JP4352951B2 - Method for producing hot-rolled steel sheet made of high carbon steel or high carbon alloy steel - Google Patents

Description

  The present invention relates to a method for producing a hot rolled steel sheet made of high carbon steel or high carbon alloy steel.For example, excellent mechanical properties can be achieved in the longitudinal direction by reliably realizing a predetermined cooling process over the entire length in the longitudinal direction. The present invention relates to a method for producing a hot-rolled steel sheet made of high carbon steel or high carbon alloy steel having substantially uniform area.

  FIG. 4 is an explanatory view schematically showing the run-out table 1 that is a part of the manufacturing process of the hot-rolled steel sheet 2. The hot-rolled steel sheet 2 is subjected to rough rolling with a roughing mill after heating the slab with a heating furnace to form a rough bar (both not shown). Then, the rough bar is finish-rolled by a finish rolling mill 3 (a tandem rolling mill consisting of 7 stands, only the final stand F7 is shown) to form a steel strip 4 having a predetermined plate thickness. A plurality of finish-rolled steel strips 4 are installed on the runout table 1 in the longitudinal direction of the steel strip 4 (left-right direction in the figure) while being transported by the run-out table 1 arranged in parallel on the downstream side of the finish rolling mill 3. A pair of upper and lower water cooling devices (5a, 5b), (6a, 6b), (7a, 7b), (8a, 8b), (9a, 9b), (10a, 10b), (11a, 11b), ( 12a, 12b) is cooled by being injected with cooling water, and is wound on a coil by a winder 13 at a predetermined winding temperature to give desired mechanical characteristics and manufactured.

  In the manufacturing process of the hot-rolled steel sheet 2, the temperature of the steel strip 4 (referred to as “finishing outlet temperature” in this specification) when the final stand F 7 of the finishing mill 3 is passed, the intermediate point of the runout table 1, and the winding The target values of the temperature of the steel strip 4 immediately before the take-up machine 13 are determined according to the mechanical properties required for the hot-rolled steel sheet 2 as a product. It is necessary to cool the steel strip 4 by the run-out table 1 so as to satisfy the target value at this point.

    However, for example, when the steel strip 4 having a C content equal to or greater than hypoeutectoid steel is cooled by the runout table 1, the water cooling by the water cooling devices (5a, 5b) to (12a, 12b) is completed and wound up on the coil. There may be a portion where the pearlite transformation has not been completed, and in such a case, a portion where the pearlite transformation has been completed and a portion where the pearlite transformation has not been completed exist in the longitudinal direction of the hot-rolled steel sheet 2. The thickness of the rolled hot-rolled steel sheet 2 varies due to mechanical properties, particularly hardness, and when various processes are performed using the hot-rolled steel sheet 2 as a raw material, Problems such as fluctuations and processing defects may occur.

  Further, in the case of a steel strip having a C content equal to or higher than that of hypoeutectoid steel to which an alloy element such as Cr is added, the variation width of the coiling temperature becomes large, so that Longitudinal mechanical characteristics, particularly hardness variations, may occur more significantly.

  For this reason, in order to suppress the dispersion | variation in the mechanical characteristic of the longitudinal direction of the hot-rolled steel plate 2, in timing with a cooling process, such as finishing exit temperature, the intermediate point of the run-out table 1, and the temperature just before the winder 13, etc. It is effective to comprehensively control the temperature of the steel strip 4 in the entire cooling process by the run-out table 1 instead of controlling the temperature of the steel strip 4 in a piecewise manner.

  FIG. 5 is a graph schematically showing four general cooling patterns A to D of the steel strip 4 between the exit of the finishing mill 3 of the run-out table 1 and immediately before the winder 13.

  The cooling pattern A in FIG. 5 is the most basic cooling pattern, and the steel strip 4 is cooled on the entire area of the run-out table 1 approximately on average (hereinafter, this cooling pattern A is referred to as “one-stage cooling”).

  After passing through the finishing mill 3, the cooling pattern B is not cooled with water for a certain period of time and starts water cooling after a certain period of time, or is slowly cooled with water for a certain period of time and sufficient after a certain period of time. Water cooling is performed (hereinafter, this cooling pattern B is referred to as “second half cooling”).

  The cooling pattern C starts water cooling immediately after passing through the finishing mill 3, stops water cooling in the middle of the run-out table 1, and thereafter performs no water cooling or loose water cooling (hereinafter, this cooling pattern is referred to as “ First half cooling ").

  The cooling pattern D starts water cooling (first half cooling; first cooling) immediately after passing through the finishing mill 3, stops water cooling in the middle of the run-out table 1 and then air cools for a predetermined time (hereinafter referred to as “intermediate air cooling”). Then, loose water cooling (second half cooling; second cooling) is performed again.

  The invention for suppressing variations in the mechanical properties in the longitudinal direction of the hot-rolled steel sheet 2 is not intended for eutectoid steel such as high-carbon steel but for high-strength steel for automobiles that require both strength and workability. For example, in Patent Document 1, cooling of a steel strip in a run-out table is divided into three zones of a quenching zone, an air cooling zone, and a control cooling zone in order from the finish rolling mill side, and predetermined cooling is performed in the first quenching zone. An invention that rapidly cools to a stop temperature, then air cools until transformation is completed in the air cooling zone, and then cools to a predetermined winding temperature in the control cooling zone and winds it around the coil. Is similar to the invention disclosed in Patent Document 1, but the cooling zone of the run-out table is divided into a temperature history control zone comprising first water cooling and first air cooling, and second water cooling and second air cooling. Inventing the temperature history zone that determines the temperature process of the steel strip, that is, the intermediate air cooling, as a region where the transformation from austenite to ferrite of the steel strip is completed, , Respectively. The cooling in these inventions is classified into the cooling pattern D described above.

  Patent Document 3 discloses hot rolling at a low finishing temperature and a high pressure reduction rate based on a predetermined relational expression related to the composition of a steel strip made of high carbon steel or high carbon alloy steel and the hot rolling conditions. The austenite grain size is refined by performing a forced cooling to a temperature range of 600 to 650 ° C., and then the air is cooled, thereby extending the residence time on the run-out table and winding the pearlite in the coil. An invention is disclosed that reduces variations in hardness in the longitudinal direction of a hot-rolled steel sheet by promoting and completing the process.

JP-A-3-277721 JP-A-8-103809 JP-A-11-131137

  The inventions disclosed in Patent Documents 1 and 2 are certainly suitable for materials that require both strength and workability, such as high-strength steel for automobiles. For example, high-carbon steel or high-carbon alloy steel When the present invention is applied to a steel strip made of high-carbon steel or high-carbon alloy steel, it takes a long time to complete the pearlite transformation. However, the pearlite transformation is not completed, and the variation in the mechanical properties in the longitudinal direction of the hot-rolled steel sheet, in particular, the variation in hardness cannot be eliminated. In the case of high-strength steel for automobiles, it is manufactured by a cooling method that actively utilizes bainite transformation to obtain material strength. However, when this method is applied to high-carbon steel or high-carbon alloy steel, the hardness increases. There is a problem of passing.

  Although the invention disclosed by Patent Document 3 is intended for eutectoid steel such as high carbon steel, for example, this invention completes pearlite transformation by air cooling, which can obtain only a small cooling rate. In the case of high carbon steel or high carbon alloy steel, it takes a long time to complete the pearlite transformation. Therefore, the pearlite transformation is not completed between the end of cooling at the run-out table and the winding of the coil. The variation in the mechanical properties in the longitudinal direction cannot be reliably eliminated. The reason for this will be described in more detail with reference to the accompanying drawings.

