JP5447310B2 - Steel for ballast tank - Google Patents

Description

  The present invention relates to a steel material for a ballast tank that is excellent in joint fatigue strength of a welded portion and excellent in corrosion resistance.

  In recent years, demands for extending the life cycle of welded structures have increased. Furthermore, there is a strong demand for reducing maintenance costs. As an example of prolonging the life cycle, for example, in the field of bridges and the like, the tendency to maintain and use for a period exceeding 100 years has become prominent.

  For this reason, in addition to the design surface and the construction surface, a longer life is also required from the material side. That is, in order to prevent fatigue damage, it is necessary to develop a technique for extending the life of a welded part as well as a technique for extending the life of a base material part of a welded structural steel. In recent years, excellent fatigue characteristics are required in a region having a longer life than the long-life region, that is, in a very long-life region.

In other words, for ultra-long-life fatigue that targets a fatigue life range where the number of repetitions of fracture exceeds 2 × 10 ⁶ times, long-life fatigue that covers the fatigue life range up to 2 × 10 ⁶ times of fracture cycles This is considered to be different from the damage behavior obtained in the fatigue test in the region. In other words, if the fatigue design of the super-long life region is advanced by extrapolating the behavior obtained in the fatigue test up to about 2 × 10 ⁶ times of fracture repetition to the welded structure, the result on the dangerous side will be obtained. There is a possibility of falling. Therefore, it is desirable to collect data for the ultra-long life region corresponding to the fatigue design in the ultra-long life region exceeding 2 × 10 ⁶ times.

  For example, the following literature discloses the fatigue fracture life of the welded portion.

  Japanese Patent Application Laid-Open No. H10-228667 discloses a metal structure of a weld heat affected zone where a fatigue crack is generated for the purpose of improving the fatigue strength of a steel plate weld. Specifically, in order to suppress the generation and propagation of fatigue cracks in the weld heat affected zone, it is effective to increase the ferrite area ratio of the weld heat affected zone structure. 15-80% is specified.

  Next, Patent Document 2 discloses a high-tensile welded steel sheet. Here, a bainite-based structure including martensite is optimal as a base material structure, and bainite exceeding an area ratio of 60% is optimal as a weld heat affected zone structure. By realizing such a structure, a high-strength welded structural steel sheet excellent in fatigue strength of a welded joint can be realized.

  Furthermore, in Patent Document 3 and Patent Document 4, steel materials related to fatigue property improvement are disclosed, and the element Sn which plays a main role in the present invention is positioned as an optional additive element, but is referred to. Yes.

  On the other hand, in the shipbuilding field, seawater is taken in and out of the ballast tank of the ship, and the temperature rise due to the deck being heated by sunlight overlaps, so that it is exposed to extremely severe corrosive environment. Therefore, in order to prevent the steel material from being corroded, the coating material is usually protected by coating with an epoxy paint or by applying an epoxy paint after applying a zinc primer on the surface of the steel material. In addition, anticorrosion is used in combination in the submerged area. However, as described above, it is difficult to maintain the corrosion resistance for a long time because of the extremely severe corrosive environment in the ballast tank. In particular, in order to maintain the structural strength of the ballast tank, beams and other structures are installed to create a complicated structure. Therefore, when the coating film deteriorates, the steel surface is easily cleaned and repainted. Is a difficult situation. In addition, it is indispensable to remove the rust of the corroded steel material during repair, but it is physically impossible to remove the rust sufficiently. For this reason, there is an urgent need to improve the long-term durability of the steel material itself.

  Regarding the steel material for ballast tanks, for example, it is disclosed in the following document.

  Patent Documents 5 to 7 disclose a steel material applicable to a ballast tank and a manufacturing method thereof. In any document, there is a description that an element Sn which plays a main role in the present invention is essential.

JP-A-9-95754 Japanese Patent Laid-Open No. 10-1743 JP 2007-197762 JP 2004-190123 A JP 2005-220394 A Japanese Unexamined Patent Publication No. 2007-270198 JP 2010-7109

  However, in these documents, fatigue damage behavior of a welded joint in an ultra-long life region is not assumed.

  That is, neither Patent Document 1 nor Patent Document 2 describes or suggests a technique related to fatigue property improvement in the ultra-long life region, which is a problem of the present invention.

  Although Patent Documents 3 and 4 have a description of adding Sn as an optional additive element, the purpose of Sn addition in Patent Document 3 is to promote the effect of densification of formed rust and the effect of lowering pH, The purpose of Sn addition in Patent Document 4 is to further suppress the corrosion resistance, particularly the progress of local corrosion in the liquid phase part. Furthermore, the fatigue characteristics aimed for improvement in Patent Document 3 and Patent Document 4 are fatigue crack growth characteristics of the base material, and there is no description about improving the fatigue characteristics of the welded part. Therefore, the dimensions are completely different from the fatigue crack generation characteristics (joint fatigue characteristics of the welded portion) of the welded joint, which is aimed to be solved by the present invention.

  In any of Patent Documents 5 to 7, there is no description that suggests a technique for improving fatigue characteristics, including a technique related to improving fatigue characteristics in the ultra-long life region, which is a problem of the present invention.

