JP7701599B2 - Continuous casting method for steel - Google Patents

Description

This application discloses a method for continuous casting of steel.

In recent years, from the viewpoint of preventing global warming, efforts to reduce _CO2 emissions have been actively undertaken in various fields. In the steel industry, the ironmaking process that uses a large amount of waste scrap as a raw material for ironmaking has attracted attention, and technological development is progressing. On the other hand, most scrap contains high concentrations of tramp elements such as Cu and Sn, and it is known that these elements are difficult to remove from molten steel.

Steels containing Cu tend to have poor hot workability. Therefore, when steel containing Cu is continuously cast under normal steel continuous casting conditions, cracks may occur on the surface of the slab. This is thought to be because when the steel is exposed to oxygen in the atmosphere during continuous casting and oxidized, liquid Cu is generated between the scale (iron oxide) and the base steel, which penetrates into the grain boundaries of the steel and reduces the interface strength (Non-Patent Document 1). In addition, Sn reduces the solubility of Cu in steel, thereby promoting the phenomenon of cracking caused by Cu, so that even steel in which Sn and Cu coexist is prone to surface cracking problems in the slab (Non-Patent Document 2).

This phenomenon is called surface embrittlement, and is believed to be caused by the fact that Cu and Sn are less easily oxidized than Fe, so that Cu and Sn concentrate in a metallic state during the scale growth process, and by the low solid solubility of Cu in Fe. On the other hand, Cu and Sn are difficult to remove during the steel refining process. In order to solve the problem of surface cracking of cast slabs due to embrittlement, it is effective to prevent Cu and Sn from being mixed into the steel, or to add Ni, an element that increases the solubility of Cu in steel. In particular, in the current recycling-based society where a large amount of scrap containing a lot of Cu is used, there is an increasing need to make Cu harmless by adding Ni. However, Ni is a rare and expensive element, and can significantly change steel properties such as mechanical properties and hardenability, so there are high expectations for a Cu and Sn harmless technology that does not rely on Ni addition or can keep the amount of Ni added to a very small amount.

In light of this background, Patent Document 1 discloses a continuous casting method as a technology for preventing surface embrittlement of the slab, in which the shape of the mold inner surface near the molten steel surface is an inverse taper shape with a reverse taper value of 2 to 10% widening downward in the direction of drawing the slab, and the shape of the mold inner surface below the reverse taper part is a forward taper shape narrowing in the direction of drawing the slab, the forward taper value being in the range of 0 to 1%, and mold flux is used that has a crystallization temperature of 900°C or less or that does not crystallize, and the contact angle between the mold flux and the steel is 70 degrees or less.

Patent Document 2 also discloses a continuous casting method that is characterized by forming a coating layer of nickel oxide on the surface of the cast piece while supplying a mold flux containing Ni oxide.

However, all of these methods aim to prevent oxidation of the slab surface by using mold flux, and depending on the type of continuous casting machine and the secondary cooling method, the adhesion of mold flux to the slab surface may not be stable, making it difficult to fully enjoy the benefits.

Patent Document 3 proposes a method for preventing surface cracks in a continuously cast slab, which is characterized by cooling the surface of the continuously cast slab at a cooling rate of 300° C./s or more from a temperature range equal to or higher than _{the Ar3} transformation point to a temperature range equal to or lower than the _Ar1 transformation point, and then reheating the surface of the continuously cast slab to a temperature range equal to or higher than the Ar3 transformation point.

Patent Document 4 also discloses a method for continuously casting steel containing 0.1 to 2.0% Cu, which includes a series of steps including a step of quenching a slab pulled out of a mold at a cooling rate of 3 to 20°C/s until the surface temperature of the slab reaches a quenching temperature T1 (°C) below the Ar3 point, a step of raising the surface temperature of the slab at the quenching temperature T1 to a reheating temperature T2 (°C) above the Ar3 point, a step of cooling the slab at the reheating temperature T2 until the surface temperature reaches a straightening temperature T3 (°C), and a step of straightening the slab at the straightening temperature T3 by applying a straightening strain amount ε (%), and the temperatures T1 to T3 and the straightening strain amount ε satisfy a predetermined formula.

"Materials Transactions" vol.43, (2002), No.3, pp.292-300 "Feram" vol.7, (2002), No.4, pp.18-22

JP 2004-202523 A Special Publication No. 2018-520004 JP 2007-245232 A JP 2015-217435 A

In conventional technology, there is room for improvement in suppressing surface cracks in the cast slab while limiting the addition of Ni to the steel during continuous casting of steel containing Cu and Sn.

