JPH11314145A - Continuous casting roll excellent in crack resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting roll excellent in crack resistance, and the manufacturing technique by applying a heating and rapid cooling heat- treatment, particularly after executing padding, with respect to the rolls for continuous casting equipment. SOLUTION: This continuous casting roll is manufactured, by which after padding a chromium base stainless steel containing 0.04-0.22% C, 0.10-0.70% Si, 0.25-1.60% Mn, 10.0-14.8% C, 0.3-1.6% Ni and 0.3-1.8% Mo on the barrel part of the roll, only the roll barrel part is heated at 910-1180 deg.C and held for >=3 min and successively, after cooling to a room temp., further, the whole roll is heated at 560-780 deg.C and after holding for >=30 min, the roll is cooled to he room temp. In this way, the obtd. structure is made to tempered sorbite and Charpy impact value in 2 mm V-notch at the room temp. is made to >=5.5 kgf.m/cm<2> .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting roll of a continuous casting facility, and more particularly to a continuous casting roll excellent in crack resistance, obtained by performing a heating / quenching heat treatment after welding overlaying, and a manufacturing technique thereof. .

[0002]

2. Description of the Related Art As shown in FIG. 8, a continuous casting apparatus of an iron making facility injects molten steel from a ladle 10 into a mold 12 through a tundish 11, which is then cooled with a water-cooled mold and a continuous casting roll. 1, the material to be cast, that is, the slab 13 is continuously manufactured by cooling and solidifying with the external spray water installed in the space 1 and pulling it out. The continuous casting roll 1 has a slab of approximately 2 m / m
While pulling and transporting while correcting the vertical state at a low speed of in or less from a vertical state to a horizontal state that allows easy post-processing,
It has a role of applying a roll pressure to the slab to finish it to a predetermined thickness. Therefore, the continuous casting roll is subjected to rapid cooling by spraying with cooling water after being in contact with the hot slab in one rotation. Is cracked and it evolves.

In particular, in recent years, when casting of a full ladle has been completed and when casting a ladle having a different composition, the molten steel is poured over the tail end of the preceding slab in the mold and the casting is continued as it is. The operation that is called casting is becoming mainstream. In this case, there is a work of laying a partition iron plate at the seam so that different types of steel do not mix in the mold. At that time, the slab must be stopped for a certain time. This condition is called a machine stop, and the presence of the machine stop makes the thermal stress more severe, and the crack growth speed is increasing.

[0004] Usually, a continuous casting roll has a diameter of 400 mm.
In the case of rolls, the depth of the crack is controlled at the critical depth of about 40 to 50 mm. However, if the crack exceeds this limit, the roll is likely to break, so it is necessary to replace the roll according to the crack depth. There is. In general, since the number of rolls in a continuous casting facility is several hundred in one production line, the cost of replacing and recycling the rolls is enormous, and extending the life of the rolls is a very important issue.

[0005] In conventional roll maintenance, it is disadvantageous in terms of cost to renew the roll including the base material. Therefore, in most cases, the repair is repeated, and the procedure is as follows. That is, first, the cracked portion was deleted according to the crack depth generated in the trunk portion, and a material equivalent to the base material was welded (underlaid) on the alloy steel roll base material for machine structure formed by forging. Above, a 13 mm chromium stainless steel with a thickness of several mm is weld-welded (upped), and then held at a temperature of about 650 ° C. for several hours and then gradually cooled, so-called stress relief annealing is performed.
Used after finishing by machining.

Another method is disclosed in Japanese Patent Publication No. 59-14101.
JP-A No. 5-2500, by performing a self-fluxing alloy spray overlay of 0.05 to 0.80 mm thickness on a chrome-based stainless steel weld overlay, a tortoiseshell formed on the outermost hard thermal spray overlay. There is a method of suppressing the development of sharp cracks due to the fine cracks. In addition, Japanese Patent Publication No. 64-48650
No., a chromium-based stainless steel weld overlay is subjected to a nitriding treatment, and a hard nitrided layer formed on the outermost layer,
There is also a method that aims at the same effect as thermal spraying. JP-A-63-4
No. 7,337, there is a method in which quenching heat treatment is performed after chrome-based stainless steel weld overlaying to increase the surface hardness to improve crack resistance and wear resistance.

[0007]

In the conventional roll, the following problems have been encountered in the chromium-based stainless steel weld overlay on the surface to ensure crack resistance. First, the most widely used method is to use stress relief annealing at a temperature of about 650 ° C. after welding overlay, but crack propagation is not sufficient and crack propagation is fast especially in lines with many machine stops, Cracks have become the limiting factor for service life, that is, repair, and have to be replaced in short cycles.