  In view of such circumstances, an object of the present invention is to provide a steel material for a ballast tank that is excellent in joint fatigue characteristics of a welded portion in an ultra-long life region and excellent in long-term corrosion resistance.

  The inventors first manufactured a welded joint having a weld length of about 400 mm, and after collecting specimens, observed the occurrence and propagation behavior of fatigue cracks in fatigue tests in detail from both macro and micro perspectives. . As a result, the following findings (a) to (g) were obtained.

(a) In the fatigue life region up to 2 × 10 ⁶ times of fracture repetition, many fatigue cracks are generated from the weld toe, and the main fracture crack is formed while repeating coalescence growth. In addition, the generation | occurrence | production position of the fatigue crack in the steel materials at this time is a welding heat affected zone of a welded joint.

(b) On the other hand, in the ultra-long life region, which covers the fatigue life region where the number of repeated fractures exceeds 2 × 10 ⁶ times, the number of occurrences of fatigue cracks is one or two in the cross section of the specimen. In addition, fatigue cracks grow very slowly at the beginning, and the life spent at that time occupies most of the entire life, and reaches fatigue fracture after repeated loading over 2 × 10 ⁶ times.

(c) As described above, the conventional fatigue design of welded joints has been studied based on fatigue characteristics evaluated based on fatigue test results with a number of repeated fractures of up to 2 × 10 ⁶ times. However, as a result of careful observation over time, it was found that fatigue cracks progress by different mechanisms in welded joints that break in the ultra-long life region with a number of repetitions of fracture exceeding 2 × 10 ⁶ times.

(d) In ultra-long-life fatigue, where the number of repetitions of fracture exceeds 2 × 10 ⁶ times, the stress applied is considerably low, so fatigue cracks occur at weld lengths from 30 mm There are about 1 or 2 spots per 50mm. The occurrence of the fatigue crack is determined by the fatigue characteristics of the weld heat affected zone. Further, the fatigue characteristics of the weld heat affected zone are greatly improved by employing a base material having an appropriate chemical composition.

  (e) The fatigue strength in the weld heat affected zone is determined by the hardness, particle size, metal structure area ratio, dislocation density, etc. of the weld heat affected zone. On the other hand, the fatigue strength influencing factors in these weld heat affected zones can be utilized by strictly controlling the chemical composition of the base steel plate. That is, by limiting the chemical composition of the base steel plate, it is possible to control the structure of the heat affected zone caused by welding.

  (f) As described above, it has been found for the first time that the fatigue characteristics of the weld heat affected zone generated during welding can be dramatically improved by strictly controlling the chemical composition of the steel material. Specifically, joint fatigue strength is improved by containing Sn, while toughness deterioration caused by containing Sn is compensated by containing one or more selected from Cr, Mo, and W. Is.

  (g) Further, the present inventors have shown that the corrosion resistance of the coated part is improved by the addition of Sn in a very severe corrosive environment where salt is present and drying and wetting are repeated, as represented by a ballast tank. In particular, it has been found that the addition of Sn is effective in improving the corrosion resistance of the coated portion on the steel surface where rust remains on the surface of the steel material during repair.

The fact that the addition of Sn is effective for improving the corrosion resistance of the coated part is that the scratched part of the coating film or the like is present in a corrosive environment where extremely severe salt exists and drying and wetting are repeated, as represented by a ballast tank. in film thickness is thin coating fragile portion, corrosion by the dry-wet repetition of FeCl ₃ solution is an essential condition, while pH is lowered by hydrolysis of Fe ^3+, and Fe ³⁺ acts as an oxidizing agent It has been found as a result of being found to be accelerated.

  Hereinafter, the mechanism of corrosion in such a corrosive environment and the effect of addition of Sn will be described.

  The corrosion reaction at this time is as follows.

As the cathode reaction, the following reaction mainly occurs.
Fe ³⁺ + e ⁻ → Fe ²⁺ (reduction reaction of Fe ³⁺ )

In addition to this reaction, the following cathode reaction also occurs.
2H ₂ O + O ₂ + 2e ⁻ → 4OH ⁻
2H ⁺ + 2e ⁻ → H ₂

On the other hand, the following anodic reaction occurs with respect to the above Fe ³⁺ reduction reaction.
Fe → Fe ²⁺ + 2e ⁻ (Fe dissolution reaction)

Therefore, the overall reaction of corrosion is as shown in the following equation (1).
2Fe ³⁺ + Fe → 3Fe ²⁺ (1)

Fe ²⁺ generated by the reaction of the above formula (1) is oxidized to Fe ³⁺ by air oxidation, and the generated Fe ³⁺ accelerates corrosion as an oxidizing agent again. At this time, the reaction rate of air oxidation of Fe ²⁺ is generally slow in a low pH environment, but is accelerated in a concentrated chloride solution, and Fe ³⁺ is easily generated. It has been found that due to such a cyclic reaction, in an environment where the amount of incoming salt is very large, Fe ³⁺ is always supplied, corrosion of steel is accelerated, and corrosion resistance is significantly deteriorated. In this way, in an environment where the amount of salt is very high, protection by a rust layer cannot generally be expected.

