JP3097537B2 - Preheating method of aluminum extrusion die - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a preheating method in which a sound hard coating is maintained on the surface of an aluminum extrusion die for a long period of time.

[0002]

2. Description of the Related Art Steel materials containing alloying elements such as Cr and Mo that form stable nitrides are hardened significantly by nitriding. The nitrided layer formed on the surface of the steel material has a Vickers hardness of 1000 to 1300, is rich in abrasion resistance, has high high-temperature hardness, and has a small decrease in hardness by reheating. For this reason, a hot working steel containing Cr is used for a hot working die used for extruding aluminum or the like, and the surface is hardened by nitriding. The nitriding layer is made of salt bath nitriding,
It is formed on the die surface by gas nitriding or the like. In the salt bath nitriding, a nitrided layer is formed in a short time of 2 to 3 hours, and the cost is low. Salt bath nitrided steel materials include:
A white layer having a thickness of 5 to 10 μm is formed on the surface. The white layer is hard and has excellent wear resistance, but has a disadvantage that it is brittle and easily peels off. In gas nitriding, a steel material is heated to a high temperature in an ammonia stream, and then gradually cooled. Ammonia decomposes in a high temperature range and becomes N and H of highly reactive active groups, and Fe,
A nitride is formed by combining with a metal element such as Cr to form a hard nitrided layer.

[0003]

The hard nitride film formed on the surface of the die is worn or peeled by repeated use of the die for extrusion, exposing the base steel. The dies with exposed base steel provide the desired wear resistance.
Another nitriding treatment is performed. Preheating of the die, which is performed prior to the extrusion, is one of the factors that promote the peeling of the nitride film. That is, since the extrusion temperature reaches a high temperature of 400 ° C. or more in hot extrusion of aluminum, the die temperature also needs to be maintained in advance similarly to this extrusion temperature. Therefore, before the extrusion process, the dies should be extruded at a temperature around the extrusion temperature.
The preheated die is preheated to 00 to 500 ° C., and the preheated die is set in an extruder. The white layer formed on the die surface is easily oxidized and deteriorated in the preheating step of the die heated to 400 ° C. or higher. As a result, the shape of the bearing of the die becomes non-uniform due to peeling or disappearance of the white layer, which adversely affects the quality of the extruded profile.

As for the peeling and disappearance of the white layer, an oxidation-resistant protective film is formed on the nitride layer by an ion beam assisted vapor deposition method as previously proposed by the present applicant in Japanese Patent Application No. 7-18631. Can be suppressed. However, this method not only increases the processing cost, but also has a disadvantage that it is difficult to perform uniform processing on the inner surface of the bearing. Also, as proposed in Japanese Patent Application No. Hei 7-191193, the atmosphere control in the preheating step can suppress the exfoliation and disappearance of the white layer, and can maintain the nitrided layer in a healthy state for a long period of time. However, in this case, heating is restricted to a narrow range of 400 to 460 ° C., and there is a problem in suppressing oxidation at a temperature exceeding this temperature range. However, when a sufficient preheating time cannot be obtained, the heating temperature may be set high to shorten the preheating time. In such a case, heating may exceed 460 ° C., and the suppression of oxidation becomes insufficient. The present inventors have proposed in Japanese Patent Application No. 7-322369 to employ vacuum heating or heating in an Ar atmosphere in order to alleviate the restriction on the temperature condition. By controlling the atmosphere, even when heated to a high temperature, peeling and disappearance of the white layer due to oxidation is suppressed, the nitrided layer is maintained in a healthy state for a long period of time, and the extrusion can be efficiently performed. .

[0005] However, in this case, a vacuum evacuation device is required, and the equipment becomes expensive. Further, since Ar gas, which is more expensive than nitrogen gas, is consumed, the cost becomes higher as a result. Therefore, the present inventors have inexpensive processing,
As a result of studying a method of easily suppressing the oxidation of the nitride layer during preheating, it was found that it is effective to apply a powder such as BN or Al in the form of a spray or paste and apply it to the bearing surface. Number P01059
0). However, in this case, unevenness occurs depending on the coating method, and the white layer may become non-uniform due to oxidation, which causes a problem in quality control. The present invention has been devised to solve such a problem, and BN, A
Combining the application of l and the like with the atmosphere control allows the white layer caused by oxidation even when heated to a high temperature of 450 ° C. or higher without requiring expensive treatment such as vacuum heating or Ar gas. It is an object of the present invention to suppress exfoliation and disappearance of a nitride layer, maintain the nitrided layer in a healthy state for a long period of time almost as in the case of vacuum heating or heating in an Ar atmosphere, and efficiently perform extrusion.

