JP2021053699A - Continuous casting process of steel - Google Patents

Abstract

To provide a continuous casting process of steel in which center segregation can be sufficiently reduced, in particular generation of a coarse segregation spot can be prevented, and delayed fracture after gas cutting can be prevented in a thick steel sheet.SOLUTION: A steel slab is obtained by continuously casting molten steel containing 0.1 mass% or more and 0.3 mass% or less of C by using a vertical bending type or curved type continuous casting machine. In the process, (A) a casting condition is controlled so that a solidified structure in a final solidified part in the thickness direction of the cast slab becomes columnar crystals with an average dendrite primary arm spacing of 2.0 mm or less on the upper surface side in the thickness direction and columnar or branched columnar crystals with an average dendrite primary arm spacing of 2.0 mm or less on the lower surface side in the thickness direction over the entire width, and (B) in the secondary cooling step, a slight pressure drop is applied to the cast slab at a pressure drop speed of 0.5 mm/min or more and 2.0 mm/min or less in the casting direction of the cast slab within a range from a position where the solid phase rate of at least the center of the thickness of the cast slab becomes 0.2 to a solidified position where solidification is completed.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for continuously casting steel using a continuous casting machine.

In the solidification process of steel, solute elements such as carbon, phosphorus, sulfur, and manganese are concentrated on the unsolidified liquid phase (molten steel) side by redistribution during solidification. This is the microsegregation formed between the dendrite trees. Due to solidification shrinkage and heat shrinkage of the slab being cast by the continuous casting machine, and bulging of the solidification shell generated between the rolls of the continuous casting machine, a gap is formed in the center of the thickness of the slab and negative pressure is generated. When it occurs, the molten steel flows toward this part. At that time, a sufficient amount of molten steel (unsolidified layer) does not exist in the central portion of the thickness of the slab at the end of solidification. Therefore, the molten steel in which the solute element is concentrated flows by the above-mentioned microsegregation, and is accumulated in the center of the thickness of the slab and solidified. The segregation spots formed in this way have a significantly higher concentration of solute elements than the initial concentration of molten steel. This phenomenon is generally called macro segregation, and is also called central segregation because of the location of the segregation spot.

By the way, industrial machinery, parts, transportation equipment (for example, power shovels, bulldozers, hoppers, bucket conveyors, rock crushers) used in fields such as construction, civil engineering, and mining are worn aggressively by rocks, sand, ores, etc. Exposed to wear such as sliding wear and impact wear. Therefore, steel used for such industrial machines, parts, and transportation equipment is required to have excellent wear resistance in order to improve the life.

It is known that the wear resistance of steel can be improved by increasing the hardness. Therefore, high-hardness steels obtained by subjecting alloy steels to which a large amount of alloying elements such as Cr and Mo are added to heat treatment such as quenching have been widely used as wear-resistant steels.

Further, in the field of wear-resistant steel sheets, it is required to prevent delayed fracture in addition to improving wear resistance. Delayed fracture is a phenomenon in which a steel sheet suddenly breaks even though the stress applied to the steel sheet is equal to or less than the yield strength. This delayed fracture phenomenon is more likely to occur as the strength of the steel sheet is higher, and is promoted by hydrogen intrusion into the steel sheet. An example of the delayed fracture phenomenon of a wear-resistant steel sheet is cracking after gas cutting. When the gas is cut, the steel sheet becomes brittle due to the intrusion of hydrogen from the combustion gas, and the residual stress after the gas cutting causes cracks several hours to several days after the cutting. Since the wear-resistant steel sheet has high hardness, it is often cut by gas, and in the wear-resistant steel sheet, delayed fracture after gas cutting (hereinafter, may be referred to as “gas cutting crack”) is often a problem.

Delayed fracture after gas cutting in a wear-resistant steel plate occurs from grain boundary fracture that occurs at the former austenite grain boundaries of the martensite structure and bainite structure. The grain boundary fracture is caused by (a) residual stress caused by gas cutting, (b) hydrogen embrittlement due to hydrogen penetrating from the cutting gas into the steel plate during gas cutting, and (c) temperature rise during gas cutting. It occurs due to the overlapping effects of temper brittleness.

