CN114951571A - Method for expanding equiaxial crystal zone in cord steel continuous casting billet - Google Patents
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/10—Supplying or treating molten metal
- B22D11/11—Treating the molten metal
- B22D11/114—Treating the molten metal by using agitating or vibrating means
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/16—Controlling or regulating processes or operations
- B22D11/18—Controlling or regulating processes or operations for pouring
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/16—Controlling or regulating processes or operations
- B22D11/22—Controlling or regulating processes or operations for cooling cast stock or mould
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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Abstract
The invention discloses a method for expanding an equiaxial crystal area in a cord steel continuous casting billet, which comprises a cord steel continuous casting process, wherein molten steel enters a crystallizer from a tundish, and an ultrasonic generator tool head is arranged at the bottom of a side hole nozzle of a square billet to carry out ultrasonic treatment on the molten steel; according to the invention, the tool head of the ultrasonic generator is arranged at the bottom of the water gap, ultrasonic waves are transmitted to molten steel in the crystallizer, the input power of the ultrasonic waves reaches the threshold value of cavitation effect generated by the molten steel, the cavitation effect is generated in the molten steel solidification process, crystal nuclei are formed near cavitation bubbles, a large number of the crystal nuclei flow onto the blank shell along with the molten steel, and the number of fine isometric crystals on the chilling layer of the blank shell is increased. Meanwhile, the strong shock wave generated in the collapse process of the cavitation bubbles can break the growing columnar crystal to form new crystal particles and grow into isometric crystals.
Description
Technical Field
The invention belongs to the technical field of metallurgy, and particularly relates to a method for expanding an equiaxed crystal area in a cord steel continuous casting billet.
Background
With the vigorous development of the automobile industry, China has become the largest tire producing country and export country worldwide, and steel cords are used as framework materials of tires, the annual demand in China exceeds 200 million tons and is on the increasing trend year by year. At present, various ferrous metallurgy enterprises at home and abroad generally adopt the cord steel wire drawing process with the carbon content of more than 0.60 percent to produce products with special purposes, and the process has the advantage of obvious economic benefit and has been widely popularized and applied.
The cord steel is the top product in the wire product, and the main difficulty is represented by: the deep processing flow of a user is long and complex, dozens of drawing and heat treatment processes are needed, meanwhile, the steel wire compression ratio exceeds 99%, the final diameter is small (reaching 0.15mm), the drawing length of each ton of steel exceeds 3000 kilometers, and the strand twisting and breaking rate of each ton of steel is required to be less than 4 times. Any minor defects on the steel wire of the cord thread may be exposed during the deep processing, which may result in wire breakage or substandard steel wire properties (mechanics, torsion, etc.).
The cord steel is mostly continuously cast by adopting a square billet, and a macrostructure in the continuously cast billet consists of 3 crystal bands from outside to inside: fine equiaxed bands (chill layers), columnar bands, central equiaxed regions, generally columnar bands tend to have more area than equiaxed regions. Because the three structures have different shapes and sizes, the sizes of the austenite generated in the casting blank and around the casting blank are not uniform in the subsequent austenitizing heating process, and the nonuniformity of austenite grains can be inherited in the subsequent production process to influence the matrix structure and the product performance of the steel. The grain boundaries of adjacent columnar crystals in the continuous casting slab are relatively straight, the combination of the crystal grains is not as firm as that of isometric crystals, and particularly, a fragile interface is often formed at the joint of the columnar crystals extending along different directions, so that the columnar crystals are easy to crack during rolling and easy to generate internal cracks. Therefore, the quantity and size of columnar crystals in the continuous casting billet of the cord steel must be controlled, the medium axialmode in the continuous casting billet is increased, and the structure and the performance of a cord steel product are improved.
At present, market competition is becoming white and more advanced, and product homogeneity is serious, so how to better control the structure in a cord steel casting blank, improve the quality of a cord steel wire rod, better meet the requirements of a drawing process with large compression ratio and a heat treatment process of a user, improve production efficiency and reduce manufacturing cost is a problem which cord steel production enterprises have to face.
