JPH0448543B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a method for cooling a slab during continuous steel casting, and an object thereof is to produce a billet with reduced segregation.

[Conventional technology]

In the production of many steel products, for example steel wires of high carbon steel, their technical properties are significantly impaired by segregation. When performing a patenting operation based on the rolling temperature of this type of steel wire, this segregation causes the formation of a brittle phase called martensite, which severely inhibits the ductility of the steel wire, in the segregated part of the ingot. invite In the production of steel products, such as steel wires of high carbon steel, if segregation occurs in an ingot or billet, when a steel wire is produced from the ingot or billet by rolling, the portion where segregation occurs becomes brittle. In the case of ingot casting, segregation is seen in the upper one-third of the ingot, so cutting and removing this part can prevent the ductility from being inhibited during rolling, but in the case of continuous casting billets, This is because segregation occurs continuously in the longitudinal direction of the cast billet (the direction in which the billet comes out of the continuous casting mold) and that segregation occurs continuously in the longitudinal direction of the cast billet (the direction in which the billet comes out of the continuous casting mold), and that the Billets, which are continuously cast and then hot rolled and expanded into rods or wires, are particularly susceptible to axial segregation during casting; Non-uniform transformation occurs during hot working and intermediate annealing heat treatment, which increases the amount of martensite in medium carbon steels, and precipitates large amounts of cementite at grain boundaries in high carbon steels, thus causing billet It is widely known that during hot rolling, a brittle phase is formed that can cause premature failure or serious deterioration of product properties [The Metallurgist and Materials Technologist, October 1980 issue]. No.
pages 569-573] and cannot be removed by cutting. Casting of so-called small slabs (the dimension of one side is
100 to 140 mm), it is required that the deformation due to rolling to make the final product is smaller than that of large slabs, so for example, the size of one side is 200 to 300 mm.
This has more negative effects than casting large slabs such as pre-billets. Considerable efforts have already been made in this field of technology to reduce segregation or its negative effects during continuous casting of rolled products. It is generally understood that the so-called spherical structure is combined with slight segregation, whereas the dendritic structure is combined with strong segregation [Stahl and Eisen, published March 23, 1978, Vol. Volume 6, pages 244-254]. The spherical tissue is
It refers to a structure in which crystals do not have a particular growth direction, but are rather randomly distributed across the cross section. 1st
The figure shows the structure of a continuously cast billet, which mostly has this type of spherical structure. On the other hand, the term dendritic structure refers to a structure in which the main growth direction of crystals occurs within the metal in a direction perpendicular to the surface of the slab. FIG. 2 shows a cutaway view of a billet having mostly dendritic structures. Efforts in this field have focused on increasing the fraction of spherical structures, as there is a belief that dendritic structures promote segregation and reduce spherical structures. Various developments have been initiated for this purpose. One direction of development is to agitate the melt within the solidifying slab to prevent the formation of dendritic structures and thereby reduce segregation [see DE-C-1783060]. Stirring action is generally achieved by a magnetic stirring device. In either case, expensive equipment is required [Metalausical Transactions, published June 1977,
pp. 243-251]. Other directions of development to achieve spherical tissue are:
This means keeping the casting temperature extremely low. In this case, difficulties arise in practice due to the tendency of the casting nozzle to become clogged (Wire Journal, August 1977, pp. 32-38).

[Problem to be solved by the invention]

Extensive research has been carried out aimed at reducing the segregation of steels containing 0.4-1.0% carbon by casting them at low temperatures or by electromagnetic stirring. Although achievable, it has been found that this reduction is not sufficient to achieve a significant improvement in the technical properties of such steels in the production of rolled wire rods.
In fact, frequent occurrence of martensite was observed when using electromagnetic stirring. Contrary to the commonly held view, especially
It has been found that in the case of steels with a carbon content of 0.4 to 1%, segregation can be significantly reduced if the steels are cooled very strongly under certain conditions. This effect can also be observed at high casting temperatures and casting speeds. The amount of reduction in segregation is sufficient to substantially improve the technical properties of rolled wire rods produced from the continuously cast billets thus obtained. Also, after controlled cooling from the rolling temperature, the occurrence of martensite at the post-heat treatment segregation sites of the rolled wire rod is decisively reduced. With extremely strong cooling, there is a risk of cracks forming on the surface of the billet or in its interior.
This problem can be solved by 0.4 to 1 wt.% when casting speed and cooling intensity are increased to improve productivity.
This is an important problem not only in steels with a carbon content of . The present invention provides a method for continuous casting of steel for steels with a carbon content of 0.4 to 1.0% by weight, by specifying the cooling rate of the billet exiting the continuous casting mold and the amount of heat removed from the billet. The main purpose is to prevent segregation from occurring. It is also an object of the present invention to particularly improve the continuous casting of small pieces, that is, the continuous casting of small pieces with a side length of up to 140 mm. According to the present invention, it is possible to prevent the formation of martensite at segregated positions during heat treatment of a wire rod rolled from a billet.

