JP6624129B2 - Steel material and method of manufacturing the same - Google Patents

Description

  The present invention relates to a steel material mainly used for a hold (a hold) of a bulk carrier and a method of manufacturing the same, and more particularly to a steel material suitable for a hold and a ballast tank of a bulk carrier carrying coal or coke, and a method of manufacturing the same. The steel material according to the present invention includes a thick steel plate, a thin steel plate, a shaped steel bar, a steel bar, and the like.

  Commercial ships are often used to transport energy resources. Bulk carriers account for about 30% of the fleet of merchant ships. A series of marine accidents occurred on these bulk carriers in the early 1990s and became an international problem. In particular, there have been numerous reports of accidents involving coal carriers, most of which were caused by damage inside the hold.

  Bulk carriers carry their cargo directly to the hold, so if the cargo is corrosive, it is susceptible to corrosion.Corrosion in the hold, especially holes in the side walls and ribs in the hold of coal carriers. It is considered that the local decrease in strength due to food is a problem. Actually, there have been reports of cases in which this pitting has progressed remarkably and cases in which the thickness of ribs for securing the strength of the ship has been extremely reduced due to pitting.

  Condensation water is likely to occur on the side walls and ribs of the bulk carrier hold where pitting occurs. The sulfur component of the coal dissolves in the place where the dew water is generated, and reacts with the dew water to generate sulfuric acid. Therefore, the inside of the hold of the coal carrier has a low pH environment in which sulfuric acid corrosion easily occurs.

  Severe sulfuric acid corrosion has also been observed on holds of coke carriers. This is because, like coal, the sulfur content in coke causes severe corrosion.

  As a countermeasure against such corrosion in the hold, a modified epoxy coating is applied to the inside of the hold with a coating thickness of about 150 to 200 μm. However, the coating often peels off due to mechanical damage due to the load, or damage or abrasion by the heavy equipment at the time of unloading the load, so that it is difficult to obtain a sufficient anticorrosion effect by the coating.

  As a further countermeasure against corrosion, methods such as periodic repainting and partial repair are adopted. However, since such a method requires a very large cost, development of a new corrosion-resistant steel has been an issue in order to reduce life cycle costs including a ship maintenance cost.

  On the other hand, the ship has a ballast tank for maintaining the stability of the ship during navigation, and when the cargo is unloaded, the ballast tank is filled with seawater. This is no exception for bulk carriers. In a ballast tank, seawater is immersed when a load is unloaded, and seawater is drained when a load is loaded.However, due to the temperature difference during the day, the environment is repeatedly dry and wet. Is a severely corrosive environment. Therefore, as a means for preventing corrosion, electrolytic protection and painting have conventionally been applied.

  However, even if the coating is applied, the coating may be damaged during the operation, or local corrosion may start from a slight defect existing in the coating. Further, although cathodic protection exerts its anticorrosion effect when submerged, it does not function sufficiently when the seawater in the ballast tank is empty with a load, and corrosion may proceed significantly. It is the actual situation that parts in such a corrosive environment are subjected to repair painting until the life of the ship and maintain corrosion resistance. Therefore, development of a steel material excellent in corrosion resistance, which can extend the period until repair painting as much as possible, is desired.

  By the way, as corrosion-resistant steel for ships, steel developed for cargo oil tanks and ballast tanks is known. Patent Literatures 1 to 3 are known as conventional techniques referring to a coal carrier. Patent Literature 1 discloses that by containing Cu: 0.05 to 1%, Ni: 0.01 to 0.5%, and Sn: 0.005 to 0.2% by mass% as a corrosion resistance improving element, corrosion in a coal / ore carrier hold can be prevented. A steel material to suppress is disclosed. Patent Document 2 discloses a steel material that suppresses corrosion in a coal / ore carrier by including Sn: 0.01 to 0.3% by mass as a corrosion resistance improving element. In Patent Document 3, as corrosion resistance improving elements, Cu: 0.05 to 0.35%, Ni: 0.02 to 0.40%, Sb: 0.01 to 0.2%, W: 0.005 to 0.5%, Nb: 0.003 to 0.025% by mass% Accordingly, a steel material that suppresses corrosion in a coal ship and a coal / ore combined ship hold is disclosed.

