JP4502949B2 - Marine steel with excellent corrosion resistance and brittle crack stopping properties - Google Patents

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, etc., particularly in environments exposed to salt and constant 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-mentioned various vessels are often corroded because they are exposed to seawater salt and constant 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 causes the steel material to corrode 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 Co and Mg in combination 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 materials according to the present invention that have solved the above-mentioned problems are: C: 0.01 to 0.2% (meaning of mass%, the same applies hereinafter), Si: 0.01 to 1%, Mn : 0.01 to 2%, Al: 0.005 to 0.1%, Co: 0.010 to 1% and Mg: 0.0005 to 0.02%, the balance being Fe And a steel material composed of inevitable impurities, which has a gist in that when the metal structure of the steel material is observed, the average grain size of ferrite grains in the region from the steel material surface to the t / 100 position is 25 μm or less. is there. However, t means the thickness (mm) of steel materials.

  In the marine steel material of the present invention, the ratio value ([Co] / [Mg]) of the Co content [Co] and the Mg content [Mg] can be adjusted to a range of 2 to 350. preferable.

  Moreover, in the marine steel material of the present invention, as required, as other elements, (a) Cu: 1.5% or less (not including 0%), Cr: 1% or less (including 0%) 1) at least one element selected from the group consisting of Ni: 2% or less (excluding 0%), Ti: 0.1% or less (not including 0%), (b) Ca: 0.02 % Or less (not including 0%), (c) Mo: 0.5% or less (not including 0%) and / or W: 0.3% or less (not including 0%), or (d) 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%) It is also effective to contain more than seed elements. 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 Co and Mg is 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 Co and Mg 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.

  Mg is an element that exhibits the effect of improving the corrosion resistance by suppressing the pH reduction in the corroded portion and suppressing the corrosion reaction. In such a component system of normal steel materials (for example, Si—Mn steel materials), such an action is porous because the generated rust is porous, and the dissolved Mg does not stay near the steel plate surface but immediately diffuses outside (for example, in seawater). It will end up. Therefore, when Mg is contained alone, the effect of improving the corrosion resistance is small. However, by including Co together with Mg, a fine surface rust film is formed, and diffusion of Mg to the outside is suppressed. Further, it is considered that the corrosion resistance can be greatly improved by a synergistic effect with the hydrolysis equilibrium reaction of dissolved Co.

  Such an effect is exhibited by controlling the contents of Co and Mo contained in the steel material to an appropriate range to be described later. The value of the content ratio of these elements ([Co] / [Mg]: mass) It is preferable to appropriately control the ratio. That is, if this value ([Co] / [Mg]) is less than 2, local corrosion is likely to be insufficiently suppressed, and if it exceeds 350, overall corrosion is likely to be insufficiently suppressed. Accordingly, the value of [Co] / [Mg] is preferably 2 to 350, more preferably a lower limit of 10, a further preferable lower limit of 20, a more preferable upper limit of 100, a still more preferable upper limit of 95, and a particularly preferable upper limit. Is 80, most preferably 60.

  As described above, in the steel material of the present invention, Co and Mg are used in combination in order to improve the corrosion resistance. However, even if Co and Mg 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 Co and Mg 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.

  That is, this is clear from the examples described later. In particular, FIG. 4 shows the average grain size 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. Shows the relationship. According to FIG. 4, it can be seen that the brittle fracture stop characteristic is improved (the numerical value is increased) as the average grain size of the ferrite grains in the region from the steel surface to the t / 100 position becomes 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. If the average particle size is 25 μm or less, it is possible to ensure Kca: 5900 N / mm ^1.5 or more at 0 ° C. in a brittle fracture propagation stop test (refer to the examples 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 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, and Al need to 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 above Co and Mg 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 (depending on the thickness of the steel material used, it is approximately 400 MPa), it is necessary to contain 0.01% or more. The minimum with preferable C content is 0.02%, More preferably, you may be 0.04% or more. However, if the content exceeds 0.2%, the toughness and weldability 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 0.16% or less.

Si: 0.01 to 1%
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, you may be 0.05% or more. However, if the content exceeds 1%, weldability and HAZ toughness deteriorate. Therefore, Si is made 1% or less. The upper limit with preferable Si content is 0.8%, More preferably, you may be 0.6% or less.

