JP4924104B2 - Method for producing high Ni content steel slab - Google Patents

Description

  The present invention relates to a method for producing a high Ni-containing steel slab using continuous casting, and in particular, by specifying the cooling rate and cooling capacity when cooling the continuous cast slab and controlling the structure of the continuous cast slab. It is intended to prevent the occurrence of cracks in the slab surface layer.

  In continuous casting of steel, as shown in FIG. 1, molten steel poured into a tundish 1 from a ladle (not shown) is passed through a smashing nozzle (not shown) installed at the bottom of the tundish 1. Continuous casting by pouring into a water-cooled continuous casting mold (hereinafter simply referred to as a mold) 2 and then continuously drawing the slab with the solidified shell formed by the mold as the outer shell while cooling down. A slab (hereinafter simply referred to as a slab) is manufactured.

In this case, the molten steel is cooled by first contacting the mold 2 to form a solidified shell. After that, the slab that has passed through the mold 2 is supported by a mold slab support device 3 including a cooling grid or a support roll immediately below the mold, and further supported by a guide roll 4-1 below the pinch roll 4- 2 is pulled in the casting direction. These casting mold direct slab support device 3, guide roll 4-1 and pinch roll 4-2 are called slab support / guide devices. The slab is supported by the slab support / guide device, thereby preventing swelling (referred to as bulging) in the thickness direction of the slab.
In the figure, number 5 is a slab cutting machine (cutter), and 6 is a slab.

As shown in FIG. 2, the slab support / guide device is provided with a spray nozzle (8 or 9), and the slab is cooled by cooling water sprayed from the spray nozzle (8 or 9). 6 is pulled out and eventually solidification to the center is completed. Then, it cut | disconnects to predetermined length with the slab cutting machine 5 installed in the machine end of the continuous casting machine, and becomes a slab. The area of the slab support / guide device where the spray nozzle (8 or 9) is located is generally called the secondary cooling zone.
In the figure, reference numeral 7 denotes a wear plate for supporting the cast piece 6 in which the solidified shell is thin and easy to bulge, particularly immediately below the mold. Reference numeral 8 denotes a conical nozzle of the spray nozzle, and 9 denotes a porous nozzle.

Up to now, continuous casting of high Ni-containing steel containing 5 mass% or more of Ni causes surface vertical cracks, horizontal cracks, corner cracks near the corners, and subsurface cracks. Various measures have been taken to prevent this.
Since vertical cracks on the surface of the slab often crack at the dendrite part in the mold, as a method of initial solidification control, use a slow cooling powder with low heat removal from the mold to obtain a difference in the thickness of the solidified shell. By reducing the size, thermal stress cracking was prevented.
In addition, lateral cracks and corner cracks in the slab are controlled by casting speed and secondary cooling so that the slab does not enter the brittle temperature range when the slab is strained in the straightening zone of a vertical or curved bending continuous casting machine. It has been prevented by adjusting.
Furthermore, regarding the subsurface cracks in the slab, it is thought that it is mainly caused by uneven cooling in the width direction of the slab, so as a secondary cooling spray, it is said that more uniform cooling is possible. The prevention has been achieved by adopting a mist spray which is a two-fluid spray of air.

  In addition, the surface crack of the general carbon steel which does not contain so much Ni occurs in the vicinity of the grain boundary or the pro-eutectoid ferrite part of the grain boundary, and the cause is that the surface temperature of the slab decreases in hot ductility. This is considered to be because the slab is corrected at around the γ → α transformation temperature (about 600 to 850 ° C.).

Therefore, in Patent Document 1, the slab surface temperature is once cooled to 650 to 700 ° C., then slowly cooled, and then reheated to 700 to 800 ° C., whereby the slab surface layer is strengthened systematically. Has proposed.
However, in this method, it is difficult to avoid the brittle temperature on the high temperature side as the recuperation temperature, and it can be applied only to the steel grade that undergoes the γ → α transformation. There was a problem that it could not be applied.

