KR102212302B1 - Nitriding method of martensite stainless steel and martensite stainless steel using the same - Google Patents

Abstract

According to one aspect of the present invention, a method for nitriding a martensitic stainless steel is provided. In the nitriding method of the martensitic stainless steel, the stainless steel is charged in a vacuum chamber and then nitrous oxide (N ₂ O) gas and ammonia (NH ₃ ) gas are alternately supplied into the chamber in a first temperature range. A pretreatment step of performing at least one unit cycle of surface activation treatment; And supplying a mixed gas of ammonia (NH ₃ ) gas and nitrogen (N ₂ ) gas into the chamber in a second temperature range relatively higher than the first temperature range to nitrify the surface-activated stainless steel. Includes steps.

Description

Nitriding method of martensite stainless steel and martensite stainless steel using the same}

The present invention relates to a method for nitriding martensitic stainless steel and a martensitic stainless steel using the same, and more particularly, a method for nitriding martensitic stainless steel using a gas nitriding method at low temperature to have excellent wear resistance and durability, and the same It relates to the used martensitic stainless steel.

Stainless steel has excellent ductility and corrosion resistance, so it is applied and used in various fields. Typically, an austenitic, martensitic, and duplex stainless steel composed of 10 wt% to 18 wt% Cr and 8 wt% to 14 wt% Ni is used. In recent years, the steel industry is seeking ways to minimize expensive alloy elements such as Co and Ni in order to reduce cost.

Among them, martensitic stainless steel is a material with reduced Ni content and increased C content, and has better corrosion resistance than alloy tool steel and mold steel, and can control properties by heat treatment, so gears and shafts, automobile parts, hydraulic parts, It is widely used industrially, such as oil and gas valves and plastic injection molds.

On the other hand, stainless steel has excellent corrosion resistance and ductility, but its hardness is low, so it is limited and used in many applications, and various studies on improving physical properties through surface treatment of stainless steel are being conducted.

In stainless steel, the Cr component, the main element, forms a nanometer-level chromium oxide layer on the surface, so it is difficult to penetrate C and N elements by the thermal diffusion surface treatment method. In addition, when the process is carried out at a specific temperature and pressure, the Cr component contained in stainless steel forms chromium precipitates such as CrC, CrN, etc., resulting in a decrease in the content of Cr in the base material, thereby reducing corrosion resistance. Therefore, it is necessary to efficiently remove the chromium oxide layer and develop a method for surface treatment of stainless steel.

Accordingly, the present invention is to solve various problems including the above problems, and there is a need to develop a process capable of performing the surface treatment of stainless steel by removing the chromium oxide layer formed on the surface of martensitic stainless steel. An object of the present invention is to provide a martensitic stainless steel having excellent wear resistance and durability. However, these problems are exemplary, and the scope of the present invention is not limited thereby.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings in order to enable those of ordinary skill in the art to easily implement the present invention.

According to an aspect of the present invention for solving the above problem, a method for nitriding a martensitic stainless steel is provided.

The nitriding method of the martensitic stainless steel is by charging the martensitic stainless steel in a vacuum chamber and then alternately supplying nitrous oxide (N ₂ O) gas and ammonia (NH ₃ ) gas into the chamber in a first temperature range. A pretreatment step of performing at least one unit cycle of activating the surface of the stainless steel at least once; And supplying a mixed gas of ammonia (NH ₃ ) gas and nitrogen (N ₂ ) gas into the chamber in a second temperature range relatively higher than the first temperature range to nitrify the surface-activated stainless steel. Step; may include.

1 is a flowchart schematically illustrating a method for nitriding a martensitic stainless steel according to an embodiment of the present invention.

Referring to FIG. 1, a method for nitriding a martensitic stainless steel according to an embodiment of the present invention is specifically, a stainless steel preparation step (S100), an atmosphere control step (S200), a pretreatment step (S300), and a nitriding treatment step ( S400) is performed.

First, the stainless steel preparation step (S100) may include charging a martensitic stainless steel into a vacuum chamber. Thereafter, as the atmosphere control step S200, before the pretreatment step, a step of supplying nitrogen (N ₂ ) gas into the chamber to control the atmosphere inside the chamber. By controlling the atmosphere inside the chamber, it is possible to uniformly heat the martensitic stainless steel.

