JPS62263855A - Method for continuous casting having little center segregation - Google Patents

Abstract

PURPOSE:To execute continuous casting for sound casting bloom material without any development of center segregation by cooling under controlling in order the surface temp. of the casting bloom along casting direction at the suitable position to the end of molten steel pool, to shrink a solidified shell. CONSTITUTION:In the continuous casting for steel, the surface temp. of the casting bloom along the casting direction from the position of 2-15m before the end of casting direction of the remaining molten steel pool to the end position of the pool is cooled forcely in order by using spray nozzle, etc., in accordance with the progressing of solidification of melt center nucleus for the casting bloom. Then, at A3 transformation temp. of steel or starting temp. TA and more of Acm transformation, and surface temp. TV or less of an effective casting bloom as shown in the equation, the casting bloom is cooled under controlling and the solidified shell of the casting bloom is shrunk, to reduce the casting bloom section. In this way, shifting of the solute concentrated molten steel toward the axial part is prevented and the center segregation is reduced. Therefore, effect for improvement of the properties, such as low temp. toughness, property to lamella tear, resistance of HIC, etc., is obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for continuous casting of steel with less center segregation, and more specifically, to reduce center segregation and improve low-temperature toughness in steel sheets caused by center segregation. The present invention relates to a method for improving the resistance to melatyers, HIC resistance, etc., and also improving the rolling fatigue life of bearing steel and the wire breakage rate and cuppy rupture rate of hard steel wire rods.

[Conventional technology]

In order to reduce the center segregation of continuously cast slabs and blooms, it is necessary to prevent the occurrence of bulging by setting an appropriate roll interval, arranging the rolls, or providing appropriate secondary cooling.

On the other hand, it is necessary to reduce the degree of superheating of the molten steel, add coolant to the mold, apply magnetic stirring to the molten steel in the mold, apply ultrasonic waves to the slab in the strand, and further improve the molten steel in the strand. Techniques to reduce center segregation by methods such as the application of electromagnetic stirring are widely used.These methods all reduce center segregation by making the cast structure equiaxed and achieving fine dispersion of solutes. It is an object.

Although reducing the degree of superheating of molten steel is effective in producing a monoaxial product, it is necessary to control the casting temperature within a narrow range, which has the drawback of inhibiting operational stability.

Addition of a coolant is also effective in producing equiaxed products, but the melting of the coolant may cause residue and a-clogging of mold slag, which has the drawback of easily causing UT defects.

The application of ultrasonic waves to slabs is effective in principle, but as an implementation technique, there is a problem of fatigue of the application roll, which makes it difficult to put it into practical use.

Taking these points into consideration, i(z magnetic stirring within the mold or within the strand has few practical drawbacks, is effective in producing equiaxed products, and is widely used.However, Although it is recognized that center segregation tends to be reduced due to the progress of equiaxed products due to electromagnetic stirring, in reality, the mechanical properties of thick steel plate products made from continuously cast slabs to which electromagnetic stirring is applied are Compared to thick steel plate products made of the same material that are not subjected to electromagnetic stirring, there is no particular improvement in m4A, and the fracture rate of hard steel wire rods made from continuous casting blooms that are subjected to electromagnetic stirring is also lower than that of materials that are not subjected to electromagnetic stirring. No significant improvement effect was observed compared to the wire rod obtained from.

Furthermore, there is a conventional technique in which the slab is lightly pressed in the final solidification region of the continuously cast IayJ slab, and the solute carbonized molten steel that causes center segregation is discharged above the slab to discharge the center segregation. (JP 54-107831) However, in continuous casting, the tip of the crater at the final solidification position is said to be very sharp, and the crater is not straight but wavy, and at such a position, the solute concentration is low. It is thought that the molten steel is confined in the four parts of the crater, resulting in the existence of point-like segregation with a high degree of segregation.

[Problem that the invention seeks to solve]

The present invention relates to continuously cast slabs or blooms,
The present invention provides a method for eliminating center segregation and producing sound slabs.

FIG. 4 shows the longitudinal cross-sectional macrostructure of a plume slab for lit1w4 wire.Since the slab was cast at a low temperature, an equiaxed product is formed over the entire area from the bottom surface to the axis on the dot side. On the other hand, on the dihedral side, the range of 150 mm from the top surface is branched columnar crystals, and the inside is equiaxed. Hollow-shaped cavities are formed intermittently in the shaft center along the casting direction. A characteristic feature of bloom slabs is that they are accompanied by V segregation near the axial center, which is different in form from the V segregation that occurs at the axial center of steel ingots.
Rather, it is a form of inverted V segregation in steel ingots! It has an island. ■Segregation occurs in an area with a convergence of approximately 100 mm around the axis.

