JP5942532B2 - Steel material with excellent corrosion resistance - Google Patents

Description

  The present invention relates to a steel material having excellent corrosion resistance used in a corrosive environment containing chloride.

  It is well known that the influence of chloride is extremely large as a factor that accelerates corrosion of steel materials. In particular, structures such as bridges in coastal areas, steel sheet piles and steel pipe piles used in harbor facilities, ship skins and ballast tanks, offshore structures, offshore wind power generation facilities, etc. are directly subject to seawater splashes. Corrosion is extremely high due to repeated exposure to wet and dry environments. Further, even in seawater, although it is not as dry and wet as the environment, it is highly corroded and may cause problems in long-term use. Although there is no splash of seawater in the beach area, corrosion is accelerated by the arrival of sea salt particles. In addition, in the inland area, corrosion by chloride is a problem everywhere, such as spraying an antifreeze containing chloride to prevent road surface freezing in winter. Furthermore, ore carriers and crude oil tanker tanks that are not directly exposed to the seawater environment but are washed with seawater are subject to corrosion by chloride remaining after washing. In addition, the crude oil tanker has a severe corrosive environment in which drain water, which is a high concentration chloride solution, is present. In addition, corrosion due to chloride is a problem in oil sand drilling and transportation facilities.

  Under these circumstances, steel materials are used in an environment where corrosion due to chloride is a problem. Corrosion occurs due to deterioration of the coating film and from thin parts such as steel edges. Because of the progress, maintenance (repainting) is essential when the structure is used for a long time. In that case, depending on the structure, it may be necessary to install a scaffold, etc., and maintenance costs become enormous, and a large amount of VOC (volatile organic compounds) that are harmful to the human body due to painting. Etc. becomes a problem. For these reasons, it has been strongly desired to develop a steel material having good corrosion resistance without being painted, or a steel material capable of extending the repainting interval.

  As steel materials excellent in corrosion resistance in such a chloride environment, steel materials with an increased Cr content as shown in Patent Document 1, steel materials with an increased Ni content as shown in Patent Document 2, and the like have been proposed. ing.

  As steel having excellent corrosion resistance without increasing Cr and Ni, for example, Patent Document 3 discloses a steel material containing P, Ni, and Mo as essential elements and added with Sb and / or Sn, and Patent Document 4 Discloses a steel material to which P, Cu, Ni, and Sb are essentially added.

  Further, Patent Document 5 discloses a steel material in which Cu is an essential element and Sb and / or Sn is added.

Japanese Patent Laid-Open No. 9-1779090 JP-A-5-51668 Japanese Patent Laid-Open No. 10-251797 JP 2002-53929 A Japanese Patent Laid-Open No. 9-25536

  However, Cr is an element that generally contributes to the corrosion resistance of steel materials. However, in steel materials with an increased Cr content as described in Patent Document 1, a certain amount of Cr is contained in steel materials in extremely severe chloride environments. However, the corrosion resistance becomes insufficient, and the steel material may not withstand long-term use. Although the effect of improving corrosion resistance can be expected even when the Ni content in the steel is increased, the steel with an increased Ni content shown in Patent Document 2 has sufficient corrosion resistance in a very severe chloride environment. There is a problem that the cost of the steel material becomes high.

  And the steel material for welded structures disclosed in Patent Document 3 with P, Ni, and Mo as essential elements and with the addition of Sb and / or Sn contains 0.03% or more of P that inhibits weldability. The weldability remains uneasy. On the other hand, the steel material disclosed in Patent Document 4, to which P, Cu, Ni, and Sb are essentially added, is only said to have good weather resistance in an environment with an incoming salt content of 0.8 mdd, and is severe enough to exceed that. There is a problem that the weather resistance is not sufficient in an environment where salt is present.

  Furthermore, the steel material disclosed in Patent Document 5, containing Cu as an essential element and added with Sb and / or Sn, is a steel material having corrosion resistance against combustion exhaust gas discharged when burning heavy oil, etc. This steel material is used in an environment greatly different from the chloride environment according to the invention. Therefore, such a steel material cannot always be applied as it is in the chloride environment of the present invention.

