JP2786759B2 - Rolling tip warpage control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of a hot-rolled steel sheet, and more particularly, to a method for controlling the warpage of a rolling tip in the production of a hot-rolled steel sheet.

[0002]

2. Description of the Related Art In the production of a hot-rolled steel sheet, as a method of controlling the warpage of the leading end of a roll, a conventional method of changing the peripheral speed of upper and lower rolls (various peripheral speed rolling) has been known. Method), a method of rolling while changing the pickup amount (pickup control method), a method of rolling by controlling the upper and lower surface temperatures by a cooling medium (asymmetric cooling method), etc., but a rolling pass earlier than the rolling pass of control start Is based on estimating the required control amount in the rolling pass from the warped shape generated in the above, and rolling.

[0003] However, since warpage is caused by various factors of asymmetric rolling in the vertical direction, the following problems occur and the control method is not satisfactory. (a) The controllable amount is small with respect to the generated warpage amount. (b) Since the necessary control amount cannot be estimated by reflecting all the factors of the asymmetric rolling, the estimation error causes warpage to remain at the tip of the plate after the rolling is completed.

[0004] In particular, in a controlled rolled steel sheet, FIG.
As shown in (1), a large warp occurs in the initial stage of the rolling pass, but in the rolling two passes before the final rolling pass, the warp rapidly changes and becomes small. This can be said to be a proof that it is difficult to control the conventional method.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a method capable of reliably controlling the warpage of a rolling tip in the production of a hot-rolled steel sheet.

[0006]

Means for Solving the Problems In order to solve the above problems, the present inventor has conducted a basic analysis on a method for controlling the warpage of a controlled rolled steel sheet, and has found an effective method. . That is, 0.09% C-0.3% Si-
The hot deformation resistance was measured by a hot working simulator in a rolling temperature range of 1.39% Mn steel (see Fig. 2), and the rigid plastic finite element method was used in consideration of the temperature distribution in the thickness direction of the rolled material. By performing deformation analysis and clarifying the warpage behavior in the rolling process, a control method that considers the influence of phase transformation was found, and the present invention was completed here.

That is, in the present invention, in the production of a hot-rolled steel sheet, the temperature of the steel sheet is controlled in a temperature range in which the temperature of the steel sheet is higher than the phase transformation temperature of the rolled material, so that the upper and lower surfaces are equalized, and thereafter, The gist of the present invention is a rolling tip warpage control method characterized by applying a warpage control method.

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

[0009]

[Action]

In a hot-rolled steel sheet, especially a control-rolled steel sheet, the temperature distribution is adjusted in the rolling line, so that the temperature distribution in the thickness direction of the rolled material is asymmetric, and as a result, the temperature of the upper and lower surfaces is different. Rolled in. In particular, under such a rolling condition, the upper surface is more cooled, so that warpage often occurs.

Therefore, when the hot-deformation resistance-temperature characteristic of the rolled material has a pattern as shown in FIG. 2, that is, when the transformation strain is developed in the phase transformation temperature region, the temperature of the upper and lower steel sheets is different. The tip warpage state was calculated by deformation analysis by rigid plastic finite element method. FIG. 3 shows an example of a warped state when the temperature of the steel sheet on the upper surface is lower than that of the lower surface by 50 ° C. in the rolling in the phase transformation temperature range.

The warping state in the case of the temperature characteristic of FIG. 3 (b) affected by the phase transformation as compared with the case of the temperature characteristic (austenitic single phase) of FIG. It changes greatly, and the amount of warpage at the end of rolling is small. This situation is very similar to the behavior of the tip warpage of the control rolled steel sheet in the actual rolling line, which means that the phase transformation of the rolled material is greatly related to the behavior of the tip warpage of the control rolled steel sheet. It is shown.

Based on this finding, the effect of warpage control was examined for the asymmetric cooling method and the different peripheral speed rolling method. For the analysis, the deformation analysis by the rigid-plastic finite element method was used as described above.

<Asymmetric Cooling Method (= Lower Surface Cooling Method)> This method is a method of controlling upward warpage by forcibly cooling the lower surface of the front end portion of a steel sheet immediately before rolling and biting. FIG. 4 shows that the temperature of the steel sheet on the lower surface immediately before the biting of the rolling is 50 ° C.
The relationship between the warpage curvature after one-pass rolling when the lower surface is forcibly cooled so as to be lowered and the average temperature of the steel sheet immediately before biting is obtained. The warpage curvature is greatly affected by the phase transformation, and exhibits a complicated nonlinear change with respect to the steel sheet average temperature immediately before biting.

