JP2001011581A - Free-cutting stainless steel excellent in outgassing characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce stainless steel provided with outgassing characteristics and excellent in corrosion resistance and machinability. SOLUTION: This steel has a compsn. contg., by weight, <=0.50% C, 0.15 to 1.00% Si, <=1.00% Mn, 0.05 to 0.50% S, 10.00 to 30.00% Cr, <=0.005% Al and 0.005 to 0.040% O, and the balance Fe with inevitable impurities, in which the weight ratio of Cr/Mn contained in sulfide base inclusions present in the steel is also controlled to >=1, and moreover, among oxide base inclusions present in the steel, at least >=50%, by the piece ratio, of oxides in which the major axis is >=10 μm in the case of being spindle or spherical, and the length is >=10 μm in the case of being linear respectively are composed of, by weight, 30 to 70% SiO2, 10 to 30% MnO and 20 to 50% Al2O3+Cr2O3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a free-cutting stainless steel having excellent outgassing characteristics.

[0002]

2. Description of the Related Art Conventionally, general-purpose SUS416, SUS43
In S-containing free-cutting stainless steels such as 0F and SUS303, S mainly forms sulfide MnS together with Mn in the steel, and the sulfide exhibits effects such as improvement in chip disposability and reduction in cutting resistance. Improve machinability. Further, S has a large effect of improving the life of a turning tool particularly over a wide cutting speed as compared with Pb or the like which is also a free-cutting element, and has a feature that it is difficult to substitute with another free-cutting element easily.

On the other hand, MnS is generally inferior in corrosion resistance to a stainless steel matrix, and has a disadvantage that it tends to be a starting point of corrosion. For this reason, the above-mentioned general-purpose S-containing free-cutting stainless steel was inferior in rust resistance as compared with steel not containing S. In addition, in contrast to free-cutting stainless steel, which is often used as a material for precision machine parts, in recent years, not only the corrosion resistance of the stainless steel material itself has been good, but also the MnS contained in the steel reacts with moisture in the air. It is also required that there be no corrosion of materials around precision machinery parts due to hydrogen sulfide gas (outgas) generated by the gas. From such a point, CAMPS-ISIJvol. 10 (1997) p
1271 etc., as an S free-cutting stainless steel with improved corrosion resistance and outgassing properties, the amount of Mn in the steel was reduced, and sulfide was changed from MnS to Cr-rich (Cr,
Steel types changed to Mn) S have been developed and put to practical use.

[0004]

However, in a free-cutting stainless steel having a normal component range, reducing Mn not only causes a change in the sulfide composition but also changes the composition of an unavoidable oxide. Let it. Conventionally, SUS41
6. In S-containing free-cutting stainless steels such as SUS430F and SUS303, it is known that MnS having a granular type and a small degree of elongation has a greater machinability improving effect.
For this reason, Japanese Patent Application Laid-Open Nos. 8-193249 and 8-2
As disclosed in Japanese Patent No. 60102, at the time of steel making, the dissolved oxygen is increased so that MnS is controlled to a granular type.

At this time, Mn silicate is often generated as an oxide in a free-cutting stainless steel having a general component range. Mn silicate, like sulfide, is rich in ductility and rarely causes a problem in product characteristics. On the other hand, when Mn is reduced to change the sulfide composition to (Cr, Mn) S, the composition of the oxide changes, and the generated oxide hardly stretches by hot working. These oxides remaining in a lump in the product not only promote tool wear during cutting and shorten the service life, but also have a very large disadvantage of being exposed on the cut surface and deteriorating the surface finish.

