KR20220130842A - Hot-dip aluminum coated steel for extremely low temperature and method for manufacturing the same - Google Patents

Abstract

According to the present invention, a cryogenic molten aluminum coated steel and a manufacturing method thereof comprise: a base iron material (10) having a Vickers hardness of 138 to 141 Hv; an Al-Fe alloy layer (20) in which molten aluminum is infiltrated on the base iron material (10) and which has a Vickers hardness of 398 to 426 Hv and contains 40 to 45 wt% Fe and the remaining of Al and other inevitable impurities; and an Al coating layer (30) which is formed on the Al-Fe alloy layer (20), has a Vickers hardness of 36 to 39 Hv, and contains 2 to 3 wt% of Fe and the remaining of Al. The cryogenic molten aluminum coated steel has excellent brittleness, scratch properties, tensile properties, cycle repetition properties, and peel properties in a cryogenic environment.

Description

Cryogenic hot-dip aluminum plated steel and its manufacturing method

The present invention relates to a cryogenic hot-dip aluminum-coated steel material and a method for manufacturing the same, and more particularly, in general, carbon steel is coated with hot-dip aluminum to prevent abruptly weakening of ductility and brittleness in a cryogenic environment and to have excellent mechanical properties and corrosion resistance. It relates to a treatment and a method for manufacturing the same.

Most metallic carbon steel materials change their metallic properties as the temperature goes down, and they are mainly found in BCC-structured metals. In a low-temperature environment, the surface of molten aluminum plated carbon steel is formed with an FCC structure of aluminum, so it has a very low brittleness. It is considered that a complementary action is possible, but since the transition temperature is different depending on the carbon content and other elements contained in the steel, it is necessary to confirm it through an impact test to understand the accurate brittleness characteristics of aluminized steel at low temperatures.

In the case of structures used at low temperatures, brittleness tends to increase in cryogenic environments, so they should be sufficiently considered when designing or manufacturing. In particular, the most important property at low temperature is toughness. Since containers and transport pipes related to LNG storage and transportation must have sufficient toughness even in low-temperature media, aluminum alloy, nickel steel, austenitic stainless steel, etc. are widely used.

Aluminum alloy has various advantages such as light weight, high strength-to-weight ratio, excellent corrosion resistance and workability as an eco-friendly and easy-renewable material. Among aluminum alloys, 5000 and 6000 series are used in ships and offshore plants exposed to seawater.

Among them, A5083 aluminum alloy, which is an Al-Mg alloy, has high strength and excellent low-temperature brittleness, and is applied to spherical shell Equators and Skirts, which are major parts of Mohs-type LNG carrier tanks.

On the other hand, low-temperature aluminum material is attracting attention as a material for storage and transportation of LNG gas. LNG gas is a natural gas with methane as its main component, which is cooled to -163°C and is a transparent cryogenic liquid. It is known to have excellent mechanical properties and corrosion resistance.

In the conventional literature related to A5083 and other alloys, it was confirmed that the yield strength and tensile strength slightly increased as the temperature was lower due to the low temperature. As a result, there is a problem in that the strength is partially low.

Patent Document 0001 contains silicon and one or more eutectic forming elements selected from the group consisting of cerium and calcium, the alloy structure is silicon and optionally magnesium, aluminum matrix, and the alloy structure is mainly silicon and optionally magnesium, L12 type lattice and 20 Secondary precipitates on Al3 (Zr, Sc) with a size of sub-nm, secondary precipitates of Al6Mn and Al7Cr, including an aluminum matrix mainly containing eutectic phases containing iron, calcium and cerium with an average particle size of 1 μm or less There is a description related to an aluminum alloy.

On the other hand, there is a known technique for a galvanized steel sheet that is inexpensive, has excellent corrosion resistance (corrosion resistance), and has a beautiful surface appearance, so that its use is gradually increasing for interior and exterior plates of automobiles.

Patent Document 0002 discloses an inhibition layer comprising an Fe-Al-based intermetallic alloy phase formed on a base steel plate; a hot-dip galvanizing layer formed on the suppression layer; and a hot-dip galvanized steel sheet having excellent low-temperature adhesion and workability including an Al-Mn-based alloy phase discontinuously formed between the suppression layer and the hot-dip galvanized layer.

