JPS62192242A - Production of continuous casting slab for steel plate having excellent internal soundness - Google Patents

Abstract

PURPOSE:To improve the internal quality of a steel plate with large thickness by forming an equi-axed crystal in the specified range at center of thickness of a solidified slab and also executing light reduction at thickness direction by one pair of faced pressing device after just finishing of the solidification. CONSTITUTION:An electromagnetic stirring device 3 is arranged below a mold 2 and also at outlet side of a continuous casting apparatus, one pair of the pressing device 7 is arranged. When the casting slab 5 is cast continuously by pouring into the mold 2 from a tundish 1, the electromagnetic stirring is given to the casting slab during existing of non-solidifying part therein by the electromagnetic stirring device 3, to form the equi-axed crystals, having at least >=30 mm thickness in center part of thickness for the solidified slab. Next, after just finishing of the solidification, the suitable quantity of the light reduction is given on the slab 8 in the specified ranges of length and width by the pressing device 7. In this way, as the center segregation and the center porosity in the slab 8 are prevented, the internal quality of the steel plate with large thickness is improved.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is a method for inexpensively manufacturing thick steel plates with less segregation in the center and excellent internal soundness without unbonded porosity by continuous casting. It is related to.

(Prior Art) In recent years, with the increase in the size of offshore structures and various pressure vessels, the thickness of the plates used has also tended to increase, and extra-thick steel plates exceeding 100 W are increasingly being used. In such extremely thick steel plates, segregation and U
Clean steel without ST defects is required. Generally speaking, in the production of steel plates, there are two methods: continuous casting slabs (hereinafter referred to as continuous casting slabs) and blooming slabs produced from steel ingots.The former is advantageous in terms of manufacturing costs, but the final product plate When the thickness exceeds 10 On+, it is difficult to stably bond the center porosity that forms at the center of the continuous cast slab with the capabilities of currently available rolling mills, and ultrasonic flaw detection testing (hereinafter referred to as UST) is difficult. ) that causes material defects such as defects.

As a countermeasure to this problem, a technique of applying pressure reduction using rolls on the exit side of the continuous casting device (Japanese Patent Application Laid-open No. 114404/1983)
However, the product board thickness is approximately 15 (IKm) as the upper limit.

Furthermore, in recent years, new processes for thick plates have been developed, such as TMC P as disclosed in Japanese Patent Publication No. 55-30047.
(Thermo Mechanical Control
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 for controlled cooling immediately after rolling. Since the hydrogen in the steel sheet is released, it is more difficult to ensure internal integrity than in the normal process, and the segregation area is particularly susceptible to hydrogen-induced cracking, increasing the tendency for UST defects to occur.

For this reason, in high-grade steels such as 80 kg high tensile strength, TMC
In the P process, a thickness of about 50 mm is the upper limit for ensuring internal soundness.

As mentioned above, when manufacturing thick steel plates using the existing rolling process, there are restrictions on the thickness that can be manufactured in both the normal process and the TMCP process, and the reason for this is: ■ Porosity remaining in the final product - ( Hydrogen-induced cracking (due to hydrogen remaining in the final product); This is due to UST defects due to the presence of segregation (which promotes hydrogen-induced cracking susceptibility), and these must be prevented in order to expand the manufacturable plate thickness. .

(Problems to be Solved by the Invention) The present invention has excellent internal soundness, which was impossible with the prior art, and as a result, excellent toughness in the center. 50 in the process
The purpose of this project is to inexpensively manufacture thick steel plates thicker than that of a dragon using a continuous casting method.

(Means for solving the problem) As mentioned above, the first cause of UST defects in the final product is the microporosity generated in the continuous cast slab.
remains on the steel plate. This microporosity
If the thickness of the product plate is thick, it is difficult to press it with existing rolling mills, and the countermeasure is to completely eliminate the occurrence of microporosity in the continuous cast slab, but this is not currently in practical use. This is not possible with the continuous casting equipment currently available.

The second cause is segregation that occurs at the center of the continuously cast slab (hereinafter referred to as center segregation).

Even if the microporosity is compressed, hydrogen remaining in the steel sheet tends to cause hydrogen-induced cracking using these as starting points, which is particularly harmful in the TMCP process where there is no time for hydrogen to dissipate.

