JP6264519B1 - Coal ships and steel for holding coal and ore ships and ships - Google Patents

Abstract

Provided with a predetermined component composition and a Sn segregation degree of less than 18 are provided a steel material for coal ship and coal / ore combined ship holding that has both excellent corrosion resistance and excellent lamellar resistance.

Description

TECHNICAL FIELD The present invention relates to a steel material for a coal ship and a coal / ore combined ship holding excellent in corrosion resistance and lamelate resistance, which can be suitably used for a coal ship formed by welding steel materials and a coal / ore combined ship hold. is there.
Moreover, this invention relates to the ship which uses said steel material.

  Merchant ships are used for the transportation of energy resources, and bulk cargo ships account for about 30% of the volume. In this bulk carrier, marine accidents occurred one after another in the early 1990s, which became an international issue. In particular, many accidents have been reported on coal ships and coal / ore combined ships, most of which were caused by damage in the hold (hereinafter also referred to as “hold”).

  Bulk cargo ships are loaded directly on the hold, so they are easily affected by corrosive loads, and corrosion in the hold, especially in the side walls and ribs in the hold of coal ships and coal / ore combined ships, is not possible. It is considered that the strength is locally reduced by eating. In fact, there have been reports of cases in which such pitting corrosion has progressed remarkably and cases in which the thickness of the ribs that ensure the strength of the ship has been extremely reduced.

  Since the side wall and rib in the hold of the bulk cargo ship where pitting corrosion occurs are in a dry and wet environment, dew condensation is likely to occur. Since the sulfur component of coal dissolves in a place where such condensed water is generated and reacts with the condensed water to generate sulfuric acid, the hold has a low pH environment in which sulfuric acid corrosion is likely to occur.

  As a countermeasure against such corrosion in the hold, a modified epoxy-based coating is applied in the hold with a coating thickness of about 150 to 200 μm. However, the coating film is often peeled off due to mechanical damage caused by coal or iron ore, or scratches or abrasion caused by heavy machinery during loading and unloading, and a sufficient anticorrosion effect cannot be obtained at that portion. As countermeasures, repainting and partial repair of the coating are regularly performed. However, such a method is very expensive. For this reason, development of the steel material suitable for the hold use of a coal ship and a coal and ore combined ship which can reduce the life cycle cost including the maintenance cost of a ship is calculated | required.

For example, Patent Documents 1 to 3 are known as conventional techniques referring to the use of such coal ships and coal / ore combined ships.
That is, in Patent Document 1,
“In terms of% by weight, C: 0.01 to 0.25%, Si: 0.05 to 0.50%, Mn: 0.05 to 2.0%, P: 0.10% or less, S: 0.0. 001 to 0.10%, Cu: 0.01 to 2.00%, Al: 0.005 to 0.10%, Mg: 0.0002 to 0.0150%, the balance being Fe and inevitable impurities Corrosion-resistant steel for shipbuilding, characterized by
Is disclosed.
In Patent Document 2,
“In mass%, C: 0.01 to 0.2%, Si: 0.01 to 1%, Mn: 0.05 to 2%, P: 0.05% or less, S: 0.01% or less, Cu: 0.05-1%, Ni: 0.01-0.5%, Sn: 0.005-0.2%, Cr: 0.1% or less and Al: 0.1% or less, Corrosion-resistant steel for holding coal and ore carriers, characterized by being composed of the balance Fe and impurities and having a Brinell hardness HBW10 / 3000 of 140 or more and 230 or less. "
Is disclosed.
In Patent Document 3,
“In mass%, C: 0.01 to 0.2%, Si: 0.01 to 1%, Mn: 0.05 to 2%, P: 0.05% or less, S: 0.03% or less, Co: 0.05% or less, Sn: 0.01 to 0.3%, Cr: 0.05% or less, and Al: 0.1% or less, the balance being Fe and impurities -Corrosion resistant steel for holding ore carriers. "
Is disclosed.

JP 2000-17381 A JP 2007-262555 JP 2008-174768 A

  By the way, steel materials developed for cargo oil tanks and ballast tanks are known as marine steel materials. However, as described above, the holding environment of coal ships and coal / ore combined ships is used for cargo oil tanks and tanks in terms of corrosive environment (temperature, humidity, corrosive substances, etc.) and mechanical damage due to contents. It is completely different from the usage environment of the ballast tank. For this reason, original material design and characteristic evaluation are required for steel materials for holding coal ships and coal / ore combined ships.

