JP7668657B2 - Ferritic free-cutting stainless steel with excellent precision cutting properties - Google Patents

Description

The present invention relates to a ferritic stainless steel that has excellent precision cutting properties and suppresses the formation of built-up edges, for use as a material cut into small parts that require corrosion resistance.

Ferritic stainless steels are used for precision equipment parts that require durability due to their corrosion resistance and low thermal expansion coefficient. In particular, precision equipment parts that are cut from polished bars made from wire rods are often used for rotating bodies, and require particularly precise cutting workability. Specifically, they require excellent tool life during cutting work and an extremely smooth cutting surface with no traces of the built-up cutting edge. Built-up edges are deposits that form and grow on the tip of a cutting tool due to adhesion of the base material; they fall off the tip of the tool during cutting and are pressed onto the cutting surface, degrading the surface properties.

So far, there has been a proposal to add B to highly corrosion-resistant ferritic stainless steel and control the ratio of oxygen to B to precipitate oxide-based inclusions containing B, resulting in a ferritic free-cutting stainless steel with machinability comparable to that of Pb (Patent Document 1).

Furthermore, a ferritic free-cutting stainless steel with 0.1% or more of boron nitride particles dispersed therein as an alternative to cutting particles of Pb or MnS has been proposed (Patent Document 2). It has also been proposed to add B to an austenitic stainless steel to which a machinability-imparting material has been added, thereby dispersing h-BN (hexagonal boron nitride) particles uniformly throughout the steel to impart machinability (Patent Document 3).

Furthermore, a ferritic free-cutting stainless steel has been proposed in which B and N have been added to ferritic stainless steel containing S as a replacement for Pb, etc., and the size and number density of the intermetallic compounds containing B, N, and S have been controlled (Patent Document 4).

JP 2008-274361 A JP 2003-129191 A Patent No. 5142289 Patent No. 5957241

Patent documents 1 to 4 do not adequately suppress built-up cutting edge marks, and the problem of smooth cutting surface properties cannot be solved, which poses problems from the perspective of precise cutting surface properties.

The inventors have found that the known techniques or combinations described in the Background Art above are unable to achieve both excellent tool life and excellent smooth surface properties with reduced built-up edge marks in highly corrosion-resistant ferritic free-cutting stainless steels that have excellent machinability.

The problem to be solved by the present invention is to provide a highly corrosion-resistant ferritic free-cutting stainless steel that can provide excellent tool life during cutting and excellent flatness in the cutting surface properties, for use in precision ferritic stainless steel parts used in highly corrosive environments where precision is required.

In order to solve the above problems, the inventors conducted various studies on S-containing ferritic free-cutting stainless steels, and discovered that by adding B and N to the S-containing free-cutting element system and controlling the amount of dissolved N by finely precipitating BN through N solution softening treatment, it is possible to suppress built-up cutting edge marks during precision cutting and obtain surface properties with excellent smoothness during cutting.

The present invention was made based on the above findings, and its gist is as follows:

(1) A ferritic free-cutting stainless steel containing, by mass%, C: less than 0.10%, Si: 0.1-2.0%, Mn: 0.1-3.0%, S: 0.02-0.40%, P: 0.10% or less, Cr: 13.0-22.0%, B: 0.001-0.01%, N: 0.015-0.08%, Al: 0.008% or less, and O: 0.015% or less, with the balance being Fe and impurities, with a solute N content of 0.010-0.060%, and with 20 or more BN-based intermetallic compounds of 0.5 μm or less per 100 ^μm2 .

(2) The ferritic free-cutting stainless steel of (1) above, characterized in that it contains, in place of a portion of the Fe, one or more of the following elements by mass: Ni: 1.5% or less, Mo: 2.5% or less, Cu: 1.5% or less, Co: 1.5% or less, and W: 2.5% or less.

(3) The ferritic free-cutting stainless steel according to (1) or (2), characterized in that it contains, in place of a portion of the Fe, one or more of the following elements by mass: Bi: 0.20% or less, Sn: 0.30% or less, Sb: 0.30% or less, Ag: 0.30% or less, and Te: 0.10% or less.

(4) A ferritic free-cutting stainless steel according to any one of (1) to (3), characterized in that, in place of a portion of the Fe, it further contains, by mass%, one or more of V: 0.8% or less, Nb: 0.5% or less, Ti: 0.5% or less, and Ta: 0.5% or less.

