JP2000254701A - Production of hot-rolled steel sheet through plate- thickness press - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a producing method for a hot-rolled sheet through a plate- thickness press, with which a long bar sheet can be used without joining the sheet bars and slabs. SOLUTION: When the plate-thickness pressing is applied to the slab continuously cast at >=30% rolling reduction in the thickness direction by using one pair of dies having inclining parts at the inlet sides and parallel parts at the outlet sides, the contacting length L in the longitudinal direction in the parallel part of the die with respect to the front tip part of the slab is set in the range of 0.2-0.4 times of the plate thickness at the inlet side. A rough-rolling is continuously applied to the slab after plate-thickness pressing and successively, a finish- rolling is applied to make the hot-rolled sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a hot-rolled steel sheet by a thickness press using a thickness press method for pressing a slab or the like in a thickness direction.

[0002]

2. Description of the Related Art In general, hot rolling of a thin sheet such as a hot-rolled steel sheet is performed by rolling a high-temperature slab to an intermediate thickness by a rough rolling mill (the rolled material in this state is called a "sheet bar"). It is rolled to the final product thickness by a finishing mill. Here, the size of the slab is limited to the size of the heating furnace for heating the slab. As a result, one converter's worth of steel is usually divided into dozens of slabs.

[0003] In a sheet bar coming out of a rough rolling mill, as in the case of ordinary rolling of a sheet, defective portions called tongues and fish tails are always formed at the leading and trailing ends, although the degree is different. By the way, "tongue" means a shape defective part where the center of the plate width at the end protrudes like a tongue, and "fish tail" means a shape defective part where both ends of the plate width at the end protrude like a fish tail. Say. Since both the tongue and the fish tail are narrower than the normal part, they are easily deformed.

[0004] If these defective portions are left untreated, further deformation proceeds in the finishing mill in the next step, which causes a rolling trouble. Therefore, the defective portions are cut and removed at the sheet bar stage. The longer the cut and removed portion (hereinafter referred to as “crop”), the lower the product yield.

[0005] The finish rolling mill is a continuous rolling mill generally consisting of several stands, and performs rolling in a state where tension is applied to a steel strip having a reduced thickness. However, the portion of the hot-rolled steel sheet that has been finish-rolled, which is about 100 m from the tip, is rolled in a state where tension does not act until the tip reaches the coiler. During this time, the running of the leading end becomes unstable due to collision with the transport rolls and lifting due to wind pressure, so that the rolling speed is generally reduced to almost half of the steady state (after reaching the coiler). Absent.

[0006] In addition, after leaving the final stand of the finish rolling mill, the shape of the rear end portion is deteriorated because the tension becomes zero. Such an unsteady portion is generally inferior in both material and shape to the unsteady portion due to a temperature drop during conveyance or uneven cooling due to a shape defect. Rolling troubles caused by these defective materials and shapes, or meandering caused by the defective shapes, decrease the equipment operation rate, and are a major obstacle to lowering the yield.

To improve the yield in finish rolling, a method of connecting a plurality of sheet bars and performing finish rolling has been developed. For example, Japanese Patent Application Laid-Open No. 4-89109 discloses a method in which the leading end of a succeeding sheet bar is sequentially joined to the trailing end of a preceding sheet bar, and finish rolling is continuously performed on a plurality of sheet bars. Proposed.

[0008] In this conventional technique, the same front and rear ends can be rolled as in the steady state, so that the yield of the front and rear ends (unsteady part) is improved. Also,
The leading end can be rolled at the same rolling speed as in the steady state (after reaching the coiler), so that the rolling efficiency is improved. Further, since a plurality of sheet bars are connected and rolled, the rolling efficiency is improved as compared with a case where rolling is performed intermittently.

Separately, a method for manufacturing a long sheet bar, such as joining a plurality of slabs or directly rolling a continuous cast slab, has been proposed. As a method of joining a plurality of slabs, JP-A-57-106403 discloses that a leading end of a succeeding slab is sequentially joined to a trailing end of a preceding slab, and the joined plurality of slabs are There has been proposed a method of continuously rolling a sheet bar using a planetary mill group.

