Manufacturing method for thick steel plates

Abstract

To provide a method for manufacturing thick steel plates that can produce thick steel plates with good shape and low residual stress throughout their entire length. [Solution] The method for manufacturing a thick steel plate according to the present invention includes a first step of accelerating the cooling of a thick steel plate obtained by hot rolling a steel material; a second step of heat-treating the accelerated-cooled thick steel plate online; a third step of measuring the longitudinal temperature distribution of the heat-treated thick steel plate; and a fourth step of hot-correcting the thick steel plate whose longitudinal temperature distribution has been measured under conditions set based on the measurement results of the longitudinal temperature distribution.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing thick steel plates.

Background Art

[0002] Thick steel plates are manufactured by hot-rolling steel materials such as slabs heated to a predetermined temperature in a heating furnace with a hot rolling mill, cooling them to room temperature, and then cutting them to predetermined dimensions. In order to increase the strength and toughness of thick steel plates, a method of accelerating cooling the thick steel plate after hot rolling is widely used following hot rolling. Furthermore, heat treatment may be performed on the thick steel plate that has been accelerated cooled for the purpose of annealing or removing residual stress. Conventionally, this heat treatment has been performed in a heat treatment furnace installed offline, but in recent years, a technique for performing heat treatment with a heating device provided on the same line as the hot rolling mill and the accelerated cooling device has been developed.

[0003] Specifically, Patent Document 1 discloses a heat treatment method for thick steel plates in which heat treatment is performed on a thick steel plate that has been accelerated cooled after hot rolling using an induction heating device, and further hot leveling is performed. Patent Document 1 also describes an embodiment in which a thermometer is installed on the outlet side of the cooling device, the temperature distribution of the thick steel plate after accelerated cooling is measured, the residual stress of the thick steel plate is estimated from the temperature distribution, and the heating conditions of the induction heating device are set. According to this method, a thick steel plate with less distortion and less residual stress can be manufactured.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the technology described in Patent Document 1, the temperature distribution in the longitudinal direction of the thick steel plate after heating by an induction heating device is not necessarily uniform, and temperature differences tend to occur between the leading edge, trailing edge, and the longitudinal center (hereinafter also referred to as the middle section). When a temperature distribution exists in the longitudinal direction of a thick steel plate, even if the plate shape of the middle section is good, the amount of warping at the leading edge and trailing edge may increase, which may hinder the efficiency of subsequent transport and processing operations. For this reason, there has been a demand for a technology that can manufacture thick steel plates with good shape and low residual stress throughout their entire length.

[0006] The present invention was made to solve the above problems, and its objective is to provide a method for manufacturing thick steel plates that can produce thick steel plates with good shape and low residual stress along their entire length. [Means for solving the problem]

[0007] The method for manufacturing a thick steel plate according to the present invention includes a first step of accelerating the cooling of a thick steel plate obtained by hot rolling a steel material; a second step of heat-treating the accelerated-cooled thick steel plate online; a third step of measuring the longitudinal temperature distribution of the heat-treated thick steel plate; and a fourth step of hot-correcting the thick steel plate whose longitudinal temperature distribution has been measured under hot-correction conditions set based on the measurement results of the longitudinal temperature distribution.

[0008] The third step may include measuring the longitudinal temperature distribution at the leading edge and the longitudinal center of the thick steel plate.

[0009] The fourth step may include a step of determining the hot straightening conditions for the thick steel plate corresponding to the measurement results of the temperature distribution in the longitudinal direction, using a model that shows the relationship between the temperature of the thick steel plate and the hot straightening conditions for the thick steel plate.

[0010] The hot straightening conditions may be the amount of indentation between the upper and lower work rolls of the hot straightening machine, or the arrangement of the amount of indentation in the longitudinal direction of the thick steel plate. [Effects of the Invention]

[0011] According to the method for manufacturing thick steel plates of the present invention, it is possible to manufacture thick steel plates that have a good shape and low residual stress throughout their entire length. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is a block diagram showing the configuration of a thick steel plate manufacturing facility to which a thick steel plate manufacturing method, which is one embodiment of the present invention, is applied. [Figure 2] Figure 2 is a block diagram showing the configuration of the hot straightening machine control device shown in Figure 1. [Figure 3] Figure 3 is a flowchart showing the flow of a hot straightening control process, which is one embodiment of the present invention. [Modes for carrying out the invention]

[0013] A method for manufacturing a thick steel plate, according to one embodiment of the present invention, will be described below with reference to the drawings.

