JP2007177302A - High-tension steel material for bessel having excellent corrosion resistance and base metal toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-tension steel material for a vessel having excellent corrosion resistance and base metal toughness in an environment exposed to salt and high temperature-high humidity caused by seawater. <P>SOLUTION: The high-tension steel material for a vessel having excellent corrosion resistance and base metal toughness has a composition comprising, by mass, 0.01 to 0.20% C, 0.01 to 1% Si, 0.01 to 2% Mn and 0.005 to 0.1% Al, and further comprising 0.01 to 1% Co and 0.0005 to 0.02% Mg, and the balance Fe with inevitable impurities, and has a structure in which the fraction of insular martensite is ≤1.0%, and the balance bainite. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-strength steel material for ships excellent in corrosion resistance and base material toughness, and more particularly, to a high-tensile steel material for ships excellent in corrosion resistance and base material toughness in an environment exposed to salinity and constant temperature and 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 consumption or dropping, severe corrosion may immediately proceed.

  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 the purpose thereof is a marine steel material excellent in corrosion resistance that can be put into practical use without being subjected to painting or cathodic protection, in particular, excellent in resistance to crevice corrosion. Another object of the present invention is to provide a marine high-tensile steel material that exhibits excellent durability against salt adhesion caused by seawater and corrosion due to a wet environment, and further exhibits excellent base material toughness.

  The marine high-strength steel material of the present invention that has achieved the above object is C: 0.01 to 0.20% (meaning mass%, the same applies hereinafter), Si: 0.01 to 1%, Mn: In addition to 0.01 to 2%, Al: 0.005 to 0.1%, Co: 0.01 to 1% and Mg: 0.0005 to 0.02%, the balance being Fe and It consists of inevitable impurities and has a gist in that the fraction of island-like martensite is 1.0% or less and the balance is a bainite structure. In this marine high-tensile steel material, it is preferable to adjust the value ([Co] / [Mg]) of the Co content [Co] and the Mg content [Mg] to a range of 2 to 350.

  Further, in the high-tensile steel material for marine use of the present invention, (A) Cu: 1.5% or less (not including 0%), Cr: 1% or less (not including 0%), Ni: 2%, if necessary. 1 or more selected from the group consisting of the following (excluding 0%) and Ti: 0.1% or less (not including 0%), (B) Ca: 0.02% or less (not including 0%) (C) Mo: 0.5% or less (excluding 0%) and / or W: 0.3% or less (excluding 0%), (D) B: 0.01% or less (0% It is also effective to include 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%), etc. The characteristics of the marine steel are further improved according to the type of component to be contained.

  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 of the present invention contains a predetermined amount of Co and Mg in combination, and has a corrosion resistance that can be put into practical use without applying coating and cathodic protection by appropriately adjusting the chemical composition and manufacturing method. In particular, it has excellent durability against crevice corrosion, and also exhibits excellent durability against adhesion of salt caused by seawater and corrosion due to wet environments. Furthermore, it has high base material toughness and is useful as a raw material for outer plates, ballast tanks, crude oil tanks, etc. in ships such as crude oil tankers, cargo ships, cargo passenger ships, passenger ships, warships and the like.

  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-mentioned problems can be realized if a predetermined amount of Co and Mg are contained in combination and the chemical composition and the 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 Co and Mg 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.

  Mg exerts the action of suppressing the pH drop in the corroded portion and suppressing the corrosion reaction to improve the corrosion resistance. By the way, in the case of normal steel materials (for example, Si—Mn steel materials), the generated rust is porous, so the dissolved Mg does not stay near the steel plate surface but immediately diffuses outside (for example, in seawater). End up. That is, if Mg is contained alone, the effect of improving the corrosion resistance by Mg is not sufficiently exhibited. However, by containing Co together with Mg, a fine surface rust film is formed and the diffusion of dissolved Mg to the outside is suppressed, so that the effect of improving the corrosion resistance of Mg is fully exhibited. Further, it is considered that the corrosion resistance can be greatly improved also by a synergistic effect with the hydrolysis equilibrium reaction of dissolved Co.

  Such an effect is exhibited by controlling Mg and Co to appropriate amounts described later. However, in order to improve the corrosion resistance more reliably, the value of the ratio of these contents ([Co] / [Mg]: mass) The ratio) is preferably controlled to 2 to 350. When the above ([Co] / [Mg]) is less than 2, local corrosion is likely to be insufficiently suppressed. More preferably, it is 10 or more, More preferably, it is 20 or more. On the other hand, ([Co] / [Mg]) exceeding 350 is not preferable because the suppression of the overall corrosion becomes insufficient. More preferably, it is 100 or less, More preferably, it is 95 or less, Especially preferably, it is 80 or less, Most preferably, it is 60 or less.

  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 0.20%>
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, it is necessary to contain 0.01% or more. However, if the content exceeds 0.20%, the toughness and weldability deteriorate. For these reasons, the C content range was set to 0.01 to 0.20%. 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 0.16% or less more preferably.

