JP2003055733A - Steel having satisfactory toughness and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel which has satisfactory toughness, and to provide a production method therefor. SOLUTION: In the steel, the cleanliness of nonmetallic inclusions with a length of >=2 μm is <=0.050%, and the average value of inclusion indexes A expressed by formula (1) as for nonmetallic inclusions with a length of <2 μm is 5,000 nm; wherein, (m) in formula (1) is the number (pieces) of nonmetallic inclusions of <2 μm present in the field at an angle of 10 μm when an optional position of the steel is observed in the above field; LX is the length (nm) of nonmetallic inclusions of <2 μm observed on the Xth when the optional position of the steel is observed in the field at an angle of 10 μm; and (n) is a coefficient satisfying 0 in the case of LX<=20 nm, 1 in the case of 0.20 nm<LX<=250 nm, 3 in the case of 250 nm<LX<=500 nm, and 4 in the case of 500 nm<LX<2 μm.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material such as a thick steel plate and a manufacturing method thereof, and more particularly to a steel material having excellent toughness and a manufacturing method capable of stably obtaining the steel material.

[0002]

2. Description of the Related Art Heretofore, various methods have been discussed for improving the resistance to breakage of steel materials such as thick steel plates used as structural members of steel structures. The methods are roughly classified into the following three (A) to (C).

(A) Improvement of cleanliness of steel material This method reduces lattice defects by reducing the content of impurity elements such as P and S contained in the steel material as much as possible to improve the purity of the steel material. The purpose is to increase the resistance to brittle fracture, the initiation and propagation of ductile cracks. For example, "Iron and Steel (64th year)", pages 740 to 748, describe that impact properties are improved by reducing the P content. Also, "Iron and Steel (No. 63
S. pp. 713), it is described that impact properties, elongation properties and drawing properties are improved by reducing MnS. In recent years, with the progress of refining technology, it has become possible to obtain steel products containing extremely few of these impurities.

However, even in the present situation, in order to carry out special cleaning, it is necessary to complicate the smelting process, for example, to carry out smelting outside the furnace, which causes a rise in cost due to increase in processing time. . For example, A- in “Iron and Steel (69th year)”
As described on page 41, in order to reduce P and S, it is necessary to add a new process, which causes a cost increase. In particular, since mass-produced products need to place importance on economic efficiency, it is not practical to adopt a method requiring such a new production process and a new process.

(B) Addition of alloying elements As a general method for improving the strength of steel materials, it is known to contain C. However, if C is contained excessively, the toughness and weldability of the steel material are significantly deteriorated. For this reason, it is widely practiced to include alloy elements such as Ni and Mo that enhance the hardenability of steel materials. However, in mass-produced products, the addition of expensive alloying elements should be avoided as much as possible from the economical viewpoint.

(C) Structure-controlled steel products often have a final structure that is not as-solidified, and are reheated, and their crystal structure is α →
It is common to go through a γ → α transformation process. It is known that refinement of the steel structure is effective for improving the toughness of the steel, but specifically, the adjustment of the slab heating temperature,
The refinement of the steel structure has been achieved by adopting TMCP (Thermo-mechanical Control Process) and by controlling the temperature during heat treatment. But TMCP
Alternatively, performing heat treatment is a factor that complicates the process and hinders productivity, and therefore it is not preferable to incorporate these processes into the manufacturing process of mass-produced products.

Recently, in addition to the above-mentioned measures for improving toughness, a method of increasing the toughness by controlling the composition of inclusions existing in the steel or refining it has been studied. For example, on page 866 of “Iron and Steel (62nd year)”, MnS and Al ₂ O
The relationship between the area ratio of No. ₃ and impact characteristics is shown, and it is pointed out that an increase in the amount of these inclusions causes a decrease in absorbed energy. The inclusions have a length of 2 μm or more that can be observed with an optical microscope, and most of them are
It is generated in molten steel and trapped in the solidified shell without floating.

[0008]

[Problems to be Solved by the Invention] The length as described above is 2
With respect to relatively large inclusions of μm or more, it has become possible to reduce the inclusions to a considerably low level due to recent advances in refining technology. However, relatively small inclusions with a length of less than 2 μm may precipitate during the solidification process, and even such minute inclusions may act as a starting point of fracture depending on the precipitation condition, and the toughness of the steel material May decrease.

