JP2011231387A - Free-cutting stainless steel material for precise processing, and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a free-cutting stainless steel material for precise processing which can meet all of excellent cutting processing accuracy, machinability, corrosion resistance, and an enviromental property at the same time, and a method of manufacturing the same.SOLUTION: The free-cutting stainless steel material for precise cutting is used for performing a shaping process by cutting at a micron meter level, and has such a structure that the free-cutting property giving material is made of h-BN particles and scattered in single state in the steel. The manufacturing method is characterized by heating the free-cutting stainless steel material for precise processing in which the h-BN particles are deposited, then rapidly cooling it to dissolve and extinguish the h-BN particles, and then, tempering it to disperse and deposite the h-BN particles again equally in the material.

Description

  The present invention relates to a free-cutting stainless steel material for precision machining that performs forming by cutting at a micrometer level.

Conventionally, stainless steel has been used as a material for machining precision parts, taking advantage of its corrosion resistance, but using stainless steel in precision machining is more difficult to cut than using ordinary steel. Therefore, it has been desired to improve the machinability.
As a stainless steel with improved machinability, sulfur free-cutting stainless steel SUS303 is widely known, but the surface after cutting becomes rough, and it is said that it is difficult to use it for precision cutting at the micrometer level. It was.
That is, conventionally, stainless steel has been considered to be incompatible with machinability and precision workability (surface roughness after cutting).

International patent application WO2008 / 016158 discloses a free-cutting stainless steel capable of simultaneously satisfying excellent machinability and corrosion resistance, and a method for producing the same. In this invention, the corrosion resistance is equivalent to that of a conventional stainless steel material, and the machinability is improved by about 25%. However, the surface roughness after cutting of this material is not disclosed.

As described above, a free-cutting stainless steel material for precision machining that has excellent surface properties after cutting and corrosion resistance and a method for producing the same have not been disclosed at present.

In view of such circumstances, an object of the present invention is to provide a free-cutting stainless steel material for precision machining that can satisfy both machinability and precision workability, and a method for producing the same.

The present invention effectively uses the properties of h-BN (hexagonal boron nitride) particles that are excellent as a solid lubricant and are chemically stable and not affected by acid or alkali. By utilizing the solid solution and re-precipitation, we found a free-cutting stainless steel material for precision machining with excellent surface properties and cutting properties and corrosion resistance after cutting, where dimensional accuracy is important, and its manufacturing method. It was made.

  Invention 1 is a free-cutting stainless steel material for precision machining that performs molding by cutting at a micrometer level, the free-cutting property imparting material is h-BN (hexagonal boron nitride) particles, and the particle size is A configuration was adopted in which spherical h-BN particles of 200 nm to 5 μm were dispersed and precipitated in a single state in steel. Here, the simple substance state refers to a state where a plurality of h-BN particles or non-metallic inclusion particles other than h-BN and h-BN particles are not aggregated.

  Invention 2 employ | adopted the structure characterized by the B content being 0.003-0.1 mass% in the free-cutting stainless steel raw material for precision processing of Invention 1.

Invention 3 employs a configuration characterized in that in the free-cutting stainless steel material for precision machining of Invention 1, the N content is equal to or higher than the B content in a molar ratio.

Invention 4 is a free-cutting stainless steel material for precision machining as set forth in any one of Inventions 1 to 3, wherein the surface roughness of the turning surface has a 10-point average roughness (Rz) of 5 μm or less. The structure characterized by having was adopted.

Invention 5 employs a configuration characterized in that in the free-cutting stainless steel material for precision machining described in Invention 4, the turning surface characteristics are obtained by turning a round bar having a diameter of 8 mm under the following conditions. .
Cutting speed: 16 m / min, cutting depth: 0.2 mm, tool feed speed: 0.08 mm / rev, tool material: M30, tool shape: equilateral triangle, with chip breaker, cutting fluid: not used.

Invention 6 is a method for producing a precision-cutting free-cutting stainless steel material according to any one of Inventions 1 to 5, wherein B is added with ferroboron or metal boron, and N is a dissolved atmosphere (argon + nitrogen). Or the structure characterized by adding to stainless steel molten steel by melt | dissolution in the pressure-reduced nitrogen was employ | adopted.

Invention 7 is a method for producing a free-cutting stainless steel material for precision machining according to any one of Inventions 1 to 5, wherein B is added by adding ferroboron or metal boron, N is added by adding a nitrogen-containing compound, and stainless steel is added. The structure characterized by adding to molten steel was adopted.