Further, the segregated portion at the center of the thickness of the steel sheet in which the grain boundary embrittlement elements Mn and P are concentrated serves as the starting point of gas cutting cracks. Further, the temperature rise at the time of gas cutting further promotes the segregation of the grain boundary embrittlement element into the former austenite grain boundary in the plate thickness center segregation portion, and as a result, the strength of the former austenite grain boundary is significantly reduced, and gas cutting Cracks are likely to occur.

Therefore, it is extremely important to reduce central segregation at the slab stage in order to suppress gas cutting cracks in thick steel sheets subjected to gas cutting such as wear-resistant steel sheets. In order to deal with this, many measures have been proposed to reduce the central segregation of slabs in the continuous casting process.

For example, as disclosed in Patent Document 1 and Patent Document 2, in a continuous casting machine, a slab at the end of solidification having an unsolidified layer is combined with a solidification shrinkage amount and a heat shrinkage amount by a slab support roll. A method of casting while gradually reducing the amount of reduction to a considerable extent has been proposed. This technique is called "light reduction" or "light reduction". In this light reduction technique, a plurality of pairs of rolls arranged in the casting direction are used to gradually reduce the volume of the unsolidified layer by gradually reducing the slab with a reduction amount commensurate with the sum of the solidification shrinkage amount and the heat shrinkage amount. Let me. This prevents the formation of voids or negative pressure portions at the center of the thickness of the slab, and at the same time prevents the flow of the concentrated molten steel formed between the dendrite trees. As a result, the central segregation of the slab is reduced.

Further, since there is a close relationship between the morphology of the dendrite structure at the center of the thickness and the central segregation, the following methods have also been proposed.

In Patent Document 3, the specific water content in a specific range in the casting direction of the secondary cooling zone of the continuous casting machine is set to 0.5 L / kg or more, and the solid phase ratio fs at the center of the thickness of the slab is 0. A method for continuous casting of steel is described in which a total reduction of 5 to 40 mm is applied in the range of 1 to 0.8. In this method, the solidified structure is refined and equiaxed by increasing the amount of specific water, thereby reducing the central segregation.

Patent Document 4 states, "The degree of superheat of the molten steel in the tundish is set to 25 ° C. or higher, and this molten steel is cast into a continuous casting mold to start solidification of the molten steel, and the slabs at the initial stage of solidification drawn from the mold are used. A continuous casting method of steel in which a static magnetic field is applied, a static magnetic field is applied to a slab immediately before final solidification, and the slab is lightly reduced is described. In this method, the solidification structure at the center of the thickness of the slab is controlled to be columnar crystals, and then a light reduction is applied to the slab to reduce the central segregation.

Japanese Unexamined Patent Publication No. 8-132203 Japanese Unexamined Patent Publication No. 8-192256 Japanese Unexamined Patent Publication No. 8-224650 Japanese Unexamined Patent Publication No. 6-608

However, the methods of Patent Document 1 and Patent Document 2 only focus on solidification shrinkage as a factor of central segregation. Therefore, although the central segregation is reduced, the strict central segregation suppression level required in recent years cannot be satisfied. Therefore, it is not possible to prevent delayed fracture of the thick steel sheet after gas cutting.

In addition, the methods of Patent Document 3 and Patent Document 4 have the following problems. As will be described later, according to the research by the present inventors, it has been clarified that the gas cutting crack occurs when the segregation spot having a predetermined segregation degree or more exceeds a predetermined size. However, in the method of Patent Document 3 in which a light reduction is applied to a slab in a solid-liquid coexisting region that has become equiaxed crystals, spot segregation is likely to occur in the gaps between adjacent equiaxed crystals, and the segregated spot is designated. It was found that the size could not be controlled below the size, and the effect of reducing central segregation, which affects gas cutting cracks, was insufficient. Further, although the method of Patent Document 4 in which a light reduction is applied to a columnar crystal slab can be expected to have a certain effect in reducing gas cutting cracks in a thick steel sheet, according to the study by the present inventors, it is simply a solidified structure. It was found that the effect of reducing central segregation, which also affects gas cutting cracks, is insufficient only by columnar crystals.

Therefore, in view of the above problems, the present invention can further sufficiently reduce central segregation, and in particular, prevent the occurrence of coarse segregation spots and prevent delayed fracture of thick steel sheets after gas cutting. It is an object of the present invention to provide a continuous casting method.