Taking these points into consideration, the present invention has found a method for effectively reducing center segregation, more effectively and economically compressing microporosity before rolling, and by using a combination of the two, This method has made it possible to manufacture thick steel plates that could not be manufactured using the above method.The feature is that when molten steel is solidified in a continuous casting machine, at least 30 mm of the center of the thickness of the solidified slab is solidified as an equiaxed product.
Immediately after solidification is completed, a light reduction is immediately applied in the thickness direction using a pair of planar pressing devices, followed by hot rolling.

(Function) The present invention will be explained in detail below. First, the reason why the central part of the continuously cast slab is made of equiaxed crystals will be explained. In a normal continuous cast slab, solidification progresses as columnar crystals develop from the periphery toward the center, and finally the columnar crystals that grow from the front and back surfaces of the slab reach the center of the slab and solidification is complete. At this time, the highly concentrated residual molten steel solidifies in the center of the slab, often resulting in segregation with extremely high component concentrations. The resulting segregation at the center of the slab becomes a starting point for hydrogen cracking and causes deterioration of toughness, as described above. According to studies by the inventors, it is effective to make the center of the slab equiaxed in order to prevent this segregation at the center of the slab. As shown in Figure 1, if the width of the equiaxed crystals is sufficiently wide, it is possible to prevent hydrogen cracking caused by center segregation, and when used in conjunction with the planar light reduction described later, UST defects can be completely prevented. can.

It is understood from FIG. 1 that the width of the equiaxed crystal should be 3011 or more. 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: JIs G 1801-1974
+6 dB defect evaluation: ○...25%<F+<50%
△...50%<F+<100% ×...100%≦F1 Defect density: γ=N/S N: Number of defects (02 is equivalent to 1 defect, △2 is ×
(Equivalent to 1 piece) S: Steel plate surface area (m2) Now, the following two methods are generally used to equiaxedly crystallize the solidified structure of a continuously cast slab. The first method is to apply electromagnetic stirring to solidify equiaxed crystals while the continuously cast slab has an unsealed portion inside, and to reduce segregation of components. In this case, the thickness of the unsolidified part is inevitably determined by the cooling method and thickness of the continuous cast slab, and not only the position of the final solidification point in the continuous casting machine but also the solidification thickness at each position in the machine can be determined empirically. If the installation position of the electromagnetic stirring equipment is determined based on this, the thickness of the equiaxed crystal zone can be controlled. That is, it is easy to ensure an equiaxed zone thickness of 3ON1 or more. Here, the slab thickness is 160 to 400 mm, and the width is 1800 to 24 mm.
In the case of a continuous casting machine of about 0.00 mm, the electromagnetic stirring is usually 400 to 9 mm.
Equipment with a capacity of about 00kVA will be used. In addition, if the equiaxed zone thickness is 30 Tsuru or more, it satisfies the conditions of the present invention.
In order to make the slab too thick, it is necessary to slow down the cooling of the slab, which reduces productivity, so the practical upper limit is 1001.
- It will be about.

The second method is to control the injection temperature of molten steel to a low temperature side. If the temperature during injection is lowered, the solidification structure tends to become coaxial, but the optimum temperature differs depending on the molten steel composition, and also depends on the type of continuous casting machine (vertical type or curved type, etc.) and operating conditions. It also varies depending on factors such as (casting speed, etc.)
・Although it cannot be defined technically, by detailed basic investigation, it is possible to control the equiaxed zone thickness of the slab according to each continuous casting machine and operating conditions.

There are two methods of equiaxed crystallization of the solidification structure, and the method using electromagnetic stirring has the disadvantage that the grain size of the crystals is somewhat coarse due to stirring, but it is possible to precisely control the thickness of the equiaxed crystal zone. There are certain advantages.

Whether to use electromagnetic stirring or low-temperature casting should be selected depending on the thick plate that will be the product, and there is no difference that will change the wood quality of the present invention.

