JP2007197760A - Marine steel with excellent corrosion resistance and brittle fracture occurrence characteristic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide marine steel which is excellent in corrosion resistance to such an extent that it can be put to practical use even without coating or electrical protection and, in addition, hardly causes brittle fracture cracks. <P>SOLUTION: The marine steel has a composition consisting of, (by mass, the same applies to the following), 0.01 to 0.2% C, 0.01 to 0.5% Si, 0.01 to 2% Mn, 0.05 to 0.5% Al, 0.010 to 1.5% Cu, 0.010 to 1% Cr, P and S in amounts controlled to ≤0.02% and ≤0.01%, respectively, and the balance Fe with inevitable impurities. When observing a metallic structure of the plane parallel to the rolling direction of the steel and perpendicular to the surface of the steel, the following are satisfied: (1) area fraction of ferrite is ≥75%; (2) average circle-equivalent diameter of ferrite grains at a position of t/2 is ≤20.0 μm; and (3) average aspect ratio of the ferrite grains at a position of t/4 is ≤2.0 (where the (t) means the thickness (mm) of the steel). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to marine corrosion-resistant steel used as a main structural material in ships such as crude oil tankers, cargo ships, cargo passenger ships, passenger ships, warships, and the like, particularly in environments exposed to salinity and high temperature and humidity due to seawater. The present invention relates to marine steel having excellent corrosion resistance.

  Steel materials used as main structural materials (for example, outer plates, ballast tanks, crude oil tanks, etc.) in the above various ships are often corroded because they are exposed to salt from seawater and high temperature and humidity. Since such corrosion may cause marine accidents such as inundation and sinking, it is necessary to apply some anticorrosion means to the steel. Conventionally well-known anticorrosion means include (A) painting and (B) cathodic protection.

  (A) Of these, coatings typified by heavy coating are likely to have coating film defects, and the coating film may be scratched by collisions in the manufacturing process, and the base steel material will be exposed. There are many. In this way, the portion where the base steel material is exposed corrodes the steel material locally and intensively, leading to early leakage of the petroleum-based liquid fuel contained therein.

  (B) On the other hand, in the anti-corrosion, it is very effective for the anti-corrosion in the part completely immersed in the sea water, but the electric circuit necessary for the anti-corrosion is formed in the part receiving the sea water splash in the atmosphere. And the anticorrosion effect may not be sufficiently exhibited. In addition, when the galvanic anode for anticorrosion is abnormally consumed or dropped and disappears, severe corrosion may proceed immediately.

  In addition to the above technique, for example, the technique of Patent Document 1 has been proposed as a means for improving the corrosion resistance of the steel material itself. In this technique, the chemical composition of the steel material is appropriately adjusted to improve the corrosion resistance. This document discloses a corrosion-resistant steel for shipbuilding that can be used even without coating. Patent Document 2 discloses a marine steel material having an improved coating film life by making the chemical composition of the steel material appropriate. With these technologies, it can be said that a certain degree of corrosion resistance can be ensured as compared with the prior art.

However, the corrosion resistance in a more severe corrosive environment is still not sufficient, and further improvement in corrosion resistance is required. In particular, corrosion in the “crevice” formed by the contact part between the foreign material and the steel material, the structural reason, the damaged part of the anticorrosion coating, etc. (hereinafter, sometimes referred to as “crevice corrosion”) becomes prominent, resulting in a long service life. However, the technologies proposed so far have insufficient corrosion resistance in these areas.
Japanese Patent Laid-Open No. 2000-17381 (Claims etc.) JP 2002-266052 A (Claims etc.)

  By the way, as a steel material for ships, it is desired to suppress the occurrence of brittle fracture cracks in order to ensure the safety of the hull even under severe use environments. This is because if a brittle fracture crack occurs, the hull itself will be destroyed. However, a marine steel material that has improved the corrosion resistance while suppressing the occurrence of brittle fracture cracks is not known.

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is excellent in corrosion resistance so that it can be put into practical use without applying coating or cathodic protection, and brittle fracture cracks are generated. It is to provide a steel material that is difficult to ship.

  Another object of the present invention is to improve the durability against crevice corrosion, among other corrosion resistances, and to provide a marine steel material that exhibits excellent durability against salt adhesion caused by seawater and corrosion due to a wet environment. It is to provide.

  The present inventors further improve the corrosion resistance so that it can be put into practical use without applying coating or cathodic protection for steel materials of a strength class (400 MPa to 500 MPa class) that are generally used as marine steel plates, In order to prevent the occurrence of brittle fracture cracks [hereinafter, sometimes referred to as brittle fracture generation characteristics or CTOD (Crac-Tip Opening Displacement) characteristics], intensive studies have been made. As a result, in order to improve the corrosion resistance of the steel material, the steel material should contain a predetermined amount of Cr and Al in combination, and the chemical composition of the steel material may be adjusted appropriately, and the brittle fracture occurrence characteristics may be improved. Found that the metallographic structure of the steel material should be appropriately controlled, and completed the present invention.

That is, the marine steel material according to the present invention that has solved the above problems is C: 0.01 to 0.2% (meaning of mass%, the same applies hereinafter), Si: 0.01 to 0.5% , Mn: 0.01 to 2%, Al: 0.05 to 0.5%, Cu: 0.010 to 1.5%, Cr: 0.010 to 1%, respectively, P: 0.0. Suppressed to 02% or less (not including 0%) and S: 0.01% or less (not including 0%), respectively, the balance being a steel material composed of Fe and inevitable impurities, parallel to the rolling direction of the steel material And when the metal structure of a surface perpendicular | vertical to a steel material surface is observed, it has a summary in the point which satisfies the following (1)-(3). However, t means the thickness (mm) of steel materials.
(1) The ferrite area ratio is 75% or more.
(2) The average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less.
(3) The average aspect ratio of ferrite grains at the t / 4 position is 2.0 or less.

  In the marine steel material of the present invention, it is preferable that the ratio value ([Cr] / [Al]) of the Cr content [Cr] and the Al content [Al] is 1 to 15. However, [Cr] / [Al] means mass ratio.

  Further, in the marine steel material of the present invention, as required, as other elements, (a) Ni: 2% or less (not including 0%), Co: 1% or less (not including 0%) And Ti: one or more elements selected from the group consisting of 0.1% or less (excluding 0%), (b) Ca: 0.02% or less (excluding 0%) and / or Mg: 0.02% or less (not including 0%), (c) Se: 0.5% or less (not including 0%), (d) Sb: 0.5% or less (not including 0%) and / or Or Sn: 0.5% or less (not including 0%), (e) B: 0.01% or less (not including 0%), V: 0.1% or less (not including 0%), and It is also effective to contain one or more elements selected from the group consisting of Nb: 0.05% or less (not including 0%), and the like. This is because the characteristics of the marine steel are further improved according to the type of component to be contained.

  In the marine steel material of the present invention, a predetermined amount of Cr and Al are contained in the steel material in combination, and by appropriately adjusting the chemical composition of the steel material, it can be put into practical use without being subjected to painting and anticorrosion. Thus, the corrosion resistance can be improved, and the occurrence of brittle fracture cracks can be prevented by optimizing the metal structure of the steel material. In particular, according to the present invention, among the corrosion resistance, marine steel that can improve durability against crevice corrosion and also exhibits excellent durability against salt adhesion caused by seawater and corrosion due to a wet environment. Can be realized. Such marine steel materials of the present invention are usefully used as materials for outer plates, ballast tanks, crude oil tanks and the like in ships such as crude oil tankers, cargo ships, cargo passenger ships, passenger ships, warships and the like.

