JP6791008B2 - Manufacturing method of carbon steel slabs and carbon steel shards - Google Patents

Description

Carbon steel slabs used as materials for steel plates with excellent hole expansion properties, which are suitable for undercarriage parts and structural materials for automobiles and the like, which are mainly pressed and used, and methods for manufacturing the carbon steel slabs. It is about.

Steel sheets used for the body structure of automobiles are required to have high press workability and strength, and many inventions have been made. However, in order to meet the demands of today's automobiles for further weight reduction and complicated shape of parts, carbon steel having better hole expansion property than before is required.
Conventionally, in the above-mentioned carbon steel, it is known that the hole expandability is deteriorated by the elongated sulfide-based inclusions such as MnS.

Therefore, for example, in Patent Document 1, by adding at least one selected from Ce and La, which are one type of REM (rare earth element), MnS-based inclusions are finely precipitated to improve the hole expandability. The technology to do is proposed. When Al is added as a deoxidizer during the production of molten steel and Ce and La are added to the place where the produced Al ₂ O ₃ is suspended, some Al ₂ O ₃ remains, but Al _{2 in} the molten steel O ₃ inclusions is reduced and decomposed, Ce, fine and hard Ce oxides by deoxidation by La, La oxides, cerium oxysulfide, and are believed to lanthanum oxysulfide are generated. As a result, MnS can be deposited on those oxysulfides and the like, and deformation of the precipitated MnS is suppressed during rolling, so that the stretched coarse MnS in the steel sheet can be significantly reduced. It is a technology that can improve the hole expandability.

Further, in order to prevent the occurrence of immersion nozzle clogging during continuous casting while maintaining the stretching prevention effect by controlling the morphology of inclusions by adding REM to the steel, a means of adding Ca at the same time is also adopted. ing.
For example, in Patent Document 2, by suppressing the REM addition concentration to 0.0060% or less and adding Ca appropriately, clogging of the immersion nozzle and the generation of a large amount of coarse low melting point inclusions are prevented. .. Further, Patent Document 3 also proposes a steel material containing any one or more of REM, Ca, and Mg.

Japanese Patent No. 5093422 Japanese Unexamined Patent Publication No. 2012-046789 Japanese Unexamined Patent Publication No. 2008-223043

By the way, in Patent Document 1, when REM is added to molten steel and continuous casting is performed, there is a problem that the immersion nozzle is easily blocked and the operation is not stable. When continuously casting REM-added steel, it is known that the refractory material of the immersion nozzle (hereinafter, simply referred to as "nozzle") reacts with REM to cause nozzle blockage. Is often taken to prevent nozzle blockage.
However, as in Patent Documents 2 and 3, simply adding REM and Ca has the presence of coarse inclusions, and it has not been possible to stably improve the hole expandability.

The present invention has been made in view of the above-mentioned situations, and is a carbon steel slab and a carbon steel slab capable of suppressing the formation of coarse inclusions and reliably improving the hole expansion property. It is an object of the present invention to provide the manufacturing method of.

As a result of diligent studies by the present inventors in order to solve the above-mentioned problems, the following findings were obtained.
The relatively coarse REM composite inclusions (REM-Ca-OS) present in the slab consist of several REM-rich phases and a CaS-based Ca-rich phase that surrounds them. Has been done. The REM composite inclusions (REM-Ca-OS) are rigid, exist in their original form upon rolling, and remain coarse.

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

The present invention has been made based on the above findings, and the carbon steel slab according to the present invention is based on 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% or less,
S; 0.0001% or more and 0.01% or less,
N; 0.01% or less,
t. O; 0.0005% or more and 0.005% or less,
Al; 0.004% or more and 2.0% or less,
Ti; 0.0001% or more and 0.20% or less,
REM; 0.001% or more and 0.02% or less,
Ca; 0.0011% or more and 0.005% or less,
The balance is composed of iron and unavoidable impurities, and REM, Ca, t. The mass% of O was changed to [REM], [Ca], and [t. When O] is set,
0.25 ≤ [Ca] / [REM] ≤ 5 ... (1)
0.0011 ≦ [Ca] −0.15 × [t. O] ≤0.005 ... (2)
The number of inclusions having a circle-equivalent diameter of 0.5 μm or more and 4 μm or less in the 1/2 thick part of the slab is 20 pieces / mm ² or more, and the circle-equivalent diameter exceeds 10 μm. It is characterized in that the number density of objects is less than 0.2 pieces / mm ² and the austenite particle size on the surface of the slab is 0.05 mm or less.

