JPH01249245A - Method for directly producing strip - Google Patents

Abstract

PURPOSE:To prevent the development of crack in a cast strip by controlling drum diameter casting velocity and molten metal surface height so as to maintain molten metal solidified time to critical solidified time or less. CONSTITUTION:In the relation between metal solidified time (t)(sec) and the critical solidified time tc(sec) showed in the equation I, the drum diameter D(m), casting velocity V(m/min) and the molten metal surface height theta (rad) are con trolled so as to satisfy the inequality t<=tc. By this method, the casting velocity V becomes higher and in this result, the solidified time (t) is shortened to the min. Therefore, the time till coming to the cast strip by separating the solidified shell from the molten metal, is shortened and the difference of growth of the solidified shells is reduced. Therefore, the solidifying shrinkage rate of the solidified shell is reduced and the development of crack in the cast strip is prevented.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for directly producing thin plates, and in particular, the present invention relates to a method for directly manufacturing thin plates, and in particular, to directly and continuously produce thin plate slabs by rapidly solidifying molten metal through a water-cooled drum (roll) or the like. This is a proposal regarding technology that belongs to the field of caster casters.

[Conventional technology]

In recent years, in the steel industry and the like, there has been active research into technology for directly and continuously casting thin slabs from molten metal by rapidly cooling and solidifying the molten metal. However, when considering this new technology, the following major issues arise: thermal stress due to shrinkage during solidification and stress from mechanical vibrations are concentrated in areas where solidification is delayed, causing vertical and horizontal cracks in the slab. It is to generate. Such cracks in slabs require a process to remove them even if they are shallow, and if they are deep, the removal of the cracks is often incomplete and the product value is lost, so it must be controlled. is an extremely important issue. In short, in the field of thin plate direct casting, whether or not cracking of cast slabs can be prevented is a major issue that determines the success of the technology.

Conventionally, as a technique for preventing slab cracking, Japanese Patent Application Laid-Open No. 59-21
No. 5256 discloses that by measuring the rolling blade applied to the solidified shell when the solidified shell is rolled by the drum, when the rolling blade is larger than a set value, the drum cooling length is shortened (or vice versa). Discloses a double-drum continuous method that continuously casts crack-free metal strips by adjusting the rolling force by moving the barrel seal so that when the rolling blade is smaller than the set value, the drum cooling length becomes longer. has been done.

In addition, Japanese Patent Application Laid-open No. 59-193740 discloses that when the drum pressing pressure exceeds a set value during gap constant control, the drum pressing pressure is controlled to a constant value until the gap becomes less than a predetermined size. A twin-drum continuous metal plate casting method is disclosed that stably casts metal plates without breakage. These known techniques all relate to techniques for preventing cracks in the slab by controlling the pressing force of the rolls when rolling down the hot slab.

Furthermore, Japanese Patent Laid-Open No. 60-184449 describes a drum-type continuous casting machine in which irregularities are formed in the form of dots on the roll surface, and a drum-type continuous casting machine equipped with a cleaning device for cleaning the drum surface. Discloses how to use it. This known technology achieves uniform cooling of the solidified shell by interposing an air layer in the contact area between the solidified shell and the drum surface, making the solidified thickness uniform in the width direction, thereby producing high-quality slabs. This is a method to aim for.

[Problem to be solved by the invention]

Each of the conventional techniques described above attempts to prevent cracking by controlling the pressing force or by controlling uniform cooling, but although these methods can reduce cracking of slabs to some extent, they may cause adverse effects in later processes. The reality is that we have not yet reached the point where we can prevent it to the extent that it will not occur. Moreover,
Regarding slab cracking in the field of direct manufacturing of thin plates,
The actual situation is that the phenomenon itself has not yet been sufficiently elucidated, and therefore the establishment of a technology for direct casting of thin sheets as soon as possible is awaited.

An object of the present invention is to establish a technique for continuously directly casting thin plates with fewer slab cracks during direct casting of steel using a drum-type thin plate direct casting machine.

[Means to solve the problem]

In order to solve the above-mentioned problem, the present inventors focused on the solidification delayed portion, which is the starting point of slab cracking, and discovered that cracking can be eliminated by eliminating the solidification delay. In other words, the shorter the time period during which a solidified shell is generated and gradually grows when the molten metal comes into contact with the cooling drum, until the solidified shell separates from the molten metal and becomes a slab, the faster the solidified shell will become. There is no difference in growth,
They also discovered that the higher the temperature at the time of completion of casting of the solidified shell, the smaller the amount of solidification shrinkage, and as a result, cracking of slabs due to thermal stress concentration can be prevented.

