JP4762926B2 - High weatherability steel with improved dense rust formation and steel structure using the same - Google Patents

Description

  The present invention relates to a weather-resistant steel material that enables non-painting of a structure in order to achieve minimum maintenance of steel structures including bridges, and in particular, improved performance of Cu-Ni high weather resistance steel About.

  The conventional weathering steel commercialized for the first time in the United States in 1933 was introduced into Japan in the 1960s, and SMA weathering steel defined in JIS G3114 (hereinafter abbreviated as JIS-SMA material). Is still widely applied to steel structures with high minimum maintenance needs such as bridges. In the JIS-SMA material, the action of forming dense protective rust mainly by Cu and Cr is utilized, and the effect of reducing the corrosion rate due to long-term exposure is manifested.

  On the other hand, when salt damage becomes severe beyond a predetermined range, Cr lowers the pH of condensed water at the steel / rust interface, thus accelerating corrosion. In order to eliminate this instability, Cr is not added, and referring to the standard of JIS-SMA material, Cu is added to increase the amount of Ni while preserving Cu to enhance the adhesion of protective rust to increase salt damage resistance. -Ni-based high weathering steel. While the main function of protective rust formed on JIS-SMA material was to improve adhesion and environmental barrier properties, the characteristic of protective rust formed on Cu-Ni high weather resistance steel is The pH control function at the rust interface is added (see Patent Document 1).

Regarding the effect of additive elements on Cu-Ni high weather resistance steel, each element of C, Si, Mn, P, S, Cu, Ni, Cr, Mo, and Ti is the Tokyo Institute of Technology Creation Project Research Group SIG1 (Study Group on Application of High Weathering Steel to Bridges), with the cooperation of Japan Iron and Steel Institute, Bridge Research Group, Weatherproof Steel Corrosion Design WG, weathering alloy shown in the following formula (I) A method for calculating the index V has been proposed (see Non-Patent Document 1). That is, as far as these components are concerned, it can be said that the high salt resistance of nickel-based high weathering steel materials can be generally grasped by the V value as a common recognition. That is, the V value is a kind of indicator that the higher the value, the higher the salt resistance of the nickel-based high weathering steel material, and the contribution of the main elements to the salt resistance is shown. In the formula (I), the V value is calculated by substituting the mass% of each element. However, there is still no established theory about the effects and influences of trace additive elements other than the elements handled in formula (I).
V = 1 / {(1.0-0.16 [C]) ・ (1.05-0.05 [Si]) ・ (1.04-0.016 [Mn]) ・ (1.0-0.5 [P]) ・ (1.0-1.9 [S]) ・(1.0-0.10 [Cu]) ・ (1.0-0.12 [Ni]) ・ (1.0-0.3 [Mo]) ・ (1.0-1.7 [Ti])} (1)

  However, sufficient weather resistance cannot be obtained with these additive elements alone, and further attempts have been made. In patent document 1, in mass%, Ti: 0.01-0.5%, La: 0.0001-0.05%, Ce: 0.0001-0.05%, Mg: 0.0001-0. In addition to containing one or more of 05%, Ti is essential for the formation of a “stable rust layer”, and the effects of La, Ce, and Mg added to iron corrosion reaction Along with this, a small amount is dissolved to become alkaline. However, the effect is not sufficient in a portion where the pH drop accompanying the progress of corrosion is remarkable. In Patent Document 2, the formation of β-FeOOH rust is suppressed by coating the steel surface with rust containing one or more of Ti, Nb, Ta, Zr, V, and Hf. Corrosion resistance is said to improve. However, when the amount of incoming salt is large, there is a problem that it is difficult to suppress the formation of β-FeOOH rust. In anticipation of the effect of the elements used in both of these documents, Patent Document 3 simultaneously adds these elements. However, as described in Patent Document 4, the effect of suppressing the formation of β-FeOOH rust due to the addition of Ta and Ce is smaller than that of Ti, and the effect is not sufficient by simply adding Ta and Ce. Absent.

Chiaki Miki, Atsushi Ichikawa, Makoto Ukai, Masahiro Takemura, Takenori Nakayama, Hiroshi Kihira: Proceedings of Japan Society of Civil Engineers, No.738 / I-64, pp.271-281, 2003. Japanese Patent No. 3568750 Japanese Patent No. 3648085 Japanese Patent Laid-Open No. 11-241139

  Therefore, the present invention provides a steel material having high weather resistance that can further improve the performance of Cu-Ni high weather resistance steel developed so far, extend maintenance intervals, and reduce maintenance costs. With the goal.

