JP3978855B2 - Optimal heating method for continuous cast slabs before hot rolling - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
In the present invention, when rolling a continuously cast slab in a hot rolling line, the optimum heating furnace is selected from a plurality of heating furnaces, and the slab is inserted, and before the continuous hot casting of the continuous casting slab to be heated and rolled. It relates to a heating method.
[0002]
[Prior art]
Conventionally, a slab cast by continuous casting is once cooled in a slab place, and then charged in a heating furnace of a hot rolling line and reheated for rolling. However, recently, in order to effectively use the retained heat of continuously cast slabs, direct feed rolling has been performed in which a continuous casting line and a hot rolling line are directly connected.
[0003]
This direct rolling is a very efficient method in terms of both energy intensity and productivity.
However, at the beginning of the introduction of this direct rolling, the problem of hot rolling cracks due to coarse γ particles due to charging in the heating furnace at a temperature higher than the γ → α transformation point, and the surface generated during hot rolling of medium carbon steel A cracking problem occurred. Therefore, with respect to the former problem, a countermeasure has been taken in which the rolling start temperature disclosed in JP-A-62-139814 is 1050 to 1300 ° C. and the reduction ratio is 20% or more. No. 62-40902 discloses a countermeasure for rolling at a slab surface temperature of 900 ° C. or higher or 700 ° C. or lower. As a result, the problem of cracking is avoided and the direct feed rolling is widely implemented.
[0004]
As a form of direct feed rolling, direct hot charge rolling (hereinafter referred to as DHCR), in which a slab is rapidly conveyed from the exit side of a continuous casting line, directly charged into a heating furnace of a hot rolling line, and rolled, is heated. Temporary placement is temporarily performed before charging, but hot charge rolling (hereinafter referred to as HCR) in which the steel is charged in a heating furnace while being hot and rolled is generally used.
[0005]
[Problems to be solved by the invention]
However, in both DHCR and HCR systems, it frequently occurred that the productivity indicated by the production tonnage per hour at the operation planning stage could not be achieved.
In a hot rolling line, productivity is often limited by the heating furnace capacity determined by the input heat amount and the heating furnace length. If all the slabs charged in the heating furnace are cold slabs and the extraction temperature is constant, the productivity of the heating furnace is stable, and the productivity of the hot rolling line is also stable. However, when charging with a hot piece, the productivity strongly depends on the change in the heat content of the slab, resulting in unstable operation.
[0006]
In particular, when carrying out the DHCR operation, for example, if the average cross-sectional temperature at the time of charging the slab into the heating furnace is 910 ° C., the productivity of the heating furnace operation of 500 ton / hr can be achieved. Line productivity can be similar. However, if a single slab with an average cross-sectional temperature of 760 ° C is inserted into the slab, this slab can only be expected to have a productivity of 250 ton / hr furnace operation. Until it is extracted, the productivity of the furnace, which can be expected to be productivity at 500 tons / hr, must be reduced to 250 tons / hr. Such a slab whose cross-sectional average temperature is greatly different from the slabs before and after the cross-sectional average temperature often occurs. For example, by detecting a precursor of an operation trouble by predicting a breakout in a continuous casting line, the casting speed and the water cooling condition in the spray may be changed. Such a change in casting conditions also occurs in the slab of the unsteady part immediately after the start of continuous casting and immediately before the end, and the average cross-sectional temperature of the slab is different from the average cross-sectional temperature of the slab in steady operation. It will be. In addition, when the casting speed is accelerated or decelerated, when replacing the tundish, immersion nozzle, etc., when the temperature of the molten steel in the tundish is high and there are concerns about operational problems, wait for the molten steel to be blown in the converter. When can I give you.
[0007]
In addition, as mentioned above, the average cross-sectional temperature of the slab fluctuates due to casting conditions, while the slab handling method after the torch cutting, the required time, the hilling method during temporary placement, the maintenance method, the forced The average temperature of the slab cross section varies greatly depending on the cooling conditions such as the presence or absence of cooling (water cooling or the like).
An object of the present invention is to propose an optimum pre-hot rolling heating method for continuously cast slabs that can ensure high productivity when rolling slabs having different cross-sectional average temperatures as described above.
[0008]
[Means for Solving the Problems]
Here, the present inventors have found that the temperature distribution of the slab varies greatly depending on the steel type, the size of the slab, the operating conditions of the continuous casting line, and the like. The knowledge that it cannot be evaluated only by the single surface temperature was obtained.
