JPH0368762B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) This invention relates to a method for cooling a hot steel plate by supplying cooling water to the surface.

(Conventional technology) Recently, in the thick plate manufacturing process, new steel types have been developed,
There is active research into thermal refining cooling processes for the purpose of reducing alloying elements and reducing heat treatment. Thermal cooling process involves controlling the heating temperature and heating time of the material,
It is also made using a technology that combines controlled rolling with forced cooling immediately after rolling. This series of controls from heating, rolling to cooling is aimed at controlling the transformed structure and improving the mechanical properties of thick steel plates, and is generally referred to as thermomechanical processing (TMCP) technology.

Among the above-mentioned processing and heat treatment technologies, heating and rolling control technologies have been developed over the past 10 years, with the elucidation of metallurgical mechanisms mainly used in the production of high-tensile line pipe materials for cold regions, and online production technologies have been established.

(Problems to be Solved by the Invention) By the way, controlled cooling technology is still insufficient in terms of temperature control technology and shape control technology. In other words, the controlled rolled steel sheet is immediately heated from the high temperature range of 700 to 850℃ to 150 to 550℃.
However, it is necessary to cool the entire steel plate uniformly while maintaining the high plate shape during cooling. However, these requirements have not yet been fully met. Therefore, for industrialization,
Research and improvement of cooling devices and cooling control methods continues.

On the other hand, if we focus on the steel plate side to be cooled, we can also see the influence of the uneven heat of the steel plate before cooling, the shape of the steel plate before cooling (shape after rolling), and the surface texture of the steel plate before cooling (scale adhesion and surface roughness). big. It has been empirically understood that these effects combine to cause temperature non-uniformity immediately after cooling and instability of the plate shape. Therefore, it is essential to achieve high precision in the cooling device and cooling control, as well as to stabilize the steel plate to be cooled in order to stabilize the uniform temperature distribution and shape of the steel plate after cooling. This invention was made in view of the above points, and aims at uniform cooling, stabilization of the shape of a steel plate, and reduction of material quality variation within the steel plate.

(Means for Solving the Problems) In the present invention, the hot steel plate is cooled by supplying cooling water to the surface of the steel plate while transporting the hot steel plate in its longitudinal direction. The hot steel plate is transferred and cooled, for example, as follows. That is, the steel plate is held between a plurality of pairs of upper and lower rollers arranged in the feeding direction, and the upper and lower rollers are rotationally driven. Further, cooling water is supplied to the upper and lower surfaces of the steel plate from the nozzles of a plurality of cooling means arranged in the feeding direction and located between pairs of adjacent upper and lower rollers, thereby cooling the steel plate.

In such a cooling method, 20μ
It has a protrusion height of mR _z or more and 100 μm _or less (R _z : JIS BO601 10-point average roughness), and its pitch is
A surface roughness of 50 μmSm or more and 500 μmSm or less (Sm: average spacing of unevenness according to ISO468) is formed on the steel sheet surface almost uniformly in the sheet width direction. The upper limit of the surface roughness formed on the steel plate surface is determined by the quality required of the steel plate, and the protrusion height is 100 μmRz _and the pitch is 500 μmSm.

One method for forming the surface roughness is to create a surface roughness with a protrusion height of 20 to _100μmRz and a protrusion pitch.
A steel plate is rolled by a rolling roll having a roughness in the range of 50 to 500 μmSm, and the surface roughness of the rolling roll is transferred to the surface of the steel plate. Instead of the rolling rolls, a pinch roll of a leveling device disposed between the rolling equipment and the cooling device, a front roll of the cooling device, or a pinch roll dedicated to adding roughness having the above-mentioned roughness may be used.

In order to maintain the required surface roughness of the steel plate, the surface roughness of the steel plate is detected before cooling, and based on the detection results, the protrusion height of the surface roughness is 20 to _100μmRz and the protrusion pitch is 50 to 500μmSm. The rolling rolls etc. are dressed so as to fit within the range. A non-contact surface roughness meter is used to detect surface roughness. In addition, due to the rolling of the steel plate, the height of the protrusions on the rolling roll is 20 to 100 μm _Rz , and the pitch of the protrusions is 50 to 500 μm S.
The surface roughness may be formed within the range of m.

(Operation) In order to help understand the operation of the present invention configured as described above, forced water cooling of a high-temperature object will be explained here.

