JP5991023B2 - Steel strip production method using continuous hot rolling equipment - Google Patents

Description

  The present invention relates to a continuous hot rolling facility that cools a hot-rolled steel strip hot-rolled to a thin plate having a thickness of 1.8 mm or less to a coiling temperature of 500 ° C. or less, and is capable of reducing variations in thickness and cooling unevenness. This is a technology related to the production of suppressed thin hot-rolled steel strip.

  In the production of a hot-rolled steel strip, a steel plate heated at a high temperature with a heating device is rolled to a desired size, and then water-cooled with a run-out table from the viewpoint of material adjustment. That is, the purpose of the water cooling is performed to adjust the material such as the intended strength and elongation mainly by controlling the precipitates and transformation structure of the steel strip. In particular, it is most important to accurately control the cooling end temperature (winding temperature) in order to produce a desired material without variation.

On the other hand, in the water cooling process, water having a low cost is often used as a cooling medium. In this case, if the cooling end temperature (winding temperature) is set low, the occurrence of temperature unevenness becomes a problem, and becomes more noticeable as the plate thickness is thinner. In addition, if the steel strip is rapidly cooled from the front end portion, head breakage at the front end portion becomes a problem, and it is not possible to cool from the front end portion to the target cooling end temperature.
The first cause of the above temperature unevenness is that the cooling water boils when it gets wet with the steel strip, but the boiling form changes at a certain temperature and the heat transfer capacity changes, and the temperature is lower than this temperature. This is because, when cooled, the cooling end temperature may not be accurately controlled.

  Here, the boiling mode when the steel strip is cooled with water will be described. Film boiling occurs when the steel strip surface temperature is high, nucleate boiling occurs in the low temperature region, and transition boiling occurs in the intermediate temperature region between the high temperature region and the low temperature region. In film boiling that occurs in a high temperature state, a vapor film is generated between the steel strip surface and the cooling water, and heat is transferred by heat conduction in the vapor film, so that the cooling capacity is low. On the other hand, in nucleate boiling that occurs in a low temperature state, the steel strip surface and cooling water are in direct contact with each other, and vapor bubbles formed by evaporation of a part of the cooling water from the steel strip surface are generated and condensed immediately by the surrounding cooling water. A complicated phenomenon such as disappearance occurs, and the cooling water is agitated due to the generation and disappearance of the vapor bubbles, so that it has a very high cooling capacity. In the intermediate temperature region between the film boiling temperature region and the nucleate boiling temperature region, a transition boiling state in which film boiling and nucleate boiling are mixed is obtained. In the transition boiling state, unlike nucleate boiling and film boiling, a phenomenon occurs in which the heat flux increases as the steel strip temperature decreases. From the viewpoint of material control, it is not preferable to change the cooling rate depending on the temperature, and if the cooling is stopped in the temperature region where the film boiling state transitions to the transition boiling state, the cooling rate is accelerated in the transition boiling region. Since it becomes high, there is a problem that the temperature of the steel strip becomes much lower than intended when the cooling control time is slightly increased.

In addition, when there is a locally low temperature region in the steel strip before cooling due to rolling or the like, the temperature deviation increases because the surface temperature shifts to transition boiling at an early timing during this cooling. To do. In a cooling device installed on a general run-out table, this transition boiling start temperature is about 500 ° C.
As a second cause of the occurrence of the temperature unevenness, there is one caused by stagnant water on the steel strip. This is because a normal run-out table is cooled by a cooling device such as a circular tube laminar or a slit laminator, especially when cooling from the upper surface, but the cooling water that has collided with the steel strip stays on the steel strip along with the steel strip. It flows out in the direction of travel. Although the cooling water on the steel strip is usually removed by draining purge or the like, the removal process is performed away from the point where the cooling water collided with the steel strip. Only the part where it is cooled down relatively much. In particular, in the case of a low temperature region of 500 ° C. or lower, this stagnant water changes from a film boiling state to a transition boiling state, so that the cooling capacity is high, and a large temperature is generated between the part where the stagnant water is on and the part where it is not. Deviation occurs. For the above reasons, if the hot-rolled steel strip is finished cooling at 500 ° C. or less, which is the transition boiling start temperature, the temperature variation in the coil increases.

  In recent years, there has been an increasing demand for materials with high strength and thin plate thickness. The plate thickness is 1.8 mm or less, and the rolled steel plate is rapidly cooled to refine the structure and control the transformation structure. Techniques for increasing the strength and workability of steel sheets are being studied. At that time, in the region where the plate thickness is thin, fluctuations in the thickness of the tip and tail ends and cooling unevenness are more likely to occur. In particular, in order to ensure the plate-passability at the tip, there is acceleration / deceleration of the plate-pass, so that the final temperature target is conventionally controlled to be 550 ° C. or higher. As a result, there is a problem that the amount of cutting off at the tip reaches as much as 100 m or more.

