WO2012014541A1 - Free-cutting stainless-steel cast product and process for producing same - Google Patents

Abstract

The present invention addresses the problem of providing a free-cutting stainless-steel cast product which can satisfy all of excellent cuttability, environmental preservation, and corrosion resistance and a process for producing the cast product. The stainless-steel cast product contains an agent for imparting free-cutting properties, and has a configuration characterized in that the agent for imparting free-cutting properties is h-BN particles and has been evenly and dispersedly precipitated in the steel. The process has a configuration characterized in that a stainless-steel cast product containing h-BN particles precipitated therein is heated and then quenched to cause the h-BN particles to disappear through formation of a solid solution and the cast product is thereafter tempered to thereby dispersedly reprecipitate h-BN particles.

Description

Free-cutting stainless steel cast steel product and manufacturing method thereof

The present invention relates to a cast stainless steel product to which a free machinability imparting material is added and a method for producing the same.

Stainless steel cast steel products where corrosion resistance is important, for example, pipe joints (elbows, sockets, nipples, etc.) used in equipment piping, vacuum equipment, analytical equipment, etc., valves, joint flanges, etc. are made by casting. Although it is manufactured, plastic products such as forging and rolling are not performed on products with complicated shapes, and cutting (mechanical) processing such as planar processing, drilling processing, and threading processing is eventually performed with the cast structure. Necessary. Since these products are required to have excellent corrosion resistance, austenitic stainless steels such as SUS304 and SUS316 are often used. However, these austenitic stainless steels are known as hard-to-cut steels whose machinability deteriorates due to work hardening in the vicinity of the cut surface because work hardening easily proceeds.
Therefore, there is a conventional free-cutting stainless steel to which S (sulfur), Pb (lead), Se (selenium), etc. are added as a free-cutting property imparting material for the purpose of improving machinability. From the viewpoint of safety and security, it is unsuitable as a joint for equipment for producing food and beverages. Se is known as an environmentally hazardous substance and has been withheld from use. In addition, SUS303, which is commercially available as sulfur-added free-cutting stainless steel, contains about 0.3% of S, which is known as an element that adversely affects corrosion resistance even in a small amount, and is a part of equipment where corrosion resistance is important. It is unsuitable for use.

Sulfur-added free-cutting stainless steel can be used only in an atmosphere that is weakly corrosive or only for the manufacture of products that do not require corrosion resistance. For these reasons, no free-cutting stainless steel product that satisfies both excellent machinability and corrosion resistance at the same time has been obtained.
Patent Document 1 discloses a free-cutting stainless steel that can simultaneously satisfy excellent machinability, corrosion resistance, and mechanical properties, and a manufacturing method thereof. In this invention, in order to satisfy all of machinability, corrosion resistance, and mechanical properties, h-BN (hexagonal boron nitride) particles are formed after erasing the cast structure by performing forging and rolling plastic working on the ingot after casting. As a result of reprecipitation, the machinability was improved by about 25% compared to the conventional stainless steel material.
However, as disclosed in Patent Document 1, although stainless steel having improved machinability by heat treatment after forging and rolling has been developed, a stainless cast steel product having improved machinability with a cast structure and its manufacture The method is not disclosed.

Therefore, the present invention is a cast steel product that does not require much importance on mechanical properties and requires a complex shape, and is capable of satisfying not only excellent machinability and environmental performance but also corrosion resistance at the same time. It is an object of the present invention to provide a machined stainless steel cast steel product and a manufacturing method thereof.

The present invention effectively uses the properties of h-BN (hexagonal boron nitride) particles that are excellent as a solid lubricant, chemically stable and not attacked by acid or alkali, and can be specified even in the state of a cast structure. The present inventors have found a stainless steel cast steel product excellent in machinability and corrosion resistance and a method for producing the same by utilizing the fact that h-BN precipitates or dissolves / reprecipitates by heat treatment.

