JP2017101273A - Toll shaft member in melting metal plating bath and its production method, and production method of melting metal plating steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a roll shaft member in a long life melting meta plating bath.SOLUTION: The roll shaft member in melting metal plating bath of the invention, includes: a first padding weld layer comprising cobalt base alloy; and a second padding weld layer comprising cobalt base alloy, which is formed on the first padding weld layer. The second padding weld layer has higher hardness than that of the first padding layer, and a plurality of carbonized tungsten particles are included in the second padding weld layer. At least a part of particles in the plurality of carbonized tungsten particles included is particles made by melting at least a part of particles in the plurality of carbonized tungsten particles added to a weld material for forming the second padding layer in the cobalt base alloy once and precipitated later.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a roll shaft member used by being immersed in a molten metal plating bath such as a hot dip galvanizing bath, a method for manufacturing the roll shaft member, and a method for manufacturing a molten metal plated steel sheet.

  As shown in FIG. 4, a hot-dip galvanized steel sheet typified by a hot-dip galvanized steel sheet has an annealed steel strip P that passes through the inside of the snout 30 and enters a molten metal bath 34 accommodated in a pot 32. After entering continuously, the sheet passing direction is changed upward by the sink roll 36, and then guided to the support roll 38, exits the molten metal bath 34, and adjusted to a predetermined plating thickness by the gas wiping nozzle. After that, it is cooled and manufactured.

  Here, the sink roll disposed by being immersed in the molten metal does not have a driving device, and rotates while the roll shaft is in contact with the bearing. Accordingly, since the roll shaft member receives sliding resistance from the bearing in the molten metal, performance such as corrosion resistance, wear resistance, and high toughness with respect to the molten metal is required for long-term use. For this reason, various studies have been made on roll shaft members disposed in a molten metal plating bath.

Patent Document 1 discloses that a dense cermet layer made of ceramic and heat-resistant and corrosion-resistant metal that is thermodynamically stable against molten Al is applied to the surface of a base metal by a thermal spraying method, a build-up welding method, a hot static method. A roll shaft member in a molten metal plating bath formed by a hydraulic press method or the like is described. Patent Document 2 discloses a molten metal in which a lower build-up layer made of a Co-based alloy and an upper build-up layer containing a Co-based alloy containing Cr ₃ C ₂ or W ₂ C are formed on the surface of a roll shaft portion. A roll shaft member in the plating bath is described.

Japanese Patent Laid-Open No. 11-61369 Japanese Utility Model Publication No. 5-69155

  However, in the technique of Patent Document 1, since the dense cermet having good wear resistance is directly formed on the base metal even though the corrosion resistance and wear resistance to the molten metal are good, the roll shaft member There is a problem that the dense cermet layer is likely to be worn or cracked when a high load is applied.

  In the technique of Patent Document 2, since a lower overlay layer having a lower hardness than the upper overlay layer is disposed between the roll shaft portion and the upper overlay layer having a high hardness, the occurrence of cracking is greater than that of Patent Document 1. Seems to be suppressed. However, when the roll shaft member receives a strong sliding resistance from the bearing during actual use, the build-up layer is not sufficiently suppressed from being worn or cracked.

  Thus, when a crack occurs in the coating layer of the roll shaft member, the molten metal penetrates into the crack and reacts with the base material of the roll shaft, damaging the base material, causing a rotation failure of the roll, It adversely affects the production and quality of steel sheets. Therefore, there has been a demand for a roll shaft member that is less likely to be worn or cracked in the coating layer even when the roll shaft member receives a strong sliding resistance from the bearing during actual use.

  In view of the above problems, an object of the present invention is to provide a roll shaft member in a long-life molten metal plating bath in which wear and cracking of a coating layer are suppressed, and a method for manufacturing the roll shaft member. Moreover, an object of this invention is to provide the manufacturing method of the molten metal plating steel plate in which long-term stable production is possible.

The gist configuration of the present invention for solving the above-described problems is as follows.
(1) a metal substrate;
A first build-up weld layer made of a cobalt-based alloy formed on the metal substrate;
A second build-up weld layer made of a cobalt-based alloy formed on the first build-up weld layer;
A roll shaft member in a molten metal plating bath having:
The second build-up weld layer has a higher hardness than the first build-up weld layer,
The second build-up weld layer includes a plurality of tungsten carbide particles,
At least some of the plurality of tungsten carbide particles included in the second build-up weld layer are at least one of the plurality of tungsten carbide particles added to the welding material forming the second build-up layer. A roll shaft member in a molten metal plating bath, wherein at least a part of the particles of the portion is once dissolved in the cobalt-based alloy and then precipitated.

