Metal founding – Including means to directly apply magnetic force to work or... – By electromagnetic means
Reexamination Certificate
2001-10-17
2003-03-11
Lin, Kuang Y. (Department: 1725)
Metal founding
Including means to directly apply magnetic force to work or...
By electromagnetic means
C164S417000, C164S466000
Reexamination Certificate
active
06530418
ABSTRACT:
TECHNICAL FIELD
This invention is concerned with continuous casting, particularly with a suitable continuous casting method and apparatus to produce highly qualified steels without segregation and porosity.
BACKGROUND ART
With regard to the continuous castings of carbon steels, low alloy steels, specialty steels and so on, more than twenty years have passed since the present vertical-bending-type continuous casting machines began to operate. And it has been said that these technologies became mature. On the other hand, the demand for the quality is increasing its severity year after year and the pressure to the cost-down also is increasing simultaneously. Aside from the problems such as breakout, etc. that often became a problem in the early period of the operating history, there still remains (1) central segregation and (2) central microporosity as the major problems concerning the quality.
The central segregation is the segregation having V characters that takes place with a periodicity in the middle of the thickness in the final solidification zone, and is generally called V segregation. The central microporosity is the microscopic void that forms in an interdendritic region also in the middle of the thickness in the final solidification zone. Summarizing these defects, they are to be called the central defects(internal defects) thereafter in this specification.
Next, the effects of the central defects on the quality of the steel products are briefly stated as follows.
(1) The case of thick plate:
Hydrogen coagulates and precipitates into these central defects, and so-called hydrogen induced cracking results during usage. Also, upon welding, the weld cracking occurs starting these defects.
(2) The cases of rod and wire:
Cracking takes place starting the microporosity during drawing.
(3) The case of thin plate:
Upon pressing or during cold rolling, banded defects form, which result from the irregularity in hardness. This irregularity is caused by the coexistence of hard and soft spots due to segregation. The above defects takes place during the solidification process of continuous casting and lead to a poor quality product. The segregation formed during the solidification process remains in final products and can not be eliminated. Tentatively, there is a method of eliminating the macrosegregation by diffusion heat treatment. However, this method is not favorable both economically and technically because a long period of heat treatment at a high temperature is required. As for the microporosity, it is possible to smash them by hot rolling. But whether or not it can completely eliminate them depends on the quantity of the porosity. Furthermore, an attention must be paid to the fact that the microporosity accompany segregation in many cases.
Like this, the central defects is the problem associated with the essence of solidification phenomena, and the present situation is that it is very difficult to solve by means for the accumulation of know-how or by means for trials and errors based improvement. Although there is some difference in degree, these central defects(internal defects) are common to all the steel grades of slabs, blooms and billets. They exist from the beginning of the continuous casting history: Thus, they are an old but at the same time a new problem.
Among the measures that have been curried out until now to improve the internal defects, several important technologies will be reviewed in the following.
(1) Prevention of the bulging
It has been said that the central segregation is formed in slabs with broad width when the solid part of the solidifying shell or the cast piece between supporting roll pitches was bulged by internal pressure of the steel melt. Although this happens by the flow of high solute concentration liquid within the solid-liquid coexisting zone which is induced by the deformation of the solidifying shell, the detailed mechanism is not clarified satisfactorily. Therefore, to reduce the bulging to as small extent as possible, such measures as shortening the roll pitches or dividing one roll into sub-rolls in the longitudinal direction have been employed. Besides, the misalignment of rolls is said to be responsible for an inter-dendritic fluid flow, thus causing the segregation. However, the internal defects can not be eliminated even if these mechanical disturbances are removed, considering the fact that the central segregation occurs even in the blooms and billets where the bulginess hardly become the problem.
(2) Strengthening of secondary cooling (please refer to Refs. (1) and (2) at the end of this specification)
This is the method of intensively cooling the vicinity of the final solidification position (the crater end) to compress the solid-liquid coexisting phase by contraction force due to thermal stress so as to compensate the solidification contraction, thereby reducing central porosity. It has been reported according to the Refs. (1) and (2) that the improvement was made to some extent.
On the other hand, the main stream at present is the method of compressing the solidifying shell and give compressive deformation to central solid-liquid coexisting phase in the vicinity of final solidification position to control the interdendritic fluid flow, thereby reducing the internal defects. This method is divided into soft-reduction and hard-reduction depending on the amount of reduction.
(3) Soft-reduction method at the last stage of solidification(please refer to Refs. (3) and (4))
With this method to improve the central segregation, the solid-liquid coexisting zone is compressed to compensate the solidification contraction which takes place continuously with the progress of solidification. With respect to the soft-reduction amount, a slope needs to be attached so as to correspond to the continuously arising solidification contraction as precisely as possible. For example, it is shown in Ref. (3) that the central segregation was improved by the real machine test of a carbon steel bloom that used the compressive crown roll with roundness attached. Also, in Ref. (4), examples are shown about theoretical estimates of reduction gradient necessary for the case of high carbon steel blooms (0.7 to 1 wt % C) with 300×500 mm section. According to the estimates, the gradient of 0.2 to 0.5 mm/m is required. However, various problems must be overcome to realize this method on a real machine, which will be stated below.
{circle around (1)} Usually, the soft-reduction is carried out in the range of a couple of meters in the vicinity of the final solidification zone, which becomes approximately 0.3 mm/m in the case of the blooms of the above Ref. (4).
This means that the inclination of 0.3 mm per 1 m needs to be attached to the solidifying shell. Thus, the reduction amount must be controlled with great accuracy by means for a multi-rolled reduction apparatus, etc.
{circle around (2)} There is a difficulty that if the amount of reduction is not enough, the effect can not be expected, and that if the amount is excessive on the contrary, the interdendritic liquid flows backward to the upstream resulting in the channel segregation (i.e. inverse V segregation).
{circle around (3)} Required amount of reduction differs depending on the operating conditions such as a steel grade, dimensions in cross section, casting speed and cooling condition. Therefore, a great amount of labor and cost is necessary for these trials and errors to find an appropriate condition even in the case of a few steel grades.
{circle around (4)} Since the soft reduction method often gives rise to the new problem of internal cracking (Ref. (5)), a consideration must be taken into to prevent this.
Thus, it is not easy to make use of this method to achieve the purpose.
(4) Continuous forging method (Refs. (7) and (8))
Next, stated is the hard-reduction method in which the solid-liquid coexisting phase in the vicinity of the final solidification zone is mechanically largely deformed thereby squeezing the high solute concentration liquid to the upstream to prevent the central segregation
Ebisu Yoshio
Sekine Kazuyoshi
EBIS Corporation
Frishauf Holtz Goodman & Chick P.C.
Lin Kuang Y.
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