JP2863013B2 - Casting and rolling method for thin slab - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates, are those related to thin slab casting-rolling how of in the steel manufacturing process developed in recent years on a global scale is underway, casting of thin slabs, in a continuous casting machine A thin slab having a thickness of 90 to 30 mm is cast, and the slab is directly sent to a hot rolling mill to produce a hot coil.

[0002]

2. Description of the Related Art In conventional slab casting and rolling in a steel manufacturing process, molten steel is cast by a continuous casting machine to 200 to 300 mm.
Cast into a thick slab, the slab is 5
It was rolled to a hot coil centered at 4 to 1 mm thick with a hot finishing mill at 5 to 30 mm.

[0003] In recent years, the development of the thin slab is currently in a vigorous stage, but various forms of the thin slab process have been published as documents. For example, Han
s-Juergen Ehrenberg et al.
(Metallurgical Plant and
Technology Vol. 3 (1989) 52)
Showed the results of Mannesmann's development of casting and rolling processes for thin slabs, and consequently showed the application image as a real process. Yujiro Nakamura et al. (Sumitomo Heavy Industries Technical Report Vol. 37, No. 110 (1989) 31)
Shows the development results and equipment configuration of the wide and thin slab continuous casting equipment.

[0004] Holleis et al. (Ste
el Times November (1989) 60
5) is a thin slab casting method of Voest-Alpine,
In addition, the daily newspaper “Steel Newspaper” (March 8, 1988)
The thin slab continuous casting method developed at Kawasaki Steel and Hitachi, Ltd. has been reported. Although there are many publications in these publications, in summary, the continuous caster and equipment configuration of the thin slab casting process can be summarized as shown in FIG.

First, a continuous caster for thin slabs is a Hans-J
uergen Ehrenbergetal. And S
Various thin slab continuous casters are shown, such as those using a fixed mold such as an MS mold, and those using a belt-type moving mold shown by Kawasaki Steel, Hitachi, Ltd., and Yujiro Nakamura. In the subsequent process, in the route A, the slab coming out of the continuous casting machine is cut into a predetermined length, and then directly charged into a heat retaining furnace having a buffer function, followed by finish rolling. Therefore, in the route A, it is indispensable that the casting timing of the continuous casting machine and the rolling timing of the rolling mill are continuous in series, which is a great feature. It is also characterized in that a slab coming out of a continuous casting machine is sent to a finishing mill without any processing (for example, bending by winding and unwinding) except for cutting.
On the other hand, in the route B, the slab coming out of the continuous casting machine for thin slabs is wound into a coil by a winder, and then charged into a heat retaining furnace as a buffer furnace, and the time required for the rolling mill is reduced. Make adjustments,
Thereafter, it is a process of being mounted on a rewinding machine and subjected to finish rolling. Therefore, the route B can adjust the casting timing by the continuous casting machine and the rolling timing by the rolling mill,
Further, it is characterized in that a thin slab having a large surface area and a large heat dissipation is intended to be avoided by coiling.

[0006]

The thin slab process disclosed in such a document is a mini mill (with an annual production of 80 to 80).
1.6 million Ton), and the rough rolling step is omitted by casting a thin slab, and at the same time, the process configuration is focused on using sensible heat during casting. For this reason, the concept of speeding up to increase production, increasing the size (particularly thickness and width) of products, and building in quality (inner surface quality, material properties) so that high-grade steel can be manufactured Is not shown. Further, when the steel sheet is cast and rolled with the process configuration as it is, it is considered that the production amount is restricted, and at the same time, a large number of cases where the rolling cannot be performed depending on the steel type and the product size occur.

The present invention seeks to provide a more complete steel sheet slab process by adding a concept necessary for product size separation and product quality assurance in consideration of the current facility technology level in addition to the above-mentioned known concept. Is what you do.

[0008]

The method of casting and rolling a thin slab according to the present invention comprises rolling a thin slab cast by a belt-type thin slab continuous casting machine in an in-line press-down facility and winding it by a coil winding device. A method in which the wound coil is retained at the Ar3 transformation point or higher, and the retained coil is rewound and finish-rolled, wherein the solidified portion of the thin slab cast by the belt-type thin slab continuous caster and downstream thereof are provided. It characterized the gradual rolling child in the line pressure equipment side vicinity within a reduction ratio of 5-50%.

