JP2017035732A - Cooling method of steel material, manufacturing method of steel material, cooling device of steel material and manufacturing facility of steel material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technology for manufacturing a steel material having uniform hardness in the thickness direction, by suppressing generation of a martensite structure in a surface layer and hardness increase of the surface layer of the steel material caused thereby.SOLUTION: In cooling method of a steel material, when cooling a steel material after hot rolling or a reheated steel material from a temperature region of Ar 3 points or more to a bainite transformation temperature region, high-pressure air and mist are jetted out simultaneously from separated nozzles to the steel material. The pressure of the high-pressure air is 0.3 MPa or higher, the average particle diameter of the mist is 0.1 mm or smaller, and the water volume density is 10-500 mL/(m-min).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a steel material cooling method, a steel material manufacturing method, a steel material cooling device, and a steel material manufacturing facility for manufacturing a steel material excellent in material uniformity by uniformly cooling the steel material.

  In the process of manufacturing a thick steel plate (plate thickness 6 mm to 200 mm) (hereinafter referred to as a steel material), for example, a slab heated by a heating furnace was subjected to hot rolling and finish rolling using a rolling mill. Then, water cooling or air cooling is performed with a cooling device to control the structure of the steel material. Alternatively, after rolling, the steel material once cooled to near room temperature, for example, is reheated to a temperature not lower than the Ar3 point, and then water cooling or air cooling is performed with a cooling device to control the steel structure. At this time, after cooling to near room temperature, it may be cut into product widths and lengths and then reheated for cooling.

  In particular, the steel material after finish rolling is cooled at a predetermined cooling rate from a temperature range of Ar3 or higher to a relatively low temperature, for example, about 300 to 450 ° C, or reheated to a temperature range of Ar3 or higher. For example, when the steel material is cooled to a temperature range of about 300 to 450 ° C. at a predetermined cooling rate, a bainite structure is obtained, and a steel material having excellent strength and toughness can be produced. In a cooling device, a technique for cooling a steel material by water cooling using spray cooling water, laminar cooling water, or the like is common.

  In recent years, a technology has been developed in which cooling water is sprayed from a large number of nozzles onto the upper and lower surfaces of steel at a high speed to cool the steel at a very high cooling rate, further refine the structure, and increase the strength of the steel material. It has been studied. In order to obtain a predetermined cooling rate at the thickness center of the steel material, it is necessary to rapidly cool the surface layer. When the surface layer is rapidly cooled, there is an advantage that the cooling in the longitudinal direction and the width direction becomes uniform, and the material of the surface layer becomes uniform.

  However, when the steel material is rapidly cooled from the surface layer, the cooling rate inevitably varies in the plate thickness direction, and material variations such as strength and toughness occur in the plate thickness direction. If there are variations in materials such as strength and toughness, depending on the application of the steel material, there may be a partial increase in hardness (surface layer), a decrease in elongation, and a deterioration in weldability. In particular, as the cooling rate increases, martensite is more likely to be generated, and the material variation appears significantly.

  Therefore, it is effective to cool the surface layer so-called slowly without excessively cooling the surface layer. Patent Documents 1 and 2 disclose techniques for performing slow cooling to lower the martensite fraction of the surface layer. Patent Document 1 discloses a method of controlling a cooling zone to a target cooling temperature by dividing the cooling zone into a cooling zone for accelerated cooling and an air cooling zone for reheating by natural heat dissipation and intermittently performing accelerated cooling. .

  In Patent Document 2, a method of intermittently performing accelerated cooling as in Patent Document 1 is proposed in which accelerated cooling is performed by water cooling to the martensite transformation start point or lower, and then the accelerated cooling is stopped and recuperated. Yes.

JP 2005-154841 A JP 2011-206793 A

  However, in the method described in Patent Document 1, the cooling water for accelerated cooling remains on the steel plate in the cooling section, the amount of recuperated heat varies in the longitudinal and width directions, and the material becomes uneven. There is.

  Moreover, in the method described in Patent Document 2, since the martensite transformation point is first cooled by accelerated cooling by water cooling, a martensitic structure is formed in the surface layer, and the ductility and toughness of the surface layer are reduced. is there.

  A method of reducing the martensite fraction by increasing the amount of recuperated and causing bainite transformation is also known, but it is difficult to make the martensite fraction zero, and the material is highly non-uniform.

  The present invention has been made in view of the above, and in the case of cooling a steel material after hot rolling, it suppresses the formation of a martensite structure in the surface layer and the accompanying increase in the hardness of the surface layer of the steel material, and is uniform in the thickness direction. An object of the present invention is to provide a technique for manufacturing a steel material having a certain hardness.

