TWI535859B - Hot Rolling Method for High Carbon Steel - Google Patents

Description

Hot rolling method for high carbon steel

The present invention relates to a hot rolling method for high carbon steel. In particular, a hot rolling method for heating a high carbon steel sheet and performing finish rolling by a heating device (hereinafter referred to as a sheet heater) provided on the inlet side of the finishing mill. In addition, the term "high carbon steel" as used in the present specification means a steel containing C (carbon) of 0.2% by mass or more and 1.0% by mass or less, as defined in JIS G4051, JISG4053, JISG4401, and JISG4404.

In general, the hot rolling line includes, for example, a heating furnace 1, a plurality of rough rolling mills 2, a plurality of finishing mills 3, a cooling zone 4, a winding device 5, and the like, as shown in FIG. Further, the hot rolling line shown in Fig. 1 is a hot rolling line used when hot rolling a steel to form a steel (hot rolled steel sheet) having a plate shape. Further, in Fig. 1, the number of roughing mills is three, and the number of finishing mills is seven.

When the steel is hot-rolled, the billet of the hot-rolled material (steel material) is heated in the heating furnace 1 to a predetermined temperature, and thereafter, the rough rolling mill 2 performs rough rolling to form a sheet, and then The finish rolling mill 3 performs rolling at a predetermined thickness. Here, the time before the sheet of the material to be rolled is bitten into the finishing mill 3 is longer than the front end of the sheet. Therefore, even if the slab is uniformly heated in the heating furnace 1, the temperature at which the sheet is bitten into the finishing mill 3, that is, measured by the finish mill inlet side thermometer 11 (indicated by an arrow in Fig. 1) The temperature is also the end of the plate is generally lower than before end. In particular, when the end portion of the sheet is rolled, the finish rolling load is increased due to such a temperature drop, whereby the rolling pressure becomes unstable, and the material to be rolled is fractured or rolled in the finishing mill 3. The material produces undesirable conditions such as shrinkage. Here, the front end of the sheet refers to the front end in the longitudinal direction (rolling direction) of the sheet. Further, the terminating end of the sheet refers to the rear end in the longitudinal direction (rolling direction) of the sheet.

In order to compensate for the temperature drop in the longitudinal direction of the sheet, the end side of the sheet is heated (hereinafter also referred to as compensation heating) before being bitten into the finishing mill, as shown in FIG. A sheet heater 10 is provided between the roughing mill 2 and the finishing mill 3. In the hot rolling line provided with the sheet heater 10, before the finishing rolling of the sheet by the finishing mill 3, the sheet heater 10 is used for heating to increase the temperature of the trailing end of the sheet, thereby reducing the temperature. The rolling load is applied, and the rolling is performed stably. Further, in the hot rolling line, there is also a case where a sizing press is set between the heating furnace 1 and the roughing mill 2 in order to carry out rolling reduction in the width direction of the steel slab, or between the heating furnace 1 and the roughing mill 2, or In order to compensate for the temperature drop in the width direction of the sheet, an edge heater is provided between the roughing mill 2 and the finishing mill 3.

The sheet heater 10 or the edge heater is generally used for a so-called difficult-to-roll material having a large rolling load, not only for heating the sheet of the material to be rolled, but also for predetermining the sheet of the predetermined conveying position. The target temperature is compensated and heated to be equal to or higher than the target temperature.

Regarding hot rolling of high carbon steel, there is also proposed a hot rolling method as disclosed in Patent Document 1. Patent Document 1 describes a hot rolling method for high carbon steel in which a sheet of high carbon steel is heated by a sheet heater provided on the inlet side of the finishing mill, and then subjected to the above finishing mill. a finisher, which is characterized in that a target temperature at a predetermined transfer position of the sheet is determined in advance and is equal to or higher than the target temperature In the manner of the above-mentioned sheet, the sheet is subjected to compensation heating; and the hot rolling method exhibits an effect of preventing the steel sheet from being broken in the finish rolling caused by the excessively low rolling temperature, thereby producing a high-quality carbon steel of good quality.