According to the carbon steel slab having this configuration, 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 is 20 even in a slab 1/2 thick portion where inclusions are likely to be coarsened. Since the number of pieces / mm is ² or more, fine REM composite inclusions (REM-Ca-OS) are sufficiently dispersed in the entire slab, and the REM composite inclusions contain S. , The formation of coarse MnS-based inclusions can be suppressed. Further, the extension of MnS can be suppressed by adhering MnS to the REM composite inclusions (REM-Ca-OS). At this time, since the average value of the austenite particle size on the slab surface is controlled to 0.05 mm or less as a representative index, the grain boundary diffusion of S rapidly progresses in the entire slab, and the REM composite intervention The amount of MnS adhering to the object (REM-Ca-OS) increases. Therefore, it is possible to surely improve the hole expandability.
In the present invention, the austenite particle size was taken as the average value of the measured values obtained by measuring 10 austenite particle sizes on the slab surface.

Further, since the number density of inclusions having a circle-equivalent diameter of more than 10 μm in the 1/2 thick part of the slab is suppressed to less than 0.2 pieces / mm ² , coarse inclusions are generated. It is suppressed, and the generation of defects caused by 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, further, by 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% or more and 2.0% or less,
Nb; 0.001% or more and 0.2% or less,
V; 0.001% or more and 1.0% or less,
W; 0.001% or more and 1.0% or less,
Zr; 0.0001% or more and 0.2% or less,
As; 0.0001% or more and 0.5% or less,
Co; 0.0001% or more and 1.0% or less,
Sn; 0.0001% or more and 0.2% or less,
Pb; 0.0001% or more and 0.2% or less,
Hf; 0.0001% or more and 0.2% or less,
It may contain one or more selected from the group consisting of.

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-mentioned carbon steel slab, and Al deoxidation is performed by adding Al to molten steel to deoxidize the molten steel. An acid step, a REM addition step of adding REM to the molten steel to which Al has been 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. It includes a continuous casting step of continuous casting after addition and a reduction step of reducing the slab in the cooling process of the continuous casting process. In the reduction step, the temperature of 1/2 thick part of the slab is the same. It is characterized in that the reduction rate is 10% or more and 50% or less in the temperature range of 1200 ° C. or more below the solidus line temperature of the carbon steel slab.
In the present invention, the reduction rate (%) in the reduction step is defined as "{(thickness of slab before reduction-thickness of slab after reduction) / thickness of slab before reduction} x 100".

According to the method for producing carbon steel slabs having this configuration, an Al deoxidizing step of adding Al to molten steel to deoxidize it is provided, so that 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 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 generated. Such molten steel is continuously cast, and the temperature of 1/2 thick part of the slab is below the solidus temperature of the carbon steel slab, and the reduction rate is 10% or more and 50% or less in the range of 1200 ° C. or more. By doing so, the austenite particle size on the surface of the slab can be controlled to 0.05 mm or less. The present invention is an invention for greatly reducing the slab during casting, and the slab during casting utilizes the fact that the temperature difference between the slab surface and the slab 1/2 part is large to reduce the slab center portion. It is an invention that enhances efficiency. Therefore, the amount of deformation of the slab 1/2 thick portion at the time of reduction is larger than that of the slab surface. Therefore, the dendrite structure, which is a solidified structure, becomes finer as the deformation amount increases to 1/2 the thickness of the slab. Since the austenite particle size is formed by coalescing several dendrite structures, the finer the dendrite structure, the finer the austenite particle size. By making the austenite grain size of the 1/2 thick part of the slab finer, it is possible to promote the grain boundary diffusion of S in the 1/2 thick part of the slab where coarse MnS is likely to be generated, thereby REM composite. The formation of MnS centered on inclusions (REM-Ca-OS) 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 are provided a carbon steel slab capable of suppressing the formation of coarse inclusions and surely improving the hole expansion property, 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 flow chart of the manufacturing method of the carbon steel slab which is an embodiment of this invention. It is explanatory drawing which shows the morphology of inclusions when the reduction is performed in the cooling process of a casting without adding REM. It is explanatory drawing which shows the morphology of inclusions when REM is added and reduction is not carried out in the cooling process of a casting. It is explanatory drawing which shows the morphology of inclusions when REM is added and reduction is carried out in the cooling process of a casting. It is an observation photograph of inclusions in Example 1-1 of this invention. It is an observation photograph of inclusions in Comparative Example 1-1.

