KR100419644B1 - A Method for Manufacturing Continuously Cast Strands from High Ni Containing Steel - Google Patents

Abstract

The present invention relates to a method for producing a cast steel of high Ni-containing steel, the object of the present invention is to control the refining conditions, continuous casting conditions and surface treatment conditions in the production of high Ni-containing steel by continuous casting, it is excellent in cast state The purpose of the present invention is to provide a method for producing a high Ni-containing cast steel that can secure the surface and internal quality.

In the present invention, 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%, remainder Fe and other unavoidable impurities, the converter is refined, degassed refined molten steel, degassed molten steel is injected into a tundish, and continuously cast by a casting machine to produce a casting cast. Degassing at a low vacuum of 2 torr or less so that the N content of the molten steel is 30 ppm or less; The molten steel in the tundish has a superheat of 13-17 ° C .; The casting speed during the continuous casting is performed at 0.90-0.93 m / min; Cooling by sprinkling cooling water on a cast piece that is continuously cast by the playing machine and exits under the mold; The surface processing is a manufacturing method of high Ni-containing steel cast steel which is performed by applying an anti-oxidant coating agent after the scarfing and grinding.

Description

A Method for Manufacturing Continuously Cast Strands from High Ni Containing Steel}

The present invention relates to a method for producing high nickel steel used as a material such as a tank for storage, and more particularly, to a method for manufacturing a high alloy steel containing a large amount of Ni into a thick plate by continuous casting.

In general, Ni is known as an element that improves low-temperature toughness. Therefore, high Ni-containing steel is LNG gas (approximately 9% Ni-containing) from propylene gas (using about 2.5% Ni-containing steel) depending on the Ni content. It is mainly used for tanks containing low temperature liquefied gas.

The high Ni-containing steel is a tank for liquefied gas storage because its main use is not to allow surface cracks to ensure safety. Contrast is much stricter. Because of this strict quality characteristics, the production mainly through ingots has been the mainstream, but nowadays, with the development of continuous casting technology, quality assurance through continuous casting is steadily being promoted worldwide.

The properties of steel grade of high Ni-containing steel are as follows: First, because austenite (γ) is the first crystal, high temperature characteristics are affected by compounds of N, P, and S that tend to precipitate at grain boundaries. (α) ↔Austenite (γ) transformation, thermal contraction and thermal expansion is about 50% larger than general steel, third, high temperature embrittlement zone is 600-900 ℃ wider than general steel (700-900 ℃), fourth, cooling Since the scale is fixed to the surface of the cast steel to promote non-uniform cooling of the surface of the cast steel, it is very easy to generate surface cracks.

Generally, secondary cooling, weak cooling, and high speed casting are advantageous to secure surface quality, but strong cooling and low speed casting are advantageous for securing internal quality. In addition, since high Ni-containing steels have unique steel grade characteristics different from those of ordinary steels, they should be cast without changing the casting conditions until the 1Heat casting is completed by setting the optimum casting conditions. The characteristics of cast surface defects generated during casting should be identified and effectively inspected and removed in cast state. As described above, when manufacturing high Ni-containing steel by continuous casting, there are many technical difficulties in controlling various conditions simultaneously.

Accordingly, the present invention is to manufacture a high-Ni-containing cast steel that can ensure excellent surface and internal quality in the cast state by appropriately controlling the refining conditions, continuous casting conditions and surface treatment conditions when manufacturing high Ni-containing steel by continuous casting. We want to provide a way to do that, and that's the purpose.

1 is a schematic diagram showing the playing equipment

Figure 2 is a photograph showing the results of the macro test of the steel according to the present invention

Figure 3 is a photograph showing the test results of the macro mark of the steel according to the comparative example

The present invention for achieving the above object is C: 0.05-0.10% by weight, 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%, remainder pre-refined to be composed of Fe and other unavoidable impurities, degassed refined molten steel, inject degassed molten steel into tundish, and continuously cast by the instrument In the method of manufacturing the cast steel,

Degassing at a low vacuum of 2 torr or less so that the N content of the molten steel is 30 ppm or less; The molten steel in the tundish has a superheat of 13-17 ° C .; The casting speed during the continuous casting is performed at 0.90-0.93 m / min; Cooling by sprinkling cooling water on a cast piece that is continuously cast by the playing machine and exits under the mold; The surface treatment relates to a method for producing a cast steel of high Ni-containing steel, characterized in that the coating is performed by applying an anti-oxidant coating agent after the scarfing and grinding.

Hereinafter, the present invention will be described in detail.