  FIG. 1 shows a continuous cooling transformation diagram (CCT line) of a eutectoid steel having a C content of 0.77% (“%” means “mass%” unless otherwise specified in the present specification). It is explanatory drawing which shows an example.

  As described above, when the temperature of the steel strip at a certain timing during cooling is controlled in a fragmentary manner, the cooling pattern of the steel strip may be as shown in pattern 1 of FIG. The hardness of the rolled steel sheet is extremely increased and the mechanical properties in the longitudinal direction vary.

  On the other hand, if the cooling rate of the steel strip is too low, the cooling pattern of the steel strip is as shown in pattern 2 of FIG. 1 and is not rapidly cooled after the hot rolling is completed. , Described as lamellar spacing), and the mechanical properties of the hot-rolled steel sheet deteriorate.

_Therefore, as in Documents 1 and 2 _simply water cooling, air cooling, is only cooled separately cooled with cooling zone, it is impossible to obtain a good tissue.
On the other hand, since the cooling pattern of the steel strip when cooled by the invention described in Patent Document 3 is the pattern 3 of FIG. 1 in the continuous cooling transformation diagram, the cooling at the run-out table is finished and the coil At first glance, it can be seen that the dispersion of mechanical properties in the longitudinal direction of the hot-rolled steel sheet can be reduced or eliminated because the pearlite transformation can be completed before the wire is wound.

  However, in the invention described in Patent Document 3, although air cooling performed in the middle of cooling is defined as completion of the pearlite transformation, it is described in Patent Document 3 under the conditions of normal runout table length and steel strip conveyance speed. When the steel strip specified in the described invention is air-cooled, the cooling rate obtained by air-cooling is considerably small, and the steel strip made of high carbon steel or high carbon alloy steel requires a long time for pearlite transformation. Inevitably, a portion that does not completely complete but passes through the cooling process in an untransformed state is inevitably generated. For this reason, the hardness variation in the longitudinal direction of the hot-rolled steel sheet occurs between this part and the part where the pearlite transformation is completely completed.

  Further, Patent Document 3 proposes finish low temperature rolling and high-pressure reduction rolling as a method for completely completing pearlite transformation on high carbon steel. However, high carbon steel has a very large deformation resistance, so that even when a normal rolling is performed, the rolling load (rolling load, rolling torque) becomes large, and the finish low temperature rolling and high pressure by this method are performed. When lower rate rolling is performed, a larger rolling load and rolling torque are generated, and in the worst case, there is a high possibility that equipment troubles such as damage to the rolling mill and the electric motor that drives the rolling mill will occur. Even if it does not lead to the facility trouble, the rolling load increases, the difference in elongation in the width direction of the steel sheet increases, the shape of the steel sheet deteriorates remarkably, leading to a rolling trouble. Therefore, in this invention, there is a problem that it is very difficult to perform stable rolling.

  As described above, even with the inventions disclosed in Patent Documents 1 to 3, the variation in the mechanical properties in the longitudinal direction of the hot-rolled steel plate made of high carbon steel or high carbon alloy steel cannot be reliably eliminated.

The present invention, the temperature of the steel strip of the hot high-carbon steel or high carbon alloy steel after rolling was defined (i) less than the outlet finish from the finishing outlet temperature below the temperature and temperature range of the A ₁ transformation start temperature than when in the temperature range up to the first cooling stop temperature exceeds performs water cooling, subjected to air cooling when in the (ii) a temperature range from below the first cooling stop temperature to the a ₁ transformation start temperature exceeds, (iii) a if performing the water cooling when in the temperature range of ₁ transformation start temperature or lower, and uniform and fine pearlite structure the structure of the hot rolled steel sheet composed of high carbon steel or high carbon alloy steel in the longitudinal direction of the substantially entire region This is based on a new and important finding that the variation in mechanical properties in the longitudinal direction of a hot-rolled steel plate made of high carbon steel or high carbon alloy steel can be substantially eliminated.

The present invention relates to the temperature of a steel strip made of high carbon steel or high carbon alloy steel having a C content of 0.50% or more and less than 1.50% obtained by finish rolling a rough bar. The measured value of temperature or the estimated value by calculation is
(A) from the finish rolling finishing outlet temperature below when in the temperature range up to the first cooling stop temperature exceeding that defines a temperature below the finishing outlet temperature and the A ₁ transformation start temperature than the region is the final structure having a pearlite First cooling at a first cooling rate of 20 ° C./second or more and 60 ° C./second or less , for example, water cooling is performed for substantially the entire region in the longitudinal direction of the steel strip,

(B) when the following first cooling stop temperature is in a temperature range of up to A ₁ transformation start temperature than is the final structure is a second cooling rate without bainite 20 ° C. / sec or higher 60 ° C. / sec or less second cooling, for example, the air cooling is performed for the longitudinal direction of the substantially whole area of the steel strip, further (c) when in a temperature range of a ₁ transformation start temperature or lower, the second cooling rate greater third cooling than After the pearlite transformation is completed by performing third cooling, for example, water cooling, at a speed of 20 ° C./second or more and 60 ° C./second or less over substantially the entire length of the steel strip, A method for producing a hot-rolled steel sheet made of high carbon steel or high carbon alloy steel, wherein the coil is wound at a coiling temperature.

  Further, the present invention provides a steel strip made of high carbon steel or high carbon alloy steel having a C content of 0.50% or more and less than 1.50% by performing finish rolling on a rough bar using a finish rolling mill. The steel strip is cooled by jetting cooling water from each of a plurality of water cooling devices arranged in the longitudinal direction of the steel strip while conveying the steel strip by a runout table arranged in parallel with the finishing mill. When manufacturing a hot-rolled steel sheet by winding a steel strip into a coil at a predetermined winding temperature, a thermometer capable of measuring the temperature of the steel strip in a water environment in the vicinity of each of a plurality of arranged water cooling devices The temperature of the steel strip is measured at a plurality of positions in the longitudinal direction with a plurality of thermometers, and the measured temperature of the steel strip is

(A) to finish rolling finishing outlet temperature below water cooling located in the vicinity of the thermometer indicating the temperature range up first cooling stop temperature exceeding that defines a temperature below the finishing outlet temperature and the A ₁ transformation start temperature than the region By operating the apparatus, the steel strip is cooled with water at a cooling rate of 20 ° C./second or more and 60 ° C./second or less over substantially the entire length direction,
(B) to stop the water-cooling unit located in the vicinity of the thermometer indicating the temperature range from below the first cooling stop temperature to the A ₁ transformation start temperature than air-cooling the steel strip in the longitudinal direction of the substantially entire region, and ( c) A high carbon characterized by operating a water-cooling device located in the vicinity of a thermometer showing a temperature range equal to or lower than the A ₁ transformation start temperature to complete pearlite transformation by water cooling in substantially the entire longitudinal direction of the steel strip. A method for producing a hot-rolled steel strip made of steel or high-carbon alloy steel.

  In this case, it is desirable that the temperature of the steel strip is repeatedly measured by a plurality of thermometers, and the water cooling device to be operated or stopped is determined based on the latest measured value of the temperature of the steel strip.

