JP7849600B2 - Manufacturing method of hot-rolled steel sheets - Google Patents

Description

This invention relates to a method for manufacturing hot-rolled steel sheets.

Steel sheets manufactured using the hot rolling process are typically wound into coils and then cooled. During the cooling process, the outer, side, and inner surfaces of the coil cool more easily than the interior, resulting in a temperature distribution along the longitudinal direction of the steel sheet. This longitudinal temperature distribution affects variations in the mechanical properties of the cooled coil, such as tensile strength (TS), r-value, yield strength (YS), uniform elongation, and fracture elongation. In particular, for high-strength steel sheets, transformation continues even after winding, so the temperature distribution during the coil cooling process has a significant impact on the material properties. Since variations in mechanical properties affect product quality, there is a need for technology to manufacture steel sheets that possess the desired mechanical properties throughout their entire length and width.

For example, Non-Patent Document 1 discloses a technique for reducing the increase in strength of the hot-rolled steel sheet at the leading and trailing ends and the widthwise ends by winding the hot-rolled steel sheet at a high temperature to promote ferrite transformation, and then rapidly water-cooling it after the transformation is complete, thereby softening the hot-rolled steel sheet along the entire length of the coil, reducing the thickness of the grain boundary oxide layer at the scale-base metal interface, and raising the winding temperature at the leading and trailing ends to a higher temperature than the winding temperature at the steady-state end.

Masayoshi Kobayashi, et al., "Improvement of Cold Rolling Characteristics of Hot-Rolled Steel Sheets for High-Tensile Steel with Cold Rolling Pressure of 980 MPa or Higher," Iron and Steel, Vol. 100 No. 5, pp. 616-624, 2014. Hiroshi Yoshida, et al., "Analysis of poor flatness after hot strip cooling," Iron and Steel, Vol. 68 No. 8, pp. 965-973, 1982.

The technology described in Non-Patent Document 1 requires winding the hot-rolled steel sheet at a high temperature, air-cooling it for a certain period to promote ferrite transformation, and then immersion water-cooling it. For example, from the standpoint of improving productivity, fewer processing steps are desirable, therefore, further technologies are needed to reduce variations in mechanical properties across the entire length and width of the steel sheet. Furthermore, the technology described in Non-Patent Document 1 is an example demonstrating the manufacturing process for a very limited type of steel and cannot be considered a widely applicable general solution.

Therefore, the present invention has been made in view of the above problems, and its object is to provide a new method for manufacturing hot-rolled steel sheets that can reduce variations in mechanical properties across the entire length and width of the steel sheet.

To solve the above problems, according to one aspect of the present invention, a method for manufacturing hot-rolled steel sheets for producing high-strength steel is provided, comprising: calculating in advance a winding temperature value for the entire length and width of the hot-rolled steel sheet so that the mechanical properties of the hot-rolled steel sheet to be manufactured reach a target value and the variation in mechanical properties is within an acceptable range; and performing at least one of heating or cooling on the hot-rolled steel sheet before winding so that the winding temperature of the hot-rolled steel sheet reaches the calculated winding temperature value for the entire length and width.

Alternatively, multiple coil positions within the axial cross-section of a hot-rolled steel sheet coil may be used as representative points for the entire length and width of the hot-rolled steel sheet. A material prediction model representing the relationship between the sheet temperature and mechanical properties of the hot-rolled steel sheet can be used to calculate the mechanical properties and winding temperature for each of these multiple coil positions at a predetermined time after the completion of finish rolling or coil winding. The winding temperature for the entire length and width can then be calculated.

The material prediction model may be expressed as a correlation equation between parameters based on plate temperature, obtained from the temperature history of the plate temperature during a period including the temperature acquisition period (which is defined as the period from the completion time of finish rolling or coil winding until a predetermined time has elapsed), and the mechanical properties measured in the manufactured hot-rolled steel plate coil.

Here, the parameter may be the sheet temperature at a predetermined time after the completion of finish rolling or coil winding within the temperature acquisition period.

Alternatively, the parameter may be the integrated temperature, which is the integral value over time of the change in plate temperature from the start temperature to the end temperature during the integration period, from a predetermined start temperature to the end temperature.

Furthermore, the parameter may also be the cumulative cumulative temperature, which is the cumulative value obtained by multiplying the change in plate temperature from the cumulative start temperature to the cumulative end temperature over a predetermined cumulative period.

In calculating the winding temperature, the axial cross-section of the hot-rolled steel sheet coil may be divided into at least one of the radial or widthwise directions, corresponding to the coil position, to set up multiple divided regions. For multiple cases where the winding temperatures of the divided regions differ, a material prediction model may be used to calculate the mechanical properties and winding temperature at a predetermined time after the completion of finish rolling or coil winding. The winding temperature of each divided region may then be determined as the calculated winding temperature for each divided region, based on the case where the evaluation function, expressed by the target value of the mechanical properties and the allowable range of variation in the mechanical properties, is minimized.

Furthermore, in calculating the winding temperature, the axial cross-section of the hot-rolled steel sheet coil may be divided into at least one of the radial or widthwise directions, corresponding to the coil position, to set up multiple divided regions. For multiple cases with different boundary positions or winding temperatures within these divided regions, a material prediction model may be used to calculate the mechanical properties and winding temperature at a predetermined time after the completion of finish rolling or coil winding. The boundary positions of the divided regions may then be determined so that the calculated mechanical properties meet the target values and the variation in mechanical properties remains within an acceptable range. The winding temperature for each divided region may then be determined accordingly.

Alternatively, in calculating the winding temperature, a table representing the correspondence between the temperature history of the entire length and width of the hot-rolled steel sheet (obtained in advance for each steel type) and the mechanical properties obtained by measuring test pieces cut from the hot-rolled steel sheet may be acquired. Based on this table, the winding temperature calculation value for the entire length and width of the hot-rolled steel sheet can be calculated such that the mechanical properties of the hot-rolled steel sheet to be manufactured reach the target value and the variation in mechanical properties is within an acceptable range.

Furthermore, when the axial cross-section of a hot-rolled steel sheet coil is divided into three sections—the inner circumference, the longitudinal center, and the outer circumference—from the radially inner side, the calculated winding temperatures for the inner and outer circumferences may be determined to be higher than those for the longitudinal center.

For example, the calculated winding temperatures may be 700-750°C for the inner circumference, 475-560°C for the longitudinal center, and 675-800°C for the outer circumference.

For the edges at both ends in the width direction of the axial cross-section of the hot-rolled steel sheet coil, a winding temperature value that ensures uniform temperature in the width direction of the sheet after a predetermined time has elapsed since the completion of coil winding may be calculated in advance. Before winding the hot-rolled steel sheet, at least one of either heating with an edge heater or cooling adjustment with an edge mask may be performed.

When winding hot-rolled steel sheets, mandrel cooling water does not need to be used.

The mechanical properties may include, for example, tensile strength.

As explained above, the present invention makes it possible to reduce variations in the mechanical properties of a steel plate across its entire length and width.

This is an explanatory diagram showing an example of a hot rolling mill according to one embodiment of the present invention, showing the equipment from the finishing rolling mill onward. This is an explanatory diagram showing an example of an analytical model for determining the temperature history of a coil. This is a schematic diagram showing a state in which an axisymmetric coil model with a plate width of 1/2 is divided into two sections in the width direction and three sections in the radial direction, thereby setting up six divided regions. This is an explanatory diagram illustrating the procedure for determining the radial boundary positions r1 and r2 in the axisymmetric coil model shown in Figure 3. This graph shows an example of the relationship between the regional ranges of the inner and outer circumferences and the predicted variation in tensile strength. Table 1 shows the estimated tensile strength (TS) for each calculation condition, and a graph illustrating the variation in the estimated tensile strength. This is an explanatory diagram illustrating the procedure for determining the boundary position w in the width direction and the winding temperature in the axisymmetric coil model shown in Figure 3. This graph shows one example of the relationship between the heating temperature T _up at the edge and the variation in tensile strength. This graph shows one example of the relationship between the boundary position w and the variation in tensile strength. Figure 3 is an explanatory diagram showing an example of the calculated winding temperature for each defined segment in the axisymmetric coil model. Figure 2 is a graph showing the results of organizing the plate temperature and tensile strength after cooling for 30 minutes at the nine coil positions indicated. This is an explanatory diagram for explaining the cumulative temperature calculated from the temperature history. Figure 2 is a graph showing one example of the relationship between integrated temperature and tensile strength at nine coil positions. Figure 2 is a graph showing one example of the relationship between cumulative temperature and tensile strength at nine coil positions.

Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. In this specification and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, thus omitting redundant explanations.

[1. Equipment configuration]
First, the equipment configuration of the hot rolling process will be explained based on Figure 1. Figure 1 is an explanatory diagram showing an example of a hot rolling equipment 1 according to this embodiment, and shows the equipment from the finishing rolling mill 30 onwards.

In the hot rolling process, the heated slab is rolled to a predetermined thickness by the hot rolling equipment 1 and then wound into a coil. The slab, heated in a heating furnace (not shown), is rolled by a roughing mill (not shown), and then rolled to a predetermined thickness by a finishing mill 30. Afterward, the steel sheet passes through a cooling equipment 40, is guided to a coiler 80 by pinch rolls 70, and is wound into a coil by a mandrel 85 at a predetermined winding temperature.

Temperature control across the entire length and width of a steel sheet during the hot rolling process is achieved by first determining the winding temperature at which the material properties, such as tensile strength (TS), r-value, yield strength (YS), uniform elongation, and fracture elongation, will be within target values. The hot rolling equipment 1 is then controlled to achieve this predetermined winding temperature. Specifically, the temperature in the longitudinal direction (thread direction) of the steel sheet is controlled by heating with a bar heater 10 installed on the entry side of the finishing rolling mill 30 and cooling with a cooling device 40 installed between the finishing rolling mill 30 and the coiler 80. The temperature in the width direction is controlled by heating with an edge heater 20 installed on the entry side of the finishing rolling mill 30 and cooling adjustment with an edge mask 55 installed on the runout table 50 between the finishing rolling mill 30 and the coiler 80, corresponding to the cooling device 40.

The hot rolling mill 1 shown in Figure 1 is equipped with a finish exit thermometer 61 at the exit of the finish rolling mill 30 to measure the finish exit temperature of the steel sheet, and a pre-winding thermometer 63 at the exit of the cooling device 40 to measure the temperature of the steel sheet before winding by the coiler 80. Based on the temperatures of the steel sheet measured by the finish exit thermometer 61 and the pre-winding thermometer 63, the control device (not shown) controls the bar heater 10, edge heater 20, cooling device 40, and edge mask 55 so that the winding temperature of the steel sheet reaches a predetermined winding temperature.

Coil C, manufactured in hot rolling mill 1, is transported to the coil yard and stored there.

[2. Method for manufacturing hot-rolled steel sheets]
Steel sheets manufactured by the hot rolling process are wound into coils and then cooled (air-cooled) during transport to and in the coil yard. Even if the temperature of the steel sheet is almost uniform along its entire length and width immediately after winding, variations occur as the cooling time increases because the outer, side, and inner surfaces of the coil cool more easily than the inside of the coil during the cooling process. This temperature distribution along the longitudinal direction of the steel sheet affects the variations in the mechanical properties of the coil after cooling. In particular, for high-strength steel sheets where transformation continues even after winding, the temperature distribution that occurs during the coil cooling process has a significant impact on the material.

Therefore, the inventors of this invention have devised a method for manufacturing hot-rolled steel sheets, which involves pre-calculating a winding temperature for the entire length and width of the hot-rolled steel sheet so that the mechanical properties of the sheet meet the target value and the variation in mechanical properties is within an acceptable range. The method then involves heating or cooling the hot-rolled steel sheet before winding, so that the winding temperature of the hot-rolled steel sheet reaches the calculated winding temperature for the entire length and width. While the required mechanical properties of the manufactured coil include tensile strength (TS), r-value, yield strength (YS), uniform elongation, and elongation at break, the following description will focus on tensile strength as an example of a mechanical property and explain the manufacturing method of hot-rolled steel sheets according to this embodiment in detail.

[2-1. Calculation of winding temperature]
First, before manufacturing the hot-rolled steel sheet, the winding temperature values for the total length and width are calculated in advance so that the mechanical properties of the hot-rolled steel sheet to be manufactured meet the target values and the variation in mechanical properties is within an acceptable range.

[2-1-1. Method for calculating winding temperature using a material prediction model]
In one example of a hot-rolled steel sheet manufacturing method according to this embodiment, a material prediction model is used to determine the winding temperature for the entire length and width of the hot-rolled steel sheet so that the mechanical properties of the hot-rolled steel sheet to be manufactured meet the target value and the variation in mechanical properties is within an acceptable range. In this method, multiple coil positions within the axial cross-section of the hot-rolled steel sheet coil are set as representative points for the entire length and width of the hot-rolled steel sheet. Then, using a material prediction model that represents the relationship between the sheet temperature and mechanical properties of the hot-rolled steel sheet, the mechanical properties and winding temperature calculation values are calculated for the multiple coil positions at the time of completion of finish rolling or a predetermined time after the completion of coil winding, and the winding temperature calculation value for the entire length and width is determined. The material prediction model used to determine the winding temperature calculation value is not particularly limited, and any well-known model may be used.

The multiple coil positions within the axial cross-section of a hot-rolled steel sheet coil, which are set as representative points for the entire length and width of the hot-rolled steel sheet, correspond to divided regions set by dividing the coil cross-section, obtained by winding the hot-rolled steel sheet into a coil from the tip to the tail, in at least one of the coil radial direction or width direction. The coil cross-section is divided into multiple sections (e.g., approximately 2 to 10,000) in at least one of the coil radial direction or width direction. For example, in the axially symmetric coil model with a sheet width of 1/2 shown in Figure 2, it is divided into 10 sections in the coil radial direction and 10 sections in the sheet width (1/2). Note that Figure 2 only shows 1/4 of the coil's circumference.

Below, we will describe two examples of methods for calculating winding temperature using a material prediction model: one method involves sequentially determining the boundary positions of the coil cross-section division regions so that the mechanical properties meet the target values and the variation in mechanical properties remains within an acceptable range, and then calculating the winding temperature for each division region; and another method involves calculating the winding temperature for each division region using an evaluation function.

(Calculation Method 1) Step-by-step calculation of boundary positions of divided regions and calculated winding temperatures First, a method for determining the calculated winding temperature of each divided region is described, while sequentially determining the boundary positions of the divided regions of the coil cross-section so that the mechanical properties become target values and the variation in mechanical properties is within an acceptable range. In this method, for multiple cases with different boundary positions of divided regions or winding temperatures, the mechanical properties and calculated winding temperatures are calculated using a material prediction model at a predetermined time after the completion of finish rolling or coil winding. Then, the boundary positions of the divided regions are determined and the calculated winding temperature of each divided region is determined so that the calculated mechanical properties become target values and the variation is within an acceptable range.

In this example, as shown in Figure 3, we consider six divided regions (s11, s12, s21, s22, s31, s32) in an axisymmetric coil model with a plate width of 1/2, divided into two in the width direction and three in the radial direction. Then, we calculate the winding temperature for each of the six divided regions so that the variation in tensile strength is minimized during the operation of the equipment.

Here, in the axisymmetric coil model, each region is referred to as the inner circumference, longitudinal center, and outer circumference from the inside out in the radial direction, and as the width center and edge in the width direction. In the example in Figure 3, divided regions s31 and s32 are the inner circumference, divided regions s21 and s22 are the longitudinal center, and divided regions s11 and s12 are the outer circumference. Also, divided regions s11, s21, and s31 are the width center, and s12, s22, and s32 are the edge. Furthermore, in Figure 3, r1 indicates the boundary position between the inner circumference and the longitudinal center, r2 indicates the boundary position between the longitudinal center and the outer circumference, and w indicates the boundary position between the width center and the edge.