  This invention is made | formed in view of the said present condition, The objective is to provide the corrosion-resistant steel materials which are excellent in the resistance with respect to corrosion by chlorides, such as seawater.

In order to solve the above-mentioned problems, the present inventors have studied in detail the corrosion promotion mechanism by chlorides. As a result, in an environment rich in chlorides, the pH of the corrosion interface decreases due to hydrolysis of Fe ³⁺ . Therefore, it discovered that corrosion was accelerated | stimulated. With conventional steel materials, the protection of the rust layer could not be expected in an environment where there are very many chlorides. However, the steel material containing a specific element forms a very highly protective rust layer. In particular, it was found that the elution of iron ions, which causes a decrease in pH at the corrosion interface, was remarkably suppressed, and as a result, it had the effect of suppressing the decrease in pH.

  As a result of examining the influence of various alloy elements on the weather resistance based on the above-mentioned concept, the present inventors have obtained the knowledge shown in the following (a) to (i).

  (a) By containing Al at a certain concentration or more, corrosion resistance is remarkably improved by forming a highly protective rust layer even in a severe chloride environment.

  (b) When Sn is compounded into Al-containing steel, a very protective rust layer is formed compared to the case where Al is contained alone. Corrosion rate is reduced.

(c) This is because Sn dissolves as Sn ²⁺ and 2Fe ³⁺ + Sn ²⁺
→ 2Fe ²⁺ and for suppressing the pH drop due to hydrolysis of Fe ³⁺ at + Sn by ⁴⁺ consisting reaction to reduce the concentration of Fe ^3+, further significantly anodic dissolution as the dissolution after ion to Sn Therefore, the corrosion resistance can be greatly improved in a small amount.

  (d) At that time, α-FeOOH and β-FeOOH are contained in the rust layer formed on the steel surface. In order to form a highly protective rust layer, the ratio of α-FeOOH to β-FeOOH (α / β ratio) is preferably 0.5 or more.

(e) In addition, when Sn exists in the steel material in a solid solution state, it is easily dissolved as Sn ²⁺ . If there are many solid solution Sn in steel materials, while being able to suppress pH fall by hydrolysis of Fe3 ⁺ more, the anodic dissolution reaction can also be suppressed more.

  (f) Addition of Cr suppresses a decrease in pH at the corrosion interface due to the effect of improving corrosion resistance in a neutral environment, and improves corrosion resistance.

(g) Like Sn, Cu,
Ni, Mo, W, and Sb are effective.

  (h) Further, when Ti and Zr are contained, the corrosion resistance is improved by reducing the inclusions that are the starting point of corrosion.

  (i) Ca and Mg increase the pH of the corrosion interface and make the corrosive environment mild.

The present invention has been completed based on the above findings, and the gist thereof is a steel material having excellent corrosion resistance as shown in the following (1) to (7) .

(1) By mass%, C: 0.01-0.25%, Si: 0.01-1.0%, Mn: 0.05-3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.15 -10.0%, Sn: 0.03 -0.5% steel, the balance is Fe and impurities, and the ratio of the solid solution Sn in Sn is 95% or more, and its surface contains α-FeOOH together with Al and Sn A steel material excellent in corrosion resistance, characterized by being covered with a rust layer and having a ratio of α-FeOOH to β-FeOOH in the protective rust layer of 0.5 or more.

(2) Further, by mass%, Cr: 7.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, Sb: 0.2% or less The steel material having excellent corrosion resistance according to the above (1) , characterized by containing the above .

(3) In addition, it has excellent corrosion resistance as described in (1) or (2) above , characterized by containing one or two kinds of Ti: 0.2% or less and Zr: 0.2% or less in mass%. Steel material.

(4) The corrosion resistance according to any one of (1) to (3) above , further comprising one or two of Ca: 0.01% or less and Mg: 0.01% or less by mass%. Excellent steel material.

(5) The above (1) to (4) , characterized by further containing one or more of Nb: 0.1% or less, V: 0.5% or less, and B: 0.01% or less by mass% The steel material excellent in corrosion resistance as described in any of the above.

