JP2001192778A - High corrosion resistance free cutting stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high corrosion resistance free cutting stainless steel used for electronic apparatus, precision mechanical parts or the like. SOLUTION: This high corrosion resistance free cutting stainless steel has a composition containing, by weight, 0.003 to 0.12% C, 0.05 to 2.00% Si, 0.05 to 1.00% Mn, 0.05 to 0.60% S and >25.00 to 32.00% Cr, in which the ratio of Mn/S is 1.8 or less, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high corrosion-resistant free-cutting stainless steel used for electronic equipment, precision machine parts and the like.

[0002]

2. Description of the Related Art In general, free-cutting stainless steel excellent in corrosion resistance and machinability is frequently used for precision machine parts. Particularly for electronic devices such as computers, it is important that not only the corrosion resistance but also the generation of hydrogen sulfide gas (outgas) from the material itself is small. Hydrogen sulfide gas is a gas generated by a reaction between sulfide in a material and moisture in the air, and has a problem of corroding copper and silver used in precision machinery.

[0003] On the other hand, the corrosion resistance required for the above-mentioned parts must be exhibited not only in the actual use environment but also in a state where the surface is relatively contaminated during cutting and after processing. That is, the steel material is very easily rusted by chips, cutting oil and the like attached to the surface in the cutting process. The occurrence of rust has a large effect on productivity, such as an increase in the rejection rate of parts, an increase in costs for sorting processed products, and the addition of a surface treatment step.

[0004]

Among the problems described above, in order to suppress outgassing, sulfide MnS usually contained in free-cutting steel is replaced with Cr-rich (Cr, M
It is known that it is effective to change the composition to n) S. On the other hand, in order to improve corrosion resistance, a surface treatment such as a passivation treatment is usually performed in many cases. But,
There is a problem that the cost is increased by performing the passivation process. Further, the passivation treatment has a disadvantage of increasing the amount of outgas.

[0005]

Means for Solving the Problems Accordingly, the present inventors have intensively developed and as a result, they have excellent corrosion resistance at room temperature without the above-mentioned cost increase passivation,
In order to obtain a high corrosion-resistant stainless steel with a small outgassing amount, it has been found that a combined control by lowering the Mn / S ratio and increasing the Cr amount is effective. Also, Mn /
It has been conventionally known that by lowering S, the concentration of Cr in the sulfide is increased, the corrosion resistance of the sulfide itself is improved, and the reaction with moisture in the air is suppressed, thereby reducing outgassing. On the other hand, it was found that the outgassing was affected not only by the sulfide composition but also by the corrosion resistance of the matrix.

That is, it has been found that even when the sulfide composition is the same, the outgas amount tends to be smaller when the matrix has higher corrosion resistance. In particular, when Cr is added in an amount exceeding 25%, the matrix has excellent corrosion resistance, and at room temperature, rusting due to the attachment of chips hardly causes a problem, so that surface treatment such as passivation is not required. That is, it has been found that, when the matrix has excellent corrosion resistance, the hydrogen sulfide generation reaction is suppressed, and the passivation treatment for increasing the outgassing becomes unnecessary, and the outgassing suppression is doubly advantageous.

The gist of the invention is as follows: (1) By weight%, C: 0.003 to 0.12%, Si:
0.05-2.00%, Mn: 0.05-1.00%,
S: 0.05 to 0.60%, Cr: more than 25.00 to 3
A highly corrosion-resistant free-cutting stainless steel characterized by 2.00%, Mn / S ratio of 1.8 or less, and the balance consisting of Fe and inevitable impurities. (2) A highly corrosion-resistant free-cutting stainless steel characterized by containing Mo: 3.00% or less in addition to the component (1).

