JP2005046852A - Method for manufacturing hot-rolled steel strip - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a hot-rolled steel strip by which the hot-rolled steel strip having reduced surface defects due to recess-shaped flaws of a work roll can be manufactured even when a hard material is taken as an object. <P>SOLUTION: In the manufacturing method for the hot-rolled steel strip, after making a slab into a rough bar by performing rough rolling with a roughing mill, the rough bar is heated with an induction heating device, successively finish-rolled with a finishing mill, and is made into the hot-rolled steel strip. In this manufacturing method, in the heating of the rough bar with the induction heating device, a local region including the nose part and its neighbor part and/or a local region including the rear end part and the neighbor part of the rough bar are heated with more weight than the regions before and after the local regions in the longitudinal direction of the rough bar, and also the heating temperature of the local regions of the rough bar is changed in accordance with the tensile strength or a surface pressure (load per unit area) of a material to be rolled and the hardness of the rolls of the finishing mill. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a hot-rolled steel strip by a hot rolling facility provided with an induction heating device for heating a rough bar between a rough rolling mill and a finish rolling mill.
[0002]
[Prior art]
In general, a hot-rolled steel strip is obtained by re-rolling a slab produced in the previous process to a predetermined temperature in a heating furnace, or directly by rough rolling without passing through the heating furnace (coarse steel having a predetermined thickness (roughness). Bar), then finish-rolled to a predetermined plate thickness at a predetermined temperature (finishing temperature) with a finishing mill composed of a plurality of stands, cooled to a predetermined temperature (winding temperature) with a runout table, and wound with a coiler. To be manufactured.
[0003]
Furthermore, as a hot rolling facility, a facility in which an induction heating device for heating the entire width of the rough bar is installed between the roughing mill and the finish rolling mill is used. The induction heating device heats the rough bar. This is to ensure the target finishing temperature of the hot-rolled steel strip on the delivery side of the finishing mill in order to obtain a required material.
[0004]
When it is going to manufacture a hot-rolled steel strip by such a method, the front-end | tip part and rear-end part of a rough bar with a remarkable temperature fall will harden | cure. When the leading and trailing ends of the rough bar are hardened, especially when the material to be rolled is a hard material, when the material to be rolled bites into the finishing mill or slips out (disengages), the work of the finishing mill A concave wrinkle may be generated on the roll. And the wrinkles which generate | occur | produced in this work roll are transcribe | transferred to the steel strip which is a to-be-rolled material, induces a surface defect, and may reduce the yield of a product remarkably.
[0005]
Therefore, when manufacturing a hard hot-rolled steel strip, finish rolling is usually performed after cutting the front end and rear end of the rough bar after rough rolling with a crop shear over a wide range of usually 300 to 500 mm. Yes. However, cutting off the front end portion and the rear end portion of the coarse bar over a wide range of 300 to 500 mm with a crop shear in this manner significantly reduces the yield.
[0006]
As a method of producing a hard hot-rolled steel strip free from surface defects due to concave wrinkles on the work roll at a high yield, the surface temperature of the coldest part of the steel slab (coarse bar) or thin slab after rough rolling is used. There is known a method of finish rolling after heating so that the temperature becomes 850 ° C. or more (for example, see Patent Document 1).
[0007]
[Patent Document 1]
Japanese Patent Laid-Open No. 10-192910
[Problems to be solved by the invention]
It has been found that the concave wrinkles of the work roll occur when the hardness of the rough bar front end and rear end becomes higher than the hardness of the work roll due to a decrease in the temperature of the front end and rear end of the coarse bar. ing. Conventionally, in order to manufacture a hard hot-rolled steel strip free from surface defects due to the concave wrinkles of the work roll, a method such as increasing the heating temperature of the coarse bar or thin slab as shown in Patent Document 1 has been adopted. It was.
