JP2007262441A - Steel for crude oil tank and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for a crude oil tank in which excellent toughness can be obtained even in the case of being subjected to high heat input welding of >100 kJ/cm, and further, local corrosion and uniform corrosion can be reduced in the case of being used in uncoated and primer-coated conditions, and to provide its production method. <P>SOLUTION: A steel stock having a composition comprising, by mass, 0.03 to 0.2% C, 0.05 to 0.8% Si, 0.5 to 2.5% Mn, ≤0.03% P, ≤0.005% S, 0.005 to 0.06% Al, 0.001 to 0.3% W, 0.005 to 0.03% Ti and 0.002 to 0.007% N, satisfying 2.0<Ti/N<5.0, and the balance Fe with inevitable impurities, and in which a W-containing oxide layer is formed on the steel surface is heated at 1,000 to 1,250°C, is thereafter subjected to hot rolling at Ar<SB>3</SB>to Ar<SB>3</SB>+150°C at a cumulative draft of ≥25% and also at a rolling finishing temperature of ≥Ar<SB>3</SB>, and is successively cooled at a cooling rate of ≤60°C/s. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a tensile strength 490 MPa class steel excellent in toughness and corrosion resistance of a high heat input weld heat-affected zone exceeding 100 kJ / cm, and a method for producing the same, and particularly relates to a material suitable for crude oil tank.

On the inner surface of the tanker's crude oil tank (same as the back of the upper deck), inert gas (about 5 vol.% For O, about 13 vol.% For CO, about 0.01 vol.% For SO) It is known that corrosive gases such as O ₂ , CO ₂ , SO _2, and H ₂ S volatilized from crude oil in the exhaust gas (boiler or engine exhaust gas having the remaining N ₂ as a representative composition) cause overall corrosion. .

Furthermore, using iron rust produced by corrosion as a catalyst, H ₂ S is oxidized and solid S is formed in layers in the iron rust, and these corrosion products easily peel off and accumulate on the bottom of the crude oil tank. In the dock inspection every 2.5 years, the repair of the upper part of the tank and the collection of the deposit are performed at great expense.

  On the other hand, in the case of a tanker's crude oil tank bottom plate, the steel material used is due to the corrosion inhibition effect of the crude oil itself and the corrosion inhibition effect of the protective film derived from crude oil (hereinafter referred to as “crude oil protection film”) produced on the inner surface of the crude oil tank. Although it was thought that no corrosion occurred in the tank, recently it became clear that a bowl-shaped local corrosion occurred on the tank bottom plate.

The causes of local corrosion are as follows: (1) Presence of flocculated water in which salts such as sodium chloride are dissolved at high concentration, (2) Separation of crude oil protective film due to excessive washing, (3) High level of sulfide in crude oil Concentration, (4) Concentration of O ₂ , CO ₂ , SO ₂ in inert gas enclosed in crude oil tank for explosion protection, (5) Participation of microorganisms, etc. are mentioned, but still clear The cause is unknown.

  The most effective method for suppressing corrosion is to coat the steel surface with heavy coating to shield the steel from the corrosive environment, but the coating on the crude oil tank has a huge application area, and about 1 in about 10 years. It is pointed out that, since repainting is required, inspection and construction are very expensive, and in the case of heavy coating, corrosion is promoted in the crude oil tank environment in the damaged part of the coating film.

  As countermeasures from the steel side, several corrosion resistant steels having corrosion resistance in a crude oil tank environment have been proposed. The corrosion-resistant steel described in Patent Document 1 is mass%, C: 0.01 to 0.3%, Si, Mn, P, S adjusted to an appropriate amount, Ni: 0.05 to 3%, It has a composition that selectively contains Mo, Cu, Cr, W, Ca, Ti, Nb, V, and B, and resistance to general corrosion and local corrosion is improved.