  In the method for producing a hot-rolled steel strip made of the high carbon steel or high carbon alloy steel according to the present invention, the high carbon steel further contains Si: 0.50% or less, Mn: 0.1% or more and 1.5. % Or less, having a steel composition consisting of the balance Fe and inevitable impurities, or the high carbon alloy steel further contains Si: 0.50% or less, Mn: 0.1% or more and 1.5% or less. In addition, Cr: 0.01% to 1.50%, Ni: 0.01% to 2.50%, W: 0.01% to 5.0%, V: 0.01% to 1. 5% or less, Ti: 0.01% or more and 0.10% or less, Nb: 0.01% or more and 0.10% or less, B: One or more of 0.0005% or more and 0.0100% or less However, it is desirable to have a steel composition consisting of the balance Fe and inevitable impurities.

  In these methods for producing a hot-rolled steel sheet according to the present invention, “air cooling” means a process of subjecting a target portion of a finish-rolled steel strip to cooling without forced cooling or water cooling. The cooling rate is about 20 ° C./second or less.

  According to the present invention, since a predetermined cooling process can be reliably realized over substantially the entire longitudinal direction, the pearlite transformation such as high carbon steel or high carbon alloy steel takes a long time to complete the coil without completing the pearlite transformation. Variations in mechanical properties such as hardness in the longitudinal direction caused by winding can be significantly reduced. Thereby, the hot rolled steel plate which consists of high carbon steel or high carbon alloy steel which has the outstanding mechanical characteristic substantially uniformly about the substantially whole area of a longitudinal direction can be manufactured.

BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out a method for producing a hot-rolled steel plate made of high carbon steel or high carbon alloy steel according to the present invention will be described in detail with reference to the accompanying drawings.
FIG. 2 is an explanatory view schematically showing a run-out table 20 for manufacturing the hot-rolled steel sheet 21 by cooling the finish-rolled steel strip 25 used in the present embodiment.

  In this embodiment, the steel strip 25 is obtained by performing finish rolling on a rough bar (not shown) using a finish rolling mill 22 including seven stands, and the C content is 0.50% or more and 1 It is made of less than 50% high carbon steel or high carbon alloy steel. In FIG. 2, the finishing mill 22 shows only the final stand F7.

  In the case where the steel strip 25 is made of high carbon steel, in addition to C: 0.50% or more and less than 1.50%, Si: 0.50% or less, Mn: 0.1% or more. It is exemplified to have a steel composition consisting of 5% or less, the balance Fe and inevitable impurities. On the other hand, when the steel strip 25 is made of high carbon alloy steel, Cr: 0.01% to 1.50%, Ni: 0.01% to 2.50%, W: 0 0.01% to 5.0%, V: 0.01% to 1.5%, Ti: 0.01% to 0.10%, Nb: 0.01% to 0.10%, B : One or more of 0.0005% or more and 0.0100% or less are included, and it is exemplified to have a steel composition composed of the balance Fe and inevitable impurities. Hereinafter, the reason why it is desirable to limit the composition of the steel strip 25 as described above will be described.

C: 0.50% or more and less than 1.50% C is an important element in the present embodiment. If the C content is less than 0.50%, the hot-rolled steel sheet does not vary in mechanical properties in the longitudinal direction even if the present invention is not applied. For this reason, the lower limit of the C content in the present embodiment is 0.50%. Moreover, since the hot-rolled steel sheet obtained as C content is 1.50% or more becomes hard and brittle, it may become impossible to perform hot rolling. Therefore, in the present embodiment, the C content is 0.50% or more and less than 1.50%.

Si: 0.50% or less Since Si can improve the hardenability of the product, Si may be added to improve the hardenability. However, this effect is saturated when the Si content exceeds 0.50%. Also, if the hardenability is too high, bainite transformation or martensitic transformation occurs during cooling after hot rolling in this embodiment, and the coil is wound around the coil while the strength is too high, or the sheet is passed through in subsequent steps. May cause trouble. Therefore, in the present embodiment, the Si content is 0.50% or less.

Mn: 0.1% or more and 1.5% or less By adding Mn, the workability during hot rolling can be improved by fixing S in steel as MnS. In addition, the hardenability of the product is improved by the presence of Mn. In order to achieve these effects, the lower limit of the Mn content is 0.1%. However, when the Mn content exceeds 1.5%, the effect is saturated, and bainite transformation or martensitic transformation occurs during cooling after hot rolling, and the coil is wound around the coil while the strength is too high. It may interfere with the threading in the process. Therefore, the Mn content is 0.1% to 1.5%.

Cr: 0.01% or more and 1.50% or less By containing 0.01% or more, Cr improves the hardenability of the product and provides temper softening resistance, which is very advantageous for heat treatment. is there. On the other hand, if the Cr content exceeds 1.50%, the effect is saturated and the hardness during hot rolling becomes too high, making it impossible to perform hot rolling or to ensure dimensional accuracy. Problems may occur. Therefore, the Cr content is set to 0.01% or more and 1.50% or less.

Ni: 0.01% or more and 2.50% or less When the Ni content is 0.01% or more, the hardenability of the product is improved. However, even if the Ni content exceeds 2.50%, this effect is saturated and the cost increases. Therefore, the Ni content is set to 0.01% or more and 2.50% or less.

W: 0.01% or more and 5.0% or less W, like Ni, contains 0.01% or more to improve the hardenability of the product. On the other hand, even if the W content exceeds 5.0%, the effect is saturated and the strength during hot rolling becomes too high, making rolling difficult. Therefore, the W content is set to 0.01% to 5.0%.

V: 0.01% or more and 1.5% or less When the V content is 0.01% or more, the temper softening resistance of the product increases. On the other hand, even if the V content exceeds 5.0%, the effect is saturated and the cost increases. Therefore, the V content is set to 0.01% to 5.0%.

Ti: 0.01% or more and 0.10% or less Ti is added to fix N as TiN in order to make an element having high affinity with N such as Cr and Nb work effectively. In order to exhibit such an effect, the Ti content is set to 0.01% or more. However, even if the Ti content exceeds 0.10%, this effect is saturated. Therefore, the Ti content is set to 0.01% or more and 0.10% or less.

Nb: 0.01% or more and 0.10% or less Nb can be refined by adding 0.01% or more, thereby improving toughness. However, the effect is saturated even if the Nb content exceeds 0.10%. Therefore, the Nb content is 0.01% or more and 0.10% or less.

B: 0.0005% or more and 0.0100% or less B By containing 0.0005% or more, the hardenability of the product can be improved. However, even if the B content exceeds 0.0100%, this effect is saturated. Therefore, the B content is limited to 0.0005% or more and 0.0100% or less.

Other than the above are Fe and inevitable impurities.
In FIG. 2, reference numeral 23 denotes a winding machine, reference numeral 24 denotes a finishing outlet thermometer for measuring the temperature of the surface of the steel strip 25 at the outlet of the finishing mill 22, and reference numeral 26 denotes winding by the winding machine 23. The winding thermometer which measures the temperature of the surface of the steel strip 25 just before is shown, the code | symbol 27 shows a temperature control apparatus, and the code | symbol 28 shows a temperature measurement processing apparatus.

  Reference numerals (29a, 29b), (30a, 30b), (31a, 31b), (32a, 32b), (33a, 33b), (34a, 34b), (35a, 35b), (36a, 36b) These show a pair of upper and lower water cooling apparatus arrange | positioned at the run-out table 20. FIG. In FIG. 2, only a total of eight water cooling devices (29a, 29b) to (36a, 36b) are described, and other water cooling devices are omitted in view of the complexity of the drawing. Further, reference numerals 29c to 36c are provided for the respective water cooling devices (29a, 29b) to (36a, 36b) for starting or stopping the water cooling devices (29a, 29b) to (36a, 36b) installed on the run-out table 20. The control valve provided one each is shown.

