KR20130123646A - Method for manufacturing of hot-rolled steel plate with excellent in flatness - Google Patents

Abstract

The present invention relates to a method for manufacturing hot-rolled steel plate with excellent flatness, and, more specifically, to a method for manufacturing hot-rolled steel sheet with excellent flatness by minimizing the defections of a plate shape which are generated when a strip is cooled on a run out table (ROT) during a hot-rolled process. Cooling devices that are disposed on the upper and lower sides respectively of the ROT, in particular, spray cooling water on the upper and lower sides of the hot-rolled steel sheet, and the quantity of cooling water is differentially applied to each section of the hot-rolled steel sheet in a process for cooling the hot-rolled steel sheet on the ROT. [Reference numerals] (AA) Existing cooling method;(BB) Cooling method of the present invention;(CC) Defective disc shape generation rate: 17.7%;(DD) Defective disc shape generation rate: 0%

Description

Manufacturing method of hot rolled steel sheet with excellent flatness {METHOD FOR MANUFACTURING OF HOT-ROLLED STEEL PLATE WITH EXCELLENT IN FLATNESS}

The present invention relates to a method of manufacturing a hot rolled steel sheet having excellent flatness, and more particularly, to excellent flatness by minimizing plate defects generated when cooling a hot rolled steel sheet in a run out table (ROT) during a hot rolling process. It relates to a method for manufacturing a hot rolled steel sheet having a.

In general, hot-rolled steel sheet is manufactured to have a desired thickness and width after reheating the slab (slab) produced by continuous casting at a high temperature, and then subjected to hot rolling processes such as rough rolling and finishing rolling.

In the hot rolling process, a slab heated in a heating furnace is rolled to a thickness of 25 to 55 mm in a roughing mill to produce a bar shape, and the bar is finally rolled in a finishing mill to 1.0. It is a process of making a hot rolled steel sheet (strip) of ˜20 mm.

As such, the hot rolled steel sheet obtained through the hot rolling process is transferred to a run out table (ROT), which is a cooling section, and cooled according to a required quality.

After the hot rolled hot rolled steel sheet is cooled by the coolant sprayed from the runout table ROT, the hot rolled steel sheet is wound in a coil shape by a predetermined length in the winder for convenient storage or movement. At this time, the hot-rolled steel sheet temperature at the time of winding according to the valve opening and closing of the injector is controlled to a target value.

Looking at the cooling method in the runout table (ROT) section for controlling the take-up temperature (CT) of the hot rolled steel sheet to the target value during the conventional hot rolling process,

First, the initial value coolant injection valve is opened and closed at the runout table ROT based on the initial setting information on the basis of when the hot rolled steel sheet enters the filament mill front end stand. Thereafter, after opening and closing the initial valve, the valve opening and closing according to the dynamic setting is performed. The dynamic valve opening and closing receives the performance value of the finishing rolling outlet temperature (FDT) and the rolling speed while the hot rolled steel sheet passes the ROT section. Opening and closing is corrected. Thereafter, in order to finely adjust the target winding temperature from the time when the winding temperature measurement is started, the valve opening and closing by feedback control is performed. When the above-described dynamic setting and feedback control are performed, the learning calculation function for the hot-rolled steel sheet cooled afterwards is performed to increase the predicted accuracy of the set equation model together with the collected performance data. At this time, initial setting, dynamic setting, feedback control and learning control are performed in connection with the winding temperature control system.

Meanwhile, in manufacturing the hot rolled steel sheet through such cooling control, when the hot rolled steel sheet is a steel pipe material having a carbon content of 0.20% or less, the flatness of the hot rolled steel sheet is very important, and the flatness of the hot rolled steel sheet for steel pipes is poor. In this case, poor storage and the occurrence of bending after printing cause productivity loss and malfunction during processing.

In the case of such a steel pipe material, a low-temperature winding design is performed at a target winding temperature of 570 ° C. in order to secure specific mechanical properties.

Conventionally, in order to secure a desired material through low temperature winding on a runout table (ROT) of about 100m, as shown in Figure 2 by operating a cooling device located in front of the runout table (ROT) to perform rapid cooling, lamina The material was secured in a short time by pouring all the upper and lower cooling water in the bank at the same ratio (for example, opening and closing the upper and lower six valves in the same manner).

