KR100340549B1 - A method for manufacturing continuously cast primary δ-ferrite stainless steel having excellent surface quality - Google Patents

Abstract

PURPOSE: A method for manufacturing continuously casting primary δ-ferrite stainless steel having excellent surface quality is provided, which can prevent linear defects formed on the surface of hot coil when hot rolling austenitic stainless steel. CONSTITUTION: In a manufacture method of continuous cast slab of austenitic stainless steel, the method is characterized in that £Cr/Ni|eq is in the range of 1.5 to 1.9 wherein, £Cr|eq=Cr+1.37Mo+1.5Si+2Nb+3Ti and £Ni|eq=Ni+0.31Mn+22C+14.2N+Cu; cooling rate from liquidus to solidus line is regulated in the range of 20 to 2400°C/min; cooling rate from peritectic reaction temperature to 950 deg.C is regulated below 180°C/min; and the content of primary δ-ferrite is regulated in the range of 2 to 6 % to the depth up to 10 mm from the surface of slab.

Description

A method for manufacturing continuously cast primary δ-ferrite stainless steel having excellent surface quality}

The present invention relates to a method for producing sound casts by continuous casting in order to prevent linear defects occurring on the surface of the hot rolled coil during hot rolling of the austenitic stainless alloy solidified with primary δ-ferrite.

Since the austenitic stainless alloy is a high alloy containing a large amount of Cr and Ni, hot workability decreases during hot rolling, and cracks are likely to occur. In particular, cracks occurring at the edge portion of the slab and the edge portion of the hot rolled sheet, which are referred to as edge cracks, cause problems such as the fact that the manufacturing rate is greatly reduced. In order to prevent such cracks, a lot of studies have been made on the cracks generated during the hot rolling process, and at present, there are fewer cases in which manufacturing is impossible due to the optimization of components and rolling conditions.

Such cracks that determine the manufacturability are rarely found in the hot rolling process and are linear defects detected after pickling and cold rolling. Since the linear defects are fatal defects in stainless steel where hot spots are produced and surface quality is important, they require an additional process of eliminating linear defects through correction processes such as property tax and grinding. Has been a factor in the price increase in the manufacturing process.

On the other hand, in order to prevent such line defects, various studies have been made from the continuous casting process to the hot rolling and annealing process. In particular, linear defects are used to prevent micro cracks in hot rolling processes.

The hot workability was secured by adjusting the steel composition so that the content of δ-ferrite determined from the relational equation was 4% or less.

For example, Japanese Patent Publication No. 2-9651 discloses a shot blast before a furnace is introduced into a slab that regulates the Si content of austenitic stainless steel in view of the improvement of the structure of the cast layer surface portion. A technique for preventing cracks is proposed by miniaturizing the grains of the surface layer of the slab recrystallized at the time of forming the processed layer. In addition, Japanese Patent Publication No. 4-48865 discloses a technique for preventing ship defects by regulating soluble Al and regulating oxygen concentration at slab heating by 0.5 to 5%. .

However, the method described above increases the process load because additional processing is required, and the δ-ferrite content present in the cast steel during continuous casting is different even when the composition is constant. It is difficult to remove the situation.

Accordingly, the present invention is to prevent the micro-cracks generated during continuous casting and hot rolling of austenitic stainless steel of the continuous casting cast of austenitic stainless steel that can prevent the micro-cracks without the process load for preventing micro-cracks The purpose is to provide a manufacturing method.

1 is a schematic diagram illustrating a process of generating and disappearing δ-ferrite

Explanation of symbols on the main parts of the drawings

1 ... primary δ-ferrite phase

Γ phase produced by the reaction

3. γ phase produced by solid state transformation

4.Liquid line temperature

5.Treating reaction temperature

6.solidus temperature

7.Temperature to complete solid state transformation

In the present invention for achieving the above object, in the method for producing a continuous cast steel cast of austenitic stainless steel solidified with primary δ-ferrite, where austenite is produced before solidification is completed,

It relates to a method for producing a continuous cast steel cast of austenitic stainless steel to control the δ-ferrite content in the range of 10mm or less from the surface layer of the cast steel to 2-6%.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is explained in full detail.

