellent in the effect of preventing deformation of sulfide inclusions during hot rolling. For this reason, it is possible to suppress the deformation of sulfide inclusions during hot rolling and effectively suppress the occurrence of characteristic anisotropy, that is, a characteristic difference between the rolling direction and the direction perpendicular thereto. it can.

In the ferritic free-cutting stainless steel of the present invention, after the hot rolling is performed in the range of 900 to 1200 ° C., the annealing treatment for 10 hours or less can be performed in the temperature range of 550 to 780 ° C. (Claim 4). .

  According to the alloy composition of the present invention, the shape of the sulfide inclusions after the annealing treatment can have an equivalent circle diameter of 2 to 50 μm and an aspect ratio of 2 to 10, and excellent free machinability. (Claim 5).

Next, the reasons for limiting each chemical component and the like in the present invention will be described below.
“About claim 1”
0.003 ≦ C ≦ 0.030
C is a typical solid solution strengthening element, but it is desirable that C be low because it has a great effect of reducing corrosion resistance and room temperature toughness. However, extremely reducing causes an increase in manufacturing cost, so that the refining technology is taken into consideration, and the content is made 0.003 to 0.030%.

0.05 ≦ Si ≦ 1.00
Si is a solid solution strengthening element and improves the strength. However, since addition exceeding 1.00% reduces hot workability, it was made 0.05 to 1.00%.

1.00 ≦ Mn ≦ 2.00
Mn forms a compound with S and Se and improves machinability. In order to obtain this effect, 1.00% or more is added. However, addition over 2.00% reduces corrosion resistance and promotes the formation of the austenite phase, so the upper limit was made 2.00%.

P ≦ 0.05
P is an impurity, segregates at the grain boundary, and increases the sensitivity to grain boundary corrosion. In addition, hot workability and toughness are reduced. For this reason, content is controlled to 0.05% or less.

0.25 ≦ S ≦ 0.40
S is a free-cutting element, and mainly forms (Mn, Cr) S in steel to improve chip cutting performance and extend tool life. Therefore, 0.25% or more is added. However, addition exceeding 0.40% remarkably reduces hot workability, so the upper limit is made 0.40%.

Cu ≦ 0.50
Cu contributes to the improvement of corrosion resistance, but is an austenite-generating element and makes the ferrite phase unstable, so its upper limit was made 0.50%.

Ni ≦ 0.50
Ni contributes to the improvement of corrosion resistance, but is an austenite-forming element and makes the ferrite phase unstable, so the upper limit was made 0.50%.

17.00 ≦ Cr ≦ 25.00
Cr is an essential element for ensuring corrosion resistance, and in order to have sufficient corrosion resistance, addition of 17.00% or more is necessary. However, if it exceeds 25.00%, the cost becomes high. Of 17.00% to 25.00%.

0.50 ≦ Mo ≦ 3.00
Mo is a solid solution strengthening element and improves strength. It also improves corrosion resistance. For this reason, 0.50% or more is added. However, since addition exceeding 3.00% reduces hot workability, the upper limit is made 3.00%.

0.0020 ≦ O ≦ 0.0500
O is an element involved in the formation of sulfides necessary for improving machinability. Specifically, O is dispersed and generated as an oxide that becomes a nucleus of sulfide formation such as MnS. MnS and the like sulfides are easily generated around the nucleus in the presence of the nucleus. Therefore, a certain amount of O is required to successfully form a sulfide such as MnS in a well dispersed state. Therefore, in the present invention, O is made 0.0020% or more.
However, if O becomes excessive, coarse oxides that are not effective for improving machinability are easily generated, so the upper limit is made 0.0500%.

0.012 ≦ N ≦ 0.050
N is a typical solid solution strengthening element, but it is desirable that N be low because it has a great effect of reducing corrosion resistance and room temperature toughness. However, extremely reducing causes an increase in manufacturing cost, so that the refining technology is taken into consideration and the content is made 0.012 to 0.050%.

0.010 ≦ V ≦ 0.050
0.001 ≦ W ≦ 0.020
0.001 ≦ Nb ≦ 0.020
0.001 ≦ Ti ≦ 0.020
V, W, Nb, and Ti form carbides to improve slidability. However, excessive addition causes an increase in cost, so the upper limit is made 0.050% for V and 0.020% for W, Nb, and Ti.

