JP7795086B2 - steel material - Google Patents

Description

The present invention relates to steel materials.

In the flue gas systems of thermal power plant boilers and waste incineration facilities, a drop in the temperature of exhaust gas causes condensed water to form from the water vapor contained in the gas. This condensed water is a weakly acidic aqueous solution containing dissolved sulfur oxides, hydrogen chloride, and carbon dioxide contained in the exhaust gas, and corrosion caused by this weak acidity can be a problem.

In addition, these components contained in exhaust gas react with oxygen and water vapor in the air to produce sulfuric acid and nitric acid, which dissolve in rainwater and lower the pH of the rainwater, resulting in acid rain. The pH of water saturated with carbon dioxide is 5.6, and rain with a pH lower than this is called acid rain. The annual average pH of rainwater across Japan is below 5. Therefore, although not as severe as in an acid dew point environment, there are concerns about corrosion by weak acids in environments where rainwater directly splashes onto the steel used in rainwater drainage facilities.

Meanwhile, in recent years, infrastructure development in Southeast Asia and subtropical regions has led to strong demand for steel for civil engineering and construction. Examples of steel for civil engineering and construction include steel sheet piles and steel pipe piles, which are used as foundations for civil engineering and construction and are partially buried underground. The soil found in these regions is weakly acidic, containing acidic sulfates that are known to be highly corrosive to steel, and there are concerns about corrosion of steel by the weak acid.

As examples of steel materials with excellent corrosion resistance in such acid corrosion environments, Patent Document 1 discloses a steel material having a metal coating on the surface exposed to the corrosive environment, and Patent Document 2 discloses a steel material having an Sn-containing layer on the surface of the steel material.

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

The steel material disclosed in Patent Document 1 has excellent corrosion resistance at machined parts in a variety of corrosive environments, but there is still room for improvement in corrosion resistance in acid dew-point corrosion environments. Furthermore, the steel material disclosed in Patent Document 2 has excellent corrosion resistance in strong acid environments with a pH of less than 2 and weak acid environments with a pH of 2 to 4, but because it requires the application of a coating containing Sn, there is still room for improvement from the perspective of manufacturability.

In addition, conventional steel materials with excellent acid dew-point corrosion resistance exhibit excellent corrosion resistance against dew-point corrosion that occurs in strongly acidic environments with a pH of 2 or less, but in weakly acidic environments with a pH of 3 to 5, there is no significant difference in corrosion resistance compared to ordinary steel.

The present invention aims to solve the above problems and provide a steel material that exhibits excellent corrosion resistance in weakly acidic environments.

The present invention was made to solve the above problems and is based on the following steel material.

(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,
The balance is Fe and impurities.
Steel material.

(2) The chemical composition is, in mass%, replacing a part of the Fe,
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 is, in mass%, replacing a part of the Fe,
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 a part of the surface of the steel material has been subjected to anticorrosion treatment.
The steel material according to any one of (1) to (3) above.

The present invention provides steel materials that exhibit excellent corrosion resistance in weakly acidic environments.

In a weakly acidic environment, corrosion of steel proceeds through the following anodic reaction:
Fe → Fe ²⁺ + 2e ⁻ (Fe dissolution reaction)

In addition, since the protective properties of a rust layer cannot be expected in a weakly acidic environment, slowing down the anodic dissolution reaction of the steel itself is effective in improving corrosion resistance. In other words, it is important to suppress the anodic dissolution reaction in a weakly acidic aqueous solution.

Furthermore, the following reduction reaction of hydrogen ions proceeds, accelerating the anodic reaction of Fe dissolution.
2H ⁺ +2e ^- →H ₂

Incorporating Sn into steel is effective in suppressing anodic dissolution reactions. However, while Sn is very effective in strongly acidic environments with a pH of less than 2, there is still room for improvement in suppressing anodic dissolution reactions in weakly acidic environments.

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

(a) In a corrosive environment, In dissolves as a cation In ³⁺ and acts as an inhibitor in an acid chloride solution, thereby suppressing corrosion.

