JP2015196193A - Continuous casting method of casting piece for steel pipe excellent in toughness and corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress an intermetallic compound as a brittle phase.SOLUTION: When a casting piece as raw material for manufacturing a steel pipe is manufactured by continuously casting an alloy comprising, by mass%, 0.01 to 0.05% of C, 0.05 to 0.5%of Si, 0.1 to 1.5% of Mn, 0.05% or less of P, 0.005% or less of S, 15 to 30% of Cr, 1 to 10% of Mo, 30 to 50% of Ni, further, 0.05 to 2% of Sn and the remaining part of Fe and impurity, a relation between a ratio I/Iof I to deposition amount Iof intermetallic compound at a center of cross-section of the casting piece in such a case that Sn is not contained in the alloy and in such a case that Sn is contained and an average Md value of the alloy defined by ΣX.(Md)[eV](Xis atomic fraction [-] of an alloy component i, (Md)is Md value [eV] of the alloy component i) and a relation between the average Md value of the alloy and inclusion Sn amount are determined and such a Sn content that the average Md value of the alloy is 0.92 or less and the ratio I/Igets to 0.8 or less is provided.

Description

  The present invention relates to a method for continuously casting austenitic stainless steel suitable for the production of oil well pipes, and more particularly to a method for continuously casting steel pipe slabs excellent in toughness and pitting corrosion.

In recent years, demand for oil and natural gas has been increasing year by year, and its importance has been increasing. On the other hand, oil and natural gas reservoirs to be newly developed have become deeper, and oil well pipes with high strength and high corrosion resistance that can withstand harsh environments have been developed. Yes. In particular, such harsh environments often contain corrosive gases such as CO ₂ or H ₂ S, and development of materials having high corrosion resistance is being promoted.

There are various steel types in oil well pipes, and the selection conditions are generally based mainly on the partial pressure and temperature of CO ₂ and H ₂ S.

For example, the addition of Cr is effective in preventing CO ₂ corrosion. This is because the addition of Cr forms a heat-resistant chromium oxide protective film on the steel surface, and the scale containing FeCO ₃ tends to cover the steel surface smoothly. Therefore, in an environment where CO ₂ partial pressure is high, 13 mass% chromium steel and duplex stainless steel containing 22 mass% chromium and 25 mass% chromium are used.

However, 13 mass% chromium steel and duplex stainless steel are weaker to H ₂ S than high nickel alloy steel, so austenitic high nickel alloy steel is suitable in the most severe environment where CO ₂ and H ₂ S coexist. ing.

  However, it is known that austenitic high nickel alloy steel causes a characteristic stress corrosion cracking phenomenon. Therefore, for the purpose of improving this stress corrosion cracking, Mo-containing high Cr-high Ni alloy (hereinafter simply referred to as “high alloy”) has been developed, and it has been developed for nuclear power generation facilities, chemical plants, offshore, etc. Used in environments with high chloride ion concentrations.

  It is known that the corrosion resistance required in such a severe corrosive environment is exhibited in a state where the above alloy elements are dissolved.

  However, it has been clarified that the high alloy targeted in the present invention contains many alloying elements such as Mo and Cr and generates an intermetallic compound which is an embrittlement phase. Since the intermetallic compound σ phase and the soot phase are embrittled phases, not only will the toughness be reduced, but in a high-concentration chloride environment, the precipitation of these intermetallic compounds may reduce pitting corrosion. It has been reported. This is because the passive film of the part becomes unstable due to the precipitation of the σ phase and the soot phase.

  Here, the σ phase is an intermetallic compound of Fe and Cr in which elements such as Mo, W, Si, Nb, and Ti are dissolved. The soot phase is an intermetallic compound composed of Fe, Cr and Mo.

  Intermetallic compounds have a crystal structure completely different from that of the parent phase and are known to exhibit unique properties, and their precipitation depends greatly on the stoichiometric ratio and phase stability. Unlike, etc., solute elements by simple diffusion cannot be organized.

