JP4821933B2 - Steel plate manufacturing method - Google Patents

Description

  The present invention relates to a manufacturing method for manufacturing a steel sheet in which surface defects such as blisters are unlikely to occur.

  In recent years, requirements for surface quality are becoming more severe for cold-rolled steel sheets for automobiles and cans or surface-treated steel sheets obtained by subjecting cold-rolled steel sheets to plating treatment. In particular, surface defects called blisters that appear as blisters in the range of several millimeters on the steel sheet surface are opened by processing, causing cracks and deterioration of corrosion resistance. If discovered, product shipment may be canceled, which causes a significant decrease in yield.

As explained in Non-Patent Document 1, cold-rolled steel sheet blisters penetrate into the steel sheet during pickling after hot rolling, and after cold rolling, non-metallic inclusions, bubbles, segregation, internal cracks in the steel sheet. This is a blister-like surface defect formed by deforming a steel sheet softened by heating.
As a method for suppressing the occurrence of such blisters, Patent Document 1 discloses a cold-rolled steel sheet made of Ti-containing ultra-low carbon steel having a specific component composition and having a predetermined thickness determined by the relationship with the component composition. A method for producing a cold-rolled steel sheet or a hot-dip galvanized steel sheet that is subjected to continuous annealing or continuous hot-dip galvanizing under predetermined conditions is disclosed.

Masanori Kowaka, "Metal Corrosion Damage and Anticorrosion Technology", Agne, 1983, p.207

Japanese Patent No. 2936963

However, although the method described in Patent Document 1 is considered to be effective for releasing hydrogen existing in the surface layer portion of the steel sheet, it may not be possible to suppress blisters generated by hydrogen diffused into the steel sheet as bubbles. .
Recently, along with stricter quality of steel sheets for automobile outer panels, defects caused by entrapment of minute bubbles and mold flux that have not been a problem until now are becoming a problem. The casting method 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 the surface layer of the base material steel plate has bubbles or mold flux defects, even if it is a small defect, the thickness of the plating layer is uneven, which appears as a streak defect on the surface, such as an automobile outer plate It cannot be used for applications with severe quality requirements.

  Therefore, the objective of this invention is providing the manufacturing method of the steel plate which can manufacture stably the high quality steel plate which is hard to produce surface defects, such as a blister.

As described above, defects in cold-rolled steel sheets called blisters penetrate into the steel sheet during pickling after hot rolling, and after cold rolling into non-metallic inclusions, bubbles, segregation, internal cracks, etc. in the steel sheet. The staying hydrogen is a blister-like surface defect that deforms the steel sheet softened by heating by volume expansion with heating during annealing to increase the 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) When manufacturing a steel plate under the above condition (4), using a continuous casting machine with a pair of upper and lower magnetic poles for braking the molten steel flow by electromagnetic force outside the mold, A slab is continuously cast by controlling the flow of molten steel in the mold by a DC magnetic field applied to each of the lower magnetic pole and a hot-rolled steel sheet obtained by rolling the slab to produce a particularly high-quality steel sheet. can do. The reason is as follows. By casting the slab while controlling the molten steel flow in the mold using a DC magnetic field as described above, it is possible to prevent mold flux defects caused by entrainment of the mold flux, and relatively large bubbles and non-metallic inclusions. Defects caused by things 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. As a starting point, a blistering defect (blister defect) due to hydrogen (H ₂ ) is caused. On the other hand, if pickling and cold rolling are performed under the conditions of (4) above, blisters caused by minute bubbles and non-metallic inclusions can be suppressed, and this effect and defect prevention by the above continuous casting method By combining the effects, it is possible to produce a high-quality steel sheet with particularly few defects.

  Also, in the above continuous casting method, by optimizing the strength of the DC magnetic field applied to the upper magnetic pole and the lower magnetic pole according to the slab width and casting speed to be cast, it is possible to reduce the bubble defects, inclusion physical defects and mold flux properties. It is possible to effectively suppress the occurrence of defects, and therefore, by using a slab cast by such a casting method, it includes blisters caused by the inclusion of extremely fine bubbles and non-metallic inclusions Further, it is possible to manufacture a high-quality steel sheet having very few surface defects due to entrapment of bubbles, non-metallic inclusions and mold flux.

