Method for predicting transformation state of hot-rolled steel sheet

Abstract

To provide a method for correctly predicting the transformation state of a hot rolled steel sheet in a process from the completion of rolling of the hot rolled steel sheet to cooling in order to prevent occurrence of coil collapse and the like.SOLUTION: Disclosed is a transformation state prediction method for predicting the transformation state of a hot rolled steel sheet in a process from the completion of rolling of the hot rolled steel sheet to cooling in a hot rolling step. The method includes: a step of acquiring an isothermal transformation diagram of a prediction-object steel type; an estimation step of actually carrying out the hot rolling step using a hot rolling facility 1 and estimating a completion time of transformation of the hot rolled steel sheet in a prescribed temperature based on the state of a coil C reeled on a coiler 4 after cooling the hot rolled steel sheet to the prescribed temperature by the hot rolling step; and a conversion step of converting the isothermal transformation diagram based on the completion time of transformation of the hot rolled steel sheet in the prescribed temperature estimated in the estimation step. By the conversion step, a time base of the isothermal transformation diagram is converted into a time base indicating the lapsed time after the completion of rolling of the hot rolled steel sheet.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for predicting a transformation state of a hot-rolled steel sheet after hot rolling in an actual operation. [Background technology]

[0002] After finish rolling in the hot rolling process, the hot-rolled steel sheet is cooled by a cooling device provided on a run-out table, which is a transport device, and wound up on a coiler. The hot-rolled steel sheet (coil) wound up on the coiler may become crushed after being removed from the coiler. In this case, when uncoiling in the next process, the coil cannot be loaded onto the unwinding device, and a rewinding process is required, which increases costs. The above-mentioned coil crushing is particularly noticeable in high-tensile steel materials (e.g., those with added Ti, Nb, etc.).

[0003] It is believed that coil collapse can be prevented by coiling the hot-rolled steel sheet after the transformation is complete. Although it is possible to predict when the transformation of high-tensile steel is complete using a material quality prediction model, for many high-tensile steels, there are no practical material quality prediction models, or even if they have been proposed, the accuracy is insufficient. In addition, the material quality prediction model predicts the material quality of the steel sheet using the initial grain size, composition, and cooling time history of the steel sheet at the discharge side of the finishing rolling mill as input values.

[0004] On the other hand, coil collapse can also be prevented when the hot-rolled steel sheet is coiled before the transformation is completed. For example, Patent Document 1 discloses a method for suppressing the change in the phase transformation rate in the longitudinal direction of the hot-rolled steel sheet, thereby suppressing coil collapse even when the phase transformation is incomplete on the hot run table. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2014-65077 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, even in the method disclosed in Patent Document 1, coil collapse occurs unless the hot-rolled steel sheet wound around the coiler is removed from the coiler after transformation is completed. In order to enable removal after transformation, it is essential to understand the transformation history of the hot-rolled steel sheet during the cooling process after the rolling of the hot-rolled steel sheet is completed. Hot-rolled steel sheets are cooled from above and below by run-out table cooling devices, then wound in a coil state, changing from a sheet state to a bulk (coil) state, so the temperature of the hot-rolled steel sheet after winding on the coiler drops slightly from the coiling temperature (CT), but does not change significantly. Therefore, a method can be considered to grasp the transformation state of the steel sheet using the isothermal transformation diagram (TTT diagram) of the relevant steel type. However, although this method is simple, predicting the completion of transformation using this diagram does not match the actual situation.

[0007] The present invention has been made in consideration of the above points, and has an object to accurately predict the transformation state of a hot-rolled steel sheet during the cooling process after the rolling of the hot-rolled steel sheet is completed, in order to prevent the occurrence of coil collapse, etc. [Means for solving the problem]

