JP2023112979A - steel - Google Patents

Abstract

To provide a steel that exhibits corrosion resistance in a weak acid environment.SOLUTION: A steel has a chemical composition consisting of, in mass%, C: 0.20% or less, Si: 1.0% or less, Mn: 3.0% or less, P: 0.050% or less, S: 0.030% or less, In: 0.005-0.20%, Al: 0.10% or less, with the balance being Fe and impurities.SELECTED DRAWING: None

Description

The present invention relates to steel materials.

In a boiler of a thermal power plant or a flue gas system of a waste incineration facility, condensed water is generated from water vapor contained in the gas as the temperature of the exhaust gas decreases. This condensed water is a weakly acidic aqueous solution in which sulfur oxides, hydrogen chloride, and carbon dioxide contained in the exhaust gas are dissolved, and corrosion due to the weak acid poses a problem.

In addition, these components contained in the exhaust gas react with oxygen and water vapor in the air to produce sulfuric acid and nitric acid, which dissolve in rainwater, lowering the pH of the rainwater and producing acid rain. The pH of water saturated with carbon dioxide is 5.6, and rain with a pH lower than this value is called acid rain. Therefore, although not as strong as in an acid dew point environment, there are concerns about an environment where rainwater is directly splashed, and corrosion of steel materials used in rainwater drainage facilities due to weak acid.

On the other hand, in recent years, in Southeast Asia and subtropical regions, the demand for steel materials for civil engineering and construction materials has been strong along with the development of infrastructure. Examples of steel materials for civil engineering and construction include steel sheet piles and steel pipe piles that are partially buried in soil and used as foundations for civil engineering and construction. The soils distributed in these areas are weakly acidic soils containing acidic sulfates, which are said to be highly corrosive to steel, and there is concern about corrosion of steels by weak acids.

As a steel material having excellent corrosion resistance in such an acid corrosion environment, for example, Patent Document 1 discloses a steel material having a coating metal on the surface exposed to the corrosive environment, and Patent Document 2 discloses a steel material. has a Sn-containing layer on its surface.

Japanese Patent Application Laid-Open No. 2001-164336 JP 2017-14577 A

The steel material disclosed in Patent Document 1 has excellent corrosion resistance to machined parts in various corrosive environments, but there is still room for improvement in terms of corrosion resistance in acid dew-point corrosion environments. In addition, the steel material disclosed in Patent Document 2 has excellent corrosion resistance in a strongly acidic environment with a pH of less than 2 and a weakly acidic environment with a pH of 2 to 4, but it is necessary to apply a coating containing Sn. There is still room for improvement from the point of view of sexuality.

In addition, conventional steel materials with excellent acid dew point corrosion resistance exhibit excellent corrosion resistance against dew point corrosion that occurs in a strong acid environment with a pH of 2 or less. There is no significant difference in corrosion resistance compared to

An object of the present invention is to solve the above problems and to provide a steel material that exhibits excellent corrosion resistance in a weakly acidic environment.

The present invention has been made to solve the above problems, and the gist of the present invention is the following steel materials.

(1) chemical composition, in mass %,
C: 0.20% or less,
Si: 1.0% or less,
Mn: 3.0% or less,
P: 0.050% or less,
S: 0.030% or less,
In: 0.005 to 0.20%,
Al: 0.10% or less,
balance: Fe and impurities,
steel.

(2) the chemical composition, instead of part of the Fe, by mass%,
Cu: 1.0% or less,
Ni: 1.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
W: 1.0% or less,
Sb: 0.30% or less,
Co: 1.0% or less,
As: 0.30% or less,
Ce: 0.50% or less,
Bi: 0.10% or less,
Se: 0.50% or less,
Pb: 0.50% or less,
Hf: 0.20% or less,
Zn: 0.10% or less,
Ga: 0.10% or less,
Sr: 0.020% or less,
Ba: 0.020% or less,
Ge: 0.10% or less,
Sc: 0.010% or less, and Sm: 0.010% or less,
It contains one or more selected from
The steel material according to (1) above.

