JP5821792B2 - Method for producing continuous cast slab of steel containing B and method for continuous casting - Google Patents

Description

The present invention also includes processes after casting, in which, when continuously cast using a vertical bending type or curved type continuous casting machine, the heat history of cooling received during casting and the strain history accompanying bending / correction are generated as starting points. The present invention relates to a method for producing a continuous cast slab of steel containing B (boron) that suppresses surface cracks of the cast slab and surface flaws of the steel sheet, and a continuous casting method thereof.

  When steel is continuously cast using a curved or vertical bending type continuous casting machine, cracks along the prior austenite grain boundaries occur on the slab surface due to tensile strain acting on the slab as it is bent or straightened. Cheap.

  One type of steel in which such cracks are particularly likely to occur is steel containing B. In particular, since B has a very high affinity with N, BN, which is the starting point of embrittlement, easily precipitates on the grain boundaries, and is inferior in hot ductility compared to other steels.

  On the other hand, B in the steel lowers the transformation point temperature and increases the hardenability of the grain boundaries, so that the steel material strength can be increased by controlling the structure of the steel material. Therefore, steel containing B in particular is often used for thick steel plate applications.

  In this way, B is one of the very useful elements for improving the characteristics of the heat affected zone, and so on, when manufacturing steel sheets for high-strength thick plates that are increasingly in demand. , One of the essential additive elements.

  However, steel containing B is prone to surface cracks in the slab and surface flaws in the steel sheet, which not only increases the frequency of surface maintenance, but also increases yield loss, lowering productivity and increasing production costs. It will lead to.

  Therefore, a method has been proposed for preventing the surface cracking of a slab or a steel plate during continuous casting or hot rolling of a thick steel plate containing B having excellent characteristics.

  For example, Patent Document 1 proposes a method for suppressing surface cracking of a continuous cast slab containing B. The method proposed in Patent Document 1 intends to suppress the austenite grain boundary precipitation of the compound containing B by restricting the B concentration and the N concentration in the steel to an appropriate control range, and to suppress the surface crack of the slab. It is what.

  Patent Document 2 also discloses lateral cracks and corner cracks along the prior austenite grain boundaries that occur when steel containing B and N is continuously cast using a vertical bending type continuous casting machine. Proposals have been made to prevent this by optimal cooling.

  However, the methods proposed in Patent Documents 1 and 2 are limited only to surface cracks that occur in the cast piece at the stage of as-cast (As Cast). Therefore, even if the surface cracks generated in the slab at the As Cast stage can be suppressed, there is a concern that surface defects will be newly revealed in the slab after heating before rolling and the steel plate after rolling.

  On the other hand, in Patent Document 3, in addition to cracks occurring in the slab at the As Cast stage, continuous cast slabs that prevent cracks occurring on the surface when heated or rolled before hot rolling and a method for manufacturing the same Has been proposed.

  However, in Patent Document 3, when the equilibrium precipitation amount of BN is specified and the appropriate range of B, N, and Ti concentration is specified, the slab is subjected to distortion due to bending and straightening during continuous casting. It is not specified that there is an appropriate slab surface temperature, nor is there any provision regarding the Al concentration causing surface cracks.

JP-A-56-80354 Japanese Patent No. 4561755 JP 2010-189712 A

  The problem to be solved by the present invention is that the suppression of surface flaws in the case of continuous casting of conventional B-containing steel was limited to only surface cracks occurring in the slab at the As Cast stage. is there. In addition, there is no disclosure about the existence of an appropriate slab surface temperature when the slab is subjected to bending and straightening during continuous casting.

  In the case of continuous casting using a vertical bending type or curved type continuous casting machine, when the slab is bent or straightened, distortion is applied to the surface of the slab. In particular, if the surface temperature of the slab when tensile strain is applied matches the embrittlement temperature region (600 to 900 ° C) of the III region, surface defects such as intergranular cracks are likely to occur in the continuous cast slab. Is generally known.

