WO2019240209A1

WO2019240209A1 - Martensitic s free-cutting stainless steel

Info

Publication number: WO2019240209A1
Application number: PCT/JP2019/023437
Authority: WO
Inventors: 勇人境沢; 雅之東城; 光司高野; 成雄福元
Original assignee: 日鉄ステンレス株式会社
Priority date: 2018-06-13
Filing date: 2019-06-13
Publication date: 2019-12-19
Also published as: JP6918238B2; CN111989418A; TWI707047B; TW202000943A; CN111989418B; KR102471016B1; JPWO2019240209A1; KR20200124294A

Abstract

This martensitic S free-cutting stainless steel contains, in terms of % by mass, 0.07-0.70% C, 0.01-1.0% Si, 0.1-1.50% Mn, 0.15-0.60% S, 0.010-0.050% P, 10-16% Cr, 0.005-0.15% N, 0.004% or less of Al, 0.0020% or less of Mg, 0.007-0.030% O, 0-1.0% Ni, and 0-3.0% Mo, the remainder comprising Fe and impurities, and contains (Mn, Cr)(S, O)-based inclusions including 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 on June 13, 2018 based on Japanese Patent Application No. 2018-112562 for which it applied to Japan, and uses the content for it here.

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

On the other hand, martensitic free-cutting stainless steel containing Bi or Sn and in which a second phase mainly composed of Cu is dispersed has been proposed in response to the market demand for eliminating Pb addition (Patent Document 4, 5).

However, the inventions described in Patent Documents 1 to 5 have not been satisfactory in terms of manufacturability and surface properties after cutting. In particular, the above precision parts have an accuracy of surface roughness Ra ≦ 0.50 μm and excellent tool resistance under industrial cutting conditions such as cutting speed ≧ 20 m / min, cutting depth ≧ 0.05 mm, feed amount ≧ 0.005 mm / rev. Abrasion is required.

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

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

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

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

First, in the casting stage, trace components are controlled so as to generate granular sulfide inclusions (clinotropic type). The sulfide inclusions in one embodiment of the present invention are (Mn, Cr) (S, O) type inclusions or (Mn, Cr, Ca, REM) (S, O, Te) type inclusions, It is characterized by increasing the deformation resistance of inclusions and reducing the aspect ratio by dissolving trace elements in sulfide inclusions. In general, rod-shaped sulfides (eutectic type) are produced at the casting stage, but such inclusions have a large aspect ratio and non-uniform 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] By mass%
C: 0.08 to 0.70%,
Si: 0.01 to 1.0%,
Mn: 0.1 to 1.50%,
S: 0.15-0.60%,
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 mass% or more of O.
[2] By mass%
Ca: 0.0005 to 0.003%,
Te: 0.010 to 0.024%,
REM: Martensitic S free-cutting stainless steel according to [1], containing one or more of 0.0005 to 0.003%.
[3] Including one or more of 0.3 mass% or more of Ca, 1 mass% or more of Te, 0.3 mass% or more of REM (Mn, Cr, Ca, REM) (S, The martensitic S free-cutting stainless steel according to [1] or [2], characterized by containing O, Te) -based inclusions.
[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: 0.1-0.5%
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 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) inclusion is 4.0 or less. steel.
[6] The martensitic S free cutting according to [3] or [4], wherein the (Mn, Cr, Ca, REM) (S, O, Te) inclusions have an aspect ratio of 4.0 or less. Stainless steel.

In one aspect of the present invention, the surface roughness (Ra) is 0.50 μm or less under an ordinary precision part cutting condition without containing Pb that adversely affects the environment, Martensitic S free-cutting stainless steel excellent in tool wear and manufacturability can be obtained. In addition, the martensitic S free-cutting stainless steel according to one aspect of the present invention includes, for example, materials for precision parts such as OA equipment and electronic equipment that require machinability and corrosion resistance, and parts such as shafts, screws, and bolts. Can be used as

In the martensitic S free-cutting stainless steel of this embodiment, the chemical components are 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 %: 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 : 0-0.024%, REM: 0-0.003%, B: 0-0.02%, Nb: 0-1.00%, Ti: 0-1.00%, V: 0-0. 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 .5% Ga: 0 contained to 0.0050% the balance being Fe and impurities, O and comprises more than 0.5 wt% (Mn, Cr) (S, O) containing inclusions.
In addition, the martensitic S free-cutting stainless steel of the present embodiment includes one or more of 0.3 mass% or more of Ca, 1 mass% or more of Te, or 0.3 mass% or more of REM. It may contain (Mn, Cr, Ca, REM) (S, O, Te) inclusions.
Furthermore, in the martensitic S free-cutting stainless steel of this embodiment, the aspect ratio of (Mn, Cr) (S, O) inclusions may be 4.0 or less.
Furthermore, in the martensitic S free-cutting stainless steel of this embodiment, the aspect ratio of (Mn, Cr, Ca, REM) (S, O, Te) inclusions may be 4.0 or less.
Below, each requirement of this embodiment is demonstrated.