As a problem with the galvanized steel sheet, since iron can diffuse into the plating layer during plating, a phenomenon in which zinc and the electrode are alloyed occurs on the surface of the welding electrode. In addition, there is a problem in that the plating layer falls off due to high-pressure and high-speed friction with the die during processing and is attached to the bead portion.

Korean Patent Publication No. 10-2020-0030035 (published date: March 19, 2020) Korean Patent Registration No. 10-2031454 (Registration Date: 2019.10.04.)

The present invention has been devised to solve the above problems, and through the ductility-brittleness transition temperature (DBTT) that considers the temperature at which the ductility and brittleness of each material rapidly change, to have an FCC structure, even in a cryogenic environment and in general carbon steel It is a technical task to provide a hot-dip aluminum plating treatment using brittleness, scratch properties, tensile properties, cycle repeatability, and peeling properties and a manufacturing method thereof.

In addition, another technical problem to be achieved by the present invention is to provide a cryogenic hot-dip aluminum plated steel material using a hot-dip aluminum plating process and a manufacturing method thereof.

The object of the present invention is not limited to the object mentioned above, and other objects not mentioned will be clearly understood by those of ordinary skill in the art to which the present invention belongs from the following description.

In order to achieve the above-described technical problem of the present invention, cryogenic hot-dip aluminum plated steel material according to an embodiment of the present invention, Vickers hardness of 138 to 141 Hv base iron base material (10); Al-Fe alloy layer 20 made of molten aluminum infiltrated and formed on the base iron base material 10, having a Vickers hardness of 398 to 426 Hv, Fe 40 to 45 wt%, and the remainder Al and other unavoidable impurities; and an Al coating layer 30 formed on the Al-Fe alloy layer 20, having a Vickers hardness of 36 to 39 Hv, and consisting of 2 to 3 wt% of Fe and the remainder Al.

The cryogenic hot-dip aluminum plated steel according to an embodiment of the present invention may satisfy the following Relations 1 and 2:

[Relational Expression 1] 48.7 ≤ Lc1 ≤ 71.8

[Relational Expression 2] 64.9 ≤ Lc2 ≤ 85.3

(In Relations 1 and 2, Lc1 and Lc2 are after exposing the cryogenic hot-dip aluminum plated steel material at -40°C to -60°C for at least 30 minutes, increasing the force at a constant rate specified in KS L ISO 20502 and ASTM C1624 It is a value measured through a scratching method (PFST: Progressive Force Scratch Test), Lc1 means the critical vertical force (N) at which cracks of the Al coating layer 30 begin to occur, and Lc2 is the Al coating layer 30 This means the force (N) that starts to peel off.)

The cryogenic hot-dip aluminum plated steel material according to an embodiment of the present invention may satisfy the following Relational Equation 3:

[Relational Expression 3] 48.7 ≤ TS ≤ 71.8

(In Relation 3, TS is the tensile strength (N/mm ² ) measured after exposing the cryogenic hot-dip aluminum plated steel material at -40°C to -60°C for 30 minutes or more.)

The present invention also includes a method for manufacturing the above-described cryogenic hot-dip aluminum plated steel.

A method for manufacturing a cryogenic molten aluminum plated steel material according to an embodiment of the present invention includes a degreasing step (S10) of immersing the base iron base material 10 in an alkaline solvent; The first water washing step (S21) of washing the surface of the degreased base iron base material 10 with water, and the acid treatment of the base iron base material 10 that has undergone the first water washing step (S21), A surface treatment step (S20) comprising a first pickling step (S22) of activating the surface, and a second washing step (S23) of washing the surface of the base iron base material 10 that has undergone the first pickling step (S22) with water (S20) ; Water-soluble flux consisting of 35 to 50% by weight of potassium chloride, 5 to 10% by weight of cryolite, 40 to 60% by weight of ammonium fluoride or aluminum fluoride, based on 100% by weight of water, the surface-treated base iron base material 10 a flux treatment step (S30) of immersing in a flux solvent added in an amount of 10 to 30% by weight under a temperature condition of 40 to 90° C. for 1 to 10 minutes; The flux-treated base iron base material (10) contains sodium chloride 25 to 35% by weight, potassium chloride 15 to 25% by weight, cryolite 20 to 30% by weight, ammonium hydrofluoride, ammonium fluoride, or aluminum fluoride 20 to 30 fluoride Plating step (S40) of immersing the molten flux composed of a component ratio of % by weight in 5 to 10% by weight based on 100% by weight of molten aluminum under a temperature condition of 680 to 750° C. for 5 to 30 minutes (S40) ; An air cooling step (S51) of air cooling the plated base iron base material 10 to 100° C. or less, and a water cooling step (S52) of removing residual heat by water cooling the base iron base material 10 that has undergone the air cooling step (S51) and , The base iron base material 10, which has undergone the water cooling step (S52), is immersed in an oxalic acid aqueous solution containing 0.5 to 10% by weight of oxalic acid based on 100% by weight of water under a temperature condition of 20 to 50℃ for 5 to 20 minutes A second pickling to remove the molten flux powder attached to the plating surface by pickling the base iron base material 10 that has undergone the oxalic acid treatment step (S53) and the oxalic acid treatment step (S53), smoothing and glossing the plating surface A post-treatment step (S50) consisting of a step (S54) and a finishing treatment step (S55) of drying the surface of the base iron base material 10 that has undergone the second pickling step (S54) after washing with water; includes; .

