JP3418738B2 - Hot rolling mill and method for producing fine-grained steel - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The claimed invention relates to a hot rolling mill capable of producing a steel sheet having a fine structure mainly composed of fine grain ferrite, and (a steel sheet for) such fine grain steel. It is about how to do it.

[0002]

2. Description of the Related Art The refinement of the structure of a steel material is well known as a means for enhancing the mechanical properties of the steel material.
If the mechanical properties of steel materials are improved, many advantages such as weight reduction of steel structures can be achieved. Many methods have been proposed so far for producing steel having a fine structure, that is, fine-grained steel, but as a typical one,
There are a large reduction rolling method and a controlled rolling method.

Of these, the large reduction rolling method is disclosed in JP-A-58-123823 and JP-B-5-65564.
It is described in the official gazette. That is, this method promotes the strain-induced transformation from the austenite (γ) phase to the ferrite (α) phase by applying a large reduction to the austenite grains, and thus the structure is refined. In the controlled rolling method of No. 3, not only can Nb (Niobium) or Ti (Titanium) be contained as a component to increase the tensile strength easily by the precipitation strengthening action of Nb and Ti, but also the recrystallization of austenite grains of Nb and Ti can be suppressed. This is a method in which the strain-induced transformation from the γ phase to the α phase is promoted when the low temperature rolling (ferrite region rolling) is performed by the action, and the ferrite grains can be made fine.

In the controlled rolling method (1), finish rolling is performed in a low temperature range (8
Since it is carried out at a temperature of not higher than 00 ° C.), the deformation resistance of the rolled material is extremely high, and therefore the load on the rolling mill is large. On the other hand, the large reduction rolling method (1) cannot be industrially carried out by a general hot strip mill as shown in Japanese Patent Publication No. 5-65564, and it is necessary to use a special rolling mill. was there. This is because, as described in each of the above-mentioned publications, it is necessary to continuously apply a high reduction rate (for example, 40% or more) that is difficult to realize with a general rolling mill.

[0005]

In order to industrially and commercially produce fine grained steel by carrying out the large reduction rolling method, in addition to the fact that a general hot strip mill type rolling mill cannot be used, There are the following issues.

(A) Since rolling is performed under a large reduction, that is, at a high reduction rate, inconvenience due to the rolling load tends to occur. In other words, the rolling load may reach the limit values (mill power limit and mechanical strength) peculiar to the rolling mill, making it impossible to carry out rolling, and also for the rolled material, the specified rolling reduction may not be realized or large edge Drops occur. The reason why the specified reduction ratio cannot be obtained is that the reduction ratio is 4 mm or less when the plate thickness on the delivery side of the rolling mill is 2 mm or less.
This is because, when the rolling load is large and the deformation resistance is high, the flatness of the roll becomes large in the case of 0% or more, and the rolling reduction cannot be improved no matter how much the rolling is performed to carry out the high pressure rolling. The reason why the edge drop becomes large is that a high load is applied near the edge (end in the width direction) of the rolled material, which does not provide a good plate profile.

(B) It is also a big problem that it is difficult to maintain the temperature of the rolled material. When rolling at a high pressure reduction rate using a plurality of stands, the temperature of the rolled material rises remarkably due to the above-mentioned heat generation of processing, and the temperature (Ar ₃ transformation point to Ar _{3 This} is because it is not easy to maintain the temperature within the range of ₃ + 50 ° C). When the rolled material is accelerated and the feed rate is increased, the strain rate is increased and the heat generated by processing is increased, so that it is particularly difficult to maintain the temperature.

C) Inconveniences related to the heat load of the rolls are likely to occur. When high-load rolling that causes a high-pressure reduction is performed, the heat generated by processing of the rolled material also increases, and the heat load of the roll increases accordingly. As a result, a thermal crown whose diameter increases at the center of the roll is likely to occur. Depending on the degree of the thermal crown, it may be difficult to solve the problem by simply cooling the roll, and the shape of the rolled material may be deteriorated to make stable striping difficult.

(2) The roll is severely worn, and the shape (crown) of the rolled material is likely to be further deteriorated. This is because in high-pressure reduction / high-load rolling, the thermal or mechanical load applied to the roll is high, and roll wear is likely to progress. At the portion in contact with the edge of the rolled material, the rolling load is high, so that the wear is particularly likely to proceed, and the profile of the rolled material, which is important for quality, tends to be significantly reduced. Further, if the roll is easily worn, the cost for maintenance such as polishing and replacement of the roll also increases.

The present invention solves the above problems relating to the hot-rolled steel sheet for fine-grain steel and enables a smooth production of the steel sheet, and a fine-grain steel production method for solving the same problem. It is intended to provide a method.

