JP2014039954A - Continuous casting cast piece for thick plate and continuous casting method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cast piece for a thick plate of miniaturizing a crystal grain over the inside from a surface of the cast piece, by miniaturizing a dendrite primary arm interval in a continuous casting cast piece.SOLUTION: The cast piece including C: 0.01-0.12%, Si: 0.01-0.6%, Mn: 0.3-2.0%, P: 0.05% or less, S: 0.008% or less, Ni: 5.0-10.0%, Al: 0.005-0.05%, N: 0.0005-0.006%, O: 0.001-0.015% and Bi: 0.0001-0.03% in maas%, having a residual part being Fe and an impurity, being 4-9 mm in an average crystal grain size and being 1.0-1.4 in the aspect ratio of a crystal grain, is continuously cast by a continuous casting method comprising a process of supplying steam and/or a particle of metal including Bi in molten steel in a tundish or molten steel in a casting mold.

Description

  The present invention relates to a slab having fine crystal grains obtained by adding a metal element in a continuous casting process of steel, a continuous casting method for the slab, and a steel plate material manufactured using the slab as a raw material. It is about.

  Steel plates for thick plates are mainly used as structural steel materials for construction, civil engineering, construction machinery, shipbuilding, pipes, tanks, marine structures and the like. Among these, steel plates for cryogenic plates used in tanks are mainly used for storing liquefied gases such as LPG (Liquid Petroleum Gas) and LNG (Liquid Natural Gas) in a cryogenic region. These liquefied gases are stored at a cryogenic temperature of −60 ° C. or lower, and particularly LNG is stored at a cryogenic temperature of −165 ° C. For this reason, the steel material for thick plates for cryogenic temperatures is required to have excellent fracture toughness at cryogenic temperatures from the viewpoint of ensuring safety. In particular, there is a demand for characteristics that can suppress the occurrence of brittle fracture even after the cold plastic working that the material receives during the production of the storage tank.

  For this purpose, it is known that it is effective to add Ni to the steel material. In particular, a steel material to which 9% of Ni is added is also defined in Japanese Industrial Standards. Here, when Ni is contained in the steel material in an amount of 5% by mass or more, the primary crystal becomes austenite, and solidification is completed in the austenite single phase. Moreover, since the difference between the liquidus temperature and the solidus temperature during solidification is as small as about 10 ° C. and solidification is completed with a large temperature gradient in the solidified shell, a large thermal stress acts on the solidified shell. It will be. For this reason, thermal stress cracks are generated on the surface of the continuous cast slab, and it is necessary to remove them, which not only reduces the yield but also requires a lot of labor and increases the manufacturing cost.

  For this problem, it is important to refine the solidification structure and crystal grains and to suppress the coarsening of the crystal grains. For example, in Patent Document 1, when continuously casting a carbon steel containing 5% by mass or more of Ni, the secondary arm interval of the dendrite of the solidified structure of the slab surface layer is controlled to be less than 40 μm, and then downstream by a continuous casting mold. Discloses a manufacturing method that eliminates cracks by controlling the cooling capacity of the secondary cooling zone installed in the factory. However, the dendrite secondary arm spacing of the solidified structure of the slab surface layer is formed in a continuous mold and depends on cooling by the mold via mold powder. The continuous casting mold oscillates up and down, and mold powder is introduced between the solidified shell and the mold to improve lubricity, but it does not necessarily flow uniformly in the mold width direction, It becomes non-uniform. Although molten steel is supplied from a nozzle immersed in the mold, the cooling state of the solidified shell is not uniform because the flow direction in the width direction of the mold and the casting method is not uniform. For this reason, the cooling is non-uniform, the thickness of the solidified shell is not constant, and it is difficult to reduce thermal stress cracking. Furthermore, in the region where the cooling is non-uniform and the cooling rate is low, the secondary arm spacing of the dendrite becomes large and it becomes difficult to satisfy the desired value.

  By the way, in order to manufacture steel plates for thick plates, the continuous cast slab as a raw material also becomes thick, and the solidification structure of the continuous cast slab becomes coarser from the slab surface layer toward the thickness center. The solidified tissue is usually dendritic and the secondary arm spacing is formed after the primary arm spacing of the dendrites is generated.

  In the case of a steel material containing 5% by mass or more of Ni, solidification is completed in an austenite single phase, and therefore it is known that the boundary of primary arm groups aligned in the direction of dendrite coincides with the boundary of crystal grains. In order to refine crystal grains, it is necessary to refine the primary arm of the dendrite. Further, in the case of steel for thick plates, it is important to secure mechanical characteristics in the thickness direction, and therefore, it is necessary to refine crystal grains even in the central portion of the thickness of the continuous cast slab. However, the technique described in Patent Document 1 does not take into account the refinement of the structure at the center of the thickness of the steel material.