Here, as a conventional continuous hot rolling facility, there are facilities described in Patent Documents 1 and 2, for example.
Patent Document 1 describes that, for the purpose of producing a thin-scale steel plate, a large-capacity cooling is performed on a run-out table so as to aim for a thin scale and uniform temperature. That is, it describes that rapid cooling is performed in order to peel off the scale generated in the idle running section and suppress the generation of scale in the cooling zone.

  Patent Document 2 describes that, for example, a steel strip having a steel strip thickness of about 3.2 mm is manufactured to produce a steel plate with little temperature variation by performing large flow density cooling in a low temperature region. Yes.

Japanese Patent Laid-Open No. 08-332514 JP 2008-110353 A

With the technique described in Patent Document 1, although temperature unevenness due to stagnant water on the upper surface of the steel strip can be reduced, cooling on the run-out table can only be performed up to 450 ° C. Moreover, there exists a possibility that the thin steel plate of 1.8 mm or less thickness targeted by this invention cannot be manufactured stably.
Further, in the technique described in Patent Document 2, it is unclear to what extent the plate thickness variation occurs when a thin plate is used. In order to perform the operation by raising the height, there is a problem that the defective portion at the tip and tail ends is generated only for the steel strip through which the plate is passed, and the yield is low.

  The present invention has been made paying attention to the above points, and an object of the present invention is to suppress defective portions at the tip and tail ends while suppressing variations in plate thickness and uneven cooling.

In order to solve the above-described problems, the present invention provides a sheet bar coiler that winds and rewinds a sheet bar after rough rolling, a leading end portion of a sheet bar trailing material that has been unwound from the sheet bar coiler, and a sheet bar preceding A joining device that joins the tail end of the material, a heating device that heats the sheet bar that is arranged and conveyed downstream of the joining device, and a steel strip that is finish-rolled to the sheet bar heated by the heating device, A finishing rolling mill, and a cooling device that cools the steel strip rolled by the finishing rolling mill while being transported by the run-out table. The cooling device includes a first cooling device on the upstream side and a second cooling device on the downstream side. using a cooling device and a continuous hot rolling facility Ru provided with,
After winding the sheet bar after rough rolling by the sheet bar coiler, the sheet bar is rewound from the sheet bar coiler, and the tail end portion and the sheet bar of the sheet bar preceding material rewound by the joining device It becomes a continuous seat bar by joining the tip of the trailing material,
When heating the continuous sheet bar with the heating device, the central portion in the longitudinal direction of the continuous sheet bar is heated to a temperature estimated to be within the target temperature range at the finish rolling mill outlet side temperature, and the continuous The both ends of the sheet bar are heated to a temperature estimated to be outside the target temperature range at the finish rolling mill exit temperature,
Furthermore, the continuous sheet bar is finish-rolled to a target plate thickness of 1.8 mm or less with the above-mentioned finish rolling mill to form a steel strip,
When cooling the steel strip with the cooling device,
In the first cooling device, water flow density of 350L / (min · ^{m 2)} or more 1200L / (min · ^{m 2)} or less of the cooling water and the steel strip surface temperature 500 ° C. in the first cooling device outlet side Cool so that
In the second cooling device , with a cooling water having a water flow density of 2000 L / (min · m ² ) or more, the steel strip surface temperature is cooled to a range of 450 ° C. or lower and 400 ° C. or higher ,
A steel strip having a tensile strength of 390 MPa or more with improved hole expansibility is manufactured .

  According to the present invention, the steel strip is continuously rolled and cooled by sequentially joining the steel strip upstream of the finish rolling mill. As a result, speed adjustment is unnecessary at the leading end of the steel strip, except for the most advanced tip and the rear end of the rearmost steel strip, and the defective portion is greatly reduced. It is possible to suppress the deterioration of the accuracy of the cooling control due to the speed fluctuation. In particular, the present invention is particularly effective because the thin plate targeted by the present invention is sensitive to temperature changes.

  Furthermore, the first cooling device and the second cooling device are provided as the cooling devices, and the first cooling device performs cooling within the temperature range of the film boiling region, and then the water amount is greatly increased and cooled by rapid cooling. The period of the transition region can be reduced or reduced to zero to achieve cooling in the nucleate boiling region. As a result, even if it is a thin plate, it is possible to suppress defective portions at the tip and tail ends while suppressing variation in thickness and cooling unevenness.