As a result of diligent studies by the present inventors in order to solve the above problems, the following findings were obtained.
[1] The solidified structure at the final solidified portion (within ± 10 mm in the thickness direction from the final solidified thickness position) is completely columnar crystals (however, branched columnar crystals may be formed on the lower surface side in the thickness direction).
[2] The dendrite primary arm spacing for forming the columnar crystals and branched columnar crystals is 2.0 mm or less on average, and [3] at least slabs after satisfying the above [1] and [2]. In the range from the position where the solid phase ratio fs at the center of the thickness is 0.2 to the solidification completion position, light reduction is applied to the slab at a reduction speed of 0.5 mm / min or more and 2.0 mm / min or less. Therefore, the major axis diameter of the segregation spot having the Mn segregation degree of 2.1 or more at the center of the slab thickness can be controlled to 200 μm or less. In the thick steel sheet manufactured from the slab in which the central segregation is further sufficiently reduced, delayed fracture after gas cutting can be prevented.

The abstract structure of the present invention completed based on the above findings is as follows.
A method for continuous casting of steel using a vertical bending type or curved type continuous casting machine.
C: A primary cooling step of primary cooling a molten steel containing 0.1% by mass or more and 0.3% by mass or less with a mold to obtain a slab in which unsolidified molten steel remains inside.
A secondary cooling step of pulling out the slab from the mold and secondary cooling the slab while supporting it with a plurality of pairs of rolls arranged in the casting direction to obtain a steel slab whose inside is completely solidified.
Have,
(A) The solidified structure in the final solidified portion in the thickness direction of the slab covers the entire width.
On the upper surface side in the thickness direction, columnar crystals with an average dendrite primary arm spacing of 2.0 mm or less are formed.
On the lower surface side in the thickness direction, while controlling the casting conditions so that the dendrite primary arm spacing becomes a columnar crystal or a branched columnar crystal of 2.0 mm or less on average.
(B) In the secondary cooling step, in the casting direction of the slab, at least in the range from the position where the solid phase ratio at the center of the thickness of the slab is 0.2 to the solidification completion position, the slab is formed. A method for continuously casting steel, characterized in that light reduction is applied at a reduction speed of 0.5 mm / min or more and 2.0 mm / min or less.

According to the continuous steel casting method of the present invention, it is possible to further sufficiently reduce central segregation, particularly prevent the occurrence of coarse segregation spots, and prevent delayed fracture of thick steel sheets after gas cutting. ..

It is a schematic diagram of the vertical bending type continuous casting machine 100 which can be used in one Embodiment of this invention. FIG. 5 is a cross-sectional view in the casting direction of a slab drawn from a mold in the vertical bending type continuous casting machine shown in FIG. It is a casting direction sectional view of the slab drawn from a mold in the curved type continuous casting machine used in another embodiment of this invention. It is a schematic diagram for demonstrating the dendrite primary arm spacing. It is a schematic diagram of a slab including a cross section perpendicular to a casting direction. It is a graph which shows the relationship between the heat quantity of molten steel and the columnar crystal ratio obtained by Experimental Example 1. FIG. It is a graph which shows the relationship between the reduction rate obtained by Experimental Example 3 and Mn average segregation degree. It is a graph which shows the relationship between the reduction rate and the segregation ratio obtained by Experimental Example 3.

The method for continuously casting steel (method for producing steel pieces) according to an embodiment of the present invention uses a vertical bending type or curved type continuous casting machine, and the molten steel is primarily cooled with a mold to be unsolidified inside. In the primary cooling step of obtaining the slab in which the molten steel remains, and in the secondary cooling while pulling out the slab from the mold and supporting the slab with a plurality of pairs of rolls arranged in the casting direction, the inside is completely solidified. It has a secondary cooling step of obtaining a steel piece.

[Continuous casting machine]
In this embodiment, a vertical bending type or curved type continuous casting machine is used. As an example, the configuration of the 2-strand type vertical bending type continuous casting machine 100 that can be used in the present embodiment will be described with reference to FIG. The continuous casting machine 100 includes a ladle 10, a tundish 11, a mold 12, a spray nozzle 13, a plurality of pairs of rolls 14, a cutting device 15, and an electromagnetic agitator 16.

