JPWO2015141154A1 - Hot rolling method for high carbon steel - Google Patents

Abstract

In hot rolling of high carbon steel, a hot rolling method capable of preventing the occurrence of steel plate cracking in the cooling process after finish rolling is provided. A high carbon steel sheet bar is heated by a sheet bar heater installed on the entry side of a finishing mill and then finish-rolled by the finishing mill. Hot-rolling method for high carbon steel, in which heating with a sheet bar heater is performed so that the amount of temperature increase at or below the predetermined upper limit value; where the amount of temperature increase at the tail end of the sheet bar is The difference between the temperature when heated by the sheet bar heater and the temperature when not heated by the sheet bar heater at the tail end of the sheet bar just before entering the finishing mill.

Description

  The present invention relates to a method for hot rolling high carbon steel. In particular, the present invention relates to a hot rolling method in which a high carbon steel sheet bar is heated and finish-rolled by a heating device (hereinafter referred to as a sheet bar heater) installed on the entrance side of the finishing mill. In addition, the high carbon steel as used in this specification refers to what is prescribed | regulated to JISG4051, JISG4053, JISG4401, JISG4404, and shall refer to the steel which contains especially 0.2 mass% or more and 1.0 mass% or less of C. .

  As shown in FIG. 1, for example, the hot rolling line is generally composed of a heating furnace 1, a plurality of rough rolling mills 2, a plurality of finishing rolling mills 3, a cooling zone 4, a winding device 5, and the like. In addition, the hot rolling line shown in FIG. 1 is a hot rolling line used when hot-rolling steel into plate-shaped steel (hot-rolled steel sheet). Moreover, in FIG. 1, the number of rough rolling mills is 3, and the number of finish rolling mills is 7.

  When hot rolling steel, a steel slab, which is a hot rolling material (steel material), is heated in a heating furnace 1 until a predetermined temperature is reached. After that, it is rolled with a finishing mill 3 so as to have a predetermined thickness. Here, the time until the sheet bar which is the material to be rolled bites into the finish rolling mill 3 is longer at the tail end of the sheet bar than at the front end. For this reason, even if the steel slab is heated uniformly in the heating furnace 1, the temperature at which the sheet bar is caught in the finishing mill 3, that is, the finishing mill entry side thermometer 11 (indicated by an arrow in FIG. 1). The measured temperature is generally lower at the tail end than at the front end of the seat bar. In particular, when rolling the tail end of the sheet bar, the increase in the finish rolling load caused by the decrease in temperature in this way makes the rolling unstable, and the material to be rolled breaks in the finish rolling mill 3, Inconveniences such as drawing occur in the material to be rolled. Here, the front end of the sheet bar refers to the front end in the longitudinal direction (rolling direction) of the sheet bar. The tail end of the sheet bar refers to the rear end of the sheet bar in the longitudinal direction (rolling direction).

  In order to compensate for such a temperature drop in the longitudinal direction of the sheet bar, the purpose is to heat the tail end side of the sheet bar before biting into the finishing mill (hereinafter also referred to as compensation heating). As shown, a sheet bar heater 10 is installed between the roughing mill 2 and the finishing mill 3. In the hot rolling line in which the seat bar heater 10 is installed, before the finish bar 3 is finished and rolled, the seat bar heater 10 is used to heat and raise the temperature of the tail end of the seat bar. The rolling load is reduced so that rolling can be performed stably. In the hot rolling line, in order to adjust the width of the slab by reducing the width of the slab, a sizing press is installed between the heating furnace 1 and the roughing mill 2, or the width direction of the sheet bar In order to compensate for this temperature drop, an edge heater may be installed between the roughing mill 2 and the finishing mill 3.

  The sheet bar heater 10 and the edge heater are generally used for so-called difficult-to-roll materials that have a particularly large rolling load, and not only uniformly heat the sheet bar that is the material to be rolled, but also the sheet bar at a predetermined conveying position. A target temperature is set in advance, and compensation heating of the seat bar is performed so as to be equal to or higher than the target temperature.

