JP2021171790A - Method for manufacturing steel plate, method for manufacturing steel pipe, apparatus for manufacturing steel plate and program - Google Patents

Abstract

To provide a method for manufacturing a steel plate which can manufacture a steel plate that suppresses a part having a problem of hardness in quality on an object surface, a method for manufacturing a steel pipe, an apparatus for manufacturing a steel plate, and a program.SOLUTION: A method for manufacturing a steel plate includes: a rolling step S3 of control-rolling a slab; a cooling step S5 of control-cooling the control-rolled steel plate; a plastic deformation step S6 of repeatedly plastically deforming the control-cooled steep plate, and thereby adjusting a stress state of a surface layer of the steel plate including the surface of the steel plate; a hardness measurement step S7 of measuring hardness of the surface performed in the plastic deformation step S6; a hardness determination step S8 of determining a part where the hardness exceeds a predetermined threshold as a hardness defective part on the basis of the measurement result in the hardness measurement step S7; and a removal step S11 of removing the hardness defective part.SELECTED DRAWING: Figure 8

Description

The present invention relates to a method for manufacturing a steel sheet, a method for manufacturing a steel pipe, a steel sheet manufacturing apparatus, and a program.

In recent years, sulfide corrosion cracking (SSC) has become a problem in steel sheets exposed to a hydrogen sulfide environment and steel products manufactured from steel sheets. It has been clarified that SSC is generated in the surface layer of a steel sheet exposed to a sulfide such as hydrogen sulfide, starting from a surface hardened portion having a hardness higher than a predetermined upper limit of hardness. Further, if the strength of the steel sheet is insufficient, the surface layer of the steel sheet may break from the surface softened portion having a hardness lower than the predetermined lower limit of hardness. Therefore, in the manufacturing stage, such a surface hardened portion and a surface softened portion (hereinafter, the surface hardened portion and the surface softened portion are collectively referred to as a surface hardness changing portion) are detected by measuring the hardness, and the surface hardness changing portion is detected. It is required to manufacture a steel plate in which is not present and a steel product using the steel plate.

As a method for measuring the hardness of the surface layer of a steel sheet, for example, a method for measuring the electromagnetic characteristics of the surface layer of a steel sheet is known. For example, in the technique described in Patent Document 1, while such electromagnetic properties show a correspondence relationship with the hardness of the steel sheet, they fluctuate so as to make a simple analytical solution impossible. A method of estimating the hardness of a steel sheet based on its characteristics has been proposed. Specifically, in the technique described in Patent Document 1, the hardness and electromagnetic characteristics of a plurality of steel samples having similar chemical compositions are measured for each electromagnetic characteristic, and these measured values are measured in the data bank, respectively. It is stored in multiple quantized groups of hardness with respect to the electromagnetic properties. Then, the multiple electromagnetic characteristics of the steel whose hardness should be estimated are measured, and compared with the quantized grouping stored in the data bank, the quantized grouping containing the measured electromagnetic characteristics is determined. After that, the hardness is estimated by comparing the results grouped according to each electromagnetic characteristic.

Special Table 9-507570 Gazette

However, the technique described in Patent Document 1 is limited to determining the grouping for each electromagnetic characteristic value, and the probability of hardness is estimated from the grouping obtained from a plurality of electromagnetic characteristic values. I could only do it. For this reason, it is not always possible to deduct the case where the electromagnetic characteristic value changes due to an influence other than hardness, and the part where the hardness changes is detected by surely deducting the influence other than hardness, and the hardness becomes a quality problem. There has been a demand for a technique for manufacturing a steel sheet in which there is no such part.

Therefore, the present invention has been made in view of the above circumstances, and is a method for manufacturing a steel sheet capable of manufacturing a steel sheet in which a portion where hardness becomes a problem in quality is suppressed on a target surface, a steel pipe It provides a manufacturing method, a steel sheet manufacturing apparatus, and a program.

The inventors measured parameters that change depending on the properties of the surface layer of the steel material under various conditions for steel materials of various steel types. As a result, the inventors have found that the parameters to be measured depend not only on the hardness of the surface layer but also on the stress state of the surface layer. Further, it was found that residual stress exists in the surface layer of the steel sheet manufactured by controlled rolling and controlled cooling, and the stress state is non-uniform depending on the location. In other words, it was found that it is necessary to eliminate the non-uniformity of the stress state on the surface layer of the steel sheet in order to accurately measure the hardness and accurately grasp and deal with the part where the hardness is a quality problem. rice field.

Based on this finding, the present invention employs the following means in order to solve the above problems. That is, in the method for producing a steel plate according to one aspect of the present invention, a rolling step of controlling and rolling a slab, a cooling step of controlling and cooling a steel plate controlled and rolled in the rolling step, and a control cooling in the cooling step are performed. At least of the plastic deformation step of adjusting the stress state of the surface layer of the steel plate including the surface of the steel plate by repeatedly plastically deforming the steel plate and the surface of the steel plate on which the plastic deformation step of the steel plate is carried out. A hardness measuring step of measuring a part of the hardness, a hardness determination step of determining a portion having a hardness exceeding a preset threshold based on the measurement result of the hardness measuring step as a hardness defective portion, and the hardness defective portion. It is provided with a removal step of removing.

According to this method, the steel sheet subjected to the rolling step and the cooling step is repeatedly plastically deformed in the plastic deformation step. As a result, in the surface layer of the steel sheet including the target surface, even if the stress having different residual stress differs depending on the location after the cooling step is in a non-uniform state, the non-uniform stress state can be eliminated. Then, in the hardness measurement step, by measuring the hardness of the target surface in a state where the non-uniformity of the stress state is eliminated in the plastic deformation step, the influence of stress is suppressed and the hardness is measured accurately. Can be done. Therefore, in the hardness determination step, the poor hardness portion can be accurately determined based on the accurately measured hardness, and the poor hardness portion can be reliably removed in the removal step.

Further, in the above-mentioned method for manufacturing a steel sheet, in the hardness measuring step, the hardness of the surface may be measured by measuring the electromagnetic characteristics of the surface.

According to this method, the hardness can be accurately measured in the hardness measuring step without damaging the surface of the steel sheet and without being affected by the stress state.

Further, in the above-mentioned method for manufacturing a steel sheet, in the plastic deformation step, it is assumed that the plastic deformation is repeatedly performed with a degree of processing in which a range of 2 mm or more in thickness from the surface to be measured in the hardness measurement step is the plastic deformation range. Is also good.

According to this method, by plastically deforming a steel material within a range of 2 mm or more from the surface, plastic deformation is caused over the entire thickness range including the surface to be measured for hardness, and a stress state is surely maintained. Can be adjusted.

Further, in the above-mentioned method for manufacturing a steel sheet, in the plastic deformation step, the plastic deformation may be repeated so that the degree of processing is 1.8 or more.

According to this method, the steel sheet is repeatedly plastically deformed so as to have a workability of 1.8 or more, so that the thickness is 2 mm or more from the surface including the surface to be measured for hardness. The stress state can be surely adjusted by plastically deforming the steel material.

