JP2017080765A - Continuous casting method for casting piece for steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for restraining formation of an embrittlement phase when continuously producing a highly corrosion-resistant duplex stainless steel.SOLUTION: When continuously casting an alloy comprising, by mass%, 0.03% or less C, 0.4% or less Si, 3.0% or less Mn, 0.04% or less P, 0.008% or less S, 0.2-2.0% Cu, 5.0-7.0% Ni, 23-30% Cr, 2.5-5.0% Mo, 1.5-4.0% W, 0.24-0.40% N, 0.030% or less Al, and 10-300 ppm by mass, a relation between a ratio I/Iof the deposition amounts Iand I of intermetallic compound at a center of cross-section of the casting piece in such a case that Bi is not contained and in a case contained and an average Md value of the alloy defined by ΣX×(Md)[eV] (Xis the atomic fraction of an alloy component i; and (Md)is the Md value [eV] of the alloy component i), and a relation between the average Md value of the alloy and included Bi amount are determined, and such a Bi content that the average Md value of the alloy is 0.97 or less and the ratio I/Igets to 0.70 or less is provided.SELECTED DRAWING: Figure 4

Description

  The present invention is a method for continuously casting a high corrosion resistance duplex stainless steel used as a steel pipe slab. Specifically, while producing high corrosion resistance duplex stainless steel, by suppressing the precipitation of intermetallic compounds such as sigma (σ) phase and chi (カ イ) phase, which are embrittled phases, while maintaining high corrosion resistance, The present invention relates to a method for continuously casting a high corrosion resistance duplex stainless steel having excellent embrittlement resistance, castability, and hot workability.

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 having high strength and high corrosion resistance that can withstand harsh environments have been developed. . In particular, in such a harsh environment, a corrosive gas such as CO ₂ or H ₂ S is often contained, and duplex stainless steel having excellent corrosion resistance and strength is frequently used.

  PRE (Pitting Resistance Equivalent: Cr + 3.3Mo + 16N) or PREW (Cr + 3.3 (Mo + 0.5W), which is a formula that adds W to PRE, as a parameter indicating the corrosion resistance of duplex stainless steel, especially pitting corrosion resistance ) + 16N) is known, and in general, the material is designed with the Cr, Mo, W and N contents adjusted to increase the PRE or PREW value.

  On the other hand, the problem with duplex stainless steels with increased Cr, Mo, and W contents is that intermetallic compounds such as σ phase and anodic phase, which lower mechanical properties and corrosion resistance, are likely to precipitate. In particular, if a solidified segregation portion at the time of casting remains, it becomes more likely to precipitate due to the formation of a brittle phase in the remaining region and an increase in the precipitation temperature. This precipitation of sigma phase and soot phase significantly hardens the material and is prone to cracking, resulting in extremely low workability, and even in the final product, the corrosion resistance and toughness around the intermetallic compound deteriorate. It becomes difficult to ensure the desired performance.

  Therefore, in the production of high corrosion resistance duplex stainless steel, it is desirable to suppress the precipitation of intermetallic compounds as much as possible, and many proposals have heretofore been made.

  For example, Patent Document 1 proposes a method for producing a highly corrosion-resistant duplex stainless steel seamless pipe having an increased content of Cr, Ni, Mo and N 2 in order to improve corrosion resistance and strength. The method proposed in Patent Document 1 defines the temperature range and treatment time of the soaking heat treatment performed before the final process as well as the reduction of the Si content, and also defines the subsequent solution heat treatment conditions. And by doing in this way, precipitating of an intermetallic compound is suppressed and it is supposed that the high intensity | strength duplex stainless steel seamless pipe which has very excellent corrosion resistance and favorable toughness can be mass-produced stably.

  In Patent Document 2, in a method of manufacturing a high-strength duplex stainless steel seamless pipe by heating the duplex stainless steel to 1110 ° C. or higher and then performing hot working, 800 + 5Cr + 25Mo after the final rolling is completed. There has been proposed a method for producing a high-strength duplex stainless steel pipe that is reheated to a temperature range satisfying + 15W ≦ T (° C.) ≦ 1150, solution treated, and then rapidly cooled.

  Thus, with respect to the precipitation of intermetallic compounds, a homogenization process is performed for the purpose of diffusing and eliminating solidification segregation that causes the precipitation of intermetallic compounds in the processes after continuous casting (Patent Document 1). ), A method of managing the heat treatment conditions (Patent Document 2) is common.

