QUENCHING APPARATUS, METHOD FOR QUENCHING, METHOD FOR MANUFACTURING COLD ROLLED STEEL SHEET, AND METHOD FOR MANUFACTURING COATED STEEL SHEET

Abstract

A quenching apparatus which produces a good cooling effect on a metal sheet, facilitates switching between a condition of performing quenching and a condition of not performing the quenching, and can suppress thermal deformation caused by thermal radiation from the metal sheet. The quenching apparatus includes a bath that contains a cooling medium in which a metal sheet is immersed and at least one pair of rolls, which are disposed such that the metal sheet running in the bath is interposed between the at least one pair of rolls in the bath, distances of which from the metal sheet are changeable, and rotate at a peripheral speed higher than or equal to a running speed of the metal sheet.

Claims

1. A quenching apparatus that cools a metal sheet, the apparatus comprising: a bath that contains a cooling medium in which the metal sheet is immersed and runs; and at least one pair of rolls disposed such that the metal sheet running in the bath is interposed between the at least one pair of rolls in the bath, the at least one pair of rolls configured to rotate at a peripheral speed higher than or equal to a running speed of the metal sheet, wherein distances from each roll of the at least one pair of rolls and the metal sheet are changeable.

2. The quenching apparatus according to claim 1, wherein the at least one pair of rolls are configured to rotate in a reverse direction to a running direction of the metal sheet.

3. The quenching apparatus according to claim 1, wherein the quenching apparatus is installed on an exit side of a soaking furnace of a continuous annealing facility.

4. A method for quenching in which a metal sheet is cooled by using a quenching apparatus including a bath that contains a cooling medium in which the metal sheet is immersed and runs, the method comprising: in a case where quenching in which the metal sheet is immersed in the cooling medium is performed: causing the metal sheet to run in the bath that contains the cooling medium, and in the bath, cooling the metal sheet by rotating at least one pair of rolls disposed such that the metal sheet running in the bath is interposed between the at least one pair of rolls at a peripheral speed higher than or equal to a running speed of the metal sheet, and, in a case where the quenching in which the metal sheet is immersed in the cooling medium is not performed: causing the metal sheet to run in the bath that does not contain the cooling medium, and disposing the at least one pair of rolls at positions farther from the metal sheet than positions where the at least one pair of rolls are disposed in the case where the quenching is performed.

5. The method for quenching according to claim 4, wherein the at least one pair of rolls are rotated in a reverse direction to a running direction of the metal sheet.

6. A method for manufacturing a cold rolled steel sheet, the method comprising: annealing a metal sheet that is the cold rolled steel sheet; and quenching the cold rolled steel sheet with the method for quenching according to claim 4.

7. A method for manufacturing a coated steel sheet, the method comprising performing a coating treatment on the cold rolled steel sheet obtained with the method for manufacturing a cold rolled steel sheet according to claim 6.

8. The method for manufacturing a coated steel sheet according to claim 7, wherein the coating treatment is one treatment selected from the group consisting of an electrogalvanizing treatment, a hot-dip galvanizing treatment, and a hot-dip galvannealing treatment.

9. The quenching apparatus according to claim 2, wherein the quenching apparatus is installed on an exit side of a soaking furnace of a continuous annealing facility.

10. A method for manufacturing a cold rolled steel sheet, the method comprising: annealing a metal sheet that is the cold rolled steel sheet; and quenching the cold rolled steel sheet with the method for quenching according to claim 5.

11. A method for manufacturing a coated steel sheet, the method comprising performing a coating treatment on the cold rolled steel sheet obtained with the method for manufacturing a cold rolled steel sheet according to claim 10.

12. The method for manufacturing a coated steel sheet according to claim 11, wherein the coating treatment is one treatment selected from the group consisting of an electrogalvanizing treatment, a hot-dip galvanizing treatment, and a hot-dip galvannealing treatment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 is a view (side view) of a quenching apparatus according to an embodiment, illustrating a state of the quenching apparatus under a condition of performing quenching.

[0031] FIG. 2 is a view (side view) of the quenching apparatus according to the embodiment, illustrating a state of the quenching apparatus under a condition of not performing the quenching.

[0032] FIG. 3 is a schematic view of a movement mechanism for rolls in the quenching apparatus according to the embodiment.

DETAILED DESCRIPTION

[0033] An embodiment will be described below with reference to the drawings. However, the disclosure is not intended to be limited to the embodiment described below.

