JPS5931407B2 - Roll for Sendzimir rolling mill and its heat treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a roll used in a Sendzimya rolling mill and a heat treatment method for the same, and particularly to a roll used in a Sendzimya rolling mill, and in particular, a roll that is suitable for application to a work roll of a Sendzimya rolling mill, or a 1.2 intermediate roll. The present invention relates to a roll for a Sendjimya rolling mill that has excellent accident resistance and a heat treatment method for the same.

The work rolls for the Sendzimya rolling mill are made of cold die steel (JIS 5KD-11 equivalent) and high speed steel, and the 1.2 intermediate rolls are made of hot die steel (JIS 5KD-11). 61 equivalent material) is used.

In addition, as for the heat treatment method, for die steel series, oil quenching is performed from the quenching temperature, followed by air cooling from just above the martensitic transformation temperature, and for high speed steel series, a method is adopted in which heat bath quenching at 400 to 450°C is followed by air cooling. has been done.

On the other hand, since these materials have good hardenability, they are hardened both inside and outside the roll, and the cooling temperature when passing through the martensitic transformation temperature of the roll surface and center during air cooling is approximately 10 to 20°C. Cooled differentially.

Therefore, the residual stress becomes a transformation stress type, that is, a stress distribution of surface tension and center compression.

The value of the tensile stress remaining on the surface is, for example, +5 to 7 kg for 60φ class and +10 to 15 kg for -1240φ class.
my? Generally, it is t.

However, the presence of such a large tensile residual stress on the surface may promote peeling accidents (hereinafter referred to as spalling) if the rolled material seizes on the roll due to a rolling error during the use of the roll. Also, when used with cracks or inclusions remaining, spalling is promoted and the so-called accident resistance is impaired.

SUMMARY OF THE INVENTION An object of the present invention is to provide a roll for a Sendzimir rolling mill that eliminates the above-mentioned conventional drawbacks and has excellent accident resistance, and a heat treatment method for the same.

The present invention provides rolls used in a Sendzimir rolling mill in which the tensile residual stress in the tangential direction on the roll surface is a small value that hardly causes cracks to develop in the longitudinal direction, or
The above objective is achieved by creating compressive stress.

In addition, in the heat treatment method for the rolls used in the Sendzimir rolling mill, during cooling from the quenching temperature, the roll surface is slowly cooled from just above the martensitic transformation temperature to room temperature so that the temperature inside and outside the roll is kept almost uniform. The tensile residual stress in the tangential direction is set to a small value that hardly causes cracks to develop in the longitudinal direction.

In the heat treatment method of the present invention, tensile residual stress is reduced or compressive stress is applied not only in the tangential direction of the roll but also in the longitudinal direction.

Furthermore, in a heat treatment method for a roll used in a Sendzimir compressor, only the roll surface layer is hardened by induction heating hardening, so that residual stress in the tangential direction of the roll surface becomes compressive stress.

This compressive stress is 2 kg/- or more, especially 5 to 1.0 kg
/ma, the object of the present invention is achieved.

The present invention will be explained in detail below.

The reason why the surface residual stress after heat treatment of the rolls for Sendzimya rolling mill becomes tensile stress is because, as mentioned above, the inside and outside are uniformly quenched, and there is a temperature difference between the inside and outside of the roll during cooling, so the surface undergoes martensitic transformation first. This is because the center begins to metamorphose later.

Therefore, by creating a temperature difference between the inside and outside of the roll during cooling, it is possible to bring the residual stress close to zero by causing martensitic transformation at the same time inside and outside the roll.

In one example of the present invention, at the time of quenching, the
Quenched in a salt bath at 0℃ to equalize the inside and outside temperatures, then transferred to an electric furnace or lime slow cooling box maintained at the same temperature, and heated for 20 minutes with almost no temperature difference inside and outside. By cooling to about 50℃ or less at a slow cooling rate of ℃/hour or less, especially 10℃/hour or less, the surface residual stress can be minimized (
It was designed to do so.

In addition, in order to make the surface into a compressive stress type, it is necessary to raise the quenching temperature only to the necessary parts from the surface, and to keep the temperature at the center below the transformation point, or even if the temperature rises to the center, the temperature at the center is lower than the surface. It is necessary to keep the temperature low so that the center transforms first and the surface transforms later.

Therefore, by employing, for example, an induction heating hardening method using electromagnetic waves with a frequency of 1 kHz or higher, the objective can be achieved.

Specifically, the steel used for the rolls for the Sendzimya rolling mill has good hardenability and is highly susceptible to quench cracking, so first the entire roll is heated to 500 to 550°C.
Then, gradually preheat to 750-800℃ using induction heating equipment, and further heat to 30-50℃ from the appropriate quenching temperature for each steel type.
It is designed to be heated by induction to a temperature higher than 10°C and then cooled by a blast cooling method.

Therefore, it is possible to easily harden only the surface layer, and the surface residual stress can be converted into compressive stress.

According to experiments, 2 kg/i? more than 5k, especially 5-10k
It was found that a compressive stress of g/mA can be generated.

