JPH1072657A - Method for modifying surface of die for aluminum extrusion and die excellent in durability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To essentially eliminate renitriding treatment, to maintain a nitriding layer to a sound state over a long period and to produce an alminum extruded shape material good in surface properties with high efficiency. SOLUTION: When reusing a die for aluminum extrusion preliminarily subjected to nitriding treatment for hot extrusion, prior to the hot extrusion, it is preliminarily heated in a nitrogen atmosphere with 20 to 500ppm oxygen concn. and is renitrided, and the die after the preliminary heating is set to an extruding machine as it is. In the preliminary heating and nitriding, the die for aluminum extrusion is held at 450 to 500 deg.C for 3 to 8hr in a nitrogen atmosphere. In the case the oxygen concn. of the nitrogen atmosphere is regulated to 20 to 500ppm, the die for aluminum extrusion is preliminarily heated and whole the nitriding layer is grown without generating the oxidation of a white layer. Since the nitriding layer grows in the priliminary stage, there is no need of renitriding, the service life of the die prolongs, and the extruded shape material good in surface properties can be produced over a long period.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a preheating method and an extrusion die in which a sound hard film 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 strongly reactive generating 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 nitrided layer formed on the surface of the die is worn or peeled off by repeated use of the die for extrusion, exposing the base steel. The die with the exposed base steel is subjected to another nitriding treatment in order to impart initial wear resistance. However, the dice become thinner and thinner as a result of the repetition of the nitriding treatment, and are eventually discarded. For this reason, a die having a longer life is obtained as the number of times of renitridation is smaller. Preheating of the die, which is performed prior to the extrusion process, is one of the factors that promote the peeling of the nitrided layer. 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. In the preheating process,
Prior to extrusion, the dies were extruded at 400
The preheated 500 ° C. die is set in the extruder.

[0004] The white layer formed on the die surface is 40
The die is easily oxidized and deteriorated in the preheating step of the die heated to 0 ° C. or higher. Oxidation and alteration of the white layer becomes more remarkable as the preheating temperature increases, but high-temperature heating is more preferable for rapid preheating. 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. The peeling / disappearance of the white layer is suppressed by forming an oxidation-resistant protective film 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. it can. 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.

Further, as described in Japanese Patent Publication No. 7-112568, the white layer can be prevented from peeling and disappearing by preheating the die in a non-oxidizing atmosphere. However, depending on atmospheric conditions, the suppression of oxidation becomes insufficient, and peeling and disappearance of the white layer is inevitable, and the life of the dice cannot be effectively extended. The present invention has been devised in order to solve such a problem, and by preheating a die immediately before extrusion in an atmosphere having a nitriding ability and performing a nitriding treatment, the nitrided layer can be sounded for a long period of time. It is intended to maintain the state and to perform the extrusion efficiently.

[0006]

According to the surface modification method of the present invention, in order to achieve the object, when a pre-nitrided aluminum extrusion die is reused for hot extrusion, it is necessary to use an oxygen extrusion die prior to hot extrusion. It is characterized in that it is preheated and renitrided in a nitrogen atmosphere having a concentration of 20 to 500 ppm, and the preheated die is directly set in an extruder. In preheating and nitriding,
The aluminum extrusion die is held at 450 to 500 ° C. for 3 to 8 hours in a nitrogen atmosphere. Oxygen concentration in nitrogen atmosphere is 20
When the concentration is regulated to 500 ppm, the aluminum extrusion die can be heated to a predetermined temperature while growing the nitride layer without causing oxidation of the white layer. The aluminum extrusion die pre-heat-treated according to the present invention has Fe ₃ O ₄ /
The X-ray intensity ratio of Fe ₃ N is adjusted to 20% or less immediately before extrusion. As a result, an extruded aluminum material having a good surface condition can be obtained without causing defects due to peeling and falling off of the white layer, and the life of the die is prolonged.

[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 heating conditions of the die in the preheating step have a significant effect on the nitrided layer. I found it. Then, by using nitrogen gas for the preheating atmosphere and regulating the oxygen concentration of the atmosphere, the oxidation of the nitride layer is suppressed, and the nitride layer is grown.
As a result, it was clarified that a profile with excellent surface quality could be obtained. That is, the die is extruded in a nitrogen atmosphere in which the oxygen concentration is regulated to 20 to 500 ppm, and the temperature of the die is set at 450 ° C near the temperature.
Preheated to ~ 550 ° C. The regulated oxygen concentration is 4
It enables high-temperature preheating of 50 ° C. or higher, and promotes diffusion of nitrogen into the die steel while preventing oxidation and disappearance of the white layer. For this reason, the preheat-treated dies have the same hardness distribution as in the case of renitriding, and the hardened surface layer having the required hardness is maintained in a healthy state for a long period of time. As a result, pre-heat treatment and re-nitriding can be performed simultaneously,
Productivity is greatly improved.