(b) The rate of hydrogen evolution reaction on an In surface is slower than that on an Fe surface. Therefore, by forming an extremely thin metallic In layer on the steel surface in the operating environment through underpotential deposition (UPD) of In ³⁺ , the anodic dissolution reaction of Fe can be significantly suppressed. This can significantly improve corrosion resistance even with a small amount of In.

(c) Unlike Sn, the effect of improving corrosion resistance due to the UPD of In ³⁺ is most effectively exhibited in the pH range of 3 to 5. Therefore, by including In, it is possible to improve corrosion resistance in a weakly acidic environment.

The present invention was made based on the above findings. Each of the requirements of the present invention will be explained in detail below.

(A) Chemical Composition The reasons for limiting the content of each element are as follows: In the following description, "%" for the content means "% by mass."

C: 0.20% or less C is an effective element for ensuring the strength of the material. However, excessive C content significantly reduces weldability. Furthermore, as the C content increases, the amount of cementite that acts as a cathode and promotes corrosion in a pH-lowering environment increases, reducing corrosion resistance. Therefore, the C content is set to 0.20% or less. The C content is preferably 0.18% or less, and more preferably 0.16% or less. To achieve the above effect, the C content is preferably 0.02% or more, and more preferably 0.05% or more.

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

Mn: 3.0% or less Mn is a low-cost element that increases the strength of steel. However, excessive Mn content deteriorates weldability and joint toughness. Therefore, the Mn content is set to 3.0% or less. The Mn content is preferably 2.5% or less, and more preferably 2.0% or less. To achieve the above effect, the Mn content is preferably 0.20% or more, and more preferably 0.40% or more.

P: 0.050% or less P is an element present as an impurity in steel. P reduces acid resistance and corrosion resistance in chloride corrosion environments, where the pH of the corrosion interface decreases. Furthermore, P reduces weldability and the toughness of the weld heat-affected zone, so the lower the content, the better. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.030% or less, and more preferably 0.010% or less, less than 0.0050%. There is no need to specify a lower limit for the P content; that is, the P content may be 0%, but excessive reductions lead to increased steelmaking costs. Therefore, the P content may be set to 0.0001% or more.

S: 0.030% or less S is an element present as an impurity in steel. S forms MnS in steel, which is the starting point for corrosion, and excessive S content significantly reduces corrosion resistance. Therefore, the S content is set to 0.030% or less. The S content is preferably 0.025% or less, and more preferably 0.020% or less. There is no need to specify a lower limit for the S content; in other words, the S content may be 0%, but excessive reduction leads to increased steelmaking costs. Therefore, the S content may be set to 0.0001% or more.

In: 0.005-0.20%
Indium dissolves as In3 ⁺ in acidic environments and acts as an inhibitor in acidic chloride solutions, suppressing corrosion. Furthermore, in a specific potential range, In3 ⁺ precipitates as a monolayer on the steel surface through UPD, significantly suppressing the anodic dissolution reaction of steel. Therefore, even trace amounts can significantly improve corrosion resistance. However, excessive In content not only saturates this effect but also degrades the toughness of the base material. Therefore, the In content is limited to 0.005 to 0.20%. The In content is preferably 0.008% or more, more preferably 0.010% or more. 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, excessive content of Al not only reduces corrosion resistance in low pH environments, thereby reducing corrosion resistance in chloride corrosion environments, but also causes nitrides to coarsen, resulting in a decrease in toughness. Therefore, the Al content is set to 0.10% or less. The Al content is preferably 0.080% or less, and more preferably 0.060% or less. 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 above chemical composition, with the balance consisting of Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial production of steel material due to various factors in the manufacturing process, including raw materials such as ore and scrap, and are acceptable within a range that does not adversely affect the present invention.

In the chemical composition of the steel material of the present invention, one or more elements selected from the following may be contained in place of a portion of the Fe, within the ranges shown below. The reasons for limiting each element are explained below.