  (2) The roll shaft in the molten metal plating bath according to (1), wherein the first build-up weld layer has a hardness of less than HRC40, and the second build-up weld layer has a hardness of HRC40 or higher. Element.

  (3) The roll shaft member in the molten metal plating bath according to (1) or (2), wherein the content of tungsten carbide particles in the second build-up weld layer is 50% by mass or less.

  (4) The roll shaft member in the molten metal plating bath according to any one of (1) to (3), wherein the second build-up weld layer is constructed in the roll axis direction.

  (5) In the molten metal plating bath according to any one of the above (1) to (4), having a thermal spray layer having a higher hardness than the second build-up weld layer on the second build-up weld layer. Roll shaft member.

  (6) The sprayed layer is a roll shaft member in a molten metal plating bath according to (5), wherein the sprayed layer is made of a tungsten carbide cermet material or a tungsten boride cermet material.

(7) a first step of overlay welding a welding material made of a cobalt-based alloy on a metal base material to form a first overlay weld layer;
On the first build-up weld layer, a second step of build-up welding a welding material made of a cobalt-based alloy and containing a plurality of tungsten carbide particles to form a second build-up weld layer;
A method for producing a roll shaft member in a molten metal plating bath having
In the second step, at least a part of the plurality of tungsten carbide particles, at least a part of which is once dissolved in the cobalt-based alloy and then precipitated, and then a roll shaft in a molten metal plating bath, Manufacturing method of member.

  (8) The manufacturing method of the roll shaft member in the molten metal plating bath according to (7), wherein the build-up welding is performed two or more times in the second step.

  (9) In the second step, build-up welding is performed in the roll axis direction. The method for manufacturing a roll shaft member in a molten metal plating bath according to (7) or (8).

  (10) In any one of the above (7) to (9), the first build-up weld layer has a hardness of less than HRC40, and the second build-up weld layer has a hardness of HRC40 or higher. A method for producing a roll shaft member in the molten metal plating bath as described.

  (11) The roll shaft member in the molten metal plating bath according to any one of (7) to (10), wherein the content of tungsten carbide particles in the second build-up weld layer is 50% by mass or less. Manufacturing method.

  (12) The above (7) to (7) further including a third step of forming a sprayed layer having a hardness higher than that of the second build-up weld layer on the second build-up weld layer after the second step. The manufacturing method of the roll shaft member in the molten metal plating bath as described in any one of 11).

  (13) The method for producing a roll shaft member in a molten metal plating bath according to (12), wherein the thermal spray material in the third step is a tungsten carbide cermet material or a tungsten boride cermet material.

  (14) Molten metal-plated steel sheet is produced using the roll shaft member in the molten metal plating bath according to any one of (1) to (6) above in a molten metal plating bath. A method for producing a metal-plated steel sheet.

  The roll shaft member in the molten metal plating bath of the present invention has a long life because wear and cracking of the coating layer are suppressed. Moreover, according to the manufacturing method of the roll shaft member in the molten metal plating bath of the present invention, it is possible to obtain a roll shaft member in the molten metal plating bath having a long life in which wear and cracking of the coating layer are suppressed. Moreover, according to the manufacturing method of the molten metal plating steel plate of this invention, long-term stable production is possible.

It is a schematic cross section of the roll shaft member 100 by one Embodiment of this invention. It is a figure explaining the state of the tungsten carbide particle in the 2nd build-up weld layer in FIG. It is a figure which shows the construction method of the overlay welding of the roll axial direction in one Embodiment of this invention. It is a schematic diagram of a molten metal plating facility. (A), (B) is sectional drawing and perspective view which respectively show the structure of the edge part of the sink roll 36 in FIG. It is a SEM photograph of the section of the 2nd build-up welding layer in example 1 of an invention.

  The roll shaft member according to the embodiment of the present invention is, for example, a sleeve used at the end of the sink roll 36 shown in FIG. Referring to FIG. 4, the annealed steel strip P passes through the inside of the snout 30 and continuously enters the molten metal bath 34 accommodated in the pot 32, and then passes through the sink roll 36. After the direction is changed upward, it is guided to the support roll 38 and exits from the molten metal bath 34, adjusted to a predetermined plating thickness by a gas wiping nozzle, and then cooled.

