JPH0678570B2 - Method for producing high-strength hot-rolled steel sheet excellent in ductility and production line thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high-strength hot-rolled steel sheet having a high tensile strength of 80 kgf / mm ² or more and excellent in ductility, and a production line thereof. It is a thing.

(B) Conventional technology Since the oil shock, the industry has made great efforts to save energy and resources. For example, in the automobile industry, reduction of fuel consumption is a top priority issue, and therefore reduction of vehicle body weight is being studied from all viewpoints. For steel materials, in particular, to strengthen the strength of hot-rolled steel sheets and cold-rolled steel sheets in order to reduce the thickness of the car body without changing the design strength by increasing the strength and to reduce the weight of the car body, Is stronger than before.

Since conventional solid solution precipitation type high strength steel sheets have a difficulty in cold workability represented by press formability due to insufficient ductility, a DP consisting of a two-phase structure of ferrite and martensite is used as an alternative. Steel (Dual Phase Steel) or a composite microstructure type high strength steel sheet having these structures as main structures has been developed. Since these high-strength steel sheets are manufactured by continuous annealing in cold-rolled steel sheets, the structure control is relatively easy to perform, whereas when the hot-rolled steel sheet is manufactured in a non-tempered state, it is extremely difficult to control the structure, and therefore the components are Various manufacturing methods have been proposed that combine the above and hot rolling conditions. For example, 1) ferrite and martensite structures (Japanese Patent Publication No. 61-
11291) 2) Ferrite and martensite, bainite structure (JP-B-62-59166) 3) Ferrite and martensite, retained austenite structure (JP-B-61-15128) 4) Ferrite and martensite, bainite, A method for producing a steel sheet having a composite structure such as a retained austenite structure (Japanese Patent Publication No. 62-35453) is disclosed. But,
Most of these are intended for strength (TS) less than 80 kgf / mm ² , and ductility (El) of about 80 kgf / mm ² or more is about 20% and TS × El is less than 2000. It is said to be insufficient for the demand for high strength and high ductility.

On the other hand, as a method for producing a high workability and high strength hot rolled steel sheet having a strength of 80 kgf / mm ² or more, a production method using retained austenite as a main structure is disclosed. For example, JP-A-60
The method of JP -43425 is hot-rolled finishing temperature Ar ₃ to Ar ₃
+50 ℃, 4 ~ in the temperature range of 450 ~ 650 ℃ after hot rolling
Hold for 20 seconds, then wind at a temperature of 350 ℃ or less, ferrite 10% or more, austenite 10% or more, balance bainite, martensite, strength is about 100 kgf / mm ²
The ductility is about 20%, but TS × El is less than 2400, and it is hard to say that the ductility is sufficient. Moreover, although no particular reference is made to the treatment of the wound hot strip coil (hereinafter referred to as coil), it is understood that it is a normal method, that is, it is allowed to cool while being wound on the coil. In order to obtain
There is no choice but to wind at temperatures as low as 200 ° C or lower. As described above, when the high-strength hot-rolled steel sheet is actually manufactured, allowing it to cool while being wound on the coil is a serious problem in manufacturing as described later.

On the other hand, in JP-A-60-184634, the hot rolling finishing temperature is set to Ar.
₃ transformation point or more, pass the Ar ₁ transformation point at a cooling rate of 20 ° C / sec or more, wind at 330 to 430 ° C, and gradually hold it for 5 minutes or more from the start of winding in this temperature range. Cold or heat retention method, as well as Ar ₁
It discloses a method of applying a total reduction of 50% or more within 1 second in a temperature range of +50 to Ar ₃ + 100 ° C. The structure obtained by this method consists of retained austenite, ferrite and bainite, and its strength and ductility are generally good. The retained austenite here is said to be "stable austenite", which is different from the structure obtained by the above-mentioned method, which is metastable austenite. Further, it is said that the winding temperature is higher than that of the above-mentioned method, and by maintaining the winding temperature as long as possible, untransformed austenite can be converted into an extremely stable one. As a concrete method, since the main part wound by the coiler is gradually cooled as if it was kept warm for a long time, it is possible to give a desired heat history without using special equipment.
However, this method is also understood to be a normal method after coiling, that is, the coil is allowed to cool as it is, and in the actual manufacturing, the same problem as the above-mentioned method occurs.