(3) the chemical composition, instead of part of the Fe, by mass%,
Ti: 0.20% or less,
Zr: 0.20% or less,
Nb: 0.10% or less,
V: 0.50% or less,
B: 0.010% or less,
Ta: 0.10% or less,
Te: 0.50% or less,
Y: 0.10% or less,
La: 0.10% or less,
Nd: 0.010% or less,
Ca: 0.010% or less,
Mg: 0.010% or less, and REM: 0.0150% or less,
It contains one or more selected from
The steel material according to (1) or (2) above.

(4) at least part of the surface of the steel material is subjected to anticorrosion treatment,
The steel material according to any one of (1) to (3) above.

ADVANTAGE OF THE INVENTION According to this invention, the steel material which exhibits the outstanding corrosion resistance in a weakly acidic environment is obtained.

In a weakly acidic environment, steel corrosion proceeds by the following anodic reaction.
Fe→Fe ²⁺ +2e ⁻ (Dissolution reaction of Fe)

In addition, since the rust layer cannot be protected in a weakly acidic environment, slowing down the anodic dissolution reaction of the steel itself is useful for improving corrosion resistance. That is, it is important to suppress the anode dissolution reaction in the weak acid aqueous solution.

Furthermore, the reduction reaction of hydrogen ions shown below proceeds, and the anode reaction of Fe dissolution is promoted.
2H ⁺ +2e ⁻ →H ₂

Containing Sn in steel is effective in suppressing the anode dissolution reaction. However, while Sn is very effective in a strongly acidic environment with a pH of less than 2, there is still room for improvement in suppressing the anodic dissolution reaction in a weakly acidic environment.

Based on this corrosion mechanism, the present inventors conducted detailed research on the relationship between various metal elements and the anodic dissolution reaction in order to improve corrosion resistance in weakly acidic environments. As a result, the following findings (a) to (c) were obtained.

(a) In becomes a cation In ³⁺ and dissolves in a corrosive environment, and suppresses corrosion by an inhibitory action in an acid chloride solution.

(b) The progress rate of the hydrogen generation reaction on the In surface is smaller than that on the Fe surface. Therefore, the anodic dissolution reaction of Fe can be greatly suppressed by forming an extremely thin metal In layer on the surface of the steel material in the use environment by underpotential precipitation (UPD) of In ³⁺ . As a result, corrosion resistance can be significantly improved even with a very small amount of In.

(c) Such an effect of improving corrosion resistance by UPD of In ³⁺ is most effectively exhibited in a pH range of 3 to 5, unlike Sn. Therefore, by including In, the corrosion resistance in a weakly acidic environment can be improved.

The present invention has been made based on the above findings. Each requirement of the present invention will be described in detail below.

(A) Chemical composition The reasons for limiting each element are as follows. In addition, "%" about content in the following description means "mass %."

C: 0.20% or less C is an effective element for ensuring strength as a material. However, if it is contained excessively, the weldability is remarkably deteriorated. In addition, as the C content increases, the amount of cementite that acts as a cathode and promotes corrosion in an environment where the pH is lowered increases, resulting in a decrease in corrosion resistance. Therefore, the C content is made 0.20% or less. The C content is preferably 0.18% or less, more preferably 0.16% or less. In order to obtain the above effects, the C content is preferably 0.02% or more, more preferably 0.05% or more.

Si: 1.0% or less Si is an element effective for deoxidation. However, an excessive content impairs the toughness of the base metal and the welded joint. Therefore, the Si content is set to 1.0% or less. The Si content is preferably 0.80% or less, more preferably 0.60% or less. In order to obtain the above effects, the Si content is preferably 0.03% or more, more preferably 0.05% or more.

Mn: 3.0% or less Mn is an element that has the effect of increasing the strength of steel at low cost. However, if it is contained excessively, the weldability deteriorates and joint toughness also deteriorates. Therefore, the Mn content is set to 3.0% or less. The Mn content is preferably 2.5% or less, more preferably 2.0% or less. In order to obtain the above effects, the Mn content is preferably 0.20% or more, more preferably 0.40% or more.

P: 0.050% or less P is an element present as an impurity in steel materials. P is an element that lowers the acid resistance, and lowers the corrosion resistance in a chloride corrosion environment where the pH at the corrosion interface is lowered. Furthermore, since it lowers the weldability and the toughness of the weld heat-affected zone, the smaller the content, the better. Therefore, the P content is made 0.050% or less. The P content is preferably 0.030% or less, more preferably 0.010% or less and less than 0.0050%. The lower limit of the P content does not need to be specified in particular, that is, the P content may be 0%, but an extreme reduction leads to an increase in steelmaking costs. Therefore, the P content may be 0.0001% or more.