  However, the soaking process is not preferable because it naturally increases the production period and increases the loss cost. Moreover, in order to prevent precipitation of intermetallic compounds in the cooling process after heat treatment, it is necessary to set the cooling start temperature to be equal to or higher than the precipitation temperature of intermetallic compounds. It becomes a structure, and new problems arise, such as adversely affecting the hydrogen embrittlement resistance.

  Then, the technique which paid its attention to the solidification process in which an intermetallic compound precipitates is described or proposed in the patent documents 3-5 instead of the technique in the process after continuous casting.

  In Patent Document 3, an increase in Cr and Mo is effective for increasing the PRE value and improving the corrosion resistance of the duplex stainless steel. However, these elements preferably promote the formation of intermetallic compounds (σ phase, etc.). It is described that it is effective to determine the contents of Cr, Mo and Si so that PSI (Phase Stability Index: Cr + 3.3Mo + 3Si) is 40 or less. It is said that no intermetallic compound is produced under the heating conditions, heat treatment conditions and welding conditions during the inter-working.

  Patent Document 4 proposes a duplex stainless steel in which the content of Mo is reduced to suppress the formation of the σ phase, and the ferrite content and PREW are in a predetermined range.

  However, if the content of Cr or Mo is reduced as in Patent Documents 3 and 4, the strength and corrosion resistance as a duplex stainless steel are impaired.

  Further, Patent Document 5 contains MM (a mixture of rare earth metals having an atomic number of 57 to 71) and / or Y 2 in a total amount of 0.0001 to 1.0% by mass to diffuse a very brittle intermetallic compound. And by slowing the deposition rate and using a small amount of RE-based composite compound or Ba oxide, further preventing the diffusion of Cr, Mo, Si, W, reducing the deposition rate of intermetallic compounds, Techniques for suppressing precipitation have been proposed.

  However, MM and Y added by the technique proposed in Patent Document 5 tend to form (composite) oxides, so that nozzle clogging is likely to occur, and stable production becomes difficult. To do is also a challenge.

Japanese Patent Laid-Open No. 4-165019 Japanese Patent Laid-Open No. 9-241746 JP-A-5-132741 Special table 2003-503596 gazette JP 2011-174183 A

  The problem to be solved by the present invention is that the techniques proposed in Patent Documents 1 and 2 are techniques for suppressing the precipitation of intermetallic compounds in the processes after continuous casting, such as heat treatment and an increase in the number of processes. Longer time and increased loss cost. In addition, the heat treatment results in a coarse structure and adversely affects the hydrogen embrittlement resistance.

  In addition, since the techniques described or proposed in Patent Documents 3 and 4 reduce the content of Cr and Mo, which greatly affects high strength and high corrosion resistance, there is a possibility that desired characteristics may not be obtained. is there.

  The technique proposed in Patent Document 5 is thought to be useful because it delays the diffusion and precipitation rate of intermetallic compounds, but it is prone to nozzle clogging and is not suitable for stable production. . In addition, the price of the additive element is high, resulting in an increase in loss cost.

  That is, in the case of the prior art in which the precipitation of intermetallic compounds is suppressed in the processes after continuous casting, the production period is prolonged and the loss cost is increased, such as heat treatment and an increase in the number of processes. In addition, regarding the technique for suppressing precipitation of intermetallic compounds in continuous casting, there has been a problem that a technique for suppressing precipitation of intermetallic compounds without affecting desired characteristics and operability has not been established.

  The present invention has been made in view of the above circumstances, and during the production of highly corrosion-resistant duplex stainless steel, precipitation of intermetallic compounds such as σ phase and slag phase, which is an embrittlement phase, is carried out by reducing the content of Cr and Mo. It aims at providing the manufacturing method of the duplex stainless steel which has the more excellent embrittlement resistance, castability, and hot workability, suppressing without reducing and maintaining high corrosion resistance.

