JPH0347664A - Method for making fine and uniformly dispersing inclusion in steel - Google Patents

Abstract

PURPOSE:To make inclusion having large size fine just before solidifying by adding Zr into a mold so as to make total Zr in a cast slab the specific concn. at the time of continuously casting carbon steel. CONSTITUTION:At first, the objective steel has a compsn. in the range of 0.3 - 2.0% Mn and 0.05 - 0.5% Si and contg. <=0.010% Al. The inclusion of component series is almost mixed material of MnO and SiO2 of molten state in the ordinary steel making temp. range. This mixed material becomes the inclusion having large size at the time of solidifying the molten steel as it is, and gives the quality adverse influence. Therefore, before solidifying, Zr is added in the mold so that the total Zr in the cast slab, i.e. the sum of Zr dissolved in the steel and Zr in the oxide becomes the concn. shown in the inequality I. Then, compound of ZrO2, MnO and SiO2 is generated at the size is made small. Therefore, even if number of the inclusions is not extremely reduced, the adverse influence to the quality is eliminated and the inclusion can be made harmless.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to the production of steel, and relates to a technique for reducing harmful inclusions and uniformly precipitating fine MnS in steel.

(Prior art) In the production of steel, deoxidation products and nonmetallic inclusions caused by secondary oxidation of molten steel have a significant negative impact on quality.
Many efforts have been made to try to solve this inclusion problem. Many methods have been proposed to solve the problem of inclusions, and these are summarized in "Nishiyama Memorial Lecture: High Cleanliness Steel" published by the Iron and Steel Institute of Japan (November 14, 1988). published on the following day). These can be roughly divided into methods that reduce the number of inclusions as much as possible, and methods that leave a certain number of inclusions but render them harmless.

Examples of the former include techniques for preventing re-oxidation during stirring during secondary refining and pouring during continuous casting; for example, on page 11 of the same book, it is stated that stirring promotes the floating and separation of deoxidized products. Furthermore, on page 13 of the same book, as techniques for preventing re-oxidation, air-insulated casting between the ladle and tundish using a long nozzle and sealing of an inert gas inside the tundish at the start of casting are described. An example of the latter is control of inclusion composition; for example, on page 15 of the same book, an example of the target inclusion composition is given.

However, as seen in recent years, in order to extremely reduce the number of inclusions to meet the strict requirements of users, the cost of removing inclusions increases exponentially as the target number of inclusions to be achieved decreases. The problem is that it increases. Depending on the method, it may actually become a source of contamination of molten steel (for example, inclusion removal using a filter).

In addition, although it is an effective method to control the composition of inclusions to make them harmless and easy to deform, considering the variation in the composition and distribution of inclusions, it is difficult to reach a composition that is easy to deform. There is a risk that large-sized inclusions may remain.

On the other hand, it is a well-known fact that harmful inclusions are larger than a certain size; for example, in small diameter boiler tubes, inclusions with a diameter of 100 ILra or more are said to be harmful.

On the other hand, the requirements for the material properties of steel materials are becoming stricter year by year, and in particular, drastic improvements in toughness are desired.Normally, during the austenite-ferrite transformation of steel, coarse ferrite precipitates from the austenite grain boundaries, resulting in a coarse structure. It is a well-known fact that the zero structure becomes coarser and the toughness decreases.
Conventionally, a method of refining the structure has been taken as a measure to improve toughness.An effective means of refining the structure is to uniformly disperse it in steel, as shown in, for example, Japanese Patent Application Laid-Open No. 81-238940. Fine ferrite (intragran) is formed within austenite grains using fine inclusions as transformation nuclei.
A method is used to refine the structure by generating an ular ferrite plate (hereinafter referred to as IFP). One of the ferrite transformation nuclei is MnS, and a large number of His
The present inventors have disclosed in Japanese Patent Application No. 83-53458 that the uniform dispersion and precipitation of N in steel leads to a finer structure. A method for uniformly depositing Is has been disclosed. However, the composition of steel materials, particularly Mn and Si, which are major alloying elements and weak deoxidizing elements, vary depending on their use, and it is necessary to determine the range of the amount of Zr added depending on these compositions.

