JP6973117B2 - Steel for mooring chains and mooring chains - Google Patents

Description

The present invention relates to steel for mooring chains and mooring chains.

Since the mooring chain is exposed to the seawater environment, it is severely corroded, and corrosion may become a problem during long-term use. The diameter of the mooring chain is measured at the time of inspection at the dock, but if it is reduced by 12% or more from the original diameter, the standard that a new chain must be switched to is set by the classification regulations. ing.

Generally, the life of a ship is about 20 years, while the life of a mooring chain is about 15 years. Therefore, at present, it is necessary to replace the mooring chain once before the ship is scrapped. Under such circumstances, it is desired that the metal material used for the mooring chain has a longer life by improving the corrosion resistance.

Japanese Unexamined Patent Publication No. 61-210153

Conventionally, industrial steel materials such as SBC300, SBC490, and SBC690 specified in JIS G 3105 (2004) are used for the mooring chain of a ship. Further, in addition to these steel materials, for example, a steel material having improved material properties by adding rare earths or the like as an arbitrary element may also be used (see Patent Document 1).

However, these steel materials have a problem that the service life of the mooring chain is short with respect to the life of the ship as described above, and the desired corrosion resistance is not sufficiently obtained.

An object of the present invention is to solve the above problems and to provide steel for mooring chains and mooring chains having excellent corrosion resistance.

The present invention has been made to solve the above problems, and the following steels for mooring chains and mooring chains are the gist of the present invention.

(1) The chemical composition is mass%.
C: 0.06 to 0.45%,
Si: 0.6% or less,
Mn: 0.3-2.5%,
P: 0.1% or less,
S: 0.05% or less,
Al: 0.1% or less,
N: 0.01% or less,
Cr: More than 0.1% and 7.0% or less,
Sn: 0-0.5%,
Sb: 0-0.5%,
Cu: 0-1.0%,
Ni: 0-5.0%,
Mo: 0-1.0%,
W: 0-1.0%,
V: 0-1.0%,
Ca: 0-0.01%,
Mg: 0-0.01%,
REM: 0-0.01%,
Nb: 0-0.1%,
Ti: 0-0.1%,
B: 0-0.01%,
Remaining: Fe and impurities,
Steel for mooring chains that satisfies the following formulas (i) and (ii).
0.01≤Sn + Sb ... (i)
1-1.31Cr-3.54Sn-3.03Sb ≦ 0.80 ・ ・ ・ (ii)
However, each element symbol in the above formula represents the content (mass%) of each element contained in the steel, and if it is not contained, it is set to zero.

(2) The chemical composition is mass%.
Cu: 0.01-1.0%,
Ni: 0.01-5.0%,
Mo: 0.01-1.0%,
W: 0.01-1.0%,
V: 0.01-1.0%,
Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%, and REM: 0.0001 to 0.01%,
Contains one or more selected from
Steel for mooring chain according to (1) above.

(3) The chemical composition is mass%.
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%, and B: 0.0001 to 0.01%,
Contains one or more selected from
The steel for mooring chains according to (1) or (2) above.

(4) A mooring chain using the steel according to any one of (1) to (3) above.

(5) The mooring chain according to (4) above, which is provided with an anticorrosion coating layer on the surface.

According to the present invention, steel for mooring chains and mooring chains having excellent corrosion resistance can be obtained.

The present inventor investigated the factors that the service life of the steel material used for the mooring chain is shorter than the service life of the ship and the desired corrosion resistance cannot be obtained.

The corrosion resistance of a steel material varies greatly depending on the corrosive environment in which the steel material is used. That is, even a steel material evaluated to have excellent corrosion resistance in a specific corrosive environment may not be able to exhibit corrosion resistance in different corrosive environments. Therefore, the present inventor first investigated the usage environment of the mooring chain.

Since the mooring chain is used for mooring vessels off the harbor, its total length may reach several hundred meters. Then, the mooring chain is pulled up on the deck when the ship is sailing, and a part of the mooring chain is left on the deck, but most of the rest is stored in a hangar called a chain locker.

Since the deck of a ship is outdoors, it is exposed to wind and rain in an atmospheric environment and is frequently exposed to seawater droplets. In such an environment, the surface of the mooring chain is frequently wetted by seawater droplets and then dried. As a result, corrosion progresses violently.