Japanese Patent Application Laid-Open No. Sho 59-92103 discloses that a slab for one converter is formed into a sheet bar by a large rolling mill, wound up as it is on a coil, and then the coil of this sheet bar is rewound and finished. A method of performing rolling has been proposed. Similarly, JP-A-59-85305 discloses that a slab cast at a high speed by a special continuous casting machine (referred to as a rotary caster) is formed into a sheet bar by rolling, and once wound into a coil box. A method of performing finish rolling has been proposed.

According to these conventional methods, the crop is cut only at the front and rear ends of the long sheet bar, and the crop is not generated for each individual slab, thereby improving the yield. Further, in these methods, the same effect as in the method of performing the finish rolling by connecting the plurality of sheet bars described above can be obtained also in the finish rolling.

[0012]

However, the above-mentioned prior art has the following problems.

First, according to the method described in Japanese Patent Application Laid-Open No. 4-89109, in order to join a plurality of sheet bars, it is necessary to cut the defective shape of the front and rear ends of the sheet bars. Therefore, the problem of a decrease in yield due to crop generation is as follows.
Still unresolved. Furthermore, the joint portion of the sheet bar has a lower strength than other portions, and may break at the joint portion during finish rolling, which may necessitate a line stop. Further, since the joining of the sheet bar is actually performed by welding, the structure of the joining portion is coarsened, which may cause a defective material or a surface crack.

In the method of joining a plurality of slabs described in Japanese Patent Application Laid-Open No. 57-106403, it is difficult to completely join the slabs in a short time because the slabs to be joined are thick. Further, even if joining can be performed in a short time, if rolling is performed under a large pressure, there is a possibility that a joining surface may be separated due to a tensile stress acting on the joining portion in addition to the hydrostatic pressure component. Therefore, it is necessary to reduce the rolling reduction, and the efficiency of the rough rolling is reduced.

Further, JP-A-59-92103,
The method of directly rolling a continuously cast slab described in JP-A-59-85305 has a problem that the efficiency of rolling is reduced due to the limitation of the casting speed. According to the latter gazette, a casting speed of 10 mpm is possible for the casting capacity (weight per unit time). However, in practice, from the viewpoint of operation and quality, there is a report that such a high-speed casting was successful. There is no.

In the method of directly rolling a continuously cast slab as in these prior arts, the rolling speed of the first stage of the rough rolling mill can be suppressed to several m / min at most due to the limitation of the casting speed. . This is about 1 rpm when the number of rolls of the rolling mill is set to be a very low speed rolling. as a result,
Since the roll of the rolling mill comes into contact with a high-temperature material of about 1200 ° C. for a long time (several seconds), there is a problem that the surface of the roll is cracked, deformed, or seized. Therefore, it is difficult to realize large-scale equipment for high-temperature materials, such as the production of hot-rolled steel sheets, regardless of the small-scale.

In the method of winding a sheet bar around a coil, when the sheet bar is applied to an ordinary thin-rolled hot rolling mill, the number of coils of the sheet bar is assumed to be several product coils. It becomes a huge coil. as a result,
It is inevitable that the coiling equipment such as the winding device becomes huge, which is a problem from the viewpoint of equipment cost, factory space, and the like.

The present invention has been made in order to solve the above-mentioned various problems, and an object of the present invention is to provide a plate capable of using a long sheet bar without joining a sheet bar or a slab. An object of the present invention is to provide a method for manufacturing a hot-rolled steel sheet by a thick press.

[0019]

According to the present invention, there is provided a method for producing a hot-rolled steel sheet by a thickness press according to the present invention, comprising a pair of continuously cast slabs having an inclined portion on an entrance side and a parallel portion on an exit side. When performing a thickness pressing process in which the rolling reduction in the thickness direction is 30% or more using the mold described above, the contact length L in the longitudinal direction of the parallel portion of the mold is determined for the tip of the slab. The thickness of the slab after this thickness press working should be continuously rough-rolled and then finish-rolled to obtain a hot-rolled steel sheet. It is characterized by.

In the present invention, the continuous casting slab is pressed in the sheet thickness direction instead of performing the rolling at the preceding stage of the rough rolling. The rolling reduction of this thickness press is set to 30% or more from the viewpoint of the occurrence rate of internal defects such as casting defects. By setting the rolling reduction to 30% or more in this way, it is possible to suppress the incidence of internal defects to 0.01% or less.

