JP5998441B2 - Steel offshore structure - Google Patents

Description

  The present invention relates to a steel offshore structure used in an environment where sea salt comes in, and more particularly to an offshore structure constructed offshore, and further to an offshore wind power generation tower.

Recently, from the viewpoint of global environmental conservation, greenhouse reduction effect of carbon dioxide is a gas (CO ₂₎ is desired, instead of the CO ₂ emissions is significant fossil fuel, CO ₂ and is noted clean energy that does not emit Yes. As such a clean energy source, there is a growing trend around the world to use renewable natural energy. Among them, wind power generation using wind power, which is renewable natural energy, has recently been actively introduced in various countries around the world because of its relatively low power generation cost. However, wind power generation has a problem that the output tends to fluctuate depending on the wind conditions.

  A region suitable for wind power generation is a place where strong winds can always be expected. However, such areas are limited on land, and in addition, wind power generation facilities have already been introduced into such areas, and there are many places where the introduction density is high. Recently, problems such as noise of wind power generation devices have become apparent, and it is difficult to construct a new wind power generation facility on land. For this reason, wind power generation is shifting from onshore to offshore.

  On the ocean, winds with high wind speed and little turbulence often blow stably. As a result, the operation time becomes longer and a large amount of wind power can be expected. Moreover, the wind speed tends to increase as the rip-off distance increases, and recently, a plan to construct an offshore wind power generation facility at a great depth off the coast is being studied. Currently, the main location is "water depth: less than 20m, offshore: within 20km", but in the future, "water depth: less than 60m, offshore: within 60km", and further "water depth: over 60m, offshore. :> 60km "is considered as the target of the study.

In order to enable offshore wind power generation offshore with such a deep water depth, various systems have been proposed for the foundation structure on which the wind power generator can be installed offshore. Recently, drilling with a large-diameter drill pit has become possible, and in the sea area where the rip-off distance is within 20 km and the water depth is less than 20 m, it is the mainstream to install a wind turbine generator on the grounded monopile foundation. .
Furthermore, in the sea area where the water depth becomes deeper than 40m, the use of foundations such as jacket type and tripile type is considered. In deeper sea areas, for example, a floating structure that has a structure as shown in Patent Document 1 and does not land on the seabed has been proposed. This floating structure is said to have a large restoring force, no inclination of the power generation device, little decrease in the amount of power generation, light weight, and production in a short time.

  The amount of wind power generation is proportional to the third power of the wind speed and the second power of the blade diameter. For this reason, the wind power generation equipment tends to increase the blade diameter and increase the size. And the tower that rotatably supports such a large blade may be a large structure (tubular body) with a height of about 80 m as a whole and a diameter of over 8 m. It is assembled by welding thick steel.

  Conventionally, even in a wind turbine generator installed on land or in the vicinity of a coast, a tower or the like constituting the wind turbine generator has been subjected to rust prevention treatment such as painting according to the environment to be used. However, wind power generators installed on the ocean are difficult to carry out maintenance and repair work such as repainting because the installation location is offshore. Improvement of the durability of the membrane is considered a major technical issue.

  Regarding the durability improvement of coating (coating film), although it is about the thin steel plate for automobiles, for example, Patent Document 2 describes a corrosion-resistant steel plate excellent in coating corrosion resistance in which a zinc-rich paint is applied to a substrate steel plate. . In the technique described in Patent Document 2, the steel sheet to be the substrate is in mass%, C: 0.001 to 0.10%, Si: 0.5% or less, Mn: 0.05 to 2.0%, and Ti and Zr in total exceeding 0.03% 0.4 %, Cu: 0.03-0.5%, Ni: 0.03-0.5%, P: 0.020-0.1%, S: 0.01% or less, Ca: 0.0005-0.02%, Al: 0.003-0.20% And Ti + Zr: more than 0.03% and less than 0.4%, Cu: 0.03-0.5%, Ni: 0.03-0.5%, P: 0.020-0.1%. In addition, the corrosion resistance of the coating film by the zinc rich paint is remarkably improved, and the rust prevention effect can be sustained for a long time. The effect is further enhanced by adding a predetermined metal salt to the zinc rich paint.

Patent Document 3 describes a steel material for coating excellent in coating durability. The steel material for coating described in Patent Document 3 contains, by mass%, C: 0.12% or less, Cu: 0.05-3.0%, Ni: 0.05-6.0%, Ti: 0.025-0.15%, Cu + Ni: 0.50% As described above, P _{CM is} 0.23% or less, and further includes Si: 1.0% or less, Mn: 2.5% or less, P: 0.05% or less, S: 0.02% or less, and Cr: 0.05% or less. . In the technique described in Patent Document 3, the Cr content is reduced as much as possible to reduce the corrosion promoting factor in the coating film defect portion, and the large amount of Cu, Ni, and Ti is used to densify the generated rust and improve the corrosion resistance. I am letting.

  Patent Document 4 describes a corrosion-resistant steel for shipbuilding having excellent corrosion resistance that contributes to extending the repair and repaint life and reducing the repair and repainting work. The technology described in Patent Document 4 relates to steel materials for ballast tanks (steel materials for ships) that are used particularly in a seawater corrosive environment, and these steel materials are used in a general offshore atmospheric environment to which marine structures are exposed. Is used in special environments with different corrosive environments. In addition, the corrosion-resistant steel for shipbuilding described in Patent Document 4 is mass%, C: 0.03-0.25%, Si: 0.05-0.50%, Mn: 0.1-2.0%, P: 0.025% or less, S: 0.01% In the following, Al: 0.01 to 0.10%, W: 0.01 to 1.0%, which is characterized by having a composition composed of the remaining Fe and inevitable impurities.