Ultrasonic waves generally refer to frequencies above 2 x 10 4 The sound wave in Hz. Power sonication is a technique that changes or accelerates the change of some physical, chemical and biological properties or states of a substance through the action of ultrasonic energy on the substance. The main content of power ultrasonography includes the generation of high-power and high-sound-intensity ultrasonic waves (including high-power ultrasonic transducers, amplitude transformers, vibration direction converters, processing tools and the like), the action mechanism of sound energy on substances and various ultrasonic processing technology applications. The basic functions of the ultrasonic wave in the medium are as follows: (l) The medium is forced to vibrate under the action of ultrasonic waves with certain frequency and sound intensity. (2) When the large-amplitude sound wave is transmitted in a medium, a periodic shock wave with a sawtooth wave surface is formed, and a series of special reactions such as local high temperature and high pressure are generated. Based on the two basic characteristics, the power ultrasonic wave can generate cavitation effect and sound when propagating in the mediumFlow effects, thermal effects, etc. Uses thereof include detection ultrasound, power ultrasound, and medical ultrasound. At present, in the field of metal smelting, ultrasonic waves are mainly used for removing inclusions in liquid and refining structures of metals in the processes of ingot casting and rolling, and mainly used in the ingot casting process of aluminum alloys (pure Al, Al-Cu alloys, Al-Si alloys, Al-Ti alloys and the like) and magnesium alloys. The method solves the problems of how to improve the quality of the cord steel continuous casting billet by adopting an ultrasonic technology in the steel continuous casting production process, particularly increasing fine isometric crystals, reducing coarse columnar crystals and improving the quality of the cord steel continuous casting billet, and has great practical significance.
CN104014754A discloses a method for producing high manganese steel by using an ultrasonic vibration continuous casting crystallizer, which comprises an electric furnace primary smelting process, a ladle furnace refining process and a tundish slab continuous casting process, wherein a horizontal continuous casting system is adopted in the tundish slab continuous casting process, molten steel refined by a ladle furnace flows through a long nozzle from the bottom of a ladle and is injected into a tundish, the molten steel in the tundish enters the horizontal continuous casting crystallizer, ultrasonic vibration is generated on the horizontal continuous casting crystallizer to gradually form the molten steel, and the molten steel is gradually solidified into continuous casting high manganese steel in a secondary cooling section through cooling. But this technique can only be used for a miniaturized horizontal continuous casting mold.
CN102181786A discloses a melting type ultrasonic vibration device of a continuous casting crystallizer, which comprises an ultrasonic transducer, an ultrasonic generator connected with the side surface of the ultrasonic transducer, and an amplitude transformer connected with the lower end of the ultrasonic transducer, wherein the lower end of the amplitude transformer is an ultrasonic vibration transmission rod inserted into molten steel of the crystallizer, the ultrasonic vibration transmission rod is made of cast steel material which is similar to the type of cast steel and is added with trace alloy elements, and the ultrasonic vibration transmission rod is allowed to melt in the molten steel in the casting process; however, the ultrasonic generator is connected to the vibration rod through the amplitude transformer and the molten steel, and the effect of the ultrasonic wave is attenuated in the transmission process.
Disclosure of Invention
The invention aims to provide a method for expanding equiaxial crystal zones in a cord steel continuous casting billet, which adopts an ultrasonic technology to increase fine equiaxial crystals, reduce coarse columnar crystals and improve the quality of the cord steel continuous casting billet.
In order to achieve the purpose, the technical scheme is as follows:
a method for enlarging equiaxed crystal areas in a continuous casting billet of cord steel comprises the following steps:
when molten steel enters a crystallizer from a tundish in the continuous casting process of the cord steel, ultrasonic treatment is carried out on the molten steel through an ultrasonic generator tool head arranged at the bottom of a side hole nozzle of a square billet.
According to the scheme, the temperature range delta T of the molten steel in the tundish is 20 ℃, and the casting temperature of the molten steel in the tundish is determined to be between T1 and T2, wherein the temperature is TL +30 ℃ and the temperature is TL +10 ℃. Ensuring that the molten steel in the crystallizer has enough temperature interval to finish ultrasonic treatment.
According to the scheme, the tool head of the ultrasonic generator is a cylinder, and the width of the molten steel in the crystallizer is not more than three times of the diameter of the cylinder. Ensuring that the molten steel in the crystallizer is in the ultrasonic treatment range.
According to the scheme, the output power of the ultrasonic generator is less than 2000W, and the frequency is 2 multiplied by 10 4 Hz～2×10 5 Hz. Ensuring that a critical threshold is reached at which the cord steel cavitates.
According to the scheme, the cord steel comprises the following chemical components in percentage by weight:
c: not less than 0.60 wt%, Si: 0.10 to 0.40 wt%, Mn: 0.30-0.60 wt%, P is less than or equal to 0.01 wt%, S is less than or equal to 0.01 wt%, Cr: 0.10 to 0.25 wt%, B: 0.0005 to 0.0015 wt%, the balance being Fe and unavoidable impurities.
According to the scheme, the size of the edge of the cross section of the continuous casting slab is within the range of 100-600 mm.
The relevant working mechanism of the invention is as follows:
c: carbon is the most important constituent element in steel, has the most obvious influence on the strength and plasticity of the wire rod, the strength of the wire rod is continuously improved along with the increase of the carbon content, but the plasticity is rapidly reduced, and meanwhile, the higher the carbon content is, the greater the difficulty in production control is, and the poorer the product quality stability is. Therefore, the carbon content of the invention is more than or equal to 0.60wt percent.
