JPH0379424B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a bright annealed (hereinafter abbreviated as BA) treated ferritic stainless steel sheet that has excellent rust resistance, particularly "stain" resistance. Generally, ferritic stainless steel is expensive
It is inexpensive because it does not contain a large amount of Ni,
In addition, it has excellent stress corrosion cracking resistance, so it is one of the steel materials that is widely used as weather-resistant exterior materials and decorative parts for vehicles. In these descriptions, among ferritic stainless steels, SUS430 series steel, which contains about 17% (hereinafter, % by weight) of Cr, has been frequently used. Although this SUS430 series steel has a fair degree of corrosion resistance in environments where it is used as exterior materials and vehicle parts, it cannot be used as rolled material or ordinary annealed material, for example, when used as decorative parts of vehicles. It did not have sufficient surface properties to be used in applications where decorativeness is required and strict checks are made on surface cleanliness, gloss, color tone, etc. Therefore, when using SUS430 series steel for such purposes, it is usually cold rolled and then subjected to ammonia decomposition gas (H ₂ + N ₂ gas),
Continuous BA treatment in non-oxidizing gas such as H ₂ gas has been used to improve the surface cleanliness and gloss of steel materials. However, even with BA-treated materials that appear to be perfect in terms of surface cleanliness, gloss, and color tone, they cannot even prevent the occurrence of "stains" during actual use. In addition,
"Stain" refers to extremely slight spot-like localized corrosion that occurs on the BA-treated surface, which has excellent surface cleanliness, gloss, and color, when exposed to the atmosphere. Visually, it can be seen as a decrease in the gloss of the steel surface. be identified. Therefore, in order to obtain a BA-treated ferritic stainless steel sheet that has excellent rust resistance and does not cause "stains" when used as an exterior material exposed to the atmosphere, we first Research aimed at elucidating the cause of "stains" on BA-treated stainless steel materials revealed that (a) BA-treated stainless steel sheets have
Various oxide coatings are formed depending on the oxidation potential, dew point, and temperature of the furnace atmosphere during treatment, but the corrosion resistance of the steel plate surface after BA treatment,
In particular, rust resistance and "stain" resistance are greatly affected by the characteristics of the formed oxide coating (scale) and the components in the steel directly beneath it, and are affected by the oxidation potential and dew point in the furnace. (b) In addition, if the oxide coating film becomes too thick and the steel surface becomes noticeably colored after BA treatment, corrosion resistance (rust resistance) also deteriorates. The S component that exists as
MnS combines with Mn to form MnS, and MnS that is exposed on the steel sheet surface has a negative impact on the formation of an oxide coating film in the initial stage of BA treatment, inhibiting the growth of a uniform and stable oxide coating film. (c) Then, BA treatment was applied to ultra-low S ferritic stainless steel under conditions where the dew point was controlled to be low (below -30°C) to form a BA oxide coating film that was invisible to the naked eye. The above (a) to (c) have significantly superior rust resistance and "stain" resistance compared to similar steels treated with BA at high dew points (-30°C or higher). ) We confirmed the phenomenon shown in (). The present inventors focused on these phenomena and developed BA
To improve the rust resistance and "stain" resistance of treated ferritic stainless steel sheets, we not only make the BA oxide coating film with excellent corrosion resistance during BA treatment, but also improve the corrosion resistance of the steel itself. IMMA (Ion
In addition to analyzing various surface coating films using a Mass Micro Analyzer (Mass Micro Analyzer), we also conducted atmospheric exposure tests to evaluate the rust resistance and "stain" resistance of various BA-treated surfaces. As a result of further research in order to realize BA-treated steel sheets with excellent properties, we found that by keeping the amount of impurity elements present in ferritic stainless steel for BA treatment as low as possible, it was possible to improve the corrosion resistance of the steel. By controlling the amount of S in the steel to 0.002% or less, preferably 0.001% or less, S is exposed on the steel plate surface.