That is, since the temperature distribution in the sheet thickness direction is asymmetric, the distribution of hot deformation resistance in the sheet thickness direction at the time of rolling bite becomes an asymmetric distribution as illustrated in FIG. Have increased hot deformation resistance. However, in the transformation temperature range, since the lower surface side where the steel sheet temperature is low starts transformation first, conversely, the hot deformation resistance on the low temperature side becomes small, and the warpage is eased or warped. Become.

In other words, the warpage curvature is considered to be the amount of warpage control at each rolling reduction (10%, 20%) in the present method. In the control using only the present method, the hot deformation resistance-temperature characteristic during transformation is obtained. It is indispensable to have a high-precision warpage control model that accurately reflects the warpage, but this may cause the sensitivity to errors due to insufficient understanding of the causes of warpage, sensor errors, etc. .

From this fact, in order to apply the present method to a controlled rolled steel sheet, it is necessary to eliminate in the austenite area the asymmetry of the temperature distribution in the thickness direction, which produces complicated nonlinearity in the phase transformation temperature area. New knowledge that it is indispensable for the control.

<Case of Different Peripheral Speed Rolling> This method is a method of controlling the warpage of the leading end of a rolled material by changing the number of rotations of the upper and lower rolls. FIG. 5 shows the state of warpage when the temperature of the steel plate on the upper surface is lower by 50 ° C. than the lower surface and the internal structure of the steel plate is a single phase (austenite) in the rolling process. The warpage at this time is upward warpage, and increases as the steel sheet temperature decreases. However, when the hot-deformation resistance-temperature characteristics of the rolled material are softened in the transformation temperature range under the influence of transformation strain as shown in FIG. 2, the rotation of the upper roll under the same rolling conditions as in FIG. The warping behavior when the number is 10% larger than that of the lower roll exhibits a complicated nonlinear change as shown in FIG.

This is due to the phase transformation, and therefore, similarly to the case of the lower surface cooling method, means that it is very difficult to control the warpage of the controlled rolled steel sheet by this method alone.

From this fact, as in the case of the asymmetric cooling method described above, in order to apply the present method to a controlled rolled steel sheet, the asymmetry of the temperature distribution in the sheet thickness direction that generates complicated nonlinearity in the phase transformation temperature region is required. We have gained a great knowledge that it is essential for stable control to be eliminated in the austenite range.

The present invention has been completed as a result of the above research and analysis. Specifically, when applying a mechanical warpage control method such as an asymmetric cooling method, a different peripheral speed rolling method, and a pickup control method, particularly, when controlling the tip warpage of a controlled rolled steel sheet, a complicated non-linearity in a phase transformation temperature region is required. The asymmetrical temperature distribution in the sheet thickness direction, which causes the behavior of linear warpage change, is eliminated in the austenite range, and then the warpage control in the phase transformation temperature range is controlled by different peripheral speed rolling methods, pickup control methods, etc. The target warpage control method is applied.

Means for eliminating the asymmetry of the temperature distribution in the plate thickness direction in the austenite region may be the following method, and usually water cooling is used. This equalizes the upper and lower surface temperatures. Ideally, the upper and lower surface temperature difference is 0 ° C.
It is sufficient if it is in a substantially non-existent state, and up to about 5 ° C. is acceptable.

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

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to confirm the effectiveness of the present invention, YP
An experiment for controlling warpage was performed on a 490 N / mm ² class steel sheet on an actual rolling line. Rolling pass schedule is 115 (mm) → 98 → 79 → 6
9 → 56 → 48 → 39 → 31 → 28 (mm) (however, only the top part), and the rolling temperature (entrance side, upper surface) at this time was 8
40 (℃) → 835 → 825 → 810 → 800 → 790 →
770 → 740 (° C.).

FIG. 7 shows the curvature of the tip of the rolled material in each pass when the rolling is performed without controlling the warpage, and it can be seen that the direction of the warp is all upward.

