Method for reducing a deviation from a target tensile strength and a target yield strength / replacement limit of rolled goods

Abstract

Methods for reducing deviations from a target tensile strength (Rm_target) and a target yield strength / equivalent yield strength (Re_target / Rp0.2_target, Rt0.5_target etc.) of rolled products (4), in particular steel sheets, produced by a plant (1) comprising at least one hot rolling mill (2) and at least one roller straightening machine (3) downstream of the hot rolling mill (2), and of single- or multi-layer rolled slabs or rolled blocks of a common steel grade, the respective steel alloy of which deviates in at least one alloying element from the target chemical composition of this steel grade due to the manufacturing process, are shown.

Description

[0001] The invention relates to a method for reducing a deviation from a target tensile strength and a target yield strength / equivalent yield strength of rolled goods, in particular steel sheets, produced by a plant having at least one hot rolling mill and at least one roller straightener arranged downstream of the hot rolling mill, single-layer or multi-layer rolled slabs or rolled blocks of a common steel grade fed from this plant, the respective steel alloy in the actual chemical composition of which deviates from the target chemical composition of this steel grade in at least one alloying element due to the production process.

[0002] It is known from EP30096896B1 that by changing at least one nominal operating parameter of a rolling mill, changes can be made to the alloy of a rolling slab to be fed to this rolling mill without having to expect a significant change in the strength of the rolled goods produced therewith, which can save alloying costs.

[0003] DE 19881711B4 discloses a method in which the nominal operating parameters of a rolling mill are adjusted to achieve the desired mechanical material parameters in the finished rolled stock. For example, the inlet temperature, the outlet temperature, and the reduction ratios of the individual rolling stands are changed. For this purpose, a microstructure observer is used to calculate the expected mechanical material parameters from at least the specified nominal operating parameters of the plant and the actual chemical composition of the rolled slab. Adjusted nominal operating parameters of a rolling mill can, however, have both a positive effect on certain mechanical material parameters and a detrimental effect on another mechanical material parameter. This limits the effectiveness of the method for achieving the desired mechanical material parameters in the finished rolled stock.

[0004] The object of the invention is therefore to create a method in which both a specified target tensile strength and a target yield strength / equivalent yield strength can be reproducibly achieved, despite alloy contents of rolled slabs or blooms of a steel alloy that vary within certain limits, for example, varying in the alloy content. Furthermore, the method should be easy to handle and also implementable in existing plants with a rolling mill and a roller straightener.

[0005] The invention solves the problem by the features of claim 1.

[0006] By calculating a model tensile strength and a model yield strength / equivalent yield strength of the rolled stock based on a mathematical model that is based at least on the nominal operating parameters of the plant specified for this steel grade, as well as on the deviating actual chemical composition of the rolled slab or ingot to be fed into the plant, all the prerequisites can be created to reproducibly reduce deviations from both a target tensile strength and a target yield strength / equivalent yield strength. To do this, it is only necessary to subsequently adjust several operating parameters of the plant if there is a difference between the model tensile strength and the target tensile strength and between the model yield strength / equivalent yield strength and the target yield strength / equivalent yield strength, in order to reduce the deviation from the target tensile strength and the target yield strength / equivalent yield strength. According to the invention, these adjusted operating parameters include: Increasing or decreasing a cooling stop temperature, in particular averaged over the thickness of the rolled stock, at which the accelerated cooling of the rolled stock is interrupted after finish rolling, as the first adjusted operating parameter on the hot rolling mill of the plant; increasing or decreasing an immersion depth of a straightening roller in the inlet, in particular of a second straightening roller in a first straightening triangle of the inlet, of the roller straightener as the second adjusted operating parameter on the roller straightener of the plant;

[0007] In contrast to the state of the art, this involves changing at least two of the plant's nominal operating parameters: the hot rolling mill and the roller leveler. Hot rolling and leveling can be performed continuously, for example, by placing the roller leveler directly downstream of the hot rolling mill, or discontinuously.

[0008] This allows for a reliable reduction of deviations, which could, for example, be due to manufacturing-related variation, from both a target tensile strength and a target yield strength / equivalent yield strength in rolled goods produced with this method. This leads to high reproducibility in achieving a specified target tensile strength and target yield strength / equivalent yield strength and is also relatively easy to implement in existing rolling mills whose operating parameters are adjustable. This also makes the process easy to handle.

[0009] It goes without saying that accelerated cooling refers to cooling more rapidly than cooling in still air. For example, accelerated cooling is quenching, particularly water quenching.