[Means to solve the problem]

In the present invention, a slab coming out of a continuous casting mold is cooled by injection of cooling liquid in a secondary cooling zone consisting of a strong cooling step in the first step and a weak cooling step in the second step. This relates to a slab cooling method during continuous casting _of steel having _a carbon _{content of}
P ₁ and P ₂ are in the first step 3 in the secondary cooling zone.
50Wh/Kg ~ 90Wh/Kg of heat is taken away, resulting in 65Wh/(Kg・min) ~ 100Wh/(Kg・min)
The subsequent second step 4 is cooled at a cooling rate of
20Wh/Kg ~ 80Wh/Kg of heat is taken away, resulting in 30Wh/(Kg・min) ~ 60Wh/(Kg・
It is characterized by being controlled to be cooled at a cooling rate of (min). In addition, the present invention provides a method for cooling a slab during continuous casting of the above-mentioned steel, including: 0.4-1.0% by weight of carbon, 0.2-1.7% by weight of manganese, 0.1-0.7% by weight of silicon, 0-1.7% by weight of chromium, 0-0.5% by weight. % of nickel and 0 to 0.3% by weight of sulfur balance and is characterized by its use in steels with iron and usual impurities.

【Example】

Figure 1 shows a sulfa print in a longitudinal section passing through the central axis of the billet, which is mostly spherical tissue.
Figure 2 shows the sulfur aprint in a longitudinal section through the central axis of the billet, which is mostly dendritic tissue.
FIG. 3 shows the microetching of a quarter slice of a billet of reinforced cooled material with a fine-grained spherical texture edge zone, and FIG. 4 schematically shows the apparatus for carrying out the method. . FIG. 4 schematically shows a continuous steel casting apparatus for carrying out the method according to the invention. Molten steel is poured from a distribution channel 1 into a continuous casting mold 2 which is vibrated and cooled, and the skin solidifies while the metal slab slowly moves downward within the mold. Two cooling steps 3, 4 are arranged at the rear of the mold, in which the slab is simultaneously sprayed with water over its entire circumference. The molten core of the metal slab is designated by 5, and the solidified skin of the slab is designated by 6. All the injected water is discharged downward and collected in the water collection pipe 7,
and is fed to the water tank 8. In the cooling steps 3 and 4, water is supplied from the water tank 8 by pumps 9 and 10 through pipes 11 and 12. A device 13 for detecting the temperature T _A of the waste water and the flow rate V _A of the waste water is connected to the injection water collection pipe 7 , and the cooling processes 3 and 4 are provided with a device 13 for detecting the temperature T A of the waste water and the flow rate V A of the waste water. Water flow rate and water pressure T ₁ , V ₁ , P ₁ or
Devices 14, 15 for detecting T ₂ , V ₂ , P ₂ are arranged. Further control and adjustment mechanisms, not shown, are provided to allow the above-mentioned quantities to be varied. One method for distributing water to both cooling processes 3 and 4 is to first adjust the flow rate of wastewater V _A
and temperature T _A are determined during the operation of both cooling steps 3 and 4, or alternatively by measuring during the operation of cooling step 3. In the usual manufacturing method of continuous casting of steel in the range of 0.4 to 1.0% carbon, for example, a slab with a square cross-section of 120 mm on a side is cast, and a steel with a carbon content of 2.4 m/
When cast at a pouring speed of min, the billet is prestressed below the mold to a pressure of typically 3 bar and a maximum of 8 bar, approximately 20-30 m ³ /h per billet.
sprayed with water at a flow rate of In the method of the invention, cooling is enhanced by increasing the heat transfer coefficient due to enhanced water cooling on the top surface of the billet. As a result, segregation is reduced. It is well known that if the cooling is too strong, there is a risk of cracks forming on the surface of the slab. These cracks were formed by casting at the above-mentioned pouring speed of 2.4 m/min and applying very strong cooling of the billet with the above-mentioned dimensions to a length of about 2 m below the mold, i.e. about 40-60 sec. This can be avoided by limiting the residence time.
