JPS5836649B2 - Method for manufacturing hot rolling mill work rolls - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a work roll for a hot rolling mill,
In particular, the present invention relates to a work roll heat treatment method suitable for use in hot rolling stainless steel.

Hot rolling mill zero work rolls have traditionally been made of cast iron.

However, due to the lack of mechanical strength, the surface of the roll tends to become rough, so it is not necessarily suitable as a material for work rolls.

In particular, in the hot rolling of stainless steel, there is a strong tendency to be reluctant to transfer the rough texture of the roll onto the rolled plate, so there has been great resistance to using cast iron work rolls.

An object of the present invention is to provide a method for manufacturing work rolls that have less rough surface than those using conventional cast iron during hot rolling of steel, particularly stainless steel.

The present invention provides a work roll for a hot rolling mill by quenching an alloy tool steel material for a hot die from an austenitic state temperature.
It is tempered at a temperature below the austenite transformation point to form a base structure of tempered martensite.

The work roll of the present invention has the advantage that the amount of surface roughness is smaller and the degree of surface roughness is finer than the nickel grain rolls conventionally used as stainless steel hot rolling work rolls.

The reason we came up with the idea of using alloy tool steel for hot dies as work rolls for hot rolling mills was to use discarded cold rolling work rolls made of tool steel for hot rolling of ordinary steel. When I tried using it, I unexpectedly got good results, so
Then I got a hint.

It was a pure idea to apply waste rolls from cold rolling work rolls to hot rolling of ordinary steel.
The alloy tool steel for tatami mat hot molds is Japanese Industrial Standard JISG44.
04 symbol SKD4, SKD5, SKD6, SKD61
, SKD62, SKT2, SKT3, SKT4, SKT
5 and SKT6 are applicable.

Among these, S has a large amount of chromium and is hard to harden.
KD6, SKD61 and SKD62 are preferable, and SKD61 and SKD are particularly suitable for hot pressure differential of stainless steel.
62 is preferred.

When using SKD61 and SKD62, the shore hardness (Hs) of the roll surface, which is generally required for work rolls for hot rolling of stainless steel, is around 80, or more specifically, the hardness is 80±5. can be easily obtained by

In addition, the quench hardening depth is 40W from the surface of the work roll.
It is possible to reach a depth of around Il, and the generally required requirement of being able to use 6071gl in diameter can be fully satisfied.

The chemical properties of SKD6, SKD61 and SKD62 are shown in Table 1.

Making the base structure of the work roll into tempered martensite is necessary in order to improve surface roughness while satisfying the conditions of surface hardness and quench hardening depth, or to get as close to the above requirements as possible. .

However, if the tempering is insufficient, the structure will contain retained austenite.

If the tempering is sufficient, no residual austenite will be produced, and the structure will become one in which carbide precipitation has begun on the base of tempered martensite.

The latter structure in which carbides are precipitated exhibits an extremely excellent effect on preventing roll roughness.

The work roll according to the present invention is manufactured by sequentially going through an ingot-forming process, a forging process, and a heat treatment process, and then finally finishing and inspecting, as in the case of general steel rolls.

The heat treatment step includes quenching and tempering, but may also include spheroidizing annealing, which may be performed before quenching in some cases.

Regarding the temperature conditions for quenching, tempering, and spheroidizing annealing, the conditions selected in JIS G4404 can be applied.

Regarding the means of quenching, it is important that the work rolls of hot rolling mills not have rough skin, but also have little wear.For stainless steel work rolls for hot rolling, in order to suppress wear, As already mentioned, a Shore hardness (Hs) of around 80 is usually required.

Such high hardness cannot be obtained unless the quenching temperature is set to a high temperature, and when the work roll is made of alloy tool steel for hot molds as in the present invention, JIS G44 is required.
As selected in No. 04, a high quenching temperature of about 1000°C is required, specifically a temperature of 1000-1050°C.

There are two ways to harden from this temperature: medium-frequency stationary heating followed by blast cooling, or low-frequency gradual heating followed by gradual blast cooling to avoid raising the temperature inside the roll. It will be done.

