JP2022183773A - Continuous casting method for steel - Google Patents

Abstract

To suppress the surface crack generated when a slab is corrected in continuous casting of a steel having high Si concentration and Mn concentration.SOLUTION: A continuous casting method for a steel that is a method for continuously casting a slab of a steel having high Si concentration and Mn concentration and a predetermined chemical composition using a continuous casting machine having a correction point includes: cooling the slab so that TA(s) defined by the following expression (1) is 30 or more, when a time during which the surface temperature of the slab is 350 to 450°C is represented by TL(s), and a time during which the surface temperature thereof is 550 to 650°C is represented by TH(s), before the slab reaches the bend correction point from the direct below of the slab; and heating the surface temperature of the slab to a temperature range of Ac3 temperature or higher again before the slab reaches the correction point. Expression (1): TA=TL×[Mn]-0.6×[Si]-1.1+TH×10-1×[Mn]-1.1. In the expression (1), [Si] and [Mn] are concentrations (mass%) of Si and Mn in the steel.SELECTED DRAWING: None

Description

The present application discloses a method for continuous casting of steel.

BACKGROUND ART In recent years, in high-strength steel materials such as thin steel sheets, many steels containing large amounts of alloying elements such as Si and Mn have been produced in order to improve mechanical properties.

However, with the addition of these alloying elements, defects such as surface cracks occur in slabs produced in continuous casting, which pose problems in terms of operation and product quality. A surface crack is a general term for crack forms such as horizontal cracks, horizontal cracks, and corner cracks.

It is known that the surface cracks that occur after the secondary cooling zone in continuous casting are along the prior austenite grain boundaries in the slab surface layer. These cracks can occur due to concentration of stress on austenite grain boundaries embrittled by precipitation of AlN, NbC, etc., and film-like ferrite generated along prior austenite grain boundaries. The form of cracks differs depending on the direction of the applied stress. Transverse cracks are caused by tensile stress in the casting direction, and longitudinal cracks are caused by tensile stress in the width direction of the slab. These cracks are particularly likely to occur in the temperature range near the phase transformation region from austenite to ferrite. Therefore, usually, a method is taken to prevent the occurrence of cracks by avoiding the temperature range where the ductility decreases (brittleness temperature range) in the bending or straightening zone where mechanical stress is applied to the surface of the cast slab. ing. However, in recent years, as the number of steel types to which various elements have been added to improve mechanical properties has increased, the number of steel types with high crack susceptibility has increased, and the above continuous casting method alone cannot necessarily prevent the occurrence of cracks.

In Patent Document 1, in the cooling process from directly below the mold in continuous casting to before the correction point, the surface layer of the cast slab is cooled to a temperature below the bainite, ferrite, or pearlite transformation start temperature in the continuous cooling transformation diagram of steel. Then, it is reheated to a temperature of Ac ₃ or higher at a heating rate of 3 ° C./s or higher and 50 ° C./s or lower, or the surface layer of the cast slab is cooled to a temperature lower than Ar ₃ -100 ° C., and then Ac ₃ A continuous casting method for steel has been proposed in which the steel is reheated to the above temperature at a heating rate of 1.4° C./s or less.

In Patent Document 2, the surface layer of the cast slab is cooled to a temperature range below the ferrite-pearlite transformation end temperature and above the bainite transformation start temperature in the continuous cooling transformation diagram of steel immediately below the mold for continuous casting. Continuous casting characterized by maintaining a temperature range below the ferrite-pearlite transformation end temperature and above the bainite transformation start temperature until it intersects with a constant rate cooling curve passing through the nose of ferrite-pearlite transformation in the continuous cooling transformation diagram. A method is proposed.

In Patent Document 3, the time T A obtained from the time T _L at which the surface temperature of the slab is between 350 and 475° C. and the time _T at which the surface temperature is between 600 and 675° C. from directly below the mold in continuous casting to the correction point _. A continuous casting method for Ni-containing low-alloy steel has been proposed, characterized in that the cast slab is cooled so that the slab becomes 60 or more, and reheated to a temperature range of Ac ₃ or more.