  In this respect, the steel material disclosed in Patent Document 1 aims to improve the corrosion resistance in a common use environment such as a ship outer plate, a ballast tank, a cargo oil tank, and an ore ship cargo hold, and evaluates the corrosion resistance of the steel material. As such, the usage environment of the cargo oil tank and the ballast tank is considered. However, Patent Document 1 does not show a corrosion test result in consideration of a holding use environment of a coal ship and a coal / ore combined ship, that is, repeated dry and wet, and a low pH environment caused by a sulfur component of coal.

  Patent Documents 2 and 3 also evaluate the corrosion resistance of steel materials in a corrosive environment simulating the use environment of ore carrier ships. Corrosion also takes into account the hold environment of coal ships and coal / ore combined ships. Test results are not shown.

Furthermore, the hold is usually configured by welding a bottom plate and a hopper plate, an upper deck back plate and a longi material, and the welded joint receives a tensile stress in the plate thickness direction. And recently, it has become clear that such welded joints have a risk of causing lamellar tears. Here, lamellate is a welded joint that receives tensile stress in the thickness direction, such as a cross joint, T joint, and corner joint, and cracks develop in the steel material in the direction parallel to the steel sheet surface due to the tensile stress. Is a phenomenon that occurs.
For this reason, steel materials for holding coal ships and coal / ore combined ships are required to have excellent lamellar resistance in addition to the above-mentioned corrosion resistance in the use environment of coal ships and coal / ore combined ships. The

  However, none of Patent Documents 1 to 3 considers the risk of lamellar tearing in welded joints, and does not consider any lamellar resistance.

The present invention has been developed in view of the above situation, and is excellent in corrosion resistance in a use environment of a hold of a coal ship and a coal / ore combined ship, and also excellent in lamellar resistance and a coal / ore combined ship An object is to provide a steel material for holding.
It is another object of the present invention to provide a ship using the above-mentioned coal ship and steel for holding a coal / ore combined ship.

Now, the inventors have conducted intensive research to solve the above problems, and have obtained the following knowledge.
(1) In order to improve the corrosion resistance in the holding environment of coal ships and coal / ore combined ships, that is, dry and wet repeated and low pH environment caused by the sulfur component of coal, together with Sn, Cu, Ni, Sb, W, Mo It is effective to add one or two or more selected from Nb and Nb in combination.
(2) On the other hand, from the viewpoint of lamellar resistance, it is effective to reduce Sn in steel and reduce Sn.

  Thus, although Sn is effective from the viewpoint of improving corrosion resistance in the use environment of coal ships and coal / ore combined ships, it is effective to reduce Sn from the viewpoint of lamellar resistance. is there. Therefore, the inventors have further studied based on the above findings in order to achieve both corrosion resistance and lamellar resistance.

as a result,
(3) By suppressing the center segregation of Sn and diffusing Sn throughout the steel as much as possible, excellent lamellar resistance can be obtained even if a predetermined amount of Sn is contained, that is, the amount of Sn is adjusted appropriately. However, if the central segregation of Sn is suppressed and Sn is diffused throughout the steel material, it is possible to achieve both corrosion resistance and lamellar resistance in the use environment of the hold of coal ships and coal / ore combined ships,
And gained knowledge.
Also,
(4) By strictly controlling the Sn amount according to the S amount, the lamellar resistance is further improved.
And gained knowledge.
The present invention has been completed through further studies based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.03-0.18%
Si: 0.01-1.50%
Mn: 0.10 to 2.00%
P: 0.030% or less,
S: 0.0070% or less,
Al: 0.005-0.100%,
Sn: 0.01-0.20% and N: 0.0080% or less,
Cu: 0.01 to 0.50%,
Ni: 0.01-0.50%,
Sb: 0.01-0.30%,
W: 0.01-0.50%
Mo: 0.01-0.50% and
Nb: 0.0010 to 0.10%
Containing one or more selected from among the above, the remainder having a component composition consisting of Fe and inevitable impurities,
Steel for holding coal ships and coal / ore ships with Sn segregation degree of less than 18.
Here, the degree of Sn segregation is defined by the following equation (1).
[Sn segregation degree] = [Sn concentration in central segregation part] / [Average Sn concentration] --- (1)

2. 2. The steel material for holding a coal ship and a coal / ore combined ship according to the above 1, wherein the S content and the Sn content in the component composition satisfy the relationship of the following formula (2).
10000 × [% S] × [% Sn] ² ≤ 1.40 --- (2)
Here, [% S] and [% Sn] are the contents (mass%) of S and Sn in the component composition, respectively.

