JP2007177286A - Steel material for vessel having excellent haz toughness and corrosion resistance upon high heat input welding - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material for a vessel having excellent corrosion resistance (particularly, durability to coercive corrosion) to be utilizable even without applying coating and electrolytic protection in an environment exposed to salt and high temperature-high humidity caused by seawater, and further having excellent HAZ (heat affected zone) toughness upon high heat input welding. <P>SOLUTION: The steel material for a vessel having excellent corrosion resistance and HAZ toughness upon high heat input welding has a composition comprising, by mass, 0.01 to 0.2% C, 0.01 to 1% Si, 0.01 to 2% Mn, 0.005 to 0.1% Al, 0.005 to 0.03% Ti and 0.003 to 0.015% N, and further comprising 0.010 to 1% Co and 0.0005 to 0.02% Mg, and the balance Fe with inevitable impurities, and in which the number of TiN based inclusions per unit area is ≥1.0×10<SP>8</SP>(pieces/mm<SP>2</SP>). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to marine corrosion resistant steel used as a main structural material in ships such as crude oil tankers, cargo ships, cargo passenger ships, passenger ships, warships, etc., particularly in environments exposed to salinity and constant temperature and humidity due to seawater. The present invention relates to a marine steel having excellent corrosion resistance and excellent HAZ (heat affected zone) toughness during high heat input welding.

  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, (a) coating, (b) cathodic protection, and the like are well known as anticorrosion means used so far.

  Of these, in coatings represented by heavy coating, there is a high possibility that coating film defects exist, 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 a steel exposed portion, the steel material corrodes locally and intensively, leading to early leakage of the petroleum-based liquid fuel contained therein.

  On the other hand, in the anti-corrosion, it is very effective for the part completely immersed in the seawater, but in the part that receives the seawater splash in the atmosphere, the electric circuit necessary for the anticorrosion is not formed, and the anticorrosion The effect may not be fully exhibited. 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 a means for improving 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. 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 under a more severe corrosive environment is still not sufficient, and further improvement in corrosion resistance is required. In particular, corrosion (so-called crevice corrosion) in the “clearance” portion formed at the contact portion between the foreign material and the steel material, the structural reason, the damaged portion of the anticorrosion coating film, etc. becomes prominent, and the life may be shortened. The techniques proposed so far have insufficient corrosion resistance in these areas.

  Patent Document 2 also discloses that the toughness of the base material and the welded portion is improved in addition to the coating film life of the steel material by adjusting the chemical component composition. In this regard, Patent Document 2 only describes adjusting the chemical component composition as a means for improving toughness. Moreover, in the Example of patent document 2, the toughness of the base material and the welding part in the small heat input welding of 5 kJ / mm is investigated.

When marine steel is used as a thick material that can be used in a ballast tank, it is necessary that the high heat input welding characteristics, particularly the HAZ toughness during high heat input welding be good. However, the technologies proposed so far have hardly obtained marine steel materials that are excellent in HAZ toughness during high heat input welding in addition to corrosion resistance.
Japanese Patent Application Laid-Open No. 2000-17381, claims, etc. JP 2002-266052 A, Claims and Examples

  The present invention has been made paying attention to the circumstances as described above, and its purpose is such that it can be put into practical use without being subjected to coating or cathodic protection in an environment where it is exposed to salt or constant temperature and humidity due to seawater. An object of the present invention is to provide a marine steel having excellent corrosion resistance (particularly durability against crevice corrosion) and excellent HAZ toughness during high heat input welding.

The marine steel material of the present invention that has achieved the above object is C: 0.01 to 0.2% (meaning of mass%, the same shall apply hereinafter), Si: 0.01 to 1%, Mn: 0.00. In addition to containing 01 to 2%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.03%, N: 0.003 to 0.015%, Co: 0.010 to 1 % and Mg: containing from .0005 to 0.02%, the balance being Fe and inevitable impurities, the number of TiN inclusions per unit area, 1.0 × 10 ⁸ (pieces / mm ²⁾ or more It has a gist in that. In this marine 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 marine steel material of the present invention, (1) Cu: 1.5% or less (not including 0%), Cr: 1% or less (not including 0%), Ni: 2% or less (if necessary) 1 or more selected from the group consisting of (not including 0%), (2) Ca: 0.02% or less (not including 0%), (3) Mo: 0.5% or less (not including 0%) ) And / or W: 0.3% or less (not including 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%), it is also effective to contain one or more selected from the group consisting of, and the characteristics of the marine steel material according to the type of component to be contained. This will be further improved.

