JP2018031055A - Cast slab and manufacturing method of cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cast slab capable of suppressing generation of surface crack during flexure or correction even in the case that a cast slab of steel containing B and Ni is manufactured by using a vertical flexure type or a curvature type continuous casting machine, and a manufacturing method of the cast slab.SOLUTION: A cast slab has a composition containing C, Si, Mn, P, S, Ni, N and B at a prescribed amount, and further one or more kind selected from REM:0.0015-0.02%, Ca:0.0015-0.0060% and Zr:0.0020-0.015% and the balance Fe and impurities, maximum concentration of S in a crystal particle diameter of 30 times or less as S concentration of whole cast slab, and the number of BN particles which can be observed at a crystal particle boundary and has particle diameter of 10-300 nm of 3000 or less per particle boundary of 1 mm when a broken surface of a test piece by conducting a tensile test at 800°C is observed.SELECTED DRAWING: None

Description

  The present invention relates to a slab made of steel containing B and Ni, which is manufactured using a vertical bending type or curved type continuous casting machine, and generation of surface cracks during bending and straightening is suppressed, and the slab. It relates to the manufacturing method.

  When Ni is added to steel, the low-temperature toughness of the steel is improved. Steel containing 0.1 to 2 mass% Ni is widely used for thick plate materials such as marine structures and liquefied natural gas tank materials. Yes. However, when Ni-containing steel is cast by a vertical bend type or curved type continuous casting machine, tensile strain acts on the slab as it is bent or straightened, so that the surface of the slab is aligned with the prior austenite grain boundaries. Cracks are likely to occur. In particular, since a crack is likely to occur when a tensile strain acts on the slab surface at 700 to 850 ° C., measures are taken to avoid the slab surface temperature from the embrittlement region by adjusting the amount of secondary cooling water. .

  On the other hand, B (boron) is only added to steel by about several tens of mass ppm, so that the transformation temperature decreases and the hardenability of the grain boundaries increases. Therefore, B is one of the important elements in the design of steel materials such as thick plates because the steel material structure can be controlled to increase the strength of the steel material. However, B, like Ni, is one of the elements that promotes surface cracking of the slab, and is particularly easily brittle at a high temperature range of about 1000 ° C.

Therefore, when steel containing both Ni and B is continuously cast, it is practically difficult to avoid the embrittlement temperature range, no matter how much the amount of secondary cooling water is adjusted, yield loss is large, and productivity is high. Leads to a decrease in production and an increase in production costs.
Therefore, there has been a demand for a method capable of preventing surface cracking of a continuous cast slab of steel containing Ni and B having excellent characteristics.

For example, Patent Document 1 discloses a technique for suppressing surface cracks in a continuous cast slab by controlling the B concentration and the N concentration within appropriate ranges.
Patent Document 2 discloses a technique for suppressing surface cracking of a continuously cast slab by optimizing cooling conditions during continuous casting of steel containing B and N.
Furthermore, Patent Document 3 discloses a technique for suppressing surface cracking of a continuously cast slab by defining the B concentration and the N concentration and defining the amount of BN equilibrium precipitation.

JP 56-080354 A Japanese Patent No. 4561755 JP 2010-189712 A

  By the way, when a continuous cast slab is manufactured using a vertical bending type or a curved type continuous casting machine, when the slab is bent or straightened, distortion is applied to the surface of the slab. In particular, when the surface temperature of the slab when tensile strain is applied matches the temperature range where the austenite transforms into ferrite, the so-called III region embrittlement temperature (700 to 850 ° C.), intergranular cracking occurs in the continuous cast slab. Surface flaws such as are easy to occur. This is due to the film-like ferrite generated along the austenite grain boundary.

  In addition, in steel containing Ni, light elements such as S are segregated at the austenite grain boundaries and the grain boundary strength is lowered, which is also a factor for promoting embrittlement. At the same time, when the slab surface is oxidized, preferential grain boundary oxidation occurs, which also promotes embrittlement. Therefore, not only the effect of solid phase transformation from austenite to ferrite but also the influence of grain boundary segregation and grain boundary oxidation of light elements, the steel containing Ni has a high susceptibility to cracking.

On the other hand, B in the steel is easily segregated at the grain boundaries, and BN precipitates at the grain boundaries from a high temperature range of about 1000 ° C., and grain boundary cracks occur from that. Therefore, since steel containing Ni and B has a wider embrittlement temperature range than general steel, it is extremely difficult to prevent surface cracking of the continuous cast slab.
From the above, in the techniques described in Patent Documents 1 to 3, the surface cracking of the continuous cast slab of steel containing Ni and B cannot be sufficiently suppressed.

  The present invention has been made in consideration of such points, and even when a steel slab containing B and Ni is manufactured using a vertical bending type or a curved type continuous casting machine. It aims at providing the slab which can suppress generation | occurrence | production of the surface crack at the time of a bending and correction, and the manufacturing method of this slab.

As a result of diligent studies to solve the above problems, the present inventors have led to a light element detoxification method and a form control method for precipitates that cause embrittlement, and surface defects generated in continuous cast slabs. A way that can be prevented has been led.
Specifically, in order to simulate the cracking susceptibility of a continuous cast slab, steels A to D having the compositions shown in Table 1 were once melted, and a high-temperature tensile test was performed on a steel having an as-cast structure. Found configuration requirements.

A steel rod with a round bar shape (φ10 × 190mm, made of forged material and not having an as-cast structure) was melted to a length of about 30 mm at the center in the longitudinal direction while maintaining the shape. The sample was cooled at a temperature of ° C / s, and the sample was continuously cooled at a temperature range of 0.4 ° C / s. In the cooling process of the sample, the sample temperature of the solidified part is in the range of 650 to 1000 ° C., and the sample is 3 × 10 ⁻⁴ s ⁻¹ , the order of which coincides with the strain rate at the time of straightening and bending that acts during continuous casting. Was pulled to break. The high temperature ductility of the steel was evaluated by the cross-sectional area reduction rate (RA) before and after the fracture defined by the formula (1).
RA = (A ₀ −A _f ) / A ₀ × 100 (%) (1)
Here, A ₀ represents the sample cross-sectional area (m ² ) before pulling, and A _f represents the sample cross-sectional area (m ² ) after fracture.

  Assuming the amount of tensile strain that the slab surface undergoes when using a vertical bending type or curved type continuous casting machine, if the RA value is 60% or more, preferably 63% or more, there is a concern about surface cracks in the slab. It matches that there is no. Further, when the RA value was less than 60%, it was a grain boundary brittle fracture surface, and when the RA value was 60% or more, it was an intragranular ductile fracture surface.

  In the case of Steel A, the reduction rate of the cross-sectional area at 650 to 950 ° C. was 20 to 55%, and a decrease in hot ductility of the steel was confirmed. As a result of observing the fracture surface of the sample after being pulled, all the fracture forms were typical austenite grain boundary cracks. Further, although the cross-sectional area reduction rate at 1000 ° C. was 63%, which was 60% or more of the allowable lower limit, it did not lead to clear ductility recovery. Therefore, it means that the cracking sensitivity of the continuous cast slab of steel containing Ni and B is large.

