JPH0776407B2 - High chrome cast iron material with graphite having excellent wear resistance and rough surface resistance - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a high chromium cast iron material mainly used as a roll material, which is excellent in wear resistance, surface roughening resistance, and seizure resistance.

(Prior Art) A high-chromium cast iron roll material is characterized by having a high hardness of M ₇ C ₃ -direction chrome carbide in the matrix structure in terms of metallurgy and having excellent wear resistance. A composite roll in which an outer layer as a rolling use layer is formed of such a high chromium roll material is widely used as a work roll for a hot strip mill.

However, the high chromium cast iron roll material has a problem that it has a low thermal conductivity and is apt to cause seizure.

Therefore, the applicant of the present invention has filed Japanese Patent Publication Nos. 61-16415 and 6
As disclosed in No. 1-16336, graphite was crystallized in a high chromium structure, and a graphite crystallized high chromium cast iron material having significantly improved seizure resistance was developed. With a rolling roll having this graphite crystallized high chromium cast iron material as a rolling use layer, conventionally,
When special steel such as stainless steel, which is said to be prone to seizure, was rolled, no seizure occurred and good results were obtained.

(Problems to be Solved by the Invention) However, in the above-described graphite crystallized high chromium cast iron material, the crystallized graphite has a large particle size of about 80 μm or more, and the graphite interval is large, so that the graphite is not worn or surface-damaged The starting point,
There is a problem of poor abrasion resistance and rough surface resistance.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a high chromium cast iron material excellent in wear resistance and surface roughening resistance without impairing seizure resistance.

(Means for Solving the Problems) The present invention made to achieve the above object has a chemical composition of C: 2.6 to 3.6% by weight%, Ni: 3.0 to 4.4% Si: 1.0 to 1.9%, and Cr: 5 to 10% Mn: 0.5 to 1.5%, Mo: 0.4 to 3.0% The balance consists essentially of Fe, and the graphite crystallized in the structure and the fine graphite precipitated by the decomposition heat treatment of the chromium carbide held at the austenite region temperature The coexistence of is a constitution of the invention.

(Function) The chemical composition of the high chromium cast iron material having the graphite of the present invention is limited for the following reasons. The unit is% by weight.

C: 2.6-3.6% C combines with Cr to form Cr carbide, and also Si, Ni described later.
Fine graphite is crystallized by the graphite forming element of. 2.
If it is less than 6%, the amount of Cr carbide is reduced and the crystallization of graphite is almost eliminated. On the other hand, if it exceeds 3.6%, the Cr content is set to 10% or less in the case of the present invention, so that the supersaturated C content is reduced.
However, due to the high concentration of Si and Ni, it is graphitized in a large amount, and the amount of crystallized graphite becomes excessive and wear resistance deteriorates.

Si: 1.0 to 1.9% Si is necessary to crystallize graphite in high chromium materials.
If it is less than 0%, this effect is hardly present, and if it exceeds 2.3%, the amount of graphite becomes excessive and wear resistance is deteriorated. Therefore, for safety reasons, it is set to 19% or less as a range in which wear resistance does not deteriorate. Regarding the addition of Si, it is better to keep the amount of Si before casting lower than the above-mentioned limited range, and to inoculate during casting to adjust the composition of the final product so that it falls within the above range in crystallization of graphite. It is valid.

Mn: 0.5-1.5% Mn is positively added together with Si for deoxidation of the molten metal. 0.
If it is less than 5%, this effect is insufficient, and if it exceeds 1.5%, mechanical properties, particularly toughness, are significantly deteriorated.

Ni: 3.0 to 4.4% Ni is positively contained in order to improve the matrix structure and crystallize graphite. If it is less than 3.0%, crystallization of graphite becomes difficult. On the other hand, if it exceeds 5.0%, the crystallized graphite becomes coarse and the amount is too large as in the case of Si. Therefore, for safety reasons, it should be 4.4% or less. In addition, the retained austenite increases, which becomes a problem in terms of resistance to rough skin.

Cr: 5-10% Cr combines with C to form Cr carbide, but if it is less than 5%, the amount of the carbide is small and the wear resistance is poor, and if it exceeds 10%, it is graphite in the composition range of Ni and Si described above. Becomes difficult to crystallize.

Mo: 0.4-3.0% Mo increases quenching and tempering resistance and enters into carbides,
It is effective in increasing the carbide hardness and at the same time improving the temper softening resistance. If the content is less than 0.4%, such an effect is small. On the other hand, if it exceeds 3.0%, the tendency toward white pig iron is strong and crystallization of graphite becomes difficult.

In addition to the above, the high chromium cast iron material of the present invention contains the balance substantially Fe.
However, since P and S generally make the material brittle, it is desirable that the content be small, and both are preferably kept to 0.1% or less. In addition, Nb and V are effective in refining the cast structure, and instead of a part of Fe, each can be contained alone or in a combination of 1.0% or less.

