JP2018193595A - Carbon steel cast slab and manufacturing method of carbon steel cast slab - Google Patents

Abstract

To provide a carbon steel cast slab capable of surely enhancing hole expansibility by suppressing variation of a composition of inclusion, and further stably conducting casting by suppressing blockage of a nozzle.SOLUTION: A carbon steel cast slab contains C, Si, Mn, P, S, N, t.O, Al, Ti, REM and Ca in prescribed ranges respectively, and the balance Fe with inevitable impurities, and has mass% of REM, Ca and t.O in ranges satisfying prescribed formulae respectively, the number density of inclusions with circle equivalent diameter of REM composite inclusion at 1/2 thickness part of the cast slab of 0.5 μm to 4 μm of 20/mmor more, the number density of inclusions with over 10 μm limited to less than 0.2/mm, and austenite particle diameter on a cast slab surface of 0.05 mm or less.SELECTED DRAWING: None

Description

  Carbon steel slabs used as raw materials for steel plates and the like excellent in hole expansibility, suitable for undercarriage parts and structural materials such as automobiles that are mainly pressed and used, and a method for producing the carbon steel slabs It is about.

Steel sheets used in automobile body structures are required to have high press workability and strength, and many inventions have been made. However, in order to meet the demands for further weight reduction and complex parts of today's automobiles, carbon steel having hole expansibility superior to conventional ones is required.
Conventionally, in the above-described carbon steel, it is known that hole expandability deteriorates due to elongated sulfide inclusions such as MnS.

Therefore, in Patent Document 1, for example, by adding at least one selected from Ce, La, which is a kind of REM (rare earth element), MnS inclusions are finely precipitated to improve hole expansibility. Techniques to do this have been proposed. Adding Al as a deoxidizer in producing a molten steel, Ce to where the generated _Al 2 _{O 3} is suspended, the addition of La, although some _Al 2 _{O 3} remains, Al ₂ in the molten steel It is believed that O ₃ inclusions are reduced and decomposed, and fine and hard Ce oxide, La oxide, cerium oxysulfide, and lanthanum oxysulfide are generated by deoxidation with Ce and La. As a result, MnS can be deposited on those oxysulfides, etc., and since deformation of the precipitated MnS is suppressed during rolling, the stretched coarse MnS in the steel sheet can be significantly reduced. This is a technique that can improve the hole expandability.

Moreover, in order to prevent generation | occurrence | production of the immersion nozzle clogging at the time of continuous casting, adding the REM in steel, and also maintaining the extending | stretching prevention effect by the form control of inclusions, the means to add together is also taken. ing.
For example, in Patent Document 2, by suppressing the REM addition concentration to 0.0060% or less and adding Ca appropriately, it is possible to prevent a large amount of clogged immersion nozzles and coarse inclusions with low melting points. . Moreover, in patent document 3, the steel materials containing any 1 type or 2 types or more of REM, Ca, and Mg are also proposed.

Japanese Patent No. 5093422 JP 2012-046789 A JP 2008-223043 A

By the way, in patent document 1, when adding REM in molten steel and carrying out continuous casting, there existed a problem that an immersion nozzle became easy to block | close and operation was not stabilized. When continuously casting REM-added steel, it is known that a refractory material of an immersion nozzle (hereinafter simply referred to as “nozzle”) reacts with REM to cause nozzle clogging. Often means are added to prevent nozzle clogging.
However, as in Patent Documents 2 and 3, when REM and Ca are simply added, coarse inclusions exist and the hole expandability cannot be stably improved.

  The present invention has been made in view of the above-described situation, and suppresses the generation of coarse inclusions, and can reliably improve the hole expandability, and the carbon steel slab. It aims at providing the manufacturing method of.

In order to solve the above-mentioned problems, the present inventors have made extensive studies and as a result, have obtained the following knowledge.
The relatively coarse REM composite inclusion (REM-Ca-OS) present in the slab is composed of several REM-rich phases and a Ca-rich phase mainly composed of CaS that surrounds it. Has been. This REM composite inclusion (REM-Ca-O-S) is hard and remains in a form as it is even after rolling, and remains coarse.

  Here, when the composition of the REM composite inclusion in the sample rapidly cooled from the molten steel and the composition of the REM composite inclusion in the slab were compared, the above Ca-rich phase was an oxide immediately after solidification, which was 1200 ° C. or higher. , It exists as a liquid phase or a soft inclusion phase, and turns into sulfide during solidification. From this, by applying a reduction at 1200 ° C. or higher, where the main component of the Ca-rich phase is an oxide, coarse REM composite inclusions (REM-Ca—O—S) are crushed and the equivalent circle diameter is 0. It was found that the inclusions were 5 μm or more and 4 μm or less, and the hole expandability in the steel sheet could be secured.

The present invention has been made on the basis of the above-mentioned knowledge, and the carbon steel slab according to the present invention is in mass%,
C: 0.03% to 0.30%,
Si: 0.08% to 2.1%,
Mn: 0.5% to 4.0%,
P: 0.05% or less,
S; 0.0001% to 0.01%,
N: 0.01% or less,
t. O: 0.0005% or more and 0.005% or less,
Al: 0.004% to 2.0%,
Ti: 0.0001% or more and 0.20% or less,
REM; 0.001% to 0.02%,
Ca; 0.0011% or more and 0.005% or less,
And the balance consists of iron and inevitable impurities, and REM, Ca, t. The mass% of O is [REM], [Ca], [t. O],
0.25 ≦ [Ca] / [REM] ≦ 5 (1)
0.0011 ≦ [Ca] −0.15 × [t. O] ≦ 0.005 (2)
And the number density of inclusions having a circle equivalent diameter of 0.5 μm or more and 4 μm or less in the slab 1/2 thick part is 20 / mm ² or more and the circle equivalent diameter exceeds 10 μm. The number density of the objects is less than 0.2 pieces / mm ² , and the austenite particle size on the slab surface is 0.05 mm or less.