By the way, in such a salinity environment, β-FeOOH is usually generated by air oxidation, but β-FeOOH is easily reduced. Therefore, an intermediate of FeOH · OH is obtained by the reaction shown in the following formula (2). Is easy to generate. And since this reaction is a counter reaction of iron dissolution reaction (the above-mentioned anode reaction: Fe → Fe ²⁺ + 2e ⁻ ), corrosion is accelerated.
2FeOOH + 2H ⁺ + 2e ⁻ → 2FeOH · OH (2)

  However, it has been found that the addition of Sn suppresses the production of β-FeOOH in a salt environment and preferentially produces α-FeOOH. Since α-FeOOH is thermodynamically stable and difficult to reduce, the reaction shown in the formula (2) does not occur, and as a result, corrosion is suppressed. In other words, α-FeOOH acts protectively and has the effect of slowing the anodic dissolution reaction of the steel itself. In particular, it is considered that Sn ions eluted only in the vicinity of the anode portion occurring in a minute region of the scratched portion of the coating film exert an effect.

Further, when Sn is added, it dissolves as Sn ²⁺ , and the concentration of Fe ³⁺ is lowered by a reaction of 2Fe ³⁺ + Sn ²⁺ → 2Fe ²⁺ + Sn ^4+, thereby suppressing the reaction of the above formula (1). Sn also has an effect of suppressing anodic dissolution.

The rust reduction reaction that occurs when the rust is wet is as shown in the above formula (2), but when wet for a long time, the reaction shown in the following formula (3) occurs.
2FeOH · OH + 2FeOOH → Fe ₃ O ₄ + 2H ₂ O (3)

  The present invention has been completed based on the above findings, and the gist thereof lies in the following steel materials for ballast tanks (1) to (5).

  (1) By mass%, C: 0.01 to 0.20%, Si: 0.03% or more and less than 0.60%, Mn: 0.5 to 2.0%, P: 0.01% or less, S: 0.005% or less, sol. Al: more than 0.006% and 0.10% or less, Sn: 0.02 to 0.40%, one or more selected from Cr, Mo and W in total of 0.03 to 1. A steel material for ballast tanks including 0%, the balance being made of Fe and impurities, and excellent in corrosion resistance and joint fatigue characteristics of welds.

  (2) Instead of a part of Fe, it contains at least one selected from Nb: 0.080% or less, Ti: 0.030% or less, and V: 0.080% or less in mass%. The steel material for a ballast tank according to (1) above, which is characterized.

  (3) The above (1), characterized in that, in place of a part of Fe, by mass%, at least one selected from Cu: less than 0.7% and Ni: 3.0% or less Or (2) steel for ballast tanks.

  (4) The steel for ballast tank according to any one of (1) to (3) above, wherein B is contained in an amount of 0.0030% or less in mass% instead of part of Fe.

  (5) Instead of a part of Fe, by mass%, Ca: less than 0.007%, Mg: 0.007% or less, Ce: 0.007% or less, Y: 0.5% or less, and Nd: 0 The steel for ballast tanks according to any one of (1) to (4) above, which contains one or more selected from 5% or less.

  ADVANTAGE OF THE INVENTION According to this invention, while being excellent in the joint fatigue characteristic of the weld part in a super-long life area, the steel material for ballast tanks which is excellent in long-term corrosion resistance is provided. Therefore, it greatly contributes to the long-term use extension and maintenance reduction of the ballast tank.

The shape and dimensions (unit: mm) of the joint fatigue test piece are shown. The appearance before and after the acceleration test is shown. (a) shows the crosscut situation before the test, and (b) shows the situation where corrosion / peeling progressed from the crosscut part after the test.

1. Hereinafter, the chemical composition of the steel material according to the present invention will be described. In addition, "%" regarding content means "mass%".

C: 0.01 to 0.20%
C is an element effective for increasing the strength of the steel, and is contained in an amount of 0.01% or more in order to obtain the strength of the steel. However, if the content exceeds 0.20%, the strength becomes too high and the base metal toughness is deteriorated, and excellent weld fatigue characteristics are not realized. That is, when the C content exceeds 0.20%, the hardness of the weld heat-affected zone becomes higher than that of the base material or the weld metal. At this time, the hardness distribution abruptly changes at the weld surplus toe that becomes the fatigue fracture starting point, and strain concentration occurs due to the material notch. The strain concentration due to the material notch overlaps with the strain concentration due to the toe shape, and significantly increases the local strain at the fracture starting point, thereby impairing the fatigue strength of the joint. For this reason, C content shall be 0.01 to 0.20%. The C content is preferably more than 0.03%. Further, the C content is preferably less than 0.15%, and more preferably less than 0.14%.