JP 2009-248792 JP 2003-171732 A JP 2000-169939 JP 2007-46148

  However, the technique described in Patent Document 1 is a technique for a foundation structure for an offshore wind power generator, and there is no description about a steel material (material) constituting the wind power generator. Moreover, in the technique described in Patent Document 2, it is essential to contain a large amount of P exceeding 0.020% by mass, and a large amount of (Ti + Zr) exceeding 0.03% by mass in total. There's a problem. In addition, in order to ensure the desired corrosion resistance in a severe usage environment with a large amount of flying sea salt particles such as on the ocean, it is necessary to contain a large amount of corrosion resistance improving elements, and there is a problem that the material cost becomes expensive. is there. Further, the technique described in Patent Document 3 requires a large Ti content of 0.025% by mass or more, and there is a problem that the low temperature toughness of the thick steel plate deteriorates. In addition, the technology described in Patent Document 3 requires a large amount of (Cu + Ni) of 0.50% by mass or more in addition to a large amount of Ti, and material costs are affected by soaring and fluctuations in raw materials. There was also a problem that.

  Furthermore, the technique described in Patent Document 4 is intended for a steel material for ballast tanks in which seawater enters and exits, and the steel material is used in an environment different from the atmospheric environment of offshore neutral chloride. Therefore, the coating type used is also different. In addition, Patent Document 4 does not mention the durability of the coating film in a marine atmospheric environment of neutral chloride, and has left a problem with respect to the durability of the coating film in an offshore atmospheric environment.

  In addition, offshore structures such as wind power generators installed on the ocean are generally large structures, and a large amount of material is required for the construction. Therefore, materials (painted steel materials) applied to offshore structures are required to have excellent corrosion resistance against the offshore atmospheric environment and are also required to be inexpensive. Therefore, as means for reducing the cost of the coated steel material as a raw material, means for reducing the amount of alloying elements added to the steel material and the like are employed. However, the consumption of paint for painting the steel surface cannot be neglected. In the case of an offshore structure, a huge amount of paint is consumed because it is a large structure. For this reason, if the consumption of paint can be reduced, a significant cost reduction can be realized. However, in the above-described conventional technology, sufficient studies have not been made for reducing the paint consumption.

  Furthermore, in the conventional rust prevention specifications (for example, C12M system defined by ISO12944), which is often used in severe corrosive environments such as offshore air environment, the coating life is about 15 years. For this reason, when a structural material is used for more than 15 years, it is necessary to perform maintenance (maintenance). However, in the case of large wind power towers installed on the ocean, maintenance and repair are difficult, so the life cycle cost (LCC) is taken into consideration and the durability of the coating is improved and the paint life is increased. It is believed that it is important to extend, reduce or eliminate the maintenance frequency of painting, etc. to achieve minimum maintenance, improve LCC, and make it maintenance-free.

The present invention solves the problems of the prior art, and can be used in an offshore air environment where sea salt comes in. Steel materials with excellent corrosion resistance that can reduce coating costs, manufacturing costs, etc., compared to the prior art. The purpose is to provide an offshore structure.
Note that the steel material applied to the offshore structure of the present invention is suitable for offshore structures, further offshore offshore structures, and offshore wind power generation, and has a tensile strength of 420 MPa or more. Use tough steel. The term “high toughness” as used herein refers to the case where the average absorbed energy at a test temperature of 0 ° C. in Charpy impact test is 27 J or more, preferably the average absorbed energy at −40 ° C. is 27 J or more. . Further, the “steel material” here includes a thick steel plate, a shape steel, a bar steel, a steel pipe and the like having a wall thickness of 30 mm or more.

In order to achieve the above-described object, the present inventors have intensively studied various factors affecting the corrosion resistance of an offshore structure used in an environment where sea salt comes in (offshore atmospheric environment).
Offshore structures are generally large structures, and some of them reach a height of 80 m, for example, towers of offshore wind power generation facilities. Therefore, the present inventors have focused on the fact that the amount of incoming sea salt particles varies in the height direction of the offshore structure, that is, the corrosive environment varies in the height direction. For example, in the case of a tower for offshore wind power generation facilities, it can be inferred that the amount of incoming sea salt particles differs greatly between the upper sea level of the tower and the highest tower 80 m above the sea level. The amount of incoming sea salt particles just above the sea level of the tower is about 0.4 to 1.5 mg / dm ² / day (hereinafter referred to as mdd), which is very high. On the other hand, it was confirmed that the amount of incoming sea salt particles at the highest part of the tower at 80m from the sea surface shows a relatively low value of about 0.01 to 0.1mdd.

  From these results, the present inventors have made conventional antirust coating specifications in the height direction of marine structures in the lower region where the amount of flying sea salt particles is high and the upper region where the amount of flying sea salt particles is low. As you can see, it was not necessary to have the same coating specifications. And in the lower area of the marine structure where the amount of incoming sea salt particles is high, it is constructed by applying painted steel with the same antirust coating specifications as before, while the marine structure where the amount of incoming sea salt particles is low. In the upper region, it was conceived that if a low-alloy corrosion-resistant steel material with excellent corrosion resistance was applied, it could be constructed with no coating specifications. It was concluded that such a configuration can reduce the painting work, can significantly reduce the painting cost, and in turn can reduce the manufacturing cost of the offshore structure and also the maintenance inspection cost.

Therefore, the present inventors further studied the influence of various alloy elements on the corrosion resistance of steel materials in an environment where sea salt comes in. As a result, they found a low alloy corrosion resistant steel material with excellent corrosion resistance that can be used without coating in the upper region of offshore structures where the amount of flying sea salt particles is low.
According to the study by the present inventors, the carbon content is reduced to less than 0.08%, and an appropriate amount of W and Nb, as well as Cr effective for improving corrosion resistance, is contained in an amount of 0.01 to 0.10%. , Cu and Ni, or one or more of Mo, V, Sb and Sn, or a steel material with a proper amount of Ti, the amount of incoming sea salt particles is 0.1 mdd or less. It has been found that the corrosion resistance is improved so that sufficient corrosion resistance can be maintained for a long time even in a relatively mild corrosive environment without painting, that is, bare use.

In addition, although the mechanism of durability improvement by W and Nb is not clear at present, the present inventors consider as follows.
By containing an appropriate amount of W, stable and dense rust is formed on the surface of the ground iron. W contained in the steel material elutes on the surface of the iron core, becomes WO ₄ ^2- ions, reacts with Fe ²⁺ ions, and forms hardly soluble Fe WO ₄ by the reaction of the following formula.
Fe ²⁺ + WO ₄ ²⁻ → Fe WO ₄
When the hardly soluble Fe WO ₄ is contained in the corrosion product (rust), the rust layer is densified, the permeation of ions and oxygen is suppressed, the amount of rust generation is reduced, rust peeling, etc. Can be remarkably prevented.

