JP2016176110A - Carbon steel slab and method for producing the carbon steel slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon steel slab in which crystal grains can be stably refined in γ solidified steel, and excellent in various properties.SOLUTION: Provided is a carbon steel slab having a composition containing prescribed amounts of C, Si, Mn, P, S, T.Al, Ca, Zr and T.O, and the balance Fe with inevitable impurities, and satisfying 0.2≤[%Ca]/[%T.O]≤0.8 and [%Zr]≥4×[%Ca]×(3.4×[%T.Al]+1.0), in which inclusions made of ZrO-CaO based composite oxide are dispersed, the average composition of the inclusions satisfies 0≤(%AlO)≤18, 0.04≤(%CaO)/(%ZrO)≤0.22, where, in the inclusions, the average value of the cross-sectional area ratio in the CaO concentration is 11% or lower is 67% or higher, and the inclusions having a size in a major axis of 1 to 10 μm are dispersed within the range of 10 to 500 pieces/mmby piece density.SELECTED DRAWING: None

Description

  The present invention relates to a carbon steel slab comprising a γ-solidified steel in which a γ-phase (γ-Fe) is crystallized as an initial crystal upon solidification, and a method for producing the carbon steel slab having a finely solidified structure. is there.

In general, carbon steel slabs made of the above-mentioned γ-solidified steel are widely used as materials for machine parts, tools, blades, and the like.
The above-described carbon steel slab is manufactured by a continuous casting method or an ingot-making method, but at the time of casting, an additive element may be concentrated in the final solidified portion to cause center segregation. When center segregation occurs, coarse carbides and sulfides are generated, which may deteriorate the characteristics of the carbon steel slab. Further, the surface crack of the carbon steel slab may occur due to the cooling of the solidification process and uneven solidification shrinkage during casting.

In order to reduce the center segregation and surface cracks in the carbon steel slab, it is known that it is effective to make the solidified structure of the carbon steel slab finer to equiaxed crystals.
As means for reducing the solidification structure of the carbon steel slab, for example, Patent Document 1 proposes a method of performing low temperature casting by lowering the molten steel temperature during casting.
Patent Document 2 proposes a method of improving equiaxed crystal ratio by electromagnetically stirring molten steel in a mold to suppress columnar crystal growth in the mold.

However, when performing low-temperature casting as in Patent Document 1, there is a problem that the nozzle through which the molten steel passes is likely to be blocked and the operation is not stable. In addition, the floatability of inclusions in the mold deteriorates, and the cleanliness of the carbon steel slab may be reduced. Furthermore, there is a possibility that the solidified structure cannot be stably refined.
In addition, when the molten steel in the mold is electromagnetically stirred as in Patent Document 2, it is possible to refine the solidification structure in the central portion of the thickness of the carbon steel slab, but the solidification near the surface layer of the slab It was difficult to refine the structure.

Therefore, in Patent Documents 3 to 6, for γ-solidified steel in which a γ-phase (γ-Fe) crystallizes as an initial crystal during solidification, sulfides that become solidification nuclei (inoculation nuclei) in the molten steel, oxidation There has been proposed a method of generating equiaxed crystals by generating inclusions such as inclusions.
Here, in Patent Document 3, as inclusions becomes solidified core (inoculation _{nuclei), MgS, ZrO 2, Ti} 2 O 3, CeO 2, Ce 2 O 3 is shown.
Further, in Patent Document 4 to 6, as inclusions becomes solidified core (inoculation nuclei), ZrO ₂ is shown.
These inclusions have high lattice matching with the γ phase (γ-Fe) and have a high inoculation effect on γ-solidified steel.

Japanese Patent Publication No. 07-84617 Japanese Patent Laid-Open No. 50-16616 JP 2001-347349 A JP 2002-321043 A JP 2002-302738 A JP 2002-363702 A

By the way, in order to increase the equiaxed crystal ratio of the slab, it is important to uniformly disperse as many inoculum nuclei as possible. In general, since oxides have poor wettability with molten steel, when oxides collide and come into contact with each other in the molten steel due to flow or the like, they aggregate and coalesce and individual oxides tend to become coarse. In addition to the decrease in the number due to agglomeration, the rising speed of the coarsened oxide in the molten steel is large, so it is easily removed from the molten steel by absorbing it by the slag on the molten metal surface. The number decreases more and more. Moreover, even if the oxide is finely dispersed in the molten steel, if it takes a long time to cast, the floating removal of the oxide proceeds and the number decreases. Thus, it is difficult to finely disperse many oxides having poor wettability in the molten steel and further in the slab. In response to this problem, Patent Document 4 shows that when Zr is added to generate ZrO ₂ as an inoculum nucleus, it is added in multiple portions, and the number density of ZrO ₂ increases in the vicinity of the addition site. It has been proposed to avoid and reduce the frequency of aggregation coalescence. However, since the wettability (aggregation coalescence property) between ZrO ₂ and molten steel is not improved, the dispersion behavior of ZrO ₂ has not been greatly improved.

Further, the crystal structure of ZrO _{2 at} the molten steel temperature is tetragonal, and the lattice constants of the bottom a-axis and the side c-axis are different (lattice constant a = b ≠ c). Since tetragonal ZrO ₂ has lower symmetry than the face-centered cubic fcc (a = b = c) of the crystal structure of γ-Fe, the effect as an inoculum nucleus on γ-Fe was limited.

  The present invention has been made in view of the above-described situation, and is made of a γ-solidified steel in which a γ-phase (γ-Fe) is crystallized as a primary crystal when solidified, and the solidified structure is sufficiently refined and the center An object of the present invention is to provide a high-quality carbon steel slab in which segregation and surface cracking are suppressed, and a method for producing the carbon steel slab.

