JP2020131203A - CONTINUOUS CASTING METHOD FOR Ni-CONTAINING LOW ALLOY STEEL - Google Patents

Abstract

To stably suppress transversal crack and crack of a cast slab surface which occur when correcting the cast slab during continuous casting of Ni-containing low alloy steel.SOLUTION: In a continuous casting method for Ni-containing low alloy steel, a time during which a surface temperature of a cast slab is between 350 and 475°C is represented by T(s), and a time during which the surface temperature is between 600 and 675°C is represented by T(s) before reaching a correction point just below a casting mold, the cast slab is cooled so that T(s), which is represented by the following formula (a): T=T+T×0.04×[Ni](in the formula (a), [Ni] represents a concentration of Ni (mass%) in a steel), becomes 60 or more, and then, the surface temperature Ac of the cast slab is reheated to a temperature range of Ac3 or more before reaching the correction point.SELECTED DRAWING: None

Description

The present application discloses a method for continuously casting Ni-containing low alloy steel.

In recent years, in products such as alloy steels for machine structural use, attempts have been widely made to improve mechanical properties by means such as containing alloying elements such as Nb, V, and Ni, or enriching C and N. It has been. When alloy steel containing these is cast using a curved or vertical bending type continuous casting machine, so-called lateral cracks and lateral cracks are likely to occur on the surface of the slab. This crack occurs because the stress acting on the surface of the slab exceeds the limit stress inherent in steel when the bending of the slab is straightened at the straightening point of the continuous casting machine. In particular, in the above alloy steel, nitrides and carbides such as AlN and NbC are likely to precipitate at the austenite grain boundaries in the secondary cooling process after being drawn from the mold. The austenite grain boundaries where these precipitates are deposited tend to be the starting points of cracks when stress is applied to the slabs.

In addition, one of the causes of surface cracking of the slab during straightening is that the as-cast γ grains are extremely large compared to the product stage, so that stress tends to concentrate on the fragile portion. Therefore, a technique has been proposed in which the surface of the slab is strongly cooled from directly below the mold to a certain temperature or less, and then reheated to a certain temperature or more to refine the austenite particles on the surface layer and then correct the austenite particles.

For example, in Patent Document 1, the surface of a slab is cooled at a cooling rate of 300 ° C./s or more from a temperature range of Ar3 or higher to a temperature range of Ar1 or lower, and then the surface temperature of the slab is again cooled. A method for preventing surface cracking of continuously cast slabs has been proposed, which comprises reheating the material to a temperature range of Ar3 or higher.

In Patent Document 2, the surface layer of the slab is cooled to a temperature lower than the bainite, ferrite or pearlite transformation start temperature in the continuous cooling transformation diagram of steel in the cooling process from directly below the mold for continuous casting to just before the straightening point. Then, the surface layer of the slab is cooled to a temperature of 3 ° C./s or higher and 50 ° C./s or lower at a heating rate of 3 ° C./s or higher, or to a temperature lower than Ar3-100 ° C., and then a temperature of Ac3 or higher. A method for continuous casting of steel has been proposed, which comprises reheating at a heating rate of 1.4 ° C./s or less.

In Patent Document 3, the surface layer portion of the 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 under the mold for continuous casting, and then the above-mentioned Continuous casting characterized by holding in a temperature range below the ferrite-pearlite transformation end temperature and above the bainite transformation start temperature until crossing a constant rate cooling curve through the nose of the ferrite-pearlite transformation in the continuous cooling transformation diagram. A method has been proposed.

Japanese Patent No. 4923650 Japanese Patent No. 5928413 Japanese Patent No. 584479

All of the techniques disclosed in Patent Documents 1 to 3 are techniques aimed at improving the ductility at the straightening point by refining the crystal grains by utilizing the phase transformation of the steel, but depending on the composition of the steel. Steel grades in which Ar3 cannot be defined without ferrite-pearlite transformation at a cooling rate corresponding to the secondary cooling zone, that is, a sufficient guideline for suppressing surface cracks cannot be obtained by the above technique, are actually produced. Examples of such steel types include Ni-containing low alloy steels. In the continuous casting of Ni-containing low alloy steel, a new technique capable of stably suppressing lateral cracks and cracks on the surface of the slab generated when the slab is straightened is required.

