JP6292039B2 - Carbon steel slab, method for producing carbon steel slab, and steel material - Google Patents

Description

The present invention relates to a carbon steel slab that has few coarse Al ₂ O ₃ clusters that cause product defects and is excellent in toughness, a method for producing the carbon steel slab, and a steel material comprising the carbon steel slab. Is.

In general, a carbon steel slab made of aluminum killed steel obtained by adding Al as a deoxidizer to molten steel is used as a material for steel materials such as thick plates. When Al is added to molten steel and deoxidation is performed, Al ₂ O ₃ is generated in the molten steel. Since this Al ₂ O ₃ is easily clustered, coarse Al ₂ O ₃ clusters may exist inside the carbon steel slab.

Here, in the steel slab or the like, product defects are known to occur due to coarse Al ₂ O ₃ clusters described above.
Conventionally, as a means for reducing coarse Al ₂ O ₃ clusters causing product defects, for example, Patent Document 1 proposes adding rare earth elements (REM) and setting solid solution REM to 1 ppm. .

On the other hand, steel materials such as thick plates are required to have excellent low-temperature toughness depending on the intended use. In order to improve the low temperature toughness, there is a method of suppressing the coarsening of crystal grains by removing coarse inclusions that become the starting point of fracture and dispersing fine inclusions.
Here, for example, in Patent Document 2, Zr and REM are added in combination to generate an oxide (an oxide containing Zr, REM, and Ca), and the size and number of the oxides are controlled. Steel materials with improved toughness have been proposed.

JP 2005-002425 A JP 2010-209433 A

Incidentally, Patent Document 1, although suppressing the occurrence of Al ₂ O ₃ clusters by the addition of REM, inclusions size is Al ₂ O ₃ of a few [mu] m, can be improved material properties such as toughness There wasn't.
Moreover, the inclusion produced | generated by patent document 2 is a comparatively coarse oxide with an equivalent circle diameter of 0.1-2.0 micrometers, and the improvement of toughness was inadequate.

The present invention has been made in view of the above-mentioned situation, and can suppress the generation of coarse Al ₂ O ₃ clusters and greatly improve toughness by dispersing and generating fine inclusions. It is an object of the present invention to provide a produced carbon steel slab, a method for producing a carbon steel slab, and a steel material comprising the carbon steel slab.

As a result of intensive studies by the present inventors in order to solve the above-mentioned problems, the presence of a small amount of Zr element in the molten steel reduces FeO that causes Al ₂ O ₃ clusters to form coarse Al _2. The knowledge that it is possible to suppress the formation of O ₃ clusters, and there are many FeO and unreacted Zr particles as fine Zr oxides, which can significantly improve the toughness of carbon steel slabs. Obtained.

The present invention has been made based on the above-mentioned knowledge, and the carbon steel slab according to the present invention is C: 0.01% or more and 0.30% or less, Si; 0.001% or more in mass%. 1.5% or less, Mn; 0.01% or more and 3.0% or less, P; 0.0010% or more and 0.050% or less, S; 0.0001% or more and 0.003% or less, Al; 0.01 % To 1.5%, t. O: 0.0001% or more and 0.004% or less, t. N: 0.0001% or more and 0.01% or less, t. Zr: 0.0003% or more and 0.004% or less, with the balance being Fe and inevitable impurities, and 50 or more Zr oxides with an equivalent circle diameter of 50 nm to 100 nm per 1 mm ² In addition, the number of Al ₂ O ₃ clusters having an equivalent circle diameter exceeding 10 μm is 5 or less per 1 mm ² .

According to the carbon steel slab of this configuration, since Al ₂ O ₃ clusters exceeding the equivalent circle diameter of 10 μm are suppressed to 5 or less per 1 mm ² , when this carbon steel slab is used, It is possible to suppress the occurrence of product defects due to Al ₂ O ₃ clusters.
Further, since there are 50 or more relatively fine Zr oxides having an equivalent circle diameter of 50 nm or more and 100 nm or less per 1 mm ² , the toughness of the carbon steel slab can be greatly improved.

