JP6818145B2 - Steel material with excellent corrosion resistance in a dew condensation environment containing sulfide and its manufacturing method - Google Patents

Description

The present invention relates to a steel material having excellent corrosion resistance in a dew condensation environment containing sulfide and a method for producing the same. More specifically, a steel material having excellent corrosion resistance used for oil tankers, crude oil tanks, etc., particularly sulfide gas. The present invention relates to a steel material having excellent corrosion resistance in a dew condensation environment containing sulfide having excellent corrosion resistance in a dew condensation environment including, and a method for producing the same.

Of the various steel materials used in ships, especially those used in crude oil tanks for oil tank ships, the environment inside the crude oil tank causes extremely serious corrosion damage. On the inner surface of the crude oil tank, various forms of corrosion progress due to volatile components in crude oil, mixed seawater, inert gas sent into the tank to prevent salt explosion in oil field salt water, and dew condensation due to internal temperature differences. Its corrosion rate is also much higher than that of a general salt water environment.

In particular, on the upper plate of the crude oil tank, hydrogen sulfide gas evaporating from the crude oil and gases such as CO ₂ , SO ₂ , and O ₂ in the inert gas input for explosion prevention are formed on the surface of the steel material due to the temperature difference. It reacts with the condensed dew and corrosion progresses due to the large amount of hydrogen sulfide and sulfur dioxide components contained. The above-mentioned dew condensation, that is, corrosion due to condensed water, is similar to atmospheric corrosion of weather-resistant steel because corrosion occurs in a thin water film, but dew condensation and drying of water due to daily range are repeated periodically. It is classified separately as dew point corrosion.

During the daytime, the internal temperature of the deck head of a crude oil transport vessel that is loaded with crude oil rises to 50 ° C, so condensation does not occur, but during night operation, the internal temperature of the deck head is about 25 ° C. Evaporated moisture is formed at the bottom of the deck head. In the case of a 300,000-ton class crude oil transport ship, a maximum of 30 tons of water condenses on the upper part of the deck head and causes corrosion, so that the corrosion due to the condensed water that has condensed cannot be ignored.

Not only that, carbon dioxide, sulfur dioxide, etc. are injected into the empty space of the tank of the crude oil transport ship to prevent the hull from exploding. When dew condensation occurs in the deck head, such a gas dissolves in water together with sulfur and hydrogen sulfide already contained in crude oil to form an atmosphere close to acidic condensed water corrosion. In general, the higher the acidity, the higher the amount of H ⁺ ions involved in the corrosion reaction, and the higher the corrosion rate.

Patent Document 1 has been proposed in order to improve the corrosion resistance of marine steel materials. However, Patent Document 1 is not sufficient for actual use in a crude oil tank because it is designed without considering corrosion by sulfide when the crude oil contains hydrogen sulfide.

Japanese Unexamined Patent Publication No. 2000-013781

An object of the present invention is to provide a steel material and a method for producing the same, which can secure excellent corrosion resistance even in a dew condensation environment containing sulfide by optimizing the steel composition and investigating the relationship between the components. ..

The problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

In the present invention, in mass%, C: 0.02 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P: 0.03% or less, S : 0.03% or less, Cu: 0.05 to 0.5%, Ni: 0.05 to 0.5%, Mo: 0.02 to 0.5%, Al: 0.1% or less, Cr: 0.05-0.5%, Ca: 0.001-0.01%, the rest consists of Fe and unavoidable impurities.
It is a steel material having a sulfide dew condensation corrosion susceptibility index represented by the formula 1 of 1.7 or more and 2.5 or less, and having excellent corrosion resistance in a dew condensation environment containing sulfide.
[Equation 1]
Sulfide Condensation Corrosion Sensitivity Index = 0.4Ca / S + 5Cr + 6Mo + 2Cu + Ni-0.5Mn
However, Ca, S, Cr, Mo, Cu, Ni and Mn are elemental contents (mass%).

