JP5369909B2 - Method for suppressing nozzle clogging of Zr-added steel and method for producing fine oxide-dispersed steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To surely restrain a nozzle-blockade of a Zr-added steel in a continuously casting with which dispersing state of ZrO<SB>2</SB>-inclusion in molten steel is made to the minute-granular diameter dispersion state without introducing large scaled facility and without lowering the productivity. <P>SOLUTION: In the circulative-flowing type vacuum-degassing apparatus, the oxide in the molten steel is controlled to Al<SB>2</SB>O<SB>3</SB>-Ti<SB>2</SB>O<SB>3</SB>series by deoxidizing with Al and Ti in a steel-making temperature zone, and to the molten steel having &le;0.008% total oxygen concentration, Zr is added so as to satisfy formula (1): 0.04&le; Zr adding quantity (kg)/ämolten steel weight (kg)&times;total oxygen concentration (%)}&le;0.07, and the Zr-added steel composed of 0.005-0.2% C, &le;0.5% Si, 0.5-2.0% Mn, &le;0.005 sol.Al, 0.005-0.03% Ti, 0.002-0.020% Zr, &le;0.008% O, 0.001-0.01% N and the balance Fe with impurities is made, and this Zr-added steel is cast. Thus, the nozzle-blockade in the casting is restrained. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

The present invention relates to a method for suppressing nozzle clogging of a Zr-added steel and a method for producing a fine oxide-dispersed steel. Specifically, the dispersion state of ZrO ₂ inclusions in the Zr-added steel is controlled to a fine particle size distribution. The present invention relates to a method for suppressing nozzle clogging of Zr-added steel and a method for producing fine oxide-dispersed steel that can suppress or eliminate nozzle clogging during continuous casting.

Oxide inclusions in steel are a phase different from a steel base material, which is inevitably generated in the manufacturing process of steel and dispersed in steel. Most oxide inclusions are generated in the molten steel stage and are more stable than carbides and nitrides even at high temperatures, and thus are attracting attention for application to steel quality improvement techniques. On the other hand, the oxide inclusions in the molten steel are stable even at high temperatures, and thus may adhere to the immersion nozzle during continuous casting and reduce its productivity. In particular, ZrO ₂ inclusions are liable to cause nozzle clogging during continuous casting, and a steel type containing this is known as a difficult-to-cast material.

In molten steel containing Zr, as described in paragraph 0003 of Patent Document 1, immersion nozzle clogging frequently occurs during continuous casting due to ZrO ₂ , making continuous casting difficult. As means for suppressing the clogging of the immersion nozzle, (a) blowing an inert gas into the immersion nozzle, (b) applying a flow by electromagnetic stirring, (c) oxidizing the immersion nozzle by heating from the outside It is known to prevent adhesion of objects.

  For example, in Patent Document 2, when molten steel is cast from a tundish through a sliding nozzle into a continuous casting mold, electromagnetic stirring is applied to the molten steel at the sliding nozzle plate or part thereof, and the convective molten steel is forced to flow. The invention which concerns on the method of preventing obstruction | occlusion of a nozzle by this is disclosed.

  In Patent Document 3, in the deoxidation treatment process of molten steel, after adding Al, Zr is added according to the free oxygen concentration in the molten steel before adding Al, so that coarse inclusions, particularly alumina clusters, are added. An invention relating to a method for refining a powder just before solidification is disclosed. According to this invention, the addition of Zr refines the alumina cluster, thereby preventing nozzle clogging.

Furthermore, in Patent Document 4, after performing an operation of adding one or more elements selected from elements having a stronger affinity for oxygen than Si in the molten steel to the molten steel as a deoxidizing element in the steelmaking temperature range, The operation 1 of adding an oxygen source capable of supplying oxygen to the steel and the operation 2 of adding one or more elements selected from elements having a stronger affinity than Si to the molten steel as a deoxidizing element are repeated at least once each. That is, the invention relating to a method for stably finely dispersing oxide in a large amount stably by optimizing the supply sequence and supply method of the oxygen source and deoxidizer to the molten steel and repeating these supply operations It is disclosed. According to the present invention, it is possible to disperse fine ZrO ₂ by repeating the operation of adding Zr and an oxygen source. Therefore, it is considered that continuous casting can be performed without causing nozzle clogging.

JP 2008-101259 A JP 61-60248 A Japanese Patent No. 3647969 JP 2007-162085 A

In order to actually implement the invention disclosed in Patent Document 2, a large-scale facility is required, and the facility cost increases.
According to the invention disclosed in Patent Document 3, although the alumina cluster is refined, the added Zr forms ZrO ₂ having a maximum particle size of 120 to 350 μm. Therefore, simply adding Zr makes Zr Nozzle clogging due to ZrO ₂ in the added steel cannot be suppressed.

Furthermore, in the invention disclosed in Patent Document 4, since it is necessary to perform an operation of adding Zr and an oxygen source a plurality of times, it takes time for melting and the melting cost increases.
The present invention has been made in view of these problems of the prior art, and by appropriately controlling the dispersion state of oxide inclusions in molten steel without the need for large-scale equipment, It is to provide a method for suppressing nozzle clogging of Zr-added steel and a method for producing fine oxide-dispersed steel that can suppress or eliminate nozzle clogging of Zr-added steel.

