JP4444924B2 - High tensile steel for marine vessels with excellent corrosion resistance and base metal toughness - Google Patents

Description

  The present invention relates to a high-tensile steel material for ships excellent in corrosion resistance and base material toughness, and particularly to a high-strength steel material for ships excellent in corrosion resistance and base material toughness in an environment exposed to salinity or high temperature and high humidity due to seawater. Is.

  Steel materials used as main structural materials (for example, outer plates, ballast tanks, crude oil tanks, etc.) in various ships are often corroded because they are exposed to salt from seawater 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.

  As conventional anticorrosion means, painting, cathodic protection and the like can be cited as general means. However, in the case of the above-described coating typified by heavy coating, there is a high possibility that a coating film defect exists, and the coating film may be damaged due to a collision or the like in the manufacturing process, so that the base steel material is often exposed. In such an exposed steel material, the steel material corrodes locally and intensively, which leads to early leakage of the petroleum-based liquid fuel.

  On the other hand, cathodic protection is very effective for the part completely immersed in seawater, but the part that receives seawater splashes in the atmosphere does not form the electrical circuit necessary for anticorrosion, and the anticorrosion effect is sufficient. May not be demonstrated. In addition, when the galvanic anode for anticorrosion disappears due to abnormal wear or dropout, severe corrosion may proceed immediately.

  In addition to the above technique, for example, a technique as disclosed in Patent Document 1 has been proposed as an improvement in the corrosion resistance of the steel material itself. This technology discloses a corrosion-resistant steel for shipbuilding that has excellent corrosion resistance by appropriately adjusting the chemical composition of the steel material and can be used even without coating. Further, Patent Document 2 discloses a marine steel material whose coating film life is improved 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, in the above technology, although improvement of corrosion resistance is being worked on, excellent base material toughness that can cope with E grade (Charpy impact test value at −20 ° C. of 55 J or more) required for marine high-tensile steel It has not been studied to be provided at the same time, and realization of a marine steel material excellent in both corrosion resistance and base metal toughness is desired.
JP, 2000-17381, A Claims etc. JP, 2002-266052, A Claims etc.

  The present invention has been made in view of the above circumstances, and its purpose is to provide corrosion resistance that can be put into practical use without painting or cathodic protection (particularly, the upper part in a ballast tank or a crude oil tank upper deck where cathodic protection does not work). Another object of the present invention is to provide a high-tensile steel material for marine vessels that is excellent in durability against crevice corrosion in a humid atmospheric atmosphere such as, and that exhibits excellent base material toughness.

  The high-tensile steel material for marine use of the present invention capable of achieving the above object is C: 0.01 to 0.2% (meaning of mass%, the same shall apply hereinafter), Si: 0.01 to 0.5%, Mn: 0.01-2%, Al: 0.05-0.5%, Cu: 0.010-1.5%, Cr: 0.010-1%, P: 0.02 % Or less (not including 0%) and S: 0.01% or less (not including 0%), respectively, the balance is made of Fe and inevitable impurities, and the fraction of island martensite is 1.1. % And the balance is a bainite structure. In this marine steel material, it is preferable to adjust the ratio value ([Cr] / [Al]) of the Cr content [Cr] and the Al content [Al] to a range of 1 to 15.

  In the high-tensile steel for marine use of the present invention, (A) Ni: 2% or less (not including 0%), Co: 1% or less (not including 0%), and Ti: 0.1%, if necessary. One or more selected from the group consisting of the following (not including 0%), (B) Ca: 0.02% or less (not including 0%) and / or Mg 0.02% or less (not including 0%) (C) Se: 0.5% or less (excluding 0%), (D) Sb: 0.5% or less (excluding 0%) and / or Sn: 0.5% or less (0% (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%) It is also effective to contain one or more selected from the group consisting of, It is improved.

  In addition, the said island-like martensite fraction shall say the value measured by the method shown in the Example mentioned later.

