JP7537400B2 - Manufacturing method of round billet - Google Patents

Description

The present invention relates to a method for manufacturing round billets.

Round billets, which are used as rolling materials, are generally manufactured by subjecting a cast piece cast by a continuous casting method to multi-stage caliber rolling to form and lengthen it. After rolling, the φ170-310 round billet is at a temperature of approximately 700°C-900°C, and is cooled to a specified temperature by air or water cooling on a cooling bed installed on the exit side of the finishing rolling mill. Examples of such cooling beds include those that use a roll conveyor (or chain conveyor) system or a walking beam system, which transport the round billet while rolling it.

In either method, the round billet that has finished rolling and arrived at the cooling bed is transported in sequence to the outlet side of the cooling bed, and then transported from the cooling bed to a storage area called a yard. At this time, the round billet, which has a relatively small cross section, is difficult to grasp with tongs having gripping claws, so it is often transported using a lifting magnet that utilizes magnetic force.
In this case, if the temperature of the round billet is high, it is not magnetic, so the round billet will not be magnetized by the lifting magnet and cannot be removed from the cooling bed. Therefore, it is necessary to quickly cool the round billet on the cooling bed to at least about 50 to 60°C, which is a temperature at which the round billet can be magnetized by the lifting magnet.

However, as mentioned above, the temperature of the round billet after rolling is about 700°C to 900°C, so depending on the composition, the round billet is quenched while being air- or water-cooled on the cooling bed. At this time, the surface of the billet, which is cooled directly by the outside air or cooling water, cools quickly, while the center of the billet, whose temperature drops due to thermal conduction, cools slower than the billet surface, resulting in a temperature difference between the surface and interior of the round billet.

For example, in steels containing many components that are quenched during cooling, such as high Cr steel or high Ni steel, the cooling rate of the surface of the round billet is faster than that of the inside, so that the martensitic transformation starts from the surface of the round billet and expands. At this time, the surface of the round billet is restrained from deformation inside, so a compressive stress σ _th occurs, and if the compressive stress σ _th exceeds the yield stress σ _YP of the material, compressive plastic strain occurs on the surface of the round billet. If the cross section is cooled to room temperature in this state, compressive plastic strain associated with the martensitic transformation remains on the surface of the round billet, so a strain difference occurs between the surface and the inside. At this time, the surface of the round billet is restrained from deformation inside, so tensile residual stress occurs on the surface and compressive residual stress occurs inside, and in some cases cracks occur from the surface.
Therefore, in order to prevent cracks due to cooling, it is necessary to reduce the plastic strain caused by the rapid progression of martensitic transformation on the surface of the round billet and to prevent tensile stress from acting on the surface of the round billet.

Various techniques have been disclosed to prevent such cracks. For example, Patent Document 1 discloses a method in which steel material is bundled and slowly cooled until the temperature at the center of the bundle reaches the martensitic transformation start temperature, thereby avoiding quenching and forming soft structures such as ferrite and pearlite. Furthermore, Patent Document 1 claims that by performing immersion water cooling after air cooling, it is possible to avoid cracks due to cooling while also reducing the cooling time.

Patent Document 2 discloses a method in which the martensitic transformation of the steel surface is completed by cooling, and then cooling is terminated just before the temperature at the center of the steel reaches the martensitic transformation start temperature. In Patent Document 2, after cooling is completed, heat is conducted from the center of the steel to the surface, causing the temperature at the center of the steel to drop and the temperature at the surface of the steel to rise. Then, martensitic transformation begins at the center of the steel, and the temperature at the surface of the steel rises due to transformation heat, causing self-tempering. It is said that this reduces distortion due to martensitic transformation and prevents cracks.

Patent Document 3 discloses a method of spraying a mixture of water mist and air blown by a fan onto a billet. Patent Document 3 claims that this method can cool the billet more quickly than conventional air cooling, and because water droplets and water flows do not come into direct contact with the billet, it is not locally cooled, and even steel types that are highly susceptible to cracking can be cooled without cracking.

JP 2017-221968 A JP 2011-6711 A JP 2008-221316 A

However, when the technology disclosed in Patent Document 1 is applied to high alloy steel with high hardenability, even if the steel material is accumulated in a bundle and slowly cooled, it is hardened and does not become a soft structure, which can lead to cracks. In addition, because the steel material is accumulated and slowly cooled until it transforms, even if immersion water cooling is performed in the subsequent process, there is a problem that it takes a very long time to cool it to a temperature where it can be magnetized with a lifting magnet.

