JP6241555B2 - Steel for ethanol storage and transport equipment - Google Patents

Description

  The present invention relates to a structural steel suitable for use in ethanol storage and transportation equipment members. That is, the steel of the present invention is suitable as a material for ethanol storage equipment members and ethanol transport equipment members. Further, the steel of the present invention relates to a structural steel excellent in ethanol corrosion resistance that can be used in a corrosive environment of ethanol containing carboxylic acid, chloride ions and water, particularly bioethanol.

  Bioethanol is mainly produced by decomposing and purifying sugars such as corn and wheat. In recent years, bioethanol has been widely used around the world as an alternative fuel for petroleum (gasoline) and as a fuel mixed with gasoline, and the amount of use tends to increase year by year. Therefore, in the process of storing and transporting bioethanol or mixing with gasoline, etc., the amount of bioethanol that has been handled is increasing, that is, pitting corrosion, especially stress corrosion. The point at which cracking (SCC) proceeds makes its handling difficult.

  In bioethanol, acetic acid and chloride ions are contained as trace impurities in the production process, and water absorption and dissolved oxygen are taken in during storage. In particular, SCC caused by bioethanol has a risk of causing a serious bioethanol leakage accident once it occurs. Therefore, SCC by bioethanol is a corrosion phenomenon that is regarded as the most problematic, and it is considered important to prevent its occurrence in operation.

  From the above, there is a drawback that bioethanol can be handled safely only in equipment that has measures for ethanol resistance, for example, tanks that use organic coating materials with excellent ethanol corrosion resistance, stainless steel, and stainless clad steel. is there. In addition, the transportation of bioethanol has a problem that a conventional pipeline for transporting oil cannot be used. Thus, the facilities that handle bioethanol remain problematic in that they require a great deal of cost.

  As an attempt to solve the above problem, for example, in Patent Document 1, as a measure against biofuel, zinc-nickel plating is applied to a tank steel material containing 5 to 25% by mass of Ni, or hexavalent on this plating. A method of performing chemical conversion treatment not containing chromium has been proposed. According to this method, it is said that the corrosion resistance in ethanol-containing gasoline is good.

  Patent Document 2 discloses corrosion resistance in which Zn-Co-Mo plating with a Co composition ratio of 0.2 to 4.0 at% on the steel sheet surface is used for handling fuel vapor such as bioethanol. Excellent steel for pipes has been proposed.

  In Patent Document 3, in mass%, Cr: 0.01 to 1.0%, Cu: 0.05 to 1.0%, Sn: 0.01 to 0.2%, and Ni: 0.01 A steel material excellent in alcohol corrosion resistance containing two or more selected from -1.0% has been reported.

  Non-Patent Document 1 examines the inhibitory effect of ammonium hydroxide on SCC (stress corrosion cracking) of steel in a simulated bioethanol solution. Non-Patent Document 1 reports that the addition of ammonium hydroxide to the simulated liquid suppresses crack propagation and mitigates SCC.

JP 2011-26669 A JP 2011-231358 A JP2013-129904A

F. Gui, J .; A. Beavers and N.W. Sridhar, Evaluation of ammonia hydroxide for mitigating stress corrosion cracking of carbon steel in fuel grade paper, NACE Corrosion Paper. 11138 (2011)

  SCC refers to a cracking phenomenon that is inherently caused by the interaction between a corrosive environment and static stress. Bioethanol SCC is often observed in facilities exposed to fluctuating load environments, and thus is considered to be essentially a corrosion fatigue phenomenon. In contrast to SCC that occurs under static stress, corrosion fatigue that occurs under dynamic stress is a severe fracture phenomenon in which cracks grow at a faster rate at a lower stress. That is, the present inventors considered that in order to prevent bioethanol SCC, it is necessary to increase the corrosion fatigue resistance in an ethanol environment.

