JP2000301302A - Continuous casting roll with low heat input - Google Patents

Abstract

(57) [Summary] In a continuous casting roll, a heat load applied to the roll is reduced, and a growth rate of a crack generated on a roll surface is reduced. SOLUTION: The roll surface is shot blasted by Ra.
Abrasion resistance, that is, unevenness of the surface is obtained by spraying a thin film of Cr ₃ C ₂ -NiCr having a hardness of 8 μm or more, which is higher in hardness than the roll body and excellent in high-temperature oxidation resistance and corrosion resistance in a thickness of 7 to 30 μm. By maintaining the metallic luster, that is, the low emissivity, while maintaining it, the contact heat transfer and the radiant heat transfer received from the counterpart high-temperature slab are reduced, and the cracks generated and propagated on the roll surface are suppressed. Before using this roll in an actual line, the roll was buffed in advance to reduce the normal temperature emissivity to ε = 0.7.
If the number is set to 5 or less, further effects can be obtained.

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 technique for reducing heat input to a roll from a high-temperature slab and suppressing cracks generated and developed in the roll.

[0002]

2. Description of the Related Art As shown in FIG. 2, a continuous casting apparatus of an iron making facility injects molten steel from a ladle 6 through a tundish 7 into a mold 8, and injects the molten steel into a mold and a continuous casting roll whose inside on the back side is water-cooled. The material to be cast, that is, the slab 5, is continuously produced by cooling with an external spray water (not shown) provided between the two and pulling it out while solidifying.

[0003] The continuous casting roll 1 is used to produce a slab of about 2 m / min.
At the following low speed, it is drawn out and conveyed while correcting it from a vertical state to a horizontal state that can be easily post-processed, and has a role of applying a roll pressure to the slab to finish it to a predetermined thickness. Therefore, as shown in FIG.
After being in contact with the hot slab 5 during rotation, it is quenched by the external spray water nozzle 9 and cracks are formed in the circumferential direction of the roll while simultaneously receiving thermal stress and mechanical bending stress for this repetition. It evolves.

[0004] In particular, in recent years, when casting of a full ladle has been completed, 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 is called a machine stop, and the presence of the machine stop makes the thermal stress even more severe, and the crack growth rate is increasing.

[0005] Usually, the limit of cracks is controlled by a roll having a diameter of about 400 mm at a crack depth of about 40 to 50 mm.
That is, if the crack exceeds this limit, the possibility that the roll is broken increases, so it is necessary to replace the roll according to the crack depth. In general, since the number of rolls of 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 an extremely important issue.

[0006] One of the effective means to reduce the thermal stress generated in the roll is to reduce the temperature of the roll. In a normal continuous casting roll, as shown in FIG. 3, a method is used in which a cooling hole 4 is formed in the center of the roll in the axial direction, and cooling water flows through the hole. As another means, as shown in Japanese Utility Model Publication No. 52-56346,
There is a method of efficiently lowering the roll temperature by providing a spiral cooling groove immediately below the roll surface and cooling the vicinity of the surface in contact with the slab as a heat input source.

[0007]

However, in the conventional cooling method aimed at improving the heat removal performance of the roll,
There were the following disadvantages. That is, the water used for cooling the rolls of the continuous casting equipment is so-called circulating water, which contains a large amount of scale generation sources such as iron oxide powder, calcium, and silica, which adheres to the inner wall of the cooling holes inside the rolls. ·grow up. Although the growth thickness of this scale is usually within several mm when used for about one year, since the composition of the scale is mainly composed of oxide, the thermal conductivity is extremely low compared to steel, and the thickness of the scale is slightly large. This leads to a decrease in cooling capacity.

In the method disclosed in Japanese Utility Model Publication No. 52-56346, the roll surface temperature can be reduced by 300 ° C. or more as compared with the case of the central cooling hole 4 when there is no scale adhesion. However, due to water quality problems, the effect of scale adhesion and growth is large, and the desired effects have not been obtained. Further, since this roll has a structure in which a cooling hole is provided in the vicinity of the surface, the size of the cooling hole is inevitably smaller than that in the case where a single hole is provided at the center due to the restriction on the strength of the roll body. Therefore, particularly in a line having poor water quality, a problem may occur in which the hole is closed at a scale that grows from the entire inner wall of the cooling hole toward the center. In addition, the roll has another drawback that the production and reproduction costs are high, and it is difficult to use the roll on a full-scale production line.