Japanese Patent No. 5928413 Japanese Patent No. 5884479 Japanese Patent Application Laid-Open No. 2020-131203

All of the above-mentioned prior arts are techniques aimed at refining crystal grains by utilizing phase transformation of steel and improving ductility at the straightening point. However, depending on the composition of the steel, there are actually produced steel types in which Ar ₃ cannot be defined at the cooling rate corresponding to the secondary cooling zone, and steel types in which surface cracks cannot be suppressed even after going through the relevant temperature history. For example, it is difficult to apply the above conventional technology to steel with high Si and Mn concentrations. In continuous casting of steel with high Si concentration and Mn concentration, in order to stably suppress surface cracks that occur when straightening the slab, a new technology for cooling the slab immediately below the mold during continuous casting. Guidance is needed.

In order to give the surface layer of the cast slab a temperature history of quenching recuperation in the secondary cooling zone immediately below the mold, refine the surface layer structure, and avoid cracking, the effect of the alloying elements on the transformation behavior must be sufficiently examined. I thought that it was necessary to consider In particular, Si concentration and Mn concentration greatly affect the transformation behavior of steel, so the influence of these elements was fundamentally investigated. Specifically, attention is paid to two temperature ranges of 350 to 450°C near the nose of bainite transformation and 550 to 650°C near the nose of ferrite-pearlite transformation, and the residence time T _L in these temperature ranges. , _TH and microstructure refinement. As a result, the slab was cooled immediately below the mold so that T _A obtained by the following formula (1) using the Si concentration and Mn concentration and the residence times T _L and T _H was equal to or greater than a predetermined value. It was found that the cast slab surface layer structure can be refined before reaching the correction point by reheating to Ac ₃ point or more.
T _A =T _L ×[Mn] ^−0.6 ×[Si] ^−1.1 +T _H ×10 ⁻¹ ×[Mn] ^−1.1 (1)

Based on the above findings, the present application as one means for solving the above problems,
% by mass, C: 0.10-0.40%, Si: 1.0-3.0%, Mn: 1.0-3.0%, Cr: 0-0.60%, Mo: 0- A method of continuously casting steel slabs having a composition of 0.600%, Ni: 0 to 0.50%, and N: 0 to 0.0250% using a continuous casting machine having a straightening point. hand,
T _L (s) is the time when the surface temperature of the slab is between 350 and 450° C. and T _H (s) is the time when the surface temperature of the slab is between 550 and 650° C., from directly below the mold to the correction point, Cooling the slab so that T _A (s) determined by the following formula (1) is 30 or more,
Then, before reaching the correction point, the surface temperature of the cast slab is restored to a temperature range of Ac ₃ or higher,
A method for continuous casting of steel is disclosed.

T _A =T _L ×[Mn] ^−0.6 ×[Si] ^−1.1 +T _H ×10 ⁻¹ ×[Mn] ^−1.1 (1)
([Si] and [Mn] in formula (1) are the concentrations (mass%) of Si and Mn in the steel.)

In the method of the present disclosure, the cast slab is, in mass%, Al: 0 to 0.10%, Ti: 0 to 0.100%, V: 0 to 0.400%, Ca: 0 to 0.0100% , Mg: 0 to 0.0100%, REM: 0 to 0.0100%, Nb: 0 to 0.050%, B: 0 to 0.0040%.

According to the method of the present disclosure, in continuous casting of steel with high Si and Mn concentrations, it is possible to stably suppress surface cracks that occur when the slab is straightened. By hot-rolling the cast slab produced by the method of the present disclosure, it is possible to obtain a steel plate and a steel slab in which the occurrence of surface cracks and the like is suppressed.