The amount of MnS can be extremely reduced, and the negative effects of MnS on oxide film formation during BA treatment are almost eliminated. Dew point temperature -
When the temperature is below 40℃, preferably below -55℃, an extremely thin layer of 50 to 300 Å is applied to the steel surface.
An oxide coating film mainly composed of Si and Cr is formed, but only when this specific coating film is formed can the rust resistance and "stain" resistance of the steel material be significantly improved. Only when the amount of S in steel is suppressed to an extremely low level does the corrosion resistance improvement effect of Cr and Ni present in the steel become extremely noticeable.
We have found that when BA and Al, or Cu, Ni and Al coexist, the effect becomes even more remarkable, resulting in extremely good results in improving rust resistance and "stain" resistance after BA treatment. I was able to obtain it. This invention was made based on the above findings, and includes a ferritic stainless steel plate containing Si: 0.01 to 5.00%, Mn: 0.01 to 5.00%, Cr: 8.0 to 20.0%, or further , Al: 0.01 to 0.20%, Cu: 0.20 to 2.00%, Ni: 0.01 to 1.50%, Mo: 0.1 to 2.0%. C as an impurity,
A component in which the content of N, P, S and O is within the following ranges: C: 0.10% or less, N: 0.05% or less, P: 0.05% or less, S: 0.002% or less, O: 0.010% or less Its rust resistance (including "stain" resistance) is greatly improved by providing a 50 to 300 Å oxide coating film containing Si and Cr on the surface by bright annealing. It is characterized by improved performance. Next, the reason why the chemical composition and oxide coating thickness of the ferritic stainless steel sheet of the present invention are limited as described above will be explained. (A) Chemical composition Si The Si component has the effect of deoxidizing the steel and forming an oxide film with excellent corrosion resistance on the steel surface together with Cr during bright annealing, but its content is 0.01%. If the content exceeds 5.00%, the workability of the steel material will deteriorate, so the content should be reduced from 0.01 to 5.00%.
It was determined that Mn The Mn component has desulfurization/deoxidation effects and hot workability improvement effects, but its content is 0.01%.
If the content is less than 5.00%, the desired effect cannot be obtained, and if the content exceeds 5.00%, no further improvement effect can be obtained. Therefore, the content was set at 0.01 to 5.00%. Cr Cr component is an important element that determines the basic corrosion resistance of the steel of the present invention, and its content
If it is less than 8.0%, it is the gist of the present invention.
Not only is the corrosion resistance improvement effect of the BA treated surface insufficient, but the corrosion resistance of stainless steel is not exhibited. On the other hand, if the Cr content exceeds 20.0%, not only will the processability deteriorate, but there will be no applications that require BA treatment, and no industrial benefits will be obtained. It was determined that C and N If large amounts of C and N are contained in steel, they increase the hardness of the steel and also have a negative effect on corrosion resistance, so their upper limits are set at 0.10% and 0.05%, respectively. Ta. In particular, if a large amount of N is contained, the effect of adding Al will be impaired, so it is preferable to keep the content as low as possible. P If a large amount of P is contained, it will have a negative effect on corrosion resistance and other properties, so the upper limit should be set.
It was set at 0.05%. O O (oxygen) is precipitated as oxide-based nonmetallic inclusions in steel, and when manufacturing steel sheets,
It not only deteriorates the surface cleanliness but also becomes the starting point for rusting. Steel types that are subjected to BA treatment are particularly required to have surface cleanliness, and when Al is added, the presence of O can cause scratches, and the content is 0.010%. These phenomena become noticeable when the content exceeds %.
It was set at 0.010% or less. S S exists as MnS in steel, but if it appears on the surface of the steel, it will be oxidized during BA treatment, inhibiting the uniform generation and growth of the oxide layer on the surface of the steel. Not only does this have a negative effect on the rust resistance after BA treatment. Figure 1 shows the relationship between the amount of S in steel and the average corrosion rate in boiling tests.