  As a conventional technique for avoiding such problems, a method of homogenizing heat treatment for the purpose of eliminating intermetallic compounds by diffusion is generally used in the processes after casting, but this method is naturally produced. This is not preferable because the construction period is prolonged and the loss cost is increased. Furthermore, long-time heat treatment leads to coarsening of crystal grains and causes cracks during processing. In view of the above problems, there is a need to omit the heat treatment step or at least shorten the heat treatment time.

  Therefore, the inventors focused on the control technology in the solidification process in which an intermetallic compound is formed, and advanced the research.

  As a control technique in the solidification process in which an intermetallic compound is formed, for example, Patent Documents 1 to 3 are proposed.

  In Patent Document 1, the total content of Cr, Mo and N 2 which improves crevice corrosion resistance is increased to 51% by mass or more, while the content of Si and Mn promotes the precipitation of intermetallic compounds. There has been proposed a method for producing an austenitic stainless steel that is excellent in crevice corrosion resistance and hot workability by reducing the amount as much as possible.

  However, when the N content is increased, the hot deformation resistance may increase, making pipe rolling difficult. Further, since the reduction of the Si content causes a problem of oxidation resistance at high temperatures, there is a limit to the reduction of the Si content.

  Further, in Patent Document 2, as in Patent Document 1, the content of Cr, Mo and N in the austenitic stainless steel is increased, and this is solidified by a powder process, so that macrosegregation, σ phase and μ phase are achieved. Techniques for suppressing the precipitation of intermetallic compounds such as are disclosed.

  However, the powder process requires special equipment and the problem of low productivity remains.

  Further, in Patent Document 3, by adding a mixture of rare earth metals having an atomic number of 57 to 71 called MM (Misch Metal) and Y 2, the solubility product in molten steel is controlled to a specific range and is intentionally generated. A method has been proposed in which the solidification structure is refined and densified using the rare earth metal composite compound as a nucleus to control segregation, thereby suppressing the formation of intermetallic compounds including the σ phase.

  The technique proposed in Patent Document 3 is considered to be a very useful means in terms of control of segregation by refinement of solidified structure and control of intermetallic compounds. However, the refinement technique proposed in Patent Document 3 is an effective means only when the primary crystal is in the ferrite phase, and can be applied to the austenitic stainless steel that is the γ single phase solidification targeted in the present invention. Can not. Also, since MM and Y are rare metals, there is a risk that the market price will fluctuate rapidly, which may make stable supply difficult.

Japanese Patent Laid-Open No. 10-60603 JP-A-6-306553 JP 2011-174183 A

  As described above, a steel piece cast for a steel pipe is required to omit a long heat treatment step or to reduce a heat treatment time from the viewpoint of process and cost reduction. However, the steel slab after casting of the high alloy that solidifies γ single phase contains a large amount of alloying elements such as Cr and Mo, so that intermetallic compounds (σ phase and soot phase) that are embrittlement phases are likely to occur, Not only is processability low, but pitting corrosion is a problem in high-concentration chloride environments such as seawater.

  On the other hand, in the prior art, a suppression technique in the process after changing the alloy content or casting is used. However, the technique by changing the alloy content may deteriorate the original material characteristics, can be used only for specific steel types, and has low versatility. Therefore, since many steel types require a long heat treatment after casting, it naturally leads to a prolonged production period and an increase in loss costs, and further a decrease in workability due to coarsening of crystal grains. Will be invited.

  The present invention has been made in view of the above-mentioned problems in the prior art, suppresses intermetallic compounds that are embrittled phases, and is suitable for steel pipes used in high-concentration chloride environments, and toughness and pitting corrosion. An object of the present invention is to provide a method for producing an austenitic stainless steel slab for a γ single-layer solidified steel pipe.