That is, the present invention
In mass%, C: 0.03% or less, Si: 0.4% or less, Mn: 3.0% or less, P: 0.04% or less, S: 0.008% or less, Cu: 0.2-2.0%, Ni: 5.0-7.0%, Cr: In addition to 23 to 30%, Mo: 2.5 to 5.0%, W: 1.5 to 4.0%, N: 0.24 to 0.40%, Al: 0.030% or less, Bi is further contained in 10 to 300 ppm by mass, the balance being Fe and In the method of continuously casting an alloy made of impurities to produce a slab that is a material for steel pipe production,
The amount of intermetallic compound precipitation I _{0 at the} center of the cross-section of the continuous cast slab when Bi 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 Bi 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 Bi content, and based on this relationship, The most important characteristic is that the Bi content is such that the average Md value of the alloy is 0.97 or less and the ratio I / I ₀ is 0.70 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]

  In the present invention, the Bi content may be added by a predetermined content by inserting a wire alloyed with Bi into the molten steel in the tundish at the time of casting.

  In the present invention, an electromagnetic stirring force is applied to at least one of the molten steel in the mold and the unsolidified molten steel in the secondary cooling zone, thereby exhibiting a higher suppression effect of precipitation of intermetallic compounds.

  The slab cast by the above-described method of the present invention has a fine structure, and changes in phase stability suppress precipitation of an intermetallic compound that is an embrittled phase.

  The continuous cast slab produced by the method of the present invention is a slab for steel pipes, in which precipitation of intermetallic compounds (σ phase and slag phase) which are embrittled phases is suppressed, and which has excellent workability. Can be obtained.

It is a figure explaining the evaluation position of the intermetallic compound of this invention. And Bi concentration, the ratio d / d ₀ solidification structure roughness, and a diagram showing the relationship between the average Md value is generated indication of intermetallic compounds. It is a figure which shows the average Md value with respect to Bi 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 found that when a predetermined amount of Bi, which is a surface active element, is contained in the molten steel in the process of continuous casting, compared to the case of not containing Bi, It was found that a duplex stainless steel slab suitable for a steel pipe excellent in workability in which precipitation of intermetallic compounds was suppressed, and the present invention 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. In a duplex stainless steel known as a complex structure, ferrite, which is a primary crystal, crystallizes and grows into dendrites, and then austenite crystallizes by a peritectic eutectic reaction, and solidification is completed in the ferrite and austenite two phases. At this time, even when the dendrite-like columnar crystal is changed to a branched columnar crystal or an equiaxed crystal in the solidification process, austenite is crystallized from the intertree portion of the structure where the solute is discharged. In addition, it is known that ferrite is transformed into austenite even in the cooling process to room temperature, and its form changes greatly.

  Thus, the solidified structure is formed due to diffusion of solute elements in the solidification process, and the solute elements are concentrated in dendrites, branched columnar crystals, and the intertree portions of the columnar crystals depending on the equilibrium partition coefficient. Since the equilibrium partition coefficient of Cr and Mo contained in the duplex stainless steel is less than 1.0, it concentrates in the tree part, and microscopically, at the grain boundary that becomes the final solidification position macroscopically, especially after austenite crystallization. As shown in FIG. 1, solidification segregation remains in the central portion that becomes the final solidified portion 1, and the solution and precipitation temperature of the embrittled phase increases in the region, thereby causing intermetallic compounds (σ phase, phase ) Is more likely to precipitate. In FIG. 1, 2 indicates a columnar crystal region formed in the outer peripheral portion, and 3 indicates a branched columnar crystal region or equiaxed crystal region between the columnar crystal region 2 and the final solidified portion 1.

Intermetallic compounds have a completely different crystal structure from the parent phase, and their 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 . Precipitation of intermetallic compounds having such a structure is due to the concentration of solute elements in the ferrite-austenite grain boundary, which is the final solidified part, and in the central part, which is the final solidified part, macroscopically. The inventors conducted a unidirectional solidification test, which will be described later, and investigated the effect of adding a trace amount of Bi, which is a surface active element, by observing the solidified structure.

  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 the observation, the inventors confirmed that the addition of Bi refined the solidified structure, and the precipitation of the intermetallic compound of the liquid phase and the σ phase was significantly suppressed.

  The following two effects can be considered as the reason why precipitation of intermetallic compounds is suppressed by adding a small amount of Bi, which is a surface active element, to the duplex stainless steel.

