EP0054867B1 - Method of cooling a continuously cast steel strand - Google Patents

Description

The invention relates to a method for cooling strands in the continuous casting of steel according to the preamble of claim 1.

A number of steel products, e.g. B. high-carbon steel wires, the technological properties are noticeably deteriorated by segregation. These segregations can also lead to the formation of brittle phases at the segregation points - often referred to as "martensite" - in the commonly used patenting of such wires from the rolling heat, which greatly reduce the tensile strength of the wire.

While in block casting the segregations are in the upper third of the block and can be removed by appropriate cropping, they are distributed over the entire length of the strand during casting and cannot be removed by cutting. Their negative effects in so-called small-format continuous casting - dimensions from 100 to 140 mm edge length - are greater than in large-format continuous casting - d. H. Pre-block formats of 200-300 mm edge length - because the deformation up to the finished rolled product is less with the small casting formats. Considerable efforts have already been made by experts to reduce the segregation in continuous casting or its negative effects on the rolled product. The general opinion here has emerged that a so-called globulitic structure is associated with low segregations, but a dendritic structure is associated with strong segregations. A globulitic structure is understood to mean a structure in which the crystals have no preferred direction of growth, but are randomly distributed over the cross section. 1 shows the structure of a continuous casting billet with a large proportion of such a globulitic structure. By contrast, a dendritic structure is understood to mean a structure in which the predominant direction of growth of the crystals runs into the metal perpendicular to the strand surface. Fig. 2 shows the micrograph of a continuous casting billet with a large proportion of dendritic structure.

Because of the perception that dendritic structures would favor segregation and reduce globulitic structures, efforts by professionals have focused on increasing the proportion of the globulitic structure. Various approaches have been taken for this purpose.

One direction of development is to prevent the formation of a dendritic structure by stirring the liquid steel in the solidifying strand and thus to reduce segregation (see for example DE-C-1783 060). The stirring effect is generally achieved by electromagnetic stirring devices. In any case, complex devices are required.

Another direction of development, to achieve a globular structure, is to keep the casting temperature very low. Difficulties arise in practice due to the fact that the pouring nozzles are clogged. In the method known from US Pat. No. 3,771,584 according to the preamble of claim 1, the strand emerging from the continuous casting mold is first intensively cooled in a secondary cooling zone by means of an sprayed-on cooling liquid, and then cooled at a reduced cooling rate. The cooling is controlled in such a way that the strand skin in the first cooling stage is of sufficient strength to withstand the metallostatic pressure - for this purpose, in the first stage a quantity of heat is generated with a cooling rate of around 82Wh / (Kg - min) of about 27.5 Wh / kg - but then in the second stage the cooling is reduced so much - at a cooling rate of 21 Wh / (kg - min) only a heat quantity of about 3.5 Wh / kg is removed - that the growth of the stem crystals in the transverse direction is impeded and the growth of the crystals with a vertical component is promoted in the core of the strand.

Extensive studies with the aim of reducing segregations by casting at low temperatures or by electromagnetic stirring in steels with 0.4 to 1.0% carbon have resulted in a slight reduction in segregations being achievable, but this reduction cannot is sufficient to achieve a noticeable improvement in technological properties in the production of wire rod from such steels. Even more frequent occurrence of martensite was observed when using electromagnetic stirring. Even a process in which only amounts of heat are removed in the secondary cooling zone in accordance with US Pat. No. 3,771,584 do not lead to the desired result in a steel with 0.4 to 1.0% carbon.

The invention has for its object to produce billets with reduced segregation in a continuous steel casting process for steels with a carbon content of 0.4 to 1.0% by weight, from which wire rod with improved mechanical and technological properties can preferably be produced. In particular, the conditions for small-format continuous casting, i. H. with dimensions up to 140 mm edge length, can be improved. It is also intended to prevent martensite from forming at the points of segregation when the wire rod rolled from a billet is tempered.

To solve this task, the kenn Drawing features of claim 1 proposed. Further refinements of the invention can be found in the subclaims.

It has been found that contrary to the prevailing opinion, steel with a carbon content of 0.4 to 1% can considerably reduce segregation if cooling is carried out in two stages within the stated limits. This effect can also be observed at high casting temperatures and casting speeds. The extent of the reduction in segregation is sufficient to substantially improve the technological properties of wire rod which is produced from a continuous casting billet obtained in this way. The occurrence of martensite at segregation points after tempering the wire rod from the rolling heat is also significantly reduced.

With two-stage cooling within the limits specified in the claims, no cracks occur on the surface of the billet or in the interior of the billet. A very fine-grained layer is formed on the billet surface, which reduces the susceptibility of the billet to the formation of cracks during rolling. The macro-etching of a quartered billet disk shown in FIG. 1 shows this fine-grained layer, which is about 4 to 10 mm on the side surfaces of the billet and up to 25 mm on the billet edges when the temperature is strongly cooled.