  In hot rolling of high carbon steel, a hot rolling method as disclosed in Patent Document 1 has been proposed. Patent Document 1 discloses a high-carbon steel hot rolling method in which a high-carbon steel sheet bar is heated by a sheet bar heater installed on the entrance side of a finishing mill and then finish-rolled by the finishing mill. A hot rolling method for high carbon steel, characterized in that a target temperature at a predetermined conveying position of the sheet bar is determined in advance, and compensation heating of the sheet bar is performed so as to be equal to or higher than the target temperature. Is described, and the effect that the steel plate cracking during finish rolling due to the rolling temperature being too low can be prevented and high-quality steel with good quality can be produced is shown.

JP 2003-275805 A

  However, when the steel is hot-rolled by the conventional hot rolling method as described above, when the steel as the rolling material is high carbon steel, when the sheet bar is heated by the sheet bar heater, the finish rolling mill 3 In the cooling zone 4 installed on the downstream side in the rolling direction, a problem (hereinafter also referred to as “steel plate cracking”) that a steel plate as a material to be rolled after finish rolling breaks may occur.

  In order to cope with such a problem that the steel plate after finish rolling breaks, for high carbon steel, a sheet bar heater is not used, and the heating temperature in the heating furnace (hereinafter also referred to as heating furnace extraction temperature) is increased. Then, the reduction of the rolling load at the tail end was studied. However, in the case of high carbon steel, when the heating temperature is increased, red scale which is a surface defect is likely to occur. Due to this limitation, there is a problem that the heating temperature of high carbon steel cannot be increased, and it is difficult to achieve both quality and sheet passing properties.

  An object of this invention is to provide the hot rolling method which can prevent generation | occurrence | production of the steel plate crack in the cooling process after finish rolling in hot rolling of high carbon steel in view of said point.

  As a result of investigating the cause of the cracking of the steel plate when the above-described high carbon steel is used as a rolling material, the inventors of the high carbon steel proceeded with the coarsening of crystal grains due to heating with the sheet bar heater, and cooling. It was found that cracking of the steel sheet occurred during cooling in the zone. Especially when heated by a high frequency induction heating type sheet bar heater, such problems often occur, and as a result of a temperature difference between the surface and the center of the sheet bar, the crystal grain size is large at the surface layer and the center. It was considered that a difference occurred, causing the above-mentioned problems.

Therefore, as a result of various studies to solve such a problem, when the amount of temperature rise of the sheet bar by heating with the sheet bar heater on the entrance side of the finishing mill reaches a certain temperature amount, It was found that the steel plate cracking occurred. When hot rolling is performed on high carbon steel, if an upper limit is set for the temperature rise in the sheet bar heater, it is necessary to raise the heating furnace extraction temperature without causing steel plate cracking in the cooling zone. It was found that high carbon steel can be manufactured without causing quality defects such as red scale.
In controlling the temperature rise amount of the seat bar, it is effective to control the temperature rise amount at the tail end of the sheet bar. And by changing the amount of temperature rise at the tail end of the sheet bar in advance, the presence or absence of cracks in the steel plate in the cooling zone provided on the downstream side of the finishing mill is examined, It is effective to make the temperature rise amount the upper limit. In addition, when a steel sheet crack occurs during normal operation, the amount of temperature rise at the tail end of the sheet bar, which was not cracked by the slab of the same steel type that was simultaneously hot rolled, It is also effective to do.

The present invention has been completed based on such findings, and the gist thereof is as follows.
[1] A hot rolling method for high carbon steel, in which a high carbon steel sheet bar is heated by a sheet bar heater installed on the entrance side of the finishing mill and then finish-rolled by the finishing mill. Hot-rolling method for high carbon steel, in which heating is performed with a sheet bar heater so that the amount of temperature increase at the tail end of the steel sheet is equal to or less than a predetermined upper limit value; The amount is the difference between the temperature when heated by the sheet bar heater and the temperature when not heated by the sheet bar heater at the tail end of the sheet bar immediately before the finishing mill entry side.

  According to the present invention, when hot-rolling high carbon steel using a sheet bar heater, it is possible to prevent the occurrence of steel plate cracking in the cooling process after finish rolling. Thereby, it is possible to stably produce a high-quality steel sheet having good quality without suppressing the heating furnace extraction temperature from being raised and causing a problem of quality problems such as occurrence of red scale.