Further, in the present invention, in mass%, C: 0.02 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P: 0.03% or less. , S: 0.03% or less, Cu: 0.05 to 0.5%, Ni: 0.05 to 0.5%, Mo: 0.02 to 0.5%, Al: 0.1% or less, Cr: 0.05 to 0.5%, Ca: 0.001 to 0.01%, the rest consists of Fe and unavoidable impurities, and the sulfide condensation corrosion susceptibility index represented by the formula 1 is 1.7 or more, 2 A method of hot rolling a steel slab of .5 or less and cooling it to manufacture a steel sheet.
The cooling is characterized by cooling at a cooling rate of 10 ° C./s or more between a cooling start temperature of Ar3 or higher and a cooling stop temperature of (Ae1-30 ° C.) to 600 ° C.
[Equation 1]
Sulfide condensation corrosion susceptibility index = 0.4Ca / S + 5Cr + 6Mo + 2Cu + Ni-0.5Mn
However, Ca, S, Cr, Mo, Cu, Ni and Mn are elemental contents (mass%).

According to the present invention, the resistance to sulfide condensation corrosion can be improved by optimizing the steel composition and satisfying the sulfide condensation corrosion susceptibility index.

It is a figure which shows the test apparatus for simulating the sulfide dew condensation test in this invention. It is a photograph which observed the test result which carried out the sulfide dew condensation corrosion experiment for 100 days by embodiment of this invention.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention have conducted research in order to solve the above-mentioned problems of the prior art. As a result, in order to increase the resistance to corrosion in a dew condensation environment containing sulfide gas, it is preferable to appropriately control the composition of each component as described below, and Ca and S which affect the sensitivity to dew condensation corrosion. , Cr, Mo, Ni, Mn and the like, and found that it is necessary to appropriately control the relationship between the components, and completed the present invention.

First, the alloy composition range of the steel material according to the present invention will be described in detail. The steel material of the present invention has C: 0.02 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P: 0.03% or less in mass%. , S: 0.03% or less, Cu: 0.05 to 0.5%, Ni: 0.05 to 0.5%, Mo: 0.02 to 0.5%, Al: 0.1% or less, Cr: 0.05 to 0.5%, Ca: 0.001 to 0.01%, and the rest consists of Fe and unavoidable impurities.

Carbon (C): 0.02-0.2% by mass
C is an element added to improve the strength, and when the content thereof is increased, the hardenability can be improved and the strength can be improved. However, as the amount of addition increases, the total corrosion resistance is inhibited and the precipitation of carbides and the like is promoted, so that the local corrosion resistance is also partially affected. In order to improve the total corrosion resistance and the local corrosion resistance, it is necessary to reduce the C content, but if C is less than 0.02% by mass, it is difficult to secure the strength and exceeds 0.2% by mass. This is not preferable as a steel for welded structures because the weldability deteriorates. Therefore, the range is preferably 0.02 to 0.2% by mass. From the viewpoint of corrosion resistance, C is more preferably 0.16% by mass or less, and further preferably 0.14% by mass or less in order to prevent casting cracks and reduce the carbon equivalent.

Silicon (Si): 0.1 to 1.0% by mass
Si must have a content of 0.1% by mass or more in order not only to act as an antacid but also to exert its role as an element for increasing the strength of steel. Further, since Si contributes to the improvement of overall corrosion resistance, it is advantageous to increase the content, but if the Si content exceeds 1.0% by mass, toughness and weldability are impaired, and scale is achieved during rolling. Since it becomes difficult to peel off, surface defects due to scale are caused. Therefore, it is preferable to limit the content to 0.1 to 1.0% by mass. In order to improve the corrosion resistance, it is more preferable to add 0.2% by mass or more of Si.

Manganese (Mn): 0.2 to 2.0% by mass
Mn is an effective component for increasing strength without reducing toughness. However, when added in an excessive amount, the electrochemical reaction rate on the surface of the steel material may be increased during the corrosion reaction to reduce the corrosion resistance. If Mn is added in an amount of less than 0.2% by mass, it is difficult to secure the durability of the structural steel material, and if the content is increased, the hardenability increases and the strength increases, but it is added in an amount exceeding 2.0% by mass. If this is done, there is a problem that weldability and corrosion resistance are lowered. Therefore, the Mn content is preferably 0.2 to 2.0% by mass.