  However, even if the embrittlement temperature range in the above-mentioned region III is avoided and no surface defects such as cracks are found in the slab immediately after continuous casting, the casting is not performed until the subsequent slab heat treatment process or rolling process. The existence of a phenomenon in which a surface defect occurs on a piece or a steel sheet has been newly clarified.

  The inventors have found that steel containing B in particular tends to reveal surface defects in the processes after casting due to the influence of continuous casting history, and the steel composition and casting conditions are controlled within an appropriate range. For example, it has been found that even a steel containing B, which causes surface defects, can prevent surface defects generated in slabs and steel sheets.

The present invention has been made based on the above findings,
% By mass
C: 0.05% or more, 0.18% or less,
Si: 0.10% or more, 0.40% or less,
Mn: 0.5% or more, 2.0% or less,
P: 0.020% or less,
S: 0.003% or less,
Ti: 0.005% or more, 0.030% or less,
Al: 0.005% or more, 0.060% or less,
B: 0.0005% or more, 0.0050% or less,
as well as,
N: 0.0015% or more, containing 0.0070% or less,
further,
Cu: 0.1% or more, 0.5% or less,
Cr: 0.2% or more, 2.0% or less,
Ni: 0.3% or more, 2.5% or less,
Mo: 0.1% or more, 0.8% or less,
V: 0.01% or more, 0.10% or less,
Nb: 0.005% or more, 0.050% or less,
Ca: 0.0005% or more, 0.0060% or less,
Among them, one or two or more of them are contained, and the balance is made of Fe and impurities, and is the method for producing a continuous cast slab of steel containing B, the main feature of which is to satisfy the following formula.
{([% N] /14.0)-([% Ti] /47.9)} × ε ≦ 0.29 × {([% Al] /27.0) + ([% B] /10.8)} + 0.0005
Where ε is the total strain (%) due to bending and straightening of the slab during continuous casting

  In order to manufacture the continuous cast slab of the present invention using a vertical bend type or curved type continuous caster, in the region where the slab is subjected to bending or straightening distortion in the continuous caster, It can be manufactured by setting the slab surface temperature in the region from the corner to a point between 100 and 250 mm at 750 ° C. or higher. This is the continuous casting method for steel containing B of the present invention.

  According to the present invention, it is possible to prevent cracks and flaws generated on the surface of a slab or a steel plate during continuous casting or hot rolling of an extremely thick steel plate containing B, and an improvement in production efficiency can be expected.

In the examples and comparative examples shown in Tables 2 and 3, {([% Al] /27.0) + ([% B] /10.8)} and {([% N] /14.0) − ([% Ti] / 47.9)} × ε is a diagram showing the relationship.

  The present invention aims to prevent cracks and flaws that occur on the surface of slabs and steel plates during continuous casting and hot rolling of very thick steel plates containing B, and within an appropriate range of steel composition and casting conditions. Realized by controlling with.

Hereinafter, the present invention will be described along with the progress from the idea of the present invention to the solution of the problem.
The inventors conducted experiments on hot ductility of steel containing B in the order of Experiment A to Experiment C. As a result, we found the mechanism of surface defects in B-containing steel that occurs in the process from continuous casting to rolling process, and derived surface defect prevention conditions.

(A) Experiment A: High temperature ductility of steel with slab structure of As Cast
Using the three types of samples shown in Table 1 below with different Ti, sol. Al, B, and N concentrations, the high temperature ductility of the steels shown below was evaluated.

  Sample 1 has a higher B and N concentration than Samples 2 and 3, and a higher sol.Al concentration. Sample 2 has the same B concentration as Sample 1, but sol.Al is the lowest of the three types. Sample 3 has the highest Ti concentration, while the B and N concentrations are the lowest.

  Three types of round bar steel (φ10 × 190mm, forged material with no As Cast structure) having the composition shown in Table 1 was melted to a length of about 30mm at the center in the longitudinal direction while maintaining the shape. Then, it was solidified as it was while continuously cooling.