In this application, as one of the means for solving the above-mentioned problems, in mass%, C: 0.1-0.4%, Si: 0.01-0.5%, Mn: 0.3-1.4. %, Cr: 0.8 to 2.0%, Mo: 0.6% or less, Ni: 0.5 to 2.0% of steel slabs, using a continuous casting machine with straightening points. a continuous casting methods Te, before reaching right under the mold to the correct point, a time in which the surface temperature of the slab is between _{350~475 ℃ T L (s),} 600~675 ℃ as T _{H (s)} of time in, the cast strip is cooled as T _a defined by the following formula _(a) (s) is more than 60, then up before reaching the correcting point, the cast A method for continuously casting a Ni-containing low alloy steel, which comprises reheating the surface temperature of a piece to a temperature range of Ac3 or higher, is disclosed.

_{T A = T L + T H} × 0.04 × [Ni] -2.32 ··· (a)
(In the formula (a), [Ni] is the concentration (mass%) of Ni in the steel.)

In the continuous casting method of Ni-containing low alloy steel of the present disclosure, the slabs are, in mass%, Al: 0.1% or less, Ti: 0.1% or less, V: 0.4% or less, Ca: 0. It has a composition of 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, Nb: 0.05% or less, B: 0.004% or less, N: 0.025% or less. You may.

According to the method of the present disclosure, lateral cracks and lateral cracks on the surface of the slab, which occur when the slab is straightened during continuous casting, can be stably suppressed. Therefore, by hot rolling the slabs produced by the method of the present disclosure, it is possible to obtain steel plates and steel slabs in which the occurrence of surface cracks and the like is suppressed.

It is a schematic diagram for demonstrating an example of the continuous casting machine adopted in the continuous steel casting method of this disclosure. It is a figure which shows an example of the TTT diagram of carbon steel. It is a figure which shows an example of the TTT diagram of the low carbon alloy steel (JIS: SCM420). T _L, which is a diagram for supplementary explanation for _{T H.} It is a figure which shows the state of the slab surface layer structure obtained by the model experiment. It is a figure which shows the relationship between the heat treatment pattern attached by the transformation point recording measuring apparatus (four master apparatus), and the obtained structure with respect to steel type N05. It is a figure which shows the relationship between the heat treatment pattern attached by the transformation point recording measuring apparatus (four master apparatus), and the obtained structure with respect to steel type N10. It is a figure which shows the relationship between the heat treatment pattern attached by the transformation point recording measuring apparatus (four master apparatus), and the obtained structure with respect to steel type N20.

The continuous casting method of the Ni-containing low alloy 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 nozzle is provided, for example, between the support rolls from directly below the mold 10 to before reaching the straightening point 20, and can inject cooling water from both sides of the slab 1. Although the 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 correction point. For example, a curved continuous casting machine may be used. The "correction point" means a point where strain is applied to correct the casting direction of the slab 1 from the curve to the horizontal direction. The correction may be performed at a plurality of places. Since the configuration itself of the continuous casting machine 100 including the mold 10, the straightening point 20, and the like may be the same as the conventionally known configuration, detailed description thereof will be omitted here.

As shown in FIG. 1, the continuous casting method of the Ni-containing low alloy steel of the present disclosure is C: 0.1 to 0.4%, Si: 0.01 to 0.5%, Mn: 0 in mass%. .3 to 1.4%, Cr: 0.8 to 2.0%, Mo: 0.6% or less, Ni: 0.5 to 2.0% Steel slab 1 is straightened. A method of continuously casting using a continuous casting machine 100 having a 20. A time during which the surface temperature of the slab 1 is between 350 and 475 ° C. before reaching the straightening point 20 from directly below the mold 10. T _L (s), as _T H (s) of time in the 600-675 ° C., cooling the slab 1 as following equation _T a defined by (a) (s) is 60 or more, then corrective The surface temperature of the slab 1 is reheated to a temperature range of Ac3 or higher before reaching the point 20.

_{T A = T L + T H} × 0.04 × [Ni] -2.32 ··· (a)
(In the formula (a), [Ni] is the concentration (mass%) of Ni in the steel.)

1. 1. Steel type In the continuous casting method of the present disclosure, C, Si, Mn, Cr and Ni are indispensably contained in the steel to be cast in addition to Fe. Further, as an optional component, for example, at least one selected from Mo, Al, Ti, V, Ca, Mg, REM, Nb, B and N may be contained. Further, as unavoidable impurities, for example, P and S may be contained.