As a result, the surface temperature of the slab is approximately 650°C to 950°C.
Self-adjusted to °C. This range (hereinafter referred to as the first
(referred to as process), approximately 65Wh/(Kg・min)
Approximately for a cooling rate of ~100Wh/(Kg・min)
50Wh/Kg to 90Wh/Kg of heat is removed from the slab. After this extremely intense cooling, the slab is approximately 30~
It is cooled with reduced intensity over a residence time of 50 seconds (for the dimensions mentioned above). In this range (hereinafter referred to as the second step), under the above conditions, the amount of heat removed in the continuous casting equipment where the slab is guided along the curve is 30Wh/(Kg・min)
Approximately for a cooling rate of ~60Wh/(Kg・min)
20Wh/Kg to 40Wh/Kg. In continuous casting equipment in which slabs are guided straight, the amount of heat removed is 20Wh/Kg to 80Wh/Kg, which is somewhat higher. If the cooling rate exceeds the upper limit, there is a risk of cracks occurring on the outer surface of the slab or inside it, and if it falls below the lower limit, a large amount of segregation may occur inside the slab. There is danger. The amount of heat removed (Wh) is determined by the amount of water sprayed and the temperature rise from intake to discharge, i.e.
Regarding the process, V ₁・C _W・(T ₁ −T _A ) and the second
The process can be determined by V ₂・C _W・(T ₂ −T _A ). Here, C _W represents the specific heat of water [1.163Wh/(℃・Kg water)]. To this amount of heat must be added the amount of heat removed by evaporation of the cooling water. This addition means that 3.5% of the sprayed water will evaporate and the water will be heated from 20°C to 100°C.
It takes 93Wh/Kg of water to heat and evaporate it, and the heat of evaporation is
This is done based on the fact that 627Wh/Kg is required. In addition to the heat released by forced convection caused by spray cooling, further amounts of heat are taken away from the slab by radiation, free convection and heat conduction (for example in the area of the guide rollers). The last two quantities of heat are negligible in continuous ingot casting equipment. The amount of heat removed by radiation depends on the surface temperature of the slab and can therefore be relatively and unconditionally reduced by increasing the intensity of the spray cooling. This amount of heat is 15 to 15% of the total amount of heat released in conventional cooling methods.
35%, but in the case of intense cooling according to the invention, it is about 6% of the total heat released in the first step and about 10% of the total heat released in the second step. Preferably, spray cooling is carried out in a closed room. In this case, the heat released by radiation is eventually released via the cooling water, and the proportion of heat released by radiation is therefore included in the value determined by the amount of water and the rise in water temperature. . In this case, therefore, in addition to the heat released via the cooling water, the amount of water typically sprayed is
There is no need to add anything other than the amount of heat removed by evaporation of the cooling water, which is within the range of 3.0-4.0%. If the casting speed or slab dimensions are changed, the unit for cooling should be Wh/(Kg・
Said changes must be taken in such a way that the cooling rate, expressed as min), and the amount of heat released in said two-stage cooling step remain approximately constant. If the slab is not bent, the second step can be prolonged and the amount of heat removed in this step can be increased accordingly. By increasing the pressure and/or flow rate of the cooling water relative to conventional systems, the amount of heat removed in the first step of the secondary cooling zone can be increased. It is economically advantageous to have a cooling water prepressure P ₁ of 15 to 30 bar. Example 0.65% C, 0.27% Si, 0.68% Mn, P