In addition, the technology disclosed in Patent Document 2 does not sufficiently reduce the strain caused by martensitic transformation in high-hardenability steels such as high alloy steels, and cracks may occur with self-tempering alone. In addition, the amount of heat generated by transformation varies depending on the composition of the steel material, but depending on the type of steel, a sufficient amount of heat generated by transformation cannot be obtained, and the strain on the surface of the steel material cannot be reduced, causing cracks.
Furthermore, the technology disclosed in Patent Document 3 does not specify cooling conditions for avoiding cracks, and cracks may occur depending on the type of steel and cooling conditions.

The present invention has been made to solve the problems of the conventional examples described above. In other words, the objective is to provide a method for manufacturing round billets that can perform rapid cooling without causing cracks in the round billet after finish rolling, even for steel types that are highly sensitive to cracking.

According to one aspect of the present invention, a method for manufacturing a round billet is provided, which includes a cooling step in which a round billet that has been subjected to finish rolling is allowed to cool on a cooling bed, and a water-cooling step in which, after the cooling step, the round billet is water-cooled by injecting cooling water from at least above the round billet using a water-cooling device, and in the cooling step, the round billet is cooled until it reaches a target water-cooling start temperature or lower.

According to one aspect of the present invention, there is provided a method for producing a round billet, which is capable of performing rapid cooling without causing cracks in the round billet after finish rolling, even for a steel type having high cracking sensitivity.
According to the present invention, while conventionally the steel was naturally cooled in a cooling bed until a predetermined temperature was reached, the cooling time can be shortened by performing water cooling, and it is also possible to prevent cracking even in the case of steel types that are highly susceptible to cracking by specifying the water cooling start temperature.

1 is a graph simulating the relationship between temperature and strain when martensitic transformation occurs. 1 is a graph simulating the relationship between temperature and distortion of a round billet at the exit side of finish rolling. 1 is a graph simulating the relationship between temperature and strain when martensitic transformation occurs during cooling after finish rolling. 1 is a graph for explaining the derivation of a target water-cooling start temperature for preventing cracking, which has led to the present invention. FIG. 2 is a schematic diagram showing the arrangement of a cooling bed, a water cooling device, and a thermometer in one embodiment of the present invention. FIG. 2 is an explanatory diagram showing the rolling of a round billet during transportation by the walking beam method. FIG. 2 is a side view showing a water-cooling device according to an embodiment of the present invention. FIG. 2 is a plan view showing a spray area of cooling water when a full cone nozzle is used as a spray nozzle. FIG. 2 is a plan view showing the spray area of cooling water when a square spray nozzle is used as the spray nozzle.

In the following detailed description, an embodiment of the present invention will be described with reference to the drawings. In the description of the drawings, identical or similar parts are given the same or similar reference numerals, and duplicate explanations will be omitted. The drawings are schematic and may differ from the actual product. In addition, the embodiments shown below are examples of devices and methods for embodying the technical concept of the present invention, and the technical concept of the present invention does not specify the materials, structure, arrangement, etc. of the components as described below. The technical concept of the present invention may be modified in various ways within the technical scope defined by the claims.

In general, depending on the cooling conditions during manufacturing, steel undergoes a phase transformation from a face-centered cubic lattice called the austenite phase to a body-centered cubic lattice such as the ferrite phase, pearlite phase, bainite phase, or martensite phase. The martensite phase is particularly hard and brittle, and is highly susceptible to cracking, making it prone to cracking. In ordinary steel, martensitic transformation occurs when the steel is rapidly cooled, but even when the cooling rate is slow, martensitic transformation becomes more likely to occur as the Cr content, an element that improves hardenability, increases. Note that when Cr is 10 wt% or more, martensitic transformation can occur even at cooling rates similar to air cooling, but when Cr is 14 wt% or more, ferrite transformation becomes more likely.

FIG. 1 shows a simulation of the relationship between temperature and strain when the austenite phase is transformed into the martensite phase. In FIG. 1, "◯" represents the temperature and strain on the surface of the round billet, and "□" represents the temperature and strain inside the round billet. In FIG. 1, T _in [°C] represents the internal temperature of the round billet, T _surf [°C] represents the surface temperature of the round billet, M _s [°C] represents the martensitic transformation start temperature, and M _f [°C] represents the martensitic transformation finish temperature. In FIG. 2 to FIG. 4, all symbols in the figures have the same meaning as in FIG. 1. As can be seen from FIG. 1, when the temperature of the round billet is reduced without phase transformation, thermal contraction occurs. On the other hand, when the temperature of the round billet is reduced with phase transformation, the body-centered cubic lattice has a larger volume than the face-centered cubic lattice due to the difference in crystal structure, and therefore the round billet expands.