According to the carbon steel slab of this configuration, the number density of inclusions having a circle equivalent diameter of 0.5 μm or more and 4 μm or less of a REM composite inclusion is 20 even in a slab ½ thick part where inclusions tend to be coarse. Since it is set to be at least ² pieces / mm ² , fine REM composite inclusions (REM-Ca-O—S) are sufficiently dispersed in the entire slab, and this REM composite inclusion contains S. , Generation of coarse MnS inclusions can be suppressed. Moreover, the extension of MnS can be suppressed by attaching MnS to this REM composite inclusion (REM-Ca-O-S). At this time, since the average value is controlled to 0.05 mm or less with the austenite grain size on the surface of the slab as a representative index, the grain boundary diffusion of the S progresses rapidly in the entire slab, and the REM composite intervention The amount of MnS adhering to the product (REM-Ca-O-S) increases. Therefore, it is possible to improve the hole expandability with certainty.
In the present invention, the austenite particle size is an average value of measured values obtained by measuring 10 austenite particle sizes on the surface of the slab.

In addition, since the number density of inclusions in which the equivalent circle diameter of the REM composite inclusions in the slab 1/2 thick part exceeds 10 μm is suppressed to less than 0.2 pieces / mm ² , generation of coarse inclusions is prevented. It is suppressed and generation | occurrence | production of the defect resulting from this inclusion can be suppressed.
Further, to produce a liquid phase during the reaction of REM compound inclusions (REM-Ca-O-S ) is _Al 2 _{O 3} based refractories and REM compound inclusions (REM-Ca-O-S ) , clogging of the nozzle Can be suppressed. Therefore, casting can be performed stably.

Here, in the carbon steel slab of the present invention, in mass%,
Mg: 0.0003% or more and 0.002% or less,
Cr: 0.001% or more and 2.0% or less,
Ni: 0.001% or more and 2.0% or less,
Cu: 0.001% to 2.0%,
Nb: 0.001% or more and 0.2% or less,
V; 0.001% to 1.0%,
W: 0.001% to 1.0%,
Zr: 0.0001% or more and 0.2% or less,
As; 0.0001% to 0.5%,
Co: 0.0001% to 1.0%,
Sn: 0.0001% or more and 0.2% or less,
Pb: 0.0001% to 0.2%,
Hf: 0.0001% to 0.2%,
One or more selected from the group consisting of:

A 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 Al is added to the molten steel to perform deoxidation. An acid step, a REM addition step of adding REM to the molten steel to which Al is added, a stirring step of adding REM and stirring for 5 minutes or more, a Ca addition step of adding Ca to the molten steel after stirring, and Ca A continuous casting step of continuous casting after the addition, and a reduction step of reducing the slab in the cooling process of the continuous casting step. In the reduction step, the temperature of the slab ½ thickness part is The carbon steel slab is characterized in that the reduction is 10% or more and 50% or less within a temperature range of 1200 ° C. or more below the solidus temperature of the slab.
In the present invention, the reduction ratio (%) in the reduction step is defined as “{(slab thickness before reduction−slab thickness after reduction) / slab thickness before reduction} × 100”.

According to the method for producing the carbon steel slab of this configuration, since the Al deoxidation step is performed in which Al is added to the molten steel to perform deoxidation, Al ₂ O ₃ is sufficiently present in the molten steel. become. By adding REM in this state, REM inclusions containing _{Al 2 O 3 (REM 2 O} 3 · Al 2 O 3) is generated number. Since it comprises a stirring step for stirring over 5 minutes with the addition of REM, REM inclusions containing _Al 2 _{O 3} a _{(REM 2 O 3 · Al 2} O 3) can be sufficiently generated.
Then, by REM inclusions containing _{Al 2 O 3 (REM 2 O} 3 · Al 2 O 3) is added to Ca in the presence sufficiently, REM composite inclusions containing REM and Ca (REM-Ca- (OS) can be stably produced. Such molten steel is continuously cast, and the temperature of the slab 1/2 thick part is not higher than the solidus temperature of the carbon steel slab, and the reduction rate is not lower than 10% and not higher than 50% in the range of 1200 ° C or higher. By doing, the austenite particle size of the slab surface can be controlled to 0.05 mm or less. The present invention is an invention that greatly reduces the slab being cast, and the slab being cast is reduced at the center of the slab by utilizing the fact that the temperature difference between the slab surface and the 1/2 part of the slab is large. It is an invention that increases efficiency. Therefore, the amount of deformation of the slab 1/2 thick part during the reduction is larger than that of the slab surface. For this reason, the dendrite structure which is a solidification structure becomes finer as the slab ½ thick part where the deformation amount increases. Since the austenite grain size is formed by combining several dendrite structures, the austenite grain size becomes finer as the dendrite structure becomes finer. By reducing the austenite grain size of the slab 1/2 thick part, it is possible to promote the grain boundary diffusion of S in the slab 1/2 thick part where coarse MnS is likely to be generated. Production of MnS with inclusions (REM-Ca-O-S) as the core is promoted, and the amount of MnS produced alone is reduced, so that coarse inclusions are reduced and the above-mentioned carbon steel slab is produced. It becomes possible to do.