The present inventors examined a method for improving the addition effect of Ce and Ta in high purity Cu—Ni high weather resistance steel. As a result, precipitated particles containing cerium are present in the steel, the number of particles having a size of 1 nm to 2 μm is 10 ³ / cm ³ or more, and 50 mol% or more of the cerium element contained in the steel. The present invention has been found out that it may be in the state of Ce (III) or Ce (IV).

  The reaction in which Fe (II) formed when steel corrodes forms rust can be described as the following formulas (2) and (3).

Fe (II) → Fe (III) + e ⁻ (2)
Fe (III) + 2H ₂ O → FeOOH + 3H ⁺ (3)
The mechanism by which the weather resistance is improved when Ce and Ta are added is not clear, but for the reasons described below, the reactions of formulas (2) and (3) are controlled, and FeOOH particles are refined. It is considered that a rust that is densified and has excellent weather resistance is formed.

  First, the reaction of formula (2) is controlled by adding Ce. When the steel material to which Ce is added corrodes, the individually dissolved metallic Ce and the precipitated Ce compound in the steel material are dissolved as Ce (III) ions in the water film formed on the steel material surface. In the atmospheric corrosion process, wetting and drying are repeated, so in the case of Cu-Ni high weathering steel, it corrodes when wet and Fe (II) ions, Ni (II) ions, Cu ions, etc. together with Ce (III) Ions dissolve. Among them, Fe (II) ions have high solubility and are difficult to change to rust fine particles necessary for forming protective rust, but are oxidized by slightly dissolved oxygen in the aqueous solution and have low solubility Fe (III) ions It becomes. In this process, the solubility of Fe ions decreases, so the formation of rust particles is chemically driven, and if nucleation sites of rust colloidal particles are present, the rust refinement and densification are greatly promoted. . In order to act as a nucleation site for colloidal grains, it is necessary that the average size of the cerium-containing particles in the steel is fine, and Ce (III) ions are effectively precipitated during dissolution. Has arrived at the present invention based on the idea that the state of cerium in steel may be Ce (III) or Ce (IV).

Next, the reaction of formula (3) is controlled by addition of Ce and Ta. If the cerium in the steel is in such a form, fine rust generation starts in the coexistence of nucleation sites and Ce (III) ions, but a structure different from Ce in order to activate the reaction at an accelerated rate. Coexistence of nucleation sites with For that purpose, Ta may be added simultaneously with Ce. Fe in the rust has a structure in which a regular octahedron in which six oxygens are coordinated around Fe is taken as a unit, and forms rust. On the other hand, Ta ₂ O _5, which is an oxide of Ta, has a shape in which a pentagonal bipyramidal portion and a strained octahedral structure portion are mixed when the arrangement of O surrounding Ta is examined. That is, in the process of forming a rust having a regular octahedron structure from a solution, Ta ₂ O ₅ becomes a nucleation site of rust, which greatly promotes rust refinement and densification. This is an effect which cannot be expected only by adding conventional Ce. Incidentally, Ta ₂ O ₅ is formed during steelmaking, it is dispersed in the steel, partially exposed on the steel surface, believed to contribute to the stabilization of rust, as described above.

  That is, in Cu-Ni high weathering steel, a protective rust having a higher density and higher adhesion can be formed by the action of Cu and Ni. However, the corrosion rate is lower than that of a conventional weathering steel, so the rust is formed. The amount of rust particles required for the layer is small, and in order to form a denser rust layer, it is necessary to make the rust particles generated by the reactions of the formulas (2) and (3) finer and denser. . When Ce and Ta are added to Cu-Ni high weathering steel at the same time, it is ionized as Ce (III) by the corrosion reaction when wet, then reacts with oxygen during drying and partially in the Ce (IV) state in the rust layer Is taken in. Ce (IV) is an ion having a strong oxidizing power. Therefore, it has the action of rapidly oxidizing Fe ions, etc. that are corroded again when wet, so it promotes nucleation of rust particles together with coexisting Ta, refines the rust particles, and forms a rust layer in the drying process Can be expected to be more precise.

In order to verify that the present invention improves long-term corrosion resistance, long-term exposure of Cu-Ni high weathering steel was performed. The relationship between the aging corrosion amount (aging corrosion depletion / mm) Y of Cu-Ni high weathering steel material and the elapsed time X is expressed by the equation (4) where As is the initial year corrosion amount and Bs is the rust stabilization index. It is known to be indicated by
Y = As · X ^Bs (4)
A small amount of Ce was added to Cu—Ni-based high weather resistance steel and subjected to a long-term exposure test, and the As value and Bs value of Formula (4) were evaluated and analyzed using the obtained data. As a result, it was found that the As value and the Bs value became smaller by the simultaneous addition of Ce and Ta. This proves that the addition of Ce and Ta to Cu-Ni high weathering steel, even in trace amounts, has the effect of assisting the formation of protective rust at the corrosion interface over a long period of time. Obtaining such verification results, the following invention was made.

