JP2011206845A - Method for continuously casting steel and method for manufacturing steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel sheet having few defects caused by the entrainment of air bubbles, nonmetallic inclusions and mold flux, and having few blisters.SOLUTION: The method uses a continuous casting machine which includes a pair of upper magnetic poles, a pair of lower magnetic poles, and an immersion nozzle having a molten steel spouting angle of 10° or higher to less than 30°. When continuously casting ultralow carbon steel while controlling the flow of molten steel by direct current magnetic fields applied to the upper magnetic poles and the lower magnetic poles respectively, the chemical composition of the ultralow carbon steel are controlled within specified ranges in consideration of an interfacial tension gradient in a concentration boundary layer formed in the front surface of a solidifying shell. The intensity of the direct current magnetic fields to be applied to the upper magnetic poles and the lower magnetic poles respectively is optimized according to the width of the slab to be cast and the casting speed. Further, the hot rolled steel sheet obtained by rolling the slab cast in the continuous casting method is pickled and cold-rolled under specified conditions.

Description

  The present invention relates to a steel continuous casting method for casting molten steel while controlling the flow of molten steel in a mold by electromagnetic force, and a method for producing a steel plate using a slab cast by this continuous casting method.

  In continuous casting of steel, molten steel placed in a tundish is poured into a continuous casting mold through an immersion nozzle connected to the bottom of the tundish. In this case, non-metallic inclusions such as alumina clusters or inert gas blown from the inner wall of the upper nozzle (nozzle due to adhesion / deposition of alumina etc.) into the molten steel flow discharged into the mold from the discharge hole of the immersion nozzle Inert gas blown in order to prevent clogging is accompanied, but if this is trapped by the solidified shell, it becomes a product defect (inclusion property defect, bubble defect). In addition, mold flux (mold powder) is caught in the upward flow of molten steel that has reached the meniscus, and this is also captured by the solidified shell, resulting in a product defect.

Conventionally, in order to prevent non-metallic inclusions, mold flux, and bubbles in molten steel from being trapped in the solidified shell and resulting in product defects, a magnetic field is applied to the molten steel flow in the mold and electromagnetic force generated by the magnetic field is used. The flow of molten steel has been controlled, and many proposals have been made regarding this technology.
For example, Patent Document 1 discloses a method of braking a molten steel flow by a DC magnetic field applied to each of a pair of upper magnetic poles and a pair of lower magnetic poles that are opposed to each other with the long side of the mold interposed therebetween. In this method, after being discharged from the discharge port of the immersion nozzle, the downflow is braked by the lower direct current magnetic field and the upward flow is braked by the upper direct current magnetic field. The non-metallic inclusions and mold flux accompanying the molten steel flow are prevented from being captured by the solidified shell.

Further, in Patent Document 2, the molten steel flow is braked by a DC magnetic field applied to each of a pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the mold long side portion as in Patent Document 1, and the upper magnetic pole Alternatively, a method of applying an alternating magnetic field on the lower magnetic pole is disclosed. In this method, the molten steel flow is braked by a DC magnetic field similar to Patent Document 1, and the cleaning effect of nonmetallic inclusions and the like at the solidified shell interface is obtained by stirring the molten steel by an AC magnetic field. .
Further, Patent Document 3 discloses a method of braking a molten steel flow by a DC magnetic field applied to each of a pair of upper magnetic poles and a pair of lower magnetic poles that are opposed to each other with a long side of the mold interposed therebetween. A method of setting the intensity ratio of the DC magnetic field between the electrode and the lower electrode within a specific numerical range is disclosed.
In Patent Documents 4 and 5, the molten steel is controlled by controlling the surface tension due to the concentration gradient of C, S, N, and O in the molten steel on the front surface of the solidified shell, that is, the surface tension is reduced to a predetermined value or less. A continuous casting method that suppresses trapping of bubbles in a solidified shell by adjusting the concentration of C, S, N, and O therein is disclosed.

Japanese Patent Laid-Open No. 3-142049 JP-A-10-305353 Japanese Patent Laid-Open No. 2008-200732 JP 2003-205349 A JP 2003-251438 A

  Recently, with the stricter quality of steel plates for automobile outer plates, defects caused by entrainment of minute bubbles, non-metallic inclusions and mold flux that have not been a problem until now are becoming a problem. The continuous casting methods as shown in Documents 1 to 3 cannot sufficiently meet such strict quality requirements. In particular, alloyed hot-dip galvanized steel sheets are heated after hot-dip plating to diffuse the iron component of the base steel sheet into the galvanized layer, and the surface layer properties of the base steel sheet contribute to the quality of the alloyed hot-dip galvanized layer. A big influence. That is, if there is a bubble defect, inclusion property defect or mold flux property defect on the surface layer of the base steel plate, unevenness occurs in the thickness of the plating layer even if it is a small defect, and it appears as a streak defect on the surface, It can no longer be used for quality demanding applications such as automotive skins.

In Patent Document 4 and Patent Document 5, no consideration is given to trapping non-metallic inclusions such as alumina clusters in a solidified shell. Although it is suggested that the trapping of bubbles in the solidified shell depends on the molten steel component, the relationship between the trapping of bubbles and the molten steel flow velocity at the molten steel-solidified shell interface has not been clarified. Cannot be captured quantitatively. This is because in the actual mold, simultaneously with the surface tension (= capturing force to the solidified shell) due to the concentration distribution of C, S, N, O, the drag due to the molten steel flow velocity also works, This is because the drag due to the molten steel flow velocity at the molten steel-solidified shell interface must also be considered when capturing non-metallic inclusions.
Therefore, the object of the present invention is to solve the above-mentioned problems of the prior art and to control the flow of molten steel in the mold by using electromagnetic force. An object of the present invention is to provide a continuous casting method of ultra-low carbon steel that can obtain not only defects due to flux but also high-quality slabs with few defects due to entrapment of fine bubbles, non-metallic inclusions and mold flux.
Another object of the present invention is to use a slab cast by such a continuous casting method and not only have few defects due to entrapment of bubbles, non-metallic inclusions and mold flux, but also very few blister defects. It is providing the manufacturing method of the steel plate which can manufacture a high quality steel plate.

  In order to solve the above-mentioned problems, the present inventors have studied various casting conditions when controlling the flow of molten steel in a mold using electromagnetic force. In the method of continuous casting of ultra-low carbon steel while the molten steel flow is damped by a DC magnetic field applied to each of the upper magnetic pole and the pair of lower magnetic poles, The mold is adjusted by adjusting the specific range in consideration of the interfacial tension gradient in the boundary layer, and by optimizing the DC magnetic field strength applied to the upper and lower magnetic poles according to the slab width and casting speed to be cast. The molten steel inside can be brought into a proper flow state in which non-metallic inclusions and bubbles are not trapped in the solidified shell and mold powder is not caught, and as a result, non-metallic inclusions that have been considered a problem in the past And mold Not only defects by Lux, microbubbles and non-metallic inclusions was found that defects due to fewer mold flux high quality cast strip is obtained. Further, in order to obtain a higher quality slab in such continuous casting, there is an optimum range of nozzle immersion depth, nozzle inner diameter, slab thickness, etc. of the immersion nozzle, and the effect of the invention is most easily manifested in that range. I found out.

By continuously casting the slab while controlling the flow of molten steel in the mold by direct current magnetic fields applied to the upper and lower magnetic poles, it is possible to prevent mold flux defects caused by mold flux entrainment and a relatively large size. Defects caused by bubbles (normally 1 mmφ or more) and non-metallic inclusions can be prevented. However, with this continuous casting method, it is difficult to reliably prevent finer bubbles and non-metallic inclusions from being trapped by the solidified shell, and surface defects caused by the inclusion of such fine bubbles and inclusions are difficult. May occur. In contrast, as described above, the chemical composition of the ultra-low carbon steel is adjusted to a specific range in consideration of the interfacial tension gradient in the concentration boundary layer on the front of the solidified shell, and also according to the slab width and casting speed to be cast. Thus, by optimizing the strength of the DC magnetic field applied to each of the upper magnetic pole and the lower magnetic pole, it is possible to suppress trapping of fine bubbles and non-metallic inclusions in the solidified shell. As described above, it is possible to prevent entrainment of mold flux and prevent trapping in the solidified shell regardless of the size of bubbles and non-metallic inclusions, and the surface caused by entrapment of bubbles and non-metallic inclusions or mold flux. High quality steel sheets with very few defects can be manufactured.
Furthermore, high-quality steel sheets with very few blisters can be obtained by pickling and cold rolling hot-rolled steel sheets obtained by rolling slabs cast by the continuous casting method as described above under specific conditions. It was found that it could be manufactured.

The present invention has been made based on these findings, and has the following gist.
[1] A pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the long side of the mold are provided outside the mold, and the molten steel discharge angle from the horizontal direction of the molten steel discharge hole is 10 ° or more and less than 30 ° Using the continuous casting machine in which the molten steel discharge hole is located between the peak position of the magnetic field of the upper magnetic pole and the peak position of the magnetic field of the lower magnetic pole, When continuously casting an ultra-low carbon steel containing 0.003% by mass or less of C while braking the molten steel flow by a DC magnetic field applied to each of the magnetic poles,
A molten steel having a chemical component with an X value defined by the following formula (1) satisfying X ≦ 5000,
X = 24989 × [% Ti] +386 147 × [% S] + 853354 × [% O] (1)
[% Ti]: Ti content in molten steel (mass%)
[% S]: S content in molten steel (mass%)
[% O]: O content (% by mass) in molten steel
Continuous casting at a casting speed of 0.75 m / min or more according to the following conditions (a) and (b)
Condition (A): When the slab width to be cast and the casting speed are the following (a) to (i), the strength of the DC magnetic field applied to the upper magnetic pole is 0.03 to 0.15 T, and is applied to the lower magnetic pole. The intensity of the DC magnetic field is 0.24 to 0.45T.
(A) Slab width of less than 950 mm and casting speed of less than 2.05 (b) Slab width of 950 mm to less than 1050 mm and casting speed of less than 2.25 m / min (c) Slab width of from 1050 mm to less than 1350 mm and casting speed of 2.35 m (D) Slab width of 1350 mm or more and less than 1450 mm and casting speed of less than 2.25 m / min (e) Slab width of 1450 mm or more and less than 1650 mm and casting speed of less than 2.15 m / min (f) Slab width of 1650 mm or more and less than 1750 mm (G) Slab width of 1750 mm or more and less than 1850 mm and casting speed of less than 1.95 m / min (h) Slab width of 1850 mm or more and less than 1950 mm and casting speed of less than 1.85 m / min (i ) The slab width is 1950 mm or more and less than 2150 mm and the casting speed is 1.75 m / min.・ Condition (b): When the cast slab width and casting speed are the following (j) to (s), the strength of the DC magnetic field applied to the upper magnetic pole is more than 0.15T and less than 0.30T, The strength of the DC magnetic field to be applied is 0.24 to 0.45T.
(J) Slab width of less than 950 mm and casting speed of 2.05 m / min or more and 3.05 m / min or less (k) Slab width of 950 mm or more and less than 1050 mm and casting speed of 2.25 m / min or more and 3.05 m / min or less (l ) Slab width of 1050 mm or more and less than 1350 mm and casting speed of 2.35 m / min or more and 3.05 m / min or less (m) Slab width of 1350 mm or more and less than 1450 mm and casting speed of 2.25 m / min or more and 3.05 m / min or less (n ) Slab width of 1450 mm to less than 1550 mm and casting speed of 2.15 m / min to 3.05 m / min (o) Slab width of 1550 mm to less than 1650 mm and casting speed of 2.15 m / min to 2.85 m / min (p ) Slab width of 1650 mm or more and less than 1750 mm and casting speed of 2.05 m / min or more and 2.65 m / min or less (q) Slab width 175 mm or more and less than 1850 mm and casting speed of 1.95 m / min or more and 2.55 m / min or less (r) Slab width of 1850 mm or more and less than 1950 mm and casting speed of 1.85 m / min or more and 2.55 m / min or less (s) Slab width 1950 mm or more and less than 2150 mm and casting speed 1.75 m / min or more and 2.55 m / min or less When the cast slab is hot-rolled into a hot-rolled steel sheet, the hot-rolled steel sheet is pickled, and then cold-rolled. The method for producing a steel sheet, wherein the time t or / and the maximum surface temperature T of the steel sheet are controlled so as to satisfy the following expression (a).
Hc / Ho> exp {−0.002 × (T + t / 100)} (a)
However, Ho: Hydrogen concentration (mass ppm) in the steel plate immediately after pickling
Hc: Critical hydrogen concentration (mass ppm) in the steel sheet immediately before cold rolling, which causes surface quality defects due to blisters, determined by cold rolling conditions
t: Time from the end of pickling to the start of cold rolling (seconds)
T: Maximum surface temperature (K) of the steel plate after the end of pickling and before the start of cold rolling (however, the surface temperature of the steel plate is the surface temperature of the steel plate when heated after the end of pickling and before cold rolling) Including)

