JPH0783946B2 - Manufacturing method of extra-thick steel plate - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to the production of an extremely thick steel sheet, and more specifically, an extremely thick steel sheet having a uniform internal quality from two or more slabs and a thickness of about 150 mm or more. The present invention relates to a method for manufacturing a low cost.

(Prior art and problems to be solved) Conventionally, extra-thick steel plates with a thickness of about 150 mm or more are forged into large steel ingots to form slabs in order to prevent uncompressed voids called zaku defects from remaining. In many cases, the slab was manufactured by hot rolling. However, the larger the steel ingot, the more segregated the top of the steel ingot becomes, resulting in a large amount of cut-off at the top, thus lowering the yield and causing an additional energy cost for forging. .

From such a background, a technique for manufacturing an extremely thick steel plate at a low cost has been developed, and, for example, a method of rolling a slab that is laminated and welded on its four circumferences (Japanese Patent Publication No. 51-14109) has been proposed. In this method, in order to improve the bondability and to secure the degree of vacuum in the space between the bonding surfaces, a part or all of the welded portions on the four circumferences are sealed by electron beam welding in a vacuum. However, for that purpose, we installed a special large vacuum chamber and an expensive electron beam welding machine,
It is necessary to use it, and there is a problem that the improvement effect on efficiency and manufacturing cost is not sufficient as compared with the conventional forging method by forging.

In addition, as a sealing means, the ends are welded to leave air holes in advance with an intermediate material (nickel plating or air absorbing material) in between, and a light pressure is applied to expel the air remaining between the mating surfaces. After cold rolling, air holes are sealed (Japanese Patent Publication Nos. 57-26870 and 59-30517), and the space between the surfaces is decompressed or replaced with an inert gas through air hole pipes. Method for sealing pipes from
No. 100890, No. 55-128390) and the like, both of which are methods of performing hot rolling thereafter on the premise that air does not exist between the overlapping surfaces by the sealing means. Similar to the sealing by electron beam welding, there are problems such as increased cost for sealing and reduced efficiency.

The present invention has been made in view of such circumstances,
It is an object of the present invention to provide a method capable of producing an extremely thick steel sheet with high efficiency and low cost by slab lap rolling.

(Means for Solving Problems) In order to achieve the above object, the inventors of the present invention, in the method of manufacturing an extra-thick steel sheet by lap rolling described above, have a cause that causes a decrease in manufacturing efficiency and an increase in manufacturing cost. In view of the fact that the degree of vacuum between the bonding surfaces has been secured or the atmosphere has been replaced before, it is possible to effectively join by a simple method without the need to secure the degree of vacuum or replace the atmosphere in advance. We have conducted intensive research to find out the conditions for obtaining it. As a result, it was found that even if air is present between the joining surfaces, if the rolling method is capable of completely expelling air during rolling, sufficient joining properties and uniformity in the plate thickness direction can be secured. Then, as a result of further research on such a rolling method, it was found that it is effective to provide air escape portions at least in the top portion and the bottom portion of the slab, and the present invention has been made here.

That is, the method for producing an extremely thick steel sheet according to the present invention is such that, when two or more slabs are stacked and pressure-bonded by hot rolling, oxides on the superposed surfaces of the slabs are removed in advance, and then the slabs are superposed and superposed. The four peripheral ends of the mating surfaces are joined, but at that time, one or more non-bonding holes are formed in at least the top part and the bottom part of the four peripheral ends of the joined surfaces, and then heat containing a reversible path is applied. It is characterized by performing hot rolling.

The present invention will be described in detail below based on examples.

As described above, in the method of the present invention, unlike the conventional method, the method of evacuating the air between the joint surfaces before rolling or replacing the air with an inert gas is not adopted, but the existence of air is confirmed. This is a prerequisite.

Therefore, the present inventors firstly, as a basic experiment, on the premise that air exists in the space between the bonding surfaces, and therefore the space is not made into a vacuum or an inert atmosphere, the bonding property after lap rolling is affected. An experiment was conducted to examine the influence of the oxide removal state of the surface, the presence or absence of a ventilable portion (eg, an air hole) between the joint surfaces and the outside air, and the arrangement position of the ventilable portion.