  With reference to FIG. 5, the structure of the edge part of the sink roll 36 arrange | positioned by being immersed in molten metal is demonstrated. The end portion of the shaft of the sink roll 36 is inserted into the cylindrical sleeve 40 and further supported by the hanger 44 through the bearing 42. The sink roll 36 does not have a driving device, and rotates with the passing of the steel strip P. At that time, the sleeve 40 is in contact with the bearing 42, and the sleeve 40 receives sliding resistance from the bearing 42. Therefore, the roll shaft member according to the embodiment of the present invention can be applied to the sleeve 40.

  Examples of the molten metal include molten zinc, molten aluminum, molten Zn—Al, and molten Al—Si, and are not particularly limited.

  The present inventors have observed in detail the damage form of the roll shaft member in the hot dip galvanizing bath. As a result, the specific gravity of the iron-based roll base material is smaller than the specific gravity of molten zinc, so buoyancy occurs in the roll in the molten zinc bath, and a strong bending stress acts on the roll shaft member in combination with the tension of the steel strip. Furthermore, the coating layer is worn and cracked due to the strong sliding resistance received from the bearing. Further, it was found that the molten zinc penetrates into the cracks of the coating layer and melts the base material having poor corrosion resistance, so that the cracks progress and finally the roll shaft member is damaged.

  From these results, the present invention was completed by intensively developing a covering material related to the selection of a build-up material having crack resistance against bending stress acting on the roll shaft member and construction conditions, etc. .

  Referring to FIG. 1, a roll shaft member 100 in a molten metal plating bath according to an embodiment of the present invention includes a metal base 10 and a first build-up weld layer 12 formed on the metal base. The second build-up weld layer 14 formed on the first build-up weld layer and the tungsten carbide cermet material or tungsten boride system formed on the second build-up weld layer And a sprayed layer 16 made of a cermet material. In the present specification, the build-up weld layer and the sprayed layer on the metal substrate are collectively referred to as “coating layer”.

  The metal base material 10 consists of materials, such as SUS316L steel, SUS410 steel, SUS420 steel, for example. The hardness of the metal substrate 10 is preferably HRC22 or less, and more preferably HRC11 or less. This is because if the hardness of the metal substrate exceeds HRC22, the metal substrate is too hard and easily cracks during use. The hardness of the metal substrate 10 is preferably HRC10 or higher.

  Here, in this embodiment, the build-up weld layer made of a cobalt-based alloy formed on the upper side of the metal base material 10 is referred to as a “first build-up weld layer”, and is more than the first build-up weld layer. The build-up weld layer made of a cobalt-based alloy, including tungsten carbide particles, and having a hardness higher than that of the first build-up weld layer 12 is defined as a “second build-up weld layer”. . The build-up weld layer formed on the metal substrate 10 is not limited to these two layers, and other build-up weld layers may be formed. When a build-up weld layer other than the first build-up weld layer 12 and the second build-up weld layer 14 is formed on the metal substrate 10, the hardness increases as the build-up weld layers are separated from the metal substrate. It is preferable to have. Thus, by gradually increasing the hardness from the metal base material of the roll shaft member to the surface, the bending stress received by the roll shaft member can be relaxed and cracking of the coating layer can be suppressed.

  Even if the build-up weld layer has two layers, it has an effect of suppressing cracking sufficiently. Although the build-up weld layer may be three or more layers, since it is inferior in workability when it becomes a multilayer, it is preferable to use two or three layers. Overlay welding is a surface treatment technique for a base material in which a metal is deposited on the surface of the base material according to the purpose of hardening, corrosion resistance, repair, regeneration, etc., and a thicker layer can be formed compared to spraying or plating.

  In this embodiment, it is preferable that all the build-up weld layers of a plurality of layers are made of a cobalt-based alloy. This is because there is a nickel-based alloy as the build-up material, but the cobalt-based alloy is superior in corrosion resistance in the molten zinc bath.

  The build-up material for forming each build-up weld layer is made from commercially available materials for various build-up alloys (for example, stellite alloy, trivalloy alloy, etc.) so that the hardness of each build-up weld layer satisfies the above conditions. Select and combine as appropriate.

  At this time, it is preferable that the first build-up weld layer 12 has a hardness of less than HRC40, and the second build-up weld layer 14 has a hardness of HRC40 or more. Wear resistance is enhanced by setting the second build-up weld layer 14 to a high hardness of HRC40 or higher. The hardness of the second build-up weld layer 14 is more preferably HRC45 or higher, and the upper limit is preferably about HRC53. If it exceeds HRC53, it is excellent in wear resistance, but cracks easily enter. The hardness of the first build-up weld layer 12 is preferably HRC35 or higher.