Further, casting of a thin slab for carrying out the method of the present invention.
ZoSo location is a belt type continuous caster, dense divided rolls apparatus, multistage inline pressure equipment, shears, coil winder, immediately transports the wound coil, Arc transform the said template piece at a predetermined position Provide coil heating equipment that can maintain heat for more than a predetermined time required for quality assurance, followed by coil rewinding equipment, and furthermore, crop sharpening equipment and edge equipment as necessary. A hot finish rolling mill equipped with a heater may be provided to perform hot finish rolling to a target product size .

[0010]

The operation of the present invention will be described below. The incoming bar thickness of current hot finishing mills is 55 to 30 mm. For this reason, in order to omit the hot rough rolling step by thin slab casting, the casting thickness cast by the thin slab continuous caster needs to satisfy the thickness of 55 to 30 mm of the finish inlet plate thickness.

However, in casting a slab in a continuous caster, immersion casting of an immersion nozzle is indispensable for measures against inclusions.
5 to 50 mm, and preferably requires a greater casting thickness. Against this background, the conditions for expanding the production volume and enabling multi-size production are as follows.

(A) As shown in Table 2 as an embodiment, the relationship between the incoming side plate thickness and the rolled product of the current hot finishing mill requires changing the finished incoming side plate thickness in accordance with the product thickness. This is unavoidable due to the current productivity of hot finish rolling and the controllability of mill strength and crown, and it is necessary to produce at least such a slab thickness according to the product thickness.

(B) According to a study by the present inventors, when a thin steel sheet is manufactured from a thin slab, the reduction ratio in hot rolling is small, so that the composition of the hot rolled sheet remains coarse, and the hot rolled sheet remains. The strength of the plate is insufficient, and the ductility and toughness are deteriorated. In addition, it was revealed that the texture was inappropriate even after cold rolling and annealing, and that the mechanical properties were also deteriorated. In order to avoid this, firstly, it was found that the rolling reduction in hot rolling was required to be 60% or more. However, for this purpose, for a hot-rolled product thickness of about 20 to 1 mm, The condition must be satisfied, and the lower limit of the slab thickness from the continuous caster is restricted here.

(C) For coil winding effective for avoiding heat dissipation due to the increased surface area due to the thinning of the slab, the thickness of the coil is limited to 80 mm from the current capacity of the winding equipment. For this reason, it is necessary that the outlet thickness of the continuous caster is 80 mm or less.

In order to satisfy these conditions and to enable casting with a belt-type thin slab continuous caster, a multi-stage in-line press-down facility is required in the continuous caster. The multi-stage in-line rolling in the present invention is to continuously perform a so-called post-solidification rolling method and a non-solidifying rolling method in which a slab is heated at a high temperature and deformation resistance is reduced in a continuous casting machine and a portion in front of the solidified completed portion is reduced.
The maximum reduction amount of about 40 mm can be secured by the combination of the uncoagulation reduction method and the post-coagulation reduction method. For this reason, the maximum casting thickness capable of providing a slab thickness of 50 mm necessary for ensuring product quality (securing a hot rolling rate of 60%) with a maximum thickness of 20 mm as a hot-rolled product is 90 mm. On the other hand, the amount of rolling that is most easily reduced by the in-line rolling method is 15 to 20 mm. However, after casting a minimum thickness of 45 to 50 mm by immersion casting, the in-line rolling results in a slab of 30 mm.

Therefore, it is impossible to roll a slab cast with a thickness of 90 to 45 mm by an in-line rolling reduction machine to a slab thickness corresponding to a target hot finish rolling thickness and to output the slab from a continuous casting machine. . Therefore, the significance of the multi-stage in-line rolling method for rolling the solidified portion and the region before and after the solidified portion in the present invention is as follows.

<Metaradical merit under post-coagulation reduction / uncoagulation reduction and reduction conditions at that time> The reduction effect and effect of the coagulation completed portion and the downstream vicinity thereof are summarized as follows.

(1) The fact that the second element present as a solid solution in steel is finely precipitated as MnS, TiC, AIN, etc.
The purpose of this reduction is to promote precipitation and shorten the retention time before rolling required for the production of a soft material in the next step.

