JPH0797664A - High corrosion resistant surface modified stainless steel - Google Patents

Abstract

PURPOSE:To provide a stainless steel excellent in corrosion resistance by controlling surface conditions, without performing high alloying. CONSTITUTION:This steel is a high corrosion resistant surface modified stainless steel in which crystal grains practically containing chromium nitride and crystal grains free from chromium nitride are finely dispersed in the surface layer part of stainless steel. In this high corrosion resistant surface modified stainless steel, the sum total of the surface areas of the crystal grains containing chromium nitride in the surface is regulated to 25-75% based on the whole surface area of the stainless steel, and further, in the crystal grains containing chromium nitride, the thickness of the existence region of chromium nitride can be regulated so that it is 2-20nm from the surface in a depth direction. By controlling the surface composition distribution of the stainless steel, its corrosion resistance can be remarkably improved and product life can greatly be prolonged, and its use under severer corrosive environments can be enabled.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to surface-modified stainless steel with high corrosion resistance. Taking advantage of its corrosion resistance, stainless steel is widely used in various exterior materials for buildings, furniture such as sinks and bath tubs, and household products such as knives and forks. INDUSTRIAL APPLICABILITY The present invention has a high utility value in all fields in which stainless steel is used, but it has a very high utility value particularly in an application field as a building construction material which requires high weather resistance.

[0002]

2. Description of the Related Art F is a typical example of stainless steel.
Ferrite-based SUS430 steel containing e-Cr (main component Fe-18 wt% Cr) as the main component and Fe-Cr-Ni
There is an austenitic SUS304 steel whose main component is (Fe-18 wt% Cr-8 wt% Ni). It is known that these stainless steels have higher corrosion resistance than ordinary steel materials by forming a passivation film containing chromium oxide as a main component on the surface thereof. Therefore, stainless steel is widely used in various exterior materials for buildings, furniture such as sinks and bath tubs, and household products such as knives and forks, which require corrosion resistance.

Particularly, recently, the number of examples used for building materials has been increasing. Building materials are exposed to severe conditions such as wind and rain and temperature cycles, and higher corrosion resistance is required. Bright annealed stainless steel is widely used for this purpose. This is because bright annealed stainless steel prevents the formation of thick oxide scale, which is found in materials that have been pickled and polished, and forms a surface passivation film with more excellent corrosion resistance. Because. However, with the expansion of applications, examples of use in the harsh environment in which rust is a problem is often found even in the above bright annealed stainless steel. Further, the demands on the corrosion resistance of users are becoming strict, and further higher corrosion resistance of stainless steel is required.

In order to solve this problem, a high-alloyed high-grade stainless steel which forms a strong passivation film by increasing the contents of Cr and Ni and the contents of quaternary elements such as Mo, compared with the above materials, For example, YUS170 (main component Fe-20 wt
% Cr-18 wt% Ni), YUS270 (main component Fe-
25 wt% Cr-13 wt% Ni-6 wt% Mo) has been developed and used (YUS is a registered trademark). However, since these highly alloyed stainless steels use a large amount of expensive raw materials such as Cr, Ni and Mo for the mother alloy itself, there is a problem that the raw material cost becomes high and the final product price is high. Therefore, instead of using high alloying,
Based on inexpensive SUS430 and SUS304, there is an increasing demand for the development of inexpensive stainless steel that has better corrosion resistance than conventional materials.

As a method of improving the corrosion resistance of stainless steel which does not depend on high alloying, for example, JP-A-54-126624.
Japanese Patent Laid-Open Publication No. 2003-187242 discloses that by controlling the composition and dew point of hydrogen gas and nitrogen gas in a bright annealing atmosphere,
An annealing method is disclosed in which nitrogen is dissolved in the surface layer of tens of μm of 304 steel to improve the corrosion resistance. However, in this prior art, it is necessary to control the dew point of the annealing atmosphere to be relatively low, and it is necessary to greatly change the composition ratio of hydrogen and nitrogen from the composition of the normally decomposed ammonia decomposition gas. This is not preferable because it causes a large increase in manufacturing cost. Further, even if the amount of solute nitrogen in the steel is unnecessarily increased, the corrosion resistance of the stainless steel cannot be drastically improved, so that it is not an essential improvement at present.

[0006]

SUMMARY OF THE INVENTION In view of the above situation, it is an object of the present invention to provide a stainless steel having excellent corrosion resistance without resorting to high alloying.