[0008] In the method in which the self-fluxing alloy is subjected to thermal spraying after the weld overlaying, a large number of small cracks are initially generated because the thermal sprayed layer is extremely hard and the thermal conductivity is lower than that of chromium stainless steel. . The fine cracks propagate into the weld overlay and reduce the stress at the crack tip on the surface of the roll, thereby suppressing the formation and propagation of sharp cracks connected linearly in the circumferential direction thereafter. . However, the thermal spray layer peels off early due to severe thermal stress. Even after the peeling, the shape of the crack generated at the initial stage and propagated to the weld overlay layer is advantageous, so that the crack is less likely to grow and propagate as a sharp crack. However, because the lifetime until the sprayed layer is peeled is short, the probability of occurrence is low, but the cracks newly generated after the peeling tend to grow as sharp cracks mixed with the squamous cracks,
The life extension allowance of the crack rate-controlled line as viewed comprehensively is about 1.3 times, and further improvement is desired.

In the method of performing the nitriding treatment after the welding overlay, chromium nitride is formed on the surface of the chromium-based stainless steel and becomes hard, but the corrosion resistance is reduced and the wear resistance is deteriorated. Further, since the crack growth rate is accelerated by the corrosion, the crack resistance is not so much improved. Therefore, nitriding of the chrome-based stainless steel weld overlay is not practical. In the method of performing quenching heat treatment after welding welding of a chromium-based stainless steel, the hardness becomes hard but the toughness is reduced, so that the crack resistance is rather deteriorated and the life determined by the crack is shortened.

[0010] None of the above rolls is satisfactory at present, and there has been a demand for the development of a continuous cast roll having better crack resistance. The present inventors have worked to solve the above-mentioned problem, and in the life in the actual line where the crack is rate-limiting, the crack depth is greatly varied among a plurality of rolls which are simultaneously used and used for the same period. We paid attention to. Therefore, it was considered important to clarify the difference between a roll with a shallow crack and a roll with a deep crack in measures to extend the service life, and the microstructure of typical rolls was analyzed. In addition, the roll is subjected to stress relief annealing for gradually cooling after holding at 650 ° C. for 5 hours after welding build-up of 0.07C-13.8Cr-1Ni-based stainless steel, and incorporating the roll into an actual continuous casting line. Used for one year and three months. Roll diameter 400mm, length 2300mm, underlay weld overlay material is J
It is an alloy steel material equivalent to IS SNCM415. The number of machine stops is about 250 times a month.

The present inventors have obtained the following findings as a result of the analysis. In the long-life roll, the crystal grains of the structure are fine and carbides are finely and uniformly precipitated. (FIGS. 1 and 2 show the microstructures of the long-life roll and the short-life roll, respectively.) The long-life roll has less segregation of Cr at the grain boundaries.
(FIGS. 3 and 4 show the degree of segregation of the long-life roll and the short-life roll, respectively).
It cannot be completely explained even now whether the structures are different as described above, but it is presumed to be probably due to some variation in manufacturing conditions. In any case, it has been concluded that if the structure of the roll having a long life can be stably formed at the time of production, the life of the roll will be prolonged.

[0012] Like a long-life roll of a real line, the fact that the crystal grains are small and carbides are finely and uniformly precipitated improves toughness and crack resistance. The reason is as follows. When the crack depth reaches a certain limit control value, the service life is reached, but the service life determined by the crack is roughly classified into two. That is, the service life is divided into a life until a crack is generated and a life until the crack progresses and reaches a limit.
If the former is called the crack initiation life and the latter is called the crack propagation life,
Crack initiation life depends on strength, and crack propagation life depends on toughness. In the continuous casting roll, since the applied stress is extremely large, early crack generation is inevitable, and the life is governed by the crack propagation life. Therefore, it is considered that the high toughness greatly contributes to the extension of the life determined by the crack.

On the other hand, since the continuous casting roll is used in an environment with a large amount of cooling water as described above, corrosion is a factor for accelerating crack growth. In general, the rate of crack growth increases with the addition of corrosion factors in addition to fatigue due to pure stress. Cr segregated at the grain boundary should exist in the form of chromium carbide, but segregation of chromium carbide at the grain boundary causes a potential difference between the grain of the structure and the grain boundary and causes local battery corrosion, which lowers corrosion resistance. Let it. Therefore, it is considered that the low segregation of Cr improves the corrosion resistance and contributes to the improvement of the corrosion fatigue strength.

