JP2011218370A - Thick plate steel, and method for continuous casting of ingot as raw material of thick plate steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide thick plate steel that has a homogeneous structure and can suppress coarsening of crystal grains at reheating, and a method for continuous casting of an ingot as a raw material.SOLUTION: (1) The thick plate steel produced from a continuously cast ingot as a raw material contains, by mass, C: 0.01-0.20%, Si: 0.02-0.5%, Mn: 0.6-3.0%, P: 0.02% or less, S: 0.002-0.008%, Ti: 0.005-0.03%, N: 0.002-0.008%, Al: 0.0005-0.05%, O: 0.0001-0.015% and Bi: 0.0001-0.03% with the remaining of Fe and impurities, and has a segregation index of 1.0-2.2, a crystal grain size index of 0.3-0.9 and a toughness index of 1.5-3.0. (2) The continuous casting method comprises inserting a metal wire containing Bi into an immersion lance immersed in molten steel to generate metal vapor in the immersion lance and supply it into the molten steel along with carrier gas.

Description

  The present invention relates to a steel plate material for a thick plate, in particular, a steel plate material for a thick plate manufactured by using a cast slab in which fine inclusions are dispersed and formed with a small amount of microsegregation of metal elements added in the continuous casting process. The present invention relates to a method for continuously casting a slab as a raw material.

  Steel plates for thick plates are mainly used as structural steel materials for structures such as architecture, civil engineering, construction machinery, shipbuilding, pipes, tanks, and marine structures. In these structures, steel materials are fixed and combined by bolting or welding. Among these, in fixing by welding, heat is applied to the thick plate base material itself, and therefore mechanical properties such as strength and toughness of the base material may be deteriorated. For this reason, many studies have been conducted on the deterioration of these mechanical properties. In particular, recently, since high heat input welding has been performed, a countermeasure for reducing the toughness of the weld affected zone (hereinafter also referred to as “HAZ”) has become a major issue. Yes.

  Many proposals have been made on the technology that focuses on the toughness of the steel material HAZ during high heat input welding.

  For example, Patent Document 1 discloses that a steel ingot or steel slab is heated to a temperature range of 1250 to 1400 ° C. so that TiN is dissolved, and then rolled or forged, or then reheated at a temperature of 1150 ° C. or lower. By doing so, a method for producing a steel material for high heat input welding is disclosed in which solute TiN is dispersed and reprecipitated as fine TiN, and the austenite grains of HAZ are refined to improve toughness.

  However, Ti nitride has a problem that the effect of improving toughness is reduced because most of it dissolves in the vicinity of the boundary with the weld metal having the highest ultimate temperature exceeding 1400 ° C in HAZ, and is required in high heat input welding. It is difficult to ensure toughness.

As a technique for improving the toughness in the vicinity of the welded portion and improving the toughness of the HAZ at the time of high heat input welding of steel, as disclosed in Patent Document 2, for example, any of TiO and Ti ₂ O ₃ A method of containing an oxide inclusion containing one or two kinds of complex crystal phases is effective.

  However, since Ti oxide tends to cause coarsening and aggregation and coalescence, Ti oxide-based coarse inclusions are generated, and when such coarse inclusions are formed, the toughness of HAZ is reduced. Problems arise.

As a technique for solving this problem, for example, Patent Document 3 discloses a technique for dispersing a Ti—Mg-based oxide. That is, there are 30 oxides / mm ² or more of oxides having a size of 0.5 to 5 μm and a total content of Ti and Mg of 15% by weight or more, and at the same time 0.05 to 0.5 μm in size. It is a steel plate having excellent weld heat-affected zone toughness with 5000 or more oxides / mm ² .

  According to Patent Document 3, in this steel sheet, Ti—Mg-based oxides can be dispersed, and good HAZ toughness can be achieved over a wide range of welding conditions and base material materials. However, in recent ultra-high heat input welding, the temperature of the HAZ becomes even higher, so it is difficult to maintain the steel structure in a fine state. Even with this technique, the improvement of toughness is sufficient. I can't say that.

In Patent Document 4, a size of 0.01 to 2.0 μm composed of a nitride formed by adding Mg and having MgO or an Mg-containing oxide as a nucleus and including or surrounding the oxide is deposited. The steel material containing 1.0 × 10 ^{5 to} 1.0 × 10 ⁸ composite particles of oxide-nitride per 1 mm ² is said to have good HAZ toughness during super-high heat input welding.

  By the way, when manufacturing the steel material for thick plates, the continuous cast slab used as a raw material also becomes thick, and the solidification structure | tissue of a continuous cast slab becomes coarse from the slab surface layer toward the thickness direction center part. The solidification structure of the continuous cast slab is usually dendritic, and the concentration of solute elements differs between the dendritic core and the intertree. For example, Mn is generally added as a solute element in order to ensure the strength and toughness of the steel plate material. This Mn has a low concentration in the dendrite tree core and a high concentration in the dendrite tree due to solute redistribution during the solidification process. Therefore, the Mn concentration varies greatly within the range of dendrites having a size of several tens of μm to several mm.

  The continuously cast slab is generally heated at about 1250 ° C., then rolled and processed into a thick plate of a desired size. In the manufacturing process of the thick plate, Mn segregation formed in the dendrite core portion and the intertree portion is not eliminated, and remains in the thick plate of the final product. As a result, the structure formed after solidification is different between the portion where the Mn concentration is high and the portion where the Mn concentration is low, resulting in non-uniformity in the structure.

  When such a structure non-uniformity exists, not only the super-high heat input welding and the large heat input welding, but also the HAZ toughness at the time of normal welding is greatly reduced. The prior art described in Patent Documents 1 to 4 does not describe anything about the structure of the steel material, and does not extend to the structure control of the steel material.