The first point is a reduction in concentration of solute elements at the grain boundaries and the final solidified part.
It is known that the concentration of the solute element depends on the roughness of the solidified structure, and by reducing this, the concentration of the solute element itself is suppressed and the 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 inventors conceived of using the Md value to organize and evaluate the suppression state of precipitation of intermetallic compounds due to Bi addition. 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 Md value of the alloy component i can be obtained by cluster calculation (a molecular orbital calculation method using a cluster model of atoms consisting of several to tens of atoms) (M. Morinaga et al., J. Phys. Soc. Jpn., 53 (1984), p. 653).

The average Md value of the alloy can be arranged from the precipitation of intermetallic compounds by calculating Χ _i by converting the composition of the grain boundary and final solidified part calculated from the initial composition and segregation ratio into atomic fraction. It is.

  That is, when the Md value is large with respect to the precipitation critical value unique to the intermetallic compound, the phase of the intermetallic compound is stabilized and is likely to precipitate. Conversely, a smaller Md value indicates a tendency to suppress precipitation.

  It has been reported that the precipitation of intermetallic compounds is also greatly affected by the temperature history, and in order to observe by experiment, a thermal history and a rolling ratio simulating pipe making and the subsequent heat treatment process were given, If it is necessary to evaluate using a reagent for revealing an intermetallic compound and the precipitation tendency of the intermetallic compound can be predicted from the as-cast structure using the Md value, it can be said to be a simpler control method.

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

  This indicates that the concentration balance of the solute elements at the grain boundaries and the final solidification zone has changed. The reason for this is that the solidification structure (dendrites, branched columnar crystals, equiaxed crystals) has changed by changing the interval. It is presumed that the amount of diffusion of the solute from the tissue axis and the amount of back diffusion from the space between the solidified tissues to the solidified tissue axis 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) Influence on final structure and mechanical properties Duplex stainless steel is a two-phase structure of ferrite and austenite, and it is generally known that the solidified structure is fine. However, the addition of a small amount of Bi makes both structures finer and has the effect of further increasing the strength.

  The present invention has been completed based on the above-described knowledge, and is a method for continuously casting the following duplex stainless steel slabs for steel pipes. In the following description, “%” means “mass%” and “ppm” means “mass ppm” unless otherwise specified.

That is, the present invention
C: 0.03% or less, Si: 0.4% or less, Mn: 3.0% or less, P: 0.04% or less, S: 0.008% or less, Cu: 0.2-2.0%, Ni: 5.0-7.0%, Cr: 23-30% , Mo: 2.5 to 5.0%, W: 1.5 to 4.0%, N: 0.24 to 0.40%, Al: 0.030% or less, and further containing 10 to 300ppm of Bi with the balance being Fe and impurities. In a method for producing a slab that is cast and becomes a material for steel pipe production,
The amount of intermetallic compound precipitation I _{0 at the} center of the cross-section of the continuous cast slab when Bi 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 Bi 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 Bi content, and based on this relationship, The Bi content is such that the average Md value of the alloy is 0.97 or less and the ratio I / I ₀ is 0.70 or less.

  The component composition of the duplex stainless steel in the present invention and the reason for limitation will be described below.

C: 0.03% or less
C is an element necessary for stabilizing the austenite phase. However, if the content is excessive, carbides are likely to precipitate and the corrosion resistance deteriorates. Therefore, in the present invention, the upper limit of the C content is set to 0.03%. A more preferred upper limit is 0.02%. In order to obtain the above effect, it is desirable to contain 0.01% or more of C 2.

Si: 0.4% or less
Si is an element necessary for deoxidation of molten steel during smelting. However, since the formation of the σ phase is promoted when the content is excessive, the upper limit of the Si content is set to 0.4% in the present invention.

Mn: 3.0% or less
Like Si, Mn is an element necessary for deoxidation of molten steel and an element effective for stabilizing the austenite phase. Mn is also an element contributing to improvement of hot workability. Furthermore, Mn has the effect of increasing the solubility of N 2. However, if the Mn content is excessive, the corrosion resistance is deteriorated. Therefore, in the present invention, the upper limit of the Mn content is set to 3.0%.

P: 0.04% or less
P is inevitably mixed as an impurity. However, if the P content is excessive, the corrosion resistance and toughness deteriorate significantly. Therefore, in the present invention, the upper limit of the content of P is set to 0.04%.

S: 0.008% or less
S, like P, is inevitably mixed as an impurity. Excessive S impairs workability, and sulfides are the starting point for pitting corrosion and deteriorate pitting corrosion resistance. Therefore, in the present invention, the upper limit of the content of S is set to 0.008%.