[0008] In order to achieve the above object, the present invention provides A transformation state prediction method for predicting a transformation state of a hot-rolled steel sheet in a process of cooling after completion of rolling of the hot-rolled steel sheet in a hot rolling process, comprising: a step of acquiring an isothermal transformation diagram of a steel of a steel type to be predicted; an estimation step of actually performing the hot rolling process, cooling the hot-rolled steel sheet to a predetermined temperature by the hot rolling process, and estimating a completion time of the transformation of the hot-rolled steel sheet at the predetermined temperature based on a state of the coil wound on a coiler; and a prediction step of estimating a completion time of the transformation of the hot-rolled steel sheet at the predetermined temperature based on the completion time of the transformation of the hot-rolled steel sheet at the predetermined temperature estimated in the estimation step. The time axis of the isothermal transformation diagram indicates the elapsed time since the rolling of the hot-rolled steel sheet is completed, and the temperature axis of the isothermal transformation diagram indicates the predetermined temperature. and a conversion step of converting the isothermal transformation diagram, When the temperature axis is replaced with the predetermined temperature and the time axis is replaced with the completion time of the transformation, and the estimated completion time of the transformation of the hot-rolled steel sheet at the predetermined temperature is superimposed on the converted isothermal transformation diagram, the completion time coincides with the isothermal holding time until the transformation is completed at a holding temperature equal to the predetermined temperature. Time axis of the isothermal transformation diagram Change It is characterized by the conversion. The present invention also provides a transformation state prediction method for predicting a transformation state of a hot-rolled steel sheet in a process of cooling after completion of rolling the hot-rolled steel sheet in a hot-rolling process, the method comprising the steps of: acquiring an isothermal transformation diagram of a steel material of a steel type to be predicted; actually performing the hot-rolling process, cooling the hot-rolled steel sheet to a predetermined temperature by the hot-rolling process, and estimating a completion time of the transformation of the hot-rolled steel sheet at the predetermined temperature based on a state of the coil wound on a coiler; and setting a time axis of the isothermal transformation diagram to indicate the elapsed time from completion of the rolling of the hot-rolled steel sheet and and a conversion step of converting the isothermal transformation diagram so that a temperature axis of the isothermal transformation diagram indicates the predetermined temperature, the predetermined temperature includes a plurality of temperatures, the estimation step estimates a completion time of transformation of the hot-rolled steel sheet at each of the plurality of temperatures, and the conversion step converts a time axis and a temperature axis of the isothermal transformation diagram so that, when the temperature axis is replaced with the predetermined temperature and the time axis is replaced with the completion time of transformation, and each point indicating the estimated completion time of transformation of the hot-rolled steel sheet at each of the plurality of temperatures is superimposed on the converted isothermal transformation diagram, the each point approximately overlaps with a curve indicating an isothermal holding time until completion of transformation. . The predetermined temperature is approximately the coiling temperature (CT). In detail, if there is a slight temperature change from immediately after the completion of cooling of the hot-rolled steel sheet until it is actually coiled, the predetermined temperature is a temperature obtained by adding the temperature change to the coiling temperature.

[0009] According to the present invention, it is possible to accurately predict the transformation state of a hot-rolled steel sheet during the cooling process after the rolling of the hot-rolled steel sheet is completed based on a diagram created from an isothermal transformation diagram, and therefore it is possible to prevent the occurrence of coil collapse, etc.

[0010] In addition, in the estimation step, for example, a plurality of coils, each of which is formed by cooling the hot-rolled steel sheet to the specified temperature and then winding it up, are prepared with different withdrawal times, which are the time until the coil is withdrawn from the coiler, and the completion time of the transformation is estimated based on the withdrawal times of each of the prepared plurality of coils and the inner diameter ratio of the length to width of the coil after it has been withdrawn from the coiler.

[0011] The estimation step may estimate a time to complete the transformation based on a change over time in a transformation rate of a surface of the wound coil.

[0013] The estimation step may, for example, estimate the time to complete the transformation based on an approximation curve obtained by regression analysis, and the regression analysis in the estimation step may use a linear function, a quadratic function, a logarithmic function, or the like.

[0014] The steel material preferably contains, in mass%, C: 0.05-0.35%, Si: 0.15-1.5%, Mn: 1.0-2.7%, P: 0.1% or less, S: 0.02% or less, Al: 0.01-1.5%, N: 0.01% or less, with the balance being Fe and unavoidable impurities. Effect of the Invention

[0015] According to the present invention, it is possible to accurately predict the transformation state of a hot-rolled steel sheet during the cooling process after the rolling of the hot-rolled steel sheet is completed, and therefore it is possible to prevent the occurrence of coil collapse. [Brief description of the drawings]

[0016] [Figure 1] FIG. 2 is an explanatory diagram showing an outline of the configuration of a hot rolling facility after a finishing mill. [Diagram 2] 1 is a flowchart for explaining a method for predicting a transformation state of a hot-rolled steel sheet in a cooling process after completion of rolling of the hot-rolled steel sheet according to a first embodiment of the present invention. [Diagram 3] FIG. 2 is a diagram showing an example of an isothermal transformation diagram, together with a cooling history from the end of rolling and a transformation completion point of a representative point of a hot-rolled steel sheet in an actual operation. [Figure 4] FIG. 2 is a diagram showing an example of representative points of a hot-rolled steel sheet. [Diagram 5] FIG. 1 is a diagram for explaining an example of a method for estimating a transformation completion time at a predetermined coiling temperature. [Figure 6] FIG. 1 is a diagram showing data used to estimate a transformation completion time at a given coiling temperature. [Figure 7] FIG. 2 is a diagram showing an example of a transformation diagram after converting an isothermal transformation diagram. [Figure 8] FIG. 4 is a diagram for explaining a transformation state predicting method according to a second embodiment of the present invention. [Figure 9] FIG. 11 is another diagram for explaining the transformation state predicting method according to the second embodiment of the present invention. [Figure 10] FIG. 11 is another diagram for explaining the transformation state predicting method according to the second embodiment of the present invention. [Figure 11] FIG. 11 is a diagram for explaining another example of a method for estimating the completion time of transformation of a hot-rolled steel sheet at a predetermined coiling temperature. [Figure 12] FIG. 4 is a diagram showing the relationship between the coil length and the position of a representative point of a hot-rolled steel sheet. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In this specification and the drawings, components having substantially the same functional configurations are denoted by the same reference numerals, and duplicated description will be omitted.

