Grain-oriented electrical steel sheet and method for manufacturing same

Abstract

A method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention comprises: a step for hot-rolling a slab to produce a hot-rolled sheet; a step for cold-rolling the hot-rolled sheet to produce a cold-rolled sheet; a step for subjecting the cold-rolled sheet to primary recrystallization annealing; and a step for subjecting the primary recrystallization annealing-completed cold-rolled sheet to secondary recrystallization annealing, wherein the primary recrystallization annealing step includes a preceding step and a subsequent step, and the amount (A) of nitriding gas introduced in the preceding step with respect to the total amount (B) of nitriding gas introduced in the primary recrystallization annealing step satisfies expression 1 below.
0.05≤[A]/[B]≤[t]  [Expression 1] (In expression 1, the amount of nitriding gas introduced is in units of Nm.sup.3/hr, and [t] represents the thickness (mm) of a cold-rolled sheet.)

Claims

1. A grain-oriented electrical steel sheet satisfying the following Expression 4:
0.06≤[D.sub.S]/[D.sub.L]≤0.1  [Expression 4] wherein [D.sub.S] represents the number of crystal grains having a particle diameter of 5 mm or less, and [D.sub.L] represents the number of crystal grains having a particle diameter of more than 5 mm.

2. The grain-oriented electrical steel sheet of claim 1, wherein: the steel sheet comprises 0.03 to 0.15 wt % of Cr.

Description

EXAMPLE

(1) A slab containing 3.15 wt % of Si, 0.045 wt % of C, 0.02 wt % of P, 0.05 wt % of Sn, 0.1 wt % of Mn, 0.005 wt % of S, 0.03 wt % of sol Al, 0.004 wt % of N, 0.08 wt % of Cr, and the balance Fe and other impurities that are inevitably contained as the other components was produced. Thereafter, a hot-rolled sheet having a thickness of 1.8 mm was produced by heating the slab at a temperature of 1180° C. for 210 minutes, and then hot-rolling the slab.

(2) After the hot-rolled sheet was heated to 1050° C., and then maintained at 950° C. for 90 seconds, the hot-rolled sheet was subjected to furnace cooling to 760° C., quenched in boiling water at 100° C., washed with acid, and then strongly cold-rolled to a thickness of 0.18 mm once.

(3) The cold-rolled sheet was subjected to simultaneous decarburization and nitridation annealing heat treatment, such that the carbon content and the nitrogen content were 30 ppm or less and 200 ppm, respectively in a mixed gas atmosphere of moist oxygen (oxidation degree about 0.6), nitrogen, and ammonia at a temperature of about 850° C. In this case, the amount of nitriding gas introduced in a preceding step and the amount of nitriding gas introduced in a subsequent step were adjusted as shown in the following Table 1, and the preceding step and the subsequent step were performed for 50 seconds and 70 seconds, respectively.

(4) Further, the crystal grain diameter and nitrogen content of the primary recrystallization annealing-completed steel sheet were analyzed and are summarized in the following Table 1.

(5) This steel sheet was finally annealed in a coil shape by applying an annealing separator MgO to the steel sheet. The final annealing was performed in a mixed atmosphere of 25 v % nitrogen and 75 v % hydrogen until 1200° C., and when the temperature reached 1200° C., the steel sheet was maintained in a 100 v % hydrogen atmosphere for 10 hours or more, and then furnace-cooled. Table 1 shows the magnetic characteristics and structural characteristics measured under each condition.

(6) For magnetism, iron loss was measured under the conditions of 1.7 Tesla and 50 Hz using a single sheet measurement method, and the magnitude of magnetic flux density (Tesla) induced under a magnetic field of 800 Nm was measured. Each magnetic flux density and iron loss value show the average under each condition.

(7) TABLE-US-00001 TABLE 1 Steel sheet after primary recrystallization annealing Magnetic [G.sub.1/4t] − [N.sub.tot] − characteristics [G.sub.1/2t] [N.sub.1/4t−3/4t] B8 W 17/50 [D.sub.S]/ Classification [A]/[B] (μm) (ppm) (Tesla) (W/Kg) [D.sub.L] Remark Invention 0.15 1.3 35 1.93 0.7 0.08 — Material 1 Invention 0.1 2 60 1.939 0.67 0.06 — Material 2 Invention 0.06 2.5 100 1.935 0.68 0.07 — Material 3 Invention 0.1 1.5 50 1.92 0.7 0.10 Cr not Material 4 added Comparative 0.25 0.5 50 1.905 0.81 0.15 — Material 1 Comparative 0.01 2.8 110 1.895 0.88 0.34 — Material 2

(8) As can be confirmed in Table 1, it can be confirmed that because Invention Materials 1 to 4 in which the nitriding gas was controlled in the primary recrystallization annealing process had the surface layer crystal grains grown appropriately and appropriate nitridation into the inside of the steel sheet, the formation of secondary recrystals of less than 5 mm was suppressed and the magnetism was excellent.

(9) In contrast, in Comparative Material 1 in which a large amount of nitriding gas was introduced in the preceding step, the surface layer crystal grains were formed too small, so that a large amount of fine secondary recrystals were formed and the magnetism also deteriorated.

(10) In addition, Comparative Material 2 in which the nitriding gas was soaked too much in the preceding step had too little nitrogen content inside the steel sheet, so that a large amount of fine secondary recrystals were formed and the magnetism also deteriorated.

(11) The present invention is not limited to the embodiments, and can be manufactured in various different forms, and those having ordinary skill in the art to which the present invention pertains will understand that the present invention can be implemented in other specific forms without changing the technical idea or essential features thereof. Therefore, it should be understood that the above-described embodiments are illustrative and not restrictive in all aspects.