[2] In the method for producing a steel plate according to [1], the hot-rolled steel plate after pickling and before cold rolling is heated to a temperature higher than the steel plate temperature immediately after the end of pickling. Method.
[3] In the method for producing a steel sheet according to [1] or [2], the molten steel in the mold of the continuous casting machine has a surface turbulent energy of 0.0010 to 0.0015 m ² / s ² and a surface flow velocity of 0.1. 30 m / s or less, The flow rate in a molten steel-solidified shell interface is 0.08-0.15 m / s, The manufacturing method of the steel plate characterized by the above-mentioned.
[4] The method for producing a steel plate according to [3], wherein the molten steel in the mold of the continuous casting machine has a surface flow velocity of 0.05 to 0.30 m / s.
[5] In the method for producing a steel sheet of [3] or [4] above, the molten steel in the mold of the continuous casting machine has a ratio A / B of the flow velocity A at the molten steel-solidified shell interface to the surface flow velocity B of 1. The manufacturing method of the steel plate characterized by being 0-2.0.
[6] In the method for producing a steel sheet according to any one of [3] to [5], the molten steel in the mold of the continuous casting machine has a bubble concentration at the molten steel-solidified shell interface of 0.008 kg / m ³ or less. A method for producing a steel sheet, comprising:
[7] In the method for producing a steel sheet according to [6] above, the slab thickness cast by the continuous casting machine is 220 to 300 mm, and the amount of inert gas blown from the inner wall surface of the immersion nozzle of the continuous casting machine is 3 to 25 NL / The manufacturing method of the steel plate characterized by the above-mentioned.
[8] A method for manufacturing a steel sheet according to any one of the above [1] to [7], wherein a nozzle immersion depth of an immersion nozzle of a continuous casting machine is 230 to 290 mm.
[9] In the method for manufacturing a steel sheet according to any one of [1] to [8] above, the nozzle inner diameter of the immersion nozzle of the continuous casting machine (however, the nozzle inner diameter at the position where the molten steel discharge hole is formed) is 70 to 90 mm. A method for producing a steel sheet, comprising:
[10] In the method for manufacturing a steel plate according to any one of [1] to [9], the opening area of each molten steel discharge hole of the immersion nozzle of the continuous casting machine is 3600 to 8100 mm ^2. Method.

[11] A pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the long side of the mold are provided outside the mold, and the molten steel discharge angle from the horizontal direction of the molten steel discharge hole is 10 ° or more and less than 30 ° Using the continuous casting machine in which the molten steel discharge hole is located between the peak position of the magnetic field of the upper magnetic pole and the peak position of the magnetic field of the lower magnetic pole, A method of continuously casting an ultra-low carbon steel containing 0.003% by mass or less of C while braking a molten steel flow by a DC magnetic field applied to each of magnetic poles,
A molten steel having a chemical component with an X value defined by the following formula (1) satisfying X ≦ 5000,
X = 24989 × [% Ti] +386 147 × [% S] + 853354 × [% O] (1)
[% Ti]: Ti content in molten steel (mass%)
[% S]: S content in molten steel (mass%)
[% O]: O content (% by mass) in molten steel
A continuous casting method for steel, characterized by continuous casting at a casting speed of 0.75 m / min or more according to the following conditions (a) and (b).
Condition (A): When the slab width to be cast and the casting speed are the following (a) to (i), the strength of the DC magnetic field applied to the upper magnetic pole is 0.03 to 0.15 T, and is applied to the lower magnetic pole. The intensity of the DC magnetic field is 0.24 to 0.45T.
(A) Slab width of less than 950 mm and casting speed of less than 2.05 (b) Slab width of 950 mm to less than 1050 mm and casting speed of less than 2.25 m / min (c) Slab width of from 1050 mm to less than 1350 mm and casting speed of 2.35 m (D) Slab width of 1350 mm or more and less than 1450 mm and casting speed of less than 2.25 m / min (e) Slab width of 1450 mm or more and less than 1650 mm and casting speed of less than 2.15 m / min (f) Slab width of 1650 mm or more and less than 1750 mm (G) Slab width of 1750 mm or more and less than 1850 mm and casting speed of less than 1.95 m / min (h) Slab width of 1850 mm or more and less than 1950 mm and casting speed of less than 1.85 m / min (i ) The slab width is 1950 mm or more and less than 2150 mm and the casting speed is 1.75 m / min.・ Condition (b): When the cast slab width and casting speed are the following (j) to (s), the strength of the DC magnetic field applied to the upper magnetic pole is more than 0.15T and less than 0.30T, The strength of the DC magnetic field to be applied is 0.24 to 0.45T.
(J) Slab width of less than 950 mm and casting speed of 2.05 m / min or more and 3.05 m / min or less (k) Slab width of 950 mm or more and less than 1050 mm and casting speed of 2.25 m / min or more and 3.05 m / min or less (l ) Slab width of 1050 mm or more and less than 1350 mm and casting speed of 2.35 m / min or more and 3.05 m / min or less (m) Slab width of 1350 mm or more and less than 1450 mm and casting speed of 2.25 m / min or more and 3.05 m / min or less (n ) Slab width of 1450 mm to less than 1550 mm and casting speed of 2.15 m / min to 3.05 m / min (o) Slab width of 1550 mm to less than 1650 mm and casting speed of 2.15 m / min to 2.85 m / min (p ) Slab width of 1650 mm or more and less than 1750 mm and casting speed of 2.05 m / min or more and 2.65 m / min or less (q) Slab width 175 mm or more and less than 1850 mm and casting speed of 1.95 m / min or more and 2.55 m / min or less (r) Slab width of 1850 mm or more and less than 1950 mm and casting speed of 1.85 m / min or more and 2.55 m / min or less (s) Slab width 1950 mm or more and less than 2150 mm and casting speed 1.75 m / min or more and 2.55 m / min or less

[12] In the continuous casting method of [11], the molten steel in the mold has a surface turbulent energy of 0.0010 to 0.0015 m ² / s ² , a surface flow velocity of 0.30 m / s or less, and a molten steel-solidified shell. A continuous casting method of steel, wherein the flow velocity at the interface is 0.08 to 0.15 m / s.
[13] The continuous casting method of [12], wherein the molten steel in the mold has a surface flow velocity of 0.05 to 0.30 m / s.
[14] In the continuous casting method of [12] or [13] above, the molten steel in the mold has a ratio A / B between the flow velocity A at the molten steel-solidified shell interface and the surface flow velocity B of 1.0 to 2.0. A continuous casting method of steel, characterized in that
[15] In the continuous casting method according to any one of [12] to [14], the molten steel in the mold has a bubble concentration at the molten steel-solidified shell interface of 0.008 kg / m ³ or less. Steel continuous casting method.
[16] The steel according to the above [15], wherein the cast slab thickness is 220 to 300 mm, and the amount of inert gas blown from the inner wall surface of the immersion nozzle is 3 to 25 NL / min. Continuous casting method.
[17] The continuous casting method according to any one of [11] to [16], wherein the nozzle immersion depth of the immersion nozzle is 230 to 290 mm.
[18] In the continuous casting method according to any one of [11] to [17], the nozzle inner diameter of the immersion nozzle (however, the nozzle inner diameter at the position where the molten steel discharge hole is formed) is 70 to 90 mm. Steel continuous casting method.
[19] The continuous casting method according to any one of [11] to [18], wherein an opening area of each molten steel discharge hole of the immersion nozzle is 3600 to 8100 mm ² .

According to the continuous casting method of steel of the present invention, the chemical composition of the ultra-low carbon steel is adjusted to a specific range in consideration of the interfacial tension gradient in the concentration boundary layer in front of the solidified shell, and the slab width and casting to be cast are adjusted. By optimizing the strength of the DC magnetic field applied to each of the upper and lower magnetic poles according to the speed, not only defects caused by non-metallic inclusions and mold flux, which have been regarded as problems in the past, but also small bubbles and A high quality slab with few defects due to non-metallic inclusions can be obtained.
In particular, a higher quality slab can be obtained by optimizing the nozzle immersion depth of the immersion nozzle, the nozzle inner diameter, and the opening area of the molten steel discharge hole.
Further, according to the method for producing a steel sheet of the present invention, it is possible to produce a high-quality steel sheet having not only few defects due to entrapment of bubbles, non-metallic inclusions, and mold flux, but also very few blisters.

The longitudinal cross-sectional view which shows one Embodiment of the casting_mold | template and immersion nozzle of a continuous casting machine with which implementation of this invention is carried out Horizontal sectional view of the mold and the immersion nozzle in the embodiment of FIG. Graph showing the relationship between the molten steel discharge angle of the immersion nozzle and the occurrence rate (defect index) of surface defects Graph showing the relationship between the X value of molten steel, the molten steel flow velocity at the molten steel-solidified shell interface, and the capture rate of non-metallic inclusions in the solidified shell In this invention method, the graph which shows the influence (influence on a mold flux defect and a bubble defect) of the nozzle immersion depth of an immersion nozzle In this invention method, the graph which shows the influence (influence on a mold flux property defect) of the nozzle inner diameter of an immersion nozzle In this invention method, the graph which shows the influence (influence on a mold flux property defect and a bubble defect) of the opening area of each molten steel discharge hole of an immersion nozzle Conceptual diagram for explaining the surface turbulent energy, solidification interface flow velocity (flow velocity at the molten steel-solidified shell interface), surface flow velocity and solidification interface bubble concentration (bubble concentration at the molten steel-solidified shell interface) of the molten steel in the mold Graph showing the relationship between surface turbulence energy and surface defect rate (number of defects) of molten steel in mold Graph showing the relationship between the surface flow velocity of molten steel in the mold and the surface defect rate (number of defects) A graph showing the relationship between the solidification interface flow velocity of molten steel in the mold (flow velocity at the molten steel-solidification shell interface) and the surface defect rate (number of defects). A graph showing the relationship between the ratio A / B between the solidification interface flow velocity A and the surface flow velocity B of the molten steel in the mold and the surface defect rate (number of defects). Graph showing the relationship between the solidified interface bubble concentration (bubble concentration at the molten steel-solidified shell interface) and the surface defect rate (number of defects) of the molten steel in the mold The graph which shows the relationship between the pickling reduction amount of a hot-rolled steel plate, and the hydrogen concentration Ho in the steel plate immediately after the end of pickling The hydrogen concentration in the hot-rolled steel sheet immediately after pickling ends Ho, when the same steel sheet surface temperature _{T 0, Ho · exp {-0.002} × (T 0 + t 1/100)} and the time from the pickling finished t ₁ Showing the relationship with the hydrogen concentration H ₁ in the steel plate at the time of passing A graph showing the relationship between the hydrogen concentration H in the steel sheet just before cold rolling and the number of blister defects generated, arranged by the finished thickness of the cold rolling.

  The continuous casting method of the present invention includes a pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the long side of the mold on the outer side of the mold (the back side of the mold side wall) and downward from the horizontal direction of the molten steel discharge hole Using a continuous casting machine including an immersion nozzle having a molten steel discharge angle α of 10 ° or more and less than 30 °, wherein the molten steel discharge hole is positioned between the peak position of the magnetic field of the upper magnetic pole and the peak position of the magnetic field of the lower magnetic pole. The continuous casting of ultra-low carbon steel is performed while the molten steel flow is damped by a DC magnetic field applied to each of the pair of upper magnetic poles and the pair of lower magnetic poles.

  In the continuous casting method as described above, as a result of investigation by the present inventor through numerical simulation, etc., as a factor (primary factor) involved in generation of bubble defects, inclusion physical defects and mold flux defects, surface turbulent energy (Involved in the generation of eddy currents near the surface), molten steel flow velocity at the molten steel-solidified shell interface (hereinafter sometimes simply referred to as “solidification interface”), and hereinafter referred to as “solidification interface flow velocity”, surface flow velocity It was found that these had an effect on the occurrence of defects. In particular, it was found that the surface flow velocity and the surface turbulent energy affect the entrainment of the mold flux, and the solidification interface flow velocity affects bubble defects and inclusion physical defects. And based on these knowledge, as a result of examining each effect | action of the upper DC magnetic field and lower DC magnetic field which were applied, the following points became clear.

(1) When a DC magnetic field is applied to the upper electrode, the upward flow of the molten steel (the upward flow generated when the jet flow from the molten steel discharge hole collides with the mold short side and reverses) is braked, and the surface velocity and surface turbulent energy Can be reduced. However, the surface flow velocity, the surface turbulent energy, and the solidification interface flow velocity cannot be controlled to an ideal state only with such a DC magnetic field.
(2) From the above points, it is considered that applying a DC magnetic field at the upper magnetic pole is effective in preventing both bubble defects, inclusion physical defects and mold flux defects, but simply applying a DC magnetic field. Applying it alone does not provide a sufficient effect. The casting conditions (casting slab width, casting speed), the application conditions of the DC magnetic field applied to the upper and lower magnetic poles are related to each other, and there is an optimum range for them. To do.
(3) In particular, in order to prevent fine non-metallic inclusions from being trapped by the solidified shell, it is difficult for non-metallic inclusions to be trapped by the solidified shell at the molten steel-solidified shell interface. Adjust to the component range (that is, a specific range considering the interfacial tension gradient in the concentration boundary layer in front of the solidified shell), and then optimize the solidification interface flow velocity by optimizing the DC magnetic field strength as described above, It is necessary to obtain a cleaning effect by the molten steel flow.