FIG. 1 is a schematic diagram showing a hot rolling line. FIG. 2 is a diagram showing the relationship between the longitudinal position of the sheet bar and the finishing mill entry side temperature when not heated by the sheet bar heater. FIG. 3 is a view showing the relationship between the longitudinal position of the sheet bar and the temperature at the finishing mill entry side.

  A preferred embodiment of the present invention will be described by taking as an example the case of hot rolling consisting of rough rolling and finish rolling on the hot rolling line shown in FIG. A high carbon steel slab obtained by continuous casting or the like is heated in a heating furnace 1 and then roughly rolled by a roughing mill 2 to obtain a high carbon steel sheet bar. The sheet bar obtained by the roughing mill 2 is heated (compensated heating) by the sheet bar heater 10 in order to compensate for the temperature drop on the finishing mill entry side.

  FIG. 2 is a schematic diagram of the temperature in the longitudinal direction of the sheet bar immediately before the finish rolling mill entry side (entrance side of the finish rolling mill closest to the roughing mill) when heating by the sheet bar heater 10 is not performed. Show. As described above, the tail end of the sheet bar has a lower temperature on the finishing mill entry side than the tip. It should be noted that the tail end and tip end of the sheet bar on which the temperature measurement is performed can be appropriately determined in consideration of the manufacturing conditions and the apparatus.

  In hot rolling using the conventional sheet bar heater 10, the sheet bar heater 10 performs compensation heating in the longitudinal direction of the sheet bar to raise the temperature of the sheet bar. The amount of temperature raised by the sheet bar heater 10 is appropriately determined so that the total length of the sheet bar is within a desired temperature range so that finish rolling can be stably performed. Here, as shown in FIG. 2, since the temperature drop at the tail end of the sheet bar is the largest among the sheet bars, the compensation heating amount at the tail end of the sheet bar becomes the maximum at the sheet bar.

  In the present invention, when performing the compensation heating by the conventional seat bar heater 10 as described above, the temperature rise amount at the tail end of the seat bar, which is the position where the temperature rise amount is maximum in the sheet bar, is determined in advance. Try to be below the upper limit.

  In FIG. 3, as an example of the hot rolling method of the present invention, in the longitudinal direction of the seat bar, a portion from the longitudinal position Lx of the seat bar to the tail end of the seat bar is heated by the seat bar heater 10 to A schematic diagram of the temperature in the longitudinal direction of the sheet bar immediately before the finishing mill entry side (entrance side of the finishing mill closest to the roughing mill) when the tail end temperature is Ta (° C.) is shown. . Here, the heating amount of the seat bar by the seat bar heater 10 is set such that Lx is a position half the seat bar length so that the temperature at the tail end of the seat bar becomes Ta (° C.). Is distributed from the center in the longitudinal direction, and the temperature of the sheet bar from Lx to the tail end is made substantially constant at Ta (° C.). FIG. 3 also shows a case where the sheet bar heater is not used for heating. In this case, the temperature at the tail end of the sheet bar is Tn (° C.).

  In the present invention, when the seat bar is heated by the seat bar heater 10, heating (compensation heating) is performed by the seat bar heater 10 so that the temperature rise at the tail end of the seat bar is equal to or less than a predetermined upper limit value. .

  As schematically shown in FIG. 3, the amount of temperature rise at the tail end of the seat bar is the temperature Tn (° C.) of the tail end of the seat bar when not heated by the seat bar heater 10 and is heated by the seat bar heater 10. The difference ΔT (° C.) (ΔT = Ta−Tn) from the temperature Ta (° C.) of the tail end of the sheet bar in this case.

The temperature Tn when not heated by the sheet bar heater 10 and the temperature Ta when heated at the tail end of the sheet bar are the temperature of the sheet bar at the exit position of the sheet bar heater 10, the size and finish of the sheet bar. It can be determined based on the distance to the rolling mill, temperature, and the like.
Usually, Ta (° C.) is a temperature measured by the finishing mill entry side thermometer 11.
Usually, Tn (° C.) is the temperature measured by the finishing mill entry-side thermometer 11 when the sheet bar is not heated. Tn (° C.) may be a value obtained by correcting the measured temperature in consideration of various operating conditions and air temperature. Further, Tn (° C.) may be obtained by calculation in consideration of manufacturing conditions and equipment.