[0011]

[SUMMARY OF] hot rolling mill according to claim 1, together with arranging the different diameters roll mill or a micro diameter roll mills multiple stand subsequent stage including the final stage, the outermost
A curtain wall type cooling means for the rolled material is arranged on the exit side of at least two stands (two stands including the final stage or more) of the latter stage including the final stage . The above-mentioned ultra-small diameter roll mill refers to a rolling mill in which a pair of work rolls each have a small diameter of less than 600 mm, and a different-diameter roll mill has a pair of work rolls whose diameters are not equal to each other. The equivalent roll diameter (average roll diameter) of the work rolls is less than 600 mm in diameter. The curtain wall type cooling means is a cooling means of a type in which a large amount of cooling water is made to flow in a laminar state from above and below like a curtain and is applied to the upper and lower surfaces of the rolled material over the entire width. The cooling means is arranged on the outlet side of the mill having two or more stands in the latter stage as described above, and the number of units used is appropriately changed depending on the type of rolled material and the like.

According to this hot rolling mill, a fine grain steel hot rolled steel sheet having a fine ferrite structure and an excellent strength balance including tensile strength, ductility, toughness and fatigue strength (see, for example, FIG. 5).
Can be produced smoothly, and commercial production of the steel sheet is also possible. The reason is as follows.

A) The different diameter roll mill having two or more stands arranged at the latter stage including the final stage or the extremely small diameter roll mill has a small equivalent roll diameter or both (a pair of) work roll diameters, and therefore a large rolling with a low rolling load, That is, rolling at a high pressure reduction rate can be performed. This is because the rolling load that brings about the same rolling reduction becomes smaller as the diameter of the work roll becomes smaller and is approximately proportional to the work roll diameter (d) (see, for example, FIG. 6). When the rolling load is small, the phenomenon that high pressure reduction rolling cannot be performed due to the flatness of the roll is eliminated, and the flat deformation amount of the rolling roll is reduced, so that the edge drop is also reduced (see, for example, FIG. 7).

B) Curtain wall type cooling means provided in the latter stage including the final stage suppresses the temperature rise due to the heat generated by the working of the rolled material during the high pressure rolling. Since the same means strongly cools the rolled material by the large amount of cooling water flowing as described above, even if the rolled material is accelerated, it is necessary to maintain the rolled material in a temperature range suitable for performing the large reduction rolling method. Is possible. Since the same means is arranged not only on the exit side of the mill of the final stand but also on the exit side of the mill of at least two stands in the latter stage, it is possible to effectively take away heat generated during rolling on the final stand and the stands up to that time, and to make it suitable. Measure the temperature.
Since the above-mentioned cooling means is provided on the outlet side of each stand, the action of stopping the grain growth of the fine structure by strongly cooling the rolled material immediately after rolling in each stand is also ensured. Since the same means applies the cooling water over the entire width of the rolled material, the rolled material can be cooled uniformly without deviation in the width direction.

That is, according to this hot rolling mill, the above-mentioned problems (a) and (b) relating to the implementation of the large reduction rolling method are solved, and fine grains can be obtained by using a general hot strip mill type rolling mill. Enables smooth manufacturing of steel sheets.
Further, if the temperature range of the rolled material is maintained at 700 to 800 ° C. (warm range) by appropriately using a curtain wall type cooling means, steel containing Nb or Ti is used as a rolled material and the above-mentioned controlled rolling method is applied. It is also possible to carry out stably (thus producing a fine-grain steel plate).

The hot rolling mill according to claim 2 further comprises:
It is characterized by arranging a plurality of CVC mills in the front stage. The CVC mill refers to a rolling mill including a roll whose outer diameter continuously changes in the axial direction and which can move in the axial direction (CVC roll). The case where the different diameter roll mill or the ultra-small diameter roll mill is also a CVC mill is included in the rolling machine according to claim 1.

In such a rolling mill, the two
The crown (shape) of the plate can be controlled such as the prevention of thermal crown by the CVC mill above the stand. In the CVC mill, the C having the outer diameter change as described above is used.
This is because the roll gap shape can be appropriately changed by moving the VC roll in the axial direction. Such a mill has a large ability to change the roll gap shape as compared with a means such as simply performing roll bending, and moreover,
Since the rolled material is thick and the front part is arranged in the center for easy crown control, it is advantageous for adjusting the crown and preventing instability of the threaded plate in the latter part where large rolling is performed.

The hot rolling mill according to claim 3 is, in particular, an equivalent roll diameter in the different diameter roll mill (average diameter of a pair of work rolls in each mill) or a roll diameter in each extremely small diameter roll mill (each work roll). The diameter) is 550 mm or less.