S: 0.030% or less S is an element present as an impurity in steel materials. S forms MnS, which is the starting point of corrosion in steel, and when the content is excessive, the corrosion resistance is remarkably lowered. Therefore, the S content should be 0.030% or less. The S content is preferably 0.025% or less, more preferably 0.020% or less. The lower limit of the S content does not need to be specified in particular, that is, the S content may be 0%, but an extreme reduction leads to an increase in steelmaking costs. Therefore, the S content may be 0.0001% or more.

In: 0.005-0.20%
In dissolves as In ³⁺ in an acidic environment, and has an inhibitory action in an acidic chloride solution to suppress corrosion. Furthermore, in a specific potential region, In ³⁺ precipitates on the surface of the steel material as a monoatomic layer by UPD, and has the effect of greatly suppressing the anodic dissolution reaction of steel. be able to. However, even if it is contained excessively, not only the above effect is saturated, but also the toughness of the base material deteriorates. Therefore, the In content is set to 0.005 to 0.20%. The In content is preferably 0.008% or more, more preferably 0.010% or more. Also, the In content is preferably 0.15% or less, more preferably 0.10% or less.

Al: 0.10% or less Al is an element effective in deoxidizing steel. However, if it is contained excessively, the corrosion resistance in a low pH environment is lowered, so that the corrosion resistance in a chloride corrosive environment is lowered. Therefore, the Al content is set to 0.10% or less. The Al content is preferably 0.080% or less, more preferably 0.060% or less. In order to obtain the deoxidizing effect of Al, the Al content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.030% or more.

The steel material according to the present invention has the chemical composition described above, with the balance being Fe and impurities. The term "impurities" as used herein refers to components that are mixed in with raw materials such as ores, scraps, etc. during the industrial production of steel materials due to various factors in the production process, and are within a range that does not adversely affect the present invention. means acceptable.

In the chemical composition of the steel material of the present invention, one or more elements selected from the following elements may be contained within the following ranges in place of part of Fe. The reason for limiting each element will be explained.

Cu: 1.0% or less Cu has the effect of improving corrosion resistance by suppressing anodic dissolution of steel in a low pH environment, so it can be contained as necessary. However, if it is contained excessively, not only the effect is saturated, but also it causes embrittlement. Therefore, the Cu content is set to 1.0% or less. In order to stably obtain the above effects, the Cu content is preferably 0.02% or more, more preferably 0.03% or more.

Ni: 1.0% or less Like Cu, Ni has the effect of improving corrosion resistance by suppressing anodic dissolution of steel in a low pH environment, so it can be contained as necessary. However, excessive content not only saturates the effect, but also leads to a significant increase in cost. Therefore, the Ni content should be 1.0% or less. The Ni content is preferably 0.80% or less. In order to stably obtain the above effects, the Ni content is preferably 0.01% or more, more preferably 0.02% or more.

Cr: 1.0% or less Cr has the effect of improving corrosion resistance, so it can be contained as necessary. However, if it is contained excessively, the acid resistance is lowered, so the corrosion resistance may be lowered in an environment with a large amount of chlorides. On the other hand, if the Cr content is 1.0% or less, no decrease in acid resistance is observed, so the Cr content is made 1.0% or less. The Cr content is preferably 0.80% or less. In order to stably obtain the effect of improving corrosion resistance, the Cr content is preferably 0.01% or more, more preferably 0.02% or more.

Mo: 1.0% or less Mo is an element that dissolves and adsorbs to corrosion products on steel materials in the form of oxygen acid ions MoO ₄ ^2- and has the effect of suppressing the arrival of chloride ions to the steel surface. Therefore, it can be included if necessary. However, an excessive content not only saturates the effect, but also significantly increases the cost of the steel material. Therefore, the Mo content is set to 1.0% or less. Mo content is preferably 0.70% or less. In order to stably obtain the above effects, the Mo content is preferably 0.01% or more, more preferably 0.02% or more.