Si: 0.03% or more and less than 0.60% Si is an element effective for deoxidation of steel. In order to obtain the effect, 0.03% or more is contained. However, when the Si content is 0.60% or more, formation of the MA structure is promoted. Since the M-A structure is extremely hard, the toughness of the welded joint is significantly deteriorated. The MA structure is a kind of island martensite formed in the bainite structure, and is an MA transformation product containing residual austenite. Therefore, in order to avoid toughness deterioration, the Si content is less than 0.60%. For this reason, Si content shall be 0.03% or more and less than 0.60%. The Si content is desirably 0.10% or more, and more desirably 0.20% or more. The Si content is preferably less than 0.50%.

Mn: 0.5 to 2.0%
Mn is an element effective for improving hardenability, and is contained in an amount of 0.5% or more in order to increase strength and improve fatigue crack growth resistance of the base material. On the other hand, if it exceeds 2.0%, the toughness deteriorates, so the upper limit of the Mn content is 2.0%. For this reason, Mn content shall be 0.5 to 2.0%. A desirable lower limit is 0.8% and a desirable upper limit is 1.6%.

P: 0.01% or less P is an impurity mixed into steel. The smaller the fracture toughness, the better. However, removing P is costly. For this reason, the allowable upper limit was set to 0.01%.

S: 0.005% or less S is an impurity mixed in steel. Since S has a high segregation rate and forms a low-melting-point substance and causes solidification cracking, it is preferable that S be as small as possible. However, it takes a cost to remove S. For this reason, the allowable upper limit was made 0.05%. The S content is preferably less than 0.004%.

sol. Al: more than 0.006% and 0.10% or less Al is an element necessary for deoxidation together with Si, and in order to obtain the deoxidation effect, sol. Al is contained. On the other hand, sol. If the Al content exceeds 0.10%, formation of the MA structure is promoted and joint toughness is deteriorated. To avoid this, sol. The Al content is 0.10% or less. For this reason, sol. The Al content is more than 0.006% and not more than 0.10%. A desirable lower limit is 0.015%, and a desirable upper limit is less than 0.05%.

Sn: 0.02 to 0.40%
Sn is positioned as an element that deteriorates the toughness of steel materials, and studies have been made in the direction of not containing as much as possible except steel materials for special applications. In contrast, the inventors conducted fatigue tests on a large number of prototype materials and evaluated the joint fatigue strength. As a result, the inventors promoted the refinement of the structure of the weld heat affected zone, which is extremely useful for improving joint fatigue strength. I found out. That is, by containing an appropriate amount of Sn in the steelmaking stage, the particle size of the weld heat affected zone can be made extremely fine, and the fatigue strength of the joint can be improved.

Sn is dissolved as Sn ²⁺ and has an action of suppressing corrosion by an inhibitor action in an acidic chloride solution. Further, by reducing Fe ³⁺ quickly and reducing the concentration of Fe ³⁺ as an oxidant, the corrosion promoting action of Fe ³⁺ is suppressed, so that the corrosion resistance in a high flying salinity environment is improved. Furthermore, since the production | generation of easily reducible (beta) -FeOOH is suppressed and the hardly reducible (alpha) -FeOOH is produced | generated preferentially, corrosion is suppressed in salty environment. Moreover, Sn has the effect | action which suppresses the anodic dissolution reaction of steel and improves corrosion resistance. For this reason, sufficient corrosion resistance is obtained even if a coating film is not provided on the steel material surface.

  Further, when one or more selected from Cr, W and Mo described later are allowed to coexist in the steel containing Sn, the corrosion resistance is improved in an environment having a high salt content.

  In order to obtain these effects, it is necessary to make the Sn content 0.02% or more. On the other hand, when Sn is contained, the toughness of the base material tends to deteriorate. In particular, when the content exceeds 0.40%, the toughness of the steel material is remarkably deteriorated, and even if a toughness recovery means for coexisting one or more selected from Cr, Mo and W is adopted, the steel material for ballast tanks Toughness is reduced to an unapplicable level. Therefore, the Sn content is 0.02 to 0.40%. A desirable lower limit is 0.03%, and a more desirable lower limit is 0.05%. A desirable upper limit is 0.30%, and a more desirable upper limit is 0.20%.

One or more selected from Cr, Mo and W: 0.03 to 1.0% in total
Cr, Mo, and W contain at least one selected from Cr, Mo, and W because it serves as a means for recovering the toughness of the steel material caused by the inclusion of Sn. . However, when the content of one or more selected from Cr, Mo and W is less than 0.03% in total, the recovery of toughness deterioration of the steel material caused by containing Sn is sufficiently achieved I can't expect. On the other hand, if the content of one or more selected from Cr, Mo and W exceeds 1.0% in total, the weldability is impaired, so that the application as a steel material for a ballast tank is greatly limited. Therefore, the content of one or more selected from Cr, Mo and W is 0.03 to 1.0% in total. The total content of these elements is preferably 0.05% or more. Further, the total content of these elements is preferably less than 0.80%, and more preferably 0.70% or less. In addition, from the viewpoint of steelmaking cost, it is preferable to contain Cr alone in excess of 0.03%.

  The steel material which concerns on this invention is a steel material which has said element and the remainder consists of Fe and an impurity. Here, the impurities are components that are mixed due to various factors of the manufacturing process including raw materials such as ore and scrap when industrially manufacturing steel materials, and in a range that does not adversely affect the present invention. It means what is allowed.