[0018] First, the configuration of the hot rolling facility according to the present invention will be described. Fig. 1 is an explanatory diagram showing an outline of the configuration of the hot rolling facility after the finishing mill. In the hot rolling equipment 1, a finishing rolling mill 2 that continuously rolls the steel sheet H discharged from a heating furnace (not shown) and rolled by a roughing mill (not shown) to a predetermined thickness, a cooling device 3 that cools the steel sheet H after finish rolling (hereinafter, hot-rolled steel sheet H) to a predetermined temperature, and a coiler 4 equipped with a mandrel 4a that winds up the cooled hot-rolled steel sheet H are provided in this order in the conveying direction of the hot-rolled steel sheet H. Between the finishing rolling mill 2 and the coiler 4, a run-out table 5 that conveys the hot-rolled steel sheet H is provided. Then, the hot-rolled steel sheet H rolled by the finishing rolling mill 2 is cooled by the cooling device 3 while being conveyed on the run-out table 5, and then wound up by the coiler 4 to be manufactured as a coil C.

[0019] When the steel sheet H / hot-rolled steel sheet H is a high-tensile steel material, the coil C manufactured as described above needs to be removed from the coiler 4 after the transformation of the hot-rolled steel sheet H wound around the coiler 4 is completed in order to prevent the coil from being crushed. For this reason, it is necessary to predict and understand the transformation state of the hot-rolled steel sheet H during the cooling process after the rolling of the hot-rolled steel sheet H is completed. In the following, the coil C may be referred to as a hot-rolled coil C or the like.

[0020] (First embodiment) FIG. 2 is a flowchart for explaining a method for predicting a transformation state of a hot-rolled steel sheet H in the process of cooling after completion of rolling, according to a first embodiment of the present invention. First, in the transformation state prediction method according to the present invention, an isothermal transformation diagram of a steel type to be predicted, i.e., data showing the isothermal transformation diagram, is obtained (Step S1). The isothermal transformation diagram shows the state of transformation when an austenitized steel material is quenched to a predetermined test temperature with a cooling pattern simulating run-out table cooling and isothermally held.

[0021] The isothermal transformation diagram can be obtained, for example, by preparing a test piece from a steel sheet obtained by carrying out a hot rolling process, i.e., a process using the above-mentioned hot rolling equipment, and then a cold rolling process, and carrying out a Formaster test using the test piece. The method of preparing the test piece for obtaining the isothermal transformation diagram is not limited to the above-mentioned example, and for example, the test piece may be prepared from a steel material that has not been subjected to a cold rolling process. In addition, if the isothermal transformation diagram of the steel type is known, it is possible to obtain it from the distributed materials instead of obtaining it by the test as described above.

[0022] FIG. 3 is an example of an isothermal transformation diagram, and also shows a cooling history and a transformation completion point, which will be described later. In the isothermal transformation diagram M1 of FIG. 3, the horizontal axis represents the isothermal holding time and the vertical axis represents the holding temperature. Each point on the isothermal transformation diagram M1 indicates the isothermal holding time required to reach a transformation rate of 10%, 30%, or 90% at each holding temperature, such as 600°C. For example, when held at 600°C, the holding time required to reach a transformation rate of 90% is approximately 20 seconds.

[0023] Moreover, on the isothermal transformation diagram M1, there are three state curves C1 to C3. The state curve C1 is a curve showing the relationship between the holding temperature at which the transformation rate is 90% and the isothermal holding time, and can be derived, for example, based on six data points at which the transformation rate is 90% on the isothermal transformation diagram M1. Similarly, state curves C2 and C3 are curves that show the relationship between the holding temperature and the isothermal holding time at which the transformation rates are 50% and 10%, respectively, and can be derived, for example, based on six data points on the isothermal transformation diagram M1 at which the transformation rates are 50% and 10%.