(1) By mass%, C: 0.03% to 0.18%, Si: 0.1% to 1.5%, Mn: 0.2% to 1.5%, P: 0.02% or less , S: 0.02% or less, Cu: 0.3% to 3%, Ni: 1.0% to 6%, Ce: 0.0001% to 0.03%, Ta: 0.0001% to 0.00%. 10 ³ weathering steel containing 05%, balance Fe and unavoidable impurities, and having precipitated particles containing cerium in the steel, the size of which is 1 nm or more and 2 μm or less / Cm ³ or more, and 50 mol% or more of the cerium element contained in the steel is in the state of Ce (III) or Ce (IV), and is a weather resistant steel with improved dense rust formation.
(2) Further, Ti: 0.001% by mass to 0.2% by mass, and the product of Ce, Ta and Ti by mass ppm [Ce] × [Ta] × [Ti] is 10 or more. The weather-resistant steel having improved dense rust formation as described in (1) above.
(3) Further, any one of P: more than 0.02% to 0.2%, Cr: 0.01% to 0.75%, Mo: 0.01% to 0.60% by mass% The weathering steel having improved dense rust formation as described in (1) or (2) above, which contains one or more kinds.
(4) Further, by mass%, Al: 0.001% to 0.08%, V: 0.001% to 0.05%, Nb: 0.001% to 0.05%, N: 0.001 % -0.010% of any one of the above, (1) to (3), the weatherproof steel with improved dense rust formation.
(5) Furthermore, Ca: 0.0001 mass% to 0.005 mass% is contained, The weather resistance which improved the dense rust formation property of any one of said (1)-(4) characterized by the above-mentioned. Steel.
(6) The dense rust-forming property as described in (5) above, wherein at least one of calcium oxide and a composite oxide of calcium and aluminum is present in the steel in an amount of 0.0001% by mass or more. Enhanced weathering steel.
(7) A steel structure in which the weathering steel according to any one of (1) to (6) is used, wherein a surface layer of the weathering steel is covered with a rust layer, and cerium in the rust layer A steel structure characterized in that 50 mol% or more of the element is in a state of Ce (III) or Ce (IV).

  According to the present invention, a steel material having high weather resistance can be produced. By applying the steel material of the present invention, the minimum maintenance of steel structures including bridges can be achieved by unpainting the structures, etc., so that the industrial utility value is great.

  The best embodiment of the present invention will be described in detail below. First, the basic steel structure will be described.

  C is an essential element for imparting a predetermined strength to the Cu—Ni high weather resistance steel. Moreover, C is an element which improves the weather resistance of steel materials as also in the formula (1). If the C concentration is less than 0.03% by mass, the strength or weather resistance is insufficient. If the C concentration is more than 0.18% by mass, there arises a problem that the toughness is lowered.

  Si is a basic element used for deoxidation during refining. Moreover, Si is an element which improves the weather resistance of steel materials as shown in Formula (1). When the Si concentration is less than 0.1% by mass, the weather resistance is insufficient. When the Si concentration exceeds 1.5% by mass, there arises a problem that toughness and weldability are lowered.

  Mn is a basic element that increases strength and improves workability. Further, Mn is an element that improves the weather resistance of the steel as shown in the formula (1). When the Mn concentration is less than 0.2% by mass, the weather resistance is insufficient. When the Mn concentration exceeds 1.5% by mass, there arises a problem that toughness and weldability are lowered.

  P is an element that greatly improves the weather resistance as shown in the formula (1), but 0.02% by mass or less is preferable when a decrease in toughness or a decrease in weldability is a problem.

  Since S is an element that lowers the weather resistance as shown in the formula (1), it is not added. Even considering the amount of contamination as an unavoidable impurity, if the S concentration exceeds 0.02 mass%, the weather resistance is insufficient.

  Cu is an element that improves the weather resistance, as described above and in formula (1). When the Cu concentration is less than 0.3% by mass, the weather resistance is insufficient. If the Cu concentration exceeds 3% by mass, problems such as hot working cracks occur.

  Ni is an element that improves the weather resistance, as described above and in formula (1). When the Ni concentration is less than 1.0%, the weather resistance is insufficient, and there is no significant difference compared to the weather resistance of the conventional JIS-SMA material. If the Ni concentration exceeds 6% by mass, the production cost increases and this is industrially problematic.

