JP2011098388A - Continuous casting method for steel and extra-thick steep plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method capable of producing a slab having stable inside quality by suppressing the occurrence of fine cracks caused by rolling reduction in the boundary between a negatively-segregated part and a bulk part. <P>SOLUTION: Regarding the continuous casting method in which a slab including an un-solidified part is subjected to rolling reduction using at least a pair of rolling reduction rolls arranged in a continuous casting machine or at the edges of the machine, and, solidified shells on both the sides in the thickness direction of the slab are press-stuck to form a negatively-segregated part in the vicinity of the central part in the thickness direction of the slab after solidification, a product between the thickness D(mm) of the negatively-segregated part in the thickness direction of the slab and the content of an S component [S](ppm) in the steel is allowed to satisfy 0<[S]×D≤60. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention suppresses the center segregation of the product and the negative segregation part at the center of the product rolling material thickness in continuous casting in which the slab including the unsolidified part is reduced using a reduction roll in the continuous casting machine or at the machine end. The present invention relates to a method for preventing the occurrence of fine cracks at the boundary between the surface and the bulk (base material) part, and an extremely thick steel sheet in which central segregation is suppressed and no such fine cracks are present.

  Conventionally, for the purpose of improving the internal quality of a continuous cast slab, a technology (hereinafter referred to as "" Many have also been proposed.

  In the patent document 1, the present inventors have also bulged a slab including an unsolidified portion, and then, in a continuous casting machine, the lower roll of the pair of reduction rolls is projected beyond the lower pass line of the slab. A continuous casting method of steel for rolling down was proposed. Under unsolidified pressure of the slab, molten steel enriched with easily segregated components such as C, Mn, P, and S is discharged to the liquid phase side by reduction, and component segregation at the center in the thickness direction of the slab is improved. Is done.

  Furthermore, the present inventors proposed improvement and stabilization of internal quality by improving segregation in the width direction of the central portion in the thickness direction in Patent Document 2 regarding unsolidification reduction of the slab.

The technique described in Patent Document 2 adjusts the reduction amount of the slab according to the degree of superheat (ΔT) of the molten steel in the tundish, and the component segregation ratio existing at both ends of the slab width direction is 1.00. The continuous casting of steel, wherein each length (W) in the slab width direction of the segregation zone of 1.20 or less is within a range satisfying the relationship represented by the following formula (A) Is the method. Here, Wo represents the slab width, and d represents the thickness of the solidified shell on the short side of the slab at the slab reduction position.
0 ≦ W ≦ 0.2 × (Wo−2 × d) (A)

  Patent Document 2 discloses a continuous casting method in which molten steel in an unsolidified portion is stirred in the slab width direction by applying electromagnetic force, and an electromagnetic stirring device for applying electromagnetic force is provided at a slab pressure reduction position. The technique of arrange | positioning in the position within 9 m toward the casting direction upstream is also described.

  On the other hand, when casting an alloy steel containing 5 to 20% by mass of Cr and 0.05 to 0.3% by mass of C or a molten steel of ferritic stainless steel, and rolling down a steel ingot or slab containing an unsolidified part When producing an intermediate material such as a billet by heating a steel ingot or slab formed with a negative segregation part in a high temperature state or after being cooled to near room temperature and then hot working In some cases, a negative segregation portion remains in these hot-worked billets, a coarse ferrite structure is generated in the remaining negative segregation portion, and fine cracks are generated along the ferrite structure. In the patent document 3, the present inventors have proposed low hydrogenation of molten steel as a method for preventing fine cracks along this ferrite structure.

  The technique described in Patent Document 3 casts molten steel of ferritic stainless steel and reduces the outer surface of the steel ingot or slab in the thickness direction while an unsolidified portion exists in the steel ingot or slab. In the casting method, a negative segregation part is formed near the center in the thickness direction inside the steel ingot or cast slab after solidification by pressing the solidified shells on both sides in the thickness direction inside the steel ingot or cast slab. And it is a casting method of the high Cr content steel characterized by making the hydrogen content rate of the said molten steel into 4 ppm or less.