W: 1.0% or less W, like Mo, dissolves and exists in the form of oxic acid ions WO ₄ ^2- , and is an element that has the effect of suppressing the arrival of chloride ions to the surface of the steel material. , can be included as required. However, an excessive content not only saturates the effect, but also significantly increases the cost of the steel material. Therefore, the W content should be 1.0% or less. The W content is preferably 0.70% or less. In order to stably obtain the above effects, the W content is preferably 0.01% or more, more preferably 0.02% or more.

Sb: 0.30% or less Sb is an element that has the effect of improving corrosion resistance in an acidic environment, and suppresses the anodic dissolution reaction of steel in a low pH environment, as well as suppressing the hydrogen gas generation reaction and the reduction reaction of Fe ³⁺ . By doing so, the corrosion resistance in a chloride environment is improved, so it can be contained as necessary. However, if it is contained excessively, the toughness is remarkably deteriorated. Therefore, the Sb content should be 0.30% or less. The Sb content is preferably 0.15% or less. In order to stably obtain the above effects, the Sb content is preferably 0.05% or more, more preferably 0.08% or more.

Co: 1.0% or less Co is an element that improves corrosion resistance in an acidic environment, so it can be contained as necessary. However, an excessive content not only saturates the effect, but also significantly increases the cost of the steel material. Therefore, the Co content should be 1.0% or less. The Co content is preferably 0.70% or less. In order to stably obtain the above effects, the Co content is preferably 0.01% or more, more preferably 0.02% or more.

As: 0.30% or less As is an element effective in improving corrosion resistance in an acidic environment, although its effect is not as remarkable as that of Sb, it can be contained as necessary. However, if it is contained excessively, the hot workability deteriorates. Therefore, the As content should be 0.30% or less. As content is preferably 0.20% or less. In order to stably obtain the above effect, the As content is preferably 0.02% or more, more preferably 0.05% or more.

Ce: 0.50% or less Ce is an element that is eluted as Ce ³⁺ in a corrosive environment and suppresses the anodic dissolution reaction of steel by its inhibitory action in a chloride solution, so it can be contained as necessary. However, excessive content causes rolling cracks. Therefore, the Ce content is set to 0.50% or less. The Ce content is preferably 0.150% or less. In order to stably obtain the above effects, the Ce content is preferably 0.005% or more, more preferably 0.010% or more.

Bi: 0.10% or less Bi is an element that improves the corrosion resistance in an acidic environment, although its effect is not as remarkable as that of Sb, so it can be contained as necessary. However, if it is contained excessively, the hot workability deteriorates. Therefore, the Bi content should be 0.10% or less. The Bi content is preferably 0.050% or less. In order to stably obtain the above effects, the Bi content is preferably 0.002% or more, more preferably 0.005% or more.

Se: 0.50% or less Pb: 0.50% or less Since Se and Pb are elements effective in improving corrosion resistance in an acidic environment, they can be contained as necessary. However, if it is contained excessively, the hot workability deteriorates. Therefore, the contents of Se and Pb are each set to 0.50% or less. The contents of Se and Pb are each preferably 0.150% or less. In order to stably obtain the above effects, the contents of Se and Pb are each preferably 0.005% or more, more preferably 0.010% or more.

Hf: 0.20% or less Hf is an element that forms oxides on the steel material surface to improve corrosion resistance, so it can be contained as necessary. Therefore, the Hf content should be 0.20% or less. The Hf content is preferably 0.10% or less. In order to stably obtain the above effects, the Hf content is preferably 0.002% or more, more preferably 0.005% or more.

Zn: 0.10% or less Ga: 0.10% or less Zn and Ga are elements that suppress the cathodic reaction on the steel material surface in an acidic environment and improve corrosion resistance, so they can be contained as necessary. However, if it is contained excessively, the toughness and weldability of the base material deteriorate. Therefore, the contents of Zn and Ga should each be 0.10% or less. The contents of Zn and Ga are each preferably 0.080% or less. In order to stably obtain the above effects, the contents of Zn and Ga are preferably 0.002% or more, more preferably 0.005% or more.

Sr: 0.020% or less Ba: 0.020% or less Sr and Ba have the effect of suppressing the decrease in pH at the interface in the corrosion reaction zone and suppressing the promotion of corrosion. can be made However, if it is contained excessively, the toughness of the base material may be lowered. Therefore, the contents of Sr and Ba should each be 0.020% or less. The contents of Sr and Ba are each preferably 0.010% or less. In order to stably obtain the above effect, the contents of Sr and Ba are each preferably 0.0005% or more, more preferably 0.0010% or more.

