Manufacturing method of hot rolled coil

Abstract

To suppress crushing of the shape of a hot-rolled coil wound by a coiler in the hot-rolling step and to obtain the quality of a hot rolled steel sheet.SOLUTION: A method for manufacturing a hot-rolled coil C formed by winding a hot rolled steel sheet H finish-rolled by a hot-rolling facility 1 by a coiler 4 comprises: starting to wind the head end of the hot rolled steel sheet H at an un-metamorphic temperature; and cooling the hot rolled steel sheet H so that the metamorphosis completion rate when completing the winding of the hot rolled steel sheet H is 90% or more. A time from when winding the head end of the hot rolled steel sheet H around the coiler 4 to when completing the winding than a time to when winding the head end of hot rolled steel sheet H around the coiler 4 preferably enhances average cooling capacity.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a manufacturing method for a hot-rolled coil in which a hot-rolled steel sheet after hot finish rolling, particularly a high-strength steel sheet such as a high-tensile material for automobiles, is finish-rolled and then wound on a coiler to form a hot-rolled coil, and relates to a manufacturing method for suppressing deformation of the coil shape after winding. [Background technology]

[0002] The hot-rolled steel sheet after the finish rolling in the hot rolling process is cooled by a cooling device installed on a run-out table (ROT), which is a conveying device, and is wound on a coiler to form a hot-rolled coil. In this hot-rolled coil, the hot-rolled coil cannot maintain its circular shape after being pulled out from the coiler, and the inner diameter may be crushed. In particular, when the hot-rolled steel sheet is a steel type with a slow transformation rate, such as high tensile steel, ferrite transformation continues even after coiling, so the hot-rolled coil after coiling is likely to lose tension and the inner diameter may be crushed.

[0003] When hot-rolled coils with crushed inner diameters are uncoiled in the next process, they cannot be loaded onto the unwinding device, and rewinding and jacking up the coils are required, resulting in time loss and increased costs due to the additional process.

[0004] In the past, attempts have been made to prevent the inner diameter of hot-rolled coils from collapsing by spray cooling inside the coiler or by using a V-skid to correct the shape, but none of these have produced the desired results.

[0005] As a method for suppressing the collapse of hot-rolled coils, for example, Patent Document 1 discloses a method for controlling the amount of volume expansion by controlling the transformation behavior over the entire length of the coil, thereby maintaining the interlaminar friction force.

[0006] As a method for homogenizing the material properties of hot-rolled steel sheet in the longitudinal direction, for example, Patent Document 2 discloses a temperature control method in which the coiling temperature control target value is continuously changed in the longitudinal direction of the coil in accordance with differences in the cooling rate in the longitudinal direction of the coil. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2014-65077 A [Patent Document 2] JP 2001-321827 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, the above-mentioned Patent Document 1 aims to suppress the collapse of the hot-rolled coil by making the phase transformation rate of the outer periphery equal to that of the inner periphery, and does not consider suppression of the strength variation of the hot-rolled steel sheet. Moreover, Patent Document 2 does not disclose a means for preventing the inner diameter collapse of the hot-rolled coil.

[0009] Conventionally, as a method for suppressing the inner diameter collapse of hot-rolled coils, cooling on the ROT, which is premised on the completion of transformation before coiling, has been proposed, for example, as described in the second embodiment of Patent Document 1. However, in the case of high tensile steel, the plate shape is likely to become wavy in the tension-free state until the start of winding, and this shape causes uneven cooling, which causes uneven hardness, which causes plate thickness variation during cold rolling. In other words, if strong cooling is simply performed before coiling, the plate thickness after cold rolling will vary, so there is a problem that it is difficult to simultaneously prevent the collapse of hot-rolled coils and ensure the quality of the steel plate after cold rolling.

