JP5482365B2 - Steel sheet cooling method, manufacturing method and manufacturing equipment - Google Patents

Description

  The present invention relates to a steel plate cooling method, a manufacturing method, and manufacturing equipment.

  In the process of manufacturing steel plates such as thick plates and thin plates by hot rolling, for example, in equipment as shown in FIG. 7, after hot rough rolling and finish rolling, the structure is controlled by water cooling or air cooling. Yes. When cooling to a relatively low temperature, for example, about 450 to 650 ° C. by water cooling, fine ferrite and bainite structure can be obtained and the strength of the steel sheet can be secured. Therefore, there is a technology for cooling the steel sheet with spray cooling water, laminar cooling water, etc. It is common.

  Further, in recent years, technological development has been actively conducted to increase the strength of a steel sheet by injecting cooling water from nozzles installed on the upper and lower surfaces of the steel sheet at a high speed to obtain a very high cooling rate and further refine the structure.

  However, when the strength of the steel sheet is increased by a high cooling rate, depending on the use of the steel sheet, the surface layer of the steel sheet becomes too hard due to rapid cooling, and the elongation may decrease or the weldability may deteriorate. As a technique for preventing this, there is a technique of Patent Document 1.

  The technique of Patent Document 1 is to reduce the deviation of the cooling stop temperature between the tip and tail ends of the steel plate for line pipes with a long rolling length, and to obtain a uniform material in the longitudinal direction of the steel plate. Is carried into the accelerating cooling device and accelerated at a constant acceleration rate until the tail end of the steel plate exits the accelerating cooling device, so that the conveying speed of the steel plate gradually increases from the steel plate tip to the tail end. .

  By doing in this way, the deviation in the longitudinal direction of the steel sheet of the cooling stop temperature is reduced, and further, intermittent cooling is performed to increase the cooling stop temperature and control it to a desired cooling rate.

JP 2005-154841 A

  The technique of Patent Document 1 can achieve an effect to some extent in a steel sheet having a relatively thin plate thickness (20 mm or less), but in a steel plate having a plate thickness of 50 mm or more, in order to cool from a predetermined cooling start temperature to a cooling end temperature, Since it is necessary to set a long cooling time, the conveyance speed must be relatively slow (for example, about 0.1 to 0.8 m / s). The slower the conveyance speed, the longer the time for cooling in the first zone, and the surface layer of the steel sheet is particularly rapidly cooled to be cooled to the martensitic transformation start point (hereinafter referred to as the Ms point) or less. . Therefore, there is a problem that the steel sheet having a thick plate thickness and a low conveyance speed cannot sufficiently suppress the increase in hardness of the steel plate surface by the technique of Patent Document 1.

  In addition, even when air cooling is performed in the next cooling zone after performing top surface cooling, the water draining roll may not completely drain the water when the shape of the steel sheet is poor. In such a case, cooling by riding water is performed on the upper surface of the steel sheet in the air cooling zone, and sufficient reheating cannot be performed, and the hardness of the surface layer cannot be reduced. Even if it is partially, if the hardness exceeds the upper limit, the quality cannot be guaranteed.

  Furthermore, if the recuperation time by air cooling is too long, the time for subsequent water cooling is insufficient and cooling may be insufficient. Conversely, if the recuperation time is too short, the steel sheet surface layer is not sufficiently tempered and hardness There is also a problem that exceeds the allowable upper limit.

  In view of the above, the present invention provides a technique for suppressing the increase in hardness of the steel sheet surface and producing a steel sheet having sufficient strength when supplying cooling water to the upper and lower surfaces of the steel sheet after hot rolling. With the goal.

  In order to solve the above problems, the present invention has the following features.

The first invention includes a plurality of cooling zones separated by a table roll and a draining roll facing the table roll, and supplies cooling water to the upper and lower surfaces of the steel sheet at a water density of 2.0 m ³ / m ² · min or more. A steel sheet cooling method using a cooling facility, wherein the steel sheet after hot rolling is heated at a temperature not lower than the Ar ₃ point, and the surface layer portion of the steel sheet in the cooling zone on the most upstream side is below the martensitic transformation start point. After accelerating cooling until the cooling zone, one or more subsequent cooling zones are set as air cooling sections, and the accelerated cooling is temporarily stopped. After reheating until the steel plate surface layer temperature reaches 640 ° C. or higher, the steel plate is cooled in the subsequent cooling zone. The steel sheet cooling method is characterized in that accelerated cooling is performed again until the plate thickness average temperature becomes 520 ° C. or less.

