JP2007107079A - Shipbuilding steel excellent in weldability and corrosion resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a shipbuilding steel excellent in weldability and corrosion resistance to such an extent that the steel can be put into practical use without being coated or galvanically protected, more particularly excellent in durability against crevice corrosion and excellent in durability against corrosion even in wet environments including salt adhesion caused by seawater, and exhibiting excellent weldability and corrosion resistance even when applied to tanks for petroleum liquid fuels. <P>SOLUTION: The shipbuilding steel comprises C: 0.01-0.30%, Si: 0.01-2.0%, Mn: 0.01-2.0%, Al:0.005-0.10%, and Se:0.005-0.050%, S: 0.030% (inclusive of 0%), with the balance of Fe and unavoidable impurities and satisfies the formula: [S]+[Se]×(32/79)≤0.030(%), wherein [S] and [Se] are the contents (mass%) of S and Se, respectively. <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., and is particularly excellent in weldability and exposed to salt and constant temperature and humidity due to seawater. The present invention relates to marine steel materials that are excellent in corrosion resistance under the environment, and also have excellent corrosion resistance and weldability that are required as materials for petroleum-based liquid fuel tanks.

  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. However, 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 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 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 in 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. So far, the proposed techniques have insufficient corrosion resistance in these areas.

  By the way, corrosion in crude oil tanker tanks (petroleum liquid fuel tanks) proceeds remarkably at the defective part of the oil coat formed on the surface of the steel plate, and this defective part is caused by the movement of crude oil and deformation of the hull during operation. Therefore, it is considered that it is repaired or newly formed. For this reason, a corrosion location does not concentrate on one certain location, but generate | occur | produces over the substantially whole surface of steel materials. Therefore, the steel material used as the raw material for the petroleum-based liquid fuel tank is required to have good corrosion resistance in a special corrosive environment in which local corrosion progresses to the entire surface. Also, in such a petroleum-based liquid fuel tank, the above-mentioned “crevice corrosion” occurs remarkably and the life of the tank may be shortened, so that it is required to have excellent crevice corrosion resistance.

  As a material for the above-described petroleum-based liquid fuel tank, for example, a technique as disclosed in Patent Document 3 has been proposed as a material having improved corrosion resistance. In this technique, corrosion-resistant steel useful as a material for a tank that stores liquid fuel has been proposed by appropriately adjusting the chemical composition. In this technique, local corrosion such as “crevice corrosion” is considered in addition to overall corrosion, and it can be said that the corrosion resistance is improved. However, these steel materials cannot be said to have sufficient corrosion resistance to withstand recent demands.

In addition, as a steel material used as a raw material for a petroleum-based liquid fuel tank, weldability typified by welding heat-affected zone (HAZ) toughness is required, but in the technology that has been proposed so far, The reality is that good weldability is not always achieved.
JP, 2000-17381, A Claims etc. JP, 2002-266052, A Claims etc. JP, 2001-214236, A Claims etc.

  The present invention has been made paying attention to the circumstances as described above, and its purpose is excellent in weldability and steel material for shipbuilding that is excellent in corrosion resistance that can be put into practical use without being subjected to painting or corrosion protection, in particular. In addition to improving durability against crevice corrosion, it also demonstrates excellent durability against salt adhesion caused by seawater and corrosion due to wet environments, and excellent weldability even when applied to petroleum liquid fuel tanks And it is providing the marine steel material which can exhibit corrosion resistance.

The marine steel material of the present invention that has achieved the above object is C: 0.01 to 0.30% (meaning of mass%, the same shall apply hereinafter), Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, Al: 0.005 to 0.10% and Se: 0.005 to 0.050%, and S: 0.030% or less (including 0%) The present invention has the gist that it satisfies the relationship expressed by the following formula (1) and the balance is made of Fe and inevitable impurities.
[S] + [Se] × (32/79) ≦ 0.030 (%) (1)
However, [S] and [Se] indicate the contents (mass%) of S and Se, respectively.