The present invention has the following features in order to achieve such an object.
[1] When cooling a steel material after hot rolling or a reheated steel material from the temperature range of Ar3 or higher to the bainite transformation temperature range, the pressure of the high pressure air and the pressure of high pressure air from separate nozzles are reduced to 0. The cooling method of the steel materials simultaneously injected to steel materials at 3 MPa or more, the average particle diameter of mist being 0.1 mm or less, and the water density is 10 to 500 mL / (m ² · min).
[2] The steel material cooling method according to [1], wherein the high pressure air injection and the mist injection are brought close to each other, the mist and the high pressure air are mixed and collide with the steel material.
[3] The method for cooling a steel material according to the above [1] or [2], wherein the steel material is cooled at a heat transfer coefficient exceeding 3000 W / (m ² K) before and / or after the high-pressure air and mist are injected onto the steel material.
[4] After hot rolling or after reheating, high pressure air and mist are applied to a steel material at a temperature of Ar3 or higher, the pressure of high pressure air from separate nozzles is 0.3 MPa or more, and the average particle diameter of mist is The manufacturing method of the steel material which injects simultaneously at 0.1-mm or less and a water density at 10-500 mL / (m < ² > * min), and cools to a bainite transformation temperature range.
[5] The method for manufacturing a steel material according to [4], wherein the injection is performed by bringing high-pressure air injection and mist injection close to each other, mixing the mist and high-pressure air, and causing the steel material to collide.
[6] The method for producing a steel material according to the above [4] or [5], wherein the steel material is cooled at a heat transfer coefficient exceeding 3000 W / (m ² K) before and / or after the high-pressure air and mist are injected onto the steel material.
[7] A high-pressure air injection nozzle that injects high-pressure air with a pressure of 0.3 MPa or more onto a steel material after hot rolling or a reheated steel material, and an average particle size of 0.1 mm or less and a water density of 10 to 500 mL / ( m ² · min), a steel material cooling device having a mist nozzle for injecting mist, and simultaneously injecting the high-pressure air injection nozzle and the mist nozzle to cool to a bainite transformation temperature.
[8] The steel material cooling device according to [7], wherein the nozzle is provided on at least one of a front surface and a rear surface of an acceleration cooling device using spray cooling water or laminar cooling water.
[9] The steel cooling device according to [8], wherein the accelerated cooling device cools the steel material with a heat transfer coefficient exceeding 3000 W / (m ² K), and any one of claims 7 to 9 And a cooling device.

  ADVANTAGE OF THE INVENTION According to this invention, the steel material which suppresses the raise of the hardness of a surface layer and has uniform hardness in the thickness direction can be manufactured.

FIG. 1 is a schematic diagram illustrating an example of a steel material production line according to an embodiment of the present invention. FIG. 2 is a schematic view showing a cooling device of the steel material production line according to the embodiment of the present invention. FIG. 3 is a diagram showing an outline of the slow cooling device according to the embodiment of the present invention. FIG. 4 is a plan view showing the arrangement of mist nozzles and air injection nozzles of the slow cooling device according to the embodiment of the present invention. FIG. 5 is a diagram showing a method for measuring the mist particle diameter. FIG. 6 is a plan view showing the arrangement of mist nozzles and air injection nozzles of the slow cooling device according to the embodiment of the present invention. FIG. 7 is a plan view showing the arrangement of mist nozzles and air injection nozzles of the slow cooling device according to the embodiment of the present invention. FIG. 8 is a plan view showing the arrangement of mist nozzles and air injection nozzles of the slow cooling device according to the embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a schematic diagram illustrating an example of a steel material production line according to an embodiment of the present invention. In this steel material production line, the slab extracted from the heating furnace 1 is subjected to rough rolling and finish rolling by a rolling mill 2 to a predetermined finished plate thickness. In addition, the temperature of the steel material after hot rolling is Ar3 point or more. After the hot rolling is completed, the steel material is transferred online to a cooling facility. The cooling facility includes a first accelerated cooling device 3, a slow cooling device 4, and a second accelerated cooling device 5. In addition, if the pre-leveler is arranged in front of the first accelerated cooling device 3 and the cooling is performed after adjusting the shape of the steel material, the shape of the steel material after cooling can be improved.

  Further, after the steel material once cooled to near room temperature after hot rolling is reheated to a temperature range of Ar3 or higher, water cooling is performed using the first accelerated cooling device 3, the slow cooling device 4, and the second accelerated cooling device 5. Alternatively, the structure of the steel material may be controlled by air cooling. At this time, after cooling to near room temperature, it may be reheated after being cut into a product width and length, and then cooled.