  In the steel material of the present invention, in order to improve the corrosion resistance, it is important to contain Cr and Al in combination, and the object of the present invention cannot be achieved without any of these components. The effects of these components will be described later. The reason why the corrosion resistance is improved by using these elements in combination can be considered as follows.

Al has an effect of forming a stable oxide anticorrosive film on the steel surface. Al ³⁺ ions corroded and dissolved in the steel are combined with dissolved oxygen and the like to become Al oxides, which are deposited on the steel surface to form a corrosion protection film. However, the anticorrosion effect by this film is not sufficient in a high chloride environment in a ship. On the other hand, Cr, like the above Al, exhibits the effect of forming a stable oxide film on the steel surface to prevent corrosion of the steel material, but Cr oxide alone cannot be said to have sufficient corrosion protection effect.

That is, the Al oxide film has a very high stability in a substantially neutral range where the pH is about 5 to 8.5, but the solubility is increased when the pH exceeds 8.5. For example, seawater to which marine steel is exposed has a pH of about 8 when it is clean, but may be alkalized to a pH of about 9.5 in sea areas where seaweed and the like are breeding. Further, at the site where the cathodic reaction of corrosion occurs, the pH tends to increase due to OH ⁻ ions generated by the reduction of dissolved oxygen. For these reasons, Al oxides in a marine environment cannot always exist stably, but rather are easily dissolved and lose their protective properties. In contrast, Cr oxide has high stability in the alkaline region, and also has an effect of lowering pH by hydrolysis equilibrium of a very small amount of dissolved Cr ions. Inhibiting the dissolution of the resin and exerting the effect of securing its protective property. Therefore, it is considered that the anticorrosive effect of the steel material is synergistically enhanced by the coexistence of Cr oxide and Al oxide in appropriate amounts.

  Such an effect is exhibited by controlling the content of Cr and Al contained in the steel material within an appropriate range described later. The value of the content ratio of these elements ([Cr] / [Al]: mass) The ratio) is preferably controlled appropriately. That is, this value ([Cr] / [Al]) is preferably in the range of 1-15. If the value of [Cr] / [Al] is less than 1, the corrosion uniformity tends to be insufficient, and if it exceeds 15, the crevice corrosion resistance is insufficient. The more preferable lower limit of the value of [Cr] / [Al] is 3, and the more preferable upper limit is 10.

  As described above, in the steel material of the present invention, Cr and Al are used in combination in order to improve the corrosion resistance. However, even if Cr and Al are used in combination, the brittle fracture occurrence characteristics cannot be improved. Therefore, the present inventors have studied in order to improve the brittle fracture occurrence characteristics without deteriorating the corrosion resistance improved by using Cr and Al together. For steel materials having a thickness t (mm), in the rolling direction. When the metal structure of the plane parallel to and perpendicular to the steel surface was observed, (1) the ferrite area ratio was 75% or more, and (2) the average equivalent circle diameter of the ferrite grains at the t / 2 position was 20. When the average aspect ratio of ferrite grains at (3) t / 4 position is 2.0 or less at 0 μm or less, it becomes clear that the brittle fracture occurrence characteristics of the steel material can be improved and the corrosion resistance is not deteriorated. It was. Hereinafter, the reason for this definition will be described in detail.

  The metal structure of the marine steel material according to the present invention is mainly composed of ferrite in order to ensure the strength of the steel material. The ferrite main body means that the ferrite fraction in the steel material is 75% by volume or more, and the ferrite area ratio may be 75% or more when the metal structure of the steel material cross section is observed. The area ratio of ferrite is preferably 80% or more, and more preferably 85% or more.

  The remainder of the metal structure is not particularly limited as long as pearlite, bainite, martensite, or the like is generated as the second phase. The area ratio of the second phase may be less than 25%, preferably less than 20%, more preferably less than 15%.

  The metal structure of the marine steel material is mainly composed of ferrite, and in order to improve CTOD characteristics, it is important to appropriately adjust both the equivalent circle diameter and the aspect ratio of the ferrite grains. That is, as a result of repeating various experiments by the present inventors, the average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less, and the average aspect ratio of the ferrite grains at the t / 4 position is 2.0 or less. Need to be. This is clear from FIG. 1 and FIG.

FIG. 1 shows the relationship between the average equivalent circle diameter, average aspect ratio, and CTOD characteristics of ferrite grains at the t / 2 position of the steel material. In FIG. 1, the X axis indicates the average equivalent circle diameter of the ferrite grains at the t / 2 position, the Y axis indicates the CTOD characteristic (δc− _{40 ° C.} ), and □ indicates the average aspect ratio of the ferrite grains at the t / 2 position. 1.4 to 1.6, ○ is the average aspect ratio of the ferrite grains at the t / 2 position is 1.7 to 2.0, and Δ is the average aspect ratio of the ferrite grains at the t / 2 position is 2.1 to 2. The results for 3 are shown.

As is apparent from FIG. 1, it can be seen that the smaller the average equivalent-circle diameter of the ferrite grains at the t / 2 position, the more the CTOD characteristic tends to be improved (the value of δc− _{40 ° C.} tends to increase). At this time, if the average aspect ratio of the ferrite grains at the t / 2 position is 1.4 to 1.6, δc− _{40 ° C.} becomes 0.20 mm or more, and the CTOD characteristics can be reliably improved, whereas t / 2 If the average aspect ratio of the ferrite grains at the position is 2.1 to 2.3, it can be seen that δc− _{40 ° C.} is less than 0.20 mm, and the CTOD characteristics cannot be improved. On the other hand, if the average aspect ratio of the ferrite grains at the t / 2 position is 1.7 to 2.0, the CTOD characteristics may or may not be improved.

Therefore, in FIG. 1, the average aspect ratio of the ferrite grains at the t / 4 position was measured for a steel material having an average aspect ratio of the ferrite grains of 1.7 to 2.0 at the t / 2 position. The result is shown in FIG. In FIG. 2, the X axis indicates the average equivalent circle diameter of the ferrite grains at the t / 2 position, the Y axis indicates the CTOD characteristic (δc− _{40 ° C.} ), and the circle indicates the average aspect ratio of the ferrite grains at the t / 4 position. 1.7 to 2.0 and ● indicate that the average aspect ratio of the ferrite grains at the t / 4 position is 2.1 to 2.2.

As is clear from FIG. 2, in order to surely improve the CTOD characteristics when δc− _{40 ° C.} is 0.20 mm or more, the average aspect ratio of the ferrite grains at the t / 4 position is 1.7 to 2.0. There is a need to. That is, even if the average equivalent circle diameter of the ferrite grains at the t / 2 position is reduced to 20.0 μm or less, if the average aspect ratio at the t / 4 position exceeds 2.0, the improvement effect of the CTOD characteristics is not much recognized. I understand that there is no.

  The reason is considered as follows. That is, in brittle fracture, the boundary between crystal grains (crystal grain boundary) serves as resistance to crack propagation. Therefore, if the grain boundaries exist densely, brittle fracture itself is less likely to occur, Even if brittle fracture occurs, propagation of cracks can be prevented if there are dense grain boundaries in the direction in which cracks propagate. However, since the ferrite grains extend in the rolling direction in the rolling process, the aspect ratio of the ferrite grains increases. Therefore, the major axis of the ferrite grains is aligned in the rolling direction, and the minor axis is easily aligned in the plate thickness direction. Therefore, although the crystal grain boundaries are densely present in the plate thickness direction, the crystal grain boundaries in the rolling direction are sparse, so that the density of the crystal grain boundaries is likely to vary, and brittle fracture is likely to occur. Further, when brittle fracture occurs, cracks are likely to propagate in the rolling direction. On the other hand, if the average equivalent circle diameter of the ferrite grains is reduced and the average aspect ratio is reduced, there is almost no variation in the density of the crystal grain boundaries, so that brittle fracture hardly occurs. The boundary acts as a resistance and can prevent crack propagation.