As a technique for suppressing the coarsening of crystal grains, a technique for steel HAZ (Heat Affected Zone) at the time of high heat input welding has been proposed. For example, Patent Document 2 includes adding Mg. the MgO or Mg-containing oxide in the nuclear, oxide inclusion or oxides of magnitude 0.01~2.0μm composed of nitrides precipitated in the neighborhood - per 1 mm ² of the composite particles of nitrides 1. It is disclosed that a steel material including ⁸ × 10 ^{5 to} 1.0 × 10 ⁸ is produced and the coarsening of crystal grains during super-high heat input welding is suppressed, thereby producing a steel material with good HAZ toughness. You can do that.

  Thus, in order to suppress the coarsening of the structure of the steel material, it is effective to add a metal element to the molten steel. Here, in order to ensure the characteristics of the steel material stably, the metal element added to the molten steel needs to be uniformly dispersed in the slab after solidification. However, in general, steel materials are often manufactured through continuous casting, and depending on the type of metal element, it is often difficult to uniformly disperse within the continuous casting slab.

  As a method of adding a metal element to molten steel, a method of adding a massive metal to a molten steel surface, a metal wire, a metal wire coated with aluminum or steel, or a wire made of an alloy containing a metal element is added. The method etc. are adopted. However, in these methods, it is difficult to stably and accurately add a metal element having a high vapor pressure or a low melting point such as Ag, Bi, Mg, Ca, Nd, or Sn. This is because when a metal element is added to the molten steel surface, the metal element is vaporized and diffused into the atmosphere in the vicinity of the molten steel surface, and it is difficult to control the amount added to the molten steel. Therefore, the addition yield decreases, and uniform addition and dispersion of metal elements becomes difficult.

  In addition, since these metal elements have a large volume expansion when vaporized, when the metal element is vaporized in the vicinity of the molten steel surface, the molten steel is severely scattered and it is difficult to ensure operational safety. Furthermore, when the melting point of the additive metal element is low, it is difficult to add a desired amount because it is softened or melted by the radiant heat of the molten steel before the addition. When a metal element having a density lower than that of the molten steel is added, the added metal is unevenly distributed only on the surface layer portion of the molten steel and does not penetrate into the molten steel, while the metal element having a higher density than the molten steel. Is added, it becomes difficult to uniformly mix the entire molten steel only by settling into the molten steel from the addition position. In addition, these metals may adhere to refractories constituting immersion nozzles for continuous casting, etc., and may clog the immersion nozzles, and stable operation of continuous casting is performed using molten steel containing these metal elements. It ’s difficult.

  Patent Document 3 discloses a method of adding Bi to molten steel, in which Bi is added to a molten steel flow that has moved out of the ladle and moved to the molten steel bath surface in the tundish. However, Bi has a low boiling point like Mg and reacts explosively when it comes into contact with the molten steel stream, and it becomes a vapor and scatters in the system, so the yield of addition is reduced and it cannot be added uniformly into the molten steel. . Therefore, it is difficult to obtain a continuous cast slab in which Bi is uniformly dispersed.

JP 2008-212972 A JP 2001-335882 A JP 2001-1116 A JP 2004-249315 A JP 2005-169404 A Japanese Patent Laying-Open No. 2005-219072

  This invention is made | formed in view of said problem, The 1st subject is to refine | miniaturize the crystal grain of steel materials (slab). A second problem is to provide a steel plate material for thick plates in which inclusions for suppressing the growth of crystal grains are finely dispersed. The third problem is to provide a continuous casting method capable of efficiently adding a metal element necessary for obtaining such steel for steel plates into molten steel and uniformly dispersing it in the continuous casting slab. is there.

  In the present application, the state where inclusions such as oxides are “finely dispersed” means that a sample collected from a continuous cast slab is observed with a SEM at a magnification of 500 to 2000 times, and 200 precipitate particles observed. It means a state in which precipitates having a mean particle diameter of 1 μm or less are dispersed.

  When the molten steel contains 5 mass% or more of Ni, solidification is completed in the austenite single phase. Therefore, in the steel material after solidification, the austenite grain size depends on the size of the dendrite primary arm group in the same growth direction. Will be determined. That is, in order to refine the austenite grains, it is considered effective to reduce the dendrite primary arm interval and reduce the dendrite group having the same growth direction. Here, the dendrite primary arm interval can be expressed by the following equation.

（ここで、λ：デンドライト一次アーム間隔（μｍ）、Ｄ：拡散係数（ｍ^２／ｓ）、σ：固液界面エネルギー（Ｊ／ｍ^２）、ΔＴ：凝固温度範囲（℃）である。）

(Where λ: dendrite primary arm spacing (μm), D: diffusion coefficient (m ² / s), σ: solid-liquid interface energy (J / m ² ), ΔT: solidification temperature range (° C.))