[0010] It is generally mentioned that tensile strength and yield strength / offset yield strength are material properties determined in uniaxial tensile tests. The equivalent yield strength is also referred to as the proof strength. The yield strength material property is primarily used for materials that, for example, do not have a pronounced yield point. In addition, the yield point is the mechanical stress up to which a material is elastically deformable. When the yield point is reached / exceeded, a plastic deformation remains. There are different definitions that depend on the residual strains considered and the type of determination in terms of a parallel displacement of the Hooke's line or a normal through the remaining strain. This results in typical examples of equivalent yield points (proof points), such as Rp0.2, Rp1.0, Rt0.5, Rt1.0, etc.

[0011] The reproducibility of the process can be further increased if the adjusted operating parameters additionally include: increasing or decreasing a cooling rate of an accelerated cooling of the rolling stock after finish rolling as a third adjusted operating parameter on the hot rolling mill of the plant;

[0012] Preferably, the cooling rate averaged over the cooling time is used here. This provides the process according to the invention with, among other things, an additional control element to counteract deviations from the target chemical composition, further improving the process.

[0013] For example, if there is a negative difference between the model tensile strength and the target tensile strength, the first operating parameter can be adjusted by decreasing it and the second operating parameter by increasing it.

[0014] It is also conceivable that in the case of a positive difference between the model tensile strength and the target tensile strength, both the first operating parameter can be adjusted by increasing it and the second operating parameter by decreasing it.

[0015] Preferably, an optimization calculation is used to calculate the increase or decrease in the first operating parameter compared to the respective nominal operating parameter and the decrease or increase in the second operating parameter compared to the respective nominal operating parameter, and optionally the increase or decrease in the third operating parameter compared to the respective nominal operating parameter, at which the differences between the model tensile strength and the target tensile strength and between the model yield strength / equivalent yield strength and the target yield strength / equivalent yield strength are the smallest. According to the invention, an efficient solution can thus be found in which both the deviation from a target tensile strength and a target yield strength / equivalent yield strength are the smallest. This can further improve the reproducibility of the process in the production of rolled goods.

[0016] For example, in the optimization calculation, the first operating parameter can be adjusted by increasing the relevant nominal operating parameter in order to lower the model tensile strength and increase the model yield strength / equivalent yield strength. The cooling stop temperature, particularly averaged over the thickness of the rolled stock, can be particularly suitable as the first operating parameter.

[0017] However, it is also conceivable to use, in particular, the cooling stop temperature averaged over the thickness of the rolled stock as the first operating parameter, to adapt the first operating parameter in the optimization calculation by lowering the relevant nominal operating parameter in order to increase the model tensile strength and lower the model yield strength / equivalent yield strength.

[0018] Preferably, the optimization calculation uses the immersion depth of a straightening roll at the inlet as a second operating parameter. This is because, in the optimization calculation, the second operating parameter is adjusted by increasing the relevant nominal operating parameter in order to increase the model tensile strength and the model yield strength / equivalent yield strength.

[0019] As an alternative to the above increase, it is conceivable that in the optimization calculation the second operating parameter is adjusted by lowering the relevant nominal operating parameter in order to thereby lower the model tensile strength and the model yield strength / equivalent yield strength.

[0020] The optimization calculation can also use the averaged cooling rate as a third operating parameter if necessary. This is achieved by adjusting the third operating parameter during the optimization calculation by increasing the corresponding nominal operating parameter in order to increase the model tensile strength and the model yield strength / equivalent yield strength.

[0021] As an alternative to the above increase, it is conceivable that in the optimization calculation the third operating parameter is adjusted by lowering the relevant nominal operating parameter in order to thereby lower the model tensile strength and model yield strength / equivalent yield strength.

[0022] For example, it may be advantageous for the method according to the invention if the optimization calculation is carried out using a mathematical optimization method. It is conceivable that this could be a discrete optimization method.

[0023] Preferably, the first operating parameter is adjusted within a range of plus / minus 100 °C to the respective nominal operating parameter (P_nominal). This allows, for example, a nominal operating parameter of the cooling stop temperature (KST_nominal) of 380 °C and the permissible control limits of plus / minus 100 °C to be adjusted between 280 °C and 480 °C.

[0024] Preferably, the second operating parameter is adjusted within a range of plus / minus 30% to the relevant nominal operating parameter (P_nominal).

[0025] The invention also has the object of creating a system with which the method according to the invention can be carried out reliably.

[0026] The invention solves the problem by claim 15.

[0027] The method according to the invention can be carried out reliably if a plant is used with a hot rolling mill, which has a cooling device with a first actuator for setting a cooling stop temperature for accelerated cooling of the rolling stock after finish rolling, with a roller straightener arranged downstream of the hot rolling mill, which has a second actuator for setting the immersion depth of a straightening roller in the inlet of the roller straightener, and with a control or regulating device which has the mathematical model and is connected to the actuators of the cooling device and the roller straightener for transmitting adapted nominal operating parameters.

[0028] For example, the cooling device has a third actuator for adjusting a cooling rate of the accelerated cooling of the rolling stock after finish rolling.