0.006 ≦ ([V] + [W] +8 [Nb] +15 [Ti]) / ([Cr] +3 [Mo]) ≦ 0.020 Equation (1)
V, W, Nb, Ti, Cr, and Mo are all carbide forming elements. By satisfying this formula (1), V, W, Nb, and Ti-based carbides can be efficiently precipitated. Particularly, the mass ratio of carbide forming elements, that is, the value of ([V] + [W] +8 [Nb] +15 [Ti]) / ([Cr] +3 [Mo]) is preferably 0.015 or more. Can ensure a sufficient surface fatigue strength.
On the other hand, if the mass ratio of the carbide-forming elements in the formula is less than 0.006, the hardness is insufficient and the surface fatigue strength decreases. On the other hand, if it exceeds 0.020, the amount of precipitation of V, W, Nb, Ti-based carbides becomes excessive, the hardness increases, and the machinability deteriorates.

4.2 ≦ [Mn] / [S] ≦ 6.0 Expression (2)
Sulfide inclusions in steel are mainly composed of MnS, but when Mn and Cr are contained together with S in steel, the inclusions contain CrS in addition to MnS.
However, since MnS has lower generation energy than CrS, MnS is preferentially generated. Here, MnS has a large machinability improving effect, but has low corrosion resistance. On the other hand, although CrS has a small machinability improvement effect compared to MnS, it has high corrosion resistance.
Formula (2) prescribes | regulates the ratio of Mn in (Mn, Cr) S, and coexistence with cutting property and corrosion resistance can be aimed at by satisfy | filling this formula (2).
In this case, if [Mn] / [S] is less than 4.2, the amount of CrS generated is greater than the appropriate balance, the inclusion hardness increases, and a sufficient machinability improvement effect cannot be obtained. .
Conversely, when [Mn] / [S] exceeds 6.0, CrS decreases and the corrosion resistance of the sulfide inclusions deteriorates significantly, which leads to deterioration of the corrosion resistance of the steel.
Therefore, in the present invention, [Mn] / [S] is set within the range of 4.2 to 6.0.

10 ≦ [S] / [O] ≦ 30 (3)
Equation (3) defines the distribution of oxides that are the core of (Mn, Cr) S. If [S] / [O] is within the range satisfying this equation (3), (Mn, Cr) S is distributed uniformly without coarse particles, so it has excellent cutting crushability and improved drill cutting performance. Can be made.
In addition, when [S] / [O] is less than 10, O is relatively increased and the oxide is coarsened, and hard oxides that are not effective for machinability are increased and the machinability is deteriorated.
On the other hand, if O exceeds 30 and O becomes relatively small, the number of oxides that become nuclei decreases, and (Mn, Cr) S that forms oxides as nuclei coarsens and the chips are sufficiently crushed. Therefore, the effect of improving machinability cannot be obtained. Therefore, in the present invention, [S] / [O] is set to 10-30.

0.05 ≦ Se ≦ 0.30
0.08 ≦ Pb ≦ 0.20
0.08 ≦ Bi ≦ 0.20
0.005 ≦ Te ≦ 0.100
These Se, Pb, Bi, and Te are free-cutting elements and improve machinability. These elements exist around sulfide inclusions, melt during hot rolling, and act like a lubricating oil between the sulfide inclusions and the substrate. As a result, it is possible to effectively prevent the sulfide-based inclusions from extending during rolling to increase the material anisotropy. Even if one of these elements is added, the effect is particularly great when two or more of these elements are added. If each element is added in excess of the upper limit, the hot workability is remarkably lowered. Therefore, Se is 0.30%, Pb and Bi are 0.20%, and Te is 0.20%. To do.

“Claim 2”
In the stainless steel of the present invention containing S in an amount of 0.25% or more, it is effective to selectively add B—Ca—Mg to improve hot workability. In particular,
0.0005 ≦ B ≦ 0.0500
0.0005 ≦ Ca ≦ 0.0100
0.0005 ≦ Mg ≦ 0.0100
Any one or two or more of them may be contained.
However, if any element is excessively contained, the hot workability is lowered. Therefore, B is 0.0500%, and Ca and Mg are 0.0100% as upper limits.

“Claim 3”
The composition of sulfide inclusions satisfies Mn: 45 to 60%, Cr: 5% or more, Mo: 0.6% or more, and sulfide inclusions in an area ratio of 0.2 to 2.5 % Distribution By making the composition and area ratio of the sulfide inclusions as described above, excellent corrosion resistance and machinability can be imparted to the steel.
When the amount of Mn in the sulfide inclusion is less than 45%, the amount of MnS decreases and the machinability deteriorates. When the Mn content exceeds 60%, the CrS content decreases and the sulfide inclusions themselves are easily corroded.
If the area ratio is less than 0.2%, the effect of improving machinability cannot be obtained. On the other hand, when the upper limit of 2.5% is exceeded, properties such as corrosion resistance and fatigue strength are deteriorated.