That is, the present invention
In mass%, C: 0.01 to 0.05%, Si: 0.05 to 0.5% or less, Mn: 0.1 to 1.5%, P: 0.05% or less, S: 0.005% or less, Cr: 15 to 30%, Mo: 1 to 10 %, Ni: 30-50% in addition to 0.05 to 2% of Sn, with the balance being a continuous casting of an alloy consisting of Fe and impurities to produce a steel pipe manufacturing material with excellent toughness and pitting resistance In a method of manufacturing a piece,
The amount of intermetallic compound precipitation I _{0 at the} center of the cross section of the continuous cast slab when Sn is not contained in the alloy, and the amount of intermetallic compound at the center of the cross section of the continuous cast slab of the alloy containing Sn The relationship between the ratio I 1 / I ₀ of the precipitation amount I and the average Md value of the alloy defined by the following formula, and the relationship between the average Md value of the alloy and the contained Sn amount, and the average Md value of the alloy The main feature is that the Sn content is such that the ratio I 2 / I ₀ is 0.8 or less.
Average Md value ＝ ΣΧ _i・ (Md) _i [eV]
Where Χ _i : atomic fraction of alloy component i [-]
(Md) _i : Md value of alloy component i [eV]

  Since the slab cast by the method of the present invention has a fine dendrite structure, precipitation of an intermetallic compound that is an embrittlement phase is suppressed, and the initial solidified γ grains are also smaller than conventional slabs. It is miniaturized.

  In the present invention, precipitation of intermetallic compounds (σ phase and soot phase), which is an embrittlement phase, is suppressed, and the initial solidified γ grains are also made finer than conventional ones. In addition to shortening the heat treatment step, the growth of austenite grains is suppressed even when reheating is performed during pipe production. Accordingly, a steel pipe slab having properties excellent in toughness and pitting resistance can be obtained.

And the Sn concentration is a diagram showing the relationship between the average Md value is generated indication of a primary dendrite arm spacing ratio d / d _0, and intermetallic compounds. It is a figure which shows the average Md value with respect to Sn density | concentration. Is a graph showing the relationship between the average Md value and the area ratio I / I ₀ of the intermetallic compound.

  As a result of intensive studies in order to solve the above-mentioned problems, the inventors have made it possible to add a predetermined amount of Sn, which is a surface active element, to the molten steel in the process of continuous casting. It has been found that slabs of austenitic stainless steel solidified by γ single-layer solidification with excellent toughness and pitting resistance suitable for steel pipes used in high-concentration chloride environments can be obtained by suppressing the precipitation of intermetallic compounds. Was completed.

Hereinafter, this knowledge will be described in detail.
A) Influence on intermetallic compounds Normally, the solidification structure of a continuous cast slab has a dendritic form. This dendrite is formed due to the diffusion of solute elements during the solidification process, and the solute elements are concentrated in the dendritic trees depending on the equilibrium partition coefficient. Since the equilibrium partition coefficient of Cr and Mo contained in the high alloy steel is less than 1.0, it concentrates in the intertree part, and intermetallic compounds (σ phase, soot phase) composed of these elements are generated in the intertree part. It is easy to do.

As described above, the intermetallic compound has a completely different crystal structure from the parent phase, and its precipitation is greatly influenced by the stoichiometric ratio. For example, the soot phase has a complex structure such as Fe ₁₈ Cr ₆ Mo ₅ . Since precipitation of intermetallic compounds having such a structure is caused by the concentration of solute elements in the intertree parts, the inventors conducted a unidirectional solidification test described later and observed Sn by observing the solidification structure. The effect of adding a predetermined amount was investigated.

  Microscope and SEM-EDX (Scanning Electron Microscope (hereinafter abbreviated as SEM) and energy dispersive X-ray detector (Energy Dispersive X-ray Detector; hereinafter abbreviated as EDX)) As a result of observation by the above, the inventors confirmed that the addition of Sn reduces the dendritic tree spacing and suppresses the precipitation of the intermetallic compounds of the cocoon phase and the σ phase.

  The following two effects can be considered as reasons why precipitation of intermetallic compounds is suppressed by adding a predetermined amount of Sn to the high alloy.

The first point is to reduce the concentration of solute elements between dendrites.
It is known that the concentration of solute elements depends on the dendrite trees, and by reducing this, concentration of solute elements itself is suppressed, and precipitation of intermetallic compounds is suppressed.

The second point is a change in phase stability of the intermetallic compound.
It has been reported that precipitation of intermetallic compounds is greatly affected by local phase stability, which means the electron orbital energy in the d orbital of each component of the alloy as one of the indicators of phase stability of multicomponent systems. A PHACOMP (Phase Computation) method has been established to predict the precipitation tendency using the Md value [eV].

