JP2000015410A - Metod for oscillating mold for continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the surface defect of a cast slab by shallowing the depth of claws caused at the time of continuously casting without adding any process by oscillating a mold based on a specific correlation. SOLUTION: The mold for continuous casting is oscillated based on the following inequality D>a*Lnb*fcx ΔTd Wherein D is the scale thickness in conversion from a removed slag thickness until a cast slab strand becomes a finish product (mm), Ln is the max. descending distance of the mold in the condition of making the drawing-out cast slab strand as the reference (mm) and (a)-(d) are constants depending on the casting condition. In the formulas I, II III, Vc is the casting velocity (mm/min), VL is the rising velocity of molten metal surface level, tc is the time of starting to satisfy the condition of Vm+ VL>Vc during oscillating the mold (min), tnr is the time satisfying Vm+VL>Vc during oscillating the mold(m), (t) is time (min), S is the mold stroke (mm), (f) is the number of oscillation (cycle/min), T is the overheat degree in the condition of making the solidus of molten steel in a tundish as the reference ( deg.C) and α is the constant (0.6-1.0).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is intended to advantageously improve the surface properties of cast slabs, particularly in the continuous casting of molten metal, by a simple method in the casting process.

In recent years, in the continuous casting of molten metal, particularly in the continuous casting of steel, hot charge rolling (HCR) and direct rolling (DHCR) have been promoted from the viewpoint of energy saving. An essential condition for carrying out HCR and DHCR is that there is no defect such as nonmetallic inclusions, surface segregation or slagging on the surface of the cast slab obtained by continuous casting or its subcutaneous surface. In order to improve the controllability of the flow of molten steel in the mold or the control of the molten steel level in the mold by changing the vibration conditions of the mold, optimizing the lubricant of the mold, and disposing an electromagnetic brake device in the mold And various other measures were taken.

However, in continuous casting, even if such measures are taken, it has not actually been possible to completely prevent the surface defects of the cast slab, and the resulting defective quality may be accompanied. Often there was. For this reason, when there is a possibility that the cast slab may be inferior in quality, H
Although it was necessary to cool the cast slab in a high temperature state for a while avoiding the implementation of CR and DHCR, or to scarf (cut) the surface at high temperature to remove surface defects in advance, As a countermeasure, heat is wasted due to the drop in temperature of the cast slab, and unnecessary work steps such as addition of a scarfing step are required. Also, the yield (good piece yield: weight of product / cast cast slab) Weight) cannot be avoided, and the effect of HCR or DHCR intended for energy saving cannot be expected.

[0004] Recently, in the casting of ultra-low carbon steel having a carbon content of 0.005 mass% or less, a discontinuous portion of a solidified structure called a "claw" immediately below an oscillation mark portion is formed.
It has become clear that nonmetallic inclusions and argon bubbles in the molten metal are trapped in the process of floating in the mold, and these become surface defects and internal defects in the final product.

As a countermeasure for reducing claws, it has been reported in "Iron and Steel Association, edited by iron and steel, Vol. 4, 1994, T165" that it is effective to reduce the negative reseat time. "Materials and Processes, Vol.4, 1991, 253. Edited by the Iron and Steel Institute of Japan" states that raising the casting temperature is effective, and furthermore, "Steelmaking Conf.Proc., AIME
(1992), P409 ”reported that the flow of molten steel at the meniscus was effective, and it was thought that the effects of casting temperature and flow velocity of molten steel were particularly large.

[0006]

However, when the casting temperature is increased to reduce the depth of the claws,
Increased heat load in the refining process may cause refractory erosion and increase the risk of breakout when the casting speed is increased. This has been a cause of quality deterioration.

An object of the present invention is to propose a novel mold vibration method which can solve the above-mentioned conventional problems which have occurred in continuous casting.

