JP6349832B2 - Continuous cast slab for thick steel plate - Google Patents

Description

  The present invention relates to a continuous cast slab for a thick steel plate cast using a curved or vertical bending type continuous casting machine.

  In recent years, low-alloy steels containing alloy elements such as Nb, V, Ni, and Cu are often used in steel plate products and the like due to demands on mechanical properties. When this low alloy steel is cast using a curved or vertical bending type continuous casting machine, surface cracks called so-called transverse cracks or lateral cracks may occur in the slab. The surface cracks of these slabs are generated when the stress generated on the surface of the slab exceeds the intrinsic limit stress of the low alloy steel when the slab is straightened by a continuous casting machine.

Generation | occurrence | production of the surface crack of a slab is further demonstrated. Hot ductility of cast piece of low-alloy steel is significantly reduced in α from solidification structure of the slab is gamma (austenite) phase (a temperature range of 600 to 850 ° C.) near the A ₃ transformation temperature for transformation to (ferrite) phase To do. That is, in the low alloy steel, the temperature range of 600 to 850 ° C. is the embrittlement temperature range.

  Further, in the low alloy steel, nitrides such as AlN and carbides such as NbC are likely to precipitate in the γ grain boundary of the slab in the secondary cooling process after being drawn from the mold. The γ grain boundaries where these nitrides and carbides are deposited tend to be the starting point of cracking when stress is applied to the slab.

  Therefore, when a curved or vertical bending type continuous casting machine is used to correct a slab of low alloy steel in a temperature range of 600 to 850 ° C., the temperature range is an embrittlement temperature range and γ grains. Due to the formation of precipitates at the boundaries, lateral cracks and lateral cracks are likely to occur in the slab.

  As a general method of suppressing the occurrence of such surface cracks, correction of the bending of the slab in a curved type or vertical bending type continuous casting machine is carried out at a temperature where the surface temperature of the slab is higher than the embrittlement temperature range or There is a method to perform at a low temperature. This method has been put into practical use and is described in, for example, Patent Documents 1 and 2.

  Patent Document 1 is directed to continuous casting of steel having a brittle temperature range of 820 to 950 ° C. In the continuous casting of steel, the method described in Patent Document 1 strongly cools the upper part of the secondary cooling zone to set the surface temperature of the slab to 650 to 700 ° C., which is lower than the embrittlement temperature range, and then gradually recovers the heat. The surface temperature of the slab at the correction point is set in the range of 700 to 800 ° C. Thereby, generation | occurrence | production of the horizontal crack of the slab surface can be prevented.

Moreover, the method described in Patent Document 2 is such that, after the slab is drawn out of the mold, the slab surface temperature is set to the A ₃ transformation temperature, which is the embrittlement temperature range, within 1 minute, and then the correction point is recovered by reheating. The slab surface temperature is set to 850 ° C. or higher.

  Among these methods, when the method described in Patent Document 2 is applied to low alloy steel, there is a problem that a thick oxide film is easily formed on the surface of the slab when strongly cooling at the upper part of the secondary cooling zone. is there. The thickness of the oxide film is likely to be non-uniform depending on the width direction of the slab and the position in the casting direction, and the slab is difficult to cool even if the secondary cooling is continued at a portion where the oxide film is particularly thick. For this reason, the surface temperature of the slab is likely to be non-uniform, and during the correction, the surface temperature is partially within the embrittlement temperature range and surface cracks may occur.

  Further, as a general method for suppressing the occurrence of surface cracks, there is a method of containing Ti in steel. It is well known that this method is effective, and is described in, for example, Patent Documents 3 and 4.

  Patent Document 3 describes a method for producing a thin steel plate having excellent press formability and surface properties. In the manufacturing method of the thin steel plate, slab is continuously cast from molten steel having a composition containing B and N in a predetermined amount without causing BN to precipitate, and then slab is hot-rolled at a predetermined temperature. Precipitate. This document describes that by further containing Ti, TiN can be precipitated and deterioration of product performance due to N can be suppressed. Further, it is described that a slab having a good surface property can be obtained by controlling the casting speed in accordance with the B, N and Ti contents.

  In the method described in Patent Document 4, the slab is cooled with cooling water having a predetermined water density immediately after being drawn out of the mold, reheated, and then cooled at a predetermined cooling rate. As a result, the microstructure of the slab has a ferrite-pearlite structure in which the γ grain boundary is unclear, thereby preventing the occurrence of lateral cracks and the like.

Japanese Patent Publication No.58-3790 Japanese Patent Laid-Open No. 9-47854 JP 2002-20836 A JP-A-9-253814

  By the way, in recent years, various high-performance steel materials such as high strength and excellent weldability have been developed in response to demands for higher strength, higher performance, and higher quality of steel. In addition, there are stricter requirements for improving productivity and reducing production costs. Along with this, slabs that had been allowed to have a certain amount of surface care and yield loss in the past are now required to be free of maintenance (to eliminate the need for maintenance) by suppressing the occurrence of surface cracks. It has become.

