JPH05339680A - Free cutting austenitic stainless steel improved in corrosion resistance and its manufacture - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance of stainless steel while its high machinability is maintained. CONSTITUTION:The objective austenitic stainless steel contains <=0.08% C, <=1.0% Si, 0.1 to 0.5% Mn, 16 to 20% Cr, 6 to 12% Ni, <=2.0% Mo and 0.05 to 0.2% S and satisfies %Mn/%S<=2.0, <=100ppm O and <=200ppm N, and the balance substantial Fe. At the time of its manufacture, finish rolling in hot rolling is executed preferably at <=1000 deg.C and >=20% reduction of area.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic stainless steel which has good machinability during machining and is suitable for applications requiring high corrosion resistance during use.

[0002]

2. Description of the Related Art Generally, stainless steel is not so good in machinability because it is sticky and has low thermal conductivity, and a tool and chips are likely to adhere to each other during cutting. In particular, austenitic stainless steel has a high degree of work hardening, which makes cutting difficult. As an improvement of this point, a certain amount of S
Free-cutting stainless steel containing (for example, SUS30
There is 3).

However, if the S content of stainless steel is increased, the corrosion resistance decreases. When using wood screws, bolts, nuts, valves, shafts and other machine parts manufactured by cutting in a corrosive atmosphere, the inherent corrosion resistance of stainless steel cannot often be expected.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide an S-containing free-cutting austenitic stainless steel having improved corrosion resistance and having both good machinability and corrosion resistance. It is also included in the object of the present invention to provide a method suitable for producing a free-cutting austenitic stainless steel having improved corrosion resistance.

[0005]

The free-cutting austenitic stainless steel with improved corrosion resistance of the present invention has a basic alloy composition of C: 0.08% or less and Si: 1.0% by weight.
Hereinafter, Mn: 0.1 to 0.5%, Cr: 16 to 20%,
Ni: 6 to 12% and Mo: 2.0% or less in addition to S: 0.05 to 0.2%,% Mn /% S ≦ 2,
It has an alloy composition of 0: 100 ppm or less, N: 200 ppm or less, and the balance being substantially Fe.

This free-cutting austenitic stainless steel is
In addition to the above basic alloy composition, one or two of Co: 0.3% or less and Cu: 2.0% or less can be contained.

This stainless steel also has, in addition to the above basic alloy composition, Se: 0.25% or less and Ca:
One or two of 0.02% or less can be contained.

Of course, these two optional alloying groups can be used together.

The method for producing free-cutting stainless steel with improved corrosion resistance of the present invention includes a step of hot-rolling stainless steel having any of the above alloy compositions, and the finish rolling step is performed at a temperature of 1000 ° C. or less. It is characterized in that it is carried out under the condition that the surface ratio is 20% or more.

[0010]

The reason why the alloy composition of the free-cutting austenitic stainless steel with improved corrosion resistance of the present invention is determined as described above is as follows.

C: 0.08% or less Since C forms a carbide of Cr and deteriorates corrosion resistance,
The lower the content, the better. However, it is disadvantageous in terms of cost to make extremely low carbon, so 0.08% was made the upper limit.

Si: 1.0% or less Si comes in as a deoxidizing agent, but if the content is high, the hot workability is lowered, so the limit is 1.
We set 0%.

Mn: 0.1 to 0.5% Mn acts as a deoxidizing agent and a desulfurizing agent, and serves as a sulfide inclusion MnS in steel to contribute to machinability, but MnS has corrosion resistance. Since it lowers, it is better to keep Mn in a small amount. From the viewpoint of deoxidizing effect, addition of 0.1% or more is required, and there is a limit of 0.5 that corrosion resistance does not decrease significantly.
%.

Cr: 16-20% In order to obtain sufficient corrosion resistance as stainless steel, Cr is required.
Must be present at 16% or more. The larger the amount is, the higher the corrosion resistance is, but if it exceeds 20%, ferrite is likely to be generated and the hot workability is impaired, so this value was made the upper limit.

Ni: 6-12% As is well known, Ni is an austenite-forming element and plays a part in corrosion resistance. If it is less than 6%, the structure is unstable and martensite is likely to be formed. A large amount of the austenite structure can be obtained more stably, but it does not make sense to meet the price even if the amount is too large.

Mo: 2.0% or less Effective for improving corrosion resistance. If it is too large, it is not preferable for hot workability. Therefore, the addition amount is 2.0% or less.

% Mn /% S ≦ 2 This ratio is preferably small from the viewpoint of corrosion resistance. If it exceeds 2, the ratio of MnS in the sulfide-based inclusions becomes high, and the corrosion resistance is significantly reduced.