  As described above, the water cooling devices (29a, 29b) to (36a, 36b) are provided in large numbers on the upper and lower surfaces of the runout table 20 provided between the finish rolling mill 22 and the winder 21. The steel strip 25 is cooled by water cooling such as cooling or spray cooling so as to pass through a predetermined temperature process, and a hot-rolled steel sheet 21 having predetermined mechanical characteristics is manufactured.

   The water cooling devices (29a, 29b) to (36a, 36b) are provided with one control valve 29c to 36c for each of the water cooling devices (29a, 29b) to (36a, 36b) as in this example. Start and stop may be configured to be subdivided and controlled, and unlike this example, one control valve is provided for each unit consisting of a plurality of cooling devices, and this unit is started and stopped. May be configured to be controlled collectively. Needless to say, the control valves 29c to 36c can be improved in resolution by providing the control valves 29c to 36c for each of the water cooling devices (29a, 29b) to (36a, 36b) as in this example. In view of the maintainability of the control valves 29c to 36c, it is preferable to control by the latter means that can reduce the number of installed control valves 29c to 36c. In general, the number of cooling devices combined into one unit may be determined as appropriate in consideration of the cost required for maintenance and the accuracy of target mechanical characteristics.

   The temperature control device 27 determines the number of water cooling devices (29a, 29b) to (36a, 36b) used by the run-out table 20 when controlling the winding temperature that has been conventionally performed. An outline of this setting procedure will be described below.

  First, when the steel strip 25 is not on the run-out table 20 and the rough bar is on the entry side of the finishing mill 22, the start and stop patterns of the water cooling devices (29a, 29b) to (36a, 36b) are initialized. There is a need. This initial setting is performed before the rough bar is finish-rolled by the finish rolling mill 22. This is based on the rolling speed of the finish rolling mill 22 with the temperature measured value or calculated value of the representative point (usually the tip) of the steel strip roughly rolled by the rough rolling mill or the temperature average value of the total length as the initial value. After calculating the temperature drop of water cooling, heat radiation, roll contact heat transfer, etc. in the finishing mill 22 to calculate the finishing outlet temperature of the steel strip 25, the calculated finishing outlet temperature of the steel strip 25 is calculated. The initial values of the water cooling devices (29a, 29b) to (36a, 36b) that can obtain the target winding temperature by increasing the number of water cooling devices according to the installation order of the cooling devices from upstream to downstream and repeatedly performing the calculation. Decide how many to use.

   Further, after the steel strip 25 has exited the finishing mill 22 and entered the run-out table 20, the outlet thermometer 24 of the finishing mill 22 pitches from the tip of the steel strip 25 at a constant length pitch or a constant time pitch. The finish outlet temperature is actually measured, and the temperature drop amount in the run-out table 20 at each measurement point (sampling point) is calculated. The steel strip 25 when each sampling point reaches directly below the winding thermometer 26 is calculated. The start and stop patterns of the water cooling devices (29a, 29b) to (36a, 36b) are determined by calculation so that the temperature becomes the target winding temperature.

  Then, this calculation is performed for all sampling points in the longitudinal direction, and by selecting the greatest common divisor of all start and stop patterns of the sampling points in the run-out table 20, the final water-cooling device (29a of the run-out table 20) 29b) to (36a, 36b) are determined. As a matter of course, the number of water cooling devices (29a, 29b) to (36a, 36b) determined by this calculation is for satisfying a desired winding temperature, and the cooling process in the runout table 20 is performed. The whole is not considered at all.

  On the other hand, this embodiment does not control only the start point and end point of the cooling process of the finish outlet temperature and the winding temperature as in the conventional case, as described later, but the first cooling in the middle of them. The entire cooling process is controlled by also controlling the stop temperature (that is, the start temperature of the second cooling) and the end temperature of the second cooling (that is, the start temperature of the third cooling).

  Moreover, since it is desirable to start cooling the steel strip 25 as soon as possible using the above-described water cooling devices (29a, 29b) to (36a, 36b) after finishing rolling, in this embodiment, Cooling is successively started sequentially from the water cooling device (29a, 29b) at the position closest to the exit of the finish rolling mill 22. Unlike the present embodiment, the activation and stop patterns of the water cooling devices (29a, 29b) to (36a, 36b) may be determined according to a predetermined order of use.

In the present embodiment, the temperature control device 27 determines a set value of a predetermined first cooling stop temperature and an A ₁ transformation start temperature that defines a second cooling time to be performed subsequently to these. Based on the entire length of the runout table 20, the entire cooling process, which is the target temperature trajectory of the steel strip 25, is determined. In this cooling, each of the water cooling devices (29a, 29b) to (36a, 36b) is continuously used. However, if each water cooling device (29a, 29b) to (36a, 36b) is used at full capacity, the steel is used. Since the temperature of the strip 25 cannot be controlled, for example, the water cooling device 29a is used in the water cooling device (29a, 29b), and the water cooling device 30b is used in the water cooling device (30a, 30b). 29b) to (36a, 36b), if one of the water cooling devices is alternately used in a staggered manner, the cooling can be continuously performed as a whole, and, for example, when the cooling rate or the cooling stop temperature is controlled. Even when the capacity is insufficient, the cooling device on the side that has not been activated can be activated to compensate for the lack of cooling capacity.

    In addition, it is not limited to this form, For example, you may make it adjust cooling capacity by increasing / decreasing the amount of cooling water of water-cooling apparatus (29a, 29b)-(36a, 36b).

  Further, if the units for starting and stopping the water cooling devices (29a, 29b) to (36a, 36b) are subdivided as much as possible as in the present embodiment, the adjustment unit for the cooling temperature drop is also subdivided. As described above, cooling should be performed in the order of installation of the water cooling devices (29a, 29b) to (36a, 36b) without paying much attention to the selection of the water cooling devices (29a, 29b) to (36a, 36b) to be used. That is, the water cooling devices (29a, 29b) to (36a, 36b) to be used can be easily set. Further, in the control at the time of cooling, the cooling temperature may be controlled to be within a certain allowable range.

In this manner, the water cooling devices (29a, 29b) to (36a, 36b) to be used are specified by the temperature control device 27.
In FIG. 2, the code | symbol 37a-37c _, 38a-38c shows the water environment thermometer which can measure the temperature of the surface of the steel strip 25 in a water environment. FIG. 2 shows six water environment thermometers 37a to 37c and 38a to 38c, and other water environment thermometers are omitted in view of the complexity of the drawing. The structure, principle, and the like of this type of water environment thermometer are disclosed in, for example, Japanese Patent Application Laid-Open No. 2003-185501, and since they are already known, description thereof is omitted here. In the following description, first, a case where control is performed using the estimated calculation value of the steel strip 25 without using the temperature measurement value of the steel strip 25 by the thermometers 37a to 37c _and 38a to 38c will be described. The temperature measurement values of the steel strip 25 according to 37a to 37c _and 38a to 38c will be described later when control is performed using the temperature measurement values.