  Therefore, in the present invention, the average equivalent circle diameter of the ferrite grains is 20.0 μm or less, and the average aspect ratio of the ferrite grains is 2.0 or less. However, in the marine steel of the present invention, the average equivalent circle diameter of the ferrite grains is The value measured at the t / 2 position is used, and the average aspect ratio of the ferrite is the value measured at the t / 4 position.

  The size of the ferrite grains is greatly influenced by the temperature, and the ferrite grains are easily coarsened as the temperature increases. Therefore, in the present invention, the average equivalent circle diameter of the ferrite grains at the t / 2 position where the temperature of the steel material is considered to be the highest is controlled.

  On the other hand, the shape of ferrite grains is greatly affected by the amount of true strain introduced during rolling. That is, as the true strain amount increases, the ferrite grains tend to be finer, but when the rolling temperature is low, the ferrite grains easily progress in the rolling direction during rolling, and the aspect ratio increases. Then, since the true strain is more likely to be introduced into the surface portion of the steel material, the average aspect ratio of the ferrite grains at the t / 4 position where the true strain amount tends to increase is controlled.

  The average equivalent circle diameter of the ferrite grains at the t / 2 position is preferably 17.5 μm or less, and more preferably 16 μm or less. The lower limit of the average equivalent circle diameter of the ferrite grains is not particularly defined and is preferably as small as possible, but is usually about 7 to 10 μm because there is a limit to the reduction. The equivalent circle diameter means the diameter of a circle when ferrite grains are converted into a circle having the same area.

  In the present invention, if the average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less, the average equivalent circle diameter of the ferrite grains at the t / 4 position is also 20.0 μm or less. As described above, since the temperature at the t / 4 position is lower than that at the t / 2 position, the coarsening of the ferrite grains is prevented. The average equivalent circle diameter of the ferrite grains at the t / 4 position is more preferably 16 μm or less, and still more preferably 14 μm or less. The lower limit of the average equivalent circle diameter of the ferrite grains at the t / 4 position is not particularly specified and is preferably as small as possible, but is usually about 7 to 10 μm because there is a limit to the reduction.

  On the other hand, the average aspect ratio of the ferrite grains at the t / 4 position is preferably 1.7 or less, and more preferably 1.5 or less. The aspect ratio of the ferrite grain means the ratio (Dl / Dt) of the grain size (Dl) in the rolling direction and the grain size (Dt) in the plate thickness direction.

  In the present invention, if the average aspect ratio of the ferrite grains at the t / 4 position is 2.0 or less, the average aspect ratio of the ferrite grains at the t / 2 position is also 2.0 or less. As described above, since the true strain amount introduced at the t / 2 position is larger than the true strain amount introduced at the t / 4 position, an increase in the aspect ratio due to the coarsening of the ferrite grains is prevented. Because. The average aspect ratio of the ferrite grains at the t / 2 position is more preferably 1.7 or less, and still more preferably 1.5 or less.

  In the marine steel material of the present invention, when the metal structure in the region from the t / 4 position to the 3t / 4 position is observed, the average equivalent circle diameter and the average aspect ratio of the ferrite grains should satisfy the above requirements. It has already been confirmed that the CTOD characteristics can be sufficiently improved. That is, the average equivalent circle diameter and the average aspect ratio of the ferrite grains in the region of 25% from the center of the steel material in the region of 25% each (that is, the region of 50% across the center of the steel material and corresponds to 50% by volume of the steel material) May satisfy the above requirements. For example, when the average equivalent circle diameter and average aspect ratio of the ferrite grains at the t / 8 position and the 7t / 8 position are calculated, the above requirements may be excluded. This is because even if the size and shape of the ferrite grains are controlled over a wide range from the t / 8 position to the 7t / 8 position (corresponding to 75% by volume of the steel material), the effect of improving the CTOD characteristics is saturated. . However, when the average equivalent circle diameter and average aspect ratio of the ferrite grains at the t / 8 position or the 7t / 8 position are calculated, the above requirement may of course be satisfied.

This is apparent from FIG. FIG. 3 is a graph showing the relationship between the location of the metal structure and the brittle fracture occurrence characteristics (δc− _{40 ° C.} ). The X-axis indicates the relative position from the center position of the steel material, where the thickness of the steel material is 100, X = 0 indicates the center position of the steel material, and X = 50 indicates the surface of the steel material. ○ in the figure indicates a limit position where a metal structure satisfying an average equivalent circle diameter of ferrite grains of 17 to 20 μm and an average aspect ratio of 1.6 to 2.0 is observed. When the metal structure at the position (steel surface side) is observed, the average aspect ratio of the ferrite grains exceeds 2.0.

  As is clear from FIG. 3 above, the average equivalent circle diameter of the ferrite grains is 17 to 20 μm on the steel material surface side beyond the position where the relative position from the center position of the steel material is 25 (that is, the t / 4 position). In addition, even if there is a metal structure satisfying an average aspect ratio of 1.6 to 2.0, it can be seen that the result of brittle fracture occurrence characteristics hardly changes and is saturated. That is, it can be understood that the brittle fracture occurrence characteristics of the entire steel material can be improved by appropriately controlling the metal structure at the t / 4 position.

  The average equivalent circle diameter and average aspect ratio of the ferrite grains can be calculated, for example, by the following procedure. First, a sample is cut out so that a surface that includes the front and back surfaces of the steel material is parallel to the rolling direction and is perpendicular to the steel surface (the front surface of the steel material), and this exposed surface is polished. And mirror finish.

  The method for polishing the exposed surface is not particularly limited. For example, polishing may be performed using a wet emery polishing paper of # 150 to # 1000 or a polishing method having an equivalent function. In addition, when performing mirror finish, an abrasive such as diamond slurry may be used.

  The mirror-finished sample is corroded using a 3% nital solution to reveal the grain boundary of the ferrite structure, and then photographed at a magnification of 100 or 400 and taken into an image analyzer. The image is captured so that the area corresponds to 1 mm × 1 mm or more at any magnification.

  Next, in the image analysis device, the ferrite grain region (area) surrounded by the grain boundary is converted into a circle having an equivalent area, and the diameter of the converted circle is defined as the equivalent circle diameter of the ferrite grain. Measure the equivalent diameter. This is measured for all observation visual fields, and the average is calculated by averaging the results.

  On the other hand, as for the aspect ratio of the ferrite grains, for the ferrite grains surrounded by the grain boundaries, the grain size Dl in the rolling direction and the grain size Dt in the plate thickness direction are measured, and the ratio of Dl to Dt (Dl / Dt) is determined. Calculate as aspect ratio. This is performed for all observation fields, and the average aspect ratio is calculated by averaging the results.

  Next, the chemical composition of the marine steel material of the present invention will be described. In the steel material of the present invention, basic components such as C, Si, Mn, Cu, P, and S must be appropriately adjusted in order to satisfy the basic characteristics as the steel material. The reasons for limiting the ranges of these components will be described below together with the effects of the Cr and Al elements.