In order to solve the above problems, the carbon steel slab according to the present invention is, in mass%, C: more than 0.50% and not more than 1.50%, Si: 0.01% or more and 1.2% or less, Mn: 0.01% to 3.0%; P: 0.0010% to 0.050%; S: 0.0001% to 0.0100%; Al: 0.001% to 0.30%; Ca: 0.0002% to 0.0040%; Zr: 0.0010% to 0.25%; O: 0.0005% or more and 0.0050% or less, the balance being Fe and inevitable impurities, O content [% T.V. O], Ca content [% Ca], Zr content [% Zr], T.O. Al content [% T.V. Al] satisfies the following formulas (1) and (2):
0.2 ≦ [% Ca] / [% T.P. O] ≦ 0.8 (1)
[% Zr] ≧ 4 × [% Ca] × (3.4 × [% T.Al] +1.0) (2)
Inclusions made of ZrO ₂ —CaO-based composite oxide are dispersed inside, and the average composition of the inclusions satisfies the following formulas (3) and (4):
0 ≦ (% Al ₂ O ₃ ) ≦ 18 (3)
0.04 ≦ (% CaO) / (% ZrO ₂ ) ≦ 0.22 (4)
Among the inclusions, the average value of the cross-sectional area ratio in a region where the CaO concentration is 11% or less is 67% or more, and the inclusions having a size of a major axis of 1 μm or more and 10 μm or less are 10 / mm ² or more in number density It is characterized by being dispersed within a range of 500 pieces / mm ² or less.

According to the carbon steel slab of this configuration, inclusions made of a ZrO ₂ —CaO-based composite oxide are dispersed inside, and the average composition of the inclusions is the above-described formulas (3) and (4) I am satisfied with the formula. As will be described later, the ZrO ₂ —CaO based composite oxide having this composition is formed by adding Ca after Al deoxidation to form a CaO—Al ₂ O ₃ based liquid phase oxide, and then adding Zr, It is formed by modifying this CaO—Al ₂ O ₃ liquid phase oxide. In the inclusion made of this ZrO ₂ —CaO-based composite oxide, the average value of the cross-sectional area ratio in the region where the CaO concentration is 11% or less is set to 67% or more. Since the region where the CaO concentration is 11% or less in this ZrO ₂ —CaO-based composite oxide is a cubic crystal, the overall lattice inclusions with the γ phase (γ-Fe) become high. Yes.
Since inclusions having a major axis of 1 μm or more and 10 μm or less made of a ZrO ₂ —CaO-based composite oxide are dispersed within a number density of 10 / mm ² or more and 500 / mm ² or less, The structure can be reliably refined. Therefore, a high quality carbon steel slab without center segregation or surface cracks can be obtained.

The method for producing a carbon steel slab according to the present invention is a method for producing a carbon steel slab for producing the above-described carbon steel slab, wherein an Al deoxidation step of adding Al to the molten steel for deoxidation, A Ca addition step of adding Ca after Al deoxidation, a Zr addition step of adding Zr after Ca addition, and a casting step of casting the molten steel whose components have been adjusted, the Al deoxidation step and the Ca addition step result, in the molten steel together with dispersing _CaO-Al 2 _{O 3} based liquid phase oxides, by the addition of Zr step, is reacted with the _CaO-Al 2 _{O 3} based liquid phase oxides and Zr, the (3 ) And the inclusions composed of a ZrO ₂ —CaO-based composite oxide having an average composition satisfying the formula (4).

In the method for producing a carbon steel slab of this configuration, an Al deoxidation step in which Al is added to molten steel to perform deoxidation, a Ca addition step in which Ca is added after Al deoxidation, and Zr is added after Ca addition. A CaO—Al ₂ O ₃ liquid phase oxide is formed by adding Ca after Al deoxidation. Since this CaO—Al ₂ O ₃ -based liquid phase oxide has better wettability with molten steel than solid phase oxide, it can be widely dispersed in molten steel.
By this _CaO-Al 2 _{O 3} based liquid phase oxides are added Ca against molten steel are dispersed, the _CaO-Al 2 _{O 3} based liquid phase oxides by reforming, gamma phase A large number of inclusions composed of the above-mentioned ZrO ₂ —CaO-based composite oxide having a high lattice matching with (γ-Fe) can be finely dispersed in the molten steel, and the inoculation effect can be reliably exhibited, The solidified structure can be reliably refined.

  As described above, according to the present invention, the solidified steel is made of a γ-solidified steel in which a γ-phase (γ-Fe) is crystallized as an initial crystal when solidified, and the solidified structure is sufficiently refined to suppress center segregation and surface cracking It is possible to provide a manufactured high quality carbon steel slab and a method for producing the carbon steel slab.

Al ₂ O ₃ is -CaO-ZrO ₂ ternary phase diagram. (Note that each component concentration is expressed in mol%. On the other hand, in this specification, it is described using mass%.) It is the elements on larger scale of FIG. It is a flowchart of the manufacturing method of the carbon steel slab which is embodiment of this invention.

  Below, the manufacturing method of the carbon steel slab and the carbon steel slab which is embodiment of this invention is demonstrated with reference to attached drawing. In addition, this invention is not limited to the following embodiment.

The carbon steel slab according to this embodiment has a composition of mass%, C: more than 0.50% and 1.50% or less, Si: 0.01% or more and 1.2% or less, Mn: 0.01 % To 3.0%, P: 0.0010% to 0.050%, S: 0.0001% to 0.0100%, Al: 0.001% to 0.30%; Ca: 0.0002% to 0.0040%; Zr: 0.0010% to 0.25%; O: 0.0005% or more and 0.0050% or less, and the balance is made Fe and inevitable impurities.
In addition, T.W. O content [% T.V. O], Ca content [% Ca], Zr content [% Zr], T.O. Al content [% T.V. Al] satisfies the following formulas (1) and (2).
0.2 ≦ [% Ca] / [% T.P. O] ≦ 0.8 (1)
[% Zr] ≧ 4 × [% Ca] × (3.4 × [% T.Al] +1.0) (2)

In the carbon steel slab according to the present embodiment, inclusions made of a ZrO ₂ —CaO-based composite oxide are dispersed inside, and the inclusions made of this ZrO ₂ —CaO-based composite oxide are The average composition satisfies the following formulas (3) and (4).
0 ≦ (% Al ₂ O ₃ ) ≦ 18 (3)
0.04 ≦ (% CaO) / (% ZrO ₂ ) ≦ 0.22 (4)
In the present embodiment. Inclusions having a major axis of 1 μm or more and 10 μm or less made of a ZrO ₂ —CaO-based composite oxide are dispersed within a number density of 10 / mm ² or more and 500 / mm ² or less.