As a result of detailed investigation of the deposits on the inner wall of the immersion nozzle after casting the molten steel added with Zr, the deposits are composed of an oxide mainly composed of ZrO ₂ inclusions and a metal. Most of the oxides have a particle diameter (equivalent circle diameter) larger than about 5 μm, and few oxides having a particle diameter (equivalent circle diameter) of 1 μm or less were hardly observed. From this result, the present inventors, even ZrO ₂ inclusions considered prone to nozzle clogging, because the particle size is less fine inclusions 1μm not adhere to the immersion nozzle, ZrO ₂ in the molten steel Even if the amount of inclusions is the same, the nozzle clogging does not occur when the dispersion state of the ZrO ₂ inclusions is a fine particle size distribution, that is, the ZrO ₂ inclusions in the molten steel even if the steel is Zr-containing. It was found that casting can occur without causing nozzle clogging depending on the dispersion state.

The present inventors have intensively studied the nozzle clogging behavior of Zr-containing steel from a viewpoint different from the conventional state of the distribution of oxide inclusions, and in the molten steel stage for each of the conditions where the nozzle was not clogged and the conditions where the nozzle was clogged When the dispersion state of the ZrO ₂ inclusions in each of the slab stages was investigated, the dispersion amount of the small ZrO ₂ inclusions having a particle diameter of 1 μm or less was observed between the condition where the nozzle was not closed and the condition where the nozzle was closed. A clear difference was observed.

As a result of further intensive studies, the present inventors have obtained knowledge (1) to (3) listed below regarding the refinement behavior of ZrO ₂ inclusions in molten steel.
(1) Oxygen source Generally, oxygen sources in molten steel include dissolved oxygen, oxygen present as oxide inclusions, refractories, and oxygen in slag. When an alloying element is added to molten steel, the production path of oxide inclusions differs depending on the affinity of the alloying element with oxygen, and the contribution of the oxygen source is also different. That is, it is considered that a large amount of fine oxide inclusions are produced when new oxide inclusions are generated by addition of alloying elements using dissolved oxygen as an oxygen source. On the other hand, when oxide inclusions are used as an oxygen source, a substitution reaction that usually forms a complex oxide or replaces metal elements occurs, and the particle size distribution of the original oxide inclusions is reduced. In general, oxide miniaturization does not occur.

When the present inventors use Zr as a deoxidizer and optimize the addition conditions of Zr, oxygen in the oxide inclusions once enters the molten steel even when the oxide inclusions are used as an oxygen source. In order to follow the dissolution route, it was experimentally confirmed that the fine ZrO ₂ inclusions having a particle size of 1.0 μm or less increased rapidly. It is a phenomenon peculiar to Zr addition that the oxygen inclusion in the oxide inclusions is once dissolved. Even if Al or Ti is added instead of Zr under the same conditions, the fineness observed in Zr addition is observed. No sudden increase in oxide inclusions is observed.

For this reason, when the oxide diameter inclusions are refined by the addition of Zr, the oxygen source of the fine ZrO ₂ inclusions to be generated is an oxide system that exists in the molten steel before the addition of Zr in addition to the dissolved oxygen Since it becomes oxygen in inclusions, it is necessary to consider the total oxygen concentration before adding Zr.

(2) dispersing conditions the inventors of the ZrO ₂ inclusions, the result of study in detail with respect to factors that determine the dispersion behavior of the ZrO ₂ inclusions, miniaturization of ZrO ₂ inclusions, and the total oxygen concentration in the molten steel It has been found that the relationship with the amount of Zr to be added occurs under conditions that generally satisfy the stoichiometric ratio of ZrO ₂ . That is, when the total oxygen concentration in the molten steel is too much with respect to the amount of Zr to be added, the ZrO ₂ inclusions that are produced easily aggregate, so the effect of refining the ZrO ₂ inclusions is low. In addition, if the amount of Zr to be added is too large relative to the total oxygen concentration, a substitution reaction with the original oxide inclusions occurs, so the effect of miniaturizing the ZrO ₂ inclusions is low, When the addition amount is too small, the effect of refining ZrO ₂ inclusions is limited.

On the other hand, the present inventors have found that the amount of inclusions of ZrO ₂ inclusions is also affected by the composition of inclusions before addition of Zr, and the inclusion composition before addition of Zr contains a large amount of SiO ₂ , MnO, and the like. In this case, it was found that no new ZrO ₂ inclusions were generated via dissolved oxygen, and ZrO ₂ inclusions were generated by a substitution reaction with the existing inclusions. On the other hand, when Zr was added once and Zr was further added in a state where the inclusion composition was ZrO ₂ , the effect of generating new ZrO ₂ was low.

Therefore, the composition of inclusions before adding Zr is Al ₂ O ₃ or Ti generated by deoxidation by Al or Ti having deoxidizing power between Si and Mn and Zr in the steelmaking temperature range. to the ₂ O ₃ as a main component, i.e. it is necessary to control the molten steel oxide _Al 2 _O 3 _-Ti 2 _{O 3} system. Here, “controlling the oxide in molten steel to the Al ₂ O ₃ —Ti ₂ O ₃ system” means that the quantitative analysis result of the obtained component is converted to oxide in the semi-quantitative analysis result by EDS. In other words, inclusions in which the sum of% Al ₂ O ₃ and% Ti ₂ O ₃ exceeds 70% of the total amount is 90% or more of the number of inclusions investigated. Considering the deoxidized state before addition of Zr, it is considered that the Ti oxide exists as Ti ₂ O ₃ , so even when TiO or TiO ₂ is found as the Ti oxide, the Ti concentration is set to Ti _2. It shall be handled in terms of O ₃ concentration.

