JP2003019546A - Method for preventing warpage of continuously casting slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent generation of warpage in an actual caster by precisely predicting the warping amount of a cast slab regardless of an operational condition in a continuous casting. SOLUTION: ΔTdu=(ΔTd-ΔTu) of difference between ΔTd=(Td-Tdo) and ΔTu=(Tu-Tuo), is obtained with the calculation of a solidified heat- conductivity. Wherein, Tdo and Tuo are the respective average temperatures at the lower half part side and the upper half part side in the thickness of the cast slab before cutting off in the end part of a continuous caster, and Td and Tu are the respective average temperatures at the lower half part side and the upper half part side just after cutting off the cast slab carried out from the caster end part. At least one among the surface temperature, a casting speed, a cut-off length and the thickness of the cast slab before cutting off, is adjusted so that the warping amount of the cast slab assumed based on the ΔTdu, the cut-off length L and the thickness H of the cast slab, and a linear expansion coefficient α, is included in the preset permissible range.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing warpage of a slab in continuous casting, particularly in continuous casting of steel. 2. Description of the Related Art As shown in FIG. 1 (b), if warpage occurs in a continuous cast slab, the subsequent processing such as cutting and unloading is hindered. In order to suppress the occurrence of warpage, various measures have conventionally been taken. For example, in Japanese Patent Application Laid-Open No. Hei 6-339761, in order to prevent warpage of the continuous cast slab, the secondary cooling zone is set so that the difference in surface temperature between the upper surface side and the lower surface side of the continuous cast slab falls within a certain value. A method for controlling the amount of cooling water in the inside has been proposed. However, in the above method, the relationship between the warpage of the continuous cast slab and the surface temperature difference between the upper and lower surfaces of the slab cannot be uniquely obtained, and the amount of cooling water with respect to the upper and lower surfaces of the slab is intentionally different. In the case where continuous casting is performed with the above-described method, the effect cannot be exhibited. That is, as disclosed in, for example, JP-A-2-182347, JP-A-58-3704, JP-A-8-52555, etc., a continuous casting of a wire, an extremely thick steel plate, or a material for a seamless pipe is performed. In manufacturing, in order to shift the final solidification position from the center of the thickness and prevent defects caused by component segregation and shrinkage holes in the center, intentionally differentiating the cooling of the upper and lower surfaces of the slab There is a case where casting, but in such a case, since the warpage of the slab can not be accurately predicted, it is difficult to make the process, or it can be operated only within a limited range, There is a problem that even if it is carried out, it does not reach the effect of completely preventing defects. [0004] Regardless of the operating conditions of continuous casting, if the amount of warpage of a continuous cast slab can be accurately predicted, from the viewpoint of operation or quality improvement,
Even if special cooling is performed, warpage of the slab after exiting the machine end of the continuous casting machine can be prevented. The present invention advantageously satisfies the above-mentioned demands, and establishes a technique for accurately predicting the warpage of a continuous cast slab, and based on this technique, can effectively prevent the slab warpage. The aim is to propose a method. [0005] That is, the present invention provides an average temperature Tuo, Tdo of the upper half side and the lower half side of the slab thickness before cutting at the machine end of the continuous casting machine, respectively. Differences ΔTu, ΔTd (ΔT) from the average temperatures Tu, Td of the upper half side and the lower half side immediately after cutting the slab taken out from the end.
Tu = Tu−Tuo, ΔTd = Td−Tdo) The difference Tdu (Δ
Td−ΔTu) is obtained by solidification heat transfer calculation.