  As the 1st acceleration cooling device 3 and the 2nd acceleration cooling device 5, the cooling device which supplies spray cooling water or laminar cooling water can be used.

  In the present invention, the front surface of the first accelerated cooling device 3, the rear surface of the second accelerated cooling device 5, or at least one place between the first accelerated cooling device 3 and the second accelerated cooling device 5 is loosened. A cooling device 4 may be arranged. In the example of FIG. 1, a slow cooling device 4 is disposed between the first accelerated cooling device 3 and the second accelerated cooling device 5.

  As will be described later, the slow cooling device 4 simultaneously injects high-pressure air and mist-like cooling water (hereinafter referred to as mist), and preferably cools the steel while mixing.

  In the present invention, since the mist-like water is injected simultaneously with air in the slow cooling device 4, the mist is injected into the steel material and then blown off by the air, so that it is difficult for the mist to remain in the steel material. Moreover, since the time required for evaporation becomes shorter as the water droplet diameter is smaller, a more stable cooling behavior can be realized than the steel material collision.

In the slow cooling device 4, it is preferable to cool the steel material under the condition of a heat transfer coefficient of 100 to 3000 W / (m ² K). When the heat transfer coefficient is less than 100 W / (m ² K), the cooling rate becomes very slow, so that the surface layer hardness and the center hardness are lowered, and when performing slow cooling after rapid cooling, the heat is restored by heat conduction from the inside. Therefore, a part of the material is transformed into a structure other than the target structure such as ferrite and pearlite, and the variation in hardness in the thickness direction may increase. If the heat transfer coefficient exceeds 3000 W / (m ² K), the surface layer is transformed into a martensite structure, and the toughness and ductility are impaired.

  FIG. 2 is a diagram showing a cooling facility of the steel material production line according to the embodiment of the present invention. The first accelerated cooling device 3, the slow cooling device 4, and the second accelerated cooling device 5 are each divided into a plurality of zones. In order to achieve a target cooling rate, the cooling water (in the slow cooling device 4, high-pressure air is used). Mist) and the amount of cooling medium injection can be controlled for each zone.

  The first accelerated cooling device 3, the slow cooling device 4, and the second accelerated cooling device 5 are each provided with an upper surface header that cools the upper surface of the steel material and a lower surface header that cools the lower surface of the steel material. Thermometers 63 and 64 are provided on the front surface and the rear surface of the slow cooling device 4, respectively, to control the temperature of the steel material during the slow cooling.

  Using the cooling zone and the injection amount of the cooling medium, the value calculated by the difference method or the like is compared with the actual value measured by the thermometers 63 and 64, and if there is a difference, the learning coefficient is used and the heat transfer coefficient The steel material temperature history is controlled by correcting.

  One or more thermometers (not shown) are arranged on the front surface, the rear surface and the middle of the first accelerated cooling device 3, and the front surface, the rear surface and the middle of the second accelerated cooling device 5, respectively, and the steel materials measured by the thermometer. Based on the temperature, quality control of the steel material, setting of the zone for injecting the cooling medium, and control of the flow rate of the cooling water are performed. Each thermometer may be a thermometer that measures only one point of the steel material, or may be a thermometer that can measure the temperature distribution in the width direction and the longitudinal direction.

  The table roll 61 and the draining roll 62 have a function of removing the cooling water remaining on the steel material.

  1 and 2 show an example in which the slow cooling device 4 is arranged between the first accelerated cooling device 3 and the second accelerated cooling device 5, but the first accelerated cooling device 4 is arranged in accordance with a desired thermal history. Either one or both of the cooling device 3 and the second accelerated cooling device 5 may not be arranged. In addition, when the first acceleration cooling device 3 and the second acceleration cooling device 5 are arranged and cooling by the first acceleration cooling device 3 and the second acceleration cooling device 5 is not necessary so as to cope with various heat histories. Is configured to close the water system valve that supplies the cooling water of each cooling device, stop the injection of the cooling water, and allow the steel material to pass through the first accelerated cooling device 3 and the second accelerated cooling device 5. May be.

Also, the first accelerated cooling device 3 and / or the second accelerated cooling device 5 is arranged, and in at least one of the first accelerated cooling device 3 and the second accelerated cooling device 5, a heat transfer coefficient of 3000 W / (m ² K). It is good also as a structure which cools steel materials in excess. Thus, by performing accelerated cooling before and / or after slow cooling, while reducing the time required for cooling, the generation of martensite structure in the surface layer and the accompanying increase in hardness of the surface layer of the steel material is suppressed, and the thickness direction It is possible to produce a steel material having a uniform hardness.

  Next, the outline of the slow cooling device 4 will be described. Drawing 3 is a figure showing the outline of the slow cooling device of the steel material production line concerning an embodiment of the invention.

