JPS63188401A - Cooling method for billet manufactured by continuous casting method - Google Patents

Abstract

PURPOSE:To prevent generation of surface flaws and to reduce a truck time by quickly cooling only the surface layer when a surface layer temp. of a hot billet exactly drops down to the A3 transformation temp. and reheating and rolling the billet after once bringing the surface temp. to be lower than the A1 transformation temp. CONSTITUTION:The surface layer of a hot billet 3 manufactured by a continuous casting method is quickly cooled from a surface temp. equal to the A3 transformation temp. by many upper nozzles 7, lower nozzles 8, and horizontal nozzles connected with a cooling water pipe 5. After the surface temp. is once brought to be a temp. lower than the A1 transformation temp., the billet 3 is reheated and rolled in the conventional method. In that case, the surface temp. means the temp. of a position of 30 mm inside from the billet surface.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for cooling slabs using a continuous casting method.

[Conventional techniques and problems] When rolling slabs produced by continuous casting, it is necessary to reheat the slabs in the previous process. It is preferable to reheat the slab at the highest possible temperature, and it is considered an economical method to immediately reheat it when the surface temperature of the slab falls below the A3 transformation point in the steel phase diagram. Theoretical shortest track
It was time. What is track time here?
This is the time from the completion of solidification of the continuous cast slab to the start of reheating in the heating furnace.

In the past, this was sufficient to achieve the purpose in the case of continuously cast slabs made of extremely common materials, but as the frequency of manufacturing aluminum killed steel and products with trace amounts of special elements increased, Manufacturers have been faced with a new problem: minute surface flaws, so-called pine needle flaws, which frequently occur on the surface of rolled intermediate products. This means that at the above-mentioned theoretical shortest track time, the Table 1Im degree of the slab is the A3 transformation point and A1 in the steel phase diagram.
Since it is in the middle of the transformation point and has two phases of α and γ, pro-eutectoid ferrite and grain boundary precipitates precipitate at grain boundaries, causing grain boundary embrittlement, and due to the synergistic effect with thermal strain caused by reheating. Subsurface cracks occur, which open during rolling and become surface flaws. Therefore, traditionally, in such cases, manufacturers would
Efforts have been made to prevent flaws by waiting for the surface temperature of the slab to drop below the A1 point and then reheating it.

This is clearly a disadvantageous method from a thermoeconomic point of view, and the purpose of the present invention is to provide a method for cooling slabs using a continuous casting method that is thermoeconomically advantageous and prevents the occurrence of defects. That is.

[Means for solving the problem] In order to solve the above problems, the method provided by the present invention is as follows.

A high-temperature slab produced by continuous casting is
Table 1: From 1m degrees just below the 3 transformation point, only the surface layer is rapidly cooled with cooling water, and once the surface layer temperature of the slab is lowered to below the A1 transformation point, it is reheated using the same method as before. This is a method of cooling slabs using a continuous casting method, which is characterized by rolling.

However, the surface temperature referred to here is the temperature of the slab at a position 30 layers below the surface of the slab, and hereinafter this is referred to as surface layer temperature.

[Operation] The operation when the slab is rapidly cooled as described above is as follows. That is, as mentioned above, just below the A3 transformation point, steel is in the two-phase region of α and γ, but when the surface layer is rapidly cooled from this point using the above method, the temperature of that part decreases and finally reaches below the A3 transformation temperature. The crystal structure there is an α structure, that is, a metal structure consisting only of ferrite. When this ferrite is subsequently reheated, it becomes a new γ structure with a different grain size, that is, austenite, so the pro-eutectoid ferrite and precipitates that were at the original γ grain boundaries are replaced by a new γ structure, that is, austenite crystal grains. It does not collect in a field and does not cause the above-mentioned flaws. Since this cooling is performed only to a small depth of the surface layer, a short cooling time is sufficient, and therefore the internal temperature does not drop much, and the slab as a whole retains more heat than in the conventional method. Since it is reheated in the same state, the amount of heat required for reheating is small, and furthermore, subsequent rolling can be carried out without the occurrence of surface flaws. That is, the track time and reheating time of the slab are shortened, and the defects that occur in the conventional method are no longer generated.

[Example] An example of an apparatus used to carry out the method of the present invention will be described with reference to the drawings.

Figures 1 and 2 show a cooling device 1 for rapidly cooling slabs.
, which shows a front view and a side view of a heating furnace (not shown). A cooling water pipe 5, a header 6 that is connected to the ε pipe 5 and distributes water uniformly to each nozzle, and a number of upper nozzles 1 and lower nozzles 8 that are connected to the header 6 and spray water onto the slab 3. It consists of a horizontal nozzle 9, and a water collection tank 10 that receives excess water dripping from the slab 3 out of the injected cooling water. The hood 4 has at its top an exhaust duct 12 equipped with a forced fan 11 for receiving a large amount of water vapor generated by cooling the slab and discharging it to the outside. Water collection tank 1
0 has a water pump 13 and a water level gauge 14 that automatically drain water when the water level accumulated therein reaches a certain level, and when the water level exceeds a specified value, the pump 13 is activated. Drainage water is sent to a reservoir. The cooling water piping 5 is a conduit that sends water from a water source, and the piping 5 is provided with an orifice 15, a flow rate regulating valve 16, and a flow chamber meter 17, and has a water tank necessary and sufficient to quickly cool the slab 3. is sent to header 6. The header 6 branches in four directions, vertically and horizontally, with respect to the slab 3, and extends in the direction in which the slab 3 moves. A large number of nine nozzles 7.8 and 9 are provided that branch from the header 6, and the spray from these nozzles covers the entire surface of the cast slab 3, so that the surface layer of the cast slab 3 is rapidly and uniformly cooled.