As one of the means for solving the above problems, the present application provides:
In mass%,
C: 0.03% or more and 0.20% or less,
Si: 0.01% or more and 1.00% or less,
Mn: 0.10% or more and 2.50% or less,
P: 0.040% or less,
S: 0.030% or less,
Cu: 0.05% or more and 0.50% or less,
Sn: 0% or more and 0.025% or less,
Ni: 0% or more and 0.05% or less,
Al: 0.005% or more and 0.100% or less,
Cr: 0% or more and 0.50% or less,
Mo: 0% or more and 0.05% or less, and
N: 0.0150% or less,
A method for continuously casting a steel slab containing a steel material having a straightening point, the method comprising the steps of:
Cooling the surface temperature of the slab removed from the mold to _Ar1 temperature or lower;
Raising the surface temperature of the slab cooled to the Ar ₁ temperature or lower to the Ac ₃ temperature or higher; and
The temperature of the surface of the slab, which has been raised to or above the _Ac3 temperature, is set to or below a temperature Tb°C expressed by the following condition at the straightening point;
A method for continuous casting of steel is disclosed, comprising:

When [Cu] + 4 [Sn] ≦ 0.15: Tb = 1050
When 0.15<[Cu]+4[Sn]≦0.25: Tb=1000
When 0.25<[Cu]+4[Sn]≦0.35: Tb=950
When 0.35<[Cu]+4[Sn]: Tb=900
Here, [Cu] and [Sn] are the Cu and Sn contents (mass%) in the steel.

In the method of the present disclosure, the steel comprises, in mass %,
Ti: 0% or more and 0.020% or less,
V: 0% or more and 0.20% or less,
Nb: 0% or more and 0.030% or less,
Zr: 0% or more and 0.010% or less,
Ca: 0% or more and 0.010% or less,
Mg: 0% or more and 0.010% or less,
REM: 0% or more and 0.010% or less, and
B: 0% or more and 0.0040% or less,
may contain one or more of the above.

The technology disclosed herein makes it easier to suppress surface cracks on the slab during continuous casting of steel containing Cu and Sn while limiting the amount of Ni added to the steel.

1 illustrates a schematic diagram of an example of a continuous casting machine. Schematic diagram of thermal processing pattern in a model experiment using a high-temperature tensile apparatus. 1 shows the relationship between the amount of Cu contained in the steel, the tensile temperature in a model experiment, and the presence or absence of surface cracks. 1 shows the relationship between the amount of Cu and Sn contained in the steel, the tensile temperature in a model experiment, and the presence or absence of surface cracks. Schematic diagram of thermal processing pattern in a model experiment using a high-temperature tensile apparatus. 1 shows the relationship between the amount of Cu and Sn contained in the steel, the tensile temperature in a model experiment, and the presence or absence of surface cracks. Schematic diagram of thermal processing pattern in a model experiment using a high-temperature tensile apparatus. 1 shows the relationship between the amount of Cu and Sn contained in the steel, the tensile temperature in a model experiment, and the presence or absence of surface cracks.

The continuous casting method of steel of the present disclosure will be described with reference to FIG. 1. In FIG. 1, cooling spray nozzles and the like are omitted for clarity. The cooling spray nozzles are provided, for example, between the support rolls between directly below the mold 10 and before the straightening point 20, and can spray cooling water from both sides of the slab 1. Although a vertical bending type continuous casting machine 100 is illustrated in FIG. 1, the continuous casting method of the present disclosure can be applied to any continuous casting machine having a straightening point. For example, a curved type continuous casting machine may be used. The "straightening point" refers to a point where distortion is applied to correct the casting direction of the slab 1 from a curved direction to a horizontal direction. Straightening may be performed at multiple points. The configuration of the continuous casting machine 100 equipped with the mold 10, the straightening point 20, etc. may be the same as the conventionally known configuration, so a detailed description will be omitted here.

As shown in FIG. 1, the method for continuous casting of steel according to the present embodiment includes the following steps:
In mass%,
C: 0.03% or more and 0.20% or less,
Si: 0.01% or more and 1.00% or less,
Mn: 0.10% or more and 2.50% or less,
P: 0.040% or less,
S: 0.030% or less,
Cu: 0.05% or more and 0.50% or less,
Sn: 0% or more and 0.025% or less,
Ni: 0% or more and 0.05% or less,
Al: 0.005% or more and 0.100% or less,
Cr: 0% or more and 0.50% or less,
Mo: 0% or more and 0.05% or less, and
N: 0.0150% or less,
The method for continuously casting a steel slab 1 containing Fe and impurities by using a continuous casting machine 100 having a straightening point 20,
Cooling the surface temperature of the slab 1 extracted from the mold 10 to the _Ar1 temperature or lower;
The temperature of the surface of the slab 1 cooled to the Ar ₁ temperature or lower is increased to the Ac ₃ temperature or higher; and
The temperature of the surface of the cast 1, which has been raised to the _Ac3 temperature or higher, is set to a temperature Tb°C or lower expressed by the following condition at the correction point 20;
Includes.

When [Cu] + 4 [Sn] ≦ 0.15: Tb = 1050
When 0.15<[Cu]+4[Sn]≦0.25: Tb=1000
When 0.25<[Cu]+4[Sn]≦0.35: Tb=950
When 0.35<[Cu]+4[Sn]: Tb=900
Here, [Cu] and [Sn] are the Cu and Sn contents (mass%) in the steel.

