JP7614503B2 - Steel and method for producing steel - Google Patents

Description

The present invention relates to steel and a method for manufacturing steel, and in particular to a slab of Al-deoxidized steel, or a steel material manufactured by rolling the slab.

A technology has been developed to change the properties of inclusions to reduce their impact on quality and material properties, making them essentially harmless. MnS formed in steel elongates during rolling, which causes poor toughness of the steel. For example, in hot-rolled high-tensile steel and steel pipes, a technology has been put into practical use in which the precipitation of MnS, which is easily elongated, is suppressed by Ca treatment to modify the inclusions to a Ca sulfide composition, which is less elongated, and the maximum length in the rolling direction is reduced to improve toughness. Most of the steels normally manufactured are Al-deoxidized steels. When Ca is contained in Al-deoxidized steel as described above, the oxide-based inclusions in the steel become CaO-Al ₂ O ₃ -based oxides. However, even CaO-Al ₂ O ₃ -based oxides of some compositions have low melting points (for example, CaO-Al ₂ O ₃ binary oxides with 35 to 60 mass % CaO are liquid phase simple substances at 1600°C), and are soft inclusions, so they are easily elongated by rolling. Even if the Ca treatment can prevent MnS, such low melting point oxides may elongate and become harmful to the steel material.

Patent Document 1 discloses an invention in which, when deoxidizing molten steel, after adding Al, the amount of generated _Al2O3 is calculated using the value of free _oxygen in the molten steel before and after the addition of Al, and one of Zr, Ca and Mg, which are stronger deoxidizing elements than Al, is added up to the amount that can completely reduce this, thereby making alumina clusters fine by utilizing a reduction reaction.

Steel cords, which need to be prevented from breaking during wire drawing, are manufactured using silicon deoxidation to avoid hard alumina inclusions. The deoxidation conditions are then changed to control the composition of inclusions with a low melting point, and the ductility of the inclusions is increased, reducing the proportion of the cross-sectional area of the inclusions in the cross-sectional area of the wire. To achieve this, for example, a ladle refining process is used to control the inclusion composition by utilizing the equilibrium reaction between slag and metal. However, the long processing times and high costs mean that there are limitations to achieving this in mass-produced steel.

Patent Document 2 discloses a wire rod having an average oxide composition of SiO ₂ : 70% or more, CaO + Al ₂ O ₃ : less than 20%, and ZrO ₂ : 0.1 to 10% by weight. SiO ₂ inclusions with a high SiO ₂ content and high melting point, which are "hard inclusions", are finely dispersed when an appropriate amount of ZrO ₂ is compounded therewith. If SiO ₂ , CaO, and Al ₂ O ₃ are allowed to coexist with a certain range of ZrO ₂ , the size of the oxides becomes fine and the inclusion composition (oxide composition) becomes uniform, and the oxides can be made extremely small without attempting to lower the melting point. Since ZrO ₂ is contained in the refractory and flux, Zr does not need to be added. By adding Zr, the average composition of the oxides already mentioned can be adjusted to a desired range relatively easily.

Japanese Patent Application Publication No. 9-287015 International Publication No. WO99/67437

T.Murakami, H.Fukuyama, T.Kishida, M.Susa and K.Nagata: Metall. Trans., vol.31B(2000), p.25

As described above, when Ca is contained in Al-deoxidized steel, the oxide-based inclusions in the steel become CaO-Al ₂ O ₃ -based oxides. CaO-Al ₂ O ₃ -based oxides have a low melting point and are soft inclusions, so they are easily elongated by rolling.

An object of the present invention is to provide a steel and a method for producing the steel, which can prevent a decrease in the toughness of the steel material caused by CaO-Al ₂ O ₃ elongated during rolling.

The present invention was devised to efficiently prevent the elongation of CaO-Al ₂ O ₃ in Al-deoxidized steel containing Ca. In the present invention, the order of deoxidization is devised to prevent elongation by forming a specific oxide form in which a phase containing CaO and Al ₂ O ₃ and a phase containing CaO and ZrO ₂ are mixed, typically a complex oxide of CaO and ZrO ₂ dispersed in CaO-Al ₂ O ₃ inclusions, by devising a deoxidization sequence. Since the complex oxide of CaO and ZrO ₂ is hard, the CaO-Al ₂ O ₃ inclusions in which it is mixed have the property of being difficult to elongate.

In Patent Document 1, only one of Zr, Ca, and Mg is added, which is different from the inclusions of the present invention. In addition, the addition order is different from that of the present invention, as Zr is added after Al.

Patent Document 2 describes a Si-deoxidized steel, which is different from the present invention. In addition, the average oxide composition must be SiO ₂ : 70% or more, CaO + Al ₂ O ₃ : less than 20%, ZrO ₂ : 0.1 to 10%, and since it contains SiO ₂ at 70% or more, it is different from the inclusions of the present invention. Since the present invention is directed to Al-deoxidized steel, the inclusions after the casting do not contain a large amount of SiO ₂ .

