JP2002115033A - Free-cutting ferritic stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide free-cutting ferritic stainless steel having excellent machinability and hot workability equal to those of the conventional Pb- containing one without adding harmful Pb. SOLUTION: This free-cutting ferritic stainless steel has a composition containing, by weight, <=0.08% C, 0.05 to 1.00% Si, 0.05 to 3.00% Mn, <=0.05% P, 0.03 to 0.50% S, 0.02 to 0.25% Bi, <=1.50% Cu, <=1.50% Ni, 12.0 to 35.0% Cr, <=0.035% N, <=0.003% B and 0.0030 to 0.0300% O, and the balance substantially Fe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a free-cutting ferritic stainless steel, and more particularly to a free-cutting ferritic stainless steel without adding Pb.

[0002]

2. Description of the Related Art For example, in electronic equipment, automobiles, office equipment and other various fields,
As a part that requires corrosion resistance and is machined by cutting, Cr-containing ferritic stainless steel with low work hardening and sufficient corrosion resistance with excellent machinability has been used to improve productivity and ensure processing accuracy. I have.

In this ferritic stainless steel,
Free-cutting elements such as S and Pb are added to improve machinability. Particularly in recent years, with the increasing number of parts requiring high processing accuracy, rather than adding a certain element alone, More than one kind of element is added in a complex form.

[0004] Here, Pb is an element effective for improving the drill piercing property by its melting embrittlement effect, and high machinability can be secured by adding Pb alone or in combination with S. However, since Pb is harmful to the human body, the content of Pb has been recently restricted, and there is a demand for a steel type that does not add Pb and that ensures high machinability.

[0005]

DISCLOSURE OF THE INVENTION The present invention has been made under the above circumstances, and has excellent cutting performance equivalent to that of conventional free-cutting ferritic stainless steel containing Pb without adding Pb. An object of the present invention is to provide a ferritic stainless steel having properties. The free-cutting ferritic stainless steel according to claim 1 has a C: ≦ 0.08% and a Si: 0.05 to
1.00%, Mn: 0.05 to 3.00%, P: ≤ 0.05%, S: 0.03 to
0.50%, Bi: 0.02 to 0.25%, Cu: ≤ 1.50%, Ni: ≤ 1.50
%, Cr: 12.0-35.0%, N: ≤ 0.035%, B: ≤ 0.003%,
O: 0.0030 to 0.0300%, with the balance substantially consisting of Fe.

According to a second aspect, in the first aspect, one or two types of Mo and W are further added, based on a weight basis, of Mo: 0.50.
~ 3.00%, W: 0.25 ~ 1.50%.

[0007] Claim 3 relates to any one of Claims 1 and 2, wherein one or two of Se and Te are used on a weight basis.
Se: 0.02 to 0.40%, Te: 0.005 to 0.10%.

[0008] In a fourth aspect of the present invention, any one of the first to third aspects further comprises one or two of Ca and Mg at a Ca and Mg content of 0.0005 to 0.0100% by weight. It is characterized by the following.

[0009]

The ferritic stainless steel containing Pb has excellent machinability due to the melting embrittlement of Pb having a low melting point as described above.
However, there is a problem that adversely affects the environment. Therefore, in the present invention
Bi is used in combination with S instead of Pb, that is, Bi and S are added in combination.

[0010] Here, Bi is an element having a low melting point like Pb, and improves the machinability of ferritic stainless steel by its melting embrittlement action. Has such a function
When Bi is added in combination with S, the machinability of ferritic stainless steel can be effectively increased by their synergistic action.

However, the problem is that Bi is an element that greatly reduces hot workability. That is, if Bi is combined with S in an attempt to enhance the machinability, the hot workability necessarily decreases. In order to solve this problem, the inventors of the present invention have conducted research and found that it is effective to regulate the amount of B in steel to a certain level or less.

For example, in austenitic stainless steel, it is said that the addition of B enhances hot workability. Furthermore, in ferritic stainless steel, B
Is believed to be useful for hot workability or at least harmless.

However, these findings are based on processing at a relatively low strain rate (for example, about 50 mm / sec strain rate). However, in actual rolling, etc., the strain rate is higher than this, and when working at such a high strain rate, the study of the present inventor has revealed that B greatly impairs hot workability. It turned out.

Therefore, the present inventor focused on regulating the B content to a certain level or less, and conducted a test.
Even when Bi: 0.02 to 0.25% is added in combination with% to 0.50%, it has been found that the required machinability and hot workability can be secured by regulating B to 0.003% or less. . The present invention has been made based on such knowledge, and is based on the concept of adding B and S as free-cutting elements in combination and regulating B to a low level.

It should be noted that what has been confirmed this time is that the regulation of B under the combination of S and Bi can suppress the reduction in hot workability due to the addition of Bi, for example, those containing no Bi, such as S It is not clear what the effect of regulation of B would be under the combination of Pb and Pb. In this sense, the combined addition of S and Bi and the regulation of B constitute an integral part of the present invention.

