JPH0663083B2 - Stainless steel having excellent scratch resistance and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a stainless steel characterized in that its surface is not easily scratched.

Since stainless steel has excellent corrosion resistance and has a clear luster and color tone, it is often used for decorative applications such as automobile parts, home electric appliance parts and building parts. Naturally, rust resistance, which is difficult to rust, is an important characteristic for such applications, but at the same time, the property that scratches are unlikely is also an important factor. That is, even if the flaws are minute, if the flaws are attached, they are easily noticeable because of the excellent surface gloss, and the commercial value is lowered. In addition, depending on the product, even if it is finer than the defect itself, there is a problem that a so-called “smear” such as a detached metal powder or a detached piece caused by the defect may hinder the function of the product. For example,
In the center core material and shutter material of the floppy disk, the flaw itself reduces the commercial value, but rather there is a concern that the magnetic recording may be disturbed by the fine metal powder generated by the flaw.

The present invention has excellent corrosion resistance and surface defects such as decorative parts of products whose product value is reduced by such surface defects and parts of products which are at risk of indirectly impairing the function of the product due to the defects. It is used for applications that require difficulty.

(Prior Art) Hardness of surface flaws can be translated into scratch resistance. Corrosion resistance is almost proportional to the hardness of the surface. Therefore, in the conventional applications where surface flaws are disliked, quench-hardening stainless steel has been used, or work-hardened stainless steel has been used. However, these quench-hardening stainless steels and work-hardening materials have a high hardness, so a corresponding effect was found in scratch resistance, but since processing becomes extremely difficult, it can be applied to any application parts. It wasn't technology.

By the way, it is clear that the surface scratch resistance, ie, scratch resistance, is naturally related to the surface hardness, although it is related to the hardness, and the bulk hardness is not directly related. Based on this viewpoint, it has been attempted to cure only the surface layer by surface treatment to secure workability and improve scratch resistance. For example, Cr plating has often been used because the plating layer exhibits almost the same level of hardness as the quench-hardened material, and the surface treatment is a nearly completed technology. Furthermore, due to recent advances in PVD and CVD technologies, application of surface-treated materials whose surface is coated with a hard ceramics film such as TiN or SiC has been considered (Journal of Japan Institute of Metals, Volume 23, No. 10 (1984)). , P812). However, although these methods are sufficiently effective from the viewpoint of scratch resistance, the processing cost is inevitably high, and the PVD and CVD methods limit the size. In addition to the drawbacks of, the Cr plating is a treatment that eliminates the reason for using stainless steel.

On the other hand, from the viewpoint that the scratch resistance is directly related to the hardness of the surface, surface quenching or carburization or nitriding of only the surface layer can be considered. However, in general, in the case of stainless steel, although carbide carbide / nitride of Cr is generated and the purpose of surface hardening can be achieved for some time because Cr is contained in the steel, the metallic Cr on the surface that maintains corrosion resistance is largely However, it was not applied because the corrosion resistance, which is essential to carbon steel, deteriorates to the same level as carbon steel.

(Problems to be Solved by the Invention) As described above, it cannot be said that the stainless steel with improved scratch resistance and the method of manufacturing the same have never existed until now. However, both methods have many drawbacks such as deterioration of corrosion resistance and workability, and extremely high cost. Therefore, it is rarely used for a part such as a decorative part that is not directly related to the functional characteristics, and is inevitably used only for a part that may slightly impair its function.

The present invention is to provide a stainless steel which is inexpensive and has excellent scratch resistance without deteriorating the characteristics of the stainless steel, and a method for producing the same. For this reason, in the present invention, from the viewpoint that the scratch resistance is governed by the hardness of the surface, how inexpensively and how to harden the surface without degrading the characteristic function of the base material as stainless steel. It was made as a result of examination.

(Means for Solving the Problems) In order to cure the surface layer, a method of applying strain only to the surface layer and a method of coating or mixing the hard substance only on the surface layer are conceivable. However, the method using the surface layer strain is difficult to apply because the strain cannot be applied only to the surface layer as the scratch resistance is improved. Therefore, it was examined to improve the scratch resistance by coating the surface layer with a hard substance or mixing them in a complex manner. The present invention is based on this idea.

As a method of coating the surface layer with the hard substance, there are methods such as plating, PVD, and CVD, which are very expensive as described above. These methods are techniques of covering the surface with a hard material.
Of course, the scratch resistance is improved by the method of covering the entire surface.

