KR102259444B1 - Steel material for crude oil tanker and crude oil tanker - Google Patents

Abstract

Provided is a steel material for crude oil tankers having a predetermined component composition and having a Sn segregation degree of less than 18, in which excellent front surface corrosion resistance and local corrosion resistance, and excellent lamellar resistance are compatible.

Description

Steel materials for crude oil tankers and crude oil tankers {STEEL MATERIAL FOR CRUDE OIL TANKER AND CRUDE OIL TANKER}

The present invention relates to a crude oil tank (oil tank part) of a crude oil tanker formed by welding steel materials, in particular, the upper deck back surface (ceiling part) or upper side wall where the entire surface corrosion occurs, and the bottom plate of the oil tank part where local corrosion (pitting) occurs It relates to a steel material for crude oil tankers excellent in corrosion resistance and lamellar tier resistance, which can be preferably used in any of.

Moreover, this invention relates to the crude oil tanker formed using said steel materials.

It is known that front corrosion occurs in the inner surface of the crude oil tank of a crude oil tanker, especially the steel used for the upper deck back surface and side wall upper part.

The cause of this front corrosion is:

(1) Repetition of dew condensation and drying (dry and wet) on the surface of the steel sheet due to the temperature difference between day and night;

(2) Inert gas enclosed for explosion-proof in the crude oil tank (O ₂ : about 4 vol%, CO ₂ : about 13 vol%, SO ₂ : about 0.01 vol%, the remainder N ₂ Boiler or engine having a representative composition _{of O 2} , CO ₂ , SO ₂ contained in the exhaust gas of

(3) Infiltration of corrosive gases such _{as H 2} S volatilized from crude oil into the dew water,

(4) Residue of seawater used for cleaning crude oil tanks

And the like. These are also known from the fact that sulfate ions and chloride ions are detected in strongly acidic dew condensation water in a solid-line poison test performed normally every 2.5 years.

_{In addition, when H 2} S is oxidized using iron rust produced by corrosion as a catalyst, solid S is produced in a layered form in the iron rust. The iron rust produced in such a layered form of solid S peels off easily and falls off, and is deposited on the bottom part of a crude oil tank. Therefore, in the poison inspection of a crude oil tanker, it spends a great amount of money, and it is the present situation that repair of the upper part of a crude oil tank, and collection|recovery of the sediment of a tank bottom are performed.

On the other hand, conventionally, in steel materials such as crude oil tank bottom plates, corrosion does not occur due to the corrosion inhibitory action of crude oil itself or the corrosion inhibitory action of the crude oil-derived protective coat (oil coat) formed on the inner surface of the crude oil tank. was thought

However, recent studies have revealed that pneumatic local corrosion (pitting) occurs in the steel materials of the crude oil tank bottom plate.

The causes of such local corrosion are:

(1) the presence of coagulated water in which salts such as sodium chloride are dissolved in high concentrations;

(2) separation of the oil coat due to excessive washing;

(3) high concentration of sulfides contained in crude oil;

(4) High concentration _{of O 2} , CO ₂ , SO ₂ etc. in explosion-proof inert gas dissolved in dew water,

And the like. In fact, high concentrations of chloride ions and sulfate ions were detected as a result of analyzing the water retained in the crude oil tank at the time of the solid line poison test.

It is effective to coat the steel surface and cut off the steel materials from the corrosive environment for prevention of the overall corrosion and local corrosion of the inner surface of the crude oil tank described above.

However, the coating operation of the crude oil tank has a huge application area, and repainting is required about once every 10 years due to deterioration of the coating film. For this reason, in the painting operation of a crude oil tank, an enormous cost arises in inspection and painting. Moreover, when a coating film is damaged, it is pointed out that corrosion is encouraged rather in the corrosive environment of a crude oil tank in such a damaged part.

Therefore, even if it does not paint, development of the steel materials which can prevent full surface corrosion and local corrosion of the inner surface of a crude oil tank is desired.

As such steel materials, for example, in Patent Document 1,

"In mass %, C: 0.01 to 0.3%, Si: 0.02 to 1%, Mn: 0.05 to 2%, P: 0.05% or less, S: 0.01% or less, Ni: 0.05 to 3%, Mo: 1% or less , Cu: 1% or less, Cr: 2% or less, W: 1% or less, Ca: 0.01% or less, Ti: 0.1% or less, Nb: 0.1% or less, V: 0.1% or less, B: 0.05% or less and steel materials for cargo oil tanks consisting of the remainder Fe and impurities.”

is disclosed.

Moreover, 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, Ni: 0.01 to 1%, Cu: 0.05 to 2 %, Sn: 0.01 to 0.2%, Cr: 0.1% or less, Al: 0.1% or less, and the balance Fe and impurities for cargo oil tanks.”

is disclosed.

