JP7008532B2 - Cold rolling method - Google Patents

Description

The present invention relates to a cold rolling method.

When a hot-rolled hot-rolled steel sheet is cold-rolled, cracks may occur at the ends (end faces). This edge cracking is particularly likely to occur in hot rolled steel sheets containing a large amount of elements such as Mn (manganese) that improve hardenability, that is, elements that have the effect of delaying the transformation of γ-iron to α-iron. This end crack may cause breakage during cold rolling, annealing step, plating step, and other subsequent steps starting from this crack.

In the cooling process after the winding process of the hot-rolled steel sheet, the cooling speed of both ends of the hot-rolled steel sheet in the width direction is faster than that of the center of the hot-rolled steel sheet in the width direction. Therefore, the hot-rolled steel sheet containing a large amount of elements that improve the hardenability has a structure in which both ends of the hot-rolled steel sheet in the width direction contain a relatively large amount of martensite and have low ductility. Is likely to occur.

As means for reducing this end crack, (1) means for softening the hot-rolled steel sheet itself, (2) means for reducing the rolling reduction during cold rolling, and (3) both ends in the width direction with low ductility. A means for removing the portion or the like can be considered.

As for these measures, regarding (1) above, if a means for softening the hot-rolled steel sheet itself is adopted, the scale increases during hot-rolling, so that the pickling time is increased or a long time is required to remove the scale. A batch annealing process after the required hot rolling process is required, which may reduce productivity. Regarding (2) above, it is necessary to increase the rolling reduction during hot rolling in order to reduce the rolling reduction during cold rolling, but in that case, the thinness of the steel sheet that can be manufactured by hot rolling is exceeded. There is a risk of rolling. Further, regarding (3), if a means for removing both ends in the width direction is adopted, the yield may decrease due to the presence of the removed portions, which may lead to an increase in manufacturing cost.

In the prior art, a heating device that heats both edges of the hot-rolled steel sheet in order to suppress the occurrence of breakage due to ear cracking of the hot-rolled steel sheet in cold rolling, and a heating device that is more than this heating device. A method has been proposed in which cold rolling is performed while stably heating both edges of the hot rolled steel sheet by a cold rolling facility equipped with a meandering correction device arranged on the upstream side in the transport direction of the hot rolled steel sheet. (Japanese Patent Laid-Open No. 2015-139810).

JP-A-2015-139810

However, when rolling is performed with only both edges of the hot-rolled steel sheet at a high temperature as in the above-mentioned conventional technique, the deformation resistance distribution in the width direction of the hot-rolled steel sheet becomes excessive, so that after cold rolling. The shape of the hot-rolled steel sheet may be disturbed, and the operability and product quality may deteriorate.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a cold rolling method having an excellent effect of suppressing end cracks during cold rolling.

In a hot-rolled steel sheet containing a large amount of alloying elements, the cooling speed at both ends of the hot-rolled steel sheet in the width direction is faster than that at the center of the width of the coil in the cooling process by air cooling after winding after the hot-rolling process. As a result, the structure is such that cracks at the ends of the hot-rolled steel sheet are likely to occur in cold rolling. The present inventors suppress the edge cracking of the hot-rolled steel sheet by heat-treating the edge portion of the hot-rolled steel sheet to appropriately soften the structure at both ends in the width direction of the hot-rolled steel sheet. I found out what I could do.

One aspect of the present invention made to solve the above problems is a method of cold rolling a strip-shaped hot-rolled steel sheet that has been wound and cooled into a coil shape, and the hot-rolled steel sheet is obtained from a coil. The feeding step, the heating step of heating both ends of the fed hot-rolled steel sheet in the width direction to a temperature of 400 ° C. or lower at A1 point or less of the hot-rolled steel sheet material, and the hot-rolled steel sheet after the heating step. It is a cold rolling method including a pickling step of washing with an acid and a cold rolling step of cold rolling a hot rolled steel sheet after the pickling step.