<Si: 0.01 to 1%>
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 1%, weldability and HAZ toughness deteriorate. In addition, the minimum with preferable Si content is 0.02%. More preferably 0.05% or more. Moreover, the upper limit with preferable Si content is 0.8%, More preferably, it is good to set it as 0.6% 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%, It is good to set it as 0.10% or more more preferably. 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: 0.005-0.1%>
Al, like Si and Mn, is an element necessary for deoxidation and securing of strength, and if it is less than 0.005%, effects such as deoxidation cannot be sufficiently exhibited. However, if added over 0.1%, the weldability and HAZ toughness are impaired, so the Al content range was made 0.005 to 0.1%. In addition, the minimum with preferable Al content is 0.010%, It is good to set it as 0.015% or more more preferably. Moreover, the upper limit with preferable Al content is 0.09%, More preferably, it is good to set it as 0.08% or less.

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

<Mg: 0.0005-0.02%>
Since Mg exhibits a pH raising action by being dissolved, it has an action of suppressing the corrosion reaction by suppressing the pH reduction due to the hydrolysis reaction and improving the corrosion resistance in the local anode where the dissolution of iron occurs. In order to exert such effects, it is necessary to contain 0.0005% or more of Mg. However, if the Mg content exceeds 0.02%, workability and weldability deteriorate. For these reasons, the Mg content is suitably in the range of 0.0005 to 0.02%. A preferable lower limit of the Mg content is 0.0007%, and more preferably 0.0010% or more. Moreover, the upper limit with preferable Mg content is 0.018%, It is good to set it as 0.015% or less more preferably.

  The basic components in the marine steel of the present invention are as described above, and the balance is composed of Fe 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.

  Moreover, in the marine steel material of the present invention, in addition to the above components, one or more selected from the group consisting of (A) Cu, Cr, Ni and Ti, if necessary, (B) Ca, (C) Mo and / or It is also effective to contain one or more selected from the group consisting of W, (D) B, V, and Nb, and the characteristics of the marine steel are further improved according to the type of components to be contained.

<Cu: 1.5% or less (not including 0%), Cr: 1% or less (not including 0%), Ni: 2% or less (not including 0%), and Ti: 0.1% or less ( 1 or more 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 effective elements 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 that the content of any element is 0.01% or more (more preferably 0.05% or more). However, since excessive weldability, hot workability, and HAZ toughness deteriorate, Cu is 1.5% or less (more preferably 1.0% or less) and Cr is 1% or less (more preferably). 0.8% or less).

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

  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 fully exhibit the said effect, it is preferable to make it contain 0.005% or more, but when it contains exceeding 0.1% excessively, workability, weldability, and HAZ toughness will deteriorate, it is unpreferable. When Ti is contained, the more preferable lower limit is 0.008%, and the more preferable upper limit is 0.05%.

<Ca: 0.02% or less (excluding 0%)>
Ca, like Mg, has an effect of increasing pH when dissolved, and in a local anode where iron is dissolved, it suppresses a pH decrease due to a hydrolysis reaction, suppresses a corrosion reaction, and improves corrosion resistance. It is an effective element. Such an effect by Ca is effectively exerted by adding 0.0005% or more, but if it exceeds 0.02%, the workability and weldability are deteriorated. The more preferable lower limit when Ca is contained is 0.0010%, and the more preferable upper limit 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 the effect of increasing the uniformity of corrosion and suppressing perforations due to local corrosion. In particular, the effect of improving the uniform corrosivity is remarkably exhibited by the inclusion at the same time as Co. In order to exhibit such an effect, it is preferable to contain 0.01% or more in any case. However, if excessively contained, the weldability and HAZ toughness deteriorate, and the cost increases significantly. Therefore, it is preferable that Mo is 0.5% or less and W is 0.3% or less. The more preferable lower limit when Mo is contained is 0.02%, and the more preferable upper limit is 0.3%. Moreover, a more preferable lower limit when W is contained is 0.02%, and a more preferable upper limit 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 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 base material toughness and the HAZ toughness deteriorate, which is not preferable. In order to improve the strength by V, it is preferable to contain 0.003% or more, but if it exceeds 0.1%, it is not preferable because it causes deterioration of the toughness of the steel and HAZ toughness. In order to increase the strength by Nb, it is effective to contain 0.003% or more. However, if it exceeds 0.05%, it is not preferable because it causes deterioration of toughness and HAZ toughness of steel materials. 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 0.0090% for B, 0.07% for V, and 0.03% 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 1.0% 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 with vTrs of −40 ° C. or lower, it is necessary to suppress the MA fraction to 1.0% or lower. More preferably, the MA fraction is 0.7% 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 anticorrosion. When such anticorrosion coating is applied, the corrosion resistance of the coating film itself (coating corrosion resistance) is also good as shown in the examples described later.