Japanese Laid-Open Patent Publication No. 2003-82435 Japanese Laid-Open Patent Publication No. 2007-270196

By the way, the crude oil tank of a crude oil tanker normally welds a bottom plate, a hopper plate, an upper deck back plate, a vertical member, etc., In the weld joint, tensile stress is received in the plate|board thickness direction. In such a welded joint, it has recently become clear that there is a risk of lamellar formation. Here, the lamellar is a weld joint that receives tensile stress in the plate thickness direction, such as a cross joint, a T joint, and a corner joint. This is a phenomenon that occurs.

For this reason, it is calculated|required by the steel materials for crude oil tankers that it is excellent also in lamellar tier resistance in addition to the corrosion resistance with respect to the full surface corrosion and local corrosion of the inner surface of a crude oil tank mentioned above.

In this regard, in the steel materials of Cited Document 1, mechanical properties such as lamellar resistance are not considered at all. In addition, also in the steel materials of Cited Document 2, no consideration is given to lamelat resistance at all.

In this way, in Cited Documents 1 and 2, the risk of generating lamellar tiers in welded joints is not considered at all, and for this reason, when the steel materials of Cited Documents 1 and 2 are used for the crude oil tanks of actual crude oil tankers, , there is a concern that lamellar tiers may occur in the welded joint.

The present invention has been developed in view of the current situation described above, and has excellent corrosion resistance against local corrosion in the upper deck back surface or side wall upper part of the crude oil tank inner surface of a crude oil tanker, and local corrosion in the bottom plate, and also has excellent corrosion resistance. An object of the present invention is to provide steel materials for crude oil tankers which are also excellent in lamellar properties.

Moreover, this invention aims at providing the crude oil tanker which uses the steel materials for crude oil tankers mentioned above.

By the way, the inventors repeated earnest research for the solution of the said subject, and acquired the following knowledge.

(1) Addition of Sn and reduction of S are effective for the improvement of the corrosion resistance in the local corrosion environment in the bottom plate of the crude oil tank of a crude oil tanker, ie, pitting environment (hereinafter also referred to as local corrosion resistance).

(2) For improvement of corrosion resistance (hereinafter also referred to as total surface corrosion resistance) in a front corrosion environment in the upper deck back surface or upper side wall of the crude oil tank of a crude oil tanker, Cu, Ni, Sb, W, Mo and It is effective to add 1 type or 2 or more types selected from Si in combination.

(3) On the other hand, from the viewpoint of lamellar resistance, it is effective to reduce the amount of S in the steel and to reduce Sn.

In this way, from the viewpoint of improving the corrosion resistance (front surface corrosion resistance and internal corrosion resistance) in the corrosive environment of the crude oil tank inner surface of the crude oil tanker, the addition of Sn is effective, but from the viewpoint of lamellar resistance, Sn is reduced it is valid Then, based on said knowledge, the inventors repeated examination so that corrosion resistance and lamellar-resistance might be compatible further.

As a result,

(4) When central segregation of Sn is suppressed and Sn is diffused as much as possible throughout the steel material, excellent lamellar resistance is obtained even when Sn is contained in a predetermined amount, that is, central segregation of Sn while appropriately adjusting the amount of Sn was suppressed and Sn was diffused throughout the steel materials, the knowledge that corrosion resistance and lamellar resistance in the corrosive environment of the inner surface of the crude oil tank of a crude oil tanker were compatible.

In addition,

(5) By strictly controlling the amount of Sn according to the amount of S, it was found that the lamellar tear resistance was further improved.

The present invention has been completed through further studies based on the above findings.

That is, the configuration of the gist of the present invention is as follows.

1. in mass %,

C: 0.03 to 0.18%;

Mn: 0.10 to 2.00%;

P: 0.030% or less;

S: 0.0070% or less;

Al: 0.001 to 0.100%,

Sn: 0.01 to 0.20% and

N: while containing 0.0080% or less,

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

Si: 0.01 to 1.50%

It contains one or two or more selected from among, and the remainder has a component composition consisting of Fe and unavoidable impurities,

Steel for crude oil tankers with Sn segregation of less than 18.

Here, the Sn segregation degree is defined by the following formula (1).

[Sn segregation degree] = [Sn concentration in central segregation region]/[average Sn concentration] --- (1)

2. Steel materials for crude oil tankers as described in said 1 with which S content and Sn content in the said component composition satisfy|fill the relationship of following formula (2).

10000 × [%S] × [%Sn] ² ≤ 1.40 --- (2)

Here, [%S] and [%Sn] are contents (mass %) of S and Sn in a component composition, respectively.

3. The said component composition is further by mass %,

Cr: 0.01 to 0.50% and

Co: 0.01 to 0.50%

Steel materials for crude oil tankers as described in 1 or 2 above, containing 1 type or 2 types selected from among.