The molten steel M is housed in the ladle 10 located at the top of the continuous casting machine. The molten steel M is poured from the bottom of the ladle 10 into the tundish 11 located below the ladle 10. After that, the molten steel M is poured from the bottom of the tundish 11 into the mold 12 via the immersion nozzle, and the primary cooling of the molten steel is performed in the mold 12.

In order to guide the slab S drawn from the mold 12 from the vertical direction to the horizontal direction and prevent the slab S from being deformed by the static iron pressure, a plurality of pairs of rolls 14 are formed along a curve such as an arc or a hyperbola. Be arranged. A part of the roll 14 has a function as a pinch roll for pulling out the slab S. With reference to FIG. 2, the slab S drawn vertically downward from the mold 12 is bent in the upper straightening band 20B after passing through the vertical band 20A, kept in a curved state in the curved band 20C, and then in the lower part. It is bent back into a flat plate in the straightening band 20D and passes through the horizontal band 20E. An unsolidified portion of molten steel exists inside the slab S from directly below the mold to the horizontal band, and the roll 14 is arranged so as to support the surface of the slab S from directly below the mold to almost the entire length of the horizontal band. Spray nozzles 13 are located between rolls adjacent to each other in the casting direction, and cooling water is sprayed from these spray nozzles 13 onto the slab S to perform secondary cooling of the slab. A plurality of spray nozzles are actually arranged between each roll, but in FIG. 1, a part of the spray nozzles is schematically represented by a line segment connecting the plurality of nozzles.

On the downstream side of the horizontal band, a cutting device 15 such as a gas torch for cutting the solidified slab S and a hydraulic cutting is provided. The slab cut by the cutting device 15 is discharged from the continuous casting machine 100 and conveyed to the rolling device.

The curved continuous casting machine shown in FIG. 3 can also be used in this embodiment. In the vertical bending type continuous casting machine, since the slab is pulled out vertically downward from the mold, the inner wall surface of the mold 12 is flat. However, in the case of a curved continuous casting machine, a curved mold 21 is used in order to pull out the slab S in an arc shape from the mold. Since the inner wall surface of the mold 21 is curved, the curved slab is sent out, and the lower straightening band 20D performs the bending back straightening. In the case of the curved type, unlike the case of the vertical bending type, there is no bending process in the upper straightening band.

[Component composition of molten steel]
In the present embodiment, the component composition of the molten steel shall contain C: 0.1% by mass or more and 0.3% by mass or less. This is because, in the case of the amount of C in this range, the type of the solidified structure of the final solidified portion (thickness center portion) changes depending on the casting conditions, so that it is necessary to optimize the casting conditions in order to obtain columnar crystals. Since there is no steel type in which central segregation does not matter in thick steel sheets, the component composition of molten steel is not particularly limited as long as the above C content is satisfied. : 0.1 to 0.3%, Si: 0.01 to 1.0%, Mn: 0.3 to 3.0%, P: 0.025% or less, S: 0.02% or less, Cr: It contains 0.01 to 2.00% and Al: 0.001 to 0.100%, and optionally Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Mo. : 0.01 to 1.0%, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.050%, B: 0.0001 to 0.0100%, and V: 0.001 to It can have a component composition containing one or more selected from 1.0% and the balance being Fe and unavoidable impurities.

[Coagulation structure in the final coagulation part]
In the present embodiment, the solidified structure in the final solidified portion in the thickness direction of the slab is formed into columnar crystals having an average dendrite primary arm spacing of 2.0 mm or less on the upper surface side in the thickness direction over the entire width, and on the lower surface side in the thickness direction. It is important to control the dendrite primary arm spacing so that the average columnar crystal or branched columnar crystal is 2.0 mm or less.

[[Coagulation structure: columnar crystals]]
The solidified structure of the final solidified portion (thickness center portion) depends on the C content of the molten steel and the casting conditions. When a general vertical bending type or bending type continuous casting machine is used, in low carbon steel having a C content of less than 0.1% by mass, the solidification structure of the final solidified portion has a thickness unless special control is applied. Columnar crystals are formed on the upper surface side in the direction, and branched columnar crystals are formed on the lower surface side. On the other hand, in the medium-high carbon steel of C: 0.1% by mass or more and 0.3% by mass or less, which is the target of the present embodiment, the solidification structure of the final solidified portion has casting conditions (mainly molten steel superheat degree ΔT). Depending on the casting conditions, the upper surface side in the thickness direction may be columnar crystals, the lower surface side may be branched columnar crystals, or the entire thickness center may be equiaxed crystals. .. In addition, a coagulated tissue may be formed as if these tissues were mixed.