  The prior art described above is a method of adding a metal element to molten steel as a method of suppressing the coarsening of the structure of the heated steel material. In order to stably secure the properties of the steel material, it is necessary that the metal element added to the molten steel is uniformly dispersed in the slab after solidification. In general, a steel material (slab) is often manufactured through a continuous casting method, and depending on the type of metal element, it is often difficult to uniformly disperse it in a continuously cast slab or the like.

  As a method of adding a metal element to molten steel, a method in which a massive metal element is added to the surface of the molten steel, a wire made of a simple metal element, a wire in which the metal element is coated with aluminum or steel, or a metal element is contained. The method of adding with the wire produced with the alloy to do is employ | adopted.

  However, it is difficult to accurately and stably add a metal element having a high vapor pressure or a low melting point such as Ag, Bi, Mg, Ca, Nd and Sn by using these methods. This is because when these metal elements with high vapor pressure are added to the molten steel, they are vaporized in the vicinity of the molten steel surface and diffused into the atmosphere, making it difficult to control the amount added to the molten steel. This is because it is difficult to add uniformly because the addition yield also decreases.

  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, making it difficult to ensure operational safety. Furthermore, when the melting point of the added metal element is low, it is difficult to add a predetermined amount because it is softened or melted by the radiant heat of the molten steel before the addition.

  In 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, and conversely, a metal having a higher density than the molten steel. In the case of adding an element, it is difficult to uniformly mix (disperse) the entire molten steel only by settling in the molten steel from the addition position. In addition, these metals are likely to adhere to the refractory used during continuous casting and block the immersion nozzle, and it is difficult to carry out stable operation of continuous casting using molten steel containing these elements. is there.

  Patent Document 5 discloses a method of adding Bi to molten steel, in which Bi is added to the molten steel flow that has moved out of the ladle and moved to the molten steel bath surface in the tundish. Bi, like Mg, has a low boiling point, reacts explosively when in contact with the molten steel stream, and becomes a vapor and scatters in the atmosphere, so the addition yield is low, and it is difficult to add uniformly into the molten steel. In this method, it is difficult to uniformly disperse the continuous cast slab.

  In Patent Document 6, in a method of adding a Pb, Bi, Pb / Bi-containing substance to molten steel in a ladle by injection using a lance, gas is ejected from a porous plug at the bottom of the ladle, and the bottom of the ladle Techniques for preventing sedimentation of Pb and Bi-containing materials in the hood are disclosed. Even when this technique is used, when Pb or Bi having a low melting point or boiling point is added, an explosive reaction occurs when these metals come into contact with the molten steel, and the addition to the molten steel becomes non-uniform and the yield is also increased. However, it is difficult to disperse uniformly in the continuous cast slab. It is also difficult to arbitrarily adjust the formation of reaction products of Pb and Bi added to the molten steel.

Japanese Patent Publication No.55-26164 JP-A-61-79745 Japanese Patent Laid-Open No. 11-124652 JP 2001-335882 A JP 2001-1116 A JP-A-9-13119 JP 2004-249315 A JP 2005-169404 A Japanese Patent Laying-Open No. 2005-219072

W. Kurz and D.J. Fisher, “Fundamentals of Solidification”, Trans Tech Publications Ltd., (Switzerland), 1998, p. 256

  This invention is made | formed in view of said problem, The 1st subject is to make a structure uniform by reducing the microsegregation of the structure | tissue of steel materials. The second problem is to provide a steel plate material for thick plates in which inclusions are finely dispersed for the purpose of suppressing the growth of austenite grains during reheating, including HAZ in super-high heat input or large heat input welding. . The third problem is a continuous casting method capable of efficiently adding a metal element necessary for obtaining the above steel for a thick plate into molten steel and uniformly dispersing it in a continuous cast slab such as a slab. It is to provide.

  The evaluation of the HAZ toughness at the time of super-high heat input welding or large heat input welding of a thick plate is based on the premise that the structure of the thick plate base material is uniform. However, no literature has been found so far regarding the relationship between the uniformity of the structure of the thick plate base metal and the HAZ toughness.

  The inventors of the present invention have clarified that the structure of the thick plate base material manufactured under normal operating conditions is not uniform while studying the improvement of the HAZ toughness of the thick plate. This study also revealed that the structure uniformity of the thick plate base material is related to the HAZ toughness. As for the structure of the plank, the final structure is usually determined through a hot rolling process. And it became clear that this final structure is deeply related to the solidified structure during continuous casting.

  The solidification structure of the continuous cast slab has a dendritic form, and segregation of solute elements occurs in the dendrite's core and intertree parts. Mn, which is a solute element with a significant degree of segregation, decreases in concentration at the dendrite tree core as the solidification progresses, and increases at the portion between trees, so that segregation occurs in the slab. This segregation does not disappear even after the hot rolling process, and remains in the product.

  In the structure of the thick plate as the final product, ferrite is formed in a region corresponding to the dendrite core of the previous slab, and bainite or pearlite is formed in a region corresponding to the dendrite. This is due to segregation of Mn formed linearly between the dendrite trees. Since ferrite is a soft phase and bainite and pearlite are hard phases, the mechanical properties of the thick plate become non-uniform when linear structural non-uniformity is formed in the thick plate. If such non-uniform mechanical properties are present in the steel sheet, the HAZ toughness is significantly reduced because microscopic stress concentration occurs when subjected to an impact as in the Charpy test.

  In the prior art and literature, the above-described non-uniform structure has not been studied, and mechanical properties such as super-high heat input welding of thick plates or HAZ toughness during high heat input welding due to the non-uniform structure. There is no finding that it decreases significantly.

  Therefore, based on this knowledge, the present inventors consider that in order to improve the mechanical properties of the thick plate base material, first, it is essential to reduce the segregation accompanying the formation of the dendritic structure of the continuous cast slab. The reduction of segregation was studied.

