JP2792755B2 - Hearth roll manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION This invention is, each for the heat treatment of steel plate or the like
Of hearth rolls used in various types of furnaces, especially electronic
The present invention relates to a method for manufacturing a hearth roll by beam welding .

[0002]

2. Description of the Related Art Generally, large and thick cast materials have a low solidification rate and therefore have a coarse grain size after casting. It is also known that even small castings have a pronounced tendency due to chemical components, such as austenitic stainless steel. In the grain boundaries of the coarse crystal grains, many gas components such as impurities, hydrogen, oxygen, and nitrogen are present. Therefore, when electron beam welding is performed to join such a casting material, these impurities intervening in the crystal grain boundaries and gas components such as hydrogen, oxygen, and nitrogen are gasified, expanded, and segregated at the weld. Blowholes occur. For this reason, the beam is poor and the weld becomes unhealthy. Therefore, electron beam welding is not suitable for a casting having a coarse crystal grain size, and is currently performed by MIG welding or TIG welding.
The application of the electron beam welding to iron-based materials has been limited to, for example, rolled materials and forged materials having a small crystal grain size.

As an application example of MIG welding of a casting material having a coarse crystal grain size, there are hearth rolls used in various heating furnaces, soaking furnaces, preheating furnaces, etc. for heat-treating steel sheets. Conventionally, the following method has been known as a method for manufacturing this kind of hearth roll. First, the parts that make up the hearth roll are:
The shaft, axle, and barrel are made of a heat-resistant cast material having heat resistance and oxidation resistance so as to withstand the atmosphere during use. Typical manufacturing methods and materials are as follows: the shaft is SUS310 material by forging, and the axle is SCH1 by sand casting or die casting.
9 materials etc., the barrel is relatively heat-resistant such as preheating furnace, annealing furnace, etc.
For equipment with low requirements for oxidation resistance, use Y
US materials are used, and SCH22 materials and the like are used as high alloy members by centrifugal casting for equipment requiring high heat resistance and oxidation resistance, such as a heating furnace and a soaking furnace.

[0004] The above-mentioned components are assembled by welding as described below. First, the welding between the shaft and the axle will be described. The shaft is manufactured by forging, and the axle is manufactured by sand casting or die casting. Both members are subjected to V-groove processing at the joint, and then MIG welding and TIG welding are performed.

Next, welding of the axle and the barrel will be described. The barrel is manufactured by centrifugal casting as described above. At this time, a mold is used as a mold for centrifugal casting,
In order to prevent seizure between the molten metal and the mold during casting, coating is performed on the inner surface of the mold. In general, a water-soluble coating material widely used in centrifugal casting rolls is used as a coating material.

Here, the procedure of applying this water-soluble coating material to the inner surface of the mold is performed as follows.
First, the mold is heated, and then a water-soluble mold material is applied to the inner surface of the heated mold. Thus, on the inner surface of the mold,
After the coating material is applied and dried, the melt is cast into a mold to produce a barrel. After the coating material is applied and dried on the inner surface of the mold, before the molten metal starts to be cast into the mold, time is required due to the incorporation of the mold into the centrifugal casting facility, a wait for adjusting the components of the molten metal, and the like. And the coating material that has been coated and dried in advance absorbs moisture. As the gas components to be absorbed and their amounts, for example, 300 to 500 PPM of nitrogen, 3 to 4 hydrogen
PPM, oxygen 300 PPM, etc. Therefore, if the thickness of the coating material to be applied to the mold is too thin, and the molten metal is cast into the mold, the gas absorbed by the coating material enters the molten metal, causing air bubbles and the like in the product. Defects occur. Therefore, the thickness of the water-soluble coating material applied is set to about 3 mm so that the gas absorbed by the coating material does not lose a place to escape and also takes into consideration the ease of application. On the other hand, if the thickness is too small, uniform coating is difficult, and the coating material may be partially peeled off from the mold, which is not preferable.

[0007] As described above, when the molten metal is cast into a mold coated with a thickness of about 3 mm, the cast molten metal is gradually cooled because of the heat insulating effect of the coating material, and the structure after cooling has a coarse crystal grain size. Austenitic structure. Therefore, when electron beam welding is carried out for welding the axle and the barrel having an austenitic structure with a coarse grain size after casting, welding of a casting material with a coarse grain size is performed because the beam passage is poor and the weld becomes unsound. MIG welding and T which are generally applied as methods
It is constructed by IG welding.

[0008]

The above-mentioned conventional hearth
MIG welding and TIG welding, which are used as a welding method for a casting material having a coarse crystal grain size, such as welding of a barrel of a roll and an axle, have the following problems.

In MIG welding and TIG welding, the heat input during welding is high, so that components having a relatively low melting point, such as NbO ₂ , contained in the base metal are melt-precipitated, and micro cracks are formed at the boundary of the welding line. Occurs as If the welding heat input during welding is reduced in order to prevent this, the base metal and the deposited metal will have poor fusion, so that a healthy welded portion cannot be obtained and the welding time will be increased. In addition, a welding groove such as a V groove is required for welding, and thus a large amount of filler metal is required. Therefore, the residual stress after welding was high and heat treatment was required after welding.

