JP2003073726A - METHOD FOR MANUFACTURING LOW Ti STEEL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method which can produce low Ti steel, without aggravating a material yield, and without transporting molten steel to other location in the middle of a manufacturing process. SOLUTION: The method for manufacturing the steel comprises an EAF process for melting scrap in an electric furnace, and a LF (ladle furnace) process for smelting the molten steel in a ladle, after transferring it to the ladle. The above LF process is divided into a low basicity smelting process (an LF process 2) under a condition in which slag is made to have relatively low basicity, and a high basicity smelting (an LF process 3) under a condition in which the slag is made to have relatively high basicity. In the LF process 2, scale is added to the molten steel, to separate Ti in the molten steel by surfacing it in the slag through a reaction of Ti with oxygen of the scale, and to reduce Ti in the molten steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low-Ti steel suitable for application to a bearing steel, particularly a bearing steel for a bearing which requires quietness.

[0002]

2. Description of the Related Art Conventionally, bearings, particularly bearings for audio equipment, which are required to be quiet, have been regarded as a problem that abnormal noise is generated during rotation. Has been. In this type of quietness-required bearing, for example, as shown in FIG. 4, the microphone 20 is rotated while rotating the bearing 200 in an acoustic chamber.
A test for collecting sound and measuring sound is performed in 2 and those that generate noise at that time are disqualified. The source of this abnormal noise is T such as hard TiN existing on the surface of the bearing component.
It is believed that the cause is i inclusions.

Therefore, this problem can be solved by reducing the Ti content of the steel at the time of manufacturing the bearing steel and reducing the amount of Ti inclusions such as TiN. However, it is not easy to reduce the amount of Ti inclusions at the time of manufacturing the bearing steel, and particularly to control it to a very low level of several ppm order, which is required recently.

Heretofore, bearing steel has heretofore been manufactured as follows. First, scrap as an iron raw material is melted in an electric furnace such as an arc furnace, and the molten steel is tapped from the electric furnace and transferred to a ladle, where a slag forming agent such as CaO and CaF ₂ and a deoxidizer and necessary alloys. After adding components and desulfurization and deoxidation refining, molten steel is cast, for example, continuous casting.

In this case, Ti is picked up in the molten steel from the refractory material of the electric furnace or ladle, or the molten steel is also brought from the slag brought into the ladle from the electric furnace or the slag attached to the ladle. Ti is picked up inside. Even if these can be sufficiently prevented, Ti will be picked up in the molten steel from the alloy components added in the refining stage.

For example, what is called high carbon bearing steel is Cr
Is contained in steel at about 1.5% (weight%). If Cr is added to the molten steel as an alloy component in the refining process, Ti, which is unavoidably contained as an impurity in the molten steel, enters the molten steel.

As a method of reducing Ti in steel, Ti is
It is conceivable that a large amount of oxygen is blown in the melting process in the electric furnace by utilizing the high affinity of oxygen with oxygen. In this case, Ti in the molten steel reacts with the oxygen spewed,
It becomes Ti oxide (TiO ₂ ) and floats and separates in the slag, and accordingly Ti in the molten steel decreases.

However, in this case, Fe in the molten steel also becomes an oxide and floats and separates. Therefore, in the case of this method, the yield of the material decreases. In addition, this method cannot deal with the problem that Ti is picked up in the molten steel from the refractory material of the ladle and the problem that Ti is picked up in the molten steel from the added alloy component.

As another method, after adding an alloy such as Cr to molten steel discharged from an electric furnace, the molten steel is carried by a crane or the like to a place where oxygen blowing equipment is installed, and oxygen blowing is performed there to remove the molten steel. It is also possible to reduce Ti.
However, in this case, in addition to the problem that the yield of the iron raw material decreases due to oxygen sparging, in the middle of the actual steelmaking line, when the molten steel is once carried to the place where the oxygen sparging equipment is installed and oxygen sprinkling is performed, In the meantime, there are various restrictions such as the temperature of molten steel decreasing and the subsequent casting cannot be performed smoothly, and it is difficult to actually adopt this method.

[0010]

The method for producing a low Ti steel according to the present invention has been devised to solve such a problem. Thus, the invention of claim 1 is a method for producing steel through a step of melting scrap in an electric furnace and a step of transferring molten steel to a ladle and refining in the ladle. It is characterized in that scale is added to the molten steel in the step and Ti of the molten steel reacts with oxygen of the scale to float and separate Ti in the slag to reduce the Ti of the molten steel.

A second aspect of the present invention is characterized in that, in the first aspect, the addition of the scale is performed before the final addition of the deoxidizing agent.

A third aspect of the present invention is the method according to any one of the first and second aspects, wherein the ladle refining is performed under low basicity refining under the condition that the slag has a relatively low basicity, and the slag is subjected to relative refining. It is characterized in that the scale is added to molten steel in the step of refining with high basicity under the condition that the degree of basicity is high, and in the step of refining with low basicity.

