JPH0377944B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for detecting abnormalities and serious abnormal dents due to solidification shrinkage of a slab in a mold during continuous casting. [Prior Art] Abnormalities that have conventionally been said to occur in a mold during continuous casting operations include seizure of molten steel to the inner wall of the mold and entrapment of large inclusions in the solidified shell. Each of these has been found to be a cause of breakout, and in order to detect such abnormalities, for example, Japanese Patent Application Laid-Open Nos. 1982-115960, 1982-115961, and 1982-115962 have been proposed. A method of detecting various abnormalities while the solidified shell passes through a mold has been proposed in Japanese Patent Publication No. The need for these proposals is due to the fact that breakout poses a significant hurdle in terms of both equipment and manufacturing costs. However, according to research conducted by the present inventors, there is breakout caused by transverse cracking of the solidified shell, particularly transverse cracking of the short shell of the mold, and no detection method has been established or studied to date. Furthermore, research into the relationship between transverse cracks and breakouts revealed that even if breakouts do not occur, the dents that cause the transverse cracks that occur on the surface of the slab, especially the transverse cracks of the short shell, are It has become clear that cracking occurs first, and even if breakout does not occur, cracking occurs during subsequent width rolling of the slab. The present invention is directed to horizontal or vertical depressions due to solidification shrinkage of the solidified shell in the mold, which is specific to steel types in the peritectic region with a C content of 0.08 to 0.15%, that is, horizontal or vertical depressions that cause cracking during width rolling, and the mold. The present invention provides a method for detecting horizontal concavities accompanied by horizontal cracks that break when the mold exits the mold, based on a temperature measurement pattern on the inner wall of the mold. As a method for detecting a known casting abnormality that causes a temperature change pattern similar to the occurrence of each abnormal dent, as disclosed in Japanese Patent Application Laid-Open No. 57-115961, there is a method for detecting a breakout caused by the entrapment of large inclusions in a solidified shell. Prediction technology exists. This prediction method, that is, the method for detecting abnormalities in the solidified shell, is based on a pattern in which the temperature values measured over time at the upper and lower parts once drop, then rise, and then return to the original temperature. This detects the presence of continuous large inclusions. [Problems to be Solved by the Invention] However, as a result of various experimental studies conducted by the present inventors, firstly, the temperature change during the entrainment of large inclusions slightly exceeds the temperature change range during normal casting. In contrast, the temperature change (amount, rate of change) when a horizontal concavity accompanied by a horizontal crack occurs is extremely large;
Second, even in the same casting, there are localized abnormal horizontal and vertical dents on the surface layer of the solidified shell in the mold that are connected to cracks during width rolling after casting, and breakouts immediately after exiting the mold. Thirdly, there is a tendency for shell abnormalities of various sizes to occur repeatedly, such as severe abnormal horizontal concavities accompanied by horizontal cracks that pose a risk of
Temperature changes due to fluctuations in the hot water level during normal operation are smaller in temperature change (amount and rate of change) than the temperature changes when horizontal depressions occur due to horizontal cracking.
Due to the fact that the temperature change at the end of casting does not rise again and there is a clear difference in the pattern, etc., the above-mentioned conventional technology can distinguish between inclusion inclusions, abnormal vertical dents, horizontal dents, and serious abnormal lateral dents. Furthermore, it is difficult to distinguish whether the change is due to a change in the level of the molten metal during casting, a change in the drawing speed, or a change at the end of casting, and it has become clear that accurate measures cannot be taken due to the occurrence of false detections. Ta. The present invention deals with changes in the casting state, breakage in the mold due to seizure of the condensed shell in the mold, inclusions entrained in the surface layer of the solidified shell in the mold, etc. The purpose of this invention is to accurately detect the abnormal horizontal dents, vertical dents, and severe abnormal lateral dents that occur as a result. [Means for solving the problem] The present invention for achieving the above object has C=
When continuously casting molten steel containing 0.08 to 0.015%, the mold wall temperature at a predetermined position below the molten metal surface of the continuous casting mold drops by 20°C or more and less than 30°C from the normal predetermined value, and subsequently falls to the above predetermined value. If the rate of change in increase exceeds 2°C/sec, vertical or horizontal dents associated with cracking during width rolling after casting will occur in the surface layer of the solidified shell in the mold. It is characterized by detecting that [Operation] The temperature T at a predetermined position on the inner wall of the mold, which changes in temperature in response to the change in the surface temperature of the solidified shell in the mold during continuous casting, shows fluctuations as shown in FIGS. 1a and 1b. In this case, the embedded position of the thermocouple in the mold inner wall that detects the inner wall temperature at a predetermined position below the mold molten metal surface is at least 20 mm above the molten metal level in the mold, and in this example 700 mm or less, although it depends on the length of the mold. , at a depth of 1 mm or more and 30 mm or less from the inner wall surface of the mold. FIG. 1a shows a temperature change observed by the present inventors during a casting abnormality accompanied by a temperature drop. Temperature changes in the mold wall have characteristics when inclusions are involved, when abnormal dents occur in the slab, and when lateral dents accompanied by horizontal cracks occur. As is clear from this, it is possible to identify various abnormalities by measuring the temperature change of the solidified shell at at least one point on the inner wall of the mold below the mold surface and based on the characteristics of the temperature change. In other words, the type of abnormality in the slab observed by slab observation,
By correlating the degree, position, etc. with the characteristics of the temperature change pattern, we found that the amount of temperature change when the horizontal dents accompanied by the above-mentioned horizontal cracks, which cause breakouts, occur is extremely large and shows a temperature drop change of more than 30°C, and the temperature change after casting and rolling. The generation of the above-mentioned abnormal dents, which cause surface defects such as cracks, is a change of 20°C or more and less than 30°C. When inclusions are involved, the change is about 10°C or less. Furthermore, while the rate of temperature change when horizontal dents and abnormal dents accompanying lateral cracks occur is as large as about 2° C./second or more, it shows a smaller value when inclusions are involved. Figure 1b shows temperature changes due to other variations under normal operation. As can be seen from this figure, the rate of temperature change due to mold level fluctuations and casting speed fluctuations is smaller than in the case of horizontal dents accompanied by lateral cracks and abnormal dent formation, and based on these characteristics, casting abnormalities and operational fluctuations are From the temperature change (amount, rate of change), horizontal dents accompanied by lateral cracks and abnormal dents are accurately detected by distinguishing them from other types of dents. By sometimes stopping the slab from being pulled out for a predetermined period of time, the shell thickness of the horizontal dents with horizontal cracks is increased to prevent breakouts due to horizontal dents with horizontal cracks in the slab, and also to detect abnormal dents. In this case, the slab drawing speed is temporarily reduced to increase the shell thickness of the abnormal recess to prevent the abnormal recess from expanding, thereby suppressing the occurrence and expansion of surface defects such as cracks during rolling after casting. Table 1 shows the above-mentioned conditions such as casting abnormalities and operational abnormalities, the corresponding temperature changes, and alarms.
Examples of operational actions, etc. are shown. [Example] FIG. 4 shows the configuration of an apparatus for carrying out one embodiment of the present invention. This is a device configuration that detects horizontal dents accompanied by lateral cracks and abnormal dents. A thermocouple 2 is embedded in a predetermined position below the hot water level on the inner wall of the mold 1, and this
Measure the temperature at one point in the drawing direction. The temperature signal T _S of the thermocouple 2 is amplified and A/D converted (T _L :° C.) by the input device 3. Using this read signal T _L , an abnormality detection/judgment device (microcomputer) 4
performs arithmetic processing for abnormality determination.