As described above, it has been widely practiced to pay attention to relatively large inclusions and improve the toughness of the steel material by reducing the amount of precipitation. A method for preventing the deterioration of toughness due to a material has not been studied. Further, if the amount of such inclusions deposited can be limited by a simple and inexpensive means, as shown in the above (A) to (C),
Since there is no need to make new capital investment or addition of alloying elements, it is useful for manufacturing steel products, such as mass-produced products, where importance is attached to economic efficiency.

The present invention has been made to solve the above-mentioned problems, and in addition to stricter quantitative restrictions on relatively large inclusions having a length of 2 μm or more,
The purpose of the present invention is to provide a steel material having good toughness by determining the condition of relatively small inclusions having a length of less than 2 μm. Another object of the present invention is to provide a manufacturing method for obtaining a steel material having good toughness by simply and inexpensively limiting the precipitation amount of inclusions to a range that does not adversely affect the toughness of the steel material. .

[0011]

DISCLOSURE OF THE INVENTION The gist of the present invention is a steel material shown in the following and and a manufacturing method of the steel material shown in the following.

For non-metallic inclusions having a length of 2 μm or more, the cleanliness measured by the method specified in JIS G 0555 is 0.050% or less, and for non-metallic inclusions having a length of less than 2 μm, Inclusion index A expressed by equation (1)
A steel material having good toughness, characterized in that the average value of is less than 5000 nm.

[Equation 5]

However, m is 10 μm at an arbitrary position of the steel material.
2 μm present in the visual field when observed in the angular visual field
Is the number of non-metallic inclusions (number), L _X is the length (nm) of the non-metallic inclusions less than 2 μm observed at the Xth position when observing an arbitrary position of the steel material in a visual field of 10 μm square. , N
Is 0 when L _X ≦ 20 nm, 20 nm <L _X ≦ 250 n
The coefficient is 1 when m, 3 when 250 nm <L _X ≦ 500 nm, and 4 when 500 nm <L _X <2 μm.

% By mass, C: 0.02 to 0.20%, Si: 0.60
% Or less, Mn: 0.20 to 2.00%, P: 0.030% or less, S: 0.0
A steel material having good toughness as described above, which contains 10% or less and Al: 0.06% or less.

The molten steel is subjected to an inert gas blowing treatment which satisfies the condition represented by the following formula (2), or a vacuum refining treatment which satisfies the condition represented by the following formula (3). A method for producing a steel material having good toughness as described in the above item 1 or 2, which is characterized by performing continuous casting after the execution.

[Equation 6]

[Equation 7]

However, G in the above equation (2)₁In molten steel
Inert gas flow rate (NL / min), H₁Is inactive
Distance from the tip of the noble gas blowing nozzle to the molten steel surface
(M), S₁Is the ladle molten steel amount (ton), D₁Is the inner diameter of the ladle
(M), t₁Indicates the inert gas blowing time (min)
And G in the formula (3) above_TwoIs an inert material used for molten steel reflux
Gas flow rate (NL / min), S_TwoIs the ladle molten steel amount (ton),
D_TwoIs the inner diameter of the immersion pipe (m), t _TwoIs the vacuum processing time (mi
n) is shown.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION In the steel material of the present invention, first, the length is
It is necessary to limit the cleanliness (hereinafter simply referred to as “cleanliness”) measured by the method specified in JIS G 0555 for non-metallic inclusions of 2 μm or more to 0.050% or less.

By reducing the precipitation amount of such a relatively large inclusion, the toughness and ductility of the steel material can be improved. As will be described later, the present invention aims to obtain a steel having excellent toughness as compared with a conventional steel material by defining conditions for relatively small inclusions of less than 2 μm. However, the cleanliness is 0.050
If it exceeds%, the effect of improving toughness cannot be sufficiently obtained even if the precipitation amount of relatively small inclusions is limited. Therefore, the cleanliness should be limited to 0.050% or less.