[0009]
However, in the method of increasing the heating temperature of the entire slab disclosed in Patent Document 1, the fuel consumption rate is deteriorated, and there may be a problem in quality due to scale and Cu soot. Further, even in the method of heating the front end portion and the rear end portion of the coarse bar, the results of the study by the present inventors show that the heating at 850 ° C. shown in Patent Document 1 is sufficient to generate concave wrinkles on the work roll. Could not be prevented. In particular, when the work roll is made of a material having a low hardness, it has been almost impossible to prevent the work roll from being wrinkled.
[0010]
The object of the present invention is to solve the problems of the prior art as described above, and to produce a hot-rolled steel strip with few surface defects caused by concave wrinkles of the work roll, even when a hard material is targeted. It is to provide a method that can be used.
[0011]
[Means for Solving the Problems]
The features of the present invention for achieving such an object are as follows.
[0012]
(1) After roughly rolling a slab with a roughing mill into a rough bar, the rough bar is heated with an induction heating device, and then the rough bar is finish-rolled with a finishing mill to form a hot-rolled steel strip. In the method of manufacturing a steel strip, in the heating of the rough bar by the induction heating device, the local region including the front end portion of the rough bar and the vicinity thereof and / or the local region including the rear end portion and the vicinity thereof is changed to the rough bar. Heating in a longer direction than the region before and after the local region in the longitudinal direction, and changing the heating temperature of the local region of the coarse bar according to the tensile strength of the material to be rolled and the hardness of the roll of the finish rolling mill A method for producing a hot-rolled steel strip.
[0013]
(2) After the slab is roughly rolled with a roughing mill to form a rough bar, the rough bar is heated with an induction heating device, and then the rough bar is finish-rolled with a finishing mill to form a hot-rolled steel strip. In the method of manufacturing a steel strip, in the heating of the rough bar by the induction heating device, the local region including the front end portion of the rough bar and the vicinity thereof and / or the local region including the rear end portion and the vicinity thereof is changed to the rough bar. While heating in a longitudinal direction more than the region before and after the local region, the heating temperature of the local region of the coarse bar is set to the hardness of the roll of the finishing mill and the material to be rolled to each rolling stand of the finishing mill. A method for producing a hot-rolled steel strip, wherein the hot-rolled steel strip is changed according to a surface pressure acting on a front end portion or a rear end portion of a material to be rolled when biting or slipping off.
[0014]
(3) When the leading end or the rear end of the rough bar described in (1) or (2) above does not crop the leading and trailing end of the rough bar before finish rolling, the leading end or the rear end of the rough bar; When crop cutting, it is the crop cutting scheduled portion that will be the leading end or rear end of the material to be rolled after crop cutting, and when dividing rough bar, it will be divided cutting scheduled to be the leading end or rear end of the material to be rolled after rough bar splitting A method for producing a hot-rolled steel strip, characterized by being a part.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
1 and 2 show an embodiment of a method for producing a hot-rolled steel strip of the present invention, and FIG. 1 is a side view showing an embodiment of a hot rolling facility provided for the implementation of the present invention. FIG. 2 is an explanatory view showing an example of the temperature distribution of the coarse bar in the method of the present invention.
[0016]
The hot rolling facility shown in FIG. 1 includes a roughing mill 2 and an induction heating device 3 composed of an edge heater and a bar heater for heating the rough bar 1 rolled by the roughing mill 2, and a rough bar after heating. A finishing mill 5 that sequentially rolls into a hot-rolled steel strip, and thermometers 6, 7, and 8 respectively installed on the entry side and the exit side of the induction heating device 3 and the exit side of the finish rolling mill are provided.
[0017]
When carrying out the present invention using the above equipment, the temperature distribution of the coarse bar 1 rolled by the coarse rolling mill 2 is measured by the thermometer 6, and based on the measurement result, the induction heating device 3 The local region including the front end portion of the bar and the vicinity thereof and / or the local region including the rear end portion and the vicinity thereof are heated more weightly than the regions before and after the local region in the longitudinal direction of the coarse bar.
