JPH07314006A - Method for hot-rolling joined slab - Google Patents

Abstract

PURPOSE:To provide a hot-rolling method which enables pressurized joining of materials having high ratio of thickness to width of sheet bars and reduces breaking at the time of rolling after preliminary joining. CONSTITUTION:In continuous hot rolling in which the bars are joined before continuous finish rolling, >=5% of the cross-sectional area including both edge parts of the rear end part of a preceding material and the tip part of the succeeding material is preliminarily joined so that the max. gap between the end parts to be joined is <=5mm and rolling just after joining is executed using hot-rolling rolls having a relationship of Mt-Et>=100mum between the max. roll diameter (Mt) and the min. roll diameter (Et). Further, at the time, when rolling just after joining the sheet bars is executed at 35-60% draft, the adhesion by pressing wide materials are enabled and breaking is eliminated. In this way, even when they are sheet bars having high ratio of thickness to width, adhesion by pressing is enabled reducing the preliminarily joined area and the wide material is manufactured with a large rolling mill.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolling method for joining hot-rolled materials for joining steel strips (sheet bars) to make them continuous in the field of hot rolling.

[0002]

2. Description of the Related Art As a joining method for continuous hot rolling, a method is known in which the trailing end portion of the preceding rolled material and the leading end portion of the trailing rolled material are butted against each other and pressed and joined together. As this joining method, Japanese Patent Laid-Open No. 61-144203 proposes a method of pre-joining both end portions in the width direction, followed by rolling at a reduction rate of 20% or more and pressure bonding. In addition, JP-A-5-
Japanese Laid-Open Patent Publication No. 104107 proposes a method in which both ends of a preceding material and a following material are welded and then pressure-bonded by changing the crown ratio of the plate material before and after rolling.

[0003]

However, in any case, since the preliminary joining portion is only the both width edges, it often breaks during the rolling immediately after the preliminary joining or the subsequent rolling. In particular, in the joining of wide sheet bars, the joining force with respect to the entire cross-sectional area was small, so that it was not possible to withstand the tension in continuous rolling.

An object of the present invention is to solve the above problems and to provide a manufacturing method which does not cause breakage due to rolling even when joining wide sheet slabs.

[0005]

The gist of the present invention for solving the above-mentioned problems is that in hot rolling continuous rolling in which bar joining is performed before continuous finishing rolling, the maximum gap at the joining end is 5 mm or less. As described above, 5% or more of the cross-sectional area is pre-joined including both the trailing edge of the preceding material and the leading edge of the trailing material, and rolling immediately after joining is performed by rolling the maximum roll diameter (Mt ) And the minimum roll diameter (Et), Mt-Et ≧
A hot rolling method for a joined steel slab is characterized by rolling using a hot rolling roll having a relationship of 100 μm.

At this time, if the rolling of the sheet bar immediately after joining is carried out at a rolling reduction of 35 to 60%, the joining becomes stronger.

[0007]

When the present invention hot-rolls the steel material having both width edges joined, the thinner the sheet bar,
It was also noticed that the wider the width, the more likely it is to break by rolling after joining. Therefore, when a rolling experiment after joining was performed with the joining area ratio kept constant and the thickness-width ratio varied, the joining strength decreased as the thickness-width ratio increased, as shown in FIG.
It was found that it would break.

Observation of this fractured surface revealed that the fractured material was not joined in any part other than the preliminary joining. Then, when the elongation margin of the tail of the preceding material was measured by another experiment, it was found that the greater the thickness width ratio, the smaller the elongation margin at the central portion of the plate width and the poorer the joint at the unbonded portion. Therefore, the inventors of the present invention have made extensive studies on a method of increasing the expansion margin at the center of the plate width of the tail. As a result, it was found that by changing the shape of the rolling roll after the pre-bonding to a convex curve, the elongation margin was increased. The convexity of the roll for obtaining an effective elongation margin is Mt−Et ≧ 100 μm between the maximum roll diameter (Mt) and the minimum roll diameter (Et) of the steel sheet rolling portion, which is a constituent feature of the present invention. And one of them.

However, this effect does not occur if the strength at both edges of the width due to the preliminary joining is small. This is because there is a binding force to both steel pieces that are joined to both edges,
This is because the central portion of the width is pressed. In order to increase the crimping area of the unbonded portion in the center of the width, pre-bonding does not require welding at one point on both edges but pre-welding of at least 5% or more of the cross-sectional area is required. As a result, the central portion of the plate width can be stretched and pressed against the central portion to form a strong bonded state.

Further, in order to crimp the central portion by this method, the gap between the joint ends must be suppressed to 5 mm or less at the maximum. If the pressure is higher than this, the pressure bonding force becomes weak, and it is impossible to expect an increase in the bonding strength. As a joining method for effectively exhibiting the above-mentioned effects, a method such as arc welding or laser joining is more effective than an electric heating method in which a smaller welding area is more effective.

In the usual rolling of a material having a width of about 1 m or less, this method can be used for rolling without problems. However, as the plate width becomes wider, the joining area at both edges of the width cannot be sufficiently obtained, or the amount of stretching of the plate at the center of the width is insufficient, so the joining force is small and there is a risk of breakage. Therefore, in addition to the above-mentioned structural requirements, the bonding force can be increased by increasing the rolling reduction of the first pass to 35% or more.

The upper limit of the rolling reduction is set to 60% because
This is because as the width becomes wider, the rolling load becomes higher, and if the width exceeds this range, there is a risk of roll damage.

[0013]

Example A bar joining experiment was conducted using a 7-stand tandem rolling mill. This is shown in Table 1. No. For Nos. 1 to 4, the thickness ratio, the rolling reduction immediately after joining, and the roll convexity were made the same, and the tests were carried out by changing the end shape of the joined portion and the joining method. No. whose end shape exceeds a gap of 5 mm 1,2
Fracture occurred during rolling regardless of the joining method. If this is made flat, the welding speed in arc welding is slow, so the pre-bonding rate is low and the temperature immediately after rolling is also reduced.
Since the range of the present invention was maintained, no breakage occurred during rolling.

No. In No. 7, since the thickness width ratio is large and the rolling reduction is as small as 30%, the binding force at both width edges is weak, and No. In Roll No. 8, the roll convexity was as small as 50 μm, so the stretching in the width center portion was small, and all were broken. In all the other examples, all were within the scope of the present invention, and fracture due to rolling did not occur.

[0015]

[Table 1]

[0016]

As described above, according to the present invention, even a sheet bar having a large thickness / width ratio can be pressure-welded with a small pre-bonding area, and a wide material can be manufactured by a large rolling mill. It will be possible.

[Brief description of drawings]

FIG. 1 is a diagram showing a change in bonding strength (breaking limit) depending on a thickness width ratio.

Claims (2)

[Claims] 1. In hot rolling continuous rolling in which bar joining is performed before continuous finishing rolling, both edges of the trailing edge of the preceding material and the leading edge of the following material are set so that the maximum gap between the joining edges is 5 mm or less. 5% or more of the cross-sectional area including the portion is preliminarily joined, and the rolling immediately after joining is performed such that Mt-Et ≧ 100 μm between the maximum roll diameter (Mt) and the minimum roll diameter (Et) of the steel sheet rolling portion. A method for hot-rolling a joined steel slab, which comprises rolling using a hot-rolling roll having a groove. 2. The hot rolling method for joining steel pieces according to claim 1, wherein the joining steel pieces are rolled at a rolling reduction of 35 to 60%.