(2) Similarly, since the rolling is performed at a high temperature of about 1200 ° C., the precipitation of MnS is greatly promoted, and
Avoiding embrittlement cracking in the second region of steel existing at ~ 1200 ° C to prevent embrittlement cracking during coil winding and the like.

(3) By applying a pressure of about 30% at about 1200 ° C. to a thick product such as a pipe material, recrystallization of a coarse γ of several mm occurs to several hundred μ. , To improve toughness.

Specific conditions are exemplified as follows. (1) AI-K steel for continuous annealing: The above (1)
Purpose In the temperature range where the cross-sectional average temperature is 1200 ° C or less,
50% rolling. (2) Cold-rolled steel sheet for processing other than the above-mentioned ... (1) Purpose Rolling after solidification with a rolling reduction of 5% or more in a temperature range where the average cross-sectional temperature is 1300 to 1000 ° C. (3) Material of Mn / S <20: Effect of the above (2) Rolling with a rolling reduction of 5% or more under the condition that the surface temperature is 1200 ° C or more.

As an effect of this, for example, the processing A
In the I-K steel, the following effects can be obtained by performing the reduction after solidification at an average cross-sectional temperature of 1250 ° C. and a reduction of 10%. Rankford value (average value): about 0.1 to 0.2
Elongation: Improvement of about 1 to 2% Yield stress: Reduction of about 1 to 2 kg / mm ² Next, the effects of reducing the unsolidified portion near the solidification completed portion are summarized as follows. Since the portion immediately before the solidification is in a semi-solid state, (1) rolling can be performed with a low load, and (2) rolling can be performed while applying tension to the slab between the rolling rolls to suppress the flow in the semi-solid portion. Component segregation can be prevented, and (3) reduction and elimination of center porosity, (4) elimination of center cracks, and (5) miniaturization of a solidified structure by formation of equiaxed crystals.

That is, the center porosity of (3) is caused by volume condensation that occurs when solidifying from a liquid to a solid during the solidification process of a slab, but is reduced by compensating for the shrinkage volume by rolling, and is thus eliminated. It is.

Regarding the center crack of (4), when local confinement occurs when solidifying from liquid to solid at the crater end (solidification completion point) as in (3), molten steel is not supplied to that part, and It occurs as a shape or plane crack. This is volume-compensated by the rolling and, at the same time, the upper and lower solidified portions are pressed.

With regard to (5), solidification nuclei are generated by the reduction, and the formation of equiaxed crystals is promoted. As a result, the coarse solidified structure moves in the direction of miniaturization.

As a specific condition, in a continuous casting machine, a slab having a solidification ratio of 0.5 to 1.0 mm is subjected to a total reduction amount of one minute by a plurality of reduction rolls (multi-stage in-line reduction rolls). Roll down to 0.5-2.0 mm. As a result, the effects shown in Table 1 can be obtained.

[0027]

[Table 1]

Next, in the belt-type thin slab continuous caster of the present invention, a function that the short side can move in synchronization with the belt is preferable. This is indispensable from the viewpoint of ensuring the quality of the slab and operability, but the width of the thin slab of 30 to 90 mm cannot be largely changed by rolling. For this reason, it is necessary to perform the forming corresponding to various required product widths at the casting stage in the continuous casting machine. Therefore, 600 to 2200 of a thin steel plate manufactured by a width rolling method in which a known vertical roll is arranged.
30-90m for the current product width range of mm
In the case of the m cast slab, it is necessary to temporarily stop the casting of the continuous casting machine for each lot of the product width and change the casting width.

However, this problem is solved in the present invention by
SUMMARY OF THE INVENTION This problem is solved by introducing the inventions described in JP-A-183775, JP-A-63-264252 and JP-A-63-264251 into the continuous casting process.

As described above, according to the present invention, the above-described multi-stage in-line rolling equipment and the continuous caster can produce a product size within a range that cannot be dealt with by a conventionally known process, with good internal quality, and achieve high quality and dramatically. Increase production.

