KR100548728B1 - Method for heat treatment of high strength invar alloy - Google Patents

Abstract

The present invention facilitates the production of wire rods with sound microstructure by effectively degrading the coarse Mo solid solution precipitate in the invar alloy ingot material for high-strength transmission line and has the effect of improving the quality of the Invar alloy product. It relates to a heat treatment method of.

In the present invention, by weight%, C: 0.15-0.45%, Si: 0.1-0.35%, Mn: 1.5% or less, Cr: 0.3-2.0%, Ni: 33-38%, Co: 5% or less, Mo: 0.5 ˜4.5%, V: 0.1-0.5%, Nb: 0.3% or less, making an ingot using an Invar alloy composed of the remaining Fe and other unavoidable impurities. The ingot was first homogenized at 1250-1300 ° C. Provided is a heat treatment method of a high strength Invar alloy, characterized in that a two-stage homogenization treatment is performed at 1150 ° C. for a second homogenization treatment.

Description

Heat treatment method of high strength invar alloy {METHOD FOR HEAT TREATMENT OF HIGH STRENGTH INVAR ALLOY}

Figure 1 is a photograph showing the decomposition state of the precipitate after homogenizing heat treatment at 1130, 1200, 1280 ℃ inva alloy ingot material according to the present invention.

The present invention relates to a heat treatment method for the Inba alloy ingot material used for the core wire of the high-strength transmission line, and more particularly to a heat treatment method for removing coarse precipitates in the Inba alloy ingot material.

Invar alloys have a lower coefficient of thermal expansion than other steels and have been widely used in electronic components such as precision measuring instruments, thermostatic bimetals, and shadow masks of color TVs. Since the invention of Invar alloy, Colin, Mo, W, Ti, Al, Cr, C, etc. have been added to the basic composition for the last 100 years to compensate for the elastic change due to temperature changes. Various types of Invar alloys have been developed and used, including Intel (Metelinvar), which combines reinforcement effects, and Fixinvar, which offsets the temporal / dimensional instability, one of the problems of existing Invar alloys. have.

Recently, the demand for electricity is increasing rapidly due to the high economic growth, and the demand for power supply in suburban cities is increasing due to the overcrowding of cities due to industrial expansion. Is required. In order to increase the amount of transmission, it is necessary to install transmission towers for the construction of new transmission lines. In this case, it is difficult to install new transmission lines due to excessive installation costs due to the increase of land price and land price caused by residentialization in urban suburbs. .

As an alternative to the increase in power transmission requirements, there is a method of increasing power transmission using existing power transmission lines.However, when power is increased by boosting an existing power transmission line, the temperature of the wire increases and sagging of the wire due to thermal expansion. There is a risk that the height required for safety from ground level cannot be obtained when high voltage current passes. In developed countries, ACIR (Aluminum Conductor) using invar alloy (Invar, 36% Ni-Fe-Co) wire with low thermal expansion coefficient (1/10 of carbon steel) instead of high carbon steel is used in advanced countries. Invar Reinforced) Power transmission lines are used to carry out boosted power transmission by utilizing existing transmission lines.

Therefore, the Invar alloy for high-strength high-voltage transmission lines should have a low coefficient of thermal expansion of 10 ^-6 and a high strength of 1000 MPa or more in the hot rolling state. In order to realize such high strength, many researchers have been trying to achieve strength improvement by solid solution strengthening and work hardening by adding Co, Mo, Ti, Cr, C, V, and Nb to the basic invar composition. However, due to the influence of these additional elements, not only carbides that contribute to the strength improvement during casting, but also coarse Mo solid solutions are formed, causing cracks in the hot wire rolling and cold drawing processes that are followed. There was a problem.

The present invention is to solve the problems of the prior art as described above, it is an object of the present invention to provide a method for effectively decomposing or removing the coarse Mo solid solution precipitate present in the Invar alloy casting material by a heat treatment control method.

Hereinafter, the present invention for achieving the above object will be described in detail.

In the present invention, by weight%, C: 0.15-0.45%, Si: 0.1-0.35%, Mn: 1.5% or less, Cr: 0.3-2.0%, Ni: 33-38%, Co: 5% or less, Mo: 0.5 It is a method of performing a two-stage homogenization heat treatment as follows for the Invar alloy composed of ~ 4.5%, V: 0.1-0.5%, Nb: 0.3% or less, the remaining Fe and other unavoidable impurities.