The present invention has been made based on these findings, and has the following gist.
[1] as a template outside, with obtain Bei the lower magnetic pole of the upper magnetic pole and a pair of a pair of opposite sides of the mold long side portions, a downward molten steel discharge angle from the horizontal direction of the molten steel discharge hole 10 ° or more 30 ° comprising a submerged nozzle below, using a continuous casting machine in which the molten steel discharge hole is positioned between the peak position of the magnetic field of the lower magnetic pole and the peak position of the magnetic field of the upper magnetic pole, the upper magnetic pole and a pair of said pair Casting the slab by braking the molten steel flow with a direct current magnetic field applied to the lower magnetic pole, setting the casting speed to 0.75 m / min or more, and continuously casting the steel according to the following conditions (a) and (b) And
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 less than 950 mm and casting speed less than 2.05
(B) Slab width of 950 mm or more and less than 1050 mm and casting speed of less than 2.25 m / min.
(C) Slab width of 1050 mm or more and less than 1350 mm and casting speed of less than 2.35 m / min
(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 and casting speed of less than 2.05 m / min
(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) Slab width of 1950 mm or more and less than 2150 mm and casting speed of less than 1.75 m / min
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 strength of the DC magnetic field to be set is 0.24 to 0.45T.
(J) The slab width is less than 950 mm and the casting speed is 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 or more and less than 1550 mm and casting speed of 2.15 m / min or more and 3.05 m / min or less
(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) The slab width is 1650 mm or more and less than 1750 mm and the casting speed is 2.05 m / min or more and 2.65 m / min or less.
(Q) Slab width of 1750 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) The slab width is 1850 mm or more and less than 1950 mm and the casting speed is 1.85 m / min or more and 2.55 m / min or less.
(S) Slab width of 1950 mm to less than 2150 mm and casting speed of 1.75 m / min to 2.55 m / min
When the cast slab is hot-rolled to form a hot-rolled steel sheet, the hot-rolled steel sheet is pickled, and then cold-rolled, the time t or / and the maximum of the steel sheet is satisfied so that the following equation (1) is satisfied. A method for producing a steel sheet, wherein the surface temperature T is controlled.
Hc / Ho> exp {−0.002 × (T + t / 100)} (1)
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] A method for producing a steel sheet according to [1], wherein the hot-rolled steel sheet after pickling and before cold rolling is heated to a temperature higher than the steel sheet temperature immediately after the end of pickling.

[3] A method for producing a steel sheet according to [1] or [2], wherein the nozzle immersion depth of the immersion nozzle is 230 to 290 mm.
[4] In the manufacturing method according to any one of [1] to [3] , the inner diameter of the immersion nozzle (however, the inner diameter of the nozzle at the position where the molten steel discharge hole is formed) is 70 to 90 mm. Manufacturing method.
[5] A method for manufacturing a steel sheet according to any one of the above [1] to [4] , wherein an opening area of each molten steel discharge hole of the immersion nozzle is 3600 to 8100 mm ² .
[6] In the manufacturing method according to any one of [1] to [5 ] above, the molten steel in the mold has a surface turbulent energy of 0.0010 to 0.0015 m ² / s ² and a surface flow velocity of 0.30 m / s. Hereinafter, the manufacturing method of the steel plate characterized by the flow rate in a molten steel-solidified shell interface being 0.08-0.15 m / s.

[7] The method for producing a steel sheet according to [6] , wherein the molten steel in the mold has a surface flow velocity of 0.05 to 0.30 m / s.
[8] In the manufacturing method of [6] or [7 ] above, the molten steel in the mold has a ratio A / B between the flow velocity A and the surface flow velocity B at the molten steel-solidified shell interface of 1.0 to 2.0. A method for producing a steel sheet, comprising:
[9] In the manufacturing method according to any one of [6] to [8] , 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. Manufacturing method.
[10] In the manufacturing method of [9 ] above, a steel plate characterized in that 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. Production method.

According to the steel sheet manufacturing method of the present invention, a high-quality cold-rolled steel sheet that is less likely to cause defects such as blisters can be stably manufactured.
Moreover, according to the slab width and casting speed to be cast, the slab was cast by optimizing the strength of the DC magnetic field applied to each of the upper magnetic pole and the lower magnetic pole, and a hot-rolled steel sheet obtained by rolling this slab was used. In some cases, producing high-quality steel sheets with very few surface defects due to entrapment of bubbles and non-metallic inclusions or mold flux, including blisters resulting from entrapment of extremely small bubbles and non-metallic inclusions Can do.