  Patent Literatures 4 to 6 are known as prior arts that refer to ballast tanks. In Patent Document 4, as a corrosion resistance improving element, Cu: 0.05 to 0.50% and W: 0.01 to 0.05 are added by mass%, and further, Ge, Sn, Pb, As, Sb, Bi, Te, and Be are added. There is disclosed a corrosion-resistant low-alloy steel containing one or two or more kinds added at 0.01 to 0.2%. Patent Document 5 discloses that a corrosion-resistant paint is applied to a low-alloy corrosion-resistant steel material to which P: 0.03 to 0.10%, Cu: 0.1 to 1.0%, and Ni: 0.1 to 1.0% by mass% are added as corrosion resistance improving elements. A coated ballast tank is disclosed.

  Patent Document 6 contains, as a corrosion resistance improving element, one or two selected from W: 0.01 to 0.5% and Mo: 0.02 to 0.5% by mass%, and Sn: 0.001 to 0.2% and Sb: contains one or two kinds selected from 0.01 to 0.2%, and Cu: less than 0.05%, Ni: less than 0.05%, Cr: less than 0.05%, Co: less than 0.05%, A marine corrosion-resistant steel material that suppresses coating film swelling is disclosed.

  Patent Document 7 discloses that as an element for improving corrosion resistance, Cu: 0.01 to 2.00% and Mg: 0.0002 to 0.0150% by mass% are essential elements, and Ni, Cr, Mo, W, Ca, Sb, and Sn are selectively used. Disclosed are corrosion-resistant steel materials for ships that can be contained and exhibit excellent corrosion resistance in use environments such as ship outer panels, ballast tanks, cargo oil tanks, and coal carrier cargo holds.

JP 2007-262555 A JP 2008-174768 A JP 2015-157969 A JP-A-48-050921 JP 07-034197 A JP 2009-46750 JP JP 2000-17381 A

  However, Patent Documents 1 and 2 are steel materials designed on the assumption of a corrosive environment in a hold of a coal / ore carrier, and do not consider corrosion on the ballast tank side. Furthermore, although the corrosion resistance under the coating film simulating the usage environment of a coal carrier is evaluated, it is assumed that the coating film is likely to peel off due to mechanical damage by coal, coke, etc. No evaluation tests have been conducted. Patent Document 3 does not consider corrosion on the ballast tank side.

  Patent Document 4 does not consider the corrosion inside the hold when coal, coke, or the like is loaded. In addition, the corrosion resistance in the presence of a coating film such as an epoxy-based paint that is generally applied in a ballast tank has not been studied, and the improvement of the corrosion resistance in the presence of such a coating film has been separately discussed. Needs to be considered. In the steel material of Patent Document 5, P is added in a relatively large amount of 0.03 to 0.10% in order to improve the corrosion resistance of the base metal, and there is a problem in terms of weldability and weld toughness. The steel of Patent Document 6 does not consider corrosion inside the hold when coal, coke, or the like is loaded. Although the addition of Mg is essential for the steel material of Patent Document 7, there is a problem that the mechanical properties of the steel material are not stable because the steelmaking yield is not stable.

  In many ships, one steel sheet has a structure on one side of the bulk carrier hold side and the opposite side on the ballast tank side. On the other hand, the side wall of a bulk carrier may be a single hull (single hull), and the inside of the hold and seawater are separated only by one steel material. The back of the upper deck of the ballast tank is not in contact with the hold. However, there is a demand for small ships (even some large ships) to be constructed of the same steel plate in order to avoid the complexity of building ships.

  However, as described above, it suppresses corrosion in the bulk carrier hold, especially in coal and coke, and at the same time, suppresses corrosion in ballast tanks. That is, simultaneously exhibits corrosion resistance in two different corrosive environments. The solution was unknown. In order to study the solution, it is important to consider the corrosion environment peculiar to the hold of a coal carrier or a coke carrier, and to evaluate the corrosion of the steel material in a state where the coating film is peeled off and there is no coating film. Further, in a ballast tank, corrosion from a defective portion of the coating film in the presence of the coating film is important.

  The present invention has been made in view of the above-mentioned circumstances, and is a corrosion-resistant steel material for ships that simultaneously exhibits excellent corrosion resistance in two different corrosive environments in a hold of a bulk carrier carrying coal and coke and in a ballast tank. The purpose is to provide.

Generally, a ship is used after being provided with an anticorrosion coating. However, the hold operating environment of coal carriers and coke carriers is in a situation where paint is easily peeled off due to mechanical damage of coal and coke, and steel materials are repeatedly wet and dry and low pH environment due to SO ₄ ^2- ions leaching from coal and coke. Exposed below. Therefore, the inventors have attempted to develop a steel material that can exhibit corrosion resistance even after the anticorrosion coating film on the surface of the steel material inside the hold is peeled off.

