JP4653629B2 - Method for producing Ti-containing chromium-containing molten steel - Google Patents

Description

The present invention relates to a method for producing a Ti-containing chromium-containing molten steel capable of stably producing a steel slab having an increased ratio of equiaxed crystals.

Conventionally, as a method of melting chromium-containing molten steel using a converter, for example, hot metal that has been subjected to pre-treatment of de-Si, de-S, and de-P is charged into the converter, and a chromium source is added to the hot metal Then, oxygen is blown out from the top blowing lance, decarburization refining is performed until the carbon concentration becomes 0.1 mass% or more and 0.7 mass% or less, and then secondary refining is employed. And the steel piece is manufactured by continuous casting from the chromium-containing molten steel manufactured by this method. In addition, as the equiaxed crystal ratio in the steel slab is higher, for example, the workability of the product plate and the surface property can be improved.
Therefore, the following method is disclosed as a typical cast structure refinement technique.
For example, in Patent Document 1, C: 0.015 mass% or less, Si: 0.01 to 1.0 mass%, Mn: 1.0 mass% or less, P: 0.04 mass% or less, S: 0 0.02 mass% or less, Cr: 16-32 mass%, Ti: 0.003-0.3 mass%, Al: 0.001-0.15 mass%, N: 0.003-0.015 mass%, O: A composite inclusion of Al inclusions and Ti inclusions having a size of 0.3 to 5 µm is dispersed in steel composed of 0.001 to 0.006 mass%, the balance Fe and inevitable impurities. And a method for producing a ferritic stainless steel in which the grain size of the equiaxed crystal structure portion of the steel piece is 3 mm or less and the equiaxed crystal ratio is 40% or more is disclosed.
Patent Document 2 contains Al: 0.002 to 0.02 mass%, Mg: less than 0.0005 mass%, and solidification temperature and casting temperature of molten steel, Ti amount in molten steel, N amount And a method of continuously casting molten steel under the condition that the temperature determined based on the Cr content satisfies the prescribed formula.
Then, Patent Document 3, Cr: 9 to 30 wt%, upon which smelted ferritic stainless steel containing composite TiN with MgO-Al ₂ O ₃ inclusions in the nucleus in the slab, Ti Ingredient adjustment so that the concentration product of N and N is 0.0007 to 0.004, and the common logarithm log (a _o ) of the oxygen activity a _{o in} the molten steel is −5.0 to −3.0. A method of performing casting and then casting is disclosed.

Japanese Patent Laid-Open No. 11-323502 JP 2002-30395 A JP 2004-43838 A

However, the conventional method still has the following problems to be solved.
Although the method of patent document 1 and patent document 2 is prescribed | regulated about each component range in molten steel, it is not prescribed | regulated regarding the production | generation process of the inclusion in molten steel. For this reason, there is a problem that it is difficult to stably maintain the equiaxed crystal ratio, which is the fine crystal structure of the steel slab, at a high level.
Moreover, in the method of patent document 3, it is necessary to perform the oxygen activity measurement in the molten steel using an oxygen sensor at least once or more at the component adjustment stage of the molten steel, and the alloy addition amount is directly calculated. This makes it difficult to adjust the component containing the deoxidizer only within the range predicted in advance. In addition, since an oxygen sensor is used, running costs are high, which is uneconomical.

The present invention has been made in view of such circumstances, and the ratio of equiaxed crystals can be stably maintained at a high level by a simple method. For example, it is possible to improve the workability of the product plate and the surface properties. An object of the present invention is to provide a method for producing a Ti-containing chromium-containing molten steel.

The method for producing a Ti-containing chromium-containing molten steel according to the present invention in accordance with the above object comprises adding a chromium source to the hot metal that has been subjected to pretreatment, performing a primary refining process in which a molten molten steel is produced by blowing acid treatment, In the method for producing chromium-containing molten steel, which performs secondary refining to produce molten steel by adjusting the components contained in the crude molten steel,
In the secondary refining, the composition of the slag covering the molten steel after the finish decarburization treatment is defined as slag basicity (CaO (mass%) / SiO ₂ (mass%)) : 2.2 or more and 3.2 or less, After controlling to Al ₂ O ₃ : 20 mass% or more and 30 mass% or less, MgO: 12 mass% or more and 22 mass% or less, Cr ₂ O ₃ : 1.0 mass% or less, respectively, Ti source is added to the molten steel .
In the method for producing a Ti-containing chromium-containing molten steel according to the present invention, the Ti source is preferably either one or two of pure Ti scrap and a Ti—Fe based alloy.