[0018]
Specifically, heating of the local region including the front end portion of the rough bar and the vicinity thereof and / or the local region including the rear end portion thereof and the vicinity thereof is performed so that the temperature of the portion indicated by hatching in FIG. 2 is increased. Do. By heating these portions of the coarse bar where the temperature is most lowered, the deformation resistance of the material can be reduced.
[0019]
Then, the temperature distribution of the coarse bar after heating is measured by the induction heating device 3 with the thermometer 7 and it is confirmed that the temperature distribution shown in FIG. 2 is obtained. Thereafter, the coarse bar is finish-rolled with a finish rolling mill 5 to produce a hot-rolled steel strip. The temperature distribution of the hot-rolled steel strip after finish rolling is measured by a thermometer 8.
[0020]
In the present invention, the induction heating device causes the local region including the front end portion of the coarse bar and the vicinity thereof and / or the local region including the rear end portion and the vicinity thereof to be disposed before and after the local region in the longitudinal direction of the coarse bar. The reason why the heating is performed more heavily than the area is as follows.
[0021]
A concave wrinkle generated in the work roll of the finishing mill when the leading end of the rough bar is caught in the finishing mill is generally referred to as a top mark, and when the trailing end of the rough bar comes out of the finishing mill, A concave wrinkle generated in a work roll is generally called a tail mark. These top marks and tail marks generated in the finishing mill are transferred to a steel strip as a material to be rolled to induce surface defects. In other words, in the case of the top mark, it is transferred to the steel strip after this material, and in the case of the tail mark, it is transferred to the steel strip after the next material.
[0022]
Here, the top mark is generated by impact when the leading end of the rough bar hardened due to the temperature drop is caught in the finishing mill, and the tail mark finishes the rear end of the rough bar hardened due to the temperature drop. It occurs because of rolling with a rolling mill.
[0023]
Therefore, in order to prevent the occurrence of the top mark and the tail mark, as described above, the front end portion of the coarse bar and the vicinity thereof, the rear end portion and the vicinity thereof are more than the regions before and after those regions in the longitudinal direction of the coarse bar. Heat with weight. By performing such heating, it is possible to reduce the deformation resistance of the material front and rear end portions, suppress the occurrence of concave wrinkles on the work roll, and manufacture a hot-rolled steel strip with few surface defects. At this time, both the front end portion and the rear end portion may be heated according to the amount of temperature decrease at the front end portion and the rear end portion of the coarse bar, or either the front end portion or the rear end portion of the coarse bar is heated. May be.
[0024]
Next, the local region including the front end portion of the coarse bar and its vicinity and / or the local region including the rear end portion thereof and the vicinity thereof, which are to be heated more heavily than other portions in the longitudinal direction of the coarse bar, will be described below. explain.
[0025]
Usually, a shear 4 is installed between the induction heating device 3 and the finish rolling mill 5 and cut by the shear 4 with the shape defective portion and the poor quality portion of the front end portion and rear end portion of the coarse bar as a crop. There is a case. When performing such crop cutting, the crop cutting scheduled portions at the leading end and the trailing end of the rough bar are the leading and trailing ends of the rough bar after cutting, respectively. Heat.
[0026]
In some cases, the shear bar 4 may divide the coarse bar into a plurality of coarse bars. Since this position becomes the forefront and rearmost ends of the coarse bar after the cutting, the planned cutting portion for the coarse bar division. When there is, there is heating centering around the part to be cut.
[0027]
On the other hand, when the leading end and the rear end of the coarse bar are not cut by the shear, the leading end and the rear end of the coarse bar are the leading edge and the trailing end, respectively. Heat to center.
[0028]
In the present invention, the heating temperature of the local region including the front end portion of the coarse bar and the vicinity thereof and / or the local region including the rear end portion and the vicinity thereof, the tensile strength of the material to be rolled and the hardness of the roll of the finish rolling mill It changes according to.
