KR20040035993A - Method for hot rolling high strength steel having small thickness deviation of width direction - Google Patents

Abstract

PURPOSE: A hot rolling method for reducing edge drop by forming dog bone in the rough rolling process and finish rolling rough rolled bar so that the dog bone is utilized for compensating thickness of an edge part of strip. CONSTITUTION: In a hot rolling method comprising rough rolling and finish rolling consisting of widthwise rolling and thicknesswise rolling, the method for hot rolling high tensile strength strip having less widthwise thickness deviation comprises a step of obtaining a rough rolled bar on which dog bone is formed by performing dummy rolling in the final thicknesswise rolling after performing the final widthwise rolling a slab for high tensile strength strip in the rough rolling; and a step of finish rolling the dog bone formed rough rolled bar so that thickness of the edge part is compensated by the dog bone, wherein finish rolling rolls having minus crown are used in the finish rolling, and wherein the dog bone formed on the rough rolled bar has a size of 5.46 mm or less.

Description

Method for hot rolling high strength steel having small thickness deviation of width direction}

The present invention relates to a hot rolling method that can reduce the thickness deviation (crown) of a high tensile strength steel sheet, and more particularly, to form a dog bone in the rough rolling process, and to roughly roll the rough rolling bar to form a dog bone. The present invention relates to a hot rolling method for reducing edge drop by utilizing edge thickness compensation of a plate.

In general, the use of high-strength steel is increasing to reduce the weight and safety of the vehicle body. Accordingly, the quality requirements for high-tensile steel for automobiles are also becoming strict, and in particular, the crown, which is a thickness deviation in the width direction, is very important.

Crown, which is a thickness deviation in the width direction, is defined as a difference in thickness (C) at the center of strip (Strip, 10) and an arbitrary point thickness (A, B) from the edge, as shown in FIG. It is set at 25mm from.

[Relationship 1]

Crown = C- (A + B) / 2

The crown of the hot rolled steel sheet is different depending on the demand of the demand, and the closer to "0" when the cold rolled material is not used, the better. In the hot rolling process, the thickness variation (crown) in the width direction is particularly managed. That is, since it is difficult to set the crown to "0" due to the characteristics of the hot rolling process, the target crown is set. The target crown of a general steel is 40 micrometers, and the target crown of a high tension steel for automobiles is 60 micrometers, and manages the thickness deviation of the width direction. High tensile strength for automobiles is higher than general steel, so even if rolling bars of the same thickness as ordinary steel, the rolling force is greater in finishing mills, so the roll bending deepens and the edge of the plate is further rolled. There are many edge drops. Accordingly, in the high tensile strength steel sheet, the crown is increased relative to the general steel, so that the target crown is managed high.

As shown in FIG. 2, the control of the amount of the crown in the hot rolling process is performed by crossing the axes of the upper work roll 12 and the lower work roll 14 to control the principle of a pair cross (FIG. 2A) and a bender ( Bender) is controlled using the facility (Fig. 2B). Moreover, the method of adjusting the initial crown provided to a work roll is also employ | adopted.

Table 1 below shows the hot rolling conditions of conventional steel and high tensile steel.

division Filament rolling stand of hot rolling process Remarks F1 F2 F3 F4 F5 F6 F7 Initial Crown (μm) General steel -100 -100 -200 -200 -200 -150 -150 - High strength steel 0 +50 +50 -50 -50 -50 -50 - Pair Cross Angle General steel - - 0.6 0.6 0.6 0.5 0.5 - High strength steel - - 0.9 0.9 0.9 0.8 0.8 Max Recommended Bender (㎏f / f㎠) General steel 140 140 130 130 120 120 110 - High strength steel 190 190 180 180 170 170 170 Max Recommended

In Table 1, in the case of general steel, an initial crown is applied to the work roll, and a minus (-) crown of which the roll diameter of the work roll center part is smaller than the edge part is given. This is because the temperature of the plate to be rolled during finishing rolling during hot rolling is a high temperature of 870 ~ 1010 ℃ usually a thermal crown is formed by thermal expansion of the work roll. In consideration of this, in order to secure the target plate crown and shape, the initial crown (Initial Crown) to the work roll to give a negative (-) crown. Typically, the provision of the initial crown is calculated according to the mill force, the sheeting ability, the material of the work roll, and the characteristics of each rolled stand.

