JPH08225835A - Rolling of thick steel plate having hihg toughness - Google Patents

Abstract

PURPOSE: To provide a rolling condition at the start of rerolling in order to prevent camber at finish rolling when a thick steel plate excellent in low temp. toughness is produced in high productivity. CONSTITUTION: For the steel which completes hot rough rolling at >=850 deg.C and has >=40mm plate thickness before finish rolling, the cooling, in which upper/rear water quantity ratio is set taking into account of the temp. difference in upper/rear surface before cooling start, is executed so that the condition of upper/rear surface at the start of finish rolling is turned to (upper surface average temp. >= rear surface average temp.). Further, the rolling reduction from a first pass to third pass at the start of finish rolling is set to <=10mm, the finish rolling is completed so that a completing temp. of finish rolling is set in a range of 700-850 deg..

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolling method for producing a thick steel sheet having excellent toughness with high productivity.

[0002]

2. Description of the Related Art Controlled rolling has been widely used as a method for producing a steel sheet having excellent toughness at low temperatures. When this controlled rolling method is used, the rolling temperature during rolling can be precisely controlled, and the final rolling finish temperature can be set directly above the Ar ₃ temperature to make the microstructure finer and to improve the toughness significantly.

In the conventional controlled rolling method, as shown in FIG. 1, the rolling work is temporarily stopped at a transfer thickness of a predetermined thickness with respect to the product sheet thickness t, and then air cooling is performed to set a target temperature T ₁ at that time. After cooling to a predetermined temperature T ₂ , rolling was restarted and the finish temperature at the time of final rolling when the product plate thickness t was adjusted according to the target toughness level.

Unlike the conventional rolling method, the conventional controlled rolling in which a long temperature waiting is performed in the middle of rolling to improve the toughness described above is different from the ordinary rolling method. Since the temperature is cooled from T ₁ to T ₂ , the rolling operation becomes discontinuous and the rolling efficiency is lowered.

[0005]

In order to reduce the rolling efficiency and improve the toughness at low temperature in such a conventional controlled rolling method, the rolling is temporarily stopped in a specific pass during rolling,
It is effective to intensively perform water cooling on the steel material. However, in the case of manufacturing by such a method, in order to smoothly perform the finish rolling thereafter, it is necessary to optimize the temperature difference between the upper and lower surfaces of the steel material before the finish rolling and the rolling conditions during the finish rolling. is there.

[0006]

The present invention has been made to solve the above problems, and its gist is to: (1) finish rolling in which hot rough rolling is completed at 850 ° C. or higher. For steel materials with a plate thickness of 40 mm or more, the ratio of upper and lower surfaces before and after cooling is considered so that the condition of the upper and lower surface temperature difference at the start of finish rolling is upper surface average temperature ≧ lower surface average temperature. The cooling amount is set to 10 mm or less from the first pass to the third pass when the finish rolling is restarted, and the finish rolling is finished so that the finish rolling end temperature is in the range of 700 to 850 ° C. The method for rolling a thick steel sheet having high toughness is characterized in that Further, following the treatment of (1), after completion of rolling, the cooling rate is continuously 60 ° C./s or less,
A rolling method for a thick steel sheet having high toughness is characterized by cooling to a temperature of 650 ° C. or lower, and then tempering at an Ac ₁ transformation point or lower.

[0007]

When the thick steel plate material before finish rolling is cooled, the average cooling rate in the plate thickness direction is increased, and the high temperature residence time is greatly shortened. Then, the present inventors made a steel plate having a product plate thickness of 22 mm as a trial using the steel shown in A of Table 1 under the same conditions except for the presence or absence of cooling before finish rolling, and evaluated the toughness. The result is shown in FIG.

FIG. 2 shows the slag thickness 15 for steel A in Table 1.
0 mm, heating temperature 1150 ° C., transfer thickness 44 mm, rolling finishing temperature 805 ° C., thick steel plate (1) of the conventional example that was not cooled during rolling, and example of the present invention that was cooled during rolling under the same conditions It showed about (2). It can be seen that the toughness is improved by about −30 ° C. in vTrs, although the manufacturing conditions are the same except for the cooling during the process. This is considered to be due to the refinement of the structure before transformation due to the reduction of the high temperature residence time.

