DE102016109489A1 - Method for improving the wear behavior of plant components in the further processing of high-alloy steels and plant for processing these high-alloy steels - Google Patents

Abstract

A method for improving the wear behavior of equipment components in the manufacture and further processing of materials having an increased tendency to micro-segregation, such as high alloy steels, in which casting and hot working takes place in one line, comprising a casting machine with a strand or ribbon guide, coolant for cooling, and Means for cutting to length and units for reheating, in which the cast high-alloy steel is conveyed by means of roller conveyors, with intensive cooling of the cast product in the casting plant taking into account subsequent cooling devices. The cooling of the cast product is controlled such that the highest temperature in the cross section of the cast product (perpendicular to the casting direction), the zero toughness temperature (ZDT) at the end of the casting machine and before the first forming step, but at least before mechanical cutting of the cast product by means of a pair of scissors, at least temporarily for the respective processing step, falls below. An apparatus for producing a cast product from materials having an increased tendency for in-line micro-segregation using the method comprising a casting machine, a strand or ribbon guide, cooling for cooling the cast product, straightening and cutting means, and means for straightening, cutting and transporting the cast product, wherein intensive cooling of the cast product in the strand or tape guide of the device taking into account subsequent cooling means and the cooling of the cast product is controllable such that the highest temperature in the cross section of the cast product, the toughness temperature at the end of the casting machine and before the first forming step , the cast product at least temporarily for the respective processing step, is undershot.

Description

The present invention relates to a method for improving the wear behavior of equipment components in the manufacture and further processing of castings from materials having a stronger segregation tendency, such as high alloy steels, in which casting and hot working take place in one line.

The production of materials with an increased tendency to segregation, especially in high alloy steels and new manganese steels with> 7% manganese, high carbon steels, phosphorus and boron alloy steels in one line, for example in a CSP process, strip casting such as BCT Process or the twin-roll casting process, could not be produced in the past with sufficient operational reliability and due to the required product qualities also not cost-effective. It has been shown that even the aggregates for reheating castings from the o.g. Steels, such as CSP furnace induction heating Hubbalken-, blast furnace, etc. in which the cast product is transported by means of roller conveyors are exposed to particularly high wear, which further complicates an economical production and further processing of such steels.

Because even on system components such as knives occurs during the cutting process on a material transfer to the blade surface. The material transfer is due to the liquid films in the structure or on the grain boundaries of the cast product. These liquid films are due to segregations that occur during dendritic solidification (transcrystalline or globulitic). This material transfer ultimately leads to increased wear.

At the same time, a brittle fracture or a hot crack may occur from the cutting edge when cutting the cast product, which runs in planes other than the cut edge and may also lead to the breaking out of material from the cast product. Such non-straight, non-planar geometries of cut edges on the cut product degrade the quality of the product and may complicate subsequent rolling, for example, when threaded into the mill stand.

Also, rolls and rolls such as e.g. Furnace rolls are affected during reheating of the cast product, even if near-surface areas have liquid films that have arisen by segregation. When the near-surface annealed regions reflow, material transfer from the cast product to the roll or roll surfaces can occur, which in turn leads to increased wear of these equipment components. The result is costly repairs associated with downtime in which the plant can not produce and a lower quality of the product produced.

Likewise, the behavior in forming, e.g. Rolling, bending, etc. adversely affected, since the connection at the grain boundaries is not given and thereby on and also create cracks, which are not permitted and cause a devaluation of the material.

The state of the art for controlling the liquid sump ("sump tip") in a casting plant assumes that in a continuous casting process, the liquid sump is within the supported length of the casting machine, if the metallo or ferrostatic pressure is no longer a change in geometry, such as Increasing the strand thickness ("whale", "bulging" of the strand), leads. The temperature of the last solidifying section ("core") of the cast product is controlled so that the liquid sump is within the supported length of the caster ("sump tip control").

Often the term "sump point" or "final point of soldification in the machine" is used, which usually refers to the coordinates (in relation to the casting direction and the cross-section of the cast product) containing the last solidifying volume element.