  The zinc-nickel plating disclosed in Patent Document 1 is considered effective for improving the corrosion resistance. However, since such Zn—Ni plating requires processing by electroplating, there is no problem in a small fuel tank for automobiles, for example. However, electroplating cannot be applied to large structures, for example, thick steel materials having a plate thickness of 3 mm or more that are applied to storage tanks or line pipes having a storage amount of 1000 kL or more because the processing cost is enormous. In addition, when plating defects occur, pitting corrosion tends to proceed on the part, and corrosion fatigue is likely to occur, which is not sufficient from the viewpoint of pitting corrosion resistance and corrosion fatigue resistance.

  The Zn—Co—Mo plating disclosed in Patent Document 2 also needs to be treated by electroplating, so that it can be applied to thick steel materials of large structures for the same reason as Patent Document 1. Can not. Also, for the same reason as in Patent Document 1, it cannot be said that it is sufficient from the viewpoint of pitting corrosion resistance and corrosion fatigue resistance.

  The steel material disclosed in Patent Document 3 is effective in pitting corrosion resistance, but corrosion fatigue resistance is not considered. Therefore, it cannot be said that the steel material disclosed in Patent Document 3 satisfies the ethanol corrosion resistance required for an actual structure.

  Furthermore, according to the description in Non-Patent Document 1, the addition of an inhibitor certainly alleviates a corrosion phenomenon such as corrosion fatigue, but the effect is not sufficient. This is because the inhibitor adsorbs on the surface and exerts its effect, but its adsorption behavior is greatly influenced by the surrounding pH and the like. For this reason, when corrosion occurs locally, the case where adsorption | suction cannot fully occur may occur. In addition, there is a risk of contamination due to the outflow of the inhibitor, and addition of the inhibitor is not a suitable countermeasure against corrosion.

  As described above, the anticorrosion method by plating is not suitable for large structures, and the addition of an inhibitor is insufficient due to variations in the corrosion reduction effect on the structural steel surface. For this reason, steel having excellent corrosion resistance, particularly corrosion fatigue resistance in a bioethanol environment containing carboxylic acid, chloride ions, and water as impurities has been eagerly desired for use in ethanol storage and transportation facilities.

  An object of the present invention is to solve such problems of the prior art and to provide structural steel for ethanol storage and transportation equipment members such as steel pipes having excellent ethanol corrosion resistance that can be used in a bioethanol environment. To do. Here, “excellent in ethanol corrosion resistance” means excellent in corrosion fatigue resistance in an ethanol environment containing carboxylic acid, chloride ion, and water as impurities.

  In order to solve the above-mentioned problems, the present inventors have conducted intensive research toward the development of steel for ethanol storage and transportation equipment exhibiting excellent corrosion fatigue resistance in a bioethanol environment. As a result, it is effective to contain Mo and W in suppressing corrosion fatigue in a bioethanol environment, and it is effective to contain Sb and / or Sn and further Al in addition to Mo and W. I understood it. In addition, the present inventors have found that the corrosion fatigue resistance is remarkably improved by reducing the N content. Note that these effects can be effectively applied to SCC under a static load environment where the stress conditions are milder. The present invention has been completed after further studies based on the above findings, and the gist thereof is as follows.

[1] By mass%
C: 0.02-0.3%,
Si: 0.01 to 1.0%,
Mn: 0.1 to 2.0%,
P: 0.003 to 0.03%,
S: 0.01% or less,
Al: 0.005 to 0.100%,
N: 0.0010 to 0.010% is contained, and the content ratio of Al and N satisfies 2.0 ≦ Al / N ≦ 70.0,
Furthermore, it contains at least one selected from the group of W: 0.010 to 0.5% and Mo: 0.010 to 0.5%,
And ethanol storage and transportation containing at least one selected from the group of Sb: 0.01 to 0.5% and Sn: 0.01 to 0.3%, the balance consisting of Fe and inevitable impurities Steel for equipment.

[2] Further, by mass%,
Cu: 0.05 to 1.0%,
Cr: 0.01-1.0% and Ni: 0.01-1.0%
The steel for ethanol storage and transportation equipment according to [1], which contains at least one selected from the group of [1].