  That is, the inventors have developed a test method that simulates the environment inside the hold of a coal carrier and a coke carrier, and studied the effect of each alloy element using the test method. At the same time, the present inventors have developed a test method that simulates the environment in the ballast tank, by testing the effect of each alloy element that suppresses the progress of corrosion from the coating defect on the ballast tank. Was considered. As a result, they found elements that effectively contribute to the improvement of the corrosion resistance of steel in both the environment inside the hold of coal and coke carriers and the environment inside the ballast tank. That is, the present invention has been completed based on the above novel findings and after further studies, and the gist configuration thereof is as follows.

1. In mass%,
C: 0.040% or more and 0.200% or less,
Si: 0.01% or more and 0.50% or less,
Mn: 0.10% to 2.00%,
P: 0.035% or less,
S: 0.010% or less,
Al: 0.003% or more and 0.100% or less,
W: 0.010% or more and 0.500% or less,
Sb: 0.010% to 0.300%,
Sn: 0.010 to 0.300% and
Nb: 0.003% or more and 0.025% or less,
N: a steel material containing 0.0010% or more and 0.0080% or less, with the balance being Fe and a component composition of unavoidable impurities,
The amount of solid solution W in W is 0.005% or more;
A steel material in which the amount of solute Nb in Nb is 0.002% or more.

2. The component composition further comprises:
In mass%,
Ti: 0.001% or more and 0.030% or less,
Zr: 0.001% or more and 0.030% or less and
V: The steel material according to 1 above, containing one or more selected from 0.002% or more and 0.200% or less.

3. The component composition further comprises:
In mass%,
3. The steel material according to 1 or 2 above, containing Ca: 0.0002% or more and 0.0050% or less.

4. The component composition further comprises:
In mass%,
B: The steel material according to any one of 1 to 3 above, containing 0.0002% to 0.0050%.

5. The component composition further comprises:
In mass%,
Cu: less than 0.04% and
Ni: The steel material according to any one of the above items 1 to 4, which contains one or more kinds selected from less than 0.04%.

6. The component composition further comprises:
In mass%,
Co: 0.010% to 0.500%,
Mo: 0.010% to 0.500% and
Cr: The steel material according to any one of the above 1 to 5, containing one or more selected from 0.010% to 0.200%.

7. The component composition further comprises:
In mass%,
REM: 0.0002% or more and 0.015% or less,
Y: 0.0001% or more and 0.1% or less and
Mg: The steel material according to any one of the above 1 to 6, containing one or more selected from 0.0002% to 0.015%.

8. 8. The steel material according to any one of 1 to 7, having a zinc primer coating on the surface.

9. 8. The steel material according to any one of the above items 1 to 7, having an epoxy-based coating film on the surface.

10. 8. The steel material according to any one of 1 to 7 above, having a zinc primer coating film and an epoxy coating film on the surface.

11. A method for producing a steel material, comprising heating a steel material having the component composition according to any one of the above 1 to 7, and thereafter performing hot rolling.
The heating temperature of the steel material is 1050 ° C. or higher,
A method for producing a steel material, wherein a cooling rate after finish rolling in the hot rolling is 10 ° C./s or more.

  ADVANTAGE OF THE INVENTION According to this invention, the steel material for ships which shows excellent corrosion resistance both in two different corrosive environments in the hold of a bulk carrier carrying coal or coke and in the ballast tank can be provided.

  Hereinafter, a steel material according to an embodiment of the present invention will be described. First, the reasons for limiting the component composition of the steel material will be described. In this specification, "%" representing the content of each component element means "% by mass" unless otherwise specified.

C: 0.040% or more and 0.200% or less
C is an element effective for increasing the strength of steel. In the present invention, C is contained in an amount of 0.040% or more to secure the strength. On the other hand, when C is contained in excess of 0.200%, the weldability and the weld heat affected zone toughness are reduced. Therefore, the C content is in the range of 0.040% or more and 0.200% or less. It is preferably in the range of 0.050% to 0.180%, and more preferably in the range of 0.060% to 0.160%.

Si: 0.01% or more and 0.50% or less
Since Si is an element added as a deoxidizing agent and increases the strength of steel, it is contained in the present invention in an amount of 0.01% or more. However, if the content of Si exceeds 0.50%, the toughness of the steel is deteriorated. Therefore, the upper limit of the amount of Si is set to 0.50%. It is preferably in the range of 0.05% to 0.40%, more preferably in the range of 0.10% to 0.35%.

