JP2010280933A - Method for manufacturing calcium-treated steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a HIC (hydrogen induced cracking) resistant steel, having inclusion composition controlled at high grade. <P>SOLUTION: Before completing an RH-treatment of the refined molten steel, mass-concentration of S contained in this molten steel is set to ≤10 ppm and mass-concentration of T.[O] is set to ≤40 ppm, and based on the T.[O] analyzed value in the molten steel after completing this RH-treatment, Ca mass-quantity added into this molten steel, is adjusted so as to satisfy an expression: A(kg/t)=B×T.[O]+0.02 and an expression: 0.003≤B≤0.006. Wherein, A: Ca additional mass-quality (kg/t), B: coefficient and T.[O]: oxygen analyzed value (ppm) in the molten steel before adding Ca after completing the RH-treatment. Before completing the RH-treatment by refining the molten steel, the components contained in this molten steel, may have mass-concentration of 0.03-0.07% C and 1.1-1.5% Mn. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for producing Ca-treated steel having HIC resistance.

In recent years, the quality requirements of steel materials have become stricter, but the demand for steel materials with high HIC cracking susceptibility has increased as the use environment has become severe and the strength has increased, particularly in HIC-resistant steels represented by line pipes. ing. HIC is generated starting from MnS-based inclusions and linear crushed inclusions that are elongated during rolling. MnS inclusions are produced by the reaction of Mn and S in the steel segregated during solidification. In order to suppress the formation of MnS, it is important to lower S in steel in the molten steel stage, and to fix S by using Ca as a CaO—Al ₂ O ₃ —CaS system by adding Ca. The linearly crushed inclusions are Al ₂ O ₃ cluster inclusions generated during deoxidation of the molten steel. The _Al 2 _{O 3} is the cluster inclusions suppressing the molten steel in the _Al 2 _{O 3} clusters with _CaO-Al 2 _{O 3} system by the addition of Ca, it is important to change the spherical its form from the cluster. CaO—Al ₂ O ₃ inclusions are not crushed during rolling due to their spherical shape and low melting point. From these facts, the addition of Ca to molten steel is effective in reforming Al ₂ O _{3 in} molten steel and suppressing the formation of MnS, and is an important process for producing a steel material having excellent HIC resistance.

However, even if the inclusions are CaO-Al ₂ O ₃ -CaS system and spherical, if the composition is high CaS and high CaO concentration, the inclusions are crushed and cracked during rolling. Need to control.

Many Ca addition methods have been proposed so far.
In Patent Document 1, the following two methods are shown as specific methods for determining the Ca addition amount according to the formulas (a) and (b).

SCI = ([Ca] + 0.0095 × [C]) / ([O] +0.6 [S] × [Mn]) (...)
0.8 ≦ SCI ≦ 2.3 (...)
The first method is a method of analyzing the steel composition after operation and manufacturing, accumulating data in advance, and determining the addition amount of Ca from the obtained results. This method has the advantage of not requiring special analysis equipment. The second method is a method in which the molten steel composition is quickly analyzed in the middle of the steelmaking process, and the amount of Ca added is determined for each process from the formulas (a) and (b) and the analysis results. This method has the advantage that Ca treatment can be performed accurately while avoiding external factors that occur during the manufacturing process.

However, the inclusion composition of the steel material obtained by this method is such that the composition of CaO—Al ₂ O ₃ —CaS inclusions contained in the steel is CaO: 30 to 60% by mass%, CaS: less than 10%, and the balance : a _Al 2 _{O 3} principal composition relatively Flexible. In addition, even in the second method, there is only an explanation that “the rapid analysis of the molten steel composition is performed during the steelmaking process”, so the accuracy of the [O] concentration used in the calculation of the formula (a) is However, there is still room for improvement.

Patent Document 2 discloses a method for controlling the Ca concentration so as to satisfy the formulas (c) and (d).
[% Ca] x [% S] ^0.28 ≤ 3.5 x 10 ^-4 (...)
1 ≦ {[% Ca] − (0.18 + 130 [% Ca]) × [% O]} / 1.25 / [% S] (D)
When adding Ca-Si wire, in order to keep the Ca, S, and O components in the steel within the target range, the S component concentration after the desulfurization treatment is investigated by rapid analysis, and the O component concentration after the degassing treatment is set to 15 ppm, for example. It is assumed that the input amount is determined by determining the target calcium composition range from the above formulas (c) and (d).

