KR100825571B1 - A continuous casting process of the steel containing high Ni for LNG tanks - Google Patents

Abstract

A continuous casting method of high nickel-containing steel for LNG tanks is provided.

In the method of the present invention, in refining molten steel containing high Ni, degassing the refined molten steel, injecting the degassed molten steel into a tundish and a molder, and continuously casting the slab by a machine, Degassing molten steel for 20 minutes or more at low vacuum so that its N content is 30 ppm or less; Continuously casting the degassed molten steel in a tundish while controlling the degree of superheating in a range of 13 to 17 ° C., at a speed of 1.17-1.19 m / min; And sprinkling and cooling the slab exiting the mold by the continuous casting by cooling the coolant in a secondary cooling stand, and playing a roll gap of a pass line through which the slab passes from the bottom of the mold. The solid-thickness / slave-thickness ratio in the cabin should be tapered at 0.19-0.20 mm / m until 0.9, and the roll gap of the passline through which the slab passes near solidification is 2.75 in the section with solidification fraction 0.3-0.7. Taper to ~ 2.85mm.

Roll gap, center segregation, continuous casting, high Ni-containing steel

Description

A continuous casting process of the steel containing high Ni for LNG tanks

1 is a partial configuration diagram of a continuous casting device

Figure 2 is a photograph showing the comparison of the test results with the surface crack marks of the steel in this embodiment,

    2 (a) is a steel of the present invention,

    2 (b) is a macro photograph of the cast steel surface of the comparative steel.

Figure 3 is a photograph showing the comparison of the test results of the central segregation macro of the steel in the present embodiment,

     3 (a) shows the present invention steel,

     Figure 3 (b) is a photograph of the central segregation macro of the comparative steel.

Figure 4 is a graph showing the results of the center segregation macro test by casting speed

Figure 5 is a graph showing the results of the center segregation macro test according to the amount of taper when casting at the casting speed performed in the present invention in the section of the solid phase ratio 0.3 ~ 0.7

The present invention relates to a continuous casting method of high-nickel-containing steel for LNG tanks, and more particularly, high-nickel-containing steel that can ensure excellent surface and internal quality in a cast state by appropriately controlling refining conditions and continuous casting conditions. The present invention relates to a method for producing a thick plate by continuous casting.

In general, Ni is an element that improves low temperature toughness. Therefore, high Ni-containing steel is mainly used from propylene gas (approximately 2.5% Ni-containing steel) to LNG gas (approximately 9% Ni-containing steel) depending on the Ni content. It is mainly used for tanks containing low temperature liquefied gas. As such, high-Ni-containing steels are not allowed to have surface cracks to ensure safety because their main use is liquefied gas storage tanks. It should be much stricter.

Therefore, due to such strict quality characteristics, the steel that has been refined mainly in converter operation, external refining and degassing was made into ingots, and then the ingots were rolled into a plate in the process of forming into a final product having a predetermined thickness. . However, when a high Ni-containing alloy steel plate is manufactured by the ingot-fragmentation process, there is a problem in that the productivity is not good due to the solidification pipe generated during the solidification process and the segregation rate of the product after the ingot rolling, which does not reach 50% due to the central segregation in the ingot. there was.

In order to solve the above problems, studies are being conducted continuously to manufacture high Ni-containing steel through continuous casting. The main factors to be considered when manufacturing high Ni-containing steel by this continuous casting method are as follows. same.

That is, first, because austenite (γ) is the primary crystal, high temperature characteristics may be affected by compounds of N, P, and S that tend to precipitate at grain boundaries. (α) ↔ austenite (γ) transformation, heat shrinkage and thermal expansion is about 50% larger than general steel, third, high temperature embrittlement area is wider than 600 ~ 900 ℃ range than general steel (700 ~ 900 ℃), fourth In this case, it is very easy to generate surface cracks because the scale adheres to the surface of the cast when it is cooled, thereby promoting uneven cooling of the surface of the cast.

Therefore, in order to secure the surface quality of the cast through the continuous casting process, secondary cooling weak cooling and high speed casting are advantageous. On the contrary, in terms of securing the quality of the casting, in contrast, cold cooling and low speed casting are advantageous. In addition to securing surface quality through secondary cooling, weak cooling and high speed casting, the internal quality should also be secured by adjusting the roll gap in the pass line of the appropriate instrument. In addition, high Ni-containing steels have to be set to the optimum casting conditions because of their characteristics different from those of ordinary steel, so that castings can be completed without changing casting conditions until the 1Heat casting is completed.