1 is a schematic diagram for explaining an example of a continuous casting machine employed in the steel continuous casting method of the present disclosure; FIG. It is a figure for supplementary description about T _L and T _H. FIG. 2 is a photographic diagram showing an example of the state of a cast slab surface layer structure obtained by a model experiment. FIG. 4 is a diagram showing the relationship between the heat treatment pattern by the Formaster device and the structure obtained for steel type A. FIG. FIG. 4 is a diagram showing the relationship between the heat treatment pattern by the Formaster device and the structure obtained for steel type B. FIG. FIG. 4 is a diagram showing the relationship between the heat treatment pattern by the Formaster device and the structure obtained for steel type C. FIG. FIG. 10 is a diagram showing the relationship between the heat treatment pattern by the Formaster device and the structure obtained for steel type D. FIG.

The continuous casting method for steel of the present disclosure will be described with reference to FIG. In FIG. 1, the cooling spray nozzle and the like are omitted for the sake of clarity. The cooling spray nozzles are provided, for example, between support rolls from directly below the mold 10 to before reaching the straightening point 20, and can spray cooling water from both sides of the slab 1. Although FIG. 1 illustrates a vertical bending type continuous casting machine 100, the continuous casting method of the present disclosure is applicable to any continuous casting machine having a straightening point. For example, a curved continuous casting machine may be used. The term "correction point" refers to a point at which strain is applied to correct the casting direction of the cast slab 1 from the curved direction to the horizontal direction. Note that correction may be performed at a plurality of locations. Since the configuration itself of the continuous casting machine 100 including the mold 10, the correction point 20, etc. may be the same as the conventionally known configuration, detailed description thereof is omitted here.

As shown in FIG. 1, the steel continuous casting method of the present disclosure is, in mass%, C: 0.10 to 0.40%, Si: 1.0 to 3.0%, Mn: 1.0 to A steel slab 1 having a composition of 3.0%, Cr: 0 to 0.60%, Mo: 0 to 0.600%, Ni: 0 to 0.50%, and N: 0 to 0.0250% , a method of continuously casting using a continuous casting machine having a correction point 20, wherein the surface temperature of the slab 1 is between 350 and 450 ° C. from directly below the mold 10 to before reaching the correction point 20. The time is T _L (s), the time at 550 to 650 ° C. is T _H (s), and the cast slab 1 is cooled so that T _A (s) determined by the following formula (1) is 30 or more, Next, before reaching the correction point 20, the surface temperature of the cast slab 1 is reheated to a temperature range of Ac ₃ or higher.

T _A =T _L ×[Mn] ^−0.6 ×[Si] ^−1.1 +T _H ×10 ⁻¹ ×[Mn] ^−1.1 (1)
([Si] and [Mn] in formula (1) are the concentrations (mass%) of Si and Mn in the steel.)

1. Steel Type In the continuous casting method of the present disclosure, the steel to be cast essentially contains C, Si and Mn in addition to Fe. At least one selected from Cr, Mo, Ni, N, Al, Ti, V, Ca, Mg, REM, Nb and B may be included as an optional element. Moreover, P and S may be included as impurities, for example.

1.1 C: 0.10-0.40%
C is the most basic element that affects not only static strength of steel but also fatigue strength, toughness and ductility. Too little C may result in insufficient static strength and fatigue strength of the steel. In this regard, the lower limit of the C content may be 0.10% by mass or more, or 0.15% by mass or more. Moreover, if there is too much C, the toughness of steel tends to deteriorate. In this regard, the upper limit of the C content may be 0.40% by mass or less or 0.35% by mass or less.

1.2 Si: 1.0 to 3.0%
Si is an important element having a large solid-solution strengthening ability next to C. When obtaining high-strength steel, the concentration of Si is made high. Specifically, the lower limit of the Si content may be 1.0% by mass or more or 1.1% by mass or more. On the other hand, if the Si content is too high, there is a risk of deteriorating toughness and workability. In this regard, the upper limit of the Si content may be 3.0% by mass or less or 2.5% by mass or less.

1.3 Mn: 1.0-3.0%
Mn is an important element for improving hardenability and securing hardness to the inside of parts even when the cooling rate is insufficient. When obtaining high-strength steel, the concentration of Mn is made high. Specifically, the lower limit of the Mn content may be 1.0% by mass or more or 1.5% by mass or more. On the other hand, too much Mn may deteriorate the toughness and workability. In this regard, the upper limit of the Mn content may be 3.0% by mass or less or 2.5% by mass or less.