Here, in the present embodiment, inclusions made of the above-described ZrO ₂ —CaO-based composite oxide are dispersed in the molten steel, so that these inclusions act as inoculation nuclei during solidification, Equiaxial crystallization of carbon steel slab is aimed at. That is, in the carbon steel piece of this embodiment, since the inclusions composed of the fine ZrO ₂ —CaO-based composite oxide are dispersed as described above, the solidified structure is sufficiently refined. Become.
Hereinafter, inclusions consisting of ZrO ₂ -CaO-based composite oxide having a vaccination nucleus, and describes a method of dispersing the inclusions.

In the present embodiment, in order to disperse the inclusions composed of the above-described ZrO ₂ —CaO-based composite oxide in the molten steel, in the intermediate stage of forming the inclusions, the liquid phase oxide (the surface is coated with the liquid phase). Oxide having a high liquid phase ratio). The liquid phase oxide has better wettability with the molten steel than the solid phase oxide, and has a low agglomeration and coalescence property in the molten steel, so it is easy to maintain a finely dispersed state. Then, by modifying the liquid phase oxide in a sufficiently finely dispersed state to obtain inclusions serving as inoculation nuclei, inclusions (inoculation nuclei) consisting of the finally obtained ZrO ₂ —CaO-based composite oxide are obtained. ) Is surely finely dispersed.

In this embodiment, in order to finally finely disperse inclusions composed of ZrO ₂ —CaO-based composite oxide using three types of deoxidizing elements Al, Ca, and Zr, Is stipulated.
As for the addition order, first, Al deoxidation is performed, then Ca deoxidation is performed, and finally Zr deoxidation is performed. The _CaO-Al 2 _{O 3} type oxide produced after Ca deoxidation by a liquid phase oxidation product, and suppress aggregation coalescence of _CaO-Al 2 _{O 3} based oxide, thereby finely dispersed.

Here, in order to obtain the above-mentioned liquid phase oxide after Ca deoxidation, the amount of Ca is defined by the following formula (1).
0.2 ≦ [% Ca] / [% T.P. O] ≦ 0.8 (1)
If the amount of Ca that satisfies the lower limit of formula (1) is added and deoxidized, the liquid phase ratio is high (50% or more) CaO—Al ₂ O ₃ system oxide (CaO—Al ₂ O ₃ system liquid phase oxide) ) Will be generated. If it is less than the lower limit of the formula (1), the liquid phase ratio of the CaO—Al ₂ O ₃ -based oxide generated after Ca deoxidation is low, and the surface is not covered with the liquid phase and the solid phase is exposed on the surface. For this reason, aggregation and coalescence of CaO—Al ₂ O ₃ -based oxide easily proceeds, and the CaO—Al ₂ O ₃ -based oxide cannot be finely dispersed. Then, there is a possibility that the inclusion composed of the ZrO ₂ —CaO based composite oxide finally obtained by modifying this CaO—Al ₂ O ₃ based oxide cannot be finely dispersed.
The upper limit of formula (1) is effective as an inoculation nucleus by suppressing the generation of CaZrO ₃ (composite oxide in which CaO and ZrO ₂ have a molar ratio of 1: 1) that does not have the inoculation effect of the γ phase, which occurs after addition of Zr. In order to promote the formation of ZrO ₂ . That is, in the range exceeding the (% Ca) / (% O ) = 0.8 in CaZrO _3, CaZrO ₃ in place of ZrO ₂ is generated, not be obtained inoculum effect.

Further, as inoculum nucleus for gamma-Fe, ZrO ₂ in Patent Document 3 to 6 it has been proposed. The crystal structure of ZrO _{2 at} the molten steel temperature is tetragonal, and the lattice constants of the bottom a-axis and side c-axis are different (lattice constant a = b ≠ c). The lattice matching with γ-Fe is good at the bottom of the tetragonal crystal, and the lattice constant of the bottom of ZrO ₂ is close to the lattice constant of γ-Fe. However, since tetragonal crystals have lower symmetry than the face-centered cubic fcc (a = b = c) of the crystal structure of γ-Fe, there is a limit to the effect as seeding nuclei for γ-Fe.
On the other hand, ZrO _2, when the solid solution of CaO, CaO-stabilized _ZrO 2 (CaO _stabilized Zirconia; hereinafter referred to as CSZ) is known to be produced in 1140 ° C. or higher. Since the crystal structure of CSZ is cubic, the inoculation effect on γ-Fe is higher than that of ZrO ₂ which is tetragonal. CaO solid solution amount varies with temperature. For example, when the molten steel temperature common 1600 ° C., when the solid solution 5 to 11% of CaO mass%, it is CaO-ZrO ₂ two yuan which CSZ generated It can be seen from the system phase diagram.

In the present embodiment, cubic CSZ having a high inoculation effect on γ-Fe is generated. Therefore, the amount of Zr is defined by the expression (2).
4 × [% Ca] × (3.4 × [% T.Al] +1.0) ≦ [% Zr] (2)
Formula (2) shows that it is necessary to increase the amount of Zr as the amount of Ca and the amount of Al increase. This equation (2) was obtained from a simulation by thermodynamic calculation.
FIG. 1 and a partially enlarged view of FIG. 2 show an Al ₂ O ₃ —CaO—ZrO ₂ ternary phase diagram at 1600 ° C. (T. Murakami, H. Fukuyama, T. Kishida, M. Susa and K. et al. Nagata: Metal.Matter.Trans.B, 2000, vol.31B, pp.25-33, which was partially added for explanation). Referring to both figures, an example of a typical change in oxide composition is described schematically in the case of adding Zr to the molten steel CaO-Al ₂ O ₃ based liquid phase oxide is present. In the original drawing, CSZ is expressed as Css (ss = solid solution, meaning a ZrO ₂ phase in which CaO is dissolved), CaZrO ₃ is expressed as CZ, and tetragonal ZrO ₂ is expressed as Tss. Has been. In FIG. 1 and FIG. 2, the concentration of each component is displayed in mol%. On the other hand, in this specification, unless otherwise specified, it is described using mass% display.

