JP2008133536A - Steel having excellent corrosion resistance for ship use - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for ship use, exhibiting corrosion resistance on a level capable of practical use in a high corrosive environment even without being subjected to coating and electrolytic protection, having excellent durability particularly to crevice corrosion, and further exhibiting excellent corrosion protection and durability even to corrosion generated by the repetition of salt adhesion and humid environments caused by sea water. <P>SOLUTION: The steel having excellent corrosion resistance for ship use is composed of a steel material comprising prescribed amounts of C, Si, Mn and Al, respectively, and also, comprising one or more kinds among A group elements (Mg, Ca, Sr, Ba) by 0.0005 to 0.020% and B group elements (Ti, Zr, Hf) by 0.005 to 0.20% in such a manner that the ratio between the total content of the A group elements (A) and the total content of the B group elements (B), (A)/(B), lies in the range of 0.01 to 1, and the balance Fe with inevitable impurities, and in which sulfide based inclusions and oxide based inclusions including the A, B group elements by prescribed amounts and having prescribed sizes are present by 200 to 2,000 pieces per 1 mm<SP>2</SP>in the cross section in the rolling direction. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to marine steel materials with excellent corrosion resistance that are used as main structural materials in ships such as crude oil tankers, cargo ships, cargo passenger ships, passenger ships, warships, and the like, and are particularly exposed to salt and high-temperature and high-humidity derived from seawater. The present invention relates to a marine steel material that exhibits excellent corrosion resistance even in a highly corrosive environment.

  Steel materials used as the main structural materials (such as outer plates, ballasted tanks, crude oil tanks, etc.) used when building various ships as described above are exposed to salt and high-temperature and high-humidity derived from seawater. Often suffers from corrosion damage. Since such corrosion may lead to marine accidents such as inundation and sinking, it is necessary to take anti-corrosion measures for steel materials. Anti-corrosion coating and cathodic protection are generally implemented as such anti-corrosion measures. is there.

  Of these, anti-corrosion coatings represented by heavy coatings are difficult to eliminate coating film defects, and the coating film is often damaged by collisions with other components during production, resulting in damage to the coating film. In the exposed steel material, local and intensive corrosion of the steel material proceeds in a highly corrosive environment, and there is a risk of leading to early leakage of petroleum liquid fuel and the like.

  On the other hand, the anti-corrosion exhibits a very excellent anti-corrosion effect for a part that is completely immersed in seawater. However, in parts that receive seawater splashes in the atmosphere, an electrical circuit necessary for anticorrosion is not formed, and a satisfactory anticorrosion effect is often not exhibited. Moreover, if the anode for passing the anticorrosion current disappears due to abnormal consumption or dropout, severe corrosion proceeds immediately.

  In addition to these anticorrosion techniques, for example, a technique as disclosed in Patent Document 1 has been proposed as one that improves the corrosion resistance of a steel material covered with an anticorrosion film. This technology relates to a steel material for cargo oil tanks, and aims to improve corrosion resistance by appropriately adjusting the chemical composition and steel structure of the steel material and the size and distribution density of inclusions contained in the steel material. Further, Patent Document 2 improves the corrosion resistance in the environment in the crude oil tank by optimizing the chemical composition of the steel material, the composition, size and number of sulfide inclusions and oxide inclusions in the steel. At the same time, steel for crude oil tanks with improved HAZ (welding heat affected zone) toughness by high heat input welding has been disclosed. With these improved technologies, it has become possible to secure a certain level of corrosion resistance compared to conventional materials. .

However, the corrosion resistance in a more severe corrosive environment is still not sufficient, and further improvement in corrosion resistance is required. In particular, so-called crevice corrosion rapidly progresses at the `` crevice '' part caused by the contact between foreign material and steel material, structural causes, damage to the anticorrosion coating, etc. Does not satisfy the corrosion resistance against these corroded parts.
JP 2003-82435 A (Claims etc.) Japanese Unexamined Patent Application Publication No. 2004-169048 (claims, etc.)

The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is to exhibit a level of corrosion resistance that is practical in a highly corrosive environment without anticorrosion coating or electrocorrosion protection, and particularly against crevice corrosion. An object of the present invention is to provide a marine steel material that is excellent in durability and exhibits excellent anti-corrosion durability against corrosion caused by salt adhesion caused by seawater and repeated wet environments. Above all, to provide steel materials for ships that exhibit excellent corrosion resistance even when used as a structural material for tanks that come in direct contact with petroleum-based fuels such as crude oil as tanker materials that transport petroleum-based fuels such as crude oil. It is in.