In the billet manufacturing process, the surface of the billet, which is in direct contact with the cooling medium (air, water, etc.), is at a lower temperature than the inside, where the temperature drops due to thermal conduction, and so a temperature difference occurs within the cross section of the round billet. Figure 2 shows the relationship between temperature and distortion of the round billet at the finish rolling exit side. As can be seen from Figure 2, a temperature difference has already occurred between the surface and the inside of the round billet at the finish rolling exit side, and in this case, residual stress occurs at room temperature due to the difference in thermal contraction. Note that if the billet is only allowed to cool on a cooling bed after finish rolling, the thermal contraction of the inside is greater than that of the surface, and the surface and inside mutually restrict deformation, resulting in compression of the surface and tension of the inside residual stress at room temperature. Note that when a round billet with a diameter of φ170 to φ310 is allowed to cool on a cooling bed, the temperature difference between the surface and the inside is 30°C to 250°C.

On the other hand, when the round billet after finish rolling is quenched by water cooling or when a steel with high hardenability is cooled, a phase transformation from the austenite phase to the martensite phase, which is hard, brittle, and highly susceptible to cracking, occurs as described above. Figure 3 shows the relationship between temperature and strain when martensite transformation occurs during cooling after finish rolling. When a steel with high hardenability is cooled on a cooling bed, the cooling rate of the surface is faster than that of the inside of the round billet, so martensite transformation progresses and expands from the surface first. At this time, the surface of the round billet is restrained from deformation inside, so a compressive stress σ _th [MPa] occurs, and if the stress exceeds the yield stress σ _YP [MPa] of the material, a compressive plastic strain occurs on the surface.

If the inside of the cross section is cooled in this state until it reaches room temperature, compressive plastic strain associated with martensitic transformation remains on the surface of the round billet, resulting in a difference in strain between the surface and the interior. Since the deformation of the surface of the round billet is restricted inside, the residual stress is tensile on the surface and compressive inside, and in some cases cracks may occur from the surface.
In other words, the inventors discovered that if the difference in strain between the surface and the interior that occurs during martensitic transformation during cooling is kept below a certain value, compressive plastic strain will not occur on the surface of the round billet, and cracks will not occur, leading to the completion of the present invention. The process is shown below.

Fig. 4 is a graph for explaining the derivation of the target water-cooling start temperature for preventing cracks, which has led to the present invention. In Fig. 4, _Tc [°C] represents the target water-cooling start temperature. When water-cooling is performed in the transformation temperature range of _Ms [°C] to _Mf [°C], the maximum value _Δεmax [-] of the strain difference generated between the surface and the interior of the round billet is expressed by formula (1) from Fig. 4.
△ε _max = (ε _f −ε _c ) ...(1)
ε _f : Strain at the end of transformation [-]
ε _c : Strain at the start of water cooling [-]

At this time, the value obtained by multiplying Δε _max by Young's modulus is the compressive stress σ _th [MPa] generated on the surface of the round billet, and is expressed by formula (2).
σ _th =E・△ε _{max ...} (2)
σ _th : Maximum value of thermal stress generated during water cooling [MPa]
E: Young's modulus [MPa]

As described above, when the compressive stress σ _th of the surface generated during martensitic transformation exceeds the yield stress σ _YP of the material, compressive plastic strain occurs on the surface, and the surface becomes tensile stress at room temperature. In other words, if the compressive stress σ _th of the surface generated during martensitic transformation does not reach the yield stress σ _YP of the material, compressive plastic strain does not occur on the surface, and the surface becomes compressive stress at room temperature, making it possible to prevent cracking. Therefore, in order to prevent cracking, it is necessary to satisfy the relationship σ _YP > σ _th , which is the formula (3) obtained from the formulas (1) and (2).
σ _YP > E(ε _f −ε _c ) ...(3)

In order to calculate the target water-cooling start temperature _Tc that prevents cracking from the formula (3), the yield stress _σYP of the material (round billet), Young's modulus E, and the relationship between temperature and strain are required. The yield stress _σYP and Young's modulus E of the material may be obtained in advance from a stress-strain diagram obtained by performing a tensile test at each temperature, for example, by a method specified in JIS G0567 High-temperature tensile test method for steel materials and heat-resistant alloys. Furthermore, the relationship between temperature and strain may be obtained by controlling the temperature via a thermocouple welded to the test piece using a hot working reproduction test device (for example, Thermec Master manufactured by Fuji Electric Industrial Co., Ltd.), and measuring the expansion and contraction of the test piece in advance using a displacement meter during the process. From the relationship between temperature and strain obtained in this way, the target water-cooling start temperature _Tc that satisfies the formula (3) may be read and determined.

