JP2003034817A - Method of manufacturing clean steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a highly clean steel which does not cause cracks in wiring and is superior in fatigue strength, by reducing an amount of hard non-metallic inclusions which unavoidably come from ladle refractory in a smelting step. SOLUTION: This method for manufacturing the clean steel by repeating the several cycles of an operation on a silicon killed steel, one cycle of which consists of taking molten steel produced in a converter or an electric furnace into a ladle to perform a secondary refining, discharging the smelted molten steel out of the ladle, continuously casting it, returning the ladle to the converter or the electric furnace, and taking the molten steel again, while using several ladles in combination, is characterized by making only the ladle which received only the silicon killed steel or the rimmed steel last time, to receive the silicon killed steel, until the repetition finishes.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength steel wire excellent in cold drawability, and particularly useful for producing a clean steel useful as a material for a steel for an ultrafine wire tire cord or a steel for a high-strength spring. It is about the method.

[0002]

2. Description of the Related Art In ultra-fine steel wire drawn to 0.1 to 0.5 mm by cold working and spring steel materials requiring high fatigue strength, hard non-metallic inclusions present in the steel material are used. It is necessary to reduce it as much as possible. From such a point of view, as the steel material used for the above-mentioned applications, it is general to use highly clean steel in which the presence of the non-metallic inclusions is reduced as much as possible.

Various techniques have been proposed so far from the viewpoint of reducing hard non-metallic inclusions in steel materials. For example, Japanese Patent Publication No. 6-74484 and 6-7
No. 4485 stipulates the composition and abundance of non-metallic inclusions so that the non-metallic inclusions are easily stretched or broken during cold working and are controlled to be soft that does not substantially cause steel fracture. It has been proposed to improve the cold workability and fatigue strength of steel materials.

However, even if the composition of the non-metallic inclusions is controlled to be soft, the inclusions that actually become the starting points of wire breakage and fatigue during cold working are the same as the previous charge that adhered to the ladle during the manufacturing process. Hard inclusions such as alumina-based inclusions, spinel-based inclusions, zircon-zirconia-based inclusions, forsterite-based inclusions that are inevitably mixed in from slag and ladle refractory Will cause wire breakage and fatigue even if the amount is small.

In fact, 95% or more of the hard inclusions, which are considered to be present in the steel, account for 95% or more of the inclusions observed in the fracture surface of the wire rod which is broken during cold drawing. It was found from the results that alumina-based inclusions and zircon-zirconia-based inclusions occupy most of them. On the other hand, the proportion of soft inclusions observed on the fracture surface is
Almost nothing. Therefore, it is extremely important to minimize the amount of these hard inclusions in order to realize a wire rod having excellent cold drawability.

From the viewpoint of reducing the amount of alumina inclusions that are inevitably mixed in from ladle refractories, etc., for example, JP-A-62-203647 and JP-A-267658,
Various techniques such as JP-A-6-212237 and JP-B-7-103416 have been proposed. These techniques basically aim to reduce alumina inclusions by using a ladle, a tundish, a nozzle, etc. lined with a non-alumina refractory material. Also,
As a non-alumina refractory specifically used, a zircon / zirconia refractory is used.

These techniques are effective methods from the viewpoint of reducing alumina inclusions. However, it is not possible to prevent the mixing of alumina caused by the slag of the previous charge that has adhered to the inside of the ladle, and instead of the alumina-based inclusions, hard zircon / zirconia-based inclusions are used as the inclusions from the ladle refractory. Since inclusions are mixed in, it cannot be a fundamental solution for reducing non-metallic inclusions.

On the other hand, it is also attempted to reduce hard nonmetallic inclusions by adjusting the composition of slag used in the step of refining molten steel. For example, JP-A-6-
No. 330147 indicates that when the secondary refining in the ladle is performed, Ca
O, SiO ₂ , CaF ₂ as main components, CaO / SiO ₂
(Basicity): 1.2-2.0 and Al ₂ O _3: 0.5 to
There is also proposed a technique for highly cleaning steel materials by forming slag adjusted to 2.0%. In such a technique, the slag basicity is set as high as possible, and the Al ₂ O ₃ existing in the slag is reduced to suppress the enrichment of Al ₂ O _{3 in the} non-metallic inclusions and It is intended to lower the melting point and increase the ductility of inclusions. However, even such a technique cannot be a fundamental solution in the situation where non-metallic inclusions from the ladle refractory are mixed, and there is still room for improvement.