[0007]

The conventional high-corrosion-resistant high-grade stainless steel has an increased content of alloy components such as Cr, Ni, and Mo as described above. That is, this is an improvement in corrosion resistance by changing the bulk composition of stainless steel itself. However, since the corrosion phenomenon is an interfacial reaction that occurs at the interface between a solid and a gas or a solid and a liquid, originally, the control of the surface condition of the stainless steel should realize further high corrosion resistance.

From this point of view, the present inventors have controlled the surface crystal state of the stainless steel surface by various treatments, and have investigated the relationship between it and the corrosion resistance in detail.
As a result, it was discovered that it is effective to improve the corrosion resistance by finely dispersing crystal grains substantially containing chromium nitride and crystal grains not containing chromium nitride in the surface layer of stainless steel. As used herein, "substantially" means that chromium nitride is clearly observed in the crystal grains of the former and chromium nitride is observed in the crystal grains of the latter by measuring means such as Auger photoelectron spectroscopy and X-ray photoelectron spectroscopy. It is a state in which it is not observed at all, or is infinitesimal even if it is observed.
Although the mechanism for improving the corrosion resistance according to the present invention has not been clarified so far, the inventors of the present invention have found that the local cell action is substantially generated between the crystal grains in which chromium nitride exists and the crystal grains in which chromium nitride does not exist. It is considered that the corrosion resistance is improved by the occurrence of.

There is no particular limitation on the method for dispersing and forming crystal grains substantially containing chromium nitride and crystal grains not containing chromium nitride on the surface of stainless steel. For example, several methods such as surface nitriding annealing by annealing in an atmosphere containing nitrogen and surface plasma nitriding using nitrogen plasma are effective.

The chemical composition of the stainless steel targeted by the present invention is not particularly limited. It is possible to target all types of steel such as ferritic stainless steel, austenitic stainless steel, and ferritic-austenitic duplex stainless steel.

The high-corrosion-resistant surface-modified stainless steel according to the present invention will be described in detail below with reference to the drawings. Stainless steel is a polycrystalline body, and many crystal grains are present on its surface. Elements such as Fe, Cr, and Ni are present in these crystal grains. In the present invention, in the outermost layer portion of these crystal grains, the substantially corroded crystal grains of Cr and the non-nitrided crystal grains are dispersed to improve the corrosion resistance. Here, the details of the present invention are described with a dew point of −45 ° C.
In a 75% hydrogen + 25% nitrogen atmosphere controlled at 1200 ° C
An example of SUS304 steel subjected to surface nitriding annealing for 15 seconds will be described. The main annealing condition is the same atmosphere as the normal bright annealing, but the main feature is that the annealing temperature is slightly high. The distribution state of the elements near the surface was measured by Auger photoelectron spectroscopy, and the binding state was measured by X-ray photoelectron spectroscopy. The distribution measurement in the depth direction of each measurement method was performed while Ar sputtering was performed.

According to the Auger photoelectron spectroscopy measurement results of the surface of this material, crystal grains having a large amount of nitrogen (hereinafter referred to as crystal grain A
It was found that there are dispersed crystal grains containing less nitrogen (hereinafter referred to as crystal grains B). Ordinary bright annealing treatment (eg 1100 ° C in the same atmosphere as above)
In the SUS304 steel annealed for 20 seconds (hereinafter referred to as the comparative material), there was no difference in the amount of nitrogen due to the crystal grains, and it was present almost uniformly in the vicinity of the surface.

FIG. 1 shows X-ray photoelectron spectroscopic spectra of crystal grains A, crystal grains B, and nitrogen 1s and chromium 2p as comparative materials. In the case of the comparative material, since a difference due to crystal grains in the X-ray photoelectron spectroscopy spectrum is not recognized, a typical example is shown. From this figure, first, in crystal grains A and B, nitrogen 1
It can be seen that the intensity level of s is several times stronger than the former. That is, nitrogen is segregated in some crystal grains. Furthermore, in the former, most of the nitrogen is present in the state of being chemically bound, and in the latter, it is found that the nitrogen is mainly in the adsorptive state. Considering together with the chromium 2p spectrum, it can be seen that chromium nitride is selectively formed in the crystal grain A. On the other hand, in the comparative material, nitrogen almost uniformly exists in the vicinity of the surface in a solid solution state, and chromium mainly forms an oxide. Therefore, the dispersed presence of chromium nitride as in the present invention is not recognized. Further, as a result of investigating the distribution of chromium nitride in the depth direction, it was found that chromium nitride exists only in a region of about 10 nm from the surface inside the crystal grain A.