(5) A ferritic free-cutting stainless steel according to any one of (1) to (4), characterized in that, in place of a portion of the Fe, it further contains, by mass%, one or more of Mg: 0.010% or less, Ca: 0.010% or less, Hf: 0.010% or less, and REM: 0.050% or less.

(6) A method for producing ferritic free-cutting stainless steel according to the present invention, comprising a heating step of heating a slab having any one of the chemical compositions described in (1) to (5) to 1150 to 1330°C, a processing step of subjecting the heated slab to hot rolling or hot working, a batch annealing step of holding the slab at 700 to 900°C for 60 to 300 minutes, and strand annealing, holding the slab in the temperature range of 700 to 900°C and cooling at a cooling rate of 1°C/s or more.

According to the present invention, by appropriately adjusting the components and applying heat treatment to control the metal structure and intermetallic compounds in ferritic free-cutting stainless steel, it is possible to obtain a good cutting tool life and excellent flat cutting surface properties with reduced built-up cutting edge marks, providing a material with excellent corrosion resistance that is suitable for precision parts.

Each of the requirements of the present invention will be explained below. In the following explanation, "%" means "mass %" unless otherwise specified.

<Essential composition of steel>
The highly corrosion-resistant ferritic free-cutting stainless steel of the present invention is intended for parts that generally require durability and a low coefficient of thermal expansion, and is based on highly corrosion-resistant ferritic stainless steel.

C can be contained in an amount of less than 0.10% to ensure the strength of the base material. However, adding more than 0.10% produces a large amount of Cr carbide, which not only deteriorates corrosion resistance but also deteriorates the cutting surface properties and tool life. Therefore, it is limited to less than 0.10%. Preferably, it is 0.08% or less.

0.015% N is added to obtain BN-based intermetallic compounds in the N solid solution softening treatment described below, and to ensure the amount of dissolved N and suppress the formation of built-up cutting edge marks. However, if it is added in excess of 0.08%, not only will corrosion resistance deteriorate due to Cr nitrides, but the cutting surface properties will also deteriorate. For this reason, it is limited to 0.08% or less. The preferred range is 0.02-0.06%.

Si is contained at 0.1% or more to suppress the formation of coarse inclusions that deoxidize and deteriorate the cutting surface properties. However, if it is contained at more than 2.0%, it hardens and promotes adhesion of the base material to the tool, promoting the formation of built-up cutting edge marks. Therefore, it is limited to 2.0% or less. The range of 0.2 to 1.0% is preferable.

Mn is added at 0.1% or more to suppress the formation of coarse inclusions that degrade the cutting surface properties by deoxidizing, and to form sulfides to ensure good tool life. However, if it is added at more than 3.0%, it will harden by solid solution hardening and the tool life will deteriorate. Therefore, it is limited to 3.0% or less. The preferred range is 0.2 to 2.0%.

S is added at 0.02% or more to form sulfides and ensure good tool life. However, if the content exceeds 0.40%, the formation of built-up cutting edge marks on the tool during precision cutting processing becomes significant, and it becomes impossible to prevent built-up cutting edge detachment marks on the cutting surface. Therefore, it is limited to 0.40% or less. The preferred range is 0.03 to 0.30%.

P is mixed in as an unavoidable impurity from the raw materials, but if it is contained in excess of 0.10%, not only does it deteriorate corrosion resistance due to grain boundary segregation, but it also significantly reduces manufacturability. For this reason, it is limited to 0.10% or less. Preferably, it is 0.05% or less.

Cr is contained in an amount of 13.0% or more in order to obtain the corrosion resistance of stainless steel, a low thermal expansion coefficient, and a soft ferritic structure with good machinability. However, if it is contained in an amount exceeding 20.0%, it becomes prone to seizure with the tool, and it becomes impossible to ensure the cutting surface properties. Therefore, it is limited to 22.0% or less. The preferred range is 15.0 to 20.0%.

When added together with N, B forms fine BN during N solution softening treatment, suppressing adhesion of the base material to the tool surface and preventing built-up edge marks on the cutting surface. For this reason, 0.001% or more is added. However, adding more than 0.0100% generates coarse borides, which not only promotes the formation of built-up edges but also significantly deteriorates manufacturability. For this reason, it is limited to 0.0100% or less. The preferred range is 0.0020-0.0070%.

Aluminum may be added for deoxidation, but if it is added in excess of 0.008%, coarse inclusions will form and the surface properties will deteriorate. Therefore, it is limited to 0.008% or less. Preferably, it is 0.006% or less.