In the present specification, "inclusion" means JI.
In addition to the inclusions specified in SG 0555, it is defined to include carbides and nitrides generally called precipitates. For example,
Includes Mn carbide, precipitated Cu, Cr carbonitride, Mo carbonitride, V carbonitride, B carbonitride, and the like.

The term "length of inclusion" means the length of the inclusion in the longest direction. Here, for relatively large inclusions with a length of 2 μm or more,
It can be measured by observation with an optical microscope, and for relatively small inclusions with a length of less than 2 μm, which will be described later, by observation with an SEM (scanning electron microscope) or TEM (transmission electron microscope). can do.

In the steel material of the present invention, it is further necessary to limit the average value of the inclusion index A represented by the following formula (1) for non-metallic inclusions having a length of less than 2 μm to 5000 nm or less. However, m in the following formula (1) is 1 at any position of the steel material.
The number of non-metallic inclusions (pieces) less than 2 μm existing in the 0 μm square visual field, L _X, is the _Xth position when observing an arbitrary position of the steel material in the 10 μm square visual field. Observed length of non-metallic inclusions less than 2 μm (n
m) and n are 0 when L _X ≦ 20 nm, 20 nm <L _X
A coefficient is 1 when ≦ 250 nm, 3 when 250 nm <L _X ≦ 500 nm, and 4 when 500 nm <L _X <2 μm.

[Equation 8]

When setting the condition of a relatively small inclusion having a length of less than 2 μm, the behavior of toughness cannot be accurately grasped only by focusing on the amount of precipitation. That is,
Increasing the size (longer) of the inclusions causes more stress concentration and plastic strain concentration when an external force is applied, which adversely affects the toughness of the steel material. Therefore, it is necessary to evaluate the toughness behavior by weighting according to the size of the inclusions.

Based on this finding, the present inventors have rigorously investigated the relationship between the size of the small inclusions precipitated in the steel material and the toughness of the steel material, and as a result, the inclusions represented by the above formula (1) By limiting the average value of the index A to 5000 nm or less,
The present invention has been completed in which the toughness of steel materials can be stably maintained at a high level.

The "average value of inclusion index A" means SEM for 100 or more fields randomly selected in the steel material.
Or using TEM, present in each field of view 2
The length of the inclusions less than μm is measured, the inclusion index A is calculated from the above formula (1), that is, the product of the length of each inclusion and the corresponding coefficient (n) is obtained, The inclusion index A was calculated by calculating the sum of the products, and the average value of the inclusion index A in all measured visual fields is shown.

The steel material of the present invention has a mass% of C: 0.02 to 0.
20%, Si: 0.60% or less, Mn: 0.20 to 2.00%, P: 0.030
% Or less, S: 0.010% or less and Al: 0.06% or less are desirable. The reasons for limiting each element will be described below.

C: 0.02 to 0.20% C is the most effective element for increasing the strength of steel and is an inexpensive element. If the C content is less than 0.02%, it becomes necessary to incorporate other elements to ensure the strength,
Increases costs. On the other hand, if the content exceeds 0.20%, the weldability and toughness of the steel material are significantly reduced. Therefore, the C content is preferably 0.02 to 0.20%.

Si: 0.60% or less Si is an element effective for improving the strength of steel,
This effect can be obtained if the steel contains a very small amount, so that it may be at the impurity level. In order to obtain a greater effect, it is desirable to contain 0.05% or more. However, if the content exceeds 0.60%, the toughness of the steel material is significantly impaired. Therefore, the Si content is 0.
It is preferably 60% or less.

Mn: 0.20 to 2.00% Mn is an element effective for ensuring the strength of the steel material. In order to obtain this effect, it is desirable to contain 0.20% or more. However, if the content exceeds 2.00%, the weldability and toughness are significantly reduced. Therefore, the Mn content is 0.20
It is desirable to be ~ 2.00%.

P: 0.030% or less P is an impurity element and lowers the toughness of the steel material. Therefore, the P content should be as small as possible. Therefore,
It is desirable to limit the P content to 0.030% or less.

S: 0.010% or less S is an impurity element and deteriorates the toughness of the steel material. Therefore, its content should be as small as possible. Therefore,
It is desirable to limit the S content to 0.010% or less.

