KR102471016B1 - Martensitic S free-cutting stainless steel - Google Patents

Abstract

This martensitic S free-cutting stainless steel contains, in mass%, C: 0.08 to 0.70%, Si: 0.01 to 1.0%, Mn: 0.1 to 1.50%, S: 0.15 to 0.60%, P: 0.010 to 0.050%, Cr : 10 to 16%, N: 0.005 to 0.15%, Al: 0.004% or less, Mg: 0.0020% or less, O: 0.007 to 0.030%, Ni: 0 to 1.0%, Mo: 0 to 3.0%. The additive contains a (Mn, Cr)(S, O)-based inclusion composed of Fe and impurities and containing 0.5% by mass or more of O.

Description

Martensitic S free-cutting stainless steel

The present invention relates to martensitic S free-cutting stainless steel (martensitic S-containing free-cutting stainless steel).

This application claims priority based on Japanese Patent Application No. 2018-112652 filed on June 13, 2018, the contents of which are incorporated herein by reference.

Among parts such as OA equipment and electronic equipment, precision parts produced by cutting require high dimensional accuracy and good surface properties on the cut surface in addition to chip disposability during cutting. As a material that meets these requirements, there is SUS420F containing 0.15% or more of S, or martensitic free-cutting stainless steel containing Pb, Se, or Te alone or in combination to further improve machinability (Patent Documents 1 to 3). .

On the other hand, in response to market demand to abolish the addition of Pb, martensitic free-cutting stainless steels containing Bi or Sn and dispersing a second phase mainly composed of Cu have been proposed (Patent Documents 4 and 5).

However, in the inventions described in Patent Literatures 1 to 5, satisfactory ones have not been obtained in terms of manufacturability or surface properties after cutting. In particular, the precision parts require excellent tool wear resistance with precision of surface roughness Ra ≤ 0.50 μm under industrial cutting conditions such as cutting speed ≥ 20 m/min, cutting ≥ 0.05 mm, feed rate ≥ 0.005 mm/rev.

Japanese Patent Publication No. 7-56064 Japanese Unexamined Patent Publication No. 2001-152298 Japanese Patent No. 5135918 Japanese Patent No. 6194696 Japanese Patent No. 4502519

The present invention was made in view of the above circumstances, and under industrial cutting conditions of precision parts, surface roughness (Ra): excellent surface accuracy of 0.50 μm or less can be obtained, tool wear resistance and manufacturability are also excellent, and Pb It is an object to provide a martensitic S free-cutting stainless steel that does not contain.

In one aspect of the present invention, it has been clarified that machinability, particularly surface roughness after cutting, can be improved by controlling the composition of inclusions by controlling minor components and uniformly dispersing MnS. Detailed knowledge is as follows.

In order to improve the surface roughness, it is effective to reduce the built-up edge formed on the cutting edge of the tool during cutting. When the built-up edge occurs, the surface roughness is deteriorated because irregularities different from the contour of the cutting edge of the tool are generated during cutting. In one aspect of the present invention, the formation of built-up edges is suppressed by reducing the aspect ratio of inclusions in steel.

First, in the casting step, trace components are controlled so as to generate granular sulfide-based inclusions (flat crystals). Sulfide-based inclusions in one aspect of the present invention are (Mn, Cr) (S, O)-based inclusions or (Mn, Cr, Ca, REM) (S, O, Te)-based inclusions, trace elements are sulfides It is characterized in that the deformation resistance of the inclusion is increased and the aspect ratio is reduced by dissolving in the system inclusion. In addition, rod-shaped sulfide (eutectic type) is generally generated in the casting step, but these inclusions have a large aspect ratio and become non-uniform in shape, leading to deterioration of surface roughness.

One aspect of the present invention has been made based on the above findings, and the gist thereof is as follows.

[1] In mass%,

C: 0.08 to 0.70%;

Si: 0.01 to 1.0%;

Mn: 0.1 to 1.50%;

S: 0.15 to 0.60%;

P: 0.010 to 0.050%;

Cr: 10 to 16%;

N: 0.005 to 0.15%;

Al: 0.004% or less;

Mg: 0.0020% or less;

O: 0.007 to 0.030%;

Ni: 0 to 1.0%;

Mo: 0 to 3.0%;

Ca: 0 to 0.003%;

Te: 0 to 0.024%;

REM: 0 to 0.003%,

B: 0 to 0.02%,

Nb: 0 to 1.00%,

Ti: 0 to 1.00%;

V: 0 to 0.50%;

Ta: 0 to 0.5%;

W: 0 to 0.5%;

Co: 0 to 1.00%;

Zr: 0 to 0.020%;

Cu: 0 to 3.0%;

Sn: 0 to 0.5%;

Sb: 0 to 0.5%;

Ga: 0 to 0.0050% is contained,

The balance consists of Fe and impurities,

A martensitic S free-cutting stainless steel characterized by containing (Mn, Cr)(S, O)-based inclusions containing 0.5% by mass or more of O.