Cu: 0.2-2.0%
Cu is an element particularly effective for improving the corrosion resistance in a low pH environment, for example, H ₂ SO ₄ or H ₂ S environment, which is considered to have low reducing ability. In order to acquire these effects, it is necessary to contain Cu 0.2% or more. However, when excessively contained, not only the hot workability is deteriorated, but also precipitation of intermetallic compounds is promoted. Therefore, in the present invention, the upper limit of the Cu content is set to 2.0%.

Ni: 5.0-7.0%
Ni is an essential element for stabilizing the austenite phase. However, if the content is low, the amount of ferrite becomes too large and the characteristics as a duplex stainless steel are lost. In addition, since the solid solubility of N 2 in ferrite is small, when the amount of ferrite increases, nitrides are likely to precipitate and the corrosion resistance deteriorates. Therefore, in the present invention, the lower limit of the Ni content is set to 5.0%. On the other hand, when the Ni content is excessive, precipitation of intermetallic compounds is facilitated and toughness is deteriorated. Therefore, in the present invention, the upper limit of the Ni content is set to 7.0%.

Cr: 23-30%
Cr is an essential element for ensuring corrosion resistance and strength. However, if the content is low, corrosion resistance sufficient for so-called super duplex stainless steel cannot be obtained. Therefore, in the present invention, the lower limit of the Cr content is set to 23%. On the other hand, when the content is excessive, precipitation of intermetallic compounds becomes remarkable, which causes a decrease in hot workability as well as a decrease in corrosion resistance. Therefore, in the present invention, the upper limit of the Cr content is set to 30%.

Mo: 2.5-5.0%
Mo is an element having an effect of improving pitting corrosion resistance. It is also an element useful for increasing the strength of steel. However, if the content is less than 2.5%, the effect cannot be obtained. On the other hand, when the content is excessive, it causes precipitation of intermetallic compounds like Cr. Therefore, in the present invention, the upper limit of the Mo content is set to 5.0%.

W: 1.5-4.0%
W is an element that causes less precipitation of intermetallic compounds than Mo and improves corrosion resistance, particularly pitting corrosion resistance and crevice corrosion resistance. Further, it is an element that is very effective for increasing the strength of steel. If W is appropriately contained, high corrosion resistance can be secured without increasing the contents of Cr, Mo, and N. Therefore, in the present invention, the lower limit of the content of W is set to 1.5%. On the other hand, even if W is excessively contained, the effect of improving the corrosion resistance is saturated. Therefore, in the present invention, the upper limit of the content of W is 4.0%.

N: 0.24-0.40%
N is a strong austenite-forming element, and is an element effective for improving the thermal stability and corrosion resistance of duplex stainless steel and increasing the strength. In order to achieve an appropriate balance between the ferrite phase and the austenite phase, it is necessary to appropriately contain N 2 in relation to the contents of the ferrite-forming elements Cr and Mo. N, like Cr, Mo and W, also has the effect of improving the corrosion resistance of the alloy. Therefore, in the present invention, the lower limit of the N 2 content is set to 0.24%. On the other hand, when the content is excessive, the toughness and corrosion resistance of the steel are deteriorated due to defects due to the occurrence of blowholes, nitride formation due to the thermal effect during welding, and the like. Therefore, in the present invention, the upper limit of the N 2 content is set to 0.40%.

Al: 0.030% or less
Al is an element used for deoxidizing molten steel during refining, but if nitride (AlN) is formed, the toughness may be reduced. Therefore, in the present invention, the lower limit of the Al content is 0.030%.

Bi: 10-300ppm
Bi plays an important role in the present invention. When the alloy contains Bi, the solidification structure of the slab becomes finer, and the structure of the slab becomes uniform even in the alloy, which is prone to microsegregation, and precipitation of intermetallic compounds such as desired σ phase and slag phase occurs. The effect of suppressing is acquired. In order to obtain this effect, a Bi content of 10 ppm or more is necessary. However, if the Bi content exceeds 300 ppm, the embrittlement in the hot working of the slab becomes a problem, although it is a small amount, and also causes an increase in cost. Therefore, in the present invention, the upper limit of the Bi content Was set to 300 ppm.