1.1 C
C is the most basic element that affects not only the static strength of steel but also fatigue strength, toughness and ductility. If C is less than 0.1% by mass, the static strength and fatigue strength are insufficient. Therefore, the lower limit is set to 0.1% by mass or more. Further, if it exceeds 0.4% by mass, the toughness deteriorates. Therefore, the upper limit is set to 0.4% by mass or less.

1.2 Si
Si is an important element having the second largest solid solution strengthening ability after C. If Si is less than 0.01% by mass, sufficient strength cannot be obtained. Therefore, the lower limit is set to 0.01% by mass or more. Further, if it exceeds 0.5% by mass, the toughness and workability are significantly deteriorated. Therefore, the upper limit is set to 0.5% by mass or less.

1.3 Mn
Mn is an important element for improving hardenability and ensuring hardness even inside parts even when the cooling rate is insufficient. If Mn is less than 0.3% by mass, the required strength cannot be secured. Therefore, the lower limit is set to 0.3% by mass or more. If it exceeds 1.4% by mass, the toughness and workability deteriorate. Therefore, the upper limit is set to 1.4% by mass or less.

1.4 Cr
Like Mn, Cr is a useful element for improving the hardenability of steel and is one of the important elements constituting low alloy steel. If it is less than 0.8% by mass, this effect cannot be sufficiently obtained. Therefore, the lower limit is set to 0.8% by mass or more. Further, if it exceeds 2.0% by mass, the effect is almost saturated, which leads to an increase in cost. Therefore, the upper limit is set to 2.0% by mass or less.

1.5 Mo
Mo is an element that causes so-called secondary hardening, which hardens steel during tempering by finely precipitating the carbonitride, and is also effective in improving fatigue strength. In addition, the effect of improving hardenability is also great. However, if it exceeds 1.5% by mass, undissolved carbides tend to remain during quenching heat treatment, which may deteriorate toughness. In order to sufficiently suppress the deterioration of toughness, the upper limit is preferably 0.6% by mass or less. The lower limit is not particularly limited, but is 0% by mass or more, preferably 0.004% by mass or more.

1.6 Ni
Ni is effective in ensuring strength and toughness, and has a great effect of improving hardenability. By setting the amount of Ni to 0.5% by mass or more, this effect becomes more remarkable. On the other hand, if it exceeds 2.0% by mass, the effect is saturated and the cost increases. Therefore, the upper limit is set to 2.0% by mass or less.

1.7 Al
Al is the most widely used element for deoxidizing purposes, and has the effect of producing AlN to suppress coarsening of crystal grains. However, when it exceeds 0.1 wt%, nozzle clogging during casting due to agglomeration if may occur in Al ₂ O _3, a defect of the Al ₂ O ₃ or the like remaining in the steel or degrade the performance It may occur. Therefore, the upper limit is preferably 0.1% by mass or less. The lower limit is not particularly limited, but is 0% by mass or more, preferably 0.016% by mass or more.

1.8 Ti
Like Al, Ti is an element capable of forming a nitride, has excellent thermal stability, and maintains the effect of suppressing grain grain coarsening up to a higher temperature. However, if it exceeds 0.1% by mass, TiN tends to grow coarsely, which may reduce the fatigue strength. Therefore, the upper limit is set to 0.1% by mass or less. The lower limit is not particularly limited, but is 0% by mass or more, preferably 0.002% by mass or more.

1.9 V
V is an element capable of forming a nitride like Ti and Al, and is used for improving the strength. However, if it exceeds 0.4% by mass, the VN tends to grow coarsely, which may reduce the fatigue strength. Therefore, the upper limit is preferably 0.4% by mass or less. The lower limit is not particularly limited and may be 0% by mass, but when it is 0.002% by mass or more, the effect of improving the strength can be easily obtained.

1.10 Ca
Ca has the effect of modifying Al ₂ O ₃ and suppressing the coarsening of oxide-based inclusions. However, if it exceeds 0.01% by mass, it may form coarse oxide-based inclusions containing CaO-Al ₂ O ₃ as a main component and serve as a base point for fatigue failure. Therefore, the upper limit is preferably 0.01% by mass or less. The lower limit is not particularly limited and may be 0% by mass, but when it is 0.0002% by mass or more, the effect of suppressing oxide coarsening can be easily obtained.