(3) Order of addition In the production of fine ZrO ₂ inclusions by Zr, it is necessary to deoxidize the molten steel with Al and Ti before adding Zr. However, once the ZrO ₂ inclusions are generated, even if an alloy having a weaker deoxidizing power than Zr is added in the steelmaking temperature range, it dissolves without forming a new oxide. The distributed state does not change.

As a result of further investigation based on these findings (1) to (3), the present inventors first deoxidized the molten steel before adding Zr with Al and Ti in order to generate fine ZrO ₂ inclusions. By making the inclusion composition in the molten steel Al ₂ O ₃ —Ti ₂ O ₃ system by optimizing the total oxygen concentration in the molten steel and adding an appropriate amount of Zr corresponding to the total oxygen concentration, and then casting The present invention has been completed by discovering that casting can be performed without causing nozzle clogging, and that it becomes possible to produce Zr-added steel, which is a fine oxide-dispersed steel.

The present invention controls the oxide in molten steel to Al ₂ O ₃ —Ti ₂ O ₃ system by deoxidizing with Al and Ti in the steelmaking temperature range in the reflux type vacuum degassing apparatus, and the total oxygen in the molten steel In a molten steel having a concentration of 0.008% or less (in this specification, “%” means “mass%” unless otherwise specified), the formula (1): 0.04 ≦ Zr addition amount (kg) / Zr is added in an amount satisfying (molten steel mass (kg) × total oxygen concentration (%)) ≦ 0.07, C: 0.005% or more and 0.2% or less, Si: 0.5% or less, Mn : 0.5% or more and 2.0% or less, sol. Al: 0.005% or less, Ti: 0.005% or more and 0.03% or less, Zr: 0.002% or more and 0.020% or less, O: 0.008% or less, N: 0.001% or more, 0 It is a Zr-added steel containing 0.01% or less, the balance being Fe and impurities, and casting this Zr-added steel.

From another point of view, the present invention controls the oxide in molten steel to Al ₂ O ₃ —Ti ₂ O ₃ system by deoxidizing with Al and Ti in the steelmaking temperature range in the reflux vacuum degassing apparatus, And, to the molten steel in which the total oxygen concentration in the molten steel is 0.008% or less, the formula (1): 0.04 ≦ Zr addition amount (kg) / (molten steel mass (kg) × total oxygen concentration (%)) ≦ 0 C: 0.005% to 0.2%, Si: 0.5% or less, Mn: 0.5% to 2.0%, by casting after adding Zr in an amount satisfying 0.07, sol. Al: 0.005% or less, Ti: 0.005% or more and 0.03% or less, Zr: 0.002% or more and 0.020% or less, O: 0.008% or less, N: 0.001% or more, 0 This is a method for producing a fine oxide-dispersed steel, characterized by producing a steel containing 0.01% or less and the balance being Fe and impurities.

According to the present invention, since the dispersion state of the ZrO ₂ inclusions in the molten steel can be made a fine particle size distribution, it is possible to continuously perform without introducing a large-scale facility and reducing productivity. The Zr-added steel, which is a fine oxide-dispersed steel, can be produced while reliably suppressing the nozzle blockage of the Zr-added steel during casting.

It is explanatory drawing which shows typically an example of the oxygen analyzer in steel used when implementing the method which concerns on this invention. It is a graph which shows the castable area | region which does not produce nozzle obstruction | occlusion in the relationship between the total oxygen concentration before Zr addition, and Zr addition amount / (molten steel mass x total oxygen concentration). Is a graph showing the frequency distribution of the particle diameter of the ZrO ₂ inclusions in cast strip.

Hereinafter, modes for carrying out the present invention will be described.
The present invention includes (i) a deoxidation step with Al and Ti in a reflux type vacuum degassing apparatus, (ii) a Zr addition step, and (iii) a casting step in this order.

(I) Deoxidation step with Al and Ti in a recirculation type vacuum degassing apparatus In the present invention, first, in a recirculation type vacuum degassing apparatus, deoxidation with Al and Ti in a steelmaking temperature range is performed to remove oxides in molten steel. The Al ₂ O ₃ —Ti ₂ O ₃ system is controlled, and the total oxygen concentration in the molten steel is 0.008% or less. Here, the “steel-making temperature range” means a temperature range in which an operation in the steel-making process is performed, and specifically means a temperature range from 1550 ° C. to 1700 ° C.

For example, using a ladle having a capacity of 300 tons, an oxygen source or a deoxidizing element is added to molten steel discharged from a converter in a reflux-type vacuum degassing apparatus.
In this melting, after a refining process such as dephosphorization or decarburization using a refining vessel such as a converter, a secondary raw material is introduced into the outgoing steel stream that is poured from the upper part of the ladle, thereby reducing weak Mn deoxidation. The Mn weak deoxidized molten steel is subjected to a reflux operation with Al addition and oxygen top blowing using a reflux-type vacuum degassing apparatus. The amount of Al added and the amount of top blown oxygen at this time are appropriately adjusted according to the molten steel temperature and oxygen concentration to be increased. By this recirculation operation, the molten steel components are adjusted to Al: 0.0003% or more and 0.0050% or less, O: 0.0020% or more and 0.0150% or less, and Al ₂ O ₃ is suspended in the molten steel. It becomes.

Here, Ti is added to the molten steel heated in the reflux type vacuum degassing apparatus. By adding Ti, inclusions in the molten steel (oxide in the molten steel) are controlled to the Al ₂ O ₃ —Ti ₂ O ₃ system. The Ti addition amount may be appropriately adjusted with 0.008% as the upper limit depending on the oxygen concentration to be controlled.