[0009]

In the various prior arts proposed so far, hard inclusions that are said to have the most adverse effect on the workability such as cold drawing, that is, adhere to the ladle during the manufacturing process. Hard inclusions such as alumina-based inclusions and zircon-zirconia-based inclusions that are inevitably mixed from the slag and refractory previously charged cannot be completely eliminated, and this is a fundamental solution. The reality is that they do not.

The present invention has been made under such circumstances, and its purpose is to reduce hard non-metallic inclusions which are inevitably mixed from ladle refractory or the like in the refining stage, and to expand It is an object of the present invention to provide a useful method for producing a highly clean steel which is excellent in fatigue strength without being broken during wire drawing or breaking during product use.

[0011]

Means for Solving the Problems The method of the present invention capable of achieving the above-mentioned object is that molten steel produced in a converter or an electric furnace is subjected to secondary refining by receiving steel in a ladle, and the molten steel after refining After the ladle is discharged from the ladle and continuously cast, the ladle is returned to the converter or the electric furnace and the molten steel is received. When the silicon-killed steel is continuously repeated several times, until the end of this repetition, the ladle that receives the silicon-killed steel has only the silicon-killed steel or the rimmed steel as the steel type to be received by the previous charge. It has the gist of operating in such a manner.

In the above method of the present invention, when using a ladle whose refractory lining is an alumina refractory, control is performed so that the Al ₂ O ₃ concentration in the adhered slag on the inner surface of the ladle is 70% or less. It is preferable to operate after that. Further, as a concrete means for controlling the Al ₂ O ₃ concentration of the adhered slag on the inner surface of the ladle to 70% or less, the top slag composition is set to basicity (CaO / SiO ₂ ): 1 to 3 and , Al ₂
The O ₃ concentration may be controlled to 10% or less.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied from various angles from the viewpoint of reducing hard inclusions in highly clean steel as much as possible. The origin of the alumina inclusions and zirconia / zircon inclusions mixed in is not only the refractory material lined in the ladle, but also the layer formed by the reaction of the slag and the refractory material that separates and falls off during refining It was also clarified that it was mixed and suspended in the inside, and it was found that the number depends on the material of the inner surface of the ladle.

Based on the above findings, further investigations have been made on specific means for reducing hard non-metallic inclusions as much as possible. As a result, as the simplest means, molten steel produced in a converter or an electric furnace is subjected to secondary refining by receiving steel in a ladle, and the molten steel after refining is discharged from the ladle and continuously cast, When the ladle is returned to the converter or the electric furnace again and the molten steel is received in one cycle, if the ladle is specialized as described above, the alumina system is revolutionary. The present invention has been completed by finding that the number of inclusions can be reduced and spinel-type and zirconia-zircon-type inclusions which cause other wire breakages can be reduced as much as possible. The refining method is not limited in any way, and any one of an out-of-furnace refining method (a ladle refining method such as the LF method) and a circulating degassing method (RH method) centering on vacuum degassing can be used. It also includes.

In the present invention, dedicating a ladle means that
For example, when a ladle receives molten steel from a converter, it is used in a cycle in which molten steel is discharged to continuous casting through secondary refining and returned to the converter to receive the next charge of molten steel. This means that the pan is specialized for each steel type. That is, in performing the operation as described above, a plurality of ladles are combined and circulated, and a certain steel type is continuously and repeatedly operated a plurality of times, but until the repetition is completed, The ladle is operated with the same steel type received by the ladle.

The "steel type" here is basically distinguished by the chemical composition of the molten steel, but it should also be distinguished by the deoxidized state of the molten steel. Even if they are different, they can be treated as the same kind of steel if they do not carry out contradictory deoxidation. However, in the present invention, the steel type to be targeted as the case where the effect is most exerted is limited to the silicon killed steel, and the ladle that receives the silicon killed steel is the steel type that the steel type received by the previous charge is the silicon killed steel. It is operated as if it were steel or rimmed steel only.