(6) The steel material having excellent corrosion resistance according to any one of the above (1) to (5) , further comprising REM: 0.01% or less by mass%.

(7) The steel material having excellent corrosion resistance according to any one of the above (1) to (6), wherein at least a part of the surface of the steel material is coated with an anticorrosion film .

  ADVANTAGE OF THE INVENTION According to this invention, the corrosion-resistant steel materials excellent in the corrosion resistance in the environment containing a chloride can be provided.

  The steel material to which the present invention is applied is not particularly limited, but is preferably a structural steel material, particularly a steel material used as a structural material in marine structures, port facilities, ships, construction / civil engineering structures, automobiles, railways, and the like. is there.

  The form of the steel material is not particularly limited, and may be any form including a plate, a bar, a shaped steel, a pipe, a cast product, etc. In addition to a steel pipe used for a line pipe or a pipe, a steel pipe pile, a steel sheet pile, a reinforcing bar, etc. It can also be applied to steel materials of various shapes.

  The present invention will be described in detail below. The “%” display of the content of each element means “mass%”.

(A) About the chemical composition of steel materials In this invention, the reason for prescribing | regulating the chemical composition of steel materials is as follows.

C: 0.01-0.25%
C is an element necessary for ensuring strength as a material, and a content of 0.01% or more is necessary. However, if the content exceeds 0.25%, the weldability is significantly reduced. In addition, as the C content increases, the amount of cementite that acts as a cathode and promotes corrosion in an environment where the pH is reduced increases, so the corrosion resistance decreases. For this reason, the upper limit was made 0.25%. The lower limit value of C is preferably 0.02%, and more preferably 0.03%. The upper limit value of C is preferably 0.18%, and more preferably 0.16%.

Si: 0.01-1.0%
Si is an element necessary for deoxidation, and in order to obtain a sufficient deoxidation effect, it is necessary to contain 0.01% or more. However, if the content exceeds 1.0%, the toughness of the base material and the welded joint is impaired. Therefore, the Si content is set to 0.01 to 1.0%. The lower limit of Si is preferably 0.03%, more preferably 0.05%. The upper limit of Si is preferably 0.8%, and more preferably 0.6%.

Mn: 0.05-3.0%
Mn is an element having an effect of increasing the strength of steel at a low cost, and a content of 0.05% or more is necessary to obtain this effect. However, if the content exceeds 3.0%, the weldability deteriorates and the joint toughness also deteriorates. Therefore, the Mn content is set to 0.05 to 3.0%. The lower limit value of Mn is preferably 0.2%, more preferably 0.4%. The upper limit value of Mn is preferably 2.5%, more preferably 2.0%.

P: 0.05% or less
P exists as an inevitable impurity in steel. P is an element that lowers the acid resistance, and lowers the corrosion resistance in a chloride corrosive environment where the pH of the corrosion interface is lowered. Further, since the weldability and the toughness of the heat affected zone are lowered, the smaller the content, the better. For this reason, the P content is limited to 0.05% or less. It is preferably 0.04% or less, and more preferably less than 0.03%.

S: 0.01% or less
S is unavoidably present as an impurity in the steel. S forms MnS as a starting point of corrosion in the steel, and when its content exceeds 0.01%, the corrosion resistance decreases significantly. For this reason, the S content is limited to 0.01% or less. It is preferably 0.008% or less, and more preferably 0.006% or less.

Al: Over 0.03% and 10.0% or less
Al is the most important element in the present invention, and if it contains more than 0.03%, a protective rust layer (α-FeOOH) is formed in a chloride environment. When the rust layer is formed, a certain amount of Al is taken into the protective rust layer. Although the details of the form are unknown, it is presumed that the protective rust layer (α- (Fe, Al) OOH) is formed in a form in which Fe is replaced with Al. As a result, the corrosion resistance is remarkably improved. On the other hand, even if the content exceeds 10.0%, the effect is saturated. Therefore, the Al content is more than 0.03% and 10.0% or less. The lower limit value of Al is preferably 0.1%, more preferably 0.5%. The upper limit value of Al is preferably 9.0%, more preferably 8.0%.