The steel to which the method for producing cast steel of the present invention can be applied is high Ni-containing steel, and the carbon content is preferably 0.05-0.10%, the Ni content is 3.5-9.5%, and the soluble Al is 0.020-0.030%, N Steels controlled to 30 ppm or less are preferred. Namely, 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 Steel grades composed of%, balance Fe and other unavoidable impurities.

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

In the present invention using the steel composition as described above, the molten steel is degassed for 20 minutes or more at low vacuum in the refining process so that the N content is 30 ppm or less.

If the hydrogen content is high, it causes cracks in the cast steel or the product over time, so that the degassing treatment is sufficiently performed at low vacuum. At this time, the degassing treatment is preferably performed for 20 minutes or less at 2 torr or less.

In addition, in the present invention, the degassed molten steel is injected into the tundish, wherein the degree of superheat of the molten steel is in the range of 13-17 ° C.

Superheat degree of the molten steel is controlled in the range of 13-17 ℃. The management of molten steel superheat during continuous casting is very important to ensure the internal quality and castability of cast steel. If the molten steel superheat is too high, the central segregation is likely to be inferior due to the development of columnar structure and the reduction of equiaxed structure. On the contrary, if it is too low, the cleanliness of the steel may be deteriorated due to clogging of the nozzle and insufficient inclusion separation.

In particular, the steel as in the present invention is a steel grade γ is the primary, so there is almost no Mushy Zone, the columnar tissue, which is a vulnerable tissue, develops, and thus, the internal quality of the molten steel is high. It is desirable to manage at a rather low 13-17 ° C.

In the present invention, the casting speed is carried out at 0.90-0.93m / min during continuous casting.

In the continuous casting, the casting speed is preferably in the range of 0.90-0.93 m / min, which is lower than that of general steel, to ensure good internal quality. When the low speed casting is performed, the internal quality of the cast is improved by making molten steel flow and casting cooling uniform within the mold and minimizing bulging at the bottom of the mold by securing sufficient solidification shell.

At this time, the mold to be used may be applied to a conventional one, in consideration of the degree of solidification shrinkage during cooling, it is more preferable to use a mold given a taper of 1.05-1.10%.

In addition, in the present invention, the cooling water is sprayed on and cooled by a cast piece that is continuously cast by the above-described machine and exits under the mold.

The cooling water is sprayed on the slab leading to the secondary cooling stand, and generally determines the amount of cooling water in consideration of the thickness and speed of the performance. In particular, in the present invention, in order to secure good cast surface quality, it is necessary to refine the austenitic (γ) particles by performing cold cooling directly under the mold, and cooling conditions for controlling the uniform cooling in the slab width direction and the temperature for passing the calibration point to 900 ° C or more. This is preferred. For this purpose, 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). In addition, in the end of the casting is usually the end of the casting after deceleration to increase the operation stability for the operation stability, in the case of the present invention because the surface and internal quality tends to deteriorate according to the change in the casting speed, the change in the casting speed even at the end of casting It is desirable to cast at constant speed without.

On the other hand, the amount of cooling water in the secondary cooling zone can be controlled so as to satisfy such cooling conditions, for example, the cooling in the secondary cooling zone can be performed with an amount of 0.38-0.43 l / kg.

In addition, in the present invention, the surface of the cast steel is subjected to the scarfing and grinding, and then coated with an antioxidant coating agent.

The cast pieces passed through the continuous casting machine under the conditions of the present invention as described above may partially have defects on the surface thereof, and are usually covered with an oscillation mark and scale. Therefore, an antioxidant is applied to minimize the occurrence of scale surface scale in the furnace for inspection and removal of defects in the cast state to prevent surface defects in the thick plate product and to prevent the plate scale defect. At this time, in order to remove the surface defects and to apply the antioxidant, it is preferable to perform grinding after scarfing. The scarfing and grinding is preferably done front. Moreover, the said antioxidant can use a conventional thing, It is more preferable to use what has alumina as a main component.

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

Example

After the high Ni-containing steel having the composition as shown in Table 1 after the converter is subjected to degassing for 20 minutes at low vacuum of 2torr or less and the tundish molten steel superheat is managed at 13-17 ℃ under the conditions shown in Table 2 Continuous casting yielded a cast having a width of 1600 mm. At this time, the player used was a conventional player as shown in Figure 1, the taper of the mold was given 1.05-1.08%.