  As can be seen from the equation (1), the dendrite primary arm interval λ depends on the solid-liquid interface energy σ, and λ can be reduced by reducing this σ. Therefore, the present inventors have devised a method of adding Bi, which is a surface active element, to molten steel for the purpose of reducing the solid-liquid interface energy.

On the other hand, in order to suppress the coarsening of the crystal grains once formed, it is necessary to disperse fine oxides stable at high temperatures in the steel material or to deposit nitrides (TiN or the like) on the oxides. It is valid. As the oxide, Mg-containing oxides such as MgO and MgO.Al ₂ O ₃ which are fine and stable at high temperatures are effective.

  Here, when a metal element having a high vapor pressure or a low melting point such as Mg or Bi is added to the molten steel, the added metal comes into contact with the molten steel or receives radiant heat from the molten steel, or melts or vaporizes. To do. If the added metal is melted or vaporized before or in contact with the molten steel, it is difficult to uniformly add the added metal into the molten steel with a high yield.

  In order to uniformly add the metal element into the continuous cast slab, the method of adding to the molten steel in the tundish near the continuous cast mold or the molten steel in the continuous cast mold is optimal. In the past, the present inventors disclosed in Patent Documents 4 to 6 a method of adding a vapor of a metal element or a compound of a metal element into molten steel in a tundish or molten steel in a continuous casting mold. By these methods, it became possible to add a metal element or a compound of a metal element to molten steel uniformly and with a good yield.

As described above, the present inventors examined a continuous casting method for efficiently and uniformly adding a metal element into a continuous casting slab in producing a steel plate for a thick plate with fine crystal grains, The following knowledge (a) to (c) was obtained and the present invention was completed.
(A) In order to refine the crystal grains of the steel material for the thick plate, the dendrite primary arm interval, which is the solidification structure of the continuous slab, may be reduced. For this purpose, it is effective to add Bi which is a surface active element.
(B) It is effective to disperse fine inclusions in the continuous cast slab in order to suppress the coarsening of the crystal grains of the steel sheet for thick plate. In order to generate fine inclusions, it is effective to add, for example, Mg or REM.
(C) When a metal element having a high vapor pressure or a low melting point is added to the molten steel, the added metal comes into contact with the molten steel or receives radiant heat from the molten steel to be melted or vaporized. If the metal element melts or vaporizes before or at the moment of addition to the molten steel, it is difficult to add the added metal uniformly and with good yield to the molten steel. To solve these problems and add metal elements uniformly into the continuous casting slab, add metal element vapor to the molten steel in the tundish near the continuous casting mold or the molten steel in the continuous casting mold. The method to do is the best.

This invention is made | formed based on said knowledge, The summary exists in the continuous casting method shown to following (1)-(4) and the continuous casting method shown to (5) and (6).
(1) A continuously cast slab for producing a thick plate, the slab being in mass%, C: 0.01 to 0.12%, Si: 0.01 to 0.6%, Mn : 0.3-2.0%, P: 0.05% or less, S: 0.008% or less, Ni: 5.0-10.0%, Al: 0.005-0.05%, N: 0.0005 to 0.006%, O: 0.001 to 0.015%, Bi: 0.0001 to 0.03%, the balance is Fe and impurities, and the average crystal grain size is 4 to A continuous cast slab having a diameter of 9 mm and an aspect ratio of crystal grains of 1.0 to 1.4.
(2) Further, by mass%, Cu: 2.0% or less, Cr: 1.5% or less, Mo: 0.5% or less, V: 0.1% or less, B: 0.005% or less The continuous cast slab according to (1), comprising one or more of the following.
(3) The continuous cast slab according to (1) or (2), further comprising at least one of Nb: 0.1% or less and Ti: 0.1% or less in mass%.
(4) Further, by mass%, Ca: 0.0001 to 0.005%, Mg: 0.0001 to 0.005%, REM: 0.0001 to 0.005%, (1) to The continuous cast slab according to any one of (3).
(5) A continuous casting method for producing the slab according to any one of (1) to (4) above, wherein Bi, Mg, Ca and the like are contained in the molten steel in the tundish or the molten steel in the mold. A continuous casting method comprising a step of supplying metal vapor and / or particles containing REM.
(6) The continuous casting method according to (5), wherein the vapor and / or particles of metal are supplied into the molten steel together with the carrier gas via the immersion lance.

  In the present invention, the “average crystal grain size” is a value determined as follows. That is, a sample is taken from a region of ± 10 mm in the vicinity of the thickness center of the continuous cast slab, and the structure of a 20 mm square surface parallel to the slab surface is observed. A structure | tissue shall be grind | polished using the abrasive | polishing agent whose diamond abrasive grain is 6 micrometers and 1 micrometer after grinding | polishing using emery paper. Here, in order to reveal the crystal grains of the sample, etching is performed using a picric acid solution. With respect to the revealed crystal grains, the horizontal ferret diameter and the vertical ferret diameter are obtained, respectively, the average particle diameter is obtained for each of the horizontal ferret diameter and the vertical ferret diameter, and these are further averaged to obtain the average crystal grain diameter.