Thereafter, a pretreatment step (S300) is performed, and in the pretreatment step (S300), nitrous oxide (N ₂ O) gas and ammonia (NH ₃ ) gas are alternately supplied into the chamber in a first temperature range to form the stainless steel. It includes the step of performing at least one unit cycle of activating the surface. Here, the first temperature range may be 360°C to 420°C.

In addition, the pretreatment step S300 may include removing or decomposing at least a part of the passivation film formed on the surface of the martensitic stainless steel. The passivation film may include chromium oxide (Cr ₂ O ₃ ).

Finally, the nitriding treatment step (S400) is performed, and the nitriding treatment step (S400) is a mixture of ammonia (NH ₃ ) gas and nitrogen (N ₂ ) gas in a second temperature range relatively higher than the first temperature range. And nitriding the surface-activated stainless steel by supplying a mixed gas into the chamber. Here, the mixed gas may have a ratio of ammonia (NH ₃ ) gas and nitrogen (N ₂ ) gas in a range of 1:1 to 1:1.5, and the second temperature range may be 450°C to 470°C.

Meanwhile, the second temperature range is very important, which is a temperature range capable of nitriding martensitic stainless steel by decomposing at least a part of the mixed gas supplied into the chamber. If the second temperature is less than 450° C., ammonia (NH ₃ ) gas contained in the mixed gas is not decomposed, and thus smooth nitriding treatment does not proceed. On the other hand, when the second temperature exceeds 470° C., the CrN secondary phase, which is a reinforcing element in the martensitic stainless steel, is removed to weaken the strength of the martensitic stainless steel. Must be maintained within the temperature range of Preferably, the second temperature range should be maintained at 455°C to 465°C.

In addition, the pretreatment step S300 and the nitriding step S400 may include a step in which the process pressure inside the chamber is performed in the same pressure range. Here, the same pressure range may be 250 mbar to 350 mbar.

Hereinafter, the reason why the surface nitriding treatment of martensitic stainless steel is performed by the method of nitriding martensitic stainless steel of the present invention will be described in detail later with reference to FIGS. 2 and 3.

2 is a cross-sectional view schematically illustrating the structure of a general steel according to a comparative example of the present invention, and FIG. 3 is a cross-sectional view schematically illustrating the structure of a martensitic stainless steel according to an embodiment of the present invention.

Referring to FIG. 2, a first iron oxide layer 20 such as Fe ₂ O ₃ is formed on the surface of a general steel (steel, 15). The first iron oxide layer 20 is a porous layer, and it is easy for nitrogen elements to penetrate into the general steel 15 through the first iron oxide layer 20. Therefore, when the bonding force is lowered, like the ferric oxide layer 25 through surface activation treatment in a vacuum atmosphere chamber having a pressure of about 300 mbar, the nitrogen element penetrates and the surface nitriding treatment of the general steel 15 is performed. It is easily performed. Here, the nitrogen element is obtained by decomposing NH ₃ ammonia by a reduction reaction.

On the other hand, as shown in FIG. 3, a first chromium oxide layer 30 such as Cr ₂ O ₃ is formed on the surface of the martensitic stainless steel 10. The first chromium oxide layer has a thin film and is dense, so that nitrogen element is difficult to penetrate, making it very difficult to treat the surface nitriding of the martensitic stainless steel 10.

In order to solve this, in the present invention, the oxidation process of the first chromium oxide layer 30 is performed by supplying nitrous oxide (N ₂ O) gas into the chamber, and ammonia (NH ₃ ) gas is continuously supplied in turn. 1 The reduction process of the chromium oxide layer 30 is performed, but this is repeated several times in one unit cycle to remove or decompose at least a part of the first chromium oxide layer 30 to form the second chromium oxide layer 35. have.

As a part of the first chromium oxide layer 30 is removed, or a second chromium oxide layer 35 having a smaller density than the first chromium oxide layer 30 is formed, nitrogen elements can easily penetrate into the martensitic system. The surface nitriding treatment of the stainless steel 10 is easily performed. Here, the nitrogen element is obtained by decomposing NH ₃ ammonia by a reduction reaction.