[0013]

As described above, according to the present invention, scale is added to molten steel in the process of ladle refining, and Ti is floated and separated in the slag by the reaction between Ti of molten steel and oxygen of the scale. To reduce Ti. Here, the scale is mainly composed of iron oxides such as wustite, magnetite, and hematite that are generated on the surface of the base iron during hot rolling of steel or soaking of slabs, and usually by pickling or grinding. It is removed from the ground iron and discarded as it is.

The present invention effectively utilizes this scale for reducing Ti in molten steel. According to the present invention, Ti in molten steel can be satisfactorily reduced to an extremely low level without oxygen blowing. Can be reduced to.

Incidentally, FIG. 1 (A) shows a distribution ratio of Ti [ppm Ti] in molten steel and Ti (% TiO ₂ ) in slag when a scale is added to molten steel, specifically, (% TiO ₂ ).
₂ ) / [ppm Ti] (Fig. 1 (A) shows the case where 250 kg of scale is added to 80 t of molten steel), and the distribution of Ti to the slag by adding scale as shown in the same figure. The ratio is dramatically increasing.

The scale is originally disposed of, and according to the present invention, such a scale can be effectively used. Further, when the Ti content in the molten steel is reduced by adding the scale according to the present invention, there is an advantage that the iron component itself contained in the scale does not adversely affect the alloy system of the molten steel by the addition.

Here, the addition of the scale can be carried out before the final addition of the deoxidizing agent (claim 2). If the scale is added after the final addition of the deoxidizing agent, oxygen will be supplied to the molten steel again by the subsequent addition of scale even if the oxygen in the molten steel is reduced.

For example, in the case of a clean bearing steel, a steel having a low oxygen level must be finally obtained, and in this case, a strong deoxidizer Al is usually added finally. In this case, if the scale is supplied to the molten steel after the addition of the deoxidizing agent, the meaning of adding the deoxidizing agent is almost lost.

In the present invention, ladle refining includes low basicity refining under conditions where slag is relatively low basicity and high basicity refining under conditions where slag is relatively high basicity. And the addition of scale is performed in the step of refining low basicity (claim 3).

For example, bearing steel must be strongly desulfurized in the refining process. In this case, the higher the basicity of the slag, the higher the efficiency of desulfurization. On the other hand, if the basicity of the slag is high, it becomes difficult for Ti in the molten steel to float and separate in the slag.

FIG. 1 (B) shows the relationship between the basicity of slag and the distribution ratio of Ti. As shown in FIG.
The distribution ratio of i, that is, (% TiO ₂ ) / [ppmTi], increases as the basicity C / S (CaO / SiO ₂ ) decreases. Note that FIG. 1B also shows that the smaller the Ti content in the molten steel, the larger the Ti distribution ratio under the same basicity.

Since the distribution ratio of Ti varies depending on the basicity of the slag, the ladle refining is divided into the low basicity refining and the high basicity refining according to claim 3, and the low basicity refining is performed. In, the distribution ratio between Ti in the molten steel and Ti in the slag can be increased, and the Ti in the molten steel can be efficiently floated and separated in the slag to reduce the Ti in the molten steel.

On the other hand, in the subsequent refining under high basicity, desulfurization can be performed with high efficiency. Then, by adding a scale according to claim 3 in the step of refining low basicity, Ti from molten steel can be obtained as shown in FIG. 1 (A).
Can be effectively removed or reduced. Incidentally, it is desirable to add the scale here in the latter stage of the low basicity refining.

In the present invention, the scale may be added when the steel is tapped from the electric furnace regardless of whether the scale is added later.
Ultra low Ti steel, for example, the content in molten steel is a few p as described above.
When producing pm-order ultra-low Ti steel, it is desirable that Ti in molten steel be as close to zero as possible at the time of tapping from an electric furnace.

At this point, if the molten steel contains several ppm of Ti, it is feared that the Ti level in the molten steel will eventually become higher considering that Ti will be picked up in the molten steel later. To be done. Therefore, it is desirable that Ti in the molten steel be as low as possible at the time of tapping, but if Ti is found to be contained in the molten steel at that time, the scale can be added at that stage to remove the molten steel. After removing Ti, the Ti in the molten steel is brought to a state close to zero as much as possible, and this can be subjected to the subsequent ladle refining.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described below with reference to the drawings. Here, first, scrap as an iron raw material is charged together with a slag-making agent into the arc furnace (electric furnace) 10 in FIG. 2, an arc is generated between the electrode and the scrap, and the scrap is melted by the arc heat (FIG. EA in 3
Step F). In this example, the arc furnace 10 has a capacity of 80 t.
belongs to. Further, this arc furnace 10 has a tapping hole at a position eccentric from the center of the furnace, and the tapped steel from the arc furnace 10 is tapped from the tapped port at the eccentric position of the hearth. Is.

