Method for the production of a hot-rolled strip from a fine-grained steel material

CROSS-REFERENCE TO RELATED APPLICATION

[0001]This application is a national stage application, filed under 35 U.S.C. § 371, of International Patent Application PCT/EP2022/082238, filed on Nov. 17, 2022, which claims the benefit of German Patent Application DE 10 2021 212 902.1, filed on Nov. 17, 2021.

TECHNICAL FIELD

[0002]The disclosure relates to a method for the production of a hot-rolled strip from a fine-grained steel material with a thickness of d_WB≤1.75 mm and an average ferrite grain size of g_s≤5 μm.

BACKGROUND

[0003]Steel strips are typically produced from fine-grained steel materials in a multi-step method. A hot-rolled strip is initially produced from a slab by means of a plurality of hot forming processes and then wound into a coil. Subsequently, the hot-rolled strip produced in this manner is heat-treated and/or cold-rolled to adjust the thickness, microstructure, and desired mechanical properties of the steel strip. However, on the part of consumers, there is the demand to make well-known multi-step methods more cost-effective and simpler. One option is to adjust the thickness of the steel strip along with the microstructure after hot rolling for the possible end use.

[0004]The rolling of a slab, in particular a thin slab, into a hot-rolled strip with a thickness of d_WB<1.5 mm is known from the prior art. For this purpose, the slab or thin slab is heated to a material-specific forming temperature and rolled out into a steel strip in a hot strip mill with a series of reduction passes. Subsequently, the hot-rolled strip is wound into a coil. The microstructure of the hot-rolled strip and thus the mechanical properties are determined by the cooling conditions after hot forming and cooling of the hot-rolled strip in the coil.

SUMMARY

[0005]In known methods for producing a hot-rolled strip from a fine-grained steel material cooling of the strip in the coil is so slow due to the narrow windings of the strip. A usable microstructure is not achieved immediately, and must first be produced or adjusted by a downstream additional annealing treatment. The present disclosure further develops the known methods for the production of a fine-grained steel material in such a manner that a hot-rolled steel sheet can be used directly, both in terms of its thickness and its microstructure.

[0010]After the last reduction pass before being wound into a coil, the hot-rolled strip is cooled from the hot rolling temperature T_W to a temperature T_H below the transformation temperature of the steel material, using rapid cooling, in particular a compact cooling unit. The transformation temperature T_H is the temperature at which austenite decomposition begins. An approximate transformation temperature T_H can be derived from the average chemical analysis and an associated CCT or TTT diagram. Alternatively, equilibrium models can also be used to simulate the transformation temperature T_H. The transformation temperature T_H determined in this manner may have to be adapted, 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 must be adapted so that the core and transition region of the slab are preferably taken into account in the transformation temperature T_H.

[0011]The cooling of the hot-rolled strip is effected by rapid cooling 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-rolled strip by rapid cooling begins within a period of t≤0.2 s, more preferably t≤0.1 s, after the last reduction pass.

[0012]The combination of the features of cooling rate, thickness of the hot-rolled strip and cooling to a temperature T_H below the transformation temperature of perlite, bainite and/or martensite in accordance with the invention produces a hot-rolled strip with the described yield strengths of 300 MPa to 400 MPa at a thickness of d_WB<1.5 mm and 400 MPa to 500 MPa at a thickness of d_WB≥1.5 mm and ≤1.75 mm.

[0013]The cooling of the hot-rolled strip is preferably effected 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 reduction pass. The closer the rapid cooling begins after the last reduction pass, the better grain growth can be reduced or prevented by the rapid cooling in accordance with the invention. With fine-grain steel in particular, the focus must be on suppressing undesirable grain growth.

[0014]It is preferable if the cooling to a temperature below the transformation temperature T_H is effected using water as the coolant. Water is a standardized coolant that is easy to transport and provide in terms of process technology. The cooling water used may also contain additives that modify the cooling properties of the water. Gases, in particular air, are also understood to be additives to the cooling water within the meaning of the invention. Thereby, it is irrelevant whether the gases are used to transport the cooling water or to atomize the cooling water downstream or by means of a spray nozzle, for example. Additives within the meaning 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.

[0015]Upon cooling, a relative water flow rate of V≥0.002 m³/kg, preferably V≥0.004 m³/kg, in relation to the mass flow of the hot-rolled strip, is preferably set. In this range, sufficient water is provided for the desired cooling effect without unduly burdening the water management of a hot strip mill.

[0016]A control or regulation system comprising at least one process model specifies a target value for the cooling rate prior to the last reduction pass and/or adapts it during the hot forming of the hot-rolled strip. Thereby, the process model simulates, preferably online, the microstructure development in the course of the hot rolling process on the basis of the chemical analysis of the hot-rolled strip to be rolled and other process parameters. Within the meaning of the invention, process parameters are understood to mean all process parameters that are directly or indirectly associated with the production of a hot-rolled strip in a hot strip mill. Direct process parameters include, for example, rolling speed, slab temperature, chemical analysis, or sampling. Indirect process parameters include, for example, roll wear, coolant composition, or system conditions. Simulation models that simulate a microstructure on the basis of chemical analyses and known temperature curves are known from the prior art. The control and regulation of the rolling mill determines possible temperature curves of the hot-rolled strip on the basis of existing target specifications or actual values by means of known temperature models. This is preferably effected cyclically during the ongoing process. The actual microstructure of the hot-rolled strip is also simulated cyclically from such temperature profiles using the microstructure 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 mill or the pass reduction, are adapted by the control or regulation system.