The carbon steel slab and the method for producing the carbon steel slab according to the embodiment of the present invention will be described below with reference to the attached drawings. The present invention is not limited to the following embodiments.

The carbon steel slab according to the present embodiment has 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 in mass%. 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% or more and 0.005% or less, Al; 0.004% or more and 2.0% or less, Ti; 0.0001% or more and 0.20% or less, REM; 0.001% or more and 0.02% Hereinafter, Ca; 0.0011% or more and 0.005% or less is contained, and the balance is composed of iron and unavoidable impurities, and REM, Ca, t. The mass% of O was changed to [REM], [Ca], and [t. When O] is set,
0.25 ≤ [Ca] / [REM] ≤ 5 ... (1)
0.0011 ≦ [Ca] −0.15 × [t. O] ≤0.005 ... (2)
Meet.

Further, in the carbon steel slab of the present embodiment, 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 pieces / mm ² or more. Moreover, the number density of inclusions having a circle-equivalent diameter of more than 10 μm is less than 0.2 pieces / mm ² .
In the carbon steel slab of the present embodiment, the austenite particle size on the slab surface is 0.05 mm or less.

The reason why each component and inclusions are defined as described above in the carbon steel slab according to the present embodiment will be described below.

(C: carbon)
C is the most basic element that controls the hardenability and strength of steel, and the fatigue strength is improved by forming the hardenable 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 produced, and the above-mentioned effects cannot be achieved. On the other hand, if the C content exceeds 0.30%, the workability and weldability may deteriorate.
From the above, in the present embodiment, the C content is limited to the range of 0.03% or more and 0.30% or less.

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

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

(P: Rin)
P is effective when used as a substituted solid solution strengthening element smaller than the Fe atom, but unavoidably contains 0.0010%.
Here, if the content of P exceeds 0.05%, P may segregate at the grain boundaries of austenite, the grain boundary strength may decrease, and the workability may deteriorate.
From the above, in the present embodiment, the content of P is limited to 0.05% or less.

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

(N: Nitrogen)
N forms a nitride with elements such as Al and Ti, and has an action effect of promoting the miniaturization of the base metal structure.
Here, if the N content exceeds 0.01%, coarse nitrides and the like are generated, and the hole expandability deteriorates. It should be noted that a great cost is required to reduce the N content to less than 0.0005%.
From the above, in the present embodiment, the N content is limited to 0.01% or less.

(TO: 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 deteriorates.
From the above, in the present embodiment, t. The content of O is limited to the range of 0.0005% or more and 0.005% or less. In addition, t. O (total oxygen) contains O dispersed in the slab in the state of a compound.

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

(Ti: Titanium)
Ti is an element that forms carbides, nitrides, and carbonitrides, contributes to finer crystal grains and higher strength of steel sheets, and improves hole expandability.
Here, if the Ti content is less than 0.0001%, the above-mentioned effects cannot be achieved. On the other hand, if the Ti content exceeds 0.20%, coarse carbonitride may be formed 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% or more and 0.20% or less.

(REM: rare earth element)
REM is a generic term that includes Sc, Y, and lanthanoids from La to Lu. When REM is added together with Ca, REM composite inclusions (REM-Ca-OS) are formed, and MnS adheres to the REM composite inclusions as described in Patent Document 1. This suppresses the formation of MnS to be stretched and improves the hole expandability.
Here, if the content of REM is less than 0.001%, the above-mentioned effects may not be exhibited. On the other hand, if the REM content exceeds 0.02%, coarse REM composite inclusions (REM-Ca-OS) may be formed and the hole expandability may be deteriorated.
From the above, in the present embodiment, the content of REM is limited to the range of 0.001% or more and 0.02% or less.
Since a mixture mainly containing Ce and La called mischmetal is easily available, REM is often added using mischmetal.