[0009]

SUMMARY OF THE INVENTION The present invention focuses on reducing the heat input load to a roll, whereas the prior art described above considers improving the cooling efficiency of the roll, that is, removing heat. The gist is as follows. In other words, by providing appropriate unevenness on the roll surface, and further spraying a thin film of a material having a higher hardness than the roll body and excellent in high-temperature oxidation resistance and corrosion resistance, while maintaining the wear resistance of the surface, that is, the unevenness, the metallic gloss, that is, the low By making it emissivity,
It reduces contact heat transfer and radiant heat transfer received from a counterpart high-temperature slab, and suppresses cracks that occur and propagate on the roll surface. The specific configuration is as follows.

(1) Surface Roughness R by Shot Blasting
a Roughened to 8 μm or more, and Cr ₃ C ₂ —N
A continuous casting roll with low heat input, characterized in that iCr spray coating is applied to a thickness of 7 to 30 μm. (2) The surface of the roll described in the above (1) was further buff-polished, and the normal-temperature emissivity ε measured by a total radiometer was ε =
A continuous casting roll with low heat input, characterized in that the casting roll is set to 0.75 or less.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
As shown in FIG. 4, the side of the continuous casting roll 1 that is in contact with the high-temperature slab 5 receives two types of heat input loads from the slab, namely, contact heat transfer 10 and radiant heat transfer 11. Heated to high temperature. It has been found that the temperature in the vicinity of the contact portion can be as high as 400 to 700 ° C., depending on the rotation speed of the roll. This increase in temperature causes the build-up layer on the surface of the roll to yield, and is a major cause of cracking and growth.

The conventional continuous casting roll has a surface layer of 13Cr.
After welding the stainless steel, the surface roughness is processed by a lathe to a surface roughness of about 2 to 6 μm as a center line average roughness Ra before use. Here, Ra means a reference length L extracted from the surface roughness curve in the direction of the average line, the X axis taken in the direction of the average line of the extracted portion, and the Y axis taken in the direction of the longitudinal magnification. When the curve is represented by y = f (x), the value obtained by the following formula is represented by μm.

(Equation 1) As the condition for measuring the roughness, the cut-off value λ, that is, the reference length L was 2.5 mm, and the evaluation length was 12.5 mm.

When a roll that has been lathe-processed after the above-mentioned welding overlay is used in an actual line, the unevenness of the roll surface is small.
In combination with the fact that the slab is softened at high temperatures, the contact between the roll and the slab is in a so-called solid state, the microscopic real contact area is large, and the slab to roll The heat is easily transmitted through the contact surface.

On the other hand, the color of the roll surface immediately after production, that is, after lathing, exhibits a silver-white bright metallic luster and exhibits a low emissivity. However, a few days after incorporating it into the actual line,
The area in contact with the slab undergoes high-temperature oxidation and corrosion and changes color to black or brownish, and the emissivity ε becomes extremely high,
Radiation energy is easily absorbed, and the amount of heat input by radiation is constantly increased.

Next, an embodiment of the present invention will be described with reference to FIG. As shown in FIG. 1, the present inventors, as a means for reducing the heat input load to the roll, roughen the roll surface by shot blasting as indicated by reference numeral 2 and have a high hardness and a metallic luster thereon. It has been invented that thin film spraying is performed in such a range that the roughness of the substrate is not reduced as much as possible.

Due to the roughening of the roll, the area of true contact with the slab is reduced, so that the heat transfer coefficient of the contact is reduced and the amount of contact heat transfer is reduced. C with high hardness and metallic luster in addition to roughened surface
perform r ₃ C ₂ -NiCr spraying. The roughness after thermal spraying is slightly reduced and the irregularities are reduced, but the surface hardness is increased and the abrasion resistance is improved, so that the imparted roughness (irregularities), that is, a low heat transfer coefficient, is maintained for a long time. Can be.

