KR950001934B1 - Method of manufacturing hot rolling plate - Google Patents

Abstract

The method regulates the mechanical property of the hot roll steel plate by making an air cooling space before a water cooling space when cooling the steel plate in a water cooling bank. The method controls the position of a water cooling start bank in order to make the position of the air cooling space before the water cooling space so that a constant portion of Ar3 transformations occur at both sides of the hot roll steel positioned in the air cooling space, and the Ar3 transformation occur at the center of the steel positioned in the water cooling space.

Description

Manufacturing method of hot rolled steel sheet with uniform mechanical properties in the width direction

1 is a schematic view of a conventional continuous hot rolling mill.

Figure 2 is a schematic diagram of the temperature change of the center and the width direction of the hot rolled steel sheet in the water cooling zone according to the conventional method.

Figure 3 is a schematic diagram of the temperature change of the center and the width direction of the hot rolled steel sheet in the water cooling zone according to the present invention.

4 is a graph showing changes in tensile strength and elongation in the width direction of hot rolled steel sheets produced by conventional methods and inventive methods.

* Explanation of symbols for main parts of the drawings

1: heating furnace 2: roughing mill

3: finish rolling mill 4: water cooling stand

4a: water-cooled bank 5: winding machine

The present invention relates to a method for manufacturing a hot rolled steel sheet by a continuous hot rolling facility, and more particularly, to a method for manufacturing a hot rolled steel sheet (hereinafter referred to as "hot rolled steel sheet") with uniform mechanical properties in the width direction. It is about.

In general, when the hot rolled steel sheet is manufactured by a continuous hot rolling mill, both ends of the width direction are formed of a material having higher strength and less elongation than the center part, and in particular, as the material becomes stronger, the variation in the mechanical properties of the width direction increases.

Such non-uniformity of the mechanical properties in the width direction causes plate deformation during cutting processing, non-uniform deformation during molding processing, occurrence of cracks, and the like, and has become a chronic problem of degrading product quality.

The cause of the nonuniformity in the width direction is mainly due to the cooling nonuniformity in the width direction, and the degree depends on the manufacturing conditions such as the thickness, width, rolling speed, and water cooling of the steel sheet, but the width of both ends of the width direction after hot rolling It is known that the temperature is about 50-100 ° C lower than the center. As shown in FIG. 1, the water cooling stand of the continuous hot rolling facility is arranged with tens of upper and lower water cooling banks composed of nozzles capable of discharging or intermitting a certain quantity by an electrical signal. . In Fig. 1, reference numeral 1 is a heating furnace, 2 is a rough rolling mill, 3 is a finish rolling mill, 4 is a water cooler, 4 is a water cooling bank, and 5 is a odor. Indicates. The length of the water cooler, the number of banks, and the amount of discharged water are designed to sufficiently cool various thicknesses, widths, transfer speeds, and steel grades. Therefore, the cooling water is not discharged from all water cooling banks. The number of water-cooled banks through which water is discharged to be wound up in the winder is added or subtracted. At this time, the number of the water-cooled banks from which the coolant is discharged from the exit of the finishing mill to be wound at the target winding temperature in consideration of the cooling rate by water cooling and air cooling and the finish rolling exit temperature measured at the center of the width direction of the steel sheet and the winding temperature. It is controlled to add or subtract. Therefore, the temperature change of the hot-rolled steel sheet in the water cooling zone after the normal hot rolling, as shown in FIG.

The mechanical properties of hot-rolled steel sheet are highly dependent on the cooling rate in the temperature zone where phase transformation occurs during cooling.It is well known that the higher the cooling rate, the higher the strength and the lower the elongation, while the lower the cooling rate, the lower the strength and the higher the elongation. have. Therefore, as shown in FIG. 2, in the case of the central portion in the width direction, the phase transformation starts in the water-cooling part and the phase transformation is completed in the air-cooling part. The supercooled portion at both ends of the width direction has an increase in strength and a non-uniformity in the width direction of the mechanical properties compared to the center portion.

Conventional methods for reducing the nonuniformity of the material due to the widthwise temperature unevenness include a method of providing an edge heater between the roughing mill and the finish rolling mill to compensate for the temperature by heating both ends of the steel sheet in the widthwise direction. . However, this method has the disadvantage of high equipment price and maintenance cost.

According to the present invention, when the hot rolled steel sheet is cooled in the water cooling stand, the position of the water cooling start bank in the water cooling bank is controlled appropriately so that an air cooling part is formed in front of the water cooling part unlike the conventional method, so that the mechanical properties are uniform in the width direction. To provide a rolled steel sheet, the purpose is.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention is a method of hot-rolling a steel slab, and then water-cooled in a water cooling stand having a plurality of water-cooled banks, and then wound up to produce a hot-rolled steel sheet, wherein the air-cooling portion at the water cooling stand is sheared, water-cooled. The present invention relates to a method of manufacturing a hot rolled steel sheet having a uniform mechanical property in the width direction by controlling the position of a bank at the time of water cooling to be formed at an additional rear end.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

Unlike the conventional method (FIG. 2), the present invention controls the position of the water cooling start bank so that the air cooling unit is formed in front of the water cooling unit by setting it at the rear end, that is, the winder side, as in FIG. By means of the temperature deviation formed in the width direction of the steel sheet, the part where the temperature at both ends of the width direction of the steel sheet is low is in the air-cooling part, and the part where the temperature in the central part is high is in the water-cooling part [Ar ₃ transformation: austenite (γ) → ferrite transformation into α)].