O is mixed in as an unavoidable impurity, but if it is contained in excess of 0.015%, coarse inclusions are generated, degrading the cutting surface properties. Therefore, it is limited to 0.015% or less. Preferably, it is 0.012% or less.

The amount of dissolved N contained in the ferritic free-cutting stainless steel of the invention is 0.01 to 0.06%.

Solution of N into the ferrite matrix is effective in embrittling the base material during cutting, suppressing the formation of built-up edges, and obtaining good surface properties. For this reason, the amount of solute N is limited to 0.01% or more. However, if N is dissolved in an amount exceeding 0.060%, the tool life will be adversely affected and deteriorated. For this reason, the upper limit of the amount of solute N is set at 0.06%. In the present invention, N is dissolved into the ferrite matrix by strand annealing (rapid cooling), as described below.

The metal structure of the ferritic free-cutting stainless steel of the present invention contains 20 or more BN-based intermetallic compounds of 0.5 μm or less per 100 μm ² .

BN is effective in obtaining excellent cutting surface properties by suppressing adhesion of the base material to the tool through its self-lubricating action, preventing the formation of a built-up edge. However, if coarse BN particles exceeding 0.5 μm in size are dispersed, the base material plastically deformed at the tip of the tool and BN are laminated and deposited in layers, generating and growing a built-up edge. Therefore, BN particles with a major axis of 0.5 μm or less are finely dispersed to suppress lamination and deposition. The effect is remarkable when BN particles are finely dispersed at a number density of 20 or more per 100 ^μm2 . Preferably, the number is 30 or more per 100 ^μm2 .

Optional Ingredients
The stainless steel of the present invention is composed of chemical components consisting of Fe and impurities other than the elements described above. Furthermore, in addition to the above-mentioned composition, the stainless steel may selectively contain the following elements in place of a portion of Fe.

The elements Ni, Cu and Co do not have to be added. Addition has the effect of improving the corrosion resistance and toughness of the product. However, if added in excess of 1.5%, it becomes difficult to soften during annealing, and tool life deteriorates. For this reason, the content is set to 1.5% or less. To ensure the above effects, it is preferable to set the content of each element to 0.01% or more and 1.0% or less.

The elements Mo and W do not need to be added. If they deteriorate, they have the effect of improving the corrosion resistance of the product. However, if they are added in excess of 2.5%, the effect saturates and, conversely, they deteriorate the tool life during cutting processing. For this reason, the content is set to 2.0% or less. To ensure the above effects, it is preferable to set the content of each element to 0.01% or more and 2.0% or less.

The element Bi does not have to be added. If added, it acts as a self-lubricant during cutting, suppressing the formation of built-up edges and improving the cutting surface properties. However, if it is added in an amount exceeding 0.20%, hot manufacturability will be significantly deteriorated and manufacturing will become impossible. Therefore, the content should be 0.20% or less. To ensure the above effects, it is preferable that the content of each element be 0.01% or more and 2.0% or less.

The elements Sn, Sb, and Ag do not need to be added. If added, they act as a self-lubricant during cutting, suppressing the formation of built-up edges and improving the cutting surface properties. However, if added in excess of 0.30%, hot manufacturability is significantly degraded and manufacturing becomes impossible. For this reason, the content is set to 0.30% or less. To reliably obtain the above effects, it is preferable to set the content to 0.005% or more and 0.20% or less.

The element Te does not have to be added. If added, it produces spherical sulfides, which inhibits the stacking and growth of the cutting edge and improves the cutting surface properties. However, if it is added in excess of 0.10%, hot manufacturability is significantly deteriorated and manufacturing becomes impossible. Therefore, the content is set to 0.10% or less. To reliably obtain the above effect, it is preferable to set the content to 0.005% or more and 0.05% or less.

The element V does not have to be added. Addition has the effect of improving the corrosion resistance of the product. However, if added in excess of 0.8%, coarse carbonitrides are formed, promoting the stacking and growth of the cutting edge, deteriorating the cutting surface properties and also deteriorating the tool life. For this reason, the upper limit is set to 0.8%. To reliably obtain the above effect, it is preferable for the content to be 0.05% or more and 0.5% or less.

The elements Nb, Ti, and Ta do not need to be added. Addition has the effect of improving the corrosion resistance of the product. However, if added in excess of 0.5%, coarse carbonitrides are formed, promoting stacking and growth of the cutting edge, deteriorating the cutting surface properties and also deteriorating the tool life. For this reason, the upper limit is set to 0.5% or less. To ensure the above effects, it is preferable to set the content to 0.01% or more and 0.4% or less.