Next, the entrance side inclined portion 6b and the exit side parallel portion 6
Pressing is performed using a pair of molds 6 provided with a.
The reason why the inclined portion 6b is provided on the entry side of the mold 6 is to prevent a step from occurring in the material at the end of the mold 6. As for the material that comes into contact with the inclined portion 6a on the mold entry side, the rolling reduction continuously changes from 30% or more of the parallel portion 6a to zero of the non-contact portion, so that it is possible to prevent troubles such as surface cracks due to the occurrence of steps. Can be.

By the way, in the sheet thickness pressing, similarly to the roll rolling, the center of the sheet thickness at the end portion of the material, particularly at the front end portion, protrudes forward from the front and back surfaces (generation of the bulge 22) or collapses. The outer surfaces of the ends overlap (generation of wrap 21). It is necessary to cut and remove the deformed portion as a crop at the stage of the sheet bar after the rough rolling. In particular, as shown in FIG.
If the wrap 21 is generated at the tip of the slab, it causes two plates, and it is necessary to completely remove it.

The present inventors have conducted intensive studies on the deformation of the tip of the hot slab and found that the deformation behavior of the tip changes depending on the processing conditions of the thickness press.
First, as a general tendency, when the mold taper portion 6b contacts the slab tip, the wrap 21 shown in FIG.
It has been found that when the mold parallel portion 6a comes into contact with the tip of the slab, both the wrap 21 and the bulge 22 may be generated as shown in FIG. 4C.

Further, as a result of intensive studies, the present inventors have found that the size (length in the slab longitudinal direction) of the wrap 21 is
It has been found that the size of No. 2 can be arranged using the length L (hereinafter, referred to as “contact length L”) of the tip of the slab in contact with the mold parallel portion 6a shown in FIG. That is, as shown in FIG. 5, the wrap 21 is likely to be generated in a region where the contact length L is short, and the frequency and size of the wrap 21 decrease as the contact length L increases. On the other hand, the occurrence frequency and the size of the bulge 22 increase as the contact length L increases. Therefore, by appropriately setting the contact length L, the occurrence frequency of the wrap 21 and the bulge 22 can be suppressed to a low level, and the size (the length in the pass line direction) of these unsteady deformation portions can be reduced. It can be kept low.

Further, the present inventors have conducted intensive studies and found that the deformation of the slab tip portion strongly depends not only on the contact length L but also on the thickness H of the material 20. The present inventors have completed the method of the present invention for estimating the magnitude of deformation (wrap 21 and bulge 22) of the slab tip using the contact length L and the plate thickness H based on these findings.

FIG. 5 shows the results. In FIG. 5, the horizontal axis represents the ratio L / H between the contact length and the plate thickness, and the vertical axis represents the wrap length L1 and the bulge length L2. FIG. 6 is a characteristic diagram showing the result of examining the effect on the temperature. In the figure, a white triangle indicates a case where the wrap 21 occurs, and a white square indicates a case where the bulge 22 occurs. The curve E in the figure is a bulge
2 shows a characteristic line obtained by combining the multiple occurrence regions of the wrap 21 by the least squares method, and a curve F corresponds to a characteristic line obtained by combining the multiple occurrence regions of the wrap 21 by the least squares method.

As is apparent from FIG. 5, when the ratio L / H of the contact length L to the plate thickness H decreases, the dimension L1 of the wrap 21 increases, and when the ratio L / H increases, the dimension L2 of the bulge 22 increases. Becomes longer. In the intermediate region, the wrap 21 or the bulge 22 occurs, which is presumed to be due to the variation in the temperature distribution of the material.

In the intermediate area, the wrap 21 and the bulge 2
When the range in which the occurrence frequency of both 2 is low is obtained from FIG. 5, the ratio L / H is in the range of 0.2 or more and 0.4 or less. Based on this, in the production method of the present invention, the ratio L
The thickness press working of the slab tip is controlled so that / H is in the range of 0.2 to 0.4.

When the ratio L / H is zero, that is, the tip of the slab 20 does not come into contact with the mold parallel portion 6a, and the inclined portion 6
In the case of contact with b, the occurrence frequency of the wrap 21 increases. Even in actual work, if the tip of the slab comes into contact with the inclined portion of the mold, the material 20 slips as in the case of poor biting in roll rolling, which is not preferable because the press work does not proceed smoothly. Thus, from the viewpoint of workability, it is preferable to set the ratio L / H in the range of 0.2 to 0.4 as in the method of the present invention.