C: 0.08 to 0.70%
C is necessary in order to obtain a martensite structure and obtain high strength after quenching. For this reason, C content shall be 0.08% or more. Further, from the viewpoint of machinability, the C content may be 0.12% or more. On the other hand, when an excessive amount of C is contained, coarse carbides are generated at the time of annealing, and the generation of the constituent cutting edge is promoted at the time of cutting to deteriorate the accuracy of the cutting surface. Therefore, the C content is 0.70% or less. To do. Preferably it is 0.40% or less.

Si: 0.01 to 1.0%
Si is contained for deoxidation. For this reason, Si content shall be 0.01% or more. The Si content may be 0.05% or more. On the other hand, when Si exceeds 1.0%, when stainless steel is hot-rolled into a bar wire, the scale generation at the time of rolling is promoted, and the formation of hot-rolled iron is promoted. 1.0% or less.

Mn: 0.1 to 1.50%
Mn is an element that generates inclusions together with Cr and improves machinability, particularly surface accuracy. For this reason, Mn content shall be 0.10% or more. On the other hand, when the Mn content exceeds 1.50%, the composition ratio of Mn / Cr in the inclusions becomes high, and the inclusions expand to increase the aspect ratio. Therefore, the Mn content is 1.50% or less. The Mn content may be 1.40% or less, or 1.10% or less.

S: 0.15-0.60%
S forms sulfide 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 constituent cutting edges is suppressed. For this reason, the precision of the cutting surface of steel improves. In order to obtain this effect, the S content is 0.15% or more. The S content may be 0.20% or more. On the other hand, when S exceeds 0.60%, hot workability is remarkably deteriorated. Therefore, the S content is set to 0.60% or less. S content is good also as 0.40% or less.

P: 0.010 to 0.050%
P segregates at the grain boundaries to lower the material ductility during the cutting process and improve the surface accuracy. For this reason, the P content is set to 0.010% or more. The P content may be 0.020% or more. On the other hand, if the P content exceeds 0.050%, the productivity is significantly deteriorated. Therefore, the P content is 0.050% or less.

Cr: 10-16%
Cr forms sulfide inclusions with Mn, and the aspect ratio of inclusions can be controlled by optimizing the composition ratio (Mn / Cr) of Mn and Cr in the inclusions. In order to reduce the aspect ratio and improve the accuracy of the cutting surface, the Cr content is 10% or more. The 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, and the inclusions are easily spread and the aspect ratio is increased. Therefore, the Cr content is 16% or less. The 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 increases the strength of the product. For this reason, N content shall be 0.005% or more. Preferably N is contained in an amount of more than 0.02%. However, if N is contained exceeding 0.15%, the manufacturability is remarkably deteriorated due to the formation of blow holes and the deterioration of hot workability. Therefore, the N content is 0.15% or less.
The N content may be 0.12% or less.

Al: 0.004% or less Al is used as a deoxidizing element, but forms rod-like sulfides (eutectic type) in order to form hard Al-based oxides and reduce oxygen. Therefore, the Al content is 0.004% or less. The Al content may be 0.003% or less and may be less than 0.002%. In order to exhibit the effect in the present embodiment, Al is preferably contained in an amount of 0.001% or more.

Mg: 0.0020% or less Mg is used as a deoxidizing element, but forms a rod-like sulfide (eutectic type) in order to form a hard Mg-based oxide to reduce oxygen. Therefore, the Mg content is 0.0020% or less. The Mg content may be 0.0010% or less or less than 0.0005%. In order to exhibit the effect in the present embodiment, it is preferable to contain Mg in an amount of 0.0001% or more.
By containing both Al and Mg in an amount within the range of this embodiment, granular sulfide inclusions (clinotropic type) are generated, and machinability is improved.