On the other hand, the inside of the chain locker is an environment in which the relative humidity does not fall below about 50% RH, although the value fluctuates. As can be seen from the fact that the relative humidity on the deck is about 25 RH%, it can be said that the inside of the chain locker is a relatively high humidity environment. Here, if the relative humidity is about 30 RH% or more, it is considered that a phenomenon called deliquescent occurs in which chlorides such as magnesium chloride contained in seawater take in moisture in the atmosphere.

For this reason, magnesium chloride or the like adhering to the mooring chain takes in moisture in the atmosphere, so that the surface is maintained in a slightly moist state and is always a thin water film (also referred to as "thin film water" in the following description). ) Is formed. Further, in the thin film water formed on the surface, the chloride contained in the seawater is concentrated. As described above, when the steel material is corroded in an environment where chloride is concentrated, a hydrolysis reaction of iron ions occurs at the place where iron is eluted due to the corrosion. As a result, the pH of the steel surface decreases, and corrosion further progresses.

Therefore, it was revealed that the mooring chain was used in a severely corrosive environment, whether it was stationary on the deck or stored in a chain locker. For this reason, mooring chains need to be corrosion resistant, both on the deck of the ship and in the chain locker.

Therefore, the present inventor considered that it is necessary to reproduce the environment on the deck and the environment inside the chain locker and evaluate the corrosion resistance. Then, by evaluating the corrosion resistance of the steel material by the above-mentioned corrosion test method, the steel of the present invention suitable as a steel for a mooring chain was obtained.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

1. 1. The reasons for limiting the chemical composition of each element are as follows. In the following description, "%" for the content means "mass%".

C: 0.06 to 0.45%
C is an element necessary for ensuring strength. In order to secure the strength of the mooring chain, the C content shall be 0.06% or more. However, when the C content exceeds 0.45%, the toughness of the base metal and the weld heat-affected zone is significantly reduced. Therefore, the C content is set to 0.45% or less. The C content is preferably 0.07% or more, and more preferably 0.08% or more. The C content is preferably 0.42% or less, more preferably 0.40% or less.

Si: 0.6% or less Si is an element necessary for deoxidation, but if it is contained in excess of 0.6%, the toughness of the weld heat affected zone decreases. Therefore, the Si content is set to 0.6% or less. From the viewpoint of toughness, it is desirable that the Si content is lower. In this case, the Si content is preferably 0.55% or less, more preferably 0.5% or less. On the other hand, if the Si content is excessively reduced, the deoxidizing effect cannot be sufficiently obtained. Therefore, the Si content is preferably 0.01% or more.

Mn: 0.3-2.5%
Mn is an element necessary for ensuring strength. In order to secure the strength, the Mn content shall be 0.3% or more. However, if Mn is contained in an amount of more than 2.5%, the toughness is significantly reduced. Therefore, the Mn content is set to 2.5% or less. The Mn content is preferably 0.35% or more, more preferably 0.4% or more. The Mn content is preferably 2.4% or less, more preferably 2.3% or less.

P: 0.1% or less P is an element that segregates at grain boundaries as impurities and reduces toughness. When the P content exceeds 0.1%, the toughness is remarkably lowered. Therefore, the P content is set to 0.1% or less. The smaller the P content, the more preferable. The P content is preferably 0.08% or less, more preferably 0.05% or less.

S: 0.05% or less S exists in the steel as an impurity and forms MnS. This MnS becomes a starting point of corrosion and lowers corrosion resistance. Therefore, the S content is set to 0.05% or less. The smaller the S content, the more preferable. The S content is preferably 0.045% or less, more preferably 0.04% or less.

Al: 0.1% or less Al is an element required as a deoxidizing agent, and the deoxidizing effect can be obtained by containing it. Further, Al binds to N to form AlN, thereby making the crystal grains finer. However, if Al is contained in an amount of more than 0.1%, the toughness is lowered. Therefore, the Al content is set to 0.1% or less. The Al content is preferably 0.08% or less, more preferably 0.06% or less. On the other hand, in order to stably obtain the above effect, the Al content is preferably 0.005% or more.