Al: 0.06% or less Al is an element effective for deoxidation and reduces inclusions in steel. This effect can be obtained if the steel contains a very small amount, so the content may be at the impurity level. But to get a bigger effect, 0.00
It is desirable to contain 5% or more. On the other hand, its content is 0.0
If it exceeds 6%, excessive precipitation of AlN and coarsening are caused, and the toughness of the steel material is significantly impaired. Therefore, the Al content is
It is preferably 0.06% or less.

The steel material of the present invention is not particularly limited with respect to the rest as long as it contains the above components. Therefore, the balance may be Fe and impurities, and Nb: 0.00
5 to 0.050%, V: 0.005 to 0.100%, Cu: 0.05 to 1.00%,
Ni: It may contain one or more elements such as 0.05 to 3.00%.

In the production method of the present invention, molten steel is subjected to an inert gas blowing treatment satisfying the condition represented by the following formula (2), or the condition represented by the following formula (3) is applied. Continuous casting is required after a satisfactory vacuum refining process is performed. However, G ₁ in the equation (2) is the flow rate of the inert gas blown into the molten steel (NL / min, normal liter /
Min), H ₁ is the distance from the tip of the inert gas blowing nozzle to the molten steel surface (m), S ₁ is the ladle molten steel amount (ton),
D ₁ is the ladle inner diameter (m), t ₁ is the inert gas blowing time (min), G ₂ in the above formula (3) is the inert gas flow rate (NL / min, normal liter / min), S ₂ is ladle molten steel amount (ton), D ₂ is immersion pipe inner diameter (m), and t ₂ is vacuum processing time (min).

[Equation 9]

[Equation 10]

The term "inert gas blowing treatment" used herein means a treatment of stirring molten steel and slag by introducing molten steel into a ladle and then blowing an inert gas such as Ar into the molten steel. As a result, the inclusions in the molten steel float up due to cohesive enlargement, or directly react with the slag caught in the molten steel and are absorbed in the slag and separated from the molten steel.

"Vacuum degassing treatment" means RH treatment or DH treatment.
It means a process of degassing using a vacuum chamber, such as a process. As a result, the inclusions in the molten steel are coagulated and enlarged and float up, so that they are absorbed in the slag and separated from the molten steel.

The reason for defining the above refining processing conditions will be described below with reference to the drawings. The deposition state of inclusions was investigated for the cast pieces continuously cast after the inert gas blowing treatment or the vacuum degassing treatment under the manufacturing conditions shown in Table 1 below. Here, for convenience, the value on the left side of expression (2) will be called the refining processing index B, and the value on the left side of expression (3) will be called the refining processing index C.

[0037]

[Table 1]

In the above investigation, the chemical composition was C: 0.
02 to 0.18%, Si: 0.04 to 0.52%, Mn: 0.42 to 1.65%,
P: 0.001-0.028%, S: 0.0003-0.0086%, Cu: 0-
1.00%, Ni: 0 to 2.98%, Cr: 0 to 0.86%, Mo: 0 to 0.
65%, V: 0 to 0.086%, Nb: 0 to 0.049%, Al: 0 to 0.
059%, Ti: 0 to 0.034%, B: 0 to 0.0021% and N:
Steel materials within the range of 0 to 0.0085% were used.

FIG. 1 is a diagram showing the relationship between the refining treatment index B and the cleanliness index or inclusion index A when the inert gas blowing process is performed under the manufacturing conditions shown in Table 1. Figure (a)
Indicates the relationship between the refining treatment index B and cleanliness, and (b) is
The relationship between the refining treatment index B and the inclusion index A is shown.

FIG. 2 is a diagram showing the relationship between the refining treatment index C and the cleanliness index or inclusion index A when the vacuum degassing treatment is performed under the manufacturing conditions shown in Table 1. The figure (a) shows the relationship between the refining processing index C and the cleanliness, and the (b) shows the relationship between the refining processing index C and the inclusion index A.