In this hot rolling mill, since the work roll is sufficiently thin, it is suitable for performing large reduction with a low rolling load as in the above a). In terms of strength, the roll diameter (diameter of each work roll) is 400 mm in any mill.
The above is preferable.

The hot rolling mill according to claim 4 further comprises:
It is characterized in that the work rolls of the different diameter roll mill or the extremely small diameter roll mill described above are provided with a CVC function and a bending function. What is the CVC function?
The roll whose outer diameter changes continuously in the axial direction moves in the axial direction to control the change of the roll gap shape. The bending function is to apply a bending force (bending moment) to the roll. A function that can change the roll gap shape.

The CVC function may be imparted only to the CVC roll arranged in the preceding stage, but in this way, it may also be imparted to the work roll of the different diameter roll mill or the ultra-small diameter roll mill provided in the latter stage. In this way, the control for preventing the thermal crown and the like can be performed even in the stand at the latter stage, and the plate profile of the plate as the rolled material is further improved. If you add a bending function,
Further, the shape control performance is improved.

The hot rolling mill according to claim 5 further comprises:
It is characterized in that any one of the stands is provided with a means for supplying a lubricant to the roll surface (including a means for supplying the lubricant to the surface of the rolled material and consequently to the surface of the roll).

Since the means for supplying the lubricant is included, in this hot rolling mill, wear of the roll surface is suppressed despite the high rolling load (see, for example, FIG. 8). When the wear of the rolls is reduced, the cost of maintenance (polishing of the surface, etc.) related to the rolls is reduced, and it is possible to stably produce a rolled material having a preferable plate shape (profile) for quality. Since the lubricant to be supplied is to prevent abrasion of the roll surface and not to reduce the coefficient of friction, a solid lubricant, for example, is used as the lubricant, and a lubricant between the roll surface and the rolling material in use is used. It is preferable that the friction coefficient μ of the above is about 0.28 or more. When such a coefficient of friction is secured, roll slip of the rolled material is appropriately prevented.

A hot rolling mill according to a sixth aspect of the present invention is characterized in that, particularly, the lubricant containing fine solid lubricant is supplied as the lubricant. As such a solid lubricant, for example, fine particles of calcium phosphate, mica, calcium carbonate or the like are suitable.

When such a lubricant is supplied to the roll surface after the material, particle size, content, etc. of the fine particle solid lubricant are appropriately set, the fine particles are present between the roll surface and the rolled material. Intervenes to prevent the two from directly contacting each other, and exerts a preferable action in relation to the above-described suppression of roll wear and friction with the rolled material. Since the above-mentioned fine particles are contained not in the mineral oil but in the highly viscous grease, there is no risk of the fine particles settling in the lubricant storage container. As long as precipitation does not occur, the fine particles uniformly mixed therein can be easily supplied to the roll surface together with the grease.

The hot rolling mill according to claim 7 is characterized in that a fluid jet spray for removing cooling water on the rolled material is arranged downstream of the cooling means on the exit side of the stand at the final stage. And

If a curtain wall type cooling means is used on the exit side of the last stand, a large amount of cooling water will be placed on the upper surface of the rolled material exiting the rolling mill. There are various measuring instruments on the downstream side of the rolling mill to measure the shape, dimensions, temperature, etc. of the rolled material, but if there is a large amount of water on the rolled material, accurate measurement becomes impossible. As a result, the rolling mill may not be able to operate. The fluid jet sprays described above remove such water. With this spray,
By the jetted fluid, the cooling water on the rolled material can be removed by the action of the cooling means, so that the necessary measurement can be appropriately performed on the rolled material that has left the final stand,
As a result, it becomes possible to continue the operation of the rolling mill smoothly.

In the hot rolling mill according to the eighth aspect, as the fluid jet spray, the rolled material is directed obliquely forward from above toward the rolled material (that is, not obliquely downward but obliquely downward to the upstream side). It is characterized in that a plurality of nozzles that blow out pressurized water are arranged so as to spread in the width direction.

According to a study conducted by the inventors by making various fluid jet sprays on a trial basis, cooling water on the rolled material is removed (water injected by itself is not left on the rolled material). It has been found that spraying is most preferred. Since water has a mass larger than that of gas, it is easy to apply kinetic energy and is easily available. Therefore, water is first suitable as a jet fluid. It is possible to prevent the cooling water from reaching the downstream side (the side with the measuring instrument) by blowing out the pressurized water diagonally forward, and further, to cool the entire width of the rolled material by using the nozzle that spreads in the width direction of the rolled material. The fact that water can be removed is considered to be the reason for the positive effect.