However, inclusions in steel aimed by the present invention have a CaO concentration of 40 to 80% and a relatively wide composition range. In the background, it can be inferred that there is no means for accurately obtaining the O concentration to be substituted in the equation (d), and that “15 ppm as an average value” had to be used as described above.
JP-A-9-227989 JP 2001-11528 A JP 2002-328125 A JP-A-10-311782

  As described above, there are many inventions in which an appropriate amount of Ca is added in accordance with the component concentration in molten steel to control the composition of inclusions to be generated. However, in any of the inventions, it is necessary to know the O concentration before addition of Ca. However, in any of the inventions, a method for accurately obtaining the O concentration is not shown. This is due to the fact that a method for accurately obtaining the O concentration that needs to be known for the Ca treatment has not been established until now.

  As a result, the inclusion composition in each invention can be controlled only in a relatively wide range such as 30 to 60% and 40 to 80%, and it can be applied as it is to melting HIC steel. The expected effect could not be achieved.

  Therefore, an object of the present invention is to provide a method for producing a Ca-treated steel having a highly controlled inclusion composition and excellent HIC resistance.

In order to solve these problems, it is necessary to take a sample from the molten steel before Ca addition, analyze T. [O] before Ca addition, and determine an appropriate Ca addition amount.
As a method for analyzing T. [O] in a short time and with high accuracy, the following analysis method was used.

  (i) A method in which a steel sample is placed in a graphite crucible and heated and melted in an inert gas, and the oxygen concentration in the sample is measured from the infrared absorption of one or both of the generated carbon monoxide and carbon dioxide. Use.

  (ii) Vacuum arc plasma treatment as a pretreatment to remove and clean the oxide film on the sample surface, the degree of vacuum at the start of arc plasma discharge is 5 Pa to 35 Pa, and the arc plasma output current is 15 A to 55 A Apply under conditions.

  (iii) A steel ingot taken from molten steel is machined so that the height is 1.5 mm or more and 7 mm or less and the ratio of surface area S to volume V (S / V) is 1.05 or more and 1.30 or less. A small piece obtained in this way is used as a sample.

(iv) After applying the arc plasma discharge to the sample a total of 4 times or less and a total treatment time of 0.2 seconds or more and 1.2 seconds or less,
The sample is directly heated and cleaned at a temperature higher than the temperature at the time of analysis without being brought into contact with the atmosphere, and then put into a graphite crucible that has been lowered to the temperature to be analyzed and placed on standby.

By applying this analysis method online, the T. [O] concentration before Ca treatment can be ascertained before Ca addition, and an appropriate amount of Ca added can be determined.
Specifically, it has become possible to analyze the T. [O] concentration with the accuracy of the following analysis value and the time required for the analysis.

(1) Accuracy of analysis value The error must be within ± 2 ppm for steel with an oxygen content of 50 ppm or less. The error is preferably within ± 1 ppm.

(2) Time required for analysis The time (hereinafter referred to as “required analysis time”) from when the steel ingot sample is received, through sample processing and pretreatment for cleaning, until the oxygen concentration is determined by analysis is 5 Less than a minute. Preferably it is 4 minutes or less.

Hereinafter, this analysis method will be described in detail with reference to the drawings.
FIG. 1 schematically shows a steel oxygen analyzer for carrying out the analysis 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 rapid and reproducible sample pretreatment method among the elemental technologies combined in the present invention. For example, the preparation method and apparatus for a sample for analyzing an in-metal component disclosed in Patent Document 3 may be applied. A sample can be inserted into the sample pretreatment apparatus 1 that has been previously kept in vacuum through the isolation valve 4 through the isolation valve 4 with almost no change in the degree of vacuum. Thereafter, the oxide film on the sample surface is removed in a few seconds by vacuum arc plasma treatment. In this apparatus, since the sample is automatically conveyed, the sample shape is limited to 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 oxide film is removed, the sample surface is oxidized again. Therefore, in order to remove the oxide film on the sample surface reliably, accurately and with good reproducibility, and to ensure the analytical accuracy required for the refining operation, it is necessary to perform an arc plasma treatment under the following conditions.

(a) Degree of vacuum: 5 Pa or more and 35 Pa or less. The sample surface oxide film removal reaction by vacuum arc plasma is promoted as the degree of vacuum increases, but if it exceeds 35 Pa, the reoxidation reaction accompanying the increase in the sample temperature becomes remarkable, which is not preferable. On the other hand, if it is lower than 5 Pa, the oxide film removal reaction itself does not proceed, which is not preferable. There is therefore an optimum degree of vacuum.
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 second or more and 1.2 seconds or less for one sample.