  Ce and Ta are contained in the ranges of Ce: 0.0001% by mass to 0.03% by mass and Ta: 0.0001% by mass to 0.05% by mass, respectively. When Ce and Ta are added to the steel, as described above, an epoch-making effect that cannot be found by adding each of them alone appears. When the Ce concentration is less than 0.0001% by mass, the effect of adding Ce to the weathering steel is not exhibited. If the Ce concentration is more than 0.03 mass%, production problems such as cracking occur in the hot rolling process. Further, when a small amount of Ce is added, the surface quality is stabilized in the casting process, so that the load on the subsequent hot rolling process is reduced, which may lead to a reduction in manufacturing cost. When the Ta concentration is less than 0.0001% by mass, the effect of adding Ta to the weathering steel is not exhibited. If the Ta concentration exceeds 0.05 mass%, a significant increase in effect cannot be expected, and the manufacturing cost increases.

  If Ce and Ta are put into the steel in the early stage when the oxygen concentration in the steel in the steelmaking process is high, Ce and Ta have high affinity with elements such as oxygen, so the oxide of Ce or Ta is early. However, the rate of staying in the steel is lowered, the yield is lowered, and the oxide of Ce or Ta is coarsened, and the working efficiency in rust is lowered. Therefore, in order to increase the yield of Ce and Ta in steel, it is desirable to add Ce and Ta immediately before casting at the stage where the main alloy element addition and the main deoxidation process are completed. By producing by such a method, it becomes possible to keep the average size of fine precipitated particles containing cerium small and to keep the ratio of Ce (III) or Ce (IV) in the cerium element high.

The effect is manifested when there are precipitated particles containing cerium in the steel and the number of particles having a size of 1 nm or more and 2 μm or less is 10 ³ / cm ³ or more. If the size of the particles is less than 1 nm, the structure of the precipitated particles tends to be incomplete, and the above-described action of the colloidal particles as nucleation sites is significantly reduced. In addition, when the particle size exceeds 2 μm, the specific surface area per unit mass becomes small, and the above-described action as a nucleation site of the colloidal particles is remarkably reduced. If the number of cerium-containing precipitated particles having a size of 1 nm or more and 2 μm or less is less than 10 ³ / cm ³ , the frequency of the presence of fine particles at the interface where corrosion proceeds is reduced, and colloidal particles As a nucleation site, there is a marked decrease in the action.

  Furthermore, if 50 mol% or more of the cerium element contained in the steel is in the state of Ce (III) or Ce (IV), the effect is manifested. When the ratio is less than 50 mol%, Ce (III) ions are not effectively precipitated at the time of dissolution, and the above-described formation of rust fine particles is extremely reduced. By defining the state of cerium in the steel as in the present invention, rust refinement and densification are remarkably promoted and weather resistance is greatly improved as compared with the case where Ce and Ta are simply added. . That is, it is a major feature that it has been found that there is a significant weather resistance improvement effect that is not in addition, with an action that is completely different from the addition of the effects when Ce and Ta are added individually. .

  The size of fine precipitate particles and the density in steel are measured as follows. The size of the precipitate is measured by observing the precipitate with an electron microscope or the like, and it is confirmed whether or not cerium is contained in the precipitate from the fluorescent X-ray generated when the electron beam is irradiated. This measurement is repeated, and the size and density in the steel are determined for about several hundreds of cerium-containing precipitates. As another method, only the precipitate can be extracted and the extract can be observed with an electron microscope. As a method for extracting precipitates, a method described in Non-Patent Literature: Japan Iron and Steel Institute, Joint Research Group, Steel Analysis Subcommittee: “Analytical Technology in the Japanese Steel Industry” (1982) can be used, and 10 volumes. % Acetylacetone-1 mass% tetramethylammonium chloride-methanol solution (hereinafter referred to as 10% AA solution), 4 mass% sulfosalicylic acid-1 mass% lithium chloride-methanol solution (hereinafter referred to as 4% SSA system solution) As an extraction residue, for example, by a potentiostatic electrolysis method using an organic solvent such as a halogen organic solvent method in which the steel chips are dissolved in a 6% by mass bromine-methyl acetate solution or a 14% by mass iodine-methanol solution. it can.

Confirmation that 50 mol% or more of the cerium element contained in steel is in the state of Ce (III) or Ce (IV) is possible by measuring the X-ray absorption rate in the X-ray energy region near the Ce absorption edge. It is. The sample is irradiated with X-rays, and Ce fluorescent X-rays therefrom are measured with a detector such as a semiconductor detector. This measurement is performed in a range of 5200 to 6800 eV, which is a region including 5723 eV where the CeL _III absorption edge is observed.