In the present invention, if necessary, Mo, W
Can be added in the range of Mo: 0.50 to 3.00% and W: 0.25 to 1.50% (claim 2), whereby the corrosion resistance can be further increased. Further, when the machinability is to be further enhanced, Se and Te are set to 0.02 to 0.40% Se and 0.005%.
It can be added in a range of 0.10% (claim 3).

Alternatively, if necessary, Ca, Mg may be replaced with Ca, M
g: It can be added in the range of 0.0005 to 0.0100% (claim 4). By these, the form of the inclusions can be controlled, and the hot workability can be further improved.

Next, the reasons for limiting each chemical component in the present invention will be described in detail below. C: ≦ 0.08% C precipitates carbides containing Cr and deteriorates properties such as corrosion resistance. It also acts as a solid solution strengthening element and regulates to increase the hardness of the matrix and reduce the machinability. However, since extreme reduction requires cost, the content is set to 0.08% or less. Preferably it is 0.03% or less, more preferably 0.01% or less.

Si: 0.05 to 1.00% Si is effective as a deoxidizing agent, and is contained in the present invention in an amount of 0.05% or more. However, excessive addition not only increases the precipitation of intermetallic compounds, but also increases the matrix strength and deteriorates machinability.
00% or less. However, for parts to which cold working is applied, 0.5
% Is desirable.

Mn: 0.05 to 3.00% Mn is effective for ensuring hot workability, and is contained in an amount of 0.05% or more. However, excessive addition will degrade the corrosion resistance,
3.00% or less.

P: ≤0.05% P segregates at the grain boundary and deteriorates the characteristics. The lower the better, the better, but it will increase the manufacturing cost, so it should be 0.05% or less.

S: 0.03 to 0.50% S is an essential element for ensuring machinability, and is contained in the present invention in an amount of 0.03% or more. Although S forms a compound with Mn or Cr, it is effective to increase the amount of Cr in the compound when corrosion resistance and outgas resistance (generation of hydrogen sulfide gas or the like) are particularly important.

Bi: 0.02 to 0.25% Bi is an essential element for enhancing machinability, and exhibits a synergistic effect on improving machinability in the presence of S. The machinability improves as the amount of addition increases. However, excessive addition degrades hot workability and raises the cost, so the content is made 0.25% or less. The desirable range of Bi is 0.04 to 0.25%, more preferably 0.06 to 0.20%, and most preferably 0.1 to 0.15%.

Cu: ≤ 1.50%, Ni: ≤ 1.50% Cu and Ni improve the corrosion resistance, especially in a reducing acid environment. However, the addition of more than necessary degrades the phase stability and causes an increase in hardness. Therefore, in the present invention, these addition ranges are set to the above ranges.

Cr: 12.0-35.0% Cr is an essential element for ensuring corrosion resistance. However, excessive addition lowers phase stability and deteriorates hot workability.
Therefore, in the present invention, the amount of Cr added is set in the above range. However, a desirable range is 16.0 to 24.0%.

N: ≦ 0.035% N as a solid solution strengthening element increases the hardness of the matrix and deteriorates machinability. In addition, from the viewpoint of manufacturing cost, this is set to 0.035% or less.

B: ≦ 0.003% By regulating the B content to not more than the above content, a decrease in hot workability due to the addition of Bi can be suppressed, and necessary workability can be ensured. The desirable content of B is 0.002% or less.

O: 0.0030 to 0.0300% O must be contained in an amount of 0.0030% or more to ensure machinability. However, when it is contained excessively, an oxide more than necessary is generated, and conversely, the machinability is reduced. Therefore, in the present invention, the content is set to 0.0300% or less.

Mo: 0.50 to 3.00%, W: 0.25 to 1.50% Mo and W can be added as needed to enhance corrosion resistance. However, the addition beyond necessity lowers the hot workability and also raises the cost.

Se: 0.02 to 0.40%, Te: 0.005 to 0.10% In order to improve the machinability, these Se and Te can be added as needed. However, the machinability is improved as the amount of addition increases, but the hot workability is lowered.

Ca, Mg: 0.0005 to 0.0100% Ca and Mg are effective for improving hot workability and machinability. However, excessive addition lowers the cleanliness of the steel, so the above range is set.

[0032]

Next, embodiments of the present invention will be described in detail. Table 1
50 kg of a ferritic stainless steel having the chemical composition shown in (1) was melted in an air induction melting furnace,
Hot forging to 0 mm, then annealing (750-820 ° C)
Processed.

No. H1 of Comparative Example in Table 1 contains S within the scope of the present invention, but does not substantially contain Bi. Conversely, No. H2 of the comparative example contains Bi within the scope of the present invention, but does not substantially contain S.