The elements Mg, Ca, and Hf do not need to be added. If added, they have the effect of improving hot workability. However, if added in excess of 0.010% of each, the effect saturates, and conversely, coarse oxides are generated, deteriorating the cutting surface properties. For this reason, the content is set to 0.010% or less. To ensure the above effects, it is preferable to set the content to 0.001% or more and 0.005% or less.

REM elements do not have to be added. If they are added, they have the effect of improving hot workability. However, if they are added in excess of 0.050%, the effect saturates, and conversely, coarse oxides are generated, deteriorating the cutting surface properties. Therefore, the content is set to 0.050% or less. To ensure the above effect, it is preferable to set the content to 0.001% or more and 0.005% or less. REM (rare earth elements) is a general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu), according to the general definition. They may be added alone or as a mixture.

Typical impurities contained in the stainless steel of the present invention include Zn, Pb, Ge, Se, Ag, Se, etc., and are usually mixed in at about 0.1% as impurities during the steel manufacturing process.

The impurity oxygen exists mainly as inclusions in steel, but the oxygen content of stainless steel produced by conventional refining is between 0.001 and 0.015%.

Regarding optional elements, representative ones are specified in (2) to (5) above, but elements not listed in this specification can also be included within the range that does not impair the effects of the present invention.

<<Method for producing ferritic stainless steel>>
A method for producing the ferritic stainless steel of the present invention will now be described.

In order to finely disperse BN particles of 0.5 μm or less in size at a number density of 20 or more particles ^per 100 μm2, a cast piece having the above-mentioned components is once heated to a high temperature of 1150 to 1330°C, and then hot-rolled or hot-worked to dissolve the BN. The resulting steel material (wire material, steel wire, etc.) is then subjected to batch annealing at 700 to 900°C for 60 to 300 minutes.

If the heating temperature of the slab is less than 1150°C, the BN will not dissolve and the number density mentioned above will not be met, and if it exceeds 1330°C, manufacturability will deteriorate. Therefore, the heating temperature should be 1150 to 1330°C, and from the viewpoint of dissolving BN, it should be greater than 1250°C and less than 1320°C.

In addition, if the batch annealing temperature is lower than 700°C and the time is shorter than 60 minutes, the amount of BN precipitation will decrease and the above-mentioned number density will not be met. On the other hand, if the batch annealing temperature is higher than 900°C and the time is longer than 300 minutes, the BN size will become coarse and the above-mentioned number density will not be met. Therefore, the batch annealing conditions are limited to 700 to 900°C and holding for 60 to 300 minutes, preferably 100 to 250 minutes.

In order to ensure the amount of dissolved N in the ferrite while maintaining the above-mentioned BN dispersion state, after the batch annealing, strand annealing is performed in which the temperature is held in the 700-900°C range and cooled at a rate of 1°C/s or more in the temperature range of 500°C or higher.

In normal softening annealing of ferritic stainless steel, the entire coil of wire or steel wire is held at a temperature range of approximately 650 to 950°C by batch annealing and then cooled in the furnace, so it is slowly cooled, for example, at a cooling rate of the order of 0.02°C/s. N that was in solid solution at high temperatures changes its state to nitrides during slow cooling, so in normal batch annealing, the amount of solute N is less than 0.01%.

In the present invention, after the above-mentioned annealing, the temperature is held in the 700-900°C range where the N solid solubility limit in the ferrite phase is 0.01-0.06% and BN does not form a solid solution, and strand annealing is performed in which the temperature range of 500°C or higher where nitrides form is quenched at 1°C/s or more. This makes it possible to maintain the N solid solution in the ferrite phase at 0.01-0.06% while maintaining the fine dispersion state of the BN.

Strand annealing is an annealing method in which wire or steel coil wound in a ring shape is unrolled into a straight line, heat-treated in a single straight line for a short period of time (in an atmosphere of nitrogen, Ar, ammonia decomposition gas, etc.), and then air-cooled or indirectly air-cooled. This method makes it possible to achieve a significantly faster cooling rate than batch annealing of the entire ring-shaped coil.

At this time, if the holding temperature is less than 700°C, the amount of solute N will be less than 0.01%, and if it is 900°C or higher, an austenite phase will form and a hard martensite structure will form during cooling, making it difficult to obtain sufficient machinability (cutting surface properties, tool life) as described above. Therefore, the strand annealing temperature is limited to the range of 700 to 900°C. It is preferably 750 to 850°C. The dwell time is arbitrary, but a range of 30 to 1000 seconds is preferable because it causes no thermal deformation of the material and allows heat to be applied uniformly.