Next, the function and effect of the more specific structure of the present invention will be described along with the background of its completion. In the present invention, when using a ladle whose refractory lining is an alumina refractory, it is preferable to operate by controlling the Al ₂ O ₃ concentration of the adhered slag on the inner surface of the ladle to 70% or less. It is a form. By adopting such a configuration, the slag on the wall surface of the ladle that is in contact with the molten steel can have a composition centered on Gehlenite, and the number of alumina-based inclusions can be dramatically reduced, and The zirconia-based and zircon-based inclusions, which are the causes, can be reduced to such an extent that no problem occurs.

Generally, the refractory used as the material for the inner surface of the ladle is desired to be a stable and strong refractory oxide from the viewpoint of its durability. However, since the origin of hard inclusions is a ladle refractory material, strong refractory oxides have an adverse effect on the wire drawability and fatigue strength of steel. is there. Therefore, we came up with the idea that a slag reaction layer should be attached to the inner surface of the ladle refractory to prevent mixing from the ladle refractory.

Based on such an idea, the present inventors further studied and found that Al in the slag adhering to the inner surface of the ladle
It has been found that if the ₂ O ₃ concentration is 70% or less, the inclusion of alumina-based inclusions can be suppressed to a minimum. In addition, in order to achieve such a structure, a non-alumina type refractory material is advantageous as a refractory material, but even when an alumina type refractory material is used, a non-alumina type such as zircon / zirconia type is used. It was also found that, as with the case of using quality, if harmless slag can be adhered stably, the difference will be less.

As a result of further studies conducted by the present inventors under these circumstances, the following findings were obtained. That is, alumina-based refractory is used as the material for the inner surface of the ladle, and CaO-
When slag refining is performed by adding a basic flux of SiO ₂ —Al ₂ O ₃ system, the oxide composition on the inner surface of the ladle is between the used slag and the refractory composition due to the reaction between the slag and the refractory material. It will change. The same applies to the case of zircon / zirconia type, which is a typical non-alumina type refractory. That is, in the case of zircon / zirconia system, the reaction layer with the slag composition has a low alumina concentration, but is in the high melting point composition region even if the reaction proceeds to a considerable extent, and the wire drawing of steel when mixed and suspended. The composition has a bad influence on workability and fatigue strength.

On the other hand, in the case of an alumina refractory, the slag composition has a basicity (CaO / SiO ₂ ): 0.5-1.
0, Al ₂ O ₃ concentration: 10% or less, the composition of the slag-refractory reaction layer has a low melting point composition of about 1300 ° C.
And in this case, since the melting point is rather low, the reaction layer formed on the steel surface also melts, peels and mixes during the next refining, and these are harmless, but they are always ladle refractory during refining. It was found that the high melting point alumina, which is the material, is exposed to the molten steel and becomes a source of inclusions of these inclusions.

In view of the above facts, in the present invention, an alumina refractory is used, and the slag composition is CaO / S.
When io ₂ : 1 to 3 and Al ₂ O ₃ concentration are 10% or less, the composition of the slag-refractory reaction layer is based on gehlenite (2CaO · S).
(iO ₂ · Al ₂ O ₃ ) is the center of the composition (more preferably, the composition is more centered on the lower alumina side than the grenite), and the melting point of this reaction layer is 1550.
The temperature is raised to about ℃, which causes melting and peeling during the next refining.
In addition to being prevented from being mixed, this composition has a composition that does not adversely affect the wire drawing workability and fatigue strength of the steel, as compared with the composition of the reaction layer of the zircon / zirconia-based refractory-slag. That is, the oxide having a grenite composition is the most harmless because it has a very low bending strength as compared with other oxides.