However, the present inventors have found that in the case of scratches having scratches, it is difficult for the scratches to adhere even to locally micro soft portions,
That is, they have found that it is not always necessary to cure the entire surface to improve scratch resistance. On the other hand, in the steel containing Ti or Zr, it has been experienced that the surface is likely to be nitrided, and thus bright annealing in a mixed gas of nitrogen and hydrogen is difficult. The present invention is based on these findings and experience, nitriding the surface without consuming metallic Cr,
The purpose of this is to deposit a hard nitride on the surface layer so that the hard material is mixed in the surface layer.

The nitriding treatment described in the present invention is different from the nitriding treatment, which is a commonly used term, in the target mechanism and action. The term nitriding, which is a commonly used term, means
This is a technique of infiltrating nitrogen, which is an interstitial solid solution element, into the steel, and then increasing the hardness of the martensite phase itself that has been transformed by quenching. On the other hand, the nitriding treatment aimed at in the present invention is a technique of utilizing the hardness of the nitride thus precipitated as a nitrogen source for forming a nitride, instead of making the solid solution as an interstitial solid solution. Is called. Therefore, it is not necessary to transform into the martensite phase after nitriding, but it is relatively soft depending on the hardness measurement method (such as the Rockwell hardness method with a large indentation) and has a large variation (a very small indentation is extremely small). Vickers hardness method, etc.) is a technique that is effective for the desired scratch resistance. Therefore, the structure of the matrix is rather left as ferrite or austenite to try to secure workability.

Hereinafter, description will be made based on the results of the survey test.

First, the type of additive element on the metal side that can precipitate hard nitride by nitriding treatment was examined. Cr, Ta, Fe, B, Ti, Nb, Zr and the like are known as elements that form a stable and hard nitride that easily forms a solid solution in stainless steel. In the present invention, it is an important requirement that metal-like Cr is not consumed. Therefore, it is necessary that the element is an element that is more easily nitrided than Cr ₂ N or CrN. Considering such conditions, the additive elements for forming the nitride are limited to Ti, Nb, and Zr.

Next, low C, N17C added with Ti, Nb, Zr alone or in combination.
r Stainless steel was annealed in a mixed gas atmosphere of 75% nitrogen, 25% hydrogen, and dew point of -35 ° C or less under air cooling conditions of 850 ° C for 1 min. Then, a scratch test was performed on the surface. According to JIS-K6718 (1983), the scratch test was defined as a method of scratching the measurement surface at a scanning speed of 10 mm / sec by applying a predetermined load to a diamond needle having a tip radius of curvature of 50 μm. Then, as one of the criteria for scratch resistance, the generated scratches were observed with an optical microscope at a magnification of 400 to 500, and the minimum load at which so-called breakage defects such as tears were generated instead of mere dents was measured. Figure 1 shows
FIG. 3 is a diagram showing the minimum load at which fracture defects occur in the scratch test with respect to the sum of the Ti, Nb, and Zr contents in steel. As is clear from the figure, the addition amount of the nitride-forming element in which the increase in the minimum load at which fracture defects occur was recognized was about 0.1% regardless of the type of element. It was also confirmed that the minimum load at which fracture defects occur due to an increase in the amount of addition increases, that is, the scratch resistance is improved.

Next, the limiting conditions of the present invention will be shown.

If the Cr content of the target steel is less than 10%, the basic corrosion resistance as stainless steel is insufficient, so the lower limit was made 10%. Further, if it exceeds 30%, the cost becomes expensive and there is no application except for special cases, so the upper limit was made.

Usually, in addition to Cr and Ti, Nb, Zr described later, Cn, Mn, Si, Ni, Mo, Cu, Al, N, etc. are added to the stainless steel as necessary. The effect is not affected by the presence or absence of addition of these and the amount added, and is not particularly limited.

The nitrides of Ti, Nb, and Zr have a lower limit because the effect of scratch resistance is not recognized when the compound amount is less than 1% in terms of area ratio measured and evaluated from the surface of the surface layer within at least 1 μm. However, if the content exceeds 60%, a crack is generated in the surface layer due to processing, so the upper limit was made.

The amount of Ti, Nb, and Zr added is 0.1, which is effective from Fig. 1.
% Was the lower limit. As is clear from FIG. 1, the scratch resistance is improved up to at least about 5% with the addition amount. Therefore, from the viewpoint of the effect, the upper limits of the addition amounts of these elements are not limited, but a large addition amount is not recommended. The cost is disadvantageous and the manufacturability such as hot workability is deteriorated. Therefore, the upper limit is 3%.