[0014] 〔composition〕 Figure 1 is a block diagram showing the configuration of a thick steel plate manufacturing facility to which a thick steel plate manufacturing method, according to one embodiment of the present invention, is applied. As shown in Figure 1, the thick steel plate manufacturing facility 1 to which a thick steel plate manufacturing method, according to one embodiment of the present invention, is applied is equipped with a hot rolling mill 10, a cooling device 20, an induction heating device 30, and a hot straightening machine 40 in that order from the upstream side of the continuous line. A thermometer 50 is provided between the induction heating device 30 and the hot straightening machine 40, and the thermometer 50 is connected to a hot straightening machine control device 60. The hot straightening machine 40 is connected to the hot straightening machine control device 60, and the hot straightening machine control device 60 is connected to a host computer 70.

[0015] The hot rolling mill 10 produces thick steel plates of a desired thickness and width by hot rolling a steel material such as a slab heated in a heating furnace (not shown). The thick steel plates produced in the hot rolling mill 10 are supplied to the cooling device 20. The hot rolling conditions for the thick steel plates are set appropriately by known methods according to the steel composition of the thick steel plates to be produced.

[0016] The cooling device 20 is located downstream of the hot rolling mill 10 on the same continuous line. The cooling device 20 performs accelerated cooling on the thick steel plates manufactured in the hot rolling mill 10, thereby controlling the material properties of the thick steel plates, such as increasing their strength and toughness. The accelerated cooling conditions for the thick steel plates are set appropriately using known methods according to the steel composition of the thick steel plates being manufactured.

[0017] The induction heating device 30 is composed of, for example, a solenoid-type induction heating device and is installed downstream of the cooling device 20 on the same continuous line. The induction heating device 30 performs heat treatment aimed at reducing residual stress and tempering by induction heating of the thick steel plate that has been accelerated cooling in the cooling device 20.

[0018] The hot straightening machine 40 is installed downstream of the induction heating device 30 on the same continuous line. A roller leveler may be used as the hot straightening machine 40. In a roller leveler type hot straightening machine 40, a work roll that contacts the thick steel plate, a backup roll that supports the work roll, and a frame that houses and holds the work roll and backup roll are provided opposite each other in the vertical direction. The hot straightening machine 40 is also equipped with a hydraulic cylinder that applies downward force to the upper frame. The hot straightening machine 40 straightens the thick steel plate by operating the hydraulic cylinder and loading the thick steel plate between the upper and lower work rolls while maintaining a predetermined distance between them. As described above, the roller leveler is equipped with a plurality of rolls arranged in a staggered pattern along the conveying direction of the thick steel plate, and removes distortion and obtains a flat plate shape by repeatedly applying bending stress to the thick steel plate between adjacent rolls. The hot-stretched thick steel plates are sent through a cooling bed and other means to cutting and shearing processes using slitters and shears, or to inspection processes using shape measuring machines.

[0019] The thermometer 50 is composed of, for example, a radiation thermometer and is installed above the thick steel plate in a continuous line between the induction heating device 30 and the hot straightening machine 40. The thermometer 50 measures the temperature of the thick steel plate after induction heating but before hot straightening and inputs the measured temperature information to the hot straightening machine control device 60. The thermometer 50 may be installed facing the upper surface of the steel plate at the center in the width direction of the thick steel plate. In this case, by continuing temperature measurement as the thick steel plate is transported, the temperature distribution at the center in the width direction in the longitudinal direction of the thick steel plate can be measured. The thermometer 50 may also be scannable in the width direction of the thick steel plate. In this case, by transporting the thick steel plate while scanning the thermometer 50 in the width direction of the thick steel plate, the temperature distribution in the width direction and longitudinal direction of the thick steel plate can be measured. Multiple thermometers 50 may also be installed along the width direction of the thick steel plate. In this case, the longitudinal temperature distribution can be measured at multiple locations in the width direction of the thick steel plate without scanning the thermometer 50 in the width direction of the thick steel plate. The thermometer 50 is connected to the hot straightening machine control device 60 via an electrical communication line such as the Internet or LAN, and the measured temperature information of the thick steel plate is transmitted to the hot straightening machine control device 60 via the electrical communication line.