Invention 8 is a method for producing a free-cutting stainless steel material for precision machining according to any one of Inventions 1 to 5, wherein h-BN particles are contained in the structure obtained by the method according to claim 6 or 7. By heating the non-uniformly precipitated stainless steel to a temperature of 1200 ° C. or higher and then rapidly cooling it, the h-BN particles are once dissolved out, and then tempered at a temperature of 950 to 1100 ° C. , A method for producing a free-cutting stainless steel material for precision machining, wherein the h-BN particles are again dispersed and precipitated.

Chemically stable h-BN particles that are not attacked by acid or alkali are dispersed and precipitated in a single state, and have excellent machinability as a solid lubricant, excellent cutting accuracy, and deterioration in corrosion resistance. We were able to provide a free-cutting stainless steel material for precision machining and its manufacturing method.
In particular, as shown in the examples, the surface roughness after cutting has a performance similar to or less than that of stainless steel without reworkability, and surface treatment after precision machining is almost unnecessary. .
These effects are due to the effective application of h-BN particles, which have excellent properties as solid lubricants, to free-cutting stainless steel materials for precision machining. Not only machining accuracy, corrosion resistance and free-cutting properties, Since environmentally hazardous substances such as Pb and Se are not used, it was possible to provide a free-cutting stainless steel material for precision machining that can satisfy environmental characteristics.
In addition, since the machining accuracy is excellent, it is possible to omit further machining accuracy improvement processes such as grinding and polishing, and the power of the cutting machine is reduced due to improved machinability, and electric energy consumption is reduced. Can be reduced and high-speed cutting is possible, leading to improved productivity.

The present invention has the features as described above, and an embodiment thereof will be described below.
In the production method of the present invention, the free-cutting stainless steel material for precision machining is melted by using a melting furnace for melting ordinary stainless steel, which can adjust the melting atmosphere. In this melting, ferroboron or metal boron is used as a raw material for B (boron), but ferroboron having a low melting point is technically advantageous as a melting raw material, and it is per unit weight of B (boron). Economical due to low market price.

The amount of B added is generally 0.003 to 0.1 mass% B, more preferably 0.003 to 0.03 mass% B, as a general guideline for the final B content in the precision-cutting free-cutting stainless steel material. . In addition, as a raw material of N (nitrogen), there is an addition of a nitride of an alloy element that absorbs N in a melting atmosphere or constitutes stainless steel, for example, chromium nitride, ferrochromium nitride or the like.

As for N content in the free-cutting stainless steel material for precision processing, N / B should just be 1 or more by molar ratio as a general standard. When the molar ratio of N and B in the free-cutting stainless steel material for precision machining is less than 1, the amount of dissolved B increases and the precipitation amount of h-BN effective for machinability decreases. Must be 1 or more. The N content depends on the constituent element components in the free-cutting stainless steel material for precision machining, but since B increases the activity of N, the equilibrium N concentration in the steel decreases as B increases. The component composition of SUS304 is 0.25 mass% or less except for dissolution in a pressurized N atmosphere.

  The stainless steel molten steel containing B and N produced as described above is cast into a mold to become a free-cutting stainless steel ingot for precision machining.

The free-cutting stainless steel ingot for precision machining is formed into a rod, wire, plate, or the like of a free-cutting stainless steel material for precision machining through hot working such as normal forging or rolling. The free-cutting stainless steel material for precision machining after hot working is air-cooled to room temperature.
In the free-cutting stainless steel material for precision machining, h-BN, which grows coarsely to about 20 to 30 μm, is unevenly distributed in a part of the material during the cooling process after hot working. May be generated.

H-BN deposited in the free-cutting stainless steel material for precision machining is dissolved in a relatively short time (for example, 0.5 to 1 hour at 1250 ° C) by maintaining the temperature at 1200 ° C or higher. It can exist in the matrix in a state of being decomposed into B and N. Utilizing this solid solution phenomenon of h-BN, heat treatment is performed to re-dissolve h-BN generated in the material in a non-uniform or coarse manner in the material.
In addition, since such a process is impossible when the free-cutting stainless steel material for precision processing is melted, it is necessary to perform the process at a temperature lower than the melting temperature.

By rapidly cooling in this state, a free-cutting stainless steel material for precision machining containing B and N in a solid solution state in a supersaturated state is obtained. The quenching operation may be water cooling performed on normal stainless steel, but it is necessary that the cooling rate in the temperature range in which h-BN, which will be described later, is precipitated, is a cooling rate that does not cause precipitation.