O: 0.007 to 0.030%
O coarsens the deoxidation product at the time of solidification and improves the machinability by generating granular sulfide inclusions (clinotropic type). For this reason, the O content is set to 0.007% or more. The O content may be 0.012% or more. Furthermore, it may be 0.016% or more. However, if O is contained in an amount exceeding 0.030%, hard inclusions increase and machinability is deteriorated, so the O content is set to 0.030% or less.

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

Ni: 0 to 1.0%
Ni may be contained in order to increase the hardness of the material by solid solution strengthening to prevent the formation of the constituent cutting edge and improve the surface accuracy during cutting. In that case, the Ni content is preferably 0.1% or more. However, if it exceeds 1.0%, it hardens and causes deterioration of the tool life. Therefore, the Ni content is 1.0% or less. The Ni content may be 0.8% or less. The Ni content may be 0%.

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

Ca: 0 to 0.003%
Ca improves the machinability by generating granular sulfide inclusions (clinotropic type), so Ca may be contained. Moreover, since there exists an effect which softens an oxide type inclusion and improves a tool life, you may make it contain. In order to acquire these effects, it is good to make it contain 0.0005% or more. However, when Ca is contained exceeding 0.003%, the effect is saturated and hot workability is reduced. For this reason, Ca content is made into 0.003% or less. The Ca content is more preferably 0.001% or more and 0.002% or less. Ca may be 0%.

Te: 0 to 0.024%
Te is an important element for improving the machinability, in particular, the accuracy of the cutting surface in the present embodiment, and therefore, Te may be contained. Te suppresses deformation of inclusions by dissolving at least 1% by mass in the inclusions, thereby reducing the aspect ratio. As a result, the growth of the cutting edge is suppressed and the accuracy of the cutting surface is improved. When Te is contained, the Te content is preferably 0.010% or more. On the other hand, when Te exceeds 0.024%, not only the effect is saturated, but also MnTe is formed around the inclusions, and the productivity is remarkably deteriorated. Therefore, the Te content is set to 0.024% or less. The Te content may be 0.015% or less. Te may be 0%.

REM: 0 to 0.003%
Since REM improves the machinability by generating granular sulfide inclusions (clinotropic type) like Ca, it may be included. Moreover, since there exists an effect which softens an oxide type inclusion and improves a tool life, you may make it contain. When it contains REM, it is good to make it 0.0005% or more. However, when the content of REM exceeds 0.003%, not only the effect is saturated, but also a hard REM-based oxysulfide is generated in a part of the inclusions, causing deterioration of the tool life. For this reason, REM content shall be 0.003% or less. The REM content is preferably 0.001% or more and 0.002% or less. REM may be 0%.

REM (rare earth element) is a generic name of two elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoid) from lanthanum (La) to lutetium (Lu) in the periodic table according to the general definition. Point to. 1 type may be contained independently and 2 or more types of mixtures may be sufficient.

B: 0 to 0.02%
B is an element used for improving hot workability, and may be contained in order to obtain a stable effect. However, if B is contained in an excessive amount, the B compound is precipitated and the hot workability is deteriorated, so the B content is set to 0.02% or less. The B content is preferably 0.015% or less. On the other hand, in order to acquire the said effect, it is preferable that B content is 0.0001% or more, and it is more preferable that B content is 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: 0-0.5%
Nb, Ti, V, Ta, and W may form carbonitride and have an effect of improving corrosion resistance, and thus may be contained. However, if these elements are contained in large quantities, the machinability deteriorates, so the Nb content is 1.00% or less and the Ti content is 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 0.00%. It is preferably at least 05%. 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 in order to increase the toughness of the matrix. However, if Co is contained in an excessive amount, it hardens and deteriorates the machinability, so the Co content is 1.00% or less. The 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%
Zr has the effect of improving the strength and may be contained. However, if a large amount of Zr is contained, the toughness is reduced, so the Zr content is 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 in order to increase the hardness of the material by solid solution strengthening to prevent the formation of the constituent cutting edge and to improve the surface accuracy during the cutting process. However, even if the content exceeds 3.0%, the effect is saturated and manufacturability deteriorates such as occurrence of cracks in the slab, so the Cu content is 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 may be contained in order to suppress deterioration of corrosion resistance by coexisting with a sulfide that deteriorates corrosion resistance. However, if the content of Sn and Sb exceeds 0.5%, manufacturability is deteriorated, so the Sn and Sb contents are 0.5% or less, respectively. Each of Sn and Sb contents may be 0.3% or less. On the other hand, in order to acquire the said effect, it is preferable that Sn and Sb content are 0.005% or more, respectively. The Sn and Sb contents may be 0.010% or more, respectively. Moreover, Sn and Sb content may be 0%, respectively.