N: 0.01% or less N has the effect of making crystal grains finer by combining with Al to form AlN. However, if the N content exceeds 0.01%, the toughness decreases. Therefore, the N content is set to 0.01% or less. The N content is preferably 0.008% or less, more preferably 0.006% or less. On the other hand, in order to obtain the above effect, the N content is preferably 0.001% or more.

Cr: More than 0.1% and 7.0% or less Cr has the effect of increasing the strength while suppressing the cost increase of the steel material. Further, Cr has an effect of remarkably suppressing the elution of steel in a wet environment due to seawater droplets or rainfall. Therefore, the Cr content is set to more than 0.1%. However, when the Cr content exceeds 7.0%, the toughness is remarkably lowered and the weldability is also lowered. Therefore, the Cr content is set to 7.0% or less. The Cr content is preferably 0.2% or more, more preferably 0.3% or more. The Cr content is preferably 6.5% or less, more preferably 6.0% or less.

Sn: 0 to 0.5%
Sb: 0-0.5%
0.01≤Sn + Sb ... (i)
Sn and Sb are eluted as ions in an environment where chloride is concentrated due to the formation of thin film water and the pH at the corrosive interface is lowered, and the inhibitory action significantly suppresses the dissolution reaction of steel. Therefore, the total content of Sn and Sb is 0.01% or more.

However, if the Sn content exceeds 0.5% or the Sb content exceeds 0.5%, the toughness is significantly reduced. Therefore, the contents of Sn and Sb are set to 0.5% or less, respectively. The Sn and Sb contents are preferably 0.03% or more, more preferably 0.05% or more, respectively. Further, the Sn content is preferably 0.4% or less. Further, the Sb content is preferably 0.4% or less, more preferably 0.3% or less.

In addition to the above components, the mooring chain steel according to the present invention may contain one or more elements selected from Cu, Ni, Mo, W, V, Ca, Mg, and REM.

Cu: 0-1.0%
Cu has the effect of suppressing the elution of steel and improving the corrosion resistance. In addition, since the corrosion products produced by corrosion are highly protective, they also have the effect of maintaining high corrosion resistance for a long period of time. Therefore, it may be contained as needed. However, when the Cu content exceeds 1.0%, not only the effect is saturated but also the toughness is lowered. Therefore, the Cu content is set to 1.0% or less. The Cu content is preferably 0.8% or less, more preferably 0.6% or less. On the other hand, in order to obtain the above effect, the Cu content is preferably 0.01% or more, more preferably 0.03% or more, and further preferably 0.05% or more.

Ni: 0-5.0%
Ni has the effect of significantly suppressing the elution of steel in a thin film water forming environment and improving the corrosion resistance. Therefore, it may be contained as needed. However, when the Ni content exceeds 5.0%, not only the effect is saturated, but also the cost of the steel material increases. Therefore, the Ni content is 5.0% or less. The Ni content is preferably 4.5% or less, more preferably 4.0% or less. On the other hand, in order to obtain the above effect, the Ni content is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.1% or more.

Mo: 0-1.0%
Mo has the effect of increasing the strength. Further, Mo has an action of suppressing the elution of steel by the inhibitory action of Mo that elutes in a corrosive environment to form molybdate ions. As a result, Mo has an effect of improving corrosion resistance, and may be contained as necessary.

However, when the Mo content exceeds 1.0%, not only the effect is saturated but also the toughness is significantly reduced. Therefore, the Mo content is set to 1.0% or less. The Mo content is preferably 0.8% or less, more preferably 0.5% or less. On the other hand, in order to obtain the above effect, the Mo content is preferably 0.01% or more, more preferably 0.03% or more, and further preferably 0.05% or more.

W: 0 to 1.0%
W also has the same effect as Mo. W eluted in a corrosive environment forms tungstic acid ions, which has the effect of suppressing the elution of steel. Therefore, it may be contained as needed. However, when the W content exceeds 1.0%, not only the effect is saturated but also the toughness is lowered. Therefore, the W content is set to 1.0% or less. The W content is preferably 0.8% or less, more preferably 0.5% or less. On the other hand, in order to obtain the above effect, the W content is preferably 0.01% or more, more preferably 0.03% or more, and further preferably 0.05% or more.