  Here, when correcting the bending of the slab during continuous casting, the long side surface (the surface in contact with the roll) of the slab is subepidermally (specifically, within a range of about 10 to 30 mm from the long side surface). Intergranular cracking may occur. Like the methods described in Patent Documents 1 to 4 above, various methods for suppressing the occurrence of surface cracks in the slab have been proposed and have some effects, but the subepidermal grain boundary cracks have not been studied. .

  This invention is made | formed in view of such a problem, and it aims at providing the continuous cast slab for thick steel plates by which the grain boundary crack is suppressed in the epidermis of a long side surface.

  The gist of the present invention is as follows:

(1) A continuous cast slab for a thick steel plate cast using a curved type or vertical bend type continuous caster, wherein the γ grain size of the slab is 20 to 30 mm from the long side surface. Continuous cast slabs for thick steel plates that are less than 2 mm in range.

(2) The continuous cast slab for a thick steel plate according to (1), wherein the continuous cast slab for the thick steel plate contains Ni: 0.3 to 2.5% by mass%.

(3) A continuous cast slab for a thick steel plate as described in (2) above, in mass%, further Al: 0.001% or more and less than 0.01%, and Ti: 0.005 to 0.030. A continuous cast slab for a thick steel plate, which is a Ti deoxidized steel containing 2%.

  In the continuous cast slab for thick steel plates of the present invention, γ grains are refined within a predetermined range. Thereby, it will be in the state where grain boundary cracking was controlled in the epidermis of the slab. For this reason, when hot rolling is performed on a slab, it can suppress that a flaw appears on the surface of a thick steel plate.

FIG. 1 is a structural photograph of the cross section of a slab, wherein FIG. 1A shows the case of conventional continuous casting conditions, and FIG. 1B shows the case where the cooling conditions are strengthened in the high-temperature γ-phase region. FIG. 2 is an Fe—C pseudo-binary phase diagram. FIG. 3 is a diagram showing the results of a tensile test. FIG. 4 is a diagram showing the rate of wrinkling during thick plate rolling of Al deoxidized steel and Ti deoxidized steel.

  Below, the knowledge for completing this invention and the form for implementing this invention are demonstrated. In the following description, “mass%” for the component composition of steel is also simply expressed as “%”.

1. Summary of Knowledge for Completing the Present Invention First, the present inventors have intensively studied the relationship between the state of subepidermal cracking of a slab during continuous casting and the microstructure of the slab. In addition, intensive investigations were made on the relationship between the microstructure and the high temperature ductility when a high temperature tensile test was performed using a test piece taken from a slab. Some of the details will be discussed later.

As a result of the above examination, the following findings (1) to (5) were obtained.
(1) In continuous cast slabs for thick steel plates, cracks may occur in the epidermis, and in continuous cast slabs for thick steel plates having high strength (for example, tensile strength of 580 MPa or more), subepidermal cracks may occur. Become prominent.

(2) Subepidermal cracking is a grain boundary crack that occurs along the γ grain boundary, and may occur even if there is no crack on the surface of the slab. Subepidermal cracks often occur within a range of about 10 to 30 mm from the long side surface.

(3) The probability of occurrence of subepidermal cracking increases with steel types containing 0.3% or more of Ni, and further increases with Ti deoxidized steel. When a slab having a subepidermal crack is rolled into a thick steel plate, the subepidermal crack expands due to the tensile strain accompanying the rolling, and may become wrinkles on the product surface.

(4) In the secondary cooling of the continuous casting machine, immediately after the slab exits the mold (near the mold outlet), the cooling rate near the γ single phase is increased, and the time for the γ single phase and the high temperature state is increased. Shorten. Thereby, a fine structure | tissue can be produced | generated under the epidermis of a slab.

(5) By utilizing this structure generation mechanism and making the γ particle size less than 2 mm within a range of about 20 to 30 mm from the long side surface of the slab, subepidermal cracking can be prevented.

  From these findings, it has been found that if the structure of the slab is appropriately controlled within a predetermined range, the occurrence of subepidermal cracks and flaws in the slab can be suppressed during continuous casting. It has also been found that a steel sheet in which the occurrence of surface flaws and the like is suppressed can be obtained by hot rolling the slab in which the occurrence of subepidermal cracks and the like is suppressed.

2. Outline of subepidermal crack As described above, the present inventors generated intergranular cracks in the epidermis of the long side surface of the slab obtained by continuous casting (in the range of about 10 to 30 mm from the long side surface). I found out that there is a case. This subepidermal crack occurs along the crystal grain boundary, and is caused by the distortion accompanying the bending of the slab or the correction of the bending.