Sn: 0.01-0.5%
Sn has the effect of greatly improving the corrosion resistance in a low pH chloride corrosive environment. Sn dissolves as Sn ²⁺ , 2Fe ³⁺ + Sn ²⁺ → 2Fe ²⁺ + Sn ⁴⁺
By reducing the concentration of Fe ³⁺ by the reaction, it has the effect of suppressing the pH drop due to the hydrolysis of Fe ³⁺ , and further, anodic dissolution as ions after dissolution can be significantly suppressed in Sn. Furthermore, when Sn is compounded and contained in a steel material containing more than 0.03% of the above Al, protection containing Sn, which is significantly more protective than steel materials containing Al alone, in severe chloride environments Rust layer is formed. Sn in this protective rust layer also has the effect of suppressing anodic dissolution as described above. In order to obtain these effects, it is necessary to contain 0.01% or more of Sn. On the other hand, even if Sn is contained in excess of 0.5%, the above effects are not only saturated, but the toughness of the base metal and the high heat input welded joint is deteriorated. Therefore, the Sn content is set to 0.01 to 0.5%. The lower limit value of Sn is preferably 0.02%, and more preferably 0.03%. The upper limit value of Sn is preferably 0.4%, and more preferably 0.3%.

  The steel material according to the present invention has the above-described chemical composition, with the balance being Fe and impurities. Here, an impurity means the component mixed by various factors of a manufacturing process including raw materials, such as an ore and a scrap, when manufacturing steel materials industrially.

  In addition to the above components, the steel material according to the present invention can contain one or more components selected from at least one group from the following first group to the fifth group, if necessary. Hereinafter, components belonging to these groups will be described.

  Group 1 components: Cr, Cu, Ni, Mo, W, Sb

Cr: 7.0% or less
Cr has an effect of remarkably improving the corrosion resistance in a neutral environment, and can be contained as necessary. In an environment with a large amount of chloride, the pH of the corrosion interface is lowered. However, the inclusion of Cr suppresses the elution of Fe ²⁺ remarkably, thereby reducing the production of Fe ³⁺ by air oxidation. As a result, the pH drop due to the hydrolysis of Fe ³⁺ is greatly suppressed, and the corrosion resistance is significantly improved. Furthermore, when Cr is combined with Sn, a highly protective rust layer is formed even in a severe corrosive environment with a lot of chloride. However, when Cr is contained exceeding 7.0%, the weldability is remarkably lowered. Therefore, the Cr content is 7.0% or less. Preferably it is 1.0% or less, More preferably, it is 0.8% or less. In order to effectively obtain the above effects, it is preferable to contain 0.01% or more of Cr, and more preferably 0.05% or more.

Cu: 1.0% or less
Cu has an action of improving corrosion resistance by suppressing anodic dissolution of steel in a low pH environment, and can be contained as necessary. However, if Cu is contained in excess of 1.0%, the effect is not only saturated, but also causes embrittlement. Therefore, the content is 1.0% or less. In order to effectively obtain the above effect, it is preferable to contain 0.02% or more of Cu, and more preferably 0.03% or more.

  In addition, when Cu is added to steel, Cu and Sn coexist, and rolling cracks may occur depending on the manufacturing method. In order to suppress rolling cracks, it is preferable to reduce the Cu content and reduce the ratio of Cu content to Sn. It is preferable that the Cu content is less than 0.2% and the Cu / Sn content ratio is 1.0 or less. The Cu content is more preferably less than 0.1%.

Ni: 1.0% or less
Ni, like Cu, has an effect of improving corrosion resistance by suppressing anodic dissolution of steel in a low pH environment, and can be contained as necessary. However, if the content exceeds 1.0%, not only the effect is saturated, but also the cost is significantly increased. Therefore, the content is 1.0% or less. The upper limit of Ni is preferably 0.8%. In order to effectively obtain the above effect, the content is preferably 0.01% or more, and more preferably 0.02% or more.