1.4 Cr: 0 to 0.60%
Cr is an element optionally added to improve the hardenability of steel, for example. However, if Cr is too much, even if the slab is cooled so as to satisfy the above formula (1) immediately below the mold, it is possible to stably suppress surface cracks that occur when the slab is straightened. It is likely to become difficult. In this regard, the lower limit of the Cr content may be 0% by mass, and the upper limit is 0.60% by mass or less, 0.40% by mass or less, 0.20% by mass or less, or 0.10% by mass or less. may

1.5 Mo: 0-0.600%
Mo is an element that is arbitrarily added with the aim of, for example, secondary hardening during tempering of steel, improvement of fatigue strength, improvement of hardenability, and the like. However, if Mo is too large, even if the slab is cooled so as to satisfy the above formula (1) immediately below the mold, it is possible to stably suppress surface cracks that occur when the slab is straightened. It is likely to become difficult. In this regard, the lower limit of the Mo content may be 0% by mass, and the upper limit is 0.600% by mass or less, 0.400% by mass or less, 0.200% by mass or less, or 0.100% by mass or less. may

1.6 Ni: 0 to 0.50%
Ni is an element that is arbitrarily added with the aim of, for example, ensuring the strength and toughness of steel and improving hardenability. However, if Ni is too much, even if the slab is cooled so as to satisfy the above formula (1) immediately below the mold, it is possible to stably suppress surface cracks that occur when the slab is straightened. It is likely to become difficult. In this regard, the lower limit of the Ni content may be 0% by mass, and the upper limit may be 0.50% by mass or less, 0.30% by mass or less, or 0.10% by mass or less.

1.7 N: 0 to 0.0250%
N forms nitrides in steel, for example, and can exert an effect of suppressing grain coarsening. However, if N is too much, coarsening of nitrides may be caused and the fatigue strength of the steel may be lowered. In addition, it may reduce hot ductility and cause surface defects during casting or rolling. In this regard, the upper limit of the N content may be 0.0250% by mass or less, and may be 0.0200% by mass or less from the viewpoint of steel cleanliness. The lower limit of the N content may be 0% by mass, or may be 0.0020% by mass or more.

In the continuous casting method of the present disclosure, the steel to be cast may further contain the following elements in addition to the above elements. That is, the slab 1 is, in mass%, Al: 0 to 0.10%, Ti: 0 to 0.100%, V: 0 to 0.400%, Ca: 0 to 0.0100%, Mg: 0 ~0.0100%, REM: 0-0.0100%, Nb: 0-0.050%, B: 0-0.0040%. According to the continuous casting method of the present disclosure, by cooling the slab immediately below the mold so as to satisfy the above formula (1), the Al, Ti, V, Ca, Mg, REM, and Regardless of the content of arbitrary elements such as Nb and B, it is possible to stably suppress surface cracks that occur when straightening the cast slab. That is, the elements described below do not substantially affect the validity of the above formula (1).

1.8 Al: 0 to 0.10%
Al is the most widely used element for deoxidizing purposes, and has the effect of forming AlN to suppress the coarsening of crystal grains. However, if the amount of Al is too large, problems such as nozzle clogging during casting due to agglomeration of Al ₂ O ₃ and Al ₂ O ₃ remaining in the steel deteriorating performance may occur. be. In this regard, the upper limit of the Al content may be 0.10% by mass or less or 0.05% by mass or less. The lower limit of the Al content may be 0% by mass or 0.01% by mass or more.

1.9 Ti: 0 to 0.100%
Ti, like Al, is an element capable of forming nitrides, is excellent in thermal stability, and maintains the effect of suppressing grain coarsening up to higher temperatures. However, if Ti is too much, TiN tends to grow coarsely, which may reduce the fatigue strength. In this respect, the upper limit of the Ti content may be 0.100% by mass or less. The lower limit of the Ti content may be 0% by mass, 0.001% by mass or more, or 0.002% by mass or more.

