JPH0378175B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a centrifugally cast composite roll for rolling steel, and the present invention relates to a centrifugally cast composite roll for rolling steel, and the present invention relates to a roll having an outer shell layer of high-alloy grain cast iron and an inner layer of spheroidal graphite cast iron, and which has excellent internal and external properties. It is related to the manufacturing method. [Prior Art] Among rolls for rolling steel, work rolls for hot rolling in particular are required to have various properties such as wear resistance, accident resistance, roughening resistance, and spalling resistance.
Manufacturing composite rolls using centrifugal casting has become commonplace. Furthermore, recently there has been a demand for stronger inner layers for reasons such as energy saving and compact rolling mills. The materials of hot rolling work rolls that correspond to these rolling technology trends include, for example, C: 3.0 to 3.5%, Si: 0.6 to 1.0%, Mn: 0.4 to 1.0 in weight ratio in the outer shell layer.
%, Ni: 4~5%, Cr: 1.2~2.0%, Mo: 0.2~
Using high-alloy grain cast iron with a concentration of 0.8%, the inner layer contains, for example, C: 2.5-3.5%, Si: 0.8-1.5%, Mn: 0.2
~0.6%, Ni: 0.5~1.5%, Cr: 0.1~0.6% high-grade cast iron was used, but for the inner layer, for example, C: 2.8~3.5%, Si: 1.5~2.5%, Mn:
0.2-0.8%, Ni: 0.5-2.0%, Cr: 0.1-0.6%,
High-strength spheroidal graphite cast iron with Mg: 0.02 to 0.10% has come into use. However, regarding the manufacturing method, a centrifugal casting method is used in which an outer shell layer similar to conventional inner layer high-grade cast iron is injected, and an inner layer is injected after solidification. An abnormal layer of graphite was formed, resulting in a significant decrease in mechanical strength and poor spalling resistance. Therefore, as a method for improving the boundary properties when the inner layer is made of spheroidal graphite cast iron, for example, the patent
1068098, a method of providing an intermediate layer as a third layer between the outer shell layer and the inner layer has been taken. [Problems to be solved by the invention] As mentioned above, work rolls for hot rolling are required to have tougher inner layers, and spheroidal graphite cast iron has come to be used. In the case of manufacturing, an abnormal layer of flaky graphite was formed at the boundary, the mechanical strength was significantly lowered to 20 to 30 Kgf/mm ² , and the spalling resistance was poor. The reason for this is that compared to high-grade cast iron, spheroidal graphite cast iron has a higher content of graphitizing elements C and Si, and at the boundary where graphitization is likely to occur, C contained in the outer shell layer is segregated and added due to centrifugation. Furthermore, since Mg in the inner layer, which has a spheroidizing effect, reacts with S in the outer shell layer and disappears as MgS sulfide, a large amount of graphite crystallizes and grows as flake graphite.
It is thought that this causes a decrease in material strength. In addition, since spheroidal graphite cast iron undergoes large solidification shrinkage, defects such as cavities and reverse chills are likely to occur in the center.To prevent this, it is necessary to achieve sufficient graphitization in the inner layer, but the graphite at the boundary This is contradictory to the suppression of naturalization. Further, although the method of casting an intermediate layer as the third layer can mutually solve these problems, it complicates the manufacturing process and increases costs. The purpose of the present invention is to overcome these problems and obtain a centrifugally cast composite roll using spheroidal graphite cast iron for the inner layer, which has a tough boundary and good internal properties, without complicating the manufacturing method. It is said that [Means for Solving the Problems] The present invention provides a centrifugally cast composite roll using high-alloy grain cast iron for the outer shell layer and spheroidal graphite for the inner layer.
By reducing S in the outer shell layer to 0.020% or less, which causes Mg in the inner layer to disappear as MgS at the boundary, and further securing 0.030 to 0.070% Mg in the inner layer, even if Mg disappears to some extent, the spheroidization effect can be maintained. to have,
It is possible to suppress the growth of flaky graphite at the boundary, and the inner layer is injected in parts, making the injection thickness of the first inner layer in contact with the outer shell layer as thin as 10 to 70% of the outer shell layer thickness. By diffusing Cr, which is a white metalizing element, from the outer shell layer and suppressing graphitization at the boundary, the injection flow is not inoculated into the first layer that is in contact with the outer shell layer. By doing it to the inner layer other than
It is characterized by promoting only internal graphitization without promoting boundary graphitization, and improving the internal properties while improving the boundary area. Furthermore, according to this method, the third layer does not need to be newly melted, and the working process does not become complicated. [Operation] The following items are regulated in this invention:
The reason for setting the range will be explained. First, the S of the outer shell layer
The S content and the Mg content in the inner layer are interrelated, and if the S content in the outer shell layer increases, the Mg content in the inner layer must also increase to prevent the spheroidization effect at the boundary. However, if Mg is increased beyond 0.070%, the tendency to become white inside increases, causing defects such as cavities. Therefore, we fixed the outer layer S at 0.020% or less, and set the upper limit for the inner layer Mg.
It was set as 0.070%. The lower limit of Mg, 0.040%, was set within a range that would not disrupt graphite spheroidization at the boundary. Next, regarding the thickness of the first injection of the inner layer, if it is less than 10% of the thickness of the outer shell layer, whitening due to Cr diffusion from the outer shell layer becomes excessive, and conversely, weakening of the boundary part occurs. Furthermore, since graphitization at the boundary was not significantly suppressed when the thickness exceeded 70%, the thickness was regulated at 10 to 70%. Also, regarding the second and subsequent inner layer vaccinations,
If the Si content is less than 0.05%, internal graphitization will not be sufficient, whereas if it exceeds 0.4%, it will become excessive and defects such as cracks may occur in the outer shell layer during cooling.
It was regulated at 0.05-0.40%. The present invention provides a centrifugally cast composite roll in which the outer shell layer is made of high-alloy grain cast iron and the inner layer is made of spheroidal graphite cast iron, and the properties can be improved by suppressing graphitization at the boundary and preventing disappearance of Mg, and at the same time, the spalling resistance can be improved. The inside has the function of promoting graphitization, preventing defects, and improving breakage resistance. [Example] Below, an example in which the present invention is applied to a hot-rolled post-finish roll will be described in comparison with a centrifugally cast composite roll manufactured by a conventional method. In the example of this method and the comparative example of the conventional method, as shown in FIG. A composite roll was manufactured by injecting spheroidal graphite cast iron into inner layers 2 to 5 in parts. Table 1 shows the components of the outer shell layer and inner layer in this case, divided into the present method and the conventional method.
The chemical components of this method are characterized in that, compared to conventional methods, the S content in the outer shell layer is 0.010% or less, and the Mg content in the inner layer is 0.040 to 0.060%. Next, regarding the split injection method for the inner layer, in the conventional method, the injection thickness of the first inner layer 2 that is in contact with the outer shell layer is 80% of the injection thickness of the outer shell layer, and the remaining amount is In contrast, in this method, the injection thickness of the first inner layer is 35% of the injection thickness of the outer shell layer, and the remaining amount is similarly injected from the second layer. It was divided and injected into the fourth layer. The injection flow inoculation 11 to each layer is as shown in Table 2. In the conventional method, the first inner layer was inoculated with a Si content of 0.05%, but in this method, this layer was not inoculated. Furthermore, for the second and subsequent layers, the vaccination rate was 0.07% in the conventional method, but 0.15% in this method.
and double the amount was added. Figures 2 and 3 show the microstructure at the boundary of the roll produced in this way. In contrast to the conventional method, in which flaky graphite aggregates and crystallizes, in the present method, almost none of this is observed. It is a good quality spheroidal graphite cast iron. Further, the tensile strength of the boundary portion was as shown in FIG. 4, and was improved to 48-50 Kgf/mm ² while it was 24-25 Kgf/mm ² in the conventional method.
Furthermore, as seen in the macrostructures of FIGS. 5 and 6, the internal properties of the roll are such that in the conventional method there is a reverse chill in the center, but in the present method, this is almost not observed.