[0029] The figures show, for example, the subject matter of the invention in more detail using an embodiment variant. Fig. 1 shows an inventive representation of a method for reducing a deviation from a target tensile strength and a target yield strength / equivalent yield strength of rolled stock produced with a hot rolling mill, Fig. 2 shows distribution functions for tensile strengths Rm of rolled stock of the same steel grade with and without application of the inventive method, and Fig. 3 shows distribution functions for equivalent yield strengths Rt0.5 of the Fig. 2 shown rolled goods with and without application of the method according to the invention, Fig. 4 a flow diagram of the hot rolling of the hot rolling mill and Fig. 5 a schematic representation of a roller straightening machine.

[0030] After Fig. 1The method according to the invention is illustrated, for example, for a rolled stock 4 produced using a plant 1 comprising a hot rolling mill 2 and a roller straightener 3 arranged downstream of the hot rolling mill 2. The hot rolling mill 2 has at least one rolling stand 2a for, for example, reversing hot rolling of the rolled stock 4 and at least one cooling device 2b for accelerated cooling of the rolled stock 4 after finish rolling by the at least one rolling stand 2a.

[0031] Since the chemical composition of the rolled slabs or rolling blocks fed into plant 1 deviates from the chemical target composition of this steel grade in several alloying elements due to the manufacturing process, a required target tensile strength Rm_soll and a required target equivalent yield strength Rt0.5_soll can no longer be achieved.

[0032] Although the exemplary embodiment specifically addresses the offset yield strength Rt0.5, the invention is not limited to this. According to the invention, it can also be a yield strength Re, ReH, ReL, or another offset yield strength Rp0.2, Rp1.0, Rt0.5, Rt1.0, etc. Furthermore, any combination of yield strengths and equivalent yield strengths is conceivable.

[0033] In this application example of the process, steel grade X65 is used, and the proportion of the alloying elements carbon (C), manganese (Mn), and chromium (C) is considered, for example. Other alloying elements that may also be relevant for the material properties are not considered in this example to avoid making the description of the process according to the invention unnecessarily complicated.

[0034] The target chemical composition of steel grade X65 is 0.032 wt% C (carbon), 1.65 wt% Mn (manganese), 0.18 wt% Cr (chromium), and other alloying elements. Typically, in the production of numerous heats, the deviations in the chemical composition from the target composition will be within the permissible range: C: plus / minus 0.006 wt%, Mn: plus / minus 0.05 wt%, Cr: plus / minus 0.02 wt%.

[0035] Exemplary rolled slabs W0-W5 of steel grade X65 have a chemical composition as shown in Table 1 below. Table 1: Rolled slabs supplied with variations in chemical composition rolled slab Rolled stock ΔC ΔMn ΔCr (sheet metal) % by weight % by weight % by weight W0 0(W0) 0,000 0,00 0,00 W1 A(W1) -0,004 -0,03 -0,01 W2 B(W2) -0,004 -0,03 -0,01 W3 C(W3) -0,004 -0,03 -0,01 W4 D(W4) 0,003 0,02 0,01 W5 E(W5) 0,003 0,02 0,01

[0036] A comparison of the chemical compositions of rolled slabs W1 to W5 shows the deviations from the target chemical composition exhibited by rolled slab W0. Rolled slabs W4 and W5 exhibit a different deviation in their chemical composition than rolled slabs W1-W3.

[0037] All rolled slabs are subjected to a forming process on the same plant 1 (rolling mill 2 + roller straightener 3) to obtain rolled goods or sheets 0, A, B, C, D, and E. These are steel sheets of steel grade X65 with a sheet thickness of 20 mm (millimeters). The target tensile strength Rm_target is 600 N / mm 2 and the target equivalent yield strength Rt0.5_target is 490 N / mm 2 . The rolled goods A(W1), B(W2), and C(W3) are produced from rolled slabs W1, W2, and W3, which deviate from the target chemical composition according to Table 1.

[0038] Here, the tensile strength Rm of the finished rolled stock A(W1) (without adjustment of the nominal operating parameters) is reduced by ΔRm_chem and the equivalent yield strength Rt0.5 by ΔRt0.5_chem. This is shown in Fig.1 to recognize.

[0039] Thus, rolled goods A(W1) and D(W4), namely steel sheets with a sheet thickness of 20 mm (millimeters), of the same steel grade leaving plant 1, show a comparatively high deviation from the target tensile strength Rm_soll and the target equivalent yield strength Rt0.5_soll.

[0040] The invention addresses this issue. A process was carried out on the rolling stocks B, C, and E according to the invention, namely steel sheets, to reduce the deviation from the target tensile strength Rm_soll and the target equivalent yield strength Rt0.5_soll.