  The marine steel material of the present invention contains a predetermined amount of Al and Cr in combination, and by appropriately adjusting the chemical composition and manufacturing method, corrosion resistance that can be put into practical use without coating and cathodic protection ( In particular, it exhibits excellent durability against crevice corrosion in wet air atmosphere such as the upper part of the ballast tank where the anti-corrosion protection does not work and the upper deck of the crude oil tank, etc. It is useful as a material for outer panels in ships such as cargo ships, freight passenger ships, passenger ships and warships.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, it is found that a high-strength steel material for a ship that can solve the above-described problems can be realized if a predetermined amount of Al and Cr are contained in combination and the chemical composition and manufacturing method are appropriately adjusted. Completed the invention. Hereinafter, the present invention will be described in detail.

  In the steel material of the present invention, it is important to contain Al and Cr in combination, and the object of the present invention cannot be achieved without any of these components. Although each effect in these components is mentioned later, the reason that corrosion resistance improved by using these together is considered as follows.

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

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 increases when the pH exceeds 8.5. The pH of seawater exposed to marine steel is about 8 when it is clean, but it may be alkalized to 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 the marine environment do not necessarily exist stably, but rather are easily dissolved and the protective properties due to the Al oxide film are often lost. In contrast, Cr oxide has a high stability in the alkaline region, and has the effect of lowering the pH due to the hydrolysis equilibrium of Cr ions dissolved in a small amount. Depresses dissolution and exerts the effect of securing its protection. Therefore, it is considered that the anticorrosion 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 Al and Cr to appropriate amounts described later. However, in order to improve the corrosion resistance more reliably, the value of the ratio of these contents ([Cr] / [Al]: mass) It is preferable to appropriately control the ratio. If the above ([Cr] / [Al]) is less than 1, the corrosion uniformity tends to be insufficient. More preferably, it is 3 or more. On the other hand, when ([Cr] / [Al]) exceeds 15, the crevice corrosion resistance becomes insufficient. More preferably, it is 10 or less.

  In the present invention, it is necessary to appropriately adjust components such as C, Si, Mn, Cu, P, and S in order to satisfy the basic characteristics as a steel material. The reasons for limiting the ranges of these components will be described below together with the effects of the above Al and Cr 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 as a structural member of a ship, that is, approximately 400 MPa (depending on the thickness of the steel material used), it is necessary to contain 0.01% or more. However, if the content exceeds 0.2%, the toughness and weldability deteriorate. For these reasons, the C content range was set to 0.01 to 0.2%. In addition, the minimum with preferable C content is 0.02%, More preferably, it is good to set it as 0.04% or more. Moreover, the upper limit with preferable C content is 0.18%, It is good to set it as 0.17% or less more preferably.

<Si: 0.01 to 0.5%>
Si is an element necessary for deoxidation and securing strength, and if it is less than 0.01%, the minimum strength as a structural member cannot be secured. However, if the content exceeds 0.5%, weldability and HAZ toughness deteriorate. In addition, the minimum with preferable Si content is 0.02%, More preferably, it is 0.05% or more, More preferably, it is 0.1% or more. Moreover, the upper limit with preferable Si content is 0.45%, More preferably, it is 0.4% or less.

<Mn: 0.01-2%>
Like Si, Mn is an element necessary for deoxidation and securing strength, and if it is less than 0.01%, the minimum strength as a structural member cannot be secured. However, if the content exceeds 2%, the toughness deteriorates. In addition, the minimum with preferable Mn content is 0.05%, More preferably, it is good to set it as 0.1% or more, More preferably, it is 0.3% or more. Moreover, the upper limit with preferable Mn content is 1.8%, It is good to set it as 1.6% or less more preferably.

