EP-4737604-A1 - DECARBURIZED ANNEALED STEEL SHEET FOR GRAIN-ORIENTED ELECTROMAGNETIC STEEL SHEET

Abstract

In a decarburization-annealed steel sheet for a grain oriented electrical steel sheet, an area fraction of a divided sub-domain group I in which a value of F C ÷ aveF C is 1.3 or more in an area of a divided domain of at least 100 mm × 100 mm on a sheet surface is 0.05% or more and 50% or less, and a value obtained by dividing an area fraction of Goss-oriented grains included in the divided sub-domain group I in an area of the divided sub-domain group I by an area fraction of the Goss-oriented grains included in the divided domain in an area of the divided domain is 1.3 or more.

Inventors

• NAKAMURA, SHUICHI
• HIROTA, YOSHIAKI
• ATSUMI, Haruhiko

Assignees

• Nippon Steel Corporation

Dates

Publication Date

20260506

Application Date

20240627

Claims (2)

1. A decarburization-annealed steel sheet for a grain oriented electrical steel sheet, wherein: when a region of at least 100 mm × 100 mm on a sheet surface is defined as a divided domain, and each domain obtained by subdividing the divided domain into regions of 1 mm × 1 mm is defined as a divided sub-domain; when an average grain size of grains included in the divided domain is defined as aveD in units of µm; when a number fraction of a number of grains having a grain size of 3.0 × aveD or more included in one divided sub-domain in a number of grains included in the divided sub-domain is defined as F C , and a number fraction of a number of grains having a grain size of 3.0 × aveD or more included in the divided domain in a number of grains included in the divided domain is defined as aveF C ; and when divided sub-domains in which a value of F C ÷ aveF C is 1.3 or more are collectively defined as a divided sub-domain group I, an area fraction of the divided sub-domain group I in an area of the divided domain is 0.05% or more and 50% or less, and wherein: when a deviation angle from an ideal Goss orientation based on a rotation axis parallel to a normal direction is defined as α; when a deviation angle from the ideal Goss orientation based on a rotation axis parallel to a transverse direction is defined as β; when a deviation angle of a crystal orientation measured at a measurement point on the sheet surface is represented as (α β); when an angular deviation at the measurement point is defined as ϕ = (α 2 + β 2 ) 1/2 ; when a grain having the angular deviation ϕ of 10° or less is defined as a Goss-oriented grain; and when an area fraction of Goss-oriented grains included in the divided sub-domain group I in an area of the divided sub-domain group I is defined as A I GOSS and an area fraction of Goss-oriented grains included in the divided domain in the area of the divided domain is defined as aveA GOSS , a value of A I GOSS ÷ aveA GOSS is 1.3 or more.
2. The decarburization-annealed steel sheet for the grain oriented electrical steel sheet according to claim 1, wherein: when a grain having a misorientation of 10° or less from a crystal orientation having a Σ9 coincidence site lattice relationship with the ideal Goss orientation is defined as a CSL oriented grain; and when divided sub-domains other than the divided sub-domain group I included in the divided domain are collectively defined as a divided sub-domain group II, an area fraction of CSL oriented grains included in the divided sub-domain group II in an area of the divided sub-domain group II is 10% or more and 50% or less.

Description

TECHNICAL FIELD The present invention relates to a decarburization-annealed steel sheet for a grain oriented electrical steel sheet. Priority is claimed on Japanese Patent Application No. 2023-106859, filed June 29, 2023, the content of which is incorporated herein by reference. BACKGROUND ART A grain oriented electrical steel sheet includes Si, the crystal orientation of the grains thereof closely aligns in the Goss orientation (cubic crystal {110}<001>), and the <001> orientation, which is a magnetization easy axis, is substantially aligned in the rolling direction in the steel sheet manufacturing process. Such a grain oriented electrical steel sheet is very desirable as a material for an iron core and the like of a transformer. Among the magnetic characteristics of the grain oriented electrical steel sheet, magnetic flux density and iron loss are particularly important. The magnetic flux density of the grain oriented electrical steel sheet when a predetermined magnetizing force is applied tends to increase as the degree to which the magnetization easy axes of the grains are aligned in the rolling direction of the steel sheet, that is, the orientation of the grains is higher. A magnetic flux density B8 is generally used as an index representing the magnetic flux density. The magnetic flux density B8 is a value of the magnetic flux density of the grain oriented electrical steel sheet excited at a magnetizing force of 800 A/m in the rolling direction. That is, the grain oriented electrical steel sheet having a larger value of the magnetic flux density B8 is more easily magnetized with a certain magnetizing force, the magnetic flux density becomes high, and thus it is suitable for a small-sized and highly efficient transformer. In addition, an iron loss W17/50 is generally used as an index representing the iron loss. The iron loss W17/50 is an iron loss when the grain oriented electrical steel sheet is excited by alternating current so as to be a maximum magnetic flux density of 1.7 T under the condition of a frequency of 50 Hz. The grain oriented electrical steel sheet having a smaller value of the iron loss W17/50 has a lower energy loss and is suitable for a transformer. The methods for reducing the iron loss include a method of increasing the electric resistance by containing Si, a method of reducing the thickness of the steel sheet, and a method of decreasing the grain size, which are effective for reducing the eddy-current loss, and a method of aligning the orientation of the grains which is effective for reducing the hysteresis loss. That is, when the value of the magnetic flux density B8 becomes higher, the crystal orientation is aligned with the Goss orientation, and the hysteresis loss is reduced. Thus, when the value of the magnetic flux density B8 becomes higher, the value of the iron loss W17/50 tends to be lower. On the other hand, when the growth of Goss-oriented grains is promoted in order to improve the alignment degree of the crystal orientation, the grain size tends to coarsen, and the eddy-current loss increases. Thus, when the grains are excessively coarse even when the value of B8 is high, the iron loss tends to deteriorate. As explained above, there is an antinomy. In recent years, in order to purposely refine a width of magnetic domain, a method of controlling the magnetic domain by applying strain by irradiation with laser beam, plasma jet, and the like, or by forming grooves by mechanical processing, etching, and the like has been developed. By using such a method, in the steel sheet in which the alignment degree of the crystal orientation is increased and the magnetic flux density is increased, the eddy-current loss can be reduced, and the iron loss can be sufficiently reduced even when the secondary recrystallized grain size coarsens. However, when the strain is applied into the steel sheet, magnetostriction (λP-P) increases due to the strain. In addition, when heat treatment is performed at a temperature of approximately 800°C after the magnetic domain refinement, the effect of reducing the iron loss disappears, and therefore the above steel sheet cannot be used for utilization requiring strain relief annealing at 800°C or higher after irradiation. On the other hand, in the magnetic domain refinement using the grooves, there is a problem in that the magnetic flux density B8 decreases. In general, the grain oriented electrical steel sheet is manufactured as described below. A steel sheet material (slab) including a predetermined amount of Si is subjected to hot rolling, annealing, and cold rolling to obtain a steel sheet having a desired thickness. Then, the steel sheet after cold rolling is annealed (this is also referred to as primary annealing or decarburization annealing). By this annealing, primary recrystallization occurs, and grains having a crystal orientation in which the magnetization easy axes are aligned in the rolling direction and a deviation angle from an ideal Goss