  As described above, according to the present invention, there is provided a carbon steel slab capable of suppressing the formation of coarse inclusions and reliably improving the hole expandability, and a method for producing the carbon steel slab. It becomes possible.

It is a ternary phase diagram at 1500 ° C. for explaining the composition range of inclusions in the carbon steel slab according to the embodiment of the present invention. It is a ternary phase diagram at 1500 ° C. for explaining the composition range of inclusions in the carbon steel slab according to the embodiment of the present invention. It is a flowchart of the manufacturing method of the carbon steel slab which is embodiment of this invention. It is explanatory drawing which shows the form of the inclusion at the time of rolling down in the cooling process of casting, without adding REM. It is explanatory drawing which shows the form of the inclusion in the case of adding REM and not implementing reduction in the cooling process of casting. It is explanatory drawing which shows the form of the inclusion at the time of adding REM and implementing reduction in the cooling process of casting. It is an observation photograph of the inclusion in this invention example 1-1. It is an observation photograph of the inclusion in Comparative Example 1-1.

  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 the present embodiment is C: 0.03% to 0.30%, Si: 0.08% to 2.1%, Mn: 0.5% to 4. 0% or less, P; 0.05% or less, S; 0.0001% or more and 0.01% or less, N; 0.01% or less, t. O: 0.0005% to 0.005%, Al; 0.004% to 2.0%, Ti; 0.0001% to 0.20%, REM; 0.001% to 0.02% Hereafter, Ca; 0.0011% or more and 0.005% or less is contained, the balance is made of iron and inevitable impurities, and REM, Ca, t. The mass% of O is [REM], [Ca], [t. O],
0.25 ≦ [Ca] / [REM] ≦ 5 (1)
0.0011 ≦ [Ca] −0.15 × [t. O] ≦ 0.005 (2)
Meet.

Furthermore, in the carbon steel slab according to the present embodiment, the number density of inclusions having a circle equivalent diameter of 0.5 μm or more and 4 μm or less of the REM composite inclusions in the slab ½ thick part is 20 pieces / mm ² or more. In addition, the number density of inclusions having an equivalent circle diameter exceeding 10 μm is set to be less than 0.2 pieces / mm ² .
And in the carbon steel slab which is this embodiment, the austenite particle size in the slab surface is 0.05 mm or less.

  Below, the reason which prescribed | regulated each component and inclusions as mentioned above in the carbon steel slab which is this embodiment is demonstrated.

(C: carbon)
C is the most basic element for controlling the hardenability and strength of steel, and the fatigue strength is improved by forming a hardened and hardened layer hard and deep.
Here, if the C content is less than 0.03%, the retained austenite and the low-temperature transformation phase cannot be sufficiently generated, and the above-described effects cannot be achieved. On the other hand, if the C content exceeds 0.30%, workability and weldability may be deteriorated.
From the above, in this embodiment, the C content is limited to a range of 0.03% to 0.30%.

(Si: silicon)
Si increases the number of nucleation sites of austenite during heating for quenching, suppresses the grain growth of austenite, and refines the grain size of the quenched hardened layer. Moreover, Si suppresses the formation of carbides, suppresses the decrease in grain boundary strength due to carbides, and is also effective for the generation of bainite structure, and ensures the strength of the entire material.
Here, if content of Si is less than 0.08%, the above-mentioned effect cannot be achieved. On the other hand, if the Si content exceeds 2.1%, the SiO ₂ concentration in the inclusions increases, the inclusions become coarse, and the toughness, ductility, and weldability may be reduced.
From the above, in this embodiment, the Si content is limited to a range of 0.08% to 2.1%.

(Mn: Manganese)
Mn is an element having an effect of improving the strength of steel.
Here, if the content of Mn is less than 0.5%, the above-described effects cannot be achieved. On the other hand, when the content of Mn exceeds 4.0%, ductility decreases due to segregation of Mn and increase in solid solution strengthening. In addition, the weldability and the toughness of the base material deteriorate.
From the above, in this embodiment, the Mn content is limited to a range of 0.5% to 4.0%.

(P: phosphorus)
P is effective when used as a substitutional solid solution strengthening element smaller than Fe atoms, but unavoidably 0.0010% is contained.
Here, if the content of P exceeds 0.05%, P segregates at the grain boundaries of austenite, the grain boundary strength decreases, and the workability may deteriorate.
From the above, in this embodiment, the P content is limited to 0.05% or less.

(S: sulfur)
S is contained as an impurity in the steel and easily segregates, and forms MnS-based coarse stretch inclusions to deteriorate the hole expandability.
Here, in order to reduce the S content to less than 0.0001%, a great cost is required. On the other hand, if the S content exceeds 0.01%, the remaining S concentration is high even if the S fixing effect of REM is taken into account, and the hole expandability may be deteriorated.
From the above, in this embodiment, the S content is limited to a range of 0.0001% to 0.01%.

(N: Nitrogen)
N forms nitrides with elements such as Al and Ti, and has the effect of promoting the refinement of the base material structure.
Here, if the N content exceeds 0.01%, coarse nitrides or the like are generated, and the hole expandability deteriorates. In order to reduce the N content to less than 0.0005%, a large cost is required.
From the above, in this embodiment, the N content is limited to 0.01% or less.

(T.O: total oxygen)
t. O is inevitably contained in 0.0005%. Here, t. If the O content exceeds 0.005%, coarse oxides and the like are generated, and the quality of the slab is deteriorated.
From the above, in this embodiment, t. The O content is limited to a range of 0.0005% to 0.005%. Note that t. O (total oxygen) includes O dispersed in the slab in a compound state.

