JP2006241555A - Heating method in heating furnace of medium carbon steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heating method in a heating furnace for suppressing the generation of lap flaws after rolling of an ordinary steel slab of 0.12 to 0.17(mass%) C, 0.4 to 0.6(mass%) Mn, 0.05 to 0.15(mass%) Si by setting heating conditions in the heating furnace. <P>SOLUTION: The slab is heated up to the slab surface temperature of 1,250±20°C within a range of 50±10 minutes before the extraction from the heating furnace, and thereafter, the slab is heated up to the slab surface temperature of 1,230±20°C within a range of 230±20 minutes after the insertion into the heating furnace and is then extracted. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for heating an ordinary steel slab having a carbon content of 0.12 to 0.17 (mass%) in a heating furnace, and particularly to generation of surface flaws at the edge of a hot rolled coil after rolling. The present invention relates to a heating method in a heating furnace for suppression.

  Among the surface defects of hot-rolled coils, lap defects having a form in which scales and subscales have bitten are conventionally considered to have occurred due to slab cracks before being inserted into the heating furnace. In order to suppress the generation of these defects, (1) to produce a slab with less defects by improving the operation method during slab casting, (2) to slab the slab before inserting the heating furnace, to improve the surface properties However, in the latter (2), it is necessary to cool to near room temperature in order to perform cutting, which increases the time and cost burden.

The surface flaws described above are likely to occur at the edge portion of ordinary steel containing Si or Mn, for example, depending on the steel type. The following Patent Document 1 relates to a unidirectional electromagnetic slab containing Si, and after heating the slab to 900 to 1250 ° C. in a gas combustion type heating furnace, performs rolling under a large width and then rolling a dogbone by horizontal roll rolling. A method is disclosed in which, after erasing, the steel sheet is charged into an electric heating furnace and heated to 1300 to 1450 ° C., and then subjected to rough rolling and finish rolling to prevent the ear cracking of the hot rolled coil.
Japanese Patent Laid-Open No. 3-133501

  The method disclosed in Patent Document 1 described above requires a gas combustion type heating furnace and an electric heating furnace as the heating furnace, and more steps than anything, in order to prevent the cracking of the ears.

  In the present invention, by appropriately setting the heating in the heating furnace, it is possible to suppress the occurrence of surface flaws at the edge portion of the hot rolled coil without increasing the number of steps.

  The invention according to claim 1 is a method of heating an ordinary steel slab having a carbon content of 0.12 to 0.17 (mass%) in a heating furnace for rolling. After the slab surface temperature is heated to 1250 ± 20 ° C. at a time point of 50 ± 10 minutes before, the slab surface temperature is heated to 1230 ± 20 ° C. and then extracted.

The invention according to claim 2 is characterized in that, in the invention according to claim 1, the in-furnace time of the slab is 230 ± 20 minutes,
The invention according to claim 3 is the invention according to claim 1 or 2, wherein the ordinary steel slab contains 0.05 to 0.15% Si and 0.40 to 0.60 (mass%) Mn. Features.

  According to the invention according to claim 1, it is possible to suppress generation of surface flaws after rolling an ordinary steel slab having a carbon content of 0.12 to 0.17 (mass%) only by setting heating conditions in a heating furnace. In the invention according to claim 2, the generation of wrinkles can be suppressed more reliably. Moreover, in the invention concerning Claim 3, the surface flaw generation | occurrence | production with the steel grade which a surface flaw tends to generate | occur | produce at an edge part can be suppressed.

  Using a heating furnace that has a pre-tropical zone and heats in three stages after preheating, 100 steel slabs with the composition shown in Table 1 (thickness 250 mm, width 800 to 1300 mm) are set at an extraction temperature of 1230 ° C. After the slab was inserted, after the slab was inserted, the slab surface temperature was heated to 1250 ± 20 ° C. 50 minutes before the extraction, and then the slab surface temperature was set to 1230 ± 20 ° C. before extraction. Next, after rolling and cooling the hot-rolled coil after rolling, the presence or absence of wrap wrinkles was confirmed by visual inspection. Of the 100 coils, 7 had wrap wrinkles and 93 had wrinkle wrinkles. As shown in Table 2, when the in-furnace time was less than 250 minutes, among 59 slabs, 3 had laps and 56 had no wraps, and the in-furnace time exceeded 250 minutes. Of the 41 slabs, 4 had wraps and 37 had no wraps.

  Fig. 1 shows the surface temperature of a slab inserted into a heating furnace by means of a radiation thermometer placed at the insertion port, 1 heating, 2 heating, 3 heating, and extraction port of the heating furnace. The graph shows the slab in-furnace time at the measurement point. The solid line shows the average surface temperature at each average in-furnace time in the in-furnace of the coil without lap soot, and the dotted line shows the coil with lap soot. Similarly, the average surface temperature in the in-furnace is shown, and the average in-furnace time is less than 250 minutes.

  FIG. 2 is a graph obtained in the same manner as described above for a slab having an average in-furnace time of 250 minutes or more.

  FIG. 3 is a graph showing the average temperature at each in-furnace time of a coil with and without a wrap flaw for 100 slabs including a slab having an in-furnace time of less than 250 minutes and a slab of 250 minutes or more. .

  As can be seen from FIGS. 1 to 3, it is better that the average slab surface temperature in the furnace is higher. In particular, from FIG. 3, the average surface temperature at 3 heating times is 50 ± 10 minutes before extraction. Was extracted at 1250 ± 20 ° C., the average surface temperature of the extraction port was 1230 ± 20 ° C., and the average in-furnace time was 230 ± 20 minutes.

Comparative Example About 100 normal steel slabs as in the examples, the slab extraction temperature from the heating furnace was set to 1160 ° C., and the average surface temperature of the slabs in the heating furnace was measured in the same manner as in the examples. The in-furnace time at the time of measurement was obtained, and when the in-furnace time was less than 250 minutes and when the in-furnace time was 250 minutes or more, the presence or absence of lap flaws in the hot rolled coil was examined. When the furnace time was less than 250 minutes, 68 wraps were found and 6 wraps were found. When the furnace time was 250 minutes or more, 8 of 32 wraps were found.

  As shown in Table 2, the number of wrapping flaws is smaller at 1230 ° C. than when the extraction temperature is set to 1160 ° C.

The graph which shows the slab surface temperature according to the presence or absence of the lap | wrapping flaw in the case where in-furnace time is less than 250 minutes. The graph which shows the slab surface temperature according to the presence or absence of the lap flaw in the case where in-furnace time is 250 minutes or more. The graph which shows the slab surface temperature classified according to the presence or absence of lap wrinkles about the total slab.

Claims (3)

  In a method of heating a normal steel slab having a carbon content of 0.12 to 0.17% in a heating furnace for rolling, the slab surface temperature is 50 ± 10 minutes before extraction after the slab is inserted into the heating furnace. And heating to 1250 ± 20 ° C., and then extracting after heating the slab surface temperature to 1230 ± 20 ° C.   2. A heating method in a heating furnace according to claim 1, wherein a slab in-furnace time is 230 ± 20 minutes.   The heating method in a heating furnace according to claim 1 or 2, wherein the ordinary steel slab contains 0.05 to 0.15% of Si and 0.40 to 0.60% of Mn.

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