Ga: 0 to 0.0050%
Ga may be contained in an amount of 0.0005% or more as necessary for improving cold workability. However, when Ga exceeds 0.0050%, forgeability deteriorates. Therefore, the upper limit of the Ga content is preferably 0.0050% or less. Ga may be 0%.

The martensitic S free-cutting stainless steel of this embodiment may inevitably contain Pb and Se, but the Pb content is controlled to be less than 0.03% and the Se content is less than 0.02%. Need to control.

Impurities are introduced from ore, scrap, or production environment as raw materials when industrially manufacturing steel materials, and are allowed within a range that does not adversely affect the steel materials of this embodiment. Means something.

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 can be kept small. As a result, the formation of the constituent cutting edges is suppressed, and high dimensional accuracy and good surface properties can be obtained during cutting.

In order to control the composition of inclusions, the oxygen content in the molten steel is controlled by controlling the amount of deoxidizing components such as Al and Mg during melting of the steel to be equal to or lower than the upper limit of the content of the present embodiment. Increase. Further, in the actual production, it is preferable that the basicity CaO / SiO ₂ of slag is 1.8 or less, preferably about 1.5 in AOD (or VOD). 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, (Mn, Cr) (S, O) inclusions containing 0.5% by mass or more of O can be generated as granular sulfide inclusions (clinotropic type). The aspect ratio of the inclusion at this stage is 4.0 or less, preferably 3.0 or less. The stainless steel in which inclusions were produced was when the total hot rolling area reduction (total area reduction in hot rolling) was 95% or more in the subsequent hot rolling process. However, the inclusions are not deformed, and the aspect ratio can be controlled to 4.0 or less, preferably 3.0 or less. If the aspect ratio exceeds 4.0, it is not preferable because machinability deteriorates when cutting a part or the like. The aspect ratio of the inclusion is preferably 1 or more. When the aspect ratio of inclusions is less than 1, the inclusions are very hard inclusions that are difficult to stretch, and are considered to cause cracks and surface scratches during production.

Furthermore, when one or more of Ca, Te, and REM are contained, 0.3% by mass or more of Ca, 1% by mass or more of Te, and 0.3% by mass or more of REM or more are included. (Mn, Cr, Ca, REM) (S, O, Te) type inclusions, which are composite inclusions, can be generated. The aspect ratio of the produced inclusions is 4.0 or less, preferably 3.0 or less. Since such composite inclusions have high deformation resistance, the inclusions are not deformed even when the rolling reduction is performed under the condition that the area reduction rate of hot rolling is 95% or more in the subsequent hot rolling process, The aspect ratio of inclusions can be controlled to 4.0 or less, preferably 3.0 or less, and the machinability can be greatly improved. If the aspect ratio exceeds 4.0, the machinability is lowered, which is not preferable. The aspect ratio of the inclusion is preferably 1 or more.

The martensitic S free-cutting stainless steel of the present embodiment may be a steel material after casting, may be a wire obtained by hot rolling the steel material, and is obtained by further cold-drawing the wire. It may be a steel wire, or a forged material obtained by forging a steel material after casting or a wire material after hot rolling. These steel materials, wire materials, steel wires or forged materials are steels having chemical components according to the present embodiment, and include (Mn, Cr) (S, O) -based inclusions or (Mn, Cr, Ca, REM) ( S, O, Te) inclusions are included. In addition, (Mn, Cr) (S, O) inclusions or (Mn, Cr, Ca, REM) (S, O, Te) inclusions contained in steel are inclusions that are relatively difficult to deform. Therefore, in any of the above steps, the aspect ratio is 4.0 or less.

In addition, (Mn, Cr, Ca, REM) (S, O, Te) -based inclusions are produced by containing one or more of Ca, Te, and REM. Even in this case, The martensitic S free-cutting stainless steel according to the present embodiment may contain (Mn, Cr) (S, O) inclusions.