The process according to claim 1 and 2 Ti-containing chromium-containing molten steel according Because defines a CaO / SiO ₂ of the slag, can deoxidation of molten steel sufficiently, MgO-Al ₂ O ₃ into the molten steel - Fine dispersion of TiO ₂ -based low melting point inclusions becomes possible.
Moreover, since the Al ₂ O ₃ concentration of the slag is regulated, the slag hatchability can be secured without using fluorite containing fluorine harmful to the environment, and MgO—Al ₂ O into the molten steel can be secured. Fine dispersion of _3- TiO ₂ -based low melting point inclusions becomes possible.
And since the MgO concentration of the slag is regulated, it is possible to secure the Mg source necessary for the generation of the MgO—Al ₂ O ₃ —TiO ₂ -based low melting point inclusions and improve the slag hatchability, It is possible to finely disperse MgO—Al ₂ O ₃ —TiO ₂ -based low melting point inclusions in the material.
Furthermore, since the Ti source is added to the molten steel after the composition control of the slag, the molten steel can be sufficiently deoxidized, and MgO—Al ₂ O ₃ —TiO ₂ -based low melting point inclusions are contained in the molten steel. Can be generated.
As described above, in the process of producing the steel slab by solidifying the molten steel by finely dispersing the MgO—Al ₂ O ₃ —TiO ₂ -based low melting point inclusion in the molten steel produced from the crude molten steel. A large number of fine spinel inclusions (MgAl ₂ O ₄ ) obtained by modifying low melting point inclusions with Al can be crystallized. Then, TiN is crystallized by using a large number of fine spinel inclusions as nuclei, and δFe is solidified by using TiN as a nucleus, whereby a high equiaxed crystal ratio can be obtained by a simple method. It is possible to improve the workability and surface properties of the product plate.

In particular, the method for producing a Ti-containing chromium-containing molten steel according to claim 2 is economical when a pure Ti scrap is used as the Ti source, and an inexpensive Ti source can be used. Further, when using a Ti—Fe based alloy as a Ti source, the Ti—Fe based alloy is not refined to pure Ti, so that it is more economical than using pure Ti.

Next, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention.
Here, FIG. 1 is an explanatory diagram of a method for producing a Ti-containing chromium-containing molten steel according to an embodiment of the present invention, FIG. 2 is an explanatory diagram showing the relationship between CaO / SiO ₂ and the equiaxed crystal ratio, and FIG. FIG. 4 is an explanatory diagram showing the relationship between the Al ₂ O ₃ concentration and the equiaxed crystal ratio, and FIG. 4 is an explanatory diagram showing the relationship between the MgO concentration and the equiaxed crystal ratio.

The method for producing Ti-containing chromium-containing molten steel according to one embodiment of the present invention includes adding a chromium source to the hot metal that has been subjected to pretreatment, and performing blown acid treatment (hereinafter also simply referred to as crude molten steel). This is a method of performing secondary refining to produce molten steel by adjusting the components contained in this crude molten steel after the primary refining to melt the molten steel, and in the secondary refining, the molten steel after the finish decarburization treatment After adjusting the composition of the slag covering the steel, a Ti source is added to the molten steel to produce a Ti-containing chromium-containing molten steel (hereinafter also simply referred to as molten steel). This will be described in detail below.

As shown in FIG. 1, in the manufacture of Ti-containing chromium-containing molten steel using blast furnace hot metal, most of the impure elements Si, P, and S are removed in advance in the hot metal preliminary treatment step, and the produced hot metal is converted. The converter is charged in the furnace process. In this converter process, only top blowing, only bottom blowing, or combination of top blowing and bottom blowing is used to perform rough decarburization treatment to remove carbon in the hot metal by blowing acid into the hot metal. A chromium-containing crude molten steel (for example, carbon concentration: about 0.1 mass% or more and about 0.7 mass% or less) is produced by adding a chromium source of molten chromium, an Fe—Cr alloy, or chromium ore to the hot metal.