  In continuous casting of thick plate slabs, if there is a concern that surface cracks may occur in the slab, the slab is once cooled to room temperature, the oxide scale is removed, and then the slab surface is inspected. The removal of the oxide scale is performed by cutting the slab surface with a scarf or grinding the slab surface with a grinder. When surface cracks are detected in the inspection of the slab surface, in order to remove the surface cracks, care is taken by means of welding or grinding.

  When subepidermal cracks occur, subepidermal cracks appear on the surface when the slab is maintained. If an attempt is made to remove the subepidermal crack that has appeared on the surface, the amount of care will be greatly increased. For this reason, the maintenance cost (production cost) and the lead time increase, and the yield decreases. Even if the subepidermal crack does not appear on the surface at the time of maintenance, the subepidermal crack expands when the slab is rolled to form a thick steel plate, and appears as nitric acid on the surface of the product, causing deterioration in quality.

  Therefore, if cracks occur even in the epidermis, the production cost, lead time, thermal efficiency, etc. of the slab are adversely affected in many ways, so it is necessary to establish a technique that can suppress the occurrence of epidermal cracks. .

3. Fig. 1 is a structural photograph of the cross section of the slab. Fig. 1 (a) shows the conventional continuous casting conditions, and Fig. 1 (b) shows the cooling in the high temperature γ phase region. The case where conditions are strengthened is shown respectively. In this figure, the lower end (position indicated by an arrow) is the long side surface of the slab, and the structure is photographed with respect to the cross section of the slab etched with a 10% by mass nitric acid aqueous solution. However, slab used for taking tissue photograph of FIG (a), in order to facilitate the γ grains observed, compared with the conventional continuous casting conditions, and accelerating the cooling rate in the vicinity of A ₃ transformation temperature. From FIG. 5A, it is confirmed that the γ grains are fine around the long side surface but become coarser as the distance from the long side surface increases.

  On the other hand, subepidermal cracking occurs within a range of about 10 to 30 mm from the long side surface. In general, when comparing the periphery of the long side of the slab and the internal range of about 10 to 30 mm from the long side, stress and strain applied to the slab are larger in the vicinity of the long side. However, cracks are more likely to occur. However, around the long side surface, the γ grains become fine and there are a large number of grain boundaries, so that the distortion associated with the bending and correction of the slab is dispersed in the large number of grain boundaries. On the other hand, the inner area of about 10 to 30 mm from the long side surface has few grain boundaries and concentrates on the grain boundary with little distortion. Depending on the steel type and conditions, even if there is no crack on the long side surface, there is no crack in the epidermis. Occur.

  In general, the γ grains of a steel material are refined by undergoing a transformation of γ → α → γ, or by imparting strain. In the steel manufacturing process, the above-mentioned phenomenon of γ grain refinement is utilized, and the necessary material properties are secured by controlling the machining (rolling) conditions and cooling conditions to refine the structure. . However, in a continuous casting machine, it is difficult to impart a large strain to the slab and it is difficult to cause phase transformation to the inside of the thick slab. For these reasons, it is particularly difficult to control the internal γ grain size (refinement of γ grains) with a slab produced by a continuous casting machine.

  FIG. 2 is an Fe—C pseudo-binary phase diagram. The reason why the γ grain size of steel becomes coarse in continuous casting will be described with reference to FIG.

  The γ grain size of steel rapidly becomes coarse when it becomes a γ single phase in the cooling process accompanying solidification, that is, in the high temperature γ phase region. Here, the high temperature γ-phase region means a temperature region immediately below the γ-single phase temperature and includes the γ-single-phase temperature. The high temperature γ phase region is generally a temperature region from the γ single phase formation temperature (° C.) to (γ single phase formation temperature (° C.) − 50 ° C.).

  In the high-temperature γ-phase region, the γ grain size rapidly increases because the second phase inhibits the movement of the γ grain boundary in the two-phase region before becoming the γ-single phase, whereas it becomes the γ-single phase. This is a phenomenon that occurs because the factor that hinders the movement of the γ grain boundary is eliminated. Furthermore, the movement of the γ grain boundary is governed by the diffusion rate of the solute element, and the diffusion rate increases remarkably as the temperature increases. For this reason, the γ grain size in the solidified structure of steel is almost determined while in the high temperature γ phase region. Therefore, if the residence time in the high temperature γ phase region is shortened, the γ grains can be made finer.

  Therefore, the present inventors conducted a test simulating continuous casting in order to refine γ grains inside the slab. In the casting test, the slab was cast from 200 kg of molten steel in a stationary state, and then the slab with unsolidified molten steel remaining inside was pulled out of the mold and then spray-cooled. As a result, if the secondary cooling conditions are appropriately controlled, specifically, if the cooling conditions are strengthened in the high-temperature γ phase region, the γ grains are dispersed within a predetermined range of the slab as shown in FIG. It was found that it can be miniaturized.