[0034] FIGS. 1 and 2 are diagrams (side views) of a quenching apparatus 11 according to an embodiment respectively illustrating a state of the quenching apparatus 11 under a condition of performing quenching and a state of the quenching apparatus 11 under a condition of not performing the quenching. This quenching apparatus 11 can be applied to, for example, a cooling facility installed on an exit side of a soaking furnace of a continuous annealing apparatus.

[0035] As illustrated in FIG. 1, the quenching apparatus 11 includes a bath (water bath) 5 in which a cooling medium (liquid, according to the present embodiment, water) 2 is contained, a sink roll 3 that changes a running direction of a metal sheet 1, and rolls (agitation rolls) 4 that rotate at high speed. The rolls 4 are operated in the form of a set of two rolls 4 (a roll 4a and a roll 4b). At least a pair of the rolls 4 are disposed in the bath 5. The rolls 4 agitate and remove a steam film generated on the surface of the metal sheet 1 during the quenching in which the metal sheet 1 is immersed in the cooling medium. That is, the steam film on the metal surface, which is removed by injecting the cooling medium (water) through the nozzles according to the related art, is removed by agitation with the rolls according to the disclosed embodiments, and thereby the same effect of cooling is obtained.

[0036] As illustrated in FIG. 1, the rolls 4 (the roll 4a and the roll 4b) are disposed with the metal sheet 1 running in the bath 5 (in the cooling medium 2) interposed therebetween, specifically, disposed at positions that face each other with the metal sheet 1 interposed therebetween in the bath 5. In the quenching apparatus 11 illustrated in FIG. 1, six pairs of rolls are spaced by a predetermined pitch from the top toward the bottom of the bath 5 along the running direction of the metal sheet 1. In FIG. 1, arrows illustrated in the rolls 4 (rolls 4a and rolls 4b) indicate the rotation direction of the rolls 4. Furthermore, an arrow illustrated along the metal sheet 1 indicates the running direction of the metal sheet 1.

[0037] In FIG. 1 (the condition of performing the quenching), the rolls 4 (the rolls 4a and the rolls 4b) are disposed at positions where the effect of removing the steam film generated on the surface of the metal sheet 1 when the metal sheet 1 is quenched can be obtained. Herein, the quenching refers to a process in which the metal sheet 1 heated as a result of annealing or the like is immersed in the cooling medium (a liquid, according to the present embodiment, water) so as to be cooled. Specifically, the rolls 4a and the rolls 4b are each disposed in a range between a position where an outer periphery of the roll is in contact with the metal sheet 1 and a position where the perimeter of the roll is spaced from the metal sheet 1 by smaller than or equal to 50 mm in side view of the quenching apparatus 11.

Preferably, the rolls 4 (the rolls 4a and the rolls 4b) are disposed at positions where the outer periphery of the rolls are in contact with the metal sheet 1. In this case, the most significant effect can be obtained.

[0038] When the quenching is performed, the rolls 4 (the rolls 4a and the rolls 4b) rotate at a peripheral speed higher than or equal to a running speed of the metal sheet 1. More specifically, the peripheral speed of the rolls 4 is preferably a relative speed higher than or equal to 1.0 times the running speed of the metal sheet 1. When the peripheral speed of the rolls 4 is a relative speed lower than 1.0 times the running speed of the metal sheet 1, agitating performance reduces. Thus, the effect of removing the steam film from the surfaces of the metal sheet 1 is not sufficiently obtained, and accordingly, the effect of cooling the metal sheet 1 is not sufficiently obtained. Furthermore, the peripheral speed of the rolls 4 is preferably a relative speed lower than or equal to 3.0 times the running speed of the metal sheet 1. When the peripheral speed of the rolls 4 is a relative speed lower than or equal to 3.0 times the running speed of the metal sheet 1, generation of flaws on the surface of the metal sheet 1 is easily suppressed.

