JP3074372B2 - Method of manufacturing thick steel plate with excellent toughness - 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 producing a thick steel plate having excellent toughness.

[0002]

2. Description of the Related Art Controlled rolling has been widely used as a method for producing steel sheets having excellent toughness at low temperatures. When this controlled rolling method is used, the rolling temperature during rolling is accurately controlled, and the final rolling finish temperature is set immediately above the Ar ₃ temperature, whereby the microstructure can be refined and the toughness can be significantly improved. .

In the conventional controlled rolling method, as shown in FIG. 1, the rolling operation is temporarily stopped at a transfer thickness of a predetermined thickness with respect to a product thickness t, and thereafter, the target is reduced from the temperature T ₁ at that time by air cooling. After cooling to a predetermined temperature T ₂ , the rolling was restarted, and the finishing temperature at the time of final rolling when the product thickness t was reached was controlled to be just above the Ar ₃ temperature.

[0004]

The conventional controlled rolling, in which a long temperature waiting is performed during the rolling in order to improve the toughness described above, is different from the normal rolling method, and the rolling operation is performed once during the rolling. Stopped, and the temperature of the rolled material changed from T ₁ to T ₂
, The rolling operation is discontinuous and the rolling efficiency is reduced.

In order to reduce the rolling efficiency and improve the toughness at a low temperature in the controlled rolling method, it is effective to temporarily stop the rolling in a specific pass during the rolling and to intensively perform water cooling on the steel material. is there. However, when the steel material before finish rolling is cooled to a specific thickness and width, the steel material thickness during cooling may be extremely large depending on the product sheet thickness. It is necessary to optimize the cooling conditions of the upper and lower surfaces, that is, the ratio of the amount of water and water.

According to the present invention, in order to greatly improve the conventional control rolling method having a very low rolling efficiency, the steel material before the finish rolling is cooled, and the subsequent finish rolling is smoothly performed, and the toughness is improved. An object of the present invention is to provide a method for manufacturing a thick steel plate.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem of producing a thick steel plate having improved toughness with high productivity. Finished at 850 ° C or higher
The above steel material is cooled, and the finish rolling end temperature is set to 720 to 85.
In the step of performing finish rolling at 0 ° C., upon cooling, the upper surface of the steel material is cooled by laminar flow and the lower surface is cooled by spraying, and the water content ratio (lower / upper) of the upper and lower surfaces is determined according to the steel thickness and the steel width [ Steel thickness (mm) / 30 + {Steel width (mm) -2000 (mm)} /
240] ± 2.4, and cooling the steel plate having excellent toughness.

[0008]

When the steel material before the finish rolling is cooled, the average cooling rate in the thickness direction is increased as shown in FIG. 2, and the high-temperature residence time is greatly reduced. Therefore, a steel sheet having a product thickness of 22 mm was trial-produced using the steel shown in A of Table 1 described below, under the same conditions except for the presence or absence of cooling before finish rolling, and the toughness was evaluated. The results are shown in FIG. 3, and it was found that the toughness was improved by about −30 ° C. in vTrs, despite the same manufacturing conditions except for cooling during the course. This is considered to be due to the micronization of the structure before transformation due to the shortening of the high-temperature residence time.

On the other hand, in a controlled rolled material manufactured by such a method, when the steel material before the finish rolling is cooled, the thickness of the steel material is increased depending on the condition if the product thickness is increased.
The temperature gradient in the thickness direction of the steel material during cooling becomes large, and the surface temperature of the steel slab becomes a low temperature region below the transition boiling region. As a result, the cooling capacity of the on-board water is increased, and the water-to-water ratio (lower / upper) at the time of cooling is increased to compensate for this. The tendency increases as the thickness and width of the steel material during cooling increases.

