JP4579838B2 - Marine steel with excellent corrosion resistance and brittle crack stopping properties - Google Patents

Description

  The present invention relates to a 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, and particularly in an environment exposed to salinity or 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 marine steel material, in order to ensure the safety of the hull even in a severe usage environment, it is desired to minimize the propagation region even if a brittle crack occurs. This is because if the generated brittle crack propagates over a wide area, it leads to the destruction of the hull itself. However, a steel material for marine vessels that has improved the corrosion resistance while suppressing the propagation of brittle cracks that have occurred 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 propagation of brittle cracks that have occurred. Is to provide a marine steel material in which cracks are difficult to propagate.

  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 have further improved the corrosion resistance so that it can be put into practical use without applying coating or cathodic protection for steel materials that are generally used as marine steel plates, and the steel materials in which the generated brittle cracks are difficult to propagate. Has been intensively studied to provide 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, and the chemical composition of the steel material should be adjusted appropriately, and the propagation of the brittle cracks that have occurred will be stopped. In order to (hereinafter, sometimes referred to as brittle crack stopping characteristics), the inventors have found that the metallographic structure of the steel material may be appropriately controlled, and have 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%, and P: 0.0. Suppressed to 02% or less (not including 0%) and S: 0.01% or less (not including 0%), the balance being steel material composed of Fe and inevitable impurities, and observation of the metal structure of the steel material In this case, the grit is that the average grain size of ferrite grains in the region from the steel surface to the t / 100 position is 25 μm or less. However, t means the thickness (mm) of steel materials.

  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) Sn: 0.5% or less (not including 0%), ( e) V: It is also effective to contain 0.1% or less (not including 0%). 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 furthermore, by optimizing the metal structure of the steel material, the propagation can be surely stopped even if a brittle crack occurs. 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 propagation of the brittle cracks that have occurred cannot be stopped.

  Therefore, the present inventors have studied to improve the brittle crack stopping characteristics without deteriorating the corrosion resistance improved by using Cr and Al together. As a result, when the metal structure of the steel material [thickness t (mm)] is observed, if the average grain size of the ferrite grains in the region from the steel material surface to the t / 100 position is 25 μm or less, the brittle crack stopping characteristics of the steel material It was revealed that the corrosion resistance could not be deteriorated.

  This is clear from the examples described later, and in particular, FIG. 4 shows the average grain diameter of ferrite grains and brittle crack stopping characteristics (Kca value at 0 ° C.) in the region from the steel surface to the t / 100 position. Showing the relationship. According to FIG. 4, it can be seen that the brittle fracture stopping characteristics are improved (the Kca value at 0 ° C. is increased) as the average grain size of ferrite grains in the region from the steel surface to the t / 100 position is smaller.

The reason why the brittle fracture stopping characteristics can be improved by reducing the average grain size of the ferrite grains as described above 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 a brittle crack is generated, propagation is considered to be prevented if there are dense grain boundaries in the direction in which the crack propagates. Therefore, if the ferrite grains are refined, the density of the crystal grain boundaries increases, so that the propagation can be stopped even if brittle cracks occur. And if the said average particle diameter is 25 micrometers or less, it can ensure Kca: 5900N / mm ^1.5 or more in 0 degreeC in a brittle fracture propagation stop test (refer an Example for details), and a brittle crack stop characteristic Can be improved. The average grain size of the ferrite grains is preferably 20 μm or less.

  The average grain size 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, reveals the grain boundary of the ferrite structure, and is photographed at a magnification of 100 or 400 times from the steel surface to the t / 100 position. . The photographed photograph is subjected to image analysis, the length of the ferrite grains recognized in the observation visual field in the thickness direction is measured, and the average of these is taken as the average grain diameter of the ferrite grains.

  The average grain size of the ferrite grains is observed in a region from the steel surface to the t / 100 position. If the ferrite grain size in the region from the steel sheet surface to the t / 100 position is appropriately controlled, the present inventors consider that the brittle crack stopping characteristics of the entire steel sheet as well as the steel sheet surface part are improved. This is because it became clear.

  The metal structure in the region from the steel surface to the t / 100 position is mainly composed of ferrite. The ferrite main body means that the ferrite fraction is 50% by volume or more, and the ferrite area ratio may be 50% or more when the metal structure of the cross section of the steel material is observed. The area ratio of ferrite is preferably 55% or more, and more preferably 60% 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 50%, preferably less than 45%, more preferably less than 40%.

  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 anti-corrosion 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 consists of iron and inevitable impurities (for example, O).