[0010] The present invention has been made in consideration of the above-mentioned points, and has an object to suppress deformation of the shape of a hot-rolled coil after coiling in a hot rolling process, and to ensure the quality of a hot-rolled steel sheet. [Means for solving the problem]

[0011] In order to solve the above problems, the present invention is a method for manufacturing a hot-rolled coil by winding a hot-rolled steel sheet that has been finish-rolled in a hot-rolling facility with a coiler, the hot-rolled steel sheet having an untransformed tip is formed. Austenite phase The coiling of the hot-rolled steel sheet is started at the temperature Ferrite The hot-rolled steel sheet is cooled so that the transformation completion rate is 90% or more. The cooling capacity of the hot-rolled steel sheet is higher during the period from when the tip of the hot-rolled steel sheet after finish rolling is wound around the coiler until the winding is completed than during the period from when the tip of the hot-rolled steel sheet after finish rolling is wound around the coiler. The present invention provides a method for producing a hot rolled coil, comprising the steps of:

[0012] In the method for producing a hot-rolled coil, the hot-rolled steel sheet may be cooled at an average cooling capacity of 0 to 500 L / (min m) per unit cooling length until the front end of the hot-rolled steel sheet after finish rolling is wound around a coiler, and at an average cooling capacity of 500 to 5000 L / (min m) per unit cooling length from the front end being wound around the coiler to the end of winding. In addition, it is preferable to cool the hot-rolled steel sheet so that the temperature from the front end of the hot-rolled steel sheet being wound around the coiler to the end of winding follows a cooling history that spans a 90% transformation curve in an isothermal transformation diagram of the hot-rolled steel sheet.

[0013] The isothermal transformation diagram may be obtained by actually performing a hot rolling process, estimating a completion time of transformation of the hot rolled steel sheet at a predetermined temperature based on a state of a hot rolled coil wound on a coiler after cooling the hot rolled steel sheet to a predetermined temperature by the hot rolling process, and converting a time axis of the isothermal transformation diagram corresponding to the steel type of the hot rolled steel sheet into a time axis indicating the elapsed time since the rolling of the hot rolled steel sheet is completed based on the estimated completion time of transformation of the hot rolled steel sheet at the predetermined temperature.

[0014] The cooling devices for cooling the hot-rolled steel sheet are arranged on both the upper and lower sides of the hot-rolled steel sheet, and each of the upper and lower cooling devices has a plurality of cooling banks that are independently controlled, and it is preferable that the cooling capacity from both the upper and lower sides of the hot-rolled steel sheet is equalized until the leading end of the hot-rolled steel sheet after finish rolling is wound around a coiler.

[0015] The hot-rolled steel sheet wound around the coiler may be retained within the coiler, and the hot-rolled steel sheet may be cooled by an in-coiler cooling device.

[0016] The components of the hot-rolled steel sheet are, in mass%, preferably C: 0.05-0.35%, Si: 0.15-2.0%, 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.

[0017] The hot rolling finish temperature of the hot rolled steel sheet may be 850 to 950°C, and the coiling start temperature of the hot rolled steel sheet after the leading end of the hot rolled steel sheet is wound around a coiler may be 450 to 750°C. Effect of the Invention

[0018] According to the present invention, it is possible to suppress the occurrence of inner diameter collapse of a hot-rolled coil after coiling on a coiler while ensuring the quality of the hot-rolled steel sheet. [Brief description of the drawings]

[0019] [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 graph showing an example of the operation of a cooling bank and the coiling temperature of a hot-rolled steel sheet. [Diagram 3] This is an example of an isothermal transformation diagram, in which the cooling history from the end of rolling and the transformation completion point of a representative point of a hot-rolled steel sheet in actual operation are superimposed. [Figure 4] FIG. 13 is a diagram showing an example of data used to estimate a transformation completion time at a predetermined coiling temperature. [Diagram 5] FIG. 2 is a diagram showing an example of a new isothermal transformation diagram after converting the isothermal transformation diagram. [Figure 6] FIG. 1 shows the results of Example 1, and is a diagram showing the sheet thickness after cold rolling of a comparative example and an example of the present invention. [Figure 7] FIG. 11 is a diagram showing the results of Example 2, and is a diagram showing the sheet thickness after cold rolling of a comparative example and an example of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0021] 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 for manufacturing high tensile steel such as high tensile steel for automobiles for cold rolling plating.