In the case of high strength steel, unlike the general steel, the initial crown given to the work roll is given as a positive crown. As shown in FIG. 3, since the rolling force (pressing force) of the high tensile strength steel sheet is large, the roll bending and the flat deformation of the work roll are severely shown, resulting in a large plate crown. In addition, the work roll in contact with the rolled material is subjected to plastic deformation that becomes flat. As the rolling pressure of the plate edge portion decreases with respect to the plate center portion, the flat deformation of the roll end portion decreases, resulting in an edge drop that reduces the thickness of the edge portion. The plate crown will increase. Therefore, in the conventional hot rolling process of high tensile steel sheet, unlike the general steel, the initial crown (Initial Crown) given to the work roll is changed to a positive (+) crown. In addition, by reducing the fair cross angle and setting the bender force of the work roll to a large degree, hot rolling is conducted to reduce the plate crown. In other words, if the initial crown of the work roll is negative to positive, that is, a block-shaped crown is provided, the thermal expansion during rolling will be larger than that of the negative crown. there was.

However, since the work rolls used in normal working conditions are applied by applying the same initial (-) initial crown, all of the work rolls having the initial plus (+) crown are polished for the application of the above method. There is a problem that must be prepared by, and rolling roll unit knitting should also be managed separately. That is, in order to organize a special roll unit of high-tensile steel for automobiles that require a low crown, it is necessary to collect and work a weekly customer orders, causing a problem of late delivery. In addition, in the process of preparing the working roll of the special initial crown, since the polishing amount of the working roll is greatly increased, there is an opportunity loss due to the roll unit increase. In addition, as the amount of reduction in the widthwise center portion is increased, problems such as poor sheet flowability such as a center wave generated during the rolling of the material are generated.

The present invention is to solve the problems as described above, by forming a dog bone (Bog Bone) in the width rolling of the rough rolling process, by performing a dummy rolling that does not perform the final horizontal (thickness) rolling of rough rolling Hot rolling method of high tensile strength steel sheet which can secure low crown by reducing edge drop in final rolling by applying dogbone to thickness compensation of edge of plate during rolling To provide.

1 is a schematic view of a crown with width thickness deviation

2 is a shape control equipment of the finishing mill,

2 (a) is a schematic diagram of a pair cross

2 (b) is a schematic diagram of a vendor

Figure 3 is a schematic diagram showing the work roll bending and flat deformation during rolling load addition in finishing rolling

4 is an example of a hot rolling mill

5 is a graph showing the plate crown control ability considering the flatness

6 is a schematic diagram of a dog bone formed during width rolling.

7 is a plate crown graph after finishing rolling angle stand rolling

7 (a) is an invention example

7B is a conventional example.

8 is a graph comparing the present invention and the conventional plate crown

Explanation of symbols for main parts of the drawings

10: Strip 12: Work Roll

14: lower work roll 16: flat deformation

20: roughing mill 22: first width rolling mill

24: first horizontal mill 26: second horizontal mill

28a: second rolling mill 28b: third rolling mill

30: fishing mill 50: dog bone

In the hot rolling method of the present invention for achieving the above object, in the hot rolling method consisting of rough rolling and finishing rolling consisting of width rolling and thickness rolling, the final thickness after performing the final width rolling of the high-strength steel slab in the rough rolling Obtaining rough rolling bar in which dogbone is formed by not performing rolling (dummy rolling),

Comprising a step of compensating the thickness of the edge portion with the dogbone by rolling the rough rolling bar formed with the dogbone.

Hereinafter, the present invention will be described in detail.

