JPH0853707A - Production of hic resistant steel - Google Patents

Abstract

PURPOSE:To produce a steel remarkably improved in HIC resistance, at the time of adding Ca-contg. substance to molten steel, by controlling the adding rate of Ca in Ca-contg. substance to be added by a specified inequality. CONSTITUTION:Ca-contg. substance (such as CaSi) is added to molten steel subjected to deoxidizing treatment by using metallic Al to change Al2O3 in the steel into CaO-Al2O3 inclusions. At this time, the adding rate of Ca to be added in the Ca-contg. substance (expressed in terms of Ca pure content) V (g/T/mim) is controlled by the inequality of 15<=V<=- -8.4X[S]-0.3Xa0+355.8 [where [S] denotes the S concn. (ppm) in the molten steel and a0 denotes the activity oxygen value (ppm)]. Thus, the formation of CaS and CaO inclusions and CaO-Al2O3 inclusions having a high m.p. compsn. is suppressed, and the inclusions are effectively changed into CaO-Al2O3 globular inclusions to remarkably improve the HIC resistance (hydrogen induced cracking resistance) of the steel.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a method for producing HIC resistant steel,
In particular, the present invention relates to a method for producing a steel having excellent HIC resistance, which effectively makes nonmetallic inclusions harmless by adding Ca to molten steel.

[0002]

2. Description of the Related Art Generally, molten iron supplied from a blast furnace is charged into a converter, and then oxygen is supplied into the molten iron to react with the carbon in the molten iron to form CO gas, thereby decarburizing the molten steel. Manufactured.

The molten steel thus melted in the converter is tapped in a ladle, processed in a secondary refining device, and then cast. The molten steel melted in the converter is tapped or In the secondary refining device, a deoxidizing agent is added and a deoxidizing treatment is performed. When metallic Al is used as the deoxidizer, Al is used as the deoxidation product.
₂ O ₃ (alumina) is generated, and this alumina remains even after casting. Since this alumina has a high melting point, is harder than steel, and forms a cluster, it causes various defects and adversely affects the properties or appearance of the steel sheet.

Therefore, by adding a metal Ca-containing substance to molten steel and reacting Al ₂ O ₃ in the steel with Ca, CaO--Al
It has been conventionally practiced to change to ₂ O ₃ inclusions. Since Ca has a strong reducing power, it is possible to reduce even relatively stable oxides such as Al ₂ O ₃ . In addition, since the CaO-Al ₂ O ₃ -based inclusions formed by the reaction have a low melting point,
The morphology of inclusions becomes spherical and does not crush during rolling, making it less likely to become the starting point for HIC cracking.

In the case of steel grades (for example, line pipe materials) which are required to have HIC (hydrogen induced cracking) resistance,
Not only is it necessary to change Al ₂ O ₃ into a low-melting CaO-Al ₂ O ₃ -based inclusion, but it is also necessary to suppress the formation of MnS, which is the origin of cracking. S in steel forms MnS in the solidification process, and due to segregation during casting, MnS-enriched sites occur in the slab. This MnS is elongated in the rolling direction during rolling and promotes the accumulation of hydrogen and significantly lowers the HIC resistance, so it is necessary to suppress the generation of MnS. Therefore, it has been conventionally performed to prevent hydrogen-induced cracking by adding Ca after low-sulfiding treatment of molten steel and performing morphology control of sulfide.

However, in these conventional techniques,
Under the Ca addition conditions, the amount of Ca in the molten steel becomes excessive, and CaO-Al ₂ O ₃ system inclusions with a high melting point composition (CaO / Al ₂ O ₃ weight ratio in inclusions 1.
5), CaO inclusions or CaS inclusions are generated, which causes problems such as the starting point of hydrogen-induced cracking. Therefore, even in the conventional technique, various measures have been proposed to solve these problems.

JP-A-57-9822 discloses that in order to produce a steel material for a line pipe which has a small amount of CaS cluster inclusions and is excellent in cleanliness, "the content of S in molten steel is reduced to 20 ppm or less (...) A production method including a step of adding Ca of 70 ppm or less ”is proposed.

According to this method, "S is fixed as CaS by adding Ca to the molten steel, but S has a very sensitive effect on the generation of CaS cluster inclusions, so that the concentration product of Ca · S is low. In order to do so, it is proposed to limit S: 20 ppm and Ca: 70 ppm or less.

