JPS5933169B2 - Heat treatment method for high alloy composite cast iron rolls - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an economical method for textural stabilization heat treatment of high alloy composite cast iron rolls.

High-alloy cast iron rolls for rolling rolls are generally manufactured as composite rolls by centrifugal casting or the like.

This is to reduce residual stress during casting and thermal stress during use by making the surface of the roll body high-alloy cast iron, increase the strength of the neck part, and improve economic efficiency.As shown in Figure 1, It is composed of an outer shell 1 made of high-alloy cast iron and a core material 2 made of high-grade cast iron or ductile cast iron.

Typical outer shell materials for this high-alloy composite cast iron roll are shown in the table below.

The structure of the above shell material contains many carbides and graphite (excluding chilled rolls), and is composed of martensite, bainite, and retained austenite, and its shell hardness is Hs6.
It is 5-90.

After casting, such high-alloy composite rolls are generally cooled to approximately room temperature and then loaded into a cart-type heat treatment furnace in several rolls, where they are heat-treated according to a temperature curve as shown in FIG.

That is, the entire roll was heated at 300 to 400°C for 20 to 50 minutes.
Perform heat treatment for a period of time.

The main purpose of this heat treatment is to stabilize the structure of the high alloy part of the outer shell, and the high alloy cast iron is mainly made of Ni and M.

This is because untransformed austenite remains at room temperature during as-casting due to the high content of austenite, which is made stable by heat treatment.

That is, since problems arise when residual austenite is transformed by heat during use, it is first transformed into stable bainite through heat treatment before use.

By the way, it is only the high-alloy cast iron part of the roll outer shell that requires stabilization of its structure, and the core part is mainly made of pearlite-based high-grade cast iron or ductile cast iron, so such heat treatment is not necessary. It's not really necessary.

From this point of view, the present invention aims to provide a rational and economical heat treatment method that heats only the necessary parts to obtain the desired heat treatment effect when performing structure stabilization heat treatment on this type of roll. The heat treatment method of the present invention is characterized in that, during the structure stabilization heat treatment of a high alloy composite cast iron roll, the structure stabilization heat treatment is performed by heating only the required outer shell portion made of high alloy cast iron. be.

The invention will now be described in detail with reference to an example of its implementation illustrated in FIGS. 3a and 3b.

The composite roll 3 after casting is supported on rotating rollers 4, 4' that rotate it, and in this state, the roll 3 is surrounded by a split heating device 5, 5' close to the surface of its body. At the same time, both neck portions are each covered by a cap-like heat-insulating cover 6,6'.

The heating device has left and right semicircular heating elements 5, 5' each equipped with heating means such as a group of burners on the inner surface to surround the roll 3, and each heating element 5, 5' 7 and 7', and is configured to be able to move away from the roll 3 in the left and right direction at any time.

In addition to the above-mentioned heating apparatus, a heating apparatus for use in the present heat treatment method may be, for example, one in which an electric resistance heating band is wound around the surface of the body, and the top is covered with a heat insulating belt.

When subjecting the roll 3 to the intended structure stabilizing heat treatment, it is continuously rotated at low speed by the rotating rollers 4, 4', and the required heat treatment made of high-alloy cast iron is heated by the heating devices 5, 5'. Only the surface of the body of the roll 3 is heated so that the temperature of the outer shell portion is raised to a processing temperature of 300 to 400° C. and maintained for a predetermined period of time.

At this time, of course, the roll core and both side necks are indirectly heated by heat conduction from the outer shell which is directly heated as the processing time increases, but in reality, the outer shell remains at the processing temperature for a predetermined period of time. All you have to do is operate the furnace so that it is maintained at

Note that the roll 3 is equipped with heating devices 5, 5' and a heat insulation cover 6.
6', the dissipation of its thermal energy is minimized.

In this way, the heat treatment method that heats only the required outer shell portion of the roll 3 requires far less total thermal energy than the conventional heat treatment method that uniformly heats the entire roll. Unlike processing furnaces, there is no idle operation time, and economical heat treatment can be performed.

