JP4185552B2 - Steel material with excellent corrosion resistance - Google Patents

Description

  The present invention is suitable as a material for steel structures such as civil engineering, architecture, steel towers, bridges, construction machines, steel pipes, tanks, etc., and particularly steel structures that are required to have excellent corrosion resistance in an air-polluted environment rich in sulfur oxides. The present invention relates to a steel material having excellent corrosion resistance.

  In general, steel materials are heavily corroded in heavy chemical industries, air pollution environments that contain a large amount of sulfur oxides and dusts from coal combustion, hot springs and volcanic areas with sulfur and hydrogen sulfide, etc. It is thought to be caused by a weakly acidic environment. Conventionally, alloy steel has been used as one of corrosion resistant materials suitable for such an environment.

  Steel with improved corrosion resistance by containing Cr or Ni has been known for a long time and has been put into practical use as various stainless steels. However, stainless steel usually contains 13% by mass or more of an expensive element Cr, and the material cost is very high. Therefore, it is rarely used for a structure or a structural member.

  On the other hand, as a corrosion-resistant steel material, low alloy steel that performs corrosion protection by protective rust is known in addition to stainless steel. These low alloy steels are roughly classified into a weather resistant steel containing Cr, Cu, P and the like and a seawater resistant steel containing Cr, Cu, Mo and the like. Weatherproof steel exhibits an excellent anticorrosion effect under atmospheric conditions, and seawater resistant steel exhibits excellent anticorrosion effects in seawater.

  Since these low alloy steels are cheaper than stainless steel and superior in corrosion resistance compared to ordinary steel, they are often used as structural members. However, protective rust peculiar to weathering steel is not generated in an acid rain environment or SOx environment, and desired corrosion resistance cannot be obtained.

There is also a so-called “sulfuric acid resistant steel”. This is one of the low alloy corrosion resistant steels, which is obtained by adding Cu to the steel and further adding a small amount of auxiliary elements (Sb, Sn, etc.). However, in Japan since 1975, the concentration of SO _{2 in} the atmosphere has been greatly reduced, and the severe corrosive environment caused by sulfur oxides is no longer present except in hot springs. Has not been developed.

  As described above, the addition of elements for improving corrosion resistance such as Cr, Cu, and Ni is commonly used for improving the corrosion resistance of iron. In general, the higher the added amount, the higher the corrosion resistance of these elements. However, as the added amount increases, the cutting performance, mechanical properties, and weldability often decrease, and the material cost also increases. Therefore, it is desirable to keep the element addition amount as low as possible. Thus, improvement in corrosion resistance and improvement in steel properties and cost performance are in a trade-off relationship, and many studies have been conducted to fully satisfy the two, but there is a compromise between some balance points. No.

  On the other hand, sulfuric acid dew-point corrosion steel is used in a sulfuric acid dew-point corrosive environment that is problematic in facilities such as flues, chimneys, and boiler air preheaters that are exposed to exhaust gas that burns heavy oil, coal, garbage, and the like. This is because, when a fuel containing a sulfur content is burned, SOx is generated in the exhaust gas, and this is combined with moisture in the exhaust gas to generate sulfuric acid. And when the temperature of exhaust gas falls and reaches the dew point of sulfuric acid, sulfuric acid gas condenses and corrodes steel materials. As a steel material used in such a sulfuric acid dew point corrosion environment, a sulfuric acid dew point corrosion steel material that exhibits corrosion resistance in a sulfuric acid environment has been developed. In general, low alloy steels containing Sb and Cu, which are effective in resistance to sulfuric acid corrosion, have been put into practical use. In recent years, sulfuric acid dew-point corrosion steel added with low C-low Si-Cu has been proposed (Patent Document 1).

By the way, in recent years, energy demand is rapidly increasing in the East Asian region with rapid economic development. In particular, in China, the production and consumption of coal is increasing at a rapid pace, and it is estimated that sulfur dioxide (SO ₂ ) emissions will be 20 million tons per year, more than 20 times that in Japan. This kind of air pollution in China has been transported over long distances through the air circulation, and even in Japan, areas where acid rain (pH of 4.5 or less) caused by SOx has been observed have been observed.
JP 2006-241476 A (Claim 1)

  However, in conventional steel materials, there is no acid rain having a pH of 4.5 or less, and there is no material having excellent bare corrosion resistance in an air polluted environment with a large amount of sulfur oxides. Civil engineering, architecture, steel towers, bridges, construction machinery, It was not possible to exhibit sufficiently high corrosion resistance depending on the use of steel structures such as steel pipes and tanks. In particular, steel materials used in Japan do not have sufficient corrosion resistance according to their use.

  Then, the subject of this invention aims at provision of the steel material which exhibits the outstanding corrosion resistance also in the air pollution environment where acid rain falls and there are many sulfur oxides.

