WO2010053279A2

WO2010053279A2 - High strength steel material and method for manufacturing same

Info

Publication number: WO2010053279A2
Application number: PCT/KR2009/006401
Authority: WO
Inventors: 김동삼; 김성철; 정희원
Original assignee: 일진경금속(주)
Priority date: 2008-11-07
Filing date: 2009-11-03
Publication date: 2010-05-14
Also published as: KR100922619B1; WO2010053279A3

Abstract

The present invention relates to a steel material that enables the enhancement of tensile strength, comprising a first layer consisting of ultra low carbon steel, and a second layer that is located to be in contact with said first layer, has a first surface on the side opposite to said first layer, consists of a solid solution wherein nitrogen is dissolved in said ultra low carbon steel, and has practically the same structure as that of said first layer.

Description

High strength steel and its manufacturing method

The present invention relates to a steel material and a method for manufacturing the same, and more particularly, to a high strength steel material and a method for manufacturing the same further improved strength.

Steel materials used in the automotive field, tools, and various structures are processed according to the use field through various alloy technologies and heat treatment technologies.

A representative method for improving the strength of the steel is to use a high carbon steel with a high content of carbon contained in the steel. In addition, high strength steel having a tensile strength of 400 MPa or more includes DP (Dual Phase) steel, CP (Complex Phase) steel, TRIP (TRansformation Induced Plasticity) steel, and TWIP (TWinning Induced Plasticity) steel.

By the way, in order to process the shape of the desired parts using the high-carbon steel and high-strength steel, it is necessary to select a special processing method suitable for the strength of the steel. In addition, the mold of the molding apparatus for processing it to match the strength of the steel should also be made of a higher strength. Therefore, such high carbon steels and high strength steels have a problem of lowering the productivity and cost of parts or structures using the same.

Conventionally, as another method for improving hardness and improving corrosion resistance and abrasion resistance to steel, a method of diffusing nitrogen into steel and simultaneously forming iron-nitrogen compounds on the surface of steel has been used.

However, this method reduces the amount of nitrogen consumed to form a compound with iron on the surface of the steel and diffuses into the steel, thereby improving the hardness, strength and corrosion resistance of the surface, but the overall strength of the steel is sufficient. There is a limit that cannot be improved. Thus, the steel thus formed is used for tools, engine parts, and the like, and has limitations for use in exterior parts of automobiles.

The present invention is to solve the above problems, and an object thereof is to provide a steel that can improve the strength.

In order to achieve the above object, the present invention, the first layer made of ultra-low carbon steel; And a solid solution disposed in contact with the first layer and having a first surface on the side opposite to the first layer, in which nitrogen is dissolved in the ultra-low carbon steel, the structure being substantially the same as that of the first layer. It provides a steel comprising a; second layer having.

According to another feature of the invention, at least a region adjacent to the first surface of the second layer does not contain a compound of iron and nitrogen.

According to still another feature of the present invention, the method further includes a third layer formed on the second layer and having a second surface in contact with the first surface and provided to prevent corrosion of the first surface.

According to still another feature of the present invention, the method further includes a fourth layer formed on the second layer and having a third surface in contact with the first surface and provided with a compound of iron and nitrogen.

According to another feature of the invention, at least a region adjacent to the first surface of the second layer comprises a compound of iron and nitrogen.

According to another feature of the invention, the ultra low carbon steel is a steel having a carbon content of 0.01% by weight or less (not including 0% by weight).

The present invention also provides a base material having a surface and provided with a steel having a carbon content of 0.01% by weight or less (not including 0% by weight) to achieve the above object; And a solid solution layer provided on the inside of the base material from the surface and having a solid solution of nitrogen in an invasive position of the base material and having a structure substantially the same as that of the base material.

According to another feature of the invention, at least the region adjacent to the surface of the solid solution layer does not contain compounds of iron and nitrogen.

According to another feature of the present invention, the solid solution layer further comprises a first film formed in contact with the surface to prevent corrosion of the solid solution layer.

According to still another feature of the present invention, it further comprises a second coating layer formed on the solid solution layer in contact with the surface and provided with a compound of iron and nitrogen.

According to another feature of the invention, a compound of iron and nitrogen is formed in at least a region adjacent to the surface of the solid solution layer.