[0024] 3, a cooling history from the end of rolling of a representative point of a hot-rolled steel sheet, which will be described later, i.e., a cooling history from the start of cooling (change in steel sheet temperature over time) R is shown superimposed on an isothermal transformation diagram M1. When showing the cooling history R1, the horizontal axis of the isothermal transformation diagram M1 is interpreted as the cooling elapsed time assuming that the end of rolling of the representative point is 0 seconds, and the vertical axis is interpreted as the steel sheet temperature at the representative point.

[0025] 3 also shows a point (transformation completion point) p1 indicating the transformation completion time described below at a coiling temperature of 550° C. In showing point p1, the vertical axis of the isothermal transformation diagram M1 is replaced with the coiling temperature, and the horizontal axis is replaced with the transformation completion time.

[0026] The representative points of the hot-rolled steel sheet described above will be illustrated and explained in FIG. 4. FIG. 4 is a side view of a hot-rolled coil C, and the positions of the representative points in each example are shown as positions in the radial direction of the coil. The representative point of the hot-rolled steel sheet is defined so that the transformation characteristics at that position represent the transformation characteristics of the hot-rolled coil C, and is, for example, the innermost part (TOP part) TP of the hot-rolled coil C in the figure. The outermost part (BOTTOM part) BT of the hot-rolled coil C may also be used as the representative point of the hot-rolled steel sheet. In addition, the position where the distance from the center of the hot-rolled coil C is given by (outer diameter + inner diameter) / 2, in other words, the position MID where the distance is half the coil thickness, is also used. A The representative point of the hot-rolled steel sheet may be the midpoint of the winding position from when the top part starts to be wound around the mandrel of the coiler until the bottom part is reached and the winding is completed. B In the following description, the position MID, which is (geometrically) half the coil thickness, may be used as the representative point. A is set as the representative point.

[0027] Returning to the explanation of Figure 2. In the transformation state prediction method according to the present invention, after the above-mentioned step S1, a hot rolling process is actually performed using the hot rolling equipment 1, and the completion time of transformation is estimated based on the manufactured coil (step S2). More specifically, in step S2, a hot rolling process is performed, and the hot rolled steel sheet is cooled to a predetermined coiling temperature by the hot rolling process, and then, based on the state of the coil wound on the coiler 4 of FIG. 1, the completion time of transformation of the hot rolled steel sheet after coiling at the predetermined coiling temperature is estimated (step S2). A specific example of the estimation method will be described later. Cooling to the predetermined coiling temperature is performed by a cooling device 3 provided for the run-out table 5 of FIG. 1. It does not matter whether steps S1 and S2 are performed in sequence. In the following, "the time required for transformation of the hot-rolled steel sheet after coiling" will be abbreviated to "transformation completion time".

[0028] Fig. 5 is a diagram for explaining an example of a method for estimating the transformation completion time at a predetermined coiling temperature, and Fig. 6 is a diagram showing data used for estimating the transformation completion time. As described above, the estimation of the transformation completion time is performed based on the state of the coil wound around the coiler 4 after the hot-rolled steel sheet is cooled to a predetermined coiling temperature in an actual hot rolling process. The state of the coil used to estimate the transformation completion time is, for example, the degree of crushing of the coil immediately after the hot-rolling process is actually performed and the coil is pulled out from the coiler, i.e., the aspect ratio of the inner diameter of the coiler, as described below. In addition, as described below, the state of the coil used to estimate the transformation completion time may be the transformation rate of the surface of the coil wound around the coiler after the hot-rolling process is actually performed.

[0029] In the method of estimating the transformation completion time in Fig. 5, first, a hot-rolled steel sheet is cooled to a predetermined coiling temperature by an actual hot rolling process, and then coiled on a coiler. A plurality of samples are manufactured / prepared by changing the time of removal from the coiler, and the inner diameters in the horizontal and vertical directions immediately after removal from the coiler are measured for each sample (step S11). The removal time is the elapsed time from the end of rolling of a representative point of the hot-rolled steel sheet to the time of removal from the coiler. In step S11, for example, samples are manufactured with a coiling temperature of 550°C, and the removal time from the end of rolling to the coil removal time of the representative point is 60 seconds, 120 seconds, and 180 seconds, and the above-mentioned inner diameter is measured for each sample.

[0030] After measuring the inner diameter, the aspect ratio of the inner diameter of each sample immediately after it is pulled out of the coiler is calculated (step S12). The aspect ratio of the inner diameter of the coil refers to the size of the inner diameter of the coil in the vertical direction relative to the size of the inner diameter of the coil in the horizontal direction. By carrying out the above-mentioned calculations, data showing the relationship between the unwinding time at the above-mentioned predetermined coiling temperature and the aspect ratio of the inner diameter can be obtained. An example of a scatter diagram of the acquired data is shown in Figure 6. In the scatter diagram P1 of Figure 6, the horizontal axis represents the extraction time, and the vertical axis represents the aspect ratio of the inner diameter.

