JPH02213413A - Production of high strength hot-rolled steel strip having excellent ductility and production line thereof - Google Patents

Abstract

PURPOSE:To obtain a high strength hot-rolled steel strip having good ductility by hot-rolling the specific composition of steel under the specific condition and cooling this while further recoiling after cooling and coiling. CONSTITUTION:The steel having compsn. contg. 0.20-0.60wt.% C, 0.5-2.0% Si, 0.5-2.0% Mn, <=0.015% P, <=0.015% S and the balance of Fe with inevitable impurities is hot-rolled at <850 deg.C and >=700 deg.C finishing temp. Successively, this is cooled at >=10 deg.C/sec the average cooling velocity and coiled at <450 deg.C and >=350 deg.C. During 5-120min after coiling, the high temp. coil 1 is fitted to a pay-off reel 2 and recoiled and cooled at <=300 deg.C with a cooling liquid injected from spray nozzles 4a, 5a. Shape of the cooled strip is adjusted with rolls 6 for skinpass rolling and the surface temp. is detected with a surface temp. detector 7. A flow rate adjusting valve 14 is controlled so as to come to the set temp. to control the flow rate of the cooling liquid injected from upper part and lower part cooling devices 4, 5.

Description

Detailed description of the invention (a) Industrial application field The present invention relates to a method for manufacturing a high-strength hot-rolled steel sheet having a tensile strength of 80 kgf/- or more and extremely excellent ductility, and a manufacturing line thereof. It is.

(b) Conventional technology Since the oil shock, great efforts have been made in industry to conserve energy and resources. For example, in the automobile industry, reducing fuel consumption has become a top priority, and therefore studies are being carried out from all perspectives to reduce vehicle weight. Regarding steel materials, in particular, there is a demand for higher 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. is stronger than ever.

Conventional solid-solution precipitation type high-strength steel sheets have difficulties in cold workability, represented by press formability, due to insufficient ductility.As an alternative, DP, which has a two-phase structure of ferrite and martensite, has been developed. Steel ((lual Phas)
e steel) or a composite structure type high-strength steel sheet having these structures as the main structure has been developed. These high-strength steel sheets are manufactured by continuous annealing in cold-rolled steel sheets, which makes it relatively easy to control the structure, whereas when hot-rolled steel sheets are manufactured without heat treatment, it is extremely difficult to control the structure. Various manufacturing methods have been proposed that combine hot rolling conditions.For example, if classified by constituent structure, l) ferrite and martensite structures (Japanese Patent Publication No. 11291/1982), 2) ferrite, martensite, and bainite structures. (Japanese Patent Publication No. 62-59166) 3) Ferrite, martensite, retained austenite structure (Japanese Patent Publication No. 62-59166)
(Japanese Patent Publication No. 1-15128) 4) A method for producing a steel sheet with a composite structure such as ferrite, martensite, bainite, or retained austenite structure (Japanese Patent Publication No. 62-35453) is disclosed. but,
Most of these are intended for strength (TS) of less than 80 kgf/mj, and even those of 80 kgf/- or more have a ductility (E7) of about 20% and a TSXfEZ of less than 2000. It is said that this is insufficient to meet the demands for increased ductility.

On the other hand, as a method for manufacturing a highly workable, high-strength hot rolled steel sheet having a strength of 80 kgf/- or more, a manufacturing method using retained austenite as the main structure has been disclosed. For example, in the method described in JP-A No. 60-43425, the finishing temperature of hot rolling is Ar, ~Ar*+50°C, and the temperature after finishing hot rolling is 450 ~
Hold in the temperature range of 650℃ for 4-20 seconds, then 350℃
It is coiled at a temperature below C, and contains 10% or more of ferrite, 10% or more of austenite, and the balance is bainite and martensite.The strength is about 100 kgf/- and the ductility is about 20%, but the TSXEl is less than 2400. can be,
Even with this, the ductility cannot be said to be sufficient, and although there is no particular mention of the treatment of the wound hot strip coil (hereinafter referred to as coil), the usual method is used, i.e., it is left to cool while being wound into a coil. In order to obtain the desired structure, the winding temperature must be as low as 200° C. or lower, as is clear from the examples. In this way, when actually manufacturing a high-strength hot rolled steel sheet, leaving it to cool while being wound into a coil poses a major manufacturing problem, as will be described later.