The cryogenic molten aluminum plated steel material according to the present invention described above has advantages of excellent brittleness, scratch properties, tensile properties, cycle repeatability, and peeling properties in a cryogenic environment.

In addition, the present invention has an FCC structure of aluminum in a cryogenic environment, so it is excellent in low-temperature brittleness, and the base iron base material 10 after the plating process; Al-Fe alloy layer 20; and Al coating layer 30; It has excellent cryogenic scratch properties due to the complementary action between the livers.

In addition, the present invention can solve problems such as high cost and SCC (Stress Corrosion Cracking) of welding parts for austenitic stainless steels, which are currently widely used in relation to LNG carriers whose orders have been rapidly increasing in recent years, and are relatively inexpensive and mechanical for a long time. It is possible to provide a plated steel material having secured reliability related to physical properties.

1 is a graph showing the thermal shock energy of each metal according to the temperature.
2 is a SEM photograph showing a cross section of a cryogenic hot-dip aluminum plated steel material according to an embodiment of the present invention.
3 is a view showing the results of EDX analysis of a cross section of a cryogenic hot-dip aluminum plated steel material according to an embodiment of the present invention.
4 is a view showing the EDX analysis result of the Al-Fe alloy layer 20 of the cryogenic molten aluminum plated steel material according to an embodiment of the present invention.
5 is a view showing the EDX analysis result of the Al coating layer 30 of the cryogenic molten aluminum plated steel material according to an embodiment of the present invention.
6 is a SEM photograph showing a cross section of a hot-dip aluminum-coated steel material after a cycle test of a cryogenic hot-dip aluminum-coated steel material according to an embodiment of the present invention.
7 is a process flowchart of a method for manufacturing a cryogenic hot-dip aluminum plated steel material according to an embodiment of the present invention.

Advantages and features of embodiments of the present invention, and methods of achieving them, will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, and only these embodiments allow the disclosure of the present invention to be complete, and common knowledge in the technical field to which the present invention pertains It is provided to fully inform those who have the scope of the invention, and the present invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout.

In describing the embodiments of the present invention, if it is determined that a detailed description of a well-known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted. In addition, the terms to be described later are terms defined in consideration of functions in an embodiment of the present invention, which may vary according to intentions or customs of users and operators. Therefore, the definition should be made based on the content throughout this specification.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The present applicant has studied for a long time to apply aluminum plated steel to a cryogenic environment. 1 is a graph showing the thermal shock energy of each metal according to the temperature. As a result, as shown in Figure 1, the ductile-brittle transition temperature (DBTT: Ductile Brittle Transition Temperature), which is a temperature at which the ductility and brittleness of each material rapidly change, is mainly in the metal of the BCC structure, and it is well in the metal of the FCC structure. It was confirmed that it did not appear. Mild steel had a BCC structure, and DBTT was found to be -60 °C, while Al metal had an FCC structure, and it was confirmed that DBTT did not appear well. Through this, the present applicant came to apply for the present invention by judging that the hot-dip aluminum plated steel material is brittle with the FCC structure of aluminum in a cryogenic environment, and that a complementary action is possible during plating.