Like the above methods, in the normal method in which the coil is allowed to cool while being wound, the coil after being wound has a difference in the cooling state between the outside and inside of the coil, which impedes the uniformity of the tissue. As a result, the material quality is also affected, which is a factor that significantly impairs yield and cost. In particular, in the case of a composite structure type high-strength hot-rolled steel sheet in which the strength and ductility are controlled by the structure, it is extremely difficult to obtain a desired material evenly in the entire coil.

(C) Problems to be Solved by the Invention As described above, it is said that the ductility is not sufficient when the strength is 80 kgf / mm ² or more, and the method of allowing the coil to cool while being wound. As a result, the nonuniformity of the structure and material due to the temperature distribution inside the coil after winding is unavoidable. Particularly, in the case of a composite structure steel sheet having retained austenite as a main structure, which is secured by concentrating C into untransformed austenite by utilizing bainite transformation,
As described above, there is a problem that not only the structure and the material but also the yield, the cost, etc. are greatly affected, which is extremely economically disadvantageous.

Therefore, the present invention has been made in consideration of the above-mentioned circumstances and was repeatedly devised to improve the ductility of high-strength hot-rolled steel sheets of 80 kgf / mm ² or more, and solves the problems of the above-mentioned invention, and is extremely good. It proposes a method for producing a high-strength hot-rolled steel sheet having excellent ductility and a production line thereof.

(D) Means for Solving the Problems The present invention is directed to the components of steel and hot rolling conditions in a continuous hot rolling machine, particularly hot rolling finishing temperature, cooling conditions from finishing hot rolling to winding, winding temperature, and further. A method and a production line for producing a high-strength hot-rolled steel sheet excellent in ductility by organically combining the cooling conditions of the hot strip coil after winding and the equipment and the production line for cooling while winding the coil. It is intended to be provided, and the main points are as follows.

(1) C: 0.20 to 0.60%, Si: 0.5 to 2.0%, Mn: 0.5 to 2.0%, P: ≤ 0.015%, S: ≤ 0.015% by weight% From steel consisting of balance Fe and unavoidable impurity elements , In the process of manufacturing hot strip coil by continuous hot rolling machine, the hot finishing temperature is less than 850 ℃, 700 ℃ or more, followed by cooling at an average cooling rate of 10 ℃ / sec or more, less than 450 ℃, 3
Wind the coil at a coiling temperature of 50 ° C or higher, and then wind the coil up to 300 ° C within 5 minutes or more and 120 minutes from the start of winding.
A method for producing a high-strength hot-rolled steel sheet having excellent ductility, which comprises cooling while rewinding.

(2) Equipment for cooling a hot strip coil wound at a coiling temperature of less than 450 ° C and 350 ° C or more in accordance with the method described in 1 above, while rewinding it to 300 ° C or less within 5 minutes or more and 120 minutes after the start of winding. A control device capable of adjusting the flow rate of a cooling medium between the payoff reel for rewinding the hot strip coil and the tension reel line for tensioning and winding the hot strip coil to cool the hot strip coil to a set temperature. A production line for a high-strength hot-rolled steel sheet with excellent ductility, characterized in that a cooling facility equipped with the above and a temper rolling mill whose shape is adjusted under a light pressure are provided in the subsequent stage of the cooling facility in combination. .

(3) A combination of a leveling device for adjusting the shape of the hot strip coil and / or a trimming line for a cutting plate or a shearing device for cutting is provided in the subsequent stage of the cooling device described in the above item 2 for combination. A production line for high-strength hot-rolled steel sheets with excellent ductility.

(E) Operation Details of the present invention will be described. First, the reasons for limiting the components of steel targeted by the present invention are as follows.

C is an essential element for securing strength and retained austenite. The present invention is directed to transformation-induced plasticity (TR) by retained austenite.
It is necessary to increase the amount of retained austenite in order to increase the ductility by utilizing (IP), but for this purpose, C must be at least 0.20%. On the other hand, when C exceeds 0.60%, the effect on strength and ductility is saturated or lowered. Therefore, C is 0.
It was set to 20 to 0.60%.

Si is an important element for improving strength, suppressing precipitation of carbides, and promoting C concentration in austenite to stabilize austenite and increase retained austenite. Si must be at least 0.5% to secure retained austenite. When the amount of Si added is increased, the amount of retained austenite is increased through the C concentration effect,
Even if added in excess of 2.0%, the effect is saturated.
Therefore, Si is set to 0.5 to 2.0%.

