JPS61186448A - Casting mold for steel ingot having superior heat resistance - Google Patents

Abstract

PURPOSE:To obtain a casting mold for a steel ingot having superior heat resistance by specifying the amounts of C, Si, Ti and Mg and/or REM in compacted graphite cast iron (vermicular iron) and restricting the amount of spheroidal graphite precipitated to a specified percentage of less. CONSTITUTION:Compact vermicular cast iron is provided with a composition contg., by weight, 3.5-4.5% C, 1.2-1.6% Si, 0.02-0.08% Ti and 0.015-0.03% Mg and/or REM, and the amount of spheroidal graphite precipitated is restricted to <=40%. A casting mold for a steel ingot is manufactured with the compacted graphite cast iron (vermicular iron).

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a mold for steel ingots using compact vermicular graphite (hereinafter referred to as α-graphite) cast iron, and in particular, it has properties such as heat resistance such as crazing resistance and deformation resistance. This invention relates to a mold for steel ingots with excellent properties.

[Conventional technology]

Steel ingot molds (hereinafter simply referred to as molds) used in ingot making operations are subject to repeated thermal history from room temperature to high temperature ranges, so crazing is likely to occur on the inner surface of the casting. If heat history is repeatedly applied to the lasing state, a layer may be partially formed, or large cracks may occur from the beginning of use, making it impossible to use the mold as a mold. There were even times when it happened. That is, generally, ordinary cast iron or spheroidal graphite cast iron is used as the material for the mold, but molds using ordinary cast iron tend to suffer from crazing on the inner wall surface as described above (as shown in Fig. 2 as the number of times they are used increases). In addition, since vertical cracks 1 are formed from the beginning of the mold's use and develop each time it is used, it often becomes unusable at an early stage.

On the other hand, molds using spheroidal graphite cast iron have a higher degree of independent distribution of graphite in the matrix than ordinary cast iron, and the graphite is less sensitive to notches, so falling off and cracking are less likely to occur. However, since it has the characteristic of large elongation, the broken line 3 in Figure 3
As shown in , there is a drawback that the deformation during use becomes large, and it becomes difficult to extract the steel ingot after it is formed. Therefore, it is necessary to frequently correct the deformation of the mold, and it has come to be considered that it is economically disadvantageous to use spheroidal graphite cast iron as the material for the mold.

[The problem that the invention attempts to solve]

The present inventors have been conducting research for some time with the aim of improving the lifespan and durability of cast iron members that are subjected to repeated heating and cooling, and have developed a graphite form that is intermediate between flaky and spherical. (Japanese Patent Application No. 59-23564) discovered that a cast iron member with excellent crazing resistance and heat deformation resistance could be obtained by using (Cv graphite).

However, it is not enough to select Cv graphite cast iron at random; instead, it is necessary to conduct a more detailed investigation in terms of crazing, cracking, deformation, etc., and select a material that can maximize mold life and durability. Needless to say, it won't happen.

Therefore, in the present invention, the present invention was completed as a result of repeated research and tests from the viewpoint of practical use of a mold for steel ingots that has a long life and excellent durability when used as a mold material.

[Failure to solve the problem]

C: 3.50 to 4.20 wt% st: x, zo to 1.60 wt% Ti: 0.020 to o, o so wt% One or more elements selected from Mg and REM: A =0.015
The gist of the present invention lies in that the mold is formed from compact vermicular graphite cast iron with a slip ratio of ~0.03030% by weight and a precipitation ratio of spheroidal graphite of 40% or less.

[Effect]

As is generally known, high thermal conductivity, low coefficient of thermal expansion, and low modulus of elasticity lower the level of thermal stress, and as a result, effectively work to improve needle sidewall properties. By the way, among the alloy components present in cast iron, C and Si can be considered as elements that can have a large effect on crack resistance. These elements have a large effect on thermal conductivity, and increasing the amount of C is an advantageous element because it increases the thermal conductivity, but increasing the amount of Si tends to decrease the thermal conductivity and is a disadvantage. It is considered to be a natural element. Further, S and Ti are grain boundary precipitated elements, and since these are low melting point elements or metals, they are not preferable from the viewpoint of crack susceptibility due to repeated thermal history. Therefore, the presence or absence of vertical cracking during use of the mold was determined by the amount of Ti and the amount of Si.
An experiment was conducted to understand the relationship in terms of quantity, and the results shown in Figure 1 were obtained. According to this, the amount of Ti is 0.020~
5ii1 within the range of 0.080% by weight from 1.20 to 1
．． It can be seen that by setting the vertical it% within the range of 60 it%, it is possible to considerably suppress the occurrence of vertical cracks in the mold. The amount of C was determined to be in the range of 3.50 to 4.20% by weight from the viewpoint of providing an appropriate amount of graphite.