1.10 V: 0 to 0.400%
V is an element capable of forming nitrides like Ti and Al, and is used for strength improvement. However, if V is too much, VN tends to grow coarsely, which may reduce the fatigue strength. In this regard, the upper limit of the V content may be 0.400% by mass or less. The lower limit of the V content may be 0% by mass, or may be 0.001% by mass or more, or 0.002% by mass or more.

1.11 Ca: 0 to 0.0100%
Ca modifies Al ₂ O ₃ and has the effect of suppressing coarsening of oxide-based inclusions. However, if Ca is too much, rather coarse oxide-based inclusions containing CaO—Al ₂ O ₃ as a main component are formed, which may become starting points of fatigue fracture. In this regard, the upper limit of the Ca content may be 0.0100% by mass or less. The lower limit of the Ca content may be 0% by mass, 0.0001% by mass or more, or 0.0002% by mass or more.

1.12 Mg: 0-0.0100%
Like Ca, Mg has the effect of modifying Al ₂ O ₃ and suppressing coarsening of oxide inclusions. It also acts on sulfide-based inclusions and has the effect of lowering the aspect ratio. However, if Mg is too much, coarse cluster-like oxide-based inclusions containing MgO as a main component may be formed, which may become starting points of fatigue fracture. In this regard, the upper limit of the Mg content may be 0.0100% by mass or less. The lower limit of the Mg content may be 0% by mass, or may be 0.0001% by mass or more, or 0.0002% by mass or more.

1.13 REM: 0-0.0100%
REM also modifies Al ₂ O ₃ and has the effect of suppressing coarsening of oxide inclusions. However, too much REM may reduce the cleanliness of the steel and degrade the toughness of the base material. In this regard, the upper limit of the REM content may be 0.0100% by mass or less. The lower limit of the REM content may be 0% by mass, or may be 0.0001% by mass or more, or 0.0002% by mass or more. REM represents a rare earth element such as La or Ce, and any one or two or more of these REMs can be used.

1.14 Nb: 0 to 0.050%
Nb is effective in improving strength and toughness. However, if the Nb content is too large, the effect is saturated. In this regard, the upper limit of the Nb content may be 0.050% by mass or less. The lower limit of the Nb content may be 0% by mass, or may be 0.001% by mass or more, or 0.002% by mass or more.

1.15B: 0 to 0.0040%
A small amount of B has a large effect of improving the hardenability. However, if B is too much, the effect is saturated. In this regard, the upper limit of the B content may be 0.0040% by mass or less. The lower limit of the B content may be 0% by mass, or may be 0.0001% by mass or more, or 0.0002% by mass or more.

2. Secondary Cooling of Slab 1 As shown in FIG. 1 , in the continuous casting method of the present disclosure, a steel slab 1 having the above composition is continuously drawn out from a mold 10 and cast at a straightening point 20 from directly below the mold 10 . Before reaching the temperature, secondary cooling of the slab 1 is performed by spraying cooling water on the surface of the slab 1 or the like. Here, in the continuous casting method of the present disclosure, T _L (s) is the time during which the surface temperature of the cast slab 1 is between 350 and 450° C. from directly below the mold 10 to the correction point 20, 550 to It is important to cool the slab 1 so that T _A (s) determined by the above formula (1) is 30 or more, where T _H (s) is the time at 650°C.

In the continuous casting method of the present disclosure, the lower limit of the temperature range of _TL is set to 350°C, but depending on the steel type, this temperature is lower than the martensite transformation start temperature, and part or all of the structure becomes martensite. It is also possible. However, even in that case, the surface layer structure can be refined after the reverse transformation, and the desired effect can be obtained.

In the actual continuous casting machine, the temperature of the surface of the slab 1 is 350 to 450° C. and 550 to 650° C. multiple times from directly below the mold 10 to before reaching the correction point 20. It is possible to pass through. In this case, T _L and T _H are represented by the sum of the time spent passing through each temperature range. For example, in the thermal history as shown in FIG. 2, T _L and T _H can be obtained by the following equations (2) and (3).