Based on such knowledge, the present invention performs continuous casting of ultra-low carbon steel under the following conditions (A) and (B), thereby generating bubble defects, inclusion physical defects and mold flux defects. Both can be effectively suppressed.
Condition (A): The chemical composition of the molten steel (very low carbon steel) is adjusted to a specific range in consideration of the interfacial tension gradient in the concentration boundary layer in front of the solidified shell.
Condition (B): The strength of the DC magnetic field applied to each of the upper magnetic pole and the lower magnetic pole is optimized according to the slab width to be cast and the casting speed.

1 and 2 show an embodiment of a mold and an immersion nozzle of a continuous casting machine used for carrying out the present invention. FIG. 1 is a longitudinal sectional view of the mold and the immersion nozzle, and FIG. It is a figure (sectional drawing which follows the II-II line of FIG. 1).
In the figure, reference numeral 1 denotes a mold, and the mold 1 is constituted by a mold long side portion 10 (mold side wall) and a mold short side portion 11 (mold side wall) in a rectangular shape in a horizontal section.
Reference numeral 2 denotes an immersion nozzle, and molten steel in a tundish (not shown) installed above the mold 1 is injected into the mold 1 through the immersion nozzle 2. The immersion nozzle 2 has a bottom portion 21 at the lower end of a cylindrical nozzle body, and a pair of molten steel discharge holes 20 penetrates the side wall portion directly above the bottom portion 21 so as to face both mold short side portions 11. It is installed.
In order to prevent non-metallic inclusions such as alumina in the molten steel from adhering to and accumulating on the inner wall surface of the immersion nozzle 2, the inside of the nozzle body of the immersion nozzle 2 and the upper nozzle (not shown) An inert gas such as Ar gas is introduced into the gas flow path provided in the nozzle, and this inert gas is blown into the nozzle from the inner wall surface of the nozzle.

Molten steel flowing into the immersion nozzle 2 from the tundish is discharged into the mold 1 from a pair of molten steel discharge holes 20 of the immersion nozzle 2. The discharged molten steel is cooled in the mold 1 to form a solidified shell 5 and is continuously drawn below the mold 1 to form a slab. A mold flux is added to the meniscus 6 in the mold 1 as a heat insulating agent for molten steel and a lubricant between the solidified shell 5 and the mold 1.
Inert gas bubbles blown from the inner wall surface of the immersion nozzle 2 or the upper nozzle are discharged into the mold 1 from the molten steel discharge hole 20 together with the molten steel.

A pair of upper magnetic poles 3a and 3b and a pair of lower magnetic poles 4a and 4b that are opposed to each other with the long side of the mold interposed therebetween are provided on the outer side of the mold 1 (the back side of the mold side wall). The lower magnetic poles 4a and 4b are arranged along the entire width in the width direction of the mold long side portion 10, respectively.
The upper magnetic poles 3a and 3b and the lower magnetic poles 4a and 4b are, in the vertical direction of the mold 1, the peak positions of the magnetic fields of the upper magnetic poles 3a and 3b (peak positions in the vertical direction: usually the vertical center positions of the upper magnetic poles 3a and 3b). ) And the magnetic field peak position of the lower magnetic poles 4a and 4b (the peak position in the vertical direction: usually the vertical central position of the lower magnetic poles 4a and 4b). In addition, the pair of upper magnetic poles 3 a and 3 b is usually disposed at a position covering the meniscus 6.

  The molten steel discharged from the molten steel discharge hole 20 of the immersion nozzle 2 in the direction of the mold short side part collides with the solidified shell 5 generated on the front surface of the mold short side part 11 and is divided into a downward flow and an upward flow. A direct current magnetic field is applied to each of the pair of upper magnetic poles 3a and 3b and the pair of lower magnetic poles 4a and 4b. The basic action of these magnetic poles is electromagnetic acting on molten steel moving in the direct current magnetic field. Using force, the molten steel upward flow is braked (decelerated) by the DC magnetic field applied to the upper magnetic poles 3a and 3b, and the molten steel downward flow is braked (decelerated) by the DC magnetic field applied to the lower magnetic poles 4a and 4b. To do.

  In the present invention method, an immersion nozzle having a molten steel discharge angle α from the molten steel discharge hole 20, that is, a molten steel discharge angle α downward from the horizontal direction of 10 ° or more and less than 30 ° is used. When the molten steel discharge angle α is less than 10 °, even if the molten steel upward flow is braked by the DC magnetic field of the upper magnetic poles 3a and 3b, the turbulence of the molten steel surface cannot be properly controlled, and mold flux is involved. On the other hand, when the molten steel discharge angle α is increased, non-metallic inclusions and bubbles are easily carried down the mold by the molten steel descending flow and are easily captured by the solidified shell. On the other hand, when the molten steel discharge angle α is less than 30 °, Since it has been found that the molten steel flow can be optimized by direct current magnetic field control according to the method of the present invention, the immersion nozzle 2 having a molten steel discharge angle α of less than 30 ° is used in the present invention. Moreover, from the above viewpoint, the more preferable lower limit of the molten steel discharge angle α is 15 °, and the more preferable upper limit is 25 °.

FIG. 3 shows the relationship between the molten steel discharge angle α of the immersion nozzle and the rate of occurrence of surface defects (defect index). The molten steel components and the magnetic field strength, casting speed and conditions (b) described later A continuous casting test is performed under various conditions in which the slab width satisfies the scope of the present invention. The effect of the molten steel discharge angle α on the occurrence of surface defects was investigated. In this test, surface defects were continuously measured with an on-line surface defect meter for the galvannealed steel sheet, and from that, the defect appearance, SEM analysis, ICP analysis, etc. made steelmaking defects (mold flux defects and bubble properties). (Defects / inclusion physical defects) and the number of defects per 100 m of coil length was evaluated according to the following criteria to obtain a surface defect index.
3: The number of defects is 0.30 or less 2: The number of defects is more than 0.30, 1.00 or less 1: The number of defects is more than 1.00

Hereinafter, the conditions (A) and (B) mentioned above will be described in order.
-Condition (A) In the present invention, molten steel having a chemical composition satisfying C: 0.003 mass% or less and an X value defined by the following formula (1) satisfying X ≦ 5000 is used as an object of casting.
X = 24989 × [% Ti] +386 147 × [% S] + 853354 × [% O] (1)
[% Ti]: Ti content in molten steel (mass%)
[% S]: S content in molten steel (mass%)
[% O]: O content (% by mass) in molten steel

  An ultra-low carbon steel having a C content of 0.003% by mass or less is used for decarburization refining in the atmosphere in a converter and decarburization under reduced pressure in a vacuum degassing facility such as an RH vacuum degassing apparatus It is refined through refining (hereinafter referred to as “vacuum decarburization refining”). Decarburization refining does not proceed unless the dissolved oxygen concentration in the molten steel is increased to some extent. Therefore, a large amount of dissolved oxygen remains in the molten steel at the end of decarburization refining. In this way, the cleanliness of steel deteriorates when a large amount of dissolved oxygen remains, so in the melting process of ultra-low carbon steel, metal Al is added to the molten steel after the vacuum decarburization refining is completed. Is deoxidized. By this deoxidation treatment, the dissolved oxygen concentration in the molten steel is rapidly lowered, and alumina is formed as a deoxidation product.

The alumina thus produced agglomerates to form alumina clusters until molten steel is injected into the mold for casting. Most of the nonmetallic inclusions (hereinafter simply referred to as “inclusions”) contained in the molten steel are alumina clusters. When such inclusions are poured into the mold together with the molten steel and captured by the solidified shell of the slab, it becomes a surface defect of the ultra-low carbon steel slab and the quality of the slab deteriorates.
The present inventors have conducted a detailed study on the influence of the chemical composition of molten steel on the trapping of inclusions in the solidified shell and the flow rate of molten steel at the front of the solidified shell, and as a result, molten steel (C: 0.003 mass% or less) The chemical composition of the ultra-low carbon steel) is set to an X value ≦ 5000, the flow state of the molten steel is controlled according to the condition (B) described later, and the solidification interface flow velocity is optimized to trap inclusions and the like in the solidified shell. It was found that can be effectively suppressed.

The X value indicates a measure of the attractive force in the direction of the solidified shell due to the interfacial tension gradient acting on the inclusions that have entered the boundary layer of the solute elements (Ti, S, O) formed on the front surface of the solidified shell in the mold. ing.
Hereinafter, the reason why the X value is derived will be described.
As shown in the publication “Iron and Steel Vol.80 (1994)” p.527, the interfacial tension gradient K in the concentration boundary layer in front of the solidified shell, that is, dσ / dx (σ: interfacial tension, x: distance) The force F received by the inclusions in the direction of the solidified shell is expressed by the following equation (2).
F = − (8/3) × πR ² K (2)
F: Force received by inclusions (N)
π: Pi ratio
R: radius of inclusion (m)
K: Interfacial tension gradient (N / m ² )

The interfacial tension gradient K is the product of the change in interfacial tension due to the solute element concentration and the concentration gradient of the component, as shown in the following equation (3).
K = dσ / dx = (dσ / dc) × (dc / dx) (3)
Where σ: Interfacial tension between molten steel and inclusions (N / m)
x: Distance from the solidification interface (m)
dσ / dc: change in interfacial tension due to solute element concentration (N / m / mass%)
dc / dx: component concentration gradient (mass% / m)
From the solidification theory, the concentration gradient dc / dx of the component under the condition where the molten steel flow velocity exists in the mold is expressed by the following equation (4).
dc / dx = −C _O × (1−K _O ) × (V _S / D) × exp [−V _S × (x−δ) / D] (4)
However, _CO : Solute element concentration (mass%) in the molten steel before casting
K 2 _O : Partition coefficient of solute element (−)
V _S : solidification rate (m / s)
D: Diffusion coefficient of solute element in molten steel (m ² / s)
δ: Concentration boundary layer thickness (m)

In the above equation (4), when x = δ is substituted, the concentration gradient (dc / dx) at x = δ is obtained by the following equation (5).
_{dc / dx = -C O × (} 1-K O) × (V S / D) ... (5)
By substituting this equation (5) into the above equation (3), the interfacial tension gradient K indicating the scale of the force acting immediately after the inclusion enters the concentration boundary layer can be obtained by the following equation (6). .
K = (dσ / dc) × [−C _O × (1-K _O ) × (V _S / D)] (6)

Here, dσ / dc shown in the above equation (6) is shown in the publication “Handbook of Physical Properties of Molten Iron and Hot Metal” (edited by the Japan Iron and Steel Institute, 1972), and the like. Among the elements, elements that greatly affect the value of the interfacial tension gradient K are Ti (titanium), S (sulfur), and O (oxygen = dissolved oxygen), and the interfacial tension gradient K calculated using only these active elements. It was found that there is no problem in considering the trapping of inclusions in the solidified shell even if the value of is used.
In addition, the distribution coefficient K _O of the solute elements, for example, publications, "Third Edition Steel Handbook I basis" (Iron and Steel Institute of Japan, 1981) has been described in, for example, p.194, the distribution of each of the above solute element coefficient K _O was used values described in the publication "iron and steel Vol.80 (1994)" P.534.
The diffusion coefficient D is described, for example, in the publication “Handbook of Physical Properties of Molten Iron / Hot Metal” (edited by the Iron and Steel Institute of Japan, 1992), but for O and S, the publication “Iron and Steel Vol. (1994) ”p. 534 was used, and for Ti, the value described in the publication“ Iron and Steel Vol. 83 (1997) ”p. 566 was used.
The solidification rate V _S can be obtained from heat transfer calculation. V _S was calculated using 0.0002 m / s.

したがって、上記それぞれの溶質元素の界面張力の溶質元素濃度による変化ｄσ／ｄｃ、分配係数Ｋ_Ｏ、拡散係数Ｄ、鋳型内における凝固速度Ｖ_Ｓを上記（6）式に代入することにより、濃度境界層においてアルミナクラスターに働く、Ｔｉ、Ｓ及びＯによる界面張力勾配Ｋとして、24989×［％Ｔｉ］、386147×［％Ｓ］、853354×［％Ｏ］がそれぞれ得られ、これら界面張力勾配Ｋの総和がＸ値となる。 The values in Table 1 were used for dσ / dc, K 2 _O , D, and Vs of Ti (titanium), S (sulfur), and O (oxygen = dissolved oxygen).

Therefore, by substituting the change dσ / dc, the distribution coefficient K _O , the diffusion coefficient D, and the solidification rate V _S in the mold with the solute element concentration of the interfacial tension of each solute element, the concentration boundary 24989 × [% Ti], 386147 × [% S], and 853354 × [% O] are obtained as interfacial tension gradients K due to Ti, S, and O acting on the alumina clusters in the layers, respectively. The sum is the X value.