V: 0 to 1.0%
V has an effect of suppressing the elution of steel by forming vanadate ions by V eluted in a corrosive environment. Therefore, it may be contained as needed.

However, when the V content exceeds 1.0%, not only the effect is saturated but also the toughness is lowered. Therefore, the V content is set to 1.0% or less. The V content is preferably 0.8% or less, more preferably 0.5% or less. On the other hand, in order to obtain the above effect, the V content is preferably 0.01% or more, more preferably 0.03% or more, and further preferably 0.05% or more.

Ca: 0-0.01%
Ca elutes as ions and raises the pH at the corrosive interface where the pH drops. As a result, corrosion is suppressed, so that it may be contained as needed. However, when the Ca content exceeds 0.01%, not only the effect is saturated but also the toughness is lowered. Therefore, the Ca content is set to 0.01% or less. The Ca content is preferably 0.008% or less, more preferably 0.006% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably 0.0001% or more, more preferably 0.0003% or more, and further preferably 0.0005% or more.

Mg: 0-0.01%
Similar to Ca, Mg has an effect of suppressing corrosion by increasing the pH of the corrosion interface. Therefore, it may be contained as needed. However, when the Mg content exceeds 0.01%, not only the effect is saturated but also the toughness is lowered. Therefore, the Mg content is set to 0.01% or less. The Mg content is preferably 0.008% or less, more preferably 0.006% or less. On the other hand, in order to obtain the above effect, the Mg content is preferably 0.0001% or more, more preferably 0.0003% or more, and further preferably 0.0005% or more.

REM: 0-0.01%
Like Ca and Mg, REM has an effect of suppressing corrosion by increasing the pH of the corrosion interface, and therefore may be contained as necessary. However, when the REM content exceeds 0.01%, not only the effect is saturated but also the toughness is lowered. Therefore, the REM content is set to 0.01% or less. The REM content is preferably 0.008% or less, more preferably 0.006% or less.

On the other hand, in order to obtain the above effect, the REM content is preferably 0.0001% or more, more preferably 0.0003% or more, and further preferably 0.0005% or more.

Here, in the present invention, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the REM content means the total content of these elements. REM is industrially added in the form of misch metal.

In addition to the above components, the mooring chain steel according to the present invention may further contain one or more elements selected from Nb, Ti, and B.

Nb: 0 to 0.1%
Since Nb has an effect of increasing strength, it may be contained if necessary. However, if the Nb content exceeds 0.1%, the toughness decreases. Therefore, the Nb content is set to 0.1% or less. The Nb content is preferably 0.08% or less, more preferably 0.05% or less. On the other hand, in order to obtain the above effect, the Nb content is preferably 0.001% or more, more preferably 0.003% or more, and further preferably 0.005% or more.

Ti: 0-0.1%
Ti improves the toughness of the weld heat-affected zone by combining with N to form TiN. Therefore, it may be contained as needed. However, when the Ti content exceeds 0.1%, the effect is saturated. Therefore, the Ti content is set to 0.1% or less. The Ti content is preferably 0.08% or less, more preferably 0.05% or less. On the other hand, in order to obtain the above effect, the Ti content is preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.01% or more.

B: 0-0.01%
Since B has an effect of increasing strength, it may be contained if necessary. However, if the B content exceeds 0.01%, the toughness decreases. Therefore, the B content is set to 0.01% or less. The B content is preferably 0.008% or less, more preferably 0.005% or less. On the other hand, in order to obtain the above effect, the B content is preferably 0.0001% or more, more preferably 0.0003% or more, and further preferably 0.0005% or more.

In the chemical composition of the present invention, the balance is Fe and impurities. Here, the "impurity" is a component mixed with raw materials such as ore and scrap, and various factors in the manufacturing process when steel is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something.

As described above, in the present invention, it is necessary to appropriately control the Cr content in order to ensure the corrosion resistance for use on the deck. Further, in order to ensure corrosion resistance for use in a chain rocker, it is necessary to appropriately control the contents of Sn and Sb. Therefore, the component system of the present invention needs to satisfy the following formula (ii) in consideration of the interaction of these elements.