[0006]

According to the present invention, in order to achieve the object, when an extrusion die is preheated before hot extrusion, an antioxidant is applied to a bearing surface in advance, and then the atmosphere is changed from nitrogen to nitrogen. The extrusion die is heated in a replaced heating atmosphere. As the antioxidant, boron nitride or aluminum paste is used alone or in combination, and is applied to the die surface by spraying or coating. Next, it is heated in a temperature range of 400 to 500 ° C. Oxidation is suppressed even at a temperature of 500 ° C. or more, but the strength of the hot tool steel may be deteriorated. When heating,
After evacuation of the inside of the preheating furnace at room temperature, heating may be performed in a vacuum atmosphere, or heating may be performed in that atmosphere while flowing nitrogen or Ar gas into the preheating furnace after evacuation. However, this method increases the processing cost, so that as an atmosphere control method, the use of nitrogen gas is inexpensive and easy to handle.

[0007]

The present inventors have conducted investigations and studies on the nitrided layer formed on the die surface from various viewpoints, and have found that the preheating condition of the die in the preheating step has a significant effect on the nitrided layer. I found it. Then, it was clarified that by controlling the preheating atmosphere, the oxidation of the nitride layer was suppressed, and as a result, a profile having excellent surface quality was obtained.
For example, as shown in the previously proposed Japanese Patent Application No. 7-191191, a die is heated to 400 to 460 ° C. in a nitrogen atmosphere.
By preheating to a temperature close to the extrusion temperature, oxidation and disappearance of the white layer are prevented, and required abrasion resistance is maintained. Preheating in a nitrogen atmosphere is also effective in growing a nitride layer. As a result, the surface hardened layer having the required hardness is maintained in a healthy state for a long period of time, the number of times that the hardened layer can be repeatedly used for extrusion is increased, the productivity is improved, and the frequency of renitriding is reduced. However, it has been found that in the heating in a nitrogen atmosphere, a preheating temperature up to 460 ° C. is effective in preventing the oxidation and disappearance of the white layer, and that if the preheating temperature exceeds 460 ° C., the oxidation of the nitride layer is not suppressed. Was. Therefore, the atmosphere during preheating was changed to Ar, vacuum, etc., and as a result of investigating the effects of the atmosphere and the preheating temperature on the oxidation of the nitride layer,
Japanese Patent Application No. Hei 7- (reference number P010542, November 1995)
Pre-heating temperature is 460 to 5
It was found that the oxidation of the nitride layer was suppressed in the range of 50 ° C. However, vacuum evacuation is not easy to handle in terms of equipment and increases the cost. In addition, the cost is higher because Ar gas, which is more expensive than nitrogen, is consumed.

Therefore, in the present invention, BN, AlN
By coating in a nitrogen atmosphere, the same action as vacuum heating or Ar gas flow is exerted even at a preheating temperature of 460 ° C. or higher by combining the coating of the above and atmosphere control. The Al or BN powder is sprayed on the bearing surface that has been nitrided. Alternatively, a paste is applied two or three times with a brush or the like to the bearing surface that has been subjected to the nitriding treatment so as not to cause unevenness. A commercially available paint paste is used as Al, and a lubricating release material is used as BN. The preheating atmosphere is controlled, for example, by the following method proposed in Japanese Patent Application No. 7-191191. That is, the preheating atmosphere is controlled by a method of flowing nitrogen into the furnace. Specifically, with respect to 10-liter reactor, first for substitution with nitrogen in the furnace from the atmosphere during heating, the outflow of N ₂ 1 to 2 minutes at 40 to 60 l / min flow rate, and then in warm is drop the flow rate ratio from 8 to 12 l / min N ₂
Was supplied, and N ₂ was released from an exhaust port provided at another place in the furnace to flow nitrogen. Thus, the inside of the furnace was always maintained in the N ₂ atmosphere during the preheating. N ₂ from the atmosphere
Although a vacuum pumping device can be used for the replacement, the cost of the equipment and the capacity of the furnace are limited. Also,
Since there is a problem that it takes a long time to exhaust gas, the method using the N ₂ flow is simple and advantageous in cost. By controlling the preheating atmosphere and the preheating temperature in this way, the oxidation of the nitride layer is suppressed in the die preheating step. Therefore, no alteration, peeling, or the like due to oxidation occurs in the nitride layer on the bearing surface, and the necessary wear resistance is maintained. Therefore, the die life is extended and the shape quality of the obtained profile is improved.