In Comparative Example No. H3, both S and Bi are contained within the scope of the present invention, but B is contained in a large amount beyond the scope of the present invention. Further, in the comparative example
No. H4 is an example in which both S and Bi are contained within the scope of the present invention, but B is also contained in a large amount beyond the scope of the present invention. No. H5 of Comparative Example
Is a conventional form in which S and Pb are compounded, that is, Pb
This is an example of a free-cut ferritic stainless steel containing steel.

Using the test materials obtained as described above, a high-temperature high-speed tensile test, a hardness measurement, a salt spray test, a turning test, and a drill test were respectively performed. However, the high-temperature and high-speed tensile test was performed using a material as cast. The conditions for each test were as follows.

<Hot workability (high-temperature high-speed tensile test)> First, the temperature was raised to 1200 ° C. in 100 seconds and held for 60 seconds, and then the temperature was raised or lowered to each test temperature at a rate of 10 ° C./second. After holding at each temperature for 60 seconds, 800 mm /
Tensile tests were performed at a strain rate of seconds. The test piece had a parallel portion diameter of 6 mm and a total length of 110 mm. The evaluation of the hot workability was performed based on the aperture value. Table 2 shows 800-1300
The drawing shows the lowest value up to ° C.

<Measurement of Hardness> Using a 20 mm diameter material, the hardness was determined at an average value of five points.

<Corrosion resistance (salt spray test)> JIS Z 2
Based on 371, the test was conducted under the conditions of 35 ° C., 5% NaCl, and salt spray for 96 hours, and a 6-point evaluation of A, B, C, D, E, and F was made between no rust and full rust. Was. However, the order of corrosion resistance is A>B>C>D>E> F. The evaluation of the corrosion resistance (corrosion test) in Table 2 is either A or B. The evaluation of B is a level at which several spot-like spot rusts are generated. Not yet in good condition as rust. The results are obtained by preparing a test piece of φ10 mm × 50 mm from a φ20 mm material, performing polishing using an abrasive of # 320, and then performing a salt spray test. Note that n = 3.

<Machinability (turning test)> As the test conditions, a carbide P20 tool was used, and the cutting speed was 200 m / min.
The test was performed under the conditions of feed 0.15 mm, depth of cut 1.0 mm, and dry type.
m) was evaluated.

<Machinability (Drill test)> Using a tool made of SKH9 material, hole diameter φ5 mm, feed 0.07 mm, hole depth 1
The test was performed under 5 mm, dry condition, and the evaluation was performed at a cutting speed (m / min) at which the tool life (unperforable) was 1000 mm. These results are also shown in Table 2.

[0041]

[Table 1]

[0042]

[Table 2]

As can be seen from the results in Table 2, in the case of the present invention, although Pb was not added, by adding S and Bi in a combined manner, and further by regulating B to a certain level or less. In comparison with the conventional free-cutting ferritic stainless steel containing Pb shown in Comparative Example No. H5, the machinability, corrosion resistance, hardness,
It can be seen that properties similar to hot workability are obtained.

As described above, although the addition of Bi inevitably reduces the hot workability, the example of the present invention can maintain the predetermined hot workability because of the B content. Is regulated below a certain level.

FIG. 1 is a graph showing the effect of regulating B in a complex addition system of S and Bi. In FIG. 1, No. 5 and No. 6 of the example and No. 5 of the comparative example.
H3, No.H4 and No.H5 are picked up and compared. Here, No. H5 contains conventional Pb.

As is clear from FIG. 1, it can be understood that the hot workability is effectively improved by regulating the B content to a low level, particularly in a tensile test at a high strain rate.

Although the embodiment of the present invention has been described in detail, this is merely an example, and the present invention can be configured in variously modified forms without departing from the gist thereof.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a test temperature and an aperture value obtained in one example of the present invention.

Claims (4)

[Claims] C: ≦ 0.08% Si: 0.05-1.00% Mn: 0.05-3.00% P: ≦ 0.05% S: 0.03-0.50% Bi: 0.02-0.25% Cu: ≦ 1.50% Ni: ≦ 1.50% Cr: 12.0-35.0% N: ≦ 0.035% B: ≦ 0.003% O: 0.0030-0.0300% A free-cutting ferritic stainless steel characterized by being substantially composed of Fe. 2. The composition according to claim 1, further comprising one or more of Mo and W in an amount of 0.50 to 3.00% W: 0.25 to 1.50% by weight. Ferritic stainless steel. 3. The method according to claim 1, wherein Se, Te,
A free-cutting ferritic stainless steel, characterized in that one or two of the following are contained by weight: Se: 0.02 to 0.40% Te: 0.005 to 0.10%. 4. The method according to claim 1, further comprising:
A free-cutting ferritic stainless steel characterized in that one or two of a and Mg are contained in a weight basis of 0.0005 to 0.0100% of Ca and Mg.