The problem to be solved by the present invention is to stabilize the microstructure and homogenization of the weld overlay and the reduction of Cr segregation at grain boundaries in order to improve the crack resistance of the roll and extend the life. It is to provide a continuous casting roll that can be realized.

[0015]

The gist of the continuous casting roll excellent in crack resistance of the present invention is as follows. That is,
A chrome-based stainless steel having a chemical composition considering quenching and weldability is weld-welded on the surface of the roll body in contact with the material to be cast, and then quenching and tempering heat treatment is performed.

In the conventional method of manufacturing by stress relieving annealing after welding overlaying, at about 650 ° C. of stress relieving annealing, the crystal structure is at a level below the transformation point at which the crystal structure changes to austenite. -Has no effect of controlling the crystal structure. Therefore, the structure of the weld overlay is determined by the cooling conditions during the weld overlay operation. In the welding overlaying work, two or more layers are usually formed in the thickness direction while continuously arranging the weld bead. However, due to the heat effect of the preceding and succeeding beads, the bead boundary and the inside of the bead, and further the first layer・ The second layer… the organization is different. In the conventional weld overlay, it depends on the chemical composition and the preheating conditions during welding overlay.
Normally, the structure is clearly divided into ferrite and pearlite or bainite, which is a mixed phase of carbide and ferrite. In addition, there is a variation due to the influence of the temperature difference of each part in the weld overlay, and the structure becomes heterogeneous as a whole. As a result, a structure having insufficient toughness, strength and corrosion resistance is obtained, and crack resistance is inferior.

Therefore, the roll of the present invention is made of a chromium-based stainless steel material having a chemical composition satisfying both weldability and hardenability by welding, performing quenching and tempering by controlling the heating temperature and cooling conditions. This improves the toughness and corrosion resistance and significantly improves the crack resistance by making the structure dense and free from chromium carbide segregation at the grain boundaries. The features are as follows.

First, a quenching heat treatment is performed on a roll of chrome-based stainless steel which is welded. This is because the crystal structure is refined by recrystallization by heating to an appropriate temperature in a region where the crystal structure is transformed into austenite, and the carbide forming element is sufficiently dissolved in austenite. By rapidly cooling from this state to room temperature, a fine crystalline state in an austenite state is secured, and an extremely hard martensite structure in which carbide forming elements such as C are dissolved in supersaturation is obtained. Thereafter, in tempering, the temperature is raised to and maintained at an appropriate temperature equal to or lower than the austenite transformation point temperature, and the carbide-forming element dissolved in supersaturation is uniformly precipitated in the structure as a dense granular carbide to form a so-called sorbite structure. This lowers the excessively high hardness (strength) to an appropriate level and greatly improves toughness.

Depending on the chemical composition such as a large C content, the quenching / tempering heat treatment may be performed after the stress relief annealing in order to prevent the occurrence of cracking or distortion due to welding residual stress. May be better.

As a method of quenching, in the method of heating the whole of the roll in a normal furnace and then immersing it in a bath containing a cooling medium, the entire roll is oxidized and the deformation becomes large. In order to modify the layer, it is necessary to build up the welds including the shaft portions where the bearings at both ends of the roll are fitted, which greatly increases the repair cost. Therefore, as a method of performing quenching while minimizing the deformation of the roll, a method has been invented in which only the surface layer of the roll body is heated in the atmosphere by a flame or an induction heating coil and then cooled by a refrigerant.