4. The said component composition is further in mass %,

Ti: 0.001 to 0.100%,

Zr: 0.001 to 0.100%,

Nb: 0.001 to 0.100% and

V: 0.001 to 0.100%

The steel materials for crude oil tankers in any one of said 1-3 containing 1 type, or 2 or more types selected from among.

5. The component composition is further in mass%,

Ca: 0.0001 to 0.0100%,

Mg: 0.0001 to 0.0200% and

REM: 0.0002 to 0.2000%

The steel materials for crude oil tankers in any one of said 1-4 containing 1 type(s) or 2 or more types selected from among.

6. The component composition is further in mass %,

B: 0.0001 to 0.0300%

The steel materials for crude oil tankers in any one of said 1-5 containing.

7. Crude oil tanker formed using the steel materials for crude oil tankers in any one of said 1-6.

ADVANTAGE OF THE INVENTION According to this invention, it is excellent in corrosion resistance in the corrosive environment of the inner surface of a crude oil tank of a crude oil tanker, ie, both front surface corrosion resistance and internal corrosion resistance, and also excellent lamellar tier resistance is obtained.

And by using the steel materials for crude oil tankers of this invention for the crude oil tank of a crude oil tanker, it becomes possible to reduce the cost for the inspection and painting of a crude oil tank, ensuring high safety.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram of the test apparatus used for the full surface corrosion test (condensation test).
2 is a schematic diagram of a testing apparatus used for a local corrosion test (acid resistance test).

Hereinafter, the present invention will be described in detail. First, the reason for limiting the component composition of steel to the above-mentioned range in this invention is demonstrated. In addition, although the unit of content of the element in the component composition of steel is all "mass %", hereafter, unless otherwise stated, it simply shows with "%".

C: 0.03 to 0.18%

C is an element necessary for securing the strength of steel. In order to obtain such an effect, the amount of C is made into 0.03 % or more. However, when the amount of C exceeds 0.18 %, weldability and the toughness of a weld heat affected zone will fall. Therefore, the amount of C is made into the range of 0.03 to 0.18%. Preferably it is 0.04 % or more and 0.16 % or less.

Mn: 0.10 to 2.00%

Mn is an element that increases the strength of steel. In order to obtain such an effect, the amount of Mn is made into 0.10 % or more. However, when the amount of Mn exceeds 2.00 %, the toughness and weldability of steel will fall. Moreover, lamellar tier resistance also falls by the center segregation of Mn. Accordingly, the amount of Mn is in the range of 0.10 to 2.00%. Preferably it is 0.60 % or more and 1.80 % or less. More preferably, they are 0.80 % or more and 1.60 % or less.

P: 0.030% or less

P deteriorates toughness and weldability. For this reason, the amount of P is made into 0.030% or less. Preferably it is 0.025 % or less. More preferably, it is 0.015 % or less. In addition, although it does not specifically limit about a minimum, It is preferable to set it as 0.003 %.

S: 0.0070% or less

S is an important element involved in internal corrosion resistance and lamellarity resistance. That is, S is a harmful element which forms MnS which is a nonmetallic inclusion, becomes a starting point of local corrosion, and reduces local corrosion resistance. Therefore, it is preferable to reduce S as much as possible. In particular, when the amount of S exceeds 0.0080 %, a remarkable fall of internal corrosion property will be caused. Moreover, coarse MnS becomes the origin of a lamellar layer. In particular, when the amount of S exceeds 0.0070%, a significant decrease in lamellar resistance is caused. Therefore, from the viewpoint of achieving both local corrosion resistance and lamellar tier resistance, the amount of S is made 0.0070% or less. Preferably it is 0.0060 % or less. More preferably, it is 0.0050 % or less. In addition, although it does not specifically limit about a minimum, It is preferable to set it as 0.0003 %.

Al: 0.001 to 0.10%

Al is an element added as a deoxidizer, and the amount of Al is made 0.001% or more. However, when the amount of Al exceeds 0.10 %, the toughness of steel will fall. For this reason, the amount of Al is made into the range of 0.001 to 0.10%.

Sn: 0.01 to 0.20%

Sn is an element necessary for improving local corrosion resistance and total surface corrosion resistance, and is an important element involved in lamellar resistance, in other words, an element that improves corrosion resistance and reduces lamellar resistance.

That is, Sn forms a sparingly soluble film on the surface of steel in a strongly acidic local corrosive environment, such as a bottom plate of a crude oil tank, ^{and suppresses diffusion of Cl −} (chloride ions) that promotes corrosion, thereby improving corrosion resistance heightening effect. In addition, Sn suppresses diffusion of anionic species such as _{SO 4} ^2- that enters into rust on the surface of steel and promotes corrosion in a weakly acidic front surface corrosion environment such as the back surface of the upper deck of a crude oil tank, thereby improving corrosion resistance It works. These effects are expressed by making the amount of Sn into 0.01% or more. Moreover, especially in the front surface corrosion environment, such as upper deck back surface, the effect of adding Sn is large, and by making Sn amount into 0.05 % or more, Cu, Ni, Sb among Cu, Ni, Sb, W, Mo, and Si mentioned later. Even if , W, and Mo are not added, it becomes possible to express favorable corrosion resistance.