[2] In mass%,

Ca: 0.0005 to 0.003%;

Te: 0.010 to 0.024%;

REM: The martensitic S free-cutting stainless steel according to [1] characterized by containing one or more of 0.0005 to 0.003%.

[3] (Mn, Cr, Ca, REM) (S, O, Te)-based inclusions containing any one or two or more of 0.3 mass% or more Ca, 1 mass% or more Te, and 0.3 mass% or more REM The martensitic S free-cutting stainless steel according to [1] or [2], characterized in that it contains.

[4] In mass%,

B: 0.0001 to 0.02%;

Nb: 0.05 to 1.00%;

Ti: 0.05 to 1.00%;

V: 0.05 to 0.50%;

Ta: 0.1 to 0.5%;

W: 0.1 to 0.5%;

Co: 0.05 to 1.00%;

Zr: 0.001 to 0.020%;

Cu: 0.1 to 3.0%;

Sn: 0.005 to 0.5%;

Sb: 0.005 to 0.5%;

Ga: 0.0005 to 0.0050%

The martensitic S free-cutting stainless steel according to any one of [1] to [3], containing one or two or more selected from

[5] The martensitic S free-cutting stainless steel according to any one of [1] to [4], wherein the aspect ratio of the (Mn, Cr) (S, O)-based inclusion is 4.0 or less.

[6] The martensitic S free-cutting stainless steel according to [3] or [4], wherein the aspect ratio of the (Mn, Cr, Ca, REM) (S, O, Te)-based inclusion is 4.0 or less.

In one aspect of the present invention, it does not contain Pb, which adversely affects the environment, and has excellent surface precision of surface roughness (Ra): 0.50 μm or less under ordinary precision parts cutting conditions, tool wear resistance and manufacturing A martensitic S free-cutting stainless steel with excellent properties can be obtained. In addition, the martensitic S free-cutting stainless steel according to one embodiment of the present invention can be used, for example, as a material for precision parts such as OA equipment and electronic equipment requiring machinability and corrosion resistance, and as parts such as shafts, screws, and bolts. can

The martensitic S free-cutting stainless steel of the present embodiment contains, in mass%, C: 0.08 to 0.70%, Si: 0.01 to 1.0%, Mn: 0.1 to 1.50%, S: 0.15 to 0.60%, P : 0.010 to 0.050%, Cr: 10 to 16%, N: 0.005 to 0.15%, Al: 0.004% or less, Mg: 0.0020% or less, O: 0.007 to 0.030%, Ni: 0 to 1.0%, Mo: 0 to 3.0%, Ca: 0 to 0.003%, Te: 0 to 0.024%, REM: 0 to 0.003%, B: 0 to 0.02%, Nb: 0 to 1.00%, Ti: 0 to 1.00%, V: 0 to 0.50 %, Ta: 0 to 0.5%, W: 0 to 0.5%, Co: 0 to 1.00%, Zr: 0 to 0.020%, Cu: 0 to 3.0%, Sn: 0 to 0.5%, Sb: 0 to 0.5% , Ga: 0 to 0.0050%, the remainder being Fe and impurities, and (Mn, Cr)(S, O)-based inclusions containing 0.5% by mass or more of O.

In addition, the martensitic S free-cutting stainless steel of the present embodiment contains any one or two or more of 0.3 mass% or more Ca, 1 mass% or more Te, and 0.3 mass% or more REM (Mn, Cr, Ca, REM) (S, O, Te) system inclusions may be contained.

In the martensitic S free-cutting stainless steel of the present embodiment, the aspect ratio of (Mn, Cr)(S, O) system inclusions may be 4.0 or less.

In the martensitic S free-cutting stainless steel of the present embodiment, the aspect ratio of (Mn, Cr, Ca, REM)(S, O, Te) system inclusions may be 4.0 or less.

Below, each requirement of this embodiment is demonstrated.

