JPH08296001A - Austenitic stainless steel for hot water equipment - Google Patents

Abstract

PURPOSE: To produce an austenitic stainless steel having workability and corro sion resistance in the weld zone as that for hot water equipment. CONSTITUTION: The austenitic stainless steel for hot water equipment having a compsn. contg., by weight, 0.02 to 0.08% C, 2.0 to 3.5% Si, 0.4 to 2% Mn, <=0.025% P, <=0.003% S, 10.0 to 12.0% Ni, 17.0 to 20.0% Cr, 0.5 to 1.0% Mo, 2.0 to 2.5% Cu, 0.05 to 0.15% N and 0.0010 to 0.0040% Ca, and the balance Fe with inevitable impurities is obtained.

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 for hot water supply equipment heated by electricity, gas, kerosene or the like.

[0002]

2. Description of the Related Art Since SUS304 has excellent weldability and workability, its application is wide-ranging.
However, the temperature is relatively high, such as warm water obtained by heating clean water, and Cl
^- In the chloride environment containing ions, which may cause stress corrosion cracking. For example, in electric water heaters, SUS444 of ferritic stainless steel is the mainstream,
Austenitic stainless steels with good workability are required in order to reduce the processing cost of the can body.

The quality required for hot water supply equipment is stress corrosion cracking resistance in addition to workability and crevice corrosion resistance. The inventors of the present invention have made a case of corrosion of SUS304 in an electric water heater and S
From the fact-finding survey of US444, cracking of SUS304 occurs only in the gaps such as the welding gaps.
It was found that S444 has sufficient corrosion resistance except in special cases where repair welding is performed, and the point of corrosion resistance is crevice corrosion and stress corrosion cracking in the weld gap, which is equivalent to SUS304. Equipped with S crevice corrosion resistance
With the goal of being equivalent to US444 and considering that the hot water temperature is around 90 ° C, the stress corrosion cracking resistance is
The goal was to be immune to cracking up to 110 ° C. The Cl ^- ion concentration at this time was set to 200 ppm, which is the upper limit of the water quality standard for domestic clean water.

The stress corrosion cracking property is evaluated according to JIS G057.
The 42% magnesium chloride and concentrated sodium chloride solutions specified in 6 are generally used, but in these tests, stress corrosion cracking occurs from the free surface, and cracks in the above-mentioned low-concentration neutral chloride environment occur. Does not match. In order to basically improve the stress corrosion cracking resistance of austenitic stainless steel, an evaluation method that simulates the crack initiation behavior in a low-concentration chloride environment corresponding to the actual environment is essential.

The inventors of the present invention are capable of reproducing crevice corrosion from a crevice corrosion site due to residual welding stress by causing crevice corrosion in a laboratory in a low concentration chloride environment using spot welding test pieces. Found. Hereinafter, the stress corrosion cracking resistance was evaluated by a method of immersing a spot welding test piece in a solution prepared by adjusting 200 ppm of Cl ⁻ ion, which is the upper limit of the reference value of domestic water, with sodium chloride. This method can reproduce stress corrosion cracking in a naturally immersed state.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an austenitic stainless steel for hot water supply equipment which has workability comparable to SUS304 and has improved corrosion resistance in the weld gap.

[0007]

The present inventors have optimized the amounts of Ni, Cr and Mo based on SUS304 and added Si and Cu in order to improve the corrosion resistance of the weld gap of austenitic stainless steel. In addition, the present invention has been completed by further researching the addition of Ca and the like in order to improve hot workability.

That is, the gist of the present invention is to
% By weight, C: 0.02 to 0.08%, Si: 2.0
-3.5%, Mn: 0.4-2%, P: 0.025% or less, S: 0.003% or less, Ni: 10.0-12.0.
%, Cr: 17.0 to 20.0%, Mo: 0.5 to 1.
0%, Cu: 2.0 to 2.5%, N: 0.05 to 0.1
5%, Ca: 0.0010 to 0.0040% is contained,
It is an austenitic stainless steel for hot water supply equipment, which has excellent workability and is characterized by the balance Fe and unavoidable impurities and which has excellent corrosion resistance in the weld gap.

[0009]

The present inventors have found that the workability is similar to that of SUS304, the crevice corrosion resistance of welds is similar to that of SUS444, and that austenitic stainless steel that does not cause stress corrosion cracking up to 110 ° C at 200 ppm of Cl ^- ion adjusted with salt. With the aim of adding Cu and Si to SUS304 series stainless steel, and examining the influence of other elements in that case in detail, the following findings were obtained.

Regarding the corrosion resistance, 30 × 30 mm2 sheets were taken from a cold rolled annealed sheet having a plate thickness of 1 mm, and the test pieces in which the central portions were spot welded were subjected to a corrosion test in a solution of 200 ppm of Cl ⁻ ions adjusted with salt. went. The temperature was kept at 110 ° C., and the autoclave was used for immersion for 2 weeks. Figure 1 shows 10% Ni-2.5% Si-2.5%
Cr, Mo on crevice corrosion resistance and workability of Cu steel
It is a diagram showing the relationship between the amounts, and it can be seen that a Cr amount of 17.0 or more and a Mo amount of 0.5% or more are required to obtain the same corrosion resistance as SUS444. On the other hand, the workability deteriorates due to the increase of the Cr and Mo contents, and in order to secure the workability comparable to SUS304, the Cr content is 20.0% or less and the Mo content is 1.0%.
It turns out that the following needs to be done.