Ge: 0.10% or less Ge has the effect of improving corrosion resistance, so it can be contained as necessary. However, if it is contained excessively, the mechanical properties of the base material deteriorate. Therefore, the Ge content should be 0.10% or less. The Ge content is preferably 0.080% or less. In order to stably obtain the above effect, the Ge content is preferably 0.002% or more, more preferably 0.005% or more.

Sc: 0.010% or less Sc is an element that forms oxides on the steel material surface to improve corrosion resistance, so it can be contained as necessary. However, if it is contained excessively, the low temperature toughness is lowered, which is not preferable. Therefore, the Sc content should be 0.010% or less. In order to stably obtain the above effects, the Sc content is preferably 0.0001% or more.

Sm: 0.010% or less Sm has the effect of improving corrosion resistance, so it can be contained as necessary. However, if it is contained excessively, the mechanical properties of the base material deteriorate. Therefore, the Sm content should be 0.010% or less. The Sm content is preferably 0.0060% or less. In order to stably obtain the above effects, the Sm content is preferably 0.0002% or more, more preferably 0.0005% or more.

Ti: 0.20% or less Ti is an element that has the effect of suppressing the formation of MnS, which is the starting point of corrosion due to the formation of sulfides, so it can be contained as necessary. However, if it is contained excessively, not only the effect is saturated but also the cost of the steel material increases. Therefore, the Ti content should be 0.20% or less. The Ti content is preferably 0.150% or less. In order to stably obtain the above effects, the Ti content is preferably 0.001% or more, more preferably 0.005% or more.

Zr: 0.20% or less Zr, like Ti, has the effect of suppressing the formation of MnS, which is the starting point of corrosion by forming sulfides, so it can be contained as necessary. However, if it is contained excessively, not only the effect is saturated but also the cost of the steel material increases. Therefore, the Zr content should be 0.20% or less. The Zr content is preferably 0.150% or less. In order to stably obtain the above effects, the Zr content is preferably 0.001% or more, more preferably 0.005% or more.

Nb: 0.10% or less Nb is an element that increases the strength of the steel material, so it can be contained as necessary. However, an excessive content not only saturates the effect but also reduces the HAZ toughness. Therefore, the Nb content should be 0.10% or less. The Nb content is preferably 0.050% or less. In order to stably obtain the above effects, the Nb content is preferably 0.001% or more, more preferably 0.003% or more.

V: 0.50% or less V, like Nb, is an element that increases the strength of the steel material. However, excessive content not only saturates the effect but also significantly increases the cost. Therefore, the V content should be 0.50% or less. The V content is preferably 0.30% or less. In order to stably obtain the above effects, the V content is preferably 0.005% or more, more preferably 0.010% or more.

B: 0.010% or less B is an element that improves hardenability and strength, so it can be contained as necessary. However, if it is contained excessively, the effect of increasing the strength is saturated, and the tendency of deterioration of toughness of both the base material and the HAZ becomes remarkable. Therefore, the B content should be 0.010% or less. In order to stably obtain the above effects, the B content is preferably 0.0003% or more.

Ta: 0.10% or less Ta is an element that contributes to improving the strength of steel materials, and can be contained as necessary. In addition, although the mechanism is not necessarily clear, it was found that Ta also contributes to the improvement of corrosion resistance. However, if it is contained excessively, not only the effect is saturated but also the cost increases. Therefore, the Ta content should be 0.10% or less. The Ta content is preferably 0.060% or less. In order to stably obtain the above effect, the Ta content is preferably 0.001% or more, more preferably 0.005% or more.

Te: 0.50% or less Te is an element that contributes to improving the strength of the steel material, and can be contained as necessary. However, if it is contained excessively, toughness and weldability are lowered. Therefore, the Te content is set to 0.50% or less. The Te content is preferably 0.40% or less. In order to stably obtain the above effects, the Te content is preferably 0.0005% or more, more preferably 0.0010% or more.