The invention is explained in more detail with reference to four figures.

• Figur 1 einen Schwefelabdruck vom Längsschnitt durch die Mittelachse eines Knüppels mit einem großen Anteil an globulitischem Gefüge ;
• Figur 2 einen Schwefelabdruck vom Längsschnitt durch die Mittelachse eines Knüppels mit einem großen Anteil an dendritischem Gefüge ;
• Figur 3 eine Makroätzung einer geviertelten Knüppelscheibe aus verstärkt gekühltem Material mit feinkörniger globulitischer Randzone ;
• Figur4 in schematischer Darstellung eine Vorrichtung zur Durchführung des Verfahrens.

Show it

• Figure 1 is a sulfur print of the longitudinal section through the central axis of a billet with a large proportion of globulitic structure;
• FIG. 2 shows a sulfur impression from the longitudinal section through the central axis of a billet with a large proportion of dendritic structure;
• FIG. 3 shows a macro-etching of a quartered billet disk made of intensively cooled material with a fine-grained globular boundary zone;
• Figure 4 shows a schematic representation of an apparatus for performing the method.

4 schematically shows a continuous steel casting device for carrying out the method according to the invention. Liquid steel is poured from a distributor trough 1 into an oscillating, cooled continuous casting mold 2, in which the outer skin solidifies during the slow downward movement of the metal strand. Two cooling stages 3 and -4 are arranged behind the mold, in which the strand is uniformly sprayed with water over its entire circumference. The liquid sump of the metal strand is denoted by 5, the solidified strand shell by 6. The entire outflowing spray water is collected in a collecting line 7 and fed to a water tank 8. The cooling stages 3 and 4 are supplied with spray water from the collecting container 8 by means of pumps 9 and 10 via lines 11 and 12. The spray water collecting line 7 is a device 13 for detecting the temperature T _A and the water volume flow V _{A of} the wastewater and the stages 1 and 2 are devices 14 and 15 for detecting the water temperature, the water volume flow and the water pressure T _j , V ₁ , P ₁ or T ₂ , V ₂ , P _{2 assigned} at the input of the relevant stages. There are also control and regulating elements, not shown, in order to be able to change the variables mentioned. The division into the two stages is determined by measuring the water volume flow V _A and the temperature T _{A of} the waste water when the two stages 1 and 2 are in operation and once only when the stage 1 is in operation.

In the usual production of continuous casting in the carbon range of 0.4-1.0%, for example with a square format of 120 mm edge length and a speed of 2.4 m / min, the strand below the mold is sprayed with water at a water pressure of Usually 3 bar, but a maximum of 8 bar, with a water volume of about 20-30 m ³ / h and strand.

In the method according to the invention, the cooling is increased by increasing the heat transfer coefficient by intensifying the water cooling on the surface of the billets. This reduces the segregation.

As is well known, very intensive cooling leads to the risk of cracks on the surface of the strand. These cracks are avoided in that the very intensive cooling with the billet format and the mentioned casting speed of 2.4 m / min to a length of about 2 m below the mold, i. H. is limited to a dwell time of the strand of about 40 to 60 seconds. A surface temperature of the strand of about 650 ° C to 950 ° C then arises. In this area - hereinafter referred to as stage 1 - about 50 Wh / kg-90 Wh / kg, corresponding to a cooling rate of about 65 Wh / (kg. Min) - 100 Wh / (kg. Min) are withdrawn from the strand. Following this very intensive cooling, cooling is carried out with reduced intensity over a dwell time of the strand of approximately 30 to 50 seconds (in the format specified). The amount of heat withdrawn in this area - hereinafter referred to as stage 2 - for a continuous caster with curved strand guide under the specified conditions is 20Wh / kg-40 Wh / kg, corresponding to a cooling rate of 30 Wh / (kg - min) -60 Wh / (kg - min). In a continuous caster with a straight strand guide, the values for the amount of heat withdrawn are 20 Wh / kg-80 Wh / kg, i.e. slightly higher.

The amount of heat withdrawn (Wh) can be determined from the amount of water sprayed on and its temperature increase from the inlet to the outlet, ie V ₁ · C _W · (T ₁ - T _A ) for stage 1 and V ₂ · C _W · (T ₂ - T _A ) for level ₂ , where C _{W means} the specific heat of the water [1,163 Wh / (° C · kg water)]. That warmth quantity is to add a quantity of heat that is extracted by the evaporation of cooling water. The calculation is based on the fact that 3.5% of the sprayed water evaporates, 93 Wh / kg of water being required to heat the evaporated water from 20 ° C. to 100 ° C. and the heat of vaporization is 627 Wh / kg of water. In addition to the heat dissipation by forced convection caused by the spray cooling, the strand is further heat quantities by radiation, free convection and heat conduction, for. B. on leadership roles, withdrawn. The last two parts are negligible in a billet caster.