In the case of steel B to which La, which is REM (rare earth element), was added, the cross-sectional area reduction rate at 750 ° C. or higher exceeded 60%, and at 900 ° C. or higher, 80% or higher, indicating good ductility.
In the case of steel C to which Ca was added, the cross-sectional area reduction rate at 750 ° C. or higher was 60% or higher, and the ductile recovery lower limit temperature was lower than that of steel B, but better ductility than that of steel A was exhibited.
In the case of steel D to which Zr is added, the cross-sectional area reduction rate is 60% or higher from 750 ° C. or higher, particularly 90% or higher at 850 ° C. or higher, which is the best among the four types of steels A to D Good ductility was exhibited.

  And the test piece after the tension | pulling in 800 degreeC was investigated by two analysis methods. First, elemental mapping of the fracture surface was performed by Auger electron spectroscopy. Second, the fractured sample was cut in the longitudinal direction, the cut surface was observed with a scanning electron microscope, and the presence or absence of BN was observed.

  In Steel A (RA value = 33%), S segregation on the fracture surface and BN of several hundred nm size were easily observed. When S is segregated at grain boundaries, the S concentration on the grain boundaries obtained by Auger electron spectroscopy is about 2 to 3 orders of magnitude higher than the bulk composition of steel. Steel A at 800 ° C. (RA value = 33%) In this case, the S concentration on the grain boundary was 0.9%. Further, at least 30 inclusions on the grain boundary of the sample were observed and analyzed with a scanning electron microscope in a state where the prior austenite grain boundary was revealed by nital etching, and 60% or more of the inclusion was 300 nm. It confirmed that it was the following spherical BN. In particular, the observed BN was not integrated with inclusions having other compositions, but was BN itself.

  On the other hand, in steel B (RA value = 69%) and steel C (RA value = 65%), S segregation on the fracture surface was hardly observed, and the grain boundary S concentration was at most 30 times the bulk concentration. Steel B contains La, and steel C contains Ca. These elements had high affinity with S, formed sulfides and oxysulfides in steel, solid solution S was fixed, and S segregation on the fracture surface was hardly observed. Here, the element having high affinity with S is a lanthanoid called REM such as Ca and La, Ce, Nd, etc. belonging to the period IIA, and in particular, an interaction assistant coefficient (thermodynamic data) of these elements with respect to S in molten iron. Is 1 or less at 1600 ° C.

  Since most of these sulfides and oxysulfides are generated in the molten steel stage, the location exists regardless of the prior austenite grain boundaries. 30 or more random inclusions having a size of 5 to 10 μm were observed as subjects. When sulfides and oxysulfides are observed at a ratio of 50% or more, the solid solution S is fixed to a level (RA value 60% or more) at which the steel is not easily brittle at high temperature. In particular, in the case of Ca, there are few cases where sulfides and oxysulfides exist alone, and calcium aluminate-based oxides and CaS existed together. Whether it exists as a single sulfide, oxysulfide, or as a composite form integrated with other non-metals, for the solid solution S fixing effect and grain boundary crack reduction effect It does n’t matter.

Further, almost no single BN on the grain boundary with a size of several hundreds nm as observed in the steel A is observed, and BN with a size of several hundreds nm is precipitated on the sulfide or oxysulfide having a size of 5 to 10 μm. It was. Since these sulfides and oxysulfides are produced at the molten steel stage, as a result, BN precipitated on the surface of the sulfides and oxysulfides is present regardless of the former austenite grain boundaries.
Therefore, S grain boundary segregation and BN precipitation at the grain boundaries were suppressed, and a ductility recovery effect was obtained compared to Steel A. After various investigations, when the RA value is 60% or more, the proper sulfide or oxysulfide is present, the grain boundary S concentration is at most 30 times less than the bulk composition, and no BN exists at the grain boundary. There is a correlation.

  Further, in Steel D (RA value = 79%), no BN precipitation was observed, and ZrN having a size of several μm was mainly observed, which was 90% or more of 30 or more observed nitrides. Among them, in most cases, MnS having a size of several hundred nm was deposited on ZrN at a ratio of 50% or more. This means that there are conditions under which N and S can be fixed simultaneously by including Zr in the steel. In addition, ZrN may be present integrally with other non-metallic materials such as oxides, but there are no particular restrictions on the form of ZrN with respect to the ductility improving effect.

  Further, when RA is less than 60%, the grain boundary S concentration is about two to three orders of magnitude higher than the bulk composition of steel, and when RA is 60% or more, it is at most 30 times the bulk concentration. Therefore, N and S were immobilized at the same time, and a ductile recovery effect appeared compared to steel A. Also in this case, when the RA value is 60% or more, there are inclusions in which ZrN and MnS are combined and precipitated, at most 30 times the bulk composition at the maximum grain boundary S concentration, and BN exists at the grain boundaries. There is a correlation to not doing.

Therefore, it has been found that the addition of REM (rare earth element), Ca or Zr to steel containing Ni and B is effective in suppressing surface cracking of a continuous cast slab.
In order to prevent surface cracks that occur in continuous cast slabs of steel containing B and Ni, it is necessary to detoxify grain boundary segregation S, which causes embrittlement, and to suppress BN precipitation on the grain boundaries. It is important to control the surface temperature at the position corresponding to the slab thickness from the corner of the slab at the time to 750 ° C. or higher, and to control the solid solution S and the control of the precipitation form of BN.

  This invention is made | formed based on the above knowledge, and the slab which concerns on this invention is C: 0.05% or more and 0.18% or less, Si: 0.10% or more and 0.00. 4% or less, Mn: 0.5% or more and 2.0% or less, P: 0.020% or less, S: 0.0035% or less, Ni: 0.1% or more and 2.0% or less, Ti: 0.0. 005% to 0.030%, Al: 0.005% to 0.06%, N: 0.0015% to 0.007%, and B: 0.0005% to 0.0050%. 1 type selected from REM: 0.0015% to 0.02%, Ca: 0.0015% to 0.0060%, Zr: 0.0020% to 0.015% Alternatively, it contains two or more types, with the balance being composed of Fe and impurities at 800 ° C. As a result of observing the fracture surface of the test piece subjected to the tension test, the maximum concentration of S at the crystal grain boundary is 30 times or less of the S concentration of the entire slab, and the grain size observed at the crystal grain boundary is 10 nm or more. It is characterized in that the number of BN particles of 300 nm or less is 3000 or less per 1 mm length of the grain boundary.