<Al: 0.05 to 0.5%>
As described above, Al has an effect of forming a stable oxide anticorrosive film on the steel surface. When the Al content is low, the Al ³⁺ ions that have been dissolved by corrosion are scattered in the seawater and are not deposited on the surface of the steel material, and no anticorrosion film is formed. In order to exhibit a sufficient anticorrosion effect in the presence of Cr oxide, it is necessary to contain Al by 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 the proper range, weldability equivalent to that of conventional steel can be ensured even if the Al content is in the range of more than 0.1% to 0.5%. However, if the Al content exceeds 0.5%, the 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 0.06%, More preferably, it is 0.07% or more, More preferably, it is good to set it as 0.08% or more. Moreover, the upper limit with preferable Al content is 0.45%, More preferably, it is 0.4% or less, More preferably, it is good to set it as 0.35% or less.

<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. Is demonstrated. In order to exert such effects, it is necessary to contain 0.010% or more. However, if Cu is excessively contained, the weldability and hot workability deteriorate, so the content is preferably made 1.5% or less. In addition, a preferable lower limit when Cu is contained is 0.05%. Moreover, a preferable upper limit is 1.3%, More preferably, it is 1% or less.

<Cr: 0.010 to 1%>
Cr, like Al, exhibits the effect of forming a stable oxide film on the steel 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, but in order to exert such effects, it is necessary to contain 0.010% or more. . However, if it is contained excessively, weldability deteriorates, so it is necessary to make it 1% or less. In addition, the minimum with preferable Cr content is 0.05%, More preferably, it is 0.1% or more. Moreover, a preferable upper limit is 0.9%, More preferably, it is 0.8% or less.

<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. For these reasons, 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% or less.

<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 0.01%, and if it exceeds this, weldability as a steel material for ships cannot be ensured. For these reasons, 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 Fe and unavoidable impurities (for example, O, etc.), but other components that do not impair the properties of the steel (for example, Zr, N etc.) is also 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 of the present invention, in addition to the above components, if necessary, (A) Ni: 2% or less (not including 0%), Co: 1% or less (not including 0%) and Ti: 0 1 or more selected from the group consisting of 1% or less (excluding 0%), (B) Ca: 0.02% or less (excluding 0%) and / or Mg 0.02% or less (0% (Not including), (C) Se: 0.5% or less (not including 0%), (D) Sb: 0.5% or less (not including 0%) and / or Sn: 0.5% or less ( (E) B: 0.01% or less (not including 0%), V: 0.1% or less (not including 0%), and Nb: 0.05% or less (0%) It is also effective to include one or more selected from the group consisting of, and the properties of marine steels depending on the type of component to be included It is further improved.

<One or more selected from the group consisting of Ni: 2% or less (not including 0%), Co: 1% or less (not including 0%), and Ti: 0.1% or less (not including 0%) >
Ni, Co and Ti are all effective elements for improving corrosion resistance. Of these, Ni and Co are effective elements for the formation of a dense surface rust film that greatly contributes to the improvement of corrosion resistance, and in order to exert such an effect, 0.01% or more in any of the elements. It is preferable to do. However, if it is excessively contained, weldability and hot workability deteriorate, and a significant cost increase is caused. Therefore, it is preferable that Ni is 2% or less and Co is 1% or less. The more preferable lower limit when Ni is contained is 0.05%, more preferably 0.1% or more. The more preferable lower limit when Co is contained is 0.015%, and more preferably 0.03% or more. The upper limit with more preferable Ni is 1.5%, More preferably, it is 1% or less. The more preferable upper limit of Co is 0.8%, and more preferably 0.6% or less.

  Ti is an element that improves the environmental barrier properties by densifying the surface rust coating that greatly contributes to the improvement of the corrosion resistance, and also suppresses the corrosion inside the crevice and improves the crevice corrosion resistance. In order to exhibit the said effect fully, it is preferable to make it contain 0.005% or more, but when it contains exceeding 0.1% excessively, workability and weldability will deteriorate, it is unpreferable. In addition, the more preferable lower limit in the case of containing Ti is 0.008%. Moreover, a more preferable upper limit is 0.090%, and further preferably 0.05% or less.