The present invention was developed based on these findings, and in directly and continuously casting thin plate-shaped steel pieces from molten metal, the molten metal solidification time t (=60Dθ/2V) sec. Critical coagulation time t c (= ((The/2 C)/K
)''

[For production]

The present inventors conducted the following test to investigate slab cracking during direct casting of thin sheets.

First, use SO5304 stainless molten steel and apply it to the first
Casting was performed using a 0.8 m diameter twin-drum direct casting machine as shown in the figure, and cracks in the slab were investigated.

FIG. 2 shows the results of the above-mentioned crack investigation, and shows the relationship between slab thickness, casting speed, and slab cracking (quality defects) such as breathing and vertical cracking. In this test, the water-cooled drum 1°2 in the twin-roll direct casting machine shown in Fig. 1 was made of Cu, 400 mφ, 8
Two types of 00 tmφ were used. Here, 400 drunkenness φ
The coagulation coefficient is 11. o, the constant C is 0.2, the coagulation coefficient at 800 mmφ is 19.2, and the constant C is -0.25.

As can be seen from this figure, it has become clear that casting defects such as vertical cracks and bleeds can be solved by increasing the casting speed and reducing the thickness of the slab.

Therefore, in the present invention, from the general formula for solidification rate proposed by "Fe1ld"etc.;D=Kv',
In practice, when the critical slab thickness at the limit where no adverse effects occur in the subsequent process is The, since twin rolls are used,
The required critical coagulation time tc was expressed as follows.

t c = ((The/2-C)/K)” x60
(sec) (21 where; Thc: critical slab thickness (Im) = 2.5 to 3: solidification coefficient (11 to 20 tm −1lin−””) C: constant (
-1 to +1.0) From the above, in order to obtain an excellent slab at a given slab thickness (The), it is sufficient to increase the casting speed, so in the end, it is necessary to shorten the solidification time. It turns out to be a good thing. Therefore, in the present invention, the actual coagulation time t (t=60Dθ/
2 V ・(1), however, D Nidrum diameter (m), V
: Casting speed (m/m1n), θ: Molten metal surface height (rad)
), that is, it is necessary to perform casting so that t≦tC is satisfied.

Note that K, C, and Thc are drum diameter and drum material.

Although it varies depending on operating conditions such as cooling conditions and molten steel temperature, the range is defined as follows.

K -11~20mm・n1in-"",
C=-1~1.0 +The=2.5
~3. OfIB [Examples] Table 1 shows Examples A to C of the method of the present invention, as well as various comparative methods.
This is shown in comparison with E. Example A.

Both B and C are casting conditions that satisfy the method of the present invention, and the solidification time t is the critical solidification time t obtained from equation (2).
In all cases where the length was shorter than c, cracking of the cast slab was either absent or almost non-problematic. Further, as a comparative method, E is a case in which none of the casting conditions according to the present invention was satisfied, and horizontal cracks and vertical cracks occurred significantly.

Table 1 [Effects of the Invention] As explained above, according to the method of the present invention, slab cracking due to thermal stress during solidified shell growth, stress due to mechanical vibration, etc. can be completely prevented.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a twin-drum direct casting machine, and FIG. 2 is a graph showing the relationship between slab thickness, casting speed, and casting time, and the range of cast plate thickness without cracking of slabs. 1.2...Water-cooled drum patent applicant Nippon Yakin Kogyo Co., Ltd. Agent Patent attorney Jun Ogawa Tamayo Patent attorney Morio Nakamura Th Figure 2 Casting speed (m/rrlin)

Claims (1)

[Claims] 1. When directly and continuously casting a thin plate-shaped steel piece from molten metal, the molten metal solidification time t shown in the following equation (1) is expressed as the following equation (2). 1. A method for directly manufacturing a thin plate, characterized in that control is performed so that t≦t_c is satisfied in relation to the critical solidification time t_c. t=60Dθ/2V (sec) (1) t_c={(Th_c/2-C)/K}^2×60(s
ec) (2) Where; D: Drum diameter (m), θ: Molten metal level height (rad) V: Casting speed (m/min) Th_c: Critical slab thickness (mm) K: Solidification coefficient C: Constant