[Previous Technical Literature] [Patent Literature]

Patent Document 1: Japanese Patent Laid-Open Publication No. 2003-275805

However, in the case of hot rolling of steel by the conventional hot rolling method as described above, and in the case where the steel of the rolled material is high carbon steel, if the sheet is heated by the sheet heater, There is a case where the steel sheet of the material to be rolled after the finish rolling is broken in the cooling zone 4 provided on the downstream side in the rolling direction of the finishing mill 3 (hereinafter also referred to as steel sheet cracking).

In order to cope with the problem of cracking of the steel sheet after the finish rolling, the following research has been conducted: for the high carbon steel, the heating temperature in the heating furnace (hereinafter also referred to as the heating furnace extraction temperature) is increased without using the sheet heater. Thereby reducing the rolling load at the end. However, in the case of high carbon steel, if the heating temperature is raised, it becomes easy to cause surface defects, that is, red rust. Due to this limitation, there is a problem that the heating temperature of the high carbon steel cannot be increased, and it is difficult to achieve both quality and board properties.

The present invention has been made in view of the above aspects, and an object thereof is to provide a hot rolling method capable of preventing cracking of a steel sheet during cooling in a hot rolling process of high carbon steel.

The inventors of the present invention have directed to setting high carbon steel as rolling compactor as described above. In the case of the material, the cause of the cracking of the steel sheet was investigated. As a result, it was found that the high carbon steel was coarsened by heating by the sheet heater, and the steel sheet was broken when it was cooled in the cooling zone. In particular, in the case of heating by a plate heater of a high-frequency induction heating method, such a problem often occurs, and it is considered that a temperature difference occurs between the surface and the center of the sheet, and as a result, the surface portion and the central portion are grained. A large difference in diameter results in the above-mentioned undesirable condition.

Therefore, in order to solve such a problem, various studies have been conducted, and as a result, it has been found that the steel sheet in which the cooling zone occurs when the temperature rise amount of the sheet obtained by heating using the sheet heater on the inlet side of the finishing mill reaches a certain temperature or more rupture. Further, it has been found that when hot rolling is performed on the high carbon steel, if the upper limit is applied to the amount of temperature rise obtained by using the sheet heater, the steel sheet in the cooling zone does not break, and there is no need to increase the extraction temperature of the furnace. High-carbon steel can be produced by poor quality such as red rust.

When controlling the temperature rise of the above-mentioned sheet, the amount of temperature rise at the end of the control sheet is effective. Further, various changes are made in advance to the amount of temperature rise at the end of the sheet, and it is investigated whether or not the sheet is broken in the cooling zone provided on the downstream side of the finishing mill, and the temperature rise at the end of the sheet which does not cause cracking is set to the upper limit. effective. Further, in the case where the steel sheet is broken under normal operation, the temperature rise at the end of the sheet which is not broken in the billet of the same steel type which is simultaneously subjected to hot rolling is set as the upper limit of the temperature rise at the end of the sheet. Also effective.

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, wherein a sheet of high carbon steel is heated by a sheet heater provided on an inlet side of a finishing mill, and is finished by the finishing mill, and The heating by the sheet heater is performed such that the amount of temperature rise at the end of the sheet is equal to or less than a predetermined upper limit; here, the temperature rise at the end of the sheet The amount refers to the difference between the temperature at the end of the sheet immediately before passing through the inlet side of the finishing mill and the temperature when it is heated by the sheet heater, and the temperature when it is not heated by the sheet heater.

According to the present invention, it is possible to prevent the steel sheet from being broken during the cooling after the finish rolling in the case of hot rolling of the high carbon steel using the sheet heater. Thereby, it is possible to suppress the deterioration of the temperature of the extraction temperature of the heating furnace without causing defects in quality such as generation of red rust, and to stably produce a high-quality steel sheet of good quality.

1‧‧‧heating furnace

2‧‧‧Roughing mill

3‧‧‧ finishing mill

4‧‧‧Cooling area

5‧‧‧Winding device

10‧‧‧ plate heater

11‧‧‧ Finishing mill entrance side thermometer

12‧‧‧ Finishing mill exit side thermometer

Figure 1 is a schematic view showing a hot rolling line.

Fig. 2 is a view showing the relationship between the position in the longitudinal direction of the sheet and the temperature at the inlet side of the finishing mill when the sheet heater is not heated.