And the surface temperature of the slab before cutting so that the amount of warpage of the slab estimated based on the slab cut length L, the slab thickness H and the linear expansion coefficient α falls within a predetermined allowable range. , A casting speed, a cutting length, and a slab thickness are adjusted. In the present invention, the upper half side and the lower half side of the slab mean the upper half and the lower half of the slab from the center of the thickness of the slab. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below. In general, the warpage of an object occurs due to the temperature difference between the front and back surfaces, and the general idea is that the amount of warpage is proportional to the absolute difference in temperature. However, in the case of a continuous cast slab, the slab is supported by upper and lower rolls in the continuous caster, so that even if a temperature difference occurs between the front and back surfaces, the warpage phenomenon does not occur, and the end of the continuous caster exits. After that, warping starts to occur. Moreover, since the continuous cast slab has a large wall thickness, a temperature distribution occurs in the thickness direction of the cast slab. Therefore, the warpage phenomenon of the continuous cast slab is more complicated than generally thought. Therefore, the inventors have observed and analyzed many examples of warpage behavior of a slab in the case of continuous slab casting. As a result, it was found that the occurrence of warpage was not significantly related to the absolute difference between the upper and lower temperatures of the slab, but to the temperature after slab cutting based on the slab temperature at the machine end of the continuous caster. . Moreover, it has been found that the relative temperature difference based on the temperature of the upper half side and the lower half side of the slab, not the slab surface temperature, has a strong correlation with the amount of warpage. That is, the inventors have found that the amount of warpage is represented by the following equation (1). Note: y = (1 + α | ΔTdu |) H [1-cos (Lα | ΔTdu | / (2H))] / (αΔTdu) --- (1) where, y: warpage amount (0> y: upward warpage ( Concave above),
0 <y: Warpage (concave downward)) α: Coefficient of linear expansion of steel ΔTdu: Temperature difference between upper and lower portions of slab (| ΔTdu |: absolute value of ΔTdu) H: Slab thickness L: Slab length after cutting The temperature difference ΔTdu in the upper and lower portions of the slab in the above equation (1)
Is the average temperature (Tuo, Tdo) of the upper half side and lower half example of the slab length when the slab of the length L before cutting is positioned at the machine end, and the unloading from the machine end. Difference ΔTu, ΔTd (ΔTu = Tu−Tuo, ΔTd = Td−) between the average temperature (Tu, Td) of the upper half side and the lower half side immediately after cutting the cast slab.
Tdo) is the difference ΔTdu (ΔTd−ΔTu) between the upper and lower parts,
This value can be determined by solidification heat transfer calculation. The calculation of solidification heat transfer includes two-dimensional solidification taking into account the cooling in the mold, the arrangement of spray nozzles and rolls in the secondary cooling body and cooling by the cooling, and the radiation / convection heat transfer cooling, which are generally performed. A heat transfer analysis method is sufficient. Various methods have been proposed as such a two-dimensional solidification heat transfer analysis method. However, the method proposed by the present applicant in Japanese Patent Application No. 2000-387609 has a small calculation load, and has a low computational load.
Advantageously adapted for calculations in real time. Therefore, when the range of the slab warpage amount is set from the viewpoint of the equipment capacity, at which the cutting and unloading of the slab becomes impossible or difficult in operation, ΔTdu is set to fall within the range.
It is only necessary to adjust. That is, when ΔTdu obtained from the solidification analysis falls outside the preset allowable range, the amount of spray water for cooling the slab provided in front of the machine is increased or decreased so that ΔTdu falls within the preset range. Adjust it. It should be noted that there is no problem if this cooling process is performed outside the machine if the equipment permits. Further, ΔTdu may be controlled by changing the casting speed. Furthermore, if it is difficult to adjust ΔTdu by cooling treatment or changing the casting speed due to equipment, if there is enough product size, the length of the slab or the thickness of the slab is reduced. The amount of warpage can be adjusted. That is, when the upper limit value (upward warpage amount) and the lower limit value (downward warpage amount) of the slab warpage amount y allowable in operation are determined, the control range of ΔTdu is determined from the equation (1). You can do it. Here, ΔTdu is obtained online in units of slabs in consideration of operating conditions during casting by the above-described calculation method. If the value exceeds the upper limit, the cooling of the upper surface side of the slab or the casting is performed. Weak cooling of one lower surface side or a combination thereof is performed. Conversely, if the value falls below the lower limit, the upper surface of the slab may be weakly cooled, the lower surface of the slab may be strongly cooled, or a combination thereof. The extent to which the amount of cooling water is changed may be determined according to the value of ΔTdu, and the degree is determined by ΔTdu analyzed in advance offline, the amount of cooling water, casting speed, slab length, slab thickness, and casting temperature. Determined from the relationship with temperature. Therefore, it is possible to adjust the slab warpage amount by cooling with a predetermined amount of water offline without controlling online. The position where the amount of cooling water is changed in order to prevent warpage may be a position where the effect can be exerted and a position which has little influence on the final solidification position. It is within 10 m, more preferably within 5 m from the nose. EXAMPLE 1 13% Cr (TKS13CR) thickness for seamless steel pipe: 260 mm,
A vertical bending type continuous casting machine (length: 25.6m, slab cutting after slab cutting: 750 mm width:
17.5m), the ratio of the total amount of cooling water on the upper surface side and the lower surface side of the slab (= total water amount on the upper surface / total water amount on the lower surface) changes in the range of 0.1 to 1.1 (over the entire length of the secondary cooling zone) A slab with a length of 6000 to 6900 mm was manufactured while uniformly increasing and decreasing. The cooling of the slab was performed up to 5 m before the machine end. The water ratio at this time is 0.8-1.0 liter / kg-steel,
The casting speed was 0.8-1.15 m / min. At this time, the measured value of the slab warpage (a slab warpage was obtained from a photograph of the cast slab immediately after cutting from the side) was compared with the value estimated by the above equation (1). Indicated by ● inside. The coefficient of linear expansion α was 23 × 10 ⁻⁶ (1 / ° C.). For reference, as a comparative example, an estimated value obtained when the difference ΔTdus (average value of lower surface temperature−average value of upper surface temperature) of the absolute value of the absolute value of the surface temperature of the slab was replaced with ΔTdu in Expression (1). In FIG. 2, it is indicated by a mark. Note that ΔTdu, ΔT
dus Both were calculated by the method shown in the text. As shown in FIG. 2, according to the present invention, the amount of warpage estimated using ΔTdu as a parameter well corresponds to the actually measured value.
