KR20010040438A - A continuous casting method of austenitic stainless steel containing high si content - Google Patents

Abstract

PURPOSE: A continuous casting method of an austenitic stainless steel containing a high Si content is provided to prevent solidification cracks generated during the continuous casting by appropriately controlling a casting temperature in the tundish and a water-liter steel-ton in the second cooling zone. CONSTITUTION: The method comprises the process of operating by maintaining a difference (ΔT) between a temperature of molten steel (1) in a tundish (4) and a theoretical temperature of solidification in a temperature ranging from 10 to 50 deg.C, and a water-liter steel-ton in the second cooling zone (9) obtained by the following formula to 0.25 to 0.35: S=Q/(VxWxDxG) where S: water-liter steel-ton, Q: total water amount of the second cooling zone (liter/min), W: width of steel billet (m), D: thickness of steel billet, G: density (kg/m3) and V: casting speed (m/min).

Description

Continuous casting method of high Si containing austenitic stainless steel {A CONTINUOUS CASTING METHOD OF AUSTENITIC STAINLESS STEEL CONTAINING HIGH SI CONTENT}

The present invention relates to a continuous casting method of high Si-containing austenitic stainless steel, and more particularly, to a continuous casting method capable of preventing solidification cracks on the surface and inside of cast steel and preventing line defects occurring during hot rolling. .

Due to the increase in the service temperature and the increase in the service life of parts due to the high performance of the engine, the usage of stainless steel is increasing in the exhaust system of automobiles, and the grades of steel are also advanced and high performance. Most of the exhaust system mainly uses ferritic stainless steel. However, flexible pipes mainly use austenitic stainless steel because they require moldability, weldability, oxidation resistance, and fatigue resistance at high temperatures. Especially, in cold areas, NaCl or CaCl ₂ salts are sprayed on the road surface as an anti-freezing material for winter roads, and because of this, the flexible pipes are corroded. It is a trend to increase the resistance to the problem. However, high Si-containing austenitic stainless steels tend to cause severe Si, P and S segregation during the solidification process, and also have a large amount of solidification shrinkage. This is easy to occur. In particular, edge cracks, which are cracks occurring at slab edges and hot-rolled sheet edges, cause a problem of making manufacturing impossible due to a large drop in error rate. Of these, the cracks generated during the hot rolling process have been conventionally studied and reviewed for prevention, and since the components and rolling conditions are controlled at present, as a result, manufacturing is not possible.

Therefore, the crack which determines the possibility of manufacture becomes a linear defect detected after pickling in a hot rolling process. Since these defects occur in cast steel and are a fatal defect in stainless steel where surface quality is important, they must be removed through additional processes such as property taxes and grinding, resulting in a price increase. In order to prevent such line defects, various studies have been made from the continuous casting process to the hot rolling and annealing process.

The continuous casting apparatus of Figure 1, the ladle 2 containing the molten steel (1) completed in the steelmaking process, the tundish (4) to serve as a buffer (buffer) between the mold (8), to produce a cast It consists of the mold 8 and the secondary cooling stand 9. There is a shrouding nozzle 3 between the ladle 2 and the tundish 4. Inside the tundish 4 is a tundish dam 5 for controlling the flow of molten steel, an immersion nozzle 7 between the tundish 4 and the mold 8, and an immersion nozzle 7. The upper part is provided with a tundish stopper 6 for adjusting the flow rate of the molten steel. In this continuous casting process, the linear defect occurs when the solidification structure is formed unevenly in the mold 8 and the secondary cooling stand 9 in which the cast pieces are formed.

As a conventional technique for preventing such defects, there is a technique disclosed in Japanese Unexamined Patent Publication No. 2-9651 focusing on the surface layer cast structure. In this method, the slab that regulates the Si content of the austenitic stainless steel is formed by shot blasting before the furnace is heated, so that a working layer is formed on the surface of the slab. It is a technique which prevents crack generation by making it fine. In addition, Japanese Patent Publication No. 4-48865, which focuses on the scale of heating, proposes a technique to prevent linear defects by regulating the oxygen concentration of 0.5 to 5 when heating sol.Al-regulated slabs. Doing. However, it is impossible to completely eliminate the line defects by the above method.