[3] Further, by mass%,
Ca: 0.0001 to 0.02%,
Mg: 0.0001-0.02% and REM: 0.001-0.2%
The steel for ethanol storage and transportation equipment according to [1] or [2], which contains at least one selected from the group of [1].

[4] Further, by mass%,
Ti: 0.005 to 0.1%,
Zr: 0.005 to 0.1%,
Nb: 0.005-0.1% and V: 0.005-0.1%
The steel for ethanol storage and transportation equipment according to any one of [1] to [3], containing at least one selected from the group of [1] to [3].

  [5] The steel for ethanol storage and transport equipment according to any one of [1] to [4], further having a tensile strength of 825 MPa or less and a yield strength of 705 MPa or less.

  ADVANTAGE OF THE INVENTION According to this invention, the steel for ethanol storage and transportation equipment excellent in ethanol corrosion resistance which can be used also in the bioethanol environment containing carboxylic acid, a chloride ion, and water can be obtained. When the present invention is used as a storage tank or transport tank for bioethanol and steel for pipeline construction, it can be used for a longer period of time than before and accidents due to bioethanol leakage due to corrosion fatigue are avoided. Furthermore, these facilities can be provided at low cost, which is extremely useful in the industry.

  The present invention will be specifically described below.

  The reason why the component composition of the steel material is limited to the above range in the present invention will be described. In addition, the unit of element content in the component composition of the steel material is “mass%”, and hereinafter, it is simply indicated by “%” unless otherwise specified.

C: 0.02-0.3%
C is an element necessary for ensuring the strength of the steel, and is contained in an amount of at least 0.02% in order to ensure the preferred yield strength (350 MPa or more) and tensile strength (400 MPa or more) in the present invention. The amount of C is preferably 0.03% or more. On the other hand, if the amount of C exceeds 0.3%, the weldability deteriorates and restrictions are imposed during welding, so 0.3% was made the upper limit. The amount of C is preferably 0.20% or less. In the present invention, from the viewpoint of obtaining good corrosion fatigue resistance, the amount of C is more preferably 0.10% or less.

Si: 0.01 to 1.0%
Si is added for deoxidation, but if the content is less than 0.01%, the deoxidation effect is poor. On the other hand, if the Si content exceeds 1.0%, the toughness and weldability are deteriorated. 0.01 to 1.0%. In addition, about the minimum of Si amount, 0.03% is preferable, 0.05% is more preferable, and 0.20% is further more preferable. About the upper limit of Si amount, 0.7% is preferable and 0.5% is more preferable.

Mn: 0.1 to 2.0%
Mn is added to improve strength and toughness. However, if the amount of Mn is less than 0.1%, the effect is not sufficient. On the other hand, if the amount of Mn exceeds 2.0%, weldability deteriorates. The content is 0.1 to 2.0%. In addition, about the minimum of the amount of Mn, 0.3% is preferable and 0.5% is more preferable. About the upper limit of the amount of Mn, 1.6% is preferable, 1.3% is more preferable, and 1.0% is further more preferable.

P: 0.003-0.03%
Since P deteriorates toughness and weldability, the P content is limited to 0.03% or less. Since excessive reduction of P becomes disadvantageous from the viewpoint of dephosphorization cost, the lower limit of P content is 0.003%. The P content is preferably in the range of 0.003 to 0.025%, and more preferably in the range of 0.003 to 0.015%.

S: 0.01% or less S is an important element that affects the corrosion resistance of the steel of the present invention. S is inevitably contained, and when the content is increased, not only the toughness and weldability are lowered, but also inclusions such as MnS, which are starting from corrosion fatigue, are increased and the corrosion fatigue resistance is lowered. In addition, since the inclusion that becomes the starting point of corrosion fatigue also becomes a preferential anode site, pitting corrosion is also promoted. Therefore, it is desirable to reduce the S amount as much as possible, and it is acceptable if it is 0.01% or less. Note that the S amount is preferably 0.005% or less, and more preferably 0.003% or less. On the other hand, for the above reason, the lower limit of the amount of S is not particularly specified.