1. Chemical composition of steel First, the chemical composition of the steel in this embodiment will be described. In this embodiment, the steel contains, by mass%, C: 0.03% to 0.20%, Si: 0.01% to 1.00%, Mn: 0.10% to 2.50%, P: 0.040% or less, S: 0.030% or less, Cu: 0.05% to 0.50%, Sn: 0% to 0.025%, Ni: 0% to 0.05%, Al: 0.005% to 0.100%, Cr: 0% to 0.50%, Mo: 0% to 0.05%, and N: 0.0150% or less.

(C: 0.03% or more and 0.20% or less)
C is the most fundamental element that affects not only the static strength of steel, but also its fatigue strength, toughness, and ductility. If the C content is too low, no significant improvement in these properties is observed, leading to increased decarburization costs. In this regard, the C content may be 0.03% or more, or 0.05% or more. If the C content is too high, the toughness of the steel may be deteriorated. In this regard, the C content may be 0.20% or less, or 0.17% or less.

(Si: 0.01% or more and 1.00% or less)
Si is an element that can increase the strength of steel by adding it in an appropriate amount. When the Si content is 0.01% or more, such an effect is easily obtained. The Si content may be 0.05% or more, or 0.10% or more. On the other hand, if the Si content is too high, the toughness and workability of the steel may deteriorate. In this regard, the Si content may be 1.00% or less, 0.80% or less, 0.60% or less, 0.40% or less, 0.30% or less, or 0.20% or less.

(Mn: 0.10% or more and 2.50% or less)
Mn, like Si, is an element that can increase the strength of steel by proper addition. When the Mn content is 0.10% or more, such an effect is easily obtained. The Mn content may be 0.25% or more, 0.50% or more, or 0.75% or more. On the other hand, if the Mn content is too high, the toughness and workability of the steel may deteriorate. In this regard, the Mn content may be 2.50% or less, 2.00% or less, 1.75% or less, or 1.50% or less.

(P: 0.040% or less)
P is an element that promotes the occurrence of cracks on the surface of a slab during continuous casting. That is, if the P content is too high, it becomes difficult to suppress cracks on the slab. The P content may be 0.040% or less, 0.030% or less, 0.020% or less, or 0.015% or less. The lower limit of the P content is not particularly limited, and may be 0%, more than 0%, 0.001% or more, or 0.002% or more.

(S: 0.030% or less)
Like P, S is an element that promotes the occurrence of cracks on the surface of a cast slab during continuous casting. In addition, S is an element that deteriorates the bending workability of steel, so the lower the content, the better. The S content may be 0.030% or less, 0.020% or less, or 0.010% or less. The lower limit of the S content is not particularly limited, and may be 0%, more than 0%, 0.001% or more, or 0.002% or more.

(Cu: 0.05% or more and 0.50% or less)
If the Cu content is too low, the amount of liquid phase generated by oxidation of the steel material is sufficiently small, and cracks due to embrittlement are unlikely to occur, that is, the problem to be solved is unlikely to occur. The method according to the present embodiment is a technology for suppressing surface cracks on a slab during continuous casting while suppressing the addition of Ni to a steel containing a certain amount of Cu. In this regard, the Cu content may be 0.05% or more, 0.07% or more, or 0.09% or more. On the other hand, if the Cu content is too high, it adversely affects the material properties of the steel. In this regard, the Cu content may be 0.50% or less, 0.45% or less, or 0.40% or less.

(Sn: 0% or more and 0.025% or less)
Sn is desirably not mixed in as much as possible because it greatly lowers the liquid phase stabilization temperature and widens the embrittlement temperature range. If the Sn content is too high, red embrittlement cracking is likely to occur during continuous casting, and a large amount of Ni is required to suppress this. In this regard, the Sn content may be 0.025% or less, 0.022% or less, or 0.020% or less. The lower limit of the Sn content is not particularly limited, and may be 0%, may be more than 0%, may be 0.001% or more, or may be 0.002% or more.

(Ni: 0% or more and 0.05% or less)
Ni is known to have the effect of suppressing red embrittlement cracking caused by Cu and Sn, but it is an expensive element, and it is desirable to add as little as possible. If the Cu content and Sn content are within the above ranges, the cast slab surface cracking can be sufficiently suppressed by controlling the cast slab surface temperature described later, so there is no particular need to add Ni in this embodiment. The Ni content is sufficient at a concentration of the amount of Ni mixed in from scrap, for example, 0.05% or less, and may be 0.04% or less, or 0.03% or less. The lower limit of the Ni content is not particularly limited, and may be 0%, may be more than 0%, or may be 0.01% or more.