Si: silicon is an important strengthening element in steel, can obviously improve the elastic limit of the drawn steel wire, can effectively reduce the strength reduction caused by heat treatment, and simultaneously can slow down the breakage of cementite in the drawing process and improve the comprehensive mechanical property of the steel wire. The silicon content of the invention is 0.10-0.40 wt%.
Mn: manganese is a precious alloy element, combines with sulfur to generate MnS, further reduces the harm of sulfur, refines pearlite and improves the strength of steel wires, but excessively high manganese can increase the overheating sensitivity of steel materials, so that crystal grains are easy to grow during heat treatment. The Mn content of the invention is controlled to be 0.30-0.60 wt%.
P, S: phosphorus and sulfur are harmful elements in the cord steel, phosphorus is easily subjected to cold embrittlement, sulfur is easily subjected to hot embrittlement, and further, the processing conditions of steel wire drawing and heat treatment are deteriorated, so that the content of phosphorus and sulfur needs to be reduced as much as possible. The invention has P less than or equal to 0.01 wt% and S less than or equal to 0.01 wt%.
Cr: the chromium can refine pearlite lamella and improve the strength of the finished steel wire, but the excessively high chromium can improve the hardenability of the wire rod, so that abnormal structures such as martensite and the like appear in the hot rolling process, and meanwhile, the excessively small lamella can reduce the toughness of the wire rod, so that the torsion performance which is the most key index of the steel wire is deteriorated, therefore, the Cr: 0.10 to 0.25 wt%.
B: trace boron in the cord steel can inhibit the enrichment of P in grain boundaries and improve the form of inclusions, so that the cold processing performance of the wire rod can be improved, but excessive boron can weaken the bonding force of the grain boundaries and deteriorate the mechanical performance of the wire rod. Thus, invention B: 0.0005 to 0.0015 wt%.
The invention relates to the cavitation effect of ultrasonic waves. When ultrasonic waves propagate in a liquid medium, many cavitation bubbles are generated inside the liquid due to the violent reciprocating vibration of the particles. These cavitation bubbles rapidly expand and close under alternating pressure, which causes violent impact between the particles, which in turn generates thousands of atmospheres of pressure with the action of microjets. The violent interaction between the particles can promote the temperature of the liquid medium to rise suddenly and has good stirring effect on the liquid medium, so that the two originally immiscible liquids are emulsified, the dissolution of the solute is accelerated, and the chemical reaction between the two is accelerated. This effect caused by the action of the ultrasound waves in the liquid medium is referred to as the cavitation effect of the ultrasound waves.
The present invention relates to the acoustic streaming effect. When ultrasonic waves propagate in liquid, limited amplitude attenuation is generated, so that a certain sound pressure gradient is formed in the liquid from a sound source, and the liquid flows at a high speed. In the case of high-energy ultrasound, when the sound pressure amplitude exceeds a certain value. A jet of fluid may be generated in the liquid. The jet exits directly from the end face of the ultrasonic horn and forms a circular flow throughout the fluid.
The presence of the acoustic wave necessarily causes a pressure change inside the propagation medium. Pressure P at a point in the ultrasound field at a certain moment and static pressure P in the absence of ultrasound sound pressure 0 The difference is called sound pressure p in Pa, and the expression of the sound pressure p is
p＝P-P 0 ＝ρcωξ m cos(ωt-φ)＝ρcu (1)
In formula (1), ρ is the density in Kg/m 3; c is the speed of the ultrasonic wave in the medium and the unit is m/s; xi m The amplitude of a mass point in the medium is m; u is the vibration speed of mass points in the medium and has the unit of m/s; ω -2 π f is the vibration angular frequency of the sound wave, in Hz. It can be seen from the equation that the sound pressure p is proportional to the sound velocity c and the vibration frequency f.
The sound intensity of an ultrasonic wave refers to the energy transmitted per square centimeter in the direction perpendicular to the traveling wave, i.e., I — E/St. Where E is energy, s is area, and t is time. From this it can be deduced:
wherein u is m Is the particle velocity amplitude, p m Is the sound pressure amplitude.
The acoustic power is a main physical quantity reflecting the total energy in the sound field, i.e. the density epsilon of acoustic energy (acoustic energy per unit volume), and is expressed by
Experiments prove that when the width of the molten steel is less than three times of that of the ultrasonic tool rod, the cavitation bubble area of the ultrasonic wave reaches the thickness of a casting blank shell, and the effect of refining casting blank grains can be achieved.