Y: 0.10% or less La: 0.10% or less Y and La are effective in controlling the morphology of inclusions, are effective in improving ductility characteristics, and are also effective in improving the HAZ toughness of large heat input welded joints. element, it can be contained as needed. However, an excessive content causes inclusions to coarsen, adversely affecting mechanical properties, particularly ductility and toughness. Therefore, the contents of Y and La should each be 0.10% or less. The contents of Y and La are each preferably 0.060% or less. In order to stably obtain the above effects, the contents of Y and La are each preferably 0.0001% or more, more preferably 0.0050% or more.

Nd: 0.010% or less Nd is an element that contributes to improvement of toughness through refinement of the structure, and can be contained as necessary. In addition, although the mechanism is not necessarily clear, it was found that Nd also contributes to the improvement of corrosion resistance. However, if it is contained excessively, not only the effect is saturated but also the cost increases. Therefore, the Nd content should be 0.010% or less. The Nd content is preferably 0.0080% or less. In order to stably obtain the above effect, the Nd content is preferably 0.0001% or more, more preferably 0.0005% or more.

Ca: 0.010% or less Ca is an element mainly used for controlling the form of sulfide, and can be contained as necessary. In addition, it also has the effect of suppressing the decrease in pH at the interface in the corrosion-reacted part, thereby suppressing the acceleration of corrosion. However, excessive content may impair the mechanical properties. Therefore, the Ca content should be 0.010% or less. The Ca content is preferably 0.0050% or less. In order to stably obtain the above effects, the Ca content is preferably 0.0002% or more, more preferably 0.0005% or more.

Mg: 0.010% or less Like Ca, Mg suppresses a decrease in pH at the interface in the corrosion reaction zone, so it can be contained as necessary. However, if it is contained excessively, the effect saturates. Therefore, the Mg content should be 0.010% or less. The Mg content is preferably 0.0050% or less. In order to stably obtain the above effects, the Mg content is preferably 0.0002% or more, more preferably 0.0005% or more.

REM: 0.0150% or less Except for Y, Sc, La, Ce, Nd, and Sm, REM (rare earth element) has the effect of improving the weldability of steel, so it may be contained as necessary. can. However, if the content is excessive, the effect saturates, so the REM content is made 0.0150% or less. Preferably, the REM content is 0.0100% or less. In order to stably obtain the above effects, the REM content is preferably 0.0002% or more, more preferably 0.0005% or more.

Here, REM is a general term for 17 elements including 15 lanthanoid elements and Y and Sc. However, in the present invention, Y, Sc, La, Ce, Nd and Sm are defined separately as described above, so one of the elements excluding Y, Sc, La, Ce, Nd and Sm from REM The content of a seed or the total content of two or more species is called REM content.

(B) Anticorrosion Coating The steel material of the present invention described above exhibits good corrosion resistance even when used as it is. However, when the surface is subjected to anti-corrosion treatment, specifically when the surface is coated with an anti-corrosion film made of organic resin or metal, the durability of the anti-corrosion film is improved compared to conventional steel materials, and the corrosion resistance is further improved. improves.

Here, examples of the anticorrosion coating made of an organic resin include vinyl butyral-based, epoxy-based, urethane-based, and phthalic acid-based resin coatings. Further, examples of anticorrosive coatings made of metal include plated coatings such as Zn, Al, and Zn--Al, and thermal spray coatings such as Zn, Al, and Al--Mg.

The reason why the durability of the anticorrosive coating is improved is that the corrosion of the underlying steel material of the present invention is remarkably suppressed, and as a result, blistering or peeling of the anticorrosive coating due to the corrosion of the underlying steel material from the defective portion of the anticorrosive coating is suppressed. It is considered to be

(C) Manufacturing method There is no particular limitation on the manufacturing method of the steel material according to the present invention. Examples include steel plates, steel pipes, and the like, which are manufactured by subjecting an ingot having the chemical composition described above to hot rolling and, if necessary, cold rolling. There are no particular restrictions on the heating conditions for hot rolling, and ordinary conditions may be adopted.

When manufacturing a steel material, the steel is melted by a conventional method, and after adjusting the composition, the steel slab obtained by casting is hot-rolled and, if necessary, cold-rolled. After hot rolling, the steel may be water-cooled as it is, or may be air-cooled and then reheated for quenching. After hot rolling, it may be coiled. After hot rolling, cold rolling may be performed and heat treatment may be further performed.