The steel material for ships according to the present invention that has solved the above-mentioned problems is
C: 0.01 to 0.30% (meaning mass%, the same shall apply hereinafter)
Si: 0.01 to 2.0%,
Mn: 0.01 to 2.0%,
Al: 0.005 to 0.10%,
S: each containing 0.010% or less, and
Group A element (one or more selected from Mg, Ca, Sr, Ba): 0.0005 to 0.020%, and Group B element (one or more selected from Ti, Zr, Hf): 0.005-0.20%
In addition, the ratio (A) / (B) of the total content (A) of the group A element and the total content (B) of the group B element is in the range of 0.01 to 1, with the balance being Fe and inevitable The average particle size of sulfide inclusions and oxide inclusions satisfying the following requirements is 1 to 10 μm, and these are 200 to 2000 per 1 mm ^{2 of the} cross section in the rolling direction. There is a gist where there are pieces.
Sulfide inclusions: sulfide inclusions having an equivalent circle diameter of 0.5 μm or more, containing 1 to 30% in total amount of the group A elements and / or 1 to 30% in total amount of the group B elements,
Oxide inclusions: Oxide inclusions having an equivalent circle diameter of 0.5 μm or more, containing 1 to 30% of the group A elements and / or 1 to 30% of the group B elements in total.

  In the marine steel material according to the present invention, as other elements, Cr: 0.01-5.0%, Cu: 0.01-5.0%, Ni: 0.01-5.0%, Co: It may contain at least one selected from the group consisting of 0.01 to 5.0%, or B: 0.0001 to 0.010%, V: 0.01 to 0 It may contain at least one selected from the group consisting of .50% and Nb: 0.003 to 0.50%.

  The marine steel material of the present invention can be used extremely effectively as a tank material for crude oil tankers, taking advantage of its excellent corrosion resistance.

  According to the marine steel material of the present invention, the component composition of the steel material is appropriately adjusted, and in particular, a predetermined group A element (one or more of Mg, Ca, Sr, Ba) and a group B element (Ti, Zr, Hf). A suitable amount of sulfide inclusions and oxide inclusions containing a group A element or a group B element, and a predetermined range. This can provide marine steels that exhibit sufficient corrosion resistance without the need for anticorrosion coating or electrocorrosion protection.In particular, while improving durability against crevice corrosion, it is possible to prevent the adhesion of salt caused by seawater and the wet environment. It is possible to provide a marine steel material that exhibits excellent anti-corrosion durability even under severe corrosive environments caused by repetition. The marine steel material of the present invention having such characteristics is useful as a material for outer plates and ballast tanks in ships such as crude oil tankers, cargo ships, freight passenger ships, passenger ships, warships, etc., and is particularly useful as a material for crude oil tanks. is there.

  The inventors of the present invention have conducted intensive research aimed at solving the problems as described above. As a result, a group A element (one or more of Mg, Ca, Sr, Ba) and a group B element (one or more of Ti, Zr, Hf) having an effect of suppressing the decrease in pH and a function of densifying and stabilizing generated rust. )), And the content ratio of these elements can be controlled appropriately, the size and number of sulfide inclusions and oxide inclusions containing these elements can be adjusted well, and the above problems can be solved. Knowing that a high performance marine steel material can be obtained, the present invention has been completed.

  In the present invention, in order to satisfy the basic characteristics as a steel material, basic components such as C, Si, Mn, and Al need to be appropriately adjusted. First, the reasons for limiting these basic components will be clarified.

C: 0.01 to 0.30%
C is an important element for securing the structural strength of steel materials, and in order to secure the minimum strength as a structural member of a ship (approximately 400 MPa or more depending on the thickness of the steel materials) It is necessary to contain 0.01% or more. However, if the C content exceeds 0.30%, the toughness deteriorates, so it should be suppressed to 0.30% at most. The C content is more preferably 0.02% or more, further preferably 0.04% or more, 0.28% or less, and further preferably 0.26% or less.

Si: 0.01 to 2.0%
Si is an element necessary for deoxidation and ensuring strength, and if it is less than 0.01%, the strength is insufficient as a structural member. However, if the amount is too large, the weldability deteriorates, so it should be suppressed to 2.0% or less. The Si content is more preferably 0.02% or more, still more preferably 0.05% or more, and 1.5% or less, and still more preferably 1.0% or less.

Mn: 0.01 to 2.0%
Mn is also an important element for deoxidation and securing of strength, like Si, and if it is less than 0.01%, the strength is insufficient as a structural member. However, if it is too much, the toughness deteriorates, so it should be suppressed to 2.0% or less. The Mn content is more preferably 0.05% or more, further preferably 0.10% or more, 1.80% or less, more preferably 1.60% or less.

Al: 0.005-0.10%
Al, like Si and Mn, is an element necessary for deoxidation and securing strength, and if it is less than 0.005%, deoxidation is insufficient. However, if the amount is too large, weldability becomes poor, so the upper limit is made 0.10%. The Al content is more preferably 0.010% or more, further preferably 0.015% or more, 0.040% or less, and further preferably 0.050% or less.

S: 0.010% or less S is an element that adversely affects the toughness and weldability of steel materials, and is preferably suppressed as little as possible. The allowable upper limit of the S content is up to 0.010%, and if it exceeds this, it becomes impossible to ensure the weldability required for marine steel. A more preferable S content is 0.008% or less.

Group A element (at least one selected from Mg, Ca, Sr, Ba): 0.0005 to 0.020%
These elements contained in Group A have the effect of increasing the pH of the surrounding aqueous environment by leaching from the steel material, and lowering the pH due to the hydrolysis reaction of the local anode forming part where iron dissolution occurs. Suppresses the corrosion reaction and contributes to the improvement of corrosion resistance. Such an effect is effectively exhibited by containing 0.0005% or more in total. However, if the total content is too large, it adversely affects the formability and weldability, so it must be suppressed to 0.020% or less at most. A more preferable content of the group A element is 0.0008% or more and 0.015% or less in total.