In Japanese Patent Laid-Open No. 637322, since the amount of Ca added is insufficient, Al ₂ O ₃ -based inclusions have a sufficiently low melting point CaO-.
In order to solve the problems such as not changing to Al ₂ O ₃ type inclusions and the formation of CaS type inclusions due to an excessive amount of Ca added, “a molten steel stored in a ladle is used. In the case of adding Ca to the inside of the molten steel to perform the Ca treatment, a method of treating the molten steel by adding Ca so as to satisfy the following formula and performing the operation is proposed. 0.7-14 [S] <[Ca] / [O] <1.1-14 [S] where [Ca]: calcium concentration in molten steel (ppm), [S]: sulfur concentration in molten steel (%), [ O]: Oxygen concentration in molten steel (ppm).

Japanese Unexamined Patent Publication (Kokai) No. 3-79713 proposes a method for treating Ca in molten steel, characterized in that the amount of Ca added is controlled by using the following formula different from the above in order to control the amount of added Ca. There is. 0.50 ≤ [Ca] / [O] _T ≤ 1.00, [Ca] ≤ 40, [O] _T ≤ 40 where [Ca]: calcium concentration in molten steel (ppm) [O] _T : total oxygen concentration in molten steel (ppm) Here, [Ca] / [O] _T is set to 0.50 or more in order to make the CaO-Al ₂ O ₃ inclusions have a low melting point composition, and 1.00 or less prevents CaS formation. This is because

In Japanese Patent Laid-Open No. 56-98415, "2 to 8 kg / T of quick lime is added to molten steel in a tapping steel from a converter to a ladle, and Al is added to a ladle surface slag layer after steeling in an amount of 0.05 to 0.05. 0.40kg / T added,
After degassing, Ar gas was passed through the top blowing lance soaked in molten steel under the condition of 0.006 to 0.009 Nm ³ / min.
After supplying more than one minute to desulfurize the molten steel,
A steel manufacturing method characterized by adding a Ca substance corresponding to 0.125 to 0.500 kg / T as the Ca content has been proposed.

[0012] Here, the proposed Ca content is such that the concentration of [Ca] (= 0.0010 to 0.0040%) that gives the best hydrogen-induced cracking resistance to the [S] concentration of 0.002 to 0.009% of the steel after desulfurization. )
And, it was determined from the yield of Ca.

In Japanese Patent Laid-Open No. 3-183721, "To molten steel
It proposes a method for treating Ca in molten steel, characterized in that when Ca is added, the rate of Ca addition is controlled according to the C content in the molten steel. This allows the Al ₂ O ₃ inclusions when the CaO-Al ₂ O ₃ inclusions in reforming spherical by Ca, Al ₂ O
_{3 The} main purpose is to suppress CaS generation due to excess Ca with respect to modification.

[0014]

As described above, in the conventional general technique, the addition amount of Ca and the [Ca] concentration after Ca addition are specified, but even if the addition amount of Ca is specified, the addition of Ca The speed wasn't enough. Even if the amount of Ca added was appropriate, if the addition method and rate were inappropriate, the Ca concentration in the molten steel became non-uniform, and the composition morphology of inclusions in the steel could not be sufficiently modified to be harmless. The presence of these inclusions with insufficient modification has caused HIC cracking.

An object of the present invention is to provide a method for solving such a problem, and more specifically,
It is an object of the present invention to provide a method for producing a HIC resistant steel capable of further improving the HIC resistance by adding Ca under necessary and sufficient conditions.

[0016]

[Means for Solving the Problems] In order to achieve such an object, the present inventors have diligently studied the correlation between Ca addition rate and inclusion composition morphology change, and controlled the Ca addition rate under specific conditions. It was found that the HIC resistance can be greatly improved by the above, and the present invention was completed.

The gist of the present invention is that Ca is contained in molten steel.
When adding a contained substance, the Ca addition rate (Ca
This is a method for producing HIC resistant steel characterized by controlling V according to the following equation. 15 ≦ V ≦ −8.4 × [S] −0.3 × a _o +355.8 ・ ・ ・ (1) V: Ca addition rate (g / T / min) [S]: S concentration in molten steel (ppm) a _o : Activity oxygen value (ppm).

As described above, according to the present invention, when the Ca-containing substance is added, by appropriately controlling the addition rate as described above, CaS, CaO inclusions, high melting point composition
It is possible to suppress the formation of CaO-Al ₂ O ₃ -based inclusions and effectively change the inclusions into CaO-Al ₂ O ₃ -based spherical inclusions having a low melting point composition. Thus, according to the present invention, the HIC resistance of the steel material can be greatly improved.

[0019]

Next, the operation of the present invention will be described more specifically. As described above, even in the prior art, a method of controlling the Ca addition rate according to the C content in molten steel has been proposed, but in the conventional method, Al ₂ O _{3 is used.}
When the inclusions are modified with Ca to form spherical CaO-Al ₂ O ₃ -based inclusions, the main aim is to suppress CaS formation due to excess Ca with respect to the modification of Al ₂ O _3. .