The Md value of the alloy is defined by the average Md value of the following formula (1) (Morinaga et al., Iron and Steel, 71 (1985), p. 1441 to 1451, hereinafter referred to as Non-Patent Document 1).
Average Md value = ΣΧ _i · (Md) _i [eV]… (1)
Where Χ _i : atomic fraction of alloy component i [-]
(Md) _i : Md value of alloy component i [eV]

The average Md value of the alloy can arrange the intermetallic compounds by calculating Χ _i by converting the composition between the dendrite trees obtained from the initial composition and the segregation ratio into the atomic fraction.

  The inventors have found by a unidirectional solidification test described below that the alloy steel containing a predetermined amount of Sn in Non-Patent Document 1 does not exceed the critical value reported that an intermetallic compound is precipitated.

  This indicates that the concentration balance of solute elements between dendrite trees has changed. The reason for this is that the amount of solute diffused from the dendrite axis and the dendrite between dendrite trees are changed. It is presumed that the amount of back diffusion to the shaft has changed.

  As a result, the concentration balance of solute elements between dendritic trees finally changed, and it is considered that precipitation of intermetallic compounds was suppressed.

B) Effect on pitting corrosion The σ phase and the soot phase, which are intermetallic compounds, destabilize the passive film in that portion, and therefore the pitting corrosion is reduced in a high concentration chloride environment. Therefore, by containing a predetermined amount of Sn as described above, the formation of the σ phase and the soot phase is suppressed, and the pitting corrosion property is remarkably improved. Furthermore, since Sn is an element that improves weather resistance, Sn ions exert an inhibitory effect when they are included in steel materials even if, for example, accidental peeling of the passive film occurs during the construction of mining equipment. Therefore, the anodic dissolution reaction rate is remarkably reduced, so that the steel material itself has extremely high pitting corrosion.

C) Influence on crystal grains Since the growth of crystal grains largely depends on the initial γ grain size, the refinement of the initial γ grains has the effect of suppressing crystal grain coarsening due to reheating.

  The present invention has been completed on the basis of the above-mentioned knowledge, and is a method for continuously casting a slab for a high alloy steel pipe having the following γ single phase solidification. In the following description, “%” means “mass%” unless otherwise specified.

  In the present invention, “γ single-phase solidification” means primary γ solidification, and solidification is completed in the γ single phase, and then the transformation does not occur, and it reaches room temperature with almost solidified structure. Means a form of phase change including solidification in which crystallization or precipitation of an intermetallic compound such as a σ phase or a soot phase occurs.

That is, the present invention
C: 0.01 to 0.05%, Si: 0.05 to 0.5% or less, Mn: 0.1 to 1.5%, P: 0.05% or less, S: 0.005% or less, Cr: 15 to 30%, Mo: 1 to 10%, Ni: In addition to 30 to 50%, 0.05% to 2% Sn is contained, and the balance is continuously cast from an alloy consisting of Fe and impurities to produce a slab that is a material for producing steel pipes with excellent toughness and pitting corrosion. In the method
Precipitation amount I ₀ (= Χ ₀ + σ ₀ ) of intermetallic compound at the center of the cross section of the continuous cast slab when Sn is not contained in the alloy, and the continuous cast slab of the alloy containing Sn The relationship between the ratio I / I ₀ to the precipitation amount I (= I + σ) of the intermetallic compound at the center of the cross section and the average Md value of the alloy defined by the formula (1), and the average of the alloy The relationship between the Md value and the contained Sn content is determined, and the Sn content is such that the average Md value of the alloy is 0.92 or less and the ratio I / I ₀ is 0.8 or less.

  The component composition of the high alloy in the present invention and the reason for limitation will be described below.

C: 0.01-0.05%
C is a component necessary for forming carbide and ensuring high-temperature tensile strength and high-temperature creep strength necessary for a high Cr and high Ni alloy, and it is necessary to contain 0.01% or more. However, if its content exceeds 0.05%, Cr carbide increases, which may adversely affect the toughness of high Cr and high Ni alloys, so the upper limit was made 0.05%.

Si: 0.05-0.5% or less
Si is an element necessary for deoxidation of molten steel during smelting, and it is necessary to contain at least 0.05%. However, if the content is excessive, the workability of the alloy decreases, so the upper limit was made 0.5%.