Further, in the method for manufacturing a steel sheet, in the plastic deformation step, the steel sheet is repeatedly plastically deformed by performing flatness correction of the steel sheet to obtain a stress state of the surface layer of the steel sheet including the surface of the steel sheet. It may be adjusted.

According to this method, by correcting the flatness of the steel sheet, the steel sheet is repeatedly subjected to compressive plastic deformation and tensile plastic deformation, and the stress state of the surface layer of the steel sheet can be adjusted.

Further, in the above-mentioned method for manufacturing a steel plate, the steel plate may be used as a material for a steel pipe, and the hardness measuring step, the hardness determining step, and the removing step may be performed on a portion to be an inner surface of the steel pipe.

According to this method, it is possible to manufacture a steel pipe in which a portion having poor hardness, which is easily affected by corrosion, is suppressed on the inner surface portion of the steel pipe, which is feared to be corroded by the fluid flowing inside.

Further, in the above-mentioned steel pipe manufacturing method, the first pressing step of pressing the steel plate manufactured by the steel plate manufacturing method into a U shape and the O-shaped steel plate processed by the first pressing step are formed. It may be provided with a second pressing step of press working and a welding step of welding the ends of the steel plates processed in the second pressing step.

According to this method, it is possible to manufacture a steel pipe in which a portion having poor hardness, which is easily affected by corrosion, is suppressed on the inner surface portion of the steel pipe, which is feared to be corroded by the fluid flowing inside.

Further, in the steel plate manufacturing apparatus according to one aspect of the present invention, a rolling portion for controlling and rolling a slab, a cooling portion for controlling and cooling a steel plate controlled and rolled by the rolling portion, and a cooling portion for controlling and cooling the steel plate by the cooling portion. A plastic deformation imparting portion that repeatedly plastically deforms a steel plate to adjust the stress state of the surface layer of the steel plate including the surface of the steel plate, and the steel plate that has been repeatedly plastically deformed by the plastic deformation imparting portion of the steel plate. A hardness measuring unit that measures at least a part of the hardness of the surface, and a hardness determining unit that determines a portion whose hardness exceeds a preset threshold value as a hardness defective portion based on the measurement results of the hardness measuring unit. To be equipped.

According to this configuration, the steel sheet that has been rolled and cooled by the rolling portion and the cooling portion is repeatedly plastically deformed by the plastic deformation imparting portion. As a result, in the surface layer of the steel sheet including the target surface, even if the residual stress differs depending on the location after cooling by the cooling unit, the non-uniform stress state can be eliminated. Then, the hardness measuring unit measures the hardness of the target surface in a state where the non-uniformity of the stress state is eliminated by the plastic deformation imparting unit, so that the influence of stress is suppressed and the hardness is accurately measured. Be measured. Therefore, the hardness determining unit can accurately determine the poor hardness portion based on the accurately measured hardness, and can reliably remove the poor hardness portion.

Further, the program according to one aspect of the present invention applies a computer to a surface whose surface stress state is adjusted by repeatedly plastically deforming a steel plate generated by controlling rolling and controlling cooling of a slab. A hardness calculation means for calculating the hardness of the surface from the electromagnetic characteristics measured in the above manner, and a hardness determination means for determining a portion whose hardness exceeds a preset threshold value based on the hardness calculated by the hardness calculation means as a hardness defect. To function as ,.

According to the present invention, it is possible to manufacture a steel plate and a steel pipe in which a portion where hardness becomes a problem in quality is suppressed on a target surface.

It is a side view which shows typically the steel sheet manufacturing apparatus of embodiment. It is a side view which shows typically the plastic deformation imparting equipment in the steel sheet manufacturing apparatus of embodiment. It is explanatory drawing which shows the dimensional relation in the plastic deformation imparting equipment of the steel plate manufacturing apparatus of embodiment. It is a graph which showed the relationship between the plate thickness and the depth from the surface of the range of plastic deformation at each degree of processing. It is a block diagram which shows typically the hardness measuring part in the steel plate manufacturing apparatus of embodiment. It is a graph which showed typically the example of the correlation between the electromagnetic characteristic value, stress and hardness. It is a block diagram which shows the hardware structure of the hardness calculation part and the hardness determination part of an embodiment. It is a flow chart which shows the manufacturing method of the steel sheet of embodiment. It is a flow chart which shows the manufacturing method of the steel pipe of embodiment. It is a block diagram which shows typically the hardness measuring part in the steel plate manufacturing apparatus of the modification of embodiment. It is a schematic diagram of the waveform obtained in the hardness measuring part of the steel plate manufacturing apparatus of the modification of embodiment. It is a frequency distribution showing the residual stress distribution on the surface of the steel sheet, and is a frequency distribution diagram of (a) immediately before the flatness correction step and (b) immediately after the flatness correction is performed in the examples. It is a contour figure which showed the stress state of the surface of the steel sheet of an Example by an electromagnetic characteristic value, and shows (a) immediately after a cooling process, and (b) immediately after a flatness correction process.

Hereinafter, embodiments according to the present invention will be described with reference to FIGS. 1 to 9. FIG. 1 shows a steel sheet manufacturing apparatus 1 used in the steel sheet manufacturing method of the present embodiment.

As shown in FIG. 1, the steel sheet manufacturing apparatus 1 of the present embodiment has a continuous casting facility 10 for continuously casting the slab S, a reheating furnace 20 for reheating the slab S, and a steel sheet P for controlled rolling of the slab S. A rolling facility 30 (rolling section), a hot straightening facility 40 for hot-straightening the steel sheet P, and a cooling facility 50 (cooling section) for controlling and cooling the steel sheet P are provided. Further, the steel plate manufacturing apparatus 1 includes a flatness measuring unit 60 for measuring the flatness of the surface P1 of the controlled-cooled steel plate P, and a plastic deformation imparting facility 65 (plastic deformation imparting) for repeatedly plastically deforming the controlled-cooled steel plate P. Section), a hardness measuring section 70 that measures the hardness of the surface P1 of the steel plate P that has been repeatedly plastically deformed, and a hardness determining section 90 that determines a hardness defect portion W based on the measurement result by the hardness measuring section 70. Be prepared. The details will be described below.

The continuous casting facility 10 has a mold 11 into which the molten steel M is injected, and continuously produces a slab S from the mold 11. The reheating furnace 20 reheats the slab S generated by the continuous casting equipment 10 to a temperature at which the rolling equipment 30 can roll. The steel sheet manufacturing apparatus 1 is not provided with the continuous casting equipment 10, and the slab pieces manufactured in advance may be heated in the reheating furnace 20 and then rolled by the rolling equipment 30.

The rolling equipment 30 controls and rolls the slab S. The rolling equipment 30 has a rough rolling section 31 and a finishing rolling section 32. The rough rolling section 31 performs rough rolling on the slab S reheated by the reheating furnace 20. Further, the finish rolling section 32 performs finish rolling on the slab S that has been roughly rolled by the rough rolling section 31 to generate a steel plate P. The rough-rolled portion 31 has a plurality of rough-rolled rollers 31a and 31b at the top and bottom. Further, the finish rolling section 32 has a plurality of finish rolling rollers 32a and 32b at the top and bottom. Both the rough rolling section 31 and the finishing rolling section 32 control the rolling reduction ratio by the rough rolling rollers 31a and 31b and the finishing rolling rollers 32a and 32b based on the information such as the temperature of the steel plate P measured in advance. Then, controlled rolling is performed to produce the desired steel sheet P by setting the temperature, rolling ratio, etc. within a preset range. The hot straightening equipment 40 hotly straightens the steel sheet P produced by the rolling equipment 30. The hot straightening equipment 40 has a plurality of straightening rollers 40a and 40b at the top and bottom, and straightens the flatness of the steel plate P by adjusting the vertical spacing between the straightening rollers 40a and 40b.