When producing Ti-containing chromium-containing molten steel from this chromium-containing crude molten steel, even if blown acid is applied to the chromium-containing crude molten steel in the atmosphere, chromium oxidation is caused and the carbon content cannot be reduced to the target level. Therefore, in the secondary refining process, the chromium-containing crude molten steel is transferred to a ladle and, for example, blown acid decarburization (finish decarburization treatment) of the chromium-containing crude molten steel is performed with an upper blowing lance under vacuum (under reduced pressure). And decarburizing until the target C amount (for example, the carbon concentration is about 0.001 mass% or more and 0.08 mass% or less) to obtain molten steel.

And after adding the CaO source required for protection of a ladle refractory and the Si source which is a deoxidizer, and performing the deoxidation treatment of the chromium-containing molten steel, it is controlled to a predetermined slag composition. Therefore, an MgO source is added. Thus, after sufficient deoxidation of the molten steel, for example, after a time of about 2 minutes to 20 minutes has elapsed since the addition of the MgO source, the Ti source is added to the chromium-containing molten steel and its temperature is adjusted.
In this way, decarburization refining was performed, and Ti-containing chromium-containing molten steel in which MgO—Al ₂ O ₃ —TiO ₂ -based low melting point inclusions (hereinafter also simply referred to as low melting point inclusions) were finely dispersed was continuously produced. Cool and solidify in the casting process. As a result, spinel inclusions (MgAl ₂ O ₄ ) obtained by modifying low melting point inclusions with Al were crystallized, and TiN was crystallized using this as a nucleus, and δFe was solidified using TiN as a nucleus. Manufacture billets.

The predetermined slag composition to be controlled in this secondary refining process is slag basicity (CaO / SiO ₂ ): 2.2 or more and 3.2 or less, Al ₂ O ₃ : 20% by mass or more and 30% by mass or less, MgO: 12 wt% to 22 wt% or less, Cr ₂ O _3: 1.0 wt% or less.
In controlling the slag composition, since the deoxidation treatment described above is performed in advance, the slag having the above-described composition can be formed only by adding the MgO source, but if necessary, for example, by adding alumina flakes. adjustment of al ₂ O ₃ may be performed.
The adjustment of the CaO source necessary for the protection of the ladle refractory is performed, for example, by adding quick lime, and the adjustment of the Si source, which is a deoxidizer, is performed by, for example, Fe-Si alloy or softening used for heating or reduction. The addition of silica stone is performed, and the MgO source is adjusted, for example, by adding MgO powder.

Here, the reason for adjusting the slag basicity to the above-described range will be described with reference to FIG.
FIG. 2 shows the basicity of slag (C) when 8 tons of slag is placed on 150 tons of molten steel in a ladle to produce Ti-containing chromium-containing molten steel, and this is continuously cast to produce a steel slab. / S) and the equiaxed crystal ratio in the steel slab. Here, the Al ₂ O ₃ concentration of the slag is 26% by mass, the MgO concentration is 15% by mass, the molten steel temperature varies depending on the chromium content, and is preferably (solidification start temperature) + (10 to 100) ° C.
Note that the two lines shown in FIG. 2 indicate the upper limit value and the lower limit value of the variation in equiaxed crystal ratio in each slag basicity. The equiaxed crystal ratio indicates the ratio of equiaxed crystals in the crystal structure of the manufactured steel slab (hereinafter the same).