In order to solve the above problems based on the above knowledge, the steel material excellent in corrosion resistance of the present invention according to claim 1 is:
(A) In mass%, C: 0.02-0.15%, Si: 0.10-1.0%, Mn: 0.1-1.5%, S: 0.02-0.5 %, Ti: 0.02 to 0.15%, Ca: 0.0001 to 0.01% and Al: 0.01 to 0.50% as essential components, and Cu: 0.05 to 3.0 % And Ni: at least one selected from 0.05% to 6.0%, consisting of the balance Fe and unavoidable impurities, between the contents of Ni, Cu, S and Ti [(Ni + 4.5 × Cu) × S × 2500 × Ti> 5], and
(B) The surface contains S: 0.3 to 5.0% by mass, and at least one selected from Ti, Cu, Ni, Nb, Zr and V in total is 0.5 to 10.0% by mass. % Covered with rust, and
(C) A rust layer having a crystallite size of less than 50 nm determined by the X-ray diffraction method of the β-FeOOH component is formed on the surface, and the specific surface area determined by the molecular adsorption method of the rust layer is 10 m ² / g. It is the above.

  In this steel material, the content of Ni, Cu, Ti and S in the air-polluting environment has a specific relationship represented by the above formula (1), thereby making a high protective film made of a unique insoluble sulfide film. Corrosion resistance can be obtained. That is, in an air pollution environment with a lot of sulfur oxides, Ni, Cu, and Ti are generated rust in normal atmospheric corrosion (Ni, Cu: promotion of stable amorphous rust formation, promotion of α rust formation, Ti: unstable In addition to controlling β-rust generation, in the SOx environment, insoluble sulfides can be produced to produce a corrosion-resistant coating. By satisfying the condition (B), high corrosion resistance in a corrosive environment can be exhibited with good reproducibility. Furthermore, by satisfying the above condition (C), there is no flow rust or exfoliation rust even under severe corrosive environment such as chloride environment or air pollution environment with a lot of sulfur oxide, and excellent corrosion resistance. Can demonstrate.

  The steel material excellent in corrosion resistance of the present invention according to claim 2 is further La: 0.0001-0.05 mass%, Ce: 0.0001-0.05 mass%, Mg: 0.0001-0.05. Mass%, Mo: 0.05-3.0 mass%, Nb: 0.005-0.5 mass%, V: 0.01-0.5 mass%, Zr: 0.005-0.5 mass% B: 0.0003 to 0.003% by mass and W: 0.05 to 3.0% by mass.

  In this steel material, the corrosion resistance can be further improved by including a specific amount of at least one selected from La, Ce, Mg, Mo, Nb, V, Zr, B and W. Among them, La, Ce, and Mg act to suppress the pH drop at the corrosion tip and to suppress the formation of MnS that becomes the starting point of pitting corrosion and lower the weather resistance. Furthermore, Zn and Fe are stably added in the early stage of corrosion. Has the effect of corroding. Furthermore, Nb, V, Zr, Mo and B are effective for promoting the generation of protective rust, and Nb and V are effective for improving the hardenability and improving the strength. B also has a function of increasing hardenability.

  The steel material excellent in corrosion resistance of the present invention according to claim 3 has a rust generated on the surface, the fraction of the amorphous component determined by the X-ray diffraction method is 30% by mass or more, and the fraction of the β-FeOOH component Is 30% by mass or less, and the fraction of rust is α-FeOOH / γ-FeOOH> 0.6.

  In this steel material, in the rust generated on the surface, the proportion of amorphous rust effective for improving the corrosion resistance is 30% by mass or more, and the fraction of crystalline rust (β-FeOOH component) that is the starting point for the progress of corrosion. When the ratio is 30% by mass or less and the fraction of rust is α-FeOOH / γ-FeOOH> 0.6, excellent corrosion resistance is exhibited, and dense protective rust can be maintained even in an SOx environment.

  The steel material according to the present invention can exhibit excellent corrosion resistance even in an air polluted environment where acid rain with a pH of 4.5 or less falls and there are many sulfur oxides. Particularly, it is suitable as a material for steel structures such as civil engineering, architecture, steel towers, bridges, construction machines, steel pipes, tanks and the like, and particularly steel structures that are required to have excellent corrosion resistance in an air pollution environment with a lot of sulfur oxides.

  Hereinafter, the steel material excellent in corrosion resistance according to the present invention (hereinafter referred to as “the steel material of the present invention”) will be described in detail.

  The steel material of the present invention contains, by mass%, C, Si, Mn, S, Ti, Ca and Al as essential components, and contains at least one selected from Cu and Ni, and is composed of the balance Fe and inevitable impurities. And having a specific relationship between the contents of Ni, Cu, and Ti. Hereinafter, the numerical range of the content of each component constituting the steel material of the present invention, the reason for limiting the numerical range, and the relationship between the contents of Ni, Cu, and Ti will be described.

