JP4586314B2 - Manufacturing method of hot-rolled steel sheet - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a hot-rolled steel sheet.
[0002]
[Prior art]
In the manufacturing process of the hot-rolled steel sheet, the slab is heated to a predetermined temperature in a heating furnace, and the heated slab is rolled to a predetermined thickness with a roughing mill to form a rough bar. A hot-rolled steel strip having a predetermined thickness is finished and rolled in a finish rolling mill consisting of the above, and after the hot-rolled steel strip is cooled in a cooling device on a run-out table, a hot-rolled coil is obtained by winding it with a coiler.
[0003]
[Problems to be solved by the invention]
However, the hot-rolled steel sheet manufactured in this way has a tendency to have a wavy variation in the sheet thickness distribution in the width direction of the steel sheet, the cause of which has not been known for a long time, and appropriate measures could not be taken.
[0004]
Accordingly, an object of the present invention is to provide a method capable of eliminating the wavy plate thickness distribution in the plate width direction of the hot rolled steel plate and producing a hot rolled steel plate having a uniform plate thickness in the plate width direction. It is in.
[0005]
[Means for Solving the Problems]
The inventors of the present invention have studied the cause of the above-described defective shape of the hot-rolled steel sheet, that is, the corrugated thickness distribution in the sheet width direction, and the prevention measures. As a result, the corrugated plate thickness distribution in the plate width direction as described above is caused by uneven cooling, and therefore it is solved by cooling the hot-rolled steel plate so that it is uniformly cooled in the plate width direction. I found out that I can do it.
[0006]
In a hot finish rolling mill, a cooling device is generally provided immediately before the rolling roll for the purpose of preventing roughing of the rolling roll and scaling defects, and the cooling device includes a header as shown in FIG. An apparatus in which a large number of injection nozzles 7b are attached to the pipe 7a is generally used. The inventors examined the cooling capacity distribution in the sheet width direction on the surface of the hot-rolled steel sheet cooled by this cooling device. The result is schematically shown in FIG. 9 and has the highest cooling ability directly under the nozzle 7b, and cooling water interferes between adjacent nozzles, resulting in a low cooling ability. In accordance with the level of cooling capability in the sheet width direction, a heat transfer coefficient distribution is generated on the surface of the hot-rolled steel sheet. In FIG. 9, the difference between the maximum heat transfer coefficient and the minimum heat transfer coefficient is Δh. . As a result, the rolling material 5 is cooled differently between the nozzle 7b immediately below and the adjacent nozzle 7b, and the temperature of the rolled material 5 is directly below the nozzle 7b than between the nozzles 7b. Decreases and the deformation resistance increases.
[0007]
In addition, when the rolled material is cooled immediately before the hot finish rolling mill roll bite using the cooling device 7 as shown in FIG. 7, a part of the cooling water directly hits the rolling roll 3, and the rolled material cooling device 7. Corresponding to the position of the nozzle, it was found that a temperature distribution (temperature unevenness) having a waveform as shown in FIG. And it turned out that the heat crown of the rolling roll 3 which generate | occur | produced corresponding to such temperature distribution turns into a concave crown right under the injection nozzle 7b. Accordingly, the portion with high deformation resistance of the rolled material is rolled at the concave crown portion, and the portion with low deformation resistance is rolled at the other portions. As a result, the distribution of the finished plate thickness in the width direction is as shown in FIG. Further, it has been found that the waveform becomes corrugated corresponding to the arrangement of the injection nozzle 7b.
[0008]
In order to solve such a problem, the present inventors investigated the temperature distribution and heat transfer coefficient in the sheet width direction of the hot-rolled steel sheet after cooling using various cooling devices having different nozzle pitches.
[0009]
The inventors use a cooling device (experimental device) having a conventional injection nozzle (see the cooling device 7 in FIGS. 6 and 7), change the nozzle pitch and flow rate, and directly adjacent to the nozzle cooling water. Experiments were conducted to determine the heat transfer coefficient between the nozzles. After heating the steel material in which the thermocouple was embedded, the steel material was cooled by being conveyed so that the place where the thermocouple was located passed directly under the nozzle or between adjacent nozzles. The heat transfer coefficient was calculated by calculating backward from the temperature change at this time. Next, attach the same nozzle pitch and nozzle header used in the above experiment with the same nozzle pitch to the actual machine, make the header pressure (or water density) the same, and immediately before the hot finish rolling mill roll bite. Rolling tests with actual machines were repeated in which rolling was performed after the surface of the rolled material was cooled. As a result, the variation (Δt) in the sheet width direction of the finished sheet thickness depends on the difference (Δh) (see FIG. 9) between the maximum heat transfer coefficient and the minimum heat transfer coefficient in the width direction of the rolled material obtained in the experiment. It became clear that The horizontal axis indicates the difference Δh between the maximum heat transfer coefficient and the minimum heat transfer coefficient in the width direction of the rolled material obtained in the experiment, and the vertical axis indicates the variation Δt in the plate width direction of the finished plate thickness obtained in the corresponding actual machine test. FIG. 2 is plotted. As a result, it was found that when Δh is 200 kcal / m ² / ° C./hr or less, there is little variation in the thickness of the finished plate in the width direction.