The cooling equipment 50 performs controlled cooling on the steel sheet P generated by the rolling equipment 30. Here, the controlled cooling is to control the structure of the steel sheet P by cooling with a cooling start temperature, a cooling stop temperature, and a cooling rate within a preset range. Specifically, controlled cooling is a process including water cooling, which has a faster cooling rate than natural cooling, accelerated cooling by air cooling, natural cooling, and slow cooling, which is slower than natural cooling, and may be heated in the middle. .. Control cooling also includes multi-stage cooling that changes the cooling rate of accelerated cooling in the middle, and intermittent cooling that stops accelerated cooling in the middle and cools naturally and restarts accelerated cooling.

In the present embodiment, the cooling equipment 50 has a plurality of upper and lower transport rollers 51a and 51b for transporting the steel plate P, and a water cooling unit 52 for water-cooling the steel plate P transported by the transport rollers 51a and 51b. The water cooling unit 52 injects cooling water onto the steel plate P. A plurality of water cooling portions 52 are provided along the transport direction above the upper surface P1a which is the surface P1 of the steel plate P transported by the transport rollers 51a and 51b, and are similarly transported below the lower surface P1b which is the surface P1. Multiple are provided along the direction. Then, the water cooling portion 52 cools the steel plate P by injecting cooling water onto the upper surface P1a and the lower surface P1b of the steel plate P.

The flatness measuring unit 60 measures the flatness of the upper surface, which is the target surface of the steel sheet. The flatness measuring unit 60 includes a transport roller 61 for transporting the steel plate and a laser range finder 62 arranged at a position separated above the upper surface for measuring the flatness of the steel plate transported by the transport roller 61. The laser range finder 62 measures the distance to the irradiation position by irradiating the upper surface of the steel plate conveyed by the transfer roller 61 with a laser and receiving the reflected light. A plurality of laser range finders 62 are provided in the width direction (that is, the paper surface depth direction) orthogonal to the transport direction, and the distance can be measured over the entire width of the steel sheet. Therefore, the steel sheet can measure the distance over the entire upper surface while being conveyed by the conveying roller 61, and the flatness of the upper surface can be measured from the distance measurement result.

In the present embodiment, the plastic deformation imparting equipment 65 is a flatness straightening portion that repeatedly plastically deforms the steel plate P by correcting the flatness of the steel plate P. As shown in FIG. 2, the plastic deformation imparting equipment 65 has a plurality of lower rollers 66 arranged below the steel plate to be conveyed and a plurality of upper rollers 67 arranged above the steel plate. The lower roller 66 and the upper roller 67 are arranged so as to be displaced in the transport direction. Further, at least one of the lower roller 66 and the upper roller 67 is provided so as to be movable up and down, whereby the distance between the lower roller 66 and the upper roller 67 can be adjusted. Then, the distance between the upper end of the lower roller 66 and the lower end of the upper roller 67 is equal to or less than the thickness of the steel plate, or the upper end of the lower roller 66 is above the lower end of the upper roller 67 (the distance dimension is a negative value). By doing so, the steel plate P meanders between the upper ends of the plurality of lower rollers 66 and the lower ends of the plurality of upper rollers 67 according to the interval, and tensile deformation and compressive deformation occur. Repeated. Then, on the surface side, the tensile stress associated with the tensile deformation and the compressive stress associated with the compressive deformation exceed the yield stress according to the size of the interval, which causes tensile plastic deformation and compressive plastic deformation on the surface layer of the steel plate P. That is, the plastic deformation imparting equipment 65 alternately and repeatedly performs tensile plastic deformation and compressive plastic deformation on the surface layer portion including the surface of the steel sheet P. As a result, the residual compressive stress generated in the surface layer portion including the surface P1 of the steel sheet P after the controlled cooling is relaxed.

As described above, the thickness of the plastic deformation region generated in the surface layer portion can be expressed by the degree of processing K. The workability K is the ratio of the total thickness of the steel sheet P to the thickness of the plastic deformation region. The degree of processing by meandering between the upper ends of the plurality of lower rollers 66 and the lower ends of the plurality of upper rollers 67 as described above is shown as follows.

That is, the curvature k of the steel plate P meandering between the lower ends of the plurality of upper rollers 67 is obtained by Hibino's practical formula (see Reference 1) and the formula (1) from FIG.
Reference 1: Japan Society for Plastic Processing, "Correcting-Straightening of Metal Products-Technology to Straighten Sheet, Tube and Others-", Corona Co., Ltd., 19 January 20, 2014, p. 29-89

However, k: curvature m: constant (= 6: see Reference 1)
d: Intermesh (pushing amount) (mm)
L: Roll half pitch (mm)
Is. The intermesh d is, for example, the value obtained by adding the plate thickness t of the steel plate P to the height of the upper end of the lower roller 66 (position coordinates in the vertical direction) and the height of the lower end of the upper roller 67 (position coordinates in the vertical direction). ) And the difference. Further, the roller half pitch L is 1/2 of the distance between the centers in the transport direction of the adjacent lower rollers 66, and is the distance between the centers of the adjacent lower rollers 66 and the upper roller 67 in the transport direction, and is a bending beam. Corresponds to the length of.

Further, in the steel plate P meandering between the lower ends of the adjacent upper rollers 67, the yield curvature ky indicating the plastically deformed portion is similarly obtained as in the equation (2).

However, ky: yield curvature sy: yield stress (MPa)
t: Plate thickness (mm)
E: Young's modulus (MPa)

And, in general, a hardness defect may occur at least in a range of about 0.25 mm from the surface. The range in which the hardness of the surface layer of the steel material P can be changed is a range of 0.5 mm in thickness from the surface P1. When the change in hardness in the range of 0.5 mm from the surface P1 is measured based on the measurement of the electromagnetic characteristics described later, the penetration depth of the eddy current generated inside the steel plate P in order to measure the electromagnetic characteristics is , It is necessary to be at least 0.5 mm or more from the surface P1. On the other hand, the influence of the residual stress can affect not only the range of 0.5 mm from the surface P1 which is the measurement range but also the residual stress state in the range adjacent to the range. Therefore, in order to eliminate the influence of the residual stress around the surface hardness change portion, it is desirable to relax the residual stress at least in the range of 2.0 mm from the surface P1.

