JP5076518B2 - Method for producing galvannealed steel sheet - Google Patents

Description

The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet, and particularly, to improve the flow of molten steel in a mold by applying a magnetic field, which is suitable for use in producing an alloyed hot-dip galvanized steel sheet used for an automobile outer plate. About.

  Alloyed hot-dip galvanized steel sheets are excellent in weldability, paintability, and corrosion resistance after painting, and are widely used in automobile bodies, home appliances, building materials, and the like. This alloyed galvannealed steel sheet is also used for an automobile outer plate, but the plating unevenness leads to a poor appearance and causes a problem of deteriorating the product value.

  The alloyed hot dip galvanized steel sheet is obtained by heating and hot diffusing the iron component of the base plate into the galvanized layer, and the galvannealed layer changes depending on the surface state of the base plate. Therefore, although various causes of plating defects are conceivable, for example, Patent Document 1 discloses a technique for adjusting the texture by paying attention to the texture after rolling.

Japanese Patent Laid-Open No. 2001-294977

  However, a method for fundamental improvement from the slab manufacturing stage before rolling has not been proposed.

  The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to fundamentally improve plating defects from the slab manufacturing stage before rolling.

In the production of the galvannealed steel sheet according to the present invention, as illustrated in FIG. 1 (vertical sectional view) and FIG. 2 (horizontal sectional view), the long side 10a of the rectangular mold 10 having the long side 10a and the short side 10b is illustrated. A continuous casting method of steel in which a magnetic field is generated by a magnetic pole 12 disposed on the back surface of the opposite side wall of the steel plate and the flow of molten steel 8 supplied from the immersion nozzle 6 into the mold 10 is controlled by the magnetic field. The magnetic pole 12 disposed on the upper side of the substrate is applied with a direct current magnetic field of 0.03T to 0.15T at the center of the mold width at the position above the immersion nozzle discharge hole 6a and at the center of the mold thickness, and the solidified shell ( 14a) during the dwell time 0-10 seconds, to use a slab 14 produced by the continuous casting method of steel to cool the slab 14 through the template 10 with the heat flux from 1,500,000 to 3,000,000 W / ^{m 2} And solved the above problems It is.

  The solidified shell residence time is obtained as D / Vc by dividing the height D of the mold 10 by the casting speed Vc.

  Here, the DC magnetic field at the center of the mold width at the position above the immersion nozzle discharge hole and at the center of the mold thickness is because the entrainment of the mold flux is caused by the vortex generation of the molten steel flow at the center of the mold thickness, This is because it was considered appropriate for the purpose of suppressing this eddy with a DC magnetic field.

  Moreover, the lower limit of the magnetic field strength of the DC magnetic field is set to 0.03T. If the magnetic field strength is less than 0.03T, the braking effect by the DC magnetic field is insufficient, the molten metal surface fluctuation is large, and the mold flux 16 is involved. This is because defects occur.

  On the other hand, the upper limit of the magnetic field strength is set to 0.15 T. When the magnetic field strength exceeds 0.15 T, braking by the DC magnetic field is excessively applied, the molten steel flow velocity becomes slow, the temperature becomes uneven, and the low flow velocity region This is because the portion solidifies rapidly, components such as Mn, C, and P are concentrated due to segregation, the components become non-uniform, the solidified structure becomes non-uniform, and a streak pattern is generated.

Moreover, the lower limit of the heat flux of cooling is 1.5 million W / m ² because if the heat flux is less than 1.5 million W / m ² , the heat flux of the molten steel 8 is too low, as shown in FIG. This is because, as illustrated, the growth of the solidified shell 14a is delayed and a thinned portion is generated.

On the other hand, the upper limit of the heat flux is set to 3 million W / m ² because when the heat flux exceeds 3 million W / m ² , the heat flux of the molten steel 8 is too high, as illustrated in FIG. This is because the solidified shell thickness grows abnormally.

An example of the relationship between the heat flux and the shell thickness variation is shown in FIG.