Mn is an austenite stabilizing element and is an element that improves strength like C and Si. Further, Mn has an effect of delaying bainite transformation. The present invention is to secure retained austenite by utilizing bainite transformation, and is closely related to not only the components but also the cooling conditions after hot rolling. It is greatly affected by cooling conditions. Therefore, Mn must be determined from the strength and the amount of retained austenite in consideration of these cooling conditions, but at least 0.5% is necessary, and even if added in excess of 2.0%, its effect saturates and Can not be expected to be effective. Therefore, Mn is set to 0.5 to 2.0%.

P is an element that reduces toughness. In particular, steel with high C and high Si content tends to deteriorate in toughness, so it is necessary to reduce it as much as possible. However, considering the cost for dephosphorization, P
Was 0.015% or less.

S is an element that forms MnS, increases the anisotropy of the steel sheet, and reduces the toughness and press workability. Especially when the strength is high, the effect of deterioration of toughness and press workability is likely to appear strongly, so it is necessary to make it as low as possible like P, but considering the cost for desulfurization, S is 0.015%. Below.

The above reasons for limiting the components are closely related to the hot rolling conditions and the cooling conditions after winding, which will be described in detail below.

First, the steel adjusted to the above composition is hot-rolled by a usual method, and the finish rolling temperature is 700 ° C or higher and lower than 850 ° C. Reducing the hot rolling finishing temperature is extremely effective for increasing the strength and toughness by fine graining, but since the effect is concealed below 700 ° C by the subsequent cold electrode treatment, the lower limit is set at 700 It was set to ℃ or higher. on the other hand,
When the hot rolling finishing temperature is 850 ° C or higher, the austenite grains become too large, which is not only disadvantageous in securing the retained austenite but also deteriorates the toughness, so the upper limit is 850 ° C.
Less than

Subsequently, it is cooled at an average cooling rate of 10 ° C / sec or more, and wound at less than 450 ° C and 350 ° C or more. In a normal continuous hot rolling machine, it is air-cooled immediately after it exits the final stand of the finishing rolling mill, then water-cooled on a hot run table, and subsequently air-cooled and wound as a coil. The temperature is measured at. By average cooling rate is meant the cooling rate between these temperature measurements. It is necessary to determine this average cooling rate in relation to the composition, but the precipitation of ferrite during cooling promotes the concentration of C in untransformed austenite, is effective in stabilizing austenite, and pearlite precipitates. It is necessary to slow down as much as possible. Therefore, the average cooling rate is set to 10 ° C / sec or more. When the average cooling rate is increased, the temperature at the end of the plate width becomes significantly lower than that at the center of the plate width, and the material is likely to be uneven. Therefore, although the upper limit of the average cooling rate is not set,
It is desirable to set it to 100 ° C / sec or less.

Further, when the coiling temperature is 450 ° C. or higher, after coiling, the bainite transformation starts quickly, pearlite is likely to occur, and it becomes difficult to secure retained austenite. Therefore, the upper limit is set to less than 450 ° C. On the other hand, when the winding temperature is lower than 350 ° C, hardening and strength increase due to bainite or martensite remarkably deteriorates ductility, so the lower limit is set to 350 ° C or higher. The winding temperature range is closely related to the cooling condition of the coil after winding, as will be described later.

In the prior art, the structure is controlled mainly by cooling from finish hot rolling to coil winding, and cooling and temperature history have a complicated pattern. Furthermore, after winding, the coil is left to cool as it is, so that it gradually cools, the pearlite transformation or bainite transformation ends, and residual austenite is hardly recognized. Also, in order to avoid the end of these transformations, the winding temperature must be extremely low, as a result, martensite is likely to occur,
Also in this case, almost no retained austenite remains. It is extremely difficult to secure the retained austenite by the conventional method of allowing the coil to cool as it is after being wound in this way. Therefore, the present inventors have made various studies to solve this problem.