[0009]

【Example】

Example 1 Prior to extrusion, the dies are heated to a temperature near the extrusion temperature, usually in an electric furnace. The effect of the atmosphere and temperature of the heating furnace was investigated as preheating conditions at this time. Also, in order to evaluate the oxidation resistance of the nitrided layer by heating assuming the die preheating, a 20 m
m, a test piece of length 40mm and thickness 15 mm H _RC 48
And then subjected to salt bath nitriding at 570 ° C. for 2.5 hours. An Al paste and BN were applied two to three times to the obtained nitrided surface. A commercially available paint for Al paste and a lubricating release material for BN were used. The atmosphere used was a nitrogen atmosphere instead of the conventional air atmosphere. That is, in order to replace the inside of the furnace with nitrogen from the atmosphere during heating, N ₂ was supplied at a flow rate of 50 L / min, and after confirming that the inside of the furnace was sufficiently replaced with nitrogen, the temperature was raised. During heating, N ₂ at a flow rate of 10 l / min
, And N ₂ was released from an exhaust port provided at another place in the furnace. Thereby, the inside of the furnace was always maintained in the N ₂ atmosphere. The preheating temperature was set in the range of 450 to 490C. The specimen was held at each preheating temperature in this range for 5 hours, and the nitride layer on the surface of the test piece was examined by X-ray diffraction, and the microstructure and hardness distribution of the cross section of the test piece were measured.

FIG. 1 and FIG. 2 show the results of measuring the structural change of the nitride layer when heated in a nitrogen atmosphere after Al paste and BN spraying, respectively. Looking at the diffraction pattern of the nitrided layer that has not been heated and has been subjected to the nitriding treatment, a carbonitride of Fe and a slight oxide peak are detected. After applying Al paste to this nitrided layer,
The sample heated at 450 ° C. in a nitrogen atmosphere showed almost the same diffraction pattern as in the case of no heating, and almost no decrease in the nitride peak or increase in the oxide peak was detected.
In the case of heating at 490 ° C., almost the same tendency was shown, though there was some peak shift. This result shows that the oxidation of the nitride layer at 490 ° C. is effectively suppressed by applying the Al paste, as compared with the heating in a nitrogen atmosphere proposed by the present inventors in Japanese Patent Application No. 7-191193. It is shown that it is. Therefore, the oxidation of the nitrided layer is suppressed even at a high temperature of 460 ° C. or more by using not only the heating atmosphere of nitrogen but also the application of the Al paste, and the surface state is almost the same as that immediately after the preheating even after the preheating. You can see that it is maintained.

When heating in a nitrogen atmosphere after the application of BN, the same tendency is exhibited as shown in FIG. 2, and oxidation of the nitride layer is suppressed even by heating at a high temperature of 460 ° C. or higher. Al,
In order to confirm the effects of the application of BN and the control of the atmosphere, the cross-sectional microstructure was observed. As shown in FIG.
Even if a heat treatment at 490 ° C. for 5 hours was performed in a nitrogen atmosphere after Al paste application and BN spray application, the white layer of the nitrided layer was maintained in a healthy state immediately after the nitriding treatment. Further, the nitrided layer after heating was much more uniform than when heated in air. From these results, it was confirmed that the combined use of the application of the antioxidant and the control of the heating atmosphere suppressed the oxidation of the nitride layer when the die was preheated at a temperature of 500 ° C. or less. Further, the hardness distribution of the nitride layer preheated according to the present invention was measured. FIG.
As can be seen from the measurement results, when the coating was heated to 450 ° C. after the application of the Al paste, the hardness distribution was almost the same as that of non-heating. On the other hand, when heated to 490 ° C., the hardness increases over a deep range to the inside. In this regard, it can be said that the higher the preheating temperature of the die, the more remarkable the effect of the present invention. Also, A
When l and BN are combined and applied to the nitride layer, oxidation of the nitride layer is suppressed even at a higher temperature. For the composite coating, a method of applying an aluminum paste after spraying boron nitride, a method of spraying boron nitride after applying the aluminum paste, a method of simultaneously applying a mixture of boron nitride and aluminum, and the like can be employed. .