[0010]

The gist of the present invention for solving the above-mentioned problems is as follows. (1) is composed of a centrifugal casting steel barrel and cast iron axle and forged axis, the barrel and the axle, in the manufacturing method of the hearth roll comprising a axle and shaft welded respectively, prior to production of the barrels, After forming a coating film by thermal spray coating on the inner surface of the mold for manufacturing the barrel, casting by a centrifugal casting method, the entire surface of the barrel body, the depth of the barrel chill crystal having a thick section of the welded joint of the barrel. Integrally formed with
And joining the barrel and the axle by electron beam welding. (2) A chill crystal is formed at the weld of the axle and the barrel
(1) The method for producing a hearth roll according to (1) .

[0011]

In manufacturing <br/> forming barrel that make up the [operation and EXAMPLES hearth rolls, after forming a thin coating to the inner surface of the mold by spraying the mold wash, by centrifugal casting, molten metal is cast,
By being rapidly cooled, the barrels, chill crystals depth has a thickness cross-section of the joint <br/> manual welding of barrels are formed integrally, the crystal grain size fine at the weld Therefore, the inclusion of gas components such as impurities, hydrogen, oxygen, and nitrogen in the grain boundaries of the fine crystal grains is extremely reduced. Thus, even when electron beam welding is applied, sound welding can be performed without the gas components being gasified and expanded, segregating in the welded portion, and the beam passing is not deteriorated. In addition, the heat input during welding is low, and there is no occurrence of micro cracks caused by conventional MIG welding or TIG welding. Furthermore, since the volume of the molten metal in the weld (joint) portion is extremely small, the residual stress after welding is low, so that heat treatment after welding is not required, deterioration of the material due to heat treatment is prevented, and the life of the hearth roll is correspondingly reduced. improves. In addition, electron beam welding has remarkable functions and effects, such as a high welding speed, so that the production (welding) time is greatly reduced.

As one embodiment of the present invention, a hearth roll having a barrel diameter of 800 mm shown in FIG. 1 was welded and manufactured.
In FIG. 1, the hearth roll includes a shaft 1, an axle 2, and a barrel 3. The shaft 1 is forged with SUS310 material, the axle 2 is cast with SCH19, and the barrel 3 is a spray-coated mold with a mold material of about 1 mm. Manufactured by casting.

The method of thermal spray coating on a mold was carried out as follows. That is, after the mold was preheated, the powder material for the mold was charged into a CO ₂ -acetylene gas burner and applied by spraying. Therefore, no water is contained in the coating. The barrel 3 was cast by the centrifugal casting method using the above-mentioned mold. FIG. 2 shows the state of formation of the chill crystal 4 in the barrel 3 in this case. As is apparent from the figure, a chill crystal 4 having a depth of about 28 mm was formed over the entire thick section of the joint welded portion of the barrel 3 due to the cooling promoting effect of the mold and the thin-walled thermal spray coating mold.

Axle 2 and barrel 3 manufactured as described above
Were welded and constructed in a vacuum by electron beam welding, which is the method of the present invention. FIG. 3 shows the state of welding of the cross section of the welded portion. As is clear from the figure, the welded portion is limited to an extremely narrow region and a sound structure without welding defects such as microcracks and blowholes is obtained. Was.

As described above, according to the method of the present invention, it is possible to perform electron beam welding of a casting having a coarse crystal grain size, and a high-quality joint can be manufactured at low cost. Further, the following effects were found by using the thermal spray coating mold as compared with the case of using the conventional water-soluble coating mold. Since the adhesion to the mold after thermal spraying is strong, there is no peeling after coating, and the coating thickness can be reduced. In addition, there was no moisture absorption after casting, so that blowhole defects during casting could be suppressed, and cooling of the molten metal after casting was promoted due to its higher thermal conductivity than conventional water-soluble casting materials.

Next, in order to confirm the operation and effect of the present invention,
As a comparative example, the barrel 3 was molded using a conventional water-soluble mold, the thickness of the mold was about 3 mm, the barrel 3 was cast, and the axle 2 and the barrel 3 were constructed by conventional MIG welding. . FIG. 4 shows the state of formation of the chill crystals 4 in the barrel 3 in this comparative example. As is clear from the figure, the depth of the chill crystals on the outer surface is as shallow as about 5 mm.

FIG. 5 shows the state of welding of the cross section of the welded portion of the comparative example. This comparative example is an example in which the welding heat input was reduced to prevent micro cracks during welding.
As a result, poor fusion 7 into the base material and poor fusion 8 in the welding beam occur.