(d) Number of treatments: The total number of treatments per 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 species, Ar is preferable from the viewpoint of reliably replacing the gas in the connecting pipe in consideration of the specific gravity difference with air and preventing reoxidation of the sample after processing, and from an economical viewpoint. . In the apparatus configuration disclosed in Patent Document 3, the pretreated sample is dispensed 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, an apparatus configuration as shown in FIG. 1 was adopted.

  In the apparatus configuration of the present invention, the oxygen analyzer 2 is disposed at a position close to the floor surface, and cleaning inside the analyzer 2 hinders 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, and during the 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.

  Among the elemental technologies combined in the present invention, as a method for obtaining an analysis sample more easily and quickly than a steel ingot collected from molten steel, the height (thickness) produced by cutting the steel ingot collected from molten steel is 1.5 mm or more. A punched cylindrical piece is used as a sample for a slice of 7 mm or less. Specifically, for example, an analysis sample adjustment method and apparatus disclosed in Patent Document 4 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 sample bottom is “1.05 ≦ S / V ≦ It is necessary to ensure a shape satisfying 1.30 ".

  The reason for this is not fully understood at this time, but it corresponds to the fact that the position suitable for efficient processing is limited in the spatial distribution of the arc plasma depending on the shape of the arc processing part such as the electrode shape. It is guessed.

  Among the elemental technologies combined in the present invention, an oxygen analyzer based on the operating principle of heating and melting in an inert gas-infrared absorption method was selected as a highly accurate method for analyzing oxygen in steel. 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 “empty baking” process is performed in which only the crucible is preheated at a temperature slightly higher than at the time of analysis. The “blank” treatment can reduce the influence of oxygen, carbon monoxide or carbon dioxide generated from the graphite crucible changing the analytical value. 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 that is 100 ° C. or more higher than the temperature at the time of analysis and for 15 seconds or more.

  In addition, in a commercially available oxygen analyzer, first, a sample is taken into the analyzer, and the atmosphere around the sample is replaced with helium gas as a carrier gas. carry out. 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 “blank” process in advance, and loading the sample that has been pre-cleaned in a state where the analyzer can analyze, according to the required analysis time. Speeding up 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 weight. As a result of evaluating the change in the sample weight 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 reduced to 0.5 to 1.0 g. On the other hand, it has been found that the error is negligible for practical use. Therefore, when carrying out the present invention, the analysis sample obtained by machining and previously weighed was subjected to vacuum arc plasma treatment and inserted into the oxygen analyzer as it was without being brought into contact with the atmosphere.

Based on the above knowledge, the following present invention was completed.
(1) Before refining the molten steel and finishing the RH treatment of the molten steel, the mass concentration of S contained in the molten steel is 10 ppm or less, the mass concentration of T. [O] is 40 ppm or less, and the RH treatment Based on the T. [O] analysis value of the molten steel after completion of the process, the Ca mass added to the molten steel is adjusted so as to satisfy the expressions (1) and (2), Manufacturing method.
A (kg / t) = B × T. [O] +0.02 ... (1)
0.003 ≦ B ≦ 0.006 (2)
A: Ca addition mass (kg / t)
B: Coefficient T. [O]: Analytical value of oxygen concentration in molten steel (ppm) before Ca addition after completion of RH treatment

  (2) Before refining the molten steel and finishing the RH treatment, the components contained in the molten steel are C: 0.03% or more and 0.07% or less in terms of mass concentration, and Mn: 1.1% or more and 1.5%. The method for producing Ca-treated steel according to (1) above, characterized in that:

(3) In the method for producing a Ca-treated steel described in (1) or (2) above, the T. [O] analysis of the molten steel after the completion of the RH treatment is collected from the molten steel and solidified. As a method of analysis using a sample, a steel sample is put in a graphite crucible and heated and melted in an inert gas, and the generated infrared absorption of carbon monoxide and / or carbon dioxide is used to determine whether the sample contains A method for measuring oxygen concentration,
As a pretreatment for removing and cleaning the oxide film on the surface of the sample, vacuum arc plasma treatment is performed under the conditions of a vacuum degree at the start of arc plasma discharge of 5 Pa to 35 Pa and an arc plasma output current of 15 A to 55 A.
It was obtained by machining a steel ingot taken from molten steel so that the height was 1.5 mm or more and 7 mm or less and the ratio of surface area S to volume V (S / V) was 1.05 or more and 1.30 or less. Using a small piece as a sample,
After the arc plasma discharge is applied to the sample a total of 4 times or less and a total treatment time of 0.2 seconds or more and 1.2 seconds or less,
Use the method for oxygen analysis in steel, which is heated and cleaned directly at a temperature higher than the temperature at the time of analysis without bringing it into contact with the atmosphere, and then put into a graphite crucible that has been lowered to the temperature to be analyzed and placed on standby. A method for producing Ca-treated steel.

