JPS609588B2 - Pitting corrosion resistant low alloy steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pitting corrosion-resistant low alloy steel that is used in places where pitting corrosion is a problem, such as water supply and drainage pipes, tank bottoms and side plates where seawater or fresh water is involved.

Steel horizontal structures undergo various types of corrosion in seawater and freshwater environments, and the form and severity of corrosion vary depending on the environmental conditions and the shape of the structure. Traditionally, the forms of corrosion in steel can be broadly divided into general corrosion and pitting corrosion (or localized corrosion), but pitting corrosion in particular poses a very serious problem because it progresses several times faster than general corrosion. The causes of pitting corrosion are complex, but are basically related to the composition of the steel, the surface condition of the steel, and the environment in which it is used. For example, it is said that when only a few percent of AI is added to steel, the amount of corrosion itself decreases in seawater, but pitting corrosion becomes more likely to occur on the surface. In addition, corrosion concentrates on steel surfaces with mill scale or cracks on painted steel surfaces in seawater or fresh water, resulting in deep pitting corrosion. When a steel structure is buried in coastal reclaimed land, if the distribution of sand in contact with the steel is uneven, the concentration of oxygen in the air will occur, resulting in oxygen concentration batteries and severe pitting corrosion. In order to prevent this kind of pitting corrosion, conventional methods have been taken such as applying cathodic protection, painting more thoroughly, using painting and cathodic protection together, and periodically repairing. , cathodic protection has a limited lifespan and requires time to replace, and cathodic protection cannot be used for structures that cannot be replaced.

Furthermore, even if the paint is applied thickly to reduce chipping, it is extremely difficult to completely prevent scratches that occur during transportation and use, and pitting corrosion will concentrate on the scratched areas. Examples of this include water pipes, seawater and freshwater supply and drainage pipes, tank bottom plates, and foundation piles of offshore structures. If the steel plate of a water supply and drainage pipe or tank bottom plate penetrates due to pitting corrosion, it can lead to a serious leakage accident. In addition, conventional corrosion-resistant low alloy steels include weathering steel, seawater resistant steel,
There are commercially available materials such as sulfuric acid-resistant steel, and the effects of steel components on corrosion resistance are well known, but pitting corrosion resistance is not mentioned.

For example, weathering steel is a composition system in which additive elements C↓Cr, Si, P, Ni, Mo, etc. are appropriately combined to improve corrosion resistance in the atmosphere. Seawater-resistant steel is Si-N based in Japanese Patent Publication No. 46-4269, Cr-AI based in Barrel Publication No. 49-52117, and Cr in Japanese Patent Publication No. 49-25527.
-Ah-Cu-Ni system is the basic component, and commercially available seawater resistant steels also have Cr, N, Si, Cu, etc. added as main elements as appropriate. In sulfuric acid-resistant steel, based on Cu addition, Cr? Sb and Nj are used together, and since P is harmful, it is reduced as much as possible. All of the low-alloy corrosion-resistant steels mentioned above are aimed at the amount of corrosion (uniform corrosion over the entire surface), and pitting corrosion is not considered.

However, as mentioned earlier, pitting corrosion is likely to occur on the back of the tank bottom plate and on painted steel structures in seawater or freshwater, and adding seawater resistance-improving elements such as Cr and AI to copper will actually promote pitting corrosion. As mentioned earlier, there are cathodic protection and coating methods to prevent pitting corrosion, but they have many drawbacks, so in order to compensate for them, it is necessary to develop inexpensive low-alloy steel that is less prone to pitting corrosion. It has become necessary.

In view of this situation, the inventors of the present invention have studied steel with excellent pitting corrosion resistance for many years, and found that the coexistence of Si and Cr in steel, which contribute to improving corrosion resistance in weathering steel and seawater resistant steel, actually causes pitting corrosion. It is necessary to limit the content of Sb, Sn, and A based on Cu-P.
It has become clear that the addition of s is extremely effective in improving pitting corrosion resistance.

The present invention was made based on such knowledge, and
The gist of this is that the sum of Si and Cr in steel is 0.20%.
Limited to the following, CO. 005-0.25%, Mn 2% or less, PO. 01-0.15%, Cuo. 05-0.40
% and one or more of Sb, Sn, As, Se each 0.0
05 to 0.15%, or further contains Ni, Mo, Ti.
One or more of Nb, Zr, V, Ta, 0.05 to 1.0% each for Ni and Mo, Ti, Nb, Zr, V,
The pitting corrosion resistant low alloy steel contains 0.003 to 0.1% of Ta, with the balance being iron and unavoidable impurities.

The present invention will be explained in detail below.

Figure 1 shows the results of an accelerated reproduction test to examine the amounts of Sj and Cr that affect the pitting corrosion resistance of steel in soil.

