JP2000117405A - Method for continuously casting billet and apparatus therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce C segregation in a cast billet and pro-eutectoid cementite in a wire rod by forming the solidified thickness on the upper surface and the lower surface in the cross section of the cast billet to a specific dimension before straightening the bent cast billet and natural-cooling for at least specific time during the interval from the start of straightening to the completion of solidifying. SOLUTION: In the casting of the billet having <=160 mm square, the solidified thicknesses on the upper surface and the lower surface in the cross section of the cast billet are secured to be >=30 mm, and the cast billet is naturally cooled for >=100 sec. Desirably, when the last solidified position satisfies the formula II: L-Lb>100×Vc/60 and the formula III: L=(d/2)2×Vc/K2, the natural cooling zone having the length or longer shown with the formula I:L=100×Vc/60, is arranged on and after the start of straightening. In these formulas, Lc is the min. length (m) of the natural cooling zone, Vc is casting velocity (m/min), L is the solidification completing length (m) decided with the formula III, Lb is the distance (m) from the start of casting to the straightening point, (d) is the thickness (mm) of the cast billet and K is the solidifying factor, which is different according to the casting machine and ordinarily 30 (mm/min0.5). Further, light rolling reduction is executed at the portion which is >=0.2 solid phase ratio at the center of the cast slab.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for preventing the deterioration of C segregation occurring when casting a slab in continuous casting.

[0002]

2. Description of the Related Art In the steel industry, slabs have been manufactured by continuous casting for more than 20 years to save energy.

[0003] At that time, a problem is segregation of the concentrated molten steel that accumulates inside the slab. When the component concentration of the segregated portion is high, for example, in the case of a wire manufactured from billets or blooms, breakage occurs due to a difference in hardness when the wire is drawn.

[0004] This tendency is particularly remarkable when the carbon concentration is high. The reason is that, when a wire rod is produced after casting a billet, if proeutectoid cementite is present, cracks occur during drawing from the starting point, leading to disconnection.

In order to improve the segregation, for example, in the center segregation in billet casting, for example, by lowering the casting temperature as much as possible, the center of the slab is made equiaxed to disperse the C segregation. After that, some academic papers on wire drawing have been reported.

However, when trying to actually adjust the casting temperature, the probability that casting trouble will occur due to the low temperature increases. Examples of casting troubles include clogging of nozzles, solidification of steel on the inner surface of the billet mold, making casting impossible, and the like.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a continuous casting method for reducing C segregation in billet slabs and reducing proeutectoid cementite in a wire rod.

[0008]

The gist of the present invention is as follows. (1) In continuous billet casting of 160 mm square or less, the solidification thickness of the upper surface and the lower surface in the cross section of the slab before bending the slab is secured to 30 mm or more, and at least between the start of the bending and the completion of the solidification. A continuous casting method for billets, which is left to cool for 100 seconds or more. (2) The continuous casting method of billet according to the above (1), wherein light reduction is performed at a portion where the slab center solid phase ratio is 0.2 or more. (3) When the final solidification position satisfies the conditions of the following formulas (2) and (3), a cooling zone longer than the length shown by the following formula (1) is provided after the start of bending back. Lc = 100 * Vc / 60 (1) L-Lb> 100 * Vc / 60 (2) L = (d / 2) ² * Vc / K ² (3) Minimum length of the cold zone (m) Vc is the casting speed (m / min) L is the solidification completion length (m) determined by equation (3) Lb is the distance from the start of casting to the point of bending back (m) d is the casting length Piece thickness (mm) K depends on solidification coefficient and casting machine, but usually 20 to 30 (mm)
/ min ^0.5 ) (4) When the final solidification position satisfies the conditions of the following formulas (2) and (3), return the cooling zone and light pressure lower zone that are longer than the length shown by the following formula (1). Billet continuous casting equipment characterized by being provided after the start Lc = 100 * Vc / 60 (1) L-Lb> 100 * Vc / 60 (2) L = (d / 2) ² * Vc / K ² ( 3) Here, Lc is minimum length of cooling zone + length of light reduction zone (m) Vc is casting speed (m / min) L is solidification completion length determined by formula (3) (m) Lb is casting Distance from start to return point (m) d is thickness of slab (mm) K is solidification coefficient, usually 20 to 30 (mm)
/ min ^0.5 ) In this case, the upper and lower surfaces of the slab section are defined as
It means the upper surface and the lower surface of the cross section of the slab in a state where the slab after casting is horizontally bent back after passing through the mold, and indicates a surface which receives a large stress at the time of bending back.

Hereinafter, the present invention will be described in detail.

The present invention is directed to a continuous billet casting machine for producing cast pieces having a size of 160 mm square or less, which can omit a sizing process, which is effective for energy saving, and which casts in a curved shape and performs bending back. To

The inventor first analyzed the components of segregation in a slab related to proeutectoid cementite of a wire. As a result, it can be understood that the site where C segregation occurs in the slab is affected not only by segregation at the center but also by solidification cracking (hereinafter referred to as internal cracking) that is split at an intermediate portion of the slab thickness.
These measures were considered.