Further, the corrosion resistant steel described in Patent Document 2 is mass%, C: 0.00 to 0.2%, and Si, Mn, P, S and Cu adjusted to appropriate amounts: 0.01 to 1.5%. , Al: 0.001-0.3%, N: 0.001-0.01%, Mo: 0.01-0.2% or W: 0.01-0.5% Contains, exhibits excellent overall corrosion resistance and local corrosion resistance, and can suppress the formation of corrosion products including solid S.
JP 2003-82435 A JP 2004-204344 A

  However, when the above-mentioned corrosion resistant steel for crude oil tanks is applied to crude oil tanks, it exhibits overall corrosion resistance when used in the upper part of crude oil tanks, but is resistant to local corrosion when used on the bottom plate of crude oil tanks (hereinafter “ “Local corrosion resistance” is not sufficient.

  In addition, the toughness of the weld heat-affected zone of flux-copper backing welding (hereinafter also referred to as FCB welding), which is applied to the manufacture of crude oil tank bottom plate and has a welding heat input exceeding 100 kJ / cm, is not necessarily sufficient. Was not limited.

  Therefore, the present invention provides excellent corrosion resistance against local corrosion that occurs on the bottom plate of the crude oil tank and overall corrosion that occurs on the top plate and side plate of the crude oil tank, and high heat input welding in which the welding heat input exceeds 100 kJ / cm. An object of the present invention is to provide a steel for crude oil tanks having a tensile strength of 490 MPa that is excellent in weld heat-affected zone toughness.

  In order to achieve the above-mentioned problem, the present inventors first extract factors involved in local corrosion of the bottom plate of the crude oil tank, combine these factors, conduct a corrosion test, and detect the local corrosion that occurs in the bottom plate of the crude oil tank. The knowledge about the controlling factors and the corrosion mechanism was obtained.

  That is, as a result of earnest examination about the influence of various alloy elements on the occurrence of local corrosion, the addition of W makes the rust layer formed on the steel sheet surface very dense in an environment used as a steel material for crude oil tanks, It has been found that the corrosion resistance and overall corrosion resistance are improved.

  Next, the strength of the base metal and the toughness of the high heat input welding heat-affected zone are examined. In the steel having the above characteristics, the carbon equivalent Ceq is 0.25 to 0.40% and the Ti / N is 2.0 to 5. When the martensite, which is an embrittlement structure, is reduced to 0 and coarsening of the austenite grains in the weld heat affected zone is suppressed by dispersing a large amount of TiN, which has a high melting temperature, It has been found that the toughness of the heat-affected zone of the high heat input welding can be improved as well as the strength characteristics of the base metal without deteriorating the overall corrosivity.

The present invention has been completed on the basis of the above-described findings and further studies, that is, the present invention
1. mass%,
C: 0.03-0.2%,
Si: 0.05 to 0.8%,
Mn: 0.5 to 2.5%,
P: 0.03% or less,
S: 0.005% or less,
Al: 0.005 to 0.06%,
W: 0.001 to 0.3%,
Ti: 0.005 to 0.03%,
N: 0.002 to 0.007%,
Ceq (= C + Mn / 6 + (Cr + Mo + V) / 5 + Ni / 15, provided that C, Mn, Cr, Mo, V are contained (mass%): 0.25 to 0.40%,
For crude oil tanks satisfying 2.0 <Ti / N <5.0, having a composition comprising the balance Fe and inevitable impurities, and having an oxide layer containing W formed on the steel surface steel.
In addition to the steel composition described in 2.1, in mass%,
Ni: 1.5% or less Cr: 1.0% or less Mo: 1.0% or less V: 0.2% or less Nb: 0.1% or less B: One or more of 0.005% or less Contains steel for crude oil tanks.
In addition to the steel composition described in 3.1 or 2, in mass%,
Sn: 0.005-0.3%
Sb: 0.005 to 0.3%
A crude oil tank steel containing one or more of the following.
In addition to the steel composition according to any one of 4.1 to 3, further in mass%,
Steel for crude oil tanks containing one or more of Ca: 0.005% or less REM: 0.02% or less and Mg: 0.005% or less.
After heating the steel material having the composition described in any one of 5.1 to 4 to 1000 to 1250 ° C., the cumulative rolling reduction in the temperature range of Ar _{3 to} Ar ₃ + 150 ° C. is 25% or more, and the rolling finish A method for producing steel for a crude oil tank, characterized by performing hot rolling at a temperature of Ar ₃ or higher and subsequently cooling at a cooling rate of 60 ° C./s or lower.
6). 6. The method for producing steel for a crude oil tank according to 5, wherein tempering is further performed at 400 ° C. or more and Ac ₁ transformation point or less after cooling.
The steel for crude oil tanks which applied the primer coating containing Zn to the surface of the steel for crude oil tanks as described in any one of 7.1 thru | or 4.
A crude oil tank using the steel for a crude oil tank described in 8.7.