The test was conducted using steel having the composition shown in Table 1 in the Examples described below (the subscripts in Figure 1 represent the steel types in Table 1) at PH: 2.0.
This was done by immersion in a boiling sulfuric acid solution containing 0.01M NaCl for 6 hours. In addition, Figure 2 is a diagram showing the relationship between the amount of S in steel and the number of rusting points in the dry-wet cyclic test.
% saline solution for 30 minutes and then drying for 30 minutes, which was repeated 500 times. Both the results shown in Figures 1 and 2 show that as the amount of S in steel decreases, better corrosion and rust resistance can be obtained, but the effect is lower than S content: 0.0020%. , is especially noticeable when the content is 0.0010% or less, and the content is 0.0020% or less.
%, it is clear that the condition becomes extremely bad. As described above, if the S content exceeds 0.002%, the corrosion resistance and rust resistance will be extremely deteriorated, so the upper limit of the S content was set at 0.002%. Al, Cu, Ni, and Mo These components have the effect of improving the corrosion resistance and rust resistance of steel, so one or more of these components are added as necessary when it is desired to further improve the above characteristics. The reason for limiting the amount of each component added is as follows. (i) Al The Al component has the effect of deoxidizing the steel and forming an oxide film with excellent corrosion resistance on the surface of the steel material together with Cr and Si during bright annealing. If the content is less than 0.01%, the effect of forming an oxide film with excellent corrosion resistance on the surface of the steel material during bright annealing cannot be said to be sufficient. The content was set at 0.01 to 0.20% because the properties of the metal deteriorated. (ii) Cu The Cu component has the effect of imparting excellent rust and acid resistance to steel, and also improves pitting corrosion resistance and crevice corrosion resistance, but if its content is less than 0.20%, it has the effect of imparting excellent rust and acid resistance to steel. On the other hand, if the content exceeds 2.00%, not only will no further improvement effect be observed, but the hot workability will be significantly impaired, so the content should be reduced to 0.20%. ~
2.00% was set. The effect of Cu addition is remarkable in ultra-low sulfur steel, and in order to fully demonstrate the effect of Cu and obtain satisfactory properties, the amount of S in the steel must be increased.
It is necessary to keep it below 0.0020%. (iii) Ni Ni component has excellent rust resistance as well as Cu.
It has the effect of imparting acid resistance and also has the effect of improving pitting corrosion resistance and crevice corrosion resistance. The effect of Ni addition is more pronounced when it is added in combination with Cu and Al. However, the content is 0.01%
If the content is less than 1.50%, the desired effect cannot be obtained, and on the other hand, if the content exceeds 1.50%, it will only impair the economic efficiency of the steel material and no further improvement effect can be obtained.
It was set at 0.01-1.50%. (ii) Mo Mo is an additive element that has the effect of significantly increasing the corrosion resistance of ferritic stainless steel, and has a great effect on improving rust resistance, acid resistance, crevice corrosion resistance, and pitting corrosion potential. However, if the content is less than 0.1%, sufficient effects cannot be obtained for the above action;
If the content exceeds the
It was set at 0.1-2.0%. (B) Thickness of oxide coating film Even if the thickness of the oxide coating film is less than 50 Å, it has a certain degree of corrosion prevention effect, but especially in harsh environments such as industrial areas, It was not possible to obtain sufficient corrosion protection effect, and on the other hand, the film thickness
If the thickness exceeds 300 Å, coloring on the surface of the steel sheet becomes noticeable and also leads to a decrease in corrosion resistance, so the thickness of the oxide coating film was set at 50 to 300 Å. The steel plate of the present invention is produced by melting steel with a desired composition according to a conventional method and rolling it under the following conditions: Furnace atmosphere: non-oxidizing gas, Furnace dew point: -40 to -75°C , Furnace temperature: 750 to 950° C. The material is manufactured by BA treatment in a bright annealing furnace under the following conditions, and the BA treatment conditions will be explained in more detail. (a) Atmosphere inside the furnace The atmosphere inside the furnace needs to have a low oxygen potential that allows heat treatment at high temperatures while suppressing oxidation of the stainless steel surface, and it needs to be filled with non-oxidizing gas. Although vacuum is most desirable, it is extremely difficult to implement on an industrial scale, so ammonia decomposition gas (AX gas, H ₂ −
_N2 gas), hydrogen gas, CO gas, etc. are used. Ammonia decomposition gas generally contains 100 ppm or less of undecomposed ammonia, and in actual operation it is usually kept to 10 ppm or less. Undecomposed ammonia is produced
It has a negative effect on the BA coating. (b) Dew point in the furnace The dew point in the furnace is determined to prevent the formation of Fe oxide scale on the stainless steel surface undergoing BA treatment.