Mo: 1.0% or less
Mo is an element that has the effect of dissolving and adsorbing to rust in the form of oxyacid ion MoO ₄ ^2- and suppressing the transmission of chloride ions in the rust layer, so it can be included as needed . However, if the content exceeds 1.0%, the effect is not only saturated, but the cost of the steel material is significantly increased. Therefore, the Mo content is 1.0% or less. The upper limit of Mo is preferably 0.7%. In order to effectively obtain the above effects, the content is preferably 0.01% or more, and more preferably 0.02% or more.

W: 1.0% or less
W, like Mo, dissolves and exists in the form of oxyacid ions and suppresses the permeation of chloride ions in the rust layer, so it can be included as necessary. However, if the content exceeds 1.0%, the effect is not only saturated, but the cost of the steel material is significantly increased. Therefore, the W content is 1.0% or less. The upper limit value of W is preferably 0.7%. In order to effectively obtain the above effects, the content is preferably 0.01% or more, and more preferably 0.02% or more.

Sb: 0.2% or less
Sb is an element with excellent acid resistance, which suppresses the anodic dissolution reaction of steel in a low pH environment and improves the corrosion resistance in a chloride environment by suppressing the hydrogen gas generation reaction and the reduction reaction of Fe ³⁺ . , If necessary. However, if the content exceeds 0.2%, the toughness is remarkably deteriorated. Therefore, the Sb content is 0.2% or less. The upper limit of Sb is preferably 0.15%. In order to effectively obtain the above effects, the content is preferably 0.01% or more, and more preferably 0.02% or more.

  Second group of ingredients: Ti, Zr

Ti: 0.2% or less
Ti has the effect of suppressing the formation of MnS, which is the starting point of corrosion due to the formation of sulfides, and can be contained as necessary. However, if the content exceeds 0.2%, not only the effect is saturated but also the cost of the steel material increases. Therefore, the Ti content is 0.2% or less. The upper limit of Ti is preferably 0.15%. In order to effectively obtain the above effects, the content is preferably 0.001% or more, and more preferably 0.005% or more.

Zr: 0.2% or less
Zr has the effect of suppressing the formation of MnS, which is the starting point of corrosion, by forming a sulfide as in the case of Ti, and can be contained as necessary. However, if the content exceeds 0.2%, not only the effect is saturated but also the cost of the steel material increases. Therefore, the Zr content is 0.2% or less. The upper limit of Zr is preferably 0.15%. In order to effectively obtain the above effects, the content is preferably 0.001% or more, and more preferably 0.005% or more.

  Group 3 components: Ca, Mg

Ca: 0.01% or less
Ca exists in the form of oxides in the steel and has the effect of suppressing the reduction of the pH at the interface in the corrosion reaction part and suppressing the promotion of corrosion, so it can be contained as necessary. However, if the content exceeds 0.01%, the effect is saturated. Therefore, the Ca content is 0.01% or less. The upper limit of Ca is preferably 0.005%. In order to effectively obtain the above effect, the content is preferably 0.0002% or more, and more preferably 0.0005% or more.

Mg: 0.01% or less
Mg, like Ca, suppresses the decrease in pH at the interface in the corrosion reaction part, and can be contained as necessary. However, if the content exceeds 0.01%, the effect is saturated. Therefore, the Mg content is 0.01% or less. The upper limit of Mg is preferably 0.005%. In order to effectively obtain the above effect, the content is preferably 0.0002% or more, and more preferably 0.0005% or more.

  Group 4 components: Nb, V, B

Nb: 0.1% or less
Since Nb is an element that increases the strength of the steel material, it can be contained as necessary. However, if the content exceeds 0.1%, the effect is saturated, so the Nb content is 0.1% or less. The upper limit of Nb is preferably 0.05%. In order to effectively obtain the above effect, the content is preferably 0.001% or more, and more preferably 0.003% or more.

V: 0.5% or less
V, like Nb, is an element that increases the strength of steel, and, like Mo and W, dissolves in the form of oxyacid ions and suppresses the transmission of chloride ions in the rust layer. Since it has, it can be made to contain as needed. However, when the content exceeds 0.5%, not only the effect is saturated but also the cost is remarkably increased. Therefore, the V content is 0.5% or less. The upper limit value of V is preferably 0.3%. In order to effectively obtain the above effect, the content is preferably 0.005% or more, and more preferably 0.01% or more.