  The second invention is a method for cooling a steel sheet according to the first invention, wherein the steel sheet has a thickness of 50 mm or more, and the conveying speed of the steel sheet passing through the cooling facility is 1.0 m / s or less. It is.

  The third invention cools the steel sheet by the method for cooling a steel sheet described in the first or second invention, and includes a width direction thermometer provided in the cooling zone entrance side and the air cooling section on the most upstream side. By measuring the temperature in the width direction of the upper surface of the steel sheet, by comparing the measured temperature in the width direction of the upper surface of the steel sheet with the cooling start temperature and the reheat temperature of the steel sheet surface layer portion in the air cooling section, and the respective reference temperatures, It is a method for producing a steel sheet, wherein it is determined whether or not the hardness of the steel sheet surface layer after cooling is within an allowable value.

The fourth invention includes a plurality of cooling zones separated by a table roll and a draining roll facing the table roll, and supplies cooling water to the upper and lower surfaces of the steel sheet at a water density of 2.0 m ³ / m ² · min or more. A steel sheet cooling facility, a width direction thermometer installed on the inlet side of the cooling facility, and the cooling zone installed in the third or fourth cooling zone from at least the upstream side of the plurality of cooling zones. A width direction thermometer that measures the temperature in the width direction of the steel sheet upper surface when the air cooling section is in the air cooling section, the cooling start temperature from the temperature of the steel sheet upper surface measured by these width direction thermometers, and the recuperation temperature of the steel sheet surface layer in the air cooling section And a surface layer hardness pass / fail judgment means for judging whether or not the hardness of the steel sheet surface layer portion after cooling is within an allowable value by comparing each reference temperature with each reference temperature. Equipment.

  By using the thick steel plate cooling method, production method, and production equipment of the present invention, it is possible to produce a steel plate that suppresses the increase in hardness of the steel plate surface and has sufficient strength. Moreover, it can be ensured that the hardness of the manufactured steel sheet surface layer portion is within an allowable value.

It is a side view of the cooling equipment concerning one embodiment of the present invention. It is a side view of the front half part of the cooling equipment which concerns on one Embodiment of this invention. It is a figure which shows an example of the time change of the temperature measurement value when a steel plate is cooled with the installation of this invention. It is a figure which shows an example of the width direction temperature distribution when a steel plate is cooled with the installation of this invention. It is a figure which shows an example of the temperature history when a steel plate is cooled with the installation of this invention. It is a figure which shows the dispersion | variation in the hardness of the steel plate surface layer when it cools with the installation of this invention, and the conventional installation. It is a figure explaining the outline of a thick plate rolling line.

  Hereinafter, an example of an embodiment of the present invention in a plate rolling process will be described with reference to the drawings.

FIG. 7 is a schematic view showing an example of a thick plate rolling line used for carrying out the present invention.
The slab extracted from the heating furnace is subjected to rough rolling and finish rolling by a rolling mill to a predetermined finishing temperature and finishing plate thickness, and then conveyed to an accelerated cooling facility online. It is suitable for the steel plate shape after cooling to perform accelerated cooling after adjusting the shape of the steel plate through a pre-leveler before cooling. In the accelerated cooling facility, a cooling facility for flowing film-like cooling water along the conveying direction is arranged on the upper surface side of the steel plate, and a cooling facility for supplying rod-shaped cooling water is arranged on the lower surface side of the steel plate.

  FIG. 1 is a side view showing the whole cooling equipment in one embodiment of the present invention.

  Many cooling zones with one zone between the table rolls 4 and 4 are provided, and cooling headers 1 and 2 are arranged above and below each zone. There are width direction thermometers 5a, 5b, 5c above the entry side of the first cooling zone, the third or fourth cooling zone, and the exit side of the final zone, respectively, and measure the width direction temperature of the upper surface of the steel sheet being conveyed. .

