DE102021212902A1 - Process for producing a hot strip from a fine-grain steel material - Google Patents

Abstract

The invention relates to a method for producing a hot strip from a fine-grain steel material with a thickness dWB≦1.75 mm and an average ferrite grain size of gs≦5 μm. A previously necessary heat treatment of the hot strip to adjust the mechanical properties is avoided by an optimized rolling and cooling strategy.

Description

Area:

The invention relates to a method for producing a hot strip from a fine-grained steel material with a thickness d _WB ≦1.75 mm and an average ferrite grain size of g _s ≦5 μm.

State of the art:

Steel strips are usually manufactured from fine-grained steel materials in a multi-step process. A hot strip is first produced from a slab by means of several hot forming operations and then wound up into a coil. The hot strip produced in this way is then heat-treated and/or cold-rolled, thereby adjusting the thickness, the structure and the desired mechanical properties of the steel strip. On the part of the consumer, however, there is a requirement to make the known multi-step process more economical and simpler. One possibility is to adjust the thickness of the steel strip as well as the structure after hot rolling for the possible end use.

The rolling of a slab, in particular a thin slab, into a hot strip with a thickness d _WB <1.5 mm is known from the prior art. To do this, the slab or thin slab is heated to a material-specific forming temperature and rolled into a steel strip in a hot strip mill with a series of acceptance passes. The hot strip is then wound into a coil. The microstructure of the hot strip and the associated mechanical properties are set by the cooling conditions after hot forming and the cooling of the hot strip in the coil.

The disadvantage of this known method is that the cooling in the coil is so slow due to the narrow windings of the strip that the usable microstructure is not reached immediately and must first be produced or set by an additional subsequent annealing treatment. The object of the invention is therefore to further develop the known methods for producing a fine-grained steel material such that a hot-rolled steel sheet can be used directly both in terms of its thickness and its structure.

Object of the invention:

• • Erwärmen eines Vorproduktes, insbesondere einer Bramme oder Dünnbramme, auf die Umformtemperatur;
• • Warmwalzen des Vorproduktes in einer Warmbandstraße zu einem Stahlband mit mehr als 2 Abnahmestichen, insbesondere mit mehr als 5 Abnahmestichen;
• • Aufwickeln des Warmbandes zu einem Coil

hergestellt.The object of the invention is achieved by a method with the features of claim 1 and a hot strip with the features of claim 10 or claim 11. A hot strip made of a fine-grain steel material is obtained at least through the work steps:

• • heating a preliminary product, in particular a slab or thin slab, to the forming temperature;
• • Hot rolling of the preliminary product in a hot strip mill to form a steel strip with more than 2 acceptance passes, in particular with more than 5 acceptance passes;
• • Winding of the hot strip into a coil

manufactured.

After the last acceptance pass, before being wound up into a coil, the hot strip is cooled from the hot rolling temperature Tw to a temperature T _H below the transformation temperature of the steel material using rapid cooling, in particular compact cooling. The transformation temperature T _H is the temperature at which austenite decomposition begins. An approximate transition temperature T _H can be read from the mean chemical analysis and an associated ZTU or ZTA diagram. Alternatively, equilibrium models can also be used to simulate the transition temperature T _H . The transformation temperature T _H determined in this way may have to be adjusted, since the segregation of the chemical elements during solidification can cause local deviations in the chemical analysis. This then shifts the local decomposition temperature and can thus shift it to higher or lower local transformation temperatures. The transformation temperature T _H is to be adjusted in such a way that the core and transition area of the slab are preferably taken into account at the transformation temperature T _H .

The cooling of the hot strip by rapid cooling takes place at a relative cooling rate a _R of a _R ≧600 K/(s·mm), more preferably a _R ≧800 K/(s mm). In addition, the cooling of the hot strip by rapid cooling begins within a period of t≦0.2 s, more preferably t≦0.1 s, after the last acceptance pass.

The inventive combination of the features cooling rate, thickness of the hot strip and cooling to a temperature T _H below the transformation temperature of pearlite, bainite and/or martensite produces a hot strip with the described yield strength of 300 MPa to 400 MPa with a thickness d _WB <1.5 mm and 400 MPa to 500 MPa with a thickness d _WB ≥ 1.5 mm and ≤ 1.75 mm.