Al: 0.005 to 0.100%
Al is added as a deoxidizer, but if the content is less than 0.005%, the toughness decreases due to insufficient deoxidation. On the other hand, when the content of Al exceeds 0.100%, when welding, the toughness of the weld metal part is lowered, so the Al content is limited to 0.100% or less.

Further, Al has a function of further enhancing the acid resistance improving effect of Sb and Sn described later. That is, Al ³⁺ ions eluted as the base material dissolves in the anode undergoes a hydrolysis reaction with water present in a small amount in bioethanol, so that the pH at the anode site is lowered, and Sb oxide and Sn oxide described later are produced. The formation of is promoted. This effect is manifested by the inclusion of 0.005% or more of Al. On the other hand, if the Al content exceeds 0.100%, the pH drop at the anode site is remarkably accelerated, resulting in excessively low pH, and the effect of improving the corrosion resistance by promoting the formation of Sb oxide and Sn oxide is sufficiently obtained. Disappear. From the viewpoint of achieving both toughness and corrosion fatigue resistance, the lower limit of the Al content is preferably 0.010%, more preferably 0.015%, and even more preferably 0.020%. Similarly, the upper limit of the Al content is preferably 0.070%, more preferably 0.060%, and even more preferably 0.050% or less.

N: 0.0010 to 0.010%, 2.0 ≦ Al / N ≦ 70.0
N is an important element affecting the corrosion fatigue resistance in the steel of the present invention. By reducing the N content, the formation of coarse nitrides is suppressed, and the corrosion fatigue life is improved. On the other hand, if the N content exceeds 0.010%, the formation of coarse AlN is promoted, and the effect of improving the corrosion fatigue resistance by Al described above cannot be sufficiently obtained, and the coarse AlN itself is the starting point of corrosion fatigue. As a result, the corrosion fatigue susceptibility increases. For this reason, the N content is limited to 0.010% or less. Note that the N content is preferably 0.007% or less, and more preferably 0.005% or less. N also has an important function in order to stably obtain the above-described effect of improving corrosion fatigue resistance by Al. That is, lowering the pH by hydrolysis of Al ³⁺ ions brings about an improvement in corrosion fatigue resistance by promoting the formation of Sb oxide and Sn oxide, but if the pH drops excessively, the total corrosion fatigue resistance deteriorates. there's a possibility that. Here, in the N steel consumes H ⁺ with the anodic dissolution, the formation of the NH ₄ ^+, shows inhibiting buffering action excessive pH reduction. In order to obtain this buffering action, it is necessary to contain at least 0.0010% of N. Therefore, the lower limit of the N content is set to 0.0010%. The lower limit of the N amount is preferably 0.0015%.

In addition, as described above, Al and N are greatly related to the formation of AlN and the effect of improving the corrosion fatigue resistance by Al, and it is necessary to make the Al amount / N amount (mass ratio) in steel materials appropriate. Is important. When the amount of Al is too much with respect to the amount of N, that is, when it exceeds 70.0, the formation rate of AlN is remarkably increased, resulting in coarsening of AlN. In addition, the buffering action due to the formation of NH ₄ ⁺ cannot catch up. Therefore, the upper limit of the amount of Al / N is 70.0. The upper limit with preferable Al amount / N amount is 50.0, and a more preferable upper limit is 20.0. On the other hand, when the amount of Al / N is less than 2.0, most of the Al in the steel exists as AlN, and Al ³⁺ ions are not sufficiently generated due to the anodic dissolution of the base material. That is, the acid resistance improvement effect of Sb and Sn by Al cannot be sufficiently obtained. Therefore, the lower limit of the amount of Al / N is 2.0. A preferable lower limit of the amount of Al / N is 3.0, and a more preferable lower limit is 5.0.