Accordingly, the present invention provides a continuous casting method of high Si-containing austenitic stainless steel that can prevent the solidification crack generated during continuous casting by appropriately controlling the casting temperature in the tundish and the specific amount in the secondary cooling zone, The purpose is.

1 is a schematic view showing a continuous casting apparatus

* Description of the symbols for the main parts of the drawings *

One… Molten steel 2. Ladle 3... Shrouding nozzle 4... Tundish

5... Tundish dam 6... Tundish stopper 7... Immersion nozzle

8… Mold 9... Secondary cooling stand

The present invention for achieving the above object, in the continuous casting method of high Si-containing austenitic stainless steel solidified with primary delta ferrite and formed austenite phase during the solidification process, the difference between the molten steel temperature and the theoretical solidification temperature in the tundish ( ΔT) in the range of 10 to 50 ° C., and the formula S = Q / (V _X W _X D _X G) : The specific water quantity in the secondary cooling zone determined by the slab width (m), D: slab thickness (m), G: density (kg / m 3), and V: casting speed (m / min) is 0.25 to 0.35. The present invention relates to a continuous casting method of high Si-containing austenitic stainless steel, which is maintained and operated.

Hereinafter, the present invention will be described in detail.

In general, austenitic stainless steels in which solidification starts with a primary delta ferrite phase and an austenite phase is formed during the solidification process have a [Cr / Ni] eq. Value in the range of 1.5 to 1.8. At this time, [Cr / Ni] eq. Is obtained from the following Hammar and Svensson relational formula.

Also, in continuous casting of high Si-containing austenitic stainless steel, since the position of the coil linear defect occurring during hot rolling of the continuous casting cast is related to the crack of the cast, it is necessary to prevent the solidification crack of the cast during the continuous casting.

In the present invention, in the case of manufacturing high Si-containing austenitic stainless steel by continuous casting, it is possible to prevent the solidification crack of the cast during continuous casting by investigating the correlation between the occurrence of coil linear defect and cast crack in annual casting It's about doing.

Metallurgical examination of the causes of solidification cracks in the cast during continuous casting may occur due to a large amount of segregation of solute elements and a decrease in elongation due to coarse formation of a solidified structure (dendrite). The coagulation crack due to segregation of the solute element is caused by the solute element being segregated between dendrites and expanding the mush zone during the coagulation process. In this case, segregation of solutes should be reduced during the coagulation process to prevent coagulation cracks. On the other hand, the coarse solidification structure has a low high temperature strength, so solidification cracks occur at the interface of the solidification structure when the external stress is applied.

Therefore, in order to suppress the coagulation crack of the cast cast it is necessary to reduce the amount of segregation of solute elements and to form a fine coagulation structure, the following method will be described in detail.

The temperature range from the liquidus temperature to the solidification temperature, the solidification temperature, is a section in which solutes are concentrated. If the cooling rate is slowed in this section, the solutes diffuse and the content of solutes concentrated in the liquid phase decreases. Therefore, the generation of coagulation cracks is suppressed. However, in this case, there is a problem that the size of the dendrites becomes coarse. On the other hand, if the cooling rate is increased, the size of the dentite is finer, so that there is an effect of suppressing the coagulation crack, but the diffusion of the solute element is delayed.

From the above, it is determined that the solidification crack of the cast during continuous casting can be adjusted by the cooling conditions during the solidification process, in the present invention is set to the molten steel superheat in the tundish to control the solidification crack generated in the cast during continuous casting It is to control the casting temperature and the specific water quantity in the secondary cooling zone. In addition, by setting a limit range of the casting temperature and the non-quantity, to solve the quality and operation problems other than the solidification crack that may occur when the casting temperature and non-quantity control.

Accordingly, the present invention is to operate by adjusting the difference between the molten steel temperature and the theoretical solidification temperature (ΔT) in the tundish in the range 10 ~ 50 ℃, the specific amount in the secondary cooling zone to 0.25 ~ 0.35 (ℓ / ㎏) will be.

The difference between the molten steel temperature and the theoretical solidification temperature in the tundish has a lower limit of 10 ° C. and an upper limit of 50 ° C., and when ΔT exceeds an upper limit of 50 ° C., the solidification rate becomes slow during coagulation and the coagulation structure becomes coarse. Because of this, casting cracks are likely to occur during solidification cracks and hot rolling. On the other hand, if ΔT is lower than the lower limit of 10 ° C., the lubricating ability of the mold powder is degraded during the playing process, thereby causing inclusion defects in the cast steel.