As shown in FIGS. 1 and 2, in order to keep the cleanliness of the steel material at 0.050% or less, the refining treatment index B is 0.7.
It is necessary to set the refining treatment index C to 5 or more or 3.0 or more. On the other hand, in order to make the inclusion index A 5000 nm or less, the refining treatment index B is 1.0 or more or the refining treatment index C is
Must be 4.0 or higher. Therefore, in order to satisfy the inclusion condition defined in the present invention, the refining treatment index B is 1.0
The refining treatment index C must be 4.0 or more. That is, the above formula (2) needs to be satisfied when the inert gas is blown into the molten steel in the ladle, and the formula (3) must be satisfied when the vacuum degassing process is performed. There is.

In the process of injecting the inert gas into the molten steel in the ladle, the effect of reducing inclusions is high even in the initial stage, but when the processing time t ₁ is less than 1 minute, the refining conditions are the above (2). Even if the formula) is satisfied, it is not possible to obtain a steel material that satisfies the inclusions defined in the present invention. Therefore, the processing time t ₁ needs to be 1 minute or more. On the other hand, in the vacuum degassing process, the effect of reducing inclusions in the initial stage is low,
When the processing time t ₂ is less than 2 minutes, the refining conditions are as described above.
Even if the formula (3) is satisfied, it is impossible to obtain a steel material that satisfies the inclusions defined in the present invention. This is because the slag on the molten steel surface of the ladle that has been sucked up into the vacuum tank is caught in the initial stage of the treatment, and it cannot be discharged to the outside of the slag vacuum tank unless the treatment is performed for 2 minutes or more. Therefore, the processing time t ₂ needs to be 2 minutes or more.

In the vacuum degassing process, the degree of vacuum is
If it exceeds 150 torr, it may not be possible to obtain a steel material that satisfies the inclusions defined in the present invention. Therefore, the vacuum degree in vacuum degassing must be 150 torr or less.

Here, in order to obtain the steel material of the present invention, an inert gas blowing treatment satisfying the above formula (2) or a vacuum degassing treatment satisfying the above formula (3) is performed as a refining treatment immediately before continuous casting. Just go. For example, when performing vacuum degassing treatment twice before continuous casting, even if the condition expressed by the above formula (3) is not satisfied in the first vacuum degassing treatment, the second vacuum degassing treatment is performed. The steel material of the present invention can be obtained if the condition represented by the above formula (3) is satisfied in the degassing treatment. Similarly, before continuous casting, when performing a vacuum degassing treatment after performing an inert gas blowing treatment, or when performing an inert gas blowing treatment after performing a vacuum degassing treatment, both It suffices that the refining treatment immediately before continuous casting satisfies the refining conditions of the present invention, in the former case, the vacuum degassing treatment should satisfy the above formula (3), and in the latter case, an inert gas blowing treatment. Is above
It suffices if expression (2) is satisfied.

In the steel material manufacturing method of the present invention, the continuous casting method is not particularly limited, but the casting speed is 0.4
Within the range of up to 2.0 m / min, the steel material of the present invention can be obtained. When the degree of superheating of molten steel in the tundish (molten steel temperature-liquid phase temperature) was less than 15 ° C, some of the materials did not satisfy the inclusion condition of the present invention. It is considered that this is because the viscosity of the molten steel increased because the molten steel temperature was low, and the inclusions in the molten steel could not rise in the tundish.
Therefore, it is desirable to limit the temperature to 15 ° C or higher. On the other hand, if the casting temperature is too high, problems such as melting loss of refractory, deterioration of operational stability, and rising cost of molten steel temperature rise may occur.Therefore, the degree of superheating of molten steel in the tundish should be 50 ° C or less. It is desirable to limit it. Further, if the specific water content is within the range of 0.2 to 2.0 L / (molten steel · kg), the steel material of the present invention can be obtained.

[0046]

Example A molten steel was subjected to an inert gas blowing treatment or a vacuum degassing treatment under the conditions shown in Table 2, and then continuously cast slabs were rolled under the conditions shown in Table 3 to obtain test steel sheets. In addition, Table 4 shows the chemical composition of the molten steel before continuous casting in terms of ladle chemical composition values. For plate rolling, see Table 3
The conditions were the same except the conditions shown in.