Mechanism for refining continuous casting billet grains by ultrasonic waves
As is known from the theory of metal solidification, nucleation begins when the molten steel is cooled to a nucleation onset temperature, and then nuclei satisfying growth conditions grow against the direction of heat flow. The condition of grain refinement is that a large amount of crystal nucleus formation and large supercooling degree required by growth are arranged in a liquid phase at the front edge of a solidification interface, and the ultrasonic wave achieves the effect of grain refinement by promoting the nucleation of the molten steel and increasing the supercooling degree of the molten steel. Meanwhile, the ultrasonic waves have cutting and crushing effects on dendritic crystals and columnar crystals in growth, so that not only are coarse columnar crystals eliminated or reduced, but also the crushed crystal grains are effective crystal nuclei in the molten steel and grow into isometric crystals. In addition, the ultrasonic wave is introduced into the molten steel, so that violent forced convection is formed inside the molten steel, the cooling speed of the molten steel is improved, and the growth of crystal grains is inhibited. The effect of refining grains of the cord steel continuous casting billet is better.
Mechanism for promoting molten steel crystal nucleation by ultrasonic wave
The main functions of the power ultrasound on metal solidification are cavitation and acoustic current stirring, and after the ultrasonic wave is guided to molten metal steel, cavitation is generated. During the formation and growth of cavitation bubbles, the size of the cavitation bubbles is rapidly increased, so that the liquid in the cavitation bubbles is evaporated and absorbs heat from the periphery, the temperature of molten metal on the surfaces of the cavitation bubbles is reduced, local supercooling is caused, crystal nuclei are formed near the cavitation bubbles, and the nucleation rate of the molten metal is increased; the strong shock wave generated during collapse of the cavitation bubbles will break the growing crystal into new crystal particles.
When the ultrasonic input power is less than a certain critical value, the influence on the grain size is small, and once the input ultrasonic power exceeds the critical value, the influence on the grain size by slightly increasing the ultrasonic power is very obvious. Analysis shows that one of the main reasons for refining the crystal grains by the ultrasonic waves is the cavitation effect formed in the molten steel, and only if the sound pressure formed by the ultrasonic waves in the molten steel is higher than a cavitation threshold value, cavities can be generated, and the cavitation effect is generated. Different molten steel has different cavitation threshold values, the larger the molecular binding force, the larger the surface tension or the larger the viscosity of the molten steel is, the higher the cavitation threshold value is, and then the power density must be increased. The input power reaches the critical threshold value of cavitation generated by the molten steel, the cavitation effect is formed in the molten steel, the corresponding solidification initial temperature is reduced, and the nucleation process is greatly promoted.
The liquid metal crystallization driving force satisfies the following conditions:
in the formula: Δ G-driving force for crystallization, J/m. Δ H-latent heat of fusion, J/m. Tm-theoretical crystallization temperature, K. T is the actual temperature of the molten steel, K.
The lower the actual molten steel temperature is, the larger the difference value of free energy of liquid and solid phases is, namely the larger the phase change driving force is, so that the nucleation is facilitated. When the ultrasonic treatment is adopted, the nucleation site of the alloy at the early stage of solidification is not only at the continuous casting shell, but the temperature of the micro-area is sharply reduced due to the cavitation effect. When these micro-regions satisfy the temperature conditions required for nucleation, a large number of nuclei are generated in these regions, wherein some small nuclei are melted by the thermal fluctuation caused by the acoustic current, and large nuclei are preserved to increase the number of nucleation cores.
Meanwhile, when ultrasonic waves are introduced into the molten steel, cavitation nuclei existing in the molten steel vibrate under the action of a sound field, and bubbles suddenly close after rapidly growing. During the bubble closing process, the bubbles have not yet collapsed. With further increase in pressure within the bubble, the bubble shrinkage is accompanied by a decrease in radius. Once the pressure reaches a certain value, the bubble breaks instantaneously. At this time, a sharp shock wave is formed in the molten steel when the bubble is closed, and this is accompanied by a local high pressure in the microcell. The pressure developed at the cell wall is, according to the rayleigh formula:
P k -pressure created instantaneously, atm. The experimental research shows that the pressure generated by cavitation is 4000 atm. Such high pressures will have a large effect on the nucleation rate of the domains.
Under high pressure, the nucleation rate of the molten steel is.
In the formula:
θ — wetting angle, degree. a-geometric constant, for a spherical crystal,
σ k -cavitation bubble breaking instant interfacial tension, N. V s Atomic volume of solid, mm 3 . Δ h-latent heat of solidification, kg/m.
Activation energy at high pressure, J. R g Gas constant, 8.314J/mol.k.
The nucleation rate at normal pressure is:
comparing the formulas (6) and (7), the nucleation rate I of the micro-region under the cavitation pressure can be obtained k The relationship between nucleation rate I and normal pressure is:
comparing the effect of different processes on nucleation requires determining the degree of variation in the respective activation energies for the different processes. Research shows that the activation energy during solidification represents a potential barrier to be overcome for liquid atoms to solid atoms to cross. Therefore, the smaller the activation energy, the smaller the potential barrier, and the less difficult it is for the liquid atom to cross over to the solid atom, and the easier it is to nucleate. Activation energy of solid-liquid conversion process is
ΔG 0 ＝G S -G L ＝ΔVdp-ΔSdT(9)
In the formula: g S Gibbs free energy of the solid phase, J; g L Gibbs free energy of the liquid phase, J.