[0008]

SUMMARY OF THE INVENTION The present invention relates to a continuous casting mold for supplying molten metal to a continuous casting mold and continuously drawing a slab strand from one end of the casting mold. A continuous casting method of molten metal, characterized by vibrating according to the following conditions. Serial ^{_{D> a * L n b *}} f C * ΔT d ... (1) Here, D: slab strand slab thickness conversion scale thickness to be removed to a final product (mm) a, b, c, d: Constants depending on casting conditions (steel type, slab strand size, mold flux properties, immersion nozzle use conditions)

(Equation 4)

V _L : απSfcos (2πf · t) = αV _m (3)

V _m : descending speed of mold πSfcos (2πf · t) (4) V _c : casting speed (mm / min) _VL : molten metal rising speed (mm / min) t ₀ : during vibration of mold while when the condition of V _m + V _L> V _c starts to be satisfied (min) t _nr: during oscillation of the mold V _m + V _L> V _c is satisfied time (min) t: time (min) S : Mold stroke (mm) f: Vibration frequency of the mold (cycle / min) ΔT: Superheat degree (° C) based on solidus temperature of molten steel in tundish α: Constant [empirical value expressed by (0.6 to 1.0) ]

[0009]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, the slab size,
As a result of various investigations on the mold vibration conditions affecting the claw depth when the physical properties of the mold flux and the use conditions of the immersion nozzle are constant, the degree of superheat ΔT in the tundesh based on the solidus temperature of the molten metal to be cast is increased. besides to, the greater the maximum lowered distance L _n of the mold (mm) is small relative to the solidified shell in the withdrawal of the slab strand in the case of considering the influence of the frequency and the melt surface variation rate of the mold, the cast slab of it than the depth of the nail to be observed directly under Oscillation marks we have found to be a shallow, here, Tsumefuka of D _t can be expressed by the following equation.

[0010] ^{_{D t = a * L n b}} * f C * ΔT d

[0011]

(Equation 7)

V _L : απSfcos (2πf · t) = αV _m

V _m : πSfcos (2πf · t)

V _c : casting speed (mm / min) _VL : molten metal rising speed (mm / min) t ₀ : time (min) at which the condition of V _m + V _L > V _c starts to be satisfied during vibration of the mold t _nr : time during which V _m + V _L > V _c is satisfied during vibration of the mold (min) t: time (min) S: mold stroke (mm) f: mold frequency (cycle / min) ΔT: Superheat degree (° C) based on the solid phase temperature of molten steel in the tundish α: Constant [experimental value expressed by (0.6 to 1.0)]

[0013] Thus, lowering speed of the mold above L _n is the maximum value, i.e., most template lowering speed of the apparent adversely affect V _m + V _L (melt surface displacement and the mold displaced Tsumefuka of 180 degrees (Corresponding to a phase shift) can be expressed by the following equation.

[0014]

V _mm = V _m + V _L = (1 + α) πsfcos (2πft)

It is considered that the fluctuation of the mold level in the mold and the vibration of the mold are synchronized, and a relative mold stroke based on the position of the mold level can be calculated as (1 + α) s, and α has an average value of 0. 8 can be used.

The constants a to d of parameters obtained by performing a multiple regression analysis on the relationship between the above factors and the claw depth are in the range of general casting conditions, for example, ultra-low carbon steel, low carbon steel, SUS30.
4, a molten metal such as _{SUS430, V c = 0. 8~2} .
3 m / min, s = 2 to 8 mm, f = 60 to 200 cpm, negative strip time (t _n ) = 0 to 0.33 s, ΔT =
15-120 ° C, shape of immersion nozzle: downward 25-35
°, immersion depth: 230 mm, mold size: thickness 200-2
A = 75.9, b = 0.415, c = −0.76 at 60 mm and 1000 mm to 1800 mm, respectively.
1, d = −0.405.

The regression equation obtained using such values is shown in FIG.
As is clear from the comparison between (a) and (b), the negative strip time (t), which is a conventionally proposed parameter of the oscillation mark depth, is known.
There is a stronger correlation with nail depth than n).

On the other hand, as the depth of the nails existing in the surface layer of the cast slab increases, the surface defects (deoxidation products, mold flux, bubbles, etc.) of the cold-rolled steel sheet increase. Is made shallower than the total thickness of the oxide scale generated in the heating furnace and annealing furnace and removed in the pickling process (scale thickness in terms of slab thickness) D (mm) to reduce the surface of the cold rolled steel sheet. It is possible to remarkably reduce defects, and for that purpose, it is necessary to set casting conditions that satisfy the following formula, whereby surface defects of products can be effectively reduced.

[0019] _{D> t = a * L n} b * f C * ΔT d

Here, the constants a, b, c and d are the results of the measurement of the nail depth and the factors according to the present invention under the constant casting conditions (steel type, cooling condition of mold, physical properties of mold flux, immersion nozzle). Can be easily obtained from the multiple regression analysis, and D can be calculated from the yield until the cast slab becomes the final product.