C: 0.08 to 0.70%

C is necessary to obtain high strength by obtaining a martensitic structure after quenching. For this reason, C content is made into 0.08 % or more. Also, from the viewpoint of machinability, the C content may be 0.12% or more. On the other hand, if an excessive amount of C is contained, coarse carbides are generated during annealing, which promotes the formation of built-up edges during cutting and deteriorates the precision of the cut surface. Therefore, the C content is set to 0.70% or less. Preferably it is 0.40% or less.

Si: 0.01 to 1.0%

Si is contained for deoxidation. For this reason, Si content is made into 0.01 % or more. Si content may be 0.05% or more. On the other hand, if Si exceeds 1.0%, scale generation during rolling is promoted when stainless steel is hot rolled to form a bar, and hot rolling marks are promoted, so the Si content is set to 1.0% or less.

Mn: 0.1 to 1.50%

Mn is an element that forms inclusions together with Cr to improve machinability, particularly surface accuracy. For this reason, Mn content is made into 0.10 % or more. On the other hand, when the Mn content exceeds 1.50%, the composition ratio of Mn/Cr in the inclusions increases, and the inclusions are transformed to increase the aspect ratio. Therefore, the Mn content is made 1.50% or less. The Mn content may be 1.40% or less, or 1.10% or less.

S: 0.15 to 0.60%

S forms sulfide-based inclusions, and stress concentrates on the inclusions during cutting. In addition, cracks are generated starting from inclusions in the shear deformation region at the time of chip generation, and the growth of the built-up edge is suppressed. For this reason, the precision of the cutting surface of steel improves. In order to acquire this effect, S content is made into 0.15 % or more. 0.20% or more of S content may be sufficient. On the other hand, when S is contained exceeding 0.60%, hot workability deteriorates remarkably. Therefore, the S content is made 0.60% or less. The S content may be 0.40% or less.

P: 0.010 to 0.050%

P segregates at grain boundaries, reduces material ductility during cutting, and improves surface precision. For this reason, P content is made into 0.010 % or more. The P content may be 0.020% or more. On the other hand, when the P content exceeds 0.050%, manufacturability deteriorates remarkably. Therefore, the P content is made 0.050% or less.

Cr: 10 to 16%

Cr forms sulfide-based inclusions together with Mn, and the aspect ratio of the inclusions can be controlled particularly by optimizing the composition ratio (Mn/Cr) of Mn and Cr in the inclusions. In order to reduce the aspect ratio and improve the precision of the cut surface, the Cr content is set to 10% or more. Cr content may be 12% or more. However, when a large amount of Cr is contained, the composition ratio of Mn/Cr in the inclusions becomes too small, the inclusions tend to be transformed, and the aspect ratio increases. Therefore, Cr content is made into 16 % or less. Cr content may be 15% or less.

N: 0.005 to 0.15%

N dissolves in the matrix, embrittles the matrix in the cutting temperature range, and enhances the strength of the product. For this reason, the N content is made 0.005% or more. Preferably, N is contained in an amount of more than 0.02%. However, when N is contained in excess of 0.15%, productivity deteriorates markedly due to generation of blowholes and deterioration of hot workability. Therefore, the N content is made 0.15% or less.

The N content may be 0.12% or less.

Al: 0.004% or less

Although Al is used as a deoxidizing element, since it forms a hard Al-based oxide and becomes low-oxygenated, rod-shaped sulfide (eutectic type) is formed. Therefore, the Al content is made 0.004% or less. The Al content may be 0.003% or less and may be less than 0.002%. What is necessary is just to contain Al in the quantity of 0.001% or more in order to express an effect in this embodiment.

Mg: 0.0020% or less

Although Mg is used as a deoxidizing element, rod-shaped sulfides (eutectic type) are formed in order to form hard Mg-based oxides and reduce oxygenation. Therefore, Mg content is made into 0.0020% or less. The Mg content may be 0.0010% or less and may be less than 0.0005%. What is necessary is just to contain Mg in the quantity of 0.0001% or more in order to express an effect in this embodiment.

By containing both Al and Mg in an amount within the range of the present embodiment, granular sulfide-based inclusions (flattened) are generated, and machinability is improved.

O: 0.007 to 0.030%

O improves machinability by coarsening the deoxidation product at the time of solidification and generating granular sulfide-based inclusions (flattened). For this reason, the O content is made 0.007% or more. The O content may be 0.012% or more. Moreover, 0.016% or more may be sufficient. However, since hard inclusions increase and machinability deteriorates when O is contained exceeding 0.030%, O content is made 0.030% or less.

The martensitic S free-cutting stainless steel of the present embodiment consists of Fe and impurities other than the above elements. However, elements described below other than the above may be selectively contained within a range that does not impair the effect of the technical features of the present embodiment. The reason for the limitation is described below. The lower limit of these elements is 0%.