Generally, efforts are made to reduce the amount of coloration as much as possible from the perspective of suppressing coloring. BA
Since the surface of the steel plate after treatment is noticeably discolored and the rust resistance is also significantly deteriorated, it is preferable to set the upper limit to -40°C. On the other hand, if the dew point is controlled below -75℃, the growth of the oxide coating film containing Si and Cr, which improves the rust resistance on the surface of the BA-treated steel sheet, will not be sufficient, so the lower limit should be set at -75℃. It is desirable to do so. (c) Temperature in the furnace If the temperature in the furnace is less than 750°C, the solution treatment, which is the purpose of annealing, will be insufficient. On the other hand, if the temperature exceeds 950°C, the coarsening of crystal grains will become noticeable. Furthermore, since sensitization is a problem, the temperature in the furnace during annealing is set at 750 to 950℃.
It is better to set it as Next, the present invention will be explained using Examples and in comparison with Comparative Examples. Examples First, steel plates 1 to 39 having the compositions shown in Table 1 were manufactured by a normal melting and rolling method. Next, from each steel plate, dimensions: 0.8t x 30W x 50L
Two test pieces were cut out and subjected to the following conditions: Furnace atmosphere: ammonia decomposition gas (AX gas), Furnace temperature: 830℃, Furnace dew point temperature: -58℃, Holding time: 15 minutes.
BA treatment was applied, and an oxide coating film mainly composed of Si and Cr was formed on the surface with the film thickness as shown in Table 1. I got 39. In addition, comparison steel plate

【table】

【table】

[Table] 26 shows the furnace dew point temperature of the BA treatment conditions.
Comparative steel plate 21 had a furnace dew point temperature of -35°C among the BA treatment conditions. The * mark in Table 1 indicates that the chemical composition of the steel sheet or the thickness of the oxide coating film is outside the range of the present invention. Next, these test pieces were subjected to an atmospheric exposure test, and the results obtained are also shown in Table 1. The atmospheric exposure test was conducted in the Amagasaki area for one month from June 21, 1980 to July 22, 1982, and the evaluation was performed according to the standards shown in Table 2. Furthermore, apart from this, a boiling test and a dry-wet cycling test were also conducted on the steel plate specimen before BA treatment. The boiling test used two test pieces with dimensions: 3t x 20W x 40L, containing 0.01M NaCl adjusted to pH 2.0.