<Ca: 0.02% or less (not including 0%) and / or Mg 0.02% or less (not including 0%)>
Ca and Mg have an effect of increasing pH when dissolved, and in a local anode where dissolution of iron occurs, it is effective to suppress a decrease in pH due to a hydrolysis reaction to suppress a corrosion reaction and to improve corrosion resistance. It is an element. Such an effect is effectively exhibited by setting it to 0.0005% or more when any element is contained. However, if excessively contained, workability and weldability deteriorate, so the upper limit of each of Ca and Mg is preferably 0.02%. The more preferable lower limit when these elements are contained is 0.001%, the more preferable upper limit is 0.015%, and further preferably 0.01% or less.

<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. Such an action makes it difficult for local pH changes to occur, so that the corrosion uniformity is improved. In particular, the above-mentioned effect (local corrosion inhibitory effect) is effectively exhibited in a “gap portion” where mass transfer is limited and local pH reduction is likely to occur. In order to ensure the corrosion resistance required in such an environment, the Se content is preferably 0.005% or more. However, if the content exceeds 0.5%, workability and weldability deteriorate. A more preferable lower limit of the Se content is 0.008%, and more preferably 0.010% or more. Moreover, the upper limit with more preferable Se content is 0.45%, More preferably, it is good to set it as 0.40% or less.

<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. In order to exert such an effect, it is preferable that the content is 0.01% or more when any element is contained. However, since workability and weldability deteriorate when contained excessively, it is preferable to make it 0.5% or less in the case of containing either Sb or Sn. A more preferable lower limit when these elements are contained is 0.02%, and a more preferable upper limit is 0.4%.

<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 types>
Depending on the site of application to the ship, further enhancement of strength may be necessary, but these elements are effective elements for further improving the strength. Among these, B is an element effective for improving the hardenability and increasing the strength, and in order to exert the effect, it is preferable to contain 0.0001% or more. However, if an excessive amount of B exceeding 0.01% is contained, the toughness of the base material deteriorates, which is not preferable. In order to improve the strength by V, the content is preferably 0.003% or more, but if it exceeds 0.1%, it is not preferable because it causes toughness deterioration of the steel material. Further, in order to increase the strength with Nb, it is effective to contain 0.003% or more, but if it exceeds 0.05%, it is not preferable because it causes toughness deterioration of the steel material. More preferable lower limits of these elements are 0.0003% for B, 0.005% for V, and 0.005% for Nb. The more preferable upper limit is 0.009% for B, 0.07% for V, and 0.045% for Nb.

  In order to combine high strength and excellent base material toughness, it is necessary that the metal structure has an island-like martensite fraction in the entire structure of 1.1% or less and the balance is a bainite structure. In high-strength steels mainly composed of bainite structure, martensite-Austenite constituent (hereinafter sometimes referred to as “MA”), which is a hard phase, is likely to form, which becomes the starting point of fracture and adversely affects the base material toughness. It is because it exerts. FIG. 1 is a graph showing the relationship between the fraction of island-like martensite (MA fraction) and vTrs (fracture surface transition temperature), and is a summary of the experimental results of Examples described later. From this, it can be seen that in order to obtain a steel material exhibiting excellent base material toughness of vTrs: −40 ° C. or lower, it is necessary to suppress the MA fraction to 1.1% or lower. More preferably, the MA fraction is 0.8% or less.

  The “remaining bainite structure” as used in the present invention means that the bainite structure occupies 90% or more of the entire structure, and besides the bainite structure, other structures that can be inevitably formed in the manufacturing process (ferrite, pearlite). , MA) is intended to contain 10% or less in total.