As a result, the weld overlay of chromium stainless steel undergoes appropriate quenching and tempering heat treatment to form a tempered sorbite structure with almost no deformation of the entire roll, and the structure becomes denser and tougher. Significantly improved.
In addition, since the segregation of Cr at the grain boundaries is reduced, the promotion of crack propagation involving corrosion can be significantly reduced. The gist of the technical features of the present invention described above is as follows. (1) By weight%, C: 0.04 to 0.22%, Si: 0.
10 to 0.70%, Mn: 0.25 to 1.60%, C
r: 10.0 to 14.8%, Ni: 0.3 to 1.6%,
Mo: After chromium stainless steel containing 0.3 to 1.8%, the balance being Fe and unavoidable impurity components being welded on the roll body, only the roll body is 910 to 1180 ° C.
And hold for more than 3 minutes, then cool to room temperature,
Further, after the whole roll is heated to 560 to 780 ° C. and held for 30 minutes or more, it is cooled to room temperature to obtain a tempered sorbite structure in which ferrite and granular carbide are finely and uniformly distributed. Charpy 2mmV
Continuous casting roll with notch impact value of 5.5 kgf · m / cm ² or more and excellent crack resistance. (2) The overlay material of the roll described in (1) further contains Nb: 0.5.
A continuous casting roll having excellent crack resistance, characterized by being a chromium stainless steel containing up to 2.0%. (3) The overlay material of the roll according to (1) or (2) further comprises one or more of V, Ti or Al in a total amount of 0.1%.
A continuous casting roll excellent in crack resistance, characterized by being a chromium-based stainless steel containing 01 to 0.30%. (4) The heating to 910 to 1180 ° C. in (1) may be performed by using one or more of a plurality of torches having a structure in which a plurality of torches directed radially of the roll are arranged in a ring-shaped holder slightly larger than the outer diameter of the roll. It is performed by surrounding the roll with a flame burner and moving the flame ring burner in the roll axis direction,
After that, one or more cooling devices having a structure in which a ring or a plurality of nozzles are provided in the ring-shaped header tube in the direction of the roll radius in the rear of the flame ring burner are installed. Or, while spraying water or air from the nozzle, move at the same time as the flame ring burner, or treat it by allowing it to cool naturally, and then heat it to 560-780 ° C. by heating the entire roll with a heating furnace. A continuous casting roll with excellent crack resistance, characterized in that the cooling is performed by forced air cooling using a fan, natural cooling, or cooling in a furnace. Manufacturing method. (5) The heating to 910 to 1180 ° C. in (4) is replaced with flame burner heating, an induction heating coil is provided in a ring-shaped holder slightly larger than the outer diameter of the roll, the roll is surrounded, and the induction heating ring coil is heated. A method for producing a continuous cast roll having excellent crack resistance, wherein the roll is processed by moving in the roll axis direction.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The features of the present invention and the grounds for limiting the numerical range will be described below. As a weld hardfacing material, as a hardenable stainless steel, the composition is expressed by weight%, C: 0.04
０．２0.22%, Si: 0.10 to 0.70%, Mn:
0.25 to 1.60%, Cr: 10.0 to 14.8%,
Ni: 0.3 to 1.6%, Mo: 0.3 to 1.8%, limited to the balance consisting of Fe and unavoidable impurity components. The basis for limiting the components is as follows.

C: 0.04 to 0.22% C is an element that controls hardenability and plays an important role in mechanical properties such as strength. C enlarges the austenite region in the quenching temperature range and makes it easy to obtain a complete martensitic structure after quenching. When C is less than 0.04%, the ratio of martensite is low in the quenching completion state, and so-called complete quenching is not performed. If it exceeds 0.22%, the possibility of weld cracking increases, and even when heated to a quenching heating temperature, C cannot be completely dissolved in austenite, carbides remain at grain boundaries, and toughness and corrosion resistance decrease. I do. Therefore, the content of C is set to 0.04 to 0.22%.

Si: 0.10 to 0.70% Si has a role as a deoxidizing agent for the weld metal and also has an effect of increasing the strength. To achieve the effect, at least 0.10% or more of Si must be added. is necessary. On the other hand, as the content increases, the austenite region in the quenching temperature range is slightly narrowed, and the hardenability is impaired. In addition, welding cracks are likely to occur.
This becomes remarkable when it exceeds 0.70%. Therefore, the content of Si is set to 0.10 to 0.70%.

Mn: 0.25 to 1.60% Mn fixes S, which is an unavoidable impurity, as MnS, eliminates its harm, and increases the strength. However, if the content is too large, the toughness is reduced. If it is less than 0.25%, no improvement in strength can be expected, and if it exceeds 1.60%, the toughness is significantly reduced. Therefore, the content of Mn is set to 0.25 to 1.60%.

Cr: 10.0 to 14.8% Cr is homogeneously dissolved in the structure, the surface of the roll in contact with oxygen is made of chromium oxide to improve corrosion resistance, and the content of C contained in stainless steel and Within the range corresponding to the content of other carbide forming elements and the carbon affinity, chromium carbide is formed and uniformly precipitated in the structure to increase the strength, particularly the high-temperature strength. If the Cr content is less than 10.0%, corrosion resistance and strength cannot be ensured, and both wear resistance and crack resistance decrease. Corrosion resistance and strength improve as the content increases, but if it exceeds 14.8%, the austenite region in the quenching temperature range is narrowed, so that quenching also becomes difficult. Therefore, the content of Cr is 10.0 to 14.8.
%.

Ni: 0.3 to 1.6% Ni is an element that expands the austenite region in the quenching temperature range and improves the quenchability. It also improves corrosion resistance. This effect cannot be expected if Ni is 0.3% or less.
If it exceeds 1.6%, the hardenability will be too good, the martensitic transformation end temperature will decrease, and even if it is cooled to normal temperature by quenching, it will be incomplete quenching in which residual austenite remains. This retained austenite is not preferable because it causes sintering cracks. Therefore, the content of Ni is set to 0.3 to 1.6%.