Ni: 0 to 1.0%

Ni may be contained because Ni increases the hardness of the material by solid solution strengthening, prevents generation of built-up edges, and improves surface accuracy during cutting. In that case, it is preferable that the Ni content is 0.1% or more. However, when it exceeds 1.0%, it hardens and causes deterioration of tool life. Therefore, the Ni content is made 1.0% or less. The Ni content may be 0.8% or less. Ni content may be 0%.

Mo: 0 to 3.0%

Mo is an element that improves corrosion resistance and may be contained. However, when a large amount of Mo is contained, it hardens and causes deterioration of tool life. For this reason, Mo content is made into 3.0 % or less. Mo content may be 2.0% or less. On the other hand, in order to obtain the said effect, it is preferable that Mo content is 0.1 % or more. Mo content may be 0%.

Ca: 0 to 0.003%

Ca improves machinability by forming granular sulfide-based inclusions (sulfide-type inclusions), so it may be contained. Moreover, since it also has the effect of softening oxide-type inclusions and improving tool life, you may contain it. In order to acquire these effects, what is necessary is just to contain 0.0005% or more. However, when Ca is contained exceeding 0.003%, the effect is saturated and the hot workability is lowered conversely. For this reason, Ca content is made into 0.003 % or less. As for Ca content, it is more preferable that it is 0.001 % or more and 0.002 % or less. Ca may be 0%.

Te: 0 to 0.024%

Te is an important element in order to improve machinability, especially the precision of the cutting surface in this embodiment, so you may contain it. By dissolving Te in an amount of 1% by mass or more in the inclusions, deformation of the inclusions is suppressed and the aspect ratio is reduced. As a result, the growth of the built-up edge is suppressed, and the precision of the cutting surface is improved. As for Te content in the case of containing Te, it is preferable that it is 0.010 % or more. On the other hand, when Te is contained in excess of 0.024%, the effect is not only saturated, but also MnTe is formed around the inclusions, resulting in markedly deteriorating manufacturability. Therefore, Te content is made into 0.024 % or less. Te content may be 0.015% or less. Te may be 0%.

REM: 0 to 0.003%

Since REM improves machinability by forming granular sulfide-based inclusions (sulfide-type inclusions) similarly to Ca, it may be contained. Moreover, since it also has the effect of softening oxide-type inclusions and improving tool life, you may contain it. In the case of containing REM, it may be 0.0005% or more. However, when REM is contained in excess of 0.003%, the effect is not only saturated, but also hard REM-based oxysulfides are generated in part of the inclusions, causing deterioration of tool life. For this reason, the REM content is made 0.003% or less. It is preferable that REM content is 0.001% or more and 0.002% or less. REM may be 0%.

REM (rare earth element), according to a general definition, refers to two elements of scandium (Sc) and yttrium (Y), and a generic name for 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu) in the periodic table. . You may contain individually by 1 type, and 2 or more types of mixtures may be sufficient.

B: 0 to 0.02%

B is an element used to improve hot workability, and since a stable effect is obtained, it may be contained. However, if B is contained in an excessive amount, the compound of B precipitates and the hot workability deteriorates, so the B content is set to 0.02% or less. It is preferable that B content is 0.015% or less. On the other hand, in order to obtain the above effect, the B content is preferably 0.0001% or more, and the B content is more preferably 0.0002% or more. B may be 0%.

Nb: 0 to 1.00%

Ti: 0 to 1.00%

V: 0 to 0.50%

Ta: 0 to 0.5%

W: 0 to 0.5%

Nb, Ti, V, Ta, and W form carbonitrides and have an effect of improving corrosion resistance, so they may be contained. However, when these elements are contained in large amounts, machinability deteriorates, so the Nb content is made 1.00% or less, and the Ti content is made 1.00% or less. Further, the V content is 0.50% or less, the Ta content is 0.5% or less, and the W content is 0.5% or less. On the other hand, in order to obtain the above effect, the Nb content is preferably 0.05% or more, the Ti content is preferably 0.05% or more, and the V content is preferably 0.05% or more. Further, the Ta content is preferably 0.1% or more, and the W content is preferably 0.1% or more. Nb, Ti, V, Ta, and W may be 0%.

Co: 0 to 1.00%

Co may be contained because it enhances the toughness of the matrix. However, if Co is contained in an excessive amount, it hardens and the machinability deteriorates, so the Co content is made 1.00% or less. Co content may be 0.60% or less. On the other hand, in order to acquire the said effect, it is preferable that Co content is 0.05 % or more. Co may be 0%.