In order to achieve the above object, the present invention also provides a step of forming a base material having a surface and provided with ultra-low carbon steel; Forming a solid solution layer on the inside of the base material from the surface, in which nitrogen is dissolved in the invasive site of the base material and having a structure substantially the same as that of the base material; Covering the surface and forming an oxide film formed of iron oxide; And removing the oxide film.

According to another feature of the invention, the formation of the solid solution layer and the oxide film proceeds simultaneously.

According to another feature of the invention, the step of forming the solid solution layer and the oxide film, at least one salt selected from the group consisting of KNO ₃ , KNO ₂ , Ca (NO ₃ ) ₂ , NaNO ₃ and NaNO ₂ . Preparing a molten salt; Heating the molten salt; Immersing the base material in the molten salt; And quenching the base material from the molten salt.

According to another feature of the invention, after removing the oxide film, further comprising the step of forming a first film formed on the solid solution layer in contact with the surface and prevents corrosion of the solid solution layer.

According to another feature of the invention, after removing the oxide film, further comprising the step of forming a second coating layer formed on the solid solution layer in contact with the surface and provided with a compound of iron and nitrogen.

According to another feature of the invention, the base material is steel having a carbon content of 0.01% by weight or less (not including 0% by weight).

According to the present invention as described above, the strength can be further improved by forming a nitrogen solid solution layer having a sufficient depth inside the steel. Accordingly, there is an effect that can be applied to various fields, such as lightweight high-strength automobile parts and various structural materials.

In addition, since the structure of the second layer in which nitrogen is diffused is the same as that of the first layer, the same physical properties can be obtained due to the uniform structure, whereby cracks and fractures can be prevented.

In addition, since the iron-nitrogen compound is not formed on the surface, a larger amount of nitrogen can diffuse into the steel, thereby forming a thicker layer of nitrogen-solubilized layer in the steel.

By forming the third layer or the fourth layer on the surface, the surface of the second layer can be prevented from being corroded, and when the fourth layer is formed, wear resistance can be increased.

By employing noncyanide salts in the river, nitrogen can be used to solve environmental problems and reduce treatment costs.

In addition, since the strength is increased after the first molding using the ultra low carbon steel, the formability, productivity, and the like of the part can be improved.

The content of carbon may be included in an amount of 0.01% by weight or less to further increase the thickness of the second layer formed.

1 is a cross-sectional view of a steel according to an embodiment of the present invention,

2 is a graph showing the tensile strength increase rate of the steel according to the carbon content of the base material,

3 is a cross-sectional photograph of the first layer of FIG.

4 is a cross-sectional photograph of the second layer of FIG.

5 is a photograph showing a cross section of a second layer when the base material immersed in molten salt is annealed according to a preferred embodiment of the present invention;

6 is a graph showing the difference in tensile strength between the case of quenching and quenching the base material immersed in the molten salt according to an embodiment of the present invention,

7 is a cross-sectional view of the steel produced according to the production method of the present invention,

8 is a cross-sectional view of a steel according to another preferred embodiment of the present invention,

9 is a cross-sectional view of a steel according to another preferred embodiment of the present invention.

The terms or words used in this specification and claims are not to be construed as being limited to their ordinary or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best describe their invention. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

Therefore, the embodiments described in the specification and the drawings shown in the drawings are only the most preferred embodiment of the present invention and do not represent all of the technical idea of the present invention, various modifications that can be replaced at the time of the present application It should be understood that there may be equivalents and variations.

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

1 is a cross-sectional view showing a cross section of a steel according to an embodiment of the present invention.

Referring to FIG. 1, a steel according to a preferred embodiment of the present invention includes a first layer 1 and a second layer 2 positioned adjacent to the first layer 1.

The first layer 1 is a portion in which a base metal made of steel is kept as it is. It is preferable that this 1st layer 1 does not contain an iron-nitrogen compound. The first layer 1 may be one in which a very small amount of nitrogen is added as an alloying element for a special use.

As the first layer 1 as described above, it is preferable to use ultra low carbon steel having a carbon content of 0.01% by weight or less (not including 0). This is because the first layer 1 serves as a base material for the formation of the second layer 2, which will be described later, so that the second layer 2 can be formed more easily as described later.

The second layer 2 is a portion provided with a solid solution layer in which nitrogen is dissolved in an interstitial site of iron, which is a main element of the base steel. The second layer 2 has a first surface 21 on the opposite side to the first layer 1, and Fe ₂ N, Fe _{3 on} the inside and the first surface 21 of the second layer 2. There is no iron-nitrogen compound such as N, Fe ₄ N.