[0006]

SUMMARY OF THE INVENTION The above-mentioned problem is solved.
As a result, the inventors proceeded with intensive development to eliminate oxides.
Softening so as to stretch hot during composition change
Is necessary, and the range of the alloying element amount is limited,
Controlled to a specific composite composition containing i, Cr, Al, etc.
In this case, a soft oxide that stretches hot at a low melting point
And found. The gist of the invention is as follows: (1) C: 0.50% or less, Si: 0.15% by weight
1.00%, Mn: 1.00% or less, S: 0.05-
0.50%, Cr: 10.00-30.00%, Al:
0.005% or less, O: 0.005 to 0.040%, remaining
Part consists of Fe and unavoidable impurities, and in steel
Weight ratio of Cr and Mn in existing sulfide inclusions
An oxide having Cr / Mn of 1 or more and present in steel
Long in case of spindle type or spherical type of inclusions
In the case of a diameter and a linear shape, the length must be 10 μm or more.
Oxides of at least 50% by number
Is 30% ≦ SiO by weight%_Two≦ 70%, 10% ≦ M
nO ≦ 30%, 20% ≦ Al_TwoO _Three+ Cr_TwoO_Three≤50
% Free cutting with excellent outgassing characteristics
Stainless steel.

(2) By weight%, C: 0.50% or less, S
i: 0.15 to 1.00%, Mn: 1.00% or less,
S: 0.05 to 0.50%, Cr: 10.00 to 30.
00%, Al: 0.005% or less, Ca: 0.0005
０．０１0.01%, O: 0.005 to 0.040%, balance is Fe and unavoidable impurities, and the weight ratio of Cr to Mn contained in the sulfide-based inclusions present in the steel is Cr.
/ Mn is 1 or more, and among the oxide-based inclusions present in the steel, at least the number of oxides each having a long diameter in the case of a spindle-shaped or spherical one and a length of 10 μm or more in the case of a linear one. 20% ≦ SiO ₂ ≦ 70%, 10% ≦ Al ₂ O ₃
+ Cr ₂ O ₃ ≦ 50%, 10% ≦ CaO ≦ 50%, a free-cutting stainless steel having excellent outgassing characteristics.

(3) As described in (1) or (2) above, Ni: 20.00% or less, Mo: 3.0 by weight%.
0% or less, Cu: 4.00% or less, Ti: 1.00% or less, Nb: 1.00% or less, N: 0.50% or less, B:
0.02% or less, Se: 0.30% or less, Te: 0.3
A free-cutting stainless steel excellent in outgas characteristics, characterized by containing one or more of 0% or less and Pb: 0.30% or less.

Hereinafter, the present invention will be described in detail. In the present invention, Cr and Mn contained in the sulfide-based inclusions
And the oxide-forming elements Si, M
The addition amount of n, Al, and Ca is regulated, and the oxide is SiO ₂ −
The composite type of MnO—Cr ₂ O ₃ —Al ₂ O ₃ (—CaO) is characterized by changing to a softness that can be stretched hot. Therefore, these oxides can be rendered harmless by stretching by rolling and cutting and surface finishing. As a result, it is possible to obtain a stainless steel having a small amount of so-called outgas, which is a phenomenon in which the inclusions in the steel material react with H ₂ O to generate hydrogen sulfide gas, and having excellent machinability and finish of the cut surface. .

[0010] Included in sulfide inclusions present in steel
The reason why the weight ratio Cr / Mn of Cr and Mn is 1 or more is as follows.
If it is less than 1, the corrosion resistance of the sulfide-based inclusions decreases and the rust resistance increases.
And outgassing characteristics are deteriorated. Also in steel
Among the oxide-based inclusions that exist, some are spindle-shaped or spherical.
In the case of, the length is 1
50% in at least the number ratio of oxides of 0 μm or more
The above are in weight%, 30% ≦ SiO_Two≦ 70%,
10% ≦ MnO ≦ 30%, 20% ≦ Al_TwoO_Three+ Cr_Two
O_Three≦ 50% range, or 20% ≦ SiO_Two≦ 70
%, 10% ≦ Al _TwoO_Three+ Cr_TwoO_Three≤50%, 10%
In the range of ≦ CaO ≦ 50%, the oxide is hot-rolled.
It is not possible to deteriorate the cutting surface finish of the product
Absent. If the oxide composition is out of this range, it will be near the rolling temperature.
Is poorly deformed or does not stretch at all, and the surface properties may deteriorate.
Sprinkle.