In the experiment, S55C (S: 0.014%) containing 0.55% C was used, two slabs with a thickness of 150 mm were overlapped, MIG welding was performed around the joint surface, and this was heated to 1250 ° C, then a thickness of 150 mm was obtained. Hot rolled. The other conditions are as shown in Table 1.

An ultrasonic flaw detection (UT) test was performed on the obtained rolled material, and inclusions were measured to check the cleanliness. These results are also shown in Table 1. In the measurement of inclusions, the position of the t / 5 part was selected as the measurement position in order to avoid the influence of the center segregation of the joining surface at the t / 2 (t: plate thickness) part and the original slab.

From the UT results shown in Table 1, it is found that it is necessary to first remove the oxide on the surface of the slab to be the bonding surface in order to ensure good bonding in a wide area of the rolled plate.

In addition, from the measurement results of inclusions shown in the same table, when there are no air holes (No. 1, No. 3) and even when there are air holes, only the top part (No. 6) shows Regarding cleanliness,
It can be seen that the cleanliness of the t / 2 joint surface is inferior to the t / 5 position. When a product with such a cleanliness distribution is used in a mold, etc., when drilling or polishing is performed, the finishing accuracy at the joint differs in the plate thickness direction, causing problems in terms of uniformity, and as a product, It becomes useless.

On the other hand, when holes are drilled in both the top and bottom of the slab (No. 7 and No. 8), uniformity in the plate thickness direction is ensured in terms of cleanliness. This is also clear when comparing the microstructures of the t / 2 joint surface, and when the oxide is removed and only the top part is perforated, as shown in FIG. When the amount of inclusions on the joint surface W is larger than at other positions in the plate thickness direction and the microstructure is discontinuous on the joint surface, when holes are drilled in both the top and bottom portions. In addition, as shown in FIG. 4, the microstructure is continuous even at the joint surface. The inclusions were manganese silicate-based inclusions.

From the above experimental results, the oxide on the joint surface was removed in advance,
It was found that when air holes were formed in at least both the top part and the bottom part of the joint of the slab, the best jointability and cleanliness uniformity in the plate thickness direction were obtained. The reason for this is considered as follows. That is, during hot rolling, the slab is first bitten into the rolling mill from the top side and then passed through, then conveyed in the opposite direction and then bitten from the bottom side to be joined, but undergoes the first-stage reduction. It is considered that the air in the joint surface space is expelled from the hole in the bottom portion at that time, and the remaining air is expelled from the hole in the top portion at the time of the next pressure reduction that is sent back.

Even if a reversible pass is performed by cold rolling, such a result cannot be expected. This is because although air is once expelled in cold rolling, it is difficult to completely expel air because the mating surfaces simply contact each other and metallurgical bonding cannot be obtained. Produces the same result. In addition, there is a possibility that some air may again invade the mating surfaces after rolling. Therefore, it is considered that the cold rolling does not have the effect of completely expelling air and the effect of preventing air intrusion by metallurgical bonding.

Based on the results of further examination of various conditions based on the above findings, the method of the present invention controls the following conditions.

First, the slab to be used is not limited in thickness, size, steel type, number of sheets, etc., but it is necessary to remove oxides on the surfaces to be bonded in advance in order to secure good bondability. Grinding, shot blasting or the like is sufficient as the removing method, but other methods may be used. In addition, the roughness of the joint surface is about the level of maintenance of the grinder, and sufficient jointability can be obtained by hot rolling in the subsequent operation.

The slab from which the oxide on the surface has been removed is superposed so that the surfaces to be joined are superposed, and then the four peripheral ends of the superposed surfaces are entirely joined by a suitable joining means. As the joining means, various welding methods such as MIG welding and covered arc welding may be used, as well as forge welding and pressing, and any method may be used as long as the joining strength is obtained to some extent.

However, in order to ensure the uniformity of the structure of the joint surface and other parts in the plate thickness method, among the joints of the four circumferences of the slab,
It is important to have unbonded holes at least in both the top and bottom portions. In order to form the non-bonding holes in this way, one or a plurality of air holes may be formed during or after the bonding, and the size of each hole should be about 10 mm or more. At that time, it is preferable that at least one hole is provided at the center in the width direction of the top portion and the bottom portion. As another method of forming a non-bonding hole, there is a method of leaving a non-bonded portion in the shape of a hole at the time of bonding with the same size as that of the hole, and the same effect as when forming a hole is obtained.