FIG. 2 is a side view showing the first half portion of the cooling facility according to the embodiment of the present invention. This cooling equipment is provided with a plurality of cooling zones partitioned by a table roll 4 and a draining roll 3 facing the table roll 4 at least in the first half thereof. The cooling water can be supplied from the cooling headers 1 and 2 disposed above and below the respective zones to the upper and lower surfaces of the steel sheet at a water density of 2.0 m ³ / m ² · min or more. Here, the water density is set to 2.0 m ³ / m ² · min or more when the water density is less than 2.0 m ³ / m ² · min. This is because product materials such as strength and elongation may vary due to mixing. In the example of the cooling equipment shown in FIG. 2, the cooling equipment for flowing the film-like cooling water along the conveying direction is arranged on the upper surface side of the steel plate, and the cooling equipment for supplying the rod-shaped cooling water is arranged on the lower surface side of the steel plate. The cooling method of the cooling facility of the present invention is not limited to this.

  In addition, a width direction thermometer 5a that measures the temperature in the width direction of the upper surface of the steel plate before the start of cooling is provided on the entry side of the cooling facility, that is, the entry side of the first cooling zone. A width direction temperature 5b capable of measuring the width direction temperature of the upper surface of the steel sheet when the zone is an air cooling section is provided. These width direction thermometers 5a and 5b may be capable of simultaneously measuring the temperature in the entire width direction of the steel sheet, or may be of a scanning type that measures the temperature in the entire width direction while moving in the width direction. And although details will be described later, it is determined whether or not the hardness of the surface layer portion of the steel sheet after cooling is within an allowable value from the width direction temperature of the upper surface of the steel sheet measured by the width direction thermometers 5a and 5b. Means 7 are provided.

Then, the steel plate cooling method of the present invention, by using a cooling facility having a plurality of cooling zones, such as shown in FIGS. 1 and 2, the steel sheet after hot rolling, the Ar ₃ point or more temperature In the most upstream cooling zone, accelerated cooling is performed until the surface layer of the steel sheet reaches a temperature below the martensitic transformation start point (Ms point), and the subsequent cooling zone is used as an air cooling section without supplying cooling water. Accelerated cooling is interrupted and reheated until the steel plate surface layer temperature reaches 640 ° C. or higher, and cooling water is supplied in the subsequent cooling zone to accelerate cooling again until the steel plate thickness average temperature reaches 520 ° C. or lower. . Details will be described below.

The steel sheet having a predetermined size by hot rolling is transported into the cooling facility after the shape is corrected by the pre-leveler. Here, the cooling start temperature is set to a temperature of Ar ₃ points or more. This is because if the cooling start temperature is less than the Ar ₃ point, coarse ferrite partially transforms before starting cooling and causes a decrease in strength. Note that the cooling start temperature is confirmed from the temperature measured by the width direction thermometer 5a on the cooling equipment entry side to determine whether the temperature is equal to or higher than Ar ₃ points in the entire width.

Next, the steel sheet transported into the cooling facility has a steel sheet surface layer portion by cooling water supplied at a water density of 2.0 m ³ / m ² · min or more in the most upstream cooling zone, that is, the first cooling zone. Rapid cooling is performed until the Ms point is reached. Since a large amount of cooling water is supplied, the cooling rate at the initial stage of cooling at a steel sheet surface layer depth of 1 mm is 100 ° C./s or more.

  Here, the reason why the steel sheet surface layer portion is rapidly cooled until the surface layer portion is equal to or lower than the Ms point is to cause martensitic transformation uniformly on the entire surface of the steel plate. Incidentally, in cooling of a steel plate having a large plate thickness and a low conveying speed, it is impossible to transform the surface layer portion over the entire surface of the steel plate. This is because if the conveyance speed is low, the cooling time in the first cooling zone becomes long and it is difficult to control the decrease in the surface temperature, and the method of applying cooling water, the scale state of the steel sheet surface, and the like differ depending on each part. .