B: 0.01% or less
Since B is an element that improves hardenability and increases strength, it can be contained as necessary. However, when the content of B exceeds 0.01%, the effect of increasing the strength is saturated, and the tendency of deterioration in toughness becomes remarkable in both the base material and HAZ. Therefore, the B content is 0.01% or less. In order to effectively obtain the effect of increasing the hardenability and strength, the content is preferably 0.0003% or more.

Group 5 ingredients:
REM

REM: 0.01% or less
REM (rare earth element) has an effect of improving the weldability of steel, and can be contained as necessary. However, if the content exceeds 0.01%, the effect is saturated, so the REM content is 0.01% or less. The upper limit of REM is preferably 0.005%. In order to effectively obtain the above effect, the content is preferably 0.0002% or more, and more preferably 0.0005% or more.

  Here, REM is a general term for 17 elements including Y and Sc in addition to 15 elements of lanthanoid, and one or more of these elements can be contained. The content of REM means the total content of these elements.

(B) About the amount of Al in the protective rust layer, the amount of Sn and the amount of α-FeOOH By containing Al in the rust layer, extremely high protection is exhibited in a chloride environment. The greater the amount of Al in the protective rust layer, the better. In the steel material in the present invention, the amount of Al in the protective rust layer is preferably 1.5 to 2000 mg / m ^{2 in} terms of metal.

By containing Sn in the rust layer, remarkably high protection is exhibited in a chloride environment. The larger the amount of Sn in the protective rust layer, the better. In the steel material in the present invention, the Sn content in the protective rust layer is preferably 0.5 to 100 mg / m ^{2 in} terms of metal.

  In the protective rust layer, β-FeOOH is generated in addition to α-FeOOH which is stable rust. In order to form a rust layer with extremely high protection in a chloride environment, the ratio of α-FeOOH to β-FeOOH (α / β ratio) needs to be 0.5 or more. The α / β ratio is preferably 1.0 or more, and more preferably 1.5 or more. The α / β ratio can be determined by quantitatively analyzing α-FeOOH and β-FeOOH in the rust layer by X-ray diffraction measurement.

In order to set the α / β ratio in the rust layer to 0.5 or more, the average amount of incoming salt is 0.05 to 10 mg / dm ² / day (mdd) and the relative humidity is 76% or more. What is necessary is just to corrode in the environment where a ratio becomes 5% or more on an annual average.

(C) About solid solution Sn In this invention, it is preferable that Sn added to steel materials is solid solution in a high ratio. In particular, when the ratio of the solid solution Sn in Sn is 95.0% or more, protective rust is formed, and sufficient corrosion resistance can be secured. As mentioned earlier, it is Sn ions dissolved by corrosion that improve corrosion resistance, so if Sn is contained in poorly soluble precipitates and the solid solubility in steel decreases, the corrosion resistance improving action Is not enough. Moreover, since the amount of Sn which contributes to protective rust formation will decrease when the ratio of solid solution Sn is low, formation of protective rust tends to become insufficient.

In addition, in order to make the ratio of solute Sn 95% or more, for example, a slab that suppresses the content of S and O that easily form a compound with Sn is suppressed to 1100 to 1200 ° C. depending on the composition of the steel. After heating at a degree, hot rolling is performed under conditions where the rolling reduction per rolling pass is 3% or more and the rolling finishing temperature is about 700 to 900 ° C. After rolling, it can be produced by cooling in the atmosphere or by cooling at a cooling rate of 5 ° C./s or more in a temperature range from a temperature of Ar ₃ or higher to at least about 550 ° C. In addition, the temperature here is the temperature of the steel material surface.

(D) About anticorrosion film Even if it uses the steel material of this invention demonstrated above as it is, it shows favorable corrosion resistance. However, when the surface is coated with an anticorrosion coating made of an organic resin or metal, the durability of the anticorrosion coating is improved and the corrosion resistance is further improved as compared with conventional steel materials.