Fig. 3 is a view showing the relationship between the position in the longitudinal direction of the sheet and the temperature on the inlet side of the finishing mill.

In the preferred embodiment of the present invention, a case where hot rolling including rough rolling and finish rolling is performed by the hot rolling line shown in Fig. 1 will be described as an example. The steel slab of high carbon steel obtained by continuous casting or the like is heated in the heating furnace 1, and then subjected to rough rolling by the roughing mill 2 to be formed into a sheet of high carbon steel. The sheet obtained by the roughing mill 2 is heated (compensated heating) by the sheet heater 10 to compensate for the temperature drop on the inlet side of the finishing mill.

In Fig. 2, it is shown that when the heating by the sheet heater 10 is not performed, the inlet side of the finishing mill (the inlet side of the finishing mill closest to the side of the roughing mill) is passed. Schematic diagram of the temperature in the longitudinal direction of the front sheet. As described above, the temperature at the trailing end of the sheet is lower at the inlet side of the finishing mill than at the leading end. In addition, the tail end and the front end of the sheet for temperature measurement can be appropriately determined in consideration of manufacturing conditions, equipment, and the like.

In the conventional hot rolling using the sheet heater 10, the sheet heater 10 performs the compensation heating in the longitudinal direction of the sheet to raise the temperature of the sheet. The amount of temperature rise by the sheet heater 10 is appropriately determined so that the finish rolling can be stably performed so that the entire length of the sheet is within a desired temperature range. Here, as shown in Fig. 2, the temperature at the trailing end of the sheet drops to the maximum among the sheets, so the amount of compensation heating at the trailing end of the sheet is the largest in the sheet.

In the present invention, when performing the conventional compensation heating by the sheet heater 10, the temperature rise amount at the end of the sheet, which is the position where the temperature rise amount is the largest in the sheet, is equal to or less than the predetermined upper limit value. .

In Fig. 3, as an example of the hot rolling method of the present invention, in the longitudinal direction of the sheet, the portion from the longitudinal direction position Lx of the sheet to the trailing end of the sheet is heated by the sheet heater 10. A graph showing the temperature in the longitudinal direction of the sheet immediately before the end of the finishing mill (the inlet side of the finishing mill closest to the side of the roughing mill) when the temperature at the end of the sheet is set to Ta (°C). Here, as the heating amount of the sheet obtained by the sheet heater 10, Lx is set to 1/2 of the length of the sheet so that the temperature at the trailing end of the sheet becomes Ta (° C.). And the inclination is distributed from the center portion in the longitudinal direction of the sheet, so that the temperature of the sheet from Lx to the end is substantially fixed to Ta (°C). Furthermore, in Fig. 3, the case where heating is not performed by the sheet heater is also shown. In this case, the temperature at the trailing end of the sheet is Tn (°C).

In the present invention, when the sheet heater is heated by the sheet heater 10, the amount of temperature rise at the trailing end of the sheet is equal to or less than a predetermined upper limit value. In the formula, heating by the sheet heater 10 (compensation heating) is performed.

As schematically shown in Fig. 3, the temperature rise at the trailing end of the sheet is the temperature Tn (°C) of the trailing end of the sheet which is not heated by the sheet heater 10, and is heated by the sheet. The difference ΔT (°C) (ΔT = Ta - Tn) between the temperatures Ta (°C) of the trailing ends of the sheets when the heater 10 is heated.

Further, the temperature at the end of the sheet is not heated by the sheet heater 10, and the temperature Ta at the time of heating by the sheet heater 10 can be based on the position of the outlet side of the sheet heater 10. The temperature of the sheet and the size of the sheet, or the distance from the finishing mill, the temperature, etc. are obtained.

In general, Ta (°C) is the temperature measured by the finisher side thermometer 11 of the finishing mill.

Further, in general, Tn (°C) is a temperature measured by the finisher side thermometer 11 when the sheet is not heated. Tn (° C.) may be a value obtained by correcting the measured temperature in consideration of various operating conditions, temperature, and the like. Further, Tn (° C.) can also be obtained by calculation in consideration of manufacturing conditions, equipment, and the like.