【table】

〔Effect of the invention〕

As explained above, when manufacturing a centrifugally cast composite roll using high-alloy grain cast iron for the outer shell layer and spheroidal graphite cast iron for the inner layer, the present invention involves injecting the inner layer in parts and injecting the first layer thinly. This suppresses graphitization near the boundary, reduces the S content in the outer shell layer, and reduces residual
Securing Mg promotes spheroidization, which has the effect of preventing agglomeration of flaky graphite generated at the boundary and improving tensile strength. In addition, by applying injection flow inoculation to the second and subsequent inner layers, it is possible to suppress graphitization at the boundary while promoting graphitization only in the center, which has the effect of preventing defects such as nesting and reverse chill. be.

[Brief explanation of drawings]

Fig. 1 is a diagram for explaining the centrifugal casting method in the present invention, and Fig. 2 shows high-alloy grain cast iron in the outer shell layer.
Figure 3 is a photograph of the boundary microstructure of a roll manufactured by the conventional method using spheroidal graphite cast iron for the inner layer. Figure 3 is a photograph of the boundary microstructure of a roll manufactured by this method. Figure 4 is the boundary between the conventional method and this method. FIG. 5 shows a cross-sectional macrostructure sketch of a roll manufactured by the conventional method, and FIG. 6 shows a cross-sectional macrostructure sketch of a roll manufactured by the present method. 1...Outer shell layer, 2...1st inner layer, 3...2nd inner layer, 4...3rd inner layer, 5...4th inner layer,
6...Outer mold, 7...Inner mold, 8...Mold, 9...
... injection funnel, 10 ... ladle, 11 ... injection stream inoculant.

Claims (1)

[Claims] 1. In a centrifugally cast composite roll in which the outer shell layer is made of high-alloy grain cast iron and the inner layer is made of spheroidal graphite cast iron, the S content of the high-alloy grain cast iron in the outer shell layer is 0.020 by weight.
% or less, and furthermore, the Mg content of the spheroidal graphite cast iron in the inner layer is 0.030 to 0.070%, and the inner layer is injected in parts, and the injection thickness of the first inner layer in contact with the outer shell layer is 10% of the outer shell layer thickness. A method for manufacturing a centrifugally cast composite roll, characterized in that the inner layer molten metal other than the first layer is inoculated with an amount of Si of 0.05 to 0.40%.