That is, the gist of the present invention is as follows.
[1] In mass%, Si: 0.05 to 2.0%, Mn: 0.10 to 2.00%, S: 0.0100% or less, Al: 0.003 to 0.021%, Ca: more than 0% and 0.0020% or less, Zr: 0.002% or more and less than 0.01%, T.O: 0.0050% or less,
Among inclusions in steel with a major axis of 0.5 μm or more,
In the average composition of each of the inclusions, when _SiO2 is 1.5 mol% or less, and the contents (mol%) of _CaO , _Al2O3 , and _ZrO2 are (%CaO), (% _Al2O3 ), and (% _{ZrO2 )} , respectively, relative to the total of the three components being 100%,
27/55×(% _{Al2O3 )} +18≦(%CaO)≦23/27×(% _{Al2O3 )} +50 (1)
Among the inclusions that satisfy
(% _{Al2O3 )} ≦(% _ZrO2 )+30 (2)
And 5≦(% _ZrO2 ) (3)
While satisfying the above,
The steel is characterized in that the number ratio of inclusions having a morphology in which two phases, a phase containing CaO _{and Al2O3} and a phase containing CaO and ZrO2 _, are mixed together as main phases is 50% or more.
[2] The steel according to [1], wherein the steel is a cast steel or a steel material.
[3] To molten steel containing, by mass%, Mn: 0.1 to 2.0% and S: 0.0100% or less,
The method for producing a steel according to [1] or [2], characterized in that, before Al deoxidation, Si-Zr deoxidation is performed by simultaneously adding Si and a trace amount of Zr, followed by Al deoxidation and then Ca treatment.

In the present invention, by devising the deoxidation order in an Al-deoxidized steel containing Ca, the steel has a morphology in which two main phases, a phase containing CaO and Al ₂ O ₃ and a phase containing CaO and ZrO _2, are mixed, and the ratio of the number of inclusions having a specific oxide composition (specific oxide) is set to 50% or more. Since the composite oxide of CaO and ZrO ₂ is hard, inclusions consisting of this composite oxide and a CaO-Al ₂ O ₃ -based oxide phase have the property of being difficult to elongate.

This is a CaO-Al ₂ O ₃ -ZrO ₂ system phase diagram at 1600°C. The range of components of inclusions satisfying the formulas (1) to (3) of the present invention is shown on a (%CaO)-(%Al ₂ O ₃ )-(%ZrO ₂ ) ternary phase diagram. FIG. 1 is a diagram showing the temperature and rolling reduction pattern of a processing formaster test. 1A to 1C are diagrams showing typical morphologies of inclusions, where (A) is an example of the present invention, (B) is Comparative Example 8, and (C) is Comparative Example 9.

Composition and morphology of inclusions in steel
In the molten steel processing of Al-deoxidized steel containing Ca, deoxidation was performed by various methods, and then the steel was cast. The cast pieces were subjected to a Formasta processing (reduction temperature 1100°C, reduction rate 79%) to produce steel materials, and the composition and morphology of nonmetallic inclusions contained in the steel materials were investigated. In some levels, Si-Zr deoxidation was performed by simultaneously adding Si and a small amount of Zr before Al deoxidation, and then Al deoxidation was performed, followed by Ca treatment. In the investigation, inclusions (including oxides, sulfides, and oxysulfides) with a major axis of 0.5 μm or more were selected using a SEM (5000x), and elemental analysis was performed using an EDS (energy dispersive X-ray analyzer) attached to the SEM to clarify the distribution state of elements in each inclusion (element mapping) and calculate the average composition of each inclusion. In calculating the average composition of each inclusion, the results of elemental analysis of inclusions that were confirmed to contain oxygen showed that Si was present as SiO ₂ , Al was present as Al ₂ O ₃ , and Zr was present as ZrO _2. For Ca, the CaO content was calculated based on the analytical values of the S content and O content of the inclusions.

Since this is an Al-deoxidized steel containing Ca, a representative example of the inclusions observed is one in which the matrix is CaO-Al ₂ O _3- based. As mentioned above, such CaO-Al ₂ O _3- based inclusions are soft inclusions. Here, we attempt to form inclusions that contain a large amount of CaO-Al ₂ O ₃ -based oxide phase while still having hard properties.

First, attention is paid to the average composition of each of the inclusions. The method of calculating the average composition is as described above, and in calculating the average composition of the inclusions after elemental analysis by EDS, the results of elemental analysis of inclusions in which oxygen was confirmed to be contained were that Si was present as _SiO2 , Al was _present as _Al2O3 , and Zr was present as _ZrO2 . For Ca, a mass balance was calculated based on the analytical values of the S content and O content of the inclusions to calculate the CaO content.

First, since the target is Al-deoxidized steel containing Ca, the target is inclusions with an average composition of each inclusion containing 1.5 mol% or less SiO2. _{The SiO2 content (} mol%) was calculated assuming the sum of CaO (+CaS), _SiO2 , _Al2O3 , and _ZrO2 to be 100 mol%.

Secondly, in the average composition of each of the inclusions, the contents (mol%) of CaO, Al ₂ O ₃ and ZrO ₂ are (% CaO), (% Al ₂ O ₃ ) and (% ZrO ₂ ), respectively, relative to the total of the three components being 100%, and attention is paid to the relationship between these three components.

FIG. 1 shows a CaO-Al ₂ O ₃ -ZrO ₂ system phase diagram at 1600° C. (Non-Patent Document 1).
The hatched region in Figure 1, marked as Liq., is the low melting point composition region that is a complete liquid phase at 1600°C. Hereinafter, oxides in this composition region will be referred to as "low melting point oxides". Since rolling is generally performed at 1250°C or less, the oxides are in a solid phase during rolling, but are soft and very easy to elongate. Inclusions that elongate long in the rolling direction are harmful because they reduce the strength and toughness of the steel.