  In the present invention, a marine steel material having excellent corrosion resistance that can be put into practical use without being subjected to painting and anticorrosion by containing a predetermined amount of Co and Mg in combination and appropriately adjusting the chemical composition. Could be manufactured. Furthermore, the marine steel material of the present invention can exhibit good HAZ toughness even with high heat input welding by including a large number of TiN inclusions. Such marine steel materials are useful as raw 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.

  As a result of intensive studies by the present inventors, it is possible to achieve a good corrosion resistance by adding a predetermined amount of Co and Mg in a marine steel material and adjusting the chemical composition appropriately, and TiN. It has been found that the presence of a large number of system inclusions can improve the HAZ toughness during high heat input welding, and the present invention has been completed.

  In the steel material of the present invention, it is important to first contain Co and Mg in combination, and none of these components can achieve the excellent corrosion resistance that is the object of the present invention. Although each effect of these components will be described later, the reason why the corrosion resistance is improved by using these components in combination can be considered as follows.

  Mg exerts the action of suppressing the corrosion drop by suppressing the pH drop at the corroded portion and improving the corrosion resistance. In such a component system of a normal steel material (for example, Si—Mn steel material), such an action is porous because the generated rust is porous, and the dissolved Mg immediately diffuses to the outside (for example, in seawater) without staying in the vicinity of the steel plate surface. 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. In addition, it was 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 to an appropriate amount described later, but the value of the ratio of these contents ([Co] / [Mg]: mass ratio) should also be appropriately controlled. Is preferred. 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 not sufficiently suppressed. Therefore, the value of [Co] / [Mg] is desirably about 2 to 350. The lower limit of the value of [Co] / [Mg] is preferably 10, more preferably 20, and the upper limit is preferably 100, more preferably 95, still more preferably 80, and particularly preferably 60.

Furthermore, in this invention, HAZ toughness is improved by making many TiN type compounds exist in steel materials. In the first place, the HAZ toughness is lowered by welding because the γ grains in the portion are coarsened and become brittle due to the thermal cycle received by the steel during welding. Therefore, when a large number of fine TiN inclusions are present, the coarsening of γ grains can be suppressed by the pinning effect. In the present invention, the number of TiN inclusions per unit area of the steel material is preferably 1.0 × 10 ⁸ (pieces / mm ² ) or more, more preferably 5.0 × 10 ⁸ (pieces / mm ² ) or more. More preferably, it is 1.0 × 10 ⁹ (pieces / mm ² ) or more.

  The TiN-based inclusion in the present invention means that both Ti and N are present in the inclusion at 0.3% or more. Does the inclusion contain 0.3% or more of Ti and N? It can be determined, for example, by an energy dispersive detector (EDX). The “number of TiN inclusions per unit area” in the present invention is a value measured at a position (t / 4) that is a quarter of the total thickness t in the depth direction from the steel surface. The number of inclusions can be measured, for example, by observing a steel material with a transmission electron microscope (TEM). In the present invention, the value of the number is an average of the values of the numbers observed at five or more locations with an observation magnification of the microscope of 60,000 times or more and an observation field of view of 1.5 μm × 1.5 μm or more. This is because by increasing the observation magnification, the number of inclusions can be measured more accurately, and by widening the observation field and increasing the number of observations and taking the average, the number of inclusions depending on the observation location can be increased. This is because variations can be reduced.

In order to increase the number of TiN inclusions per unit area to 1.0 × 10 ⁸ (pieces / mm ² ) or more, the molten steel is rapidly cooled to precipitate a large number of fine TiN inclusions. . Therefore, the cooling rate from 1500 ° C. to 1000 ° C. when the molten steel is cooled and solidified is 1.0 × 10 ⁻³ (° C./second) or more, preferably 1.0 × 10 ⁻² (° C./second). ) Or more is recommended. This cooling rate can be adjusted, for example, by changing the amount of cooling water and the cooling method of the continuous casting machine.