3. The component composition is further mass%,
Cr: 0.01 to 0.50% and
Co: 0.01-0.50%
3. The steel material for holding a coal ship and a coal / ore combined ship according to the above 1 or 2, containing one or two selected from among them.

4). The component composition is further mass%,
Ti: 0.001 to 0.100%,
Zr: 0.001 to 0.100% and V: 0.001 to 0.100%
The coal ship according to any one of the above 1 to 3, containing one or more selected from among the above, and a steel material for holding a coal / ore combined ship.

5. The component composition is further mass%,
Ca: 0.0001 to 0.0100%,
Mg: 0.0001-0.0200% and
REM: 0.0002 to 0.2000%
The coal ship according to any one of the above 1 to 4, which contains one or more selected from among the above, and a steel material for holding coal / ore combined ship.

6). The component composition is further mass%,
B: 0.0001-0.0300%
The steel material for holding a coal ship and a coal / ore combined use ship according to any one of 1 to 5 above.

7). A ship using the coal ship according to any one of 1 to 6 and a steel material for holding coal / ore combined ship.

ADVANTAGE OF THE INVENTION According to this invention, the steel material for coal ship and coal / ore combined use hold which is excellent in the corrosion resistance in the use environment of a hold of a coal ship and coal / ore combined use ship, and is excellent also in lamellar resistance.
And by applying the steel material for holding a coal ship and a coal / ore combined ship according to the present invention to the hold of a ship, it becomes possible to reduce the cost required for the inspection and painting of the hold while ensuring high safety. .

  Hereinafter, the present invention will be specifically described. First, the reason why the composition of steel is limited to the above range in the present invention will be described. In addition, although the unit of element content in the component composition of steel is “mass%”, hereinafter, it is simply indicated by “%” unless otherwise specified.

C: 0.03-0.18%
C is an element that increases the strength of steel, and the C content is 0.03% or more in order to ensure a desired strength (490 to 620 MPa). However, when the C content exceeds 0.18%, the weldability and the toughness of the weld heat affected zone are deteriorated. Accordingly, the C content is in the range of 0.03 to 0.18%. Preferably they are 0.05% or more and 0.16% or less.

Si: 0.01 to 1.50%
Si is an element added as a deoxidizer. Si is also an effective element for increasing the strength of the steel. In order to secure a desired strength, the Si content is 0.01% or more. However, if the Si content exceeds 1.50%, the toughness of the steel is lowered. Therefore, the Si content is in the range of 0.01 to 1.50%. Preferably they are 0.03% or more and 1.00% or less. More preferably, it is 0.04% or more and 0.50% or less.

Mn: 0.10 to 2.00%
Mn is an element that increases the strength of steel, and in order to ensure the above-described desired strength, the amount of Mn is set to 0.10% or more. However, if the amount of Mn exceeds 2.00%, the toughness and weldability of the steel will decrease. Further, the lamellar resistance is also lowered by the central segregation of Mn. Therefore, the Mn content is in the range of 0.10 to 2.00%. Preferably they are 0.60% or more and 1.80% or less. More preferably, it is 0.80% or more and 1.60% or less.

P: 0.030% or less P deteriorates toughness and weldability. Therefore, the P content is 0.030% or less. Preferably it is 0.025% or less. More preferably, it is 0.015% or less. The lower limit is not particularly limited, but is preferably 0.003%.

S: 0.0070% or less S is an important element involved in lamellar resistance. That is, S forms coarse MnS which is a non-metallic inclusion, and this MnS becomes the starting point of lamellar tear. In particular, if the amount of S exceeds 0.0070%, the lamellar resistance is greatly reduced. Therefore, the S amount is 0.0070% or less. Preferably it is 0.0060% or less. More preferably, it is 0.0050% or less. The lower limit is not particularly limited, but is preferably 0.0003%.

Al: 0.005-0.100%
Al is an element added as a deoxidizer, and the Al content is 0.005% or more. However, if the Al content exceeds 0.100%, the toughness of the steel decreases. For this reason, Al amount is taken as 0.005 to 0.100% of range.