  According to the slab of this configuration, as a result of observing the fracture surface of the test piece that was subjected to the tensile test at 800 ° C., the maximum concentration of S at the grain boundary is 30 times or less of the S concentration of the entire slab. , S grain boundary segregation is sufficiently suppressed. Further, since the number of BN particles having a particle size of 10 nm or more and 300 nm or less observed at the crystal grain boundary is 3000 or less per 1 mm length of the grain boundary, excessive coarse BN particles are present at the crystal grain boundary. Absent. For this reason, the grain boundary strength is ensured, and the occurrence of surface cracks can be suppressed.

In the slab of the present invention, Cu: 0.1% to 0.5%, Cr: 0.2% to 2.0%, Mo: 0.1% to 0.8%, One or two or more selected from V: 0.01% to 0.1% and Nb: 0.005% to 0.05% may be contained.
By adding the above-mentioned elements, it is possible to further improve the properties of the slab such as strength, weldability, weather resistance, and the like. Therefore, it is preferable to add them as necessary.

The slab manufacturing method according to the present invention is a slab manufacturing method for manufacturing the above-described slab using a vertical bending type or a curved type continuous casting machine, and is in mass%, and C: 0.05%. 0.18% or less, Si: 0.10% or more and 0.4% or less, Mn: 0.5% or more and 2.0% or less, P: 0.020% or less, S: 0.0035% or less, Ni : 0.1% to 2.0%, Ti: 0.005% to 0.030%, Al: 0.005% to 0.06%, N: 0.0015% to 0.007% And B: 0.0005% to 0.0050%, Cu: 0.1% to 0.5%, Cr: 0.2% to 2.0%, Mo if necessary : Selected from 0.1% to 0.8%, V: 0.01% to 0.1%, Nb: 0.005% to 0.05% Contain one or two or more that, the molten steel the balance being Fe and impurities, the sulfur concentration in the molten steel [% S], the total oxygen concentration in the molten steel T. [% O], when the REM concentration in molten steel is [% REM] and the atomic weight of _REM is M _REM , [% REM] / M _REM ≧ 0.3 × ([% S] /32.06+T. REM is added so as to satisfy [% O] /16.1), and the oxide and oxysulfide having a particle size of 1 μm or more whose REM content is controlled to 30 mol% or more are dispersed, In the region where bending or straightening strain is applied, the slab surface temperature at a position corresponding to the slab thickness distance from the corner of the long side surface of the slab is 750 ° C. or more.

According to the method for producing a slab having this configuration, the sulfur concentration in the molten steel is [% S], and the total oxygen concentration in the molten steel is T.P. [% O], when the REM concentration in molten steel is [% REM] and the atomic weight of _REM is M _REM , [% REM] / M _REM ≧ 0.3 × ([% S] /32.06+T. Since REM is added so as to satisfy [% O] /16.1), S in the molten steel reacts with REM to form a compound, thereby suppressing the segregation of S to the grain boundaries. it can.
Moreover, since the oxide and oxysulfide having a particle size of 1 μm or more whose REM content is controlled to 30 mol% or more are dispersed, BN can be precipitated using the oxide and oxysulfide as a precipitation site, Precipitation of BN at the crystal grain boundary can be suppressed.
Further, in the region where bending or straightening strain is applied to the slab, the slab surface temperature at a position equivalent to the slab thickness from the corner of the long side surface of the slab is 750 ° C. or higher. It is possible to suppress the occurrence of surface cracks in the slab at the part or the correction part.

Moreover, the manufacturing method of the slab which concerns on this invention is a manufacturing method of the slab which manufactures the above-mentioned slab using a vertical bending type | mold or a curved type continuous casting machine, Comprising: In mass%, C: 0. 05% to 0.18%, Si: 0.10% to 0.4%, Mn: 0.5% to 2.0%, P: 0.020% or less, S: 0.0035% or less Ni: 0.1% to 2.0%, Ti: 0.005% to 0.030%, Al: 0.005% to 0.06%, N: 0.0015% to 0.007 %: B: 0.0005% to 0.0050%, Cu: 0.1% to 0.5%, Cr: 0.2% to 2.0% as necessary Mo: 0.1% or more and 0.8% or less, V: 0.01% or more and 0.1% or less, Nb: 0.005% or more and 0.05% or less It contains one or two or more kinds-option, the molten steel the balance being Fe and impurities, the sulfur concentration in the molten steel [% S], the total oxygen concentration in the molten steel T. When [% O], the Ca concentration in the molten steel is [% Ca], and the atomic weight of _Ca is M _Ca , [% Ca] / M _Ca ≧ 0.3 × ([% S] /32.06+T. Ca is added so as to satisfy [% O] /16.1), and oxides and oxysulfides having a particle size of 1 μm or more whose Ca content is controlled to 30 mol% or more are dispersed, In the region where bending or straightening strain is applied, the slab surface temperature at a position corresponding to the slab thickness distance from the corner of the long side surface of the slab is 750 ° C. or more.

According to the method for producing a slab having this configuration, the sulfur concentration in the molten steel is [% S], and the total oxygen concentration in the molten steel is T.P. When [% O], the Ca concentration in the molten steel is [% Ca], and the atomic weight of _Ca is M _Ca , [% Ca] / M _Ca ≧ 0.3 × ([% S] /32.06+T. Since Ca is added so as to satisfy [% O] /16.1), S in the molten steel reacts with Ca to form a compound, thereby suppressing the segregation of S to the grain boundaries. it can.
Moreover, since the oxide and oxysulfide having a particle diameter of 1 μm or more whose Ca content is controlled to 30 mol% or more are dispersed, BN can be precipitated using the oxide and oxysulfide as a precipitation site, Precipitation of BN at the crystal grain boundary can be suppressed.
Further, in the region where bending or straightening strain is applied to the slab, the slab surface temperature at a position equivalent to the slab thickness from the corner of the long side surface of the slab is 750 ° C. or higher. It is possible to suppress the occurrence of surface cracks in the slab at the part or the correction part.

Furthermore, the manufacturing method of the slab which concerns on this invention is a manufacturing method of the slab which manufactures the above-mentioned slab using a vertical bending type | mold or a curved type continuous casting machine, Comprising: In mass%, C: 0. 05% to 0.18%, Si: 0.10% to 0.4%, Mn: 0.5% to 2.0%, P: 0.020% or less, S: 0.0035% or less Ni: 0.1% to 2.0%, Ti: 0.005% to 0.030%, Al: 0.005% to 0.06%, N: 0.0015% to 0.007 %: B: 0.0005% to 0.0050%, Cu: 0.1% to 0.5%, Cr: 0.2% to 2.0% as necessary , Mo: 0.1% to 0.8%, V: 0.01% to 0.1%, Nb: 0.005% to 0.05%, In the molten steel containing one or more selected, with the balance being composed of Fe and impurities, the nitrogen concentration in the molten steel is [% N], the boron concentration in the molten steel is [% B], When the Zr concentration is [% Zr] and the atomic weight of _Zr is M _Zr , [% Zr] / M _Zr ≧ 0.3 × ([% N] /14.01 − [% B] /10.81 Zr is added so as to satisfy (2), and composite inclusions having a particle size of 500 nm to 5 μm containing ZrN and MnS are dispersed at a rate of 30 pieces / mm ² or more per unit area, and bending or straightening distortion is applied to the slab. In the region where the slab is loaded, the slab surface temperature at a position corresponding to the slab thickness equivalent distance from the corner of the long side surface of the slab is 750 ° C. or more.