  In order to eliminate segregation of solute elements such as Mn in the dendritic tree, it is effective to promote diffusion of solute elements.

The Fourier law is known for heat conduction, and the Fourier number Fr = α · t / x ² is derived from the theoretical analysis result of heat conduction in a semi-infinite solid. Here, α: thermal diffusion coefficient (m ² / s), t: time (s), x: heat transfer distance (m).

By applying this Fourier number Fr to the solidification structure and element diffusion in the solidification process of steel, the Fourier number Fr = D · t / λ ² generally used as an index representing the effect of diffusion can be obtained. Here, D: diffusion coefficient of solute (cm ² / s), t: diffusion time (s), λ: diffusion distance (cm).

  Using the Fourier number Fr, the following examination was performed on the effect of changing the operation factor in actual operation. The greater the Fourier number Fr, the greater the diffusion effect. Therefore, increasing the Fourier number Fr can promote diffusion of solute elements and reduce segregation.

  As can be seen from the above equation, in order to increase the Fourier number Fr, it is necessary to increase the diffusion coefficient D or the diffusion time t or decrease the diffusion distance λ.

  First, the diffusion coefficient D is a function of the temperature T and can be increased by increasing the temperature T. When the temperature T is increased during operation, the temperature of the heating furnace is increased. However, since the normal operating temperature is about 1050 to 1350 ° C., raising the temperature beyond this will not only significantly increase the cost, but also reduce the yield with an increase in the amount of scale generated during heating. The surface properties of the steel sheet after rolling due to the deterioration of the surface properties of the slab. Therefore, it is practically difficult to increase the temperature T, that is, increase the diffusion coefficient D in actual operation.

  Secondly, increasing the diffusion time t extends the charging time into the heating furnace during operation. Assuming that the normal heating time is several hours, it is estimated that several times as long as it is necessary to eliminate segregation of solute elements. If the charging time is extended in the current operation, the production efficiency is greatly reduced, so it is practically difficult to increase the time t.

Third, the diffusion distance λ corresponds to the dendrite primary arm spacing found in continuous cast slabs. Research on the primary arm spacing of dendrites has been conducted conventionally, and according to Non-Patent Document 1, it can be expressed by the following equation (1).
λ∝ (D × σ × ΔT) ^0.25 (1)
Here, λ: dendrite primary arm interval (μm), D: diffusion coefficient (m ² / s), σ: solid-liquid interface energy (J / m ² ), ΔT: solidification temperature range (° C.).

  From the equation (1), it can be seen that the primary arm interval λ of the dendrite depends on the solid-liquid interface energy σ, and if this σ can be reduced, λ decreases. If λ can be reduced, the Fourier number Fr can be increased, diffusion of solute elements during heating of the continuous cast slab can be promoted, and segregation can be reduced.

  Accordingly, the present inventors have conceived a method of adding Bi, which is a surface active element, to the molten steel for the purpose of reducing the solid-liquid interfacial energy σ between the molten steel and the dendrite.

Next, to suppress the coarsening of crystal grains during super-high heat input welding or high heat input welding, it is possible to disperse fine oxides stable at high temperatures in steel materials, and to form nitrides on the oxides. It is possible by precipitating. As such an oxide, an Mg-containing oxide such as MgO or MgO.Al ₂ O ₃ which is fine and stable at high temperatures is effective. In addition, it is effective to suppress the coarsening of crystal grains by precipitating TiN on these oxides.

  When a metal element having a high vapor pressure or a metal element having 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. Vaporize. If metal elements are melted or vaporized before or at the moment of addition to the molten steel, it is difficult to add these metal elements uniformly to the molten steel with good yield. Further, 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 in the continuous cast mold is optimal.

  So far, the present inventors have added a vapor of a metal element or a compound of a metal element in molten steel in a tundish or a continuous casting mold in Patent Document 7, Patent Document 8 and Patent Document 9 according to the previous application. Proposed method to do. By these methods, it has become possible to add a metal element or a compound of a metal element uniformly into molten steel with good yield.

  The present inventors examined a steel plate for thick plates having good HAZ toughness during high heat input welding, and in producing such a steel plate for thick plates, a metal element was placed in a continuous cast slab such as a slab. The present invention was completed by studying a continuous casting method for efficiently and uniformly adding, obtaining the following findings (a) to (d).

(A) In order to improve the HAZ toughness during super-high heat input welding or high heat input welding of steel plates for thick plates, it is essential to make the structure of the thick plates uniform. To make the structure of the thick plates uniform What is necessary is just to eliminate the segregation of the dendrite core part and the intertree part of the continuous cast slab for the thick plate, the segregation index described later is 1.0 to 2.2, and the toughness index is 1.5 to 3.0. Just do it. For this purpose, it is effective to add Bi, which is a surface active element for making dendrites finer, into the molten steel.

(B) In order to improve the HAZ toughness during super-high heat input welding or high heat input welding of steel plates for thick plates, fine inclusions are dispersed in the steel material to suppress the growth of austenite crystal grains during reheating. It is effective to set the crystal grain size index described later to 0.3 to 0.9. In order to generate fine inclusions, it is effective to add Mg into the molten steel.

(C) In order to further enhance the effect of (b), it is effective to add one or more of Ca, Sr and Ba into the molten steel.

(D) When adding a metal element having a high vapor pressure or a metal element having a low melting point such as Bi described in (a) above to the molten steel, these added metals are in contact with the molten steel or It melts or vaporizes by receiving radiant heat from molten steel. If the metal elements are melted or vaporized before being added to the molten steel or at the moment of addition, it is difficult to add these metal elements to the molten steel uniformly and with a high yield. In order to solve such problems and to uniformly add a metal element into a continuous cast slab, a method of adding a vapor of metal element to a molten steel in a tundish near a continuous cast mold or in a continuous cast mold Is the best.