In the present embodiment, by appropriately selecting the casting conditions, the solidified structure of the final solidified portion is controlled to be columnar crystals on the upper surface side in the thickness direction and columnar crystals or branched columnar crystals on the lower surface side in the thickness direction over the entire width. To do. It is said that the solidified structure is determined by Hunt's relational expression, and the smaller the number of equiaxed nuclei and the larger the temperature gradient at the tip of the solidified interface, the more likely it is to form columnar crystals. Then, through diligent research by the inventors, it was discovered that the greater the amount of heat of molten steel when injecting molten steel into a mold, the more likely it is to form columnar crystals. It is considered that this is because as the amount of heat of molten steel increases, remelting of equiaxed nuclei occurs and the number of nuclei decreases. Here, the amount of heat of molten steel Q is represented by Q = (heat capacity of molten steel C) × (injection amount q per unit time) × (degree of superheat of molten steel ΔT). The injection amount q (kg / min) per unit time is a value obtained by Vc (m / min) × slab width (m) × slab thickness (m) × density (kg / m ^3). The molten steel superheat degree ΔT is (molten steel temperature in tundish) − (liquidus temperature). Under other conditions as well, columnar crystallization can be promoted by suppressing the formation of equiaxed nuclei. Regarding agitation in the mold, the more the application is not applied or the strength is weakened, the easier it is for columnar crystals to be formed. As for the stirring in the strand, columnar crystals are likely to be generated by not applying the stirring. It is also effective to reduce the amount of precipitates that can be the nuclei of equiaxed crystals such as TiN for the inoculated nuclei.

The type of coagulated tissue can be determined by the following method. Hydrochloric acid corrosion is applied to the cross section of the slab (steel slab) perpendicular to the casting direction. The central portion of the thickness of this cross section (within ± 10 mm in the thickness direction from the final solidification thickness position) is photographed with a projector. Based on the image thus obtained, the type of coagulated tissue is identified.

[[Dendrite primary arm spacing: 2.0 mm or less on average]]
In the present embodiment, in the solidified structure of the final solidified portion, the dendrite primary arm spacing for forming columnar crystals and branched columnar crystals is set to 2.0 mm or less on average. Even if an appropriate light pressure is applied to the slab, the discharge of concentrated molten steel to the front surface of the solidification interface cannot be prevented in the progress of solidification. At the final stage of solidification, the concentrated molten steel in front of the solidification interface solidifies, so that some segregation occurs. At that time, how the segregation is distributed depends on the dendrite primary arm spacing. When the dendrite primary arm spacing is large, coarse segregation spots are formed. In this case, since the Mn concentration of the main shaft of the dendrite is low, the segregation degree as a whole is not high, but segregation spots having a major axis diameter of more than 200 μm where gas cutting cracks can occur are generated. When the dendrite primary arm spacing is 2.0 mm or less on average, the occurrence of such coarse segregation spots can be sufficiently suppressed. The lower limit of the dendrite primary arm spacing is not particularly limited, but due to restrictions on casting conditions, the dendrite primary arm spacing is approximately 0.5 mm or more on average.

The dendrite primary arm spacing can be controlled by casting conditions, especially secondary cooling conditions such as specific water volume. Specifically, as the amount of specific water increases, the dendrite primary arm spacing can be reduced. In addition to the secondary cooling conditions, the dendrite primary arm spacing can be reduced by adding a surfactant element such as Bi.

The dendrite primary arm spacing can be measured by the following method. The central part of the cross section of the slab (steel piece) perpendicular to the casting direction (within ± 10 mm in the thickness direction from the final solidification thickness position) is corroded with picric acid and photographed with a projector. The image thus obtained is processed to measure the dendrite primary arm spacing. As shown in FIG. 4, the “dendrite primary arm spacing” means the _{distance λ 1} between the centers of the dendrite primary arms adjacent to each other in the width direction of the slab. In the present embodiment, the dendrite primary arm spacing is measured over the entire width of the central portion of the thickness, and the average value thereof is calculated.