(Al: Aluminum)
Al is an element added to promote deoxidation of molten steel.
Here, when the Al content is less than 0.004%, the deoxidation cannot be sufficiently performed. On the other hand, if the Al content exceeds 2.0%, coarse inclusions (Al ₂ O ₃ clusters) are generated, and the quality of the slab may be deteriorated.
From the above, in this embodiment, the Al content is limited to a range of 0.004% to 2.0%.

(Ti: Titanium)
Ti is an element that forms carbides, nitrides, carbonitrides, contributes to refinement of crystal grains and high strength of steel sheets, and improves hole expansibility.
Here, if the content of Ti is less than 0.0001%, the above-described effects cannot be achieved. On the other hand, if the Ti content exceeds 0.20%, coarse carbonitrides are produced, and the hole expandability may be deteriorated.
From the above, in the present embodiment, the Ti content is limited to the range of 0.0001% to 0.20%.

(REM: rare earth element)
REM is a generic name including Sc, Y, and lanthanoids from La to Lu. REM is added together with Ca to form a REM composite inclusion (REM-Ca-O-S), and MnS adheres to this REM composite inclusion as described in Patent Document 1. As a result, the generation of stretched MnS is suppressed, and the hole expandability is improved.
Here, if content of REM is less than 0.001%, there exists a possibility that there may be no above-mentioned effect. On the other hand, when the REM content exceeds 0.02%, coarse REM composite inclusions (REM-Ca-O-S) are formed, and the hole expandability may be deteriorated.
From the above, in the present embodiment, the REM content is limited to a range of 0.001% to 0.02%.
REM is often added using misch metal because a mixture mainly containing Ce and La called misch metal is easily available.

(Ca: calcium)
When Ca is added together with REM, it forms REM composite inclusions (REM-Ca-O-S) as described above, suppresses the generation of stretched MnS, and improves the hole expandability. Moreover, since a liquid phase is produced at the time of the reaction of the Al ₂ O ₃ refractory constituting the immersion nozzle and the REM composite inclusion, the clogging of the nozzle is suppressed.
Here, if the Ca content is less than 0.0011%, the above-described effects may not be achieved. On the other hand, if the Ca content exceeds 0.005%, CaO—Al ₂ O ₃ -based liquid phase inclusions are formed in the molten steel, and the inclusions may be coarsened to deteriorate the hole expansibility.
From the above, in this embodiment, the Ca content is limited to a range of 0.0011% to 0.005%.

([Ca] / [REM])
If the ratio (Ca) / (REM) between the Ca content (Ca) and the REM content (REM) in the inclusion is within the range of 0.25 to 2.5, the immersion nozzle It is known that a liquid phase can be generated during the reaction with Al ₂ O ₃ constituting, and nozzle clogging can be suppressed. At this time, by adjusting [Ca] / [REM] in the molten steel within a range of 0.25 or more and 5 or less, (Ca) / (REM) is 0.25 or more and 2.5 or less in the generated inclusions. It is possible to control within the range.
Therefore, in this embodiment, the content of REM and the content of Ca in the steel are defined so as to satisfy the following formula (1).
0.25 ≦ [Ca] / [REM] ≦ 5 (1)

([Ca] -0.15 × [t.O])
In order to prevent nozzle clogging, as described above, it is important that (Ca) / (REM) is in the range of 0.25 to 2.5 in the inclusion. In order to control the composition of inclusions within the above-mentioned range, it is important to satisfy the above-mentioned formula (1) as one of the conditions. From the laboratory experiment, it is necessary to adjust the Al content in the inclusions in order to prevent the nozzle clogging by generating a phase and completely becoming a liquid phase and coarsening in molten steel. all right. In addition, it turned out that the average Al content in the inclusion at the time of equilibrium has an approximately proportional relationship with the oxygen content in the molten steel. From this, (Ca) and (Al) in inclusions were defined by the relationship between Ca and t.O.
Here, when [Ca] −0.15 × [t.O] is less than 0.0011, there is a large variation in inclusion composition, and the risk of nozzle clogging is high. On the other hand, when [Ca] -0.15 × [t.O] exceeds 0.005, the REM composite inclusion (REM-Ca-O-S) may become a liquid phase in the molten steel and become coarse. .
Therefore, in this embodiment, the content of Ca and the content of t.O in the steel are defined so as to satisfy the following expression (2).
0.0011 ≦ [Ca] −0.15 × [t.O] ≦ 0.005 (2)

Here, in the ternary phase diagram of CaO—AlO _1.5 —CeO _{1.5 at} 1500 ° C. shown in FIG. 1A, the regions A and B are compositions in which inclusions react with the Al ₂ O ₃ of the nozzle to cause nozzle clogging. It is a range. Region C is a composition range in which inclusions become a liquid phase in molten steel and become coarse. Region D is a composition range in which generation of stretched inclusions of MnS and CaO—Al ₂ O ₃ cannot be suppressed.
And the remaining area | region E becomes a composition range which can suppress the expansion of a MnS type inclusion while preventing the coarsening of an inclusion, and can suppress the obstruction | occlusion of a nozzle. In the present embodiment, the abundance ratio of (REM) and (Ca), which is the main component of the REM composite inclusion (REM-Ca-O-S), is defined by the above formula (1). Since almost all REM and Ca present in the molten steel exist as REM composite inclusions (REM-Ca-O-S), as shown in FIG. 1B, by adjusting the equation (1), REM) and (Ca) can be controlled to an abundance ratio. Further, the (Al ₂ O ₃ ) concentration in the inclusion is defined by the expression (2), and the inclusion composition is adjusted to be stably within the range of the region E. Oxygen in the steel is almost all Al ₂ O ₃ except for REM and Ca oxide, and is taken into the REM composite inclusion (REM-Ca-O-S). By controlling the Al content in the REM composite inclusion (REM-Ca-O-S) to a low level, the liquid phase ratio of the REM composite inclusion (REM-Ca-OS) is suppressed, and by aggregation coalescence Prevents coarsening.