Zr: 0 to 0.020%

Since Zr has an effect of improving strength, you may contain Zr. However, since toughness is reduced when a large amount of Zr is contained, the Zr content is made 0.020% or less. On the other hand, in order to sufficiently obtain the effect of improving the strength, the Zr content is preferably 0.001% or more. Zr may be 0%.

Cu: 0 to 3.0%

Cu may be contained because it increases the hardness of the material by solid solution strengthening, prevents the generation of built-up edges, and improves the surface precision during cutting. However, even if it is contained in excess of 3.0%, the effect is saturated and manufacturability deteriorates, such as cracking of cast slabs, so the Cu content is set to 3.0% or less. On the other hand, in order to acquire the said effect, it is preferable that Cu content is 0.1 % or more. Cu may be 0%.

Sn: 0 to 0.5%

Sb: 0 to 0.5%

Sn and Sb suppress deterioration of corrosion resistance by coexisting with sulfides that deteriorate corrosion resistance, so they may be contained. However, since manufacturability deteriorates when Sn and Sb contain more than 0.5%, Sn and Sb content is made into 0.5% or less, respectively. The content of Sn and Sb may be 0.3% or less, respectively. On the other hand, in order to obtain the above effect, it is preferable that the Sn and Sb contents are each 0.005% or more. The content of Sn and Sb may be 0.010% or more, respectively. In addition, 0% may be sufficient as Sn and Sb content, respectively.

Ga: 0 to 0.0050%

Ga may be contained in an amount of 0.0005% or more as needed to improve cold workability. However, forgeability deteriorates when Ga exceeds 0.0050%. Therefore, what is necessary is just to make the upper limit of Ga content into 0.0050 % or less. Ga may be 0%.

Although the martensitic S free-cutting stainless steel of the present embodiment inevitably contains Pb and Se in some cases, the Pb content needs to be controlled to less than 0.03% and the Se content needs to be controlled to less than 0.02%.

In addition, impurities mean that they are mixed from ore as a raw material, scrap, manufacturing environment, etc. when manufacturing steel materials industrially, and are permitted within a range that does not adversely affect the steel materials of the present embodiment.

In this embodiment, it is important to control the composition of inclusions. When the deformation resistance of the inclusions increases, the aspect ratio of the inclusions after rolling the martensitic S free-cutting stainless steel according to the present embodiment into a wire rod can be reduced. As a result, formation of built-up edges is suppressed, and high dimensional accuracy and good surface properties are obtained during cutting.

In order to control the composition of the inclusions, the oxygen content in the molten steel is increased by controlling the amount of deoxidizing components such as Al and Mg to the upper limit of the content in the present embodiment or less at the time of melting the steel in the refining step. Moreover, in AOD (or VOD) in actual machine manufacture, it is preferable to set slag basicity CaO/SiO ₂ to 1.8 or less, preferably about 1.5. After completion of the refining, the oxygen content in the molten steel can be increased by an operation in which no deoxidizing components such as Al and Mg are added. As a result, it is possible to generate (Mn, Cr)(S, O)-based inclusions containing 0.5% by mass or more of O as granular sulfide-based inclusions (flat crystalline). The aspect ratio of the inclusions at this stage is 4.0 or less, preferably 3.0 or less. In the stainless steel in which inclusions are formed, in the subsequent hot rolling step, the inclusions are not deformed even when the total hot rolling reduction ratio (the sum of the reduction ratios in hot rolling) is rolled under the condition of 95% or more, and the aspect ratio is the target. It can be controlled to 4.0 or less, preferably 3.0 or less. If the aspect ratio exceeds 4.0, the machinability deteriorates when cutting parts or the like, which is not preferable. The aspect ratio of the inclusions is preferably 1 or more. When the aspect ratio of an inclusion is less than 1, the inclusion is considered to be a very hard inclusion that is difficult to stretch, and causes cracks or surface flaws during production.

In addition, when one or more of Ca, Te, and REM are contained, (Mn, Cr), which is a composite inclusion containing at least one of 0.3 mass% or more Ca, 1 mass% or more Te, and 0.3 mass% or more REM, , Ca, REM) (S, O, Te)-based inclusions can be generated. The aspect ratio of the generated inclusion is 4.0 or less, preferably 3.0 or less. Since these composite inclusions have high deformation resistance, the inclusions do not deform even when rolling is performed under the condition that the hot rolling area reduction ratio is 95% or more in the subsequent hot rolling step, and the aspect ratio of the inclusions is 4.0 or less, preferably 3.0. It can be controlled to the following, and machinability can be improved significantly. When the aspect ratio exceeds 4.0, the machinability deteriorates, which is not preferable. The aspect ratio of the inclusions is preferably 1 or more.

