JP5397154B2 - Melting method of steel material for oil pipes with high strength and high corrosion resistance - Google Patents

Description

  By controlling the composition range of the inclusions contained in the steel material to an appropriate narrow width, the present invention can produce a steel material for oil well pipes that enables production of high strength and high corrosion resistance oil well pipes without reducing productivity. It relates to the manufacturing method.

  In recent years, quality requirements for steel materials have become stricter, and in particular, demands for oil well pipe steel materials have become more severe. In other words, there is a demand for the development of steel materials for producing high strength and high corrosion resistance grade oil well pipes necessary for oil field development.

  Oil well pipe steels have been demanded not only for hydrogen-induced crack resistance (HIC resistance) but also for sulfide stress crack resistance (SSC resistance) because the use environment has become harsher than before. Yes. In the production of conventional oil well pipes, there is a method in which Ca is not added in order to impart SSC resistance, SSC generation due to dissolution of Ca-based oxysulfides is prevented, and heat treatment is performed multiple times in order to obtain high strength. I have been. However, this method has a problem that productivity is deteriorated because the heat treatment process takes time. Therefore, there is a demand for melting steel that can produce a high-strength, high-corrosion-resistant oil well pipe without reducing productivity.

  In the melting of conventional steel materials for high strength and high corrosion resistance oil well pipes, in order to provide SSC resistance in addition to HIC resistance, firstly in the refining process, the molten steel is subjected to two heat treatments. The production of Ca-based oxysulfide is suppressed by adding Ca to the molten steel without adding Ca to the molten steel, and continuously producing such molten steel to produce steel for oil country tubular goods. . Thereby, a steel material having high strength and high corrosion resistance can be obtained.

  However, in this method, it is not possible to enjoy the advantage of controlling the form of inclusions by Ca treatment of molten steel, and it is necessary to perform two heat treatments after pipe making, so the productivity of oil well pipes is reduced. Resulting in. In addition, when only a single heat treatment is performed online to prevent a decrease in productivity, variations in strength and corrosion resistance occur.

In Patent Document 1, in addition to C: 0.05 to 0.35%, Si: 0.02 to 0.5%, Mn: 0.5 to 2%, Ca is 0.0005 to 0.008%. The content of S, O, and Ca is 0.005 to 0.1% Al and the balance is Fe and impurities.
1.0 ≦ (% Ca) (1-72 (% O)) / 1.25 (% S) ≦ 2.5
So that the deoxidation product is a complex inclusion of (CaO) _m (Al ₂ O ₃ ) _n and the molecular composition ratio is in the range of m / n <1, and it has high toughness with excellent sour resistance A steel plate for a sewn steel pipe is disclosed. However, in this method, the composition range of inclusions is relatively wide. Since the analysis method of oxygen in steel is not clarified, it is presumed that the composition range is widened, and there is room for improvement from the viewpoint of improving product quality by controlling inclusions.

In Patent Document 2, when Ca is added, 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 determined as, for example, As 15ppm,
[% Ca] × [% S] 0.28 ≦ 3.5 × 10 ^-4 (i)
1 ≦ {[% Ca]-(0.18 + 130 [% Ca]) × [% O]} / 1.25 / [% S] ・ ・ ・ ・ (ii)
A steel melting method is disclosed in which the target calcium composition range is determined from these two formulas to determine the input amount.

  However, inclusions in steel aimed by the present invention have a CaO content of 40 to 80% by mass, and this also has a relatively wide composition range. In the background, there was a lack of means for accurately obtaining the O concentration to be substituted into the above equation (ii), and as described above, “15 ppm as an average value” (Patent Document 2, paragraph 0017) had to be used. I can guess that there is.

  Patent Document 3 describes a steel for high-strength steel pipes that controls the component composition of non-metallic inclusions in steel, particularly reduces the influence of carbonitrides to improve sour resistance, and a method for producing the same. Yes. In the method, the oxygen content in steel is 0.002% or less, the CaO content in inclusions is 30 to 80%, and the like, the N content in steel and the CaO content in inclusions are It is necessary to control the ratio to 0.28 ≦ [N] / (% CaO) ≦ 2.0. However, in order to control in such a manner, a method is described in which the ratio of the N content in steel and the amount of Ca added to molten steel is within a certain range. Further, it is described that the oxygen content in steel is preferably in the range of 0.0003 to 0.0015%. However, it does not describe a method for controlling the oxygen content or a method for checking the content online. Therefore, it is considered that there is still room for improvement regarding the control of the CaO content in inclusions in the method.