Sn: 0.01-0.20%
Sn is an element necessary for improving the corrosion resistance in the use environment of the hold of coal ships and coal / ore combined ships, and is an important element involved in the lamellar resistance. Specifically, Sn is an element that improves the corrosion resistance while lowering the lamellar resistance.
That is, Sn forms a poorly soluble coating on the surface of steel in a dry and wet repeated and low pH corrosive environment of coal ships and coal / ore combined ships. At the same time, Sn is taken into the rust on the surface of the steel material and suppresses diffusion of anion species such as SO ₄ ²⁻ that promotes corrosion. Thereby, corrosion resistance improves. Such an effect is manifested when the Sn content is 0.01% or more. Preferably it is 0.02% or more.
On the other hand, Sn is easily segregated at the center of the steel material, and in such a segregated part, the hardness is remarkably increased, so that the lamellar resistance is deteriorated. In particular, when the Sn content exceeds 0.20%, the lamellar resistance is greatly deteriorated. Therefore, from the viewpoint of ensuring the lamellar resistance, the Sn content is 0.20% or less. Preferably it is 0.15% or less. More preferably, it is 0.10% or less.

N: 0.0080% or less Since N is a harmful element that lowers toughness, it is desirable to reduce it as much as possible. In particular, when the N content exceeds 0.0080%, the toughness is greatly reduced. Therefore, the N content is 0.0080% or less. Preferably it is 0.0070%. The lower limit is not particularly limited, but is preferably 0.0005%.

Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Sb: 0.01 to 0.30%, W: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and Nb: 0.0010 to 0.10% More than seeds
Cu, Ni, Sb, W, Mo, and Nb are elements that improve the corrosion resistance in the use environment of hold of coal ships and coal / ore combined ships by adding together with Sn.
As described above, Sn is an element effective for improving corrosion resistance, but cannot be contained in a large amount from the viewpoint of lamellar resistance. For this reason, in order to achieve both corrosion resistance and lamellar resistance in the holding environment of coal ships and coal / ore combined ships, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Sb: 0.01 to 0.30%, It is necessary to contain one or more selected from W: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and Nb: 0.0010 to 0.10%.
Here, Cu, Ni, Sb and Nb are released from the steel surface as Cu ²⁺ , Ni ²⁺ , Sb ⁵⁺ and Nb ⁴⁺ as the corrosion progresses, and the corrosion product is made dense, thereby making the steel interface ( Suppresses permeation of corrosive anions such as SO ₄ ²⁻ to the interface between the rust layer and the steel. In addition, W and Mo are liberated as WO ₄ ²⁻ and MoO ₄ ²⁻ , respectively, are taken into rust, impart cation selective permeability to rust, and corrosive anions such as SO ₄ ²⁻ at the steel interface. Is electrically suppressed.
These effects become apparent when the above-described anticorrosive action of Sn coexists, and the Cu, Ni, Sb, W, and Mo contents are each 0.01% or more and the Nb contents are 0.0010% or more. However, if any element is contained in a large amount, the weldability and toughness are deteriorated, which is disadvantageous from the viewpoint of cost.
Therefore, Cu content is in the range of 0.01 to 0.50%, Ni content is in the range of 0.01 to 0.50%, Sb content is in the range of 0.01 to 0.30%, W content is in the range of 0.01 to 0.50%, Mo content is in the range of 0.01 to 0.50%. The range and Nb content are in the range of 0.0010 to 0.10%.
Preferably, Cu amount is 0.02% or more and 0.40% or less, Ni amount is 0.02% or more and 0.40% or less, Sb amount is 0.02% or more and 0.25% or less, W amount is 0.02% or more and 0.40% or less, Mo amount is 0.02% or more and 0.40% or less, Nb amount is 0.0020% or more and 0.08% or less.

Further, as described above, the mechanism for decreasing lamellar resistance by Sn is different from the mechanism for decreasing lamellar resistance by S. However, the reduction of lamellar resistance due to S and Sn acts synergistically. For this reason, from the viewpoint of further improving the lamellar resistance, it is preferable that the relationship of the following formula (2) is satisfied with respect to the S and Sn contents.
10000 × [% S] × [% Sn] ² ≤ 1.40 --- (2)
Here, [% S] and [% Sn] are the contents (mass%) of S and Sn in the component composition, respectively.

