JPS62134101A - Production of thick steel plate having excellent internal soundness - Google Patents

Abstract

PURPOSE:To obtain a steel plate having excellent internal soundness by executing rapping right after the solidification of a molten steel poured into a casting mold and subjecting the steel ingot to light reduction by a planar pressurizing device having a pair of upper and lower parallel pressurizing surfaces, then to hot rolling. CONSTITUTION:Rapping is executed right after the solidification of the molten steel and the steel ingot 1 is placed horizontally and is subjected to light reduction by the planar pressurizing device 2 disposed above and below the ingot. In this case, the length L of the pressurizing surfaces of the device 2 is required to satisfy L/t>=1.0. The ingot is thereafter subjected to hot rolling, by which the internal soundness is assured and the deformation of the steel ingot is obviated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing a thick steel plate with excellent internal soundness.

[Prior Art] In recent years, as offshore structures and various pressure vessels have become larger, the thickness of the plates used has also tended to increase, and there is a demand for plates exceeding 200 mm. When such an extremely thick steel plate is rolled with an existing rolling mill, the rolling effect is insufficient, and the microporosity generated in the steel ingot remains in the steel plate, causing υST defects. As a countermeasure to this problem, the Japanese Patent Application Publication No. 52-94649 has already been published.
．． 55-106601, Special Publication No. 53-12258. 53-12259, etc., have been proposed to improve the rolling technology, and some effects have been obtained. The thickness limit has not yet been raised significantly, and the demands of customers have not been fully met.

Furthermore, in recent years, TM has become a new process for thick plates.
CP (Thermo Mechanical Co.
In this process, controlled rolling is performed, so the rolling temperature is low and the rolling effect on porosity crimping is smaller, and the rolling process is water-cooled immediately after rolling for controlled cooling. As a result, hydrogen in the steel plate is released, making it more difficult to ensure internal soundness than in the normal process, and for high-grade steels such as 80 kg high tensile steel, the upper limit for ensuring internal soundness is about 50 mm.

[Problems to be solved by the present invention] The purpose of the present invention is to manufacture a steel plate with excellent internal soundness, which has not been possible with conventional techniques, and has a thickness of 150 mm or more in the normal process and 50 mm or more in the TMCP process. This is what I did.

[Means for Solving the Problem] As mentioned above, the main reason why UST defects occur in the final product is that microporosity generated in the steel ingot remains in the steel plate. If the product plate is thick, it is difficult to compress this microporosity using existing rolling mills.The countermeasures are to either completely eliminate the occurrence of microporosity in the steel ingot, or to compress it before rolling. There is only one. The former is almost impossible with the current ingot forging method, and the latter involves preliminary forging of a slab using large forging equipment, but it requires very expensive forging equipment and requires heating of the steel ingot. There are also significant cost disadvantages such as lower yields and lower yields.

In consideration of this point, the present invention has discovered a more effective and economical method of compressing microporosity before rolling. In other words, the feature is that after the molten steel injected into the spinning mold solidifies, it is immediately removed from the mold, and a planar pressurizing device with a pair of upper and lower parallel pressurizing surfaces immediately applies light reduction in the thickness (short piece) direction of the steel ingot. The process involves hot rolling after adding .

[Action Co. The operation of the present invention will be explained below.

The present invention has the following features compared to conventional slab pre-forging. That is, (1) since the pressure is reduced immediately after solidification, the temperature of the fi lump is the highest among all the manufacturing processes, and reduction is easy.

(2) At the same time, the center has the highest temperature immediately after solidification, and the front and back surfaces have a temperature gradient that is slightly cooler.
Even under the same pressure, the microporosity in the center is easily crimped.

(3) Therefore, microporosity can be crimped with a slight light reduction, and this can be achieved using a small-scale, economical reduction device.

(4) Furthermore, due to the light reduction, there is little deformation of the steel ingot, and the decrease in yield is also small.

(5) Needless to say, heating costs are unnecessary.

Now, with regard to the surface pressure applied immediately after die cutting, which is a feature of the present invention, the inventors have experimentally discovered through studies that the following conditions are necessary.

That is, 1) The length Lmm of the pressing surface of the planar pressing device needs to satisfy L/l≧1.0, where the thickness (short piece) of the steel ingot is tIIlm (FIG. 1). The reason for this is L/l.
When L/l is 0, the penetration of the reduction into the center of the steel ingot is insufficient, but when L/l exceeds 1.0, the effect of reduction is sufficiently spread to the center of the steel ingot, and the center porosity is crimped. In FIG. 1, the defect density γ on the vertical axis was determined from the results of flaw detection on a steel plate under the following conditions.

UST flaw detection sensitivity: JIS00801. -1974+6d
B Defect evaluation 20...25% F work<SO%Δ...50
%<Fl<100% ×...100%≦F□ Defect density: γ=N/S N:Δ Number of defects (02 is equivalent to 61, 62 is ×
(Equivalent to 1 piece) S: Steel plate surface area (m') It is advantageous for operation that the width (B) of the planar pressurizing device is equal to or larger than the steel ingot width, but if B/L≧1, This does not impair the effects of the present invention.

2) Furthermore, as for the reduction amount Δhmm by the planar pressurizing device, it was confirmed from the experimental results that it is effective when Δh/l≧0.01 (FIG. 2). Here, all Δh is applied by one pressurization, but the effect does not change even if the pressure is applied in multiple times.

The steel ingot that has been lightly rolled in this way is then rolled into a slab at a blooming factory, as in the conventional process.
In addition to being rolled into extra-thick steel plates at a plate mill, the top of the hot ingot is cut and immediately rolled into final products at a plate mill. In either case, since the center porosity is crimped while the steel ingot is lightly rolled down, no UST defects occur in the final product.

Figure 3 shows an overview of an example of equipment. Reference numeral 1 denotes a steel ingot (a portion of original thickness t and a portion whose thickness has been reduced by Δh after being subjected to light reduction) is placed horizontally and subjected to light reduction by planar pressure devices 2 arranged above and below.

[Example] Examples of the present invention are shown in Table 1 along with comparative examples.

A steel plate manufactured using a steel ingot subjected to light surface reduction after solidification of molten steel based on the present invention has a thickness of 150 mat or more in a normal process,
In the TMCP process, excellent internal soundness is shown even at lengths of 50 mm or more. On the other hand, the comparative example lacks one of the conditions and therefore has a UST defect.

[Effects of the Invention] As explained above, according to the present invention, it is possible to achieve a process that cannot be achieved using only the conventional rolling method, and when preliminary forging of the slab is performed, there are many problems such as equipment costs, yield reduction, and heating costs. , 150mm or more in normal process, 50mm in TMCP process
Ensuring the internal integrity of extremely thick steel plates of mm or more can be economically advantageously solved.

[Brief explanation of drawings]

Figure 1 shows the ratio (
The relationship between L/1) and the internal soundness (defect density) of the steel plate is shown. Figure 2 shows the ratio (Δ
'h/t) and the internal soundness (defect density) of the steel plate. FIG. 3 shows an outline of the planar pressurizing device.

Claims (1)

[Claims] The steel ingot method is characterized in that after the molten steel completes solidification in the mold, it is immediately removed from the mold, immediately subjected to a light reduction in the thickness direction using a pair of parallel planar pressurizers, and then hot rolled. , a method for manufacturing thick steel plates with excellent internal soundness