Further, the present inventors have conducted detailed investigations on hot-rolled products and cold-rolled products manufactured by the known route A process shown in FIG. 1, and found that there are many scale flaws on the product surface. Was. The reason is that in this process, the scale existing on the surface of the slab cannot be completely removed by the current descaling equipment before finish rolling. In the conventional thick continuous casting process, Vertical Scale Breeze is added to the scale of the slab.
After applying a process with an aker (VSB) or the like to make it easy to peel off from the iron base material, the scale is removed by a descaling facility. However, in the A route process,
The slab undergoes descaling without any processing after casting. The present inventors have found that the scale on the as-cast slab is deeply penetrated, centering on the grain boundary portion. At the same time, it has been found that it is advantageous to apply bending strain or tensile / width-direction rolling strain to the slab surface in order to easily peel such a scale. Conventional V
Although SB is effective from this point, in the thin slab process, the slab is easily buckled in the width direction when VSB is applied because the slab is thin. To solve this problem, we have:
It has been found that it is most effective to wind the slab immediately after casting as shown in the B route of FIG.

The purpose of the B route in the known literature is as follows:
Although the heat dissipation of the cast slab was avoided and the sensible heat was used effectively, the present invention emphasizes that the winding and rewinding functions are indispensable for perfect descaling.

Further, the present invention is to unwind the slab wound at the exit point of the continuous casting machine before the finish rolling, as the best method for completely descaling before starting the hot finish rolling. It has been found that the unwound slab is quickly descaled. Therefore, it is desirable that the rewinding equipment and the descaling equipment be as close as possible to the finishing mill.

The following is means for setting the material properties of the product to target properties. That is, in the thin slab process, as shown in (b) above, since the rolling reduction of the hot rolling is small, the structure of the hot-rolled sheet remains coarse, and depending on the type of steel, the strength or ductility and toughness are insufficient. The necessary means for that is to secure the above-mentioned rolling reduction. However, for steel types that require high ductility and drawability, further control of precipitate morphology such as MnS, and Ti,
It is necessary to maintain a solid solution of a precipitation hardening element such as Nb.

In this regard, the present inventors have determined that the form of these precipitates is controlled and the solid solution is maintained, after the continuous cast slab is wound by a winding device at the discharge side of the continuous caster, and then the front surface of the finishing mill is rolled. It was found that the control was possible by controlling the retention temperature and retention time during rewinding with. The retention time is determined by the Ar of the material.
It is based on the fact that the temperature is not less than _three points, and that the temperature and the retention time are different depending on the steel type. According to conventional known literature, after winding the slab, during unwinding, no additional purpose was added except utilization as a buffer heat storage furnace during continuous casting-rolling,
The present invention is intended to make it possible to hold the coil in the heat retention furnace at a required temperature and for a required time according to the type of steel, and to control the form of the precipitate and maintain the solid solution required for the type of steel. It is.

In the thin continuous casting, since the cooling rate is low,
The second element in the steel is frozen in a solid solution state, but precipitation also occurs in a finely dispersed state. In this case, even if it is in a solid solution state, it often becomes a fine precipitate by winding and holding after hot rolling, so that the material after hot rolling hardens. Therefore in order to create a soft cold-rolled steel sheet is retention at a high temperature of at least A ₃ transformation point, MnS to finely precipitated at a high temperature, TiC, grown coarse precipitates such as AIN, the effect on the strength It is necessary to lower it.

Therefore, the following conditions (A) and (B)
Examples are the subject. (A) AI-K steel coil for continuous annealing The entire length and entire cross section of the coil are kept at 1050 ° C. or more for 15 minutes or more in a heat retaining furnace. (B) The entire length and entire cross section of the steel coil for a cold steel sheet for processing other than those described above can be maintained at 1050 to 1200 ° C. for 10 minutes or more in a heat retaining furnace.

As a result, as shown in the specific example [2], a soft cold-rolled steel sheet having a low yield stress and a high Rank Ford value (average value) can be obtained.

Therefore, the heat retention furnace is not a simple buffer furnace, but has an active heat retention / heating function. Then, as shown in FIG. 2, the temperatures of the low temperature portions of the edge portion and the top slab portion which are present even as a result of the simulation with the moving time of only 3 to 5 minutes are kept high, and the temperature of the entire cross section is maintained. To make the material characteristics of the product uniform over the entire length and width.