In the case of a CaO-Al ₂ O ₃ binary system that does not contain ZrO ₂ , that is, on the Al ₂ O ₃ -CaO axis on the _right side of the phase diagram, the "low melting point oxide" region is from 50 mol% CaO to 73 mol% CaO, which is the CA phase with a molar ratio of CaO and Al ₂ O ₃ of 1:1. It is widely known that Al ₂ O ₃ -CaO oxides in this composition range in the CaO-Al _{2 O 3 binary system have a low melting point and are easily stretched during rolling, which is harmful. However, the low melting point oxide region of the CaO-Al 2 O 3} -ZrO ₂ ternary system is somewhat expanded to the low CaO high Al ₂ O ₃ side when ZrO ₂ is contained, so the mol% CaO of the hatched low melting point oxide region in Figure 1 is in the range from 45 to 73 mol% CaO.

On the lower CaO/high Al ₂ O ₃ side of the low melting point oxide on the upper left side of FIG. 1, a hard solid phase CA ₂ phase (a composite oxide with a molar ratio of CaO and Al ₂ O ₃ of 1:2) with a melting point exceeding 1600°C is mixed. On the higher CaO/low Al ₂ O ₃ side of the low melting point oxide and on the higher CaO concentration side of 73 mol% CaO-27 mol% Al ₂ O ₃ on the lower right side of FIG. 1, a hard solid phase CaO phase with a melting point exceeding 1600°C is also mixed. That is, in the compositions on both sides of the low melting point oxide indicated as Liq. in FIG. 1, a low melting point oxide and a hard solid phase are mixed. Therefore, the degree of elongation during rolling is lower than that of a single low melting point oxide, and the degree of harmfulness is significantly lower.

For the above reasons, when CaO-Al ₂ O ₃ system oxides are generated by Ca treatment of Al deoxidized steel, it is widely practiced to avoid low melting point oxide compositions (50 to 73 mol% CaO in the case of a CaO-Al ₂ O ₃ binary system not containing ZrO ₂ ), that is, to control the composition to be lower CaO side or higher CaO side than the low melting point oxide.

However, in reality, there are cases where it is not possible to avoid all oxide compositions other than low-melting oxides due to fluctuations in operating conditions. Therefore, the present invention aims to reduce the harmfulness and render harmless the above-mentioned low-melting oxides that are completely liquid at 1600°C or higher and are very easy to elongate by controlling the composition. For this reason, the present invention targets oxides of 45 to 73 mol% CaO, which are low-melting oxides of the CaO-Al ₂ O ₃ -ZrO ₂ ternary system. The present invention targets not only the case where all observed inclusions are low-melting oxides, but also the case where the oxide composition is widely distributed as a whole and some oxides are the above-mentioned low-melting oxide composition. This is because the harmfulness of the entire steel can be reduced by suppressing the elongation of all or some of the low-melting oxides.

The composition control range targeted by the present invention will be explained using the (% CaO)-(% Al ₂ O ₃ )-(% ZrO ₂ ) ternary phase diagram shown in FIG.
First, the following formula (1) is defined.
27/55×(% _{Al2O3 )} +18≦(%CaO)≦23/27×(% _{Al2O3 )} +50 (1)
In the (%CaO)-(%Al ₂ O ₃ )-(%ZrO ₂ ) ternary phase diagram shown in FIG. 2, the range defined by the above formula (1) is the part surrounded by the thick line in FIG. 2. The left side of formula (1) is a formula for excluding the white part in the upper left of FIG. 2. In the white part in the upper left, a hard solid phase _CA2 phase (a composite oxide with a molar ratio of CaO and Al ₂ O ₃ of 1:2) with a melting point exceeding 1600°C is mixed in with the low melting point oxide phase. Therefore, the degree of elongation during rolling is lower and the harmfulness is lower than that of a single low melting point oxide. The right side of formula (1) is a formula for excluding the white part in the lower right of FIG. 2. In the white part in the lower right, a hard solid phase CaO phase with a melting point exceeding 1600°C is mixed in. Therefore, the degree of elongation during rolling is lower and the harmfulness is lower than that of a single low melting point oxide. When (%ZrO ₂ )=0%, i.e., (%CaO)=100-(%Al ₂ O ₃ ) (on the (%CaO)-(%Al ₂ O ₃ ) axis in Figure 2), the range of formula (1) indicates a low-melting point oxide region of 45 to 73 mol% CaO in terms of the CaO-Al ₂ O ₃ binary system.
Therefore, with regard to the individual average component composition of the inclusions, inclusions falling within the range encompassed by the above formula (1) (the bold line portion in Figure 2) in the (%CaO)-(%Al ₂ O ₃ )-(%ZrO ₂ ) ternary phase diagram are taken as "target inclusions".

Next, the hatched areas in Figure 2 will be explained in more detail. Note that the component concentrations shown in parentheses in the formulas are in mol%.