(Ca: calcium)
When Ca is added together with REM, it forms a REM composite inclusion (REM-Ca-OS) as described above, suppresses the formation of MnS to be stretched, and improves the hole expandability. Further, since a liquid phase is generated during the reaction between the Al ₂ O ₃ refractory constituting the immersion nozzle and the REM composite inclusions, the nozzle blockage is suppressed.
Here, if the Ca content is less than 0.0011%, the above-mentioned effects may not be achieved. On the other hand, if the Ca content exceeds 0.005%, CaO-Al ₂ O ₃ system liquid phase inclusions may be formed in the molten steel, and the inclusions may become coarse and deteriorate the hole expandability.
From the above, in the present embodiment, the Ca content is limited to the range of 0.0011% or more and 0.005% or less.

([Ca] / [REM])
From the results of laboratory experiments, if the ratio (Ca) / (REM) of the Ca content (Ca) to the REM content (REM) in the inclusions is in the range of 0.25 or more and 2.5 or less, the immersion nozzle It is known that a liquid phase can be generated during the reaction with Al ₂ O ₃ which constitutes the above, and nozzle clogging can be suppressed. At this time, by adjusting [Ca] / [REM] in the molten steel within the range of 0.25 or more and 5 or less, (Ca) / (REM) in the generated inclusions is 0.25 or more and 2.5 or less. It is possible to control within the range of.
Therefore, in the present embodiment, the REM content and the Ca content in the steel are defined so as to satisfy the following equation (1).
0.25 ≤ [Ca] / [REM] ≤ 5 ... (1)

([Ca] −0.15 × [t.O])
In order to prevent nozzle blockage, it is important that (Ca) / (REM) is in the range of 0.25 or more and 2.5 or less in the inclusions as described above. In order to control the composition of inclusions within the above range, it is important to satisfy the above equation (1) as one of the conditions, but at the same time, the generated inclusions are liquid when reacting with the nozzle. From laboratory experiments, it is necessary to adjust the Al content in the inclusions in order to generate a phase, prevent nozzle blockage, and prevent the molten steel from becoming a completely liquid phase and becoming coarse. all right. It was found that the average Al content in the inclusions at equilibrium has a roughly proportional relationship with the oxygen content in the molten steel. From this, (Ca) and (Al) in the inclusions were defined by the relationship between Ca and t.O.
Here, when [Ca] −0.15 × [t.O] is less than 0.0011, the inclusion composition varies widely and there is a high risk of nozzle clogging. On the other hand, when [Ca] −0.15 × [t.O] exceeds 0.005, the REM composite inclusions (REM-Ca—OS) become a liquid phase in the molten steel and may become coarse. ..
Therefore, in the present embodiment, the Ca content and the t.O content in the steel are defined so as to satisfy the following equation (2).
0.0011 ≤ [Ca] -0.15 x [t.O] ≤ 0.005 ... (2)

Here, in the CaO-AlO _1.5- CeO _1.5 ternary phase diagram at 1500 ° C. shown in FIG. 1A, the regions A and B have a composition in which inclusions react with Al ₂ O ₃ of the nozzle to cause nozzle blockage. The range. Further, region C is a composition range in which inclusions become a liquid phase in molten steel and become coarse. The region D is a composition range in which the formation of stretching inclusions of MnS and CaO-Al ₂ O ₃ cannot be suppressed.
The remaining region E is a composition range in which the coarsening of inclusions can be prevented, the stretching of MnS-based inclusions can be suppressed, and the clogging of the nozzle can be suppressed. In the present embodiment, the abundance ratio of (REM) and (Ca), which are the main components of the REM composite inclusion (REM-Ca-OS), is defined by the above-mentioned equation (1). Almost all of REM and Ca present in the molten steel are present as REM composite inclusions (REM-Ca-OS). Therefore, as shown in FIG. 1B, by adjusting to the equation (1), the region E ( The abundance ratio of REM) and (Ca) can be controlled. Further, the concentration of (Al ₂ O ₃ ) in the inclusions is defined by the equation (2), and the inclusion composition is adjusted so as to be stable within the range of the region E. Oxygen in steel becomes Al ₂ O ₃ except for oxides of REM and Ca, and is taken into the REM composite inclusions (REM-Ca-OS). By controlling the Al content in the REM composite inclusions (REM-Ca-OS) to a low level, the liquid phase ratio of the REM composite inclusions (REM-Ca-OS) is suppressed, and by aggregation and coalescence. It prevents coarsening.