First, a CaO—Al ₂ O ₃ -based liquid phase inclusion is generated before adding Zr, and then a small amount of Zr is added to form a CaO—Al ₂ O ₃ —ZrO ₂ -based liquid phase inclusion (in FIG. 1). Point a). As the amount of Zr is increased, the liquid phase inclusions continue to decrease, while the complex oxide CaZrO ₃ having a molar ratio of CaO to ZrO ₂ of 1: 1 begins to be formed (point b). CaZrO ₃ is a solid phase at the molten steel temperature, and also has a low lattice matching with γ-Fe, and therefore has no inoculation effect on γ-Fe. As the amount of Zr is further increased, the liquid phase inclusions are further reduced and CaZrO ₃ is increased, and eventually CSZ starts to be formed (point c). When the amount of Zr increases until the formula (2) is satisfied, CSZ increases until it accounts for 2/3 or more of the volume of the oxide. When Zr is further added, the two-phase region of CSZ and CZ (point d) is reached, and the region changes to a CSZ single-phase region (point e) and a region where tetragonal ZrO ₂ is formed (point f).
According to FIG. 1, there is also a narrow composition region (region including the point g) where the two phases of CaO—Al ₂ O ₃ —ZrO ₂ -based liquid phase inclusions and CSZ are in equilibrium.

If an amount of Zr that satisfies the formula (2) is added, a ZrO ₂ —CaO-based composite oxide in which CSZ occupies 2/3 or more of the cross-sectional area is generated, and a high inoculation effect is obtained for γ-Fe. can get. In order to obtain a high inoculation effect as a whole inclusion, CSZ needs to occupy 2/3 or more.
If the amount does not satisfy Expression (2), a sufficient amount of CSZ cannot be obtained. In that case, when the remaining part is mainly (i) CaO—Al ₂ O ₃ —ZrO ₂ -based liquid phase inclusion, (ii) composite oxidation in which CaO and ZrO ₂ are formed at a molar ratio of 1: 1. In the case of the product CaZrO ₃ , there are four types: (iii) mixing of the liquid phase and CaZrO ₃ , and (iv) tetragonal ZrO ₂ . Here, since CaZrO ₃ has low lattice matching with γ-Fe as described above, there is almost no inoculation effect on γ-Fe.

The final inclusion distributed in the slab thus obtained has the following composition.
0 ≦ (% Al ₂ O ₃ ) ≦ 18 (3)
0.04 <(% CaO) / (% ZrO ₂ ) ≦ 0.22 (4)
Both show the conditions of the average composition of the entire inclusion when CSZ accounts for 2/3 or more. In the above-mentioned inclusions, depending on the amount of Ca and other conditions, in addition to CSZ, (i) a case where CaO—Al ₂ O ₃ —ZrO ₂ -based liquid phase inclusions remain and (ii) CaZrO ₃ When left, (iii) When liquid phase inclusions and CaZrO ₃ remain mixed, (iv) Tetragonal ZrO ₂ may remain, but from (i) to the upper limit of formula (3), from (ii) The upper limit of formula (4) and the lower limit of formula (4) are defined from (iv).

First, it demonstrates from the upper limit of Formula (3). (I) It was defined from the maximum value of the average concentration of Al ₂ O ₃ of inclusions in the two-phase coexistence region of CSZ and liquid phase inclusions (region including the point g in FIG. 1). The solid solubility limit of Al ₂ O _{3 in} CSZ is about 1%. The maximum concentration of Al ₂ O ₃ in the liquid phase inclusions other than CSZ is 52%. As the cross-sectional area ratio of CSZ of total inclusions becomes low, the average concentration (% _Al 2 _{O 3)} of _Al 2 _{O 3} contained in the entire inclusions combined CSZ and liquid inclusions is increased. The value of (% Al ₂ O ₃ ) is maximum when the cross-sectional area ratio of CSZ is 2/3, which is the minimum, and is calculated as 2/3 × 1% + 1/3 × 52% = 18%. This value is acceptable for the cross-sectional area ratio of CSZ to ensure a minimum of 2/3, which is the maximum value of the average concentration of _{Al 2 O 3 (% Al 2} O 3). The lower limit is 0 because it may not contain Al ₂ O ₃ .

Next, the upper limit of Expression (4) will be described. (Ii) It is defined from the case where CaZrO ₃ occupies 1/3 or more in the two-phase coexistence region of CSZ and CaZrO ₃ (the region including the point d in FIGS. 1 and 2). The solid solubility limit of CaO in CSZ in molten steel is about 11%, and the proportion of CaO in CaZrO ₃ is 31.3%. The ratio of the average CaO concentration (% CaO) and the average ZrO ₂ concentration (% ZrO ₂ ) as a whole inclusion becomes maximum when the cross-sectional area ratio of CSZ is 2/3, which is the minimum (2/3 × 11%). + 1/3 × 31.3%) / (2/3 × 89% + 1/3 × 68.7%) = 0.22. This value is the maximum value of (% CaO) / (% ZrO ₂ ) allowed in order to secure 2/3, which is the minimum sectional area ratio of CSZ.

As for the lower limit of formula (4), a large amount of Zr is added, and (iv) in the two-phase coexistence region of CSZ and tetragonal ZrO ₂ (region including the point f in FIGS. 1 and 2) It is defined from the case where ZrO ₂ of occupies 1/3. T.A. Murakami et al. According to the literature, the equilibrium composition of CSZ and tetragonal ZrO ₂ has a range (thick lines La and Lb in FIG. 2), but CA ₆ (CaO · 6Al ₂ O ₃ ) -tetragonal ZrO ₂ —CSZ The value of equation (4) calculated by the composition of tetragonal ZrO ₂ and CSZ (point h in FIG. 2) at the three-phase equilibrium takes the minimum value. At this time, the solid solution amount of CaO in tetragonal ZrO ₂ is 0.22%, and the solid solution amount of CaO in CSZ is 5.8%. When the formula (4) is calculated, (1/3 × 0.22 + 2/3 × 5.8) / (1/3 × 99.78 + 2/3 × 94.2) = 0.04.