In addition, in this specification, "over the entire width" does not mean the total width between both ends in the width direction of the slab in the cross section perpendicular to the casting direction of the slab, but extends in the width direction of the slab. It shall mean the total width between the final solidification thickness positions (the total width between the triple points, see FIG. 5).

[Giving under light pressure]
In the present embodiment, it is important to give the slab at the end of solidification a light reduction under predetermined conditions after forming the solidified structure in the final solidified portion as described above. Specifically, in the casting direction of the slab, the solidification completion position starts from the position where at least the solid phase ratio fs at the center of the thickness of the slab (hereinafter, also simply referred to as “central solid phase ratio fs”) is 0.2. In the range up to (CE position), light reduction is applied to the slab at a reduction speed of 0.5 mm / min or more and 2.0 mm / min or less.

Here, the central solid phase ratio fs is obtained by a one-dimensional heat transfer solidification calculation. The enthalpy method and the equivalent specific heat method are known for heat transfer / solidification calculation, but any method may be used. The central solid phase ratio fs is calculated by the following equation. This formula is not based on a strict metallurgical definition, but it is often used for simplicity.
fs = (T-TL) / {(1-k) · (T-TM)}
T: Central temperature TL: Liquidus temperature TM: Pure iron melting point k: Solute partition coefficient

Reduction speed: 0.5 mm / min or more and 2.0 mm / minute or less When the reduction speed is less than 0.5 mm / minute, the amount of solidification shrinkage cannot be compensated and the flow of concentrated molten steel cannot be suppressed. , Central segregation is not improved. Further, when the reduction speed exceeds 2.0 mm / min, the amount of reduction becomes excessive and the concentrated molten steel flows back to the upstream side, and as a result, segregation called reverse V segregation occurs, so that the central segregation is also improved. Not done. Therefore, in the present embodiment, the reduction speed is set to 0.5 mm / min or more and 2.0 mm / min or less.

Central solid phase ratio fs: 0.2 to 1.0 (CE position)
Furthermore, the timing of applying light pressure is also important for reducing central segregation. Even if the light reduction is started after the central solid phase ratio fs exceeds 0.2, the central segregation is already being formed, so that the effect of reducing the central segregation is small. Further, the central segregation occurs when the microsegregation flows and locally gathers, but the liquid phase does not move downstream from the CE position where fs is 1.0. Therefore, light reduction is not required on the downstream side of the CE position where fs is 1.0. Therefore, in the present embodiment, light reduction is performed in a range from at least a position where fs is 0.2 to a position where fs is 1.0.

Under light reduction, in a plurality of pairs of rolls facing each other across the slab, the roll interval of each pair (this interval is referred to as "roll opening degree") is set so as to be gradually narrowed toward the downstream in the casting direction. By doing so, it can be done.

In the continuous steel casting method according to the embodiment of the present invention described above, it is possible to appropriately control the solidification structure at the center of the thickness and then apply a light reduction to the slab at the end of solidification. Therefore, it is possible to manufacture high-quality steel pieces (slabs) with sufficiently reduced central segregation. That is, it is possible to sufficiently suppress the generation of coarse segregation spots at the slab stage, which causes gas cutting cracks in the thick steel sheet. As a result, from the steel pieces obtained in the present embodiment, it is possible to manufacture a thick steel sheet in which gas cutting cracks do not occur even under severe cutting conditions.

Specifically, the major axis diameter of the segregation spot having the Mn segregation degree of 2.1 or more at the center of the slab thickness can be controlled to 200 μm or less. Here, the "major axis diameter of the segregation spot" means the length in the width direction of the segregation spot as seen in the cross section of the steel piece perpendicular to the casting direction. In the cross section of the steel piece perpendicular to the casting direction, the Mn concentration is measured using an electron probe microprobe analyzer (EPMA) device in the width direction (measurement range) at 20 mm in the thickness direction centering on the final solidified part. analyse. The Mn segregation degree is obtained by dividing the Mn concentration within the measurement range by the Mn concentration of crude steel. The segregation spot is specified from the measurement point cloud having the Mn segregation degree of 2.1 or more. For all the identified segregation spots, the length in the width direction is calculated. The analysis conditions are a voltage of 25 kV, a current of 1.5 μA, an integration time of 50 msec, and a beam diameter of 10 μm.