Mn: 0.1 to 1.5%
Mn is an element necessary for deoxidation of molten steel, like Si. In order to obtain a deoxidation effect, a content of 0.1% or more is necessary. However, when the Mn content exceeds 1.5%, hot workability deteriorates. Therefore, in the present invention, the Mn content is set to 0.1 to 1.5%. A more preferable range is 0.5 to 1.0%.

P: 0.05% or less
P is inevitably mixed as an impurity. Since excessive P impairs workability, the upper limit was made 0.05% in the present invention. A desirable upper limit is 0.025%.

S: 0.005% or less
S, like P, is inevitably mixed as an impurity. Since excessive S impairs workability, the upper limit is set to 0.005% in the present invention. A desirable upper limit is 0.001%.

Cr: 15-30%
Cr is an effective element for improving the hydrogen sulfide corrosion resistance represented by stress corrosion cracking resistance in the presence of Ni. However, if the content is less than 15%, the effect cannot be obtained. On the other hand, if the content exceeds 30%, the above effect is saturated, which is not preferable from the viewpoint of hot workability. Therefore, in the present invention, the appropriate range for the Cr content is 15-30%.

Mo: 1-10%
Mo is an element having an effect of improving pitting corrosion resistance. However, if the content is less than 1%, the effect cannot be obtained. On the other hand, since the effect is saturated even if Mo exceeding 10% is contained, the upper limit was made 10% in the present invention.

Ni: 30-50%
Ni is an element having an action of improving hydrogen sulfide corrosion resistance. However, if the content is less than 30%, a Ni sulfide film is not sufficiently formed on the outer surface of the alloy, so that the effect of containing Ni cannot be obtained. On the other hand, even if Ni exceeding 50% is contained, the effect is saturated, so that an effect corresponding to the alloy cost cannot be obtained and the economic efficiency is impaired. Therefore, in the present invention, the appropriate range of Ni content is set to 30 to 50%.

Sn: 0.05-2%
Sn plays an important role in the present invention. When the alloy contains Sn, the solidification structure of the slab becomes finer, and the structure of the slab becomes uniform even in the alloy that tends to cause microsegregation. The effect of improving pitting corrosion resistance is obtained. In order to acquire the said effect, 0.05% or more of Sn content is required. However, if the Sn content exceeds 2%, embrittlement during hot working of the slab becomes a problem, so the Sn content is desirably 2% or less.

  By the way, although Sn is known as a low melting point metal, the melting point of pure Sn is 232 ° C. and the boiling point is 2602 ° C., which is higher than the temperature of molten steel, so that it is agglomerated in the tundish as pure Sn, or It is possible to input in granular form. However, in order to reduce the variation of Sn concentration in the molten steel, it is added by manufacturing SnNi wire as 75% Sn-25% Ni and inserting it into the molten steel in the tundish at the time of casting. did.

  The inventors added Sn to the molten steel in the process of continuous casting by the above method, and by adding a predetermined amount of Sn to the steel ingot, the dendrite structure was refined, and the precipitation of the intermetallic compound that was an embrittled phase. The reduction was found by the following unidirectional solidification test.

(Test conditions for unidirectional solidification test)
Unidirectional solidification tests are performed on ingots with a diameter of 15 mm and a height of 50 mm and ingots containing 0, 0.07, 0.15, 0.34, and 0.76% Sn, and ingots containing no Sn went. Cooling was performed only from the bottom of the cylinder, and the cooling rate was 5 to 15 ° C / min in accordance with the cooling rate of round billet continuous casting.

  The obtained ingot was etched under the following conditions, and the solidified structure was observed. At the time of observation, about 10 primary dendrite arm intervals extending substantially parallel to the axial direction in a longitudinal section passing through the axial center of the cylinder were measured, and an arithmetic average value was taken as the primary dendrite arm interval of each ingot.

  Furthermore, ten dendrite ridges were randomly selected from the surface of the longitudinal section of the sample after etching, and the average Md value was calculated by measuring the atomic fraction with SEM-EDAX. Since only the σ phase is known as the precipitation critical value of the intermetallic compound in the austenitic stainless steel, which is the γ single phase of interest in the present invention, the σ phase was evaluated in the present invention.