An example of the prior art of manufacturing a high Ni-containing steel plate by such a continuous casting process may be Korean Patent Application No. 1997-74573. In the above patent application, while casting the flux having a basicity of 0.9 to 1.2 on the molten steel surface while adjusting the short side taper of the mold to the range of 1.05 to 1.10%, the casting speed is 0.8 to 1.0 m / min and the secondary cooling ratio A continuous casting method is proposed, with yields ranging from 0.40 to 0.45 l / kg.

In addition, in the Republic of Korea Patent Application No. 2000-77025, the casting speed is 1.0 ~ 1.1m / min, and the non-quantity in the secondary cooling zone 30 ~ 35, 330 ~ 340, 300 ~ 310, 220 respectively A method of playing high alloy steel for cryogenic LNG tanks which is continuously cast in the range of ~ 230, 130-150, 240-260 l / kg is presented.

However, there are many difficulties in manufacturing the high-Ni-containing steel having excellent surface quality and internal quality by the continuous casting method with the casting conditions proposed in the related art.

Accordingly, the present invention is to solve the above problems of the prior art, and when manufacturing high Ni-containing steel by continuous casting, by optimizing the refining conditions and continuous casting conditions, etc. to ensure excellent surface and internal quality in the cast state It is an object of the present invention to provide a continuous casting method of high nickel-containing steel for LNG tanks.

The present invention for achieving the above object, by weight% C: 0.05 ~ 0.10%, Si: 0.15 ~ 0.35%, Mn: 0.50 ~ 0.80%, P: 0.005% or less, S: 0.003% or less, soluble Al: 0.020 ~ 0.060%, Ni: 3.5 ~ 9.5%, balance iron and other impurity elements are refined with converter, degassed refined molten steel, inject degassed molten steel into tundish and molder In continuous casting of cast iron by                     

Degassing the refined molten steel for 20 minutes or more at a low vacuum so that its N content is 30 ppm or less; Continuously casting the degassed molten steel in a tundish while controlling the degree of superheating in a range of 13 to 17 ° C., at a speed of 1.17-1.19 m / min; And sprinkling and cooling the slab exiting the mold by the continuous casting by cooling the coolant in a secondary cooling stand, and playing a roll gap of a pass line through which the slab passes from the bottom of the mold. The solid-thickness / slave-thickness ratio in the cabin should be tapered at 0.19-0.20 mm / m until 0.9, and the roll gap of the passline through which the slab passes near solidification is 2.75 in the section with solidification fraction 0.3-0.7. It relates to a continuous casting method of high Ni-containing steel for LNG tanks, characterized in that to taper to ~ 2.85mm.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

1 is a partial configuration of a continuous casting apparatus, which is largely ladle (1), tundish (3), mold (5), pass line (7), secondary cooler (9) and roll gap adjustment It can be seen that the device 11 is configured.

First, the steel that can be applied to the reproducing casting method of the high Ni-containing steel for LNG tank of the present invention is a Ni content of 3.5 to 9.5% of the steel, generally C: 0.05 to 010% by weight, Si: 0.15 to 0.35 %, Mn: 0.50 to 0.80%, P: 0.005% or less, S: 0.003% or less, soluble Al: 0.02 to 0.06%, Ni: 3.5 to 9.5%, balance Fe and other unavoidable impurities Can be mentioned.

Since such high Ni steels are sensitive to surface cracks, it is desirable to manage soluble Al and N as low as possible to minimize embrittlement factors caused by precipitates of other impurities.

In the present invention, after the molten steel having the composition described above is prepared in the converter step, the molten steel is degassed for 20 minutes or more at low vacuum in the refining process so that the N content in the molten steel is 30 ppm or less. This is because if the nitrogen content is high, it causes cracks in the cast or the product over time, so that the degassing treatment is sufficiently performed at low vacuum.

At this time, Preferably it is degassing 20 minutes or more in the vacuum degree of 2 torr or less.