Furthermore, when calculating the winding temperature, a maximum value for the winding temperature is set. This maximum value can be arbitrarily set based on past operational performance; for example, it can be set to 700°C. If, after performing the procedures described below, the winding temperature calculations for all divided regions cannot be set so that the mechanical characteristics meet the target values and the variation in mechanical characteristics is within an acceptable range, the maximum value for the winding temperature may be changed, and the winding temperature calculations for each divided region may be recalculated.

In calculating the winding temperature for each divided region, first, the radial boundary positions r1 and r2 are determined. Here, as shown in Figure 4, the division in the width direction is not considered, and three radial regions are considered: the inner circumference (s31, s32), the longitudinal center (s21, s22), and the outer circumference (s11, s12). As initial values, the winding temperatures of the inner circumference (s31, s32) and outer circumference (s11, s12) are set to the maximum value, and the winding temperature of the longitudinal center (s21, s22) is set to a temperature lower than the maximum value. This is because the coil is cooled on three sides during the cooling process. In other words, physically, the coil is cooled from the surface, so the longitudinal center cools more slowly and becomes hotter than the inner and outer circumferences. By setting the winding temperatures of the outer and inner circumferences higher than the winding temperature of the longitudinal center, the temperature of the coil in the longitudinal direction after cooling tends to become more uniform. For example, the initial winding temperature of the inner circumference (s31, s32) and outer circumference (s11, s12) is set to 700°C, and the initial winding temperature of the longitudinal center (s21, s22) is set to 630°C.

Then, using radial boundary positions r1 and r2 as parameters, the tensile strength at a predetermined time after the completion of coil winding across the entire length and width of the coil is predicted for multiple cases in which boundary positions r1 and r2 are varied, using a material prediction model and a coil cooling model. Here, the tensile strength prediction calculation can be performed using nine points (Pe1 to Pe3, Pq1 to Pq3, Pc1 to Pc3) within the coil cross-section of the axisymmetric coil model with a plate width of 1/2 shown in Figure 2 as representative points of the entire length and width of the coil. In Figure 2, points Pe1 to Pe3 indicate the positions of the outermost, middle, and innermost parts of the coil in the radial direction on the coil side surface. Points Pq1 to Pq3 indicate the positions of the outermost, middle, and innermost parts of the coil in the radial direction at a point 1/4 of the plate width towards the center (also called the quarter section) from the coil side surface. Points Pc1 to Pc3 indicate the positions of the outermost, middle, and innermost parts of the coil in the radial direction at the center of the coil plate width.

Furthermore, to determine the radial boundary positions r1 and r2, the variation in tensile strength between the innermost circumference and the middle section, and between the outermost circumference and the middle section, is calculated at the center of the plate width. The variation in tensile strength may be expressed as the difference between the maximum and minimum values, or as the standard deviation or variance. Then, from multiple cases, the boundary position r1 that minimizes the variation in tensile strength between the innermost circumference and the middle section, and the boundary position r2 that minimizes the variation in tensile strength between the outermost circumference and the middle section are determined.

Figure 5 shows an example of the relationship between the regional ranges of the inner and outer circumferences and the predicted variation in tensile strength. In Figure 5, the regional ranges of the inner and outer circumferences are represented by the ratio of the radial length to the coil thickness A for each region. The variation in tensile strength is shown as the difference between the maximum and minimum values. In the example in Figure 5, when the inner circumference region is 20%, the variation in tensile strength between the innermost circumference and the middle section is minimized, and when the outer circumference region is 30%, the variation in tensile strength between the outermost circumference and the middle section is minimized. From these results, boundary position r1 is determined to be at a position 20% of the coil thickness A from the innermost circumference of the coil, and boundary position r2 is determined to be at a position 30% of the coil thickness A from the outermost circumference of the coil.

After determining the radial boundary positions r1 and r2, the optimal winding temperature at the longitudinal center is then determined. While the initial winding temperature at the longitudinal center (e.g., 630°C) was used to predict tensile strength when determining the radial boundary positions r1 and r2, here the optimal winding temperature at the longitudinal center is determined for the determined radial boundary positions r1 and r2. Using the winding temperature at the longitudinal center as a parameter, the tensile strength at a predetermined time after completion of coil winding across the entire length and width of the coil is predicted for multiple cases where the winding temperature at the longitudinal center is varied, using a material prediction model and a coil cooling model. The range of change in the winding temperature at the longitudinal center can be arbitrarily determined, for example, between 630°C and 510°C.

For example, under five calculation conditions (Cases 1 to 5 in Table 1, described later) where the winding temperature of the inner circumference (s31, s32) and outer circumference (s11, s12) was set to 700°C, and the winding temperature of the longitudinal center was set between 510 and 630°C, numerical analysis was used to calculate the estimated plate temperature 30 minutes after coil cooling and the tensile strength (TS) of the manufactured steel plate.

Numerical analysis of the coil cooling model was performed using finite element analysis with an axisymmetric coil model with a plate width of 1/2, as shown in Figure 2. The coil specifications were a plate width of 1000 mm, a plate thickness of 2.5 mm, and a plate length of 1000 m. The ambient temperature was assumed to be 15°C, and the heat transfer coefficient was assumed to be 20 W· ^{m⁻² K⁻¹} for natural convection, σ (= 5.67 × ^10⁻⁸ W· ^{m⁻² K⁻⁴} ) for Stefan-Boltzmann constant, and 0.6 for emissivity. Since the plate thickness is sufficiently small compared to the plate width and length, it was assumed that there was no temperature distribution in the thickness direction.

The tensile strength was predicted using a correlation formula obtained by analyzing the plate temperature after 30 minutes of cooling and the tensile strength (TS) at the nine coil positions shown in Figure 2. The correlation formula was obtained by first analyzing the plate temperature after 30 minutes of cooling at each coil position and the measured tensile strength (TS) of these coils actually manufactured in a hot rolling mill. The plate temperature after 30 minutes of cooling, obtained through numerical analysis, was substituted into the correlation formula to obtain an estimated tensile strength of the manufactured steel sheet.

Table 1 below shows the plate temperatures at the outermost (Pc1), middle (Pc2), and innermost (Pc3) parts of the coil plate width 30 minutes after coil cooling for each calculation condition. It also shows the innermost temperature difference (the temperature difference between the middle and innermost parts) and the outermost temperature difference (the temperature difference between the middle and outermost parts). Figure 6 shows the estimated tensile strength (TS) and the variation in estimated tensile strength for each calculation condition shown in Table 1. The variation in estimated tensile strength indicates the difference between the tensile strength of the outermost part and the tensile strength of the middle part.

As shown in Table 1 and Figure 6, it can be seen that as the winding temperature of the middle section increases and the difference between the winding temperature of the outermost and innermost sections decreases, the variation in tensile strength (deviation of the tensile strength of the outermost section relative to the middle section) also increases. For example, suppose the target value of the tensile strength of the hot-rolled steel sheet is 580 to 620 MPa as the allowable range for tensile strength, and the deviation of the tensile strength of the outermost section relative to the middle section is within 40 MPa as the variation in tensile strength. In order to satisfy the allowable range from the viewpoint of the target value of tensile strength, as shown in Figure 6, the winding temperature of the middle section, i.e., the longitudinal center section, must be 560°C or less. Also, in order to satisfy the allowable range from the viewpoint of tensile strength variation, as shown in Figure 6, the winding temperature of the middle section, i.e., the longitudinal center section, must be 560°C or less. Therefore, it is sufficient to set the optimal value for the winding temperature of the longitudinal center section to 560 to 510°C. For example, in order to reduce the variation in tensile strength, the optimal value for the winding temperature of the longitudinal center section may be set to 510°C.

Furthermore, when determining the optimal winding temperature at the longitudinal center, if the tensile strength does not meet the allowable range, the maximum value of the winding temperature may be changed as described above, and the radial boundary positions r1 and r2 may be determined again to find the optimal winding temperature at the longitudinal center.

After determining the radial boundary positions r1 and r2 and calculating the radial winding temperature, the next step is to determine the widthwise boundary position w and winding temperature. As mentioned above, the coil is cooled on three sides during the cooling process. That is, the coil is cooled from the sides, resulting in a lower temperature at the edges compared to the center of the sheet width. Therefore, the sheet temperature at the edges is preheated using edge heaters installed on the entry side of the finishing mill, cooling devices installed on the runout table between the finishing mill and the coiler, and edge masks. Thus, the widthwise division is performed in the heated edge region, and the boundary position w is determined accordingly.