In the cooling process of the sample 1-3, in the range of sample temperature 600 to 1000 ° C. of its freezing unit, 3 × 10 ^-4 sec of strain speed and digits during bending straightening-acting during continuous casting is generally matches ^{- At 1} , the sample 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 following formula (1).
RA = {(A0−Af) / A0} × 100 (%) (1)
Here, A0 represents the cross-sectional area of the sample before tension (m ² ), and Af represents the cross-sectional area of the sample after fracture (m ² ).

  Assuming the amount of tensile strain applied to the slab surface when using a vertical bending type or curved type continuous casting machine, if the RA value is 60% or more, there is a concern that surface cracks may occur in the slab during casting. There is no.

Only Sample 1 exhibited a shrinkage in the cross section of 20 to 40% in the range of 700 to 850 ° C., confirming a decrease in hot ductility of the As Cast structure steel. This is a typical example of embrittlement in the III region.
On the other hand, Sample 2 and Sample 3 were confirmed to have a high cross-sectional shrinkage of 70 to 98% in the range of 700 to 850 ° C. and good high temperature ductility in As Cast.

  Therefore, from these results, even in steel containing B, surface cracking does not always occur in the continuous cast slab at the generally known embrittlement temperature of region III (600-900 ° C). It became clear.

(B) Experiment B: High temperature ductility of steel that is not melted and kept at high temperature Experiment B is different from Experiment A that simulates surface cracks associated with bending and straightening of a slab during continuous casting, and the high temperature of steel. The purpose is to investigate the influence of the heating process of the slab after casting on the ductility, that is, the influence of the thermal strain acting when the slab is heated.

For sample 2 and sample 3 where no embrittlement was observed in the As Cast structure of experiment A, the temperature was raised to 1200 ° C at which the sample did not melt, and the solution treatment was performed for 2 to 6 hours. Thereafter, a high-temperature tensile test was performed in the range of 600 to 1000 ° C. at three strain rates of 10 ⁻⁴ , 10 ⁻³ , and 10 ⁻² sec ⁻¹ .

  As a result of evaluating the cross-sectional shrinkage rate from the cross-sectional area of the sample before and after the fracture, the cross-sectional shrinkage rate reached 70% or more regardless of the above conditions, even if it is only an influence of heat treatment or rolling process not affected by continuous casting It was confirmed that surface cracks are unlikely to occur.

(C) Experiment C: High temperature ductility of steel after heat treatment after applying 10% strain to steel with As Cast structure

  Samples 2 and 3 were prepared by melting and solidifying the sample in the same manner as in Experiment A, and then preparing samples that passed through the two levels of conditions 1 and 2 below. After the temperature was raised again to 0 ° C. and held for 2 hours, a second tensile strain was applied while the temperature was lowered to the range of 700 to 850 ° C. to break the sample.

・ Condition 1:
A tensile amount of 600 ° C. or higher, less than 750 ° C., and a strain rate of 3 × 10 ⁻³ sec ⁻¹ gave a strain amount (pre-strain) of 10% that did not lead to the sample breaking.

・ Condition 2:
A tensile amount of 750 ° C. or more, less than 1000 ° C., and a strain rate of 3 × 10 ⁻⁴ sec ⁻¹ gave a strain amount (pre-strain) of 10% that did not lead to the sample breaking.

  Under condition 2 where pre-strain was applied at 750 to 1000 ° C, both sample 2 and sample 3 had a cross-sectional shrinkage rate of 75% or more regardless of the second pulling temperature, and the effect of pre-strain was completely observed. There wasn't.

  However, in condition 1 where pre-strain was applied at 600 ° C. or more and less than 750 ° C., the cross-sectional shrinkage rate rapidly decreased to 30 to 50% by applying tensile strain at the second tensile temperature only for sample 2, and pre-strain was applied. For the first time, the existence of conditions under which high-temperature ductility deteriorates due to the effects of both strain application and heat treatment was confirmed. On the other hand, in the case of Sample 3, even if prestraining was applied at 600 ° C. or higher and lower than 750 ° C., the effect of applying tensile strain at the second tensile temperature was not observed at all.