From the above, it is possible to reduce the claw depth by performing continuous casting at a casting temperature that does not impose a burden on the refractory, or under a meniscus molten steel flow rate at which mold flux is not involved. it can.

[0022]

EXAMPLES C: 0.0015 mass% (hereinafter simply referred to as%), Si: 0.01%, Mn: 0.05%, P: 0.
030%, S: 0.007%, Al: 0.035%, T
Ultra low carbon steel (solid phase temperature 1525 ° C) with i: 0.030% and Nb: 0.003% was molded flux using a vertical bending type continuous casting facility with a vertical length of 2.3m. Viscosity: 4 poise (1300 ° C), solidification temperature: 980
Degree, temperature in the tundish of molten metal: 1555
℃ ~1560 ℃, immersion nozzle: two-hole discharge angle of downward 25 ° of the molten steel jet by the formula, immersion depth 230 mm, casting speed _{V c:. 2 0m / min} , stroke of mold oscillation S: 4 mm, the frequency f : 160cpm: width 1560m
m, cast slab 260 mm thick, then HC
R (Surface temperature of slab when charged into heating furnace: 750-85
0 ° C, heating temperature: 1150 ° C, furnace time: 110 to 13
0 min) and finally cold-rolled to a thickness of 1.5 mm. The resulting cold-rolled steel sheet has an incidence of surface defects (severes, slivers, etc.) (= defect occurrence length / coil length × 10
0) (the total thickness of the scale in terms of the slab up to the cold-rolled coil was 0.7 mm, and the estimated value of the nail depth in the continuous casting according to the present invention was 0.19 to 0.55 mm).

Further, in order to conduct a similar investigation on the comparative material, a conventional method of high-temperature casting (casting temperature: 157) was used.
(0 ° C.), from the viewpoint of preventing breakout, continuous casting was performed at a maximum casting speed of 1.8 m / min, a stroke S of mold vibration of 7.8 mm, and a frequency f of 140 cpm. The estimated value of the nail depth in continuous casting performed under these conditions was 0.95 mm.

As a result, the rate of occurrence of surface defects of the cold-rolled steel sheet was 0.05% in the case of casting according to the present invention, whereas it was 0.25% in the comparative material. It has been confirmed that the quality of the product can be significantly improved. In the present invention, since the casting temperature could be lowered by about 10 ° C., the casting speed was reduced to 0.2 m / min.
As a result, the productivity can be improved.

[0025]

According to the present invention, surface defects which have been unavoidable in continuous casting can be reduced without adding an extra step, so that defect-free products can be stably manufactured under HCR or DHCR. it can.

[Brief description of the drawings]

FIG. 1 (a) is a diagram showing a relationship between an actually measured nail depth and an estimated nail depth, and FIG. 1 (b) is a diagram showing a relationship between a negative strip time tn and an actually measured nail depth.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Kenichi Sorimachi 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Corp. (72) Inventor Hiroshi Nishikawa 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki (72) Inventor Masaki Takashi 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term (reference) 4E004 AD10 MA02

Claims (1)

[Claims] 1. A casting operation in which a molten metal is supplied to a continuous casting mold and a slab strand is continuously drawn from one end of the casting mold, wherein the continuous casting mold is vibrated according to the following conditions. A continuous casting method for molten metal. Here serial ^{_{D> a * L n b *}} f C * ΔT d, D: slab strand slab thickness conversion scale thickness to be removed to a final product (mm) L _n: cast being withdrawn Maximum descent distance of mold based on one strand (mm) a, b, c, d: Constants depending on casting conditions (steel type, size of strand strand, physical properties of mold flux, operating conditions of immersion nozzle) V _L : απSf cos (2πf · t) = αV _m V _m : mold down speed πSf cos (2πf · t) V _c : casting speed (mm / min) V _L : metal surface rising speed (Mm / min) t ₀ : Time (min) at which the condition of V _m + V _L > V _c starts to be satisfied during vibration of the mold t _nr : V _m + V _L > V _c is satisfied during vibration of the mold Time (min) t: Time (min) S: Mold stroke (mm) f: Vibration frequency of the mold (cycle / min) ΔT: Superheat degree (° C) based on solid phase temperature of molten steel in tundish α: Constant [Experience value indicated by (0.6 to 1.0)]