Spalling resistance evaluation method and spalling resistance evaluation device

Abstract

This spalling resistance evaluation method is a method for evaluating the spalling resistance of a steel material used for work rolls used for skin pass rolling of zinc plated steel sheet, and includes: a stress application step for repeatedly applying stress to the steel material by causing a rolling element pressed against the surface of the steel material to roll in one direction while supplying hydrogen to the steel material by applying a current or a voltage to the steel material in an electrolytic solution; and an evaluation step for evaluating the spalling resistance of the steel material by measuring the service life until peeling occurs at the surface of the steel material in the stress application step.

Description

Spalling resistance evaluation method and spalling resistance evaluation device

[0001] The present disclosure relates to a spalling resistance evaluation method and a spalling resistance evaluation device for evaluating the spalling resistance of steel materials used in work rolls for skin-pass rolling of zinc-based coated steel sheets. In particular, the present disclosure relates to a spalling resistance evaluation method and a spalling resistance evaluation device that can accurately evaluate the spalling resistance of steel materials. This application claims priority based on Japanese Patent Application No. 2024-029418, filed February 29, 2024, the contents of which are incorporated herein by reference.

[0002] 1A and 1B are diagrams illustrating the spalling that occurs during skin-pass rolling of zinc-based coated steel sheets. FIG. 1A illustrates skin-pass rolling. FIG. 1B illustrates the cause of spalling. As shown in FIG. 1A , in a hot-dip galvanizing line, lightly reduced skin-pass rolling is performed on coated steel sheets (zinc-based coated steel sheets). In skin-pass rolling, water (e.g., industrial water) is sprayed onto the work roll surface to prevent adhesion of zinc-based powder produced by the zinc-based coated steel sheets. However, it is known that continuous use of work rolls in such a water-lubricated environment can cause micro-defects (fine pits and blister defects) to occur on the work roll surface. These micro-defects can initiate cracks that propagate inside the work roll and ultimately lead to spalling (a phenomenon in which the work roll surface peels off).

[0003] This spalling is said to occur when the work roll and zinc come into contact in a water-lubricated environment, as shown in the left diagram of Figure 1B. This causes a local battery to form between the surface of the iron (Fe) constituting the work roll and the zinc (Zn). This causes some of the hydrogen (H) produced by the cathodic reaction to penetrate into the work roll, accelerating the propagation of cracks, as shown in the right diagram of Figure 1B (see, for example, Non-Patent Document 1).

[0004] Generally, used work rolls are reused after surface grinding and dulling. However, if spalling occurs, they are discarded, resulting in significant damage. For this reason, work rolls that can suppress the occurrence of spalling by omitting subzero treatment or adjusting the tempering temperature in the work roll manufacturing process have been developed. However, this reduces the hardness of the work roll and shortens the replacement cycle. On the other hand, Non-Patent Document 1 proposes a high-hardness work roll with excellent spalling resistance by forming carbonitrides in the steel, which can serve as hydrogen trapping sites. In order to develop high-hardness work rolls with excellent spalling resistance to be used in skin-pass rolling of zinc-based coated steel sheets, a method is needed that can accurately evaluate the spalling resistance of steel materials used for work rolls.

[0005] Patent Document 1 proposes a method for evaluating the spalling resistance of a steel material by applying a potential equivalent to the natural potential of zinc to the steel material in a solution containing an electrolyte, bringing a member into contact with the steel material, causing relative motion of the steel material, and measuring the time until a crack occurs in the steel material. The relative motion of the steel material in the method described in Patent Document 1 is vibration (reciprocating sliding) with respect to the contacting member. However, since an actual work roll rotates in one direction, and the rolling contact force with the steel sheet causes compressive and shear forces to act simultaneously, leading to spalling, it is difficult to say that the method described in Patent Document 1 can simulate an actual environment, and the spalling resistance of the steel material cannot be accurately evaluated.