Cu: 1.0% or less Cu has the effect of suppressing the anodic dissolution of steel in a low pH environment, thereby improving corrosion resistance, and can be added as needed. However, excessive addition not only saturates the effect, but can also cause embrittlement. Therefore, the Cu content is set to 1.0% or less. To stably obtain the above effect, the Cu content is preferably set to 0.02% or more, and more preferably 0.03% or more.

Ni: 1.0% or less Like Cu, Ni has the effect of improving corrosion resistance by suppressing the anodic dissolution of steel in a low pH environment, and can therefore be added as needed. However, excessive Ni content not only saturates the effect but also leads to a significant increase in costs. Therefore, the Ni content is set to 1.0% or less. The Ni content is preferably set to 0.80% or less. To stably obtain the above effect, the Ni content is preferably set to 0.01% or more, and more preferably 0.02% or more.

Cr: 1.0% or less Cr has the effect of improving corrosion resistance, so it can be added as needed. However, excessive addition reduces acid resistance, which may result in reduced corrosion resistance in chloride-rich environments. On the other hand, a Cr content of 1.0% or less does not result in a reduction in acid resistance, so the Cr content is set to 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, and more preferably 0.02% or more.

Mo: 1.0% or less Mo is an element that dissolves and adsorbs to corrosion products on the steel in the form of oxyanions MoO ₄ ^2- , thereby inhibiting chloride ions from reaching the steel surface, and can be added as needed. However, excessive addition not only saturates the effect, but also significantly increases the cost of the steel. Therefore, the Mo content is set to 1.0% or less. The Mo content is preferably set to 0.70% or less. To stably obtain the above effects, the Mo content is preferably set to 0.01% or more, and more preferably 0.02% or more.

W: 1.0% or less Like Mo, W dissolves and exists in the form of oxyanions WO ₄ ^2- , and is an element that has the effect of inhibiting chloride ions from reaching the steel surface, so it can be added as needed. However, if it is added in excess, not only will the effect saturate, but the cost of the steel will also increase significantly. Therefore, the W content is set to 1.0% or less. The W content is preferably set to 0.70% or less. In order to stably obtain the above effect, the W content is preferably set to 0.01% or more, and more preferably 0.02% or more.

Sb: 0.30% or less Sb is an element that improves corrosion resistance in acidic environments. It suppresses the anodic dissolution reaction of steel in low pH environments and improves corrosion resistance in chloride environments by suppressing the hydrogen gas generation reaction and the reduction reaction of Fe3 ⁺ . Therefore, it can be added as needed. However, excessive Sb content significantly deteriorates toughness. Therefore, the Sb content is set to 0.30% or less. The Sb content is preferably set to 0.15% or less. To stably obtain the above effects, the Sb content is preferably set to 0.05% or more, and more preferably 0.08% or more.

Co: 1.0% or less Co is an element that improves corrosion resistance in acidic environments, so it can be added as needed. However, if it is added in excess, not only will the effect saturate, but the cost of the steel will also increase significantly. Therefore, the Co content is set to 1.0% or less. The Co content is preferably set to 0.70% or less. In order to stably obtain the above effect, the Co content is preferably set to 0.01% or more, and more preferably 0.02% or more.

As: 0.30% or less Although the effect of As is not as significant as that of Sb, it is an element that is effective in improving corrosion resistance in acidic environments, and therefore can be added as needed. However, excessive addition of As reduces hot workability. Therefore, the As content is set to 0.30% or less. The As content is preferably set to 0.20% or less. In order to stably obtain the above effects, the As content is preferably set to 0.02% or more, and more preferably set to 0.05% or more.

Ce: 0.50% or less Ce is an element that dissolves as Ce3 ⁺ in a corrosive environment and suppresses the anodic dissolution reaction of steel by acting as an inhibitor in a chloride solution, so it can be added as needed. However, excessive addition can cause rolling cracks. Therefore, the Ce content is set to 0.50% or less. The Ce content is preferably 0.150% or less. To stably obtain the above effects, the Ce content is preferably 0.005% or more, and more preferably 0.010% or more.