On the other hand, in JP-A No. 60-184634, the hot rolling finishing temperature is Ar, the transformation point or higher, and the cooling rate is 20°C/sec or more to pass through the Ar + transformation point, and the temperature is 330 to 430°C.
A method of performing slow cooling or heat retention so that the temperature range is kept in this temperature range for 5 minutes or more from the start of winding, and in addition, in the latter stage of hot rolling, Ar, +50~^r, +100
A method is disclosed in which a total reduction of 50% or more is applied within -t-1 seconds in a temperature range of "C".The structure produced by this method consists of retained austenite ferrite and bainite, and has a high strength.

Ductility is generally good. The retained austenite here is said to be "stable austenite", which is different from the structure obtained by the method described above, which is metastable austenite. Furthermore, the coiling temperature is higher than in the above-mentioned method, and by keeping the coiling temperature for as long as possible, it is possible to convert untransformed austenite into extremely stable austenite. Specifically, the main part wound up with a coiler is gradually cooled, similar to when it is kept warm for a long time, so it is possible to provide the desired thermal history without the use of special equipment.

However, this method is also understood to be a normal method after winding, that is, the coil is left to cool as it is, and in actual manufacturing, the same problems as the above-mentioned method occur.

In conventional methods such as these methods, in which the coil is left to cool while being wound, there is a difference in the cooling state between the outer part and the inside of the coil after it has been wound, which impairs the uniformity of the structure. This has an effect on the quality of the material and has become a factor that significantly impairs yield and cost. In particular, in the case of high-strength hot-rolled steel sheets with a composite structure in which strength and ductility are controlled by the constituent structure, it is actually extremely difficult to obtain a uniform desired material quality throughout the coil.

(c) Problems to be Solved by the Invention As mentioned above, it is said that the ductility is still insufficient in the case where the strength is 80 kgf/- or more. However, non-uniformity in structure and material due to temperature distribution within the coil after winding is unavoidable. In particular, in composite steel sheets whose main structure is retained austenite, which is obtained by concentrating C into untransformed austenite using bainite transformation, as mentioned above, this has a significant impact not only on the structure and material quality but also on yield, cost, etc. This poses a problem that is extremely disadvantageous economically.

Therefore, in view of the above-mentioned circumstances, the present invention provides a
The invention has been developed after repeated studies to improve the ductility of the above-mentioned high-strength hot-rolled steel sheets, and it solves the problems of the above-mentioned invention, as well as a method for manufacturing high-strength hot-rolled steel sheets having extremely good ductility, and a manufacturing line thereof. This is what we propose.

(d) Means for Solving the Problems The present invention is directed to the composition of steel, the hot rolling conditions in a continuous hot rolling mill, especially the finishing temperature of hot rolling, the cooling conditions from finish hot rolling to coiling, and the coiling temperature. We have developed a method and production line for manufacturing high-strength hot-rolled steel sheets with excellent ductility by organically combining the cooling conditions for the hot strip coil after winding, the equipment and production line that cools the coil while unwinding it. The main points of what we intend to provide are as follows.

(1) In weight% C: 0.20-0.60%.

Si: 0.5-2.0%.

Mn: 0.5-2.0%.

P: ≦0.015%.

S: ≦0.015%.