Group B element (at least one selected from Ti, Zr, and Hf): 0.005 to 0.20%
These elements included in Group B are useful for improving the crevice corrosion resistance by densifying the surface rust film that contributes greatly to the improvement of corrosion resistance and improving the environmental barrier properties, and also suppressing the corrosion inside the crevice. It is an element. In order to exert such an action effectively, it is necessary to contain 0.005% or more in total. However, if too much, workability and weldability are deteriorated, so the total content is suppressed to 0.20% or less. A more preferable content of the group B element is 0.008% or more and 0.15% or less in total.

  In the present invention, the total content (A) of the A group element and the B group element are used in order to synergistically exhibit the pH lowering suppressing action by the A group element and the dense and stabilizing action of the surface rust film by the B group element. It is important to properly control the ratio with the total content (B), and the total content of both is adjusted so that the ratio (A) / (B) is in the range of 0.01 to 1.0. By doing so, it becomes possible to improve comprehensive corrosion resistance. A more preferable ratio of (A) / (B) is 0.02 or more and 0.8 or less.

  The essential constituent elements of the marine steel according to the present invention are as described above, and the balance is an impurity (for example, P, O) that is inevitably mixed in from a steelmaking raw material (iron ore, secondary raw material, scrap, etc.) or a manufacturing process. , N, W, Mo, etc.), but these are also allowed to be contained in trace amounts as long as they do not impair the properties of the steel material. However, if these allowable components are too much, they will adversely affect toughness and the like, so at most 0.5% or less, more preferably about 0.1% or less should be suppressed.

  In the present invention, further physical properties can be improved by positively containing the following elements as other elements. Hereinafter, it supplements about those elements and the addition effect.

1 selected from the group consisting of Cr: 0.01-5.0%, Cu: 0.01-5.0%, Ni: 0.01-5.0%, Co: 0.01-5.0% Seed and above Cr, Cu, Ni and Co are all elements that contribute to improving the corrosion resistance of steel. Among these, Cr and Cu have an action of promoting the formation of a dense surface rust film that greatly contributes to the improvement of corrosion resistance, and these effects are effectively exhibited by adding 0.01% or more of each. However, if excessively contained, the weldability and hot workability will be adversely affected, so each should be suppressed to 5.0% or less. More preferable contents when Cr or Cu is contained are 0.05% or more and 4.50% or less, respectively.

  Moreover, Ni and Co have the effect | action which stabilizes the precise | minute surface rust film | membrane which contributes largely to an improvement in corrosion resistance, and especially contribute to improvement of corrosion resistance after coating by suppressing the progress of corrosion under the coating film. These effects are effectively exhibited by containing 0.01% or more of each, but if it is too much, weldability and hot workability are deteriorated, so it should be suppressed to 5.0% or less. More preferable contents when Ni or Co is contained are 0.05% or more and 4.50% or less, respectively.

One or more types selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50%. In some cases, further strengthening is required, and these elements all contribute to further improvement in strength. Among these, when B is contained in an amount of 0.0001% or more, the hardenability is improved and the strength is increased. V contributes to strength improvement by containing 0.01% or more, but if it exceeds 0.50%, it also deteriorates toughness. Nb effectively acts to improve the strength by containing 0.003% or more, but if it exceeds 0.50%, the toughness of the steel is also deteriorated. In addition, when the more preferable lower limit of these elements is shown, B is 0.0003%, V is 0.02%, and Nb is 0.005%. Moreover, if a more preferable upper limit is shown, B is 0.0090%, V is 0.45%, and Nb is 0.45%.

  The above is the reason for limiting the chemical components. In the present invention, the sulfide-type inclusions and oxide-type inclusions in steel contain the predetermined group A element and group B element, and the size and number thereof. Proper control is important for obtaining excellent corrosion resistance. Hereinafter, these inclusions will be described.

  In an aqueous environment containing chloride ions, sulfide inclusions gradually dissolve in water and become the starting point of corrosion. However, it has been found that when a predetermined amount of the group A element and the group B element are contained in the sulfide inclusions, the dissolution of the sulfide inclusions in an aqueous environment containing chloride ions can be suppressed. Further, in a corrosive environment containing sulfur content derived from petroleum (such as elemental sulfur and hydrogen sulfide gas), if a predetermined amount of the above group A element and group B element is contained in the oxide inclusions, It promotes the dissolution of physical inclusions and promotes their dissolution in water to raise the pH of the aqueous environment. As a result, it was confirmed that the pH decrease due to the hydrolysis reaction in the local anode where iron dissolution occurred was suppressed to suppress the corrosion reaction and contribute to the improvement of the corrosion resistance.

  Therefore, in the present invention, the sulfide inclusions and oxide inclusions contain an appropriate amount of the elements contained in Group A and Group B, so that the dissolution of sulfide inclusions can be suppressed and the oxide inclusions can be reduced. It is intended to increase the corrosion resistance by significantly exhibiting the pH increase due to dissolution and the corrosion inhibition effect associated therewith.