  By the way, Bi is known as a low-melting-point metal. Pure Bi has a melting point of 274 ° C. and a boiling point of 1564 ° C., which is lower than the temperature at which molten steel is discharged. Therefore, it is desirable to add BiNi wire alloyed with Ni for the purpose of raising the melting point and boiling point as 75% Bi-25% Ni and insert it into the molten steel in the tundish at the time of casting.

  The inventors refined the solidification structure of the steel and made it brittle by adding Bi to the molten steel in the process of continuous casting by the above method and adding a small amount (10 ppm or more and 300 ppm or less) of Bi to the steel ingot. It was found by the following unidirectional solidification test that precipitation of intermetallic compounds as phases can be reduced.

  Further, the inventors have made an electromagnetic stirring force to act on at least one of the molten steel in the mold and the unsolidified molten steel in the secondary cooling zone with respect to the molten steel containing Bi from the actual machine test described later. It was found that the precipitation of intermetallic compounds can be effectively reduced even with a small amount of Bi added.

  The electromagnetic stirring conditions are all aimed at increasing the number density of nuclei by melting the solidified structure. In the case of molten steel in the mold, a coil for electromagnetic stirring is installed at a depth of 300 mm from the molten metal surface, and the solidified shell Casting is performed in the range where the stirring flow velocity at the front is 40 to 50 cm / s. On the other hand, the electromagnetic stirring in the secondary cooling zone is applied so that a coil is installed directly under the mold and the stirring flow velocity on the front surface of the solidified shell is approximately the same as when the molten steel in the mold is electromagnetically stirred. The electromagnetic stirring in the secondary cooling zone has the effect of expanding the supercooling region by low-temperature casting, in addition to the increase in the number density of nuclei due to the above-mentioned solidification structure cutting, and contributes to the increase in equiaxed crystal ratio.

  The result is shown in FIG. 3 as described later. When Bi concentration is 40 ppm, the average Md value when electromagnetic stirring is not applied is about 0.9 eV, but when electromagnetic stirring is applied only to one of the molten steel in the mold and the unsolidified molten steel in the secondary cooling zone. The average Md value was 0.7 eV, and the average Md value was about 0.5 eV when electromagnetic stirring was applied to both the molten steel in the mold and the unsolidified molten steel in the secondary cooling zone.

  The effect is that by applying electromagnetic stirring, the structure of the dendrites of ferrite, which is the initial solidification structure, is divided, and the number density of very fine nuclei is increased, thereby increasing the equiaxed crystal ratio. This is probably because the solidification segregation to the central portion that is the final solidification portion 1 shown in FIG. 1 is reduced, resulting in a more isotropic structure.

(Test conditions for unidirectional solidification test)
Unidirectional solidification tests were performed on ingots having a diameter of 15 mm and a height of 50 mm and ingots containing Bi, 11 ppm, 23 ppm, 38 ppm, and 52 ppm, and ingots containing no Bi. 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 continuous casting.

  The obtained ingot was etched under the following conditions, and the solidified structure was observed. The solidified tissue is a two-phase tissue, and comparison of solidified tissues by dendrite arm interval measurement is not possible. Therefore, in observation, for each observation field of the sample after etching, 10 lines perpendicular to the casting direction were drawn, and the arithmetic average value was obtained by counting the number of lines with a certain length crossing the grain boundary. It was set as the solidification structure roughness of each ingot.

  Furthermore, 20 ferrite-austenite grain boundaries were randomly selected from the surface of the longitudinal section of the sample after etching, and the Md value was calculated by measuring the atomic fraction with SEM-EDAX.

(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. 2 is a graph showing the relationship between the Bi concentration and the solidified structure roughness ratio, and the average Md value, which is an index of generation of intermetallic compounds. In FIG. 2, the main axis of the vertical axis shows the ratio d / d ₀ of the solidified structure roughness d to the solidified structure roughness d ₀ of the ingot containing Bi, and the second axis shows the average Md value. In FIG. 2, the ratio of solidified structure roughness is indicated by black circles, the average Md value is indicated by white circles, and the average Md reported as the critical value of occurrence of intermetallic compounds (σ phase) in duplex stainless steel. The value (= 0.97) is indicated by a broken line. In the present invention, the average Md value is 0.97 or less. Thereby, in the present invention, the σ phase that is an intermetallic compound is hardly generated, and cracking during processing caused by the σ phase can be prevented.