In the process of manufacturing a hot strip coil using a continuous hot rolling mill from steel consisting of the balance Fe and unavoidable impurity elements, the hot finishing temperature is lower than 850 °C and 700 °C or higher, and the average cooling rate is 10 °C/sec or higher. Cooling, winding into a coil at a winding temperature of less than 450°C and 350°C or more, and then cooling the wound coil while unwinding it to 300″C or less within 5 minutes or more and within 120 minutes from the start of winding. A method for producing a high-strength hot-rolled steel sheet with excellent ductility.

(2) Equipment that cools the hot strip coil, which has been wound at a winding temperature of less than 450°C and above 350°C, while unwinding it to 300°C or less within 5 minutes and 120 minutes from the start of winding according to the method described in 1 above. A control is provided between a payoff reel that unwinds the hot strip coil and a tensile reel that winds the hot strip coil under tension, allowing the flow rate of a cooling medium to be adjusted in order to cool the hot strip coil to a set temperature. Production of a high-strength hot-rolled steel sheet with excellent ductility characterized by a combination of a cooling facility equipped with a cooling device and a temper rolling mill that adjusts the shape with light rolling at the downstream stage of the cooling facility. line.

(3) A combination configuration is provided in which a leveler equipment for adjusting the shape of the hot strip coil and/or a finishing line of shearing equipment for cutting plates or strips are arranged after the cooling equipment described in the preceding paragraph 2. A production line for high-strength hot-rolled steel sheets with excellent ductility.

(e) Function The details of the present invention will be described. First, the reasons for limiting the class of components targeted by the present invention are as follows.

C is an essential element for ensuring strength and retained austenite. The present invention focuses on the transformation-induced plasticity (Transformation Indus) caused by retained austenite.
In order to increase the ductility by utilizing ced plasticity (TRIP), it is necessary to increase the amount of retained austenite, and for this purpose, a minimum of 0.20% of C is required. On the other hand, when C exceeds 0.60%, the effect on strength and ductility is saturated or reduced. Therefore, C is 0
．． The content was set at 20% to 0.60%.

St improves strength and suppresses carbide precipitation,
It is an important element for stabilizing austenite and increasing retained austenite by promoting C concentration in austenite. In order to ensure retained austenite, at least 0.5% of St is required. When the amount of St added is increased, the amount of retained austenite is increased through the C concentration effect, but even if it is added in excess of 2.0%, the effect is saturated. Therefore, St? , tO, 5 to 2.0%.

Mn is an austenite stabilizing element, and C15t
Similarly, it is an element that improves strength. In addition, FIn has the effect of delaying bainite transformation. The present invention attempts to secure retained austenite by utilizing bainite transformation, and it is closely related not only to the ingredients but also to the cooling conditions after hot rolling. In particular, retained austenite is greatly influenced by the cooling conditions after being wound into a coil. Therefore, Mn must be determined from the strength and amount of retained austenite, taking these cooling conditions into consideration, but it must be at least 0.
5% is necessary, and even if it is added in excess of 2.0%, the effect will be saturated and no further effect can be expected. Therefore,
Mn was set at 0.5 to 2.0%.

P is an element that reduces toughness. In particular, since the toughness of high C1 and high St content steel tends to deteriorate, it is necessary to keep it as low as possible, but in consideration of the cost for dephosphorization, P is set to 0.015% or less.

S is an element that forms MnS, increases the anisotropy of the steel sheet, and reduces toughness and press workability. Especially when the strength is high, the influence of deterioration of toughness and press workability tends to be strong, so like P, it is necessary to keep it as low as possible, but considering the cost of desulfurization, S should be set at 0. 015
% or less.

The reasons for limiting the above components are closely related to the hot rolling conditions and the cooling conditions after coiling, which will be detailed below.