(Etching conditions)
Etching solution: 10% by volume oxalic acid aqueous solution Etching method: Electrolytic etching Etching solution temperature: Room temperature Etching time: 60 to 180 seconds

FIG. 1 is a graph showing the relationship between the Sn concentration, the primary dendrite arm spacing ratio, and the average Md value, which is an index of generation of intermetallic compounds. In FIG. 1, the primary dendrite arm interval d is shown on the main axis of the vertical axis, the ratio d / d ₀ to the primary dendrite arm interval d ₀ on the ingot containing Sn is shown, and the average Md value is shown on the second axis. In FIG. 1, the primary dendrite arm spacing ratio is indicated by a black circle, and the average Md value is indicated by a white circle.

  FIG. 1 shows that the higher the Sn concentration, the smaller the primary dendrite arm spacing ratio of the high alloy and the finer the dendrite structure. This is considered to be because Sn is an element having an effect of lowering the solid-liquid interface energy of the high alloy, and by adding a predetermined concentration, Sn is effective in reducing the primary dendrite arm interval.

  Further, in the sample containing no Sn from FIG. 1, the average Md value reported as an occurrence critical value of the intermetallic compound exceeds 0.92, whereas the average Md value of the sample containing Sn at a predetermined concentration is less than 0.92. Therefore, it can be said that the addition of Sn is effective in suppressing precipitation of intermetallic compounds.

In FIG. 1, a linear relationship was observed between the average Md value and the Sn concentration. When the relationship between the average Md value and the Sn concentration was determined, the following equation (2) was obtained.
Average Md value = [Sn%] x (-0.1768) + 0.9335 (2)
This was used for the continuous casting test described later.

  In order to confirm the effect of the continuous casting method of the steel pipe slab of the present invention, a continuous casting test of γ single-phase solidified high alloy steel was carried out under the following casting conditions, and the results were evaluated.

(Casting conditions)
Casting speed: 0.5 m / min Mold inner diameter: φ360mm
Added Sn alloy: NiSn (Use 75% Sn-Ni outer diameter φ10mm wire)
Addition position of Sn alloy: in tundish

  Table 1 below shows the component compositions of the steels of Examples 1 to 5 that were continuously cast under the conditions specified in the present invention and Comparative Example 1 that deviated from the conditions specified in the present invention. Table 2 below shows the Sn concentration in the slab after continuous casting, the dendrite arm spacing ratio, the area ratio of the intermetallic compound, and the average Md value obtained using the above equation (2). Furthermore, a tensile test piece having an outer diameter of 8 mm is collected from the vicinity of the center of the cross section of the slab, and the result of the tensile test at 1300 ° C. (drawing value) and the corrosion index are shown together.

The dendrite arm spacing ratio d / d ₀ in Table 2 is obtained as follows using a test piece taken from a surface perpendicular to the casting direction at a position 50 mm from the center of the cross section of the slab to the outer diameter surface layer side. It was. First, after performing the above-described etching on the test piece, the primary dendrite arm interval is measured. The value obtained by arithmetically averaging the measured values is used as the primary dendrite arm interval d of the slab, and the ratio d / d ₀ (reduction rate) with the primary dendrite arm interval d _{0 of} steel not added with Sn (Comparative Example 1). Is calculated.

Moreover, observation of the intermetallic compound was performed as follows.
First, after re-polishing the specimen taken from the surface perpendicular to the casting direction to the center of the cross section of the slab to the mirror surface, Murakami's reagent (10% KOH + 10% K ₃ [Fe Etching was performed with (CN) ₆ ] + balance H ₂ O). Thereafter, the metal structure of the cross section of the slab was observed, and the metal when Sn was added on the basis of the area ratio I ₀ of the intermetallic compound (slag phase and σ phase) of the steel not added with Sn (Comparative Example 1). The generation index of the intermetallic compound obtained by taking the ratio of the area ratio I of the intermetallic compounds (slag phase and σ phase) was evaluated as the area ratio I / I ₀ (reduction rate). At this time, 30 fields of images with an area of 0.64 × 0.46 mm per field of view were observed with an image analyzer, and the average area ratio of intermetallic compounds (slag phase and σ phase) was used as the representative area ratio of each test condition. .