  Here, since the draining roll 3 is installed with the same pitch as the table roll 4 of a conveyance line, the length per zone is about 0.8-1.2 m similarly to the distance between table rolls. When the thickness of the steel plate is as thick as 50 mm or more and the conveying speed is slow, for example, at 0.2 m / s, water cooling is performed for 4 to 6 s per zone. Therefore, the temperature of the steel sheet surface layer becomes the Ms point or less simply by passing through the first water cooling zone.

  Next, in the second to fourth cooling zones, the cooling water is not supplied as an air-cooling section, and is reheated until the surface layer portion of the steel sheet reaches 640 ° C. or higher. By reheating the steel plate surface layer portion to 640 ° C. or higher, the surface layer portion is tempered, and an increase in the hardness of the surface layer can be suppressed. Here, if the steel sheet surface layer is reheated to 640 ° C. or more, the air cooling section may be one zone, but usually two zones or more are required. In addition, the air cooling section may be 4 zones or more. However, if the air cooling section is longer than necessary, the subsequent water cooling time is shortened. Therefore, the air cooling section has a sufficient number of zones necessary for reheating the steel sheet surface layer to 640 ° C. or higher. As appropriate, it is preferable to select two or three zones.

  In addition, drainage with the draining roll 3 between the water cooling zone and the air cooling zone is not complete, and in some cases, cooling water leaks from the upstream water cooling zone. do not do. Therefore, it is necessary to confirm that the temperature of the upper surface of the steel plate is reheated to a target temperature or more by always measuring it with a width direction thermometer installed above the steel plate. Therefore, in the present invention, the width direction thermometer 5b capable of measuring the temperature in the width direction of the upper surface of the steel sheet is installed in the air cooling section, and the surface temperature of the steel sheet reheated in the air cooling section is measured. The installation position is preferably at least the third or fourth cooling zone because there is a high possibility that the recuperation is not yet sufficient in the second cooling zone. In most cases, the water cooling is 1 zone and the air cooling is 3 zones. Furthermore, installation at two or more locations costs equipment costs, but if it is important for quality assurance, it may be installed at two or more locations including the fifth and subsequent cooling zones. In addition, since there is no problem of cooling water leakage from the upstream water cooling zone on the lower surface side of the steel plate, there is no need to bother reconfirming the temperature after reheating with a width direction thermometer.

  And cooling water is again supplied in the cooling zone after an air cooling area, and it cools until the plate | board thickness average temperature of a steel plate becomes 520 degrees C or less. This is to ensure the intended strength and toughness. Note that the lower limit of the cooling stop temperature may be appropriately determined depending on the target strength and toughness. For example, when high strength and toughness are required, the average thickness may be 400 ° C. Alternatively, if the required strength and toughness are not severe, the lower limit of the cooling stop temperature is easier to control if it is set a little higher, so it may be set to 460 ° C.

  FIG. 3 shows an example of the temperature history of the steel plate when the cooling pattern shown in FIG. 2 is applied, and shows the surface layer depth of 1 mm, the plate thickness average, and the plate thickness center. At a surface layer depth of 1 mm, the water cooling in the first cooling zone is below the Ms point (for example, 380 ° C.), but it rises to 660 ° C. due to the subsequent recuperation. An increase in surface hardness can be suppressed.

On the other hand, even in the portion including the plate thickness center portion, even if heat diffusion is performed during recuperation in the air cooling section after water cooling, it is still higher than the Ar ₃ transformation point. Therefore, since the transformation is started for the first time during the water cooling restarted after air cooling, if the cooling capacity of the water cooling is high, the structure becomes finer and high strength and high toughness can be obtained.

  FIG. 4 is an example of a temperature distribution measured by the width direction thermometer 5b. The maximum temperature and the minimum temperature are extracted by data processing from the measurement effective width excluding the edge portion and output.