  According to the present invention, a steel material having excellent overall corrosion resistance and local corrosion resistance in a steel plate for transportation of crude oil or a storage tank for crude oil can be produced at low cost and has excellent high heat input weld toughness. As a result, the manufacturing process can be simplified and the safety can be improved, resulting in a remarkable industrial effect.

Hereinafter, the component composition and production conditions will be described. The (%) indication regarding the component shall mean mass%.
[Ingredient composition]
C: 0.03-0.2%
C is an element useful for obtaining the strength required for structural steel. However, if less than 0.03%, the effect is poor. On the other hand, if over 0.2%, the toughness of welds and weld crack resistance are poor. Since it degrades the property, it is limited to the range of 0.03 to 0.2%. In addition, Preferably, it is 0.03 to 0.16%.
Si: 0.05 to 0.8%
Si acts as a deoxidizer and requires at least 0.05% for steelmaking. Moreover, Si contributes to the improvement of corrosion resistance by forming a corrosion protection film in an acidic environment. On the other hand, if it exceeds 0.8%, the toughness of the base metal deteriorates and island martensite is generated in the heat-affected zone of the high heat input welding, and the toughness is significantly deteriorated. Limited to range. In addition, Preferably, it is 0.2 to 0.6%.

Mn: 0.5 to 2.5%
Mn is added to increase the strength of the steel. In order to secure a tensile strength of 490 MPa or more, 0.5% or more is necessary. On the other hand, if it exceeds 2.5%, the base metal and the weld heat affected zone deteriorate significantly, so 0.5 to 2.5% Limited to the range. In addition, Preferably, it is 0.6 to 1.6%.

P: 0.03% or less P, if the content exceeds 0.03%, deteriorates the toughness of the welded portion, so is suppressed to 0.03% or less. In addition, excessive P reduction raises the refining cost and is economically disadvantageous, so 0.005% or more is desirable.

S: 0.005% or less S is a harmful element that degrades the toughness of the base metal and the welded part, and forms MnS of non-metallic inclusions to cause local corrosion and reduce local corrosion resistance. Yes, it is desirable to reduce as much as possible. If the content exceeds 0.005%, this tendency becomes remarkable, so the content is made 0.005% or less.

Al: 0.005-0.06%
Al acts as a deoxidizer and requires 0.005% or more in steelmaking. However, if it exceeds 0.06%, the toughness of the base material decreases, and at the same time, it is mixed into the weld metal during welding. In order to deteriorate the toughness, the content is limited to 0.005 to 0.06%. In addition, Preferably, it is 0.005 to 0.05%.

W: 0.001 to 0.3%
When W is added, WO ₄ ²⁻ ions formed in a corrosive environment exhibit a barrier effect against anions such as chloride ions, and insoluble FeWO ₄ is formed to suppress the progress of corrosion. Furthermore, the rust layer formed on the steel plate surface is very densified by containing W.

The addition of W suppresses the progress of the general corrosion and the local corrosion in the corrosive environment where H ₂ S and Cl ⁻ are present due to the chemical and physical action. A steel material for crude oil tanks with excellent corrosion properties can be obtained.

  In addition, when using the primer together, Zn in the primer is taken into the rust layer densified by containing W, and a composite oxide of W or Zn centering on Fe is formed, so that the surface of the steel plate is extended for a long time. Zn can survive.

  If the effect described above is less than 0.001%, a sufficient effect cannot be obtained, and if it exceeds 0.3%, the effect is saturated and the base metal and weld toughness are lowered. It limited to the range of 001-0.3%.

Ti: 0.005 to 0.03%
Since Ti has a strong affinity for N and precipitates as TiN during solidification, it contributes to suppressing the austenite grain coarsening in the weld heat affected zone or to increasing the weld heat affected zone as a ferrite transformation nucleus. % Or more. On the other hand, if it exceeds 0.03%, the TiN particles become coarse and the above-mentioned effect cannot be expected, so the content is made 0.005 to 0.03%. In addition, Preferably, it is 0.007 to 0.020%.