  In general, as shown in FIG. 1A, the structure of a slab obtained by continuous casting becomes coarse as the distance from the long side surface increases. On the other hand, the structure of the slab according to the present invention is refined in a predetermined range in the thickness direction from the long side surface (specifically, a range within 20 to 30 mm from the long side surface). For example, as shown in FIG. 1B, a region in which γ grains are miniaturized is observed, and the miniaturized region extends in a band shape along the long side surface in the middle of the thickness direction from the long side surface. On the other hand, on the long side surface side of the miniaturized region and the inside side of the miniaturized region, the γ grains become coarse as the distance from the long side surface increases. Thus, the γ grains are discontinuously refined within a predetermined range of the slab.

4). Confirmation of cracking prevention effect by refinement of γ grain size A high temperature tensile test was performed using a slab having such a structure, and the effect of improving high temperature ductility by refinement of γ grain size was evaluated. The tensile test apparatus used for the test was a high-frequency induction heating type and had a heating mechanism that was a cold crucible type.

  In order to produce a test piece, the slab is cast from a 200 kg molten steel in a laboratory in a stationary state, the slab with the unsolidified molten steel remaining inside is drawn out of the mold, and then spray-cooled (secondary cooling) did. Two slabs with different secondary cooling conditions were obtained, and test pieces (Sample 1 and Specimen 2) were produced from these slabs. The slab dimensions were 400 mm wide, 180 mm thick, and 400 mm long. Table 1 shows the component composition of the slab of this test. The γ single phase temperature of the steel shown in Table 1 was 1478 ° C.

  In casting the slab for the specimen 1, the secondary cooling conditions were set following conventional continuous casting, and both the first long side surface and the second long side surface were gradually cooled. As a result, the slab was formed into a normal coarse structure under the epidermis of both long sides.

  In the casting of the slab for the specimen 2, the secondary cooling condition is strengthened. Specifically, when the position 20 mm from the first long side surface becomes the γ single phase temperature, the cooling rate at the position is high. The cooling conditions were adjusted so that On the other hand, the second long side surface was gradually cooled. As a result, a fine γ grain structure was generated from the first long side surface at a predetermined position in the second slab, and a normal coarse structure was formed under the surface of the second long side surface.

  Since it is difficult to directly measure the cooling rate inside the slab, the cooling rate when the γ single phase temperature was reached at a position 20 mm from the first long side surface was estimated by heat transfer solidification analysis.

  From each of the slabs thus obtained, a cylindrical test piece having a diameter of 10 mm was produced by cutting using a portion inside 20 mm from both long side surfaces. At that time, the axis of the cylindrical test piece was set to be parallel to the annealed surface of the slab.

  Moreover, about the cross section of the slab etched with 10 mass% nitric acid aqueous solution, it image | photographed by 10 time magnification using the optical microscope, and measured the (gamma) particle size using the image | photographed photograph. In the measurement of the γ particle diameter, first, a line parallel to the first long side surface and having a length of 50 mm was drawn at a position of 20 mm from the reference surface. Subsequently, the number of γ grain boundaries crossing the line was measured, and the γ grain size was determined by dividing the length of the line by the number of γ grain boundaries.

  Table 2 shows the test material classification, the secondary cooling condition of the slab, the cooling rate when the position 20 mm from the first long side surface becomes the γ single phase temperature, and the 20 mm position from the first long side surface. The γ particle size is shown.

  In the tensile test, the test piece (Sample 1 or Sample 2) was heated from room temperature to the test temperature, held in that state for 60 seconds, and then subjected to a tensile test. In the heating of the test piece, in order to reduce the change in γ particle size (coarsening) accompanying the temperature increase as much as possible, the test piece was directly heated to the test temperature without temporarily holding the temperature in the middle. Both the test material 1 and the test material 2 were subjected to a tensile test a plurality of times while changing the test temperature at 600 to 1000 ° C.

  FIG. 3 is a diagram showing the results of a tensile test. This figure shows the relationship between the test temperature and the aperture (RA, unit%). From the figure, it can be confirmed that the specimen 2 with fine γ grains has a better drawing than the specimen 1 with coarse γ grains. That is, the high-temperature ductility of the specimen 2 having a fine γ grain size is clearly improved. This is because stress is dispersed in a specimen having a fine γ grain size when strain is applied in a tensile test.

6). Continuous cast slab for thick steel plate of the present invention The continuous cast slab for thick steel plate of the present invention is cast using a curved or vertical bending continuous caster. Moreover, the γ particle size of the slab is less than 2 mm in the range of 20 to 30 mm from the long side surface. That is, it means that the γ grains are in a refined state within a range of 20 to 30 mm from the long side surface. If γ grains are made finer in this way, in a continuous casting machine, even if distortion occurs due to bending of the slab or correction of bending, there are many grain boundaries. The distortion caused by the is dispersed.