FIGS. 4 and 1 show the results of calculating the relationship between the plate thickness t (mm) and the depth h (mm) from the surface plastically deformed by the plastic deformation applying equipment 65, which is a plastic deformation applying portion, for each degree of processing. Is shown. As shown in FIGS. 4 and 1, by setting the degree of processing to 1.8 or more, the thickness from the surface P1 to the thickness in the range of thickness t of 10 mm or more including the thickness range of a general thick plate is included. Residual stress can be relaxed by causing plastic deformation in the range of 2.0 mm. Therefore, the degree of processing is preferably 1.8 or more. Further, in order to relax the residual stress without damaging the steel sheet P due to plastic deformation, the degree of processing is more preferably 5.0 or less. In this way, the target workability is determined, and based on the thickness of the steel plate P, the yield stress of the material of the steel plate P, the Young's modulus, and the equations (1) to (3), the intermesh d is used. The positions of the lower roller 66 and the upper roller 67 in the determined plastic deformation imparting facility 65 may be determined.

Further, the plastic deformation imparting equipment 65 may adjust the distance between the lower roller 66 and the upper roller 67 according to the flatness measurement result by the flatness measuring unit 60. The plastic deformation imparting equipment 65 does not provide the flatness measuring unit 60, and repeatedly plastically deforms the steel plate P by performing flatness correction on the steel plate P regardless of the measurement result of the flatness measuring unit 60. It may be applied.

The hardness measuring unit 70 measures the hardness of the surface P1 of the steel sheet P that has been repeatedly plastically deformed. In the present embodiment, the hardness defect of the upper surface P1a of the surface P1 of the steel plate P becomes a problem, so the hardness measuring unit 70 measures the hardness of the upper surface P1a.

As shown in FIG. 5, the hardness measuring unit 70 of the present embodiment is an apparatus for measuring hardness based on a parameter that changes depending on the properties of the surface layer of the steel sheet P. The parameters of the present embodiment include, for example, parameters that are input to generate a magnetic field when a magnetic field is applied to the surface layer of the steel plate P and that change under the influence of the properties of the surface layer, and the magnetic field. Includes a parameter whose value is measured according to the properties of the surface layer by multiplying. When a magnetic field is applied to the surface layer of the steel sheet P, the parameters that change under the influence of the properties of the surface layer are collectively referred to as electromagnetic characteristics, and the values of the obtained parameters are referred to as electromagnetic characteristic values. Hereinafter, a case where the parameter that changes depending on the properties of the surface layer of the steel sheet P is the electromagnetic characteristic of the surface layer of the steel sheet P will be described.

The hardness measuring unit 70 includes a parameter measuring unit 700 that measures the electromagnetic characteristic value of the surface layer of the steel plate P, and a hardness calculation unit 720 that obtains the hardness of the surface layer of the steel plate P based on the electromagnetic characteristic value measured by the parameter measuring unit 700. To be equipped. Here, the hardness in the present embodiment includes hardness quantified by various tests. For example, Vickers hardness by Vickers hardness test, Brinell hardness by Brinell hardness test, Knoop hardness by Knoop hardness test, Rockwell hardness by Rockwell hardness test, and the like. Further, these hardnesses do not have to be values measured by a test method for measuring each hardness, and if the correlation is known in advance, the measured values are obtained based on the results measured by the rebound type tester. Alternatively, the measured value itself obtained in the rebound type test may be used as an index of hardness. In the following, it will be described as an example of measuring Vickers hardness, and will be simply referred to as hardness.

As shown in FIG. 5, in the present embodiment, the parameter measuring unit 700 is an apparatus for measuring an electromagnetic characteristic value obtained from, for example, a BH loop of a steel plate P. The BH loop is correlation data showing the relationship between the strength H of the magnetic field periodically applied to the surface layer of the steel sheet P and the magnetic flux density B generated on the surface layer of the steel sheet P by the applied magnetic field. The parameter measuring unit 700 includes a magnetizer 710, an oscillator 712, an exciting power supply 713, a magnetic field calculation unit 714, a detection coil 715, a magnetic flux density calculation unit 716, a BH loop calculation unit 717, and an electromagnetic characteristic value detection unit. It is equipped with 718.

The magnetizer 710 is arranged above the upper surface P1a with a gap between the surface layer portion of the steel plate P to be measured, that is, the surface layer portion including the upper surface P1a. The magnetizer 710 has a yoke 711A and an exciting coil 711B. The U-shaped yoke 711A has a body portion 711b and a pair of tip portions 711a formed at both ends of the body portion 711b. The pair of tip portions 711a are arranged so that the tip surfaces serving as magnetic poles face the surface P1 of the surface layer of the steel plate P to be measured. The exciting coil 711B is wound around each of the tip portions 711a. With such a configuration, the yoke 711A has a strength H corresponding to the magnitude of the alternating current on the surface layer of the steel plate P arranged at a position facing the tip portion 711a due to the alternating current flowing through the exciting coil 711B. A magnetic field can be generated.

The oscillator 712 outputs a signal having a frequency corresponding to the frequency of the target alternating current. The exciting power supply 713 outputs an alternating current corresponding to the frequency of the signal received from the oscillator 712 to the exciting coil 711B. Further, the exciting power supply 713 can set the magnitude of the output AC current, that is, the amplitude of the AC current. The magnetic field calculation unit 714 detects the magnitude of the alternating current output from the exciting power supply 713 to the exciting coil 711B, and based on the magnitude of the detected alternating current, the number of turns of the exciting coil 711B stored in advance, and the like, the steel plate P The strength H of the magnetic field generated on the surface layer of is calculated. The magnetic field calculation unit 714 outputs the calculated magnetic field strength H to the BH loop calculation unit 717.

The detection coil 715 is wound around at least one tip portion of the pair of tip portions 711a so as to surround the tip surface serving as a magnetic pole. The magnetic flux Φ generated in the gap between the magnetic pole and the surface P1 of the steel plate P changes depending on the magnetic field generated by the magnetizer 710 and the state of the surface layer of the steel plate P. Then, a voltage is generated in the detection coil 715 by electromagnetic induction according to the time change of the magnetic flux Φ. The magnetic flux density calculation unit 716 detects the voltage generated in the detection coil 715, and calculates the magnetic flux density B from the detected voltage, the number of turns of the detection coil 715 obtained in advance, the cross-sectional area of the detection coil 715, and the like. .. The magnetic flux density calculation unit 716 outputs the calculated magnetic flux density B to the BH loop calculation unit 717.

The BH loop calculation unit 717 sets the magnetic field strength H and the magnetic flux density B based on the magnetic field strength H output from the magnetic field calculation unit 714 and the magnetic flux density B output from the magnetic flux density calculation unit 716. Compute the BH loop that shows the relationship. FIG. 6 shows an example of a BH loop calculated by the BH loop calculation unit 717. The electromagnetic characteristic value of the surface layer of the steel plate P to be measured can be obtained by the BH loop as shown in FIG. Specifically, examples of the electromagnetic characteristic values include residual magnetic flux density Br, coercive force Hc, magnetic permeability μ, and the like. The residual magnetic flux density Br is the magnetic flux density B at the point R2 where the magnetic field strength H is reduced to zero from the point R1 where H is maximized in the BH loop. Further, the coercive force Hc indicates the strength of the magnetic field at the point R3 where the direction of the magnetic field is further reversed and the magnetic flux density B becomes zero. Further, the magnetic permeability μ indicates the rate of change of the magnetic flux density B with respect to the magnetic field strength H when the magnetic field strength is changed by ΔH when the magnetic field strength H is arbitrary. The electromagnetic characteristic value is not limited to the residual magnetic flux density Br, the coercive force Hc, and the magnetic permeability μ, and is not limited to this as long as it is an electromagnetic characteristic value detected by a change in the strength of the magnetic field. In the present embodiment, the electromagnetic characteristic value detection unit 718 extracts, for example, the coercive force Hc. However, the present invention is not limited to this, and the electromagnetic characteristic value detecting unit 718 may use the residual magnetic flux density Br, the magnetic permeability μ, or the like instead of the coercive force Hc, and may detect a plurality of types of electromagnetic characteristic values. The electromagnetic characteristic value detection unit 718 outputs the extracted electromagnetic characteristic value to the hardness calculation unit 720.