Japanese Patent Laid-Open No. 02-290947 Japanese Patent Laid-Open No. 2001-11528 JP 2009-120899 A JP 2002-328125 A JP-A-10-311782

  As explained above, when manufacturing steel materials for oil-well pipes with high strength and high corrosion resistance, the conventional method in which Ca is not added cannot enjoy the advantages of Ca treatment of molten steel, and requires multiple heat treatments after pipe making. There is a problem that the productivity of the oil well pipe decreases. In addition, the methods disclosed in Patent Documents 1 to 3 leave room for improvement from the viewpoint of narrowing the inclusion composition.

  The present invention has been made in view of the above problems, and by controlling the inclusion composition within a narrow appropriate range by treating the molten steel with Ca, a high-strength, high-corrosion-resistant oil well without reducing productivity. It is an object of the present invention to provide a method for melting a steel material for oil country tubular goods that makes it possible to manufacture a pipe.

  In the present invention provided to solve the above problems, the molten steel is adjusted to a predetermined component range before the end of the secondary refining of the molten steel, and the molten steel is transferred to the tundish of the continuous casting machine after the end of the secondary refining. By the time the injection is started, the total oxygen content (T. [O]) in the molten steel is measured quickly and accurately. [O] An optimum amount of Ca is added to the pan or tundish according to the measured value, and the composition of inclusions contained in the steel material is controlled within a narrow appropriate range.

That is, the present invention is as follows.
(1) Secondary refining is performed on the molten steel after completion of primary refining, and addition of Ca to the molten steel after completion of secondary refining is performed to control inclusions, so that C: 0. 15% or more and 0.35% or less, Si: 0.10% or more and 1.5% or less, Mn: 0.10% or more and 2.0% or less, P: 0.025% or less, S: 0.002% or less Al: 0.005% or more and 0.100% or less, Ca: 0.0005% or more and 0.0035% or less, N: 0.008% or less, O (oxygen): 0.002% or less, and the balance A method of melting high strength and high corrosion resistance steel for oil well pipes having a chemical composition comprising Fe and impurities, wherein the mass concentration of S contained in the molten steel is 20 ppm or less, and T.O. The mass concentration of [O] is 20 ppm or less, and after completion of the secondary refining, the T.O. A sample for mass concentration analysis of [O] is taken, and then the analytical value is obtained before the molten steel is injected into the tundish of a continuous caster, and the added amount of Ca is determined by the analytical value. Is added within the range of the A value obtained by substituting into the following formula (1), and a high strength and high corrosion resistance steel material for oil country tubular goods is melted.
A = B x T. [O] + 0.025 ... (1)
0.0015 ≦ B ≦ 0.0045 ... (2)
Here, A: Mass of added Ca per ton of molten steel (unit: kg / t)
B: Coefficient
T. [O]: Mass concentration analysis value of oxygen in molten steel (unit: ppm)

(2) Instead of a part of Fe contained in the steel material, it contains one or more component elements selected from one or more of the following groups (a) to (c): The method for melting the steel material for oil well pipe according to (1) above, which has high strength and high corrosion resistance.
(A) In mass%, Ti: 0.20% or less, Cr: 1.5% or less, Mo: 1.0% or less, Nb: 0.05% or less, V: 0.30% or less, W: 1 0.0% or less, B: 0.0030% or less and Zr: 0.10% or less (b) mass%, Mg: 0.005% or less (c) mass%, Ni: 0.30% or less, and Cu : 0.40% or less

  (3) The CaO content in inclusions contained in the steel material is controlled to 30% by mass or more and 55% by mass or less by the above-described Ca addition method. The method for melting a steel material for high strength and high corrosion resistance oil-based pipe as described in 2).