  In the present invention, “aspect ratio of crystal grains” means a value obtained by dividing the major axis of a crystal grain by the minor axis. In the present invention, “metal vapor and / or particles” means metal particles present as liquid or solid particles due to insufficient metal vapor and / or evaporation, or metal particles formed by condensation of metal vapor. Means. Here, “metal” includes pure metal and metal alloys. “Supplying metal vapor and / or particles into the molten steel” means, in other words, supplying metal vapor and particles from below the molten steel surface.

  According to the present invention, the surface active element Bi can be uniformly dispersed in the molten steel, the dendrite primary arm spacing in the continuous cast slab is made finer, and the crystal grains are made finer from the surface to the inside of the slab. It is possible to provide a cast slab for thick plate. Or according to this invention, Mg, Ca, REM, etc. can be disperse | distributed uniformly in molten steel, and the slab for thick plates which carried out the fine dispersion of the inclusion for suppressing the growth of a crystal grain is provided. be able to.

It is the schematic for demonstrating the continuous casting method of this invention which concerns on one Embodiment.

<Continuous cast slab for thick plate manufacturing>
The continuous cast slab for producing thick plates according to the present invention is in mass%, C: 0.01 to 0.12%, Si: 0.01 to 0.6%, Mn: 0.3 to 2.0%. , P: 0.05% or less, S: 0.008% or less, Ni: 5.0 to 10.0%, Al: 0.005 to 0.05%, N: 0.0005 to 0.006%, O: 0.001 to 0.015%, Bi: 0.0001 to 0.03%, the balance is Fe and impurities, and these components are uniformly dispersed, so that the average crystal grain The diameter is 4 to 9 mm and the aspect ratio of crystal grains is 1.0 to 1.4.

C: 0.01 to 0.12%
C is an element effective for securing strength and toughness. If the content is less than 0.01%, the above effect cannot be obtained sufficiently. On the other hand, if the content exceeds 0.12%, the toughness of the base material decreases. For the above reason, the appropriate range of the C content is set to 0.01 to 0.12%. A more preferable range of the C content is 0.03 to 0.09%.

Si: 0.01 to 0.6%
Si is an effective element for increasing the strength of steel. If the content is less than 0.01%, the strength of the base material cannot be ensured. On the other hand, if the content exceeds 0.6%, the weldability decreases. For the above reason, the appropriate range of the Si content is set to 0.01 to 0.6%. A more preferable range of the Si content is 0.03 to 0.3%.

Mn: 0.3 to 2.0%
Mn is an element effective for increasing the strength of steel and ensuring toughness. In order to acquire said effect, while the content needs to be 0.3% or more, when the content exceeds 2.0%, toughness will be impaired. For the above reason, the appropriate range of Mn content is set to 0.3 to 2.0%. A more preferable range of the Mn content is 0.5 to 1.5%.

P: 0.05% or less P is an element that deteriorates the ductility, toughness, and workability of the steel material, so its content is limited to 0.05% or less, more preferably 0.03% or less.

S: 0.008% or less S is present in steel as an impurity element, and if it is too much, it promotes center segregation of a continuous cast slab or forms MnS inclusions that cause brittle fracture. In particular, if it exceeds 0.08%, the toughness of the base material decreases, so the content is limited to 0.08% or less, more preferably 0.003% or less.

Ni: 5.0 to 10.0%
Ni is an important element necessary for securing toughness as a cryogenic steel. In order to ensure toughness as a cryogenic steel, it is necessary to contain 5.0% or more. The higher the Ni content, the better the toughness at cryogenic temperatures, but if it exceeds 10%, the production cost increases. For the reason described above, the appropriate range of Ni content is set to 5.0 to 10.0%. A more preferable range of the Ni content is 5.5 to 9.0%.

Al: 0.005 to 0.05%
Al is a deoxidizing element of molten steel, and at the same time, has the effect of suppressing the precipitation of cementite and improving the stabilization of austenite during tempering. Furthermore, Al combines with N in the steel to produce AlN, and has the effect of suppressing the austenite grain coarsening when the slab is heated. In order to obtain such an effect, it is necessary to contain 0.005% or more. However, if the content exceeds 0.05%, the toughness is deteriorated. For this reason, the appropriate range of Al content was made 0.005-0.05%. A more preferable range of the Al content is 0.007 to 0.040%.