An example of the measurement result is shown in FIG. At the Ce L _III absorption edge, the absorptance increases rapidly as the energy increases. The value of this energy depends on the valence of Ce in the rust. Ce (III) is about 1 to 4 eV higher than Ce in the metal state, and Ce (IV) is about 4 to 7 eV higher than Ce in the metal state. Furthermore, when Ce is combined with oxygen, characteristic peaks are observed for Ce (III) and Ce (IV), respectively. Therefore, it can be easily detected that Ce in the rust is a mixed state of Ce (III) and Ce (IV). For each curve, the measured absorption curve is expressed as the sum of the Ce metal curve and the Ce (III) and Ce (IV) standard (Ce ₂ O ₃ and CeO ₂ ) curves. If the weight is determined, the respective ratios can be obtained. It is preferable that 50 mol% or more of the cerium element contained in the steel determined by this method is in the state of Ce (III) or Ce (IV).

  As a simple method for confirming that 50 mol% or more of the cerium element contained in steel is in the state of Ce (III) or Ce (IV), the state of Ce (III) or Ce (IV) is finely precipitated. It can be determined by calculation from the amount of fine precipitates obtained by the above-described fine precipitate extraction method and the amount of cerium in steel. However, since the state of cerium may change during processing, it is desirable to verify by a direct method such as the X-ray absorption method described above.

  In addition to the above, if the surface layer of the steel is covered with a rust layer so that 50 mol% or more of the cerium element in the rust is in the state of Ce (III) or Ce (IV), the weathering steel Can be increased. This is because if the steel plate surface is already covered with such a rust layer, there will already be enough nucleation sites for colloidal grains, and as described above, rust refinement and densification are greatly reduced. Because it promotes. When the ratio of Ce (III) or Ce (IV) of cerium in rust is less than 50 mol%, Ce (III) ions do not effectively precipitate during dissolution, and the formation of the aforementioned rust fine particles is chemically Driven extremely little.

Confirmation that 50 mol% or more of the cerium element in the rust is in the state of Ce (III) or Ce (IV) is possible by measuring the X-ray absorption rate in the X-ray energy region near the Ce absorption edge. is there. In the case of rust, a method of collecting the rust and sandwiching it in an X-ray transmission film or the like and irradiating the sample with X-rays is convenient. Then, the X-ray intensity (I ₀ ) before transmission of the sample and the X-ray intensity (I) after transmission are measured with a detector such as an ion chamber, and the sample thickness is included by μt = −ln (I / I ₀ ). The absorption rate μt can be obtained. The rest is performed in the same manner as the analysis of cerium in the steel described above.

  In order to cover the surface of steel with the rust layer of the present invention, 20 or more cycle tests in which a humidity process with a relative humidity of 40% RH or more and a drying process with a relative humidity of 34% RH or less are alternately repeated for 1 hour or more are repeated. Do. It is more desirable to carry out the wet process and the dry process in a cycle of about 3 to 10 hours each, so that the number of repetitions is about 10 to 100 days. If necessary, water or an aqueous solution such as diluted seawater is sprayed on the steel surface at the beginning of one cycle.

An example of one cycle is shown below.
[1] Temperature = 28.0 ° C. and relative humidity = 90% RH. Holding time = 4 hours and 36 minutes [2] The temperature is changed continuously from the final state of [1] to temperature = 49.0 ° C. and relative humidity = 32% RH over a period of time = 2 hours and 22 minutes.
[3] From the final state of [2], temperature = 54.0 ° C. and relative humidity = 25% RH are continuously changed over time = 1 hour 40 minutes.
[4] From the final state of [3], the temperature is changed continuously over a period of 1 hour and 22 minutes from 55.0 ° C. to a relative humidity of 24% RH.
[5] The temperature is continuously changed from the final state of [4] to temperature = 54.0 ° C. and relative humidity = 25% RH over a period of time = 1 hour 22 minutes.
[6] From the final state of [5], the temperature is continuously changed from 49.0 ° C. and relative humidity to 32% RH over a period of 1 hour and 40 minutes.
[7] From the final state of [6], temperature = 28.0 ° C. and relative humidity = 90% RH are continuously changed over a time = 2 hours 22 minutes.
[8] Temperature = 28.0 ° C, relative humidity = 90% RH. Retention time = 4 hours 36 minutes [9] Temperature = 28.0 ° C, relative humidity = 95% RH. Holding time = 4 hours 00 minutes The basic configuration of the present invention has been described above, but the present invention can further improve the weather resistance by the following manner.