[0031] After the data is acquired, a regression analysis is performed on the acquired data to acquire an approximation curve of the data (step S13). The scatter plot P1 shows an approximation curve C11 obtained by the regression analysis. In this example, the regression analysis is performed using a logarithmic function, but the regression analysis may be performed using other functions such as a linear function or a quadratic function.

[0032] Then, based on the approximation curve obtained by the regression analysis, the extraction time that satisfies the predetermined conditions, i.e., the extraction time t p (Step S14), and the calculated extraction time t p is estimated as the transformation completion time at a given coiling temperature (e.g., 550°C) at which multiple samples were manufactured.

[0033] Returning to the explanation of Figure 1. In the transformation state prediction method according to the present invention, the isothermal transformation diagram acquired in step S1 is converted based on the transformation completion time at the above-mentioned predetermined coiling temperature estimated in step S2 (step S3).

[0034] FIG. 7 is a diagram showing an example of a new TTT diagram obtained by converting an isothermal transformation diagram. A new TTT diagram M2 in Fig. 7 shows state curves C1' to C3' that represent the relationship between the coiling temperature, the elapsed time from the end of rolling of a representative point of the hot-rolled steel sheet, and the predicted transformation rate of the representative point. In the new TTT diagram M2, the vertical axis represents the coiling temperature, and the time axis represents the elapsed time from the end of rolling of the representative point of the hot-rolled steel sheet.

[0035] In Fig. 7, the cooling history R1 and the transformation end point p1 of the representative point of the hot-rolled steel sheet are shown superimposed on a new TTT diagram M2, as in Fig. 3. When showing the cooling history R1 and the transformation end point p1, the same interpretation as in Fig. 3 is used.

[0036] In the conversion of step 3, the time axis of the isothermal transformation diagram is converted so that, for example, the estimated transformation completion time at the above-mentioned predetermined coiling temperature coincides with the isothermal holding time until the transformation completion at a holding temperature equal to the above-mentioned predetermined coiling temperature in the original isothermal transformation diagram. For the conversion of the time axis, a linear function of the converted time f(x)=ax+b, where x is the time before conversion, can be used. It should be noted that the conversion of the temperature axis in step S3 does not involve substantial conversion. The method of converting the time axis of the isothermal transformation diagram will be described in more detail below.

[0037] Here, it is assumed that in step S2, the transformation completion time for the coiling temperature of 550° C. was estimated, and that the estimated transformation completion time was 270 seconds. In the isothermal transformation diagram M1 in Fig. 3, the isothermal holding time at which the transformation rate reaches 90% at the holding temperature (550°C) equal to the coiling temperature at which the transformation completion time is estimated is about 27 seconds, and in this example, this isothermal holding time is set as the isothermal holding time until transformation completion. The reason for this will be described later.

[0038] In step S3, the time axis is converted using a linear function f(x)=ax+b (x: time before conversion, f(x): time after conversion) so that the transformation completion time (270 seconds) at a coiling temperature of 550°C estimated in step S2 coincides with the isothermal holding time (27 seconds) until transformation completion at an isothermal holding temperature of 550°C in the isothermal transformation diagram M1.

[0039] In this example, the two coincide when a = 10 and b = 0, so the result of transforming the time axis so that f(x) = 10x is the new TTT diagram (transformation diagram) M2 in Figure 6. In the TTT diagram M2, as described above, the horizontal axis represents the elapsed time from the end of rolling of the representative point of the hot-rolled steel sheet, and the vertical axis represents the coiling temperature. Therefore, based on the new TTT diagram M2 in Fig. 7, the progress state of the transformation of the hot-rolled steel sheet from the end of rolling of the representative point of the hot-rolled steel sheet can be grasped, and therefore, for example, the coil can be held in the coiler until a predetermined transformation completion time and then pulled out, thereby preventing the occurrence of coil crushing.

[0040] In the above example, when converting the isothermal transformation diagram, the isothermal holding time at which the transformation rate reaches 90% in the original isothermal transformation diagram is considered to be the isothermal holding time until the transformation is completed, because it is difficult to measure the time at which the transformation is completed, i.e., the time at which the transformation rate reaches 100%. However, in the present invention, the TTT diagram is redefined based on the transformation completion point estimated from the results of actual operation, i.e., the new TTT diagram described above is obtained, so that in the new TTT diagram, the state curve with a transformation rate of 90% can be considered as the state curve with a transformation rate of 100%.