Phosphorus (P): 0.03% by mass or less P is an impurity element, and if its content exceeds 0.03% by mass, not only the weldability is significantly reduced, but also the toughness is deteriorated, so the content is set to 0. It is preferable to limit it to 0.03% by mass or less.

Sulfur (S): 0.03% by mass or less S is also an impurity element, and if its content exceeds 0.03% by mass, there is a problem that the ductility, impact toughness and weldability of steel deteriorate. Therefore, it is preferable to limit the content to 0.03% by mass or less. In particular, S easily reacts with Mn to form stretching inclusions like MnS, and the pores existing at both ends of the stretching inclusions serve as the starting points of local corrosion, so the content thereof is 0.01% by mass or less. It is more preferable to limit to.

Copper (Cu): 0.05 to 0.5% by mass
When Cu is contained in an amount of 0.05% by mass or more together with Ni, the elution of Fe is delayed and it is effective in improving the total corrosion resistance and the local corrosion resistance. However, if it exceeds 0.5% by mass, Cu in a liquid state dissolves in the grain boundaries during slab production and causes a red hot brittleness phenomenon that causes cracks during hot working, so the content is 0.05. It is preferably ~ 0.5% by mass. Since surface cracks generated during slab production interact with the C, Ni, and Mn contents, the frequency of surface cracks varies depending on the content of each element, but the Cu content is best set to 0.5% by mass or less. preferable.

Nickel (Ni): 0.05 to 0.5% by mass
When Ni is contained in an amount of 0.05% by mass or more like Cu, it is effective in improving the total corrosion resistance and the local corrosion resistance. When added together with Cu, it also has the effect of reacting with Cu to suppress the formation of a Cu phase having a low melting point and suppress red hot brittleness. Ni is an element that is also effective in improving the toughness of the base metal. However, Ni is an expensive element, and adding more than 0.5% by mass is disadvantageous in terms of economy and weldability, so the content should be 0.05 to 0.5% by mass. Is preferable.

The effect of Ni on the improvement of corrosion resistance is not higher than that of Cu. Therefore, rather than adding a large amount of Ni to improve corrosion resistance, it is more preferable to contain Ni in an amount of Cu content or more and 1.5 times or less of the Cu content in order to suppress surface cracking due to the addition of Cu. It is more preferable to limit the content to 0.3% by mass or less.

Molybdenum (Mo): 0.02-0.5% by mass
Mo is an element that contributes to the improvement of corrosion resistance and strength, and it is necessary to add 0.02% by mass or more in order to exert its effect. However, in order for Mo to improve corrosion resistance, it must be solid-solved in the steel material. That is, the solid-dissolved Mo improves the corrosion resistance to condensed water containing hydrogen sulfide, but the Mo contained in excess of the solid-solution limit reacts with S to form Mo ₂ S, which lowers the corrosion resistance. If Mo is added in an excessive amount, the corrosion resistance to condensed water containing hydrogen sulfide is lowered. Therefore, the upper limit is preferably 0.5% by mass. Further, the precipitate of Mo acts to improve the strength, but the coarsely precipitated Mo may cause local corrosion of steel, so it is more preferable to add 0.1% by mass or less.

Aluminum (Al): 0.1% by mass or less Al is an element added for deoxidation, reacts with N in steel to form AlN, and refines austenite crystal grains to improve toughness. It is an element that causes. However, if it is excessively contained in excess of 0.1% by mass, inclusions are formed in the coarse oxide in the steelmaking process, and due to the characteristics of the aluminum system, stretch inclusions that are crushed during rolling and elongated during rolling are formed. The formation of such stretched inclusions promotes the formation of pores around the inclusions, and such pores act as a starting point for local corrosion, thus inhibiting local corrosion resistance. Therefore, the Al content is preferably 0.1% by mass or less. Even if Al is added, the deoxidizing effect can be obtained by other deoxidizing elements such as Si, so that the lower limit of Al is not particularly limited. However, in order to expect the deoxidizing effect of Al, it is preferable to add at least 0.001% by mass or more of Al.