At least one W selected from the group of W: 0.010 to 0.5% and Mo: 0.010 to 0.5% is an element effective for improving corrosion fatigue resistance. W forms oxyacid ions as a corrosion product, similar to Mo, so when a crack that is the starting point of stress corrosion cracking occurs, the corrosion product quickly adsorbs to the crack tip, reducing the anode reaction activity. , Has the function of suppressing the progress of cracks. In addition, by incorporating W into the oxide film on the surface of the steel material, the dissolution resistance of the oxide film in an acidic environment due to the carboxylic acid contained as an impurity in bioethanol is improved, reducing uneven corrosion, It also has the effect of reducing pitting corrosion. However, if the W content is less than 0.010%, the effects of improving corrosion fatigue resistance and pitting corrosion resistance are not sufficiently exhibited. On the other hand, if the W content exceeds 0.5%, the cost becomes disadvantageous, so the W content is set to 0.010 to 0.5%. The lower limit of the amount of W is preferably 0.05%, more preferably 0.08%. In order to prevent an increase in cost, the upper limit of the W amount is preferably 0.3%. The upper limit of the amount of W is more preferably 0.2%.

  Mo is an element effective for improving corrosion fatigue resistance. Mo forms oxyacid ions as corrosion products, so when a crack that becomes the starting point of corrosion fatigue occurs, the corrosion product immediately adsorbs to the crack tip, lowers the anode reaction activity, and causes the crack to progress. Has a function to suppress. In addition, by incorporating Mo into the oxide film on the steel surface, the dissolution resistance of the oxide film in an acidic environment due to carboxylic acid contained as an impurity in bioethanol is improved, reducing non-uniform corrosion, It also has the effect of reducing pitting corrosion. However, if the Mo content is less than 0.010%, the effects of improving corrosion fatigue resistance and pitting corrosion resistance are not sufficiently exhibited. On the other hand, if the Mo content exceeds 0.5%, the cost becomes disadvantageous, so the Mo content is set to 0.010 to 0.5%. The lower limit of the amount of Mo is preferably 0.05%, more preferably 0.08%. Furthermore, in order to prevent an increase in cost, the upper limit of the Mo amount is preferably 0.4%, and more preferably 0.3%.

  In addition, in this invention, it is preferable to contain the said W and Mo from a viewpoint of obtaining favorable corrosion fatigue resistance.

At least one Sb selected from the group of Sb: 0.01 to 0.5% and Sn: 0.01 to 0.3% is an element that improves acid resistance, and is important in the steel of the present invention. It is an element that improves corrosion fatigue resistance. In particular, it is an effective element for suppressing crack propagation at the tip of a corrosion fatigue crack, which is a low pH environment. Sb remains and concentrates at the anode site as an oxide as the base material dissolves in the anode. As a result, the anode part is protected, the progress of the dissolution reaction is remarkably suppressed, and the corrosion fatigue resistance is improved. However, if the Sb content is less than 0.01%, the effect is poor. On the other hand, if the Sb content exceeds 0.5%, there are restrictions in terms of steel production, so the Sb content is 0.01 to 0.5. % Range. The lower limit of the Sb amount is preferably 0.02%, more preferably 0.05%. The upper limit of the amount of Sb is preferably 0.4%, more preferably 0.30%.

  Sn, like Sb, is an element that improves acid resistance, and is an important element for improving corrosion fatigue resistance in the steel material of the present invention. In particular, it is an effective element for suppressing crack propagation at the tip of a corrosion fatigue crack, which is a low pH environment. Sn remains and concentrates at the anode site as an oxide as the base material is dissolved in the anode. As a result, the anode part is protected, the progress of the dissolution reaction is remarkably suppressed, and the corrosion fatigue resistance is improved. However, if the content is less than 0.01%, the effect is poor. On the other hand, if the Sn content exceeds 0.3%, there are restrictions in terms of steel production, so the Sn content is 0.01 to 0.3%. The range. Note that the lower limit of the Sn content is preferably 0.02%, and more preferably 0.05%. The upper limit of the Sn content is preferably 0.30%, and more preferably 0.15%.