Therefore, in order to examine appropriate cooling conditions in the case of manufacturing by cooling in the middle of rolling, the ratio of water and water is varied with respect to various steel thicknesses and widths of steel, and the ratio of water during finish rolling is changed. The rolling situation was investigated. FIG. 4 shows the results. Under conditions where the water / water ratio was too large, rolling was impossible and rolling was impossible. On the other hand, under the condition that the water / water ratio was too small, the warpage was reversed.

As discussed above, under various conditions, the water / water ratio is determined according to the steel thickness and the steel width at the time of cooling [steel thickness (mm) / 30 + ｛steel width (mm) -2000 (mm). ｝ /
240] ± 2.4, it has been found that a rolling pass immediately thereafter can be made possible, and that there is an excellent effect that warpage can be reliably prevented.

The hot rough rolling temperature is set to 850 ° C. or higher because it is assumed that rolling is performed in a temperature range in which austenite is recrystallized. In addition, the finish rolling end temperature is set in a range of 720 to 850 ° C. because rolling in an unrecrystallized region is essential for improving toughness.

[0013]

EXAMPLES Examples of the present invention are shown below together with comparative examples. As the components of the test steel, the components used as typical structural steels are used, and the chemical components of the steel used in this example are shown in Table 1. Tables 2 and 3 show the production conditions, rolling conditions and the obtained toughness.
(Continued in Table 2-1) and Table 4 (Continued in Table 2-2).

[0014]

[Table 1]

[0015]

[Table 2]

[0016]

[Table 3]

[0017]

[Table 4]

Steel Nos. 1 and 5 of the present invention shown in Tables 2 to 4
In Nos. 7, 11, 13, and 19, the temperature waiting time at the transfer thickness is greatly reduced and the toughness is also greatly improved as compared with the conventional controlled rolled material which does not perform cooling during the same conditions. Further, the ratio of the upper and lower water amounts during cooling during the cooling was also controlled to an appropriate value, and the shape during rolling was good. On the other hand, each of the comparative examples has a problem, and steel numbers 2, 4, 6, 8, 10, 12, 14, 16, and 18 for which cooling is not performed in the middle.
In the case of the present invention, the temperature waiting time at the transfer thickness was longer and the productivity and toughness were both deteriorated, as compared with the present invention example under the same conditions where cooling was performed halfway.

Steel No. 9, which had a large water / water ratio during the cooling during the course, was warped downward and could not be rolled. Steel numbers 3, 15, and 17 whose water / water ratio was smaller than the appropriate value
Warpage was greatly increased, warpage could not be corrected, and rolling was impossible.

[0020]

As described above, in order to manufacture a thick steel plate having excellent low-temperature toughness, when performing intermediate cooling at the transfer thickness, the steel slab before finish rolling is performed based on the cooling method of the present invention. In the case of cooling, the warpage in the subsequent finish rolling can be reliably prevented by greatly changing the water ratio in accordance with the slab width. By applying the present invention, a thick steel plate having excellent toughness can be manufactured with high productivity without having to wait for unnecessary temperatures, and the effect in this field is extremely large.

[Brief description of the drawings]

FIG. 1 is a diagram showing a temperature transition of a steel sheet during a temperature waiting before finish rolling.

FIG. 2 is a diagram showing a temperature transition of a steel sheet during rolling.

FIG. 3 is a view showing an effect of improving toughness by cooling in the middle.

FIG. 4 is a diagram showing a relationship between a steel thickness, a steel width, and a water / water ratio at the time of intermediate cooling.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) B21B 1/38 B21B 45/02

Claims (1)

(57) [Claims] 1. A steel material having a thickness of 40 mm or more, which has been subjected to hot rough rolling at 850 ° C. or more, is cooled, and a finish rolling end temperature is set to 7
In the step of performing finish rolling at 20 to 850 ° C., upon cooling, the upper surface of the steel material is cooled by laminar flow, and the lower surface is cooled by spraying. T [Steel thickness (mm) / 30 + {Steel width (mm) -2000 (mm)} /
240] A method for producing a thick steel plate having excellent toughness, characterized in that the steel plate is cooled in a range of ± 2.4.