The above equation (2) means that the influence of Sn amount on the lamellar resistance is much larger than the influence of S amount. That is, strictly managing Sn means that it is particularly important in securing the lamellar resistance.
Here, 10000 × [% S] × [% Sn] ² is more preferably 1.20 or less. The lower limit of 10000 × [% S] × [% Sn] ² is not particularly limited, but is preferably 0.001.
Needless to say, in order to suppress the lamellar tear, it is assumed that both the S amount and the Sn amount are limited to the above-described range.

Although the basic components have been described above, the elements described below can be appropriately contained in the steel material for holding a coal ship and a coal / ore combined use ship according to the present invention.
One or two selected from Cr: 0.01 to 0.50% and Co: 0.01 to 0.50%
Cr and Co, with the progress of corrosion proceeds to rust layer, by blocking the entry into rust layer of corrosive anions SO ₄ ^2-like, SO ₄ to the interface between the rust layer and base iron Suppresses the concentration of corrosive anions such as ^2- , thereby contributing to further improvement of corrosion resistance.
Such an effect cannot be sufficiently obtained when the Cr content or the Co content is less than 0.01%. On the other hand, if the Cr content or Co content exceeds 0.50%, the toughness of the weld zone is deteriorated. Cr is an element that causes a hydrolysis reaction and lowers the pH in the corroded part. That is, if Cr is added excessively, the total corrosion resistance may be deteriorated.
Therefore, when Cr and Co are contained, the amount is in the range of 0.01 to 0.50%. Preferably they are 0.02% or more and 0.30% or less. More preferably, it is 0.03% or more and 0.20% or less.

One or more selected from Ti: 0.001 to 0.100%, Zr: 0.001 to 0.100% and V: 0.001 to 0.100%
Ti, Zr and V can be added singly or in combination from the viewpoint of securing desired strength. However, if any element is excessively contained, toughness and weldability are deteriorated. For this reason, when Ti, Zr and V are contained, the amounts are all in the range of 0.001 to 0.100%. Preferably it is 0.005% or more and 0.050% or less.

Ca: 0.0001-0.0100%, Mg: 0.0001-0.0200% and REM: One or more selected from 0.0002-0.2000%
Ca, Mg and REM can be added alone or in combination from the viewpoint of improving the toughness of the weld. However, if any of these elements is excessively contained, the toughness of the weld is deteriorated. Also, the cost increases. Therefore, when Ca, Mg and REM are contained, the Ca amount is in the range of 0.0001 to 0.0100%, the Mg amount is in the range of 0.0001 to 0.0200%, and the REM amount is in the range of 0.0002 to 0.2000%.

B: 0.0001-0.0300%
B is an element that improves the hardenability of the steel material. Moreover, B can be contained from a viewpoint of ensuring desired intensity | strength. From such a viewpoint, it is effective to set the B amount to 0.0001% or more. However, when B is contained excessively, especially when the amount of B exceeds 0.0300%, the toughness is greatly deteriorated. Therefore, when B is contained, the amount is in the range of 0.0001 to 0.0300%.

  Components other than the above are Fe and inevitable impurities.

As described above, the component composition of the steel material for holding the coal ship and the coal / ore combined use ship of the present invention has been described. In the steel material for holding the coal ship and the combined use of the coal and ore ship, the Sn segregation degree is controlled as follows. It is extremely important.
Sn segregation degree: less than 18
Due to Sn center segregation, the hardness of the segregated portion is greatly increased. And such a segregation part becomes a starting point of lamellar tear generation. In other words, in order to ensure excellent lamellar tear resistance in the component composition containing Sn, it is important to suppress the center segregation of Sn and suppress the increase in hardness of the segregated portion. From such a viewpoint, the Sn segregation degree is set to less than 18. Preferably it is less than 16. More preferably, it is 15 or less. The lower limit is not particularly limited, but is preferably 2.