  In addition, the inclusion of an appropriate amount of Nb enables generation of a fine ferrite structure and fine dispersion of carbide (NbC). Further, it is considered that the inclusion of Nb forms a heterogeneous interface with the ferrite phase, delays the generation of cementite that adversely affects the corrosion resistance, and refines the corrosion resistance of the steel material significantly. Since carbides typified by cementite act as a cathode in a corrosive environment, it can be said that it is preferable to finely disperse the carbides from the viewpoint of improving corrosion resistance. Furthermore, it has also been found that when an appropriate amount of Cr is contained, the density of the generated rust increases.

  Note that the steel material having the above composition can be used without coating in a relatively mild corrosive environment where the amount of sea salt particles is 0.1 mdd or less. However, when used without painting, rust generated on the surface is washed away by rainwater and becomes flow rust. If such flow rust is formed, the structure, for example, the aesthetics of the offshore wind power generation tower, particularly the aesthetics of the lower part of the tower, is impaired. Then, when importance was attached to the aesthetics of a structure, it came to the mind that it would be good to apply a commercially available rust stabilization auxiliary processing agent to unpainted steel materials. In particular, when considering changes over time, it is possible to prevent or greatly reduce the occurrence of flow rust, thereby maintaining the aesthetics of the structure, especially for structures with painted areas such as offshore wind power generation towers. I got the knowledge that it would be possible to preserve the beauty.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) A steel-made marine structure used in an environment where the amount of incoming sea salt particles differs in the height direction of the structure, wherein a predetermined boundary value is set for the amount of incoming sea salt particles, The region where the amount of flying sea salt particles exceeds the boundary value is a lower region, the region of the structure where the amount of flying sea salt particles is less than or equal to the boundary value is an upper region, and the predetermined boundary value is 0.1mdd as below, the lower region is formed by a painted steel having a coating film on the surface, whereas the upper region, consists of unpainted steel having the treatment film stabilizing auxiliary processing agent rust on the surface The unpainted steel material is, by mass%, C: less than 0.08%, Si: 0.75% or less, Mn: 2.0% or less, P: 0.030% or less, S: 0.030% or less, Al: 0.01 to 0.05%, N: 0.010% or less, W: 0.50-1.0%, Nb: 0.010-0.200%, Cr: 0.01-0.10% , Cu: 0.05 to 0.50%, Ni: 0.05 to 0.50%, one or two kinds selected from steel, and a steel material having a composition consisting of the balance Fe and inevitable impurities Offshore structures.

( 2 ) The steel-made marine structure according to ( 1 ), wherein the contents of P and S in the composition are, by mass%, P: 0.010% or less and S: 0.0020% or less.
( 3 ) In ( 1 ) or ( 2 ), in addition to the above composition, in terms of mass%, Mo: 0.01 to 0.50%, V: 0.05 to 1.0%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.30% A steel-made marine structure characterized by having a composition containing one or more selected from among the above.

( 4 ) In any one of ( 1 ) to ( 3 ), in addition to the above composition, the steel marine structure further contains Ti: 0.005 to 0.025% by mass%.
( 5 ) In any one of ( 1 ) to ( 4 ), the composition is expressed by the following formula (1): Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
(Here, C, Mn, Si, Ni, Cr, Mo, V: content of each element (mass%))
A steel marine structure characterized by having a carbon equivalent Ceq defined by the formula of 0.40 or less.

( 6 ) In any one of ( 1 ) to ( 5 ), the composition is expressed by the following formula (2): _PCM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: content of each element (mass%))
Steel made marine structures weld crack sensitivity index P _CM THAT defined is characterized in that the composition satisfies 0.30 or less.

(7) In any one of (1) to (6), the lower region is made of a coated steel material provided with a C5M-based coating film conforming to the provisions of ISO12944 on the surface of the base steel material. Steel offshore structure.
(8) The steel material offshore structure according to any one of (1) to (7), wherein the steel material offshore structure is a steel material offshore structure.

( 9 ) The steel offshore structure according to ( 8 ), wherein the steel offshore structure is an offshore wind power generation tower.

  According to the present invention, the coating specifications of a steel marine structure are changed in the height direction of the structure in a marine environment where sea salt is flying, and in particular, the height where the amount of flying sea salt particles is 0.1 mdd or less. In the direction area (upper area), it is possible to make it a non-painting specification, and there is a remarkable industrial effect that the painting cost of offshore structures can be reduced as compared with the conventional one. In addition, according to the present invention, it is possible to reduce the occurrence of flow rust, which is a concern when using unpainted steel, and in particular, it is possible to suppress a decrease in the appearance of the structure when changes over time are taken into consideration.

  Moreover, according to this invention, there also exists an effect that the fall of the life cycle cost (LCC) by the minimum maintenance of an offshore structure is realizable.

It is explanatory drawing which shows typically the shape of the corrosion test piece used in the Example, and the position of an antirust coating process.

  On the coast and further offshore, the wind speed is higher than on land and the wind containing a lot of salt is blowing, but the amount of sea salt particles that fly (the amount of sea salt particles) varies depending on the height from the sea level. doing. Since the corrosion of the steel material is promoted by the incoming salt content, that is, the incoming sea salt particles, the severity of the corrosive environment can be roughly distinguished by the amount of incoming sea salt particles. It is said that the severity of corrosion changes greatly when the amount of flying sea salt particles is 0.1 to 0.4 mdd, and the amount of flying sea salt particles: 0.1 mdd is used as an index for determining the coating specifications. In the offshore structure, the coating specifications are changed in the height direction depending on the amount of incoming sea salt particles.

  The marine structure according to the present invention is made of steel, and in the height direction region (upper region) where the amount of flying sea salt particles is 0.1 mdd or less, the steel material is unpainted, that is, used barely, and the amount of flying sea salt particles is In the height direction area (lower area) exceeding 0.1 mdd, the steel is coated. As a result, the painting of offshore structures can be reduced, the painting cost can be greatly reduced, and the life cycle cost (LCC) can be reduced by the minimum maintenance.