[0006] Furthermore, Patent Document 2 proposes a method for conducting a rolling fatigue test on steel for bearings after supplying hydrogen to the steel in advance. This method could be applied to evaluating the spalling resistance of steel used in work rolls for skin-pass rolling of zinc-based coated steel sheets, but it is difficult to obtain stable test results because the hydrogen concentration in the steel is released during the test and gradually decreases.

[0007] Japanese Patent No. 5081138 Japanese Patent Application Laid-Open No. 2023-103360

[0008] Nobuyuki Fujitsuna and four others, "CGL Skin-Pass Roll Steel with Excellent Spalling Resistance," Kobe Steel Techniques, December 2005, Vol. 55, No. 3, pp. 33-36

[0009] The present disclosure has been made to solve the problems of the conventional techniques as described above, and has an object to provide a spalling resistance evaluation method and a spalling resistance evaluation device that can accurately evaluate the spalling resistance of steel materials.

[0010] In order to solve the above problems, the present disclosure employs the following configuration.

[0011] (1) A first aspect of the present disclosure is a method for evaluating the spalling resistance of a steel material used in a work roll that is used in skin-pass rolling of a zinc-based plated steel sheet, the spalling resistance evaluation method comprising: a stress application step of repeatedly applying stress to the steel material by applying a current or a voltage to the steel material in an electrolytic solution, thereby causing rolling elements pressed against the surface of the steel material to roll in one direction while supplying hydrogen to the steel material; and an evaluation step of evaluating the spalling resistance of the steel material by measuring the life until peeling occurs on the surface of the steel material in the stress application step.

[0012] (2) A second aspect of the present disclosure is an apparatus for evaluating the spalling resistance of a steel material used in a work roll that is used in skin-pass rolling of a zinc-based plated steel sheet, the spalling resistance evaluation apparatus comprising: a power supply unit that applies a current or a voltage to the steel material in an electrolyte; rolling elements; a support unit that supports the rolling elements so that they can roll; a drive unit that rotates the support unit in one direction while the rolling elements are pressed against the surface of the steel material, thereby applying repeated stress to the steel material; and a detection unit that detects the occurrence of peeling on the surface of the steel material by rotating the support unit in one direction while the drive unit rotates the support unit in one direction while applying repeated stress to the steel material by applying a current or a voltage to the steel material using the power supply unit, thereby supplying hydrogen to the steel material.

[0013] (3) In the spalling resistance evaluation device described in (2) above, the power supply unit may include a counter electrode for the steel material, and the counter electrode may be composed of a plurality of electrodes arranged at equal intervals around the steel material or a single electrode arranged to surround the steel material.

[0014] According to the present disclosure, the spalling resistance of steel materials can be accurately evaluated.

[0015] 2A is a diagram for explaining skin-pass rolling, which is a schematic diagram for explaining spalling that occurs in skin-pass rolling of a zinc-based plated steel sheet; FIG. 2B is a diagram for explaining the cause of spalling; FIG. 2C is a plan view schematically showing the general configuration of a spalling resistance evaluation device according to an embodiment of the present disclosure; FIG. 2D is a cross-sectional view taken along the arrow AA in FIG. 2A; and FIG. 2E is a diagram showing an example of test results obtained by evaluating the spalling resistance of a steel material used in a work roll used in skin-pass rolling of a zinc-based plated steel sheet, using a spalling resistance evaluation device according to an embodiment of the present disclosure.

[0016] Hereinafter, a spalling resistance evaluation device according to an embodiment of the present disclosure will be described.

[0017] 2A and 2B are diagrams schematically illustrating the overall configuration of a spalling resistance evaluation device according to this embodiment. FIG. 2A is a plan view, and FIG. 2B is a cross-sectional view taken along the line AA in FIG. 2A . The drive unit 40 and the detection unit 50 are omitted from FIG. 2A . The dimensions, scale, and shapes of the components illustrated in FIGS. 2A and 2B may differ from the actual dimensions. As shown in FIGS. 2A and 2B , the spalling resistance evaluation device 100 according to this embodiment is an apparatus for evaluating the spalling resistance of a steel material S used in a work roll for skin-pass rolling of a zinc-based coated steel sheet, and includes a power supply unit 10, a rolling element 20, a support unit 30, a drive unit 40, and a detection unit 50. The spalling resistance evaluation device 100 according to this embodiment also includes a housing 1 and a base 60. The steel material S according to this embodiment is made of the same steel type as the work roll and has an annular shape with a circular opening in the center in a plan view.