  In addition, in this invention, it is preferable to contain the said Sb and Sn from a viewpoint of obtaining favorable corrosion fatigue resistance.

  Among the components described above, in the present invention, a combination of a high-speed surface protection effect by Mo oxyacid ions and W oxyacid ions and a strong surface protection effect by Sb oxide and Sn oxide are combined. is important. That is, when the corrosion fatigue crack growth rate is high, the formation of Sb oxide and Sn oxide at the crack tip does not naturally catch up, and the Sn and Sb crack surface protecting action cannot be obtained. However, when Mo and W coexist, a rapid surface protection action by Mo oxyacid ions and W oxyacid ions at the crack portion works first. As a result, the crack growth rate decreases, and the formation of Sb oxide and Sn oxide at the crack tip catches up. As a result, the crack tip is covered with a strong surface protective layer composed of two layers of an oxyacid ion layer and an oxide layer, and corrosion fatigue is remarkably suppressed. Here, in order to promote the formation of Sb oxide and Sn oxide, it is important to control the amount of Al and reduce the amount of N. Since the reduction of the N amount contributes to the reduction of the corrosion fatigue starting point, a superimposed effect of improving the corrosion fatigue resistance can be obtained.

  The basic components have been described above, but in the present invention, other components described below can be appropriately contained as necessary.

At least one Cu, Cr, Ni selected from the group of Cu: 0.05-1.0%, Cr: 0.01-1.0% and Ni: 0.01-1.0% is bioethanol It is an effective element for improving the corrosion fatigue resistance in an acidic environment due to carboxylic acid contained as an impurity. However, when the content is small, there is no effect. On the other hand, if the content exceeds 1.0%, there is a restriction in terms of steel production, so the Cu content is 0.05 to 1.0%, Cr content is contained. The amount is 0.01 to 1.0%, and the Ni content is 0.01 to 1.0%. The upper limit of the Cu content is preferably 0.5%, more preferably 0.2%. The upper limit of the Cr content is preferably 0.5% and more preferably 0.2%. The upper limit of the Ni content is preferably 0.5% and more preferably 0.2%.

At least one selected from the group of Ca: 0.0001 to 0.02%, Mg: 0.0001 to 0.02% and REM: 0.001 to 0.2% As described above, MnS is pitting corrosion, Ca, Mg, and REM are effective elements from the viewpoint of controlling the morphology and dispersion of sulfides in steel, which are harmful as a starting point of corrosion fatigue. This effect cannot be sufficiently obtained when the content is small. On the other hand, when the content is large, Ca, Mg, and REM itself are coarse inclusions, which become starting points for pitting corrosion and corrosion fatigue. Therefore, the Ca content is in the range of 0.0001 to 0.02%, the Mg content is in the range of 0.0001 to 0.02%, and the REM content is in the range of 0.001% to 0.2%. The lower limit of the Ca content is preferably 0.001%. The upper limit of the Ca content is preferably 0.005%. The lower limit of the Mg content is preferably 0.001%. The upper limit of the Mg content is preferably 0.005%. The upper limit of the REM content is preferably 0.030%.

At least selected from the group of Ti: 0.005-0.1%, Zr: 0.005-0.1%, Nb: 0.005-0.1% and V: 0.005-0.1% In order to improve the mechanical properties of the type 1 steel, one or more types selected from Ti, Zr, Nb, and V may be contained. Any of these elements has a poor content effect if the content is less than 0.005%, whereas if the content exceeds 0.1%, the mechanical properties of the welded portion deteriorate, so the content of each element is The range was 0.005 to 0.1%. In addition, about each element, Preferably content is 0.005 to 0.05% of range.

  In the steel material of the present invention, components other than those described above are Fe and inevitable impurities. Furthermore, as long as the effects of the present invention are not impaired, the inclusion of components other than those inevitably included is not rejected.