Fig. 1 shows the main components of 0.35% C, 1.50% Si, 1.35% Mn hot rolled steel sheet with a thickness of 2mm.
It is a graph showing changes in retained austenite (γ _R ), tensile strength (TS) and total elongation (El) in the case of performing a three-stage or three-stage heat treatment. Use a salt bath for heat treatment, hold at 850 ° C for 2 minutes as shown in Fig. 2, and then hold at 400 ° C.
Bake in the bath for 1 minute to 180 minutes and hold for 400 minutes at 400 ℃, then immediately transfer to the bath at 350 ℃ to 250 ℃, hold for 180 minutes, and from various holding time to room temperature Air cooled. In the diagram of Fig. 1, ○ indicates that the temperature was kept at 400 ℃, and ● indicates that the temperature was kept at 400 ℃ for 15 minutes and then 350 ℃.
Change to 300 ° C., and ▲ to 250 ° C., as is clear from the figure, El changes markedly in response to changes in γ _R , but TS Has not changed significantly. In particular, El tends to decrease when the retention time exceeds 6 minutes, but when it is retained for 15 minutes and immediately transferred to a low temperature bath, the lower the temperature, the less change occurs, and it was retained at 400 ° C for a long time. On the other hand, high El is maintained. That is, it is clear that if the wound coil can be unwound and cooled to a temperature lower than the winding temperature, retained austenite can be stably and easily obtained, and high ductility can be obtained, and the conventional problems are solved. I found out that it would be done.

The present invention has been made on the basis of such findings, and a coil wound under the above conditions can be wound 5
Cool to 300 ° C or less while rewinding within 120 minutes or more.

The time from the start of winding to the start of cooling depends on the components and the winding temperature, but if it is less than 5 minutes, the untransformed austenite transforms to martensite due to insufficient stabilization, and the strength becomes high. Less and less ductile. Further, this time needs to be determined in consideration of the arrangement and ability of the apparatus, and in consideration of these factors, the lower limit of the time from the start of cooling after winding to 5 minutes or more. On the other hand, if the time from the start of winding to the start of cooling exceeds 120 minutes, bainite transformation proceeds further, retained austenite decreases, and sufficient ductility cannot be obtained, so the upper limit of the time to start cooling is 120 minutes. did.

Further, if the cooling temperature is 300 ° C. or lower, preferably 250 ° C. or lower, reduction of retained austenite generated due to the retention effect immediately after winding can be prevented, and high ductility can be obtained. Therefore, the upper limit is set to 300 ° C or lower.

In a normal continuous hot rolling machine, the coil is taken up from the coiler after being wound up, conveyed by a conveyor, a ram lift truck, a crane, etc., and left to cool in the cooling floor as it is for several tens of hours. Therefore, in such a usual method, the manufacturing conditions of the present invention, particularly the cooling conditions after winding, cannot be satisfied, the transformation is completed during this period, and the residual austenite hardly remains.

In contrast to the natural cooling method in which the coil is allowed to cool while being wound, there are cooling media such as forced air cooling, water spray cooling, and submersion cooling as methods for shortening the coil cooling time, but these are already known. ing. These methods are aimed at reducing the construction cost, improving the complexity of production control, and improving productivity because the natural cooling method requires a long time for cooling and thus requires a vast cooling floor. . However, in both cases, the cooling is performed in the state of the coil, and the cooling time is shorter than that of the natural cooling method, but it takes several hours or more, and the temperature difference between the inside and outside of the coil tends to be rather large. Furthermore, JP-A-52
-54603, the coil wound in a high temperature state is mounted on a rewinding machine, and then unwound while applying a cooling liquid to the outer surface of the coil, and further, on a plate on a subsequent table of the rewinding machine. A method of applying a cooling liquid to the lower surface to cool it is disclosed. This method aims at shortening the cooling time and reducing the cooling bed or the coil stock yard, and has the same purpose as the above-mentioned method such as immersion cooling in water.
In addition, this method is aimed at simply cooling quickly, so cooling water is injected and cooling is started from the state of being rolled before unwinding, and the ultimate cooling temperature is 60 ° C.
It is remarkably low as below, and the cooling temperature and cooling time are not particularly controlled. Therefore, none of these known methods positively control the tissue and material,
These methods cannot be applied to the manufacturing method of the present invention, particularly the cooling method after winding.

Therefore, in order to satisfy the manufacturing conditions of the present invention, it is necessary to arrange a rewind cooling facility capable of starting cooling to 300 ° C. or lower within 5 to 120 minutes after winding by a continuous hot rolling machine. A coil wound at a coiling temperature of less than 450 ° C and 350 ° C or more in a continuous hot rolling machine is 300 minutes within 5 minutes or more and 120 minutes after starting the winding.
The rewinding cooling equipment of the present invention that starts cooling to ℃ or less, in order to cool the hot strip coil to a set temperature between the line of the payoff reel for rewinding the coil and the line of the tension reel for applying tension and winding up. The cooling equipment is equipped with a control device that can adjust the flow rate of the cooling medium.