  Therefore, the continuous cast slab for thick steel plates according to the present invention is in a state where grain boundary cracking is suppressed under the surface of the slab (in the range of about 10 to 30 mm from the long side surface). Moreover, since generation | occurrence | production of a subepidermal crack is suppressed, when performing hot rolling to a slab, it can suppress that a flaw appears on the surface.

  In addition, what is necessary is just to suppress a surface crack by the conventional method. Here, the surface crack is mainly caused by distortion applied to the surface of the slab. Further, as described above, the conventional slab is fine as the γ grains are fine around the long side surface, but becomes coarse as the distance from the long side surface increases. Therefore, in the conventional slab, even if the periphery of the long side surface is not cracked due to the refinement of the grain size, the grain size of the subepidermal portion becomes the starting point of cracking, and the surface crack is propagated to the long side surface May be formed. With respect to the surface cracks formed in this way, the continuous cast slab for thick steel plates of the present invention can be expected to have a suppression effect because there is a region in which γ grains are refined within a predetermined range of the slab.

  Moreover, the continuous cast slab for thick steel plates of the present invention can be obtained by setting the cooling rate of the high-temperature γ-phase region to about 3 ° C./s or more at a position 20 mm from the long side surface, as will be described later. Is possible. This slab can reduce the sensitivity of subepidermal cracking. Therefore, even when an oxide film having a non-uniform thickness is formed on the surface of the slab as in the case of low alloy steel, the occurrence of cracks can be suppressed.

  The reason why the fine range of γ grains is defined as being 30 mm from the long side surface is that the range in which grain boundary cracking is likely to occur is from the long side surface to 30 mm. On the other hand, the fine range of γ grains is defined from the position 20 mm from the long side surface, as will be described later, in continuous casting, the γ grains are refined on the long side surface side (around the long side surface) from the 20 mm position. It is difficult to do.

  Further, as will be described later, if the γ grains are refined in the range of 20 to 30 mm from the long side surface, the sensitivity of subepidermal cracking can be significantly reduced. Thereby, a grain boundary crack can be suppressed in the whole range of about 10-30 mm from a long side surface.

  The γ particle size in the range of 20 to 30 mm from the long side surface is preferably less than 2 mm, more preferably less than 1.5 mm, from the viewpoint of further suppressing subepidermal cracking. On the other hand, since the secondary cooling is limited, the γ particle size is preferably 0.3 mm or more.

In the continuous cast slab for a thick steel plate of the present invention, the γ particle size in the range of 20 to 30 mm from the long side surface is determined by the following procedure.
(1) A cross section of a slab etched with a 10% by mass nitric acid aqueous solution is photographed at a magnification of 10 using an optical microscope.
(2) In the photograph taken, a line parallel to the long side surface and having a length of 50 mm is drawn from the long side surface to the 20 mm position and the 30 mm position, respectively.
(3) The number of γ grain boundaries across each line is measured, and the γ grain size is determined by dividing the total length of the lines by the total number of γ grain boundaries.

7). 2. Method for Refinement of Gamma Particle Size in the Subcutaneous In general, in continuous casting of steel, molten steel is cast into a water-cooled copper mold, and a cast piece in which only the skin portion is solidified is pulled out from the mold. The drawn slab is gradually solidified by being cooled (secondary cooling) while being supported by a roll. The thickness of the solidified skin portion at the outlet of the mold is approximately 15 to 20 mm, although it varies depending on the casting speed and the cooling conditions in the mold. In such a solidification process, the position where the γ single phase temperature is reached becomes a solid-liquid interface portion in steels having a C content higher than the peritectic point. Moreover, in steel with C content lower than a peritectic point, it becomes several mm inside from a solid-liquid interface.

  From the results of the casting test described above, it became clear that a fine γ grain structure can be generated in a predetermined range of the slab by enhancing the secondary cooling immediately after leaving the mold. When this result was examined in detail, at a position 20 mm from the long side surface, the cooling rate in the high-temperature γ phase region was set to about 3 ° C./s or more, so that the γ grains were fined at a position 20 mm from the long side surface. It became clear that we could do it.

  If the cooling rate in the high-temperature γ phase region is about 3 ° C./s or more at a position 20 mm from the long side surface, even if the cooling rate falls inside the 20 mm position, the γ particle size suddenly reaches the original size. Never come back. For this reason, the state in which the γ particle size is fine is maintained while gradually coarsening, even inside the 20 mm position. Therefore, if the cooling rate at a position 20 mm from the long side surface is set to about 3 ° C./s or more, the γ particle size can be refined within the range of 20 to 30 mm from the long side surface.

  As described above, the subepidermal grain boundary cracking occurs within a range of 10 to 30 mm from the long side surface. The present inventors have clarified that it is possible to refine the γ particle size within the range of 20 to 30 mm from the long side surface, and accordingly, the sensitivity of intergranular cracking in the epidermis can be significantly reduced. .