N: 0.0005 to 0.006%
N is an element that contributes to the stabilization of austenite. N is an element necessary for reacting with Al and Ti to precipitate AlN and TiN. However, when the content exceeds 0.006%, the toughness decreases. On the other hand, since it is impossible to remove N completely industrially, the lower limit is preferably set to 0.0005% in consideration of the range that can be reduced in actual operation. For this reason, the appropriate range of N content was made into 0.0005 to 0.006%. A more preferable range of the N content is 0.001 to 0.004%.

O: 0.0001 to 0.015%
O is an element necessary for forming an oxide. If the amount is less than 0.0001%, the number of oxides is insufficient. On the other hand, if it exceeds 0.015%, the amount of oxides increases and the toughness decreases. For this reason, the appropriate range of O content was made 0.0001 to 0.015%. A more preferable range of the O content is 0.001 to 0.0050%.

Bi: 0.0001 to 0.03%
Bi is the most important element in the present invention. Bi is an element that acts as a surface active element in the solidification process of steel and has the effect of refining the dendritic solidification structure. If it is less than 0.0001%, this effect of miniaturization is almost absent. On the other hand, if it exceeds 0.03%, coarse Bi oxide is generated and toughness is lowered. In addition, since Bi is a surface active element, it has an effect of promoting the precipitation of TiN, MnS, etc. on the Mg-containing oxide described later, and is coarsened by precipitating on finely dispersed Mg oxide. Easy TiN, MnS, etc. can be finely dispersed. The effect is scarcely less than 0.0001%. For the above reason, the appropriate range of Bi content is set to 0.0001 to 0.03%. A more preferable range of Bi content is 0.0005 to 0.0100%.

  The slab according to the present invention has the above composition. The slab according to the present invention is manufactured by performing continuous casting while uniformly dispersing a predetermined metal (in particular, a metal having a high vapor pressure or a metal having a low melting point) in molten steel as will be described later. This makes it possible, for example, to uniformly disperse Bi, which is a surface active element, in the molten steel, miniaturizing the dendrite primary arm spacing in the continuous cast slab, and miniaturizing crystal grains from the surface to the inside of the slab Can be made. That is, in the Ni-containing slab where the primary crystallized phase is austenite and solidification is completed in the austenite single phase in the solidification process, the dendrite primary arm interval can be refined from the slab surface to the inside, and the mechanical characteristics It is excellent and it can be set as the slab which does not have a crack etc. on the surface.

  The slab according to the present invention preferably further contains the following components.

Cu: 2.0% or less Cu, if contained, is an element having an effect effective in improving hardenability and precipitation strengthening. However, when the Cu content exceeds 2.0%, the hot workability of steel decreases. Therefore, in the present invention, the content is limited to 2.0% or less. Particularly preferably, it is 1.3% or less.

Cr: 1.5% or less Cr is an element that exhibits an effective action for improving the hardenability and the strength of the base material by precipitation strengthening when contained, but its content exceeds 1.5%. And the tendency for toughness and weldability to deteriorate is recognized. Therefore, in the present invention, the content is limited to 1.5% or less. Especially preferably, it is 1.0% or less.

Mo: 0.5% or less Mo is an element that exhibits an effective action for improving hardenability and strength when contained, but if its content exceeds 0.5%, Deterioration of toughness and ductility as well as deterioration of weldability become apparent. Therefore, in the present invention, the content is limited to 0.5% or less. Particularly preferably, it is 0.3% or less.

V: 0.1% or less V is an element that has the effect of improving the strength by generating carbides and nitrides if contained, but if its content exceeds 0.1%, it increases toughness. Reduce. Therefore, in the present invention, the content is limited to 0.1% or less. Particularly preferably, it is 0.08% or less.

B: 0.005% or less B, if contained, increases hardenability, but has the effect of reducing solid solution N by generating BN and improving the toughness of the HAZ part, but its content is When the content exceeds 0.005%, coarse boride precipitates in the steel, which deteriorates the toughness of the steel. Therefore, in the present invention, the content is limited to 0.005% or less. Particularly preferably, it is 0.004% or less.

Nb: 0.1% or less Nb is an element that has the effect of improving the strength by generating carbides and nitrides when contained, but if its content exceeds 0.1%, On the other hand, since coarse carbides and nitrides are formed, the toughness is reduced. Therefore, in the present invention, the content is limited to 0.1% or less. Particularly preferably, it is 0.08% or less.

Ti: 0.1% or less Ti is an effective element that mainly precipitates carbonitrides and contributes to improvement of the strength of the base metal by its precipitation strengthening action, but its content exceeds 0.1% and is high. As a result, coarse precipitates and inclusions are formed in the steel, thereby reducing the toughness of the steel. Therefore, in the present invention, the content is limited to 0.1% or less. Particularly preferably, it is 0.07% or less.