  Thus, even with a thin plate, it is possible to perform cooling control with high accuracy while reducing the yield.

It is a schematic diagram of the continuous hot rolling facility which concerns on embodiment based on this invention. It is a schematic diagram explaining an oblique laminar type rapid cooling equipment. It is a figure explaining winding temperature at the time of cooling processing, without joining each coil (steel strip). It is a figure explaining coiling | winding temperature at the time of joining and cooling each coil (steel strip) sequentially. It is a figure which shows plate | board thickness and temperature at the time of applying this invention. It is a figure which shows the plate | board thickness and temperature in the case of a comparative example.

Next, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram illustrating a continuous hot rolling facility according to an embodiment of the present invention.
As shown in FIG. 1, the continuous hot rolling facility of the present embodiment has a heating furnace 1, a rough rolling mill 2, a sheet bar coiler 3, a joining device 4, and a high frequency induction heating device 5 from the upstream side toward the downstream side. The finish rolling mill 6, the run-out table 11, and the winding device 12 are arranged in this order.

  A first cooling device 7 on the upstream side and a second cooling device 8 on the downstream side are provided along the runout table 11, and the two cooling devices 7 and 8 pass through the runout table 11. In the meantime, the steel strip 9 is cooled. In the present embodiment, a case will be described in which the first cooling device 7 is configured by a circular laminar facility 7 and the second cooling device 8 is configured by an oblique laminar type rapid cooling facility 8.

The heating furnace 1 heats the slab to a preset slab temperature.
The rough rolling machine 2 roughly rolls the heated slab to a preset plate thickness.
The sheet bar coiler 3 winds and unwinds a sheet bar (steel strip 9) formed by rough rolling.
The joining device 4 joins the tail end portion of the preceding material and the tip end portion of the succeeding material sequentially by butt welding. As a result, the coil is continuously conveyed to the finishing mill and the cooling device.

The high frequency induction heating device 5 is joined by the joining device 4 so that the temperature in the longitudinal center (steady portion) excluding both ends of the continuous steel strip 9 is within the target temperature range at the finish rolling exit temperature. Heat to the estimated temperature. In addition, the temperature at the finishing input which becomes the temperature preset at the finishing rolling exit temperature can be obtained by the conveying speed or the like. The high-frequency induction heating device 5 heats a portion including the outermost winding and the innermost winding when the sheet bar is wound up after joining and before finish rolling over the entire width.

The finish rolling mill 6 finish-rolls to a target plate thickness of 1.8 mm or less.
Circular tube laminar facility 7, by water flow density injects 350L / (min · ^{m 2)} or more 1200L / (min · ^{m 2)} or less of the cooling water from the nozzle, the steel strip 9 being conveyed, circular tube laminar Cooling is performed so that the steel strip surface temperature on the exit side of the facility 7 is 500 ° C. The cooling control is performed by adjusting the cooling amount based on the temperature on the exit side of the finishing mill 6 and the steel plate temperature.

The rapid cooling equipment 8 cools the steel strip 9 to be conveyed so that the steel strip surface temperature is 450 ° C. or less by injecting cooling water having a water flow density of 2000 L / (min · m ² ) or more from the nozzle. To implement.
As shown in FIG. 2, the rapid cooling equipment 8 is provided with a group of nozzles 10 that are paired with an interval in the conveying direction of the steel strip 9. Each group of nozzles 10 is set such that the injection axis is inclined by an inclination angle θ toward the nozzle group that forms a pair with the vertical direction. The inclination angle θ is set in the range of, for example, 30 degrees to 60 degrees, preferably 30 degrees to 50 degrees. However, it arrange | positions so that the jet water flow from the nozzle 10 group which opposes may not cross | intersect.

Thus, the cooling water sprayed from each pair of nozzles 10 is moved toward the other nozzle group 10 after colliding with the steel strip surface from an oblique direction inclined in the steel plate conveyance direction. As a result, the cooling water sprayed from each of the paired nozzles 10 collides with the steel strip surface and then collides with each other in the steel plate conveying direction on the steel strip surface, and is discharged in the steel strip width direction by the collision. . This eliminates or significantly reduces the water remaining on the steel strip surface.
In addition, the shape of the cooling water injected from each nozzle of the circular tube laminar equipment 7 or the rapid cooling cooling equipment 8 may be laminar or jet flow. There is no particular limitation as long as the water flow density can be secured.