Mo: 0.3-1.8% Mo is an element that improves hardenability, facilitates martensitic transformation, and segregates carbides at grain boundaries in the structure after quenching and tempering. In addition, by uniformly depositing as molybdenum carbide, it contributes to prevention of temper embrittlement and improves strength, particularly high-temperature strength. To ensure this effect, at least 0.3% or more is required. 1.
If it exceeds 8%, the toughness decreases. Therefore, the content of Mo is set to 0.3 to 1.8%.

[0029] In addition to the above weld overlay material, Nb: 0.2 to
If chromium-based stainless steel containing 2.0% is used, the crack resistance can be further improved. The purpose of adding Nb and the grounds for limiting the numerical range will be described below. Nb: 0.5 to 2.0% Nb is an element having a stronger bonding force with C than Cr and other carbide forming elements. Therefore, during the cooling of the bead during welding overlay, it becomes niobium carbide and precipitates to fix C. As a result, the formation of a martensite phase after welding is completely suppressed, and a structure composed of a fine ferrite phase and a niobium carbide phase in which other carbide forming elements are dissolved is formed, thereby preventing welding cracks. Further, niobium carbide finely precipitated after melting into austenite again by subsequent quenching and tempering improves strength, particularly high-temperature strength. Further, it suppresses the precipitation of chromium carbide in the grain boundaries and in the grains, and prevents a decrease in corrosion resistance and a decrease in toughness due to a decrease in metallic Cr. If Nb is less than 0.5%, this effect is difficult to appear. Up to 2.0%, the effect is sufficiently exhibited. Therefore, the content of Nb is set to 0.5 to 2.0%.

Further, one or more of V, Ti or Al is added to the above-mentioned weld overlay material in a total amount of 0.01 to 0.30.
%, A further improvement in crack resistance can be expected. The crystal grains that are recrystallized and refined just above the austenite transformation temperature during heating for quenching,
Attempts to re-agglomerate with increasing temperature. When a small amount of V is added, fine vanadium carbide uniformly precipitates in the structure, and this becomes the core of austenite grains during austenite transformation during heating during quenching, and a large number of austenite grains are generated. Become. Due to the small size of the initial recrystallized grains, even if the temperature is further increased in the austenite region and the grains are aggregated, the resulting grains are much smaller than usual. On the other hand, in order to perform complete quenching, it is necessary to sufficiently dissolve the carbide forming element in the austenite grains, and it is necessary to heat the austenite grains to a temperature just above the transformation point. Therefore, the fact that the crystals are still small even if they are aggregated to some extent is preferable for obtaining a fine structure after quenching and tempering.

According to the same principle as V, Ti is combined with fine titanium carbide or nitrogen / oxygen present in a trace amount in the weld metal to form titanium nitride and titanium oxide.
It becomes the nucleus of recrystallization and makes the crystal finer. Al also becomes aluminum nitride and aluminum oxide to refine the crystal. The content thereof is required to be at least 0.01% as a total addition amount of one or more kinds in order to be used as a nucleus of recrystallization. If it exceeds 0.30%, toughness is reduced, and welding defects are liable to occur. Therefore, the content of V is 0.01 to 0.30%.
And

Further, the present inventors can evaluate the toughness required for improving the crack resistance of the chrome-based stainless steel weld overlay by a Charpy 2 mmV notch impact value at room temperature. It has been confirmed by a simulator test and an actual line test that a life of at least 5.5 kgf · m / cm ² can be expected to be at least twice as long as the crack life of a conventional typical weld overlay roll. Therefore, the impact value was 5.5.
kgf ・ m / cm ² or more.

The thickness of the chrome-based stainless steel weld overlay having the above chemical components is preferably 3 to 15 mm. As the thickness of the chrome-based stainless steel weld overlay increases, the thickness of the layer with excellent toughness, strength and corrosion resistance increases, and the life to cracks increases. And a thickness range of 3 to 15 mm.

When the heating temperature for quenching is 910 ° C. or lower, the carbide forming element does not form a solid solution in the austenite phase and the crystal grains do not become fine. Conversely, if the temperature exceeds 1180 ° C., crystal grains once refined grow and become coarse. The holding time at the quenching temperature may be maintained for 3 minutes or more in a state where the deepest part in the thickness direction of the chrome-based stainless weld overlay has reached a predetermined temperature.