C is an element effective for improving the strength of steel, and is an element effective for securing a strength of 390 to 630 N / mm grade ² or higher, but when the C content exceeds 0.15 mass%. Deteriorates the weldability and bare weather resistance of steel. On the other hand, when the C content is less than 0.02% by mass, it is difficult to ensure the strength. From this viewpoint, the C content is 0.02 to 0.15 mass%, preferably 0.04 to 0.10 mass%.

  Si is an element effective for deoxidation and solid solution strengthening of molten steel, and also has an effect of promoting the formation of a dense stable rust layer and improving corrosion resistance such as bare weather resistance. However, when the Si content is less than 0.10% by mass, these effects are insufficient. Moreover, when Si content exceeds 1.0 mass%, weldability will fall. From such a viewpoint, the Si content is 0.10 to 1.0 mass%, preferably 0.2 to 0.8 mass%.

Mn is an element effective for improving the steel strength, and is an element useful for securing a strength of 390 to 630 N / mm grade ² or higher in place of C, but the Mn content is 1.5% by mass. When exceeding, MnS will produce | generate in large quantities in steel and there exists a possibility of causing deterioration of corrosion resistance, such as a bare weather resistance. Moreover, if Mn content is less than 0.1 mass%, it will become difficult to ensure steel strength. From such a viewpoint, the Mn content is 0.1 to 1.5% by mass, preferably 0.3 to 1.3% by mass.

  Conventionally, S is an element that is desirable to be reduced because too many elements cause corrosion starting points such as FeS and MnS. However, in the steel of the present invention, insoluble sulfides in the SOx environment due to the coexistence of Cu, Ni and Ti. Is an element effective in improving corrosion resistance. However, if the content exceeds 0.5% by mass, the mechanical properties deteriorate, and FeS and MnS, which are the starting points of corrosion, are produced in large amounts in the steel, thereby inhibiting the formation of the stable rust layer, Corrosion resistance may be deteriorated. Further, when Ni or the like is excessively contained, a reaction with S causes a NiS compound having a low melting point to precipitate at the grain boundary of the weld metal, thereby easily deteriorating the ductility of the grain boundary of the solidified metal. From this viewpoint, if the S content is 0.5% by mass or less, there is an advantage that a larger amount of Ni can be contained without precipitating the low melting point NiS compound. Then, S content is 0.02-0.5 mass%, Preferably it is 0.01-0.3 mass%.

  P is an element that has the effect of preventing the entry of chloride ions into the rust generated on the surface of the steel material and forming a dense stable rust layer to improve the corrosion resistance. For this reason, the conventional weathering steel is required to contain about 0.05% by mass or more and about 0.15% by mass or less in order to exhibit the effect of improving the corrosion resistance. On the other hand, the excessive P content exceeding about 0.05 mass% significantly deteriorates the weldability. On the other hand, in this invention, since a precise | minute stable rust layer can be formed by containing Ti etc., excessive content of P is unnecessary. Therefore, in consideration of improvement in weldability, when P is contained, the P content is preferably 0.001 to 0.15% by mass. Here, P is an element which is usually reduced in the manufacturing process of steel and inevitably remains in the steel. However, in the present invention, P is an element effective for improving the corrosion resistance. Therefore, in the manufacturing process of the steel material of the present invention, the degree of reduction of P may be relaxed so that the residual amount of P becomes larger.

  Generally, Cr is an element effective for improving the corrosion resistance as added to stainless steel. However, it adversely affects air chloride environments and beach environments. In such an environment, the perforation resistance is particularly improved by reducing the Cr content. In order to improve the perforation resistance, the local corrosion resistance, and the corrosion resistance in a salt environment, it is particularly effective to reduce the Cr content. From this viewpoint, the steel material of the present invention contains Cr. In this case, the Cr content is preferably 0.1% by mass or less. Furthermore, it is desirable to make Cr-free (Cr content is reduced to 0), but it is economically expensive to make it excessively small. In addition, Cr is an inevitable component derived from scrap that is often used as part of the raw material in the steel manufacturing process. Therefore, in the present invention, it is not desirable to positively add like the weathering steel defined in JIS-SMA. Therefore, it is preferable to make it less than 0.02 mass%.

  Ti, like Cu and Ni, has a beneficial effect of densifying the generated rust and promoting the formation of a stable rust layer, and also has very excellent corrosion resistance. It is an important essential additive element. In particular, when Cu and Ni are added together as an element that suppresses the formation of β-FeOOH that is characteristically produced in the beach / marine environment, an excellent effect is exhibited. It also has the advantage of cleaning the steel. Such an effect can be obtained by addition of 0.02% by mass or more, but if the addition exceeds 0.03% by mass, the effect is remarkably increased. However, even if excessive addition is performed, the effect tends to saturate, which is not preferable economically. Further, since there is a possibility that it may be harmful in the case of welding, the upper limit of Ti is 0.15% by mass. Moreover, although the said matter is a case of the corrosion product of steel materials, there exists an effect which a compactness improves by containing Ti also in the corrosion product of zinc. Therefore, it acts on the steel itself and the corrosion products of iron and zinc and has the effect of improving the corrosion resistance, so it is also an extremely important element from this viewpoint. Therefore, the Ti content is 0.02 to 0.15 mass%, preferably 0.03 to 0.10 mass%.