FIG. 2 shows a reduced statistical distribution of semi-macro analysis values in the vicinity of the axis of a double-sized slab. From this, the central segregation and the adjacent negative segregation zone are clearly recognized. From this figure, the area where negative segregation begins to occur is 40° from the axis.
com range. In other words, 80m around the axis
It can be seen that there is bulk solute movement in a region with a width of m. Under the actual casting conditions in which the slab was cast, the area where this solute movement occurs is 8 points from the tip (crater end) of the molten steel pool inside the slab when viewed from the slab position along the casting direction.
~ion to the leading edge of the molten steel pool. Furthermore, this range naturally changes if the slab cross-sectional dimensions, casting speed, cooling conditions, etc. change, but under actual continuous casting conditions, from 2 to 15 m in front of the leading edge of the molten steel pool to the leading edge of the molten steel pool. This corresponds to a range of . It is clear from the surface analysis of V segregation that such movement of the solute C-treated molten steel occurs along the V segregation line as shown in FIG. Furthermore, it is clear from metallurgical observations and extensive numerical calculations that this movement of solute C-treated molten steel is caused by the suction force accompanying the solidification contraction of the remaining molten steel in the molten steel pool. can do.

Therefore, in order to prevent center segregation, the solute carbonized molten steel generated by the suction force caused by the solidification shrinkage of the remaining molten steel in the molten steel pool near the axis of the slab (in the case of slabs, the thickness center) The goal is to prevent movement.

[Means for solving problems]

The first stage of technology to solve the above problems is to continuously cast steel from a position 2 to 15 m before the leading edge of the residual molten metal pool in the casting direction to the pool's leading edge position. In accordance with the progress of solidification of the liquid core core of the slab, the surface temperature of the slab along the slab is adjusted to be below the A3 transformation temperature of the steel or the starting temperature Ta of Acm 15 and below the effective slab surface temperature Tv shown in the following formula. The solidified slab shell is contracted by sequential forced cooling to a certain temperature.

It is characterized by casting with a reduced cross section of the slab.

Ta + (TN - Ta) Xo, 3=Tv However.

TN: Surface temperature of the slab due to natural cooling after exiting the pinch rolls Ta: Surface temperature of the slab to obtain the average cooling of the solidified shell necessary to compensate for solidification shrinkage 1 [Function] The above technical means Since the shrinkage caused by the solidification of the molten steel, that is, the amount of shrinkage caused by the solidification of the liquid core of the slab, is compensated for by shrinking and deforming the solidified slab shell, the movement of the H-quality concentrated molten steel toward the axial center is compensated for by shrinking and deforming the solidified slab shell. Figure 3 shows the amount of shrinkage, that is, compressive deformation strain caused by the solidification shrinkage of the remaining molten steel in the molten steel pool, which acts as a deterrent, and is calculated for various casting conditions such as slab cross-sectional area, pouring speed, and cooling water ratio. It is within the shaded area.

When attempting to compensate for the shrinkage of the liquid core core of the slab by applying external shrinkage to the solidified slab shell, the amount of contraction that effectively acts on the liquid core core also depends on the method of applying the external shrinkage force. However, r becomes low to 1/2 to 1/10 of the application ↑. This can be expressed as the total efficiency η1.For example, if a billet slab with a partial modulus ratio of about 1.4 is subjected to shrinkage strain in the thickness direction, the total efficiency is η = 0.2.
It was found through experimental verification that

As a means of applying compressive strain to the solidified slab shell, the surface of the y1 piece is forcibly cooled, and the volume shrinkage due to the solidification of the liquid core core of the slab is reduced by the equivalent amount of acid so that the compressive strain required by the average temperature of the solidified shell is obtained. By decreasing the amount by 111, it is possible to completely compensate for the shrinkage strain caused by the solidification of the remaining molten steel liquid core, and to prevent the solute-enriched molten steel from moving along the ■-shaped segregation line.

For example, in the case of bloom slabs, the molten steel pool protrudes significantly beyond the pinch rolls, and this area is usually not water cooled and is allowed to reheat. Figure 5 shows the distance from the meniscus of the slab and its surface temperature.

The slab surface temperature TN due to natural cooling after exiting the N097 pinch rolls shown in FIG. 5 has a small recuperation state. Even in the region after exiting the pinch rolls, solidification of the molten steel liquid core progresses steadily and solidification shrinkage occurs, but the 'PJ piece solidified grains tend to expand due to reheating.

Therefore, the suction of the solute C-treated molten steel along the segregation line is further promoted, which in turn accelerates the degree of center segregation.

Therefore, as shown in Figure 1, by installing a sprayer in the first position up to the leading edge of the molten steel pool, the solidified shell of the slab is forcibly cooled, the solidified shell shrinks, and the molten steel liquid core solidifies. Compensating for Shrinkage - In order to reduce center segregation using this method, it is necessary to understand the amount of solidification shrinkage at the position where the spray is placed and the surface temperature that must be maintained.