FIG. 6 shows the results of the embodiment of the present invention and the comparative example.
The measurement result of the residual stress before and after the heat treatment of the weld is shown. As is clear from the figure, the residual stress of the embodiment of the present invention before and after the heat treatment was 10 to 1 in both the circumferential direction and the axial direction.
As low as 5 kgf / mm ² , there is no problem in use. Therefore, heat treatment after electron beam welding can be omitted. In the comparative example, the residual stress before heat treatment was about 30 kgf /
mm ^2, a high heat treatment axial direction about 50 kgf / mm ² is required, about 30 kgf / mm ² in the axial direction even after the heat treatment is left.

FIG. 7 shows the results of the embodiment of the present invention and the comparative example.
4 shows the results of a hardness test of a weld. As is clear from the figure,
The hardness of the heat affected zone of the embodiment of the present invention is slightly higher than the hardness of both members, and its range is limited to a narrow range. The hardness of the weld is the hardness of both members. In the comparative example, the hardness of the heat affected zone is considerably higher than the hardness of both members, and the range is wide. Also, the hardness of the weld is lower than that of both members.

Next, the barrel diameter of the barrel 3 is 650, 800, 1
A 200 mm hearth roll was prepared in the same manner as in the previous embodiment.
After forming a chill crystal in the welded portion, it was welded and manufactured by electron beam welding, and was actually supplied to an actual furnace as a hearth roll for a heat treatment furnace for a steel sheet. As a result, in each case, there was no defect in the welded portion after production, the soundness was good, and the durability during use was excellent.

In this embodiment, when forming the chill crystal 4 in the welded portion of the barrel 3, a mold material and a mold thickness are selected as means for controlling the cooling rate of the molten metal after casting.
In other words, a 1 mm thick thermal spray coating was performed on the inner surface of the mold,
The method for forming the chill crystal 4 is not limited to this. For example, after casting with a conventional water-soluble coating mold, barrel 3
Heat or quench the whole or only the joint welded part and chill crystal 4
May be formed.

Although the barrel 3 has been described above, the formation of the chill crystal 4 of the other members is not limited to the use of a material having a large cooling effect for a chill or a sand-type aggregate in other sand-type cast materials. Crystal 4 may be formed. The range of formation of the chill crystal 4 may be such that the entire casting material is chilled as in this embodiment or may be partially chilled.

[0023]

Conventionally, when manufacturing hearth rolls,
Although electron beam welding could not be applied due to the large crystal grain size after casting, the application of electron beam welding became possible by forming chill crystals in the joint welds as in the present invention . As a result, the present invention has the following effects.

The application of electron beam welding has made it possible to produce a high-quality hearth roll with low welding heat input and no occurrence of microcracks in the welded portion. Low welding residual stress eliminates the need for heat treatment to remove residual stress after welding, preventing material deterioration due to heat treatment
This extends the life of the hearth roll. Electron beam welding is compared to conventional MIG welding and TIG welding
All welding speeds are high, which significantly reduces production time.
You.

[Brief description of the drawings]

FIG. 1 shows a cross section of a hearth roll according to an embodiment of the present invention.

FIG. 2 shows the state of formation of chill crystals in a barrel according to an embodiment of the present invention.

FIG. 3 shows a state of welding of a cross section of a welded portion according to an embodiment of the present invention.

FIG. 4 shows a state of formation of chill crystals in a barrel of a comparative example which is a conventional method.

FIG. 5 shows a state of welding of a cross section of a welded portion of a comparative example which is a conventional method.

FIG. 6 shows the measurement results of residual stress before and after heat treatment of the example of the present invention and a comparative example which is a conventional method.

FIG. 7 shows the results of a hardness test of a welded part between an example of the present invention and a comparative example which is a conventional method.

[Explanation of symbols]

 1 Axis 2 Axle 3 Barrel 4 Chill Crystal 5 Granular Crystal 6 Segregation Layer 7 Insufficiency of Penetration into Base Material 8 Insufficiency of Fusion in Weld Bead

──────────────────────────────────────────────────続 き Continued on the front page (72) Katsuhiro Furukawa, Inventor 46-59 Ohara Nakahara, Tobata-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation Machinery & Plant Business Department (56) References JP-A-63-231014 (JP) JP-A-2-55680 (JP, A) JP-A-2-107749 (JP, U) JP-A-59-9003 (JP, Y2) JP-A-48-38488 (JP, Y1) 33-9110 (JP, B1) (58) Fields investigated (Int. Cl. ⁶ , DB name) B23K 15/00 B22D 27/04 C21D 1/00, 9/56 F27B 9/24

Claims (2)

(57) [Claims] 1. A is composed of a centrifugal casting steel barrel and cast iron axle and forged axis, the barrel and the axle, in the manufacturing method of the hearth roll comprising a axle and shaft welded respectively, for the production of the barrels First, after forming a coating film by thermal spray coating on the inner surface of the mold for manufacturing the barrel, casting by centrifugal casting method, the entire surface of the barrel body, the depth of the chill crystal having a thick section of the joint welding portion of the barrel And an axle are formed integrally with the barrel by electron beam welding. 2. A chill crystal is formed in a weld portion of an axle with a barrel.
The method for manufacturing a hearth roll according to claim 1 formed .