[0017] On the other hand, the thermal spray coating has a metallic luster and thus has a low emissivity ε, so that the absorptance of the radiant heat received from the slab is reduced and the radiant heat input is reduced. Cr ₃ C ₂ -NiCr spray coating high temperature oxidation resistance, since very excellent corrosion resistance, the conventional incorporation into the real line as 13Cr stainless steel, black-brown like proceeds, oxidation and corrosion of the very short time And the emissivity does not increase.

The above-mentioned effects of providing unevenness and reducing the emissivity are as follows.
According to experiments and analysis by the present inventors, it is roughly as follows. 0.08C-13Cr-1N as conventional roll
After i-type stainless steel was weld overlayed and subjected to stress relief annealing at 650 ° C. × 4 hours, the roll was processed to Ra 2.8 μm by lathe processing, and the roll of the present invention was manufactured by the same procedure as the above method. Then, a roughening treatment by shot blast is performed, and then Cr ₃ C ₂ -20% Ni
A roll having a Cr sprayed film having a thickness of about 12 μm and buffing was performed to make the roughness Ra 11.5 μm.

The surfaces of these two rolls were partially pressed at a linear pressure of 30 kgf / mm with a press having a flat pressing surface to crush the surfaces to some extent. The linear pressure of 30 kgf / mm is a value at the same level as the linear pressure generated by the reaction force that the roll receives from the slab at the horizontal portion of the actual line. The profile of the surface of the prototype roll was measured, and the true contact length at which the roll would contact the slab was read from the chart and compared. As a result, it was found that the true contact length of the roll of the present invention with respect to the conventional roll can be reduced to about ／. Since the amount of contact heat transfer is proportional to the contact area, the amount of heat transfer can be reduced to 2/3 even if the contact width in the roll circumferential direction is considered to be the same. When the roll surface temperature reduction effect is estimated and calculated from the heat transfer amount reduction effect,
It was found that a temperature reduction of about 84 ° C. was possible.

On the other hand, when the emissivity of the roll of the present invention was measured, it was ε = 0.68. The data of the conventional roll measured about 10 days after the start of use of the actual line was about ε = 0.95. As a result of estimating the amount of radiant heat transfer received by the roll based on this difference in emissivity, it was found that a roll surface temperature reduction of 52 ° C. could be expected.

Here, the meaning of the emissivity ε is as follows. That is, the energy incident by radiant heat transfer is 1
The object (perfect black body) that absorbs 00%
The magnitude of the radiation energy absorbed per unit time (referred to as radioactivity) is defined as Eb. On the other hand, when the radioactivity on the roll surface is E, the emissivity is a value represented by ε = E / Eb. Note that the radioactivity E and Eb change depending on the temperature, that is, the change in the wavelength of the radiant energy, but the ratio emissivity ε does not change with temperature if it is a normal industrial material called a gray body. Since the metal material used for the roll is a gray body and the emissivity ε is considered to hardly change with temperature, the emissivity was defined as a value measured at room temperature. The instrument used for the measurement is called a total radiometer (thermopile).

Since the proof stress of the 13Cr stainless steel of the roll surface layer is rapidly reduced particularly at a high temperature of 500 ° C. or more, the plastic strain can be suppressed, that is, the crack can be suppressed even in the temperature range as described above in the present invention. The effect of delaying the onset and progress of the phenomenon is very large.

In the present invention, the reason why shot blasting is performed after thermal spraying by making the sprayed coating thicker to provide unevenness in the sprayed coating layer is as follows. In other words, in this method, the thickness of the coating film exceeds 50 μm at least at a thick portion. The thermal spraying is performed by a so-called high-speed flame spraying machine, and the adhesion to the base is mainly mechanical coupling,
Not as strong as a self-fluxing alloy sprayed coating. It is a well-known fact that the thicker the sprayed coating, the easier it is to peel off, and under severe conditions of thermal stress such as continuous casting rolls,
If the thickness is large, it is easy to peel off. Therefore, as long as the required function can be satisfied, the thinner the film, the better the durability. Needless to say, the cost for improving the surface of the present invention is significantly lower.

