JP2786772B2 - Tip Warpage Control Method in Hot Rolling - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling tip warpage in hot rolling, and more particularly, to a method for controlling tip warpage by a method of controlling a temperature difference between upper and lower surfaces.

[0002]

2. Description of the Related Art The tip warpage generated at the leading end of a rolled material in hot rolling is mainly caused by the fact that the temperature of the upper surface of the rolled material is lower due to descaling water or the like in the rolling process. Is known to be caused by a temperature deviation in the plate thickness direction caused by lowering the temperature than the temperature.

Conventionally, one of the methods of controlling such tip warpage is described below.
One method is to control the temperature difference between the upper and lower surfaces of the rolled material by cooling the lower surface of the leading end of the rolled material during rolling.

In this conventional method, a means for controlling the temperature difference between the upper and lower surfaces of the control path from the warp shape before the even-numbered path of the control path is employed. That is, when the upper surface temperature is low, or when the tip warpage before the even-numbered pass of the control path is the upward warpage, the lower surface is cooled so that the upper and lower surface temperatures become the same.

However, in this conventional method, the effect cannot always be exerted due to the transformation plasticity, and instead, the warping of the control path may be promoted and increased.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a method for effectively controlling the tip warpage in a tip warpage control method for cooling the lower surface of the leading end of a rolled material during hot rolling. It is assumed that.

[0007]

In order to solve the above-mentioned problems, the present inventor studied the cause of tip warping even when there is no difference between the upper and lower surfaces in the conventional warpage control method, and further solved the problem. As a result of intensive studies on the present invention, the present invention has been completed here.

That is, the present invention provides a water-cooling device for water-cooling the lower surface of a rolled material on the rolling mill entry side, means for calculating the temperature at each position in the plate thickness direction of the rolled material, In the hot rolling line having the means for determining the lower surface temperature, when performing the tip warpage control for cooling the lower surface of the front end portion of the rolled material at the time of rolling, the lower surface temperature range at the time of performing the lower surface cooling is different from that at the time of performing the lower surface cooling. in a temperature range where the lower surface temperature of the rolled material tip becomes more Ac ₃ point of the rolled material during rolling after heat and gist tip warpage control method in hot rolling, which comprises applying a lower surface cooling Things.

Hereinafter, the present invention will be described in more detail.

[0010]

[Action]

The present inventor has found an effective method by conducting various studies and investigations on the relationship between the phase transformation of a rolled material and the warpage of the tip. The following describes the results of basic experiments carried out for that purpose. explain.

FIG. 1 shows the hot deformation resistance of a 0.09% C-0.3% Si-1.39% Mn steel in the rolling temperature range measured by a hot working simulator. When the hot deformation resistance of the rolled material exhibits such temperature characteristics, that is, when it exhibits deformation plasticity in the phase transformation temperature range,
The state of tip warpage when lower surface cooling was performed on rolled materials having different upper and lower surface temperatures in each rolling temperature range was calculated by analysis using rigid-plastic finite elements. An example is shown in FIG.

FIG. 2 shows that the rolled material whose lower surface temperature is 50 ° C. higher than the upper surface temperature in each rolling temperature range is subjected to lower surface cooling so that the difference in upper and lower surface temperatures during rolling is eliminated, ie, in the thickness direction. Is obtained by calculating the curvature of tip warpage when one-pass rolling is performed so that the temperature distribution is symmetrical with respect to the center of the sheet thickness, and the curvature when rolling without lower surface cooling under the same conditions for comparison. It is. In FIG. 2, the warpage curvature when the lower surface is cooled greatly changes with respect to the rolling temperature under the influence of the phase transformation. It turns out that it is presenting. The cause is as follows.

That is, when the lower surface cooling is stopped, the lower surface temperature does not become lower than Ar ₃ points of the rolled material, and when the lower surface cooling is stopped, the lower surface temperature becomes lower than Ar ₃ points. In the temperature range where the temperature of the rolled material is ₃ points or more, the rolled material is a single phase of γ (austenite), and when there is no temperature difference between the upper and lower surfaces, the distribution of hot deformation resistance in the thickness direction is shown in FIG. As shown in FIG. 2, since it is symmetric with respect to the plate thickness direction, warpage is prevented.