(Number density of inclusions in the center of the slab)
Most of the REM composite inclusions are present throughout the carbon steel slab as inclusions having a circle-equivalent diameter of 0.5 μm or more and 4 μm or less. In the carbon steel slab according to the present embodiment, when the number density of inclusions having a circle-equivalent diameter of 0.5 μm or more and 4 μm or less is 20 pieces / mm ^{2 or} less, the number of the above-mentioned REM composite inclusions is small. MnS cannot be sufficiently attached to the REM composite inclusions, and the effect of suppressing the stretching of MnS by the REM composite inclusions cannot be exhibited. Therefore, there is a possibility that the hole expandability cannot be sufficiently improved.
Further, when the number density of inclusions having a circle-equivalent diameter of more than 10 μm is 0.2 pieces / mm ² or more, the hole expandability is deteriorated by the coarse inclusions, and defects caused by the coarse inclusions are present. It may occur.
Therefore, in the present embodiment, the number density of inclusions having a circle-equivalent diameter of 0.5 μm or more and 4 μm or less is 20 pieces / mm ² or more, and the number density of inclusions having a circle-equivalent diameter of more than 10 μm. Is less than 0.2 pieces / mm ² .

(Austenite particle size)
In the continuously cast slab after continuous casting, the crystal structure formed during cooling after solidification can be observed. Above all, the smaller the particle size of austenite above the temperature at which MnS is generated, the easier it is to promote the diffusion of S, which is preferable. Here, the austenite particle size means the particle size of the austenite phase formed when the slab temperature is in the austenite region in the process of cooling the solidified slab 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 is observed in a cross section perpendicular to the growth direction of dendrite, which is a solidified structure. However, the region where the dendrite directions are the same can be evaluated as the austenite particle size.
The austenite grain size was evaluated at the surface layer of the slab, which had a small amount of deformation when reduced immediately after solidification. Since the temperature of the central portion of the plate thickness is higher than that of the slab surface, the amount of deformation is large and the plate becomes fine. If the austenite particle size on the surface of the slab exceeds 0.05 mm, the intergranular diffusion of S does not proceed sufficiently during solidification, the amount of adhered MnS generated decreases, and the hole expandability may deteriorate. .. Therefore, in the present embodiment, the average value of the austenite particle diameter on the surface of the slab is set to 0.05 mm or less.
The lower limit of the austenite particle size is not particularly limited, but the lower limit is about 0.0005 mm on average depending on the method for producing a slab according to the present invention.
The austenite particle size was taken as the average value of the measured values obtained by measuring 10 austenite particle sizes on the surface of the slab.

<Manufacturing method of carbon steel slabs>
Next, the method for producing the carbon steel slab according to the present embodiment will be described with reference to the flow chart of FIG.

(Melting process S01)
First, in terms of 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 A molten steel containing% or less, 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 is prepared.

(Al deoxidation step S02)
Next, a deoxidizing agent such as Al is added to the molten steel, and deoxidizing treatment is performed so that the t.O content is 0.0005% or more and 0.005% or less. Since the O content is the total oxygen concentration of the dissolved oxygen and the oxygen in the inclusions, the Al content can be set within the range of 0.004% or more and 2.0% or less after this step. Prediction adjustment is possible within the range of normal operation technology, including the effect of floating removal of inclusions. If necessary, a desulfurizing agent may be added to perform the finish desulfurization treatment. Further, an optional additive element such as Cr, Ni, or Cu may be added in this step.

(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 Mg O · Al ₂ O ₃ react with REM, and then also react with S to form a large number of REM-Al-OS inclusions.
Then, after adding REM, stirring is carried out for 5 minutes or more. As a result, the above-mentioned REM-Al-OS inclusions are sufficiently formed.

(Ca addition step S05)
Next, Ca is added to the molten steel so as to be 0.0011% or more and 0.005% or less in the slab while considering the equations (1) and (2) and also considering the Ca addition yield. Then, the REM-Al-OS inclusions react with Ca, and a large number of REM composite inclusions (REM-Ca-OS) are formed.