(Experimental Example 1)
As a solidification structure control experiment, using a vertical bending type continuous casting machine, C: 0.20%, Si: 0.3%, Mn: 0.70%, P: 0.008%, S in mass%. : 0.003%, Cr: 0.50%, Al: 0.030%, Cu: 0.01%, Ni: 0.02%, Mo: 0.18%, Nb: 0.015%, Ti: A molten steel having a component composition containing 0.030% and V: 0.01% and having the balance of Fe and unavoidable impurities was cast. The slab thickness is 250 mm and the casting width is 2100 mm. As conditions, the molten steel heat quantity Q can be varied by varying the molten steel superheat degree ΔT in the range of 10 to 45 ° C. and varying the casting speed Vc in the range of 0.95 to 1.4 m / min. By shaking, the ratio of columnar crystals to the center of the thickness of the slab (within ± 10 mm in the thickness direction from the final solidification thickness position) was investigated. The columnar crystal ratio was defined by the ratio of columnar crystals in the central part of the thickness by investigating the section between the triple points mentioned above. FIG. 6 shows the relationship between the calorific value Q of molten steel in the mold and the ratio of columnar crystals at the center of the thickness. It can be seen that the solidified structure can be completely formed into columnar crystals with Q ≧ 1 MW. However, since this threshold value depends on the amount of nucleation, it differs depending on the electromagnetic stirring conditions and the presence or absence of precipitates such as TiN. That is, when casting steels having different conditions, it is necessary to newly determine the threshold value of Q.

(Experimental Example 2)
A molten steel having the same composition as that of Experimental Example 1 was cast using a vertical bending type continuous casting machine. The slab thickness is 250 mm and the casting width is 2100 mm. Table 1 shows the casting speed Vc, molten steel superheat degree ΔT, molten steel heat quantity Q, specific water amount in secondary cooling, reduction speed when performing light reduction, central solid phase ratio at the start of reduction, and end of reduction at each level. Indicates the central solid phase ratio of. At each level, the central solid phase ratio and CE position were determined by heat transfer calculation. Among the segment groups located in the range of 24 to 30 m from the meniscus, the central solid phase ratio at the start and end of the reduction was controlled by performing light reduction in a predetermined segment. The reduction rate was controlled by the reduction gradient of the light reduction segment. As a matter of course, casting conditions other than these are unified at all levels.

The type of solidified structure at the center of the thickness of the slabs obtained at each level was determined by the method described above, and the results are shown in Table 1. In addition, the dendrite primary arm spacing in the solidified structure at the center of the thickness of the slabs obtained at each level was measured by the method described above, and the average value in the width direction is shown in Table 1. When the solidified structure is equiaxed, it is difficult to measure the dendrite primary arm spacing, so this is omitted here.

At each level, segregation spots having a Mn segregation degree of 2.1 or more in the center of the thickness of the slab were identified by the method described above, and the number of segregation spots having a major axis length of more than 200 μm was counted. Is shown in Table 1.

After heating the slabs obtained at each level in a heating furnace, they were hot-rolled and further heat-treated to produce wear-resistant steel sheets. Then, the steel sheet was gas-cut under severe conditions of a cutting speed of 30 cm / min without preheating or slow cooling, and one day after cutting, the presence or absence of cracks was investigated by a color inspection. The results are shown in Table 1.

As is clear from Table 1, in Invention Examples 1 to 3, no coarse segregation spots were generated, and no gas cutting cracks were generated in the thick steel sheet. On the other hand, in Comparative Examples 1 to 6, coarse segregation spots were generated, and gas cutting cracks were generated in the thick steel sheet.

(Experimental Example 3)
According to the research by the present inventors, it was clarified that the segregation reducing effect when light pressure is applied varies depending on the solidified structure. The segregation form targeted by the present invention is a segregation spot having a Mn segregation degree of 2.1 or more and a major axis diameter of more than 200 μm, but this is a relatively high concentration, a small particle size, and the final stage of solidification. Since it is a segregated spot as generated in, the solidified structure has a great influence.