Invention 1 is a cast stainless steel product to which a free machinability imparting material is added, wherein the free machinability imparting material is h-BN (hexagonal boron nitride) particles, and the particle size is 200 nm to 10 μm. The spherical h-BN particles were uniformly dispersed and precipitated in the steel.

Invention 2 is a method for producing a cast stainless steel product of Invention 1, characterized by controlling the cooling rate in the temperature range of 1250 ° C. to 850 ° C. during solidification of casting and dispersing and precipitating h-BN particles. The configuration to adopt was adopted.

Invention 3 is a method for producing a cast stainless steel product of Invention 1, wherein the cast stainless steel on which h-BN particles are precipitated is heated and then rapidly cooled to cause the h-BN particles to dissolve and disappear. A configuration characterized by re-dispersing and precipitating the h-BN particles by returning is employed.

Invention 4 is a method for producing a cast stainless steel product of Invention 1, wherein a temperature range of 1250 ° C. to 850 ° C. during casting solidification is rapidly cooled to obtain a cast structure in which no h-BN particles are precipitated, and then 950 A configuration characterized in that h-BN particles are dispersed and precipitated by performing a tempering heat treatment at a temperature of ˜1100 ° C. is adopted.

Invention 5 is a method for producing cast stainless steel products of Inventions 2 to 4, wherein the addition amount of B (boron) is 0.003 to 0.5 mass%, and the addition amount of N (nitrogen) A configuration characterized in that the ratio N / B is 1 or more was adopted.

Invention 6 is a method for producing a cast stainless steel product of Invention 5, wherein B is added to ferroboron or metal boron, and N is added to the molten stainless steel by setting the melting atmosphere to (argon + nitrogen) or reduced pressure nitrogen. The structure characterized by doing is adopted.

Invention 7 is a method for producing a cast stainless steel product of Invention 5, characterized in that B is added to the molten stainless steel by addition of ferroboron or metal boron, and N is added by a nitrogen-containing compound or ferroalloy. The configuration to adopt was adopted.

H-BN particles that are chemically stable and not affected by acids or alkalis are uniformly dispersed and precipitated, and h-BN, which has excellent characteristics as a solid lubricant, improves machinability and deteriorates corrosion resistance. We could provide no stainless steel cast steel product and its manufacturing method.
The invention 1 can satisfy not only excellent machinability and environmental performance but also corrosion resistance.

These effects are due to the effective application of h-BN particles, which have excellent properties as solid lubricants, to cast stainless steel, using not only machinability but also environmentally hazardous substances such as Pb and Se. Therefore, it was possible to realize a cast stainless steel product that satisfies the environmental characteristics and does not cause deterioration of the corrosion resistance.

In addition, the improvement of machinability can reduce the power of the cutting machine, the carbon dioxide emission can be reduced by reducing the electric energy consumption, and the productivity can be expected to be improved because high-speed cutting is possible.

A photograph of the formation and distribution of precipitates by SEM observation of the developed material 1. A photograph of the formation and distribution of precipitates by SEM observation of the developed material 3. The photograph of the production | generation and distribution state of the precipitate by the fracture surface SEM observation of the comparative material 2. FIG. The graph which showed the relationship between the cutting speed and cutting force resultant force in the turning of the development material 4 and the comparison materials 1 and 2. FIG. The graph which showed the relationship between the cutting speed and cutting force resultant force in the turning of the development materials 1 to 3 and the comparative material 3. The graph which showed the result of the corrosion test by the sulfuric acid corrosion test method (JIS G 0591) of the development material 4 and the comparison materials 1 and 2.

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 melting of the cast stainless steel is performed using a melting furnace for melting ordinary stainless steel, in which the melting atmosphere can be adjusted. In this melting, ferroboron or metal boron is used as the raw material for B. However, ferroboron having a low melting point is technically advantageous as the melting raw material, and economical because the price per unit weight of B is low. It is.

The amount of B added is such that the final B content in the cast stainless steel product is generally 0.003 to 0.5 mass%, more preferably 0.01 to 0.2 mass%.