(Operation other)
This is a sheet bar obtained by roughly rolling a slab heated to a predetermined temperature in a heating furnace 1 with a roughing mill 2, winding a sheet bar formed by the rough rolling with a sheet bar coiler 3, and rewinding from the sheet bar coiler 3. The leading end portion of the succeeding material is joined to the tail end portion of the preceding material by the joining device 4. Subsequently, the portion including the outermost winding and the innermost winding when the sheet bar is wound up after joining and finish rolling is heated by the high frequency induction heating device 5 over the entire width, thereby finishing the entire length of the coil. Finishing rolling is continuously performed by the finishing mill 6 with the delivery temperature within the target temperature range.

With the above equipment configuration, stable plate-passability on the run-out table 11 can be ensured for the second and subsequent coils while suppressing fluctuations in plate thickness.
Thereafter, the tube laminar equipment 7 is used to cool to 500 ° C., which is the lower limit value of the film boiling region or near the lower limit value, and then the oblique laminar type rapid cooling equipment is used to rapidly cool to 450 ° C. or less. .

  In normal run-out cooling, transition boiling starts at about 500 ° C., and the heat flux increases as the temperature decreases. At this time, in the present embodiment, as the heat transfer characteristics, the amount of cooling water is increased, and the transition boiling start temperature and the temperature for maintaining nucleate boiling are increased. In this way, when the steel strip temperature is up to about 500 ° C., normal run-out cooling is performed by the circular tube laminar equipment 7, and the cooling water amount is increased in the temperature range below that to cool all in the nucleate boiling region. If controlled, transition boiling does not occur in the cooling device, but only during a small period, and it is possible to suppress the occurrence of temperature deviation after cooling and the decrease in accuracy of the cooling stop temperature.

That is, the first cooling device 7 is cooled to about 500 ° C. using an existing run-out cooling device, and the second cooling device 8 is rapidly used at a water density of 2000 L / (min.m ² ) for reliably performing nucleate boiling. By cooling, it becomes possible to cool so as to avoid the transition boiling region.
Further, in the quenching cooling equipment 8 as the second cooling device, a laminar or jet water flow such as a jet flow ejected from a nozzle through a slit or a circular pipe is opposed to form an oblique laminar type so that it can be opposed in the direction of conveying the steel plate. Due to the collision of the cooling water flow on the steel strip surface, the cooling water is discharged so as to flow in the width direction of the steel strip 9, and the stagnant water is removed from the steel strip surface at the cooling position. As a result, a draining device is not required, and both cooling and draining can be performed.

Here, when the coil of the sheet bar is not joined, as shown in FIG. 3, defective portions are generated at the front end portion and the tail end portion of each coil. In addition, in the control method for performing the hot rolling process for each coil as described above, conventionally, in the case of a thin plate having a plate thickness of 1.8 mm or less, from the viewpoint of stability of the plate passing properties of the tip and tail ends. Increase the temperature target and control.
On the other hand, in this embodiment, as shown in FIG. 4, the tip of the first coil has the same temperature target as the conventional one, but the temperature target can be kept constant from the tail end of the first coil. It should be noted that the tail end of the final material is passed through with the temperature target raised.

  Thereby, in this embodiment, the unsteady part of a front-end | tip end can be decreased by continuous rolling. For example, when making a conventional 2-coil material, the unsteady portion at the tip and tail ends occurs four times. However, it is possible to suppress to twice by using continuous rolling. By using the method of this embodiment, it is possible to suppress the unsteady portion of the tip / tail end portion and reduce the number of times of performing the target temperature increase at the tip / tail end portion as in the prior art. Moreover, as a result of reducing acceleration / deceleration of the plate passing speed, stable plate passing is possible on the run-out table 11 that performs cooling. This also makes it possible to suppress defective portions.

As described above, in the present embodiment, even when the sheet steel strip 9 has a plate thickness of 1.8 mm or less and is set to a winding temperature of 450 ° C. or less, the plate thickness variation and cooling unevenness are suppressed stably. Since the thin hot-rolled steel strip 9 can be manufactured and the number of unsteady portions can be reduced, it can be expected that defective portions are suppressed.
Here, the present invention is suitable for the production of a steel plate having a thickness of 1.8 mm or less and having particularly high temperature sensitivity. For example, it is suitable for a steel plate having a tensile strength (TS) of 390 MPa or more, such as a high-tensile material.

The reason is as follows.
In a steel sheet having a tensile strength (TS) of 390 MPa or more, when the coiling temperature is lower than 500 ° C., a bainite structure appears and the strength is rapidly increased as the temperature decreases. When the temperature is lower than 400 ° C., a martensite structure is formed. Thus, in the coiling temperature range of 400 ° C. to 500 ° C. including the coiling temperature of 450 ° C. of the present invention, the steel sheet having a tensile strength (TS) of 390 MPa or more becomes the bainite structure region.