Japanese Patent No. 4218383 Japanese Patent Application No. 2008-116548 Japanese Patent No. 3671868

  As described above, the present inventors have proposed a continuous casting method in Patent Document 1 and Patent Document 2 in which center segregation is improved by unsolidification reduction of a slab. As a result of repeated tests for stabilizing the casting by these processes, the present inventors subsequently reduced the rolling at the boundary between the negative segregation portion and the bulk (base material) portion formed in the central portion in the thickness direction. It has been found that there is a tendency for fine cracks (hereinafter, also simply referred to as “fine cracks”) to occur. Moreover, as a result of testing about various conditions in order to suppress this fine crack, it discovered that it was insufficient only to make the hydrogen content rate of molten steel into a low value as described in patent document 3. FIG.

  The present invention has been made in view of this problem, and the problem is that the center segregation is improved by the unsolidification reduction of the continuous cast slab, and the fineness due to the reduction at the boundary between the negative segregation portion and the bulk portion. An object of the present invention is to provide a continuous casting method capable of producing a slab having a stable internal quality while suppressing the occurrence of cracks, and a very thick steel plate having a stable internal quality.

  As a result of repeated tests to suppress fine cracks, the present inventors have found that it is necessary to set the thickness of the negative segregation part and the content of the S component in the steel to appropriate values.

  The present invention has been completed on the basis of the above-mentioned findings, and has the gist of the following (1) steel continuous casting method and the following (2) extra-thick steel plate.

(1) By squeezing a slab including an unsolidified portion using at least one pair of squeezing rolls arranged in a continuous casting machine or at the end of the slab, and pressing the solidified shells on both sides in the thickness direction of the slab , A continuous casting method in which a negative segregation part is formed in the vicinity of the central part in the thickness direction of the cast slab after solidification, and the thickness D (mm) of the negative segregation part in the thickness direction of the slab and the S component content in the steel A continuous casting method characterized in that the product [S] · D of [S] (ppm) satisfies the following formula (1).
0 <[S] · D ≦ 60 (1)

(2) By squeezing the slab including the unsolidified portion using at least one pair of squeezing rolls arranged in the continuous casting machine or at the end of the slab, and pressing the solidified shells on both sides in the thickness direction of the slab The slab manufactured by a continuous casting method in which a negative segregation part is formed in the vicinity of the central part in the thickness direction of the slab after solidification is a very thick steel plate obtained by rolling down, and An extra-thick steel plate, wherein the product [S] · Dp of the thickness Dp (mm) of the negative segregation part and the S component content [S] (ppm) in the steel satisfies the following formula (2).
0 <[S] · Dp ≦ 60 / α (2)
Here, α represents the total reduction ratio (dimensionless number) (≡ slab thickness / thickness of extra heavy steel plate) when the slab is reduced to extra heavy steel plate.

In the extra-thick steel plate (2), the number of MnS having a maximum length of 1 μm or more in a portion other than the negative segregation portion is preferably ² / mm ² or less.

  In the continuous casting method and extra-thick steel plate of (1) and (2), the slab is mass%, C: 0.05 to 0.3%, Si: 0.05 to 0.4%, Mn: 0.2-2%, P: 0.020% or less, S: 0.003% or less, and sol. Al: 0.1% or less, Mo: 1.5% or less, Ni: 1.5-3.0%, Cr: 5% or less, Cu: 1.5% or less, Ti: 0.00%. It is preferably an alloy steel containing one or more of 1% or less, Nb: 0.1% or less, and V: 0.1% or less, with the balance being Fe and impurities.

  In the present invention, “negative segregation” means the content C (mass%) of components such as C, Mn, P, and S at an arbitrary position of a slab or extra-thick steel plate, and the average content of each component during casting. (Hereinafter also referred to as “base material average content”) A state where the segregation ratio (C / Co), which is a value divided by Co (mass%), is smaller than 1, and the content C at that position is the average base material content. It means lower than the rate Co. In the description of the present specification, “positive segregation” means a state where the segregation ratio is larger than 1, and means that the component content C at that position is higher than the base material average content Co.

In the description of the present specification, the “solid phase ratio” means the ratio of the solid phase to the total amount of the solid phase and the liquid phase at the center of the slab. Further, the “number of inclusions” means the number of inclusions having a maximum length of 1 μm or more per 1 mm ² in a cross section of a cast slab or extra-thick steel plate unless otherwise specified.

  In the following description, “mass%” representing the component composition of steel is also simply expressed as “%”.