When cooling water is supplied to the surface of a heated steel plate, the cooling water cools the steel plate while boiling. Citing conventional knowledge, such cooling can be explained using a model as shown in FIG. FIG. 1 is a diagram comparing a cooling curve and a boiling curve in boiling cooling. The characteristics of the cooling curve in boiling cooling are gradual cooling, rapid cooling,
and slow cooling. The cooling curve at a point in a hot object is determined by the object's dimensions, the object's thermal conductivity, and the boundary conditions (heat transfer coefficient) at the object's surface. The reason why the cooling curve in boiling cooling passes through the three stages described above is that the boiling curve is composed of three regions: a film boiling region, a transition boiling region, and a nucleate boiling region.

That is, in the cooling curve and the boiling curve, the following correspondences are established: high temperature region...film boiling region, medium temperature region...transition boiling region, low temperature region...nucleate boiling region. However, strictly speaking, the first
As shown in the figure, for example, the minimum heat flux point
There is a slight deviation between q _nio and the quench point. Furthermore, as shown in Figure 1, rapid cooling occurs when the surface temperature of the object falls below the minimum heat flux point q _nio temperature, so the subsequent cooling time is short. Therefore, the film boiling region and minimum heat flux point conditions play an important role in the cooling curve or cooling time.

Parameters that affect the film boiling region and minimum heat flux point conditions include water temperature, water flow rate, pressure, surface properties of the object to be cooled (scale adhesion, roughness, scale wettability, etc.) size, thermal conductivity, and gravity. Examples include acceleration.

Figure 2 shows how each parameter affects the results. For example, when the cooling curves differ as shown by the solid line and the dashed-dotted line in FIG. 2, it is important to understand what on the boiling curve and which parameter changed.

The above is an overview of conventional knowledge regarding cooling.

Therefore, if the cooling stop temperature in controlled cooling is in the intermediate temperature range (350 to 550°C), there will be variations in cooling capacity at transition boiling (behavior of q _nio and q _BO in Figure 1).
is an important point. Especially the minimum heat flux point q _nio
is also the point at which the vapor film begins to collapse (≒ quench point), meaning that its behavior has a significant impact on the cooling capacity.

Next, the height of the protrusions formed on the steel plate before cooling is
20-100μmR _z and protrusion pitch 50-500μmSm
The effect of the surface roughness, that is, the surface irregularities, will be explained below.

According to basic research, the fundamental cause of uneven hardening and deformation is that the cooling curve varies depending on the location and location of the steel sheet due to the physical and chemical properties of the heat transfer surface. Therefore, the basic and essential solution is to create a stable heat transfer surface whose cooling curve does not vary with location and time.

When cooling hot steel sheets, industrial water, which undergoes a boiling phenomenon, is usually used as a refrigerant. Therefore, the end temperature of film boiling q _BO (critical heat flux point) is likely to vary depending on the physical or chemical properties of the heat transfer surface. Research has revealed that the end temperature of film boiling can be stabilized by forming relatively regular and dense uneven protrusions on the heat transfer surface. That is,
As shown in FIG. 3, these projections penetrate the vibrating vapor film on the heat transfer surface and come into direct contact with the refrigerant liquid. As a result, the end temperature of film boiling settles within a narrow range, the cooling curve becomes stable, and the end temperature of cooling becomes approximately constant.

The reason for this, when considered from a heat transfer engineering perspective, is as follows.

When the temperature of the cooling water is around 20 to 30°C, the thickness of the steam film is about several μm to about 10 μm, and the cooling water surface on the steam film vibrates or pulsates. Therefore, if there are relatively regular and dense uneven protrusions on the heat transfer surface, the protrusions will break through the vapor film layer and enter the cooling water layer, and the protrusions will be locally caused by solid-liquid contact or the Finn effect. The surface temperature decreases and becomes the core of vapor film collapse. At this time, if the pitch of the protrusions is too small, the thickness of the vapor film formed on the solid surface due to the surface energy of the liquid becomes apparently large, inhibiting solid-liquid contact, and furthermore, the fin effect becomes small.

If the uneven protrusions are relatively regular and dense, many nuclei for vapor film collapse are formed, and the vapor film collapses uniformly over the entire surface almost simultaneously, resulting in a transition to transition boiling.
As a result, the cooling end temperature is stabilized within an extremely narrow range and follows the same temperature history.