The slight temperature compensation at the edge of the slab can also be made by an edge heater installed on the front surface of the hot finish rolling. The installation of the edge heater becomes an effective quality assurance facility if it is installed as required depending on the production amount, steel type and steel type configuration.

The main technical means of the present invention have been described above. Other means such as a dense split roll for preventing bulging and cooling equipment in a continuous casting machine, a slab shearing equipment, a dummy bar drawing apparatus, a descaling equipment, etc. Adopts a technique well known in the industry.

The hot finishing rolling mill to which the present invention is applied may be a continuous multi-stage finishing rolling mill widely used by an integrated steelmaker or a single rolling equipment such as a Steckel mill. In particular, in the former, it is a natural premise to add necessary equipment as a finishing mill such as a crop shear, a run-out table, and a coiler.

[0043]

Embodiments of the present invention will be described below. The embodiment of the present invention is shown in FIG.
And the process in which the following equipment is arranged in connection with it. That is, a thickness of 90 to 30 mm and a width of 600 to 220
In the belt type thin slab continuous caster 1 for casting a 0 mm slab,
For example, the moving short side (not shown), which moves in synchronization with the belt and can change the slab width, and the dense dividing roll 4 for preventing bulging and ensuring sufficient slab quality in the secondary cooling zone and the cooling equipment (air A water cooling device) 2 and an in-run pressure reduction device 3 for rolling the slab thickness to a multi-stage in the solidified portion and the vicinity thereof before and after the solidified portion according to the target finish rolling thickness. , Quickly synchronized with the moving speed of the slab,
In addition, a shearing equipment 5 capable of cutting a slab to a predetermined length, a dummy bar pulling-out device 7, a single-stage or several-stage coil winding device 6 and a coil transfer machine (lifting type) 9 are arranged.
Furthermore, it has a soaking / heating function of receiving the coil wound by the coil winding device 6 behind the equipment from the coil transfer machine 9 and immediately or from the insulated coil box freight car 14, A tunnel-type coil heating furnace 8 capable of holding the slab at or above the Ar ₃ transformation point and for a predetermined time or more necessary for quality assurance is provided. The coil is rewound by being mounted on a coil rewinding facility 11 arranged on the entry side of a hot finishing mill 10 equipped with a scaling facility and an edge heater installed as required. And delay table 12
This is a thin slab casting and rolling process in which descaling is performed on the delivery side, edge heating is performed as necessary, and then hot finish rolling is performed to a target product size. In addition, on the front surface of the hot finish rolling mill, the arrangement of equipment added to the finish rolling such as the bar joining equipment and the width adjusting equipment other than the equipment having the above functions does not hinder at all.

The casting conditions in the continuous casting machine 1 of FIG. 3 are as follows. Casting thickness: 50 mm, coil thickness: 50 to 35 mm Casting width: 700 to 2100 mm Casting speed: max 10 m / min Machine length: 30 m Inline reduction capacity: max 15 mm Steel type: AI-K 63-1837
No. 57, JP-A-63-264252, wherein an in-line reduction rolling-down facility 3 comprises a solidification-completed section composed of three tension-rollable segments having a rolling capacity of max 400 Ton, and an unsolidified section and a solidified section before and after the solidified section. , Which has a rolling capacity of max 15 mm.

[1] Production capacity In the equipment shown in FIG. 3, the production amount when the casting thickness of 50 mm is wound as it is and rolled by a hot finish rolling mill, and the production thickness is 50 mm but the casting thickness is rolled by in-line reduction. The production amount when the slab coil thickness was rolled to 50 to 30 mm according to the thickness was verified by the production results in actual operation. Table 2 shows the thickness of the slab coil according to the thickness of the hot-rolled product in this case. And the product width in this case was manufactured separately at the time of casting to 700 to 2100 mm.

[0046]

[Table 2]

Looking at the production results, when the slab coil thickness is 50 mm single size, 30,000 Ton / month When the slab coil thickness is adjusted to 50-30 mm, it becomes 370,000 Ton / month, and the production volume for thin slab process by installing in-line reduction Is about 10 times or more, and it is verified that the steel type to which the thin slab process is applied dramatically increases.