In the non-patent literature ( Kh Kim, Tj. Yeo, KH Oh, DN Lee: "Effect of carbon and sulfur in continuously cast strand on longitudinal surface cracks", ISIJ International, Vol. 36 (1996), no. 3, pp. 284-289 and YM Won, Kh Kim, Tj. Yeo, KH Oh: "Effect of Cooling Rate on ZST, LIT and ZDT of Carbon Steel near Melting Point", ISIJ International Vol. 38 (1998), no. 10, pp. 1093-1099 ) characteristic temperatures in the solidification or in the partially solidified area ("mushy zone") are described and correlated with a "fraction solid". The zero toughness temperature (ZDT, fractional solid approx. F _s = 0.99), the zero-strength temperature (ZST, fractional solid approx. F _s = 0.80) and the zero make-up temperature (Liquid Impenetrable Temperature, LIT, fraction solid approx. F _s = 0.90.

LIT and ZDT are used to characterize the hot crack sensitive temperature interval of a material. Between these temperatures, the permeability of the partially solidified area is so low that a make-up of melt is no longer possible and, for example, porosity and hot cracks can occur. Between the ZDT and LIT even small strains are sufficient to a material separation or a To cause hot crack. The ZDT is also identified with the temperature at which the coalescence (or coherency temperature) of the secondary dendrite arms occurs.

The selection of the process parameters in a sump tip control (eg casting speed, secondary cooling) is usually done for flat products so that the "fraction solid" in the core between f _s = 0.90 (temperature less LIT) and f _s = 1.0 and this is located within the strand guide. The range around f _s = 0.95 is preferred. For steels that do not segregate significantly, the extension of the temperature range between f _s = 0.95 and f _s = 0.99 is usually less than 50K. Deviating from this, the extent of the temperature range increases with severely seething steels greater than 50K to over 150K. Hereby, the term is thus defined as strong or reinforced segregating for the material such as a high-alloy steel.

Experience shows that such a silt tip regulation does not lead to a bulging after strand guidance. Further, the problems described above do not occur with a flame cut, which is common in conventional continuous slab casting.

In conventional continuous slab casting, using the ZDT as the definition of the sump tip in a high-sifting steel would have the disadvantages of too long strand guidance or too low casting speed or too little discharge or too much water to be provided for secondary cooling. In addition, a shear cut with a sump tip control leads to the above-described defects or damage to the cast product and to the plant components.

The object of the present invention is to provide a safe and inexpensive process for the production of materials, e.g. high-alloyed steels, with an increased tendency to segregations, so as to enable large-scale production of such grades in a line consisting of casting and hot forming. Furthermore, a device for carrying out the method should be provided.

This object is achieved on the basis of the preamble in conjunction with the characterizing part of claim 1. Advantageous developments are the subject of dependent claims.

The method according to the invention is characterized in that intensive cooling of the cast material takes place in the strand guide of the casting plant and taking into account the casting plant downstream cooling devices, wherein the cooling of the cast material is controlled such that the highest temperature in the cross section of the cast material (perpendicular to the casting direction) the zero toughness temperature (ZDT) at the end of the casting plant and before the first forming step, but at least before a mechanical cutting to length of the cast material by means of a pair of scissors, falls below. In temporary forming, e.g. when cutting paper, this temperature must be at least in the area that is influenced by the paper cutting / forming, in addition, before and behind lying areas in Gießstrang can for the purpose of saving energy in the wider area of the system when processing at a higher temperature level remain.

This ensures that the cast product (the cast material) is completely penetrated when reaching the scissors and at the grain boundaries also no liquid films are more. Thus, a paper cut also no longer causes aggregation on the shear blade or hot cracks or brittle fractures in the shear area.

It is advantageous if the temperature profile in the cross section is balanced (low temperature difference between surface and core) in order to reduce the load on the shears or the forming unit. Below the ZDT, the liquid films have virtually disappeared, so that no material transfer or hot crack / brittle fracture can occur.

The ZDT can be experimentally determined experimentally by hot tensile testing after previous melting by measuring the fracture necking. Here, the ZDT represents the temperature at which Brucheinschnürungswerte greater than zero can be achieved for the first time.

