JPH0219447A - Ferritic stainless steel - Google Patents

Abstract

PURPOSE:To form a protective film having superior characteristic dense adhesive strength and to improve corrosion resistance by incorporating specific trace amounts of Cu to a ferritic stainless steel. CONSTITUTION:As a material for heat-transfer tube for heat exchanger in the nuclear power plant, a ferritic stainless steel having a composition containing, by weight, 0.002-0.03% C, <1.0% Si, <1.0% Mn, 16.0-20.05 Cr, <0.5% Mo, <0.6% Ni, <0.030% N, <0.05% Co, 0.01-0.5% Cu, and Ti or Nb by 6X(C+N) to 0.75% is used. Since C and N are fixed by the action of Ti or Nb and the precipitation of metal carbide or nitride in the grain boundaries and its attendant occurrence of intergranular corrosion can be prevented and, further, a dense protective film is formed by the addition of Cu to prevent the corrosion of heat-transfer tube, the safety and reliability of the nuclear power plant can be improved over a long period.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Field of Application) The present invention relates to the improvement of ferritic stainless steel, which is used in heat exchanger tubes for heat exchangers in nuclear power plants, for example, and is highly corrosion resistant.

(Prior art) For example, feed water heaters used in nuclear power plants,
Heat exchanger tubes made of austenitic stainless steel, which have excellent corrosion resistance, have traditionally been used in heat exchangers such as moisture separation heaters. This austenitic stainless steel is highly resistant to corrosion, so it is preferred and often used in ultra-large heat exchangers.

However, austenitic stainless steel.

(In an environment containing 1-ions, there is a high risk of intragranular stress corrosion cracking, and stress corrosion cracking may occur due to Ca- ions when seawater leaks from the condenser. Also, Ni in the alloy component and a trace amount of C coexisting mainly with Ni.
O is eluted and carried into the reactor, where it changes into Co'' and C01l' respectively.These have long half-lives and have problems such as increasing radiation dose, but they have excellent stress corrosion cracking resistance and corrosion resistance. There has been a demand for heat exchanger tube materials with low Ni and Co contents.

(Problems to be Solved by the Invention) In response to this requirement, ferritic stainless steel has recently been found to be promising as a material for heat transfer tubes, and studies have been progressing toward its practical application. Ferritic stainless steel has higher thermal conductivity and better stress corrosion cracking resistance than austenitic stainless steel, and has extremely low Ni and Co contents, so it is a promising material for heat exchanger tubes. However, the corrosion resistance is inferior to that of austenitic stainless steel, and there is a problem in that the cladding, which is mainly composed of Fe, increases. Although Fe has a shorter half-life than CoCo, it is a factor that generates Fe5g and increases the radiation dose, so it must be kept as low as possible, and a solution to this problem is currently being sought.

The present invention was made to improve the above-mentioned drawbacks of conventional materials, and its purpose is to provide a ferritic stainless steel that has excellent corrosion resistance in the high-temperature pure water environment of an atomic couplant. be.

[Structure of the invention]

(Means for Solving Problem W) The present inventors have focused on the fact that corrosion resistance can be greatly improved by adding a small amount of an alloying component to carbon steel, and have developed a ferritic stainless steel to which a small amount of an alloying component has been added. As a result of examining corrosion resistance, it was discovered that addition of Cu improves corrosion resistance, leading to the present invention.

That is, 1 the present invention has C0.002 to 0.0 on a weight basis.
30%, Si 1.0% or less, Mn 1.0% or less, C
r16.0-20.0%, Mo 0.5% or less, Ni
0.6% or less, N 0.030% or less, Ti or Nb 6x (C+N) ~ 0.75%, Co 0.05
It is a ferritic stainless steel containing 0.01 to 0.5% of Cu, with the balance consisting of Fe and incidental impurities.

(Function) Figure 1 of the examples described later shows 18 cr ferritic stainless steel (Examples 1 to 3) with varying Cu content.
It is a graph showing the change over time in corrosion reduction of conventional 18 Cr ferritic stainless steel (comparative example). According to this graph, the corrosion resistance of the material according to the present invention is significantly improved compared to the conventional material, and the effect of Cu on corrosion resistance is significant.

That is, the present invention was achieved based on the new knowledge that the corrosion resistance of ferritic stainless steel is improved by adding a vll amount of Cu, and the drawback of poor corrosion resistance of conventional materials has been improved.

Next, the reason why the chemical composition of the ferritic stainless steel of the present invention is limited as described above will be explained. In the following description, "1%" in the composition is based on weight unless otherwise specified.

C0.002-0.030%: C is an element that significantly increases the stress corrosion susceptibility of ferritic stainless steel in a high-temperature pure water environment, and the lower the amount, the more preferable it is.By setting C to 0.030% or less, The upper limit is set at 0.03% to prevent stress corrosion cracking.
shall be. However, the purpose can be achieved by adding Ti, a stabilizing element, which will be described later.

On the other hand, C is an essential element to improve hardenability, tensile strength and yield strength, and the C content is 0.002%.
If C is less than 0.0%, sufficient strength cannot be obtained. Therefore, the lower limit of C is set to 0.002%.