However, such rapid cooling is advantageous for securing the material, but in the runout table (ROT), in the phase transformation from austenite to ferrite (+ pearlite) during cooling, the edge part and the center in the width direction are rapidly cooled. A negative transformation speed difference is caused, and thus a temperature difference occurs at both the edge and the center.

That is, by cooling, the crystal structure is deformed during phase transformation (FCC-BCC), thereby causing volume expansion, and cooling is continued even after phase transformation in the width direction, thereby causing shrinkage due to the coefficient of thermal expansion. For this reason, there arises a problem that plastic deformation occurs even by a small force that is less than the yield stress of the material during phase transformation.

More specifically, as shown in FIG. 1, when cooling is performed on the ROT by a conventional method, a transformation speed difference occurs in the width direction of the hot rolled steel sheet, and the edge portion of the hot rolled sheet is relatively compared to the center portion. Since the transformation speed is high, volume expansion due to phase transformation occurs earlier than the center portion in the width direction edge portion. Accordingly, the plastically deformed edge portion acts as a restraining force in the volume expansion of the center portion, resulting in a plate shape defect of the hot-rolled steel sheet.

In order to reduce such plate shape defects due to uneven expansion and contraction due to transformation speed difference in phase transformation during cooling, conventionally, both ends of the hot rolled steel sheet are reduced by reducing the main quantity of the rapid cooling device and the upper lamina bank cooling device. We tried to reduce the temperature difference during cooling of the edge part and center part.

However, simply reducing the amount of water supplied to the cooling device does not completely eliminate the plate shape defects occurring on the runout table ROT, and thus requires fundamental improvement.

One aspect of the present invention is to provide a cooling method capable of securing a plate shape having excellent flatness by minimizing a difference in strip width direction transformation speed in a runout table (ROT) section.

According to an aspect of the present invention, a runout table is obtained by hot rolling a steel slab including, by weight, C: 0.15 to 0.20%, Mn: 0.30 to 0.50%, Si: 0.02 to 0.04%, and P: 0.01 to 0.02%. In the method for producing a hot rolled steel sheet cooled at (ROT),

When the hot rolled hot rolled steel sheet enters the runout table (ROT), firstly air-cooling at 5 to 10 ° C./s to 860 to 820 ° C .; First water cooling the first air-cooled hot rolled steel sheet at 60 to 70 ° C / s to 780 to 740 ° C; Second air cooling the first water-cooled hot rolled steel sheet at 5 to 10 ° C./s to 740 to 700 ° C .; And a second water cooling step of the second air-cooled hot rolled steel sheet at 590 to 550 ° C. at 20 to 30 ° C./s.

According to the present invention, when the hot rolled steel sheet hot rolled on the runout table (ROT) is cooled, a phenomenon in which the plate shape is deteriorated by the proposed cooling method can be prevented, thereby improving product quality and productivity.

1 is a result of measuring the degree of volume expansion occurring in the edge portion and the center portion in the width direction of the hot rolled steel sheet cooling on the runout table (ROT).
2 is a diagram illustrating a cooling device on a runout table ROT.
3 is a diagram showing the division of the cooling method according to the comparative example (A) and the invention example (B) when cooling in the runout table (ROT).
4 is a result of measuring the temperature behavior, cooling rate and transformation rate for each cold section by applying the cooling method of Comparative Example (A) and Inventive Example (B) shown in FIG.
5 is a result of measuring the flatness of the hot-rolled steel sheet to which each cooling method shown in FIG. 3 is applied.

The present inventors have studied in depth to solve the problem of poor plate shape when cooling the strip in the runout table (ROT), in particular, when cooling the steel pipe material having a carbon content of 0.20% or less in the ROT, when cooling the strip in the ROT By differently applying the cooling method according to the temperature section, it is possible to minimize the transformation speed difference between the edge part and the center part in the width direction of the strip, thereby confirming that a hot rolled steel sheet having excellent flatness can be obtained. The present invention has been completed.

Hereinafter, as one aspect of the present invention, a strip cooling method in a runout table (ROT) of a hot rolling process will be described.