The degassed molten steel is injected into the tundish 3, as shown in Figure 1, for continuous casting, in this case, the superheat degree of the molten steel should be controlled in the range of 13 ~ 17 ℃. Because molten steel superheat management is very important to ensure the internal quality and castability of cast steel during continuous casting. If the molten steel superheat is too high, it is highly likely to cause central segregation due to the development of columnar tissue and the reduction of equiaxed tissue. On the contrary, if it is too low, the cleanliness of the steel may deteriorate due to clogging of the nozzle and insufficient inclusion separation.

Particularly, the present invention is a steel grade that is the first of the earliest Mingga, so there is almost no Mushy Zone, so that the columnar tissue, which is a vulnerable organization, develops and is highly susceptible to weak internal quality. It is preferable to manage at 17 degreeC.

The molten steel loaded into the tundish is continuously cast in the continuous casting machine. In the present invention, the casting speed is limited to 1.17-1.19 m / min during the continuous casting to ensure good cast quality.

In detail, when the high speed casting is performed, the surface temperature of the slab of the unbending section may be out of the range of 600 to 900 ° C., which is a brittle region in which slab cracks occur well. In this case, however, in order to prevent the occurrence of cast cracks, the surface temperature of cast steel is required to be 970 to 1000 ° C, which is about 70 ° C lowered by the contact between the caster roll and the heat, and thus 970 ° C compared to the brittle area 900 ° C. The above temperature is required to be secured. In consideration of this point, in the present invention, the continuous casting speed of the cast steel is limited to 1.17-1.19 m / min, which means that if the casting speed is less than 1.17 m / min, the surface temperature of the cast steel cannot be secured by more than 970 ° C. If the casting speed is higher than 1.19m / min, the surface temperature of cast steel can be secured more than 1000 ℃, so it is advantageous in terms of surface cracking, but the formation of solidification shell is so small that the breakout due to iron static pressure of molten steel is likely to occur. .

Next, in the present invention, the castings continuously cast at the continuous casting speed and exited from the mold 5 are sprinkled with cooling water in a secondary cooling stand.

In general, the amount of cooling water sprayed on the cast steel is determined in consideration of the thickness and speed of the casting product. In particular, in order to secure the surface quality of the cast steel as in the present invention, the austenite particles are formed by cold cooling directly under the mold. Miniaturization is required, and cooling conditions that can control the uniform cooling in the slab width direction and the calibration point passing temperature to 970 ° C or more are preferable. The passing point temperature refers to the temperature at the unbending part of the vertical bending machine, and the reason why this temperature is important is that cracks are likely to occur due to stress during unbending, and the surface temperature of the cast steel maintains a close relationship with the crack generation. Because there is.

To this end, it is desirable to increase the amount of water sprayed directly under the mold, and to cool the water not to spray water from a calibration point before a certain distance (for example, 5 m). And usually in the end of the casting to reduce the speed after the operation to increase the casting finish, the surface and the internal quality is deteriorated by the change in the casting speed, so it is preferable to cast at a constant speed without changing the casting speed even at the end of the casting.

In order to satisfy the above cooling conditions, more preferably, cooling is performed while controlling the amount of cooling water in the secondary cooling zone 9 in the range of 0.42 to 0.46 liter / kg.

Meanwhile, in the present invention, the roll gap of the pass line (Pass Line) 7 through which the cooled slab passes is tapered from 0.19 to 0.20 mm / m from 0.9 to 0.19 to 0.20 mm / m. In the case of high speed casting at 1.18-1.19m / min, slab bulging occurs, so that the bulging is effectively controlled by adjusting the roll gap of the pass line.

In the present invention, when the amount of taper is larger than 0.19-0.20 mm / m, the friction force between the cast and the roll is large, which may make it difficult to pull out the cast. When the taper is excessively large, cracks in the cast may be caused. On the other hand, if the taper amount is less than 0.19-0.20 mm / m, the bulging becomes difficult to control effectively.

In the present invention, in adjusting the roll gap of the pass line 7, the solid phase thickness / casting thickness ratio in the player from the bottom of the mold is based on a point of 0.9, which is a solidification proceeds considerably if it exceeds 0.9 and separate bulging occurs. It is not necessary to taper because it does not need to be tapered, but on the contrary, the roll life is shortened by friction, whereas when it is less than 0.9, it is not preferable because the amount of taper is insufficient to control bulging.

In addition, in the present invention, the roll gap of the pass line 7 through which the cast passes through the vicinity of the solidification is completed, and a taper of 2.75 to 2.85 mm is used in the section where the solidification fraction is 0.3 to 0.7 using the roll gap adjusting device 11. Do as possible.