[0039] Although the rotation direction of the rolls 4 (the rolls 4a and the rolls 4b) is not limited, for more efficiently improving the agitating performance for agitating the steam film on the surface of the metal sheet 1, the rolls 4 are preferably rotated in a reverse direction to the running direction of the metal sheet 1 (see FIG. 1). Furthermore, the maximum height roughness Rz of the surfaces of the rolls 4 (the rolls 4a and the rolls 4b) is preferably greater than or equal to 10 ?m. When the maximum height roughness Rz of the roll surface is greater than or equal to 10 ?m, the agitating performance is improved. Thus, the effect of removing the steam film from the surface of the metal sheet 1 is more improved, and accordingly, a better effect of cooling is easily obtained. Furthermore, the maximum height roughness Rz of the surfaces of the rolls 4 (the rolls 4a and the rolls 4b) is preferably smaller than or equal to 50 ?m. When the maximum height roughness Rz of the roll surface is lower than or equal to 50 ?m, generation of flaws on the surface of the metal sheet 1 is easily suppressed. Herein, the maximum height roughness Rz of the roll surface is defined by JIS B 0601 (2001) and can be measured by a measurement method described in JIS B 0633. The measurement method may be a stylus type or a non-contact type. The quenching apparatus 11 may include a control device (not illustrated) that controls the rotation speed and the rotation direction of the rolls 4 (the rolls 4a and the rolls 4b) as described above. The rotation speed and the rotation direction of the rolls 4 may be controlled by controlling output of main machine motors 45 that rotate and drive the rolls 4 (see FIG. 3) by using the control device with spindles 44 interposed therebetween.

[0040] Furthermore, a roll diameter of the rolls 4 (the rolls 4a and the rolls 4b) is preferably greater than or equal to 50 mm. When the roll diameter is smaller than 50 mm, bending is likely to occur in the rolls 4 due to a reaction force from the metal sheet 1. Furthermore, the roll diameter of the rolls 4 (the rolls 4a and the rolls 4b) is preferably smaller than or equal to 250 mm. When the roll diameter is smaller than or equal to 250 mm, the agitating performance is improved. Thus, the effect of removing the steam film from the surface of the metal sheet 1 is more improved, and accordingly, a better effect of cooling is easily obtained.

[0041] Although the number of rolls disposed in the bath 5 is not limited, it is required that at least a pair of rolls be disposed such that the metal sheet 1 is interposed between the pair of rolls. Furthermore, a plurality of rolls are preferably disposed on each of the front surface side and the rear surface side of the metal sheet 1. With this configuration, the steam film can be more uniformly and more reliably removed, and accordingly, stable cooling power is obtained. In order to obtain the same cooling power on the front and rear surfaces of the metal sheet 1, the numbers of rolls disposed on the front surface side and the rear surface side of the metal sheet 1 are preferably the same, and each of the rolls disposed on the front surface side is preferably paired with a corresponding one of the rolls disposed on the rear surface side. The number of rolls to be disposed is preferably greater than or equal to three pairs. When the number of rolls to be disposed is greater than or equal to three pairs, the steam film on the surface of the metal sheet 1 can be more uniformly and more reliably removed, and accordingly, stable cooling power is easily obtained. Although an upper limit of the number of rolls 4 to be disposed is not particularly defined, the number of rolls to be disposed is preferably smaller than or equal to ten pairs. When the number of rolls to be disposed is smaller than or equal to ten pairs, it is preferable in terms of the cost. The rolls may be in contact or not in contact with each other. The quenching apparatus according to the disclosed embodiments agitates the cooling medium (the liquid) with the rolls to obtain the effect of removing the steam film from the surface of the metal sheet, and accordingly, obtain a better effect of cooling on the metal sheet. Thus, it is not required that cooling water injection nozzles be installed in the bath of the quenching apparatus.

[0042] Regarding the material of the rolls 4 (the rolls 4a and the rolls 4b), it is sufficient that the rolls 4 (the rolls 4a and the rolls 4b) be formed of a material that has strength to withstand the reaction force of the metal sheet 1. Examples of the material of the rolls 4 include, for example, SUS304, SUS310, ceramic, and so forth.

[0043] FIG. 2 illustrates a state of the quenching apparatus 11 under the condition of not performing the quenching. The difference between FIG. 1 and FIG. 2 is only the state of the quenching apparatus 11 (the state of performing the quenching and the state of not performing the quenching), and the basic configuration of the quenching apparatus 11 is the same. Accordingly, elements corresponding to those of the quenching apparatus 11 illustrated in FIG. 1 are denoted by the same reference numerals so as to omit the detailed description thereof. In the quenching apparatus 11 in the state illustrated in FIG. 2, the cooling medium (water) 2 contained in the bath 5 in the state illustrated in FIG. 1 is discharged, and the bath 5 is empty. In the case of not performing the quenching, the cooling medium having been contained in the bath 5 may be completely discharged from the bath 5 or left in the bath 5 to such a degree that the metal sheet 1 is not immersed.

