Hot rolling method for steel plate

Abstract

To provide a hot rolling method for a steel plate that can perform dimension conversion of a steel plate in a hot state and a steel plate in a cold state, with high accuracy, and control dimensions of the plates with high accuracy.SOLUTION: In a hot rolling method for a steel plate, distortion εN in a plate thickness direction, distortion εC in a plate width direction and distortion εL in a plate length direction, of a steel plate are calculated, using a target product plate thickness of a hot-rolled steel plate at an ordinary temperature, a final-pass predicted temperature in finish rolling, a density in a γ-phase calculated from the final-pass predicted temperature and components included in the steel plate, a density in an α-phase at an ordinary temperature, and a density distribution of alloy components; a target plate thickness at a final-pass output side is calculated from the distortion εN, εC and εL and the target product plate thickness of the hot-rolled steel plate at the ordinary temperature; and at least either of a predicted plate width at the ordinary temperature and a predicted plate length at the ordinary temperature is calculated, when the plate thickness at the final-pass output side matches the target plate thickness.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for hot rolling a steel sheet. [Background technology]

[0002] In the hot rolling process of steel sheets, in order to obtain a desired dimensional accuracy of the hot-rolled steel sheet at room temperature, the thickness of the hot steel sheet is optimized. For this reason, technology for measuring or estimating the thickness of the hot steel sheet with high accuracy has been studied.

[0003] For example, Patent Document 1 discloses a hot thickness measuring device that uses radiation to measure the hot thickness of a steel sheet. This hot thickness measuring device has a function of calibrating a correction formula used for thickness calculation based on the linear absorption rate based on the hot thickness measurement result, the surface temperature measurement result during the hot thickness measurement, the band structure prediction result, the back stress prediction result of the material, and the thickness data measured in the cold state of the same material. This reduces the variation in thickness measurement by converting the plate thickness in the hot state to the plate thickness in the cold state, taking into account the influence of the back stress on transformation strain and thermal strain and the influence of the band structure. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2017-90083 A [Non-patent literature]

[0005] [Non-Patent Document 1] J. Miettinen, “Calculation of solidification-related thermophysical properties for steels”, Metallurgical and Materials Transactions B, Vol. 28B, pp. 281-297 (1997) [Non-Patent Document 2] Takayuki Otsuka, et al., "Microstructure and heat treatment effect on transformation strain in steels part 2 modelling", Materials Science and Technology, Volume 35, 2019, [searched on June 26, 2023], Internet <URL:https: / / doi.org / 10.1080 / 02670836.2018.<1548111>

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the technology described in the above Patent Document 1, since the transformation strain and thermal strain with anisotropy change depending on the casting conditions and rolling conditions of the material, the hot plate thickness measured according to the influence of anisotropy is corrected. However, since the technology described in the above Patent Document 1 is a method for improving the measurement accuracy of the plate thickness, it is desirable to consider the influence of anisotropy not only on the plate thickness but also on the cold values in the plate thickness and plate width. This is because the volume changes due to thermal expansion and contraction accompanying temperature changes and density changes accompanying transformation are isotropic, but the volume change due to plastic deformation is not isotropic.

[0007] Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a hot rolling method for steel sheets that can perform high-precision dimensional conversion between a hot steel sheet and a cold steel sheet and control the plate dimensions with high precision.

Means for Solving the Problems

[0008] In order to solve the above problems, according to an aspect of the present invention, there is provided a hot rolling method for steel sheets, which uses the target product plate thickness of a hot-rolled steel sheet at room temperature, the predicted temperature of the final pass in finish rolling, the density of the γ phase calculated from the predicted temperature of the final pass and the components contained in the steel sheet, the density of the α phase at room temperature, and the concentration distribution of alloy components to obtain the strain ε in the thickness direction of the steel sheet , the strain ε in the width direction of the plate C ​and strain in the plate length direction ε L Calculate the strain ε N , ε C and ε L and the target product thickness of the hot-rolled steel sheet at room temperature, a target thickness at the final pass delivery side is calculated, and at least one of a predicted width at room temperature and a predicted length at room temperature is calculated when the thickness at the final pass delivery side is the target thickness. T and the length at room temperature L Tair So, ε=L Tair / L T This is the value calculated as -1.