In the present embodiment, a number of water cooling devices (29a, 29b) are arranged on the runout table 20 in the longitudinal direction of the steel strip 25 while the steel strip 25 is conveyed by the runout table 20 provided in parallel to the finishing mill 22. ) To (36a, 36b) to cool by injecting cooling water from each. Further, a plurality of thermometers 37a to 37c _and 38a to 38c capable of measuring the temperature of the steel strip 25 in the water environment are arranged in the vicinity of the plurality of water cooling devices (29a, 29b) to (36a, 36b). Keep it. And the temperature of the surface of the steel strip 25 is measured in the several position of a longitudinal direction with several thermometer 37a-37c _, 38a-38c. The measured value of the temperature of the surface of the steel strip 25 is sent to the temperature control device 27 via the temperature measurement processing device 28.

  The temperature control device 27 predicts and calculates the finishing outlet temperature based on the sheet thickness reduction schedule, the rolling speed setting value, etc. in the finishing mill 22 with the detected value of the exit thermometer (not shown) of the rough rolling mill as an initial value. To do.

In addition, it is desirable that the finishing outlet temperature is not less than the Ae ₃ transformation point from the viewpoint of reliably controlling the cooling process before and after the A ₁ transformation start point after hot rolling in the austenite region. This is because by setting the finishing outlet temperature to be equal to or higher than the Ae ₃ transformation point, the austenite-ferrite transformation that occurs during finish rolling can be suppressed, so that the material in the longitudinal direction can be made uniform easily. The finishing outlet temperature is preferably a temperature just above the Ae ₃ transformation point.

Temperature control device 22 of the present embodiment, estimation calculation of the temperature of the steel strip 25, the following finish rolling of the finishing outlet temperature was determined in advance to a temperature of the finishing outlet temperature less than and A ₁ transformation start temperature than the region By operating a water cooling device (for example, (29a, 29b) to (30a, 30c)) located in the vicinity of a thermometer (for example, 38a) in a temperature range up to the first cooling stop temperature, the steel strip 25 is made to operate. On the other hand, the first cooling (water cooling) is performed over substantially the entire area in the longitudinal direction.

The temperature control device 27 of the present embodiment, estimation calculation of the temperature of the steel strip 25, the finishing outlet temperature below when in the temperature range up to the first cooling stop _temperature, the thermometer 38a~38c The first cooling stop temperature is controlled to a target value by a water cooling device in the vicinity. Water cooler (e.g. (32a, 32b) ~ (33a , 33b)) is located in the vicinity of the thermometer in a temperature range of up to _{A 1} transformation start temperature than the following first cooling stop temperature (e.g. 37a) to a stop Then, the second cooling (air cooling) is performed on the steel strip 25 in substantially the entire longitudinal direction.

Further, the temperature control device 27, a water cooler estimation calculation value of the temperature of the steel strip 25 is located in the vicinity of the thermometer in the temperature range of _{the A 1} transformation starting temperature or less (e.g., 37c) (e.g. (34a, 34b) (36a, 36b)) is operated, and the steel strip 25 is subjected to the third cooling (water cooling) in substantially the entire longitudinal direction thereof, thereby completing the pearlite transformation.

As described above, in this description, an estimated calculation value is used as the temperature of the steel strip 25. However, such an estimation calculation inevitably includes an error. Therefore, the temperature of the steel strip 25 may be measured by using the thermometers 37a to 37c _and 38a to 38c instead of the estimated calculation value.

That is, in order to grasp the _first cooling stop temperature and the A1 transformation start temperature with higher accuracy, as described above, the water environment thermometers 37a to 37a that can measure the temperature of the surface of the steel strip 25 in the water environment. 37 c and 38 a to 38 c are used, and the water cooling device downstream of the thermometer whose detected value of the thermometers 38 a to 38 c indicates the first cooling stop temperature is stopped. The first cooling than thermometer showing the stop temperature at the downstream side of the water environment thermometers 37 a - 37 c, the upstream side of the water-cooling unit from a thermometer showing the detection value A ₁ transformation start temperature of 38a~38c On the other hand, cooling is performed using a water cooling device on the downstream side. According to this, since it is not necessary to perform the temperature calculation is eliminated, it can be identified more first cooling stop temperature with high accuracy, the start position and start position of the third cooling of A ₁ transformation start temperature and a second cooling. However, in order to carry out this method, it is necessary to install a plurality of expensive water environment thermometers 37a to 37c and 38a to 38c, which increases costs. For this reason, it is desirable to properly use the above two types of means according to the accuracy of characteristics required for the hot-rolled steel sheet 21.

  The measurement of the temperature of the steel strip 25 by the thermometers 37a to 37c and 38a to 38c is repeatedly performed at an appropriate time interval, and the latest measurement value is input to the temperature control device 27. In order to improve the control accuracy, it is desirable to control the start and stop of the above-described water cooling devices (29a, 29b) to (36a, 36b) based on the measured values.

  Thus, in the present embodiment, the temperature of the steel strip 25 made of high carbon steel or high carbon alloy steel having a C content of 0.50% or more and less than 1.50% is estimated by calculation, or the temperature It calculates | requires by measuring by the total 37a-37c and 38a-38c.

The estimating calculation value or measured value obtained is from finishing the outlet temperature of finish rolling below, lower than the finishing outlet temperature and the temperature range to the first cooling stop temperature exceeding that defines the temperature range of the A ₁ transformation start temperature than At a certain time, the first cooling at the first cooling rate, specifically the water cooling, at which the final structure having pearlite is obtained is performed over substantially the entire length of the steel strip 25 in the longitudinal direction. Here, the first cooling stop temperature is, for example, first cooling stop temperature = A ₁ transformation start temperature + plate thickness × α (where α = 5.4 [Mn] +14.5 [Si] +8.5 [Cr] +8.5 [Ni]) is exemplified.

  If the first cooling is performed up to a temperature range lower than the first cooling stop temperature, the steel strip 25 is hardened, and the variation of the material tends to increase. The first cooling is desirably performed up to a temperature range of 700 ° C. or lower. If the first cooling is stopped in a temperature range exceeding 700 ° C., the pearlite transformation is delayed, and the pearlite transformation is not completed on the hot run table 20. Variation of the increase.

The cooling rate of the first cooling is desirably 20 ° C./second or more and 60 ° C./second or less. When the cooling rate of the first cooling is less than 20 ° C./second, the progress of the pearlite transformation is delayed, and the pearlite transformation is not completed on the hot run table 20. Variation in the longitudinal direction is likely to increase. Moreover, since the temperature controllability deteriorates when the first cooling rate exceeds 60 ° C./second, bainite is likely to partially precipitate. On the other hand, by setting the cooling rate of the first cooling to 20 ° C./second or more and 60 ° C./second or less, the formation of bainite can be prevented while promoting the A ₁ transformation. In other words, this cooling rate is: This is the cooling rate at which pearlite transformation is started when continuous cooling is performed from the eutectoid point in eutectoid steel.

Then, the estimated calculation value or measured value obtained is, when the following first cooling stop temperature is in a temperature range of up to A ₁ transformation start temperature than the first to the final tissue at a second cooling rate without bainite 2 cooling, for example, air cooling, is performed on substantially the entire length of the steel strip 25 in the longitudinal direction.

Here, A ₁ transformation start temperature, the components of the steel strip 25, rolling conditions more may be empirically properly set the cooling conditions and the like. For example, using a function f defined by the following equation obtained empirically, A ₁ transformation start temperature (° C.) = F ([C], [Mn], [Si], [Cr], Tf, CR, VR) can be calculated. Here, A ₁ transformation start temperature, the amount of the additive element, cooling start temperature, cooling rate further vary by thickness, in the present embodiment, a large number of data when performing an actually cool the formula Based on.