The present invention is directed to a high strength steel slab. As the high tensile strength, the tensile strength is 50kgf / cm ^{2 It is} preferably suitable for the tensile strength of 60kgf / cm ² or more commonly used for automobiles.

Figure 4 is an example of a hot rolling mill, the plate crown control ability in consideration of the flatness when performing hot rolling in such a hot rolling mill is known, Figure 5 corresponds to this. As shown in FIG. 5, the flatness dead band region is wide in the front stands F1 to F3, and the flatness dead zone is narrow in the rear stands F4 to F7. Therefore, the front stand is used to increase the fair cross angle as much as possible to induce a low crown, and in the rear stage stands F4 to F7 having a small plate thickness change, a bender is used to maintain a low crown. In FIG. 5, when the amount of change in the plate crown ratio δ is "+", a lot of waves occur in the center of the plate (indicated by medium wave in FIG. 5), and in "-", the wave is indicated on both edges of the plate. ) A lot occurs.

As can be seen from Figure 5, the low crown is most preferably secured in the finishing mill shear stand. In view of this aspect, the inventors of the present invention can utilize the dogbone of the rough-rolled bar in the front stand to compensate for the edge thickness of the plate if the rough-rolled bar on which the dogbone is formed is mapped, and the low crown can be maintained in the rear stand. The present invention has been completed by paying attention to the fact that it exists.

Typically, high-strength steel slabs are hot rolled in a hot rolling mill as shown in FIG. Determination of the width rolling amount in such a rolling mill is managed for each stand, which is set in consideration of the facility capacity. Most of the width rolling amount is performed in the first width rolling mill 22, and the horizontal width spreading amount generated during the second horizontal (thickness) rolling 26 in the second width rolling mill 28a and the third width rolling mill 28b. The control is performed in such a manner as to control. In addition, in determining the final target width of the rough rolling side, the final target width of each rolling mill is determined according to the rough rolling width control margin by considering the width variation in the finishing mill 30 based on the order width of the customer. When the plate width is determined, the width rolling amount is determined according to the load distribution for each stand and the load distribution for each pass.

In the present invention, in the hot rolling step, the final rolling of the rough rolling step is performed, and then, the final rolling of the rough rolling step, that is, the horizontal rolling is not performed (called a dummy rolling) to obtain a rough rolling bar having a dogbone. In other words, the rough rolling mill is a reversible rolling mill which determines the number of passes in accordance with slab, and rolls them. Since the slab is rolled in thickness after width rolling every one pass, the dog bone 50 is rolled smoothly by spreading width. At this time, in the present invention, by rolling the dummy roll that does not perform horizontal rolling in the second horizontal mill 26 after rolling in the final pass, the dogbone 50 formed during the final rolling is obtained.

6 shows a dogbone 50. In the rough rolling process 20, after the width rolling is performed in the vertical rolling mill 28 which performs the width rolling, a dog bone 50 having a thickened end portion of the bar 10 is formed, and the amount of the dog bone 50 is formed. This can be seen through Equations 1 and 2 below.

Here, hb: thickness of the rough rolling bar in the state where the dogbone is formed

ho: Thickness of rough rolling bar of the part where dog bone is not formed

de: Amount of rolling

For example, if the thickness (ho) of the rough rolling bar is 31mm, the vertical rolling amount (de) is 15mm Substituting this equation (1), the thickness (hb) of the rough rolling bar in the dogbone state is 35.46 mm. Therefore, the dogbone production amount hb-ho is 4.46 mm.

According to the present invention, the larger the amount of dogbone produced, the more advantageous in terms of the thickness compensation of the edge portion. However, when the amount of dogbone produced in the rough rolling bar exceeds 5.46 mm, the plate-bearing amount due to the edge wave is worse during the finishing rolling process. It is preferable to be mm or less.

Rough rolling of the roughly rolled bar formed by the dogbone by rough rolling as described above in accordance with a conventional method has the effect of increasing the thickness of the edge portion in the finishing mill 30 to reduce the final plate crown after finishing rolling.