[0047]

[Table 2]

[0048]

[Table 3]

[0049]

[Table 4]

Using the test steel sheet thus obtained, the length was 2 μm.
For inclusions of m or more, the cleanliness measured by the method specified in JIS G 0555, the average number of inclusions present in a 10 μm square (100 fields of view) and the length of inclusions having a length of less than 2 μm, The average value of the inclusion index A is shown in Table 5.

The cleanliness factor is 1 / thickness of the wall thickness of the test steel plate, which is cut in parallel with the rolling direction.
Specimen 4 was mirror-polished with a diamond paste and then measured by the method specified in JIS G 0555. Further, the average value of the inclusion index A is a transmission electron microscope using a carbon replica for a 1/4 part of the wall thickness, with a cross section cut in parallel with the rolling direction from the above test steel plate as a test surface. Measure the length of inclusions present in any 10 μm square with and measure the number and length of inclusions in each length group (group less than 20 nm, 20 nm or more 25
Group of less than 0 nm, group of 250 nm or more and less than 500 nm and 500
Each group was classified into each group (nm or more and less than 2 μm), the inclusion index A was obtained by substituting it in the above formula (1), and the same operation was repeated to calculate the average value of the inclusion index A for 100 visual fields.

However, in Table 5 below, since it is not possible to describe the lengths of all the inclusions measured, the average number for each length group (when the total number of the number per 100 visual fields is Y, (Y / 100) and the average length for 100 fields in each length group are described.

[0053]

[Table 5]

Notch Charpy impact test pieces (2 mm V notch test pieces specified in JIS Z 2202) were cut out from the C direction of the above-mentioned test steel plate, and about 1/4 of the wall thickness,
Table 6 shows the results of measuring the upper shelf energy and the fracture surface transition temperature by performing the Charpy impact test at different temperatures.

[0055]

[Table 6]

As shown in Table 4 above, all of the steels used in the examples were melted as structural 40-50kg steels, and as a group of steels having almost the same chemical composition, No. 1 to 7 groups and steel No. 8 to 14 groups were prepared.

As shown in Tables 2 and 3 above, steel
No. 1 to 5 are all steel materials subjected to vacuum degassing treatment satisfying the condition represented by the formula (3) of the present invention, continuous casting, and plate rolling, and steel No. 6 And 7 are steel materials that have been vacuum-degassed under conditions that deviate from the conditions represented by the formula (3) of the present invention, and then subjected to continuous casting and plate rolling. In addition, as shown in Table 5 above, the steel Nos. 1 to 5 have cleanliness
In addition to being 0.050% or less, the average value of the inclusion index A represented by the formula (1) of the present invention was 5000 nm or less. on the other hand,
Although the cleanliness of steel Nos. 6 and 7 is 0.050% or less, the average value of the inclusion index A represented by the formula (1) of the present invention is 5
It exceeds 000 nm and does not satisfy the conditions specified in the present invention. Here, as shown in Table 6 above, Steel Nos. 1 to 5 satisfying the conditions of the present invention were compared with Steel Nos. 6 and 7 not satisfying the conditions of the present invention. Both the surface transition temperatures were excellent.

As shown in Tables 2 and 3 above, steel
No. 8 to 12 are all steel materials which were subjected to an inert gas blowing treatment satisfying the condition represented by the formula (2) of the present invention, followed by continuous casting and plate rolling, and steel No. 13 and 14
Is a steel material which has been subjected to an inert gas blowing treatment under conditions deviating from the conditions represented by the formula (2) of the present invention, and then subjected to continuous casting and plate rolling. Also, as shown in Table 5 above,
Steel Nos. 8 to 12 have a cleanliness of 0.050% or less and an average value of the inclusion index A represented by the formula (1) of the present invention is 5
It was 000 nm or less. On the other hand, Steel Nos. 13 and 14 have the cleanliness of 0.050% or less, but the average value of the inclusion index A represented by the formula (1) of the present invention exceeds 5000 nm,
The conditions specified in the present invention were not satisfied. Here, as shown in Table 6 above, steel N satisfying the conditions of the present invention
o.8 to 12 are steel Nos. 13 and 14 which do not satisfy the conditions of the present invention.
Compared with, the upper shelf energy and the fracture surface transition temperature were both excellent.