In the present invention, Ti is within a range of 0.001 mass% to 0.2 mass%, and the product [Ce] × [Ta] × [Ti] of each mass ppm of Ce, Ti, Ta is 10 ([[ ppm] ³ ) or more. If it does in this way, a weather resistance can be improved further. This is presumably because in the oxide, Ti is in the (IV) valence and Ta is in the (III) valence state, so that Ce is easily in the (III) valence + (IV) valence state. The crystal structure of each oxide is such that Ce (III) is a La ₂ O ₃ structure, Ce (IV) is a fluorite structure, Ti (IV) is an anatase or rutile structure, and Ta is a pentagon. It is a structure combining both pyramids and strained octahedron. Since rust has a structure based on a regular octahedron, it is considered that these additive elements become nucleation sites in the process of rust formation from the solution, and greatly promote rust refinement and densification. Thus, since the addition effect of Ce, Ti, Ta is expressed by the mutual synergistic action, it was discovered that the effect can be evaluated by the concentration product, and when each concentration of Ce, Ti, Ta is expressed in mass ppm, It has been found that the lower limit value at which the effect appears is 10.

  In the present invention, any of P: more than 0.02 mass% to 0.2 mass%, Cr: 0.01 mass% to 0.75 mass%, Mo: 0.01 mass% to 0.60 mass%, By including one or more kinds, the weather resistance is further improved.

  P greatly improves the weather resistance, but it must be limited as described above when the toughness and weldability must be sufficiently taken into consideration. However, in the case where deterioration in toughness or weldability does not become a major problem, it is desirable that the content is more than 0.02% by mass and 0.2% or less because the effect of improving weather resistance can be used. On the other hand, if added over 0.2%, adverse effects such as a reduction in toughness and a decrease in weldability become remarkable, and this is not practical.

    As noted in the formula (1), Cr is an element that improves the corrosion resistance of steel in areas where the weather resistance is not too high. Assuming that the salt damage is not so severe, the upper limit is set to 0.75 mass% as a range that does not adversely affect the rust stabilization performance. However, depending on the site of the structure, salt may easily accumulate. Therefore, the content is desirably 0.3% by mass or less, and more desirably 0.1% or less.

  Mo is an element that improves the weather resistance as shown in Formula (1), and expresses an effect that is not in the single effect of Mo due to the effect of the present invention. In order to reliably exhibit the effects of the present invention, the content is preferably 0.01% by mass or more and 0.60% by mass or less.

  Al: 0.001% by mass to 0.08% by mass, V: 0.001% by mass to 0.05% by mass, Nb: 0.001% by mass to 0.05% by mass, N: 0.001% by mass By containing any one or more of ˜0.010 mass%, the weather resistance is further improved. Al, V, and Nb do not inhibit the Ce and Ta addition effect, but are considered to have an assisting action. These elements can also be used to increase the strength of weathering steel. Further, N is combined with the added aluminum to effectively act on the austenite grain size, thereby improving the mechanical properties of the weathering steel. In addition, N undergoes chemical changes to ammonium ions, nitrite ions, etc. during the corrosion process, so that the effect of increasing the pH of the corrosion interface and the effect of anti-corrosion inhibitors are exhibited, and the surface of the steel is more reliably passivated and weather resistant steel. Helps stabilize rust. A large amount of addition deteriorates toughness and weldability and places an excessive load on the manufacturing process.

  Furthermore, corrosion resistance can be kept high by containing 0.0001 mass%-0.005 mass% of Ca. This is because when covered with dense protective rust, there is an effect of increasing the pH of the corrosion interface when wet. Ca addition does not inhibit the effect of Ce addition to improve the long-term weather resistance of Cu-Ni high weather resistance steel, and assists rust stabilization by increasing the pH.

  Of the inclusions in the steel formed by the addition of Ca, the effect of raising the pH is the calcium oxide, and the next is the utilization of the complex oxide of calcium and aluminum. Therefore, it is desirable that at least one of calcium oxide or a composite oxide of calcium and aluminum is present in the steel. As a method for the confirmation, there can be used a method in which steel is electrolyzed in a non-aqueous solvent, only inclusions are taken out, and the structure is determined by X-ray diffraction or an electron microscope.

  In the steel structure manufactured using the weathering steel of the present invention described above, the surface layer of the weathering steel is covered with the rust layer, and 50 mol% or more of the cerium element in the rust layer is Ce (III) or Ce. Since it is in the state of (IV) and has high weather resistance even without painting, it becomes possible to make minimum maintenance of steel structures including bridges.

  Examples of the present invention will be described below, but the conditions adopted in the examples are one example of conditions for confirming the feasibility and effects of the present invention, and the present invention is limited to this example. is not. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the present invention.

  After melting and producing steel ingots having chemical components described in Tables 1 and 2, rolled sheets were produced by hot rolling at a heating temperature of 1100 ° C. A plurality of sheets having a predetermined size were cut out from the rolled sheet, and the surface was ground with water-resistant abrasive paper to a particle size of 600th finish. The sample was cut out, the precipitate was observed in the structure with an electron microscope, and it was confirmed from the fluorescent X-rays generated when the electron beam was irradiated whether the deposit contained cerium. This measurement was repeated, and the number of those having a size of 1 nm or more and 2 μm or less was measured.