As stated above, if the center part is equiaxed, it is possible to prevent center segregation of the slab, and from this point of view it is effective in preventing UST defects, but equiaxed crystallization in the center part will inevitably occur. Promotes the occurrence of center porosity. Therefore, countermeasures against center porosity are required, and the following method is effective for this. That is, immediately after solidification of the slab is completed, a light pressure is immediately applied in the thickness direction using a planar pressure device having a pair of upper and lower pressure surfaces, and the method has the following characteristics.

■ Since the steel is rolled down immediately after solidification, the temperature of the steel is the highest in the entire manufacturing process and rolling down is easy.

■ The center has the highest temperature immediately after solidification, and there is a temperature gradient where the temperature is slightly lower on the front and back surfaces, so the microporosity in the center is easily compressed even under the same pressure.

(2) Therefore, it is possible to reduce the microporosity by a slight light reduction, and this can be achieved using a small-scale, economical reduction device.

Now, although the outline of the surface pressure applied immediately after solidification, which is a feature of the present invention, is shown in Fig. 4, the inventors' study revealed that
It has been experimentally found that the following conditions are necessary.

That is, 1) the length L) of the pressure surface of the planar pressure device must satisfy L/l≧1.0, where t is the thickness of the continuous cast slab (FIG. 2). The reason for this is that when L/l is <1.0, penetration of the reduction into the center of the continuous casting slab is insufficient;
This is because when /l exceeds 1.0, the rolling effect is sufficiently spread to the center of the continuous casting slab, and the center porosity is compressed. It is advantageous for operation that the width (B) of the planar pressurizing device is equal to or larger than the width of the continuous casting slab, but as long as B/L≧1, the effects of the present invention are not impaired.

2) Furthermore, regarding the amount of reduction (Δh) by the planar pressurizing device, it was confirmed from the experimental results that it is effective when Δh/l≧0.01 (FIG. 3). Here, Δh is all 1
Although it is applied by applying pressure once, the effect remains the same even if the pressure is applied in multiple times.

The continuous cast slabs that have been lightly rolled in this way are then cooled and rolled into extra-thick steel plates at a plate factory, as in the conventional process, or they are immediately turned into final products in a hot lump at a plate factory. Rolled. In some cases, the material goes through a similar process and then goes through the TMCP process at a plate factory. FIG. 4 shows an outline of the equipment for the method of the present invention.

(Examples) Examples of the present invention are shown in Tables 1 and 2 together with comparative examples. The steel plate manufactured according to the present invention exhibits excellent internal soundness even when the plate thickness is 100 Tn or more in the normal process and 501 Tn or more in the TMCP process. On the other hand, the comparative example had UST defects due to lack of one of the conditions. In other words, anything based on nature is UST.
Result: Good, defect density (γ) 20, but
All of the comparative examples were judged as defective and defect density (γ) not 20 by UST.

(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 extended 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. In particular, in the present invention, since there is no segregation in the central part, which is highly susceptible to hydrogen cracking, dehydrogenation heat treatment, which has been indispensable in the past, can be omitted, and it is possible to manufacture heat-treated steel sheets with excellent internal soundness at low cost. In addition, it is also possible to employ processes such as real charging or direct rolling, and from this aspect as well, a large energy saving effect can be obtained.

[Brief explanation of drawings]

Figure 1 shows the relationship between the width of equiaxed crystals at the center of a continuously cast slab and the internal soundness (defect density) of the steel plate. Figure 2 shows the length L of the pressurizing surface of the planar pressurizing device and the thickness t of the continuous cast slab.
The relationship between the ratio (L/l) and the internal soundness (defect density) of the steel plate is shown. Figure 3 shows the ratio (
The relationship between Δh/l) and the internal soundness (defect density) of the steel plate is shown. FIG. 4 shows an outline of the apparatus of the present invention, where ■ = tundish, 2: mold, 3: electromagnetic stirring device, 4: drawing roll, 5: slab, 6: pressure surface, 7: pressure device, 8 It's Nislav.

Claims (1)

[Claims] When solidifying molten steel in a continuous casting device, at least 30 mm of the center of the thickness of the solidified slab is solidified as equiaxed crystals,
Immediately after solidification is completed, a method for manufacturing a continuously cast slab for thick steel plates with excellent internal soundness is characterized in that the slab is lightly rolled down in the thickness direction by a pair of planar pressure devices.