JP2015161011A - High nitrogen solid solution stainless steel and method for forming mold material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stainless steel capable of forming a nitrated layer having comparatively large depth, high nitrogen concentration and high hardness on the surface of the stainless steel.SOLUTION: The high nitrogen solid solution stainless steel has a nitrided layer formed by performing plasma nitriding on the surface of the stainless steel to generate phase transformation and having a depth of 5 μm or more (preferably 20 μm or more and more preferably 50 μm or more) from the surface, a nitrogen concentration of 10% or more (preferably 20% or more and more preferably 40% or more) and a hardness of 1000 Hv or more (preferably 1500 Hv or more and more preferably 1700 Hv or more).

Description

  The present invention relates to a technique for hardening by dissolving nitrogen on the surface of stainless steel.

Since stainless steel is excellent in corrosion resistance, it is widely used in various applications such as machine parts to chemical production plants, but has a drawback of low hardness and poor wear resistance. For this reason, studies have been made to increase the surface hardness by dissolving nitrogen in stainless steel.
For example, Non-Patent Document 1 discloses a technique for solidly dissolving nitrogen by a high-temperature pressurized gas method. Non-Patent Document 2 discloses a technique for low-temperature nitriding the surface of austenitic stainless steel by a combined treatment of DC plasma nitriding and carburizing.
Knowing near future steel materials Internet URL: http://www.nims.go.jp/stx-21/jp/publications/stpanf/pdf/tainetsu2.pdf Search date: February 16, 2014 Low temperature plasma nitriding and carburizing treatment of austenitic stainless steel Internet URL: http://tri-osaka.jp/densi_kannkoubutu/syoho/TRI25(2011)29.pdf Search date: February 16, 2014

According to the method of Non-Patent Document 1, nitrogen can be dissolved in the entire material. However, since the nitriding concentration is only about 1 mass%, the demand for increasing the hardness of the surface layer of the material can be met. There was no problem.
In addition, according to the method of Non-Patent Document 2, the introduced nitrogen distribution is as small as 10% or less, and the distribution tends to monotonously decrease in the depth direction, so the surface portion of stainless steel is stably nitrided at a high concentration. There was a problem that I could not.

The present invention has been devised in order to solve the above-mentioned conventional problems. It is a first object of the present invention to provide a stainless steel in which a nitride layer having a relatively deep, high nitrogen concentration and high hardness is formed on the surface of the stainless steel. One purpose.
A second object is to provide a technique for applying this high nitrogen solid solution stainless steel as a mold material.

In order to achieve the first object, the high nitrogen solid solution stainless steel according to the present invention has a surface portion of a nitrided layer formed by phase transformation by dissolving nitrogen at a high concentration on the surface of the stainless steel by plasma nitriding. It is characterized by being formed.
The nitride layer is formed, for example, at a depth of 5 μm or more, preferably 20 μm or more, more preferably 50 μm or more from the surface.
The nitride layer has a nitrogen concentration of, for example, 10 at% or more, preferably 20 at% or more, more preferably 40 at%.
The nitride layer has a hardness of, for example, 1000 Hv or higher, preferably 1500 Hv or higher, more preferably 1700 Hv or higher.

  In order to achieve the second object, a method for forming a mold according to the present invention includes a step of arranging a mask in a predetermined pattern on a surface of stainless steel, a plasma nitriding treatment on the surface of the stainless steel, and a mask Forming a nitride layer on the portion where no metal is disposed, removing the mask, and subjecting the surface of the stainless steel to sand blasting, laser processing, etc., and forming a recess in a portion where the nitride layer is not formed It consists of a process.

In the case of the high nitrogen solid solution stainless steel according to the present invention, a highly uniform nitride layer having a high nitrogen concentration and a uniform hardness is formed from the surface to a relatively deep region, and thus has excellent wear resistance.
For this reason, for example, after forming a mask pattern on the surface and selectively forming a nitrided layer and a non-nitrided portion, the non-nitrided portion having a low hardness is removed by sandblasting, laser irradiation, etc. It becomes possible to apply.

FIG. 1 is a schematic diagram showing the structure of an RF-DC low-temperature plasma nitriding apparatus 10 according to the present invention. The vacuum chamber 12, a DC bias 14 disposed in the vacuum chamber 12, and the DC bias 14 are mounted thereon. A stainless steel workpiece 16, a heater 18 mounted in the DC bias 14, a pair of RF electrodes 20, and an RF oscillator 22 with an output of 2 MHz disposed outside the vacuum chamber 12 are provided.
Although not shown, outside the vacuum chamber 12, a control device for the RF oscillator 22, a control device for the DC bias, and a cooling device are installed.