In the description of the present invention, the term "cryogenic environment" means a condition that directly or indirectly affects the steel at a temperature of -40 ℃ or less, or -40 to -200 ℃.

In the description of the present invention, the term “room temperature” means a natural temperature that is not heated or reduced, for example, about 15° C. to 35° C., more specifically about 20° C. to 25° C., more specifically may mean a temperature of about 25 °C.

2 is a SEM photograph showing a cross section of a cryogenic hot-dip aluminum plated steel material according to an embodiment of the present invention. 3 is a view showing the results of EDX analysis of a cross section of a cryogenic hot-dip aluminum plated steel material according to an embodiment of the present invention.

As shown in Figure 2, the cryogenic molten aluminum plated steel material according to an embodiment of the present invention is basically a base iron base material (10); Al-Fe alloy layer 20 formed on the base iron base material (10); and Al-Al coating layer 30 formed on the Fe alloy layer 20 .

The base iron base material 10 can penetrate a predetermined depth (about 80 ㎛ or more) into the base iron base material 10 in the cryogenic molten aluminum plated steel manufacturing method to be described later, and through the method for manufacturing the molten aluminum plating steel It may have a high Vickers hardness. The base iron base material 10 may be composed of 0.05 wt% or less of P (excluding 0 wt%), 0.05 wt% or less of S (excluding 0 wt%), and the balance Fe.

The Al-Fe alloy layer 20 is formed by penetrating molten aluminum on the base iron base material 10 . 2 and 3 , the Al-Fe alloy layer 20 may have a thickness of about 80 to 120 μm. In addition, in the component analysis, the Al-Fe alloy layer 20 has a section in which the Al component and the Fe component are mixed.

As shown in FIG. 3 , the Al coating layer 30 has Al as a main component, and is formed on the outside of the cryogenic molten aluminum plated steel material according to the present invention. In addition, as shown in FIG. 3 , the Al coating layer 30 may have a thickness of about 30 to 50 μm.

4 is a view showing the EDX analysis result of the Al-Fe alloy layer 20 of the cryogenic molten aluminum plated steel material according to an embodiment of the present invention.

As shown in FIG. 4 , the Al-Fe alloy layer 20 may include 40 to 45 wt% Fe, and 55 to 60 wt% Al. On the other hand, in the Al-Fe alloy layer 20, some trace components of the base iron base material 10 may be eluted or transferred through a cryogenic molten aluminum plating steel material manufacturing method to be described later, and other unavoidable impurities such as P, S, etc. may include more.

5 is a view showing the EDX analysis result of the Al coating layer 30 of the cryogenic molten aluminum plated steel material according to an embodiment of the present invention.

As shown in FIG. 5 , the Al coating layer 30 may be composed of 2 to 3 wt% of Fe and the remainder Al.

Meanwhile, the aluminum plating adhesion amount of the Al coating layer 30 may be about 250 to 290 g/m2. The aluminum plating adhesion amount of the Al coating layer 30 is expressed by Equation 1 below. The aluminum plating adhesion amount of the Al coating layer 30 may be measured according to 7.3 sodium hydroxide method of KS D0229.

[Equation 1]

A = ((W1-W2)/S)×10 ⁶

(In Equation 1, A is the adhesion amount of the aluminum plating (g/m2), W1 is the weight before removing the plating layer (g), W2 is the weight after removing the plating layer (g), and S is the Al coating layer 30) is the surface area (mm2) of

In addition, the surface roughness (Ra) of the Al coating layer 30 may be 25 to 40 ㎛. Table 1 below contains the results of measuring the surface roughness values of the Al coating layer 30 of the cryogenic molten aluminum plated steel according to the present invention using a portable surface roughness measuring device.

[Table 1]

In addition, in the cryogenic molten aluminum plated steel material according to an embodiment of the present invention, the Vickers hardness of the base iron base material 10 is 138 to 141 Hv, and the Vickers hardness of the Al-Fe alloy layer 20 is 398 to 426 Hv, and the Vickers hardness of the Al coating layer 30 may be 36 to 39 Hv.

In detail, the hardness of the cryogenic hot-dip aluminum plated steel according to the present invention was measured by the method of KS B0811 using a Vickers hardness tester, and the measured values are listed in Table 2 below.