When B and N dissolved in supersaturation are tempered at a temperature of 800 ° C. to 1150 ° C., h-BN dissolved in the solution heat treatment is precipitated again. When tempering is performed at around 800 ° C., the difference between the equilibrium solubility of B and N at this temperature and the supersaturated solubility is large, and since the diffusion rate of B and N at this temperature is slow, the travel distance that can be diffused is small. Since nucleation of h-BN occurs preferentially over nuclear growth for one factor, it can be seen that very fine h-BN precipitates uniformly throughout the material. Conversely, when tempering near 1150 ° C., nucleation of h-BN occurs preferentially over nucleation, contrary to tempering near 800 ° C., so that precipitation of h-BN grown to a large particle size occurs. It can be seen.

Therefore, the selection of the tempering temperature is important for precipitating h-BN having a particle size and a distribution state that provides good machinability. As a result of the trial experiment, the tempering temperature at which a particle size and a distribution state such that the machinability is good is obtained is preferably in the range of 950 to 1100C.
Moreover, when the temperature at the time of hot working is performed in a state where h-BN is dissolved, it is possible to make a state where B and N are dissolved in supersaturation by quenching after hot working. It is. In the case of such processing temperature conditions, it goes without saying that the heat treatment for h-BN solid solution at a temperature of 1200 ° C. or higher becomes unnecessary.

  Further, regarding the tempering holding time, the higher the temperature, the faster the diffusion rate of B and N, so that a shorter time is required, and the range is 0.5 to 3 hours, preferably 1 to 2 hours. Since this tempering heat treatment can also serve as a solution heat treatment performed on general stainless steel, cooling is performed at a cooling rate performed during the solution heat treatment.

  When the content of B is changed from 0.003 mass% to 0.1 mass%, if the content is 0.003 mass% or less, there is no noticeable effect on machinability, and the content of B exceeds 0.1 mass%. This is because the precipitation of BN increases the tendency of a plurality of h-BN particles to aggregate and the machinability is greatly improved, but adversely affects the surface roughness.

  The reason why N / B is 1 or more in the molar ratio of N is that if it is less than 1, reprecipitation of h-BN during heat treatment of B and N, which are solid solution in supersaturation, cannot be sufficiently achieved. This is because plastic processing becomes difficult because B exists excessively.

(Example 1) A commercially available austenitic stainless steel (SUS304) round bar (weight 2 kg) was melted as a melting raw material using a cold crucible flotation melting furnace. The component composition (mass%) of the melting raw material was 0.06% C, 0.28% Si, 1.33% Mn, 0.035% P, 0.025% S, 8.05% Ni, 18.39. % Cr. At the time of melting, 0.07 MPa of N was sealed in a vacuum induction melting furnace, and the N concentration in the molten steel was adjusted. After melting, a predetermined amount of commercially available ferroboron (19.2 mass% B) is added to the molten metal, the B concentration is adjusted, melted down in a low-pressure N atmosphere, held at 1600 ° C. for 10 min, and solidified in a cold crucible And ingots were produced.
The ingot was forged and rolled at 1200 ° C., processed into a 14 mm square bar, and air-cooled. The rod was held at 1250 ° C. for 0.5 hours, then cooled with water, and further heat treated by holding at 1100 ° C. for 1 hour and then water cooling.

Table 1 shows the analytical values of the newly developed steel.
Further, as the comparative material 1, the commercially available SUS304 stainless steel used for the melting raw material of Example 1 and the commercially available sulfur free-cutting SUS303 stainless steel as the comparative material 2 were both cut out from a round bar having a diameter of 55 mm and surface roughness was obtained. A test sample was obtained.
Table 1 shows analysis values (unit: mass%) of B, N, and S of the melted material.

As a surface roughness evaluation test, the surface roughness (10-point average roughness Rz) of a round bar sample cut out from the sample was measured using a scanning laser microscope. Each sample was turned to a diameter of 7.6 mm under the same turning conditions (cutting speed, cutting depth, tool feed speed). The final cutting conditions were cutting speed: 16 m / min, cutting depth: 0.2 mm, tool feed speed: 0.08 mm / rev, tool material: M30, tool shape: equilateral triangle, chip breaker, cutting fluid: non It was use. The measurement results of the surface roughness are shown in Table 2.