[2] Casting speed 10 mm with the equipment shown in FIG.
/ Min, casting thickness 50m cast to a slab width of 1200mm
m slab is reduced to 35 by in-line reduction
After making the coil thickness of 10 mm, a 10 m long slab was cut by a running cutter, the coil was charged into the box-type heating furnace, and immediately rolled to a product thickness of 5.0 mm by a hot rolling mill. The test which simulated the A route of No. 1 (this is A
3), the coil winding device 6 of FIG.
Winding into n coils, immediately rewinding the winding device and rewinding, taking a slab of 10m length in the stationary part by cutting machine while running, immediately charging the box type heating furnace, and immediately hot rolling 1 was simulated by rolling to a product thickness of 3.0 mm with a machine (this is referred to as the B route), and further, after being wound into a 40-ton coil by the coil winding device 6 in FIG. Put the heat insulation cover and hold for 20 minutes, then rewind the winding device to reverse, take a slab of 10 m length in the stationary part by the cutting machine while running, immediately put it in the box heating furnace, Immediately with hot rolling mill, product thickness5.
Tests of three routes of a test simulating the present invention rolled to 0 mm (referred to as a route of the present invention) were performed for each 10 slabs, and surface defects were investigated and mechanical properties after cold rolling and annealing were investigated. Detailed production conditions and production results in this case are as shown in Tables 3 and 4.

[0049]

[Table 3]

[0050]

[Table 4]

As a result, as described above, since the A-root material cannot sufficiently remove the scale generated at the time of casting, the scale flaw generation rate in the cold-rolled sheet is high.

On the other hand, the scale flaws of the root material of the present invention and the root material of the present invention obtained by winding and rewinding and processing the coil are drastically reduced, and a good product is obtained.

The effect of maintaining the coil at a high temperature as described above can be obtained by comparing the root material of the present invention with the root material of the invention. That is, since the B route material and the route material of the present invention were AI-K, a difference in the retention time at 1050 ° C. for 20 minutes was given. However, the precipitation of MnS could be controlled, and the Rankford value (average value) was reduced. It has been found that remarkably improved, elongation is improved, and a soft material having a low yield stress, which is said to be difficult to manufacture by a thin slab process, can be manufactured.

[0054]

As described above, the thin slab process of the present invention eliminates the production constraints associated with the product size, so that the production volume of the thin slab process is dramatically increased and the scale can be sufficiently peeled off during rolling. The surface defect problem resulting from the process problem is eliminated. Furthermore, the quality of the product is improved, and a steel type requiring high workability can be manufactured with a soft material which is particularly problematic in the thin slab process, and the applicable steel type is significantly increased.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a known thin slab manufacturing process.

FIG. 2 is a diagram showing an example of a temperature change of a slab coil from casting to before rolling.

FIG. 3 is a diagram showing an example of a thin slab casting / rolling facility of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Belt-type thin slab continuous caster 2 Cooling equipment 3 In-line press-down equipment 4 Dense split rolls 5 Shearing equipment 6 Coil winding device 7 Dummy bar pull-out device 8 Tunnel type coil heating furnace 9 Coil transfer machine 10 Hot finishing press release machine 11 Coil unwinding equipment 12 Delay table 13 Transfer machine 14 Insulated coil box wagon

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Tsutomu Nabe, Oita, Oita, Oita, Nishi-no-su, 1 Nippon Steel Corporation Oita Works (72) Inventor Kazuaki Esaka, Oita, Oita, O-Ita, 1-Nishi-no-Su (56) References JP-A-58-122107 (JP, A) JP-A-62-187505 (JP, A) JP-A 1-130859 (JP, A) JP-A Sho 59 -7464 (JP, A) JP-A-62-270257 (JP, A) Edited by the Iron and Steel Institute of Japan, "Iron and Steel Manufacturing Process (Third Volume) Processing (2)" Maruzen (47-9-30, p. 47) p. 606-609

Claims (1)

(57) [Claims] 1. A thin slab cast by a belt-type thin slab continuous caster is rolled by an in-line reduction device, then wound by a coil winding device, and the wound coil is retained at or above the Ar3 transformation point. In the method in which the coil is rewound and the finish rolling is performed, the solidification completed portion of the thin slab cast by the belt type thin slab continuous caster and a portion near the downstream side thereof are reduced by 5%.
The line pressure gradually cast and rolling method for thin slab characterized that you rolling in facilities within the 50%.