  (4) T. of molten steel after the secondary refining is completed. As a method for analyzing the mass concentration of [O], a sample steel ingot is collected from the molten steel and used as a sample for analyzing the concentration of contained components. The steel sample is placed in a graphite crucible and heated and melted in an inert gas. A method for measuring the oxygen concentration in the sample from the infrared absorption of one or both of generated carbon monoxide and carbon dioxide, wherein a vacuum arc is used as a pretreatment to remove and clean the oxide film on the sample surface. With respect to the steel ingot collected from the molten steel under the conditions that the degree of vacuum at the start of the arc plasma discharge is 5 Pa to 35 Pa and the arc plasma output current is 15 A to 55 A, the height is 1.5 mm to 7 mm. Hereinafter, a small piece obtained by machining so that the ratio (S / V) of the surface area S to the volume V is 1.05 or more and 1.30 or less is used as a sample, and the arc plasma discharge is performed. Is applied to the sample for a total of 4 times or less and the total treatment time is 0.2 seconds or more and 1.2 seconds or less. Any one of the above (1) to (3), characterized by using a method for analyzing oxygen in iron and steel, which is heated and cleaned at a high temperature and then charged into a graphite crucible which has been lowered to a temperature to be analyzed and placed on standby. 2. A method for melting a steel material for high strength and high corrosion resistance oil-based pipes as described in the section.

  In the present invention, after the refining of the molten steel, the T.D. in the molten steel is started after the molten steel is injected into the tundish of the continuous casting machine. [O] The T. Since the optimum amount of Ca is added to the molten steel according to the [O] value, it is possible to optimize the CaO concentration in the inclusions in a short time. Therefore, it is possible to stably melt a steel material that can produce an oil well pipe having high strength and high corrosion resistance without requiring a step of reducing productivity such as performing heat treatment twice after pipe making. it can.

It is a figure which shows typically the oxygen analysis equipment in the steel which concerns on this invention. T.A. It is a graph which shows the relationship between [O] and Ca addition amount. T.A. It is a graph which shows the relationship between [O] and CaO content in inclusions.

The method for melting the steel material for high strength and high corrosion resistance oil well pipe of the present invention will be described below.
1. Below, the chemical composition of the steel material for oil country tubular goods according to the present invention will be described. In the following description, “%” indicating the content of a steel material component means mass%.

C: 0.15% or more and 0.35% or less C is an element necessary for ensuring the strength of the steel pipe. If it is less than 0.15%, the hardenability is insufficient, and the tempering temperature is lowered. It is difficult to ensure the performance. On the other hand, if it exceeds 0.35%, firing cracks occur and the toughness is further deteriorated. Therefore, the appropriate range of the C content is set to 0.15% or more and 0.35% or less. Desirably, it is 0.20% or more and 0.30% or less.

Si: 0.10% to 1.5% Si is added for the purpose of deoxidation. It also contributes to an increase in strength by increasing the temper softening resistance. For the purpose of deoxidation, it is necessary to contain 0.10% or more. On the other hand, if the content exceeds 1.5%, the hot workability becomes extremely poor, so the upper limit was made 1.5%. Desirably, it is 0.10% or more and 0.50% or less.

  However, Si is an element which affects the activity of Ti in steel. For this reason, as will be described later, when Ti is contained in an amount of 0.005% or more, if the Si content exceeds 1.0%, the Ti activity is increased too much to suppress the formation of TiN. Can not do. Therefore, the appropriate range of the Si content is 0.10% or more and 1.0% or less in the case of a steel material containing 0.005% or more of Ti. Moreover, the preferable range of Si content in that case is 0.10% or more and 0.50% or less.

Mn: 0.10% or more and 2.0% or less Mn is an element that increases the hardenability of the steel and is effective in securing the strength of the steel pipe. If it is less than 0.10%, both strength and toughness decrease due to insufficient hardenability. On the other hand, if the content exceeds 2.0%, segregation increases and the toughness decreases, so the upper limit was made 2.0%. Desirably, it is 0.20% or more and 1.0% or less.

P: 0.025% or less P is unavoidably present in steel as an impurity. If it exceeds 0.025%, it segregates at the grain boundary to lower the toughness, so it is made 0.025% or less. The lower the better.