  In addition, the total oxygen concentration in the molten steel by this deoxidation process is such that the amount of Mn and Si added at the time of steel removal from the converter, the oxygen up-blowing accompanying the temperature raising operation using the oxidation heat of Al, etc. It can be adjusted by adding an alloy amount including Ti, which is performed before adding Zr. In addition, the total oxygen concentration in the molten steel can be adjusted by controlling the degree of vacuum, the flow rate of the circulation, the circulation time, and the like with a reflux type vacuum degassing apparatus.

  The total oxygen concentration in the molten steel before addition of Zr is preferably as low as possible from the viewpoint of preventing nozzle clogging, but the upper limit is set to 0.008% in consideration of improvement in steel material properties due to oxide inclusions and nozzle clogging behavior.

(Ii) Zr addition process In this invention, Zr is added to the molten steel which passed through the deoxidation process mentioned above, before ending a reflux type vacuum degassing process. In the present invention, Zr can be added as metal Zr or as an alloy with one or more other metals such as an FeZr alloy in which Zr and Fe are mixed at a certain ratio. Moreover, as a form of addition, it is possible to add as a lump, a granular form, or a wire.

  The amount of Zr added satisfies the formula (1): 0.04 ≦ Zr added amount (kg) / (molten steel mass (kg) × total oxygen concentration (%)) ≦ 0.07. Specifically, the amount of Zr added was calculated by calculating a total oxygen concentration before adding Zr by collecting a sample for rapid analysis after adding Ti and analyzing it on the spot by a rapid analyzer. It is determined by substituting this value into equation (1).

Hereinafter, this rapid analysis method will be described with reference to the drawings.
FIG. 1 is an explanatory diagram schematically showing an example of an in-steel oxygen analyzer 10 used when carrying out the method according to the present invention.

  In order to achieve a short time and high accuracy analysis required for the analysis method according to the present invention, vacuum arc plasma treatment was selected as a sample pretreatment method with high speed and high reproducibility. For example, the adjustment method and apparatus for the metal component analysis sample disclosed in Japanese Patent Application Laid-Open No. 2002-328125 may be applied. In FIG. 1, a sample is inserted into the sample pretreatment apparatus 1 that has been previously maintained in a vacuum through the isolation valve 4 from the pretreatment sample input port 3 with almost no change in the degree of vacuum. Thereafter, the oxide film on the surface of the sample is removed in a few seconds by vacuum arc plasma treatment. In this apparatus 10, in order to automatically convey a sample, the shape of the sample is a cylinder or a block (a rectangular parallelepiped). Since the sample is placed on the sample stage and processed, the surface in contact with the sample stage is not processed. Therefore, it is necessary to invert the sample for processing. That is, it is necessary to discharge at least twice for one sample. When the number of discharges increases, the sample is heated for a long time, and once the surface of the sample from which the oxide film has been removed is oxidized again. Therefore, arc plasma treatment is performed under the following conditions (a) to (d) in order to remove the oxide film on the surface of the sample reliably, accurately and with good reproducibility, and to ensure the analysis accuracy required for the refining operation.

  (A) Degree of vacuum: 5 Pa or more and 35 Pa or less. The higher the degree of vacuum, the more the oxide film removal reaction on the surface of the sample by the vacuum arc plasma is promoted. However, when the degree of vacuum exceeds 35 Pa, the reoxidation reaction accompanying the temperature rise of the sample becomes remarkable, which is not preferable. On the other hand, when the degree of vacuum is lower than 5 Pa, the oxide film removal reaction itself does not proceed, which is not preferable. It is more preferable to have a pressure control mechanism for controlling the opening and closing of the vacuum exhaust valve and the gas introduction valve so that the degree of vacuum is maintained at a constant value during processing.

(B) Arc plasma output current: 15A or more and 55A or less.
(C) Processing time: The total processing time is 0.2 seconds or more and 1.2 seconds or less for one sample.

(D) Number of treatments: The total number of treatments for one sample is 4 or less.
The treated sample is finally put into the graphite crucible through the pretreated sample inlet 5 arranged in the analyzer 2 without being brought into contact with the atmosphere. The sample pretreatment chamber and the sample inlet of the analyzer are connected by a connecting tube 8 whose inside is replaced with vacuum or an inert gas. As the inert gas, Ar is selected from the viewpoint of surely replacing the gas in the connecting pipe 8 in consideration of the specific gravity difference with air to prevent reoxidation of the sample after processing, and from an economical viewpoint. preferable. In the apparatus configuration disclosed in Japanese Patent Laid-Open No. 2002-328125, the pretreated sample is discharged and then transferred to a separate oxygen analyzer. However, since the object of the present invention requires quickness, the sample pretreatment device 1 and the oxygen analyzer 2 are arranged vertically above and below, and freely fall in the connecting tube 8 to transfer the sample. That is, the apparatus configuration shown in FIG. 1 was adopted.

  In this apparatus configuration, the oxygen analyzer 2 is disposed at a position close to the floor surface, and cleaning inside the analyzer 2 hinders the maintenance work of the apparatus such as replacement of the impurity adsorbent in the gas. Therefore, the entire apparatus incorporated in the gantry 6 is placed on the lifter 7 so that it can be raised and lowered. During this operation, the entire apparatus is raised to ensure workability. The driving method of the lifter 7 is not particularly limited. However, since the weight of the entire apparatus is considerable, an automatic hydraulic type is preferable from the viewpoint of operability. Moreover, it is desirable to cover the movable part of the lifter 7 with a stretchable material and to have a structure in consideration of safety so that an operator is not caught.