  The environment where the amount of flying sea salt particles exceeds 0.1 mdd is a severe corrosive environment, and in the present invention, the steel surface is coated, that is, in the lower region where the amount of flying sea salt particles exceeds 0.1 mdd. Painted steel is used. In steel offshore structures, coating is applied to protect the steel to prevent corrosion, but the coating specifications according to the operating environment are defined for each standard. . In the lower region in the present invention, a coated steel material coated with a coating specification corresponding to the use environment is used for the steel material as a base material. Further, the steel material used as the base material may be a steel material having mechanical properties such as strength and toughness adapted to the use environment, and is not particularly limited. It is more preferable to use a steel material having excellent durability.

  For example, ISO 12944 defines C5M as the strictest atmospheric corrosion category, and describes the coating specifications conforming thereto. In the offshore structure of the present invention, it is preferable to apply the coating specification according to this rule for the lower region. That is, it is preferable that the coating specification conforms to C5M defined in ISO 12944 as the coating specification of the lower region close to the sea surface where the amount of flying sea salt particles exceeds the boundary value.

  The coating conforming to C5M specified in ISO 12944 includes an undercoat layer of inorganic zinc coating film 40 to 80 μm (dry film thickness), an intermediate coating layer of epoxy resin coating film 400 to 600 μm (dry film thickness), and polyurethane. It can be set as the coating comprised from the overcoat layer of resin coating film 50-100micrometer (dry film thickness). The undercoat layer of the inorganic zinc coating film may be omitted, and an intermediate coating layer and an overcoating layer of an epoxy glass flake coating film having excellent corrosion resistance may be used.

In the steel marine structure according to the present invention, NOROS STANDARD M-501 and ISO 20340 also have provisions for protective coatings, and may be applied to the steel marine structure according to the present invention. . Needless to say, the steel material to be the base material is subjected to a blasting treatment of about Sa: 21/2 before coating.
In addition, as described above, the coating steel used as the base material in the height direction region (lower region) where the amount of flying sea salt particles exceeds 0.1 mdd is not particularly limited, and is usually used for offshore structures. For example, the steel may be EN10025-compliant steel (AP12W-50 equivalent steel). By applying C5M coating specifications based on ISO 20340 to these steel materials, a coating life of 15 years is expected to be secured. Further, by using a steel material having excellent corrosion resistance as the coated steel material, the coating thickness can be reduced. Needless to say, a steel material having excellent low-temperature toughness is used particularly when the usage environment is low.

On the other hand, the environment where the amount of sea salt particles is 0.1 mdd or less, for example, the environment where the amount of sea salt particles is 0.1 mdd or less in the upper part of the ocean tower above 20 m above the sea surface, which is relatively mild corrosion for steel. Since it is an environment, if the steel material to be used is appropriately selected, sufficient corrosion resistance can be maintained for a long time even without painting, that is, bare use.
In the marine structure according to the present invention, the steel material used without coating in the height direction region (upper region) where the amount of incoming sea salt particles is 0.1 mdd or less is C: less than 0.08%, Si: 0.75 %: Mn: 2.0% or less, P: 0.030% or less, S: 0.030% or less, Al: 0.01 to 0.05%, N: 0.010% or less, W: 0.50 to 1.0%, Nb: 0.010 to 0.200% , Cr: 0.01 to 0.10%, Cu: 0.05 to 0.50%, Ni: 0.05 to 0.50% or one selected from Mo, or Mo: 0.01 to 0.50% V: 0.05 to 1.0%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.30% or less, and / or Ti: 0.005 to 0.025%, and / or A steel material having a composition composed of the remaining Fe and inevitable impurities is used.

Hereinafter, the reasons for limiting the composition of steel materials used without coating will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: Less than 0.08% C is an element that solidifies to increase the strength of the steel material and combines with carbide-forming elements to form carbides. In order to ensure the desired strength, it is desirable to contain 0.02% or more. On the other hand, cementite and other carbides form a heterogeneous interface with the ferrite phase and can easily become a cathode in corrosive environments where sea salt comes in, and promote corrosion, so can it be reduced as much as possible from the viewpoint of corrosion resistance and coating life extension? It is desirable to finely disperse. On the other hand, if it is excessively contained in an amount of 0.08% or more, it becomes difficult to ensure desired corrosion resistance. For this reason, C was limited to a range of less than 0.08%. In addition, Preferably it is 0.03 to 0.06%.

Si: 0.75% or less
Si is an element that acts as a deoxidizing agent and increases the strength by solid solution. In order to secure a desired strength, Si is desirably contained in an amount of 0.20% or more. Si, when oxidized during heating, generates firelite at the interface between the scale and the scale, increasing the adhesion between the scale and the scale, but a large content exceeding 0.75% reduces toughness. At the same time, the interface thickness between the base iron and the scale increases too much during hot rolling, and cracking and peeling of the scale become noticeable during roller correction and press correction, resulting in reduced productivity. For this reason, Si was limited to 0.75% or less. In addition, since excessive content of Si causes hardening of a steel material and a fall of workability, it is preferable to set it as 0.60% or less.

Mn: 2.0% or less
Mn is an element having a function of increasing the strength of the steel material by solid solution and improving the toughness. Further, Mn combines with S to form MnS, and suppresses the formation of harmful FeS and suppresses the adverse effects of S in the steel surface and in steel. In order to obtain such an effect, it is desirable to contain 0.5% or more of Mn. On the other hand, if the content exceeds 2.0%, the weldability is lowered and the machinability is lowered. For this reason, Mn was limited to the range of 2.0% or less. In addition, Preferably it is 0.5 to 1.6%.

P: 0.030% or less P has the effect of increasing the strength of the steel material, but segregates at the grain boundaries, lowers the corrosion resistance, further lowers the base metal toughness, weldability and welded portion toughness, and secondary processing brittleness Cause deterioration of the workability of the steel material. For this reason, it is desirable to reduce P as much as possible, but excessive reduction raises the refining cost. For this reason, it is desirable to set it as about 0.001% or more. Further, such a decrease in the base metal toughness, welded portion toughness and workability becomes remarkable when the content exceeds 0.030%. For this reason, P was limited to 0.030% or less. It is preferably 0.010% or less. More preferably, it is 0.006% or less.