First, in order to prevent internal cracks, it has been found that cracks can be prevented by ensuring a solidification thickness of 30 mm or more at the time of bending back. The relationship between the casting length and the solidification thickness can be determined using heat transfer calculations. The relationship is between the casting speed and the amount of cooling water (the amount of water required to cool a 1 kg slab). Based on the calculation result, the relationship between the cooling water amount and the casting speed for making the solidified thickness at the time of bending back 30 mm or more can be obtained.

For example, when a slab of 130 mm square is cast by using oil lubrication and then bent back with a casting length of 7 m,
An example of the relationship between the cooling water amount and the casting speed when the solidification thickness is 30 mm or more at the time of bending back is as follows: Cooling water amount> A * casting speed + B (4) Here, A = 1.5 B = −3 This relationship can be easily obtained by numerical calculation using the difference method. The calculation result is obtained by calculating the thickness of the solidified shell from the crack generation position at the bending return point, and adjusting the calculation result by measuring the billet surface temperature to estimate the coefficient with an accuracy of about 10%.

The coefficients A and B differ depending on the casting machine, and also differ between the oil casting and the powder casting. However, it is considered that the relationship of the formula (4) holds regardless of the casting machine.

In order to secure a solidified thickness of 30 mm or more at the time of bending back as a measure for preventing internal cracks, it is effective to increase the amount of cooling water or lower the casting speed, as is apparent from equation (4). However, when the amount of cooling water was extremely increased or the casting speed was decreased, pro-eutectoid cementite due to C segregation was generated at the wire rod stage. In this case, the C segregation rate (= max C concentration of segregated portion / base material C concentration) of the wire was about 1.8.

Next, measures for reducing the C segregation were examined.

According to the scientific literature, in the case of solidification of aluminum and copper, the lower the cooling rate, the smaller the volume of eutectic formed in the segregated part, which is that the copper concentrated at the interface during solidification becomes a solid. It is believed that as a result of diffusion in the liquid, the amount of concentration in the liquid phase was reduced, and segregation was reduced. However, there was no knowledge about the effect of cooling during solidification on the proeutectoid cementite of the wire.

The inventors studied how the time from the stop of cooling to the solidification affects the C segregation.
As a result, the time from the stop of water cooling to the solidification is 2 hours.
In the case of 0 second, the degree of C segregation in the wire was 1.6, but when the time from the stop of water cooling to the solidification was 130 seconds, it was reduced to 1.3.

The degree of segregation of the wire is EPMA,
The concentration of the component was measured, and a value obtained by dividing the maximum concentration by the C concentration obtained by chemically analyzing the entire wire was used.

The degree of segregation for preventing proeutectoid cementite is C
When the concentration is 0.9%, it is about 1.3, and when the C concentration is lower than this, there is a possibility that it is relaxed. However, when this value is set as a limit value, as shown by a solid line in FIG. According to the analysis from the experimental data, it is necessary that the time from when the water cooling is stopped to when the solidification is completed is 100 seconds or more.

By the above measures, internal cracks in the cast slab and C segregation at the center could be reduced. On the other hand, segregation in which components are concentrated in an oblique line shape called V-shaped segregation may remain in the slab. The V-shaped segregation is caused by the flow of molten steel caused by solidification shrinkage during solidification. This increases the area of the segregated portion, which also affects the segregation of the wire. In order to prevent this, the part where the central solid phase ratio was 0.2 or more was lightly reduced. The length of the cooling zone was defined as the length from the return of the bend to the side where the light pressure was released. Since the actual coagulation completion position changes due to the light pressure reduction, it is difficult to quantify the actual coagulation completion position. Figure 1 shows the relationship between the time from the start of cooling in this case and the passage through the exit side under light pressure and the degree of segregation.
, Which are almost on the same line as the solid line. This allows
When the V-shaped segregation in the billet was reduced and the segregated grains were converted into a circle, those having about 4 mm before light reduction were reduced to 3 mm or less. As a result, the segregation component other than proeutectoid cementite in the wire, for example, the probability of disconnection due to P segregation was significantly reduced.
Preferably, V-shaped segregation can be surely reduced by performing light reduction on a portion where the center solid fraction of the slab is 0.2 or more and 0.8 or less.

Here, the ratio of the center solid phase = (liquidus temperature−temperature at the center of the slab) / (liquidus temperature−solidus temperature),
The temperature at the center of the slab is calculated from the amount of heat removed by spray cooling in the mold and thereafter to the slab.