  According to the continuous casting method of the present invention, since it is possible to prevent the occurrence of microcracking in the vicinity of the center portion in the thickness direction of a slab having a negative segregation portion that has undergone unsolidification reduction, steady reduction casting with stable internal quality is possible. Pieces can be manufactured with high yield and productivity. Moreover, the extra-thick steel plate of the present invention has high internal quality.

It is a figure which shows the outline of the longitudinal cross-section of a vertical bending type continuous casting machine. It is a cross-sectional view of the slab which shows the cutting position of the sample for mapping analysis. It is a cross-sectional view of the rolling sample showing the cut-out position of the micro sample, the figure (a) shows the entire cross-section of the slab, and the figure (b) shows the observation surface of the sample. It is a figure which shows the number of the MnS inclusion in the rolling sample which the micro crack generate | occur | produced. It is a figure which shows the number of the MnS inclusion in the rolling sample in which the fine crack did not generate | occur | produce. It is a figure which shows the relationship between product [S] * D of S component content rate [S] in steel, and thickness D of the negative segregation part of a slab, and the fine crack generation defect rate. It is a figure which shows the relationship between the number of the MnS inclusion of the bulk part after implementation of a dehydrogenation process countermeasure, and a fine crack defect generation index.

In the continuous casting method of the present invention, a slab including an unsolidified portion is reduced by using at least one pair of reduction rolls disposed in the continuous casting machine or at the end of the slab, and solidified shells on both sides in the thickness direction of the slab. Is a continuous casting method in which a negative segregation part is formed in the vicinity of the central part in the thickness direction of the cast slab after solidification, and the thickness D (mm) of the negative segregation part in the slab thickness direction and steel A product [S] · D of medium S component content [S] (ppm) satisfies the following formula (1).
0 <[S] · D ≦ 60 (1)

1. Basic Configuration of Continuous Casting Method FIG. 1 is a diagram schematically showing a longitudinal section of a vertical bending type continuous casting machine for carrying out the present invention.

  Molten steel 4 is supplied to the tundish 1a from a ladle (not shown). Molten steel 4 injected from the tundish 1a through the immersion nozzle 1b to form a molten steel surface (meniscus) 2 in the mold 3 is sprayed from the mold 3 and a group of secondary cooling spray nozzles (not shown) below the mold 3. The slab 8 is formed by forming a solidified shell 5 by cooling with spray water. The slab 8 is pulled out while being supported by the guide roll group 6 including the driven roll 6 a and the driving roll 6 b while holding the unsolidified portion 10 therein, and is squeezed by the roll pair 7. The position where the reduction roll pair 7 is installed may be either the inside of the continuous casting machine or the end on the downstream side in the casting direction.

  The guide roll 6 group is arranged so that the interval in the thickness direction of the slab 8 can be controlled to a predetermined value. Therefore, as shown in FIG. 1, after bulging when the unsolidified portion 10 exists inside the slab 8, the thickness t of the central portion in the width direction is made larger than the short side length t0 of the slab 8. It is also possible to reduce the central portion in the width direction by the reduction roll 7.

  At the start of continuous casting, a head of a dummy bar (not shown) is inserted into the bottom of the bottomless mold 3 to form a temporary bottom, and then molten steel 4 is poured into the mold 3 from the immersion nozzle 1. Then, when the molten steel surface 2 in the mold 3 reaches a preset position and the solidified shell 5 having a predetermined thickness is formed, the dummy bar starts to be extracted, and the extraction speed is increased to increase the predetermined casting speed. Transition to (steady state).

  The dummy bar is a jig that connects the unit blocks by pin connection. The method of inserting the dummy bar into the mold 3 includes a method of inserting from the top (top insertion method) and a rear side of a pinch roll (not shown) arranged on the downstream side in the casting direction with respect to the rolling roll pair 7. There is a method (bottom insertion method), and a dummy bar head located at the upper end of the dummy bar is arranged in the mold 3. And after casting starts and it transfers to a steady state, it is taken out diagonally upward on the back | latter stage side of a pinch roll.