〔Effect of the invention〕

As described above, the present invention distinguishes the surface defects of the slab, particularly the occurrence of horizontal dents accompanied by horizontal cracks in the solidified shell, and the occurrence of abnormal dents from other abnormalities, and distinguishes them from other abnormalities while the slab is still in the mold. Since it detects accurately, there is no occurrence of cracks during slab rolling, and
It becomes possible to prevent breakouts caused by horizontal dents accompanied by horizontal cracks, resulting in great effects such as improved continuous casting operation rate and improved rolling retention.

[Brief explanation of drawings]

Figures 1a and 1b are graphs showing the results of measuring mold inner wall temperature changes, and Figures 2a and 2b are graphs showing temperature changes when a horizontal dent accompanied by horizontal cracking occurs in the slab. It is a perspective view showing the shape of the slab. FIG. 3 is a graph showing upper and lower temperature measurement data when a lateral dent accompanied by a lateral crack occurs. Fourth
The figure is a block diagram showing the configuration of an apparatus for implementing the present invention in one embodiment, and FIG. 5 is a block diagram showing the configuration of an apparatus for implementing the present invention in another embodiment. 1: Slab, 2: Thermocouple, 3: Input device, 4: Abnormality detection and judgment device, 5: Comprehensive judgment processing device, 11:
Reference value calculator, 12: first determination circuit, 13: second
determination circuit, 14: third determination circuit, 15: fourth determination circuit, 16: fifth determination circuit, 17: sixth determination circuit, 18: seventh determination circuit, 19: eighth determination circuit,
20: Abnormal dent detection circuit, 21: Temperature change rate detection circuit, 22: Casting speed control device, 23a to 23
c: computing unit, 24: casting speed control device, 25: cooling time setting device.

Claims (1)

[Claims] 1. When continuously casting molten steel containing 0.08 to 0.015% C, the mold wall temperature at a predetermined position below the molten metal surface of the continuous casting mold is 20°C or more than the normal predetermined value.
℃, followed by an increase toward the above-mentioned predetermined value, and the rate of increase exceeds 2℃/sec.
A method for detecting abnormal depressions due to solidification shrinkage of a cast slab in a mold, the method comprising detecting the occurrence of vertical or horizontal depressions associated with cracks during width rolling after casting. 2. The method for detecting abnormal concaveness due to solidification shrinkage of a slab in a mold according to claim 1, wherein the predetermined value is a time series average value of the most recent past. 3. The method for detecting abnormal depressions due to solidification shrinkage of a slab in a mold according to claim 1, wherein the predetermined value is a value set based on operating conditions. 4 When continuously casting molten steel containing 0.08 to 0.15% C, the mold wall temperature at a predetermined position below the molten metal surface of the continuous casting mold drops by 30°C or more from the normal predetermined value, and subsequently falls to the above predetermined value. If the rate of increase exceeds 2°C/sec, a lateral dent accompanied by a lateral crack that will lead to rupture after exiting the mold has occurred in the surface layer of the solidified shell within the mold. A method for detecting abnormal dents due to solidification shrinkage of a cast slab in a mold. 5. The method for detecting abnormal concaveness due to solidification shrinkage of a slab in a mold according to claim 4, wherein the predetermined value is a time series average value of the most recent past. 6. The method for detecting abnormal depressions due to solidification shrinkage of a slab in a mold according to claim 4, wherein the predetermined value is a value set based on operating conditions.