However, by controlling the Ca addition rate in this way,
HIC resistance is not improved simply by suppressing the formation of S inclusions. Oxide inclusions with high melting point composition (e.g. CaO, 12CaO
(3Al ₂ O ₃ etc.), regardless of its form, becomes a cluster-like inclusion that is crushed during rolling and elongated in the rolling direction, and becomes the starting point of HIC cracking.

Therefore, in the present invention, HIC resistance is provided.
In order to improve the properties, not only the suppression of the formation of CaS-based inclusions, but also the suppression of the formation of oxide-based inclusions with a high melting point composition. Therefore, according to the present invention, by controlling the Ca addition rate according to the above equation according to the activity oxygen value in the molten steel and the sulfur concentration, the formation of CaS-based inclusions is suppressed,
Furthermore, the oxide-based inclusions are effectively added to the low melting point composition (in the inclusions
By modifying the CaO / Al ₂ O ₃ weight ratio from 0.54 to less than 1.5), the HIC resistance is significantly improved.

FIG. 1 is a schematic diagram for explaining the outline of the Ca addition mode in the present invention.
CaSi blown into the molten steel 14 inside the ladle 12 from the lance is in the high-speed injection high Ca region near the injection position from the lance.
CaO and CaS are produced by each reaction of + O → CaO and Ca + S → CaS. It should be noted that the molten steel 14 has an upward flow in the central portion and a downward flow in the peripheral region.

As shown in FIG. 1, if the addition rate of the Ca-containing substance is too high, the Ca content in the vicinity of the injection position from the immersion lance is increased.
Ca concentration rises locally. Like this in molten steel
When regions with extremely high and low Ca concentrations are generated, inclusions generated or modified when Ca is added have various compositions and morphologies, and HIC resistance such as CaS inclusions and oxide-based inclusions with high melting point compositions. Many inclusions that adversely affect the power generation are generated.

Therefore, it is extremely difficult to suppress the formation of these inclusions only by controlling the amount of Ca added, and further improvement in HIC resistance cannot be expected. It can be seen that the addition rate of Ca must be further regulated in order to suppress the formation of inclusions that adversely affect the HIC resistance.

Further, the conditions for forming CaS inclusions are as follows:
It is defined by Ca concentration, S concentration and active oxygen value, while the composition form of oxide inclusions depends on Ca concentration in molten steel and active oxygen value. Therefore, in order to suppress the formation of CaS-based inclusions and at the same time control the composition morphology of oxide-based inclusions, it is necessary to control the Ca concentration in molten steel, the active oxygen value, and the S concentration. As described above, controlling the Ca concentration when Ca is added is extremely disadvantageous in controlling inclusions, and the method of controlling the Ca addition rate is advantageous.

Here, the present inventors have found that active oxygen in molten steel,
As a result of investigating the relationship between the S concentration, the Ca addition rate and the HIC resistance, the Ca addition rate was controlled according to the following equation.
It was found that it is possible to manufacture steel with high HIC resistance by suppressing the formation of S inclusions and effectively changing the oxide inclusions to a low melting point composition. V ≦ −8.4 × [S] −0.3 × a _o +355.8 (2) V: Ca addition rate (g / T / min) [S]: S concentration in molten steel (ppm) a _o : Activity Oxygen value (ppm).

On the other hand, when the Ca addition rate is reduced to less than 15 g / T / min, the Ca content in the molten steel is more
The evaporation rate becomes faster and the Ca concentration in the molten steel cannot be increased. As a result, the Ca concentration in the steel becomes low, and MnS is produced during solidification, which significantly reduces the HIC resistance.

Therefore, by controlling the Ca addition rate according to the above-mentioned formula (1), it is possible to manufacture a steel excellent in HIC resistance. 15 ≦ V ≦ −8.4 × [S] −0.3 × a _o +355.8 ・ ・ ・ (1) V: Ca addition rate (g / T / min) [S]: S concentration in molten steel (ppm) a _o : Activity oxygen value (ppm).

The Ca-containing substance used in the present invention may be any as long as Ca is contained, and Ca-Si, Fe-
Alloys such as Ca, Ca-Al, Ca-Ni, and Fe-Ca-Ni can be used. In addition, a mixture of Ca-containing metal powder and another metal powder, or a mixture obtained by pressure molding after mixing may be used.