First, a heat transfer analysis of the slab was performed, and the solidification shrinkage strength was determined from the solidification profile at the spray location using the following equation (1).

A cross section of the solidified shell is schematically shown in FIG.

(1) The shell profile fsL is the rotation MTh(4
Assume that

The shell profile of the solid fraction f(, f; +t is congruent between different solid fractions.

Under the assumption of 1- below, the volumetric shrinkage rate η during which the slab progresses from X[ to fsi: Solid phase ratio in the i-th region vi: Volume surrounded by planes Xi + In order to compensate for volumetric shrinkage, the following equation (2) must hold true.

ηm=ηn+1−ηn (2) where, ηm: contraction rate of solidified IM shell ηnun solidification contraction rate in layer ηn++: solidification number Iii rate in n+1 layer. The solidification shrinkage rate ηm is expressed by the following equation (3), and by substituting the difference in the volume shrinkage rate of each layer obtained using the above equation (1), IM
Obtain the uniform temperature drop F ΔT of the solid shell.

...(3) Here, L: Distance from the axis of the slab to fs = 1 α: Tensile coefficient ΔT: Average temperature drop of the solidified shell, calculated by the above equation (3) The relationship between the average temperature of the shell and the surface temperature of the slab is expressed by the following equation (4) (H, S
GAR9LAII, J, G, JAEGER: “Ga
nductian of beat in 5olids
”, Oxford University Pres.
s.

5th edition 1958 P, 99
~100), and the surface temperature is maintained at Ta, x/l=1
So, assuming that it is maintained at Tl, the temperature profile is as follows (
5) It is approximated as shown in Eq.

Therefore, by controlling the surface temperature of the slab to Ta determined by equation (5), volume shrinkage during solidification, which causes center segregation, can be compensated for.

Figure 5 shows 0.8%C10, 24%Si, 0.8%Mn,
An example of the surface temperature of a bloom slab containing 0.010% F, 0.007% S, and 0.03% A after exiting the pinch roll during normal operation is shown. The surface temperature of the slab as it was allowed to cool naturally was the temperature TN shown by the broken line. The slab temperature during natural cooling is the slab surface temperature TN shown by this broken line.
In this case, the solid line indicates the slab surface temperature Ta at which the solidification-a-monouniform cooling degree required to compensate for the solidification shrinkage noise shown in FIG. 3 can be obtained.

It has been confirmed through experiments that in order to reduce center segregation of a continuous slab, the surface temperature of the slab after exiting the pinch rolls should be in the shaded area in FIG.

The limit temperature TV in the shaded area may be higher than the L temperature Ta, and the allowable range is the difference between the temperature TN shown by the broken line in the figure and the E temperature Ta shown by the solid line. This is based on experimental results showing that it is effective even when the temperature is 30% higher. The experimental results are shown in Figure 7. Figure 7 shows the central segregation ratio (C/Co), where C is the composition of the MJi center, and this CO is the molten steel before casting (for example, in the tumble push).
The composition is

Next, the lower limit temperature of the slab surface temperature shown in Fig. 5 is 0.8%.
C, 0.24%Si, 0.8%Mn, 0.0IO%P,
Change from γ phase to (α+Fe5C) phase in steel containing 0.007% S and 0.03% A love 1! The reason why the lower limit of the slab surface temperature, which is the starting temperature TA (this steel type = 690°C), is defined as the transformation temperature T^ is as follows. Steel contracts when cooled, but it's strange! When the temperature reaches the warm temperature, it changes to W tension. Therefore, if there is concentrated molten steel at the slab axis, cooling the slab surface and shrinking the slab solidified shell will compensate for the shrinkage during partial solidification (11) and reduce center segregation. However, when the surface of the slab reaches a temperature exceeding the temperature of the steel type, it expands, and the carbonized molten steel is attracted, promoting center segregation. Therefore, it is necessary to maintain the surface temperature higher than the transformation temperature T^.

As detailed above, the molten steel pool is the most advanced! i
When spraying is installed in a range to water-cool the surface of a slab, the surface temperature is set to the temperature difference between the surface temperature of the slab and the conventional method to obtain the uniform cooling degree of the solidification a-money required to compensate for the solidification shrinkage bar. Center segregation can be reliably reduced by controlling the temperature within a temperature range with the upper limit being the surface temperature that has shifted to a high temperature by 30% and the lower limit being the A3 transformation or Acmf7JQ initiation temperature of the steel type to be water cooled.

It is preferable to confirm the A3 or Acm transformation start temperature of each steel type, that is, the volumetric expansion start temperature, by differential 8 dilatation measurement.

[Example] Bloom slab for broken line with slab cross section 560 x 400 mm (c
= o, so%) at a casting speed of 0.55 m/min.

Continuous casting was carried out at a water ratio of 0.55 m/kg.