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, you may be 0.10% or more. However, if the content exceeds 2%, the toughness deteriorates. Therefore, Mn is 2% or less. The upper limit with preferable Mn content is 1.80%, More preferably, it is 1.60% or less.

Al: 0.005 to 0.1%
Al is also necessary for deoxidation and securing strength in the same manner as Si and Mn, and if it is less than 0.005%, the deoxidation effect cannot be obtained. Therefore, Al is made 0.005% or more. The minimum with preferable Al content is 0.010%, More preferably, you may be 0.015% or more. However, since addition exceeding 0.1% impairs weldability and HAZ toughness, the upper limit of the amount of Al added is set to 0.1%. The upper limit with preferable Al content is 0.09%, More preferably, it is 0.08% or less.

Co: 0.01 to 1%
Co is an indispensable element for forming a dense surface rust film that greatly contributes to improving the corrosion resistance of steel in a high salinity environment. In order to exert such an effect, the Co content needs to be 0.01% or more. The minimum with preferable Co content is 0.015%, More preferably, you may be 0.020% or more. However, if the content exceeds 1%, weldability and HAZ toughness deteriorate. For these reasons, the upper limit of the Co content is set to 1%. The upper limit with preferable Co content is 0.8%, More preferably, it is good to set it as 0.6% or less.

Mg: 0.0005 to 0.02%
Since Mg exhibits a pH raising action by being dissolved, it has a function of suppressing a pH reduction due to a hydrolysis reaction in a local anode where iron is dissolved, thereby suppressing a corrosion reaction and improving corrosion resistance. In order to exert such an effect, it is necessary to contain 0.0005% or more of Mg. A preferable lower limit of the Mg content is 0.0007%, and more preferably 0.0010% or more. However, if it exceeds 0.02%, workability and weldability are deteriorated. For these reasons, the upper limit of the Mg content is set to 0.02%. The upper limit with preferable Mg content is 0.018%, More preferably, it is good to set it as 0.015% or less.

  The basic components in the marine steel of the present invention are as described above, and the balance is composed of iron and inevitable impurities (for example, P, S, O, etc.), but does not impair the properties of the steel other than these. Some components (eg, 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 Cu, Cr, Ni and Ti, (b) Ca, (c) It is also effective to contain one or more elements selected from the group consisting of Mo and / or W, (d) B, V, and Nb, etc., and the characteristics of the marine steel material according to the type of components to be contained. This will be further improved. The reasons for limiting the ranges of these components will be described below.

Cu: 1.5% or less (not including 0%), Cr: 1% or less (not including 0%), Ni: 2% or less (not including 0%), Ti: 0.1% or less (0 One or more elements selected from the group consisting of Cu, Cr, Ni and Ti are all effective elements for improving corrosion resistance.

  Among these, Cu and Cr are elements effective for forming a dense surface rust film that greatly contributes to the improvement of corrosion resistance, like Co. In order to exhibit such an effect, it is preferable to contain 0.01% or more of all. In the case where Cu and Cr are contained alone or in combination, a more preferred lower limit of each is 0.05%. However, if excessively contained, weldability, HAZ toughness, and hot workability deteriorate, so it is preferable that Cu be 1.5% or less and Cr be 1% or less. A more preferable upper limit of the Cu content is 1.0%, and a more preferable upper limit of the Cr content is 0.8%.

  Ni is an element effective for stabilizing a dense surface rust film that greatly contributes to the improvement of corrosion resistance. In order to exert such an effect, Ni is preferably contained in an amount of 0.01% or more. A more preferable lower limit when Ni is contained is 0.05%. However, when the Ni content is excessive, weldability and hot workability deteriorate. In addition, excessive addition increases the cost significantly. Therefore, Ni is preferably 2% or less. A more preferable upper limit when Ni is contained is 1.5%.

  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. In order to ensure the corrosion resistance required in such an environment, it is preferable to contain 0.005% or more. A more preferable lower limit when Ti is contained is 0.008%. However, if the content exceeds 0.1%, the workability, weldability, and HAZ toughness are deteriorated. Therefore, Ti is preferably 0.1% or less. A more preferable upper limit when Ti is contained is 0.05%.