T _L =t ₄ -t ₃ (2)
T _H =(t ₂ −t ₁ )+(t ₆ −t ₅ ) (3)

As a method for cooling the slab 1 in the secondary cooling zone, in addition to the method of injecting cooling water using the cooling spray nozzle described above, a method using airflow, a method of allowing to cool without providing special cooling equipment, etc. is also valid. Furthermore, a method of cooling by combining these methods may be used. The cooling rate of the slab 1 is not particularly limited, and desired effects can be exhibited at any cooling rate.

3. Reheating of slab 1 In the continuous casting method of the present disclosure, after austenite is decomposed in the secondary cooling zone _, the surface temperature of the slab 1 is is reheated to a temperature above ₃ Ac. This reheating is essential for obtaining a so-called reverse-transformed structure, in which the surface layer structure of the slab 1 is made into a fine austenitic structure. If the recuperation temperature is less than Ac ₃ , there remain places where reverse transformation does not occur. Since such a structure has the effect of the as-cast structure, which tends to crack due to corrective strain, it is effective to reheat to Ac ₃ or more to form an austenite single-phase structure to suppress the occurrence of cracks. In addition, if the surface temperature of the slab 1 is once restored to Ac ₃ or higher before reaching the correction point 20, the hot ductility of the surface of the slab 1 is kept high after that, so at the correction point 20 Surface cracks are not a problem even if the temperature drops.

By reheating to Ac ₃ , the surface layer structure of the slab 1 is modified, and in combination with appropriate secondary cooling, a slab with less surface cracks can be obtained. From the viewpoint of further suppressing variations in surface temperature and structure in the slab 1, the maximum temperature after reheating may be Ac ₃ +30° C. or higher. If the recuperation temperature is too high, the austenite grains may coarsen again. In this regard, the maximum temperature after reheating may be 1200° C. or less.

Incidentally, _Ac3 can be measured using a transformation point recording measuring device (Formaster device) or the like. Alternatively, _Ac3 can also be specified using the following formula (4) proposed in a prior document (Tatsuro Kunitake: Heat Treatment, 43, p. 100 (2003)).

Ac ₃ = (32[Si]+17[Mo])-(231[C]+20[Mn]+40[Cu]+18[Ni]+15[Cr])+912 (4)
([Si], [Mo], [C], [Mn], [Cu], [Ni], and [Cr] in formula (4) represent the concentration (mass %) of each component.)

The heat recovery on the surface of the slab 1 is a phenomenon that 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 heat recovery of the surface of the slab 1 can be performed relatively easily by relaxing the cooling in 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 rate (heating rate) of the slab 1 is not particularly limited, and the desired effect can be exhibited at any reheating rate.

As described above, according to the continuous casting method of the present disclosure, in continuous casting of steel with high Si concentration and Mn concentration, surface cracks of the slab that occur when the slab is straightened can be stably suppressed. can be done. In the present application, the "surface of the slab" does not necessarily mean the entire surface of the slab. That is, at least the portion of the surface of the slab where surface cracks are desired to be suppressed is cooled immediately below the mold so that the above formula (1) is satisfied, and then reheated to a temperature of Ac ₃ or higher.

Hereinafter, the effects and the like of the technology of the present disclosure will be described in more detail while showing examples, but the technology of the present disclosure is not limited to the following examples.

1. Model experiment 1
In order to clarify the conditions for sufficiently obtaining the effect of refining the cast slab surface layer structure by secondary cooling and reheating, a model experiment was carried out using a transformation point recording measuring device (Formaster device).

For each of the steel types A to D having the compositions shown in Table 1 below, after heating to 1400 ° C. with a Formaster device, quenched with helium gas to a predetermined temperature of 350 to 750 ° C. The quenched sample was cooled for 30 to 3000 seconds. After isothermally maintained, it was heated to 900° C. at 20° C./s and cooled to room temperature at 0.4° C./s. A cross section of the obtained sample was etched with nital and observed with an SEM.

