KR20140138770A - Steel having acid dew corrosion resistance, and exhaust gas flow path constituent member - Google Patents

Abstract

The present invention provides a steel which is a steel based on a normal steel, which improves the sulfuric acid dew point corrosion resistance without resorting to the addition of Sb, and preferably further improves the hydrochloric acid dew point corrosion resistance. The corrosion resistant dew point corrosion steel according to the present invention is characterized by containing, by mass%, 0.005 to 0.200% of C, 0.20 to 0.80% of Si, 0.05 to 1.50% of Mn, 0.002 to 0.020% of P, 0.005 to 0.015% of S, 0.10 to 0.50%, Ni: 0.05 to 0.30%, Al: 0.005 to 0.100%, Mo: 0 to less than 0.010%, and the balance Fe and impurities. Particularly, when the corrosion resistance of the hydrochloric acid dew point is emphasized, the Mo content in the steel is set to 0.005 to 0.030 mass%.

Description

TECHNICAL FIELD [0001] The present invention relates to a corrosion resistant steel pipe having a corrosion resistant steel pipe,

Called "sulfuric acid condensation" occurs at a temperature lower than the dew point of the gas at the surface of the member in contact with the gas containing sulfur oxide or hydrogen chloride. When the member is a metal, corrosion may occur due to condensation water containing sulfuric acid, which may be a problem. This acid corrosion in the condensed water is referred to as " sulfuric acid dew point corrosion " in the present specification. TECHNICAL FIELD The present invention relates to a steel to which a resistance against sulfuric acid dew point corrosion is imparted, and an exhaust gas passage component member using the same.

The combustion exhaust gas of a thermal power plant mainly consists of water, sulfur oxides (sulfur dioxide, sulfur trioxide), hydrogen chloride, nitrogen oxide, carbon dioxide, nitrogen, oxygen and the like. Particularly, even if sulfur oxide is contained in the exhaust gas even at 1 ppm, the dew point of the exhaust gas often reaches 100 ° C or higher, and condensation of sulfuric acid is apt to occur. A material excellent in sulfuric acid dew point corrosion resistance is used for a metal member constituting such a flow path of the exhaust gas (for example, a duct wall of a fire pipe passage, a member constituting a chimney, a dust collector member and a heat exchange member for utilizing exhaust gas heat) It needs to be applied.

Patent Document 1: Japanese Patent Publication No. 43-14585 Patent Document 2: JP-A-2003-213367

Sb-added steel is known as a steel in which sulfuric acid dew point corrosion resistance is improved (Patent Documents 1 and 2). However, Sb is an expensive element, which causes an increase in the cost of the steel material, and is unstable in terms of raw material procurement when a large amount of Sb is consumed as a steel raw material. Further, the addition of Sb lowers the hot workability of the steel. In addition, the level of toxicity of Sb to the human body is not necessarily clarified, and it is preferable for safety to avoid the use of Sb as far as possible in consideration of dissolution of metal elements due to corrosion.

On the other hand, stainless steels generally have good acid resistance, but depending on the concentration and temperature of the acid, the corrosion sometimes proceeds more easily than the Sb-added steel. In other words, stainless steel is not expensive and is not a perfect material for sulfuric acid dew point corrosion.

In view of the above-described phenomenon, the present invention has been made to solve the problems of improving sulfuric acid dew point corrosion resistance without resorting to addition of Sb, preferably corrosion resistance to hydrochloric acid contained in the condensation water Corrosion resistance of hydrochloric acid dew point).

As a result of detailed studies, the inventors of the present invention have found that sulfuric acid dew point corrosion resistance can be improved when the content of impurity elements P and S is strictly controlled in a narrow range in the Cu-added steel. Further, it was found that the corrosion resistance (hydrochloric acid dew point corrosion resistance) against hydrochloric acid contained in the condensed water can be improved without impairing sulfuric acid dew-point corrosion resistance when a trace amount of Mo is contained. In other words, it has been clarified that there exists a "solution" of a composition range capable of achieving the above object in a steel made of a general steel component element containing no special element such as Sb. The present invention has been completed based on this new knowledge.

In order to achieve the above object, in the present invention, there is provided a steel sheet comprising 0.005 to 0.200% of C, 0.20 to 0.80% of Si, 0.05 to 1.50% of Mn, 0.002 to 0.020% of P, 0.005 to 0.015% of S, (Corrosion resistant corrosion resistant steel) comprising 0.10 to 0.50% of Cu, 0.05 to 0.30% of Ni, 0.005 to 0.100% of Al, 0 to less than 0.010% of Mo, and the balance of Fe and impurities. In particular, when the hydrochloric acid dew point corrosiveness is emphasized, it is preferable that the Mo content in the steel is 0.005 to 0.030 mass%.

Further, in the present invention, the exhaust gas passage constituent member constituting a part of the combustion exhaust gas flow path of the coal partial power plant, which is exposed to the exhaust gas and forms a surface on the surface, to provide.

Here, the exhaust gas channel constituent member means a member constituting a structure (for example, a duct or a chimney) of the exhaust gas channel, and a member (for example, a member of a dust collector or a heat exchanger) disposed in the exhaust gas channel. The member of the heat exchanger is, for example, a "cooling fin" attached to a tube through which a fluid receiving heat flows.