[0018] Hereinafter, each of the components included in the spalling resistance evaluation device 100 according to this embodiment will be specifically described.

[0019] [Housing 1] The housing 1 is a bottomed housing with an open top, and is made of, for example, stainless steel (SUS304). The housing 1 of this embodiment is circular in plan view because the steel material S placed inside has an annular shape. The steel material S is placed inside the housing 1 in a state where it is immersed in the electrolyte E contained inside the housing 1.

[0020] [Base 60] The base 60 is placed at the bottom of the housing 1, and the steel material S is placed on the base 60. The base 60 is formed from an insulator (for example, zirconia) to prevent hydrogen from being supplied to anything other than the steel material S (for example, the bottom of the housing 1).

[0021] [Power Supply Unit 10] The power supply unit 10 includes an electrolyte E, an adjustment unit 11, and a counter electrode 12, and applies a current or voltage to the steel material S in the electrolyte E. For example, a sodium hydroxide aqueous solution is used as the electrolyte E. The electrolyte E is contained in the housing 1 to a height at least such that the steel material S and the counter electrode 12 are immersed in the electrolyte E. In the example shown in FIGS. 2A and 2B , the electrolyte E is contained to a height such that the steel material S, the counter electrode 12, and a portion (lower portion) of the rolling element 20 are immersed, but it is also possible to contain the electrolyte E up to the same height as the housing 1. The adjustment unit 11 adjusts the current or voltage applied to the steel material S. For example, a potentio-galvanostat is used as the adjustment unit 11. The adjustment unit 11 is connected to the steel material S and the counter electrode 12 for the steel material S by conductors C1 and C2, respectively. The counter electrode 12 of this embodiment is composed of a plurality of electrodes (three in the example shown in FIG. 2 ), and each of the plurality of electrodes constituting the counter electrode 12 is connected in parallel to the adjustment unit 11 by a conductor C2. As the counter electrode 12, for example, a platinum plate is used. In a preferred embodiment, the counter electrode 12 of this embodiment is composed of a plurality of electrodes arranged at equal intervals around the steel material S, so that the hydrogen concentration on the surface of the steel material S is uniformed, and the spalling resistance of the steel material S can be evaluated more accurately. Note that, if there is no obstacle in terms of installation space, it is also possible to uniform the hydrogen concentration on the surface of the steel material S even if a counter electrode 12 composed of a single electrode arranged to surround the steel material S is used.

[0022] [Rolling element 20] The rolling element 20 has a shape that allows it to roll on the steel material S. In this embodiment, a sphere is used as the rolling element 20. The rolling element 20 is made of an insulator (for example, ceramic such as silicon nitride) to prevent hydrogen from being supplied. The insulator has an electrical resistance R of 10 ６ [Ω] or more. In the present embodiment, as a preferred aspect, a plurality of rolling elements 20 (three in the example shown in FIGS. 2A and 2B ) are arranged at equal intervals. The shape of the rolling elements 20 is not limited to a sphere, but may also be a cylinder, a cone, or a truncated cone.

[0023] [Support portion 30] The support portion 30 supports the rolling element 20 so that it can roll. Specifically, a recess (not shown) for attaching the rolling element 20 is formed on the underside of the support portion 30, and a part (upper portion) of the rolling element 20 is attached to this recess so that it can roll. The support portion 30 has an annular shape with the same dimensions as the steel material S in a plan view, and is formed from, for example, a copper alloy, steel, or synthetic resin.