(Al: 0.005% or more and 0.100% or less)
Al is an element used for deoxidation purposes. If the Al content is too low, it is difficult to obtain the effect of deoxidation. In this regard, the Al content may be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, if the Al content is too high, problems such as nozzle clogging during casting and deterioration of the performance of the steel due to oxide-based inclusions remaining in the steel are likely to occur. In this regard, the Al content may be 0.100% or less, 0.075% or less, or 0.050% or less.

(Cr: 0% or more and 0.50% or less)
Cr is a useful element for increasing the strength of steel. However, if the Cr content is too high, not only will the effect be saturated, leading to an increase in cost, but the Ar ₁ temperature will be low, making it difficult to refine the surface structure of the cast slab. In this regard, the Cr content may be 0.50% or less, 0.40% or less, 0.30% or less, or 0.20% or less. The lower limit of the Cr content is not particularly limited, and may be 0%, more than 0%, or 0.01% or more.

(Mo: 0% or more and 0.05% or less)
Mo is an element that enhances hardenability. If the Mo content is too high, the Ar ₁ temperature will be low, similar to Cr, and it will be difficult to refine the surface structure of the cast slab. In this regard, the Mo content may be 0.05% or less, 0.04% or less, 0.03% or less, or 0.02% or less. The lower limit of the Mo content is not particularly limited, and may be 0%, more than 0%, or 0.01% or more.

(N: 0.0150% or less)
N is an element that affects the mechanical properties of steel, reduces hot ductility, and is also an element that causes surface defects during continuous casting or hot rolling. N is mainly removed in the degassing process of secondary refining. The N content may be 0%, but from the viewpoint of reducing refining costs, it may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, or may be 0.0040% or more. On the other hand, if the N content is too high, it will lead to coarsening of nitride-based inclusions, which will cause a decrease in the fatigue strength of the steel. In this regard, the N content may be 0.0150% or less. In addition, from the viewpoint of the cleanliness of the steel, the N content may be 0.0080% or less.

In this embodiment, the steel may further contain one or more of the following elements to achieve the desired properties of the product. Since all of these elements sensitize III region embrittlement, the method according to this embodiment is particularly effective when casting steel containing these elements. That is, in this embodiment, the steel may contain, by mass%, one or more of Ti: 0% to 0.020%, V: 0% to 0.20%, and Nb: 0% to 0.030%.

In this embodiment, the steel may further contain one or more of the following optional elements: Zr: 0% to 0.010%, Ca: 0% to 0.010%, Mg: 0% to 0.010%, REM: 0% to 0.010%, and B: 0% to 0.0040%.

(Ti: 0% or more and 0.020% or less)
Ti not only has a deoxidizing effect like Al, but also forms nitrides with high thermal stability, which can refine the structure of steel in a heating furnace. On the other hand, if the Ti content is too high, the amount of nitride precipitates produced increases, and cracking sensitivity due to III region embrittlement increases. In addition, there is a risk of frequent nozzle clogging due to oxides during casting. In this regard, the Ti content may be 0.020% or less, or 0.015% or less. The lower limit of the Ti content is not particularly limited, and may be 0%, may be more than 0%, may be 0.001% or more, or may be 0.005% or more.

(V: 0% or more and 0.20% or less)
V, like Ti, is an element that generates nitrides and is used to improve strength. On the other hand, if the V content is too high, the nitrides tend to grow coarsely, which causes a decrease in fatigue strength. In this regard, the V content may be 0.20% or less, or 0.10% or less. The lower limit of the V content is not particularly limited, and may be 0%, may be more than 0%, or may be 0.01% or more.

(Nb: 0% or more and 0.030% or less)
Nb is an element that generates nitrides, etc., like Ti. In addition, a small amount of Nb has the effect of significantly increasing the strength of steel. On the other hand, if the Nb content is too high, not only will the above effect saturate, but it will also cause frequent cracking during casting. In this regard, the Nb content may be 0.030% or less, or 0.020% or less. The lower limit of the Nb content is not particularly limited, and may be 0%, may be more than 0%, or may be 0.001% or more.

(Zr: 0% or more and 0.010% or less)
When added appropriately, Zr has the effect of forming oxides that become inoculation nuclei for the solidification structure and increasing the equiaxed crystal ratio. On the other hand, if the Zr content is too high, coarse oxide-based inclusions are formed, which not only become the starting point of fatigue fracture but also may cause clogging of the nozzle used to supply molten steel to the mold. In this regard, the Zr content may be 0.010% or less, or 0.005% or less. The lower limit of the Zr content is not particularly limited, and may be 0%, more than 0%, or 0.001% or more.

(Ca: 0% or more and 0.010% or less)
Ca has the effect of modifying Al ₂ O ₃ and suppressing the coarsening of oxide-based inclusions. On the other hand, if the Ca content is too high, coarse oxide-based inclusions mainly composed of CaO-Al ₂ O ₃ are formed, which may become the starting point of fatigue fracture. In this respect, the Ca content may be 0.010% or less, or 0.005% or less. The lower limit of the Ca content is not particularly limited, and may be 0%, more than 0%, or 0.001% or more.