Mn: 0.10% or more and 2.00% or less
Mn is contained in an amount of 0.10% or more because it can increase the strength of steel. However, when Mn is contained in excess of 2.00%, the toughness and weldability of the steel are reduced, so the upper limit of the amount of Mn is set to 2.00%. Preferably, it is in the range of 0.50% to 1.60%. More preferably, it is in the range of 0.70% to 1.60%.

P: 0.035% or less
P is a harmful element that reduces the base metal toughness of steel, but reducing P leads to an increase in manufacturing costs. Therefore, from the viewpoint of base metal toughness and manufacturing cost, the P content is set to 0.035% or less. Preferably it is 0.020% or less, more preferably 0.010% or less. Since it is difficult to reduce the content to less than 0.001% on an industrial scale, the content of 0.001% or more is allowable.

S: 0.010% or less
Since S is a harmful element that deteriorates the toughness and weldability of steel, it is preferable to reduce it as much as possible. Preferably it is 0.006% or less, more preferably 0.003% or less. Since it is difficult to reduce the content to less than 0.0005% in industrial scale production, the content of 0.0005% or more is allowable.

Al: 0.003% or more and 0.100% or less
Al is contained as a deoxidizing agent in an amount of 0.003% or more. However, since an amount exceeding 0.100% adversely affects weld toughness, the Al content is set to 0.100% or less. Preferably it is 0.015% or more and 0.050% or less, more preferably 0.015% or more and 0.040% or less.

W: 0.010% or more and 0.500% or less
W is the product of WO ₄ ^2-ions, Cl from SO ₄ ^2-ions and seawater exuded from coal and coke ^- suppresses the diffusion of the base steel surface of the ion, and a dense corrosion products Suppresses the diffusion of water, oxygen, SO ₄ ^2- ions and Cl ⁻ ions to the surface of the iron base. Thereby, W can improve corrosion resistance in both environments on the hold side and the ballast tank side. This effect is exhibited at 0.010% or more. However, when the content exceeds 0.500%, not only the effect is saturated, but also the cost increases. Therefore, the W amount is set to be in a range of 0.010% to 0.500%. Preferably, it is in the range of 0.020% to 0.200%. More preferably, it is in the range of 0.030% or more and 0.150% or less.

Solid solution W: 0.005% or more
W has the above-described action of improving corrosion resistance, but W exists as a solid solution W or a precipitate such as a carbide in steel. Among them, solid solution W contributes to the improvement of corrosion resistance. Since the corrosion resistance is exhibited when the amount of solid solution W is 0.005% or more, the amount of solid solution W is set to 0.005% or more. Preferably it is 0.010% or more and 0.50% or less. It is more preferably at least 0.020%. Here, in order to make the solid solution W 0.005% or more, it is necessary to make the amount of W added to steel 0.007% or more and to set the cooling rate after hot finish rolling to 10 ° C / s or more.

Sb: 0.010% to 0.300%, Sn: 0.010% to 0.300%
Both Sb and Sn are concentrated near the base iron in a low pH environment when steel contains 0.010% or more as an alloying element. Since Sb and Sn have a large hydrogen overvoltage, the hydrogen generation reaction is suppressed in the portion where Sb and Sn are deposited, and the corrosion resistance is improved. On the hold side, a low pH environment is created by SO ₄ ²⁻ ions leaching from coal or coke, and on the ballast tank side, a low pH environment is created at the anode portion at the coating defect. Therefore, the corrosion resistance effect of Sb and Sn is exhibited in both environments. This effect is manifested at 0.010% or more, but if it exceeds 0.300%, the toughness is reduced, so that both Sb and Sn are in the range of 0.010% to 0.300%. Preferably it is in the range of 0.020% or more and 0.250% or less. More preferably, it is in the range of 0.030% or more and 0.120% or less.

Nb: 0.003% or more and 0.025% or less
Nb densifies the corrosion products and suppresses the diffusion of SO ₄ ^2- ions leached from coal and coke and Cl ⁻ ions derived from seawater to the surface of the iron base. This improves the corrosion resistance of the steel. In order to obtain this effect, it is necessary to contain 0.003% or more of Nb. On the other hand, even if Nb is contained in an amount exceeding 0.025%, the effect is saturated. Therefore, the Nb content is in the range of 0.003% or more and 0.025% or less. Preferably, it is in the range of 0.005% to 0.020%. More preferably, it is in the range of 0.007% to 0.020%.