Chromium (Cr): 0.05 to 0.5% by mass
Cr is an element that forms an oxide film containing Cr on the surface of a steel material in a corrosive environment to increase corrosion resistance. In order to obtain the effect of corrosion resistance due to the addition of Cr, it is necessary to contain 0.05% by mass or more. However, if Cr is excessively contained in excess of 0.5% by mass, the toughness and weldability are adversely affected. Therefore, the content is preferably 0.05 to 0.5% by mass in mass%.

Calcium (Ca): 0.001 to 0.01% by mass
Ca reacts with Al, Si, and O in molten steel to form a composite oxide, and then reacts with S to form CaS. Such CaS inclusions dissolve in water in a dew condensation environment to raise the pH of the steel material surface, thereby suppressing the electrochemical reaction of the steel material, promoting the formation of a safe phase, and improving the corrosion resistance characteristics. In order to improve the corrosion resistance of Ca, it is necessary to add at least 0.001% by mass, but if it exceeds 0.01% by mass, there is a problem that the refractory is melted and damaged during the steelmaking process. Therefore, the content is preferably 0.001 to 0.01% by mass. Further, in order to secure the sulfide dew condensation corrosion susceptibility index, it is more preferable to add 0.002% by mass or more.

In addition to the above components, the rest consists of Fe and unavoidable impurities. However, the addition of other alloying elements is not excluded within the range not deviating from the technical idea of the present invention.

On the other hand, the steel material of the present invention preferably satisfies the sulfide dew condensation corrosion susceptibility index defined by the formula 1 from 1.7 to 2.3.
[Relationship formula 1]
Sulfide condensation corrosion susceptibility index = 0.4Ca / S + 5Cr + 6Mo + 2Cu + Ni-0.5Mn
However, Ca, S, Cr, Mo, Cu, Ni and Mn are the contents (mass%) of the corresponding element.

Ca, Cr, Mo, Cu, Ni, and Mn are factors that affect the corrosion resistance effect in the sulfide dew condensation environment depending on the amount added. The effect of each of these components on corrosion resistance was quantitatively derived, and the relationship between them was shown by Equation 1. When the sulfide dew condensation corrosion susceptibility index defined by the formula 1 is 1.7 to 2.5, excellent corrosion resistance can be ensured in the corresponding environment.

The steel material of the present invention having the above-mentioned advantageous composition can utilize the ordinary knowledge in the technical field to which the present invention belongs without excessively repeating the experiment, if the person has ordinary knowledge in the technical field to which the present invention belongs. Can be easily manufactured. However, the present invention proposes a more advantageous manufacturing method found by the inventor of the present invention, for example, a method for manufacturing the steel sheet.

That is, the method for producing a steel material of the present invention is a method for producing a steel material by hot rolling by a usual method and then cooling, and the cooling start temperature is Ar3 temperature or higher and the cooling stop temperature is (Ae1-). It is characterized in that cooling is performed at a cooling rate of 10 ° C./s or more so as to be in the range of 30 ° C.) to 600 ° C. Hereinafter, the cooling conditions of the present invention will be described.

Cooling section: Cooling from Ar3 or higher (Ae1-30 ° C) to 600 ° C According to the experimental results of the present inventors, when Mo added to obtain a favorable effect in the present invention forms a large amount of precipitates, It has an adverse effect on total corrosion or local corrosion. On the contrary, when Mo is excessively dissolved, it adversely affects the corrosion resistance in the environment containing hydrogen sulfide. Therefore, it is necessary to appropriately control the ratio of Mo that forms a precipitate to Mo that dissolves in a solid solution. However, since Mo tends to form a precipitate at a temperature between 700 and 550 ° C, Mo is a precipitate. It is necessary to cool a part of the section quickly so as not to form a solid solution, and slowly cool the remaining part so as not to excessively dissolve.