In the present invention, since the deformation of the tip of the slab can be controlled by the pressing conditions, a better shape than rough rolling can be expected. In general, the shape of the tip of the slab after rolling greatly changes due to the temperature distribution of the slab. When the corner of the slab is in an overheated state, a wrap 21 occurs, and conversely, the surface temperature of the slab decreases. In this case, the occurrence of the bulge 22 cannot be avoided. Therefore, in the present invention, when the corner portion of the slab 20 is in an overheated state, the contact length L is set to be longer to suppress the generation of the wrap 21 and to reduce the wrap size L1.
When the surface temperature of the slab 20 decreases, the contact length L is set to be short, and the occurrence of the bulge 22 is suppressed, and the bulge size L2 is suppressed.

[0031]

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a diagram showing an outline of equipment used in a method of manufacturing a hot-rolled steel sheet by a thickness press according to an embodiment of the present invention. The slab 20 continuously cast by the continuous casting machine 1 is heated to a target temperature range by the heating device 13, and is subjected to thickness press working in the roughing equipment 2 via the heat retaining device 9.
The sheet bar 20A is further roughly rolled by the rough rolling machine 7 to form a sheet bar 20A. After the temperature of the sheet bar 20A is adjusted by the heat retaining device 11 and the heating device 12, the sheet bar 20A is introduced into the finishing mill 3, and is finish-rolled to a target thickness to form a steel strip. further,
The steel strip is finally passed through the cutting machine 4 to the coilers 5a, 5b.
To be wound up.

The roughing equipment 2 includes a plate thickness press having a pair of upper and lower dies 6, a heat retaining device 10, and a rough rolling mill 7. The long continuous cast slab 20 is press-forged in the thickness direction by the mold 6 and is roughly rolled by the rough rolling machine 7 while being kept at a predetermined temperature by the heat retaining device 9. The pressing in the thickness direction is repeatedly performed while the hot slab 20 is intermittently fed at a predetermined feed amount f. Here, the slab feed amount f is determined based on conditions described later.

FIG. 2 shows the forging draft (%) on the horizontal axis.
FIG. 4 is a characteristic diagram showing a result obtained by examining the correlation between the rolling reduction and the internal defect occurrence rate at the time of plate thickness pressing for various materials, with the internal defect occurrence rate (%) taken on the vertical axis. As the material, a continuous cast slab having a thickness of 100 mm and a thickness of 200 mm was used. For a 100mm thick slab, the rolling reduction is 10%,
Each of which was changed to 20% and the as-cast state were used. The incidence of internal defects was determined by a normal metallographic examination (macro corrosion method). In the figure, curve A shows the result of a slab with a thickness of 100 mm as continuously cast, and curve B
Is the result of a 200mm thick slab with continuous casting,
Curve C shows the result of a 100 mm thick slab rolled at a rolling reduction of 10%, and curve D shows the thickness of 100 mm rolled at a rolling reduction of 20%.
The results for mm slabs are shown. As is clear from the figure, it was found that when the rolling reduction was 30% or less for any of the materials, the internal defect generation rate became lower than the allowable value of 0.01%.

Next, the length (contact length L) of the portion where the material and the mold are in contact with each other will be defined with reference to FIG.

The tip of the slab 20 having a thickness H is inserted between a pair of upper and lower molds 6. At this time, the feed amount f of the slab 20 is controlled such that the slab 20 comes into contact with the mold flat portion 6a by the contact length L from the corner C at the tip of the slab. The control of the slab feed amount f is performed by a control device (not shown). As a result, the tip of the slab is pressed by the mold flat portion 6a by the contact length L, thereby suppressing the generation of the wrap 21 and the bulge 22 and the length L1,
L2 is minimized.

FIG. 4A is a schematic view showing a wrap generated at the end of the slab by the press working, FIG. 4B is a schematic view showing a bulge generated at the end of the slab by the press working, and FIG. FIG. 4 is a schematic view showing a wrap and a bulge which are compositely generated at an end of a slab by press working. When the wrap 21 occurs, as shown in FIG. 4A, the corner portion C at the tip of the slab becomes the foremost portion. However, when the bulge 22 occurs and when the wrap 21 and the bulge 22 occur, As shown in FIGS. 4B and 4C, the leading end of the slab extends forward of the pass line.
Is not at the forefront of the slab.