(Number density of inclusions in the center of the slab)
Most of the REM composite inclusions exist in the entire carbon steel slab as inclusions having an equivalent circle diameter of 0.5 μm to 4 μm. In the carbon steel slab according to the present embodiment, when the number density of inclusions having an equivalent circle diameter of 0.5 μm or more and 4 μm or less is less than 20 / mm ² , the number of the above-mentioned REM composite inclusions is small, MnS cannot sufficiently adhere to the REM composite inclusion, and the effect of suppressing the extension of MnS by the REM composite inclusion cannot be exhibited. For this reason, there exists a possibility that hole expansibility cannot fully be improved.
Further, when the number density of inclusions having an equivalent circle diameter exceeding 10 μm is 0.2 pieces / mm ² or more, the hole expandability deteriorates due to the coarse inclusions, and defects due to the coarse inclusions are present. May occur.
Therefore, in this embodiment, the number density of inclusions having an equivalent circle diameter of 0.5 μm or more and 4 μm or less of the REM composite inclusion is 20 pieces / mm ² or more and the number density of inclusions having an equivalent circle diameter of more than 10 μm. Is less than 0.2 pieces / mm ² .

(Austenite particle size)
In the continuous cast slab after continuous casting, the crystal structure formed during cooling after solidification can be observed. Among these, the smaller the austenite particle size above the temperature at which MnS is generated, the more preferable it is because the diffusion of S is facilitated. Here, the austenite grain size means the grain size of the austenite phase formed when the slab temperature is in the austenite region in the process of cooling the slab after completion of solidification during continuous casting. Since the slab after casting is cooled via the ferrite transformation region, the austenite phase is often not observed, but the austenite grain size of the slab observes a cross section perpendicular to the growth direction of dendrites, which are solidified structures. And the area | region where the direction of a dendrite is the same can be evaluated as an austenite particle size.
The austenite grain size was evaluated at the slab surface layer where the amount of deformation during rolling immediately after solidification was small. The center portion of the plate thickness is higher in temperature than the slab surface, so the amount of deformation is large and fine. When the austenite grain size on the slab surface exceeds 0.05 mm, the grain boundary diffusion of S does not proceed sufficiently during solidification, and the amount of adhered MnS is reduced, which may deteriorate the hole expandability. . Therefore, in this embodiment, the average value of the austenite particle size on the slab surface is set to 0.05 mm or less.
In addition, although there is no restriction | limiting in particular in the minimum of an austenite particle size, About 0.0005mm is a minimum in an average value also with the manufacturing method of the slab which concerns on this invention.
Moreover, the austenite particle diameter was made into the average value of the measured value which measured ten austenite particle diameters of the slab surface.

<Method for producing carbon steel slab>
Next, the manufacturing method of the carbon steel slab which is this embodiment is demonstrated, referring the flowchart of FIG.

(Melting step S01)
First, in mass%, C; 0.03% or more and 0.30% or less, Si; 0.08% or more and 2.1% or less, Mn; 0.5% or more and 4.0% or less, P; 0.05 %, S; 0.0001% or more and 0.01% or less, N; 0.01% or less, Ti; 0.0001% or more and 0.20% or less.

(Al deoxidation step S02)
Next, a deoxidizer such as Al is added to the molten steel, and deoxidation treatment is performed so that the t.O content is 0.0005% or more and 0.005% or less. Since the content of O is the total oxygen concentration of the dissolved oxygen and the oxygen in the inclusions, the content of O is within the range of 0.004% or more and 2.0% or less. Predictive adjustments can be made within the range of normal operation technology, including the effects of floating removal of inclusions. In addition, you may add a desulfurization agent as needed and may perform a finishing desulfurization process. Moreover, you may add arbitrary additional elements, such as Cr, Ni, Cu, at this process.

(REM addition step S03 / stirring step S04)
Next, REM is added to the molten steel so as to be 0.001% or more and 0.02% or less in the slab. Then, Al ₂ O ₃ and MgO · Al ₂ O ₃ react with REM, and then react with S to form a large number of REM-Al—O—S inclusions.
And stirring is implemented for 5 minutes or more after REM addition. Thereby, the above-mentioned REM-Al-O-S inclusion is sufficiently formed.

(Ca addition step S05)
Next, considering the formulas (1) and (2), the Ca addition yield is also taken into account, and Ca is added to the molten steel so as to be 0.0011% or more and 0.005% or less in the slab. Then, REM-Al-O-S inclusions and Ca react to form a large number of REM composite inclusions (REM-Ca-O-S).

(Continuous casting process S06 and reduction process S07)
Next, the above-mentioned molten steel is poured into a mold of a continuous casting apparatus to continuously cast carbon steel slabs. At this time, rolling is performed at a rolling rate of 10% or more and 50% or less in a temperature range where the temperature of the slab 1/2 thick part is not higher than the solidus temperature of the carbon steel slab and is 1200 ° C. or higher. Thereby, a coarse REM compound inclusion (REM-Ca-OS) is crushed.

  Here, the forms of the molten steel, the slab after solidification, and the inclusions in the product will be described with reference to FIGS. 3 to 5, (a) is in the molten steel, (b) is in the solidification process (at the time of rolling), (c) is the slab after solidification, (d) is the product in the product after rolling the slab. It shows the form of the object.