Providing regular and appropriate surface roughness to the surface using such a mechanism is effective as a means for achieving uniform cooling. In addition, the surface roughness is determined when the protrusion height is 20μmRz _or more and the pitch is 50μmSm.
Must be above. On the other hand, the upper limit of the protrusion height is determined by the quality required of the steel plate, and the upper limit is 100 _μmRz . In addition, the upper limit of the pitch of the protrusions is 500 μmSm because if the pitch is too large, the surface energy of the vapor film generated on the solid surface will inhibit the Finn effect (the fusion of solid and liquid). It is.

(Example) FIG. 4 is an example in which the average cooling rate (° C./S) was determined by arbitrarily changing the surface roughness _Rz and Sm of the cooled steel plate. According to Figure 4, it is presumed that the heat transfer area increases due to the surface irregularities, and that the convex portions explained in Figure 3 break through the vapor film, causing localized solid-liquid contact. . It can also be seen that when the surface roughness exceeds a certain level, fluctuations in cooling capacity become insensitive.

Figure 5 shows the protrusion pitch Sm of the surface roughness of the cooling steel plate.
This figure shows the difference in heat transfer coefficient when the height _Rz is arbitrarily changed while Rz is kept constant. In other words, the influence of the protrusion height _Rz varies greatly in the film boiling region of the high temperature region (≧500°C). In addition, the protrusion height is 30μmR _z
Above that, the change in the heat transfer coefficient is small. Based on Figure 5, the target cooling stop temperature is calculated using the heat transfer coefficient curve above 500°C with protrusion heights of _30μmRz and _5μmRz .
Estimated at 450℃ (based on heat conduction difference equation),
When the plate thickness is 15 mm and the water density is 0.65 m ³ /m ² min,
A difference in cooling stop temperature of approximately 120°C occurs. In other words,
5μmR _z with a small protrusion height is less likely to be cooled,
The cooling stop temperature is about 570℃ (450+120℃), which is a high temperature shutdown.

FIG. 6 shows a cooling rate curve determined by experiment using the cooling stop temperature as a parameter. The experimental conditions were: plate thickness 15 mm, water density 0.65 m ³ /m ² min,
and the cooling start temperature is 750°C. It can be seen from Figure 6 that the cooling rate increases when the protrusion height R _z is as large as possible, and the protrusion height R _z is approximately 20 μm.
Above that, the change in cooling rate is small. Conversely, when the protrusion height R _z becomes less than 20 μm, the cooling rate decreases rapidly. In other words, this means that the larger the protrusion height _Rz is, the smaller the cooling stop temperature deviation is.

On the other hand, the vapor film thickness δ that occurs in the film boiling region is defined by Zuber (“On the Stability of Boiling Heat”).
Transfer”Transactions of the ASME P711
(1957-4) N. Zuber), Berenson (“Film
Boiling Heat Transfer from a Horizontal
Surface”Journal of Heat Transfer P 351
(1961-8) PJ Berenson) etc.
As shown in FIG. 7, the vapor film thickness δ is determined by the steel plate surface temperature and water temperature. The point at which the vapor film begins to collapse (q _nio ) is approximately 13 μm due to thermal balance, and at that time the surface temperature and water temperature are approximately 600 °C and 25 °C.
It is. This agrees well with the quench point (≒q _nio ) in the inventors' cooling experiments. According to Figure 7, the effect of water temperature is approximately 0.3μ on the vapor film thickness δ.
Although the temperature is small at m/10°C, the quench point corresponds to approximately 50°C/10°C in terms of the steel plate surface temperature. In other words, the quench point changes by 50°C for a water temperature of 10°C. Also,
It can be seen from FIG. 7 that the vapor film thickness δ is about 10 to 15 μm when the steel plate surface temperature is in the range of 300 to 800°C. Considering this using the surface roughness model shown in FIG. 3, the vapor film thickness δ is 10 to 15 μm. and,
Since the protrusion height of the surface roughness is R _z ≈2 δ μm, when this vapor film thickness δ is converted into the protrusion height R _z of the surface roughness, it becomes 20 to 30 μm. This is a quantitative explanation of the concept in Figure 3 mentioned earlier, and is similar to Figure 6.
This confirms the phenomenon shown in the figure.

In addition, FIG. 8 is a diagram in which the quench points were determined in the cooling experiment, and each quench point is written in FIG. 7.

Judging from the cooling theory and basic cooling experiment data described above, the conditions for reducing the temperature variation within the steel plate when cooling is stopped midway are the surface roughness and protrusion height of the entire surface of the steel plate (including the front and back surfaces). but
20-100μmR _z and protrusion pitch 50-500μmSm
It can be seen that it is important to have a uniform surface roughness within the range of .