Next, the grounds for limiting the numerical values of the present invention will be described. The reason for setting the roughness after the shot blast treatment to Ra 8 μm or more is that if it is less than this, a substantial effect of reducing the true contact area cannot be obtained. The type of thermal spray coating is Cr ₃ C ₂ −
NiCr is used because the coating has the highest hardness and the lowest emissivity among materials having a certain degree of hardness and a metallic luster. Thickness of thermal spray coating is 7 ~
The reason for limiting the thickness to 30 μm is that if the thickness is less than 7 μm, the retention of unevenness is poor. The reason for setting the thickness to 30 μm or less is that if the thickness exceeds 30 μm, peeling of the coating film is likely to occur, and irregularities after thermal spraying will be too small.

The emissivity of the roll of the present invention is about .epsilon. = 0.8 when sprayed as it is, and the reduction in the emissivity of the conventional roll after use of the actual line is small. However, when the roll of the present invention is incorporated in an actual line, a moderate lubrication and polishing action with the slab during operation causes a metallic luster to appear and the emissivity further decreases. Therefore, the thermal spraying may be performed as it is, but it is more preferable to roughen the surface by buffing in advance so as not to make the irregularities too small, since the emissivity can be further reduced from the beginning. The emissivity after buffing is set to 0.75 or less because the difference in emissivity that can be expected to significantly extend the service life in an actual line in which the roll life greatly varies due to the variation in environmental conditions is 0.
This is because 94 is 0.75.

[0026]

An embodiment of the present invention will be described with reference to Table 1. Table 1 shows the results of examining the performance of a roll manufactured by a conventional method and a roll having a surface formed under various conditions in order to consider the configuration of the present technology by measuring roughness, emissivity, and the like. . No. 1 is a roll conventionally used as a conventional roll. After welding build-up of 0.08C-13Cr-1Ni-based stainless steel, stress relief annealing at 650 ° C. × 4 hours was performed, and finishing was performed with a lathe. Things. No.2
Is measured 10 days after incorporating the No. 1 roll into the actual line. No.3 is No.1 roll with shot blast. No.4-9 are three kinds of thermal spraying with hardness higher than 13Cr stainless steel and metallic luster.
No. 3 roll was coated with a thin film and further buffed. No.10-11 are N
o. Measured after incorporating rolls 8 to 9 into an actual line and using them for 240 days.

The conditions for the trial production are as follows. (1) Roll: 400 mm in diameter (2) Shot blast: Use of alumina ceramic particles (3) Buffing condition: Bundling a 0.2 mm diameter nylon wire formed by embedding silicon carbide ceramic particles, width 17 mm, diameter 150 mm Was used. The particle size is equivalent to # 80. The brush was set on a carriage on a lathe and polished two times at a rotation speed of 500 rpm and a feed of 10 mm. The number of rotations of the roll during polishing is 60
rpm.

The conventional rolls No. 1 and No. 2 are formed with lathes with a constant pitch and small irregularities in a lathe processing state after production, and have a surface roughness Ra of about 4 to 6 μm. The color tone of the surface before using the actual line is silvery white and radiant, and the emissivity is extremely low at 0.25. However, N
As can be seen from the results of o.2, the roll is inferior in oxidation resistance and corrosion resistance.
The emissivity rose to 0.94, and it became extremely easy to absorb radiant heat. No. 3 is obtained by subjecting a conventional roll to a shot blast treatment, and the surface roughness is Ra14.
Sufficient irregularities of 4 μm were formed.

Next, three types of thermal spray coatings were further formed on the roll. Sprayed materials are NiCr, Tribaloy, and C
It was selected three r ₃ C ₂ -NiCr. The thickness of each coating was 10 to 15 μm. The emissivity of No. 4 NiCr was relatively low even when sprayed, and was good. However, the hardness was low, so buffing was performed in No. 5, and the sprayed coating was partially exposed. In addition, whether or not the underlayer was exposed was confirmed using a copper sulfate solution. Copper sulfate reacts with iron and changes color, so if you wet the polishing marks and change color,
This is because it can be determined whether or not the base weld overlay, that is, the stainless steel containing the iron component is exposed.