  In order to obtain the above structure, a steel material satisfying the above component composition is used. After the hot rolling in the production process is finished, the finish rolling finish temperature is not higher than the bainite transformation end temperature (Bf) and not higher than the martensitic transformation start temperature (Ms). It is recommended to cool to a temperature range at 6 ° C./s or more. Thus, by controlling the cooling rate to Bf or less, the structure can be mainly composed of bainite. On the other hand, by controlling the cooling rate to Ms or more, MA in which untransformed austenite is a hard phase. Can be sufficiently suppressed. The finish rolling end temperature is the temperature at the t (plate thickness) / 4 portion at the end of finish rolling determined in the manner shown in the examples described later.

  Examples of the method for cooling at the speed include direct quenching and accelerated cooling, but it is recommended to adopt a direct quenching method capable of realizing a theoretical limit cooling rate.

  Moreover, the temperature of the heating performed in the case of hot rolling should just be 950-1200 degreeC, and the finish rolling completion | finish temperature at the time of hot rolling (temperature of the t / 4 site | part calculated | required in the way shown in the Example mentioned later) is 800- Controlling the temperature within the range of 900 ° C. is preferable because both high strength and high toughness can be achieved.

  The high-tensile steel material for marine use 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 It can also be used in combination with other anticorrosion methods such as heavy duty anticorrosion coating, zinc rich paint, shop primer and the like. 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 Table 1 below were melted in a converter and hot-rolled on a continuously cast slab (slab thickness as shown in Table 2) to produce a steel plate having a thickness shown in Table 2. .

  Bf (Bainite transformation end temperature) and Ms (Martensite transformation start temperature) shown in Table 1 are respectively obtained by creating a CCT curve by a processing for master test. Specifically, the processed formaster specimen is heated to 1100 ° C. and held for 10 seconds, then processed at 1000 ° C. with a cumulative rolling reduction of 25%, further processed at 800 ° C. with a cumulative rolling reduction of 25%, and then 700 The cooling rate from 0 ° C. was changed in 7 steps between 1 to 100 ° C./s, and the temperature at which the volume change during cooling was measured to determine the transformation temperature. Further, the final structure was identified by observing the cooled structure and measuring the Vickers hardness. From these results, a CCT curve was created to obtain Bf and Ms.

Table 2 shows the temperature of heating performed at the time of hot rolling, finish rolling end temperature (temperature at the t / 4 portion at the end of finish rolling), cooling rate after finish rolling, and cooling end temperature at the cooling rate. In Table 2, the surface temperature (measured value) at the end of finish rolling is also shown for reference. The temperature at the t / 4 portion at the end of the finish rolling is determined in the following manner (1) to (6).
(1) In the process computer, based on the atmospheric temperature from the start of heating to the end of heating and the in-furnace time, the heating temperature at an arbitrary position in the thickness direction from the front surface to the back surface of the steel slab is calculated.
(2) Using the calculated heating temperature, calculate the rolling temperature at any position in the plate thickness direction based on the rolling pass schedule during rolling and the cooling method between the passes (water cooling or air cooling). Rolling while calculating using a suitable method.
(3) The steel sheet surface temperature is measured using a radiation type thermometer installed on the rolling line (however, calculation is also performed on a process computer).
(4) The steel plate surface temperature measured at the start of rough rolling, at the end of rough rolling and at the start of finish rolling is collated with the calculated temperature on the process computer.
(5) When the difference between the calculated temperature at the start of rough rolling, at the end of rough rolling and at the start of finish rolling and the above measured temperature is ± 30 ° C or more, recalculate so that the measured surface temperature matches the calculated surface temperature. The calculated temperature on the process computer.
(6) The above calculated temperature is corrected to determine the finish rolling finish temperature at the t / 4 part.

  Using the steel sheet obtained as described above, the metal structure was observed, and the mechanical properties (tensile properties, impact properties) and corrosion resistance were evaluated.