[0022] In the hot rolling equipment 1, a finishing rolling mill 2 that continuously rolls a steel sheet that has been discharged from a heating furnace (not shown) and then rolled by a roughing mill (not shown) to a predetermined thickness, a cooling device 3 that cools the hot-rolled steel sheet H after finish rolling 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 (hereinafter, ROT) 5 that conveys the hot-rolled steel sheet H is provided. 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 ROT 5, and then wound up by the coiler 4 to manufacture a hot-rolled coil C.

[0023] The cooling devices 3 are provided above and below the hot-rolled steel sheet H, and each includes multiple stages of cooling banks 11. Each cooling bank 11 is provided with multiple cooling nozzles, and the opening and closing of valves of each cooling bank 11 is controlled independently by a control device (not shown).

[0024] In general, the basic idea of ​​suppressing cooling unevenness caused by the shape of the hot-rolled steel sheet H is to improve the shape of the hot-rolled steel sheet H. However, high-tensile steel (hereinafter, high-tensile steel) has high hot strength, and it is difficult to control the shape of the hot-rolled steel sheet H at the exit of the hot-rolling process. That is, the shape of the hot-rolled steel sheet H is unstable due to free tension from the time it leaves the finishing rolling mill 2 until it is wound around the coiler 4. If the hot-rolled steel sheet H is cooled in this state, the cooling unevenness occurs, the temperature becomes non-uniform, and the shape becomes worse. It is well known that in order to increase the transformation speed of high-tensile steel, rapid cooling from the stage before ROT5 is effective. However, high-tensile steel in particular is prone to become corrugated in the longitudinal direction immediately after finish rolling. Due to the difference in the contact state between the cooling water and the hot-rolled steel sheet H at the peaks and valleys, a difference occurs in the heat transfer coefficient α, and this difference leads to cooling unevenness, which is a factor that causes hardness fluctuations in the hot-rolled steel sheet H.

[0025] Therefore, in the present invention, we have focused on a method of reducing cooling unevenness by improving the cooling method. From the time when the hot-rolled steel sheet H leaves the finishing rolling mill 2 until the tip end is wound around the coiler 4, while passing through the ROT 5 under free tension, the shape of the hot-rolled steel sheet H remains the shape at the exit of the finishing rolling mill 2. At this time, the cooling capacity is reduced so as to maintain the state of the untransformed austenite phase. Then, after the tip end of the hot-rolled steel sheet H reaches the coiler 4 and winding starts, tension is generated in the hot-rolled steel sheet H, and the hot-rolled steel sheet H becomes flat. In this state, the cooling capacity is increased to increase the transformation completion rate of the hot-rolled steel sheet H at the time of winding completion, thereby suppressing the inner diameter collapse of the hot-rolled coil.

[0026] That is, first, the amount of water injected into the cooling device 3 is reduced until the leading end of the hot-rolled steel sheet H after finish rolling is wound around the coiler 4, or until the defective shape portion of the leading end of the steel sheet has finished passing through the cooling device 3. In the cooling device 3, for example, the cooling bank 11 on the finishing rolling mill side is used for rapid cooling, and the cooling bank 11 further downstream is used for slow cooling, and when the amount of water injected is reduced, only the cooling bank 11 for slow cooling is operated to reduce the cooling capacity. Specifically, for example, the average cooling capacity until the leading end of the hot-rolled steel sheet H after finish rolling reaches the coiler 4 is set to 0 to 250 L / (min m) per unit cooling area in the cooling devices 3 provided above and below the ROT 5, respectively.2 ), 0~500L / (min m 2 ) This cooling is performed so that the temperature of the hot-rolled steel sheet H is higher than the transformation point until the leading end of the hot-rolled steel sheet H reaches the coiler 4. At this time, it is preferable to perform cooling with the same heat transfer coefficient α of the cooling devices 3 above and below the ROT 5 in order to prevent the hot-rolled steel sheet H from becoming corrugated. In this specification, the transformation refers to ferritic transformation. The transformation point here refers to a point at which a high transformation rate occurs, and corresponds to, for example, a 90% transformation curve in the isothermal transformation diagram of the hot-rolled steel sheet H.