  Ca is an element having an important role in corrosion of a coating film defect portion. In other words, Ca is an element that has an action of buffering the pH drop in the coating film defect, and exhibits alkalinity due to a small amount of dissolution accompanying the corrosion reaction of iron in the progress of corrosion under the coating film (at the tip of the anode dissolution tip). Solution pH buffering effect) element. For this reason, Ca is an element which has the effect | action which suppresses crevice corrosion in a coating-film defect part, raises pH at the time of melt | dissolution, and suppresses crevice corrosion. That is, if there is Ca that raises the pH when iron is dissolved at the tip of corrosion, the progress of crevice corrosion can be suppressed. In this respect, the content of Ca is 0.0001 to 0.01% by mass, preferably 0.0003 to 0.005% by mass.

  Al is an element that has the effect of further promoting the formation of a stable rust layer and further improving the corrosion resistance by being combined with Ti. Al also has an effect of improving weldability. Furthermore, Al is an element effective for improving the toughness of steel by capturing solid solution oxygen as a deoxidizing element of molten steel and preventing the occurrence of blowholes. Al forms an oxide on the surface layer, but the Al oxide particles are small, form a very stable and dense scale with few voids, and contribute to laser cutting properties. When the Al content is less than 0.01% by mass, these effects cannot be sufficiently obtained. On the other hand, when the Al content exceeds 0.5% by mass, the corrosion resistance by promoting the formation of the stable rust layer is described above. The improvement effect is saturated, and conversely, the weldability is deteriorated, and the toughness of the steel is deteriorated due to an increase in alumina inclusions. From such a viewpoint, the Al content is preferably 0.01 to 0.5% by mass, particularly preferably 0.1 to 0.5% by mass.

  Further, the steel material of the present invention contains at least one selected from Cu and Ni in addition to C, Si, Mn, S, P, Cr, Ti, Ca and Al, and consists of the balance Fe and inevitable impurities. Is.

  In the steel material of the present invention, Cu is an element effective for improving corrosion resistance and weldability. In other words, Cu is an electrochemically noble element than iron, and is an element that has the effect of densifying the rust generated on the steel surface, promoting the formation of a stable rust layer, and improving the corrosion resistance such as weather resistance. is there. It also contributes to improved weldability. Further, although part of the surface layer is an oxide, many are effective elements for improving the laser cutting property by concentrating in a solid solution state, densifying the surface scale, and improving the adhesion. When the Cu content is less than 0.05% by mass, the improvement in corrosion resistance is insufficient, and when the Cu content exceeds 3.0% by mass, the effect of improving the corrosion resistance is saturated. Therefore, there is a possibility of causing embrittlement of the material (hereinafter also referred to as hot work brittleness) during processing such as hot rolling. From these viewpoints, in order to more reliably suppress the occurrence of the hot work brittleness, the Cu content is 0.05 to 3.0% by mass, preferably 0.3 to 1.5% by mass. is there.

  Ni is an element effective for improving corrosion resistance and weldability. Ni is an element having the effect of densifying the rust generated on the steel surface, promoting the formation of a stable rust layer, and improving the corrosion resistance such as weather resistance, as in the case of Cu. It also contributes to improved weldability. Furthermore, Ni also has the effect of suppressing the hot work brittleness. Therefore, by containing Ni together with Cu, a synergistic effect of improving corrosion resistance and suppressing hot work brittleness can be expected. Ni, like Cu, partially becomes an oxide in the surface layer, but most of it concentrates in a solid solution state, densifies the surface scale, and improves the laser cutting property by improving the adhesion. When the Ni content is less than 0.05% by mass, the corrosion resistance is insufficiently improved. On the other hand, when the Ni content exceeds 6.0% by mass, the solid-liquid coagulation temperature range in the complete austenite structure is widened and reduced. It promotes the segregation of melting point impurity elements to the dendrite grain boundaries and reacts with S to precipitate a low melting point NiS compound at the grain boundaries of the weld metal, thereby degrading the ductility of the grain boundaries of the solidified metal. Adversely affects weld hot cracking. From these viewpoints, when Ni is contained, the Ni content is 0.05 to 6.0 mass%, preferably 0.5 to 5.0 mass%, more preferably 1.0 to 3.0 mass%. It is.