Solid solution Nb: 0.002% or more
In order to improve the corrosion resistance by adding Nb, it is important that the content is in the above range, and that the solid solution amount of Nb in the steel is in an appropriate range. In order for Nb in steel to exhibit a corrosion resistance effect, it is necessary for Nb to elute into the rust layer in the form of oxygen acid or oxide along with corrosion of the steel material. On the other hand, when Nb carbides, nitrides, and carbonitrides are formed, the amount of solid solution Nb around the Nb decreases, thereby reducing the effect of corrosion resistance. In order to exhibit corrosion resistance, the amount of dissolved Nb needs to be 0.002% or more. Preferably it is 0.003% or more and 0.020% or less, more preferably 0.005% or more. Here, in order to make the solid solution Nb 0.002% or more, it is necessary to set the heating temperature of a steel material such as a slab to 1050 ° C. or more.

In addition, on the hold side where coal or coke is stacked, the corrosion resistance can be further improved by forming Cu ₂ Sb, which is an intermetallic compound between Cu and Sb. However, on the ballast tank side, the corrosion resistance deteriorates after coating in the absence of the zinc primer, and therefore, Cu is not actively added in the present invention. In the present invention, in order to ensure corrosion resistance in both environments, it is most important that a predetermined amount of W, Sb, Sn, and Nb be simultaneously contained, and that a predetermined amount of solute W and solute Nb be simultaneously contained. It is.

N: 0.0010% or more and 0.0080% or less
Since N is an element that lowers toughness, it is desirable to reduce it as much as possible. However, it is industrially difficult to reduce it to less than 0.0010%. On the other hand, if the content exceeds 0.0080%, remarkable deterioration of toughness is caused. Therefore, in the present invention, the N content is in the range of 0.0010% to 0.0080%. Preferably it is 0.0015% or more and 0.0060% or less, more preferably 0.0020% or more and 0.0050% or less.

  The basic components of the present invention have been described above. The balance other than the above components is Fe and inevitable impurities, but other elements described below can be appropriately contained as needed.

Ti: 0.001% or more and 0.030% or less, Zr: 0.001% or more and 0.030% or less, and V: One or more types selected from 0.002% or more and 0.200% or less.
Ti, Zr, and V are all elements that increase the strength of steel, and can be selectively contained depending on the required strength. In order to obtain such an effect, it is necessary to contain Ti and Zr at 0.001% or more and V at 0.002% or more. However, if the content of Ti and Zr exceeds 0.030%, and the content of V exceeds 0.200%, the toughness decreases. Therefore, when Ti, Zr and V are included, the content is within the above ranges, respectively. I will make it.

Ca: 0.0002% or more and 0.0050% or less
Ca has the effect of controlling the inclusion morphology and can enhance the ductility and toughness of the steel. This effect appears when the amount of Ca is 0.0002% or more. On the other hand, when Ca exceeds 0.0050%, coarse inclusions are formed and the toughness of the base material is deteriorated. Therefore, the Ca content is in the range of 0.0002% to 0.0050%. Preferably, it is in the range of 0.0005% to 0.0040%. More preferably, it is in the range of 0.0010% to 0.0030%.

B: 0.0002% or more and 0.0050% or less
B is an element that increases the strength of steel, and can be selectively contained depending on the required strength. Such an effect appears at 0.0002% or more. However, if the content exceeds 0.0050%, the toughness is reduced, so the B content is set to 0.0002% or more and 0.0050% or less. Preferably it is 0.0003% or more and 0.0025% or less, more preferably 0.0005% or more and 0.0015% or less.

One or more selected from Cu: less than 0.04% and Ni: less than 0.04%
Cu and Ni have an effect of making corrosion products such as Fe ₃ O ₄ dense on the hold side where coal or coke is loaded, and improving corrosion resistance. The densified corrosion product layer acts as a protective film, H ₂ O, transmission to O _2, SO ₄ ^2-like base iron surface corrosion factor is suppressed, thereby improving the corrosion resistance of steel. Further, by containing Cu in combination with Sb, Cu ₂ Sb, which is an intermetallic compound, is formed, and the corrosion resistance is improved. However, since both Cu and Ni deteriorate the coating corrosion resistance in the absence of the zinc primer, it is preferable to reduce their contents as much as possible from the viewpoint of coating corrosion resistance. However, it is an element that cannot be avoided as inevitable impurities when scrap or the like is used. Then, the inventors examined the allowable ranges of these elements, and found that if both Cu and Ni are less than 0.04%, there is almost no effect on the coating corrosion resistance and it is acceptable. More preferably, both are less than 0.02%, and still more preferably less than 0.01%.