[0027] After the leading end of the hot-rolled steel sheet H reaches the coiler 4 and coiling starts, the amount of water injected is preferentially increased from the cooling bank 11 on the finishing mill 2 side among the multiple cooling banks 11, so that the desired coiling temperature and cooling speed can be achieved according to the hot-rolled steel sheet H. Specifically, the cooling capacity per unit cooling area in the cooling devices 3 provided above and below the ROT 5 is set to 250 to 2500 L / (min m 2 ), 500~5000L / (min m 2 ) by this cooling. After the leading end of the hot-rolled steel sheet H is wound around the coiler 4, the temperature of the hot-rolled steel sheet H is lower than the transformation point at which a high transformation rate occurs. In other words, the hot-rolled steel sheet H is cooled so that the temperature from when it is wound around the coiler 4 until the winding is completed follows a cooling history that crosses the 90% transformation curve in the isothermal transformation diagram of the hot-rolled steel sheet H. This cooling control is performed, for example, so that the transformation completion rate of the hot-rolled steel sheet H at the time of completion of winding is 90% or more, that is, the ferrite phase is 90% or more.

[0028] Fig. 2 shows an example of operation of the cooling bank 11 for rapid cooling, which is closest (upstream) to the finishing rolling mill 2 of the cooling device 3. The valve is closed for about 200 m from when the leading edge of the hot-rolled steel sheet H leaves the finishing rolling mill 2 until it starts to wrap around the coiler 4, and when it wraps around the coiler 4, the valve is opened to start cooling (rapid cooling). Between 0 and 200 m, the cooling bank 11 for slow cooling provided downstream is in operation, and after 200 m, the other cooling banks 11 for rapid cooling gradually open their valves and operate in accordance with the desired cooling capacity.

[0029] According to this embodiment, the rewinding process of the hot rolled coil due to the collapse of the inner diameter of the coil has been reduced from 10% to 5%. As a result, the burden of the jacking up work during rewinding due to the collapse of the inner diameter of the coil has been reduced.

[0030] The isothermal transformation diagram is a diagram showing the change in steel structure with temperature and time, specifically, a diagram showing 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. The isothermal transformation diagram can be obtained, for example, by preparing a test piece from a steel plate obtained by a hot rolling process and performing a Formaster test using the test piece. If the steel type is known, it can also be obtained from existing materials. In the present invention, an existing isothermal transformation diagram obtained for the corresponding steel type may be used, but there are cases where the transformation point obtained at the laboratory level differs from the transformation point in the actual process, and in order to perform more accurate control, it is preferable to use an isothermal transformation diagram corrected according to the actual situation and control at the transformation temperature in the corrected isothermal transformation diagram.

[0031] An example of how to obtain the corrected isothermal transformation diagram will be described below.

[0032] First, data showing a normal isothermal transformation diagram of the steel type of interest is obtained. The isothermal transformation diagram may be obtained by, for example, a Formaster test as described above, or, if the steel type is known, it may be obtained from an existing isothermal transformation diagram.

[0033] Fig. 3 is an example of an isothermal transformation diagram. In this original isothermal transformation diagram M1, the horizontal axis indicates the isothermal holding time and the vertical axis indicates the holding temperature, and the cooling history and transformation completion point, which will be described later, are also shown. Each point on the isothermal transformation diagram M1 indicates the isothermal holding time at each holding temperature, such as 600°C, until the transformation rate reaches 10%, 50%, and 90%. For example, when held at 600°C, the holding time until the transformation rate reaches 90% is about 20 seconds.

[0034] Also, three state curves C1 to C3 are shown on the isothermal transformation diagram M1. 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 items on the isothermal transformation diagram M1 at which the transformation rate is 90%. Similarly, the state curves C2 and C3 are curves showing the relationship between the holding temperature at which the transformation rate is 50% and 10%, respectively, and can be derived, for example, based on six data items on the isothermal transformation diagram M1 at which the transformation rate is 50% and 10%.