(Inclusion size)
Since inclusions in the carbon steel slab are not subjected to processing such as rolling, the inclusions are almost spherical, and the ratio of major axis / minor axis is mostly about 3 or less. For this reason, it is possible to prescribe | regulate the size of an inclusion with the long diameter of an inclusion.
Here, when the major axis is less than 1 μm, the effect as an inoculum nucleus is low, and the solidified structure cannot be refined. In addition, it becomes difficult to observe at a magnification of 1000 times using an optical microscope. On the other hand, if the major axis exceeds 10 μm, the number of inclusions cannot be ensured. For this reason, in this embodiment, the number density of inclusions whose major axis is 1 μm or more and 10 μm or less is defined.

(Number density of inclusions)
When the number density of inclusions of the above-mentioned size is less than 10 pieces / mm ² , it is insufficient for generating equiaxed crystals over a wide range in the slab, and the solidified structure cannot be refined. On the other hand, when the number density of inclusions of the above size exceeds 500 / mm ² , the number of inclusions is too large, which may cause product defects. Moreover, there exists a possibility that workability and toughness may deteriorate.
From the above, in this embodiment, the number density of inclusions having a major axis of 1 μm or more and 10 μm or less is defined within a range of 10 pieces / mm ² or more and 500 pieces / mm ² or less. In order to ensure that the above-described effects are achieved, the number density is preferably in the range of 20 / mm ^{2 to} 200 / mm ² .

  Moreover, the reason which prescribed | regulated each element amount of the carbon steel slab in this embodiment below is shown.

(C: carbon)
C is an element having an effect of improving the strength of the carbon steel slab. Here, when the C content exceeds 0.50%, a γ-solidified steel in which a γ-phase (γ-Fe) is crystallized as an initial crystal when solidified is obtained. Here, if the C content exceeds 1.50%, workability and weldability may be deteriorated.
From the above, in the present embodiment, the C content is regulated within the range of more than 0.50% and less than 1.50%.

(Si: silicon)
Si is an element having an effect of promoting deoxidation of molten steel and improving the strength of the carbon steel slab. Here, when the content of Si is less than 0.01%, the above-described effects cannot be achieved. On the other hand, if the Si content exceeds 1.2%, workability and weldability may be deteriorated.
From the above, in the present embodiment, the Si content is defined within the range of 0.01% to 1.2%.

(Mn: Manganese)
Mn is an element having an effect of improving the strength and toughness of the carbon steel slab. Here, when the content of Mn is less than 0.01%, the above-described effects cannot be achieved. On the other hand, if the Mn content exceeds 3.0%, the weldability is lowered.
From the above, in the present embodiment, the Mn content is regulated within the range of 0.01% to 3.0%.

(P: phosphorus)
P is an element that affects the toughness of the carbon steel slab, but unavoidably 0.0010% is contained. Here, when the content of P exceeds 0.050%, the center segregation becomes intense, and the toughness of the carbon steel slab deteriorates.
From the above, in the present embodiment, the P content is defined within the range of 0.0010% to 0.050%.

(S: sulfur)
S is an element that affects the toughness of the carbon steel slab, but unavoidably 0.0001% is contained. Here, if the S content exceeds 0.0100%, coarse inclusions are generated, and the toughness of the carbon steel slab is significantly deteriorated.
From the above, in the present embodiment, the S content is defined within the range of 0.0001% to 0.0100%.

(T.Al: Aluminum)
Al is an element generally added for deoxidizing molten steel. Here, T.W. If the Al content is less than 0.001%, sufficient deoxidation cannot be performed. On the other hand, T.W. If the Al content exceeds 0.30%, coarse inclusions (Al ₂ O ₃ clusters, AlN) are generated, and the quality of the carbon steel slab may be deteriorated.
From the above, in the present embodiment, T.I. The Al content is specified in the range of 0.001% to 0.30%. In the present invention, T.I. The content of Al (total aluminum) is a total amount including aluminum contained in the carbon steel slab in a compound state in addition to aluminum in the molten steel.

(Ca: calcium)
Ca has two important actions in the present invention.
(1) First, it is a ZrO ₂ —CaO-based composite oxide serving as an inoculum nucleus, particularly a CSZ-forming element responsible for inoculation. As described above, CSZ is cubic and has a higher inoculation effect on γ-Fe than ZrO ₂ which is tetragonal. The ZrO ₂ —CaO-based composite oxide produced in this embodiment has a sufficiently high inoculation effect because 2/3 or more of the observed cross-sectional area is a tetragonal CSZ.
(2) It is an element that exerts an important influence in order to finely disperse a large number of ZrO ₂ —CaO-based composite oxides finally produced in molten steel (inside the slab). That is, by adding Ca after Al deoxidation, a CaO—Al ₂ O ₃ liquid phase oxide is produced in the molten steel. If Zr is added thereafter, the ZrO ₂ —CaO based composite oxide is added to the molten steel. Fine dispersion can be ensured. This is because the liquid phase oxide has better wettability with the molten steel than the solid phase oxide, so that it does not easily aggregate and coalesce. Thus, before the addition of Zr, a CaO—Al ₂ O ₃ liquid phase oxide that hardly aggregates and coalesces is generated, and by modifying this, a large number of ZrO ₂ —CaO composite oxides are contained in the molten steel, It can be finely dispersed.

Here, if the content of Ca is less than 0.0002%, a CaO—Al ₂ O ₃ liquid phase oxide cannot be generated. And CSZ cannot be generated. On the other hand, if it exceeds 0.0040%, the CaO content of the composite oxide produced after the addition of Zr becomes high, and the content of CSZ having an inoculation action decreases, and the entire ZrO ₂ —CaO based composite oxide As the inoculation effect becomes insufficient.
For the reasons described above, in the present embodiment, the Ca content is specified in the range of 0.0002% or more and less than 0.0040%.