0.012%, S 0.013%, Cu 0.05%, Cr 0.02%
Steel containing 0.01% and Mo was cast by continuous casting. The pouring temperature in distribution channel 1 of the continuous casting equipment was 1530°C, which exceeded the melting temperature by 50°C. The steel was cast into square cross-section slabs with a side length of 120 mm in a continuous caster with curved slab guides. One of the slabs cast by this device was cooled in a secondary cooling zone with two cooling steps 3, 4. The casting speed was 2.5 m/min. The first step (cooling step 3) is to perform enhanced cooling in the casting direction of the slab from the mold 2.
It extended over a length of 1.9m. This length is
This corresponds to the residence time of a slab of 46 seconds. The slab was then cooled in front of the injection nozzle with water at a flow rate of 31 m ³ /h, which was prepressured to a pressure P ₁ of 22 bar. As a result, the heat transfer coefficient (due to convection and radiation) at the surface of the slab ranges from 1500W/ ^m2・K) to 1700W/
(m ²・K), which is 91Wh/(Kg・min)
This corresponds to a cooling rate of 70Wh/Kg and a heat dissipation amount of 70Wh/Kg. As a result, the ratio of heat taken away by radiation is 3.9Wh/Kg, calculating the degree of heat radiation as ε = 0.8.
That is 5.6%. This was followed by a second step (cooling step 4) with a 1.6 m length of low cooling rate water cooling corresponding to a residence time of 38 sec. In this case, the feed prepressure P ₂ in front of the nozzle was 7 bar and the water flow rate was 12 m ³ /h. The heat transfer coefficient is 800W/( ^m2・K) ~ 900W/( ^m2・K), the cooling rate is 47Wh/(Kg・min), and the amount of heat removed is 30Wh/Kg, which is 2.8Wh/Kg. , i.e. 9.4
It was accompanied by a radiant part of %. In addition, in the above embodiment, the temperature of the water at the entrance of steps 3 and 4 is
T ₁ and T ₂ were 20°C, and the wastewater temperature T _A was 35°C. For comparison, the distribution channel 1 of the continuous casting equipment
Another of the slabs cast in parallel from one of the molds placed in the
It was cooled with water at a flow rate of m ³ /min. This amount of water was also sprayed in the secondary cooling zone for a residence time of 46 seconds. This amount of water is 50Wh/(Kg・
min) or 9.7Wh/
Kg, i.e. 38Wh/Kg with 25.5% radiant part
corresponds to the amount of heat dissipated. Heat transfer coefficient is approximately 500W/
(m ² ·K) ~ 700W/(m ² ·K). The material was formed into 5.5 mm rolled wire rods on a twin-screw rod mill. Micrographic examination of rolled wire rods and testing of wire rods in cut planes and evaluation by Richtreihe of Bekeert yields on average a value of 0.6 for the reinforced cooled material according to the invention, and For materials cooled by conventional methods, a value of 1.4 was produced on average. Wires made from reinforced cooled billets were free of martensite, whereas wires made from normally cooled billets were found to have 12% more martensite. The material produced according to the invention has a tensile strength of 1050 N/mm ² and was drawn to a diameter of 2.3 mm in a six-stage drafting machine in wire rolling. After stretching, the material produced according to the invention had a tensile strength of 1743 N/mm ² and was able to withstand 23 bends along a radius of 7.5 mm, while the comparative material could only be bent 17 times. I couldn't stand it. Finally, the material is processed without annealing.
It was cold rolled in one pass to a thickness of 1.7mm. No rejects occurred in the strongly cooled material, but the normally cooled material no longer had sufficient technical properties after 1.7 mm cold rolling. The difference in quality is that the uniform directional elongation of the band made of the material produced according to the invention is 2.9%;