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. 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. Graph showing the relationship between the molten steel discharge angle of the immersion nozzle and the occurrence rate (defect index) of surface defects 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

  In the method for producing a steel sheet of the present invention, 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 peak position of the magnetic field of the upper magnetic pole and the magnetic field of the lower magnetic pole are provided. Continuous casting of steel using a continuous casting machine in which the molten steel discharge hole is located between the peak positions of the steel plate and the molten steel flow is braked by a DC magnetic field applied to each of the pair of upper magnetic poles and the pair of lower magnetic poles. To cast a slab. And this slab is hot-rolled to make a hot-rolled steel sheet, and after pickling this hot-rolled steel sheet, it is cold-rolled to make a cold-rolled steel sheet.

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 manufacturing method of the steel sheet of the present invention, the slab cast by the continuous casting method described above is hot-rolled to form a hot-rolled steel sheet, and after pickling this hot-rolled steel sheet, it is cold-rolled. The time t or / and the maximum surface temperature T of the steel sheet are controlled so as to satisfy the following expression (1).
Hc / Ho> exp {−0.002 × (T + t / 100)} (1)
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 1 shows the pickling equipment in which five pickling tanks are arranged in series. The hot-rolled steel sheet is pickled under various conditions, and the amount of pickling of the steel sheet and the hydrogen concentration Ho in the steel sheet immediately after the end of pickling are shown. The result of the investigation is shown. FIG. 3 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 1, no dependency on pickling loss due to pickling conditions is 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 loss 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. It was measured H _1, Table 2 results. From the results in Table 2, 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, the hydrogen concentration H ₁ (mass ppm), and the 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 4 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 3 were obtained. Based on this result, FIG. 5 shows the relationship between the hydrogen concentration H in the steel sheet immediately before the 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 (1) Formula may be satisfied.
Hc / Ho> exp {−0.002 × (T + t / 100)} (1)
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 (preferably, a specific continuous casting method described later) is used, but for the reason described in (5) above. , Including blisters caused by entrapment of extremely fine bubbles and non-metallic inclusions (hereinafter sometimes referred to simply as “inclusions”), and defects caused by entrapment of bubbles and non-metallic inclusions or mold flux are extremely Fewer high quality steel sheets can be produced.

  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 (1). Also, 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 equation (1) can be shortened, so 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.

  In the present invention, the slab is continuously cast while controlling the molten steel flow in the mold by the DC magnetic field applied to each of the upper magnetic pole and the lower magnetic pole, and the hot rolled steel sheet obtained by rolling the slab is used. By casting the slab while controlling the flow of molten steel in the mold using a DC magnetic field as described above, it is possible to prevent flux defects caused by entrainment of mold flux, and relatively large bubbles and non-metallic inclusions. Can prevent defects. However, in this continuous casting method, it is difficult to reliably prevent finer bubbles and non-metallic inclusions from being trapped by the solidified shell, and blisters resulting from the inclusion of such fine bubbles and inclusions are generated. There is a fear. On the other hand, if pickling and cold rolling are performed under the conditions of the present invention, blisters caused by fine bubbles and non-metallic inclusions can be suppressed, and this effect and the defect prevention effect by the continuous casting method are combined. Therefore, it is possible to manufacture a high-quality steel sheet having particularly few defects.

  In the above continuous casting method, by optimizing the strength of the DC magnetic field applied to the upper magnetic pole and the lower magnetic pole according to the slab width and casting speed to be cast, It is possible to effectively suppress the occurrence of both, therefore, by using a slab cast by such a casting method, including blisters caused by entrapment of extremely fine bubbles and non-metallic inclusions, It is possible to manufacture a high-quality steel sheet with very few surface defects due to entrapment of bubbles, non-metallic inclusions and mold flux.

Hereinafter, the continuous casting method will be described.
As a result of investigation by the present inventor regarding the continuous casting method through numerical simulations, as a factor (primary factor) involved in the generation of bubble defects, inclusion physical defects and mold flux defects, surface turbulent energy (near the surface) ), The molten steel flow rate at the molten steel-solidified shell interface (hereinafter sometimes simply referred to as “solidification interface”), the surface flow velocity, and the like. Has been found to affect 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.

Specifically, if the strength of the DC magnetic field applied to the upper magnetic pole and the lower magnetic pole is optimized according to the slab width and casting speed to be cast, basically as in (I) and (II) below. I found it good.
(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 the DC magnetic field as described above, the surface turbulent energy, the solidification interface flow velocity and the surface flow velocity are controlled within an appropriate range to prevent the occurrence of bubble defects, inclusion physical defects and mold flux 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, the surface turbulent energy, the solidification interface flow velocity and the surface flow velocity are controlled within appropriate ranges, thereby preventing the occurrence of bubble defects and mold flux defects.