  ADVANTAGE OF THE INVENTION According to this invention, the composition of the inclusion in steel can be controlled, and manufacture of the steel material excellent in HIC resistance is attained.

It is a figure which shows typically the oxygen analysis equipment in the steel which concerns on this invention. It is a graph which shows the relationship between T. [O] mass concentration and Ca addition amount affecting HIC generation. It is a graph which shows the relationship between T. [O] mass concentration in molten steel, and CaO concentration in inclusions. It is a graph which shows the influence of HIC generation | occurrence | production by Ca addition amount in a comparative example and this invention Example. It is a graph which shows the influence of HIC generation | occurrence | production by the CaO density | concentration in an inclusion in a comparative example and this invention Example.

Hereinafter, the best mode of a method for producing a HIC-resistant steel using the oxygen rapid analysis technique according to the present invention will be described with reference to the drawings.
Maintaining the spherical without fracturing during rolling inclusions inclusions low-melting composition CaO concentration of CaS concentration of 10 wt% less and inclusions is 60 mass% or less than 50 mass% in CaO-Al ₂ O ₃ inclusions. By controlling the inclusions in such a range, the HIC resistance of the steel material becomes extremely high.

The gist of the present invention is (1) and (2) a method for producing Ca-treated steel, and (3) the method for producing Ca-treated steel according to (1) and (2) above using the T. [O] analysis method. is there.
(1) Before the composition of inclusions contained in the steel is CaS: less than 10 mass% and CaO: 50 mass% or more and 60 mass% or less, the molten steel is refined and the RH treatment of the molten steel is finished. The mass concentration of S contained in the molten steel is 10 ppm or less, the mass concentration of T. [O] is 40 ppm or less, and based on the T. [O] analysis value of the molten steel after the completion of the RH treatment. A method for producing a Ca-treated steel, wherein the mass of Ca added to the molten steel is adjusted so as to satisfy the expressions (1) and (2).
A (kg / t) = B × T. [O] +0.02 ... (1)
0.003 ≦ B ≦ 0.006 (2)
A: Ca addition mass (kg / t)
B: Coefficient T. [O]: Analytical value of oxygen concentration in molten steel (ppm) before Ca addition after completion of RH treatment
(2) Before refining the molten steel and finishing the RH treatment, the components contained in the molten steel are C: 0.03% or more and 0.07% or less in terms of mass concentration, and Mn: 1.1% or more and 1.5%. The method for producing Ca-treated steel according to (1) above, characterized in that:
(3) A sample is taken from the molten steel before Ca addition, and T. [O] is determined in a short time and with accuracy in order to determine an appropriate amount of Ca addition by analyzing T. [O] before Ca addition. The method for producing Ca-treated steel according to (1) or (2) above, wherein the oxygen analysis method having the characteristics (i) to (iv) described above is used as a method for well analyzing.

  Here, before finishing the RH treatment, the components contained in the molten steel should be C: 0.03% to 0.07% and Mn: 1.1% to 1.5% by mass concentration. The reason for the feature is as follows.

  The C concentration is 0.03% or more for securing the strength of the steel material, and 0.07% or less for suppressing deterioration of the center segregation. The Mn concentration is 1.1% or more in order to ensure the strength of the steel material. However, if Mn exceeds 1.5%, MnS generation becomes active, so it is 1.5% or less.

  The oxygen concentration in steel is the sum of oxygen in oxide inclusions and oxygen dissolved in steel. Since the dissolved oxygen in the molten steel deoxidized with Al in advance is very small, the oxygen concentration in the analyzed steel is considered to be an oxide inclusion, and there is no problem.

  The CaO concentration in inclusions depends on the oxygen concentration in steel, the S concentration in steel, and the amount of Ca added. If the S concentration in steel is> 10 ppm, a large amount of Ca must be added to suppress MnS formation. However, when a large amount of Ca is added to molten steel having a high S concentration, it is difficult to suppress the formation of CaS. Therefore, when the S concentration in the steel is> 10 ppm, the inclusion composition cannot be controlled by adding Ca. Therefore, the S concentration in the molten steel before Ca addition needs to be 10 ppm or less.

  Table 1 separately analyzes the RH-treated molten steel sample offline, and organizes the relationship between the component mass concentration and Ca addition amount in the molten steel and the CaO mass concentration, CaS mass concentration, and HIC resistance test results in inclusions. Show. The HIC resistance test was performed under the NACE conditions described later.