On the other hand, Sn tends to segregate in the center of steel materials, and since hardness remarkably increases in such a segregation part, lamellar tier resistance deteriorates. In particular, when the amount of Sn exceeds 0.20%, the lamellar tier resistance is greatly deteriorated. Therefore, from the viewpoint of ensuring lamellar tier resistance, the amount of Sn is made 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 which reduces toughness, it is preferable to reduce it as much as possible. In particular, when the amount of N exceeds 0.0080%, the decrease in toughness becomes large. Therefore, the amount of N is made 0.0080% or less. Preferably it is 0.0070 %. In addition, although it does not specifically limit about a minimum, It is preferable to set it as 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 Si: 0.01 to 1.50% 1 type or more selected from

Cu, Ni, Sb, W, Mo, and Si are elements which improve corrosion resistance in front surface corrosion environments, such as the upper deck of the crude oil tank of a crude oil tanker.

As described above, although Sn is an element effective for the improvement of corrosion resistance, it cannot be contained in a large amount from the viewpoint of lamellar resistance. Therefore, in order to obtain excellent corrosion resistance in the front corrosion environment of the upper deck of crude oil tanks of crude oil tankers, Cu: 0.01-0.50%, Ni: 0.01-0.50%, Sb: 0.01-0.30%, W: 0.01-0.50% , Mo: 0.01 to 0.50% and Si: 0.01 to 1.50%, it is necessary to contain one or two or more selected from the group consisting of.

Here, Cu, Ni and Sb are each ^{liberated from the steel surface as Cu 2+} , Ni ²⁺ and Sb ³⁺ with the progress of corrosion, ^{and are coupled with S 2 -} as a corrosion factor, CuS, NiS, Sb ₂ S ₃ is formed. As a result, the permeation of ^{S 2 -} to the steel interface is suppressed. In addition, W, Mo and Si are each _{liberated as WO 4} ^2- , MoO ₄ ^2- and SiO ₄ ^4- , enter into rust to impart cation selective permeability to rust, and SO ₄ ^2- I to the steel interface It electrically suppresses permeation of corrosive ^{anions, such as S2-.}

These effects are manifested when the anticorrosive action of Sn described above coexists, and the amounts of Cu, Ni, Sb, W, Mo, and Si are each expressed at 0.01% or more. However, when a large amount of any element is contained, weldability and toughness are deteriorated, and it becomes disadvantageous also from the viewpoint of cost.

Therefore, the amount of Cu is in the range of 0.01 to 0.50%, the amount of Ni is in the range of 0.01 to 0.50%, the amount of Sb is in the range of 0.01 to 0.30%, the amount of W is in the range of 0.01 to 0.50%, and the amount of Mo is in the range of 0.01 to 0.50%. The range and the amount of Si are made into a range of 0.01 to 1.50%.

Preferably, the amount of Cu is 0.02% or more and 0.40% or less, the amount of Ni is 0.02% or more and 0.40% or less, the amount of Sb is 0.02% or more and 0.25% or less, the amount of W is 0.02% or more, 0.40% or less, and the amount of Mo Silver is 0.02% or more and 0.40% or less, and the amount of Si is 0.01% or more and 1.00% or less.

Moreover, as described above, the mechanism for lowering the lamellar resistance by Sn is different from the mechanism for lowering the lamellar resistance by S. However, the reduction in lamellarity resistance caused by S and Sn acts synergistically with each other. For this reason, it is preferable to satisfy|fill the relationship of following formula (2) with respect to content of S and Sn from a viewpoint of further improving lamellar tear resistance.

10000 × [%S] × [%Sn] ² ≤ 1.40 --- (2)

Here, [%S] and [%Sn] are contents (mass %) of S and Sn in a component composition, respectively.

Formula (2) listed above means that the influence of the amount of Sn on the lamellar tier resistance is very large compared to the effect of the amount of S. That is, it means that controlling Sn strictly is especially important in ensuring lamellar tier resistance.

Here, 10000 x [%S] x [%Sn] ² is more preferably 1.20 or less. Although it does not specifically limit about the lower limit of 10000*[%S]*[%Sn] ^{2, It is preferable to set it as 0.001.}

In addition, in suppressing lamellar tear, it goes without saying that it is a premise that both the S amount and the Sn amount are limited to the above-described ranges.

As mentioned above, although the basic component was demonstrated, in the steel materials for crude oil tankers of this invention, the element mentioned below can be made to contain suitably.