(Issue 8 to be solved by the invention) In view of the above problems, the present invention provides a method for making inclusions harmless.
It is an object of the present invention to provide a method for making large-sized inclusions fine just before solidification, and a method for uniformly dispersing a large amount of fine MnS in steel regardless of the steel types having different Mn and Si compositions.

(Means for Solving the Problems) The present inventors have developed a method for refining and uniformly dispersing inclusions in steel, which have a strong affinity with oxygen, have a large apparent specific gravity when combined with oxygen, and are uniform. Focusing on Zr, which has a strong tendency to be dispersed, an experiment was conducted to confirm this effect. As a result, the inclusion size distribution changed with the addition of Zr (200p
We were able to confirm the effect of a significant shift to smaller particle sizes compared to before (Fig. 1), and along with this,
We have discovered that MnS can also be finely dispersed.

The outline of the present invention is shown below. First, as a method for refining inclusions in steel, the present invention has a Mn content of 0.3 to 2.0.
%, Kass Mira 0.05-0.5% range near! J.
When casting carbon steel with a composition in which A9 is 0.010% or less using a continuous casting machine, Zr is added to the total Z in the slab.
This is a method in which r (the sum of Zr dissolved in the steel and Zr in the oxide) is added into the mold at a concentration expressed by the following formula. This method is performed while stirring the inside using electromagnetic force.

0.07X (Mzr/Wa2)
C2(O))≦C(Zr)≦l
z) X (CI(0)-C2(O)) where Mzr:
Zr atom fi(91,2)Moz:02 molecule 1(
32) C (Zr): Total Zr in the slab 5 degrees (ppm) C
1(0): Oxygen concentration in molten steel before adding Mll and Si (pp■) C2(O): Oxygen concentration in molten steel after adding Mn and Si and M as necessary (pp■) Next As a method for uniformly dispersing MnS,
n 0.3 to 2.0%, Si 0.05 to 0.5%,
When adding Zr to molten steel after adding Mn and Si in carbon steel containing 0.012 or less M, Zr
The purpose is to adjust the amount added within the range shown by the following formula.

0, IOX (Mzr/Moz)
C2(O))≦C(Zr)≦0.30× (Mzr/
Moz) X (CI(0)-C2(O)) where Mz
r: Atomic weight of Zr (91,2) Moz: Molecular weight of 02 (32) C(Zr): Total Zr concentration in slab (pp) C
1(0): Dissolved oxygen concentration before adding Mn and Si (ppm) Dissolved oxygen concentration before adding C2(O) + Zr (ppm
) (Function) Next, the present invention will be described in detail. First, an invention related to miniaturization of inclusions will be explained.

First, the target steel is our 0.3~2. Oz, Ka Si is 0.05~0.5 $c7) i surrounding near, Ka AQ is O,
OIO! It is a steel with the following composition. The reason why Mn and Si have such a composition is that, as described later, in the present invention, most of the inclusions in the molten steel before adding Zr must be a mixture of MnO and Si02 in the molten state. This is because there is. With the above Mn and Si compositions, these conditions can be satisfied.

On the other hand, the reason why A9 is limited to less than o, otox is as follows. Compared to Zr, A9 has three times the amount of oxygen combined with the same weight, and even a small amount of M is present in molten steel.
O and 5102 are reduced, and the inclusions are mostly Aq20.
Only 3 will be available. If a large amount of A11203 is generated, it forms clusters and becomes large in size, so adding Zr eliminates the effect of making inclusions finer. Therefore, the M content was set at 0.010% or less as a condition under which A1103 would not be produced in large amounts.

Therefore, there is no need to add anything for mosquitoes, but
If it is added as necessary, it is desirable to add it between after the addition of Mn and Si and before the addition of Zr. Other components do not have a significant influence on the effects of the present invention even if they are not particularly regulated.

The reason why the size distribution of inclusions shifted to a smaller size by adding Zr is considered to be due to the following mechanism. That is, the inclusions of the component system of the present invention are mostly a mixture of MnO and SiO2 in a molten state in the normal steelmaking temperature range. If this mixture is allowed to solidify steel as it is, it will form large-sized inclusions, which will have a negative impact on quality. However, when Zr is added as in the present invention before solidification, Zr
It becomes a compound of O2, MnO, and SiO2, and its size becomes smaller. This is because the added Zr reacts with oxygen in the molten steel to form ZrO2, and this ZrO2 collides with the mixed inclusions of MnO and Si02 in the molten state due to the flow of the molten steel, making them fine. This is due to the two functions of directly reacting with the MnO and 5in2 mixed inclusions and reducing them into fine particles.