2. Relationship between the fine crack, the thickness of the negative segregation part of the slab and the S component content in the steel In the continuous casting method of the present invention, the solidified shells on both sides in the thickness direction of the slab are pressure-bonded. A negative segregation part is formed near the center of the slab in the thickness direction, and the product of the thickness D (mm) of the negative segregation part in the slab thickness direction and the S component content (ppm) in the steel [S] · D (Ppm · mm) is set so as to satisfy the following expression (1). Thereby, generation | occurrence | production of the fine crack in the boundary part of the negative segregation part formed in the thickness direction center part vicinity of a slab and a bulk part can be prevented.
0 <[S] · D ≦ 60 (1)

Further, the extra-thick steel plate of the present invention has a product [S] · Dp (ppm · ppm) of the thickness Dp (mm) of the negative segregation portion in the thickness direction of the extra-thick steel plate and the S component content [S] (ppm) in the steel. mm) is a steel plate that satisfies the above-mentioned formula (2) and has a thickness of 100 mm or more. As a result, it is possible to obtain a very thick steel plate having excellent internal quality without a fine crack at the boundary with the bulk portion.
0 <[S] · Dp ≦ 60 / α (2)

  Here, α represents the total reduction ratio (dimensionless number). Specifically, α is a value obtained by dividing the thickness of the slab before rolling by the thickness of the extra-thick steel plate after rolling, and satisfies α ≧ 1. Since there is a relationship of α = D / Dp, the equation (2) can be obtained by dividing each side of the equation (1) by α.

3. Steel type of slab suitable for the present invention In the present invention, C: 0.05-0.3%, Si: 0.05-0.4%, Mn: 0.2-2% , P: 0.020% or less, S: 0.003% or less, and sol. Al: 0.1% or less, Mo: 1.5% or less, Ni: 1.5-3.0%, Cr: 5% or less, Cu: 1.5% or less, Ti: 0.00%. An alloy steel containing one or more of 1% or less, Nb: 0.1% or less, and V: 0.1% or less, with the balance being Fe and impurities is preferable. The effect of the present invention for preventing fine cracks in the slab is particularly effective in this alloy steel.

  Below, the test conducted in order to complete this invention and the test conducted in order to confirm the effect of this invention are demonstrated.

1. Test method 1-1. Casting Test Method A casting test was conducted using the vertical bending type continuous casting machine shown in FIG. A pair of reduction rolls 7 of the continuous casting machine was installed at a position 21.5 m downstream from the molten steel surface 2 in the mold 3 in the casting direction. The diameter of each rolling roll was 470 mm, and the maximum rolling force was 5.88 × 10 ⁶ N (600 tf).

  The steel types used in the casting test are: C: 0.05 to 0.3%, Si: 0.05 to 0.4%, Mn: 0.2 to 2%, P: 0.020% or less, S: 0 0.003% or less, sol. Al: 0.1% or less, Mo: 1.5% or less, Ni: 1.5-3.0%, Cr: 5% or less, Cu: 1.5% or less, Ti: 0.00%. An alloy steel containing one or more of 1% or less, Nb: 0.1% or less, and V: 0.1% or less, with the balance being Fe and impurities. Using this alloy steel, a slab having a thickness of 300 mm and a width of 2250 mm was produced. The casting speed was constant at 0.70 m / min, and the secondary cooling specific water amount was 0.40 to 0.58 L / kg-steel. Moreover, the molten steel superheat degree ((DELTA) T; the value which reduced the liquidus temperature of this alloy steel from the temperature of molten steel) in the tundish was 30-40 degreeC.

  In practicing the present invention, it goes without saying that the hydrogen content of the molten steel is preferably as small as possible. The present inventors set the hydrogen content of molten steel to 1.3 ppm or less when performing a series of tests.

  In the continuous casting apparatus shown in FIG. 1, even when the slab 8 is bulged, it can be cast in the same manner as when bulging is not performed. Even when the slab thickness is changed by bulging the slab 8, heat transfer and solidification calculations are performed under various conditions of changing the casting speed according to the thickness of the central portion of the slab 8 in the width direction. The casting speed condition where the solid phase ratio becomes a predetermined value at the position of the rolling roll pair 7 is calculated, and casting is performed under this casting speed condition.

  In the casting test, when the steady solidified portion of the slab 8 including the unsolidified portion 10 having a predetermined solid phase ratio reached the position of the reduction roll pair 7, the reduction by the upper reduction roll was started. After the start of reduction, the amount of protrusion of the lower reduction roll upward from the lower pass line 11 of the slab 8 becomes the reduction amount of the slab by the lower reduction roll.