In the present invention, various changes are made in advance to the temperature rise amount at the end of the sheet: ΔT, and it is investigated whether or not cracking of the steel sheet occurs in the cooling zone 4 provided on the downstream side of the finishing mill, and it is determined that cracking does not occur. T upper limit. Further, when the sheet is heated by the sheet heater, the sheet is heated so that the amount of temperature rise at the trailing end of the sheet is equal to or less than a predetermined upper limit of ΔT.

In addition to the above method, when the steel sheet is broken under normal operation, the temperature rise at the end of the sheet which is not broken in the billet of the same steel type which is simultaneously subjected to hot rolling can be determined as the upper limit of ΔT.

On the other hand, the lower limit of the amount of temperature rise at the end of the sheet can be appropriately determined in consideration of manufacturing conditions, equipment, and the like.

By setting such a hot rolling method of the present invention, the sheet heater 10 can be compensated and heated to increase the temperature of the tail end portion of the sheet, thereby suppressing breakage in the finishing mill and suppressing cooling. The steel plate of Zone 4 is broken.

In addition, in Fig. 3, heating by the sheet heater 10 is started from the center portion in the longitudinal direction of the sheet, but the method of heating the sheet is not limited to this method. When it is necessary to use the compensation heating of the plate heater at the entire length of the plate, the heating amount at the tail end of the plate may be equal to or less than the upper limit of ΔT, or the end of the plate may be heated at the end of the plate. The temperature rise amount is equal to or less than the upper limit of ΔT, and only the tail end portion of the sheet is locally heated.

Further, in the method of the present invention, an upper limit is imposed on the amount of temperature rise of the sheet. Therefore, it is considered that depending on the rolling conditions or the type of the sheet, the temperature may not be sufficiently raised to such an extent that it can be prevented from being broken in the finishing mill. In such a case, for example, if the upper limit of the temperature rise amount of the sheet determined in advance is insufficient to suppress the breakage in the finishing mill or the material in the longitudinal direction of the sheet, the billet before rough rolling can be shortened. The length is shortened by shortening the length of the sheet itself, thereby reducing the amount of temperature drop at the end portion of the sheet before finish rolling.

Moreover, as the heating method of the sheet heater 10, various heating methods such as a heating method using induction heating and a heating method using a burner can be applied, and in order to control the temperature increase amount as described above, it is preferable to use an induction heating method.

In the present invention, the slab heating temperature in the heating furnace is not particularly limited, and may be appropriately determined in consideration of production conditions, equipment, and the like. The furnace extraction temperature can be set, for example, to 1050 to 1150 °C.

[Example 1]

High carbon using the composition (% by mass) of the steel mark A shown in Table 1 The steel slab of steel (S45C specified in JIS G4051) is hot rolled by the hot rolling line shown in Fig. 1 . The slab used has a thickness of 260 mm and a width of 1150 mm. After the slab is heated in a heating furnace to the furnace extraction temperature shown in Table 2, the width is set to 1065 mm by a sizing press (not shown). The rough rolling was carried out to obtain a sheet having a thickness of 40 mm and a width of 1065 mm. Regarding the obtained sheet, in the example of the present invention (rolling number 1), the upper limit of the temperature rise at the end of the sheet was set to 40 ° C, and the sheet heater was used for heating. Rolling. Further, in comparison, when the heating is performed by the sheet heater at a temperature increase exceeding the upper limit (rolling number 2), and the temperature at which the furnace is extracted is increased, the sheet heater is not heated. At the time (rolling number 3), finish rolling is also performed. Further, when heating by the sheet heater, as shown in FIG. 3, the temperature rise amount is obliquely distributed from the central portion to the trailing end portion in the longitudinal direction of the sheet to increase the temperature of the end of the sheet. Heating was carried out in such a manner as to become the temperature of Table 2. Furthermore, the plate heater is set to a high frequency induction heating method. Further, the end of the sheet was 3 m from the rear end in the longitudinal direction (rolling direction) of the sheet (the total length of the sheet after the rough rolling was 60 m).