Referring to FIG. 1, second build-up weld layer 14 is assumed to include a plurality of tungsten carbide particles 20A and 20B. The content of tungsten carbide in the second build-up weld layer 14 is preferably 50% by mass or less, and more preferably 40% by mass or less. It is because a crack may generate | occur | produce at the time of construction of a build-up weld layer when it exceeds 50 mass%. The content of tungsten carbide in the second build-up weld layer 14 is preferably 25% by mass or more in order to ensure the effect of adding tungsten carbide particles. By including the tungsten carbide particles in the second build-up weld layer 14, resistance to melting and wear resistance to the molten metal is improved. In the present invention, “tungsten carbide particles” is a concept including all of WC particles, W ₂ C particles, and particles containing both WC and W ₂ C.

  Here, with reference to FIG. 1 and FIG. 2, at least some of the plurality of tungsten carbide particles contained in the second build-up weld layer 14 are welded to form the second build-up layer 14. At least a part of at least some of the plurality of tungsten carbide particles added to the material is once dissolved in the cobalt-based alloy and then precipitated. In FIGS. 1 and 2, tungsten carbide particles at least partially dissolved in the cobalt-based alloy and then precipitated are indicated by reference numeral 20B, and other tungsten carbide particles are indicated by reference numeral 20A. Once the tungsten carbide particles are once dissolved in the cobalt-based alloy, they are dispersed and precipitated in the subsequent precipitation process. When the hard tungsten carbide particles are discretely positioned in the second build-up weld layer 14, the hardness thereof becomes uniform, so that the wear resistance of the second build-up weld layer 14 is improved. Further, when the hardness becomes uniform, it becomes possible to form a sprayed layer on the second build-up weld layer 14 and to further extend the life. Tungsten carbide particles 20A are particles that did not dissolve in the cobalt-based alloy, or particles in which only a portion of the surface of the particles was dissolved in the cobalt-based alloy and only the central portion remained.

  Here, the state of the tungsten carbide particles in the second build-up weld layer can be grasped by observing the cross section of the second build-up weld layer with a scanning electron microscope (SEM). FIG. 6 is an SEM photograph of a portion corresponding to the second build-up weld layer. The plurality of tungsten carbide particles in the second build-up weld layer in FIG. 6 are at least partially or entirely in the cobalt-based alloy from the surface of the tungsten carbide particles added to the welding material forming the second build-up layer. Is dissolved once and then precipitated. Each of the precipitates has a different shape, and a part thereof has a dendrite shape. Whether or not the tungsten carbide particles are once dissolved and precipitated in the cobalt-based alloy can be determined from the shape of the particles in the SEM photograph.

  When the welding material is held for a certain time at a temperature higher than the melting point (1270 to 1300 ° C.) of the cobalt-based alloy and then cooled for a certain time, a dissolution-precipitation phenomenon of tungsten carbide particles in the cobalt-based alloy occurs. As a method of simply realizing such a situation, there is a method of performing build-up construction of two or more rounds when forming the second build-up weld layer.

  The size of the tungsten carbide particles contained in the welding material is preferably an average particle size of 80 to 280 μm, and the shape is preferably spherical or block-shaped.

  In this embodiment, it is desirable that the second build-up weld layer is constructed in the roll axial direction C as shown in FIG. Overlay welding is usually performed on a cylindrical member in a spiral shape in the circumferential direction. However, in roll shaft members in hot metal plating baths where bending stress acts, damage to the overlay of the overlay is the starting point. In many cases, cracks occur. On the other hand, if build-up welding is performed in the roll axis direction, even if the overlapped portion of the build-up weld layer is damaged, even if bending stress acts on the roll shaft member, it is damage in the roll axis direction. There is no immediate crack formation. The first build-up weld layer may be applied in the roll axial direction or in the circumferential direction of the roll.

  In the present embodiment, the thermal spray layer 16 is formed on the second build-up weld layer 14. As the thermal spray material, a tungsten carbide cermet material or a tungsten boride cermet material is preferable, and specific examples include WC-12Co, WC-17Co, and WB-30% CoCr. The sprayed layer of tungsten carbide cermet or tungsten boride cermet has a very high hardness (HRC 68 or higher) and high corrosion resistance, but when directly sprayed onto a metal substrate, the sprayed layer tends to crack. However, in the present embodiment, since there is the second build-up weld layer 14 in which the hardness is made uniform by dissolving and depositing the tungsten carbide particles once under the thermal spray layer 16, the thermal spray layer 16 is cracked. Does not occur, and the life of the roll shaft member can be greatly extended.