The amount of Zr to be added was determined as follows.

First, the relationship between the total amount of Zr in the steel and the particle size of inclusions in the steel is shown in FIG. 2. In this case, when the amount of Zr exceeds 400 ppH, the particle size of the inclusions tends to increase. This seems to be caused by the following mechanism.

Before adding Zr, mixed inclusions of MnO and 5in2 exist in the molten steel. The added Zr reduces this, but as the amount of Zr added increases, all the mixed inclusions of MnO and 5iQ2 are reduced and disappear, leaving only ZrO2.In the case of only ZrO2, the specific gravity are larger than the mixed inclusions, and inclusions with larger grain sizes also remain in the steel. Therefore, the limit of the effect of reducing inclusions by adding Zr is the amount of Zr at which all MnO and 5i02 in molten steel are reduced, and this is expressed by the following formula.

Go(Zr)=(Nzr/Moz) X(C+(0)−
C2(O)) where Mzr: atomic weight of Zr (91,2
) Molecular weight of MO2:02 (32) Go(Zr)sZrfa degree (ppm).

C+ (0): Oxygen concentration (pp) in molten steel before adding Mn and Si.

C2(O): Oxygen concentration (pp) in molten steel after adding Mn, Si, and M if necessary.

On the other hand, regarding the lower limit of Zr content, the total Zr in the steel mentioned above
From the experimental results investigating the relationship between the amount and the particle size of inclusions in steel,
There was a refinement effect up to a Zr amount of 0.07X (:o(Zr)).

Therefore, the amount of Zr that has the effect of refining inclusions is determined by adding Zr to the total amount of Zr in the steel (
When expressed as the sum of Zr dissolved in m and Z in the oxide, the following formula is obtained.

0.07 X (Mzr/Moz) X (G+ (0
) −C2(O))≦C(Zr)≦l X(Mzr/M
oz) X((+(0) C2(O)) where Mzr
: Zr atomic weight (91,2) MO2:02 molecule 1 (
32) G (Zr): Total Zr concentration in slab (ppm) C
:1(0): Oxygen concentration in molten steel before adding Mn and Si (pp■) C2(O): Oxygen concentration in molten steel after adding Mn and Si and M as necessary (pp■) Next Regarding the timing of adding Zr, it is desirable to solidify it quickly before the inclusions that have become fine due to the addition of Zr are sufficiently agglomerated, but with current continuous casting machines, adding in the mold is the closest condition. From this point of view,
It is more effective to apply it in a continuous thin slab caster, which takes less time to completely solidify.

The flow of molten steel to cause the Zr02 to collide with the mixed inclusions of MnO and 5r02 can be achieved by using thermal convection or injection flow into the mold, but it is possible to make the collision more effective and to uniformly remove the fine inclusions. In order to disperse Zr, it is preferable to add Zr before stirring the inside of the mold using electromagnetic force.

Next, a method for uniformly dispersing MnS in steel will be explained.

First, as in the case above, the target steel is Mn 0
．． It is a steel having a composition in which the Si content is 0.05 to 0.5% and the resistance is 0°01z or less.

The reason why Mu and Si have such a composition is that in the present invention, most of the inclusions in the molten steel before adding Zr must be in a molten state and in the form of MnO.

This condition can be satisfied if the composition is as described above. On the other hand, the composition range for 1M was chosen for the following reasons: 0 As is well known, M binds three times as much oxygen at the same weight as Zr, and even a small amount of M can cause it to dissolve in molten steel. The existing MnO.5i02 is reduced and most of the inclusions become AQ203, so as a condition that a large amount of M2O3 is not generated, the mosquito content is set to 0. It was set at less than O1$. Before adding Zr, Mn0esio2 mixed inclusions are present in the molten steel due to the addition of Mn and Si. In molten steel, 1lInO.5i02 floats due to agglomeration and coalescence, and the number decreases.