The relationship between the X value and the trapping rate of inclusions in the solidified shell was examined by a casting test using molten steel having various compositions. In this test, when the solidification interface flow velocity in the mold is 0.01 m / s, 0.08 m / s, 0.10 m / s, and 0.15 m / s, the X value and inclusions at the respective solidification interface flow rates. The relationship with the capture rate was investigated. Here, the inclusion capture rate is the value obtained by dividing the inclusion index in the solidified shell by the inclusion index in the molten steel, as shown in the following formula (7), and the trapping per unit inclusion concentration: Is a value indicating the frequency of
α = I / A (7)
Where α: Inclusion trapping rate (-)
I: Inclusion index (−) in the solidified shell
A: Inclusion index in molten steel (-)

The inclusion index is a measurement of the major and minor axes of inclusions with an optical microscope to calculate the area as an ellipsoid, and the total sum of the observed inclusion areas is divided by the measured area. This is an index indicating how much inclusions are included in the unit measurement area. The inclusion index in the molten steel can be calculated by measuring the inclusion in the sample collected from the molten steel.
The above test results are shown in FIG. 4, and it can be seen that when the X value ≦ 5000, trapping of inclusions in the solidified shell can be suppressed by giving a certain solidification interface flow velocity. Further, such an effect is significant when the X value ≦ 4000, particularly the X value ≦ 3000. Therefore, by setting the chemical composition of the molten steel to an X value ≦ 5000 (preferably X value ≦ 4000, particularly preferably X value ≦ 3000), and providing a solidification interface flow velocity under the condition (B) described later, inclusions (particularly, minute Trapping in the solidified shell, etc., can be appropriately prevented. In addition, from the restriction | limiting on the chemical composition of molten steel (ultra-low carbon steel), about 2000 X value becomes a substantial minimum normally.

  The chemical composition of the molten steel cast by the present invention is not particularly limited as long as the C content is 0.003% by mass or less and the X value ≦ 5000, but the effect of the present invention is obtained particularly effectively. From the viewpoint, as chemical components other than C, Si: 0.05% by mass or less, Mn: 1.0% by mass or less, P: 0.05% by mass or less, S: 0.015% by mass or less, Al: 0.010-0.075 mass%, containing Ti: 0.005-0.05 mass%, and further containing Nb: 0.005-0.05 mass%, with the balance being Fe and Chemical components consisting of inevitable impurities are preferred.

Hereinafter, the reasons for limiting the chemical components will be described.
C deteriorates the workability of the thin steel sheet when its content increases. For this reason, when carbide forming elements such as Ti and Nb are added, the elongation and deep drawability excellent as IF steel (Interstitial-Free steel) can be obtained to 0.003 mass% or less.
Si is a solid solution strengthening element, and if the content is large, the workability of the thin steel sheet deteriorates. In consideration of the influence on the surface treatment, it is preferable to set the upper limit to 0.05% by mass.
Mn is a solid solution strengthening element and increases the strength of the steel, but on the other hand, it lowers the workability, so it is preferable that the upper limit is 1.0% by mass.

P is a solid solution strengthening element and increases the strength of the steel. However, if it exceeds 0.05 mass%, workability and weldability deteriorate, so 0.05 mass% is preferable as the upper limit.
S causes cracking during hot rolling and generates A-based inclusions that lower the workability of the thin steel sheet. Therefore, the content is preferably reduced as much as possible. For this reason, 0.015% by mass Is preferably the upper limit.
Al functions as a deoxidizing agent, and it is preferable to contain 0.010% by mass or more in order to obtain a deoxidizing effect. However, since adding Al more than necessary causes an increase in cost, its content is 0.010%. It is preferable to set it as -0.075 mass%.
Ti fixes C, N, and S in the steel as precipitates and improves workability and deep drawability, but the effect is poor when the content is less than 0.005 mass%. On the other hand, since Ti is also a precipitation strengthening element, if the content exceeds 0.05 mass%, the steel sheet becomes hard and workability deteriorates. For this reason, it is preferable that Ti content shall be 0.005-0.05 mass%.

  Nb fixes C, N, and S in the steel as precipitates and improves workability and deep drawability in the same manner as Ti, but its effect is poor when the content is less than 0.005 mass%. On the other hand, since Nb is a precipitation strengthening element, if the content exceeds 0.05% by mass, the steel sheet becomes hard and workability deteriorates. For this reason, it is preferable that Nb content shall be 0.005-0.05 mass%.

-Regarding condition (B) In casting of molten steel having the above-described chemical component (X value ≤ 5000), the strength of the DC magnetic field applied to each of the upper magnetic pole and the lower magnetic pole is basically determined according to the slab width and casting speed to be cast. In particular, it has been found that optimization should be performed as in the following (I) and (II).
(I) "Slab width-casting speed" region in which casting speed set corresponding to each slab width is relatively small: Since the throughput amount is relatively small, the jet velocity from the molten steel discharge hole of the immersion nozzle is also relative. Small. For this reason, since the upward flow (reversal flow) is also reduced, the strength of the DC magnetic field of the upper magnetic pole for braking the upward flow is relatively reduced. On the other hand, by suppressing the inclusions and bubbles accompanying the downward flow from diving downward, changing the flow of downward molten steel upward, increasing the solidification interface flow velocity in the region above the lower magnetic field, In order to prevent inclusions and bubbles from being trapped by the solidified shell, the DC magnetic field strength of the lower magnetic pole is sufficiently increased. By applying a DC magnetic field as described above under the condition that the chemical composition of the molten steel is X value ≦ 5000, the surface turbulence energy, solidification interface flow velocity and surface flow velocity are controlled within the appropriate ranges, and bubble defects and intervening Prevents the occurrence of physical property defects and mold flux property defects.

(II) "Slab width-casting speed" region where casting speed set corresponding to each slab width is relatively large: Since the throughput amount is relatively large, the jet velocity from the molten steel discharge hole of the immersion nozzle is also relative It ’s big. For this reason, since the upward flow (reversal flow) also increases, the strength of the DC magnetic field of the upper magnetic pole for braking the upward flow is relatively increased. On the other hand, similarly to the above (I), non-metallic inclusions and bubbles accompanying the downward flow are suppressed from entering in the downward direction, and the downward flow of the molten steel is changed upward, in the region above the lower magnetic field. In order to prevent non-metallic inclusions and bubbles from being trapped by the solidified shell, the strength of the DC magnetic field of the lower magnetic pole is sufficiently increased. By applying the DC magnetic field as described above under the condition that the chemical composition of the molten steel is X value ≦ 5000, the surface turbulence energy, the solidification interface flow velocity and the surface flow velocity are controlled within the proper ranges, and the bubble defects and mold Prevents the occurrence of flux defects.

  In the method of the present invention, the casting speed is set to 0.75 m / min or more from the viewpoint of productivity, and further applied to the upper magnetic poles 3a and 3b and the lower magnetic poles 4a and 4b according to the slab width to be cast and the casting speed. By optimizing the strength of the direct current magnetic field as shown in the following conditions (a) and (b), the mold flux to the solidified shell 5 that causes mold flux defects, bubble defects, and inclusion physical defects In the same way, the trapping of entrainment and the trapping of microbubbles (mainly bubbles of inert gas blown from the inner wall surface of the immersion nozzle) and inclusions are suppressed.

Condition (A): When the slab width to be cast and the casting speed are the following (a) to (i), the strength of the DC magnetic field applied to the upper magnetic pole is 0.03 to 0.15 T, and the DC applied to the lower magnetic pole The strength of the magnetic field is 0.24 to 0.45T.
(A) Slab width of less than 950 mm and casting speed of less than 2.05 (b) Slab width of 950 mm to less than 1050 mm and casting speed of less than 2.25 m / min (c) Slab width of from 1050 mm to less than 1350 mm and casting speed of 2.35 m (D) Slab width of 1350 mm or more and less than 1450 mm and casting speed of less than 2.25 m / min (e) Slab width of 1450 mm or more and less than 1650 mm and casting speed of less than 2.15 m / min (f) Slab width of 1650 mm or more and less than 1750 mm (G) Slab width of 1750 mm or more and less than 1850 mm and casting speed of less than 1.95 m / min (h) Slab width of 1850 mm or more and less than 1950 mm and casting speed of less than 1.85 m / min (i ) The slab width is 1950 mm or more and less than 2150 mm and the casting speed is 1.75 m / min. Mitsuru

  Although the molten steel flow discharged from the immersion nozzle 2 collides with the solidified shell on the short side of the mold, a reversal flow upward and a downward flow occur, but each of the above-described (a) to (i) When the casting speed set corresponding to the slab width is relatively small (compared to the condition (b)), the throughput rate is relatively small, so the jet velocity from the molten steel discharge hole of the immersion nozzle is also Relatively small. For this reason, since the upward flow (reversal flow) is also reduced, the strength of the DC magnetic field of the upper magnetic poles 3a and 3b for braking the upward flow is relatively reduced. On the other hand, non-metallic inclusions and bubbles accompanying the downward flow are suppressed from entering the downward direction, and the flow of the molten steel is changed upward to increase the solidification interface flow velocity in the region above the lower magnetic field. Thus, in order to prevent non-metallic inclusions and bubbles from being trapped by the solidified shell, the strength of the DC magnetic field of the lower magnetic poles 4a and 4b is sufficiently increased. In particular, by applying a DC magnetic field as described above under the condition that the chemical composition of the molten steel is an X value ≦ 5000 and imparting a solidification interface flow velocity to the molten steel, a solidified shell is obtained even for fine inclusions and bubbles. Can be prevented appropriately.

In the above cases (a) to (i), when the strength of the DC magnetic field of the upper magnetic poles 3a and 3b is less than 0.03T, the effect of braking the molten steel upward flow by the DC magnetic field is insufficient and the molten metal surface fluctuation is large. Entrainment is likely to occur. On the other hand, when the strength exceeds 0.15 T, the cleaning effect due to the molten steel ascending flow decreases, so that non-metallic inclusions and bubbles are easily trapped by the solidified shell.
Further, when the strength of the DC magnetic field of the lower magnetic poles 4a and 4b is less than 0.24T, the braking effect of the molten steel descending flow due to the DC magnetic field is insufficient, so that non-metallic inclusions and bubbles associated with the molten steel descending flow are lowered. It will sink in the direction and will be easily captured by the solidified shell. On the other hand, when the strength exceeds 0.45 T, the cleaning effect due to the molten steel descending flow is reduced, so that non-metallic inclusions and bubbles are easily captured by the solidified shell.

Condition (b): When the slab width to be cast and the casting speed are the following (j) to (s), the strength of the DC magnetic field applied to the upper magnetic pole is more than 0.15T and not more than 0.30T and applied to the lower magnetic pole. The intensity of the DC magnetic field is 0.24 to 0.45T.
(J) Slab width of less than 950 mm and casting speed of 2.05 m / min or more and 3.05 m / min or less (k) Slab width of 950 mm or more and less than 1050 mm and casting speed of 2.25 m / min or more and 3.05 m / min or less (l ) Slab width of 1050 mm or more and less than 1350 mm and casting speed of 2.35 m / min or more and 3.05 m / min or less (m) Slab width of 1350 mm or more and less than 1450 mm and casting speed of 2.25 m / min or more and 3.05 m / min or less (n ) Slab width of 1450 mm to less than 1550 mm and casting speed of 2.15 m / min to 3.05 m / min (o) Slab width of 1550 mm to less than 1650 mm and casting speed of 2.15 m / min to 2.85 m / min (p ) Slab width of 1650 mm or more and less than 1750 mm and casting speed of 2.05 m / min or more and 2.65 m / min or less (q) Slab width 175 mm or more and less than 1850 mm and casting speed of 1.95 m / min or more and 2.55 m / min or less (r) Slab width of 1850 mm or more and less than 1950 mm and casting speed of 1.85 m / min or more and 2.55 m / min or less (s) Slab width 1950 mm or more and less than 2150 mm and casting speed 1.75 m / min or more and 2.55 m / min or less

  When the casting speed set corresponding to each slab width is relatively large as compared with (j) to (s) above (compared with the condition (A)), the throughput amount is inevitably relative. Therefore, the jet velocity from the molten steel discharge hole of the immersion nozzle is relatively high. For this reason, the upward flow (reversal flow) also increases, and therefore the strength of the DC magnetic field of the upper magnetic poles 3a and 3b for braking the upward flow is relatively increased. On the other hand, as in the case of condition (b), non-metallic inclusions and bubbles accompanying the downward flow are prevented from entering downward, the downward flow of molten steel is changed upward, and above the lower magnetic field. By increasing the solidification interface flow velocity in the region, the strength of the DC magnetic field of the lower magnetic poles 4a and 4b is sufficiently increased so that non-metallic inclusions and bubbles are not trapped by the solidified shell. In particular, by applying a DC magnetic field as described above under the condition that the chemical composition of the molten steel is an X value ≦ 5000 and imparting a solidification interface flow velocity to the molten steel, a solidified shell is obtained even for fine inclusions and bubbles. Can be prevented appropriately.

In the above cases (j) to (s), when the strength of the DC magnetic field of the upper magnetic poles 3a and 3b is 0.15T or less, the braking effect of the molten steel upward flow due to the DC magnetic field is insufficient and the molten metal surface fluctuation is large. Entrainment is likely to occur. On the other hand, when the strength exceeds 0.30 T, the cleaning effect due to the molten steel ascending flow is reduced, so that nonmetallic inclusions and bubbles are easily trapped by the solidified shell.
Further, when the strength of the DC magnetic field of the lower magnetic poles 4a and 4b is less than 0.24T, the braking effect of the molten steel descending flow due to the DC magnetic field is insufficient, so that non-metallic inclusions and bubbles associated with the molten steel descending flow are lowered. It will sink in the direction and will be easily captured by the solidified shell. On the other hand, when the strength exceeds 0.45 T, the cleaning effect due to the molten steel descending flow is reduced, so that non-metallic inclusions and bubbles are easily captured by the solidified shell.