[0029]
That is, the higher the tensile strength of the material to be rolled, the higher the hardness of the material to be rolled in proportion, and the greater the possibility that the leading end and the trailing end of the coarse bar will be harder than the hardness of the work roll. It is necessary to increase the heating temperature of the local region including the front end portion of the coarse bar and the vicinity thereof and / or the local region including the rear end portion and the vicinity thereof.
[0030]
In addition, the lower the hardness of the roll of the finish rolling mill, the greater the influence of the hardness of the material to be rolled, and there is a possibility that the leading end and the trailing end of the coarse bar will be harder than the hardness of the work roll. Therefore, it is necessary to increase the heating temperature of the local region including the front end portion of the coarse bar and the vicinity thereof and / or the local region including the rear end portion and the vicinity thereof.
[0031]
Furthermore, in the present invention, the heating temperature of the local region including the front end portion of the rough bar and the vicinity thereof and / or the local region including the rear end portion thereof and the vicinity thereof is determined according to the tensile strength of the material to be rolled and the workpiece of the finish rolling mill. It is preferable to change not only the hardness of the roll but also the surface pressure (load per unit area) that is expected when the rolls of each rolling stand of the finishing mill are bitten. This embodiment will be specifically described below using a top mark as an example.
[0032]
For example, as shown in FIG. 3, the relationship between the surface pressure and the work roll hardness at the time of biting the rolling roll where the top mark occurs is obtained in advance. Then, for each rolling stand, the roll hardness is compared with the expected surface pressure. When the relationship is the top mark generation region shown in FIG. 3, the surface pressure is such that the surface pressure is not generated. The temperature of the rolling material tip is calculated, and the temperature at which the rough bar tip is heated is obtained. At this time, it is preferable that the lower limit temperature of the necessary heating temperature for preventing the top mark to be generated is calculated and the tip of the coarse bar is heated based on the calculated value because there is little loss during temperature increase.
[0033]
Here, the expected load P of the material to be rolled is calculated from the tensile strength TS, temperature T, width w, rolling reduction amount Δh, and work roll diameter R of the material to be rolled, as shown in the following formula (1). The
[0034]
P = f (TS, T, w, Δh, R) (1)
Here, the function f may be obtained theoretically or may be determined by regression of experimental data. Further, it is known that the surface pressure p (load per unit area) is expressed by the following equation (2).
[0035]
p = P / {√ (R · Δh) × w} (2)
Here, √ (R · Δh) is the projected contact arc length.
Furthermore, as shown in FIG. 3, the condition of the surface pressure p at which the top mark is not generated is expressed by the following expression (3) with respect to the roll hardness Hs.
[0036]
p ≦ αHs + β (3)
However, α and β are constants and can be obtained experimentally.
[0037]
That is, formulas (1) to (3) are determined in advance, the surface pressure p at which no top mark is generated with respect to the roll hardness Hs is obtained from the formula (3), and the predicted load P at that time is calculated from the formula (2). And the temperature T of the to-be-rolled material used as such a load P can be calculated | required from (1) Formula. Then, the top mark can be prevented from being generated by heating the tip of the rough bar so that the temperature of the tip of the material to be rolled in the rolling stand becomes the temperature T thus obtained.
[0038]
Although the top mark has been described above, the tail mark is also determined for the tail mark by obtaining the temperature at which the rear end of the coarse bar should be heated by the same method and heating the rear end of the coarse bar to that temperature. Can be prevented.
[0039]
【Example】
A hot-rolled steel strip according to the method of the present invention was manufactured for a hard material having a tensile strength of TS = 80 (kgf / mm ² ) using the hot rolling equipment provided for carrying out the present invention shown in FIG. . A high chrome material having a roll hardness of 70 (HS) was used for the work roll of the finishing mill.