[0024] [Drive Unit 40] The drive unit 40 is equipped with a motor (not shown) and a uniaxial stage (not shown) and is configured to be able to move up and down and rotate around a rotation axis N. A support unit 30 is attached to a flange 41 located at the bottom of the drive unit 40. As the drive unit 40 moves up and down and rotates, the support unit 30 moves up and down and rotates together with the drive unit 40. Therefore, as the drive unit 40 moves down, the support unit 30 also moves down, and the rolling elements 20 supported by the support unit 30 are pressed against the surface of the steel material S. The magnitude of the contact stress between the rolling elements 20 and the steel material S can be adjusted by adjusting the vertical position of the drive unit 40. Furthermore, when the drive unit 40 rotates in one direction (e.g., clockwise) with the rolling elements 20 pressed against the surface of the steel material S, the support unit 30 also rotates in the same direction, and the rolling elements 20 pressed against the surface of the steel material S roll in the same direction. This allows repeated stress to be applied to the steel material S.

[0025] [Detection Unit 50] The detection unit 50 detects the occurrence of peeling on the surface of the steel material S. Specifically, the power supply unit 10 applies a current or voltage to the steel material S to supply hydrogen to the steel material S, and the drive unit 40 rotates the support unit 30 in one direction to repeatedly apply stress to the steel material S, thereby causing the detection unit 50 to detect the occurrence of peeling on the surface of the steel material S. When peeling occurs on the surface of the steel material S, it is considered that the torque around the rotation axis N of the drive unit 40 will be larger than in a state where no peeling occurs. Therefore, for example, a torque meter that detects the torque around the rotation axis N of the drive unit 40 is used as the detection unit 50. If the torque value detected by the torque meter is larger than a predetermined threshold value, it can be determined that peeling has occurred on the surface of the steel material S.

[0026] According to the spalling resistance evaluation device 100 having the configuration described above, a stress application step can be performed in which a current or voltage is applied to the steel material S in the electrolytic solution E using the power supply unit 10, the rolling elements 20, the support unit 30, and the drive unit 40, thereby causing the rolling elements 20 pressed against the surface of the steel material S to roll in one direction while supplying hydrogen to the steel material S, thereby repeatedly applying stress to the steel material S. In addition, an evaluation step can be performed in which the spalling resistance of the steel material S is evaluated by measuring the life (elapsed time, etc.) until peeling occurs on the surface of the steel material detected by the detection unit 50.

[0027] FIG. 3 shows an example of test results obtained by using the spalling resistance evaluation device 100 according to this embodiment to evaluate the spalling resistance of a steel material S used in a work roll for skin-pass rolling of a zinc-based coated steel sheet. The horizontal axis of FIG. 3 represents the life from the start of the test until spalling occurs (spadding life), expressed as the number of cycles of the rolling elements 20. In this embodiment, three rolling elements 20 are provided, so the number of cycles of the rolling elements 20 is three per one rotation of the support part 30. The vertical axis of FIG. 3 represents the maximum value of the contact stress (Hertzian contact stress) between the rolling elements 20 and the steel material S, which is adjusted by adjusting the vertical position of the drive part 40. The test results obtained by using the spalling resistance evaluation device 100 according to this embodiment, plotted with "△" in FIG. 3, were obtained by previously investigating the current capable of supplying the maximum amount of hydrogen that can be generated by bimetallic contact between the steel material S and zinc (current density: 5 A / m in the example shown in FIG. 3). ２ The results were obtained under conditions in which the investigated current was applied by the power supply unit 10 and the support unit 30 was rotated in one direction at a rotational speed of 500 rpm. For comparison, in Fig. 3, results obtained under conditions in which lubricating oil was used instead of the electrolytic solution E and no current or voltage was applied to the steel material S by the power supply unit 10 are plotted with "◯". Also, results obtained under conditions in which industrial water containing zinc peeled off from a zinc-based plated steel sheet was used instead of the electrolytic solution E and no current or voltage was applied to the steel material S by the power supply unit 10 (conditions simulating an actual environment) are plotted with "□".