[0041] According to the method according to the invention, a model tensile strength Rm_mod and a model equivalent yield strength Rt0.5_mod of the rolling stock are first calculated on the basis of a mathematical model.

[0042] Input data for the mathematical model include the chemical composition of the rolled slab to be fed into Plant 1, as well as the operating parameters P of Plant 1 (hot rolling mill 2 and downstream roller leveler 3). This means that with the target chemical composition of the rolled goods and the nominal operating parameters P_nominal of Plant 1 specified for this steel grade, the mathematical model calculates the target tensile strength Rm_soll as the tensile strength Rm_mod and the target equivalent yield strength Rt0.5_soll as the equivalent yield strength Rt0.5_mod. The influence of the individual operating parameters stored in the model (KST, KR, ETE, ...) as well as the chemical composition of the steel grade is represented in the mathematical model in the form of functions. These can be linear functions, but also higher-order functions, whose parameters have been determined based on theoretical models, test results, and measurement results from the production of actual sheets. In the Figure 1 In the example shown, this function of the tensile strength and equivalent yield strength for the cooling stop temperature in the considered temperature range (from 300 °C to 400 °C) is approximated as a linear function.

[0043] In order to reduce the deviation from the target tensile strength Rm_soll and the target equivalent yield strength Rt0.5_soll_for a specific actual chemical composition of the rolled slabs to be fed to plant 1, the tensile strength Rm_mod and equivalent yield strength Rt0.5mod are calculated for these actual chemical compositions using the mathematical model and the nominal operating parameters P_nenn of plant 1.

[0044] The results are then compared with the nominal tensile strength Rm_soll and the nominal equivalent yield strength Rt0.5_soll. If there is a difference between the model tensile strength Rm_mod and the nominal tensile strength Rm_soll, and between the model equivalent yield strength Rt0.5_mod and the nominal / equivalent yield strength Rt0.5_soll, the nominal operating parameters P_nominal are adjusted to reduce the deviation from the nominal tensile strength Rm_soll and the nominal equivalent yield strength Rt0.5_soll. This means that in the further course of the procedure, the specified nominal operating parameters P_nominal are no longer used, but rather the adjusted operating parameters P_adapt.

[0045] According to the invention, the first adjusted operating parameter P_adapt is a cooling stop temperature KST averaged over the thickness of the rolled stock, which is increased or decreased. The cooling stop temperature KST is the temperature at which the accelerated cooling of the rolled stock is interrupted after finish rolling, which is Fig. 4 can be seen.

[0046] As a second adapted operating parameter P_adapt, according to the invention an immersion depth ETE of a straightening roller of the rollers 5a, 5b, 5c referred to as a straightening triangle in the inlet 3a of the roller straightening machine 3 is increased or decreased, which in Fig. 5 can be seen. Preferably, the immersion depth ETE of a second straightening roll 5b is increased or decreased in a first straightening triangle of the inlet 3a. The rolled stock 4, which has been finish-rolled to a final thickness D, enters this inlet 3a with this thickness D. As shown in Fig. 5As can be seen, the roller straightening machine 3 has two rows 3b, 3c of straightening rollers 5a to 5i.

[0047] The upper row 3c of the upper straightening rollers 5b, 5d, 5f, 5h is inclined, which can be seen from the fact that the plane E2 with the axes of rotation of the upper straightening rollers 5b, 5d, 5f, 5h intersects with the, in particular horizontal, plane E1 with the axes of rotation of the lower straightening rollers 5a, 5c, 5e, 5g, 5i of the lower row 3b.

[0048] Due to this inclination, for example, the deformation induced by the first straightening rolls 5a, 5b, 5c is relatively strong and steadily decreases in the subsequent straightening rolls 5d to 5i. As it passes through, the rolled stock 4 is bent alternately upwards and downwards, causing it to be subjected to alternating bending stresses, resulting in alternating tensile and compressive stresses on the top and bottom surfaces of the rolled stock.

[0049] With this combination of the adjustment of both operating parameters, namely adjusted cooling stop temperature KST_adapt and adjusted immersion depth ETE_adapt, deviations in more than one mechanical material parameter of the finished rolled goods can be reduced despite the different chemical composition of the fed rolled slabs.

[0050] In addition, a third operating parameter can also be adjusted. For this purpose, a cooling rate KR averaged over the cooling time t_KR is used for the accelerated cooling of the rolling stock (beginning at the cooling start temperature K_start and ending at the cooling stop temperature KST, as in Fig. 4 shown) after finish rolling. This adjustment of the operating parameters must be carried out again for each batch of rolled slabs with a different deviation from the target chemical composition.