  The slow cooling device 4 has a mist nozzle 41 and an air injection nozzle 42, and is configured to inject mist and high-pressure air simultaneously onto a steel material. The mist nozzle 41 and the air injection nozzle 42 are respectively provided above and below the steel material.

  The mist nozzle 41 ejects mist-like water. The mist nozzle 41 may be a one-fluid nozzle that ejects only water, or may be a fluid nozzle that ejects water and air simultaneously. The mist nozzle 41 of FIG. 3 shows a two-fluid nozzle as an example. The mist nozzle 41 of FIG. 3 mixes and sprays the water stored in the water tank 44 with the air supplied from the air supply header 43.

  The size of the mist is 0.1 mm or less in average particle size, more preferably 0.05 mm or less, further preferably 0.02 mm or less, and further preferably 0.01 mm or less. The smaller the mist particle size, the more difficult it is to condense the mist on the steel material, and it is possible to reduce the non-uniform temperature of the steel material due to non-uniform cooling water remaining in the steel material.

  In the case of a two-fluid nozzle, the size of the mist is generally determined by the nozzle shape and the air / water ratio (ratio of air volume to water volume). About management of the magnitude | size of mist, it is preferable to manage an air-water ratio with the mist shape actually used.

  The distance from the mist nozzle 41 to the steel material is preferably 10 mm or more and 500 mm or less. When the distance from the mist nozzle 41 to the steel material is less than 10 mm, there is no escape space after the mist has evaporated. This is because the mist that has lost its escape is condensed and becomes large water droplets and adheres to the steel material, and water droplets remain on the surface of the steel material, causing uneven temperature. In addition, if the spray distance from the mist nozzle 41 to the steel material exceeds 500 mm, the mist does not reach the steel material, so that the cooling capacity is lowered and the mist is difficult to adhere to the steel material.

The water density of the mist is 10 to 500 mL / (m ² · min). If the water density of the mist is less than 10 mL / (m ² · min), the heat transfer coefficient of the mist is very small, the steel material cannot be sufficiently cooled, and the internal strength of the steel material is reduced. Further, if the water density of the mist exceeds 500 mL / (m ² · min), the mist cannot be removed after the cooling is completed, and residual water is generated, so that the cooling stop temperature control becomes difficult.

  In addition, you may comprise so that the water which cooled steel materials may be collect | recovered, and after filtering and water temperature adjustment, it may return to the water tank 44 and circulate. Furthermore, an electric heater or steam may be blown into the water tank to heat the water tank temperature in order to obtain a temperature necessary for mist injection. Further, a heater may be attached to the pipe and heated.

  The water temperature may be managed by installing a thermometer 45 in the immediate vicinity of the water tank 44 or in the immediate vicinity of the mist nozzle 41 as shown in FIG. However, since the water temperature changes when passing through the pipe, it is preferable to arrange the water temperature at a position closest to the mist nozzle 41. The temperature of water sprayed from the mist nozzle 41 is 30 ° C. or higher, and the higher the temperature, the better.

The air injection nozzle 42 is supplied with air from the air supply header 43 and injects high-pressure air. A pressure gauge is installed immediately before the air injection nozzle 42 to measure the pressure of the air supply header 43. The air injection nozzle 42 injects air with an injection pressure of 0.3 MPa or more. This is because if the air injection pressure is less than 0.3 MPa, the mist is not sufficiently accelerated and the mist does not collide with the steel material, resulting in a decrease in cooling capacity.
The amount of high pressure air is ^preferably 140L / (m 2 · min) (nor) ^or, if it 140L / (m 2 · min) lower than (nor), is to create a high pressure air above 0.3MPa It can be difficult.

  FIG. 4 is a plan view showing the arrangement of mist nozzles and air injection nozzles of the slow cooling device according to the embodiment of the present invention. In order to ensure that the mist injected from the mist nozzle 41 reaches the steel material, it is preferable to arrange an air injection nozzle 42 around the mist nozzle 41 as shown in FIG.

  It is preferable that the high pressure air injection and the mist injection are close to each other, and the distance between the nozzle centers of the mist nozzle 41 and the air injection nozzle 42 is 200 mm or less. If the distance between the centers of the nozzles exceeds 200 mm, the mist is not accelerated by the high-pressure air, so the steel material collision speed of the mist is reduced. In addition, uneven cooling tends to occur in the width direction. In addition, the distance between the nozzle centers of the adjacent air injection nozzles 42 may be set to be approximately the same as the distance between the nozzle centers of the mist nozzle 41 and the air injection nozzle 42.