Further, in the present embodiment, the estimated calculation value or measured value of the temperature of the steel strip 25, the temperature range the second cooling to A ₁ transformation start temperature, for example, a heated target portion of the steel strip 25 quiet such an air cooling for cooling and allowed to stand in the air, by performing in the longitudinal direction of the substantially entire region of the steel strip 25, the temperature rise due to the a ₁ transformation has started currently (the transformation heat generated by the start of the a ₁ transformation) Since it can be confirmed, the start time of the third cooling can be accurately determined.

Furthermore, in the present embodiment, the estimated calculation value or measured value of the temperature of the steel strip 25, when in the temperature range of the A ₁ transformation starting temperature below, at greater than the second cooling rate third cooling rate The pearlite transformation is completed by performing the third cooling, for example, water cooling for substantially the entire region of the steel strip 25 in the longitudinal direction.

  The cooling rate of the third cooling is desirably 20 ° C./second or more and 60 ° C./second or less. This is because a fine pearlite structure can be obtained reliably by performing the third cooling at a cooling rate in this range. If the cooling rate of the third cooling is less than 20 ° C./second, the pearlite transformation may not be completed completely. On the other hand, if it exceeds 60 ° C./second, the controllability of the coiling temperature is deteriorated.

  In the present embodiment, the pearlite transformation is completed on the run-out table 20 in this way. And the steel strip 25 which completed the pearlite transformation is wound around a coil by predetermined winding temperature, and it is set as a hot rolled sheet steel. The coiling temperature is preferably 400 ° C. or more and 600 ° C. or less in order to prevent the formation of bainite while obtaining fine pearlite and to ensure workability in the next step.

  Thus, in the present embodiment, the final pearlite structure is refined by performing the first cooling (water cooling) as soon as possible after finishing rolling. Then, the first cooling is temporarily stopped at the first cooling stop temperature that is a predetermined pearlite nose temperature, and the second cooling (air cooling) is performed until the timing of the pearlite transformation starts. A bainite structure is surely prevented from excessively increasing the hardness. Then, after the pearlite transformation is started, the third cooling (water cooling) is started at the same time to complete the pearlite transformation, and then the coil is wound around the coil.

According to the present embodiment, since the cooling rate after finish rolling is set to 20 ° C./second or more and 60 ° C./second or less and the first cooling is performed up to the first cooling stop temperature, after hot finish rolling in the austenite region from a ₁ can be reliably controlled as desired the cooling process in the vicinity of transformation start temperature, it is possible to promote the a ₁ transformation while preventing the occurrence of bainite. Further, according to this embodiment, it is possible to ensure proper cooling time by the second cooling, the start of the third cooling performed subsequently to check accurately the temperature rise due to the start of the A ₁ transformation The timing can be determined accurately.

  Furthermore, according to the present embodiment, the third cooling is subsequently performed at the cooling rate of 20 ° C./second or more and 60 ° C./second or less based on the start timing of the third cooling thus determined. Finally, a uniform fine pearlite structure can be obtained.

  Therefore, although the cooling pattern of the present embodiment is similar to the cooling pattern D in FIG. 5, this cooling pattern is at least in two points: a condition for stopping the second cooling and a condition for starting the third cooling. Different from D.

  For this reason, since the hot-rolled steel sheet 21 manufactured according to the present embodiment is manufactured by reliably realizing a predetermined cooling process over the entire length of the steel strip 25 in the longitudinal direction, the high-carbon steel requires a long time for pearlite transformation. Even if it is made of steel or high carbon alloy steel, the variation in mechanical properties such as hardness in the longitudinal direction, which occurs because the pearlite transformation is wound up without completing the pearlite transformation, has been significantly reduced, It has excellent mechanical properties substantially uniformly over substantially the entire region in the longitudinal direction. For this reason, the hot-rolled steel sheet manufactured according to the present embodiment has substantially eliminated variations in the mechanical properties in the longitudinal direction, and this secondary processing is performed even when subjected to secondary processing such as cold rolling. It can be done reliably and easily.

Next, the calculation flow of such temperature control calculation by the temperature control device 27 will be described in detail with reference to FIG.
In the temperature control device 27, the amount of temperature drop of the rough bar and the steel strip 25 in each cooling process is the thickness of the target rough bar and steel strip 25, the finish rolling speed of the rough bar, and the finish outlet of the steel strip 25. target value of the temperature, the first cooling stop temperature, _{a 1} transformation start temperature set value that defines the time of the second cooling, the target value of the winding temperature, water-cooling device (29a, 29b) ~ (36a , 36b ), For example, using a steel sheet temperature drop calculation formula defined by the following formula (1).

  Tout = Tinexp {− (α / cρh) · (L / V)} (1)

  In this equation (1), Tout represents the outlet temperature of the water cooling devices (29a, 29b) to (36a, 36b), Tin represents the inlet temperature of the water cooling devices (29a, 29b) to (36a, 36b), and α Represents the heat transfer coefficient of the water cooling devices (29a, 29b) to (36a, 36b), c represents the specific heat of the steel strip 25, ρ represents the density of the steel strip 25, and h represents the thickness of the steel strip 25. L indicates the length of the water cooling devices (29a, 29b) to (36a, 36b), and V indicates the rolling speed of the coarse bar.

Specifically, the temperature control device 27 sequentially determines usage patterns from the water cooling devices (29a, 29b) according to the calculation flow shown in FIG.
In step (hereinafter abbreviated as “S”) 1 in FIG. 3, the thickness h, specific heat c, density ρ of the rough bar, which is material information of the rough bar subjected to rough rolling, and the rolling speed, which is rolling information. V, the length L of the water cooling devices (29a, 29b) to (36a, 36b) is input to the temperature control device 27. Then, the process proceeds to S2. In the present embodiment, in order to facilitate understanding, the lengths of all the water cooling devices (29a, 29b) to (36a, 36b) are the same.

In S2, a temperature control device 27, as the cooling conditions, the target value of the first cooling stop temperature TSaim, _{A 1} transformation start temperature set value TA1aim, the target value TCaim the coiling temperature is input. The cooling stop temperature tolerance .DELTA.TS, A ₁ transformation start temperature tolerance DerutaTA1, tolerance ΔTC of the winding temperature is also input. Then, the process proceeds to S3.

  Here, the smaller the allowable values ΔTS, ΔTA1, and ΔTC, the higher the calculation accuracy, but the longer the calculation time. Conversely, the larger the allowable values ΔTS, ΔTA1, and ΔTC, the lower the calculation accuracy, but the shorter the calculation time. . Therefore, the allowable values ΔTS, ΔTA1, and ΔTC may be appropriately set in consideration of the required accuracy and the computer capability necessary for the calculation.

  In S3, the finish outlet temperature TF is determined by setting calculation or the like of another finish rolling mill 22, for example, as shown in the following formula (2): water cooling, air cooling (including thermal radiation) of the finishing mill 22; Based on the temperature drop of contact heat transfer.