Next, the hardness calculation unit 720 will be described. As shown in FIG. 7, the hardness calculation unit 720 is a functional unit realized by executing a program by a control unit 80 including a processor 800 such as a CPU (Central Processing Unit) connected by a bus and a memory 810. Is. The hardness calculation unit 720 calculates the hardness based on the correlation between the hardness stored in advance in the storage unit 82 and the electromagnetic characteristic value, and the electromagnetic characteristic value measured by the hardness measuring unit 70.

In addition, all or a part of each function of the hardness calculation unit 720 may be realized by ASIC (Application Specific Integrated Circuit), PLD (Programmable Logic Device), FPGA (Field Programmable Gate Array), or the like. .. The program may be recorded on a computer-readable recording medium. The computer-readable recording medium is, for example, a flexible disk, a magneto-optical disk, a portable medium such as a ROM or a CD-ROM, or a storage device such as a hard disk built in a computer system. The program may be transmitted over a telecommunication line.

The output unit 81 outputs various information. The output unit 81 outputs, for example, the hardness at an arbitrary position on the surface of the steel sheet P measured by the parameter measuring unit 700 and calculated by the hardness calculation unit 720. Further, the position information may be acquired and the position information and the information related to the hardness may be displayed in association with each other. Further, the information on the hardness may be visually indicated by numerical values or colors on the plane showing the steel plate P based on the position information and the hardness. The output unit 81 includes, for example, a display device such as a CRT (Cathode Ray Tube) display, a liquid crystal display, or an organic EL (Electro-Luminessense) display. The output unit 81 may be configured as an interface for connecting these display devices to its own device.

The hardness determination unit 90 shown in FIG. 1 is a functional unit realized by executing a program by the control unit 80 shown in FIG. 7 in the same manner as the hardness calculation unit 720. The hardness determination unit 90 determines whether or not the position where the hardness is measured is the hardness defect portion W based on the hardness measured by the hardness measurement unit 70. In the present embodiment, a surface hardened portion having a hardness higher than a predetermined upper limit of hardness is detected on the target upper surface P1a of the surface P1 of the steel plate P. That is, the hardness determination unit 90 compares the upper limit value stored in advance in the storage unit 82 with the hardness measured by the hardness measuring unit 70, and if the measured hardness exceeds the upper limit value, the position is set. It is determined that the hardness is poor part W. The hardness determination unit 90 may detect a surface softening portion whose hardness is lower than a predetermined lower limit of hardness on the target upper surface P1a of the surface P1 of the steel plate P. The hardness determination unit 90 shall detect at least one surface hardness change portion of the surface hardened portion and the surface softened portion as the hardness defective portion W. The hardness determination unit 90 causes the output unit 81 to display the portion determined as the hardness defect portion W together with the position information.

In the steel plate manufacturing apparatus 1 of the present embodiment, the hardness measuring unit 70 and the hardness determining unit 90 are used in the continuous casting equipment 10, the reheating furnace 20, the rolling equipment 30, the hot straightening equipment 40, the cooling equipment 50, and the plasticity. Although it has been described as a line integrated with the deformation imparting equipment 65, the present invention is not limited to this. The steel plate P that has been repeatedly plastically deformed is transported to another line, and the hardness measuring unit 70A and the hardness determining unit 90 configured as devices in the other line carry out the hardness measuring step S7 and the hardness determining step S8, which will be described later. You may. Further, the plastic deformation applying equipment 65 is also set as a separate line, and the steel plate P cooled by the cooling equipment 50 is transported to another line, and the plastic deformation applying equipment 65 described later in the separate line is used for the plastic deformation step (flatness straightening step). ) S6 may be carried out.

Further, the annealing step may be performed on the steel sheet P after the controlled cooling is performed. When the annealing step is carried out, an annealing equipment is provided between the cooling equipment 50 and the flatness measuring unit 60, and when the flatness measuring unit 60 is not provided, the cooling equipment 50 and the flatness of the plastic deformation imparting unit 60 are provided. It may be provided between the straightening portion 65 and the straightening portion 65. Further, an internal defect detection step for detecting an internal defect in the manufactured steel sheet P may be performed. When the internal defect detection step is carried out, for example, the internal defect detection equipment is provided after the cooling equipment 50 when the annealing step is not carried out, and after the annealing equipment when the annealing step is carried out. Examples of the internal defect detection equipment include an ultrasonic flaw detection test device that detects internal defects by oscillating ultrasonic waves with respect to the steel sheet P and detecting reflected waves.

Further, in the above, the flatness is corrected as a means for repeatedly plastically deforming the steel sheet P, but the present invention is not limited to this. If the stress state of the surface layer of the steel plate P can be adjusted by repeating compressive plastic deformation and tensile plastic deformation of the steel plate P, for example, three-point bending with an oil press machine is repeatedly performed by other means. The plastic deformation imparting portion may be realized.

Next, the method for manufacturing the steel sheet of the present embodiment will be described. FIG. 8 shows a flow chart of a method for manufacturing a steel sheet of the present embodiment. As shown in FIG. 8, the method for producing a steel sheet of the present embodiment includes a casting step S1 for continuously casting the slab S, a reheating step S2 for reheating the slab S, and a steel sheet P for controlled rolling of the slab S. The rolling step S3, the hot straightening step S4 for hot straightening the steel sheet P, and the cooling step S5 for controlling and cooling the steel sheet P are provided. Further, the method for manufacturing the steel plate P of the present embodiment includes a plastic deformation step S6 in which the controlled and cooled steel plate P is repeatedly plastically deformed, and a hardness measurement for measuring the hardness of the surface P1 of the flattened steel plate P. A step S7 and a hardness determination step S8 for determining a hardness defect portion W based on the hardness measurement result are provided. Further, the method for producing a steel plate of the present embodiment includes a surface polishing step S9 for polishing the surface of a portion determined to be a poor hardness portion W and a hardness confirmation step S10 for confirming the hardness of the poorly hardened portion W whose surface has been polished. A removal step S11 for removing the poor hardness portion W based on the result of the hardness confirmation, a hardness reconfirmation step S12 for reconfirming the hardness after the removal step S11, and a thickness confirmation step S13 for measuring the thickness of the steel plate P. It is provided with S14. The details will be described below.

In the casting step S1, the slab S is generated by the continuous casting equipment 10. Further, in the reheating step S2, the reheating furnace 20 reheats the slab S generated by the continuous casting equipment 10 to a temperature at which the rolling equipment 30 can roll. It should be noted that the casting step S1 may not be carried out, and the slab pieces manufactured in advance may be heated in the reheating furnace 20 and then rolled in the rolling step S3 described below.