First, the steel adjusted to the above composition is hot-rolled using a normal method, and the finish rolling temperature is 700℃ or higher, 85''C.
less than Lowering the hot rolling finishing temperature is extremely effective in increasing strength and toughness through grain refinement, but the effect is hidden below 700°C due to the subsequent cooling treatment, so the lower limit is On the other hand, if the hot rolling finishing temperature is 850°C or higher, the austenite grains will become too large, which will not only be disadvantageous in securing retained austenite but also deteriorate the toughness, so the upper limit is set at 850°C. less than

Subsequently, cooling was performed at an average cooling rate of 10°C/sec or more, and 4
Wind up at a temperature below 50°C and above 350°C. In a normal continuous hot rolling mill, immediately after exiting the final stand of the finishing rolling mill, it is air cooled, then water cooled on a hot run table, and then air cooled and wound as a coil. Temperature measurements are taken at Average cooling rate means the cooling rate between these temperature measurements.

This average cooling rate needs to be determined in relation to the components, but the precipitation of ferrite during cooling promotes the concentration of C in untransformed austenite, which is effective in stabilizing austenite, and pearlite precipitates. It is necessary to slow down as much as possible. Therefore, the average cooling rate is 10℃/s
ee or higher.

When the average cooling rate is increased, the temperature at the ends of the sheet width becomes significantly lower than that at the center of the sheet width, which tends to cause non-uniformity of the material.

Therefore, no upper limit is set for the average cooling rate, but 10
It is desirable that the temperature be 0° C./sec or less.

Further, when the winding temperature is 450° C. or higher, the coiled steel undergoes a rapid start of recurrent bainite transformation, and furthermore, pearlite tends to form, making it difficult to secure retained austenite. Therefore, the upper limit was set to less than 450°C. On the other hand, the winding temperature is 350
If the temperature is less than 350°C, hardening due to bainite or martensite and increase in strength will be significant and ductility will be impaired, so the lower limit is set to 350°C or higher. Note that this winding temperature range is closely related to the cooling conditions of the coil after winding, as will be described later.

In the conventional technology, structure control is mainly performed by cooling between finish hot rolling and coil winding, and the cooling and temperature history has a complicated pattern. Furthermore, after winding, the coil is allowed to cool as it is, so it is slowly cooled, pearlite transformation or bainite transformation is completed, and residual austenite is hardly recognized. Further, in order to avoid the completion of these transformations, the winding temperature must be extremely low, and as a result, martensite is likely to be formed, and in this case, almost no residual austenite remains. With the conventional method of leaving the coil to cool after winding it, it is extremely difficult to ensure retained austenite. Therefore, the inventors of the present invention have conducted various studies in order to solve this problem.

Figure 1 shows that the main components are 0.35%C11, 50%Si,
Retained austenite (T, ) and tensile strength (TS) when a 1.35%Mn hot rolled steel plate with a thickness of 2mm0 is subjected to two or three stages of heat treatment to correspond to the coiling temperature.
and changes in total elongation (k). A salt bath was used for heat treatment at 850 °C as shown in Figure 2.
After holding for 2 minutes at
After holding for 1 minute, it was immediately transferred from 350°C to a 250°C bath and held for up to 180 minutes, during which it was air cooled to room temperature for various hold times.

In the diagram in Figure 1, the O mark is when the temperature is kept at 400 °C, the ・ mark is when the temperature is changed to 350 °C after being held at 400 °C for 15 minutes, the Δ mark is the same 300 °C, and the Δ mark is 250 "C".
However, as is clear from the figure, d is T
8. However, TS does not change significantly. In particular, J tends to decrease when the retention time exceeds 6 minutes, but when the temperature is maintained for 15 minutes and then immediately transferred to a lower temperature bath, the lower the temperature, the less the change occurs, and it is maintained at 400℃ for a long time. high J for things
is maintained. In other words, 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, thus solving the conventional problems. It turned out that it would be done.

It was.

The present invention has been made based on such knowledge, and a coil wound under the above conditions is unwound and cooled to 300° C. or less within 5 minutes to 120 minutes from the start of winding.