When REM is not added to the molten steel, as shown in FIG. 3, inclusions 30 mainly composed of calcium aluminate (CaO—Al ₂ O ₃ ) are present in the molten steel in addition to CaS. The inclusions 30 are stretched during rolling and rolling, and the hole expandability is reduced in the rolled product.

On the other hand, when REM is added to the molten steel, as shown in FIGS. 4 and 5, the inclusion 40 made of a coarse REM composite oxide exists in the molten steel. The inclusion 40 has a structure in which a liquid Ca rich phase 42 surrounds a solid phase REM rich phase 41. Here, the Ca rich phase 42 is mainly composed of calcium aluminate (CaO—Al ₂ O ₃ ).

And some CaO of calcium aluminate is replaced with CaS at the time of solidification. Then, the Ca-rich phase 42 is mainly composed of liquid phase calcium aluminate (CaO—Al ₂ O ₃ ) and hard solid phase CaS. Here, in the case where no reduction is performed in the cooling process of casting, as shown in FIG. 4, there is a coarse REM composite inclusion (REM-Ca-O-S) having a coarse structure in the slab. become. This coarse REM composite inclusion (REM-Ca-O-S) is mainly composed of hard solid CaS in the Ca-rich phase 42, so it is not crushed by subsequent rolling and remains in the product as it is. However, the hole expandability is reduced.

On the other hand, when the above-described reduction step S07 is performed in the continuous casting step S06 as in the present embodiment, the solid phase REM rich phase 41 is surrounded around the solid phase REM rich phase 41 as shown in FIG. Since the reduction is performed in a state in which the Ca-rich phase 42 mainly composed of the liquid phase calcium aluminate (CaO—Al ₂ O ₃ ) is surrounded, the inclusions 40 can be finely crushed. Thereby, the inclusion 40 present in the product can be made finer, and the hole expandability of the product is improved.

According to the carbon steel slab of the present embodiment configured as described above, the number density of inclusions whose equivalent circle diameter of the REM composite inclusion of the slab 1/2 thick part is 0.5 μm or more and 4 μm or less. Is 20 pieces / mm ² or more.
And the production | generation of coarse MnS can be suppressed because MnS adheres to the finely disperse | distributed REM compound inclusion (REM-Ca-OS), and the extending | stretching of MnS at the time of rolling can be suppressed. Therefore, it is possible to improve the hole expandability with certainty.

In addition, since the number density of inclusions having an equivalent circle diameter exceeding 10 μm is suppressed to less than 0.2 pieces / mm ² , the formation of coarse inclusions is suppressed, and the hole expandability deteriorates. While being able to suppress, generation | occurrence | production of the defect resulting from a coarse inclusion can be suppressed.
Further, to produce a liquid phase during the reaction of _Al 2 _{O 3} based refractories and REM compound inclusions (REM-Ca-O-S ), clogging of the nozzles is suppressed. Therefore, casting can be performed stably.

  Furthermore, in the carbon steel slab according to the present embodiment, since the austenite grain size on the slab surface is finely controlled to be 0.05 mm or less, the grain boundary diffusion of S can be promoted, and the REM composite intervening By attaching MnS to the product (REM-Ca-O-S), it becomes possible to reduce the stretched inclusions.

Further, according to the method of producing a carbon steel slabs are present embodiment is provided with the Al deoxidation step S02 to perform the deoxidation by adding Al against the molten steel, Al ₂ O ₃ in molten steel There will be enough.
Since they have REM addition step S03 of adding REM Thereafter, REM inclusions containing _{Al 2 O 3 (REM 2 O} 3 · Al 2 O 3) is generated. Further, is provided with the stirring step S04 stirring over 5 minutes with the addition of REM, REM inclusions containing _Al 2 _{O 3} a _{(REM 2 O 3 · Al 2} O 3) can be produced reliably.

Then, after the REM addition step S03 and stirring step S04, since it has a Ca addition step S05 of adding Ca, REM inclusions containing _{Al 2 O 3 (REM 2 O} 3 · Al 2 O 3) is sufficiently In this state, Ca is added, and REM and Ca-containing REM composite inclusions (REM-Ca-O-S) can be stably generated.
Furthermore, since it has a continuous casting step S06 and a reduction step S07, coarse REM composite inclusions (REM-Ca-O-S) can be crushed by reduction, and the above-mentioned carbon steel slab can be produced. It becomes.

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.
For example, it has been described that the O and S concentrations are adjusted by adding a deoxidizing agent and a desulfurizing agent such as Al. However, the present invention is not limited to this, and the O and S concentrations are adjusted by other means. Also good.

Further, the carbon steel slab of the present invention may further contain Mg: 0.0003% or more and 0.002% or less as an additive element in mass%. This Mg has the effect of suppressing the clustering of Al ₂ O ₃ . Here, if the Mg content is less than 0.0003%, the above-described effects may not be achieved. On the other hand, if the Mg content exceeds 0.002%, Mg may cause the refractory to melt. Therefore, when it contains Mg, it is preferable to make content of Mg into the range of 0.0003% or more and 0.002% or less.

  Further, the carbon steel slab of the present invention is further added as an additive element in mass%, Cr: 0.001% to 2.0%, Ni: 0.001% to 2.0%, Cu; 0 0.001% to 2.0%, Nb; 0.001% to 0.2%, V; 0.001% to 1.0%, W; 0.001% to 1.0%, Zr 0.0001% to 0.2%, As; 0.0001% to 0.5%, Co; 0.0001% to 1.0%, Sn; 0.0001% to 0.2% Pb; 0.0001% or more and 0.2% or less, Hf; 0.0001% or more and 0.2% or less, and one or two or more selected from the group consisting of:

  All of these elements are contained as necessary for improving the strength and toughness of the steel sheet, and the REM composite inclusion (REM-Ca-O-) which is a basic feature of the present invention. S) is produced stably and finely, so that the hole expandability is surely improved and the effect of suppressing nozzle blockage during continuous casting is not affected.