Further, when cooling is started at a temperature of Ar3 or less, Cu segregates into pearlite and corrosion due to the galvanic pair of pearlite and ferrite further accelerates the corrosion. Therefore, cooling needs to be started at a temperature of Ar3 or higher, and cooling is carried out to Ae1-30 ° C. or lower, which is a temperature at which pearlite is not formed but precipitates such as Mo are properly formed. There is a need. In addition, if the temperature drops too much due to cooling, Mo will not be properly precipitated and will dissolve excessively. Therefore, Mo will combine with S to form Mo ₂ S in a condensed atmosphere containing hydrogen sulfide, resulting in Mo ₂ S. There is a risk of deteriorating the corrosion resistance of steel materials. Therefore, cooling needs to be completed at a temperature of 600 ° C. or higher.

Cooling rate: 10 ° C./s or more When the cooling rate is low, the time required for passing through the temperature range in which Mo precipitates are easily formed increases as described above, so that the precipitates may be excessively formed. There is. Therefore, the cooling rate needs to be 10 ° C./s or higher. Since there is no problem in achieving the object of the present invention even if the cooling rate is high, it is not necessary to set an upper limit of the cooling rate. However, in order to apply a very high cooling rate, the upper limit can be set to 50 ° C./s in consideration of the fact that the cooling equipment capacity is limited.

Hereinafter, examples of the present invention will be described in detail. The following examples are for facilitating the understanding of the present invention and are not intended to limit the present invention.

(Example)
After providing molten steel having the composition as shown in Table 1 (mass%, the rest being Fe and unavoidable impurities), a steel slab was produced by continuous casting. Subsequently, the produced steel slab was hot-rolled under normal conditions and then cooled under the conditions shown in Table 2.

As can be seen from Table 1, each of the invention steels means a steel sheet having a composition satisfying the component range specified in the present invention. However, Comparative Steels 1, 5 and 6 show the case where the elements selected as essential additive elements in the present invention such as Mo, Cu and Cr are not added. Further, although the comparative steels 2, 3, 4, 7 and 8 have the essential elements added, the sulfide dew condensation corrosion susceptibility index represented by the formula 1 is less than 1.7 or more than 2.5, as will be described later. , Indicates a case where the required range is not satisfied. Such a component of the comparative steel has a significantly lower corrosion resistance than the invention steel and cannot prevent the steel material from corroding in a sulfide condensation corrosion environment, so that the durability may be lowered and the replacement cycle may be shortened. is there.

Table 3 shows the results of measuring the sulfide condensation corrosion susceptibility index and the corrosion rate of the invention steel and the comparative steel. The corrosion rates shown in Table 3 are the results measured using the apparatus shown in FIG. That is, as shown in FIG. 1, in order to simulate a sulfide dew condensation environment, after filling a closed container with distilled water, corrosive gases such as SO ₂ , H ₂ S, CO ₂ , and O ₂ are added to the distilled water. Was continuously filled. Then, a 60 mm × 20 mm × 5 mm size test piece for measuring the corrosion rate was sanded with # 600 sandpaper and then placed on the upper part of the closed container. The lid of the closed container has a gas inlet, outlet and heating / cooling water circulation system, and after sealing, the container is installed in a constant temperature bath, and (50 ° C, 20 hours) → (25 ° C, 4 hours). The temperature cycle of was carried out for 100 days. The gas injected into the test apparatus is a gas simulating the sulfide condensation and corrosion environment on the upper deck of the crude oil tank, and has the following composition.
Gas composition: by _{volume%, 5% O 2 -15%} CO 2 -0.011% SO 2 -0.055% H 2 S- remainder _{N 2}

After performing a corrosion test for 100 days, the rust was removed with a corrosion product removing solution, and then the mass loss of each test piece was divided by the surface area of the initial test piece. For relative comparison, the corrosion rate of the comparative steel 1 was set to 100, and the relative corrosion rate is shown in Table 3.