Observation examples are shown in FIGS. The tissue confirmed by the cross-sectional observation consisted of the tissue A shown in FIG. 3(A) and/or the tissue B shown in FIG. 3(B). As is clear from FIG. 3(A), in the structure A, fine granular ferrite on the order of several tens of μm is observed, grain boundary ferrite is unclear, and the structure is considered to be a structure with low crack sensitivity. On the other hand, as is clear from FIG. 3(B), in the structure B, no granular ferrite was observed, and grain boundary ferrite was partially observed.

4 to 7 show the relationship between the temperature pattern of the heat treatment by the Formaster apparatus and the obtained structure for each of the steel types A to D. FIG. 4 is for steel type A, FIG. 5 is for steel type B, FIG. 6 is for steel type C, and FIG. In FIGS. 4 to 7, the case where the obtained structure was A on the entire surface is indicated by “◯”, the case where the obtained structure was a mixed phase of A and B by “Δ”, and the case where the obtained structure was on the entire surface by “X”. rice field. As is clear from FIGS. 4 to 7, the temperature conditions under which the structure A was obtained differed greatly depending on the steel type. In other words, it was found that appropriate temperature conditions for obtaining a structure with low crack susceptibility cannot be determined unless the effects of Si and Mn concentrations are considered. From this model experiment, it can be seen that the residence time in two temperature ranges where bainite transformation at 350-450° C. and ferrite-pearlite transformation at 550-650° C. occur are important for austenite decomposition. Based on this, a model experiment was conducted in order to understand the transformation behavior when the slab surface layer crosses over both of these two temperature ranges in actual continuous casting.

2. Model experiment 2
Steel grades A, B, and D, which are the same as in model experiment 1, were heated to 1400°C using a Formaster device, held at 400°C for a predetermined time, and further held at 600°C for a predetermined time. °C at 20 °C/s and then cooled at 0.4 °C/s to room temperature. Table 2 below shows the relationship between each holding time at 400°C and 600°C and the structure obtained. In Table 2 below, the case where the obtained structure was A on the entire surface was indicated as "◯", the case where the obtained structure was a mixed phase of A and B as "△", and the case where the entire surface was B was indicated as "×". .

As is clear from the results shown in Table 2, the samples held at 400 ° C. for 40 seconds were held at 625 ° C. for 40 seconds or less for steel type A, 40 to 120 seconds for steel type B, and 40 to 120 seconds for steel type D. exhibited the overall structure A in 240 to 600 seconds, while the sample held at 400 ° C. for 20 seconds was held at 600 ° C. for 40 to 120 seconds for steel type A and 120 to 360 seconds for steel type B. Seconds, steel grade D exhibited the overall structure A at 240 to 600 seconds. At this time, it is presumed that coarse austenite transforms into bainite during holding at 400°C, and untransformed austenite transforms into ferrite-pearlite during holding at 600°C. That is, when the sum of the amount of bainite transformation and the amount of ferrite-pearlite transformation balances with the amount of austenite before transformation, the decomposition of austenite is completed. be done.

Relationship between residence times T _L (s) and T _H (s) in temperature ranges of 350-450°C and 550-650°C and cast slab surface layer structure for steel grades with high Si and Mn concentrations Various model experiments were conducted in the same manner as above. As a result, when T _A (the total value of the holding time in the transformation temperature range) (s) shown in the following formula (1) is 30 or more, the subsequent reheating and cooling of Ac ₃ or more , it was found that a structure with low cracking susceptibility was generated.

T _A =T _L ×[Mn] ^−0.6 ×[Si] ^−1.1 +T _H ×10 ⁻¹ ×[Mn] ^−1.1 (1)
([Si] and [Mn] in formula (1) are the concentrations (mass%) of Si and Mn in the steel.)