FIG. 2 shows 18% Cr-10% Ni-0.7%.
For Mo steels, Si and Cu affect stress corrosion cracking resistance
It shows the effect of quantity. The amounts of Si and Cu that do not cause stress corrosion cracking are 2.0% or more. next,
The reason for defining the component range of each alloy element will be described. C: C does not affect the stress corrosion cracking resistance, but if it is made high, Cr carbides are precipitated during welding and intergranular corrosion is likely to occur, so it is preferably low. Therefore, the content is set to 0.02 to 0.08%, which can be easily reached by an ordinary steelmaking method.

Si: Si improves stress corrosion cracking resistance by coexistence with Cu. The stress corrosion cracking resistance improves with the amount of Si added up to 3.5%, but if added in excess of this amount, there is no great effect, and the target stress corrosion cracking resistance up to 110 ° C is obtained. 2% or more is required, so it is set to 2 to 3.5%. Mn: Mn requires 0.4% or more for deoxidation and desulfurization during melting, but if added in a large amount, it deteriorates crevice corrosion resistance, so the upper limit is 2% or less within the specification range of SUS304. And

It is known that P: P hinders the stress corrosion cracking resistance, and a lower value is desirable. In the present invention, Si,
Since the adverse effect of P is reduced by the combined addition of Cu, the content is set to 0.025% or less.
Since the target stress corrosion cracking resistance may be within the range that can be achieved by a normal steelmaking method, it is preferably 0.020 to 0.0
25% is good.

S: S does not affect the stress corrosion cracking property, but is harmful to hot workability and crevice corrosion.
It was 003% or less. Cr: Cr, together with Mo, is an essential element for maintaining the corrosion resistance, but since the workability deteriorates with the addition amount, it was set to 17.0 to 20.0%. Ni: Ni does not affect the stress corrosion cracking resistance in the amount of SUS304 level, but is made 10.0 to 12.0% in order to maintain the austenite phase and ensure workability.

Mo: Mo improves crevice corrosion resistance, but deteriorates workability. S in the above amounts of Cr and Ni
In order to obtain crevice corrosion resistance comparable to US444, 0.5% or more is required, while if it exceeds 1%, workability comparable to SUS304 cannot be obtained, so the upper limit was made 1.0%. Cu: Cu has no effect unless added to the stress corrosion cracking resistance in an amount of about 2.0% or more, and if Si is contained in an amount of 2.0% or more, it has no effect. 2.
It was set to 5%.

N: N has no effect on the stress corrosion resistance and improves the crevice corrosion resistance, but on the other hand it hardens the steel and deteriorates the workability, so it was set to 0.05 to 0.15%. Ca: 0.0010 to 0.0040% in order to compensate for the deterioration of hot workability due to the addition of Si, Mo, and Cu within the above S amount range and to secure hot workability comparable to SUS304.

Due to the combination and limitation of the above steel components,
An austenitic stainless steel having workability, crevice corrosion resistance, and stress corrosion cracking resistance required for hot water supply equipment can be obtained.

[0018]

EXAMPLES The present invention will be described based on examples. Table 1 shows the components and evaluation results of the steel of the present invention (example steel) and the comparative steel.
The results are shown in Table 2 (continued from Table 1). After melting in a vacuum melting furnace,
Casting a 45 kg ingot and hot rolling it to a plate thickness of 5 m
After making a hot-rolled sheet of m, it was held at 1150 ° C. for 1 minute and then heat-treated by water cooling. Subsequently, after pickling, a cold-rolled sheet having a thickness of 1 mm was subjected to a heat treatment of heating at 1100 ° C. for 1 minute and then water cooling. The hot workability was evaluated by observing the edge cracking condition at the end of the hot rolled sheet, and the workability was evaluated by a tensile test. The corrosion resistance was evaluated by using a test piece in which two pieces of the same steel having a 30 mm square were stacked and spot-welded in the central part,
l ^- was carried out by immersing two weeks to 200ppm aqueous solution. The evaluation of crevice corrosion was performed by punching out the nugget portion of spot welding, opening the crevice surface, and comparing the degree of corrosion with SUS444. For stress corrosion cracking, the presence or absence of cracking was evaluated with a microscope of 500 times.

[0019]

[Table 1]

[0020]

[Table 2]

The steel types Nos. 1 to 5 of the steel of the present invention have workability of S.
US304 equivalent and crevice corrosion resistance equivalent to SUS444,
Moreover, it has the quality required for hot water supply equipment, such as no stress corrosion cracking.

[0022]

INDUSTRIAL APPLICABILITY As described above, the steel of the present invention is an austenitic stainless steel having the workability, crevice corrosion resistance and stress corrosion cracking resistance required for hot water supply equipment, and its industrial advantage is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between crevice corrosion resistance and workability and the amounts of Cr and Mo.

FIG. 2 is a diagram showing a relationship between stress corrosion cracking resistance and amounts of Si and Cu.

Claims (1)

[Claims] 1. By weight%, C: 0.02 to 0.08%, Si: 2.0 to 3.5%, Mn: 0.4 to 2%, P: 0.025% or less, S: 0.003% or less, Ni: 10.0 to 12.0%, Cr: 17.0 to 20.0%, Mo: 0.5 to 1.0%, Cu: 2.0 to 2.5%, N: 0.05 to 0.15%, Ca: 0.0010 to 0.0040%, balance F
e and unavoidable impurities, an austenitic stainless steel for hot water supply equipment.