1.11 Mg
Like Ca, Mg 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. However, if it exceeds 0.01% by mass, coarse cluster-like oxide-based inclusions containing MgO as a main component may be formed, which may serve as a base point for fatigue fracture. Therefore, the upper limit is preferably 0.01% by mass or less. The lower limit is not particularly limited and may be 0% by mass, but when it is 0.0002% by mass or more, the effect of suppressing the coarsening of oxide-based inclusions can be easily obtained.

1.12 REM
REM also has the effect of modifying Al ₂ O ₃ and suppressing the coarsening of oxide-based inclusions. However, if it exceeds 0.01% by mass, the cleanliness of the steel may be lowered and the toughness of the base metal may be deteriorated. Therefore, the upper limit is preferably 0.01% by mass or less. The lower limit is not particularly limited and may be 0% by mass, but when it is 0.0002% by mass or more, the effect of suppressing the coarsening of oxide-based inclusions can be easily obtained. Here, the REM represents a rare earth element such as La or Ce, and any one type or two or more types of REM can be used.

1.13 Nb
Nb is effective in improving strength and toughness. However, if it exceeds 0.05% by mass, the effect is saturated. If the Nb content is too high, cracks may occur during casting. Therefore, the upper limit is preferably 0.05% by mass or less. The lower limit is not particularly limited and may be 0% by mass, but when it is 0.001% by mass or more, the strength improving effect and the toughness improving effect can be easily obtained.

1.14 B
B has a large hardenability improving effect with a small amount. However, if it exceeds 0.004% by mass, the effect is saturated. If the B content is too high, cracks may occur during casting. Therefore, the upper limit is preferably 0.004% by mass or less. The lower limit is not particularly limited and may be 0% by mass, but when it is 0.0002% by mass or more, the effect of improving hardenability can be easily obtained.

1.15 N
N produces nitrides such as TiN and AlN, and exerts an effect of suppressing grain grain coarsening. However, if it exceeds 0.025% by mass, the nitride may become coarse and the fatigue strength may be lowered. In addition, the hot ductility is lowered, which may cause surface defects during casting or rolling. Therefore, the upper limit is preferably 0.025% by mass or less. From the viewpoint of steel cleanliness, 0.02% by mass or less is more preferable. The lower limit is not particularly limited, but is 0% by mass or more, preferably 0.0036% by mass or more.

2. 2. Secondary cooling of the slab 1 As shown in FIG. 1, in the continuous casting method of the present disclosure, the steel slab 1 having the above composition is continuously drawn from the mold 10 and reaches the straightening point 20 from directly below the mold 10. By then, the surface of the slab 1 is secondarily cooled by injecting cooling water or the like. Here, in the continuous casting method of the present disclosure, the time during which the surface temperature of the slab 1 is between 350 and 475 ° C. is set to _TL (s), 600 to before reaching the straightening point 20 from directly below the mold 10. the time in the 675 ° C. as _T H (s), it is important to cool the slab 1 so that the formula _T a defined by (a) (s) is 60 or more.

In continuous steel casting, surface cracks in the slab are likely to occur along the austenite (or former austenite) grain boundaries of the coarse cast structure. This is due to the preferential formation of inclusions or soft ferrite on the austenite (or former austenite) grain boundaries with temperature drop. In order to prevent this, the slab is cooled in the secondary cooling zone directly under the mold to transform the structure of the slab surface layer into ferrite-pearlite or bainite, and then reheat to Ac3 or higher to reverse transformation. A technique for refining the austenite structure on the surface layer of the slab has been developed.

For example, as described in Patent Document 1 described above, Ar1 is set as the cooling target temperature in the secondary cooling zone for the purpose of transforming the surface layer structure of the slab into ferrite-pearlite or bainite (hereinafter referred to as decomposition of austenite as necessary). , Or a specific temperature has been proposed. However, the above technique was proposed mainly for carbon steel. Since low alloy steel containing a predetermined amount of Cr or the like tends to delay the ferrite-pearlite transformation as compared with carbon steel, there is a problem that slab cracking cannot be sufficiently suppressed by the above technique.