[0010]
The present invention has been made based on such findings, and the features thereof are as follows.
[0011]
(1) In a method of manufacturing a hot-rolled steel sheet that is rolled after cooling the surface of the rolled material immediately before the hot finish rolling mill roll bite, the cooling is performed with the highest heat transfer coefficient and the lowest heat transfer in the width direction of the rolled material. A method for producing a hot-rolled steel sheet, characterized in that the difference from the rate is 200 kcal / m ² / ° C./hr or less.
[0012]
(2) The method for producing a hot-rolled steel sheet according to (1) above, wherein the rolled material is cooled with a slit laminator immediately before the hot finish rolling mill roll bite.
[0013]
(3) The method for producing a hot-rolled steel sheet according to (1) above, wherein the rolled material is cooled by mist cooling immediately before the hot finish rolling mill roll bite.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a rolled material cooling device 1 for cooling a rolled material 5 immediately before a roll bite of a hot finish rolling mill to be used in the practice of the present invention.
[0015]
The rolled material cooling device 1 can cool the rolled material so that the difference Δh between the maximum heat transfer coefficient and the minimum heat transfer coefficient in the width direction of the rolled material 5 is 200 kcal / m ² / ° C./hr or less. It has a cooling means.
[0016]
As such cooling means, for example, (1) means for cooling by slit laminar, (2) means for cooling by mist cooling, (3) means for cooling by roll cooling, (4) means for cooling by superheated liquid jet cooling (5) Means for cooling by forced air cooling can be considered.
[0017]
In FIG. 1, a rolling roll cooling device 2 is for cooling the rolling roll 3, and in this example, a header arranged along the axial direction facing the rolling roll and an appropriate length along the header. And a plurality of nozzles provided at regular intervals. In addition, components normally used before and after the rolling roll 3 such as a roll cooling water draining wiper 4, a side guide and a stripper guide, and a hot lubrication device (not shown) are not disturbed. It can be used at all within the range. However, the drainage wiper 4 for roll cooling water must always have a width that covers the plate width of the rolled material 5.
[0018]
FIG. 3 shows a case where the slit laminator cooling means (1) is used. FIG. 3 is a view taken along the line AA in FIG. This cooling means is composed of a header 1a disposed along the width direction of the rolled material 5 at a position immediately before the roll bite of the hot finish rolling mill, and a slit nozzle 1b provided along the longitudinal direction of the header 1a. The slit laminar 1c is ejected from the slit nozzle 1b.
[0019]
Since the slit laminator 1c discharged from the slit nozzle originally has no cooling capacity distribution in the width direction of the rolled material 5, the rolled material 5 does not generate a corrugated temperature distribution in the width direction of the rolled material 5 or the rolling roll 3. Can be rolled. Water or warm water is used as the fluid.
[0020]
When the mist cooling means (2) is used, the mist cooling is a method of cooling by jetting water from a nozzle together with a high-speed air flow. There are cases where air is used for the purpose of accelerating the liquid droplets to increase the cooling capacity, and air is used for the purpose of finely controlling the liquid cooling capacity without making a very large momentum and finely controlling the cooling capacity. The latter is sometimes referred to as fog cooling, but is included in the mist cooling of the present invention. Since there is almost no interference between nozzles, it is suitable for the cooling means of the present invention. A technique such as a slit mist nozzle that ejects mist from a slit is also known, and can be used as the cooling means of the present invention.
[0021]
The case where the roll cooling means (3) is used is shown in FIGS. 4 is a side view, and FIG. 5 is a view taken along the line AA in FIG.
[0022]
Immediately before the roll bite of the hot finish rolling mill, a water-cooled roll 6 using roll cooling that cools the surface of the rolled material 5 is provided by pressing the water-cooled roll 6 whose interior is water-cooled against the rolled material 5.
[0023]
In the cooling method of FIGS. 4 and 5, a specific temperature distribution due to cooling hardly occurs in the width direction of the rolled material 5. Since the rolling roll 3 is not in contact with the water-cooled roll 6, the corrugated temperature distribution does not occur in the width direction as in the prior art. Embodiments of the rolling roll cooling device 2 and the roll cooling water draining wiper 4 are the same as those shown in FIG.
[0024]
When the superheated liquid jet cooling means (4) is used, the superheated liquid jet cooling is a method in which superheated water is produced from waste heat and used for cooling. When superheated water is ejected from the nozzle, due to sudden boiling under reduced pressure, part of the liquid is self-evaporated and the liquid becomes finer, resulting in a high-speed vapor droplet two-phase flow spreading over a wide area. In this cooling method, a high-speed two-phase flow is collided with a high-temperature rolling material to perform cooling. Power can be reduced as compared with mist cooling using air, and a wide range of injections and more uniform cooling are possible.