Furthermore, if the roll 3 is heated and heat treated while rotating, a uniform heat treatment effect can be obtained without any temperature unevenness over the entire circumference of the roll body surface, that is, a roll with a uniform structure can be obtained. This is effective in shortening the heat treatment time, compared to the conventional heat treatment method in which the roll had to be heated and maintained for that much more in order to equalize the temperature of each part.

A roll that has been heat-treated only on the necessary parts as described above can be processed into a product as is, but the diameters of the roll body and neck are different, and the outer shell of the body is partially heat-treated. This may cause a volume change and residual strain at the base of the neck.

Therefore, in order to be safer, after the above heat treatment, the entire roll is loaded into a conventional heat treatment furnace before the temperature of the roll body decreases, and the entire roll is subjected to strain relief heat treatment. can be administered.

When loading this roll, it may be loaded simultaneously with other rolls, and since the purpose is to remove distortion, the processing time is approximately 10 hours (based on a body diameter of 800φ and a unit weight of 10 tons or less). ) is sufficient.

Next, specific examples of the heat treatment method of the present invention will be described.

<Example> Body diameter 800φ, body length 178011, outer shell thickness 60mm,
A composite roll for a hot strip mill finishing work roll having the following chemical composition was heat treated by the method of the present invention in which only the surface of the shell was heated using the LPG burner one-type heating device as shown in FIG. 3 described above.

However, the heat treatment conditions for the roll are as follows: Roll rotation speed: 1 rotation/min Heating temperature: 400°C (body surface) Heating holding time: 8 hours (wet the outer shell position of 60 dragons from the surface, and this part temperature is 380°
After the above heat treatment, when the surface temperature reached 80° C., the roll was further placed in a conventional strain relief heat treatment furnace, and the entire roll was subjected to strain relief heat treatment.

As a result of measuring the outer shell structure, mechanical properties, and hardness of the roll heat-treated as described above, almost the same results as those obtained by the conventional heat treatment method were obtained, confirming that the desired heat treatment effect was obtained.

At this time, conventional heat treatment methods require a treatment time of 20 to 50 hours, whereas 8 hours is sufficient for the present method, which is an efficient and economical heat treatment method.

The heat treatment method of the present invention described in detail above can be applied to at least Ni
For high-alloy cast iron rolls, the outer shell is made of high-alloy cast iron containing Mo and Mo as alloying elements and in which untransformed austenite remains until room temperature when cast, and the core is made of high-grade cast iron or ductile cast iron mainly based on pearlite. Since only the outer shell portion is heated to 300 to 400°C and a structure stabilizing heat treatment is performed to transform the outer shell base structure into a structure consisting of bainite and martensite, conventional structure stabilizing heat treatment method (heat treatment of the entire roll) is performed. This method has the effect of being able to perform structure stabilizing heat treatment more efficiently and economically than the method of heating in a furnace).

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the structure of a composite roll, FIG. 2 is a diagram showing a heat treatment temperature curve for high-alloy cast iron, and FIGS. They are a sectional view and a front view. 1... Outer shell, 2... Core material, 3...
・Composite roll to be heat treated, 4, 4'...
Rotating roller, 5, 5'... Heating device, 6, 6
'... Heat insulation cover, 7, 7'... Traveling trolley.

Claims (1)

[Claims] 1 A high-alloy cast iron roll whose outer shell is made of high-alloy cast iron that contains at least Ni and Mo as alloying elements and in which untransformed austenite remains at room temperature during as-casting, and whose core is made of high-grade cast iron or ductile cast iron mainly based on pearlite. In contrast, a high-alloy composite cast iron roll characterized in that only the outer shell portion is heated to 300 to 400°C to perform a structure stabilizing heat treatment to transform the outer shell base structure into a structure consisting of bainite and martensite. Heat treatment method.