Furthermore, the inventors determined the target water-cooling start temperature T _c [° C.] at which cracking is prevented for a highly hardenable alloy steel containing 10 wt. % or more of Cr and consisting mainly of Fe, Ni and Cr by the above-mentioned method, and derived a regression equation based on the components, resulting in equation (4). In other words, in this embodiment, equation (4) may be used as the target water-cooling start temperature T _c [° C.] at which cracking is prevented.
T _c <369-345[C]-20[Mn]-0.06[Si]-9.1[Cr]
-19[Mo]-78[Cu]...(4)
T _c :Target water cooling start temperature [℃]
[X]: Chemical composition of X [mass%]

<Manufacturing method of round billets>
A method for manufacturing a round billet according to one embodiment of the present invention includes a rolling step, a natural cooling step, and a water cooling step.
In the rolling process, a slab cast by continuous casting is subjected to multi-stage caliber rolling to produce a round billet with a predetermined cross-sectional shape. The diameter of the round billet is φ170 mm or more and φ310 mm or less. The method for producing a round billet according to this embodiment is not particularly limited to the type of steel used for the round billet, but is suitable for steel types with high hardenability, and is even more suitable for alloy steels with high hardenability containing 10 wt% or more of Cr and mainly composed of Fe, Ni and Cr.

In the cooling process, the round billet rolled in the rolling process, that is, the round billet finally rolled by the finishing rolling mill, is cooled (air-cooled) on a cooling bed. The round billet cooled in the cooling process is immediately after the finish rolling, and the surface temperature is high, about 700°C to 900°C. In the cooling process, the round billet is cooled until the temperature (surface temperature) becomes equal to or lower than the above-mentioned target water-cooling start temperature T _c . Note that the lower limit of the surface temperature of the round billet after cooling in the cooling process is not particularly limited, but it is preferable to set it to 300°C or lower from the viewpoint of shortening the time required for cooling. The target water-cooling start temperature T _c may be set from the formula (3) using the yield stress σ _YP of the material, the Young's modulus E, and the relationship between temperature and strain, or may be set so as to satisfy the formula (4) from the chemical composition of the round billet.

5 shows an example of a cooling bed 2 and a water cooling device 3 in which the round billet 1 is subjected to the natural cooling process and the water cooling process, respectively, in this embodiment. As shown in FIG. 5, the round billet 1 that has been subjected to the rolling process is transported to the cooling bed 2. Next, the round billet 1 is natural cooled while being transported on the cooling bed 2. Furthermore, the temperature (surface temperature) of the round billet 1 is measured by a thermometer 4 provided on the exit side (water cooling device side) of the cooling bed 2. Then, when the temperature of the round billet 1 becomes equal to or lower than the target water cooling start temperature T _c , the round billet 1 is transported to the water cooling device 3 where the water cooling process is performed. Note that, when the temperature of the round billet 1 exceeds the target water cooling start temperature T _{c , natural cooling on the cooling bed 2 is continued until the temperature becomes equal to or lower than the target water cooling start temperature T c .}

In the cooling process, it is preferable to use a roll conveyor or a walking beam to roll the round billet 1 while cooling it. When rolling multiple round billets continuously, the round billets 1 may be arranged adjacent to each other on the cooling bed 2. In this case, the adjacent sides of the round billets 1 are difficult to cool due to the influence of mutual radiation heat transfer, and the temperature becomes higher than the upper and lower parts of the round billet 1, so the temperature of the round billet 1 becomes uneven in the circumferential direction. If water cooling is performed in this state, it is difficult to eliminate the unevenness in the circumferential temperature before water cooling, and the temperature becomes uneven in the circumferential direction even during or after water cooling. This may cause the round billet 1 to warp or bend, or make it difficult to magnetize it with a lifting magnet.

Therefore, by using a roll conveyor or a walking beam system for transporting the round billet 1 while rolling it, as the transport method of the cooling bed 2, the temperature of the round billet 1 in the circumferential direction can be made uniform. FIG. 6 shows the rolling of the round billet 1 by the walking beam system. In the walking beam system, the round billet 1 is supported on a fixed rake 21, which is a fixed support base (FIG. 6(A)), and then the movable rake 22 is raised, moved horizontally, and lowered to transport it to the outlet side of the cooling bed 2 (FIGS. 6(B) to 6(E)). In this transport system, the round billets 1 are adjacent to each other at a certain point in time, and the position of the part that was difficult to cool (the position indicated by "x" in FIG. 6) changes to the upper surface direction or the lower surface direction by rolling. Therefore, at the next point in time, the effect of radiation heat transfer is reduced and it becomes easier to cool. Therefore, by repeating this while rolling and transporting the round billet 1, it is possible to make the temperature of the round billet 1 uniform in the circumferential direction when it reaches the inlet side of the water cooling device 3. Furthermore, if the target water-cooling start temperature _Tc that satisfies formula (4) has not yet been reached when the round billet 1 reaches the inlet side of the water-cooling device 3, the round billet 1 can be transported back and forth on the cooling bed to wait until the target water-cooling start temperature _Tc is reached, while also making the temperature of the round billet 1 uniform in the circumferential direction.