Here, the runout table (ROT) is a means for cooling the hot rolled steel sheet, the runout table (ROT) section is usually formed in a length of about 100m to secure the cooling time of the hot rolled steel sheet, cooling equipment that can spray the coolant Is provided. The cooling apparatus is provided with an injector for pouring coolant and a laminar bank for supplying coolant to the injector.

The method for manufacturing a hot rolled steel sheet cooled by a hot rolled steel slab according to the present invention in a runout table (ROT), when the hot rolled hot rolled sheet enters the runout table (ROT), the first air cooling to 860 ~ 820 ℃ ; First water cooling to 780˜740 ° C .; Second air cooling to 740˜700 ° C .; And 590 to 550 ° C. in a second water cooling step.

In particular, the present invention is based on the specific component of the steel pipe material (% by weight, 0.15% ≤ [C] ≤0.20%, 0.30% ≤ [Mn] ≤0.50%, 0.02% ≤ [Si] ≤0.04%, 0.01% ≤ [ In order to minimize the plate shape defects which are mainly generated in the hot rolled steel sheet having P] ≤0.02%), the cooling method is used to reduce the difference in the transverse transformation speed of the hot rolled steel sheet having the above component system and the cooling deviation of the upper and lower surfaces of the strip. It is characterized by applying differential for each temperature section.

Hereinafter, a process of cooling a hot rolled hot rolled steel sheet in a runout table (ROT) according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

In the cooling process for cooling the hot rolled hot rolled steel sheet according to the invention in the runout table (ROT),

As shown in FIG. 3, when the hot rolled hot rolled sheet enters the first air cooling section of the runout table ROT, the temperature of the hot rolled sheet is cooled to 5 to 10 ° C./s so as to cool down to 860 to 820 ° C. from the hot finish rolling temperature. Primary air cooling at speed.

Unlike the conventional hot rolled steel sheet which has been rapidly cooled as soon as it enters the runout table, air cooling without cooling the cooling water is intended to reduce the cooling deviation of the edge portions and the center portion in the width direction of the hot rolled steel sheet. .

Conventional early rapid cooling was to reduce the material deviation by securing the material of the steel sheet preferentially, but in this case, as soon as cooling starts, cooling deviation occurs at the edges and the center of the hot rolled steel sheet in the width direction. The problem that the plate shape becomes poor after finishing the process arose. In general, when the coolant is poured into the hot rolled steel sheet, a phenomenon in which the coolant flows from the center portion in the width direction to the edge portions occurs. As a result, the cooling proceeds more rapidly at the edge portion than at the center portion, resulting in a cooling deviation in the width direction of the hot-rolled steel sheet.

In order to solve this problem in the related art, by performing the initial rapid cooling to air cooling, it is possible to prevent the cooling deviation from occurring in the width direction of the hot rolled steel sheet during the initial cooling.

After performing air cooling until the temperature of the hot rolled steel sheet is in the range of 860 ~ 820 ℃, 60 ~ 70 ℃ / s to the temperature (780 ~ 740 ℃) at the time when the transformation of the hot rolled steel sheet primary water Cool at cooling rate.

At this time, it is preferable to apply the amount of cooling water poured into the upper surface of the hot rolled steel sheet from the upper portion of the runout table ROT less than the amount of cooling water poured into the lower surface of the hot rolled steel sheet. As described above, adjusting the amount of cooling water poured in the upper and lower portions of the runout table ROT is to apply the same amount of cooling water to the upper and lower surfaces of the hot rolled steel sheet in consideration of the flow rate difference of the runout table ROT cooling apparatus. For sake.

Considering the technical common sense, the coolant injected from the top of the runout table (ROT) flows along the edge at the center of the upper surface after contacting the upper surface of the hot rolled steel sheet, but the coolant injected from the lower portion of the hot rolled steel sheet After contacting the lower surface it falls directly in the direction of gravity. Accordingly, even when the same amount of cooling water is poured, more cooling water comes into contact with the upper surface of the hot-rolled steel sheet than the lower surface, resulting in cooling deviations between the upper and lower surfaces of the hot-rolled steel sheet during cooling.