First, as described above, the present invention relates to the average composition of each of the inclusions in steel with a major axis of 0.5 μm or more, and takes up as "target inclusions" in the range encompassed by the above formula (1) (the bold line portion in FIG. ₂ ) in the (%CaO)-(%Al ₂ O ₃ )-(%ZrO 2 ) ternary phase diagram as the "target inclusions." When (%ZrO ₂ )=0, low-melting-point oxides of 45 to 73 mol% CaO calculated as a CaO-Al ₂ O ₃ binary system are included in this range.
Among the "target inclusions" included in this range, the inclusions limited to high melting point inclusions essentially contain _ZrO2 , and therefore the following formula (3) is defined.
5≦(% _ZrO2 ) (3)
Further, in order to exclude a component composition close to the (CA+CA ₂ ) phase on the high Al ₂ O ₃ side of the low melting point oxide phase of the CaO-Al ₂ O ₃ system oxide phase, the following formula (2) is defined.
(% _{Al2O3 )} ≦(% _ZrO2 )+30 (2)

Next, attention will be paid to the morphology of each individual inclusion.
At a level where Si-Zr deoxidation was performed by simultaneously adding Si and a small amount of Zr before Al deoxidation, followed by Al deoxidation and then Ca treatment, it was possible to obtain inclusions having a morphology in which two phases, a phase containing CaO and Al ₂ O ₃ and a phase containing CaO and ZrO _2, were mixed as the main phases. As a representative example, inclusions were observed in which the matrix was CaO-Al ₂ O ₃ and a phase containing CaO and ZrO ₂ was dispersed in the matrix. The phase containing CaO and ZrO ₂ is presumed to be one or both of a CaO-ZrO ₂ compound and ZrO ₂ in which CaO is solid-dissolved. Although the phase containing CaO and Al ₂ O ₃ (CaO-Al ₂ O ₃ oxide phase) is soft, the phase containing CaO and ZrO ₂ is hard, so an inclusion in which these two phases are mixed and present can be expected to be hard. The same is true for an inclusion in which the matrix is CaO-Al ₂ O ₃ and the phase containing CaO and ZrO ₂ is dispersed in the matrix. Therefore, one of the conditions for an inclusion to be included in the "target inclusion" defined above and still be a hard inclusion is that the inclusion has a form in which the two phases, a phase containing CaO and Al ₂ O ₃ and a phase containing CaO and ZrO _2, are mixed together (including the case where they are dispersed) as the main phases. Here, _the phase containing CaO and _Al2O3 is defined as a region _containing 5 mol% or more of CaO and _Al2O3 , and 80 mol% or more in total, with the sum of the mol% of the three components CaO, _Al2O3 , and _ZrO2 being 100%, and the phase containing CaO and _ZrO2 is defined as a region containing 5 _mol % or more of CaO and _ZrO2 , and 80 mol% or more in total. "These two phases are the main phases" means that the total area ratio of the two phases in each inclusion is 67% or more.

Among the phases containing CaO and ZrO ₂ , the CaO-ZrO ₂ compound is called the "CZ phase" and the ZrO ₂ phase in which CaO is dissolved is called the "Css phase". In the (%CaO)-(%Al ₂ O ₃ )-(%ZrO ₂ ) ternary phase diagram shown in FIG. 2, the hatched area is a multi-phase region in which a phase containing low-melting CaO and Al ₂ O ₃ and a phase containing CaO and ZrO ₂ (mainly CaO-ZrO ₂ compound (CZ phase) and ZrO ₂ phase (Css phase) in which CaO is dissolved) are mixed. As mentioned above, the low-melting oxide phase is a completely liquid phase at 1873K, and is a solid phase during rolling, which is generally performed at 1250°C or less, but is soft. Therefore, the low melting point oxide phase containing CaO and Al ₂ O ₃ alone is easily stretched during rolling, which is harmful. On the other hand, the hard CZ phase and Css phase are hard at the rolling temperature and are very difficult to deform. Therefore, if the low melting point oxide phase containing CaO and Al ₂ O ₃ and the phase containing CaO and ZrO ₂ are mixed as the main phases, the CZ phase and Css phase are mixed in the low melting point oxide phase, so that the stretching and deformation as a whole can be effectively inhibited. In this way, the oxide having the component composition of the hatched part of the phase diagram is more sufficiently suppressed from stretching and deformation than the low melting point oxide alone. The CZ phase and Css phase are generally distributed inside the low melting point oxide phase, but they have the same overall deformation suppressing effect not only inside but also when attached to the periphery. Furthermore, soft and easily elongated MnS may adhere to a portion of the low-melting-point oxide phase in which the CZ phase and the Css phase are mixed, but for the same reason, deformation and elongation of the inclusions as a whole, including MnS, can be suppressed.

As described above, among the selected "target inclusions" having a major axis of 0.5 μm or more, _SiO2 of 1.5 mol% or less, and satisfying formula (1), those that satisfy formulas (2) and (3) and have the above-mentioned inclusion morphology were defined as "specific inclusions", and the number ratio of the "specific inclusions" contained in the "target inclusions" was defined as the "specific inclusion ratio", and the elongation state of the inclusions during rolling and the occurrence of cracks during rolling were evaluated. As a result, it was revealed that when the specific inclusion ratio is 50% or more, the elongation of the inclusions during rolling is suppressed and the occurrence of cracks during rolling is reduced.