N: 0.002 to 0.007%
N binds to Ti and precipitates as TiN to suppress the coarsening of austenite grains in the HAZ or contribute to the toughening of the HAZ as a ferrite transformation nucleus. In order to secure the necessary amount of TiN having such an effect, it is necessary to make it 0.002% or more. On the other hand, if it exceeds 0.007%, in the region heated to the temperature at which TiN dissolves during welding, the amount of solute N increases and the toughness decreases remarkably, so it is limited to 0.002 to 0.007%. .

Ceq (= C + Mn / 6 + (Cr + Mo + V) / 5 + Ni / 15, where C, Mn, Cr, Mo, V, Ni are contents (mass%))
If Ceq is less than 0.25%, the hardenability at the time of rolling and accelerated cooling is insufficient, and a tensile strength of 490 MPa or more cannot be secured. On the other hand, when Ceq exceeds 0.40%, the base metal toughness and the HAZ toughness are lowered, so the content is limited to the range of 0.25 to 0.40%.

Ti / N (However, Ti and N content (mass%)
If Ti / N is 2.0 or less, the amount of TiN necessary to improve the weld heat affected zone toughness cannot be secured. On the other hand, when Ti / N is 5.0 or more, since the base metal toughness and the weld heat affected zone deteriorate due to the generation of TiC particles and the coarsening of TiN, Ti / N is more than 2.0 to less than 5.0. Limited to range.

  The above is the basic component system of the present invention. Depending on the desired characteristics, Ni: 1.5% or less, and / or Cr: 1% or less, Mo: 1% or less, V: 0.2% or less, One or more selected from Nb: 0.1% or less, B: 0.005% or less, and / or Sn: 0.005-0.3%, Sb: 0.005-0 1 or more selected from 3% and / or Ca: 0.005% or less, REM: 0.02% or less, Mg: 1 selected from 0.005% or less It can contain seeds or two or more.

Ni: 1.5% or less Ni is an element that can increase strength while maintaining high toughness, and has little influence on HAZ toughness. Therefore, Ni is a useful element for achieving both high strength and high HAZ toughness. It is. However, if it exceeds 1.5%, the effect will be saturated and it will be economically disadvantageous. In addition, Preferably it is 0.1 to 1%.

One or more selected from Cr: 1% or less, Mo: 1% or less, Nb: 0.1% or less, V: 0.2% or less, B: 0.005% or less Cr, Mo , Nb, V, and B are all elements that contribute to improving the strength of the steel, and one or more elements are selected and added.

The Cr content is preferably 0.05% or more, but if it exceeds 1%, the toughness of the weld heat-affected zone is deteriorated, so when added, it is desirable to limit it to 1% or less.
Mo is added together with W to improve the general corrosion resistance and local corrosion resistance, and further helps to form a dense rust layer by the combined effect of W and Sn or Sb, thereby strengthening the corrosion resistance. Add to improve properties. However, if it exceeds 1%, the base material toughness and the weld heat-affected zone toughness are adversely affected. Therefore, when Mo is added, the content is preferably made 1% or less.

  Nb is preferably 0.005% or more, but if it exceeds 0.1%, the base metal toughness and the weld heat affected zone toughness are deteriorated. Is desirable.

V is preferably contained in an amount of 0.01% or more. However, if it exceeds 0.2%, the weld heat affected zone toughness is deteriorated.
B has the effect of increasing the strength of the steel through improving hardenability. On the other hand, if the content exceeds 0.005%, the hardenability is remarkably increased and the toughness and ductility of the base metal are deteriorated. In addition, Preferably, it is 0.0003 to 0.002%.

One or more selected from Sn: 0.005-0.3%, Sb: 0.005-0.3% Sn and Sb are both improved in overall corrosion resistance and local corrosion resistance It is an element that contributes to the above and is added according to the desired characteristics.

  Sn has the effect of suppressing the corrosion in an acidic environment by forming a dense rust layer by the combined effect with W, and is added when improving this property. However, addition of 0.005% or less has no effect, and addition of 0.3% or more leads to deterioration of hot workability, base material and HAZ toughness. %.