  The present invention has been made on the basis of the above findings (a) to (d), and the gist thereof is the steel material for thick plates shown in the following (1), (2), (3) and (4) And the continuous casting method of steel shown in (5) and (6).

(1) A steel plate material manufactured using a continuously cast slab as a raw material, and in mass%, C: 0.01 to 0.20%, Si: 0.02 to 0.5%, Mn : 0.6-3.0%, P: 0.02% or less, S: 0.002-0.008%, Ti: 0.005-0.03%, N: 0.002-0.008% , Al: 0.0005 to 0.05%, O: 0.0001 to 0.015% and Bi: 0.0001 to 0.03%, with the balance being Fe and impurities, and a line using EPMA The segregation index, which is a value obtained by dividing the maximum value of the Mn content measured in the analysis by the average Mn content, is 1.0 to 2.2, and the value of the average crystal grain size is other than Bi without containing Bi. The component composition of is the average crystal grain size of the standard steel material, which is a steel for thick plates manufactured using a continuously cast slab that satisfies the above conditions. The toughness index, which is a value obtained by dividing the absorption energy measured by the Charpy test by the absorption energy of the reference steel material, is 1.5 to 3. A steel material for thick plates, which is zero.

(2) The steel material for thick plates according to (1) above, which contains Mg: 0.0001 to 0.005% by mass% instead of part of the Fe.

(3) Instead of a part of the Fe, by mass%, Ca: 0.0001 to 0.005%, Sr :: 0.0001 to 0.005%, and Ba: 0.0001 to 0.005% The steel material for thick plates according to the above (1) and (2), characterized by containing one or more of them.

(4) Instead of a part of the Fe, by mass%, Cu: 0.05 to 1.5%, Ni: 0.05 to 5.0%, Cr: 0.02 to 1.0%, Mo : 0.02 to 1.0%, Nb: 0.005 to 0.05%, V: 0.005 to 0.1% and B: One or more of 0.0004 to 0.004 The steel material for thick plates according to (1) to (3) above.

(5) A continuous casting method for producing a slab as a raw material for the steel plate material according to (1) to (4), wherein the molten steel in the tundish or the molten steel in the continuous casting mold is used. By inserting a metal wire or rod containing Bi or a metal wire or rod containing one or more of Bi, Mg, Ca, Sr and Ba into the immersed lance soaked in the immersion lance A continuous casting method characterized in that metal vapor and / or metal particles are generated in the step and the metal vapor and / or metal particles are supplied into the molten steel together with a carrier gas.

(6) A continuous casting method for producing a slab as a raw material for the steel plate material according to (1) to (4), wherein the molten steel in the tundish or the molten steel in the continuous casting mold is used. A continuous casting method characterized by supplying a metal wire or rod containing Bi, or a metal wire or rod containing Bi or one or more of Mg, Ca, Sr and Ba.

  In the present invention, “metal vapor and / or metal particles” means metal vapor and / or metal particles present as liquid or solid particles due to insufficient evaporation, or metal formed by condensation of metal vapor. Means particles. The “metal” includes both pure metals and alloys.

  Further, in the description of the present specification, “precipitates are finely dispersed” means that a sample collected from a hot-rolled steel sheet is observed with a SEM (scanning electron microscope) at a magnification of 500 to 2000 times, and the observed precipitation. It means a state in which precipitates having an average value per 200 maximum lengths of product particles of 1 μm or less are dispersed. “Large heat input welding” and “very large heat input welding” refer to welding with heat inputs of approximately 20 kJ / mm and 50 to 100 kJ / mm, respectively.

  In the following description, “mass%” for the composition of steel is simply expressed as “%”.

  In the steel plate for thick plate of the present invention, segregation is reduced, the transformation structure is uniform in size and phase, and fine inclusions are dispersed, which suppresses the austenite grain coarsening during reheating. It is possible to obtain high HAZ toughness even when high heat input welding is performed.

  Further, the continuous casting method of the present invention efficiently adds an appropriate amount of a metal element necessary for producing a slab as a raw material for the above-described steel plate material to the molten steel, and continuously cast slab such as slab. It is an optimum continuous casting method that can be uniformly dispersed therein.

It is a figure which shows the method of continuous casting, supplying a metal wire through the immersion lance and supplying the molten steel in a tundish directly.

  As described above, the steel plate material of the present invention is a steel plate material manufactured using a continuously cast slab as a raw material, and is in mass%, C: 0.01 to 0.20%, Si: 0.02 to 0.5%, Mn: 0.6 to 3.0%, P: 0.02% or less, S: 0.002 to 0.008%, Ti: 0.005 to 0.03%, N: 0.002 to 0.008%, Al: 0.0005 to 0.05%, O: 0.0001 to 0.015% and Bi: 0.0001 to 0.03%, with the balance being Fe And the segregation index, which is a value obtained by dividing the maximum value of the Mn content measured by line analysis using EPMA by the average Mn content, is 1.0 to 2.2, and the average crystal grain size is A steel plate material manufactured using a slab that does not contain Bi and has a component composition other than Bi that satisfies the above conditions as a raw material. The crystal grain size index, which is a value divided by the average crystal grain size of the reference steel material is 0.3 to 0.9, and the value obtained by dividing the absorbed energy measured by the Charpy test by the absorbed energy of the reference steel material. A steel material for a thick plate characterized by having a certain toughness index of 1.5 to 3.0. Hereinafter, the contents of the present invention will be described in more detail.