The atmosphere of the nitriding treatment was set to the lower limit because nitrogen does not proceed if the partial pressure of nitrogen is less than 0.2 atm without ammonia. The nitriding process of course proceeds even in a pressurized atmosphere exceeding 1.2 atm of nitrogen, but 1.2 atm is set as the upper limit because equipment for pressurization is required and it is not necessarily advantageous from the viewpoint of effective use of nitrogen gas itself. did. Further, addition of ammonia gas or the like, which is recognized in the conventional nitriding treatment for promoting the nitriding treatment, can be used without any problem, and is similarly effective for improving the treatment speed. When the amount of ammonia gas is 0.002 to 0.5 atm, the required amount of nitrogen is reduced, but the nitriding does not proceed if the partial pressure of nitrogen is less than 0.01 atm. The upper limit is 1.0 atmospheric pressure.

The nitriding temperature does not nitrid below 500 ° C, or even if nitriding, the rate is very slow, so the lower limit is set.If it exceeds 1200 ° C, nitriding proceeds but the quality of the material deteriorates due to coarsening of crystal grains. In addition to being connected to, there is a cost disadvantage, so the upper limit was set.

(Operation) As described above, the scratch resistance is greatly improved by mixing hard nitrides in the surface layer in the present invention. That is, since nitrides such as TiN are much harder than the hardness of martensite and the like, they are very effective against scratches such as scratches even if the entire surface is not covered.

(Example) 0.2% Ti-added 17Cr steel and 0.3% Zr-added 17Cr shown in Table 1
Steel, 0.8% Ti-added 18Cr-8Ni steel and Ti-free 17Cr steel with a thickness of 0.5 mm were treated with nitrogen gas having a purity of 99.99% or more.
99.99% or higher purity hydrogen gas and 9 at dew point -40 ° C or lower
It heat-processed in the mixed gas adjusted using the ammonia gas of 9.9% or more of purity. After that, the amount of nitride measured in the surface layer and the measurement result of scratch resistance are shown in Table 2 together with the heat treatment conditions. 1μ by measuring and evaluating from the surface
The present invention steel according to the present invention method in which the nitride TiN is compounded in an area ratio of 1% or more in the surface layer within m, the minimum load that causes so-called fracture flaw is higher than the comparative steel, and the scratch resistance is excellent. You can see that Further, in the comparative method in which the nitrogen partial pressure was less than 0.2 atm or the heat treatment temperature was less than 500 ° C., the progress of nitriding was not observed and the scratch resistance was not improved. In the comparison method in which the heat treatment temperature was more than 1200 ° C, the scratch resistance was found to be satisfactory without any problem in the state as it was, but the crystal grains were coarsened and could not be used as a material.

(Effects of the Invention) As described above, the present invention makes it possible to obtain stainless steel that is inexpensive and has excellent scratch resistance. As a result, stainless steel, which has excellent scratch resistance, can be used in large quantities even for parts for decorative applications, which were rarely used in the past, and industrial and social benefits are extremely large. .

Further, such a stainless steel excellent in scratch resistance according to the present invention, simply containing Ti or Zr in the steel,
It can be manufactured by a simple method of heat treatment in a gas containing nitrogen. Furthermore, this heat treatment
For example, it is possible to combine with recrystallization annealing after cold rolling, and the conditions are such that it can be easily incorporated into a conventional manufacturing process, so that some industrial effects cannot be measured.

It is obvious that the present invention can be expected to have the effect regardless of the shape of stainless steel such as a thin plate, a thick plate, a wire rod or a steel pipe.

[Brief description of drawings]

FIG. 1 is a diagram showing the minimum load at which fracture defects occur due to the above-mentioned scratch test with respect to the Ti, Nb or Zr content in steel.

Claims (3)

[Claims] 1. A stainless steel containing Cr in an amount of 10% or more and 30% or less. Ti, Z is formed in a surface layer having a depth of at least 1 μm.
Stainless steel excellent in scratch resistance, which is characterized by compounding at least 1% or more and 60% or less of an area ratio measured and evaluated from the surface of one or more nitrides of r and Nb. 2. A Cr content of 10% or more and 30% or less.
0.1% to 3.0%, Zr: 0.1% to 3.0%, Nb: 0.1
% Or 3.0% or less of one or two kinds of stainless steel is heat-treated at a temperature of 500 ° C to 1200 ° C in an atmosphere in which the partial pressure of nitrogen is 0.2 atm to 1.2 atm And a method for producing stainless steel having excellent scratch resistance. 3. A Cr content of 10% to 30%, and Ti:
0.1% to 3.0%, Zr: 0.1% to 3.0%, Nb: 0.1
% Or more and 3.0% or less of one or two kinds of stainless steel and 500 ° C in an atmosphere in which the partial pressure of ammonia is 0.002 atm or more and 0.5 atm or less, and the nitrogen partial pressure is 0.01 atm or more and 1.0 atm or less. A method for producing stainless steel having excellent scratch resistance, which comprises performing heat treatment at a temperature of not less than 1200 ° C.