Method for nitriding grain-oriented electrical steel sheet

Abstract

Provided is a method for nitriding a grain-oriented electrical steel sheet which is very useful in obtaining excellent magnetic properties with no variation, that enables generating glow discharge between positive electrodes and negative electrodes disposed in a nitriding zone and irradiating the generated plasma to a strip to perform appropriate nitriding.

Claims

1. A method for nitriding a grain-oriented electrical steel sheet comprising plasma nitriding a strip by glow discharge using an apparatus after cold rolling and before secondary recrystallization annealing during producing a grain-oriented electrical steel sheet, the apparatus comprising: a nitriding zone for nitriding the strip; a cooling zone for cooling the strip; and an optional heating zone provided upstream of the nitriding zone for heating the strip, wherein the nitriding zone contains positive electrodes for glow discharge provided opposite to the strip and negative electrodes for glow discharge provided proximate to the strip between the positive electrodes and the strip, and glow discharge is generated between the positive electrodes and the negative electrodes and the generated plasma is applied to the strip to nitride the strip.

2. The method according to claim 1, wherein the nitriding zone is kept under reduced pressure.

3. The method according to claim 2, wherein at least one of the heating zone and the cooling zone is kept at a state with a lower degree of pressure reduction compared to the nitriding zone and reduced pressure compared to atmospheric pressure.

4. The method according to claim 1, further comprising an upstream atmosphere adjusting zone provided between the heating zone and the nitriding zone, and a downstream atmosphere adjusting zone provided between the nitriding zone and the cooling zone.

5. The method according to claim 4, wherein the upstream atmosphere adjusting zone and the downstream atmosphere adjusting zone are each divided into multiple air chambers where the degrees of pressure reduction are individually adjustable.

6. The method according to claim 5, wherein the degrees of pressure reduction of the air chambers in the upstream atmosphere adjusting zone are gradually increased toward the nitriding zone, while the degree of pressure reduction of the air chambers in the downstream atmosphere adjusting zone are gradually decreased toward the cooling zone.

7. The method according to claim 1, wherein the inside of the nitriding zone is divided into multiple zones in the width direction of the strip to allow individual controls of nitriding inside each divided zone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the accompanying drawings:

(2) FIG. 1 schematically shows a preferable example of the nitriding apparatus of the disclosure.

(3) FIG. 2 shows a preferable example of a plasma nitriding device according to the disclosure.

(4) FIG. 3 schematically shows another example of the nitriding apparatus of the disclosure.

DETAILED DESCRIPTION

(5) Our methods and components will be described in detail below.

(6) FIG. 1 schematically shows a preferable example of the nitriding apparatus of the disclosure. In the figure, a heating zone is labeled 1, a nitriding zone is labeled 2, and a cooling zone is labeled 3. Further, a strip (steel sheet) continuously passing inside the nitriding apparatus with a structure comprising the aforementioned components is labeled 4. The heating zone may be provided when required and is not always necessary.

(7) In the disclosure, a strip 4 is subjected to plasma nitriding by glow discharge in the above nitriding zone 2.

(8) FIG. 2 shows a preferable example of a plasma nitriding device according to the disclosure. In the figure, positive electrodes for glow discharge disposed opposite to the strip 4 are labeled 5, pinch rolls are labeled 6, and negative electrodes disposed near the strip 4 are labeled 7.

(9) The inner part of the nitriding zone 2 is filled with nitrogen gas and hydrogen gas as nitrogen sources.

(10) A voltage is applied between the positive electrodes 5 and the negative electrodes 7 to generate glow discharge, and by irradiating the strip 4 with the plasma generated during glow discharge generation, the surface of the strip 4 is subjected to nitriding.

(11) As described above, in the disclosure, it is necessary for the plasma generated between the positive electrodes and the negative electrodes to pass through the negative electrodes and reach the strip. Therefore, the negative electrodes must be perforated or mesh-like.

(12) When performing the above nitriding, the strip is preferably heated to a temperature of 400° C. or higher.

(13) Further, the inside of the nitriding zone is preferably kept under a reduced pressure.

(14) Further, although the heating zone and the cooling zone have a lower degree of pressure reduction compared to the nitriding zone, it is preferable for them to be kept in a state with reduced pressure compared to atmospheric pressure, and by doing so, heat exchange due to convection tends to proceed, and heating and cooling efficiency can be improved.

(15) The inside of the nitriding zone is preferably depressurized to around 0.5 torr to 10 torr which is a preferable glow discharge condition, and the heating zone and the cooling zone are preferably depressurized, with a lower degree of pressure reduction, to around 30 torr to 500 torr.

(16) Next, FIG. 3 shows an upstream atmosphere adjusting zone 8-1 and a downstream atmosphere adjusting zone 8-2 with a nitriding zone 2 in between.

(17) In this case, each of the upstream atmosphere adjusting zone 8-1 and the downstream atmosphere adjusting zone 8-2 is preferably divided into multiple air chambers where the degrees of pressure reduction are individually adjustable. In a preferable construction, the degrees of pressure reduction of the air chambers in the upstream atmosphere adjusting zone 8-1 are gradually increased toward the nitriding zone 2, while the degree of pressure reduction of the air chambers in the downstream atmosphere adjusting zone 8-2 are gradually decreased from the nitriding zone 2 toward the cooling zone 3.

(18) As the seal between each zone and each air chamber, conventionally known airtight seals may be used, such as rolls, seal pads and the like.

(19) In a preferable structure, the inside of the nitriding zone is divided into multiple zones in the width direction of the strip where nitriding can be performed individually inside each divided zone. By adopting such structure, it is possible to effectively eliminate non-uniformity in nitridation in the width direction of the strip, such as excessive nitriding of the edges due to edge effects.

(20) The heating zone can be omitted if it is disposed in a continuous line for performing other necessary treatment and the strip is already heated, or if the heating by plasma irradiation at the time of plasma nitriding is sufficient.

(21) Further, in a case where another treatment is performed after plasma nitriding with the strip at a heated state, the cooling zone may be disposed after the zone for such treatment.

(22) Further, the nitriding apparatus disclosed herein may be an independent apparatus that continuously performs only nitriding, or be attached to a processing line for performing another treatment, and in the case of a continuous line, it may be attached to the optimal place considering conditions including efficiency.

(23) In the disclosure, the strip which is the material to be treated is not particularly limited and, as long as it is a grain-oriented electrical steel strip, any conventionally known strip is applicable.

REFERENCE SIGNS LIST

(24) 1 Heating Zone 2 Nitriding Zone 3 Cooling Zone 4 Strip (Steel Sheet) 5 Positive Electrode 6 Pinch Roll 7 Negative Electrode 8-1 Upstream Atmosphere Adjusting Zone 8-2 Downstream Atmosphere Adjusting Zone