In the water cooling process, the round billet cooled in the natural cooling process is cooled (water-cooled) using cooling water in a water cooling device 3. In the water cooling process, cooling is performed until the round billet 1 reaches a temperature at which it can be magnetized. Fig. 7 shows an example of the configuration of the water cooling device 3. The water cooling device 3 shown in Fig. 7 has a plurality of upper nozzles 31 and a plurality of lower nozzles 32.
The multiple upper nozzles 31 are provided above the transported round billet 1 and are spray nozzles that spray cooling water from above onto the round billet 1. The multiple upper nozzles 31 form a nozzle row lined up at predetermined intervals in the longitudinal direction of the round billet 1, and in the water cooling device 3, multiple nozzle rows are provided lined up in the transport direction of the round billet 1.

The multiple lower nozzles 32 are spray nozzles that are provided below the transported round billet 1 and spray cooling water onto the round billet 1 from below. Similar to the upper nozzles 31, the multiple lower nozzles 32 form a nozzle row arranged at a predetermined interval in the longitudinal direction of the round billet 1, and in the water cooling device 3, multiple nozzle rows are arranged in the transport direction of the round billet 1. In the following description, the upper nozzles 31 and the lower nozzles 32 are collectively referred to as spray nozzles.

In such a water cooling device 3, cooling water is sprayed from an upper nozzle 31 and a lower nozzle 32 onto the transported round billet 1, thereby water-cooling the round billet 1. The method of transporting the round billet 1 in the water cooling process is not particularly limited, but may be the same as that of the cooling bed 2 in which the natural cooling process is performed. Also, the water cooling device 3 may be provided at a part of the exit side of the cooling bed 2 in the transport direction.
The water cooling device 3 preferably uses spray cooling as a cooling method. It is also preferable to install a thermometer 5 on the outlet side of the water cooling device 3 in order to check whether the round billet 1 has reached a temperature at which it can be magnetized by the lifting magnet.

When cooling water is sprayed from the upper nozzle 31 installed above the round billet 1, the cooling water comes into contact with the upper surface of the round billet 1, and then flows down along the circumferential direction of the round billet 1 according to gravity to reach the lower surface. As a result, it is possible to cool the entire circumference of the round billet 1 from the upper surface to the lower surface only by water cooling from above the round billet 1. For this reason, the spray nozzle provided in the water cooling device 3 may be only the upper nozzle 31. It is preferable to provide both the upper nozzle 31 and the lower nozzle 32, since spray cooling from the top and bottom of the round billet 1 can shorten the cooling time while cooling the circumferential direction more uniformly. Usually, the round billet 1 is often large, with a length of several tens of meters in the longitudinal direction, so it is preferable to provide multiple spray nozzles in the longitudinal direction, but if the entire length of the round billet 1 can be cooled with one spray nozzle, it is also possible to provide one spray nozzle in the longitudinal direction of the round billet 1.

The spray nozzle is preferably a nozzle with a rectangular spray pattern. Figures 8 and 9 show schematic diagrams of a round billet 1 being cooled by a water cooling device 3, viewed from above. Figure 8 shows a case where a full cone nozzle with a circular cooling water spray pattern is used as the upper nozzle 31, while Figure 9 shows a case where a square spray nozzle with a rectangular cooling water spray pattern is used as the upper nozzle 31. The hatched areas in Figures 8 and 9 are the cooling water spray areas.

As shown in FIG. 8, when a full cone nozzle is used as the upper nozzle 31, there may be localized non-cooled areas where the cooling water does not hit the round billet 1 compared to the area (spray area) where the cooling water sprayed from the upper nozzle 31 hits directly, resulting in temperature unevenness. On the other hand, as shown in FIG. 9, when a square nozzle is used as the upper nozzle 31, the non-cooled areas where the cooling water sprayed from the upper nozzle 31 does not hit the round billet 1 directly can be eliminated, allowing for uniform cooling. In addition, when using a full cone nozzle or an oval nozzle with an elliptical spray pattern, in order to reduce temperature unevenness, it is sufficient to arrange the spray nozzles closely or use a nozzle with a sufficiently large spray area, but when considering installation costs and maintainability, it is preferable to use a square nozzle.