Here, dimensions of the wrap 21 and the bulge 22 are defined in order to quantitatively evaluate the sectional shape of the slab tip. Here, in each case, the measurement is performed with the above-mentioned slab tip corner portion C as a starting point. In the case of the wrap 21, the length L1 of the overlapping portion is measured toward the inside of the slab 20, and in the case of the bulge 22, the length L2 of the portion projecting toward the outside of the slab is measured. When the wrap 21 and the bulge 22 occur, the lengths L1 and L2 are measured.

When the corner C at the tip of the slab is in an overheated state, the wrap 21 is likely to be generated.
Is set to be longer to suppress the generation of the wrap 21 and to reduce the wrap size L1. On the other hand, when the surface temperature of the slab is reduced, the bulge 22 is likely to be generated. Therefore, the contact length L is set to be short, the generation of the bulge 22 is suppressed, and the bulge size L2 is reduced.

According to the above-described embodiment, the crop loss can be greatly reduced, and the yield of the product has been greatly improved.

[0041]

According to the present invention, a continuously cast slab is subjected to press working in the thickness direction and then continuously rolled to form a sheet bar without joining the sheet bar and the slab. A long sheet bar can be obtained. In the press working, the rolling reduction can be increased as compared with the rolling, so that the occurrence rate of internal defects can be reduced.

Further, in the thickness press working, by appropriately setting the dimensions of the contact portion between the mold and the material,
Since the occurrence of shape defects due to the deformation of the tip of the slab can be reduced, the crop cutting yield in the subsequent sheet bar stage is improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing an outline of equipment used for a method of manufacturing a hot-rolled steel sheet by a thickness press according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a correlation between a forging draft (%) and an internal defect occurrence rate (%).

FIG. 3 is a schematic diagram for defining a contact length L at which a mold contacts a material (slab).

4A is a schematic view showing a wrap generated at the end of the slab by the press working, FIG. 4B is a schematic view showing a bulge generated at the end of the slab by the press working, and FIG. FIG. 4 is a schematic diagram showing a wrap and a bulge which are generated in a composite manner at an end.

FIG. 5 is a characteristic diagram showing a correlation between the length of a tip portion of a slab in contact with a parallel portion of a mold and the shape of the tip portion.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Continuous casting machine, 2 ... Rough processing equipment, 3 ... Finish rolling mill, 4
... Cutting machine, 5a, 5b ... coiler, 6 ... die, 6a ... parallel part (flat part), 6b ... taper part (inclined part), 7 ... rough rolling machine, 8, 9, 10, 11 ... heat retention device , 12 ... heating device,
13: heating furnace, 20: continuous casting slab, 20A: sheet bar, 21: wrap, 22: bulge.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Sadaka Masuda 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Takashi Nishii 1 Shinnakaharacho, Isogo-ku, Yokohama-shi, Kanagawa Prefecture Stone Kawashima-Harima Heavy Industries Co., Ltd. (72) Inventor Masao Mikami 1st place, Shin-Nakahara-cho, Isogo-ku, Yokohama-shi, Kanagawa Ishikawa Kawashima-Harima Heavy Industries Co., Ltd. No. 1, Nakahara-cho Ishi Kawashima-Harima Heavy Industries, Ltd. Yokohama Engineering Center (72) Inventor Shiro Nagata No. 1, Shin-Nakahara-cho, Isogo-ku, Yokohama, Kanagawa Prefecture Ishi Kawashima-Harima Heavy Industries, Ltd. Yokohama Engineering Center F-term (reference 4E002 AB03 AD02 AD03 AD04 BC05 BD01 CA01 CA06 CA09 4E087 BA13 CA03 CA32 CB01 DA02

Claims (1)

[Claims] 1. A plate having a rolling reduction of 30% or more in a thickness direction of a continuously cast slab using a pair of dies having an inclined portion on an entrance side and a parallel portion on an exit side. At the time of performing the thickness pressing, the contact length L in the longitudinal direction of the parallel portion of the mold is set to be equal to 0.
A thickness press wherein the slab after the thickness press working is continuously rough-rolled, and subsequently subjected to finish rolling to obtain a hot-rolled steel sheet. Production method of hot rolled steel sheet.