The fine grained steel manufacturing method according to claim 9, double
Using a rolling mill with several stands arranged in tandem, heated steel sheets are cooled with a curtain wall type before and after the work rolls (that is, the upstream side and the downstream side of the work rolls) in the stand of the latter stage including the final stage.
While it cooled by retirement unit, subsequent Stan including the final stage
Cumulative strain in de characterized in that the rolling to be 0.9 or more. The term “distortion” means the difference between the thickness h ₀ of the steel plate on the entrance side of each stage and the thickness h ₁ on the exit side divided by the average thickness of both ε = (h ₀ −h ₁ ) / {(H ₀ + h ₁ ) / 2}. The term “cumulative strain” refers to the strain in each of the latter three stands (there may be two stands) among the above stands (ignoring the stand upstream from them because their influence is small). Weighted integration considering the strength of the effect on the strain, and letting ε _n , ε _n-1 , and ε _n-2 be the strains at the final stage and the preceding and previous stages, respectively, ε _c = ε _n It is assumed that ε _c represented by + ε _n-1 / 2 + ε _n-2 / 4. The heating temperature of the steel sheet is, for example, A when performing the above-mentioned large reduction rolling method.
It is preferable that the temperature is maintained in the range of r ₃ transformation point to Ar ₃ + 50 ° C., and in the case of performing the controlled rolling method, in the temperature range of 700 to 800 ° C.

According to this method, it is possible to smoothly produce a fine grain steel hot rolled steel sheet having a fine ferrite structure.
While rolling at a high-pressure reduction rate with a cumulative strain of 0.9 or more in the latter-stage stand, the steel sheet is cooled by the curtain wall type cooling means before and after the latter-stage work roll, so that heat generated during rolling is taken away. The reason is that the rolled material just after rolling can be cooled strongly and the grain growth of the fine structure can be stopped while maintaining an appropriate temperature. When rolling with cumulative strain of 0.9 or more is performed in this way, the crystal grain size of the ferrite structure in the steel sheet becomes as fine as about 4 μm or less.

The fine grain steel manufacturing method according to claim 10 uses the hot rolling mill according to any one of claims 1 to 8 and includes a final stage.
It is characterized in that the steel sheet is rolled so that the cumulative strain in the latter stand is 0.9 or more. Again, "strain" and "cumulative strain" refer to ε and ε _c above.

The hot rolling mill according to any one of claims 1 to 8 makes it possible to smoothly produce a fine-grain steel hot-rolled steel sheet having a fine ferrite structure as described above. Using such a rolling mill, the rolled material is appropriately heated to operate each mill, cooling means, etc., and the cumulative strain is 0.9 or less in the latter stand having a strong influence on the structure. If the steel sheet is rolled as described above, the crystal grain size of the ferrite structure is 4 μm.
It is possible to obtain a preferable fine-grained steel having a size of about m or less.

The method for producing fine-grained steel according to claim 11 is
In particular, it is characterized in that the steel plate is cooled to 20 ° C. or more per second immediately after leaving the final stand. By doing this, the rolled material immediately after rolling is strongly cooled,
It is possible to accurately stop the growth of crystal grains after rolling and obtain a fine grain steel having a preferable fine structure.

The method for producing fine-grained steel according to claim 12 is characterized by rolling a steel sheet having a carbon content of 0.5% or less and an alloying element content of 5% or less.

The fine-grained steel sheet having such a composition is widely used because of its balance of mechanical properties (general versatility in terms of tensile strength and ductility) and high weldability, and has a relatively low price. Therefore, it is considered to be in very high demand because it is easily available and has recyclability. Therefore, a steel sheet with such a content of components has a high social contribution and is accompanied by sufficient economic rationality for its production. Generally, when the amount of C (carbon) increases, the amount of ferrite decreases and the steel mainly contains pearlite, but according to the method of the invention, the amount of ferrite can be increased even if the amount of C is the same. Up to 0.5%, a structure mainly composed of ferrite can be obtained. Although the method of the invention results in effect or without alloying elements other than C in the rolled material, Ar ₃ transformation point ~A
In order to keep the temperature range of r ₃ + 50 ° C. between 700 and 900 ° C. which is the optimum temperature for hot working, it is preferable to adjust the transformation point temperature by the total amount of alloying elements. However, if the total content of alloying elements exceeds 5%, Ar ₃
The transformation point becomes too low and it becomes difficult to obtain fine grains.