  Further, the method for producing a carbon steel slab according to the present invention is a method for producing the above-described carbon steel slab, wherein Zr is added to the molten steel stored in the container after the addition of Al. Zr addition step is performed, and in the Zr addition step, Zr is added so that the Zr concentration of 50 cm immediately below the molten metal surface of the Zr addition portion becomes 15 ppm or less 30 seconds after the addition of Zr. It is said. Since Zr temporarily remains in the molten steel surface layer part due to surface tension when Zr is added, the Zr concentration in the molten steel from the molten metal surface to the depth of 50 cm from the molten metal surface is the highest. Therefore, in order to prevent the molten steel sample from being affected by the molten metal surface and causing fluctuations in the component, etc., it was decided to measure the component by collecting molten steel 50 cm immediately below the molten metal surface of the Zr-added portion.

According to the method for producing a carbon steel slab of this configuration, in the Zr addition step of adding Zr to the molten steel stored in the container, the Zr concentration of 50 cm immediately below the molten metal surface of the Zr addition portion is 30 seconds from the addition of Zr. Since it is set as the structure which adds Zr so that it may become 15 ppm or less later, a trace amount Zr element will exist in molten steel. With this Zr element, FeO that causes Al ₂ O ₃ cluster generation can be reduced, and clustering (coarseness) of Al ₂ O ₃ can be suppressed.

  In addition, since the Zr concentration is 15 ppm or less 30 seconds after the addition of Zr, FeO and unreacted Zr do not grow, and there are many fine Zr oxides with an equivalent circle diameter of 50 nm to 100 nm. It becomes possible to produce a carbon steel slab excellent in toughness. Since the amount of Zr oxide produced increases 30 seconds after the addition of Zr, in the present invention, the addition of Zr is controlled so that the Zr concentration after 30 seconds from the addition of Zr is 15 ppm or less. Yes. The Zr concentration after 30 seconds from the addition of Zr can be controlled to 15 ppm or less by dividing the addition of Zr in small portions or by stirring the molten steel in the container and changing the average reflux time. It becomes possible.

Here, in the method for producing a carbon steel slab of the present invention, it is preferable that an average reflux time of the molten steel in the container is 120 seconds or more and 360 seconds or less. The average reflux time indicates the time until the molten steel component in the container becomes uniform. The average reflux time is determined by the size of the container and the presence or absence of a molten steel stirring function.
According to the method for producing a carbon steel slab of this configuration, since the average reflux time of the molten steel in the container is 120 seconds or more, oxygen is less likely to be caught in the molten steel by stirring, and the coarse oxide Occurrence can be suppressed. On the other hand, since the average reflux time of the molten steel is 360 seconds or less, the stirring force is sufficiently ensured, there is no region where the Zr concentration is locally high, and the Zr oxide is coarsened. Can be suppressed.

Further, the steel material according to the present invention is characterized by being formed by processing the above-described carbon steel slab.
Steel of this arrangement is equivalent circle diameter 50nm or 100nm following Zr oxide with the presence 1 mm ² per 50 or more, a circle more than equivalent diameter 10μm _Al 2 _{O 3} Carbon steel clusters is less than 5 per 1 mm ² Since the slab is used, toughness can be greatly improved and the occurrence of product defects due to coarse Al ₂ O ₃ clusters can be suppressed. Examples of the steel material include a thick plate material obtained by hot rolling the carbon steel slab and a thin plate material obtained by performing cold rolling.

As described above, according to the present invention, the generation of coarse Al ₂ O ₃ clusters can be suppressed and the toughness of the carbon steel slab, the carbon steel slab manufacturing method, and the It is possible to provide a steel material made of a carbon steel slab.