According to the method for producing a slab of this configuration, the nitrogen concentration in the molten steel is [% N], the boron concentration in the molten steel is [% B], the Zr concentration in the molten steel is [% Zr], and the atomic weight of Zr is when the _{M Zr, [% Zr] /} M Zr ≧ 0.3 × - since the addition of ([% N] /14.01 [% B] /10.81) so as to satisfy the Zr N in the molten steel reacts with Zr to form ZrN, and the generation of BN can be suppressed.
Further, since ZrN exists together with MnS, it is possible to disperse 30 / mm ² or more of composite inclusions containing ZrN and MnS and having a particle size of 500 nm or more and 5 μm or less per unit area.
Further, in the region where bending or straightening strain is applied to the slab, the slab surface temperature at a position equivalent to the slab thickness from the corner of the long side surface of the slab is 750 ° C. or higher. It is possible to suppress the occurrence of surface cracks in the slab at the part or the correction part.

  According to the present invention, even when a steel slab containing B and Ni is produced using a vertical bending type or a curved type continuous casting machine, the occurrence of surface cracks during bending and straightening is suppressed. It is possible to provide a slab that can be manufactured and a method for manufacturing the slab.

It is a schematic explanatory drawing of the continuous casting apparatus used in embodiment of this invention. In the examples, the relationship between [% α] / Mα and ([% S] /28.09+T. [% O] /16.01), where α is an element having a high affinity for S (REM, Ca) It is a graph which shows. In Example, [% _{Zr] / M} Zr and - is a graph representing the relationship between ([% N] /14.01 [% B] /10.81).

  Below, the slab which is embodiment of this invention and the manufacturing method of a slab are demonstrated. In addition, this invention is not limited to the following embodiment.

The slab according to this embodiment has a composition of mass%, C: 0.05% to 0.18%, Si: 0.10% to 0.4%, Mn: 0.5% or more. 2.0% or less, P: 0.020% or less, S: 0.0035% or less, Ni: 0.1% or more and 2.0% or less, Ti: 0.005% or more and 0.030% or less, Al: 0.005% or more and 0.06% or less, N: 0.0015% or more and 0.007% or less, and B: 0.0005% or more and 0.0050% or less, and REM: 0.0015% 0.02% or less, Ca: 0.0015% or more and 0.0060% or less, Zr: One or more selected from 0.0020% or more and 0.015% or less, and the balance is Fe And impurities. Furthermore, in the present embodiment, Cu: 0.1% to 0.5%, Cr: 0.2% to 2.0%, Mo: 0.1% to 0.8% as necessary. V: 0.01% or more and 0.1% or less, Nb: 0.005% or more and 0.05% or less.
Below, the reason which prescribed | regulated each component is demonstrated.

(C: 0.05% or more and 0.18% or less)
C is generally known as an element having a great influence on the strength of steel, and if it is less than 0.05%, it is difficult to obtain a predetermined strength for applications such as high-strength thick steel plates. If the C concentration exceeds 0.18%, the hardness will be extremely high and cause new flaws. Therefore, a special process is required for heat treatment, and as a thick steel plate for hardening the weld and heat affected zone The required weldability is impaired. For these reasons, the C concentration range is defined as 0.05% or more and 0.18% or less. The lower limit of the C concentration is preferably 0.08% or more, and the upper limit of the C concentration is preferably 0.16% or less.

(Si: 0.10% to 0.4%)
In general, Si is an element effective for reducing the oxygen concentration in steel as a deoxidizing element in the steel production process, and has an effect of strengthening steel. Continuous casting in a state where the molten steel is not sufficiently deoxidized causes bubbles to be generated in the steel, resulting in product defects, and sometimes causing breakout and inability to operate. For this reason, the lower limit of the Si content is 0.10% or more. On the other hand, when the Si content exceeds 0.4%, striped martensite is generated, and there is a problem that the HAZ toughness is deteriorated during welding. Therefore, the upper limit is defined as 0.4% or less, but more preferably less than 0.3%.

(Mn: 0.5% to 2.0%)
Mn is an element that generally has a great influence on the strength of steel, but if it is less than 0.5%, it is difficult to obtain sufficient strength as a high-strength thick steel plate. On the other hand, if it exceeds 2.0%, the strength is remarkably strengthened due to the solid solution strengthening, making it difficult to adjust the strength of the product. Further, since Mn is concentrated at the center segregation portion, unevenness in strength occurs in the cast slab and the thick steel plate after rolling. For this reason, the Mn concentration range is defined as 0.5% or more and 2.0% or less. The lower limit of the Mn concentration is preferably 0.8% or more, and the upper limit of the Mn concentration is preferably 1.8% or less.

(P: 0.020% or less)
P is one of the impurity elements inevitably contained in the steel and is preferably low. P segregates significantly because the equilibrium partition coefficient at the solid-liquid interface during solidification is small. For this reason, there is a concern that various product characteristics may be adversely affected. In the segregation part, the melting point also decreases remarkably, so the concentrated part melts during rolling and may lead to product defects. Therefore, the upper limit of the content is set to 0.020% or less. In order to prevent various problems in the segregated portion, it should preferably be less than 0.010%.

(S: 0.0035% or less)
S is one of impurity elements inevitably contained in the steel and is preferably as low as possible. S is not only an element that segregates remarkably because the equilibrium distribution coefficient at the solid-liquid interface after solidification is small, but also lowers the melting point in the segregated part in the same manner as P, and causes surface defects particularly during rolling. For this reason, the upper limit was made 0.0035% or less. Under conditions that are more demanding than high strength steel and the like, the upper limit of the S content is preferably 0.0020% or less.

(Ni: 0.1% to 2.0%)
Ni has the effect of improving the toughness as well as improving the strength of the steel by solid solution strengthening. In order to obtain these effects, it is necessary to add 0.1% or more, but even if added over 2.0%, the effect is saturated and there is also an adverse effect that the weldability is deteriorated. Therefore, the Ni concentration range is defined as 0.1% or more and 2.0% or less. Note that the lower limit of the Ni concentration is preferably 0.3% or more, and the upper limit of the Ni concentration is preferably 1.8% or less.