In general, the poor internal quality at the center of continuous casting cast is a liquid containing concentrated solute elements and impurities remaining in the solid zone in accordance with the volume shrinkage that occurs during the transformation from liquid to solid phase at the end of continuous casting. This is because the volume of the volume shrinks to fill the center segregation, or if the liquid thickened steel being sucked is smaller than the volume reduction, it exists as pores. This central segregation and pore generation is increased by increasing the relatively high liquid coexistence zone at a high casting speed, as described above, the present invention performs a high-speed casting to ensure a good casting quality, so that the casting internal quality than the low-speed casting May be worse than that.                     

Therefore, in the present invention, in order to appropriately control the central segregation and pores generated during the transformation from the liquid phase to the solid phase, the pass gap (7) roll gap is set so that the section having a solidification fraction of 0.3 to 0.7 has a taper of 2.75 to 2.85 mm. Do it. That is, the present invention is to compensate for the volume shrinkage that occurs during the transformation from the liquid phase to the solid phase with a roll gap taper to suppress central segregation and pore generation.

In detail, in the present invention, it is preferable to give a roll gap taper in the section of the solid phase ratio of 0.3 to 0.7. In the present invention, if the roll gap taper is given before the solid phase ratio of 0.3, the liquid phase has good fluidity, and there is no compensating effect on volume shrinkage. This is because a roll gap taper after a solid phase ratio of 0.7 may cause an internal crack. In addition, the amount of taper is limited to 2.75 ~ 2.85mm, which means that if less than 2.75mm, the center segregation may remain due to lack of compensation for volume shrinkage, and if it exceeds 2.85mm, the amount of shrinkage will be exceeded due to excessive taper. This can cause segregation.

By continuous casting under the above conditions, it is possible to obtain an excellent cast steel for producing high Ni steel without surface cracks. However, such high Ni steel has a problem that surface defects may occur due to an oxidizing scale after rolling due to an oxidative atmosphere in a heating furnace during subsequent thick plate rolling.

For this purpose, it is more preferable to uniformly coat the anti-oxidant coating agent after grinding the surface of the cast piece passed through the continuous period.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

The high Ni-containing steel having the composition shown in Table 1 was degassed for at least 20 minutes at a low vacuum of 2torr or less after the converter was blown. Then, the degassed molten steel was charged into a tundish, and the tundish molten steel superheat was managed at 13 to 17 ° C. The molten steel was continuously cast under the conditions shown in Table 2 to obtain a cast having a width of 1600 to 1900 mm. .

At this time, the player used was a player that can adjust the roll gap of the pass line as shown in Figure 1, the taper of the mold was given as 1.05 ~ 1.08%.

On the other hand, a part (sample) was taken from the cast pieces obtained by continuous casting, and the surface and internal quality were inspected by a macro test, and the results are shown in Table 2 below.

 Steel grade                    Chemical composition (% by weight) C Si Mn P S S.Al Ni N (ppm) a 0.061 0.27 0.64 0.005 0.002 0.022 9.08 33 b 0.065 0.258 0.67 0.005 0.002 0.024 9.12 28 c 0.061 0.257 0.65 0.005 0.001 0.015 8.99 25 d 0.066 0.23 0.65 0.005 0.001 0.019 9.16 23 e 0.064 0.23 0.7 0.005 0.001 0.022 9.05 29 f 0.059 0.244 0.63 0.004 0.001 0.021 9.1 27 g 0.058 0.242 0.63 0.004 0.001 0.021 9.3 25

Steel grade Casting speed (mpm) Specific quantity (ℓ / kg) Final casting condition Calibration well temperature (℃) Primary taper amount (mm / m) 2nd taper (mm)   Cast quality Central segregation index Surface Defect Index Inventive Example 1 a 1.18 0.42 Constant casting 970 0.19 2.75 0 0 Inventive Example 2 a 1.17 0.44 Constant casting 990 0.2 2.8 0 0 Inventive Example 3 b 1.18 0.42 Constant casting 980 0.19 2.8 0 0 Inventive Example 4 c 1.19 0.46 Constant casting 1000 0.2 2.85 0 0 Comparative Example 1 e 0.9 0.46 Constant casting 870 0 0 0.7 ~ 0.8 0.5-0.7 Comparative Example 2 f 0.9 0.44 Constant casting 900 0 0 0.9-1.1 0.6 ~ 0.9 Comparative Example 3 g 0.93 0.44 Deceleration, Acceleration 910 0 0 1.0 ~ 1.2 0.3 ~ 0.5 Comparative Example 4 g 0.95 0.44 Deceleration, Acceleration 920 0 0 1.0-1.3 0.2-0.4