(Mg: 0% or more and 0.010% or less)
Mg, like Ca, has the effect of modifying Al ₂ O ₃ and suppressing the coarsening of oxide-based inclusions. It also acts on sulfide-based inclusions and has the effect of lowering the aspect ratio. On the other hand, if the Mg content is too high, coarse cluster-like oxide-based inclusions mainly composed of MgO are formed, which may become the starting point of fatigue fracture. In this regard, the Mg content may be 0.010% or less, or 0.005% or less. The lower limit of the Mg content is not particularly limited, and may be 0%, may be more than 0%, or may be 0.001% or more.

(REM: 0% or more and 0.010% or less)
REM also has the effect of modifying _Al2O3 and suppressing the coarsening of oxide-based inclusions. On the other hand, if the REM content is too high, the cleanliness of the steel _may be reduced and the toughness of the steel may be deteriorated. In this respect, the REM content may be 0.010% or less, or 0.005% or less. The lower limit of the REM content is not particularly limited, and may be 0%, more than 0%, or 0.001% or more. In this application, REM refers to rare earth elements such as La and Ce, and any one or two or more of these REMs may be used.

(B: 0% or more and 0.0040% or less)
A small amount of B has the effect of improving the mechanical properties of steel. On the other hand, if the B content is too high, the effect is saturated and cracks are likely to occur during casting. In this regard, the B content may be 0.0040% or less, or 0.0030% or less. The lower limit of the B content is not particularly limited, and may be 0%, more than 0%, or 0.0001% or more.

In this embodiment, the steel may contain the above-mentioned components, with the balance being Fe and impurities. Impurities are components that are mixed in due to various factors in the steel manufacturing process, including raw materials such as ore and scrap.

2. Secondary cooling of cast slabs When Cu-containing steel is oxidized, red embrittlement occurs in the range of approximately 1050°C to 1200°C, and the lower limit temperature is expanded downward by an increase in Cu concentration and the coexistence of Sn. Therefore, during continuous casting using a continuous casting machine having a straightening point such as a curved or vertical bending type, it is considered possible to prevent red embrittlement cracking on the slab surface by avoiding this temperature range at the straightening point where a large strain is applied to the slab surface.

However, if the surface temperature of the slab at the straightening point drops, it enters the embrittlement temperature range (so-called III region embrittlement) caused by precipitates that precipitate along the gamma grain boundaries when the slab is cooled and film-like ferrite, increasing the susceptibility to cracking. The embrittlement temperature is generally in the range of 700°C to 950°C, depending on the composition of the steel. In other words, when casting steel containing Cu and Sn using a normal continuous casting method, it is necessary to control the surface temperature of the slab at the straightening point within a very narrow range to avoid cracking, and depending on the composition of the steel being cast, there may be no appropriate straightening point temperature, making it difficult to avoid cracking.

The inventors have thoroughly investigated the above problems and discovered a method to fundamentally prevent both red embrittlement cracking and III region cracking by optimizing the secondary cooling conditions during continuous casting to create a cast slab surface structure that is less susceptible to cracking, and by properly controlling the correction point temperature, even if liquid Cu forms between the scale and the base steel.

Specifically, the structure of the slab surface layer is transformed into ferrite-pearlite or bainite in the secondary cooling zone, and then reheated to Ac ₃ or higher to cause reverse transformation, thereby refining the austenite structure. This not only prevents III region cracking, but also increases the number of liquid Cu-based alloy infiltration sites, thereby significantly reducing the crack depth. Furthermore, by controlling the correction point temperature to a value or lower determined according to the Cu and Sn concentrations of the steel, red embrittlement cracking can be prevented without excessively cooling the slab. As described above, in the continuous casting method according to this embodiment, it is important that, from just below the mold 10 to the correction point 20, the surface temperature of the slab 1 is cooled to the A ₁ temperature or lower, and then raised to the Ac ₃ temperature or higher, and the surface temperature of the slab 1 at the correction point is set to the following temperature Tb (° C.) or lower according to the Cu content [Cu] (mass %) and Sn content [Sn] (mass %) of the steel. In the continuous casting method according to the present embodiment, since surface cracking of the cast slab can be suppressed by the above-mentioned mechanism without increasing the solubility of Cu in steel, it is not necessary to add Ni to the steel, or the amount of Ni added to the steel can be made extremely small.

When [Cu] + 4 [Sn] ≦ 0.15: Tb = 1050
When 0.15<[Cu]+4[Sn]≦0.25: Tb=1000
When 0.25<[Cu]+4[Sn]≦0.35: Tb=950
When 0.35<[Cu]+4[Sn]: Tb=900

2.1 Cooling to Ar ₁ temperature or less, then raising to Ac ₃ temperature or more As described above, in this embodiment, the structure of the slab surface layer is transformed to ferrite-pearlite or bainite in the secondary cooling zone, and then reheated to Ac ₃ or more to cause reverse transformation, thereby refining the austenite structure. This not only prevents III region cracking, but also increases the number of liquid Cu-based alloy infiltration sites, thereby significantly reducing the crack depth. If one or both of the above conditions are not satisfied, a highly ductile reverse transformation structure cannot be obtained in the entire slab surface layer. That is, in order to suppress cracking, it is necessary to cool to Ar ₁ or less in the cooling process after mold withdrawal (referred to as step 1), and then reheat to Ac ₃ or more before reaching the correction point in the subsequent reheating process (referred to as step 2).