(Ti: 0.005% to 0.03%)
Ti improves the strength of the steel and fixes N in the steel as TiN, which also affects the generation of BN. This also has the effect of preventing slab surface cracks during bending and straightening of continuously cast slabs. In order to obtain such an effect, addition of 0.005% or more is necessary. However, if the content exceeds 0.03%, a large number of carbides are generated, which reduces the toughness of the weld heat affected zone and causes coarse TiN to be generated. For this reason, it is specified as 0.005% or more and 0.03% or less. From the viewpoint of stably suppressing both the surface crack of the slab and the deterioration of the surface properties based on TiN, the lower limit of the Ti concentration may be 0.010% or more and the upper limit of the Ti concentration may be 0.020% or less. preferable.

(Al: 0.005% to 0.06%)
Al is one of the elements effective for reducing the oxygen concentration in steel as a deoxidizing element. The content required for deoxidation is 0.005% or more. If it is less than 0.005%, sufficient desulfurization in the smelting process becomes difficult. On the other hand, when Al is added excessively, AlN is likely to be generated, and it causes cracking of the slab surface, so 0.06% or less is preferable. Note that the lower limit of the Al concentration is preferably 0.007% or more, and the upper limit of the Al concentration is preferably 0.04% or less.

(N: 0.0015% or more and 0.007% or less)
N is an element that inevitably penetrates into steel when it is melted in an air atmosphere such as a converter, and is a constituent element of BN. In steel materials, it is an element that forms nitrides with Ti and the like, and these nitrides have an effect of refining crystal grains as pinning particles in the process of hot working, and thus affect the mechanical properties of the steel materials. Therefore, the concentration needs to be 0.0015% or more. On the other hand, as described above, these nitrides are dynamically precipitated at the austenite grain boundaries during continuous casting, which causes slab surface cracks, so the upper limit is made 0.007% or less. From the viewpoint of reliably exhibiting the pinning effect of the structure and preventing toughness deterioration due to the formation of coarse carbon / nitride in the center of the slab, the lower limit of the N concentration is 0.002% or more, N The upper limit of the concentration is preferably 0.004% or less.

(B: 0.0005% or more and 0.0050% or less)
B is added as a component that increases the hardenability of the grain boundaries, controls the structure of the steel material, and increases the strength of the steel material. B is highly effective when added in a small amount, but the lower limit is 0.0005% or more in order to achieve high strength of 700 MPa to 1200 MPa. On the other hand, if adding over 0.0050%, the effect is saturated and the toughness is also lowered, so the upper limit is made 0.0050% or less. From the viewpoint of controlling the microstructure of the thick steel plate and clearly expressing the effect of addition of B, it is preferable that the lower limit of the B concentration is 0.0010% or more and the upper limit of the B concentration is 0.0040% or less.

  Moreover, in order to achieve high strength of 700 MPa or more and to develop other characteristics such as weldability and weather resistance, one or more selected from Cu, Cr, Mo, V, and Nb shown below are used. It is added.

(Cu: 0.1% to 0.5%)
Cu improves the hardenability of the steel. For that purpose, addition of 0.1% or more is necessary, but when it exceeds 0.5%, not only the effect becomes excessive, but also the hot workability of the steel material decreases. In addition, since it is an element which induces a surface crack called a star crack at the time of continuous casting, when adding 0.2% or more of Cu, it is necessary to add Ni at a concentration of 1/3 or more.

(Cr: 0.2% or more and 2.0% or less)
Cr has the effect of increasing the strength and toughness of steel. For that purpose, addition of 0.2% or more is necessary. When high-strength specifications such as 80 kg class or more are required, it becomes a semi-essential additive element. On the other hand, if it exceeds 2.0%, problems such as weld cracking occur. When emphasizing weldability for the same reason, the upper limit of Cr concentration should be 1.5% or less.

(Mo: 0.1% to 0.8%)
Mo improves the hardenability of the steel sheet and contributes to an increase in strength. Like Cr, when high-strength specifications such as 80 kg class or more are required, it becomes a semi-essential additive element. In order to obtain this effect, addition of 0.1% or more is necessary. However, Mo is an expensive element and not only leads to an increase in cost, but if added over 0.8%, a hardened phase such as a bainite or martensite phase is generated, deteriorating hot workability and weldability. The upper limit is 0.8% or less.

(V: 0.01% to 0.1%)
V is an effective element for increasing the strength of steel by forming a solid solution and a carbonitride in ferrite in the steel. For that purpose, it is necessary to add 0.01% or more. However, if the content of V exceeds 0.1%, the precipitation state in the weld heat affected zone changes and adversely affects toughness. Moreover, since it will precipitate as VN inside a slab and will cause a slab surface crack if it adds excessively, an upper limit shall be 0.1% or less.

(Nb: 0.005% or more and 0.05% or less)
Nb is an element that forms carbonitrides in steel to increase the strength of the steel and is effective in improving toughness. Therefore, it is necessary to add 0.005% or more. In particular, it is used for controlling the microstructure of the steel sheet by controlling solid solution and precipitation in TMCP (Thermo-Mechanical Control Process). In order to obtain this effect, it is necessary to add 0.005% or more. However, if it exceeds 0.05%, it does not dissolve at the time of heating, and the structure cannot be controlled. Moreover, when it adds excessively, it will precipitate as NbC inside a slab, and will cause a slab surface crack. Therefore, the Nb concentration is defined as 0.005% or more and 0.05% or less.

  And in this embodiment, in order to suppress the surface crack of the slab by fixing the solid solution S and controlling the form of the nitride, one or more selected from REM, Ca, Zr shown below are used. It is added.

(REM: 0.0015-0.02%)
REM (rare earth element) means one or more metal elements selected from lanthanoids (La, Ce, etc., 15 elements having atomic numbers 57 to 71), and in particular, among Ce, La, Pr, or Nd One or more elements are applicable. The effect of adding REM appears at 0.0015% or more. However, since REM is expensive and the effect is saturated even if it is added excessively, it is preferable to set it to 0.02% or less from the viewpoint of cost-effectiveness, and further, a new immersion nozzle is blocked. The problem will also occur. The lower limit of the REM concentration is preferably 0.003% or more, and the upper limit of the REM concentration is preferably 0.018% or less.

(Ca: 0.0015% or more and 0.0060% or less)
From the viewpoint of preventing surface cracks including B and Ni, Ca needs to be added in an amount of 0.0015% or more. Even if added over 0.0060%, the effect is saturated and not only increases the production cost, but also may cause new problems such as nozzle clogging. % Or less. The lower limit of Ca concentration is preferably 0.0020% or more, and the upper limit of Ca concentration is preferably 0.0050% or less.

(Zr: 0.0020 to 0.015%)
From the viewpoint of preventing surface cracks including B and Ni, it is necessary to add 0.0020% or more of Zr. Even if added over 0.015%, the effect is saturated and not only the manufacturing cost is increased, but the merit is reduced. The lower limit of the Zr concentration is preferably 0.0030% or more, and the upper limit of the Zr concentration is preferably 0.013% or less.

  Other than the elements described above, Fe and impurities. Here, the “impurity” is a mixture of ore as a raw material in industrial production of steel materials, an elution component from scrap, manufacturing equipment, or the like, and may be contained within a range that does not adversely affect performance.