  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 basic characteristics. The reasons for limiting the ranges of these components will be described below together with the effects of the respective elements of Ti, N, Co and Mg.

[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, about 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 0.16% or less more preferably.

[Si: 0.01 to 1%]
Si is a necessary element for deoxidation and securing strength, and the minimum strength as a structural member cannot be secured unless it is less than 0.01%. However, if the content exceeds 1%, weldability and HAZ toughness deteriorate. In addition, the minimum with preferable Si content is 0.02%. 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%]
Mn is also necessary for deoxidation and securing strength in the same manner as Si, 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 1.80%, More preferably, it is good to set it as 1.60% or less.

[Al: 0.005 to 0.1%]
Al is also necessary for deoxidation and securing of strength in the same manner as Si and Mn, and if less than 0.005%, there is no effect on deoxidation. However, if added over 0.1%, the weldability and HAZ toughness are impaired, so the range of Al addition amount is set to 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 0.090%, It is good to set it as 0.080% or less more preferably.

[Ti: 0.005 to 0.03%]
Ti is an element that densifies the surface rust coating, which greatly contributes to the improvement of corrosion resistance, improves its environmental barrier properties, suppresses corrosion inside the crevice, and improves crevice corrosion resistance. Ti also forms a nitride, thereby exhibiting a pinning effect that prevents the coarsening of γ grains in the HAZ during welding. In order to ensure such corrosion resistance and HAZ toughness, it is preferable to contain 0.005% or more. However, if it exceeds 0.03%, excessively dissolved Ti will increase, and HAZ toughness will decrease. In addition, workability and weldability are also deteriorated. The more preferable lower limit when Ti is contained is 0.008%, and the more preferable upper limit is 0.025%.

[N: 0.003 to 0.015%]
N forms TiN together with Ti and exhibits a pinning effect, thereby improving HAZ toughness. If the amount of N is less than 0.003%, the amount of TiN produced is insufficient, and the effect of improving HAZ toughness is small. On the other hand, when it exceeds 0.015%, the solid solution N increases too much, and the HAZ toughness deteriorates. A more preferable lower limit when N is contained is 0.004%, and a more preferable upper limit is 0.010%.

[Co: 0.010 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 0.010% or more. However, if the content exceeds 1%, weldability and HAZ toughness deteriorate. For these reasons, the Co content was set to 0.010 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 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 an action 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 effects, Mg needs to be contained in an amount of 0.0005% or more. However, if it exceeds 0.02%, workability and weldability are deteriorated. For these reasons, the Mg content is suitably in the range of 0.0005 to 0.02%. The minimum with preferable Mg content is 0.0007%, More preferably, it is 0.00
It is good to contain 10% 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 iron and inevitable impurities (for example, P, S, O, etc.), but to the extent that does not impair the properties of the steel other than these. These components (for example, Zr) are also acceptable. However, these allowable components are preferably suppressed to about 0.1% or less because the toughness may be deteriorated when the amount thereof is excessive.

  In addition to the above components, the marine steel material of the present invention may include (1) one or more selected from the group consisting of Cu, Cr and Ni, (2) Ca, (3) Mo and / or W, ( 4) It is also effective to contain one or more selected from the group consisting of B, V and Nb, etc., and the characteristics of the marine steel will be further improved according to the type of component 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%) ]
Cu, Cr and Ni are all effective elements for improving the corrosion resistance. Among these, Cu and Cr are elements that are effective for forming a dense surface rust film that contributes greatly to the improvement of corrosion resistance like Co. In order to exert such an effect, it is preferable to contain 0.01% or more of all. However, if excessively contained, the weldability, hot workability and HAZ toughness deteriorate, so Cu: 1.5 % Or less and Cr: 1% or less are preferable. When one or both of Cu and Cr are contained, the more preferable lower limit of each is 0.05%, and the more preferable upper limit is 1.0% for Cu and 0.8% for Cr.

  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, it is preferably contained in an amount of 0.01% or more. However, if the Ni content is excessive, weldability and hot workability are deteriorated, and further, the cost is significantly increased. The more preferable lower limit when Ni is contained is 0.05%, and the more preferable upper limit is 1.5%.