In the untreated state, dP is 0 when liquid atoms are aggregated to form a solid phase. In the ultrasonic treatment, high pressure higher than 100atm is generated due to the cavitation effect, and the effect of the pressure on the activation energy needs to be considered. At the moment of cavitation bubble closure, dP is present<0, so that the activation energy of the atom under the action of the ultrasonic field is less than that of the atom under the untreated condition, i.e. Δ G'>ΔG' k . According to the formula (8), the nucleation rate in the solidification process after the ultrasonic treatment is improved.
The analysis shows that when the input ultrasonic power reaches a certain value, a special cavitation effect is formed in the molten steel, wherein the generated pressure change and temperature chilling have positive effects on the increase of the nucleation number at the initial stage of solidification, and the nucleation rate is greatly improved.
(II) mechanism for increasing supercooling degree of molten steel by ultrasonic waves
According to the Clausius-Clapeyron equation, it can be obtained that:
in the formula: t-theoretical crystallization temperature, deg.C. Δ H-latent heat of phase change, kg/m. Δ V-volume change, m 3 。
The equations (6-14) can be integrated by
In the formula, T km Theoretical crystallization temperature of metal under ultrasonic wave, DEG C
The equations (6-16) can be simplified as:
after ultrasonic treatment, the theoretical crystallization temperature of the metal is increased, and the theoretical crystallization temperature of the metal under the ultrasonic condition is obtained by combining the formula (5):
the difference of the external environment in the metal solidification process has influence on the actual crystallization temperature of the alloy. Analysis shows that the solidification temperature of the molten steel gradually decreases as the ultrasonic power increases. The supercooling degree of the metal solidification is the difference between the theoretical crystallization temperature and the actual solidification temperature of the metal, so that the supercooling degree of the molten steel is increased by introducing ultrasonic waves, the generation of a large number of crystal nuclei is promoted, and the refining of a metal solidification structure is facilitated. Experiments prove that when the width of the molten steel is more than three times that of the ultrasonic tool rod, the grain refining effect of the ultrasonic waves on the edge of the casting blank is not central, and the refining effect is good.
The invention relates to the influence of ultrasonic treatment on columnar crystals. In the solidification process, the impact force generated when the ultrasonic cavitation bubbles are closed also has great influence on the appearance of the growing dendrite. The ultrasonic wave is continuously introduced, the impact force has a circulatory impact effect on the dendritic crystal, most dendritic crystal cells are smashed and converted into granules and ellipsoids, secondary dendritic crystals are obviously reduced, and the structure is more uniform. When ultrasonic wave is introduced into molten steel, tens of thousands of tiny cavitation bubbles are generated by the molten steel due to vibration, the cavitation bubbles grow in a negative pressure zone formed by the longitudinal propagation of the ultrasonic wave, a positive pressure zone is rapidly closed, the cavitation bubbles are compressed and stretched under the alternating positive and negative pressure, when the cavitation bubbles are compressed until the cavitation bubbles collapse, strong shock waves are generated in the molten steel to break initial coarse crystals and growing dendritic crystal cells, one part of the coarse crystals and the growing dendritic crystal cells are continuously remelted, the rest particles can become effective nucleation particles, and the particles are uniformly dispersed in the molten steel under the stirring action of acoustic flow, so that external particles are increased, the nucleation rate is improved, and fine isometric crystal structures can be obtained without adding any modifier.
The present invention relates to a mechanism for increasing the cooling rate by ultrasound. When ultrasonic waves propagate in liquid, limited amplitude attenuation is generated, so that a certain sound pressure gradient is formed by the medium from a sound source, and the liquid flows rapidly. The flow causes rapid transport of particles and changes in the temperature field of the liquid metal.
Under normal solidification conditions, liquid metal buoyancy flow and solidification contraction cause natural convection to cause convective cooling of the metal. However, when ultrasonic waves are introduced into liquid metal, violent forced convection is formed inside the metal, and the violent forced convection changes a temperature field and a solute concentration field in front of a solidification interface and influences the shape of a solidification structure. The intensity of forced convection cooling can reach tens of times it compared to natural convection cooling.
Under normal solidification, heat convection causes various parts with different temperatures in the molten steel to be mixed with each other, and heat transfer is carried out after macroscopic motion of the molten steel is caused. According to the newtonian cooling model, the cooling rate of the solidification process by convection is:
in the formula: t is 0 -initial temperature of the molten steel. C p -specific heat coefficient.