First, the initial value of the boundary position w is set, and the tensile strength over the entire length and width of the coil is determined using a material prediction model and a coil cooling model, with the edge heating temperature T- _up as a parameter. The boundary position w may be expressed as, for example, the distance from the side of the coil. The initial value of the boundary position w can be set arbitrarily, for example, to 62.5 mm. As a result, as shown in Figure 7, the divided regions s12, s22, and s32 are set as heating regions with a higher heating temperature T- _up than the divided regions s11, s21, and s31, respectively. The range of change in the edge heating temperature T- _up can be determined arbitrarily, for example, to 0 to 75°C. Then, for multiple cases in which the edge heating temperature T- _up is changed, the tensile strength over the entire length and width of the coil is determined using a quality prediction model.

Figure 8 shows an example of the relationship between the edge heating temperature T- _up , obtained from this tensile strength prediction, and the variation in tensile strength. Here, the variation in tensile strength is shown as the difference between the maximum and minimum values of tensile strength at representative points in the coil side and the center of the plate width region. In finite element analysis, the temperature can be determined for each integration point of the region-divided elements, and the tensile strength can be predicted. In this case, the variance or standard deviation of the tensile strength obtained for each integration point of each element may be used as the variation in tensile strength. In the example in Figure 8, the variation in tensile strength decreases as the edge heating temperature T- _up increases. From this, the equipment limit of 75°C can be determined as the edge heating temperature T- _up .

Then, with the edge heating temperature T _up set to 75°C, the tensile strength over the entire length and width of the coil is determined using a material prediction model and a coil cooling model, with the boundary position w as a parameter. The range of change in the boundary position w can be arbitrarily determined, for example, from 62.5 to 187.5 mm. Then, for multiple cases in which the boundary position w is changed, the tensile strength over the entire length and width of the coil is determined using a quality prediction model. Figure 9 shows an example of the relationship between the boundary position w and the variation in tensile strength obtained from this tensile strength prediction. Here, the variation in tensile strength is shown as the difference between the maximum and minimum values of the tensile strength at representative points in the coil side and the center region of the plate width. Note that if the tensile strength is determined for each integration point of the elements divided into regions by finite element analysis, the variance or standard deviation of the tensile strength may be used as the variation in tensile strength. In the example in Figure 9, the variation in tensile strength decreases as the boundary position w increases. From this, the equipment limit of 187.5 mm can be determined as the boundary position w.

Following this procedure, for example, as shown in Figure 10, the winding temperature calculation values for each of the six divided regions (s11, s12, s21, s22, s31, s32) are obtained by dividing the coil cross-section of an axisymmetric coil model with a plate width of 1/2 into two sections in the width direction and three sections in the radial direction. In this way, the winding temperature calculation values for each divided region can be determined while sequentially determining the boundary positions of the divided regions of the coil cross-section so that the mechanical characteristics meet the target values and the variation in mechanical characteristics is within an acceptable range.

(Calculation Method 2) Calculation of Winding Temperature Calculation Values for Each Divided Region Using an Evaluation Function Next, we will explain a method for determining the winding temperature calculation values for each divided region set in the coil cross-section such that the mechanical properties meet the target values and the variation in mechanical properties is within an acceptable range, using an evaluation function. In this method, for multiple cases with different winding temperatures for the divided regions, a material prediction model is used to calculate the mechanical properties and winding temperature calculation values at a predetermined time after the completion of finish rolling or after the completion of coil winding. The winding temperature of each divided region in the case where the evaluation function, expressed by the target value of the mechanical properties and the acceptable range of variation in the mechanical properties, is minimized is determined as the winding temperature calculation value for each divided region.

In the method described above for calculating the boundary positions of the divided regions and the winding temperature values stepwise, for example, in an axisymmetric coil model with a plate width of 1/2 as shown in Figure 3, the coil cross-section is divided into two in the width direction and three in the radial direction, resulting in six divided regions. Multiple divided regions are pre-set by dividing the coil in at least one of the radial or width directions. However, it is possible to set even more divided regions in the coil cross-section. For example, the coil cross-section may be divided evenly into 100 sections in the width direction and 100 sections in the radial direction, resulting in 10,000 divided regions. Even when the number of divided regions increases, it is still possible to calculate the boundary positions of the divided regions and the winding temperature values stepwise as described above. However, it is also possible to use an optimization method to calculate the winding temperature values for each divided region so that the mechanical characteristics reach the target value and the variation in mechanical characteristics remains within an acceptable range.

For example, multiple cases are prepared in which the winding temperature of each divided region is set in the range of 800 to 400°C in 25°C increments. Next, for each case, an analysis is performed using a material prediction model to predict the mechanical properties at the time of completion of finish rolling or a predetermined time after coil winding completion. Then, from the predicted mechanical properties, the winding temperature of the case where the mechanical properties meet the target value and the variation in mechanical properties is within an acceptable range, and where the evaluation function represented by the error from the target value of the mechanical properties and the variation in mechanical properties is minimized, is taken as the calculated winding temperature value for each divided region. The optimization method used to perform this process is not particularly limited, but for example, random search or a genetic algorithm may be used.

Thus, the method of determining the winding temperature for each divided region where the evaluation function is minimized using an optimization technique allows for easy calculation of the winding temperature over the entire length and width of the coil, regardless of whether the number of divided regions is small (as shown in Figure 3) or numerous. This ensures that the mechanical characteristics meet the target values and that variations in mechanical characteristics remain within an acceptable range.

The material prediction model used in the two methods for calculating the winding temperature described above is not particularly limited. For example, the relationship (correlation equation) between the sheet temperature and mechanical properties of hot-rolled steel sheets obtained by the following method may be used as the material prediction model to calculate the winding temperature.

The calculation of winding temperature using a correlation formula involves first setting the period from the completion of finish rolling or coil winding to a predetermined time elapsed as the temperature acquisition period. During this period, the plate temperature across the entire length and width of the coil is acquired as a temperature history. Based on the mechanical properties measured at multiple locations on the manufactured coil and the parameters derived from the plate temperature at those locations obtained from the acquired temperature history, a correlation formula between mechanical properties and parameters is determined. This correlation formula becomes the material prediction model. Using the obtained correlation formula, mechanical properties can be predicted from the values of the parameters based on the plate temperature.

Here, the parameter based on the plate temperature during the coil cooling process refers to information regarding the plate temperature during the coil cooling process, which affects the coil's material. Such a parameter may be, for example, the plate temperature itself after a predetermined time has elapsed since the start of coil cooling, or it may be the integral value of the change in plate temperature over time during the coil cooling process. Using such a parameter, an appropriate correlation equation representing the relationship with the coil's mechanical properties (e.g., tensile strength (TS)) can be obtained. Below, we will explain three methods for using the coil's plate temperature as a parameter: Method A, Method B, Method C, and Method C, Method B using the integral value obtained by multiplying the change in coil's plate temperature by cumulative time (also referred to as "cumulative integrated temperature").

(Method A: Using the coil plate temperature as a parameter)
For example, as shown in Figure 11, a correlation can be observed between the plate temperature at nine coil positions 30 minutes after the start of coil cooling and the measured tensile strength (TS) of the steel plate, which can be expressed by a linear function, for example. Therefore, using the plate temperature at multiple positions of the coil at a predetermined time after the completion of coil winding as a parameter based on the coil plate temperature, the relationship with the mechanical properties of the coil after it has cooled to room temperature can be expressed. The correlation formula between the parameter and the coil material is determined for each type of steel.