One or two selected from Cr: 0.01 to 0.50% and Co: 0.01 to 0.50%

Cr and Co migrate into the rust layer along with the progress of corrosion ^{and block the intrusion of Cl − into the rust layer, thereby suppressing the concentration of Cl −} at the interface between the rust layer and the base iron, thereby contributing to the improvement of corrosion resistance. . In addition, when a Zn-containing primer is applied to the surface of a steel material, Cr and Co form a complex oxide with Zn and the like centering on Fe, making it possible to maintain Zn on the surface of the steel sheet for a long period of time, thereby further improving corrosion resistance make it Such an effect becomes especially remarkable in the part which comes into contact with the liquid containing the high-concentration salt separated from the crude oil fraction, such as the bottom plate of the crude oil tank of a crude oil tanker especially. That is, when a Zn-containing primer treatment is applied to steel, and this steel is used for a portion in contact with a liquid containing a high concentration of salt separated from crude oil, in the steel containing Cr or Co, these elements are contained Compared with steel materials that are not made, corrosion resistance is greatly improved.

Such an effect is not sufficiently obtained when the amount of Cr or the amount of Co is less than 0.01%. On the other hand, when the amount of Cr or the amount of Co exceeds 0.50%, the toughness of the weld is deteriorated. Moreover, about Cr, as an element which raise|generates a hydrolysis reaction, the pH in a corrosion part is reduced. That is, when Cr is added excessively, there also exists a possibility that corrosion resistance as a whole may deteriorate.

Accordingly, when Cr and Co are contained, the amounts thereof are both in the range of 0.01 to 0.50%. Preferably it is 0.02 % or more and 0.30 % or less. More preferably, they are 0.03 % or more and 0.20 % or less.

Ti: 0.001 to 0.100%, Zr: 0.001 to 0.100%, Nb: 0.001 to 0.100%, and V: 0.001 to 0.100% one or more selected from

Ti, Zr, Nb, and V may be added alone or in combination from the viewpoint of securing desired strength. However, if any element is contained excessively, toughness and weldability will deteriorate. For this reason, when Ti, Zr, Nb, and V are contained, the amount is all made into the range of 0.001 to 0.100 %. Preferably it is 0.005 % or more and 0.050 % or less.

Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0200%, and REM: 0.0002 to 0.2000% one or more selected from the group consisting of

Ca, Mg and REM can be added alone or in combination from the viewpoint of improving the toughness of the weld zone. However, when any element is contained excessively, the toughness deterioration of a weld part will be caused on the contrary. Also, the cost increases. Therefore, when Ca, Mg and REM are contained, the amount of Ca is in the range of 0.0001 to 0.0100%, the amount of Mg is in the range of 0.0001 to 0.0200%, and the amount of REM is in the range of 0.0002 to 0.2000%.

B: 0.0001 to 0.0300%

B is an element which improves the hardenability of steel materials. Moreover, B can be contained from a viewpoint of ensuring desired intensity|strength. From such a viewpoint, it is effective to set the amount of B to 0.0001% or more. However, when B is contained excessively, especially when the amount of B exceeds 0.0300 %, significant deterioration of toughness will be caused. Accordingly, when B is contained, the amount thereof is in the range of 0.0001 to 0.0300%.

Components other than the above are Fe and unavoidable impurities.

As mentioned above, although the component composition of the steel materials for crude oil tankers of this invention was demonstrated, in the steel materials for crude oil tankers of this invention, it is very important to control Sn segregation degree as follows.

Sn segregation degree: less than 18

Due to central segregation of Sn, the hardness of the segregation portion is greatly increased. And such a segregation part becomes a starting point of lamellar formation. That is, in order to secure excellent lamellar resistance properties in the component composition containing Sn, it is important to suppress the central segregation of Sn to suppress the increase in hardness of the segregated portion. From such a viewpoint, the Sn segregation degree is set to less than 18. Preferably less than 16. More preferably, it is 15 or less. Although it does not specifically limit about a minimum, It is preferable to set it as 2.

Incidentally, the Sn segregation degree as used herein refers to the average Sn concentration obtained by line analysis of an electron beam microanalyzer (hereinafter referred to as EPMA) in a cross section cut parallel to the rolling direction of the steel material (the cross section perpendicular to the steel material surface). is the ratio of the concentration of Sn in the central segregation to .

Specifically, when the thickness of the steel is t (mm) and the width (the direction perpendicular to the rolling direction and thickness direction of the steel) is W (mm), first, the cross section cut parallel to the rolling direction of the steel (steel) In the face area (that is, the face area including the central position in the thickness direction of the steel material) of the thickness direction of the steel: (0.5 ± 0.1) × t, the rolling direction: 15 mm, the beam diameter: EPMA surface analysis of Sn is performed on the conditions of 20 micrometers and pitch: 20 micrometers. In addition, EPMA surface analysis of Sn is performed in three cross-sectional views of the positions of 1/4xW, 1/2xW, and 3/4xW.

