JPS61273201A - Production of continuously cast slab for thick steel plate having excellent internal non-defectiveness - Google Patents

Abstract

PURPOSE:To expand the production thickness of an extra thick steel plate having excellent internal non-defectiveness by providing a panel-shaped pressurizing device on the outlet side of a continuous casting device and subjecting the central region of the thickness of a slab to surface rolling down. CONSTITUTION:The panel-shaped pressurizing device 2 is provided on the outlet side of the continuous casting device. The size of the pressurizing surfaces 2a thereof is made slab width W in the width B in order to pressurize the entire width of the slab 3. The Length L is determined from L/t which is the ratio between said length and the slab thickness (t) and the condition L/t<=1 is satisfied. The central region of the thickness of the continuously cast slab having the structure before ferrite transformation is subjected to surface rolling down by the device 2. The center porosity in the central part of the thickness of the slab is thereby effectively press-welded and the continuously cast slab for the thick steel plate having the excellent internal non-defectiveness is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention is a method for manufacturing a continuously cast slab for thick steel plates that ensures internal soundness and is inexpensive.

(Prior Art) In recent years, with the increase in the size of offshore structures and various pressure vessels, extra-thick steel plates with a thickness of over 100 mm have come to be used in large numbers. Generally, in the production of steel plates, there are methods of using continuous casting slabs (hereinafter referred to as continuous casting slabs) and blooming slabs produced from steel ingots, but the former is advantageous in terms of production costs.

However, if the final product board thickness exceeds 100 plates,
With the capabilities of rolling mills currently in use, it is difficult to stably compress the center porosity that forms at the center of continuous cast slabs, resulting in material defects such as ultrasonic flaw detection (UST) defects. There are many things. As a countermeasure for this, there is a technique (Japanese Unexamined Patent Application Publication No. 55-114404) in which rolling is applied by rolls on the outlet side of the continuous casting apparatus, but the upper limit of the thickness of the product plate is about 150I1 m.

On the other hand, the use of high heat input welding is popular from the perspective of reducing the number of welding operations, and therefore the steel materials used are required to have a low carbon equivalent in order to ensure toughness after high heat input welding. In the normal production process of thick steel plates, lowering the carbon equivalent brings about a decrease in strength, so in the production of such steel plates, processing heat treatment (controlled rolling and This process is followed by controlled cold rolling, which ensures strength and toughness even in low carbon equivalent steel sheets.

One of the problems with this processing heat treatment is that cooling is performed immediately after rolling, so if a large amount of hydrogen exists in the slab, there is no time for the hydrogen to dissipate until the steel plate is completely cooled, resulting in a relatively thin product. Even if it is thin (plate thickness 6 to 1200 mm), hydrogen-based UST defects are likely to occur. For this reason, in reality, a method is adopted in which a continuous cast slab is either left as it is or after being subjected to blooming to reduce its thickness to some extent and then subjected to a long dehydrogenation heat treatment to reduce the hydrogen content in the slab before rolling. .

However, this dehydrogenation treatment of continuously cast slabs is very disadvantageous in terms of cost, and the scale of implementation is currently being expanded to reduce costs. It cannot be used in fuel-saving methods (hereinafter referred to as hot charging) or in processes such as direct rolling from continuous casting equipment.

(Problems to be Solved by the Invention) The purpose of the present invention is to provide a method for manufacturing a continuous casting slab for extra-thick steel plates with a thickness exceeding 100111 mm, which has excellent internal soundness that has not been possible with conventional techniques. The aim is to increase the thickness of the plate and provide an inexpensive manufacturing method.

(Means for solving the problem) In order to achieve the above objectives, it is necessary to remove center porosity, which impairs the integrity of the final product. Therefore, it is difficult to prevent this from occurring, and as mentioned above, it is also difficult to perform crimping using a rolling mill that is currently in practical use. The present invention has solved these problems, and its characteristics are that the width (W) is greater than the slab width and the length (L) is
) is the ratio to the slab thickness (t), and a planar pressurizing device having a pair of upper and lower pressurizing surfaces satisfying the condition of L/t≧1 is provided, and the pressurizing device is used to remove the structure before ferrite transformation. The point is to reduce the surface pressure in the center area of the plate thickness of the continuously cast slab.

As mentioned above, it is impossible to prevent center porosity from occurring at the center of continuous casting slabs with the continuous casting equipment currently in use, and various new continuous casting equipment are being considered for this purpose. However, it has not been put into practical use. Therefore, in order to ensure the soundness of the final product, it is necessary to press the center porosity at the processing stage. When using , the penetration of the reduction to the center of the plate thickness is insufficient, and the center porosity cannot be crimped.