S: 0.002% or less Since S is a constituent element of sulfide inclusions which is a problem in HIC-resistant steel, its content is preferably low. If the S content exceeds 0.002% and Ca is added, the content of CaS in the inclusions increases, which may cause problems. Therefore, the S content needs to be 0.002% or less. The preferable range of the content is 0.001% or less.

O (oxygen): 0.002% or less O content means the total oxygen content (T. [O]) including oxygen contained in oxide inclusions, and is a measure of the amount of inclusions. Concentration. If the content is higher than 0.002%, the amount of inclusions becomes too large, and it becomes difficult to suppress the generation of HIC in high-strength steel. The lower the O content, the fewer oxide inclusions, so the lower the content, the better.

N: 0.008% or less N is inevitably present in steel. N combines with Al, Ti and Nb to form a nitride. In particular, if a large amount of AlN or TiN precipitates, the toughness, SSC resistance, and HIC resistance are adversely affected, so 0.008% or less is preferable.

Al: 0.005% or more and 0.100% or less Al is an element having a strong deoxidizing action, and is an important element for reducing oxygen in steel. If the content is less than 0.005%, the deoxidation action is insufficient and the amount of inclusions cannot be reduced sufficiently. On the other hand, when the Al content is higher than 0.100%, the deoxidation effect is saturated and the generation of sulfide is promoted. Therefore, the appropriate range of the Al content is set to be 0.005% or more and 0.100% or less. The preferable range of the content is 0.008% or more and 0.040% or less. In addition, the display of Al in this invention means acid-soluble Al (sol.Al).

Ca: 0.0005% or more and 0.0035% or less Ca is an element having an effective action for modifying sulfide inclusions and spheroidizing alumina inclusions. If the Ca content is less than 0.0005%, these effects cannot be obtained, and the generation of HIC due to MnS and alumina clusters cannot be suppressed. Further, when the Ca amount is less than this, there is no problem of SSC caused by Ca-based nonmetallic inclusions, so the lower limit of Ca is 0.0005%. On the other hand, when the content rate exceeds 0.0035%, a CaS cluster may be generated. Therefore, the appropriate range of the Ca content is set to 0.0005% or more and 0.0035% or less. A preferable range of the content is 0.0008% or more and 0.002% or less.

  The above is the essential adjustment component of the steel material for steel pipes and the adjustment range thereof in the present invention, and the balance thereof is Fe and impurities, but depending on the use and use environment of the steel material, the following (a) to (c) groups One or more component elements selected from one or more groups of the above can be contained in place of a part of the Fe. That is, (a) group: Ti, Cr, Mo, Nb, V, W, B and Zr, (b) group: Mg, (c) group: Ni and Cu. The elements of each group described above may or may not be contained, but when they are contained, their effects can be obtained by containing them in the following content ranges.

The elements of group (a) mainly have the effect of improving the strength, hardenability, toughness and the like of steel.
Ti: 0.20% or less Ti is an element that precipitates as TiN in the steel and has an effect of improving the toughness of the steel. However, excessive addition of Ti causes coarsening of precipitated TiN. Therefore, the Ti content needs to be 0.2% or less. From the viewpoint of ensuring toughness, the content is preferably 0.005% or more. For the above reason, the Ti content is preferably 0.005% or more and needs to be 0.20% or less.

Cr: 1.5% or less Cr is an element useful for enhancing hardenability, but may not be contained when the minimum hardenability is ensured by other elements. However, it is advantageous to contain it when manufacturing a thicker steel pipe. When it is contained, if the Cr content is 0.10% or more, there is an effect of improving hardenability and temper softening resistance. Further, when the content exceeds 1.5%, the toughness deteriorates. Therefore, when Cr is contained, the content is made 0.10% to 1.5%. Desirably, it is 0.20% or more and 1.2% or less.

Mo: 1.0% or less Mo is preferably contained for the purpose of enhancing hardenability and temper softening resistance when producing a thick steel pipe. It also has the effect of improving the sour resistance performance. In the case of containing it, the effect does not appear if it is less than 0.10%, so it is desirable to contain it by 0.10% or more. Moreover, since toughness will deteriorate when it exceeds 1.0%, it is good to set it as 1.0% or less. Desirably, it is 0.20% or more and 0.80% or less.