  Further, the sample pretreatment device 1 and the oxygen analyzer 2 are connected in preparation for the case where the connected oxygen analyzer 2 cannot be used due to a failure or when a preprocessed sample waiting for analysis is analyzed by another oxygen analyzer. An outlet 9 for a pretreated sample is provided in the middle of the tube 8.

  In addition, as a method for obtaining an analysis sample more easily and quickly than a steel ingot collected from molten steel, a height (thickness) produced by cutting a steel ingot collected from molten steel is 1.5 mm to 7 mm. A punched cylindrical piece is used as a sample. Specifically, for example, an analysis sample adjustment method and apparatus disclosed in JP-A-10-311782 may be applied. In order to remove the oxide film on the sample surface reliably, accurately and with good reproducibility, the ratio S / V of the surface area S to the volume V calculated from the diameter and height of the bottom surface of the sample satisfies the following formula (2). Ensure such a shape.

1.05 ≦ S / V ≦ 1.30 (2)
Furthermore, as a highly accurate method for analyzing oxygen in steel, an oxygen analyzer based on the principle of heating and melting in an inert gas-infrared absorption method was selected. In this analysis method, a graphite crucible serving as a sample holder and a sample deoxidation reagent (carbon) supply source is used.

  Prior to analysis, in order to remove oxygen and contamination adsorbed on the surface of the crucible, a so-called baking process is performed in which only the crucible is heated in advance at a slightly higher temperature than during the analysis. By this baking process, the influence of oxygen, carbon monoxide or carbon dioxide generated from the graphite crucible changing the analytical value can be reduced. When analyzing oxygen in steel with a commercially available oxygen analyzer, a crucible, that is, a sample is usually heated to a temperature of about 1800 ° C. to 2200 ° C. In order to realize the high analysis accuracy required in the present invention, for example, the heating may be performed at a temperature 100 ° C. or more higher than the temperature at the time of analysis and for 15 seconds or more.

  In addition, with a commercially available oxygen analyzer, first, the sample is taken into the analyzer, and the atmosphere around the sample is replaced with the helium gas, which is a carrier gas. To do. Therefore, it takes a relatively long time until the analytical value is determined after the sample is introduced. Replacing the crucible, cleaning the electrodes, and performing the baking process in advance, and introducing the sample that has been pre-cleaned and ready for analysis by the analyzer, speeds up the required analysis time. Can be realized.

  Usually, in the oxygen analysis, in order to convert the detected gas amount into the oxygen concentration in the sample, it is necessary to accurately weigh the sample. As a result of evaluating the change in mass of the sample before and after the vacuum arc plasma treatment, although there was some variation depending on the shape of the sample and the degree of surface oxidation, the weight loss was about 1 mg at most, so the sample weight was 0.5 to 1.0 g. It has been found that the error is negligible for practical use. Therefore, when carrying out the present invention, an analytical sample obtained by machining and previously weighed is subjected to a vacuum arc plasma treatment and directly inserted into an oxygen analyzer without being brought into contact with the atmosphere.

Next, the reasons for limiting the total oxygen concentration before adding Zr, the added amount of Zr, and the mass of molten steel will be described.
FIG. 2 is a graph showing a castable region in which nozzle clogging does not occur in the relationship between total oxygen concentration before Zr addition and Zr addition amount / (molten steel mass × total oxygen concentration).

  In the present invention, the total oxygen concentration before Zr addition and the amount of Zr addition / (molten steel mass × total oxygen concentration) are inseparable, and if neither of them is satisfied, fine dispersion of the oxide occurs. Nozzle blockage occurs.

That is, even if the Zr addition amount is appropriate, if the total oxygen concentration before Zr addition is high, nozzle clogging occurs. At this time, it can be seen that many ZrO ₂ inclusions adhere to the inner wall of the closed nozzle, and if the total oxygen concentration is too high, deterioration of castability cannot be prevented. For this reason, the upper limit of the total oxygen concentration before addition of Zr is set to 0.008%.

Further, even when the total oxygen concentration before the addition of Zr was appropriate, the nozzle was clogged even when the amount of Zr addition was large.
Further, when the amount of Zr added was small, the nozzle was clogged or the nozzle was clogged. It can be seen that many ZrO ₂ inclusions adhere to the inner wall of the nozzle at this time, and the effect of refining ZrO ₂ inclusions due to the addition of Zr is not sufficiently exhibited.

For the above reasons, in the present invention, the value {Zr addition amount (kg) / (molten steel mass (kg) × total oxygen concentration (mass%))} is set to 0.04 or more and 0.07 or less.
The following describes the frequency distribution of the particle diameter of the ZrO ₂ inclusions present invention and comparative example embodiment. FIG. 3 is a graph showing the frequency distribution of the particle diameter of the ZrO ₂ inclusions in the slab for the inventive example and the comparative example in the graph of FIG.

As shown graphically in FIG. 3, in the present invention example, 1.0 .mu.m in the following areas higher number density of ZrO ₂ inclusions, it can be seen that ZrO ₂ inclusions are finely dispersed. This indicates that the dispersion state of the ZrO ₂ inclusions is in a fine particle size distribution state, that is, a dispersion state in which the number of ZrO ₂ inclusions larger than 5 μm causing nozzle clogging is small by the manufacturing method according to the present invention. Is.