The test method is to put earth and sand to a depth of 10 cm in a fence temperature tank kept at 40q0, and horizontally place 50 cm of soil in the middle of the earth and sand and 100 m of sand in length.
In this method, test specimens with a thickness of 3 sails were lined up and watered with seawater once a week. The test pieces were manufactured by melting pure iron with Si and Cr added respectively, and rolling the steel to the above dimensions. After the test was completed one month later, the specimen was pulled up to remove only the rust, and the pitting corrosion was measured at 10 points from the deepest point using a micro-shielding device, and the average was taken as the pitting depth.

The pitting depth is a value compared with the pitting depth of pure iron as 100. As is clear from this figure, the amount of Si + Cr is 0.2
Exceeds 0%. When this happens, the depth of pitting corrosion increases rapidly. Usually, Si is added to steel as a deoxidizing agent during steel manufacturing, and Si is added to improve the strength of the material, but it has been found that it is better to reduce it as much as possible to prevent pitting corrosion. This seems to be particularly severe when Si and Cr coexist. Cr is normally contained in steel as an impurity, but it should be reduced as much as possible in order to improve pitting corrosion resistance. Cr is used as an additive element for corrosion-resistant steel as mentioned above because it significantly improves the corrosion resistance of steel in seawater, freshwater, and the atmosphere where there is sufficient dissolved oxygen and air. The pitting corrosion resistance of steel is severely degraded in the sand on the back side of the tank bottom plate. The mechanism by which pitting corrosion occurs is that in steel containing Si and Cr, a rust layer enriched with Si and Cr forms uniformly on the steel surface in an environment with sufficient dissolved oxygen, whereas in an environment with little dissolved oxygen, a rust layer containing concentrated Si and Cr forms uniformly on the steel surface. In this case, a rust layer with many holes is formed, and the holes become anode.
This causes a corrosion cell to form between the rust layer (cathode) and the corrosion concentrates in the notch, resulting in pitting corrosion. As mentioned above, it is necessary to limit the content of Si and Cr in order to improve the pitting corrosion resistance, but since their effects are almost the same, the total amount was selected as the main content.

As shown in Figure 1, if the amount exceeds 0.20%, the pitting corrosion resistance deteriorates significantly, so this was set as the limit value.
It is desirable that the amounts of i and Cr do not each exceed 0.1%. Next, we will explain the reasons for limiting the other component elements.

First, C is an important element that affects the material quality of steel, but it also affects pitting corrosion resistance.

The lower the C content, the better the pitting corrosion resistance, but if it is less than 0.005%, it is difficult to manufacture as a practical steel, and if it exceeds 0.25%, there will be problems in welding the steel material, so the C content should be 0.005 to 0. 25
% range. Mn has almost no effect on pitting corrosion resistance, so it was determined based on the deoxidizing element in steelmaking and material strength. However, if it exceeds 2%, the material strength increases, but the quality deteriorates, so it was determined to be less than this. .

P in steel is the element that most improves pitting corrosion resistance.

The pitting corrosion resistance mechanism of P is that when pitting corrosion progresses, P concentrates inside the pitting corrosion and acts as an inhibitor (corrosion suppressant), which prevents the progress of pitting corrosion, but if it is less than 0.01%, the effect is small. If P exceeds 0.15%, the rutting and welding properties of the steel will be significantly deteriorated, so the range of P is determined to be 0.01 to 0.15%. Further, as described above, the pitting corrosion occurrence site becomes an anode, and the inside of the pitting corrosion becomes weakly acidic, promoting self-dissolution of iron, but Cu serves to prevent this.

The appropriate addition range of Cu is 0.05 to 0.40%,
If it is less than 0.05%, it has no effect on pitting corrosion resistance, and if it exceeds 0.40%, scratches are likely to occur on the surface of the steel during rolling, and the pitting corrosion resistance will not be improved. By adding one or more of Sb, Sn, As, and Se, they coexist with Cu to form an intermetallic compound and improve pitting corrosion resistance, but the optimal range is 0.005 to 0.15% each, and less than 0.005% each. This has no effect on pitting corrosion resistance, and even if the content exceeds 0.15%, no improvement in pitting corrosion resistance can be expected. In addition, when two or more types are added, it is desirable that the total amount does not exceed 0.20% from the viewpoint of ballability and weldability. Ni
, Mo is also an element that improves pitting corrosion resistance by adding one or two types, but it is sufficient in the range of 0.05 to 1.0% each, and when two or more types are added, the total is 2.0% or less is desirable, and even if it exceeds it, the performance will not improve despite the cost increase. Adding one or more of Tj, Nb, Zr, V, and Ts has the effect of fixing C and increasing pitting corrosion resistance, but the amount of each is 0.003 to 0.1%, which is sufficient.
When two or more types are added, it is preferable for the content not to exceed 0.15% in terms of material quality.

