JPS5934213B2 - Online processing heat treatment method and equipment for steel plate rolling - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention performs processing heat treatment of steel thick plates very efficiently in terms of time, thermoeconomics, and space on-line during steel plate rolling. The present invention relates to an online processing heat treatment method and apparatus for steel plate rolling, which prevents non-uniformity of mechanical properties in the width direction of the plate.

For example, high-strength steel plates are increasingly required to have high strength and high toughness in recent years, and at the same time, there is a tendency for plate thickness to increase.

In manufacturing thick steel plates such as high-strength steel plates,
Accelerated cooling after hot rolling has been widely proposed.

That is, in order to effectively utilize the heat held by the steel sheet after hot rolling, the steel is accelerated cooled at a temperature of 3°C/sec or more from a temperature of Ar1 or higher to a temperature of 650°C or lower. It has been proposed that
Publication No. 5-4111, Japanese Patent Publication No. 49-16010,
(See Japanese Patent Application Laid-Open No. 123921/1984, etc.).

This changes the structure of the steel from a ferrite-pearlite structure to a fine-grained ferrite-bainite structure with high dislocation density by cooling and heating, and
This is an extremely excellent method that suppresses the precipitation of these substances during transformation and allows them to precipitate effectively after transformation, and can significantly increase strength without impairing toughness.

However, when accelerated cooling after hot rolling as described above is applied to a wide material, the cooling rate at the edge in the width direction (hereinafter simply referred to as the edge of the sheet) is lower than that at the center in the width direction (hereinafter simply referred to as the center of the sheet). It is thicker than the ``naru''.

This is thought to be because water injected onto the upper surface of the steel plate during accelerated cooling remains in the soil, flows in the width direction of the plate, and falls through the edge of the plate.

However, the temperature of the steel sheet at the end of the sheet is lower than that at the center of the sheet when accelerated cooling is stopped.

This is because the temperature of the steel sheet immediately after rolling tends to be lower at the ends of the sheet than at the center of the sheet, and as mentioned above, the cooling rate at the ends of the sheet is faster than at the center of the sheet. The cause is to become.

Therefore, when the above-mentioned accelerated cooling after hot rolling is applied to a wide material, edge waves are generated at the end of the sheet, and the mechanical properties are non-uniform in the width direction of the sheet.

Such edge waves and non-uniformity of mechanical properties in the width direction of the sheet lead to a decrease in yield. It tends to increase with

For example, Fig. 1 shows a position 5 mm from the center of the plate edge in the plate thickness direction (hereinafter referred to as the plate edge 5 mm position) of a steel plate with a plate thickness of 25 mm and a plate thickness of 300 mm and a length of 15,000 rran obtained by rolling described later. The following table shows the results obtained by calculating the temperature change at the center position in the thickness direction (hereinafter referred to as the center of the plate thickness) and the center position in the thickness direction (hereinafter referred to as the center of the plate thickness) in the heat treatment process shown below. .

That is, after heating the slab to 1200°C, it is heated to 900°C.
790 by performing controlled rolling with a cumulative reduction rate of 60% as follows.
A steel plate of the above-mentioned size obtained by finishing rolling at ℃ is placed at the center of the plate thickness, and the temperature at the center of the plate thickness is increased from 780℃ to 60℃.
Down to 0℃, heat transfer coefficient 1200 Kcal/lr?・hr
・Accelerated cooling at 'C' and then air cooling (cooling). In the figure, the triangle mark indicates the board edge 5rrrrIL position, and the circle mark indicates the center position of the board.

From the figure, the average cooling rate at the center of the plate thickness during accelerated cooling is 5.6'c/8ee, and the average cooling rate at the center of the plate thickness at the 5 m position of the plate edge is 5.6'c/8ee. Although the average cooling rate is approximately twice that at the center of the plate, it is clear that the temperature at the time of stopping the accelerated cooling has decreased to approximately 400°C. It can be seen that the temperature difference between the center of the plate width and the edge of the plate at the time of stopping is approximately 200° C., and this difference is still large and unresolved even during air cooling (standing cooling) after stopping the accelerated cooling.