In the present invention, the amount of temperature increase at the tail end of the sheet bar: ΔT is variously changed in advance, and the occurrence of cracks in the steel sheet in the cooling zone 4 provided on the downstream side of the finish rolling mill is examined. The upper limit value of ΔT is determined. When the seat bar is heated by the seat bar heater, the seat bar is heated so that the amount of temperature rise at the tail end of the seat bar is equal to or less than a predetermined upper limit value of ΔT.
In addition to the above-described method, when a steel plate crack occurs in a normal operation, the amount of temperature rise at the tail end of the sheet bar that is not cracked by a slab of the same steel type that is simultaneously hot rolled can be determined as the upper limit value of ΔT.
On the other hand, the lower limit of the temperature rise at the tail end of the seat bar can be appropriately determined in consideration of manufacturing conditions, equipment, and the like.

  By using such a hot rolling method of the present invention, the sheet bar heater 10 compensates and heats the sheet bar, raises the temperature of the tail end of the sheet bar, and suppresses breakage in the finishing mill. Moreover, the steel plate cracking in the cooling zone 4 can be suppressed.

  In FIG. 3, heating by the seat bar heater 10 is started from the longitudinal center of the seat bar, but the heating method of the seat bar is not limited to such a method. When compensation heating with the seat bar heater is required over the entire length of the seat bar, the sheet bar may be heated over the entire length of the sheet bar so that the temperature rise at the tail end of the seat bar is less than or equal to the upper limit of ΔT. Only the tail end of the bar may be heated locally.

  In the method of the present invention, an upper limit is set for the amount of temperature rise of the seat bar. Therefore, depending on the rolling conditions and the type of the sheet bar, there may be a case where the temperature cannot be raised sufficiently to the extent that breakage in the finishing mill can be suppressed. In such a case, for example, when the upper limit value of the temperature rise of the sheet bar determined in advance is insufficient for securing the material in the longitudinal direction of the sheet bar in the finish rolling mill, The length of the slab before rolling can be shortened, the length of the sheet bar itself can be shortened, and the amount of temperature drop at the tail end of the sheet bar before finish rolling can be reduced.

In addition, as the heating method of the seat bar heater 10, various types such as induction heating and burner can be applied. However, as described above, in order to control the temperature rise, the induction heating method is used. Is preferred.
In the present invention, the slab heating temperature in the heating furnace is not particularly limited, and can be appropriately determined in consideration of manufacturing conditions, equipment, and the like. The heating furnace extraction temperature can be set to, for example, 1050 to 1150 ° C.

  Using a slab of high carbon steel (S45C defined in JIS G4051) having a component composition (mass%) of steel symbol A shown in Table 1, hot rolling was performed in the hot rolling line shown in FIG. The thickness of the slab used was 260 mm and the width was 1150 mm. After heating the slab to the heating furnace extraction temperature shown in Table 2 with a heating furnace, the width was set to 1065 mm with a sizing press (not shown), and then rough rolling was performed. A sheet bar having a thickness of 40 mm and a width of 1065 mm was obtained. In the example of the present invention (rolling number 1), the obtained sheet bar was heated by a sheet bar heater with the upper limit of the temperature rise at the tail end of the sheet bar being 40 ° C. and subjected to finish rolling. In addition, as a comparison, when heated with a sheet bar heater at a temperature increase exceeding the above-described upper limit (rolling number 2), when heating furnace extraction temperature is increased and not heated with a sheet bar heater (rolling number 3) Also, finish rolling was performed. When heating with a seat bar heater, as shown in FIG. 3, the temperature rise amount is distributed from the longitudinal center to the tail end of the seat bar so that the temperature rise at the tail end of the seat bar is expressed. Heated to a temperature of 2. The sheet bar heater was a high frequency induction heating type. The tail end of the sheet bar is 3 m from the rear end in the longitudinal direction (rolling direction) of the sheet bar (the total length of the sheet bar after rough rolling is 60 m).

  Table 2 shows the temperature of the steel sheet measured by the finish rolling mill thermometer 12 (indicated by an arrow in FIG. 1). The sheet bar was rolled into a hot-rolled steel sheet with a finish rolling mill, cooled in a cooling zone, and wound up with a winding device. The thickness of the finish-rolled steel sheet was 3.25 mm, the width after finish rolling was 1065 mm, and the winding temperature was 670 ° C. Table 2 shows the results of hot rolling.