  In an ethanol solution containing 0.02 mmol / L or more of carboxylic acid, 0.02 mg / L or more of chloride ion, and 0.05 vol% or more of water, particularly in a pitting portion and a crack tip portion under a low pH environment. Will be exposed. Therefore, in addition to the occurrence of pitting corrosion and cracks, embrittlement cracks due to secondary hydrogen can be superimposed. In order to suppress the hydrogen embrittlement susceptibility of the steel, it is preferable that the steel of the present invention has a tensile strength of 825 MPa or less and a yield strength of 705 MPa or less.

  The steel of the present invention is suitable for use in ethanol storage and transportation facilities. The steel of the present invention is a steel excellent in ethanol corrosion resistance that can be used in a corrosive environment of ethanol containing carboxylic acid, chloride ions and water, particularly bioethanol.

  In the present invention, the carboxylic acid is an aliphatic carboxylic acid and has 1 to 5 carbon atoms. In the present invention, the ethanol storage and transport equipment refers to equipment for storing, transporting, transporting, accumulating, distributing, collecting, blending, etc. ethanol. Examples of the equipment include a tank, a steel pipe, a tanker, piping, a pipeline, a nozzle, and a valve. The shape of the steel for ethanol storage and transportation equipment of the present invention can be selected as appropriate, but is preferably a steel plate. The preferable thickness (wall thickness) of the steel of the present invention is 1 to 50 mm, more preferably 3 to 50 mm, and further preferably 5 to 50 mm.

  Next, the suitable manufacturing method of this invention steel material is demonstrated.

  The molten steel having the above component composition is melted in a known furnace such as a converter or an electric furnace, and is made into a steel material such as a slab or billet by a known method such as a continuous casting method or an ingot forming method. In addition, vacuum degassing refining or the like may be performed at the time of melting.

  The component adjustment method of molten steel should just follow a well-known steel smelting method.

  Next, when the steel material is hot-rolled to a desired size and shape, it is preferably heated to a temperature of 1000 to 1350 ° C. When the heating temperature is less than 1000 ° C., the deformation resistance is large, and hot rolling tends to be difficult. On the other hand, heating exceeding 1350 ° C. may cause generation of surface marks, increase scale loss, and increase fuel consumption. The heating temperature is more preferably in the range of 1050 to 1300 ° C. In addition, when the temperature of a steel raw material is the range of 1000-1350 degreeC from the first, you may use for hot rolling as it is, without heating.

  In hot rolling, the finish temperature of hot finish rolling is usually optimized. The hot finish rolling end temperature is preferably 600 ° C. or higher and 850 ° C. or lower. When the finish temperature of hot finish rolling is less than 600 ° C., the rolling load increases due to an increase in deformation resistance, which may make it difficult to perform the rolling. On the other hand, if the temperature exceeds 850 ° C., the desired strength may not be obtained. The cooling after the hot finish rolling is preferably air cooling or accelerated cooling at a cooling rate of 150 ° C./s or less. The cooling stop temperature for accelerated cooling is preferably in the range of 300 to 750 ° C. Note that, after cooling, reheating treatment may be performed.

  Next, examples of the present invention will be described. In addition, this invention is not limited only to these Examples. In the description of the examples, Table 1-1 and Table 1-2 are collectively referred to as Table 1. Table 2-1 and Table 2-2 are collectively referred to as Table 2.

  The molten steel having the composition shown in Table 1 was made into a slab by continuous casting after melting in a vacuum melting furnace or after melting in a converter. Then, after heating to 1230 ° C., hot rolling was performed under the condition of finish rolling end temperature: 850 ° C. to obtain a 15 mm thick steel plate.

  A micro tensile test piece (parallel portion 6 mmφ × 25 mm) was taken in the C direction (width direction) of the steel plate thus obtained, and subjected to a tensile test at room temperature in accordance with the provisions of JIS Z 2241, yield strength (YS) and Tensile strength (TS) was determined. The results are shown in Table 1.

  Furthermore, the corrosion fatigue test described below was performed.