Next, a method for imparting uniform surface roughness to the front and back surfaces of the steel plate will be explained.

When measuring the surface roughness of a steel plate in the example of thick plate rolling, as shown in Figure 9, it is 5 to 45 μm R _z and 100 to 300 μm Sm.
Some degree of surface roughness is observed. This approximately corresponds to the surface roughness of the work roll of a finishing mill. In other words, the work roll surface roughness is printed on the steel plate. Therefore, controlling the surface roughness of the work roll leads to controlling the desired steel plate surface roughness.

FIG. 10 shows an example in which the protrusion height _Rz of the roll surface roughness immediately after the work rolls were rearranged was actually measured using the formula in the roll body length direction.

However, as is well known, between the reassembly and disassembly of the upper and lower work rolls (also called one roll chance), in order to roll various steel types and sizes (especially widths), there is a The wear is not uniform, and the surface roughness is also not uniform.

FIG. 11 shows an example of roll wear and roll surface roughness during roll removal. Naturally, the roll surface hardness and material, the plate width of the threaded material,
Work roll surface roughness differs depending on the steel type. Furthermore, the surface roughness differs between the upper and lower rolls. 1st
If there is a deviation in the surface roughness in the length direction of the roll body, as in the surface roughness at points a, b, and c in Figure 1, a temperature deviation will occur in the sheet width direction during cooling, as described above. This temperature deviation causes the generation of residual stress in the steel sheet at room temperature, leading to deterioration of the shape of the steel sheet.

When the steel plate surface temperature distribution after cooling is actually measured for the steel plates with rolling widths A (narrow material) and B (wide material) shown in FIG. 11, the temperature profile in the width direction of the steel sheet is as shown in FIG. 12. This temperature deviation in the width direction of the plate is 20
If the temperature exceeds ~30°C, the shape of the steel sheet after cooling deteriorates in proportion to the temperature deviation. Therefore, in order to avoid deterioration of the shape of the steel plate during the thick plate rolling to cooling process, the following method is adopted.

(1) Utilize the rolling tonnage to roll surface roughness characteristics shown in FIG. That is, first, 1000 tons or more of rolled material without controlled cooling is rolled. The plate width of the rolled material is wider than the plate width of the material to be controlled and cooled.
From this, it can be seen that the rolling roll is relatively uniform in the lengthwise direction of the roll body within the rolling plate width without controlled cooling, with a protrusion height of 20 to 100 μmRz _and a protrusion pitch of 50 to 500 μm.
A surface roughness of mSm is formed. Next, the material to be forcibly cooled is rolled using the rolling roll,
Cooling. The surface roughness of the rolling rolls is directly transferred to the rolled steel plate. As a result, the temperature profile in the width direction of the cooled steel sheet after cooling is relatively uniform, and the shape of the steel sheet is also good.

(2) If the plate width of the control coolant varies greatly, the method (1) above has limitations. In this case, a method is adopted to actively make the surface roughness in the width direction of the plate uniform.

Figure 13 shows a cooling device 5 behind the finishing rolling mill 1.
The figure shows a plate rolling-cooling facility equipped with
A surface roughness control device 3 is placed immediately after the finishing mill 1. During the free time between the preceding slab S to be rolled and the next slab S, the surface roughness control device 3 controls the upper and lower work rolls 2 to have a uniform surface roughness in the length direction of the roll body at an arbitrary timing. give.

This makes it possible to obtain a desired roll surface roughness (rough skin).

The above description focuses on the finish rolling work roll 2, but for example, a hot leveler roll before the steel plate S enters the cooling device 5, a pre-stage roll of the cooling device, and a protrusion height of 20 to 100 μm on the steel plate.
It is also possible to provide the same function with a dedicated pinch roll or the like for the purpose of uniformly adding a surface roughness with a protrusion pitch of 50 to 500 μmSm at mR _z .

(3) Also, FIG. 13 shows a surface roughness meter 4 that detects the surface roughness of the front and back surfaces of the steel plate. Based on the surface roughness information from this surface roughness meter 4 (preferably a scanning type in the plate width direction), the surface roughness control device 3 is controlled, and at the same time, the flow to the upper and sewage flow ratio control of the cooling device 5 is controlled. Eid rewards are also possible. Furthermore, the sewage flow ratio control reduces the strain in the width direction of the steel plate S. Moreover, by utilizing the above-mentioned relationship between surface roughness and cooling capacity (for example, FIG. 4), the front and back surfaces of the steel plate can be cooled symmetrically.