The present invention has been made on the basis of the above-mentioned knowledge. When a continuously cast slab is rolled in a hot rolling line having three heating furnaces having different heating capacities , the continuously cast slab is obtained. The pieces are divided into three levels according to the number of heating furnaces based on the heat content immediately before charging the heating furnace, and a heating furnace having a smaller heating capacity is selected and charged for a slab having a higher heat content. By doing so, the above problems were solved.
[0009]
At that time, the heating capacity of each of the plurality of heating furnaces is made different to facilitate selection of the optimum heating furnace.
In addition , it is preferable to estimate the heat content of the slab from casting conditions and cooling conditions in continuous casting.
The casting conditions are determined by the steel type, slab size and casting speed, and the conditions determined by the cooling water spray conditions in each spray zone, and the cooling conditions are cut after the continuous cast material is cast into the mold. It determined Me Rukoto preferably between elapsed time and handling means from being elapsed time and cut until that until it reaches the furnace.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
A continuous casting line to which the present invention is applied and a hot rolling line directly connected to the continuous casting line will be described with reference to FIG. FIG. 1A is a schematic side view of a continuous casting line. FIG. 1B is a schematic plan view of the entry line of the hot rolling line. Here, the point A shown in FIG. 1A and the point A shown in FIG. 1B are connected to form a continuous line.
[0011]
The continuous casting line shown to Fig.1 (a) is demonstrated.
The molten steel 15 refined in a refining furnace such as a converter is cast from a tundish 11 into a mold 10 via a ladle 12. Then, the mold 10 is drawn downward with the shell portion of the surface layer solidified. The drawn continuous cast material 2 is in a state where only the shell portion is solidified immediately below the mold 10 and the inside remains as molten steel. Then, the first spray zone 4, the second spray zone 5, and the third spray zone 6 are sequentially spray-cooled with cooling water and solidify to the inside. The continuous cast material 2 solidified to the inside is straightened by a straightening device 7 and cut by a cutting device 8 to obtain a slab 1 having a desired length. During this time, in order to stabilize the quality of the continuous cast material 2, it is desirable to keep the casting speed and cooling conditions in each spray as constant as possible. However, it is difficult to keep the casting conditions constant for the reasons described above.
[0012]
Here, in FIG. 1 (a), the continuous casting material 2 is tracked from the time when the continuous casting material 2 is cast into the mold 10 by a tracking system (not shown). The spray conditions, casting speed, and the like are always collected, and when the slab 1 is cut by the cutting device 8, the corresponding data is transferred as slab data for each sheet.
[0013]
The main data items to be transferred are the following items.
(1) slab dimensions (slab width, slab thickness, slab length)
(2) Steel grade (3) Cooling water spray conditions in each spray zone (This condition allows estimation of heat removal from the slab by spraying.)
(4) Elapsed time from mold to cutting device (The amount of heat removed due to natural cooling can be estimated and calculated from this elapsed time.)
(5) Cooling conditions after the torch cutting, such as “slab handling method, required time, hilling method for temporary placement, care method, presence of forced cooling (water cooling, etc.), etc.” (It is possible to determine the command for the cooling condition, and it is possible to correct the command based on the results, and thereby the heat removal amount can be estimated.)
Here, various data such as the slab surface temperature are also collected, but since they are not directly related to the present invention, description thereof is omitted here.
[0014]
Next, the entry side of the hot rolling line shown in FIG.
The slab 1 that has been directly fed from the continuous casting line is usually charged in the DHCR furnace 21 as it is in direct rolling, and is slightly reheated in the DHCR furnace 21 and carried out to the rolling entry side conveying table 27. . The unloaded slab is conveyed to the rolling line 28, and is wound up as a hot coil through rough rolling and finish rolling.
[0015]
Here, a plurality of heating furnaces are usually installed on the entry side of the hot rolling line.
These are the DHCR furnace 21 described above, the HCR furnace 22 for reheating the temporarily placed slab, the cold slab heating furnace 23 for charging and reheating the slab as a cold piece, and the like. The slab that is not directly sent to the DHCR furnace 21 is carried out to an offline table 24, and is transported to a storage place (not shown) by a transport facility such as a tongue (not shown) and temporarily stored, or forced cooling (water cooling) for maintenance and surface inspection. Etc.) is performed. Moreover, it may be conveyed directly or indirectly to the HCR furnace charging table 25 and the cold slab heating furnace charging table 26.