On the other hand, in a temperature range where the lower surface temperature is lower than the Ar ₃ point of the rolled material when the lower surface cooling is stopped and the lower surface temperature does not become higher than the Ac ₃ point due to reheating from the lower surface cooling to the rolling, the rolled material is In the vicinity of the lower surface, a region that is not reversely transformed, that is, an α (ferrite) phase region remains, and the hot deformation resistance of the α phase is obtained in that region. In such a case, under the same temperature, the hot deformation resistance of the γ phase is larger than the hot deformation resistance of the α phase, so even if the upper and lower surface temperatures are the same, the hot deformation resistance near the lower surface is the γ phase. Since it becomes smaller than the upper surface and the hot deformation resistance in the thickness direction has an asymmetric distribution as illustrated in FIG. 4, an upward warpage occurs.

As described above, the occurrence of the warpage when the lower surface cooling is applied becomes more remarkable in the steel type in which the hot deformation resistance is greatly reduced due to the transformation as illustrated in FIG. On the contrary, the cooling promotes the warpage and increases it.

From the results of the above basic experiments, the lower surface cooling
It has been found that it is indispensable to perform the process in a temperature range in which the lower surface temperature of the rolled material at the time of rolling after the lower surface cooling is equal to or higher than the Ac _three points of the rolled material.

According to the present invention, the lower surface is cooled under the above-mentioned conditions. To this end, at least a water cooling device for water-cooling the lower surface of the rolled material at the entry side of the rolling mill and each position in the thickness direction of the rolled material are provided. It is needless to say that it is necessary to have a means for calculating the temperature in the above and a means for calculating the amount of decrease in the lower surface temperature due to the lower surface cooling.

Here, the water cooling device for water-cooling the lower surface of the rolled material and the means for calculating the temperature in the sheet thickness direction of the rolled material may be the same as those in the related art.

The means for determining the lowering amount of the lower surface temperature due to the lower surface cooling is not particularly limited.

The amount of temperature rise at which the heat is recovered after the lower surface cooling is stopped is calculated by calculating the heat transfer using a difference method or the like, or by calculating the lower surface temperature at the time of lower surface cooling and the lower surface temperature at the time of reheating after rolling under various cooling conditions. The relationship can be easily predicted based on data obtained by experiments.

Next, an embodiment of the present invention will be described.

[0023]

EXAMPLES In order to confirm the effectiveness of the present invention, SM52 was used.
A warpage control experiment was performed using steel 0. The Ar ₃ point determined by the chemical composition of this steel type is 752 ° C., and the Ac ₃ point is 840. Experiments were performed on three conditions, that is, when the lower surface cooling was not applied as Comparative Example 1, when the conventional method was applied as Comparative Example 2, and when the present invention was applied. In each case, the heating conditions and the rolling pass schedule (

[Table 1] ) Are almost the same.

The experiment was conducted using a reverse rolling mill. In order to control the top end in the longitudinal direction of the rolled material, even-numbered passes (No. 2, No. 4, No. 6) shown in Table 1 were performed. ,
No. 8 and No. 10) were considered only.

[0025]

Comparative Example 1 FIG. 6 shows the amount of warpage of the tip warpage generated in each even-numbered pass when lower surface cooling is not applied, and the rolling temperature in each even-numbered pass (the lower surface temperature of the rolled material).
Are: No. 2: 910 ° C., No. 4: 890 ° C., No. 6: 8
65 ° C., No. 8: 840 ° C., No. 10: 810 ° C. In the case of this example, the amount of warpage generated in the last tenth pass is 2
00 mm.

[0026]

Comparative Example 2 FIG. 7 shows the amount of warpage of the tip warpage generated in each even-numbered pass when the lower surface cooling according to the conventional method is applied.