1. 1. Range of component composition of steel and reasons for limitation 1-1. Essential element C: 0.01 to 0.20%
C is an element effective for ensuring the strength and toughness of the steel sheet. If the C content is less than 0.01%, sufficient strength and toughness cannot be ensured. On the other hand, if the C content exceeds 0.20%, the toughness of the base material and the HAZ decreases. Therefore, in the present invention, the appropriate range of the C content is set to 0.01 to 0.20%.

Si: 0.02 to 0.5%
If the Si content is less than 0.02%, the strength of the base material cannot be secured, so the lower limit was made 0.02%. Moreover, since weldability will fall when Si content rate exceeds 0.5%, an upper limit was made into 0.5%. Therefore, in the present invention, the appropriate range of the Si content is set to 0.02 to 0.5%.

Mn: 0.6 to 3.0%
Mn is an effective element for increasing the strength of the steel sheet and ensuring toughness. In order to increase the strength of the steel sheet and ensure toughness, the Mn content must be 0.6% or more. On the other hand, if the Mn content exceeds 3.0%, the toughness is impaired. Therefore, in the present invention, the appropriate range of the Mn content is set to 0.6 to 3.0%.

P: 0.02% or less P is an element that deteriorates the ductility, toughness, and workability of the steel sheet. Therefore, the P content is limited to 0.02% or less.

S: 0.002 to 0.03%
S has the effect of promoting the formation of ferrite in the crystal grains by forming MnS inclusions and the like. If the S content is less than 0.0025%, there is almost no effect of generating ferrite, so 0.002% was made the lower limit. However, if the S content exceeds 0.03%, the ductility of the steel sheet is lowered, so 0.03% was made the upper limit. For the above reason, the appropriate range of the S content is set to 0.002 to 0.03%.

Ti: 0.005 to 0.03%
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. If the Ti content is less than 0.005%, the effect of improving the strength of the base metal by the precipitation strengthening action of carbonitride is not sufficient, while if the Ti content exceeds 0.03%, Coarse precipitates and inclusions are formed to reduce the toughness of the steel. For the above reason, the appropriate range of Ti content is set to 0.005 to 0.03%.

N: 0.002 to 0.008%
N is an element necessary for reacting with Ti to precipitate TiN. However, if the N content exceeds 0.01% and the toughness of the steel decreases, the upper limit of the N content is set to 0.008%. However, since it is impossible to remove N from steel completely industrially, the lower limit of the N content is set to 0.002% in consideration of the range that can be reduced in actual operation.

Al: 0.0005 to 0.05%
Al is a deoxidizing element of molten steel, and the content is preferably low in order to suppress the formation of Ti-containing oxides. However, if the Al content exceeds 0.05%, the production of Ti-containing oxides and Mg-containing oxides decreases. Moreover, in order to leave some oxygen in the molten steel and ensure toughness, the lower limit of the Al content is set to 0.0005%.

O: 0.0001 to 0.015%
O is an element necessary for forming an oxide. When the O content is less than 0.0001%, the number of oxides is insufficient, so the lower limit of the O content is set to 0.0001%. In addition, when the O content exceeds 0.015%, the amount of oxide increases and the toughness of the steel decreases, so the upper limit of the O content was set to 0.015%.

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. When the Bi content is less than 0.0001%, there is almost no effect of refining the solidified structure, so the lower limit of the Bi content is set to 0.0001%. Further, when the Bi content exceeds 0.05%, coarse Bi oxide is generated and the toughness of the steel is reduced, so the upper limit of the Bi content is set to 0.03%.

  Further, since Bi is a surface active element, it has an effect of promoting the precipitation of TiN, MnS, etc. on the Mg-containing oxide. Therefore, although TiN, MnS, etc. are easily coarsened, they can be finely dispersed by being precipitated on finely dispersed Mg oxide. Since the effect of fine dispersion of TiN, MnS, etc. is hardly present when the Bi content is less than 0.0001%, the lower limit of the Bi content is set to 0.0001%. For the above reason, the appropriate range of Bi content is set to 0.0001 to 0.03%.

1-2. Arbitrary element Instead of a part of Fe, the following first to fourth optional elements may be contained.

1-2-1. First optional element Mg: 0.0001 to 0.005%
Mg is an important additive element after Bi. Oxygen in molten steel reacts with added Mg to produce Mg oxide. As the Mg oxide, in addition to MgO alone, an oxide containing MgO and one or more of Al ₂ O ₃ and Ti ₂ O ₃ is generated. In order to produce these oxides, the Mg content needs to be 0.0001% or more. However, when the Mg content exceeds 0.005%, the amount of coarse oxide inclusions in the steel increases, and the toughness of the steel decreases. For the above reason, the appropriate range of the Mg content is set to 0.0001 to 0.005%.

1-2-2. Second optional element Ca: one or more of 0.0001 to 0.003%, Sr: 0.0001 to 0.003%, and Ba: 0.0001 to 0.003%

Ca: 0.0001 to 0.003%
Ca reacts with oxygen in molten steel to generate Ca oxide. As Ca oxide, in addition to CaO alone, an oxide containing one or more of CaO, Al ₂ O ₃ , Ti ₂ O _{3 and the} like is generated. These oxides are finely dispersed in the steel. In order to obtain the effect of finely dispersing the oxide, the Ca content needs to be 0.0001% or more. However, when the Ca content exceeds 0.003%, the amount of coarse oxide inclusions in the steel increases and the toughness of the steel decreases. For the above reason, the appropriate range of the Ca content is set to 0.0001 to 0.003%.

Sr: 0.0001 to 0.003%
Sr reacts with oxygen in the molten steel to produce Sr oxide. As the Sr oxide, an oxide containing SrO and one or more of Al ₂ O ₃ and Ti ₂ O ₃ in addition to SrO alone is generated. These oxides are finely dispersed in the steel. In order to obtain the effect of finely dispersing the oxide, the Sr content must be 0.0001% or more. However, if the Sr content exceeds 0.003%, the amount of coarse oxide inclusions in the steel increases and the toughness of the steel decreases. For the above reason, the appropriate range of Sr content is set to 0.0001 to 0.003%.