As described above, when the coiling temperature is less than 500 ° C., the strength rapidly increases as the temperature decreases, that is, a slight temperature fluctuation is directly connected to the strength fluctuation, and thus a technique capable of suppressing the temperature fluctuation as in the present invention. By adopting, it becomes possible to produce a more targeted steel sheet.
In addition, by controlling the coiling temperature in the region of the bainite structure, the hole expandability can be increased as compared with the case of controlling in the region of the pearlite structure or the martensite structure.

  In addition, when manufacturing the thin steel strip 9 having a thickness of 1.8 mm or less, when cooling is performed only by the first cooling device 7 or the second cooling device 8, the tip end portion and the tail end portion The coiling temperature is likely to cause hunting even in the stationary part except for. In particular, the hunting becomes noticeable when the coiling temperature is set to 500 ° C. or lower. In this case, the yield is lowered.

Next, the production of the hot-rolled steel strip 9 will be described with respect to the embodiment of the present invention.
A slab having a thickness of 260 mm was heated to 1200 ° C. in the heating furnace 1, then rolled to 28 mm by the roughing mill 2, and further rolled to 1.6 mm by the finishing mill 6.
FIG. 5 shows the finish rolling mill 6 outlet side plate thickness and temperature history at the time of winding (when two coils are joined) when continuous rolling is performed and a circular tube laminar and oblique laminar type rapid cooling equipment is used. . Moreover, as a comparative example, FIG. 6 shows the history of the outlet side plate thickness and the temperature history during cooling using only the circular tube laminator by the conventional rolling method.

As can be seen from FIG. 5, when the present invention is applied, both the thickness and temperature are better than the conventional material, the variation can be reduced, and the tip and tail ends are cut off less clearly. It is.
On the other hand, in the case of the comparative material of FIG. 6, it is clear that the thickness and temperature deviation of the tip and tail ends are large and the number of defective parts increases as compared with the inventive material.

  Thus, it has been found that application of the present invention can reduce variations in temperature and plate thickness and can also reduce defective portions at the tip and tail ends.

DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Coarse rolling mill 3 Sheet bar coiler 4 Joining device 5 High frequency induction heating device 6 Finishing rolling mill 7 1st cooling device (circular tube laminar equipment)
8 Second cooling device (rapid cooling equipment)
9 Steel strip 10 Nozzle (second cooling device)
11 Runout Table 12 Winding Device

Claims (1)

A sheet bar coiler that winds and unwinds the sheet bar after rough rolling;
A joining device that joins the leading end of the seat bar trailing material unwound from the seat bar coiler and the tail end of the seat bar preceding material;
A heating device that heats a sheet bar that is arranged and conveyed downstream of the joining device;
A finish rolling mill that finish-rolls the sheet bar heated by the heating device into a steel strip ;
A cooling device that cools the steel strip rolled by the finish rolling mill during conveyance by the run-out table,
The cooling apparatus uses a first cooling device upstream, a second cooling device and Ru between successive hot rolling facility provided with the downstream side,
After winding the sheet bar after rough rolling by the sheet bar coiler, the sheet bar is rewound from the sheet bar coiler, and the tail end portion and the sheet bar of the sheet bar preceding material rewound by the joining device It becomes a continuous seat bar by joining the tip of the trailing material,
When heating the continuous sheet bar with the heating device, the central portion in the longitudinal direction of the continuous sheet bar is heated to a temperature estimated to be within the target temperature range at the finish rolling mill outlet side temperature, and the continuous The both ends of the sheet bar are heated to a temperature estimated to be outside the target temperature range at the finish rolling mill exit temperature,
Furthermore, the continuous sheet bar is finish-rolled to a target plate thickness of 1.8 mm or less with the above-mentioned finish rolling mill to form a steel strip,
When cooling the steel strip with the cooling device,
In the first cooling device, water flow density of 350L / (min · ^{m 2)} or more 1200L / (min · ^{m 2)} or less of the cooling water and the steel strip surface temperature 500 ° C. in the first cooling device outlet side Cool so that
In the second cooling device , with a cooling water having a water flow density of 2000 L / (min · m ² ) or more, the steel strip surface temperature is cooled to a range of 450 ° C. or lower and 400 ° C. or higher ,
A method for producing a steel strip by continuous hot rolling equipment, comprising producing a steel strip having a tensile strength of 390 MPa or more with enhanced hole expansibility .

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