(3) In addition to the above components (1) and (2), Al: 0.0001-0.020%, Ca: 0.
0005-0.010%, Mg: 0.0005-0.0
High corrosion-resistant free-cutting stainless steel containing one or more of 10%. (4) In addition to the components (1) to (3), O: 0.0
A highly corrosion-resistant free-cutting stainless steel containing 0.05 to 0.040%. (5) In addition to the components (1) to (4), Se: 0.
03 to 0.30%, Te: 0.01 to 0.10%, P
b: 0.03 to 0.30%, Bi: 0.03 to 0.30
%, A high corrosion-resistant free-cutting stainless steel characterized by containing one or more of them.

(6) In addition to the components (1) to (5), Ti: 0.02 to 1.00%, Nb: 0.02 to
1.00%, V: 0.02 to 1.00%, W: 0.02
High corrosion-resistant free-cutting stainless steel comprising one or more of 1.00% and 1.00%. (7) In addition to the above components (1) to (6), N: 0.0
A highly corrosion-resistant free-cutting stainless steel characterized by containing one or two or more of the following components: 0.05 to 0.10% and B: 0.001 to 0.010%.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the composition of the components according to the present invention will be described below. C: 0.003 to 0.12% C is an element that increases strength, but is also an element that deteriorates corrosion resistance. If it is less than 0.003%, the productivity will be significantly reduced during steel refining. On the other hand, if it exceeds 0.12%, hot workability is deteriorated due to formation of an austenite phase, and effective C
Since the corrosion resistance is reduced by decreasing the amount of r, the range is set to 0.003 to 0.12%.

Si: 0.05-2.00% Si is effective as a deoxidizing agent in steel making. However, 0.
If it is less than 05%, the effect as a deoxidizer is weak,
If it exceeds 2.00%, the annealing hardness will be increased and workability will be impaired. Therefore, the range is 0.05 to 2.00.
%. Mn: 0.05 to 1.00% Mn is used as a deoxidizing agent during steelmaking. Further, it reacts with S to form sulfide. However, if it is less than 0.05%, it is insufficient to exhibit the above function, and if it exceeds 1.00%, the Mn concentration in the sulfide becomes too high, impairing corrosion resistance and outgassing properties.

S: 0.05 to 0.60% S forms sulfide and is extremely effective in improving machinability.
The machinability improvement effect increases with the amount, but if less than 0.05%, the effect is small, and if it exceeds 0.60%, the effect is saturated, and hot workability is deteriorated. .
05 to 0.60%.

Cr: more than 25.00 to 32.00% Cr is a basic element of stainless steel and forms an oxide film to provide corrosion resistance. In addition, it is a ferrite forming element,
The corrosion resistance of the matrix is improved according to the amount added. In addition, the improvement of the corrosion resistance of the matrix is effective in reducing the outgas amount even with the same sulfide composition, and the outgas suppression effect is doubled because the passivation treatment can be omitted.
However, if the content is 25.00% or less, these effects are hardly obtained, and if it exceeds 32,000%, in an indoor environment, excessive corrosion resistance, high cost, and brittleness are apt to occur.

Mo: 3.00% or less Mo has an effect of improving corrosion resistance. However, the addition of a large amount increases the cost, so the upper limit was made 3.00%. Al: 0.0001 to 0.020% Al is a strong deoxidizing element, and is used by changing the addition amount according to the purpose at the time of refining. When controlling the composition of the oxide to improve the stretchability, a relatively low concentration of Al is aimed at, but 0.00
If the content is less than 01%, no effect is recognized, and when it is necessary to reduce the oxygen content in the steel by strengthening deoxidation for improving cleanliness, a relatively high concentration is aimed at, but when it exceeds 0.020%, the effect is almost saturated.

Ca: 0.0005 to 0.010% Ca is a strong deoxidizing element, and controls the oxide composition and is added for the purpose of improving oxide ductility and improving machinability. However, less than 0.0005% has little effect,
It is difficult to add more than 0.010%, so 0.010%
% As the upper limit. Mg: 0.0005 to 0.010% Mg is a powerful deoxidizing element, controls the oxide composition, suppresses the sulfide form, and improves the machinability. However, if the content is less than 0.0005%, the effect is small, and
% Is difficult, so the upper limit is 0.010%.