  In addition, although this invention demonstrated as an example the steel material manufacturing line, this invention is not restricted to this, It can also apply to a thin plate manufacturing line, a shape steel manufacturing line, and a pipe manufacturing line.

  Next, a method for cooling a steel material according to the present invention will be described.

  The present invention can be applied to a method for cooling a steel material that does not produce a hard phase (martensite structure) on the surface layer. As an example of such a cooling method, there is a method in which a steel material after hot rolling or a reheated steel material is cooled from a temperature at an Ar3 point or higher (austenite phase temperature range) to a bainite transformation temperature range. However, if cooling is performed to a temperature range lower than the martensitic transformation start point, a temperature distribution is generated in the thickness direction, the surface layer is excessively cooled, and the ductility and toughness of the surface layer are significantly reduced. Therefore, it is necessary to stop the cooling at a temperature equal to or higher than the martensitic transformation start point. Moreover, if the water used for accelerated cooling remains on the steel material, the cooling rate increases due to the residual water and martensitic transformation occurs. Therefore, it is preferable to remove residual water using the table roll 61 and the draining roll 62. Further, as an example, the method of removing the residual water by the table roll 61 and the draining roll 62 has been described, but the residual water may be removed by other methods such as air purge.

  Normally, in the process of cooling steel with water, in a high temperature range, a vapor film is formed between the steel and cooling water, and the film boiling state in which heat is transferred through the vapor film becomes dominant. ing. This film boiling state has a smaller cooling capacity than the nucleate boiling state in which the cooling water is in direct contact with the steel material. However, when the steel material temperature is lower than 450 ° C., the vapor film generated by boiling cannot be maintained, and a nucleate boiling place where the steel material and the cooling water are in direct contact with each other is generated.

  In such a nucleate boiling place, the cooling rate increases rapidly, so that the surface layer of the steel material falls below the martensitic transformation start point before the bainite transformation occurs, and the surface layer may become a martensitic structure. Therefore, in order to transform the surface layer into a bainite transformation in the range of 300 to 450 ° C., for example, it is preferable to perform slow cooling. And in this invention, in order to perform slow cooling, it cools by mist, makes the mist boil easily by making the particle size of mist fine or raising the water temperature of mist. By making the mist easily boiled, the film boiling state is easily stabilized even in a region of 450 ° C. or lower, and stable slow cooling can be performed.

  In the present invention, it is preferable to carry out slow cooling in the temperature range of 300 to 500 ° C., which is a temperature range in which the nucleate boiling state is likely to change, and it is important that the bainite transformation is maintained in the temperature range until it is finished. is there. For example, after rapidly cooling to 300 ° C. or less, recuperating and using slow cooling, the temperature is maintained, or after rapidly cooling to 400 ° C., slowly cooling to control the recuperation to a temperature range of 300 to 500 ° C. Any temperature pattern of temperature holding and slow cooling can be performed. Moreover, since the residual water of the steel material can be reduced by injecting air simultaneously with the mist, nucleate boiling is less likely to occur, and more stable slow cooling is possible. Accelerated cooling by water cooling, such as laminar cooling or spray cooling, may be performed in a high temperature range where film boiling occurs or in a low temperature range after bainite transformation is completed.

  After the surface bainite transformation is completed, accelerated cooling can be performed again in order to increase the hardness inside the steel material.

  Here, whether or not the bainite transformation of the surface layer has been completed may be determined as follows. The temperature range causing the bainite transformation and the transformation completion time are obtained in a laboratory by conducting a test using a hot working simulator such as a working for master. And the temperature of steel materials is measured by the thermometers 63 and 64 arrange | positioned before and behind the slow cooling apparatus 4 at the time of manufacture of steel materials. Then, it may be determined whether the bainite transformation has been completed from the temperatures measured by the thermometers 63 and 64 and the passing time of the slow cooling device 4.

  The “surface layer” used in the present invention refers to a portion having a depth of 3 mm or less from the steel surface. At a depth of more than 3 mm from the surface, the cooling rate does not increase from the surface. Therefore, when a martensite structure is not formed on the surface layer, martensite transformation does not occur at a position deeper than 3 mm.

Hereinafter, a description will be given based on examples.
The steel production line described in FIGS. 1 and 2 was used to produce a steel material for shipbuilding having a tensile strength of 600 MPa class and an upper limit of surface layer Vickers hardness of Hv220, and the effect of the present invention was verified. The composition of the steel material is mass%, C: 0.06%, Si: 0.15%, Mn: 1.85%, P: 0.005%, S: 0.0005%, Cr: 0.2%, Mo: 0.2%, Nb: 0.05%, The balance was Fe and inevitable impurities. The structure of the slow cooling device 4 used is as shown in FIG. In this example, a steel material of a steel type having a tensile strength of 600 MPa was used. The Ar3 point of this steel material is 724 ° C., and the bainite transformation temperature is between 300 ° C. and 450 ° C. depending on the cooling rate.