The martensitic S free-cutting stainless steel of the present embodiment may be a steel material after casting, a wire material obtained by hot rolling a steel material, or a steel wire obtained by cold drawing a wire material again, or a steel material after casting or a wire material after hot rolling. A forged material may be used. These steel materials, wire rods, steel wires, or forged materials are steels having chemical components according to the present embodiment, and contain (Mn, Cr) (S, O)-based inclusions or (Mn, Cr, Ca, REM) (S, O, Te ) system inclusions are included. In addition, since (Mn, Cr) (S, O)-based inclusions or (Mn, Cr, Ca, REM) (S, O, Te)-based inclusions contained in steel are inclusions that are relatively difficult to deform, any of the above Even in the step, it has an aspect ratio of 4.0 or less.

Further, by containing one or more of Ca, Te, and REM (Mn, Cr, Ca, REM) (S, O, Te)-based inclusions are generated, but even in this case, the martensite according to the present embodiment is formed. The system S free-cutting stainless steel may contain (Mn, Cr)(S, O) system inclusions.

Note that (Mn, Cr)(S, O)-based inclusions containing 0.5% or more of O are inclusions containing all of Mn, Cr, S, and O and having an O concentration of 0.5% or more.

In addition, (Mn, Cr, Ca, REM) (S, O, Te)-based inclusions include all of Mn, Cr, S, and O, and contain 1% of Ca of 0.3% or more, Te of 1% or more, and REM of 0.3% or more. It is an inclusion containing a species or two or more species. In addition, (Mn, Cr, Ca, REM) (S, O, Te)-based inclusions may contain 0.5% or more of O.

The respective amounts of O and Te in the inclusions are preferably 10% or less. Each amount of Ca and REM in the inclusion is preferably 20% or less.

The composition of these inclusions is analyzed by an energy dispersive X-ray analyzer (EDS) attached to the scanning electron microscope (SEM). In the case where all of Cr, Mn, S, and O are detected from the inclusions specified by SEM, and O is contained at 0.5% by mass or more, the inclusions are regarded as (Mn, Cr)(S, O)-based inclusions. Further, when all of Mn, Cr, S, and O are detected from a specific inclusion by SEM, and one or two or more of 0.3 mass% or more Ca, 1 mass% or more Te, and 0.3 mass% or more REM are detected, The inclusions are (Mn, Cr, Ca, REM) (S, O, Te)-based inclusions. Whether or not these inclusions are mixed may be determined by specifying and analyzing 10 or more inclusions, and then confirming whether or not the inclusions are mixed from the result.

In addition, the aspect ratio of the inclusions was determined by optical microscope observation using a sample supplied to SEM-EDS, 10 fields of view at 100 times magnification, and a diameter horizontal to the rolling direction circumscribed to the inclusions (horizontal ferret diameter). and the diameter perpendicular to the rolling direction (vertical ferret diameter) is measured by an image analysis method. The ratio of the horizontal ferret diameter/vertical feret diameter of each inclusion is calculated as an aspect ratio, and the average value of the aspect ratios of all inclusions is taken as the aspect ratio of the sample. In the case where the above two types of inclusions are included, the aspect ratios of all the inclusions may be averaged.

As described above, the martensitic S free-cutting stainless steel of the present embodiment contains S as a free-cutting element and is excellent in machinability. When this steel is used as a steel wire, it can be suitably used, for example, as a raw material for precision parts such as OA equipment and electronic equipment requiring machinability and corrosion resistance, and as a raw material for parts such as screws and bolts.

Example

By melting 150 kg of alloy raw material in a vacuum melting furnace and controlling the amount of deoxidizing components such as Al and Mg to the upper limit of the content of the present embodiment or less, casting in a mold with a diameter of 200 mm while the oxygen content in the molten steel remains high. did. After that, it was heated at 1200°C, followed by hot forging and processing to a diameter of 70 mm. Next, it annealed (air-cooled) at 780 degreeC for 1 hour, and peeled to a diameter of 66 mm. Subsequently, it was processed into a diameter of 10 mm by hot extrusion equivalent to rolling a steel bar. After pickling, it was then annealed at 780°C for 1 hour, followed by air cooling (5°C/s) (total hot rolling area reduction: 98%). Subsequently, cold drawing was performed to φ6 mm, and the obtained wire rod was held in a furnace at 780° C. for 3 minutes to perform strand annealing (cooling was rapid cooling). Finally, the wire rod was processed with a pendulator to obtain a polished bar having a diameter of 5.5 mm. Each evaluation test was performed using this polishing rod as a raw material for evaluation. In the steel components shown in Tables 1 to 3, Pb was less than 0.03% and Se was less than 0.02%.