In order to confirm that 50 mol% or more of the cerium element contained in steel is in the state of Ce (III) or Ce (IV), the X-ray absorption rate is measured in the X-ray energy region near the Ce absorption edge. It was. The sample was cut out, the sample was irradiated with X-rays, and the fluorescent X-rays of Ce from the sample were measured with a semiconductor detector. The measurement was performed in a range of 5200 to 6800 eV, which is a region including 5723 eV where the Ce L _III absorption edge is observed. Ce metal, Ce ₂ O ₃ , and CeO ₂ were prepared as standard substances for Ce (0), Ce (III), and Ce (IV), and the absorption curves measured for actual samples were prepared. The ratio of Ce (III) or Ce (IV) in the steel was determined by comparing with the curve of. The measurement results of each sample are shown in Tables 3 and 4.

About a part of sample, the rust layer was formed in the surface with the following method. The sample was cut into a size of 50 × 50 × 3 mm, and a cycle test was performed by dripping salt water. The front side of the sample and the other side are masked with a tape within a range of 3 mm from the end, with the exposed surface facing upward, and artificial seawater (NaCl: 2.35 mass%, Salt solution containing MgCl ₂ : 1.07% by mass, Na ₂ SO ₄ : 0.41% by mass, CaCl ₂ : 0.15% by mass, adjusted to pH = 8.2) diluted 12.7 times. 0.80 ml was added dropwise. Then, the sample was hold | maintained in the dry-wet cycle [1]-[9] mentioned above. Next, the diluted artificial seawater was dropped again in the same manner as the first time. Then, it was again held in the dry and wet cycle [1]-[9] (total 24 hours). Such dripping and the wet and dry cycle of [1]-[9] were repeated 5 times. Thereafter, the wet and dry cycle of [1]-[9] was repeated 27 times without dripping. And the sample was taken out and the state of the cerium element in the rust produced | generated on the surface layer was evaluated by the X-ray absorption rate method mentioned above.

  A sample was cut into a size of 150 × 50 × 3 mm and subjected to a long-term exposure test. The exposure test is conducted for 10 years, samples are collected at each of 3, 5, 7, and 10 years, rust is removed using the pickling method described in Non-Patent Document 1, and the amount of mass loss of the sample is determined. Corrosion loss was calculated. The place of exposure is Futtsu City, Chiba Prefecture, with an annual average salinity of 0.21 mdd. Using the obtained data, the As value and Bs value of the formula (4) were determined by regression analysis. Tables 3 and 4 show the comparison results of the corrosion amount after 100 years calculated from the formula (4) together with the As value and the Bs value. In Tables 3 and 4, in order to clarify the effect of the present invention for each Ni addition amount level, the corrosion amount after 100 years of each Example and Comparative Example is shown by the ratio (f). That is, in order to clarify the effect of additive elements other than Ni, the ratio (f) is based on the corrosion amount of the reference material for each Ni addition level. The reference materials having Ni addition amounts of 1.2, 3.0, 4.0 to 5.0, and 5.5% by mass are Comparative Examples R2, R9, R14, and R17, respectively.

  When Ca is contained in the sample component, the steel is subjected to constant potential decomposition in a 10% by volume acetylacetone-1 mass% tetramethylammonium chloride-methanol solution to take out only inclusions, and the parent phase is obtained by the constant potential decomposition. The amount of inclusions was quantified from the amount dissolved. Moreover, the structure and composition of the inclusions taken out were observed and analyzed with a transmission electron microscope and an energy dispersive X-ray analyzer, and calcium oxide and a composite oxide of calcium and aluminum were identified. The results are shown in Tables 3 and 4. A case where it is confirmed that 0.0001 mass% or more of calcium oxide or a complex oxide of calcium and aluminum is present in the steel is indicated by ◯.

  As can be seen from the results of Tables 3 and 4, the corrosion amount after 100 years of the inventive example is a standard for each Ni addition level, and the ratio (f) compared to Comparative Examples R2, R9, R14, and R17, respectively. Therefore, the effect of the present invention can be clearly confirmed.

In Tables 3 and 4, * 1) to * 3) show definitions of the meaning of the following symbols.
* 1) ◎: More than 105 pieces / cm ³ , ◯: 103 to 105 pieces / cm ³ , x: less than 103 pieces / cm ³
* 2) ○: The ratio of Ce (III) or Ce (IV) of the cerium element contained in the steel is 50 mol% or more, and x: less than 50 mol%.
* 3) ○: The presence of calcium oxide or a composite oxide of calcium and aluminum has been confirmed by electron microscope observation. X: Presence of calcium oxide or composite oxide of calcium and aluminum could not be confirmed by observation with an electron microscope.