As a specific heating means for quenching, one or a plurality of flame burners having a structure in which a plurality of torches directed in the radial direction of the roll are provided in a ring-shaped holder slightly larger than the outer diameter of the roll are prepared. I do. This method encloses the roll and moves the flame ring burner in the axial direction of the roll, or provides an induction heating coil in a ring-shaped holder that is slightly larger than the outer diameter of the roll, and surrounds the roll with this. Move to. The former is more advantageous in terms of manufacturing cost.

Next, it is practically difficult to heat the entire roll in a furnace during quenching. This is because heating in a high temperature range causes the entire roll to deform and oxidize, so it is necessary to build up the entire shaft, including the shaft for setting the bearing, and machine the entire roll, greatly increasing the manufacturing cost. Because it will. The cooling of the quenching is performed by a forced cooling method using a liquid or gas, usually water or air, or by natural air cooling. The cooling method is based on the premise that the structure is completely martensite, and is appropriately selected from air cooling and water cooling, and the flow rate at that flow rate, according to the hardenability determined by the chemical composition.

As a specific cooling means for quenching, as shown in FIGS. 5 and 6, a ring-shaped header pipe is provided in the radial direction of the roll behind the flame ring burner 2 or the induction heating coil ring 8 as shown in FIGS. A cooling device 3 having a structure provided with a slit or a plurality of nozzles is prepared. Liquid or gas, specifically, water or air is injected from a slit or a nozzle of the cooling device, or, as shown in FIG.
Or let it cool naturally.

Next, if the heating temperature of the tempering is lower than 560 ° C., it is not preferable because the tempering temperature is affected. Further, since the roll surface temperature in the actual production line is about 600 ° C., a tempering temperature higher than this temperature level is required. If the tempering temperature exceeds 780 ° C., softening due to aggregation of precipitated carbides proceeds, and the hardness is excessively reduced. The alloy steel for machine structural use, which is usually a roll base material, is 6
It is tempered at about 50 ° C., and if heated further, the base material is softened, and the bending strength of the entire roll is reduced. Therefore, the heating temperature for tempering is 600
It is preferable that the heat treatment be performed in a temperature range from ℃ to the tempering temperature of the roll base material, that is, substantially in the range of 600 to 650 ℃. The holding time of tempering needs to be 30 minutes or more in order to precipitate carbides as necessary and sufficiently.

As a specific heating means for tempering, it is preferable to put the whole roll in a furnace and raise the temperature. This is because the heating temperature is not as high as in quenching, so that there is no need to worry about heating during quenching, and this is economically advantageous. Further, the cooling of the tempering is performed by forced cooling with an air fan, natural air cooling, or cooling in a furnace. In the quenching heat treatment, in order to prevent bending deformation of the roll, it is preferable to perform heating / cooling processing while slowly rotating the roll. Hereinafter, the present invention will be described in more detail based on examples.

[0040]

[Examples] With respect to a typical overlay material of a conventional continuous casting roll and a roll produced by applying the technology of the present invention, a mechanical property confirmation test, a crack resistance evaluation test by a simulator, and an actual production line. Perform a comparative test, strength,
The toughness and crack resistance were investigated. Table 1 shows the manufacturing specifications of the overlay materials used in the survey, that is, the chemical components of the weld overlay metal and the thermal spray overlay metal and the method of heat treatment. Table 2 shows the mechanical properties of these materials and the simulator and actual line. 3 shows the results of the evaluation of the crack resistance.

[0041]

[Table 1]

[0042]

[Table 2]

Table 1 shows ten kinds of materials. Nos. 1 to 4 are materials according to the prior art, and No. 1 is a chromium-based stainless steel weld overlay, subjected to stress relief annealing, .2 is a chrome-based stainless steel weld overlay, subjected to stress relief annealing, and then a nickel-based self-fluxing alloy is sprayed and melted. No.3 is a chrome-based stainless steel weld overlay. No. 4 was obtained by performing salt bath nitriding after removal annealing, and No. 4 was subjected to stress relief annealing after chromium-based stainless steel weld overlay, followed by quenching heat treatment. The technology of the present invention is shown in Nos. 5 to 10,
After welding welding of chromium-based stainless steel, stress relief annealing was performed, and quenching and tempering heat treatments were performed by the methods shown in the table. The specific method of quenching is shown in FIGS.