In the cold rolling method, by heating both ends of the hot-rolled steel sheet wound in a coil shape in the width direction before cold rolling, cracks occur in both ends in the width direction during cold rolling. It is possible to secure an appropriate ductility. As a result, it is possible to accurately suppress the occurrence of end cracks during cold rolling or the like. In particular, since both ends in the width direction are heated to a temperature of 400 ° C. or higher below the A1 point of the hot-rolled steel sheet material, martensite in the microstructure at both ends in the width direction generated when cooling after the hot rolling is ensured. Can be tempered to martensite. Therefore, it is possible to more reliably secure the ductility of both ends of the hot-rolled steel sheet in the width direction, and the effect of suppressing edge cracking during cold rolling is excellent. Further, since the cold rolling method can suppress end cracks in cold rolling at a rolling reduction rate equal to or higher than the conventional one, it is performed by removing the portion where end cracks are likely to occur in order to reduce the risk of end cracks. Yield loss can be significantly reduced.

It is preferable that the heating in the heating step is performed only in the regions on both ends in the longitudinal direction of the hot-rolled steel sheet. As a result, the heating cost can be reduced by partially heating only the regions on both ends in the longitudinal direction where cracks at the ends are likely to occur. The running cost can be reduced as compared with the case where the entire end face of the coiled hot-rolled steel sheet is heated all at once.

The length in the longitudinal direction in the regions on both ends in the longitudinal direction is 50% or less of the total length from the tip portion in the plate-passing direction of the hot-rolled steel sheet to the tail end portion, and from the tail end portion to the tip end portion. It is preferably 20% or less of the total length. By limiting the heating region in the heating step to the above range, the effect of reducing the heating cost can be enhanced.

The cold rolling method of the present invention is excellent in the effect of suppressing edge cracking of the hot rolled steel sheet.

It is a schematic diagram which shows the process of one Embodiment of the cold rolling method of this invention. It is a graph which shows the temperature change in the heating process of an Example.

Hereinafter, embodiments of the cold rolling method according to the present invention will be described in detail.

The cold rolling method of the embodiment of the present invention is a method of cold rolling a strip-shaped hot-rolled steel sheet that has been wound into a coil and cooled. The cold rolling method includes a feeding step of feeding the hot-rolled steel sheet from a coil, a heating step of heating both ends of the hot-rolled steel sheet in the width direction to a predetermined temperature, and heat after the heating step. It is provided with a pickling step of washing the hot-rolled steel sheet with acid and a cold-rolling step of cold-rolling the hot-rolled steel sheet after the pickling step, and the cold-rolled steel sheet is manufactured through each step. .. Further, the cold rolling method may further include a winding step of winding the steel sheet after the cold rolling step into a coil shape.

FIG. 1 is a schematic view showing a process of the cold rolling method of the present embodiment. In the cold rolling method of the present embodiment, for example, a cold rolling apparatus 1 having a cold rolling line as shown in FIG. 1 can be mentioned. In this cold-rolled line, a coil 3 of a hot-rolled steel sheet that has been wound and cooled on one reel is unwound, and one end of the hot-rolled steel sheet 2 is wound on the other reel to wind the hot-rolled steel sheet 2. 2 travels between both reels in the plate direction R. Further, in the cold rolling apparatus 1, the heating apparatus 5, the pickling tank 6, and the continuous rolling mill 10 are arranged in this order during the process of rewinding the strip-shaped hot-rolled steel sheet that has been wound into a coil and cooled. Have been placed.

[Feeding process]
In the feeding step, the strip-shaped hot-rolled steel sheet 2 wound into a coil and cooled is fed in the plate direction R.