A die for extruding aluminum according to the present invention uses a hot tool steel such as SKD61 or SKD62 as a die base material and removes oxides generated by quenching and tempering to remove the surface of the hot tool steel. Is formed with a nitride layer. Iron oxide is formed on the surface of the hot work tool steel after quenching and tempering. This oxide is Fe ₃ O ₄ ,
Is a Fe ₂ O _3, etc., is affected by the oxygen concentration in the atmosphere during the chemical treatment and nitriding treatment to be performed as a subsequent step, to increase. In the process of preheating the nitrided die to about 500 ° C. at the time of extruding aluminum, the oxide on the surface tends to further increase. Iron oxide has low adhesion to a substrate and is more easily worn than iron nitride Fe ₃ N. As a result, the aluminum is exfoliated or abraded during the extrusion of aluminum, causing defects on the surface of the extruded aluminum material. In order to suppress such defects, it is important to control iron oxide. In this connection, the present inventors have considered that the Fe ₃ O ₄
It has been found that it is effective to control the X-ray intensity ratio of / Fe ₃ N to 20% or less immediately before extrusion.
No. 85618.

When the X-ray intensity ratio of Fe ₃ O ₄ / Fe ₃ N is 20
% To maintain the oxygen concentration at 20 to 500 p.
It is effective to set a die preheated in an extruder as it is in a nitrogen atmosphere regulated to pm. When the die is preheated in an atmosphere having an oxygen concentration of 20 to 500 ppm, the surface layer of the die is maintained at an X-ray intensity ratio of Fe ₃ O ₄ / Fe ₃ N of 20% or less, and the peeling and disappearance resistance of the white layer is improved. . As a result, the life of the die is prolonged, and the productivity of the extrusion process is improved. In the preheating according to the present invention, a method of flowing nitrogen into the furnace is employed. 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 the supplies N ₂ dropped the flow rate ratio from 8 to 12 liters / min, the N ₂ was discharged from the exhaust port provided elsewhere in the furnace, thereby a flow of nitrogen. Thereby, the inside of the furnace is always maintained in the N ₂ atmosphere during the preheating. It can also be used evacuation device to substitution of the N ₂ from the atmosphere, on costly in equipment, the volume of the furnace is limited. Further, since there is a problem that it takes a long time for evacuation, the method using the N ₂ flow is simple and advantageous in cost.

The preheating temperature is set at 450 to speed up preheating.
~ 500 ° C. The high-temperature preheating promotes the diffusion of nitrogen and is also effective for growing a nitrided layer. At this time, since the oxygen concentration in the atmosphere is regulated to 500 ppm or less, the white layer does not peel or disappear. Oxygen concentration 500p
If the atmosphere is not adjusted to not more than pm, oxidation of the nitride layer is rapidly promoted, and there is a possibility that shape quality is adversely affected.
On the other hand, if the preheating temperature does not reach 450 ° C., the oxidation is suppressed, but the time required for the preheating becomes long, and the extrusion temperature may be too low to perform good extrusion. However, in a nitrogen atmosphere having an oxygen concentration of less than 20 ppm, the generated Fe ₃ N crystal does not grow at random and the growth direction is constant, so that the wear resistance performance is deteriorated. By controlling the preheating atmosphere and the preheating temperature in this manner, not only oxidation of the nitride layer is suppressed in the die preheating step, but also an effective hard nitride layer grows. 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.

The oxygen concentration specified in the present invention is 20 to 500 p.
The conditions of pm and the X-ray intensity ratio of Fe ₃ O ₄ / Fe ₃ N of 20% or less were determined from many experimental results,
This is an effective index for preventing oxidation and disappearance of the nitride layer formed on the die surface. For example, an air atmosphere (oxygen concentration: about 20%) and a nitrogen atmosphere (oxygen concentration: 50 to
60 ppm), when the die was preheated, Fe ₃ O ₄ /
The X-ray intensity ratio of Fe ₃ N changed as shown in FIG.
Here, the peak intensity of Fe ₃ O ₄ was measured at about 36 ° (including a part of Fe ₂ O ₃ ), and the peak intensity of Fe ₃ N was measured at about 44 °, and Fe ₃ O ₄ / Fe was measured.
_The 3N X-ray intensity ratio was calculated. FIG. 1 shows that Fe ₃ O ₄ /
The oxidation tendency of the nitride layer is separated from the boundary of the X-ray intensity ratio of Fe ₃ N of 20%. In any atmosphere, up to 410 ° C., Fe ₃ O ₄ / Fe ₃ compared to the unheated die surface layer
Although the X-ray intensity ratio of N is increased, in the heating in the air atmosphere where the preheating temperature is higher than 410 ° C., the oxidation of the nitride layer proceeds with the increase of the preheating temperature.