And in the slab which is this embodiment, as a result of observing the fracture surface of the test piece which performed the tensile test at 800 degreeC, the maximum density | concentration of S in a crystal grain boundary is 30 times or less of the S density | concentration of the whole slab. In addition, the number of BN particles having a particle diameter of 10 nm or more and 300 nm or less observed at the crystal grain boundary is set to 3000 or less per 1 mm length of the grain boundary.
That is, in this embodiment, segregation of S at the grain boundaries and precipitation of BN particles having a grain size of 10 nm to 300 nm are suppressed, and the grain boundary strength is sufficiently ensured.

Next, the manufacturing method of the slab which is this embodiment mentioned above is demonstrated using a vertical bending type | mold or a curved type continuous casting machine.
In this embodiment, the slab 1 which is this embodiment mentioned above is manufactured using the continuous casting machine 10 shown in FIG.

A continuous casting machine 10 shown in FIG. 1 includes a slab support roll group including a water-cooled mold 11, an immersion nozzle 12 that supplies molten steel to the water-cooled mold, and a plurality of slab support rolls 21 positioned below the water-cooled mold 11. 20 is provided.
In addition, in the continuous casting machine 10 which is this embodiment, the vertical part 14 which draws out the slab 1 pulled out from the water-cooled mold 11, and the bending part 15 which bends the slab 1, and the bent slab 1 are used. The vertical bending type continuous casting machine has a straightening part 16 that bends back and a horizontal part 17 that transports the slab 1 in the horizontal direction.

  The water-cooled mold 11 has a cylindrical shape having a rectangular hole, and the slab 1 having a cross section matching the shape of the rectangular hole is pulled out. For example, the long side length of the rectangular hole (corresponding to the width of the cast piece 1) is 500 to 2500 mm, and the short side length of the rectangular hole (corresponding to the thickness of the cast piece 1) is 100 to 600 mm. However, the present invention is not limited to this.

The slab support roll group 20 is located in the pinch roll part 24 located in the vertical part 14, the bending roll part 25 located in the bending part 15, the straightening roll part 26 located in the straightening part 16, and the horizontal part 17. A horizontal roll unit 27.
In the continuous casting machine 10, tensile strain is applied to the slab 1 at the bending portion 15 and the correction portion 16.

  Here, in this embodiment, in the bending part 15 and the correction | amendment part 16 where tensile strain is loaded with respect to the slab 1, the slab in the position of the slab thickness equivalent distance from a corner in the long side surface of the slab 1 The surface temperature is 750 ° C. or higher. For example, when the width of the slab 1 is 500 mm and the thickness of the slab 1 is 100 mm, the surface temperature at a position of 100 mm from the width end of the long side surface of the slab 1 is 750 ° C. or more. It is.

In the present embodiment, the strength of crystal grain boundaries is ensured by means as described below, and the surface cracks of the slab 1 at the bending portion 15 and the correction portion 16 are suppressed.
Hereinafter, a method for manufacturing a slab according to this embodiment is divided into three cases: (1) when REM is added, (2) when Ca is added, and (3) when Zr is added. I will explain.

(1) In the case of REM addition Since REM has a high affinity with S, a compound of REM and S is generated by adding REM to molten steel. As a result, S is fixed and segregation of S at the crystal grain boundary can be suppressed. In addition, by adding REM to the molten steel, oxides and oxysulfides are generated. Since these oxides and oxysulfides become precipitation sites for BN particles, the crystal grains of BN particles It is possible to suppress the precipitation to the boundary.

Here, in this embodiment, in order to suppress the segregation of S and the precipitation of BN particles at the crystal grain boundaries, the amount of REM added is adjusted within the following range.
The sulfur concentration in the molten steel is [% S], and the total oxygen concentration in the molten steel is T.V. When [% O], the REM concentration in the molten steel is [% REM], and the atomic weight of the _REM is M _REM ,
[% REM] / M _REM ≧ 0.3 × ([% S] /32.06+T. [% O] /16.1)
REM is added to satisfy

Thereby, it becomes possible to disperse oxides and oxysulfides having a particle size of 1 μm or more whose REM content is controlled to 30 mol% or more. When slabs are observed, 50% or more of the observed oxides and oxysulfides having a particle size of 1 μm or more are oxides and oxysulfides whose REM content is controlled to 30 mol% or more. Preferably there is.
As described above, by adding an appropriate amount of REM, segregation of S at the grain boundaries and precipitation of BN grains at the grain boundaries are suppressed, and the grain boundary strength is ensured. It becomes possible to suppress the surface crack of the piece 1.

(2) When Ca is added Since Ca, like REM, has a high affinity with S, the addition of Ca into molten steel produces a compound of Ca and S. Segregation can be suppressed. Further, when Ca is added to the molten steel, oxides and oxysulfides are generated. Since these oxides and oxysulfides become precipitation sites of BN particles, the BN particles are crystal grains. It is possible to suppress the precipitation to the boundary.

Here, in this embodiment, in order to suppress the segregation of S and the precipitation of BN particles at the crystal grain boundaries, the addition amount of Ca is adjusted within the following range.
The sulfur concentration in the molten steel is [% S], and the total oxygen concentration in the molten steel is T.V. When [% O], the Ca concentration in the molten steel is [% Ca], and the atomic weight of _Ca is _MCa ,
[% Ca] / M _Ca ≧ 0.3 × ([% S] /32.06+T. [% O] /16.1)
Ca is added so as to satisfy the above.

Thereby, it becomes possible to disperse oxides and oxysulfides having a particle size of 1 μm or more whose Ca content is controlled to 30 mol% or more. When slabs are observed, 50% or more of oxides and oxysulfides having a particle size of 1 μm or more observed are oxides and oxysulfides whose Ca content is controlled to 30 mol% or more. Preferably there is.
As described above, by adding an appropriate amount of Ca, segregation of S at the grain boundaries and precipitation of BN grains at the grain boundaries are suppressed, and the grain boundary strength is ensured. It becomes possible to suppress the surface crack of the slab 1 at the portion 16.

(3) When Zr is added Zr reacts with N to produce ZrN. Thereby, the production | generation of BN can be suppressed and precipitation to the crystal grain boundary of BN particle | grains can be suppressed. Moreover, since this ZrN is complex-precipitated with MnS, S in the molten steel is fixed. Thereby, it is possible to suppress the segregation of S at the crystal grain boundaries.