C: 0.01 to 0.2%
C is an element necessary for ensuring the strength of the material. In order to obtain the minimum strength required as a structural member of a ship (approximately 400 MPa depending on the thickness of the steel used), it is necessary to contain 0.01% or more. The minimum with preferable C content is 0.02%, More preferably, it is 0.04%. However, when it contains excessively exceeding 0.2%, toughness and weldability will deteriorate. For these reasons, the upper limit of the C content is set to 0.2%. The upper limit with preferable C content is 0.18%, More preferably, it is 0.17%.

Si: 0.01 to 0.5%
In addition to having a deoxidizing action, Si is an element necessary for securing strength, and if it is less than 0.01%, the minimum strength as a structural member cannot be secured. Accordingly, Si is set to 0.01% or more. The minimum with preferable Si content is 0.02%, More preferably, it is 0.05%, More preferably, it is 0.10%. However, even if the content exceeds 0.5%, weldability and HAZ toughness deteriorate. Therefore, Si is 0.5% or less. The upper limit with preferable Si content is 0.45%, More preferably, it is 0.4%.

Mn: 0.01-2%
Mn also has a deoxidizing action similar to Si and is an element necessary for ensuring strength. If it is less than 0.01%, the minimum strength as a structural member cannot be secured. Therefore, Mn is 0.01% or more. The minimum with preferable Mn content is 0.05%, More preferably, it is 0.10%, More preferably, it is 0.3%. However, even if the content exceeds 2%, the toughness deteriorates. Therefore, Mn is 2% or less. The upper limit with preferable Mn content is 1.8%, More preferably, it is 1.6%.

Al: 0.05-0.5%
As described above, Al has an effect of forming a stable oxide anticorrosive film on the steel surface. When the Al content decreases, the corrosion-dissolved Al ³⁺ ions are scattered in the seawater and are not deposited on the surface of the steel material, so that an anticorrosion film is not formed. In order to exhibit a sufficient anticorrosion effect in the presence of Cr oxide, Al needs to be contained in an amount of 0.05% or more. In the case of a normal steel material, if the Al content exceeds 0.10%, there is a problem in weldability such as a decrease in the toughness of the welded portion, but C, Si, P like the steel material of the present invention. By setting S and S within an appropriate range, weldability equivalent to that of conventional steel can be ensured even when the Al content is in the range of more than 0.1% to 0.5%. However, if the Al content exceeds 0.5% and becomes excessive, weldability is impaired. For these reasons, the Al content range was set to 0.05 to 0.5%. In addition, the minimum with preferable Al content is 0.060%, More preferably, it is 0.070%, More preferably, it is 0.080%. Moreover, the upper limit with preferable Al content is 0.45%, More preferably, it is 0.4%, More preferably, it is 0.35%.

Cu: 0.010 to 1.5%
Cu is an element effective for forming a dense surface rust film that greatly contributes to the improvement of corrosion resistance. In addition, the dense rust film formed by containing Cu and the stable oxide anticorrosive film in which Al oxide and Cr oxide coexist synergistically enhance the protection of the base material, and have excellent corrosion resistance. Will be demonstrated. In order to exhibit such an effect, it is necessary to contain 0.010% or more. A preferable lower limit of the Cu content is 0.05%. However, if excessively contained, weldability and hot workability deteriorate, so the content is made 1.5% or less. The upper limit with preferable Cu content is 1.3%, More preferably, it is 1.0%.

Cr: 0.010 to 1%
Cr, like Al, exhibits the effect of forming a stable oxide film on the surface and preventing corrosion of the steel material. In the present invention, as described above, the coexistence of Al oxide and Cr oxide dramatically improves the corrosion resistance of the steel material. In order to exert such an effect, 0.010% or more. It is necessary to contain. The minimum with preferable Cr content is 0.05%, More preferably, it is 0.10%. However, if it is contained excessively, weldability deteriorates, so it is necessary to make it 1% or less. The upper limit with preferable Cr content is 0.9%, More preferably, it is 0.8%.

P: 0.02% or less (excluding 0%)
P is an element that deteriorates toughness and weldability, and the content is preferably suppressed as much as possible. The allowable upper limit of the P content is 0.02%, and if it exceeds this, weldability as marine steel cannot be ensured. Therefore, the P content is set to 0.02% or less. In addition, the upper limit with preferable P content is 0.018%, More preferably, it is 0.015%.

S: 0.01% or less (excluding 0%)
S, like P, is an element that deteriorates toughness and weldability, and the content is preferably suppressed as much as possible. The allowable upper limit of the S content is up to 0.01%, and if it exceeds this, weldability as marine steel cannot be ensured. Therefore, the S content is set to 0.01% or less. In addition, the upper limit with preferable S content is 0.008%.

  The basic components in the marine steel of the present invention are as described above, and the balance is composed of iron and unavoidable impurities (for example, O, etc.). For example, Zr, N, etc.) are acceptable. However, these allowable components should be suppressed to about 0.1% or less because their toughness deteriorates when the amount is excessive.

  Further, in the marine steel material of the present invention, in addition to the above components, if necessary, (a) one or more elements selected from the group consisting of Ni, Co, and Ti, (b) Ca and / or Mg, It is also effective to contain one or more elements selected from the group consisting of (c) Se, (d) Sb and / or Sn, (e) B, V, and Nb. Accordingly, the characteristics of the marine steel are further improved. The reasons for limiting the ranges of these components will be described below.

(A) Ni: 2% or less (not including 0%), Co: 1% or less (not including 0%), and Ti: 0.1% or less (not including 0%) One or more elements Ni, Co, and Ti are all effective elements for improving corrosion resistance.

  Among these, Ni and Co are effective elements for forming a dense surface rust film that greatly contributes to the improvement of corrosion resistance. However, when the content of Ni or Co becomes excessive, weldability and hot workability deteriorate. In addition, excessive addition increases the cost significantly. Therefore, Ni is preferably 2% or less, and Co is preferably 1% or less. A more preferable upper limit when Ni is contained is 1.5%, and more preferably 1%. A more preferable upper limit when Co is contained is 0.8%, and further preferably 0.6%. Ni and Co are both preferably contained in an amount of 0.01% or more. The more preferable lower limit when Ni is contained is 0.05%, and more preferably 0.10%. The more preferable lower limit when Co is contained is 0.015%, and more preferably 0.03%.

  Ti is an element that densifies the surface rust film that greatly contributes to the improvement of corrosion resistance and improves its environmental barrier properties, and also suppresses corrosion inside the crevice and improves crevice corrosion resistance. However, if the content exceeds 0.1%, workability and weldability are deteriorated. Therefore, Ti is preferably 0.1% or less. A more preferable upper limit when Ti is contained is 0.090%, and more preferably 0.05%. It is preferable to contain Ti 0.005% or more. A more preferable lower limit when Ti is contained is 0.008%.

(B) Ca: 0.02% or less (not including 0%) and / or Mg: 0.02% or less (not including 0%)
Ca and Mg are effective elements for improving corrosion resistance by exhibiting a pH raising action by being dissolved, suppressing a pH drop due to a hydrolysis reaction at a local anode where iron is dissolved, suppressing a corrosion reaction. However, if Ca and Mg are contained in excess of 0.02%, workability and weldability are deteriorated. A more preferable upper limit when Ca is contained is 0.015%, more preferably 0.01%. A more preferable upper limit when Mg is contained is 0.015%, more preferably 0.01%. Both Ca and Mg are preferably contained in an amount of 0.0005% or more. A more preferred lower limit when Ca is contained is 0.0010%, and a more preferred lower limit when Mg is contained is 0.0010%.