[0035] 3, a cooling history R1 from the end of rolling of a representative point of a hot-rolled steel sheet, which will be described later, i.e., a change in steel sheet temperature over time from the start of cooling, 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.

[0036] 3, a transformation completion point p1, which indicates the transformation completion time described below at a coiling temperature of 550° C., is superimposed on the isothermal transformation diagram M1. When 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.

[0037] The representative point of the hot-rolled steel sheet is defined as a point whose transformation characteristics represent the transformation characteristics of the hot-rolled coil C, and can be, for example, the innermost or outermost part of the hot-rolled coil C, or a position between the outermost and innermost parts in the radial direction, i.e., a position that is half the coil thickness, etc. In the following explanation, the position that is geometrically half the coil thickness is defined as the representative point.

[0038] After obtaining the data showing the isothermal transformation diagram, the hot rolling process is actually performed using the hot rolling equipment 1, and the completion time of the transformation is estimated based on the manufactured hot rolled coil C. More specifically, the 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 the completion time of the transformation of the hot rolled steel sheet after coiling at the predetermined coiling temperature is estimated based on the state of the hot rolled coil C wound around the coiler 4 in FIG. 1. 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 in FIG. 1.

[0039] FIG. 4 is an example of a diagram showing data used for estimating the transformation completion time. The state of the coil used for estimating the transformation completion time is, for example, the aspect ratio of the inner diameter of the hot-rolled coil C immediately after the hot-rolled coil C is actually pulled out of the coiler 4 after the hot-rolled steel sheet is cooled to a predetermined coiling temperature by the actual hot-rolling process and then wound around the coiler 4. In this case, first, a plurality of samples of the hot-rolled coil C are manufactured by changing the pull-out time from the coiler 4, and the inner diameters in the horizontal and vertical directions immediately after the hot-rolled coil C is pulled out of the coiler 4 are measured for each sample. The pull-out time is the elapsed time from the end of rolling of a representative point of the hot-rolled steel sheet to the pull-out from the coiler 4. For example, samples are manufactured with a coiling temperature of 550° C. and pull-out times of 60 seconds, 120 seconds, and 180 seconds from the end of rolling of the representative point to the pull-out of the hot-rolled coil C, and the above-mentioned inner diameter is measured for each sample. After measuring the inside diameter, the aspect ratio of the inside diameter of each sample is calculated immediately after it is unwound from the coiler 4. The aspect ratio of the inside diameter of the coil refers to the size of the inside diameter of the coil in the vertical direction relative to the size of the inside diameter of the coil in the horizontal direction. In this way, data showing the relationship between the unwound time and the aspect ratio of the inside diameter at a given coiling temperature can be obtained.

[0040] An example of a scatter plot of the acquired data is shown in FIG. 4. In the scatter plot P1 in FIG. 4, the horizontal axis represents the extraction time, and the vertical axis represents the aspect ratio of the inner diameter. After the data is acquired, a regression analysis is performed on the acquired data to acquire an approximation curve of the data. 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 also be performed using other functions such as a linear function or a quadratic function. Then, based on the approximation curve obtained by the regression analysis, an extraction time that satisfies a predetermined condition, i.e., an extraction time t at which the aspect ratio of the inner diameter of the coil is 1, is calculated. p Calculate the 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.

[0041] Thereafter, the initially acquired isothermal transformation diagram is converted based on the transformation completion time at the above-mentioned predetermined coiling temperature. FIG. 5 is a diagram showing an example of an improved isothermal transformation diagram obtained by converting the isothermal transformation diagram. The improved isothermal transformation diagram M2 shown in FIG. 5 shows state curves C1' to C3' that represent the relationship between the coiling temperature, the elapsed time since the end of rolling of the representative point of the hot-rolled steel sheet, and the predicted transformation rate of the representative point. In the new isothermal transformation diagram M2, the vertical axis is the coiling temperature, and the time axis is the elapsed time since the end of rolling of the representative point of the hot-rolled steel sheet. In FIG. 5, similarly to FIG. 3, the cooling history R1 and the transformation completion point p1 of the representative point of the hot-rolled steel sheet are superimposed on the new isothermal transformation diagram M2. When showing the cooling history R1 and the transformation completion point p1, the same interpretation as in FIG. 3 is performed.