In addition, the continuous casting method of the present invention described above can be regarded as two continuous casting methods such as the following (i) and (ii) that are defined according to the slab width and the casting speed.
(I) A pair of upper magnetic poles and a pair of lower magnetic poles facing each other with the long side of the mold interposed therebetween are provided outside the mold, and the molten steel discharge angle downward from the horizontal direction of the molten steel discharge hole is 10 ° or more and less than 30 °. Using the continuous casting machine in which the molten steel discharge hole is located between the peak position of the magnetic field of the upper magnetic pole and the peak position of the magnetic field of the lower magnetic pole, A method of continuously casting an ultra-low carbon steel containing 0.003% by mass or less of C while braking a molten steel flow by a DC magnetic field applied to each of magnetic poles,
A molten steel having a chemical component with an X value defined by the following formula (1) satisfying X ≦ 5000,
X = 24989 × [% Ti] +386 147 × [% S] + 853354 × [% O] (1)
[% Ti]: Ti content in molten steel (mass%)
[% S]: S content in molten steel (mass%)
[% O]: O content (% by mass) in molten steel
The casting speed is 0.75 m / min or more, the slab width and casting speed are any of the following conditions (a) to (i), the strength of the DC magnetic field applied to the upper magnetic pole is 0.03 to 0.15 T, the lower part A continuous casting method for steel, characterized in that continuous casting is performed by setting the strength of a DC magnetic field applied to a magnetic pole to 0.24 to 0.45 T.
(A) Slab width of less than 950 mm and casting speed of less than 2.05 (b) Slab width of 950 mm to less than 1050 mm and casting speed of less than 2.25 m / min (c) Slab width of from 1050 mm to less than 1350 mm and casting speed of 2.35 m (D) Slab width of 1350 mm or more and less than 1450 mm and casting speed of less than 2.25 m / min (e) Slab width of 1450 mm or more and less than 1650 mm and casting speed of less than 2.15 m / min (f) Slab width of 1650 mm or more and less than 1750 mm (G) Slab width of 1750 mm or more and less than 1850 mm and casting speed of less than 1.95 m / min (h) Slab width of 1850 mm or more and less than 1950 mm and casting speed of less than 1.85 m / min (i ) The slab width is 1950 mm or more and less than 2150 mm and the casting speed is 1.75 m / min. Mitsuru

(Ii) A pair of upper magnetic poles and a pair of lower magnetic poles facing each other with the long side of the mold interposed therebetween are provided outside the mold, and the molten steel discharge angle downward from the horizontal direction of the molten steel discharge hole is 10 ° or more and less than 30 °. Using the continuous casting machine in which the molten steel discharge hole is located between the peak position of the magnetic field of the upper magnetic pole and the peak position of the magnetic field of the lower magnetic pole, A method of continuously casting an ultra-low carbon steel containing 0.003% by mass or less of C while braking a molten steel flow by a DC magnetic field applied to each of magnetic poles,
A molten steel having a chemical component with an X value defined by the following formula (1) satisfying X ≦ 5000,
X = 24989 × [% Ti] +386 147 × [% S] + 853354 × [% O] (1)
[% Ti]: Ti content in molten steel (mass%)
[% S]: S content in molten steel (mass%)
[% O]: O content (% by mass) in molten steel
The casting speed is 0.75 m / min or more, the slab width and the casting speed are any one of the following conditions (j) to (s), the strength of the DC magnetic field applied to the upper magnetic pole is more than 0.15 T and not more than 0.30 T, A continuous casting method of steel, characterized in that continuous casting is performed with the strength of the DC magnetic field applied to the lower magnetic pole being 0.24 to 0.45 T.
(J) Slab width of less than 950 mm and casting speed of 2.05 m / min or more and 3.05 m / min or less (k) Slab width of 950 mm or more and less than 1050 mm and casting speed of 2.25 m / min or more and 3.05 m / min or less (l ) Slab width of 1050 mm or more and less than 1350 mm and casting speed of 2.35 m / min or more and 3.05 m / min or less (m) Slab width of 1350 mm or more and less than 1450 mm and casting speed of 2.25 m / min or more and 3.05 m / min or less (n ) Slab width of 1450 mm to less than 1550 mm and casting speed of 2.15 m / min to 3.05 m / min (o) Slab width of 1550 mm to less than 1650 mm and casting speed of 2.15 m / min to 2.85 m / min (p ) Slab width of 1650 mm or more and less than 1750 mm and casting speed of 2.05 m / min or more and 2.65 m / min or less (q) Slab width 175 mm or more and less than 1850 mm and casting speed of 1.95 m / min or more and 2.55 m / min or less (r) Slab width of 1850 mm or more and less than 1950 mm and casting speed of 1.85 m / min or more and 2.55 m / min or less (s) Slab width 1950 mm or more and less than 2150 mm and casting speed 1.75 m / min or more and 2.55 m / min or less

Hereinafter, particularly preferable casting conditions in which the effects of the invention are most easily manifested in the method of the present invention will be described.
First, the nozzle immersion depth of the immersion nozzle 2 is preferably 230 to 290 mm. Here, the nozzle immersion depth refers to the distance from the meniscus 6 to the upper end of the molten steel discharge hole 20.
This nozzle immersion depth affects the effect of the present invention when the flow rate and flow rate of the molten steel discharged from the immersion nozzle 2 change even if the nozzle immersion depth is too large or too small. Moreover, since the flow state of the molten steel in the mold is greatly changed, it is difficult to appropriately control the molten steel flow. That is, when the nozzle immersion depth is less than 230 mm, the molten steel surface (meniscus) directly fluctuates when the flow rate or flow velocity of the molten steel discharged from the immersion nozzle 2 changes, and the surface disturbance increases, and the mold flux On the other hand, if it exceeds 290 mm, when the flow rate of molten steel fluctuates, the flow rate downwards tends to increase, and the non-metallic inclusions and bubbles tend to become deeper.

FIG. 5 shows the result of investigating the influence of the nozzle immersion depth of the immersion nozzle 2 (influence on mold flux defects and bubble defects) in the method of the present invention, and shows the molten steel in the molten steel discharge hole of the immersion nozzle. Discharge angle α: 15 °, slab width: 1200 mm, slab thickness: 260 mm, casting speed: 1.8 m / min, DC magnetic field strength of upper magnetic pole: 0.12 T, DC magnetic field strength of lower magnetic pole: 0.38 T The test results according to the casting conditions are shown. Other casting conditions are: immersion nozzle inner diameter: 80 mm, opening area of each molten steel discharge hole of the immersion nozzle: 4900 mm ² (70 mm * 70 mm), amount of inert gas blown from the inner wall of the immersion nozzle: 12 L / min, mold used Flux viscosity (1300 ° C.): 0.6 cp.
About the cast slab, using an ultrasonic flaw detector, measure the number of bubble defects and mold flux defects having a particle size of approximately 80 μm or more present at a depth of 2 to 3 mm on the surface of the slab, and the degree of defect occurrence Is indexed. According to FIG. 5, it can be seen that, in the method of the present invention, in particular, by setting the nozzle immersion depth of the immersion nozzle 2 to 230 to 290 mm, bubble defects and mold flux defects are more effectively reduced. .

  The nozzle inner diameter of the immersion nozzle 2, that is, the nozzle inner diameter at the position of the molten steel discharge hole 20 is preferably 70 to 90 mm. When alumina or the like partially adheres to the inner side of the immersion nozzle 2, drift may occur in the molten steel discharged from the immersion nozzle 2 (symmetry of the flow velocity in the width direction is worse), and the inner diameter of the nozzle is less than 70 mm. Then, in such a case, there is a possibility that the drift becomes extremely large. When such an extreme drift occurs, the molten steel flow in the mold cannot be properly controlled. On the other hand, the amount of molten steel flowing through the immersion nozzle 2 is adjusted by adjusting the opening of the sliding nozzle above the immersion nozzle 2, but if the nozzle inner diameter exceeds 90 mm, there may be a portion where the molten steel is not filled inside the nozzle. In this case, too, an extreme drift similar to the above occurs, and there is a possibility that the molten steel flow in the mold cannot be properly controlled.

FIG. 6 shows the results of investigating the influence of the nozzle inner diameter of the immersion nozzle 2 (influence on the mold flux property defect) in the method of the present invention. , Slab width: 1300 mm, slab thickness: 260 mm, casting speed: 2.5 m / min, DC magnetic field strength of upper magnetic pole: 0.16 T, DC magnetic field strength of lower magnetic pole: 0.38 T Is shown. Other casting conditions are: nozzle immersion depth of the immersion nozzle: 260 mm, opening area of each molten steel discharge hole of the immersion nozzle: 4900 mm ² (70 mm * 70 mm), amount of inert gas blown from the inner wall surface of the immersion nozzle: 12 L / min The viscosity of the mold flux used (1300 ° C.): 0.6 cp.
For the cast slab, an ultrasonic flaw detector was used to measure the number of mold flux defects having a particle size of approximately 80 μm or more present at a depth of 2 to 3 mm on the surface of the slab, and the degree of defect occurrence was indexed. Is. According to FIG. 6, in the method of the present invention, it can be seen that mold flux defects are more effectively reduced by setting the inner diameter of the immersion nozzle 2 to 70 to 90 mm.

The opening area of each molten steel discharge hole 20 of the immersion nozzle 2 is preferable to be 3600～8200Mm ^2. The opening area of the molten steel discharge hole 20 affects the effect of the present invention. If the opening area of the molten steel discharge hole 20 is too small, the flow velocity of the molten steel discharged from the molten steel discharge hole 20 becomes too large and the opening is reversed. This is because if the area is too large, the molten steel flow velocity is too small, and in any case, it becomes difficult to optimize the flow velocity of the molten steel flow in the mold.
FIG. 7 shows the results of examining the influence of the opening area of each molten steel discharge hole of the immersion nozzle 2 (influence on mold flux property defect and bubble defect) in the method of the present invention. Molten steel discharge angle α: 15 °, slab width: 1300 mm, slab thickness: 260 mm, casting speed: 2.0 m / min, DC magnetic field strength of upper magnetic pole: 0.14 T, DC magnetic field strength of lower magnetic pole: The test result by the casting condition of 0.38T is shown. The other casting conditions were: nozzle immersion depth of the immersion nozzle: 260 mm, immersion nozzle inner diameter: 80 mm, amount of inert gas blown from the inner wall surface of the immersion nozzle: 12 L / min, viscosity of the mold flux used (1300 ° C.): 0 .6 cp.

About the cast slab, using an ultrasonic flaw detector, measure the number of bubble defects and mold flux defects having a particle size of approximately 80 μm or more present at a depth of 2 to 3 mm on the surface of the slab, and the degree of defect occurrence Is indexed. According to FIG. 7, in the method of the present invention, in particular, by setting the opening area of each molten steel discharge hole 20 of the immersion nozzle 2 to 3600 to 8200 mm ² , bubble defects and mold flux defects are more effectively reduced. You can see that

Other preferable casting conditions are as follows.
The mold flux used preferably has a viscosity at 1300 ° C. of 0.4 to 10 cp. If the viscosity of the mold flux is too high, smooth casting may be hindered. On the other hand, if the viscosity of the mold flux is too low, the mold flux is likely to be caught.
In order to carry out the present invention, each DC magnetic field of the upper magnetic pole and the lower magnetic pole is determined using a control computer based on the slab width to be cast, the casting speed, the molten steel discharge angle downward from the horizontal direction of the molten steel discharge hole of the immersion nozzle, etc. The direct current value to be applied to the coil is obtained from a preset comparison table or mathematical expression, and the direct current is applied to the upper magnetic pole and the lower magnetic pole to automatically control the intensity of the direct current magnetic field. preferable. The casting conditions used as the basis for obtaining the above current values include the immersion nozzle immersion depth (however, the distance from the meniscus to the top of the molten steel discharge hole), the slab thickness, and the amount of inert gas blown from the inner wall of the immersion nozzle. May be added.