(C) Se: 0.5% or less (excluding 0%)
Se is an element that exerts an action of suppressing the corrosion reaction by suppressing the pH drop of the site where the corrosion dissolution reaction is occurring and improving the corrosion resistance. Inclusion of such Se makes it difficult for local pH changes to occur, and thus has an effect of improving corrosion uniformity. In addition, in the “gap portion” where the mass transfer is restricted and the pH is likely to decrease, the effect (local corrosion inhibition effect) is effectively exhibited for the reason described above. However, if the content exceeds 0.5%, workability and weldability deteriorate. For these reasons, the Se content was set to 0.005 to 0.50%. The upper limit with more preferable Se content is 0.45%, More preferably, it is 0.4%. Se is preferably 0.005% or more. The more preferable lower limit of the Se content is 0.008%, and more preferably 0.01%.

(D) Sb: 0.5% or less (not including 0%) and / or Sn: 0.5% or less (not including 0%)
Sb and Sn are elements that enhance the corrosion resistance by promoting the effect of densification of rust produced by Cu, Ni, Ti, etc. and the effect of lowering the pH by Se, Ca, Mg, etc. However, if excessively contained, workability and weldability deteriorate, so Sb and Sn are each preferably 0.5% or less. A more preferred upper limit when Sb is contained is 0.4%, and a more preferred upper limit when Sn is contained is 0.4%. Both Sb and Sn are preferably contained in an amount of 0.01% or more. A more preferred lower limit when containing Sb is 0.02%, and a more preferred lower limit when containing Sn is 0.02%.

(E) B: 0.01% or less (not including 0%), V: 0.1% or less (not including 0%), and Nb: 0.05% or less (not including 0%) One or more elements selected from the group In marine steel materials, higher strength may be required depending on the site of application, but B, V, and Nb are elements necessary for such strength improvement.

  Among these, B is an element which improves hardenability and acts effectively on strength improvement. However, if the content exceeds 0.01%, the base material toughness and the HAZ toughness deteriorate, which is not preferable. Therefore, B is preferably 0.01% or less. A more preferred upper limit is 0.009%. B is preferably contained in an amount of 0.0001% or more. A more preferred lower limit is 0.0003%.

  V is an element that effectively acts on strength improvement. However, if it exceeds 0.1% and is contained excessively, it will lead to deterioration of the toughness of steel and HAZ, which is not preferable. Therefore, V is preferably 0.1% or less. More preferably, it is 0.07% or less. V is preferably contained in an amount of 0.003% or more. A more preferred lower limit is 0.005%.

  Nb is an element that effectively acts on strength improvement. However, if it exceeds 0.05% and is contained excessively, the toughness deterioration of the steel material and the deterioration of the HAZ toughness are caused. Therefore, Nb is preferably 0.05% or less. More preferably, it is 0.045% or less. It is preferable to contain Nb 0.003% or more. A more preferred lower limit is 0.005%.

  The marine steel material according to the present invention satisfies the above requirements, and the production method thereof is not particularly limited. For example, if the method shown below is adopted, the marine steel material satisfying the above requirements can be produced. it can. That is, the average equivalent circle diameter of the ferrite grains at the t / 2 position was 20.0 μm or less, and the average aspect ratio of the ferrite grains at the t / 4 position was 2.0 or less. The slab may be heated to 1000 to 1200 ° C., then roughly rolled, then immediately forced-water cooled to cool to the austenite non-recrystallization temperature range, and then finish-rolled in this austenite non-recrystallization temperature range. In the following, description will be given in order.

  It is preferable that the temperature which heats a slab shall be 1000-1200 degreeC. This is to reversely transform austenite in order to refine the ferrite structure obtained after rough rolling and subsequent cooling (natural cooling or forced water cooling). That is, normally, the ferrite is reversely transformed into austenite by heating to about 900 ° C., but it is effective to roll and recrystallize the austenite structure in order to refine the ferrite structure after rolling. Therefore, although the lower limit of the recrystallization temperature of austenite depends on the chemical composition of the steel material, it is usually 850 to 900 ° C. Therefore, in order to roll and recrystallize the austenite structure above this lower temperature, the heating temperature is 1000 ° C. It is good to be the above. Preferably it shall be 1050 degreeC or more. The heating temperature is preferably controlled by calculating an average temperature (calculated value) in the thickness direction of the steel slab using a process computer.

  However, if the temperature exceeds 1200 ° C., the initial austenite structure becomes too coarse, and it is difficult to sufficiently refine the austenite structure even if the austenite structure is rolled and recrystallized. Also, heating at a high temperature is uneconomical in terms of energy. Therefore, the heating temperature is preferably 1200 ° C. or lower. More preferably, it shall be 1100 degrees C or less.

  The heated slab may be roughly rolled at a cumulative reduction rate of 50% or more in the austenite recrystallization temperature range. This is because the austenite structure can be recrystallized by rough rolling at a cumulative reduction ratio of 50% or more in the recrystallization temperature region of austenite, and the ferrite structure after rolling can be refined. That is, even if the cumulative reduction ratio in the recrystallization temperature range of austenite is less than 50%, the ferrite grains can be refined by increasing the cumulative reduction ratio in the austenite non-recrystallization temperature range described later. However, if the ferrite grains are coarsened at the time of starting rolling in the austenite non-recrystallization temperature range, the metal structure finally obtained is coarse ferrite even if rolled appropriately in the austenite non-recrystallization temperature range. This is because it tends to be a mixed grain state in which grains and fine ferrite grains are mixed. When in a mixed grain state, the CTOD characteristics tend to be difficult to stabilize. Therefore, to sufficiently refine the austenite structure in the austenite recrystallization temperature region, it is recommended that the cumulative rolling reduction in the austenite recrystallization temperature region be 50% or more.

  The cumulative rolling reduction is preferably as large as possible, and as the cumulative rolling reduction increases, the equivalent circle diameter of the ferrite grains can be reduced to about 25 to 30 μm. However, even if the cumulative rolling reduction in the recrystallization temperature range of austenite exceeds 70%, the effect is almost saturated, so the cumulative rolling reduction may be about 70% or less.

  The temperature range at which the rough rolling is performed is the austenite recrystallization temperature range, but this temperature range varies somewhat depending on the chemical composition of the steel material. However, since the lower limit of the recrystallization temperature of austenite is usually about 850 to 900 ° C., the cumulative rolling reduction in the temperature range of 900 ° C. or higher may be adjusted to the above range. However, if the rolling temperature range is too high, the austenite grains grow faster following the recrystallization after rolling, so that it may not be possible to effectively reduce the size. However, although such grain growth occurs at 1000 ° C. or less, if the rolling is performed again within 60 seconds after the end of rolling, the structure in a finer state than the structure before rolling can be maintained. Therefore, the rolling start temperature is preferably 1000 ° C. or less.

The cumulative rolling reduction is as follows when the steel piece thickness at 1000 ° C. is t ₀ and the steel piece thickness at 900 ° C. is t _{1 at} the t / 2 position of the steel piece (calculated value): It can be calculated by a formula.
Cumulative rolling reduction (%) = [(t ₀ −t ₁ ) / t ₀ ] × 100 (a)

However, when the rough rolling start temperature is lower than 1000 ° C., the steel slab thickness at the start of the rough rolling is t _0, and when the rough rolling start temperature exceeds 1000 ° C., the temperature at the t / 2 position of the steel slab. The above-mentioned cumulative rolling reduction is calculated with the steel slab thickness at 1000 ° C. as t ₀ . On the other hand, when the rough rolling end temperature does not reach 900 ° C. (when it exceeds 900 ° C.), the steel slab thickness at the end of rough rolling is t _1, and when the rough rolling end temperature is lower than 900 ° C., 900 the billet thickness at ℃ calculates the cumulative rolling reduction as t _1.