Preferred forms of the method are presented in claims 2 to 9 which are dependent on claim 1. The hot strip is preferably cooled from the hot-rolling temperature T _W to a temperature below the transformation temperature T _H within a distance of ≦6 m, preferably ≦4 m, after the last acceptance pass. The closer the rapid cooling begins after the last acceptance pass, the better grain growth can be reduced or prevented by the rapid cooling according to the invention. In the case of fine-grain steel in particular, attention must be paid to suppressing unwanted grain growth.

It is preferred if the cooling to a temperature below the transformation temperature T _H takes place using water as the coolant. Water is a standardized coolant here, easy to transport and to provide in terms of process technology. The cooling water used can also contain additives that modify the cooling properties of the water. Gases, in particular air, are also understood as additives to the cooling water in the sense of the invention. It is irrelevant whether the gases are used to transport the cooling water or to atomize the cooling water after or by means of a spray nozzle, for example. Additives in the sense of the invention can also be chemical substances that are suitable for modifying the boiling point or other physical or chemical properties of the cooling water.

A relative water volume flow of V≧0.002 m ³ /kg, preferably V≧0.004 m ³ /kg, based on the mass flow of the hot strip, is preferably set during cooling. Sufficient water is provided in this area for the desired cooling effect without overburdening the water management of a hot strip mill.

A controller or regulator having at least one process model specifies a target value for the cooling rate before the last acceptance pass and/or adjusts it during the hot forming of the hot strip. The process model simulates, preferably online, the microstructure development during the hot rolling process on the basis of the chemical analysis of the hot strip to be rolled and other process parameters. In the sense of the invention, process parameters are understood to mean all process parameters that are directly or indirectly connected to the production of a hot strip in a hot strip mill. Direct process parameters are, for example, the rolling speed, slab temperature, chem. Analysis or sample acceptance, indirect process parameters are, for example, roll age, cooling water composition or plant conditions. Simulation models that simulate a structure based on chemical analyzes and known temperature profiles are known from the prior art. The control and regulation of the rolling train determines possible temperature profiles of the hot strip on the basis of existing target specifications or actual values using known temperature models. This preferably takes place cyclically during the running process. The actual structure of the hot strip is also cyclically simulated from these temperature profiles by the structure model. If the actual microstructure deviates from the target microstructure, the target specifications, for example the intensity of the cooling at different points in the rolling train or the pass reduction, are adjusted by the control or regulation.

Furthermore, the process model uses an optimization algorithm to determine the target value for the cooling rate to be set, with which the target structure, in particular the ferrite grain size, is reached. Such a control or regulation improves the setting of the mechanical properties of the finished hot strip through the targeted setting of the structure development in the course of the hot rolling process. The controller can better compensate and optimize possible fluctuations with the help of the process model.

It is preferred if a structure sensor determines the structure composition of the hot strip and the process model determines the measured actual structure composition when determining the target value of the cooling speed taken into account. The use of a microstructure sensor at one point within the hot strip mill makes it possible not only to determine possible microstructure developments on the basis of a chemical analysis, but also to take into account an actual state of the microstructure when predicting the microstructure development. This makes the target value determination for the cooling rate more accurate and the deviation smaller.

The hot strip consists preferably of a steel material with the analysis • C: 0.05% to 0.20%, preferably 0.05% to 0.10% • Si: 0.01% to 0.50%, preferably 0.05% to 0.20% • Mn: 0.30% to 2.20%, preferably 0.40% to 1.80% • Al: 0.015% to 0.075%, preferably 0.015% to 0.035% • N: 0.000% to 0.050%, preferably 0.001% to 0.025% • Nb: 0.00% to 0.10%, preferably 0.01% to 0.06% • Ti: 0.00% to 0.12%, preferably 0.01% to 0.10% • V: 0.00% to 0.10%, preferably 0.01% to 0.06% • Mon: 0.00% to 0.35%, preferably 0.01% to 0.10% • Approx: 0.005% to 0.035%, preferably 0.005% to 0.025% • Remainder Fe and unavoidable impurities during production.

Such a steel material is particularly suitable due to its transformation behavior and basic mechanical properties.

The Al/N ratio is preferably between 1 and 10, more preferably between 1 and 8. An Al/N ratio adjusted in this way reduces both the edge cracks during the solidification of the slab and the sensitivity to edge cracks in the first forming passes in the rolling train.