(Hot rolled steel sheet)
The hot-rolled steel sheet 2 is a strip-shaped steel sheet that has been subjected to a hot-rolling process. In this hot rolling step, the hot rolled steel sheet 2 is formed by heating and rolling the slab. Specifically, first, the slab is heated in a range of 900 ° C. or higher and 1200 ° C. or lower using a heating furnace, and the primary scale generated at this time is removed by a desscaler. Next, the heated slab is roughly rolled in a temperature range of 900 ° C. or higher and 1300 ° C. or lower, and then the secondary scale generated on the surface is removed by a desscaler. Further, the rough-rolled slab is finish-rolled at 800 ° C. or higher and 1100 ° C. or lower to obtain a hot-rolled steel sheet 2.

The composition of the hot-rolled steel sheet 2 is not particularly limited, but has, for example, carbon, silicon, manganese, phosphorus, sulfur, chromium, nickel, molybdenum and copper, and a composition in which the balance is iron and unavoidable impurities. Further, in the cold rolling method, the hot rolled steel sheet 2 which is wound into a target coil and cooled has a hardenability multiple F of 20 or more represented by the following formula (1). It exerts a better effect when it has a composition in which edge cracks are likely to occur during interrolling. The steel sheet having a hardenability multiple F of 20 or more is prone to edge cracking during cold rolling, but this can be achieved by performing the cold rolling method on a steel sheet having a hardenability multiple F of 20 or more. Even if it is on a steel plate, edge cracking can be accurately suppressed. In the following formula (1), C, Si, Mn, P, S, Cr, Ni, Mo and Cu are carbon element, silicon element, manganese element, phosphorus element, sulfur element, chromium element and nickel in the steel plate, respectively. It means the content (mass%) of elements, molybdenum elements and copper elements.

F = (1 + 1.5 × (0.9-C)) × (1 + 0.64 × Si) × (1 + 4.1 × Mn) × (1 + 2.83 × P) × (1-0.62 × S) × (1 + 2.33 × Cr) × (1 + 0.52 × Ni) × (1 + 3.14 × Mo) × (1 + 0.27 × Cu) ... (1)

In the hot rolling step, the hot rolled steel sheet 2 is rolled so as to have a desired thickness (finishing thickness). Specifically, it is preferable to roll a slab having a predetermined thickness (for example, 230 mm) so that the finishing thickness is 1.2 mm or more and 6.0 mm or less. The upper limit of the finish thickness is more preferably 4.5 mm. Further, the lower limit of the finish thickness is more preferably 2.0 mm. If the finish thickness exceeds the upper limit, rolling becomes insufficient, and the strength of the hot-rolled steel sheet 2 may not be sufficiently obtained. On the other hand, if the finish thickness does not satisfy the above lower limit, the amount of processing in the hot rolling process becomes too large, and the equipment cost may become excessive.

The strip-shaped steel sheet after the hot rolling process is wound on a reel. The winding temperature of the hot-rolled steel sheet 2 is not particularly limited, but may be, for example, 500 ° C. or higher, or 650 ° C. or higher. The coil 3 of the hot-rolled steel sheet after winding is cooled (air-cooled) to room temperature.

(Payout)
The reel of the coil 3 of the hot-rolled steel sheet, which is wound and cooled in a coil shape, is supported by a support device (not shown) so as to be able to rotate about its axial direction. Specifically, the coil 3 of the hot-rolled steel sheet is rotated around the central axis of the reel by the support device, and the hot-rolled steel plate 2 is fed out in the plate-passing direction R.

[Heating process]
The heating step is a step of heating both ends of the hot-rolled steel sheet 2 unwound from the coil by the heating device 5 in the width direction, and is performed before the pickling step. In the cold rolling method, by heating both ends in the width direction of the hot rolled steel sheet 2, both ends in the width direction of the hot rolled steel sheet 2 have appropriate ductility, and in the subsequent cold rolling step. Edge cracking can be suppressed. Further, if the heating step is performed after the pickling step, there are disadvantages that an oxide film is generated on the surface of the hot-rolled steel plate 2 and a cooling facility after the heating step is required. In the method, since the heating step is performed before the pickling step, there is no possibility that such a demerit will occur.