In a particularly advantageous embodiment of the present invention, the ZDT may be calculated using a micro-segregation model. This model provides the ZDT temperature to the temperature calculation model of the automation of the casting plant (control model). This control model regulates or controls the cooling water quantity of the cooling of the casting plant (secondary cooling) or of a tertiary cooling unit so that the condition T _{cross section, max} <ZDT is fulfilled. The microsegregation model as part of the computational model calculates the solid phase fraction at least as a function of chemical composition, eg, by means of the Scheil-Gulliver model and possibly as a function of secondary dendrite arm spacing and cooling rate.

To avoid surface defects such as cracks, the surface temperature of the surface should be Casting product in the directional range of a continuous casting or strip casting higher than 850 ° C. Therefore, the new cooling must be arranged after or far in front of the straightening area. Usually, the coolant intensity is reduced at a greater distance from the mold. In the method according to the invention, this is deviated from and intensive cooling is carried out in order to lower the core temperature in good time before reaching the shears or the forming unit so that no liquid films are present on the grain boundaries in the structure of the cast product.

In a plant concept in which the strand is cast by means of a Zweirollengießmaschine or a strip casting machine, the invention also relates to the areas in which the band deflected, or placed in loops, transversely divided by scissors and should be rolled directly.

To realize the intensive cooling, the water pressure, the amount of water, if necessary, the water / air ratio, the nozzle type or the nozzle spacing in the cooling can be adjusted so that thereby an increased heat dissipation can be realized.

It may be necessary that the cut-out cast product (the cast material) must be reheated for subsequent processing steps. This takes place in a heating unit. Here, the surface and possibly also the core of the cast product can again exceed the ZDT and liquid films can form again between the grain boundaries in the structure of the cast product.

The cast product is understood to mean the strand or strip that is made from the cast material. Wherein this cast material has an increased tendency to segregation or micro segregation during its cooling / heating or exceeding the ZDT.

To prevent material transfer to rollers or rollers, the surface or, to avoid the occurrence of cracks, must also be cooled down to the core, below the ZDT. For this purpose, special cooling devices are provided after the heating unit (for example ovens or induction heating). These cooling devices are preferably provided in the working direction immediately behind the heating unit and before the inlet further processing units such as rolling stands. The cooling devices are controlled by the controller so that they can cool the surface of the cast product, depending on its transport speed, below the ZDT in a very short time interval; but so that the further processing in the warm state can be ensured.

In order to be able to adjust the microsegregation model and thus the control of the cooling as accurately as possible, the chemical composition altered by macrosegregation (changed by segregation at most) can also be taken into account in the last solidifying region of the cross section of the cast product.

This can be done on the one hand by simulation of macrosegregations with CFD software (Computational Fluid Dynamics, with a coupled calculation of solidification, heat and mass transport) or by the determination and application of segregation indices.

The latter can be determined experimentally by means of a cross-sectional sample of the cast product (perpendicular to the casting direction, "pickling wheel") of a specific material grade with a limited chemical composition. The concentrations c _{i of} an element i measured there in the region of the maximum segregations can be set in relation to the nominal concentrations c _{0, i of} the chemical elements and thus segregation indices K _i = C _i / C _{0, i} can be determined.

It can be assumed that these indices are relatively constant for a certain material quality with limited process parameters of the casting process in question. With the segregation coefficients, the nominal composition c _i = c _{0, i, actual} · k _i currently to be cast can be corrected for a melt and passed to the microsegregation model.

Apparatus for the production of castings from materials with a greater tendency to segregation, such as high alloy steels, in line using the method according to claims 1 to 11, comprising a caster, a strand or strip guide, a cooling for cooling the cast material , Straightening and cutting means, as well as means of transport for straightening, cutting and transporting the cast material, wherein an intensive cooling of the cast material in the strand or strip guide of the casting and taking into account the casting plant subsequent cooling devices, wherein the cooling of the cast material is controllable in that the highest temperature in the cross section of the cast material (perpendicular to the casting direction) is the zero toughness temperature (ZDT) at the end of the casting machine and before the first forming step, but at least before mechanical cutting of the cast material s by means of a pair of scissors, at least temporarily for the respective processing step, is below.

Casting in the context of the invention, a device is understood, with a tundish from which the material, consisting of a molten steel with an increased tendency for segregation or micro segregation, abandoned; a continuous mold with a primary cooling in which the melt can be cast into a strand / strip, a strand / strip guide with a secondary cooling, in which the cast material can be cooled down to reach the ZDT, as well as arranged in the strand guide directing means. Downstream of the casting plant may be further means for cooling the cast material, as well as means for cutting to length and reheating units in which the cast material can be conveyed by means of roller conveyors. All together forms a plant or forms a plant concept.