The added Zr has a stronger deoxidizing power than the already contained Mn and Sl, so it reduces a part of 1Mn0.5i02 to become ZrO2, forms a complex oxide with MnO.SiO2, and becomes fine. Spread. The reason for this is that ZrO2 has a specific gravity of approximately 2 compared to MnO・5i02.
The specific gravity of the composite oxide is larger than that of MnO.SiO2 alone, and the floating speed is lower. Therefore, the number of MnO@5i02 remaining in the molten steel without flotation separation is greater than in the case where Zr is not added. As a result, Mn precipitates on MnO.5i02 during solidification and cooling.
It is thought that the number of S's will increase. Zr concentration is 0
．． 10XO1zr/Moz) X(CI(0)-C2(
If it is less than O)), the amount of ZrO2 produced is also small;
Since the rate at which complex oxide is formed becomes low, the effect becomes insufficient.

Therefore, this was set as the lower limit. As the amount of Zr added increases, the proportion of MnO/S in each composite oxide increases.
The proportion of i02 decreases, and finally M in the composite oxide
All nO.5i02 is reduced to only ZrO2. When MnO.5i02, which is a preferential precipitation site, is completely reduced, the precipitation rate of MnS decreases rapidly. Further Zr
As the amount of ZrO2 added increases, the number of individual ZrO2 particles increases, forming clusters, which adversely affects the quality. Zr concentration is 0.30×
(Mzr/ Moz)X (CI(0)-Cz(0)
) corresponds to this case. Therefore, the Zr concentration at this time is set as the upper limit.

Note that the dissolved oxygen concentration C+ before adding Mn and Si
(0) (ppm) and the dissolved oxygen concentration C2(O) (PP11) before adding Zr are measured using, for example, an oxygen sensor, and the amount of Zr added is adjusted to the target concentration considering the yield of Zr. decided on. The timing of adding Zr can be effective even if it is added in a ladle or in a ladle, but after adding Zr, it is desirable to solidify it quickly before the composite oxide aggregates. In this case, a method of adding in a mold is also considered.

In addition, in rapid solidification processes such as continuous thin slab casting and twin roll casting processes, swirling is the process of shortening the time from adding a deoxidizer to solidification as much as possible, and the effects of the present invention are particularly demonstrated. can.

(Examples) Example 1 An experiment was conducted in which steel was produced by high-frequency induction heating and Zr was added to it. First, add C, Mn, and Si to pure iron.
Zr was added to obtain the composition shown in the table, and then Zr was added to obtain molten steel with a Zr concentration of 2θQPpII. The particle size distribution, composition, and morphology of the inclusions before and after the inclusions were investigated using an X-ray microanalyzer and a scanning electron microscope.As for 0M, no particular addition was made, but the analysis results showed that o, oosx
It was below. The oxygen concentration in the molten steel before the addition of Mn and Si was 200 ppm, and the oxygen concentration in the molten steel after the addition of Mn and Si was 80 ppm.

Therefore, C(Zr)-0,50X(Mzr/Not)
x(ci(o)-C2(O)).

As a result, the composition of the inclusions was MnO and 810% before adding Zr.
However, after adding Zr, Zr
A composite inclusion of O2 and Mn01Si02 was observed. The size distribution of these inclusions is shown in FIG. 1, and it can be seen from this figure that the addition of Zr significantly shifts the size distribution of the inclusions toward smaller grain sizes.

As a result of detailed investigation of changes in the morphology of inclusions, the present inventors found that the size of inclusions decreases due to ZrO2 colliding with mixed inclusions of MnO and 5i02, and Zr reducing mixed inclusions of MnO and SiO2. It was found that the size had become smaller.

Example? When casting steel with the same composition as shown in Table 1 using a vertical bending type continuous casting machine, Zr is added to the mold and the total amount of Zr in the slab is 200.
It was added so that it became ppm. In this case too, M was not added, but the result of one analysis was 0.007! Met. Mn,
The oxygen concentration in the molten steel before the addition of Si was 220 ppst, and the oxygen concentration in the molten steel after the addition of Mn and Si was 90 ppm.

Jutte, C (Zr) = 0.54X (Mzr/Mo2)
X(CI(0) C2(O)).

The results are shown in Figure 3. In the actual test, the particle size distribution of inclusions shifted to the smaller side, similar to the results in the laboratory, and the effect of finer inclusions due to Zr addition was evident. .