The hot strip temperature of the hot strip is preferably at least 50° C., more preferably at least 30° C. and at most 100° C., above the Ae3 temperature of the alloy of the hot strip before the last acceptance pass before rapid cooling. This ensures that the formation of ferrite in the hot strip does not take place until rapid cooling begins and that the more easily deformable austenite is present during the forming of the hot strip in the roll stands.

Furthermore, the object of the invention is achieved by a hot strip that is produced by a method according to one of claims 1 to 9. The finished hot strip has a hot strip thickness of d _WB <1.5 mm, preferably 1.2 mm and a tensile strength R _m of 300 to 400 mPa and a yield point _Re ≥340 mPa. Furthermore, the object of the invention is achieved by a hot strip, produced by a method according to one of claims 1 to 7, wherein the hot strip has a hot strip thickness of d _WB ≤ 1.75 mm, preferably <1.4 mm and a tensile strength R _m of 400 to 500 mPa and a yield strength of _Re ≥ 340 mPa.

• 1: Beispiel einer Gießwalzanlage;
• 2: Beispielhafter Temperaturverlauf eines Warmbandes aus einem Feinkornwerkstoff im Prozessverlauf; und
• 3: Umformgrade im Prozessverlauf
• 4:
  1. a) herkömmliches Gefüge eines Feinkornstahls;
  2. b) Gefüge mit erfindungsgemäßem Verfahren.

Three figures accompany the description of the invention.

• 1 : Example of a casting and rolling plant;
• 2 : Exemplary temperature profile of a hot strip made of a fine-grain material during the course of the process; and
• 3 : degrees of deformation in the course of the process
• 4 :
  1. a) conventional structure of a fine-grain steel;
  2. b) Microstructure using the method according to the invention.

The invention is described in detail below with reference to the figures. The same technical elements are provided with the same reference symbols in all figures.

1 shows the schematic structure of a casting and rolling plant for the production of a hot strip. The continuous casting plant 1 produces a slab or thin slab from a liquid melt. In the first furnace 2, the slab is heated to the temperature before the first tapping in the roughing train 3. Between the roughing train 3 and the finishing stands 5 there is another equalizing furnace 4. Behind the last finishing rolling stand 4, 5 there is a compact cooling system 6 according to the invention. This is able to provide sufficient cooling liquid to set the desired cooling speed of the strip. After cooling, the strip is rolled up on the coiler 6 to form a coil.

2 shows a diagram with the temperature curve a of a hot strip from the first tapping in the roughing stand to winding up into a coil in the coiler. Starting from a tapping temperature of 1,120°C, the hot strip is rolled in several steps to a thickness of ≤ 1.6 mm. Without further influences, a temperature profile according to curve a is established. The furnace between the roughing stands and the first pass in the finishing stand is not used to heat up the pre-strip, but to homogenize the temperature between the core and the outer layer of the pre-strip. After the last forming step in the last finishing stand, the finished hot strip is cooled down to a temperature of ≤ 520°C by a compact cooling system. This is followed by cooling down to the coiling temperature of 150° C. by laminar cooling. Table 1: Analysis of fine-grain material in % by weight chemical element C si Mn P S Cr no A N Mon Ti V Nb Approx Salary 0.069 0.06 0.92 0.012 0.003 0.04 0.03 0.041 0.005 0.01 0.001 0.001 0.047 0.02

Table 1 shows an example of an analysis of a fine-grain material. The degrees of deformation that are technically possible in this analysis in the individual acceptance passes and an example of the actual degree of deformation are shown in the diagram 3 shown. It can be seen here that the forming work can essentially take place in the first four stands. The possible degrees of deformation then decrease, with this having a positive effect on the tolerances of the finished hot strip. Thereby, this method can adjust and maintain a fine grain from the beginning of the working.

4 a) and 4 b) show microsections of a rolled hot strip. Both hot strips consist of the same alloy, ie they are rolled from slabs from a single batch. Figure a) shows the cut of a conventionally produced hot strip. FIG. b) shows the cut of a hot strip produced according to the invention. Both hot strips were each wound into a coil after the hot rolling. A comparison of the microstructures shows that the method according to the invention produces a significantly finer grain directly after the hot rolling. The mean grain boundary is 5.5 μm in figure a) and 4.4 μm in figure b). The microstructure set in FIG. b) allows the hot strip to be used directly without further subsequent heat treatment.