Co: 0.010% or more and 0.500% or less, Mo: 0.010% or more and 0.500% or less, and Cr: 0.01% or more and 0.200% or less.
Co, Mo, and Cr are all elements that increase the strength of steel, and can be selectively contained as necessary. Such an effect is manifested at 0.010% or more for Co, Mo, and Cr. However, if the content of Co and Mo exceeds 0.500%, and if the content of Cr exceeds 0.200%, the toughness decreases, so that , Mo, and Cr are contained in the above ranges.

REM: 0.0002% to 0.015%, Y: 0.0001% to 0.1%, and Mg: One or more selected from 0.0002% to 0.015%
REM (rare earth element), Y, and Mg are elements that are effective in improving the toughness of the weld heat affected zone, and can be selectively added as needed. This effect is obtained when REM is 0.0002% or more, Y is 0.0001% or more, and Mg is 0.0002% or more. However, if the content exceeds REM: 0.015%, Y: 0.1%, and Mg: 0.015%, the toughness is rather reduced. Therefore, REM, Y, and Mg are preferably added with the above values as the upper limits, respectively.

  In the component composition of the present invention, components other than the above are Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not refused.

Next, a preferred method for producing a steel material according to the present invention will be described.
It is preferable to smelt the molten steel having the above component composition by a commonly known method such as a converter or an electric furnace, and to form a steel material such as a slab or a billet by a generally known method such as a continuous casting method or an ingot casting method. Needless to say, a process such as ladle refining and vacuum degassing may be added to the molten steel.

  The heating temperature of the steel material has a correlation with the solid solution amount of Nb. By setting the heating temperature of the steel material to 1050 ° C. or higher, a required amount of Nb in the steel can be secured, and as a result, the corrosion resistance can be improved. Therefore, after the steel material is heated to a temperature of 1050 to 1250 ° C., the material is hot-rolled to a desired size or shape, or heated when the temperature of the steel material is high enough to allow hot rolling. It is preferable to perform hot rolling to a steel material having a desired size and shape immediately without any heat or to the extent of soaking.

  In order to control the solid solution W to 0.005% or more, the amount of W added to the steel is set to 0.010% or more, and the cooling rate after hot finish rolling in the hot rolling process is set to 10 ° C / s or more. It is essential that

  In the hot rolling, in order to ensure strength, it is preferable to optimize the hot finish rolling end temperature and the cooling rate after the hot finish rolling are completed. The cooling after the finish rolling is preferably performed by air cooling or accelerated cooling at a cooling rate of 10 ° C./s or more and 150 ° C./s or less. After cooling, a reheating treatment may be performed. Other manufacturing conditions may be in accordance with a general method of manufacturing a steel material.

  Note that solid solution W which exhibits an effect on corrosion resistance tends to be gradually precipitated and reduced as W carbide during heating or cooling. However, air cooling after rolling hardly affects the amount of solid solution W. However, since the solid solution W decreases in the reheating treatment after the rolling and cooling, the reheating treatment should be avoided as much as possible in order to secure corrosion resistance.

Further, in the present invention, rust with excellent corrosion resistance generated on the steel material surface on the ballast tank side is quickly formed when the zinc primer is applied to the steel material surface. In the presence of zinc primer, Zn in the zinc primer acts as a sacrificial corrosion inhibitor for the underlying steel and elutes, forming Zn-based corrosion products such as basic zinc chloride and zinc oxide. This acts as a core of the Fe ₃ O ₄ rust component, and promotes the formation of a fine rust layer by the corrosion resistance improving element of the present invention.

  Therefore, in order to quickly form a dense rust layer with excellent corrosion resistance, it is preferable to apply a zinc primer to the steel material surface on the ballast tank side, but this is a case where the zinc primer is not applied to the steel material surface. However, the present invention does not deny the formation of a fine rust layer.

  In order to guarantee this high corrosion resistance on the ballast tank side, the corrosion resistance of the zinc coated steel material exposed to the salt water immersion or the repeated environment including the spray environment and the salt water immersion or the salt water spray simulating the dry and wet repeated environment is evaluated. Is preferred.

  Next, examples of the present invention will be described. In addition, this invention is not limited only to the following Examples.

  Steel containing the component composition shown in Table 1 was melted in a vacuum melting furnace and then made into an ingot, or was melted in a converter and made into a slab by continuous casting. Next, the ingot or the slab was charged into a heating furnace, heated to 1150 ° C., and then hot-rolled into a steel sheet having a thickness of 25 mm.