In the rolling step S3, the slab S is controlledly rolled by the rolling equipment 30. In the present embodiment, in the rolling step S3, a rough rolling step S3a in which rough rolling is performed on the reheated slab S and a finish rolling in which the rough rolled slab S is finished rolled to produce a steel plate P. Step S3b is carried out. In the hot straightening step S4, the hot straightening equipment 40 straightens the flatness of the steel sheet P produced by the controlled rolling in the hot. In the cooling step S5, the cooling equipment 50 performs controlled cooling on the controlledly rolled steel sheet P.

In the plastic deformation step S6 of the present embodiment, a flatness straightening step is carried out in which the flatness of the steel plate P is corrected to repeatedly plastically deform the steel plate P to adjust the stress state of the surface layer of the steel plate P. In the plastic deformation step S6, first, the flatness measuring unit 60 measures the flatness on the target upper surface P1a of the surface P1 of the steel plate P (step S6a). Then, based on the measurement result, the flatness of the steel plate P is corrected by the plastic deformation imparting equipment 65 (step S6b). As a result, the steel plate P can be straightened according to the flatness immediately after being cooled by the cooling equipment 50. Therefore, the steel sheet P is repeatedly plastically deformed along with the correction of the flatness, and the stress state of the surface layer portion including the surface P1 of the steel sheet P can be adjusted to make it uniform. As described above, the flatness measurement may be omitted and only the flatness correction of the steel plate P may be performed.

In the hardness measuring step S7, the hardness measuring unit 70A measures the hardness of the upper surface P1a, which is the surface P1 of the target steel sheet P. In the hardness measuring step S7 of the present embodiment, the hardness is measured based on a parameter that changes depending on the properties of the surface layer of the steel sheet P. Examples of the parameters of the present embodiment are electrical characteristic values, such as residual magnetic flux density Br, coercive force Hc, and magnetic permeability μ as described above. The hardness measurement method is not limited to the method of measuring the hardness based on the parameters changing depending on the properties of the surface layer of the steel sheet P as described above, and the hardness is directly measured as in the rebound type hardness test. It may be used as a test method.

In the hardness determination step S8, the hardness determination unit 90 determines the presence or absence of the hardness defective portion W on the measured upper surface P1a based on the measured hardness. The algorithm for determining the hardness defect portion W is as described above. If the presence of the poor hardness portion W is not confirmed, the method for manufacturing the steel sheet of the present embodiment is completed, and the steel sheet P is taken out as a product.

In the surface polishing step S9, in order to directly measure the hardness and confirm the hardness in the hardness confirmation step S10 described later, the surface of the portion determined to be the poor hardness portion W is polished to remove the scale of the surface. Examples of the polishing method include a method of polishing the surface with a grinder. When polishing the surface with a grinder, for example, P100 to 180 (JIS R6001) is adopted as the particle size of the polishing fine powder used for the polishing material. By carrying out the surface polishing step S9, the scale existing on the surface is removed from the surface P1 of the steel sheet P, for example, in the range of about 0.01 to 0.1 mm in depth.

In the hardness confirmation step S10, the hardness is confirmed again at the position where the hardness defect portion W is determined and the scale of the surface P1 of the steel sheet P is removed (step S10a). In this embodiment, it is carried out by a test method for directly measuring hardness such as a rebound type hardness test. As a result, the hardness can be measured more accurately in a narrower range than in the case of indirectly measuring the hardness. As in the hardness measuring step S7, the hardness may be indirectly measured by a measuring method. Further, when the hardness is indirectly measured, the surface polishing step S9 may be omitted. When the hardness defective portion W is not confirmed by the measurement result in the hardness confirmation step S10 (step S10b: NO), the steel sheet manufacturing method is completed and the steel sheet P is taken out as a product. On the other hand, when the poor hardness portion W is confirmed (step S10b: YES), the process proceeds to the removal step S11.

In the removal step S11, the hardness defective portion W confirmed in the hardness confirmation step S10 is removed. Specifically, grinding is performed by a grinding device at a position where a hardness defect portion W exists. Examples of the grinding method include a method of grinding the surface with a grinder. When the surface is ground with a grinder, for example, P36 to 60 (JIS R 6010) is adopted as the particle size of the grinding fine powder used for the abrasive. Thereby, the steel material causing the poor hardness can be removed from the surface P1 of the steel plate P, for example, in the range of about 0.1 to 0.5 mm in depth. Since poor hardness may occur at least in the range of about 0.25 mm from the surface, it is preferable to grind in the range of at least 0.25 mm from the surface. Then, after the removal, the hardness reconfirmation step S12 is carried out.

In the hardness reconfirmation step S12, the hardness is reconfirmed at a position where the hardness defective portion W is determined and the steel material on the surface P1 is removed (step S12a). In this embodiment, it is carried out by a test method for directly measuring hardness such as a rebound type hardness test. As in the hardness confirmation step S10, the hardness measurement step S7 may be carried out by a measuring method for indirectly measuring the hardness. When the poor hardness portion W is not confirmed by the measurement result in the hardness reconfirmation step S12 (step S12b: NO), the process proceeds to the thickness confirmation step S13. On the other hand, when the poor hardness portion W is confirmed (step S12b: YES), the process proceeds to the thickness confirmation step S14.

In the thickness confirmation step S13, the thickness of the steel sheet P for which it has been confirmed that the poor hardness portion W has been removed is measured (step S13a). The range for measuring the thickness may be the entire steel sheet P, or may be only the position where the poor hardness portion W is removed in the removal step S11. As a measuring method, for example, an ultrasonic flaw detection test is used. The thickness of the steel plate P at the measurement position can be measured by transmitting ultrasonic waves from one surface with an ultrasonic flaw detection tester and measuring the reflected waves reflected by the other surface. As a result of the measurement, when the thickness of the steel sheet P is equal to or greater than the preset reference value (step S13b: YES), the method for manufacturing the steel sheet is completed and the steel sheet P is taken out as a product. On the other hand, as a result of the measurement, when the thickness of the steel plate P is less than the preset reference value (step S13b: NO), it is determined that the product is defective.

Further, in the thickness confirmation step S14, the thickness of the steel sheet P confirmed to have the poor hardness portion W remaining is measured (step S14a). The range for measuring the thickness may be the entire steel sheet P, or may be only the position where the poor hardness portion W is ground in the removal step S11. The measuring method is the same as the thickness confirmation step S13. As a result of the measurement, when the thickness of the steel plate P is equal to or greater than the preset reference value (step S14b: YES), the removal step S11 is performed again. On the other hand, as a result of the measurement, when the thickness of the steel plate P is less than the preset reference value (step S14b: NO), it is determined that the product is defective.

In the above method for manufacturing a steel sheet, an annealing step of annealing the steel sheet P after the cooling step S5 may be performed. Further, an internal defect detection step for detecting an internal defect in the manufactured steel sheet P may be performed. When the internal defect detection step is carried out, if the annealing step is not carried out, it is carried out after the cooling step S5, and if the annealing step is carried out, it is carried out after the annealing step. As described above, examples of the inspection method carried out in the internal defect detection step include an ultrasonic flaw detection test.