In summary of the above findings, FIG. 2 shows a schematic diagram of the distribution of chromium nitride near the surface of SUS304 stainless steel according to the present invention. The shaded portion in the figure is the region in which chromium nitride is substantially present. Crystal grains containing chromium nitride (crystal grains A) and crystal grains containing almost no chromium nitride (crystal grains B) are dispersed on the surface. Also,
Chromium nitride existing region in the depth direction is about 10 nm from the surface
Is limited to. SUS that has been subjected to normal bright annealing treatment (for example, 1100 ° C. for 20 seconds in the same atmosphere as above)
In 304 steel, nitrogen exists almost uniformly in the vicinity of the surface in a solid solution state, and chromium mainly forms an oxide,
The presence of dispersed chromium nitride is not recognized.

Next, the surface nitriding-annealed SUS304 is used.
The corrosion resistance of steel and ordinary bright annealed SUS304 steel will be described. The corrosion resistance test was performed by measuring the pitting corrosion generation potential in a 3.5% NaCl aqueous solution at 25 ° C. The voltage was raised at a rate of 20 mV / min. FIG. 3 shows the polarization curve of this measurement. As shown in this figure, in the conventional bright-annealed SUS304 steel, a sharp increase in current is observed at about 0.6 V, and pitting occurs. In contrast, pitting corrosion does not occur in the present invention. The increase in current density from around 1.2 V in this case is due to generation of oxygen gas by electrolysis of water. As described above, according to the present invention, the corrosion resistance of stainless steel can be remarkably improved.

Next, the appropriate range of the surface distribution of these crystal grains will be described. The surface abundance of crystal grains having chromium nitride on the surface of stainless steel can be expressed by the ratio of the total surface area S of crystal grains containing chromium nitride to the total surface area S ₀ of stainless steel: S / S ₀ . In the present invention, S / S ₀ is preferably in the range of 0.25 or more and 0.75 or less. If it is 0.25 or less, the amount of nitrided crystal grains is small, and if it is 0.75 or more, the amount of non-nitrided crystal grains is small. Further, it is desirable that the crystal grains having chromium nitride and the crystal grains having almost no chromium nitride are dispersed so as to be adjacent to each other as much as possible.

Next, the depthwise distribution of chromium nitride in the above-mentioned nitrided crystal grains will be described. It is desirable that the region where chromium nitride exists has a thickness of 2 nm or more and 20 nm or less from the outermost surface layer portion. If the thickness is less than 2 nm, the amount of chromium nitride present is too small to contribute to the improvement of corrosion resistance. On the other hand, in the case of a stainless steel having a nitrided layer of 20 nm or more, the problem of intergranular corrosion occurs, and the corrosion resistance is rather deteriorated.

As described above, the high-corrosion-resistant surface-modified stainless steel according to the present invention is characterized in that crystal grains of chromium nitride that are preferentially generated are dispersed on the surface of the stainless steel. It is essentially different from nitrided stainless steel. For example, Japanese Patent Application Laid-Open No. 5-125558 discloses a technique of "stainless steel having a nitrogen-containing compound and excellent in rust resistance", which is characterized by simply having a nitrogen-containing compound in the surface oxide film. However, it is not a technique for controlling the distribution state of chromium nitride near the surface as in the present invention.

[0019]

EXAMPLES Examples of the present invention will be described below. [Example 1] SUS304 stainless steel and SUS4
Surface nitriding annealing of 30 stainless steel was performed under various conditions, and the relationship between the state of surface nitriding and corrosion resistance was investigated. The nitriding annealing conditions are as follows. Atmosphere: 75% hydrogen-25% nitrogen (dew point -45 ° C) Temperature: 1185 ° C Annealing time: 10 seconds to 100 seconds Under the above conditions, the surface of stainless steel is nitrided in a reducing atmosphere. The degree of nitriding was measured by Auger electron spectroscopy and X-ray photoelectron spectroscopy. The distribution measurement in the depth direction of each measurement method was performed while Ar sputtering was performed.

According to the result of Auger photoelectron spectroscopy measurement on the surface of the present material, crystal grains having a large amount of nitrogen (crystal grain A) and crystal grains having a small amount of nitrogen (crystal grain B) are dispersed on the surface. I found out that The height of the Auger peak of the crystal grain A was about 4 times that of the crystal grain B. In addition, according to the X-ray photoelectron spectroscopy measurement result, the crystal grain A
The integrated intensity of associative nitrogen 1s in the vicinity of 396 eV was 12 times or more that of the crystal grain B.

Further, it has been found that the region where chromium nitride is formed by the main annealing treatment does not change significantly from the surface to the depth direction of about 4 nm to 6 nm. On the other hand, it was found that the amount of crystal grains containing chromium nitride (S / S ₀ ) can be controlled in a wide range by changing the annealing time.
The corrosion resistance test was performed by measuring the pitting corrosion generation potential in a 3.5% NaCl aqueous solution at 25 ° C. The voltage is 20
It was raised at a rate of mV / min.