<Observation of metal structure>
The fraction of island martensite was measured as follows. That is, a sample was taken from the steel sheet so that the cross section including the front and back surfaces of the steel sheet, which was parallel to the rolling direction and perpendicular to the steel sheet surface, could be observed. . Then, t (plate thickness) / 4 portion was photographed with an optical microscope at a magnification of 1000 times (1 visual field size: 60 μm × 80 μm), and the main structure was bainite (B) or ferrite (F) + pearlite ( P), if the main organization is bainite, the photograph taken is taken into an image analysis device, processed in black and white, and then the area ratio of the white portion (MA) was obtained. .
The above measurement was performed for any three visual fields, and the average value was taken as the MA fraction.

<Evaluation of mechanical properties>
[Evaluation of tensile properties]
A JIS Z 2201 No. 4 test piece was taken from the t / 4 part of each steel plate in a direction perpendicular to the rolling direction, and subjected to a tensile test according to the procedure of JISZ 2241 to measure the tensile strength (TS). And YP of 390 MPa or more and TS of 510 MPa or more (YP: ship class standard value of 390 MPa class steel material) were evaluated as having high tension and having tensile properties as marine steel material.

[Evaluation of base metal toughness]
A V-notch test piece of JIS Z 2202 was taken from the t / 4 part of each steel plate, and subjected to a Charpy impact test in the manner of JISZ 2242, and the fracture surface transition temperature (vTrs) was measured. And the thing whose vTrs is -40 degrees C or less was evaluated as being excellent in base-material toughness [The ship E grade steel material specification value (55J or more at -20 degrees C can be ensured stably)].

  These results are also shown in Table 2.

<Evaluation of corrosion resistance>
[Sample preparation]
The 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. 4, 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.

  Furthermore, a test piece C (FIG. 5) was prepared in which tar epoxy resin coating (undercoating: zinc rich primer) having an average thickness of 250 μm was applied to the entire surface. As shown in FIG. 5, in order to investigate the degree of corrosion progress when the coating material for anticorrosion is scratched and the base steel material is exposed, cut scratches reaching the base material on one side of the resin-coated steel plate (length: 100 mm, width) : About 0.5 mm) was formed with a cutter knife.

  For each of the following corrosion test A and corrosion test B, the experiment No. A corrosion test was conducted by the following method using five test pieces A, B and C, respectively.

[Corrosion test method]
A corrosion test was conducted in which a moist air atmosphere such as the upper part of the ballast tank where the cathodic protection does not act was simulated and the sea salt particles were adhered and kept in a wet state (corrosion test A). 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, overall corrosion resistance and corrosion uniformity were evaluated using the test piece A, and crevice corrosion resistance was evaluated using the test piece B.

In addition, simulating the corrosive environment of the upper deck in the crude oil tank, the test piece was installed horizontally in a test tank maintained at a temperature of 50 ° C, composition: 5vol% O ₂ -10vol% CO ₂ -0.01vol% SO _The test piece was corroded by aeration of ₂ -0.3 vol% H ₂ S corrosive gas at 1 L / min (corrosion test B). 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 was 6 months. In addition, 5.0 mL of actual seawater was added dropwise to the test surface every month. In this test, overall corrosion resistance and corrosion uniformity were evaluated using the test piece A, and coating corrosion resistance was evaluated using the test piece C.
(A) For test piece A, the change in weight before and after the test is converted into an average thickness reduction amount D-ave (mm), and the average value of five test pieces is calculated. 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. In addition, the weight measurement and the plate thickness measurement after the test were performed after removing corrosion products such as iron rust by the cathodic electrolysis method [JIS K8284] in an aqueous solution of diammonium hydrogen citrate.
(B) 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 device to evaluate crevice corrosion resistance.
(C) For the test piece C (with cut flaws) subjected to the coating treatment, the swollen width of the coating film in the direction perpendicular to the cut flaws is measured with a caliper, and the maximum value of five test pieces is defined as the maximum swollen width. The coating corrosivity was evaluated by this maximum swollen width.

  Table 3 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 Table 4 below.

  Tables 1 to 4 can be considered as follows (note that the following numbers indicate the experiment numbers in Tables 2 and 4).