[0041] Second embodiment 8 to 10 are diagrams for explaining a transformation state predicting method according to a second embodiment of the present invention. The first and second embodiments are different in the estimation target of the transformation completion time and the conversion method of the isothermal transformation diagram.

[0042] In the first embodiment, the transformation completion time at one winding temperature is estimated, but in the present embodiment, the transformation completion time at two winding temperatures is estimated. Here, the estimation of the transformation completion time was performed for two coiling temperatures, 550°C and 600°C.

[0043] In Fig. 8, point p1 indicating the transformation completion time estimated at a coiling temperature of 550°C and point p2 indicating the transformation completion time estimated at a coiling temperature of 600°C are superimposed on an isothermal transformation diagram M3 together with a cooling history R1 of a representative point of the hot-rolled steel sheet at a coiling temperature of 550°C and a cooling history R2 of a representative point of the hot-rolled steel sheet at a coiling temperature of 600°C. Note that when showing points p1 and p2 and the cooling histories R1 and R2, the same interpretation as in Fig. 3 is used.

[0044] In the phase prediction method of the second embodiment, when converting the isothermal transformation diagram M3, not only the time axis but also the temperature axis of the isothermal transformation diagram M3 is converted based on the transformation completion times estimated at each of the two winding temperatures. For example, as shown in FIG. 10 described later, when points p1 and p2 indicating the transformation completion times estimated at the above two coiling temperatures are superimposed on a new TTT diagram M4 obtained by conversion, the time axis and temperature axis are converted so that the points p1 and p2 approximately overlap with a curve C1″ indicating the isothermal holding time until transformation completion in the new TTT diagram M4.

[0045] In the above-mentioned conversion of the time axis and the temperature axis, for example, when the time and temperature before conversion are x and y, respectively, a linear function is used such that the converted time f(x) = ax + b and the converted temperature f(y) = cy + d, and when points p1 and p2 are superimposed on the isothermal transformation diagram M3 as described above, the sum of the absolute values ​​of the following distances is minimized.

[0046] The sum of the above-mentioned distances is the sum of a distance L1 from an approximation line C21 for data showing the isothermal transformation diagram where the transformation rate is 90% and the holding temperature is 550°C or less to a point p1 of the estimated transformation completion time at a winding temperature of 550°C in the same figure, and a distance L2 from an approximation line C22 for data showing the isothermal transformation diagram where the transformation rate is 90% and the holding temperature is 600°C or more to a point p2 of the estimated transformation completion time at a winding temperature of 600°C in the same figure, as shown in FIG.

[0047] The result of converting the time axis and temperature so as to minimize the sum of the above distances is the new TTT diagram M4 in FIG. By performing such a conversion, it is possible to more accurately grasp the progress state of transformation of the hot-rolled steel sheet after the rolling of the representative point of the hot-rolled steel sheet is completed. In FIG. 9, the same interpretation as in FIG. 3 is made, and points p1, p2 and cooling histories R1, R2 are shown superimposed on a TTT diagram M4 in the same manner as in FIG.

[0048] In this example, two transformation completion times are estimated, but three or more may be estimated, and the isothermal transformation diagram may be converted based on three or more transformation completion times. Furthermore, the method of converting the temperature axis and the time axis based on the transformation completion times estimated for a plurality of winding temperatures may be a method other than the above.

[0049] FIG. 11 is a diagram for explaining another example of a method for estimating the time to complete transformation of a hot-rolled steel sheet at a predetermined coiling temperature. Unlike the examples of Figures 4 and 5, the transformation completion time estimation method of this example estimates the transformation completion time at a specified winding temperature based on the change over time in the transformation rate of the surface of a coil wound at the above winding temperature.

[0050] Specifically, the hot-rolled steel sheet is cooled to a predetermined coiling temperature and then coiled on a coiler to manufacture a coil. During the manufacturing process, the transformation rate of the outermost surface layer of the coil during or after coiling is measured at regular intervals (e.g., every 60 seconds) after the transformation rate measurement point (outermost circumference) completes rolling. This makes it possible to obtain data showing the relationship between the transformation rate of the surface layer of the outermost circumference of the coil during or after winding and the elapsed time from the completion of rolling at the measurement point of the transformation rate. An example of a scatter diagram of the acquired data is shown in Figure 11. In the scatter diagram P2 in Figure 11, the horizontal axis represents the elapsed time, and the vertical axis represents the transformation rate.

[0051] After the data is acquired, a regression analysis is performed on the acquired data. The scatter plot P2 shows an approximation curve C31 obtained by the regression analysis. In this example, the approximation curve C31 is obtained by performing the regression analysis using a logarithmic function, but the regression analysis may be performed using other functions such as a linear function or a quadratic function.