Nb: 0.05% or less Nb is effective in refining austenite grains by suppressing the growth of crystal grains by the pinning effect during reheating in the offline heat treatment process. In the case where it is contained, if less than 0.01%, the effect does not appear, so it is desirable to contain 0.01% or more. Desirably, it is 0.015% or more and 0.05% or less.

V: 0.30% or less V is an element effective for improving the SSC resistance. When V is contained, it has an effect of increasing strength by strengthening secondary precipitation, and when a predetermined strength is obtained, it can be tempered at a higher temperature, which contributes to improvement of SSC resistance. Further, since the solubility of VC in the austenite region is large, it is completely dissolved at the time of in-line quenching, and does not cause variations in strength. If the content is less than 0.01%, the effect is not obtained. If the content exceeds 0.30%, the toughness is greatly deteriorated. Therefore, when it contains, it is 0.01% or more and 0.30% or less. Desirably, it is 0.02% or more and 0.20% or less.

W: 1.0% or less W, like Mo, can improve hardenability and obtain high strength, and has the effect of increasing temper softening resistance and improving SSC resistance. . If the content exceeds 1.0%, the effect is saturated, so the upper limit of the content is preferably 1.0%.

B: 0.0030% or less When B is contained, the hardenability is remarkably improved. Therefore, the required strength can be ensured by adding B when producing a thick steel pipe. When it contains, if it exceeds 0.0030%, carbonitride tends to precipitate at the grain boundary, which causes toughness deterioration. For this reason, the upper limit is preferably set to 0.0030%.

Zr: 0.10% or less When Zr is contained, N, which is an impurity in the steel, is fixed as a nitride like Ti, so that the hardenability of B can be sufficiently secured. Moreover, since it is difficult to form carbides, it does not cause variations in strength. On the other hand, if the content exceeds 0.10%, inclusions increase and the toughness decreases, so the upper limit of the content is preferably 0.10%.

The elements in group (b) mainly have an inclusion modification effect.
Mg: 0.005% or less When Mg is contained, it reacts with S in the steel to produce a sulfate in the molten steel. This sulfated oxide is spherical after rolling and does not extend in the rolling direction. For this reason, it has the effect | action which improves the impact property of a rolling orthogonal direction, and also suppresses hydrogen induced cracking. However, if the content exceeds 0.005%, the amount of inclusions in the steel is increased, the cleanliness is lowered, and various performances are lowered. Therefore, the content is preferably 0.005% or less.

The elements of group (c) have the action of suppressing the entry of hydrogen mainly in a hydrogen sulfide environment.
Ni: 0.30% or less Ni has an action of suppressing entry of hydrogen into steel in a hydrogen sulfide environment. In order to obtain the effect, the above effect can be obtained by containing 0.10% or more of Ni. However, when the content exceeds 0.30%, the effect of suppressing hydrogen penetration is saturated. Therefore, when Ni is contained, the content is preferably 0.30% or less. Moreover, it is preferable to make the content rate into the range of 0.10% or more.

Cu: 0.40% or less Cu, like Ni, has an action of suppressing hydrogen intrusion into steel in a hydrogen sulfide environment. In order to obtain the effect, the above effect can be obtained by containing 0.10% or more of Cu. However, when its content exceeds 0.40%, it melts at a high temperature and lowers the strength of the grain boundaries. Therefore, when Cu is contained, the content is made 0.40% or less. It is good. Moreover, it is preferable to make the content rate into the range of 0.10% or more.

2. Ca addition amount and T.I. Regarding the relationship with [O] In the steel material melting method according to the present invention, in order to obtain a steel material having the above-described chemical composition, a so-called RH vacuum degassing apparatus is used after primary refining in a converter or the like. In addition to performing secondary refining (in this invention, this refining process is called “secondary refining”) to adjust the component content in the molten steel, in particular, before the end of the secondary refining, the content of S contained in the molten steel. The mass concentration is 20 ppm or less. The mass concentration of [O] is reduced to 20 ppm or less. S and T. in molten steel. [O] constitutes inclusions (sulfides / oxides) containing Ca by addition of Ca, so the content thereof is the amount of Ca addition necessary for properly controlling the composition of inclusions. It has a big impact. Therefore, in order to suppress variations in the composition of inclusions in the steel and to reduce the amount of inclusions themselves, S and T. in the molten steel before adding Ca are included. It is essential to reduce [O].