1-2. 2. Evaluation method of slab component segregation FIG. 2 is a cross-sectional view of a slab showing the cut-out position of the sample for mapping analysis. A slab sample having a length of 150 mm in the casting direction was cut out from the slab obtained by the above casting test. And after observing the macro structure of the cross section of a slab, the MA sample for mapping analysis by EPMA (henceforth "MA analysis") was cut out from the position shown in FIG.

  The MA sample is a rectangular parallelepiped having a length of 100 mm in the slab thickness direction, a length of 40 mm in the casting direction, and a thickness (length in the slab width direction) of 9 mm, and from one short side of the slab to the slab width direction. 1/4, 1/2, and 3/4 positions of the slab width W (referred to as “1 / 4W”, “1 / 2W”, and “3 / 4W” in FIG. 5, respectively), and both short sides It was cut out from a total of five locations where segregation components tend to concentrate at the center in the slab thickness direction on the side (both are denoted as “end portions” in FIG. 2).

  The field of view for performing the MA analysis was a rectangular range of 50 mm in the slab thickness direction including the center of the slab thickness direction of the MA sample and 20 mm in the slab width direction. After MA analysis was performed with a beam diameter of 50 μm to obtain the Mn component distribution in the above-mentioned area, line analysis was performed with a width of 2 mm in the slab thickness direction, and the Mn content C at the center of the slab thickness direction was determined. The component segregation ratio (C / Co) was calculated by dividing the value of C by the average content ratio Co of Mn at the time of casting. Hereinafter, in this example, when simply referred to as “negative segregation part”, it refers to a negative segregation part of Mn.

1-3. Method for evaluating the amount of MnS inclusions

  FIG. 3 is a cross-sectional view of the extra-thick steel plate showing the cut-out position of the sample for scanning electron microscope observation. FIG. 3 (a) shows the entire cross-section of the extra-thick steel plate, and FIG. An observation surface is shown.

  The present inventors also performed MA analysis on a very thick steel plate (hereinafter also referred to as “rolled sample”) as a product obtained by rolling a slab. As a result, MnS inclusions having a maximum length of about 10 μm were observed around the negative segregation part. Moreover, there were a steel plate in which fine cracks occurred around the negative segregation part and a steel plate in which fine cracks did not occur.

  Therefore, detailed investigation of inclusions using SEM (Scanning Electron Microscope) was performed as follows. A micro sample 13 (length in the steel plate thickness direction: 35 mm, length in the steel plate width direction: 10 mm) shown in FIG. ) 12a and negative segregation part 12b were collected. After the micro sample 13 was polished, gold was vapor-deposited on the surface, and the inclusions were observed using SEM at the bulk part, the bulk-negative segregation boundary on the top side, and the center of the negative segregation part in the thickness direction of the steel sheet. And conducted a survey. FIG. 6B shows three lines observed using the SEM.

  Observation of the micro sample was performed at a magnification of 250 times, scanning with a steel plate thickness direction of about 350 μm (0.35 mm), and a steel plate width direction with a sample width of 10 mm. All the inclusions detected by observation were observed with magnification of the field of view from 1000 times to a maximum of 3000 times, and quantitative analysis was performed. After determining the type of inclusions based on the result of quantitative analysis, the number of inclusions of MnS was obtained, and this number was divided by the observation visual field area of 0.35 mm × 10 mm and converted to the number per unit area. .

  With such a procedure, the number of MnS inclusions in the rolled sample in which fine cracks occurred in the negative segregation part and the bulk part, bulk-negative segregation boundary part, and negative segregation part of the rolled sample that did not occur was investigated.

  FIG. 4 is a diagram showing the number of MnS inclusions in a rolled sample in which fine cracks occurred, and FIG. 5 is a diagram showing the number of MnS inclusions in a rolled sample in which fine cracks did not occur. From FIG. 4 and FIG. 5, there is a tendency that both the bulk sample and the boundary portion have more MnS inclusions than the negative segregation portion in the rolled sample in which fine cracks occurred and the rolled sample in which fine cracks did not occur. It was. However, as can be seen from FIG. 4 and FIG. 5, the absolute amount of inclusions of MnS was larger in the rolled sample in which the microcrack was generated than in the rolled sample in which no microcrack occurred.