[0037]

BEST MODE FOR CARRYING OUT THE INVENTION One mode for carrying out the invention is shown in FIGS. FIG. 1 is a side view conceptually showing the overall arrangement of the hot rolling mill A. FIG. 2 is a schematic view for explaining the CVC function of the mill 1 at the front stage of the rolling mill A, and FIG. 3 is a side view showing details of the mill 6 at the final stage of the rolling mill A and the like. is there.

The hot rolling mill A shown in FIG. 1 is a finish rolling mill, which has a heating furnace and a rough rolling mill on the upstream side (not shown), and a runout table and a winding machine on the downstream side (not shown). Machines etc. are arranged. This hot rolling mill A is configured as follows so that a fine grain steel hot rolled steel sheet having a fine ferrite structure can be manufactured by continuously rolling a rolled material (steel sheet) P roughly rolled upstream. ing.

First, the so-called C is used as the first three stands.
VC mills 1, 2 and 3 are arranged in tandem. The frontmost CVC mill 1 has work rolls 1a and 1 as shown in FIG.
b and the backup rolls 1c and 1d are configured as a quadruple rolling mill.
It has a crown (CVC, that is, a continuous change in diameter) as shown in FIG. Work roll 1a ・ 1b
Can be simultaneously moved (shifted) vertically in opposite axial length directions as shown in FIGS. 2 (b) and 2 (c), whereby the positional relationship between rolls, that is, the roll gap can be adjusted. is there. The diameter of the work rolls 1a and 1b is 700 mm, and the maximum shift amount is 100
mm. The other two-stage CVC mills 2 and 3 are no different from the front-stage CVC mill 1 in terms of such configuration and function.

The reason why the CVC mills 1, 2, and 3 are arranged in the preceding stage is to keep the crown (shape) of the rolled material P suitable. Later stage different diameter roll mills 4, 5, 6 (described later)
In such cases, since thermal crowns and the like due to heat generation during rolling are likely to occur, these CVC mills 1
-By 2 and 3, the plate crown is corrected in advance to reduce the intermediate drawing of the rolled material P.

So-called different diameter roll mills 4, 5 and 6 are arranged in tandem as the subsequent three stands. All six stands including the above-mentioned CVC mills 1, 2, and 3 have an equal stand spacing of 5.5 m. The different diameter roll mill 4 corresponding to the fourth stand counting from the CVC mill 1 has work rolls 4a and 4b and a backup roll 4c as shown in FIG.
The work rolls 4a and 4b having different diameters are used as the work rolls 4a and 4b. Then, of the work rolls 4a and 4b, only the lower large-diameter roll 4b is rotationally driven by a motor or the like (not shown), and the upper small-diameter roll 4a is rotatable and no driving force is applied. And Since a bender (not shown) is attached to the work rolls 4a and 4b, the work rolls 4a and 4b can be bent. Also, each work roll 4a, 4b has a CV
A C function is also provided, and both can be moved in the axial direction within the range of 100 mm in each of the forward and reverse directions. The work roll 4a has a diameter of 480 mm, the work roll 4b has a diameter of 600 mm, and the equivalent roll diameter which is an average of the two is 54 mm.
It is 0 mm. With respect to such a configuration and function, the other two different-diameter roll mills 5 and 6 at the rear side are not different from the different-diameter roll mill 4 described above.

These three-stand roll mills with different diameters 4.5
In No. 6, since the equivalent roll diameter is small and the shearing force acts on the rolled material P because it drives only one work roll (4b etc.), the rolling ratio is high even with a relatively low rolling load ( For example, rolling with a reduction rate of 50%) can be performed.
For this reason, large reduction rolling or the like for forming a fine ferrite structure in the rolled material P can be performed with a small rolling load, and since the rolling load is small, inconvenience due to roll flattening or edge drop does not occur. The diagram X3 in FIG. 6 is
Thickness of 2.3m in the different diameter roll mill 6 of the 6th stand
Steel plate with m and width of 730 mm (composition is C: 0.16%, S
When i (0.22%, Mn: 0.82%) is rolled as a rolled material P at the same reduction rate (48%), it is shown how the rolling load changes with the change of the equivalent diameter of the work roll. It is a thing. In addition, the line X5 in FIG. 7 shows a case where the constant-diameter roll mills 5 and 6 (the diameters of the work rolls 5a and 6a are 480 mm, 5b and 6b are 600 mm, and the equivalent roll diameters of the mills are 540 mm). It represents an edge drop that occurs when rolling the same rolled material P as in the above case. Note that the line X4 in FIG. 7 shows, for comparison, edge drops when the same rolled material P is rolled with the work rolls having the same diameter (medium-scale diameter of 600 mm) instead of different diameters.