[0025]
When the forced air cooling means (5) is used, the forced air cooling is a method of cooling by injecting gas onto the surface of the rolled material. Sometimes called blast cooling or forced air cooling. This technology is widely used in the iron making process. If sprayed from the slit nozzle, the rolled material can be uniformly cooled in the width direction. As the gas to be blown, not only air but also a gas species such as nitrogen gas that suppresses scale generation can be used.
[0026]
In the present invention, the cooling methods (1) to (5) described above can be used in any combination. Further, any cooling device may be used in the present invention as long as it has a uniform cooling capacity distribution in the width direction of the rolled material 5.
[0027]
In the hot rolling method of the present invention, the occurrence of a heat crown of the rolling roll 3 when the surface of the rolled material 5 is cooled immediately before the roll bite of the hot finish rolling mill is prevented. There is a basic effect of making the finished plate thickness uniform, and the surface of the rolling roll 3 is reduced by cooling the rolled material 5 immediately before the roll bite of the hot finish rolling mill to lower the surface temperature. Further, the growth of secondary scale is suppressed, and the descaling effect is added to reduce the scale defect.
[0028]
【Example】
An example in which the present invention is applied to a hot finish rolling mill will be described. The hot finish rolling mill of this example includes a finish rolling mill group consisting of seven rolling mills, and finish rolls a rough bar having a thickness of 30 to 40 mm with a finish rolling mill, and the finish rolling is 1.2 to 10 mm. Manufactures hot-rolled steel sheets. A rolling material cooling device 7 as shown in FIGS. 6 and 7 was installed immediately before the first to fifth roll bites of the finishing mill.
[0029]
When rolling was performed with this facility, the finished plate thickness in the width direction of the rolled material became corrugated corresponding to the arrangement of the nozzles 7b of the rolled material cooling device 7. Here, the width direction nozzle arrangement pitch of the nozzles 7b of the rolled material cooling device 7 is 130 mm, and the number of nozzles is 15. In the case of a hot-rolled steel sheet with a finishing plate thickness of 3 mm, the variation in the finishing plate thickness in the plate width direction reached 15 μm at the maximum.
[0030]
Therefore, as shown in FIGS. 1 and 3, when the rolled material was cooled with a slit laminator, the corrugated pattern disappeared from the finished thickness in the width direction of the hot-rolled steel sheet.
[0031]
【The invention's effect】
By applying the present invention, the surface of the rolled material can be uniformly cooled in the width direction of the rolled material immediately before the hot finish rolling mill roll bite, thereby reducing the roughness of the rolling roll and the scale property defects, The finished plate thickness in the plate width direction of the rolled material can be made uniform.
[Brief description of the drawings]
FIG. 1 is a side view showing a rolled material cooling device for cooling a rolled material immediately before a roll bite of a hot finish rolling mill to be used in the practice of the present invention. FIG. 3 is a graph showing an example of the relationship between variation (Δt) and the difference (Δh) between the maximum heat transfer coefficient and the minimum heat transfer coefficient in the width direction of the rolled material. FIG. 4 is a side view showing the rolling material cooling device by the roll cooling means of the present invention. FIG. 5 is an arrow view taken along the line AA in FIG. A side view showing a general cooling device FIG. 7 is a side view showing a rolling material cooling device for cooling the rolled material immediately before a roll bit of a prior art hot finish rolling mill. FIG. 8 is a cooling of the rolled material. Showing the axial temperature distribution of rolling rolls and the finished plate thickness distribution in the sheet width direction of the rolled material [Fig. 9] Figure [EXPLANATION OF SYMBOLS] indicating the Do distribution of the heat transfer coefficient generated by the cooling device (cooling capacity)
DESCRIPTION OF SYMBOLS 1 Rolling material cooling device 1a Rolling material cooling header 1b Rolling material cooling nozzle 1c Slit laminator 2 Rolling roll cooling device 3 Rolling roll 4 Drain wiper 5 Rolling material 6 Water cooling roll 7 Rolling material cooling device 7a Rolling material cooling header 7b Rolling material cooling nozzle

Claims (3)

In the method of manufacturing a hot-rolled steel sheet that is rolled after cooling the surface of the rolled material immediately before the hot finish rolling mill roll bite, the cooling is performed between the maximum heat transfer coefficient and the minimum heat transfer coefficient in the width direction of the rolled material. A method for producing a hot-rolled steel sheet, wherein the difference is 200 kcal / m ² / ° C./hr or less. The method for producing a hot-rolled steel sheet according to claim 1, wherein the rolled material is cooled by a slit laminator immediately before the hot finish rolling mill roll bite. The method for producing a hot-rolled steel sheet according to claim 1, wherein the rolling material is cooled by mist cooling immediately before the hot finish rolling mill roll bite.

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