The above high chromium cast iron material has a grain size of 10
Fine granular graphite of about 40 μm can be crystallized.
Such fine-grained graphite is unlikely to become a starting point of wear and roughening and can obtain good characteristics, but seizure resistance is insufficient because the amount of crystallization is relatively small. Therefore, in the present invention, in order to secure good seizure resistance without impairing wear resistance and surface roughness, chromium carbide is decomposed and heat treated to deposit fine graphite in the matrix. The particle size of the precipitated graphite is about 5 μm or less, and the particle spacing is about 10 μm, and it is possible to improve not only seizure resistance but also crack resistance without deteriorating properties such as wear resistance. it can.

After the casting, the decomposition heat treatment is performed by holding the austenite region temperature for several hours to several tens of hours. Casting strain is also removed by the heat treatment. After the heat treatment, an appropriate heat treatment is continuously performed.

(Example) The high chromium cast iron material having graphite of the present invention is mainly used as an outer layer material which is a used layer of a rolling composite roll.
As the inner layer (axial core) material, a cast iron material having high toughness such as high-grade cast iron or ductile cast iron or a cast steel material such as graphite cast steel is appropriately used.

Further, as the method for producing the composite roll, after casting the outer layer by the centrifugal force casting method, the centrifugal force casting mold having the outer layer is erected to form a stationary mold, and the inner layer material molten metal is contained therein. There is a method in which the outer layer and the inner layer are welded and integrated by pouring, and it is generally applied because it is simple.

When welding the outer and inner layers, Cr from the outer layer to the inner layer
In order to prevent the mixture of the above, an intermediate layer may be provided between the outer layer and the inner layer. By providing the intermediate layer, not only the toughness of the inner layer can be secured, but also the quality of the welded portion between the outer layer and the inner layer can be improved.

The composite roll cast as described above is subjected to the above-described carbide decomposition heat treatment for reducing residual stress and graphite precipitation, and then subjected to heat treatment for transforming retained austenite into a thermally stable phase. . Usually, after the carbide decomposition heat treatment, air-cooling quenching or spray water quenching quenching is performed at a controlled cooling rate, and then strain-relieving or tempering heat treatment is performed by holding at 500 to 600 ° C, and the base is treated with secondary carbide and pearlite, or A method of using a mixed phase of tempered martensite and pearlite is adopted.

The high chrome material of the present invention can be applied not only as a roll material but also as a roller material in rolling auxiliary equipment, for example, as an outer layer material of a hollow cylindrical roller such as a hot run table roller.

Next, specific examples will be described. Product body diameter φ680m
A composite roll having a length of m, a body length of 1800 mm and a total length of 4200 mm was manufactured as follows.

(1) As the outer layer, a high-chromium cast iron melt having the chemical composition (% by weight) shown in Table 1 below was used, and this was cast in a mold rotating on a centrifugal casting machine at a casting temperature of 1410 ° C. and a wall thickness of 80 mm (casting Weight included 2 ^T 400 kg) Cast.

(2) The outer layer was completely solidified 23 minutes after the start of casting.

(3) After that, the mold was erected vertically, and ductile cast iron was cast as an axial core material at 1365 ° C from the upper part to completely fill the inside of the mold, and then the upper end was covered with a feeder heat insulating material.

(4) After completely cooling, the roll material was taken out from the mold, and after being held at 1000 ° C. × 10 hr for carbide decomposition and strain relief heat treatment, it was forcibly air-cooled to 550 ° C., held at 550 ° C. × 15 hr, and then cooled to room temperature. Then, it machined and obtained the product roll.

(5) As a result of ultrasonic flaw detection of the body of the product roll, it was confirmed that the outer layer and the shaft core were completely welded. The results of observing the structure of the outer layer with an optical microscope are shown in FIGS. 1 and 2. FIG. 1 is the one after the heat treatment, and FIG. 2 is the one before the heat treatment, both at a magnification of 50 times. As is clear from FIG. 1, a state in which relatively fine crystallized graphite and fine precipitated graphite of about 5 μm coexist due to heat treatment is observed.

(Effects of the Invention) As described above, according to the high chromium cast iron material having the graphite of the present invention, since fine grain crystallized graphite and finely dispersed precipitated graphite coexist in the matrix structure, seizure resistance is improved. Without deteriorating, it is possible to effectively prevent the deterioration of wear resistance and surface roughness due to the presence of coarse-grained graphite, and fine or fine-grained graphite effectively prevents the development of cracks. It is also possible to improve the crack resistance, and thus the breakage resistance. Further, since the present invention material has a relatively low Cr content, when the present invention material and the tough material are compounded, there is an advantage that the deterioration of the toughness of the tough material due to the mixing and diffusion of Cr can be relatively slight. .

[Brief description of drawings]

FIG. 1 and FIG. 2 are photographs of a metal structure in a manufacturing process of a high chromium cast iron material having graphite according to an embodiment (both have a magnification of 50).
FIG. 1 shows the structure after the graphite precipitation heat treatment, and FIG. 2 shows the structure before the heat treatment.

Claims (1)

[Claims] 1. Chemical composition by weight% C: 2.6-3.6%, Ni: 3.0-4.4% Si: 1.0-1.9%, Cr: 5-10% Mn: 0.5-1.5%, Mo: 0.4-3.0% The balance consists essentially of Fe and is characterized by the coexistence of graphite crystallized in the structure and fine graphite precipitated by the decomposition heat treatment of the chromium carbide held at the austenite region temperature. High-chromium cast iron material with excellent graphite properties.