The subject will be described in detail using a TTT diagram. FIG. 2 shows an example of a TTT diagram of carbon steel, and FIG. 3 shows an example of a TTT diagram of low carbon alloy steel (JIS: SCM420). In the TTT diagram of carbon steel, the ferrite-pearlite transformation proceeds in a short time in a temperature range of 700 ° C. or lower, whereas in the TTT diagram of low carbon alloy steel, the transformation progress changes greatly depending on the temperature. It is known that ferrite-pearlite transformation is unlikely to occur in low-carbon alloy steel due to the influence of Cr and Mo contained in it, and especially at 500 to 600 ° C., bainite transformation is also delayed, resulting in such a shape. That is, for example, in carbon steel, even in a temperature range in which austenite decomposition proceeds rapidly, it is generally possible that decomposition does not proceed sufficiently depending on the steel type. Therefore, during continuous casting of steel, the austenite structure on the surface layer of the slab is refined by quenching and reheating the slab in the secondary cooling zone directly under the mold, and casting is performed in order to avoid cracking at the straightening point. It is indispensable to adopt a cooling method according to the metallurgical properties of the steel type.

On the other hand, various data books are known for the TTT diagram, but most of them are based on the austenitization temperature of 1000 ° C. or lower as shown in FIGS. 2 and 3. Since the transformation behavior during slab cooling largely depends on the austenite particle size, these data books cannot be used as they are. The present inventors have a result of our intensive studies in light of the transformation behavior of low alloy steel having the above composition has the above formula in the secondary cooling zone the T defined by _{(a) A} (s) 60 or more It was found that the surface structure of slabs of Ni-containing low alloy steel having the above composition can be effectively transformed.

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 may be lower than the martensitic transformation start temperature, and part or all of the structure may become martensite. Is also possible. However, even in that case, the surface layer structure can be miniaturized after the reverse transformation, and a desired effect can be obtained.

In the continuous casting machine of the actual machine, the surface temperature of the slab 1 is 350 to 475 ° C. and 600 to 675 ° C. multiple times before the slab 1 reaches the straightening point 20 from directly under the mold 10. It is possible to pass. In this case, T _L and T _H are represented by the sum of the times passed through the respective temperature regions. For example, in the thermal history as shown in FIG. 4, _TL and _TH can be obtained by the following equations (b) and (c).

T _L = t _4- t ₃ ... (b)
_TH = (t _2- t ₁ ) + (t ₆ −t ₅ ) ・ ・ ・ (c)

As a method of 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 of using an air flow, a method of allowing the slab to cool without any special cooling equipment, etc. Is also valid. Further, a method of cooling by combining these may be used. The cooling rate of the slab 1 is not particularly limited, and a desired effect can be exhibited at any cooling rate.

3. 3. Reheating of slab 1 In the continuous casting method of the present disclosure, after decomposing austenite in the secondary cooling zone, the surface temperature of slab 1 is reheated to a temperature of Ac3 or higher by the time the correction point 20 is reached. This reheating is indispensable for obtaining a so-called reverse transformation structure in which the surface layer structure of the slab 1 is made into a fine austenite structure. When the reheat temperature is less than Ac3, a place where reverse transformation does not occur remains. Since such a structure has the influence of the structure as it is cast, which is prone to cracks due to the correction strain, it is effective to reheat the structure to Ac3 or higher to form an austenite single-phase structure in order to suppress the occurrence of cracks. If the surface temperature of the slab 1 is once reheated to Ac3 or higher by the time the straightening point 20 is reached, the hot ductility of the surface of the slab 1 is maintained high thereafter, so that the temperature at the straightening point 20 is maintained. Surface cracking does not matter even if the temperature decreases.

By reheating to Ac3, the surface structure of the slab 1 is modified, and a slab with less surface cracking can be obtained in combination with appropriate secondary cooling. From the viewpoint of further suppressing the variation in the surface temperature and structure in the slab 1, it is preferable that the maximum temperature after reheating is Ac3 + 30 ° C. or higher. Further, if the reheating temperature is too high, the austenite crystal grains may be coarsened again, so the temperature is preferably 1200 ° C. or lower.

Ac3 can be measured using a metamorphosis point recording measuring device (four master device) or the like. Alternatively, Ac3 can be specified using the following formula (d) proposed in the prior art (Tatsuro Kunitake: Heat Treatment, 43, p. 100 (2003)).