It should be noted that the additive components other than Ca are basically adjusted before the end of the secondary refining, but some components may be added to the molten steel before the addition of Ca or with the addition of Ca. Absent.
Conventionally, as described in Patent Documents 1 to 3, as a method for improving the sour resistance performance of a steel pipe, the composition of inclusions in steel is controlled by addition of Ca, and the influence of carbonitrides is further reduced. Methods are known.

  By adding Ca to the molten steel, MnS contained in the molten steel changes to Ca inclusions. For this reason, HIC resulting from MnS is suppressed and the HIC resistance performance of the steel material is improved. However, inclusions containing Ca, specifically, Ca-based oxidation and sulfide inclusions (oxysulfide inclusions) such as Ca—Al—O—S, Ca—S, and Ca—S—O are included. Since an object also causes HIC, excessive addition of Ca rather lowers the HIC resistance performance of the steel material. For example, as disclosed in Patent Document 3, when the CaO content in inclusions is not within the appropriate range, the formation of aggregate carbonitride cannot be suppressed, leading to the generation of HIC. Is also accepted.

Further, the cause of SSC generation includes pitting corrosion due to dissolution of Ca oxysulfide and corrosion due to galvanic action due to TiN carbonitride.
By appropriately controlling the CaO content in the inclusions, it is possible to control not only the Ca-based oxysulfide but also the content and form of the TiN-based carbonitride and also the dispersion state.

  Thus, the amount of Ca added to the molten steel is an important operating factor in producing a steel material having high sour resistance, and is obtained as the composition of inclusions is appropriately controlled by appropriate operation of the amount added. It is expected that the crack resistance of steel can be improved.

  By the way, when Ca is added to the molten steel, S and T. in the molten steel. Reacts with [O] to produce Ca-based oxysulfide inclusions. Therefore, in order to appropriately control the component composition of the generated inclusions, chemical components in the molten steel, particularly S and T. Depending on the content of [O], an appropriate amount of Ca must be added.

  Of these, T.W. Except for [O], the mass concentration of chemical components in the molten steel can be known with high accuracy in a short time within a few minutes by taking a sample and analyzing it. However, T.W. Regarding [O], a method for accurately knowing in a short time has not been known.

  Since the Ca value (Ca content) necessary for achieving the target inclusion composition is determined by the components of the molten steel (including inclusions in the molten steel), the T.O. [O] By measuring the concentration, determining an appropriate Ca addition amount based on the measured value, and adding the appropriate amount of Ca before the start of continuous casting, it is possible to produce a steel with an optimal inclusion composition It becomes possible.

3. T. T. et al. Rapid analysis of [O] In the method for melting steel according to the present invention, in order to obtain a steel having the above-described chemical composition, a sample is taken from the molten steel after completion of secondary refining such as RH treatment. [O] is analyzed quickly and accurately, and the amount of Ca added to the molten steel is determined based on the analysis value. Although it is assumed that this Ca addition is performed in the tundish of the continuous casting facility, it may be added to the molten steel in the ladle using a wire or the like if there is a time allowance.

  Since the time from the end of the secondary refining of the molten steel to the start of the supply of the molten steel to the tundish of the continuous casting facility is generally about 5 to 15 minutes. As a method of analyzing [O] with high accuracy, the rapid analysis method of oxygen in steel, which was developed in parallel in the present invention, is used.

The analysis method will be described in detail below 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 method and apparatus for preparing a sample for analyzing an in-metal component disclosed in Patent Document 4 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 analysis 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 4, the preprocessed 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, the 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 5 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 that satisfies “≦ 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.

4). In the present invention, the amount of Ca added to the molten steel in the tundish after the secondary refining (hereinafter, simply referred to as “Ca addition amount”) has been completed. Later, a sample was taken from the molten steel. Based on the value obtained by analyzing [O], it is set as the range of the A value obtained by the following formula (1).

A = B x T. [O] + 0.025 ... (1)
0.0015 ≦ B ≦ 0.0045 ... (2)
Here, A: Ca addition mass per molten steel mass (t) (unit: kg / t),
B: coefficient,
T. [O]: Oxygen concentration analysis value of molten steel after completion of secondary refining and before Ca addition (unit: ppm).