Lubricating agents are arranged for the work rolls of all the mills 1 to 6 of the six stands.
The means is, for example, reference numerals 5e, 5f, 6e, 6 in FIG.
It is composed of an injection port facing the surface of the work roll like f, a pump for feeding the lubricant to the injection port, and the like. As the lubricant, a grease containing a fine solid lubricant such as calcium phosphate, mica, calcium carbonate or the like is used. By mixing these solid fine particles, the friction coefficient μ between each work roll and the rolled material P when the lubricant is used is about 0.2.
It will be as high as 8.

If the above lubricant is used, the above-mentioned fine particles are present between the roll surface and the rolled material P to prevent direct contact between the roll and the rolled material, so that the wear of the roll surface is suppressed. , The shape of the rolled material P is long and is easily maintained well.
Since the solid fine particles are contained in the grease instead of the mineral oil, there is an advantage that the solid fine particles are supplied so that they are always uniformly dispersed on the roll surface. Figure 8
The effect of reducing the wear of the roll by the use of the lubricant is shown. The line X6 shows the case where the lubricant is not used, and the line X7
Indicates when the lubricant is used. The horizontal axis of FIG. 8 represents the load on the work roll, and the vertical axis represents the wear amount of the work roll.

Curtain wall type cooling means 7A, 7B, 7C are arranged on the respective outlet sides of the three-stand roll mills 4, 5, 6 of three stands arranged in the latter stage. As shown in FIG. 3, the cooling means 7B has a curtain-like shape (curtain wall-like shape and a thickness of 10 mm or more) in a laminar state in which a large amount of room-temperature cooling water is directed toward the entire width surface of the rolled material P from the upper and lower headers 7Ba and 7Bb. And the optimum thickness is 16 mm)
The rolled material P is strongly cooled. The amount of cooling water can be adjusted within a range of 100 to 500 m ³ / h per unit width (1 m) of the rolled material P, and the temperature drop of the rolled material P due to cooling is 20 ° C. /
More than sec. Usually (including the case of the following examples)
350 m ³ / h of cooling water is used per unit width,
In this case, the temperature drop of the rolled material P is 1 product of plate thickness x velocity
60 to 80 ° C / sec at 200 mm · mpm
(Around 40 ° C./sec including the temperature rise due to heat generated by processing). The other cooling means 7A and 7C have the same configuration and function as described above.

By using such cooling means 7A, 7B and 7C, the temperature rise of the rolled material P due to heat generation during processing during rolling is suppressed and rolling is carried out within a temperature range suitable for the large reduction rolling method or the controlled rolling method. While keeping the material P, it is possible to suppress the grain growth of the fine structure after rolling. Even in the run-out table (not shown) on the downstream side of the hot rolling mill A in FIG. 1, the rolled material P is cooled with cooling water at a rate of 10 ° C./sec or more to prevent grain growth. is doing.

In the hot rolling mill A of FIG. 1, the water jet spray 8 is arranged on the exit side of the different diameter roll mill 6 which is the final stage stand, separated from the curtain wall type cooling means 7C by several hundred mm to 1 m. ing. This is for removing the cooling water on the upper surface of the rolled material P by the cooling means 7C. As shown in FIG. 3, 10 kg of this spray 8 is directed obliquely forward from the upper side toward the rolled material P so that the angle with the upper surface of the rolled material P is 65 ° (or within a range of 50 to 80 °). 3 / min of pressurized water at around 1 / cm ²
A plurality of nozzles 8a for blowing out 00 liters are provided (a total of four in this example. As shown in FIG. 3, they are arranged at intervals in the length direction of the rolled material P and also at intervals in the width direction thereof. ). Each nozzle 8a blows out water so as to spread in the width direction of the rolled material. The spread angle of the rolled material P in the width direction is 15 to 30 °, and the spread angle in the length direction is 1 to 10.
Is good (21 ° and 3 ° in the examples).
If such a water jet spray 8 is used, the cooling means 7
Since the cooling water placed on the rolled material P can be smoothly removed by the action, it becomes possible to appropriately perform various measurements on the rolled material P after rolling by various measuring devices on the downstream side thereof.

In addition, for the work rolls of each stand, as shown in FIG. 3, roll cooling water jet nozzles (for example, reference numerals 5i, 5j, 6i, 6j) and a draining plate (for example, reference numeral) for removing cooling water by the nozzles 5g ・ 5h ・
6g and 6h) are also arranged.

[0049]

EXAMPLE An example of hot rolling using the rolling mill A described above is shown below.