  As shown in FIG. 1, the slab extracted from the heating furnace 1 was roughly rolled by a rolling mill 2 and further subjected to finish rolling to a plate thickness of 20 mm and a plate width of 4.5 m. The surface temperature of the steel material measured immediately after finish rolling, that is, the finish temperature was 800 ° C.

  In the inventive examples 1 to 4 and the comparative examples 1 to 4, after the finish rolling, the mist and the high-pressure air are simultaneously applied by the slow cooling device 4 without passing through the pre-leveler and performing the first accelerated cooling in the first accelerated cooling device 3. The steel material was sprayed for slow cooling. The slow cooling start temperature is set to 780 ° C., which is higher than the Ar3 point (724 ° C.) of the test steel, and the slow cooling end temperature is 300 to 350 ° C. except for Comparative Example 2, and Comparative Example 2 has an insufficient water density and is desired. Since the cooling rate could not be obtained, the temperature was set to 450 to 500 ° C. In addition, the conveyance speed of steel materials was 12.5 mpm. After completion of the slow cooling, the second accelerated cooling device 5 was not subjected to accelerated cooling, and was allowed to cool until the surface layer temperature became 100 ° C. or lower.

In Inventive Examples 5 to 17 and Comparative Examples 5 to 7, after finish rolling, the first accelerated cooling with a heat transfer coefficient of 6500 W / (m ² K) is performed in the first accelerated cooling device 3 through the pre-leveler, Mist and high-pressure air were simultaneously jetted onto the steel material by the slow cooling device 4 to perform slow cooling. The accelerated cooling start temperature was 780 ° C. higher than the Ar3 point (724 ° C.) of the test steel, the accelerated cooling end temperature was 450 to 500 ° C., and the slow cooling end temperature was 300 to 350 ° C. In addition, the conveyance speed of steel materials was 12.5 mpm. After the end of the slow cooling, the second accelerated cooling apparatus 5 again performed the accelerated cooling with a heat transfer coefficient of 3200 W / (m ² K), and the second accelerated cooling was performed until the surface layer temperature became 100 ° C. or lower.

  Here, the nozzle arrangement of the slow cooling device is the arrangement shown in FIG. Here, the interval between the nozzles was 50 mm.

Table 1 shows air injection pressure, mist average particle diameter, water density, and mist water temperature, which are conditions during slow cooling.
Here, a method for measuring the mist particle diameter will be described with reference to FIG. Under the nozzle 71, a shutter 72 and an oil adhesion plate 73 coated with oil 74 are arranged in this order from the nozzle 71 side. The shutter 72 has a hole or slit 72a. The shutter 72 is movable in the horizontal direction. Mist is ejected from the nozzle 71 to move the shutter 72 in the horizontal direction at high speed. Thereby, of the mist ejected from the nozzle 71, the mist 75 that has passed through the hole or slit 72 a reaches the oil adhesion plate 73. The particle diameter of the mist can be measured by observing the mist 75 that has reached the oil adhering plate 73 with a magnifying glass or a microscope, and performing image processing. In this example, 500 or more mists 75 that reached the oil adhesion plate 73 were observed for each condition, and the average particle diameter was referred to as “mist average particle diameter”.

  Samples of the manufactured steel material were cut out and the following hardness measurements were performed. The obtained results are shown in Table 1.

  “Hardness variation” in Table 1 is the difference between the maximum value and the minimum value among the obtained hardnesses measured in accordance with JIS Z 2244 in the width direction, the longitudinal direction, and the thickness direction of the steel material. Is within the range of Hv40 is indicated by “◯”, and those exceeding the range of Hv40 are indicated by “x”.

The surface hardness was measured by the following method. The Vickers hardness of the cross section perpendicular to the rolling direction was measured according to JIS Z 2244.
The sample is 10 mm from the longitudinal ends of the thick steel plate, 1/4 position in the longitudinal direction, 10 mm position in the width direction plate thickness direction at 1/2 position, 1/4 position in width, 1/2 position meter. The sample is collected, and the hardness is measured at a pitch of 1 mm in the thickness direction, and the highest hardness among the obtained hardness is shown in Table 1 as the surface layer hardness.

  In Examples 1 to 17 of the present invention, the surface layer hardness Hv (98N) was Hv 219 at the maximum, and the hardness Hv 220 of the allowable upper limit of the hardness was satisfied. Further, in Examples 1 to 17 of the present invention, the variation in hardness was within the range of Hv40.