In the example A strand, 23 to 29 m from the meniscus
The spray piping shown in FIG. 1 was installed in the area shown in FIG. 1 for spray cooling, and the slab surface temperature T5 shown by the solid line in FIG. 5 was obtained.

Under the present Pt construction conditions, the leading edge of the molten steel pool is at a position 28.5 m from the meniscus, and this position will be moved to the front (towards the meniscus) by performing additional spray cooling based on the present invention. However, it was separately confirmed that the spray cooling zone was within the spray cooling zone and that the effects of the present invention were not impaired.

As a comparative example, the same spray piping as for the A strand was used for the B strand, the amount of spray cooling water was intentionally made twice that of the A strand, and the slab surface temperature 1f1 was made to fall below the A3 transformation point about 4 m after the start of cooling. Another Comparative Example C strand was cast using conventional methods.

Plume slabs of each strand cast in this way were collected as samples to investigate center segregation, and
A PC steel wire was produced by drawing, and the cementite rating and Cubby fracture rate were measured. The results are shown in Table 1. Compared to the comparative example, the variation in center segregation C/Co (standard deviation However, in the example,
It was not recognized and I got 10 points.

1st Table 11J: *O: None l: Occurs at one grain boundary 2: Occurs at grain boundaries 3: Occurs at all grain boundaries Also, the cuppy fracture surface ratio is 6.5% for C strand and 12.2% for B strand. However, in the example, it was 0.

Thus, it is clear that the present invention is extremely superior.

Cementite Jt point is the rolled material (8 to 10 mm) before wire drawing.
A cross section of φ) was corroded with a saturated aqueous solution of picrin l'ltj, and the state of cementite precipitation on the cross section was observed using a microscope 1i! !
Based on our observations, the scores are graded into four grades: Oll, 2.3.

For example, a score of O indicates that there is no precipitation of cementite, a score of 4 indicates that cementite precipitation occurs in a network, and a score of 1 and 2 indicates that precipitation occurs, but only intermittently.

In addition, the cuppy fracture surface ratio is expressed as the ratio of cup-shaped fracture surfaces among the 7 wires by tensile testing a drawn wire material (7 strands of 4 mm diameter) and determining the shape of the wave surface.

In addition, under the above-mentioned I-building conditions, 0.45%C10.24%
Si, 1.10%Mn, 01010%P, 0,007%
The method of the present invention was applied and tested on steel containing S. The surface temperature transition at that time is shown in the eighth factor, and the example is located between the A1 transformation start temperature of 712°C and Ta of this steel type.

The steel type used in this experiment was rolled to a diameter of 40 mm and hardened, but martensite was generated due to center segregation and cracks could occur in the center.

Table 2 shows the center segregation results of slabs and the incidence of cracking after quenching.

Table 2 1) Investigated by 11 rope drawing wave test after quenching [Effects of the invention] The present invention reduces center segregation of continuously cast slabs and blooms, improves low-temperature toughness, lamellar tear resistance, HIC resistance
The transfer fatigue life of bearing steel, wire breakage rate and cuppy rupture rate of hard steel wire rods can be determined in the direction of resistance.

[Brief explanation of drawings]

Figure 1 is a layout diagram of the spray nozzle to reduce center segregation, Figure 2 is a graph showing semi-macro segregation near the axis of the slab, and Figure 3 is shrinkage due to solidification of the remaining molten steel in the molten steel pool. A graph showing the compressive deformation strain necessary to compensate for silicon, Fig. 4 is a photograph of the metal structure showing the macrostructure of hard steel wire bloom (magnification: 0.25x), Fig. 5 shows the χ flow side of the present invention. Figure 6 is a schematic diagram of the cross section of the solidified shell; Figure 7 is a graph showing the experimental results for determining the effective temperature range for center segregation; Figure 8 is 0.45. It is a graph showing the surface temperature transition when the present invention is applied to %C steel. l... Solidified shell 2... Molten steel pool 3... Pinch roll 4... Roller table 5... Spray nozzle

Claims (1)

[Claims] 1. In continuous casting of steel, the surface temperature of the slab along the casting direction from a position 2 to 15 m before the leading edge of the remaining molten metal pool in the casting direction to the pool's leading edge position is measured. As the liquid core solidifies, the steel's A_3 transformation temperature or Ac
It is characterized by successively forced cooling to a temperature above the m-transformation start temperature T_A and below the effective slab surface temperature T_V shown by the following formula to shrink the solidified slab shell and reduce the cross section of the slab before casting. Continuous casting method with less center segregation. Ta + (T_N - T_a) x 0.3 = T_V However, T_N: Surface temperature of slab due to natural cooling after exiting the pinch rolls T_a: Casting temperature to obtain average cooling of solidified shell necessary to compensate for solidification shrinkage amount Single surface temperature