T = T0−ΔT
ΔT = ΔTw + ΔTa + ΔTr−ΔTq
ΔTw = hw (T−Tw) · tw / (c · ρ · H)
ΔTa = ha (T−Ta) · ta / (c · ρ · H)
ΔTr = hr (T−Tr) · tr / (c · ρ · H)
ΔTq = G · η / (c · ρ · H)
(2)

  In equation (2), T represents the temperature of the coarse bar, T0 represents the initial temperature of the coarse bar, ΔT represents the temperature drop of the coarse bar, and ΔTw is determined by a water cooling device installed in the finishing mill 22. Indicates the amount of temperature drop, ΔTa indicates the amount of temperature drop due to air cooling during finish rolling, ΔTr indicates the amount of temperature drop due to contact with the work roll, ΔTq indicates the amount of temperature increase due to processing heat generation during finish rolling, hw, ha, and hr indicate heat transfer coefficients for water cooling, air cooling, and rolling roll, respectively, Tw, Ta, and Tr indicate water temperature, air temperature, and rolling roll temperature, respectively, tw, ta, and tr indicate water cooling, air cooling, and rolling, respectively. This is the time required and is determined from the speed pattern of the rolling mill and the transport table. C represents the specific heat of the coarse bar, ρ represents the density of the coarse bar, and H represents the thickness of the coarse bar. Further, G represents the rolling torque, and η represents the rate at which the rolling torque changes to processing heat generation.

Then, the finishing outlet temperature TF calculated in this way is set as the initial temperature for the subsequent calculation, and the process proceeds to S4.
In S4 to S10, the water cooling device to be used is in the order of the water cooling devices (30a, 30b), (31a, 31b),... Sequentially located downstream from the first water cooling device (29a, 29b). To decide. In addition, it is not limited to this procedure, You may determine the use order of water-cooling apparatus (29a, 29b)-(36a, 36b) according to a predetermined use order.

  That is, in S4 and S5, the temperature drop by each of the water cooling devices (29a, 29b) to (36a, 36b) is calculated, and the outlet temperature of the first water cooling device (29a, 29b) is expressed by the above-described equation (1). Calculate according to The water cooling heat transfer coefficients of the water cooling devices (29a, 29b) to (36a, 36b) at this time are obtained using the following equation (3). Then, the process proceeds to S6.

α _W = A (W ^B / T ^C ) (1-D × T _W ) V ^E (3)

In this equation (3), W is shown a water density, T is indicates the temperature of the steel strip 24, T _W denotes a cooling water temperature, V is shown a rolling speed. Α _W is a water-cooling heat transfer coefficient, which varies depending on the type of the water-cooling devices (29a, 29b) to (36a, 36b), and A to E are parameters at that time.

  In S6 to S10, whether or not the temperature of the steel strip 24 at the outlet of the water cooling device (29a, 29b) is within the allowable range of the first cooling stop temperature, that is, whether or not | Tout−TSaim | ≦ ΔTS Check whether or not. And if not, it will transfer to S7 and S8, and will perform the same calculation repeatedly also about each water cooling apparatus after a water cooling apparatus (30a, 30b).

  When the temperature of the steel strip 25 at the outlet of the Nc1st water cooling device falls within the allowable range of the first cooling stop temperature, the process proceeds to S10. Thereby, the 1st-Nc 1st water cooling apparatus is determined as a water cooling apparatus used for the 1st cooling. Then, the process proceeds to S11.

  That is, in S1 to S10, the steel strip 25 is removed from the finishing mill 25 by the temperature control device 27 and enters the run-out table 20 after the steel strip 25 using the outlet thermometer 24 of the finishing mill 22. The temperature drop amount in the run-out table 20 at each measurement point (sampling point) at which the finishing outlet temperature is measured from the tip of 25 at a constant length pitch or a constant time pitch is calculated, and each sampling point is a winding thermometer. 26, the start and stop patterns of the water cooling devices (29a, 29b) to (36a, 36b) are calculated so as to reach the target winding temperature when reaching just below 26, and this calculation is performed for all the longitudinal directions of the steel strip 25. By performing the sampling point, the greatest common divisor of the start and stop patterns of all sampling in the run-out table 20 is obtained. Set the start and stop patterns of the final runout table 20 by, outputs a control signal to the control valve 29c~36c based on the start and stop patterns is performed with water cooling. The calculation in the longitudinal direction is sequentially performed for the entire region from the front end portion to the tail end portion of the steel strip 25 at every sampling timing of the finishing outlet temperature.

  In S11, the most upstream water cooling device among the water cooling devices that perform the second cooling performed subsequently is determined as the (Nc1 + 1) th water cooling device downstream of the Nc1st one determined in S10. Then, the process proceeds to S12.

In S12 to S17, it is determined how many water cooling devices from the most upstream (Nc1 + 1) th water cooling device set in S11 are used for the second cooling.
Specifically, in S12, the temperature cooling start temperature by each water cooling device after the (Nc1 + 1) th water cooling device is calculated by Tout = Tinexp (− (αair / cρh) tail). In this equation, αair is an air cooling heat transfer coefficient. Then, the process proceeds to S13.

In S13, the temperature of the steel strip 25 at the outlet of the first (Nc1 + 1) th water-cooling device, whether to reach the _{A 1} transformation start temperature, | by whether ≦ DerutaTA1 is satisfied, | Tout-TA1aim To check.

  If | Tout−TA1aim | ≦ ΔTA1 is not satisfied, the process proceeds to S14 and S15, and the same calculation is repeated for the temperature of the steel strip 25 at the outlet of the (Nc1 + 2) th and subsequent water cooling devices.

Then, the temperature of the steel strip 25 at the outlet of the first (Nc1 + Nstop) th water-cooling unit, | Tout-TA1aim | if ≦ DerutaTA1 is within the allowable range of filled and _{A 1} transformation start temperature is later to S17 . Accordingly, the number of water cooling devices used for the second cooling is Nstop.

  In S18, the most upstream water cooling device among the water cooling devices that perform the third cooling performed subsequently is set as the (Nc1 + Nstop + 1) th water cooling device. Then, the process proceeds to S19.

In S <b> 19 to S <b> 23, it is determined how many consecutive water cooling devices for performing the third cooling from the (Nc1 + Nstop + 1) th water cooling device to the water cooling device.
That is, in S19, the temperature drop by each water cooling device is calculated using the above-described equations (1) and (3). Then, the process proceeds to S20.

  In S20, whether or not a predetermined coiling temperature is reached at the outlet of the (Nc1 + Nstop + 1) -th water cooling device is checked by | Tout−TCaim | ≦ ΔTC. If the temperature of the steel strip 25 at the outlet of the (Nc1 + Nstop + 1) th water cooling device is not within the allowable range of the predetermined coiling temperature, the process proceeds to S21 and S22, and the same applies to each of the (Nc1 + Nstop + 2) th water cooling devices. Repeat the calculation.

  When the temperature of the steel strip 25 at the outlet of the (Nc1 + Nstop + Nc2) th water cooling device satisfies | Tout−TCaim | ≦ ΔTC and reaches a predetermined winding temperature, the process proceeds to S23. Accordingly, the number of water cooling devices specified for the third cooling is NC2.

  In the present embodiment, as described above, the water cooling device that performs the first cooling is determined as the first to Nc1th water cooling devices, and the water cooling device that performs the second cooling is the (Nc1 + 1) th to The (Nc1 + Nstop) th water cooling device is determined, and the water cooling device for performing the third cooling is determined as the (Nc1 + Nstop + 1) th to (Nc1 + Nstop + Nc2) th water cooling device, and rolling is performed at the set rolling speed. Done.

  Here, the calculation flow shown in FIG. 3 is for the case where the coiling temperature is also controlled to a predetermined target value. However, depending on the specifications of the hot-rolled steel sheet 21 to be manufactured, particularly high accuracy is required for management of the coiling temperature. In such cases, it is advantageous to complete the pearlite transformation as soon as possible. Therefore, in such a case, the third cooling may be performed using all the water cooling devices after the second cooling is completed.

