Continuous casting method for steels

Abstract: A continuous casting method which comprises casting molten steel into a cast piece with a circular cross section and applying soft reduction treatment of a reduction ratio from 1.0 to 3.0 % to the cast piece with a flat part of a roll at a position of a solid phase ratio from 0.2 to 0.8 before the molten steel in the center portion of the cast piece is solidified completely.

Description: This invention relates to a continuous casting method applicable for steels such as bearing steels and spring steels according to the features as laid out in claim 1, which is based upon EP-A- 0 211 422. The continuous casting process has remarkable advantages in the high yield rate and in the high productivity because a blooming process is not required, and is widely utilized as a casting method which is possible to continuously manufacture the final cast piece such as a slab, a bloom, a billet and so on directly from molten steel. However, in a case of casting the molten steel through the continuous casting process, carbon, phosphorus, sulfur and the like have a tendency to segregate (concentrate) in the center part of the cast piece, accordingly there has been a problem in that it is difficult to introduce the continuous casting process for steels containing chemical elements easy to segregate, especially for the steels containing a large amount of carbon, such as bearing steels, spring steels and so on. As a method for inhibiting the segregation, the method to apply soft reduction against the cast piece drawn out from the mould is well known. The segregation and the concentration phenomena of the chemical elements such as carbon in the center part of the cast piece is considered to be caused by the solidification of the molten steel proceeding toward the center part from the outer peripheral part of the cast piece, in particular, it is considered that flowing of the concentrated molten steel is produced by solidification shrinkage at the time a liquid phase remaining in the center part of the cast piece is finally solidified, and linked up with the concentrated segregation of carbon and the like. The aforementioned soft reduction treatment is to prevent the flowing of the concentrated molten steel caused by the solidification shrinkage and to inhibit the segregation at the time of solidification by compressively deforming the unsolidified portion in the center of the cast piece at least an amount corresponding to the volume reduction at the time of solidification. However, in the soft reduction treatment, the cast piece is applied with the compressive deformation in the state where the unsolidified portion remains in the center part of the cast piece, therefore the soft reduction treatment is sometimes accompanied with a risk of generation of cracks at a front surface of the solidification according to magnitude of tensile stress produced by the deformation of unsolidified interface of the molten steel. The cracks generated at the front surface of the solidification brings penetration into the cracks of the molten steel concentrated with carbon, sulfur, phosphorus and so on at the front surface of the solidification, and may be a defect harmful as much as center segregation in the finished products. EP-A-0211422 discloses a continuous casting method wherein the thickness of a strand is continuously reduced in its unsolidified region (generally called a soft reduction treatment). The document further discloses casting the metal into a bloom having a rectangular cross section. It is necessary to apply a soft reduction to a cast piece effectively so that high tensile stress may not be generated at the unsolidified interface of the molten steel and the unsolidified center portion of the cast piece may change in the volume (compressive deformation) sufficiently in order to prevent the aforementioned cracks at the soft reduction treatment. Therefore, it has been hoped to establish the technical method possible to realize the effective soft reduction treatment without generating the cracks. This invention is made in order to solve his kind of problem of the prior art. The present invention provides a continuous casting method for steels comprising: pouring molten steel into a water cooled mould through an upper opening of the mould; solidifying the molten steel to form a cast piece in the mould and drawing continuously the cast piece through a lower opening of said mould at the same time, wherein said cast piece is subjected to soft reduction treatment with a flat part of a roll at a position of a solid phase ratio from 0.2 to 0.8 before the molten steel in the center part of the cast piece is completely solidified, and wherein said cast piece has a circular cross section, and the flat part of the roll forms a plane on a depressed surface of the cast piece. Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which: FIG.1 is a graph illustrating distribution of the carbon concentration on a cross section of a cast piece obtained in an embodiment according to this invention in comparison with that of a cast piece obtained without soft reduction; FIG. 2 is a graph illustrating distribution of the carbon concentration on a cross section of a cast piece obtained in another embodiment according to this invention in comparison with that of a cast piece obtained without soft reduction; FIGS.3A and 3B are perspective views illustrating testing methods for confirming an effect of soft reduction in the continuous casting method according to this invention; FIG.4 is a graph illustrating results obtained through the testing method shown in FIGS.3A and 3B; and FIG.5 is a schematic view illustrating a position to be applied with the soft reduction in the continuous casting method according to this invention. In the continuous casting, the molten steel undergoes a change in state from perfect liquid phase A to perfect solid phase C through solid-liquid mixed phase B as shown in FIG.5. In the continuous casting process according to this invention, the solid phrase ratio is defined as a weight ratio occupied by the solid phase on a cross section of the cast piece (the solid phase ratio 1 means the state of perfect solid phase). It is possible to estimate the solid phase ratio from the thermal distribution on the cross section of the cast piece obtained according to the heat transfer calculation based on data such as specific heat and thermal conductivity of the cast piece, the temperature of the molten steel, for example. Then the continuous casting method according to this invention is characterized by subjecting to the soft reduction treatment against the cast piece cast into a shape having a circular cross section with the flat part of the roll R at the position of the solid phase ratio from 0.2 to 0.8 in the solid-liquid mixed phase as shown in FIG.5. The roll having the flat part means a roll possible to form a plane on the depressed surface of the cast piece with the circular cross section, and the roll is applicable without distinction of the shape so long as it is possible to form a plane on the contact surface of the cast piece. As mentioned above, in the soft reduction treatment, it is necessary to apply the soft reduction to the cast piece effectively so as to sufficiently vary the volume of the unsolidified center portion (compressive deformation) without producing the high tensiled stress on the unsolidified interface of the molten steel for preventing the cast piece from the internal cracks. In order to find appropriate conditions for the soft reduction treatment, the inventors carried out stress-strain analysis at the time of applying the soft reduction with the flat part of the roll R against a cast piece D having a circular cross section (350mm diameter) and a cast piece E having a square cross section (350mm square) shown in FIGS.3A and 3B through the finite element method using computer, and results were obtained as shown in FIG.4. Additionally, symbol F denotes the unsolidified portion, and symbol G denotes the completely solidified portion of the cast pieces D and E in FIGS. 3A and 3B. In graph A of FIG.4, the axis of abscissas shows a reduction ratio applied to the cast piece, and the axis of ordinates shows volume reduction of the unsolidified center portion with an index. The reduction ratio means the decrease percentage of area on the cross section of the cast piece before and after the soft reduction. The other side, in graph B and graph C of FIG.4, the tensile stress generated on the unsolidified interface of the molten steel in the directions of X-axis (perpendicular to the axis of the cast piece) and Y-axis (axial direction of the cast piece) are plotted as ordinates against the reduction ratio as abscissas. The results show that the reduction ratio required for obtaining the some volume reduction at the unsolidified center portion of the cast piece is remarkably different between the cases of the cast piece D having the circular section and the cast piece E having the square cross section, and the reduction ratio to be applied on the cast piece D having the circular cross section is not required so much in comparison with that to be applied on the cast piece E having the square cross section. Furthermore, when the volume reduction is equivalent at the unsolidified center portion between the cast pieces D and E, it is found that the tensile stress generated at the unsolidified interface of the molten steel in the cast piece D having the circular cross section is remarkably low as compared with that in the cast piece E having the square cross section from the results shown in graphs B and C of FIG.4. In view of the aforementioned results, it is seen that it is possible to reduce the volume of the unsolidified center portion effectively at the same time of controlling the tensile stress generated at the unsolidified interface of the cast piece very low by the compression with low reduction ratio when the cast piece is cast into the shape having the circular cross section and subjected to the low reduction with the flat part of the roll. This invention is made on basis of the above-mentioned findings, it is possible to prevent the cast piece from the segregation of carbon and the like at the same time of controlling the generation of the tensile stress in the cast piece in a low level and preventing the cast piece from the generation of