S: 0.030% or less S, in a composition containing Mn, forms MnS which is a soluble nonmetallic inclusion. MnS becomes a starting point of corrosion in an environment where sea salt comes in and reduces corrosion resistance. S is an element that reduces toughness and weldability. For this reason, it is desirable to reduce S as much as possible, but it is acceptable if it is 0.030% or less. In addition, excessive reduction increases the refining cost, so 0.0005% or more is desirable. In addition, Preferably it is 0.0050% or less, More preferably, it is 0.0020% or less.

Al: 0.01-0.05%
Al is an element that acts as a deoxidizer and refines crystal grains. In order to acquire such an effect, it is made to contain 0.01% or more. On the other hand, if the content exceeds 0.05%, oxide inclusions increase and the cleanliness of the steel material decreases. For this reason, Al was limited to 0.01 to 0.05%.
N: 0.010% or less N is dissolved to increase the strength of the steel material. However, if contained in a large amount, the steel material is hardened and the toughness and weldability are reduced. Furthermore, in the case of containing Ti, N can also be used as an effective element for forming TiN and improving the toughness of the high heat input welding heat-affected zone. Further, from the viewpoint of lowering weldability, it is desirable to reduce N as much as possible. However, excessive reduction raises the refining cost, so it is preferable to be about 0.001% or more. On the other hand, if it is contained in a large amount exceeding 0.010%, the steel material becomes hard, toughness is lowered, and weldability is lowered. For this reason, N was limited to 0.010% or less. In addition, Preferably it is 0.005% or less.

W: 0.50-1.0%
W is an important element in the present invention, and by containing W, the formed corrosion product (rust) is densified and the permeation of ions and oxygen is suppressed, contributing to improvement in corrosion resistance. thinking. As corrosion of the steel progresses, W elutes to become WO ₄ ²⁻ ions in the corrosion product (rust) and reacts with Fe ^{2 +} ions to form poorly soluble FeWO ₄ . By containing hardly soluble FeWO _{4 in the} corrosion product (rust), the rust layer becomes dense, the permeation of chloride ions and oxygen is suppressed, the anode reaction is suppressed, and the amount of rust generation is reduced. Become.
In order to obtain such an effect, the content of 0.50% or more is required. On the other hand, if the content exceeds 1.0%, the steel becomes hard and the toughness decreases. For these reasons, W is limited to the range of 0.50 to 1.0%. In addition, Preferably it is 0.50 to 0.80%.

Nb: 0.010-0.200%
Nb is an important element in the present invention, and binds to C and precipitates as fine NbC. Along with this, fine Nb-based carbides are formed, the structure becomes finer, and the carbides are finely dispersed to suppress the formation of coarse cementite and other carbides that form a heterogeneous interface that becomes the starting point of corrosion. Will improve. In addition, the inclusion of Nb suppresses the formation of coarse carbides, refines the crystal grain size of the parent phase (ferrite phase), and improves toughness. In order to obtain such an effect, a content of 0.010% or more is required. On the other hand, if the content exceeds 0.200%, the corrosion resistance improving effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous and leads to hardening of the steel material. For this reason, Nb was limited to 0.010 to 0.200% of range. In addition, Preferably it is 0.010 to 0.020%.
It is.

Cr: 0.01-0.10%
Cr partially replaces Fe in cementite and has a function of forming a solid solution in cementite to form (Fe, Cr) ₃ C and finely dispersing cementite (carbide). Suppresses the formation of coarse carbides such as cementite that cause corrosion. For these reasons, it is considered that the Cr content effectively acts to form a uniform, fine and dense rust layer, and Cr is limited to the range of 0.01 to 0.10%.

One or two selected from Cu: 0.05 to 0.50%, Ni: 0.05 to 0.50%
Both Cu and Ni contribute significantly to the improvement of corrosion resistance in an environment where sea salt particles fly by incorporating them together with W. In order to obtain such an effect, it is desirable to contain 0.05% or more of both Cu and Ni. However, if both Cu and Ni exceed 0.50%, a large amount of expensive alloy elements are contained, and a reduction in material cost can be expected. Disappear. For these reasons, Cu: 0.05 to 0.5%, Ni: 0.05 to 0.5%, respectively.

The above components are basic components. In addition to these basic compositions, Ti: 0.005 to 0.025% and / or Mo: 0.01 to 0.50%, V: 0.05 to 1.0%, Sn: 0.01 to 0.50 %, Sb: One or more selected from 0.01 to 0.30% can be selected and contained as necessary.
Ti: 0.005-0.025%
Ti increases the strength of steel materials and contributes to the improvement of toughness by refining the structure through the formation of nitrides. Furthermore, Ti is an element that has a dense structure of rust to be generated and has an effect of improving corrosion resistance, and can be contained as necessary. In order to acquire such an effect, it is desirable to contain 0.005% or more, but when it contains more than 0.025%, coarse nitride (TiN) is formed in the steel, and the toughness tends to be lowered. For this reason, when it contains, it is preferable to limit Ti to 0.005 to 0.025% of range. In addition, More preferably, it is 0.0005 to 0.020%.

Mo: 0.01 to 0.50%, V: 0.05 to 1.0%, Sn: 0.01 to 0.50%, Sb: One or more selected from 0.01 to 0.30%
Mo, V, Sn, and Sb are all elements that improve the corrosion resistance, and can be selected as necessary and contained in one or more. Mo, V, Sb, Sn, and Cr all contribute to the formation of a dense and stable rust layer and contribute to the improvement of the corrosion resistance of the steel material. In order to obtain such effects, it is desirable to contain Mo: 0.01% or more, V: 0.05% or more, Sn: 0.01% or more, and Sb: 0.01% or more. On the other hand, if the content exceeds Mo: 0.50%, V: 1.0%, Sn: 0.50%, and Sb: 0.30%, the workability is lowered and the manufacturability of the steel is impaired. For this reason, when it contains, it is preferable to limit to Mo: 0.01-0.50%, V: 0.05-1.0%, Sn: 0.01-0.50%, Sb: 0.01-0.30%, respectively. More preferably, Mo is 0.05 to 0.20%, V is 0.10 to 0.20%, Sn is 0.10 to 0.30%, and Sb is 0.10 to 0.20%.