Therefore, in the present invention, the inclusions contained in steel are defined as follows.
Among inclusions in steel with a major axis of 0.5 μm or more,
In the average composition of each of the inclusions, when _SiO2 is 1.5 mol% or less, and the contents (mol%) of _CaO , _Al2O3 , and _ZrO2 are (%CaO), (% _Al2O3 ), and (% _{ZrO2 )} , respectively, relative to the total of the three components being 100%,
27/55×(% _{Al2O3 )} +18≦(%CaO)≦23/27×(% _{Al2O3 )} +50 (1)
Among the inclusions that satisfy
(% _{Al2O3 )} ≦(% _ZrO2 )+30 (2)
And 5≦(% _ZrO2 ) (3)
While satisfying the above,
In the morphology of the inclusions, the ratio by number of inclusions having a morphology in which two phases, a phase containing CaO and Al ₂ O ₃ and a phase containing CaO and ZrO _2, are mixed as main phases is 50% or more.

Steel composition
The steel components of the present invention are defined as follows, where % means mass %.

Zr: 0.002% or more, less than 0.010% In the present invention, Zr is an important element for preventing the toughness of steel from decreasing by suppressing the elongation of low-melting-point CaO-Al ₂ O _3. That is, by distributing CaO.ZrO ₂ (a compound with a molar ratio of 1:1, shown as CZ in the phase diagram) and 18 to 20 mol% CaO-ZrO ₂ (a cubic _{ZrO 2} phase in which CaO is dissolved, shown as Css = cubic solid solution in the phase diagram. Solid solution means a solid solution phase) in the low-melting-point CaO-Al 2 O ₃ oxide, these phases all have a high melting point, i.e., are hard and difficult to deform, and therefore have the effect of suppressing the elongation of the entire CaO-Al ₂ O ₃ oxide.
If the Zr content is less than the lower limit of 0.002%, _ZrO2 is not formed, and the desired CaO- _{Al2O3 -ZrO2 composition} is not obtained. Therefore, the lower limit is set to 0.002%.
On the other hand, if Zr is 0.01% or more, the oxide composition will deviate from the target. In addition, coarse _ZrO2 will be generated and will combine with _the existing _Al2O3 , causing nozzle blockage and reducing toughness by becoming the starting point of cracks due to their large size. For this reason, Zr is limited to less than 0.010%.

Al: 0.003-0.021%
Al is important as a deoxidizing element for molten steel. As a result of Al deoxidization, Al _{2 O 3} is generated, and since it has a lower density than molten steel, _it floats up from the molten steel and is removed from the molten steel. Coarse Al ₂ O ₃ particles of about 100 μm or more float to the surface and are removed from the molten steel, while finer Al ₂ O ₃ particles remain and form an Al ₂ O ₃ —ZrO ₂ composite oxide.
However, when the Al content is less than 0.0030%, deoxidation is insufficient. If deoxidation is insufficient, a large amount of dissolved oxygen remains. As a result, when other deoxidizing elements are added, In order to obtain a sufficient deoxidizing effect, 0.0030% or more of Al is added to reduce the dissolved oxygen and the generated oxides are reduced. It is necessary to sufficiently float and remove Al ₂ O ₃ .
On the other hand, when Al is added in an amount exceeding 0.021%, Zr is completely reduced and no _ZrO2 remains.

Ca: more than 0% and 0.002% or less Ca is added to prevent the formation of MnS. However, if the content exceeds 0.002%, a large amount of coarse low melting point oxides (CaO-Al ₂ O ₃ ) are formed, which are elongated during rolling and impair toughness and workability, so the content is limited to 0.002% or less. The lower limit of Ca is preferably 0.0005%.

Si: 0.05-2.0%
In the present invention, by performing Si-Zr deoxidation before Al deoxidation, SiO ₂ -ZrO ₂ composite oxides and SiO ₂ ·ZrO ₂ +ZrO ₂ are dispersed. This is because the low melting point SiO _2- containing oxide Therefore, we take advantage of the fact that it is difficult to cluster and easy to distribute.
In addition, although silicon is an element that contributes to improving the strength of steel materials through solid solution strengthening, if the silicon content is less than 0.05%, sufficient effect cannot be obtained. On the other hand, if the silicon content is too large, the toughness decreases. , with a maximum of 2.0%.

Mn: 0.10-2.00%
Mn contributes to improving strength by solid solution strengthening, but if the content is less than 0.10%, sufficient effect cannot be obtained. On the other hand, if the content exceeds 2.00%, the workability decreases, so the upper limit is set to 2.00%. Let us assume that.

S: 0.0100% or less From the viewpoint of preventing the formation of MnS, the S content is preferably as small as possible, so it is limited to 0.0100% or less. There is no particular lower limit. If the S content is high, the amount of free MnS, which is easily elongated during rolling, increases, and deterioration of the material quality is unavoidable, so it is usually controlled to 0.001% or less.

T.O: 0.0050% or less Since O is an element that generates oxides, it is necessary to reduce its content as much as possible. In particular, if T.O exceeds 0.0050%, coarse oxides are formed, and if they remain, they are likely to cause cracks and surface defects. Therefore, the content of T.O is set to 0.0050% or less.

The following ingredients are not essential, but it is preferable to limit their content for various reasons.

C: 0.003-0.3%
In order to ensure the strength of the steel material, the content is set to 0.003% or more. If the content exceeds 0.3%, the toughness decreases, so the upper limit is set to 0.3%.

P: 0.012% or less P is prone to segregation, particularly at grain boundaries, which causes a decrease in toughness, so the lower the content, the better. However, taking into account the cost of removing P, the upper limit is set at 0.012%.

The preferred steel composition of the present invention contains each element in the above content range, with the remainder being Fe and impurities. Furthermore, the following components may be contained in place of a portion of Fe.