It is a flowchart which shows the manufacturing method of the carbon steel slab which is embodiment of this invention. It is explanatory drawing of the Zr addition process in the manufacturing method of the carbon steel slab which is embodiment of this invention. In a Zr addition process, it is explanatory drawing which shows an effect | action at the time of adding Zr so that Zr density | concentration of 50 cm right under the molten metal surface of a Zr addition part may be 15 ppm or less 30 seconds after addition of Zr. In a Zr addition process, it is explanatory drawing which shows an effect | action at the time of adding Zr so that Zr density | concentration of 50 cm right under the molten metal surface of a Zr addition part may exceed 15 ppm 30 seconds after addition of Zr.

  Hereinafter, a carbon steel cast piece, a method for producing a carbon steel cast piece, and a steel material, which are embodiments of the present invention, will be described with reference to the accompanying drawings. In addition, this invention is not limited to the following embodiment.

The carbon steel slab according to the present embodiment has a composition in mass%, C: 0.01% or more and 0.30% or less, Si; 0.001% or more and 1.5% or less, Mn; 0.01 % To 3.0%, P; 0.0010% to 0.050%, S; 0.0001% to 0.003%, Al; 0.01% to 1.5%, t. O: 0.0001% or more and 0.004% or less, t. N: 0.0001% or more and 0.01% or less, t. Zr: 0.0003% or more and 0.004% or less, with the balance being Fe and inevitable impurities.
Below, the reason which prescribed | regulated each component is demonstrated.

(C: carbon)
C is an element having an effect of improving the strength of the carbon steel slab. Here, if content of C is less than 0.01%, the above-mentioned effect cannot be achieved. On the other hand, if the C content exceeds 0.30%, the HAZ toughness deteriorates, and the weldability may also deteriorate.
From the above, in this embodiment, the C content is limited to a range of 0.01% to 0.30%.

(Si: silicon)
Si is an element having an effect of promoting deoxidation of the molten steel 10 and improving the strength of the carbon steel slab. Here, if content of Si is less than 0.001%, the above-mentioned effect cannot be achieved. On the other hand, if the Si content exceeds 1.5%, the HAZ toughness deteriorates and the weldability may also deteriorate.
From the above, in this embodiment, the Si content is limited to a range of 0.001% to 1.5%.

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

(P: phosphorus)
P is an element that affects the toughness of the carbon steel slab, but unavoidably 0.0010% is contained. Here, when the content of P exceeds 0.050%, coarse inclusions are generated, and the toughness of the carbon steel slab is deteriorated.
From the above, in this embodiment, the P content is limited to a range of 0.0010% to 0.050%.

(S: sulfur)
S is an element that affects the toughness of the carbon steel slab, but unavoidably 0.0001% is contained. Here, if the content of S exceeds 0.003%, coarse inclusions are generated, and the toughness of the carbon steel slab deteriorates.
From the above, in this embodiment, the S content is limited to a range of 0.0001% to 0.003%.

(Al: Aluminum)
Al is an element added to promote deoxidation of the molten steel 10. Here, if the content of Al is less than 0.01%, sufficient deoxidation cannot be performed. On the other hand, if the Al content exceeds 1.50%, coarse inclusions (Al ₂ O ₃ clusters 20) are likely to be generated, and the quality of the carbon steel slab may be deteriorated.
From the above, in this embodiment, the Al content is limited to a range of 0.01% to 1.50%.

(T.O: oxygen)
O is inevitably contained in an amount of 0.0001%. Here, if the O content exceeds 0.004%, coarse oxides and the like are generated, and the quality of the carbon steel slab deteriorates.
From the above, in this embodiment, the content of O is limited to the range of 0.0001% to 0.004%. Note that t. O (total oxygen amount) is a total amount including O dispersed in the carbon steel slab in a compound state.

(T.N: nitrogen)
N is inevitably contained in 0.0001%. Here, if the N content exceeds 0.01%, coarse nitrides or the like are generated, and the quality of the carbon steel slab deteriorates.
From the above, in this embodiment, the N content is limited to a range of 0.0001% to 0.01%. Note that t. N (total nitrogen amount) is a total amount including N dispersed in the carbon steel slab in a compound state.