Next, from the EPMA surface analysis, a position with the highest Sn concentration in each cross-sectional view is selected, and at each of the positions in the thickness direction of the steel, under the conditions of beam diameter: 5 µm and pitch: 5 µm, Sn Perform EPMA line analysis. In addition, in implementation of an EPMA line|wire analysis, the area|region up to 25 micrometers from the front and back surfaces of steel materials is excluded.

Then, the maximum value of the Sn concentration (mass concentration) for each measurement line is obtained, and these average values are taken as the Sn concentration (mass concentration) in the central segregation portion, and the Sn concentration in the central segregation portion is the arithmetic average value of all the measured values of the measurement line. Let the value divided by the average Sn concentration (mass concentration) be Sn segregation degree.

In other words,

[Sn segregation degree] = [Sn concentration in central segregation region]/[average Sn concentration]

to be.

As described above, the steel material for crude oil tanker of the present invention suppresses the central segregation of Sn, that is, the Sn segregation degree indicating the degree of central segregation of Sn from the viewpoint of securing excellent lamellar resistance properties. It is very important to control below. Here, even if the component composition is the same, the Sn segregation degree changes greatly according to manufacturing conditions. For this reason, in order to suppress center segregation of Sn, it is very important to control the manufacturing method of steel materials appropriately.

Hereinafter, the suitable manufacturing method of the steel materials for crude oil tankers of this invention is demonstrated.

That is, the steel material of the present invention is prepared by melting steel adjusted to the above-described component composition using a known refining process such as a converter, electric furnace, vacuum degassing, and the like, followed by a continuous casting method or an ingot-ingot rolling method. It can be manufactured by making a steel raw material (slab) into a steel raw material (slab) and then hot rolling after reheating this steel raw material as needed to make a steel plate, a section steel, etc. In addition, the thickness of steel materials is although it does not specifically limit, Preferably it is 2-100 mm. More preferably, it is 3-80 mm. More preferably, it is 4-60 mm.

Here, in the case of continuous casting, it is preferable to set the casting speed (drawing speed) to 0.3-2.8 m/min. If the casting speed is less than 0.3 m/min, the operating efficiency deteriorates. On the other hand, when the casting speed exceeds 2.8 m/min, surface temperature non-uniformity occurs, the molten steel supply to the inside of the slab becomes insufficient, and the center segregation of Sn is promoted. From a viewpoint of suppressing central segregation of Sn, More preferably, they are 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, it is preferable to perform a light-reducing method in which a cast slab at the end of solidification having an unsolidified layer is cast while being gradually reduced by a reduction roll group with a total reduction amount and reduction speed equivalent to the sum of the amount of solidification shrinkage and heat shrinkage.

Next, when hot-rolling the above-mentioned steel raw material to a desired dimensional shape, it is preferable to heat at the temperature of 900 degreeC - 1350 degreeC. When the heating temperature is less than 900°C, the deformation resistance is large, and hot rolling becomes difficult. On the other hand, when heating temperature exceeds 1350 degreeC, a surface mark will generate|occur|produce, or a scale loss or a fuel source unit will increase.

In particular, since diffusion of Sn in the central segregation portion is promoted as the heating temperature is higher, it is advantageous from the viewpoint of securing lamellarity resistance. From such a viewpoint, it is more preferable that heating temperature shall be 1030 degreeC or more.

In addition, it is preferable that the holding time in the said heating temperature shall be 60 min or more. Thereby, diffusion of Sn in the central segregation portion is sufficiently promoted. More preferably, it is 150 min or more. In addition, although it does not specifically limit about an upper limit, It is preferable to set it as 1000 min.

In addition, when the temperature of a steel raw material is the case originally in the range of 1030-1350 degreeC, and it was hold|maintained for 60 min or more in the temperature range, you may apply to hot rolling as it is, without heating. Moreover, a reheating process, acidity, and cold rolling may be given to the hot-rolled sheet obtained after hot rolling, and it is good also as a cold-rolled sheet of predetermined thickness.

In hot rolling, it is preferable to make the finish rolling completion|finish temperature into 650 degreeC or more. When the finish rolling end temperature is less than 650°C, the rolling load increases due to an increase in the deformation resistance, making it difficult to perform rolling.

Cooling after hot rolling may be either air cooling or accelerated cooling. However, in order to obtain higher strength, it is preferable to perform accelerated cooling.

Here, when performing accelerated cooling, it is preferable that a cooling rate shall be 2-100 degreeC/s and a cooling stop temperature shall be 700-400 degreeC. That is, when the cooling rate is less than 2°C/s and/or when the cooling stop temperature is more than 700°C, the effect of accelerated cooling is small, and sufficient strength enhancement may not be achieved in some cases. On the other hand, if the cooling rate is more than 100°C/s and/or the cooling stop temperature is less than 400°C, the toughness of the steel may decrease or the shape of the steel may be deformed. However, this is not always the case when heat treatment is performed in the post-process.

