JPS6238408B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing compact vermicular graphite cast iron (hereinafter abbreviated as CV graphite cast iron) using blast furnace hot metal as a raw material, and particularly relates to a method for improving productivity and stably producing CV graphite cast iron. It is something. The basic material properties required for cast iron parts that are used repeatedly between high and low temperatures, such as molds for steel ingots and other general heat-resistant containers, include crazing resistance and thermal shock resistance. . In order to satisfy these material properties, improvements in the shape of graphite and improvements in the base have been made in the past, but the present invention was researched with a focus on the shape of graphite. Regarding the shape of graphite, it is roughly divided into flaky graphite and spheroidal graphite, but in the former, free graphite is connected over a wide range in the base, so cracks propagate quickly through the flaky graphite. They have a tendency to crack and tend to deteriorate with repeated use, which is a drawback. Furthermore, since the former has a tensile strength of 5 to 10 Kg/mm ² and an elongation of only about 1% or less, it is necessary to increase the effective cross-sectional area in terms of strength design, resulting in an increase in weight.
On the other hand, in the case of the latter type of spheroidal graphite, free graphite is distributed in the matrix in the form of spheres, so although it has the advantage of less crack propagation and structural deterioration due to high-temperature oxidation, its tensile strength is only 40Kg/mm ² More than 10% growth
For this reason, there is a problem in that the amount of deformation due to repeated heating and cooling increases and dimensional accuracy decreases. For this reason, for example, in the case of a mold for a steel ingot, it becomes necessary to repeatedly adjust the dimensions, and this tendency is particularly noticeable in large molds. The inventors of the present invention have long been concerned about the above-mentioned circumstances, and have thought that it is necessary to improve the casting life and durability of cast iron parts that are frequently heated and cooled. We have been carrying out research with the immediate challenge of improving this. As a result, they found that if the graphite form is intermediate between flaky and spherical (CV graphite cast iron), cast iron parts with excellent crazing resistance and thermal shock resistance can be obtained, and they have previously presented this at an academic conference. Methods for manufacturing CV graphite cast iron include (1) adjusting the progress of spheroidization by adjusting the amount of Mg, which is an element that promotes spheroidization; and (2) adjusting the progress of spheroidization by adding Ti and Al, which are elements that inhibit spheroidization. , A method of aiming for an intermediate shape between flaky and spherical by using it in combination with Mg and adjusting the amount of addition. (3) Adjusting the progress of spheroidization using elements with weak spheroidization ability (Ca and REM) There are ways to do this. Among these, method (2) is for example
The CV graphite generation area is expanded compared to method (1), so it can be evaluated as an extremely stable manufacturing method. However, Ti
Since Al is originally contained in large amounts in the returned waste, the added Ti and Al are superimposed on these initial Ti and Al, and the spheroidization inhibiting effect is exerted more than necessary, resulting in the production of CV graphite. It also has the disadvantage of being unstable. Furthermore, if a large amount of Ti is incorporated into cast iron and it becomes a flaky graphite structure, the risk of cracking due to thermal history becomes even more pronounced, and this is not necessarily a good method. For this reason, the present inventors had concerns about the conventional method using Ti. On the other hand, the present inventors have been considering a method for producing CV graphite cast iron from blast furnace hot metal for some time, but since TiO operation is increasingly being carried out for the purpose of protecting the blast furnace cast bed, we are particularly looking at hot metal at the final stage of the blast furnace. In order to avoid the above-mentioned drawbacks caused by Ti, it was considered necessary to take strong countermeasures against this problem. In other words, as part of the component adjustment operation of blast furnace hot metal, which is the raw material, we focus on adjusting Ti, and perform oxygen blowing according to the Ti level of the hot metal. We once again considered a method of manufacturing CV graphite cast iron by adding Mg-Ti alloy. If this method
The spheroidization-promoting effect of Mg and the spheroidization-inhibiting effect of Ti were successfully exerted in an antagonistic manner, and it became possible to almost stably obtain an intermediate morphology between flaky and spherical shapes. The Mg added in this method is consumed by strongly combining with the S in the hot metal, and in order to retain the amount of Mg necessary to form CV graphite, the hot metal must be desulfurized in advance to reduce the amount of S. Along with letting
It was necessary to add an extra amount of Mg after taking into account the amount of Mg that binds to residual S. Therefore, the hot metal was desulfurized and Ti removed beforehand, and then the Mg-Ti alloy was added to it.
Since it is necessary to subject it to Ti treatment, and it is also necessary to set a fairly severe target for Ti reduction in this treatment, it is not possible to ensure sufficient productivity due to the relationship with blowing equipment and capacity. There was also the problem that the blowing cost increased and was passed on to the product price. The present inventors believed that it was necessary to establish a new manufacturing method that does not have these drawbacks, and investigated a method that can achieve CV conversion more efficiently and at lower cost. The present invention, which has been completed as a result, uses desulfurized blast furnace hot metal as a raw material, divides it into two parts, subjects one side to Ti removal treatment, adjusts the composition as necessary, and then divides it into two parts. The first point is that the raw materials are combined with the hot water, and the second point is that the Ti level of the combined hot water is 0.02 to 0.08% (meaning of weight %, the same applies hereinafter).
The third important point is to add the Mg-REM alloy so that the following conditions are satisfied. 1.97 [S] + 0.022% ≧ [Mg + REM] ≧ 2.0 [S] + 0.012% (In the above, [S] is the weight% of S in the hot water,
[Mg+REM] means the total addition percentage of Mg and REM to the combined melt, respectively.) The method of the present invention will be explained step by step below, but depending on the case, we will explain the Mg-Ti alloy addition method (the above-mentioned hot metal total deTi method). ), the features of the present invention will be clarified by comparing them. FIG. 1 is an explanatory diagram schematically showing the entire process of the present invention. Hot metal tapped from a blast furnace 1 is charged into a pretreatment furnace 2 and subjected to desulfurization treatment. Then, this is divided into two parts and one part is placed in the blowing furnace 3 to remove Ti by blowing, but at the same time, decarburization and temperature rise proceed as a side effect, and both reducing Si and Mn are oxidized and reduced. Therefore, it is also possible to adjust the composition by adding alloying elements in consideration of the intended use of the cast product. The alloying elements added are
Ferroalloys such as Fe-Si and Fe-Mn are exemplified, but the type of alloying element is not limited at all, and the composition can be recovered by the mixing described later, so such composition adjustment is not necessary at all. Sometimes it is necessary. Furthermore, even if the composition is adjusted by adding an alloying element, the alloying element may be added to the untreated molten metal or combined metal itself, which will be described later, and all of these modifications are included in the present invention. The molten metal that has been subjected to the Ti-free treatment (alloying elements have been added as necessary) is poured into the ladle 4 and mixed with the molten metal that has been waiting without being treated. ing. The method of combining the hot water is completely free, and one example is pouring it into the ladle 5 as shown in Figure 1.
This can be said to be a very convenient method because it allows you to use the REM placement method. The method of adding Mg-REM can also be changed freely, such as by injection method. Table 1 shows examples of the present invention, and also includes a conventional Mg-Ti alloy addition method as a comparative example, but these experiments were conducted using hot metal of the same composition as raw material and CV graphite cast iron of the same composition. This shows the results of a model experiment planned in advance to obtain the following results.
In addition, in the comparative example shown in Table 1 (B), the entire amount was treated to remove Ti, so the blowing conditions for removing Ti were relatively gentle (O ₂ : 5 to 15 Nm ³ /ton), and the conditions shown in Table 1 (B) were In the example shown in C), the blowing conditions for removing Ti were slightly strengthened (O ₂ :20 to 25 Nm ³ /ton) because a part of the steel was removed from Ti and mixed with untreated steel. Therefore, the C and Ti levels in Example (C) are lower than those in Comparative Example (B), and conversely, the temperature is slightly higher and the temperature is stopped. The blending ratio of (A) and (C) in section (E) was 0.6:0.4.