  The temperature at which the rough rolling is performed is preferably based on the temperature calculated by calculating the temperature at the t / 2 position using a process computer when the thickness of the steel material is t (mm). This is to represent the temperature inside the steel material.

  However, the recrystallized grain size of austenite tends to saturate the refinement effect by rough rolling if the cumulative rolling reduction is 50% or more as described above.

  In addition, compared with the temperature of t / 2 position (calculated value), the temperature of the steel sheet surface (actually measured value) is lower by about 50 to 70 ° C. when the steel material thickness is 150 mm, and when the steel material thickness is 100 mm. Is about 40-50 ° C. lower. Further, the temperature (measured value) on the surface of the steel sheet is lower by about 30 to 40 ° C. when the thickness of the steel material is 150 mm than the temperature (calculated value) at the t / 4 position, and when the thickness of the steel material is 100 mm. Is about 25-30 ° C. lower. Therefore, the temperature at which the rough rolling is performed may be controlled using the temperature of the steel sheet surface (measured value) or the temperature at the t / 4 position (calculated value) as a reference in consideration of such a temperature difference.

  After rough rolling at a cumulative reduction ratio of 50% or more in the recrystallization temperature range of austenite, it is preferable to immediately perform forced water cooling to cool to the austenite non-recrystallization temperature range. This is because even if rolling is performed in the recrystallization temperature range of austenite and the crystal grains are refined, if left as it is (or air-cooled), austenite grains grow due to temperature energy, and the austenite structure becomes coarse.

  The time from the end of rough rolling to the start of water cooling should be as short as possible, for example, within 60 seconds. In addition, productivity is also improved by shortening the time until water cooling is started.

  After cooling to the austenite non-recrystallization temperature range, it is recommended to finish-roll with a true strain of 0.5 or more in the austenite non-recrystallization temperature range. This is because the ferrite grains can be further refined by finish rolling in the austenite non-recrystallization temperature range. That is, since the metal structure obtained by rolling in the austenite recrystallization temperature range is an austenite structure having an average particle size of about 25 to 30 μm, ferrite grains obtained by air cooling the steel material as it is or by forced cooling. The average equivalent-circle particle diameter of is no more than about 25 μm. Therefore, CTOD characteristics cannot be improved sufficiently. On the other hand, if the finish rolling is performed in the austenite non-recrystallization temperature range, strain is introduced into the ferrite grains, so that the ferrite grains can be further refined.

  In this finish rolling, the true strain is preferably 0.5 or more. If the true strain amount is less than 0.5, the ferrite grains may be insufficiently refined, and the CTOD characteristics may not be sufficiently improved. The larger the true strain amount, the better. The larger the amount, the smaller the ferrite grains.

The austenite non-recrystallization temperature range is a temperature range where the austenite structure is not recrystallized even when the steel material is rolled. This temperature range varies somewhat depending on the chemical composition of the steel material, but in the present invention, the final strain is introduced with a true strain introduced at a temperature of 850 ° C. or lower at the t / 2 position of the steel slab being 0.5 or higher. However, if the temperature range of finish rolling becomes too low, the flatness (ie, aspect ratio) of the ferrite grains tends to be extremely large, and the CTOD characteristics tend to deteriorate. Therefore, the finish rolling end temperature is preferably “Ar3 transformation point + 40 ° C.” or higher. The temperature of the Ar3 transformation point can be calculated by the following equation (b) based on the content of chemical components contained in the steel material.
Ar3 transformation point (° C.) = 868−369 × [C] + 24.6 × [Si] −68.1 × [Mn] −36.1 × [Ni] −20.7 × [Cu] −24.8 × [Cr] + 190 × [V]
... (b)
However, [] has shown content (mass%) of each element.

The above-mentioned true strain amount is as follows when the temperature (calculated value) at the t / 2 position of the steel slab is t ₂ at the 850 ° C. thickness and the steel slab thickness at the finish rolling finish temperature is t ₃ ( c) can be calculated by the equation
True strain = ln (t ₂ / t ₃ ) (c)

However, when the finish rolling start temperature is lower than 850 ° C., the steel slab thickness at the start of finish rolling is t _2, and when the finish rolling start temperature exceeds 850 ° C., the temperature at the t / 2 position of the steel slab. There calculates the true strain billet thickness at 850 ° C. as t _2. On the other hand, when the finish rolling end temperature does not reach “Ar3 transformation point + 40 ° C.” (in the case of “Ar3 transformation point + 40 ° C.”), the steel slab thickness at the end of finish rolling is t ₃ , and the finish rolling finish temperature Is less than “Ar3 transformation point + 40 ° C.”, the true strain is calculated with the steel piece thickness at “Ar3 transformation point + 40 ° C.” as t ₃ .

  The temperature at which the finish rolling is performed is based on both temperatures calculated by calculating the temperatures at the t / 2 position and the t / 4 position using a process computer when the thickness of the steel slab is t (mm). And This is because if the temperature of only one of the positions is managed, the metal structure at the unmanaged position may not be appropriately controlled.

  However, when the thickness of the steel material is about 10 to 30 mm, the temperature difference between the temperature inside the steel plate (temperature at the t / 4 position or t / 2 position) and the surface temperature of the steel plate is at most about 10 to 15 ° C. Therefore, in consideration of such a temperature difference, the surface temperature (measured value) of the steel sheet may be managed as a reference (for example, “850 ° C.−temperature difference”, “Ar 3 transformation point + 40 ° C.−temperature difference”).

  In addition, when managing by the surface temperature of the steel material, if rolling is started immediately after water cooling, the temperature difference between the surface and the center of the steel material is large, so the circle equivalent diameter and aspect ratio of the ferrite grains can be strictly controlled. It becomes difficult. Therefore, in this case, it is desirable to start rolling after 60 seconds or more have passed after water cooling. By leaving it for 60 seconds or more, the temperature difference between the surface and the inside of the steel sheet (that is, the t / 4 position or t / 2 position) becomes small. This is because it can be controlled. That is, if left for 60 seconds or more and the thickness of the steel material is 100 mm, the temperature difference between the surface temperature of the steel material (measured value) and the temperature at the t / 2 position (calculated value) is about 40 ° C. The temperature difference between the surface temperature (measured value) and the temperature at the t / 4 position (calculated value) is about 25 ° C. When the thickness of the steel material is 60 mm, the temperature difference between the surface temperature of the steel material (actual value) and the temperature at the t / 2 position (calculated value) is about 30 ° C., the surface temperature of the steel material (actual value) and t The temperature difference from the temperature at the / 4 position (calculated value) is about 20 ° C.

  What is necessary is just to cool in accordance with a conventional method after completion | finish of finish rolling. The cooling method is not particularly limited, and air cooling or forced cooling may be used. The cooling rate at this time is not particularly limited, but the present inventors have confirmed that the size of the ferrite grains is not affected as long as it is about 4 ° C./second or less.

  The steel material for shipbuilding of the present invention basically exhibits excellent corrosion resistance even if it is not coated, but if necessary, the tar epoxy resin paint shown in the examples below, or other than that It can be used in combination with other anticorrosion methods such as heavy duty anticorrosion coating, zinc rich paint, shop primer, and anticorrosion. When such anticorrosion coating is applied, the corrosion resistance of the coating film itself (coating corrosion resistance) is also good as shown in the examples described later.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steel materials having the chemical composition shown in the following Table 1-1 or Table 1-2 (the balance is Fe and inevitable impurities) were melted in a converter and continuously cast to obtain a slab. The obtained slab was hot-rolled under the conditions described later to produce various steel plates. In Table 1-1 and Table 1-2, the value of the content ratio of Cr and Al contained in the steel sheet ([Cr] / [Al]: mass ratio) was calculated and shown together. Moreover, based on content of the chemical component contained in a steel plate, the temperature of Ar3 transformation point was computed from the said (b) formula, and the value was also shown collectively.