The heat treatment method of the Invar alloy of the present invention is preheated at 1000-1100 ° C. for a predetermined time in order to secure a uniform temperature throughout the large invar alloy ingots prepared as described above, and at the time of decomposition treatment of Mo solid precipitates. First homogenization at 1250-1300 ° C and second homogenization at 1100-1150 ° C for faster redissolution, effectively disintegrating the coarse Mo solids precipitate in the Invar alloy casting and subsequently hardening It comprises a heat treatment method for depositing fine Mo ₂ C carbide beneficial for strengthening by.

The inventors of the present invention conducted the research to effectively remove coarse precipitates by using an invar alloy casting material for a high-voltage power transmission line, and confirmed the following facts.

(1) Complete redissolution of coarse precipitates occurs at a holding time of 2 hours in the temperature range of 1250-1300 ° C.

(2) Even when kept in the temperature range of 1100-1150 ° C for a long time, a considerable amount of coarse precipitates remain and the strength is higher than that maintained in the temperature range of 1250-1300 ° C.

Therefore, paying attention to the above two facts, it has been concluded that proper control of the homogenization heat treatment procedure can achieve both the complete decomposition of the coarse precipitate and the strengthening effect at the same time.

The present invention is characterized by a heat treatment method that can achieve both the complete decomposition of the coarse precipitates and the strengthening effect at the same time in the invar alloy ingot material for high-voltage transmission lines, the following describes the composition limitation range and heat treatment method of the alloy to be applied do.

[Scope of application of the alloy to be applied]

C: 0.15 to 0.45 wt%

The carbon content in the Invar alloy is a factor determined by the contents of the carbide forming elements Mo, V, Nb and Cr, and forms various carbides by combining with various carbide forming elements such as V, Cr, Mo, and Nb. As a necessary basic component, the addition of less than 0.15% by weight is difficult to form a sufficient amount of carbide, and when added in excess of 0.45% by weight lowers the thermal expansion characteristics, it is preferable to limit the content to 0.15 to 0.45% by weight.

Si: 0.1 ~ 0.35 wt%

Said Si is a component added as a deoxidizer and contributes to the improvement of hardness by further increasing carbide precipitation. If the content of Si is less than 0.1% by weight, there is almost no inherent deoxidation effect of Si, and if it is added in excess of 0.35% by weight, the coefficient of thermal expansion on the high temperature side is increased, so that the content is preferably limited to 0.1 to 0.35% by weight. .

Mn: 1.5 wt% or less

The Mn is an austenite stabilizing element, and when added in excess of 1.5% by weight, the thermal expansion coefficient increase transition temperature is lowered, it is preferable to limit the content to 1.5% by weight or less.

Cr: 0.3-2.0 wt%

The Cr is a component having a very high strength improvement effect in the Fe-Ni-based alloy, and when added below 0.3 wt%, the reinforcing effect is small, and when added in excess of 2.0 wt%, the thermal expansion property is inhibited. It is preferable to limit the weight percentage.

Ni: 33-38 wt%

Ni has the lowest coefficient of thermal expansion at 36% by weight. If less than 33% by weight or more than 38% by weight is added to the Invar alloy is difficult to use for the transmission line, it is preferable to limit the content to 33 to 38% by weight.

Co: 5.0 wt% or less

Co has the effect of lowering the coefficient of thermal expansion without changing the thermal expansion coefficient increase transition temperature by substituting a part of Ni. However, if it exceeds 5.0% by weight, the effect is saturated and the price is very expensive, so it is preferable to limit the content to 5.0% by weight or less.

Mo: 0.5-4.5 wt%

The Mo is a component added to form a carbide of the M ₂ C form, but has the effect of reinforcing the Fe-Ni-based alloy, but less than 0.5% by weight is less effective, when added in excess of 4.5% by weight is saturated In order to increase the raw material cost, it is preferable to limit the content to 0.5 to 4.5% by weight.

V: 0.1-0.5 wt%

The V is added for the formation of MC-type carbide, if less than 0.1% by weight it is difficult to secure sufficient carbide, when it is added more than 0.5% by weight is left in the matrix to reduce the hot workability of the matrix, It is preferable to limit it to 0.1-0.5 weight%.

Nb: 0.3 wt% or less

Nb is added to form MC in the same manner as V, and when added in an amount of 0.1 wt% or more, it remains in the matrix and degrades hot workability of the matrix. Thus, the content is preferably limited to 0.3 wt% or less.

In addition to the above compositions, the remainder is composed of Fe and other unavoidable impurities.