When making the above knowledge into equipment, it is necessary to determine the minimum length of the cooling zone. This requires a distance for cooling for 100 seconds, as shown in equation (1) from the above knowledge. In addition, as a necessary condition at this time, it is necessary to satisfy a condition that solidification is not completed in the cooling zone. This is (3)
If the length obtained by subtracting the distance from the solidification completion length to the bending return point shown in the equation is larger than the length of the cooling zone (Equation (2)), it is good.

[0024]

Next, embodiments of the present invention will be described based on examples.

[0025]

EXAMPLE A high carbon steel 130 mm square billet was cast using a billet continuous casting machine. Bending back is performed at a casting length of 7 m. The carbon concentration is 0.4-0.9%.

For example, when producing a slab of 120 to 130 mm by casting using oil or casting using powder, the left side of the line 1 as shown in FIG. The cooling conditions were such that the thickness was 30 mm or more, and no cracks occurred.

Next, the relationship between the time required for solidification to be completed after cooling was started by changing the casting speed and the degree of C segregation in the wire was examined. Due to the longer time corresponding to FIG.
The segregation degree was reduced, and no proeutectoid cementite was generated in the region of 100 seconds or more, and the wire was satisfactory.

In the range of 100 seconds or less, the occurrence of proeutectoid cementite was observed.

Further, after providing a cooling zone of 4.5 m, a low pressure lowering zone is provided.
2m was installed. The casting speed was in the range of 2.8 m / min to 3.2 m / min. In this case, the solid fraction in the light reduction zone is about 0.2 to 0.5 on the entrance side, but the solid fraction on the exit side is unclear due to the light reduction, but is close to 1. Value.

In this case, the cooling time was from 121 seconds to 139 seconds. In each case, no proeutectoid cementite was generated and no breakage due to P segregation was generated. In addition, the position of the low pressure zone is shifted to the upstream side, the cooling zone is set up 2 m, then the low pressure zone is installed 2 m, and the casting speed is 2.4 m / min. Is less than 100 seconds, 0.9%
In the case of casting molten steel with a C concentration of 1, proeutectoid cementite was observed.

[0031]

According to the present invention, a technique for reducing C segregation of a high carbon steel wire rod in billet continuous casting at a high casting speed has been achieved. The advantage of the present invention is that, compared to the conventional method of producing a billet through a lumping process after manufacturing in a bloom, the lumping process can be omitted, and the energy required for the production can be reduced. There is.

[Brief description of the drawings]

FIG. 1 shows the time from the start of cooling to the completion of solidification and C in the wire.
Diagram showing the relationship between the degree of segregation (solid line in the figure) and the relationship between the time from the start of cooling to the exit side under light pressure and the degree of C segregation (● in the figure)

FIG. 2 is a diagram showing a state of occurrence of a slab crack in a relationship between a casting speed and a specific water amount.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Hiroshi Oba Kimitsu, 1 Kimitsu City Inside Nippon Steel Corporation Kimitsu Works (72) Inventor Jun Fukuda 1 Kimitsu, Kimitsu City Nippon Steel Corporation Kimitsu F term in steel works (reference) 4E004 KA13 MC07 NB02

Claims (4)

[Claims] 1. In continuous casting of billets of 160 mm square or less, the solidification thickness of the upper surface and the lower surface in the cross section of the slab before the slab is bent is secured to 30 mm or more, and from the start of the bending to the completion of solidification. And continuously cooling the billet for at least 100 seconds. 2. The continuous casting method for billets according to claim 1, wherein light reduction is performed at a portion where the slab center solid phase ratio is 0.2 or more. 3. When the final solidification position satisfies the conditions of the following formulas (2) and (3), it is necessary to provide a cooling zone having a length equal to or longer than the length represented by the following formula (1) after the start of bending back. Features billet continuous casting equipment. Lc = 100 * Vc / 60 (1) L-Lb> 100 * Vc / 60 (2) L = (d / 2) ² * Vc / K ² (3) where Lc is the minimum length of the cooling zone (m) Vc is the casting speed (m / min) L is the solidification completion length determined by equation (3) (m) Lb is the distance from the start of casting to the return point (m) d is the thickness of the slab (mm) K varies depending on the solidification coefficient and casting machine, but is usually 20 to 30 (mm
/ min ^0.5 ) When the final solidification position satisfies the conditions of the following formulas (2) and (3), after a cooling zone having a length equal to or longer than the length represented by the following formula (1) is provided after the start of bending back, A billet continuous casting device characterized by installing a light pressure lowering zone. Lc = 100 * Vc / 60 (1) L-Lb> 100 * Vc / 60 (2) L = (d / 2) ² * Vc / K ² (3) where Lc is the minimum length of the cooling zone + Light reduction zone length (m) Vc is casting speed (m / min) L is solidification completion length (m) determined by formula (3) Lb is distance from casting start to bending back point (m) d is casting Piece thickness (mm) K depends on solidification coefficient and casting machine, but usually 20 to 30 (mm)
/ min ^0.5 )