  Moreover, in the steel material for shipbuilding of this invention, (a) Ca: 0.0005-0.0040%, (b) Cu: 0.01-5.0%, Cr: 0.01-5.0 as needed. %, Co: 0.01 to 5.0%, Ni: 0.01 to 5.0%, and Ti: 0.005 to 0.20%, (c) La: 0 One or more selected from the group consisting of .0005 to 0.15%, Ce: 0.0005 to 0.15% and Mg: 0.0005 to 0.015%, (d) Mo: 0.01 to 5. 0%, (e) Sb: 0.01-0.5%, As: 0.01-0.5%, Sn: 0.01-0.5%, Bi: 0.01-0.5% and Te: one or more selected from the group consisting of 0.01 to 0.5%, (f) B: 0.0001 to 0.010%, V: 0.01 to 0.50% And Nb: It is also effective to contain one or more selected from the group consisting of 0.003 to 0.50%, etc., and the characteristics of the marine steel materials are further improved according to the type of components to be contained become.

  Even when the marine steel material of the present invention is used as a material for a petroleum-based liquid fuel tank, it exhibits excellent corrosion resistance and good weldability in the corrosive environment.

  In the marine steel material of the present invention, while containing the Se content while appropriately controlling in relation to S, by appropriately adjusting the chemical component composition, it is possible to prevent deterioration of weldability due to Se content, It is possible to realize marine steel with excellent corrosion resistance that can be put to practical use without painting or cathodic protection, especially to improve durability against crevice corrosion, as well as against salt adhesion caused by seawater and corrosion due to wet environments. A marine steel material exhibiting excellent durability can be realized, and even when used as a material for a petroleum-based liquid fuel tank, excellent corrosion resistance is exhibited even in a corrosive environment. Such steel materials for ships are useful not only as outer plates in ships such as crude oil tankers, cargo ships, cargo passenger ships, passenger ships, warships, but also as materials for ballast tanks, crude oil tanks, and the like.

  The present inventors have made further studies for the purpose of realizing a marine steel material having excellent corrosion resistance. As part of the research, if a predetermined amount of Se is contained and the chemical composition is appropriately adjusted, the durability against crevice corrosion will be improved, and the adhesion of salt caused by seawater and corrosion due to wet environments will be improved. However, it has been found that a marine steel material exhibiting excellent durability can be realized and its technical significance has been recognized, and has been filed earlier (Japanese Patent Application No. 2004-191759). However, it has been found that depending on the content of Se, the HAZ toughness deteriorates.

  As factors governing HAZ toughness, the magnitude of fracture propagation energy, the origin of fracture, and the like can be considered. Among these, the propagation energy of fracture is considered to be related to the HAZ structure, and the origin of fracture is considered to be related to the presence of island martensite and inclusions in the HAZ. In the marine steel material to be used in the present invention, low carbon steel is basically used, and since the carbon dioxide arc welding method and the submerged arc welding method are applied, the HAZ structure is ferrite + pearlite and the ferrite grain size is 100 to 100. Since it is 200 micrometers, it is thought that there is not so much influence on HAZ toughness. Also, island martensite hardly occurs under the above conditions.

  For these reasons, it was considered that the factors that most affect the HAZ toughness include inclusions in the steel, and in particular, the presence of MnS. Further, when Se is contained for improving the corrosion resistance, Mn—S—Se inclusions are likely to be generated, which greatly affects the HAZ toughness.

  Accordingly, the present inventors have repeatedly studied for the purpose of realizing a marine steel material that can exhibit excellent corrosion resistance while maintaining good HAZ toughness. As a result, if the contents of S and Se are appropriately controlled and controlled so as to satisfy a predetermined relational expression [the relation of the above formula (1)], inclusions that affect the HAZ toughness are reduced. As a result, it was found that a steel material for shipbuilding capable of solving the above-mentioned problems was realized, and the present invention was completed.

  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 Se and S.

C: 0.01 to 0.30%
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 0.30%, the toughness deteriorates. For these reasons, the C content range was set to 0.01 to 0.30%. 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 0.28%, More preferably, it is good to set it as 0.26% or less.

Si: 0.01 to 2.0%
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 2.0%, the weldability deteriorates. In addition, the minimum with preferable Si content is 0.02%, More preferably, it is good to set it as 0.05% or more. Moreover, the upper limit with preferable Si content is 1.80%, It is good to set it as 1.60% or less more preferably.

Mn: 0.01 to 2.0%
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 2.0%, 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: 0.005-0.10%
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 0.10%, the weldability is impaired, so the range of the amount of Al added is set to 0.005 to 0.10%. 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 0.040%, It is good to set it as 0.050% or less more preferably.