On the other hand, in the new controlled rolled material for producing a steel sheet by the method of performing cooling during the rolling and performing the rolling again in this way, it is necessary to secure the shape of the steel material at the time of finish rolling and to ensure the stable production of the steel material. Is important for. Therefore, in order to examine the appropriate pre-rolling conditions when manufacturing by the method of cooling during rolling and then rolling, the temperature difference between the upper and lower surfaces was changed for various steel thicknesses and widths. Then, the state of the shape of the rolling during the finish rolling was investigated. The result is shown in Figure 3.
Shown in Under the condition that the average temperature of the upper surface ≥ the average temperature of the lower surface, the warpage occurred and rolling was impossible. The upper and lower temperatures and the lower surface temperature are average values obtained by simultaneously measuring the entire steel sheet before finish rolling with a thermotracer.

As discussed above under various conditions, the average temperature of the upper and lower surfaces before finish rolling is set to the average temperature of the upper surface ≧ the average temperature of the lower surface, and the reduction from the first pass to the third pass when restarting the finish rolling is performed. It has been known that the thickness of 10 mm or less has an excellent effect that the rolling pass immediately after that is possible and the warp can be surely prevented. If the thickness of the sheet to be cooled before finish rolling is 40 mm or less, the temperature waiting time during controlled rolling will inevitably become short, so that it is possible to maintain sufficient productivity without performing cooling during rolling. For this reason, the plate thickness to be cooled before finish rolling was set to 40 mm or more. Further, the hot rough rolling temperature is set to 850 ° C. or higher because it is premised that rolling is performed in a temperature range where austenite is recrystallized. Further, the finish rolling finish temperature is set to a range of 720 to 850 ° C. because rolling in a non-recrystallized region is essential to improve toughness.

[0011]

EXAMPLES Examples of the present invention are shown below together with comparative examples. The chemical composition of the steel used in this example is shown in Table 1 as the composition of the test steel using the composition as a typical structural steel.

[0012]

[Table 1]

Table 2 shows the manufacturing conditions, rolling conditions and the obtained toughness. Steel Nos. 1, 5, 7, 1 of Examples shown in Table 2
Nos. 1, 13, and 19 have greatly shortened the temperature waiting time at the transfer thickness and the toughness, as compared with the conventional controlled rolled material which does not perform the intermediate cooling under the same conditions. Further, the temperature difference between the upper and lower surfaces before the start of finish rolling and the amount of reduction from the first pass to the third pass during finish rolling are also within proper ranges,
The shape during rolling was also good.

[0014]

[Table 2]

[0015]

[Table 3]

On the other hand, the comparative examples have their respective problems, and steel Nos. 2, 4, 6, 8, 10,
Nos. 16, 18 and 20 had a long temperature waiting time at the transfer thickness, and both productivity and toughness were deteriorated, as compared with Examples under the same conditions in which cooling was performed halfway. Further, although the intermediate cooling was carried out, Steel Nos. 3, 9, 15 and 17 in which the average temperature difference between the upper and lower surfaces before the start of finish rolling became less than 0 ° C. significantly warped,
Warp correction was also impossible and rolling was impossible, and the temperature difference between the upper and lower surfaces satisfied the predetermined conditions, but the amount of reduction from the first pass to the third pass at the start of finish rolling did not fall below 10 mm 12,14. Was also unrollable.

[0017]

As described above, in order to produce a thick steel sheet having excellent low temperature toughness, cooling is performed midway at the transfer thickness, and when finishing rolling is performed, the finishing method is performed based on the cooling method of the present invention. By properly controlling the temperature difference between the upper and lower surfaces before rolling, it is possible to reliably prevent warpage in the subsequent finish rolling. By using this method, a thick steel plate having excellent toughness can be manufactured with high productivity without waiting for unnecessary temperature, and therefore the effect in the field is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing a temperature transition during rolling in conventional controlled rolling.

FIG. 2 is a diagram showing an influence on toughness with and without cooling before finish rolling.

FIG. 3 is a diagram showing a relationship between upper and lower surface temperature differences between steel plate widths and steel plate warpage.

Claims (2)

[Claims] 1. A steel material having a plate thickness of 40 mm or more before finish rolling, which has been subjected to hot rough rolling at 850 ° C. or higher, has a condition that a temperature difference between upper and lower surfaces at the start of finish rolling is an upper surface average temperature ≧ lower surface average temperature. In this way, cooling is performed by setting the ratio of the amount of water supplied to the upper and lower surfaces before the start of cooling, and the reduction amount from the first pass to the third pass when restarting finish rolling is 10 mm.
Within the range, the finish rolling is finished so that the finish rolling finish temperature is in the range of 700 to 850 ° C. A rolling method for a thick steel sheet having high toughness. 2. After the rolling is finished, it is continuously cooled to a temperature of 650 ° C. or lower at a cooling rate of 60 ° C./s or lower, and then,
The method for rolling a thick steel sheet having high toughness according to claim 1, wherein tempering is performed at an Ac ₁ transformation point or lower.