Cu: 0.1-1.5%
Ni: 0.1-10.0%
Cr: 0.1-10.0%
Mo: 0.05-1.5%
Cu, Ni, Cr, and Mo are elements that improve the hardenability of steel. By including Cu, Ni, and Cr at 0.1% or more, and Mo at 0.05% or more, the strength is improved. If Cu and Mo exceed 1.5%, and Ni and Cr exceed 10.0%, toughness and workability may be impaired. Therefore, Cu should be limited to 0.1 to 1.5%, and Ni and Cr should be limited to 0. .1 to 10%, and Mo is limited to the range of 0.05 to 1.5%.

Nb: 0.005-0.1%
V: 0.005-0.3%
Ti: 0.001-0.25%
Nb, V, and Ti are elements that improve the strength of steel by precipitation strengthening. When Nb and V are contained at 0.005% or more, and Ti is contained at 0.001% or more, the strength improving effect is achieved. However, Nb If Nb is added in excess of 0.1%, V in excess of 0.3%, or Ti in excess of 0.25%, toughness may be impaired. Therefore, Nb should be added in an amount of 0.005-0.1%, V in an amount of 0.005-0. 3%, and Ti is limited to the range of 0.001 to 0.25%.

B: 0.0005-0.005%
B is an element that improves the hardenability of steel and increases its strength. When it is contained in an amount of 0.0005% or more, it shows a strength improving effect. However, when it is added in an amount exceeding 0.005%, it increases the amount of B precipitates, which reduces toughness. Therefore, the content is limited to the range of 0.0005 to 0.005%.

The steel of the present invention specifically refers to a slab that has been cast (e.g., continuous casting), or a steel product obtained by rolling the slab.

<Production Method>
The production of the steel (cast or steel material) of the present invention is characterized in that the components in the steel are adjusted to Mn: 0.1 to 2.0%, S: 0.0100% or less, Si and a trace amount of Zr are simultaneously added to perform Si-trace amount of Zr deoxidation, Si: 0.05 to 2.0%, Zr: 0.002 to 0.01%, Al deoxidation is performed to adjust Al: 0.003 to 0.021%, and Ca is added to adjust Ca: more than 0% and 0.0020% or less. The addition of a trace amount of Zr here means addition such that the Zr content in the steel is 0.002 to 0.01%. The simultaneous addition of Si and Zr is preferably true simultaneous addition, but it is permissible to add Si first and then add Zr quickly without intentionally leaving a time interval. For example, when the alloy addition hopper of the secondary refining equipment is separate for Si and Zr, this corresponds to the case where Si is added first, and then the hopper for adding Zr is immediately opened to add the alloy. In general, the time from switching the hopper until the next alloy is added does not exceed one minute. On the other hand, if Zr is added first, _ZrO2 is generated first, and even if Si is added after that, Si cannot reduce the already generated _ZrO2 , so _SiO2 - _{ZrO2-based composite oxides are not generated. Therefore, the property of SiO2-ZrO2 - based} composite oxides that is difficult to cluster and easy to disperse cannot be utilized, and the inclusions do not become fine.

By carrying out Si-trace Zr deoxidation before Al deoxidation, the SiO2- _Zr oxides are finely dispersed by utilizing the property of SiO2 _- containing inclusions that are less likely to cluster and are easier to disperse, and therefore the _{Al2O3 - ZrO2 oxides} produced by reducing Si through Al _deoxidation are also finely dispersed. Then, Ca treatment is carried out to produce inclusions having a morphology in which two main phases, a phase containing CaO and _Al2O3 and a phase containing CaO and ZrO2 _, are mixed together, typically inclusions in which CaO· _ZrO2 (+ _{ZrO2 )} is dispersed in inclusions whose matrix is CaO- _Al2O3 .

By performing Si-trace Zr deoxidation before Al deoxidation, _SiO2 - _ZrO2 oxides are finely dispersed (<5μm). If Al deoxidation is then performed, the Si in the inclusions is reduced to form an _Al2O3 - _ZrO2 system (<10μm). _Al2O3 and _ZrO2 do not form a compound, but form _{an Al2O3} + _{ZrO2 two} -phase structure that contains _ZrO2 inside. Because _{SiO2 - ZrO2} is fine, even if multiple _{ZrO2 particles} are contained within _Al2O3 , they do not become coarse inclusions (> _15μm ) that adversely affect the material. Then, Ca treatment is carried out to create an oxide form in which a phase containing CaO and Al ₂ O ₃ and a phase containing CaO and ZrO ₂ are mixed, typically CaO-Al ₂ O ₃ + CaO.ZrO ₂ (+ZrO ₂ ) in which high-melting point CaO-ZrO ₂ -based composite oxide is dispersed, thereby preventing elongation and enabling the production of a steel material with improved toughness.

(Manufacturing of steel billets)
1 kg of electrolytic iron was melted in an Ar atmosphere in a high-frequency induction melting furnace. The amount of dissolved oxygen before the addition of the deoxidizing element was 0.0200%. After adding C, Mn, P, S, and other elements, Si and Zr were added simultaneously, and then Al and Ca were added in that order, and the furnace was turned off and cooled. In Comparative Example 8, Al was added first, C, Si, Mn, P, S, Ca, and other elements were added, and then Zr was added. In Comparative Example 9, Al was added first, C, Si, Mn, P, S, and other elements were added, and then Zr was added, and finally Ca was added. The steel slab after furnace cooling corresponds to the steel slab. The component composition of the slab is shown in Table 1.