Further, the above-mentioned method for manufacturing a steel sheet includes, but is not limited to, a surface polishing step S9, a hardness confirmation step S10, a hardness reconfirmation step S12, and thickness confirmation steps S13 and S14. For example, if the presence of the poor hardness portion W is not confirmed after the hardness determination step S8 is performed, the manufacturing method of the steel sheet P is completed, and if the presence of the poor hardness portion W is confirmed, the portion is also considered. The removal step S11 may be carried out to complete the method for manufacturing the steel sheet P.

As described above, in the steel sheet manufacturing method and the steel sheet manufacturing apparatus 1 of the present embodiment, the steel sheet P subjected to the rolling step S3 and the cooling step S5 is repeatedly plastically deformed in the plastic deformation step S6. Be given. More specifically, in the above embodiment, the steel sheet P is repeatedly plastically deformed by the flatness straightening step. As a result, in the surface layer portion of the steel sheet P including the upper surface P1a which is the target surface P1, the stress is not uniform even if the residual stress of the steel sheet P after the cooling step S5 is different depending on the location. The uniform state can be eliminated. Then, in the hardness measuring step S7, the hardness is measured with respect to the target surface P1 in a state where the non-uniformity of the stress state is eliminated in the plastic deformation step S6, so that the influence of the stress is suppressed and the hardness is accurately measured. Can be measured. Therefore, in the hardness determination step S8, the hardness defect portion W can be accurately determined based on the accurately measured hardness, and the hardness defect portion W can be reliably removed in the removal step S11. Therefore, it is possible to manufacture the steel sheet P in which the portion where the hardness becomes a problem in quality is suppressed on the target surface P1.

Further, by measuring the electromagnetic characteristics of the surface P1 of the steel plate P in the hardness measuring step S7, the surface P1 of the steel plate P whose hardness is to be measured is not damaged and is not affected by the stress state. Hardness can be measured accurately.

Further, the steel plate P manufactured by such a method for manufacturing the steel plate P is used as the material of the steel pipe Q, and the hardness measuring step S7, the hardness determination step S8 and the removing step S11 are performed on the inner surface Q1 of the steel pipe Q (FIG. 9 and the following). By implementing the part that becomes (see), that is, the upper surface P1a of the steel sheet P, the inner surface Q1 portion of the steel pipe Q, which is concerned about corrosion by the fluid flowing inside, has a hardness defect that is easily affected by corrosion. It is possible to manufacture a steel pipe Q in which the portion W is suppressed.

Then, the method for manufacturing the steel sheet P may be carried out as one step of the method for manufacturing the steel pipe to manufacture the steel pipe Q. That is, as shown in FIG. 9, in the method for manufacturing the steel pipe Q of the present embodiment, the steel sheet manufacturing process S20 for manufacturing the steel sheet P by the method for manufacturing the steel sheet P, the first pressing process S21, and the second press. A step S22 and a welding step S23 are provided. In the first pressing step S21, the steel sheet P manufactured by the above method for manufacturing a steel sheet is pressed into a U shape to form a U-shaped intermediate material P'. At that time, the upper surface P1a of the steel sheet P on which the plastic deformation step S6, the hardness measurement step S7, the hardness determination step S8, and the removal step S11 were carried out becomes the inner surface Q1 when the steel pipe Q is formed, in other words, a concave curved surface. As such, the steel plate P is processed into a U shape. Further, in the second pressing step S22, the U-shaped intermediate material P ′ processed in the first pressing step S21 is pressed into an O shape to form an O-shaped intermediate material P ″. Further, in the welding step S23, the portions of the O-shaped intermediate material P ″ processed in the second pressing step S22, which are the ends of the steel plates P, are welded together by the welding line Q2.

According to such a method for manufacturing a steel pipe, it is possible to manufacture a steel pipe Q in which the poor hardness portion W, which is easily affected by corrosion, is suppressed in the inner surface Q1 portion of the steel pipe Q, which is feared to be corroded by the fluid flowing inside. Can be done. Further, in the above-mentioned steel pipe manufacturing method, a UOE steel pipe can also be manufactured by carrying out a diameter-expanding step of expanding the diameter of the steel pipe Q after the welding step S23.

Further, in the above, the flatness is corrected as a method of repeatedly plastically deforming the steel sheet P, but the present invention is not limited to this. If the stress state of the surface layer of the steel plate P can be adjusted by repeating compressive plastic deformation and tensile plastic deformation of the steel plate P, for example, three-point bending with an oil press machine is repeatedly performed by another method. The plastic deformation imparting portion may be realized.

Further, in the hardness measurement step S7, the BH loop is measured for the steel sheet P to be inspected, and the electromagnetic characteristic value obtained from the BH loop is obtained to calculate the hardness, but the hardness is not limited to this. No. 10 and 11 show the hardness measuring unit 70A in the steel sheet manufacturing apparatus 1A of the modified example used in the hardness measuring step S7.

As shown in FIG. 10, the hardness measuring unit 70A of the steel sheet manufacturing apparatus 1A of this modified example is an eddy current flaw detector. That is, the hardness measuring unit 70A includes an inspection probe 851, an oscillator 852, a bridge 853, a phase shifter 854, an amplifier 855, a synchronous detector 856, a waveform generation unit 857, and an electromagnetic characteristic value detection unit 858. Has. The inspection probe 851 has a measuring coil 851a. The oscillator 852 generates a reference signal having a predetermined frequency and outputs it to the bridge 853 and the phase shifter 854. The bridge 853 converts a minute impedance change of the measuring coil 851a into a voltage and outputs it to the amplifier 855. The amplifier 855 amplifies the signal output from the bridge 853 and outputs it to the synchronous detector 856. The phase shifter 854 generates a phase-shifted signal while maintaining the frequency of the reference signal, and outputs the signal to the synchronous detector 856. The synchronous detector 856 synchronously detects the signal output from the amplifier 855 by the signal output from the phase shifter 854, extracts the DC component, and outputs it to the waveform generation unit 857. The waveform generation unit 857 generates a waveform as shown in FIG. 11 based on the signal output from the synchronous detector 856. The electromagnetic characteristic value detection unit 858 detects, for example, the vortex phase δ from the waveform generated by the synchronous detector 856 as the electromagnetic characteristic value. Then, the electromagnetic characteristic value detection unit 858 outputs the vortex phase δ, which is the electromagnetic characteristic value, to the hardness calculation unit 720. In the hardness calculation unit 720, the correlation between the hardness stored in advance in the storage unit 82 (see FIG. 7) and the vortex phase δ, which is an electromagnetic characteristic value, and the vortex phase δ measured by the hardness measurement unit 70A. The hardness is calculated based on.

As described above, the device for measuring the electromagnetic characteristic value is not limited to the device for detecting the BH loop, and the eddy current flaw detection test device as in the present embodiment may be used as the device for measuring the electromagnetic characteristic value. Further, the device for detecting the BH loop and the eddy current flaw detection test device may be combined, or the device other than the device for detecting the BH loop and the eddy current flaw detection test device may be used. It is applicable to any device capable of measuring at least an electromagnetic characteristic value in which a magnetic field is generated on the surface layer of the steel sheet P and a different response is obtained depending on the difference in hardness.