When the slab 1 pulled out of the mold 10 is cooled to a temperature of Ar ₁ or less, various methods such as water cooling and gas-liquid two-phase cooling can be used. In this embodiment, there is no particular limit to the cooling rate of the surface of the slab 1, and a cooling rate of, for example, 1°C/s or less is also acceptable. However, from the viewpoint of facilitating subsequent reheating, i.e., re-increasing the slab surface temperature, without applying external energy, the cooling rate may be 3°C/s or more. On the other hand, if the cooling rate is excessively high, the temperature distribution on the slab surface may become uneven, which may promote the occurrence of cracks due to thermal stress. In this respect, the cooling rate may be 20°C/s or less.

The reheating of the surface of the slab 1 cooled to a temperature of Ar ₁ or less occurs when the amount of heat transferred from the inside of the slab 1 exceeds the amount of heat released from the surface of the slab 1. The reheating of the surface of the slab 1 can be performed relatively easily by relaxing the cooling of the secondary cooling zone. Alternatively, a heat source or high-frequency induction heating equipment may be arranged around the casting line to heat the surface. The reheating speed (temperature rise speed) of the slab 1 is not particularly limited, and any reheating speed will produce the desired effect. In addition, the surface of the slab 1 may be reheated to Ac ₃ or more, but from the viewpoint of further suppressing the variation in the surface temperature and structure of the slab 1, the maximum temperature after reheating may be Ac ₃ +30°C or more. If the reheating temperature is too high, there is a risk that the austenite grains will become coarse again. In this regard, the maximum temperature after reheating may be 1200°C or less.

_Ar1 and _Ac3 can be measured using a transformation point recorder (Formaster device) or the like. Alternatively, _Ar1 and _Ac3 can be specified using the following formulas (a) and (b) proposed in a prior document (Tatsuo Kunitake: Heat Treatment, 43, p. 99 (2003)).

Ar ₁ =(52C+122Si+66Cu+6Cr)-(65Mn+36Ni+58Mo)-73.2/log((Ac3-500)/v)+713 (a)
Ac ₃ =(32Si+17Mo)-(231C+20Mn+40Cu+18Ni+15Cr)+912 (b)
Here, v in the formula (a) is the average cooling rate (°C/s) from _Ac3 to the cooling target temperature.

2.2 Temperature at the straightening point When the straightening point 20 of the continuous casting machine is in a temperature range where Cu and Sn are liquid, the liquid Cu and Sn may penetrate into the austenite grain boundary when subjected to straightening strain. The greater the penetration depth, the greater the crack susceptibility. On the other hand, when the amount of liquid Cu and Sn is small, they do not penetrate deeply into the grain boundary even when the slab is deformed, and cracks are less likely to occur. In this respect, in this embodiment, the surface temperature of the slab 1 at the straightening point 20 is set to a temperature Tb or lower specified according to the amount of Cu and Sn, thereby preventing the occurrence of cracks due to red embrittlement. Furthermore, due to the effects of steps 1 and 2 described above, it is possible to simultaneously prevent III region cracks, which are a concern when the straightening point temperature is lowered. In addition, when the continuous casting machine has multiple straightening points 20, the surface temperature of the slab 1 may be set to Tb or lower at the straightening point 20 closest to the meniscus.

The lower limit of the surface temperature of the slab 1 at the straightening point 20 is not particularly limited, and may be any temperature that does not cause problems in the operation of continuous casting. For example, the surface temperature of the slab 1 at the straightening point 20 may be 500°C or higher. The surface temperature of the slab 1 at the straightening point 20 may be controlled by the cooling conditions for the slab 1 after reheating until it reaches the straightening point 20, the roll temperature at the straightening point 20, etc.

According to the inventor's knowledge, when focusing only on the amount of Cu contained in the steel, it is difficult to set a straightening point temperature that can adequately prevent cracks caused by red embrittlement. In contrast, in this embodiment, as described above, the temperature of the slab surface at the straightening point is specified taking into account not only the amount of Cu but also the amount of Sn. According to the inventor's knowledge, by specifying the upper limit Tb of the temperature at the straightening point according to the value of the sum of the amount of Cu and four times the amount of Sn ([Cu] + 4 [Sn]), cracks caused by red embrittlement can be adequately prevented.