[Table] As is clear from Table 1 and its explanation, in the present invention, 40% of the desulfurized hot metal that is the raw material is removed from Ti.
We succeeded in obtaining a cast product with almost the same composition as the comparative example simply by subjecting it to treatment, and the results of microscopic observation of both the specimen structure and the solid structure showed that it was a good CV graphite cast iron. confirmed. Furthermore, the Mg-Ti alloy used for CV treatment and
The composition of the Mg-REM alloy was as shown in Table 2.

[Table] The effects of the present invention have been substantially confirmed through the basic examples shown in Tables 1 and 2. However, as understood from the above explanations, special attention must be paid to the By combining the results of various experiments, it was found that the Ti level should be adjusted to within the range of 0.02 to 0.08% after mixing. If Ti is less than 0.02%, the antagonizing effect of Mg and REM on promoting the spheroidization of graphite is insufficient, and spheroidization tends to proceed too much.
If it exceeds 0.08%, the schistose tissue will increase and the intended purpose of CV conversion will not be achieved. A more preferable range is 0.03 to 0.06%. Therefore, the allocation of raw materials to be subjected to Ti removal treatment from the desulfurized hot metal, as well as the Ti removal target for the raw materials, must be comprehensively judged in consideration of the Ti content of the hot metal, and can be uniformly determined. isn't it. Next, we will explain the appropriate amount of Mg-REM alloy to be added. First, we conducted a preliminary experiment on the technology of adding Mg-Ti alloy or Mg-REM alloy after desulfurizing and deTining the entire amount, and we will show the results from that. .