[0017]Furthermore, by means of an optimization algorithm, the process model determines the target value for the cooling rate to be adjusted at which the target microstructure, in particular the ferrite grain size, is achieved. Such a control or regulation system improves the adjustment of the mechanical properties of the finished hot-rolled strip through the targeted adjustment of the microstructure development in the course of the hot-rolling process. Possible fluctuations can be better compensated for and optimized by the control system with the help of the process model.

[0018]It is preferable if a microstructure sensor determines the microstructure composition of the hot-rolled strip and the process model takes the measured actual microstructure composition into account upon determining the target value for the cooling rate. The use of a microstructure sensor at a point within the hot strip mill makes it possible not only to determine possible microstructural developments on the basis of a chemical analysis, but also to take into account an actual state of the microstructure upon the prediction of the microstructural development. As a result, the determination of the target value for the cooling rate is more accurate and the deviation is smaller.

[0019]The hot-rolled strip preferably consists of a steel material with the analysis

[0020]This type of steel material is particularly suitable due to its transformation behavior and basic mechanical properties.

[0021]The Al/N ratio is preferably between 1 and 10, more preferably between 1 and 8. An Al/N ratio adjusted in this manner reduces both edge cracks upon the solidification of the slab and edge crack sensitivity in the first forming passes in the rolling mill.

[0022]The hot-rolled strip temperature of the hot-rolled strip prior to the last reduction pass prior to rapid cooling is preferably at least 50° C., more preferably at least 30° C. and a maximum of 100° C., above the Ae3 temperature of the alloy of the hot-rolled strip. As a result, it is ensured that ferrite formation in the hot-rolled strip is only effected at the beginning of rapid cooling and that the more easily formable austenite is present in the rolling stands upon the forming of the hot-rolled strip.

[0023]Furthermore, the object of the invention is achieved by a hot-rolled strip that is produced by a method in accordance with one of the claims 1 to 9. The finished hot-rolled strip has a hot-rolled strip thickness of d_WB<1.5 mm, preferably 1.2 mm, a tensile strength R_m of 300 to 400 mPA and a yield strength of R_e≥340 mPA. Furthermore, the object of the invention is achieved by a hot-rolled strip produced by a method in accordance with one of the claims 1 to 7, wherein the hot-rolled strip has a hot-rolled strip thickness of d_WB≤1.75 mm, preferably <1.4 mm, and has a tensile strength R_m of 400 to 500 mPA and a yield strength of R_e≥340 mPA.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1: Example of a casting-rolling line.

[0025]FIG. 2: Exemplary temperature curve of a hot-rolled strip made of a fine-grain material in the course of the process.

[0026]FIG. 3: Degrees of deformation in the course of the process.

[0027]FIG. 4a: Conventional microstructure of a fine-grained steel.

[0028]FIG. 4b: Microstructure with the method in accordance with the disclosure.

DETAILED DESCRIPTION

[0029]The invention is described in detail below with reference to the figures. Identical technical elements are marked with the same reference signs in all figures.

[0030]FIG. 1 shows the schematic structure of a casting-rolling line for the production of a hot-rolled strip. The continuous casting line 1 produces a slab or thin slab from a liquid melt. In the first furnace 2, the slab is heated to the temperature prior to the first pass in the pre-rolling mill 3. A further equalizing furnace 4 is located between the pre-rolling mill 3 and the finishing stands 5. A compact cooling unit 6 in accordance with the invention is arranged behind the last finishing rolling stand 4, 5. This is able to provide sufficient cooling liquid to adjust the desired cooling speeds of the strip. After cooling, the strip is rolled up into a coil on the reel 6.

[0031]FIG. 2 shows a diagram with the temperature curve a of a hot-rolled strip from the first pass in the roughing rolling stand until winding into a coil in the reel. Starting from a passing temperature of 1,120° C., for example, the hot-rolled strip is rolled in a plurality of steps to a thickness of ≤1.6 mm. Without any further influences, a temperature curve is thereby established, for example according to curve a. The furnace between the roughing rolling stands and the first pass in the finishing rolling stand is not used to heat the roughing strip, but to homogenize the temperature between the core and the outer layer of the roughing strip. After the last forming step in the last finishing rolling stand, the finished hot-rolled strip is cooled to a temperature≤520° C. by a compact cooling unit. Subsequently, cooling to an exemplary coiling temperature of 150° C. is effected using laminar cooling.

[0032]Table 1 shows an example of an analysis of a fine-grained material. The technically possible degrees of deformation in the individual reduction passes in this analysis and an exemplary actual degree of deformation are shown in the diagram in FIG. 3. It can be seen here that the forming work can substantially be effected in the first four stands. The possible degrees of deformation then decrease, which has a positive effect on the tolerances of the finished hot-rolled strip. As a result, this method can adjust and maintain a fine grain from the beginning of the forming.

[0033]FIG. 4a) and FIG. 4b) show microstructure cross-sections in each case of a rolled hot-rolled strip. Both hot-rolled strips consist of the same alloy, i.e. they are rolled from slabs of a single batch. FIG. 4a) shows the cross-section of a conventionally produced hot-rolled strip. FIG. 4b) shows the cross-section of a hot-rolled strip produced in accordance with the invention. In each case, both hot-rolled strips were thereby wound into a coil after hot rolling. A comparison of the microstructure cross-sections shows that the method in accordance with the invention produces a significantly finer grain directly after hot rolling. The average grain size amounts to 5.5 μm in FIG. 4a) and 4.4 μm in FIG. 4b). The microstructure shown in FIG. b) allows the hot-rolled strip to be used directly without further downstream heat treatment.