Further, as a concrete means for making the above-mentioned gehlenite, a ladle using a refractory material containing an alumina refractory material as a main component is used, and a top slag (upper slag) at the time of refining the ladle is used. Basic composition (CaO / Si
O ₂ ): 1-3, Al ₂ O ₃ concentration: If controlled to 10% or less, 70% Al ₂ O _{3 on the} inner surface of the ladle at the next charging stage.
The following slag reaction layers can be deposited.

However, the above-mentioned slag control is performed from the ladle.
It suffices if it is done before taking out the molten steel.
The slag composition at the time of pot refining is basic (CaO / SiO₂):
0.5-1.0, Al₂O₃Concentration: Controlled below 10%
Refining to control the composition of slag-based and deoxidized inclusions.
Not included, CaO-containing substances before the molten steel is taken out after the refining
The quality of slag is adjusted to basicity (CaO / Si
O₂): 1-3, Al₂O ₃Concentration: Controlled to within 10%
Control to increase the adhesion of slag to the ladle,
l₂O₃To attach slag with a concentration of 70% or less
You can also

According to the present invention, the concentration of Al ₂ O ₃ in the slag adhering to the inner surface of the ladle is uniformly set to 70% or less (more preferably 50% or less), and the slag is stably fed to the ladle. Wire drawing of steel is made possible by the composition that is adhered to the inner surface and the layer formed as a result of reacting the slag with the alumina refractory does not mix due to peeling, etc. It has become possible to increase workability and fatigue strength.

According to such a method of the present invention, zircon
Since no refractory such as zirconia is used as a material that comes into contact with the molten steel, other zirconia inclusions that cause disconnection can be reduced to the extent that problems do not occur.

The present invention has been made on the assumption of highly clean steel useful as a raw material for steel for tire cords, steel for high strength springs and the like, and the type of steel is not particularly limited. For example, C: 0.4 as a basic component
1.3%, Si: 0.1 to 2.5%, Mn: 0.2 to
The alloy containing 1.0% is preferable, and if necessary, other alloy elements Ni: 0.01 to 1.0%, Cu:
One containing at least one selected from the group consisting of 0.01 to 1.0% and Cr: 0.01 to 1.5% can be mentioned as a preferable steel material having enhanced cold drawability.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any modification of the design of the present invention can be made without departing from the spirit of the preceding and following paragraphs. It is included in the technical scope.

[0029]

Example The following experiment was conducted at the laboratory level.
First, 500 kg of molten steel (composition composition: C: 0.8%, Si: 0.25%, M
(n: 0.6%) was melted, steel was received in a ladle exhibiting various refractory inner surfaces, various fluxes were added to adjust components, perform electrode heating, perform argon bubbling, and perform slag refining.

The molten steel thus produced is cast through a non-alumina type tundish, and the obtained ingot is cast,
It was hot rolled into a wire having a diameter of 3 to 10 mm. While measuring the number of hard inclusions (the number of inclusions of 20 μm or more) in these wire rods, the wire rods were drawn under the same conditions to evaluate the wire drawing workability and the fatigue strength. These measuring methods and evaluation criteria are as follows.

(Number of hard inclusions of 20 μm or more) 1500 g of the hot-rolled wire to be cut is cut at intervals of about 100 g, scales are removed, and the steel is placed in a warm nitric acid solution at about 90 ° C. Dissolved in acid. This solution is sieve mesh 10 μm
The hard inclusions (alumina, zirconia, zircon) extracted on the filter paper were subjected to composition analysis by EPMA and the number thereof was counted. This was calculated by converting the number into 50 g of steel.

(Drawing workability) Evaluation of drawing workability was carried out using a test drawing machine. That is, the wire rod (5.5 mmφ) after hot rolling is subjected to primary wire drawing to 2.5 mmφ, heat treatment (air patenting), and then secondary wire drawing to 0.
8 mmφ, followed by heat treatment (lead patenting) and brass plating, wet drawing to 0.15 mmφ, and conversion to the number of wire breakages per 10 t of steel wire for evaluation.