C: 0.16%, Si: 0.22%, M
n: 0.82% (without other significant amount of components) for a steel having a chemical composition, with a rolling mill A, a thickness of 2.3
A steel plate having a width of 3 mm and a width of 730 mm was subjected to three conditions (Example 1).
~ 3). Table 1 below shows pass schedules (rolling conditions) for Example 1, and Table 2 shows pass schedules for Examples 2 and 3. Table 3 shows
Curtain wall type cooling means 7A in each of Examples 1 to 3
The use conditions of 7B and 7C, and Table 4 are the finishing temperatures of the rolled material P measured behind the mill 6 at the final stage for each of Examples 1 to 3. In each table, "rough bar" represents a rough rolling mill,
"F1" to "F6" represent the first to sixth stage mills 1 to 6, respectively. The rolling speed was not particularly limited, and the rolling speed commonly used in general hot strip mills (for example, 7 to 9 m / sec) was adopted.

[Table 1]

[Table 2]

[Table 3]

[Table 4]

Table 5 shows the ferrite grain size and mechanical properties of the hot-rolled steel sheet (rolled material P) obtained in each of Examples 1 to 3. In this table 5, "TS"
Is tensile strength, "YP" is the yield point, and "EL" is elongation. In Table 5, the main rolling conditions shown in Tables 1 to 3 are also shown.

[Table 5]

As shown in Table 5, Examples 2 and 3 in which the cumulative distortion (the above-mentioned weighted integrated value ε _c ) was set to 0.92
Then, it was possible to obtain a steel sheet having a ferrite structure with a grain size of around 4 μm and excellent in mechanical properties. Example 3 using curtain wall type cooling means 7A to 7C on the output side (rear surface) of the three stands (F4 to F6) in the latter stage
In some cases, a steel sheet having a ferrite grain size of about 4 μm or less and excellent mechanical properties was obtained.

FIG. 4 is a graph showing the relationship between the grain size (grain size D (μm) raised to the power of −½) and the yield point regarding the crystal grains of the ferrite structure of the steel sheets obtained in Examples 1 to 3. It was done. As shown in the figure, when the cumulative strain in the latter three stands is set to 0.65 (group X2 in FIG. 4), the grain size is 0.43 or less (particle size 5.4 μm or more) and the yield point is sufficient. However, when the cumulative strain is set to 0.92, the grain size becomes about 0.5 (particle size is about 4 μm), and the yield point becomes higher than 45 kg / mm ² .

FIGS. 5 (a), (b), and (c) are photographs of the crystal structures of the steel sheet obtained in Example 3, taken near the upper surface, near the center of the sheet thickness, and near the lower surface, respectively. . A fine ferrite structure having a grain size on the order of 3 μm is formed in any portion within the plate thickness.

[0055]

The hot rolling mill described in claim 1 is a general hot strip mill type rolling mill,
It is possible to smoothly produce a fine-grain steel hot-rolled steel sheet having a fine ferrite structure, and also enable commercial production of the steel sheet. According to the hot rolling mill of the second aspect, it is possible to suppress the generation of a thermal crown and stabilize the threading. The hot rolling mill described in claim 3 is particularly preferable in reducing the rolling load.

In the hot rolling mill according to the fourth aspect, control for preventing the thermal crown and the like can be performed in the stand at the latter stage, and the profile of the plate as a rolled material becomes good.

According to the hot rolling mill of the fifth aspect, abrasion of the roll surface is suppressed, so that a rolled material having an excellent shape can be stably manufactured. There is also an advantage that the roll slip of the rolled material is reduced. The hot rolling mill of claim 6 is particularly advantageous in terms of the above-described suppression of roll wear and friction with the rolled material. There is an advantage in that the lubricant can be supplied smoothly.

The hot rolling mill according to claim 7 enables necessary measurement of the rolled material by appropriately removing the cooling water on the upper surface of the rolled material exiting the final stand.
As a result, smooth operation of the rolling mill is realized. The hot rolling mill according to claim 8 is particularly excellent in the ability to remove the cooling water on the rolled material.

According to the method for producing fine-grain steel according to the ninth aspect, it is possible to smoothly produce a fine-grain steel hot-rolled steel sheet having a fine ferrite structure. It is possible to make the crystal grain size of the ferrite structure finer to about 4 μm or less.

According to the fine grain steel manufacturing method of the tenth aspect,
By using the hot rolling mill described in claims 1 to 8, it becomes possible to smoothly manufacture a fine grain steel hot rolled steel sheet having a fine ferrite structure.

According to the eleventh aspect of the present invention, in the method for producing fine-grained steel, the rolled material immediately after rolling is strongly cooled, the growth of crystal grains after rolling is accurately stopped, and fine-grained steel having a preferable microstructure is obtained. Bring

The fine grain steel manufacturing method according to the twelfth aspect of the invention produces a useful steel sheet, which makes a high social contribution and enables rational production commensurate with cost.