Furthermore, in the present invention, the above-described components are contained in the above-described range, and the carbon equivalent Ceq is limited to 0.40 or less and / or Pcm is limited to 0.30 or less, so that the weldability and the weld heat affected zone toughness are reduced. It is preferable from the viewpoint of ensuring.
The carbon equivalent Ceq is expressed by the following formula (1): Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
(Here, C, Mn, Si, Ni, Cr, Mo, V: content of each element (mass%))
In the calculated value using the formula defined, weld cracking sensitivity index P _CM, the following equation (2) _{P CM = C + Si / 30} + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ‥‥ (2)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: content of each element (mass%))
Each of the values calculated using the formula defined in is used. When not containing the element contained in (1) type | formula and (2) type | formula, the content of the said element shall be calculated as zero.

When the carbon equivalent Ceq exceeds 0.40, the strength (hardness) of the welded portion becomes too large, and a weld crack is likely to occur. Note that Ceq is preferably 0.35 or less from the viewpoint of setting the hardness of the weld metal portion to 2 times or less of the base metal hardness. Further, the welding crack sensitivity index P _CM increases beyond 0.30, the higher the susceptibility to cracking, weld cracking occurs frequently. In addition, P _CM is preferably 0.25 or less.

The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities include O: 0.0020% or less, Ca: 0.050% or less, and B: 0.002% or less.
Next, a preferable structure of the steel material used without coating in the present invention will be described.
The steel material used without coating in the present invention has the above-described composition, and further has a structure composed of a ferrite phase of 70% or more and a second phase other than the ferrite phase in the area ratio with respect to the entire structure. Is preferred.

Ferrite phase: 70% or more The steel material used uncoated in the present invention preferably has a structure composed of a ferrite phase of 70% or more and a second phase other than ferrite in terms of the area ratio relative to the entire structure.
In the composition range of the steel material described above, the steel material exhibits various structures such as a ferrite single phase, a mixture of a ferrite phase and a pearlite phase, a bainite phase, a martensite phase, or a mixed structure thereof. According to the study by the present inventors, it has been found that in a corrosive environment where sea salt comes in like an offshore wind tower, the structure having the best corrosion resistance is a ferrite phase. For this reason, in the steel material used without coating in the present invention, it is preferable that the structure is a structure mainly composed of a ferrite phase with an area ratio of 70% or more with respect to the entire structure.

  In a corrosive environment where sea salt comes in, in the case of a structure other than a single phase of ferrite phase, the origin of corrosion is mainly the heterogeneous interface between carbide and matrix (ferrite phase), especially coarse as represented by cementite. It is an interface between carbide and ferrite phase, and from the viewpoint of corrosion resistance, it is preferable to have a ferrite phase single phase structure. However, in the ferrite phase single phase, it is possible to impart desired characteristics, for example, desired high strength to the steel material. It becomes difficult. Therefore, in the present invention, considering the balance between corrosion resistance and other characteristics, the ferrite phase is limited to 70% or more in terms of the area ratio with respect to the entire structure. In addition, Preferably it is 80 to 95%. Even if the ferrite phase is 80% or more in terms of the area ratio relative to the entire structure, if the carbides are coarsened, sufficient corrosion resistance cannot be improved. It is preferable that the second phase other than is finely and uniformly distributed.

The remainder (second phase) other than the ferrite phase is preferably cementite, pearlite phase, bainite phase, residual austenite phase, martensite phase, or the like. When the second phase is present in a large amount exceeding 30%, the corrosion resistance is lowered. The second phase is preferably 15% or less.
Moreover, in the steel material used without coating in the present invention, it is more preferable that the ferrite phase has a fine structure having a crystal grain size number of 7.0 or more. By refining the structure (ferrite crystal grains), as a result, the crystal grain boundaries can be lengthened, and carbides that are likely to precipitate at the crystal grain boundaries are also finely dispersed. The finer the crystal grains, the finer the carbides can be dispersed. In consideration of the load of the manufacturing process, the crystal grain size number may be 6.0 or more. In the present invention, as the crystal grain size number, a value calculated in accordance with JIS G 0551 is used.

Next, a preferred method for producing a steel material used without coating in the present invention will be described.
As a method for producing a steel material having the above-described composition, any known method can be applied and is not particularly limited.
When the steel material is a thick steel plate, the molten steel having the above composition is melted by a generally known melting method such as a converter, and after being made a steel material such as a slab by a conventional method such as a continuous casting method, It is preferable that the steel material is heated, subjected to thick plate rolling (hot rolling), cooled, or further corrected to obtain a thick steel plate having a desired size and shape. In thick plate rolling, characteristics such as desired strength and toughness can be imparted by applying controlled rolling, controlled cooling and the like in addition to normal rolling.

  When the steel material is a thick steel plate, for example, after heating the steel material to 1000 to 1200 ° C., the cumulative rolling reduction in the temperature range of 700 to 1000 ° C. is 60% or more, and the rolling end temperature is 650 ° C. or more. It is preferable to perform thick plate rolling to form a thick steel plate, and then cool to 300 ° C. or less at a cooling rate of 5.0 ° C./s or less, but it is needless to say that the invention is not limited to this. Instead of controlling the cooling conditions after rolling, heat treatment may be performed after cooling to room temperature. Heat treatment conditions include heating temperature: 300 to 700 ° C., and then cooling to room temperature at a cooling rate in the range of 1.0 to 30 ° C./s, or air cooling after heating to ensure the desired strength and toughness. It is preferable to do.

  Further, when the steel material is a shape steel, the molten steel having the above composition is melted by a generally known melting method such as a converter, and is made into a steel material such as bloom by a conventional method such as a continuous casting method. After that, it is preferable that the steel material is heated, subjected to shape steel rolling (hot rolling), cooled, or further straightened to obtain a shape steel such as H-shaped steel or steel sheet pile having a desired dimensional shape. Any of the usual rolling methods can be applied to shape steel rolling. The same applies to the case where the steel material is a steel bar, and it is preferable to adjust the manufacturing conditions so that the steel bar has a desired dimensional shape, strength, and toughness by applying steel bar rolling under normal conditions.