O: 0.005 to 0.040% Most of O becomes an oxide at room temperature and is often treated as an impurity. However, free cutting steel is actively used because the aim is to reduce the hot deformability of sulfide-based inclusions and to maintain a sulfide shape suitable for cutting even small-diameter products. In this case, less than 0.005% is less effective,
If it exceeds 0.040%, the effect is saturated and the amount of oxides as impurities increases.

Se: 0.03 to 0.30% Se, like S, reacts with Mn and Cr to form nonmetallic inclusions, thereby improving machinability. However, if the content is less than 0.03%, the effect is not sufficient, and if it exceeds 0.30%, the effect is saturated and hot workability deteriorates. Te: 0.01 to 0.10% Te has an effect of improving machinability by forming inclusions, like S and Se. However, if the content is less than 0.01%, the effect is small, and if it exceeds 0.10%, the effect is saturated and the hot workability deteriorates.

Pb: 0.03 to 0.30% Pb exists alone in the steel or dispersed in a form adhering to inclusions, and is melted during cutting to improve the effect as a lubricant and to improve the chip crushability. Has the effect. However, the addition of less than 0.03% is insufficiently effective, and the addition of more than 0.30% results in saturation of the machinability improving effect and inhibition of hot workability. Bi: 0.03 to 0.30% Bi, like Pb, is a low melting point metal dispersed in steel and has an effect on improving machinability. However, if it is less than 0.03%, it is insufficient. If it exceeds 0.30%, the effect is saturated and the hot workability is deteriorated.

Ti: 0.02 to 1.00% Ti contributes to improvement of corrosion resistance by forming Ti carbonitride.
However, if the content is less than 0.02%, the effect is weak, and if it exceeds 1.00%, the effect is saturated. Nb: 0.02 to 1.00% Nb is a strong carbonitride forming element, forms Nb carbonitride, suppresses the formation of Cr carbide, and improves corrosion resistance. However, if the content is less than 0.02%, the effect is weak, and if it exceeds 1.00%, the effect is saturated.

V: 0.02 to 1.00% V forms carbonitrides and is advantageous for improving corrosion resistance. However, if the content is less than 0.02%, the effect is small, and if it exceeds 1.00%, the effect is saturated. W: 0.02 to 1.00% W forms carbonitrides and is advantageous for improving corrosion resistance. However, if the content is less than 0.02%, the effect is small, and if it exceeds 1.00%, the effect is saturated.

B: 0.001 to 0.010% B is added as an element for improving hot workability. However, if less than 0.001%, the effect is small.
Addition of more than 0.01% impairs hot workability, so that 0.1% is added.
010% is the upper limit. N: 0.001 to 0.010% N is a solid solution strengthening element, but if its content is less than 0.001%, its effect is not sufficient, and if it exceeds 0.010%, the corrosion resistance and toughness are reduced. 0.001 quantity range
-0.010%.

Mn / S ratio of 1.8 or less By decreasing the Mn / S ratio, the amount of Mn in the sulfide is reduced, while the amount of Cr is increased and the corrosion resistance of the sulfide itself is improved. For this reason, the reaction with the moisture in the air is suppressed, and the outgas is reduced. For this purpose, the Mn / S ratio needs to be 1.8 or less.