FIG. 8 is a conceptual diagram showing surface turbulent energy, solidification interface flow velocity (flow velocity at the molten steel-solidified shell interface), surface flow velocity, and solidification interface bubble concentration (bubble concentration at the molten steel-solidified shell interface) of the molten steel in the mold. is there.
The surface turbulent energy of molten steel is a spatial average value of k values obtained by the following equation, and is defined by a flow simulation of numerical analysis by a three-dimensional k-ε model defined by fluid dynamics. At this time, an inert gas (for example, Ar) blowing speed in consideration of the molten steel discharge angle of the immersion nozzle, the nozzle immersion depth, and volume expansion should be considered. For example, the volume expansion rate is 6 times when the inert gas blowing rate is 15 NL / min. That is, the numerical analysis model is a model that takes into account the nozzle blowing lift effect by coupling the momentum, the continuity equation, the turbulent k-ε model, and the magnetic Lorentz force. (Reference: Based on the description of the two-equation model on p.129- of the “Computational Fluid Dynamics Handbook” (issued on March 31, 2003))

  The solidification interface flow velocity (molten steel flow velocity at the molten steel-solidified shell interface) is the spatial average value of the molten steel flow velocity at a position 50 mm below the meniscus and at a solid fraction fs = 0.5. Here, regarding the solidification interface flow velocity, the solidification latent heat and heat transfer should be taken into consideration, and the temperature dependence of the molten steel viscosity should be taken into consideration. According to detailed calculations by the present inventors, it was found that the solidification interface flow rate at a solid phase ratio of fs = 0.5 corresponds to a flow rate of 1/2 in dendrite inclination measurement (fs = 0). That is, if fs = 0.5 in calculation and the solidification interface flow rate is 0.1 m / s, the solidification interface flow rate of the slab dendrite inclination angle (fs = 0) is 0.2 m / s. The solidification interface flow velocity at the dendrite inclination angle (fs = 0) of the slab is the measurement of the solidification interface flow velocity at the position of the solid phase ratio fs = 0 on the solidification front surface. Here, the dendrite tilt angle is the tilt angle of the primary branch of dendrites extending in the thickness direction from the surface with respect to the normal direction to the slab surface. (Reference: Iron and steel, 61st year (1975), No.14 "Relationship between large inclusions in continuous slab and columnar crystal growth direction", p.2982-2990)

The surface flow velocity is a spatial average value of the molten steel flow velocity on the molten steel surface (bath surface). This is also defined by the aforementioned three-dimensional numerical analysis model. Here, the surface flow velocity coincides with the drag measurement value by the dip rod, but in this definition, it is the area average position, and can be calculated by numerical calculation.
Specifically, numerical analysis of surface turbulent energy, solidification interface flow velocity and surface flow velocity can be performed as follows. In other words, as a numerical analysis model, a model that takes into account the momentum, continuity equation, and turbulence model (k-ε model) coupled to magnetic field analysis and gas bubble distribution is used. Can be obtained. (Reference: Based on Fluent 6.3 User Manual (Fluent Inc. USA))

The surface turbulent energy greatly affects the entrainment of the mold flux. When the surface turbulent energy increases, the entrainment of the mold flux is likely to occur, and the mold flux property defect increases. On the other hand, if the surface turbulent energy is too small, the mold flux is not sufficiently hatched. FIG. 9 shows the relationship between the surface turbulent energy and the surface defect rate (the number of defects per 1 m of coil length measured by the same method as in the examples described later). Flow velocity: 0.08 to 0.15 m / s, surface flow velocity: 0.05 to 0.30 m / s, coagulation interface bubble concentration: 0.008 kg / m ³ or less. According to FIG. 9, when the surface turbulent energy is in the range of 0.0010 to 0.0015 m ² / s ² , the entrainment of the mold flux is effectively suppressed, and there is no problem with the mold flux hatching.

The surface flow rate also has a great influence on the mold flux entrainment. When the surface flow rate is increased, mold flux entrainment tends to occur, and mold flux defects increase. FIG. 10 shows the relationship between the surface flow velocity and the surface defect rate (the number of defects per 1 m of coil length measured by a method similar to the example described later). Other conditions are the surface turbulent energy. : 0.0010 to 0.0015 m ² / s ² , solidification interface flow velocity: 0.08 to 0.15 m / s, solidification interface bubble concentration: 0.008 kg / m ³ or less. According to FIG. 10, when the surface flow velocity is 0.30 m / s or less, the entrainment of mold flux is effectively suppressed. Therefore, the surface flow velocity is preferably 0.30 m / s or less. If the surface flow velocity is too small, a region in which the temperature of the molten steel decreases is generated, and the operation becomes difficult because it promotes partial solidification of the wolf and the molten steel due to insufficient melting of the mold flux. For this reason, it is preferable that the surface flow velocity is 0.05 m / s or more.

The solidification interface flow rate has a great influence on the trapping of bubbles and inclusions by the solidified shell. When the solidification interface flow rate is small, the bubbles and inclusions are easily trapped by the solidification shell, and bubble defects and the like increase. On the other hand, if the solidification interface flow rate is too large, the generated solidified shell is re-dissolved and inhibits the growth of the solidified shell. In the worst case, it leads to a breakout, and the suspension of operations causes a fatal problem in productivity. FIG. 11 shows the relationship between the solidification interface flow velocity and the surface defect rate (the number of defects per 1 m of coil length measured by the same method as in the examples described later). Energy: 0.0010 to 0.0015 m ² / s ² , surface flow velocity: 0.05 to 0.30 m / s, coagulation interface bubble concentration: 0.008 kg / m ³ or less. According to FIG. 11, when the solidification interface flow rate is in the range of 0.08 to 0.15 m / s, trapping of bubbles by the solidified shell is effectively suppressed, and productivity such as breakout due to growth inhibition of the solidified shell is achieved. Does not cause a problem.

The ratio A / B between the solidification interface flow velocity A and the surface flow velocity B affects both the trapping of the bubbles and the entrainment of the mold flux. Sexual defects increase. On the other hand, if the ratio A / B is too large, the mold powder is likely to be entrained, and mold flux defects increase. FIG. 12 shows the relationship between the ratio A / B and the surface defect rate (the number of defects per 1 m of coil length measured by the same method as in the examples described later). Flow energy: 0.0010 to 0.0015 m ² / s ² , surface flow velocity: 0.05 to 0.30 m / s, solidification interface flow velocity: 0.08 to 0.15 m / s, solidification interface bubble concentration: 0.008 kg / M ³ or less. According to FIG. 12, the surface quality defect becomes particularly good when the ratio A / B is 1.0 to 2.0. Therefore, the ratio A / B between the solidification interface flow velocity A and the surface flow velocity B is preferably 1.0 to 2.0.

From the above mentioned point, the molten steel in a fluid state in the mold, the surface turbulent energy: 0.0010~0.0015m ² / ^{s 2,} the surface flow rate: 0.30 m / s or less, molten steel - flow velocity in the solidified shell interface : It is preferable that it is 0.08-0.15 m / s. The surface flow velocity is more preferably 0.05 to 0.30 m / s, and the ratio A / B between the solidification interface flow velocity A and the surface flow velocity B is preferably 1.0 to 2.0.
Another factor involved in the generation of bubble defects is the bubble concentration at the molten steel-solidified shell interface (hereinafter simply referred to as “solidified interface bubble concentration”), and this solidified interface bubble concentration is appropriately controlled. As a result, the trapping of the bubbles at the solidification interface is more appropriately suppressed.
The coagulation interface bubble concentration is defined as the above-described numerical calculation, with the bubble concentration having a diameter of 1 mm at a position 50 mm below the meniscus and the solid phase ratio fs = 0.5. Here, the calculated number N of bubbles to be blown into the nozzle is N = AD ⁻⁵ , A is the blowing gas velocity, and D is the bubble diameter (reference: ISIJ Int. Vol. 43 (2003), No. 10). , P. 1548-1555). The blowing gas speed is generally 5 to 20 Nl / min.

The solidification interface bubble concentration has a great influence on the trapping of bubbles, and when the bubble concentration is high, the amount of bubbles trapped in the solidification shell increases. FIG. 13 shows the relationship between the solidification interface bubble concentration and the surface defect rate (the number of defects per 1 m of coil length measured by the same method as in the examples described later). Flow energy: 0.0010 to 0.0015 m ² / s ² , surface flow velocity: 0.05 to 0.30 m / s, solidification interface flow velocity: 0.08 to 0.15 m / s. According to FIG. 13, when the solidification interface bubble concentration is 0.008 kg / m ³ or less, the amount of bubbles trapped in the solidification shell is suppressed to a low level. Therefore, the solidification interface bubble concentration is preferably 0.008 kg / m ³ or less.
The solidification interface bubble concentration can be controlled by the slab thickness to be cast and the amount of inert gas blown from the inner wall surface of the immersion nozzle. It is preferable to be 25 NL / min or less.

  The molten steel discharged from the molten steel discharge hole 20 of the immersion nozzle 2 is accompanied by bubbles, and if the slab thickness is too small, the molten steel flow discharged from the molten steel discharge hole 20 is directed to the solidified shell 5 on the long side of the mold. Approaching, the solidification interface bubble concentration becomes high, and the bubbles are easily trapped at the solidification shell interface. In particular, when the slab thickness is less than 220 mm, even if the electromagnetic flow control of the molten steel flow as in the present invention is performed, it is difficult to control the bubble distribution for the reasons described above. On the other hand, when the slab thickness exceeds 300 mm, the productivity of the hot rolling process is lowered. For this reason, it is preferable that the slab thickness cast is 220-300 mm.

  When the amount of inert gas blown from the inner wall surface of the immersion nozzle 2 increases, the concentration of bubbles in the solidified interface increases, and bubbles are easily trapped at the solidified shell interface. In particular, when the inert gas blowing rate exceeds 20 NL / min, even if the electromagnetic flow control of the molten steel flow as in the present invention is performed, it is difficult to control the bubble distribution for the reasons described above. On the other hand, if the amount of inert gas blown is too small, nozzle clogging is likely to occur, and on the contrary, the flow rate is difficult to control because the drift is increased. For this reason, it is preferable that the amount of inert gas blown from the inner wall surface of the immersion nozzle 2 is 3 to 25 NL / min.

Next, the manufacturing method of the steel plate using the slab cast by the continuous casting method of the present invention described above will be described.
As mentioned earlier, cold rolled steel sheet defects called blisters penetrate into the steel sheet during pickling after hot rolling, and after cold rolling, non-metallic inclusions, bubbles, segregation, internal cracks, etc. in the steel sheet. Hydrogen staying at the site is a blister-like surface defect that deforms a steel sheet softened by heating by volume expansion with heating during annealing to increase pressure.
As a result of studying the relationship between the generation of such blisters and the pickling conditions and cold rolling conditions of the hot-rolled steel sheet and the slab to be used, the following knowledge was obtained.

(1) The hydrogen concentration Ho in the hot-rolled steel sheet immediately after the end of pickling has a good correlation with the pickling reduction amount of the hot-rolled steel sheet. The concentration Ho can be determined.
(2) The hydrogen concentration H ₁ (mass ppm) in the hot-rolled steel sheet at the time point p when time t ₁ (seconds) has elapsed after the end of pickling is the hydrogen concentration Ho ( (Ppm by mass) and the maximum surface temperature T ₁ (K) of the steel sheet up to the time point p after the end of pickling can be expressed by the following equation (i). Therefore, the time t ₁ in the following formula (i) is set as “time t from the end of pickling to the start of cold rolling”, and the maximum surface temperature T _{1 is set} to “the maximum of the steel sheet after the end of pickling and before the start of cold rolling”. If the surface temperature is T ”, the hydrogen concentration H in the steel sheet immediately before cold rolling can be obtained.
_{H 1 = Ho · exp {-0.002} × (T 1 + t 1/100)} ... (i)

(3) Whether or not the surface quality of the steel sheet due to blistering occurs is determined by the hydrogen concentration H in the steel sheet immediately before cold rolling and the cold rolling conditions (reducing conditions). Depending on the cold rolling conditions, the blister The “critical hydrogen concentration Hc in the steel sheet immediately before cold rolling” at which surface quality failure (failed surface quality) occurs due to is determined.
(4) From the above, the time t from the end of pickling to the start of cold rolling so that the hydrogen concentration H in the steel sheet immediately before cold rolling determined by the above formula (i) does not become the critical hydrogen concentration Hc. Also, by controlling the maximum surface temperature T of the steel sheet, it is possible to suppress the generation of blisters and prevent the occurrence of surface quality defects (surface quality failure) due to blisters.

(5) By casting the slab by the above-described continuous casting method of the present invention, defects caused by the inclusion of non-metallic inclusions and mold flux (so-called sliver defects) can be reduced, and the defects caused by the inclusion of minute bubbles are also reduced. However, it is difficult to surely prevent entrapment of finer bubbles (for example, those having a bubble diameter of 5 mm or less) and inclusions. Causes blistering defects (blister defects) due to hydrogen (H ₂ ). For such problems, a hot-rolled steel sheet obtained by combining the continuous casting method of the present invention with the method of (4) above, that is, rolling a slab cast by the above-described continuous casting method of the present invention. By pickling and cold rolling under the conditions of (4) above, surface defects caused by entrapment of bubbles, inclusions and mold flux, including extremely fine bubbles and inclusions of inclusions, are caused. Very few high quality steel sheets can be produced.

The manufacturing method of the steel sheet of the present invention based on such knowledge is a hot-rolled steel sheet obtained by hot rolling the slab cast by the continuous casting method of the present invention described above, and after pickling the hot-rolled steel sheet, In the hot rolling, the time t or / and the maximum surface temperature T of the steel sheet are controlled so as to satisfy the following formula (a).
Hc / Ho> exp {−0.002 × (T + t / 100)} (a)
However, Ho: Hydrogen concentration (mass ppm) in the steel plate immediately after pickling
Hc: Critical hydrogen concentration (mass ppm) in the steel sheet immediately before cold rolling, which causes surface quality defects due to blisters, determined by cold rolling conditions
t: Time from the end of pickling to the start of cold rolling (seconds)
T: Maximum surface temperature T (K) of the steel sheet after the end of pickling and before the start of cold rolling (however, this steel sheet surface temperature is the surface temperature of the steel sheet when the steel sheet is heated after the end of pickling and before cold rolling) including.)
The steel sheet manufacturing method as described above is particularly carried out in a pickling and cold rolling continuous line (PPCM line, PPCM; Pickling and Profile-Control Cold Mill) in which pickling and cold rolling are continuously performed. It is effective. This is because blisters are particularly likely to occur in the steel sheet produced in such a PPCM line.