Here, in this embodiment, in order to suppress the segregation of S at the crystal grain boundaries and the precipitation of BN grains at the crystal grain boundaries, the amount of Zr added is adjusted within the following range.
When the nitrogen concentration in the molten steel is [% N], the boron concentration in the molten steel is [% B], the Zr concentration in the molten steel is [% Zr], and the atomic weight of _Zr is M _Zr ,
[% Zr] / M _Zr ≧ 0.3 × ([% N] /14.01 − [% B] /10.81)
Zr is added so as to satisfy

As a result, it is possible to disperse composite inclusions containing ZrN and MnS and having a particle size of 500 nm or more and 5 μm or less at a rate of 30 pieces / mm ² or more per unit area. In addition, when slab is observed, it is preferable that 50% or more of the composite inclusions having a particle diameter of 500 nm or more and 5 μm or less observed are composite inclusions containing ZrN and MnS.
As described above, by adding an appropriate amount of Zr, segregation of S at the grain boundaries and precipitation of BN grains at the grain boundaries are suppressed, and the grain boundary strength is ensured. It becomes possible to suppress the surface crack of the slab 1 at the portion 16.

  As described above, according to the embodiment of the present invention, even when the steel slab 1 containing B and Ni is manufactured using the vertical bending die continuous casting machine 10, the bending portion 15 and It becomes possible to suppress generation | occurrence | production of the surface crack of the slab 1 in the correction part 16. FIG.

As mentioned above, although embodiment of this invention was described concretely, this invention is not limited to this, In the range which does not deviate from the technical idea of the invention, it can change suitably.
In the present embodiment, the vertical bending type continuous casting machine 10 has been described as an example. However, the present invention is not limited to this, and a curved type continuous casting machine may be used.

Below, the confirmation experiment conducted in order to confirm the effect of invention is demonstrated.
A 2.5 ton high frequency induction furnace was used to melt 2.5 tonnes of molten steel subjected to preliminary combined deoxidation with Si and Mn, and then the molten steel was transferred from the high frequency induction furnace to the ladle by top pouring. In the ladle, metal Al was charged in advance for the molten steel to be transferred, and Al was dissolved in the molten steel by pouring the molten steel.

  At this time, if necessary, elements having a high affinity with S (REM, Ca) or Zr were also introduced in the same procedure, and finally adjusted to the molten steel composition to be cast. The molten steel in the ladle was poured through a tundish into a vertical bending type continuous casting machine having a vertical portion length of 1.3 m to obtain a slab having a thickness of 100 mm and a width of 500 mm. The casting speed is 0.70 to 1.20 m / min, and the specific water amount for secondary cooling is 0.7 to 1.6 L / kg-steel. Further, the surface temperature of the slab on the L surface (the long side surface on the top side) was measured with a radiation thermometer disposed between the rolls immediately before the slab bending side and immediately before the correction side. The temperature measuring position defined in the present invention is a position corresponding to the slab thickness from the corner, and corresponds to a position 100 mm from the corner in this embodiment.

In addition, in the slab C cross section (in the sense of a slab transverse section, this example corresponds to 500 mm × 100 mm), a slab width of 1/4 and a 10 mm × 20 mm field of view from the L-layer side surface layer to the 10 mm position in the thickness direction This was observed using a scanning electron microscope. 50 or more inclusions having a size of 5 to 10 μm were randomly observed. At this time, an oxide and an oxysulfide having a particle size of 1 μm or more whose REM content is controlled to 30 mol% or more, or an oxide and an oxysulfide having a particle size of 1 μm or more whose Ca content is controlled to 30 mol% or more. Must be present in a proportion of at least 50% by this observation method. In addition, when the observed sulfide or oxysulfide is integrated with other non-metals, point analysis is performed on the sulfide or oxysulfide portion with EDS, and Ca or REM It was confirmed whether the content of was 30 mol% or more.
On the other hand, in the same procedure, 30 or more nitrides were randomly observed, and 10 μm or less of ZrN was present in a proportion of 90% or more, and 50% or more of which was less than 1 μm of MnS on ZrN. Is simultaneously observed, it is defined as including a precipitate in which ZrN and MnS are combined.

After removing the scale from the surface of the obtained slab and pickling, both the L side and F side of the slab (L side = long side on the top side, F side = long side on the ground side) On the other hand, the presence or absence of cracks was visually observed by a dyeing penetrant flaw test defined in JIS Z2343, a so-called color check method. The degree of surface cracking was evaluated in three stages (0, 1, 2) of values (index of wrinkles) that index the degree of surface flaws. When the haze index is 0 (zero), no haze can be confirmed on the surface of the slab, indicating that the hull is sound. When the flaw index is 1, the number of flaws per slab unit length is as small as 10 pieces / m or less, and if the surface is turned with a grinder with a maximum of about 3 mm, it can be easily removed by maintenance, and there is no practical problem. It is. When the flaw index is 2, flaws are found on the entire surface of the slab (the number of ridges per unit length of the slab is about 30 to 40 / m), and both sides are not easily handled with a grinder turning of about 3 mm. This is a level that cannot be completely removed. Alternatively, cracks with a depth of 3 mm or more are remarkably observed in the slab corner, and in order to use the slab in the next process, both ends of the slab must be cut by about 30 mm, which is accompanied by a significant decrease in yield.
Therefore, the flaw occurrence index 0, 1 corresponds to a level that does not hinder the surface quality of the slab, and the flaw index 2 corresponds to a level that is practically unacceptable.

  The results of Invention Examples 1 to 5 and Comparative Examples 1 to 5 to which REM and Ca having high affinity with S are added are shown in Table 2 and FIG. The unit is mass%, and the balance of each steel is Fe and impurities.

In FIG. 2, the sulfur concentration in the molten steel is [% S], and the total oxygen concentration in the molten steel is T.V. When [% O], the concentration of elements having high affinity with S (REM, Ca) in the molten steel is [% α], and the atomic weight of elements having high affinity with S (REM, Ca) is M _α , It represents the relationship between [% α] / Mα and ([% S] /28.09+T. [% O] /16.01).
In Inventive Examples 2 and 3, no wrinkle was observed (habit index 0), and it was very healthy. In the examples 1, 4, and 5 of the present invention, the wrinkle index 1 was not a level that would have a negative effect on productivity and yield, as long as slight surface care was required. In particular, compared with the case where the soot index was 0, the concentration of α in the oxide or oxysulfide was slightly lower, and so the generation of soot could not be prevented completely. Comparative Examples 1, 2, and 4 do not satisfy the formulas defined in claims 3 and 4, and the concentration of elements (REM, Ca) having a high affinity for S in the oxide or oxysulfide is less than 30 mol%. Therefore, even if the surface temperature during bending and straightening was 750 ° C. or higher, cracking could not be suppressed. Moreover, although the comparative examples 3 and 5 satisfy | fill the formula prescribed | regulated by Claim 3, 4, since the slab surface temperature at the time of correction was less than 750 degreeC, the crack generate | occur | produced.

  Next, Table 3 and FIG. 3 show the results of Invention Examples 6 to 8 and Comparative Examples 6 to 8 to which Zr was added. The unit is mass%, and the balance of each steel is Fe and impurities.