When the slag basicity is less than 2.2, the molten steel cannot be sufficiently deoxidized, so the Mg source and the Al source necessary for forming the MgO—Al ₂ O ₃ —TiO ₂ -based low melting point inclusions Is insufficient, and the composition of the low melting point inclusion cannot be controlled. As a result, the fine dispersion of the low melting point inclusion is hindered, and as shown in FIG. 2, the equiaxed crystal ratio of the manufactured steel piece rapidly decreases, and the refractory that contacts the ladle or other molten steel is protected. The effect is diminished, and the cost of the refractory life is reduced.
On the other hand, when the slag basicity exceeds 3.2, the fine dispersion of the low melting point inclusions is hindered with the deterioration of the slag hatchability, and the equiaxed crystal ratio of the steel slab decreases as shown in FIG. Begin to. Moreover, the generated slag processing load increases due to an increase in the amount of chromium-containing slag generated, and further, there arises a problem that leads to refractory wear due to an increase in eluted MgO accompanying an increase in the amount of slag generation.
From the above, the slag basicity is set to 2.2 or more and 3.2 or less, but the lower limit is preferably 2.5, and the upper limit is preferably 3.0.

Next, the reason why the Al ₂ O ₃ concentration of the slag is adjusted to the above range will be described with reference to FIG.
This FIG. 3 shows the concentration of Al ₂ O ₃ when Ti-containing chromium-containing molten steel is produced by placing 8 tons of slag on 150 tons of molten steel in a ladle and continuously casting this to produce a steel slab. And the equiaxed crystal ratio in the steel slab. Here, the basicity of the slag is 2.6, the MgO concentration is 15% by mass, and the molten steel temperature is the same as the above-described conditions of FIG.
Note that the two lines shown in FIG. 3 indicate the upper limit value and the lower limit value of the variation in equiaxed crystal ratio at each Al ₂ O ₃ concentration.

When the Al ₂ O ₃ concentration is less than 20% by mass, the slag hatchability deteriorates, and fine dispersion of the target MgO—Al ₂ O ₃ —TiO ₂ -based low melting point inclusion is inhibited. As shown in FIG. 3, the equiaxed crystal ratio of the manufactured steel slab decreases rapidly. For this reason, in order to improve the equiaxed crystal ratio, it is necessary to use fluorite containing fluorine harmful to the environment.
On the other hand, when the Al ₂ O ₃ concentration exceeds 30 mass%, the properties of the low melting point inclusions change, and as shown in FIG. The amount of saturated MgO increases, leading to elution of MgO in the refractory, so the protective effect of the refractory in contact with the ladle or other molten steel is diminished, and the cost deteriorates due to a decrease in the refractory life or the addition of Al ₂ O ₃ Invite.
From the above, the Al ₂ O ₃ concentration of the slag is set to 20% by mass or more and 30% by mass or less, but the lower limit is preferably 25% by mass.

The reason why the MgO concentration of the slag is adjusted to the above range will be described with reference to FIG.
FIG. 4 shows the MgO concentration and steel slab when a Ti-containing chromium-containing molten steel is produced by placing 8 tons of slag on a 150-ton molten steel in a ladle and continuously casting this to produce a steel slab. The relationship with the equiaxed crystal ratio is shown. Here, the basicity of the slag is 2.6, the Al ₂ O ₃ concentration is 26% by mass, and the molten steel temperature is the same as the conditions shown in FIGS.
Note that the two lines shown in FIG. 4 indicate the upper limit value and the lower limit value of the variation in equiaxed crystal ratio at each MgO concentration.

When the MgO concentration is less than 12% by mass, the Mg source for producing the MgO—Al ₂ O ₃ —TiO ₂ -based low melting point inclusion is insufficient, so that the low melting point inclusion is not finely dispersed. As shown, the equiaxed crystal ratio of the manufactured steel slab decreases rapidly.
On the other hand, when the MgO concentration exceeds 22% by mass, the hatchability of the slag deteriorates with a rapid rise in the melting point, and the fine dispersion of the MgO—Al ₂ O ₃ —TiO ₂ -based low melting point inclusion is inhibited, As shown in FIG. 4, the equiaxed crystal ratio of the manufactured steel slab begins to decrease.
From the above, the MgO concentration of the slag is set to 12% by mass or more and 22% by mass or less, but the lower limit is preferably 15% by mass, and the upper limit is preferably 20% by mass.