N 0.030% or less: Like C, N is an element that significantly increases the stress corrosion cracking susceptibility of ferritic stainless steel, and the lower the amount, the better. Since the occurrence of stress corrosion cracking can be prevented by reducing N to 0.030% or less, the upper limit is set to 0.030%.

Ti or Nb 6X (C1N) ~ 0.75%:T
Since i or Nb fixes C and N, it prevents sensitization during welding and has the effect of suppressing intergranular stress corrosion cracking, but if it is less than 6X (C + N)%, a sufficient effect cannot be obtained. The lower limit is set to 6X(C+N)%. On the other hand, 0.
Addition of more than 75% causes a decrease in toughness, so the upper limit is 0.
．． It shall be 75%.

SL 1.0% or less, Mn 1.0% or less: Si and M
n is added as a deoxidizing agent and a desulfurizing agent, respectively;
Both Si and Mn are 1.0% or less. S exceeds the upper limit
Addition of i reduces toughness, and addition of Mn has the effect of increasing hardenability, but excessive addition reduces corrosion resistance.

Cr 16.0-20.0%: Cr is an essential component for improving the corrosion resistance of ferritic stainless steel. 16 against general corrosion in high temperature pure water environment
．． If it is less than 0%, there is insufficient corrosion resistance, so the lower limit is set to 16.
0%. On the other hand, when the Cr content exceeds 20.0%,
The upper limit is limited to 20.0% because there is a risk of 475°C embrittlement that is unique to ferritic stainless steel.

Mo 0.5% or less: Like Cr, if the Mo content increases, 475° C. embrittlement may occur, so the upper limit is limited to 0.5%.

Cu 0.01-0.5%: Cu is a particularly important element among the constituent elements of the ferritic stainless steel according to the present invention, and reduces initial corrosion.
0.0 to form a Cu-containing protective film with good adhesion. Add 1-0.5% O.

Cu exhibits its effect in the presence of Cr, but o, o
Since the effect is not sufficient if only i% is added, the lower limit is set to 0.01%. However, even if it is added in excess of 0.5%, no further effect can be expected.

The upper limit is set to 0.5%.

Ni 0.6% or less: Ni is an austenite phase forming element, and as the content increases, the γ loop width of the Fe-Cr alloy phase diagram widens, making it easier for martensite to transform and form in the weld heat affected zone. , to cause weld cracking and to generate a radiation source of Co, the lower the amount of aN, the better.
By setting i to 0.6% or less, transformation formation of martensite in the weld heat affected zone can be prevented, so the upper limit is set to 0.6%.

Coo, 0.5% or less: Co is an austenite phase forming element like Ni, and is an element that generates C060, which is a radiation source.

The lower the amount, the better, but since there is no effect up to 0.05%, the upper limit is set at 0.05%.

It is preferable that impurities incidentally included when adding the above components and Fe as the main component be as small as possible.

The ferritic stainless steel of the present invention is produced by first mixing and melting various raw material metals under vacuum or atmospheric pressure, and after deoxidizing, obtaining a molten ferritic stainless steel having substantially the above composition, which is then ingot-formed and rolled. By applying this, it becomes a material for heat transfer tubes.

The material made in this manner is subjected to drawing, heat treatment, fin processing, etc. depending on the final use, and is formed into feed water heaters, moisture separation heaters, heat exchanger tubes, etc.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

(Example) Four types of 18Cr ferritic stainless steels having the compositions shown in Table 1 (numbers in the table mean weight percent) were melted and ingot-formed, rolled, and heat treated. , a material test piece was obtained.

(Left below) The material test pieces obtained in this manner were tested under conditions corresponding to the most corrosive areas in an actual moisture separation heater: temperature of 200°C, humidity of 11%, and flow rate of 30 m/see.
Corrosion tests were conducted under steam conditions. The obtained results are shown in FIG. 1. Looking at the results shown in FIG. 1, all of the materials according to the present invention have superior corrosion resistance than the conventional materials (comparative examples). In particular, in Example 3, which has the highest Cu content, the corrosion loss was less than 1/2 that of the conventional material, and the corrosion resistance was significantly improved, indicating that the effect of adding Cu is significant.

In the above examples, the material was evaluated with its bare metal surface, but it goes without saying that the corrosion resistance can be further improved by applying a bleed film treatment, and the manufacturing method is not particularly limited. %N.

〔Effect of the invention〕

As mentioned above, the present invention provides a ferritic stainless steel with significantly improved corrosion resistance compared to conventional materials with the addition of a small amount of relatively inexpensive elements. Therefore, if used for heat exchanger tubes such as feed water heaters and moisture separation heaters, it will have excellent SCC resistance and low Co
At the same time, the amount of Fe granules in the water supply is reduced, which brings about industrially effective effects such as improving the safety and reliability of nuclear power plants and the like.

[Brief explanation of the drawing]

FIG. 1 is a graph showing a comparison of corrosion test results between conventional materials and materials according to the present invention. Agent Patent Attorney Noriyuki Chika Yudo Ken Daishimaru

Claims (1)

[Claims] Weight ratio: C0.002-0.030%, Si1.0% or less, Mn1.0% or less, Cr16.0-20.0%, M
o0.5% or less, Ni0.6% or less, N0.030% or less, Ti or Nb6×(C+N)~0.75%, C
o0.05% or less, Cu0.01-0.5%, balance Fe
and incidental impurities.