  First, when the contents of Al, Cu and Cr do not satisfy the range defined in the present invention (No. 2 to 6), the overall corrosion resistance is slightly improved compared to the conventional steel (No. 1). There is no improvement in corrosion uniformity, crevice corrosion resistance, and paint corrosion resistance.

  Furthermore, no. In Nos. 12 and 13, since cooling at a recommended cooling rate was performed to a temperature range lower than Ms, the MA fraction was large and excellent base metal toughness could not be secured. No. In No. 14, cooling at a rate lower than the recommended cooling rate was completed at a temperature higher than Bf, so ferrite was generated and the desired strength was not obtained. No. No. 16 was cooled at a rate lower than the recommended cooling rate, so that ferrite was generated and the strength was insufficient.

  In contrast, a steel material that satisfies the requirements defined in the present invention has excellent base material toughness and excellent corrosion resistance. That is, it can be seen that a material containing appropriate amounts of Al, Cu, and Cr has improved overall corrosion resistance, corrosion uniformity, crevice corrosion resistance, and coating corrosion resistance due to the synergistic effect of these elements. It is considered that such an improvement in corrosion resistance is synergistically contributed by the protective action of a stable oxide anticorrosive film in which Al oxide and Cr oxide coexist and a dense rust film formed by containing Cu. .

  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, Mg, etc. (No. 10, 11, 19-23, etc.) It can be seen that the corrosivity is greatly improved. In particular, the inclusion of Ca and Mg has been confirmed to improve corrosion uniformity and crevice corrosion resistance (No. 20, 23, etc.), and the local pH lowering action by these elements can be improved locally. It is presumed that corrosion was suppressed.

  In addition, by containing Ni, Co or Ti, the effect of improving the coating corrosion resistance is recognized (No. 19, 22, etc.), and the progress of corrosion at the scratches on the coating film is prevented by the synergistic effect of the rust densification action of these elements. It is inferred that

  Furthermore, it is clear that the corrosion resistance is significantly improved by containing Se (No. 29, 30 etc.), and the local pH change suppression effect by Se is improved in the corrosion resistance against local corrosion such as crevice corrosion. It is thought that it contributes to. It can also be seen that various corrosion resistances are significantly improved by appropriately adjusting the value of ([Cr] / [Al]).

It is a graph which shows the relationship between the fraction (MA fraction) of island-like martensite, and vTrs (fracture surface transition temperature). It is explanatory drawing which shows the external appearance shape of the test piece A used for the corrosion resistance test. It is explanatory drawing which shows the external appearance shape of the test piece B used for the corrosion resistance test. It is explanatory drawing which shows the external appearance shape of the test piece C used for the corrosion resistance test.

Claims (6)

C: 0.01 to 0.2% (meaning 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%, P: 0.02% or less (excluding 0%) and S: 0.01% or less (0 %), The balance is made of Fe and inevitable impurities, and the ratio of Cr content [Cr] to Al content [Al] ([Cr] / [Al]) is 1 10 a and, island martensite fraction is high-tensile steel for ship and balance excellent in corrosion resistance and the base material toughness, which is a bainite structure at 1.1% or less. Furthermore, Ni: 2% or less (not including 0%), Co: 1% or less (not including 0%), and Ti: 0.1% or less (not including 0%) marine high-tensile steel material according to claim 1 containing more. The marine high-tensile steel material according to claim 1 or 2 , further comprising Ca: 0.02% or less (not including 0%) and / or Mg 0.02% or less (not including 0%). Furthermore, Se: 0.5% or less (0% is not included) The high-tensile steel material for ships according to any one of claims 1 to 3 . Furthermore, Sb: 0.5% or less (not including 0%) and / or Sn: 0.5% or less (not including 0%), the ship height according to any one of claims 1 to 4. Tensile steel. Furthermore, B: 0.01% or less (not including 0%), and V: claim 1-5 containing one or more selected from 0.1% or less (not including 0%) good Li Cheng group The high-tensile steel material for ships according to any one of the above.