[Heat treatment method]

In the present invention, the alloy prepared as described above is first preheated for a predetermined time at 1000-1100 ℃ in order to ensure a uniform temperature throughout the large ingot. Subsequently, the first homogenization treatment at 1250-1300 ° C. and the second homogenization treatment at 1100-1150 ° C. in order to achieve complete redissolution of the coarse Mo solid solution precipitate are performed. A heat treatment for effectively distributing carbides effective for improvement is performed.

The reason for the first homogenization treatment in the homogenization treatment temperature range is 1250-1300 ° C. is less than 1250 ° C., in order to re-dissolve the coarse Mo solid solution precipitate produced during casting, it is difficult to apply practically, and 1300 ° C. This is because it is difficult to apply in the field and re-dissolves even carbides effective for improving strength.

The second homogenization temperature is 1100-1150 ℃. If the temperature is less than 1100 ℃, the load of the roll or press hammer increases due to the temperature drop during hot wire rolling or forging. Since it is difficult to nucleate the carbide to help improve the strength to cause a problem to secure the strength in the future, the hot working temperature is preferably limited to 1100 ~ 1150 ℃.

Hereinafter, the present invention will be described through examples.

Example 1

Three kinds of steels having the composition as shown in Table 1 below were prepared in a 50 kg ingot using a vacuum melting furnace. The prepared ingot was preheated at 1000 to 1100 ° C. for 2 hours in a heat treatment furnace, and then maintained at 1130, 1200, and 1280 ° C. for 3-9 hours for homogenization.

1 is a photograph showing the state of coarse precipitates when the homogenization treatment for 1 hour at 1130, 1230, 1280 ℃ after preheating using the ingot material. As shown in FIG. 1, when the homogenization treatment is performed at 1130 ° C., a large amount of precipitates remain undecomposed, whereas when the homogenization treatment is performed at 1280 ° C., almost all precipitates are dissolved again and disappeared.

<Table 1>

division Ingredient (% by weight) C Si Mn Cr Ni Co Mo V Nb Fe Alloy 1 0.32 0.15 0.28 0.81 36.2 2.15 2.28 0.12 0.09 Remainder Alloy 2 0.28 0.26 0.81 1.10 33.8 1.43 4.14 0.32 0.26 Remainder Alloy 3 0.16 0.31 1.20 1.85 37.7 4.16 3.76 0.41 0.18 Remainder

Example 2

Three kinds of steels having the composition as shown in Table 1 above were prepared in a 50 kg ingot using a vacuum melting furnace. The prepared ingot was preheated at 1000 to 1100 ° C. for 2 hours in a heat treatment furnace, and then maintained at 1130, 1200, and 1280 ° C. for 3-9 hours for homogenization.

<Table 2>

Homogenization temperature (℃) Mechanical properties Yield strength (MPa) Tensile Strength (MPa) Elongation (%) 1130 424 1071 39 1230 425 1058 40 1280 422 1062 43

Table 2 summarizes the results of room temperature tensile test of the hot-rolled sheet in the temperature range of 1100 ~ 1150 ℃ after the homogenization treatment using the ingot material. As shown in FIG. 1, when the homogenization treatment was performed at 1280 ° C., the high strength of 1000 MPa or more was obtained even after the coarse precipitate was almost disappeared, and the elongation was slightly improved. This is because the first homogenization treatment at 1250 ~ 1300 ℃ presented in the present invention, the second homogenization treatment at 1100 ~ 1150 ℃, which is a hot working temperature, almost completely removes coarse precipitates, and the strength can be obtained at the same level as before. This confirms the result.

As described above, the present invention is to effectively decompose the coarse Mo solid solution precipitate in the invar alloy ingot material for high-strength transmission line to facilitate the production of wire rods with a healthy microstructure and to improve the quality of the Invar alloy products There is.

Claims (2)

By weight%, C: 0.15-0.45%, Si: 0.1-0.35%, Mn: 1.5% or less, Cr: 0.3-2.0%, Ni: 33-38%, Co: 5% or less, Mo: 0.5-4.5% , V: 0.1-0.5%, Nb: 0.3% or less, the ingot is made of an Invar alloy composed of the remaining Fe and other unavoidable impurities, and the ingot is first homogenized at 1250-1300 ° C. for 3-9 hours. Heat treatment method of high strength Invar alloy, characterized in that the two-stage homogenization treatment to secondary homogenization treatment for 3 to 9 hours at 1100-1150 ℃. The method of claim 1, wherein the preheating treatment is performed at 1000-1100 ° C. for 2 hours before the two-step homogenization of the ingot material.

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