Se: 0.005 to 0.050%
Se exerts the action of suppressing the pH drop at the site where the dissolution reaction of corrosion occurs to suppress the corrosion reaction and improve the corrosion resistance. Inclusion of such Se makes it difficult for local pH changes to occur, and thus has an effect of improving corrosion uniformity. In addition, in the “gap portion” where the mass transfer is restricted and the pH is likely to decrease, the effect (local corrosion inhibition effect) is effectively exhibited for the reason described above. In order to ensure the corrosion resistance required in such an environment, the Se content needs to be 0.005% or more. However, Se generates MnSe inclusions (or Mn-S-Se inclusions) and degrades HAZ toughness. Therefore, if it is excessively contained in excess of 0.050%, weldability deteriorates. Moreover, workability will also deteriorate. Therefore, the Se content needs to be 0.005 to 0.050%. In addition, the minimum with preferable Se content is 0.006%, More preferably, it is good to set it as 0.008% or more. Moreover, the upper limit with preferable Se content is 0.030%, It is good to set it as 0.020% or less more preferably.

S: 0.030% or less (including 0%)
S produces inclusions with Mn at the melting stage, and this is extended by rolling. Although this inclusion does not have a great influence in the state of a steel plate having a high base toughness, in the HAZ in which the base toughness is reduced, this inclusion causes a brittle fracture. Therefore, in order to maintain the HAZ toughness satisfactorily, it is necessary to use a component system that suppresses the generation of MnS, and for that purpose, the S content needs to be suppressed to at least 0.030% or less. The S content is preferably 0.025% or less.

[S] + [Se] × (32/79) ≦ 0.030 (%)
Even if the content of Se and S is appropriate, if the total content thereof exceeds a predetermined amount, it becomes Mn-S-Se inclusions and deteriorates the HAZ toughness. It is necessary to satisfy the relationship.

  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, O, etc.), but other than these, the properties of the steel are not impaired. Components (eg, Zr, N, etc.) are 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, the marine steel material of the present invention may include (a) Ca: 0.0005 to 0.0040%, (b) Cu: 0.01 to 5.0%, Cr: 0.0. One or more selected from the group consisting of 01 to 5.0%, Co: 0.01 to 5.0%, Ni: 0.01 to 5.0% and Ti: 0.005 to 0.20%; c) one or more selected from the group consisting of La: 0.0005 to 0.15%, Ce: 0.0005 to 0.15% and Mg: 0.0005 to 0.015%, (d) Mo: 0 0.01-5.0%, (e) Sb: 0.01-0.5%, As: 0.01-0.5%, Sn: 0.01-0.5%, Bi: 0.01- One or more selected from the group consisting of 0.5% and Te: 0.01-0.5%, (f) B: 0.0001-0.010%, V: 0.01-0 One or more selected from the group consisting of 50% and Nb: 0.003 to 0.50% may be included, and the characteristics of the steel material for shipbuilding are further improved depending on the type of component to be included. It will be.

Ca: 0.0005 to 0.0040%
Ca is an element effective for promoting the spheroidization of inclusions, and the HA toughness is improved by spheroidizing the inclusions. In order to exhibit such an effect, it is preferable to contain 0.0005% or more. However, if the Ca content is excessive, CaO inclusions are generated and the cleanliness of the steel material is lowered. Therefore, the content is preferably 0.0040% or less. In addition, the more preferable minimum of Ca content is 0.0030%, and a more preferable upper limit is 0.0035%.

Cu: 0.01-5.0%, Cr: 0.01-5.0%, Co: 0.01-5.0%, Ni: 0.01-5.0% and Ti: 0.005- One or more selected from the group consisting of 0.20% Cu, Cr, Co, Ni and Ti are all effective elements for improving corrosion resistance. Among these, Cu, Cr and Co are effective elements for forming a dense surface rust film that greatly contributes to the improvement of corrosion resistance. Co is an effective element in a high salinity environment. In order to exert the effect of these elements, it is preferable to contain 0.01% or more of all, but if contained excessively, weldability and hot workability deteriorate, so 5.0% or less. It is preferable to do. A more preferable lower limit when Cu, Cr and Co are contained is 0.05%, and a more preferable upper limit is 4.50%.

  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 deteriorate, so 5.0% or less is preferable. The more preferable lower limit when Ni is contained is 0.05%, and the more preferable upper limit is 4.50%.

  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. In order to ensure the corrosion resistance required in such an environment, it is preferable to contain 0.005% or more. However, if it exceeds 0.20%, workability and weldability are deteriorated. . The more preferable lower limit when Ti is contained is 0.008%, and the more preferable upper limit is 0.15%.