When the particle size distributions of the ZrO ₂ inclusions of the present invention example and the comparative example are compared, when the ZrO ₂ inclusions as seen in the present invention example are miniaturized, the number density in a fine region of 1 μm or less The increase is remarkable. For this reason, whether or not the ZrO ₂ inclusion distribution is a fine particle size distribution is determined by the number density of the region of 1.0 μm or less. In addition, in addition to the fact that fine ZrO ₂ inclusions of 0.5 μm or less are considered to be irrelevant to the nozzle clogging behavior, the measurement conditions with a microscope are taken into consideration, and in the present invention, 0.5 μm or more and 1.0 μm or less. The number density of ZrO ₂ inclusions is counted.

  In addition, before and after the addition of Zr, the oxide dispersion state can be confirmed by collecting a molten steel sample in a reflux-type vacuum degassing apparatus for adding Zr and a tundish as the next step, and using the accuracy method shown below. .

[Accuracy method]
With respect to the collected sample, the composition of the oxide is examined by a scanning electron microscope and an energy dispersive X-ray microanalyzer attached thereto, and the number of oxide inclusions is examined by a confocal laser microscope.

The composition of the oxide is examined by mirror-polishing the sample surface and randomly extracting 10 oxide inclusions in the center of the polished surface per sample.
The composition of inclusions before addition of Zr is the total amount of% Al ₂ O ₃ and% Ti ₂ O ₃ when the quantitative analysis results of the obtained constituent components are converted into oxides in the semi-quantitative analysis results by EDS. The inclusion composition is defined as Al ₂ O ₃ —Ti ₂ O ₃ inclusions when the inclusions exceeding 70% of the total number is 90% or more of the number of inclusions investigated. In consideration of the deoxidized state before the addition of Zr, the Ti oxide is considered to exist as Ti ₂ O ₃ , so even when TiO or TiO ₂ is found as the Ti oxide, the Ti concentration is set to Ti _2. It shall be handled in terms of O ₃ concentration.

On the other hand, the inclusion composition after the addition of Zr was investigated in the semi-quantitative analysis result by EDS when inclusion of% ZrO ₂ exceeding 90% was obtained when the quantitative analysis result of the obtained component was converted to oxide. When the number of inclusions is 90% or more, the inclusion composition is defined as ZrO ₂ . In the present invention, the inclusion composition after addition of Zr is all ZrO ₂ .

The number of oxides is determined by counting the number of inclusions present in the center of the mirror-polished sample using a confocal laser microscope using a violet laser having a wavelength of 408 nm as a light source. In this measurement by a microscope, a field of view of 128 μm wide and 96 μm long can be photographed as a digital image of 1024 pix × 768 pix by using a 100 × objective lens. The area of one pixel under this photographing condition is 0.015625 μm ² , and an element composed of 13 pixels or more corresponding to a circle-equivalent diameter of 0.5 μm existing in one visual field is determined as an inclusion. At this time, those composed of 12 pixels or less are excluded because they may be an indivisible wrinkle or dirt from the oxide. In addition, oxides such as Al ₂ O ₃ —Ti ₂ O ₃ and ZrO ₂ are observed at 0 to 50 gradations in a 256 gradation gray scale depending on reflection luminance when measured with a confocal laser microscope. The sulfides and nitrides measured simultaneously are observed with about 80 to 120 gradations depending on the difference in reflection luminance, while the matrix (spectroscopic surface) is observed with 180 gradations or more. Since these can be easily separated by using image processing software, it is possible to count oxides including Al ₂ O ₃ —Ti ₂ O ₃ before addition of Zr, and after addition of Zr. Only ZrO ₂ can be counted. In addition, the measurement visual field is obtained by measuring 100 visual fields of 0.0123 mm ² per field, that is, measuring 1.23 mm ² continuously with an electric stage, and processing an image obtained by image processing software. The number of oxide inclusions of 0.5 μm or more is counted.

(Iii) Casting step After Zr is added in the Zr addition step described above, components such as Mn are finely adjusted as necessary, so that C: 0.005% or more and 0.2% or less, Si: 0.5% or less, Mn: 0.5% or more and 2.0% or less, sol. Al: 0.005% or less, Ti: 0.005% or more and 0.03% or less, Zr: 0.002% or more and 0.020% or less, O: 0.008% or less, N: 0.001% or more, 0 A Zr-added steel containing 0.01% or less, the balance being Fe and impurities, and by casting this Zr-added steel in a continuous casting step, a continuous cast slab having a predetermined thickness (for example, 250 mm) Is done.

  The reason for limiting the composition of this Zr-added steel in the present invention will be described. The Zr and O contents defined in the present invention mean the contents in the slab stage after casting in the casting process, and the O content before adding Zr is 0.008 as described above. % Or less.

[C: 0.005% to 0.2%]
C is contained in an amount of 0.005% or more in order to ensure the strength of the base material. However, if the C content is too large, the toughness at the time of welding is remarkably lowered, so that it is 0.2% or less.

[Si: 0.5% or less]
Si can be used for preliminary deoxidation of molten steel, and is useful for securing the strength of the base material. However, if the Si content is too large, the weldability deteriorates, so the content is made 0.5% or less.

[Mn: 0.5% to 2.0%]
Mn can be used for preliminary deoxidation of molten steel and is indispensable for ensuring the strength of the base material, so it is contained in an amount of 0.5% or more. However, if the Mn content is too large, problems such as promoting the center segregation of the slab occur, so the content is made 2.0% or less.

