JP2601585B2 - Manufacturing method of controlled cooling steel sheet - 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 of manufacturing a controlled cooling steel sheet, and more particularly to a method for reducing a temperature variation within a sheet surface or between sheets after accelerated cooling of a thick plate.

[0002]

2. Description of the Related Art In hot rolling, a steel sheet is cooled after rolling. In a controlled cooling type steel sheet manufactured by applying controlled cooling, it is important to uniformly control the temperature of the steel sheet after cooling is stopped. It is a problem.

That is, a material deviation occurs due to a temperature deviation in the plate surface with respect to the target cooling stop temperature, and if the deviation exceeds an allowable range, the material becomes defective. Therefore, the heat treatment may be performed again offline.
However, performing the heat treatment again eliminates the merit of producing a high-quality steel plate at low cost and online, which the controlled cooling steel plate has. In addition, the variation in the temperature of the steel sheet when the cooling is stopped also causes a shape defect.

[0004]

Conventionally, as a method for equalizing the temperature of a steel sheet after cooling is stopped, a cooling control method in which a water amount crown is provided in a width direction, a shielding device for a steel sheet end, and an oblique nozzle are employed. Therefore, a method for achieving a uniform steel plate temperature distribution has been adopted.

[0005] However, when the cooling rate at the time of controlled cooling increases, a large temperature variation that cannot be solved by the uniform cooling technology developed and developed by the above-described cooling device itself may occur.

For example, even if dozens of steel sheets of the same size are manufactured under the same heating, rolling, and cooling conditions, the cooling stop temperature in the sheet surface of some of the steel sheets is higher than the target temperature. Since the variation exceeds the allowable range, there is a problem that a material defect or a shape defect occurs.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to reduce the temperature variation within the control cooling steel sheet after the cooling of the steel sheet is stopped. It is an object of the present invention to provide a method capable of stabilizing the above.

[0008]

Means for Solving the Problems The present inventors have conducted various research and investigations on the manufacturing conditions of the controlled cooling steel sheet in order to solve the above-mentioned problems. The scale peeling behavior during cooling is significantly different between the scale layer generated in the portion where the steel sheet temperature is higher than the Ac ₃ point and the scale layer generated in the portion in the temperature range of Ac _{1 to} Ac ₃ points. Therefore, it has been found that the scale thickness varies in the plane of the plate and that uneven cooling occurs, and the present invention has been completed based on such findings.

Accordingly, the present invention is, when manufacturing a steel sheet by applying controlled cooling after hot rolling finished, your Keru controlled cooling start temperature to a rolled sheet after finish rolling, skid during the heating before hot rolling after the temperature of the position corresponding to the section was controlled so as to be sure <br/> coercive than _c3 point a, it is an gist manufacturing method of controlled cooling the steel sheet, characterized in that initiating the controlled cooling .

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

First, the findings that led to the present invention will be described.

For the accelerated cooling steel sheet in which the temperature variation has occurred after the cooling stop, the normal portion (stop temperature 430 to 470)
℃) and high temperature part (stop temperature 600-620 ℃)

[Table 1] As shown in the figure, the scale thickness in the high temperature part is much smaller than the scale thickness in the normal part (20 to 30 μm).
Therefore, it was found that the influence of scale was extremely large as a cause of the occurrence of uneven cooling.

Further, the relationship between the scale and the cooling behavior was investigated in detail. In the experiment, a test piece of a round bar having a thermocouple attached at the center was heated and held in an air atmosphere, and then immersed in a water tank to examine a cooling behavior. The results of this experiment are shown in FIGS. In FIG. 3, stainless steel and carbon steel
C. and 700.degree. C., and immersed in a water bath to examine the cooling behavior. Heat holding time, the time from the start of cooling to the start of the actual temperature drop at the center of the test piece, that is,
When the cooling response time (τ) was examined, if the heating holding time was long, in other words, if the surface scale thickness of the test piece was large, the cooling response time was short. The relationship between the scale thickness and the cooling effect is shown in FIG. 4 in which the carbon steel is heated and held at 700 ° C. for 1 minute and 30 minutes, and then cooled. In the case of the response time (τ ₂ )), a gentle cooling curve is shown. In the case of the cooling holding time of 30 minutes (indicated by the cooling response time (τ ₁ )), a smooth cooling curve is obtained. Cooling is performed, and at a temperature 8 minutes after the start of cooling, the temperature difference (ΔT) between the two is about 100 ° C., and the scale thickness greatly contributes to the cooling effect, in other words, the scale thickness is a cause of uneven cooling. It will be understood that

Further, after the round bar test piece is heated and held at a constant temperature for various heating holding times by a hot working simulator,
Pressure was applied at a rolling reduction of 10% (see FIG. 5), and the scale peeling state on the side surface was observed in the manner shown in FIG. 6 to examine the scale peelability. As a result, as shown in FIG. 7, when the holding time is 3 minutes (see FIG. 7B), the peeling state is partially uneven in the phase transformation region (Ac _{1 to} Ac ₃ points). I have.