One or more elements selected from the group consisting of La: 0.0005 to 0.15%, Ce: 0.0005 to 0.15% and Mg: 0.0005 to 0.015% were dissolved by corrosion. It has an action of suppressing pH reduction due to Fe ion hydrolysis, and also promotes rust densification by Cu or the like contained if necessary, and further enhances local pH reduction suppression action by Se. Such an effect is effectively exhibited by containing at least 0.0005% of one or more of these elements. However, if La and Ce are contained in excess of 0.15% and Mg exceeds 0.015%, workability and weldability are deteriorated. In addition, a more preferable lower limit when containing La and Ce is 0.0010%, and a more preferable upper limit is 0.10%. Moreover, a more preferable lower limit when Mg is contained is 0.0010%, and a more preferable upper limit is 0.010%.

Mo: 0.01-5.0%
Mo has the effect of increasing the uniformity of corrosion and suppressing perforations due to local corrosion. In particular, by containing Cu, Cr, Co, etc. at the same time, a remarkable uniform corrosion improvement effect is exhibited. In order to exhibit these effects, Mo is preferably contained in an amount of 0.01% or more. However, if excessively contained, the weldability is deteriorated, so 5.0% or less is preferable. The more preferable lower limit when Mo is contained is 0.02%, and the more preferable upper limit is 4.50%.

Sb: 0.01-0.5%, As: 0.01-0.5%, Sn: 0.01-0.5%, Bi: 0.01-0.5% and Te: 0.01- One or more elements selected from the group consisting of 0.5% are elements that enhance corrosion resistance by promoting rust densification by Cu or the like and pH lowering action by La or the like. In order to exert such an effect, it is preferable to contain 0.01% or more in any case. However, if excessively contained, workability and weldability deteriorate, so 0.5% or less is preferable. The more preferable lower limit when these elements are contained is 0.02%, and the more preferable upper limit is 0.40%.

In one or more marine steel materials selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50% , depending on the portion to be applied In some cases, higher strength is required, but these elements are necessary for strength improvement. Of these, B is contained in an amount of 0.0001% or more, which improves the hardenability and is effective in improving the strength. However, excessively soliciting exceeding 0.010% deteriorates the base material toughness, which is not preferable. . V is effective for improving the strength by containing 0.01% or more, but if it exceeds 0.50%, it is not preferable because it causes toughness deterioration of the steel material. Nb is effective for improving the strength by containing 0.003% or more, but if it exceeds 0.50% and it is contained excessively, the toughness of the steel will be deteriorated. More preferable lower limits of these elements are 0.0003% for B, 0.02% for V, and 0.005% for Nb. The more preferable upper limit is 0.0090% for B, 0.45% for V, and 0.45% 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 of the coating film itself (coating corrosion resistance) is also good as shown in the examples described later.

  In addition, the steel material of the present invention can realize a marine steel material that exhibits excellent durability against salt adhesion caused by seawater and corrosion due to a wet environment, but when used as a material for petroleum-based liquid fuel tanks. Even if it exists, it will exhibit excellent corrosion resistance even in the corrosive environment.

  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.

Example 1
Steel materials having the chemical composition shown in the following Tables 1 and 2 were melted in a converter, and various steel plates were produced by continuous casting and hot rolling. The obtained steel plate was cut and subjected to surface grinding to finally produce a test piece having a size of 300 × 300 × 25 (mm) (test piece A). The external shape of the test piece A is shown in FIG.

  Further, as shown in FIG. 2, four small test pieces of 60 × 60 × 5 (mm) are brought into contact with a large test piece of 300 × 300 × 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 10 mmφ hole 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 M8 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, 2, the test piece A, the test piece B, and five test pieces C were used for the corrosion test, respectively. The corrosion test method (actual ship exposure test) at this time is as follows.

[Corrosion test method]
The prepared specimens (each specimen A to C) were attached to the inner bottom plate, wall and upper deck of the VLCC crude oil tanker, and after five years of normal operation, the corrosion status of each specimen was investigated. . Five test pieces A and B were exposed on the bottom plate and the back of the deck, and five test pieces A and C were exposed on the wall surface.