(Zr: zirconium)
Zr is an element necessary for generating inclusions made of a ZrO ₂ —CaO-based composite oxide that becomes a solidification nucleus (inoculation nucleus) of the γ phase (γ-Fe).
Here, when the content of Zr is less than 0.0010%, there is a possibility that CSZ in the ZrO ₂ —CaO-based composite oxide is not sufficiently generated. On the other hand, if the content of Zr exceeds 0.25%, coarse oxides are generated, and the workability and toughness of the carbon steel slab may be significantly deteriorated.
From the above, in the present embodiment, the Zr content is regulated within the range of 0.0010% or more and 0.25% or less.

(TO: Oxygen)
O is inevitably contained in 0.0005%. Here, T.W. If the O content exceeds 0.0050%, coarse oxides and the like are generated, and the quality of the carbon steel slab is deteriorated.
From the above, in the present embodiment, T.I. The O content is limited to a range of 0.0005% to 0.0050%. In the present invention, T.I. The content of O (total oxygen) is a total amount including O contained in the carbon steel slab in an oxide state in addition to dissolved oxygen (free oxygen) in the molten steel.

  Next, the manufacturing method of the carbon steel slab which is this embodiment is demonstrated with reference to the flowchart of FIG.

(Melting step S01)
First, molten steel melted in a converter or the like is prepared. And the steel component except Ca and Zr is adjusted. In order to stabilize the component value (yield) at the time of this component adjustment, Al deoxidation may be performed in multiple steps. Thus, first, component adjustment excluding Ca and Zr and Al deoxidation are performed. In this molten steel, inclusions mainly composed of Al ₂ O ₃ exist.

(Ca addition step S02)
Next, Ca is added. In order to levitate and remove coarse Al ₂ O ₃ produced after Al deoxidation from molten steel, it is preferable to add Ca after 1.5 minutes or more have elapsed since Al was added.
By adding Ca, Al ₂ O ₃ remaining in the molten steel is modified into a CaO—Al ₂ O ₃ liquid phase oxide. Therefore, Ca is contained in an amount satisfying the formula (1) according to the amount of O. The amount of Ca at this time is not the amount added but the content in steel after the yield. Since this CaO—Al ₂ O ₃ -based liquid phase oxide has good wettability with molten steel, it is easily finely dispersed in the molten steel. In order to sufficiently react Ca and Al ₂ O ₃ to form a CaO—Al ₂ O ₃ liquid phase oxide, it is preferable to stir the molten steel for 1.5 minutes or more after the Ca addition is completed. However, when stirring for a long time, aggregation and coalescence proceed even for a liquid phase oxide, so the stirring time is preferably within 3 minutes at the longest. The form and method of Ca to be added are not particularly limited, and commonly used methods such as CaSi alloy wire addition and CaSi alloy powder injection can be used.

(Zr addition step S03)
And Zr is added to molten steel. As a result, the CaO—Al ₂ O ₃ -based liquid phase oxide generated in the Ca addition step S02 and finely dispersed in the molten steel is modified into a ZrO ₂ —CaO-based composite oxide. The amount of Zr is contained in an amount that satisfies the formula (2) according to the amount of Al and the amount of Ca. In order to sufficiently react Zr and CaO—Al ₂ O ₃ -based liquid phase oxide, it is preferable to stir the molten steel for 1.5 minutes or more after the addition of Zr. However, since agglomeration and coalescence proceeds when stirring for a long time, the stirring time is preferably within 3 minutes at the longest.

(Casting process S04)
The molten steel thus obtained is cast by a continuous casting machine. At that time, the time from the addition of Zr to the end of casting is within 6 hours from the addition of Zr to the end of casting. By doing in this way, inclusions in the molten steel having a major axis size of 1 μm or more and 10 μm or less made of ZrO ₂ —CaO-based complex oxide are within a range of 10 / mm ² or more and 500 / mm ² or less in number density. It can be.
Here, when the time from the addition of Zr to the end of casting exceeds 6 hours, the ZrO ₂ —CaO-based composite oxide formed in the Zr addition step S03 floats on the surface of the molten steel, and functions as an inoculation nucleus. There is a risk of disappearing.

According to the carbon steel slab of the present embodiment configured as described above, inclusions made of ZrO ₂ —CaO-based composite oxide are dispersed therein, and the average composition of the inclusions is Since the above formulas (3) and (4) are satisfied, 2/3 or more of the inclusions are occupied by cubic CZS, and the inclusions as a whole are γ phase (γ-Fe) and The lattice matching of is high. Therefore, this inclusion becomes a solidified nucleus, and the solidified structure can be reliably refined.

Further, in the present embodiment, inclusions having a major axis size of 1 μm or more and 10 μm or less made of a ZrO ₂ —CaO-based composite oxide are dispersed within a number density of 10 / mm ² or more and 500 / mm ² or less. Therefore, the solidified structure can be reliably miniaturized. Therefore, a high quality carbon steel slab without center segregation or surface cracks can be obtained.

Moreover, in this embodiment, since it has Ca addition process S02 which adds Ca after Al deoxidation, and Zr addition process S03 which adds Zr after Ca addition, by Ca addition process S02, in molten steel Al ₂ O ₃ can be modified to form a CaO—Al ₂ O ₃ liquid phase oxide. Since this CaO—Al ₂ O ₃ -based liquid phase oxide has better wettability with molten steel than solid phase oxide, it can be widely dispersed in molten steel.
Then, reformed in Zr addition step S03, by _CaO-Al 2 _{O 3} based liquid phase oxides are added Zr against molten steel are dispersed, the _CaO-Al 2 _{O 3} based liquid phase oxides In addition, a large number of inclusions made of the above-mentioned ZrO ₂ —CaO-based composite oxide having a high lattice matching with the γ phase (γ-Fe) can be finely dispersed in the molten steel, ensuring the inoculation effect. This makes it possible to reliably refine the solidified structure.