  Patent Document 2 proposes a method in which the slab surface layer portion is cooled at a cooling rate of 10 ° C./s or more from the austenite single-phase region temperature or more to make the austenite grain size of the surface layer fine. In this case, although the austenite grain size is surely small, an increase in cost is unavoidable because He gas is used as a cooling medium, and it is not suitable as a production process in the steel industry.

Patent Document 3 describes a method in which the surface layer is similarly cooled to the Ar ₃ transformation point or lower, then reheated to the Ar ₃ transformation point or more, and the surface temperature is corrected to a temperature of less than 850 ° C.
However, similarly to Patent Document 1, this method has a problem that it can be applied only to a steel type that undergoes γ → α transformation.

Patent Document 4 describes a method of preventing cracking by transforming the surface layer by cooling and holding the surface layer portion of the slab at a temperature of 300 to 500 ° C. for 1 minute or longer.
However, this method is a technique in which the slab is cooled after being cut with a cutter, and is not intended to control the surface structure of the slab in the continuous casting machine, so it is not possible to prevent cracking in the continuous casting machine. It was possible.

Japanese Patent Publication No.58-3790 JP 63-63559 A JP 2002-86252 A JP-A-5-329505

  The present invention has been developed in view of the above-mentioned present situation, and effectively suppresses surface vertical cracks, lateral cracks, corner cracks, subsurface cracks, and the like of slabs that are likely to occur during continuous casting of high Ni-containing steels. The object is to propose an advantageous production method for high Ni-containing steel slabs that can be prevented.

That is, the gist configuration of the present invention is as follows.
(1) When continuously casting carbon steel containing 5 mass% or more of Ni, the secondary arm spacing of the dendrite of the solidified structure of the slab surface layer is controlled to less than 40 μm, and then the secondary installed downstream from the continuous casting mold In a partial zone of the cooling zone, using a multi-hole nozzle or conical nozzle, the water density immediately below the spray is 3000 L / (minm2) or more, and the average heat transfer coefficient of the slab surface in the secondary cooling zone is 1000 kcal / A method for producing a high Ni-containing steel slab characterized by performing strong cooling of (m 2 · hr · ° C.) or more and then correcting the slab.

(2) In the case of spray cooling in a partial zone of the secondary cooling zone, when cooling the slab using a porous nozzle, a flow rate for removing the vapor film on the slab surface is secured. The manufacturing method of the high Ni content steel slab of Claim 1.

  According to the present invention, by optimizing the cooling conditions in the mold and the secondary cooling zone immediately below the mold, and the accompanying structure control of the slab surface layer part, it is possible to eliminate surface cracks in the slab in high Ni-containing steel. It is possible to omit slab care and to omit processes after rolling. As a result, even in a high Ni-containing steel with high cracking sensitivity, a high-quality slab can be stably produced without causing operational troubles, which is extremely useful industrially.

Hereinafter, the present invention will be specifically described with reference to the drawings.
First, control of the cooling rate of the initial solidified shell will be described. First, cooling control in the mold was performed by changing the thickness of the mold into which molten steel was cast in a laboratory experiment. Specifically, in order to obtain slow cooling, that is, to obtain an initial solidification rate in which the dendrite secondary arm spacing is 40 μm or more, the thickness of the mold is increased, while strong cooling, that is, the dendrite secondary arm spacing is 40 μm. The mold thickness was reduced to obtain an initial solidification rate of less than. That is, in the case of the strong cooling experiment, the thickness of the mold was set to 5 mm, while in the case of the slow cooling experiment, the thickness of the mold was set to 15 mm.

  Moreover, when measuring the secondary arm space | interval of a dendrite, it solidified and measured as it was, without providing secondary cooling after that. In the measurement, since the purpose of the present invention is to control cracks in the surface layer within 10 mm from the surface, the average value of the secondary arm spacing of 20 dendrites within the surface layer within 10 mm from the surface is obtained. The distance between the dendritic secondary arms of the present invention was used.