  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 (c) Se, (d) Sn, (e) V, and the like, and the characteristics of the marine steel will be further improved according to the type of components to be contained. 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) Sn: 0.5% or less (excluding 0%)
Sn is an element that enhances the corrosion resistance by promoting the generated rust densification effect due to Cu, Ni, Ti and the like, and the pH lowering effect due to Se, Ca, Mg and the like. However, if excessively contained, workability and weldability deteriorate, so Sn is preferably 0.5% or less. A more preferable upper limit when Sn is contained is 0.4%. Sn is preferably contained in an amount of 0.01% or more. A more preferable lower limit when Sn is contained is 0.02%.

(E) V: 0.1% or less (excluding 0%)
In the case of marine steel materials, higher strength may be required depending on the part to be applied, but V is an element necessary for such strength improvement.

  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%.

  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.

  In order to reduce the average grain size of ferrite grains in the region from the steel surface to the t / 100 position to 25 μm or less, the slab obtained by casting is heated and rough-rolled, then air-cooled or forced-cooled, and then austenite-recycled. What is necessary is just to finish-roll the true strain amount to 0.5 or more in the crystallization temperature range, the austenite non-recrystallization temperature range, or the two-phase temperature range.

  This is because the ferrite grains can be refined by setting the temperature range of the finish rolling to an appropriate temperature range. That is, since the metal structure obtained by air cooling or forced cooling after rolling according to a conventional method without temperature control is a ferrite structure having an average grain size of about 35 μm or more, the brittle crack stopping property cannot be sufficiently improved. . On the other hand, if the finish rolling is performed in an appropriate temperature range, the ferrite grains can be further refined. In particular, if the finish rolling is performed in a two-phase temperature range, the ferrite grains can be directly deformed, so that a large true strain is introduced and the ferrite grains can be further refined.

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

In addition, the temperature range which performs the said finish rolling changes a little with the chemical component composition of steel materials. Therefore, in the present invention, finish rolling is performed with the true strain introduced in the region where the surface temperature of the steel material is 900 ° C. or less being 0.5 or more. However, if the temperature range of finish rolling becomes too low, the work embrittlement of the ferrite structure becomes remarkable, and the brittle crack stopping property tends to be lowered. Therefore, the finish rolling end temperature is preferably “Ar 3 transformation point −40 ° C.” or higher. The temperature of the Ar3 transformation point can be calculated by the following equation (1) 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] (1)
However, [] has shown content (mass%) of each element.

The true strain amount can be calculated by the following equation (2), where t _{s is} the thickness of a steel slab when the surface temperature of the steel is 900 ° C., and t _f is the thickness of the steel slab at the finish rolling finish temperature.
True strain _{= ln (t s / t f} ) ... (2)

However, if the finish rolling start temperature is below 900 ° C. calculates the true strain steel thickness during finish rolling start as t _s. In the case where the finish rolling start temperature exceeds 900 ° C., the surface temperature of the steel material to the billet thickness at 900 ° C. with t _s.

  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 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.

  Steels having the chemical composition shown in 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 said Formula (1), and the value was also shown collectively.

  The obtained slab was roughly rolled after heating. After the rough rolling, air cooling or forced water cooling was performed, and then finish rolling was performed. As finish rolling conditions, finish rolling finishing temperatures (surface temperatures) are shown in Tables 2-1 and 2-2 below. Moreover, the true strain amount introduced in the region where the surface temperature of the steel material is 900 ° C. or less was calculated from the above formula (2), and the results are shown in Tables 2-1 and 2-2 below. The surface temperature is an actual measurement value measured using a radiation thermometer installed on the rolling line.

  After the finish rolling, cooling was performed under the conditions shown in Table 2-1 or Table 2-2. In addition, the cooling rate shown in Table 2-1 or Table 2-2 is an average value from cooling start temperature to 500 degreeC.

  Next, the average particle diameter of the ferrite grains was calculated for the obtained steel sheet by the following procedure. The results are shown in Table 2-1 and Table 2-2 below.

[Measurement procedure of average particle size]
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, and after revealing the grain boundary of the ferrite structure, the region from the steel surface to the t / 100 position was photographed at a magnification of 100 times or 400 times, The photograph was 6 cm × 8 cm (ie, 100 × corresponds to 600 μm × 800 μm, and 400 × corresponds 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. The photographed photograph is subjected to image analysis, the length of the ferrite grains recognized in the observation visual field in the thickness direction is measured, and the average of these is taken as the average grain diameter of the ferrite grains.

