RU2234389C2 - Cord steel continuous casting method - Google Patents

Abstract

FIELD: ferrous metallurgy, namely continuous casting of cord steel.

SUBSTANCE: method comprises steps of cooling casting in mold; radial drawing of continuous cast billet at forced cooling of it by water in secondary cooling zone; dressing ingot and cooling it in air; during casting process setting cooling intensity of casting in mold and casting speed while taking into account thermal stress of billet according to condition that values of maximum thermal stresses in continuously cast billet along cross section of solidified skin at outlet of mold, at outlet of secondary cooling zone and in completely solidified ingot are the same and they are no more than 0.9 of limit strength of cast steel at predetermined temperature.

EFFECT: manufacture of cord steel with enhanced physical and mechanical properties at high casting rate up to 0,8 m/min.

1 dwg, 1 ex

Description

The invention relates to metallurgy, to continuous casting of cord steel, mainly using continuous radial casting of billets.

A known method of continuous casting of billets, including the supply of the melt to the mold for primary cooling, the vertical drawing of a continuously cast billet from the mold into the secondary forced water cooling zone at an overpressure of 0.2 MPa, cooling in air, followed by gas cutting into dimensional slabs / 1 /.

The method has high performance, however, due to the fact that the technology does not take into account the maximum thermal stresses in the continuously cast billet at the exit of the mold, at the exit of the water cooling zone and in the fully hardened ingot, in the latter it is possible to form macro- and microprojections on surface and in the center, which requires additional mechanical or fire treatment of bloom, or leads to rejection of the workpiece.

The closest technical solution adopted as a prototype is a method of manufacturing cord steel by continuous casting of the melt into a mold, cooling castings in a mold, radial drawing of a continuously cast billet with forced water cooling, straightening an ingot with cooling in air, followed by gas separation into measured ingots / 2 /.

The method allows to obtain cord steel with an initial casting speed of 0.5-0.55 m / min and a final casting speed of 0.58-0.6 m / min, however, it allows at high speeds of 0.8-0.9 m / min castings breakthrough of liquid metal, the formation of internal defects in the form of hot cracks and friable due to insufficient or excessive cooling of the surface of a continuously cast billet.

The basis of the invention is the task of manufacturing cord steel with a high speed of up to 0.8 m / min casting with improved physical and mechanical properties.

The problem is achieved in that in the method of continuous casting of cord steel, including cooling the casting in the mold, radial drawing of the continuously cast billet with its forced water cooling in the secondary cooling zone and straightening the ingot with its cooling in air, according to the invention, the intensity of the casting cooling of the casting in the mold and the casting speed are set taking into account the thermally stressed state of the workpiece, provided that the values of the maximum thermal stresses in continuously cast billet over the cross section of the hardened crust at the exit from the mold, at the exit from the secondary cooling zone and in the fully hardened ingot are respectively equal to each other and do not exceed 0.9 of the tensile strength of cast steel at a given temperature.

The drawing shows a graph of the distribution of the maximum thermal stresses in a continuously cast billet at the exit of the mold (1), at the exit of the secondary water cooling zone (2) and in the fully hardened ingot (3) over the cross section of the hardened crust.

The method of continuous casting of cord steel is carried out on a continuous casting machine of the type CCM-3 BMZ radial type with a vertical rectilinear mold.

Liquid steel from the casting ladle is fed into an intermediate tank equipped with a metal flow control mechanism, and into the mold for cooling the casting, from which radially extracting continuously cast billets with forced water cooling by flat torch nozzles in the secondary water cooling zone of the ZVO is performed at a given speed to form a hard shell up to 45 ... 50 mm thick at a temperature on the surface of the ingot 1100 ... 1150 ° С. Behind the secondary cooling zone there is a casting arc for the gradual radial bending of the ingot with a radius of curvature of 10 m. After that, the ingot is moved to a horizontal position by straightening it with cooling in air. Next, a continuous ingot is cut into blooms of equal length.

During the casting process, the cooling rate of the casting in the mold, in the secondary water cooling zone and the casting speed are set taking into account the thermal and thermal - thermally stressed state of the workpiece, provided that the maximum thermal stresses in the continuously cast workpiece over the cross section of the hardened crust at the exit of the mold, at the exit from the secondary water cooling zone and in the fully hardened ingot, respectively, are equal to each other and do not exceed 0.9 of the tensile strength of the cast steel whether and are related by the following relation at a given temperature to exclude the possibility of surface and internal defects in the workpiece

where σ is the current thermal stress in the solid crust of the casting, MPa;

β is the linear expansion coefficient of cast steel;

σ _CR - tensile strength of cast steel, MPa;

E is the modulus of elasticity, MPa;

ν is the Poisson's ratio;

φ is the relative thickness of the fictitious layer of the hard crust;

X = x / ξ is the relative coordinate of the solid phase;

ξ is the current thickness of the hard crust, m;

x is the current coordinate in the cross section of the hard crust, m;

T _S , T _C - solidus and ambient temperatures (walls of the mold, cooling water and air), respectively;

- фиктивный слой твердой фазы;

- a fictitious layer of a solid phase;

λ (T) is the thermal conductivity of this brand at temperature TC, W / mK;

α is the heat transfer coefficient on the surface of the workpiece, W / (m ² K)).