Ac3 = (32 [Si] +17 [Mo])-(231 [C] +20 [Mn] +40 [Cu] +18 [Ni] +15 [Cr]) +912 ・ ・ ・ (d)
([Si], [Mo], [C], [Mn], [Cu], [Ni], and [Cr] in the formula (d) represent the concentration (mass%) of each component.)

The reheat of 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 reheating of the surface of the slab 1 can be relatively easily performed 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 rate (heating rate) of the slab 1 is not particularly limited, and a desired effect can be exhibited at any reheating rate.

The examples shown below show an example of the steel continuous casting method of the present disclosure. The continuous steel casting method of the present disclosure is not limited to the examples shown below.

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

A sample having the steel composition shown in Table 1 below was heated to 1400 ° C. to form an austenite structure having an average particle size of 1.5 mm or more, and then rapidly cooled to various temperatures of 350 to 750 ° C. in a helium gas stream. The rapidly cooled sample was held isothermal for 30 to 20,000 seconds and then reheated to 910 ° C. at 20 ° C./s to make the whole tissue austenite and then cooled to room temperature at 0.2 ° C./s. The cross section of the obtained sample was corroded with a nital solution and observed by SEM.

An example of observation is shown in FIG. In the sample in which austenite was presumed to have been decomposed by quenching and temperature maintenance, as shown in Photo A on the left of FIG. 5, coarse austenite grain boundaries were not observed, and a large number of ferrites were dispersed (hereinafter, this form). The organization is referred to as "organization A"). On the other hand, in the sample in which austenite is presumed not decomposed, as shown in Photo B on the right of FIG. 5, not only dispersed ferrite but also ferrite was hardly observed on the old austenite grain boundaries (hereinafter, this structure is referred to as "this structure" Tissue B ").

For each of the steel types N05, N10, and N20, the relationship between the heat treatment pattern attached by the transformation point recording measuring device (four master device) and the obtained structure is shown in FIGS. 6, 7, and 8. At 350 to 475 ° C., the holding time for obtaining the tissue A on the entire surface of the sample was short, and all were 60 seconds or less. On the other hand, at 500 to 700 ° C., the holding time for obtaining the structure A greatly differed depending on the steel type. Although the tissue A was obtained in this temperature range, a retention time of 5 to 125 times that of 350 to 475 ° C. was required.

2. 2. Model experiment 2
From the above model experiment, it is considered that there are two temperature regions in which the decomposition of coarse austenite proceeds relatively rapidly, 350 to 475 ° C and 600 to 675 ° C, and bainite (martensite) is considered to be in those temperature regions, respectively. May be included), presumed to be ferrite and pearlite. Following the above experiment, the present inventors carried out the following experiment in order to understand the transformation behavior when the slab surface layer straddles both of these two temperature ranges in an actual continuous casting machine.

A sample similar to the above model experiment was heated to 1400 ° C. to form an austenite structure having an average particle size of 1.5 mm or more, then rapidly cooled to 425 ° C. in a helium gas stream, and maintained at an isothermal temperature for 15 seconds or 45 seconds. It was subsequently heated to 625 ° C. at 20 ° C./s and kept isothermal for 40-5400 seconds. Then, the tissue was further reheated to 910 ° C. at 10 ° C./s to make the whole tissue austenite, and then cooled to room temperature at 0.2 ° C./s. The cross section of the obtained sample was corroded with a nital solution and observed by SEM.

The relationship between the heat treatment pattern and the obtained structure is shown in Table 2 below. In Table 2 below, "x" means that the entire surface was organization B, "△" means that organization A and organization B were mixed, and "○" means that the entire surface was organization A. It means that.

As is clear from the results shown in Table 2, the sample held at 425 ° C. for 45 seconds exhibited the entire structure A in the steel type N05 for 80 seconds or more and in the steel type N20 for 1800 seconds or more. The sample held at 425 ° C. for 15 seconds was subsequently held at 625 ° C. for 240 seconds for steel type N05 and 5400 seconds for steel type N20. At this time, it is presumed that the coarse austenite is transformed into bainite while being held at 425 ° C., and the untransformed austenite is transformed into ferrite-pearlite while being held at 625 ° C. That is, it is considered that the decomposition of austenite is completed when the sum of the bainite transformation amount and the ferrite-pearlite transformation amount is balanced with the austenite amount before transformation, and the tissue A is exhibited during slow cooling through the subsequent reheating to 910 ° C. ..