The fact that this uniform directional elongation is only 1.8% for the comparative material is also remarkable. The segregation coefficients and mechanical engineering values of the wires produced from these charges can be directly compared with the values mentioned above, both for the strongly cooled material and for the comparison material. The method according to the invention has a carbon content of 0.4 to 1.0% by weight.
%, manganese 0.2-1.7%, silicon 0.1-0.7%,
It is particularly useful for steels with 0-1.7% chromium, 0-0.5% nickel, 0-0.3% sulfur, balance iron and unavoidable impurities. In the present invention, in the first and second steps of the secondary cooling zone during continuous casting, the temperatures of the cooling liquid T ₁ , T ₂ ,
By controlling the liquid volumes V ₁ and V ₂ and the pressures P ₁ and P ₂ , the cooling rate of the slab (Wh/Kg・min) and the amount of heat taken from the slab (Wh/Kg) can be controlled. This is clear from the following rationale. Here is the temperature T,
The flow rate V is determined as follows. T ₁ : Temperature of the injection water introduced in the first process (cooling process 3 in Figure 4) T ₂ : Temperature of the injection water introduced in the second process (cooling process 4 in Figure 4) T _A : Cooling Temperature of cooling water discharged from the cooling chamber V ₁ : Flow rate of water introduced into the first step V ₂ : Flow rate of water introduced into the second step V _A : Cooling water discharged from the cooling chamber Flow rate T ₀₁ : Surface temperature of the billet entering the second process T ₀₂ : Surface temperature of the billet at the exit of the second process when cooling water is not injected in the second process The measuring device is a fluid thermometer. , thermoelements, flowmeters, etc. can be used, and an optical pyrometer can be used to measure the surface temperature of the billet. Next, the amount of heat removed from the billet is calculated using the following formula. Q=Q _K1 +Q _K2 +Q _STR +Q _R …(1) Here, Q: Total amount of heat removed in the secondary cooling zone Q _K1 : Amount of heat removed by the injected water in the 1st process Q _K2 : Injected water in the 2nd process Amount of heat taken away by Q _STR : Amount of heat taken away by radiation Q _R : Amount of heat taken away when the billet guide roller contacts the billet Cooling performed by jetted water is usually performed in a closed space (cooling room), so The amount of heat taken away by radiation Q _STR and the amount of heat Q _R taken away by the guide roller (extremely small) are transferred to the jet water in the cooling chamber. Assuming that Q _STR and Q _R in each of the first step and the second step are the same amount of heat, it can be set as follows: Q=Q ₁ +Q ₂ (2). Here, Q ₁ : Total amount of heat removed in the first process Q ₂ : Total amount of heat removed in the second process Even if the equation is simplified as in equation (2), the amount of heat removed by radiation in the first and second steps is The amount of heat is
Since only 6 to 10% of the total heat is removed in the secondary cooling zone, the error is less than 3%. Next, when calculating the total amount of heat removed in the secondary cooling zone, water evaporation must be taken into account. Here, if V _D is the flow rate of the evaporated jet water, then V _D = V ₁ + V ₂ − V _A Therefore, Q = Q _E + Q _ED + Q _D …(3) Here, Q _E : The amount of jet water that does not evaporate. Amount of heat required for heating Q _ED : Amount of heat required to raise the temperature of the evaporated amount of the injected water from the temperature at the time of introduction to the evaporation temperature Q _D : Enthalpy of the evaporated amount of the injected water That is, the fourth aspect of the present invention Since the water injected in the first and second steps shown in the figure is usually supplied from the same water source, the values of the water introduction temperatures T ₁ and T ₂ are the same. Therefore, since the introduction temperature T _E of the injection water introduced into the first step and the second step is T _E =T ₁ =T ₂ or more, equation (3) becomes as follows. Q〓＝V _A・C _W (T _A −T _E ) ＋V〓 _D・C _W・(100−T _E )＋V〓 _D・H _D …(4) Here, the amount of heat transferred per unit time Q〓 The unit is watt (W), the flow rate V〓 _D , V〓 _E is expressed in (/h), and C _W is the specific heat of the injected water (1.163Wh/