A method of measuring the average corrosion rate (g/m ² ·hr) per 6 hours in a H ₂ SO ₄ solution was adopted. Also,
For the dry-wet cyclic test, we prepared a test piece with dimensions: 3t x 30W x 70L, and after rough buffing the surface,
The process of immersing in a 3% NaCl aqueous solution at 35℃ for 30 minutes and drying for 30 minutes was repeated 500 times, and the number of rusting points (pieces/100cm ² )
We adopted a method of measuring. These test results are also shown in Table 1. From the results shown in Table 1, steels whose chemical composition and BA treatment oxide coating thickness are within the range of the present invention show good results in all tests. , chemical composition or
It is clear that any BA-treated oxide coating film thickness outside the range of the present invention shows unfavorable results. In particular, the steel sheet of the present invention, in which the S concentration in the steel is extremely low sulfided to 0.0020% or less, has a lower average corrosion rate than comparative steels that have the S content in the steel at the same level as conventional steel. % or less is clearly effective. Furthermore, depending on the amount of S in the steel, it can be confirmed that the number of rusting points increases in the dry-wet cyclic test.In particular, in the comparison steel, multiple rusting points combine to form a large rusting point. There was a remarkable tendency to Therefore, we were able to reconfirm that the corrosion resistance and rust resistance of these steel sheet materials themselves greatly affect the atmospheric exposure test results of BA-treated steel sheets. As mentioned above, the steel sheet material is made to have extremely low sulfurization, the effects of trace elements in the steel are fully exhibited, and appropriate bright annealing treatment conditions are selected to form an oxide coating film by specific bright annealing. According to the present invention, which has achieved a rust-resistant steel plate that is significantly superior to conventional steel plates, it can be used at a low price to supply parts that require strict rust-resistant performance, such as building exterior materials and vehicle decorative parts. Moreover, industrially useful effects can be brought about, such as being able to respond with materials that have characteristics with sufficient margins.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the average corrosion rate and S concentration in steel in a boiling test, and FIG. 2 is a diagram showing the relationship between rusting points and S concentration in steel in a dry-wet cyclic test.

Claims (1)

[Claims] Contains 1% by weight of Si: 0.01 to 5.00%, Mn: 0.01 to 5.00%, Cr: 8.0 to 20.0%, with the remainder consisting of Fe and inevitable impurities, and C as an unavoidable impurity,
The contents of N, P, S, and O are respectively: C: 0.10% or less, N: 0.05% or less, P: 0.05% or less, S: 0.002% or less, O: 0.010% or less, and the surface Si and
A ferritic stainless steel sheet with excellent rust resistance characterized by having a 50 to 500 Å oxide coating film containing Cr. 2% by weight, contains Si: 0.01 to 5.00%, Mn: 0.01 to 5.00%, Cr: 8.0 to 20.0%, and further contains Al: 0.01 to 0.20%, with the remainder being Fe and unavoidable impurities. C as an unavoidable impurity,
The contents of N, P, S, and O are respectively: C: 0.10% or less, N: 0.05% or less, P: 0.05% or less, S: 0.002% or less, O: 0.010% or less, and the surface Si and
A ferritic stainless steel sheet with excellent rust resistance characterized by having a 50 to 500 Å oxide coating film containing Cr. 3. Contains Si: 0.01~5.00%, Mn: 0.01~5.00%, Cr: 8.0~20.0%, and further contains Cu: 0.20~2.00%, Ni: 0.01~1.50%, Mo: 0.1~ 2.0%, containing one or more of the following, with the remainder consisting of Fe and unavoidable impurities, and C as an unavoidable impurity,
The contents of N, P, S, and O are respectively: C: 0.10% or less, N: 0.05% or less, P: 0.05% or less, S: 0.002% or less, O: 0.010% or less, and the surface Si and
A ferritic stainless steel sheet with excellent rust resistance characterized by having a 50 to 500 Å oxide coating film containing Cr. 4. Contains Si: 0.01 to 5.00%, Mn: 0.01 to 5.00%, Cr: 8.0 to 20.0%, and further contains Cu: 0.20 to 2.00%, Ni: 0.01 to 1.50%, Mo: 0.1 to 4% by weight. 2.0%, one or more of the following, and Al: 0.01 to 0.20%, with the remainder consisting of Fe and unavoidable impurities, and C as an unavoidable impurity.
The contents of N, P, S, and O are respectively: C: 0.10% or less, N: 0.05% or less, P: 0.05% or less, S: 0.002% or less, O: 0.010% or less, and the surface Si and
A ferritic stainless steel sheet with excellent rust resistance characterized by having a 50 to 500 Å oxide coating film containing Cr.