  Although the thickness of the 1st build-up weld layer 12 and the thickness of the 2nd build-up weld layer 14 are not specifically limited, It can be about 1-2 mm and 2-4 mm, respectively. Further, the thickness of the thermal spray coating 16 is not particularly limited, but can be about 50 to 200 μm.

  If the roll shaft member 100 of the present embodiment is used in a molten metal plating bath to produce a molten metal plated steel sheet, the member has a long life, and thus long-term stable production is possible.

  The invention examples and comparative examples of the present invention will be described below.

(Invention Example 1)
On the outer periphery of the sleeve base material (SUS316L steel), a weld welding is performed in the circumferential direction by a PTA method using a cobalt-based alloy (Stellite alloy) as a welding material, and a first build-up weld layer having a thickness of 1 mm is formed on the surface of the base material. Formed. The plasma arc current was 150 to 180 A, the plasma arc voltage was 22 to 26 V, the feed rate was 9 to 13 cpm, and the interpass temperature was 150 to 200 ° C. Overlay welding (circumferential direction) was applied while gradually shifting in the roll axis direction so that the entire surface was applied.

  Subsequently, on the outer periphery of the first build-up weld layer, build-up welding is performed in the roll axis direction by the PTA method using a cobalt-based alloy (Trivalloy alloy) containing 35% by mass of tungsten carbide particles as a welding material. A second build-up weld layer having a thickness of 2 mm was formed on the surface of the build-up weld layer. The plasma arc current was 130 to 170 A, the plasma arc voltage was 22 to 26 V, the feed rate was 12 to 16 cpm, and the interpass temperature was 600 to 700 ° C. Tungsten carbide particles having a particle size of 45 to 150 μm (average particle size of about 100 μm) were used. Overlay welding (in the roll axis direction) was applied while gradually shifting in the circumferential direction so that the entire surface was applied, and was performed twice.

  The hardness of the first build-up weld layer was HRC38, and the hardness of the second build-up weld layer was HRC49. Further, when the cross section of the second build-up weld layer was observed with an SEM, it was found that at least some of the plurality of tungsten carbide particles were once dissolved in the cobalt-based alloy and then precipitated. confirmed. An SEM photograph is shown in FIG.

(Comparative Example 1)
The first build-up weld layer is formed in the same manner as in Invention Example 1, and the build-up weld layer in the circumferential direction is applied only once by using the same welding material as in Invention Example 1, and the second build-up weld layer is formed. Formed. When the cross section of the second build-up weld layer was observed with an SEM, tungsten carbide particles that were once dissolved and precipitated in the cobalt-based alloy were not confirmed.

(Comparative Example 2)
The first build-up weld layer is formed in the same manner as in Invention Example 1, and the build-up welding in the roll axis direction is applied only once by using the same welding material as in Invention Example 1. Formed. When the cross section of the second build-up weld layer was observed with an SEM, tungsten carbide particles that were once dissolved and precipitated in the cobalt-based alloy were not confirmed.

(Invention Example 2)
The first build-up weld layer and the second build-up weld layer are formed in the same manner as in Invention Example 1, and atmospheric pressure plasma spraying is performed on the outer periphery of the second build-up weld layer using WC-17Co cermet as a spraying material. A 100 μm thick sprayed layer was formed on the surface of the second build-up weld layer. The hardness of the WC-17Co cermet sprayed layer was HRC70. Further, when the cross section of the second build-up weld layer was observed with an SEM, it was found that at least some of the plurality of tungsten carbide particles were once dissolved in the cobalt-based alloy and then precipitated. confirmed.

(Comparative Example 3)
The first build-up weld layer is formed in the same manner as in Invention Example 1, and the build-up welding in the roll axis direction is applied only once by using the same welding material as in Invention Example 1. Formed. Subsequently, thermal spraying was performed in the same manner as in Invention Example 2. However, the surface of the sprayed layer was cracked and could not be used. When the cross section of the second build-up weld layer was observed with an SEM, tungsten carbide particles that were once dissolved and precipitated in the cobalt-based alloy were not confirmed.

(Evaluation method of life of roll shaft member)
The roll shaft member produced in each invention example and comparative example was attached to the end of the sink roll of the hot dip galvanizing facility shown in FIG. 4 and used for 30 days in the hot dip galvanizing line. Thereafter, the roll shaft member was visually checked for wear and cracks.