  The obtained slab was heated to the heating temperature shown in the following Table 2-1 or Table 2-2, and then roughly rolled under the conditions shown in the following Table 2-1 or Table 2-2. Table 2-1 or Table 2-2 shows the rough rolling start temperature (t / 2 position) and the rough rolling end temperature (t / 2 position). As reference values, the surface temperature of the steel sheet and the temperature at the t / 4 position are also shown. The surface temperature is an actual value measured using a radial thermometer installed on the rolling line, and the temperature at the t / 4 position and the temperature at the t / 2 position are calculated values calculated using a process computer. is there.

  Tables 2-1 and 2-2 show the thickness of the steel sheet at the end of the rough rolling, and the cumulative rolling reduction in the range where the temperature (calculated value) at the t / 2 position is 1000 to 900 ° C. The result computed using (a) Formula is shown.

Incidentally, the hitting to calculate the cumulative rolling reduction, the t _0, the temperature of the t / 2 location at the start of rough rolling is 1000 if falls below ° C. The plate thickness at the start rough rolling, the 1000 ° C. or higher In this case, the plate thickness at 1000 ° C. is substituted, and for t ₁ , when the temperature at the t / 2 position at the end of rough rolling is 900 ° C. or less, the plate thickness is 900 ° C., and when it is higher than 900 ° C. The thickness was calculated by substituting the thickness at the end of rolling.

  After the rough rolling, cooling was performed by the method shown in Table 3-1 or Table 3-2, and then finish rolling was performed. Tables 3-1 and 3-2 show the time from the end of rough rolling to the start of cooling. In addition, the time from the end of cooling (at the end of rough rolling when not cooled by water) to the start of finish rolling is also shown.

  As the conditions for finish rolling, the thickness at the start of finish rolling, the finish rolling start temperature (t / 4 position), and the finish rolling end temperature (t / 4 position) are shown in Tables 3-1 and 3-2, respectively. As reference values, the surface temperature of the steel sheet and the temperature at the t / 2 position are also shown. In addition, the temperature (calculated value) at the t / 2 position is 850 ° C. or lower, and the true strain amount introduced into the temperature range until the finish rolling is finished is also shown.

  After the finish rolling, cooling was performed under the conditions shown in Table 4-1 or Table 4-2. The cooling rates shown in Table 4-1 or Table 4-2 are average values from the cooling start temperature to 500 ° C.

The temperature at the t / 4 position and the temperature at the t / 2 position of the steel slab during hot rolling were controlled by the following procedure.
1. Using a process computer, calculate the heating temperature at any position (eg, t / 4 position or t / 2 position) from the front surface to the back surface of the steel slab based on the ambient temperature from the start of heating to the end of heating and the in-furnace time. To do.
2. Using the calculated heating temperature, rolling is performed while calculating the rolling temperature at an arbitrary position in the plate thickness direction based on the data of the rolling pass schedule during rolling and the cooling method (water cooling or air cooling) between passes.
3. The surface temperature of the steel sheet is measured using a radiation type thermometer installed on the rolling line. However, the theoretical value is also calculated in the process computer.
4). The surface temperature of the steel sheet measured at the start of rough rolling, at the end of rough rolling, and at the start of finish rolling is collated with a calculated temperature calculated from a process computer.
5). If the difference between the calculated temperature and the measured temperature is ± 30 ° C or more, recalculate the calculated surface temperature so that it matches the measured temperature to obtain the calculated temperature on the process computer. The calculated temperature is used as it is.
6). Next, this temperature is used to manage the rolling temperature in the region to be controlled.

  The average equivalent circle diameter and average aspect ratio of the ferrite grains were calculated for the obtained steel sheet by the following procedure. The results are shown in Tables 4-1 and 4-2 below.

[Measurement procedure of equivalent circle diameter and aspect ratio]
A sample is cut out so that a surface including the front surface and the back surface of the steel plate is parallel to the rolling direction and perpendicular to the steel surface (the front surface of the steel material), and the exposed surface is polished. Mirror finish. For polishing the exposed surface, polishing was performed using wet emery polishing paper of # 150 to # 1000, and then mirror-finished using diamond slurry as an abrasive.

  The mirror-finished sample was corroded using a 3% nital solution to reveal the grain boundary of the ferrite structure, and then photographed at a magnification of 100 times or 400 times to obtain a photograph of 6 cm × 8 cm (that is, 100 Double is equivalent to 600 μm × 800 μm, and 400 × is equivalent to 150 μm × 200 μm). The 6 cm side of the photo corresponds to the plate thickness direction, and the 8 cm side corresponds to the rolling direction. This was taken into the image analysis apparatus so that the area corresponds to 1 mm × 1 mm or more at any magnification.

  Next, in the image analysis device, the ferrite grain region (area) surrounded by the grain boundary is converted into a circle having an equivalent area, and the diameter of the converted circle is defined as the equivalent circle diameter of the ferrite grain. The equivalent diameter was measured. This was measured for all observation visual fields, and the average equivalent circle diameter was calculated by averaging the results.

  On the other hand, as for the aspect ratio of the ferrite grains, for the ferrite grains surrounded by the grain boundaries, the grain size Dl in the rolling direction and the grain size Dt in the plate thickness direction are measured, and the ratio of Dl to Dt (Dl / Dt) is determined. Calculated as aspect ratio. This was performed for all observation visual fields, and the average aspect ratio was calculated by averaging the results.

  In addition, the measurement position of the circle equivalent diameter and the aspect ratio of the ferrite grain was set at the t / 2 position and the t / 4 position when the thickness of the steel material was t (mm). When the magnification is 100 times, the number of observation fields is at least 6, and when it is 400 times, the number of observation fields is at least 35.

  Moreover, when calculating the average equivalent circle diameter and aspect ratio of the ferrite grains, the ferrite area ratio in the metal structure was also measured. The results are shown in the following Tables 4-1 and 4-2.

  The obtained steel sheet was examined for corrosion resistance and brittle fracture occurrence characteristics according to the following procedure.

<About corrosion resistance>
The obtained steel plate was cut and subjected to surface grinding to finally produce a test piece having a size of 100 × 100 × 25 (mm) (test piece A). The external shape of the test piece A is shown in FIG.

  Further, as shown in FIG. 5, four 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 test piece A), A test piece B having a clearance was formed. The small test piece and the large test piece for forming the gap were steel materials having the same chemical composition, and the surface finish was the same as that of the test piece A. Then, a hole of 5 mmφ was formed in the center of the small test piece, and a screw hole was made on the base material side (large test piece side), and fixed with an M4 plastic screw.

  Further, a test piece C (FIG. 6) on which the tar epoxy resin coating (undercoat: zinc rich primer) having an average thickness of 250 μm was applied to the entire surface was also used. Then, in order to investigate the degree of corrosion progress when the base steel material is exposed due to scratches on the anticorrosion coating film, the cut surface reaching the base on one side of the test piece C (length: 100 mm, width: about 0) 0.5 mm) was formed with a cutter knife.

  About the test material of each chemical component composition shown in the said Table 1-1 or Table 1-2, the test piece A, the test piece B, and five test pieces C were used for the corrosion test, respectively. The corrosion test method at this time is as follows.