  From this, the inventors have a phenomenon that the ductility is reduced even when the surface crack caused by receiving tensile strain in a specific temperature range does not occur in the slab stage having the As Cast structure, for example. I found

  That is, it has been clarified that surface cracks due to casting history may occur in processes after casting such as a heating process because tensile strain acts on the surface of the slab during continuous casting.

  Therefore, for sample 2, in Experiment C, (1) a sample that only gave 10% strain at 700 ° C, and (2) 2 hours at 1200 ° C after applying 10% strain at 700 ° C A retained sample was made.

  As a result of weighing nitrides (AlN, BN, TiN) in steel by electrolytic extraction analysis, in (1), only TiN and AlN were present at about 10 mass ppm and about 30 mass ppm, respectively. On the other hand, in (2), 15 mass ppm with the same level of TiN was present, but AlN decreased and BN was newly present at 30 mass ppm.

  Furthermore, as a result of performing qualitative analysis of B by time-of-flight secondary ion mass spectrometry (TOF-SIMS) on the same parallel plane as the tensile direction of the sample after fracture in which a decrease in hot ductility was observed in Experiment C, Clear B segregation was confirmed at the prior austenite grain boundaries.

  From the above, in the steel containing B, even if no cracks were found in the slab stage immediately after continuous casting, which was affected by tensile strain in the region III, the slab was removed by the heating process or rolling process before rolling. Alternatively, the inventors have elucidated the mechanism of cracking on the surface of a steel sheet as follows.

As a result of tensile deformation at a strain rate of approximately 10 ⁻⁴ to 10 ⁻³ sec ⁻¹ at 600 ° C. or more and less than 750 ° C., AlN is generated on the prior austenite grain boundaries by dynamic precipitation. Once again, when the austenite phase is heated and held in a thermodynamically stable high temperature range, once generated AlN is decomposed.

  However, diffusion of the solute B in the austenite to the grain boundary occurs simultaneously, and the diffusion rate of B in the austenite is 2 to 5 orders of magnitude higher than that of other solute elements. Accordingly, BN of several μm or less is precipitated on the austenite grain boundary using AlN as one nitrogen source. If an excessive amount of BN is present on the austenite grain boundaries, thermal or mechanical tensile strains can easily act as starting points for grain boundary cracks.

  Therefore, {([% N] /14.0)-([% Ti] /47.9)} to suppress precipitation of AlN during continuous casting and to suppress BN precipitation at the austenite grain boundaries in the subsequent heating and rolling processes. × ε ≦ 0.29 × {([% Al] /27.0) + ([% B] /10.8)} + 0.0005 satisfying the relationship of Al, B, N, Ti and total strain ε ( %).

  Furthermore, in order to prevent dynamic precipitation of AlN during continuous casting, it is desirable that the surface temperature of the slab be 750 ° C. or higher when bending or straightening the slab in a vertical bending mold or a curved continuous casting machine, The temperature is preferably 770 ° C or higher. At this time, [% sol.Al] represents the mass% concentration of acid-soluble Al.

The present invention has been completed based on the experimental results (a), (b), and (c), and the results of the examination thereof.
% By mass
C: 0.05% or more, 0.18% or less,
Si: 0.10% or more, 0.40% or less,
Mn: 0.5% or more, 2.0% or less,
P: 0.020% or less,
S: 0.003% or less,
Ti: 0.005% or more, 0.030% or less,
Al: 0.005% or more, 0.060% or less,
B: 0.0005% or more, 0.0050% or less,
as well as,
N: 0.0015% or more, containing 0.0070% or less,
further,
Cu: 0.1% or more, 0.5% or less,
Cr: 0.2% or more, 2.0% or less,
Ni: 0.3% or more, 2.5% or less,
Mo: 0.1% or more, 0.8% or less,
V: 0.01% or more, 0.10% or less,
Nb: 0.005% or more, 0.050% or less,
Ca: 0.0005% or more, 0.0060% or less,
A method for producing a continuous cast slab of steel containing B, characterized in that one or more of them are contained, the balance is Fe and impurities, and satisfies the following formula.
{([% N] /14.0)-([% Ti] /47.9)} × ε ≦ 0.29 × {([% Al] /27.0) + ([% B] /10.8)} + 0.0005
Where ε is the total strain (%) due to bending and straightening of the slab during continuous casting

  First, the chemical composition of the continuous cast slab of steel containing B of the present invention will be described. In the following description, “%” indicating the chemical composition of steel means “% by mass” unless otherwise specified.