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, untransformed austenite will not be sufficiently stabilized and will transform into martensite, increasing its strength, but residual austenite will and low ductility. Furthermore, this time must be determined by taking into consideration the arrangement and capacity of the equipment, and taking these factors into account,
The lower limit of the time to start cooling was set 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 will proceed further, residual austenite will decrease, and sufficient ductility will not be obtained. did.

Furthermore, the cooling temperature is below 300"C, preferably 250"C.
If the temperature is below 0.degree. C., it is possible to prevent the residual austenite generated by the retention effect immediately after winding from decreasing, and high ductility can be obtained. Therefore, the upper limit was set to 300°C or less.

In a typical continuous hot rolling mill, the coil is taken out from the coiler after being wound, transported by a conveyor, ram lift truck, crane, etc., and left to cool as a coil on a cooling bed for several tens of hours. Therefore, such a conventional method cannot satisfy the manufacturing conditions of the present invention, especially the cooling conditions after winding, and during this time the transformation is completed and almost no residual austenite remains.

In contrast to this natural cooling method, in which the coil is left to cool while being wound, there are cooling media such as forced air cooling, water spray cooling, and underwater immersion cooling that shorten the cooling time of the coil, but these are not already known. ing. These methods aim to reduce construction costs, reduce the complexity of production management, and improve productivity, since natural cooling takes a long time to cool and therefore requires a large cooling bed. . However, in both methods, the coil is cooled, and although the cooling time is shorter than the natural cooling method, it takes several hours or more, and the temperature difference between the inside and outside of the coil tends to increase.

Furthermore, in Japanese Patent Application Laid-Open No. 52-54603, the coil that has been wound and remains in a high temperature state is attached to an unwinding machine, and then the coil is unwound while applying a cooling liquid to the outer surface of the coil. A method of cooling the plate by applying a cooling liquid to the upper and lower surfaces of the plate on a table is disclosed.

This method aims to shorten the cooling time and reduce the size of the cooling bed or coil dockyard, and has the same purpose as the above-mentioned methods such as underwater immersion cooling. In addition, since this method is aimed at simply cooling down quickly, cooling water is injected and cooling starts from the unrolled state before unwinding, and the ultimate cooling temperature is 60 degrees Celsius or less. In addition, the cooling temperature and cooling time are not particularly controlled.Therefore, none of these known methods actively controls the structure and material, and these methods cannot be used in the production method of the present invention. In particular, it cannot be applied to a cooling method after winding.

Therefore, in order to satisfy the manufacturing conditions of the present invention, it is necessary to provide unwinding cooling equipment that can start cooling to 300° C. or less within 5 to 120 minutes after winding in a continuous hot rolling mill. Coiling temperature less than 450"C in continuous hot rolling mill, 3
The unwinding cooling equipment of the present invention starts cooling a coil wound at 50° C. or higher to 300° C. or lower within 5 minutes or more and 120 minutes from the start of winding. The hot strip coil is cooled to a set temperature between the lines of the tension reel for winding. (The cooling equipment is equipped with a control device that can adjust the flow rate of the cooling medium.)

The cooling conditions for this unwinding cooling equipment should be determined from the viewpoint of controlling the material quality as described above, but there are restrictions from the manufacturing equipment, and the time from the start of winding to the start of cooling is particularly constrained. . Normally, depending on the coil unit weight and hot rolling speed, it takes about 1 to 3 minutes from the start of winding until the coil is removed from the winding device and can be carried out. Therefore, when installing cooling equipment in conjunction with the winding device, the minimum time from the start of winding to the start of cooling must be set in consideration of handling time such as carrying out the removed coil and installing it in the cooling equipment. was set to 5 minutes.

However, when cooling control is performed independently using this cooling equipment, the number of steps increases, which is not preferable in terms of manufacturing cost. Therefore, the present invention proposes a combination with a finishing line such as a temper rolling or a leveler, a slitter, or a shear, which is a normal downstream process, that is, a
) A temper rolling mill that adjusts the shape under light rolling, or 2) A finishing line that includes leveler equipment that adjusts the coil shape and/or shearing equipment that cuts plates or strips. By creating a production line that does this, problems related to production costs can be solved.