The present invention is to investigate the correlation between the occurrence location of the coil wire defect and the cast structure when the cast steel is hot rolled when the austenitic stainless steel is solidified with primary δ-ferrite and austenite is formed before the solidification is completed. This is to control the surface structure of the slab during continuous casting. The austenitic stainless steel solidified with primary δ-ferrite and produced austenite before solidification is in the range where [Cr / Ni] eq is in the range of 1.5 to 1.9, and [Cr / Ni] eq is the following Hammer and Svensson relation Obtained from

In the metallurgical examination of the linear defects occurring on the coil surface during hot rolling, there are two types of linear defects during hot rolling, which occur at the δ / γ interface and at the γ grain boundary. In case of the δ / γ interface, when δ-ferrite remains in the hot rolling process, the elongation of δ-ferrite and γ phase is different during hot rolling, and the cracking occurs due to the decrease in hot workability at the δ / γ interface. In order to prevent cracking, it is required to control the δ-ferrite from the slab state to less. On the other hand, δ-ferrite is directly related to hot workability, but since δ-ferrite acts as a nucleation site for reclining when heated, it also affects growth and coarsening of γ grains, thereby increasing the bulgal growth of γ grains. In some cases, cracking may occur at the γ grain boundary. Therefore, in order to remove the influence by δ-ferrite, it is necessary to finely distribute δ-ferrite in the surface layer portion of the cast piece and to prevent abnormal generation of γ grains when heated. The present invention is characterized in that for controlling the δ-ferrite content in the range of 10mm or less in the surface layer of the cast steel to 2-6%. When δ-ferrite content in the surface layer of continuous casting slab is less than 2%, cracks and red-brittleness are induced in the slab surface freely, and δ-ferrite completely disappears during hot rolling, which delays γ recrystallization. The lip grows irregularly and causes linear defects during hot rolling. In addition, when the δ-ferrite content of the continuous cast slab surface portion is distributed by 6% or more, δ-ferrite remains in a hot rolled condition, and linear defects occur at the δ / γ interface during hot rolling.

In order to prevent cracking during hot rolling, it is necessary to finely distribute the δ-ferrite in the surface portion of the slab. For this purpose, formation and disappearance of the δ-ferrite in the austenitic stainless steel should be understood. The process for this can be explained through a schematic diagram of the solidification process shown in FIG.

Step 1: During solidification, the δ-ferrite (1) is generated from the liquidus temperature (4) to the primary, and as the temperature drops, the δ-ferrite (1) grows to the reaction temperature (5) through coarsening.

Step 2: From the crystallization reaction temperature (5) to the solidified solidus temperature (6), the γ phase (2) is generated by the crystallization reaction, and the δ-ferrite (1) disappears.

Step 3: After solidification is completed, δ-ferrite (1) is transformed into γ phase (3) by solid phase diffusion.

Therefore, the temperature range from the liquidus temperature (4) to the trapping reaction temperature (5) is a section in which the δ-ferrite is produced and grown, and if the cooling rate is increased, the size of the δ-ferrite initially formed becomes fine. Although the content is small, slower cooling rate increases the size of δ-ferrite and increases the content. After completion of coagulation, the δ-ferrite → γ transformation is a step in which the δ-ferrite is extinguished by the diffusion reaction. Since the maximum temperature that can proceed is 950 ℃ (7) it can be finely distributed δ-ferrite by slowing the cooling rate in the temperature range from the reaction reaction temperature (5) to 950 ℃ (7). From the above description, since the content of δ-ferrite remaining in the slab during continuous casting can be controlled by the cooling conditions after solidification and solidification, the present invention controls the surface layer δ-ferrite of the slab during continuous casting by cooling rate. It was presented how to. However, in the process of controlling the cooling rate, the limit and range of the cooling rate are set because it may cause quality and operation problems other than δ-ferrite. Therefore, the present invention controls the cooling rate from the liquid line 4 to the tableting reaction temperature 5 at 20-2400 ° C./min, and the cooling rate from the tableting reaction temperature 5 to 950 ° C. 180 is 180. It is characterized by controlling the δ-ferrite content in 2 ~ 6% in the range of 10mm or less in the surface layer of the cast by controlling to less than ℃ / min. When the cooling rate is set at 2400 ° C./min or more, the time for diffusion of the solidified segregation formed between dendrite during continuous casting decreases, thereby causing a duty free crack on the surface of the cast steel.