[0009] The chemical composition of the steel sheet may be, in mass %, C: 0.1-0.25%, Si: 0.5-7.0%, Mn: 1.0-10.0%, Cr: 0.01-15.0%, Ni: 0.01-10.0%, and the balance: Fe and impurities.

[0010] The concentration distribution of at least one of C, Si, Mn, Ni, and Cr may be used as the concentration distribution of the alloy component.

[0011] Strain ε N , ε C and ε L may be calculated using the following formulas (a) and (b). N , A LC is the correction coefficient, ρ α (T air ) is the density of the α phase at room temperature, ρ γ (T) is the density of the gamma phase at temperature T.

[0012]

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[0013] Correction factor A N , A LC A N ×A LC ×A LC =1 may be satisfied. Effect of the Invention

[0014] As described above, according to the present invention, the dimensional conversion between a hot steel sheet and a cold steel sheet can be performed with high accuracy, and the sheet dimensions can be controlled with high accuracy. [Brief description of the drawings]

[0015] [Figure 1] 1 is an example of a micrograph showing the microstructure of the surface of a steel material. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and the drawings, components having substantially the same functional configurations are designated by the same reference numerals, and duplicated explanations will be omitted.

[0017] When converting the dimensions of hot steel sheet to cold steel sheet, it is important to predict with high accuracy the thermal strain that occurs with temperature changes during rolling and the transformation strain that occurs with phase transformation. Thermal strain generally has little dependence on the composition of the steel sheet and is not anisotropic. On the other hand, transformation strain is dependent on the composition of the density of each phase. For this reason, transformation strain varies greatly depending on the steel type. In addition, it has been revealed that transformation strain behaves differently in the thickness, width, and length directions because anisotropy occurs due to the influence of microsegregation.

[0018] In general, alloying elements and impurities contained in steel are distributed unevenly. When observing the microstructure of steel, the uneven distribution of alloying elements and impurities can be recognized, for example, by differences in color that appear in the micrograph. Figure 1 shows an example of a micrograph showing the microstructure of the surface of a steel. In the micrograph shown in Figure 1, the horizontal direction corresponds to the plate length direction of the steel, and the vertical direction corresponds to the plate thickness direction of the steel. In the micrograph shown in Figure 1, there are white parts and black parts that appear in a band shape along the horizontal direction (i.e., the plate length direction), and there is a difference in the concentration of the contained components in each part. The temperature range where phase transformation occurs is different in the parts with different concentrations of the components. In other words, anisotropy occurs due to microsegregation, and different behavior is exhibited in the plate thickness direction, plate width direction, and plate length direction, resulting in non-uniform deformation.

[0019] Therefore, the inventors of the present application have studied how to convert the dimensions of a hot steel sheet to that of a cold steel sheet, taking into consideration the strains that occur in the thickness, width, and length directions. Specifically, laboratory tests simulating an actual machine were carried out using several types of steel materials having the chemical compositions shown in Table 1 below, and a conversion formula for converting the dimensions of a hot steel sheet to that of a cold steel sheet was studied.

[0020] In this study, a subject material, which is a steel material suitable for application of the hot rolling method for steel sheet according to one embodiment of the present invention, and a comparative material, which is a steel material other than the subject material, were examined, as shown in the following Table 1. The chemical composition of the steel sheet, which is the subject material, is, in mass%, C: 0.1-0.25%, Si: 0.5-7.0%, Mn: 1.0-10.0%, Cr: 0.01-15.0%, Ni: 0.01-10.0%, and the balance: Fe and impurities.