As a method of adding N, there is an addition of a nitride of an alloy element that absorbs N in a melting atmosphere or constitutes stainless steel, such as chromium nitride or ferrochrome nitride.

The N content in the stainless steel cast steel product should be N / B of 1 or more in terms of a molar ratio as a general guide. If the molar ratio of N to B in the stainless steel cast steel is less than 1, the amount of dissolved B increases and the precipitation amount of h-BN effective for machinability decreases, so it is necessary to increase N / B as much as possible. There is. Although the N content depends on the constituent elements of the cast stainless steel, since B increases the activity of N, the equilibrium N concentration in the steel decreases as B increases. In the component composition of SUS304, the N content 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 obtained by controlling the cooling rate in the precipitation temperature region of h-BN from 1250 ° C. to 850 ° C. during the solidification process in the mold. It is possible to deposit and distribute h-BN spherical particles having a particle size of 200 nm to 10 μm.

In a cast stainless steel product, h-BN, which has grown coarsely to about 20 to 30 μm, may be unevenly distributed in a part of the material depending on the cooling rate during the cooling process after casting. To avoid this,
(1) Control the cooling rate after casting.
(2) Use a mold mold.
(3) Consider the mold design such as pouring position and shape of the feeder.
And so on.

H-BN precipitated in stainless steel cast steel products decomposes into B and N at a temperature of 1200 ° C or higher for a relatively short time (for example, 0.5 to 1 hour at 1250 ° C). Solid solution in the cast steel matrix. In addition, since such a process is impossible when a cast stainless steel product is melted, it is necessary to process at a temperature lower than the melting temperature.
By rapidly cooling from this state, a cast stainless steel product containing B and N in a solid solution state in a supersaturated state is obtained. The quenching operation may be water cooling performed on ordinary stainless steel, but the cooling rate in the temperature range in which h-BN is deposited, which will be described later, needs to be a cooling rate that does not cause precipitation.

When tempering B and N dissolved in supersaturation at a temperature of 800 ° C. to 1150 ° C., h-BN dissolved in the solution heat treatment again precipitates. 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 because the diffusion rate of B and N at this temperature is slow, the migration distance that can be diffused is small. Due to these two factors, nucleation of h-BN occurs preferentially over growth, and it can be seen that very fine h-BN precipitates uniformly throughout the material. Conversely, when tempering near 1150 ° C, h-BN grows preferentially over nucleation, contrary to tempering near 800 ° C, so that coarsely grown h-BN precipitates. It is done.

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 grain size and distribution state that provides good machinability are obtained is preferably in the range of 950 to 1100 ° C.

In addition, if the casting conditions are such that cooling to this tempering temperature can be performed at once after casting, h-BN does not precipitate, and B and N are in a solid solution state, 1200 ° C. or higher. It goes without saying that the heat treatment for h-BN solid solution at the above temperature becomes unnecessary.

Furthermore, regarding the tempering holding time, the higher the temperature, the faster the diffusion rate of B and N, so that it takes only a short time. The range is 0.5 to 3 hours, preferably 1 to 2 hours. Quench quickly to stop h-BN growth.

Example 1
A commercially available austenitic stainless steel (SUS304) round bar (weight 18 kg) was melted as a melting raw material using a vacuum induction melting furnace. The component composition (mass%) of the melting raw material is 0.04% C, 0.30% Si, 1.00% Mn, 0.030% P, 0.024% S, 8.09% Ni, 18.05. % 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, held in a weakly reduced N atmosphere for 20 minutes, heated at 1600 ° C., poured into a cast iron mold, An ingot was manufactured. In the developed materials 1 to 3, the B content target values are 0.02 mass%, 0.05 mass%, and 0.1 mass%, respectively, and the N content target value is 0.2 mass%.

Comparative material 1 was solidified under the same melting and casting conditions except that the melting raw material was melted in an Ar atmosphere (cast steel corresponding to SUS304 cast material and not containing h-BN), comparative material 2 Comparative material 1 containing 0.3 mass% of S (corresponding to SUS303 cast material, sulfur-added free-cutting stainless steel cast steel), as comparative material 3, N is developed in comparative materials 1 to 3 Similarly, 0.2 mass% contained was melted.