Work roll 2 instead of surface roughness of steel plate S
The surface roughness of the steel plate S may be approximately determined by directly measuring the surface roughness of the steel plate S.

FIG. 14 shows the variation ΔT in the steel plate temperature after forced cooling due to the difference in roll surface roughness in an embodiment of the present invention. It is possible to industrially produce a coolant with small variations in steel plate temperature and a well-controlled steel plate shape. The variation ΔT in the temperature of the steel plate is the temperature deviation value of the data from the radiation thermometer 6 installed at the rear of the cooling device 5 at 5 points each in the longitudinal direction and width direction of the steel plate, 25 points in total.

(Effects of the Invention) As described above, according to the present invention, variations in the temperature of a steel plate after forced cooling are reduced, and a coolant with good control of the shape of the steel plate can be industrially manufactured.

[Brief explanation of drawings]

Figure 1 is a diagram comparing the cooling curve and boiling curve in boiling cooling, Figure 2 is a diagram showing the influence of cooling conditions on the cooling curve and boiling curve, and Figure 3 is a schematic diagram of the heat transfer surface. Figure 4 is a diagram showing the relationship between the surface roughness R _z a and Smb of the cooling steel plate and the average cooling rate, and Figure 5 is a graph showing the steel plate surface temperature when the surface roughness R _z of the cooling steel plate is used as a parameter. Figure 6 is a diagram showing the relationship between surface roughness protrusion height _Rz and cooling rate when the cooling stop temperature is used as a parameter. Figure 7 is a diagram showing the relationship between cooling rate and heat transfer coefficient. A diagram showing the relationship between the steel plate surface temperature and steam film thickness when water temperature is used as a parameter. Figure 8 is an example of a cooling curve that shows the change in quench point depending on water temperature. Figure 9 is a steel plate surface roughness curve. Figure 10 is a diagram showing rolling tonnage to roll surface roughness _Rz characteristics, Figure 11 is a diagram showing an example of roll wear and roll surface roughness Rz during roll removal, Figure 12 is a diagram showing an example of roll wear and roll surface roughness _Rz during roll removal. The figure is a diagram showing the temperature profile in the plate width direction, FIG. 13 is an equipment configuration diagram showing an example of thick plate rolling-cooling equipment for carrying out the method of the present invention, and FIG. 14 is an example of the present invention. FIG. 3 is a diagram showing variations in temperature of a steel sheet after cooling due to differences in surface roughness of rolls. 1... Finish rolling mill, 2... Work roll, 3
...Surface roughness control device, 4...Surface roughness meter, 5
……Cooling system.

Claims (1)

[Claims] 1. The height of the protrusions on the surface of the heated steel sheet immediately after controlled rolling is 20 to 100 _μmRz , and the pitch of the protrusions is 50 to 500 μmS.
1. A method for cooling a hot steel sheet, which comprises forming a surface roughness in the range of m, and forcibly cooling the hot steel sheet from a high temperature range of 700 to 850°C to 150 to 550°C with water. 2 The height of the protrusion is 20 to _100μmRz and the pitch of the protrusion is
The cooling method according to claim 1, characterized in that the surface roughness of the hot steel plate is formed using a roll whose surface roughness is adjusted to a range of 50 to 500 μmSm. 3 The surface roughness of the heated steel sheet after controlled rolling is detected before forced cooling, and based on the detection results, the surface roughness of the rolling roll is determined as follows: the protrusion height is 20 to 100 μm _Rz and the protrusion pitch is 50 to 500 μm Sm. After dressing the surface of the rolling roll so that it is within the range of 700~
A cooling method for hot steel sheets characterized by forced cooling using water from a high temperature range of 850°C to 150 to 550°C. 4. The cooling method according to claim 3, wherein the surface roughness of the mill roll is detected in advance and the surface of the mill roll is dressed based on the detection result. 5 Before performing controlled cooling, a hot steel plate that is wider than the hot steel plate that should be cooled is heated to 1000 mm by uncontrolled cooling.
After rolling more than a ton, the protrusion height on the rolling roll is 20 ~
A surface roughness with a protrusion pitch of 50 to 500 μm _Sm is formed using 100 μmRz, and the hot steel plate is rolled with this rolling roll, and the hot steel plate after rolling is heated from a high temperature range of 700 to 850°C.
A cooling method for hot steel sheets characterized by forced cooling with water between 150 and 550°C.