An example in which this cooling device 1 is used will be described as follows.

First, as mentioned above, this water cooling device 1 is installed on the charging table 2 immediately in front of the heating furnace, and is based on the dimensions of the plume to be used (400 x 520 cross section, unit layer), and the injection range of cold W water. 405Hx520Wx11,0OOL
(1 position), and the amount of water jetted is set to 0 as the adjustable range.
~250λ/min.

- A flow rate regulating valve 16 was installed, and the setting range of the injection time was 0 to 70 seconds. During implementation, the amount of water jetted is 1
The injection time was set to 40 seconds at 70ffi/min. When the above bloom is rapidly cooled by injection under such cooling conditions, the surface layer temperature before water cooling is 850°C, and the surface temperature after water cooling is H'lf.
The A degree was 550°C, and the surface temperature had dropped by 300°C.

To explain this situation conceptually using Fig. 3 and comparing it with the conventional method, the temperature of the slab 3 gradually decreases over time due to natural cooling from the high temperature (top left of the figure) where the slab 3 has completely solidified. is drawn as a gentle curve toward the bottom right of the figure. The two horizontal straight lines schematically represent the A3 and As transformation temperatures. Compared to the track time of conventional method (2) in which the slab 3 is air-cooled to below the A1 transformation point, in the case of this embodiment (2), the temperature is reduced to below the A1 transformation point in a very short time, so the track time is significantly shortened. It is clear that Note that ■ is a curve when reheating is performed immediately from the theoretical track time.

Comparing the case of water cooling in this way (method of the present invention) and the case of no water cooling, the fourth
As shown in the figure, as the track time decreases, the number of defects increases significantly when water cooling is not used, but when water cooling is performed using the method of the present invention, the number of defects increases regardless of the track time. is hardly recognized.

As another example, when continuous casting slabs of aluminum, silicon, and killed steel were water-cooled, round billets with the same diameter were produced using the conventional method (in the figure, 8 long pieces refer to billets with a length of 10 m). A monthly statistical comparison of the average number of flaws on the billet surface when manufacturing a material with water spraying of the present invention method compared to the conventional method shows that It can be seen that there are fewer scratches. Even with this method, an increase in the number of scratches is observed for pellet diameters of 210 m or more, but this is due to an increase in surface scratches caused by causes other than the above-mentioned scratches.

[Effects of the Invention] To summarize the effects of the present invention described above, the effects of the present invention can be summarized as follows. These are prevention of surface flaws in rolled products manufactured from slabs produced by the c-casting method, shortening of track time, and resulting reduction in fuel consumption.

First, regarding surface defects, the method of the present invention is equally effective when rolling slabs made of materials having two-phase regions of α and γ. In the case of high manganese steel, and also in the case of materials to which titanium and/or boron are added, remarkable effects are observed.

Next, regarding the track time, if only the surface layer of the slab is rapidly cooled using the method of the present invention, then reheated and rolled, the track time can be significantly shortened without increasing the surface defects of the product. Since the method of the present invention rapidly cools only the surface layer of the slab, the total heat capacity of the slab when it is charged into the heating furnace increases compared to the conventional method.

The charging temperature to the heating furnace substantially increases, and the fuel consumption rate and heating time can be reduced, and in long-term implementation using the method of the present invention, the fuel consumption rate is lower than that of the conventional method. It was reduced by about 13% compared to . The results are as follows.

Method Lowest processing rate per month Fuel consumption, 1,000 kcal Conventional method 47,034 245.6 Method of the present invention
39,379 213.6 difference
32.0 Furthermore, in addition to the direct effect of saving fuel costs, the method of the present invention also equalizes the charging temperature to the heating furnace, and also reduces the possibility of removing defects in the subsequent process. It can be said that the great hidden effect of the method of the present invention is that the manufacturing process can be kept under stable control because the work is greatly eased.

[Brief explanation of the drawing]

The first factor is a front view showing the main features of the water cooling device that performs rapid cooling according to the present invention, FIG. 2 is a side view, FIG. 3 is a conceptual diagram showing the relationship between slab temperature and time, and FIG. FIG. 5 is a relational diagram showing the correlation between track time and flaws, and is a comparison diagram of the method of the present invention and the conventional method, showing the occurrence of flaws in billets according to billet size. Applicant's representative Patent attorney Takehiko Suzue

Claims (1)

[Claims] When the surface temperature of the hot slab immediately after being manufactured by the continuous casting method becomes just below the A_3 transformation temperature,
A method for cooling a slab using a continuous casting method, characterized by rapidly cooling only the surface layer of the slab with cooling water, once bringing the temperature of the surface layer below the A_1 transformation temperature, and then reheating and rolling.