  As mentioned above, although the carbon steel slab and the method for producing the carbon steel slab according to the embodiment of the present invention have been specifically described, the present invention is not limited to this and does not depart from the technical idea of the invention. The range can be changed as appropriate.

In the following, the results of experiments conducted to confirm the effects of the present invention will be described.
The molten steel melted in the 250 t converter was deoxidized by adding Al in a vacuum degassing apparatus and the components of the molten steel were adjusted.
And in this invention example 1-11 and comparative example 21-31, CaSi alloy wire addition was performed to the molten steel after Al deoxidation, Ca was added, and Zr was added after that.
In Comparative Example 21, Ca was added after Al deoxidation, but Zr was not added.
In Comparative Example 22, Zr was added without adding Ca to the molten steel after Al deoxidation.
In Comparative Example 31, Zr was added after Al deoxidation, and then Ca was added.

  As described above, molten steel having the composition shown in Table 1 was obtained. Component analysis samples were collected in a tundish. By continuously casting the molten steel, a carbon steel slab (slab) having a rectangular cross section with a width of 1250 mm and a thickness of 250 mm was produced.

(Observation of coagulated tissue)
In a cross section perpendicular to the casting direction of the obtained carbon steel slab (slab), a solidified structure (dendritic structure) is detected with a picric acid solution, and each position of 1/4 width, 1/2 width, and 3/4 width is detected. The equiaxed crystal ratio (ratio of equiaxed crystal zone thickness to slab thickness) was measured. A columnar crystal structure with a uniform growth direction develops from the slab surface layer toward the center of the thickness, and an equiaxed crystal with a random growth direction is generated at the center of the thickness. The reason why the columnar crystal growth directions are aligned is that dendrites whose preferential growth orientation matches or is close to the macroscopic heat flow direction preferentially grow. Thus, the columnar crystal zone and the equiaxed crystal zone can be distinguished by the directionality of the structure (primary branch of dendrite). In addition, the equiaxed crystal zone includes many granular crystals having an almost spherical shape (the ratio of major axis / minor axis is close to 1) and branched dendrites having an elongated shape (major axis / minor axis ratio of 3 or less). Is more than 3). The ratio of equiaxed crystal zones is shown in Table 2.

(Composition of inclusions)
In the cross section perpendicular to the casting direction of the obtained carbon steel slab (slab), an observation sample is taken from a 1/4 thickness position of 1/4 width, and this observation sample is observed by SEM, and the major axis is 1 μm or more and 10 μm or less. 20 inclusions were arbitrarily selected. With the EDS (energy dispersion measurement) apparatus attached to the SEM, the entire composition of each inclusion was determined with the range covering the entire inclusions as the measurement range. Then, the total composition of the 20 inclusions measured was averaged to obtain (% Al ₂ O ₃ ) and (% CaO) / (% ZrO ₂ ). Here, the composition indicated in parentheses () indicates an inclusion composition.

(Cross sectional area of CZS phase in inclusions)
The cross-sectional area ratio occupied by the CSZ phase inside the inclusions was determined by image processing of the reflected electron image. Since the reflected electron image appears brighter as the atomic number (in the case of a compound, the average atomic number obtained by adding the product of the atomic number and mass concentration of the constituent atoms to the total element) increases, the CSZ phase is discriminated. it can. It was confirmed in advance by EDS that the target brightness phase was a CSZ phase. In addition, instead of the reflected electron image, the element concentration distribution of the entire inclusion may be measured by EPMA (element mapping), the CSZ phase may be specified, and the cross-sectional area ratio may be obtained.

(Number density of inclusions / maximum diameter of slab inclusions)
The observation sample was observed with an optical microscope at a magnification of 1000 times and 60 fields of view, and the number density of inclusions having a major axis of 1 μm to 10 μm was calculated. Moreover, the maximum major axis of the observed inclusion was determined. Table 2 shows the maximum long diameter and number density of inclusions.

(Product 疵)
According to a conventional method, the obtained carbon steel slab (slab) is heated to 1,050 to 1,200 ° C. and hot-rolled to produce a hot-rolled sheet having a thickness of 3 mm, and visually checked for wrinkles having a length of 10 mm or more. Inspected with. A case where the number of ridges per 100 m was less than 1 was judged as “◯”, and one or more pieces were judged as “x”. The evaluation results are shown in Table 2.

In Comparative Example 21, since no Zr was added, no CSZ phase was observed in the slab, and CaO—Al ₂ O ₃ liquid phase oxide was distributed. Since this oxide had low lattice matching with γ-Fe and no inoculation effect, the equiaxed crystal ratio was as low as 5%.

In Comparative Example 22, Zr was added without adding Ca after Al deoxidation. Since Zr was added in a state where no CaO—Al ₂ O ₃ -based liquid phase oxide was generated, CSZ was not generated, and ZrO _{2 that} was tetragonal was generated, and the number density in the slab was 10 / It was less than mm ^2. Therefore, the equiaxed crystal ratio remained at 10%.

In Comparative Example 23, the amount of Ca after Al deoxidation was 0.0001% below the lower limit, and as a result, the value of formula (1) was also lower than the lower limit. For this reason, the CaO—Al ₂ O ₃ -based oxide became a solid phase, the aggregation coalescence progressed, and the number decreased. Therefore, the number of ZrO ₂ —CaO based composite oxides produced after Zr deoxidation was less than 10 / mm ² and the equiaxed crystal ratio was 10%. Moreover, there were few CSZ phases and the cross-sectional area ratio was less than 67%.

In Comparative Example 24, the amount of Ca after Al deoxidation was 0.0045% exceeding the upper limit, and as a result, the values of the formulas (1) and (4) exceeded the upper limit, and thus were generated after Zr deoxidation. (% CaO) in the ZrO ₂ —CaO composite oxide was high, and a large amount of CaZrO ₃ composite oxide was produced. On the other hand, the cross-sectional area ratio of the CSZ phase having a high inoculation effect on γ-Fe is less than 67%, and the inoculation effect as a whole inclusion composed of the ZrO ₂ —CaO-based composite oxide is reduced. % Did not reach.