Further, these Ca-containing substances include CaO, Al ₂ O ₃ ,
CaF ₂ , CaO-Al ₂ O ₃ , CaO-CaF ₂ , and CaO-Al ₂ O ₃ -CaF ₂ flux may be mixed and used. The effect of mixing and adding these fluxes is described in JP-A-54-37019 and JP-B-59.
-22765, JP 64-75622 A, JP 3-A
As described in Japanese Patent No. 47910.

According to a preferred embodiment of the present invention, the Ca purity in the Ca-containing substance used when Ca is added is 50% or less, particularly 40%.
The following is desirable. Since Ca is highly reactive, if the purity is too high, the reaction at the time of addition becomes vigorous, causing a problem of splash generation. On the other hand, if the purity is too low, the amount of metal components other than Ca increases, and the basic unit of the Ca-containing substance required when adding the required amount of pure Ca becomes too large.

Further, it is desirable that the amount of added Ca or the basic unit of Ca (calculated as Ca pure content) be 1.0 kg / T, particularly 0.5 kg / T or less. The reason is that if the Ca addition amount becomes excessive, the Al ₂ O ₃ -based inclusions that should react will decrease or disappear even if the Ca addition rate is reduced, and the CaO concentration of the CaO-Al ₂ O ₃ -based inclusions will be high. This is because it becomes too much. Furthermore, if the amount of Ca added is too large, the formation of CaS-based inclusions cannot be sufficiently suppressed.

On the other hand, the effect is sufficiently exhibited when the amount of added Ca or the unit amount of Ca (calculated as pure Ca) is 0.1 kg / T or more. The reason is that if the amount of Ca added is too low, Al ₂ O
_This is because the CaO—Al ₂ O ₃ -based inclusions are not fully converted into the ₃ -based inclusions, and the alumina cluster-based inclusions remain. Furthermore, if the amount of Ca added is too low, MnS which adversely affects the HIC resistance performance is generated in the region where Mn is concentrated due to center segregation during casting.

Next, the method of adding Ca is not particularly limited in the present invention. For example, it is customary to carry out the powdery Ca-containing substance through an immersion lance using Ar gas, which is an inert gas, as a carrier gas. Even in the present invention, Ca may be added by the same method. At this time, it is desirable to provide a load cell in the powder hopper and manage the powder supply amount per unit time within a narrow width. Similarly, an iron-coated wire containing a Ca-containing substance may be added into the molten steel from the upper surface of the molten steel. With regard to the addition of the wire, the delivery length per unit time can be accurately determined by the driving device, and the Ca addition rate can be easily calculated from the weight per unit length of the wire.

If possible, a Ca-containing substance in a molten state may be added. Furthermore, although a Ca-containing substance may be added from the alloy hopper, it is usually difficult to control the addition speed to a narrow width, so it is possible to provide a continuous cutting device or the like to control the addition speed to a narrow width. desirable.

Here, a series of steps of the method for melting steel having excellent HIC resistance in the present invention will be described as follows. It should be understood that the present invention is not particularly limited to these, and many other modifications are possible. That is, the hot metal that has been appropriately pretreated is decarburized in a converter, and the obtained molten steel is tapped in a ladle.

A vacuum degassing method (RH
Method, tank degassing method, etc.) is preferable. This is to prevent hydrogen-induced cracking by reducing the hydrogen concentration in the steel. If it is possible to reduce the hydrogen concentration of steel by performing slab gradual cooling after casting, and product gradual cooling after rolling, it is possible to omit this melt dehydrogenation process or reduce the melt dehydrogenation process. is there. However, in the case of slab gradual cooling and product gradual cooling, it takes time to reduce the hydrogen concentration, which is disadvantageous in terms of shortening the delivery time of the product.

After desulfurization treatment in the ladle,
A Ca-containing substance is added, and the CaO-Al ₂ O ₃ -based inclusions generated there are float-separated by a vacuum degassing method (RH method, tank degassing method, etc.), and then a Ca-containing substance is further added in the ladle. You may. The vacuum degassing method is suitable for separating inclusions, but it is because Ca dissolved in molten steel is easily vaporized due to vacuum treatment.

Further, it is desirable that the atmosphere during addition of the Ca-containing substance is a closed atmosphere to shut off the atmosphere in order to prevent secondary oxidation. If necessary, an inert gas atmosphere may be used. Further, the atmospheric pressure is usually atmospheric pressure, but may be slightly increased or decreased.
However, when the Ca-containing substance is added under reduced pressure, it is desirable that the atmospheric pressure is 100 torr or more, considering that vaporization of Ca is easy due to high vapor pressure of Ca.