Specifically, to determine the correlation equation, the temperature change of the steel plate during the cooling process is obtained for multiple coils of a single steel type, each with a different initial plate temperature. The plate temperature can be determined by numerical analysis using a computer or by actual measurement. For example, when determining the temperature by numerical analysis, the input value is the winding temperature of the steel plate, and an analysis simulating the coil cooling process in an actual machine is performed using the analysis model shown in Figure 2, either as a finite element analysis or a difference method analysis. This allows for the determination of the temperature change (temperature history) over the entire length and width of the coil during the cooling process, at least for a predetermined time, from the completion of coil winding to the completion of cooling. Alternatively, when measuring the temperature change during the coil cooling process, the temperature of the steel plate can be measured using, for example, a thermocouple.

Here, the temperature acquisition period for obtaining the temperature history is a predetermined time from the completion time of finish rolling or the completion time of coil winding until a predetermined time has elapsed, and is set appropriately depending on the type of steel. The length of the temperature acquisition period corresponds to the time during which transformation may occur during the coil cooling process, and is usually set to about 5 to 60 minutes, for example, about 30 minutes. The plate temperature from the completion time of finish rolling to the start of coil winding can be obtained based on known methods (for example, Non-Patent Document 2).

Furthermore, for multiple coils with different plate temperatures at the start of cooling, the cooling process is actually carried out, and the tensile strength of the coils after cooling to room temperature is measured. Tensile strength is measured at multiple locations on the coil. For example, as shown in the analysis model in Figure 2, tensile strength can be measured at the outermost, middle, and innermost parts of the coil, specifically at the side, quarter, and center of the plate width. Increasing the number of measurement locations for tensile strength can improve the accuracy of the resulting correlation equation. To further improve the accuracy of the correlation equation, it is advisable to measure the tensile strength at the outermost and innermost parts of the coil, where the plate temperature changes significantly during the cooling process.

Next, the relationship between the coil plate temperature at a predetermined time after coil winding completion (obtained through numerical analysis or actual measurement) and the measured coil tensile strength is determined. Specifically, for each tensile strength measurement location, the plate temperature at a predetermined time after the start of cooling (obtained through numerical analysis or actual measurement) is associated. Then, a correlation equation representing this relationship is determined based on the plate temperature and tensile strength at multiple locations. This correlation equation is expressed as an approximation, and can be represented, for example, as a linear function correlation equation (y = -0.6137x + 1049.7) as shown in Figure 11. Note that the correlation equation may be a linear function, a higher-order function of quadratic or higher, an exponential function, a logarithmic function, or a power function; the form of the regression equation is not limited. Such approximation equations are determined in advance for each type of steel.

By using the correlation formula obtained in this manner, corresponding to the hot-rolled steel sheet being manufactured, the mechanical properties of the hot-rolled steel sheet can be determined from the sheet temperature at a predetermined time after the completion of finish rolling or coil winding.

(Method B: Using the integrated temperature of the change in coil plate temperature as a parameter)
When using the integral value of the change in plate temperature over time during the coil cooling process (integrated temperature) as a parameter based on the plate temperature of the coil, the correlation equation between the parameter and the tensile strength can be determined in the same way as in method A described above.

Specifically, to determine the correlation formula, the temperature change (temperature history) during the cooling process of multiple coils with different initial temperatures for a single steel type is obtained through computer-based numerical analysis or actual measurement. Furthermore, these multiple coils with different initial temperatures are actually cooled, and the tensile strength of the coils after they have cooled to room temperature is measured. These processes can be carried out in the same manner as in Method A described above.

Next, we calculate the integral value (integrated temperature) of the change in the coil plate temperature over time, which is a parameter based on the coil plate temperature.

First, the start temperature Tsum_s and end temperature Tsum_e are set to calculate the cumulative temperature. The start temperature Tsum_s and end temperature Tsum_e are used commonly for the temperature history at each tensile strength measurement location. The setting of the start temperature Tsum_s and end temperature Tsum_e is done by arbitrarily setting these values and calculating the integral value (cumulative temperature) of the change in plate temperature over time from the start temperature Tsum_s to the end temperature Tsum_e for each tensile strength measurement location's temperature history. Then, the coefficient of determination of degrees of freedom (CRO) is calculated for the relationship between the cumulative temperature at multiple points and the measured tensile strength at the initially set and determined start and end temperatures (Tsum_s and Tsum_e). Mathematical optimization methods such as linear programming and nonlinear programming are then used to modify the start and end temperatures (Tsum_s and Tsum_e) to maximize the CRO, thereby determining a common start and end temperature (Tsum_s and Tsum_e) that results in the maximum CRO.

After setting a common integration start temperature Tsum_s and integration end temperature Tsum_e, the integrated temperature is then calculated for the temperature history at each tensile strength measurement location. The integrated temperature is the value obtained by accumulating the difference ΔT(t) (= Tsum-s-T(t)) between the coil plate temperature T(t) at time t and the integration start temperature Tsum_s during the integration period from the integration start temperature Tsum_s to the integration end temperature Tsum_e. Specifically, this can be expressed by the following equation (1). In equation (1), Δt is the time interval for acquiring the plate temperature (plate temperature acquisition period).

Figure 12 shows the temperature histories A, B, and C obtained at different positions on the coil. In the graphs for each temperature history A, B, and C, the horizontal axis represents the time from the start of cooling (i.e., the completion of coil winding), and the vertical axis represents the coil plate temperature. Compared to the coil position where temperature history A was obtained, the cooling at the coil position where temperature history B was obtained was slower, and the cooling at the coil position where temperature history C was obtained was faster. The integrated temperature TT is the integral value of the change in coil plate temperature ΔT(t) over time, calculated by setting a common integration start temperature Tsum_s and integration end temperature Tsum_e for these temperature histories A, B, and C. Compared to the integrated temperature TT of temperature history A, the integrated temperature TT of temperature history B is larger, and the integrated temperature TT of temperature history C is smaller.

By calculating the integrated temperature of the coil at each tensile strength measurement location in this manner, the relationship between the integrated temperature of the coil and the measured tensile strength of the steel plate is determined. That is, for each tensile strength measurement location, the integrated temperature calculated from the temperature history obtained by numerical analysis or actual measurement is associated. Then, a correlation equation representing the relationship between the integrated temperature and tensile strength at multiple locations is obtained. The correlation equation is expressed as an approximation, and can be expressed as a linear function, for example, as shown in Figure 13. The correlation equation in Figure 13 (y = -0.0016x + 804.1) has a correlation with a coefficient of determination of degrees of freedom ( ^R² ) of 0.88. Note that the correlation equation may be a higher-order function of second order or higher, an exponential function, a logarithmic function, or a power function, and the form of the regression equation is not limited. Such approximation equations are determined in advance for each type of steel.

Furthermore, by using a correlation formula corresponding to the hot-rolled steel sheet being manufactured, the mechanical properties of the hot-rolled steel sheet can be determined from the integrated coil temperature obtained from the sheet temperature at the time of completion of finish rolling or a predetermined time after the completion of coil winding.

(Method C: When using cumulative integrated temperature, which is obtained by multiplying the change in coil plate temperature by the cumulative time, as a parameter.)
Method C is a modification of Method B, and uses the cumulative value (cumulative cumulative temperature) obtained by multiplying the change in plate temperature during the coil cooling process by the cumulative time as a parameter based on the plate temperature of the coil.

In the calculation process for cumulative temperature, first, as in Method B, a common integration start temperature Tsum_s and integration end temperature Tsum_e are set for the temperature history. Then, based on the set common integration start temperature Tsum_s and integration end temperature Tsum_e, the cumulative temperature is calculated for the temperature history at each tensile strength measurement location. The cumulative temperature is the value accumulated over the integration period from the integration start temperature Tsum_s to the integration end temperature Tsum_e, calculated by multiplying the difference ΔT(t) between the coil plate temperature T(t ₎ at time t and the integration start temperature Tsum_s by the cumulative time t _a (= t - t _s ) from the integration start time t s to time t when the integration start temperature Tsum_s was reached. Specifically, it can be expressed by the following equation (2).