  In this casting method, it is preferable to use 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 °. 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 °, It was found that the molten steel flow can be optimized by direct current magnetic field control. 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. 6 shows the relationship between the molten steel discharge angle α of the immersion nozzle and the occurrence rate of surface defects (defect index). The magnetic field strength, casting speed and slab width under the conditions (A) and (B) described later are as follows. A continuous casting test is performed under various conditions satisfying the preferred range, and the continuously cast slab is hot-rolled and cold-rolled into a steel plate, which is subjected to alloying hot-dip galvanizing treatment, and a molten steel discharge angle α This is a study of the effect of the surface on the occurrence of surface defects. 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

  In this casting method, the casting speed is set to 0.75 m / min or more from the viewpoint of productivity. Further, the casting speed is 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, respectively. 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. By applying the DC magnetic field as described above, even a fine inclusion or bubble can be appropriately prevented from being captured by the solidified shell.

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. By applying the DC magnetic field as described above, even a fine inclusion or bubble can be appropriately prevented from being captured by the solidified shell.

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 described above can also be grasped as two continuous casting methods such as the following (i) and (ii) defined according to the slab width and the casting speed.
(I) In continuous casting of a slab, using a continuous casting machine equipped with an immersion nozzle 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, at a casting speed of 0.75 m / min or more. The slab width and casting speed are any one of the following conditions (a) to (i), the intensity of the DC magnetic field applied to the upper magnetic pole is 0.03 to 0.15 T, and the intensity of the DC magnetic field applied to the lower magnetic pole is Continuous casting method for continuous casting as 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

(Ii) In continuous casting of slabs, using a continuous casting machine equipped with an immersion nozzle 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, at a casting speed of 0.75 m / min or more. The slab width and casting speed are any one of the following conditions (j) to (s), the intensity of the DC magnetic field applied to the upper magnetic pole is more than 0.15T and not more than 0.30T, and the intensity of the DC magnetic field applied to the lower magnetic pole Continuous casting method for continuously casting at 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 effect is most easily manifested in the continuous casting method 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. 7 shows the results 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. 7, in the method of the present invention, it can be seen that, particularly when the nozzle immersion depth of the immersion nozzle 2 is 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. 8 shows the result 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, and the molten steel discharge angle α of the immersion steel discharge hole α: 15 °. , 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. 8, 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. 9 shows the results of examining the effect of the opening area of each molten steel discharge hole of the immersion nozzle 2 (influence on mold flux defects and bubble defects) 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. 9, 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 continuous casting in the present invention, a control computer is used to set in advance the DC current value to be applied to each DC magnetic field coil of the upper magnetic pole and lower magnetic pole based on the slab width to be cast and the casting speed. It is preferable to automatically control the strength of the DC magnetic field applied to each of the upper magnetic pole and the lower magnetic pole by obtaining the reference table or formula and applying the DC current. Also, the casting conditions based on which the current value is obtained include the molten steel discharge angle and immersion depth from the horizontal direction of the molten steel discharge hole of the immersion nozzle (however, the distance from the meniscus to the upper end of the molten steel discharge hole), slab thickness Alternatively, an inert gas blowing amount from the inner wall surface of the immersion nozzle may be added.

FIG. 10 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. 11 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. 11, 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 the mold flux does not have a problem.

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. 12 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 the same method as in the examples 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. 12, 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. 13 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 Examples described later). Other conditions are surface turbulence. 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. 13, 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. 14 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 a method similar to the example 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. 14, the surface quality defects are 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. 15 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. 15, 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.

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.
Further, some of the comparative examples were carried out without braking the molten steel flow by applying the DC magnetic field as described above.

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

  The molten steel was continuously cast under the conditions shown in Tables 4 to 13. The continuously cast slab was hot-rolled to obtain a hot-rolled steel sheet (thickness 2.8 to 4.5 mm), pickled with hydrochloric acid, and then cold-rolled to obtain a cold-rolled steel sheet. Among the examples, the steel sheet temperature T (the maximum surface temperature of the steel sheet after the end of pickling and before the start of cold rolling) was 346 K or less, the examples in which heating after pickling was not performed, the ones exceeding 346 K were Ar It is the Example which performed the heating (heating of a coil) with the heating furnace of gas atmosphere. The hydrogen concentration Ho in the steel sheet immediately after the end of pickling was obtained in advance based on the actual pickling reduction amount, for example, by previously obtaining the relationship between the pickling reduction amount and the hydrogen concentration as shown in FIG. Further, the critical hydrogen concentration Hc was determined based on data as shown in FIG. 5 in accordance with the thickness of the hot-rolled steel plate and the finished thickness of the cold rolling.