[Table 2]

As shown in Table 2, the Al coating layer 30 has a relatively low hardness, while the Al-Fe alloy layer 20 is uniquely confirmed to have a higher strength value than the base iron base material 10. can Such a base material (10); Al-Fe alloy layer 20; And Al coating layer 30; including; cryogenic molten aluminum plated steel can significantly improve the cryogenic mechanical properties such as brittleness, tensile strength, bending strength, thermal shock in a cryogenic environment.

In addition, the cryogenic hot-dip aluminum plated steel material according to an embodiment of the present invention may satisfy the following Relations 1 and 2:

[Relational Expression 1] 48.7 ≤ Lc1 ≤ 71.8

[Relational Expression 2] 64.9 ≤ Lc2 ≤ 85.3

(In Relations 1 and 2, Lc1 and Lc2 are after exposing the cryogenic hot-dip aluminum plated steel material at -40°C to -60°C for at least 30 minutes, increasing the force at a constant rate specified in KS L ISO 20502 and ASTM C1624 It is a value measured through a scratching method (PFST: Progressive Force Scratch Test), Lc1 means the critical vertical force (N) at which cracks of the Al coating layer 30 begin to occur, and Lc2 is the Al coating layer 30 This means the force (N) that starts to peel off.)

In detail, the scratch test of the cryogenic molten aluminum plated steel material according to the present invention is to measure the adhesion between the coating layer and the base material, and is a test method to compare and evaluate the degree of adhesion by the friction force and sound wave change on the surface. KS L ISO 20502 and ASTM C1624 are stipulated in KS L ISO 20502 and ASTM C1624, and the measurement methods include the method of scratching while increasing the force at a constant speed (PFST: Progressive Force Scratch Test), and each time the fracture occurs while increasing the vertical force step by step at different positions. There are methods such as the Constant Force Scratch Test (CFST) and the Multi Pass Scratch Test (MPST), which repeatedly perform a scratch test at the same location with a constant force below the critical vertical force, but the general PFST method, and the test conditions are listed in [Table 3] below.

[Table 3]

First, the scratch test of the cryogenic hot-dip aluminum plated steel according to the present invention was measured at room temperature, and the results are listed in Table 4 below.

[Table 4]

Next, the scratch test of the cryogenic hot-dip aluminum plated steel material according to the present invention was measured after sufficiently exposed to -40 ℃, -50 ℃, -60 ℃ temperature for 30 minutes or more, and the results are shown in Tables 5, 6, and Tables below. 7 is included.

[Table 5] : -40℃ scratch test measurement

[Table 6]: -50℃ scratch test measurement

[Table 7] : -60℃ scratch test measurement

As shown in Tables 4 to 7, the scratch test results measured at room temperature and the scratch test results measured at -40°C, -50°C, -60°C temperatures are equal to or better than the room temperature test results was confirmed to have These results indicate that the cryogenic hot-dip aluminum plated steel according to the present invention has an FCC structure of aluminum in a cryogenic environment, so it is judged to be excellent in low-temperature brittleness, and after the plating process, the base iron base material 10; Al-Fe alloy layer 20; and Al coating layer 30; It is judged that it is the result of excellent cryogenic scratch properties due to the complementary action between the livers.

In addition, the cryogenic hot-dip aluminum plated steel material according to an embodiment of the present invention may satisfy the following Relational Equation 3:

[Relational Expression 3] 48.7 ≤ TS ≤ 71.8

(In Relation 3, TS is the tensile strength (N/mm ² ) measured after exposing the cryogenic hot-dip aluminum plated steel material at -40°C to -60°C for 30 minutes or more.)

In detail, the tensile test of the cryogenic hot-dip aluminum plated steel according to the present invention was conducted with a 13B type test piece of KS B 0801 with a width of about 12.5 mm and a gage distance of 50 mm, and the test method was KS B 0802 with a tensile speed of 25 mm It proceeded at /min, and the tensile strength and yield strength values were calculated by proceeding in the form including the plating layer.

First, the tensile test of the cryogenic hot-dip aluminum plated steel according to the present invention was measured at room temperature, and the results are listed in Table 8 below.

[Table 8]

Next, the tensile test of the cryogenic hot-dip aluminum plated steel according to the present invention was measured after sufficiently exposed to temperatures of -40°C, -50°C, and -60°C for at least 30 minutes, and the results are shown in Tables 9, 10, and Tables below. 11 is included.