The occurrence of such a temperature difference causes thermal stress to act on the steel plate (so even if the strain is corrected with a hot leveler after accelerated cooling, compressive stress will eventually be created in the center of the width of the steel plate. Tensile stress acts on each end, generating edge waves.

Furthermore, as mentioned above, if there is a large difference in the cooling rate and the temperature at the time of stopping accelerated cooling between the center of the width of the steel plate and the edge of the plate, the mechanical properties in the direction of the thickness of the steel plate will become non-uniform. It will be done.

Therefore, the inventor of the present invention conducted research to solve the above-mentioned problems, and as a result, when performing online processing heat treatment during thick plate rolling of steel, the present inventors discovered that after hot rolling, a steel plate having a temperature above the Ar point is ,6
If accelerated cooling is performed at a cooling rate of 3° C./sec or more to a temperature of 50° C. or lower, and then straightened and reheated to a temperature in the range of 500° C. to the Acm point before or after the straightening, by accelerated cooling, Even if there is a large temperature difference between the edge of the steel plate and the center of the width of the steel plate, the edge of the steel plate will be greatly affected by heat due to reheating and the temperature will rise rapidly. It is possible to make the temperature difference between
It is possible to prevent the occurrence of edge waves and uneven mechanical properties in the direction of the plate thickness due to uneven temperature distribution in the direction of the plate thickness, and to effectively utilize the heat of the steel plate immediately after hot rolling. Then, accelerated cooling is performed online, followed by reheating to a predetermined temperature range and straightening. Therefore, the steel plate is hot rolled, acceleratedly cooled, cooled to around room temperature, and then reheated. Conventional thick plates that require straightening, etc., have major disadvantages in terms of thermoeconomics and operation (e.g., the need for a large cooling bed and the need to wait for the cooling to a specified temperature). Compared to the process heat treatment in rolling, the process heat treatment can be performed much more efficiently in terms of time, thermoeconomics, and space, and such process heat treatment can be carried out on the exit side of the hot rolling mill. They found that it is possible to carry out the process by installing an accelerated cooling device, followed by a reheating furnace and a straightening machine in the reverse order.

This invention was made based on the above findings, and the reason why the reheating temperature is numerically limited as described above will be explained below.

That is, in Figure 2 A and the mouth, the slab is 1150.
After heating to ℃, controlled rolling was performed at a cumulative reduction rate of 65% at 900℃ or less, and the rolling was completed at 780℃, resulting in a plate thickness of 20mm, plate thickness of 2500rrtjn, and length of 15000mm.
The steel plate was placed at the center of the plate thickness, and the temperature at the center of the plate thickness was increased from 760°C to 550°C at 5°C/S.
Plate thickness when accelerated cooling with EE, then reheating by holding at each temperature for 4 minutes (after accelerated cooling, time until charging into the reheating furnace is 20 seconds), and then air cooling (air cooling) The mechanical properties of the center part of the board width and the ends of the board at the center position are
Indications are given for the reheating temperature.

In the figure, the black marks indicate the edges of the board, and the open marks indicate the center of the board width.

From the figure. If the steel plate is heated to a temperature above Ac1 point by reheating, the edge waves will disappear by straightening after reheating, but the strength will decrease and the toughness will be 160℃ and 600℃. It can be seen that the loss is greater than when reheating.

This is because when a steel plate is reheated to a temperature above AC1 point, part or all of the structure transforms into austenite, so the fine structure obtained by controlled rolling returns to its original state and the subsequent air cooling causes it to transform into austenite. This is because it becomes a normal ferrite-pearlite structure.

On the other hand, when a steel plate is reheated at a temperature of 500°C or lower, the properties of the base material are not impaired and it has high strength and toughness, but even after straightening after reheating, the edge The waves could not be sufficiently extinguished, and moreover,
The difference in strength between the center portion of the board width and the end portions of the board has not been completely resolved.

As described above, the reheating temperature may be 500° C. or higher from the viewpoint of edge wave straightening, but in consideration of the characteristics of the base material, it must be Ac 1 point or lower.