[0051] Table 2 shows the calculated mechanical material parameters for sheets that can be manufactured from rolled slabs W0-W5. Table 2: Mechanical material parameters of the rolled goods sheet metal Operating parameters (P) Model tensile strength Rm_mod [N / mm 2< ] Model equivalent yield strength Rt0.5_mod [N / mm 2< ] Difference to the target tensile strength Rm_soll [N / mm 2< ] Difference to the target equivalent yield strength Rt0.5_soll [N / mm 2< ] 0(W0) P_nominal 600 490 0 0 A(W1) P_nominal 587,7 477,7 -12,3 -12,3 B(W2) P_adapt1 600,3 494,2 0,3 4,2 C(W3) P_adapt2 600,1 491,2 0,1 1,2 D(W4) P_nominal 609,3 499,3 9,3 9,3 E(W5) P_adapt3 600,4 490,3 0,4 0,3

[0052] The rolled slab W0 has no deviation from the target chemical composition in its chemical composition and therefore the calculation of the mathematical model with the nominal operating parameters P_nominal also results in no deviation in the calculated mechanical material parameters.

[0053] The rolled slabs W1 and W4 are also not subjected to any adjustment of the operating parameters P, but are manufactured or calculated with the nominal operating parameters P_nenn, which leads to the calculated mechanical material parameters listed in Table 2 for the plates A(W1) and D(W4).

[0054] Due to the deviation of the actual chemical composition from the target chemical composition, the rolled slabs W2, W3 and W5 are subjected to a correction of the operating parameters according to the invention, which leads to the calculated mechanical material parameters listed in Table 2 for the sheets B(W2), C(W3) and E(W5).

[0055] Table 3 shows various operating parameters P of the plant associated with the rolled goods / sheets 0(W0) - E(W5). Table 3: Operating parameters P of the plant with rolling mill and roller straightener Operating parameters (P) sheet metal Cooling stop temperature (KST) [°C] Immersion depth in the inlet (ETE) [mm] Cooling rate (CR) [°C / s] P_nominal 0(W0) 380 5,4 25 P_nominal A(W1) 380 5,4 25 P_adapt1 B(W2) 350 6,9 25 P_adapt2 C(W3) 350 5,9 29 P_nominal D(W4) 380 5,4 25 P_adapt3 E(W5) 405 4,9 23

[0056] The sheets A(W1) and D(W4) are manufactured with the nominal operating parameters P_nenn, the sheets B(W2) and C(W3) and E(W5) are manufactured with the adapted values ​​P_adapt1, P_adapt2 and P_adapt3 respectively.

[0057] As can be seen from Table 2, sheets B(W2) and C(W3) exhibit significantly smaller deviations from the specified target tensile strength Rm_soll and the specified target equivalent yield strength Rt0.5_soll than is the case with sheets A(W1), even though the deviations from the target chemical composition are the same. This applies analogously to sheet E(W5) compared to sheet D(W4).

[0058] This is due to the operating parameters P_adapt of plant 1, comprising rolling mill 2 and roller straightener 3, adapted according to the invention.

[0059] These adjustments were made according to the method according to the invention as follows: In the case of rolled slabs W2 and W3, the deviation of the chemical composition from the target chemical composition leads to a tensile strength reduced by ΔRm_chem and an equivalent yield strength reduced by ΔRt0.5_chem, as shown in Fig. 1Therefore, there is a negative difference between the model tensile strength Rm_mod calculated according to the invention and the target tensile strength Rm_soll. Likewise, there is a negative difference between the model equivalent yield strength Rt0.5_mod and the target equivalent yield strength Rt0.5_soll.

[0060] The method according to the invention therefore reduces the first operating parameter of the nominal operating parameters P_nenn, namely the cooling stop temperature KST_nenn, for sheet B(W2) by 30 °C to the adapted cooling stop temperature KST_adapt, namely to 350 °C.

[0061] This adaptation to the cooling stop temperature KST_adapt increases the model tensile strength Rm_mod by an amount ΔRm_KST, namely by 3.6 N / mm 2< . However, this reduces the model equivalent yield strength Rt0.5_mod by an amount ΔRt0.5_KST, namely by 3 N / mm 2< , as shown in Fig. 1 to recognize.

[0062] To compensate for this, the second operating parameter, namely the immersion depth ETE_adapt of a straightening roll, is adjusted in comparison with the respective nominal operating parameter P_nenn by increasing the immersion depth ETE of the nominal operating parameter P_nenn. This increases the model equivalent yield strength Rt0.5_mod by an amount ΔRt0.5_ETE, namely 19.5 N / mm 2< . Fig. 1 As can be seen, the value ΔRt0.5_ETE is significantly higher than the value ΔRt0.5_KST. Furthermore, the model tensile strength Rm_mod increases by an amount ΔRm_ETE, namely by 9 N / mm 2< .