First, a single-axis round bar tensile test piece (parallel portion dimension: length 25.4 mm × diameter 3.81 mmφ) was cut out from the steel plate, and the parallel portion was polished with a count equivalent to 2000 finish. Thereafter, ultrasonic degreasing was performed for 5 minutes in acetone, air-dried, and attached to a low strain rate tensile tester. A solution in which water: 10 ml, methanol: 5 ml, acetic acid: 56 mg, and NaCl: 13.2 mg were added to ethanol: 985 ml was used as a bioethanol simulation solution. The cell covering the single-axis round bar tensile test piece is filled with bioethanol simulation liquid, and the maximum stress is measured in the tensile axis direction of the single-axis round bar tensile test piece based on the yield strength (YS) measured before the test. Fluctuating stress with a yield strength of 110% and a minimum stress of yield strength of 10% was applied at a cycle of 8.3 × 10 ⁻⁴ Hz for a maximum of 240 hours.

  In the evaluation, first, the presence or absence of breakage during the test period was confirmed. Next, about the uniaxial round bar tensile test piece which did not fracture | rupture, the test piece was taken out after the test, the external appearance observation with the microscope was implemented, and the presence or absence of the crack was confirmed. About the test piece by which the crack was confirmed, the cross section of the tensile axis direction was observed, and the cross-section maximum crack length was measured. The corrosion fatigue resistance was evaluated according to the following criteria. About crack length less than 20 micrometers, it was judged that the crack progress was slow and the risk that the corrosion fatigue fracture in an implementation equipment would be low (pass).

◎: No crack ○: Micro crack (crack length less than 20μm)
Δ: Cracked (crack length 20 μm or more)
X: Breaking Table 2 shows the obtained results.

  As is clear from Table 2, it can be seen that all of the inventive examples clearly improved the degree of corrosion fatigue cracks in the bioethanol simulated liquid. On the other hand, all the comparative examples whose component compositions deviated from the scope of the invention were broken or had a large degree of corrosion fatigue cracks.

  From the comparison between the inventive example and the comparative example, the improvement effect of the present invention is clear. Further, according to the Auger spectroscopic analysis performed on the crack tip of the invention example in which the crack occurred, the concentrated layer of the oxyacid ion forming element (W or Mo) and the concentration of the oxide forming element (Sn or Sb) are formed at the crack tip. It was confirmed that a surface layer divided into two layers was formed. That is, in the invention example, the crack tip was protected by the strong protective layer.

Claims (4)

% By mass
C: 0.02-0.3%,
Si: 0.01 to 1.0%,
Mn: 0.1 to 2.0%,
P: 0.003 to 0.03%,
S: 0.01% or less,
Al: 0.005 to 0.100%,
N: 0.0010 to 0.010% is contained, and the content ratio of Al and N satisfies 2.0 ≦ Al / N ≦ 70.0,
Furthermore, it contains at least one selected from the group of W: 0.010 to 0.5% and Mo: 0.010 to 0.5%,
And it contains at least 1 sort (s) selected from the group of Sb: 0.01-0.5% and Sn: 0.01-0.3%, and the remainder consists of Fe and an unavoidable impurity, 825 MPa or less tension | tensile_strength A steel for ethanol storage and transportation equipment having strength and yield strength of 705 MPa or less . In addition,
Cu: 0.05 to 1.0%,
Cr: 0.01-1.0% and Ni: 0.01-1.0%
The steel for ethanol storage and transportation equipment according to claim 1, comprising at least one selected from the group consisting of: In addition,
Ca: 0.0001 to 0.02%,
Mg: 0.0001-0.02% and REM: 0.001-0.2%
The steel for ethanol storage and transportation equipment according to claim 1 or 2, which contains at least one selected from the group consisting of: In addition,
Ti: 0.005 to 0.1%,
Zr: 0.005 to 0.1%,
Nb: 0.005-0.1% and V: 0.005-0.1%
The steel for ethanol storage and transportation equipment according to any one of claims 1 to 3, comprising at least one selected from the group of (1).