On the other hand, there are rolls that have internal holes and rolls that do not have internal holes, and when considering the stress generated during roll operation, it is said that rolls without internal holes are more advantageous.

When a roll is made to have non-directed holes in medium-frequency stationary heating, the temperature at the center of the roll increases, resulting in an increase in tensile stress due to quenching.

For this reason, it is necessary to heat rapidly and keep the time at the quenching temperature as short as possible.

However, if this is done, the temperature variation during quenching becomes large, and the surface hardness and hardening depth become uneven.

In other words, it is no longer suitable as a work role.

On the other hand, the low-frequency gradual heating method can eliminate temperature variations during quenching even when using a roll without holes.

In the case of low-frequency gradual heating, it is heated rapidly and the retention time in the austenitic state is very short.
There is a concern that it is difficult to dissolve carbide into the base.

In order to eliminate this concern, it is desirable to perform a preheat treatment to facilitate solid solution of the carbide in the matrix.

The method of preheat treatment consists of heating the roll in its entirety to quench it and then tempering it.

During hardening, it is better to apply a decarburization inhibitor to the entire surface of the roll.

The quenching folding temperature and tempering temperature conditions in the pretreatment are the same as those of the subsequent low frequency gradual heating quenching and tempering.

Blast cooling is used as a countermeasure against water or oil quenching, which can result in excessive quenching and cracking.

This blast cooling method is carried out, for example, by sending compressed air using a compressor and blowing it onto the work rolls.

In order to smoothly transition from low frequency gradual heating means to blast cooling means, install a cooling pipe under the low frequency heating coil and connect an air hose to this pipe.
It is possible to send compressed air using a compressor.

If it is thought that the cooling effect cannot reach the inside of the work roll sufficiently with the cooling pipe alone, it is desirable to further cool the entire roll using a jet fan or the like to suppress heat recovery from inside the roll.

The tempering temperature after blast cooling is JISG4.
The temperature range of 550 to 650° C. selected in No. 404 can be applied, but the temperature range may be changed somewhat depending on how much the target surface hardness of the roll is set.

Example An alloy tool steel equivalent to SKD62 having the chemical content shown in Table 2 is ingotted and subjected to a forging process and a heat treatment process to form a diameter of 4.
A work roll with non-directed holes with a length of 20 m and a body length of 1500 m was produced.

Then, 200 tons of stainless steel SUS420 was rolled using a hot rolling four-stand top roll.

The rolling operation was performed at a relatively low temperature of around 700°C.

The rolling reduction ratio was 35 to 40.

As for the heat treatment, first, spheroidizing annealing was performed, and then preheat treatment was performed to perform quenching and tempering.

After spheroidizing annealing and pretreatment, rough machining and medium machining were performed, respectively.

Spheroidizing annealing must be performed by heating and maintaining the austenitic state temperature, and in this example, 88
A two-stage cycle was performed in which the temperature was once held at 0°C, then cooled to 700°C, and the temperature was held constant again at that temperature, followed by furnace cooling.

The hardness after washing was 31 to 32 in Shore hardness Hs.

Pre-heat treatment involves heating the entire roll in an electric furnace to an austenitic temperature, then oil quenching it to 650°C.
This was done by tempering it.

Since it is oil quenched, there was concern that tensile stress would be generated in the center of the roll, leading to defects, but it was found that as a countermeasure to this, there would be no problem if the final cooling temperature was controlled at 250°C or higher. Therefore, the roll surface is 250
When the temperature reached ℃, it was transferred to the same 250 ℃ gas furnace and cooled in the furnace.

During quenching, a decarburization inhibitor was applied to the entire surface of the roll.

The structure after the preheat treatment was tempered martensite, and it was confirmed that carbides that had once melted into the matrix during quenching and heating were precipitated.

The quenching after the preheat treatment is carried out by gradual low-frequency heating followed by gradual blast cooling.As shown in FIG. Place the holder or the combination, connect the air hose to this cooling pipe 4, and connect the compressor (pressure 5~
5. This was done by supplying compressed air at a rate of 5 Kt/cr/t).

