JPH06264176A - Low alloy steel excellent in seawater corrosion resistance - Google Patents

Abstract

PURPOSE:To produce a low alloy steel excellent in seawater corrosion resistance in the field requiring seawater corrosion resistance, such as water gate of weir at river-mouth, marine structure, and ballast tank of tanker. CONSTITUTION:This low alloy steel has a composition consisting of <=0.10% C, 0.1-1.0% Si, >2.0-5.0% Mn, 0.02-0.08% P, <=0.003% S, 0.1-0.4% Cu, 0.5-5.0% Cr, and the balance essentially Fe and further containing, if necessary, one or >=2 kinds among 0.03-0.30% Ti, 0.03-0.5% Ni, and 0.03-0.5% Mo.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low alloy steel excellent in seawater resistance in a field requiring seawater resistance such as a floodgate of an estuary weir or a marine structure or a ballast tank of a tanker.

[0002]

2. Description of the Related Art Recently, there has been a great demand for materials having excellent seawater resistance in sluices of estuaries and other offshore structures. However, conventional seawater resistant steels have a corrosion rate in seawater that is not ordinary steel. By comparison, seawater resistance is not enough at about 1/2, and heavy-corrosion coating materials of ordinary steel and stainless steel or ordinary steel have been used while cathodic-preventing.

For example, although seawater resistant steels such as those described in "Handbook of Corrosion Protection Technology", edited by Corrosion Protection Association, published by Nikkan Kogyo Shimbun, Ltd. in 1986, page 388 are commercially available, these have sufficient seawater resistance performance. The results of using these seawater resistant steels are extremely small. Further, even with a heavy-duty anticorrosion coating material, long-term durability of the coating is a problem.

Cr, P, Si, Al, Mo, Ni and Be have been conventionally known as elements effective in corrosion resistance of steel to seawater. First, the one disclosed in JP-A-49-52117 contains Cr and Al, and the corrosion resistance is improved by the Cr-Al-based carbide generated thereby. In addition, JP-A-6
In JP-A-2-274050, JP-A-2-138440 and JP-A-2-138441, corrosion resistance is improved by adding a large amount of Al. However, the addition of a large amount of Al remarkably deteriorates the hot workability of steel due to the formation of intermetallic compounds, so that practical application is difficult from this aspect.

By the way, according to the descriptions of the above publications,
Mn in steel combines with S to form MnS, which causes the steel to become brittle. Therefore, it is suppressed to 1.0% or 2.0% or less as a harmful element.

[0006]

In the present invention, S is reduced as much as possible to suppress the formation of MnS, so that Mn, which is an element originally having seawater corrosion resistance, is effectively used. The purpose of the present invention is to supply a seawater-resistant low-alloy steel, which is much more excellent in seawater resistance than seawater-resistant steel and can withstand barely long-term use even when used barely.

[0007]

In order to solve the above problems, the inventors of the present invention conducted tests using low alloy steels with various addition amounts of alloying elements to affect the seawater resistance of the alloying elements. The impact was examined. As a result, it is possible to suppress the generation of MnS, which adversely affects the corrosion, by suppressing S to an extremely low level and, if necessary, adding a predetermined amount of Ti having an affinity for S larger than Mn. It turned out that the property of can be utilized. Then, it was found that when Mn, which has been suppressed as much as possible to make the steel brittle, is added to a level higher than a normal seawater resistant steel level, the seawater resistance is significantly improved. If Mn is added too much, the seawater resistance is impaired, but if it is added up to 5.0%, the seawater resistance is improved.

Based on the above findings, in the present invention, the S content in steel is suppressed to an extremely low level, while M
The amount of n added was increased. Then, as a concrete means for solving the problem, the following seawater-resistant low alloy steel was developed.

1 C: 0.10% or less, Si: 0.1-
1.0%, Mn: more than 2.0% and 5.0% or less, P: 0.0.
02-0.08%, S: 0.003% or less, Cu: 0.
Low alloy steel with 1 to 0.4% and Cr: 0.5 to 5.0% and the balance consisting essentially of Fe and unavoidable impurities and excellent in seawater resistance.

2 Further, Ti: 0.03 to 0.30%
The low alloy steel excellent in seawater resistance according to 1, characterized by containing.

3. The low alloy steel having excellent seawater resistance according to 1 or 2, further containing Ni: 0.03 to 0.5%.

4. A low alloy steel excellent in seawater resistance according to any one of 1 to 3, further containing Mo: 0.03 to 0.5%.

[0013]

[Operation] The reason for limiting the content of each alloying element in the low alloy steel of the present invention will be described below. C has the effect of increasing the strength of the steel, but in the present invention, Cr that increases the strength is added, and it is not necessary to add a large amount. Further, in order to suppress the formation of carbide, lower C is preferable. Therefore, C is added in an amount of 0.1% or less.

Si is necessary as a deoxidizing element, and
It is an effective element that improves seawater resistance. Therefore 0.
It is necessary to contain at least 1%, but if it exceeds 1.0%, the toughness and hot workability of the welded portion will deteriorate, so 0.1
It was set to 1.0%.

Mn is an effective element that secures mechanical properties of steel and improves seawater resistance. However, if the amount is less than 2.0%, the effect is small, and if it exceeds 5.0%, the seawater resistance deteriorates. Therefore, it is set to more than 2.0% and 5.0% or less.

P is an element that deteriorates the weldability, and if its amount exceeds 0.08%, the adverse effect becomes remarkable. However, P is an effective element that improves seawater resistance. Therefore, less than 0.02% is insufficient. From the above, 0.02 to 0.08% was added.