  The steel material according to the present invention may further contain one or more selected from Nb, Ti, V, Cu, B, Ca, Mg, Ce, Y and Nd in addition to the above elements.

These components can be classified into the following four groups.
(1) 1st group: One or more types selected from Nb: 0.080% or less, Ti: 0.030% or less, and V: 0.080% or less.
(2) Second group: one or more selected from Cu: less than 0.7% and Ni: 3.0% or less.
(3) Third group: B: 0.0030% or less.
(4) Fourth group: selected from Ca: less than 0.007%, Mg: 0.007% or less, Ce: 0.007% or less, Y: 0.5% or less, and Nd: 0.5% or less One or more.

  The reason why these elements may be contained and the contents at that time are as follows.

(1) First group Nb: 0.080% or less Nb can be contained as necessary. If contained, the hardenability is increased, so that it is effective in improving the strength and suppressing the fatigue crack growth of the base material. In addition, there is an effect of improving toughness through a fine graining action. However, if the content exceeds 0.080%, the toughness deteriorates, so the upper limit is made 0.0800%. Desirably, it is 0.060% or less. In addition, in order to acquire the effect by containing Nb stably, it is desirable to make it contain 0.005% or more.

Ti: 0.030% or less Ti can be contained as necessary. If contained, it is effective in improving strength and suppressing fatigue crack growth of the base material. However, if the content exceeds 0.030%, the toughness deteriorates, so the upper limit is made 0.030%. Desirably, it is 0.020% or less. In addition, in order to acquire the effect by containing Ti stably, it is desirable to make it contain 0.005% or more.

V: 0.080% or less V can be contained as necessary. If contained, it is effective in improving strength and suppressing fatigue crack growth of the base material. In particular, in the thick material, the characteristic improvement by containing V becomes remarkable. However, if the content exceeds 0.080%, the toughness deteriorates, so the upper limit is made 0.080%. Desirably, it is 0.070% or less. In addition, in order to acquire the effect by containing V stably, it is desirable to make it contain 0.005% or more.

(2) Second group Cu: less than 0.7% Cu can be contained as necessary. If contained, it has the effect of increasing the strength of the steel. However, if the content is 0.7% or more, the toughness of the steel deteriorates, so the Cu content is less than 0.7%. Desirably, it is 0.48% or less. In addition, in order to obtain the effect by containing Cu stably, it is desirable to make content of Cu 0.1% or more.

Ni: 3.0% or less Ni can be contained as necessary. If contained, it has the effect of increasing the strength of the steel. It is also effective in suppressing fatigue crack growth. However, if the content exceeds 3.0%, there is no increase in strength sufficient to meet the cost increase due to the Ni contained, and the fatigue crack growth inhibiting effect of the base material is not seen, so the upper limit is 3.0% And Desirably, it is 2.5% or less. In addition, in order to acquire the effect by containing Ni stably, containing 0.2% or more is desirable.

(3) Third group B: 0.0030% or less B can be contained as necessary. If contained, it has the effect of increasing the strength and improving the fatigue crack growth resistance of the base material by significantly increasing the hardenability. However, if its content exceeds 0.0030%, the toughness deteriorates, so 0.0030% is made the upper limit. Desirably, it is 0.0025% or less. In addition, in order to acquire the effect by containing B stably, containing 0.0003% or more is desirable.

(4) Fourth group Ca: less than 0.007% Ca can be contained as necessary. If contained, it contributes to toughness improvement through refinement of the structure. However, if the amount of Ca inclusions becomes excessive, the toughness deteriorates, so the Ca content is less than 0.007%. Desirably, it is 0.003% or less. In addition, in order to acquire the effect by containing Ca stably, containing 0.0015% or more is desirable.

Mg: 0.007% or less Mg can be contained as necessary. If contained, it contributes to toughness improvement through refinement of the structure. However, if the content exceeds 0.007%, the amount of Mg inclusions becomes excessive and the toughness deteriorates, so 0.007% is made the upper limit. Desirably, it is 0.003% or less. In addition, in order to acquire the effect by containing Mg stably, 0.0005% or more containing is desirable.

Ce: 0.007% or less Ce can be contained if necessary. If contained, it contributes to toughness improvement through refinement of the structure. However, if the content exceeds 0.007%, the amount of Ce inclusions becomes excessive and the toughness is deteriorated, so 0.007% is made the upper limit. Desirably, it is 0.003% or less. In addition, in order to acquire the effect by containing Ce stably, containing 0.0005% or more is desirable.

Y: 0.5% or less Y can be contained as necessary. If contained, it contributes to toughness improvement through refinement of the structure. However, if the content exceeds 0.5%, the amount of Y inclusions becomes excessive and the toughness is deteriorated, so 0.5% is made the upper limit. Desirably, it is 0.05% or less. In addition, in order to obtain the effect by containing Y stably, containing 0.01% or more is desirable.