3. Supplementary Note: The method according to the present embodiment is intended to be used mainly for continuously casting molten steel using scrap as a raw material, but it can also be applied without any problem to continuous casting using molten steel adjusted to a predetermined composition using iron ore or reduced iron without using scrap. The method according to the present embodiment can also be applied to any shape of cast piece, such as slab, rectangular bloom, or round bloom, as long as the casting is performed using a continuous caster having a straightening point. In the method according to the present embodiment, the continuous casting conditions (casting speed, etc.) other than the cooling conditions, reheating conditions, and straightening point temperature are not particularly limited.

As described above, according to the continuous casting method of the present embodiment, when steel containing Cu and Sn is continuously cast, it is possible to suppress III region cracking and surface cracking of the slab caused by red embrittlement while suppressing the addition of Ni to the steel. In the present application, the "surface of the slab" does not necessarily mean the entire surface of the slab. In other words, it is sufficient to perform cooling to Ar ₁ or less, reheating to Ac ₃ or more, and control the temperature at the straightening point for at least a portion of the surface of the slab where surface cracking is desired to be suppressed.

The following is an example of the present invention. The present invention is not limited to this example of conditions. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

1. Model Experiments Model experiments were conducted using a high-temperature tensile device to clarify the optimal conditions for obtaining sufficient crack prevention effects through secondary cooling and reheating of the cast piece pulled out of the mold, and through the strain application temperature at the straightening point.

A 100 mm long sample made of steel having the composition shown in Table 1 was heated to 1400°C in air to form an austenitic structure with an average grain size of 1.5 mm or more, and then quenched in a helium gas flow. The cooling rate was -10°C/s, and the cooling temperature was either the Ar ₁ temperature or a temperature 20°C higher than that. After the target temperature was reached, the sample was immediately reheated to the tensile temperature. After the specified temperature was reached, the sample was held for 60 s and then stretched. The thermal processing pattern is shown in Figure 2. The cooling conditions, reheating conditions, and tensile conditions are as follows:

Cooling condition: Cool down to Ar ₁ temperature at -10°C/s

Reheating conditions: Reheat at 5°C/s to 825-1075°C (in 25°C increments)

Tension conditions: tension speed 0.01/s, tension length 5 mm

After hot tension, the deformed parts of the test pieces were observed to see if there were any cracks. The results are shown in Figures 3 and 4. In Figure 3, the horizontal axis is the Cu concentration (mass%), and in Figure 4, the horizontal axis is the sum of the Cu concentration (mass%) and four times the Sn concentration (mass%). As is clear from Figures 3 and 4, it is clear that not only the Cu concentration but also the Sn concentration has an effect on cracking. After careful consideration of the effect of Sn on red embrittlement, it was found that it was best to express it as the sum of the Cu concentration and four times the Sn concentration ([Cu] + 4[Sn]), as shown in Figure 4, and therefore this relationship was used hereafter.

Figure 4 shows that cracking susceptibility is high in the high temperature range of 900 to 1050°C and in the low temperature range of less than 875°C. These are thought to correspond to red embrittlement and region III embrittlement associated with selective oxidation, respectively.

In order to avoid III region embrittlement and prevent cracking, it is essential not only to transform austenite into a structure such as ferrite-pearlite or bainite by cooling, but also to obtain a so-called reverse transformation structure, which is a fine austenite structure before the straightening point is reached. If the reheating temperature is less than Ac ₃ , there will remain areas where reverse transformation does not occur. Regarding the level at which cracks were observed despite cooling to Ar ₁ , it is presumed that reheating to Ac ₃ temperature or higher was not performed before reaching the tensile temperature, and the influence of the coarse structure before cooling remains, making it more likely to exhibit cracks due to straightening strain. Therefore, it was assumed that reheating to Ac ₃ or higher to form an austenite single-phase structure is effective in suppressing cracking.

To confirm the above, the thermal processing pattern of the hot tensile test was changed to the pattern shown in Figure 5, and cracks were evaluated again. As a result, as shown in Figure 6, the cracks that had occurred below 875°C were not confirmed with this thermal processing pattern.

To confirm further, samples A-5, A-6, A-7, and A-8, which showed cracks over a wide tensile temperature range, were subjected to tensile processing starting at 1400°C using the thermal processing pattern shown in Figure 7. As a result, as shown in Figure 8, not only was it not possible to suppress cracks thought to be caused by III region embrittlement, but cracks thought to be caused by red embrittlement also appeared at lower temperatures. The reason for this is presumably that the austenite grains remained coarse by the time of tension, and the liquid Cu-based alloy infiltration site penetrated deeper, increasing the cracking sensitivity.