The cooling conditions by the rewind cooling facility should be determined from the viewpoint of controlling the material as described above, but are subject to restrictions from the manufacturing equipment, and particularly the time from the start of winding to the start of cooling is greatly restricted. . Usually, depending on the coil unit weight and the hot rolling speed, it takes about 1 to 3 minutes from the start of winding until the winding is completed and the coil can be removed from the winding device and carried out. Therefore, when arranging the cooling equipment in relation to the winding device, consider the handling time from unloading the coil to mounting it on the cooling equipment, and consider the lower limit of the time from the start of winding to the start of cooling. Was set to 5 minutes.

However, if the cooling control is independently performed by this cooling facility, one step is added, which is not preferable in terms of manufacturing cost. Therefore, the present invention is combined with a tempering rolling or a leveling line such as a leveler, a slitter, or a shear, which is a normal lower step, that is, in the latter stage of the cooling equipment,
1) Temper rolling mill that adjusts the shape under light pressure, or 2)
Problems in production cost due to the production line characterized by the combination of the leveling equipment for adjusting the shape of the coil and / or the adjusting line of the shearing equipment for cutting or cutting Is the solution.

Incidentally, these temper rolling mills or refining lines are generally premised on threading at room temperature, and when applied to the present invention, the coiling temperature is less than 450 ° C and is high as 350 ° C or higher. , Bearings, rubber rolls, etc. should be protected against heat and fire.

An embodiment of this rewinding cooling equipment and a production line in which this is combined with a temper rolling mill will be described in detail with reference to the drawings.

FIG. 3 is an explanatory view of a rewind cooling facility and a production line that is configured by combining it with a temper rolling mill. In this drawing, 1 is a high temperature coil after being wound by a continuous hot rolling machine, 2
Is a payoff reel, 3 is an unrolled strip, 4 is an upper cooling device, 5 is a lower cooling device, and these cooling devices are from one row or several rows of headers 4b, 5b having many spray nozzles 4a, 5a attached. A cooling liquid such as water or emulsion is sprayed onto the unwound strip, 6 is a pair of temper rolling rolls, 7 is a strip surface temperature detector, and 9 is a coil of the cooled strip. Is. 8 is a tension reel, 10 is a coolant pipe, 11 is a flow meter built into the pipe, 12 is a temperature setter, which sets the target temperature of the strip, and the deviation between the set temperature and the temperature from the surface temperature detector. , 13 is a flow rate setting device for controlling the optimum flow rate by the signal from the temperature setting device, and 14
Is a flow control valve. In addition, various bearings, rolls, and piping were wrapped with a heat insulating material as necessary to prevent heat and fire.

The rewinding cooling equipment configured as described above is installed near the winding device of the continuous hot rolling machine. Actually, the equipment near the winding device was modified. The high temperature coil of 350 to 450 ° C. removed from the winder is promptly conveyed by the conveyor and the ram lift truck, and mounted on the payoff reel 2 of the cooling equipment. The mounted high-temperature coil 1 is unwound, and is cooled to 300 ° C. by the cooling liquid jetted from the spray nozzles 4a and 5a through the headers 4b and 5b of the upper cooling device 4 and the lower cooling device 5.
Cooled below. At this time, the strip end gets cold,
It is important to make the temperature distribution uniform in the width direction of the strip because it tends to be in a medium stretch state. Further, it is desirable that the upper and lower cooling devices are capable of not only cooling with a cooling liquid such as water or emulsion but also high pressure air or mist cooling. The shape of the cooled strip is adjusted by the temper rolling roll 6, the strip surface temperature is detected by the surface temperature detector 7, and the flow rate adjusting valve 14 is controlled by the temperature setting device 12 and the flow rate setting device 13 so as to reach the set temperature. In this way, the flow rates of the cooling liquids injected from the upper cooling device 4 and the lower cooling device 5 are controlled, and the strip temperature is maintained at the set temperature.

(F) Examples Next, the effects of the present invention will be described more specifically with reference to Examples.

A steel having the components shown in Table 1 was formed into a slab by an ordinary method, heated, rough-rolled, and continuously hot-rolled to a plate thickness of 2.0 mm. Table 2 shows the main hot rolling conditions such as the finishing temperature and the winding temperature on the delivery side of the continuous hot rolling machine, the average cooling rate and the like, and the rewinding condition after winding the coil. Here, the coil after winding is promptly conveyed by a conveyor and a ram lift truck, and rewinding cooling is performed by water spray using cooling equipment combined with a temper rolling mill as shown in FIG. did. Further, for shape adjustment, light reduction was appropriately applied and temper rolling was performed. For comparison, a conventional method of cooling the coil as it is after being wound up was also performed.