According to the present invention, it is possible to provide a steel which further improves sulfuric acid dew-point corrosion resistance or hydrochloric acid dew-point corrosion resistance without adding Sb. The steel is made of steel elements that are generally used, and does not contain any special element, so the raw material cost is low. In addition, deterioration in hot workability due to addition of special elements is also avoided. In addition, it is advantageous in terms of safety because it does not use Sb which is concerned about toxicity to human body. Therefore, the present invention is particularly useful for constructing a combustion exhaust gas flow path in a coal separating power plant.

1 is a graph illustrating the influence of the P content on the corrosion rate in an aqueous sulfuric acid solution.
2 is a graph illustrating the effect of the S content on the corrosion rate in an aqueous sulfuric acid solution.
4 is a graph illustrating the effect of the Mo content on the corrosion rate in an aqueous sulfuric acid solution.
3 is a graph illustrating the influence of the Mo content on the corrosion rate in an aqueous hydrochloric acid solution.

According to the detailed examination by the inventors, the sulfuric acid dew point corrosion resistance can be improved by strictly adjusting the content of impurity elements P and S in Cu-added steel. The addition of a trace amount of Mo can further improve the corrosion resistance of the hydrochloric acid dew point. The mechanisms for improving the sulfuric acid dew point corrosion resistance and the hydrochloric acid dew point corrosion resistance are not necessarily clarified sufficiently, but the following knowledge can be obtained at present.

(1) Cu is effective for forming a poorly soluble CuS coating, and the coating particularly enhances the resistance to sulfuric acid.

(2) Reduction of P inhibits corrosion of grain boundaries because it cleans ferrite and old austenite grain boundaries.

(3) Since the amount of sulfide inclusions in the steel is reduced due to the reduction of S, the interface between the inclusions likely to corrode and the steel is reduced, and the corrosion rate is reduced. However, if the S content is excessively low, the CuS film is hardly formed and the corrosion reduction amount increases inversely.

(4) As the Mo content increases, the sulfuric acid resistance is lowered. However, the sulfuric acid dew point corrosion resistance is most improved in the region where a small amount of Mo is added.

(5) On the other hand, by the inclusion of Mo, the corrosion potential shifts to the noble side in a noble side, and the hydrochloric acid resistance improves. In addition to sulfuric acid resistance, there is a content range of Mo capable of improving hydrochloric acid resistance.

[Sulfuric acid dew point corrosion resistance]

Figs. 1, 2, and 3 illustrate the influence of the P content, the S content, and the Mo content on the corrosion rate in the sulfuric acid aqueous solution, respectively. This immersing test was carried out under the conditions of a sulfuric acid concentration of 40 mass%, a temperature of 60 占 폚 and a soaking time of 6 hours, which is a very strict condition for assuming combustion gas of heavy oil (coal). The used steels were found to contain P, S, and S in a ratio of S: 0.008 to 0.010 mass%, P: 0.010 to 0.012 mass%, P: 0.010 to 0.012 mass%, and S: 0.008 to 0.010 mass% , And the content of the remaining elements other than Mo are all within the range of the present invention.

The corrosion rate of the conventional acid dew point corrosion resistant steel containing Sb, Cu and Mo under the sulfuric acid immersion test conditions is in the range of approximately 10 to 20 mg / cm 2 / h. As can be seen from Figs. 1, 2, and 3, when the P content is 0.020 mass% or less, the S content is 0.005 to 0.015 mass%, and the Mo content is 0 to 0.030 mass% Of sulfuric acid dew point corrosion is obtained.

[Corrosion resistance of hydrochloric acid dew point]

FIG. 4 illustrates the influence of the Mo content on the corrosion rate in an aqueous hydrochloric acid solution. The test conditions were a hydrochloric acid concentration of 1 mass% and a temperature of 80 캜, and the immersion time was 6 h. As can be seen from Fig. 4, the hydrochloric acid resistance is drastically improved by adding a small amount of Mo, and the hydrochloric acid resistance at the Mo content of 0.050 mass% or more is as good as that of the conventional Sb-added steel. Therefore, in applications where importance is given to simultaneous improvement of sulfuric acid dew point corrosion resistance and hydrochloric acid dew point corrosion resistance, the Mo content may be set in the range of 0.005 to 0.030 together with the result of Fig.

[Component elements]

The constituent elements of the steel according to the present invention will be described. "%" With respect to the elemental elements means% by mass.

C has a small influence on the sulfuric acid dew point corrosion resistance and is made 0.005 to 0.200% in order to secure the strength as a general structural material.

Since Si has an action to improve sulfuric acid corrosion resistance, a content of 0.20% or more is ensured. However, the excessive addition of Si lowers the descaling property at the time of hot rolling, resulting in an increase in scale defects. It is also a factor to lower the weldability. As a result of various studies, the Si content is limited to 0.80% or less.