Ba: 0.0001 to 0.003%
Ba reacts with oxygen in the molten steel to generate Ba oxide. As the Ba oxide, in addition to BaO alone, an oxide containing BaO and one or more of Al ₂ O ₃ and Ti ₂ O ₃ is generated. These oxides are finely dispersed in the steel. In order to obtain the effect of finely dispersing the oxide, the Ba content needs to be 0.0001% or more. However, when the Ba content exceeds 0.003%, the amount of coarse oxide inclusions in the steel increases and the toughness of the steel decreases. For the above reason, the appropriate range of the Ba content is set to 0.0001 to 0.003%.

1-2-3. Third optional element Cu: 0.05-1.5%, Ni: 0.05-5.0%, Cr: 0.02-1.0%, Mo: 0.02-1.0%, Nb : One or more of 0.005-0.05%, V: 0.005-0.1% and B: 0.0004-0.004%

Cu: 0.05 to 1.5%
If Cu is contained, it is an element having an effect effective in improving hardenability and precipitation strengthening. However, when the Cu content is less than 0.05%, there is no effect of improving hardenability and effect of precipitation strengthening. On the other hand, when the Cu content is higher than 1.5%, the hot workability of steel is lowered. For the above reason, the range of the Cu content when Cu is contained is set to 0.05 to 1.5%.

Ni: 0.05-5.0%
Ni is an element having an action of improving the toughness of the base material if contained. However, when the Ni content is less than 0.05%, there is no effect of improving the toughness of the base material. On the other hand, if the Ni content exceeds 5.0%, the hardenability becomes excessive and adversely affects the toughness of the steel. Therefore, the range of Ni content when Ni is included is set to 0.05 to 5.0%.

Cr: 0.02-1.0%
When Cr is contained, it is an element that exhibits an effective action for improving the hardenability and for improving the strength of the base metal by precipitation strengthening. However, if the Cr content is less than 0.02%, there is no effect of improving hardenability and effect of precipitation strengthening. On the other hand, when the Cr content is higher than 1.0%, there is a tendency for the toughness and weldability of steel to deteriorate. Therefore, the range of Cr content when Cr is contained is set to 0.02 to 1.0%.

Mo: 0.02-1.0%
Mo is an element that exhibits an effective action for improving hardenability and strength when contained. However, if the Mo content is less than 0.02%, the hardenability improving effect and the strength improving effect are not clear. On the other hand, when the Mo content exceeds 1.0%, the toughness and ductility of the steel and the weldability deteriorate. Therefore, the range of the Mo content when Mo is contained is set to 0.02 to 1.0%.

Nb: 0.005 to 0.05%
Nb is an element that has the effect of improving the strength of steel by forming carbides and nitrides when contained. However, if the Nb content is less than 0.005%, the effect of improving the strength of steel due to the formation of carbides and nitrides is not clear. On the other hand, when the Nb content is higher than 0.05%, coarse carbides and nitrides are formed in the steel, so that the toughness is reduced. For the above reason, the range of Nb content when Nb is contained is set to 0.005 to 0.05%.

V: 0.005 to 0.1%
V is an element that has the effect of improving the strength of steel by forming carbides and nitrides when contained. However, if the V content is less than 0.005%, the effect of improving the strength of steel due to the formation of carbides and nitrides is not clear. On the other hand, when the V content exceeds 0.1%, the toughness of the steel is reduced. For the above reason, the range of the V content when V is contained is set to 0.005 to 0.1%.

B: 0.0004 to 0.004%
When B is contained, it has the effect of increasing the hardenability and reducing the content of solid solution N by generating BN and improving the toughness of HAZ. However, if the B content is less than 0.0004%, the effect of increasing hardenability and the effect of improving the toughness of HAZ are not clear. However, if the B content exceeds 0.004%, coarse borides precipitate in the steel, which deteriorates the toughness of the steel. For the above reason, the range of the B content when B is contained is set to 0.0004 to 0.004%.

2. Continuous casting method As described above, the continuous casting method of the present invention includes a Bi-containing metal wire or rod, or Bi, in a dipping lance immersed in molten steel in a tundish or molten steel in a continuous casting mold, and By inserting a metal wire or rod containing one or more of Mg, Ca, Sr and Ba, metal vapor and / or metal particles are generated in the immersion lance, and the metal vapor and / or metal particles are This is a method of supplying the molten steel together with the carrier gas.

  By adding Bi and Mg, Ca, Sr, and Ba by such a method, appropriate amounts of these elements can be efficiently added to the molten steel and uniformly dispersed in the continuous cast slab.

  As an apparatus for carrying out this continuous casting method, for example, as described in the examples described later, a tundish and a dipping for supplying molten steel in the tundish to the continuous casting mold provided in the lower part of the tundish Nozzle, continuous casting mold located below the tundish, immersion lance for supplying metal wire or rod to the molten steel in the tundish, or supplying metal wire or rod to the molten steel in the continuous casting mold A continuous casting apparatus having an immersion lance, a wire or rod supply device for supplying a wire or rod into the hole of the immersion lance, and a gas supply device for supplying a carrier gas into the immersion lance is preferred.

  Moreover, you may supply the said metal wire or rod directly from the wire or rod supply apparatus to the molten steel in a tundish, or the molten steel in a continuous casting mold.

  In order to confirm the effects of the continuous cast slab and the continuous casting method of the present invention, the following tests were conducted and the results were evaluated.