  It is difficult to make the γ grain size finer for a portion on the longer side surface side (specifically, a range within 10 to 20 mm from the long side surface). This is because the longer side surface portion has entered the high temperature γ phase region in the mold and it is difficult to increase the cooling rate in the mold so as to affect the particle size.

  From these facts, the continuous cast slab for thick steel plate of the present invention can be obtained by setting the cooling rate in the high-temperature γ phase region to about 3 ° C./s or more at a position 20 mm from the long side surface. It is.

  As is clear from FIG. 2, the γ single phase temperature varies greatly depending on the steel type, particularly the C content. Therefore, in order to produce fine γ grains depending on the steel type, the temperature range (high temperature γ) should be controlled. Needless to say, the phase is different. On the other hand, the lower limit of the temperature range (high temperature γ phase range) where the cooling rate should be controlled can be appropriately set according to the γ single phase temperature and the required γ particle size, for example, (γ single phase temperature) (° C.)-50 ° C.).

8). Component Composition The continuous cast slab for thick steel plates of the present invention is not particularly limited in the component composition, and can be, for example, low alloy steel. As the low alloy steel, for example, Al deoxidized steel, Ti deoxidized steel or Si deoxidized steel can be adopted.

  The continuous cast slab for a thick steel plate of the present invention preferably contains Ni: 0.3 to 2.5%. Further, the slab is preferably Ti deoxidized steel containing Al: less than 0.01% and Ti: 0.005-0.030%. The reasons for the provision are described below.

  FIG. 4 is a diagram showing the rate of wrinkling during thick plate rolling of Al deoxidized steel and Ti deoxidized steel. The figure shows the result of investigation of the generation rate of flaws, which is considered to be caused by subepidermal cracking on the steel plate surface during thick plate rolling, for each steel type against representative steel types of Al deoxidized steel and Ti deoxidized steel. is there. In the same figure, data is arranged and shown by the Ni content of each representative steel type.

  From the figure, it can be confirmed that the soot generation rate is significantly increased in Ti deoxidized steel compared to Al deoxidized steel. Moreover, in any of the Al deoxidized steel and the Ti deoxidized steel, the rate of soot generation increases as the Ni content increases. More specifically, in both the Al deoxidized steel and the Ti deoxidized steel, when the Ni content is 0.3% or more, the soot generation rate becomes significant. For this reason, when the Ni content is 0.3% or more, the frequency of occurrence of subepidermal cracks in the slab also significantly increases. Therefore, if the γ grain size of the slab is refined within a predetermined range when the Ni content is 0.3% or more, the effect of suppressing the subsurface cracking of the slab and the appearance of defects during rolling The effect of suppressing is also remarkable.

  In particular, when Ti is deoxidized steel and Ni is contained in an amount of 0.5% or more, not only the soot generation rate is remarkably increased, but also serious cracks are increased, resulting in scrap.

  Below, the reason for limitation of content of each element is demonstrated.

Ni: 0.3-2.5%
Ni has the effect of improving the strength of steel by solid solution strengthening, and also has the effect of improving toughness. If the Ni content is less than 0.2%, these effects cannot be obtained. On the other hand, when the Ni content exceeds 2.5%, not only the effect of improving the strength and toughness is saturated, but also the adverse effect of deteriorating the weldability occurs. Moreover, as shown in FIG. 4, the wrinkles considered to be caused by subepidermal cracking increase remarkably when Ni is contained in an amount of 0.3% or more. Furthermore, in the case of Ti deoxidized steel, when Ni is contained in an amount of 0.3% or more, the sensitivity of soot is significantly increased. Therefore, the Ni content is preferably 0.3 to 2.5%. Since the soot generation rate increases more remarkably when the Ni content is 0.5% or more, the present invention is particularly effective when the Ni content is 0.5% or more. For this reason, the Ni content is more preferably 0.5% or more. On the other hand, the upper limit with more preferable Ni content is 1.0%.

Al: 0.001% or more and less than 0.01% Al is one of the elements effective for reducing the O (oxygen) content in steel and is widely added to steel as a deoxidizing element. . However, in the case of Ti deoxidized steel, since Al has a stronger deoxidizing power than Ti, there is an upper limit for the Al content in Ti deoxidized steel. Therefore, in the case of Ti deoxidized steel, the Al content is preferably less than 0.01%, and more preferably less than 0.006%. Although depending on the properties required for steel, when the production of Al ₂ O ₃ adversely affects the properties of the product, the Al content is preferably less than 0.003%.

  On the other hand, Ti deoxidized steel also inevitably contains Al with the addition of other component elements. Moreover, the process which adds Al once may be performed from a viewpoint of utilizing the effect which reduces oxygen content, and a viewpoint which adjusts molten steel temperature. Therefore, the lower limit of the Al content is preferably 0.001% or more.