(Evaluation of cast pieces and steel products)
From the steel slab formed above, a sample (diameter 8 mm x height 12 mm) was taken from a sound part without defects such as shrinkage cavities, and a Formaster test was performed. Compression was performed at a reduction temperature of 1100°C, a reduction rate of 79%, and a strain rate of 10 s ^-1 . The temperature and reduction pattern of the Formaster test are shown in Figure 3. The compressed sample corresponds to the steel material of the present invention. The compressed sample was cut in a cross section along the diameter direction and polished, and the polished surface was used as the observation surface.

The inclusions were observed with a SEM (5000x magnification). 50 inclusions (including oxides, sulfides, and oxysulfides) with a major axis of 0.5 μm or more were randomly selected, and inclusions in which SiO ₂ was 1.5 mol% or less in the average composition of each inclusion and which satisfied the above formula (1) were selected as "target inclusions". Next, the number ratio of "specific inclusions" to the "target inclusions" (specific inclusion ratio (%) = number of specific inclusions / number of target inclusions x 100) was confirmed and recorded in the "number ratio (%) of specific inclusions" column in Table 2. As described above, "specific inclusions" refer to "target inclusions" that satisfy formulas (2) and (3) and have a form in which the two phases, CaO and Al ₂ O ₃ and CaO and ZrO _2, are mixed together (including the case where they are dispersed) among the "target inclusions". In the EDS analysis, the phase containing CaO and Al ₂ O ₃ is defined as a region containing 5 mol% or more of CaO and Al ₂ O ₃ , and 80 mol% or more in total, with the sum of the mol% of _{the three} components CaO, Al ₂ O ₃ , and ZrO 2 being 100%, and the phase containing CaO and ZrO ₂ is defined as a region containing 5 mol% or more of CaO and ZrO ₂ , and 80 mol% or more in total. The term "main phases are these two phases" means that the total area ratio of the two phases in each inclusion is 67% or more. The molecular weights of CaO, Al ₂ O ₃ , and ZrO ₂ were calculated to be 56 g/mol, 102 g/mol, and 123 g/mol, respectively.
At some levels, inclusions were evaluated not only in the steel but also in the slab. As a result, it was confirmed that at the same level, the "specific inclusion ratio" of the slab and the steel was almost the same value.

In addition, in Table 2, the average composition of the inclusions of each steel sample is shown for reference. In the examples and comparative examples 3, 4, 5, and 6, 10 inclusions containing CaO-Al ₂ O ₃ and CaO-ZrO ₂ were randomly selected, and the oxide composition (mol%) was calculated based on the elemental analysis value of the EDS (energy dispersive X-ray analyzer) attached to the SEM, with the total of the four components CaO, Al ₂ O ₃ , ZrO ₂ , and SiO ₂ being 100%, and the average value of the 10 inclusions was shown in Table 2. In addition, in comparative example 1, there was no inclusion containing CaO-Al ₂ O ₃ and CaO-ZrO ₂ , so CaO-Al ₂ O ₃ was selected, and the oxide composition (mol%) was calculated based on the total of the four components CaO, Al ₂ O ₃ , ZrO ₂ , and SiO ₂ being 100%, and the average value of the 10 inclusions was shown. In Comparative Examples 2, 7, 8, and 9, there were less than 10 samples containing CaO-Al ₂ O ₃ and CaO-ZrO ₂ , so one sample containing CaO-Al ₂ O ₃ and CaO-ZrO ₂ was selected, and the oxide composition (mol %) was calculated and recorded assuming that the total of the four components, CaO, Al ₂ O ₃ , ZrO ₂ , and SiO _2, was 100%.

The aspect ratio (= (maximum length) ÷ (maximum thickness)) of the inclusions in each steel sample was investigated. The aspect ratio was calculated as the average of 10 inclusions and is shown in the "Aspect Ratio" column in Table 2. An aspect ratio of 3 or less was used as the criterion for non-stretching.

In addition, the presence or absence of cracks of 1 mm or more in length on the surface of the compressed sample was evaluated using a loupe (magnification 8x), and the results are shown in Table 2. The absence of cracks was set as the pass criterion.

The steel materials according to the present invention 1 to 17 in Tables 1 and 2 satisfy the conditions of the present invention. The morphology of the inclusions in the present invention examples is shown in FIG. 4(A) as a schematic diagram. The number ratio of the specific inclusions among the inclusions was 50% or more. By making the ratio of the specific inclusions 50% or more, the oxide morphology in which a phase containing CaO and Al ₂ O ₃ and a phase containing CaO and ZrO ₂ are mixed, typically CaO-Al ₂ O ₃ +CaO.ZrO ₂ (+ZrO ₂ ) morphology in which high melting point CaO.ZrO ₂ composite oxide is dispersed, is predominant, so that the aspect ratio of the inclusions in the steel material after compression is low, 3 or less, and elongation can be prevented. In addition, no cracks were generated.

Comparative examples 1 to 9 in Tables 1 and 2 are comparative examples.

Comparative Example 1 is an example where Zr was not added, and Comparative Example 2 is an example where Zr was outside the lower limit. In both cases, the oxide did not contain hard _ZrO2, so CaO- _{Al2O3 ,} which has a low melting point and is easy to elongate, was generated alone, and the aspect ratios were large, at 10 and 9, and the elongation was achieved.
In Comparative Example 3, which was outside the upper limit of Zr, coarse _ZrO2 was generated, and cracks penetrating the thickness direction were generated in the compressed sample.