・ C: 0.05% or more, 0.18% or less Generally, C is 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. Become. On the other hand, if the C concentration exceeds 0.18%, the hardness becomes extremely high and causes 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 Impairs necessary weldability. Therefore, in the present invention, the concentration range of C is defined as 0.05% or more and 0.18% or less.

・ Si: 0.10% or more, 0.40% or less
Si is an effective element for reducing the oxygen concentration in steel as a deoxidizing element in the steel manufacturing process, and has an effect of strengthening steel. Continuous casting in a state where the molten steel has not been sufficiently deoxidized causes bubbles to form in the steel, resulting in defects in the product, and sometimes causing breakout and inability to operate. Therefore, in the present invention, it was defined as 0.10% or more. However, when the content exceeds 0.40%, striped martensite is generated, and there is a problem that HAZ toughness is deteriorated during welding. Therefore, in the present invention, the upper limit is set to 0.40%, but more preferably less than 0.30%.

・ Mn: 0.5% or more, 2.0% or less
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%, strength strengthening is remarkable due to solid solution strengthening, and it becomes difficult to adjust the strength of the product. Further, since Mn is concentrated at the center segregation part, it causes unevenness in strength in the cast slab and the thick steel plate after rolling. Therefore, in the present invention, the Mn concentration range is defined as 0.5% or more and 2.0% or less.

・ P: 0.020% or less
P is one of the impurity elements inevitably contained in the steel, and it is desirable that the content is small. P is segregated remarkably 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 thickening part melts during rolling, which may lead to product defects. Therefore, in the present invention, the upper limit of the content is set to 0.020%. In order to prevent various problems in the segregation part, it is desirable to make it less than 0.010%.

・ S: 0.003% or less
Like P, S is one of the impurity elements inevitably contained in steel, and it is desirable that it be as small as possible. In addition, since the equilibrium distribution coefficient at the solid-liquid interface after solidification is small, not only is the element segregated remarkably, but the melting point is lowered at the segregated portion, and it causes surface flaws particularly during rolling. Therefore, in the present invention, 0.003% is made the upper limit. In the more demanding conditions such as high strength steel, the upper limit is preferably 0.002%.

・ Ti: 0.005% or more, 0.030% or less
Since Ti improves the strength of the steel and fixes N in the steel as TiN, it also affects the generation of AlN and BN, which are the main points of the present invention. 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 0.030%, a large number of carbides are generated, which deteriorates the toughness of the heat affected zone and causes coarse TiN to be generated. For this reason, in this invention, it prescribed | regulated as 0.005% or more and 0.030% 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, it is desirable that the content be 0.010% or more and 0.020% or less.

・ Al: 0.005% or more, 0.060% or less
Al is also an effective element 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, if it is added excessively, AlN is likely to be formed, which causes cracks on the slab surface and contradicts the object of the present invention. Therefore, in the present invention, the content is made 0.060% or less.

・ B: 0.0005% or more, 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 0.0005% or more must be added in order to achieve a high tensile strength of 700 MPa to 1200 MPa. However, if added over 0.0050%, the effect is saturated and the toughness is lowered, so the upper limit was made 0.0050% in the present invention. From the viewpoint of controlling the microstructure of the thick steel plate and clearly expressing the addition effect of B, it is desirable that the content be 0.0010% or more and 0.0040% or less.

・ N: 0.0015% or more, 0.0070% 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 AlN and BN focused in the present invention. In steel materials, it is an element that forms nitrides with Ti and the like. 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 steel materials. For this reason, in the present invention, the concentration is defined as 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. Therefore, in the present invention, the upper limit is set to 0.0070%. 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, it is desirable to be 0.0020% or more and 0.0040% or less .