Ti: 0.005-0.030%
Ti is an element that generally improves the strength of steel. In addition, it is an element that fixes N in steel as TiN. By containing Ti in steel, it is possible to suppress surface cracks of the slab when bending or correcting the slab in a continuous casting machine. Effect is obtained. However, when casting Ti deoxidized steel, Ti ₂ O ₃ is produced in the molten steel prior to the production of TiN, and Ti in the steel is consumed by O. For this reason, the amount consumed for the production | generation of TiN reduces, and the effect which suppresses generation | occurrence | production of the surface crack of a slab may not be acquired.

  Nevertheless, Ti-deoxidized steel is used because, for example, if the oxide in the steel is Ti oxide, advantageous effects such as improved welding characteristics can be obtained. These effects cannot be obtained when the Ti content is less than 0.005%. On the other hand, when the Ti content exceeds 0.030%, a large number of Ti carbides are generated, and there is a problem that the HAZ toughness is lowered during welding. Therefore, in the case of Ti deoxidized steel, the Ti content is preferably 0.005 to 0.030%. A more preferable lower limit of the Ti content is 0.008%. A more preferable upper limit of the Ti content is 0.020%.

  Other component elements are optional and not particularly specified. However, when Nb is contained, when B is contained, the incidence of subepidermal cracking increases when the N content is high. Specifically, when one or more of Nb: 0.005 to 0.050%, B: 0.0005 to 0.0050%, and N: 0.0035 to 0.0070% are contained, the epidermis Since the occurrence rate of cracks increases, the effect of preventing epidermal cracks according to the present invention becomes significant.

  In order to confirm the effect of suppressing subepidermal cracking by the continuous cast slab for the thick steel plate of the present invention, a continuous casting test was conducted and the result was evaluated.

1. Test Method In this test, a slab having a thickness of 300 mm and a width of 2300 mm was cast from molten steel using a vertical bending type continuous casting machine having a vertical portion length of 2.5 m. The molten steel was blown in a converter with a capacity of 270 t, then subjected to ladle treatment and RH treatment as secondary refining, and the molten steel after these treatments was used for casting. The casting speed was 0.65 m / min or 0.75 m / min.

  In the continuous casting machine, the slab was secondary cooled by dividing the zone in the casting direction. In order to change the γ particle size in the range of 20 to 30 mm from the long side surface, by adjusting the cooling conditions in the zone adjacent to the mold and positioned in the vertical part, the γ at the position 20 mm from the long side surface The cooling rate when reaching the single phase temperature was controlled to 0.5 to 8.2 ° C./second. In order to maintain this cooling rate in the high-temperature γ-phase region, specifically, the cooling conditions are set so that the position 20 mm from the long side surface is maintained (γ single-phase temperature −50 ° C.). It was adjusted. At that time, since it is difficult to directly measure the cooling rate inside the slab, the cooling rate in the high-temperature γ-phase region at a position 20 mm from the long side surface was estimated by heat transfer solidification analysis.

  In this test, two types of steels, Steel A and Steel B, were used. Steel A was an Al deoxidized steel containing 0.6% Ni and had a γ single phase temperature of 1485 ° C. Steel B was a Ti deoxidized steel containing 0.7% of Ni, and the γ single phase temperature was 1484 ° C. Table 3 shows the component compositions of Steel A and Steel B.

  A sample with a observable cross-section was taken from the slab after casting, subjected to a dye penetrant inspection (die check), and subsurface subcutaneous cracks of the entire width of the slab were confirmed by visual observation. Thereafter, in order to observe the microstructure, a sample was taken from the slab and subjected to night etching. The sample was photographed at a magnification of 10 using an optical microscope, and the γ particle size was measured using the photographed photographs. The measurement of the γ particle size was carried out by the same procedure as the measurement of the γ particle size in “4. Confirmation of cracking prevention effect by refinement of γ particle size”.

  Moreover, the slab heated at 1200 degreeC was supplied and rolled to the rolling machine for thick plates, and the thick steel plate of thickness 30mm and width 2800mm was obtained. About the obtained thick steel plate, the wrinkles resulting from a subepidermal crack were confirmed visually.

The meanings of the symbols in the “Evaluation of subepidermal cracking” column in Table 4 are as follows:
◯: Indicates that the skin was excellent without causing subepidermal cracking.
X: Subcutaneous cracking occurred, indicating that it was not possible.

The meanings of the symbols in the column “Evaluation of steel plate defects” in Table 4 are as follows:
◯: It indicates that it was excellent without wrinkles due to subepidermal cracking.
(Triangle | delta): It shows that the wrinkles resulting from a subepidermal crack generate | occur | produced and were possible to the extent which can be repaired.
X: Wrinkles caused by subepidermal cracks occurred to the extent that they could not be repaired, indicating that they were not possible.

  Table 4 shows the classification of each test, the steel type, the casting speed, the cooling rate at the γ single phase temperature at the position 20 mm from the long side surface, the slab correction temperature, the γ particle size at the position 20 mm from the long side surface, the slab. The evaluation of subepidermal cracking and the evaluation of wrinkles of the steel sheet are shown.