2. 2. Relationship between inclusions of MnS and S component content [S] in steel 2-1. Knowledge Regarding Inclusions of MnS The present inventors have studied based on the knowledge about the casting method of high Cr-containing steel obtained in Patent Document 3, and obtained the following knowledge.

  C is an austenite (hereinafter referred to as “γ”) stabilizing element. In addition, the slab that has been squeezed has a reduced C content in the negative segregation portion, and becomes rich in ferrite (hereinafter referred to as “α”). Therefore, the slab that has been reduced exhibits a state where the α-rich structure constituting the negative segregation part is sandwiched between the γ structures constituting the bulk part.

  In high-temperature slabs, hydrogen is dissolved in both γ and α. However, as the temperature decreases in the subsequent cooling process, the hydrogen solubility in γ and α decreases, and hydrogen that is finally in solid solution may reach the solid solution limit. Hydrogen atoms that have reached the solid solution limit in the negative segregation part having a low hydrogen solubility and a high hydrogen diffusion rate diffuse to the bulk part through a bulk-negative segregation boundary having a higher hydrogen solubility than the negative segregation part. However, since the diffusion rate of hydrogen in the bulk part is smaller than that in the negative segregation part, hydrogen atoms that move from the negative segregation part toward the bulk part cannot be diffused into the bulk part at the bulk-negative segregation boundary. It remains as hydrogen gas in the fine MnS present. And when this slab is hot-worked, the pressure of the hydrogen gas in MnS will become high and a crack will generate | occur | produce in a slab.

  In response to this finding, the inventors have stated that “the amount of MnS produced is proportional to the average content [S] of the S component in the steel, and the amount of hydrogen diffusing from the negative segregation part to the bulk part is that of the negative segregation part. We hypothesized that it is proportional to the thickness, and focused on the thickness of the negative segregation part.

2-2. Investigation of the effect of negative segregation thickness on slab cracking In order to investigate the effect of negative segregation thickness on slab cracking, two levels of conditions shown in Table 1 differing in the amount of reduction and specific water The test was performed by changing the thickness of the negative segregation part of the slab. The obtained slab was subjected to MA analysis and the thickness of the negative segregation part was measured. As a result, the average in case 1 was 6.01 mm and in case 2 was 10.9 mm, and the thickness of the negative segregation part can be controlled. did it. Also, each slab was rolled and cut to obtain a plurality of rolled cutting materials, and each rolled cutting material was examined for occurrence of fine cracks. The number of rolled cut materials) / (number of rolled cut materials) × 100 (%)) was 0.0% in case 1 and 54.7% in case 2.

FIG. 6 is a diagram showing the relationship between the product [S] · D of the S component content [S] in steel and the thickness D of the negative segregation portion of the slab, and the fine crack occurrence defect rate. Under the casting conditions shown in Table 1, the S component content [S] (ppm) in the steel is further varied, and the product [S] · D of [S] and the thickness D (mm) of the negative segregation portion of the slab. The relationship between (ppm · mm) and the incidence of fine crack defects was investigated. As a result, as shown in FIG. 6, it was found that the occurrence of fine cracks was suppressed when [S] · D was 60 ppm · mm or less, and the fine cracks markedly increased when it exceeded 60. This is the basis of the following formula (1) included in the invention according to the above (1).
0 <[S] · D ≦ 60 (1)

Moreover, when the thickness of the negative segregation part in the extra-thick steel plate after the slab is reduced at the total reduction ratio α is Dp, the relationship between D and Dp is expressed by the following equation (a).
D = α · Dp (a)

By substituting the expression (a) into the expression (1), the following expression (b) is obtained, and by dividing each side of the expression (b) by α, the following expression (2) is obtained. This is the basis of the following formula (2) included in the invention according to the above (2) for the extra-thick steel plate.
0 <[S] · α · Dp ≦ 60 (b)
＜ 0 <[S] · Dp ≦ 60 / α (2)

3. The relationship between the number of MnS inclusions and the occurrence of fine cracks In order to further suppress the occurrence of fine cracks, the present inventors have performed dehydrogenation treatment that reduces hydrogen in steel by diffusion, and S that becomes MnS as CaS. A casting and rolling test was performed in which CaSi treatment for fixing and reducing MnS was performed. A sample was taken from the ultra-thick steel plate having a thickness of 100 mm or more thus obtained, and the number of MnS inclusions having a maximum bulk-negative segregation boundary portion of 1 μm or more was observed using the SEM described above. Counted by law.