Therefore, in the present invention, in order to apply the same flow rate to the upper surface and the lower surface of the hot-rolled steel sheet during the first water cooling, it is preferable to supply more cooling water from the lower portion than the upper portion of the runout table (ROT), for example, four upper portions when opening and closing the valve. The lower six valve openings can be applied.

After the hot rolled steel sheet is cooled to the transformation start temperature by the primary water cooling, the hot rolled steel sheet is secondly air cooled again to induce phase transformation. At this time, the air cooling is performed at a cooling rate of 5 ~ 10 ℃ / s until the temperature of the hot rolled steel sheet is in the range of 740 ~ 700 ℃.

As such, when air cooling is performed at the start of transformation during cooling, the transformation time from austenite to ferrite may be delayed.

Conventionally, the rapid transformation was induced by performing cold cooling at the start of transformation, but in this case, the transformation speed is relatively faster at the edge portion than the center portion of the hot rolled steel sheet, so that the difference in the width transformation speed is different. Occurred. That is, when phase transformation is accompanied by cooling, the crystal structure is deformed (FCC-BCC) and volume expansion occurs. In the width direction, both edges of the width direction are caused by phase transformation before the center. Afterwards, cooling is continued to cause shrinkage due to the coefficient of thermal expansion. At this time, the plastically deformed edge portion acts as a restraining force in the volume expansion of the center portion, which has a relatively slow transformation speed.

As such, the volume expansion caused by the phase transformation occurs at different points in time along the width direction of the hot-rolled steel sheet, resulting in a problem of poor plate shape.

However, in the present invention, by performing air cooling at the start of the transformation during cooling in the runout table (ROT), it delays the time of transformation from austenite to ferrite (+ pearlite) as a whole, and in addition, the edge portion in the width direction of the hot rolled steel sheet It is possible to reduce the transformation speed difference between the center and the center part. In other words, it is possible to minimize the difference in the transformation speed of the hot rolled steel sheet in the width direction which is a fundamental cause of poor plate shape of the hot rolled steel sheet.

After the phase transformation is completed by the secondary air cooling, the hot rolled steel sheet is secondly water-cooled again and cooled at a cooling rate of 20 to 30 ° C./s to 590 to 550 ° C.

At this time, when the cooling water is poured in the cooling apparatus, the amount of cooling water poured in the upper portion is applied less than the amount of cooling water in the lower portion of the runout table ROT as in the case of the first water cooling.

In addition, in the hot rolling process, the rolling speed is differently applied according to the thickness of the hot rolled steel sheet, but in the cooling process, it is preferable to apply a cooling water pattern required differently according to the rolling speed, and in particular, the transformation speed delay effect in the second water cooling section. It is desirable to apply a doubling pattern in order to continuously secure this condition. As shown in (B) of FIG. 4, it is possible to effectively minimize the cooling deviation due to rapid cooling in a section involving transformation by applying the cooling rate relatively slowly as the pouring pattern is differentially applied.

Specifically, for example, the valve opening and closing, if the thickness of the hot-rolled steel sheet is less than 6.0mm when the valve opening and closing the upper two, four valve opening and closing, if the thickness is more than 6.0mm and less than 9.0mm three upper It is desirable to apply the bottom five valve openings. In addition, when the thickness is more than 9.0mm, it is preferable to apply four upper and six lower valve openings and closings, and in this case, very small rapid cooling (for example, one upper and one lower valve) in the first air cooling section for the purpose of securing the material. May be additionally applied.

As described above, by setting the air-cooling section during cooling in the runout table (ROT) to secure the material of the hot-rolled steel sheet, the edge portion and the center portion in the width direction of the hot-rolled steel sheet by conventional rapid cooling It is possible to improve the transformation speed difference at.

When the secondary water cooling is completed, the subsequent compensation cooling section may control the winding temperature CT by performing compensation cooling through opening and closing the valve of the cooling device in order to secure a target final winding temperature CT. . At this time, the water injection pattern of the cooling water is not particularly limited, but the upper and lower six valve opening and closing can be applied, which can be applied differently depending on the material.

Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are only illustrative of the present invention in more detail and do not limit the scope of the present invention.