After blast cooling, the entire roll was cooled with a jet fan to suppress recuperation from the inside.

The quenching temperature was 1030° C., and the feed speed of the work roll 1 was 60 M/min.

Tempering is done once at 510℃ and twice at 520℃, total 3 times.
I went around.

The first two times are aimed at decomposing retained austenite, and the second time is aimed at creating a tempered martensite structure.

Figure 2 shows the hardness distribution of the roll after tempering.

The shore hardness (Hs) of the work roll is 81 to 8 on the surface.
2, and the center part had a value of about 40, which is almost the same as that at the time of the preheat treatment.

The hardened layer reached a depth of 508 mm from the surface of the body.

In addition, the structure after tempering was satisfactory, with carbide precipitated on the base of tempered martensite.

The residual stress on the surface of the body was measured in the roll axis direction using an X-ray method, and the result was a compressive residual stress of 30 KP/1.

The roll was used as a 4-stand roll in a hot rolling mill, and 200 tons of SUS 420 was rolled and assembled. The surface roughness of the roll was examined, and the center part of the body was found to be in good condition with no roughness. , Slight roughness was observed at the end of the roll body, where both ends of the strip passed.

The rough skin was a collection of fine pine needle patterns.

However, when compared to when a conventional cast iron-based nickel grain roll was used, the amount of surface roughness was significantly less, and the extent of the roughness was extremely fine.

In other words, the condition of rough skin was significantly improved.

As described above with respect to the embodiments, according to the present invention, it is possible to obtain a work roll for a hot rolling mill with less rough surface than when a conventional cast iron roll is used.

Further, according to the present invention, it is possible to improve surface roughness resistance while ensuring other requirements that a work roll should have, namely the surface hardness and quench hardening depth necessary to maintain a high level of wear resistance. can.

Therefore, it is particularly suitable as a work roll for hot rolling, such as stainless steel, which should avoid roughening of the roll surface.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing the relationship between distance from the roll surface and hardness. 1... Roll, 2... Low frequency furnace, 3... Heating coil, 4... Cooling pipe, 5... Roll rotation direction,
6...Roll movement direction.

Claims (1)

[Scope of Claims] 1. In a method for manufacturing a work roll for a hot rolling mill, the work roll is made of alloy tool steel for a hot die, and after being forged into a roll shape, the temperature of the austenitic state is reduced by low-frequency gradual heating. Production of a hot rolling mill work roll characterized in that it is heated to , blast-cooled and quenched, and then tempered at a temperature below the austenite transformation point to provide a base structure of tempered martensite. Law. 2. The method of manufacturing a hot rolling mill work roll according to claim 1, wherein the work roll is a holeless roll selected from symbols SKD6, SKD61, and SKD62 of JIS G4404. 3. After forging alloy tool steel for hot molds into a roll shape, it is heated to an austenite state temperature by low-frequency progressive heating, then blast-cooled and quenched, and then tempered at a temperature below the austenite transformation point. In a method for manufacturing a hot rolling mill work roll which is treated to give a matrix structure of tempered martensite, the entire roll is heated to an austenitic state temperature before the low frequency progressive heating and then quenched to form a carbide in the austenite matrix. 1. A method for producing a work roll of a hot rolling mill, comprising a pre-heat treatment step consisting of a step of dissolving the austenite into a solid solution and a step of tempering from a temperature below the austenite transformation point. 4. After forging alloy tool steel for hot molds into a roll shape, the entire roll is heated to an austenitic state temperature and then quenched to form a solid solution of carbides in the austenite base, and tempered from a temperature below the austenite transformation point. After performing pre-heat treatment consisting of the following steps, it is then heated again to the austenite state temperature by low-frequency gradual heating, blast-cooled and quenched, and then tempered at a temperature below the austenite transformation point to form tempered marten. A method for manufacturing a hot rolling mill work roll that provides a site base structure, comprising a spheroidizing annealing step of heating and holding at an austenite state temperature before the preheat treatment step. How to make rolls.