[Ca: 0.02% or less (excluding 0%)]
Ca, like Mg, exhibits an effect of increasing pH when dissolved, and is an element effective in improving corrosion resistance by suppressing pH reduction due to hydrolysis reaction in the local anode where iron is dissolved and suppressing corrosion reaction. is there. Such an effect by Ca is preferably 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, when it is contained simultaneously with Co, a remarkable effect of improving uniform corrosion is exhibited. In order to exert such an effect, it is preferable to contain 0.01% or more in all cases. However, if excessively contained, the weldability and HAZ toughness deteriorate, and further, the cost increases. 0.5% or less, and W is preferably 0.3% or less.
When one or both of Mo and W are contained, the more preferable lower limit of each is 0.02%, and the more preferable upper limit is 0.3% for Mo and 0.2% for W.

[B: selected from the group consisting of 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
In marine steel materials, higher strength may be required depending on the site to be applied, but these elements are elements necessary for strength improvement. Of these, B is preferably 0.0001% or more, and hardenability is improved and effective in improving the strength. However, when B exceeds 0.01%, the base material toughness and HAZ toughness are improved. Since it deteriorates, it is not preferable. V is preferably effective to improve the strength by containing 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 the HAZ toughness. . Nb is preferably effective in improving the strength by containing 0.003% or more, but if it exceeds 0.05%, Nb causes the deterioration of the toughness of the steel and the HAZ toughness. 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.

  The marine steel material 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 (painting corrosion resistance) of the coating film itself is also good as shown in the following examples.

  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 as a matter of course, and appropriate modifications are made within a range that can meet the purpose described above and below. Of course, it is also possible to implement them, and they are all included in the technical scope of the present invention.

[1. Steel sheet manufacturing]
Steels having the chemical composition shown in Table 1 below were melted by an ordinary melting method, and then the molten steel was directly slabd by a continuous casting machine and subjected to hot rolling to produce a steel plate. At this time, by changing the cooling rate from 1500 ° C. to 1000 ° C. (described as “cooling rate” in Table 4) and solidifying the molten steel, the number of TiN inclusions per unit area ( "TiN-based inclusion"). This cooling rate was adjusted by changing the amount of cooling water and the cooling method of the continuous casting machine. The steel plate thus obtained was cut and surface ground to prepare test pieces for the following corrosion resistance and weldability evaluation.

[2. Evaluation of corrosion resistance]
A test piece having a size of 100 × 100 × 25 (mm) was produced from the steel sheet obtained as described above (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 above-mentioned test piece A), and the clearance is obtained. A test piece B in which a part was formed was produced. 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 reaching the base on one side of the test piece C (length: 100 mm, width: about 0) 0.5 mm) was formed with a cutter knife.

  About the test material of each chemical component composition shown in the said Table 1, the test piece A, the test piece B, and five test pieces C were used for the corrosion test, respectively. The method of the corrosion test 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. 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.

  (1) For test piece A, the weight change before and after the test is converted into an average thickness reduction amount D-ave (mm), the average value of five test pieces is calculated, and the entire surface corrosivity of each specimen 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.

  (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 swollen width) are as follows: As shown in Table 2. The results of the corrosion test are shown in Table 3 below.

  These results can be considered as follows. No. containing only one of Co or Mg. Nos. 2, 3 and No. in which the content of Co or Mg is less than the lower limit specified in the present invention. Nos. 4 and 5 are No. containing neither Co nor Mg. Compared to one, the overall corrosion resistance or corrosion uniformity is slightly improved. 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, the improvement effect is not recognized by crevice corrosion resistance and the maximum swollen width, which is insufficient as the corrosion resistance of marine steel materials.

  Among these, in the test materials to which Cu, Ni or Cr was added, the effect of reducing the maximum swollen width of the coated test materials was recognized (No. 9, 10 or 24), and rust densification of these elements was observed. It is presumed that the corrosion progress was suppressed by acting on the rust stabilization of the cut part. Ca has an effect of increasing crevice corrosion resistance (No. 12, 16, 17, etc.), and it is considered that Ca further strengthened the suppression of pH reduction in the crevice and reduced corrosion. Furthermore, it can be seen that the addition of Mo and W is very effective in improving the corrosion uniformity and paint swellability (No. 26 to 28, etc.). No. As is clear from the results of 25, 28, 29, 30 and the like, various values of corrosion resistance are greatly improved by appropriately adjusting the value of ([Co] / [Mg]).