Further, in order to make the influence of N on the machinability and corrosion resistance the same, the N partial pressure of the melting atmosphere was adjusted to 0.005 MPa, and the developed material 4 having the same N level as the comparative materials 1 and 2 was melted. The same material as the developed materials 1 to 3 was used as the melting raw material, and the target value of the B content was set to 0.05 mass% as in the developed material 2. Table 1 shows analytical values (unit: mass%) of B, N, and S after casting of the melted materials (developed materials 1 to 4, comparative materials 1 to 3).

The ingot that had been cast and solidified was subjected to an h-BN solution heat treatment that was held in a resistance heating type electric furnace at a temperature of 1250 ° C. for 0.5 hours in an air atmosphere and then cooled with water outside the furnace. Using a resistance heating type electric furnace, the water-cooled ingot was kept at 1050 ° C. for 1 hour in an air atmosphere, and then h-BN precipitation heat treatment was carried out by water cooling outside the furnace. This heat treatment was performed under the same conditions for both the developed material and the comparative material in order to make the conditions of the cast structure the same.

1 is a developed electron 1, FIG. 2 is a developed material 3, and FIG. 3 is an SEM (scanning electron microscope) photograph showing the formation and distribution of precipitates in the comparative material 2. FIG. A round bar having a diameter of 3.6 mm was cut out from the sample after heat treatment, a notch was made in a part thereof, this part was bent and broken, and the fracture surface was observed with an SEM. This SEM is equipped with an EDS (energy dispersive X-ray analysis) device. Using this EDS device, elemental analysis of non-metallic inclusions that can be observed on the fracture surface was performed, and the inclusion species were identified. .

In FIG. 1, it was observed that several 200-μm or smaller h-BN particles (indicated by solid arrows) were dispersed on the entire fracture surface in the 200 × field of view at the bottom of the dimple. In addition, several spherical 10 μm MnS (indicated by broken line arrows) produced by a small amount of sulfur and manganese contained in the raw material for dissolution were also observed.

In FIG. 2, it was observed that h-BN particles of 10 μm or less were dispersed throughout the fracture surface in the 200 × field of view as in FIG. In FIG. 2, the number of h-BN particles is approximately 10 times that of the developed material 1 in proportion to the B concentration. From the shape of the h-BN particles observed in FIGS. 1 and 2, it can be considered that B and N dissolved in supersaturation in the molten steel are precipitated as h-BN in the solidification process.

In the case of the comparative material 2 in FIG. 3, a large number of spherical MnS of about 10 to 30 μm were observed at the bottom of the fracture surface dimple over the entire 200 × field of view. These MnS particles are also observed in FIG. 1 and are considered to be crystallized when the steel is in a molten state from the particle size and shape.

As a machinability evaluation test, FIG. 4 shows a result of a turning test performed on a round bar sample cut from a cast steel sample after heat treatment of the comparative materials 1 and 2 and the developed material 4. As shown in Table 1, these three materials have almost the same N content as about 0.06 mass%. FIG. 4 shows that h-BN particles or MnS particles dispersed in the materials have an effect on the machinability. Shows the impact. The cutting test conditions were a cutting depth of 1.0 mm, a tool feed of 0.1 mm / rev, a tool material M30 (without chip breaker), no cutting oil, and a turning speed in the range of 12 to 200 m / min. It is a measured value.
Compared to Comparative Material 1 (corresponding to SUS304 cast material), the developed material 4 has a cutting force reduction of 20% in the medium cutting speed region and 11% in the high speed cutting region, and the machinability is greatly improved.
The comparative material 2 is a sulfur-added free-cutting stainless steel cast steel. Compared with this, the developed material is inferior in machinability, but the comparative material 2 cannot be used when corrosion resistance is important. Only the developed material can satisfy both good machinability and corrosion resistance, and the superiority of the developed material cannot be denied.