[0059]

EFFECTS OF THE INVENTION The steel material of the present invention is subject to stricter quantitative restrictions than before with respect to relatively large inclusions having a length of 2 μm or more, and the conditions for relatively small inclusions having a length of less than 2 μm. It is a fixed steel material with good toughness. Further, according to the method of the present invention, without making new capital investment, without adding an expensive alloy element, simply and inexpensively, the precipitation amount of inclusions is limited to a range that does not adversely affect the toughness of the steel material. Thus, a steel material having good toughness can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a refining treatment index B and a cleanliness index or an inclusion index A when an inert gas blowing treatment is performed under the manufacturing conditions shown in Table 1, where (a) is a refining treatment index B. And (b) show the relationship between the refining treatment index B and the inclusion index A.

FIG. 2 is a diagram showing a relationship between a refining treatment index C and a cleanliness index or an inclusion index A when vacuum degassing treatment is performed under the manufacturing conditions shown in Table 1, and (a) is a refining treatment index C and The relationship with cleanliness is shown, and (b) shows the relationship between refining treatment index C and inclusion index A.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/58 C22C 38/58 F term (reference) 4K013 AA07 AA09 BA14 CA01 CA02 CC01 CE01 CE02 CF13 DA03 DA05 DA12 DA13 FA02

Claims (3)

[Claims] 1. A non-metallic inclusion having a length of 2 μm or more
Cleanliness measured by the method specified in JIS G 0555
Is less than 0.050% and the length is less than 2 μm
Of the inclusion index A represented by the following equation (1)
Good toughness characterized by an average value of 5000 nm or less
Good steel material. [Equation 1] However, the definition of the symbol in the above formula (1) is as follows.
is there. m: When observing an arbitrary position of steel with a 10 μm square field of view
The number of non-metallic inclusions less than 2 μm present in the field of view
several) L_X: When observing an arbitrary position of steel with a 10 μm square field of view
X-th observed length of non-metallic inclusions less than 2 μm
(Nm) n: L_X0 ≦ 20 nm, 20 nm <L_X≤250 nm
When, 1,250nm <L _X3,500 when ≦ 500 nm
nm <L_X4 when <2 μm. 2. In mass%, C: 0.02 to 0.20%, Si: 0.60%
Below, Mn: 0.20 to 2.00%, P: 0.030% or less, S: 0.010
% Or less and Al: 0.06% or less, with a length of 2 μm or more
Method specified in JIS G 0555 for non-metallic inclusions
Cleanliness measured by 0.050% or less and long
In the following formula (1) for non-metallic inclusions with a dimension of less than 2 μm
The average value of the inclusion index A represented is 5000 nm or less.
Steel material with good toughness characterized by [Equation 2] However, the definition of the symbol in the above formula (1) is as follows.
is there. m: When observing an arbitrary position of steel with a 10 μm square field of view
The number of non-metallic inclusions less than 2 μm present in the field of view
several) L_X: When observing an arbitrary position of steel with a 10 μm square field of view
X-th observed length of non-metallic inclusions less than 2 μm
(Nm) n: L_X0 ≦ 20 nm, 20 nm <L_X≤250 nm
When, 1,250nm <L _X3,500 when ≦ 500 nm
nm <L_X4 when <2 μm. 3. A molten steel after being subjected to an inert gas blowing treatment satisfying the condition represented by the following formula (2), or
The continuous casting is performed after performing a vacuum refining process that satisfies the condition represented by the following formula (3).
Alternatively, the method for producing a steel material having good toughness according to 2 above. [Equation 3] [Equation 4] However, the definitions of the symbols in the above formula (2) or (3) are as follows. G ₁ : Flow rate of inert gas blown into molten steel (NL / min) H ₁ : Distance from tip of inert gas blow nozzle to molten steel surface (m) S ₁ : Ladle molten steel amount (ton) D ₁ : Ladle inner diameter (m) t ₁ : Inert gas blowing time (min) G ₂ : Inert gas flow rate (NL / min) used for molten steel circulation S ₂ : Ladle molten steel amount (ton) D ₂ : Dipping pipe Inner diameter (m) t ₂ : Vacuum processing time (min)