From the results of the above model experiments, it is considered that in order to suppress III region cracking and surface cracking of a cast slab caused by red embrittlement while suppressing the addition of Ni to the steel during continuous casting of steel containing Cu and Sn, it is necessary to satisfy the following requirements.
(1) Cooling the surface temperature of the slab pulled out of the mold to _Ar1 temperature or lower. (2) Raising (reheating) the surface temperature of the slab cooled to _Ar1 temperature or lower to _Ac3 temperature or higher. (3) Reducing the surface temperature of the slab heated to _Ac3 temperature or higher to a temperature Tb°C or lower at the straightening point, which is expressed by the following condition:

When [Cu] + 4 [Sn] ≦ 0.15: Tb = 1050
When 0.15<[Cu]+4[Sn]≦0.25: Tb=1000
When 0.25<[Cu]+4[Sn]≦0.35: Tb=950
When 0.35<[Cu]+4[Sn]: Tb=900
Here, [Cu] and [Sn] are the Cu and Sn contents (mass%) in the steel.

2. Continuous casting test The validity of the results of the above model experiment was confirmed by a continuous casting test. Specifically, molten steel having the composition shown in Table 2 was melted in an electric furnace, and a slab having a width of 2000 mm and a thickness of 250 mm was produced using a continuous casting test machine (5-point correction type) with a radius of curvature of 12.0 m. The casting speed was 0.8 to 1.5 m/min. The slab pulled out of the mold was quenched in a spray cooling device with a zone length of 1.0 m installed directly below the mold. After passing through the zone, the amount of water in the secondary cooling spray was adjusted to control the recuperation, thereby varying the quenching temperature T1, the recuperation temperature T2, and the temperature T3 at the correction point (closest to the meniscus). The slab was cut into a length of 5.0 ± 0.2 m using a gas cutting machine, and then the surface was observed.

The surface temperature of the slab was measured using multiple radiation thermometers installed on the outer periphery of the curved section inside the continuous casting machine. A heat transfer solidification analysis was performed using these actual measured values along with heat removal conditions using cooling water and rolls to determine the surface temperature distribution of the slab. Based on this surface temperature distribution of the slab, the minimum quenching temperature T1, the maximum reheating temperature T2, and the straightening point temperature T3 were obtained. T1, T2, and T3 were all taken as the surface temperatures of the widthwise center of the long side surface on the outer periphery of the curved section. It was confirmed that there was no deviation of 20°C or more between the surface temperatures calculated by heat transfer calculations and the actual measured values obtained from the radiation thermometers.

Ar ₁ and Ac ₃ were calculated based on the above formulas (a) and (b). The cooling rate in step 1 was calculated based on an estimate of v=15° C./s based on heat transfer calculations.

The test results are shown in Table 3. Surface cracks were evaluated as follows: no cracks were marked with an O, 10 or fewer cracks per meter of slab length were marked with a △, and neither were marked with an X.

As is clear from the results shown in Table 3, in all of the levels 1 to 14, which satisfied the above requirements (1) to (3), cast slabs with good surface quality and no cracks were obtained. On the other hand, in the levels 15 to 18, 19 to 22, and 23 to 36, the cooling temperature in the secondary cooling of the cast slab did not reach Ar ₁ or lower, the temperature after cooling until the straightening point did not reach Ac ₃ or higher, and the straightening point temperature was not Tb or lower, and all of them exhibited cracks on the cast slab surface.

1: slab 10: mold 20: straightening point 100: continuous casting machine

Claims (2)

In mass%,
C: 0.03% or more and 0.20% or less,
Si: 0.01% or more and 1.00% or less,
Mn: 0.10% or more and 2.50% or less,
P: 0.040% or less,
S: 0.030% or less,
Cu: 0.05% or more and 0.50% or less,
Sn: 0.001 % or more and 0.025% or less,
Ni: 0% or more and 0.05% or less,
Al: 0.005% or more and 0.100% or less,
Cr: 0% or more and 0.50% or less,
Mo: 0% or more and 0.05% or less, and
N: 0.0150% or less,
A method for continuously casting a steel slab containing a steel material having a straightening point, the method comprising the steps of:
Cooling the surface temperature of the slab removed from the mold to _Ar1 temperature or lower;
Raising the surface temperature of the slab cooled to the Ar ₁ temperature or lower to the Ac ₃ temperature or higher; and
The temperature of the surface of the slab, which has been raised to or above the _Ac3 temperature, is set to or below a temperature Tb°C expressed by the following condition at the straightening point;
2. A method for continuous casting of steel, comprising:
When [Cu] + 4 [Sn] ≦ 0.15: Tb = 1050
When 0.15<[Cu]+4[Sn]≦0.25: Tb=1000
When 0.25<[Cu]+4[Sn]≦0.35: Tb=950
When 0.35<[Cu]+4[Sn]: Tb=900
Here, [Cu] and [Sn] are the Cu and Sn contents (mass%) in the steel. The steel comprises, in mass %,
Ti: 0% or more and 0.020% or less,
V: 0% or more and 0.20% or less,
Nb: 0% or more and 0.030% or less,
Zr: 0% or more and 0.010% or less,
Ca: 0% or more and 0.010% or less,
Mg: 0% or more and 0.010% or less,
REM: 0% or more and 0.010% or less, and
B: 0% or more and 0.0040% or less,
Contains one or more of the following:
The method of claim 1.

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