In addition, there is no problem if cold wire drawing or other processing is performed between the batch annealing and strand annealing steps described above. Batch annealing and strand annealing may be performed multiple times, but the final heat treatment must be strand annealing.

As explained above, the present invention provides a ferritic free-cutting stainless steel that can suppress the formation and growth of built-up edges that deteriorate the cutting surface properties during cutting, and is used for precision parts with a small thermal expansion coefficient that are precision cut.

Example 1
Steels having the chemical compositions shown in Tables 1 and 2 were melted at 1600°C in a 65 kg vacuum melting furnace and then cast into a mold. After heating to 1200°C, the steels were hot worked into wires having a diameter of 9 mm, batch annealed at 850°C for 120 minutes, cold drawn to a diameter of 6.3 mm, and strand annealed at 850°C for 200 seconds (in ammonia decomposition gas). The wires were then drawn into polished bars having a diameter of 6.0 mm using a drawing machine to prepare materials for cutting.

The bars thus obtained were evaluated for the number density of BN compounds (number per unit area), surface properties after peripheral cutting, and tool life using the evaluation methods described below. The results are shown in Tables 3 and 4. Table 3 shows the evaluation results for the steel of the present invention, and Table 4 shows the evaluation results for the comparative steel.

"Intermetallic compound (BN compound)"
The longitudinal section of the rod was embedded in resin and mirror-polished, and the precipitates were extracted electrolytically with an alcohol-based etchant, and then a replica sample was made by carbon deposition. The size and distribution of the precipitates were then examined using a transmission electron microscope. Five fields of view were observed over an area of ^{100 μm2} , and the number density of BN compounds with a major axis of 0.5 μm or less per 100 ^μm2 was determined.

"Cutting surface properties"
Tool used: Carbide P type, cutting edge R 0.4 mm, cutting speed: 50 m/min, feed rate: 0.02 mm/rev., depth of cut: 0.1 mm, cutting oil (mineral oil): with feed rate of 0.05 mm/rev. Under the following precision cutting conditions, the outer circumference of the wire rod was cut in the circumferential direction, and the surface after cutting was observed with a 100x magnifying glass and evaluated for the presence or absence of clear built-up edge marks. When clear built-up edge marks were observed, they were evaluated as ×, when minute built-up edge marks were observed, they were evaluated as ◯, and when they were not clearly observed, they were evaluated as ◎.

"Tool life"
Tool used: Carbide P type, cutting edge R 0.4 mm, cutting speed: 200 m/min, feed rate: 0.02 mm/rev., depth of cut: 0.11 mm, cutting oil (mineral oil): with feed rate of 0.05 mm/rev. The outer circumference of the wire rod was cut in the circumferential direction under the precision cutting conditions of a depth of cut ≦ 0.3 mm, and the condition of the tool after cutting for 30 minutes was examined. If the flank wear amount of the tool after use, including boundary wear and knitting wear, was 20 μm or less, it was evaluated as ◎, if it was more than 20 μm and less than 50 μm, it was evaluated as ○, and if it was more than 50 μm, it was evaluated as ×.

"Solute N content"
1 g of the material was electrolyzed in maleic anhydride, and the precipitate was extracted using 0.2 μm mesh filter paper. The extract was then dissolved in an acid solution and the amount of N precipitated as nitrides in the precipitate was determined by atomic absorption spectrometry. The amount of dissolved N was calculated by subtracting the amount of N in the precipitate from the total nitrogen amount in the steel material.

Inventive Examples 1 to 43 in Table 3, the number density of BN with a major axis of 0.5 μm or less is 20 or more per 100 ^μm2 , the amount of solute N is 0.01 to 0.06%, and all of them show excellent cutting surface properties and tool life.

On the other hand, Comparative Examples 1 to 9 and 11 to 33 in Table 4 are outside the composition range, BN number density range, and solute N amount range of the present invention, and were unable to satisfy all of the excellent cutting surface properties and tool life. Comparative Example 10 had a low Cr content, and the BN number density range, material hardness range, and solute N amount range were within the range of the present invention, and the cutting surface properties and tool life were excellent, but the corrosion resistance was insufficient.