  FIG. 7 is a diagram showing the relationship between the number of MnS inclusions in the bulk part after the implementation of the dehydrogenation process countermeasure and the microcrack defect occurrence index. The occurrence of fine crack defects was evaluated by the fine crack defect index. The microcrack defect index (dimensionless number) is a three-stage index (2: removal of wrinkles (fine cracks) by maintenance) that evaluates the result of microcracking evaluated by dye penetration inspection on the cut surface of an extremely thick steel plate product. Impossible, 1: removal of wrinkles by care, 0: no wrinkles).

From FIG. 7, the present inventors show that the smaller the number of MnS inclusions, the smaller the fine crack defect index, that is, the number of MnS inclusions represented by the bulk-negative segregation boundary portion and the occurrence of fine crack defects. We found that there is a correlation between Furthermore, from these results and the results shown in FIGS. 2 and 3, it was found that the occurrence of fine cracks can be prevented by setting the number of MnS inclusions having a maximum length of 1 μm or more to 2 pieces / mm ² or less. It was. Based on these findings, the inventors have proposed the invention described in (3) above.

  According to the continuous casting method of the present invention, it is possible to prevent the occurrence of microcracking near the center in the thickness direction of a slab having a negative segregation portion, which has been subjected to unsolidified reduction during steady state reduction, and thus stable internal quality. It is possible to manufacture a slab having a high yield and productivity.

  Therefore, the method of the present invention is a technique that can be applied to a steel sheet as a continuous casting method that can exert an excellent effect on the production of a slab for a steel product for an extremely thick product having a thickness of 100 mm or more. .

  Moreover, the extra-thick steel plate of the present invention has high internal quality.

1a: tundish, 1b: immersion nozzle, 2: molten steel surface (meniscus),
3: mold, 4: molten steel, 5: solidified shell, 6: guide roll group,
6a: guide roll (driven roll), 6b: guide roll (drive roll),
7: Pair of rolling rolls, 8: Slab, 10: Unsolidified part, 11: Lower pass line,
12: Extra-thick steel plate, 13: Micro sample

Claims (4)

After solidification, the slab containing the unsolidified part is reduced using at least one pair of reduction rolls arranged in the continuous casting machine or at the end of the slab, and the solidified shells on both sides in the thickness direction of the slab are pressed. A continuous casting method in which a negative segregation portion is formed in the vicinity of the center portion in the thickness direction of the slab,
The product [S] · D of the thickness D (mm) of the negative segregation part in the slab thickness direction and the S component content [S] (ppm) in the steel satisfies the following formula (1). A continuous casting method characterized by the above.
0 <[S] · D ≦ 60 (1) After solidification, the slab containing the unsolidified part is reduced using at least one pair of reduction rolls arranged in the continuous casting machine or at the end of the slab, and the solidified shells on both sides in the thickness direction of the slab are pressed. A slab manufactured by a continuous casting method in which a negative segregation part is formed in the vicinity of the center of the slab in the thickness direction.
The product [S] · Dp of the thickness Dp (mm) of the negative segregation part in the thickness direction of the extra-thick steel plate and the S component content [S] (ppm) in the steel satisfies the following formula (2): Extra heavy steel sheet.
0 <[S] · Dp ≦ 60 / α (2)
Here, α represents the total reduction ratio (dimensionless number) (≡ slab thickness / thickness of extra heavy steel plate) when the slab is reduced to extra heavy steel plate. The extra-thick steel plate according to claim 2, wherein the number of MnS having a maximum length of 1 µm or more in a portion other than the negative segregation portion is 2 pieces / mm ² or less.   The slab is in mass%, C: 0.05 to 0.3%, Si: 0.05 to 0.4%, Mn: 0.2 to 2%, P: 0.020% or less, S: 0 0.003% or less and sol. Al: 0.1% or less, Mo: 1.5% or less, Ni: 1.5-3.0%, Cr: 5% or less, Cu: 1.5% or less, Ti: 0.00%. 2. The alloy steel according to claim 1, comprising one or more of 1% or less, Nb: 0.1% or less, and V: 0.1% or less, with the balance being Fe and impurities. The extra-thick steel plate according to claim 2 or 3, or a continuous casting method.

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