[Sol. Al: 0.005% or less]
Al is an important element having a strong deoxidizing action. However, when a large amount of Al is contained, an Al ₂ O ₃ cluster that promotes nozzle clogging is formed, and this cluster may remain even when Zr is added. The Al content is 0.005% or less.

[Ti: 0.005% to 0.03%]
Ti is an element having a strong deoxidizing action like Al, and precipitates as TiN in the steel to improve the inertia during welding. For this reason, Ti content shall be 0.005% or more. On the other hand, if the Ti content is too large, the solid solution Ti increases and the inertia decreases, so the content is made 0.03% or less.

[Zr: 0.002% to 0.020%]
Zr is one of the most important elements in the present invention and is an indispensable element for oxide miniaturization. ZrO ₂ is useful as a nucleus for formation of intragranular ferrite, and can improve the fertility of the steel material. In order to acquire this effect, it contains 0.002% or more. On the other hand, if Zr is too much, the cleanliness of the steel material is worsened, so the content is made 0.020% or less.

[O (oxygen): 0.008% or less]
O, along with Zr, is one of the most important elements in the present invention, and is directly related to the oxide dispersion amount and size. In the present invention, although a certain amount of O needs to be contained in the steel material for the formation of fine oxides, if O is contained more than necessary, the cleanliness of the base material is deteriorated, so the O content is 0.008% or less. Since the present invention controls the dispersion state of the ZrO ₂ inclusions to a state in which the particle size distribution is fine, the O content may be small, but there is a limit to industrial reduction. From such a viewpoint, the O content in the present invention is desirably 0.003% or more and 0.005% or less.

[N: 0.001% to 0.01%]
If N is too small, TiC that deteriorates fertility is generated, so the N content is 0.001% or more. On the other hand, if the amount of solute N is too large, the fertility is lowered, so the N content is 0.01% or less.

  Each of the above-described elements is an element related to deoxidation that is directly related to the clogging behavior of the nozzle. In addition to these elements, P and S are inevitably included as impurities in the actual steel material. Contained. Hereinafter, P and S will be described.

[P: 0.04% or less]
P is an impurity element inevitably mixed in the steel material. Although it is preferable from the viewpoint of the characteristics of the material, it is preferable to make the P content 0.04% or less in consideration of productivity because the extreme reduction is accompanied by a corresponding increase in cost.

[S: 0.01% or less]
S is also an impurity element inevitably mixed in the steel material. The S content is preferably as small as possible because the segregation rate is high. However, since the extreme reduction is accompanied by a corresponding increase in cost, the S content is set to 0.01% or less in consideration of productivity.

  The fine oxide-dispersed steel produced according to the present invention can be one or two selected from Cr, Ni, Cu, Mo, V, Nb and B as an optional additive element, instead of a part of Fe. It may contain seeds or more, and by containing these elements, mechanical properties such as strength or toughness can be improved. These optional additive elements will be described.

[Cr: 1% or less]
Cr improves the hardenability of the base material. In order to acquire this effect, it is desirable to make it contain 0.05% or more. However, if the Cr content exceeds 1%, the inertia is lowered, so the Cr content is 1% or less.

[Ni: 6% or less]
By adding an appropriate amount of Ni, the base material strength and the inertia are improved without degrading the weldability. If the Ni content is 0.1% or more, the effect of improving hardenability can be obtained in addition to the above-described effects. Therefore, it is desirable to contain Ni by 0.1% or more. However, since Ni is very expensive and excessive addition loses economic efficiency, the Ni content is set to 6% or less.

[Cu: 1.5% or less]
Cu improves the base material strength without deteriorating the base material inertia. In order to acquire this effect, it is preferable to contain 0.1% or more. However, if the Cu content exceeds 1.5%, the hardenability of the steel is excessively increased and the inertia and the like are deteriorated, so the Cu content is set to 1.5% or less.

[Mo: 0.8% or less]
Mo improves the base material strength and inertia. In order to acquire this effect, it is preferable to make it contain 0.05% or more. However, if the Mo content exceeds 0.8%, the weld is excessively cured, so the Mo content is set to 0.8% or less.

[V: 0.1% or less]
V improves the strength of the base metal by precipitating carbonitride during tempering. In order to acquire this effect, it is preferable to contain 0.005% or more. However, if the V content exceeds 0.1%, the performance improvement effect of the base material strength is saturated and the cost increases, so the V content is set to 0.1% or less.

[Nb: 0.05% or less]
Nb refines the base material structure and improves the mechanical properties of the base material. In order to acquire this effect, it is preferable to contain 0.004% or more. However, if the Nb content exceeds 0.05%, the base material inertia deteriorates, so the Nb content is set to 0.05% or less.

[B: 0.002% or less]
B improves hardenability and improves the mechanical properties of the base material. In order to acquire this effect, it is preferable to contain 0.0003% or more. However, if the B content exceeds 0.002%, characteristics such as weldability deteriorate, so the B content is set to 0.002% or less.

The balance other than those described above is Fe and impurities.
As described above, the Zr-added steel produced by the present invention is a fine oxide-dispersed steel in which fine ZrO ₂ is dispersed, and is mainly used as a thick steel plate. The main applications of this steel plate are suitable for building materials, shipbuilding and offshore structures.

According to the present invention, since the dispersion state of the ZrO ₂ inclusions in the molten steel can be made a fine particle size distribution, it is possible to continuously perform without introducing a large-scale facility and reducing productivity. Fine oxide-dispersed steel can be produced while reliably suppressing nozzle blockage of the Zr-added steel during casting.