[0042] In the conversion of the isothermal transformation diagram, the time axis of the isothermal transformation diagram is converted. For example, the conversion is performed so that the estimated transformation completion time at the above-mentioned specified coiling temperature coincides with the isothermal holding time until the transformation completion at a holding temperature equal to the above-mentioned specified coiling temperature in the original isothermal transformation diagram. For the conversion of the time axis, a linear function can be used such that the time after the conversion f(x)=ax+b when the time before the conversion is x. Note that no substantial conversion of the temperature axis is involved.

[0043] The method of converting the time axis of the isothermal transformation diagram will be described in more detail below. Here, in the above-mentioned estimation of the completion time of transformation, it is assumed that the transformation completion time for a coiling temperature of 550°C is estimated, and the estimated transformation completion time is 270 seconds. In the isothermal transformation diagram M1 of FIG. 3, the isothermal holding time at which the transformation rate is 90% at the holding temperature (550°C) equal to the coiling temperature at which the transformation completion time is estimated is about 27 seconds. In this example, this isothermal holding time is set as the isothermal holding time until the completion of transformation. 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 estimated transformation completion time at the coiling temperature of 550°C (270 seconds) coincides with the isothermal holding time until the completion of transformation at the isothermal holding temperature of 550°C in the isothermal transformation diagram M1 (27 seconds). In this example, since the two coincide at a = 10 and b = 0, the result of converting the time axis so that f(x) = 10x is the corrected isothermal transformation diagram (transformation diagram) M2 shown in Figure 5.

[0044] As described above, in the isothermal transformation diagram M2 shown in Fig. 5, 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 isothermal transformation diagram M2 in Fig. 5, the progress state of the transformation of the hot-rolled steel sheet after the end of rolling of the representative point of the hot-rolled steel sheet can be accurately grasped.

[0045] The conversion method for obtaining the corrected isothermal transformation diagram is not limited to the above example. In the above example, the transformation completion time at one winding temperature is estimated, but the transformation completion time at two winding temperatures may be estimated. In that case, not only the time axis of the isothermal transformation diagram but also the temperature axis is converted based on the transformation completion time estimated at each of the two winding temperatures. That is, for example, the time axis and temperature axis are converted so that when points p1 and p2 indicating the estimated transformation completion times at the two winding temperatures are superimposed on a new isothermal transformation diagram obtained by conversion, the points p1 and p2 are approximately superimposed on a curve indicating the isothermal holding time until the completion of transformation in the new isothermal transformation diagram. For example, when the time and temperature before conversion are x and y, respectively, a linear function is used such that the time after conversion f(x)=ax+b and the temperature after conversion f(y)=cy+d, and the sum of the absolute values ​​of the following distances is minimized when the points p1 and p2 are superimposed on the isothermal transformation diagram M3 as described above. Alternatively, the isothermal transformation diagram may be converted based on three or more transformation completion times.

[0046] Furthermore, the method of estimating the completion time of transformation of a hot-rolled steel sheet at a predetermined coiling temperature is not limited to the above example. For example, the completion time of transformation at the above coiling temperature may be estimated based on the change over time in the transformation rate of the surface of a hot-rolled coil C coiled at a predetermined coiling temperature. In this case, data is obtained that indicates the relationship between the transformation rate of the outermost surface layer of the coil during or after coiling and the elapsed time from the completion of rolling at the measurement point of the transformation rate, and a regression analysis is performed on the obtained data. Then, based on the approximation curve obtained by the regression analysis, the elapsed time at which the transformation rate becomes 100% is calculated, and the calculated elapsed time is estimated as the transformation completion time at the predetermined coiling temperature at which the coil was manufactured.

[0047] 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.

[0048] Next, preferred compositions of the hot-rolled steel sheet H according to the embodiment of the present invention and reasons for limiting the components thereof will be described below.