12 and 13 show examples of the present invention. FIG. 12 shows the residual stress of the upper surface P1a of the steel plate P immediately before the plastic deformation step S6 is carried out by the plastic deformation imparting equipment 65 when the steel plate P is manufactured by the steel plate manufacturing apparatus 1 of the above embodiment. It is the result of the measurement, and (b) is the result of measuring the residual stress of the upper surface P1a of the steel plate P immediately after the plastic deformation step S6 is carried out. X65 (yield stress 450 MPa) was used for the steel sheet P. Further, the thickness of the steel plate P of this embodiment is 30 mm. Further, the degree of processing by the flatness straightening performed on the steel sheet P in the plastic deformation step S6 is 2.2. The residual stress was measured by the X-ray stress measurement method. Negative values indicate compressive stress and positive values indicate tensile stress. Further, FIG. 12 shows a frequency distribution shown for each stress range, and shows the frequency for each band of 50 MPa, such as a band of less than -50 MPa and -100 MPa or more, and a band of less than -100 MPa and -100 MPa or more. As shown in FIG. 12A, the residual stress before the plastic deformation step S6 was -225 MPa on average, and the standard deviation was 77.5 MPa. On the other hand, as shown in FIG. 12B, by carrying out the plastic deformation step S6, the entire upper surface P1a of the steel sheet P is brought into a slight compressive stress state with an average of 0.1 MPa, and the standard deviation is 35.4 MPa, which is standard. The deviation could be within the range of 40 MPa. As a result, it was confirmed that homogenization could be achieved in a slight compressive stress state, and that the hardness could be accurately measured in the subsequent hardness measurement step S7 without being affected by the stress.

FIG. 13 shows the result of measuring the incremental magnetic permeability (Ω) as the electromagnetic characteristic value of the upper surface P1a of the steel sheet P by the hardness measuring unit 70A by contouring on the upper surface P1a. The incremental magnetic permeability is a value indicating the rate of change at B = 0 in the BH loop shown in FIG. 6 obtained from the measurement result. If H = 0, it does not matter whether it is H = + Hc or H = −Hc. FIG. 13A shows the measurement result immediately after the cooling step S5 is performed. Further, FIG. 13B shows the measurement results immediately after the plastic deformation step S6 is carried out. As shown in FIG. 13A, immediately after the cooling step S5 is performed, the measurement result of the incremental magnetic permeability on the upper surface P1a of the steel sheet P appears as a large unevenness. This is because the stress state of the surface layer portion including the upper surface P1a of the steel sheet P is non-uniform due to the influence of the steps up to the cooling step S5, and the measurement result of the incremental magnetic permeability also varies due to the influence of the stress. It depends. On the other hand, as shown in FIG. 13B, immediately after the plastic deformation step S6 is carried out, the stress state of the surface layer portion including the upper surface P1a of the steel sheet P is made uniform with a slight compressive stress state by flatness correction. As a result, there is no variation in the measurement results of the incremental magnetic permeability without being affected by stress. Therefore, even a slight change in the incremental magnetic permeability due to the influence of hardness can be detected.

Although the embodiments and examples of the present invention have been described in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes design changes and the like within a range that does not deviate from the gist of the present invention. Is done.

1, 1A Steel plate manufacturing equipment 30 Rolling part 50 Cooling part 65 Plastic deformation imparting equipment (Plastic deformation imparting part)
70, 70A Hardness measurement unit 90 Hardness determination unit P Steel plate P1 Surface Q Steel pipe Q1 Inner surface S Slab S3 Rolling process S5 Cooling process S6 Plastic deformation process S7 Hardness measurement process S8 Hardness determination process S9 Removal process S21 First press process S22 Second Pressing process S23 Welding process W Hardness defective part

Claims (9)

A rolling process that controls and rolls slabs,
A cooling process for controlling and cooling the steel sheet controlled and rolled in the rolling process, and
A plastic deformation step of adjusting the stress state of the surface layer of the steel sheet including the surface of the steel sheet by repeatedly plastically deforming the steel sheet controlled and cooled in the cooling step.
A hardness measuring step of measuring the hardness of at least a part of the surface of the steel sheet on which the plastic deformation step of the steel sheet is carried out, and a hardness measuring step of measuring the hardness of at least a part of the surface of the steel sheet.
Based on the measurement result of the hardness measurement step, a hardness determination step of determining a portion having a hardness exceeding a preset threshold value as a hardness defective portion,
A method for manufacturing a steel sheet, which comprises a removing step of removing a portion having poor hardness. The method for manufacturing a steel sheet according to claim 1, wherein in the hardness measuring step, the hardness of the surface is measured by measuring the electromagnetic characteristics of the surface. The steel plate according to claim 1 or 2, wherein in the plastic deformation step, the steel plate is repeatedly plastically deformed at a degree of processing in which a range of 2 mm or more in thickness from the surface to be measured in the hardness measurement step is the plastic deformation range. Production method. The method for manufacturing a steel sheet according to any one of claims 1 to 3, wherein in the plastic deformation step, plastic deformation is repeatedly performed so that the degree of processing is 1.8 or more. The first to fourth aspects of the plastic deformation step, wherein the flatness of the steel sheet is corrected to repeatedly plastically deform the steel sheet to adjust the stress state of the surface layer of the steel sheet including the surface of the steel sheet. The method for manufacturing a steel sheet according to any one of the items. The steel plate is used as a material for steel pipes.
The method for manufacturing a steel sheet according to any one of claims 1 to 5, wherein the hardness measuring step, the hardness determining step, and the removing step are carried out on a portion to be an inner surface of the steel pipe. A first pressing step of pressing a steel sheet manufactured by the method for manufacturing a steel sheet according to any one of claims 1 to 6 into a U shape, and
A second press step of pressing the steel sheet processed in the first press step into an O shape, and a second press step.
A method for manufacturing a steel pipe, comprising a welding step of welding the ends of the steel sheet processed in the second pressing step. A rolling section that controls and rolls slabs,
A cooling unit that controls and cools the steel sheet that has been controlledly rolled by the rolling unit,
A plastic deformation imparting portion that repeatedly plastically deforms the steel sheet that has been controlled and cooled by the cooling unit to adjust the stress state of the surface layer of the steel sheet including the surface of the steel sheet.
A hardness measuring unit for measuring the hardness of at least a part of the surface of the steel sheet that has been repeatedly plastically deformed by the plastic deformation imparting portion of the steel sheet.
A steel sheet manufacturing apparatus including a hardness determination unit that determines a portion whose hardness exceeds a preset threshold value as a hardness defective portion based on the measurement result of the hardness measurement unit. Computer,
The hardness of the surface is calculated from the electromagnetic characteristics measured for the surface whose surface stress state has been adjusted by repeatedly plastically deforming the steel sheet generated by controlling rolling and controlling cooling of the slab. Hardness calculation means,
A hardness determining means for determining a portion where the hardness exceeds a preset threshold value based on the hardness calculated by the hardness calculating means as a hardness defect.
A program to function as.

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