In FIG. 3, the nitrogen concentration in the molten steel is [% N], the boron concentration in the molten steel is [% B], the Zr concentration in the molten steel is [% Zr], the atomic weight of Zr is MZr, and [% Zr] / MZr and ([% N] /14.01-[% B] /10.81).
In Example 7 of the present invention, no generation of wrinkles was observed (wrinkle index 0), which was very healthy. In addition, Invention Examples 6 and 8 (having index 1) were not at a level that would adversely affect productivity and yield as long as minor surface maintenance was possible. Compared with the condition with a wrinkle index of 0, the surface temperature was slightly lower during correction, so that the wrinkle index was 1. In Comparative Examples 6 and 7, the formula defined in claim 5 was not satisfied, ZrN was not recognized as a nitride present in the steel, and BN was observed. Moreover, although the comparative example 8 satisfied the formula prescribed | regulated by Claim 5, since the slab surface temperature at the time of correction was less than 750 degreeC, the crack generate | occur | produced.

  From the above, according to the example of the present invention, it was confirmed that occurrence of surface cracks in the slab can be suppressed.

1 Slab 10 Continuous casting machine

Claims (5)

In mass%, C: 0.05% to 0.18%, Si: 0.10% to 0.4%, Mn: 0.5% to 2.0%, P: 0.020% or less S: 0.0035% or less, Ni: 0.1% or more and 2.0% or less, Ti: 0.005% or more and 0.030% or less, Al: 0.005% or more and 0.06% or less, N: 0.0015% or more and 0.007% or less, and B: 0.0005% or more and 0.0050% or less,
Further, one or two selected from REM: 0.0015% to 0.02%, Ca: 0.0015% to 0.0060%, Zr: 0.0020% to 0.015% Containing the above, with the balance being composed of Fe and impurities,
As a result of observing the fracture surface of the test piece subjected to the tensile test at 800 ° C., the maximum concentration of S at the crystal grain boundary is 30 times or less of the S concentration of the entire slab, and the grains observed at the crystal grain boundary A cast slab characterized in that the number of BN particles having a diameter of 10 nm or more and 300 nm or less is 3000 or less per 1 mm length of the grain boundary.   Further, Cu: 0.1% to 0.5%, Cr: 0.2% to 2.0%, Mo: 0.1% to 0.8%, V: 0.01% to 0.8%. The slab according to claim 1, comprising one or more selected from 1% or less and Nb: 0.005% or more and 0.05% or less. A slab manufacturing method for manufacturing a slab according to claim 1 or 2, using a vertical bending mold or a curved continuous casting machine,
In mass%, C: 0.05% to 0.18%, Si: 0.10% to 0.4%, Mn: 0.5% to 2.0%, P: 0.020% or less S: 0.0035% or less, Ni: 0.1% or more and 2.0% or less, Ti: 0.005% or more and 0.030% or less, Al: 0.005% or more and 0.06% or less, N: 0.0015% or more and 0.007% or less, and B: 0.0005% or more and 0.0050% or less. If necessary, Cu: 0.1% or more and 0.5% or less, Cr: 0.005% or less. 2% to 2.0%, Mo: 0.1% to 0.8%, V: 0.01% to 0.1%, Nb: 0.005% to 0.05% 1 type or 2 types or more of the molten steel having a composition consisting of Fe and impurities in the balance,
The sulfur concentration in the molten steel is [% S], and the total oxygen concentration in the molten steel is T.V. When [% O], the REM concentration in the molten steel is [% REM], and the atomic weight of the _REM is M _REM ,
[% REM] / M _REM ≧ 0.3 × ([% S] /32.06+T. [% O] /16.1)
REM is added to satisfy
Disperse oxides and oxysulfides having a particle size of 1 μm or more whose REM content is controlled to 30 mol% or more,
In a region where bending or straightening strain is applied to the slab, the slab surface temperature at a position corresponding to the slab thickness distance from the corner of the long side surface of the slab is 750 ° C. or more. A method for producing a slab. A slab manufacturing method for manufacturing a slab according to claim 1 or 2, using a vertical bending mold or a curved continuous casting machine,
In mass%, C: 0.05% to 0.18%, Si: 0.10% to 0.4%, Mn: 0.5% to 2.0%, P: 0.020% or less S: 0.0035% or less, Ni: 0.1% or more and 2.0% or less, Ti: 0.005% or more and 0.030% or less, Al: 0.005% or more and 0.06% or less, N: 0.0015% or more and 0.007% or less, and B: 0.0005% or more and 0.0050% or less. If necessary, Cu: 0.1% or more and 0.5% or less, Cr: 0.005% or less. 2% to 2.0%, Mo: 0.1% to 0.8%, V: 0.01% to 0.1%, Nb: 0.005% to 0.05% 1 type or 2 types or more of the molten steel having a composition consisting of Fe and impurities in the balance,
The sulfur concentration in the molten steel is [% S], and the total oxygen concentration in the molten steel is T.V. When [% O], the Ca concentration in the molten steel is [% Ca], and the atomic weight of _Ca is _MCa ,
[% Ca] / M _Ca ≧ 0.3 × ([% S] /32.06+T. [% O] /16.1)
Ca is added so as to satisfy
Disperse oxides and oxysulfides having a particle size of 1 μm or more with Ca content controlled to 30 mol% or more,
In a region where bending or straightening strain is applied to the slab, the slab surface temperature at a position corresponding to the slab thickness distance from the corner of the long side surface of the slab is 750 ° C. or more. A method for producing a slab. A slab manufacturing method for manufacturing a slab according to claim 1 or 2, using a vertical bending mold or a curved continuous casting machine,
In mass%, C: 0.05% to 0.18%, Si: 0.10% to 0.4%, Mn: 0.5% to 2.0%, P: 0.020% or less S: 0.0035% or less, Ni: 0.1% or more and 2.0% or less, Ti: 0.005% or more and 0.030% or less, Al: 0.005% or more and 0.06% or less, N: 0.0015% or more and 0.007% or less, and B: 0.0005% or more and 0.0050% or less. If necessary, Cu: 0.1% or more and 0.5% or less, Cr: 0.005% or less. 2% to 2.0%, Mo: 0.1% to 0.8%, V: 0.01% to 0.1%, Nb: 0.005% to 0.05% 1 type or 2 types or more of the molten steel having a composition consisting of Fe and impurities in the balance,
When the nitrogen concentration in the molten steel is [% N], the boron concentration in the molten steel is [% B], the Zr concentration in the molten steel is [% Zr], and the atomic weight of _Zr is M _Zr ,
[% Zr] / M _Zr ≧ 0.3 × ([% N] /14.01 − [% B] /10.81)
Zr is added so as to satisfy
A composite inclusion having a particle size of 500 nm or more and 5 μm or less containing ZrN and MnS is dispersed at a rate of 30 pieces / mm ² or more per unit area,
In a region where bending or straightening strain is applied to the slab, the slab surface temperature at a position corresponding to the slab thickness distance from the corner of the long side surface of the slab is 750 ° C. or more. A method for producing a slab.

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