[0044] As illustrated in FIG. 2, in the case of not performing the quenching, the position of each of the rolls 4 (the rolls 4a and the rolls 4b) is changed to a position spaced farther from the metal sheet 1 than the position in the case of performing the quenching (in other words, the spacing between the rolls 4a and the rolls 4b is increased). That is, the distances between the rolls 4 according to the present embodiment and the metal sheet 1 can be changed. Accordingly, the rolls 4 include a drive mechanism (a motor) that moves the rolls 4 to positions space from the metal sheet 1. A technique is not particularly limited as long as the rolls 4 can be moved as described above. However, when the responsivity is considered, electrical type is more preferable. As an example, a movement mechanism of the rolls 4 (the rolls 4a and the rolls 4b) is illustrated in FIG. 3. FIG. 3 (a) is a bird's eye view when the movement mechanism is looked down from above, and FIG. 3 (b) is a side view of part of the movement mechanism seen from the side. Examples of the movement mechanism of the rolls 4 include, for example, a mechanism that changes the distances between the rolls 4 (the rolls 4a and the rolls 4b) and the metal sheet 1 as in the embodiment illustrated in FIG. 3. In this mechanism, arms (arms with a linear guide) 6 connected to bearing mechanisms 43 at both the ends of each rotation shaft 41 of the rolls 4 (the rolls 4a and the rolls 4b) are installed so as to surround the outer periphery of the bath 5, and, with electrical jacks 7, the rolls 4 (the rolls 4a and the rolls 4b) are moved entirely with the arms 6 in the horizontal direction. The arms 6 are installed in the rolls 4 (the rolls 4a and the rolls 4b) on both the front and rear surface sides of the metal sheet 1 and moved in the horizontal direction. Thus, the movement of the rolls 4 in the horizontal direction (opening and closing of the spacing between the rolls 4a and the rolls 4b) can be performed. In order to suppress leakage of the cooling medium from the bath 5, a sealing mechanism 42 (mechanical seal) may be installed as appropriate in a region where the rotation shafts 41 of the rolls 4 and the bath 5 are connected to each other.

[0045] In FIG. 2 (the condition of not performing the quenching), in order to reduce thermal radiation from the metal sheet 1, it is sufficient that the rolls 4 (the rolls 4a and the rolls 4b) be spaced away from the metal sheet 1. At this time, it is preferable that the rolls 4a and the rolls 4b be spaced from the metal sheet 1 as much as possible. It is also preferable that the distances between the rolls 4a and the metal sheet 1 and the distances between the rolls 4b and the metal sheet 1 be greater than or equal to 200 mm. Specifically, as illustrated in FIG. 2, it is preferable that the distances between the rolls 4a and the metal sheet 1 and the distances between the rolls 4b and the metal sheet 1 be positions spaced by greater than or equal to 200 mm in the horizontal direction. When the rolls 4 are spaced from the metal sheet 1 as described above, damage to the rolls 4 caused by flapping of the metal sheet can be avoided. In the state illustrated in FIG. 2, operation may be performed while the rotation of the rolls 4 is stopped.

[0046] As described above, in the quenching apparatus 11 according to the present embodiment, the distances between the rolls 4 (the rolls 4a and the rolls 4b) and the metal sheet 1 can be changed depending on the condition whether the quenching is performed (FIG. 1 or 2). When the quenching is performed, the rolls 4 are moved close to the metal sheet 1, and the steam film of the metal sheet 1 is agitated and removed by the rolls 4 to cool the metal sheet 1 so as to ensure sufficient cooling power. Thus, according to the disclosed embodiments, the need for installing nozzles to cool the metal sheet 1 can be dropped. Furthermore, when the quenching is not performed, the rolls 4 can be moved away from the metal sheet 1 to suppress thermal deformation of the rolls 4 due to thermal radiation from the metal sheet 1. In addition, the need for installing reflectors or consideration of deformation of the nozzles in the bath is dropped. Thus, when the quenching is performed, a good cooling effect on the metal sheet 1 is obtained. In addition, for example, when the condition for the metal sheet 1 heated in a continuous annealing facility is switched between the condition of performing the quenching and the condition of not performing the quenching, the need for the following work is dropped: opening a furnace of the continuous annealing facility; removing members such as cooling water injection nozzles that is disposed in the bath of the quenching apparatus for preventing thermal deformation caused by thermal radiation from the metal sheet 1; and installing reflectors or the like to prevent thermal deformation in the bath. Accordingly, the switching is facilitated. Thus, both the metal sheet required to be quenched and the metal sheet not required to be quenched can be manufactured with a single quenching apparatus with high productivity.