In the following description, the actual measurement value of the hydrogen concentration of the steel plate is a value obtained by heating the steel plate to 800 ° C. and analyzing hydrogen released from the steel plate with a mass spectrometer.
Table 2 shows that in a pickling facility in which five pickling tanks are arranged in series, the hot-rolled steel sheet is pickled under various conditions, the amount of pickling reduction of the steel sheet and the hydrogen concentration Ho in the steel sheet immediately after the end of pickling. The result of the investigation is shown. FIG. 14 shows the relationship between the pickling loss and the hydrogen concentration Ho in the steel sheet immediately after the end of pickling based on the results. The pickling conditions include acid concentration, pickling temperature and time, but as shown in Table 2, the dependency of pickling loss by pickling conditions is not observed. This is presumably because the pickling loss changes depending on the surface condition (scale thickness, etc.) of the steel sheet before pickling. On the other hand, the hydrogen concentration Ho in the steel sheet immediately after the end of the pickling shows a good correlation with the pickling reduction as shown in FIG. Therefore, the hydrogen concentration Ho in the steel sheet immediately after the end of pickling can be obtained based on the pickling loss.

The hydrogen concentration Ho in the hot-rolled steel sheet immediately after the end of pickling and the steel sheet surface temperature T ₀ are measured, and the hydrogen concentration in the steel sheet at the time when time t ₁ has passed since the end of pickling. was measured H _1, the results in Table 3 were obtained. From the results in Table 3, hydrogen was released over time from the hot-rolled steel sheet that had been pickled, and the hydrogen concentration Ho (mass ppm) in the hot-rolled steel sheet, also the hydrogen concentration H ₁ (mass ppm), time t It was found that ₁ (second) and the steel sheet surface temperature T ₀ (K) are approximately related by the following equation (ii). Figure 15 shows the relationship between the hydrogen concentration _{H 1} in the steel sheet at the time has elapsed _{Ho · exp {-0.002 × (T} 0 + t 1/100)} and the time from the pickling finished _{t 1.} Here, the reason why the hydrogen concentration H ₁ in the steel sheet is influenced not only by the time t ₁ but also by the steel sheet surface temperature T ₀ immediately after the end of the pickling is that the amount of hydrogen released depends on the steel sheet temperature, particularly the highest temperature reached. It is influenced (dominated), and under the above test conditions, the steel sheet temperature (the highest temperature reached) was the highest immediately after the end of pickling.
_{H 1 = Ho · exp {-0.002} × (T 0 + t 1/100)} ... (ii)
Therefore, when the steel sheet is heated to a temperature higher than the steel sheet temperature immediately after the end of the pickling after the end of the pickling and before the start of cold rolling, the steel sheet surface temperature T ₀ of the above formula (ii) This is the steel plate surface temperature (attainable maximum temperature). This is because the amount of hydrogen released from the hot-rolled steel sheet that has been pickled is influenced (dominated) by the maximum temperature reached by the steel sheet as described above.

From the above, the hydrogen concentration H ₁ (mass ppm) in the hot-rolled steel sheet at the time point p after time t ₁ (seconds) has elapsed after the end of pickling is the hydrogen concentration in the hot-rolled steel sheet immediately after the end of pickling. The relationship between Ho (ppm by mass) and the maximum surface temperature T ₁ (K) of the steel sheet between the end of pickling and the time point p was found to be expressed by the following equation (i). Therefore, the time t ₁ in the following formula (i) is set as “time t from the end of pickling to the start of cold rolling”, and the maximum surface temperature T _{1 is set} to “the maximum of the steel sheet after the end of pickling and before the start of cold rolling”. If the surface temperature is T ”, the hydrogen concentration H in the steel sheet immediately before cold rolling can be obtained.
_{H 1 = Ho · exp {-0.002} × (T 1 + t 1/100)} ... (i)

On the other hand, whether or not surface quality defects due to blisters occur depends on the hydrogen concentration H in the steel sheet immediately before cold rolling and the cold rolling conditions (reduction conditions). Depending on the cold rolling conditions, the surface quality due to blisters It was found that the “critical hydrogen concentration Hc in the steel sheet immediately before cold rolling” at which a defect (failed surface quality) occurs was determined.
When a hot-rolled steel sheet having a thickness of 4 mm is rolled into various finished sheet thicknesses (final sheet thickness of cold rolling) by cold rolling, the hydrogen concentration H in the steel sheet immediately before cold rolling and the cold rolling When the finished plate thickness and the number of blister defects were investigated, the results shown in Table 4 were obtained. Based on this result, FIG. 16 shows the relationship between the hydrogen concentration H in the steel sheet immediately before cold rolling and the number of blister defects generated by the finished thickness of the cold rolling.

According to this, when the hydrogen concentration H in the steel plate immediately before cold rolling exceeds a certain value, it can be seen that blister defects rapidly increase. It can also be seen that the smaller the finished thickness of the cold rolling (that is, the larger the amount of cold rolling reduction), the smaller the value of the hydrogen concentration H at which blister defects increase rapidly. This is considered to be because the higher the hydrogen concentration H in the steel plate immediately before the cold rolling, and the larger the reduction amount in the cold rolling, the greater the increase in the internal pressure of the hydrogen retained in the steel plate. It is done. Here, in general, when the number of blister defects exceeds about 0.0350 × 10 ⁻² / m, surface quality defects due to blister defects become apparent, so “occurrence of surface quality defects due to blisters” (surface quality) For example, the number of blister defects can be more than 0.0350 × 10 ⁻² / m.

From the above points, it was found that the “critical hydrogen concentration Hc in the steel sheet immediately before cold rolling” in which surface quality defects due to blisters occur can be determined according to the cold rolling conditions (reduction conditions). . Specifically, the critical hydrogen concentration Hc in the steel sheet immediately before the cold rolling can be determined according to the finished sheet thickness determined by the cold rolling reduction ratio. For example, when the thickness of the hot-rolled steel sheet is 4 mm, the critical hydrogen concentration Hc in the steel sheet can be determined as follows according to each finished sheet thickness in the cold rolling based on the result of FIG. it can.
Finished sheet thickness in cold rolling Critical hydrogen concentration Hc in steel sheet
1.8mm 0.030 mass ppm
1.5mm 0.025 mass ppm
1.2mm 0.020 mass ppm

From the above, according to the cold rolling conditions, the time t from the end of pickling to the start of cold rolling and the maximum of the steel plate so that the hydrogen concentration in the steel plate immediately before cold rolling does not become the critical hydrogen concentration Hc. By controlling the surface temperature T, the occurrence of surface quality defects due to blisters can be prevented. For this reason, in this invention, when pickling a hot-rolled steel plate and cold-rolling, time t or / and the maximum surface temperature T of a steel plate are controlled so that the following (a) Formula may be satisfied.
Hc / Ho> exp {−0.002 × (T + t / 100)} (a)
However, Ho: Hydrogen concentration (mass ppm) in the steel plate immediately after pickling
Hc: Critical hydrogen concentration (mass ppm) in the steel sheet immediately before cold rolling, which causes surface quality defects due to blisters, determined by cold rolling conditions
t: Time from the end of pickling to the start of cold rolling (seconds)
T: Maximum surface temperature T (K) of the steel sheet after the end of pickling and before the start of cold rolling (however, this steel sheet surface temperature is the surface temperature of the steel sheet when the steel sheet is heated after the end of pickling and before cold rolling) including.)
In such a method of the present invention, as described above, it is necessary to predetermine the “critical hydrogen concentration Hc in the steel sheet immediately before cold rolling” according to the cold rolling conditions (reducing conditions). Moreover, it is preferable to obtain | require in advance also about the relationship between the amount of pickling and the hydrogen concentration Ho in the steel plate immediately after the end of pickling.
Further, as shown in the examples described later, the smaller the Ho · exp {−0.002 × (T + t / 100)} value with respect to the Hc value, the greater the effect of improving blister defect occurrence. = Hc value−Ho · exp {−0.002 × (T + t / 100)} value) of 0.005 or more, the occurrence of blister defects is particularly remarkably suppressed. Therefore, the Hc value−Ho · exp {−0.002 × The (T + t / 100)} value is particularly preferably 0.005 or more.

  As the hot-rolled steel sheet, a hot-rolled slab cast by the above-described continuous casting method of the present invention is used. For the reason described in (5) above, extremely fine bubbles and inclusions are used. It is possible to manufacture a high-quality steel sheet that includes blisters caused by entrainment and has very few surface defects caused by entrainment of bubbles and inclusions or mold flux.

  In order to carry out the method of the present invention, for example, the pickled steel sheet is left in a coiled state at room temperature, and cold rolling is performed after a time t that satisfies the above formula (a). Moreover, if the hot-rolled steel sheet after pickling is heated to increase the maximum surface temperature T of the steel sheet, the time t that satisfies the above formula (a) can be shortened, so that it can be applied to the PPCM line, improving productivity. Can be planned. For heating the hot-rolled steel sheet, gas burner heating, electric heater heating, high-frequency induction heating, etc. can be applied, but since cold rolling is performed thereafter, heating should be performed in an inert gas atmosphere in which the oxygen partial pressure is controlled. Is preferred. In addition, when applied to a PPCM line, the line speed can be adjusted by using a looper that can change the distance between rolls.

[Example 1]
A continuous casting machine as shown in FIG. 1 and FIG. 2, that is, a pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the long side of the mold on the outside of the mold (on the back side of the mold side wall) Using a continuous casting machine in which the molten steel discharge hole of the immersion nozzle is located between the peak position of the magnetic field of the magnetic pole and the peak position of the magnetic field of the lower magnetic pole, a direct current magnetic field applied to each of the pair of upper magnetic poles and the pair of lower magnetic poles About 300 tons of aluminum killed ultra-low carbon steel was cast by a continuous casting method that brakes the molten steel flow.

Ar gas is used as the inert gas blown from the immersion nozzle, and the amount of Ar gas blown is adjusted within a range of 5 to 12 NL / min according to the opening of the sliding nozzle so that the nozzle is not blocked. did. The specifications of the continuous casting machine and other casting conditions are as follows.
The specifications of the continuous casting machine and other casting conditions are as follows.
-Molten steel discharge angle α of the molten steel discharge hole of the immersion nozzle: 15 °
・ Immersion depth of immersion nozzle: 230 mm
-Shape of molten steel discharge hole of immersion nozzle: rectangular shape of size 70mm x 80mm-Immersion nozzle inner diameter: 80mm
-Opening area of each molten steel discharge hole of the immersion nozzle: 5600 mm ²
-Viscosity of mold flux used (1300 ° C): 2.5 cp

Molten steel having chemical components shown in Table 5 was continuously cast under the conditions shown in Tables 6 to 15.
The chemical composition of the molten steel is the analysis value of the sample taken from the molten steel at the end of refining in the RH vacuum degassing unit. The total oxygen concentration of the molten steel is collected from the molten steel in the tundish before the injection into the mold. The chemical analysis value of this sample was used.
The continuously cast slab was hot-rolled and cold-rolled into a steel plate, and this steel plate was subjected to alloying hot-dip galvanizing treatment. About this alloyed hot-dip galvanized steel sheet, surface defects are continuously measured with an on-line surface defect meter, and sliver defects (mold flux defects, bubble defects) by defect morphology (appearance), SEM analysis, ICP analysis, etc. , Inclusion physical property defects) and blister defects were determined, and the defects after Zn plating were evaluated according to the following criteria based on the number of defects per 1 m of coil length. The results are also shown in Tables 6 to 15.
○: Defect number 0.01 or less △: Defect number over 0.01, 0.05 or less ×: Defect number over 0.05, 0.10 or less XX: Defect number over 0.10 For the examples in which a slab having a slab width exceeding 1700 mm was cast, data by simulation based on the results of an actual machine was shown.

[Example 2]
Using the same equipment and method as in Example 1 (continuous casting machine, Ar gas blowing conditions, mold flux conditions, etc.), the molten steel of No. 2 chemical component in Table 5 was continuously cast under the conditions shown in Table 16.
The continuously cast slab was hot-rolled, pickled and cold-rolled into a steel plate, and this steel plate was subjected to alloying hot-dip galvanizing treatment. Among the examples shown in Table 16, in the examples No. 1 to No. 3 and No. 9 to No. 11, after the pickling, the sample was allowed to stand at room temperature for the time t shown in the table, and then cooled. Hot rolling was performed. On the other hand, in other examples, a PPCM line in which an electric heater type heating furnace is installed between the pickling equipment and the cold rolling equipment is used. It heated to surface temperature T, and cold-rolled after that.