For these steel sheets, the mechanical properties of the base material, that is, tensile properties and impact properties (absorbed energy at −20 ° C., vE− ₂₀ measured by Charpy impact test) were investigated. In addition, the toughness of the weld corresponds to the heat history at the 1 mm position of the weld heat affected zone (1 mm from the fusion line to the base metal side) in the welded joint when the welding heat input was 150 kJ / cm for submerged arc welding. After applying the reproducible heat cycle, the absorbed energy vE ₀ at 0 ° C. was measured by the Charpy impact test.

The corrosion resistance was evaluated by tests under the following three conditions.
Evaluation test 1: Simulated test of the corrosive environment in the hold of a bulk carrier carrying coal or coke A test piece of 5 mmt x 50 mmW x 75 mmL was taken from the steel plate, the surface of which was shot blasted, and the scale and oil content of the surface were measured. Was removed. Using this surface as a test surface, the corrosion resistance of the steel material after the coating film was peeled off was evaluated. After coating the back surface and the end surface with a silicone seal, fit it in an acrylic jig, spread 5 g of coal on it, and use a thermo-hygrostat to atmosphere A (temperature 60 ° C., relative humidity 95%, 20 hours). A temperature / humidity cycle of atmosphere B (temperature 30 ° C., relative humidity 95%, 3 hours) and transition time 0.5 hour was given for 84 days. Here, the symbol “⇔” means repetition.

5 g of coal was weighed, immersed in 100 ml of distilled water at room temperature for 2 hours, filtered, and diluted to 200 ml with a coal leachate having a pH of 3.0.
In this embodiment, by conducting a test under the above-described conditions, a temperature and humidity environment and a dew condensation state that greatly affect the corrosion in the hold of the coal carrier and the coke carrier are simulated. After the test, the rust of each test piece was peeled off using a rust peeling solution, and the weight loss of the steel material was measured to obtain the corrosion amount. Moreover, the generated maximum pit depth was measured using a depth meter. W, Sb, Sn, Nb, solid solution W and the content of solid solution Nb were all outside the scope of the present invention. Comparative Example B1 had a corrosion reduction amount and a maximum pitting depth of 100. It is shown as a relative ratio. If the amount of corrosion reduction and the maximum pit depth shown by this relative ratio are 70% or less, it can be said that the corrosion resistance is excellent.

Evaluation test 2: Corrosion environment simulation test at a place where the submersion time is relatively long in the ballast tank A test piece of a test piece of 3 mmt x 70 mmW x 150 mmL was sampled from the steel plate, and the surface of the test piece was shot blasted. To remove scale and oil on the surface. Thereafter, a test piece coated with only about 15 μm of zinc rich primer, a test piece further coated thereon with a coating of a modified epoxy resin paint (about 320 μm), and a test piece coated with only a modified epoxy resin paint (about 320 μm) , And three types of test pieces were prepared.

  For the two types of test specimens coated with the modified epoxy resin paint, scratches with a length of 80 mm reaching the surface of the base steel with a cutter knife from the top of the coating were formed in a single letter, and corrosion under the following conditions was performed. After the test, the scratch resistance: the swelling area of the coating film generated per 10 mm was calculated to evaluate the corrosion resistance. The swelling area of the coating film was represented by a relative ratio with the swelling area of the coating film of B1 as a comparative example being 100%. If the swelling area of the coating film represented by this relative ratio is 70% or less, it can be said that the coating corrosion resistance is excellent.

  In addition, for the test specimen coated with zinc rich primer only, after the corrosion test, the coating film and the rust were peeled off with a coating film remover and a rust remover, and the weight change (corrosion amount) after the corrosion test was evaluated. Was. The change in the corrosion weight of B1 as a comparative example was set to 100, and the relative weight ratio was shown. If the change in corrosion weight represented by this relative ratio is 70% or less, it can be said that the corrosion resistance is excellent.

  In the corrosion test of the evaluation test 2, the salt water immersion (50 ° C. artificial seawater immersion) for 7 days → the dry / wet repetition test (60 ° C., relative humidity 30%, 4 hours @ 50 ° C., relative humidity 95%, 2 hours) for 7 days A cycle test was performed for 52 cycles. By conducting the test under the above conditions, the saltwater immersion environment and the dry / wet repetitive environment were combined at the part where the submersion time was relatively long in the ballast tank, that is, at the part except the part near the upper deck on the ballast tank side. It simulates the environment.