Based on equation (2) above, the cumulative integrated temperature of the coil at each tensile strength measurement position is calculated, and the relationship (correlation equation) between the cumulative integrated temperature of the coil and the measured tensile strength of the steel plate is determined. Figure 14 shows an example of the relationship between the cumulative integrated temperature of the coil and the tensile strength. The correlation equation obtained by method C can also be expressed as an approximate equation, and can be expressed as a linear function, for example, as shown in Figure 14. The correlation equation in Figure 14 (y = -0.000005x + 781.99) has a correlation with a coefficient of determination of degrees of freedom ( ^R² ) of 0.95. Note that the correlation equation may be a higher-order function of second order or higher, an exponential function, a logarithmic function, or a power function, and the form of the regression equation is not limited.

Furthermore, by using a correlation formula corresponding to the hot-rolled steel sheet being manufactured, the mechanical properties of the hot-rolled steel sheet can be determined from the cumulative integrated temperature of the coil, which is obtained from the sheet temperature at the time of completion of finish rolling or a predetermined time after the completion of coil winding.

[2-1-2. Method for calculating winding temperature using the correspondence between temperature history and mechanical properties]
In another example of the manufacturing method for hot-rolled steel sheets according to this embodiment, a table is obtained that shows the correspondence between the temperature history of the entire length and width of the hot-rolled steel sheet, which has been acquired in advance for each steel type, and the mechanical properties obtained by measuring test pieces cut from the hot-rolled steel sheet. Based on this table, a calculated winding temperature value for the entire length and width of the hot-rolled steel sheet is calculated such that the mechanical properties of the hot-rolled steel sheet to be manufactured reach the target value and the variation in mechanical properties is within an acceptable range.

First, for each steel type, a correspondence relationship is obtained between the temperature history acquired over the entire length and width of multiple hot-rolled steel sheets and the mechanical properties obtained by measuring test specimens cut from the hot-rolled steel sheets. This correspondence relationship is determined for each steel type. For each steel type, hot-rolled steel sheets are manufactured under multiple different temperature histories, and tensile tests are performed on test specimens cut from the manufactured sheets to measure their tensile strength. This allows the relationship between tensile strength and temperature history to be obtained for each steel type.

Then, by identifying the temperature history at each position of the hot-rolled steel sheet where the mechanical properties meet the target values and the variation in mechanical properties is within the acceptable range, the winding temperature can be determined from the temperature history.

[2-1-3. Method for calculating winding temperature through experiments using actual equipment]
The calculated winding temperature for the entire length and width, where the mechanical properties meet the target values and the variation in mechanical properties is within an acceptable range, may be determined through trial-and-error experiments on the actual machine. When determining the winding temperature through experiments, for example, the following two experiments should be conducted for each type of steel.

(Experiment 1)
First, using a single steel type, multiple levels of heating or cooling are performed on the hot-rolled steel sheet before winding. At this time, the heating and cooling patterns performed are recorded. Next, the winding temperature of the entire length and width during winding is measured using a pre-winding thermometer. The pre-winding thermometer may be, for example, a thermocouple. At least one pre-winding thermometer is required in the width direction of the sheet, and it may be installed in a way that allows measurement of the sheet temperature at the center of the width, for example. The pre-winding thermometer measures the sheet temperature at multiple positions in the coil diameter direction. For example, the sheet temperature may be measured at three locations: the innermost part of the coil, the center, and the outermost part. The measurement positions for the sheet temperature in the coil diameter direction should be set appropriately according to the target values of the mechanical properties.

Furthermore, the mechanical properties (e.g., tensile strength) of multiple coils manufactured using each heating and cooling pattern are measured after cooling to room temperature. The measurement location for the mechanical properties can be, for example, the same location where the plate temperature was measured using a pre-winding thermometer. Then, for multiple divided regions established by dividing the coil cross-section in at least one direction (radial or widthwise), the temperature range for winding the coil is determined such that the measured mechanical properties reach the target value and the variation in mechanical properties is within an acceptable range. For example, the temperature range for winding the coil is determined for each of the three divided regions: the inner circumference, the center, and the outer circumference.

(Experiment 2)
After determining the winding temperature range in multiple divided regions using Experiment 1, multiple levels of heating or cooling are performed on a single steel type before winding the hot-rolled steel sheet. Then, a heating and cooling pattern is determined that results in the winding temperature in each divided region being within the temperature range determined in Experiment 1.

By conducting Experiment 1 and Experiment 2 for each type of steel, it is possible to calculate the winding temperature for the entire length and width of the coil.

As explained above, winding temperature can be calculated using various methods. However, the method for calculating winding temperature is not limited to the methods described above.

Considering that the outer, side, and inner surfaces of a coil cool more easily than the interior during the coil cooling process, Table 1 and Figure 6 indicate that it is desirable to wind the easily cooled regions (i.e., the outer, side, and inner surfaces of the coil) at high temperatures. Therefore, the calculated winding temperatures for the inner and outer parts of the coil should be determined to be higher than the calculated winding temperature for the longitudinal center. The inner part of the coil is the leading edge of the hot-rolled steel sheet, and in the shortest case, it is the area equivalent to one full turn of the innermost part of the coil. The outer part of the coil is the trailing edge of the hot-rolled steel sheet, and in the shortest case, it is also the area equivalent to one full turn of the outermost part of the coil. The longitudinal center is the area other than the inner and outer parts. For example, the calculated winding temperatures could be set as follows: inner part 700-750°C, longitudinal center 475-560°C, and outer part 675-800°C.

Furthermore, during the coil cooling process, the sides of the coil cool more easily than the inside of the coil. Therefore, for the edges in the width direction, it is preferable to determine the winding temperature calculation value such that the plate temperature in the width direction becomes uniform after a predetermined time (e.g., 30 minutes) has elapsed since the completion of coil winding. The edge of the coil (hot-rolled steel sheet) is the region from the side of the coil (the end of the hot-rolled steel sheet) toward the center in the width direction, up to a predetermined distance. For example, when the plate width is 1000 mm, the edge may be defined as the region from 62.5 mm, 125 mm, or 187.5 mm from the side of the coil (the end of the hot-rolled steel sheet). The region between two edge sections in the width direction of the coil (hot-rolled steel sheet) is defined as the width center.

The degree of uniformity of the sheet temperature in the width direction after a predetermined time has elapsed since the completion of coil winding varies depending on the type of steel, and should be determined according to the variation in the mechanical properties required for the hot-rolled steel sheet. By determining the calculated winding temperature of the edge portion in the width direction such that the sheet temperature is uniform after a predetermined time has elapsed since the completion of coil winding, the mechanical properties of the hot-rolled steel sheet can be made more uniform across its entire length and width.

[2-2. Manufacturing of Hot-Rolled Steel Sheets]
After calculating the winding temperature for the entire length and width of the hot-rolled steel sheet to be manufactured, the hot-rolled steel sheet is manufactured by heating or cooling it before winding, so that the winding temperature of the hot-rolled steel sheet becomes the calculated winding temperature for the entire length and width.

As explained with reference to Figure 1, temperature control across the entire length and width of the steel sheet during the hot rolling process is achieved by first determining the winding temperature at which the mechanical properties of the material, such as tensile strength (TS), r-value, yield strength (YS), uniform elongation, and fracture elongation, are within target values, and then controlling the hot rolling equipment 1 to achieve the predetermined winding temperature. Specifically, the temperature in the longitudinal direction (thread direction) of the steel sheet is controlled by heating with a bar heater 10 installed on the entry side of the finishing rolling mill 30 and cooling with a cooling device 40 installed between the finishing rolling mill 30 and the coiler 80. The temperature in the width direction is controlled by heating with an edge heater 20 installed on the entry side of the finishing rolling mill 30 and cooling adjustment with an edge mask 55 installed on the runout table 50 between the finishing rolling mill 30 and the coiler 80, corresponding to the cooling device 40.

The control device (not shown) of the hot rolling mill 1 controls the bar heater 10, edge heater 20, cooling device 40, and edge mask 55 based on the temperature of the steel sheet measured by the finishing exit thermometer 61 and the pre-winding thermometer 63, so that the winding temperature of the steel sheet reaches a predetermined winding temperature (i.e., the calculated winding temperature). In this way, by performing at least one of heating or cooling on the hot-rolled steel sheet before winding, it is possible to manufacture hot-rolled steel sheets with low variation and desired mechanical properties.