As can be confirmed from Table 2, the sulfide condensation corrosion susceptibility index proposed in the present invention is more than 1.7 and less than 2.3 in the state where no corrosion-resistant elements such as Mo, Cu, and Cr are added or not sufficiently added. It was found that the relative corrosion rate was up to twice as high as that of the invention steel when the above range was not satisfied. Such a phenomenon occurred in the whole comparative steel to varying degrees. It is determined that this is because the sulfide condensation corrosion susceptibility index presented in the present invention was not satisfied.

On the other hand, FIG. 2 is a photograph of the test pieces of the invention steels 1 to 7 and the comparative steels 1 to 8 observed after the sulfide dew condensation corrosion experiment was carried out for 100 days. As described above, in the case of the invention steels 1 to 7 in which the sulfide dew condensation corrosion susceptibility index satisfies the range of 1.7 or more and 2.5 or less, which is the range proposed by the relational expression 1 of the present invention, the corrosion product is bright and dense in color. However, in the case of the other comparative steels 1 to 8, it can be confirmed that the corrosion products are so porous and dark in color that they can be distinguished with the naked eye.

As described above, as can be confirmed from Table 3 and FIG. 2, in order to prevent sulfide condensation corrosion, the sulfide condensation corrosion susceptibility index proposed in the present invention must be satisfied, and sulfide condensation corrosion susceptibility is satisfied. If the index is not satisfied, sufficient corrosion resistance for stable use of the steel material in the relevant environment cannot be ensured, and the life of the relevant structure cannot be ensured.

Claims (6)

By mass%, C: 0.02 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P: 0.03% or less, S: 0.03 % Or less, Cu: 0.05 to 0.5%, Ni: 0.05 to 0.5%, Mo: 0.02 to 0.5%, Al: 0.1% or less, Cr: 0.05 to 0.5%, Ca: 0.001-0.01%, the rest consists of Fe and unavoidable impurities.
A steel material having excellent corrosion resistance in a dew condensation environment containing sulfide, which has a sulfide dew condensation corrosion susceptibility index represented by the formula 1 of 1.78 or more and 2.5 or less.
[Relationship formula 1]
Sulfide condensation corrosion susceptibility index = 0.4Ca / S + 5Cr + 6Mo + 2Cu + Ni-0.5Mn
However, Ca, S, Cr, Mo, Cu, Ni and Mn are the contents (mass%) of the corresponding element. The steel material having excellent corrosion resistance in a dew condensation environment containing the sulfide according to claim 1, wherein the Ni component contains Cu content or more and 1.5 times or less of Cu content. The steel material having excellent corrosion resistance in a dew condensation environment containing the sulfide according to claim 1, wherein Ca is 0.002 to 0.01%. By mass%, C: 0.02 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P: 0.03% or less, S: 0.03 % Or less, Cu: 0.05 to 0.5%, Ni: 0.05 to 0.5%, Mo: 0.02 to 0.5%, Al: 0.1% or less, Cr: 0.05 to 0.5%, Ca: 0.001 to 0.01%, the rest consists of Fe and unavoidable impurities, and the sulfide dew corrosion susceptibility index represented by Equation 1 is 1.78 or more and 2.5 or less. Is a method of hot rolling and cooling to manufacture steel sheets.
The cooling is performed in a sulfide-containing dew condensation environment characterized by cooling at a cooling rate of 10 ° C./s or higher between a cooling start temperature of Ar3 or higher and a cooling stop temperature of (Ae1-30 ° C.) to 600 ° C. A method for manufacturing steel materials with excellent corrosion resistance.
[Relationship formula 1]
Sulfide condensation corrosion susceptibility index = 0.4Ca / S + 5Cr + 6Mo + 2Cu + Ni-0.5Mn
However, Ca, S, Cr, Mo, Cu, Ni and Mn are the contents (mass%) of the corresponding element. The method for producing a steel material having excellent corrosion resistance in a dew condensation environment containing a sulfide according to claim 4, wherein the Ni component contains Cu content or more and 1.5 times or less of Cu content. The method for producing a steel material having excellent corrosion resistance in a dew condensation environment containing sulfide according to claim 4, wherein Ca is 0.002 to 0.01%.