In order to effectively exhibit such an action, the sulfide group inclusions and oxide inclusions contain 1 to 30% of the total amount of the A group element and / or 1 to 30% of the total group B element, respectively. In addition, among these sulfide inclusions and oxide inclusions, those having an equivalent circle diameter of 0.5 μm or more must be controlled within an appropriate range. That is, in the present invention, sulfide inclusions and oxide inclusions containing an appropriate amount of the above group A element and / or group B element and having an equivalent circle diameter of 0.5 μm or more have an average particle size of 1 to 1, respectively. It is extremely important that 10 to ²⁰⁰ μm and each of them be 200 to 2000 per 1 mm ^{2 of the} steel cross section in order to effectively exhibit both of the above characteristics and to exhibit a high level of corrosion resistance.

  For the sulfide-based and oxide-based inclusions, a sample surface obtained by mirror-polishing the cross section of the test steel material to about 1 μm with, for example, diamond paste or the like is observed with a scanning electron microscope (SEM). The inclusions of these inclusions are confirmed by elemental analysis by X-ray analysis (EDX), and among the S, O, and N contents contained in the inclusions, the one with the largest S (sulfur) content is determined as a sulfide inclusion ( For example, see FIG. 1), and the one with the highest O (oxygen) content was identified as an oxide inclusion (see FIG. 2, for example).

  If the content of the group A element and group B element contained in these sulfide inclusions and oxide inclusions is less than 1%, the corrosion resistance improving effect at the level intended by the present invention is not exhibited. When the excess amount exceeds 30%, the mechanical properties of the steel materials are deteriorated. Therefore, the contents of Group A elements and Group B elements contained in these inclusions are determined to be in the range of 1 to 30%, respectively. A more preferable content is 2% or more and 28% or less, and further preferably 3% or more and 25% or less.

  By the way, if the particle size of the sulfide inclusions is too small, the fine sulfide inclusions are dispersed all over the surface, so that overall corrosion is likely to occur and the corrosion resistance is lowered. Conversely, if the particle size of the sulfide inclusions is too large, the mechanical properties of the steel material will deteriorate. If the particle size of oxide inclusions is too small, the effect of increasing the pH due to dissolution in an aqueous atmosphere will be insufficient, and it will not be possible to suppress the decrease in pH due to the hydrolysis reaction at the local anode where the dissolution of iron occurs. It is difficult to achieve a satisfactory corrosion resistance improvement effect. On the other hand, if the particle size of the oxide inclusions is too large, the mechanical properties of the steel material deteriorate as in the case of sulfide inclusions. Therefore, the sulfide inclusions and oxide inclusions should be adjusted to an average particle diameter (average value of equivalent circle diameter) of 1 to 10 μm, and more preferably 2.0 to 9 It is good to set it as the range of 0.0 micrometer.

  The particle size of these sulfide inclusions and oxide inclusions is determined by mirror-polishing the cross section of the test steel (for example, up to about 1 μm with diamond paste) and then polishing the polished surface with a scanning electron microscope (SEM). And select 100 arbitrary circles whose equivalent circle diameter is considered to be approximately 0.5 μm or more from each inclusion by image analysis to obtain the equivalent circle diameter, and the average value is determined as the average particle diameter. did.

Next, with regard to the number of these inclusions, if the number of sulfide inclusions is too small, the individual sulfide inclusions become too large, which impairs the mechanical properties of the steel material, and even if the number itself is too large. Similarly, the mechanical properties of the steel material deteriorate. In addition, when the number of oxide inclusions is insufficient, the pH increasing action due to dissolution in the aqueous environment described above is reduced, and the local anode where iron is dissolved and the pH lowering suppressing action due to hydrolysis reaction are not effective. If the number is too large, the mechanical properties of the steel will deteriorate. Therefore, in the present invention, the number of sulfide inclusions and oxide inclusions of the size (equivalent to a circle equivalent diameter of 0.5 μm or more) observed per 1 mm ^{2 of} an arbitrary cross section of the steel material is determined to be 200 to 2000, respectively. It was. A more preferable number is 500-1500.

  The number of sulfide inclusions and oxide inclusions as used herein refers to mirror polishing of the cross section of the test steel (for example, up to about 1 μm with diamond paste), and then polishing the polished surface with a scanning electron microscope (SEM) And the average value was obtained from the number of sulfide inclusions and oxide inclusions of the above-mentioned size (equivalent circle diameter of 0.5 μm or more) observed in an arbitrary 10 field of view. .

  Next, the manufacturing method of the marine steel material according to the present invention will be described.

  The steel material of the present invention can be produced by the following method. That is, secondary refining including component adjustment and temperature adjustment is performed using a RH vacuum degassing apparatus on molten steel melted by a conventional method using a converter or electric furnace and put out in a ladle. In the present invention, the group A elements and the group B elements constituting the sulfide inclusions and oxide inclusions are added during the secondary refining and adjusted so as to have a predetermined component composition. In the secondary refining process, it is also possible to perform processing using an apparatus other than RH such as de-S processing by LF (ladder refining) if necessary. As a deoxidation type in secondary refining, killed steel is preferably used from the viewpoint of obtaining a steel material excellent in mechanical properties and weldability, and Al killed steel is more preferably used. After secondary refining, steel ingots are made by conventional casting methods such as continuous casting and ingot casting.