Example

Steel having the component composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by continuous casting under the conditions shown in Table 2. After reheating these steel slabs to 1150 degreeC, it hold|maintained under the conditions shown in Table 2, finish rolling completion temperature: 800 degreeC hot-rolling was performed, and the steel plate of plate|board thickness: 40 mm was obtained. In addition, cooling after hot rolling was made into water cooling (accelerated cooling) of cooling rate:10 degreeC/s, and cooling stop temperature:550 degreeC.

And the Sn segregation degree in the obtained steel plate was calculated|required by the said method. A result is written together in Table 2.

In addition, with respect to the steel sheet obtained as described above, in the following way, a front corrosion test (condensation test) simulating the environment of the upper deck of a crude oil tank of a crude oil tanker and a local corrosion test (acid resistance test) simulating the bottom plate environment each was performed.

(1) Front corrosion test (condensation test)

In order to evaluate the corrosion resistance (front corrosion resistance) with respect to the front surface corrosion in the upper deck back surface of the crude oil tank of the crude oil tanker, for each of the steel plates of No. 1-58, from the position of the surface 1 mm, the width 25 mm × the length A square piece of 60 mm × thickness 5 mm was cut out, and the surface was polished with an emery paper of number 600. Next, the back surface and the end surface were sealed with a tape so as not to be corroded, the test piece 1 was prepared, and the entire surface corrosion test was performed using the corrosion test apparatus shown in FIG. This corrosion test apparatus is comprised by the corrosion test tank 2 and the temperature control plate 3 . Water 6 maintained at a temperature of 30° C. is injected into the corrosion test tank 2, and 13 vol% CO ₂ , 4 vol% O _{2 is introduced into the water 6 through the introduction gas pipe 4 .} , 0.01 vol% SO ₂ , 0.05 vol% H ₂ S, and the balance N ₂ are introduced, thereby filling the corrosion test tank 2 with supersaturated water vapor, and a corrosive environment on the back surface of the upper deck of the crude oil tank is reproducing Then, the test piece (1) is set on the back surface of the corrosion test tank (2), and the test piece (1) is passed through a temperature control plate (3) having a built-in heater and a cooling device at 25°C x 1.5 hours + 50 The temperature change which makes 1 cycle of °C x 22.5 hours was applied repeatedly for 21, 49, 77, and 98 days, and dew condensation water was generated on the surface of the test piece (1), and it was made to cause total corrosion. In addition, in FIG. 1, the code|symbol 5 shows the exhaust gas pipe extended from the test tank.

After the above-mentioned corrosion test, the rust on the surface of each test piece was removed, the mass feeling due to corrosion was obtained from the mass change before and after the test, and from this value, it was converted into an amount of plate thickness decrease per year (corrosion rate on one side) from this value. Then, the predicted wear and tear after 25 years is calculated from the value of the 4 test period, and when the amount of corrosion is 2.0 mm or less, the corrosion resistance is good (○), and when it is more than 2.0 mm, the corrosion resistance is evaluated as poor (x) did.

(2) local corrosion test (acid resistance test)

In order to evaluate the corrosion resistance (local corrosion resistance) in the local corrosive environment (pitting) in the bottom plate of the crude oil tank of the crude oil tanker, for the steel plates of No. 1-58, respectively, from the position of the surface 1 mm, A square piece of 25 mm in width × 60 mm in length × 5 mm in thickness was cut out, the surface was polished with an emery paper of number 600, and a test piece was prepared.

Next, a 10 mass % NaCl aqueous solution was prepared using concentrated hydrochloric acid to have a Cl ion concentration of 10 mass %, pH: 0.85, and suspended through a 3 mm diameter hole in the upper part of the test piece through a cloth thread, About each test piece, the corrosion test made to immerse in 2 L test solution for 168 hours was done. In addition, the test solution was previously heated and maintained at 30 degreeC, and it exchanged with the new test solution every 24 hours.

The apparatus used for the said corrosion test is shown in FIG. This corrosion test apparatus is an apparatus of a double structure of a corrosion test tank 8 and a constant temperature tank 9, and the test solution 10 enters the corrosion test tank 8, and the test piece 7 is placed therein. ) is suspended and immersed. The temperature of the test solution 10 is maintained by adjusting the temperature of the water 12 placed in the constant temperature tank 9 .

After the above-mentioned corrosion test, after removing the rust generated on the surface of the test piece, the mass difference before and after the test was obtained, and this difference was divided by the total surface area to determine the amount of plate thickness reduction per year (corrosion rate on both sides). As a result, the case where the corrosion rate was 1.0 mm/y or less was evaluated as good (circle) in internal corrosion property, and the case where the corrosion rate was 1.0 mm/y second was evaluated as poor (x) in local corrosion property.

In addition, the following points evaluated lamellarity resistance.