In the steel material of the present invention, it is essential that the contents of Ni, Cu, S and Ti have a relationship represented by the following formula (1). When the contents of Ni, Cu, S, and Ti satisfy the relationship of the formula (1), S and Ni, Cu, and Ti form a corrosion resistance improving film made of an insoluble sulfide film in an SOx environment, High corrosion resistance can be obtained.
(Ni + 4.5 × Cu) × S × 2500 × Ti> 5 (1)
In the steel material of the present invention, (a) promotion of formation of highly protective amorphous rust, (b) promotion of formation of alpha rust that is highly protective and thermodynamically stable, (c) in a chloride environment The production of β rust, which is characteristically generated and deteriorates the corrosion resistance, (d) the corrosion resistance improvement coating that promotes the corrosion resistance under the air pollution environment (SOx environment) containing a large amount of sulfur oxide and dust, Demonstrate a synergistic effect. Therefore, the present inventors repeated various experiments, and the above formula (1) defines the amount of each element required to satisfy all the items (a) to (d).

  In the steel material of the present invention, Ni, Cu and Ti control the generation of rust in normal atmospheric corrosion (Ni, Cu: promotion of stable amorphous rust formation, promotion of α rust formation, Ti: suppression of unstable β rust formation) ) And S content in the range of 0.02 to 0.5% by mass, coexisting with Ni, Cu, Ti, a large amount of sulfur oxide and dust accompanying heavy chemical industrial zone and coal combustion In the air pollution environment (SOx environment), a corrosion-resistant film made of an insoluble sulfide can be generated, and excellent corrosion resistance can be obtained.

  Furthermore, the steel material of the present invention preferably contains at least one selected from La, Ce, Mg, Mo, Nb, V, Zr, W and B for further improvement of corrosion resistance.

  La, Ce, and Mg act to suppress the pH drop at the corrosion tip and to suppress the generation of MnS that becomes the starting point of pitting corrosion and lower the weather resistance, and also stably corrodes Zn and Fe at the initial stage of corrosion. Has an effect. Therefore, when La, Ce and Mg are contained, the La content is 0.0001 to 0.05 mass%, the Ce content is 0.0001 to 0.05 mass%, and the Mg content is 0.0001 to 0.05 mass%. It is preferable to set it as 0.05 mass%.

  Furthermore, Mo, Nb, V, Zr and B are effective for promoting the generation of protective rust, and Nb and V are effective for improving the hardenability and improving the strength. B also has a function of increasing hardenability. Therefore, from the viewpoint of improving corrosion resistance, when Mo, Nb, V, Zr or B is contained, the Mo content is 0.05 to 3.0 mass% and the Nb content is 0.005 to 0.5 mass%. The V content is preferably 0.01 to 0.5 mass%, the Zr content is preferably 0.005 to 0.5 mass%, and the B content is preferably 0.0003 to 0.003 mass%, more preferably. Has a Mo content of 0.1 to 1.0 mass%, an Nb content of 0.005 to 0.10 mass%, a V content of 0.01 to 0.20 mass%, and a Zr content of 0.005. -0.10 mass%, B content is 0.0003-0.0030 mass%.

  W is also an element effective for improving corrosion resistance. When W is contained, the W content is preferably 0.05 to 3.0% by mass.

  Further, in the steel material of the present invention, an effect of improving the corrosion resistance by forming an insoluble sulfide film in an SOx environment where acid rain falls is obtained, so that Be, As, Sb, Bi, Ge, Sn, Pb, It is preferable to contain 0.002 to 0.2 mass% in total of at least one selected from Se, Te, Zn and Cd. Instead of forming sulfides that are insoluble in water and acid in steel instead of Mn (Cr, Ti, V, etc.), these elements elute into the corrosive environment as a result of corrosion and form on the steel surface. It is thought that an insoluble sulfide film is formed.

  As described above, the steel material of the present invention contains C, Si, Mn, S, P, Cr, Ti, and Ca as essential components, and further contains at least one selected from Cu and Ni, with the remainder being Fe and inevitable. And a specific relationship represented by the formula (1) between the contents of Ni, Cu, and Ti, and, if necessary, the La, Ce, Mg, Mo, Nb, By containing at least one selected from V, Zr, W and B, a protective film made of a unique insoluble sulfide film can be formed and high corrosion resistance can be obtained. That is, in an air pollution environment with a lot of sulfur oxides, Ni, Cu, and Ti are generated rust in normal atmospheric corrosion (Ni, Cu: promotion of stable amorphous rust formation, promotion of α rust formation, Ti: unstable In addition to controlling β-rust generation, in the SOx environment, insoluble sulfides can be produced to produce a corrosion-resistant coating.