[0047] The disclosed embodiments are preferably applied to a method for manufacturing a steel sheet. In particular, when the quenching apparatus according to the disclosed embodiments is installed on the exit side of a soaking furnace of a continuous annealing facility through which steel sheets of different steel types continuously pass, the quenching apparatus can be more effectively used. Furthermore, the quenching apparatus can also be used for shape correction of the metal sheet by adjusting installation positions of the rolls and disposition relationship with the metal sheet running in the bath of the quenching apparatus.

[0048] Examples of the above-described steel sheet include a cold rolled steel sheet, and a hot-dip galvanized steel sheet, an electrogalvanized steel sheet, a hot-dip galvannealed steel sheet, and the like that are made by performing a surface treatment on the cold rolled steel sheet. The disclosed embodiments are preferably applied to a method for manufacturing a cold rolled steel sheet in which an annealed cold rolled steel sheet is quenched and to a method for manufacturing a coated steel sheet in which the cold rolled steel sheet is further subjected to a coating treatment. The coating treatment is at least one treatment selected from an electrogalvanizing treatment, a hot-dip galvanizing treatment, a hot-dip galvannealing treatment, and the like. The disclosed embodiments are not limited to examples of manufacturing steel sheets but are applicable to manufacturing of metal sheets in general other than steel sheets.

EXAMPLES

[0049] Examples of the disclosed embodiments are described. However, the disclosure is not intended to be limited to the examples described below.

[0050] In the present examples, a cooling apparatus used after high-tensile steel sheets having a thickness of 0.8 to 2.3 mm had undergone a treatment in a continuous annealing facility was changed. Specifically, according to the present examples, examples are described in which a quenching apparatus was installed on the exit side of the continuous annealing facility, the above-described high-tensile steel sheets were annealed in the continuous annealing facility, and, after that, a steel sheet (coil) of a tensile strength target of 780 to 1470 MPa and a steel sheet (coil) of a tensile strength target of 340 to 590 MPa were manufactured with the same quenching apparatus.

RELATED-ART EXAMPLE

[0051] As a related-art example, a quenching apparatus described in Patent Literature 2 was installed on the exit side of the continuous annealing facility. In order to obtain the mechanical properties of the steel sheet of a tensile strength target of 780 to 1470 MPa, the quenching (water quenching) was performed with the above-described quenching apparatus, and 200 pieces of the coil of a tensile strength target of 780 to 1470 MPa were manufactured. Then, 200 pieces of the coil of a tensile strength target of 340 to 590 MPa were manufactured. In so doing, since the water quenching was not required, water in the water bath of the above-described quenching apparatus was discharged, reflectors for protection were installed for water injection nozzles installed in the water bath. The steel sheets having undergone continuous annealing were run in the water bath of the quenching apparatus in this state, and 200 pieces of the coil of a tensile strength target of 340 to 590 MPa were manufactured. As a result, the reflectors underwent thermal deformation.

COMPARATIVE EXAMPLE

[0052] As a comparative example, the above-described quenching apparatus 11 (excepting that the rolls 4 (the rolls 4a and the rolls 4b) were forcibly fixed so as not to rotate) was installed on the exit side of the continuous annealing facility. In order to obtain the mechanical properties of the steel sheet of a tensile strength target of 780 to 1470 MPa, the quenching (water quenching) was required. Accordingly, water was poured into the water bath 5 as illustrated in FIG. 1, and cooling was performed without rotating the rolls 4. At this time, the positions of the rolls 4 (the rolls 4a and the rolls 4b) were positions where the rolls 4 were in contact with the steel sheet running in the water bath 5. Under these conditions, 200 pieces of the coil of a tensile strength target of 780 to 1470 MPa were manufactured. Out of the 200 pieces of the coil, 120 pieces of the coil were unable to exhibit the mechanical properties due to insufficient cooling. Then, continuously, 200 pieces of the coil of a tensile strength target of 340 to 590 MPa were manufactured in a state in which the rolls 4 (the rolls 4a and the rolls 4b) were spaced from the steel sheet by 500 mm in the horizontal direction and the water was discharged from the water bath 5 as illustrated in FIG. 2. As a result, it was confirmed that damage such as thermal deformation due to heat effect from the steel sheet was not caused, and the target tensile strength was obtained. In this comparative example, when the pieces of the coil of a tensile strength target of 780 to 1470 MPa were manufactured, a sufficient cooling effect was unable to be obtained, and a yield and the productivity reduced.