(3) Evaluation of lamellarity resistance

In accordance with the ClassNK Steel Wire Rules and Inspection Guidelines (Part K, Chapter 2), No. 1-58 steel sheets obtained as described above were subjected to a tensile test in the sheet thickness direction (Z direction) of the steel sheet, The reduction of area (RA) was calculated. And based on the calculated cross-sectional shrinkage ratio (RA), the following criteria evaluated lamellar tear resistance.

◎ (Passed, especially excellent): 70 or higher

○ (Pass): 35 or more and less than 70

△ (Failure): 25 or more but less than 35

× (failed): less than 25

Table 2 shows the evaluation results of (1) to (3). In addition, in the case of the comprehensive evaluation in Table 2, when all of the above evaluations (1) to (3) are "○" or "double-circle", "Pass", and one in the evaluations from (1) to (3). Even if there was a "△" or "X", it was regarded as "failed".

[Table 1]

[Table 1 (continued)]

As shown in Table 2, all of the invention examples have excellent overall corrosion resistance, local corrosion resistance, and further excellent lamellar tear resistance.

On the other hand, in the comparative example, sufficient properties were not obtained with respect to at least one of the total surface corrosion resistance, the local corrosion resistance and the lamellarity resistance.

That is, in Comparative Examples Nos. 42, 48, and 52, since the amount of S exceeds the upper limit, sufficient characteristics cannot be obtained with respect to the internal corrosion resistance and lamellarity resistance.

Moreover, in Comparative Examples No. 43, 47, and 50, since the amount of Sn exceeds the upper limit, sufficient characteristics cannot be obtained with respect to lamellarity resistance.

In Comparative Example No. 44, the amount of S exceeds the upper limit, and the predetermined amounts of Cu, Ni, Sb, W, Mo and Si are not contained. Therefore, the total corrosion resistance, the local corrosion resistance and the lamellar tier resistance are sufficient. properties are not obtained.

In Comparative Example No. 45, since the amount of Sn was less than the lower limit, sufficient characteristics were not obtained with respect to total corrosion resistance and local corrosion resistance.

In Comparative Example No. 46, since the amount of S and the amount of Sn exceeded the upper limits, sufficient characteristics were not obtained with respect to the local corrosion resistance and lamellarity resistance.

Since Comparative Example No. 49 does not contain predetermined amounts of Cu, Ni, Sb, W, Mo, and Si, sufficient characteristics cannot be obtained with respect to corrosion resistance on the entire surface.

In Comparative Example No. 51, since the amount of S exceeded the upper limit and the amount of Sn was less than the lower limit, sufficient characteristics were not obtained with respect to total surface corrosion resistance, local corrosion resistance, and lamellar tear resistance.

In Comparative Examples Nos. 53 to 56, since the degree of Sn segregation exceeded the upper limit, sufficient characteristics were not obtained with respect to lamellarity resistance.

1, 7: test piece
2, 8: corrosion test tank
3: temperature control plate
4: introduction gas pipe
5: exhaust gas pipe
6, 12: water
9: constant temperature bath
10: test solution
11: Cheonjamsa Temple

Claims (7)

in mass %,
C: 0.03 to 0.18%;
Mn: 0.10 to 2.00%;
P: 0.030% or less;
S: 0.0070% or less;
Al: 0.001 to 0.100%,
Sn: 0.01 to 0.10% and
N: while containing 0.0080% or less,
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
Si: 0.01 to 1.50%
It contains one or two or more selected from among, and the remainder has a component composition consisting of Fe and unavoidable impurities,
A steel material for crude oil tankers with a Sn segregation degree of 12 or less.
Here, the Sn segregation degree is defined by the following formula (1).
[Sn segregation degree] = [Sn concentration in central segregation region]/[average Sn concentration] --- (1) The method of claim 1,
Steel materials for crude oil tankers in which S content and Sn content in the said component composition satisfy|fill the relationship of following formula (2).
10000 × [%S] × [%Sn] ² ≤ 1.40 --- (2)
Here, [%S] and [%Sn] are contents (mass %) of S and Sn in a component composition, respectively. The method according to claim 1 or 2,
The said component composition contains at least 1 group selected from the following (A)-(D), the steel materials for crude oil tankers.
(A) in mass %,
Cr: 0.01 to 0.50% and
Co: 0.01 to 0.50%
1 or 2 types selected from
(B) in mass %,
Ti: 0.001 to 0.100%,
Zr: 0.001 to 0.100%,
Nb: 0.001 to 0.100% and
V: 0.001 to 0.100%
1 type or 2 or more types selected from
(C) in mass %,
Ca: 0.0001 to 0.0100%,
Mg: 0.0001 to 0.0200% and
REM: 0.0002 to 0.2000%
1 type or 2 or more types selected from
(D) in mass %,
B: 0.0001 to 0.0300% The crude oil tanker formed using the steel materials for crude oil tankers of Claim 1 or 2. A crude oil tanker using the steel materials for crude oil tankers of Claim 3. delete delete

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