Furthermore, the steel material structure of the present invention is basically a mixed structure of ferrite and pearlite, but for example, a required strength of 390 to 630 N / mm class ² or more as a structural material as a structure such as a bridge. In order to ensure strength and toughness and to have excellent corrosion resistance, the ferrite content is preferably 90% or more. As the amount of ferrite increases and the steel structure approaches the ferrite phase monolayer, the steel structure itself is less likely to form a micro battery, and the corrosion resistance such as bare weather resistance is improved. Therefore, the steel structure is more preferably 95% or more of ferrite.

Here, the rust and the rust layer on the surface of the steel material of the present invention will be described below.
The surface of the steel material of the present invention is covered with rust containing S and at least one selected from Ti, Cu, Ni, Nb, Zr and V. This produces dense and fine α-FeOOH rust and amorphous rust in a salt corrosion environment and an air-polluted environment rich in sulfur oxides, and the generation of β-FeOOH is suppressed as much as possible, resulting in high corrosion resistance. Can be obtained with good reproducibility. That is, if the steel surface or steel material rust layer contains or exists the aforementioned S and at least one selected from Ti, Cu, Ni, Nb, Zr and V, then the steel material surface or steel material rust layer Rust generated in the atmosphere is generated by these elements as fine and dense α-FeOOH rust or amorphous rust even in a salt corrosion environment or an air pollution environment. , Β-FeOOH is suppressed as much as possible.

  The effect of these Ti and S on rust formation is that during the rust formation / growth stage, ions, colloidal fine compound particles or fine precipitates (Ti or Ti ions are produced by oxidation, hydrolysis, etc.) Affected by the form of Ti hydroxides, oxyhydroxides, oxides, or other biological elements), disturbing the crystal structure of rust, suppressing growth, and rust defects It is presumed that the starting point of corrosion and peeling is suppressed by the effect of filling the part and the formation of sulfide.

  In the rust covering the surface of the steel material, the S content is 0.3 to 5.0 mass%. When S is contained in the rust layer, the rust layer itself is dense and has high corrosion resistance. Although this mechanism is not clear, it is considered that the insoluble sulfide on the surface of the steel material and fine and fine iron rust containing S, Ni, Cu, and Ti have a synergistic effect, and the corrosion resistance is further improved. If the S content in the rust on the surface is less than 0.3% by mass, the effect of improving the corrosion resistance is not exhibited, and if it exceeds 5% by mass, the corrosion starts, and the corrosion resistance may be deteriorated.

  Furthermore, in the surface rust, the total content of S and at least one selected from Ti, Cu, Ni, Nb, Zr and V is 0.5 to 10.0% by mass. Further, in the rust covering the surface, the lower limit of the total content of S and at least one selected from Ti, Cu, Ni, Nb, Zr and V is preferably 2.0% by mass or more, Furthermore, it is preferable to set it as 3.0 mass% or more.

Moreover, in the rust layer formed on the surface of the steel material of the present invention, the crystallite size of the β-FeOOH component obtained by the X-ray diffraction method is less than 50 nm. Furthermore, the specific surface area calculated | required by the molecular adsorption method of the said rust layer is 10 m < ² > / g or more. As a result, even in severe corrosive environments such as chloride environments and air polluted environments with a lot of sulfur oxides, flow rust and peeling rust are not generated, and excellent corrosion resistance is maintained and the appearance is also maintained. . In particular, in a salt corrosion environment, the stabilization of the rust layer depends on the presence of β-FeOOH rust, but the inventors of the present invention particularly have a crystallite size of β-FeOOH rust and a specific surface area of rust particles of the rust layer. It is a factor that determines the improvement in corrosion resistance. When the crystallite size of β-FeOOH rust exceeds 50 nm, a peeled rust layer is easily formed. Therefore, it is useful to make the crystal size of β rust less than 50 nm. In other words, it is not possible to achieve a significant improvement in corrosion resistance simply by reducing the crystallite size of the rust particles constituting the rust layer, regardless of the type of rust. Lowering the size is important for improving corrosion resistance.

  In the present invention, the main component of rust is α-FeOOH and / or amorphous rust. Of these, especially amorphous rust forms an extremely fine and dense stable rust layer than crystalline rust, and even if a "defect part" as a rust film is formed, the amorphous rust part Has a “defect repair function” for filling this hole. Therefore, the higher the proportion of amorphous rust in the iron rust (the degree of amorphousness), the higher the corrosion resistance. For this reason, in this invention, it is preferable that the fraction of the amorphous component calculated | required by the X ray diffraction method shall be 30 mass% or more in the rust produced | generated on the steel material surface.

On the other hand, other rust, especially crystalline rust such as β-FeOOH, even if the proportion of the amorphous or α-FeOOH in the rust is high, this rust serves as a starting point to promote corrosion. It is necessary to suppress as much as possible. For this reason, in this invention, it is preferable to make the fraction of (beta) -FeOOH component calculated | required by the X ray diffraction method of the rust produced | generated on the steel material surface below 30 mass%. When the fraction of the rust amorphous component is less than 30% by mass and the fraction of the β-FeOOH component (β rust) exceeds 30% by mass, the α-FeOOH, β-FeOOH, γ-FeOOH and Since the crystalline rust component of Fe ₃ O ₄ increases and the rust on the surface of the steel material does not form a dense stable rust layer, there is a possibility that the high corrosion resistance of the steel material cannot be guaranteed.