[0049] The hot-rolled steel sheet according to the embodiment of the present invention preferably contains, in mass%, C: 0.05-0.35%, Si: 0.15-2.0%, 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. Note that, hereinafter, mass% in the composition will simply be referred to as %.

[0050] C is added as an element that increases the strength of steel and stabilizes the retained austenite that 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%, ductility, weldability, toughness, etc. are significantly deteriorated. Therefore, the C content is preferably 0.05% or more and 0.35% or less.

[0051] Silicon is a useful element for increasing the strength of steel sheets by solid solution strengthening. Silicon also inhibits the formation of cementite, and therefore has the effect of promoting the concentration of carbon 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 exhibited, and if the content is more than 2.0%, 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 2.0% or less.

[0052] Mn is an element effective in 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.

[0053] 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, so the P content is preferably 0.1% or less.

[0054] S is an impurity element that deteriorates hot workability and toughness, and it is desirable to reduce the S content, so the S content is preferably 0.02% or less.

[0055] 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 is less than 0.01%, these effects are not expressed, and if it is added in excess of 1.5%, the toughness deteriorates, so the Al content is preferably 0.01% or more and 1.5% or less.

[0056] 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.

[0057] In the present invention, it is preferable that the hot rolling finish temperature of the hot rolled steel sheet H is 850 to 950° C., and the coiling start temperature of the hot rolled steel sheet H after the leading end of the hot rolled steel sheet H is wound around the coiler 4 is 450 to 750° C. The coiling start temperature is usually measured, for example, about 40 m upstream from the coiler 4.

[0058] In the present invention, as a cooling method after the leading end of the hot-rolled steel sheet H is wound around the coiler 4, a cooling device within the coiler may be provided in addition to the cooling device 3 on the ROT 5.

[0059] Although the preferred embodiment of the present invention has been described above, 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 revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally belong to the technical scope of the present invention. EXAMPLES

[0060] [Example 1] The plate thickness after cold rolling was examined for an example of the present invention in which the cooling capacity was reduced only for the head of the hot-rolled steel plate after finish rolling, and for a comparative example in which the cooling capacity was reduced for both the head and tail of the hot-rolled steel plate.

[0061] The horizontal axis in Fig. 6 indicates the length of the hot-rolled steel sheet H, with the right side being the head portion and the left side being the tail portion of the hot-rolled steel sheet H. The upper portion of Fig. 6 indicates a comparative example, and the lower portion is an example of the present invention, and the vertical axis indicates the sheet thickness after cold rolling. The cooling method of the present invention significantly reduces the hunting of the sheet thickness at the head portion after cold rolling, and it has been possible to achieve both suppression of collapse of the hot-rolled coil and quality of the steel sheet after cold rolling.

[0062] [Example 2] In the hot rolling facility 1 shown in Fig. 1, 17 cooling banks 11 were provided on the upper and lower sides of the cooling device 3, and cooling was performed with the number of valves and flow rate of the cooling banks 11, #2 and #3 on the front side (upstream side) and #16 and #17 on the rear side (downstream side), as shown in Table 1. In the present invention example, until the tip of the hot-rolled steel sheet H starts to wrap around the coiler 4, a total of 28.5 m of cooling water was used, as shown in Table 1. 3 / min, with a ratio of upper and lower flow rates of 1.15, and thereafter, the number of cooling banks 11 in operation was increased to reach a predetermined coiling temperature. 3 Cooling was performed with a flow rate of 100 / min and a ratio of the upper and lower flow rates of 0.26.

[0063] [Table 1]

[0064] The horizontal axis in Fig. 7 indicates the length of the hot-rolled steel sheet H, with the left side being the head portion and the right side being the tail portion of the hot-rolled steel sheet H. The upper portion of Fig. 7 indicates a comparative example, and the lower portion an example of the present invention, and the vertical axis indicates the sheet thickness after cold rolling. By manufacturing using the cooling method of the present invention, hunting in the sheet thickness at the head portion after cold rolling was significantly suppressed, and it was possible to achieve both suppression of collapse of the hot-rolled coil and quality of the steel sheet after cold rolling. [Industrial Applicability]