Nd: 0.5% or less Nd can be contained as necessary. If contained, it contributes to toughness improvement through refinement of the structure. However, if the content exceeds 0.5%, the amount of Nd inclusions becomes excessive and the toughness is deteriorated, so 0.5% is made the upper limit. Desirably, it is 0.05% or less. In addition, in order to acquire the effect by containing Nd, 0.01% or more containing is desirable.

2. Steel Material Production Method The steel material for ballast tank according to the present invention can be produced, for example, by the following procedure using a known hot rolling facility or a known hot rolling facility and a known heat treatment facility.

  A cast slab having the above chemical composition is heated to 1000 ° C. to 1250 ° C. and then hot-rolled. Then, when cooling this, in the cooling step, accelerated cooling is performed so that the average cooling rate between 650 ° C. and 400 ° C. is 5 ° C./second or more, preferably 5 to 25 ° C./second. Stop at a temperature below ℃. Thereafter, the cooling is finished so that the recuperated temperature range becomes 70 ° C. or less. Here, the recuperated temperature range means the difference between the temperature reached when cooling is stopped and the temperature when the surface temperature rises and stabilizes due to the heat inside the steel plate after cooling stops.

  When the heating temperature of the casting slab is less than 1000 ° C., the ferrite fraction increases and the fatigue crack growth rate of the base material increases. If it exceeds 1250 ° C, the structure becomes coarse and the toughness deteriorates. When the average cooling rate between 650 ° C. and 400 ° C. in the cooling process is less than 5 ° C./second, the ferrite fraction increases and the fatigue crack growth rate of the base material increases. At this time, a preferable average cooling rate is 25 ° C./second or less. When the recuperation temperature range from the accelerated cooling stop to the end of cooling exceeds 70 ° C, the dislocation density decreases and the fatigue crack growth rate of the base metal increases. When the accelerated cooling stop temperature exceeds 400 ° C., the ferrite fraction increases and the fatigue crack growth rate of the base material increases. A preferred stop temperature is 350 ° C. or higher. The ferrite fraction of the ferrite formed through the above process is preferably 40% or less.

  In order to reduce the recuperation temperature range, it is desirable to reduce the temperature difference between the steel plate surface layer and the center portion during cooling and to end the phase transformation of at least the surface layer portion at the end of cooling. In order to reduce the temperature difference between the steel sheet surface layer and the central portion, it is preferable to increase the cooling rate at the subsequent stage from the preceding stage of the cooling zone. In order to complete the phase transformation of the surface layer portion when the accelerated cooling is stopped, the accelerated cooling stop temperature is preferably 400 ° C. or lower.

3. Use form of steel material When using the steel material of this invention, it is desirable to coat | cover the surface with an anticorrosion film | membrane. The anticorrosion film used in the present invention means a film applied for the purpose of anticorrosion of steel materials. Specifically, when used for a ballast tank, various types of rust stabilization coatings (including a chemical conversion treatment system and a coating system) well known in weathering steel materials; Zn plating, Al plating, Zn-Al plating Corrosion-proof coatings such as Zn sprays, Al sprays, etc .; general anti-corrosion coatings such as vinyl butyral, epoxy, urethane, phthalic acid, and so-called C-based and I-based coatings Is included. Even when any anticorrosion film is applied, excellent weather resistance and high anticorrosion performance can be exhibited. Moreover, a general-purpose anticorrosion paint can be used, and the effect is also exhibited when the anticorrosion paint is applied with a so-called Zn-based primer as a base. The film thickness or adhesion amount of these anticorrosion films is not particularly limited, and may be within a normal range. Further, the steel material may be a rusted steel material. That is, in the situation where the surface rust cannot be completely removed at the time of repair, especially the painted part exhibits corrosion resistance, so even if the rust cannot be completely removed with keren etc. Even if it is painted, the life can be extended significantly. This is because the scratched part is likely to become an anode when it is painted, and especially when it is painted in a rusted state, the phenomenon that the pH drops locally becomes remarkable, so the performance of this steel material is demonstrated. it is conceivable that.

  Moreover, you may give the cathodic protection by a sacrificial anode to the submerged part in a ballast tank. Also in this case, the steel material of the present invention is suitable because it has an effect of suppressing hydrogen penetration during cathodic protection.

  Steel having the chemical composition shown in Table 1 was melted in a converter to produce a slab, and a steel plate having a thickness of 12 to 80 mm was produced by the above-described suitable production method.

  The manufactured steel sheet was embedded in resin so that the cut-out cross section became the test surface, mirror-polished, then corroded with nital, observed in the center of the cross section with an optical microscope, and the microstructure was observed The tissue (phase) was identified. As a result, it was found that all the steel plates of the present invention had a ferrite fraction of 40% or less.

  In order to evaluate the properties as a steel plate base material, the tensile properties and impact properties of each steel plate were investigated. That is, the tensile test is performed at room temperature by collecting a No. 10 tensile test piece based on the description of JIS Z 2201 (1998) having a parallel part diameter of 12.5 mm, and yield strength (YS) and tensile strength. (TS) was measured. In addition, said tensile test piece was extract | collected in parallel with the rolling direction from the site | part of 1/4 thickness from the surface of a steel plate to the plate thickness direction. However, for thin steel plates with a thickness of 12 mm and 16 mm, specimens cannot be collected from the above-mentioned sites, so that the tensile specimen is replaced with a small specimen with a parallel part diameter of 8.5 mm and a distance between gauge points of 25 mm. evaluated.