(Fatigue Strength) Wire rod after hot rolling (5.5 m)
mφ), low temperature annealing (LA) → cold wire drawing (Drawing
4.8 mmφ) → Oil temper [Oil quenching and lead bath (about 450 ° C) tempering continuous process] → Simple strain relief annealing (blueing: about 400 ° C) → Shot peening → Strain relief annealing, and then as a test piece A 4.8 mmφ × 650 mm wire was sampled, and in a Nakamura-type rotary bending tester, test stress: nominal stress of 940 MPa, rotation speed: 4000 to 50
00 rpm, number of discontinuations: 2 × 10 ⁷ times, and among the damaged ones, breakage of inclusions, breakage rate = breakage of inclusions / (breakage of inclusions + number of cancels) × 100
It was evaluated by (%).

Example 1 The same pan was used, and the same silicon killed steel as the steel of the present invention, which was the subject steel of the present invention, was used for precharging (Experiment No.
3, 4), and rimmed steel with no deoxidizer added (Experiment N
o. 5) treated and aluminum pre-charged aluminum killed steel (Experiment No. 1), aluminum silicon killed steel (Experiment N)
o. The above experiment was conducted by comparing the case where 2) was treated.

The results are shown in Table 1 below together with the base material of the pot, the basicity of the pre-charged slag, and the concentration of alumina on the inner surface of the pot before the steel was charged this time (concentration of alumina in the adhered slag). At this time, regarding the alumina concentration on the inner surface of the pot before receiving the steel, two places on the side wall surface (the center in the height direction) before receiving the steel,
5 mm was sampled in the depth direction from one location on the center surface of the bottom portion, each sample was crushed and then measured by fluorescent X-ray analysis, and the average value of the three points was taken. As is clear from Table 1 below, by dedicating the ladle to the same steel type for pre-charging or steel without using a deoxidizer (Experiment No. 3 to 5),
It can be seen that the cleaning of the steel is highly achieved and the properties of the wire are improved.

[0036]

[Table 1]

[0037]Example 2 Slag basicity (CaO / SiO₂) 1, alumina concentration
Is kept constant at 10%, and the alumina concentration on the outermost surface before steel receiving and the steel
The relationship of material properties was investigated. The result is the base material of the pot
Quality, alumina concentration on the inner surface of the pot before steel receiving, slag basicity,
Al in slag ₂O₃It is shown in Table 2 below together with the concentration. Also,
Based on this result, the concentration of alumina on the inner surface of the pan and
Figure 1 shows the relationship between the number of hard inclusions and the number of wire breaks.
Figure 2 shows the relationship, and Figure 3 shows the relationship between the number of hard inclusions and the breakage rate.
Show each.

As is clear from these results, the number of hard inclusions was reduced by setting the alumina concentration on the inner surface of the ladle before steel receiving to 70% or less (Experiment No. 16 to 25) (steel). It is understood that the cleaning of the wire is achieved) and the characteristics of the wire are improved.

[0039]

[Table 2]

Example 3 An experiment in which the alumina concentration in the slag was set to 5% and the slag basicity was continuously changed to 2 charges under a constant condition of 90% alumina concentration on the inner surface of the pot before steel receiving ( Performed the previous charge, this time charge) and investigated the change in the alumina concentration on the inner surface of the pan, and the second charge (current charge)
Each of the above properties was evaluated for the material. The results are shown in Table 3 below together with the base material of the pot, the concentration of alumina on the inner surface of the pot before receiving steel, the slag basicity, and the Al ₂ O ₃ concentration in the slag. In addition, based on this result, the relationship between the slag basicity and the alumina concentration on the inner surface of the pot after one charge is shown in FIG.
Fig. 5 shows the relationship between the concentration of alumina on the inner surface of the pan before steel receiving and the number of hard inclusions, Fig. 6 shows the relationship between the number of hard inclusions and the number of wire breaks, and Fig. 7 shows the relationship between the number of hard inclusions and the breakage rate. , Respectively.

As is clear from these results, by setting the slag basicity to 1 to 3 (more preferably 1 to 2), the alumina concentration on the inner surface of the pot becomes 70% or less, and the steel material is highly cleaned. It can be seen that the above has been achieved and the characteristics of the wire rod have been improved.