Next, we examined the crazing resistance and deformation resistance, and conducted experiments to find conditions to minimize the number of corrections due to deformation. As a result, the precipitation ratio of spheroidal graphite structure in cast iron was 40% or less. (The remaining 60% or more should have a composition having a C■ graphite structure), and it was found that by doing so, the number of repairs and repair costs can be reduced, making it a preferable material for molds. In addition, in order to obtain such a graphite structure, Mg and REM, which are graphite spheroidizing elements, should be blended in a total amount of 0.015 to 0,000% by weight, as will be explained later in practice. Through chemical tests, we were able to clarify 5.

〔Example〕

A mold for a steel ingot was made using a material having the composition shown in Table 1, and a practical application test was conducted for the purpose of improving the cracking resistance of the mold. The service life in the table indicates the number of times the mold can be used until it becomes unusable. The cast iron shown in the conventional example is one in which the Si content is varied by increasing the content of 1゛i. In all of these examples, large vertical cracks occurred in the mold bases, and although the cracks were repaired by welding, the service life was short at 96 to 123 cycles.

The cast iron shown in Conventional Example ■ has a low Ti content (high Si content). In these examples as well, mold cracks occur as often as in the conventional example, and the life of the mold is also the same as in Conventional Example I. It was also short.

On the other hand, those shown in the column of Examples of the present invention have a Ti ratio of 0.020 to 0.080% by weight and low melt LC suppression L.
, Si content was in the range of 1.20 to 1.60% by weight. As a result, no cracking was observed in the mold, and the number of times it was used, which is an indicator of service life, was considerably increased compared to the conventional example and (2). It was confirmed that it is effective in improving cracking resistance.

Next, Table 2 shows the results of testing and observing the deformation of the mold by varying the precipitation rate of the spheroidal graphite structure in cast iron in order to investigate the deformation resistance of the mold. In the conventional example, the total amount of Mg4-REM, which is a graphite spheroidizing element, is set at 0.030% or more directly, and the precipitation ratio of spheroidal graphite, 111a, in the mold is 60%.
The cast iron has the above composition (the remaining composition is a CV graphite structure). The lifespan of conventional molds is 136 to 151 times, but in both cases deformation during use becomes large, which hinders the work of removing the steel ingot and requires rework to correct the deformation of the mold. Ta.

On the other hand, in the example of the present invention, the total amount of Mg+yLEM is 0.030
% or less, and cast iron having a composition having a precipitation ratio of spheroidal graphite structure of 40% or less (the remaining 60% or more is Cv graphite) is used. In the example of the present invention, the life cycle is 130~
137, which was inferior to the conventional example, and there was little mold deformation during use, so there was no need for modification.

Next, we investigated the impact of crazing on the inner wall surface that occurs during use of the mold, the repair of the mold when the inner wall of the mold falls off, or the mold is deformed, on the lifespan and cost of the mold. The results shown in Table 3 were obtained.

The molds that satisfy the conditions of the present invention are inferior to the conventional molds with a spheroidal graphite composition of 60% or more in terms of life, and the molds of the present invention are inferior to the conventional molds in terms of peeling resistance (resistance to falling off of mold walls). Inferior. However, the mold deformation rate of the present invention is better, and regarding the mold deformation site, the present invention is better in that it does not require modification, but the conventional example has a spheroidal graphite structure of 60% or more. In this case, it is necessary to modify the grinding process using a grinder, and furthermore, the workability of extracting the steel ingot is poor and a considerable amount of time and effort is required.

As a result of considering these factors, if the repair cost of a mold with 100% flaky graphite structure (regular cast iron g & mold) is set as 100, it is possible to reduce the cost by 20 to 30% in the example of the present invention, and when extracting the steel ingot. I found out that this work can be done efficiently.

〔Effect of the invention〕

By setting the Ti and Si amounts in the cast iron material used in the mold for steel ingots within the range of the present invention, it is possible to suppress the occurrence of cracks in the mold for steel ingots, and improve the quality of the mold. It was possible to extend the number of uses, that is, the lifespan.

In addition, the value of Mg4-REM ii in cast iron is 0.015~
By setting the precipitation ratio of spheroidal graphite structure in C■ graphite cast iron to 40% or more as 0.030%, it was possible to reduce the mold cost by reducing mold corrections due to wall fall-off in the deformed area. .

[Brief explanation of drawings]

Figure 1 is a graph showing the frequency of cracking in the mold with respect to changes in Si+'rikt, Figure 2 is a schematic side view showing cracks and falling off of the mold for steel ingots, and Figure 3 is the deformation and shape of the mold. FIG. 1... Cracking, 2... Falling off of the inner wall of the mold, 3... Deformation of the mold.

Claims (1)

[Claims] C: 3.50 to 4.20% by weight Si: 1.20 to 1.60% by weight Ti: 0.020 to 0.080% by weight One or more selected from Mg and REM Element: 0.0
A mold for a steel ingot having excellent heat resistance, characterized in that it is made of compact vermicular graphite cast iron having a content of 15 to 0.030% by weight and a precipitation ratio of nodular graphite of 40% or less.