[0063] Thus, the deviation from the nominal equivalent yield strength Rt0.5_soll and nominal tensile strength Rm_soll of sheet B(W2) can be significantly reduced by adjusting the operating parameters P_adapt1, as shown in Table 2. In comparison, according to Table 2, a significant deviation from the nominal equivalent yield strength Rt0.5_soll and nominal tensile strength Rm_soll can be seen for sheet A(W1), manufactured using nominal operating parameters P_nenn.

[0064] A further improvement can be achieved by adjusting the operating parameters P_adapt2, as shown for sheet C(W3). Here, a third operating parameter is additionally adjusted: the cooling rate KR_adapt averaged over the cooling time is adjusted by increasing the nominal cooling rate KR_nenn of the nominal operating parameters P_nenn. This increases the model tensile strength Rm_mod by an amount ΔRm_KR, namely by 5.8 N / mm 2< . This also increases the model equivalent yield strength Rt0.5_mod by an amount ΔRt0.5_KR, as shown in Fig. 1 can be seen, namely by 10 N / mm 2 < . Sheet C(W3) therefore comes even closer to the target equivalent yield strength Rt0.5_soll and the target tensile strength Rm_soll than sheet B(W2). Furthermore, by adding the correction for the third operating parameter, the magnitude of the change in the second operating parameter can be reduced.

[0065] In the present example, the correction of the second operating parameter, namely the immersion depth ETE_adapt of the second straightening roll, is 1.5 mm in case B(W2), provided the third operating parameter, namely the cooling rate, is not adjusted. In contrast, the correction of the second operating parameter for C(W3) is 0.5 mm, since the third operating parameter is also adjusted here. This can also have a beneficial effect on other product properties.

[0066] In the case of rolled slab W5, the deviation of the chemical composition from the target chemical composition leads to an increased tensile strength by ΔRm_chem and an increased equivalent yield strength by ΔRt0.5_chem. Therefore, there is a positive difference between the model tensile strength Rm_mod calculated according to the invention and the target tensile strength Rm_soll. Likewise, there is a positive difference between the model equivalent yield strength Rt0.5_mod and the target equivalent yield strength Rt0.5_soll.

[0067] The method according to the invention therefore increases the first operating parameter of the nominal operating parameters P_nenn, namely the cooling stop temperature KST_nenn, for sheet E(W5) by 25 °C to the adapted cooling stop temperature KST_adapt, namely to 405 °C. This adaptation to the cooling stop temperature KST_adapt reduces the model tensile strength Rm_mod by an amount ΔRm_KST, namely by 3.0 N / mm 2<, and increases the model equivalent yield strength Rt0.5_mod by an amount ΔRt0.5_KST of 2.5 N / mm 2<. In addition, the second operating parameter, namely the immersion depth of a straightening roll ETE_nenn, is adapted to ETE_adapt in comparison with the relevant nominal operating parameter P_nenn. This reduces the model tensile strength by an amount ΔRm_ETE of 3.0 N / mm 2< and the model equivalent yield strength Rt0.5_mod by an amount ΔRt0.5_ETE, namely by 6.5 N / mm 2< .The adjustment of the third operating parameter, the mean cooling rate, results in a model tensile strength of 2.9 N / mm 2< reduced by the amount of ΔRm_KR and a model equivalent yield strength of 5.0 N / mm 2< reduced by the amount of ΔRt0.5_KR.

[0068] It should also be noted that the deviations of the individual alloying elements can be both positive and negative. This means that contributions to the respective strength deviations can have different signs, which is taken into account accordingly in the mathematical model. In certain cases, this can lead to the influences of individual alloying elements on the mechanical properties compensating each other, making correction unnecessary.

[0069] With the inventive adjustment of the nominal operating parameters P_nenn, the standard deviation in production can be significantly reduced, as shown in the Figures 2 and 3 can be seen. Figures 2or 3 normal distributions resulting from plotting a relative frequency hn(Rm) or hn(Rt0.5) of measured values ​​for tensile strength Rm or equivalent yield strength Rt0.5 over a scale in N / mm 2<. These measurements were carried out according to ASTM Standard A370 (tensile test at room temperature) on a large number of sheets (greater than 1000) with the sample direction transverse to the rolling direction without the method according to the invention and with the method according to the invention.

[0070] As in Fig. 2 and 3As can be seen, the standard deviation in the production of a variety of rolled products, namely steel sheets with a sheet thickness of 20 mm of the same steel grade, namely X65, with unchanged nominal operating parameters P_nominal is significantly higher than is the case with production with adjusted operating parameters P_adapt1 and P_adapt2. Furthermore, an improvement in the standard deviation is evident between the adjusted operating parameters P_adapt1, in which the first and second operating parameters KST, ETE were adjusted, and P_adapt2, in which the first, second, and third operating parameters KST, ETE, KR were adjusted.