The heating device 5 is not particularly limited as long as it can heat only both ends of the hot-rolled steel sheet 2 in the width direction, and a burner, an induction heating device, or the like is used.

The hot-rolled steel sheet 2, which is wound into a coil and cooled, has a reduced ductility due to the presence of a martensite structure due to retransformation. By heating the structure below the A1 transformation point, the ductility of both ends of the hot-rolled steel sheet 2 in the width direction can be suitably secured, and cracking at the ends can be suppressed.

The upper limit of the heating temperature in the heating step is the A1 point of the hot rolled steel sheet material, preferably 700 ° C., more preferably 600 ° C. When the upper limit of the heating temperature is within the above range, martensitic transformation can be reliably suppressed, and suitable ductility of both ends in the width direction of the hot-rolled steel sheet 2 can be more reliably ensured. The A1 point of the hot-rolled steel sheet material, which is the upper limit of the heating temperature, means the temperature at which the crystal lattice of the hot-rolled steel sheet starts to transform from the body-centered cubic lattice to the face-centered cubic lattice of austenite when heated. .. If the heating temperature exceeds the A1 point of the hot-rolled steel sheet material, martensitic transformation occurs when the material is cooled, which may be the starting point of cracking depending on the cooling rate.

The lower limit of the heating temperature is 400 ° C, more preferably 500 ° C. If the heating temperature is less than the above lower limit, the degeneration of both ends in the width direction is not sufficiently performed, and the ductility of both ends in the width direction may decrease.

The range in the width direction of both ends in the width direction to be heated in the heating step is not particularly limited, but a portion (including the end face) 1 mm or less from the end face (side end face) of the strip-shaped hot-rolled steel sheet 2 is preferable. It is more preferably a portion of 5 mm or less from the end face, and further preferably a portion of more than 5 mm from the end face. However, if the range in the width direction of both ends in the width direction to be heated is widened, the heating cost may increase unnecessarily.

The heating range in the longitudinal direction of the hot-rolled steel sheet in the heating step is not necessarily limited, but it is preferably performed only in the regions on both ends in the longitudinal direction of the hot-rolled steel sheet. As a result, only the regions on both ends in the longitudinal direction, where cracks at the ends are likely to occur, are partially heated, so that the heating cost can be reduced. The running cost can be reduced as compared with the case where the entire end face of the coiled hot-rolled steel sheet is heated all at once.

Further, the length in the longitudinal direction in the region on both ends in the longitudinal direction is preferably 50% or less, more preferably 20% or less, from the tip end portion to the tail end portion of the hot-rolled steel sheet 2 in the plate-passing direction R. preferable. Further, the length in the longitudinal direction in the regions on both ends in the longitudinal direction is preferably 20% or less, more preferably 10% or less of the total length from the tail end portion to the tip end portion. By limiting the heating region in the heating step to the above range, the effect of reducing the heating cost can be enhanced.

After the heating step, the hot-rolled steel sheet 2 is cooled. The cooling means is not particularly limited, and known means such as air cooling and air blow can be used.

[Pickling process]
In the pickling step, the hot-rolled steel sheet 2 after the heating step is washed with the acid in the pickling tank 6. By performing the pickling step, the oxide film on the surface of the hot-rolled steel sheet 2 formed in the heating step is melted and removed in the pickling tank 6. Further, since the non-uniform heat generated in the heating step in the width direction of the steel plate is made uniform in the pickling tank 6, the rolling rolls of the continuous rolling mill 10 in the cold rolling step after the pickling step are not contaminated, and the rolling rolls are not contaminated. In the cold rolling process, shape defects due to non-uniform deformation resistance in the width direction of the steel plate are suppressed.