In FIG. 3, the structure | tissue photograph of an actual slab is shown.
The photograph in Fig. 3 shows the cross-section of the slab (C-section in the width direction) ground and polished and then corroded with picric acid to extract the solidified structure. In this case, FIG. 5B shows the case of strong cooling. In general, the smaller the secondary arm spacing of the dendrite, which is a solidified structure, indicates that the cooling rate is faster.In fact, in the case of FIG. (A), the secondary arm spacing of the dendrite is 46 μm, whereas FIG. In this case, the dendrite secondary arm interval was 38 μm.

Thus, next, cast slabs with various secondary arm intervals of dendrites were produced, and the relationship between the secondary arm intervals and cracks of the slabs was investigated.
As a result, it was found that in high Ni-containing steels containing 5 mass% or more of Ni, the difference in cracking morphology occurs with the dendrite secondary arm interval being 40 μm as a threshold value.
That is, if the dendrite secondary arm interval is 40 μm or more, it means that the cooling rate during solidification is slow, whereas if the dendrite secondary arm interval is less than 40 μm, the cooling rate during solidification is fast. It has been clarified that since the starting point of nucleation increases, the solidified structure becomes finer, the strength of the slab increases, and it becomes difficult to break.

  As described above, it has been found that it is effective to reduce the dendrite secondary arm interval to less than 40 μm in order to prevent cracking of the cast slab. Even if the cooling conditions in the secondary cooling zone, which will be described later, are appropriate, cooling with the mold is an important factor that determines the initial solidification. Therefore, the control of the initial solidification is evaluated by the secondary arm interval of the dendrite. It is extremely important to do.

According to the Steel Handbook, etc., an equation for converting the cooling rate from the secondary arm spacing of this dendrite has also been proposed. However, for the high Ni-containing steel that is the subject of the present invention, the correct conversion equation is not currently available. It has not been issued.
However, according to experiments by the inventors on this point, in order to control the secondary arm spacing of the dendrite in the slab surface layer, i.e., the area from the slab surface to 10 mm, to less than 40 μm, the mold powder has strong heat removal. It has been found that it is effective to use one.

(1) Generally, it is important to reduce the interfacial thermal resistance between the mold and the slab for the heat transfer behavior between the mold and the slab. To reduce the interfacial thermal resistance, The surface roughness of the surface that contacts and solidifies the powder should be reduced. For that purpose, a vitrifying powder in which crystals do not precipitate may be used. In general, when the cooling rate is high, crystals may not precipitate, so when the powder once melted is cooled at 5 ° C./min, the peak of crystal precipitation is not detected by DTA (Differential Thermal Analysis) or the like. Use powder.
(2) In addition to the interfacial thermal resistance, if the thickness of the powder film flowing between the mold and the slab is thin, the heat removal ability naturally increases. For this reason, in order to reduce the thickness of the powder film, it is important to use a high-viscosity powder, and it is preferable to use a powder of 2 poise or more.
(3) It is particularly preferable to use a mold powder that satisfies both the above (1) and (2).

Next, based on FIG. 2, the cooling procedure of the slab in the secondary cooling zone just under a casting_mold | template is demonstrated.
In the continuous casting machine, there are a so-called support roll type in which a roll is installed immediately below the mold and the slab is supported by the roll, and a system in which the slab is fixed on the surface, that is, a cooling grid is used. The cooling grid is composed of a wear plate 7 and a secondary cooling spray (8 or 9). In this cooling grid method, the area of the thin shell directly under the mold is not fixed with a wire like a roll, but the surface of the slab is fixed with a surface, so the slab thickness directly under the mold is thin and bulging is likely to occur. Even in the area, there is an advantage that stable operation is possible.

  In the case of this cooling grid method, since the wear plates 7 are arranged in a staggered manner, the cooling in the width direction is uniform in the total continuous casting machine, but strictly speaking, the wear plates 7 are installed in an area where the wear plates 7 do not exist. Cooling is performed by a secondary cooling spray. Originally, when viewed from the slab, the average water density is the value divided by the area including the wear plate 7, but when looking at the cooling capacity alone, the actual cooling capacity in the area immediately below the spray nozzle Needless to say, this has a big influence.