S is an element that adversely affects the corrosion resistance.
This is because non-metallic inclusions MnS, which are the starting points of corrosion, are generated. Therefore, the smaller the content of S, the better, and its content is set to 0.003% or less. Also, Mn
The higher the / S ratio, the less MnS is produced,
Since the production of r) S increases, the preferential production of (Mn, Cr) S becomes remarkable and the seawater resistance is further improved by setting the Mn / S ratio to 700 or more. This is because when MnS dissolves in seawater, it becomes S ²⁻ , HS ⁻ , H ₂ S, etc., which promotes the melting of steel, but (Mn, Cr) S has a low solubility and is significantly more stable than MnS. This is because it is unlikely to become the starting point of corrosion.

Cu is an effective element that enhances the seawater resistance of steel and suppresses the growth of local corrosion such as pitting corrosion. If the amount is less than 0.1%, the effect is insufficient, while
If it exceeds 0.4%, the hot workability deteriorates. In addition, Cu is particularly effective for improving the corrosion resistance of the welded portion. For that purpose, the addition of 0.1% or more is essential. For the above reasons, Cu is added in an amount of 0.1 to 0.4%.

Cr is one of the basic elements for improving seawater resistance. However, if the amount is less than 0.5%, the effect is not sufficient, while if it exceeds 5.0%, the susceptibility to pitting corrosion of steel increases. Therefore 0.5-
It was set to 5.0%. In particular, the steel according to the present invention has an extremely small amount of S and hardly produces MnS which is the starting point of pitting corrosion.
No pitting corrosion occurs with the addition of more than 0% and up to 5.0%. This is because when Cr is contained in an amount of more than 3%, the starting point is less likely to occur than MnS which is the starting point of pitting corrosion (Mn,
This is because the formation of Cr) S is prioritized, and the corrosion form shifts from the pitting type to the general corrosion type. However, if it exceeds 5%, it becomes a passive type, and pitting corrosion starting from inclusions such as oxides is likely to occur instead of general corrosion.

By adding Ti, most of S is fixed as TiS, and MnS, which is the starting point of seawater corrosion, is eliminated to improve seawater resistance. If it is less than 0.03%, the effect is not produced, and even if it is added in an amount of more than 0.30%, the improvement of the seawater resistance improving effect is not recognized, so the content was made 0.03 to 0.30%.

Ni is an element exhibiting the same effect as Cu, and in particular, pitting corrosion of which pH is lowered like that of a local anode enhances corrosion resistance in a long portion and suppresses the progress of local corrosion. C
Coexistence with u is extremely effective. That amount
If it is less than 0.03%, its effect is insufficient, while
If it exceeds 0.5%, the hot workability deteriorates. Therefore Ni
The amount was set to 0.03 to 0.5%.

Like Cr, Mo is an effective element that improves the seawater resistance of steel. Mo dissolves as MoO _x ^2- ions and acts as an inhibitor at the local anode. However, if the amount is less than 0.03%, the effect is not sufficient, and if the amount exceeds 0.5%, the effect is saturated and it is not economical. Therefore, Mo is 0.03 ~
The addition was 0.5%.

[0023]

[Examples] Low alloy steels and carbon steels having the chemical compositions shown in Table 1 were melted in an atmospheric high-frequency furnace at 20 kg, forged, hot-rolled and then heat-treated to obtain a 7 mm thick plate, and the test of the dimensions shown in FIG. The piece is processed and manufactured, and the depth of LWL-1.0m in seawater (LWL = Low
It was immersed in Water Level = average low water surface level) and exposed to a splash zone, and a half-year test was performed, and the corrosion rate was determined from the corrosive raw materials before and after the test. The results are also shown in Table 1.

[0024]

[Table 1]

The steels 1 to 15 of the present invention have a corrosion rate in seawater of 0.015 mm / y or less, which is significantly smaller than that of the comparative steels 16 to 28. Further, even in the splash zone, in the present invention, it is 0.10 mm / y or less, whereas in the comparative materials, it is 0.20 mm / y or more, and the steels of the present invention are all excellent in seawater resistance in the environment. Can be said.

No. 1, 7, 25, 26, 27,
As the result of No. 28 is plotted in FIG. 2, it is clear that the corrosion resistance shows the maximum at the amount of Mn within the range of the present invention.

[0027]

EFFECT OF THE INVENTION The present invention provides a material that is more excellent in corrosion resistance than conventional seawater-resistant steel as a constituent material for a portion where seawater resistance is required, such as various offshore structures that have been recently developed. Can be supplied. Needless to say, in the present invention, the present material also has excellent corrosion resistance in other environments in which corrosion by chlorides like seawater poses a problem.

[Brief description of drawings]

FIG. 1 is a view showing a test piece used for a corrosion test by immersion in seawater and exposure to seawater droplets.

FIG. 2 is a graph showing the relationship between the Mn content in a test piece and the corrosion rate of the test piece in a corrosion test of immersion in seawater.

Claims (4)

[Claims] 1. C: 0.1% or less, Si: 0.1-
1.0%, Mn: more than 2.0% and 5.0% or less, P: 0.0.
02-0.08%, S: 0.003% or less, Cu: 0.
Low alloy steel with 1 to 0.4% and Cr: 0.5 to 5.0% and the balance consisting essentially of Fe and unavoidable impurities and excellent in seawater resistance. 2. The low alloy steel excellent in seawater resistance according to claim 1, which further contains Ti: 0.03 to 0.30%. 3. The low alloy steel excellent in seawater resistance according to claim 1 or 2, further containing Ni: 0.03 to 0.5%. 4. The composition according to claim 1, further comprising Mo: 0.03 to 0.5%.
A low alloy steel having excellent seawater resistance as described in the item.