Meanwhile, according to another aspect of the present invention, a martensitic stainless steel is provided.

The martensitic stainless steel is manufactured by the above-described nitriding method of martensitic stainless steel, and in the vicinity of the surface of the martensitic stainless steel, Fe ₂₄ N ₁₀ phase and α'-N phase having a hexagonal crystal structure ( expanded martensite phase).

According to the embodiment of the present invention made as described above, by applying a gas nitriding process to martensitic stainless steel used in various industrial fields requiring corrosion resistance and physical properties, a solid solution nitride layer is formed on the surface of martensitic stainless steel. By forming, it is possible to provide a method for nitriding a martensitic stainless steel having excellent wear resistance and durability, and a martensitic stainless steel using the same, since the hardness of the surface is improved by more than two times than the conventional while maintaining the existing corrosion resistance. Of course, the scope of the present invention is not limited by these effects.

1 is a flowchart schematically illustrating a method for nitriding a martensitic stainless steel according to an embodiment of the present invention.
2 is a cross-sectional view schematically illustrating the structure of a general steel according to a comparative example of the present invention.
3 is a cross-sectional view schematically illustrating the structure of a martensitic stainless steel according to an embodiment of the present invention.
4 is a diagram schematically illustrating a process condition for nitriding a martensitic stainless steel according to an embodiment of the present invention.
5 is a graph analyzing the Vickers hardness of martensitic stainless steel samples according to Examples and Comparative Examples of the present invention.
6 is a photograph of analyzing a microstructure of a martensitic stainless steel sample according to Examples and Comparative Examples of the present invention using an optical microscope.
7 is a photograph of analyzing a microstructure of a martensitic stainless steel sample according to an embodiment of the present invention using a scanning electron microscope.
8 is a graph analyzing the nitrogen concentration of martensitic stainless steel samples according to Examples and Comparative Examples of the present invention.
9 and 10 are graphs for analyzing the crystal structure of martensitic stainless steel samples according to Examples and Comparative Examples of the present invention using an X-ray diffraction analyzer.
11 is a graph showing an analysis of the coefficient of friction of martensitic stainless steel samples according to Examples and Comparative Examples of the present invention using a wear tester.
12 is a photograph of analyzing the surface of the sample shown in FIG. 11 with an optical microscope.
13 is a graph for comprehensively comparing and analyzing the amount of wear, surface hardness and friction coefficient of the sample shown in FIG. 11.

Hereinafter, various embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely describe the present invention to those of ordinary skill in the art, and the following examples may be modified in various other forms, and the scope of the present invention is as follows. It is not limited to the examples. Rather, these embodiments are provided to make the present disclosure more faithful and complete, and to completely convey the spirit of the present invention to those skilled in the art.

Hereinafter, embodiments for aiding understanding of the present invention will be described. However, the following experimental examples are only to aid understanding of the present invention, and the present invention is not limited to the following examples.

<Example>

A martensitic stainless steel sample according to an embodiment of the present invention was prepared. The example sample was a martensitic stainless steel, and an AISI 410 material (hereinafter, specimen) having a size of Φ30×10 mm was prepared. The chemical composition of the specimen is summarized in Table 1 below.

ingredient
(wt%) C Si Mn P S Cr Ni Fe AISI 410 <0.15 <1.0 <1.0 <0.04 <0.03 12.5 0.3 Bal.

The specimen was subjected to tempering after quenching to obtain a base metal hardness of 480HV _0.1 . The heat-treated specimen was sequentially polished up to #1,500 using SiC sandpaper and then loaded into a vacuum chamber. Thereafter, as shown in FIG. 4, a nitridation treatment process according to an embodiment of the present invention was performed. In the nitriding process, first, in order to uniformly heat the entire specimen, nitrogen (N ₂ ) gas as an atmospheric gas is supplied into the chamber at a pressure of 1,100 mbar and 390° C. to control the atmosphere inside the chamber. I did.