  According to the present invention as described above, it is possible to provide a ferritic free-cutting stainless steel excellent in machinability, corrosion resistance and surface fatigue strength, and a method for producing the same.

It is the figure which showed the influence of the mass ratio of S and O which has on a drill lifetime.

Next, embodiments of the present invention will be described in detail below.
50 kg of steel having a chemical composition (mass%) shown in Table 1 was melted using a high frequency induction furnace, and then cooled to prepare each ingot.
Next, each ingot was heated to 1100 ° C., and hot forging and hot rolling were performed to produce a Φ28 mm round bar. And this was subjected to a 4Hr annealing process between 750 and 800 ° C., and cutting and polishing were performed on each test piece shape.

<Drill life test>
In the drill life test, a Φ5 mm drill (SKH51) was used, and a maximum cutting speed (VL1000) capable of cutting (drilling) by 1000 mm was obtained without using lubricating oil, and evaluated according to the following criteria.
A: VL1000 is 70.0 m / min or more B: VL1000 is 60.0 m / min or more and less than 70.0 m / min C: VL1000 is less than 60.0 m / min

<Sulphide composition evaluation>
In the sulfide composition evaluation, EPMA was used to determine the average composition of Mn, Cr and Mo in the sulfide inclusions. Specifically, the characteristic X-ray intensity of a standard sample with a clear element concentration is compared with the characteristic X-ray intensity of the sulfide inclusions of each example / comparative example, quantitative analysis is performed, and evaluation is performed according to the following criteria did.
○: The composition of Mn, Cr and Mo contained in the sulfide inclusions satisfies all of the following:
The composition of Mn is 45-60 mass%
Cr composition of 5% by mass or more
Mo composition is 0.6 mass% or more x: Other than the above

<Sulfide morphology evaluation>
The sulfide morphology was observed with an optical microscope, and 10 fields of view were observed at a magnification of 100 ×, and the area ratio and aspect ratio of sulfide inclusions (long axis length / short axis length of sulfide inclusions) by image analysis. The equivalent circle diameter was determined and evaluated according to the following criteria.
○: The area ratio of sulfide inclusions is 10% or more, the aspect ratio is 2 to 10, and the equivalent circle diameter is 2 to 50 μm.

<Surface fatigue strength>
The surface fatigue strength was determined by a fatigue strength test using a roller pitting tester. In the test, the large roller was pressed against the test piece with a predetermined surface pressure under the conditions of large roller: SUJ2 (crowning 150R), rotation speed: 1500 rpm, lubricating oil: transmission oil, oil temperature: 90 ° C., slip ratio: −60% The 10 ⁷ times strength (limit strength at which pitting occurs on the surface of the test piece) was measured as surface fatigue strength. The surface fatigue strength of each example and comparative example was evaluated according to the following criteria using the ratio when the surface fatigue strength of SUS430F (Comparative Example 2) was 1, that is, the surface fatigue strength ratio.
A: Surface fatigue strength ratio is 1.5 or more B: Surface fatigue strength ratio is 1.3 or more and less than 1.5 C: Surface fatigue strength ratio is less than 1.3 These results are shown in Table 2.

Table 2 shows the following.
In Comparative Example 1, the V amount is less than the lower limit value of the present invention, and the expression (1) is below the lower limit value 0.006. For this reason, the surface fatigue strength ratio is low, the judgment of evaluation is “C”, and it is not suitable for use as a sliding member. Further, the amount of Mn is small, and the expression (2) is below the lower limit value 4.2. For this reason, the amount of Cr in the sulfide inclusions increases, and the evaluation of the sulfide composition is “x”. In this case, while the inclusion hardness increases, the amount of MnS produced decreases, and a sufficient machinability improvement effect is not obtained, and the evaluation of the drill life is “C”.

Comparative Example 2 is a ferritic free-cutting stainless steel SUS430F defined by JIS, and is a criterion for determining surface fatigue strength. In Comparative Example 2, the amount of V is small, and the formula (1) is below the lower limit of 0.006, which is not suitable for use as a sliding member. Further, since the amount of S, which is a free-cutting element, is below the lower limit of 0.25% of the present invention, a sufficient machinability improving effect is not obtained and the evaluation of the drill life is “C”.
Further, this Comparative Example 2 does not contain any of Se, Pb, Bi, and Te, the sulfide inclusions are elongated during rolling and the aspect ratio exceeds 10, and the evaluation of the sulfide form is “×”. Is. For this reason, we are anxious about the anisotropy arising in the characteristic of a material.