Evaluation test 3: Corrosion environment simulation test at a place where the water immersion time is relatively short in the ballast tank The preparation and evaluation of the test piece are the same as those of the evaluation test 2.
In the corrosion test of Evaluation Test 3, salt spray (35 ° C., 5% NaCl solution spray, 2 hours) → drying (60 ° C., relative humidity 25%, 4 hours) → wet (50 ° C., relative humidity 95%, 2 hours) ) Was performed for 252 cycles. By conducting the test under the above conditions, a simulated environment in which the saltwater spray environment and the dry / wet repeated environment are combined in a location where the submersion time is relatively short in the ballast tank, that is, in the vicinity of the upper deck on the ballast tank side are doing.

In the present invention, the amount of solute W and the amount of solute Nb effective for corrosion resistance were determined by subtracting the amount of precipitation determined from extraction residue analysis from the total content of steel.
Table 2 shows the results of the mechanical property investigation, and Table 3 shows the results of the corrosion resistance test.
As shown in Table 2, both the invention examples and the comparative examples exhibited good base material mechanical properties and welded impact properties. For the tensile properties for evaluating the base metal mechanical properties, the yield stress YS is 315 MPa or more, the tensile strength TS is 440 MPa or more, and the elongation El is 19% or more, and the impact properties are -20 ° C by Charpy impact test. The absorption energy vE _{-20 at the point} of 31 J or more was determined to be good. Regarding the impact characteristics of the welded portion, the absorption energy vE ₀ at 0 ° C. by the Charpy impact test was determined to be good when it was 34 J or more.

  However, as shown in Table 3, there was a significant difference in corrosion resistance. That is, the corrosion test result of the invention example was 70% or less and good corrosion resistance was shown with respect to B1 as the comparative example, whereas the corrosion test result was 70% or more in the comparative example, which was insufficient as the corrosion resistance. there were.

  According to the steel material according to the present invention, in two different corrosive environments in a hold and a ballast tank of a bulk carrier carrying coal or coke, both exhibit excellent corrosion resistance, so that the frequency of repainting and switching of the steel material is performed. Can be reduced.

Claims (11)

In mass%,
C: 0.040% or more and 0.200% or less,
Si: 0.01% or more and 0.50% or less,
Mn: 0.10% to 2.00%,
P: 0.035% or less,
S: 0.010% or less,
Al: 0.003% or more and 0.100% or less,
W: 0.010% or more and 0.500% or less,
Sb: 0.010% to 0.300%,
Sn: 0.010 to 0.300% and
Nb: 0.003% or more and 0.025% or less,
N: 0.0010% or more and 0.0080% or less, the balance is Fe and a steel material having a component composition of unavoidable impurities,
The amount of solid solution W in W is 0.005% or more;
A steel material in which the amount of solute Nb in Nb is 0.002% or more. The component composition further comprises:
In mass%,
Ti: 0.001% or more and 0.030% or less,
Zr: 0.001% or more and 0.030% or less and
V: The steel material according to claim 1, containing one or more selected from 0.002% or more and 0.200% or less. The component composition further comprises:
In mass%,
The steel material according to claim 1, comprising Ca: 0.0002% or more and 0.0050% or less. The component composition further comprises:
In mass%,
B: The steel material according to any one of claims 1 to 3, containing from 0.0002% to 0.0050%. The component composition further comprises:
In mass%,
Cu: less than 0.04% and
The steel material according to any one of claims 1 to 4, wherein the steel material contains one or more selected from Ni: less than 0.04%. The component composition further comprises:
In mass%,
Co: 0.010% to 0.500%,
Mo: 0.010% to 0.500% and
The steel material according to any one of claims 1 to 5, wherein the steel material contains one or more kinds selected from Cr: 0.010% or more and 0.200% or less. The component composition further comprises:
In mass%,
REM: 0.0002% or more and 0.015% or less,
Y: 0.0001% or more and 0.1% or less and
The steel material according to any one of claims 1 to 6, wherein the steel material contains one or more kinds selected from Mg: 0.0002% or more and 0.015% or less.   The steel material according to any one of claims 1 to 7, having a zinc primer coating on the surface.   The steel material according to any one of claims 1 to 7, having an epoxy-based coating film on a surface.   The steel material according to any one of claims 1 to 7, having a zinc primer coating film and an epoxy coating film on the surface. In producing a steel material according to any one of claims 1 to 10, wherein the amount of solute W in the W is 0.005% or more and the amount of solute Nb in the Nb is 0.002% or more. 7. A method for producing a steel material, comprising heating a steel material having the component composition according to any one of (7) and thereafter performing hot rolling.
The heating temperature of the steel material is 1050 ° C. or higher,
A method for producing a steel material, wherein a cooling rate after finish rolling in the hot rolling is 10 ° C./s or more.