FIG. 5 shows the relationship between the cutting speed and the resultant force of cutting resistance in the turning of the samples of the developed materials 1 to 3 and the comparative material 3 (without B addition) with different B addition amounts. As shown in Table 1, these four materials have almost the same N content as about 0.2 mass%, and FIG. 5 shows the effect of B addition on machinability. In the developed material in which B was added to the comparative material 3 having a B concentration of 0 mass%, a decrease in cutting resistance was observed in most cutting speed regions, and an improvement in machinability due to the addition of B was recognized.

As an evaluation test of the corrosion resistance of the stainless steel, the results of a corrosion resistance test performed on the sample after the heat treatment of the cast material are shown in FIG. Since N contributes to the corrosion resistance of stainless steel, a comparison was made for materials with the same N concentration level. FIG. 6 shows the result of the corrosion test by the sulfuric acid corrosion test method (JIS G 0591). The test conditions were immersed in boiling 5% H ₂ SO ₄ for 6 hours continuously, and compared with the amount of corrosion obtained by dividing the weight loss by the initial surface area of the sample.
When the corrosion resistance with and without h-BN was compared, the difference between the developed material 4 and the comparative material 1 was hardly observed, and no deterioration of the corrosion resistance was observed. On the other hand, in the comparative material 2, the amount of corrosion is remarkably increased as compared with the developed material 4 and the comparative material 1, indicating that the comparative material 2 cannot be used when corrosion resistance is required.

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 described above in detail, this invention makes it possible to easily provide a stainless steel cast steel product that is environmentally friendly and has improved machinability without deteriorating corrosion resistance, and is excellent in various processing fields using stainless steel cast steel products. Was able to bring about usability.

International Publication No. 2008/016158

Claims (7)

1. A stainless steel cast steel product to which a free-cutting property imparting material is added, wherein the free-cutting property imparting material is h-BN (hexagonal boron nitride) particles and has a spherical shape with a particle size of 200 nm to 10 μm. A cast stainless steel product characterized in that h-BN particles are uniformly dispersed and precipitated in the steel.
2. 2. The method for producing a cast stainless steel product according to claim 1, wherein the cooling rate is controlled in a temperature range of 1250 ° C. to 850 ° C. during casting solidification, and h-BN particles are dispersed and precipitated. Manufacturing method for stainless steel cast steel products.
3. The method for producing a cast stainless steel product according to claim 1, wherein the stainless steel in which h-BN particles are non-uniformly precipitated in the cast structure is heated to a temperature of 1200 ° C or higher and then rapidly cooled. A method for producing a cast stainless steel product, characterized in that h-BN particles are dispersed and precipitated again by eliminating solid solution of BN particles and then performing a tempering heat treatment at a temperature of 950 to 1100 ° C.
4. A method for producing a cast stainless steel product according to claim 1, wherein a temperature range from 1250 ° C. to 850 ° C. at the time of casting solidification is rapidly cooled to obtain a cast structure in which h-BN particles are not precipitated, A method for producing a cast stainless steel product, characterized in that h-BN particles are dispersed and precipitated by performing a tempering heat treatment at a temperature of 1100 ° C.
5. 5. The method for producing a cast stainless steel product according to claim 2, wherein the addition amount of B (boron) is 0.003 to 0.5 mass%, and the addition amount of N (nitrogen) is a molar ratio. A method for producing a cast stainless steel product, wherein N / B is 1 or more.
6. 6. The method for producing a cast stainless steel product according to claim 5, wherein B is added to the molten stainless steel by adding ferroboron or metal boron, and N is a molten atmosphere (argon + nitrogen) or reduced pressure nitrogen. A method for producing a cast stainless steel product characterized by the above.
7. 6. The method for producing a cast stainless steel product according to claim 5, wherein B is added to the molten stainless steel by addition of ferroboron or metal boron, and N is added by a nitrogen-containing compound or ferroalloy. Manufacturing method for stainless steel cast steel products.

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