[0071] According to Fig. 2The standard deviation of the tensile strength Rm is approximately 15 N / mm 2 < when the nominal operating parameters are applied. When using adapted operating parameters P_adapt1, this standard deviation can be reduced to approximately 14 N / mm 2 < and when using adapted operating parameters P_adapt2 to approximately 12 N / mm 2 <.

[0072] The strength value at the peak of the normal distribution, however, remains approximately constant, regardless of whether the method according to the invention is used or not, because, on the one hand, the target chemical composition corresponds to the arithmetic mean of the respective permissible upper and lower limits of the alloying elements, and, on the other hand, the deviation of the actual chemical composition from the target chemical composition also exhibits a normal distribution. The strength value of the peak of the measured normal distribution approximately corresponds to the value calculated by the mathematical model with the target chemical composition and nominal operating parameters P_nom.

[0073] A significant improvement is also evident after Fig. 3 to the standard deviation of the equivalent yield strength Rt0.5. Fig. 3The standard deviation of the equivalent yield strength Rt0.5 is approximately 17 N / mm 2 < when the nominal operating parameters are applied. When applying adapted operating parameters P_adapt1, this standard deviation can be reduced to approximately 16 N / mm 2 < and when applying adapted operating parameters P_adapt2 to approximately 13 N / mm 2 <.

[0074] This adaptation can be solved in a structurally simple way and can be carried out reproducibly by a mathematical optimization calculation.

[0075] With the help of the optimization calculation, for example with the help of discrete iterations, the increase or decrease in the first operating parameter in comparison with the respective nominal operating parameter and the decrease or increase in the second operating parameter in comparison with the respective nominal operating parameter and optionally the increase or decrease in the third operating parameter in comparison with the respective nominal operating parameter can be calculated quickly and easily in order to find the smallest possible difference between the model tensile strength Rm_mod and the target tensile strength Rm_soll and between the model equivalent yield strength Rt0.5_mod and the target equivalent yield strength Rt0.5_soll.

[0076] During the optimization calculation, it is also possible, for example, to specify the weighting to be assigned to the deviations of the model values ​​from the target values. In this exemplary embodiment, both the deviation from the equivalent yield strength and the deviation from the tensile strength were each weighted with 0.5. The sum of the weightings must equal 1. Furthermore, in this exemplary embodiment, only results were permitted that lead to model tensile strengths Rm_mod and model equivalent yield strengths Rt0.5_mod that are greater than the target tensile strength Rm_soll or the target yield strength / equivalent yield strength Rt0.5_soll.

[0077] For example, the rolled stock 4 produced by plant 1 may be a heavy plate (flat product according to DIN EN 10079), for example, made of a steel alloy. Such a heavy plate may have a sheet thickness in the range of 8 to 150 mm, in particular in the range of 25 to 120 mm.

[0078] In general, it is noted that "in particular" can be translated into English as "more particularly." A feature preceded by "in particular" is considered an optional feature that can be omitted and thus does not constitute a limitation, for example, of the claims. The same applies to "vorzugsweise," which translates into English as "preferably."

Claims

1. Method for reducing a deviation from a target tensile strength (Rm_soll) and a target yield strength / equivalent yield strength (Re_soll / Rp0.2_soll, Rt0.5_soll etc.) of rolled goods (4), in particular steel sheets, and single-layer or multi-layer rolled slabs or rolled blooms of a common steel grade, which are produced by a plant (1) which has at least one hot rolling mill (2) and at least one roller straightening machine (3) downstream of the hot rolling mill (2), and whose respective steel alloy, in its actual chemical composition, differs from the nominal chemical composition of this steel grade in at least one alloying element due to the manufacturing process, using a plant (1) which has at least one hot rolling mill train (2) and at least one roller straightening machine (3) arranged downstream of the hot rolling mill train (2), a model tensile strength (Rm_mod) and a model yield strength / equivalent yield strength (Rt0.5_mod) of the rolling stock (4) is calculated, and subsequently, if there is a difference between the model tensile strength (Rm_mod) and the target tensile strength (Rm_soll) and between the model yield strength / equivalent yield strength (Rt0.5_mod) and the target yield strength / equivalent yield strength (Rt0.5_soll), several operating parameters of the nominal operating parameters (P_nenn) of the plant (1) are adjusted in order to thereby determine the deviation from the target tensile strength (Rm_soll) and the target yield strength / equivalent yield strength (Rt0.5_target), wherein the adjusted operating parameters comprise: increasing or decreasing a cooling stop temperature (KST), in particular averaged over the thickness of the rolling stock, at which the accelerated cooling of the rolling stock (4) is interrupted after finish rolling, as the first adjusted operating parameter on the hot rolling mill (2) of the plant (1); increasing or decreasing an immersion depth (ETE) of a straightening roller in the inlet (3a), in particular of a second straightening roller in a first straightening triangle of the inlet (3a), of the roller straightener (3) as the second adjusted operating parameter on the roller straightener (3) of the plant (1);.