Further, when Ca is added, it usually reacts with S to easily generate CaS. Since CaS is spherical and finely dispersed, it may be used in free-cutting steel for mechanical structural members that needs to suppress anisotropy in mechanical properties. However, CaS has extremely strong reactivity with moisture,
It has an adverse effect on the outgassing properties as with MnS. Therefore, by limiting the lower limit of the amount of O and fixing Ca as an oxide, the amount of Ca existing as sulfide is limited.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the content of each component element of the present invention will be described below. C: 0.50% or less C is an element necessary for increasing the strength. However, in order to improve the corrosion resistance, a smaller amount is better.
50%. Si: 0.15 to 1.00% Si is an element necessary as a deoxidizing element during steel making, and is also necessary for forming an oxide into a suitable composition. However, if it is too large, it will increase the annealing hardness and impair workability,
The range was 0.15 to 1.00%.

Mn: 1.00% or less Mn is a deoxidizing element like Si, and is important as an element affecting the sulfide composition in the present component system. However, if the content is too high, the Mn concentration in the sulfide increases, and the corrosion resistance and outgas characteristics are deteriorated. Therefore, the upper limit is set to 1.00%. S: 0.05 to 0.50% S forms a sulfide and is extremely effective in improving machinability.
If the content is too large, the machinability improving effect is saturated and the hot workability is deteriorated. Therefore, the range is set to 0.05 to 0.50%.

Cr: 10.00-30.00% Cr is a basic element of stainless steel and imparts corrosion resistance. In the present component system, it reacts with S to have a high Cr low M
It forms n-type sulfide and improves the corrosion resistance and outgassing properties of the sulfide itself. Therefore, the range is from 10.00 to
30.00%. Al: 0.005% or less Al is a powerful deoxidizing element, and at the same time, has a large effect on the oxide composition. When it contains an appropriate amount, Al is a composite oxide excellent in hot ductility even if the Mn concentration is low. Generate If the amount is too large, the oxide becomes a hard oxide of Al ₂ O ₃ single phase, and the deformability during hot working is reduced. Reduces the amount of dissolved oxygen during production and deteriorates machinability. Therefore,
The upper limit was set to 0.005%.

Ca: 0.0005 to 0.01% Ca is an extremely powerful deoxidizing element, and at the same time, has a great influence on the oxide composition. A composite oxide suitable for finishability is produced. If the content is too large, the oxide becomes a hard oxide having a high CaO concentration and the deformability during hot working is reduced, so that it is exposed in a large area on the cutting surface of the product, deteriorating the surface finish, and reducing the dissolved oxygen. Causes a reduction in machinability. Therefore,
The range was 0.0005 to 0.01%.

O: 0.005 to 0.040% O becomes excessively fine due to excessive addition of deoxidizing elements during steelmaking, so that sulfides are easily stretched hot and become too fine. The shape becomes inappropriate for improving the machinability. In addition, when Ca is added, Ca, which is a deoxidizing element, does not react with O to form a sulfide having a high Ca concentration, so that desired corrosion resistance and outgassing characteristics cannot be obtained. If it is too large, the effect of improving machinability is saturated, and the amount of oxide only increases. Therefore,
The range was 0.005 to 0.040%.

Ni: 20.00% or less Ni is a strong austenite-forming element and is mainly added to austenitic stainless steel and some martensitic steels. More than a certain amount is required for non-magnetic applications,
Further, it has the effect of improving the corrosion resistance to non-oxidizing acids. However, the upper limit was set to 20.00% in terms of cost. Mo: 3.00% or less Mo is an element necessary for improving corrosion resistance. However, if it is too much, an embrittlement phase is precipitated, and the corrosion resistance and mechanical properties are reduced. Therefore, the upper limit is set to 3.00%.