Ca: 0.02% or less (excluding 0%)
Ca, like Mg, exhibits an effect of increasing pH by dissolving, and suppresses a corrosion reaction by suppressing a pH decrease due to a hydrolysis reaction in a local anode where iron is dissolved, and is an element effective for improving corrosion resistance. It is. Such an effect by Ca is effectively exhibited by containing 0.0005% or more. A more preferable lower limit when Ca is contained is 0.0010%. However, if over 0.02% is contained, workability and weldability are deteriorated. Therefore, Ca is preferably 0.02% or less. A more preferable upper limit when Ca is contained is 0.015%.

Mo: 0.5% or less (not including 0%) and / or W: 0.3% or less (not including 0%)
Mo and W have an effect of increasing the uniformity of corrosion and suppressing perforation due to local corrosion. In particular, when these elements are contained simultaneously with Co, a remarkable effect of improving uniform corrosion is exhibited. In order to exhibit such an effect, it is preferable to contain 0.01% or more of any element. A more preferred lower limit when Mo is contained is 0.02%, and a more preferred lower limit when W is contained is 0.02%. However, if it is contained excessively, weldability and HAZ toughness are deteriorated, and the cost is greatly increased. Therefore, it is preferable that Mo is 0.5% or less and W is 0.3% or less. A more preferred upper limit when Mo is contained is 0.3%, and a more preferred upper limit when W is contained is 0.2%.

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 Marine steel materials may require higher strength depending on the site to which they are applied, but B, V and Nb are elements necessary for such strength improvement.

  Among these, B is an element which improves hardenability and effectively acts on strength improvement, and is preferably contained in an amount of 0.0001% or more. A more preferred lower limit is 0.0003%. 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 0.0090%.

  V is an element that effectively acts to improve strength, and is preferably contained in an amount of 0.003% or more. A more preferred lower limit is 0.005%. 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.

  Nb is an element that effectively acts to improve strength, and is preferably contained in an amount of 0.003% or more. A more preferred lower limit is 0.005%. However, if it exceeds 0.05% and is contained excessively, the toughness deterioration of the steel material and the deterioration of the HAZ toughness will be caused. Therefore, Nb is preferably 0.05% or less. More preferably, the content is 0.03% or less.

  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, in order to make the average grain size of ferrite grains in the region from the steel surface to t / 100 position 25 μm or less, the slab obtained by casting is heated and rough-rolled, then air-cooled or forcedly cooled, What is necessary is just to finish-roll the true strain amount as 0.5 or more in the austenite recrystallization 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] + 29.6 × [Mo] + 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 ratio of Co and Mo contained in the steel sheet ([Co] / [Mg]: 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. 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.

  Tables 2-1 and 2-2 show finish rolling finish temperatures (surface temperatures) as finish rolling conditions. 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.

  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, 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]
First, a combined cycle corrosion test was conducted by simulating a marine environment and repeating a seawater spray test and a constant temperature and humidity test.

In the seawater spray test, the specimen (each test piece A to C) was tilted at an angle of 60 ° from the horizontal, and was placed in a test tank, and 35 ° C artificial seawater (salt water) was sprayed in the form of a mist. Spraying of salt water was continuously performed. At this time, in the test tank, the spray amount was adjusted in advance so that 1.5 ± 0.3 mL of artificial seawater was collected at an arbitrary position per hour on a circular dish having an area of 80 cm ² installed horizontally.

  In the constant temperature and humidity test, a test material (each test piece A to C) was installed at an angle of 60 ° from the horizontal in a test tank adjusted to a temperature of 60 ° C. and a humidity of 95%.

  The seawater spray test was performed for 4 hours and the constant temperature and humidity test was performed for 4 hours as one cycle, and these were alternately performed to corrode the specimen. The total test time was 6 months.

  (1) For the test piece A, the mass change before and after the test 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 ratio (bulging area rate) of the coating film swollen area in the surface which formed the cut flaw after a test was measured. The swollen area ratio was determined by a lattice point method (lattice interval 1 mm). That is, an average value of five test pieces was obtained by defining a swelling area ratio by dividing the number of lattice points where swelling was observed by the total number of lattice points. In addition, the swollen width of the coating film in the direction perpendicular to the cut flaw was measured with calipers, and the maximum value of five test pieces was defined as the maximum swollen width.