  Here, examples of the anticorrosion coating made of an organic resin include vinyl butyral, epoxy, urethane, and phthalic acid resin coatings. Examples of the anticorrosion coating made of metal include a plating coating such as Zn, Al, and Zn—Al, and a thermal spray coating such as Zn, Al, and Al—Mg.

  The durability of the anticorrosion coating is improved because the corrosion of the steel material of the present invention as a base is remarkably suppressed, and the swelling and peeling of the anticorrosion coating due to the corrosion of the base steel material from the defective portion of the anticorrosion coating is suppressed. It is thought that.

  What is necessary is just to perform the process covered with said anti-corrosion film by a normal method. In addition, it is not always necessary to apply an anti-corrosion coating to the entire surface of the steel material, and only one surface of the steel material as a surface exposed to the corrosive environment, if it is a steel pipe, only the outer surface or the inner surface, that is, at least a part of the steel material surface is subjected to anticorrosion treatment Good.

47 types of steel were melted using a vacuum melting furnace to form a 50 kg steel ingot, and then hot forged by a normal method to produce a block having a thickness of 60 mm.

  Tables 1 and 2 show the chemical composition of the prepared blocks. In addition, the ratio of the solid solution Sn in Sn (Sn solid solubility) is shown.

Sn forms a compound with O (oxygen) and S, and forms tin oxide (SnO, SnO ₂ etc.) and tin sulfide (SnS, SnS ₂ ). Therefore, when these compounds are formed in the steel, the solute Sn in the steel material decreases, so when melting the steel, except for steel No. 46, deoxidation and desulfurization are performed as follows. Control was performed to adjust the solute Sn in the steel.

  That is, in the deoxidation, Si and Mn were added in advance at the initial stage of melting to carry out preliminary deoxidation, the dissolved oxygen concentration was adjusted to 100 ppm or less, and Al was added to perform deoxidation again. At this time, Ti having a deoxidizing effect was added together with Al if necessary. On the other hand, desulfurization was performed by adding quick lime together with a slag modifier to slag formed by melting.

  Next, the block was heated at 1120 ° C. for 1 hour, and then hot-rolled, finished to a thickness of 20 mm at 850 ° C., and then allowed to cool to room temperature in the air to obtain a steel plate.

  A test piece having a width of 60 mm, a length of 100 mm, and a thickness of 3 mm was taken from each steel plate having a thickness of 20 mm and subjected to a test in a chloride environment. For some steel types, an anti-corrosion film of about 200 μm was formed by spray coating with a modified epoxy paint, and a cross barb was put in the anti-corrosion film to expose a part of the metal, and subjected to the same corrosion test.

  For the corrosion test, an exposure test in the coastal area was used. A three-year exposure test was conducted in an extremely harsh environment with splashes of seawater on the coast of the Okinawa region.

After the exposure test, the rust layer on the surface of each test piece was removed, and the thickness reduction was measured. In addition, after the collected rust sample is dried in a desiccator for more than a week, quantitative analysis of rust constituent compounds is performed by powder X-ray diffraction method using ZnO powder (made by Wako Pure Chemical Industries, particle size of about 5μm) as an internal standard substance. went. A sample for powder X-ray diffraction was mixed with ZnO at a constant weight ratio (30% in this example) with respect to the rust weight collected in advance, and mixed with an agate mortar so that rust and ZnO were uniformly dispersed. The X-ray diffraction measurement was performed using a RU200 model manufactured by Rigaku Denki Co., with a Co target, a voltage-current of 30 kV-100 mA, and a scanning speed of 2 ° / min. Α-FeOOH, γ-FeOOH (manufactured by Rare Metallic), Fe ₃ O ₄ (manufactured by High-Purity Chemical), and β-FeOOH synthesized by hydrolyzing an FeCl ₃ aqueous solution at 100 ° C are used as standard reagents. Using the calibration curve prepared in this way, quantitative analysis was performed from the intensity of the obtained X-ray diffraction pattern. The amounts (mass%) of β-FeOOH and α-FeOOH in the rust thus determined were compared. Furthermore, rust samples collected from a certain area were dissolved in concentrated hydrochloric acid, and the amounts of Al and Sn were measured by ICP analysis.