[0016]
Also in this hot rolling line, a tracking system (not shown) functions, and tracking is performed for each slab. The time until each slab is charged into the heating furnace is collected as actual results, and at the same time, a predictive calculation is performed based on the operation schedule and the like.
In other words, the tracking system can accurately predict the elapsed time from casting into the mold of each slab to cutting and reaching the heating furnace.
[0017]
As described above, since the heat content can be accurately predicted for each slab, the slab is distributed to each heating furnace according to the level of the heat content, and the level at which the slab is directly sent to the DHCR furnace 21. The heating furnace was selected according to the level at which it was reheated in the HCR furnace 22, completely or forcedly cooled as completely off-line, and reheated again in the cold slab heating furnace 23, and the charging process was performed.
[0018]
Here, the slabs charged in the HCR furnace 22 and the cold slab heating furnace 23 are each reheated and then transported to the rolling entry-side transport table 27 while seeing the timing of unloading of the slabs sent directly to the DHCR furnace 21. Alternatively, the slabs directly sent to the DHCR furnace 21 may be carried out collectively after the carrying out process is completed.
[0019]
【Example】
The present invention was applied to a slab of standard SPHD low carbon steel.
Here, the slab has a width of 1300 mm, a thickness of 260 mm, and a length of 9000 mm.
The target furnace extraction target temperature of the slab is 1050 ° C. in terms of the average cross-sectional temperature, which is 160 Mcal / ton in terms of heat content.
[0020]
When operating at a heating furnace capacity of 500 ton / hr, the heat content of the slab when the slab is sent directly to the DHCR furnace 21 must be in the range of 120 Mcal / ton to 140 Mcal / ton. This is level 1. When charging into the HCR furnace 22, it is 60 Mcal / ton to 120 Mcal / ton. This is level 2.
In the case of 60 Mcal / ton or less, it is completely off-line, and batch processing is performed together with other cold cast slabs after completion of direct rolling. This is level 3.
[0021]
FIG. 2 is a graph showing the heat content for each slab, with the slab number on the horizontal axis and the heat content on the vertical axis. Here, in the slab No. 1, since the heat start amount is slightly small and level 2 because of the start of operation start, it is sent to the HCR furnace. Each heat content from slab No. 2 to slab No. 7 is stable within level 1 that can be directly fed to a DHCR furnace. However, for casting slab No. 6, because a casting instruction was added due to abnormal casting conditions, it was predicted that it would be level 3 due to a temperature drop during temporary placement and maintenance, so offline batch It was decided to carry out the process. Moreover, in casting slab No. 8 to slab No. 10, the casting speed in the mold is reduced in order to wait for the end of the converter blowing, so slab No. 8 to slab No. 10 are not included. Since the amount of heat is small and the heat content is level 2, it is for the HCR furnace. After slab No. 11, the heat content again reached level 1 and remained stable.
[0022]
As described above, by selecting a heating furnace using the present invention, it was possible to stably perform an operation with a heating furnace capacity of 500 ton / hr in direct rolling.
[0023]
【The invention's effect】
When the present invention is applied, it is possible to avoid a decrease in productivity due to fluctuations in the heat content of the slab and maintain high productivity in direct rolling.
[Brief description of the drawings]
FIG. 1 is a schematic diagram for explaining a line to which the present invention is applied;
(A) is a schematic side view of a continuous casting facility.
(B) is a plane schematic diagram of the entrance side of the hot rolling line which performs direct feed rolling.
FIG. 2 is a graph showing the heat content for each slab number of a slab to which the present invention is applied.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Cast piece 2 Continuous casting material 3 Pinch roll 4 1st spray zone 5 2nd spray zone 6 3rd spray zone 7 Straightening device 8 Cutting device
10 Mold
11 Tundish
12 ladle
15 Molten steel
20 Transport table
21 DHCR furnace
22 HCR furnace
23 Cold slab heating furnace
24 Offline table
25 HCR furnace charging table
26 Cold slab heating furnace charging table
27 Rolling entry side transfer table
28 Rolling line

Claims (1)

When the continuously cast slab is rolled in a hot rolling line having three heating furnaces having different heating capacities , the continuously cast slab is heated based on the heat content immediately before charging the heating furnace. It is divided into three levels according to the number of furnaces, and the slab with higher heat content is selected and charged with a heating furnace having a smaller heating capacity. Way .

Images