[Corrosion test method]
(Corrosion test A)
A corrosion test was carried out in which a moist air atmosphere such as the upper part of the ballast tank where the anti-corrosion protection does not act was simulated and sea salt particles were adhered and kept in a moist state. Specifically, 7.5 mL of real seawater collected in Kakogawa City, Hyogo Prefecture, was dropped almost uniformly onto the test surface, and the dried test piece was subjected to a constant temperature and humidity test at a temperature of 50 ° C. and a humidity of 95% RH. It was installed horizontally in the tank and corroded. The test time was 6 months, and 5.0 mL of actual seawater was added dropwise to the test surface every month. In this test, the test piece A and the test piece B were used to evaluate the general corrosion resistance, the corrosion uniformity, and the crevice corrosion resistance.

(Corrosion test B)
Simulating the corrosive environment of the upper deck in the crude oil tank, placing the test piece horizontally in a test tank maintained at a temperature of 50 ° C., composition: 5 vol% O ₂ -10 vol% CO ₂ -0. The test piece was corroded by passing a corrosive gas of 01 volume% SO ₂ -0.3 volume% H ₂ S through 1 L / min. At this time, the humidity was controlled to 98% RH or higher so that the inside of the test tank was always saturated with water vapor, and the wet state was maintained. The test time is 6 months. In this test, 5.0 mL of actual seawater was additionally dropped on the test surface every month. In this test, the test piece A and the test piece C were used to evaluate the general corrosion resistance, the corrosion uniformity, and the paint corrosion resistance.

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

  Table 5 below shows evaluation criteria for the overall corrosion resistance (D-ave), corrosion uniformity (D-max / D-ave), crevice corrosion resistance (D-crev), and coating corrosion resistance (maximum swollen width). Street. The corrosion test results are shown in Tables 6-1 and 6-2 below.

<About brittle fracture characteristics>
The brittle fracture occurrence characteristics were evaluated based on the results of a crack tip opening displacement test (CTOD test) defined by WES1108 (established on February 1, 1995) issued by the Japan Welding Association (WES). As the test piece, P.I. of WES1109 (established in 1995) was used. 6 “standard three-point bending test piece” shown in FIG. 6 was used. The test temperature was −40 ° C., and δc− _{40 ° C.} (mm) was measured. In this invention, the case where (delta) c- ₄₀ degreeC is 0.20 mm or more is set as a pass. The results of the CTOD test are shown in Tables 6-1 and 6-2 below.

  These results can be considered as follows. Even when the content of Al, Cu and Cr does not satisfy the appropriate range specified in the present invention (No. 2 to 6), the overall corrosion resistance is slightly improved compared to the conventional ordinary steel (No. 1). Yes. However, there is no improvement in corrosion uniformity and paint corrosion resistance.

  On the other hand, those containing appropriate amounts of Al, Cu, and Cr (Nos. 7 to 53) are all greatly improved in overall corrosion resistance due to the synergistic effect resulting from the addition of these elements. It can be seen that the crevice corrosion resistance and the paint corrosion resistance are also improved. It is thought that the protective action of the stable oxide corrosion protective film in which Al oxide and Cr oxide coexist and the dense rust film formed by containing Cu synergistically contribute to such corrosion resistance improvement. It was.

  Of these, in addition to the combined use of Al, Cu and Cr, by further containing an element for improving corrosion resistance such as Ni, Co, Ti, Ca and Mg (No. 16 to 53), the overall corrosion resistance of the steel material is greatly It can be seen that it has improved. In particular, by including Ca and Mg, improvement in corrosion uniformity and crevice corrosion resistance has been recognized (No. 20-23, No. 26-29, etc.), and suppression of local pH reduction of these elements It is presumed that local corrosion was suppressed by the action.

  Further, by containing Ni or Co, the effect of improving the coating corrosion resistance was recognized (No. 16 to 19 etc.), and the progress of corrosion at the scratches on the coating film was prevented by the synergistic effect of the rust densification action of these elements. Inferred.

  Furthermore, it is clear that the corrosion resistance is significantly improved by containing Se (No. 44 to 47, etc.), and the local pH change suppression effect by Se is improved in the corrosion resistance against local corrosion such as crevice corrosion. It was thought that it contributed to. No. As is apparent from the results of 7 to 15 and the like, it can be seen that by appropriately adjusting the value of ([Cr] / [Al]), various corrosion resistances are greatly improved.

  However, among those containing appropriate amounts of Cr and Al (Nos. 7 to 53), the average equivalent circle diameter of the ferrite grains at the t / 2 position exceeds 20.0 μm, or the ferrite at the t / 4 position. An example in which the average aspect ratio of the grains exceeds 2.0 is inferior in brittle fracture occurrence characteristics.

  On the other hand, an example in which the average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less and the average aspect ratio of the ferrite grains at the t / 4 position is 2.0 or less is a brittle fracture occurrence characteristic. It turns out that it is excellent.

FIG. 1 is a graph showing the relationship between the average equivalent circle diameter of ferrite grains at the t / 2 position, the average aspect ratio of ferrite grains at the t / 2 position, and CTOD characteristics. FIG. 2 is a graph showing the relationship between the average equivalent circle diameter of ferrite grains at the t / 2 position, the average aspect ratio of ferrite grains at the t / 4 position, and CTOD characteristics. FIG. 3 is a graph showing the relationship between the location of the metal structure and the brittle fracture occurrence characteristics (δc− _{40 ° C.} ). FIG. 4 is an explanatory view showing the external shape of the test piece A used in the corrosion resistance test. FIG. 5 is an explanatory view showing the external shape of the test piece B used in the corrosion resistance test. FIG. 6 is an explanatory view showing the external shape of the test piece C used in the corrosion resistance test.

Claims (7)

C: 0.01 to 0.2% (meaning mass%, the same shall apply hereinafter)
Si: 0.01 to 0.5%,
Mn: 0.01-2%
Al: 0.05-0.5%
Cu: 0.010 to 1.5%,
Cr: each containing 0.010 to 1%,
P: 0.02% or less (excluding 0%) and S: 0.01% or less (not including 0%), respectively,
The balance is steel made of Fe and inevitable impurities,
Excellent corrosion resistance and brittle fracture occurrence characteristics characterized by satisfying the following (1) to (3) when the metallographic structure of the plane parallel to the rolling direction of the steel material and perpendicular to the steel material surface is observed. Marine steel.
(1) The ferrite area ratio is 75% or more.
(2) The average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less.
(3) The average aspect ratio of ferrite grains at the t / 4 position is 2.0 or less.
However, t means the thickness (mm) of steel materials.   The marine steel material according to claim 1, wherein a ratio value ([Cr] / [Al]) of the Cr content [Cr] and the Al content [Al] is 1 to 15. As other elements,
Ni: 2% or less (excluding 0%),
The marine vessel according to claim 1 or 2, comprising one or more selected from the group consisting of Co: 1% or less (not including 0%) and Ti: 0.1% or less (not including 0%). Steel material. As other elements,
The marine steel material according to any one of claims 1 to 3, containing Ca: 0.02% or less (not including 0%) and / or Mg: 0.02% or less (not including 0%). As other elements,
The steel material for ships according to any one of claims 1 to 4, containing Se: 0.5% or less (not including 0%). As other elements,
The marine steel according to any one of claims 1 to 5, which contains Sb: 0.5% or less (not including 0%) and / or Sn: 0.5% or less (not including 0%). As other elements,
B: 0.01% or less (excluding 0%),
7. One or more selected from the group consisting of V: 0.1% or less (not including 0%) and Nb: 0.05% or less (not including 0%) The steel for marine described in 1.