[0021] [Table 1]

[0022] In the laboratory test, each of the steel materials shown in Table 1 above was heated and roughly rolled to a plate length of 250 mm, plate width of 100 mm, and plate thickness of 30 mm, and then finish-rolled by three rolling mills to produce steel plates with a plate thickness of 5 mm at room temperature. At this time, the inlet temperature was adjusted so that the rolling in the final pass of the finish rolling was performed at a plate temperature of 900°C. Note that if the plate thickness at room temperature is about 2 to 10 mm, the plate temperature in the plate thickness direction can be considered to be uniform.

[0023] Based on the strip temperature at the final pass exit measured in the laboratory test, the thickness direction strain ε N , strain in the plate width direction ε C , strain in the plate length direction ε L The subscripts N, C, and L indicate the thickness direction, width direction, and length direction, respectively. Table 2 shows the strain ε N , ε C , ε L The values ​​of each strain ε are shown in Table 2. N , ε C , ε L The calculated values ​​of ε are shown in the following comparative example, assuming that the thickness, width and length directions are all the same, using the following formula (1), and in the following example, assuming that the thickness, width and length directions are all the same, using the following formulas (2a) and (2b). Table 2 also shows the strains ε obtained by measuring the steel plates manufactured in the laboratory test. N , ε C , ε L The measured values ​​are shown.

[0024]

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[0025] In the above formula (1), α α , α γ , β is the coefficient, T air [℃] is room temperature, T Ar [℃] is the transformation temperature, and T [℃] is the sheet temperature at the exit of the final pass (corresponding to the "predicted final pass temperature"). The calculated values ​​shown in Table 2 are γ =2.2×10 -5 , α α=1.5×10 -5 ,β=0.0044,T Ar = 700℃, T air = 20℃, T = 900℃. Ar is generally a value with a range, but since the value of the coefficient β changes depending on each transformation temperature, in reality, defining it at any one point in the transformation temperature range does not affect the calculation of the strain value.

[0026]

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[0027] In the above formula (2a) and formula (2b), A N , A LC is the correction coefficient derived from the structure of the steel material, ρ α (T air ) is the density of the α phase at room temperature, ρ γ (T) is the density of the gamma phase at temperature T. Correction coefficient A N is the strain in the thickness direction ε N is the correction coefficient for LC is the strain in the plate width direction, ε C and strain in the plate length direction ε L Here, the anisotropy in the width direction and the length direction is considered to be the same, and the same correction coefficient is used, but different correction coefficients may be set in the width direction and the length direction. α , ρ γ Alternatively, the density ρ may be calculated by adjusting the coefficients of the J. Miettinen formula: α , ρ γ In the following formula by J. Miettinen, C M P [mass%] represents the solid solution concentration of the M component in the P phase.

[0028]

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[0029] In addition, when the anisotropy in the plate width direction and plate length direction becomes larger than that in the plate thickness direction, the strain in the plate width direction ε C and strain in the plate length direction ε L The absolute value of the strain in the thickness direction ε N In this case, the correction coefficient A LC is the correction coefficient A N The calculated values ​​shown in Table 2 are Ar = 700℃, T air = 20℃, A N =0.8, A LC = 1.12.

[0030] [Table 2]

[0031] Based on the results shown in Table 2 above, the strain in the thickness direction ε N , strain in the plate width direction ε C , strain in the plate length direction ε L When the calculated values ​​of the comparative examples were compared with the calculated values ​​of the examples, the differences from the measured values ​​were all about the same for the comparative steel materials Nos. 1, 3, 5, 6, 8, 10, and 12. In other words, for the comparative materials, the above formula (1), formula (2a), and formula (2b) can be used as a conversion formula for converting the dimensions of the steel plate in a hot state to those in a cold state with the same degree of accuracy.