After the introduction of the ultrasonic wave, considering the strong cooling effect of the acoustic flow on the molten steel, equation (14) is modified to include:
in the formula: i-ultrasonic waveStrength, W/mm 2 (ii) a μ — absorption coefficient of the medium; v-volume of medium, mm 3 。
The general solution for equation (15) is:
taking the differential to obtain
The expression (17) is a relational expression of the influence of forced convection on the cooling rate of molten steel in the case of ultrasonic treatment. From equation (17), it can be seen that the cooling rate dT/dT is proportional to the intensity I, but the cooling effect of the ultrasonic acoustic flow decreases exponentially as the treatment time increases. To increase the cooling rate in the solidification process of molten steel, the input ultrasonic power must be increased, and forced convection cooling of the molten steel is intensified. With increasing power, the solidification time of the metal becomes progressively shorter. By integrating equation (17), the temperature decrease Δ T of the molten steel in time T can be obtained as:
when the sound pressure amplitude exceeds a certain value along with the increase of the ultrasonic power, the jet of the fluid can be generated in the molten steel. The jet flow directly leaves the end surface of the ultrasonic amplitude transformer and forms turbulent flow in the whole fluid to stir the molten steel continuously. The violent turbulence of the molten steel improves the transmission rate of molten steel particles, and increases the heat diffusion in the solidification process of the molten steel, so that the solidification rate of the molten steel is increased, and the near-equilibrium solidification time of the molten steel is shortened, so that the acoustic flow accelerates the cooling process of the molten steel, inhibits the growth of crystal grains, and finally influences the size of the crystal grains.
Compared with the prior art, the invention has the following beneficial effects:
according to the invention, the tool head of the ultrasonic generator is arranged at the bottom of the water gap, ultrasonic waves are transmitted to molten steel in the crystallizer, the input power of the ultrasonic waves reaches the threshold value of cavitation effect generated by the molten steel, the cavitation effect is generated in the molten steel solidification process, crystal nuclei are formed near cavitation bubbles, a large number of the crystal nuclei flow onto the blank shell along with the molten steel, and the number of fine isometric crystals on the chilling layer of the blank shell is increased. Meanwhile, the strong shock wave generated in the collapse process of the cavitation bubbles can break the growing columnar crystal to form new crystal particles and grow into isometric crystals.
Molten steel is in a crystallizer, and a large number of crystal nuclei generated by ultrasonic waves are gathered near a casting shell to generate a large number of isometric crystals. After the ultrasonic treatment, the growing columnar crystals are crushed to form new crystal nuclei, and the crystals grow to form isometric crystals. In addition, the sound flow effect of the ultrasonic wave can also improve the cooling speed and the cooling effect of the continuous casting billet and inhibit the growth of crystal grains.
The method carries out ultrasonic treatment on the molten steel in the crystallizer, increases fine isometric crystals through process control, reduces or eliminates coarse columnar crystals in the continuous casting billet, and improves the uniformity of the structure and the performance of the product for subsequent production.
Drawings
FIG. 1: the invention discloses a schematic diagram of a device for enlarging an equiaxed crystal area in a cord steel continuous casting billet;
FIG. 2: in the ultrasonic treatment, a picture of crystal nucleus is formed near the cavitation bubble;
FIG. 3: the invention provides a comparison effect diagram of the cord steel cast billet structure after ultrasonic treatment;
wherein, 1, the tundish is added; 2-molten steel; 3-a side hole water gap of a square billet; 4-a crystallizer; 5-side hole of water gap; 6-a tool head; 7-cavitation bubbles; 8-continuous casting of the shell.
Detailed Description
The following examples further illustrate the technical solutions of the present invention, but should not be construed as limiting the scope of the present invention.
The specific embodiment provides a method for enlarging equiaxed crystal areas in a continuous casting billet of cord steel, which comprises the following steps:
referring to fig. 1, in the continuous casting of a billet of cord steel, molten steel 2 flows from a tundish 1 into a mold 4 through a submerged billet side hole nozzle 3, and the molten steel flowing out of the nozzle side hole 5 forms a circulating flow field in the mold 4. A tool head 6 of an ultrasonic generator is arranged at the bottom of a side hole water gap 3 of a square billet, ultrasonic waves are sent to molten steel 2 in a crystallizer 4, the input power of the ultrasonic waves reaches a threshold value of cavitation effect generated by the molten steel, the cavitation effect is generated in the molten steel solidification process, crystal nuclei (shown in attached figure 2) are formed near generated cavitation bubbles 7, a large number of crystal nuclei flow onto a continuous casting billet shell 8 along with the molten steel, and a large number of fine isometric crystals are generated near the billet shell. Meanwhile, the strong shock wave generated in the collapse process of the cavitation bubbles 7 can break the growing columnar crystal to form new crystal particles which grow into isometric crystals. In addition, the sound flow effect of the ultrasonic wave can also improve the cooling speed and the cooling effect of the continuous casting billet and inhibit the growth of columnar crystals. In the ultrasonic treatment area, the solidification structure is greatly changed, including grain refinement, suppression of columnar crystal growth, improvement of isotropy of grains and reduction of segregation.