℃/water), _HD is the amount of heat to evaporate water (627Wh/
). The amount of heat transferred per unit time Q〓 ₁ , Q〓 ₂ in each of the first step and the second step can be calculated by stopping the supply of cooling water in the second step and measuring the amount of heat per unit time in the first step. The amount of heat transferred Q〓 can be calculated, and then the amount of heat transferred per unit time Q〓 in the second step can be calculated from the above formula (2). In the above calculation, a part of the amount of heat transferred by radiation in the second process is included in Q〓 in the measurement after stopping the supply of cooling water in the second process, so the calculation result of Q〓 is This will be slightly higher than the actual value. However, if we can measure the surface temperature of the billet in the second step and calculate the amount of heat transferred by radiation from the law of heat conduction, we can calculate the measured value of the billet surface temperature and the model of heat transfer by radiation. You can make corrections. Using the method explained above, calculations were performed based on the following sample 1. Sample 1 Chemical composition of steel: 0.62%C, 0.26%Si, 0.64%
Mn, 0.012%P, 0.008%S, 0.008%Cu, 0.04
%Cr, 0.01%Mo, remaining Fe and normally recognized impurities Billet casting conditions: Continuous casting mold called "Conquist S" with 4 curved molds Billet dimensions...120mm x 120mm (cross section) Casting speed 2.5m /min Casting temperature 1532℃ 1st process: Cooling water pressure 22 bar Flow rate 31 m ³ /h Cooling time 46 s 2nd process: Cooling water pressure 7 bar Flow rate 12 m ³ /h Cooling time 46 s 1st process, 2nd process Measurement results: Temperature of introduced cooling water (T _E ) 22.2°C Temperature of discharged cooling water (T _A ) 34.3°C Flow rate of water discharged from the cooling chamber 41.5 m ³ /h Only the first process was cooled (the first (No water injection in 2nd process)
Measurement results: Temperature of introduced cooling water (T _E ) 22.2°C Temperature of discharged cooling water (T _A ) 35.2°C Flow rate of water discharged from the cooling chamber 29.2 m ³ /h Surface temperature of billet at the entrance of the second process 755°C Surface temperature of the billet at the exit of the second process: 1090℃ Calculation result: Amount of heat removed: Q〓=41500・1.163・12.2 +1500・1.163・77.8+1500・627 Q〓=1665049W 1st process: Q〓=29900・1.163・13.0 +1100・1.163・77.8+1100・627 Q〓 ₁ = 1241288W Amount of heat calculated by the model (heat amount due to radiation and reflux through the billet guide roller in the second process section): 75430W Therefore, Q〓 ₁ is as follows will be corrected accordingly. Q〓 ₁ = 1241288W−75430W = 1165858W 2nd process: Q〓 ₂ = Q〓−Q〓 ₁ = 1665049−1165858 Q〓 ₂ = 499191W Amount of heat removed per unit weight of billet: 1st process 70.05Wh/Kg 2nd Process 29.99Wh/Kg Amount of heat removed per billet unit weight/unit time: 1st process 91.37Wh/Kg・min 2nd process 47.35Wh/Kg・min As is clear from the above, in the secondary cooling zone during continuous casting , in each of the first step and the second step, the temperature of the injection water introduced (T _E = T ₁
= T ₂ ), flow rate V ₁ , V ₂ of the injection water and pressure P ₁ , P ₂
By controlling the billet cooling rate (Wh/Kg・
min) and the amount of heat removed from the billet (Wh/Kg)
It is clear that the temperature, flow rate, pressure, etc. can be easily determined from the well-known heat transfer theory if the cooling rate (Wh/Kg·min) and the amount of heat (Wh/Kg) are determined. Table 1 summarizes the above calculation results and shows them together with the Beckert & Co. index.