[0052] Based on the approximation curve C31 obtained by the regression analysis, the elapsed time t p Calculate the elapsed time t p is estimated as the time to complete transformation at a given coiling temperature (e.g., 550°C) at which the coil was manufactured.

[0053] The transformation completion time calculated / estimated by the above method is for the bottom part of the hot-rolled steel sheet / hot-rolled coil. However, in the transformation state prediction method of this example, the position half the geometric coil thickness is used as the representative point of the hot-rolled steel sheet, so the transformation completion time of the representative position is obtained by interpolation from the transformation completion time of the bottom part. Incidentally, if the representative position is the bottom part, it can be used as it is.

[0054] This allows the time of completion of transformation to be estimated based on one sample / coil. The part where the transformation ratio is measured is the outermost layer of the hot-rolled steel sheet being wound around the coiler, but since the outermost layer of the hot-rolled steel sheet is constantly replaced with a new hot-rolled steel sheet until the end of winding, it is considered that the transformation ratio of the outermost layer of the hot-rolled steel sheet during winding does not change significantly. Therefore, the measurement of the transformation ratio may be started after the end of winding the coil.

[0055] The high-tensile steel to be predicted by the transformation state prediction method of the present invention preferably contains, in mass%, C: 0.05-0.35%, Si: 0.15-1.5%, Mn: 1.0-2.7%, P: 0.1% or less, S: 0.02% or less, Al: 0.01-1.5%, N: 0.01% or less, with the remainder being Fe and unavoidable impurities. The reasons for limiting the composition of the high tensile steel will be explained below. Note that hereinafter, mass% in the composition will simply be abbreviated as %.

[0056] C: C is added as an element to increase the strength of steel and stabilize the retained austenite, which improves ductility. If it is less than 0.05%, it is difficult to ensure a tensile strength of 980 MPa or more, and if it is added in excess of 0.35%, it significantly deteriorates ductility, weldability, toughness, etc. Therefore, the C content is preferably 0.05% or more and 0.35% or less.

[0057] Si: Si is a useful element for increasing the strength of steel sheets through solid solution strengthening. In addition, Si suppresses the formation of cementite, and therefore has the effect of promoting the concentration of C in austenite during bainite transformation, making it an essential element for forming retained austenite after annealing. If the content is less than 0.15%, these effects are not expressed, and if it is added in excess of 1.5%, the peelability of scale formed during hot rolling is significantly deteriorated and the wettability of the plating is significantly impaired, so the Si content is preferably 0.15% or more and 1.5% or less.

[0058] Mn: Mn is an element effective for improving hardenability. If it is less than 1.0%, the effect of improving hardenability is not fully expressed, and if it is added in excess of 2.7%, toughness is deteriorated. Therefore, the Mn content is preferably 1.0% or more and 2.7% or less.

[0059] P: P is an impurity element that segregates at grain boundaries to reduce grain boundary strength and deteriorate toughness, and it is desirable to reduce this content. Therefore, the P content is preferably 0.1% or less.

[0060] S: S is an impurity element that deteriorates hot workability and toughness, and it is desirable to reduce the S content. Therefore, the S content is preferably 0.02% or less.

[0061] Al: Al is an element that acts as a deoxidizer. It is also a ferrite stabilizing element like Si, and can be used as a substitute for Si. If the content of Al is less than 0.01%, the effect is not exhibited, and if it is added in excess of 1.5%, the toughness deteriorates, so the content of Al is preferably 0.01% or more and 1.5% or less.

[0062] N: N forms coarse nitrides and deteriorates bendability and hole expandability, so the amount added must be kept to a minimum. This is because this tendency becomes more pronounced when N exceeds 0.01%, and it also causes blowholes during welding, so it is better to have less N. For this reason, the N content is preferably 0.01% or less.

[0063] Furthermore, the high tensile steel preferably contains, by mass%, one or more of Ti: 0.005-0.5%, Nb: 0.005-0.5%, Cr: 0.005-0.5%, Mo: 0.005-0.5%, Ni: 0.005-0.5%, Cu: 0.0003-0.030%, B: 0.0003-0.030%, Ca: 0.0003-0.030%, Mg: 0.0003-0.030%, and REM (rare earth elements): 0.0003-0.030%.

[0064] In the above example, a linear function is used for the conversion of the time axis and the temperature axis in the conversion of the isothermal transformation diagram, but a quadratic function or a logarithmic function may be used instead of the linear function.

[0065] Incidentally, as can be seen from the above explanation, the coil width has no effect on the present invention. However, since the coil representative position changes when the coil length changes, if the coil length is different, it is possible to deal with the change in the coil length by changing the transformation completion time in accordance with the change in the representative point position shown in Figure 12.

