JPS6358214B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a fully austenitic stainless steel that exhibits excellent corrosion resistance under usage conditions in which it comes into contact with seawater, and that exhibits no deterioration in corrosion resistance even in welded parts. The term fully austenitic stainless steel referred to herein means a type of stainless steel that remains in an austenitic state no matter what kind of heat treatment it undergoes. Stainless steel is generally widely used as a corrosion-resistant steel, but it is still problematic because it often suffers from localized corrosion such as pitting and crevice corrosion in environments with high concentrations of chlorides, such as seawater. However, there is a problem in that intergranular corrosion occurs, especially in welded parts, making it impractical. A typical austenitic stainless steel
SUS304 and SUS316 are also subject to limitations in use due to intergranular corrosion, stress corrosion, and pitting corrosion. Austenite-ferrite duplex steel, represented by SUS329J1, has excellent seawater resistance, but the welded part has a ferrite single-phase structure, so it has no corrosion resistance at all. When a small tank was made from SUS329J1 steel and used to circulate natural seawater at room temperature, the material showed complete corrosion resistance, but the welded parts developed corrosion that penetrated the welded parts within a year. On the other hand, in the case of ferritic steels, high Cr-Mo steels are becoming increasingly popular with C, N, and S, which are harmful to corrosion resistance, as steelmaking technology improves.
etc. can now be manufactured in extremely small amounts,
Although it has become possible to use it in seawater environments, it still lacks reliability in terms of corrosion resistance. In addition, there are manufacturing difficulties due to the special smelting process, and there are disadvantages in terms of price. Typically, as a material used in a seawater environment,
It is necessary to have corrosion resistance under conditions of 50°C or higher, workability (press formability), and weldability.
There is an improved Cr-Ni-Mo fully austenitic steel that satisfies these requirements to a large extent; for example, in Japanese Patent Publication No. 49-40331, C: 0.001-0.2% Si: 0.1-5.0% Mn: A steel is disclosed that consists of 0.25-10.0% Cr: 15.0-25.0% Ni: 8.0-30.0% Mo: 4.0-6.0% Cu: 0.01-8.0% N: 0.1-0.35% with the balance being Fe. In general, high alloy steel containing a large amount of Mo tends to form intermetallic compounds rich in Cr and Mo, and the concentration of Cr and Mo decreases near the precipitated metal compounds, causing deterioration in corrosion resistance. Said Special Public Service No. 49-40331
The specification teaches that the addition of N prevents the precipitation of metal compounds (probably a typo or misprint of "intermetallic compound") and delta ferrite, but there is no specific description of its corrosion resistance. figure,
Regarding its mechanical properties, there is no concrete empirical description other than mentioning that it has a yield strength of at least 30 Kg/mm ² and an elongation of at least 35% after heating to 1000° C. or higher followed by rapid cooling. This steel still has insufficient mechanical properties and corrosion resistance, as shown in the comparative examples described later. As a result of our efforts to further improve this, the present inventors discovered that Al, La, Ce, or
By adding La, Ce, B and, if necessary, Cu, a steel with further improved corrosion resistance and mechanical properties was obtained. According to the present invention, in weight%, C: 0.04% or less,
Si: 1.5% or less, Mn: 2.0% or less, Cr: 18-25%,
Ni: 20-30%, Mo: 4-8%, N: 0.01-0.3
%, Al: 0.02% or less, La + Ce: 0.01 to 0.06%, the balance consists of Fe and inevitable impurities,
and %Cr+%Mo+1.5×%Si/%Ni+30×(%C+%N)+0
A fully austenitic stainless steel having a composition satisfying the relationship: .5×%Mn<1.08 and having excellent corrosion resistance in welded parts is provided. According to the present invention, the above steel further includes 0.01% or less B, or 0.01% or less B and 0.3 to 3% Cu.