(Continuous casting process S06 and reduction process S07)
Next, the above-mentioned molten steel is injected into a mold of a continuous casting apparatus to continuously cast carbon steel slabs. At this time, the reduction rate is 10% or more and 50% or less in the temperature range from the solidus temperature of the carbon steel slab or less to 1200 ° C. or more in the temperature of the 1/2 thick part of the slab. As a result, the coarse REM composite inclusions (REM-Ca-OS) are crushed.

Here, the morphology of molten steel, solidified slabs, and inclusions in the product will be described with reference to FIGS. 3 to 5. In FIGS. 3 to 5, (a) is in molten steel, (b) is a solidification process (during rolling), (c) is a slab after solidification, and (d) is a product after rolling the slab. It shows the form of an 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, inclusions 40 made of a coarse REM composite oxide are present in the molten steel. The inclusion 40 has a structure in which the Ca-rich phase 42 of the liquid phase surrounds the REM-rich phase 41 of the solid phase. Here, the Ca-rich phase 42 is mainly composed of calcium aluminate (CaO-Al ₂ O ₃ ).

Then, at the time of solidification, a part of CaO of calcium aluminate is replaced with CaS. Then, the Ca-rich phase 42 is mainly composed of CaS in a hard solid phase from calcium aluminate (CaO-Al ₂ O ₃ ) in the liquid phase. Here, when the reduction is not performed in the cooling process of the casting, as shown in FIG. 4, a coarse REM composite inclusion (REM-Ca-OS) having a coarse structure is present in the slab. become. Since this coarse REM composite inclusion (REM-Ca-OS) is mainly composed of solid-phase CaS in which the Ca-rich phase 42 is hard, it is not crushed by subsequent rolling and remains coarse in the product. However, the hole expandability is reduced.

On the other hand, when the above-mentioned reduction step S07 is carried out in the continuous casting step S06 as in the present embodiment, as shown in FIG. 5, in the casting process, the periphery of the solid phase REM rich phase 41 is Since the reduction is performed in a state in which the liquid phase calcium aluminate (CaO-Al ₂ O ₃ ) is surrounded by the Ca-rich phase 42 as the main component, the inclusions 40 can be finely crushed. As a result, the inclusions 40 existing in the product can be miniaturized, and the hole expandability of the product is improved.

According to the carbon steel slab of the present embodiment having the above configuration, 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 1/2 thick portion. Is 20 pieces / mm ² or more.
Then, by adhering MnS to the finely dispersed REM composite inclusions (REM-Ca-OS), the formation of coarse MnS can be suppressed, and the stretching of MnS during rolling can be suppressed. Therefore, it is possible to surely improve the hole expandability.

Further, since the number density of inclusions having a diameter equivalent to a circle exceeding 10 μm is suppressed to less than 0.2 pieces / mm ² , the formation of coarse inclusions is suppressed, and the hole expandability is deteriorated. It is possible to suppress the occurrence of defects caused by coarse inclusions.
Further, since a liquid phase is generated during the reaction between the Al ₂ O ₃ refractory and the REM composite inclusion (REM-Ca-OS), the nozzle blockage is suppressed. Therefore, casting can be performed stably.

Further, in the carbon steel slab of the present embodiment, since the austenite particle size on the slab surface is finely controlled to be 0.05 mm or less, it is possible to promote the intergranular diffusion of S, and the REM composite intervention. It is possible to attach MnS to an object (REM-Ca-OS) to reduce stretching inclusions.

Further, according to the method for producing carbon steel slabs according to the present embodiment, since the Al deoxidizing step S02 for adding Al to the molten steel to deoxidize it is provided, Al ₂ O ₃ is contained in the molten steel. It will exist 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 Ca is added in the state of being present in REM, and REM composite inclusions (REM-Ca-OS) containing REM and Ca can be stably produced.
Further, since the continuous casting step S06 and the reduction step S07 are provided, the coarse REM composite inclusions (REM-Ca-OS) are crushed by reduction, and the above-mentioned carbon steel slab can be produced. It becomes.

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 above, the present invention is not limited to this and does not deviate from the technical idea of the invention. It can be changed as appropriate within the range.
For example, it has been described that the O and S concentrations are adjusted by adding a deoxidizer such as Al and a desulfurizing agent, but the present invention is not limited to this, and the O and S concentrations are adjusted by other means. May be good.