  In order to achieve high strength with a tensile strength of 700 MPa or more, and to develop other characteristics such as weldability and weather resistance, it is necessary to add one or more of the following elements.

・ Cu: 0.1% or more, 0.5% or less
Cu improves the hardenability of 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. Therefore, in the present invention, it is defined as 0.1% or more and 0.5% or less. However, since Cu is an element that induces a surface crack called a star crack during continuous casting, when adding 0.2% or more, it is necessary to add Ni at a concentration of 1/3 or more.

・ Cr: 0.2% or more, 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 a high-strength specification with a tensile strength per 1 mm ^{2 of} 780 N class or higher is required, it is a semi-essential additive element. On the other hand, when 2.0% or more is added, problems such as weld cracking occur. Therefore, in the present invention, it is defined as 0.2% or more and 2.0% or less. However, if the weldability is important for the above reasons, the upper limit should be 1.5%.

・ Ni: 0.3% or more, 2.5% or less
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 0.3% or more, but even if added over 2.5%, the effect is saturated and weldability is deteriorated. Therefore, in the present invention, it is defined as 0.3% or more and 2.5% or less.

・ Mo: 0.1% or more, 0.8% or less
Mo improves the hardenability of the steel sheet and contributes to an increase in strength. Like Cr, it is a semi-essential additive element when high-strength specifications with a tensile strength per mm ² of 780 N class or higher are required. In order to obtain this effect, addition of 0.1% or more is necessary. However, Mo is an expensive element, which 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. Therefore, in the present invention, it is defined as 0.1% or more and 0.8% or less.

・ V: 0.01% or more, 0.10% or less
V is an effective element for increasing the strength of steel by forming solid solution and carbonitride in ferrite in steel. Therefore, it is necessary to add 0.01% or more. However, if the content of V exceeds 0.10%, the precipitation state in the weld heat affected zone changes and adversely affects toughness. Moreover, when it adds excessively, it will precipitate as VN inside a slab, and will cause a slab surface crack. Therefore, in the present invention, it is defined as 0.01% or more and 0.10% or less.

・ Nb: 0.005% or more, 0.050% or less
Nb is an element that forms carbonitrides in steel and is effective in increasing the strength of steel and improving toughness. For that purpose, it is necessary to add 0.005% or more. It is also used to control the microstructure of steel sheets by controlling solid solution and precipitation, especially in TMCP (Thermo-Mechanical Control Process). In order to obtain this effect, it is necessary to add 0.005% or more. However, if it contains 0.050% or more, it does not dissolve at the time of heating, and the microstructure cannot be controlled. Moreover, when it adds excessively, it will precipitate as NbC inside a slab, and will cause a slab surface crack. For this reason, in this invention, it prescribed | regulated as 0.005% or more and 0.050% or less.

  Furthermore, Ca, unlike other constituent elements, does not have a significant effect on the material properties of steel, but has the effect of preventing nozzle clogging during continuous casting, and may be added for this purpose. In addition, when Ca is added to steel, the effect of reducing the S concentration and preventing the formation of MnS is obtained, so it may be added to control the form of sulfide.

-Ca: 0.0005% or more, 0.0060% or less To obtain the above effect, Ca needs to be added by 0.0005% or more. On the other hand, even if added over 0.0060%, the effect is saturated and not only increases the manufacturing cost but also promotes nozzle clogging. Therefore, in the present invention, it is defined as 0.0005% or more and 0.0060% or less.

  In addition to the above elements, if necessary, other elements such as W, S, Se, Te, rare earth metals, and Mg may be added to the steel in accordance with other requirements in terms of material characteristics. The effect of prevention does not change.

  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.

  In the present invention, in addition to the chemical composition of steel, {([% N] /14.0) − ([% Ti] /47.9)} × ε ≦ 0.29 × {([% Al] /27.0) + ([% B] /10.8)}+0.0005, Al, B, N, Ti concentrations satisfying the relationship, and total strain ε (%) accompanying bending and straightening are specified.