1. Test conditions 1-1. Casting conditions Molten steel components: C, Si, Mn, P, S, N, Al, Ti, Nb, V, Cr, Mo, Cu, Ni, REM, Zr and B are listed in Table 1 described later. Using molten steel prepared for composition, Bi, Mg, Ca, Sr and Ba were added by the following addition method to prepare the composition shown in Table 1. Molten steel temperature: 1570 ° C. (molten steel temperature in tundish)
Continuous casting mold size: width 1400mm x thickness 250mm
Casting speed: 1.0 m / min Metal addition method: A metal wire made of an additive metal having a diameter of 3 mm is inserted into the immersion lance. Additive metal types: Bi, Mg, Ca, Sr and Ba
Addition position: in tundish Carrier gas: Argon gas 10 L / min Gas pressure before lance: 0.05 MPa

  In this test, continuous casting was performed by changing the molten steel components to produce a continuous cast slab. The component composition of the molten steel cast in the test of the present invention example is shown in the columns of Invention Examples 1 to 12 in Table 1, and the component composition of the molten steel cast in the comparative example test without adding Bi is shown in Table 1. It showed in the column of the comparative examples 1-3 inside.

  Invention Examples 1 to 12 are examples in which all of the above essential elements are contained within the specified range of the present invention. Invention Examples 1 and 2 are examples that do not contain any of the above-mentioned first to third optional elements, Invention Example 3 is an example that contains the first optional element in a prescribed range, and Invention Example 4 is the first example. Examples containing 1 and 2 optional elements in specified ranges, Invention Examples 5 to 7 and 11 include Examples containing 1 and 3 optional elements in specified ranges, Invention Examples 8 to 10 and 12 Is an example containing the first to third optional elements within a specified range.

  Comparative Examples 1 to 3 are examples in which all of the above essential elements other than Bi are contained within the specified range of the present invention. Comparative Example 1 is an example that does not contain any of the first to third optional elements described above, Comparative Example 2 is an example that contains the first optional element in a specified range, and Comparative Example 3 is the first to third optional elements. It is an Example containing an arbitrary element in a specified range.

  FIG. 1 is a view showing a method of continuous casting while supplying a metal wire to molten steel in a tundish through an immersion lance. The molten steel 1 supplied from the ladle 3 to the tundish 2 is poured into the continuous casting mold 8 through the immersion nozzle 6 and forms a solidified shell 7 while being drawn downward to form a slab. A metal wire 50 containing an additive metal element is inserted into the hole of the immersion lance 4 immersed in the molten steel 1 in the tundish 2 at a predetermined speed.

  The upper end of the immersion lance 4 is connected to a metal wire feeder 5. A wire reel 51 is loaded in the metal wire feeder 5, and the metal wire 50 is inserted and supplied into the immersion lance 4 by a wire feeding roll 52 whose feeding speed is controlled by a wire feeding speed control device 53. Is done. A carrier gas 54 whose flow rate and pressure are controlled by a flow rate control valve 56 that is actuated by a command of a flow rate pressure controller 57 based on the numerical value of the pressure gauge 55 is introduced into the metal wire feeder 5 and is immersed together with the metal wire 50. Supplied into the lance 4.

1-2. Steel plate production conditions Using a continuous cast slab obtained by a continuous casting test as a raw material, a thickness of 40 to 100 mm by a production method of any one of a controlled rolling / controlled cooling method, a quenching / tempering method, and a direct quenching / tempering method A steel plate was produced. The manufacturing method and thickness of each steel plate in each example were as shown in Table 2.

1-3. Evaluation condition of segregation index A sample was taken in parallel with the surface from a position of 1/4 of the thickness in the thickness direction from the surface of the steel plate obtained by any of the above methods, and this sample was Mn-containing using EPMA. The rate was measured by line analysis. The beam diameter at the time of analysis using EPMA was 1 μm, and the length for performing line analysis was 20 mm.

  And the value which remove | divided the maximum value of the measured Mn content rate by the average Mn content rate of the steel plate was defined as the segregation index. The closer the segregation index value is to 1.0, the closer the Mn content in the steel sheet is, and the closer the crystal grain structure of the steel sheet is.

  The average composition (including Mn content) of the steel sheet is the result of measuring chips by collecting chemicals from the steel sheet.

1-4. Reproduction HAZ test implementation condition and crystal grain size index evaluation condition In addition, a sample having a diameter of 10 mm and a length of 100 mm was taken from a 1/4 position in the thickness direction from the surface of the steel sheet, and this sample was used to reproduce the HAZ. A test was conducted.

  The reproduction HAZ test was performed in an Ar gas atmosphere using a high-frequency induction heating apparatus, and the central region 10 mm in the length direction of the sample was heated. Heating was performed from room temperature to 1450 ° C. in 30 seconds, held for 60 seconds, and then the heated portion was rapidly cooled using He gas. This heating condition corresponds to high heat input welding.

  After cooling, the sample from which the heated portion was cut was polished with emery paper, and then further polished with a polishing agent having diamond abrasive grain diameters of 6 μm and 1 μm. In order to reveal the crystal grains of the sample, the polished sample was etched using a nital solution or a picric acid solution.

  The average crystal grain size of the revealed crystal grains was determined for the samples of each Example, and the value obtained by dividing the average crystal grain size of the sample of Comparative Example 1 to which no metal element was added was defined as the crystal grain size index. The smaller the crystal grain size index value is, the greater the effect of suppressing the coarsening of crystal grains.

1-5. Evaluation condition of toughness index Furthermore, a sample (test piece) having a length of 10 mm square × 100 mm was taken from a position of 1/4 in the thickness direction from the surface of the steel sheet, and a reproduced HAZ test and a Charpy test were performed using this sample. I did it.