Furthermore, in the manufacturing of hot-rolled steel sheets, the use of mandrel cooling water may be omitted when winding the hot-rolled steel sheets. The mandrel is a device that winds the manufactured hot-rolled steel sheets into a coil, and it becomes very hot during this process. To prevent seizing of the sliding parts of the mandrel, the inside of the mandrel is cooled with mandrel cooling water. However, using mandrel cooling water can excessively cool the inner surface of the coil. In operations where the outer and inner circumferences of the coil, which are easily cooled, are wound at higher temperatures than the middle section, excessive cooling of the inner surface of the coil can affect the mechanical properties of the coil and lead to significant variations. Therefore, by omitting the use of mandrel cooling water when winding the hot-rolled steel sheets, excessive cooling of the inner surface of the coil can be suppressed, reducing its impact on mechanical properties.

The above describes a method for manufacturing a hot-rolled steel sheet according to one embodiment of the present invention. According to this embodiment, the winding temperature values for the entire length and width of the hot-rolled steel sheet are calculated in advance so that the mechanical properties of the sheet meet the target values and the variation in mechanical properties is within an acceptable range. Before winding, at least one of heating or cooling is performed on the hot-rolled steel sheet so that its winding temperature matches the calculated winding temperature values for the entire length and width. This makes it possible to manufacture a hot-rolled steel sheet that has the desired mechanical properties and low variation throughout its entire length and width.

To verify the effectiveness of the hot-rolled steel sheet manufacturing method of the present invention, 1,000 steel sheets were rolled on an actual machine under the following rolling conditions, and the tensile strength of the manufactured hot-rolled steel sheets was measured as a mechanical property.

(Rolling conditions)
Steel type: C mass%: 0.001 to 1.0
Thickness of the plate at the entry side of the finishing rolling mill: 2.0 to 20 mm
Reduction ratio (each stand): 10-50%
Plate temperature at the first stand entry side of the finishing rolling mill: 800-1100°C

As a comparative example, hot-rolled steel sheets were manufactured by setting the heating and cooling patterns in the hot-rolling equipment based on experience, as is done in conventional operations. In Examples 1 to 6, hot-rolled steel sheets were manufactured based on the manufacturing method of hot-rolled steel sheets according to the above embodiment. In Example 1, the winding temperature was calculated through experiments using an actual machine. In Examples 2 to 5, the winding temperature was calculated using a material prediction model. As parameters based on the coil plate temperature, Examples 2 and 3 used the coil plate temperature (Method A above), Example 4 used the accumulated temperature (Method B above), and Example 5 used the cumulative accumulated temperature of the coil (Method C above). In Example 6, the winding temperature was calculated using the correspondence between temperature history and mechanical properties. The number of divided regions of the coil cross-section (i.e., the number of representative points for the winding temperature calculation) set in Examples 1 to 6 is as shown in Table 2.

Table 2 shows the material accuracy rates for the comparative examples and Examples 1-10. The material accuracy rate represents the percentage of hot-rolled steel sheets out of 1000 manufactured sheets where the mechanical properties of the manufactured sheets met the target values within the defined divisional area, and where the variation was within the acceptable range. In Table 2, the numbers following "Length:" and "Width:" in the material accuracy rate [%] indicate the number of locations where the material was measured. For example, "Length: 6, Width: 3" means that the material was measured at 6 locations in the longitudinal direction and 3 locations in the width direction, for a total of 18 locations. The measurement locations were the midpoints of the longitudinal and width directions within each divisional area.

Table 2 shows that in the comparative examples, the percentage of samples meeting the allowable tensile strength was at most about 60%, while in Examples 1 to 6, the percentage of samples meeting the allowable tensile strength was 80% or higher in all divided regions. In Example 3, where the divided regions were set corresponding to the tensile strength measurement locations, the material accuracy rate was 100% at all measurement locations. Furthermore, as shown in Examples 7 to 10, similar effects were obtained for mechanical properties other than tensile strength, such as r-value, yield strength (YS), uniform elongation, and fracture elongation.

Although preferred embodiments of the present invention have been described in detail above with reference to the attached drawings, the present invention is not limited to these examples. It is clear to any person with ordinary skill in the art to which the present invention pertains that various modifications or alterations can be conceived within the scope of the technical idea described in the claims, and these, too, are naturally understood to fall within the technical scope of the present invention.

For example, in the above embodiment, the case was described in which the temperature of the steel sheet is controlled by heating using a bar heater and edge heater installed on the inlet side of the finishing rolling mill, and cooling using a cooling device and edge mask installed on the runout table between the finishing rolling mill and the coiler. However, the installation locations of the heating and cooling devices are not limited to this example.

Furthermore, the following configurations are also included within the technical scope of the present invention.
(1)
A method for manufacturing hot-rolled steel sheets for producing high-strength steel,
In advance, calculate the winding temperature values for the total length and width such that the mechanical properties of the hot-rolled steel sheet to be manufactured meet the target values, and the variation in said mechanical properties is within an acceptable range.
A method for manufacturing a hot-rolled steel sheet, comprising heating or cooling the hot-rolled steel sheet before winding it, such that the winding temperature of the hot-rolled steel sheet becomes the calculated winding temperature for the entire length and width.
(2)
Multiple coil positions within the axial cross-section of a hot-rolled steel sheet coil are used as representative points for the entire length and width of the hot-rolled steel sheet.
A method for manufacturing a hot-rolled steel sheet as described in (1) above, comprising using a material prediction model that represents the relationship between the sheet temperature and mechanical properties of the hot-rolled steel sheet, calculating the mechanical properties and the calculated winding temperature for each of the multiple coil positions at a predetermined time after the completion of finish rolling or after the completion of coil winding, and determining the calculated winding temperature for the entire length and width.
(3)
The aforementioned material prediction model is
The period from the time of completion of finish rolling or coil winding until a predetermined time has elapsed is defined as the temperature acquisition period.
A method for manufacturing a hot-rolled steel sheet as described in (2) above, wherein the method is expressed by a correlation equation between parameters based on the plate temperature, obtained from the temperature history of the plate temperature during a period including the temperature acquisition period, and the mechanical properties measured in the manufactured hot-rolled steel sheet coil.
(4)
The method for manufacturing a hot-rolled steel sheet according to (3) above, wherein the parameter is the sheet temperature at a predetermined time after the completion of finish rolling or coil winding within the temperature acquisition period.
(5)
The method for manufacturing a hot-rolled steel sheet as described in (3) above, wherein the parameter is the integrated temperature, which is the integral value over time of the change in the plate temperature from the integrated start temperature to the integrated end temperature during an integrated period from a preset integrated start temperature to an integrated end temperature.
(6)
The method for manufacturing a hot-rolled steel sheet as described in (3) above, wherein the parameter is the cumulative cumulative temperature, which is the cumulative value obtained by multiplying the amount of change in the plate temperature from the cumulative start temperature by the cumulative time during the cumulative period from a preset cumulative start temperature to a cumulative end temperature.
(7)
Corresponding to the coil position, the axial cross-section of the hot-rolled steel sheet coil is divided in at least one of the radial or widthwise directions to set up multiple divided regions.
For multiple cases where the winding temperature of the divided region differs, the material prediction model is used to calculate the mechanical characteristics and the calculated winding temperature at the time of completion of the finish rolling or a predetermined time after the completion of the coil winding.
A method for manufacturing a hot-rolled steel sheet according to any one of (2) to (6) above, wherein the winding temperature of each divided region in the case in which the evaluation function expressed by the target value of the mechanical properties and the allowable range of variation of the mechanical properties is minimized is determined as the calculated winding temperature of each divided region.
(8)
Corresponding to the coil position, the axial cross-section of the hot-rolled steel sheet coil is divided in at least one of the radial or widthwise directions to set up multiple divided regions.
For multiple cases with different boundary positions or winding temperatures of the divided region, the mechanical characteristics and the calculated winding temperature are calculated using the material prediction model at the time of completion of finish rolling or a predetermined time has elapsed since the completion of coil winding.
A method for manufacturing a hot-rolled steel sheet according to any one of the above (2) to (6), wherein the boundary position of the divided region is determined such that the calculated mechanical properties become target values and the variation in the mechanical properties is within an acceptable range, and the calculated winding temperature value of each divided region is determined.
(9)
A table is obtained that shows the correspondence between the temperature history of the entire length and width of the hot-rolled steel sheet, which has been acquired in advance for each type of steel, and the mechanical properties obtained by measuring test pieces cut from the hot-rolled steel sheet.
A method for manufacturing a hot-rolled steel sheet as described in (1) above, comprising calculating the winding temperature for the total length and width of the hot-rolled steel sheet to be manufactured, based on the table, such that the mechanical properties of the hot-rolled steel sheet to be manufactured meet the target value and the variation in the mechanical properties is within an acceptable range.
(10)
When the axial cross-section of a hot-rolled steel sheet coil is divided into three parts from the radially inner side: the inner circumference, the longitudinal center, and the outer circumference,
A method for manufacturing a hot-rolled steel sheet according to any one of (1) to (9) above, wherein the calculated winding temperature values of the inner circumference and the outer circumference are determined to be higher than the calculated winding temperature value of the longitudinal center.
(11)
The method for manufacturing a hot-rolled steel sheet according to (10) above, wherein the calculated winding temperature is 700 to 750°C for the inner circumference, 475 to 560°C for the longitudinal center, and 675 to 800°C for the outer circumference.
(12)
Regarding the edges at both ends in the width direction of the axial cross-section of a hot-rolled steel sheet coil,
The winding temperature value at which the temperature in the width direction of the board becomes uniform after a predetermined time has elapsed since the completion of coil winding is calculated in advance.
A method for manufacturing a hot-rolled steel sheet according to any one of the above items (1) to (11), wherein at least one of heating with an edge heater or cooling adjustment with an edge mask is performed before winding the hot-rolled steel sheet.
(13)
A method for manufacturing a hot-rolled steel sheet according to any one of the above items (1) to (12), wherein no mandrel cooling water is used when winding the hot-rolled steel sheet.
(14)
The method for manufacturing a hot-rolled steel sheet according to any one of the above items (1) to (13), wherein the mechanical property is tensile strength.