[Table 9] : -40℃ tensile test measurement

[Table 10] : -50℃ tensile test measurement

[Table 11] : -60℃ tensile test measurement

As shown in Tables 8 to 11, the tensile test results measured at room temperature and the tensile test results measured at -40°C, -50°C, and -60°C are equivalent to or better than the room temperature test results. was confirmed to have These results show that the cryogenic hot-dip aluminum plated steel according to the present invention has an FCC structure of aluminum in a cryogenic environment, so it is judged to have excellent low-temperature toughness, and after the plating process, the base iron base material 10; Al-Fe alloy layer 20; and Al coating layer 30; It is judged that it is the result of excellent cryogenic tensile properties due to the complementary action between the livers.

In addition, the cryogenic hot-dip aluminum plated steel material according to an embodiment of the present invention may have excellent low-temperature peeling properties. In detail, after repeating 100 times between a stress of about 95% of the average room temperature average yield strength value of the cryogenic hot-dip aluminum plated steel according to the present invention and a load of 50N, exposure to temperatures of -40℃, -50℃, -60℃ The low-temperature peeling properties through the post-adhesion test were evaluated.

6 is a SEM photograph showing a cross section of a hot-dip aluminum-coated steel material after a cycle test of a cryogenic hot-dip aluminum-coated steel material according to an embodiment of the present invention. As shown in Figure 6, the cryogenic hot-dip aluminum plated steel according to an embodiment of the present invention did not generate any cracks or pinholes even after the low-temperature peeling test, and it was confirmed that the intact shape and structure were maintained before the initial experiment.

In addition, the present invention includes a method for manufacturing the above-described cryogenic hot-dip aluminum plated steel.

7 is a process flowchart of a method for manufacturing a cryogenic hot-dip aluminum plated steel material according to an embodiment of the present invention.

As shown in Figure 7, the cryogenic hot-dip aluminum plated steel manufacturing method according to an embodiment of the present invention,

A degreasing step (S10) of immersing the base iron base material 10 in an alkaline solvent;

The first water washing step (S21) of washing the surface of the degreased base iron base material 10 with water, and the acid treatment of the base iron base material 10 that has undergone the first water washing step (S21), A surface treatment step (S20) comprising a first pickling step (S22) of activating the surface, and a second washing step (S23) of washing the surface of the base iron base material 10 that has undergone the first pickling step (S22) with water (S20) ;

Water-soluble flux consisting of 35 to 50% by weight of potassium chloride, 5 to 10% by weight of cryolite, 40 to 60% by weight of ammonium fluoride or aluminum fluoride, based on 100% by weight of water, the surface-treated base iron base material 10 a flux treatment step (S30) of immersing in a flux solvent added in an amount of 10 to 30% by weight under a temperature condition of 40 to 90° C. for 1 to 10 minutes;

The flux-treated base iron base material (10) contains sodium chloride 25 to 35% by weight, potassium chloride 15 to 25% by weight, cryolite 20 to 30% by weight, ammonium hydrofluoride, ammonium fluoride, or aluminum fluoride 20 to 30 fluoride Plating step (S40) of immersing the molten flux composed of a component ratio of % by weight in 5 to 10% by weight based on 100% by weight of molten aluminum under a temperature condition of 680 to 750° C. for 5 to 30 minutes (S40) ; and

An air cooling step (S51) of air cooling the plated base iron base material 10 to 100° C. or less, and a water cooling step (S52) of removing residual heat by water cooling the base iron base material 10 that has undergone the air cooling step (S51) and , The base iron base material 10, which has undergone the water cooling step (S52), is immersed in an oxalic acid aqueous solution containing 0.5 to 10% by weight of oxalic acid based on 100% by weight of water under a temperature condition of 20 to 50℃ for 5 to 20 minutes A second pickling to remove the molten flux powder attached to the plating surface by pickling the base iron base material 10 that has undergone the oxalic acid treatment step (S53) and the oxalic acid treatment step (S53), smoothing and glossing the plating surface A post-treatment step (S50) consisting of a step (S54) and a finishing treatment step (S55) of drying the surface of the base iron base material 10 that has undergone the second pickling step (S54) after washing with water; includes; .