[Brief description of drawings]

FIG. 1 is a side view conceptually showing the overall arrangement of a hot rolling mill A, which is one mode for carrying out the invention.

2 (a), (b), and (c) are schematic diagrams for explaining the CVC function of the mill 1 in the preceding stage of the rolling mill A of FIG.

FIG. 3 is a side view showing details of a final stage mill 6 and the like of the rolling mill A.

FIG. 4 is a diagram showing a relationship between a grain size and a yield point regarding crystal grains of a ferrite structure in a manufactured steel sheet.

5 (a), (b), and (c) are photographs of crystal structures of the manufactured steel sheet near the upper surface, near the center of the plate thickness, and near the lower surface, respectively.

FIG. 6 is a diagram showing a relationship between an equivalent diameter of a work roll and a rolling load.

FIG. 7 is a diagram showing the effect of reducing edge drops in a different diameter roll mill.

FIG. 8 is a diagram showing the effect of reducing the wear of the roll surface when a lubricant is used.

[Explanation of symbols]

A hot rolling mill 1, 2, 3 CVC mill 4/5 6 different diameter roll mill 7A / 7B / 7C Curtain wall type cooling means 8 water spray

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Ichiro Takeshi 1-16-1 Funamachi, Taisho-ku, Osaka City Nakayama Steel Works Ltd. (72) Inventor Masanori Takahashi 3-1-1 Higashikawasaki-cho, Chuo-ku, Kobe-shi, Hyogo No. 1 Kawasaki Heavy Industries Co., Ltd.Kobe Factory (72) Inventor Akio Adachi 3-1-1 Higashikawasaki-cho, Chuo-ku, Kobe-shi, Hyogo Prefecture Kawasaki Heavy Industries Ltd. Kobe Factory (72) Inventor Shinji Takaoka Chuo-ku, Kobe-shi, Hyogo Prefecture 3-1, 1-1 Higashikawasaki-cho Kawasaki Heavy Industries Ltd. Kobe factory (56) Reference JP 2003-10905 (JP, A) JP 2003-94101 (JP, A) JP 2003-94102 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) B21B 1/22 B21B 1/26 B21B 35/00 B21B 45/02

Claims (12)

(57) [Claims] 1. A curtain wall type cooling means for a rolled material is provided in which a plurality of stands of different diameter roll mills or ultra-small diameter roll mills are arranged in a rear stage including a final stage, and at least two mills in the rear stage including the final stage exit the mill. The hot rolling mill is characterized in that. 2. The hot rolling mill according to claim 1, wherein a CVC mill having a plurality of stands is arranged in the preceding stage. 3. The equivalent roll diameter in the different diameter roll mill or the roll diameter in the ultra-small diameter roll mill is 5.
It is 50 mm or less, 1 or 2 characterized by the above-mentioned.
The hot rolling mill described in. 4. The hot rolling mill according to claim 1, wherein the work rolls of the different diameter roll mill or the extremely small diameter roll mill have a CVC function and a bending function. 5. The hot rolling mill according to claim 1, wherein a means for supplying a lubricant to the roll surface is attached to any of the stands. 6. The hot rolling mill according to claim 5, wherein as the lubricant, fine solid lubricant contained in grease is supplied. 7. A fluid jet spray for removing cooling water on the rolled material is arranged downstream of the cooling means on the exit side of the stand at the final stage, according to any one of claims 1 to 6. The hot rolling mill described. 8. The fluid jet spray according to claim 7, wherein a plurality of nozzles for ejecting pressurized water are arranged so as to spread obliquely forward in the width direction of the rolled material toward the rolled material from above. Hot rolling mill. 9. A pressure in which a plurality of stands are arranged in tandem.
With caster, a heated steel sheet, a car in front and behind the work roll in the latter stage of the stand including the final stage
While cooling by the ten wall type cooling means ,
A method for producing fine-grained steel, which comprises rolling so that the cumulative strain in a stand in the subsequent stage including is 0.9 or more. 10. A hot rolling mill according to any one of claims 1 to 8 is used to roll a steel sheet such that the cumulative strain in a stand at the rear stage including the final stage is 0.9 or more. Method for producing fine grain steel. 11. The method for producing fine-grained steel according to claim 9, wherein cooling of the steel sheet immediately after leaving the final stand is 20 ° C. or more per second. 12. The fine-grain steel according to claim 9, wherein a steel sheet having a carbon content of 0.5% or less and an alloying element content of 5% or less is rolled. Production method.