Therefore, the cooling capacity value of the portion where the wear plate 7 does not exist (the region where the water of the cooling spray shown in FIG. 2 is circled) is defined as the local water density, and the heat transfer coefficient value of the region is defined as the local heat transfer coefficient value. Defined as heat transfer coefficient.
The average heat transfer coefficient needs to be calculated in consideration of the area of the wear plate. Generally, since the cooling from the wear plate is small, the average heat transfer coefficient is smaller than the local heat transfer coefficient. Further, the local cooling, that is, the cooling between the wear plates may be a water spray that is one fluid, or a mist that is a two-fluid mixture of water and air. Appropriate cooling method should be selected according to each needs.

  In general, water spray or mist spray is sprayed from one nozzle tip so that cooling water is uniformly applied to the entire cooling area. However, for cooling, there is not a single nozzle tip, but a cooling method using a multi-hole nozzle having a plurality of holes, such as a household shower, is advantageous. Of course, if spray arrangement is possible, a plurality of sprays may be arranged in the gap between the wear plates.

Next, the heat transfer coefficient on the slab surface will be described.
It is virtually impossible to measure the correct heat transfer coefficient by incorporating a thermocouple into the slab of an actual continuous caster. Therefore, an experiment for evaluating only the cooling capacity was performed according to the following procedure.
First, molten steel adjusted to the specified composition is melted in a melting furnace in the required amount, then cast into an ingot (cross-sectional area: 100 mm x 120 mm) of about 25 kg, and after the surface layer: 15 mm shell has hardened, One side of the ingot was released and cooled with spray. In this case, thermocouples were installed at positions 10, 20, and 30 mm from the surface in a direction perpendicular to the surface of the ingot to be released, and the heat transfer coefficient was calculated from the temperature history of these thermocouples. At this time, since the temperature of the slab exceeded 1500 ° C., the R type was adopted as the thermocouple. Moreover, the slab surface was cooled by a water spray that is one fluid, and it was performed by two methods: a method of cooling from one commonly used nozzle and a method of cooling by a porous nozzle. As the cooling width, when the water spray was a conical nozzle, a nozzle capable of cooling a conical region having a diameter of 90 mm was used. The perforated nozzle used was a 90 × 90 mm plate with a hole diameter of 3 mm arranged at a pitch of 30 mm. Therefore, if the experiment is carried out with an appropriate water pressure, the actual cooling area is 90 × 90 mm, and it can be said that the cooling area is almost the same as that of the conical nozzle. And this area was used as an area at the time of calculating | requiring each local water amount density.

  In addition, since the present invention contemplates cooling directly under the mold, the cooling surface is perpendicular to the slab surface in order to simulate the vertical portion of the continuous casting machine. In addition, the cooling time was set to 60 seconds because a part of the secondary cooling zone to be corrected with the actual machine was cooled.

  In the experiment, cooling started from the moment when the thermocouple 10mm from the surface reached 1200 ° C, and it was taken into a personal computer every 0.1 seconds. After the experiment, the heat transfer coefficient of each thermocouple was calculated from the measured temperature history. did. This heat transfer coefficient is three points in the depth direction of the thermocouple, but in the cooling area of about 90mm square, only one thermocouple is installed, and the heat transfer coefficient of the present invention is the local heat transfer coefficient. I asked for.

It should be noted that in the case of an actual machine, as can be seen from FIG. 2, the wear plate portion is not cooled by the local heat transfer coefficient obtained in the above-described laboratory experiment. Therefore, it is necessary to adopt an average heat transfer coefficient.
Hereinafter, a method for calculating the average heat transfer coefficient will be briefly described. First, the area ratio of the wear plate and the heat transfer coefficient of the wear plate within the zone area of the cooling grid are obtained, and the average heat transfer coefficient in the entire zone is calculated. The heat transfer coefficient of the wear plate portion was found in a similar experiment to be about 300 kcal / (m ² · hr · ° C.), and the present invention was applied to an actual continuous casting machine as shown in the examples described later. At the time, the proportion of the wear plate in the cooling grid was 53%. Therefore, the average heat transfer coefficient was calculated using this ratio.
Further, since the heat transfer coefficient changes depending on the temperature, in the present invention, the cooling capacity was compared using the heat transfer coefficient at 850 ° C., which is estimated to be close to the average surface temperature of the continuous cast slab.