  With respect to the steel material subjected to the anticorrosion treatment, the maximum corrosion depth of the anticorrosive coating ridge was measured. The test results are shown in Tables 3 and 4. Here, “corrosion weight loss” is an 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. Further, the “corrosion depth” is the maximum value of the depth from the steel material surface of the paint ridge.

  The following method was used as a method for determining protective rust formation. When the protective rust layer was sufficiently formed, “◯” was displayed. When sufficient formation of the protective rust layer was remarkable, it was indicated as “特 に”. On the other hand, when formation of the protective rust layer was insufficient, “x” was displayed.

Using the results of exposure tests 1, 2, and 3 years, the test period is x, the corrosion weight loss or the corrosion depth is y,
y = A ・ x ^B・ ・ ・ (1)
When the obtained coefficient B is 0.8 or less, it is assumed that a protective rust layer is formed. Note that when the protective rust is not formed, the corrosion weight loss or corrosion depth increases linearly with the test period, so the coefficient B is close to 1.

  Furthermore, the rust produced | generated by the test material was quantitatively analyzed by said X-ray-diffraction measurement, the ratio ((alpha) / (beta) ratio) with respect to (beta) -FeOOH of produced | generated α-FeOOH was calculated | required, and it combined with Table 3 and 4 and showed.

As is apparent from the results of Tables 3 and 4, in Comparative Steel Nos. 50 to 53 , at least one of the Al content, Sn content, and α / β ratio in the steel was low. The corrosion depth was large, and a sufficient protective rust layer was not formed. That is, since steel No. 50 has low Al content in steel, sufficient protective rust was not formed. Steel No. 51 did not contain Sn in the steel, and the α / β ratio was low, so that sufficient protective rust was not formed. Steel No. 52, like Steel No. 50, had a low Al content in the steel, so sufficient protective rust was not formed. Steel No. 53 has a low Al content in the steel, does not contain Sn in the steel, and has a low α / β ratio, so that sufficient protective rust was not formed .

On the other hand, in steel Nos . 1 to 4 , 7, 8 , 10 to 16 , 18 to 20 , 22 to 27 , and 29 to 45 of the present invention example, the Al content, the Sn content, and the ratio of solute Sn in the steel , Α / β ratio was satisfied, and sufficient formation of the protective rust layer was remarkable. This is because the α / β ratio is as high as 1.7 to 3.5, the production of β-FeOOH is suppressed, and a large amount of α-FeOOH is produced. Steel Nos. 9, 17, 28 and 46 are reference examples.

  As described above, according to the present invention, it is possible to provide a corrosion-resistant steel material having excellent corrosion resistance in an environment containing chloride.

Claims (7)

  In mass%, C: 0.01-0.25%, Si: 0.01-1.0%, Mn: 0.05-3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.15-10.0%, Sn: 0.03-0.5% Is a protective rust layer comprising the balance Fe and impurities, and having a solid solution Sn ratio in Sn of 95% or more, the surface of which is a protective rust layer containing α-FeOOH together with Al and Sn. A steel material excellent in corrosion resistance, characterized in that the ratio of α-FeOOH to β-FeOOH in the protective rust layer is 0.5 or more.   In addition, by mass%, Cr: 7.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, Sb: 0.2% or less The steel material having excellent corrosion resistance according to claim 1.   Furthermore, steel material excellent in corrosion resistance according to claim 1 or 2, characterized by containing one or two of Ti: 0.2% or less and Zr: 0.2% or less in mass%.   The steel material having excellent corrosion resistance according to any one of claims 1 to 3, further comprising one or two of Ca: 0.01% or less and Mg: 0.01% or less in mass%.   Furthermore, it contains the 1 type (s) or 2 or more types of Nb: 0.1% or less, V: 0.5% or less, and B: 0.01% or less in the mass%, In any one of Claim 1 to 4 characterized by the above-mentioned. Steel material with excellent corrosion resistance.   The steel material having excellent corrosion resistance according to any one of claims 1 to 5, further comprising REM: 0.01% or less by mass%. The steel material excellent in corrosion resistance according to any one of claims 1 to 6, wherein at least a part of the surface of the steel material is coated with an anticorrosion film .