[0032] On the other hand, for the steel materials No. 2, 4, 7, 9, and 11, which are the target materials, the use of the above formulas (2a) and (2b) rather than the above formula (1) as the conversion formula for converting the dimensions of the steel plate in the hot state to that in the cold state results in a better understanding of at least the strain ε in the plate width direction. C and strain in the plate length direction ε L The calculated values ​​of ε were close to the measured values. For steels No. 2, 4, 7, and 11, the strain ε in all three directions was N , ε C , ε L The calculated values ​​were closer to the measured values ​​when the above formulas (2a) and (2b) were used than when the above formula (1) was used.

[0033] Based on the results of Table 2, the average error (=Σ|ε 計算値 -ε 測定値 The accuracy was calculated as 0.00388 for the comparative example and 0.00105 for the working example for the target steel materials Nos. 2, 4, 7, 9, and 11, indicating that the accuracy was improved. In addition, the accuracy was 0.00584 for the comparative example and 0.00585 for the other steel types (i.e., steel materials Nos. 1, 3, 5, 6, 8, 10, and 12, which are comparative materials), indicating that the accuracy was almost the same.

[0034] From these results, by using formulas (2a) and (2b) which take into account the anisotropy in the thickness, width and length directions, respectively, the strain ε N , ε C , ε L It can be seen that the strain ε can be calculated with high accuracy. N , ε C , ε L If it is possible to obtain the strain ε N , ε C , ε L Using this value, the dimensions of the steel sheet in the hot state and the steel sheet in the cold state can be converted with high accuracy.

[0035] From the above considerations, in the hot rolling method of a steel sheet according to an embodiment of the present invention, first, a target product thickness of the hot rolled steel sheet at room temperature, a predicted final pass temperature in finish rolling, a density of the γ phase calculated from the predicted final pass temperature and the components contained in the steel sheet, a density of the α phase at room temperature, and a concentration distribution of the alloy components are used to calculate a strain ε in the thickness direction of the steel sheet. N , strain in the plate width direction ε C and strain in the plate length direction ε L Calculate.

[0036] The target product thickness of the hot-rolled steel sheet at room temperature is, in other words, the thickness of the hot-rolled steel sheet in a cold state. The final pass predicted temperature is the temperature of the steel sheet at the roll bite exit side of the rolling mill at the last stage of finish rolling, and may be the temperature immediately after the rolling by the rolling mill is completed. The final pass predicted temperature may be a value obtained in advance by, for example, a setup calculation. The density of the gamma phase and the density of the alpha phase can be calculated, for example, by using the above-mentioned J. Miettinen formula.

[0037] The concentration distribution of the alloying elements can be measured using any measuring device. For example, an Electron Probe Micro Analyzer (EPMA) or the like may be used to scan the surface of the steel material to be analyzed in one direction to obtain the concentration distribution of the alloying elements to be measured. The concentration distribution of the alloying elements is almost the same for the same steel type. Therefore, it is sufficient to measure the concentration distribution of the alloying elements at least once for each steel type, and it is not necessary to measure it each time.

[0038] Here, the strain in each direction ε N , ε C , ε L The strain can be calculated with high accuracy by using the above formulas (2a) and (2b). In order to use the above formulas (2a) and (2b), the concentration distribution of at least one alloy component in the chemical composition of the steel is required. In this case, it is preferable to use the concentration distribution of at least one of C, Si, Mn, Ni, and Cr.

[0039] Then, the degree of segregation of the alloying element is calculated based on the obtained concentration distribution of the alloying element. The degree of segregation is calculated based on the maximum concentration ρ max and the average concentration ρ mean Ratio to (=ρ max / ρ mean )

[0040] Once the degree of segregation is determined, the timing of phase transformation can be determined from the degree of segregation and the average concentration. The timing of phase transformation may be determined experimentally using thermodynamic calculations, or may be determined from a transformation diagram. Then, from the timing of phase transformation, the correction coefficient A in the above formulas (2a) and (2b) is calculated by mechanical calculations. N , A LC The mechanical calculation can be performed using known techniques (for example, Non-Patent Document 2). Note that the correction coefficient A N , A LC A N ×A LC ×A LC = 1. This is based on the principle of constant volume in plastic deformation.