The invention will be explained in more detail below with reference to an exemplary embodiment with reference to the accompanying drawings. Show it:

1 the ZDT control and integration of the micro-segregation model into the temperature control in a graph;

2 a plant concept with a cooling unit for cooling the core in front of the shears and cooling the surface of a cast product before the further treatment;

3 the arrangement of cooling within the strand guide a vertical strip casting.

Like in the 1 a microsegregation model is shown 10 used to determine the zero ductility temperature (ZDT) depending on the chemical composition of a melt. The microsegregation model 10 as part of a rule model calculates the solid phase fraction as a function of chemical composition, for example using the Scheil-Gulliver model. This results in a temperature value at which the cast product is completely solidified and no liquid films are present at the grain boundaries of the solidified structure. This temperature is called T _soll 11 denotes and is at the same time the ZDT. The determined ZDT is used to control or control the cooling and / or the transport speed. The cooling must be controlled in such a way that the ZDT is reached or undershot before the first forming step, but at least before any mechanical cutting of the cast product (the cast material) by means of scissors. This can be a dynamic control process that is continuously monitored and controlled.

The 2 shows an exemplary plant concept 12 with cooling units 13 for cooling the core of a strand 14 , The strand 14 or the cast product consisting of the cast material, is made of a molten steel 16 of a certain chemical composition with an increased tendency to segregation by pouring from a ladle 17 via an intermediate vessel 18 in a continuous casting mold 19 educated. Important for the cooling concept is that before and after straightening the strand 14 is cooled intensively. Too intense cooling during straightening would increase the risk of cracking the strand 14 mean. The intensive cooling of the strand 14 (of the cast material) takes place before and after straightening in the region of the strand guide (not shown) by means of the cooling units 13 cooling down to a temperature which is equal to or just below the calculated ZDT. In any case, it is important that the ZDT before cutting off with the scissors 15 is reached. A cooling of the surface of the strand 14 (of the cast material) is after a heating unit 20 intended. Due to the heating, a liquid film can again form on the grain boundaries by exceeding the ZDT at the surface or into the core. This liquid film would lead in the subsequent operations to aggregations of system parts, or to cracks during rolling. To prevent this is in the working direction another cooling 21 provided, which cools the surface, or the core, of the strand on the ZDT, before this example, in a multi-stage rolling stand 22 is introduced. At the end of the "in line" process is the rolled strand 14 to a coil 23 coiled.

3 is a detail view after 2 , Here it is intended, within a range 24 , which lies in front of the straightening area in the strand guide, to cool intensively, as indicated by the arrow 25 is hinted at. After judging is another area 26 provided in the strand 14 is intensively cooled again within the strand guide, as indicated by the arrow 27 is hinted at. After the second intense cooling in the area 26 the ZDT should be reached for the special cast product so that it can easily be separated with scissors to the desired length.

LIST OF REFERENCE NUMBERS

10

 Mikroseigerungsmodell

11

T _shall

12

 investment concept

13

 cooling units

14

 strand

15

 scissors

16

 molten steel

17

 ladle

18

 intermediate vessel

19

 ended mold

20

 heating unit

21

 further cooling

22

 rolling mills

23

 coil

24

 first cooling area

25

 arrow

26

 second cooling area

27

 arrow

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• Kh Kim, Tj. Yeo, KH Oh, DN Lee: "Effect of carbon and sulfur in continuously cast strand on longitudinal surface cracks", ISIJ International, Vol. 36 (1996), no. 3, pp. 284-289 [0009]
• YM Won, Kh Kim, Tj. Yeo, KH Oh: "Effect of Cooling Rate on ZST, LIT and ZDT of Carbon Steel near Melting Point", ISIJ International Vol. 38 (1998), no. 10, pp. 1093-1099 [0009]

Claims (14)