After exposure for 5 years, the test piece A was subjected to removal of corrosion products such as iron rust by the cathodic electrolysis method in diammonium hydrogen citrate aqueous solution [JIS K8284]. For test piece B, the small test piece for forming the gap was removed, and the corrosion products were removed in the same manner.
(1) For test piece A, the weight change before and after the test is converted into the average thickness reduction D-ave (mm), the average value of the five test pieces is calculated, and the overall 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.
(2) For test piece B, the maximum crevice corrosion depth D-crev (mm) on the large test piece side was measured using a stylus type three-dimensional shape measuring apparatus.
(3) For test piece C (with cut flaws) that has been subjected to coating treatment, measure the swollen width (mm) of the coating film perpendicular to the cut flaws with a vernier caliper, and determine the maximum swollen width of the five test pieces. Defined.

  Evaluation criteria for the above-mentioned overall corrosion resistance (average plate reduction: D-ave), corrosion uniformity (D-max / D-ave), crevice corrosion resistance (D-crev), and coating corrosion resistance (maximum swollen width) are It is as shown in Table 3 below. The corrosion test results are shown in Table 4 below.

  Moreover, about each steel plate base steel plate, the weldability (HAZ toughness) was evaluated by the following method. The results are also shown in Table 4 below.

[HAZ toughness]
Double-sided submerged arc welding (SAW) with a heat input of 7 KJ / mm was performed (X groove), and Charpy impact test pieces (JIS Z 2204 No. 4) were collected from the HAZ part, and the impact absorption energy vE ₀ at 0 ° C. was determined. (Triple test). A vE ₀ of 41 J or more was considered acceptable.

Based on these results, the relationship between the value {[S] + [Se] × (32/79)} on the left side of the equation (1) and the impact absorption energy vE ₀ is shown in FIG. 4 (component system 1 in Table 1). To 3) and FIG. 5 (component systems 4 to 6 in Table 2). By controlling the value of [S] + [Se] × (32/79) to an appropriate range, good HAZ toughness is obtained. It can be seen that it is secured. Moreover, these steel plates (Nos. 2 to 5 and 7 to 26 in Table 4) exhibit good corrosion resistance and can be suitably used as crude oil tank corrosion resistant steel.

It is explanatory drawing which shows the external appearance shape of the test piece A used for the corrosion resistance test of the Example. It is explanatory drawing which shows the external appearance shape of the test piece B used for the corrosion resistance test of the Example. It is explanatory drawing which shows the external appearance shape of the test piece C used for the corrosion resistance test of the Example. 7 is a graph showing the relationship between {[S] + [Se] × (32/79)} and impact absorption energy vE ₀ in component systems 1 to 3 in Table 1. 7 is a graph showing the relationship between {[S] + [Se] × (32/79)} and impact absorption energy vE ₀ in component systems 4 to 6 in Table 2.

Claims (8)

C: 0.01-0.30% (meaning of mass%, the same applies hereinafter), Si: 0.01-2.0%, Mn: 0.01-2.0%, Al: 0.005-0. 10% and Se: 0.005 to 0.050%, S: 0.030% or less (including 0%), and satisfying the relationship of the following formula (1), A marine steel material excellent in weldability and corrosion resistance, wherein the balance is made of Fe and inevitable impurities.
[S] + [Se] × (32/79) ≦ 0.030 (%) (1)
However, [S] and [Se] indicate the contents (mass%) of S and Se, respectively.   The marine steel material according to claim 1, further comprising Ca: 0.0005 to 0.0040%.   Furthermore, Cu: 0.01-5.0%, Cr: 0.01-5.0%, Co: 0.01-5.0%, Ni: 0.01-5.0% and Ti: 0.0. The marine steel material of Claim 1 or 2 containing 1 or more types chosen from the group which consists of 005 to 0.20%.   Furthermore, it contains at least one selected from the group consisting of La: 0.0005 to 0.15%, Ce: 0.0005 to 0.15% and Mg: 0.0005 to 0.015%. 4. The marine steel material according to any one of 3 above.   Furthermore, the marine steel material in any one of Claims 1-4 containing Mo: 0.01-5.0%.   Further, Sb: 0.01 to 0.5%, As: 0.01 to 0.5%, Sn: 0.01 to 0.5%, Bi: 0.01 to 0.5%, and Te: 0.0. The marine steel material according to any one of claims 1 to 5, comprising one or more selected from the group consisting of 01 to 0.5%.   Furthermore, it contains at least one selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50% and Nb: 0.003 to 0.50%. The marine steel material according to any one of 6.   The steel material for ships according to any one of claims 1 to 7, which is used as a material for a petroleum liquid fuel tank.

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