[0020] The hot straightening machine control device 60 is composed of an information processing device such as a personal computer, and is connected to the hot straightening machine 40 and the host computer 70 via a telecommunication line. The hot straightening machine control device 60 controls the hot straightening machine 40 by using various information on the steel material to be processed obtained from the host computer 70. FIG. 2 is a block diagram showing the configuration of the hot straightening machine control device 60. The hot straightening machine control device 60 functions as a representative temperature calculation unit 61, a correction condition determination unit 62, and a hot straightening machine control unit 63 when an arithmetic processing device such as a CPU in the information processing device executes a computer program. The functions of these units will be described later. Note that the host computer 70 may execute some or all of the functions of the representative temperature calculation unit 61, the correction condition determination unit 62, and the hot straightening machine control unit 63. In other words, a part of the hot straightening machine control device 60 may be constituted by the host computer 70. Also, the functions of the hot straightening machine control device 60 may be distributed and arranged in a plurality of information processing devices connected to each other via a telecommunication line.

[0021] Return to FIG. 1. The host computer 70 is composed of an information processing device such as a business computer or a process computer. The host computer 70 stores various information on the steel material corrected by the hot straightening machine 40.

[0022] In the manufacturing equipment 1 for thick steel plates having such a configuration, the hot straightening machine control device 60 executes the hot straightening control process shown below to manufacture a thick steel plate having a good shape over the entire length and a small residual stress. Hereinafter, the operation of the hot straightening machine control device 60 when executing the hot straightening control process will be described with reference to the flowchart shown in FIG. 3.

[0023] 〔Hot straightening control process〕 FIG. 3 is a flowchart showing the flow of the hot straightening control process according to an embodiment of the present invention. The flowchart shown in FIG. 3 starts at the timing when the operation of the manufacturing equipment 1 for thick steel plates is started, and the hot straightening control process proceeds to the process of step S1.

[0024] In step S1, the representative temperature calculation unit 61 calculates the representative temperature of the thick steel plate to be used for setting the hot straightening conditions, based on the temperature information of the thick steel plate measured by the thermometer 50. Specifically, the representative temperature calculation unit 61 calculates the average temperature of the steel plate between a position 500 mm from the tip and a position 2000 mm from the tip (hereinafter also simply referred to as the tip temperature) as the representative temperature of the longitudinal end of the thick steel plate, especially the longitudinal tip. The reason for excluding the position up to 500 mm from the tip is that the shape of the thick steel plate is often non-steady, such as tongue-shaped or fishtail-shaped, near the tip, and extreme temperature drops may occur.

[0025] On the other hand, the area 2000 mm away from the tip can be considered a steady-state temperature zone. Therefore, the representative temperature calculation unit 61 calculates the average temperature of the thick steel plate between the position 2000 mm from the tip and the position 2000 mm from the tail end as the average temperature of the longitudinal center (hereinafter also referred to as the middle section temperature). Both the tip temperature and the middle section temperature are calculated by finding the average temperature of the widthwise center position at each position in the longitudinal direction. This is because the warping of the thick steel plate is due to the temperature distribution in the longitudinal direction, and the influence of the widthwise temperature distribution is relatively small. Alternatively, the tip temperature and middle section temperature may be calculated by averaging the widthwise temperatures at each position in the longitudinal direction over the longitudinal direction. This allows the average temperature, which also takes into account the influence of the widthwise temperature distribution, to be reflected in the hot straightening conditions.

[0026] When hot straightening thick steel plates, the appropriate values ​​for straightening conditions such as the roll gap, which is the distance between the upper and lower work rolls of the hot straightening machine 40, depend on the temperature of the thick steel plate. Therefore, it is ideal to change the hot straightening conditions as needed according to the temperature of the thick steel plate measured immediately before hot straightening. However, due to the limitations of the response speed of the hydraulic cylinder that controls the roll gap in the hot straightening machine 40, it is not practical to change the hot straightening conditions moment by moment in response to temperature changes in the thick steel plate. In contrast, by setting the hot straightening conditions using the average temperature of the longitudinal end (especially the tip) and the average temperature of the longitudinal center of the thick steel plate as representative values, a good steel plate shape can be obtained after hot straightening. With this, the process of step S1 is completed, and the hot straightening control process proceeds to the process of step S2.

[0027] In step S2, the straightening condition determination unit 62 determines the hot straightening conditions for the thick steel plate based on the representative temperature of the thick steel plate calculated in step S1. In this embodiment, the straightening condition determination unit 62 inputs the representative temperature of the thick steel plate calculated in step S1 to a model that shows the relationship between the representative temperature of the thick steel plate and the hot straightening conditions for the thick steel plate, which has been determined in advance. The straightening condition determination unit 62 then uses the hot straightening conditions for the thick steel plate output from the model as the hot straightening conditions for the thick steel plate corresponding to the representative temperature of the thick steel plate calculated in step S1. With this, the process of step S2 is completed, and the hot straightening control process proceeds to step S3.