The present invention will be described more specifically with reference to examples.
Using a ladle with a capacity of 300 tons, Zr was added using a reflux vacuum degassing apparatus to produce fine oxide dispersed steel. Table 1 shows the amount of molten steel in this example, the molten steel temperature before adding Zr, the total oxygen concentration before adding Zr, the composition of the oxide before adding Zr, the amount of Zr added (kg) / the weight before adding Zr (molten steel weight). (Kg) × total oxygen concentration (mass%)) in the sample collected by tundish, that is, {(number density of ZrO ₂ inclusions of 0.5 to 1.0 μm) / (1.0 μm in the molten steel stage) Ratio of ZrO ₂ inclusions exceeding}, castability, and {(number density of ZrO ₂ inclusions of 0.5 to 1.0 μm) / (ZrO ₂ inclusions exceeding 1.0 μm) (Number density of objects)}. The amount of Zr added, the total oxygen concentration, the oxide composition, and the number density were determined by the method described above.

Table 2 shows the chemical composition before Zr addition, and Table 3 shows the chemical composition of the slab.
The “Zr addition amount” in Table 1 indicates the mass actually added to the molten steel as a pure Zr component in consideration of the quality of the alloy and the addition yield. “Castability” was determined by monitoring the casting speed and the sliding nozzle opening. That is, the casting speed at the beginning of casting is set to 1.0 m / min, and the case where the casting can be completed without lowering the casting speed by 30% or more from the casting speed at the start of casting by adjusting the opening of the sliding nozzle or the like is defined as “no nozzle clogging”. When the casting speed was reduced by more than 30% from the casting start time, but the casting was completed as “closed”, and the casting had to be interrupted even when the sliding opening was maximized. Was “nozzle blockage”.

  Test numbers 1 to 6 in Table 1 are test results of the present invention examples that satisfy all the requirements defined in the present invention, and test numbers 7 to 10 are test results of comparative examples that do not satisfy the requirements defined in the present invention. is there.

As shown in Tables 1 to 3, since test numbers 1 to 6 are suitable for inclusion composition, total oxygen concentration, and Zr addition amount before Zr addition, the dispersion state of ZrO ₂ accompanying Zr addition is the particle size. However, the fine particle size distribution was controlled, and nozzle clogging did not occur.

On the other hand, in Test Nos. 7 to 10, since the total oxygen concentration before Zr addition or the Zr addition amount deviates from the conditions defined in the present invention, the ZrO ₂ inclusions are not finely dispersed even when Zr addition is added. Castability deteriorated.

From the above results, {(number density of ZrO ₂ inclusions of 0.5 to 1.0 μm) / (number density of ZrO ₂ inclusions exceeding 1.0 μm) at the molten steel stage of the molten steel containing Zr in the present invention] } Is 2.0 or more, and {(number density of ZrO ₂ inclusions of 0.5 to 1.0 μm) / (1. The ratio of the number density of ZrO ₂ inclusions exceeding 0 μm)} was found to be 1.75 or more.

According to the present invention, since the dispersion state of the ZrO ₂ inclusions in the molten steel can be made a fine particle size distribution, it is possible to continuously perform without introducing a large-scale facility and reducing productivity. It can be seen that the fine oxide-dispersed steel can be produced while reliably suppressing nozzle blockage of the Zr-added steel during casting.

DESCRIPTION OF SYMBOLS 1 Pretreatment apparatus 2 Oxygen analyzer 3 Pretreatment sample inlet 4 Isolation valve 5 Pretreatment specimen inlet 6 Base 7 Lifter 8 Connection pipe 9 Pretreatment specimen intermediate outlet

Claims (2)

In the reflux-type vacuum degassing apparatus, the oxide in the molten steel is controlled to be Al ₂ O ₃ —Ti ₂ O ₃ system by deoxidizing with Al and Ti in the steelmaking temperature range, and the total oxygen concentration in the molten steel is set to 0. Zr in an amount satisfying the formula (1) is added to molten steel set to 008% by mass or less, and C: 0.005% or more and 0.2% or less, Si: 0.5% or less, Mn: 0 by mass%. .5% to 2.0%, sol. Al: 0.005% or less, Ti: 0.005% or more and 0.03% or less, Zr: 0.002% or more and 0.020% or less, O: 0.008% or less, N: 0.001% or more, 0 A method for suppressing nozzle clogging of a Zr-added steel, characterized in that the Zr-added steel contains 0.01% or less, the balance being Fe and impurities, and the Zr-added steel is cast.
0.04 ≦ Zr addition amount (kg) / (molten steel weight (kg) × total oxygen concentration (mass%)) ≦ 0.07
(1) In the reflux-type vacuum degassing apparatus, the oxide in the molten steel is controlled to be Al ₂ O ₃ —Ti ₂ O ₃ system by deoxidizing with Al and Ti in the steelmaking temperature range, and the total oxygen concentration in the molten steel is set to 0. By casting after adding Zr in an amount satisfying the formula (1) to molten steel with 008% or less, C: 0.005% or more and 0.2% or less, Si: 0.5% or less , Mn: 0.5% to 2.0%, sol. Al: 0.005% or less, Ti: 0.005% or more and 0.03% or less, Zr: 0.002% or more and 0.020% or less, O: 0.008% or less, N: 0.001% or more, 0 A method for producing a fine oxide-dispersed steel, comprising producing 0.01% or less of steel, the balance being Fe and impurities.
0.04 ≦ Zr addition amount (kg) / (molten steel weight (kg) × total oxygen concentration (mass%)) ≦ 0.07
(1)

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