When melting a steel material having the above-described component composition, S and T. in the molten steel before adding Ca are added. The CaO mass concentration in inclusions is 30% or more and 55% or less by adding Ca in the range of the A value obtained by the equation (1) to molten steel in which the mass concentration of [O] is reduced to 20 ppm or less. It can be controlled to be narrow. In this case, the inclusions in the steel are CaO, CaS, etc. in addition to the composite inclusions mainly composed of CaO.Al ₂ O _3. The concentration of the CaO component in all of these inclusions is It means that it is within the range of 30% or more and 55% or less.

By setting the components of the steel material to the above-described predetermined component range and the Ca addition amount within the range of the formula (1), it is possible to stably obtain a steel having high SSC resistance.
As described above, by appropriately controlling the CaO content in inclusions, it is possible to control not only the Ca-based oxysulfides but also the content and form of TiN-based carbonitrides as well as the dispersion state. However, before adding Ca, S and T in the molten steel. [O] is reduced, and T.sub. If the addition amount is determined based on [O], the CaO content in inclusions can be accurately controlled within the appropriate range. That is, even when Ca is added, SCC can be effectively suppressed.

  Hot metal desulfurization using a mechanical stirring type hot metal desulfurization unit (KR), hot metal dephosphorization using a converter-type dephosphorization furnace, decarburization using a converter, and final dephosphorization, followed by a molten steel circulation type Secondary refining was performed using a vacuum degasser (RH). The melting amount was 240 t per charge. After this secondary refining, a seamless steel pipe with an outer diameter of 244.5 mm and a wall thickness of 13.8 mm is manufactured by a Mannesmann mandrel mill type pipe manufacturing process through a round mold continuous casting machine, and quenched into the steel pipe. The tempering heat treatment was performed only once under appropriate conditions, and the SSC resistance was compared.

  In the embodiment of the present invention, S and T. in the molten steel are finished before the secondary refining using the RH is finished. [O] was reduced to 20 ppm or less, a sample was taken from the molten steel immediately after the molten steel reflux in the RH was completed, and a rapid analysis of oxygen in the steel was immediately performed by the method described above. Basically, the analysis value of the total oxygen mass concentration in the sample was obtained within 10 minutes from the end of the molten steel recirculation, but the time required until the analysis value was found was 10 minutes after the continuous refining process was completed. In some cases, it was within 6 minutes for convenience. However, in either case, the analysis concentration error with respect to the total oxygen concentration confirmed later after detailed analysis separately off-line was within ± 3 ppm. Then, Ca was added in the range of the above-described formula (1) in the tundish of the continuous casting machine. Note that Ca was added by a method of adding a wire containing a CaSi alloy having a pure Ca content of 30%.

The chemical composition of the steel material to be evaluated is as shown in Table 1 below. In addition, the unit of content in Table 1 is mass%, and the remainder is Fe and impurities.
An example of evaluation results according to the prior art and the present invention regarding the corrosion resistance of the slab is shown in Table 2 and FIGS.

  Here, test no. 1 to 5, the T.F. Without analyzing [O], the necessary addition amount of Ca was estimated based on conventional experience, and the Ca amount was added to the molten steel in the tundish. On the other hand, test no. In 6 to 17, T.F. [O] was analyzed, Ca addition amount was determined based on the analysis value and the above formulas (1) and (2), and the addition amount of Ca determined by the method was added. Therefore, the T.O. The numerical value in the [O] column indicates the test No. About 1-5, the result by the off-line analysis after completion | finish of a test means test No.1. For samples 6 to 17, T.P. of samples collected after completion of RH treatment. It means the result obtained by analyzing [O] by the above method in 6 to 10 minutes. In Table 2, “CaSi addition amount” means the addition amount per 1 ton of molten steel of the above CaSi alloy, and “Ca addition amount” means the addition amount per 1 t of molten steel in the added CaSi alloy. means.