Cu: 4.00% or less Cu mainly forms a solid solution in an austenite ground to improve corrosion resistance.
Effective for improving cold workability. However, excessive addition deteriorates hot workability, so the upper limit was made 4.00%. Ti: 1.00% or less Ti is advantageous in improving corrosion resistance by forming Ti carbides and in improving strength by precipitation of intermetallic compounds. But,
If the content is too large, the material becomes brittle, so the upper limit is made 1.00%.

Nb: 1.00% or less Nb is a strong carbide-forming element and forms Nb carbide to suppress the formation of Cr carbide and improve corrosion resistance. However, if the content is too large, the hot workability deteriorates. Therefore, the upper limit is set to 1.00%. N: 0.50% or less N is useful for improving corrosion resistance and strength. However, excessive addition causes defects in the steel ingot.
0%.

B: 0.02% or less B is an appropriate addition element and particularly improves the hot workability of austenitic stainless steel. However, if the content is too large, it will adversely affect the performance. Therefore, the upper limit is set to 0.02%. Se: 0.30% or less Se, like S, reacts with Mn, Cr and the like to form nonmetallic inclusions to improve machinability. However, excessive addition saturates the effect and deteriorates hot workability.
30%.

Te: 0.30% or less Te, like S and Se, improves the machinability by forming inclusions. However, excessive addition saturates the effect and deteriorates hot workability, so the upper limit was made 0.30%. Pb: 0.30% or less Pb exists alone or dispersed in a form adhering to inclusions in steel, and is dissolved during cutting to have an effect as a lubricant and an effect of improving chip crushability. Excessive addition causes saturation of the machinability improving effect and impairs hot workability, so the upper limit was made 0.30%.

[0022]

Embodiments of the present invention will be specifically described below. In a vacuum induction furnace, 19Cr ferritic stainless steel was melted. After adjusting this component except Si, Mn, Al, and Ca, the composition of the chemical component added by changing the addition amount of the above elements variously is shown in Table 1. 160m diameter of these components
The m ingot was forged at 1200 ° C. to a diameter of 15 mm, cut out in parallel with the forging direction, and the form of inclusions was observed using an optical microscope and an electron microscope. Arbitrarily select ten oxide-based inclusions having a length of 10 μm or more in the longitudinal direction, and use the major-minor diameter ratio (major axis / minor axis) as an index to indicate the minor axis length and the degree of deformation of the oxide-based inclusion. Was measured. Table 2 shows the results. Further, the composition of the same oxide was investigated using an energy dispersive X-ray analyzer (EDX).

Further, a forged material having a diameter of 15 mm was subjected to 800 ° C.
After annealing for 2 hours, the sample was subjected to corrosion resistance and outgassing tests. Corrosion resistance test is 20 ° C at 90% relative humidity
The temperature was raised and lowered from −2 hours to 50 ° C. for 4 hours by repeating 20 cycles, and the state of rusting on the test piece surface was observed. For outgas test, test specimen, pure water and silver plate at 80 ℃
In 20 hours after the change in the color of the silver plate,
The generation amount of hydrogen sulfide gas was evaluated. That is, hydrogen sulfide reacts with silver to change the silver plate to black-brown, and the degree of discoloration increases with an increase in the amount of hydrogen sulfide outgas.

In addition, a drill piercing test was performed as a machinability evaluation. Thrust 414N, peripheral speed 18.7m / m using SKH51 straight shank drill with a diameter of 5mm
Under a constant thrust of constant in and a constant peripheral speed, a hole was drilled perpendicular to the cross section of the test piece having a diameter of 15 mm, and the machinability was evaluated by the time (seconds) required to reach a depth of 10 mm. That is, the better the machinability, the shorter the drilling time.

[0025]

[Table 1]

[0026]

[Table 2]

Table 1 shows the results of various tests. N shown in Table 1
o1 to 8 are examples of the present invention, and Nos. 9 to 16 are comparative examples. Nos. 1 to 8 have excellent corrosion resistance and outgassing properties while ensuring good machinability because the compositions of sulfide and oxide are appropriately controlled, and the oxide is stretched, so that the surface properties Is also good. On the other hand, Comparative Example No. 9
Is based on a conventional steelmaking method, and since Al and Ca are not added, the generated oxide is close to SiO ₂ , remains in a lump without being stretched by hot working, and reduces surface finish. When Al is excessively added as in Comparative Example No. 10,
As the amount of O is greatly reduced, the oxide produced is Al ₂ O ₃
Become the subject. For this reason, although the amount of oxides decreases, the machinability deteriorates due to the poor sulfide shape.