  In Comparative Examples 1 and 3 to 7, the surface layer hardness exceeded Hv220 and the hardness variation was outside the range of Hv40, and the target surface layer hardness and hardness variation could not be obtained. In Comparative Example 2, since the water density was small, the desired cooling rate was not obtained, and the slow cooling end temperature was high, the steel structure was not a desired bainite monolayer, but a pearlite mixed structure. As a result, the surface hardness was within the allowable range, but the hardness varied.

Further, as Conventional Example 1, a steel material was manufactured using the method described in Patent Document 1. Specifically, the steel material was cooled by one accelerated cooling device having a heat transfer coefficient of 6500 W / (m ² K) without using a slow cooling device. In the accelerated cooling device, water cooling was performed in the first, third, and fifth to eleventh zones out of all eleven zones, and air cooling was performed in the second and fourth zones. The surface layer hardness was Hv400 at the maximum, greatly exceeding the allowable upper limit of hardness Hv220, and the hardness variation was large.

As Conventional Example 2, a steel material was manufactured using the method described in Patent Document 2. Specifically, the steel material was cooled from one accelerated cooling device having a heat transfer coefficient of 6500 W / (m ² K) without using a slow cooling device. The cooling start temperature is set to 785 ° C., the steel surface layer is cooled to 350 ° C. or less by the first water cooling, the water cooling is stopped and reheated, the reached temperature after the reheating is set to 650 ° C., and the cooling stop temperature is set to the maximum temperature of 420 ° C. The minimum temperature was 380 ° C. As a result, the maximum surface hardness was Hv300, which was lower than that of Conventional Example 1, but did not become the target Hv220 or less, and the hardness variation was large.

  When the cross section of the surface layer of the present invention examples and comparative examples in Examples 1 and 2 and Table 1 and the comparative example, 1/4 t, and the central part of the plate thickness was observed with an optical microscope, the conditions of the present invention example were bainite structures at all positions However, in the conventional examples 1 and 2 and the comparative examples 1 and 3 to 7, at least 10% or more of the martensite structure is observed on the surface layer, and in the comparative examples 1, 2 and 5, pearlite is observed at least 10% or more at the 1/4 t position. It was.

  In Comparative Example 2, since the water density was too small and a desired cooling rate could not be obtained, a pearlite structure was generated in the vicinity of the plate thickness ¼. In Comparative Examples 1 and 5, since the air injection pressure was too small, the surface layer was supercooled by the mist, and a martensite structure was generated in the surface layer. In Comparative Examples 3, 4, 6, and 7, since the mist average particle size was too large or the water density was too large, the surface layer was supercooled by the mist, and a martensite structure was generated in the surface layer. As described above, in Comparative Examples 1 to 7, the variation in hardness was large.

  In the present invention, the martensite fraction of the surface layer does not necessarily have to be 0%, and if it is less than 10%, the effect of the present invention can be obtained in which uniform and sufficient hardness is obtained in the thickness direction. .

  Next, the effect by arrangement | positioning of the mist nozzle 41 and the air injection nozzle 42 was implemented.

  The steel production line described in FIGS. 1 and 2 was used to produce a steel material for shipbuilding having a tensile strength of 600 MPa class and an upper limit of surface layer Vickers hardness of Hv220, and the effect of the present invention was verified. The structure of the slow cooling device 4 used is as shown in FIG. As the steel material, the same steel material having a tensile strength of 600 MPa as in Example 1 was used.

As shown in FIG. 1, the slab extracted from the heating furnace 1 was roughly rolled by a rolling mill 2 and further subjected to finish rolling to a plate thickness of 20 mm and a plate width of 4.5 m. The surface temperature of the steel material measured immediately after finish rolling, that is, the finish temperature was 800 ° C.
After the finish rolling, the first accelerated cooling device 3 performs the first accelerated cooling with a heat transfer coefficient of 3200 W / (m ² K) through the pre-leveler, and then the mist and high-pressure air are simultaneously injected onto the steel by the slow cooling device 4. Then, slow cooling was performed. In the present invention example to which the present invention is applied, the accelerated cooling start temperature is 780 ° C. higher than the Ar3 point of the test steel, the accelerated cooling end temperature is 450 to 500 ° C., and the slow cooling end temperature is 300 to 350 ° C. did. In addition, the conveyance speed of steel materials was 12.5 mpm. After the end of the slow cooling, the second accelerated cooling device 5 again performed the accelerated cooling with a heat transfer coefficient of 5000 W / (m ² K), and the second accelerated cooling was performed until the surface layer temperature became 100 ° C. or lower.