  As shown in FIG. 2 under the conditions shown in Table 1, the influence of T. [O] concentration and Ca addition amount on the generation of HIC was investigated. The area was clearly organized. Therefore, the range of the appropriate Ca addition amount was determined from the analyzed T. [O] concentration as shown in Equation (1) and Equation (2). Moreover, as shown in FIG. 3, by making it within the range of the Ca addition amount determined by the equation (1), it is possible to control the CaO concentration in the inclusions to 50 mass% or more and 60 mass% or less. It was found that the HIC resistance is extremely improved.

  The Ca or Ca-containing substance added to the molten steel may be any material such as elemental metal Ca or Ca alloy such as CaSi and FeCa or an alloy composed of three or more kinds of elements including Ca. Methods for adding Ca include an injection method in which Ca or a Ca-containing material is blown into molten steel together with a carrier gas, and a wire feeder method in which Ca or Ca-containing material is filled in an iron wire and added to the molten steel together with the wire. Good.

  In the present invention example, after the steel is removed from the converter, the deoxidized and desulfurized molten steel is RH-treated, a sample is taken after the RH treatment, and the total oxygen is rapidly analyzed using the rapid analysis method described above. The amount of Ca determined by the formulas (1) and (2) was added. For Ca addition, a wire feeder method filled with a CaSi alloy of 30 mass% Ca-70 mass% Si was used.

  The results are shown in Table 2 and FIGS. A sample was cut out from the obtained thick plate, and the form and composition of inclusions were examined by a scanning method using a scanning EPMA and energy dispersive X-ray analyzer, and an HIC resistance evaluation test was performed. The HIC resistance evaluation test was performed under the NACE conditions shown below.

NACE conditions Temperature: 24.8 ± 2.8 ° C
pH: Max4.5
_{Solution: 5% NaCl + 0.8% CH} 3 COOH
Time: 96 hours the concentration of H ₂ S: _{H 2} S saturated H ₂ S flow rate: 100~200cc / min
As shown in Table 2, in the present invention, CaO mass concentration in inclusions: 50 to 60% and CaS mass concentration: less than 10% were reliably controlled, and generation of HIC was suppressed.

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 (3)

Before refining the molten steel and finishing the RH treatment of the molten steel, the mass concentration of S contained in the molten steel is set to 10 ppm or less, and the mass concentration of T. [O] is set to 40 ppm or less.
And based on the T. [O] analysis value of the molten steel after finishing the RH treatment,
A method for producing a Ca-treated steel, comprising adjusting the mass of Ca added to the molten steel so as to satisfy the expressions (1) and (2).
A (kg / t) = B × T. [O] +0.02 ... (1)
0.003 ≦ B ≦ 0.006 (2)
A: Ca addition mass (kg / t)
B: Coefficient T. [O]: Analytical value of oxygen concentration in molten steel (ppm) before Ca addition after completion of RH treatment   Before refining the molten steel and finishing the RH treatment, the components contained in the molten steel are C: 0.03% to 0.07% and Mn: 1.1% to 1.5% by mass concentration. The manufacturing method of Ca processing steel of Claim 1 characterized by the above-mentioned. A method for producing a Ca-treated steel according to claim 1 or 2,
The T. [O] analysis of the molten steel after completion of the RH treatment described above was performed using a sample collected from the molten steel and solidified.
In addition, as an analysis method, a steel sample is placed in a graphite crucible and heated and melted in an inert gas, and the oxygen concentration in the sample is measured from the infrared absorption of one or both of the generated carbon monoxide and carbon dioxide. A way to
As a pretreatment for removing and cleaning the oxide film on the surface of the sample, vacuum arc plasma treatment is performed under the conditions of a vacuum degree at the start of arc plasma discharge of 5 Pa to 35 Pa and an arc plasma output current of 15 A to 55 A.
It was obtained by machining a steel ingot taken from molten steel so that the height was 1.5 mm or more and 7 mm or less and the ratio of surface area S to volume V (S / V) was 1.05 or more and 1.30 or less. Using a small piece as a sample,
After the arc plasma discharge is applied to the sample a total of 4 times or less and a total treatment time of 0.2 seconds or more and 1.2 seconds or less,
Use the method for oxygen analysis in steel, which is heated and cleaned directly at a temperature higher than the temperature at the time of analysis without bringing it into contact with the atmosphere, and then put into a graphite crucible that has been lowered to the temperature to be analyzed and placed on standby. A method for producing Ca-treated steel.

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