When only Ca was added as in Comparative Example No. 11, the generated oxide was mainly CaO—SiO ₂ ,
It remains without being stretched by hot working. Further, in the case of Comparative Example No. 12 in which the amount of Si was small, the generated oxide was Cr ₂ O ₃ −
MnO, which remains without deformation due to its hardness. Even in the case of Comparative Example No. 13 containing appropriate amounts of Al and Ca, if the amount of Si is small, the concentration of Si in the oxide decreases, and the composition does not become optimal for hot ductility. Comparative Example No. 14 had a high Mn and an appropriate oxide composition, but had a high Mn concentration in the sulfide.
Poor corrosion resistance and outgassing properties, no advantage over conventional steel. Comparative Example No. 15 has a small amount of S and has no effect of improving machinability. Comparative Example No. 16 had an appropriate oxide composition,
Since the amount of O is small, the sulfide is significantly elongated by hot working, so that the machinability is poor.

[0029]

As described above, the present invention has an extremely excellent effect that it is possible to provide a material having excellent machinability with improved corrosion resistance and outgas characteristics by controlling the sulfide form. .

Claims (3)

[Claims] C: 0.50% or less, Si: 0.15 to 1.00%, Mn: 1.00% or less, S: 0.05 to 0.50%, Cr: 10 0.00 to 30.00%, Al: 0.005% or less, O: 0.005 to 0.040% Sulfide having a balance of Fe and inevitable impurities and having a major axis of 10 μm or more in steel. At least 90% or more of the number-based inclusions have a weight ratio Cr / Mn of Cr and Mn contained in the sulfide-based inclusions of 1 or more and oxide-based inclusions present in steel. Among them, the composition of a spindle-shaped or spherical one having a long diameter, and a linear one having a length of at least 10 μm and at least 50% or more in terms of the number ratio of the oxides, is represented by weight%.
And 30% ≦ SiO ₂ ≦ 70%, 10% ≦ MnO ≦ 30
%, 20% ≦ Al ₂ O ₃ + Cr ₂ O ₃ ≦ 50%, a free-cutting stainless steel having excellent outgassing characteristics. 2. In% by weight, C: 0.50% or less, Si: 0.15 to 1.00%, Mn: 1.00% or less, S: 0.05 to 0.50%, Cr: 10 0.00 to 30.00%, Al: 0.005% or less, Ca: 0.0005 to 0.01%, O: 0.005 to 0.040% The balance consists of Fe and unavoidable impurities, and steel For a sulfide-based inclusion having a major axis of at least 90% in number ratio of at least 10 µm present therein, the weight ratio Cr / Mn of Cr and Mn contained in the sulfide-based inclusion is 1 or more, In addition, among oxide-based inclusions present in steel, the composition of a spindle-shaped or spherical-shaped oxide having a long diameter, and a linear-shaped oxide-based inclusion having a length of 10 µm or more and at least 50% or more by number of oxides. But weight%
And 20% ≦ SiO ₂ ≦ 70%, 10% ≦ Al ₂ O ₃ +
A free-cutting stainless steel having excellent outgassing characteristics, wherein Cr ₂ O ₃ ≦ 50% and 10% ≦ CaO ≦ 50%. 3. The method according to claim 1, further comprising: Ni: 20.00% or less; Mo: 3.00% or less; Cu: 4.00% or less; Ti: 1.00% or less in terms of% by weight. Nb: 1.00% or less, N: 0.50% or less, B: 0.02% or less, Se: 0.30% or less, Te: 0.30% or less, Pb: 0.30% or less Free-cutting stainless steel with excellent outgassing characteristics, characterized by containing one or more species.