In addition, these temper rolling mills or finishing lines are generally assumed to pass the sheet at room temperature, and when applicable to the present invention, the winding temperature is as high as less than 450°C and 350°C or more. It is necessary to take heat and fire prevention measures for bearings, rubber rolls, etc.

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

FIG. 3 is an explanatory diagram of a production line in which an unwinding cooling facility and a temper rolling mill are combined.

In this drawing, 1 is a high-temperature coil after being wound in a continuous hot rolling mill, 2 is a payoff reel, 3 is an unrolled strip, 4 is an upper cooling device, and 5 is a lower cooling device. The device consists of one or several rows of headers 4b, 5b equipped with a large number of spray nozzles 4al''5a, which spray a cooling liquid such as water or emulsion onto the unwound strip. 7 is a strip surface temperature detector.

9 is a coil of cooled strip. 8 is a tension reel, 10 is a cooling liquid pipe, 11 is a flow meter built into the pipe, and 12 is a temperature setting device, which sets the target temperature of the strip and measures the deviation between the set temperature and the temperature measured by the surface temperature detector. 13 is a flow rate setting device for controlling the flow rate to an optimum flow rate based on a signal from a temperature setting device, and 14 is a flow rate regulating valve. In addition, various bearings, rolls, and piping were wrapped with heat insulating material as necessary to provide heat and fire protection.

The unwinding cooling equipment configured as described above is installed near the winding device of the continuous hot rolling mill. In reality, the equipment closest to the winding device was modified. The high-temperature coil of 350 to 450"C removed from the winder is immediately transported by a conveyor and ram lift truck, and installed on the payoff reel 2 of the cooling equipment.The installed high-temperature coil 1 is unwound. Headers 4b and 5b of the upper cooling device 4 and the lower cooling device 5
The cooling liquid is sprayed from the spray nozzles 4a and 5a via the cooling liquid to a temperature below 300°C. At this time, it is important to make the temperature distribution uniform in the width direction of the strip because the ends of the strip tend to cool and become stretched in the middle. In addition, it is desirable that this upper and lower cooling device is capable of cooling not only with a cooling liquid such as water or an emulsion, but also with high-pressure air or mist. 7 detects the strip surface temperature, and the temperature setting device 12 and flow rate setting device 13 control the flow rate regulating valve 14 so that the temperature reaches the set temperature.
The flow rate of the cooling liquid injected from the lower cooling device 5 is controlled to maintain the temperature of the strip at the set temperature.

(f) Examples Next, the effects of the present invention will be explained in more detail using examples.

Steel with the components shown in Table 1 is made into a slab by the usual method, heated, rough rolled, and then continuously hot rolled to a thickness of 2, Otrim.
Finished. The main hot rolling conditions such as the finishing temperature and coiling temperature at the exit side of the continuous hot rolling machine, the average cooling rate during that period, and the unwinding conditions after coiling are shown in Table 2. Here, the coil after winding is quickly transported by a conveyor and ram lift truck, and the coil is unwound and cooled.
As shown in FIG. 3, cooling was performed by water spray using a cooling equipment combined with a supplementary rolling mill. Further, in order to adjust the shape, light reduction was appropriately applied and skin pass rolling was performed.

For comparison, a conventional method of leaving the coil to cool after winding was also conducted.

Further, Table 2 shows these material properties. Retained austenite was measured using X-ray diffraction method, and tensile test was conducted using JI
Tensile strength and total elongation were determined using a S13 B test piece. In addition, the value of TSXIIJ is also shown in the same table.

As is clear from Table 2, NQ of the present invention, 1 to 7
The material has a tensile strength of about 80 kgf/- or more, a total elongation of about 20% or more, and a TSXFJ of 2500 or more, indicating an extremely good material.