  The obtained steel ingot is heated to a temperature range of 1100 to 1200 ° C. and then hot-rolled to obtain a desired size and shape. At this time, it is preferable that the end temperature of the hot rolling is 750 to 850 ° C., and the cooling rate in the temperature range up to 500 ° C. after the end of the hot rolling is controlled to a range of 0.1 to 15 ° C./s.

  To adjust the size and number of the sulfide inclusions and oxide inclusions, which are particularly important constituent elements in the present invention when carrying out the above production method, to the preferred range described above, It is important to add the group B element during secondary refining. If these elements are added during secondary refining, and the addition amount is adjusted so that the ratio of the group A element to the group B element is within a predetermined range, the group A in the sulfide inclusions and oxide inclusions. An element and a group B element can be contained, and the size and number of inclusions can be controlled within a suitable range. If component adjustment is performed by adding Group A elements or Group B elements prior to secondary refining, too many inclusion constituent elements or coarse inclusions are likely to be generated, and the size and number of inclusions are described above. It becomes impossible to fit within the preferred range.

It is also important to adjust the slab (steel ingot) heating temperature before hot rolling to 1100 to 1200 ° C. When the heating temperature exceeds 1200 ° C., the number of inclusions increases even if the chemical components are appropriate, and the object of the present invention cannot be achieved.

  The marine steel material of the present invention basically exhibits excellent corrosion resistance even if it is not coated due to the above characteristics, but if necessary, it is represented by a modified epoxy resin paint as shown in the examples below. It can also be used in combination with other anticorrosion methods such as various anticorrosion coatings, zinc rich paints, shop primers, and cathodic protection. When such an anticorrosion coating is applied, the corrosion resistance (coating corrosion resistance) of the coating film itself is excellent as shown in the examples below.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and is suitable as long as it can meet the purpose described above and below. It is also possible to carry out the invention with modifications, and these are all included in the technical scope of the present invention. In the following examples, “%” means “% by mass” unless otherwise specified.

Example 1
[Sample material]
Continuous casting after steel materials having the chemical composition shown in Tables 1 and 2 below are melted in a converter according to the addition timing of Group A elements and Group B elements shown in Table 3 and subjected to dephosphorization and desulfurization by secondary refining. With slab. Next, the obtained slab was reheated as shown in Table 3 and then hot-rolled to obtain a steel sheet containing the average particle size and number of sulfide inclusions and oxide inclusions shown in Table 4. Obtained.

  The cross section in the rolling direction of each test steel sheet was mirror-polished using a 1 μm diamond paste, and then the size and number of sulfide inclusions and oxide inclusions contained in the test material were measured using an SEM (scanning electron microscope). : Magnification 5000 times) and EDX (energy dispersive X-ray fluorescence analyzer).

That is, the chemical composition of each inclusion is measured by EDX at five arbitrarily selected locations, and SEM observation is performed for sulfide inclusions and oxide inclusions satisfying predetermined contents of Group A elements and Group B elements. The average particle diameter (average value of equivalent circle diameter; a) and the average value (N) of the number of existing particles per 1 mm ² were obtained. FIG. 1 shows an example of a secondary electron image of a sulfide inclusion by SEM, an example of an EDX analysis chart of an arrow portion of the electron image, and FIG. 2 shows a secondary electron image of an oxide inclusion by SEM. An example of the EDX analysis chart of the arrow part of the electronic image is shown.

  Further, each steel plate obtained was cut and subjected to surface grinding to finally produce a test piece A having a size of 100 × 100 × 25 (mm) [FIG. 3 (A)], and FIG. ) 4 small test pieces of 20 × 20 × 5 (mm) are brought into contact with a large test piece of 100 × 100 × 25 (mm) (the same as the above-mentioned test piece A) to form a gap portion. Test piece B was prepared. Steel materials having the same chemical composition were used for the small test piece and the large test piece for forming the gap, and the surface finish was the same as that of the test piece A. Then, a hole with a diameter of 5 mm was made in the center of the small test piece, a screw hole was made on the base material (large test piece) side, and fixed with a plastic screw.

  Further, a test piece C [FIG. 3C] in which a modified epoxy resin coating (undercoat: zinc rich primer) with an average thickness of 250 μm was applied to the entire surface of the test piece having the same size as the test piece A was used. On one side of the test piece C, a cut flaw (length: 100 mm, width: reached to the base material with a cutter knife) in order to investigate the degree of corrosion progress when the coating film for anticorrosion is damaged and the base steel material is exposed. About 0.5 mm).

The test piece A, the test piece B, and the test piece C were used five by five, respectively, and the corrosion test was performed by the following method.