[Table] In Table 3, the pretreated molten metals are divided into groups according to the S content, and Groups 2 and 3 are classified according to the amount of added alloy. In addition, the MT series in the same table indicates the Mg-Ti alloy addition group, and the MR
The series indicates an addition group of Mg-REM alloys. First of all, looking at Group 1, in MT-1, even if the upper limit of the standard addition amount is added, 95
While more than % of tissues were converted to CV, MR-
In No. 1, graphite was already completely spheroidized (SG) even though the same amount was added. In group 2, the amount of molten S is increasing, so it is consumed by S.
The amount of Mg should also increase, and the reference amount has been shifted to a larger value, but according to the results of adding the upper limit in the same way, the CV conversion rate is over 95% in MT-2, but 90% in MR-2. More than % of the particles were spheroidized. On the other hand, Group 3 targets the same molten metal, Mg--Ti alloys with lower limit stiffness, and Mg--Ti alloys with an amount below the lower limit.
Although REM alloy is added, both CV
Graphite was obtained. To summarize these experimental results, in the case of Mg-Ti alloys, all showed good CV conversion rates within the standard addition amount set according to the amount of S, but in the case of Mg-REM alloy, the CV conversion rate was good within the standard addition amount set according to the amount of S. If you do not set it to the lower limit or below, the CV rate will worsen,
At the upper limit of the standard addition amount, almost all graphite appeared to be spheroidized. Therefore, in order to find out the appropriate amount of Mg-REM alloy added to the amount of S in the combined hot water method of the present invention,
A similar experiment was conducted using a 40:60 mixture of Ti-treated and untreated molten metal. In other words, it is divided into groups according to the S content, as in the previous example.
When the relationship between the amount added and the CV conversion rate was determined for each MT series and MR series, the results shown in Table 4 were obtained. In addition, "substantive FC" in the same table means that flaky graphite was observed in the substantive tissue.

[Table] As you can see from the group, there is a good
The MR series that gives the CV rate has a lower additive level than the MT series, and in the group, as with MR-3 in Table 3, by reducing the amount of Mg-REM alloy additive. We have succeeded in improving CV conversion rate. In this way, we learned that the appropriate addition range for Mg-REM alloy addition is slightly lower than that for the Mg-Ti alloy addition method, so we varied the molten metal S amount in order to clarify this range. 95% on sample
When we determined the appropriate range that would give the above CV rate, we obtained the results shown in Figure 2. As shown on the horizontal axis in Figure 2, Mg and REM have a spheroidizing effect.
This is because we thought that the effect would be on the same level, and this was also confirmed experimentally. In conclusion, it was found that the appropriate addition range for Mg + REM is extremely narrow, and is limited to the range shown by 1.97 [S] + 0.022% ≧ [Mg + RE] ≧ 2.0 [S] + 0.012%. was determined to be a necessary condition of the present invention. Since the present invention is constructed as described above, it is not necessary to subject the entire amount of the desulfurized molten metal to the Ti removal treatment, which contributes to improving productivity, and also allows the user to know the appropriate addition range of Mg-REM alloy, albeit within a narrow range. Since the amount required was small, it became possible to reliably produce CV graphite cast iron without incurring high costs.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the flow of the present invention, and FIG. 2 is a graph showing the appropriate addition range of Mg-REM alloy.

Claims (1)

[Claims] 1 Desulfurized blast furnace hot metal is used as a raw material, divided into two parts, subjected to Ti removal treatment, and then adjusted in composition as necessary, and then combined with the other untreated raw material. A compact vermitter characterized by adjusting the Ti level of the combined hot water to fall within the range of 0.02 to 0.08% by weight, and adding Mg-REM alloy so as to satisfy the following conditions. Method of manufacturing graphite cast iron. 1.97 [S] + 0.022% ≧ [Mg + REM] ≧ 2.0 [S] + 0.012% (In the above formula, [S] is the weight% of S in the hot water,
[Mg + REM] means the total weight percentage of Mg and REM added to the combined water, respectively)