[Cold rolling process]
The cold rolling step is a step of cold rolling the hot rolled steel sheet 2 after the pickling step. The continuous rolling mill 10 can be configured such that a plurality of pairs of cold rolled rolls are arranged in the plate passing direction R of the hot rolled steel plate 2. The hot-rolled steel sheet 2 is sandwiched between the rolling rolls of the continuous rolling mill 10 during the sheet-passing, so that cold rolling is performed. As the continuous rolling mill 10, a known rolling mill can be used, and for example, a reverse rolling mill that repeatedly rolls with one mill can be used.

In the main cold rolling step, the hot rolled steel sheet 2 after the pickling step is cold rolled at a predetermined reduction ratio. The lower limit of the rolling reduction in this cold rolling step is preferably 20%, more preferably 30%. If this reduction rate does not satisfy the above lower limit, the strength and thinning of the steel sheet may be insufficient. On the other hand, as the upper limit of this reduction rate, 70% is preferable, and 60% is more preferable. If the reduction rate exceeds the upper limit, the processing limit of the steel sheet may be exceeded. Further, in the cold rolling method, the heating step is performed before the cold rolling step, and the ductility of both ends of the hot rolled steel sheet 2 in the width direction is sufficiently ensured, so that the rolling reduction in the cold rolling step is high. Within the above range, cracks at the ends of the steel sheet are unlikely to occur. Here, the reduction ratio is (h0-h1) / when the plate thickness before cold rolling is h0 and the plate thickness after cold rolling (final stage when two or more stages of rolling are performed) is h1. It means the rate of change of the plate thickness represented by h0. The larger the value of the rolling reduction, the larger the rate of change in the plate thickness due to cold rolling.

Further, as the lower limit of the true strain in the cold rolling step, 0.20 is preferable, and 0.35 is more preferable. If this true strain does not satisfy the above lower limit, the strength and thinning of the steel sheet may be insufficient. On the other hand, as the upper limit of this true strain, 1.20 is preferable, and 0.90 is more preferable. If the reduction rate exceeds the upper limit, the processing limit of the steel sheet may be exceeded. The true strain is a value of ln (H / h).

[Winding process after cold rolling process]
In the winding step after the cold rolling step, the steel sheet after the cold rolling step is wound on a reel to form a coil 11 of the cold rolled steel sheet.

(advantage)
In the cold rolling method, the ductility of both ends in the width direction is appropriately increased by heating both ends in the width direction of the hot rolled steel sheet wound in a coil shape as described above before cold rolling. As a result, the effect of suppressing end cracks in the cold rolling process is excellent.

(Other embodiments)
It should be considered that the embodiments disclosed this time are exemplary in all respects and not restrictive. The scope of the present invention is not limited to the configuration of the above embodiment, but is indicated by the scope of claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims. To.

In the above embodiment, the heating step, the pickling step, and the cold rolling step are performed during the step of rewinding the strip-shaped hot-rolled steel plate that has been wound into a coil and cooled, but the pickling step has been performed. After the step, the hot-rolled steel plate may be rewound and a cold-rolled step may be carried out as a separate step.

Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

(Preparation of test piece)
A molten steel having the following composition was melted by a normal melting method and hot-rolled using a hot-rolling machine to produce a hot-rolled steel sheet. Next, it was wound at 650 ° C. to obtain a hot-rolled steel sheet coil. Next, it was allowed to cool to room temperature in the atmosphere. After that, the hot-rolled steel sheet was unwound from the coil, and the test number No. of 50 mm × 500 mm was obtained. 1 to No. 5 test pieces were obtained. Table 1 shows the plate thickness H of each test piece.