On the other hand, oxygen concentration: 50 to 60 ppm
It can be seen that when preheating is performed in a nitrogen atmosphere, the oxidation of the nitride layer is suppressed without increasing the X-ray intensity ratio of Fe ₃ O ₄ / Fe ₃ N even at a preheating temperature exceeding 410 ° C. When the preheating temperature becomes 450 ° C. or higher, Fe ₃ O ₄ / Fe ₃ N
In contrast, the X-ray intensity ratio has decreased, and the nitride layer has grown. When the preheating temperature does not reach 410 ° C., the influence of the difference in atmosphere does not appear on the X-ray intensity ratio of Fe ₃ O ₄ / Fe ₃ N. This indicates that the oxidation of the nitride layer is not affected by the oxygen concentration of the atmosphere. The oxygen concentration of the atmosphere has a significant effect on the oxidation reaction of the white layer. When the oxygen concentration is 500 ppm or less, the preheating temperature can be raised to around 500 ° C. even when nitrogen is used as the atmospheric gas, and rapid preheating and nitriding can be performed. On the other hand, when the oxygen concentration exceeds 500 ppm, the oxidation and disappearance of the white layer becomes remarkable, and the X-ray intensity ratio of Fe ₃ O ₄ / Fe ₃ N exceeds 20%, and may be 30% or more.

[0013]

【Example】

Example 1: Hot tool steel SKD as a die material for extrusion
Using 61, width 20mm, length 40mm and thickness 15
The mm of the test piece was tempering the H _RC 48. One test piece was subjected to salt bath nitriding at 570 ° C. × 2.5 hours (single nitriding). Other test pieces were further subjected to the same nitriding treatment to obtain a renitrided material (twice nitriding). The remaining two test pieces are
According to the present invention, a pre-heat treatment was performed at 450 ° C. and 490 ° C. in a nitrogen atmosphere having an oxygen concentration of 50 ppm, respectively. The hardness distribution in the depth direction from the surface of each test piece was measured. FIG.
As can be seen from the measurement results, although the surface hardness of the test piece preheated in a nitrogen atmosphere was slightly lower, the depth of the hardened layer was almost the same as that of the test piece nitrided once and twice. Absent. From this, when the once-nitrided die is heated at a high temperature in a nitrogen atmosphere, it shows almost the same curing curve as that of the twice-nitrided die, indicating that the re-nitriding process can be replaced by preheating in a nitrogen atmosphere.

Example 2 A dice having a bearing length of 5 mm and a slit width of 40 mm × 3 mm, the surface of which was modified by nitriding, was used for hot extrusion of Al alloy JIS 6063. The extrusion conditions were as follows: die temperature 450 ° C., extrusion speed 4
It was set to 0 m / min. In the die according to the present invention, after performing the salt bath nitriding treatment, 20 billets were extruded. Then, without renitriding, it was preheated to 490 ° C. in a nitrogen atmosphere having an oxygen concentration of 50 ppm. The preheated die was used again for extrusion, and 50 billets were extruded. As a comparative example, a die renitrided by a conventional cycle was used for extrusion under similar conditions. Each die after extrusion was washed with alkali and the bearing surface was observed.
As a result, it was found that both of them had almost the same surface state. From these results, it can be seen that the preheating in the nitrogen atmosphere according to the present invention can also be used for renitriding.

[0015]

As described above, in the present invention, when the extrusion die is preheated to a temperature near the extrusion temperature prior to the hot extrusion, the oxygen concentration is 20 to 500 p.
By heating the dice in a nitrogen atmosphere adjusted to pm, a nitride layer is grown, and a healthy hard nitride layer is maintained on the die surface. Therefore, wear of the bearing surface is suppressed, the life of the die is extended, and the surface roughness of the obtained extruded profile does not increase with the extrusion length,
An extruded profile with good surface properties is obtained. Further, the preheating in the nitrogen atmosphere can omit the conventional renitriding treatment, so that the lead time is shortened and the production efficiency is improved.

[Brief description of the drawings]

FIG. 1 shows a heating atmosphere and a preheating temperature of F of a die surface layer.
Effect of e ₃ O ₄ / Fe ₃ N on X-ray intensity ratio

Fig. 2 Effect of nitriding and pre-heating on the surface hardened layer of die

Claims (3)

[Claims] When a pre-nitrided aluminum extrusion die is reused for hot extrusion, the die is preheated and renitrided in a nitrogen atmosphere having an oxygen concentration of 20 to 500 ppm prior to hot extrusion. A method for modifying the surface of an aluminum extrusion die, wherein the method is directly performed in an extruder. 2. The temperature is raised to 450 to 500 ° C. in a nitrogen atmosphere.
The method for modifying the surface of an aluminum extrusion die according to claim 1, wherein the aluminum extrusion die is held and preheated for 8 hours. 3. An aluminum extrusion die in which the X-ray intensity ratio of Fe ₃ O ₄ / Fe ₃ N on the surface of the die is adjusted to 20% or less immediately before extrusion by the preheating according to claim 1 or 2.