In addition, Sn segregation degree here is the average obtained by the line analysis of an electron beam microanalyzer (henceforth EPMA) in the cross section (cross section perpendicular | vertical to the steel material surface) cut | disconnected in parallel with the rolling direction of steel materials. This is the ratio of the Sn concentration in the central segregation part to the Sn concentration.
Specifically, when the thickness of the steel material is t (mm) and the width (the rolling direction of the steel material and the direction perpendicular to the thickness direction) is W (mm), the steel material is first cut parallel to the rolling direction of the steel material. In the thickness direction of the steel material of the cross section (cross section perpendicular to the steel material surface): (0.5 ± 0.1) × t, rolling direction: 15 mm surface area (that is, the surface area including the center position in the thickness direction of the steel material) EPMA surface analysis of Sn is performed under the conditions of beam diameter: 20 μm and pitch: 20 μm. In addition, Sn's EPMA surface analysis is performed in three cross-sectional visual fields at positions of 1/4 × W, 1/2 × W, and 3/4 × W.
Next, select the position with the highest Sn concentration in each cross-sectional field from the above EPMA surface analysis, and perform the EPMA line analysis of Sn under the conditions of beam diameter: 5 μm and pitch: 5 μm in the thickness direction of the steel at each position. carry out. In conducting the EPMA line analysis, the area from the front and back surfaces of the steel material to 25 μm is excluded.
Then, the maximum value of the Sn concentration (mass concentration) is obtained for each measurement line, and the average value of these values is taken as the Sn concentration (mass concentration) of the central segregation part. The value obtained by dividing by the average Sn concentration (mass concentration), which is the arithmetic average value, is the Sn segregation degree.
That is,
[Sn segregation degree] = [Sn concentration in central segregation part] / [Average Sn concentration]
It is.

As described above, the steel material for holding a coal ship and a coal / ore combined ship according to the present invention suppresses the center segregation of Sn, that is, exhibits the degree of center segregation of Sn, from the viewpoint of securing excellent lamellar resistance properties. It is very important to control the degree of Sn segregation below a predetermined value. Here, the degree of Sn segregation varies greatly depending on manufacturing conditions even if the component composition is the same. For this reason, in order to suppress the center segregation of Sn, it is very important to appropriately control the steel material manufacturing method.
Hereinafter, the suitable manufacturing method of the steel material for coal ship of this invention and coal and ore combined use ship hold | maintenance is demonstrated.

That is, the steel material of the present invention is prepared by melting a steel adjusted to the above-described component composition using a known refining process such as a converter, electric furnace, vacuum degassing, etc., and continuous casting or ingot-bundling rolling. It can be manufactured by using a steel material (slab) by the method, and then re-heating the steel material as necessary, followed by hot rolling to obtain a steel plate or a shaped steel. The thickness of the steel material is not particularly limited, but is preferably 2 to 100 mm. More preferably, it is 3 to 80 mm. More preferably, it is 4 to 60 mm.
Here, in the case of continuous casting, the casting speed (drawing speed) is preferably 0.3 to 2.8 m / min. When the casting speed is less than 0.3 m / min, the operation efficiency is deteriorated. On the other hand, when the casting speed exceeds 2.8 m / min, surface temperature unevenness occurs, and the supply of molten steel to the inside of the slab becomes insufficient, which promotes center segregation of Sn. From the viewpoint of suppressing the center segregation of Sn, it is more preferably 0.4 m / min or more and 2.6 m / min or less. More preferably, it is 1.5 m / min or less.
In addition, a light reduction method in which an end-solidified slab having an unsolidified layer is cast while being gradually reduced by a reduction roll group at a reduction amount and a reduction speed corresponding to the sum of the solidification shrinkage and the heat shrinkage. It is preferable to carry out.

Next, when hot rolling the steel material to a desired size and shape, it is preferable to heat to a temperature of 900 ° C to 1350 ° C. When the heating temperature is less than 900 ° C., the deformation resistance is large and hot rolling becomes difficult. On the other hand, when the heating temperature exceeds 1350 ° C., surface marks are generated, scale loss and fuel consumption increase.
In particular, the higher the heating temperature, the more the diffusion of Sn in the central segregation part is promoted, which is advantageous from the viewpoint of securing the lamellar resistance. From such a viewpoint, the heating temperature is more preferably 1030 ° C. or higher.
Further, the holding time at the heating temperature is preferably 60 min or longer. Thereby, the diffusion of Sn in the central segregation part is sufficiently promoted. More preferably, it is 150 min or more. The upper limit is not particularly limited, but is preferably 1000 min.