The steel components determine the liquidus temperature TL of different cord steel, in the continuous casting process of the cord steel, molten steel enters a crystallizer from a tundish, the temperature interval Delta T of the continuous casting molten steel in the tundish is 20 ℃, the pouring temperature of the molten steel in the tundish is respectively determined to be between T1 and TL +30 ℃ and T2 and TL +10 ℃, and the molten steel in the crystallizer has enough temperature interval to complete ultrasonic treatment.
Specifically, the output power of the ultrasonic generator is less than 2000W, and the frequency is 2 multiplied by 10 4 Hz～2×10 5 Hz. Ensuring that a critical threshold is reached at which the cord steel cavitates.
Example 1
The cord steel comprises the following chemical components in percentage by weight: c: not less than 0.601%, Si: 0.15%, Mn: 0.38%, P is less than or equal to 0.01%, S is less than or equal to 0.01%, Cr: 0.13%, B: 0.0006 percent, and the balance of Fe and inevitable impurities. Specification of continuous casting billets: the cross dimension of the casting blank is 160mm multiplied by 180 mm.
(1) In the continuous casting process of the cord steel, molten steel enters a crystallizer from a tundish, the temperature interval Delta T of the continuous casting molten steel in the tundish is 20 ℃, the pouring temperature of the molten steel in the tundish is respectively set to be 1505-1485 ℃, and the molten steel in the crystallizer is ensured to have enough temperature interval to finish ultrasonic treatment;
(2) the molten steel is stirred by the cavitation effect and the acoustic current of the ultrasonic generator. The ultrasonic generator emits ultrasonic waves to the molten steel through a tool head which is arranged at the bottom of a molten steel nozzle of the crystallizer and is immersed in the tundish, and the diameter of the tool rod exceeds 1/3 of the thickness of the square billet crystallizer, so that the molten steel in the crystallizer is ensured to be in the ultrasonic treatment range;
(3) the output power of the ultrasonic generator is 300W, and the frequency is 2.5 multiplied by 10 4 Hz, ensuring that the critical threshold value of the cavitation effect generated by the cord steel is reached;
(4) molten steel is in a crystallizer, and a large number of crystal nuclei generated by ultrasonic waves are gathered in a chilled layer of a casting blank to generate a large number of isometric crystals. After the ultrasonic treatment, the growing columnar crystals are crushed to form new crystal nuclei, and the crystals grow to form isometric crystals. In addition, the sound flow effect of the ultrasonic wave can also improve the cooling speed and the cooling effect of the continuous casting billet and inhibit the growth of crystal grains.
Example 2
The cord steel comprises the following chemical components in percentage by weight: c: 0.73% or more, Si: 0.23%, Mn: 0.42 wt%, P is less than or equal to 0.01%, S is less than or equal to 0.012%, Cr: 0.24 wt%, B: 0.0013 percent, and the balance of Fe and inevitable impurities. Specification of continuous casting billets: the size of the edge part of the cross section of the casting blank is within the range of 320mm multiplied by 420 mm.
(1) In the continuous casting process of the cord steel, molten steel enters a crystallizer from a tundish, the temperature interval Delta T of the continuous casting molten steel in the tundish is 25 ℃, the casting temperature of the molten steel in the tundish is respectively set to be 1500-1495 ℃, and the molten steel in the crystallizer has enough temperature interval to complete ultrasonic treatment;
(2) the molten steel is stirred by the cavitation effect and the sound flow of the ultrasonic generator. The ultrasonic generator emits ultrasonic waves to the molten steel through a tool head which is arranged at the bottom of a molten steel nozzle of the crystallizer and is immersed in the tundish, and the diameter of the tool rod exceeds 1/3 of the thickness of the square billet crystallizer, so that the molten steel in the crystallizer is ensured to be in the ultrasonic treatment range;
(3) ultrasonic generator outputPower 2300W, frequency 2.2X 10 4 Hz, ensuring that the critical threshold value of the cavitation effect generated by the cord steel is reached;
(4) molten steel is in a crystallizer, and a large number of crystal nuclei generated by ultrasonic waves are gathered in a chilled layer of a casting blank to generate a large number of isometric crystals. After the ultrasonic treatment, the growing columnar crystals are crushed to form new crystal nuclei, and the crystals grow to form isometric crystals. In addition, the sound flow effect of the ultrasonic wave can also improve the cooling speed and the cooling effect of the continuous casting billet and inhibit the growth of crystal grains.