2. Method according to claim 1, characterized in thatthe adjusted operating parameters additionally comprise: increasing or decreasing a cooling rate (KR), in particular averaged over the cooling time, of an accelerated cooling of the rolling stock (4) after finish rolling as a third adjusted operating parameter on the hot rolling train (2) of the plant (1); 3. Method according to claim 1 or 2, characterized in that If the difference between the model tensile strength (Rm_mod) and the target tensile strength (Rm_soll) is negative, both the first operating parameter must be adjusted by decreasing it and the second operating parameter must be adjusted by increasing it.

4. Method according to claim 1 or 2, characterized in that If the difference between the model tensile strength (Rm_mod) and the target tensile strength (Rm_soll) is positive, both the first operating parameter can be adjusted by increasing it and the second operating parameter can be adjusted by decreasing it.

5. Method according to one of claims 1 to 4, characterized in thatWith the aid of an optimization calculation, the increase or decrease in the first operating parameter in comparison with the relevant nominal operating parameter (P_nominal) and the decrease or increase in the second operating parameter in comparison with the relevant nominal operating parameter (P_nominal) and optionally the increase or decrease in the third operating parameter in comparison with the relevant nominal operating parameter (P_nominal) are calculated at which the differences between the model tensile strength (Rm_mod) and the target tensile strength (Rm_soll) and between the model yield strength / equivalent yield strength (Rt0.5_mod) and the target yield strength / equivalent yield strength (Rt0.5_soll) are the lowest.

6. Method according to claim 5, characterized in thatin the optimization calculation, the first operating parameter is adjusted by increasing it compared to the relevant nominal operating parameter (P_nenn) in order to reduce the model tensile strength (Rm_mod) and increase the model yield strength / equivalent yield strength (Rt0.5_mod).

7. Method according to claim 5, characterized in that in the optimization calculation, the first operating parameter is adjusted by lowering the relevant nominal operating parameter (P_nenn) in order to increase the model tensile strength (Rm_mod) and lower the model yield strength / equivalent yield strength (Rt0.5_mod).

8. Method according to claim 5, 6 or 7, characterized in that in the optimization calculation, the second operating parameter is adjusted by increasing the relevant nominal operating parameter (P_nenn) in order to increase the model tensile strength (Rm_mod) and the model yield strength / equivalent yield strength (Rt0.5_mod).

9. Method according to claim 5, 6 or 7, characterized in thatin the optimization calculation, the second operating parameter is adjusted by lowering the relevant nominal operating parameter (P_nenn) in order to lower the model tensile strength (Rm_mod) and the model yield strength / equivalent yield strength (Rt0.5_mod).

10. Method according to claim 5, 6, 7, 8 or 9, characterized in that in the optimization calculation, the third operating parameter is adjusted by increasing the relevant nominal operating parameter (P_nenn) in order to increase the model tensile strength (Rm_mod) and the model yield strength / equivalent yield strength (Rt0.5_mod).

11. Method according to claim 5, 6, 7, 8 or 9, characterized in that in the optimization calculation, the third operating parameter is adjusted by lowering the relevant nominal operating parameter (P_nenn) in order to lower the model tensile strength (Rm_mod) and the model yield strength / equivalent yield strength (Rt0.5_mod).

12. Method according to one of claims 5 to 11, characterized in thatthe optimization calculation is carried out using a mathematical, in particular discrete, optimization method.

13. Method according to one of claims 1 to 12, characterized in that the first operating parameter is adjusted within a range of plus / minus 100 °C to the relevant nominal operating parameters (P_nenn) and the second operating parameter is adjusted within a range of plus / minus 30 % to the relevant nominal operating parameters (P_nenn).

14. Method according to one of claims 2 to 13, characterized in that the third operating parameter is adjusted within a range of plus / minus 30% to the relevant nominal operating parameter (P_nominal).

15. Plant for carrying out the method according to one of claims 1 to 14, with a hot rolling mill (2) which has a cooling device with a first actuator for setting a cooling stop temperature (KST) for accelerated cooling of the rolling stock (4) after finish rolling, with a roller straightening machine (3) arranged downstream of the hot rolling mill (2), which has a second actuator for setting the immersion depth (ETE) of a straightening roller in the inlet (3a) of the roller straightening machine (3), and with a control or regulating device which has the mathematical model and is connected to the actuators of the cooling device and the roller straightening machine (3) for transmitting adapted nominal operating parameters (P_nenn).

16. Plant according to claim 15, characterized in that the cooling device has a third actuator for setting a cooling rate (KR) of the accelerated cooling of the rolling stock (4) after finish rolling.

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