A part (test piece) was taken from the obtained cast and the surface and internal quality were inspected by a macro test, and the results are shown in Table 2 below.

division Chemical composition (% by weight) C Si Mn P S S.Al Ni N (ppm) Steel grade a 0.067 0.26 0.66 0.0040 0.0010 0.027 9.26 26 Steel grade b 0.058 0.27 0.69 0.0043 0.0010 0.056 9.23 34 Steel grade c 0.060 0.26 0.65 0.0035 0.0013 0.047 9.37 36 Steel grade d 0.066 0.25 0.65 0.0040 0.0010 0.030 9.33 36

division Applicable Steel Casting speed (m / min) Specific quantity (l / kg) Final casting condition Calibration point temperature (℃) Cast quality index Internal Defect Index Surface Defect Index Inventive Example 1 Steel grade a 0.90 0.43 Constant casting 900 0 0.2 Inventive Example 2 Steel grade a 0.90 0.40 Constant casting 920 0 0.2 Inventive Example 3 Steel grade b 0.93 0.38 Constant casting 940 0.1 0.4 Comparative Example 1 Steel grade d 0.90 0.38 Deceleration, Acceleration 940 0.1-0.3 0.3-0.8 Comparative Example 2 Steel grade c 0.95 0.35 Deceleration, Acceleration 1000 0.5-0.7 0.7-1.0 Comparative Example 3 Steel grade c 0.95 0.35 Constant casting 1000 0.5-0.7 0.8

As can be seen in Table 2, the surface defects were relatively reduced, and the internal quality was also good in the case of the invention in which the casting speed and the secondary cooling were performed such that the calibration point passing temperature was around 900 ° C. On the contrary, in the comparative example in which the casting speed was increased and the secondary cooling was weakly cooled, the surface and the internal quality were relatively poor.

In Table 2, the results of the macro test of the steel according to Inventive Example (1) and Comparative Example (2) are shown in FIGS. 2 and 3. Figure 2 showing the macro test results of the invention example did not have an internal crack, but in Figure 3 showing the macro test results of the comparative example it can be seen that the internal crack is generated as a weak tissue in which the columnar tissue is very developed. The surface quality is also a comparative example of the chill zone (chill zone) of the surface is less than the comparative example, the surface crack depth is less than about 5mm in the case of the invention, but in the case of the comparative example up to about 20mm.

On the other hand, there was a difference in the occurrence of surface crack defects in the thick plate according to the method of slab cleaning, but the surface solidification chill zone was completely removed by excessive scaffolding, and the columnar texture became the slab surface, and the thick plate was heated in the furnace. The surface quality of the slab was better when the surface of the slab was trimmed in a manner that leaves a large number of slab surface chills even though some surface defects remain rather than the surface becoming weak due to grain boundary oxidation.

That is, this fact can be easily seen by inspecting the quality of the surface of the thick plate after the surface is trimmed by the method as shown in Table 3 using the cast pieces obtained in Examples (1) and Comparative Examples (1,2). there was. The inspection results of the surface quality are shown in Table 3 below as an index of occurrence of surface crack defects.

division How to Care Cast Heavy Plate Surface Crack Defect Index Inventive Example 1 Full stripping (2mm) → Front grinding (1mm) → Antioxidant coating → Thick plate rolling 0.1 Comparative Example 1 Front scarfing (7mm) → Front grinding (2mm) → Antioxidant coating → Thick plate rolling 0.4 Comparative Example 2 Front grinding (2mm) → Antioxidant coating → Thick plate rolling 0.3

As described above, when the cast steel is manufactured by the method of the present invention, it is possible to produce high Ni-containing steel by the continuous casting method, not the conventional method of the ingot method, and the obtained cast steel is not only the interior but also the surface quality It is an excellent thick plate material.

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%, The converter is refined to be composed of the balance Fe and other unavoidable impurities, degassed refined molten steel, injected degassed molten steel into a tundish, continuously cast by a machine, and then processed into a surface to manufacture a cast steel. In the method, Degassing at a low vacuum of 2 torr or less so that the N content of the molten steel is 30 ppm or less; The molten steel in the tundish has a superheat of 13-17 ° C .; The casting speed during the continuous casting is performed at 0.90-0.93 m / min; Cooling by sprinkling cooling water on a cast piece that is continuously cast by the playing machine and exits under the mold; The surface treatment is a method of manufacturing a cast steel of high nickel-containing steel, characterized in that the coating is carried out by applying an anti-oxidant coating agent after the scarfing and grinding. The method of claim 1, The mold is a method of producing a cast steel of high nickel-containing steel, characterized in that the tapered of 1.05-1.10%. The method of claim 1, The process of sprinkling and cooling the cooling water is a method for producing cast steel of high nickel-containing steel, characterized in that by the cooling water in the amount of 0.38-0.43l / kg in the secondary cooling zone.