[0028] The above model can be obtained, for example, by the following procedure (1) to (3): (1) Read the actual data of hot straightening conditions for thick steel plates stored in the upper computer 70. (2) From the read actual data, extract the group of actual data in which the shape of the thick steel plate after hot straightening meets a predetermined standard. (3) From the extracted group of actual data, obtain a model such as a mathematical formula that shows the relationship between the representative temperature of the thick steel plate and the hot straightening conditions by regression calculation. The above procedure (1) to (3) may be executed by the hot straightening machine control device 60, or it may be executed by an information processing device other than the hot straightening machine control device 60, and the hot straightening machine control device 60 may acquire the obtained model. Alternatively, it may be a system that performs database modeling processing on the extracted group of actual data and outputs the hot straightening conditions. The model may be divided according to the steel type (steel composition), size (combination of plate thickness and plate width), and required specifications of the steel plate shape of the target thick steel plate. By providing a model for each of these divisions, more accurate hot straightening conditions can be provided more quickly.

[0029] Examples of the above performance data include the steel type (steel composition), size (plate thickness, plate width), temperature before hot straightening (typical temperature), hot straightening conditions, and plate shape after hot straightening (warp, flatness, etc.) of the thick steel plate. Examples of hot straightening condition data include the amount of indentation between specific upper and lower work rolls of the hot straightening machine 40, or the arrangement of the indentation amounts in the longitudinal direction of the thick steel plate (roll gap pattern). In addition, in a typical roller leveler type hot straightening machine, the spacing between the upper and lower work rolls, which are arranged in a staggered pattern in the transport direction (longitudinal direction) of the thick steel plate, is narrowest at the entry side (upstream side) and widens towards the downstream side. Therefore, the amount of indentation between the upper and lower work rolls at the entry side (upstream side) may be used as the hot straightening condition. The amount of indentation corresponds to the amount by which the thick steel plate is actually bent. For example, in the case of pressing with upper work rolls, the pressing amount refers to the dimension obtained by subtracting the thickness of the steel plate being straightened from the distance between the line connecting the upper surfaces of two adjacent lower work rolls and a line parallel to this line and passing through the lower surface of the upper work rolls. Similarly, in the case of pressing with lower work rolls, the pressing amount refers to the dimension obtained by subtracting the thickness of the steel plate being straightened from the distance between the line connecting the lower surfaces of two adjacent upper work rolls and a line parallel to this line and passing through the upper surface of the lower work rolls.

[0030] In step S3, the hot straightening machine control unit 63 hot straightens the thick steel plate by controlling the hot straightening machine 40 based on the hot straightening conditions determined in step S2. Specifically, first, the hot straightening machine control unit 63 starts the straightening process from the leading edge of the thick steel plate according to the hot straightening conditions determined based on the leading edge temperature. Then, when the thick steel plate reaches a position of 2000 mm from the leading edge of the thick steel plate, the hot straightening machine control unit 63 changes the hot straightening conditions to those corresponding to the middle section temperature. The timing of changing from the straightening conditions corresponding to the leading edge temperature to those corresponding to the middle section temperature is determined by considering the response speed characteristics of the hydraulic cylinder of the hot straightening machine 40 and the transport speed of the thick steel plate in the hot straightening machine 40. It is sufficient if the change in hot straightening conditions is completed before the position of 2000 mm from the leading edge of the thick steel plate reaches the work roll of the hot straightening machine 40. In addition, although the above explanation described changing the hot straightening conditions from the leading edge to the middle section of the thick steel plate, when the straightening position shifts from the middle section to the trailing edge of the thick steel plate, the hot straightening conditions may also be changed from the hot straightening conditions suited to the middle section to the hot rolling conditions suited to the trailing edge, similar to the shift from the leading edge to the middle section. This completes the process in step S3, and the series of hot straightening control processes are finished.

[0031] As is clear from the above description, in the hot straightening control process, which is one embodiment of the present invention, the representative temperature calculation unit 61 calculates the tip temperature and middle temperature of the thick steel plate based on the temperature information of the thick steel plate measured by the thermometer 50, and the straightening condition determination unit 62 determines the hot straightening conditions of the thick steel plate based on the tip temperature and middle temperature of the thick steel plate. Then, the hot straightening machine control unit 63 hot straightens the thick steel plate by controlling the hot straightening machine 40 based on the hot straightening conditions determined in the process of step S2. As a result, it is possible to manufacture a thick steel plate with a good shape and low residual stress along its entire length.