In shape steel rolling and bar rolling, the cooling condition after rolling depends on the thickness of the steel material, but the steel material having desired characteristics can be cooled to 300 ° C or less at a cooling rate of 1.0 ° C / s or less. It is preferable to obtain.
In the upper region of the steel structure offshore structure of the present invention, a rust stabilizing auxiliary treatment agent is applied to the steel material used without painting. As the rust stabilization auxiliary treatment agent, there are agents that have a proven record as a rust stabilization auxiliary treatment agent for weatherproof steel, for example, Captain Coat M (NETIS registration number: QS-020027), Eras (e-RUS), etc. Examples include commercially available weathering steel chemicals.

  These commercially available weathering steel chemicals are protective rust formation accelerators for weathering steels, and these chemicals are usually applied to weathering steels for long-term atmospheric exposure. The dense protective rust formed below can be preferentially formed in a short time. In the present invention, for example, such a commercially available chemical for weathering steel is applied as a rust stabilizing auxiliary treatment agent to the surface of uncoated steel used in the upper region of the marine structure made of the steel of the present invention. Accordingly, it is possible to prevent the occurrence of flow rust observed when the steel material is used without painting, and to shorten the period in which the color tone unevenness until the formation of the protective rust occurs. For example, when using such commercially available chemicals for weathering steel as rust stabilization aids, they may be used in accordance with the method of using each chemical. For example, in the case of Captain Coat M, a film thickness of about 45 μm is recommended.

Even in an environment where the amount of flying sea salt particles is large, even if the above-mentioned commercially available weathering steel chemicals are used as a rust stabilizing auxiliary treatment agent, the same dense protective rust as in the environment where the amount of flying salt is low. Can be formed in a short time, and the occurrence of flow rust can be prevented. When the steel material to be used is an unpainted steel material containing W as in the present invention, the effect is particularly remarkable.
Using each of the steel materials described above, an upper region and a lower region are configured to form an offshore structure having a predetermined size and shape. Examples of the offshore structure of the present invention include those constructed in the coastal area or offshore structures constructed on the sea far away from the coast, and in particular, wind power generation towers.

  For example, an offshore wind power generation tower is used by welding a thick steel plate manufactured by the above-described manufacturing process, manufacturing a plurality of large diameter pipes having a diameter of, for example, 8.0 m, and welding or bolting them together. . And the predetermined | prescribed coating is given to the lower area | region which uses the steel material for coating, On the other hand, only a rust stabilization auxiliary processing agent is apply | coated to an upper area | region, without coating, and it is set as the tower for wind power generation.

Example 1
A steel plate (slab) having the composition shown in Table 1 is heated to 1050 to 1130 ° C, the cumulative reduction in the temperature range of 800 ° C or higher is 75% or higher, and the rolling end temperature is 660 ° C or higher. Rolling was performed, and after completion of thick plate rolling, cooling was performed at a cooling rate (air cooling) of 5.0 ° C./s or less to obtain a thick steel plate (plate thickness: 50 mm). Table 2 shows the rolling conditions. Some thick steel plates were subjected to heat treatment (tempering treatment) under the conditions shown in Table 2.
The obtained thick steel plate was subjected to a structure observation, a tensile test, and an impact test. The test method was as follows.

(1) Microstructure observation A specimen for microstructural observation is collected from the obtained thick steel plate, the cross section in the rolling direction (L cross section) is polished, corroded with the nital liquid, and the optical microscope ( (Magnification: 400 times) was used to observe the tissue, and five fields of view were imaged. The obtained structure was subjected to image processing, and the type of structure (differentiation of the second phase other than the ferrite phase and the ferrite phase) and the structure fraction (area% with respect to the entire structure) were obtained. Further, the crystal grain size number was calculated in accordance with the provisions of JIS G 0551.

(2) Tensile test JIS No. 5 tensile test specimen was taken from the position of 1/4 thickness of the obtained thick steel plate so that the tensile direction would be perpendicular to the rolling direction, and in accordance with the provisions of JIS Z 2241. Tensile tests were performed, and tensile properties (yield strength YS, tensile strength TS, elongation El) were measured.
(3) Impact test V-notch test specimens were collected from the position of 1/4 thickness of the obtained steel plate so that the specimen length direction was perpendicular to the rolling direction, and in accordance with the provisions of JIS Z 2242 The Charpy impact test was performed, and the absorbed energy vE ₀ (J) and vE ₋₄₀ (J) at test temperatures: 0 ° C. and −40 ° C. were obtained. In addition, 3 each was implemented with each steel plate, the arithmetic mean of the obtained absorbed energy was calculated | required, and the toughness of each steel plate was evaluated.

  Next, a test material (size: 100 × 70 mm) was collected from the obtained thick steel plate, and the oxidized scale on the surface of the thick steel plate was removed by shot blasting. Note that the shot blast treatment was RZ: 50 μm and Sa: 21/2. From the test material after the shot blasting treatment, an air exposure test piece was collected so that the surface of the thick steel plate became the exposed surface. Using this atmospheric exposure test piece, an atmospheric exposure test was conducted to evaluate the corrosion resistance without coating. The test method was as follows.

(4) Corrosion resistance The collected atmospheric exposure test piece (10mm thickness x 100mm x 70mm) is 3 years atmospheric exposure test (test period) in an area where the amount of flying sea salt particles is 0.10mdd which is below the specified boundary value. The average temperature was 16 ℃). In the air exposure test, the test piece was tilted 60 ° from the horizontal, and the test surface was placed facing the south. On the exposed surface of the test piece, a rust stabilizing auxiliary treatment agent (Captain Coat M (manufactured by JFE Steel Co., Ltd.)) was applied to a film thickness of 45 μm. A specimen was also prepared, and three specimens were prepared for each.