A mixed gas of N ² and H ^{2 as} a raw material is introduced into the inside from the air supply port (not shown) of the vacuum chamber 12, and a high frequency is applied between the RF electrodes 20 and 20 and simultaneously a DC bias voltage is applied, When the heater 18 is further powered and the inside of the vacuum chamber 12 is heated to 450 degrees Celsius or lower, the mixed gas becomes plasma, and as shown in FIG. 2, high-density nitrogen ions and NH radicals are generated, and nitrogen atoms are processed. It penetrates the surface of the object 16.

In the case of this apparatus 10, since RF plasma and DC plasma can be controlled independently, nitriding can be performed in a wide high pressure range (meso pressure region) of 20 Pa to 1 kPa.
The density of nitrogen ions and NH radicals in this meso pressure range is 5 × 10 ¹⁷ m ⁻³ or more, and nitriding is performed in the high nitrogen ion / high NH radical state. Nitrogen atoms can be introduced as solute atoms.
Also, unlike conventional plasma devices, input / output power matching is performed in the frequency domain, so input energy is not wasted and it is quickly input into plasma. As a result, it is possible to stably dissolve nitrogen in the surface of the workpiece 16 without unevenness.

When the above processing is continued for a predetermined time, nitrogen is dissolved at a high concentration on the surface of the workpiece 16.
Fig. 3 shows AES (Auger Electron Spectroscopy) analysis of a SUS420 (martensitic stainless steel) sample that was plasma-nitrided at 450 degrees Celsius for 2 hours by nitriding with high-density RF-DC plasma. It is a graph which shows a result.
As shown in the drawing, a high nitrogen concentration of 30 to 40 at% is realized from the surface to the nitriding tip (around 4 μm in depth).
That is, it is considered that the high nitrogen concentration is constant from the surface to the tip of nitriding, and solute nitrogen is regularly introduced into the vacancy positions of the crystal lattice of stainless steel.
By extending the processing time to 2 hours or more, a nitride layer can be formed to a depth of 50 μm or more and with a substantially equal nitrogen concentration.

On the other hand, since the XRD peak of chromium / iron nitride cannot be observed at all, it can be said that all of these high-concentration nitrogen is dissolved in stainless steel.
Thus, since chromium and iron nitrides are desired to precipitate, the original characteristics of stainless steel are not adversely affected.

FIG. 4 is a graph showing the results of XRD analysis performed on the sample.
The main peak of the SUS420 material, which is martensitic stainless steel, originally corresponds to the (110) plane, which is the closest dense surface of the body-centered cubic lattice crystal.
On the other hand, the plasma high nitrogen solution treatment described above causes distortion in the c-axis direction rather than in-plane distortion including the a-axis, and transforms to an austenite phase having a (111) plane parallel to the original (110) plane. . That is, since it becomes an austenite single phase, it turns out that very high concentration nitrogen is dissolved in stainless steel.
On the other hand, chromium and iron nitride peaks are not detected.

FIG. 5 shows the results of a micro-Vickers hardness test with a rolling load of 200 g when a Φ25 mm disk-shaped SUS420J2 test piece (thickness: 5 mm) 30 is low-temperature nitridated at 425 degrees Celsius for 4 hours. .
As shown in the figure, the hardness of the central portion of the test piece 30 was 1750 Hv on average, and the hardness of its peripheral portion was 2400 Hv on average.

FIG. 6 shows the results of hardness measurement on a rectangular SUS420J2 test piece (thickness: 5 mm) 40 in which square nitriding sections 42 and square non-nitriding sections 44 are alternately arranged. is there.
As shown in [Measurement results] in the figure, the hardness at the nitriding section 42 (position (1), position (2), position (4), position (6), position (7)) is non-nitriding treatment. Compared with part 44 (position (3), position (5)), it is significantly improved.

The test piece 40 is prepared by the following procedure.
(1) A square mask (carbon tape) is affixed at a position where the non-nitriding portion 44 is intended to be formed on the surface of the rectangular SUS420J2 plate.
(2) The low temperature nitriding treatment is performed on the SUS420J2 plate at 425 degrees Celsius for 4 hours.
(3) Finally, the above mask is removed.
FIG. 7 shows the measurement result of nitrogen element by EDX (energy dispersive X-ray analyzer) on the surface of the nitriding section 42, and a nitride layer is formed to a depth of about 5 μm from the surface of the SUS420J2 test piece. The situation is shown.

FIG. 8 shows the result of measuring the average surface roughness (Ra) for the same test piece 40 as shown in FIG.
As shown in [Measurement Result] in the figure, the Ra value in the nitriding unit 42 is significantly increased (nearly 100 nm) as compared to the non-nitriding unit 44.