Further, the temperature of molten steel in the ladle may be compensated by raising the temperature of the molten steel by supplying oxygen or by electric heating after the steel is fed from the converter and before or after desulfurization treatment in the ladle. Generally, in the desulfurization process, it takes a long processing time and the amount of heat released during the process is large, so that the tapping temperature rises.Therefore, the tapping temperature decrease can be measured by including the heating step after tapping in this way. May be. Next, the function and effect of the present invention will be described more specifically by way of examples.

[0041]

[Examples] Sixteen kinds of molten steel, which had the basic composition shown in Table 1 and were changed in S concentration and activity oxygen value respectively, as shown in Table 2 described later, which were housed in a 250-ton ladle, Ca
The contained substances were added at different addition rates, and the effect of Ca addition rate on HIC crack initiation and inclusion composition was investigated for the obtained steel.

Here, the molten steel before the addition of Ca was treated by the following process. The hot metal supplied from the blast furnace was subjected to hot metal treatment (hot metal desulfurization, hot metal dephosphorization, or both), and then the hot metal was poured into a converter and blown. After the converter was blown, the molten steel was tapped into a ladle and various alloys were added. The Ca-containing substance was added after desulfurization by molten steel desulfurization treatment in a ladle.

The addition of Ca to the molten steel is carried out under normal pressure
The Ca-containing substance was subjected to immersion lance using Ar gas as an inert gas as a carrier gas. At this time, a load cell was provided in the powder hopper, and the powder supply amount per unit time was controlled within the target value ± 10 kg / min. Ca at this time
As shown in Table 2, 16 kinds of addition rates were changed according to the molten steel composition.

Next, the molten steel to which this Ca addition treatment was applied was cast by a continuous casting apparatus, a sample was cut out from the obtained slab, and the composition of inclusions and the HIC test were conducted in the following manner.

(1) Relationship between Ca addition rate and CaO-Al ₂ O ₃ -based inclusion composition Powdered Ca-Si alloy (1 kg / T) containing about 30% Ca was continuously added from the immersion lance. did. The inclusion composition in the obtained slab was investigated. In the case of the example of the present invention, the Ca addition rate was controlled by the load cell as described above. FIG. 3 shows the analysis results of the composition of inclusions in Examples and Comparative Examples shown in Table 2. The horizontal axis is Run No. shown in Table 2.

When the Ca addition rate is controlled according to the present invention, the inclusions become CaO-Al ₂ O ₃ -based inclusions (Ca / Al ₂ O ₃ weight ratio in the inclusions of 0.54 or more and less than 1.5) having a low melting point composition. Has become. On the other hand, as shown in Comparative Examples, when the Ca addition rate was not controlled according to the present invention, CaO and CaS inclusions were generated, and CaO-Al ₂ O ₃ inclusions also had a high melting point composition. It turned out that

As described above, when the Ca addition rate is controlled according to the present invention, inclusions are efficiently modified to form CaO-Al ₂ O ₃ type inclusions having a low melting point composition that is harmless to HIC resistance. .

(2) Relationship between Ca Addition Rate and HIC Crack Occurrence Rate A HIC test using a NACE test solution was conducted on each test steel (slab). The results of this HIC test are summarized in Table 2.

As can be seen from Table 2, according to the present invention Ca
In Run No. 1 to 8, which shows the case where the addition rate was controlled, HI was used.
Although the rate of occurrence of C cracking could be suppressed to a low level, Ca addition was performed under conditions other than that, and there was a high probability in Run Nos. 9 to 16 showing the case where the Ca addition rate was not controlled according to the present invention.
HIC crack occurred. In Run Nos. 11 and 15, HIC cracking occurred although CaS-based inclusions were suppressed. From this, it is understood that not only the suppression of the formation of CaS-based inclusions as described in the present invention but also the composition control of the oxide-based inclusions is essential for improving the HIC resistance.

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

EFFECTS OF THE INVENTION According to the present invention, hydrogen-induced cracking does not occur, which contributes to stabilization of the quality of steel products.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of a mode of adding a Ca-containing substance.

FIG. 2 is a graph showing the relationship between Ca addition rate and HIC cracking.

FIG. 3 is a graph showing the relationship between the Ca addition rate and the composition of inclusions.

Claims (1)

[Claims] 1. When adding a Ca-containing substance to molten steel, Ca
A method for producing a HIC resistant steel, characterized in that the Ca addition rate (calculated as a Ca content) V of the contained substance is controlled according to the following formula. 15 ≦ V ≦ −8.4 × [S] −0.3 × a _o +355.8 V: Ca addition rate (g / T / min) [S]: S concentration in molten steel (ppm) a _o : Activity oxygen value (ppm) )