[0028] As shown in Figure 3, compared to the results obtained using the lubricating oil plotted with "◯", the results obtained using the spalling resistance evaluation device 100 according to this embodiment plotted with "△" show that peeling occurred in a short time. Also, compared to the results obtained using industrial water containing zinc plotted with "□", the results obtained using the spalling resistance evaluation device 100 according to this embodiment show that peeling occurred in about the same time. From the above results, it can be said that the spalling resistance evaluation device 100 according to this embodiment can evaluate the spalling resistance of the steel material S as accurately as in an actual environment.

[0029] According to the spalling resistance evaluation device 100 of this embodiment, hydrogen is supplied to the steel material S by applying a current or a voltage to the steel material S in the electrolytic solution E. In other words, hydrogen is continuously supplied to the steel material S electrochemically. Then, since stress is repeatedly applied to the steel material S while applying a current or a voltage to the steel material S (while continuously supplying hydrogen) thereto, the problem of a gradual decrease in the hydrogen concentration in the steel material S during repeated application of stress does not occur, as in the method described in Patent Document 2.

[0030] Furthermore, according to the spalling resistance evaluation device 100 of this embodiment, the rolling elements 20 pressed against the surface of the steel material S are rolled in one direction, thereby repeatedly applying stress to the steel material S. Therefore, unlike the method described in Patent Document 1, in which a member is brought into contact with the steel material S and the steel material S slides back and forth, it is possible to simulate an environment close to the actual environment in which spalling occurs in actual work rolls. Typically, in steel material S subjected to rolling contact, cracks propagate along with plastic flow on the surface of the steel material S. The amount of plastic flow differs between cases with and without reversal of rolling contact, with the amount of plastic flow being greater without reversal (i.e., unidirectional rolling). Furthermore, cracks generated by unidirectional rolling propagate at the position of maximum shear stress, and their shape depends on plastic flow. On the other hand, cracks generated by repeated reversal (bidirectional rolling) inhibit plastic flow and crack propagation, extending the life by more than 10 times compared to unidirectional rolling. It is known that an actual work roll does not have a reversing motion and depends on the maximum shear stress and plastic flow. Therefore, the spalling resistance evaluation device 100 according to this embodiment is capable of accurately evaluating the spalling life and further shortening the test time, compared to the method described in Patent Document 1.

[0031] According to the present disclosure, the spalling resistance of steel materials can be accurately evaluated.

[0032] REFERENCE SIGNS LIST 1... Housing 10... Power supply unit 11... Adjustment unit 12... Counter electrode 20... Rolling element 30... Support unit 40... Drive unit 50... Detection unit 60... Base 100... Spalling resistance evaluation device E... Electrolyte S... Steel material

Claims

1. A method for evaluating the spalling resistance of a steel material used in work rolls used in skin-pass rolling of zinc-based plated steel sheets, comprising: a stress application step in which a rolling element pressed against the surface of the steel material rolls in one direction while supplying hydrogen to the steel material by applying an electric current or voltage to the steel material in an electrolytic solution, thereby repeatedly applying stress to the steel material; and an evaluation step in which the life until peeling occurs on the surface of the steel material in the stress application step is measured to evaluate the spalling resistance of the steel material.

2. An apparatus for evaluating the spalling resistance of steel used in work rolls used in skin-pass rolling of zinc-based plated steel sheet, comprising: a power supply unit that applies current or voltage to the steel in an electrolyte; rolling elements; a support unit that supports the rolling elements so that they can roll; a drive unit that rotates the support unit in one direction with the rolling elements pressed against the surface of the steel, thereby applying repeated stress to the steel; and a detection unit that detects the occurrence of peeling on the surface of the steel by rotating the support unit in one direction with the drive unit while applying repeated stress to the steel by applying current or voltage to the steel using the power supply unit to supply hydrogen to the steel.

3. The spalling resistance evaluation device according to claim 2, wherein the power supply unit is provided with a counter electrode for the steel material, and the counter electrode is composed of a plurality of electrodes arranged at equal intervals around the steel material or a single electrode arranged to surround the steel material.

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