(Evaluation results)
It was confirmed that the roll shaft member of Invention Example 1 had a lifetime of at least 30 days because no abrasion occurred but no cracks occurred. On the other hand, the roll shaft members of Comparative Example 1 and Comparative Example 2 were cracked after use, and it was found difficult to use over 30 days. It was confirmed that the roll shaft member of Invention Example 2 could extend its life to 30 days or more since there was no wear or cracking.

  The roll shaft member in the molten metal plating bath of the present invention can suppress the occurrence of wear and cracks even when subjected to strong sliding resistance from the bearing when using the actual machine. Production can be done.

DESCRIPTION OF SYMBOLS 100 Roll shaft member 10 Metal base material 12 1st build-up weld layer 14 2nd build-up weld layer 16 Thermal spray layer 20A Tungsten carbide particle 20B Tungsten carbide particle which precipitated once melt | dissolved 30 Snout 32 Pot 34 Molten metal bath 36 Sink Roll 38 Support roll 40 Sleeve 42 Bearing 44 Hanger C Roll axial direction P Steel strip

Claims (14)

A metal substrate;
A first build-up weld layer made of a cobalt-based alloy formed on the metal substrate;
A second build-up weld layer made of a cobalt-based alloy formed on the first build-up weld layer;
A roll shaft member in a molten metal plating bath having:
The second build-up weld layer has a higher hardness than the first build-up weld layer,
The second build-up weld layer includes a plurality of tungsten carbide particles,
At least some of the plurality of tungsten carbide particles included in the second build-up weld layer are at least one of the plurality of tungsten carbide particles added to the welding material forming the second build-up layer. A roll shaft member in a molten metal plating bath, wherein at least a part of the particles of the portion is once dissolved in the cobalt-based alloy and then precipitated.   The roll shaft member in a molten metal plating bath according to claim 1, wherein the first build-up weld layer has a hardness of less than HRC40, and the second build-up weld layer has a hardness of HRC40 or higher.   The roll shaft member in a molten metal plating bath according to claim 1 or 2, wherein the content of tungsten carbide particles in the second build-up weld layer is 50% by mass or less.   The roll shaft member in the molten metal plating bath according to any one of claims 1 to 3, wherein the second build-up weld layer is constructed in the roll axis direction.   The roll shaft member in the molten metal plating bath according to any one of claims 1 to 4, further comprising a thermal spray layer having a hardness higher than that of the second build-up layer on the second build-up weld layer.   6. The roll shaft member in a molten metal plating bath according to claim 5, wherein the sprayed layer is made of a tungsten carbide cermet material or a tungsten boride cermet material. A first step of depositing a welding material made of a cobalt-based alloy on a metal base material to form a first overlay layer;
On the first build-up layer, a second step of build-up welding a welding material made of a cobalt-based alloy and containing a plurality of tungsten carbide particles to form a second build-up layer;
A method for producing a roll shaft member in a molten metal plating bath having
In the second step, at least a part of the plurality of tungsten carbide particles, at least a part of which is once dissolved in the cobalt-based alloy and then precipitated, and then a roll shaft in a molten metal plating bath, Manufacturing method of member.   In the said 2nd process, build-up welding is a manufacturing method of the roll shaft member in the hot-dip metal-plating bath of Claim 7 which constructs more than 2 rounds.   The method for producing a roll shaft member in a molten metal plating bath according to claim 7 or 8, wherein build-up welding is performed in the roll axis direction in the second step.   The molten metal plating bath according to any one of claims 7 to 9, wherein the first build-up weld layer has a hardness of less than HRC40, and the second build-up weld layer has a hardness of HRC40 or higher. Manufacturing method of middle roll shaft member.   The manufacturing method of the roll shaft member in the molten metal plating bath as described in any one of Claims 7-10 whose content of the tungsten carbide particle in a said 2nd build-up weld layer is 50 mass% or less.   12. The method according to claim 7, further comprising a third step of forming a sprayed layer having a hardness higher than that of the second build-up layer on the second build-up weld layer after the second step. A method for producing a roll shaft member in a molten metal plating bath according to claim 1.   The method for producing a roll shaft member in a molten metal plating bath according to claim 12, wherein the thermal spray material in the third step is a tungsten carbide cermet or a tungsten boride cermet.   A method for producing a molten metal plated steel sheet, comprising producing a molten metal plated steel sheet using the roll shaft member in the molten metal plating bath according to any one of claims 1 to 6 in the molten metal plating bath. .