In this way, the steel of the present invention limits harmful elements such as Si and Cr, and at the same time contains compound additions of elements that improve pitting corrosion resistance such as Cr, P, Sb, Sn, AS, Se, or Ni and Mo, or further contains Ti, Nb, etc. , Zr, V, Ta, etc. to improve overall pitting corrosion resistance.

The steel of the present invention can be manufactured by a conventional method using ordinary steelmaking equipment, and even when manufacturing steel containing volatile components such as Sb, Sn, As, Se, etc., alloying elements such as these can be used alone. Alternatively, in the state of an alloy, by a conventional method such as a ladle addition method,
Since it can be added very easily, no special equipment is required.

Next, the effects of the present invention will be explained in more detail using examples. Table 1 is a table showing the compositions and pitting corrosion resistance test results of conventional steel, comparative steel, and steel of the present invention.

Two types of pitting corrosion resistance tests were adopted: the same soil corrosion test as the test shown in Figure 1 described above, and the pitting corrosion test for scratched parts of the paint film.
For the latter, the surface of the test piece (plate thickness: 3.0 mm, length: low, 50 mm) was ground and then coated with tar-epoxy, the coating thickness was adjusted to 200 A, and an artificial scratch was created in the center of the test piece. The test pieces were prepared and immersed in artificial seawater at room temperature for three months to cause pitting corrosion. At the end of the test, the test pieces were taken out, the remaining paint film and rust on the surface were removed, and the pitting corrosion that had formed under the scratches was removed. The depth was measured. As a measurement method, in the earth and sand corrosion test, the IQ point at which pitting was deeper than the naked eye was measured with a micrometer, and in the paint film pitting test, the maximum pitting depth was measured using a cross-sectional photograph. The pitting corrosion resistance test value in Table 1 is No. This is a value when the pitting corrosion depth of No. 1 steel is set as 100, and the larger the value, the worse the pitting corrosion resistance is. No. 1 in Table 1. 1 is conventional commercially available ordinary carbon steel, No. 2~No. Comparative steel No. 4 has a Si+Cr content of more than 0.15%, a low P content, and therefore poor pitting corrosion resistance. No. 5～Side.
21 is the steel of the present invention, but the ship. 1~M. It has significantly better pitting corrosion resistance than No. 4. The best material among these is No. It is a low C-low Si-Cu-P system of No. 18 and has a pitting depth that is about 1/3 that of conventional steel. As is clear from the table above, the steel of the present invention has excellent pitting corrosion resistance compared to conventional steel (ordinary carbon steel). It can be used in environments containing fresh water, such as the bottom plates of large oil storage tanks, water supply and drainage pipes, and the foundations of offshore structures.

S ship ship S ■ ship Money column ship

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the amount of Si+Cr in steel and pitting corrosion resistance in a soil corrosion test. Ta-no-zu

Claims (1)

[Claims] 1. The sum of Si and Cr content is limited to 0.20% or less, and C0
．． 005-0.25%, Mn 2% or less, P0.01-0
．． 15%, Cu0.05-0.40% and Sb, Sn
, As, and Se in an amount of 0.005 to 0.15% each, with the balance being iron and unavoidable impurities. 2 Limit the sum of Si and Cr content to 0.20% or less, and reduce CO
．． 005-0.25%, Mn 2% or less, P0.01-0
．． 15%, Cu0.05-0.40% and Sb, Sn
, As, Se, 0.005 to 0.15% each;
Further, one or both of Ni and Mo are each 0.05 to 1.
Pitting corrosion resistant low alloy steel containing 0%, the balance consisting of iron and unavoidable impurities. 3 Limit the sum of Si and Cr content to 0.20% or less, and
．． 005-0.25%, Mn 2% or less, P0.01-0
．． 15%, Cu0.05-0.40% and Sb, Sn
, As, Se, 0.005 to 0.15% each;
Furthermore, one or more of Ti, Nb, Zr, V, and Ta each have 0.
A pitting corrosion resistant low alloy steel containing 0.003 to 0.1%, with the balance consisting of iron and unavoidable impurities. 4 Limit the sum of Si and Cr content to 0.20% or less, and reduce CO
．． 005-0.25%, Mn 2% or less, P0.01-0
．． 15%, Cu0.05-0.40% and Sb, Sn
, As, Se, 0.005 to 0.15% each;
Further, one or both of Ni and Mo are each 0.05 to 1.
0%, each of one or more of Ti, Nb, Zr, V, and Ta.
A pitting corrosion resistant low alloy steel containing 0.003 to 0.1% with the remainder being iron and unavoidable impurities.