A steel containing further improved corrosion resistance is provided. The present invention provides a completely austenitic stainless steel having excellent corrosion resistance, in which not only the base metal but also the weld zone becomes completely austenitic by adjusting the ratio of Ni equivalent to Cr equivalent. The role of each component in the steel of the present invention and the reason for limiting the content are as follows. C: Although it is better to have a lower C content in terms of pitting resistance and corrosion resistance, the upper limit was set to 0.04% in consideration of manufacturability. Si: It is better to contain a large amount of Si in terms of stress corrosion cracking, but it is harmful to weldability and workability.
For this reason, the upper limit was set at 1.5%. Mn: Mn is necessary for deoxidizing and improving hot workability, or as an austenite stabilizing element, but too much will deteriorate corrosion resistance, so the upper limit was determined to be 20%. Cr: Cr is extremely important for corrosion resistance, including pitting corrosion resistance of stainless steel. If it is less than 18%, the pitting corrosion resistance and corrosion resistance will not be sufficient, so a higher content is preferable, but if it exceeds 25%, the workability of the steel will deteriorate and a large amount of Ni will be added to maintain the austenitic structure. This increases the price and makes manufacturability difficult. Ni: Ni is effective in improving corrosion resistance, especially in suppressing active dissolution, but in the case of the steel of the present invention, in order to maintain the structure in the austenitic phase and the Cr/Ni equivalent ratio to be less than 1.08, It is determined in consideration of the relationship with the amount of N added. A minimum content of about 20% is required in relation to the Cr content. There is no need to limit the upper limit, but it is limited to 30% for economic reasons. Cu: Addition of Cu is effective in increasing acid resistance in high Cr, high Ni, and high Mo. 0.3% for that purpose
If it is less than 3%, the effect is small, and if it exceeds 3%, it is harmful to hot workability, so for applications requiring acid resistance, it is added in an amount of 0.3 to 3%. Mo: Mo, along with Cr, Ni, and N, is effective in improving the corrosion resistance of stainless steel, especially pitting corrosion resistance. If it is less than 4%, sufficient corrosion resistance cannot be obtained, and if it is more than 8%, hot workability is inhibited. N: N, together with Cr, Ni, and Mo, is effective in improving the corrosion resistance of stainless steel, especially pitting corrosion resistance. It is also an important element in the steel of the present invention, and it is desirable to contain it in a large amount for corrosion resistance and austenite phase stability, but 0.5% is the limit of solid solution in the steel, and 0.3% in consideration of manufacturability. %. La+Ce: In the steel of the present invention, La+Ce is used to improve corrosion resistance and hot workability, and to reduce sulfide formation.
Add 0.01%. However, if excessively contained, it remains in the steel as non-metallic inclusions and impairs corrosion resistance, so the upper limit is 0.06%. B: Adding a small amount of B to steel helps improve hot workability, especially hot cracking properties. However, if it exceeds 0.01%, the hot cracking resistance will be adversely affected. Al: Al is added to prevent La+Ce from oxidizing. If added in a large amount, Al ₂ O ₃ type inclusions will be generated, which will deteriorate the corrosion resistance and workability of the steel, so the amount should be limited to 0.02%, which corresponds to the amount of La+Ce added. Cr equivalent/Ni equivalent ratio The formation of the above-mentioned intermetallic compounds that deteriorate the corrosion resistance of steel is suppressed in a composition in which the Cr equivalent/Ni equivalent ratio is less than 1.08. This will be demonstrated later in the detailed description of the invention. In the present invention, La and Ce have the same effect, and the available raw material is usually a mixture (crude alloy) of La and Ce, so it is written as La+Ce. In the steel of the present invention, P and S are contained in amounts normally allowed for this type of steel, with P up to about 0.04%.
S may be included up to about 0.03%. The present invention will be specifically explained below using Examples.