  The target of the tensile properties of the steel plate base material was YS of 235 MPa or more.

Impact test based on JIS Z 2202 (1998), performed Charpy impact test were taken V-notch test piece width 10 mm, was determined Charpy absorbed energy value _V E _-5 at -5 ° C.. In addition, said Charpy impact test piece was extract | collected in parallel with the rolling direction from the site | part of 1/4 thickness from the surface of a steel plate to the plate thickness direction. However, the thin steel plates having a thickness of 12 mm and 16 mm were collected from the center of the thickness of the steel plate in parallel with the rolling direction without forming a quarter portion.

The goal of the Charpy absorbed energy value _V E _-5 steel sheets, 100 J or more for the base material, the joint was 50 J.

  Subsequently, the steel sheet was cut into an appropriate size and welded. A V-notch test piece was prepared from the welded steel plate for impact tests in the same manner as described above, and a load non-transmission type cross welded joint specimen was prepared. Details of the welding conditions are shown in Table 2. Welding was performed without preheating, but the welded joint was not tested for those in which weld cracking occurred.

  A small test specimen was adopted to increase the repetition rate in the fatigue test so that fatigue data in the ultra-long life range could be collected efficiently. FIG. 1 shows the shape and dimensions (unit: mm) of the load non-transmission type cross welded joint specimen. In the test body shape, the repetition rate can be ensured at about 10 Hz, and the number of repetitions per day is about 860,000 times. The rib plate thickness of the cross joint was aligned with the plate thickness of the main plate, and the height of the rib plate was twice the rib plate thickness. In addition, the test body was produced with the steel plate thickness less than 20 mm with the steel plate thickness kept.

  Also, for steel plates with a thickness exceeding 20 mm, the thickness effect on the joint fatigue characteristics is eliminated, and the joint fatigue characteristics of the steel itself are purely evaluated. The thickness was 20 mm. Here, the reason for reducing the thickness on one side is that, in general, fatigue cracks are generated in the welded joint from the surface of the steel sheet, so that the surface of the test steel sheet remains. The steel plate surface side, that is, the black skin side, is used as the evaluation part. The shape was adjusted to reduce the stress concentration, and the tensile residual stress generated by welding in the jet chisel was reduced. As a result, in all the specimens, fatigue cracks occurred from the steel sheet surface, that is, from the black skin side.

  The fatigue test was conducted using an electrohydraulic closed loop fatigue tester. A plurality of testing machines having a load capacity of ± 10 tons to ± 50 tons were used. The test piece was mounted on the testing machine with a hydraulic chuck or a bolt-fastened jig, and a repeated load was applied. The repeated stress waveform was a sin wave, the maximum stress was fixed at 350 MPa, and the stress range (= maximum stress-minimum stress) was set by changing the minimum stress. Here, the reason why the maximum stress is fixed at a high level, which is close to the yield stress, is to compensate for the residual welding stress released when processing into a small test piece with the external force of the fatigue test. By adopting such a stress waveform, fatigue characteristics equivalent to those of an actual structure or a large welded structure model can be reproduced while evaluating fatigue strength using a small specimen. During the fatigue test, the displacement at the gripping part of the specimen is dynamically monitored, and when the displacement at the maximum load increases by 1 mm during the fatigue test compared to the maximum displacement at the start of the fatigue test, The number of repetitions at that time was defined as the fracture life. In addition, when the displacement was increased by 1 mm and the test was completed, about half the area of the evaluation cross section was damaged by fatigue.

At least three specimens were manufactured from each of the prepared steel plates, the stress range was appropriately set, and the fatigue fracture life was measured by a joint fatigue test. Then, for each steel material, an SN line having a fracture probability of 50%, that is, an SN average line, is derived by experiment, and the fatigue strength (time strength) at 1 × 10 ⁷ repetitions is read using the diagram. The intensity was 10 ⁷ times.

Target value of the Charpy absorbed energy value _V E _-5 of the welded joint was not less than 50 J. The target value of the fatigue strength of the welded joint was set to 100 MPa or more.

Moreover, the front and back surfaces of these steel materials were mechanically ground, and test pieces having a thickness of 3 mm × width of 60 mm × length of 100 mm were cut out. The obtained test piece was evaluated by SAE (Society of Automotive Engineers) J2334 test. The SAE J2334 test is an accelerated test conducted under the following conditions, and the corrosion form is said to be similar to the atmospheric exposure test (Hiroo Nagano, Masato Yamashita, Hitoshi Uchida: Environmental Materials Science, Kyoritsu Shuppan (2004) , P.74). This test simulates a severe corrosive environment in which the amount of incoming salt exceeds 1 mdd, and further simulates a ballast environment in which dry and wet conditions are repeated. Wetting: 50 ° C., 100% RH, 6 hours, salt adhesion: 0.5% by weight NaCl, 0.1% by weight CaCl ₂ , 0.075% by weight NaHCO ₃ aqueous solution, 0.25 hour, drying: 60 ° C. 50% RH, 17.75 hours was defined as one cycle (24 hours in total). After 160 cycles of SAE J2334 test, the rust layer on the surface of each test piece was removed, and the thickness reduction amount was measured. The test results are shown in Table 3. “Corrosion weight loss” in Table 3 is the average thickness reduction amount of the test piece, and is calculated using the weight reduction before and after the test and the surface area of the test piece.