In addition, in the method of manufacturing a cryogenic molten aluminum plated steel material according to an embodiment of the present invention, in the plating step (S40), immersing the base iron base material 10 under the conditions of Equation 1 below is the Al-Fe alloy described above. It penetrates well into the layer 20 and can improve brittleness, scratch properties, tensile properties, cycle repeatability, and peeling properties at cryogenic temperatures.

[Equation 1]

18,770 < LMP < 20,000

(In Equation 1, LMP = T(logt _r + C), where T is the temperature of the molten plating solution (K), t _r is the immersion time (hr), and C is the constant 20.)

In addition, in the method for manufacturing a cryogenic molten aluminum plated steel material according to an embodiment of the present invention, in the post-processing step (S50),

The air cooling step (S51) is a step of air cooling the base iron base material 10 that has undergone the plating step (S40), and the base iron base material 10 lifted in the aluminum molten metal through the plating step (S40) is the lowest temperature Since its mechanical properties are significantly changed according to a sudden temperature change of 500°C or more, it is sufficiently cooled in air until it is 100°C or less in air.

The water cooling step (S52) is a step of water cooling the carbon steel pipe that has undergone the air cooling step (S51), and the residual heat of the base iron base material 10 that has been subjected to the air cooling step (S51) is removed by water cooling to minimize thermal deformation. do.

The oxalic acid treatment step (S53) is a step of selectively eluting the surface of the Al coating layer 30 formed on the base iron base material 10 that has undergone the water cooling step (S52), and the base iron base material that has undergone the water cooling step (S52). (10) was immersed in an aqueous oxalic acid solution containing 0.5 to 10 wt% of oxalic acid (C ₂ H ₂ O ₄ ) based on 100 wt% of water under a temperature condition of 20 to 50 ° C. for 5 to 20 minutes to form an Al coating layer ( 30), the molten flux powder attached to the surface is primarily removed, and at the same time, the surface of the Al coating layer 30 is selectively eluted.

The reason for limiting the amount of oxalic acid added to the aqueous oxalic acid solution to 0.5 to 10 wt% is that when the amount of oxalic acid added is less than 0.5 wt%, the surface elution of the aluminum plating layer hardly occurs, and when the amount of oxalic acid is more than 10 wt% This is because the aluminum plating layer 30 itself rapidly melts away.

In addition, the reason that the temperature of the oxalic acid aqueous solution is limited to 20 ~ 50 ℃ is that when the temperature of the oxalic acid aqueous solution is 20 ℃ or less, it takes a lot of time for the surface of the aluminum plating layer to selectively elute, and the temperature of the oxalic acid aqueous solution is 50 ℃ or more In this case, the time for selectively eluting the surface of the Al coating layer 30 can be shortened, but this is because the Al coating layer 30 itself rapidly melts.

When the surface of the Al coating layer 30 is selectively eluted in the oxalic acid treatment step (S53), the surface of the Al coating layer 30 becomes porous and the thickness of the aluminum oxide layer formed on the surface of the Al coating layer 30 is increased. It is possible to further enlarge, as described above, if the thickness of the aluminum oxide layer formed on the surface of the Al coating layer 30 is further increased, the hardness, corrosion resistance, abrasion resistance, etc. of the plating surface can be significantly improved.

As described above, the conventional steel and galvanized steel are very difficult to apply to the piping, structures, etc. of the LNG vessel in the cryogenic environment because the ductility and brittleness change rapidly in the cryogenic environment, but the base material 10; Al-Fe alloy layer 20; and an Al coating layer 30 formed on the Al-Fe alloy layer 20, having a Vickers hardness of 36 to 39 Hv, and consisting of 2 to 3 wt% of Fe and the remainder Al; By including the aluminum-plated steel material, it can be confirmed that brittleness, scratch properties, tensile properties, cycle repeatability, peeling properties can be greatly improved in a cryogenic environment.

In the above description, various embodiments of the present invention have been presented and described, but the present invention is not necessarily limited thereto. It will be readily appreciated that branch substitutions, transformations and alterations are possible.