In addition, while controlling the above casting temperature conditions, the secondary cooling zone specific water is controlled to 0.25 ~ 0.35ℓ / kg to prevent the solidification crack.

Reasons for setting non-quantity limits are as follows. If the specific amount of secondary cooling zone is increased, the coagulated structure can be finely formed.However, if the non-aqueous water exceeds 0.35ℓ / kg, the segregation of impurities in the solidified tissue during the continuous casting decreases the time for diffusion. Furnace cracks are generated, and cracks and residual stresses due to thermal stress during reheating of the cast steel are excessively generated on the surface, and surface cracks are generated during grinding of the cast steel. If the specific water content is less than 0.25 L / kg, the solidification structure becomes coarse to cause the solidification crack, and the strength of the slab solidification shell during casting decreases, causing cracks due to the slab bulging. Therefore, it is preferable to control the secondary cooling zone specific water quantity to 0.25-0.35 L / kg.

Hereinafter, the embodiment of the present invention will be described in more detail.

(Example)

[Cr / Ni] eq. As shown in Table 1 using a high Si-containing austenitic stainless steel having a 1.64, and is produced as a continuous casting cast by applying the molten steel superheat and cooling water ratio as shown in Table 2 below It was. Investigation of defects of the prepared cast steels are shown in Table 2 below.

Cr (WT.) Ni (WT.) Si (WT.) Mn (WT.) P (WT.) S (WT.) C (WT.) N (WT.) [Cr / Ni] eq. 19.42 13.3 3.42 0.58 0.013 0.001 0.052 0.023 1.64

Molten steel superheat at tundish (℃) Secondary cooling amount (ℓ / ㎏) Defective degree of cast steel Inventive Example One 10 0.32 ○ 2 48 0.27 ○ Comparative example One 20 0.23 △ 2 30 0.37 △ 3 8 0.33 X 4 55 0.35 X 5 52 0.38 X ○: none, △: fine, X: severe

As can be seen in Table 2, Inventive Examples (1) and (2), which satisfy all of the control conditions of the present invention, did not generate bulging and solidification cracks on the surface of the continuous cast steel and the hot rolled coil.

However, in the comparative example (1), since the non-aqueous range was out of the conditions of this invention, bulging generate | occur | produced in a slab and the crack generate | occur | produced in the slab surface duty-free. As a result, excessive scale was formed during hot rolling, causing linear defects on the hot rolled coil. Comparative Example (2) is a case in which the specific quantity is higher than the scope of the present invention, bulging (buzzing) did not occur during casting, but the thermal stress on the surface of the cast steel severely acted to cause cracking. In Comparative Example (3), because the molten steel superheat in the tundish was lower than the range of the present invention, melt defects occurred in the mold powder, resulting in a lack of lubrication between the mold and the solidification layer. At the end of the casting, a large surface defect such as a scab was caused on the surface of the cast. In Comparative Example 4, the casting temperature was cast under the condition of the present invention. Therefore, coarse coagulation structure was formed on the surface of the cast steel, and solidification cracks were generated at the grain boundaries. Also, bulging occurred during continuous casting, causing unstable operation such as hunting of the surface. Comparative Example (5) is a case where the casting temperature and the specific quantity deviate from the standard values of the present invention, causing cracks in the continuous casting cast and the hot rolled coil.

According to the present invention as described above, it is possible to prevent the solidification crack generated during continuous casting and hot rolling without a process load, it is possible to manufacture a high corrosion resistance austenitic stainless steel excellent in quality.

Claims (1)

In the continuous casting method of high Si-containing austenitic stainless steel solidified by primary delta ferrite and forming an austenite phase during the solidification process, the difference between molten steel temperature and the theoretical solidification temperature (ΔT) in the tundish is in the range of 10 to 50 ° C. , Formula S = Q / (V _X W _X D _X G) (S: specific quantity (ℓ / ㎏), Q: total quantity of secondary cooling stand (ℓ / min), W: slab width (m), D: slab High Si characterized by operating by maintaining the specific water quantity in the secondary cooling zone determined by the thickness (m), G: density (kg / m 3), V: casting speed (m / min) at 0.25 to 0.35. Continuous casting method of austenitic stainless steel.