These results, as a condition for exhibiting entire tissue A, T _A of formula (a) below has been found to be at least 60 seconds.
_{T A = T L + T H} × 0.04 × [Ni] -2.32 ··· (a)
(In the formula (a), _{T L:} time the sample is in the 475 ° C. or less 350 ° C. or higher, _{T H:} sample is a time in the 600 ° C. or higher 675 ° C. or less, [Ni] is the concentration of Ni in the steel (mass %).)

It has been confirmed that the above relational expression also holds in experiments carried out using other Ni-containing low alloy steels. That is, in terms of mass%, C: 0.1 to 0.4%, Si: 0.01 to 0.5%, Mn: 0.3 to 1.4%, Cr: 0.8 to 2.0%, When a steel having a composition of Mo: 0.6% or less and Ni: 0.5 to 2.0% is targeted for casting, the slab is secondarily cooled so as to satisfy the above formula (a). The surface layer structure of the slab can be miniaturized, and cracks on the surface of the slab at the straightening point can be suppressed.

As shown in the above formula (a), in a steel having a high Ni content, it takes a very long time to decompose austenite only in the temperature range of 600 to 675 ° C. That is, when producing such a steel grade, it is desirable to emphasize the temperature range of 350 to 475 ° C. from the viewpoint of productivity.

3. 3. Actual machine test A molten steel having the composition shown in Table 3 below was melted by a converter-LF process, and a slab having a size of 220 mm × 256 mm was cast in a curved continuous casting machine having a radius of curvature of 12.0 m. The casting speed is 1.0 to 1.6 m / min. The slabs drawn from the mold were rapidly cooled by a spray quenching device having 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 the reheat. The slab was cut to a length of 5.0 ± 0.2 m with a gas cutting machine and then subjected to surface observation.

The temperature of the slab surface is the temperature at the center of the slab L surface calculated by heat transfer solidification calculation. It was verified that the temperature calculated by the heat transfer solidification calculation was sufficiently high accuracy by comparing with the data of the slab surface temperature system installed in the continuous casting machine. Further, the value of Ac3, which is the target of the maximum temperature reached after reheating, was specified by the above formula (d).

The surface cracks of the obtained steel pieces were visually observed. The table below shows the survey results together with the time when the slab surface temperature was between 350 to 475 ° C, 475 to 600 ° C, and 600 to 675 ° C in the cooling zone, and the maximum temperature from the start of reheating to the correction point. Shown in 4. In Table 4 below, the evaluation of surface cracks was "○" for those without cracks, and "○" for those with a crack depth of less than 0.5 mm and a number of cracks of 10 or less per slab. Those that do not correspond to "Δ" are indicated as "x".

In Examples 1 to 9, slabs having good surface quality without cracking were obtained. On the other hand, the highest and when the T _A represented by the formula (a) as in Comparative Examples 1 to 9 and less than 60 seconds, since the start of recuperation as in Comparative Example 10 to 13 up to the correct point When the temperature was lower than Ac3, cracks were exhibited on the surface of the slab.

1 slab 10 mold 20 straightening point 100 continuous casting machine

Claims (2)

By mass%, C: 0.1 to 0.4%, Si: 0.01 to 0.5%, Mn: 0.3 to 1.4%, Cr: 0.8 to 2.0%, Mo: A method of continuously casting steel slabs having a composition of 0.6% or less and Ni: 0.5 to 2.0% using a continuous casting machine having a straightening point.
Before reaching right under the mold to the correct point, a time in which the surface temperature of the slab is between 350~475 ℃ _T L (s), as _T H (s) of time in the 600-675 ° C., the cast strip is cooled as defined by the following formula _(a) T _a (s) is 60 or more,
Next, the surface temperature of the slab is reheated to a temperature range of Ac3 or higher before reaching the correction point.
Continuous casting method for Ni-containing low alloy steel.
_{T A = T L + T H} × 0.04 × [Ni] -2.32 ··· (a)
(In the formula (a), [Ni] is the concentration (mass%) of Ni in the steel.) The slabs are mass%, Al: 0.1% or less, Ti: 0.1% or less, V: 0.4% or less, Ca: 0.01% or less, Mg: 0.01% or less, REM. : 0.01% or less, Nb: 0.05% or less, B: 0.004% or less, N: 0.025% or less.
The method for continuously casting a Ni-containing low alloy steel according to claim 1.