[3. Weldability test]
A test piece D having a thickness of 50 (mm) was prepared from the steel sheet obtained as described above. The number of TiN inclusions per unit area of this test piece was measured as follows, and subjected to a weldability test in order to evaluate HAZ toughness.

(Measurement of the number of TiN inclusions)
The number of TiN inclusions was measured by observing the test piece D with TEM and EDX attached to the TEM. Specifically, the position of t / 4 (12.5 mm from the surface) is observed from the surface of the test piece in the depth direction by TEM at an observation magnification of 60,000 times and an observation field of view of 1.5 μm × 1.5 μm. Among the inclusions present at the position, check for TiN inclusions containing 0.3% or more of Ti and N by EDX, measure the number of them, and calculate the number per unit area (mm ² ). The average value of the number of TiN inclusions was obtained from the five measurement locations. The results are shown in Table 4.

(Weldability test)
Specimen D was heated to 1400 ° C. and held for 50 seconds, and then subjected to a heat cycle in which the temperature from 800 ° C. to 500 ° C. was cooled in 400 seconds (corresponding to welding at a heat input of 60 kJ / mm). Three test pieces were collected. Using this JIS No. 4 test, a V Charpy impact test at −40 ° C. was performed, and the average value of three absorbed energies (vE ₋₄₀ ) was obtained. The results are shown in Table 4. In this weldability test, it was evaluated that a material having vE- ₄₀ of 55 J or more is excellent in HAZ toughness.

FIG. 4 is a graph showing the relationship between the cooling rate from 1500 ° C. to 1000 ° C. and the number of TiN-based inclusions per unit area of the steel material. The number of TiN-based inclusions per unit area of the steel material and vE- ₄₀ A graph showing the relationship is shown in FIG. As shown in Table 4 and FIG. 4, by setting the cooling rate from 1500 ° C. to 1000 ° C. to 1.0 × 10 ⁻³ (° C./second) or more, the TiN inclusions per unit area of the steel material number to be a 1.0 × 10 ⁸ (pieces / mm ²⁾ or more. Further, as shown in Table 4 and FIG. 5, when the number of TiN inclusions is 1.0 × 10 ⁸ (pieces / mm ² ) or more, vE- ₄₀ is 55 J or more, and high heat input welding is performed. Even (corresponding to welding of 60 kJ / mm), the HAZ toughness is good.

It is explanatory drawing which shows the external appearance shape of the test piece A used for the corrosion test. It is explanatory drawing which shows the external appearance shape of the test piece B used for the corrosion test. It is explanatory drawing which shows the external appearance shape of the test piece C used for the corrosion test. It is a graph which shows the relationship between the cooling rate from 1500 degreeC to 1000 degreeC, and the number of TiN type inclusions. It is a graph which shows the relationship between the number of the TiN inclusions per unit area of steel materials, and vE- ₄₀ .

Claims (6)

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%, Ti : 0.005 to 0.03%, N: 0.003 to 0.015%, Co: 0.010 to 1% and Mg: 0.0005 to 0.02%, the balance Consists of Fe and inevitable impurities,
A marine steel material excellent in corrosion resistance and HAZ toughness during high heat input welding, wherein the number of TiN inclusions per unit area is 1.0 × 10 ⁸ (pieces / mm ² ) or more.   2. The marine steel material according to claim 1, wherein the value ([Co] / [Mg]) of the Co content [Co] and the Mg content [Mg] is 2 to 350. 3.   Furthermore, Cu: 1.5% or less (not including 0%), Cr: 1% or less (not including 0%), Ni: 2% or less (not including 0%) The marine steel material of Claim 1 or 2 containing the above.   Furthermore, the marine steel material in any one of Claims 1-3 containing Ca: 0.02% or less (0% is not included).   The marine steel according to any one of claims 1 to 4, further comprising Mo: 0.5% or less (excluding 0%) and / or W: 0.3% or less (excluding 0%). .   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 marine steel material according to any one of claims 1 to 5, comprising one or more selected.

Images