The wire rod is embedded in a resin so as to observe a cross-sectional image in the longitudinal direction including the center line, mirror polishing is performed, and the composition of the inclusion is determined using a scanning electron microscope (SEM) with an attached energy dispersive X-ray analyzer (EDS). analyzed by When all of Cr, Mn, S, and O were detected from a specific inclusion by SEM, and O was contained at 0.5% by mass or more, the inclusion was regarded as a (Mn, Cr)(S, O)-based inclusion. Further, when all of Mn, Cr, S, and O are detected from a specific inclusion by SEM, and one or two or more of 0.3 mass% or more Ca, 1 mass% or more Te, and 0.3 mass% or more REM are detected, The inclusions were made into (Mn, Cr, Ca, REM) (S, O, Te) system inclusions. Whether or not these inclusions were mixed was confirmed by specifying and analyzing 10 or more inclusions, and based on the result, whether or not the inclusions were mixed. In Table 4 and Table 5, the composition ratio of inclusions is shown.

The aspect ratio of the inclusions was determined by using a sample supplied to SEM-EDS, photographing 10 fields of view at a magnification of 100 times by optical microscope observation, and measuring the diameter horizontal to the rolling direction circumscribed to the inclusions (horizontal pellet diameter) and rolling The diameter perpendicular to the direction (vertical Feret diameter) was measured by an image analysis method. The ratio of the horizontal ferret diameter/vertical ferret diameter of each inclusion was calculated as an aspect ratio, and the average value of the aspect ratios of all inclusions was taken as the aspect ratio of the sample. The results are shown in Table 6 and Table 7. In Table 6 and Table 7, when the above two types of inclusions are included, a value obtained by averaging the aspect ratios of all the inclusions is expressed as the aspect ratio of the sample.

The surface roughness after cutting the outer periphery of the wire rod was evaluated by the center line average roughness (Ra) of the cut surface. Cutting is a turning process, the material is carbide class P, and a tool with an edge R of 0.4 mm is used, cutting speed is 50 m/min, feed rate is 0.02 mm/rev, and cutting is 0.1 mm while applying cutting oil (mineral oil). it was cut

Surface roughness Ra was measured on the sample after 15 minutes of turning. For the measurement, a contact-type roughness measuring instrument was used, each of 5 points was measured at a standard length of 2.5 mm, and the average value was used as the measured value. In this embodiment, when the surface roughness Ra was 0.50 μm or less, it was judged to be good. The results are shown in Table 6 and Table 7.

In addition, the tool life was evaluated by the time until the average wear amount of the relief surface reached 0.2 mm, and if the average wear amount of the relief surface was less than 0.2 mm in 15 minutes of machining, it was set as life achievement. That is, when the average wear amount of the relief surface was less than 0.2 mm in 15 minutes of machining, the tool life was long and the machinability was evaluated as excellent. When the average wear amount of the relief surface was 0.2 mm or more in 15 minutes of machining, it was evaluated that the tool life was short and the machinability was poor. The results are shown in Table 6 and Table 7.

Manufacturability was evaluated by a high-temperature tensile test. A hot ductility evaluation test piece with a diameter of 10 mm was taken from the center of the forged material with a diameter of 70 mm and the middle part of the surface in the longitudinal direction of the round bar. Manufacturability was evaluated by the throttle value after tensile fracture under conditions of a test temperature of 1000°C and a tensile speed of 10 mm/s. The shape of the test piece at this time is phi 10 mm x 100 mm. Manufacturability was said to be manufacturability achieved when the throttle value at 1000°C was 50% or more. That is, when the throttle value at 1000°C was 50% or more, it was evaluated as excellent in manufacturability. A case where the throttle value at 1000°C was less than 50% was evaluated as inferior in manufacturability. The results are shown in Table 6 and Table 7.

Sample Nos. 1 to 49 are steels of the present invention (examples of the present invention), and sample Nos. 50 to 65 are comparative steels (comparative examples).

* mark in the table indicates that the value is out of the range of the present embodiment.