Example 3 An experiment was conducted in which 1 kg of molten steel was melted by high-frequency induction heating and Zr was added. After melting pure iron and adjusting the composition at 1570℃, M! 1. Zr was added after Si addition. Note that the dissolved oxygen concentration C1 (0
) was 200 ppm in both cases. After adding Zr, it was held for 30 seconds and then allowed to cool and solidify in the crucible.

A sample was taken from the obtained steel ingot, and the distribution and number of MnS was investigated using an X-ray microanalyzer. Table 2 shows the chemical analysis values of the steel ingot and C2(O), G(Zr)/(
(Mzr/)Io2)×(C1(O) C2(O))
), Figure 4 shows the number of MnS, C(Zr)/((
Mzr/Moz)X ((+(0) -Cz(0)))
0, which shows the relationship between MnS and MnS. Under the conditions of the present invention, a sufficient number of MnS can be obtained, whereas with the comparative material, it is insufficient.

(Effects of the Invention) As is clear from the above examples, the present invention reduces the size of inclusions in steel, so it is not necessary to drastically reduce the number of inclusions by spending a lot of effort and cost. There will be no negative impact on quality. Therefore, the present invention is an effective means for rendering inclusions harmless. Furthermore, according to the present invention, in a wide range of Mu and Si concentrations, fine MnS
As a result, it not only increases the amount of intragranular ferrite produced and improves the toughness of the weld HAZ, but also improves the toughness of the weld HAZ.
For example, improvements in properties can be expected in electromagnetic steel as well.

Further, as described above, the effects of the present invention are tremendous in a rapid solidification process such as a thin slab continuous casting process.

[Brief explanation of drawings]

Figure 1 shows the change in the particle size distribution of inclusions due to the addition of Zr (200 ppH), (a) before the addition of Zr, (b) before the addition of Zr.
After R addition, Figure 2 shows the relationship between Zr concentration and inclusion particle size, and Figure 3 shows the relationship between inclusion particle size and number of inclusions for Zri filler material and comparative material. It is a diagram. Figure 4 shows C(
Zr)/((Mzr/Mo2) X(C+(0) C
FIG. 2 is a diagram showing the relationship between 2(O))l and the number of MnS.

Claims (1)

[Claims] (1) Mn is 0.3 to 2.0%, and Si is 0.05 to 2.0%.
When casting carbon steel with a composition of Al in the range of 0.5% and 0.010% or less using a continuous casting machine, the total Zr in the slab (Zr dissolved in the steel and Zr in the oxide) is Z
1. A method for refining inclusions in molten steel, characterized by adding Zr into a mold so that Zr (sum of r) has a concentration expressed by the following formula. 0.07×(Mzr/Mo_2)×(C_1(O)-C
_2(O)≦C(Zr)≦1×(Mzr/Mo_2)×
(C_1(O)-C_2(O)) where Mzr: Atomic weight of Zr (91.2) Mo_2: Molecular weight of O_2 (32) C(Zr): Total Zr concentration in slab (ppm) C_
1(O): Oxygen concentration in molten steel (ppm) before adding Mn and Si C_2(O): Oxygen concentration in molten steel (ppm) after adding Mn and Si and, if necessary, Al (2) Weight % Mn is 0.3 to 2.0% and Si is 0
．． 0.05% to 0.5%, and Al content of 0.010%
% or less, the total Zr in the slab (the sum of Zr dissolved in the steel and Zr in the oxide) is added to the molten steel so that the concentration is expressed by the following formula: ni Z
A method for uniformly dispersing inclusions in steel, characterized by adding r and casting continuously or into an ingot. 0.10×(Mzr/Mo_2)×(C_1(O)-C
_2(O))≦C(Zr)≦0.30×(Mzr/Mo
_2)×(C_1(O)-C_2(O)) where Mzr
: Atomic weight of Zr (91.2) Mo_2: Molecular weight of O_2 (32) C(Zr): Total Zr concentration in slab (ppm) C_
1(O): Dissolved oxygen concentration before adding Mn and Si (
C_2(O): Dissolved oxygen concentration before adding Zr (ppm)
m) (3) The method according to claim (1) or (2), wherein the method is carried out while stirring the inside of the mold using electromagnetic force.