O: 100 ppm or less It is preferable that the amount of O is low for both corrosion resistance and hot workability. As a limit that can be relatively easily achieved by ordinary refining and does not hinder corrosion resistance and hot workability, 100
Defined ppm.

N: 200 ppm or less It is generally said that N improves the corrosion resistance. However, when the amount of N in sulfur free-cutting steel is increased, the corrosion resistance is rather deteriorated. At the same time, it was found that hot workability was severely deteriorated and neither rolling nor forging was possible. The allowable limit without such adverse effects is 200 ppm.

The two groups of optionally added alloy components have the following functions, respectively, and their compositions are limited for the following reasons.

Co: 0.3% or less and Cu: 2.0%
Either one or two of the following will contribute to further improvement in corrosion resistance, so they are added as desired. This action of Co is based on the formation of stable double carbides. If the respective upper limits are exceeded, the hot workability deteriorates, so the addition amount up to 0.3% or 2.0% is selected.

Se: 0.25% or less and Ca: 0.0
When 1% or 2% or less of 2% or less is desired to further improve machinability, one or both are added. From the consideration of not impairing the hot workability, the upper limit of each addition amount is set as above.

In the manufacturing method of the present invention, the finishing step of hot rolling is carried out under the conditions of a temperature of 1000 ° C. or less and a surface reduction rate of 20% or more in order to refine the sulfide inclusions. Mn content is kept to 0.5% or less, and% M
The amount of MnS in the inclusions contained in the free-cutting austenitic stainless steel of the present invention is relatively small due to the alloy composition in which the ratio of n /% S is kept to 2 or less. MnS tends to be easily deformed or aggregated at high temperatures, but by lowering the finish rolling temperature and applying a strong deformation above a certain limit, the MnS particles become finer and remain in the steel as they are. Exists in. This makes it difficult for the corrosion starting from the MnS particles to proceed, which helps improve the corrosion resistance.

[0024]

EXAMPLE Steels having the alloy compositions shown in Table 1 were melted, ingot-cast, then slab-rolled and then hot-rolled. Table 2 shows the temperature and surface reduction rate in the finish rolling step of hot rolling.

A machinability test piece was sampled from the rolled material, a cutting test (longitudinal turning) was performed under the conditions shown in Table 3, and the amount of tool wear was measured to evaluate the machinability. At the same time, a corrosion resistance test piece was sampled, a corrosion test was performed by immersing the test piece in 5% sulfuric acid for 6 hours, and the weight loss rate during the corrosion test was measured to evaluate the corrosion resistance. The results are shown in Table 2.
Shown together with.

As is clear from the data in Table 2, the stainless steel having the alloy composition according to the present invention is superior in both machinability and corrosion resistance to those having an alloy composition outside the scope of the invention. Those containing Co or Cu have high corrosion resistance and S
Machinability is even better for those containing e and Ca. Also, those with finish rolling conditions selected according to the invention have better corrosion resistance than those without.