The obtained cold-rolled steel sheet was subjected to an 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 total number of defects per 1 m of coil length.
○: The number of defects is 0.01 or less △: The number of defects is more than 0.01, 0.05 or less ×: The number of defects is more than 0.05, 0.10 or less, or The number of defects is 0.05 or less, but the number of blister defects is more than 0.0350 × 10 ⁻² / m XX: The number of defects is more than 0.10

For blister defects, the number of defects per 1 m of coil length was evaluated according to the following criteria.
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 and 0.0250 × 10 ⁻² or less Δ: The number of blister defects is 0.0250 × 10 ⁻² or more, 0.0350 × 10 ⁻² or less ×: blister defect number exceeding 0.0350 × 10 ^{−2 The} above evaluation results are shown in Table 4 to Table 13. Also shown. In addition, about the Example which casts the slab whose slab width exceeds 1700 mm, the data by the simulation based on the result of an actual machine were shown.

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 21 Bottom 20 Molten steel discharge hole

Claims (10)

The mold outer, with obtain Bei the lower magnetic pole of the upper magnetic pole and a pair of a pair of opposite sides of the mold long side portions, immersed in the horizontal direction of the molten steel discharge hole downward of the molten steel discharge angle of less than 10 ° or more 30 ° a nozzle, using a continuous casting machine which is located said molten steel discharge hole during the peak position of the magnetic field and the peak position of the magnetic field of the upper magnetic pole the lower magnetic pole, the lower magnetic pole of the upper magnetic pole and a pair of said pair Casting the slab by braking the molten steel flow by each applied DC magnetic field, setting the casting speed to 0.75 m / min or more, and performing continuous casting of steel 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 less than 950 mm and casting speed less than 2.05
(B) Slab width of 950 mm or more and less than 1050 mm and casting speed of less than 2.25 m / min.
(C) Slab width of 1050 mm or more and less than 1350 mm and casting speed of less than 2.35 m / min
(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 and casting speed of less than 2.05 m / min
(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) Slab width of 1950 mm or more and less than 2150 mm and casting speed of less than 1.75 m / min
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 strength of the DC magnetic field to be set is 0.24 to 0.45T.
(J) The slab width is less than 950 mm and the casting speed is 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 or more and less than 1550 mm and casting speed of 2.15 m / min or more and 3.05 m / min or less
(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) The slab width is 1650 mm or more and less than 1750 mm and the casting speed is 2.05 m / min or more and 2.65 m / min or less.
(Q) Slab width of 1750 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) The slab width is 1850 mm or more and less than 1950 mm and the casting speed is 1.85 m / min or more and 2.55 m / min or less.
(S) Slab width of 1950 mm to less than 2150 mm and casting speed of 1.75 m / min to 2.55 m / min
When the cast slab is hot-rolled to form a hot-rolled steel sheet, the hot-rolled steel sheet is pickled, and then cold-rolled, the time t or / and the maximum of the steel sheet is satisfied so that the following equation (1) is satisfied. A method for producing a steel sheet, wherein the surface temperature T is controlled.
Hc / Ho> exp {−0.002 × (T + t / 100)} (1)
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 method for producing a steel sheet according to claim 1 or 2, wherein the immersion depth of the immersion nozzle is 230 to 290 mm. The method for manufacturing a steel sheet according to any one of claims 1 to 3 , wherein 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. The manufacturing method of the steel plate according to any one of claims 1 to 4 , wherein an opening area of each molten steel discharge hole of the immersion nozzle is 3600 to 8100 mm ² . 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 manufacturing method of the steel plate in any one of Claims 1-5 characterized by the above-mentioned. The method for producing a steel sheet according to claim 6 , wherein the molten steel in the mold has a surface flow velocity of 0.05 to 0.30 m / s. In the mold molten steel, molten steel - the ratio A / B between the flow rate A and the surface velocity B of the solidification shell interface of the steel sheet according to claim 6 or 7, characterized in that 1.0 to 2.0 Production method. The method for producing a steel plate according to any one of claims 6 to 8 , 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 method for producing a steel sheet according to claim 9 , 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.

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