  Sb, like Sn, has a function of suppressing the corrosion in an acidic environment by forming a dense rust layer by the combined effect with W, and is added when this property is improved. However, addition of 0.005% or less has no effect, and addition of 0.3% or more causes saturation of the effect and deterioration of the base metal and weld heat affected zone toughness. Limited to 3%.

One or more selected from Ca: 0.005% or less, REM: 0.02% or less, and Mg: 0.005% or less Ca, REM, and Mg are all elements that contribute to toughness improvement Add according to the desired properties.

  Ca is a useful element that improves toughness through refinement of crystal grains, and is preferably 0.001% or more, but the effect is saturated even if it exceeds 0.005%. Has an upper limit of 0.005%.

  The REM content is preferably 0.002% or more, but the effect is saturated when it exceeds 0.02%. Therefore, when it is added, the upper limit is 0.02%.

  Mg is a useful element that improves toughness through refinement of crystal grains, and is preferably 0.001% or more, but the effect is saturated even if it exceeds 0.005%. Has an upper limit of 0.005%.

[Production conditions]
In the description, the “° C.” display relating to the temperature means a temperature at half the plate thickness.

Steel material heating temperature: 1000 ° C-1250 ° C
A steel material of a slab or steel slab having the composition described above is prepared from molten steel by a generally known method such as a converter, electric furnace, vacuum melting furnace, etc., and reheated to 1000 ° C to 1250 ° C. If the reheating temperature is less than 1000 ° C, the deformation resistance in hot rolling is high, and the amount of rolling reduction per pass cannot be increased. Therefore, the number of rolling passes increases and the rolling efficiency decreases, and the steel material ( In some cases, the casting defect in the slab cannot be crimped. On the other hand, if the reheating temperature exceeds 1250 ° C, surface flaws are likely to occur due to the scale during heating, and the care load after rolling increases, so the range is 1000 to 1250 ° C.

Hot rolling: The cumulative rolling reduction in the temperature range of Ar _{3 to} Ar ₃ + 150 ° C. is 25% or more, and the rolling finishing temperature Ar ₃ or more, the cumulative rolling reduction in the temperature range of Ar _{3 to} Ar ₃ + 150 ° C. is less than 25%. In addition, the recrystallization of the reheated austenite grains is not promoted by recrystallization, and the introduction of strain into the unrecrystallized austenite is insufficient. The toughness of the material decreases.

For this reason, the cumulative rolling reduction in the temperature range of Ar _{3 to} Ar ₃ + 150 ° C. is set to 25% or more. In the case of an extremely thick steel plate having a plate thickness exceeding 70 mm, it is desirable to secure at least one rolling pass with a reduction rate of 15% or more per pass for the zaku pressure bonding.

When the rolling end temperature is less than Ar ₃ , the deformation resistance increases, so the rolling load increases and the burden on the rolling mill increases. Moreover, in order to reduce the thick material to a rolling temperature lower than Ar ₃ , it is necessary to wait in the middle of rolling, which greatly hinders productivity. Therefore, the rolling end temperature is set to Ar ₃ or higher.

  After the end of rolling, cooling is performed at a cooling rate of 60 ° C./s or less. Cooling methods for obtaining a cooling rate of 60 ° C./s or less include air cooling and accelerated cooling, and are appropriately selected according to desired mechanical characteristics. If the cooling rate exceeds 60 ° C./s, it becomes difficult to control the temperature depending on the position of the steel sheet, resulting in material variations. The cooling rate is an average cooling rate obtained by averaging the cooling rates at the respective positions in the thickness direction.

Ar ₃ can be obtained by the following equation.
_{Ar 3 = 868-396C + 25Si-68Mn} -36Ni-25Cr-30Mo
(However, C, Si, Mn, Ni, Cr, Mo content (mass%))
In the present invention, the steel sheet may be cooled to room temperature and then reheated and tempered. The tempering treatment is performed at a heating temperature of 400 ° C. or higher and Ac ₁ or lower to improve toughness. If Ac ₁ is exceeded, strength will be reduced.