FIG. 5 shows the steel plate longitudinal direction distribution of the temperature processed by the method described in FIG. 4 with respect to the temperature distribution measured by the three width direction thermometers 5a, 5b, and 5c. It shows the temperature, the lowest temperature at the time of recuperation in the air cooling section, and the highest and lowest temperatures at the end of cooling (stop). As shown in FIG. 5, if the minimum temperature at the start of cooling exceeds the lower limit of the allowable value over the entire length, it is guaranteed that the entire surface of the steel sheet has started cooling from Ar ₃ points or more. If the minimum temperature during recuperation in the air-cooled section exceeds the lower limit of the allowable value over the entire length, it is ensured that the surface layer portion of the entire surface of the steel sheet has been tempered and the increase in hardness could be suppressed. If the maximum temperature when cooling is stopped is below the allowable upper limit and the minimum temperature is above the allowable lower limit, it is guaranteed that the intended strength and toughness can be secured.

Therefore, in the embodiment shown in FIG. 2, surface layer hardness acceptance / rejection determination means 7 is provided. The surface layer hardness pass / fail judgment means 7 takes in the width direction temperature measurement value of the upper surface of the steel sheet by the width direction thermometers 5a and 5b, and the cooling start temperature measured by the width direction thermometer 5a is equal to or higher than the reference temperature (Ar ₃ points). It is determined whether or not it is a temperature, and whether or not the temperature after reheating of the steel sheet surface layer portion in the air cooling section measured by the width direction thermometer 5b is equal to or higher than a reference temperature (640 ° C.). If it is satisfied, the hardness of the steel sheet surface layer after cooling is determined to be within the allowable value.

  On the other hand, for example, if a part where the recuperation temperature is not sufficient is measured, it is judged that the part is a defective part whose hardness increase of the steel sheet surface exceeds the allowable value, and the part is cut off before shipping to guarantee the quality. It is also possible to temper only the surface layer of the portion by using a known technique such as radiant heating or induction heating.

  Thus, according to the present invention, the surface temperature after recuperation can be confirmed over the entire surface of the steel sheet by measuring the steel sheet temperature distribution over the entire width and length during the air cooling after the first water cooling. Thus, it can be ensured that the increase in hardness of the surface layer of the obtained product is suppressed.

  In addition, the steel plate surface layer part which suppresses hardness by this invention means the part of the depth within 1 mm or more and 3 mm or less from the steel plate surface. The reason why the portion of the outermost layer less than 1 mm is not evaluated is that it is not appropriate as a representative value of the surface hardness of the product due to the adhesion of scale or the effect of decarburization. Further, when the depth exceeds 3 mm from the surface layer, it takes time for heat conduction from the surface, and the cooling rate is not so high, so the increase in hardness is not a problem. By the way, the temperature of the 1 mm depth of the surface layer can be easily obtained by a general heat conduction temperature analysis if the measured value of the steel sheet surface temperature before cooling start and after recuperation after cooling and the water cooling time are grasped.

  Hereinafter, as an embodiment of the present invention, a case of cooling a steel plate for shipbuilding having a yield stress of 355 MPa class and an upper limit of surface Vickers hardness of Hv350 in the plate rolling process will be described with reference to the drawings.

  In the thick plate rolling equipment schematically shown in FIG. 7, the slab extracted from the heating furnace is formed by a rolling mill, subjected to tenter rolling, then rough rolled, and further subjected to finish rolling to obtain a plate thickness of 80 mm, The plate width was 4.5 m. The surface temperature of the steel sheet measured immediately after finish rolling, that is, the finish temperature was 800 ° C. Thereafter, accelerated cooling was performed in an accelerated cooling facility through a pre-leveler. The cooling start temperature is 780 ° C., the cooling end temperature is 420 ° C., and the cooling conditions are as follows. In addition, the conveyance speed of the steel plate was 12.5 mpm.

  As an example of the present invention, the cooling equipment shown in the above embodiment was used.

  As shown in FIG. 2, this cooling facility performed water cooling in the most upstream first cooling zone with one zone between table rolls (distance is 1 m). Thereafter, 3 zones were reheated as an air cooling section, and water cooling was performed again in the remaining 8 zones.

In each water cooling zone, cooling water was supplied at a water density of 2.0 m ³ / m ² min on the upper surface of the steel plate and 2.5 m ³ / m ² min on the lower surface of the steel plate.