The (Mn, Cr) (S, O) 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. .
The (Mn, Cr, Ca, REM) (S, O, Te) inclusions include all of Mn, Cr, S, and O, 0.3% or more of Ca, 1% or more of Te, and It is an inclusion containing one or more of 0.3% or more of REM. Further, the (Mn, Cr, Ca, REM) (S, O, Te) type inclusions may contain 0.5% or more of O.
Each amount of O and Te in the inclusion is 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 a scanning electron microscope (SEM). When all of Cr, Mn, S, and O are detected from the inclusion specified by SEM and 0.5 mass% or more of O is contained, the inclusion is (Mn, Cr) (S, O). System inclusions. In addition, Mn, Cr, S, and O are all detected from the inclusions specified by SEM, and one of 0.3% by mass or more of Ca, 1% by mass or more of Te, and 0.3% by mass or more of REM. Alternatively, when two or more types are detected, the inclusions are (Mn, Cr, Ca, REM) (S, O, Te) type inclusions. Whether or not these inclusions are mixed may be determined by identifying and analyzing 10 or more inclusions, and confirming whether or not the inclusions are mixed from the result.

In addition, the aspect ratio of inclusions was measured using a sample subjected to SEM-EDS, and 10 fields of view were taken at a magnification of 100 by observation with an optical microscope. The diameter was horizontal in the rolling direction circumscribing the inclusions (horizontal ferret diameter). The diameter perpendicular to the rolling direction (vertical ferret diameter) is measured by image analysis. The ratio of the horizontal ferret diameter / vertical ferret 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. When the 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 this embodiment contains S as a free-cutting element and has excellent machinability. When this steel is used as a steel wire, for example, it can be suitably used as a material for precision parts such as OA equipment and electronic equipment that require machinability and corrosion resistance, and a material for parts such as screws and bolts.

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 below the upper limit of the content of this embodiment, the oxygen content in the molten steel remains high. And cast into a mold having a diameter of 200 mm. Then, it heated at 1200 degreeC and then processed by hot forging to a diameter of 70 mm. Next, it was annealed at 780 ° C. for 1 hour (air-cooled) and peeled to a diameter of 66 mm. Subsequently, it processed into the diameter of 10 mm by the hot extrusion equivalent to rolling of steel bar. Pickling and then annealing again at 780 ° C. for 1 hour and air cooling (5 ° C./s) was performed (total hot rolling area reduction: 98%). Subsequently, it cold-drawn to (phi) 6 mm, and the obtained wire was again hold | maintained in the furnace at 780 degreeC for 3 minute (s), and strand annealing (cooling was rapidly cooled) was performed. Finally, the wire rod was processed with a drawing machine to obtain a polishing rod having a diameter of 5.5 mm. Each polishing test was carried out using this polishing bar as a 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 is embedded in a resin so as to observe a cross section in the longitudinal direction including the center line, mirror-polished, and the composition of inclusions is analyzed by energy dispersive X-ray analysis attached to a scanning electron microscope (SEM). Analyzed by instrument (EDS). When all of Cr, Mn, S, and O are detected from the inclusion specified by SEM and 0.5 mass% or more of O is contained, the inclusion is (Mn, Cr) (S, O). System inclusions were used. In addition, Mn, Cr, S, and O are all detected from the inclusions specified by SEM, and one of 0.3 mass% or more of Ca, 1 mass% or more of Te, and 0.3 mass% or more of REM. Alternatively, when two or more kinds are detected, the inclusions are (Mn, Cr, Ca, REM) (S, O, Te) type inclusions. Whether or not these inclusions were mixed was determined by analyzing 10 or more inclusions, and the results confirmed whether or not inclusions were mixed. Tables 4 and 5 show the composition ratio of inclusions.

The aspect ratio of inclusions was measured using a sample subjected to SEM-EDS, and was observed with an optical microscope at 10 magnifications at a magnification of 100. The horizontal diameter in the rolling direction circumscribing the inclusions (horizontal ferret diameter) and rolling The diameter perpendicular to the direction (vertical ferret 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 Tables 6 and 7. In Tables 6 and 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 was evaluated by the centerline average roughness (Ra) of the cutting surface. Cutting is turning, material is P type carbide, cutting edge R is 0.4mm, cutting speed is 50m / min, feed rate is 0.02mm / rev, cutting depth is 0.1mm. Cutting was performed while applying oil (mineral oil).

The surface roughness Ra was measured on a sample after turning for 15 minutes. For the measurement, a contact-type roughness measuring machine was used, and the average length was measured at 5 points each with a reference length of 2.5 mm. In this embodiment, it was determined that the surface roughness Ra was good when it was 0.50 μm or less. The results are shown in Tables 6 and 7.