  In addition, amorphous rust may not be generated depending on the environment. In that case, rust is compared with α rust (stable; easily generated by acid) and γ rust (unstable; easily generated by neutral). It is effective to evaluate the protection. Therefore, in the steel material of the present invention, the fraction of rust is preferably α-FeOOH / γ-FeOOH> 0.6. Here, in this invention, the high corrosion resistance of the rust produced | generated on the steel material surface means the corrosion resistance of the steel material in a SOx environment. Therefore, in order to guarantee this high corrosion resistance, it is necessary to evaluate the corrosion resistance of the steel material based on the evaluation results in the atmospheric exposure of the steel material for 0.5 years or in the acid rain spray test simulating the atmospheric environment.

In the present invention, as a means for measuring the degree of amorphousness of rust, it is disclosed in “Quantification of iron rust components by powder X-ray diffraction method and its application” of “Corrosion protection 95C-306 (pages 341 to 344)”. The produced powder X-ray diffraction method is effective. In this document, quantification of the iron rust component on the surface of the steel material is attempted by a powder X-ray diffraction method for weathering steel material, and the higher the ratio of amorphous rust in the iron rust (amorphous degree), It supports the corrosion resistance improvement model that becomes a dense stable rust layer. As a more specific powder X-ray diffraction method, a powder obtained by mixing a rust sample taken from steel with a constant mass ratio of CaF ₂ or ZnO as an internal standard is identified by a normal X-ray diffraction method. Each crystalline rust component is quantified from the integral intensity ratio of each unique diffraction peak of each type of rust and the calibration curve of each rust component determined in advance, and each of these crystals is calculated from the total amount of rust. The ratio of the amorphous component is calculated by subtracting the amount of the rust component. This is because the integral intensity ratio of the diffraction peaks of the amorphous component itself is difficult to obtain and is difficult to quantify.

Next, the manufacturing method of the steel material of this invention is demonstrated. The steel material of this invention can be manufactured with the manufacturing method of a normal thick steel plate. That is, after a steel is continuously cast or melted by an ingot-making method, hot working such as ingot rolling or hot forging or thick plate rolling is performed to produce a predetermined product plate thickness. These hot working conditions and conditions of cooling and heat treatment after hot working are the properties of steel such as strength of 390 to 630 N / mm grade ² or higher as a structural material of a bridge. It is determined appropriately according to requirements and specifications. Therefore, in addition to normal hot working, a method for ensuring mechanical properties such as the above-mentioned strength may be selected after ensuring low alloying or low carbon equivalent to ensure weldability. For example, forced cooling such as accelerated cooling after hot working or controlled rolling may be performed so that the steel structure of the present invention has a ferrite content of 90% or more. In addition, the heat treatment after the hot working is appropriately subjected to rolling on-line direct quenching (DQ) or off-line quenching and tempering (QT) as necessary.

  Hereinafter, the present invention will be specifically described by comparing an example satisfying the requirements of the present invention with a comparative example not satisfying the requirements of the present invention. The following examples are representative, and the present invention is not limited to these examples.

(Examples 1 to 21)
Each steel ingot having the chemical composition shown in Table 1 was dissolved in the atmosphere at the laboratory level. A 45 kgf square mold for thin plate was used as the mold, and ingots having the chemical composition shown in Table 1 were produced. Next, these ingots were roughly rolled. Heating conditions were 1100 ° C x 30 minutes, no hot rolling pass schedule was specified, and the finishing thickness was 25 mm thick x width x length (where “width in width” specifically specifies the width of the plate This means that the width corresponding to the rolling is maintained, and the length is also the same. Thereafter, gas cutting was performed to prepare a plate material having a length of 300 mm for each material. Next, finish rolling was performed. The hot rolling conditions were heating 1100 ° C. × 2 hr, the number of passes was 5 passes, and the finishing temperature was 900 ° C. ± 50 ° C. The finishing size was 6 mm thick × the width × length, the cooling rate was 70 ° C./sec, and the stop temperature was 650 ° C. Thereafter, the furnace was cooled in a holding furnace: 600 ° C. × 60 minutes to produce a steel plate. Test pieces were collected from these steel plates.

  Furthermore, from the collected test pieces, test materials having a size of 50 mm × 50 mm width × 3 mm thickness were prepared, and corrosion resistance was evaluated by the following test.