Mn is effective for adjusting the strength of steel and also has an action of preventing hot stiffness by S, so that the content of Mn is secured to not less than 0.05%. 0.30% or more is more effective, and 0.50% or more may be controlled. However, the presence of a large amount of Mn may cause a decrease in corrosion resistance. The Mn content is allowed up to 1.50%, and may be controlled to be 1.20% or less, or 1.00%.

P deteriorates the corrosion resistance, hot workability and weldability, so it is limited to 0.020% or less, and more preferably 0.018% or less. In order to further improve the sulfuric acid corrosion resistance, the reduction of the P content is effective. However, excessive reduction may increase the steelmaking load (steelmaking load) and increase the cost, so that the content should be 0.002% or more.

S deteriorates the corrosion resistance and hot workability, and therefore is limited to 0.015% or less. However, with regard to sulfuric acid dew point corrosion resistance, it was found that when the S content is reduced, the corrosion rate is reversely increased (FIG. 2). This is considered to be due to the contribution of the CuS film to the improvement of the sulfuric acid resistance in the case of the object steel according to the present invention containing no Cr. When the S content is decreased, formation of the CuS film becomes insufficient . As a result of various studies, it is very effective that the S content is 0.005% or more.

Cu is effective for improving sulfuric acid corrosion resistance, and it is necessary to secure a content of 0.10% or more. However, excessive Cu content causes a deterioration in hot workability, and therefore is limited to 0.50% or less.

Ni has an action of suppressing the deterioration of the hot workability due to the addition of Cu and ensures a content of 0.05% or more. It is more effective to set it to 0.10% or more. However, Ni is a factor that deteriorates the sulfuric acid corrosivity, so it is limited to 0.30% or less.

Al is an element necessary for deoxidation at the time of steelmaking, and is set to a content of 0.005% or more. 0.010% or more is more effective. However, Al is a factor for lowering hot workability and is therefore limited to 0.100% or less.

Since Mo is a very effective element for improving the hydrochloric acid resistance as described above, it may be added when the hydrochloric acid dew point corrosiveness is emphasized as necessary. In order to sufficiently exhibit the hydrochloric acid-improving effect, it is effective to secure an Mo content of 0.005% or more (Fig. 4). However, if the Mo content is increased, the sulfuric acid dew point corrosion resistance is lowered. Therefore, when Mo is added, the content is 0.030% or less. On the other hand, in order to stably realize particularly excellent sulfuric acid dew point corrosion resistance, it is preferable to control the Mo content in the range of 0 to less than 0.010 mass%.

Example

The steel described in Table 1 was dissolved (solvent), and a hot-rolled steel sheet (blank material) having a thickness of 2.0 mm was produced by a conventional method. The test pieces cut out from each of the test pieces were subjected to a sulfuric acid immersion test and a hydrochloric acid immersion test under the same conditions as those in the case of obtaining the plots of FIGS. 1, 2, 3, and 4 (described above). The sulfuric acid dew point corrosion resistance was evaluated as? (Good) when the corrosion rate in the sulfuric acid immersion test was 20 mg / cm2 / h or less, and the other was rated poor (poor). The corrosion resistance of the hydrochloric acid dew point was evaluated by the following criteria: ⊚ (excellent), 4 캜 to 20 mg / cm 2 / h (good), and the others having a corrosion rate of 4 mg / Poor).

A JlS13B test piece was prepared from the cast slabs of the steels shown in Table 1 and subjected to a high temperature tensile test at three levels of 850 DEG C, 90 DEG C, and 950 DEG C according to JIS GO567. In the test, the entire parallel portion of the test piece was heated in the atmosphere using an infrared heating furnace, and after reaching a predetermined temperature and holding for 10 minutes, a tensile load was applied so that the tensile rate was 5 mm / min to break the test piece. The temperature of the test piece was measured by a thermocouple connected to the center of the parallel portion and controlled within a predetermined temperature range of +/- 10 DEG C.

(Hot workability: good) and that the brittle fracture surface was confirmed at any temperature was evaluated as? (Hot workability: somewhat poor).

These results are shown in Table 2.

As can be seen from Tables 1 and 2, the steel having the composition specified in the present invention is excellent in sulfuric acid dew point corrosion resistance and has a good Mo content. 21, 22 and 23 showed good resistance to hydrochloric acid dew point corrosion, and they all had no problem in hot workability.

On the other hand, No. 1 containing Sb, Cu, and Mo. 29 (which corresponds to conventional corrosion durability against corrosion durability), the sulfuric acid durability against corrosion was good but the hot workability was poor. In addition, 27 was poor in hot workability because the amount of Ni added was small.

Claims (3)

0.005 to 0.015%, S: 0.005 to 0.015%, Cu: 0.10 to 0.50%, Ni: 0.05 to 0.5% 0.30%, Al: 0.005 to 0.100%, Mo: 0 to less than 0.010%, balance Fe and impurities. The acid-resistant dew point corrosion steel according to claim 1, wherein the Mo content is 0.005 to 0.030%. A member using a steel plate made of the steel according to any one of claims 1 and 2, characterized in that, in a flow path of a combustion exhaust gas of a coal separating power plant, a part constituting a portion exposed to the exhaust gas, Exhaust gas flow path constituent member.

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