From Table 2, the surface roughness of the developed free-cutting stainless steel material for precision machining is improved compared to that of the comparative material 1 (SUS304), and the surface roughness compared to SUS303 of the free-cutting stainless steel of the comparative material 2 The roughness has been reduced to about 1/3, which is much improved. This is because, in the developed steel, the fine h-BN particles are dispersed in a single pair state, whereas in the comparative material 2, the MnS particles as the free-cutting imparting substance are coarse and extend like needles. This is due to the formation of a metal structure. In FIG. 1, the SEM observation photograph of the broken cross section of the sample cut out from the development steel 2 was shown. FIG. 1 (a) shows a heat treatment in which water is cooled after holding at 1250 ° C. for 0.5 hour, and further, water cooling is carried out after holding at 1100 ° C. for 1 hour, and FIG. 1 (b) is held at 1250 ° C. for 0.5 hour. Then, it is cooled with water, and further subjected to a heat treatment of holding at 850 ° C. for 2 hours and then cooling with water. All the white spherical particles in the figure were found to be h-BN particles by EDS analysis. In the observation surface of FIG. 1 (a), it is recognized that h-BN particles are dispersed in a single state at 3 μm or less, and the h-BN particle diameter is smaller when precipitated at a low reprecipitation temperature. It was observed that there was a tendency to

  FIG. 2 shows a SEM observation photograph of the cross section of the sample of the comparative material 2 (SUS303). It was confirmed by EDS analysis that the arrows indicate the elongated MnS particles having a diameter of several μm and a length of several tens of μm, which are present in the form of fibers in the steel. Table 2 also shows the surface roughness corresponding to the shape of the dropped MnS because the elongated MnS particles appear on the processed surface and fall off during the cutting process.

Of course, the present invention is not limited to the above examples, and it goes without saying that various aspects are possible in detail.

  As explained in detail above, the present invention makes it possible to easily provide a free-cutting stainless steel material for precision machining that has excellent cutting accuracy and machinability, as well as excellent corrosion resistance and environmental resistance. It was possible to bring excellent usability to the various processing fields used.

In the SEM observation photograph of the folded cross section of the developed steel sample, (a) is the heat treatment condition of 1250 ° C., water cooling after holding for 0.5 hour, 1100 ° C., water cooling after holding for 1 hour, (b) is the heat treatment condition, Water cooling after holding at 1250 ° C. for 0.5 hour and water cooling after holding at 850 ° C. for 2 hours. The SEM observation photograph of the broken cross section of the sample of the comparative material 2. FIG.

JP 2002-38238 A JP 2001-234298 A International patent application WO2008 / 016158

Claims (8)

A free-cutting stainless steel material for precision machining that performs cutting with a micrometer-level surface roughness, and spherical h-BN particles having a particle size of 200 nm to 5 μm are dispersed as a free-cutting imparting material in a single state. A stainless steel material for precision machining. The stainless steel material for precision machining according to claim 1, wherein the B content is 0.003 to 0.1 mass%. The stainless steel material for precision machining according to claim 1, wherein the N content is equal to or higher than the B content in molar ratio. The free-cutting stainless steel material for precision machining according to any one of claims 1 to 3, which has a turning surface characteristic in which the 10-point average roughness (Rz) of the surface roughness of the turning surface is 5 μm or less. Stainless steel material for precision machining. 5. The precision machining free-cutting stainless steel material according to claim 4, wherein the turning surface characteristics are obtained by turning a round bar having a diameter of 8 mm under the following conditions. Stainless steel material.
Cutting speed: 16 m / min, cutting depth: 0.2 mm, tool feed speed: 0.08 mm / rev, tool material: M30, tool shape: equilateral triangle, with chip breaker, cutting fluid: not used. The method for producing a free-cutting stainless steel material for precision machining according to any one of claims 1 to 5, wherein B is an addition of ferroboron or metal boron, and N is an inert gas that dissolves the raw stainless steel. And a method of producing a free-cutting stainless steel material for precision machining, characterized by adding nitrogen or reduced-pressure nitrogen. A method for producing a free-cutting stainless steel material for precision machining according to any one of claims 1 to 5, wherein B as a raw material of BN is added to the molten stainless steel by adding ferroboron or metal boron, and N is a nitrogen-containing compound. A process for producing a free-cutting stainless steel material for precision machining, characterized in that B and N are added to molten stainless steel by the addition of. It is a manufacturing method of the free-cutting stainless steel raw material for precision processing in any one of Claims 1-5, Comprising: h-BN particle | grains are heterogeneous in the structure | tissue obtained by the method of Claim 6 or 7. By heating the precipitated stainless steel to a temperature of 1200 ° C. or higher and then rapidly cooling it, the h-BN particles are once dissolved to disappear, and then subjected to a tempering heat treatment at a temperature of 950 to 1100 ° C. A method for producing a free-cutting stainless steel material for precision machining, wherein BN particles are dispersed and precipitated again.