The radiation component depends on the strand surface temperature and therefore decreases relatively and absolutely with increasing spray cooling intensity. In the case of the strong cooling according to the invention, it is approximately 6% of the total heat dissipation in the first stage and approximately 10% in the second stage, while it is 15 to 35% of the total heat dissipation in the case of conventional cooling.

The spray cooling is preferably carried out in a closed chamber. In this case, the radiation portion of the heat dissipation is ultimately dissipated via the cooling water and is therefore included in the values determined from the amount of water and the increase in water temperature. In this case, all that needs to be taken into account for the values determined via the removed cooling water is only the amount of heat removed by evaporation of the cooling water, which is generally between 3.0 and 4.0% of the amount of water sprayed on.

If one changes to other casting speeds or to other continuous casting formats, the cooling must be adapted so that the cooling rate in Wh / (kg - min) and the heat quantities dissipated in the two cooling stages remain approximately constant.

If the strand is not straightened, stage 2 can be extended and the amount of heat extracted in this stage can be increased.

The high amounts of heat withdrawn in the first stage of the secondary cooling zone are achieved by increasing the pressure and / or the amount of cooling water compared to the usual way of working. A pre-pressure P _{1 of} the cooling water of 15-30 bar appears to be economically advantageous.

The structure of the continuous casting material produced in this way has a high proportion of dendritic structure, roughly corresponding to FIG. 2.

The edge zone of the billets produced in this way has - as Fig. 3 shows - an extremely fine-grained "globulitic" structure. The thickness of the edge zone is at least 4 mm compared to usually 1 mm. This ensures that the billets are much more resistant to the formation of cracks under high stresses during rolling, since the dendritic structure, which is sensitive to tears at the grain boundary, does not reach as far to the surface.

If one rolls the continuous cast billets produced in this way with a carbon content of 0.4-1%, for example, into wire rod, it is found that the segregations have been significantly reduced compared to the known procedure described at the beginning. In the case of wires with the carbon contents mentioned, the segregations in the wire rod are usually assessed according to a reference number from Bekaert. The average value of the guide number for 5.5 mm wire in the named carbon range can be reduced from about 1.1 to 0.6 by the described procedure. With the tempering from the rolling heat, with the usual manganese content of the steel up to 0.9% and the usual cooling rate up to 15 ° C / sec, there is no longer any «martensite at the remaining segregation points of the wire produced in this way.

The technical progress lies in the fact that in this way a wire rod can be produced from small-format continuous casting with low segregation, which can be deformed at high drawing speeds and which after drawing has high values in the so-called bending test and in the so-called torsion test, i.e. H. has good plastic and elastic behavior. This wire rod can be tempered at high cooling rates from the rolling heat without the brittle phase called "martensite" forming at the segregation points.

The material is also less prone to cracking on the surface than normal continuous casting material due to the reinforced globulitic edge zone under high stresses during rolling.

Embodiment

A steel with 0.65% C, 0.27% Si, 0.68% Mn, 0.012% P, 0.013% S, 0.05% Cu, 0.02% Cr and 0.01% Mo was cast in a continuous casting . The casting temperature in distributor 1 of the continuous casting installation was 1,530 ° C. and was thus 50 ° C. above the liquidus point. The steel was cast in a continuous caster with a curved strand guide to form square strands with an edge length of 120 mm. One strand of this plant was cooled in a secondary cooling zone with two stages 3 and 4. The casting speed was 2.5 m / min. The first stage 3 of increased cooling extended from the mold 2 in the casting direction of the strand over a length of 1.9 m, corresponding to a residence time of the strand of 46 seconds. Here the strand was at a pre-pressure P ₁ of 22 bar in front of the spray nozzles a water volume of 31 m ³ / h cooled. This results in a heat transfer coefficient (by convection and radiation) of 1,500 W / (m2. K) to 1,700 W / (m2. K) on the surface of the strand. This corresponds to a cooling down Speed of 91 Wh / (kg - min) and an extracted amount of heat of 70 Wh / kg. The proportion of the amount of heat extracted by radiation here is calculated with an emissivity of ε = 0.8 3.9 Wh / kg, that is 5.6%. This was followed by a second stage 4 with reduced water cooling of a length of 1.6 m corresponding to a dwell time of 38 sec. Here the admission pressure P ₂ was 7 bar in front of the nozzle and the water quantity was 12 m ³ / h. The heat transfer coefficient here was 800 W / (m ² · K) to 900 W / (m ^2. K), the cooling rate was 47 Wh / (kg. Min) and the amount of heat removed was 30 Wh / kg, with a radiation component of 2.8 Wh / kg, that is 9.4%.