Of course, after induction heating and hardening, tempering is performed at a low temperature of 150 to 250°C or a high temperature of 450 to 530°C.

Examples will be described below.

Carbon content 1,51% by weight, silicon content 0.44% by weight, manganese content 0.51% by weight, chromium content 1
1.76% by weight, molyfden content 0.88% by weight, vanadium content 0.71% by weight, balance substantially iron, diameter 133mm, length 1200m
Experiments were conducted by applying various heat treatments to the cylindrical roll material.

That is, the sample with sample number 1 was subjected to the same heat treatment as the conventional one, and the whole body was heated at 1030°C, then oil quenched, and then heated to 280°C.
The temperature was raised above ℃ and air-cooled.

In addition, sample No. 2 was heated to 1030°C, then heated to 3°C.
It was quenched in a heat bath at 50°C, and after 20 minutes, it was transferred to a round electric furnace with an inner diameter of 1200 mm and a depth of 2100 mm maintained at 350°C, and the cooling rate was 5°C/hour when passing the martensitic transformation temperature. It was cooled in a furnace so that

Furthermore, the sample No. 3 was pretreated by heating to 1030°C and oil-cooling to 650°C.
The sample was charged into induction heating equipment, first the surface temperature was kept at 750°C, and the sample was gradually preheated at a feed rate of 100 mm/min.
It was heated gradually at a surface temperature of 1070° C. and a feed rate of 80 mm for 1 minute, and at the same time, the lower part of the heating coil was blast cooled downward at 45° C. for quenching.

In addition, for all samples, after quenching, 510~
Tempering was performed three times at 515° C. for 1.5 hours.

The surface residual stress of each sample measured by the resistance wire strain gauge method was as follows.

That is, in the conventional sample No. 1, the residual stress in the tangential direction was +4.8 kg/ma, and the residual stress in the axial direction was +4.8 kg/ma.
5.7 kg/ma, for sample number 2,
For sample number 3, the residual stress in the tangential direction is +0.3 kg/mA, the residual stress in the axial direction is 0.9 kg/ma,
The residual stress in the tangential direction was -17.1 kg/myA, and the residual stress in the axial direction was -16.5 kg/m4.

Note that the symbol - in residual stress indicates compressive residual stress.

As is clear from the experimental results, according to the heat treatment method of the present invention, the residual stress on the roll surface becomes a small value that hardly causes cracks to develop in the longitudinal direction, or the residual stress in the tangential direction on the roll surface becomes compressive stress. can do.

The roll was cut at the center and the hardness distribution inside and outside was measured, and the results are shown in FIG.

Sample number 1.2 shows almost uniform hardness both inside and outside the roll, but sample number 3, which was produced by induction heating, has a hardness of around Hs65 at the center, indicating that the effect of temperature gradient quenching has decreased. It is noticeable.

Note that when 1 kHz electromagnetic waves are used in induction heating, it is possible to reliably harden 5 to 15 mm from the surface.

If this frequency is below 1 kHz, there is a risk that the center will be heated.

As explained above, the present invention provides rolls used in the Sendzimya rolling mill so that the tensile residual stress in the tangential direction on the roll surface becomes a small value or compressive stress that hardly causes cracks to develop in the longitudinal direction. Therefore, it has the excellent effect of significantly improving spalling resistance and cracking resistance, that is, accident resistance.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between heat treatment conditions and hardness distribution inside and outside the roll.

Claims (1)

[Scope of Claims] 1. In a roll used in a Sendzimir rolling mill having residual stress in the tangential direction on the roll surface, the residual stress has a tensile residual stress that hardly causes cracks to develop in the longitudinal direction;
A roll for a Senzimya rolling mill characterized by having compressive stress. 2 The tensile residual stress in the tangential direction of the roll surface is 2
A roll for a Sendzimya rolling mill according to claim 1, wherein the roll is less than or equal to kg/-. 3 The residual stress in the tangential direction of the roll surface is 2 kg/
A roll for a Sendzimya rolling mill according to claim 1, which has a compressive stress of - or more. 4. In the heat treatment of the rolls used in the Sendzimya rolling mill, during the cooling from the quenching temperature, the rolls are slowly cooled from just above the martensitic transformation temperature to around room temperature, so that the temperature inside and outside the rolls is kept almost uniform, and the roll surface is A method for heat treating rolls for a Sendzimya rolling mill, characterized in that tensile residual stress in the tangential direction is set to a small value that hardly causes cracks to develop in the longitudinal direction. 5. The method for heat treating rolls for a Sendzimya rolling mill according to claim 4, wherein the cooling rate from just above the martensitic transformation temperature to room temperature is 20° C./hour or less. 6. A Sendzimya rolling mill characterized in that in the heat treatment of the rolls used in the Sendzimya rolling mill, only the roll surface layer is quenched by an induction heating quenching method so that residual stress in the tangential direction on the roll surface becomes compressive stress. Heat treatment method for rolls. 7 The depth of the surface layer to which the hardening is applied is 5 to 15 mm.
A method for heat treating rolls for a Sendzimya rolling mill according to claim 6.