(T. Zr: zirconium)
Zr has an effect of reducing FeO that causes generation of the Al ₂ O ₃ cluster 20. Moreover, it has the effect of forming the fine Zr oxide 27 and improving toughness. Here, if the content of Zr is less than 0.0003%, the above-described effects may not be achieved. On the other hand, if the Zr content exceeds 0.004%, coarse carbonitrides may be generated.
From the above, in this embodiment, the Zr content is limited to a range of 0.0003% to 0.004%. Note that t. Zr (total Zr amount) is a total amount including Zr dispersed in the carbon steel slab in a compound state.

In the carbon steel slab according to the present embodiment, 50 or more Zr oxides 27 having an equivalent circle diameter of 50 nm or more and 100 nm or less exist per 1 mm ^2, and Al ₂ O ₃ clusters 20 having an equivalent circle diameter of more than 10 μm are formed. The number is 5 or less per 1 mm ² .
That is, the number of coarse Al ₂ O ₃ clusters 20 is small, and a relatively large amount of relatively fine Zr oxide 27 is dispersed.

Next, the manufacturing method of the carbon steel slab which is this embodiment is demonstrated with reference to FIG. 1 and FIG.
First, in mass%, C; 0.01% or more and 0.30% or less, Si; 0.001% or more and 1.5% or less, Mn; 0.01% or more and 3.0% or less, P; 0.0010 % To 0.050%, t. N: Molten steel 10 containing 0.0001% or more and 0.01% or less is prepared (melting step S01).

  Then, a deoxidizing agent such as Al and a desulfurizing agent are added to the molten steel 10 and deoxidized so that the dissolved oxygen concentration is 0.0001% or more and 0.004% or less, and the S concentration is 0.0001% or more. The desulfurization treatment is performed until it becomes 0.003% or less (deoxidation treatment and desulfurization treatment step S02). At this time, the Al content is set within a range of 0.01% to 1.5%.

Next, after adding Al and desirably 2 minutes or more, Zr is added (Zr addition step S03). Here, in the Zr addition step S03, the amount of Zr added is adjusted so that the total Zr concentration falls within the range of 0.0003% to 0.004%.
And in this Zr addition process S03, as shown in FIG. 2, Zr is added so that the Zr density | concentration of 50 cm right under the hot_water | molten_metal surface of the Zr addition part 12 may be 15 ppm or less 30 seconds after addition of Zr. That is, Zr is added so that the Zr concentration at 50 cm immediately below the molten metal surface of the Zr added portion 12 having the highest Zr concentration 30 seconds after the addition of Zr is 15 ppm or less. In order to make the Zr concentration of 50 cm immediately below the molten metal surface of the Zr addition part 12 after 30 seconds 15 ppm or less, assuming that the average reflux time of the molten steel 10 is t seconds and the molten steel mass is M, per Zr The amount of Zr added may be adjusted so that the amount of Zr added ≦ 0.000015 × M × 30 / t, and added at intervals of 30 seconds.

In the present embodiment, the Zr addition is divided into a plurality of times to reduce the amount of Zr added once, and the average reflux time is controlled by stirring the molten steel 10, so that The Zr concentration of 50 cm is set to 15 ppm or less.
In addition, in Zr addition process S03, it is preferable to make Zr density | concentration of 50 cm right under the molten metal surface of the Zr addition part 12 into 10 ppm or less.

By this Zr addition step S03, generation of coarse Al ₂ O ₃ clusters 20 having an equivalent circle diameter exceeding 10 μm is suppressed, and many fine Zr oxides 27 having an equivalent circle diameter of 50 nm to 100 nm are generated.
Specifically, as shown in FIG. 3, a part of the added Zr 26 a (26) reacts with the FeO binder 23 that bonds the Al ₂ O ₃ particles 21, 21, thereby causing Al ₂ O _3. The generation of the cluster 20 is suppressed. The FeO binder 23 and unreacted Zr26b (26) are oxidized in the molten steel 10 to become Zr oxide. The formation of this Zr oxide increases after 30 seconds from the addition of Zr. Here, in this embodiment, the Zr concentration of 50 cm immediately below the molten metal surface of the Zr addition part 12 is 15 ppm or less 30 seconds after the addition of Zr, and the amount of unreacted Zr26b is small, so that the Zr oxide is coarsened. In addition, a large number of fine Zr oxides 27 having an equivalent circle diameter of 50 nm to 100 nm are generated.