In addition, when the temperature of the steel material is originally in the range of 1030 to 1350 ° C. and kept in the temperature range for 60 minutes or more, the steel material may be subjected to hot rolling as it is without being heated. Further, the hot-rolled sheet obtained after hot rolling may be subjected to reheating treatment, acidity, and cold rolling to obtain a cold-rolled sheet having a predetermined thickness.
In hot rolling, it is preferable that the finish rolling finish temperature is 650 ° C. or higher. When the finish rolling finish temperature is less than 650 ° C., the rolling load increases due to an increase in deformation resistance, making it difficult to perform the rolling.

Cooling after hot rolling may be either air cooling or accelerated cooling, but accelerated cooling is preferred when higher strength is desired.
Here, when performing accelerated cooling, it is preferable that the cooling rate is 2 to 100 ° C./s and the cooling stop temperature is 700 to 400 ° C. That is, when the cooling rate is less than 2 ° C./s and / or the cooling stop temperature exceeds 700 ° C., the effect of accelerated cooling is small, and sufficient strength may not be achieved. On the other hand, if the cooling rate exceeds 100 ° C./s and / or the cooling stop temperature is less than 400 ° C., the toughness of the steel material may be reduced, or the shape of the steel material may be distorted. However, this is not the case when heat treatment is performed in the subsequent process.

Steel having the component composition shown in Table 1 (the balance is Fe and inevitable impurities) was melted in a vacuum melting furnace or converter, and a steel slab was obtained by continuous casting under the conditions shown in Table 2. These steel slabs were reheated to 1150 ° C. and held under the conditions shown in Table 2, and hot rolling at a finish rolling finish temperature of 930 ° C. was performed to obtain a steel plate having a plate thickness of 30 mm. The cooling after hot rolling was water cooling (accelerated cooling) at a cooling rate of 10 ° C./s and a cooling stop temperature of 550 ° C.
And the Sn segregation degree in the obtained steel plate was calculated | required by the above-mentioned method. The results are also shown in Table 2.

In addition, the steel plate obtained as described above is subjected to a corrosion test simulating the use environment of coal ships and coal / ore combined ships in the following manner, and the hold use of coal ships and coal / ore combined ships is used. The corrosion resistance in the environment was evaluated.
(1) Evaluation of corrosion resistance Samples of 5mmt x 50mmW x 75mmL were collected from No. 1 to 60 steel plates obtained as described above, and the surface was shot blasted to remove surface scale and oil. did. Using this surface as a test surface, the corrosion resistance of the steel material after peeling the coating film was evaluated. After coating the back and end surfaces with silicone seals, they are fitted into acrylic jigs, and 5 g of coal is laid on top of them. Atmosphere A (temperature: 60 ° C, relative humidity: 95%, 20 hours) ) Ambient B (temperature: 30 ° C., relative humidity: 95%, 3 hours), each transition time: 0.5 hour temperature and humidity cycle was given for 84 days. Here, the symbol “⇔” is used to mean repetition. The coal was weighed 5 g and immersed in 100 ml of distilled water at room temperature for 2 hours, then filtered and diluted to 200 ml, and the coal exudate having a pH of 3.0 was used. Here, by conducting the test under the above conditions, the corrosive environment of the bottom inner plate of the coal ship and the coal / ore combined ship is simulated.
After the test, using a rust remover, the rust of each test piece was peeled off, and the amount of mass loss before and after the corrosion test of each test piece was measured. Moreover, the maximum pitting corrosion depth in each test piece was measured using the depth meter. And, based on No. 53 to which Sn, Cu, Ni, Sb, W, Mo and Nb are not added as a base steel, the corrosion resistance is as follows based on the ratio of mass reduction and maximum pitting depth to this base steel. Evaluated.
○ (Pass): The ratio of mass reduction and maximum pitting depth to the base steel is less than 70%. △ (Fail): One of the ratio of mass reduction and maximum pitting depth to the base steel. One is 70% or more and less than 80%, and the other is less than 80%. X (failed): At least one of the ratio of the mass reduction amount and the maximum pitting depth to the base steel is 80% or more.