  From Table 4, Example 1 of the present invention used Steel A, which is an Al deoxidized steel, and the casting speed was 0.75 m / min. Further, when the cooling rate at the γ single phase temperature at the position 20 mm from the long side surface was 7.5 ° C./second, the γ particle size at the 20 mm position from the long side surface was 0.83 mm, and the epidermis No cracking occurred. When this slab was rolled, no wrinkles were observed in the steel plate. Further, when the γ particle size at 30 mm position from the long side surface was confirmed by a photograph, it was almost the same as the γ particle size at 20 mm position.

  On the other hand, in Comparative Example 1, steel A of the same steel type was used and the casting speed was also 0.75 m / min, but the cooling rate when the γ single-phase temperature at the position 20 mm from the long side surface was reached. Was changed to 0.5 ° C./second. As a result, the γ particle size at a position 20 mm from the long side surface was 2.7 mm. Subepidermal cracks did not occur in the range from the center of the width of the slab to the position about 100 mm inside from the corner (end in the width direction) on both long sides, but on the long side on the inner R side Intergranular cracks with a length of about 5 to 10 mm were found at about 50 mm from the corner. On the other hand, no epidermal crack occurred on the long side surface on the outer R side even near the corner.

  Here, the “long side surface on the inner R side” means a long side surface located on the inner side when the slab is bent by a continuous casting machine. The “long side surface on the outer R side” means a long side surface located on the outside when the slab is bent by a continuous casting machine.

  Regarding the surface temperature at the time of correction that has a large effect on surface cracks and subepidermal cracks, in Comparative Example 1, it was about 860 ° C. at the center of the width of the slab, and was almost equivalent to Example 1 of the present invention. Compared with the central portion of the width, the temperature in the vicinity of the corner is about 110 ° C., but no significant difference was found between Comparative Example 1 and Invention Example 1. In Comparative Example 1, no long epidermal crack leading to the epidermis was observed. When this slab was rolled after confirming and removing the surface flaws, flaws with a bent shape were generated in a portion within 100 mm from the corner (end in the width direction).

  Furthermore, in Example 2 of the present invention, steel B, which is a Ti deoxidized steel, was used and cast at a casting speed of 0.65 m / min. When the cooling rate at the γ single phase temperature at the position 20 mm from the long side surface was 8.2 ° C./sec, the γ particle size at the 20 mm position from the long side surface was 1.05 mm, and the subepidermal crack was Did not occur. When this slab was rolled, no wrinkles were observed in the steel plate. Further, when the γ particle size at 30 mm position from the long side surface was confirmed by a photograph, it was almost the same as the γ particle size at 20 mm position.

  On the other hand, in Comparative Example 2, steel B of the same steel type was used and the casting speed was also 0.65 m / min, but the cooling rate when the γ single phase temperature at the position 20 mm from the long side surface was reached. Was changed to 0.5 ° C./second. As a result, the γ particle size at a position 20 mm from the long side surface was 3.4 mm. Subepidermal cracks were found to have grain boundary cracks with a length of about 5 to 10 mm regardless of the position of the long side surface of the slab. Subepidermal cracks occurred on the long side surface of the inner R side and the outer R side of the slab, but the outer R side was less severe. In addition, the cracks remained in the epidermis, and no long ones that connected to the long side were found. This slab was also rolled after confirming and removing surface flaws. As a result, wrinkles with a bent shape were scattered on the surface of the steel sheet.

  From these facts, it has been clarified that the occurrence of subepidermal cracking of the slab can be suppressed by refining the γ grains within the range of 20 to 30 mm from the long side surface of the slab. Moreover, it became clear by using the slab in which generation | occurrence | production of subepidermal crack was suppressed that it can suppress that a flaw appears on the surface when performing hot rolling to a slab.

  The continuous cast slab for a thick steel plate of the present invention has suppressed subepidermal grain boundary cracking, and can suppress the appearance of wrinkles on the surface when subjected to hot rolling. For this reason, it can utilize effectively in manufacture of a thick steel plate.

Claims (3)

A continuous cast slab for a thick steel plate cast using a curved or vertical bending type continuous casting machine,
The γ grain size of the slab is 0.83 mm or more and less than 2 mm in the range of 20 to 30 mm from the long side surface. Intergranular cracking is suppressed in the range of about 10 to 30 mm from the long side surface of the slab. It was, continuous casting slab of a steel plate.   It is a continuous cast slab for thick steel plates of Claim 1, Comprising: The continuous cast slab for thick steel plates which contains Ni: 0.3-2.5% by the mass%. It is a continuous cast slab for thick steel plates according to claim 2, and further contains, by mass%, Al: 0.001% or more and less than 0.01% and Ti: 0.005 to 0.030%. Continuous cast slab for thick steel plate, which is Ti deoxidized steel.