Since the second layer 2 does not contain an iron-nitrogen compound, nitrogen can diffuse from the first surface 21 to a sufficient depth, so that the thickness t1 of the second layer 2 is sufficiently thick. The tensile strength can be greatly improved.

The thickness t1 of the second layer 2 may be appropriately adjusted according to the desired tensile strength. In other words, when a relatively high tensile strength is required, the thickness t1 of the second layer 2 is thick, and when a relatively low tensile strength is required, the thickness t1 of the second layer 2 is thinly formed. You can do

The second layer 2 as described above is a solid solution layer in which nitrogen diffuses from the surface of the base material and is dissolved in an iron intrusion site.

Like the first layer 1, the second layer 2 is also preferably made of ultra low carbon steel having a carbon content of 0.01% by weight or less (not including 0). This is because the second layer 2 is a solid solution layer formed by diffusion of nitrogen from the surface of the base material as described above. When the carbon content exceeds 0.01% by weight, the thickness t1 of the second layer 2 is increased. It is because it is not possible to obtain a sufficiently thick, and thus it is difficult to improve the tensile strength of the steel.

Figure 2 shows the tensile strength increase rate of the steel according to the carbon content of the base material. At this time, the tensile strength increase rate represents the increase rate of the tensile strength before forming the second layer (2) and after forming the second layer (2). In FIG. 2, steels having a carbon content of 0.002 wt%, 0.008 wt%, 0.01 wt%, 0.015 wt%, 0.05 wt%, and 0.1 wt%, respectively, were immersed in NaNO ₃ molten salt at 650 ° C. for 12 hours as described below. After quenching with cooling water, the oxide film on the surface was removed, and then subjected to tensile test.

As can be seen in Figure 2, when the carbon content is less than 0.01% by weight, the tensile strength increase rate of the steel is more than 100%, that is, the tensile strength of more than twice the tensile strength of the base material, but the carbon content is 0.01% by weight If exceeded, the increase in tensile strength is shown, but less than 100%, and the profit for forming the second layer 2 for the increase in the tensile strength falls.

This phenomenon occurs because the position where carbon is employed in iron is similar to the position where nitrogen is employed in iron, and the atomic size of carbon and nitrogen is similar, This is because nitrogen is less likely to be dissolved by the amount of carbon. However, the present invention is not necessarily limited to those reasons, and the thickness t1 of the second layer 2 is lowered and the tensile strength improvement rate of the steel is also reduced for other complex and unknown reasons. May appear.

In this invention, it is preferable that the said 1st layer 1 and the 2nd layer 2 are provided so that it may have substantially the same structure. In this specification, the fact that the tissues of the first layer 1 and the second layer 2 are substantially the same means that the tissue of the first layer 1 and the tissue of the second layer 2 have the same morphology without discontinuity. Means

3 and 4 are optical micrographs of the first layer 1 and the second layer 2, and as shown in FIGS. 3 and 4, it can be seen that they have the same structure. As a result, the steel material has a homogeneous structure over the first layer 1 and the second layer 2, and thus, may have the same physical properties and prevent cracking or fracture.

Next, an example of the formation method of the said 2nd layer 2 is demonstrated.

First, molten salt is prepared. The molten salt is a non-cyanide molten salt, more specifically NaNO ₃ , NaNO ₂ , KNO ₃ , KNO ₂ , which is not a conventional cyanide (CN), that is, molten salt such as KCN, NaCN containing cyan-based, And at least one salt selected from the group consisting of Ca (NO ₃ ) ₂ .

This salt bath is maintained at a constant temperature in the range of 400 ° C to 800 ° C.

Then, the salts in the salt bath is subjected to the nitrogen production reaction as follows.

Formula 1 below shows the nitrogen formation reaction of the molten salt bath of NaNO ₃ , NaNO ₂ .

Formula 1

Formula 2 below shows the nitrogen formation reaction of the molten salt bath of KNO ₃ , KNO ₂ .

Formula 2

Formula 3 below shows the nitrogen formation reaction of a molten salt bath of Ca (NO ₃ ) ₂ .

Formula 3

As described above, NO and NO ₂ are generated in the molten salt bath. As described below, NO and NO ₂ generate activated nitrogen N in accordance with the reaction with iron to cause nitrogen diffusion into the base metal.