  After the atmospheric exposure test is completed, the corrosion product formed on the surface of the test piece that is not coated with rust stabilization aid (untreated) is removed using an aqueous solution of diammonium citrate, and the test piece is weighed and tested. The weight after the test was calculated, and the difference from the pre-test weight measured in advance before the test was calculated to determine the corrosion weight loss, and the corrosion rate (um / year) was obtained. The case where the corrosion rate was less than 50 μm / year was evaluated as ○, and the other cases were evaluated as ×.

In order to evaluate the degree of flow rust of the exposed sample (test piece), as shown in FIG. 1, a 20 mm rust-proof coating (shaded area) was applied to the exposed surface at the bottom of the test piece. The coating was a white C5M rust-proof coating.
Next, after the test was completed, the state of occurrence of flow rust was evaluated based on whether or not there was flow rust in the white painted portion formed on the lower part of the exposed surface of the test piece. The case where the flow rust in the white painted portion was in the range of 0 to 5% on average in terms of area ratio was evaluated as ◯, the case where it exceeded 5 to 20% or less was evaluated as Δ, and the case where it exceeded 20% was evaluated as ×. The area ratio of flow rust was calculated by image processing.

  The obtained results are shown in Table 3.

  Thick steel plates with composition ranges that meet unpainted specifications (examples of the present invention) all have the desired high strength and excellent toughness, and corrosive environment (air exposure environment) where the amount of incoming sea salt particles is 0.10 mdd or less. ) Shows a corrosion rate of less than 50 μm / year, and it is clear that no particular repair is required even if it is used without a predetermined repair interval (15 years) or without painting. On the other hand, the comparative example that falls outside the composition range that conforms to the unpainted specification of the present invention has a corrosion rate of 50 μm / year or more and poor corrosion resistance.

In addition, in the present invention example in which the rust stabilization assisting agent is applied, it can be inferred that the generation of flow rust is greatly reduced, and the marine structure can be prevented from deteriorating the appearance of the lower region due to flow rust. On the other hand, in the comparative example in which the rust stabilizing auxiliary treatment agent is not applied, the flow rust area ratio exceeds 20% in the yearly exposure test, and the most conspicuous lower part of the tower material (for example, the area below 10 m from the sea surface) The white painted part is contaminated with brown flow rust, and there is a concern that the aesthetics of the marine structure will deteriorate.
(Example 2)
Assuming that an offshore wind power generation tower with an outer diameter of 8.0mφ and a sea height of 80m is installed at a position of 5.0km from the coast and a water depth of about 30 to 45m, the amount of incoming sea salt particles will exceed 0.1mdd The boundary height varies depending on the installation environment. For example, when the height at which the amount of flying sea salt particles exceeds 0.1 mdd is 20 m or less, the height at which the amount of flying sea salt particles becomes 0.1 mdd or less exceeds 20 m. Therefore, the lower region where the amount of flying sea salt particles exceeds 0.1 mdd is coated on the steel surface with a total of 500 μm (inorganic sink: 50 μm thickness, epoxy resin coating: 200 μm thickness, two layers, urethane resin coating : 50 μm thickness). On the other hand, it is assumed that the upper region where the amount of flying sea salt particles is 0.1 mdd or less is applied with a rust stabilizing auxiliary treatment agent (Capten Coat M (manufactured by JFE Steel): film thickness 45 μm). The cost of painting was estimated.

  In addition, the coating cost when the whole area in the height direction on the sea was the same as that of the lower region was used as a comparative example, and was estimated to be the standard (1.0). As a result, it was estimated that the coating cost would be more than 50% cheaper than the standard, including a reduction in work man-hours, by making the upper area unpainted (more than 70% of the total length). Even if the increase in alloying elements in unpainted steel is subtracted, it is cheaper by 7% or more.

Claims (9)

A steel-made marine structure used in an environment where the amount of incoming sea salt particles differs in the height direction of the structure, wherein a predetermined boundary value is set for the amount of incoming sea salt particles, and the incoming The region where the amount of sea salt particles exceeds the boundary value is the lower region, the region of the structure where the amount of incoming sea salt particles is the boundary value or less is the upper region, and the predetermined boundary value is 0.1 mdd or less as the lower region is formed by a painted steel having a coating film on the surface, whereas, the upper region is composed of unpainted steel having the treatment film stabilizing auxiliary processing agent rust on the surface, the Unpainted steel is mass%,
C: Less than 0.08%, Si: 0.75% or less,
Mn: 2.0% or less, P: 0.030% or less,
S: 0.030% or less, Al: 0.01 to 0.05%,
N: Including 0.010% or less, W: 0.50-1.0%, Nb: 0.010-0.200%,
Cr: 0.01-0.10%
In addition, the steel material further contains one or two selected from Cu: 0.05 to 0.50% and Ni: 0.05 to 0.50%, and has a composition composed of the remaining Fe and inevitable impurities. Characteristic steel offshore structure.   2. The steel offshore structure according to claim 1, wherein the P and S contents in the composition are, by mass%, P: 0.010% or less and S: 0.0020% or less.   In addition to the above-mentioned composition, one or more selected from Mo: 0.01 to 0.50%, V: 0.05 to 1.0%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.30% in mass% The steel-made marine structure according to claim 1, wherein the marine structure is made of steel.   The steel offshore structure according to any one of claims 1 to 3, further comprising Ti: 0.005 to 0.025% by mass% in addition to the composition. 5. The steel-made marine structure according to claim 1, wherein the composition has a carbon equivalent Ceq defined by the following formula (1) satisfying 0.40 or less.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Here, C, Mn, Si, Ni, Cr, Mo, V: Content of each element (mass%) The composition, the following (2) steel-made marine structure according to any one of claims 1 to 5 weld crack sensitivity index P _CM to be defined, characterized in that the composition satisfies 0.30 or less in Formula.
Serial _{P CM = C + Si / 30} + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ‥‥ (2)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%) 7. The steel-made marine structure according to claim 1, wherein the lower region is made of a coated steel material provided with a C5M-based coating film in accordance with ISO 12944 on the surface of the base steel material. object.   The steel offshore structure according to any one of claims 1 to 7, wherein the steel offshore structure is a steel offshore structure.   9. The steel offshore structure according to claim 8, wherein the steel offshore structure is an offshore wind power generation tower.

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