[0042]

[Table 3]

Example 4 Refining and casting were performed under the following two conditions, and the characteristics of the obtained wire were evaluated in the same manner as above.

(Invention) Refining conditions for pre-charge were as follows: slag basicity (CaO / SiO ₂ ) = 0.75, Al ₂ O ₃ concentration in slag = 10% or less, and then calcination lime was added. Slag basicity (CaO / SiO ₂ ) = 2,
Casting after adjusting the Al ₂ O ₃ concentration in the slag to 10% or less,
After that, steel was received and CaO / SiO ₂ = 0.75, Al ₂ O ₃
It was refined and cast at a concentration of 10% or less.

(Comparative Example) The refining conditions for the precharge were set to Ca
O / SiO ₂ = 0.75, Al ₂ O ₃ = 10% or less for refining, then casting without adding burned lime, and then receiving steel, CaO / SiO ₂ = 0.75, Al ₂ O ₃ = 10%
It was refined and cast below.

The results are shown as base material of the pot, alumina concentration on the inner surface of the pot before receiving steel, basicity of slag, Al ₂ O _{3 in} slag.
Although it is shown in Table 4 below together with the concentration, even if a low basicity slag is used during refining, by adjusting the basicity of the slag after refining to 1 to 3 (more preferably 1 to 2), It can be seen that cleaning has been achieved and the properties of the wire have been improved.

[0047]

[Table 4]

[0048]

EFFECTS OF THE INVENTION The present invention is configured as described above, and hard inclusions that are inevitably mixed from ladle refractory or the like in the refining stage are reduced as much as possible, and cracks may occur during wire drawing. It was possible to realize a useful method for producing highly clean steel with excellent fatigue strength.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the concentration of surface alumina in a pot and the number of hard inclusions determined based on Example 2.

FIG. 2 is a graph showing the relationship between the number of hard inclusions and the number of disconnections determined based on Example 2.

FIG. 3 is a graph showing the relationship between the number of hard inclusions and the breakage rate obtained based on Example 2.

FIG. 4 is a graph showing the relationship between the slag basicity determined based on Example 3 and the concentration of alumina on the inner surface of a pot after one charge.

FIG. 5 is a graph showing the relationship between the concentration of alumina on the inner surface of a pan before receiving steel and the number of hard inclusions, which was determined based on Example 3.

FIG. 6 is a graph showing the relationship between the number of hard inclusions and the number of disconnections determined based on Example 3.

FIG. 7 is a graph showing the relationship between the number of hard inclusions and the breakage rate obtained based on Example 3.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Ikuo Hoshikawa             1 Kanazawa Town, Kakogawa City, Hyogo Prefecture             To Steel Works, Kakogawa Works (72) Inventor Toyoshi Takimoto             1 Kanazawa Town, Kakogawa City, Hyogo Prefecture             To Steel Works, Kakogawa Works F-term (reference) 4K013 AA07 BA14 CF01 CF13 FA05                       FA06                 4K070 AA02 AB11 AC02 AC14 AC15                       AC16 BC01 BC02 CC03 EA02                       EA03

Claims (3)

[Claims] 1. A molten steel produced by a converter or an electric furnace is received in a ladle for secondary refining, the molten steel after the refining is discharged from the ladle and continuously cast, and then the ladle is aforesaid. When repeating the operation of returning the steel to the converter or electric furnace and receiving the molten steel as one cycle, by repeatedly using a plurality of ladles in combination and repeating it repeatedly for silicon killed steel, this repetition is repeated. Until the end, the ladle that receives the silicon-killed steel is operated such that the steel type that is received by the previous charge is only silicon-killed steel or rimmed steel. . 2. When using a ladle whose refractory lining is an alumina refractory, the concentration of Al ₂ O ₃ in the adhered slag on the inner surface of the ladle is 70% or less (meaning mass%, the same applies hereinafter) or less. The manufacturing method according to claim 1, which is operated by controlling so that 3. The top slag composition is adjusted to basicity (CaO /
SiO ₂ ): 1-3 and Al ₂ O ₃ concentration: 10
The manufacturing method according to claim 2, which is controlled to be not more than%.