Example 2
Steel A was melted at 1600°C in a 65 kg vacuum melting furnace and cast into a mold. After heating to 1100-1360°C, it was hot worked into a wire rod of φ9 mm, softened at 650-1000°C for 30-500 minutes, cold drawn to φ6.3 mm, and strand annealed at 650-950°C for 30-1500 seconds (in ammonia decomposition gas). Then, it was drawn into a polished bar of φ6.0 mm using a drawing machine to prepare a material for finish cutting. Then, the number density (number per unit area) of the BN compound, the surface properties after peripheral cutting, and the tool life were evaluated. The results are shown in Table 5.

Inventive Examples 44 to 51 in Table 5, the number density of BN with a major axis of 0.5 μm or less was 20 or more per 100 ^μm2 , and the amount of solute N was 0.01 to 0.06%, and all of them showed excellent cutting surface properties and tool life.

On the other hand, Comparative Examples 34 to 41 in Table 5 are outside the composition range, BN number density range, and solute N content range of the present invention, and were unable to satisfy both excellent cutting surface properties and tool life.

As is clear from the above examples, the present invention can provide a ferritic free-cutting stainless steel that has excellent cutting surface properties with excellent flatness during precision cutting and tool life, and can significantly improve the durability of precision parts with a small thermal expansion coefficient in severe corrosive environments, making it extremely useful in industry.

Claims (6)

In mass percent,
C: less than 0.10%,
Si: 0.1-2.0%,
Mn: 0.1 to 3.0%,
S: 0.02-0.40%,
P: 0.10% or less,
Cr: 13.0-22.0%,
B: 0.001-0.01%,
N: 0.015-0.08%,
Al: 0.008% or less; O: 0.015% or less; and the balance being Fe and impurities.
The amount of solute N is 0.010 to 0.060%;
A ferritic free-cutting stainless steel characterized in that there are 20 or more BN-based intermetallic compounds of 0.5 μm or less per 100 ^μm2 ,
The outer periphery of the φ6.0 mm wire rod made of the ferritic free-cutting stainless steel is
Tool used: Carbide P type, cutting edge R0.4 mm, cutting speed: 50 m/min, feed rate: 0.02 mm/rev., cutting depth: 0.1 mm, cutting oil (mineral oil): yes
When the cutting process was performed in the circumferential direction under the precision cutting process conditions, and the surface after cutting was observed with a 100x magnifying glass, no clear cutting edge marks were observed.
The outer periphery of the φ6.0 mm wire rod made of the ferritic free-cutting stainless steel is
Tool used: Carbide P type, cutting edge R0.4 mm, cutting speed: 200 m/min, feed rate: 0.02 mm/rev., cutting depth: 0.11 mm, cutting oil (mineral oil): yes
The tool is cut in the circumferential direction under the precision cutting conditions, and the flank wear amount of the tool after cutting for 30 minutes is 50 μm or less, including boundary wear and knot wear.
The present invention relates to a ferritic free-cutting stainless steel . A part of the Fe is replaced with further mass%
Ni: 1.5% or less,
Mo: 2.5% or less,
Cu: 1.5% or less,
The ferritic free-cutting stainless steel according to claim 1, further comprising at least one of Co: 1.5% or less and W: 2.5% or less. A part of the Fe is replaced with further mass%
Bi: 0.20% or less,
Sn: 0.30% or less,
Sb: 0.30% or less,
The ferritic free-cutting stainless steel according to claim 1 or 2, further comprising at least one of Ag: 0.30% or less, and Te: 0.10% or less. A part of the Fe is replaced with further mass%
V: 0.8% or less,
Nb: 0.5% or less,
The ferritic free-cutting stainless steel according to any one of claims 1 to 3, characterized in that it contains one or more of Ti: 0.5% or less, and Ta: 0.5% or less. A part of the Fe is replaced with further mass%
Mg: 0.010% or less,
Ca: 0.010% or less,
The ferritic free-cutting stainless steel according to any one of claims 1 to 4, characterized in that it contains one or more of Hf: 0.010% or less, and REM: 0.050% or less. A method for producing the ferritic free-cutting stainless steel according to any one of claims 1 to 5, comprising the steps of:
A heating step of heating a slab having the chemical composition according to any one of claims 1 to 5 to 1150 to 1330 ° C.;
a processing step of subjecting the heated cast piece to hot rolling or hot working;
A batch annealing process at 700 to 900°C for 60 to 300 minutes.
The method for producing the ferritic free-cutting stainless steel of the present invention is characterized by comprising strand annealing in which the temperature is maintained in the range of 700 to 900°C and cooled at a cooling rate of 1°C/s or more.