Reference List

1

continuous caster

2

Oven

3

roughing mill

4

Oven

5

finishing stand

6

compact cooling

7

reel

Claims (11)

Process for producing a hot strip from a fine-grain steel material with a thickness d _WB and an average ferrite grain size of g _s ≤ 5 µm, the hot strip produced having a thickness d _WB < 1.5 mm and a yield point of 300 MPa to 400 MPa and with a thickness d _WB ≥ 1.5 mm and ≤ 1.75 mm has a yield strength of 400 MPa to 500 MPa, in which at least the following steps are carried out: - Heating a preliminary product, in particular a slab or thin slab, to the forming temperature; - Hot rolling of the preliminary product in a hot strip mill with more than 2 acceptance passes, in particular with more than 5 acceptance passes, to form a hot strip with a thickness of d _WB < 1.5 mm or a thickness d _WB ≥ 1.5 mm and ≤ 1.75 mm ; - Winding the hot strip into a coil; characterized in that - the hot strip after the last acceptance pass before being wound up into a coil with rapid cooling, in particular compact cooling, from the hot rolling temperature Tw to a temperature T _H below the transformation temperature of the hard phases, in particular pearlite, bainite and/or martensite, is cooled; and - the hot strip is cooled by rapid cooling at a relative cooling rate a _R of a _R ≧600 K/(s·mm), preferably a _R ≧800 K/(s·mm); - The cooling of the hot strip by rapid cooling begins within a period of ≦0.2 s, preferably ≦0.1 s, after the last acceptance pass. procedure after claim 1 , characterized in that the hot strip is cooled from the hot rolling temperature Tw to a temperature below the transformation temperature T _H within a distance of ≤ 6 m, preferably ≤ 4 m, after the last acceptance pass. Method according to one of the preceding claims, characterized in that the cooling to a temperature below the transformation temperature T _H takes place with water as coolant. procedure after claim 3 , characterized in that during the cooling, a relative volume flow of water V ≥ 0.002 m ³ /kg, preferably V ≥ 0.004 m ³ /kg, based on the mass flow of the hot strip, is set. Method according to one of the preceding claims, characterized in that - a controller or regulator, having a process model, specifies a target value for the cooling rate before the last acceptance pass and/or adjusts it during hot forming; and - the process model based on the chemical analysis of the hot strip and other process parameters simulates the structure development during the hot rolling process; and - the process model uses an optimization algorithm to determine the target value for the cooling rate with which the target microstructure, in particular the ferrite grain size, is reached. procedure after claim 5 , characterized in that - a microstructure sensor determines the microstructure of the hot strip; and - the process model takes into account the measured actual microstructure composition when determining the desired value for the cooling rate. Method according to one of the preceding claims, characterized in that the hot strip consists of a steel material with the analysis • C: 0.05% to 0.20%, preferably 0.05% to 0.10% • Si: 0.01% to 0.50%, preferably 0.05% to 0.20% • Mn: 0.30% to 2.20%, preferably 0.40% to 1.80% • Al: 0.015% to 0.075%, preferably 0.015% to 0.035% • N: 0.000% to 0.050%, preferably 0.001% to 0.025% • Nb: 0.00% to 0.10%, preferably 0.01% to 0.06% • Ti: 0.00% to 0.12%, preferably 0.01% to 0.10% • V: 0.00% to 0.10%, preferably 0.01% to 0.06% • Mon: 0.00% to 0.35%, preferably 0.01% to 0.10% • Approx: 0.005% to 0.035%, preferably 0.005% to 0.025% • Remainder Fe and unavoidable impurities during manufacture. consists. procedure after claim 7 , characterized in that the Al/N ratio is between 1 and 10, preferably between 1 and 8. Method according to one of the preceding claims, characterized in that the hot strip temperature of the hot strip before the last acceptance pass before rapid cooling is at least 50°C, preferably at least 30°C and at most 100°C, above the Ae3 temperature of the alloy of the hot strip. Hot strip produced by a method according to any one of Claims 1 until 9 , characterized in that - the hot strip has a hot strip thickness of d _WB <1.5 mm, preferably ≤1.2 mm; and - has a tensile strength R _m of 300 MPa to 400 MPa and a yield strength R _e ≥ 340 MPa. Hot strip produced by a method according to any one of Claims 1 until 9 , characterized in that - the hot strip has a hot strip thickness of d _WB ≤ 1.75 mm, preferably ≤ 1.4 mm; and - has a tensile strength R _m of 400 MPa to 500 MPa and a yield strength R _e ≥ 340 MPa.

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