Thereafter, in order to remove the chromium oxide on the surface of the specimen, at a pressure of 300 mbar and at 390° C., nitrous oxide (N ₂ O) gas was supplied into the chamber and the oxidation process was performed for about 30 minutes. Subsequently, ammonia (NH ₃ ) gas was supplied into the chamber under the same pressure and temperature conditions, and the reduction process was performed for about 30 minutes. The nitrous oxide (N ₂ O) gas and ammonia (NH ₃ ) gas were alternately supplied to the inside of the chamber, and the oxidation and reduction processes were repeated to reduce the chromium oxide formed on the surface of the test piece, thereby activating the surface reaction. .

After the pretreatment, the temperature was raised to about 460°C at the same pressure, and then ammonia (NH ₃ ) gas and nitrogen (N ₂ ) gas were mixed in a ratio of 1:1 and supplied into the chamber, followed by nitriding treatment for about 10 hours. To prepare the example samples.

On the other hand, in order to compare this, a comparative example sample was prepared in which the nitriding treatment was not performed on the same specimen.

After that, after etching with a solution of 95 ml of ethanol, 5 ml of hydrochloric acid and 1 g of picric acid, the surface hardened layer and the cross-sectional structure were observed with an optical microscope and a scanning electron microscope. I did. In addition, surface hardness and cross-sectional hardness were measured (100gf, 10 seconds) using a Micro Vickers Hardness Tester. X-ray diffraction analysis of the surface-hardened layer was performed using a Cu-Kα target to detect diffraction angles in the range of 20° to 90° to perform phase analysis on the surface of each test piece, and to analyze the formation behavior of the nitride layer, GD -OES was used to investigate the cross-sectional concentration distribution of nitrogen.

In addition, using a Ball-On-Disk abrasion tester, the surface friction and wear characteristics (friction coefficient and amount of wear) of each sample were investigated. Here, as the wear test ball, a 3 mm size ball made of SiC material was used, and a load was applied to the ball and the surface of each sample with a load of 5N. The track radius was set to 5 mm, and abrasion test was performed under dry conditions. After the wear test, the coefficient of friction of each sample was measured, and the width of the wear track formed on the sample surface was observed through an optical microscope. In addition, the amount of wear loss of the ball was calculated and compared. The wear amount of the lost ball was compared by calculating the volume of the loss amount as shown in Equation (1) below.

Equation (1): V=πd ⁴ /64r

(Where d is the diameter of the lost area of the wear ball (mm), V is the volume of the loss amount (㎣), and r is the radius of the spherical wear ball)

5 is a graph analyzing the Vickers hardness of martensitic stainless steel samples according to Examples and Comparative Examples of the present invention.

Referring to FIG. 5, it is a cross-sectional hardness measurement result of an Example sample and a Comparative Example sample of the present invention. First, in the case of the comparative example sample, a uniform hardness value was measured at about 490HV _0.1 for both the surface and the core. On the other hand, in the case of the example sample, a high hardness value of _0.1 to 1,250HV was measured. In addition, in the case of the example sample, the hardness increased from the core to the surface due to the penetration of nitrogen, and the hardness value was measured to be high up to 50 μm. However, the hardness decreased sharply from about 80㎛ to 580HV _0.1 , and after 100㎛, a hardness of about 500HV _0.1 similar to the base metal hardness was measured.

6 is a photograph showing a microstructure analysis of a martensitic stainless steel sample according to an exemplary embodiment and a comparative example of the present invention using an optical microscope, and FIG. 7 is a photograph showing a martensitic stainless steel sample according to an exemplary embodiment of the present invention. This is a picture of microstructure analysis using a scanning electron microscope.

In the case of the comparative example sample shown in FIG. 6(a), a tempered martensite structure was observed by quenching and tempering (Q&T) heat treatment, but in the case of the example sample shown in FIG. 6(b), the surface It could be confirmed that a layer of about 50㎛ was clearly distinguished. Example A compound having a size of 1 μm to 2 μm could be observed on the surface layer of the sample (see FIG. 7 ).

8 is a graph analyzing the nitrogen concentration of martensitic stainless steel samples according to Examples and Comparative Examples of the present invention.