FIG. 4 collectively shows the above results. On the right side of this figure, the pitting corrosion potential of the conventional stainless steel is shown for comparison. As can be seen from this figure, in SUS304 steel, pitting corrosion did not occur in the region where S / S ₀ was 0.25 or more and 0.75 or less, and high corrosion resistance that could not be obtained with conventional materials was achieved. Further, in SUS430 steel, 0.25 or more and 0.
In the region of 75 or less, highly alloyed stainless steel YUS1
Corrosion resistance comparable to 70 was achieved.

Example 2 SUS304 steel was subjected to surface plasma nitriding treatment in a mixed gas plasma of nitrogen and hydrogen, and the relationship between the state of surface nitriding and corrosion resistance was investigated. The main conditions of this processing are as follows. Plasma nitriding gas: Nitrogen 40 Pa + hydrogen 13 Pa Nitriding temperature: 300 ° C. to 700 ° C. Nitriding time: 30 minutes Under the above conditions, the surface of stainless steel is nitrided in a reducing plasma atmosphere. The degree of nitriding was measured by Auger electron spectroscopy and X-ray photoelectron spectroscopy. The distribution measurement in the depth direction of each measurement method was performed while Ar sputtering was performed.

According to the Auger photoelectron spectroscopy measurement results of the surface of this material, crystal grains having a large amount of nitrogen (crystal grains A) and crystal grains having a small amount of nitrogen (crystal grains B) are dispersed on the surface. I found out that The height of the Auger peak of the crystal grain A was about 3 times that of the crystal grain B. In addition, according to the X-ray photoelectron spectroscopy measurement result, the crystal grain A
The integrated intensity of associative nitrogen 1s in the vicinity of 396 eV was 10 times or more that of crystal grain B. Further, it was found that the amount of crystal grains having chromium nitride (the above S / S ₀ ) did not significantly change from about 0.4 to 0.6 by the main annealing treatment. On the other hand, it was found that the range from the surface where chromium nitride is formed to the depth direction can be controlled in a wide range by changing the nitriding temperature. The corrosion resistance test is
It was performed by measuring the pitting corrosion potential in a 3.5% NaCl aqueous solution at 25 ° C. The voltage was raised at a rate of 20 mV / min. The above results are shown together in FIG. On the right side of this figure, the pitting corrosion potential of the conventional stainless steel is shown for comparison. As can be seen from the figure, in the SUS304 steel, pitting corrosion did not occur in the region where chromium nitride was present in the range of 2 nm to 20 nm, and high corrosion resistance that could not be obtained with conventional materials was achieved.

[0025]

According to the present invention, the corrosion resistance of stainless steel can be greatly improved without resorting to high alloying.
This means that not only the product life of stainless steel can be significantly extended at a low manufacturing cost, but also it can be used in a more severe corrosive environment. INDUSTRIAL APPLICABILITY As described above, the present invention provides a very useful technique for application fields of stainless steel in general, and particularly in the field of building materials, and greatly contributes to the development of these fields.

[Brief description of drawings]

FIG. 1 is an X-ray photoelectron spectroscopy spectrum of a crystal grain A, a crystal grain B according to the present invention, and nitrogen 1s and chromium 2p as comparative materials.

FIG. 2 is a schematic view of chromium nitride distribution on a surface and a cross section near the surface of stainless steel according to the present invention.

FIG. 3 shows pitting potential measurement polarization curves of the present invention and a comparative material.

FIG. 4 is a surface area ratio of crystal grains in which chromium nitride exists (S
/ S ₀ ) and the pitting potential.

FIG. 5 is a graph showing the relationship between the surface depth of a region in which chromium nitride is present and the pitting potential.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Kihira 20-1 Shintomi, Futtsu City, Chiba Shin Nippon Steel Co., Ltd. Technology Development Division

Claims (2)

[Claims] 1. A high corrosion resistant surface modification characterized in that, in a surface layer portion of stainless steel, crystal grains substantially containing chromium nitride and crystal grains not containing chromium nitride are finely dispersed. Stainless steel. 2. The total surface area of crystal grains containing chromium nitride on the surface is 25 with respect to the total surface area of stainless steel.
% Or more and 75% or less, and in the crystal grains in which chromium nitride is present, the thickness of the region in which chromium nitride is present is 2 nm or more and 20 nm or less in the depth direction from the surface thereof. Corrosion resistant surface modified stainless steel.