Thereafter, the base metal made of steel having a carbon content of 0.01% by weight or less as described above is immersed in the salt bath for a predetermined time, for example, 1 minute to 24 hours.

At this time, the base material is preferably molded into a shape desired by the user. That is, since the base material is an ultra low carbon steel having a carbon content of 0.01% by weight or less, the tensile strength is small, the ductility is high, and the workability is very excellent. It is very easy to mold to the desired shape using such ultra-low carbon steel. In addition, the life of the mold of the molding apparatus can be long.

When the simply formed base material is immersed in the salt bath, NO and NO ₂ generated in the salt bath react with Fe on the surface of the base material as shown in Chemical Formulas 4 to 9 below.

Formula 4

Formula 5

Formula 6

Formula 7

Formula 8

Formula 9

Equations 1 to 6 below calculate the Gibbs free energy of Formulas 4 to 9, respectively. Calculation of the Gibbs free energy is shown in R.C. Weast (Ed.), Handbook Chemistry and Physics, 49th ed., The Chemical Rubber co., 1968, P.D-22.

Equation 1

Equation 2

Equation 3

Equation 4

Equation 5

Equation 6

Referring to Equations 1 to 6 above, all Gibbs-free energy values (ΔG °) have a negative value within a temperature range of 400 ° C to 800 ° C (absolute temperature 673.25K to 1073.25K). Therefore, it can be seen that Formulas 4 to 9 corresponding to the equations are spontaneous reactions within the above temperature range.

Therefore, in the end, NO and NO ₂ reacted with Fe form a compound of Fe-O, that is, iron oxide, on the surface of the base material according to the above formulas 4 to 9, and generate the generator N to interstitial site inside the iron (Interstitial site) The N is diffused to strengthen the solid solution.

Meanwhile, nitrogen (N ₂ ) and oxygen (O ₂ ) are further present in the molten salt bath, and the nitrogen and oxygen react with iron as shown in Chemical Formulas 10 to 13, respectively.

Formula 10

Formula 11

Formula 12

Formula 13

The Gibbs free energy (ΔG °) of the above formulas 10 to 13 may be obtained by ΔH ° -TΔS °, which may be obtained by Equations 7 to 10, respectively. The calculation of the Gibbs-free energy below is from the Thermodynamics of Materials, David V. Ragone.

Equation 7

Equation 8

Equation 9

Equation 10

Equations 7 to 10 are all within a temperature range of the present invention ΔG ₇ ° has a positive value of 20,000 or more, ΔG ₈ ° to ΔG ₁₀ ° all have a negative value of -200,000 or less. Therefore, Formula 10 in Formulas 10 to 13 is an involuntary reaction, and Formulas 11 to 13 are spontaneous. Accordingly, the iron-nitrogen compound to be produced according to Formula 10 is not generated in the second layer, which is the solid solution layer of nitrogen, according to the present invention, unless specially treated as described above.

After treating the base material in a salt bath in the range of 400 ℃ to 800 ℃ as described above is rapidly cooled with water or oil.

When the base material treated in the molten salt bath is quenched, the structure of the second layer 2, which is the nitrogen solid solution layer, can be obtained as shown in FIG. 4, so as to be the same as the structure of the base material (see FIG. 3). have. FIG. 5 is an optical micrograph showing the structure of the second layer 2 when the base material treated in the molten salt bath is left to cool at room temperature and gradually cooled. As can be seen in Figure 5, when slowly cooling the base material treated in the molten salt bath, the tissue of the second layer, which is a solid solution layer, appears needle-like structure unlike the second layer (1). In this case, therefore, it is difficult to obtain crack and fracture prevention effects due to the homogeneous tissue as described above.

In addition, the tensile strength of the steel is also significantly different depending on the cooling conditions of the base material treated in the molten salt bath. 6 is a result of tensile test after removing two IF steels with a carbon content of 0.002% by weight in molten NaNO ₃ at 650 ° C. for 5 hours and removing the scale after quenching (I) and slow cooling (II), respectively. It is shown. At this time, the condition of quenching (I) means cooling with general cooling water, and the condition of cooling (II) means leaving it to stand at room temperature. IF steel, which is the base material, has a tensile strength of 300 MPa. In the case of quenching (I), the tensile strength increased to 750 MPa, but in the case of quenching (II), the tensile strength of the base steel IF steel was 540 MPa (I). It can be seen that the falls significantly.