That is, judging from the warping state of No. 6 pass, the lower surface cooling was applied in No. 8 pass, and the temperature difference between the upper and lower surfaces of the rolled material was measured. As a result, 53 ° C. (upper surface temperature: 845 ° C.) , Lower surface temperature: 792 ° C). Therefore, in order to eliminate the temperature difference between the upper and lower surfaces, the lower surface of the leading end of the rolled material was water-cooled for 3.8 seconds at a water density of 0.7 m ³ / m ² · min. The lower surface temperature when the lower surface cooling was stopped at that time was 685 ° C (<
Ar ₃ point), a lower surface temperature at the time of rolling (No.10 path) after heat recuperation was 795 ℃ (<Ac ₃ point). As a result, No. 8
In the rolling of the pass, the warpage is 570 mm, and the final No.
On the 10th pass, a warpage of 500 mm occurred.

[0028]

FIG. 8 shows the amount of warpage of the tip warpage generated in each even-numbered pass when the present invention is applied.

That is, as a result of measuring the temperature difference between the upper and lower surfaces of the rolled material in the No. 2 pass, 51 ° C. (upper surface temperature: 850)
° C, lower surface temperature: 901 ° C). After the lower surface cooling necessary to eliminate this temperature difference was performed, the lower surface temperature at which the Ar ₃ point or less became lower than the Ar ₃ point when the lower surface cooling was stopped was reduced to Ac ₃ during rolling after reheating.
Only the No. 2 pass can secure a rolling temperature higher than the point.

Therefore, in order to eliminate the temperature difference between the upper and lower surfaces at 51 ° C. in the No. 2 pass, the lower surface of the leading end of the rolled material was water-cooled at a water density of 0.7 m ³ / m ² · min for 4.0 seconds. The lower surface temperature at the time of stopping the lower surface cooling at that time is 739 ° C (<Ar ₃ points), and the lower surface temperature during rolling after reheating (No. 4 pass) is 853 ° C.
(> Ac ₃ points).

As a result, the amount of warpage after the No. 2 pass is smaller than in the case of Comparative Example 1 and Comparative Example 2, and the amount of warp in the final No. 10 pass is 40.
mm and very small.

[0032]

As described in detail above, according to the present invention,
In the tip warpage control method for cooling the lower surface of the front end of the rolled material during hot rolling, the tip warpage can be effectively controlled even when there is no difference between the upper and lower surfaces.

[Brief description of the drawings]

FIG. 1 is a diagram showing temperature characteristics of hot deformation resistance.

FIG. 2 is a diagram showing the relationship between the upper surface temperature of a rolled material and the warpage curvature.

FIG. 3 is a diagram showing a hot deformation resistance distribution in a plate thickness direction.

FIG. 4 is a diagram showing a hot deformation resistance distribution in a thickness direction.

FIG. 5 is a diagram showing temperature characteristics of hot deformation resistance.

FIG. 6 is a diagram showing the amount of warpage in each even-numbered pass in Comparative Example 1.

FIG. 7 is a diagram showing the amount of warpage in each even-numbered pass in Comparative Example 2.

FIG. 8 is a diagram showing the amount of warpage in each even-numbered pass in the example of the present invention.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-51-74959 (JP, A) JP-A-1-241314 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B21B 37/28 B21B 37/44 B21B 45/02

Claims (1)

(57) [Claims] 1. A water-cooling device for water-cooling the lower surface of a rolled material on the entry side of a rolling mill, means for calculating a temperature at each position in the thickness direction of the rolled material, and means for calculating a lowering amount of the lower surface temperature due to the lower surface cooling. In the hot rolling line having, when performing the tip warpage control to cool the lower surface of the rolled material tip portion at the time of rolling, the lower surface temperature range when performing lower surface cooling, the rolling after recuperation when performing lower surface cooling A method for controlling tip warpage in hot rolling, wherein lower surface cooling is applied in a temperature range in which the lower surface temperature of the leading end of the rolled material at the time is equal to or higher than the Ac _three points of the rolled material.