In the example of the present invention (rolling number 1), hot rolling could be performed without generating steel plate cracks in the cooling zone, and there was no occurrence of red scale, and good quality steel plates could be produced. On the other hand, in the comparative example (rolling number 2) in which the temperature rise exceeded the range of the present invention and was too high, a steel plate crack occurred in the cooling zone. Moreover, in the comparative example (rolling number 3) in which the heating temperature in the heating furnace itself was increased without performing heating with the sheet bar heater, although the steel plate cracking did not occur in the cooling zone, a red scale was generated. did.
The presence or absence of the occurrence of cracks in the steel plate was judged by passing through a recoil line and conducting a visual inspection. When there was a crack in the edge portion in the width direction of the steel plate having a length of 10 mm or more, if there was a crack in the steel plate, it was determined that there was no crack in the steel plate if there was no crack in the edge portion having a length of 10 mm or more. Also in Example 2 and later, the presence or absence of steel plate cracking was determined based on the same criteria.

Based on the results of Example 1 above, the upper limit value of the temperature rise at the tail end of the seat bar was set to 40 ° C. For the six slabs of steel symbol A, under the same conditions as in Example 1 above, except that the upper limit of the temperature rise at the tail end of the seat bar was changed to 40 ° C. (repeated execution) and 35 ° C. Hot rolling was performed (rolling numbers 4 to 9). The results are shown in Table 3.

  As a result, in any case, it was possible to perform hot rolling without generating a steel plate crack in the cooling zone, and it was possible to produce a steel plate with good quality without occurrence of red scale. From the above, by setting the upper limit of the temperature rise at the tail end of the seat bar in advance, it is possible to stably produce a high-quality steel sheet with good quality without causing problems such as red scale. Could be manufactured.

  Using a slab of high carbon steel (S45C as defined in JIS G4051) with a component composition (mass%) of steel symbols B to E shown in Table 4, the upper limit value of the temperature rise at the tail end of the seat bar is 50 ° C 40 ° C. and 35 ° C., and hot rolling was performed under the same conditions as in Example 1 except that. Table 5 shows the results of hot rolling.

  In the example of the present invention (rolling numbers 10 to 14) in which the upper limit of the temperature rise at the tail end of the sheet bar is 40 ° C. and 35 ° C., hot rolling is performed without causing steel plate cracks in the cooling zone. It was possible to produce a steel plate of good quality with no red scale. On the other hand, in the comparative example (rolling number 15) in which the temperature rise exceeded the range of the present invention and was too high, a steel plate crack occurred in the cooling zone.

Based on the results of Example 1, the amount of temperature increase at the tail end of the seat bar was set to 40 ° C., which is the upper limit value. The heating position by the sheet bar heater is from the longitudinal direction 1/3 part of the sheet bar to the tail end part (rolling number 16), from the longitudinal direction 2/3 part of the sheet bar to the tail end part (rolling number 17), Hot rolling was performed under the same conditions as in Example 1 except that the heating was performed by distributing the respective slopes.

  As a result, in any case, it was possible to perform hot rolling without generating a steel plate crack in the cooling zone, and it was possible to produce a steel plate with good quality without occurrence of red scale. From the above, by setting the upper limit of the temperature rise at the tail end of the seat bar in advance, it is possible to stably produce a high-quality steel sheet with good quality without causing problems such as red scale. Could be manufactured.

DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Coarse rolling mill 3 Finishing mill 4 Cooling zone 5 Winding device 10 Sheet bar heater 11 Finishing mill entrance side thermometer 12 Finishing mill exit side thermometer

Claims (1)

A high carbon steel sheet bar is heated by a sheet bar heater installed on the entry side of a finishing mill and then finish-rolled by the finishing mill. Hot-rolling method for high carbon steel, in which heating with a sheet bar heater is performed so that the amount of temperature increase at or below the predetermined upper limit value; where the amount of temperature increase at the tail end of the sheet bar is The difference between the temperature when heated by the sheet bar heater and the temperature when not heated by the sheet bar heater at the tail end of the sheet bar just before entering the finishing mill.

Images