  Also, spray a general-purpose epoxy resin paint (Banno 200: manufactured by China Paint Co., Ltd.) to a coating thickness of 150 μm and cut the cloth with a vinyl chloride cutter until the steel surface is scratched. Cut test evaluation was performed. Test evaluation Furthermore, in order to simulate repair, 10 cycles of SAE J2334 test were conducted in advance, rust was formed on the surface, and then the rusted steel material (residual rust steel material) that was cleansed with a wire brush was similarly applied to general purpose epoxy. Resin paint (Banno 200: manufactured by China Paint Co., Ltd.) is spray-coated so that the coating thickness is 150 μm, and the cross is cut with a vinyl chloride cutter until the steel surface is scratched. went. After these tests, the maximum corrosion depth was measured using a point micrometer.

  FIG. 2 is an appearance before and after the acceleration test. (a) shows the crosscut situation before the test, and (b) shows the situation where corrosion / peeling progressed from the crosscut part after the test.

  Table 3 also shows the test results of the base material and the welded joint. Judgment criteria for base material strength, base material toughness, weldability, joint toughness, joint fatigue characteristics, and comprehensive evaluation shown in Table 3 are as follows.

[Base material strength YS (MPa)]: 315 or more, ○: 235 or more and less than 315, ×: less than 235.
[Base material toughness _V E _-5 (J)] A: 120 or more, B: 100 or more and less than 120, X: less than 100.
[Weldability] A: No preheating required, x: Preheating required.
[Joint Toughness _V E _-5 (J)] A: 100 or more, ○: 50 or more and less than 100, x: less than 50.
[Fitting fatigue characteristics: fatigue limit (MPa)] ◎: 120 or more, ○: 100 or more and less than 120, ×: less than 100.
[Corrosion resistance: Average thickness reduction of unpainted steel (mm)]
◎: 0.5 or less, ○: 0.5 to 0.7 or less, ×: 0.7 or more [Corrosion resistance: Blasting steel material + Paint scratch maximum corrosion depth (mm)]
◎: 0.1 or less, ○: 0.1 to 0.2 or less, ×: 0.2 or more [Corrosion resistance: Rust remaining steel material + maximum scratch depth of coating scratches (mm)]
◎: 0.1 or less, ○: 0.1 to 0.2 or less, ×: 0.2 or more [Overall evaluation] ◎: All of the above five test results are ◎.
○: The above five test results are ◎ or ○.
X: Among the above five test results, there is at least one x.

  All the steel sheets defined in the present invention have YS of 120 MPa or more, exhibit excellent joint fatigue strength and excellent corrosion resistance in the ultra-long life region, and are suitable as steel for ballast tanks. On the other hand, the steel sheet that does not satisfy the present invention does not satisfy the target value that the fatigue strength of the welded joint is YS 100 MPa or more, or does not satisfy at least one of the target values such as base material strength and toughness.

The steel material for ballast tanks of the present invention is excellent in the joint fatigue characteristics of the welded portion in the ultra-long service life region and in the long-term corrosion resistance. Therefore, it greatly contributes to the long-term use extension and maintenance reduction of the ballast tank.

Claims (5)

  In mass%, C: 0.01 to 0.20%, Si: 0.03% or more and less than 0.60%, Mn: 0.5 to 2.0%, P: 0.01% or less, S: 0 0.005% or less, sol. Al: more than 0.006% and 0.10% or less, Sn: 0.02 to 0.40%, one or more selected from Cr, Mo and W in total of 0.03 to 1. A steel material for ballast tanks including 0%, the balance being made of Fe and impurities, and excellent in corrosion resistance and joint fatigue characteristics of welds.   Instead of a part of Fe, it contains at least one selected from Nb: 0.080% or less, Ti: 0.030% or less, and V: 0.080% or less in mass%. The steel material for ballast tanks according to claim 1.   It replaces with a part of Fe and contains 1 or more types selected from Cu: less than 0.7% and Ni: 3.0% or less by mass%, The claim 1 or 2 characterized by the above-mentioned. Steel for ballast tanks.   The steel material for ballast tank according to any one of claims 1 to 3, characterized in that, instead of a part of Fe, B: 0.0030% or less is contained in mass%.   Instead of a part of Fe, in mass%, Ca: less than 0.007%, Mg: 0.007% or less, Ce: 0.007% or less, Y: 0.5% or less, and Nd: 0.5% The steel material for a ballast tank according to any one of claims 1 to 4, wherein the steel material contains at least one selected from the following.