  Further, when calculating the average grain size of the ferrite grains, the metal structure in the region from the steel surface to the t / 100 position was observed, and the ferrite area ratio was also measured simultaneously. As a result, the area ratio of ferrite in the metal structure was 50% or more.

  When the magnification is 100 times, the number of viewing fields is at least 6, and when it is 400 times, the number of viewing fields is at least 35.

  Next, the obtained steel sheet was examined for corrosion resistance and brittle crack stopping characteristics by 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. 2, 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. 3) on which an entire thickness of 250 μm thick tar epoxy resin coating (undercoating: zinc rich primer) was applied was 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 (length: 100 mm, width: about 0) is reached on one side of the test piece C. 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 above-mentioned test piece A and 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 the 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 4-1 and 4-2 below.

<About brittle crack stopping characteristics>
The brittle crack arresting properties are the same for four specimens by changing the temperature range in accordance with the brittle fracture propagation stopping test specified by the steel type qualification test method (established on March 31, 2003) issued by the Japan Welding Association (WES). went. The results were plotted on a 1 / T-Kca graph (X-axis is 1 / T, Y-axis is Kca), and the intersection of 4 approximate curves and 273K was Kca at 0 ° C. T means the brittle fracture stop temperature (unit: K).

The calculation results are shown in Tables 4-1 and 4-2 below. FIG. 4 shows the relationship between the average grain size of the ferrite grains and the brittle crack stopping characteristics in the region from the steel material surface to the t / 100 position. In addition, in this invention, the case where Kca in 0 degreeC is 5900 N / mm ^1.5 or more is set as a pass.

  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. 7-18, 23-38, 41-44, 47-52) are all resistant to the entire surface due to the synergistic effect of the addition of these elements. It can be seen that the corrosion resistance is greatly improved, and the corrosion uniformity, crevice corrosion resistance and 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.

  Among these, in addition to the combined use of Al, Cu and Cr, by further containing an anticorrosion improving element such as Ni, Co, Ti, Ca, Mg, etc. (No. 14-18, 23-38, 41-44, 47- 52), it can be seen that the overall corrosion resistance of the steel material is greatly improved. In particular, the inclusion of Ca and Mg has been confirmed to improve corrosion uniformity and crevice corrosion resistance (No. 25-30, etc.), and local corrosion due to the local pH lowering action of these elements. Is presumed to be suppressed.

  Further, by containing Ni or Co, an effect of improving the coating corrosion resistance was recognized (No. 14 to 18 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. 47, 48, 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 11 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 a suitable amount of Cr and Al in combination (No. 7-18, 23-38, 41-44, 47-52), the ferrite grains in the region from the steel surface to the t / 100 position Examples where the average particle size exceeds 25 μm are inferior in brittle crack stopping properties.

  On the other hand, it can be seen that the example in which the average grain size of ferrite grains in the region from the steel surface to the t / 100 position satisfies 25 μm or less is also excellent in brittle crack stopping characteristics.

FIG. 1 is an explanatory view showing the external shape of a test piece A used in a corrosion resistance test. FIG. 2 is an explanatory view showing the external shape of the test piece B used in the corrosion resistance test. FIG. 3 is an explanatory view showing the external shape of the test piece C used in the corrosion resistance test. FIG. 4 is a graph showing the relationship between the average grain size of ferrite grains and brittle crack stopping characteristics in the region from the steel surface to the t / 100 position.

Claims (5)

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,
The ratio of the Cr content [Cr] and the Al content [Al] ([Cr] / [Al]) is 1 to 15,
When the metal structure of the steel material is observed, the metal structure in the region from the steel material surface to the t / 100 position has a ferrite fraction of 50% by volume or more and an average grain size of ferrite grains of 25 μm or less. Marine steel with excellent corrosion resistance and brittle crack stopping characteristics.
[However, t means the thickness (mm) of the steel material. ] As other elements,
Ni: 2% or less (excluding 0%),
The marine steel material according to claim 1, comprising at least one selected from the group consisting of Co: 1% or less (not including 0%) and Ti: 0.1% or less (not including 0%). As other elements,
The marine steel according to claim 1 or 2, containing Ca: 0.02% or less (excluding 0%) and / or Mg: 0.02% or less (excluding 0%). As other elements,
Se: 0.5% or less (excluding 0%) and
The marine steel material according to any one of claims 1 to 3, containing Sn: 0.5% or less (not including 0%) . As other elements,
V: Steel material for ships according to any one of claims 1 to 4 , containing 0.1% or less (not including 0%).