In Table 2, the mechanical properties were confirmed by J
The hardness, yield point, and Charpy impact value were confirmed by a mechanical test specified in IS. The number of test N is 3
For each item, the measurement was performed 5 times (points) and the average value was shown. In addition,
The hardness was measured at five points at a pitch of 1 mm in the depth direction starting from a point 1 mm deep from the surface. For No. 2 and No. 3 which are subjected to thermal spraying and nitriding, the hardness was measured from the interface between the thermal sprayed or nitrided layer and the average value measured at five points along the surface near the outermost surface. The average value of the values measured at five points at a pitch of 1 mm in the depth direction starting from the point entering the 2 mm weld overlay layer side is shown in parentheses. For the tensile test piece for obtaining the yield point and the Charpy test piece for measuring the impact value, the test piece was cut out from the deeper side with the position at a depth of 2 mm from the surface as the test piece surface. In addition, the thickness of the No. 2 spray coating layer is 0.5 mm.

Next, a crack resistance evaluation test using a simulator was carried out using a test device simulating a roll of a real line shown in FIG. After heating the specific part A, the test machine is rotated by 180 degrees and stopped to cool the part A with water,
The facing specific part B is heated. And after a certain time,
Rotate 180 degrees again in the same direction to heat part A. The rotation / stop at every 180 degrees as described above is periodically repeated to apply thermal stress to the opposing specific portions A and B. At the same time, a hydraulic cylinder for applying a bending load and a fixed part (reaction force receiving roller) are installed, and bending stress is applied to the roll. A cooling hole is provided at the center of the test roll, and the inside of the test roll is water-cooled. Hereinafter, the test conditions will be described in detail. (1) Test roll: diameter: 120 mm, roll base material: JIS: S45
C, underlay material: none, weld overlay thickness: 10 mm, thermal spray overlay thickness: (No. 2) 0.5 mm (2) Heating / cooling method of test piece: heating by high frequency induction heating coil, lower part Cool by immersing in water tank (3) Heating temperature / cycle: 700 ° C × 40 seconds, 180 ° rotation at 4 rpm (4) Bending stress: 22 kgf / mm ² (5) Temperature monitoring: Check with thermoviewer Evaluation Was performed with the number of cycles (N1) until crack generation and the number of cycles (N2) until the crack depth reached 10 mm. The crack generation timing was confirmed by performing a color check at regular cycle intervals. The crack depth is measured at regular intervals with a crack meter, which is a crack depth measuring instrument of the potential difference measurement method, the test is terminated at the crack depth of 10 mm, and after the test, the roll crack is cut and the depth is measured. To correct the cycle number. In addition, the test was performed three times for each roll overlay material, and the average value was described in the table.

As a comparison of the crack life, the relative ratio of N2 was calculated and evaluated with No. 1 as a reference, that is, 1.0. Finally, an outline of the comparison test on the actual line will be described. The roll used for the trial in the actual line was manufactured under the following conditions.・ Roll base material: JIS, SNCM415 equivalent material (manufactured by forging) ・ Roll dimensions: 400mm in diameter, 2300mm in length ・ Lower overlay: weld overlay of component equivalent to roll base material, thickness 20mm ・ Overlay layer (upper overlay) ) Thickness: ・ Weld overlay: 10 mm ・ Sprayed overlay: (No. 2) 0.5 mm As a measure to prevent roll deformation during quenching of No. 5 to 10 overlay materials, set the roll to 5 rpm. Hardening was performed while rotating.

Under the above conditions, three rolls were manufactured, and the rolls were assembled into an actual line for trial use. The confirmation period is about six months,
The number of ladle cups is 5000 (charge).
The evaluation was performed by measuring the maximum value of the crack depth of the roll after using 5000 charges, calculating the average value of each of the three rolls, and Table 2
It described in. As a comparison of the crack life, the reciprocal of the ratio of the crack depth was calculated with No. 1 as a reference, that is, 1.0, and evaluated as the relative ratio of the crack life. The crack depth was measured using a crack meter.

FIG. 10 shows the Charpy impact values in Table 2 and
3 shows the correlation of the ratio of the crack lifespans. This figure
Charpy 2mmV Notch impact value is 5.5kgf ・ m
/cm ^Two13 chromium-based material
Compared to rolls annealed stainless steel after stress build-up after stress build-up
In addition, it can be seen that a crack life of more than twice can be expected. Book
Stainless weld overlay on rolls manufactured by the inventive technology
A photograph of the tissue is shown in FIG. This is N in Tables 1 and 2.
o.5. Ferrite and black parts appear white
Fine granular carbide whose visible part is precipitated by tempering
The overall structure is extremely dense and homogeneous.
You. Examination of the surface distribution of Cr in this structure revealed that
No Cr segregation was observed.