  In the present invention, in addition to the above metal elements, Mg, Ca, REM, etc. are uniformly dispersed in the molten steel to obtain a slab in which oxides for suppressing the growth of crystal grains are finely dispersed. Can do. Thereby, it can be set as the slab which is further excellent in a mechanical characteristic and does not have a crack on the surface.

Ca: 0.0001 to 0.005%
Ca reacts with oxygen in molten steel to generate Ca oxide. The Ca oxide, may also be produced as a composite oxide containing one or more such CaO and _{Al 2 O 3, Ti 2 O} 3 in addition to CaO alone. In order to finely disperse these oxides in steel, it is necessary to contain 0.0001% or more of Ca. However, if the content exceeds 0.005%, the amount of coarse oxide inclusions in the steel increases and the toughness decreases. For the above reason, the appropriate range of the Ca content is set to 0.0001 to 0.005%. A more preferable range of the Ca content is 0.004 to 0.003%.

Mg: 0.0001 to 0.005%
Mg reacts with oxygen in the molten steel to produce Mg oxide. The Mg oxides, may also be produced as a composite oxide containing one or more other to MgO and _Al 2 _O 3, etc. _Ti 2 _{O 3} of MgO alone. In order to finely disperse these oxides in steel, it is necessary to contain 0.0001% or more of Mg. However, if the content exceeds 0.005%, the amount of coarse oxide inclusions in the steel increases and the toughness decreases. For the above reason, the appropriate range of Mg content is set to 0.0001 to 0.005%. A more preferable range of the Mg content is 0.0003 to 0.003%.

REM: 0.0001 to 0.005%
REM reacts with oxygen in molten steel to produce REM oxide. As the REM oxide, in addition to the REM oxide alone, it can be generated as a composite oxide containing REM oxide and one or more of Al ₂ O ₃ , Ti ₂ O _{3 and the} like. In order to finely disperse these oxides in steel, it is necessary to contain REM 0.0001% or more. However, if the content exceeds 0.005%, the amount of coarse oxide inclusions in the steel increases and the toughness decreases. For the above reason, the appropriate range of the content is set to 0.0001 to 0.005%. A more preferable range of the REM content is 0.0003 to 0.003%.

  Thus, in the slab according to the present invention, the metal or non-metallic inclusions are finely dispersed, so that the dendrite primary arm interval can be refined from the slab surface to the inside, and in the steel structure. The growth of crystal grains can be suppressed, and a slab having excellent mechanical properties and no cracks on the surface can be obtained.

  The slab according to the present invention has an average crystal grain size of 4 to 9 mm and a crystal grain aspect ratio of 1.0 to 1.4.

  Here, the average crystal grain size is preferably as small as possible. However, in order to make the average crystal grain size less than 4 mm, the amount of the added metal is excessively increased and the manufacturing cost is increased. Therefore, it was set to 4 mm or more. Moreover, when an average crystal grain size exceeds 9 mm, there exists a possibility that the surface crack of a slab may arise.

  The aspect ratio of the crystal grain is a value obtained by dividing the major axis of the crystal grain by the minor axis, and the closer the aspect ratio is to 1.0, the more isotropic the crystal grain is. That is, when the aspect ratio is 1.0 to 1.4, the crystal grains are isotropic, and the mechanical properties of the slab are uniform without depending on the directionality. Even if an external force acts, surface cracks are less likely to occur. In addition, when the crystal grains are isotropic, the extension of cracks is isotropic, and the property of stopping the propagation of cracks, so-called arrestability, is improved. This is because if the anisotropy of the crystal grains is large, cracks easily extend in the direction of the major axis of the crystal grains.

  In the present invention, in order for the average crystal grain size and aspect ratio of the crystal grains in the slab to be within a predetermined range, the additive metal element needs to be uniformly dispersed in the slab. For example, by producing a slab by the continuous casting method according to the present invention shown below, it becomes possible to obtain a slab in which the additive metal element is uniformly dispersed from the surface to the inside, and the average grain size of the crystal grains And a slab whose aspect ratio satisfies the above range.

<Continuous casting method>
The continuous cast slab according to the present invention can be obtained, for example, by the following continuous casting method. That is, the continuous casting method according to the present invention includes a step of supplying metal vapor and / or particles containing Bi, Mg, Ca and / or REM into molten steel in a tundish or molten steel in a mold. It is characterized by.

  As a method of adding Bi, Mg, Ca, and REM to molten steel, a method in which a massive metal is introduced into the molten steel surface, a metal wire, a metal wire coated with aluminum or steel, or an alloy containing a metal element is used. There may be a method of adding the wire by using a wire. However, in these methods, it is difficult to stably and accurately add a metal element having a high vapor pressure or a low melting point. This is because when a metal element is added to the molten steel surface, the metal element is vaporized and diffused into the atmosphere in the vicinity of the molten steel surface, making it difficult to control the amount added to the molten steel. . Therefore, the addition yield decreases, and uniform addition and dispersion of metal elements becomes difficult.