On the other hand, according to the conventional concept, the amount of warpage estimated using ΔTdus as a parameter and a large difference from the measured value were exhibited. From this, it has been found that it is difficult to adjust the amount of warpage to a desired range with the conventional concept. Embodiment 2 In order to make ΔTdu calculated and calculated before casting close to zero as much as possible, and to prevent slab warpage, a caster is provided just before the end of the continuous casting machine.
Casting was performed while performing additional cooling by a spray nozzle at the upper and lower positions of 2.5 to 3 m. That is, the warpage of the slab was set so as to be in the range of +10 mm to 0 mm (that is, slightly upward warpage). The additional cooling water amount at this time was, specifically, ΔTdu was 0 to 13 ° C. The casting speed at this time was 0.9 m / min, and the slab length was 6900 mm. Case a (approximately +20 mm) warpage (upward warpage) occurs without additional cooling (ΔTdu at this time is calculated by two-dimensional solidification heat transfer calculation)
(It was estimated to be 35 ° C.), and additional cooling was performed so that ΔTdu was 13 ° C., and as shown by case A in FIG. 3, the amount of warpage could be reduced to about 5 mm. Approximately -5mm without additional cooling at other casting timings
B (downward warpage) occurs in case b (Δ
Tdu was estimated to be −12 ° C. in the two-dimensional solidification heat transfer calculation). When additional cooling was performed so that ΔTdu was set to 0 ° C., the amount of warpage was increased as shown by case B in FIG. Was reduced to about 2 mm. The result is shown in FIG. As shown in the figure, the case slab warpage is improved in case A and the case B in case b, and it can be seen that the slab warpage can be effectively prevented by the method of the present invention. Thus, according to the present invention, in continuous casting, warpage of an actual machine can be effectively reduced by accurately predicting the warpage of a slab regardless of operating conditions. Can be prevented. Therefore, according to the present invention, the vertical cooling difference is set large, the final solidification position is shifted from the center of the thickness to prevent defects caused by component segregation and shrinkage holes in the center, a wire or an extremely thick steel sheet. Since a continuous casting material for seamless pipes can be manufactured without any operational troubles, the incidence of defects in final products can be greatly reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a conceptual diagram of a warpage phenomenon of a slab. FIG. 2 is a graph showing a relationship between an amount of warpage estimated according to the present invention and an actually measured value. FIG. 3 is a graph showing a relationship between ΔTdu estimated according to the present invention and a measured value of a warpage amount.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Takayuki Kashiwa             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba             Inside the Iron Corporation Chiba Works (72) Inventor Nishikori regular             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba             Inside the Iron Corporation Chiba Works F-term (reference) 4E004 MC01 MC09 NC01

Claims (1)

Claims 1. An average temperature Tuo, T of upper and lower halves of a slab thickness before cutting at an end of a continuous casting machine.
and the difference Δ between the average temperatures Tu and Td of the upper and lower halves immediately after cutting of the slab discharged from the machine end.
Tu, ΔTd (ΔTu = Tu−Tuo, ΔTd = Td−T
do), the difference Tdu (ΔTd−ΔTu) is obtained by solidification heat transfer calculation, and the ΔTdu and the amount of warpage of the slab estimated based on the slab cut length L, the slab thickness H and the linear expansion coefficient α are calculated as follows. A continuous cast slab characterized by adjusting at least one of the surface temperature of the slab before casting, the casting speed, the cutting length and the slab thickness so as to fall within a predetermined allowable range. Warp prevention method.