The manufactured alloyed hot-dip galvanized steel sheet is continuously measured for surface defects using an on-line surface defect meter, and from that, sliver defects (mold flux defects, bubbles, etc.) are determined by defect morphology (appearance), SEM analysis, ICP analysis, etc. Discriminating defects, inclusion physical property defects) and blister defects, and the defects after Zn plating (total defects after Zn plating) were evaluated on the same basis as in [Example 1] based on the number of defects per 1 m of coil length. Moreover, the blister defect was evaluated on the following reference | standard among the defects after Zn plating. The results are also shown in Table 16.
A: The number of blister defects is 0.0200 × 10 ⁻² or less ○: The number of blister defects is more than 0.0200 × 10 ⁻² , 0.0250 × 10 ⁻² or less Δ: The number of blister defects is 0.0250 × More than 10 ⁻² , 0.0350 × 10 ⁻² or less ×: The number of blister defects exceeds 0.0350 × 10 ⁻²

No. 1 to No. 16 in this example satisfy the continuous casting conditions of the present invention. On the other hand, No.2, No.3, No.5, No.7, No.8, No.10, No.11, No.13, No.15, No.16 are the production conditions of the steel sheet of the present invention. While No. 1, No. 4, No. 6, No. 9, No. 12, and No. 14 do not satisfy the formula (a). According to this example, it can be seen that those satisfying the formula (a) which is the production condition of the steel sheet of the present invention can more effectively suppress the occurrence of blister defects.
In addition, the smaller the Ho · exp {−0.002 × (T + t / 100)} value relative to the Hc value, the greater the effect of improving blister defect generation. In particular, the Hc value and Ho · exp {−0.002 × (T + t / 100) )} Value is 0.005 or more, it can be seen that the occurrence of blister defects is particularly suppressed.

DESCRIPTION OF SYMBOLS 1 Mold 2 Immersion nozzle 3a, 3b Upper magnetic pole 4a, 4b Lower magnetic pole 5 Solidified shell 6 Meniscus 10 Mold long side 11 Mold short side 20 Molten steel discharge hole 21 Bottom

Claims (19)

A submerged nozzle having a pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the long side of the mold on the outside of the mold, and having a molten steel discharge angle of 10 ° or more and less than 30 ° downward from the horizontal direction of the molten steel discharge hole Using the continuous casting machine in which the molten steel discharge hole is located between the peak position of the magnetic field of the upper magnetic pole and the peak position of the magnetic field of the lower magnetic pole, and each of the pair of upper magnetic poles and the pair of lower magnetic poles. While continuously casting ultra-low carbon steel containing 0.003% by mass or less of C while braking the molten steel flow by the applied DC magnetic field,
A molten steel having a chemical component with an X value defined by the following formula (1) satisfying X ≦ 5000,
X = 24989 × [% Ti] +386 147 × [% S] + 853354 × [% O] (1)
[% Ti]: Ti content in molten steel (mass%)
[% S]: S content in molten steel (mass%)
[% O]: O content (% by mass) in molten steel
Continuous casting at a casting speed of 0.75 m / min or more according to the following conditions (a) and (b)
Condition (A): When the slab width to be cast and the casting speed are the following (a) to (i), the strength of the DC magnetic field applied to the upper magnetic pole is 0.03 to 0.15 T, and is applied to the lower magnetic pole. The intensity of the DC magnetic field is 0.24 to 0.45T.
(A) Slab width of less than 950 mm and casting speed of less than 2.05 (b) Slab width of 950 mm to less than 1050 mm and casting speed of less than 2.25 m / min (c) Slab width of from 1050 mm to less than 1350 mm and casting speed of 2.35 m (D) Slab width of 1350 mm or more and less than 1450 mm and casting speed of less than 2.25 m / min (e) Slab width of 1450 mm or more and less than 1650 mm and casting speed of less than 2.15 m / min (f) Slab width of 1650 mm or more and less than 1750 mm (G) Slab width of 1750 mm or more and less than 1850 mm and casting speed of less than 1.95 m / min (h) Slab width of 1850 mm or more and less than 1950 mm and casting speed of less than 1.85 m / min (i ) The slab width is 1950 mm or more and less than 2150 mm and the casting speed is 1.75 m / min.・ Condition (b): When the cast slab width and casting speed are the following (j) to (s), the strength of the DC magnetic field applied to the upper magnetic pole is more than 0.15T and less than 0.30T, The strength of the DC magnetic field to be applied is 0.24 to 0.45T.
(J) Slab width of less than 950 mm and casting speed of 2.05 m / min or more and 3.05 m / min or less (k) Slab width of 950 mm or more and less than 1050 mm and casting speed of 2.25 m / min or more and 3.05 m / min or less (l ) Slab width of 1050 mm or more and less than 1350 mm and casting speed of 2.35 m / min or more and 3.05 m / min or less (m) Slab width of 1350 mm or more and less than 1450 mm and casting speed of 2.25 m / min or more and 3.05 m / min or less (n ) Slab width of 1450 mm to less than 1550 mm and casting speed of 2.15 m / min to 3.05 m / min (o) Slab width of 1550 mm to less than 1650 mm and casting speed of 2.15 m / min to 2.85 m / min (p ) Slab width of 1650 mm or more and less than 1750 mm and casting speed of 2.05 m / min or more and 2.65 m / min or less (q) Slab width 175 mm or more and less than 1850 mm and casting speed of 1.95 m / min or more and 2.55 m / min or less (r) Slab width of 1850 mm or more and less than 1950 mm and casting speed of 1.85 m / min or more and 2.55 m / min or less (s) Slab width 1950 mm or more and less than 2150 mm and casting speed 1.75 m / min or more and 2.55 m / min or less When the cast slab is hot-rolled into a hot-rolled steel sheet, the hot-rolled steel sheet is pickled, and then cold-rolled. The method for producing a steel sheet, wherein the time t or / and the maximum surface temperature T of the steel sheet are controlled so as to satisfy the following expression (a).
Hc / Ho> exp {−0.002 × (T + t / 100)} (a)
However, Ho: Hydrogen concentration (mass ppm) in the steel plate immediately after pickling
Hc: Critical hydrogen concentration (mass ppm) in the steel sheet immediately before cold rolling, which causes surface quality defects due to blisters, determined by cold rolling conditions
t: Time from the end of pickling to the start of cold rolling (seconds)
T: Maximum surface temperature (K) of the steel plate after the end of pickling and before the start of cold rolling (however, the surface temperature of the steel plate is the surface temperature of the steel plate when heated after the end of pickling and before cold rolling) Including)   The method for producing a steel sheet according to claim 1, wherein after hot pickling, the hot rolled steel sheet before cold rolling is heated to a temperature higher than the steel sheet temperature immediately after the end of pickling. The molten steel in the mold of the continuous casting machine has a surface turbulent energy of 0.0010 to 0.0015 m ² / s ² , a surface flow velocity of 0.30 m / s or less, and a flow velocity at the molten steel-solidified shell interface of 0.08 to It is 0.15 m / s, The manufacturing method of the steel plate of Claim 1 or 2 characterized by the above-mentioned.   The method for producing a steel sheet according to claim 3, wherein the molten steel in the mold of the continuous casting machine has a surface flow velocity of 0.05 to 0.30 m / s.   The molten steel in the mold of the continuous casting machine is characterized in that the ratio A / B of the flow velocity A and the surface flow velocity B at the molten steel-solidified shell interface is 1.0 to 2.0. The manufacturing method of the steel plate of description. The method for producing a steel sheet according to any one of claims 3 to 5, wherein the molten steel in the mold of the continuous casting machine has a bubble concentration at the molten steel-solidified shell interface of 0.008 kg / m ³ or less.   The slab thickness cast by the continuous casting machine is 220 to 300 mm, and the amount of inert gas blown from the inner wall surface of the immersion nozzle of the continuous casting machine is 3 to 25 NL / min. A method of manufacturing a steel sheet.   The method for producing a steel sheet according to any one of claims 1 to 7, wherein the nozzle immersion depth of the immersion nozzle of the continuous casting machine is 230 to 290 mm.   The method for producing a steel sheet according to any one of claims 1 to 8, wherein the nozzle inner diameter of the immersion nozzle of the continuous casting machine (however, the nozzle inner diameter at the position where the molten steel discharge hole is formed) is 70 to 90 mm. Method for producing a steel sheet according to any one of claims 1 to 9 the opening area of each molten steel discharge hole of the immersion nozzle of a continuous casting machine, characterized in that the 3600~8100mm ^2. A submerged nozzle having a pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the long side of the mold on the outside of the mold, and having a molten steel discharge angle of 10 ° or more and less than 30 ° downward from the horizontal direction of the molten steel discharge hole Using the continuous casting machine in which the molten steel discharge hole is located between the peak position of the magnetic field of the upper magnetic pole and the peak position of the magnetic field of the lower magnetic pole, and each of the pair of upper magnetic poles and the pair of lower magnetic poles. A method of continuously casting an ultra-low carbon steel containing 0.003% by mass or less of C while braking a molten steel flow by an applied DC magnetic field,
A molten steel having a chemical component with an X value defined by the following formula (1) satisfying X ≦ 5000,
X = 24989 × [% Ti] +386 147 × [% S] + 853354 × [% O] (1)
[% Ti]: Ti content in molten steel (mass%)
[% S]: S content in molten steel (mass%)
[% O]: O content (% by mass) in molten steel
A continuous casting method for steel, characterized by continuous casting at a casting speed of 0.75 m / min or more according to the following conditions (a) and (b).
Condition (A): When the slab width to be cast and the casting speed are the following (a) to (i), the strength of the DC magnetic field applied to the upper magnetic pole is 0.03 to 0.15 T, and is applied to the lower magnetic pole. The intensity of the DC magnetic field is 0.24 to 0.45T.
(A) Slab width of less than 950 mm and casting speed of less than 2.05 (b) Slab width of 950 mm to less than 1050 mm and casting speed of less than 2.25 m / min (c) Slab width of from 1050 mm to less than 1350 mm and casting speed of 2.35 m (D) Slab width of 1350 mm or more and less than 1450 mm and casting speed of less than 2.25 m / min (e) Slab width of 1450 mm or more and less than 1650 mm and casting speed of less than 2.15 m / min (f) Slab width of 1650 mm or more and less than 1750 mm (G) Slab width of 1750 mm or more and less than 1850 mm and casting speed of less than 1.95 m / min (h) Slab width of 1850 mm or more and less than 1950 mm and casting speed of less than 1.85 m / min (i ) The slab width is 1950 mm or more and less than 2150 mm and the casting speed is 1.75 m / min.・ Condition (b): When the cast slab width and casting speed are the following (j) to (s), the strength of the DC magnetic field applied to the upper magnetic pole is more than 0.15T and less than 0.30T, The strength of the DC magnetic field to be applied is 0.24 to 0.45T.
(J) Slab width of less than 950 mm and casting speed of 2.05 m / min or more and 3.05 m / min or less (k) Slab width of 950 mm or more and less than 1050 mm and casting speed of 2.25 m / min or more and 3.05 m / min or less (l ) Slab width of 1050 mm or more and less than 1350 mm and casting speed of 2.35 m / min or more and 3.05 m / min or less (m) Slab width of 1350 mm or more and less than 1450 mm and casting speed of 2.25 m / min or more and 3.05 m / min or less (n ) Slab width of 1450 mm to less than 1550 mm and casting speed of 2.15 m / min to 3.05 m / min (o) Slab width of 1550 mm to less than 1650 mm and casting speed of 2.15 m / min to 2.85 m / min (p ) Slab width of 1650 mm or more and less than 1750 mm and casting speed of 2.05 m / min or more and 2.65 m / min or less (q) Slab width 175 mm or more and less than 1850 mm and casting speed of 1.95 m / min or more and 2.55 m / min or less (r) Slab width of 1850 mm or more and less than 1950 mm and casting speed of 1.85 m / min or more and 2.55 m / min or less (s) Slab width 1950 mm or more and less than 2150 mm and casting speed 1.75 m / min or more and 2.55 m / min or less The molten steel in the mold has a surface turbulent energy of 0.0010 to 0.0015 m ² / s ² , a surface flow velocity of 0.30 m / s or less, and a flow velocity at the molten steel-solidified shell interface of 0.08 to 0.15 m / s. It is s, The continuous casting method of steel of Claim 11 characterized by the above-mentioned.   The continuous casting method of steel according to claim 12, wherein the molten steel in the mold has a surface flow velocity of 0.05 to 0.30 m / s.   14. The steel according to claim 12, wherein the molten steel in the mold has a ratio A / B of the flow velocity A at the molten steel-solidified shell interface to the surface flow velocity B of 1.0 to 2.0. Continuous casting method. The continuous casting method for steel according to any one of claims 12 to 14, wherein the molten steel in the mold has a bubble concentration at the molten steel-solidified shell interface of 0.008 kg / m ³ or less.   The continuous casting method of steel according to claim 15, wherein the slab thickness to be cast is 220 to 300 mm, and the amount of inert gas blown from the inner wall surface of the immersion nozzle is 3 to 25 NL / min.   The continuous casting method for steel according to any one of claims 11 to 16, wherein the immersion depth of the immersion nozzle is 230 to 290 mm.   The continuous casting method of steel according to any one of claims 11 to 17, wherein a nozzle inner diameter of the immersion nozzle (however, a nozzle inner diameter at a position where a molten steel discharge hole is formed) is 70 to 90 mm. The continuous casting method of steel according to claim 11, wherein an opening area of each molten steel discharge hole of the immersion nozzle is 3600 to 8100 mm ² .

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