  On the other hand, in the Zr addition step S03, when the Zr concentration of 50 cm immediately below the molten metal surface of the Zr addition part 12 is added to exceed 15 ppm 30 seconds after the addition of Zr, as shown in FIG. 4, unreacted Zr26b Therefore, a relatively coarse Zr oxide 28 having an equivalent circle diameter of 500 nm or more is generated, and the number of fine Zr oxides 27 having an equivalent circle diameter of 50 nm or more and 100 nm or less is reduced. Therefore, in this embodiment, the addition of Zr is performed so that the Zr concentration in the region A 50 cm immediately below the molten metal surface of the Zr addition part 12 falls within the above-mentioned range.

Moreover, in this embodiment, the average recirculation | reflux time of the molten steel 10 is made into the range of 120 second or more and 360 second or less.
When the average reflux time of the molten steel 10 is less than 120 seconds, the stirring force is too strong, and oxygen may be involved in the molten steel 10 to generate coarse oxides. On the other hand, when the average reflux time of the molten steel 10 exceeds 360 seconds, the stirring force is weak, there is a region where the Zr concentration is locally high, the Zr oxide is coarsened, and the equivalent circle diameter is 50 nm to 100 nm. There is a possibility that the Zr oxide 27 may not be sufficiently present. Therefore, in this embodiment, the average reflux time of the molten steel 10 is defined as described above.

The molten steel 10 thus obtained is cast by a continuous casting machine (casting step S04).
Thereby, 50 or more Zr oxides 27 having an equivalent circle diameter of 50 nm or more and 100 nm or less exist per 1 mm ^2, and the Al ₂ O ₃ cluster 20 having an equivalent circle diameter of 10 μm or more is 5 or less per 1 mm ^2. A slab is obtained.
Moreover, steel materials, such as a thick plate material, are manufactured by processing, such as hot rolling, with respect to this carbon steel slab.

According to the carbon steel slab of the present embodiment configured as described above, the number of Al ₂ O ₃ clusters 20 exceeding the equivalent circle diameter of 10 μm is suppressed to 5 or less per 1 mm ^2. In the steel material obtained by processing the steel slab, it is possible to suppress the occurrence of product defects due to the coarse Al ₂ O ₃ cluster 20.
In addition, since there are as many as 50 or more relatively fine Zr oxides 27 having an equivalent circle diameter of 50 nm to 100 nm per 1 mm ² , the toughness of the carbon steel slab can be greatly improved.

Further, in the method for producing a carbon steel slab according to the present embodiment, in the Zr addition step S03 in which Zr is added to the molten steel 10, the Zr concentration of 50 cm immediately below the molten metal surface of the Zr addition part 12 is 30 seconds from the addition of Zr. as will be 15ppm or less, since a configuration of adding Zr, the Zr elements, _Al 2 _{O 3} can be reduced to FeO to be produced causes the cluster 20, clustering of _Al 2 _{O 3} (coarsening ) Can be suppressed. In addition, the Zr oxide produced by oxidation of unreacted Zr is refined, and a large amount of fine Zr oxide 27 having an equivalent circle diameter of 50 nm or more and 100 nm or less can be present, and carbon steel having excellent toughness. A slab can be manufactured.

  Moreover, in this embodiment, since the average reflux time of the molten steel 10 is 120 seconds or more and 360 seconds or less, oxygen is hardly involved in the molten steel 10 by stirring, and the generation of coarse oxides is suppressed. In addition, the stirring force is sufficiently ensured, there is no region where the Zr concentration is locally high, and the coarsening of the Zr oxide can be suppressed. As a result, a large number of fine Zr oxides 27 having an equivalent circle diameter of 50 nm or more and 100 nm or less can be present.