  Al and B, which have a high affinity with N, form nitrides at the austenite grain boundaries, which makes the steel more susceptible to embrittlement. Furthermore, these Al and B nitrides are more likely to precipitate dynamically when tensile strain acts. Therefore, compared with these Al and B, the amount of solid solution N can be reduced by adding Ti which has a large affinity with N and easily forms nitrides from a high temperature range of about 1300 to 1400 ° C. Even when tensile strain is applied, the formation of nitrides of Al and B that cause embrittlement can be suppressed.

  When the continuous casting slab of steel containing B of the present invention is continuously cast using a vertical bending type or curved type continuous casting machine, the slab is subjected to bending or straightening distortion in the continuous casting machine. Then, continuous casting can be performed by setting the slab surface temperature in the region from the corner of the slab to a point between 100 and 250 mm to be 750 ° C. or higher. This is the continuous casting method of the present invention.

  In the continuous casting method of the present invention, the slab surface temperature in the region from the corner of the slab to a point between 100 and 250 mm within the region where the slab is subjected to bending or straightening distortion in the continuous casting machine is 750 ° C. or higher. This is because it is easy to become brittle when tensile strain acts in a temperature range of 600 ° C. or higher and lower than 750 ° C. Moreover, it is because the area | region from the corner of a slab to the point between 100-250 mm tends to be in an overcooling state compared with the center part of a slab.

  The area from the corner of the slab to the point between 100 and 250 mm varies depending on the width of the mold used for continuous casting. If the mold width is 1800 mm, the area from the corner of the slab to 100 mm, the mold width is In the case of 2300 mm, the region is 250 mm from the corner of the slab, and in the case where the mold width is between 1800 mm and 2300 mm, the region is obtained by a linear function between 100 mm and 250 mm from the corner of the slab.

Next, examples of the present invention will be described.
Molten steel blown in a converter with a capacity of 270 tons is treated with a ladle and RH, then cast with a vertical bending type continuous casting machine with a vertical section length of 2.5 m, and a thickness of 250 mm or 300 mm, width 1800- A 2300 mm slab was obtained. The composition of the steel that has been cast and rolled is shown in Tables 2 and 3 below. The unit is mass%, and the balance of each steel is Fe and impurities.

  The casting speed is 0.45 to 1.20 m / min, and the specific water amount for secondary cooling is 0.7 to 0.8 liter / kg-steel. The slab surface temperature is a region from the corner of the slab to a point between 100 and 250 mm, and is non-contact at the position of the bending part and the correction part on the surface in the drawing direction of the slab (inside the arc) Measured with a type radiation thermometer.

  The obtained slabs are stacked in layers, covered with a gradual cooling cover, gradually cooled to room temperature over about 150 hours, the surface is turned with a grinder about 1 to 2 mm, and the dye penetration test specified in JIS Z 2343 The presence of cracks was visually inspected by a test, the so-called color check method.

  Further, this slab was heated to 1150 to 1250 ° C. and hot-rolled with a thick plate rolling mill under a condition that the finishing temperature was about 800 ° C. to obtain a thick steel plate having a thickness of 50 mm. The surface defect after rolling was visually observed on the surface of the plate cooled after rolling.

  The degree of flaws including surface cracks was evaluated in four levels (0, 1, 2, 3) of values (flaw index) that indexed the degree of occurrence of surface flaws.

  A haze index of 0 (zero) means that there was no flaw in both the slab and the steel plate. Moreover, when the iron index is 1, the number of iron slabs in the slab is so small that it can be easily removed by care, and the steel slab has no slag.

  On the other hand, when the crack index is 2, cracks are found in the As Cast stage after casting. In this case, rolling was not performed. Furthermore, a crack index of 3 is a case where there is no defects in the slab, and defects are found in the steel plate stage.

  Therefore, the wrinkle occurrence index 0, 1 corresponds to a level that does not hinder the product, and the wrinkle index 2, 3 corresponds to a level that cannot be used as a product.