Furthermore, the lamellar tear resistance was evaluated in the following manner.
(2) Evaluation of lamellar resistance
In accordance with the ClassNK Steel Ship Rules and Inspection Procedures (Part K, Chapter 2), the No. 1-60 steel plates obtained as described above are subjected to a tensile test in the plate thickness direction (Z direction). The aperture value (RA) was calculated. Based on the calculated aperture value (RA), the lamellar resistance was evaluated according to the following criteria.
◎ (pass, especially excellent): 70 or more ○ (pass): 35 or more and less than 70 △ (failure): 25 or more and less than 35 × (failure): less than 25

  The evaluation results of (1) and (2) are also shown in Table 2. In addition, the overall evaluation in Table 2 is “Pass” when the evaluations of (1) and (2) are all “◯” or “◎”, and at least one of the evaluations of (1) and (2). A case where “△” or “×” is present is regarded as “fail”.

As shown in Table 2, all of the inventive examples have both excellent corrosion resistance and lamellar resistance.
On the other hand, in the comparative example, sufficient characteristics are not obtained for at least one of the corrosion resistance and the lamellar resistance.

That is, in Comparative Examples No. 50 and 52, the amount of S exceeds the upper limit, and since a predetermined amount of Cu, Ni, Sb, W, Mo and Nb is not contained, sufficient corrosion resistance and lamellar resistance are sufficient. Characteristics are not obtained.
In Comparative Examples Nos. 51, 55, and 58, the Sn amount exceeds the upper limit, so that sufficient properties are not obtained for the lamellar resistance.
In Comparative Example No. 54, since the amount of S and the amount of Sn exceed the upper limit, sufficient properties are not obtained for the lamellar resistance.
In Comparative Examples Nos. 56 and 60, since the amount of S exceeds the upper limit, sufficient characteristics for lamellar resistance are not obtained.
Since Comparative Example No. 57 does not contain a predetermined amount of Cu, Ni, Sb, W, Mo, and Nb, sufficient characteristics are not obtained with respect to corrosion resistance.
In Comparative Example No. 59, the amount of S exceeds the upper limit and the amount of Sn is lower than the lower limit, so that sufficient characteristics are not obtained with respect to corrosion resistance and lamellar resistance.
In Comparative Examples Nos. 61 to 64, the Sn segregation degree exceeds the upper limit, so that sufficient characteristics are not obtained for the lamellar resistance.

Claims (7)

% By mass
C: 0.03-0.18%
Si: 0.01-1.50%
Mn: 0.10 to 2.00%
P: 0.030% or less,
S: 0.0070% or less,
Al: 0.005-0.100%,
Sn: 0.01-0.20% and N: 0.0080% or less,
Cu: 0.01 to 0.50%,
Ni: 0.01-0.50%,
Sb: 0.01-0.30%,
W: 0.01-0.50%
Mo: 0.01-0.50% and
Nb: 0.0010 to 0.10%
Containing one or more selected from among the above, the remainder having a component composition consisting of Fe and inevitable impurities,
Steel for holding coal ships and coal / ore ships with Sn segregation degree of less than 18.
Here, the degree of Sn segregation is defined by the following equation (1).
[Sn segregation degree] = [Sn concentration in central segregation part] / [Average Sn concentration] --- (1) 2. The steel material for holding a coal ship and a coal / ore combined ship according to claim 1, wherein the S content and the Sn content in the component composition satisfy a relationship represented by the following formula (2).
10000 × [% S] × [% Sn] ² ≤ 1.40 --- (2)
Here, [% S] and [% Sn] are the contents (mass%) of S and Sn in the component composition, respectively. The component composition is further mass%,
Cr: 0.01 to 0.50% and
Co: 0.01-0.50%
The steel material for holding a coal ship and a coal / ore combined ship according to claim 1 or 2, containing one or two kinds selected from among them. The component composition is further mass%,
Ti: 0.001 to 0.100%,
Zr: 0.001 to 0.100% and V: 0.001 to 0.100%
The coal ship according to any one of claims 1 to 3, which contains one or more selected from among the above, and a steel material for holding coal / ore combined ship. The component composition is further mass%,
Ca: 0.0001 to 0.0100%,
Mg: 0.0001-0.0200% and
REM: 0.0002 to 0.2000%
The coal material according to any one of claims 1 to 4, comprising one or more selected from among the above, and a steel material for holding a coal / ore combined ship. The component composition is further mass%,
B: 0.0001-0.0300%
A steel material for holding a coal ship and a coal / ore combined ship according to any one of claims 1 to 5. A ship using the coal ship according to any one of claims 1 to 6 and the steel material for holding coal / ore combined ship.