In FIG. 4, the local heat transfer coefficient calculated | required by the above-mentioned laboratory experiment using a conical nozzle and a porous nozzle as a water spray nozzle is shown by the relationship with the water amount density of each nozzle.
From the figure, it is understood that the water density is preferably 3000 L / (min · m ² ) or more in the case of the multi-hole nozzle. Further, when the pressure is 4000 L / (min · m ² ) or more, the cooling capacity is further increased, but the cooling capacity is not improved as the water density is increased. Accordingly, the water density is 3000 L / (min · m ² ) or more, preferably 3000 to 4000 L / (min · m ² ).
In this respect, even with a conical nozzle, if the amount of water is increased, cooling similar to that of a porous nozzle can be obtained, but a very large amount of water is required. Therefore, it is desirable to use a porous nozzle with excellent cooling capacity. If only an area of 90 mm × 90 mm is cooled, there is no problem with a single water spray, but from the viewpoint of cooling uniformly in the width direction, more uniform cooling can be achieved by using a porous nozzle.

As for the amount of water, if the amount of water is excessively reduced in the porous nozzle, the flow velocity that collides with the surface of the slab is eventually reduced, and the vapor film on the surface of the slab cannot be removed.
As can be seen from the results in FIG. 4, when a multi-hole nozzle is used, a water density of 3000 L / (min · m ² ) or more is necessary for securing a flow rate.

Further, when the solidified structure was observed, the high Ni-containing steel targeted in the present invention is γ single-phase solidified and therefore does not undergo the γ → α transformation. However, when the slab was macro-etched, a refined phase was observed on the surface layer. When estimated from three thermocouples installed in the slab thickness direction, it was found that the boundary temperature of the refined layer thickness was the martensitic transformation temperature. Therefore, it is effective to cool to a martensitic transformation in a steel type that does not undergo the γ → α transformation.
FIG. 5 shows an example (macrophotograph) of a solidified structure whose surface layer has been refined by martensitic transformation.

In addition, as high Ni content steel made into object by this invention, all will be suitable if it is carbon steel which contains 5 mass% or more of Ni in steel.
However, since an iron-nickel alloy containing Ni in a large amount exceeding 45 mass% is less in demand and lacks the merit of continuous casting, the upper limit of Ni content is preferably about 45 mass%.
Other components are not particularly limited, but typical components and preferred contents thereof are described as follows.
C: 0.03-0.07 mass%, Si: 0.1-0.5 mass%, Mn: 0.3-0.8 mass%, P: 0.01 mass% or less, S: 0.01 mass% or less, Al: 0.005-0.006 mass%.

As the mold _{powder, SiO 2: 30~45mass%, CaO} : 20~40mass%, Al 2 O 3: 1~20mass%, Na 2 O: 0.5~15mass%, Li 2 O: less than 4 mass%, F : Less than 10 mass%, a composition of CaO / SiO ₂ <1.2, and a viscosity of 2 poise or more are advantageously suitable.

  As described above, according to the present invention, when cooling from between the wear plates in the cooling grid portion directly below the mold, a predetermined water content density is ensured to make the solidified structure of the slab surface layer: 10 mm fine. This makes it possible to obtain a slab that does not crack in the surface layer. That is, a high-quality slab can be stably produced without causing any trouble in operation.

  Short side (corresponding to slab thickness): 250 mm, long side (corresponding to slab width): 1750 mm, using a continuous casting machine with a cooling grid directly under the mold, steel materials 1, 2 and 3 shown in Table 1 The component molten steel was continuously cast. At this time, casting was performed at a casting speed of 0.7 m / min, a superheat degree of molten steel of 32 ° C., and various changes in secondary cooling. In the cooling grid portion, a porous nozzle was incorporated in one strand, and the other strand was cast using a conventionally used conical nozzle.