Molten steel K from the arc furnace 10 is received by the ladle 12 at the time of tapping. At that time, with the molten steel K, the arc furnace 10
Since some of the slag inside may come into the ladle 12, the slag-off process (SO process in FIG. 3) is performed after tapping. In this SO process, as shown in FIG. 2, the ladle 12 is tilted and the slag S on the surface is scraped out by the manipulator 13 to the slag receiver 15.

Then, the ladle 12 is covered with a furnace lid having an electrode, and a slag generation step (LF step in FIG. 3) is performed as a pre-step of ladle refining in a ladle refining apparatus (LF refining apparatus) 16.
Is carried out. Here, only CaO and CaF ₂ as a slag forming agent are added without adding alloy components and the like, and only slag is generated. Its purpose is as follows.

The molten steel K received in the ladle 12 may be partially carried with the slag from the arc furnace 10 (carry-in slag) even if the SO process is carried out. There is a possibility that the slag with a pot that had adhered to the molten steel enters the molten steel K. In this case, Ti contained in the slag
Is picked up in molten steel.

Therefore, in the LF as a pre-process, the slag is simply recreated, whereby the carry-in slag and the slag with a pot are combined with the newly generated slag S, and the slag S is removed to remove the slag S. The amount of Ti picked up from molten steel K into molten steel K is minimized. The removal of the slag S at this time is performed by performing the SO process using the manipulator 13 shown in FIG.

Next, following the previous step, (ladle refining device 1
6) is used to perform low basicity refining under the condition of making the slag low basicity (LF step in FIG. 3). Specifically, here, Cr, Si, and Mn as alloying components are added in the form of Cr, Si, and Mn alloys containing impurities, particularly Ti content is small, and CaO, CaF ₂ , and C as slag forming agents. A high-purity C powder having a low Ti content is added as a source.

In this LF step, a relatively small amount of the slag forming agent is added as compared with the high basicity refining described later. This L
In the F step, an additive with a small amount of Ti as an impurity is selected so that Ti is not picked up as much as possible in the molten steel of the ladle 12.

In this low basicity refining (LF step), S
Silicon is deoxidized by the i alloy, and Ti in the molten steel K is efficiently floated and separated in the slag by the low basicity slag. In this low basicity refining (LF step), scale is added in the latter stage, and Ti is floated and separated as TiO ₂ in the slag by the reaction between oxygen in the scale and Ti in the molten steel K.

After that, the SO step using the manipulator 13 shown in FIG. 2 is performed again, and subsequently, the high basicity refining (LF step) shown in FIG. 3 is performed. Here, Al, which is a strong deoxidizing agent, and CaO as a slag forming agent in the initial stage,
CaF ₂ is added, and subsequently, alloy components are added if necessary.

In this high basicity refining (LF step), a larger amount of CaO, Ca is used as a slag-forming agent than in the case of low basicity refining.
F ₂ is added, whereby the slag is maintained at a high basicity. This also results in strong desulfurization and deoxidation by addition of Al as a strong deoxidizer.

The molten steel K that has undergone the high basicity refining (LF step) is subsequently degassed in vacuum by the RH vacuum degassing device 18, and hydrogen, oxygen and nitrogen in the molten steel K are degassed. After that, while the molten steel maintains the predetermined temperature, the next continuous casting 20 is performed, and the cast pieces are continuously taken out.

[0037]

EXAMPLES Next, specific examples of low Ti steel production will be described in detail below. A low Ti steel (SUJ2 steel) having the composition shown in Table 1 was manufactured according to the process shown in FIGS. 2 and 3. That is, first, the melting process of steel using the arc furnace 10 (the EAF process in FIG. 3)
Was carried out. At this time, along with scrap as an iron raw material, a high-purity C powder having a very small Ti content as a C source and CaO (2000 kg) as a slag forming agent were used in the arc furnace 10.
It was charged into a vessel and dissolved. In this arc furnace 10, molten steel 80t was melted.

[0038]

[Table 1]

Subsequently, the molten steel K inside the arc furnace 10 was tapped from the furnace bottom to the ladle 12. The molten steel temperature during tapping is 1680.
It was ℃. Then, following this, ladle 1 with molten steel K
Slag-off process (S to remove slag that has entered inside 2)
O step) was performed, and then the LF step was performed. In this LF process, only the slag forming agent was added to heat up. Specifically, here, CaO 500kg, CaF ₂ 150k
g was added to the molten steel K, and the temperature was raised to 1600 ° C.