Therefore, using a vertical bending continuous casting machine, molten steel having the same composition as that of Experimental Example 1 was cast under various casting conditions. The slab thickness is 250 mm and the casting width is 2100 mm. Specifically, as the first group, the conditions are that the casting speed Vc = 1.4 m / min, the degree of superheat of molten steel ΔT = 45 ° C., the amount of heat of molten steel Q = 1.98 MW, and the amount of specific water in secondary cooling = 1.92 L / kg. Then, the solidification structure at the center of the thickness of the slab becomes columnar crystals (upper surface side: columnar crystals, lower surface side: branched columnar crystals), and as the second group, the casting rate Vc = 1.4 m / min, molten steel overheating. Under the conditions of degree ΔT = 10 ° C., heat quantity of molten steel Q = 0.44 MW, and specific water amount in secondary cooling = 1.92 L / kg, the level at which the solidified structure at the center of the thickness of the slab became equiaxed was set. In each group, various levels of tests were performed with varying reduction velocities applied to the slabs. At any level, the central solid phase ratio at the start of light reduction was 0.2, and light reduction was continued until the solidification completion position. In the first group, the dendrite primary arm spacing was 2.0 mm or less on average at any level.

FIG. 7 shows the relationship between the reduction rate and the Mn average segregation degree (the value obtained by dividing the average Mn concentration within the measurement range described above by the Mn concentration of the crude steel), and FIG. 8 shows the reduction rate and the segregation ratio (described above). The relationship with (the ratio of the area where the Mn segregation degree exceeds 2.1) in the measurement range of is shown. In FIGS. 7 and 8, the plot of ▲ shows the level at which the solidified structure at the center of the thickness of the slab becomes equiaxed, and the plot of ● shows the columnar crystal of the solidified structure at the center of the thickness of the slab. Indicates the level to be.

As is clear from FIG. 7, it can be seen that there is an appropriate range for the reduction speed. Further, at the same reduction rate, the central segregation can be reduced when the solidified structure is columnar crystals. Further, as is clear from FIG. 8, the area ratio in which the Mn segregation degree exceeds 2.1 is reduced in the range of the appropriate reduction speed in the case of columnar crystals, whereas it is almost in the case of equiaxed crystals. Not reduced. Therefore, it was found that no improvement effect was observed for segregation in the high concentration range even if light reduction was applied with equiaxed crystals. Even in the case of equiaxed crystals, the average segregation degree is reduced because the area with a relatively low segregation degree is reduced.

From the above results, it is necessary to control the solidified structure at the center of the thickness into columnar crystals and to apply a light reduction in an appropriate range in order to suppress gas cutting cracks.

According to the continuous steel casting method of the present invention, it is possible to further sufficiently reduce central segregation, particularly prevent the occurrence of coarse segregation spots, and prevent delayed fracture of thick steel sheets after gas cutting. .. Therefore, the thick steel plate produced from the steel pieces produced according to the present invention is suitably used for applications such as wear-resistant steel plates.

100 continuous casting machine (vertical bending type)
10 Ladle 11 Tandish 12 Mold 13 Spray nozzle 14 Roll 15 Cutting device 16 Electromagnetic stirrer 20A Vertical band 20B Upper straightening band 20C Curved band 20D Lower straightening band 20E Horizontal band 21 Curved mold M Molten steel S slab

Claims (1)

A method for continuous casting of steel using a vertical bending type or curved type continuous casting machine.
C: A primary cooling step of primary cooling a molten steel containing 0.1% by mass or more and 0.3% by mass or less with a mold to obtain a slab in which unsolidified molten steel remains inside.
A secondary cooling step of pulling out the slab from the mold and secondary cooling the slab while supporting it with a plurality of pairs of rolls arranged in the casting direction to obtain a steel slab whose inside is completely solidified.
Have,
(A) The solidified structure in the final solidified portion in the thickness direction of the slab covers the entire width.
On the upper surface side in the thickness direction, columnar crystals with an average dendrite primary arm spacing of 2.0 mm or less are formed.
On the lower surface side in the thickness direction, while controlling the casting conditions so that the dendrite primary arm spacing becomes a columnar crystal or a branched columnar crystal of 2.0 mm or less on average.
(B) In the secondary cooling step, in the casting direction of the slab, at least in the range from the position where the solid phase ratio at the center of the thickness of the slab is 0.2 to the solidification completion position, the slab is formed. A method for continuously casting steel, characterized in that light reduction is applied at a reduction speed of 0.5 mm / min or more and 2.0 mm / min or less.

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