Furthermore, if the water flow rate from the spray nozzle exceeds 2000 L/ ^m2 min, the cooling capacity remains almost unchanged, so from the economical point of view, the water flow rate is preferably 2000 L/ ^m2 min or less. If the water flow rate from the spray nozzle is less than 20 L/ ^m2 min, the cooling speed is almost the same as air cooling, so no improvement in productivity can be expected. For this reason, the water flow rate from the spray nozzle is preferably 20 L/ ^m2 min or more.

In addition, after the cooling water sprayed from the lower nozzle 32 comes into contact with the underside of the round billet 1, it falls from the surface of the round billet 1 due to gravity. For this reason, even if the upper nozzle 31 and the lower nozzle 32 have the same water volume density, the lower nozzle 32 tends to have a lower cooling capacity than the upper nozzle 31. Therefore, when installing the upper nozzle 31 and the lower nozzle 32, it is preferable to set the water volume density of the lower nozzle 32 to 1.5 to 1.8 times that of the upper nozzle 31.

Although the present invention has been described above with reference to specific embodiments, it is not intended that the invention be limited by these descriptions. By referring to the description of the present invention, other embodiments of the present invention including various modifications in addition to the disclosed embodiment will be apparent to those skilled in the art. Therefore, it should be understood that the embodiments of the invention described in the claims also include embodiments including these modifications described in this specification, either alone or in combination.

The present invention will be described below with reference to examples for a better understanding of the present invention, but the examples are not intended to limit the present invention in any way.
In the embodiment, the round billet 1 was cooled using the cooling equipment shown in FIG. 5. The water cooling device 9 was provided at the exit side of the cooling bed 8, and the thermometer 4 was provided at the entrance side of the water cooling device 3. The cooling bed 2 had a total length (length in the conveying direction) of about 50 m, and the length of the water cooling device 3 in the conveying direction was about 5 m. The conveying method of the cooling bed 8 was a walking beam method. That is, in the embodiment, the round billet 1 was cooled over a distance of about 45 m from the entrance side of the cooling bed 2 to the entrance side of the water cooling device 3. The water cooling device 3 has an upper nozzle 31 and a lower nozzle 32. The upper nozzle 31 and the lower nozzle 32 form nozzle rows above and below the round billet 1, each of which is composed of 11 spray nozzles arranged in a row in the longitudinal direction of the round billet 1. In addition, five nozzle rows arranged in the conveying direction are provided above and below the round billet 1. One nozzle row is configured to be able to cool four round billets 1.

In the embodiment, the spray nozzle was replaced with a square spray nozzle or a full cone nozzle, and water cooling was performed while adjusting the flow rate of the cooling water. The temperature of the cooling water was set to 15°C. The round billet 1 is transported to the outlet side of the cooling bed while rolling on a walking beam. In addition, a thermometer 5 is provided at the outlet side of the cooling bed 2 (the outlet side of the water cooling device 3).

Table 1 shows the results when a round billet 1 with a diameter of 207 mm and a length of 10 m made of steel type A (composition by mass: C: 0.165%, Mn: 0.3%, Si: 0.5%, Cr: 10%, Mo: 0.8%, Cu: 0.02%) or steel type B (composition by mass: C: 0.3%, Mn: 0.3%, Si: 0.5%, Cr: 11%, Mo: 1%) was cooled in the equipment shown in Figure 5. Note that in all conditions, four round billets 1 were used, and they were arranged adjacent to each other on the cooling bed 2. In addition, the surface temperature of both steel types A and B when they reached the cooling bed 2 was 850°C, and the temperature at which they could be magnetized by a lifting magnet was 60°C.

In the embodiment, the target water-cooling start temperature _Tc was determined from formula (4), and after the temperature of the round billet 1 measured by the thermometer 4 reached the target water-cooling start temperature _Tc , the round billet 1 was water-cooled by the water-cooling device 3. In addition, "above" and "upper and lower" in the column of cooling method in Table 1 mean that the round billet 1 was water-cooled from above and from the upper and lower directions of the round billet 1, respectively, and the injection pattern is described in parentheses. In addition, the column of water flow density in Table 1 lists the water flow density of the nozzles, and when the round billet 1 was water-cooled from the upper and lower directions, the water flow density of the upper nozzle 31 is listed in the first line, and the water flow density of the lower nozzle 32 is listed in the second line.