By using the steel material having the above composition, hot rolling, cooling, and tempering under the above-described conditions, an oxide layer containing W is formed on the surface of the steel sheet, and the corrosion resistance is improved. Steel plates for crude oil tanks with excellent toughness can be easily manufactured.
Furthermore, the steel of the present invention has the effect of extending the coating life, and when used as a steel material for crude oil tanks of tankers, the application of a primer containing Zn increases the local corrosion resistance and overall corrosion resistance. Can be improved.

  In general, since the surface of a steel plate is coated with a primer after shot blasting, a coating thickness of a certain level or more is required to cover the entire surface. The coating amount of the primer containing Zn is preferably 15 μm or more because the local corrosion resistance and the overall corrosion resistance are remarkably improved.

  From the standpoint of local corrosion resistance and overall corrosion resistance, there is no upper limit on the coating amount. However, as the primer becomes thicker, the cutting property, weldability and economy are worsened, so the upper limit is in the range up to 100 μm. preferable.

  As described above, according to the present invention, even when a primer or paint is used in combination, as a steel plate for various containers constituting an oil tank depot, a tank for transporting crude oil, or a tank for storing crude oil. Can be used widely and cheaply.

  Steel materials prepared in the composition shown in Table 1 by the converter-ladder refining-continuous casting method were made into thick steel plates by hot rolling under various conditions-accelerated cooling or air cooling, and further tempering. Table 2 shows hot rolling-accelerated cooling or air cooling, and further tempering conditions and the thickness of the obtained steel sheet.

  About each obtained thick steel plate, the JIS4 tension test piece was extract | collected from plate | board thickness 1/2 position. For some thick steel plates, JIS 1A full-thickness tensile specimens were collected. A tensile test was carried out in accordance with the standard of JIS Z 2241 to investigate the tensile properties.

In addition, JIS No. 4 V-notch impact test specimens were collected from the position of 1/4 of the thickness of each steel plate obtained in accordance with JIS Z 2202, and Charpy impact test was conducted in accordance with JIS Z 2242. The absorbed energy (vE- ₂₀ ) at -20 ° C was calculated and the base metal toughness was evaluated.
In addition, a joint test plate (plate thickness × width 400 × length 1000 (mm)) was collected from each of the obtained thick steel plates, the two pieces were butted together, and a welded joint was produced by FCB welding. The groove shape was a Y groove, and welding was performed at a heat input of 100 kJ / cm or more.

A JIS No. 4 V-notch impact test piece with the notch position as the bond part was collected from the position of the obtained welded joint with a thickness of 1/4, and Charpy at a test temperature of 0 ° C. in accordance with the provisions of JIS Z 2242. An impact test was conducted to determine the absorbed energy (vE ₀ ) at 0 ° C. of the joint bond part, and the joint toughness was evaluated.

  Further, from each of the obtained thick steel plates, a corrosion test plate (plate thickness × 50 × 50 (mm)) was sampled and subjected to shot blasting on the surface. 1. An inorganic zinc primer is not applied; What coated the inorganic type zinc primer (film thickness: 20 micrometers) was created, and the sludge containing the crude oil component extract | collected from the actual tanker was apply | coated uniformly, respectively, and it was set as the corrosion test piece.

  The corrosion test was performed by immersing these test pieces in the test solution 6 of the corrosion test apparatus for one month. FIG. 1 shows a block diagram of the corrosion test apparatus. The corrosion test apparatus is a double-type apparatus composed of a corrosion test tank 2 and a thermostatic chamber 3, and the corrosion test tank 2 is a test capable of generating local corrosion similar to the local corrosion that occurs in an actual crude oil tank bottom plate. The liquid is injected (hereinafter referred to as “test liquid 6”).

As the test liquid 6, artificial seawater specified in ASTM D1141 was used as a test mother liquid, and a mixed gas adjusted to a partial pressure ratio of 5% O ₂ + 10% H ₂ S was introduced into the test mother liquid. N ₂ gas was used as an inert gas for adjusting the balance of the mixed gas.

  The mixed gas adjusted by the inert gas is referred to as “introducing gas 4”. The temperature of the test solution 6 was maintained at 50 ° C. by adjusting the temperature of the water 7 put in the thermostat 3. Since the introduction gas 4 is continuously supplied, the test liquid 6 is constantly stirred by the bubbles 8. After the test, rust generated on the surface of the test piece was removed, the corrosion form was visually observed, and the local corrosion depth was measured with a dip meter.