[0065] INDUSTRIAL APPLICABILITY The present invention can be applied to the manufacture of hot-rolled coils that suppress the collapse of the inner diameter of the hot-rolled coils, and is particularly useful in the case of high-tensile steel materials. [Explanation of symbols]

[0066] 1. Hot rolling equipment 2 Finishing rolling mill 3 Cooling device 4. Coiler 4a Mandrel 5 Runout Table 11 Cooling Bank C Hot rolled coil H Hot rolled steel plate M1, M2 isothermal transformation diagram

Claims

1. A method for manufacturing a hot rolled coil by winding a hot rolled steel sheet that has been finish-rolled in a hot rolling facility with a coiler, Coiling is started when the leading end of the hot-rolled steel sheet is at a temperature of an untransformed austenite phase, and the hot-rolled steel sheet is cooled so that the ferrite transformation completion rate at the time of completion of coiling of the hot-rolled steel sheet is 90% or more; A method for manufacturing a hot-rolled coil, characterized in that the cooling of the hot-rolled steel sheet is performed with a higher average cooling capacity during the period from when the tip of the hot-rolled steel sheet after finish rolling is wound around the coiler until the winding is completed than during the period from when the tip of the hot-rolled steel sheet after finish rolling is wound around the coiler until the winding is completed.

2. The method for manufacturing a hot-rolled coil according to claim 1, characterized in that the cooling of the hot-rolled steel sheet is performed with an average cooling capacity of 0 to 500 L / (min m) per unit cooling length until the tip of the hot-rolled steel sheet after finish rolling is wound around a coiler, and with an average cooling capacity of 500 to 5000 L / (min m) per unit cooling length from the tip being wound around the coiler until winding is completed.

3. 3. The method for manufacturing a hot-rolled coil according to claim 1, characterized in that the temperature of the hot-rolled steel sheet from when the front end of the hot-rolled steel sheet is wound around a coiler to when the winding is completed follows a cooling history that spans a 90% transformation curve in an isothermal transformation diagram of the hot-rolled steel sheet.

4. The isothermal transformation diagram is A hot rolling process is actually performed, and the hot rolled steel sheet is cooled to a predetermined temperature by the hot rolling process, and then the hot rolled coil is wound on a coiler, based on the state of the hot rolled coil, estimating the completion time of the transformation of the hot rolled steel sheet at the predetermined temperature; 4. The method for producing a hot-rolled coil according to claim 3, characterized in that the time axis of an isothermal transformation diagram corresponding to the steel type of the hot-rolled steel sheet is converted into a time axis indicating the elapsed time since the rolling of the hot-rolled steel sheet is completed, based on the estimated completion time of transformation of the hot-rolled steel sheet at the predetermined temperature.

5. The method for manufacturing a hot-rolled coil according to any one of claims 1 to 4, characterized in that cooling devices for cooling the hot-rolled steel sheet are arranged on both upper and lower sides of the hot-rolled steel sheet, and each of the upper and lower cooling devices has a plurality of cooling banks that are independently controlled, and the cooling capacity from both the upper and lower sides of the hot-rolled steel sheet is equalized until the tip of the hot-rolled steel sheet after finish rolling is wound around a coiler.

6. The method for manufacturing a hot-rolled coil according to any one of claims 1 to 5, characterized in that the hot-rolled steel sheet wound around the coiler is retained in the coiler, and the hot-rolled steel sheet is cooled by an in-coiler cooling device.

7. The composition of the hot-rolled steel sheet is, in mass%, C: 0.05-0.35%, Si: 0.15-2.0%, Mn: 1.0 to 2.7%, P: 0.1% or less, S: 0.02% or less, Al: 0.01-1.5%, N: 0.01% or less and the balance consisting of Fe and unavoidable impurities.

8. The hot-rolled coil manufacturing method according to any one of claims 1 to 7, characterized in that the hot-rolled steel sheet has a hot-rolling finish temperature of 850 to 950 ° C., and the hot-rolled steel sheet has a coiling start temperature of 450 to 750 ° C. after the front end of the hot-rolled steel sheet is wound around a coiler.

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