  In order to further secure the strength of the steel sheet, Cr can be contained in the steel as necessary. In order to obtain this effect, 0.01% or more of Cr may be added to the steel. However, if a large amount of Cr is contained, the balance between strength and ductility deteriorates, so the upper limit is 2.0%. The lower limit of the Cr concentration is 0.001% due to the influence of mixing from scraps and the like. Ni and Cu are elements that improve the hardenability and increase the strength of the steel, and any of them can be contained in the steel in the range of 0.001% to 2.0% as necessary.

  Nb, W, and V are elements that form carbides, nitrides, and carbonitrides with C or N, promote fine graining of the base material structure, and improve toughness. Therefore, you may add 0.01% or more of Nb in steel. However, even if a large amount of Nb is added and the Nb concentration exceeds 0.2%, the effect of refining the structure of the base material is saturated and the manufacturing cost only increases, so the upper limit is 0.2%. is there. The lower limit of the Nb concentration is 0.001% due to the influence of mixing from scraps and the like. Similarly, W and V may be added in the range of 0.01% to 1.0%. These elements also have a lower concentration limit of 0.001%.

  Zr is an element that spheroidizes sulfides and improves the toughness of the base metal, so 0.001% or more may be added to the steel. However, if added in a large amount in steel, the cleanliness of the steel is impaired and the ductility deteriorates, so the upper limit is 0.2%. The lower limit of the Zr concentration is 0.0001%.

  Furthermore, when scrap or the like is used as a raw material, As, Co, Sn, Pb, and Hf may inevitably be mixed. In order to prevent these elements from adversely affecting the mechanical properties of the steel sheet, it is preferable to limit the concentration of each element as follows. The upper limit of As concentration is 0.5%, and the upper limit of Co concentration is 1.0%. Further, the upper limit of the concentration of Sn, Pb, Hf is 0.2%. Note that the lower limit of the concentration of these elements is 0.0001%.

  In the following, the results of experiments conducted to confirm the effects of the present invention will be described.

Example 1
The molten steel adjusted to the molten steel components described in Table 1 was cast into a mold having a casting width of 2000 mm and a slab thickness of 240 mm using a vertical bending continuous casting apparatus. The solidus temperature in this steel type is 1459 ° C.

  A reduction roll was provided at a position after completion of solidification during continuous casting, and the reduction was performed while controlling the temperature and reduction rate at the center of the plate thickness shown in Table 2. The temperature at the center of the plate thickness at the time of rolling was controlled by changing the solidification completion position by changing the casting speed from 0.5 m / min to 1.5 m / min. The plate thickness center temperature at the reduction position was calculated by a slab heat transfer simulation during continuous casting using the thermal fluid analysis software FLUENT. In addition, it has been confirmed in advance that the temperature measurement result at the center portion of the slab thickness matches the heat transfer simulation result.

  Further, the measurement of the austenite grain size of the slab was taken from the surface of the slab, 50 mm in the casting direction, 50 mm in the width direction, and 10 mm in the thickness direction, and after grinding 0.1 mm in the thickness direction from the black skin surface, Mirror polishing was performed. Then, it was immersed in a saturated aqueous picric acid solution at room temperature for 60 seconds, and the crystal grain size was observed. A region in which the directions of the dendrite, which is a solidified structure, are the same is defined as one austenite grain, and the equivalent circle diameter of the crystal grain was measured. Ten austenite grains were investigated, and the average value was defined as the austenite grain size of the slab.

  Moreover, the observation photograph of the inclusion of this invention example 1-1 is shown in FIG. 6, and the observation photograph of the inclusion of Comparative Example 1-1 is shown in FIG.

  In Comparative Example 1-1 in which the rolling start temperature was equal to or higher than the solidus temperature, the concentrated molten steel was extruded by the rolling, and the aggregated coalescence was promoted by the concentrated molten steel, so that the inclusions became coarse.

  In Comparative Example 1-2, in which the reduction start temperature was 1150 ° C., the inclusions were not crushed even after the reduction, and remained coarse. This is presumably because the Ca-rich phase changed from a low-melting-point calcium aluminate phase to CaS as the temperature became lower, which made it difficult to crush.

  In Comparative Example 1-3 in which the reduction ratio was 5%, the inclusions were not crushed even after the reduction, and the inclusions remained coarse. Moreover, the austenite particle size was 0.05 mm or more, and the amount of MnS adhering to the inclusions was small.

  On the other hand, the rolling start temperature is from the solidus temperature or lower to 1200 ° C., and in this example where the rolling reduction is 10% to 50%, the inclusions are crushed and coarse inclusions are reduced, The number density of inclusions having an equivalent circle diameter of REM composite inclusions of 0.5 μm or more and 4 μm or less increased.

(Example 2)
Next, based on the result of Example 1, the influence of the component composition of the carbon steel slab was confirmed.
The additive elements were added by the procedure shown in FIG. 2 so that the molten steel components described in Table 3 were obtained. That is, after adding Al by RH, an Fe-Si-REM alloy was added by RH as REM. After stirring for 5 minutes or more after REM addition, Ca-Si was added with a wire. And the obtained molten steel was cast with the continuous casting machine. In the cooling process during continuous casting, the slab during continuous casting was reduced by 20% while the temperature of the slab 1/2 thick part was below the solidus temperature of the carbon steel slab and above 1200 ° C. . After casting, an observation sample was collected from a ½ thick part of the obtained slab, and the number density of inclusions having a circle equivalent diameter exceeding 10 μm in particle diameter and 0.5 μm or more and 4 μm or less was calculated. The evaluation results are shown in Table 4.