In addition, when the cooling rate is less than 20 ℃ / min, the content of the δ-ferrite produced initially increases and in order to satisfy this cooling rate, the quantity of primary and secondary cooling should be reduced during the reworking operation. Due to the slow heat transfer of the cast during casting, the strength of the slag solidification shell (shell) is reduced to cause the phenomenon of the bulging (casting) of the cast to cause a deterioration of operation and quality. In addition, when the cooling rate at the 950 ° C. (7) section is more than 180 ° C./min at the reaction temperature (5), the δ-ferrite-γ transformation process is slowed down and the δ-ferrite content remaining in the slab becomes high. Therefore, the cooling rate in the 950 ℃ section at the reaction temperature is required to be 180 ℃ / min or less.

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

EXAMPLE

Cast steel was prepared by continuous casting under the cooling conditions shown in Table 3 for 304 stainless steel composition as shown in Table 2. Each produced cast was rolled, wound and rolled up under normal hot rolling and rolling conditions, and then pickled to investigate surface defects of the coil to evaluate the occurrence of linear defects, and the results are shown in Table 3.

As shown in Table 3, the invention (A ~ C) according to the present invention was able to obtain a good product without bulging, cracking and linear defects on the surface of the continuous casting cast and hot rolled coils.

On the other hand, in the comparative material (D), the cooling rate range from the liquidus temperature to the trapping reaction temperature range was out of the conditions of the present invention. The content of δ-ferrite was 4.1%, which was good, and no linear defects occurred on the hot rolled coil. . However, due to the duty-free cracking on the surface of the continuous cast slab to reduce the error rate of the cast steel, and also requires a process to remove the cracks of the cast slab by polishing, the process load is taken during production. Comparative material (E) was cast in a cooling condition of the cooling reaction between the reaction temperature of 950 ℃ faster than the present invention, the cast quality was good, but the δ-ferrite content is present at 7.4%, the line defect on the hot rolled coil This occurred. In the comparative material (F), the cooling rate of the trapping reaction temperature range was lower than the lower limit at the liquidus temperature, so slab bulging occurred during continuous casting, and operation instability such as shunting occurred. In the comparative material (G), the cooling rate of the trapping reaction temperature range was lower than the lower limit at the liquidus temperature, and slab bulging occurred during continuous casting, and operation instability such as water surface vibration occurred. In addition, the comparative material (G) is a problem that causes both the cooling rate of the reaction temperature range at the liquidus temperature and the cooling rate from the reaction temperature to 950 ° C out of the standard value of the present invention cause cracks in the continuous casting cast and hot rolled coils Is sitting down. Through the above embodiment, the present invention confirmed that the surface cooling rate condition of the cast steel during the performance of the production can not only secure an appropriate δ-ferrite content, but also obtain a continuous continuous cast quality, which enables stable continuous casting operation.

As described above, the present invention has the effect of preventing the surface micro-cracks of the hot-rolled coil without the process load for preventing the micro-cracks during continuous casting and hot rolling of austenitic stainless steel.

Claims (1)

In the method for producing a continuous cast steel cast of austenitic stainless steel solidified with primary δ-ferrite, where austenite is produced before solidification is completed, The austenitic stainless steel is [Cr] eq = Cr + 1.37Mo + 1.5Si + 2Nb + 3Ti and [Cr / Ni] eq, defined as [Ni] eq = Ni + 0.31Mn + 22C + 14.2N + Cu, is from 1.5 to 1.9; And Δ in the range of 10mm or less from the surface layer of the cast steel by controlling the cooling rate from the liquidus to the solidus to 20 ~ 2400 ℃ / min, and controlling the cooling rate from the reaction reaction temperature to 950 ℃ below 180 ℃ / min. -Method of manufacturing cast steel of primary δ-ferritic stainless steel with excellent surface quality, characterized by controlling ferrite content to 2 ~ 6%

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