  The reproduction HAZ test was performed in an Ar gas atmosphere using a high-frequency induction heating apparatus, and the central region 10 mm in the length direction of the sample was heated. Heating was performed from room temperature to 1450 ° C. in 30 seconds, held for 60 seconds, and then the heated portion was rapidly cooled using He gas.

  A notch was made in the central portion in the length direction of the test piece subjected to the reproduction HAZ test, and a Charpy test was performed in an atmosphere at a temperature of 0 ° C. to obtain the absorbed energy. Absorbed energy was determined for the test piece of each example, and the value divided by the absorbed energy of the test piece of Comparative Example 1 was defined as the toughness index. A larger toughness index indicates higher absorbed energy and better toughness.

2. Test Results The steel sheets produced under the above conditions were evaluated for the above three types of items (segregation index, crystal grain size index, and toughness index). The evaluation results are shown in Table 2 together with the production conditions.

  In each of Comparative Examples 1 to 3, the segregation index was 2.44 or more, which was a value larger than the upper limit of the specified range (1.0 to 2.2) of the present invention. On the other hand, in Invention Examples 1 to 12, 1.01 to 1.14, which are within the specified range of the present invention, the Mn content in the steel sheet and the crystal grain structure of the steel sheet had excellent uniformity.

  The crystal grain size index was 1.00 or more in Comparative Examples 1 to 3, which was a value larger than the upper limit of the specified range (0.3 to 0.9) of the present invention. On the other hand, in Examples 1-12 of the present invention, 0.33 to 0.85, which is within the specified range of the present invention, the effect of suppressing the coarsening of crystal grains was sufficiently obtained.

  The toughness index was 1.00 or more in Comparative Examples 1 to 3, which was a value smaller than the lower limit of the specified range (1.5 to 3.0) of the present invention. On the other hand, Examples 1 to 12 of the present invention were 1.56 to 2.84, which were within the specified range of the present invention, and had good toughness.

  In the steel plate for thick plate of the present invention, segregation is reduced, the transformation structure is uniform in size and phase, and fine inclusions are dispersed, which suppresses the austenite grain coarsening during reheating. It is possible to obtain high HAZ toughness even when high heat input welding is performed.

  In addition, the continuous casting method of the present invention efficiently adds an appropriate amount of a metal element necessary for obtaining a slab to be a raw material for the steel plate material to the molten steel, and the slab or the like is continuously cast into the continuous cast slab. This is the optimum continuous casting method for uniform dispersion.

  Therefore, the steel plate for the thick plate of the present invention is a structural steel material having mechanical properties such as the strength and toughness of the stable base metal after welding, and the continuous casting method of the present invention includes the steel plate for the thick plate. Each can be widely applied as a continuous casting method for casting a slab as a raw material.

1: molten steel, 2: tundish, 3: ladle, 4: immersion lance,
5: Metal wire feeder, 50: Metal wire, 51: Wire reel
52: Wire feeding roll, 53: Wire feeding speed control device,
54: Carrier gas, 55: Pressure gauge, 56: Flow control valve, 57: Flow pressure controller,
6: immersion nozzle, 7: solidified shell, 8: continuous casting mold

Claims (6)

It is a steel for a thick plate manufactured using a continuously cast slab as a raw material, and in mass%, C: 0.01 to 0.20%, Si: 0.02 to 0.5%, Mn: 0.00. 6-3.0%, P: 0.02% or less, S: 0.002-0.008%, Ti: 0.005-0.03%, N: 0.002-0.008%, Al: 0.0005-0.05%, O: 0.0001-0.015% and Bi: 0.0001-0.03%, with the balance consisting of Fe and impurities,
The segregation index, which is a value obtained by dividing the maximum value of the Mn content measured by line analysis using EPMA by the average Mn content, is 1.0 to 2.2,
The value of the average grain size is divided by the average grain size of the standard steel material, which is a steel plate for thick plates manufactured using a continuously cast slab that does not contain Bi and that satisfies the above conditions for components other than Bi. The crystal grain size index, which is the value obtained, is 0.3 to 0.9,
A steel material for thick plates, wherein the toughness index, which is a value obtained by dividing the absorbed energy measured by the Charpy test by the absorbed energy of the reference steel material, is 1.5 to 3.0.   It replaces with a part of said Fe and contains Mg: 0.0001-0.005% by the mass%, The steel plate material for thick plates of Claim 1 characterized by the above-mentioned.   Instead of a part of the Fe, by mass%, Ca: 0.0001 to 0.005%, Sr :: 0.0001 to 0.005% and Ba: 0.0001 to 0.005% The steel material for thick plates according to claim 1 or 2, characterized by containing the above.   Instead of a part of the Fe, by mass%, Cu: 0.05 to 1.5%, Ni: 0.05 to 5.0%, Cr: 0.02 to 1.0%, Mo: 0.00. 02-1.0%, Nb: 0.005-0.05%, V: 0.005-0.1% and B: containing one or more of 0.0004-0.004 The steel material for thick plates according to any one of claims 1 to 3. A continuous casting method for producing a slab as a raw material for a steel plate for a thick plate according to any one of claims 1 to 4,
In a dipping lance immersed in molten steel in a tundish or molten steel in a continuous casting mold, a metal wire or rod containing Bi, or Bi, and one or more of Mg, Ca, Sr and Ba are contained. By inserting metal wire or rod,
A continuous casting method, wherein metal vapor and / or metal particles are generated in the immersion lance, and the metal vapor and / or metal particles are supplied into the molten steel together with a carrier gas. A continuous casting method for producing a slab as a raw material for a steel plate for a thick plate according to any one of claims 1 to 4,
Metal wire or rod containing Bi, or metal wire or rod containing Bi, and one or more of Mg, Ca, Sr and Ba in molten steel in tundish or molten steel in continuous casting mold A continuous casting method characterized in that

Images