  According to the present invention, it is possible to uniformize the solidified structure, particularly the solidified structure of the surface layer, and prevent the appearance defect of the galvannealed steel sheet.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

In the first embodiment of the present invention, as shown in FIGS. 1 and 2, a magnetic field is generated by the magnetic pole 12 disposed on the back surface of the opposing side wall of the long side 10a of the rectangular mold 10 having the long side 10a and the short side 10b. When the flow of the molten steel 8 supplied from the immersion nozzle 6 into the mold 10 is controlled by the magnetic field, the mold width at the position above the immersion nozzle discharge hole 6a is set by the magnetic pole 12 arranged at the upper part of the immersion nozzle 6. in and position of the central mold thickness at the center, by applying a DC magnetic field 0.03T～0.15T, and, between the solidified shell residence time 0-10 seconds, the heat flux from 1,500,000 to 3,000,000 W / ^{m 2} The slab 14 is cooled by the heat flux through the mold 10.

  The present invention is a steel of C: 0.001 to 0.005 wt%, Si: 0.034 wt% or less, Mn: 0.03 to 0.3 wt%, P: 0.01 to 0.08 wt% In the production of a hot dip galvanized steel sheet having a composition, it has a particularly good effect as a countermeasure against the occurrence of defects such as uneven plating.

  Therefore, in the examples, the test was performed with molten steel containing components of C: 0.002 wt%, Si: 0.02 wt%, Mn: 0.1 wt%, and P: 0.05 wt%. The results are shown in the following table.

  Here, the “Sleeper-like defect index” is obtained by visually inspecting the outer surface of the coil after finishing the slab into a hot-dip galvanized steel sheet and indexing the occurrence of the sleeper-like defect. The length of the sleeper-like defect extending in the longitudinal direction was measured, and the sum was divided by the total length of the inspected coil.

  Also, the “Plating unevenness evaluation index” is obtained by visually inspecting the outer surface of the coil after finishing the slab into a hot dip galvanized steel sheet, and indexing the occurrence of defects caused by uneven plating. The length of the streak defect due to the plating unevenness extending in the longitudinal direction of the coil was measured, and the total sum was divided by the total length of the inspected coil.

  In the examples, four locations were sampled in the width direction of a slab having a width of 1500 mm and a thickness of 220 mm, a cross section was subjected to EDX analysis, a sulfur distribution was investigated, and a solidified shell thickness was evaluated.

  Here, the thickness of the solidified shell was confirmed by a sulfur printing method (S printing method). That is, at a certain point during solidification, sulfur was added into the mold, the slab was disassembled, the distribution state of sulfur was confirmed by observing the cross section of the slab, and the growth state of the solidified shell was confirmed.

As a result, as shown in FIG. 4, the heat flux from 1,500,000 to 3,000,000 W / ^{m 2,} relative to the shell thickness 10 mm, it exhibited a good dispersion of within the range of ± 2 mm. On the other hand, at 1.5 million W / m ² or less or 3 million W / m ² or more, the variation was ± 4 mm, which was a failure.

  In the first embodiment, the DC magnetic field is applied only by the magnetic pole 14 disposed on the top of the immersion nozzle 6. However, the magnetic pole is also provided below the immersion nozzle 6 as in the second embodiment shown in FIG. 16 and a magnetic field having the same magnetic field condition as that of the upper magnetic field may be applied by the magnetic pole 16.

Vertical sectional view showing a configuration around a mold for carrying out the present invention Similarly, a horizontal sectional view along the line II-II in FIG. Sectional drawing which shows the relationship between the heat flux of cooling, the growth delay of a solidified shell, and abnormal growth for demonstrating the principle of this invention Similarly, a diagram showing the relationship between cooling heat flux and solid shell thickness variation Sectional drawing which shows the structure around the casting_mold | template for implementing 2nd Embodiment of this invention

Explanation of symbols

6 ... Submerged nozzle 6a ... Nozzle discharge hole 8 ... Molten steel 10 ... Mold 10a ... Long side 10b ... Short side 12, 16 ... Magnetic pole 14 ... Slab 14a ... Solidified shell

Claims (1)

Continuous casting of steel that generates a magnetic field with magnetic poles arranged on the back side of the opposing side wall of the long side of a rectangular mold having a long side and a short side, and controls the flow of molten steel supplied from the immersion nozzle into the mold by the magnetic field A method,
Applying a DC magnetic field of 0.03T to 0.15T at the center of the mold width at the position above the immersion nozzle discharge hole and at the center of the mold thickness by the magnetic pole disposed on the top of the immersion nozzle; and
Between solidified shells residence time 0-10 seconds, characterized by using a cast slab produced by continuous casting method of steel to cool a heat flux from 1.5 to 3 million cast piece through a mold in W / m ² A method for producing a galvannealed steel sheet.

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