Figure 7 shows the change in the plate crown of each stand (F1 ~ F7) in the finish rolling mill, the dog when rolling the rough bar bar formed with the dog bone 50 in each stand (F1 ~ F7) of the finishing mill 30 The effect of the bone 50 on the plate crown is shown. The edge of the edge during the rolling in the front stand (F1 ~ F3), the dogbone is used for the edge drop compensation, and the plate crown after rolling in the rear end (F4 ~ F7), the conventional material (Fig. 7b, no dogbone) It can be seen that it is better than the rough bar).

According to the present invention, hot rolling conditions can have a negative crown of a finishing roll as in the conventional general steel. Of course, even if applied to the rolling of the roll having a flux crown in a conventional high tensile strength steel sheet is effective in reducing the crown.

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

EXAMPLE

The dogbone was produced in the rough rolling process of the high strength steel slab of Table 2, and finishing rolling was performed. The slab width was 1180 mm, the target width was 1087 mm, and the target thickness was 2.61 mm.

Chemical composition (% by weight) C Mn Si Sol.Al Nb P S 0.07 1.5 0.1 0.015 0.045 <0.023 <0.003 Tensile Strength 60kgf / cm ²

That is, after performing 5-10 mm width rolling in the 2nd width rolling mill 28a and the 3rd width rolling mill 28b, 4-10 mm thickness rolling is performed in the 2nd horizontal rolling mill 26. As shown in FIG. The width rolling process is set to three passes in total, and 14 mm width rolling in one pass in the second and third width rolling mills 28, horizontal rolling in the second horizontal rolling mill 26, and then two or three width rolling mills 28 again. 15mm width rolling in two passes, horizontal rolling in the second horizontal mill 26, and 16mm width rolling in the final pass, where the final thickness rolling is performed after 16mm width rolling. Finish rolling was performed in the state in which the dogbone was formed. The results are shown in Table 3.

division Crown (C- (A + B) / 2 in Figure 1) Wedge (A-B in FIG. 1) Measuring position 25 mm from the edge 50 mm from the edge 70 mm from the edge 25 mm from the edge 50 mm from the edge 70 mm from the edge Conventional example 79 69 52 5 6 7 Inventive Example 42 37 28 -8 -7 -5 Conventional example is the conventional high-tensile steel rolling conditions in Table 1, the rough rolling bar without the dogbone invention example is conventional general steel rolling conditions in Table 1, finishing the rough rolling bar on which the dogbone is formed

As can be seen from Table 3, the crown of the invention example was greatly reduced.

9 shows the crown of the hot rolled plate obtained by hot rolling according to the method of the present invention and the conventional method. Since the initial roll of the work roll does not have to be processed with a positive (+), the center wave generated by excessive thermal expansion is reduced, and good mail flow is realized.

As described above, according to the present invention, the dog bone formed during the width rolling in the rough rolling process is subjected to dummy rolling, which does not perform horizontal rolling in the final pass, so that the dog bone is applied during the finishing rolling operation. By using the edge thickness compensation of the plate to reduce edge drop in final rolling, it is possible to secure a low crown to prepare a roll having a separate positive crown. There is no need, and there is a useful effect that can work by adding a high-tensile steel material for automobiles whenever necessary without having to concentrate the rolling unit.

Claims (3)

In the hot rolling method consisting of rough rolling and finishing rolling consisting of width rolling and thickness rolling, After the final width rolling of the high-strength steel slab in rough rolling, the final thickness rolling is subjected to dummy rolling to obtain a rough rolling bar formed with a dogbone, Compensating the thickness of the edge portion with the dogbone by rolling the rough rolling bar on which the dogbone is formed, the hot rolling method of the high tensile strength steel sheet having a small width variation in the width direction. The hot rolling method of claim 1, wherein the finishing rolling uses a finishing rolling roll having a negative crown. The hot rolling method of claim 1, wherein the dog bone of the rough bar has a thickness of 5.46 mm or less.