Further, * 4) shows an example that becomes a reference value of the following stabilization index Bs.
* 4) Inventive Examples 1 to 20, 46 to 52 and Comparative Examples R1 to R8 and R19 containing 1.2% Ni obtain the ratio (f) using Comparative Example R2 as a reference value for the rust stabilization index Bs. It was. Inventive Examples 21 to 34 and Comparative Examples R9 to 13 containing 3% Ni contain Comparative Example R9, Inventive Examples 35 to 43 containing 4 to 5% Ni and Comparative Examples R14 to R16 and R18 represent Comparative Example R14. In the inventive examples 44 and 45 containing 5.5% Ni, the ratio (f) was determined using the comparative example R17 as the reference value of the rust stabilization index Bs.

  For each of the steel materials (thickness 10 mm) having the chemical components of Invention Examples 1 and 21 in Table 1 and Comparative Examples R2 and R9 in Table 2, four structures simulating a bridge as shown in FIG. It was left for 5 years at a position of 10 m. The ratio of the state of Ce (III) or Ce (IV) in the cerium element in the rust layer on the side surface portion of the beam of each structure was measured by the method described in the text. It was confirmed that the steel materials of 21 components were 62 mol% and 71 mol%, respectively. On the other hand, in the steel materials of the components of Comparative Examples R2 and 9, the ratio of Ce (III) or Ce (IV) in the cerium element in the rust layer was 10 mol% or less.

  Further, when the corrosion amount of the side surface portion of the beam of each structure is compared for each same Ni addition amount level, the steel material of the component of Invention Example 1 of 1.2% Ni is the corrosion amount of the steel material of the component of Comparative Example R2. Is 0.77 times (ie, the ratio f = 0.77), and the steel material of the component of Invention Example 21 of 3.0% Ni is 0.85 times (ie, the ratio) of the steel material corrosion amount of the component of Comparative Example R9. f = 0.85). Therefore, it was found that the amount of corrosion was reduced in the inventive example compared to the comparative example when compared at the same Ni addition level. The ratio f was in good agreement with the As and Bs values obtained in Tables 3 and 4 and the result calculated at X = 5 (years) in the equation (4).

Measurement results of the absorption rate μt near the Ce absorption edge for rust and standard substances (Ce ₂ O ₃ , CeO ₄ ). Outline of the structure simulating a bridge.

Claims (7)

% By mass
C: 0.03% to 0.18%,
Si: 0.1% to 1.5%,
Mn: 0.2% to 1.5%
P: 0.02% or less,
S: 0.02% or less,
Cu: 0.3% to 3%,
Ni: 1.0% to 6%,
Ce: 0.0001% to 0.03%,
Ta: 0.0001% to 0.05%
A weather resistant steel comprising the balance Fe and unavoidable impurities,
There are precipitated particles containing cerium in the steel, the number of which is 1 nm or more and 2 μm or less is 10 ³ / cm ³ or more, and 50 mol% or more of the cerium element contained in the steel is A weather-resistant steel with improved dense rust formation, characterized by being in the state of Ce (III) or Ce (IV).   Further, Ti: 0.001% by mass to 0.2% by mass, and the value of the product [Ce] × [Ta] × [Ti] of each mass ppm of Ce, Ta, and Ti is 10 or more. The weather-resistant steel according to claim 1, which has improved dense rust formation.   Furthermore, in mass%, any one or more of P: more than 0.02% to 0.2%, Cr: 0.01% to 0.75%, Mo: 0.01% to 0.60% The weatherable steel with improved dense rust formation according to claim 1 or 2, characterized in that Furthermore, by mass%, Al: 0.001% to 0.08%, V: 0.001% to 0.05%, Nb: 0.001% to 0.05%, N: 0.001% to 0 .010%
The weatherproof steel with improved dense rust formation property according to any one of claims 1 to 3, characterized by containing at least one of them.   Furthermore, Ca: 0.0001 mass%-0.005 mass% are contained, The weatherproof steel which improved the dense rust formation property of any one of Claims 1-4 characterized by the above-mentioned.   6. The weather resistance with improved dense rust formation according to claim 5, wherein at least one of calcium oxide and calcium-aluminum composite oxide is present in the steel in an amount of 0.0001% by mass or more. steel.   A steel structure using the weathering steel according to any one of claims 1 to 6, wherein a surface layer of the weathering steel is covered with a rust layer, and 50 mol of cerium element in the rust layer A steel structure characterized in that at least% is in the state of Ce (III) or Ce (IV).

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