  In addition, when the exit temperature of the finishing mill 22 is not controlled as prescribed, the rolling speed of the finishing mill 22 is changed or the heating device is installed on the entry side of the finishing mill 22. Can be cooled more stably by controlling the temperature using this heating device.

  C: 0.53, Mn: 0.8%, Si: 0.22%, Cr: 0.07%, a rough bar having a steel composition consisting of the balance Fe and inevitable impurities is finished by a hot finish rolling mill. It rolled and it was set as the steel strip of board thickness: 3.0mm and board width: 1000mm. The finishing outlet temperature was 840 ° C. After that, it is cooled with water to 650 ° C. at a cooling rate of 50 ° C./second, air-cooled to 625.8 ° C., then cooled with water at a cooling rate of 50 ° C./second, and wound on a coil at a winding temperature of 590 ° C. A steel plate was produced (invention example). The plate passing speed was 450 mpm.

  On the other hand, as a comparative example, the rough bar having the steel composition described above was finish-rolled by a hot finish rolling mill to obtain a steel strip having a plate thickness of 3.0 mm and a plate width of 1000 mm. The finishing outlet temperature was 840 ° C. And it cooled at the cooling rate of 10-20 degreeC / sec, and wound up by the coil at the coiling temperature of 630 degreeC, and manufactured the hot rolled sheet steel (conventional example).

And about the coil obtained by this invention example and the prior art example, the dispersion | variation (DELTA) HRB of the hardness of a longitudinal direction was measured.
As a result, the longitudinal hardness variation ΔHRB of the coil of the present invention example could be suppressed to less than half of the longitudinal hardness variation ΔHRB of the conventional example coil.

  Further, the variation Δt in the longitudinal winding temperature in the conventional example was 60 ° C. (± 30 ° C.), but the variation in the longitudinal winding temperature Δt in the present invention example was 30 ° C. (± 15 ° C.). Met.

  For this reason, according to the example of the present invention, since a predetermined cooling process can be reliably realized over substantially the entire lengthwise direction, the time required for the pearlite transformation is long, so that the pearlite transformation is not completed and is wound on the coil. Thus, the variation in mechanical properties such as hardness in the longitudinal direction can be significantly reduced. As a result, it was possible to produce a hot-rolled steel sheet made of a high carbon alloy steel having excellent mechanical properties substantially uniformly over substantially the entire region in the longitudinal direction.

It is explanatory drawing which shows an example of the continuous cooling transformation diagram (CCT diagram) of eutectoid steel whose C content is 0.77%. It is explanatory drawing which shows typically the run-out table for cooling the finish-rolled steel strip used by embodiment, and manufacturing a hot-rolled steel plate. It is a flowchart which shows the calculation flow of the temperature control calculation by a temperature control apparatus. It is explanatory drawing which shows typically the runout table which is a part of manufacturing process of a hot-rolled steel plate. It is a graph which shows typically four general cooling patterns AD of the steel strip between the exit of the finishing mill of a run-out table and just before a winder.

Explanation of symbols

25 Steel strip 21 Hot-rolled steel plate 20 Runout table 22 Finishing mill 23 Winding machine 24 Finishing exit thermometer 26 Winding thermometer 27 Temperature control device 28 Temperature measurement processing device (29a, 29b) to (36a, 36b) Water cooling device 29c to 36c Control valve 37a to 37c Water environment thermometer

Claims (3)

The temperature of the steel strip made of high carbon steel or high carbon alloy steel having a C content of 0.50% by mass or more and less than 1.50% by mass obtained by performing finish rolling on the rough bar,
The following the rolling finishing outlet temperature the partition, when in the temperature range up to the first cooling stop temperature exceeds that defined temperature range on the outlet temperature less than and A ₁ transformation start temperature than the partition may obtain a final tissue having a pearlitic Water cooling, which is the first cooling at a first cooling rate of 20 ° C./second or more and 60 ° C./second or less , is performed over substantially the entire region in the longitudinal direction of the steel strip,
Wherein when the first cooling stop temperature below a certain temperature range up the A ₁ transformation start temperature than, the final structure to the second cooling a 20 ° C. / sec or less is a second cooling rate without bainite the cooling is performed in the longitudinal direction of the substantially entire region of the steel strip, further when in the a ₁ transformation start temperature below the temperature range, than the cooling rate of the second is a major third cooling rate 20 ° C. After the pearlite transformation is completed by performing water cooling, which is the third cooling at a temperature of not less than 60 ° C./second and not more than 60 ° C./second, over substantially the entire longitudinal direction of the steel strip, the steel strip is heated at a predetermined winding temperature. A method for producing a hot-rolled steel sheet made of high-carbon steel or high-carbon alloy steel, which is wound on a coil. The temperature of the steel strip made of high carbon steel or high carbon alloy steel having a C content of 0.50% by mass or more and less than 1.50% by mass obtained by performing finish rolling on the rough bar,
The following finishing exit temperature on rolling the partition, when in the temperature range up to the first cooling stop temperature exceeding that defines the temperature range up to the outlet temperature less than and A ₁ transformation start temperature than the partition in the longitudinal direction of the steel strip Substantially water-cooled at a cooling rate of 20 ° C / second or more and 60 ° C / second or less ,
Wherein when the first cooling stop temperature below a certain temperature range up the A ₁ transformation start temperature than the air-cooled to substantially the entire area in the longitudinal direction of the steel strip, is further to the A ₁ transformation start temperature below the temperature range Sometimes, the high carbon steel or the high carbon alloy steel is characterized by winding the steel strip around a coil at a predetermined winding temperature after completing the pearlite transformation by water-cooling substantially the entire longitudinal direction of the steel strip. A method for producing a hot-rolled steel sheet. The steel bar is made of high carbon steel or high carbon alloy steel having a C content of 0.50 mass% or more and less than 1.50 mass% by performing finish rolling on the rough bar using a finish rolling mill, The steel is cooled by spraying cooling water from each of a plurality of water-cooling devices arranged in the runout table in the longitudinal direction of the steel strip while being conveyed by a runout table arranged in parallel in the finishing mill. When manufacturing a hot-rolled steel sheet by winding a strip into a coil at a predetermined winding temperature,
A plurality of thermometers capable of measuring the temperature of the steel strip in a water environment are arranged in the vicinity of each of the plurality of arranged water cooling devices, and the temperature of the steel strip is adjusted in the longitudinal direction by the plurality of thermometers. The temperature of the steel strip measured at multiple locations
Wherein from less than or equal to the finish-rolling finishing outlet temperature, located in the vicinity of the thermometer indicating the temperature range up first cooling stop temperature exceeds that defined temperature range on the outlet temperature less than and A ₁ transformation start temperature than the partition By operating a water cooling device, the steel strip is cooled with water at a cooling rate of 20 ° C./second or more and 60 ° C./second or less over substantially the entire length direction,
Above air cooling the steel strip by stopping the water cooling apparatus in the longitudinal direction of substantially the entire area located in the vicinity of the thermometer indicating the temperature range from below the first cooling stop temperature to the A ₁ transformation start temperature exceeds, and characterized in that to complete the pearlitic transformation further the water cooled in the longitudinal direction of the substantially entire region of the steel strip by operating the water-cooling unit located in the vicinity of the thermometer indicating the temperature range of the a ₁ transformation starting temperature below A method for producing a hot-rolled steel strip made of high-carbon steel or high-carbon alloy steel.

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