By carrying out this LF step, the slag brought in from the arc furnace 10 together with the molten steel K and the slag with a pot attached to the ladle 12 are combined with the newly generated slag S, and The slag S combined in the subsequent slag-off step (SO step) was separated and removed.
By carrying out this LF process, the pickup of Ti from the slag S to the molten steel K can be reduced as much as possible. It should be noted that this LF process is not intended to actively perform deoxidation and desulfurization refining.

After carrying out the LF process and the subsequent SO process, the LF process of FIG. 3 was carried out. This LF process is a low basicity refining under the condition that the slag S has a low basicity. In this LF step, first, high-purity Cr, Mn alloy and C powder containing few impurities such as Ti as alloy components were added at the beginning. At the same time, a Si alloy containing few impurities such as Ti is added as a deoxidizing agent, and CaO500 is added as a slag forming agent.
kg and CaF ₂ 150 kg were added.

In this LF step, the amount of the slag-forming agent added is smaller than in the subsequent LF step, and the slag S produced has a relatively low basicity. As a result, Ti in the molten steel K
Reacts with the slag forming agent to efficiently float and separate in the slag S. In this LF step, 250 kg of scale was added after the addition of the alloy components, and then held for 15 minutes or more, or the electrodes were energized, and then the LF step was terminated.

After the LF process, the SO process was performed again, and then the LF process was performed. This LF process is a high basicity refining under the condition that the slag has a high basicity. In this LF process, the strong deoxidizer, Al
Was added to 60 kg, and CaO, CaF ₂ as a slag forming agent.
1000 kg of CaO and 400 kg of CaF ₂ were added. In this LF step, deoxidation and desulfurization refining are strongly carried out by adding Al and by virtue of the high basicity of the slag.

In this LF step, alloy addition for component adjustment was also performed, and Al was additionally added. After the LF process, the RH vacuum degassing device 18 shown in FIG.
Was used to perform the RH vacuum degassing process. RH here
The degassing process was performed for 60 minutes. Then, the molten steel K is shown in FIG.
It was conveyed to the continuous casting apparatus 20 shown in FIG. The transition of the components in the molten steel K through each of these processes is also shown in FIG.

For comparison, instead of adding the scale in the LF step of the above-mentioned embodiment, the molten steel is conveyed to the place where the oxygen-sparing equipment is located before the end of the LF, and the oxygen-sparging is carried out there. Therefore, a manufacturing process for reducing Ti in the molten steel was carried out. As a result, Ti was only 10 ppm.

As can be seen from these comparisons, by producing the low-Ti steel according to this example, it is possible to obtain the scale of the scale even if the molten steel is not transported to the place where the oxygen blowing unit is provided during the process of the molten steel production. The final addition of T in molten steel
It can be seen that i can be reduced to an extremely low level, specifically 5 ppm or less.

Although the embodiment of the present invention has been described in detail above, this is merely an example. For example, in the above embodiment, LF and L
F and LF are implemented, but in some cases LF
It is also possible to omit, and to carry out the LF process after the LF, and to add the scale to the molten steel in the LF process. The present invention can be applied to steel types other than the above bearing steel, and can be implemented in various modifications without departing from the scope of the invention.

[Brief description of drawings]

FIG. 1A is a diagram illustrating an effect of adding scale. (B): Basicity of slag and (% TiO ₂ ) / [ppmT
It is a figure showing the relationship with i].

FIG. 2 is a diagram showing an embodiment of a method for producing low Ti steel according to the present invention.

FIG. 3 is a diagram showing one embodiment of a method for producing low Ti steel according to the present invention and transition of molten steel components.

FIG. 4 is a diagram showing an example of a method of acoustic testing of a bearing.

[Explanation of symbols]

10 arc furnace 12 ladle K molten steel S slug

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/50 C22C 38/50 F term (reference) 4K013 AA09 BA01 BA05 BA08 BA09 BA11 CD02 CE01 CF03 CF12 CF13 DA03 DA09 DA12 DA14 EA02 EA03 EA09 EA18 EA19 EA20 EA28 EA30 FA06 4K014 CA04 CB01 CB05 CD12

Claims (3)

[Claims] 1. A step of melting scrap in an electric furnace,
In a method for producing steel through the step of transferring molten steel to a ladle and refining in the ladle, in the step of ladle refining, a scale is added to the molten steel to react Ti of the molten steel with oxygen in the scale. A method for producing a low Ti steel, characterized in that Ti is floated and separated in the slag by means of the method to reduce Ti in the molten steel. 2. The low T according to claim 1, wherein the addition of the scale is performed before the final addition of the deoxidizing agent.
i Steel manufacturing method. 3. The ladle refining according to claim 1, wherein the ladle refining is a low basicity refining under the condition that the slag is relatively low basicity, and the slag is relatively high basicity. The method for producing low-Ti steel, characterized in that the scale is added to molten steel in the step of refining with high basicity under the conditions described below.