In addition, when the Cr ₂ O ₃ concentration in the slag exceeds 1.0% by mass, the molten steel cannot be sufficiently deoxidized, and the fine dispersion of the MgO—Al ₂ O ₃ —TiO ₂ -based low melting point inclusions is reduced. Inhibit. Incidentally, does not define the lower limit of the Cr ₂ O ₃ concentration, if Cr ₂ O ₃ concentration of 1.0 wt% or less, the fine of MgO-Al ₂ O ₃ -TiO ₂ based low-melting inclusions This is because the dispersion is not affected.
Furthermore, when the addition of the Ti source is performed before the control of the slag composition described above, the deoxidation is not sufficiently performed, and the Mg source for generating the MgO—Al ₂ O ₃ —TiO ₂ low melting point inclusions In addition, since the Al source is insufficient, TiO ₂ is mainly used to inhibit fine dispersion of the low melting point inclusions.
In addition, as Ti source added to molten steel, any 1 or 2 of a pure Ti scrap and a Ti-Fe-type alloy is used. The addition amount of this Ti source is determined according to the Ti concentration (for example, about 0.10 mass% or more and 0.25 mass% or less) in the manufactured product.

Next, examples carried out for confirming the effects of the present invention will be described.
In the primary refining process, the hot metal that had been pretreated was treated in a converter to produce a crude molten steel, and the crude molten steel was transferred to a ladle at 150 tons. In the secondary refining process, finish decarburization treatment, CaO addition, Si reduction treatment, slag composition adjustment, Ti source addition, and temperature adjustment of the molten steel (for example, about 1600 ° C.) A chromium-containing molten steel was produced.
The molten steel was solidified by continuous casting to produce a steel piece, and the equiaxed crystal ratio was investigated. The target equiaxed crystal ratio of the steel slab is 80% or more. The results are shown in Table 1 and Table 2, respectively.

Table 1 shows examples and Table 2 shows comparative examples.
In Examples 1 to 5 in Table 1, the composition of the slag covering the molten steel is defined as slag basicity (CaO / SiO ₂ : C / S): 2.2 or more and 3.2 or less, Al ₂ O ₃ : 20 After controlling to mass% or more and 30 mass% or less, MgO: 12 mass% or more and 22 mass% or less, and Cr ₂ O ₃ : 1.0 mass% or less, the Ti concentration of the molten steel to be manufactured becomes 0.15 mass%. Thus, it is the result of adding pure Ti scrap which is a Ti source. Among these, in Examples 2 to 4, slag basicity, Al ₂ O ₃ , and MgO are used under particularly preferable conditions (slag basicity: 2.5 to 3.0, Al ₂ O ₃ : 25 mass% to 30 mass%. Hereinafter, the results are respectively controlled to MgO: 15% by mass or more and 20% by mass or less.

On the other hand, Comparative Examples 6 and 7 in Table 2 are results in which the slag basicity is outside the above-described range. In Comparative Examples 8 to 10, the slag Al ₂ O ₃ concentration is outside the above-described range, and in Comparative Example 10, the slag basicity is also a result of lowering the lower limit of the above-described range. In Comparative Examples 11 and 12, the MgO concentration of the slag is outside the above range, and in Comparative Example 14, the Cr ₂ O ₃ concentration of the slag exceeds the above range. In Comparative Example 13, slag basicity, Al ₂ O ₃ , MgO, and Cr ₂ O ₃ satisfy the above-mentioned ranges, but the addition timing (timing) of pure Ti scrap is controlled by the composition of slag. It is the previous result.

As is clear from Examples 1 to 5, by defining the slag composition and the addition timing of pure Ti scrap, the equiaxed crystal ratio of the manufactured steel slab was 80% or more, and the target value was achieved. In addition, the slag hatching property, that is, the slag fluidity is good (for example, the additive on the slag is easy to enter into the slag), and it is prominent in the slag line of the ladle refractory, that is, the slag existence region. No dent was confirmed.
In particular, in Examples 2 to 4 in which slag basicity, Al ₂ O ₃ , and MgO were defined as particularly preferable conditions, the equiaxed crystal of the steel slab became 98% or more.