The charging time of the reheating furnace varies depending on the set temperature of the reheating furnace and the thickness and width of the steel plate, but if it is less than 1 minute,
It is difficult to bring the temperature at the edge of the plate close to the temperature at the center of the width of the plate.

FIG. 3 shows a schematic configuration diagram of an online processing and heat treatment apparatus to which the present invention is applied.

As shown in the figure, 1 is a heating furnace, 2 is a rough rolling mill, 3 is a finishing mill, 4 is an accelerated cooling device, 5 is a reheating furnace, and 6 is a hot leveler. heated to a predetermined temperature by a rough rolling mill 2, finished rolled by a finishing mill 3, acceleratedly cooled by an accelerated cooling device 4, reheated by a reheating furnace 5, and then strain-straightened by a hot leveler 6. It is now possible to proceed to the finishing process, etc.

Note that the hot leveler 6 may be disposed between the accelerated cooling device 4 and the reheating furnace 5 to reheat the steel plate after straightening the strain.

Note that the cooling capacity of the accelerated cooling device 4 must be able to cool a steel plate at a temperature of 3°C/see or higher, but specifically, the In the case of 3℃/sec~20℃/see
It is desirable to be able to cool it down.

The reheating furnace 5 can be heated by a high-frequency or low-frequency induction method, or a gas burner method. As a heating method, it is preferable to heat from the edge of the steel plate, and the furnace length is set so that the steel plate is stopped. When heating, it is necessary to secure enough space for the length of the steel plate, but when heating is performed while moving the steel plate, it is not necessarily necessary to secure enough space for the length of the steel plate.

FIG. 4 shows a schematic configuration diagram of another online processing heat treatment apparatus to which the present invention is applied (in the figure,
The same parts as in FIG. 3 are indicated by the same reference numerals).

As shown in the figure, this online processing heat treatment equipment
Next to the heating furnace 1, a rough rolling mill 2, a finishing rolling mill 3, an accelerated cooling device 4, and a hot leveler 6 are arranged in sequence, and a reheating furnace 5 is provided in a bypass line 7 on the exit side of the hot leveler 6. The steel plate strain-straightened by the hot leveler 6 passes through the bypass line I and is reheated by the reheating furnace 5, and then returns to the original line for a refining process. It has become.

With such a configuration, for example, when combined with rolling that does not require reheating, reheating can be performed without passing through the bypass line 7 while other steel plates are being rolled and conveyed to the finishing line. Therefore, it is possible to equalize the temperature distribution in the direction of the thickness of the steel plate, without reducing productivity (in addition, the line length is too long and there is no space available). ).

Next, examples will be explained.

Example 1 After heating a slab to 1100°C, controlled rolling was performed at a cumulative reduction rate of 60 at 900°C or lower and the rolling was completed at 790°C, with a thickness of 25 and a width of 3000 rrvnz.
A steel plate with a length of 15,000 ran was placed at the center of the plate thickness, and the temperature at the center of the plate thickness was increased from 780°C to 6.
Heat transfer coefficient 1200Kcal/7 to 00℃? Z2・hr
・Accelerated cooling at °C (average cooling rate is 5.6 °C/5ec) L
, then air cooled for 20 seconds, and then the temperature inside the furnace was set to 700.
When the sample was charged into a reheating furnace set at .degree. C., the temperature change at the center of the plate thickness at the same position and at the plate end 5rrtm position was calculated.

The results are shown in FIG.

In the figure, a circle mark indicates the center position of the plate, and a triangle mark indicates the position of the plate edge 5fi.

From the figure, it is clear that the temperature difference at the same position became 20° C. or less after 2 minutes and 30 seconds of charging into the reheating furnace, which is an extremely small temperature difference.