The carbon steel slab, the carbon steel slab manufacturing method, and the steel material according to the embodiment of the present invention have been specifically described above, but the present invention is not limited to this, and the technical idea of the invention is not limited thereto. Changes can be made as appropriate without departing from the scope.
For example, in the deoxidation treatment and desulfurization treatment step S02, it has been described that the dissolved oxygen concentration and the S concentration are adjusted by adding a deoxidizer and a desulfurizer such as Al, but is not limited thereto. The dissolved oxygen concentration and the S concentration may be adjusted by other means.

In the following, the results of experiments conducted to confirm the effects of the present invention will be described.
First, the amount of molten steel shown in Table 1 is expressed in terms of mass%, C; 0.12%, Si; 0.2% or less, Mn; 1.5% or less, P; 0.0010%, S; 0.0007. %, Al; adjusted to 0.03% component.
Zr was added to this molten steel by setting the total Zr addition amount, the Zr addition amount per addition, and the addition interval.
In addition, the molten steel was stirred so that the average reflux time of the molten steel in the vessel was the value shown in Table 1.

  Next, 30 seconds after the addition of Zr, an analytical sample was taken from the molten steel having a radius of 50 cm and a depth of 50 cm around the Zr addition site, and the Zr concentration was measured. The results are shown in Table 1.

The cross section of the carbon steel slab obtained by casting this molten steel was observed, and the number of Zr oxides having an equivalent circle diameter of 50 nm to 100 nm in 1 mm ² was measured. In addition, the observation sample was extract | collected from the 1/4 thickness part of the slab. The measurement results are shown in Table 1.

The obtained carbon steel slab was rolled at 1150 ° C. to 25 mm. After rolling, heat treatment was performed at 940 ° C. to 950 ° C. for 1 hour, and this was used as a test material.
The number of coarse alumina clusters having a circle equivalent diameter of 10 μm or more in 1 mm ² was evaluated for the obtained specimens. The evaluation results are shown in Table 1.

vTrs (Charpy fracture surface transition temperature) was tested using test pieces taken from 1/2 thickness, and the result was used as a representative value for each steel plate. The test was conducted in accordance with JIS Z 2242 (2005) “Charpy impact test method for metal materials” using a 2 mm V notch Charpy impact test piece. The temperature at the fracture rate was measured.
The number of Al ₂ O ₃ clusters was confirmed by collecting a sample for observation from the obtained carbon steel slab and observing it at 400 times using an optical microscope with a field number of 40 or more.

In Examples 1 to 5, the number of Zr oxides having an equivalent circle diameter of 50 nm or more and 100 nm or less in 1 mm ² is 100 or more, and many fine Zr oxides are dispersed. confirmed. Moreover, the Charpy fracture surface transition temperature was low and the toughness was sufficient.
On the other hand, in Comparative Examples 1 to 5, the number of Zr oxides having an equivalent circle diameter of 50 nm or more and 100 nm or less in 1 mm ² was less than 50. Moreover, the Charpy fracture surface transition temperature was higher than that of the Examples, and the toughness was insufficient.
In addition, about the number of coarse alumina clusters 10 micrometers or more, the big difference was not recognized by Examples 1-5 and Comparative Examples 1-5.

  Thus, even when the amount of Zr added was the same, it was confirmed that the number of Zr oxides having an equivalent circle diameter of 50 nm to 100 nm could be increased by adjusting the amount of Zr added per time. .

  Next, Zr was added by the Zr addition method of Example 1 to melt molten steel having the components shown in Table 2. 25 kg of this molten steel was cast to produce a slab having a width of 100 mm, a thickness of 100 mm, and a height of 300 mm. The obtained slab was rolled at 1150 ° C. to 25 mm. After rolling, heat treatment was performed at 940 ° C. to 950 ° C. for 1 hour, and this was used as a test material.