As shown in FIG. 7, the quenched steel is formed with a second layer 2, which is a layer in which nitrogen is dissolved, is formed on the first layer 1, and the upper surface of the first surface 21 of the second layer 2 is formed. As a result, an oxide film 22 formed of iron oxide is formed. Since the formation of the oxide film 22 is a spontaneous reaction as described above, and the formation of the iron-nitrogen compound is an involuntary reaction, the base metal has a nitrogen solid solution layer in which nitrogen is dissolved in preference to the iron-nitrogen compound. The layer 2 is formed, and the oxide film 22 is formed on the surface.

On the other hand, the oxide film 22, which is a Fe-O compound formed on the surface of the base material, may be similarly decomposed, or may be decomposed even when the Fe-N compound is formed. At this time, decomposition takes place as in the following formulas (14) and (15).

Formula 14

Formula 15

The decomposition energy at this time can be obtained by the following Equations 11 and 12, respectively. The calculation of Gibbs-free energy at the time of decomposition is as follows. Metallurgical Thermo-chemistry 5th ed. -O. According to Kubaschewski and C.B.Alcock.

Equation 11

Equation 12

Equations 11 and 12 in the temperature range of the present invention ΔG ₁₁ ° has a negative value, ΔG ₁₂ ° has a positive value. Therefore, Chemical Formula 14 is a spontaneous reaction, and Chemical Formula 15 is an involuntary reaction. According to Formula 15, the Fe-O compound does not spontaneously decompose and forms a Fe-O compound, that is, an oxide film 22. Since the Fe-N compound spontaneously decomposes according to Formula 14, a trace amount of the Fe-O compound spontaneously decomposes. Even if the Fe-N compound is formed, it is spontaneously decomposed.

As such, the iron-nitrogen compound does not exist at least in the vicinity of the surface of the second layer 2 and the oxide film 22, and even if present, the amount is extremely small. The formation of the second layer 2 and the oxide film 22 as described above occurs simultaneously.

As described above, in the process of forming the second layer 2 according to the preferred embodiment of the present invention, the formation of the oxide film 22 is essentially accompanied, and the formation of the oxide film 22 causes the second layer 2 to be formed. The degree of formation can be estimated.

Next, the oxide film 22 is removed through a surface descaling operation to complete the steel as shown in FIG. 1.

As described above, since nitrogen does not react with iron to form a compound on the surface of the steel, a larger amount of nitrogen can diffuse into the iron infiltration site, thereby deepening the diffusion depth of nitrogen. The thickness t1 of the second layer 2 can be made thicker than the conventional method of forming the iron-nitrogen compound layer on the surface. Since the thickness t1 of the second layer 2 is somewhat proportional to the temperature of the molten salt bath and the treatment time, when the thickness of the second layer 2 is to be formed thickly, the allowable temperature range of the temperature of the molten salt bath is increased. It is desirable to increase the maximum to and increase the treatment time.

On the other hand, the nitrogen diffused to the surface of the base material is a generator N generated according to the above formulas (4) to (9), the amount of nitrogen to be diffused into the steel increases when the N is more increased. However, in the above Chemical Formulas 4 to 9, the amount of N is related to the amount of NO and NO ₂ reacted with Fe.

Therefore, hot air may be blown into the molten salt bath during the above treatment to allow nitrogen and oxygen in the air to form NO and NO ₂ to participate in the reaction as in Chemical Formulas 4 to 9 above. In addition, a separate gas containing NO and NO ₂ may be blown into the molten salt bath.

An example of a method of forming the second layer 2 has been described above, but the above-described second layer may be formed by other methods in addition to the above method.

In the case of the second layer 2 as described above, since the first surface 21 is exposed to the outside, the steel may be vulnerable to corrosion.

Thus, as can be seen in FIG. 8, a second surface 31 is provided on the second layer 2 in contact with the first surface 21 and is provided to prevent corrosion of the first surface 21. The third layer 3 can be further formed.

The third layer 3 prevents corrosion of the second layer 2, and a phosphate coating may be used, but is not limited thereto. The third layer 3 may be formed on the first surface 21 of the second layer 2. Any material capable of preventing corrosion of the second layer 2 while excellent in adhesive force may be applied, such as plating and painting.

9 illustrates another embodiment of the present invention.