The molten steel has the following main compositions and other unavoidably contained impurities. The A1 point of the molten steel is 723 ° C.
C: 0.22% by mass
Si: 1.13% by mass
Mn: 2.23% by mass

(Heating process)
Using an induction heating device as the heating device, the test piece No. 2-No. About No. 5, the position 0 mm from the end face was heated so as to have a temperature of A1 point or less. The temperature of the heated portion of the test piece during heating was measured by a thermocouple attached to the center of the side end face of the test piece. Table 1 shows the ultimate temperature of each test piece. After each test piece reached a predetermined temperature, it was cooled by air cooling or air blowing. The air blow conditions were such that cooling could be performed from 500 ° C. to 100 ° C. or lower within 10 seconds. As an example of the temperature change of cooling by air cooling and air blow, the test piece No. 2 and No. The temperature change in the heating step of No. 3 is shown in FIG.

(Pickling process)
In the pickling step, hydrochloric acid was used and pickling was performed until the scale was removed.

(Cold rolling process)
After the pickling step, the test piece No. 1 to No. No. 5 was cold-rolled until the ultimate plate thickness h (mm) shown in Table 1 was reached. Test piece No. 1 to No. Table 1 shows the ultimate plate thickness h, true strain, and reduction rate of 5.

[evaluation]
(End crack resistance)
Test piece No. after the cold rolling process. 1 to No. The presence or absence of cracks at the end of 5 was visually confirmed. The results of these evaluations are shown in Table 1.

As shown in Table 1 above, the test piece No. 1 in which the cooled strip-shaped hot-rolled steel sheet was heated to a temperature of 400 ° C. or higher at A1 point or lower. 2-No. In No. 5, no end crack (crack on the end face) was detected even though the reduction rate was 70% higher than the normal reduction rate. On the other hand, the test piece No. that was not subjected to the above heating step. In No. 1, the reduction rate was No. 1 of the test piece. 2-No. Edge cracking was detected even though it was 50%, which was smaller than 5. From these results, it was shown that the cold rolling method can effectively suppress end cracks.

Since the cold rolling method of the present invention can accurately suppress cracking of the end face as described above, it can be suitably used when manufacturing a cold rolled steel sheet.

1 Cold-rolled equipment 2 Hot-rolled steel sheet 3 Hot-rolled steel sheet coil 5 Heating device 6 Pickling tank 10 Continuous rolling machine 11 Cold-rolled steel sheet coil

Claims (2)

It is a method of cold-rolling a strip-shaped hot-rolled steel sheet that has been wound into a coil and cooled.
The feeding process of feeding the hot-rolled steel sheet from the coil and
A heating step of heating both ends of the drawn hot-rolled steel sheet in the width direction to a temperature of 400 ° C. or higher at A1 point or less of the hot-rolled steel sheet material, and
A pickling step of cleaning the hot rolled steel sheet after the heating step with acid, and
It is equipped with a cold rolling process for cold rolling the hot rolled steel sheet after the pickling process .
The thickness of the hot-rolled steel sheet is 2.0 mm or more and 4.5 mm or less.
The heating in the heating step is 50% or less of the total length from the tip portion of the hot-rolled steel sheet in the plate-passing direction toward the tail end portion, and 20% or less of the total length from the tail end portion toward the tip portion. Do,
A cold rolling method in which the rolling reduction in the cold rolling step is 20% or more and 70% or less . The composition of the hot-rolled steel sheet is carbon, silicon, manganese, phosphorus, sulfur, chromium, nickel, molybdenum and copper, and the balance is iron and unavoidable impurities, which is a hardenability multiple represented by the following formula (1). The cold rolling method according to claim 1, wherein F is 20 or more.
F = (1 + 1.5 × (0.9-C)) × (1 + 0.64 × Si) × (1 + 4.1 × Mn) × (1 + 2.83 × P) × (1-0.62 × S) × (1 + 2.33 × Cr) × (1 + 0.52 × Ni) × (1 + 3.14 × Mo) × (1 + 0.27 × Cu) ... (1)
In the above formula (1), C, Si, Mn, P, S, Cr, Ni, Mo and Cu are carbon element, silicon element, manganese element, phosphorus element, sulfur element, chromium element and nickel in the steel plate, respectively. It means the content (mass%) of elements, molybdenum elements and copper elements.

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