Bi: 0.10% or less Although the effect of Bi is not as significant as that of Sb, Bi is an element that improves corrosion resistance in acidic environments, and therefore can be added as needed. However, excessive addition of Bi reduces hot workability. Therefore, the Bi content is set to 0.10% or less. The Bi content is preferably set to 0.050% or less. In order to stably obtain the above effects, the Bi content is preferably set to 0.002% or more, and more preferably set to 0.005% or more.

Se: 0.50% or less Pb: 0.50% or less Se and Pb are elements effective in improving corrosion resistance in acidic environments, so they can be added as needed. However, excessive addition of these elements deteriorates hot workability. Therefore, the Se and Pb contents are each set to 0.50% or less. The Se and Pb contents are preferably set to 0.150% or less. In order to stably obtain the above effects, the Se and Pb contents are preferably set to 0.005% or more, and more preferably 0.010% or more.

Hf: 0.20% or less Hf is an element that forms oxides on the surface of the steel material to improve corrosion resistance, so it can be added as needed. Therefore, the Hf content is set to 0.20% or less. The Hf content is preferably set to 0.10% or less. In order to stably obtain the above effects, the Hf content is preferably set to 0.002% or more, and more preferably set to 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 surface in an acidic environment and improve corrosion resistance, so they can be added as needed. However, excessive addition of these elements will deteriorate the toughness and weldability of the base metal. Therefore, the Zn and Ga contents are each set to 0.10% or less. The Zn and Ga contents are preferably set to 0.080% or less. In order to stably obtain the above effects, the Zn and Ga contents are preferably set to 0.002% or more, and 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 acceleration of corrosion, so they can be added as needed. However, excessive addition may reduce the toughness of the base material. Therefore, the Sr and Ba contents are each 0.020% or less. The Sr and Ba contents are preferably 0.010% or less. In order to stably obtain the above effects, the Sr and Ba contents are preferably 0.0005% or more, and 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 needed. However, excessive content of Ge reduces the mechanical properties of the base material. Therefore, the Ge content is set to 0.10% or less. The Ge content is preferably set to 0.080% or less. In order to stably obtain the above effects, the Ge content is preferably set to 0.002% or more, and more preferably set to 0.005% or more.

Sc: 0.010% or less Sc is an element that forms oxides on the surface of the steel material to improve corrosion resistance, so it can be added as needed. However, excessive addition is not preferable because it leads to a decrease in low-temperature toughness. Therefore, the Sc content is set to 0.010% or less. In order to stably obtain the above effects, it is preferable that the Sc content be 0.0001% or more.

Sm: 0.010% or less Sm has the effect of improving corrosion resistance, so it can be added as needed. However, excessive Sm content reduces the mechanical properties of the base material. Therefore, the Sm content is set to 0.010% or less. The Sm content is preferably set to 0.0060% or less. In order to stably obtain the above effects, the Sm content is preferably set to 0.0002% or more, and more preferably set to 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, and can be added as needed. However, if it is added in excess, not only will the effect saturate but the cost of the steel material will increase. Therefore, the Ti content is set to 0.20% or less. The Ti content is preferably 0.150% or less. In order to stably obtain the above effect, the Ti content is preferably 0.001% or more, and more preferably 0.005% or more.

Zr: 0.20% or less Like Ti, Zr has the effect of suppressing the formation of MnS, which is the starting point of corrosion, by forming sulfides, and therefore can be added as needed. However, if it is added in excess, not only will the effect saturate but the cost of the steel material will increase. Therefore, the Zr content is set to 0.20% or less. The Zr content is preferably set to 0.150% or less. In order to stably obtain the above effect, the Zr content is preferably set to 0.001% or more, and more preferably 0.005% or more.

Nb: 0.10% or less Nb is an element that increases the strength of steel, so it can be added as needed. However, if it is added in excess, not only will the effect saturate, but the toughness of the HAZ will decrease. Therefore, the Nb content is set to 0.10% or less. The Nb content is preferably set to 0.050% or less. In order to stably obtain the above effect, the Nb content is preferably set to 0.001% or more, and more preferably set to 0.003% or more.