cracks, therefore it is possible to improve the quality of the cast piece according to this invention. This invention is especially effective when it is applied to steels containing carbon not lower than 0.5 %. The continuous casting method according to this invention is especially characterized by performing the soft reduction at the position of the solid phase ratio from 0.2 to 0.8. In this invention, the soft reduction is applied on the cast piece at the position of the solid phase ratio from 0.2 to 0.8 for the reason that a liquid phase in the center portion of the cast piece loses its fluidity substantially and it is not possible to obtain the sufficient effect by the low reduction treatment even if the low reduction is applied on the cast piece at the position of solid phase ratio more than 0.8, and in contrast with this, the liquid phase occupied most of the cross section of the cast piece at the position of solid phase ratio less than 0.2 and it is not possible similarly to obtain the sufficient effect by the low reduction treatment owing to the excessive fluidity of the liquid phase. In this invention, the low reduction is performed for the purpose of preventing the cast piece from the cracks and the concentrated segregation of the chemical components such as carbon, phosphorus, sulfur and so on at the center portion of the cast piece. In this sense, it is preferable to apply the low reduction to the cast piece in a reduction ratio range of 1.0 to 3.0 %, and further preferable in the reduction ratio range of 1.5 to 2.5 %. This invention will be described below in detail on basis of the following non-limiting examples. EXAMPLE 1 Steel including 1.0 % of carbon and specified as SUJ 2 in JIS G 4805 (High Carbon Chromium Bearing Steels) was cast into a cast piece having a circular cross section with a diameter of 350 mm at an extraction speed(Vc) of 0.4 m/min through the continuous casting process. In this time, the cast piece was subjected to the low reduction treatment with a reduction ratio of 2 % at a position of a solid phase ratio(fs) from 0.4 to 0.5 using a double roll disposed with two flat rolls on the upper and lower sides. Additionally, the roll is available so long as it has a flat part even a roll having a V-shaped recess partially. As a result of analyzing the carbon content on the cross section of the obtained cast piece, a graph on the upper side in FIG.1 was obtained. Furthermore, it is confirmed that there were not internal cracks in the cast piece. The other side, a result shown in a graph on the lower side in FIG.1 was obtained by analyzing the carbon content of a cast piece cast in the same manner without applying the low reduction treatment in order to make comparison. According to the comparison between both graphs of FIG.1, it is apparent that it is possible to inhibit the center segregation without generating the internal cracks by casting the molten steel into the cast piece having the circular cross section and applying the low reduction to the cast piece with the flat roll. EXAMPLE 2 Steel including 0.6 % of carbon and specified as SUP 7 in JIS G 4801 (Spring Steels) was cast into a cast piece through the continuous casting process under the same condition as that of Example 1, and a graph on the upper side in FIG.2 was obtained as a result of analyzing the carbon content on the cross section of the obtained cast piece . Furthermore, there were not internal cracks at all. A graph on the lower side in FIG.2 shows a result of a cast piece obtained without applying the low reduction treatment. By comparing results shown in both graphs of FIG.2, it is seen that it is possible to inhibit the segregation of carbon in the center portion effectively also in the case of the steel specified as SUP 7 by casting the cast piece having the circular cross section and applying the low reduction to the cast piece using the flat roll.

Claims: A continuous casting method for steel comprising: pouring molten steel into a water cooled mould through an upper opening of the mould; solidifying the molten steel to form a cast piece in the mould and drawing continuously the cast piece through a lower opening of said mould at the same time, wherein said cast piece is subjected to soft reduction treatment with a flat part of a roll at a position of a solid phase ratio from 0.2 to 0.8 before the molten steel in the center part of the cast piece is completely solidified, characterised in that said cast piece has a circular cross section, and the flat part of the roll forms a plane on a depressed surface of the cast piece. A continuous casting method for steels as set forth in claim 1, wherein said cast piece is subjected to a soft reduction treatment of a reduction ratio from 1.0 to 3.0 %. A continuous casting method for steels as set forth in claim 2, wherein said cast piece is subjected to a soft reduction treatment of a reduction ratio from 1.5 to 2.5 %.

Inventors: HAYAKAWA SHIZUNORI, INAGAKI YOSHIO, MORII KIYOSHI, TAKAHASHI HAJIME, HAYAKAWA, SHIZUNORI, INAGAKI, YOSHIO, MORII, KIYOSHI, TAKAHASHI, HAJIME
Applicants: DAIDO STEEL CO LTD, DAIDO TOKUSHUKO KABUSHIKI KAISHA
Application year: 1995
Main classification: B22D11
IPC codes: B22D11, B21B1