[0066] Although the preferred embodiment of the present invention has been described above with reference to the accompanying drawings, the present invention is not limited to such an example. It is clear that a person skilled in the art can come up with various modified or altered examples within the scope of the ideas described in the claims, and it is understood that such examples also naturally belong to the technical scope of the present invention. [Industrial Applicability]

[0067] As described above, according to the present invention, the transformation state of a hot-rolled steel sheet during the cooling process after the rolling of the hot-rolled steel sheet can be predicted more accurately, and therefore, the present invention is useful in, for example, the steel sheet manufacturing industry using high tensile steel materials. [Explanation of symbols]

[0068] 1...Hot rolling equipment 2...Finishing rolling mill 3…Cooling device 4. Coiler 4a…Mandrel 5…Runout table

Claims

1. A transformation state prediction method for predicting a transformation state of a hot-rolled steel sheet during a cooling process after the rolling of the hot-rolled steel sheet is completed, A step of obtaining an isothermal transformation diagram of a steel material of a steel type to be predicted; An estimation step of actually performing the hot rolling process, cooling the hot rolled steel sheet to a predetermined temperature by the hot rolling process, and estimating a completion time of the transformation of the hot rolled steel sheet at the predetermined temperature based on a state of a coil wound on a coiler. A conversion step of converting the isothermal transformation diagram so that the time axis of the isothermal transformation diagram indicates the elapsed time since the rolling of the hot-rolled steel sheet is completed and the temperature axis of the isothermal transformation diagram indicates the predetermined temperature based on the completion time of the transformation of the hot-rolled steel sheet at the predetermined temperature estimated in the estimation step, a transformation state prediction method characterized in that, in the conversion step, the temperature axis is converted to the specified temperature, and the time axis is converted to the completion time of the transformation, and when the estimated completion time of the transformation of the hot-rolled steel sheet at the specified temperature is superimposed on the converted isothermal transformation diagram, the time axis of the isothermal transformation diagram is converted so that the completion time coincides with the isothermal holding time until the transformation is completed at a holding temperature equal to the specified temperature.

2. A transformation state prediction method for predicting a transformation state of a hot-rolled steel sheet during a cooling process after the rolling of the hot-rolled steel sheet is completed, comprising: A step of obtaining an isothermal transformation diagram of a steel material of a steel type to be predicted; An estimation step of actually performing the hot rolling process, cooling the hot rolled steel sheet to a predetermined temperature by the hot rolling process, and estimating a completion time of the transformation of the hot rolled steel sheet at the predetermined temperature based on a state of a coil wound on a coiler. A conversion step of converting the isothermal transformation diagram so that the time axis of the isothermal transformation diagram indicates the elapsed time since the rolling of the hot-rolled steel sheet is completed and the temperature axis of the isothermal transformation diagram indicates the predetermined temperature based on the completion time of the transformation of the hot-rolled steel sheet at the predetermined temperature estimated in the estimation step, The predetermined temperature includes a plurality of temperatures, The estimation step estimates a completion time of transformation of the hot-rolled steel sheet at each of the plurality of temperatures, The transformation state prediction method is characterized in that the conversion step converts the time axis and the temperature axis of the isothermal transformation diagram so that, when the temperature axis is converted to the predetermined temperature and the time axis is converted to the completion time of the transformation, each point indicating the completion time of the transformation of the hot-rolled steel sheet at each of the estimated temperatures is superimposed on the converted isothermal transformation diagram, the each point approximately overlaps with a curve indicating the isothermal holding time until the completion of the transformation.

3. The estimation step includes: A plurality of coils are prepared by cooling the hot-rolled steel sheet to the predetermined temperature and then winding the coil, the coils being different in pull-out time, which is the time until the coil is pulled out from the coiler; 3. The transformation state prediction method according to claim 1 or 2, characterized in that the completion time of the transformation is estimated based on the extraction time of each of the prepared coils and the inner diameter ratio of the length to width of the coil after being extracted from the coiler.

4. 3. The transformation state prediction method according to claim 1, wherein the estimation step estimates a completion time of the transformation based on a change over time in a transformation rate of a surface of the wound coil.

5. the estimation step estimates a completion time of the transformation based on an approximation curve obtained by regression analysis; 5. The transformation state prediction method according to claim 1, wherein a linear function, a quadratic function or a logarithmic function is used for the regression analysis in the estimation step.

6. The steel material comprises, in mass%, C: 0.05-0.35%, Si: 0.15-1.5%, Mn: 1.0 to 2.7%, P: 0.1% or less, S: 0.02% or less, Al: 0.01-1.5%, The transformation state prediction method according to any one of claims 1 to 5, characterized in that N is 0.01% or less, and the balance consists of Fe and unavoidable impurities.

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