10: base iron base material 20: Al-Fe alloy layer
30: Al coating layer
S10: degreasing step S20: surface treatment step
S21: first washing step S22: first pickling step
S23: second washing step S30: flux treatment step
S40: plating step S50: post-processing step
S51: air cooling stage S52: water cooling stage
S53: oxalic acid treatment step S54: second pickling step
S55 : Finishing step

Claims (4)

Soji-iron base material 10 having a Vickers hardness of 138 to 141 Hv;
Al-Fe alloy layer 20 made of molten aluminum infiltrated and formed on the base iron base material 10, having a Vickers hardness of 398 to 426 Hv, Fe 40 to 45 wt%, and the remainder Al and other unavoidable impurities; and
Formed on the Al-Fe alloy layer 20, having a Vickers hardness of 36 to 39 Hv, and an Al coating layer 30 consisting of 2 to 3 wt% of Fe and the remainder Al; comprising, cryogenic molten aluminum plated steel.
The method of claim 1,
Cryogenic hot-dip aluminum plated steel that satisfies the following Relations 1 and 2:
[Relational Expression 1] 48.7 ≤ Lc1 ≤ 71.8
[Relational Expression 2] 64.9 ≤ Lc2 ≤ 85.3
(In Relations 1 and 2, Lc1 and Lc2 are after exposing the cryogenic hot-dip aluminum plated steel material at -40°C to -60°C for at least 30 minutes, increasing the force at a constant rate specified in KS L ISO 20502 and ASTM C1624 It is a value measured through a scratching method (PFST: Progressive Force Scratch Test), Lc1 means the critical vertical force (N) at which cracks of the Al coating layer 30 begin to occur, and Lc2 is the Al coating layer 30 This means the force (N) that starts to peel off.)
The method of claim 1,
Cryogenic hot-dip aluminum-coated steel that satisfies the following relation 3:
[Relational Expression 3] 48.7 ≤ TS ≤ 71.8
(In Relation 3, TS is the tensile strength (N/mm ² ) measured after exposing the cryogenic hot-dip aluminum plated steel material at -40°C to -60°C for 30 minutes or more.)
A method for producing a cryogenic hot-dip aluminum plated steel material according to any one of claims 1 to 3,
A degreasing step (S10) of immersing the base iron base material 10 in an alkaline solvent;
The first water washing step (S21) of washing the surface of the degreased base iron base material 10 with water, and the acid treatment of the base iron base material 10 that has undergone the first water washing step (S21), A surface treatment step (S20) comprising a first pickling step (S22) of activating the surface, and a second washing step (S23) of washing the surface of the base iron base material 10 that has undergone the first pickling step (S22) with water (S20) ;
Water-soluble flux consisting of 35 to 50% by weight of potassium chloride, 5 to 10% by weight of cryolite, 40 to 60% by weight of ammonium fluoride or aluminum fluoride, based on 100% by weight of water a flux treatment step (S30) of immersing in a flux solvent added in an amount of 10 to 30% by weight under a temperature condition of 40 to 90° C. for 1 to 10 minutes;
The flux-treated base iron base material (10) contains sodium chloride 25 to 35% by weight, potassium chloride 15 to 25% by weight, cryolite 20 to 30% by weight, ammonium hydrofluoric acid, ammonium fluoride, or aluminum fluoride 20 to 30 fluoride Plating step (S40) of immersing the molten flux composed of a component ratio of % by weight in a molten plating solution containing 5 to 10% by weight based on 100% by weight of molten aluminum under a temperature condition of 680 to 750° C. for 5 to 30 minutes (S40) ;
An air cooling step (S51) of air cooling the plated base iron base material 10 to 100° C. or less, and a water cooling step (S52) of removing residual heat by water cooling the base iron base material 10 that has undergone the air cooling step (S51) and , The base iron base material 10, which has undergone the water cooling step (S52), is immersed in an oxalic acid aqueous solution containing 0.5 to 10% by weight of oxalic acid based on 100% by weight of water under a temperature condition of 20 to 50℃ for 5 to 20 minutes. Oxalic acid treatment step (S53) and the oxalic acid treatment step (S53), the base iron base material 10 is pickled to remove the molten flux powder attached to the plating surface, and a second pickling to smooth and gloss the plating surface A post-treatment step (S50) comprising a step (S54) and a finishing treatment step (S55) of drying the surface of the base iron base material 10 that has undergone the second pickling step (S54) after washing with water Method for manufacturing cryogenic hot-dip aluminum plated steel.

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