Corrosion Resistance Evaluation Test A combined cycle type accelerated lab test was conducted for 7 days. This combined cycle test consists of 2 cycles of artificial acid rain spray with pH = 3.5 for 2 hours, drying (temperature 60 ° C., humidity 40%) for 2 hours, and a humid environment (temperature 40 ° C., humidity 95%). The time setting process was defined as one cycle, and four cycles were performed per day. And after the test, after removing the rust by liquid honing, the weight was measured and the corrosion weight loss was measured. No. The corrosion amount of each test piece was standardized with the corrosion weight loss of 1 as 100.

Analysis of generated rust The crystallite size of β-FeOOH rust (Miller index: 110) was measured by an X-ray diffraction method for a rust sample collected from a test material subjected to a corrosion resistance evaluation test. For some samples, the crystallite size of γ-FeOOH rust (β rust) (020) or magnetite (220) was measured.

In addition, a rust sample collected from the test material was identified by the above-mentioned X-ray diffraction method, and the intrinsic diffraction peak of each of the three types of rust of α-FeOOH, β-FeOOH, γ-FeOOH, and Fe ₃ O ₄ Quantify each crystalline rust component from the integral strength ratio and the calibration curve for each rust component obtained in advance, and subtract the amount of each crystalline rust component from the total amount of rust to obtain amorphous Component ratio and rust fraction: α-FeOOH / γ-FeOOH was calculated.

Specific surface area of rust layer An N ₂ adsorption isotherm was determined at liquid nitrogen temperature using an automatic capacity adsorption device, and a specific surface area was determined by performing a BET plot using this adsorption isotherm.

Elemental analysis in rust In addition, the test piece is cut and the cross section is observed with a scanning electron microscope (SEM) at a magnification of about 100 to 2000 times, and the location where the iron layer on the surface is in close contact with the ground iron is arbitrarily extracted. Then, the degree of concentration (content) of the alloy elements (S, Ti, Cu, Ni, Nb, Zr, V) of the rust layer was measured by an electron beam probe microanalyzer (EPMA).

  Table 2 shows the results of the corrosion resistance evaluation test, analysis of the generated rust, measurement of the specific surface area and pore diameter of the rust layer, and elemental analysis in rust.

  As shown in Table 2, the inventive examples (Nos. 6 to 16) exhibited excellent corrosion resistance compared to the comparative examples (Nos. 1 to 5). In addition, the present invention example is not only excellent in corrosion resistance, but also has a small crystallite size of β-FeOOH rust and a large specific surface area of the rust layer. That is, the examples of the present invention have excellent corrosion resistance, the fraction of the amorphous component determined by X-ray diffraction method is 30% by mass or more, and the fraction of the β-FeOOH (β rust) component is 30% by mass or less. In many cases, the rust fraction (α-FeOOH / γ-FeOOH) exceeds 0.6. On the other hand, it can be seen that the comparative example does not satisfy any of these conditions and is inferior in corrosion resistance.

  Further, in the present invention examples (Nos. 6 to 16), effective elements such as S, Cu, Ni, and Ti were concentrated in the rust layer, and the results of the corrosion resistance test were supported.

Claims (3)

(A) In mass%, C: 0.02-0.15%, Si: 0.10-1.0%, Mn: 0.1-1.5%, S: 0.02-0.5 %, Ti: 0.02 to 0.15%, Ca: 0.0001 to 0.01% and Al: 0.01 to 0.50% as essential components, and Cu: 0.05 to 3.0 % And Ni: at least one selected from 0.05% to 6.0%, consisting of the balance Fe and unavoidable impurities, between the contents of Ni, Cu, S and Ti [(Ni + 4.5 × Cu) × S × 2500 × Ti> 5], and
(B) The surface contains S: 0.3 to 5.0% by mass, and at least one selected from Ti, Cu, Ni, Nb, Zr and V in total is 0.5 to 10.0% by mass. % Covered with rust, and
(C) A rust layer having a crystallite size of less than 50 nm determined by the X-ray diffraction method of the β-FeOOH component is formed on the surface, and the specific surface area determined by the molecular adsorption method of the rust layer is 10 m ² / g. A steel material excellent in corrosion resistance characterized by the above.   Furthermore, La: 0.0001-0.05 mass%, Ce: 0.0001-0.05 mass%, Mg: 0.0001-0.05 mass%, Mo: 0.05-3.0 mass%, Nb: 0.005 to 0.5 mass%, V: 0.01 to 0.5 mass%, Zr: 0.005 to 0.5 mass%, B: 0.0003 to 0.003 mass%, and W The steel material excellent in corrosion resistance according to claim 1, comprising at least one selected from 0.05 to 3.0% by mass.   The fraction of the rust generated on the surface is 30% by mass or more of the amorphous component determined by the X-ray diffraction method, the fraction of the β-FeOOH component is 30% by mass or less, and the fraction of rust is α- The steel material excellent in corrosion resistance according to claim 1 or 2, wherein FeOOH / γ-FeOOH> 0.6.