  In order to verify the effect of the nozzle arrangement of the slow cooling device, the ratio of the mist nozzle 41 is decreased and the ratio of the air injection nozzle 42 is increased (example 18 of the present invention, arrangement shown in FIG. 6), mist nozzle 41 and the number of air injection nozzles 42 are the same (Example 19 of the invention, arrangement shown in FIG. 7). Similarly, the number of mist nozzles 41 and the number of air injection nozzles 42 is the same. The present invention example (invention example 20, arrangement shown in FIG. 8) in which the adjacent nozzle is not necessarily the air injection nozzle 42 but has a nozzle whose center-to-center distance between the mist nozzle 41 and the air injection nozzle exceeds 200 mm. 6, 7, and 8, the interval between adjacent nozzles is 50 mm.

Table 2 shows air injection pressure, mist average particle diameter, water density, and mist water temperature, which are conditions during slow cooling. The test was conducted under the same conditions as in Example 1 except for the nozzle arrangement. Although the number of the mist nozzles 41 was changed, the water amount density was also the same as that of the first embodiment, so the flow rate injection amount per mist nozzle was changed.
The method for measuring the mist particle size, the method for measuring the hardness variation and the surface layer hardness, and the criteria for judgment are the same as in Example 1. The obtained results are shown in Table 2.

In Inventive Examples 18 and 19, the surface layer hardness Hv (98N) was Hv 183 at the maximum, sufficiently satisfying the hardness Hv 220 of the allowable upper limit of hardness. Further, in Examples 18 and 19 of the present invention, the variation in hardness was within the range of Hv40. In Inventive Example 20, the maximum hardness of the surface layer was Hv219, which was just below the allowable upper limit of hardness Hv220, and the variation was within the allowable range, but was very close to Hv38. Since the nozzle adjacent to the mist nozzle 41 is not necessarily the air injection nozzle 42 and there is a nozzle whose center distance between the mist nozzle 41 and the air injection nozzle exceeds 200 mm, the cooling becomes excessive. It is preferable to make it adjoin to the injection nozzle 42.

DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Rolling machine 3 1st acceleration cooling device 4 Slow cooling device 5 2nd acceleration cooling device 41 Mist nozzle 42 Air injection nozzle 43 Air supply header 44 Water tank 45 Thermometer 61 Table roll 62 Draining roll 63, 64 Thermometer 71 Nozzle 72 Shutter 72a Hole or slit 73 Oil adhesion plate 74 Oil 75 Mist

Claims (10)

When cooling the steel material after hot rolling or the reheated steel material from the temperature range of Ar3 point or higher to the bainite transformation temperature range,
High-pressure air and mist are jetted simultaneously onto steel from a separate nozzle at a high-pressure air pressure of 0.3 MPa or more, a mist average particle size of 0.1 mm or less, and a water density of 10 to 500 mL / (m ² · min). To cool the steel. The jet is
The method for cooling a steel material according to claim 1, wherein the high pressure air injection and the mist injection are brought close to each other, the mist and the high pressure air are mixed and collide with the steel material. The steel material cooling method according to claim 1 or 2, wherein the steel material is cooled at a heat transfer coefficient exceeding 3000 W / (m ² K) before and / or after the high-pressure air and mist are injected into the steel material. After hot rolling or after reheating, the steel material at a temperature of Ar3 point or higher,
The pressure of the high pressure air and the mist from separate nozzles is 0.3 MPa or more, the average particle size of the mist is 0.1 mm or less, and the water density is 10 to 500 mL / (m ² · min),
A method for producing a steel material that is simultaneously injected and cooled to a bainite transformation temperature range. The jet is
The manufacturing method of the steel materials of Claim 4 which makes high pressure air injection and mist injection adjoin, mixes mist and high pressure air, and collides with steel materials. The method for producing a steel material according to claim 4 or 5, wherein the steel material is cooled at a heat transfer coefficient exceeding 3000 W / (m ² K) before and / or after the high-pressure air and mist are injected into the steel material. A high-pressure air injection nozzle that injects high-pressure air having a pressure of 0.3 MPa or more onto a steel material after hot rolling or a reheated steel material, and an average particle size of 0.1 mm or less and a water density of 10 to 500 mL / (m ² · a cooling apparatus for a steel material having a mist nozzle for injecting a mist of (min), and simultaneously cooling the high-pressure air injection nozzle and the mist nozzle to cool to a bainite transformation temperature.   The steel nozzle cooling device according to claim 7, wherein the nozzle is provided on at least one of a front surface and a rear surface of an acceleration cooling device using spray cooling water or laminar cooling water. The steel cooling device according to claim 8, wherein the accelerated cooling device cools the steel material with a heat transfer coefficient exceeding 3000 W / (m ² K).   A steel production facility comprising a rolling mill and the cooling device according to claim 7.

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