Table 1 Composition of composition (% by weight, balance Fe) No. C Si Mn Cr Ni Mo S Co / Cu Se / Ca% Mn /% S Example 1 0.07 0.52 0.16 18.21 7.43 1.15 0.08 2.0 2 0.06 0.54 0.20 18.50 8.00 1.30 0.11 1.8 3 0.04 0.56 0.22 18.34 8.25 0.80 0.15 1.5 4 0.03 0.51 0.34 18.72 8.50 0.20 0.17 2.0 5 0.05 0.54 0.34 18.80 8.75 0.75 0.18 Cu 1.50 1.9 6 0.06 0.53 0.24 18.95 8.23 1.17 0.16 Cu 1.70 1.5 7 0.07 0.54 0.13 18.35 8.80 1.75 0.13 Co 0.18 1.0 Cu 1.40 8 0.07 0.53 0.20 18.42 8.90 1.35 0.17 Cu 1.35 1.2 9 0.06 0.52 0.09 18.51 9.01 1.65 0.09 Co 0.25 1.1 Cu 1.71 10 0.05 0.50 0.10 18.44 9.25 1.20 0.10 Cu 1.40 1.0 11 0.04 0.53 0.08 18.73 9.80 1.25 0.09 Cu 1.32 0.9 12 0.05 0.54 0.20 18.67 10.00 1.35 0.12 Cu 1.10 Se 0.20 1.7 13 0.01 0.55 0.21 18.59 10.20 1.33 0.14 Cu 0.90 Se 0.18 1.5 Ca 0.01 14 0.03 0.52 0.30 18.73 11.50 1.20 0.19 Co 0.18 Ca 0.02 1.6 Cu 0.80 15 0.07 0.45 0.25 18.25 6.50 1.05 0.18 Co 0.20 Se 0.15 1.4 Cu 1.00 16 0.06 0.40 0.31 18.35 7.00 1.10 0.17 Co 0.20 Se 0.17 1.8 Cu 1.20 Ca 0.01 17 0.07 0.56 0.21 18.23 8.15 0.95 0.17 1.2 18 0.05 0.55 0 .33 18.75 8.37 0.75 0.18 Cu 1.48 1.8 19 0.05 0.56 0.23 18.70 9.98 1.28 0.12 Cu 1.15 Se 0.19 1.9 Comparative example 21 0.08 0.90 0.70 18.21 8.23 0.80 0.15 4.7 22 0.09 1.00 1.00 18.50 8.70 1.20 0.22 4.5 23 0.10 1.10 1.20 18.70 9.00 1.50 0.25 4.8 24 0.10 1.00 1.80 18.45 11.50 2.20 0.30 6.0 25 0.10 1.20 2.00 18.23 12.50 3.00 0.28 7.1 table 2 finish rolling conditions machinability corrosion resistance No. Temperature Surface reduction rate Tool wear Corrosion weight loss (° C) (%) (mm) (g / m ₂ · h) Example 1 900 30 0.8 0.16 2 950 25 25 0.9 0.15 3 900 230 .8 0.13 4 870 24 0.9 0.15 5 850 23 0.9 0.12 6 900 25 25 0.8 0.11 7 900 28 0.8 0.8 0.10 8 920 27 0.9 0.13 9 910 25 0.9 0.12 10 900 900 25 0.9 0.14 11 880 20 0.9 0.12 12 870 22 22 0.6 0.13 13 870 23 0.7 0.7 0.15 14 900 25 0. 8 0.11 15 900 26 26 0.6 0.10 16 900 25 0.5 0.5 0.15 17 1050 15 1.0 1.0 0.17 18 1050 15 1.1 1.1 0.15 19 1050 15 0.8 0.8 0.15 Comparison Example 21 100 15 1.0 0.30 22 1100 15 0.9 0.32 23 1100 15 1.1 0.29 24 1100 20 0.9 0.28 25 1100 20 1.2 0.35 Table 3 cutting test conditions Engineering tools Carbide P10 feed 0.08 mm / rev. Depth of cut 1 mm cutting length 22 mm cutting speed 30 m / min. Machinability evaluation
Tool wear amount

[0028]

The free-cutting austenitic stainless steel of the present invention has a high corrosion resistance despite containing S in an amount effective for improving the machinability. Therefore, the high machinability of free-cutting stainless steel can be enjoyed when manufacturing bolts, nuts and various machine parts, and good corrosion resistance can be exhibited when these products are used. Those containing an optional alloying component show a correspondingly further improvement in corrosion resistance or machinability.

If this free-cutting austenitic stainless steel is manufactured under the finish rolling conditions according to the present invention, high corrosion resistance can be secured.

Claims (5)

[Claims] 1. C: 0.08% or less by weight, Si:
1.0% or less, Mn: 0.1 to 0.5%, Cr: 16 to
20%, Ni: 6 to 12% and Mo: 2.0% or less in addition to S: 0.05 to 0.2%,% Mn /% S.
A free-cutting austenitic stainless steel with improved corrosion resistance, having an alloy composition of ≦ 2, 0: 100 ppm or less, N: 200 ppm or less, and the balance being substantially Fe. 2. In addition to the alloy composition according to claim 1, C
Free cutting austenitic stainless steel containing one or two of o: 0.3% or less and Cu: 2.0% or less. 3. In addition to the alloy composition according to claim 1, S
Free-cutting austenitic stainless steel containing one or two of e: 0.25% or less and Ca: 0.02% or less. 4. In addition to the alloy composition according to claim 1, C
O: 0.1 to 0.3% and Cu: 2.0% or less of one or two, and Se: 0.25% or less and Ca:
Free-cutting austenitic stainless steel containing one or two 0.02% or less. 5. A finish rolling process including a step of hot-rolling the stainless steel having the alloy composition according to claim 1, wherein the finish rolling process is performed at a temperature of 1000 ° C. or less and a surface reduction ratio of 2.
A method for producing free-cutting stainless steel with improved corrosion resistance, which is characterized in that it is carried out under a condition of 0% or more.