As described above, when the first surface 21 of the second layer 2 is exposed to the outside, it is vulnerable to corrosion. In order to prevent this, and to further increase the wear resistance of the surface, the fourth layer 4 formed of the iron-nitrogen compound 4 to cover the first surface 21 of the second layer 2 after forming the second layer 2 ) Can be further formed. Thus, the fourth layer 4 has a third surface 41 on the second layer 2 in contact with the first surface 21 and is provided to prevent corrosion of the first surface 21.

The fourth layer 4 may be formed on the first surface 21 of the second layer 2 by gas nitriding. The fourth layer 4 may be one in which at least one layer of Fe ₂ N and Fe ₃ N in the ε phase or Fe ₄ N in the γ phase is present.

An iron-nitrogen compound may be formed near the first surface 21 of the second layer 2 adjacent to the fourth layer 4.

When the fourth layer 4, which is a nitride layer, is formed on the first surface 21 of the second layer 2, the surface hardness of the steel may be further increased, and the wear resistance and the corrosion resistance may be increased.

As described above, the present invention can be easily formed into a desired shape using ultra low carbon steel, and then, by increasing the tensile strength by the treatment as described above, it is possible to further maximize the mass production of steel parts.

Claims

A first layer made of ultra low carbon steel; And

It is located in contact with the first layer, has a first surface on the side opposite to the first layer, made of a solid solution of nitrogen dissolved in the ultra-low carbon steel, having a structure substantially the same as that of the first layer Steel material comprising a second layer.
The method of claim 1,

The steel material characterized in that it does not contain the compound of iron and nitrogen in the area | region which contact | connects at least the said 1st surface of the said 2nd layer.
The method of claim 2,

And a third layer formed on the second layer and having a second surface in contact with the first surface and provided to prevent corrosion of the first surface.
The method of claim 1,

And a fourth layer formed on the second layer and having a third surface in contact with the first surface and provided with a compound of iron and nitrogen.
The method of claim 4, wherein

At least a region of the second layer in contact with the first surface includes a compound of iron and nitrogen.
The method according to any one of claims 1 to 5,

The ultra low carbon steel is a steel, characterized in that the carbon content of 0.01% by weight or less (not including 0% by weight).
A base material provided with steel having a carbon content of 0.01% by weight or less (not including 0% by weight) and having a surface; And

And a solid solution layer provided on the inner side of the base material from the surface, in which nitrogen is dissolved in an invasive portion of the base material and has a structure substantially the same as that of the base material.
The method of claim 7, wherein

At least an area of the solid solution layer in contact with the surface does not contain a compound of iron and nitrogen.
The method of claim 8,

And a first film formed on the solid solution layer in contact with the surface and preventing corrosion of the solid solution layer.
The method of claim 7, wherein

Steel is characterized in that it further comprises a second coating layer formed on the solid solution layer in contact with the surface and provided with a compound of iron and nitrogen.
The method of claim 10,

Steel is characterized in that a compound of iron and nitrogen is formed in at least the region of the solid solution layer in contact with the surface.
Molding a base material provided with ultra low carbon steel and having a surface;

Forming a solid solution layer on the inside of the base material from the surface, in which nitrogen is dissolved in the invasive site of the base material and having a structure substantially the same as that of the base material;

Covering the surface and forming an oxide film formed of iron oxide; And

Removing the oxide film; manufacturing method of a steel comprising a.
The method of claim 12,

Formation of the solid solution layer and the oxide film is characterized in that proceeding at the same time.
The method of claim 13,

Forming the solid solution layer and the oxide film,

Preparing a molten salt comprising at least one salt selected from the group consisting of KNO 3 , KNO 2 , Ca (NO 3 ) 2 , NaNO 3 and NaNO 2 ;

Heating the molten salt;

Immersing the base material in the molten salt; And

Removing the base material from the molten salt and quenching the steel material.
The method according to any one of claims 12 to 14,

After removing the oxide film, forming a first film formed on the solid solution layer in contact with the surface and preventing corrosion of the solid solution layer.
The method according to any one of claims 12 to 14,

After removing the oxide film, forming a second film layer formed on the solid solution layer in contact with the surface and provided with a compound of iron and nitrogen.
The method according to any one of claims 12 to 14,

The base material is a method for producing steel, characterized in that the carbon content of 0.01% by weight or less (not including 0% by weight).