On the other hand, in Comparative Example 6, since the slag basicity is low (2.1), fine dispersion of the MgO—Al ₂ O ₃ —TiO ₂ -based low melting point inclusion is inhibited, and the equiaxed crystal ratio can achieve the target value. There wasn't. Moreover, the remarkable dent generate | occur | produced in the refractory.
Further, in Comparative Example 7, since the slag basicity is high (3.3), the fine dispersion of the low melting point inclusion is inhibited, the equiaxed crystal ratio cannot achieve the target value, and the slag hatching property is also deteriorated. did. Moreover, the remarkable dent generate | occur | produced in the refractory.
In Comparative Example 8, since the Al ₂ O ₃ concentration of slag is low (19% by mass), the hatchability of slag deteriorates, the fine dispersion of low melting point inclusions is inhibited, and the equiaxed crystal ratio reaches the target value. could not.
In Comparative Example 9, since the Al ₂ O ₃ concentration of the slag was high (33% by mass), the equiaxed crystal ratio could not achieve the target value, and a remarkable dent was generated in the refractory.

In Comparative Example 10, since the slag Al ₂ O ₃ concentration is low (7% by mass) and the slag basicity is low (1.1), the slag hatchability deteriorates and the low-melting inclusions are finely dispersed. The equiaxed crystal ratio could not achieve the target value. Moreover, the remarkable dent generate | occur | produced in the refractory.
In Comparative Example 11, since the MgO concentration of the slag is low (11% by mass), the Mg source for generating the low melting point inclusion is insufficient, the fine dispersion of the low melting point inclusion is inhibited, and the equiaxed crystal ratio is the target. The value could not be achieved.
In Comparative Example 12, since the MgO concentration of slag was high (23% by mass), the slag hatchability deteriorated, the fine dispersion of low melting point inclusions was inhibited, and the equiaxed crystal ratio could not achieve the target value. .

In Comparative Example 13, since the addition timing of pure Ti scrap is before the composition control of slag, deoxidation of the crude molten steel is not sufficiently performed, and as a result, the fine dispersion of low melting point inclusions is hindered, etc. The axial crystal ratio could not achieve the target value.
In Comparative Example 14, since the Cr ₂ O ₃ concentration of slag is high (1.2% by mass), the coarse molten steel cannot be sufficiently deoxidized, the fine dispersion of low melting point inclusions is inhibited, and the equiaxed crystal ratio is the target. The value could not be achieved.
From the above, it is possible to manufacture a Ti-containing chromium-containing molten steel for producing a steel slab in which the proportion of equiaxed crystals is stably maintained at a high level without performing oxygen activity measurement with an oxygen sensor as in the past. Was confirmed.

As described above, the present invention has been described with reference to the embodiment. However, the present invention is not limited to the configuration described in the above embodiment, and the matters described in the scope of claims. Other embodiments and modifications conceivable within the scope are also included. For example, the case where the manufacturing method of the Ti-containing chromium-containing molten steel of the present invention is configured by combining some or all of the above-described embodiments and modifications is also included in the scope of the right of the present invention.

It is explanatory drawing of the manufacturing method of Ti containing chromium containing molten steel which concerns on one embodiment of this invention. It is an explanatory diagram showing a relationship between the equiaxed Akiraritsu the CaO / SiO _2. It is an explanatory view showing the relationship between the concentration of Al ₂ O ₃ and equiaxed Akiraritsu. It is explanatory drawing which shows the relationship between MgO density | concentration and an equiaxed crystal ratio.

Claims (2)

The secondary refining is performed by adding the chromium source to the hot metal that has been pre-treated and performing the primary refining to produce the molten steel by blowing acid treatment and adjusting the components contained in the molten molten steel. In the method for producing chromium-containing molten steel,
In the secondary refining, the composition of the slag covering the molten steel after the finish decarburization treatment is defined as slag basicity (CaO (mass%) / SiO ₂ (mass%)) : 2.2 or more and 3.2 or less, After controlling to Al ₂ O ₃ : 20 mass% or more and 30 mass% or less, MgO: 12 mass% or more and 22 mass% or less, Cr ₂ O ₃ : 1.0 mass% or less, respectively, Ti source is added to the molten steel A method for producing a Ti-containing chromium-containing molten steel characterized by the above. The method for producing a Ti-containing chromium-containing molten steel according to claim 1, wherein the Ti source is one or two of pure Ti scrap and Ti-Fe alloy. .

Images