[Corrosion test method A: simulated marine environment]
First, a combined cycle corrosion test was conducted by simulating the marine environment to which the ship was exposed and repeating a seawater spray test and a constant temperature and humidity test. In the seawater spray test, the specimens (each test piece A to C) are installed in the test tank at an angle of 60 ° from the horizontal, and 35 ° C artificial seawater (salt water with a Cl concentration of 2%) is atomized. Sprayed continuously. The spray amount at this time was adjusted in advance so that 1.5 ± 0.3 mL of artificial seawater per hour was collected in a circular dish with an area of 80 cm ² installed horizontally in a test tank. In addition, the constant temperature and humidity test was performed by tilting each test material at an angle of 60 ° from the horizontal in a test tank adjusted to a temperature of 60 ° C. and a humidity of 95%. Seawater spray test: 4 hours, constant temperature and humidity test: 4 hours as one cycle, these were alternately performed to corrode the specimen. The total test time was 6 months.

  (1) For the test piece A, the mass change before and after the test was converted into an average thickness reduction amount Dave (mm), and the average value of five test pieces was calculated to evaluate the overall corrosion state of each specimen. . Further, the maximum erosion depth Dmax (mm) of the test piece A is obtained using a stylus type three-dimensional shape measuring apparatus, and Dmax / Dave is calculated from the average thickness reduction amount [Dave (mm)] to obtain corrosion uniformity. Evaluated. The weight measurement and plate thickness measurement after the test were carried out after removing corrosion products such as iron rust by a cathodic electrolysis method [JIS K8284] in a 10% diammonium hydrogen citrate aqueous solution.

  (2) For test specimen B, the presence or absence of crevice corrosion was examined by visual observation of the crevice (contact surface). If crevice corrosion was observed, the corrosion products were removed by the cathodic electrolysis method. The maximum crevice corrosion depth Dcrev (mm) was measured using a stylus type three-dimensional shape measuring device “SE3500” manufactured by Tokoro.

  (3) For test piece C (with cut flaws) subjected to coating treatment, the swollen width of the coating film in the direction perpendicular to the cut flaw is measured with a caliper, and the maximum value among the five test pieces is defined as the maximum swollen width. did.

  The evaluation criteria for the overall corrosion resistance (Dave), corrosion uniformity (Dmax / Dave), crevice corrosion resistance (Dcrev), and coating corrosion resistance (maximum swollen width) are as shown in Table 5 below.

  The results are shown in Tables 6 and 7 together with the results of Example 2 described later.

  From the results of the above experiment, it can be considered as follows.

The content and content ratio of a predetermined group A element (one or more of Mg, Ca, Sr, Ba) and group B element (one or more of Ti, Zr, Hf) do not satisfy the prescribed requirements (No. 2) To 5), or the content and content ratio of the predetermined group A element and group B element are satisfied, but the group A element and group B element are added before secondary refining, or the slab heating temperature is an appropriate temperature. If the average particle size (a) or number (N) of sulfide inclusions and oxide inclusions does not satisfy the specified value (Nos. 6 to 8), conventional materials ( Compared to No. 1), the overall corrosion resistance is slightly improved in all corrosion tests, but sufficient improvement effect is not recognized in the corrosion uniformity and crevice corrosion resistance.
The corrosion resistance of marine steel is insufficient.

  On the other hand, the content and ratio of the predetermined group A element and group B element are appropriate, and the average particle size (a) and the number (N) of sulfide inclusions and oxide inclusions are The appropriately controlled specimens (Nos. 9 to 29) show excellent corrosion resistance compared to the conventional steel (No. 1), and are excellent as marine corrosion resistant steels. I understand.

  It can also be seen that the corrosion resistance of the steel material is further improved when an appropriate amount of an element for improving the corrosion resistance such as Cu, Cr, Ni, Co or the like is contained in the steel. Among these, it can be confirmed that the overall corrosion resistance is greatly improved in the steel types to which Cu or Cr is added (for example, No. 10, 11 and the like). Moreover, in the steel type (for example, No. 17, 18 etc.) to which an appropriate amount of Ni is added, the coating corrosion resistance is greatly improved. Further, in the steel types to which Co is added (for example, Nos. 13 and 16, etc.), the corrosion uniformity is greatly improved.

  As described above, in addition to the adjustment of the chemical components, a predetermined amount of group A element and group B element are contained in an appropriate ratio, and a sulfide-based inclusion containing the predetermined group A element and group B element; By properly controlling the size and number of oxide inclusions, it is possible to obtain a marine steel material with extremely excellent anticorrosion durability.

Example 2
[Sample material]
Steel materials containing the chemical composition shown in Tables 1 and 2 and the inclusions shown in Table 4 were produced in the same manner as in Example 1 above. Next, cutting and surface grinding were performed, and the test piece (A), test piece (B), and test piece (C) of FIG. 3 were prepared in the same manner as in Example 1, and each of them was used for corrosion tests by using 5 pieces each. . The corrosion test method at this time is as follows.

[Corrosion test method B: Crude oil tank simulated environment]
Simulated crude oil tank environment, prepared specimens (each specimen A to C) were mixed with crude oil sludge collected from crude oil tanker and natural seawater collected in Kakogawa City, Hyogo, Japan at a volume ratio of 1: 1. The mixed gas of 5% O ₂ -0.5% H ₂ S-10% CO ₂ (remainder N ₂ ) in a partial pressure ratio was introduced into the test tank. The test period was one year. For test piece A, corrosion products such as iron rust were removed by a cathodic electrolysis method (JIS K8284) in a 10% aqueous diammonium hydrogen citrate solution. For test piece B, the small test piece for forming the gap was removed, and the corrosion products were removed in the same manner.