[Table] For Samples 2 to 5, the chemical composition and casting conditions of the samples are shown below, as well as the amount of heat removed, the cooling rate, and the Bekert index in Tables 2 to 5. Sample 2: Chemical composition of steel: C 0.66%, Si 0.25%, Mn 0.65
%, P 0.011%, S 0.012%,
Cu 0.07%, Cr 0.04%, Mo 0.01
%, residual Fe and impurities Casting conditions: Billet cross section: 120mm x 120mm Casting speed: 2.2m/min Casting temperature: 1530℃ First process length: 1.9m (cooling time: 52s) Second process length: 3.2 m (cooling time: 87s) Temperature at time of cooling water injection: 22.0℃

[Table] Sample 3: Chemical composition of steel: C 0.62%, Si 0.21%, Mn 0.69
%, P 0.007%, S 0.015%,
Cu 0.09%, Cr 0.06%, Mo 0.01
%, residual Fe and impurities Casting conditions: Billet cross section: 120mm x 120mm Casting speed: 2.4m/min Casting temperature: 1528℃ First process length: 1.9m (cooling time: 52s) Second process length: 3.2 m (cooling time: 87s) Temperature at time of cooling water injection: 18.0℃

[Table] Sample 4: Chemical composition of steel: C 0.67%, Si 0.21%, Mn 0.64
%, P 0.013%, S 0.011%,
Cu 0.08%, Cr 0.04%, Mo 0.01
%, residual Fe and impurities Casting conditions: Billet cross section: 120mm x 120mm Casting speed: 2.2m/min Casting temperature: 1528℃ First process length: 1.9m (cooling time: 52s) Second process length: 1.6 m (cooling time: 43s) Temperature at time of cooling water injection: 20.0℃

[Table] Sample 5: Chemical composition of steel: C 0.65%, Si 0.24%, Mn 0.65
%, P 0.009%, S 0.011%,
Cu 0.09%, Cr 0.03%, Mo 0.01
%, residual Fe and impurities Casting conditions: Billet cross section: 120mm x 120mm Casting speed: 2.6m/min Casting temperature: 1527℃ First process length: 1.9m (cooling time: 44s) Second process length: 2.5 m (cooling time: 58s) Temperature at time of cooling water injection: 20.3℃

【table】

[Operation and effects of the invention]

According to the present invention, the 0.4
A steel billet with a carbon content of ~1.0% by weight is
In the first step in the secondary cooling zone, 50-90Wh/Kg of heat is removed, resulting in 65-100Wh/(Kg・
min), and the subsequent second
The temperature of the coolant T ₁ , T ₂ and the liquid volume V ₁ , V are set so that 20 to 80 Wh/Kg of heat is removed in the process, resulting in cooling at a cooling rate of 30 to 60 Wh/(Kg・min). ₂
By controlling the pressures P ₁ and P ₂ , segregation within the casting billet can be significantly reduced. The cooling rate was changed to 100Wh/(Kg・
If the cooling rate is 65Wh/(Kg・min) or more in the first step and 60Wh/(Kg・min) or more in the second step, cracks tend to occur on the surface or inside of the billet. 30Wh/
(Kg・min) or less, there is a risk that segregation will increase. In addition, in the first step of the secondary cooling zone, the temperature, liquid volume, and pressure of the cooling liquid are adjusted to 50 to 80Wh/Kg.
of heat is taken away, resulting in 75 to 90Wh/(Kg・
min), and in the second step, 30 to 60Wh/Kg of heat is removed.
When cooling is controlled at a cooling rate of 35 to 45 Wh/(Kg·min), no cracks occur on the surface or inside of the billet. A layer of very fine particles is formed on the surface of the billet,
This layer reduces the tendency of the billet to form cracks during rolling. Micro-etching of the square slices cut from the billet is the third step.
It has a fine particle layer as shown in the figure, and when strong cooling is performed, this layer reaches an average thickness of approximately 4 to 10 mm on the side surfaces of the billet, and an average thickness of approximately 4 to 10 mm on the end surfaces of the billet. and reaches a thickness of 25mm. The structure of the continuously cast slab material cooled by the method of the present invention has a high proportion of dendritic structure, as shown in FIG. The edge area of the billet cooled in this way is
As shown in FIG. 3, it has a spherical structure of ultrafine particles. The thickness of the edge zone will be at least 4 mm, compared to 1 mm in conventional methods. The billet thus obtained has a remarkable resistance to crack initiation at high stresses during rolling operations, since the dendritic structure, which is sensitive to cracks at the grain boundaries, does not occur near the surface. There is. According to the invention, it has been found that when continuously cast billets with a carbon content of 0.4 to 1% are rolled, for example into wire rods, segregation is significantly reduced compared to the known methods mentioned at the outset. In steel wires with the abovementioned carbon contents, the segregation in the wire rod is usually evaluated according to the Bekaert index. The average value of the index for 5.5 mm wire in the carbon range mentioned above is:
It can be reduced from about 1.1 to 0.6 by the method described above. For quenching from rolling temperature, steel wires produced in this way may be used in steels with a normal manganese content of up to 0.9% and when quenching is carried out at a normal cooling rate of up to 15°C/sec. Martensite no longer occurs at the residual segregation locations. The technical inventiveness of this method is as follows. This means that wire rods with low segregation can be produced in this way from small continuous casting steels, which can be formed at high drawing speeds and which, after drawing, exhibit excellent results both in the so-called bending test and in the so-called torsion test. high value, ie good plastic and elastic properties. This rolled wire rod can be quenched from the rolling temperature at a high cooling rate without forming a brittle layer called martensite at the segregated position. The material also has a reduced tendency to crack at the surface under the high stresses of rolling operations, compared to conventional continuous cast materials, due to the reinforced thicker bulbous edge area.