[0041] In this way, the strain ε N , strain in the plate width direction ε C and strain in the plate length direction ε L can be calculated.

[0042] Strain in each direction ε N , ε C , ε L When the strain ε N , ε C and ε L From the target product thickness of the hot-rolled steel sheet at room temperature, the target thickness at the final pass delivery side is calculated, and at least one of the predicted width and the predicted length at room temperature when the thickness at the final pass delivery side becomes the target thickness can be calculated. The predicted width and the predicted length at room temperature are the predicted width and the predicted length of the hot-rolled steel sheet in the cold state. The strain in each direction here is the length L at the predicted final pass temperature. T and the length at room temperature L Tair So, ε=L Tair / L T -1. Therefore, the strain in each direction ε N , ε C , ε L can be used to convert the dimensions of the hot steel sheet to those of the cold steel sheet.

[0043] The predicted thickness at the final pass delivery side, the predicted width at room temperature, and the predicted length at room temperature thus obtained may be used in a setup calculation performed to determine rolling conditions before rolling, or may be obtained and used during rolling control during operation. By considering the influence of anisotropy on the thickness, thickness, and width, the predicted thickness at the final pass delivery side, the predicted width at room temperature, and the predicted length at room temperature can be obtained with high accuracy. In this way, according to the hot rolling method for steel plate according to this embodiment, the dimensional conversion between the hot state steel plate and the cold state steel plate can be performed with high accuracy, and as a result, the plate dimensions can be controlled with high accuracy.

[0044] Although the preferred embodiment of the present invention has been described in detail above with reference to the accompanying drawings, the present invention is not limited to such an example. It is clear that a person having ordinary knowledge in the technical field to which the present invention pertains can conceive of various modified or altered examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally belong to the technical scope of the present invention.

Claims

1. A method for hot rolling a steel sheet, comprising the steps of: The target product thickness of the hot-rolled steel sheet at room temperature, the predicted final pass temperature in finish rolling, the density of the γ phase calculated from the predicted final pass temperature and the components contained in the steel sheet, the density of the α phase at room temperature, and the concentration distribution of the alloy components are used to calculate the strain ε in the thickness direction of the steel sheet. Ｎ , strain in the plate width direction ε Ｃ and strain in the plate length direction ε Ｌ Calculate The strain ε Ｎ , ε Ｃ and ε Ｌ and a target product thickness of the hot-rolled steel sheet at room temperature, a target thickness at a final pass delivery side is calculated, and at least one of a predicted width at room temperature and a predicted length at room temperature is calculated when the thickness at the final pass delivery side becomes the target thickness.

2. The chemical composition of the steel sheet is, in mass%, C: 0.1 to 0.25%, Si: 0.5-7.0%, Mn: 1.0 to 10.0%, Cr: 0.01-15.0%, Ni: 0.01 to 10.0%, The method for hot rolling a steel sheet according to claim 1, wherein the balance is Fe and impurities.

3. The method for hot rolling a steel sheet according to claim 1 or 2, wherein the concentration distribution of the alloying components is a concentration distribution of at least one of C, Si, Mn, Ni, and Cr.

4. The strain ε Ｎ , ε Ｃ and ε Ｌ The method for hot rolling a steel sheet according to claim 1 or 2, wherein the above-mentioned is calculated using the following formula (a) and formula (b). In addition, A Ｎ , A ＬＣ is the correction coefficient, ρ α (T ａｉｒ ) is the density of the α phase at room temperature, ρ γ (T) is the density of the gamma phase at temperature T. [0010]

5. The correction coefficient A Ｎ , A ＬＣ A Ｎ ×A ＬＣ ×A ＬＣ The method for hot rolling a steel sheet according to claim 4, wherein:

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