On the other hand, Comparative Examples No. 8 and No. 11 were made by the usual method of cooling after winding, and the TS was 90k.
gf/- or more, but the retained austenite is extremely small, and the g is less than 20%. Comparative examples Nα9 and Nα10 have coiling temperatures outside the range of the present invention, and N1112
and No. 13 have components outside the scope of the present invention.

These comparative examples all have a low TSXIJ of 2000 or less.

From these examples, according to the present invention, the composition, hot rolling conditions, and coil cooling conditions after coiling are closely related and work effectively, and a high strength hot rolled steel sheet with extremely excellent ductility can be produced. It is clear that it can be manufactured.

(G) Effect of the invention As explained above, according to the present invention, the tensile strength is 80 kgf.
It is possible to produce a hot-rolled steel sheet having a strength of /- or more and extremely excellent ductility. In particular, by unwinding and cooling the coil, the material of the coil becomes uniform, which is extremely advantageous in terms of yield and cost, compared to the conventional method in which the coil is left to cool while being wound. Therefore, the present invention is industrially extremely effective.

[Brief explanation of the drawing]

Figure 1 shows primary soaking temperature 850℃-2m1n, secondary soaking temperature 4
Secondary soaking time and residual austenie when changing the soaking temperature from 00℃ and midway)! (γ,), tensile strength (T
This shows the relationship between S) and total elongation (E).
If is kept at 400℃, ・ is 400℃-1
When the temperature was changed to 350°C after holding 5ml, Δ was changed to the same <300″C, and M was changed to 250°C. FIG. 2 shows the heat treatment cycle of FIG. 1. Fig. 3 is an explanatory diagram showing an embodiment of a production line combining cooling equipment and a temper rolling mill. 1: High-temperature coil, 2: Payoff reel, 3-ni strip, 4: Upper cooling device, 4a varnish spray nozzle, 4b:
Header, 5: Lower cooling device, 5a varnish spray nozzle,
5b: header, 6:! J quality rolling roll, 7: Surface temperature detector, 8: Tension reel, 9: Cooled coil, 10: Coolant piping, 11: Flow meter, 12: Temperature setting device, 13: Flow rate setting device, 14: Flow rate adjustment valve. Gentle ℃ x 2 air cooling Figure 3

Claims (3)

[Claims] (1) C: 0.20-0.60%, Si: 0.5-2.0%, Mn: 0.5-2.0%, P:≦0.015%, S:≦ (1) Weight% In the process of manufacturing hot strip coils using a continuous hot rolling mill from steel consisting of 0.015% Fe and unavoidable impurity elements, the hot finishing temperature is lower than 850°C and higher than 700°C, followed by an average cooling rate of 10°C. Cooling at ℃/sec or more, winding into a coil at a winding temperature of less than 450℃ and 350℃ or more, and then rewinding the coil to 300℃ or less within 5 minutes or more and 120 minutes from the start of winding. A method for producing a high-strength hot-rolled steel sheet with excellent ductility, characterized by cooling the steel sheet while cooling. (2) According to the method according to claim 1, a hot strip coil wound at a winding temperature of less than 450°C and more than 350°C is cooled while being unwound to 300°C or less within 5 minutes or more and within 120 minutes from the start of winding. A control that allows the equipment to adjust the flow rate of a cooling medium between a payoff reel for unwinding the hot strip coil and a tension reel for applying tension and winding the hot strip coil in order to cool the hot strip coil to a set temperature. Production of a high-strength hot-rolled steel sheet with excellent ductility characterized by a combination of a cooling facility equipped with a cooling device and a temper rolling mill that adjusts the shape with light rolling at the downstream stage of the cooling facility. line. (3) A combination configuration in which a leveler equipment for adjusting the shape of the hot strip coil and/or a finishing line for cutting plates or shearing equipment for cutting the hot strip coil is arranged downstream of the cooling equipment according to claim 2. A production line for high-strength hot-rolled steel sheets with excellent ductility.