( Example )

Thickness for steel pipe material (% by weight, C: 0.21%, Mn: 0.467%, Si: 0.03%, Al: 0.035%, P: 0.0149%, S: 0.0018%, Cr: 0.02%, Ni: 0.01%) When cooling the 3mm hot rolled steel sheet in the runout table (ROT), as shown in Table 1 below, after applying the valve opening and closing according to the conventional cooling method (comparative example) and the cooling method (invention example) according to the present invention, the cooled Rolling data of each hot rolled steel sheet were collected, respectively, and the results are shown in FIGS. 4 and 5.

division Cooling method Comparative Example Rapid Cooling Section General cooling section (water cooling) Compensation Cooling Section #One #2 # 3 #4 # 5 # 6 # 7 #8 # 9 # 10 # 11 # 12 # 13 # 14 # 15 Top
Valve opening and closing 3 3 6 6 6 6 6 6 6 0 0 0 0 6 6 bottom
Valve opening and closing 3 3 6 6 6 6 6 6 6 0 0 0 0 6 60 Cooling rate Cold Cold - Cold 52.46 ° C / s 32.25 ° C / s - - Honor 1st air cooling
section First water cooling
section Second air cooling
section 2nd water cooling
section Compensation Cooling Section #One #2 # 3 #4 # 5 # 6 # 7 #8 # 9 # 10 # 11 # 12 # 13 # 14 # 15 Top
Valve opening and closing 0 0 4 4 0 0 0 2 2 2 2 2 2 6 6 bottom
Valve opening and closing 0 0 6 6 0 0 0 4 4 4 4 4 4 6 6 Cooling rate Air cooling Water cooling Air cooling Water cooling (slow cooling) Cold 7.99 ℃ / s 64.52 ℃ / s 6.5 ℃ / s 21.9 ℃ / s -

First, FIG. 4 is a result of measuring the temperature behavior and cooling rate for each cooling section according to the application of the cooling method of Comparative Example (A) and Inventive Example (B) shown in FIG. 3, and the cooling method of Inventive Example (B). In this case, the transformation speed difference between the edge part and the center part of the hot rolled steel sheet can be minimized in comparison with the comparative example (A) by inducing transformation delay in the phase transformation section of the hot rolled steel sheet during cooling. You can check it.

In addition, Figure 5 is a measure of the flatness of the hot-rolled steel sheet to which each cooling method is applied, it can be seen that the plate shape failure rate is 17.7% very poor plate shape when the comparative example is applied. On the other hand, in the case of applying the invention example, it can be seen that the flatness of the hot rolled steel sheet is very excellent as the occurrence rate of the plate shape defect is 0%.

When the hot rolled steel sheet is cooled in the runout table (ROT), as the phase transformation time becomes longer, the transformation starts at the center portion before the edge transformation is completed and the restraint force is weakened. Defects can be interpreted as minimized results.

Claims (3)

Hot rolled steel slab containing C: 0.15-0.20%, Mn: 0.30-0.50%, Si: 0.02-0.04%, and P: 0.01-0.02%, followed by cooling in a runout table (ROT) In the manufacturing method of the steel sheet,
When the hot rolled hot rolled steel sheet enters the runout table (ROT), firstly air-cooling at 5 to 10 ° C./s to 860 to 820 ° C .;
First water cooling the first air-cooled hot rolled steel sheet at 60 to 70 ° C / s to 780 to 740 ° C;
Second air cooling the first water-cooled hot rolled steel sheet at 5 to 10 ° C./s to 740 to 700 ° C .; And
Cooling the second air-cooled hot rolled steel sheet to 590-550 ° C. at a second water cooling at 20˜30 ° C./s
Method for producing a hot rolled steel sheet excellent in flatness comprising a.
The method of claim 1,
The first air cooling step and the second air cooling step may delay the phase transformation of the hot-rolled steel sheet, thereby reducing the difference in the transformation speed difference between the edge portion and the center portion in the strip width direction. Manufacturing method.
The method of claim 1,
The first water cooling step and the second water cooling step cool by pouring coolant from the upper and lower portions of the runout table (ROT) to the upper and lower surfaces of the strip, and the amount of cooling water injected from the upper part is lower than the amount of cooling water supplied from the lower part. Method for producing a hot rolled steel sheet having excellent flatness, characterized in that less applied.

Images