In the hot drawing test, the drawing value is calculated from the outer diameter before the test and the diameter of the fracture surface after the test in the tensile test at 1300 ° C, which is the pipe making temperature, and steel without adding Sn (Comparative Example 1) of the aperture Ra ₀ as a reference, was evaluated as the ratio Ra / Ra ₀ aperture value Ra of the steel with the addition of Sn (rate of change).

In addition, pitting corrosion resistance was evaluated according to the ferric chloride corrosion test method for stainless steel specified in JIS G0578, and immersed in a 6% by volume FeCl ₂ + 0.05N HCl aqueous solution at 50 ° C for 48 hours. The degree of corrosion was determined from the amount of weight loss before and after. At this time, the ratio P / P ₀ (rate of change) of the weight loss P of the steel to which Sn was added was evaluated as a corrosion index based on the weight loss P ₀ of the steel to which Sn was not added (Comparative Example 1).

  The Sn concentration in the slab is a value obtained as a result of investigating the Sn concentration contained in the slab by conducting a gas analysis from the chips prepared from the slab obtained in the continuous casting test.

  The average Md value is a value obtained by calculating the Md value with respect to the Sn concentration in the continuous casting test by the above equation (2).

  From Table 2, in Examples 1-5 of this invention, compared with the comparative example 1, the reduction | decrease of the area ratio of the sigma phase which is a dendrite arm space | interval and an intermetallic compound can be confirmed, and it is processed also from the value of a hot drawing test. Improvement effect was obtained. Furthermore, the effect of improving pitting corrosion resistance was confirmed from the ferric chloride corrosion test.

Furthermore, the results obtained by plotting the relationship between the area ratio I / I ₀ of the average Md value and the intermetallic compounds in Table 2 in FIG.

As shown in FIG. 3, when the average Md value reported as the generation critical value of the intermetallic compound is 0.92, the area ratio I / I ₀ of the intermetallic compound is 0.76, There was a reduction effect.

In addition, the inventors have reduced the area ratio I / I ₀ of the intermetallic compound to 0.30 or less, that is, the intermetallic compound can be significantly reduced to 1/3 or less of the comparative example, and 35% or more improvement in corrosivity. As a value showing the effect, it was found that an Sn addition amount of 1.2% or more so that the average Md value is 0.72 or less is desirable (see FIG. 2). However, as described above, the upper limit of the Sn addition amount is preferably 2% because of the problem of embrittlement during hot working of the slab.

Claims (2)

In mass%, C: 0.01 to 0.05%, Si: 0.05 to 0.5% or less, Mn: 0.1 to 1.5%, P: 0.05% or less, S: 0.005% or less, Cr: 15 to 30%, Mo: 1 to 10 %, Ni: 30-50% in addition to 0.05 to 2% of Sn, with the balance being a continuous casting of an alloy consisting of Fe and impurities to produce a steel pipe manufacturing material with excellent toughness and pitting resistance In a method of manufacturing a piece,
The amount of intermetallic compound precipitation I _{0 at the} center of the cross section of the continuous cast slab when Sn is not contained in the alloy, and the amount of intermetallic compound at the center of the cross section of the continuous cast slab of the alloy containing Sn The relationship between the ratio I 1 / I ₀ of the precipitation amount I and the average Md value of the alloy defined by the following formula, and the relationship between the average Md value of the alloy and the contained Sn amount, and the average Md value of the alloy There at 0.92, the continuous casting method of steel for insert piece, characterized in that the Sn content ratio I / I ₀ of the is 0.8 or less.
Average Md value ＝ ΣΧ _i・ (Md) _i [eV]
Where Χ _i : atomic fraction of alloy component i [-]
(Md) _i : Md value of alloy component i [eV] When the Sn is contained 1.2% by mass or more,
2. The continuous casting method for a steel pipe slab according to claim 1, wherein the alloy has an Sn concentration such that an average Md value of the alloy is 0.72 or less and the ratio I / I ₀ is 0.30 or less.

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