Table 2 shows the temperature of the steel sheet measured by the finish mill exit side thermometer 12 (indicated by an arrow in Fig. 1). After the sheet is rolled into a hot rolled steel sheet by a finishing mill, it is cooled in a cooling zone and taken up by a winding device. Further, the thickness of the finished steel sheet was 3.25 mm, the width after finish rolling was 1065 mm, and the coiling temperature was 670 °C. The results of the hot rolling are shown in Table 2.

In the example of the present invention (rolling number 1), hot rolling can be performed without causing cracking of the steel sheet in the cooling zone, and red rust is not generated, and a steel sheet of good quality can be produced. On the other hand, in the comparative example (rolling number 2) in which the amount of temperature rise exceeded the range of the present invention, the steel sheet was broken in the cooling zone. Further, a comparative example in which the heating temperature in the heating furnace itself is raised without heating by the sheet heater (rolling number 3) is not performed. In the middle, although the steel plate broke in the cooling zone, red rust was generated.

The presence or absence of the occurrence of the steel plate rupture is judged by passing the plate through the recoil line and performing a visual inspection. In the case where the edge portion of the steel sheet width direction has a crack of 10 mm or more in length, it is judged that the steel sheet is broken, and it is judged that there is no steel sheet crack in the case where the edge portion has no crack of 10 mm or more in length. After the second embodiment, the same criteria were used to judge the presence or absence of cracking of the steel sheet.

[Embodiment 2]

Based on the results of the above Example 1, the upper limit of the amount of temperature rise at the end of the sheet was set to 40 °C. For the slab of the six steel marks A, the upper limit of the temperature rise amount at the end of the sheet was changed to 40 ° C (repeated execution) and 35 ° C, and the same conditions as in the above Example 1 were carried out. Hot rolling (rolling number 4 to 9). The results are shown in Table 3.

As a result, in any case, hot rolling can be performed without causing cracking of the steel sheet in the cooling zone, and red rust does not occur, and a steel sheet of good quality can be produced. According to the above, by predetermining the upper limit of the temperature rise amount at the tail end of the sheet, the red color may not be caused. A high-carbon steel sheet of good quality is stably produced with a problem of quality such as rust.

[Example 3]

The slab of high carbon steel (S45C specified in JIS G4051) having the composition (% by mass) of the steel composition B to E shown in Table 4 is used, and the upper limit of the temperature rise at the end of the sheet is set to 50. Hot rolling was carried out under the same conditions as in the above Example 1 except for ° C, 40 ° C and 35 ° C. The results of hot rolling are shown in Table 5.

In the example of the present invention (rolling number 10 to 14) in which the upper limit of the temperature rise amount at the end of the sheet is 40 ° C and 35 ° C, hot rolling may be performed without cracking of the steel sheet in the cooling zone, and It does not produce red rust and can produce steel of good quality. On the other hand, in the comparative example (rolling number 15) in which the amount of temperature rise exceeded the range of the present invention, the steel sheet was broken in the cooling zone.

[Example 4]

Based on the results of the above Example 1, the amount of temperature rise at the end of the sheet was set to 40 ° C of the upper limit. Further, the 1/3 portion to the trailing end portion (rolling number 16) in the longitudinal direction of the sheet, and the 2/3 portion to the trailing end portion (rolling number 17) in the longitudinal direction of the sheet are respectively obliquely distributed. Hot rolling was performed under the same conditions as in the above Example 1 except that the heating position of the sheet heater was changed by heating.

As a result, in any case, hot rolling can be performed without causing cracking of the steel sheet in the cooling zone, and red rust does not occur, and a steel sheet of good quality can be produced. According to the above, by setting the upper limit of the temperature rise amount at the trailing end of the sheet in advance, it is possible to stably produce a high-quality carbon steel having a good quality without causing a problem of quality such as occurrence of red rust.

Claims (1)

A hot rolling method for high carbon steel, which is obtained by heating a sheet of high carbon steel by a sheet heater installed at an inlet side of a finishing mill, and finishing by the above-mentioned finishing mill, and Heating by the sheet heater is performed in such a manner that the amount of temperature rise at the end of the sheet is equal to or less than a predetermined upper limit; here, the amount of temperature rise at the end of the sheet means that it is about to pass through the inlet side of the finishing mill. The temperature at the end of the sheet is heated by the sheet heater and the difference between the temperature when it is not heated by the sheet heater.