Referring to FIG. 8, in the case of the comparative example sample, there was no significant change in hardness from the surface to the core, but in the case of the example sample, a high nitrogen concentration was measured at the surface part, indicating that the nitrogen concentration was high up to about 55 μm. I could confirm. It was confirmed that the surface was hardened by the solid solution of nitrogen from the results of tissue observation and hardness measurement.

9 and 10 are graphs for analyzing the crystal structure of martensitic stainless steel samples according to Examples and Comparative Examples of the present invention using an X-ray diffraction analyzer.

Referring to FIG. 9, it was confirmed that the surface measurement result of the sample of Comparative Example had a peak detected at 2θ values of 45°, 65°, and 82° (ICDD #06-0696) of the martensite (α′) phase. On the other hand, in the case of the example sample, it was confirmed that the α'-N phase (expanded martensite phase) and the Fe ₂₄ N ₁₀ phase (ICDD #73-2103) having a hexagonal crystal structure were measured.

Figure 10 is a graph for confirming the diffraction angle of the α'-N phase (expanded martensite phase) in detail, as seen that the diffraction angle was diffracted at a 44.2° angle slightly lower than 45°, so that the α'-N phase It was confirmed that (expanded martensite phase) was formed.

It can be understood that, in the example samples, a high fraction of nitrogen penetrated from 50 µm to 60 µm, and nitrogen penetrated the surface of the martensitic stainless steel by nitriding, causing the lattice to expand. It was confirmed through the experimental example of the present invention that such an over-solubility of nitrogen leads to high surface hardness and is a result of the formation of a surface compound and an α'-N phase (expanded martensite phase).

Referring again to FIGS. 2 and 3, a first iron oxide layer 20 such as Fe ₂ O ₃ is formed on the surface of steel 15. The first iron oxide layer 20 is a porous layer, and nitrogen (N ₂ ) gas can easily penetrate into the general steel 15 through the first iron oxide layer 20. Therefore, when the bonding force is lowered, like the ferric oxide layer 25, through surface activation treatment in a vacuum atmosphere chamber having a pressure of about 300 mbar, nitrogen atoms penetrate and the surface nitriding treatment of the general steel 15 is facilitated. Performed.

On the other hand, a first chromium oxide layer 30 such as Cr ₂ O ₃ is formed on the surface of the martensitic stainless steel 10. The first chromium oxide layer has a thin and dense film, so that nitrogen atoms are difficult to penetrate, making it very difficult to treat the surface nitriding of the martensitic stainless steel 10.

In order to solve this, in the present invention, the oxidation process of the first chromium oxide layer 30 is performed by supplying nitrous oxide (N ₂ O) gas into the chamber, and ammonia (NH ₃ ) gas is continuously supplied in turn. 1 The reduction process of the chromium oxide layer 30 is performed, but this is repeated several times in one unit cycle to remove or decompose at least a part of the first chromium oxide layer 30 to form the second chromium oxide layer 35. have.

Therefore, as a part of the first chromium oxide layer 30 is removed, or a second chromium oxide layer 35 having a smaller density than the first chromium oxide layer 30 is formed, nitrogen atoms are easily penetrated, and thus martensite. The surface nitriding treatment of the stainless steel 10 is easily performed.

11 is a graph showing an analysis of the coefficient of friction of martensitic stainless steel samples according to Examples and Comparative Examples of the present invention using a wear tester.

Referring to FIG. 11, as a result of the coefficient of friction measured during the abrasion test, in the case of the sample according to the comparative example, a constant friction coefficient of about 0.45 was measured even when the distance increased, and in the case of the sample according to the example, the initial friction The coefficient was measured to be the lowest, about 0.15, and it can be seen that the coefficient of friction increases as the distance increases. At 200m, all coefficients of friction were measured similarly, ranging from 0.4 to 0.45.

FIG. 12 is a photograph of analyzing the surface of the sample shown in FIG. 11 with an optical microscope, and Table 2 below summarizes the average coefficient of friction measured up to a distance of 200 m of each sample and the width and length values of worn balls and tracks.