The evaluation method of corrosion resistance is as follows.
Two tensile test pieces each having a parallel portion of 6.35φ × 25.4 mm were collected from the heat-treated steel pipe, and an SSC test was performed according to NACE TM0177 Method A. That is, a uniaxial tensile test was performed by applying a standard minimum yield stress (SYS) no 85% stress in 0.5% acetic acid + 5% saline solution at 25 ° C. saturated with 1 atmosphere of hydrogen sulfide. Those that did not break within the test period of 720 h were determined to have good SSC resistance (“◯” in the corrosion resistance column of Table 2). On the other hand, one that broke even within this test period was determined to have poor SSC resistance ("X" in the corrosion resistance column in Table 2).

  As apparent from Table 2 and FIGS. 2 and 3, conventionally, since the oxygen concentration in the molten steel before the addition of Ca is unknown, variation in Ca addition effect has occurred. For this reason, the target Ca type | system | group oxysulfide composition was not obtained and the case where SSC corrosion resistance was not satisfied as a result had arisen.

  On the other hand, in order to obtain the target composition of the Ca-based oxysulfide, the necessary amount of Ca addition is shown in FIG. By changing in accordance with [O], the target inclusion CaO concentration can be achieved as shown in FIG. 3, and SSC generation can be stably 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 (4)

By performing secondary refining on the molten steel after completion of the primary refining, and further adding Ca to the molten steel after completion of the secondary refining to control inclusions,
In mass%, C: 0.15% to 0.35%, Si: 0.10% to 1.5%, Mn: 0.10% to 2.0%, P: 0.025% or less S: 0.002% or less, Al: 0.005% or more and 0.100% or less, Ca: 0.0005% or more and 0.0035% or less, N: 0.008% or less, O (oxygen): 0. A method of melting a steel material for high strength and high corrosion resistance oil well pipes having a chemical composition comprising 002% or less and the balance being Fe and impurities,
Before finishing the secondary refining, the mass concentration of S contained in the molten steel is 20 ppm or less. The mass concentration of [O] is 20 ppm or less,
Further, after completion of the secondary refining, the T.V. Take a sample for mass concentration analysis of [O],
After that, know the analysis value before injecting the molten steel into the tundish of the continuous casting machine,
A method for melting a steel material for high strength and high corrosion resistance oil well pipes, characterized in that the addition amount of Ca is added within the range of the A value obtained by substituting the analytical value into the following formula (1).
A = B x T. [O] + 0.025 ... (1)
0.0015 ≦ B ≦ 0.0045 ... (2)
Here, A: Mass of added Ca per ton of molten steel (unit: kg / t)
B: Coefficient
T. [O]: Mass concentration analysis value of oxygen in molten steel (unit: ppm) It replaces with a part of Fe contained in the said steel materials, The 1 or more types of component element chosen from one or more groups of the following (a)-(c) group is contained, It is characterized by the above-mentioned. A method for melting a steel material for high strength and high corrosion resistance oil well pipes according to 1.
(A) In mass%, Ti: 0.20% or less, Cr: 1.5% or less, Mo: 1.0% or less, Nb: 0.05% or less, V: 0.30% or less, W: 1 0.0% or less, B: 0.0030% or less and Zr: 0.10% or less (b) mass%, Mg: 0.005% or less (c) mass%, Ni: 0.30% or less, and Cu : 0.40% or less   The CaO content in the inclusions contained in the steel material is controlled to be 30% by mass or more and 55% by mass or less by the above-described Ca addition method. A method of melting high strength and high corrosion resistance steel for steel pipes. T. of the molten steel after finishing the secondary refining. As a method of analyzing the mass concentration of [O],
A sample steel ingot is taken from the molten steel and used as a sample for analyzing the concentration of contained components,
A method of measuring an oxygen concentration in a sample from an infrared absorption of one or both of carbon monoxide and carbon dioxide generated by placing a steel sample in a graphite crucible and heating and melting in an inert gas,
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 collected 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 a method for oxygen analysis in steel that directly heats and cleans the sample at a temperature higher than the temperature at the time of analysis without bringing it into the atmosphere, and then puts it in a graphite crucible that has been lowered to the temperature to be analyzed and placed on standby. The method for melting a steel material for a high-strength, high-corrosion-resistant oil-based pipe according to any one of claims 1 to 3, characterized in that:

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