Taking into account the manifestation of the relaxation effect in the high-temperature region, the following dependence is used to refine the values of temperature stresses:

where τ is the current time; τ _rel - relaxation time:

where μ ^* is the viscosity coefficient of the material at temperatures close to the solidus temperature (μ ^* = 3.5 · 10 ⁹ Pa · s);

G = G ₀ exp [-k (TT ₀ ) / (T ^* -T ₀ )] is the shear modulus;

G ₀ is the value of the shear modulus at the control temperature T ₀ <T ^* (G ₀ = 5 · 10 ⁸ Pa, T ₀ = 700 ° C, T ^* = 1400 ° C, k = 4);

Q = 269.93 kJ / mol - activation energy;

R = 8.31 J / (mol · K) is the universal gas constant.

The maximum thermal stresses in a continuously cast billet over the cross section of the hardened crust at the outlet of the mold, at the outlet of the secondary water cooling zone and in the fully hardened ingot within the allowable voltage of 0.9 of the tensile strength of cast steel, practically gives 10% safety margin hardening crust at a given temperature to exclude the possibility of surface and internal defects in the workpiece.

Using the developed technology, a multivariate analysis of the solidification and cooling processes is carried out. As an example, the drawing shows the results of determining the maximum thermal stresses over the cross section of the hardened crust of a continuously cast ingot 0.250 × 0.300 m in size (80K steel, casting speed ω = 0.8 m / min). From the graphical dependence it follows that the nature of the distribution of thermal stresses over the cross section of the hardening crust of the workpiece is the same.

The graphs shown in Figure 1 characterize the technological feature of the method, since curves 1, 2, 3 show the distribution of maximum thermal stresses of maximum thermal stresses in a continuously cast billet at the outlet of the mold (1), at the outlet of the secondary water cooling zone of the ZVO (2) and in a fully hardened ingot (3), which are related to each other and to the tensile strength of cast steel by the ratio (1).

The dimensionless coordinate along the abscissa axis clearly demonstrates the state of the hardened crust in the continuously cast billet along the cross section of the ingot, where the coordinate is 0-surface of the ingot, and the coordinate 1.0 is the axis of the ingot.

On the abscissa axis at the exit from the mold, the thickness of the hard crust is approximately 0.23 of half the thickness of the cast billet (left dashed ordinate, along curve 1).

At the exit from the secondary water cooling zone, the thickness of the hard crust is approximately 0.48 of half the thickness of the cast billet (right dashed ordinate, along curve 2).

Throughout the entire solidification process, tensile stresses (~ 14 MPa) are observed in the layer adjacent to the two-phase zone – solidified part boundary, and compressive stresses (~ 55 MPa) are observed in the layer located near the cooled surface.

The tensile strength of cast steel from the known data accept σ _pr = 20 MPa for high temperatures (near the solidus temperature) and σ _pr = 60-70 MPa (for temperatures cooled surface) with the corresponding σ _pr interpolation in the range T _S -T _dressing .

An analysis of the distribution of thermal stresses over the cross section of the crust shows that the calculated stress values do not exceed the tensile strength over the entire cross section of the hardened crust. Based on the new technology, operating modes for casting cord steel of grades 70K, 75K, 80K, 85K are proposed: ω - 0.65-0.8 m / min for workpieces 0.250 × 0.300 m; ω - 0.6-0.7 m / min for workpieces 0.300 × 0.400 m.

The surface temperature field at the characteristic points of a continuously cast billet of 0.250 × 0.300 m at various casting speeds had the following character: at the exit from the mold 870-970 ° С, at the end of the secondary forced water cooling zone 1050 ... 1180 ° С, in the dressing zone it increases by 110 ... 140 ° С and monotonously decreases to 900 ° С, in a fully hardened ingot.

Using the proposed technology, it is possible to control the flow of cooling water and the casting speed, as well as to predict the value of compressive thermal stresses over the cross section of the workpiece during solidification, cooling, and subsequent heating in a continuous furnace of mill 850.

The new technology for casting cord steel of grades 70K, 75K, 80K, 85K allows increasing the speed of continuous casting to ω - 0.75-0.8 m / min for workpieces 0.250 × 0.300 m; ω - 0.68-0.73 m / min for workpieces 0.300 × 0.400 m without breakthroughs of molten metal and the formation of internal defects in the form of hot cracks and friability in comparison with the known one at a casting speed of 0.58-0.6 m / min.

The developed casting modes of cord steel grades have passed industrial testing in the conditions of CCM-3 BMZ.

Sources of information

1. Yu.A. Samoilovich, V.A. Timoshpolsky, I.A. Trusova, V.V. Filippov. Steel Ingot, vol. 2, Minsk, Belarusian Science, 2000, p. 367-371.

2. Yu.A. Samoilovich, V. A. Timoshpolsky, I. A. Trusova, V. V. Filippov. Steel Ingot, Vol. 2, Minsk, Belarusian Science, 2000, p. 380-383, Fig. 4-12.

Claims (1)

The method of continuous casting of cord steel, including cooling the casting in the mold, radial drawing of the continuously cast billet with its forced water cooling in the secondary cooling zone and straightening the ingot with its cooling in air, characterized in that during the casting the cooling rate of the casting in the mold and the casting speed are set taking into account the thermally stressed state of the workpiece, provided that the maximum thermal stresses in the continuously cast workpiece over the cross section of the shutter the crusts at the exit from the mold, at the exit from the secondary cooling zone and in the fully hardened ingot are respectively equal to each other and do not exceed 0.9 of the tensile strength of cast steel at a given temperature.