Table 2 shows these material characteristics. Residual austenite was measured using the X-ray diffraction method, and the tensile test was JIS
Tensile strength and total elongation were determined using a No. 13B test piece. The value of TS × El is also shown in the same table.

As is clear from Table 2, Nos. 1 to 7 of the present invention have a tensile strength of about 80 kgf / mm ² or more and a total elongation of about 20% or more.
XEl is 2,500 or more, indicating a very good material.

On the other hand, Comparative Examples No. 8 and No. 11 are based on the normal method of cooling after being wound, and TS is 90 kgf / mm.
² or more, but very little retained austenite,
l is 20% or less. In Comparative Examples No. 9 and No. 10, the winding temperature was outside the range of the present invention, and in No. 12 and No. 13, the components were outside the range of the present invention. These comparative examples
TS x El is 2000 or less, which is low.

From these examples, according to the present invention, the components and hot rolling conditions, coil cooling conditions after winding are closely related, and act effectively, high strength hot rolled steel sheet having extremely excellent ductility. It is clear that it can be manufactured.

(G) Effect of the Invention As described above, according to the present invention, the tensile strength is 80 kgf / mm ²
It is possible to manufacture a hot-rolled steel sheet having the above strength and extremely excellent ductility. In particular, by rewinding and cooling the coil, the material of the coil becomes uniform, which is extremely advantageous in terms of yield and cost, as compared with the conventional method in which the coil is allowed to cool while being wound. Therefore, the present invention is extremely effective industrially.

[Brief description of drawings]

Figure 1 shows the primary soaking temperature of 850 ℃ -2min, the secondary soaking temperature of 400 ℃, and the secondary soaking time and the amount of retained austenite (γ _R ) and tensile strength (TS ) And total elongation (El). In the figure, ○ indicates that the temperature was kept at 400 ℃, ● indicates that the temperature was 400 ℃ for 15 min, and then 35
When the temperature is changed to 0 ° C, Δ is the same as 300 ° C, and ▲ is the same when changed to 250 ° C. FIG. 2 illustrates the heat treatment cycle of FIG. FIG. 3 is an explanatory view showing an embodiment of a cooling facility and a production line in which the temper rolling mill is combined therewith. 1: Hot coil, 2: Payoff reel, 3: Strip, 4:
Upper cooling device, 4a: Spray nozzle, 4b: Header, 5: Lower cooling device, 5a: Spray nozzle, 5b: Header, 6: Roll for temper rolling, 7: Surface temperature detector, 8: Tension reel, 9: Cooled coil, 10: Coolant pipe, 11: Flow meter,
12: Temperature setting device, 13: Flow setting device, 14: Flow control valve.

Claims (3)

[Claims] 1. By weight%, C: 0.20 to 0.60%, Si: 0.5 to 2.0%, Mn: 0.5 to 2.0%, P: ≤ 0.015%, S: ≤ 0.015%, balance Fe and unavoidable impurity elements In the process of producing hot strip coils from steel by a continuous hot rolling machine, the hot finishing temperature is set to less than 850 ° C and 700 ° C or more, followed by cooling at an average cooling rate of 10 ° C / sec or more and less than 450 ° C, 3
Wind the coil at a coiling temperature of 50 ° C or higher, and then wind the coil up to 300 ° C within 5 minutes or more and 120 minutes from the start of winding.
A method for producing a high-strength hot-rolled steel sheet having excellent ductility, which comprises cooling while rewinding. 2. A winding temperature of 450 according to the method of claim 1.
Equipment for cooling the hot strip coil wound below 350 ° C and below 350 ° C while rewinding it to 300 ° C or below within 5 minutes or more and 120 minutes after the start of winding, and a payoff reel for rewinding the hot strip coil and tension. Between the lines of the tension reel for winding the hot strip coil, and a cooling device equipped with a control device capable of adjusting the flow rate of the cooling medium for cooling the hot strip coil to a set temperature, and a lighter device at the latter stage of the cooling device. A production line for a high-strength hot-rolled steel sheet having excellent ductility, which is characterized in that a temper rolling mill whose shape is adjusted under rolling is arranged and combined. 3. A combination of a leveling equipment for adjusting the shape of a hot strip coil and / or a trimming line for a cutting board or a shearing equipment for slicing is provided in the subsequent stage of the cooling equipment according to claim 2. A high-strength hot-rolled steel sheet production line with excellent ductility.