  FIG. 2 shows that the higher the Bi concentration, the smaller the ratio of solidified structure roughness of the duplex stainless steel, and the finer the solidified structure. This is an element that Bi has the effect of lowering the solid-liquid interface energy of duplex stainless steel, and even if its content is very small, it has an effect on refining primary ferrite, and the second phase austenite is also combined. This is thought to be due to the miniaturization.

  Furthermore, the sample containing Bi does not exceed the critical value for the generation of intermetallic compounds (σ phase) in the duplex stainless steel, whereas the average Md value for the sample containing Bi is less than the critical value for generation, as shown in FIG. It can be said that addition of Bi is effective in suppressing precipitation of intermetallic compounds.

In FIG. 2, a linear relationship is recognized between the average Md value and the Bi concentration. When the relationship between the average Md value and the Bi concentration is obtained, the following equation (2) is obtained.
Md value = [Bi%] x (-0.0019) + 0.9847 (2)
This was used for the continuous casting test described later.

Further, as will be described later, in the duplex stainless steel targeted by the present invention, the average Md value and the area ratio I / I ₀ obtained by the above method show the relationship shown in FIG. In the duplex stainless steel, the hot workability can be remarkably improved if the area ratio I 1 / I ₀ is 0.7 or less. Accordingly, in the present invention, the relationship between the area ratio ratio I / I ₀ and the average Md value as shown in FIG. 4 is obtained, and based on this, the average Md is such that the area ratio ratio I / I ₀ is 0.7 or less. A value is obtained, and further, based on the above equation (2), a Bi concentration such that such an average Md value is obtained is obtained, and Bi is added so as to obtain such a Bi concentration.

In this way, it is possible to obtain a desired cast piece area ratio I / I ₀ without excessive addition of Bi leading to loss cost.

  The above equation (2) is an equation when electromagnetic stirring is not applied, and when electromagnetic stirring is applied, the relationship between the Md value and [Bi%] changes. The Bi concentration may be determined based on this relational expression.

  In order to confirm the effect of the continuous casting method for steel pipe slab of the present invention, a continuous casting test of duplex stainless steel was carried out under the following casting conditions, and the result was evaluated.

(Casting conditions)
Casting speed: 0.4 m / min Mold size: width 600 mm x thickness 280 mm
Added Bi alloy: NiBi (use 75% Bi-Ni outer diameter φ10mm wire)
Bi alloy addition position: in tundish

  Table 1 below shows the component compositions of the steels of Examples 1 to 8 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. Examples 1 to 5 and Comparative Example 1 are examples in which electromagnetic stirring is not applied to the molten steel in the mold and unsolidified molten steel in the secondary cooling zone. Example 6 is an example in which electromagnetic stirring is applied only to the molten steel in the mold. Example 7 is an example in which electromagnetic stirring is applied only to unsolidified molten steel in the secondary cooling zone, and Example 8 is an example in which electromagnetic stirring is applied to molten steel in the mold and unsolidified molten steel in the secondary cooling zone. The conditions of electromagnetic stirring applied to the molten steel in the mold and the unsolidified molten steel in the secondary cooling zone are as described above.

Table 2 below shows the Bi concentration in the slab after continuous casting, the ratio d / d ₀ of the solidified structure roughness, the area ratio I / I _{0 of} the intermetallic compound, and the average Md value. Furthermore, the results (drawing ratio) of a tensile test piece having an outer diameter of 8 mm taken from the vicinity of the center part of the slab and subjected to a tensile test at 1300 ° C. are also shown.

The ratio d / d ₀ of the solidified structure roughness in Table 2 is as follows using a test piece taken from a surface perpendicular to the casting direction at a position of 50 mm from the center of the cross section of the slab to the outer surface layer side. Asked. After the above-mentioned etching was performed on the test piece, the solidified structure roughness was measured. Then, the solidification structure roughness d of the slab the arithmetic mean value with the measured values, the ratio d / d ₀ (reduction ratio) between the solidification structure roughness d ₀ of the steel without the addition of Bi (Comparative Example 1) Was calculated.

  Further, in the present invention, in the final solidified portion 1 shown in FIG. 1, a test piece is collected from a central cross section around the position of each 1/2 of the width and thickness of the cross section of the slab as a representative value. After holding at 1300 ° C. for 2 hours, a pipe-forming test was simulated, and a solution obtained by hot forging was subjected to a solution treatment at 1110 ° C. for 30 minutes.