First of all, in order to advantageously compress this center porosity, the present invention provides a pair of upper and lower planar pressure devices on the outlet side of the continuous casting device, and applies a slight surface pressure after solidification is completed and before cooling. There is a particular thing. The planar pressurizing device has a pair of surfaces parallel to the front and back surfaces of the continuous casting slab, and has a length of 201 m.
A slight surface pressure not exceeding + is applied to the continuous cast slab. At this time, if the thickness t1 width of the continuous casting slab is W, the dimensions of the pressure surface are W (=B) in the width direction of the continuous casting slab and t in the longitudinal direction.
or more (=L) is required.

Here, we will explain why the exit side of the continuous casting device is the optimal installation location for this planar pressurizing device. The center porosity of a continuously cast slab is generated at the center due to volumetric contraction of molten steel when it solidifies inside a continuous casting device. Therefore, at this stage, by applying a pressure from the outside that is commensurate with the amount of shrinkage, the center porosity can theoretically be reduced.
It is also possible to prevent the generation of

However, when molten steel is present in the center of the slab, pressure is not transmitted to the molten steel even if pressure is applied from the outside.
In reality, since the amount of molten steel fluctuates, it is practically difficult to prevent center porosity from forming according to this theory. Therefore, in order to stably press the center porosity by applying pressure from the outside, it is necessary to apply pressure after the molten steel has completely solidified, and it is appropriate to apply pressure after the exit side of the continuous casting apparatus.

In order to bond the center porosity with a more compact device and with a small pressure, it is preferable that the average temperature of the slab is high and the temperature of the center is higher than that of the surface. When considering the entire manufacturing process of extra-thick steel plates, the closest condition to this condition is the exit side of the continuous casting equipment where the center of the slab has just completed solidification. It is preferable to install the above-mentioned planar pressurizing device to press the center porosity.

For the same purpose, there is a method of applying rolling reduction with rolls on the exit side of a continuous casting device (Japanese Patent Laid-Open No. 55-114404).
However, according to studies conducted by the present inventors, the thickness that can be produced by the roll reduction method for extra-thick steel plates with excellent internal soundness remains at most 150,111 mm.

The reason for this is that with the current roll diameter (R (50011II)) and reduction amount (approximately 201 m), the penetration of the material to the center of the thickness of the slab is insufficient, and the porosity cannot be completely crimped. This is because porosity tends to remain after rolling.

Based on these considerations, the present invention has investigated surface reduction that allows the reduction to more easily permeate to the center of the slab, and created conditions to completely compress porosity at the slab stage and ensure internal soundness after plate rolling. This is what I found.

Furthermore, when considering the dehydrogenation of continuously cast slabs that are the raw material for heat-treated steel sheets, center porosity that is generated in the continuously cast slabs occurs in the center of the slabs due to volumetric contraction when molten steel solidifies. At the time this center porosity is generated, the slab is in an austenitic state with a relatively large amount of solid solution of hydrogen, so there is little hydrogen accumulation in the center porosity.

However, when the temperature of the slab decreases and transforms into ferrite, the amount of solid solution hydrogen decreases significantly, so that a high concentration of hydrogen accumulates in the center porosity. Hydrogen in steel exists in atomic form, so it is easy to dehydrogenate by heat treatment, but hydrogen that accumulates in center porosity becomes molecular, so it is difficult to dehydrogenate by heat treatment, and dehydrogenation is difficult. Heat treatment takes a long time.

The installation position of the planar pressurizing device in the present invention is based on the results of these studies.1. The optimal position for crimping the center porosity was found between the time when the center of the continuously cast slab solidifies and the time when it does not transform into ferrite.

That is, when applying surface pressure according to the present invention, by ensuring the pressure surface has sufficient dimensions, the reduction blade can easily penetrate to the center of the slab and press the center porosity. In addition, since the planar pressurizing device is only intended to press the continuous cast slab 11 and the porosity at its center, the reduction amount is small at 20 u or less, and the temperature of the continuous cast slab is just below the freezing point. In order to pressurize at a very high level (therefore, the deformation resistance is very low), after reheating a large steel ingot,
Compared to conventional forging press equipment, which requires large amounts of processing, it has the advantage of having a very small pressurizing capacity.

FIG. 1 shows a continuous casting apparatus l suitable for carrying out the present invention and a planar pressurizing apparatus 2, and the planar pressurizing apparatus 2 is located close to the outlet side of the continuous casting apparatus. has been done. The pressurizing device 2 is configured to pressurize the slab 3 at a pressurizing surface 2a using a hydraulic mechanism.

The same effect can be obtained whether the continuous casting device 1 and the planar pressurizing device 2 are installed continuously or separately. The separated type is more advantageous in terms of equipment costs and maintenance, but in this case, the center temperature of the slab 3 is less than 900°C (when the slab 3 is left to cool, the surface temperature is It is preferable to install the planar pressurizing device at a position where the temperature does not drop to (corresponding to less than 700° C.).