2 and 3, T ₁ represents the finish rolling exit temperature, T ₂ represents the water cooling start temperature, T ₃ represents the water cooling end temperature, and T ₄ represents the winding temperature.

In the present invention, preferably, the water-cooling portion is formed at a temperature section where the temperature at both ends of the hot rolled steel sheet in the width direction is equal to or less than the Ar ₃ transformation start temperature, and both ends of the hot-rolled steel sheet have a predetermined percentage of Ar ₃ transformation in the air-cooled portion. And the center portion is configured such that Ar ₃ transformation occurs only in the water cooling portion.

The water cooling unit is controlled by the water cooling start bank position, and the water cooling start bank position should be set after the position where the temperature of both ends of the width direction of the steel plate becomes lower than the phase transformation start temperature during air cooling, that is, the position A of FIG. The greater the distance between the position and the bank at the start of the water cooling, the greater the amount of transformation due to air cooling at both ends of the plate width, thus increasing the strength reduction effect at both ends of the plate width.

Therefore, the present invention applies only when the length of the water cooling section is smaller than the distance from the A position in FIG. 3 to the last bank position.

However, this limitation of the scope of application is that the respective water cooling banks increase the cooling capacity or increase the length of the water cooling stand, and the method of moving the A position forward by slowing the mailing speed or lowering the finishing rolling temperature. Can be extended.

As described above, when both ends in the width direction cause transformation in the air cooling part with a slow cooling rate, a material having low strength and a large elongation is generated, so that the length of the air cooling part, that is, the bank position at the time of water cooling is properly adjusted, so By controlling the degree of phase transformation, the strength of both ends in the width direction of the steel sheet can be lowered and the elongation can be increased to homogenize the overall mechanical properties in the width direction of the steel sheet.

Continuous hot rolling equipment to which the present invention is applied should be provided with an air cooling section that can cause phase transformation because the length and water cooling capacity of the water cooling stage are sufficiently large, and the target material is austenite to ferrite phase transformation between the exit rolling mill and the winding machine. It is a steel material having a hardening ability to control the phase transformation occurring in a given air cooling section. The reheating and hot rolling conditions are the same as in the conventional method, and the finish rolling temperature and the coiling temperature are the same as in the conventional method, but if necessary, they remain in the air-cooled zone by lowering the finish rolling pressure or lowering the flow rate in the water cooling target. By lengthening time, the phase transformation of the width direction both ends during air cooling can be adjusted.

As described above, the present invention appropriately controls the existing cooling facility based on the temperature gradient in the width direction that is naturally formed during hot rolling, thereby homogenizing mechanical properties in the width direction of the steel sheet economically without requiring any new equipment. There is an effect that can be made.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

Fe-0.094%, C-1.3%, Mn-0.23%, Si steel was hot-rolled under the hot rolling conditions as shown in Table 1, then water-cooled under the water cooling conditions as shown in Table 1, then wound up at 660 ℃ Hot rolled steel sheet was prepared.

In this case, the position of the water cooling unit in the water cooling zone is water in the conventional method by using 1-4 water cooling banks in front of the 16 water cooling banks, and 11-14 water cooling banks in the rear stage in the case of the present invention. Winding temperature was adjusted by cooling.

The tensile strength and elongation value of the hot rolled steel sheet manufactured as described above were measured according to the widthwise position, and the results are shown in FIG. 4 (a) for the hot rolled steel sheet manufactured by the conventional method. The hot rolled steel sheet produced by the above is shown in FIG.

* Water cooling stand length: 130m

Total number of water cooling banks: 16

As shown in FIG. 4, in the case of hot-rolled steel sheet manufactured by the conventional method, undesirable high-strength low elongation phenomenon is observed at both ends of the width direction of the steel sheet, whereas in the case of the hot-rolled sheet manufactured by the present invention, It can be seen that the variation in tensile strength and elongation is reduced.

Claims (2)

A method of manufacturing a hot rolled steel sheet by hot rolling a steel slab, and then cooling the water in a water cooling stage including a plurality of water cooling banks, wherein the air cooling portion is placed at the front end of the water cooling stage, and the water cooling portion is A method of producing a hot rolled steel sheet having a uniform mechanical property in the width direction, characterized in that the position of the bank at the time of water cooling is formed so as to be formed at the rear end. The method of claim 1, wherein the water-cooling portion, the temperature across the hot-rolled steel sheet is formed in the interval of less than Ar ₃ transformation starting temperature taking place the Ar ₃ transformation of certain fractions from both ends of the hot-rolled steel sheet is air-cooled unit, and the center of the water-cooling unit Method for producing a hot rolled steel sheet with uniform mechanical properties in the width direction, characterized in that only Ar ₃ transformation occurs to occur.