[0049]

The roll produced by the technique of the present invention has a denser and more homogeneous structure than conventional rolls, has excellent crack resistance without chromium segregation at grain boundaries, and has a low crack life ratio. As shown, a significant life extension effect of 2.4 to 3.6 times that of the conventional roll was obtained.

[Brief description of the drawings]

FIG. 1 shows the structure of a long-life roll having a shallow crack depth in a roll used in an actual production line, (a) a macro photograph near a crack portion, (b) a micro structure photograph, (c) a sampling position, FIG.

FIG. 2 is a view showing the structure of a short-life roll having a deep crack in a roll used in an actual production line, and (a) showing a macro photograph near a crack portion and (b) showing a micro structure photograph.

FIG. 3 is a view showing a surface distribution map photograph of Cr element by EPMA of a structure of a long-life roll having a shallow crack depth in a roll used in an actual production line.

FIG. 4 is a view showing a photograph of a plane distribution map image of a Cr element by EPMA of a structure of a short life roll having a deep crack depth in a roll used in an actual production line.

FIG. 5 is a schematic diagram showing a state where heating and cooling are performed by a flame ring burner and a cooling device.

FIG. 6 is a schematic diagram showing a state where heating and cooling are performed by an induction heating coil ring and a cooling device.

FIG. 7 is a schematic diagram showing a state in which heating is performed by a flame ring burner or an induction heating ring coil and cooling is performed by a fan.

FIG. 8 is a diagram showing a configuration of a continuous casting apparatus.

FIG. 9 is a schematic diagram showing a crack resistance evaluation simulator.

FIG. 10 is a diagram showing a correlation between crack resistance and a Charpy impact value by an actual line test.

FIG. 11 is a structural photograph of a stainless weld overlay of a roll manufactured according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Continuous casting roll 2 ... Flame ring burner 3 ... Cooling device 4 ... Fuel pipe 5 ... Refrigerant pipe 6 ... Moving direction 7 ... Wire for electricity supply 8 ... Induction heating coil ring 9 ... Cooling fan 10 ... Ladle 11 ... Tundish 12 ... Mold 13: Cast piece 14: Test roll 15: Induction heating coil

Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/58 C22C 38/58

Claims (5)

[Claims] (1) C: 0.04 to 0.22% by weight,
Si: 0.10 to 0.70%, Mn: 0.25 to 1.6
0%, Cr: 10.0 to 14.8%, Ni: 0.3 to
After chromium-based stainless steel containing 1.6% and Mo: 0.3 to 1.8%, and the balance being Fe and unavoidable impurities, was weld-welded on the roll body, only the roll body was covered with 910 to 910.
After heating to 1180 ° C and holding for 3 minutes or more, and then cooling to room temperature, the entire roll is further heated to 560 to 780 ° C and held for 30 minutes or more, and then cooled to room temperature, and ferrite and granular carbides are finely divided. A continuous cast roll excellent in crack resistance having a notch impact value of not less than 5.5 kgf · m / cm ² at room temperature with a Charpy 2 mm V notch characterized by having a tempered sorbite structure having a uniformly distributed structure. 2. A continuous casting roll excellent in crack resistance, wherein the overlay material of the roll according to claim 1 is a chromium stainless steel further containing Nb: 0.5 to 2.0%. . 3. The surfacing material for a roll according to claim 1 or 2, which is a chromium stainless steel further containing one or more of V, Ti or Al in a total amount of 0.01 to 0.30%. A continuous casting roll with excellent crack resistance. 4. Heating to 910 to 1180 ° C. according to claim 1, wherein one or more of a plurality of torches are provided in a ring-shaped holder slightly larger than the outer diameter of the roll and arranged in a radial direction of the roll. The roll is surrounded by the base flame burner, the flame ring burner is moved in the roll axis direction, and the subsequent cooling is performed on the ring-shaped header tube behind the flame ring burner in the roll radial direction. One or a plurality of cooling devices having a structure provided with a slit or a plurality of nozzles are provided, and while moving water or air from the slit or the nozzle, moving simultaneously with the flame ring burner, or Treated by letting it cool naturally, and then 560-780 ° C
Heating the whole roll into a heating furnace to raise the temperature, and the subsequent cooling is performed by forced air cooling using a fan, natural cooling, or cooling in the furnace. A method for producing a continuous casting roll having excellent crack resistance. 5. The method according to claim 4, wherein the heating to 910 to 1180 ° C. is replaced with flame burner heating, and an induction heating coil is provided in a ring-shaped holder slightly larger than the outer diameter of the roll to surround the roll. A method for producing a continuous casting roll having excellent crack resistance, characterized in that a coil is moved in a roll axis direction and treated.