Also, the tool life was evaluated by the time until the average flank wear amount reached 0.2 mm. If the average flank wear amount was less than 0.2 mm after 15 minutes of machining, the tool life was achieved. That is, it was evaluated that the tool life was long and the machinability was excellent when the average wear amount of the flank was less than 0.2 mm after 15 minutes of machining. When the average wear amount of the flank face was 0.2 mm or more after 15 minutes of machining, the tool life was short and the machinability was inferior. The results are shown in Tables 6 and 7.

Manufacturability was evaluated by a high temperature tensile test. A hot ductility evaluation test piece having a diameter of 10 mm was collected in the longitudinal direction of the round bar from the center of the forged material having a diameter of 70 mm and the middle portion of the surface. Manufacturability was evaluated by a drawing value after tensile fracture at a test temperature of 1000 ° C. and a tensile speed of 10 mm / s. The shape of the test piece at this time is φ10 mm × 100 mm. Manufacturability was achieved when the aperture value at 1000 ° C. was 50% or more. That is, when the aperture value at 1000 ° C. was 50% or more, it was evaluated that the productivity was excellent. When the aperture value at 1000 ° C. was less than 50%, it was evaluated that the productivity was inferior. The results are shown in Tables 6 and 7.

Sample No. 1 to 49 are steels of the present invention (invention examples). 50 to 65 are comparative steels (comparative examples).
* Mark in a table | surface shows that the value has remove | deviated from the range of this embodiment.
When supplementing about the composition of the inclusion of Table 4 and Table 5, any 1 type or 2 types or more of Ca, Te, and REM were detected. For 25 to 37 and 63 to 65, both (Mn, Cr) (S, O) inclusions and (Mn, Cr, Ca, REM) (S, O, Te) inclusions are included. It was. When two types of inclusions are included, the aspect ratios of (Mn, Cr) (S, O) inclusions and (Mn, Cr, Ca, REM) (S, O, Te) inclusions are as follows: , Both were 4.0 or less.
No. In 59, 60, and 62, the amount of oxygen in the inclusion composition was less than 0.5% by mass. These No. In 59, 60, and 62, (Mn, Cr) (S, O) inclusions having an aspect ratio of 4.0 or less were not included. No. In 52, the amount of Mn was out of the range of the present embodiment. No. In 55, the Cr amount was out of the range of the present embodiment. These No. In Nos. 52 and 55, (Mn, Cr) (S, O) inclusions having an aspect ratio of 4.0 or less were not included.
Samples other than the above contained (Mn, Cr) (S, O) inclusions having an aspect ratio of 4.0 or less.

No. of steel of the present invention 1-No. 49, by controlling the composition of inclusions in martensitic S free-cutting stainless steel, the surface roughness Ra after cutting becomes 0.50 μm or less, the tool wear amount is less than 0.2 mm, and the target tool life Achieved the criteria. In addition, the aperture value at 1000 ° C. was 50% or more, and the manufacturability standard was achieved. On the other hand, no. 50-No. No. 65 did not satisfy the specified range of the embodiment and did not satisfy any of the characteristics.

As is clear from the examples, according to this 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 as materials for precision parts such as OA equipment and electronic equipment that require machinability and corrosion resistance, and parts such as shafts, screws, and bolts. it can.

Claims

% By mass
C: 0.08 to 0.70%,
Si: 0.01 to 1.0%,
Mn: 0.1 to 1.50%,
S: 0.15-0.60%,
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 mass% or more of O.
% By mass
Ca: 0.0005 to 0.003%,
Te: 0.010 to 0.024%,
The martensitic S free-cutting stainless steel according to claim 1, characterized by containing one or more of REM: 0.0005 to 0.003%.
0.3% by mass or more of Ca, 1% by mass or more of Te, 0.3% by mass or more of REM including one or more of (Mn, Cr, Ca, REM) (S, O, Te 3) Martensitic S free-cutting stainless steel according to claim 1 or 2, characterized by containing system inclusions.
% By 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: 0.1-0.5%
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 claims 1 to 3, comprising one or more selected from the group consisting of:
The martensitic S free-cutting stainless steel according to any one of claims 1 to 4, wherein an aspect ratio of the (Mn, Cr) (S, O) inclusions is 4.0 or less.
The martensitic S free-cutting stainless steel according to claim 3 or 4, wherein an aspect ratio of the (Mn, Cr, Ca, REM) (S, O, Te) inclusions is 4.0 or less.