The obtained test specimen, vTrs (Charpy fracture appearance transition temperature), circle equivalent diameter 10μm or more coarse _Al 2 _{O 3} the number of clusters in 1 mm ^2, an equivalent circle diameter 50nm or more in 1 mm ² of 100nm or less The number of Zr oxides was evaluated. The evaluation results are shown in Table 2.

vTrs (Charpy fracture surface transition temperature) was tested using test pieces taken from 1/2 thickness, and the result was used as a representative value for each steel plate. The test was conducted in accordance with JIS Z 2242 (2005) “Charpy impact test method for metal materials” using a 2 mm V notch Charpy impact test piece. The temperature at the fracture rate was measured.
The number of Al ₂ O ₃ clusters and the number of Zr oxides were confirmed by collecting samples for observation from the obtained carbon steel slabs and observing them at 400 times using an optical microscope, with a field number of 40 or more.

In Comparative Example 11 in which the amount of Al was less than the range of the present invention, deoxidation was insufficient and the Zr oxide was coarsened. For this reason, the number of fine Zr oxides was small, the Charpy fracture surface transition temperature was high, and the toughness was insufficient.
In Comparative Example 12, where the amount of Al was larger than the range of the present invention, the number of coarse Al ₂ O ₃ clusters was large.

In Comparative Example 13 in which the amount of Zr was less than the range of the present invention, the generation of Al ₂ O ₃ clusters could not be suppressed, and the number of coarse Al ₂ O ₃ clusters was large.
In Comparative Example 14 in which the amount of Zr was larger than the range of the present invention, coarse Zr carbonitride was generated, the Charpy fracture surface transition temperature was high, and the toughness was insufficient.
In Comparative Example 15 in which the amount of oxygen was larger than the range of the present invention, the number of coarse Al ₂ O ₃ clusters was large.

In contrast, in Examples 11-28, less number of more circle equivalent diameter 10μm in 1 mm ² of coarse _Al 2 _{O 3} clusters with 5 or less, and an equivalent circle diameter 50nm or more in 1 mm ² The number of Zr oxides of 100 nm or less is increased to 50 or more. Moreover, the Charpy fracture surface transition temperature was −55 ° C. or lower, and the toughness was greatly improved.

From the above, according to the present invention, it is possible to suppress the formation of coarse Al ₂ O ₃ clusters by adding Zr appropriately, and toughness is greatly increased by the presence of many fine Zr oxides. It was confirmed that it was possible to improve it.

1 Molten steel pan or melting furnace (vessel)
10 Molten steel 12 Zr addition part 20 Al ₂ O ₃ cluster 27 Zr oxide S03 Zr addition process

Claims (4)

C: 0.01% or more and 0.30% or less, Si: 0.001% or more and 1.5% or less, Mn: 0.01% or more and 3.0% or less, P; 0.0010% or more 0.050% or less, S; 0.0001% or more and 0.003% or less, Al; 0.01% or more and 1.5% or less, t. O: 0.0001% or more and 0.004% or less, t. N: 0.0001% or more and 0.01% or less, t. Zr: 0.0003% or more and 0.004% or less, with the balance being Fe and inevitable impurities,
While there are 50 or more Zr oxides with an equivalent circle diameter of 50 nm or more and 100 nm or less per 1 mm ² ,
A carbon steel slab characterized in that the number of Al ₂ O ₃ clusters exceeding the equivalent circle diameter of 10 μm is 5 or less per 1 mm ² . It is a manufacturing method of the carbon steel slab which manufactures the carbon steel slab of Claim 1,
The molten steel stored in the container has a Zr addition step of adding Zr after Al addition,
In the Zr addition step, Zr is added so that the Zr concentration of 50 cm immediately below the molten metal surface of the Zr addition portion becomes 15 ppm or less 30 seconds after the addition of Zr.   3. The method for producing a carbon steel slab according to claim 2, wherein an average reflux time of the molten steel in the container is 120 seconds or longer and 360 seconds or shorter.   A steel material formed by processing the carbon steel cast slab according to claim 1.

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