JPS63177950A - Method for casting ferritic stainless steel - Google Patents

Abstract

PURPOSE:To stably mass-produce a continuously cast slab having fine recrystallized structure, which the surface defect is not developed at the time of deep draw working by cooling the surface layer of cast slab formed in both end opening mold at >=10 deg.C/sec cooling velocity from the temp. range of solidus line or higher. CONSTITUTION:Temp. detecting sensor 3 and nozzle holes 4 for injecting coolant on the inner wall face 2 at the lower part of mold 1, and introducing holes 6 for sucking the coolant in the upper part of inner wall face 2 at the lower part of mold, are arranged. When the molten steel 7 is cast into the mold 1, the extremely thin solidified shell 9 formed by heat-conducting action of the upper part of inner wall face 5, is immediately descended to the position of lower part of inner wall face 2. And, by the coolant injecting from the nozzle holes 4 for injecting the coolant and circulating to the introducing holes 6 for sucking, the surface layer part is cooled at >=10 deg.C/sec cooling velocity from the temp. range of solidus line or higher. Adjustment of the cooling velocity is executed under confirming by the temp. detecting sensors 3 and the high cooling velocity at the high temp. range can be stably secured.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention provides a steel sheet with good surface properties that allows stable production of steel sheets that do not cause surface defects such as "rough roving phenomenon" during processing. The present invention relates to a method for casting ferritic stainless steel slabs.

<Prior art and its problems> Conventionally, when deep-drawing a ferritic stainless steel sheet manufactured by rolling a continuously cast slab, rough surface undulations called "roving" frequently occur along the rolling direction. It has been pointed out that this "roving" has become a major problem because it impairs the appearance of the product.

By the way, the cause of roving is generally explained as follows.
The coarse columnar crystal structure generated during continuous casting is not sufficiently destroyed during the rolling process, and the texture remains, and the coarse grains and texture are difficult to recrystallize during hot rolling, resulting in a fine structure. It is thought that this is because anisotropy and non-uniformity appear in plastic deformation because the plastic deformation does not change. Furthermore, among ferritic stainless steels, 5US430 steel separates into a two-phase structure of ferrite (α) and austenite (γ) during hot rolling, but if α in the casting structure is coarse, it will separate from this. It was thought that these two phases produced would naturally become coarse and cause the above-mentioned inconveniences.

Therefore, it is considered that the most effective way to reduce the occurrence of surface defects such as roving phenomena is to refine the crystal grains. In order to obtain a recrystallized structure, attempts were made to employ methods such as ``warm rolling'' and ``deformed roll rolling''.

However, since the crystal grains of continuously cast slabs are coarse, it is impossible to perform only the normal hot rolling-annealing-cold-rolling-annealing process, even if "warm rolling method" or "deformed roll rolling method" is adopted. However, recrystallization is insufficient and grains are not refined, so a process of hot rolling - annealing - cold rolling - annealing - cold rolling - annealing is adopted, which further adds cold rolling and annealing to the above steps. However, this led to a significant increase in costs and was by no means a desirable method.

Therefore, in order to more easily refine the crystal grains of continuously cast slabs, it has been proposed to add alloying elements such as Ti to promote crystal nucleation, and to implement low-temperature casting and electromagnetic stirring within the mold. However, although these methods resulted in relatively fine equiaxed crystal grains inside the slab, it was difficult to prevent the surface layer of the slab from becoming coarse columnar crystals. It also showed a tendency to produce surface defects such as rough roving phenomenon during deep drawing.

<Means for Solving the Problems> From the above-mentioned viewpoint, the present inventors have developed a method for stably producing ferritic stainless steel sheets that do not produce surface defects such as rough roving during deep drawing. In order to achieve this, in particular, taking into account the fact that ``the recrystallized grain size of steel becomes finer as the initial grain size before rolling becomes smaller,'' Recognizing that microstructural refinement is essential for preventing surface defects during deep drawing, various methods have been developed to achieve continuous casting of ferritic stainless steel slabs with a fine cast structure in the surface layer during casting. As a result of conducting experiments and research from various angles,
The following findings were obtained. That is, (a) The ferrite grain size of a ferritic stainless steel slab begins to coarsen rapidly immediately after the solidus temperature (ferrite single phase temperature: Tα), and this phenomenon causes the microstructure of the surface layer of the slab to become coarser. What is hindering the development of

Figure 1 shows the cooling rate after melting 5US430 steel: 1
．． This is a graph showing the relationship between the water quenching temperature and the α grain size for samples that were cooled at 5°C/see and water quenched midway through to fix the structure, and the arrow in the figure indicates the solidus line. temperature (α single-phase temperature: Tα). This first
It is clear from the figure that the α grains begin to coarsen rapidly immediately after the temperature drops below Tα.

