JPH04367302A - Hot rolling method for billet - Google Patents

Abstract

PURPOSE:To prevent separation and breaking at a joined part caused by a tension control, especially in a finish rolling when the rear end of a preceding billet and the front end of a following billet are joined and a continuous hot rolling is carried out. CONSTITUTION:The rear end of the preceding billet for which a rough rolling is finished and the front end of a following billet are pre-joined and when the pre-joined billets are continuously hot rolled, the rolling is carried out by restricting tension to less than the tension strength at the prejoined part in the first half of the finish rolling of the above-mentioned joined billets.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] This invention is particularly useful for applying tension during finish rolling when continuous rolling is performed by connecting a preceding steel billet that has finished rough rolling to a subsequent steel billet that is subsequently conveyed. This is intended to effectively avoid separation and fracture at the joints of the plates due to.

0002

BACKGROUND OF THE INVENTION Conventionally, when hot rolling steel billets, the steel billets extracted from a heating furnace were rolled one by one, which caused various troubles as described below, especially in the finish rolling process. a) Inadequate biting of the tip of the billet into the rolling mill, b) Squeezing of the rear end of the billet, c) Trouble running the tip of the billet on the run-out table, d) Defective dimensions at the tip and rear end of the billet. To solve this problem, there are various rolling methods in which the trailing end of the preceding billet and the leading end of the succeeding billet are sequentially joined in the inlet conveyance line of the hot finishing mill, and then finish rolling is performed. There have been proposals, especially in patent application Hei 2-2039.
The method disclosed in Specification No. 92, in which at least both end regions of each steel billet are pre-welded at a predetermined ratio when joining steel billets, is extremely effective in speeding up and simplifying the joining work. It was a means.

0003

[Problems to be Solved by the Invention] By the way, the technology disclosed in the above specification sets the welding allowance to 0 at both end regions with respect to the billet width B in order to avoid separation and rupture of the pre-joint portion during finish rolling. Although it is said that it is desirable to have a bonding strength of 0.1 B or more, and a total of 0.2 B or more, in actual rolling work, it is necessary to check whether the above welding allowance is secured after the joining of steel slabs is completed. This is extremely difficult; for example, in cases where the front and rear ends of the steel billet are bent, it is not possible to secure the necessary welding allowance, and the joints may separate during finish rolling. Furthermore, when applying the technique disclosed in Japanese Patent Application Laid-Open No. 63-097307 regarding plate width control during finish rolling, such joints are difficult to avoid because there is a risk that the joints of the steel billets may separate and break. At present, it is not possible to apply sufficient tension when rolling steel billets.

[0004] Strip width control, which has been difficult when continuous rolling is performed by joining the rear end and front end of a steel bill that has gone through the rough rolling stage, can be achieved without causing separation and breakage, especially at the joints of the strips. It is an object of this invention to propose a new hot rolling method.

[0005]

[Means for Solving the Problems] The present invention pre-joins the rear end of a preceding steel piece that has been roughly rolled and the front end of a succeeding steel piece, and continuously hot-rolls these joined steel pieces. On the occasion of
This method of hot rolling a steel billet is characterized in that, in the first half of the finish rolling of the bonded billet, the tension is limited to less than the tensile strength at the pre-joined portion.

[0006]