1. Hot rolling equipment 10. Bar heater 20. Edge heater 30. Finishing rolling mill 40. Cooling system 50. Runout table 55. Edge mask 61. Finishing exit thermometer 63. Pre-winding thermometer 70. Pinch roll 80. Coiler 85. Mandrel C. Coil

Claims (12)

A method for manufacturing hot-rolled steel sheets for producing high-strength steel,
In advance, calculate the winding temperature values for the total length and width such that the mechanical properties of the hot-rolled steel sheet to be manufactured meet the target values, and the variation in said mechanical properties is within an acceptable range.
Before winding the hot-rolled steel sheet, at least one of heating or cooling is performed on the hot-rolled steel sheet so that the winding temperature of the hot-rolled steel sheet becomes the calculated winding temperature for the entire length and width .
Multiple coil positions, divided radially and widthwise within the axial cross-section of a hot-rolled steel sheet coil, are used as representative points for the entire length and width of the hot-rolled steel sheet.
A method for manufacturing a hot-rolled steel sheet, comprising using a material prediction model that represents the relationship between the sheet temperature and mechanical properties of the hot-rolled steel sheet, calculating the mechanical properties and the calculated winding temperature for each of the multiple coil positions at a predetermined time after the completion of finish rolling or after the completion of coil winding, and determining the calculated winding temperature for the entire length and width . A method for manufacturing hot-rolled steel sheets for producing high-strength steel,
In advance, calculate the winding temperature values for the total length and width such that the mechanical properties of the hot-rolled steel sheet to be manufactured meet the target values, and the variation in said mechanical properties is within an acceptable range.
Before winding the hot-rolled steel sheet, at least one of heating or cooling is performed on the hot-rolled steel sheet so that the winding temperature of the hot-rolled steel sheet becomes the calculated winding temperature for the entire length and width.
Multiple coil positions within the axial cross-section of a hot-rolled steel sheet coil are used as representative points for the entire length and width of the hot-rolled steel sheet.
Using a material prediction model that represents the relationship between the sheet temperature and mechanical properties of a hot-rolled steel sheet, the mechanical properties and the calculated winding temperature are calculated for each of the multiple coil positions at a predetermined time after the completion of finish rolling or after the completion of coil winding, and the calculated winding temperature for the entire length and width is determined.
The aforementioned material prediction model is
The period from the time of completion of finish rolling or coil winding until a predetermined time has elapsed is defined as the temperature acquisition period.
A method for manufacturing a hot-rolled steel sheet, expressed as a correlation equation between parameters based on the plate temperature, obtained from the temperature history of the plate temperature during a period including the aforementioned temperature acquisition period, and the mechanical properties measured in the manufactured hot-rolled steel sheet coil. The method for manufacturing a hot-rolled steel sheet according to claim 2 , wherein the parameter is the sheet temperature at a predetermined time after the completion of finish rolling or coil winding within the temperature acquisition period. The method for manufacturing a hot-rolled steel sheet according to claim 2 , wherein the parameter is the integrated temperature, which is the integral value over time of the change in the plate temperature from the integrated start temperature to the integrated end temperature during an integrated period from a preset integrated start temperature to an integrated end temperature. The method for manufacturing a hot-rolled steel sheet according to claim 2, wherein the parameter is the cumulative cumulative temperature, which is the cumulative value obtained by multiplying the amount of change in the plate temperature from the cumulative start temperature by the cumulative time during the cumulative period from a preset cumulative start temperature to a cumulative end temperature. Corresponding to the coil position, the axial cross-section of the hot-rolled steel sheet coil is divided in at least one of the radial or widthwise directions to set up multiple divided regions.
For multiple cases where the winding temperature of the divided region differs, the material prediction model is used to calculate the mechanical characteristics and the calculated winding temperature at the time of completion of the finish rolling or a predetermined time after the completion of the coil winding.
A method for manufacturing a hot-rolled steel sheet according to any one of claims 1 to 5, wherein the winding temperature of each divided region in the case in which the evaluation function expressed by the target value of the mechanical properties and the allowable range of variation of the mechanical properties is minimized is determined as the calculated winding temperature of each divided region. Corresponding to the coil position, the axial cross-section of the hot-rolled steel sheet coil is divided in at least one of the radial or widthwise directions to set up multiple divided regions.
For multiple cases with different boundary positions or winding temperatures of the divided region, the mechanical characteristics and the calculated winding temperature are calculated using the material prediction model at the time of completion of finish rolling or a predetermined time has elapsed since the completion of coil winding.
A method for manufacturing a hot-rolled steel sheet according to any one of claims 1 to 5, wherein the boundary position of the divided region is determined such that the calculated mechanical properties become target values and the variation in the mechanical properties is within an acceptable range, and the calculated winding temperature value of each divided region is determined. When the axial cross-section of a hot-rolled steel sheet coil is divided into three parts from the radially inner side: the inner circumference, the longitudinal center, and the outer circumference,
A method for manufacturing a hot-rolled steel sheet according to any one of claims 1 to 5 , wherein the calculated winding temperatures of the inner circumference and the outer circumference are determined to be higher than the calculated winding temperature of the longitudinal center. The method for manufacturing a hot-rolled steel sheet according to claim 8 , wherein the calculated winding temperature is 700 to 750°C for the inner circumference, 475 to 560°C for the longitudinal center, and 675 to 800°C for the outer circumference. Regarding the edges at both ends in the width direction of the axial cross-section of a hot-rolled steel sheet coil,
The winding temperature value at which the temperature in the width direction of the board becomes uniform after a predetermined time has elapsed since the completion of coil winding is calculated in advance.
A method for manufacturing a hot-rolled steel sheet according to any one of claims 1 to 5 , wherein at least one of heating with an edge heater or cooling adjustment with an edge mask is performed before winding the hot-rolled steel sheet. A method for manufacturing a hot-rolled steel sheet according to any one of claims 1 to 5 , wherein no mandrel cooling water is used when winding the hot-rolled steel sheet. The method for manufacturing a hot-rolled steel sheet according to any one of claims 1 to 5 , wherein the aforementioned mechanical property is tensile strength.