In Comparative Example 25, since the Al amount exceeded the upper limit, coarse Al ₂ O ₃ was generated after Al deoxidation, and the CaO—Al ₂ O ₃ oxide after Ca deoxidation did not become a liquid phase, but agglomerated. The coalescence progressed. The number of ZrO ₂ —CaO composite oxides after Zr deoxidation was less than 10 / mm ² , the cross-sectional area ratio of the CSZ phase was less than 67%, and the equiaxed crystal ratio was 5%. Further, due to the coarse Al ₂ O ₃ , the coarse ZrO ₂ —CaO based composite oxide was distributed in the slab, resulting in product defects.

In Comparative Example 26, Zr was less than the lower limit value, so the value of formula (4) exceeded the upper limit and did not satisfy formula (2). Therefore, the production amount of CSZ which becomes an inoculation nucleus of γ-Fe is insufficient, the cross-sectional area ratio of the CSZ phase is less than 67%, and the inoculation effect as a whole inclusion made of the ZrO ₂ —CaO-based composite oxide is obtained. Because it was low, the equiaxed crystal ratio remained at 10%.

In Comparative Example 27, the number of ZrO ₂ —CaO-based composite oxides exceeded 50 / mm ² and the equiaxed crystal ratio was 35%. However, since Zr exceeded the upper limit, the ZrO ₂ —CaO-based composite oxide became coarse and product defects occurred.

In Comparative Example 28, since the value of the formula (1) was less than the lower limit, the CaO—Al ₂ O ₃ -based oxide after Ca deoxidation did not sufficiently become a liquid phase, aggregation coalescence progressed, and Zr deoxidation progressed. Since the number of later ZrO ₂ —CaO based composite oxides was also small, the equiaxed crystal ratio was only 10%.

In Comparative Example 29, the upper limit of Formula (1) was exceeded, and as a result of the increase in (% CaO) of the ZrO ₂ —CaO-based composite oxide after Zr deoxidation, the upper limit of Formula (4) was also exceeded. Therefore, (% CaO) in the ZrO ₂ —CaO-based composite oxide produced after Zr deoxidation is high and a large amount of CaZrO ₃ is produced, while the cross-sectional area ratio of the CSZ phase having a high inoculation effect on γ-Fe is 67. The equiaxed crystal ratio did not reach 20% because the inoculation effect as a whole of the ZrO ₂ —CaO inclusions was reduced.

In Comparative Example 30, since the amount of Zr did not satisfy the formula (2), (% CaO) in the ZrO ₂ —CaO-based composite oxide generated after Zr deoxidation was high, and the cross-sectional area ratio of the CSZ phase was less than 67%. Thus, since the inoculation effect as a whole inclusion composed of the ZrO ₂ —CaO-based composite oxide was lowered, the equiaxed crystal ratio did not reach 20%.

In Comparative Example 31, Zr addition was performed after Al deoxidation, and Ca was added last, so that a stage of generating a CaO—Al ₂ O ₃ liquid phase oxide was not performed. Therefore, from Al deoxidation to Ca deoxidation, the oxide was in a solid phase, and agglomeration proceeded in the molten steel to reduce the number, so that the equiaxed crystal ratio was 10%.

On the other hand, in Inventive Example 1-11, the equiaxed crystal ratio was high and the product wrinkle was small.
From the above, according to the present invention example, in the γ-solidified steel in which the primary crystal that crystallizes when the molten steel solidifies, the solidified structure can be stably refined and excellent in various properties. It was confirmed that a carbon steel slab can be obtained.

S02 Ca deoxidation step S03 Zr addition step

Claims (2)

% By mass
C: 0.50% to 1.50% or less,
Si: 0.01% or more and 1.2% or less,
Mn: 0.01% to 3.0%,
P: 0.0010% or more and 0.050% or less,
S: 0.0001% or more and 0.0100% or less,
T.A. Al: 0.001% or more and 0.30% or less,
Ca: 0.0002% or more and 0.0040% or less,
Zr: 0.0010% or more and 0.25% or less,
T.A. O: 0.0005% or more and 0.0050% or less,
The balance is made Fe and inevitable impurities, and T. O content [% T.V. O], Ca content [% Ca], Zr content [% Zr], T.O. Al content [% T.V. Al] satisfies the following formulas (1) and (2):
0.2 ≦ [% Ca] / [% T.P. O] ≦ 0.8 (1)
[% Zr] ≧ 4 × [% Ca] × (3.4 × [% T.Al] +1.0) (2)
Inclusions made of ZrO ₂ —CaO based composite oxide are dispersed inside,
The average composition of the inclusions satisfies the following formulas (3) and (4):
0 ≦ (% Al ₂ O ₃ ) ≦ 18 (3)
0.04 ≦ (% CaO) / (% ZrO ₂ ) ≦ 0.22 (4)
Of the inclusions, the average value of the cross-sectional area ratio of the region where the CaO concentration is 11% or less is 67% or more,
A carbon steel slab characterized in that the inclusions having a major diameter of 1 μm or more and 10 μm or less are dispersed in a number density of 10 / mm ² or more and 500 / mm ² or less. It is a manufacturing method of the carbon steel slab which manufactures the carbon steel slab of Claim 1,
Al deoxidation process in which Al is added to molten steel to perform deoxidation, Ca addition process in which Ca is added after Al deoxidation, Zr addition process in which Zr is added after Ca addition, and casting in which molten steel with components adjusted is cast And having a process
While dispersing the CaO-Al ₂ O ₃ liquid phase oxide in the molten steel by the Al deoxidation step and the Ca addition step,
In the Zr addition step, the CaO—Al ₂ O ₃ -based liquid phase oxide and Zr are reacted to have a ZrO ₂ —CaO-based composite having an average composition satisfying the formulas (3) and (4). A method for producing a carbon steel slab, comprising forming the inclusion made of an oxide.

Images