[Table] Mechanical strength was measured for steel samples having the chemical compositions shown in Table 1, and corrosion tests were conducted on the base metal and welded parts. Samples A, B, C, D, and E are comparative steels, of which A and B correspond to SUS316 and SUS329J1 of JIS standards. Although the Al content of these samples has not been analyzed,
It is understood that the content is the amount that remains after being used in normal agglomeration, that is, 0.01% or less. What should be noted here is that sample A is the aforementioned special public
-It also represents steel No. 40331. This is because it states that Cu is an acceptable impurity in this steel. Samples C, D, and E are steels similar to the steel of the present invention. Samples F, G,
H, I, J, and K are steels of the present invention. Samples A and B were commercially available, and samples C to K were melted in a 100 kg electric furnace, bloomed, and cold rolled into 2 mm plates, which were solution treated at 1150°C. The solution-treated steel sample was used as a microscopic specimen using a conventional method, and its structure was observed using a microscope. As shown in Figure 1A in Sample D under 400x magnification, intermetallic compound precipitates appear quite clearly. Observation of a characteristic X-ray image using a scanning electron microscope revealed that this intermetallic compound was rich in Mo. On the other hand, in sample G that satisfies the conditions of the present invention, the first
As shown in Figure B, this precipitate did not appear at all. Similar micrographs of the weld structure of the same steel subjected to TIG welding as described below are shown in Figures C and D. In sample G, almost no precipitates are observed in the weld structure. For each sample, the relationship between the presence or absence of precipitates in the weld zone and the composition of the base metal is plotted on a so-called Schiefler phase diagram, as shown in Figure 2. As shown there, precipitation of intermetallic compounds occurs in the region where the Cr equivalent/Ni equivalent value is 1.08 or more. That is, it has been found that if the weld is in the austenite single phase region and the ratio is less than 1.08, the weld is completely austenite and there is no precipitation of intermetallic compounds. Next, assuming that it will be used in a seawater environment, the base metal and co-metal welded part of the sample steel were placed in a test solution at 50°C made by adding 50 g/FeCl ₃ to a N/20HCl aqueous solution.
Pitting corrosion resistance was tested by soaking for 48 hours. The base metal is a 29mm x 31mm (thickness 2mm) specimen, and the welding material is a flat plate that is TIG welded twice at the same position, with the weld bead in the center and the long side in the welding direction. A specimen of 31 mm x 2 mm (thickness) was prepared by polishing and removing the welding scale. The corrosion rate is evaluated as weight loss per square meter per hour (g/
^m2・hr). The results are summarized in Figure 3. According to this, the steel of the present invention has significantly better corrosion resistance in both the base metal and the welded part than the comparative steel. Furthermore, the pitting potential of the base metal and welded parts of these samples was measured potentiodynamically at a temperature of 50°C to 70°C in a 3.5% NaCl solution with the same concentration as NaCl in seawater. The results for some of the samples are shown in FIG. As the temperature increases, the pitting corrosion potential of all samples decreases, but in comparison steels B and D, whose compositions are outside the range of the present invention and whose Cr equivalent/Ni equivalent values are large and where precipitation of intermetallic compounds is significant, the weld zone There is a significant decrease in pitting potential. Even among the steels of the present invention, in steel F with a Cr equivalent/Ni equivalent value of about 1.0, the pitting potential of the weld is slightly lower when the temperature increases, but in comparative steels (for example, D), This is a significant improvement compared to the significant reduction in the pitting potential of the weld. FIG. 4 also gives suggestions regarding the selection of the composition depending on the service temperature of the steel of the present invention. That is, if the operating temperature is 50°C, steel F may be used, but if the operating temperature is 70°C, steel G must be used. To examine acid resistance, a specimen cut into 29 x 31 x 2 mm was immersed in 1% HCl and heated continuously for 6 hours in a boiling state.
A time corrosion test was conducted and the results are shown in Figure 5. Corrosion resistance is indicated by the degree of corrosion. Inventive steels G and J both have superior acid resistance compared to comparative steel A. Furthermore, when comparing G and J of the steel of the present invention,
J containing 1% Cu is superior. This makes Cu suitable for applications where acid resistance is particularly required.
The addition of is advantageous. Taking into account the forming operations in the use of this type of steel (for example, in the production of plates for heat exchangers), elongations of the material of more than 40% are required. In this regard, although the steel of the present invention contains large amounts of Cr, Mo, Ni, and N, precipitation of intermetallic products is suppressed, so as shown in Table 1 above, all
It has an elongation of more than 40%. In general, high alloy steel containing a large amount of Mo has low manufacturability.