It has been confirmed that the above formula (1) holds true also in experiments conducted using steel types other than the above steel types A to D, which have high Si and Mn concentrations. That is, in mass%, C: 0.10 to 0.40%, Si: 1.0 to 3.0%, Mn: 1.0 to 3.0%, Cr: 0 to 0.60%, Mo: When casting steel having a composition of 0 to 0.600%, Ni: 0 to 0.50%, and N: 0 to 0.0250%, the above formula (1) is 30 or more. By performing the secondary cooling, it is possible to refine the cast slab surface layer structure, and it is possible to suppress the cast slab surface cracks at the correction point.

3. Actual machine test In order to verify the validity of the model experiment described above, an actual machine test was conducted using a slab continuous casting machine. Specifically, from molten steel having the composition shown in Table 3 below, a slab having a size of 240 mm×1500 mm was cast using a curved continuous caster having a radius of curvature of 12.0 m. The casting speed is 1.0-1.4 m/min. The slab pulled out from the mold was quenched by a spray quenching device with a zone length of 1 m installed directly under the mold. After passing through the zone, the amount of water in the normal secondary cooling spray was adjusted to control reheating. The slab was cut into lengths of 5.0±0.2 m with a gas cutter, and then subjected to surface observation.

The temperature of the slab surface is the temperature at the center of the L surface of the slab calculated by heat transfer solidification calculation. It was verified that the temperature calculated by heat transfer solidification calculation has sufficiently high accuracy by comparison with the data of the slab surface temperature system installed in the continuous casting machine. Also, the value of _Ac3 , which is the target of the highest temperature after recuperation, is specified by the above formula (4).

Surface cracks in the obtained steel slabs were visually observed. Investigation results together with the time T _L during which the cast slab surface temperature was between 350 and 450° C. in the cooling zone, the time T _H during which it was between 550 and 650° C., and the maximum temperature from the start of reheating to the straightening point. are shown in Table 4 below. In Table 4 below, the evaluation of surface cracks is "○" for those with no cracks, and "○" for those whose depth of cracks is less than 0.5 mm and the number of cracks is 10 or less per slab. △”, and those that do not correspond to any of the above are indicated as “x”.

As is clear from the results shown in Table 4, the slab surface was cooled immediately below the mold so that the T _A shown in the above formula (1) was 30 or more, and then Ac ₃ before reaching the straightening point. For the slabs (Examples 1 to 9) that had been reheated up to the above, no cracks were observed on the surface, and a fine structure in which ferrite was dispersed was observed in the surface layer structure of the slab. On the other hand, when _TA shown by formula (1) is less than 30 (Comparative Examples 1, 2, 4, 5, 7 and 8), the maximum temperature from the start of reheating to the correction point is When Ac was less than ₃ (Comparative Examples 3, 6 and 9), cracks were observed on the slab surface.

1 slab 10 mold 20 straightening point 100 continuous casting machine

Claims (2)

% by mass, C: 0.10-0.40%, Si: 1.0-3.0%, Mn: 1.0-3.0%, Cr: 0-0.60%, Mo: 0- A method of continuously casting steel slabs having a composition of 0.600%, Ni: 0 to 0.50%, and N: 0 to 0.0250% using a continuous casting machine having a straightening point. hand,
T _L (s) is the time when the surface temperature of the slab is between 350 and 450° C. and T _H (s) is the time when the surface temperature of the slab is between 550 and 650° C., from directly below the mold to the correction point, Cooling the slab so that T _A (s) determined by the following formula (1) is 30 or more,
Then, before reaching the correction point, the surface temperature of the cast slab is restored to a temperature range of Ac ₃ or higher,
A method of continuous casting of steel.
T _A =T _L ×[Mn] ^−0.6 ×[Si] ^−1.1 +T _H ×10 ⁻¹ ×[Mn] ^−1.1 (1)
([Si] and [Mn] in formula (1) are the concentrations (mass%) of Si and Mn in the steel.) The cast slab, in mass%, Al: 0 to 0.10%, Ti: 0 to 0.100%, V: 0 to 0.400%, Ca: 0 to 0.0100%, Mg: 0 to 0 .0100%, REM: 0 to 0.0100%, Nb: 0 to 0.050%, B: 0 to 0.0040%,
The method of claim 1.

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