  (1) For test piece A, the weight change before and after the test was converted into an average thickness reduction amount Dave (mm), the average value of five test pieces was calculated, and the overall corrosivity of each specimen was evaluated. . Further, the maximum erosion depth Dmax (mm) of the test piece A is obtained by using a stylus type three-dimensional shape measuring apparatus, and normalized with an average thickness reduction amount [Dave (mm)] (that is, Dmax / Dave is Calculated) and evaluated the corrosion uniformity. The weight measurement and the plate thickness measurement after the test were performed after removing corrosion products such as iron rust by a cathodic electrolysis method [JIS K8284] in a 10% diammonium hydrogen citrate aqueous solution.

  (2) For test piece B, the crevice portion (contact surface) was visually observed to check for crevice corrosion. If crevice corrosion was observed, the corrosion product was removed by the cathodic electrolysis method, The maximum crevice corrosion depth Dcrev (mm) was measured using a stylus type three-dimensional shape measuring apparatus.

  (3) About the test piece C (with cut flaws) which performed the coating process, the coating film swollen width of the perpendicular | vertical direction to a cut flaw was measured with calipers, and the maximum value of five test pieces was defined as the maximum swollen width.

  The evaluation criteria for the overall corrosion resistance (Dave), corrosion uniformity (Dmax / Dave), crevice corrosion resistance (Dcrev), and coating corrosion resistance (maximum swollen width) are as shown in Table 8 below.

[Corrosion test results]
The results of the corrosion test are as summarized in Tables 6 and 7 above, including the group A element (one or more of Mg, Ca, Sr, Ba) and the group B element (one or more of Ti, Zr, Hf). The content ratio is not adjusted properly, the ratio is appropriate but the content does not meet the prescribed requirements (No. 2 to 5), or the content ratio of the A group element and the B group element Although the content satisfies the specified requirements, sulfide inclusions and oxide inclusions are added by adding Group A and Group B elements before secondary refining, or when the slab heating temperature exceeds the appropriate range. No. in which the average particle diameter (a) and number (N) of the product do not satisfy the prescribed requirements. 6-8, although the overall corrosion resistance is slightly improved compared to the conventional steel (No. 1), no improvement effect is observed in the corrosion uniformity and crevice corrosion resistance, etc. Is insufficient.

  On the other hand, the contents and content ratios of the group A element and the group B element are appropriate, and the average particle diameter (a) and number (N) of the sulfide inclusions and oxide inclusions are appropriate. Those in the range (Nos. 9 to 29) all show excellent corrosion resistance as compared with the conventional steel (No. 1), and are found to be preferable as marine corrosion resistant steels.

  Further, the steel containing a suitable amount of an element for improving corrosion resistance such as Cu, Cr, Ni, Co, etc. further improves the corrosion resistance of the steel material. Among these, the steel material (for example, No. 10, 11 etc.) to which appropriate amounts of Cu and Cr are added has greatly improved the overall corrosion resistance. Moreover, in steel materials added with Ni (for example, No. 17, 18, etc.), a significant improvement in coating corrosion resistance is recognized. Furthermore, in steel materials to which Co is added (for example, Nos. 13 and 16, etc.), the corrosion uniformity is greatly improved.

  As described above, in addition to appropriately adjusting the chemical components, a predetermined A group element and a B group element are contained to appropriately adjust the ratio thereof, and a predetermined amount of the A group element and the B group element are contained. It can be seen that marine steels with higher durability can be obtained by controlling the size and number of sulfide inclusions and oxide inclusions.

FIG. 4 shows the ratio [(A) / (B)] of the total content of group A elements (A) and the total content of group B elements (B) of each steel material based on the results of Examples 1 and 2 above. It is the graph which arranged and showed the relation of comprehensive corrosion resistance. That is, the horizontal axis represents the (A) / (B) ratio, the vertical axis represents the overall judgment of corrosion resistance (if there are two judgments, the plot is plotted between them), and the (A) / (B) ratio is 0.01. Those that do not satisfy -1.0 are "◇", and the ratio (A) / (B) falls within the range of 0.01-1.0, but either the size or the number of both inclusions deviates from the requirement. Examples that satisfy all of the requirements of the present invention are indicated by “□”.

  As is apparent from this graph, the examples satisfying the prescribed requirements of the present invention all have good results of ◯ or more in the overall evaluation of corrosion resistance.

Example 3
[Sample material]
Steel materials containing the chemical composition shown in Tables 1 and 2 and the inclusions shown in Table 4 were produced in the same manner as in Example 1 above. Next, cutting was performed to cut out a metal piece having a size of 30 × 30 × 5 (mm). The entire surface of this metal piece is polished with a wet rotary polishing machine (abrasive paper; # 600), washed with water and acetone, dried, attached with a lead wire, and 10 mm × 10 mm as shown in FIG. Apply the product name "KE-45W" [Shin-Etsu Chemical Co., Ltd., one-component RTV (Room Temperature Vulcanizing, meaning curing at room temperature)] as a coating material, and leave it to dry for more than a day at room temperature. The test piece D obtained in this way was prepared.
Further, as a sacrificial anode, the entire surface of a pure zinc plate having a size of 25 × 40 × 2 (mm) is polished with a wet rotary polishing machine (abrasive paper; # 600), washed with water and acetone, and then dried. Then, a lead wire was attached, and in the same manner as the above-mentioned test piece D, a 10 mm × 10 mm square portion was left and the covering material was covered to prepare a test piece E shown in FIG.
And the following test was done using the said test piece D and the test piece E. FIG.