V: 0.50% or less Like Nb, V is an element that increases the strength of steel. However, if it is contained in excess, not only does the effect saturate but the cost increases significantly. Therefore, the V content is set to 0.50% or less. The V content is preferably set to 0.30% or less. In order to stably obtain the above effect, the V content is preferably set to 0.005% or more, and more preferably set to 0.010% or more.

B: 0.010% or less B is an element that improves hardenability and increases strength, so it can be added as needed. However, if added in excess, the effect of increasing strength saturates, and there is a significant tendency for the toughness of both the base material and the HAZ to deteriorate. Therefore, the B content is set to 0.010% or less. In order to stably obtain the above effects, it is preferable that the B content be 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 added as needed. It has also been found that Ta contributes to improving corrosion resistance, although the mechanism is not entirely clear. However, excessive Ta content not only saturates the effect but also increases costs. Therefore, the Ta content is set to 0.10% or less. The Ta content is preferably set to 0.060% or less. To stably obtain the above effects, the Ta content is preferably set to 0.001% or more, and more preferably set to 0.005% or more.

Te: 0.50% or less Te is an element that contributes to improving the strength of steel and can be added as needed. However, excessive addition of Te reduces toughness and weldability. Therefore, the Te content is set to 0.50% or less. The Te content is preferably set to 0.40% or less. In order to stably obtain the above effects, the Te content is preferably set to 0.0005% or more, and more preferably set to 0.0010% or more.

Y: 0.10% or less La: 0.10% or less Y and La are elements that are effective in controlling the shape of inclusions, improving ductility characteristics, and also improving the HAZ toughness of high heat input welded joints, so they can be added as needed. However, excessive addition causes inclusions to coarsen, adversely affecting mechanical properties, particularly ductility and toughness. Therefore, the Y and La contents are each set to 0.10% or less. The Y and La contents are preferably set to 0.060% or less. In order to stably obtain the above effects, the Y and La contents are preferably set to 0.0001% or more, and more preferably 0.0050% or more.

Nd: 0.010% or less Nd is an element that contributes to improving toughness by refining the structure, and can be added as needed. It has also been found that Nd contributes to improving corrosion resistance, although the mechanism is not entirely clear. However, excessive Nd content not only saturates the effect but also increases costs. Therefore, the Nd content is set to 0.010% or less. The Nd content is preferably set to 0.0080% or less. To stably obtain the above effects, the Nd content is preferably set to 0.0001% or more, and more preferably 0.0005% or more.

Ca: 0.010% or less Ca is an element mainly used to control the morphology of sulfides and can be added as needed. It also has the effect of suppressing the decrease in pH at the interface in the corrosion reaction zone and suppressing the promotion of corrosion. However, excessive addition of Ca may impair mechanical properties. Therefore, the Ca content is set to 0.010% or less. The Ca content is preferably set to 0.0050% or less. In order to stably obtain the above effects, the Ca content is preferably set to 0.0002% or more, and more preferably set to 0.0005% or more.

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

REM: 0.0150% or less REM (rare earth elements), excluding Y, Sc, La, Ce, Nd, and Sm, have the effect of improving the weldability of steel, and can be added as needed. However, if added in excess, the effect saturates, so the REM content is set to 0.0150% or less. The REM content is preferably 0.0100% or less. In order to stably obtain the above effect, the REM content is preferably 0.0002% or more, and more preferably 0.0005% or more.

Here, REM is a collective term for 17 elements, including the 15 lanthanide elements plus Y and Sc. However, in the present invention, Y, Sc, La, Ce, Nd, and Sm are defined separately as described above, so the REM content refers to the content of one or the total content of two or more of the elements excluding Y, Sc, La, Ce, Nd, and Sm from REM.

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

Here, examples of corrosion-resistant coatings made from organic resins include vinyl butyral-based, epoxy-based, urethane-based, and phthalic acid-based resin coatings. Furthermore, examples of corrosion-resistant coatings made from metals include plated coatings of Zn, Al, Zn-Al, etc., and thermally sprayed coatings of Zn, Al, Al-Mg, etc.