Control of cooling in the mold was performed by using two different mold powders. The composition of the powder used is shown in Table 2. It was assumed that powder A was slowly cooled, that is, the dendrite secondary arm interval was 40 μm or more, and powder B was strongly cooled, that is, the dendrite secondary arm interval was less than 40 μm.
The viscosity of the powder is a measured value measured by a platinum pulling method at 1300 ° C. The crystallization temperature is a temperature at which the first peak of crystal precipitation expressed by DTA appears after cooling at 5 ° C./min from the molten state.

  Furthermore, for strong cooling of the cooling grid part until correction, the amount of water per nozzle was changed to 12, 25, and 48 L / min during casting. The length of the cooling grid in the casting direction, that is, the zone length is 0.79 m. As shown in FIG. 2, the cooling grid has three rows of nozzles and a staggered arrangement. The number of nozzles is 18 in the first and third rows in the width direction per side and 19 in the second row. That is, under the experimental conditions shown in Table 3, the specific water amount only in the cooling grid portion changes from 0.55 to 2.21 L / kg. However, the amount of secondary cooling water was set so that the surface temperature of the slab could be reheated without changing the secondary cooling zone at the lower part of the slab, and making it cooler than the cooling capacity of the cooling grid part. . The specific water amount is 0.6 L / kg.

For the slab, the slab cut with a cutter torch is cooled once to room temperature, cut and processed, then macro-etched, and the C cross section (cross section in the width direction) of the slab is observed to investigate the solidified structure within the surface layer of 10 mm The pass / fail was determined by the following evaluation.
×: There is a crack of 3 mm or more in the surface layer of 10 mm ... Failure △: There is a crack of 1 to 3 mm in the surface layer of 10 mm ... Failure ○: A crack of 1 mm or more in the surface layer of 10 mm No pass ... Pass: A: No cracks in the surface layer of 10 mm ... Pass Table 3 shows the results.

As shown in Table 3, the dendrite secondary arm spacing of the solidified structure of the slab surface is less than 40 μm, the water density immediately below the spray is 3000 L / (min · m ² ) or more, and By setting the average heat transfer coefficient in the next cooling zone to 1000 kcal / (m ² · hr · ° C.) or more, the solidified structure of the surface layer: 10 mm could be refined and cracking of the slab could be prevented.

It is the schematic of a continuous casting machine. FIG. 5 is a schematic view showing a structure of a cooling grid immediately below a mold of a continuous casting machine and a state in which a slab is cooled by a multi-hole nozzle or a conical nozzle. This shows the solidification structure of the slab surface layer, where (a) shows the case of slow cooling and (b) shows the case of strong cooling. It is the figure which showed the relationship between the water quantity density in a laboratory experiment, and a heat transfer coefficient. It is a macro photograph of the solidification structure | tissue which the surface layer refined | miniaturized by the martensitic transformation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Tundish 2 Mold 3 Cast slab support device 4-1 Guide roll 4-2 Pinch roll 5 Slab cutting machine (cutter)
6 Cast slab 7 Wear plate 8 Conical nozzle 9 Porous nozzle

Claims (2)

During continuous casting of carbon steel containing 5 mass% or more of Ni, the secondary arm spacing of the dendrite in the solidified structure of the slab surface is controlled to less than 40μm, and then the secondary cooling zone installed downstream of the continuous casting mold. In some zones, a porous nozzle or a conical nozzle is used, and the water density immediately below the spray is 3000 L / (min ・ m2) or more, and the average heat transfer coefficient of the slab surface in the secondary cooling zone is 1000 kcal / (m2 ・hr · ° C) or more, followed by straightening of the slab, and a method for producing a high Ni-containing steel slab. 2. The flow rate for removing the vapor film on the surface of the slab is secured when cooling the slab using a porous nozzle during spray cooling in a partial zone of the secondary cooling zone. The manufacturing method of the high Ni content steel slab of description.