Example 3
The cord steel comprises the following chemical components in percentage by weight: c: not less than 0.83%, Si: 0.39%, Mn: 0.57%, P is less than or equal to 0.008%, S is less than or equal to 0.009%, Cr: 0.24%, B: 0.0019%, and the balance of Fe and inevitable impurities. Specification of continuous casting billets: the cross dimension of the casting blank is 600mm multiplied by 600 mm.
(1) In the cord steel continuous casting process, molten steel enters a crystallizer from a tundish, the temperature interval Delta T of the continuous casting molten steel in the tundish is 15 ℃, the pouring temperature of the molten steel in the tundish is respectively set to be 1500-1483 ℃, and the molten steel in the crystallizer has enough temperature interval to complete ultrasonic treatment;
(2) the molten steel is stirred by the cavitation effect and the sound flow of the ultrasonic generator. The ultrasonic generator emits ultrasonic waves to the molten steel through a tool head which is arranged at the bottom of a molten steel nozzle of the crystallizer and is immersed in the tundish, and the diameter of the tool rod exceeds 1/3 of the thickness of the square billet crystallizer, so that the molten steel in the crystallizer is ensured to be in the ultrasonic treatment range;
(3) 1960W of output power of ultrasonic generator and 2.9X 10 of frequency 4 Hz, ensuring to reach the critical threshold value of the cord steel generating cavitation effect;
(4) a large number of crystal nuclei generated by ultrasonic waves are gathered in a chilled layer of a casting blank in the crystallizer to generate a large number of isometric crystals. After the ultrasonic treatment, the growing columnar crystals are crushed to form new crystal nuclei, and the crystals grow to form isometric crystals. In addition, the sound flow effect of the ultrasonic wave can also improve the cooling speed and the cooling effect of the continuous casting billet and inhibit the growth of crystal grains.
Comparative examples 1 and 2 were set simultaneously, corresponding to the cases of example 1 and example 2, respectively, where no ultrasonic treatment was used. The comparative effect of the cord steel billet structure after the ultrasonic treatment of example 1 compared with the billet structure without the ultrasonic treatment of comparative example 1 is shown in figure 3.
The equiaxed grain ratio, columnar grain ratio and casting segregation index of the continuous casting slabs of examples 1 to 3 were compared with those of comparative examples 1 and 2, and the results are shown in table 1.
TABLE 1
As can be seen from Table 1, the invention is compared with comparative examples 1 and 2 which adopt the technology, the isometric crystal proportion of the cord wire steel continuous casting billet is improved by about 55 percent after the technology is adopted, the segregation index is reduced by 0.14, the isometric crystal proportion can be obviously improved, the segregation index of the casting billet is reduced, and the quality of the cord wire steel is improved.
Claims (6)
1. A method for enlarging equiaxed crystal areas in a continuous casting billet of cord steel is characterized by comprising the following steps of:
when molten steel enters a crystallizer from a tundish in the continuous casting process of the cord steel, ultrasonic treatment is carried out on the molten steel through an ultrasonic generator tool head arranged at the bottom of a side hole nozzle of a square billet.
2. The method for enlarging equiaxed zones in a continuous cast strand of cord steel as set forth in claim 1, wherein the temperature range Δ T of the molten steel in the tundish is 20 ℃, and the casting temperature of the molten steel in the tundish is T1 ═ TL +30 ℃ to T2 ═ TL +10 ℃.
3. A method of enlarging equiaxed zones in a continuous cast strand of cord steel as claimed in claim 1 wherein said sonotrode tool tip is a cylinder and the width of the molten steel in the crystallizer is no more than three times the diameter of the cylinder.
4. A method of enlarging equiaxed zones in a continuous cast strand of cord steel as claimed in claim 1 wherein said ultrasonic generator has an output power of less than 2000W and a frequency of 2 x 10 4 Hz～2×10 5 Hz。
5. A method of enlarging equiaxed zones in a continuous cast strand of cord steel as claimed in claim 1 wherein said cord steel has a chemical composition in weight percent of:
c: not less than 0.60 wt%, Si: 0.10 to 0.40 wt%, Mn: 0.30-0.60 wt%, P is less than or equal to 0.01 wt%, S is less than or equal to 0.01 wt%, Cr: 0.10 to 0.25 wt%, B: 0.0005 to 0.0015 wt%, the balance being Fe and unavoidable impurities.
6. A method of enlarging equiaxed zones in a continuous cast strand of cord steel as claimed in claim 1 wherein the edge dimension of the cross section of the resulting strand is in the range of 100mm to 600 mm.
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| JP2008200692A (en) * | 2007-02-19 | 2008-09-04 | National Institute For Materials Science | Casting method |
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| JP2008200692A (en) * | 2007-02-19 | 2008-09-04 | National Institute For Materials Science | Casting method |
| CN201304474Y (en) * | 2008-10-13 | 2009-09-09 | 上海梅山钢铁股份有限公司 | Ultrasonic wave processing device for molten steel in crystallizer of conticaster |
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