Table 3 shows the mechanical properties of the base material. Each of the inventive examples has a high strength and high toughness base material characteristic of a tensile strength of 490 MPa or more and an absorbed energy vE- _{20 of} −20 ° C. of 100 J or more.

Table 4 shows the characteristics of the high heat input welded joint and the corrosion test results. In all the inventive examples, welding heat input: Even when high heat input welding exceeding 100 kJ / cm is applied, the weld heat-affected zone toughness having an absorbed energy vE ₀ of 0 ° C. at the bond portion of 100 J or more is excellent. It was confirmed that

  For local corrosion resistance, the occurrence of local corrosion is “no local corrosion: ◎”, and local corrosion occurs, but “local corrosion depth is less than 0.2 mm: ○”, “local corrosion depth is 0.2 mm or more 0” Less than .6 mm: ○ '"," local corrosion depth 0.6 mm or more and less than 1 mm: Δ ", and" local corrosion depth 1 mm or more: x ".

  In all of the inventive examples, the local corrosion resistance evaluation is good as ◎ or ○ regardless of whether or not the zinc primer is applied, and the local corrosion resistance evaluation is markedly improved when the zinc primer is applied. The steel of the present invention has a maximum depth of less than 0.5 mm even when local corrosion occurs in the unpainted state, has good local corrosion resistance, and is coated with an inorganic zinc primer. As a result, it was confirmed that the film has particularly good local corrosion resistance.

  On the other hand, in a comparative example that is out of the scope of the present invention, one or more of the base material strength, base material toughness, weld heat affected zone toughness, or local corrosion resistance does not satisfy the target value.

Corrosion test equipment (for local corrosion test)

Explanation of symbols

1 Test piece 2 Corrosion test bath 3 Constant temperature bath 4 Introduced gas 5 Exhaust gas 6 Test solution 7 Water 8 Bubble

Claims (8)

mass%,
C: 0.03-0.2%,
Si: 0.05 to 0.8%,
Mn: 0.5 to 2.5%,
P: 0.03% or less,
S: 0.005% or less,
Al: 0.005 to 0.06%,
W: 0.001 to 0.3%,
Ti: 0.005 to 0.03%,
N: 0.002 to 0.007%,
Ceq (= C + Mn / 6 + (Cr + Mo + V) / 5 + Ni / 15, provided that C, Mn, Cr, Mo, V are contained (mass%): 0.25 to 0.40%,
For crude oil tanks satisfying 2.0 <Ti / N <5.0, having a composition comprising the balance Fe and inevitable impurities, and having an oxide layer containing W formed on the steel surface steel. The steel composition according to claim 1, further in mass%,
Ni: 1.5% or less Cr: 1.0% or less Mo: 1.0% or less V: 0.2% or less Nb: 0.1% or less B: One or more of 0.005% or less Contains steel for crude oil tanks. The steel composition according to claim 1 or 2, further in mass%,
Sn: 0.005-0.3%
Sb: 0.005 to 0.3%
A crude oil tank steel containing one or more of the following. The steel composition according to any one of claims 1 to 3, further in mass%,
Steel for crude oil tanks containing one or more of Ca: 0.005% or less REM: 0.02% or less and Mg: 0.005% or less. After the steel material having the composition according to any one of claims 1 to 4 is heated to 1000 to 1250 ° C, the cumulative rolling reduction in the temperature range of Ar _{3 to} Ar ₃ + 150 ° C is 25% or more, and the rolling finish is performed. A method for producing steel for a crude oil tank, characterized by performing hot rolling at a temperature of Ar ₃ or higher and subsequently cooling at a cooling rate of 60 ° C./s or lower. 6. The method for producing steel for a crude oil tank according to claim 5, further comprising tempering at 400 ° C. or more and Ac ₁ transformation point or less after cooling. A crude oil tank steel obtained by applying a primer coating containing Zn on a surface of the crude oil tank steel according to any one of claims 1 to 4. A crude oil tank using the steel for a crude oil tank according to claim 7.

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