1-1.31Cr-3.54Sn-3.03Sb ≦ 0.80 ... (ii)
The above formula (ii) represents the corrosion resistance of the steel of the present invention, and if the formula (ii) is satisfied, sufficient corrosion resistance can be ensured as the steel for the mooring chain. On the other hand, if the formula (ii) is not satisfied, the corrosion resistance is not sufficient and it is difficult to use it as a mooring chain for a long period of time. The lvalue of the equation (ii) is preferably 0.5 or less, and more preferably 0 or less.

However, each element symbol in the above formula (ii) represents the content (mass%) of each element contained in the steel, and if it is not contained, it is set to zero.

In the present invention, after the steel is formed into the shape of a mooring chain, an anticorrosion coating (coating) may be applied to the surface. That is, an anticorrosion coating layer is formed on the surface of the mooring chain. The type of the anticorrosion coating layer is not limited, and a general one may be used. Specifically, anticorrosion plating film exemplified by Zn plating, Al plating, Zn-Al plating, etc., metal sprayed coating exemplified by Zn spraying, Al spraying, etc., vinyl butyral type, epoxy type, urethane type, phthalic acid. Examples thereof include general anticorrosion coating films exemplified by systems, fluorine-based, oil-based paints, bituminous-based, and the like.

2. 2. Manufacturing method A steel ingot having the above chemical composition is manufactured by a continuous casting method. The ingot may be made into an ingot by the ingot forming method (see JIS G 0203 (2009)). Subsequently, the obtained ingot is subjected to soaking and hot rolling (hot steel bar rolling) in the range of 800 to 1250 ° C. to obtain steel bars. In the present invention, the diameter of the steel bar is not particularly limited, but is, for example, in the range of 50 to 120 mm. Further, the finish rolling of the hot rolling is performed at 800 ° C. or higher. Subsequently, the obtained steel bar is cooled to room temperature by air cooling or air cooling.

In the present invention, heat treatment may be performed after the above cooling. Further, after the heat treatment, a surface treatment such as shot blasting may be applied. Then, using the steel bar manufactured as described above as a material, the steel bar is heated at 600 to 1100 ° C., then bent, joined by flash butt welding, and the joint is deburred to manufacture a link. This process is repeated to obtain a chain having a predetermined length. If necessary, the stud can be attached by welding. After that, quenching and tempering are performed. The temperature of the quenching treatment is 800 to 1100 ° C., the time is 10 minutes or more, and then cooling is performed by water cooling. The temperature of the tempering treatment is 450 to 750 ° C., the time is 10 minutes or more, and then it is cooled by water cooling. These steps for producing the engagement Tomechi En having a desired shape.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

After melting the steel having the chemical composition shown in Table 1, the ingots obtained by continuous casting were ingot-rolled to obtain steel pieces. Then, the steel piece was subjected to hot rolling (hot bar steel rolling). In hot rolling, the soaking temperature of the steel pieces was set to 1250 ° C., the material was rolled to a diameter of 25 mm at 1050 ° C. or higher, and air-cooled to room temperature.

Subsequently, the obtained steel bar was heat-treated. In the heat treatment, quenching treatment at 900 ° C. for 15 minutes and water cooling were performed, and then tempering treatment at 638 ° C. for 20 minutes and water cooling were performed. After the above-mentioned heat treatment, a test piece having a diameter of 20 mm × 100 mm was collected, and the surface of the test piece was shot blasted.

Then, the test piece was subjected to the corrosion test described below. In the corrosion test, a corrosion test 1 simulating the corrosion environment in the chain rocker and a corrosion test 2 simulating the corrosion environment on the deck were carried out. The corrosion test 1 was a test based on the method described in Japanese Patent Application No. 2017-240413, and in the corrosion test 2, the humidity in the drying and wetting step described later was different from that of the corrosion test 1.

Specifically, in the corrosion test, 8 cycles (about 8 weeks) were carried out, with the treatment consisting of the seawater immersion step and the drying and wetting step as one cycle. In the seawater immersion step, the test piece was immersed in artificial seawater at 35 ° C. for 15 minutes once a week (7 days). The composition of the artificial seawater used in the test was NaCl: 2.45%, MgCl ₂ : 1.11%, Na ₂ SO ₄ : 0.41%, CaCl ₂ : 0.15%, KCl: 0.07%, NaHCO ₃ : 0.02%, KBr: 0.01%. In addition,% of the composition of the said artificial seawater shows mass%.