[Operation] Fig. 1 shows an example of a hot strip mill line, in which a slab 2 heated to a predetermined temperature in a heating furnace 1 is descaled and then passed through a group of rough rolling mills 3 several times. After that, the sheet bar 4 is rolled to a desired thickness and width, and after being wound, the sheet bar 4 is kept at a uniform temperature over its entire length in a coil box 5 by recuperation. The sheet bar 4 taken out from the coil box 5 and rewound is then cut off by a crop shear 6 to remove defective parts such as fish tails, and the rear end surface of the preceding sheet bar and the front end surface of the following sheet bar are joined together by a joining device 7. The strips are butt-jointed, then rolled in a finishing mill group 8, cooled on a runout table 10, cut into strips 9 of a predetermined length by a running shear 11, and wound up in a down coiler 12. In the finishing rolling mill group 8 of such a rolling line, the results obtained by three-dimensional rolling analysis show that as shown in Fig. 2, the stress in the longitudinal direction generated in the material during roll biting is It is clear that tensile stress is acting at the center of the sheet bar, and compressive stress is acting at the center in the width direction of the sheet. Therefore, when joining the sheet bars 4 together at the entrance side of the rolling mill, it is not necessary to apply the full width of the sheet bar. It is not necessary to join the entire width, but it is possible to pre-join only the ends in the width direction where tensile stress is applied (hereinafter simply referred to as pre-joining) and to join the other parts as rolling progresses. The technique disclosed in the above-mentioned Japanese Patent Application No. 203992/1992 is based on such knowledge, but in hot rolling of such jointed steel pieces, it is particularly useful for adjusting the plate width during finish rolling. In cases where tension control was added, there was a risk that the pre-joined portion would break and separate. Figure 3 shows steel type SS4
1. After partial welding (pre-joining by butt) in the width direction of a sample with plate thickness t = 18.7 mm, plate width B = 150 mm, and length L = 300 mm, the rolling temperature was 1000℃.
This is the result of investigating the separation strength ratio when rolling three passes from
When the total reduction rate in the second pass reaches approximately 59%, the strength of the abutting surface is equivalent to that of the base metal, and when the scale of the abutting surface is 60 μm, it is difficult for new surfaces to appear due to shear deformation due to rolling, so the total When the reduction rate reaches about 74%, a bonding strength of about 30% of the base metal strength is finally obtained. By the way, in actual operation, it is difficult to always keep the scale thickness of the abutting surfaces of steel slabs to 0 μm, and it is inevitable that scales of about 10 to 60 μm will occur. Also, considering the case where tension is applied to change the width in finish rolling, the amount of width shrinkage increases rapidly by about 1/3 of the base material strength, so it is important to ensure sufficient tension control during finish rolling. In order to achieve this, it is necessary to ensure the joint strength at the pre-joint part to be at least 30% of the base metal strength, and in cases where the scale of the butting surfaces reaches 60 μm, the total reduction rate reaches approximately 74%. It is dangerous to apply tension up to that point, but by increasing the reduction rate thereafter, the strength of the joint can be brought close to the strength of the base material. For this reason, in this invention, during the first half of finish rolling, especially until the total rolling reduction reaches 74%, rolling is carried out with the tension between the stands as small as possible, especially less than the tensile strength at the pre-joint part. ,
A total reduction rate of 7 that provides a tensile strength of 30% or more of the base material strength.
When it exceeded about 4%, we decided to adjust the board width by applying tension between the stands. The tensile strength of the pre-joined part varies depending on the joining method, so the tensile strength corresponding to the rolling reduction ratio when using various joining methods is determined in advance through experiments, etc., and the tension is controlled based on this data. It is better.

[0007]

[Example] Width 1000mm, thickness 35m after rough rolling
Pre-join the sheet bars with a length of 200 mm so that the joining distance is 200 mm at both ends (joining method, patent application No. 2-203992)
After that, hot rolling was performed according to the following conditions to produce a hot strip with a thickness of 1.6 mm and a width of 1000 mm, and the separation and rupture of the pre-joined portion at the finish rolling stage was investigated.

[Table 1]

[Table 2]

As a result, in the adapted example rolled according to the present invention, no separation fracture was observed at the pre-joint part, and good rolling could be performed, whereas in the comparative example, the tension between the stands It was confirmed that since the tensile strength of the pre-joint exceeds the tensile strength of the pre-joint, separation and fracture are unavoidable, which seriously impedes the rolling operation.

[0009]

Thus, according to the present invention, there is provided a hot strip mill that pre-joins the trailing end of a roughly rolled preceding steel piece and the tip of a succeeding steel piece, and then continuously hot-rolls the joined steel piece. In endless rolling on a line, effective tension control can be performed without causing separation and breakage of the pre-joint.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the configuration of a hot strip mill line.

FIG. 2 is a diagram showing the stress distribution in the longitudinal direction of the material occurring in the material within the roll bite.

FIG. 3 is a graph showing the relationship between rolling reduction and separation breaking strength ratio in finish rolling.

[Explanation of symbols]

1 Heating furnace 2 Slab 3 Rough rolling mill group 4 Seat bar 5 Coil box 6 Cropshire 7　Joining device 8 Finishing rolling mill group 9 Strip 10 Runout table 11　Travel shear 12 Down coiler

Claims (1)

[Claims] [Claim 1] In pre-joining the rear end of the preceding steel piece that has been roughly rolled and the leading end of the succeeding steel piece, and continuously hot rolling these joined steel pieces, finishing of the above-mentioned joined steel pieces is carried out. A method for hot rolling a steel billet, characterized in that in the first half of rolling, the tension is limited to less than the tensile strength at the pre-joint part.