[Brief explanation of the drawing]

Figure 1 shows a sulfa print in a longitudinal section passing through the central axis of the billet, which is mostly spherical tissue.
Figure 2 shows the sulfur aprint in a longitudinal section through the central axis of the billet, which is mostly dendritic tissue.
FIG. 3 shows the microetching of a quarter slice of a billet of reinforced cooled material with a fine-grained spherical texture edge zone, and FIG. 4 schematically shows the apparatus for carrying out the method. . In the figure, the following symbols indicate the following parts, respectively. 1... Distribution, 2... Mold, 3... First process, 4... Second process, 5... Core, 6... Skin of slab, 7... Water collection pipe, 8... Water tank , 9, 10...
...Pump, 11,12...Pipe line, 13,14,1
5...Detection device.

Claims (1)

[Claims] 1. A slab coming out of a continuous casting mold is cooled by jetting a cooling liquid in a secondary cooling zone consisting of a strong cooling step in the first step and a weak cooling step in the second step. In the slab cooling method during continuous casting of steel with a carbon content of ~1.0% by weight, the temperature of the cooling liquid
T ₁ , T ₂ , liquid volumes V ₁ , V ₂ and pressures P ₁ , P ₂ are 50Wh/Kg to 90Wh/Kg in the first step 3 in the secondary cooling zone.
of heat is taken away, resulting in 65Wh/(Kg・min)
It is cooled at a cooling rate of ~100Wh/(Kg・min), and in the subsequent second step 4 it is cooled to 20Wh/Kg~80Wh/
Kg of heat is taken away, resulting in 30Wh/(Kg・
A method for cooling a slab during continuous casting of steel, characterized in that the cooling rate is controlled to be cooled at a cooling rate of (min) to 60Wh/(Kg・min). 2 In the first step 3 of the secondary cooling zone,
The method for cooling slabs according to claim 1, wherein a heat amount of 50Wh/Kg to 80Wh/Kg is removed. 3 In the first step 3, the cooling rate is
75Wh/(Kg・min) to 90Wh/(Kg・min) The slab cooling method according to claim 1 or 2, characterized in that the cooling rate is 75Wh/(Kg・min) to 90Wh/(Kg・min). 4 In the second step 4 of the second cooling zone,
The method for cooling a slab according to any one of claims 1 to 3, wherein a heat amount of 30Wh/Kg to 60Wh/Kg is removed. 5 In the second step 4, the cooling rate is
5. The slab cooling method according to any one of claims 1 to 4, characterized in that the cooling rate is 35Wh/(Kg・min) to 45Wh/(Kg・min). 6. A method for cooling slabs according to any one of claims 1 to 5, characterized in that the water pressure of the injection nozzle in the first step 3 is at least 15 bar. 7. A slab cooling method according to any one of claims 1 to 6, which is used in continuous casting of steel billets. 8 Used for casting steel slabs with a round, oval, rectangular or square cross section of 2500 mm ² to 20000 mm ² , and the ratio between the axial lengths of the oval cross section and the length of two sides of the rectangular cross section. Claims 1 to 7, characterized in that the ratio is at most 2:1.
The slab cooling method according to any one of paragraphs. 9 0.4-1.0 wt% carbon 0.2-1.7 wt% manganese 0.1-0.7 wt% silicon 0-1.7 wt% chromium 0-0.5 wt% nickel 0-0.3 wt% sulfur balance iron and usual impurities 9. A method for cooling a cast slab according to any one of claims 1 to 8, characterized in that the method is used for steel having a steel. 10 0.4-1.0 wt% carbon 0.3-0.9 wt% manganese 0.15-0.4 wt% silicon 0-0.25 wt% chromium 0-0.30 wt% nickel 0-0.04 wt% sulfur balance iron and usual impurities 10. The method for cooling a cast slab according to claim 9, wherein the method is used for steel having a steel.