(b) However, if a means is taken to increase the cooling rate after the solidus temperature (α single-phase temperature: Tα) to a specific value or more, the coarsening of the α particle size can be effectively prevented.

Figure 2 shows that when cooling and solidifying 50 types of molten ferritic stainless steel, the cooling rate after Tα was varied, and 1
2 is a graph showing the α grain size of samples whose structure was fixed by rapid cooling after reaching 000° C., organized by the cooling rate. From this figure 2, we can also see that the cooling rate after Tα is 10°C.
It can be seen that if it is made larger than /sec, coarsening of α grains can be prevented.

(C) Also, 5OS43 in which γ phase precipitates during solidification and cooling
In stainless steel with a ferrite diameter such as 0, the T interface plays an important role as a nucleation site for recrystallized ferrite during rolling, and if the γ phase is dispersed within the α grains, the ferrite is blocked due to the bottling effect. In addition to suppressing grain growth, fine recrystallized grains with no preferred orientation can be obtained during rolling, so in addition to measures to increase the cooling rate after Tα,
It is also preferable to perform slow cooling in the (α+γ) two-phase region in order to obtain good properties.

Figure 3 is a micrograph showing the results of an investigation into the effect of cooling rate on the precipitation state of the γ phase for SUS 430 steel. Figure 3 (1) l is 5US4.
The structure of No. 30 steel is shown when it is cooled at a cooling rate of 12° C./sec after melting (when the α+γ region is rapidly cooled). From this figure 3, the α + γ region is slowly cooled [3
Figure (a)] shows that a large number of γ phases precipitate not only at the α grain boundaries but also within the grains, whereas the α+γ region is rapidly cooled [Figure 3 (
In the case shown in b)), it can be confirmed that precipitation occurs only at grain boundaries. In this way, when slowly cooling in the α+γ2 phase region, the phase is also precipitated within the α grains, which tends to result in a fine recrystallized structure in the rolling process.

(d) For this reason, from the moment the very thin surface layer of the slab is formed in the mold during continuous casting, most of the surface layer of the slab reaches the solidus temperature (α single phase temperature :Tα
) When the surface layer is rapidly cooled at a cooling rate of 10°C/sec or higher while the temperature is above 10°C, the cast structure of the surface layer of the stainless steel slab with a ferrite diameter becomes finer, and the ferrite grains of the steel plate after rolling are also refined. Due to the miniaturization, the occurrence of surface defects such as rough roving phenomenon is suppressed as much as possible. In addition, among ferritic stainless steels, such as 5US430 steel, in which the γ phase precipitates during solidification and cooling, the α + γ two phase region is slowly cooled at a cooling rate of less than 1 sec at 10°C to separate one grain within the α grain. Even if precipitation is promoted, fine recrystallized grains are likely to occur during the rolling process and surface defects such as rough roving phenomena are less likely to occur as a ferritic stainless steel sheet manufacturing material.

(e) By the way, in a normal mold (700 to 900 nm or more in length) used in a vertical or curved continuous casting machine, the solidified shell and the mold wall are connected through the molten powder in the vicinity of the molten meniscus. Although solidification progresses in a close contact state and a sufficient cooling rate is secured, below this point, the slab separates from the mold wall due to solidification shrinkage of the molten steel and shrinkage due to the temperature drop of the slab, causing heat to be removed from the mold. There is a problem in that it is impossible to secure a high cooling rate at an early stage as described above, as an air gap is created that impairs the operation, resulting in a significant cooling delay. In the mold, only an extremely thin solidified layer is formed on the surface of the slab, and when a cooling medium is sprayed onto the slab pulled out early from the bottom of the mold to increase the cooling rate in the high temperature range, the slab breaks. Although there was an extremely high risk of outflow, as shown in Fig. 4, the continuous casting mold is equipped with a temperature sensor 3 for measuring the temperature of the surface layer of the slab and a nozzle hole 4 for blowing coolant on the lower inner wall surface 2 of the mold 1. and by providing a cooling medium suction passage hole 6 above the lower inner wall surface 2, the temperature of the surface layer of the slab reaching the lower inner wall surface can be controlled within the appropriate range mentioned above (from Tα or above). The temperature sensor 3 detects whether the temperature is within the range in which a rapid cooling effect can be expected.
The amount of coolant blown from the nozzle hole 4 for blowing the coolant can be adjusted through the temperature sensor 3 so that cooling can be performed at a cooling rate of 0°C/sec or more, and the blown coolant flows into the upper part of the mold 1. A gap is created between the inner wall surface 5 and the meniscus 8 of the molten steel 7 and the molten steel 7 blows upward from there, destabilizing the cooling in the vicinity of the meniscus 8, which is ensured by smooth discharge from the cooling medium suction passage hole 6. If a product that can prevent the breakout is used (in Fig. 4, numeral 9 indicates the solidified shell, numeral 10 indicates the cooling water passage, and numeral 11 indicates the cooling water spray nozzle), there is a risk of slab breakout. It is possible to easily secure a high cooling rate at the temperature of the cast molten steel while reliably avoiding the condition that the surface layer of the slab is cooled from a temperature of Tα or higher at a cooling rate of 10°C/sec or higher. To be able to achieve this stably.