The temperature results obtained for the full width and full length of the steel sheet were as follows. As the cooling start temperature, a temperature of 785 ° C. or higher was obtained with respect to the allowable lower limit temperature (Ar _three- point temperature) of 780 ° C. The steel sheet surface layer was cooled to a temperature of 350 ° C. or lower with respect to the allowable upper limit temperature (Ms point temperature) of 380 ° C. by the first water cooling. As the ultimate temperature after reheating, a temperature of 650 ° C. or higher was obtained with respect to the allowable lower limit temperature of 640 ° C. As the cooling stop temperature, an allowable upper limit temperature of 430 ° C. and an allowable lower limit temperature of 370 ° C., a maximum temperature of 420 ° C. and a minimum temperature of 380 ° C. were obtained.

  Since all the measured values of the temperature were within the allowable range, the quality of the entire surface of the steel sheet could be guaranteed. As a result of cutting the sample from the steel plate and measuring the hardness, the Vickers hardness Hv (load 98 N) was Hv 300 at the maximum as shown in FIG. 6 and sufficiently satisfied the allowable upper limit Hv 350 of the hardness.

  As a comparative example, the case where water cooling was performed in the first, third, and fifth to eleventh zones and the air cooling was performed in the second and fourth zones by the method described in Patent Document 1 was examined. The hardness measurement result was Hv400 at the maximum, greatly exceeding the allowable upper limit of hardness Hv350. This is because temperature recovery due to recuperation was not sufficient. In order to reduce the hardness, the C content was decreased, and in order to maintain the strength, Nb and Mn had to be added, so that the production cost was higher compared to the inventive examples.

DESCRIPTION OF SYMBOLS 1 Upper surface cooling header 2 Lower surface cooling header 3 Draining roll 4 Table roll 5 Width direction thermometer 6 Steel plate

Claims (4)

A cooling facility provided with a plurality of cooling zones separated by a table roll and a draining roll facing the table roll, and supplying cooling water to the upper and lower surfaces of the steel sheet at a water density of 2.0 m ³ / m ² · min or more was used. A method of cooling a steel sheet, wherein the steel sheet after hot rolling is acceleratedly cooled from a temperature of ₃ or more Ar points until the surface layer of the steel sheet reaches a temperature below the martensitic transformation start point in the cooling zone on the most upstream side. After that, the accelerated cooling is temporarily interrupted by setting the subsequent cooling zone or zones as an air cooling section, and reheating is performed until the steel plate surface layer temperature reaches 640 ° C. or higher, and then the steel plate thickness average temperature is 520 in the subsequent cooling zone. A method for cooling a steel sheet, characterized in that accelerated cooling is carried out again until the temperature falls below ℃.   The steel sheet cooling method according to claim 1, wherein the steel sheet has a thickness of 50 mm or more, and a conveying speed of the steel sheet passing through the cooling facility is 1.0 m / s or less.   The steel sheet is cooled by the method for cooling a steel sheet according to claim 1 or 2, and the width direction temperature of the upper surface of the steel sheet is measured using a width direction thermometer provided in the cooling zone entry side and the air cooling section on the most upstream side. By comparing the measured temperature in the width direction of the upper surface of the steel sheet with the cooling start temperature and the reheat temperature of the steel sheet surface layer portion in the air cooling section and the respective reference temperatures, the hardness of the steel sheet surface layer portion after cooling A method of manufacturing a steel sheet, wherein it is determined whether or not the value is within an allowable value. A steel sheet cooling facility comprising a plurality of cooling zones separated by a table roll and a water draining roll facing the table roll, and supplying cooling water to the upper and lower surfaces of the steel sheet at a water density of 2.0 m ³ / m ² · min or more; , A width direction thermometer installed on the inlet side of the cooling equipment, and when installed in the third or fourth cooling zone from at least the upstream side of the plurality of cooling zones, and the cooling zone is an air cooling section A width direction thermometer for measuring the width direction temperature of the upper surface of the steel sheet, a cooling start temperature from the temperature of the upper surface of the steel sheet measured by these width direction thermometers, a recuperation temperature of the steel sheet surface layer portion in the air cooling section, and respective reference temperatures A steel sheet manufacturing facility comprising a surface layer hardness pass / fail determination means for determining whether or not the hardness of the steel sheet surface layer portion after cooling is within an allowable value.

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