JP2015124093A - Aluminum molten metal resistant silicon nitride sintered compact, and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aluminum molten metal resistant silicon nitride sintered compact in which initial cracks are hard to be generated.SOLUTION: Provided is a tubular or box-shaped silicon nitride sintered compact used as a member for aluminum molten metal, the silicon nitride sintered compact is sintered with the total addition amount of the silicon nitride sintered compact as below 18 wt.%, and further, the difference between the residual stress of the outer surface of the sintered compact and the residual stress of the inner surface is controlled to 50 MPa or lower, and also, the absolute value of the residual stress is controlled to 100 MPa or lower.

Description

  The present invention relates to a silicon nitride sintered body for molten aluminum resistance used as a member for molten aluminum and a method for producing the same.

  Silicon nitride sintered bodies are excellent in corrosion resistance, mechanical properties, thermal shock resistance, wear resistance, etc., so they are used in a wide range of industrial parts such as metal melt parts, engine parts, and cutting tools. Yes. In particular, as a member for molten aluminum exposed to molten aluminum, a thermocouple protection tube, a heater tube for heating molten aluminum, stalk for supplying molten aluminum to a mold, and pouring molten aluminum into a die-cast sleeve Tubular or box-like things such as ladle are used in the automobile industry.

  In order to increase the strength, toughness and wear resistance of the silicon nitride sintered body used as such a member or part, use of a sintering aid that produces a crystalline phase with a high melting point or columnar β-type silicon nitride Various methods for increasing the residual stress after sintering have been proposed, such as the addition of a third component such as zirconia and hafnia, and the method of increasing the residual stress after sintering and reheating and quenching. ing.

  In addition, a method for improving the strength by reducing the residual stress to prevent abnormal growth of crystal grains in the structure of the silicon nitride sintered body, and the silicon nitride sintered body for use as a molten aluminum member are repeatedly used. A surface structure has also been proposed to prevent the occurrence of cracks and increase the wetting angle with the molten aluminum.

Japanese Patent Laid-Open No. 6-305834 JP 2013-75796 A JP-A-08-319166 JP 2005-179176 A JP 2005-213081 A Japanese Patent Laid-Open No. 11-180773

  Of the above proposals, leaving compressive residual stress in the sintered body is an effective method for increasing the strength and fracture toughness. However, if the shape of the silicon nitride sintered body is tubular or box-shaped, the outer surface There is a case where the residual stress on the inner surface is not uniform, and there is a problem that cracks occur due to this.

  For example, in the case of a heater tube, the inner surface continues to receive heat from a heater at 1200 to 1400 ° C, and the outer surface continues to be cooled by molten aluminum at about 680 ° C, so the temperature difference between the inner surface and the outer surface is 200 ° C or more. Become. The heater tube is used because the residual stress is relaxed by thermal expansion due to heating, and the difference between the residual stress on the inner surface and the outer surface is increased due to the difference in thermal expansion due to the temperature difference between the inner surface and the outer surface. There may be a so-called initial crack that breaks immediately after starting or after several uses.

  In order to prevent initial cracking of a silicon nitride sintered body having a tubular or box shape used as a member for molten aluminum, the present invention tests and examines the relationship between residual stress on the outer surface and inner surface and occurrence of initial cracking. The object of the present invention is to provide a silicon nitride sintered body for molten aluminum resistance that is less prone to initial cracking and a method for producing the same.

  To achieve the above object, a silicon nitride sintered body for molten aluminum resistance according to claim 1 is a tubular or box-shaped silicon nitride sintered body used as a member for molten aluminum, and the silicon nitride sintered body is The sintered additive was sintered with a total addition amount of less than 18% by weight, the difference between the residual stress on the outer surface of the sintered body and the residual stress on the inner surface was less than 50 MPa, and the absolute value of the residual stress was It is 100 MPa or less.

  A method for manufacturing a silicon nitride sintered body for resistance to molten aluminum according to claim 2 is a method for manufacturing the silicon nitride sintered body according to claim 1, wherein the cooling process is performed after sintering according to a conventional method. The method is characterized in that a treatment is performed in a temperature range of 700 to 1150 ° C. under a nitrogen gas atmosphere.

  According to the present invention, a tubular or box-shaped silicon nitride sintered body used as a member for molten aluminum, the residual stress on the outer surface and the inner surface is small, and initial cracking is difficult to occur. A ligation and a method for producing the same are provided.

  The silicon nitride sintered body for resistance to molten aluminum according to the present invention is a tubular or box-shaped silicon nitride sintered body that can be used by being immersed in molten aluminum (including molten aluminum alloy), and has a residual outer surface. The difference between the stress and the residual stress on the inner surface is smaller than 50 MPa, and the absolute value of the residual stress is 100 MPa or less.

  When the difference between the residual stress on the outer surface of the tubular or box-shaped sintered body and the residual stress on the inner surface is 50 MPa or more, cracking tends to occur at the initial stage of use. The absolute values of the residual stress on the outer surface and the residual stress on the inner surface are both preferably 100 MPa or less, and if any of the absolute values exceeds 100 MPa, initial cracking is likely to occur.

  The silicon nitride sintered body for resistance to molten aluminum according to the present invention is sintered by reaction sintering, ordinary pressure sintering, which is a general sintering method, and in a cooling process after sintering under a nitrogen gas atmosphere, Manufactured by carrying out a treatment in a temperature range of 700 to 1150 ° C.

Particular embodiments, in the course of cooling after sintering, sintered silicon nitride sintered body was cooled down to T ₁ ° C. of the temperature range of from 700 to 1,150 ° C., under a nitrogen gas atmosphere at a temperature of T ₁ ° C. Heat treatment is carried out. The holding temperature (T ₁ ° C.) is desirably 50 ° C. or more lower than the crystallization temperature of the sintering aid to be used, more preferably 100 ° C. or more. It is difficult to obtain the characteristics of the present invention at a temperature lower than 700 ° C.