That is, in FIG. 4, when molten steel 7 is poured into a mold 1, an extremely thin solidified shell is first formed by the heat removal action of the upper inner wall surface 5 of the mold. If the length is kept extremely short, about 400m (length below the meniscus: about 300m), the part of the slab where the solidified shell has just been formed will be immediately lowered to the position of the lower inner wall surface 2, and will be cooled. The cooling medium (such as He gas) that is blown from the medium injection nozzle hole 4 and returned to the cooling medium suction passage hole 6 is efficiently cooled, so it is not possible to use "solidification or cooling" as in conventional molds with both ends open. The solidified shell surface separates from the wall surface in the mold due to shrinkage, forming an air layer between both surfaces, which causes a delay in cooling.Furthermore, the temperature sensor 3 allows the temperature sensor 3 to quickly cool the slab. In addition to being able to accurately check the temperature of the surface layer, it is also easy to adjust the cooling rate, thereby stably ensuring a high cooling rate in the high temperature range of the surface layer of the slab.

Furthermore, with such a mold, the slab can be kept in the mold until a solidified shell of the desired thickness is formed, so there is no risk of breakout. Note that as the cooling medium, in addition to cooling gas such as He gas, a mixed gas of these gases and water, etc. can also be employed.

Based on the above findings, this invention has developed a ferritic stainless steel that facilitates the generation of fine recrystallized grains in the subsequent rolling process by suppressing the coarsening of ferrite grains, and that does not cause surface defects such as rough roving. It was created with the aim of "providing continuous cast slabs that can stably produce steel plates", and "when producing continuous cast slabs of ferritic stainless steel from molten steel, it is possible to produce continuously cast slabs of ferritic stainless steel that are formed in a mold with both ends open. The surface layer of the slab is heated at 10℃/s from the temperature range above the solidus temperature (Tα).
By cooling at a cooling rate of ec or higher, it is possible to prevent the coarsening of α grains and to stably mass-produce continuous cast slabs in which a fine recrystallized structure is easily obtained in the subsequent rolling process. It is characterized by the ability to prevent the occurrence of surface defects (such as rough roving phenomenon) during deep drawing of ferritic stainless steel sheets.

Here, the "slab surface layer" refers to the area up to about 20 mm from the surface of the slab at most.

In this invention, the cooling conditions for the surface layer of the slab formed from molten steel are particularly set at a cooling rate of 10'C/sec or more from a temperature range of at least the solidus temperature (ferrite single-phase temperature: Tα). The reason for this limitation is explained below.

That is, as explained earlier with reference to Fig. 1, when the temperature of the surface layer of the slab formed by solidification in the mold becomes below Tα, the tendency of coarsening of the α grains becomes remarkable. This is because if the surface layer of the slab is cooled at a fast cooling rate of 10° C./sec or more, coarsening of α grains is suppressed. If the cooling rate at this time is less than 10°C/sec, α
The effect of suppressing grain coarsening is insufficient, and it is difficult to form fine recrystallized grains in the rolling process, and steel sheets manufactured from this are susceptible to surface defects such as rough roving during deep drawing. As explained earlier, Fig. 2 is a diagram showing the relationship between the cooling rate and the α grain size after the temperature of the surface layer of the slab reaches Tα. ``If the cooling rate is 10°C/sec or more, a sufficient effect of suppressing grain coarsening can be obtained; however, if the cooling rate is 10°C/sec or more,
It is clear that if the temperature is less than ℃/sec, the α grains in the surface layer of the slab will tend to become coarser, and a fine recrystallized structure will not form during the rolling process, leading to frequent surface defects during deep drawing. It is.