  Comparative Example 3 satisfies the requirements regarding the machinability such as S amount, Mn amount, and formulas (2) and (3), and the evaluation about the drill life is “B” compared with other comparative examples. It was good. However, the amount of V is small as in Comparative Examples 1 and 2, and Equation (1) is below the lower limit of 0.006. For this reason, evaluation of a surface fatigue strength ratio is bad with "C", and is not suitable for the use as a sliding member.

  In Comparative Example 4, the value of Expression (3) exceeds the upper limit 30. In such a case, when the amount of O is too small relative to the amount of S, the number of oxides serving as nuclei decreases, so that the sulfide generated from the oxides as nuclei coarsens and the equivalent circle diameter exceeds 50 μm. The evaluation of the sulfide form was “x”. For this reason, the chips are not easily crushed, and a sufficient machinability improvement effect cannot be obtained, and the evaluation of the drill life is “C”.

On the other hand, in Examples 1 to 16, each carbide forming element is added so as to satisfy the formula (1), the evaluation of the surface fatigue strength is “A” or “B”, and the conventional ferrite type A surface fatigue strength better than that of the machined stainless steel (Comparative Example 2) is obtained. In particular, in Examples 5, 8, 12, and 16 in which the value of the expression (1) is 0.015 or more, the evaluation of the surface fatigue strength is high as “A”.
Moreover, by satisfy | filling Formula (2) and Formula (3), the composition and form of the sulfide produced | generated in steel also satisfy | fill the criterion of evaluation, and coexistence of machinability and corrosion resistance is achieved.
FIG. 1 is a graph showing the relationship between the mass ratio [S] / [O] in equation (3) and the drill life. As shown in the figure, when [S] / [O] is 18 or less, the drill life tends to be maintained high. In Examples 3, 6 to 10, and 12 to 14, the evaluation of the drill life is “A”. And high.

  Although the ferritic free-cutting stainless steel of the present invention has been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

Claims (5)

0.003 ≦ C ≦ 0.030 in mass%
0.05 ≦ Si ≦ 1.00
1.00 ≦ Mn ≦ 2.00
P ≦ 0.05
0.25 ≦ S ≦ 0.40
Cu ≦ 0.50
Ni ≦ 0.50
17.00 ≦ Cr ≦ 25.00
0.50 ≦ Mo ≦ 3.00
0.0020 ≦ O ≦ 0.0500
0.012 ≦ N ≦ 0.050
0.010 ≦ V ≦ 0.050
0.001 ≦ W ≦ 0.020
0.001 ≦ Nb ≦ 0.020
0.001 ≦ Ti ≦ 0.020
And satisfying the following formulas (1) to (3),
0.006 ≦ ([V] + [W] +8 [Nb] +15 [Ti]) / ([Cr] +3 [Mo]) ≦ 0.020 Equation (1)
4.2 ≦ [Mn] / [S] ≦ 6.0 Expression (2)
10 ≦ [S] / [O] ≦ 30 (3)
(In formula (1), formula (2), and formula (3), [] indicates the mass% of each element)
Furthermore,
0.05 ≦ Se ≦ 0.30
0.08 ≦ Pb ≦ 0.20
0.08 ≦ Bi ≦ 0.20
0.005 ≦ Te ≦ 0.100
A ferritic free-cutting stainless steel containing any two or more of the above, and having the balance of Fe and inevitable impurities. % By mass
0.0005 ≦ B ≦ 0.0500
0.0005 ≦ Ca ≦ 0.0100
0.0005 ≦ Mg ≦ 0.0100
The ferritic free-cutting stainless steel according to claim 1, further comprising any one or more of the following. The composition of sulfide inclusions is mass%,
Mn: 45-60%, Cr: 5% or more, Mo: 0.6% or more are satisfied, and
The ferritic free-cutting stainless steel according to any one of claims 1 and 2, wherein the sulfide inclusions are distributed in an area ratio of 0.2 to 2.5%.   The hot rolling is performed in the range of 900 to 1200 ° C, and then annealing is performed for 10 hours or less in the temperature range of 550 to 780 ° C. A method for producing machined stainless steel.   5. The ferritic free-cutting stainless steel according to claim 4, wherein the shape of the sulfide inclusions after the annealing treatment is an equivalent circle diameter of 2 to 50 μm and an aspect ratio of 2 to 10. 5. Production method.

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