Next, the joined slab is hot-rolled, and this rolling may be performed in a pass schedule including a reversible pass as in the case of normal slab rolling. As described above, it is preferable to include at least one pass that is engaged from the past bottom part that is engaged from the top part of the slab, and that the total reduction ratio is about 2 or more. As a result, the effect of expelling air from the joint surface and the effect of preventing air intrusion are obtained, and the bondability and the uniformity of the structure in the plate thickness direction are improved.

The present invention is suitable for manufacturing an extremely thick steel plate having a plate thickness of about 150 mm or more, but can also be applied to manufacturing a thinner steel plate.

Next, the practice cooling of the present invention will be described.

(Example) Slabs having the steel types and thicknesses shown in Table 2 were prepared, and the oxides on the joint surfaces of the slabs were removed by grinder grinding, and then the slabs were laminated in the combinations shown in the table. Then, the four peripheral ends of the slab joint surface were joined by MIG welding, manual welding or forge welding. At that time, an air hole having a flow of 10 mm was provided at the center of the top portion, the bottom portion, or the side portion during or after the joining.

After that, the normal hot rolling (1250
C., including a reversible pass) to obtain a thick steel plate. An ultrasonic deep scratch test was performed on these thick steel plates, and
The cleanliness was examined by measuring inclusions at the t / 5 portion (t: total plate thickness). The results are also shown in Table 2.

As is clear from Table 2, in each of the examples of the present invention, UT
An extra-thick steel plate having no defects and having no difference in cleanliness at the position in the plate thickness direction and having good bondability and cleanliness uniformity was obtained.

On the other hand, in Comparative Example G, since the oxide on the bonding surface was not removed, UT defects occurred. Further, in Comparative Example H, the UT defects were not present because the oxide on the joint surface was removed, but the cleanliness of the joint surface was inferior because no air holes were provided.
Similarly, in Comparative Examples I to L, the air holes are provided, but the positions thereof are not appropriate, and therefore the cleanliness of the joint surfaces is poor. Particularly in the case of the cold rolling method in which only the pass in the same direction as the pass direction for expelling air by the conventional method is applied, the process of the present invention (reversible pass) is applied to the air hole (only the position in front of the rolling direction). However, it can be seen that the cleanliness of the joint surface cannot be improved by taking Comparative Example K into consideration.

(Effects of the Invention) As described in detail above, according to the present invention, when two or more slabs are superposed and hot rolled, the oxides on the joint surfaces are removed in advance and then the end portions of the joint surfaces are joined. At that time,
One or more non-bonding holes such as air holes are provided in at least the top part and the bottom part of the four peripheral ends of the slab bonding surface,
After that, hot rolling including a reversible pass is performed, so that it is possible to obtain an extra-thick steel sheet having excellent bondability and uniformity of cleanliness in the sheet thickness direction and excellent internal quality. Moreover, the joining method does not require any special means such as electron beam welding which brings about high cost, and the non-joint portion can be formed very easily. Therefore, the workability is good, the cost is low, and the extremely thick steel sheet is highly efficient. It is possible to manufacture.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a position where an air hole is formed at an end portion of a slab joint surface, and FIGS. 2 (a), (b) and (c) are defects of an ultrasonic flaw detection test result in a thick steel plate after hot rolling. 3 and 4 are micrographs showing the microstructure of the joint surface W of the thick steel sheet after hot rolling, and FIG. 3 shows air only in the top portion. FIG. 4 shows a case where holes are opened, and FIG. 4 shows a case where air holes are formed in both the top portion and the bottom portion.

Claims (3)

[Claims] 1. When two or more slabs are stacked and pressure-bonded by hot rolling, oxides on the superposed surfaces of the slabs are removed in advance, and then the slabs are superposed to join the four peripheral ends of the superposed surfaces. However, at that time, one or a plurality of non-bonding holes are provided in at least the top part and the bottom part of the four peripheral end parts of the bonding surface, and then hot rolling including a reversible pass is performed. Manufacturing method of extra-thick steel sheet. 2. The method according to claim 1, wherein the non-bonded holes are unbonded holes. 3. The method according to claim 1, wherein the non-bonding hole is a part where a hole or the like is formed in the bonding portion.