  The evaluation criteria for the overall corrosion resistance (D-ave), corrosion uniformity (D-max / D-ave), crevice corrosion resistance (D-crev), and coating corrosion resistance (blowing area ratio and maximum blister width) are as follows. As shown in Table 3. The corrosion test results are shown in Tables 4-1 and 4-2 below.

<About brittle crack stopping characteristics>
The brittle crack arresting property is the same as that of the four specimens by changing the temperature range according to the brittle fracture propagation stopping test defined by the steel type qualification test method (established on March 31, 2000) 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 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. No. containing no Co or Mg No. 2 and 3, No. in which the content of Co or Mg is less than the lower limit specified in the present invention. In the cases of Nos. 4 and 5, the general corrosion resistance is slightly improved as compared with the conventional steel (No. 1) due to the addition effect of Co or Mg. However, no. No. 2 and No. in which the amount of Co is insufficient. In the case of 4, the improvement effect is not recognized by the corrosion uniformity and the swollen area ratio. No. No Mg is contained. No. 3 and No. with insufficient Mg content. In the case of No. 5, no improvement effect is observed in the crevice corrosion resistance and the maximum swollen width, and the corrosion resistance of the marine steel is insufficient.

  On the other hand, those containing a suitable amount of Co and Mg (Nos. 6 to 55) have a synergistic effect due to the addition of these elements and are superior in corrosion resistance to the conventional steel (No. 1). It can be seen that it is preferable as a corrosion-resistant steel for shipbuilding.

  In particular, in addition to the combined use of Co and Mg, it can be seen that the corrosion resistance of the steel material is further improved by further containing a corrosion resistance improving element such as Cu, Cr, Ni, Ti, Ca, Mo and W.

  Among these, in the test material to which Cu, Cr, Ni or Ti is added, the effect of reducing the maximum swollen width of the paint test material is recognized (No. 13-17, 20-54, etc.), It is inferred that the rust densification acts on the rust stabilization of the cut part to suppress the corrosion progress.

  In addition, Ca has an effect of increasing crevice corrosion resistance (No. 18 to 19, 25 to 29, 32 to 40, 43 to 44, 49 to 54, etc.), and Ca further strengthens suppression of pH decrease in the gap. Therefore, it is thought that corrosion was reduced.

  Furthermore, it can be seen that the addition of Mo or W is very effective in improving the corrosion uniformity and the paint swellability (No. 45 to 52, etc.).

  However, among those containing a suitable amount of Co and Mg (Nos. 6 to 55), examples in which the average grain size of ferrite grains in the region from the steel surface to the t / 100 position exceeds 25 μm are brittle cracks. Inferior stopping characteristics.

  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 (6)

C: 0.01 to 0.2% (meaning mass%, the same shall apply hereinafter)
Si: 0.01 to 1%,
Mn: 0.01-2%
Al: other than 0.005 to 0.1% respectively,
Co: 0.010 to 1% and Mg: 0.0005 to 0.02%,
The balance is steel made of Fe and inevitable impurities,
When observing the metal structure of the steel material,
A marine steel material excellent in corrosion resistance and brittle crack stopping characteristics, characterized in 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.   2. The marine steel material according to claim 1, wherein a ratio value ([Co] / [Mg]) of the Co content [Co] and the Mg content [Mg] is 2 to 350. 3. As other elements,
Cu: 1.5% or less (excluding 0%),
Cr: 1% or less (excluding 0%),
Ni: 2% or less (excluding 0%),
The marine steel material according to claim 1 or 2, comprising at least one selected from the group consisting of Ti: 0.1% or less (not including 0%). As other elements,
The steel material for ships according to any one of claims 1 to 3, containing Ca: 0.02% or less (not including 0%). As other elements,
The marine steel according to any one of claims 1 to 4, containing Mo: 0.5% or less (not including 0%) and / or W: 0.3% or less (not including 0%). As other elements,
B: 0.01% or less (excluding 0%),
6. 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 material for ships described.