[Evaluation test of sacrificial anode wear due to cathodic protection]
A tank filled with artificial seawater as a test solution, with the uncoated parts of test piece D and test piece E facing each other in parallel at a distance of 1 cm, and connecting the lead wires of test piece D and test piece E Soaked. The temperature of the test solution was 30 ° C., and the immersion period was 10 days.
Then, the corrosion product on the surface of the specimen E after immersion for 10 days is immersed in a 10% aqueous ammonium chloride solution at 70 ° C. for 5 minutes according to the corrosion product removing method described in JIS Z 2371, washed with water, and dried. Then, the weight of the test piece E (the weight of the test piece E after the test) was measured, and the weight change (wear weight) of zinc was determined from the difference from the weight of the test piece E before the test. This test was performed five times for each steel type, and the average value (average wear weight) was calculated. And ratio of the conventional steel (No. 1) and the average wear weight of the test piece E connected with the lead wire (sacrificial anode wear weight ratio; average wear weight of zinc connected to each steel material / conventional steel (No. 1) The average sacrificial anode weight was evaluated based on the average wear weight of zinc wire. The evaluation criteria for the average wear weight ratio of the sacrificial anode are as shown in Table 9. Table 10 shows the results of evaluation based on the criteria in Table 9.

[Test results]
From Table 10, the content ratio of the conventional steel (No. 1), group A element (one or more of Mg, Ca, Sr, Ba) and group B element (one or more of Ti, Zr, Hf) is appropriate. Those that are not adjusted, those whose ratios are appropriate, but whose contents do not meet the prescribed requirements (Nos. 2 to 5), or the content ratios and contents of group A elements and group B elements are prescribed. The average particle size of sulfide inclusions and oxide inclusions (by adding the A group element and B group element before secondary refining, or when the slab heating temperature exceeds the appropriate range) a) No. in which the number (N) does not satisfy the prescribed requirements. Steels Nos. 9 to 29 satisfying the provisions of the present invention are due to coupling with zinc, whereas 6 to 8 have a large amount of wear of zinc due to coupling with zinc and are evaluated as Δ or less. It is clear that the amount of wear of zinc is reduced and improved to a level of ◯ or higher.

It is a chart which shows the secondary electron image by SEM of the sulfide type inclusion in the steel materials of the present invention, and the EDX analysis result of the inclusion. It is a chart which shows the secondary electron image by SEM of the oxide type inclusion in the steel materials of the present invention, and the EDX analysis result of the inclusion. It is explanatory drawing of the test pieces A, B, and C used for the corrosion test. Graph showing the relation between the total content of group A elements (A) and the total content of group B elements (B) [(A) / (B)] and the overall corrosion resistance of the test steel obtained in the experiment It is. It is explanatory drawing of the test piece D used in Example 3. FIG. It is explanatory drawing of the test piece E used in Example 3. FIG.

Claims (4)

C: 0.01 to 0.30% (meaning mass%, the same shall apply hereinafter)
Si: 0.01 to 2.0%,
Mn: 0.01 to 2.0%,
Al: 0.005 to 0.10%,
S: each containing 0.010% or less, and
Group A element (one or more selected from Mg, Ca, Sr, Ba): 0.0005 to 0.020%, and Group B element (one or more selected from Ti, Zr, Hf): 0.005-0.20%
In addition, the ratio (A) / (B) of the total content (A) of the group A element and the total content (B) of the group B element is in the range of 0.01 to 1, with the balance being Fe and inevitable It is made of steel which is an impurity, and further, the average particle size of sulfide inclusions and oxide inclusions satisfying the following requirements is 1 to 10 μm respectively, and these are 200 to 1 mm ² of 1 mm ^{2 of the} cross section in the rolling direction. A marine steel with excellent corrosion resistance, characterized by the presence of 2000 pieces.
Sulfide inclusions: sulfide inclusions having an equivalent circle diameter of 0.5 μm or more, containing 1 to 30% in total amount of the group A elements and / or 1 to 30% in total amount of the group B elements,
Oxide inclusions: Oxide inclusions having an equivalent circle diameter of 0.5 μm or more, containing 1 to 30% of the group A elements and / or 1 to 30% of the group B elements in total.   The steel material further includes, as other elements, Cr: 0.01 to 5.0%, Cu: 0.01 to 5.0%, Ni: 0.01 to 5.0%, Co: 0.01 to 5%. The marine steel material according to claim 1, comprising at least one selected from the group consisting of 0.0%.   The steel material is at least one selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50% as another element. The marine steel material according to claim 1 or 2, comprising a seed.   The steel material for ships according to any one of claims 1 to 3, which is used as a tank material for a crude oil tanker.