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

Further, the carbon steel slab of the present invention further contains, as an additive element, Cr; 0.001% or more and 2.0% or less, Ni; 0.001% or more and 2.0% or less, Cu; 0 in mass%. .001% or more and 2.0% or less, Nb; 0.001% or more and 0.2% or less, V; 0.001% or more and 1.0% or less, W; 0.001% or more and 1.0% or less, Zr 0.0001% or more and 0.2% or less, As; 0.0001% or more and 0.5% or less, Co; 0.0001% or more and 1.0% or less, Sn; 0.0001% or more and 0.2% or less , Pb; 0.0001% or more and 0.2% or less, Hf; 0.0001% or more and 0.2% or less, and may contain one or more selected from the group.

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

Cr can be contained in the steel, if necessary, in order to further secure the strength of the steel sheet. 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 scrap and the like. Ni and Cu are elements that improve hardenability and increase the strength of steel, and both can be contained in steel in the range of 0.001% to 2.0% as needed.

Nb, W, and V are elements that form carbides, nitrides, and carbonitrides with C or N to promote fine graining of the base metal structure and improve toughness. Therefore, 0.01% or more of Nb may be added to the steel. However, even if a large amount of Nb is added and the Nb concentration exceeds 0.2%, the effect of microstructural granulation of the base material is saturated and the manufacturing cost is only increased, 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 scrap and the like. Similarly, W and V may be added in the range of 0.01% to 1.0%. The lower limit of the concentration of these elements is 0.001%.

Since Zr is an element that spheroidizes sulfide and improves the toughness of the base metal, 0.001% or more may be added to the steel. However, if a large amount is added to the 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%.

Further, when scrap or the like is used as a raw material, As, Co, Sn, Pb, and Hf may be inevitably mixed. In order for these elements not to adversely affect the mechanical properties of the steel sheet, it is preferable to limit the concentration of each element as follows. The upper limit of the As concentration is 0.5%, and the upper limit of the Co concentration is 1.0%. The upper limit of the concentrations of Sn, Pb, and Hf is 0.2%. The lower limit of the concentration of these elements is 0.0001%.

The results of experiments carried out in order to confirm the effects of the present invention will be described below.

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

A reduction roll was provided at a position after solidification was completed during continuous casting, and reduction was performed by 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 reduction 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 reduced position was calculated by slab heat transfer simulation during continuous casting using the thermo-fluid analysis software FLUENT. It has been confirmed in advance that the temperature measurement result at the center of the slab thickness and the heat transfer simulation result match.

The austenite particle size of the slab is measured by collecting a sample of 50 mm in the casting direction, 50 mm in the width direction, and 10 mm in the thickness direction from the surface layer of the slab, and after grinding 0.1 mm in the thickness direction from the surface of the black skin. Mirror polished. Then, it was immersed in a saturated aqueous solution of picric acid at room temperature for 60 seconds, and the crystal grain size was observed. A region in which the direction of dendrite, which is a solidified structure, is the same was regarded 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 taken as the austenite particle size of the slab.

Further, an observation photograph of the inclusions of Example 1-1 of the present invention is shown in FIG. 6, and an observation photograph of the inclusions of Comparative Example 1-1 is shown in FIG.

In Comparative Example 1-1 in which the reduction start temperature was equal to or higher than the solidus temperature, the concentrated molten steel was extruded by the reduction, and the concentrated molten steel promoted the aggregation and coalescence of the inclusions, 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 when reduced, and remained coarse. It is presumed that this is because the Ca-rich phase changed from the calcium aluminate phase having a low melting point to CaS as the temperature became lower, which made it difficult to crush.

In Comparative Example 1-3 in which the reduction rate was 5%, the inclusions were not crushed even when reduced, and remained coarse. In addition, 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, in this embodiment in which the reduction start temperature is from the solidus line temperature or lower to 1200 ° C. and the reduction rate is 10% to 50%, inclusions are crushed and coarse inclusions are reduced. 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 increased.