Furthermore, in the column of reciprocating transport at the inlet side of the water cooling device in Table 1, "not carried" means that the round billet 1 was not moved at the location where the temperature was measured because the round billet 1 had not yet reached the target water cooling start temperature _Tc or lower despite reaching the inlet side of the water cooling device. Furthermore, in the column of reciprocating transport at the inlet side of the water cooling device in Table 1, "carried" means that the round billet 1 was not moved at the location where the temperature was measured because the round billet 1 had not yet reached the target water cooling start temperature _Tc or lower despite reaching the inlet side of the water cooling device, and was carried out reciprocating while rolling on the cooling bed 2 until the round billet 1 reached the target water cooling start temperature _Tc . In the column of circumferential temperature deviation and longitudinal temperature deviation at the end of cooling in Table 1, the circumferential and longitudinal temperatures of the round billet 1 were measured with the thermometer 4 (eight circumferential locations and four longitudinal locations), and the difference between the maximum and minimum values was recorded. The cooling time column in Table 1 shows the time required for the round billet 1 to reach the inlet side of the cooling bed 2 and be discharged from the cooling bed 2, i.e., the time required for the billet 1 to be cooled naturally on the cooling bed 2 and water-cooled by the water-cooling device 3 to a temperature at which it can be magnetized by the lifting magnet.

In Comparative Example 1, the round billet 1 was not water-cooled on the cooling bed 8, but was allowed to cool to a temperature at which it could be magnetized by a lifting magnet. The circumferential temperature deviation and the longitudinal temperature deviation at the end of cooling were both 20° C., and the cooling time was 12 hours.
In Comparative Example 2, the round billet 1 was cooled to 350°C on the cooling bed 8, and then cooled by the method described in Patent Document 3. Specifically, 10 cooling fans spraying a mixture of water mist and air were arranged at equal intervals on one side of the side of the cooling bed 2 (the end face side of the round billet 1) to cool the round billet 1. The number of spray nozzles installed per cooling fan was 15, the amount of water was 18 L/min, and the average particle size of the water mist was 260 μm. The diameter of the cooling fan was 370 mm, and the amount of air blown was 75 m ³ /min. The circumferential temperature deviation and the longitudinal temperature deviation at the end of cooling were 35°C and 35°C, respectively, and the cooling time was 10 hours. In Comparative Example 2, the cooling time could be shortened compared to Comparative Example 1, but cracks occurred because the target water cooling start temperature T _c was too high.

In Comparative Example 3, the round billet 1 was cooled to 300°C on the cooling bed 8, and then a full cone nozzle was attached to the water cooling device 3 as the upper nozzle 31, and the round billet 1 was water-cooled from above with water at a water flow rate of 9 L/ ^m2 min. The circumferential and longitudinal temperature deviations at the end of cooling were both 40°C, and the cooling time was 10 hours. In Comparative Example 3, the round billet 1 was water-cooled with the water cooling device 3, so that the cooling time could be shortened compared to Comparative Examples 1 and 2, but cracks occurred because the target water-cooling start temperature ^Tc was too high.

Example 1 shows the result of cooling the round billet 1 from above with water at a water flow rate of 9 L/ ^m2 min after cooling the round billet 1 to 195°C on the cooling bed 2 and then attaching a full cone nozzle as the upper nozzle 31 to the water cooling device 3. The circumferential and longitudinal temperature deviations at the end of cooling were both 40°C, and the cooling time was 10 hours. In Example 1, by water-cooling the round billet 1 with the water cooling device 3, the cooling time could be shortened compared to Comparative Examples 1 and 2, and no cracks occurred because the target water-cooling start temperature _Tc was appropriate.

In Example 2, the round billet 1 was cooled to 195°C on the cooling bed 2, and then a full cone nozzle was attached to the water cooling device 3 as the upper nozzle 31, and the round billet 1 was water-cooled from above with water at a water flow rate of 20 L/ ^m2 min. The circumferential and longitudinal temperature deviations at the end of cooling were both 45°C, and the cooling time was 7 hours. In Example 2, the round billet 1 was water-cooled with the water cooling device 3, so that the cooling time could be shortened compared to Example 1, and no cracks occurred because the target water-cooling start temperature _Tc was appropriate.

In Example 3, the round billet 1 was cooled to 195°C on the cooling bed 2, and then a square spray nozzle was attached to the water cooling device 3 as the upper nozzle 31, and the round billet 1 was water-cooled from above with water at a water flow rate of 20 L/ ^m2 min. The circumferential and longitudinal temperature deviations at the end of cooling were both 35°C, and the cooling time was 7 hours. In Example 3, by using a square spray nozzle, the circumferential and longitudinal temperature deviations at the end of water cooling were improved compared to Example 2, and no cracks occurred because the target water cooling start temperature _Tc was appropriate.