Experimental example Coefficient of friction Of signs of wear and tear by the ball
Diameter (㎛) Wear track width (㎛) Comparative example 0.45 542.2 498.15 Example 0.32 597.7 520.83

Referring to Table 2 and FIG. 12, in the case of the sample according to the comparative example, a circular surface having a diameter of 542.2 μm was observed in the case of traces of wear by a ball, and the width of the track was measured to be 498.15 μm. On the other hand, in the case of the sample according to the example, the marks of wear by the balls were measured to be 597.7 μm, and the width of the track was measured to be 520.83 μm. The coefficient of friction of the sample according to the example was lower, and the diameter of the mark worn by the ball was measured the largest.

13 is a graph for comprehensively comparing and analyzing the amount of wear, surface hardness and friction coefficient of the sample shown in FIG. 11.

13, as a result of comparative analysis of the wear characteristics, compared to the friction coefficient of the sample according to the comparative example, the sample according to the example was measured as low as the friction coefficient of 0.2 to 0.4, and the wear resistance was considered to be the best. It is believed that the nitriding method according to an embodiment of the present invention is the best nitriding method for obtaining a high surface hardness and a low coefficient of friction.

As described above, as a pretreatment step by the nitriding method of martensitic stainless steel according to the embodiments of the present invention, oxidation/reduction by alternately supplying nitrous oxide (N ₂ O) gas and ammonia (NH ₃ ) gas By repeatedly performing the reaction, at least a part of the first chromium oxide layer may be removed or decomposed to form a second chromium oxide layer having a smaller density than the first chromium oxide layer.

When the first chromium oxide layer is removed or the second chromium oxide layer is formed, nitrogen atoms are easily penetrated, and the surface hardness of the martensitic stainless steel is nitrified by nitrogen diffusion, greatly improving the surface hardness compared to the prior art. I can make it. In addition, by implementing a martensitic stainless steel with an α'-N phase (expanded martensite phase) and an Fe ₂₄ N ₁₀ phase having a hexagonal crystal structure due to over-solubilization of nitrogen on the surface, the surface while maintaining the existing corrosion resistance It is possible to provide a method for nitriding martensitic stainless steel having excellent wear resistance and durability by improving the hardness of 2 times or more than the conventional one.

The present invention has been described with reference to the embodiments shown in the drawings, but these are merely exemplary, and those of ordinary skill in the art will appreciate that various modifications and equivalent other embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention should be determined by the technical spirit of the appended claims.

10: Martensitic stainless steel
15: General steel
20: first iron oxide layer
25: ferric oxide layer
30: first chromium oxide layer
35: second chromium oxide layer

Claims (9)

A unit cycle for activating the surface of the stainless steel by alternately supplying nitrous oxide (N ₂ O) gas and ammonia (NH ₃ ) gas into the chamber in a first temperature range after charging martensitic stainless steel into the vacuum chamber A pre-processing step of performing a plurality of times; And
After the pretreatment step, the stainless steel surface-activated by supplying a mixed gas of ammonia (NH ₃ ) gas and nitrogen (N ₂ ) gas into the chamber in a second temperature range relatively higher than the first temperature range. Nitriding the steel;
Including,
The pretreatment step,
And removing or decomposing at least a part of the passivation film formed on the surface of the martensitic stainless steel, wherein the passivation film contains chromium oxide (Cr ₂ O ₃ ),
The first temperature range is 360°C to 420°C, and the second temperature range is 450°C to 470°C,
The pretreatment step and the nitriding step,
Including the step of performing the process pressure inside the chamber in the same pressure range,
The same pressure range is 250 mbar to 350 mbar,
Nitriding method of martensitic stainless steel. delete delete delete The method of claim 1,
Before the pretreatment step,
Further comprising the step of supplying nitrogen (N ₂ ) gas into the chamber to control the atmosphere inside the chamber,
Nitriding method of martensitic stainless steel. The method of claim 1,
The mixed gas has a ratio of ammonia (NH ₃ ) gas and nitrogen (N ₂ ) gas in the range of 1:1 to 1:1.5,
Nitriding method of martensitic stainless steel. delete delete delete

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