No. 6.7 No. 7 tribaloys are made of NiCr and C
r ₃ C ₂ -NiCr is a coating with intermediate hardness.
Even in the case of buffing in No. 7, the emissivity did not decrease so much, and the radiation heat input reduction effect was just one step away. No.
8, 9 Cr ₃ C ₂ -20% NiCr is the coating having the highest hardness and expected to maintain the roughness. The emissivity measurement result was also at the same level as the tribaloy when sprayed as it was, but the emissivity was further reduced by 17% to 0.68 by buffing.

Further, the results of following the situation by using Cr ₃ C ₂ -20% NiCr on the actual line are Nos. 10 to 11.
It is. As can be seen from No. 10, even after a long period of 240 days from the use in the actual line, the coating shows no significant decrease in roughness and maintains the effect of reducing contact heat transfer. In addition, it was confirmed that the emissivity was reduced by about 12% from that before use by receiving an appropriate polishing action from the cast slab, and the radiation heat input suppression effect was increased.

Based on the above results, Cr ₃ C ₂ —NiCr, which is effective for both maintaining the roughness and reducing the radiant heat input, was selected as the thermal spray coating. In addition, Cr ₃ C ₂ -NiCr
In thermal spraying, there are several types of raw materials having different contents of NiCr. If the content of NiCr is in the range of 15 to 50% by weight, the effect is effectively exhibited from the viewpoints of both hardness and emissivity.

The roll of the present invention, that is, a roll having a thin film sprayed of Cr ₃ C ₂ -20% NiCr after shot blasting and a further 10 buff-polished rolls were applied to a real line and cracked for 1.5 years. Progress was tracked. The crack depth was evaluated by the average value of 10 cracks having the maximum depth of each roll. As a result of the follow-up inspection, it was confirmed that the crack depth could be reduced from 1 / 1.7 to 1 / 2.3 compared to the conventional 13Cr stainless steel weld overlay / lathe finish roll.

[0034]

[Table 1]

[0035]

As described above, according to the present invention, the replacement cycle of the rolls on the actual line can be extended at the above-mentioned reduction rate, so that the replacement work cost for repairing the crack and the build-up after removal are increased. Regeneration costs, that is, running costs can be significantly reduced.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a configuration of a surface of a continuous casting roll of the present invention.

FIG. 2 is a diagram showing a configuration of a continuous casting facility.

FIG. 3 is a schematic diagram for explaining a thermal cycle received by a continuous casting roll.

FIG. 4 is a schematic diagram for explaining heat input applied to a continuous casting roll.

[Explanation of symbols]

1: Continuous casting roll 2: Rough surface generated by shot blasting on the surface of the continuous casting roll 3: Thermal spray coating 4: Cooling hole of continuous casting roll 5: Slab 6: Ladle 7: Tundish 8: Mold 9: External spray water nozzle 10: Contact heat transfer 11: Radiant heat transfer

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Yasunori Miyamoto 1 Kimitsu, Kimitsu City Inside Nippon Steel Corporation Kimitsu Works (72) Inventor Yukihisa 1 Kimizu Kimitsu City Kimitsu Manufacturing In-house F term (reference) 3J103 AA02 AA51 EA03 EA20 FA01 FA09 FA13 FA15 GA02 GA15 HA04 HA31 HA33 4E004 LC00 4K031 AA01 AB08 BA01 CB10 CB14 CB22 CB45 FA04

Claims (2)

[Claims] 1. The surface is shot blasted with a roughness Ra of 8 μm.
m or more, and Cr ₃ C ₂ -NiCr
A continuous casting roll with low heat input, characterized in that thermal spraying is applied with a thickness of 7 to 30 μm. 2. The surface of the roll according to claim 1 is further buff-polished to obtain a normal-temperature emissivity ε measured by a total radiometer.
= 0.75 or less, a continuous casting roll with low heat input.