Example 2: A die which had been subjected to salt bath nitriding at 570 ° C. for 2.5 hours was coated with an Al paste or sprayed with BN according to the present invention and preheated in a furnace whose atmosphere was adjusted from nitrogen to nitrogen. Used for hot extrusion of the alloy. In addition, as a die, the bearing length is 5mm
And a slit size of 40 mm × 3 mm was used. The preheating was performed by holding the dice at 450 ° C. and 490 ° C. for 5 hours in a nitrogen atmosphere. As a comparative example, Al or BN
Dies, Al,
A die preheated in a nitrogen atmosphere without the application of BN was used. Using each die, an aluminum alloy A6063 having a diameter of 97 mm and a length of 150 mm was hot-extruded into a shape having a width of 40 mm and a thickness of 3 mm under the conditions of an extrusion temperature of 480 ° C and an extrusion speed of 40 m / min.

[0013] The surface roughness of the obtained extruded profile is measured,
The change of surface roughness due to the difference of die preheating condition was investigated. As a result, the extruded material is [Al, BN coating + preheating in nitrogen atmosphere] <[preheating in nitrogen atmosphere without Al and BN coating] <[Al, BN coating + preheating in air atmosphere] The surface tended to become rough in the order of. this is,
When heating to 460 ° C. or more in a nitrogen atmosphere after applying the Al paste or spraying BN, the oxidation of the white layer is suppressed, and the nitriding layer maintains the same level of adhesion as the nitriding treatment. It is considered that the abrasion resistance in the inside was also good, and as a result, the surface roughness of the extruded profile showed a substantially constant value. Thus, the preheating according to the present invention is comparable to the case of heating in a vacuum atmosphere or an Ar atmosphere, and exhibits a sufficient oxidation suppressing effect at a preheating temperature of 460 ° C. or higher.

[0014]

As described above, in the present invention, Al and BN are applied to a hard nitrided layer formed by nitriding, and then heated to a preheating temperature in a nitrogen atmosphere to thereby reduce the temperature during preheating. Prevents the nitride layer from deteriorating due to oxidation, etc., and maintains the die steel surface in a healthy hard state for a long period of time. By the combination of the application of Al and BN and the heating in a nitrogen atmosphere, the white layer of the nitride layer is maintained in the initial sound state even when the preheating temperature is set to be higher than 500 ° C., and the preheating time is shortened. Therefore, the life of the die is prolonged and the production efficiency is improved.

[Brief description of the drawings]

FIG. 1 X-ray diffraction pattern showing the effect of the Al coating applied to the nitride layer on the suppression of alteration of the white layer

Fig. 2 X-ray diffraction pattern showing the effect of the BN coating applied to the nitride layer on the suppression of alteration of the white layer

FIG. 3 shows Al and B for suppressing oxidation of a nitride layer during preheating.
Photograph showing the effect of N application and nitrogen atmosphere on the surface layer metallographic structure

FIG. 4 is a graph showing a hardness distribution when a nitride layer coated with an Al paste is heated in a nitrogen atmosphere.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) B21C 29/04

Claims (3)

(57) [Claims] When the extrusion die is preheated prior to the hot extrusion, an antioxidant is applied to the bearing surface in advance, and the extrusion die is heated in a heating atmosphere in which the atmosphere is replaced with nitrogen. Preheating method for aluminum extrusion dies. 2. A method for preheating an aluminum extrusion die, wherein boron nitride or aluminum paste is used alone or in combination as an antioxidant according to claim 1, and is applied to the die surface by spraying or coating. 3. The method according to claim 1, wherein the heating is performed at 400 to 500 ° C.
Preheating method for aluminum extrusion dies performed in the above temperature range.