In Comparative Example 4, where the Ca content was outside the upper limit, the inclusion composition deviated from the target composition, and the aspect ratio was elongated to 10.

In Comparative Example 5, where the T.O. limit was exceeded, coarse oxides were formed due to the excess O, and cracks of 1 mm or more were formed on the surface of the sample after compression.

In Comparative Example 6, in which the Al content was outside the lower limit, deoxidation was insufficient and a large amount of oxide was produced, resulting in numerous cracks of 1 mm or more on the surface of the compressed sample.
In Comparative Example 7, which was outside the upper limit of Al, the oxide did not contain ZrO ₂ , and CaO-Al ₂ O _3, which has a low melting point and is easy to elongate, was generated alone, and the aspect ratio was 9, which was a large elongation.

In Comparative Example 8, Al was added first, then Ca, and finally Zr was added, which is a different order of addition from that of the present invention. The morphology of the inclusions in Comparative Example 8 is shown in Fig. 4(B) as a schematic. Since Al and Ca were added first, CaO-Al ₂ O ₃ was formed first, and then ZrO ₂ was formed. Therefore, the oxide morphology of the present invention in which a phase containing CaO and Al ₂ O ₃ and a phase containing CaO and ZrO ₂ are mixed, typically an inclusion in which CaO-Al ₂ O ₃ oxide is dispersed with CaO-ZrO ₂ and ZrO ₂ in which CaO is solid-dissolved, was not formed, and instead, a CaO-Al ₂ O ₃ oxide alone (left side of Fig. 4(B)) and a ZrO ₂ oxide alone (right side of Fig. 4(B)) were formed.

In Comparative Example 9, Al was added first, followed by Zr, and finally Ca, which is a different order of addition from that of the present invention. The morphology of the inclusions in Comparative Example 9 is shown in FIG. 4(C) as a schematic diagram. _{Al 2} O ₃ was generated first, ZrO ₂ was generated, and then Ca was added to generate CaO-Al ₂ O ₃ and CaO.ZrO _2. Therefore, the oxide morphology of the present invention in which a phase containing CaO and Al ₂ O ₃ and a phase containing CaO and ZrO ₂ are mixed, typically an inclusion in which CaO-Al ₂ O ₃ oxide is dispersed with CaO.ZrO ₂ and ZrO ₂ in which CaO is solid-dissolved, was not generated, and instead, a CaO-Al ₂ O ₃ oxide alone (left side of FIG. 4(C)), CaO.ZrO ₂ (center of FIG. 4(C)), and a ZrO ₂ oxide alone (right side of FIG. 4(C)) were generated.

Claims (6)

In mass percent,
C: 0.003-0.3%,
Si: 0.05-2.0%,
Mn: 0.10-2.00%,
P: 0.012% or less,
S: 0.0100% or less,
Al: 0.003-0.021%,
Ca: more than 0%, less than 0.0020%,
Zr: 0.002% or more, less than 0.01%
T. O: 0.0050% or less,
and the balance being Fe and impurities,
Among inclusions in steel with a major axis of 0.5 μm or more,
In the average composition of each of the inclusions, SiO ₂ is 1.5 mol % or less, and the contents (mol %) of CaO, Al ₂ O ₃ and ZrO ₂ are each 1.5 mol % or less relative to the total of the three components, 100%.
CaO), (% Al ₂ O ₃ ), and (% ZrO ₂ ).
27/55×(% _{Al2O3 )} +18≦(%CaO)≦23/27×(% _{Al2O3 )} +50
(1)
Inclusions that satisfy the above criteria are considered to be "target inclusions."
Among the above-mentioned "target inclusions" ,
(% _{Al2O3 )} ≦(% _ZrO2 )+30 (2)
And 5≦(% _ZrO2 ) (3)
While satisfying the above,
In the form of the inclusions, inclusions having a form in which the two main phases, a phase containing CaO and Al ₂ O ₃ and a phase containing CaO and ZrO _2, are mixed are defined as "specific inclusions",
A steel characterized in that the number ratio of the "specific inclusions" contained in the "target inclusions" is 50% or more. Further, a part of the Fe is replaced with, in mass%,
The steel according to claim 1, characterized in that it contains one or more selected from the group consisting of Cu: 0.1 to 1.5%, Ni: 0.1 to 10.0%, Cr: 0.1 to 10.0%, and Mo: 0.05 to 1.5%. Further, a part of the Fe is replaced with, in mass%,
The steel according to claim 1 or 2, characterized in that it contains one or more selected from the group consisting of Nb: 0.005 to 0.1%, V: 0.005 to 0.3%, and Ti: 0.001 to 0.25%. Further, a part of the Fe is replaced with, in mass%,
The steel according to any one of claims 1 to 3, characterized in that it contains B: 0.0005 to 0.005%. The steel according to any one of claims 1 to 4 , wherein the steel is a cast steel or a steel material. In mass percent,
Mn: 0.1 to 2.0%,
S: molten steel containing 0.0100% or less,
The method for producing steel according to any one of claims 1 to 5, characterized in that, before Al deoxidation, Si-Zr deoxidation is performed by simultaneously adding Si and a trace amount of Zr, followed by Al deoxidation and then Ca treatment.

Images