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

  Steel materials having the chemical composition shown in 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 and Ms shown in Table 1 are obtained by creating a CCT curve by a machining for master test.

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, and the observation surface was mirror-polished and then corroded with a repeller corrosion solution. . 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. 3, 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. 4) 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. Then, in order to investigate the degree of corrosion progress when the coating material for anticorrosion is scratched and the base steel material is exposed, a cut scratch (length: 100 mm, width: about 0.1 mm) reaching the base on one side of the test piece C. 5 mm) was formed with a cutter knife.

  Experiment No. 2 shown in Table 2 above. For each, five test pieces A, B and C were prepared, and the corrosion test was performed by the following method.

[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. The constant temperature and humidity test was carried out by placing the test material at an angle of 60 ° from the horizontal in a test tank adjusted to a temperature of 60 ° C. and a humidity of 95%. Seawater spray test: 4 hours, constant temperature and humidity test: 4 hours as one cycle, these were alternately performed to corrode the specimen. The total test time was 6 months.
(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 apparatus.
(C) About the test piece C (with cut flaws) which performed the coating process, the ratio (swelling area ratio) 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 above general corrosion resistance (D-ave), corrosion uniformity (D-max / D-ave), crevice corrosion resistance (D-crev), and coating corrosion resistance (blowing area ratio and maximum swollen width) are as follows: As shown in Table 3. 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).

  No. containing no Co No. 2 and Co content is less than the lower limit specified in the present invention. In the case of No. 4, the overall corrosion resistance is slightly improved as compared with the conventional steel (No. 1) due to the effect of addition of Mg, but no improvement is observed except for the general corrosion resistance.

  Moreover, No. which does not contain Mg. No. 3 and the Mg content are less than the lower limit specified in the present invention. In the case of No. 5, the uniformity of corrosion is slightly improved compared to the conventional steel (No. 1) due to the effect of addition of Co, but there is no improvement except for the uniformity of corrosion. It does not have excellent corrosion resistance as a steel material.

  Furthermore, no. In Nos. 16 and 17, since cooling at a recommended cooling rate was performed to a temperature range lower than Ms, the MA fraction was large, and excellent base material toughness could not be secured. No. In No. 18, since cooling at a rate lower than the recommended cooling rate was completed at a temperature higher than Bf, ferrite was generated and the desired strength was not obtained. No. Since No. 21 was cooled at a speed lower than the recommended cooling speed, 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 is understood that a combination containing Co and Mg in an appropriate amount is superior to the conventional steel (No. 1) because of the synergistic effect of these elements, and is preferable as a marine corrosion resistant steel. . In particular, it can be seen that the corrosion resistance of the steel material is further improved by including an element for improving corrosion resistance such as Cu, Cr, Ni, Ti, Ca, Mo and W in addition to the combined use of Co and Mg.

  Among these, in the test material containing an appropriate amount of one or more selected from the group consisting of Cu, Cr, Ni and Ti, the effect of reducing the maximum swollen width of the paint test material is recognized (No. 9-11 etc.). ), It is assumed that the rust densification of these elements acts to stabilize the rust of the cut portion and suppresses the progress of corrosion. In addition, Ca has an effect of increasing crevice corrosion resistance (No. 12, 20 to 23, etc.), and it is considered that Ca further strengthens the suppression of pH reduction in the crevice and reduces corrosion. Furthermore, it can be seen that the addition of Mo and W is very effective in improving the corrosion uniformity and the paint swellability (Nos. 31 to 35). It can also be seen that various corrosion resistances are greatly improved by appropriately adjusting the value of ([Co] / [Mg]).

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 0.20% (meaning of mass%, the same applies hereinafter), Si: 0.01 to 1%, Mn: 0.01 to 2%, Al: 0.005 to 0.1%, respectively In addition, it contains Co: 0.01 to 1% and Mg: 0.0005 to 0.02%, the balance is made of Fe and inevitable impurities, and the fraction of island martensite is 1.0% or less And the balance is a bainite structure. A marine high-tensile steel material excellent in corrosion resistance and base metal toughness.   2. The high-tensile steel material for ships 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.   Further, Cu: 1.5% or less (not including 0%), Cr: 1% or less (not including 0%), Ni: 2% or less (not including 0%), and Ti: 0.1% or less The high-tensile steel material for ships according to claim 1 or 2, comprising at least one selected from the group consisting of (not including 0%).   Furthermore, the high-tensile steel material for ships according to any one of claims 1 to 3, further comprising Ca: 0.02% or less (not including 0%).   Furthermore, Mo: 0.5% or less (not including 0%) and / or W: 0.3% or less (not including 0%), the ship height according to any one of claims 1 to 4. Tensile steel.   Further, from the group consisting of B: 0.01% or less (excluding 0%), V: 0.1% or less (not including 0%), and Nb: 0.05% or less (not including 0%) The high-tensile steel material for ships according to any one of claims 1 to 5, which contains one or more selected.