When manufacturing a steel pipe, a steel plate may be formed into a tubular shape and welded, and a UO steel pipe, an electric resistance welded steel pipe, a butt welded steel pipe, a spiral steel pipe, or the like may be formed. A seamless steel pipe manufactured by subjecting a steel billet to hot extrusion or piercing-rolling is also included in the steel material of the present invention.

Moreover, the treatment for covering with the anti-corrosion film described above may be performed by a normal method. In addition, it is not always necessary to apply the anticorrosion coating to the entire surface of the steel material, and it is possible to apply anticorrosion treatment to only one surface of the steel material exposed to the corrosive environment, or only the outer surface or the inner surface of the steel pipe, that is, at least a part of the steel surface. good.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these examples.

A steel having the chemical composition shown in Tables 1 to 3 was melted into an ingot of 50 kg, and then hot forged by a normal method to produce a block having a thickness of 60 mm. Next, the block was heated at 1120° C. for 1 hour, hot rolled, finished at 850° C. to a thickness of 20 mm, and then allowed to cool to room temperature in the atmosphere to form a steel plate.

Two test pieces each having a width of 25 mm, a length of 25 mm, and a thickness of 4 mm were taken from each steel plate, and one of the test pieces was subjected to the following corrosion test simulating a weakly acidic environment. For the other test piece, a modified epoxy paint was spray-coated to form an anti-corrosion film of about 200 μm on the entire surface of the test piece. submitted for testing.

Corrosion resistance was evaluated by an immersion test in an aqueous sulfuric acid solution adjusted to pH 3. A test piece was immersed in a solution at 60° C. for 6 hours, and the reduction in plate thickness was measured. For the anticorrosion treated steel, the maximum corrosion depth of the anticorrosion film flaw was measured using a confocal laser microscope.

Table 4 shows the test results. "Corrosion weight loss" in the table is the average plate thickness reduction amount of the test piece, which is calculated using the weight loss before and after the test and the surface area of the test piece. Further, the "corrosion depth" is the maximum value of the depth from the steel material surface of the coating film flaw.

As is clear from the results in Table 4, test steel No. 1, which is a comparative example, Since the steel material No. 4 did not contain In, the corrosion weight loss was as large as 2.1 g/m ² /hr, and the corrosion depth exceeded 25.0 μm.

On the other hand, test steel No. 1, which is an example of the present invention. The steel materials of Nos. 1 to 3 and 5 to 31 all satisfy the component contents specified in the present invention, so the corrosion weight loss is 1.5 g/m ² /hr or less, and the corrosion depth is 18.0 μm or less. was getting smaller.

INDUSTRIAL APPLICABILITY The steel material according to the present invention can be used as corrosion-resistant steel that exhibits excellent corrosion resistance in a weakly acidic environment.

Claims (4)

The chemical composition, in mass %,
C: 0.20% or less,
Si: 1.0% or less,
Mn: 3.0% or less,
P: 0.050% or less,
S: 0.030% or less,
In: 0.005 to 0.20%,
Al: 0.10% or less,
balance: Fe and impurities,
steel. wherein the chemical composition, instead of part of the Fe, is in % by mass,
Cu: 1.0% or less,
Ni: 1.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
W: 1.0% or less,
Sb: 0.30% or less,
Co: 1.0% or less,
As: 0.30% or less,
Ce: 0.50% or less,
Bi: 0.10% or less,
Se: 0.50% or less,
Pb: 0.50% or less,
Hf: 0.20% or less,
Zn: 0.10% or less,
Ga: 0.10% or less,
Sr: 0.020% or less,
Ba: 0.020% or less,
Ge: 0.10% or less,
Sc: 0.010% or less, and Sm: 0.010% or less,
It contains one or more selected from
The steel material according to claim 1. wherein the chemical composition, instead of part of the Fe, is in % by mass,
Ti: 0.20% or less,
Zr: 0.20% or less,
Nb: 0.10% or less,
V: 0.50% or less,
B: 0.010% or less,
Ta: 0.10% or less,
Te: 0.50% or less,
Y: 0.10% or less,
La: 0.10% or less,
Nd: 0.010% or less,
Ca: 0.010% or less,
Mg: 0.010% or less, and REM: 0.0150% or less,
It contains one or more selected from
The steel material according to claim 1 or 2. At least part of the surface of the steel material is subjected to anticorrosion treatment,
The steel material according to any one of claims 1 to 3.