Thereafter, the temperature may be further lowered to T ₂ ° C (T ₁ ° C> T ₂ ° C ≧ 700 ° C.) in the temperature range, and maintained at a temperature of T ₂ ° C. under a nitrogen gas atmosphere. Can be repeated step by step. The holding time at each temperature is preferably about 0.5 to 3 hours in practice. If the holding temperature exceeds 1150 ° C., the residual stress increases, making it difficult to obtain the characteristics of the present invention. In addition, it is difficult to obtain the characteristics of the present invention even when the sintered body is cooled to room temperature after sintering and then heated to a temperature range of 700 to 1150 ° C. and kept at a temperature within this temperature range.

  As described above, the silicon nitride sintered body for molten aluminum resistance according to the present invention is produced by sintering by reaction sintering or normal pressure sintering, which are general sintering methods. It is preferable that the total amount of sintering aid added at the time of sintering is less than 18% by weight. When the total amount of sintering aid added is 18% by weight or more, the amount of sintering aid present at the grain boundaries increases. Thus, the difference in residual stress between the outer surface and the inner surface increases, and the absolute value thereof easily exceeds 100 MPa.

  Examples of the present invention will be described below in comparison with comparative examples to demonstrate the effects of the present invention. In addition, these Examples show one embodiment of this invention, and this invention is not limited to these.

Example 1 and Comparative Example 1
Using silicon (Si) as a starting material, as shown in Table 1, a sintering aid was blended to form a tubular closed end tube having an outer diameter of 56 mm, an inner diameter of 42 mm, and a length of 900 mm. After nitriding this molded body at a temperature of 1300 ° C., it is stored in a graphite tray and sintered for 4 hours at a sintering temperature shown in Table 1 and a nitrogen gas pressure of 1.5 atm. In the cooling process after sintering, As shown in Table 1, a heat treatment that is performed in a nitrogen atmosphere at a temperature within a temperature range of 700 to 1150 ° C. and a heat treatment that is not performed are prepared, and the outer diameter is 50 mm, the inner diameter is 38 mm, and the length is 800 mm. A sintered body (test materials 1 to 6) of the tubular closed end tube was obtained. In Table 1, those outside the conditions of the present invention are underlined.

(Measurement of residual stress):
From the obtained test material, a pipe having an outer diameter of 50 mm, an inner diameter of 38 mm, and a length of 100 is cut out and cut into a semi-cylindrical shape to prepare a residual stress measurement sample. Stress was measured. The results are shown in Table 2.

(Aluminum resistance test)
The obtained test material was held immediately above the molten aluminum at 680 ° C. for 5 minutes, then quickly immersed in the molten aluminum to a depth of 350 mm, held for 10 minutes, then pulled up at the same speed, allowed to cool to room temperature, Tests were conducted five times to check for the presence of damage and cracks by visual confirmation and hammering inspection. Those that were not damaged or cracked after 5 implementations were evaluated as acceptable (O). The results are shown in Table 2.

(Atmospheric oxidation test)
The obtained test material was kept at a temperature of 1000 ° C. in the atmosphere for 24 hours, and the state after cooling was observed. Those in which the state after cooling was normal were evaluated as acceptable (◯). The results are shown in Table 2. In Table 2, those outside the conditions of the present invention are underlined.

  As shown in Table 2, all of the test materials 1, 3, and 5 according to the present invention were free from cracks in the molten aluminum test, and the state after the atmospheric oxidation test was normal. In contrast, the test materials 2 and 4 that were not heat-treated in the cooling process after sintering, and the test material 6 in which the total amount of the sintering aid added exceeded 18% by weight were cracked in the molten aluminum resistance test. Arise. Alternatively, cracks and breakage occurred in the atmospheric oxidation test.

Example 2 and Comparative Example 2
The starting raw material was changed to the raw material shown in Table 3, and the same treatment as in Example 1 was performed to produce a sintered body (test materials 7 to 12) of a tubular closed end tube having an outer diameter of 50 mm, an inner diameter of 38 mm, and a length of 800 mm. Residual stress was measured by the same method as in Example 1, and a molten aluminum test and an atmospheric oxidation test were conducted. The results are shown in Table 4. In Tables 3 and 4, those outside the conditions of the present invention are underlined.

  As shown in Table 4, all of the test materials 7, 8, 10, and 12 according to the present invention were free from cracks in the molten aluminum test, and the state after the atmospheric oxidation test was normal. On the other hand, the test material 9 that was not heat-treated in the cooling process after sintering, and the test material 11 in which the total amount of sintering aid added exceeded 18% by weight did not cause cracks in the molten aluminum test. However, chipping and damage occurred in the atmospheric oxidation test.

Claims (2)

A tubular or box-shaped silicon nitride sintered body used as a member for molten aluminum, the silicon nitride sintered body being sintered with a total amount of sintering aid added of less than 18% by weight. A silicon nitride sintered body for aluminum-resistant molten metal, characterized in that the difference between the residual stress on the outer surface and the residual stress on the inner surface is less than 50 MPa and the absolute value of the residual stress is 100 MPa or less. A method for producing the silicon nitride sintered body according to claim 1, wherein the sintered body is sintered according to a conventional method, and then maintained in a temperature range of 700 to 1150 ° C under a nitrogen gas atmosphere in a cooling process after the sintering. The manufacturing method of the silicon nitride sintered compact for aluminum-resistant molten metal characterized by performing the process to perform.