Metal founding – Process – Shaping liquid metal against a forming surface
Patent
1996-02-06
1998-12-29
Batten, Jr., J. Reed
Metal founding
Process
Shaping liquid metal against a forming surface
164417, 164424, 164452, B22D 1112, B21B 146
Patent
active
058530436
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for continuous casting of a thin slab by reducing the thickness of a slab with liquid core, the slab having a liquid plus solid phase after having been withdrawn from a mold.
2. Description of the Related Art
In the continuous casting of thin slabs, molten steel tends to erode or clog an immersion nozzle. Therefore, it is difficult to reduce the outer diameter of an immersion nozzle beyond a certain value. The restriction on the outer diameter of the immersion nozzle further causes a lower limit on the width of the narrow side of the mold, i.e., the thickness of the slab. This applies particularly to conventional continuous casters for the manufacture of slabs with a nearly constant thickness, in which only amounts of solidification shrinkage are taken into account. Thus, it is difficult to manufacture thin slabs with conventional continuous casters.
In one known method for making a thin slab by continuous casting, the thickness of a slab is reduced by rolling when a liquid plus solid phase remains inside the slab.
For example, Japanese Patent Application Laid-open (kokai) No. 2-20650 discloses a continuous casting and rolling process which defines a total ratio of reduction with respect to the thickness of a slab in a solidifying interval. According to the process of this reference, the thickness of the slab is reduced by at least 10%, or even 70%, within the solidifying interval of the slab. This process has the drawback that unless a proper amount of reduction is shared with each reduction roller, slab quality becomes poor, and particularly, internal cracks form inside the slab.
The formation of internal cracks in a slab is greatly affected by a tensile strain applied to the slab (hereinafter simply referred to as a strain). The strain includes a strain caused by reduction of bulging using support rollers, bending strain, unbending strain, misalignment strain, strain due to thermal stress, and strain caused by reduction of a strand with liquid core. These are collectively called "internal strain".
In a continuous casting process of steels disclosed in Japanese Patent Application Laid-open (kokai) No. 3-174962, it is disclosed that internal cracks of a slab are generated when a maximum value of accumulated strain exceeds a critical strain corresponding to the specific type of steel. This accumulated strain results from the histories of the above-mentioned strains excepting strain caused by reduction with liquid core. In addition, the accumulation region for each strain corresponds to the temperature region between a zero strength temperature (ZST, at which a strain begins to occur due to stress applied to the slab in the solidifying process of strand) and a zero ductility temperature (ZDT), and that the ZST and ZDT nearly correspond to solid fractions 0.8 and 0.99, respectively.
Methods for reducing the thickness of a slab with a liquid core in a continuous caster having a curved segment, include: (a) a method using one pair of rollers, (b) a method using multiple pairs of rollers, (c) a method using connected segment frames, and (d) a method using a single segment frame.
(a) Method using one pair of rollers
This method utilizes a pair of reduction rollers (a rolling machine) or forging equipment placed just after the exit of a mold or in a horizontal section of the caster following a section for unbending the strand. See, for example, Japanese Patent Application Laid-open (kokai) Nos. 63-60051 and 3-124352.
This method has the following problems. If the amount of reduction is great and the reduction rate (or reduction incline) is fixed, the diameter of the reducing roller, the size of the press head, and the reduction force all increase, placing a significant burden on reducing equipment. On the other hand, if the roller diameter and the size of the press head are somewhat limited, the reduction rate rises, increasing the chance of generating internal cracks inside the slab. It is also noted th
REFERENCES:
patent: 4043382 (1977-08-01), Saito et al.
patent: 5488987 (1996-02-01), DiGiusto et al.
New Evaluation of Critical Strain for Internal Crack Formation in Continuous Casting, A. Yamanaka et al., ATS Steelmaking Conference (1991), pp. 627-633.
Measurement of Critical Strain for Solidification Cracking, A. Yamanaka, et al., The Minerals, Metals & Materials Society (1991), pp. 279-284.
Kanazawa Takasi
Koide Masakazu
Kumakura Seiji
Murakami Toshihiko
Okamura Kazuo
Batten, Jr. J. Reed
Sumitomo Metal Industries Ltd.
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