  FIG. 1 shows {([% Al] /27.0) + ([% B] /10.8)} and {([% N] /14.0) − ([ % Ti] /47.9)} × ε.

  Examples 1 to 7 shown in Tables 2 and 3 are the chemical composition of steel defined in claim 1 and {([% N] /14.0) − ([% Ti] /47.9)} × ε ≦ 0.29. This is an example that satisfies the relationship of × {([% Al] /27.0) + ([% B] /10.8)} + 0.0005.

  On the other hand, in Comparative Examples 1 to 5, the chemical composition of the steel satisfies the definition of claim 1, but {([% N] /14.0) − ([% Ti] /47.9)} × ε ≦ 0.29 × {( In this example, the relationship of [% Al] /27.0) + ([% B] /10.8)} + 0.0005 is not satisfied.

  From Tables 2 and 3 and FIG. 1, the chemical composition of steel and {([% N] /14.0)-([% Ti] /47.9)} × ε ≦ 0.29 × {([% Al] /27.0) + ([ % B] /10.8)} + 0.0005, it can be seen that wrinkles at a level that cannot be used as a product occur in the comparative example.

  In particular, in Example 3, it was possible to achieve a level of wrinkle occurrence index 1 that was easy to remove, but even in the same component as in Example 3, Comparative Example 4 had a large total distortion amount due to correction and bending. Therefore, {([% N] /14.0) − ([% Ti] /47.9)} × ε ≦ 0.29 × {([% Al] /27.0) + ([% B] /10.8)} + 0.0005 Since the relationship was not satisfied, the soot index was 3 levels.

  In the compositional condition of the steel shown in Example 2, the surface temperature of the region from the corner of the slab during bending and straightening to the point between 100 and 250 mm is changed. Examples 8 and 9 and Comparative Example 6 of the prescribed continuous casting method are shown in Table 4 below.

  Examples 8 and 9 are examples satisfying the provisions of claim 2, and no wrinkles were generated and no surface maintenance was required. On the other hand, Comparative Example 6 is an example that does not satisfy the provisions of claim 2. In Comparative Example 6, since the slab surface temperature during casting did not satisfy claim 2, the generation of wrinkles could not be eliminated, and surface flaws requiring maintenance remained.

  Needless to say, the present invention is not limited to the above-described examples, and the embodiments may be appropriately changed within the scope of the technical idea described in the claims.

Claims (2)

% By mass
C: 0.05% or more, 0.18% or less,
Si: 0.10% or more, 0.40% or less,
Mn: 0.5% or more, 2.0% or less,
P: 0.020% or less,
S: 0.003% or less,
Ti: 0.005% or more, 0.030% or less,
Al: 0.005% or more, 0.060% or less,
B: 0.0005% or more, 0.0050% or less,
as well as,
N: 0.0015% or more, containing 0.0070% or less,
further,
Cu: 0.1% or more, 0.5% or less,
Cr: 0.2% or more, 2.0% or less,
Ni: 0.3% or more, 2.5% or less,
Mo: 0.1% or more, 0.8% or less,
V: 0.01% or more, 0.10% or less,
Nb: 0.005% or more, 0.050% or less,
Ca: 0.0005% or more, 0.0060% or less,
A method for producing a continuous cast slab of steel containing B, wherein one or more of them are contained, the balance is Fe and impurities, and satisfies the following formula.
{([% N] /14.0)-([% Ti] /47.9)} × ε ≦ 0.29 × {([% Al] /27.0) + ([% B] /10.8)} + 0.0005
Where ε is the total strain (%) due to bending and straightening of the slab during continuous casting A method for producing the continuous cast slab according to claim 1 using a vertical bending mold or a curved continuous casting machine,
Within the region where the slab is subjected to bending or straightening distortion in the continuous casting machine, the slab surface temperature in the region from the corner of the slab to a point between 100 and 250 mm is 750 ° C. or more B A continuous casting method for steel containing steel.

Images