Subsequently, in the drying and wetting step, two steps, a drying step in which the humidity was low and a wetting step in which the humidity was high, were carried out. In the corrosion test 1, in order to simulate the environment inside the chain rocker, the drying and wetting step was carried out under the following conditions. Specifically, in the drying step, the temperature was maintained in an environment of 60 ° C. and a relative humidity of 65% RH for 4 hours, and in the wetting step, the temperature was maintained in an environment of 60 ° C. and a relative humidity of 90% RH for 4 hours.

On the other hand, in the corrosion test 2, in order to simulate the environment on the deck, the drying and wetting step was carried out under the following conditions. Specifically, in the drying step, the temperature was maintained in an environment of 60 ° C. and a relative humidity of 25% RH for 4 hours, and in the wetting step, the temperature was maintained in an environment of 60 ° C. and a relative humidity of 100% RH for 4 hours.

In both corrosion tests 1 and 2, after the seawater dipping step, the drying step and the wetting step were repeated until the next seawater dipping step. In this case, if the drying step and the wetting step are performed once, the above treatment is performed three times in one day.

After the above corrosion test, the amount of decrease in the diameter of the test piece was measured with a caliper. The test piece was measured at a position 30 mm from the upper end and the lower end and a position 50 mm from the central portion, that is, the upper end. In addition, at each of the above-mentioned positions, the size of the diameter at the position orthogonal to that position was also measured in the same manner. That is, the size of the diameter was measured at six positions in one sample, and the size of the diameter was calculated after the corrosion test from the average value. Subsequently, the difference between the original diameter size and the diameter size after the corrosion test was defined as the diameter reduction amount (mm). In the present invention, in corrosion tests 1 and 2, it was determined that the mooring chain has sufficient corrosion resistance when the amount of decrease in diameter is 0.9 mm or less.

The above test results are shown in Table 1.

As shown in Table 1, Test No. Since 1 to 20 satisfied the composition and formula (i) specified in the present invention and also satisfied the formula (ii), the corrosion resistance was good. On the other hand, the test No. 21 to 23 did not satisfy the composition or the formula (ii), resulting in inferior corrosion resistance.

Claims (5)

The chemical composition is by mass%,
C: 0.06 to 0.45%,
Si: 0.6% or less,
Mn: 0.3-2.5%,
P: 0.1% or less,
S: 0.05% or less,
Al: 0.1% or less,
N: 0.001 to 0.01 %,
Cr: More than 0.1% and 7.0% or less,
Sn: 0-0.5%,
Sb: 0-0.5%,
Cu: 0-1.0%,
Ni: 0-5.0%,
Mo: 0-1.0%,
W: 0-1.0%,
V: 0-1.0%,
Ca: 0-0.01%,
Mg: 0-0.01%,
REM: 0-0.01%,
Nb: 0-0.1%,
Ti: 0-0.1%,
B: 0-0.01%,
Remaining: Fe and impurities,
Steel for mooring chains that satisfies the following formulas (i) and (ii).
0.01≤Sn + Sb ... (i)
1-1.31Cr-3.54Sn-3.03Sb ≦ -0.18 ··· (ii)
However, each element symbol in the above formula represents the content (mass%) of each element contained in the steel, and if it is not contained, it is set to zero. The chemical composition is by mass%.
Cu: 0.01-1.0%,
Ni: 0.01-5.0%,
Mo: 0.01-1.0%,
W: 0.01-1.0%,
V: 0.01-1.0%,
Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%, and REM: 0.0001 to 0.01%,
Contains one or more selected from
The steel for mooring chains according to claim 1. The chemical composition is by mass%.
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%, and B: 0.0001 to 0.01%,
Contains one or more selected from
The steel for mooring chains according to claim 1 or 2. A mooring chain using the steel according to any one of claims 1 to 3. The mooring chain according to claim 4, wherein the surface is provided with an anticorrosion coating layer.