Supplementing the composition of the inclusions in Tables 4 and 5, for Nos. 25 to 37 and 63 to 65 in which any one or two or more of Ca, Te, and REM were detected, (Mn, Cr) (S, Both O)-based inclusions and (Mn, Cr, Ca, REM)(S, O, Te)-based inclusions were included. In the case of including two types of inclusions, the aspect ratios of (Mn, Cr) (S, O)-based inclusions and (Mn, Cr, Ca, REM) (S, O, Te)-based inclusions are all was less than 4.0.

In Nos. 59, 60, and 62, the amount of oxygen in the inclusion composition was less than 0.5% by mass. In these Nos. 59, 60, and 62, (Mn, Cr) (S, O)-based inclusions having an aspect ratio of 4.0 or less were not included. In No. 52, the amount of Mn was out of the range of this embodiment. In No. 55, the amount of Cr was outside the range of this embodiment. In these Nos. 52 and 55, (Mn, Cr) (S, O) type inclusions having an aspect ratio of 4.0 or less were not included.

Samples other than the above contained (Mn, Cr)(S, O)-based inclusions having an aspect ratio of 4.0 or less.

In No.1 to No.49, which are steels of the present invention, by controlling the inclusion composition of the martensitic S free-cutting stainless steel, the surface roughness Ra after cutting is 0.50 μm or less, and the amount of tool wear is less than 0.2 mm, which is the standard for the target tool life. has achieved Also, the manufacturability standard was achieved when the throttle value at 1000°C was 50% or more. On the other hand, comparative steels No.50 to No.65 did not satisfy the stipulated range of the embodiment and did not satisfy any characteristic.

As is clear from the examples, according to the present embodiment, martensitic S free-cutting stainless steel excellent in machinability and manufacturability can be produced without containing highly toxic Pb or the like.

The martensitic S free-cutting stainless steel of the present embodiment can be used, for example, as a material for precision parts such as OA equipment and electronic equipment requiring machinability and corrosion resistance, and as parts such as shafts, screws, and bolts.

Claims (6)

in mass percent,
C: 0.08 to 0.70%;
Si: 0.01 to 1.0%;
Mn: 0.1 to 1.50%;
S: 0.15 to 0.60%;
P: 0.010 to 0.050%;
Cr: 10 to 16%;
N: 0.005 to 0.15%;
Al: 0.004% or less;
Mg: 0.0020% or less;
O: 0.007 to 0.030%;
Ni: 0 to 1.0%;
Mo: 0 to 3.0%;
Ca: 0 to 0.003%;
Te: 0 to 0.024%;
REM: 0 to 0.003%,
B: 0 to 0.02%,
Nb: 0 to 1.00%,
Ti: 0 to 1.00%;
V: 0 to 0.50%;
Ta: 0 to 0.5%;
W: 0 to 0.5%;
Co: 0 to 1.00%;
Zr: 0 to 0.020%;
Cu: 0 to 3.0%;
Sn: 0 to 0.5%;
Sb: 0 to 0.5%;
Ga: 0 to 0.0050% is contained,
The balance consists of Fe and impurities,
A martensitic S free-cutting stainless steel characterized by containing (Mn, Cr)(S, O)-based inclusions containing 0.5% by mass or more of O. The method of claim 1, in mass%,
Ca: 0.0005 to 0.003%;
Te: 0.010 to 0.024%;
REM: 0.0005 to 0.003%;
B: 0.0001 to 0.02%;
Nb: 0.05 to 1.00%;
Ti: 0.05 to 1.00%;
V: 0.05 to 0.50%;
Ta: 0.1 to 0.5%;
W: 0.1 to 0.5%;
Co: 0.05 to 1.00%;
Zr: 0.001 to 0.020%;
Cu: 0.1 to 3.0%;
Sn: 0.005 to 0.5%;
Sb: 0.005 to 0.5%;
Ga: 0.0005 to 0.0050%
Martensitic S free-cutting stainless steel, characterized in that it contains one or two or more selected from (Mn, Cr, Ca, REM) (S, A martensitic S free-cutting stainless steel characterized by containing O, Te)-based inclusions. The martensitic S free-cutting stainless steel according to claim 1 or 2, wherein the (Mn, Cr) (S, O)-based inclusion has an aspect ratio of 4.0 or less. The martensitic S free-cutting stainless steel according to claim 3, wherein the aspect ratio of the (Mn, Cr) (S, O)-based inclusions is 4.0 or less. The martensitic S free-cutting stainless steel according to claim 3, wherein the aspect ratio of the (Mn, Cr, Ca, REM) (S, O, Te)-based inclusions is 4.0 or less.