In Example 4, the round billet 1 was cooled to 195°C on the cooling bed 2, and then square nozzles were attached to the water cooling device 3 as the upper nozzle 31 and the lower nozzle 32, and the round billet 1 was water-cooled from above and below with water densities of 20 L/ ^m2 min and 30 L/ ^m2 min. The circumferential and longitudinal temperature deviations at the end of cooling were both 30°C, and the cooling time was 6 hours. In Example 4, by performing water cooling from above and below with the square nozzles, the circumferential and longitudinal temperature deviations at the end of water cooling were improved compared to Example 3, and the cooling time was also shortened. In addition, no cracks occurred because the target water cooling start temperature _Tc was appropriate.

In Example 5, the round billet 1 was cooled to 195°C while being transported back and forth on the inlet side of the water cooling device 3 of the cooling bed 2, and then square nozzles were attached to the water cooling device 3 as upper nozzles 31 and lower nozzles 32, and the round billet 1 was water-cooled from above and below with water flow densities of 20 L/ ^m2 min and 30 L/ ^m2 min. The circumferential temperature deviation and longitudinal temperature deviation at the end of cooling were both 20°C, and the cooling time was 6 hours. In Example 5, the billet was transported back and forth on the inlet side of the water cooling device 3 until the target water cooling start temperature _Tc was reached, so the circumferential temperature deviation and longitudinal temperature difference at the end of water cooling were improved compared to Example 4, and no cracks occurred because the target water cooling start temperature _Tc was appropriate.
Comparative Examples 4 to 6 and Examples 6 to 10 are results obtained with steel type B, but the results obtained were similar to those of Comparative Examples 1 and 2 and Examples 1 to 5, so a description thereof will be omitted.

REFERENCE SIGNS LIST 1 round billet 2 cooling bed 21 fixed rake 22 moving rake 3 water cooling device 31 upper nozzle 32 lower nozzle 4, 5 thermometer

Claims (7)

a cooling step of cooling the finish-rolled round billet on a cooling bed;
a water-cooling step of water-cooling the round billet by spraying cooling water from at least above the round billet using a water-cooling device after the cooling step;
Equipped with
In the cooling step, the round billet is cooled until the temperature is equal to or lower than a target water-cooling start temperature ,
The method for manufacturing a round billet , wherein the target water-cooling start temperature is set as a value that satisfies formula (3) based on the relationship between the temperature and distortion of the round billet .
σ _YP > E(ε _f −ε _c ) ...(3)
σ _YP : Yield strength of material [MPa]
E: Young's modulus [MPa]
ε _f : Strain at the end of transformation [-]
ε _c : Strain at the start of water cooling [-] a cooling step of cooling the finish-rolled round billet on a cooling bed;
a water-cooling step of water-cooling the round billet by spraying cooling water from at least above the round billet using a water-cooling device after the cooling step;
Equipped with
In the cooling step, the round billet is cooled until the temperature is equal to or lower than a target water-cooling start temperature,
The method for manufacturing a round billet according to claim 1, wherein the target water-cooling start temperature satisfies formula (4).
Tc<369-345[C]-20[Mn]-0.06[Si]-9.1[Cr]
-19[Mo]-78[Cu]...(4)
Tc: Target water cooling start temperature [℃]
[X]: Chemical composition of X [mass%] 3. The method for producing a round billet according to claim 1 , wherein in the water-cooling step, a water flow density of the cooling water is set to 20 L/m ² min or more and 2000 L/m ² min or less. The cooling method of the water cooling device is spray cooling,
The water cooling device has a plurality of upper nozzles , which are a plurality of spray nozzles, above the round billet;
The upper nozzles are arranged in a longitudinal direction of the round billet,
The method for manufacturing a round billet according to claim 1 , wherein the injection areas of the plurality of upper nozzles cover the entire surface of the round billet as viewed from above. the water cooling device further includes a plurality of lower nozzles below the round billet, the lower nozzles being a plurality of spray nozzles;
The plurality of lower nozzles are arranged in a longitudinal direction of the round billet,
The method for manufacturing a round billet according to claim 4 , wherein the injection areas of the plurality of lower nozzles cover the entire surface of the round billet as viewed from below. The method for manufacturing a round billet according to claim 4 or 5 , wherein the spray nozzle is a spray nozzle with a square spray pattern. The cooling bed is transported by a roll conveyor or a walking beam.
7. The method for producing a round billet according to claim 1 , wherein in the cooling step, the round billet is transported while rolling in association with reciprocating transport on the cooling bed and is allowed to cool.

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