In addition, in the mold of a conventional vertical or curved continuous casting machine, the cooling rate of the surface layer of the slab in the high temperature range is 10°C/sec.
Although it is impossible to do more than that, as shown in Figure 4,
In addition to shortening the length of the mold, which is open at both ends, the inner wall is equipped with multiple sensors that can measure the temperature of the surface layer of the slab and multiple cooling medium jetting nozzles (installed between the sensors, allowing adjustment of the cooling medium jetting). The cooling rate can be adjusted by adjusting the drawing speed of the slab so that the temperature of the surface layer of the slab at the outlet is Tα or higher, and then spraying a cooling medium onto the slab directly under the mold. As mentioned earlier, it is quite possible to secure this.

In this case, if continuous casting is performed in combination with low-temperature casting and electric stirring in the mold, fine equiaxed crystal grains can be generated inside the slab, and the occurrence of the roving phenomenon can be stably suppressed. becomes possible.

Next, the present invention will be specifically explained using examples and comparing with comparative examples.

<Example> 5t with surface defects such as roving phenomenon during rolling
lS430A1 killed steel (C by weight percentage: 0.07%
, Si: 0.62%, Mn: 0.46%,
P: 0.029%, S: 0.001%,
Cr: 16.11%, Ni: 0.18%,
Mo: 0.01%, Ti: 0.007%,
AA: 0.082%, N: o, oti
5%), and a practical curved continuous casting machine (curving radius:
12.5m,) makes the cross-sectional dimensions 250 fins x 1200+
+n slabs were cast under the following two conditions and rolled using the conventional rolling method (
Hot rolling - annealing - cold rolling - annealing) and roving prevention rolling method (hot rolling - annealing - cold rolling - annealing - cold rolling - annealing) were carried out separately, and the surface roughness of the obtained steel sheet was evaluated. The occurrence of surface defects such as roving phenomenon was observed.

■ Conditions according to the method of the present invention (example of the present invention) A mold with both ends open, as shown in Fig. 4, equipped with a plurality of sensors capable of measuring the temperature of the surface layer of the slab and a plurality of coolant jet nozzles on the inner wall. (The shape and dimensions are shown in Table 1), and by increasing the amount of cooling medium ejected from the nozzle located above the sensor that detects that the temperature of the surface layer of the slab is close to Tα. The cooling rate of the surface layer of the slab was set to 10'C/s.
Adjust to ec or higher.

■ Conventional conditions (comparative example) Using a conventional mold with both ends open, the first
1. Cool the cast slab after it has been surfaced by regular water spray cooling.

The cooling rate of the surface layer of the slab at this time was confirmed by the arm spacing of the α dendrites, and the cooling rate at a position 20 m from the surface was 10°C/sec or more in the example of the present invention, whereas in the comparative example In the example, it was less than 10°C/sec.

The slabs cast under the conditions shown in the inventive example and the comparative example were
Fig. 5 shows the roving grades obtained by visual evaluation of the occurrence of roving phenomenon when rolled using the ``conventional rolling method'' and the ``roving countermeasure rolling method''.The roving grade of the slab cast by the method of the present invention is The roving grade of the slab cast by the comparative method was B' even by the roving countermeasure rolling method, whereas the rolling method gave A, and by casting with the method of the present invention, the rough surface roving phenomenon was significantly suppressed. I understand that.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between water quenching temperature and α grain size when 5US430 steel during cooling is water quenched at various temperatures to fix the structure. Figure 2 is a graph showing the relationship between water quenching temperature and α grain size. A graph showing the relationship between the cooling rate and ferrite (α) grain size after the solidus temperature (α single-phase temperature) of It is a micrograph diagram of the metal structure, and the third
Figure (al is the metal structure when cooled at a cooling rate of 2°C/sec, and Figure 3fbl is the metallographic structure when cooled at a cooling rate of 12'C/sec.
Fig. 4 is a schematic diagram showing a continuous casting situation using a mold suitable for carrying out the method of the present invention, and Fig. 5 is a comparison with the method of the present invention. 2 is a graph comparing the roving grade when a cast slab cast by the method is rolled by the conventional rolling method and the roving countermeasure rolling method. In the drawings, 1... Mold, 2... Lower inner wall surface, 3...
Temperature sensor, 5... Upper inner wall surface, 4... Nozzle hole for cooling medium injection, 6... Continuity hole for cooling medium suction, 7... Molten steel, 8... Meniscus, 9...
- Solidified shell, 10... Cooling water passage, 11... Cooling water spray nozzle.

Claims (1)

[Claims] When manufacturing continuously cast slabs of ferritic stainless steel from molten steel, the surface layer of the slab formed in a mold with both ends open is cooled from a temperature range above the solidus temperature at a cooling rate of 10 ° C / sec or more. A method for continuous casting of ferritic stainless steel.