(Example 2)
Next, based on the results of Example 1, the influence of the component composition of the carbon steel slab was confirmed.
Additive elements were added according to the procedure shown in FIG. 2 so as to obtain the molten steel components shown in Table 3. That is, after adding Al with RH, Fe-Si-REM alloy was added as REM with RH. After stirring for 5 minutes or more after adding REM, Ca-Si was added by wire. Then, the obtained molten steel was cast by a continuous casting machine. In the cooling process during continuous casting, the slab during continuous casting was reduced by 20% within the temperature range of 1200 ° C. or higher below the solidus temperature of the carbon steel slab. .. After casting, an observation sample was taken from a half-thick part of the obtained slab, and the number densities of inclusions having a circle-equivalent diameter of more than 10 μm and inclusions of 0.5 μm or more and 4 μm or less were 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. It was wound at the taking temperature to obtain a hot-rolled steel sheet having a thickness of 2.8 mm. Then, the hole expandability was evaluated in accordance with JIS Z2256: 2010. The evaluation results are also shown in Table 4.

In Comparative Example 2-1 in which the content of REM was less than the range of the present invention, the improvement of the hole expandability 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 inclusions was small, it is presumed that the formation of REM composite inclusions was small and the elongation of MnS could not be suppressed in the first place. ..
In Comparative Example 2-2 in which the content of REM was higher than the range of the present invention, the range of Eq. (1) was out of range, but since the amount of REM was too high in the first place, coarse inclusions (REM-Ca- A large amount of OS) was generated, and the improvement of hole expandability was insufficient. In addition, nozzle blockage was observed.

In Comparative Example 2-3 in which the Ca content was higher than the range of the present invention, a large amount of coarse inclusions (CaO-Al ₂ O ₃ ) were generated, and the improvement of the hole expandability was insufficient.
In Comparative Example 2-4 in which the Ca content was smaller than the range of the present invention and the range of the formula (2) was out of the range, the nozzle was blocked.

In Comparative Example 2-5, which was out of the range of the formula (1), a liquid phase was not generated during the reaction with the nozzle due to the large amount of REM, and nozzle clogging was observed.

In Comparative Example 2-6, which was outside the range of the formula (2), a large number of coarse REM composite inclusions (REM-Ca-OS) were observed. It is presumed that this was coarsened by agglomeration because inclusions were present as a liquid phase in the molten steel.

On the other hand, in Examples 2-1 to 2-22 of the present invention, which are 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 inclusions is 20. It was set to / mm ² or more, and the number density of inclusions having a circle-equivalent diameter of more than 10 μm was less than 0.2 pieces / mm ² . As a result, the hole expandability was sufficiently improved. In addition, no nozzle blockage was observed.

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

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

Claims (3)

By 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% or less,
S; 0.0001% or more and 0.01% or less,
N; 0.01% or less,
t. O; 0.0005% or more and 0.005% or less,
Al; 0.004% or more and 2.0% or less,
Ti; 0.0001% or more and 0.20% or less,
REM; 0.001% or more and 0.02% or less,
Ca; 0.0011% or more and 0.005% or less,
Containing, the balance consists of iron and unavoidable impurities,
REM, Ca, t. The mass% of O was changed to [REM], [Ca], and [t. When O] is set,
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 and
A carbon steel slab characterized by having an austenite particle size of 0.05 mm or less on the surface of the slab. In addition, in% by 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% or more and 2.0% or less,
Nb; 0.001% or more and 0.2% or less,
V; 0.001% or more and 1.0% or less,
W; 0.001% or more and 1.0% or less,
Zr; 0.0001% or more and 0.2% or less,
As; 0.0001% or more and 0.5% or less,
Co; 0.0001% or more and 1.0% or less,
Sn; 0.0001% or more and 0.2% or less,
Pb; 0.0001% or more and 0.2% or less,
Hf; 0.0001% or more and 0.2% or less,
The carbon steel slab according to claim 1, further 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.
The Al deoxidation process, in which Al is added to the molten steel to deoxidize it,
The REM addition process of adding REM to molten steel to which Al has been added, and
A stirring step of adding REM and stirring for 5 minutes or more,
Ca addition step of adding Ca to molten steel after stirring,
Continuous casting process of continuous casting after adding Ca,
In the cooling process of the continuous casting process, it is equipped with a reduction process that reduces the slab.
In the reduction step, the reduction rate is 10% or more and 50% or less within the temperature range of 1200 ° C. or more at the temperature of the solid phase temperature of the carbon steel slab or less. A characteristic method for producing carbon steel slabs.