  Originally, the surface roughness of SUS420J2 is about 10 nm, and it has the same surface roughness in the mask part and the non-mask part before nitriding treatment, but because nitrogen dissolves at a high concentration only in the non-mask part, It is considered that the surface roughness is increased because the surface is higher than the mask portion (portion where nitrogen is not dissolved) due to lattice expansion.

Here, the thickness of the nitrided layer is 5 μm, and the strain in the c-axis direction of SUS420 / single crystal caused by 40 at% solute nitrogen is 7% (theoretical estimated value).
The actual SUS420 material is polycrystalline, and the orientation ratio in the c-axis thickness direction is set to 33% by rolling and forging.
From the above, the amount of expansion deformation in the thickness direction of the actual SUS420 material can be calculated as follows.
(5000 nm) × 0.07 × 0.33 = 100 nm

  Thus, since the value obtained as a result of calculation (100 nm) and the above measurement result almost coincided, the difference in surface roughness (100 nm) was caused by the selective expansion and deformation of the non-mask part by the nitriding process. Can be considered.

Next, a method for forming a mold material by applying this selective nitriding process will be described.
First, as shown in FIG. 9 (a), a mask 52 is attached to the surface of a base material 50 made of SUS420 at a predetermined interval, and then the above nitriding treatment is performed.
As a result, nitrogen is dissolved in the non-mask portion on the surface of the base material 50 at a depth of about 5 μm, and a large number of nitride layers 54 are formed as shown in FIG. 9B.

Next, as shown in FIG. 10A, after the mask 52 is removed, a sand blast process is performed in which an abrasive material 56 such as sand is sprayed toward the surface of the base material 50.
As a result, it is possible to obtain a mold 60 in which a nitride layer 54 whose hardness has been increased by the previous nitriding treatment remains, and a large number of recesses 58 are formed in the mask portion that has not been subjected to the nitriding treatment (FIG. 10 ( b)).

  Recently, resin materials filled with various fillers and minerals are increasing, and there is a problem that the wear is severe when the mold material is formed of ordinary stainless steel, but the above-mentioned plasma nitriding treatment and sandblasting treatment Since the formed mold material has sufficient hardness on the surface, high durability can be expected.

1 is a schematic diagram showing the structure of an RF-DC low temperature plasma nitriding apparatus according to the present invention. It is a conceptual diagram which shows a mode that a nitrogen atom osmose | permeates the surface of a workpiece. It is a graph which shows the analysis result by AES with respect to the nitriding stainless steel. It is a graph which shows the analysis result by XRD with respect to the nitriding stainless steel. It is a figure which shows the result of the hardness test with respect to the disk-shaped stainless steel in which the nitriding process was performed. It is a figure which shows the measurement result of the hardness of the part by which the nitriding process was performed on the surface of stainless steel, and the hardness of the part which has not been nitrided. It is a figure which shows the measurement result of the nitrogen element by EDX in a nitriding part. It is a figure which shows the measurement result of the surface roughness of the part by which the nitriding process was performed on the surface of stainless steel, and the part which has not been nitrided. It is a figure which shows the formation process of a mold material. It is a figure which shows the formation process of a mold material.

10 Low temperature plasma nitriding equipment
12 Vacuum chamber
14 DC bias
16 Workpiece
18 Heater
20 RF electrodes
22 RF oscillator
30 specimens
40 specimens
42 Nitriding section
44 Non-nitriding part
50 base material
52 Mask
54 Nitride layer
56 abrasive
58 recess
60 Mold material

Claims (5)

  A high nitrogen solid solution stainless steel characterized in that a nitride layer by phase transformation is formed on the surface portion by solidifying nitrogen at a high concentration on the surface of stainless steel by plasma nitriding.   2. The high nitrogen solid solution stainless steel according to claim 1, wherein the nitrided layer has a depth of 5 μm or more from the surface, preferably 20 μm or more, more preferably 50 μm or more.   3. The high nitrogen solid solution stainless steel according to claim 1, wherein the nitrogen concentration of the nitrided layer is 10 at% or more, preferably 20 at% or more, more preferably 40 at% or more.   The high nitrogen solid solution stainless steel according to any one of claims 1 to 3, wherein the hardness of the nitrided layer is 1000 Hv or more, preferably 1500 Hv or more, more preferably 1700 Hv or more. Placing a mask in a predetermined pattern on the surface of stainless steel;
Performing a plasma nitriding treatment on the surface of the stainless steel, and forming a nitride layer on a portion where no mask is disposed;
Removing the mask;
Forming a recess in a non-formed portion of the nitride layer on the surface of the stainless steel;
A method for forming a mold material, comprising:

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