For comparison, the parallel runs in a first stage were cooled in a conventional manner with a water pressure of 3 bar and a water quantity of 14 m ³ / h per line. This amount of water was applied in a secondary cooling zone with a residence time of also 46 seconds. This corresponds to a cooling rate of 50 Wh / (kg - min) or a dissipated amount of heat of 38 Wh / kg, with a radiation component of 9.7 Wh / kg, i.e. 25.5%. The heat transfer coefficient was approx. 500 W / (m ² · K) to 700 W / (m ² · K).

The material was rolled out in a two-wire wire mill to 5.5 mm wire rod. An examination of the wire rod in the micrograph and evaluation of the polish according to the standard series from Bekaert showed a value of 0.6 for the material cooled more according to the invention and an average of 1.4 for the material cooled in the usual way. While the wire from intensely cooled billets was free of "martensite", "martensite" was found on 12% of the wires from normally cooled billets. The material produced according to the invention had a tensile strength of 1,050 N / mm ² and was drawn to a diameter of 2.3 mm in a wire drawing plant using a 6-stage tractor. It then had a tensile strength of 1,743 N / mm ² and could be bent 23 times over a radius of 7.5 mm, while the comparison material only had 17 bends. The material was then cold rolled to a thickness of 1.7 mm in one pressure without intermediate annealing. There were no failures with the increasingly cooled material, while the normally cooled material no longer had sufficient technological properties after cold rolling to 1.7 mm. The difference in quality is also expressed in the fact that the uniform elongation of the strip made of material produced according to the invention was 2.9%, whereas it was only 1.8% for the comparison material.

The segregation indicators and mechanical-technological values of the wires produced from these batches are directly comparable to the values described above for both the strongly cooled and the comparative material.

The method according to the invention is particularly applicable to a steel having the composition mentioned in claims 9 and 10.

Claims (10)

1. A method of cooling castings in the continuous casting of steel having a carbon content of from 0.4 to 1 %, wherein the casting issuing from the continuous casting mould is cooled in a secondary cooling region by means of a cooling fluid sprayed thereon in a two-stage procedure, firstly intensively and then with a reduced cooling speed, characterised in that the temperatures (T₁, T₂), the volume flows (V₁, V₂) and the pressures (P₁, P₂) of the cooling fluid are so selected that an amount of heat of from 50 Wh/kg to 90 Wh/kg is removed in the first stage (3) of the secondary cooling region, wherein cooling is effected with a cooling speed of from 65 Wh/(kg · min) to 100 Wh/(kg - min) and in the subsequent second stage (4), an amount of heat of from 20 Wh/kg to 80 Wh/kg is removed, with the cooling speed being from 30 Wh/(kg - min) to 60 Wh/(kg - min).

2. A method according to claim 1, characterised in that an amount of heat of from 50 Wh/kg to 80 Wh/kg is removed in the first stage (3) of the secondary cooling region.

3. A method according to claim 1 or 2, characterised in that the cooling speed in the first stage (3) is from 75 Wh/(kg - min) to 90 Wh/(kg - min).

4. A method according to one of claims 1 to 3, characterised in that an amount of heat of from 30 Wh/kg to 60 Wh/kg is removed in the second stage (4) of the cooling region.

5. A method according to one of claims 1 to 4, characterised in that the cooling speed in the second stage (4) is from 35 Wh/(kg - min) to 45 Wh/(kg - min).

6. A method according to one of claims 1 to 5, characterised in that the water pressure upstream of the spray nozzles of the first stage (3) is as least 15 bars.

7. A method according to one of claims 1 to 6, characterised by use thereof in the continuous casting of steel billets.

8. A method according to one of claims 1 to 7, characterised by the use thereof in casting elongate steel castings of a round, oval, rectangular or square cross-section of from 2500 mm² to 20 000 mm², wherein the ratio between the axes in an oval cross-section and the ratio between the sides in a rectangular cross-section is at maximum 2 : 1.

9. A method according to one of claims 1 to 8, characterised by the use thereof on a steel comprising :

0.4 to 1.0 % by weight of carbon

0.2 to 1.7 % by weight of manganese

0.1 to 0.7 % by weight of silicon

0 to 1.7 % by weight of chromium

0 to 0.5 % by weight of nickel

0 to 0.3 % by weight of sulphur

with the residue iron and usual impurities.

10. A method according to claim 9, characterised by the use thereof on a steel comprising :

0.4 to 1.0 % by weight of carbon

0.3 to 0.9 % by weight of manganese 0.15 to 0.4 % by weight of silicon

0 to 0.25 % by weight of chromium

0 to 0.30 % by weight of nickel

0 to 0.04 % by weight of sulphur

with the residue iron and usual impurities.

Images