The improved durability of the corrosion-protective coating is believed to be due to the fact that corrosion of the underlying steel material of the present invention is significantly suppressed, thereby suppressing swelling or peeling of the corrosion-protective coating caused by corrosion of the underlying steel material from defects in the corrosion-protective coating.

(C) Manufacturing Method There are no particular limitations on the manufacturing method of the steel material according to the present invention. For example, the steel material includes a steel plate, a steel pipe, etc., which are manufactured by hot rolling an ingot having the above-mentioned chemical composition, and further cold rolling it as needed. There are no particular limitations on the heating conditions when performing hot rolling, and ordinary conditions may be used.

When producing steel, steel is melted using conventional methods, and after adjusting the composition, the resulting cast steel billet is hot-rolled and, if necessary, cold-rolled. After hot-rolling, the steel may be water-cooled directly or air-cooled and then reheated and quenched. After hot-rolling, the steel may be wound into a coil. After hot-rolling, it may be cold-rolled and then further heat-treated.

When manufacturing steel pipes, steel plates may be formed into a tubular shape and welded, resulting in UO steel pipes, electric resistance welded steel pipes, forged steel pipes, spiral steel pipes, etc. Seamless steel pipes manufactured by hot extrusion or piercing rolling of steel billets are also included in the steel materials of the present invention.

The coating of the above-mentioned anticorrosion coating can be carried out using standard methods. It is not necessary to apply an anticorrosion coating to the entire surface of the steel material; it is sufficient to apply anticorrosion treatment to only one side of the steel material that is exposed to a corrosive environment, such as the outer or inner surface in the case of steel pipes, i.e., only a portion of the steel material surface.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

Steel having the chemical composition shown in Tables 1 to 3 was melted and formed into a 50 kg ingot, which was then hot forged using conventional methods to produce a 60 mm thick block. The block was then heated at 1120°C for 1 hour, hot rolled, and finished at 850°C to a thickness of 20 mm, and then allowed to cool to room temperature in the air to produce a steel plate.

Two test pieces, each measuring 25 mm wide, 25 mm long, and 4 mm thick, were then taken from each steel plate. One test piece was subjected to the corrosion test described below, simulating a weakly acidic environment. The other test piece was spray-painted with a modified epoxy paint to form a corrosion-resistant coating of approximately 200 μm over the entire surface of the test piece, and a cross-shaped scratch was made in the corrosion-resistant coating to expose part of the base metal, before being subjected to the same corrosion test.

Corrosion resistance was evaluated by immersion testing in a sulfuric acid solution adjusted to a pH of 3. Test pieces were immersed in the solution at 60°C for 6 hours, and the amount of thickness reduction was measured. For corrosion-protected steel, the maximum corrosion depth in the corrosion-protection coating defects was measured using a confocal laser microscope.

The test results are shown in Table 4. "Corrosion weight loss" in the table is the average thickness loss of the test specimen, calculated using the weight loss before and after the test and the surface area of the test specimen. Furthermore, "corrosion depth" is the maximum depth of the paint flaw from the steel surface.

As is clear from the results in Table 4, the steel material of Test Steel No. 4, which is a comparative example, does not contain In, and therefore the corrosion weight loss was large at 2.1 g/m ² /hr, and the corrosion depth exceeded 25.0 μm.

On the other hand, in the case of the steel materials of test steels Nos. 1 to 3 and 5 to 31, which are examples of the present invention, all of them satisfied the component contents specified in the present invention, and therefore the corrosion weight loss was small, at 1.5 g/m ² /hr or less, and the corrosion depth was small, at 18.0 μm or less.

The steel material of the present invention can be used as a corrosion-resistant steel that exhibits excellent corrosion resistance in weakly acidic environments.

Claims (4)

The chemical composition, in mass%, is
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,
The balance is Fe and impurities .
Used in a weakly acidic environment with a pH of 3 to 5 .
Steel material. The chemical composition contains, in mass %, replacing a part of the Fe,
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. The chemical composition contains, in mass %, replacing a part of the Fe,
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 a part of the surface of the steel material has been subjected to anticorrosion treatment.
The steel material according to any one of claims 1 to 3.