[0023]

EXAMPLES In a 100 kg vacuum melting furnace, free-cutting stainless steel having the components shown in Table 1 was melted. The ingot was forged into a steel bar having a diameter of 20 mm, subjected to an annealing treatment of 1073 K × 2 h, and then subjected to the following tests. Table 2 shows the results.
A pitting test piece, a wet test piece and an outgas test piece having a diameter of 12 mm and a length of 21 mm were prepared and subjected to the test. In the pitting corrosion test, the test piece was immersed in a 6% NaCl + 0.5% H ₂ O ₂ solution at 298 K, and evaluated by the corrosion degree after 24 hours. In the wet test, a test piece immersed in a 5% NaCl aqueous solution and then naturally dried was held at 353 K for 90 hours at a relative humidity of 90%, and after the test, the area ratio of rust spots on the test piece surface was calculated and evaluated. In the outgassing test, two types of test pieces were prepared, a test piece having a polished surface and a test piece immersed in a 50% nitric acid solution at room temperature for 1 hour after polishing. The nitric acid immersion simulates a passivation treatment. These test pieces were individually sealed together with pure water and a silver plate at 353 K, and the amount of generated hydrogen sulfide gas was evaluated based on a change in the color of the silver plate after 20 hours. That is, hydrogen sulfide reacts with silver to change the silver plate to brown, and the degree of discoloration increases with an increase in the amount of hydrogen sulfide outgas.

[0024]

[Table 1]

[0025]

[Table 2]

As shown in Table 2, the present invention is No. 1 to 15
These have inherently excellent corrosion resistance as shown in the pitting corrosion test results, and exhibit excellent corrosion resistance even in a relatively contaminated environment as seen in the results of the wet test, so that they can be used as passivation. There is no need for special surface treatment.
In addition, even when passivation is performed, good characteristics can be maintained with little outgassing.

No. 16 is SUS430F, which is more susceptible to rust and more outgas than the inventive steel. Further, when the passivation is performed, the outgas amount increases remarkably. Comparative Example No. 17 contains more Cr and Mo and has a smaller Mn / S ratio than Comparative Example No. 16, but is inferior in both corrosion resistance and outgassing characteristics to the inventive steel.
Comparative Example No. 18 has a small Mn / S ratio and a good corrosion resistance although the Cr content is smaller than that of the invention steel. However, the outgassing characteristics are inferior to those of the invention steel having the same Mn / S ratio. In Comparative Example No. 19, the Cr content was as high as that of the inventive steel, but the Mn / S ratio was high and the corrosion resistance and outgas characteristics were poor.
The same applies to Comparative Example No. 20. As in Comparative Example No. 21, when the Mn / S ratio is high even when S is reduced, the corrosion resistance and outgas characteristics are not improved.

[0028]

As described above, according to the present invention, it has become possible to provide a free-cutting stainless steel excellent in both corrosion resistance and outgassing characteristics.

Claims (7)

[Claims] C. 0.003 to 0.12%, Si: 0.05 to 2.00%, Mn: 0.05 to 1.00%, S: 0.05 to 0. High corrosion resistant free-cutting stainless steel characterized by 60%, Cr: more than 25.00 to 32.00%, Mn / S ratio is 1.8 or less, and the balance consists of Fe and unavoidable impurities. 2. In addition to the component of claim 1, Mo: 3.0
A highly corrosion-resistant free-cutting stainless steel containing 0% or less. 3. The composition according to claim 1, further comprising: Al: 0.0001 to 0.020%, Ca: 0.0005 to 0.010%, Mg: 0.0005 to 0.010%. A highly corrosion-resistant free-cutting stainless steel characterized by containing one or more of them. 4. In addition to the components of claims 1-3, O: 0.
High corrosion-resistant free-cutting stainless steel containing 005 to 0.040%. 5. In addition to the components of claims 1 to 4, Se: 0.03 to 0.30%, Te: 0.01 to 0.10%, Pb: 0.03 to 0.30%, Bi : High corrosion-resistant free-cutting stainless steel containing one or more of 0.03 to 0.30%. 6. In addition to the components of claims 1 to 5, Ti: 0.02 to 1.00%, Nb: 0.02 to 1.00%, V: 0.02 to 1.00%, W : 0.02 to 1.00%. A highly corrosion-resistant free-cutting stainless steel containing one or more of the following. 7. In addition to the components of claims 1 to 6, one or more of N: 0.005 to 0.10%, B: 0.001 to 0.010% High corrosion resistant free cutting stainless steel.