  In the present invention, when Bi, Mg, Ca, and REM are added, the growth of crystal grains seen in the slab is different from the case of no addition, and the aspect ratio, which is the ratio of major axis / minor axis of crystal grains, is different. This was the first time that it was 1.0 to 1.4.

  On the other hand, in the present invention, “metal vapor and / or particles” containing Bi, Mg, Ca and / or REM are supplied to the molten steel in the tundish or “in the molten steel” in the mold. That is, instead of supplying various metals from the upper surface of the molten steel, by supplying metal vapor and particles into the molten steel (from below the molten steel surface), while solving the above-mentioned problem of diffusion, The metal element can be uniformly dispersed therein. In particular, it is preferable to supply metal vapor or particles into the molten steel in the form described below.

  In FIG. 1, the implementation structure of the continuous casting method of this invention which concerns on one Embodiment is shown. As shown in FIG. 1, in the continuous casting method according to the present invention, first, molten steel 1 is supplied into a tundish 2 from a ladle 3. Then, one end of the immersion lance 4 is immersed in the molten steel 1 in the tundish 2. A wire feeder 5 is connected to the other end of the immersion lance 4. A wire reel 51 is loaded in the wire feeder 5, and a wire is inserted into the immersion lance 4 under the control of a wire feeding speed control device 53 by a wire feeding roll 52. On the other hand, the carrier gas 54 is introduced into the wire feeder 5 via the flow rate pressure controllers 55 to 57 and supplied into the immersion lance 4 together with the wire. As a result, the wire supplied to the immersion lance 4 is heated in the lance or brought into contact with the molten steel 1 on the outlet side of the lance, thereby becoming vapor and / or particles, which is converted into the carrier gas. While being accompanied, it is supplied into the molten steel 1 and spreads throughout the molten steel. The molten steel 1 in which the additive metal is uniformly dispersed in this manner is supplied to the mold 8 through the immersion nozzle 6 and is continuously drawn out from the mold 8 to become a slab 7.

  As described above, when adding metal to the molten steel, metal vapor and / or particles are supplied into the molten steel together with the carrier gas through the immersion lance, so that the above-mentioned problem of dissipation can be more appropriately achieved. It is possible to more uniformly disperse the metal element in the molten steel while preventing it.

  As described above, according to the present invention, Bi, which is a surface active element, can be uniformly dispersed in the molten steel, the dendrite primary arm interval in the continuous cast slab is refined, and the crystal is formed from the surface of the slab to the inside. A slab for thick plates with fine grains can be provided. Or according to this invention, Mg, Ca, REM, etc. can be disperse | distributed uniformly in molten steel, and the slab for thick plates which carried out the fine dispersion of the inclusion for suppressing the growth of a crystal grain is provided. be able to.

  In order to confirm the effect produced by the continuous casting slab and the continuous casting method of the present invention, the following continuous casting test was conducted and the results were evaluated.

<Casting conditions>
Molten steel: Component molten steel temperature shown in Table 1: 1570 ° C (molten steel temperature in tundish)
Mold size: width 1400 x thickness 250mm
Casting speed: 1.0 m / min Metal addition method: Insert φ3 mm added metal wire into the immersion lance Add metal type: Bi, Mg, Ca, Sr, Ba
Addition position: Carrier gas in tundish: Argon gas 10 L / min Pre-lance pressure: 0.05 MPa

  In this experiment, a continuous casting test was performed by changing the molten steel components. Specifically, as shown in FIG. 1, when adding Bi, Mg, Ca, Sr, and Ba, continuous casting was performed by the continuous casting method according to the present invention using a wire. In continuous casting, first, molten steel 1 was supplied from a ladle 3 into a tundish 2. Then, one end of the immersion lance 4 was immersed in the molten steel in the tundish. A wire feeder 5 is connected to the other end of the dipping lance 4. A wire reel 51 is loaded in the wire feeder 5 and dipped under the control of a wire feeding speed control device 53 by a wire feeding roll 52. A wire was inserted into the lance 4. The carrier gas 54 was introduced into the wire feeder 5 via the flow rate pressure controllers 55 to 57 and supplied into the immersion lance 4 together with the wire. Thereby, the wire supplied to the immersion lance 4 was made into steam and / or particles and supplied into the molten steel while being accompanied by the carrier gas.