Example 2 Component composition: C: 0.11 ratio, Si: 0.30 ratio,
Mn: 1.27%, P: 0.014%, S: 0.
Section 004, Nb: 0.032%, V: 0.046%, 5
A slab consisting of olAA: 0.038%, T:N: 0.0047%, and Fe and unavoidable impurities: the remainder (more than weight %) was heated to 1150°C and then heated at a cumulative reduction rate of 60% at 900°C or less. A steel plate having a thickness of 20 m, a wall thickness of 2500 mm, and a length of 15000 rra obtained by performing controlled rolling and finishing rolling at 760°C was then placed at the center of the plate thickness, and the temperature at the center of the plate thickness was 750°C. 55
In the case of accelerated cooling to 0°C at 5°C/sec and then cooling (comparative steel sheet), and after cooling for 15 seconds after the accelerated cooling, it was placed in a reheating furnace with the furnace temperature set at 600°C for 3 minutes. The mechanical properties were determined at the same position as when the steel plate was charged and then left to cool (invention steel plate), and at the center of the plate thickness at a position of 5 m from the plate edge.

The results are shown in Figure 6A. In the figure, the black mark indicates the board edge 5rrrIrL position, the white mark indicates the center position of the board, and the triangle mark at the entrance of Figure 6 indicates the TS
The values and circles indicate the TS values, and for further reference,
The TS value when cooling is allowed to occur without performing the accelerated cooling is shown as a, and the TS value is shown as b.

From the figure, the strength difference between the plate edge 5m+++ position and the plate thickness center position is 3 to 5 K for the steel plate as-is accelerated cooling (comparative steel plate).
g/rnrjt, whereas it is clear that the steel sheet that was reheated after accelerated cooling (the steel sheet of the present invention) has almost no change in strength and toughness.
Since the precipitation of Nb (C, N) and V (C, N) is promoted, it is clear that the steel sheet of the present invention has a higher concentration than the comparative steel sheet.

Note that the edge wave was also 60 mm for the comparative steel sheet, whereas it was 20 mm or less for the steel sheet of the present invention.

Conventionally, when performing accelerated cooling, the upper limit of the cooling rate and the cooling stop temperature have been set in order to prevent the generation of edge waves and to reduce as much as possible the non-uniformity of mechanical properties in the direction of the plate thickness. Although a lower limit value is defined, in the present invention, such a regulation can be eliminated by adjusting the set temperature of the reheating furnace.

Further, in all of the above embodiments, controlled rolling is performed in which hot rolling is completed at a temperature of Ar3 or higher, but the present invention is not limited to this.
Even if it is normal rolling in which no reduction is performed at all in the non-recrystallization temperature range as described in the publication, rolling with substantial reduction at a temperature of Ar3 or less (see the description in Japanese Patent Publication No. 55-115924) ) is also applicable.

As explained above, in the present invention, the heat of the steel plate immediately after hot rolling is effectively utilized to perform online accelerated cooling, then reheating to a predetermined temperature range, and straightening. The generation of edge waves and non-uniformity of mechanical properties in the direction of the plate thickness can be prevented, and furthermore, processing heat treatment can be carried out efficiently in terms of time, thermoeconomics, and location.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the temperature change of a steel plate due to conventional heat treatment. Figure 5 is a diagram showing the temperature change of a steel plate due to the heat treatment of the present invention, and Figure 6A-9 shows the mechanical properties of the steel plate after heat treatment. FIG. DESCRIPTION OF SYMBOLS 1... Heating furnace, 2... Rough rolling mill, 3... Finishing rolling mill, 4... Accelerated cooling device, 5... Reheating furnace, 6...
・Hot leveler, 7...Bypass line.

Claims (1)

[Claims] 1. Online processing heat treatment in steel thick plate rolling, in which a steel plate having a temperature of Ar1 or higher after hot rolling is acceleratedly cooled to a temperature of 650°C or lower at a cooling rate of 3°C/see or higher. In the method, after the accelerated cooling, the steel plate is straightened and 50%
1. A method for online processing heat treatment in steel plate rolling, characterized in that reheating to a temperature in the range of 0° C. to Ac1 point is performed in this order or in the reverse order. 2. In steel plate rolling, which is characterized in that an accelerated cooling device is provided on the exit side of a hot rolling mill, followed by a reheating furnace and a straightening machine, either in this order or in the reverse order. Online processing heat treatment equipment.