  Further, the obtained slab is heated to a temperature of 1150 ° C. or higher in a heating furnace, hot-rolled at a finishing temperature of about 900 ° C., cooled at an average cooling rate of 30 ° C./sec, and then wound at about 300 ° C. The coil was wound up at a coiling temperature to obtain a hot-rolled steel sheet having a thickness of 2.8 mm. And hole expansibility was evaluated based on JISZ2256: 2010. The evaluation results are also shown in Table 4.

In Comparative Example 2-1, in which the content of REM is less than the range of the present invention, the improvement of hole expansibility was insufficient. Although the number of inclusions having a circle equivalent diameter of 0.5 μm or more and 4 μm or less of the REM composite inclusion was small, it is presumed that the generation of REM composite inclusion was small in the first place and extension of MnS could not be suppressed. .
In Comparative Example 2-2 in which the content of REM was larger than the range of the present invention, it was out of the range of the formula (1), but since there was too much REM in the first place, coarse inclusions (REM-Ca- A large amount of O-S) was generated, and the improvement of hole expansibility was insufficient. In addition, nozzle blockage was observed.

In Comparative Example 2-3 in which the Ca content was larger than the range of the present invention, a large amount of coarse inclusions (CaO—Al ₂ O ₃ ) was generated, and the improvement of the hole expansibility was insufficient.
In Comparative Example 2-4 in which the Ca content was less than the range of the present invention and out of the range of the formula (2), nozzle clogging was observed.

  In Comparative Example 2-5 outside the range of the formula (1), since the amount of REM was large, a liquid phase was not produced during the reaction with the nozzle, and nozzle clogging was observed.

  In Comparative Example 2-6 outside the range of the formula (2), many coarse REM composite inclusions (REM-Ca-O-S) were observed. This is presumed that the inclusions were coarsened by agglomeration because the inclusions existed in the molten steel as a liquid phase.

On the other hand, in Invention Examples 2-1 to 2-22 within the scope of the present invention, the number density of inclusions having a circle equivalent diameter of 0.5 μm or more and 4 μm or less of the REM composite inclusion is 20 pieces. / mm is ² or more, the number density of inclusions circle equivalent diameter of more than 10μm is less than 0.2 pieces / mm ^2. Thereby, the hole expansibility was fully improved. Also, no nozzle blockage was observed.

  From the above, according to the present invention, it is possible to suppress the formation of coarse inclusions, reliably improve the hole expandability, further suppress nozzle clogging, and perform stable casting. It was confirmed that it was possible.

S02 Al deoxidation step S03 REM addition step S04 Stirring step S05 Ca addition step S06 Continuous casting step S07 Reduction step

Claims (3)

% By mass
C: 0.03% to 0.30%,
Si: 0.08% to 2.1%,
Mn: 0.5% to 4.0%,
P: 0.05% or less,
S; 0.0001% to 0.01%,
N: 0.01% or less,
t. O: 0.0005% or more and 0.005% or less,
Al: 0.004% to 2.0%,
Ti: 0.0001% or more and 0.20% or less,
REM; 0.001% to 0.02%,
Ca; 0.0011% or more and 0.005% or less,
And the balance consists of iron and inevitable impurities,
REM, Ca, t. The mass% of O is [REM], [Ca], [t. O],
0.25 ≦ [Ca] / [REM] ≦ 5 (1)
0.0011 ≦ [Ca] −0.15 × [t. O] ≦ 0.005 (2)
The filling,
Slab half the number density of the thick portion of the REM composite inclusions circle equivalent diameter 0.5μm or more 4μm following inclusions is 20 / mm ² or more and the number of inclusions circle equivalent diameter is more than 10μm The density is less than 0.2 pieces / mm ² ;
A carbon steel slab characterized in that the austenite grain size on the slab surface is 0.05 mm or less. Furthermore, in mass%,
Mg: 0.0003% or more and 0.002% or less,
Cr: 0.001% or more and 2.0% or less,
Ni: 0.001% or more and 2.0% or less,
Cu: 0.001% to 2.0%,
Nb: 0.001% or more and 0.2% or less,
V; 0.001% to 1.0%,
W: 0.001% to 1.0%,
Zr: 0.0001% or more and 0.2% or less,
As; 0.0001% to 0.5%,
Co: 0.0001% to 1.0%,
Sn: 0.0001% or more and 0.2% or less,
Pb: 0.0001% to 0.2%,
Hf: 0.0001% to 0.2%,
The carbon steel slab according to claim 1, comprising one or more selected from the group consisting of: A method for producing a carbon steel slab for producing the carbon steel slab according to claim 1 or 2,
Al deoxidation step of adding Al to the molten steel and deoxidizing;
REM addition step of adding REM to molten steel to which Al is added;
A stirring step of adding REM and stirring for 5 minutes or more;
A Ca addition step of adding Ca to the molten steel after stirring;
A continuous casting process of continuous casting after adding Ca;
A reduction process for reducing the slab in the cooling process of the continuous casting process,
In the reduction step, the reduction of the reduction ratio of 10% or more and 50% or less is performed within a temperature range of 1200 ° C. or more at a temperature of the slab 1/2 thick portion below the solidus temperature of the carbon steel slab. A method for producing a carbon steel slab characterized.