* Primary taper amount: Taper amount from solid bottom to solid thickness / casting thickness ratio up to 0.9

2nd taper amount: Taper amount of section 0.3 ~ 0.7

As can be seen in Table 2, in the case of the present invention in which the casting speed, the secondary cooling so that the cast temperature (temperature of the calibration point) is 970 ℃ or more, all the surface defects were relatively very good. On the contrary, in the comparative example of low casting speed and low cooling rate, surface quality was relatively insufficient.

Further, in Table 2, the primary taper, which is the taper amount of the roll gap of the pass line from the bottom of the mold to the solid phase thickness / casting thickness ratio of 0.9, was performed at 0.19 to 0.20 mm / m, and the roll gap of the pass line of the solid phase ratio of 0.3 to 0.7. There was no central segregation in the case of the invention example in which the end-coagulation taper amount, which is the taper amount of, was 2.75 to 2.85 mm. On the contrary, it can be seen that in the comparative example where the roll gap of the pass line is not performed, the center segregation is relatively large.

In Table 2, macro test results of the steels corresponding to Inventive Example (3) and Comparative Example (2) are shown in FIGS. 2 and 3. As shown in Figure 3, in the case of the comparative steel it can be seen that the crack cracking occurs.                     

On the other hand, Figure 4 is a graph showing the results of the center segregation macro test by casting speed showing the center segregation degree by analyzing the cast specimens in the section 0.02m / min while changing the casting speed from 1.10 to 1.30m / min, in the present invention At the casting speed of 1.17 ~ 1.19m / min, excellent quality cast without center segregation can be obtained.

In addition, Fig. 5 shows the center segregation degree of the cast steel while giving a taper of the cast steel cast at a casting speed of 1.17 to 1.19 m / min with a taper up to 4.0 mm from 2.0 mm to 0.2 mm in the pass line roll gap in the 0.3 to 0.7 section. As a graph showing the comparison, it can be seen that excellent casting quality can be obtained when the taper is 2.75 to 2.85 mm in the solid phase ratio of 0.3 to 0.7.

As described above, the present invention can manufacture a high Ni-containing steel by the continuous casting method, not the conventional ingot method, and also cast steel plate material that is excellent in interior and surface quality than the cast steel produced by the conventional casting method It can be prepared.

Claims (3)

By weight% C: 0.05 ~ 0.10%, Si: 0.15 ~ 0.35%, Mn: 0.50 ~ 0.80%, P: 0.005% or less, S: 0.003% or less, Soluble Al: 0.020 ~ 0.060%, Ni: 3.5 ~ 9.5% In the continuous casting of cast steel by using a machine by refining the converter to minimize the balance of Fe and other impurities, degassing the refined molten steel, injecting the degassed molten steel into a tundish and a molder, Degassing the refined molten steel for 20 minutes or more at a low vacuum so that its N content is 30 ppm or less; Continuously casting the degassed molten steel in a tundish while controlling the degree of superheating in a range of 13 to 17 ° C., at a speed of 1.17-1.19 m / min; And sprinkling and cooling the slab exiting the mold by the continuous casting in a secondary cooling zone. The roll gap of the pass line through which the cast steel passes is tapered from 0.19 to 0.20 mm / m from the bottom of the mold to the solid-thickness / cast thickness thickness ratio in the machine to 0.9, and the cast steel is near the solidification completion. The roll gap of the pass line passing through the tapered section of the solidification fraction (Solidification fraction) 0.3 ~ 0.7 to 2.75 ~ 2.85mm characterized in that the continuous casting method of high Ni-containing steel for LNG tanks. The method of continuous casting of a high Ni-containing steel for an LNG tank according to claim 1, wherein the anti-oxidant coating agent is uniformly coated after grinding the surface of the cast steel that has passed through the continuous cycle as described above. The method of continuous casting of high Ni-containing steel for LNG tanks according to claim 1, wherein the secondary cooling zone is sprinkled with cooling water of 0.42 to 0.46 l / kg.

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