Summarizing the above findings, when controlled cooling is performed from the phase transformation region, the scale is partially peeled off, resulting in a difference in scale thickness within the plate surface. As a result, the region where the scale thickness is small in the plate surface has a higher cooling stop temperature than the region where the scale thickness is thicker due to the cooling behavior described above. That is,
It is considered that the cooling stop temperature varies from the scale thickness variation.

[0015] With the above findings, the present invention, finish your Keru controlled cooling start temperature on the rolled sheet after rolling, the temperature of the position corresponding to the skid part during the heating before hot rolling A _c3 or more points
The control cooling is started after the control is performed so as to ensure the above-mentioned condition, whereby it is possible to prevent the cooling unevenness after the stop of the cooling in the control cooling.

That is, the rolled sheet after finish rolling has a position indicating the highest temperature and a position indicating the minimum temperature, and the position indicating the minimum temperature is set to a temperature higher than the phase transformation region (Ac _{1 to} Ac ₃ points). Then, for the above-described reason, it is possible to prevent the occurrence of the variation in the cooling stop temperature due to the variation in the scale thickness. For this purpose, since the position showing the minimum temperature is a position corresponding to the skid portion at the time of heating before hot rolling, the temperature at this position is set to _three or more Ac.

Various methods can be used to secure the temperature at the position corresponding to the skid portion at _three points of Ac or more. For example, a method of shortening the rolling cycle (pitch) can be mentioned.

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

[0019]

EXAMPLE 1 A steel sheet obtained by hot rolling to a size of 50 t × 1900 w × 7000 l (mm) was subjected to a cooling density of 0.8 m ³ / min · m ² and a cooling start temperature of 7 with an accelerated cooling device.
00 to 900 ° C, and the target cooling stop temperature 450
The mixture was water-cooled to ℃.

The relationship between the uneven cooling and the minimum cooling start temperature in the sheet thickness direction (the cooling start temperature at the position corresponding to the skid) was examined. The cooling unevenness was determined based on the maximum temperature deviation from the average cooling stop temperature. As shown in FIG. 8, the results show that the phase transformation region (Ac ₁ to
It can be seen that the degree of unevenness in cooling is smaller when cooling is started from _three points or more than when the cooling is started from _three points (Ac).

[0021]

Example 2 A steel plate for building obtained by hot rolling to a size of 50 t × 2700 w × 6500 l (mm) was rolled at a water density of 0.8 m ³ / min · m ² by an accelerated cooling device. In Example 2, the cooling start temperature at the position corresponding to the skid portion at the time of heating was set to two cases: a case where the temperature was higher than Ac ₃ points and a case where the temperature was lower than Ac ₃ points.
Tensile test specimens were taken from a position corresponding to the skid at the time of heating of the obtained steel sheet, and the strength was examined. The result

[Table 2] Shown in

[0022] As can be seen from Table 2, as compared with the case where the cooling start from the following Ac ₃ point, in the present invention example was started cooled from above ₃ points Ac, the maximum temperature deviation after cooling stopped, 24 ° C. from 98 ° C. And the tensile strength satisfies the standard value for steel plates for construction.

[0023]

As described above, according to the present invention,
In the production of the controlled cooling steel sheet, particularly in the accelerated cooling of the thick plate, it is possible to reduce the temperature variation within the sheet surface or between the sheets after cooling, so that a controlled cooling steel sheet having a stable material and shape can be obtained. Further, it is economical because there is no need to perform heat treatment again off-line.

[Brief description of the drawings]

FIG. 1 is a photograph showing a microstructure (metal structure) of a surface layer portion in a normal portion of an accelerated cooling steel sheet, (a) is an upper surface,
(B) shows the case of the lower surface.

FIG. 2 is a photograph showing a microstructure (metal structure) of a surface layer portion in a high temperature portion of an accelerated cooling steel sheet, (a) is an upper surface,
(B) shows the case of the lower surface.

FIG. 3 is a diagram showing the effect of heating and holding time on cooling behavior (response time: time from the start of immersion to the start of cooling).

FIG. 4 is a diagram showing the influence of the heating and holding time on the cooling behavior (time from the start of immersion to the stop of cooling).

FIG. 5 is a diagram showing a heating temperature and a holding time in a hot working simulator.

FIG. 6 is a diagram for explaining a procedure for determining the releasability of the scale on the side surface.

7A and 7B are diagrams showing the influence of the heating temperature and the holding time on the scale peelability. FIG. 7A shows the case where the heating holding time is 0.5 minutes, and FIG. 7B shows the case where the heating holding time is 3 minutes. .

FIG. 8 is a diagram showing a relationship between a minimum longitudinal cooling start temperature and uneven cooling.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-57-152430 (JP, A) JP-A-58-61224 (JP, A) JP-B-47-30809 (JP, B2)

Claims (1)

(57) [Claims] Upon 1. A producing apply to steel plate controlled cooling after hot rolling completion, you Keru control the rolled sheet after finish rolling
The cooling start temperature, controlled so that the temperature of the position corresponding to the skid part during the heating before hot rolling is ensured over _c3 point A
The method of manufacturing a controlled cooling steel sheet, wherein the controlled cooling is started after the cooling.