A method for improving the wear behavior of equipment components in the manufacture and further processing of castings from materials having a stronger segregation tendency, such as high alloy steels, wherein casting and hot working take place in one line comprising a casting machine with a strand or tape guide, coolant for cooling, and means for cutting to length and units for reheating, in which the cast material is conveyed by means of roller conveyors, characterized in that intensive cooling of the cast material takes place in the strand guide of the casting plant and taking into account the casting plant following cooling means, the cooling for the cast material is controlled such that the highest temperature in the cross section of the cast material (perpendicular to the casting direction) the zero toughness temperature (ZDT) at the end of the casting plant and before the first U Molding step, but at least before a mechanical cutting to length of the cast material by means of a pair of scissors, falls below. A method according to claim 1, characterized in that the ZDT for the cast material by means of hot tensile tests after previous melting by measuring the Brucheinschnürung is determined experimentally. A method according to claim 1 or claim 2, characterized in that the intensive cooling of the cast material is controlled such that the temperature profile of the cast material is balanced in cross section. A method according to claim 3, characterized in that for controlling the intensive cooling, the zero toughness temperature (ZDT) is calculated using a micro segregation model, taking into account the underlying cast material of the chemical composition of the melt. A method according to claim 4, characterized in that the calculated zero toughness temperature is transmitted to a control model for the automation of the casting plant and is taken over for the regulation of the cooling and / or the transport speed of the cast material. A method according to claim 5, characterized in that the control model, the cooling water quantity of the casting plant for the secondary cooling and tertiary cooling is controlled so that the condition T _{cross section, max} <zero toughness temperature is met. A method according to claim 6, characterized in that the micro segregation model as part of the control model calculates the solid phase content in the cast material at least as a function of chemical composition and possibly as a function of secondary Dendritenarmabstand and the cooling rate. A method according to claim 7, characterized in that to avoid surface defects, the surface temperature of the cast material in the directional range of the casting plant is higher than 850 ° C. Method according to claim 8, characterized in that the macrosegregation-changed chemical composition is taken into account in the last solidifying region of the cross-section of the cast material, either by simulation of macrosegregations with CFD software or by the determination and application of segregation indices. A method according to claim 9, characterized in that the Seigerungsindizes is determined experimentally based on a cross-sectional sample of the cast material (perpendicular to the casting direction) of a certain material quality, where measured in the region of the maximum segregations concentrations c _{i of} an element i in relation to the nominal concentrations c _{0, i of} the chemical elements are set and thus the segregation indices K _i = C _i / C _{0, i are} determined. Method according to claim 10, characterized in that with the segregation indices the nominal composition c _i = c _{0, i, current} · k _i currently to be cast is corrected and is transferred to the microsegregation model as a correction value for the determination of the ZDT. Apparatus for the production of castings from materials with a greater tendency to segregation, such as high alloy steels, in line using the method according to claims 1 to 11, comprising a caster, a strand or strip guide, a cooling for cooling the cast material, straightening devices and Ablängmittel, as well as means of transport for straightening, cutting and transporting the cast material, characterized in that intensive cooling of the cast material in the strand or strip guide of the casting and taking into account the casting plant subsequent cooling means, wherein the cooling of the cast material so controllable is that the highest temperature in the cross section of the cast material (perpendicular to the casting direction) is the zero toughness temperature (ZDT) at the end of the casting machine and before the first Forming step, but at least before a mechanical cutting to length of the cast material by means of a pair of scissors, at least temporarily for the respective processing step, is below. Means for the production of cast products from materials with an increased tendency for micro segregation in a line according to claim 12, characterized in that during temporary forming, for example during paper cutting, the zero toughness temperature (ZDT) at least in the region defined by the paper cut or the Forming is affected is achieved and next, before and behind lying areas in the casting strand, for the purpose of saving energy in the further processing of the casting strand at a temperature level above the zero toughness temperature (ZDT) remain. Device for the production of a cast product from materials with an increased tendency for micro segregation in a line according to claim 13, characterized in that in the device with a plant concept, in which the strand of the cast material is cast by means of a Zweirollengießmaschine or a strip casting machine, also the areas in which the band is deflected or looped, sheared by scissors and directly rolled, an intensive cooling of the cast material takes place in these areas, taking into account subsequent cooling devices, wherein the cooling of the cast material is controllable such that the highest Temperature in the cross section of the cast material (perpendicular to the casting direction) is below the zero toughness temperature (ZDT) before these areas.

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