  About the slab obtained by continuous casting, the sample was extract | collected from the area | region of +/- 5mm in the thickness center vicinity, and the structure | tissue of the 20 mm square surface parallel to a slab surface was observed. The structure was polished with emery paper, and further polished with a polishing agent having diamond abrasive grains of 6 μm and 1 μm. In order to reveal the crystal grains of the sample, etching was performed using a picric acid solution. The average grain size of the revealed crystal grains is obtained, and further divided by the average crystal grain size of the sample of Comparative Example 1 in Table 1 in which no metal element is added. The ratio of the average crystal grain size based on the diameter) was calculated. It means that the smaller the average crystal grain size, that is, the smaller the crystal grain size index is, the greater the effect of suppressing crystal grain coarsening. Here, in order to make the average crystal grain size less than 4 mm (the crystal grain size index is less than 0.4), the amount of added metal is large and the manufacturing cost increases, so that it is 4 mm or more (crystal grain size index is 0.4 or more) Is preferred. Further, when the average crystal grain size exceeds 9 mm (the crystal grain size index exceeds 0.9), it can be said that the effect of reducing the surface crack index of the slab is small.

  Further, the aspect ratio of the crystal grains was obtained based on the observation result of this structure. The aspect ratio is a value of the major axis / minor axis of the crystal grain. The closer the aspect ratio is to 1.0, the more isotropic the crystal grain is. If the crystal grains are isotropic, the mechanical properties of the slab are uniform without depending on the directionality, and therefore surface cracks are less likely to occur even when an external force is applied during forced bending during continuous casting. In addition, when the crystal grains are isotropic, the extension of cracks is isotropic, and the property of stopping the propagation of cracks, so-called arrestability, is improved. This is because if the anisotropy of the crystal grains is large, cracks easily extend in the direction of the major axis of the crystal grains.

  On the other hand, as a comparative example, continuous casting was performed under the condition where the above-mentioned additive metal was not added, and the same tests and investigations were performed as when the additive metal was added.

  Table 1 below shows the molten steel components according to Examples and Comparative Examples, and Table 2 shows the evaluation results. As is clear from Tables 1 and 2, it can be seen that according to the present invention, the crystal grain structure of the steel sheet is uniform and the grain coarsening can be suppressed.

  While the present invention has been described in connection with embodiments that are presently the most practical and preferred, the present invention is not limited to the embodiments disclosed herein. However, the present invention can be changed as appropriate without departing from the spirit or concept of the invention that can be read from the claims and the entire specification, and the continuous cast slab and the continuous casting method with such a change are also within the technical scope of the present invention. Must be understood as encompassed by.

  The continuous cast slab according to the present invention can be suitably used particularly as a slab for producing a thick plate containing 5% or more of Ni. According to the continuous casting method according to the present invention, Bi, which is a surface active element, can be uniformly dispersed in the molten steel, and the dendrite primary arm interval in the continuous cast slab is made fine, and from the surface of the slab to the inside. A slab for thick plates with fine crystal grains can be obtained. Or according to this invention, Mg, Ca, REM, etc. can be disperse | distributed uniformly in molten steel, and the slab for thick plates which disperse | distributed the inclusion for suppressing the growth of a crystal grain is obtained. Can do.

1: molten steel, 2: tundish, 3: ladle, 4: dipping lance, 5: wire feeder, 51: wire reel, 52: wire feeding roll, 53: wire feeding speed control device: 54: carrier gas, 55 -57: Flow rate pressure controller, 6: Immersion nozzle, 7: Slab, 8: Mold

Claims (6)

A continuously cast slab for producing a thick plate, the slab comprising:
% By mass
C: 0.01 to 0.12%,
Si: 0.01 to 0.6%,
Mn: 0.3 to 2.0%,
P: 0.05% or less,
S: 0.008% or less,
Ni: 5.0 to 10.0%,
Al: 0.005 to 0.05%,
N: 0.0005 to 0.006%,
O: 0.001 to 0.015%,
Bi: 0.0001-0.03%,
Comprising
The balance is Fe and impurities,
The average grain size is 4-9 mm,
The aspect ratio of the crystal grains is 1.0 to 1.4,
Continuous casting slab. Furthermore, in mass%,
Cu: 2.0% or less,
Cr: 1.5% or less,
Mo: 0.5% or less,
V: 0.1% or less,
B: The continuous cast slab according to claim 1, comprising at least one of 0.005% or less. Furthermore, in mass%,
Nb: 0.1% or less,
Ti: 0.1% or less,
The continuous cast slab according to claim 1 or 2, comprising at least one of the above. Furthermore, in mass%,
Ca: 0.0001 to 0.005%,
Mg: 0.0001 to 0.005%,
REM: 0.0001 to 0.005%
The continuous cast slab according to claim 1, comprising: A continuous casting method for producing the slab according to claim 1,
Supplying steam and / or particles of metal containing Bi, Mg, Ca and / or REM into molten steel in a tundish or molten steel in a mold,
Continuous casting method. The continuous casting method according to claim 5, wherein the metal vapor and / or particles are supplied into the molten steel together with a carrier gas via an immersion lance.

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