EXAMPLE

[0053] As an Example, the above-described quenching apparatus 11 was installed on the exit side of the continuous annealing facility. In order to obtain the mechanical properties of the steel sheet of a tensile strength target of 780 to 1470 MPa, the quenching (water quenching) was required. Accordingly, water was poured into the water bath 5 as illustrated in FIG. 1, and cooling of the steel sheet was performed while the rolls 4 (the rolls 4a and the rolls 4b) were rotated. At this time, the positions of the rolls 4 (the rolls 4a and the rolls 4b) were positions where the rolls 4 were in contact with the steel sheet running in the water bath 5. Here, the maximum height roughness Rz of the roll 4 surface was set to 20 ?m, and the roll diameter of the rolls 4 was set to 150 mm.

Example 1

[0054] In example 1, the rotation directions of the steel sheet and the rolls 4 (the rolls 4a and the rolls 4b) were set to be the same, and the rolls were rotated at a peripheral speed 2.0 times the steel sheet running speed (when the steel sheet was 1000 mm/s, the roll peripheral speed was 2000 mm/s; the peripheral speed of the rolls was a relative speed 2.0 times the running speed of the metal sheet 1). Under these conditions, when 200 pieces of the coil of a tensile strength target of 780 to 1470 MPa were manufactured, a good cooling effect was obtained, and the target strength was achieved in all the pieces of the coil. Then, continuously, 200 pieces of the coil of a tensile strength target of 340 to 590 MPa were manufactured in a state in which the rolls 4 (the rolls 4a and the rolls 4b) were spaced from the steel sheet by 500 mm in the horizontal direction and the water was discharged from the water bath 5 as illustrated in FIG. 2. As a result, it was confirmed that damage such as thermal deformation due to heat effect from the steel sheet was not caused, and the target tensile strength was obtained. In example 1, in switching between the condition of performing the quenching and the condition of not performing the quenching, removal of the members in the water bath 5 of the quenching apparatus 11 or installation of the reflectors in the water bath 5 was not performed. Thus, the switching was facilitated, and both the steel sheet required to be quenched and the steel sheet not required to be quenched were successfully manufactured with a single quenching apparatus with high productivity.

Example 2

[0055] In example 2, the rotation directions of the steel sheet and the rolls 4 (the rolls 4a and the rolls 4b) were set to be reverse to each other, and the rolls 4 (the rolls 4a and the rolls 4b) were rotated at the peripheral speed that is the same as the steel sheet running speed (when the steel sheet was 1000 mm/s, the roll peripheral speed was 1000 mm/s; the peripheral speed of the rolls 4 was a relative speed 2.0 times the running speed of the metal sheet 1). Under these conditions, when 200 pieces of the coil of a tensile strength target of 780 to 1470 MPa were manufactured, a good cooling effect was obtained, and the target strength was achieved in all the pieces of the coil. Then, continuously, 200 pieces of the coil of a tensile strength target of 340 to 590 MPa were manufactured in a state in which the rolls 4 (the rolls 4a and the rolls 4b) were spaced from the steel sheet by 500 mm in the horizontal direction and the water was discharged from the water bath 5 as illustrated in FIG. 2. As a result, it was confirmed that damage such as thermal deformation due to heat effect from the steel sheet was not caused, and the target tensile strength was obtained. In example 2, in switching between the condition of performing the quenching and the condition of not performing the quenching, removal of the members in the water bath 5 of the quenching apparatus 11 or installation of the reflectors in the water bath 5 was not performed. Thus, the switching was facilitated, and both the steel sheet required to be quenched and the steel sheet not required to be quenched were successfully manufactured with a single quenching apparatus with high productivity. Thus, effectiveness of the disclosed embodiments is confirmed.