Next, regarding the dimensions of the pressing surface 2a in the planar pressing device, the width (B) is the width (B) since the entire width of the slab 3 is pressed.
(W) Above. The length (L) is an important factor for exhibiting the effects of this device, and is determined from L/l, which is the ratio to the slab thickness t. In other words, there is a relationship between L/l and the UST results of the final product as shown in Figure 2.
To ensure the internal integrity of the final product, L/l should be 1
It is necessary to do more than that.

Continuously cast slabs generally used for manufacturing extra-thick steel plates are 100
It has a thickness of about 50,011 m, and the length (L) of the necessary planar pressurizing device and pressurizing surface 2a is 100,000 m.
0111~500 or more.

Note that FIG. 2 shows the manufacturing results of the final product plate with a thickness of 150 to 220 mm, and the defect density γ on the vertical axis was determined from the results of flaw detection of the steel plate under the following conditions.

UST flaw detection sensitivity: JIS G 0801-1974+
6dB defect evaluation 20...25% (F, (50%Δ...
・50% (F, (t00%
(equivalent to 1 piece) S: Steel plate surface area (m2) However, the steel plate is a slab whose center porosity is effectively and appropriately crimped by applying pressure to the center of the plate thickness using a planar pressurizing device. The process is not special, and when supplying the plate to the plate rolling line, it can be cooled once and then supplied via a heating furnace, or it can be supplied while still warm to the heating furnace, or if the temperature conditions are favorable. If so, it may be directly supplied to the rolling process.

Furthermore, the conditions for controlled rolling and controlled cooling in processing heat treatment are not limited in the present invention, but are appropriately selected and determined based on conventional knowledge depending on the components and uses. Depending on the required characteristics of the steel after rolling, quenching, tempering,
It is made into a thick steel plate through conventionally known normal processes such as tempering heat treatment such as normalizing.

Although a curved type continuous casting device is shown in Fig. 1,
Even if it is a vertical type, the present invention can be applied and the effect will not change.

(Example 1) Table 1 shows the manufacturing conditions of the examples of the present invention and comparative examples, and the UST results of the obtained steel plates.

In addition, F is shown in FIG. 1 for the examples of the present invention and comparative examples.

Pressure was applied by a planar pressing device 2 provided on the exit side of the continuous casting device 1, and Comparative Example J is data from a conventional roll rolling mill placed at the location of the pressing device. That is,
As is clear from Table 1, steels A, B, according to the invention
C shows excellent internal soundness after processing heat treatment even without long slab dehydrogenation heat treatment.

On the other hand, steels with insufficient surface reduction conditions, steels F, steels J subjected to rolling by a roll mill, steels H and I subjected to bloom breakdown rolling, and steels made from as-cast slabs into thick steel plates. Both E and G had UST defects and had poor internal soundness.

The reason why the examples of the present invention achieved such good results is that the center porosity at the center of the plate thickness was effectively compressed by the planar pressurizing device, and there was no place for hydrogen accumulation. be.

(Example 2) Table 2 shows other examples of the present invention and comparative examples.

In the embodiment of the present invention, a planar pressurizing device having the layout shown in Fig. 1 is used to pressurize the center of the plate thickness to crimp the center porosity, and through the manufacturing process shown in Note 2 of Table 2, It is made of extra-thick steel plate.

In addition, as a comparative example, E and F steels are equipped with a roll reduction device at the location where the planar pressure device is installed in the layout shown in Figure 1.
It was rolled down. G steel has a small L/l of 0.3 when using a planar reduction device.

However, as shown in Table 2, it can be seen that according to the present invention, an extremely thick steel plate with excellent internal soundness can be reliably obtained using a continuous casting slab. On the other hand, in all comparative examples, only those with poor internal soundness were obtained.

(Effects of the Invention) As explained in detail above, according to the present invention, compared to the conventional roll reduction method, by applying pressure with a planar pressure device,
Since the reduction is sufficiently applied to the center of the plate thickness and the center porosity is reliably crimped, the production thickness of extra-thick steel plates with excellent internal soundness can be increased.

Furthermore, dehydrogenation heat treatment, which has been indispensable in the past, can be omitted, making it possible to produce heat-treated steel sheets with excellent internal soundness at low cost. In addition, it is also possible to employ processes such as hot charging or direct rolling, and from this aspect as well, a large energy saving effect can be obtained.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram showing the relationship between a continuous casting device and a planar pressurizing device suitable for carrying out the present invention, and Fig. 2 shows L/l and internal soundness (defect density) of a heat-treated steel sheet. FIG. 1 □ Kami pressure 1iiff2a ÷ ″alljl-12

Claims (1)

[Claims] On the exit side of the continuous casting device, the width (W) is larger than the slab width,
and the ratio of the length (L) to the slab thickness (t), L/t≧1
A planar pressurizing device having a pair of upper and lower pressurizing surfaces that satisfies the conditions is provided, and the pressurizing device applies surface pressure to the central region of the plate thickness of a continuous casting slab having a structure before ferrite transformation. A method for manufacturing continuous casting slabs for thick steel plates with excellent internal soundness.