Observation of the metal structure of the cross section of the slab etched with Murakami's reagent (10% KOH + 10% K ₃ [Fe (CN) ₆ ] + balance H ₂ O) for the purpose of coloring the intermetallic compound. Carried out. Based on the area ratio I ₀ of the intermetallic compound (sintered phase and σ phase) of the steel not added with Bi (Comparative Example 1), the area ratio I of the intermetallic compound (sintered phase and σ phase) of the steel added with Bi The generation index of the intermetallic compound taking the ratio of was evaluated as the area ratio I 1 / I ₀ (reduction rate). At this time, 30 images of an image with an area of 0.64 mm x 0.46 mm per field of view were observed with an image analyzer, and the average area ratio of intermetallic compounds (solid phase and σ phase) was determined as the representative area ratio of each test condition. did.

Also, 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 Bi (Comparative Example 1) The ratio Ra / Ra ₀ (rate of change) of the drawing value Ra of the steel to which Bi was added was evaluated based on the drawing value Ra ₀ of the steel.

  Further, the Bi concentration in the slab is a value obtained as a result of conducting a gas analysis from the chips prepared from the slab obtained in the continuous casting test and investigating the Bi concentration contained in the slab.

  The average Md value is a value obtained by obtaining the Md value with respect to the Bi 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 ratio of a solidification structure roughness and the area ratio of an intermetallic compound can be confirmed, and it is workability also from the value of a hot drawing test. Improvement effect was obtained. In addition, in Examples 6 to 8 to which electromagnetic stirring was applied, it was found that a very low average Md value can be obtained even if the Bi concentration is very small.

  This is because by applying electromagnetic stirring, the structure of the ferrite dendrite, which is the initial solidification structure, is divided, and the number density of very fine nuclei is increased. This is thought to be due to the fact that it has become an organization. This result suggests that Bi addition amount can be reduced by applying electromagnetic stirring to the conditions of the present invention.

Furthermore, the result of plotting the relationship between the average Md value in Table 2 and the area ratio ratio I / I ₀ of the intermetallic compound is shown in FIG.

As shown in FIG. 4, when the average Md value reported as the generation critical value of the intermetallic compound is 0.97, the area ratio I / I ₀ of the intermetallic compound is 0.66, There was a reduction effect. Precipitation of the σ phase, which is an intermetallic compound, is known to have a significantly reduced impact value and extremely low toughness even with only a few percent of precipitation, and if the precipitation amount is reduced by 30% or more, In other words, if the area ratio I / I _{0 of} the intermetallic compound is 0.7 or less, significant improvement in hot workability can be expected.

Furthermore, the inventors set the average Md value as 0.85 or less, assuming that the area ratio I / I ₀ of the intermetallic compound is 0.33 or less, that is, the value that can significantly reduce the intermetallic compound to 1/3 or less of the comparative example. It was found that a Bi addition amount of 60 ppm or more is desirable (see FIG. 3). However, as described above, the upper limit of the Bi addition amount is preferably 300 ppm because of the problem of embrittlement during hot working of the slab.

Claims (3)

In mass%, C: 0.03% or less, Si: 0.4% or less, Mn: 3.0% or less, P: 0.04% or less, S: 0.008% or less, Cu: 0.2-2.0%, Ni: 5.0-7.0%, Cr: In addition to 23 to 30%, Mo: 2.5 to 5.0%, W: 1.5 to 4.0%, N: 0.24 to 0.40%, Al: 0.030% or less, Bi is further contained in 10 to 300 ppm by mass, the balance being Fe and In the method of continuously casting an alloy made of impurities to produce a slab that is a material for steel pipe production,
The amount of intermetallic compound precipitation I _{0 at the} center of the cross-section of the continuous cast slab when Bi 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 Bi 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 Bi content, and based on this relationship, A continuous casting method for a steel pipe slab, characterized in that the average Md value of the alloy is 0.97 or less and the Bi content is such that the ratio I / I ₀ is 0.70 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]   The said Bi is added by inserting the wire which alloyed Bi into the molten steel in a tundish at the time of casting, The continuous casting method of the slab for steel pipes of Claim 1 characterized by the above-mentioned.   3. The continuous casting method for a steel pipe slab according to claim 1, wherein an electromagnetic stirring force is applied to at least one of the molten steel in the mold and the unsolidified molten steel in the secondary cooling zone.

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