In particular, hot workability is often a problem. Table 2 below shows the results of the drop weight test for samples D, G, I, and J.

[Table] A drop weight test is a test in which a cylindrical test piece with a diameter of 20 mm and a height of 30 mm is cut out from a cast steel ingot, heated (in this case, heated to each temperature listed in 2), and then placed on a surface plate. By dropping a weight weighing 200 kg onto the test piece from a height of 800 mm, the height becomes approximately 10 mm.
In this test, hot workability is determined by processing the material until it becomes . La + Ce 0.01
Steels G and I of the present invention containing % or more have no difficulty in hot working, which is a hindrance in manufacturing. Furthermore, steel J of the present invention to which 0.0028% of B was added has a small degree of cracking even at high temperatures. In comparison, hot working of Comparative Steel D is impossible or extremely difficult. As described above, the steel of the present invention has excellent corrosion resistance, especially in the welded portion, when used in contact with hot seawater, and is also excellent in manufacturability and workability. The present invention provides a new and useful material to the art.

[Brief explanation of the drawing]

Figures 1A and 1B are micrographs of the structures after solution treatment of cold-rolled sheets of comparative steel and inventive steel, and C and D are 400 times magnified micrographs of the structures of welded parts of comparative steel and inventive steel. Figure 2 shows the structures of the welds of the inventive steel and comparative steel.
FIG. 3 is a diagram showing the relationship between Cr equivalent and Ni equivalent. Figure 3 shows the materials themselves and welded parts of the inventive steel and comparative steel.
FIG. 3 is a diagram showing the results of a pitting corrosion resistance test at 50°C. FIG. 4 is a diagram showing the measurement results of the pitting corrosion potential of the steel of the present invention and the comparative steel in a 3.5% NaCl solution at a hot water temperature. FIG. 5 is a diagram showing the corrosion resistance of the present invention steel and comparative steel in a boiling 1% HCl solution.

Claims (1)

[Claims] 1% by weight, C: 0.04% or less, Si: 1.5% or less,
Mn: 2.0% or less, Cr: 18-25%, Ni: 20-30%,
Contains Mo: 4 to 8%, N: 0.01 to 0.3%, Al: 0.02% or less, La+Ce: 0.01 to 0.06%, the balance consists of Fe and unavoidable impurities, and %Cr+%Mo+1.5×%Si/ %Ni+30×(%C+%N)+0
Fully austenitic stainless steel with excellent corrosion resistance in welded parts, characterized by having a composition that satisfies the relationship: .5×%Mn<1.08. 2 In weight%, C: 0.04% or less, Si: 1.5% or less,
Mn: 2.0% or less, Cr: 18-25%, Ni: 20-30%,
Contains Mo: 4 to 8%, N: 0.01 to 0.3%, Al: 0.02% or less, La + Ce: 0.01 to 0.06%, B: 0.01% or less, and the balance consists of Fe and inevitable impurities,
and %Cr+%Mo+1.5×%Si/%Ni+30×(%C+%N)+0
Fully austenitic stainless steel with excellent corrosion resistance in welded parts, characterized by having a composition that satisfies the relational expression: .5×%Mn<1.08. 3 In weight%, C: 0.04% or less, Si: 1.5% or less,
Mn: 2.0% or less, Cr: 18-25%, Ni: 20-30%,
Mo: 4-8%, Cu: 0.3-3%, N: 0.01-0.3
%, Al: 0.02% or less, La+Ce: 0.01-0.06%,
B: Contains 0.01% or less, the balance consists of Fe and unavoidable impurities, and %Cr + %Mo + 1.5 x %Si / %Ni + 30 x (%C + %N) + 0
Fully austenitic stainless steel with excellent corrosion resistance in welded parts, characterized by having a composition that satisfies the relational expression: .5×%Mn<1.08.