[0032] In the above embodiment, the thermometer 50 is installed above the thick steel plate in the continuous line between the induction heating device 30 and the hot straightening machine 40. In another embodiment of the present invention, the thermometer 50 may be installed below the thick steel plate in the continuous line between the induction heating device 30 and the hot straightening machine 40 to measure the temperature of the bottom surface (back surface) of the thick steel plate. Obtaining temperature information for the bottom surface (back surface) of the thick steel plate in addition to the temperature of the top surface (front surface) makes it possible to accurately evaluate the temperature distribution in the thickness direction of the thick steel plate, thereby contributing to improved material stability and manufacturing stability. [Examples]

[0033] In this embodiment, first, a thick steel plate was manufactured using the thick steel plate manufacturing equipment 1 shown in Figure 1, and then the thick steel plate was accelerated cooling down to a cooling stop temperature of 300°C. Next, the accelerated-cooled thick steel plate was subjected to a tempering treatment to achieve a center temperature of 650°C in the thickness direction. Next, the surface temperature of the center of the thick steel plate in the width direction was continuously measured, and the temperature distribution of the surface temperature of the center of the width direction in the longitudinal direction of the thick steel plate was measured. Next, hot straightening was performed on the thick steel plate under straightening conditions based on the measured temperature distribution. Specifically, a straightening machine with a total of 5 rolls, consisting of 3 upper rolls and 2 lower rolls, was used as the hot straightening machine. The tip temperature was defined as the average temperature from 500 mm to 2000 mm from the tip of the thick steel plate. The middle section temperature was defined as the average temperature from 2000 mm from the tip of the thick steel plate to 2000 mm from the tail end of the thick steel plate. Based on these tip and middle section temperatures, the straightening conditions determined by the hot straightening machine control device were adopted. Then, the amount of warp and flatness of the tip of the thick steel plate after hot straightening were measured. The amount of warp was determined as the distance from the extension line of the lower surface of the middle section of the thick steel plate to the lower surface of the thick steel plate at the tip. The flatness was measured according to the flatness standards of JIS G3193:2019. As a comparative example, the warp and flatness of a thick steel plate manufactured under the same hot straightening conditions for both the tip and middle sections were measured in the same manner. The measurement results for the present invention example and the comparative example are shown in Table 1.

[0034] [Table 1]

[0035] As shown in Table 1, when hot straightening was performed under hot straightening conditions corresponding to the longitudinal temperature distribution after induction heating, the tip curvature was 3 mm and the flatness was 6 mm. In contrast, in the comparative example where the hot straightening conditions were not changed according to the longitudinal temperature distribution, the tip curvature after hot leveling was 12 mm and the flatness was 7 mm. From this, it was confirmed that thick steel plates with excellent curvature and flatness can be obtained by changing the hot straightening conditions according to the longitudinal temperature distribution.

[0036] Although embodiments applying the invention made by the present inventors have been described above, the present invention is not limited by the descriptions and drawings that constitute part of the disclosure of the present invention in this embodiment. That is, all other embodiments, examples, and operational techniques made by those skilled in the art based on this embodiment are included in the scope of the present invention. [Explanation of symbols]

[0037] 1. Manufacturing equipment for thick steel plates 10 Hot rolling mill 20 Cooling device 30 Induction heating device 40 Hot straightening machine 50 thermometer 60 Hot straightening machine control device 61 Representative temperature calculation section 62 Orthodontic Condition Determination Section 63 Hot straightening machine control unit 70 Top Computers

Claims

1. The first step involves accelerated cooling of a thick steel plate obtained by hot-rolling a steel material, A second step involves heat-treating the accelerated-cooled thick steel plate online, A third step involves measuring the temperature distribution in the longitudinal direction of the heat-treated thick steel plate, A fourth step involves hot-stretching the thick steel plate whose longitudinal temperature distribution has been measured, using hot-stretching conditions set based on the measurement results of the longitudinal temperature distribution. A method for manufacturing thick steel plates, including the method described above.

2. The method for manufacturing a thick steel plate according to claim 1, wherein the third step includes measuring the longitudinal temperature distribution at the leading edge and the longitudinal center of the thick steel plate.

3. The method for manufacturing a thick steel plate according to claim 1, wherein the fourth step includes determining the hot straightening conditions of the thick steel plate corresponding to the measurement results of the temperature distribution in the longitudinal direction, using a model that shows the relationship between the temperature of the thick steel plate and the hot straightening conditions of the thick steel plate.

4. The method for manufacturing a thick steel plate according to claim 1, wherein the hot straightening conditions are the amount of indentation between the upper and lower work rolls of the hot straightening machine, or the arrangement of the amount of indentation in the longitudinal direction of the thick steel plate.

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