[0040]
First, it can be seen from FIG. 3 expressed as equation (3) that when the roll hardness is 70 (HS), the top mark is not generated if the surface pressure is 0.028 (Ton / mm ² ) or less. Therefore, using the formulas (1) and (2), calculate the temperature of the material to be rolled at which the surface pressure at the time of biting the material to be rolled becomes 0.028 (Ton / mm ² ) or less, This was performed for all rolling stands, and based on the result, the temperature of the tip of the coarse bar where no top mark was generated was determined, and it was determined to be 1050 ° C. The same calculation was performed for the rear end portion to determine the temperature of the rear end portion of the coarse bar where no tail mark was generated. Then, the local region including the front end portion of the coarse bar and the vicinity thereof and the local region including the rear end portion and the vicinity thereof are heated by the induction heating device so as to be higher than these temperatures, and then finish rolling is performed. Thus, a hot-rolled steel strip was obtained.
[0041]
As a result of performing the production of the hot-rolled steel strip according to the method of the present invention for a certain period of time, the occurrence rate of surface defects due to the concave wrinkles of the work roll was 1.0%. It was possible to decrease to.
[0042]
【The invention's effect】
As described above, according to the present invention, the heating temperature of the local region including the front end portion of the coarse bar and the vicinity thereof and / or the local region including the rear end portion and the vicinity thereof is determined as the tensile strength of the material to be rolled. By changing according to the hardness of the roll of the finishing mill, surface defects caused by the concave wrinkles of the work roll can be reduced.
[0043]
Furthermore, by considering the load (surface pressure) per unit area that acts on the leading end or the trailing end of the material to be rolled when the material to be rolled is caught in each rolling stand of the finish rolling mill or slips off the surface, Defect reduction effect goes up.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing an embodiment of a hot rolling facility provided for carrying out the present invention. FIG. 2 is a diagram showing a temperature distribution of a coarse bar showing an embodiment of a method for producing a hot rolled steel strip according to the present invention. Explanatory drawing showing an example [Fig. 3] Graph showing the relationship between surface pressure and roll hardness with respect to occurrence of top mark [Explanation of symbols]
1 Coarse bar 2 Coarse rolling mill 3 Induction heating device 4 Shear 5 Finishing mill 6 Thermometer 7 Thermometer 8 Thermometer

Claims (3)

After the slab is roughly rolled with a roughing mill to form a rough bar, the rough bar is heated with an induction heating device, and then the rough bar is finish-rolled with a finishing mill to form a hot-rolled steel strip. In the manufacturing method of
In the heating of the coarse bar by the induction heating device, the local region including the front end portion of the coarse bar and the vicinity thereof and / or the local region including the rear end portion and the vicinity thereof are arranged before and after the local region in the longitudinal direction of the coarse bar. A hot-rolled steel strip characterized in that the heating temperature of the local region of the coarse bar is changed according to the tensile strength of the material to be rolled and the hardness of the roll of the finishing mill. Manufacturing method. After the slab is roughly rolled with a roughing mill to form a rough bar, the rough bar is heated with an induction heating device, and then the rough bar is finish-rolled with a finishing mill to form a hot-rolled steel strip. In the manufacturing method of
In the heating of the coarse bar by the induction heating device, the local region including the front end portion of the coarse bar and the vicinity thereof and / or the local region including the rear end portion and the vicinity thereof are arranged before and after the local region in the longitudinal direction of the coarse bar. When the heating temperature of the local region of the coarse bar is set to be higher than that of the region, the hardness of the roll of the finish rolling mill and the material to be rolled are caught in each rolling stand of the finish rolling mill or butt A method for producing a hot-rolled steel strip, characterized by varying the surface pressure acting on the front end or rear end of the material to be rolled. The leading end or the rear end of the rough bar according to claim 1 or 2 is the leading end or the rear end of the rough bar when the front / rear end of the rough bar is not crop-cut before finish rolling, and when the crop is cut It is a crop cutting scheduled portion that becomes the leading end or rear end of the material to be rolled after crop cutting, and when it is divided into coarse bars, it is a divided cutting scheduled portion that becomes the leading end or rear end of the rolled material after rough bar division A method for producing a hot-rolled steel strip.

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