Next, in the rolled material rolled under the same rolling conditions, the upper and lower surface temperatures of the steel sheet are measured between rolling passes in which the rolling temperature range is in the austenitic range, that is, after the plate thickness is rolled from 115 mm to 98 mm. Then, the surface on the high temperature side was cooled by a cooling device installed on the entrance side of the rolling mill so as to eliminate the temperature difference.

Specifically, the lower surface temperature of the steel sheet was actually measured using a rear surface thermometer installed so as to measure the back surface temperature of the steel sheet at a position corresponding to the measurement position on the upper surface of the steel sheet. At this time, the lower surface temperature in a range of 1500 mm from the leading end of the rolled material was higher than the upper surface temperature, and the temperature difference was 50 ° C. Therefore, the lower surface of the steel sheet was cooled by applying a cooling device installed on the inlet side of the rolling mill so that the lower surface temperature of the steel sheet was the same as the upper surface.

The warping state in the rolling pass after application of the lower surface cooling is as shown in FIG. 8, and the warpage at the tip is significantly reduced. However, warpage due to other asymmetric rolling factors remains as before.

In order to roll this warp at a different peripheral speed based on the number of rotations of the upper and lower rolls in the final rolling pass calculated from the warped shape of the tip several passes before the final rolling pass, a rolled material having the same component is further used. Then, an experiment was performed under the same rolling conditions as those of the two rolled materials. At this time, the temperature difference between the upper and lower surfaces in a region of 1500 mm from the tip is 36 ° C., and the lower surface has a higher temperature. In order to eliminate this temperature difference, the water density: 0.7 m ³
/ M ² · min, lower surface cooling time: 2.7 seconds, the lower surface was cooled under water cooling conditions.

As a result, the warpage was greatly reduced as in the above-mentioned second experiment, and the tip warpage two passes before the final rolling pass was as small as 55 mm. The difference in the number of revolutions of the upper limit roll of the final rolling pass calculated from this warpage is a difference given when the number of revolutions of the upper roll is increased by 5.5% from that of the lower roll. The warpage at the time of rolling was as extremely small as 30 mm or less.

From these experiments, it was confirmed that the present invention effectively functions also for a controlled rolled steel sheet manufactured in a rolling temperature range including a phase transformation temperature.

[0032]

As described in detail above, according to the present invention,
Before applying the mechanical warpage control method in the production of hot-rolled steel sheets, the asymmetry of the temperature distribution in the thickness direction, which creates a complicated nonlinear warpage change behavior in the phase transformation temperature range, is eliminated in the austenite range. Therefore, tip warpage can be effectively prevented. It is particularly effective for the production of controlled rolled steel sheets.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram showing a warped end state of a controlled rolled steel sheet in a rolling process, and shows a relationship between an incoming side sheet thickness and a warpage amount.

FIG. 2 is a diagram showing a relationship between hot deformation resistance and temperature characteristics.

FIG. 3 is a view showing the effect of phase transformation on tip warpage during rolling in the phase transformation temperature range, and shows the relationship between the thickness of the entry side plate and the curvature of warpage.

FIG. 4 is a diagram showing the effect of phase transformation on tip warpage when the lower surface cooling method is applied, and shows the relationship between the average temperature in the sheet thickness direction immediately before rolling and the curvature of warpage.

FIG. 5 is a diagram showing the effect of phase transformation on tip warpage when the different peripheral speed rolling method is applied, and shows the relationship between the average temperature in the sheet thickness direction immediately before rolling and the curvature of warpage.

FIG. 6 is a diagram showing a tip warpage state in a temperature region including a phase transformation, and shows a relationship between an average temperature in a sheet thickness direction immediately before rolling and a curvature of warpage.

FIG. 7 is a diagram showing a state of warpage at the top in the longitudinal direction of the rolled material in a case where warpage control is not applied, and shows a relationship between the entry side plate thickness and the amount of warpage.

FIG. 8 is a view showing a state of warpage at the top of the rolled material in the longitudinal direction when warpage control is applied, and shows the relationship between the entry side plate thickness and the amount of warpage.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) B21B 37/28 B21B 37/44 B21B 45/02

Claims (1)

(57) [Claims] In the production of a hot-rolled steel sheet, by controlling the steel sheet temperature in a temperature range in which the steel sheet temperature is higher than the phase transformation temperature of the rolled material, the upper and lower surface temperatures are made equal, and then a mechanical warpage control method is provided. A rolling tip warpage control method characterized by being applied.