Continous hot-rolling facility

Abstract

A continuous hot-rolling mill includes an upper rolling unit including a plurality of rolling stands, and a lower rolling unit including a plurality of rolling stands and disposed downstream with respect to the upper rolling unit in a workpiece passing direction in which a workpiece to be rolled is passed. The two or more rolling stands of the lower rolling unit are differential or very-small-diameter rolling stands. Each of the two or more differential or very-small-diameter rolling stands of the lower rolling unit is provided with a drive motor having a capacity greater than that of any one of drive motors included in the rolling stands disposed upstream the differential or very-small-diameter rolling stands. The continuous hot-rolling mill is suitable for producing hot-rolled, fine-grained steel sheets and is excellent in sheet passing abilities for preventing the steel sheet from meandering and for rolling the steel sheet in a desired shape.

Description

TECHNICAL FIELD The present invention relates to a continuous hot-rolling mill suitable for manufacturing a hot-rolled fine-grained steel sheet of fine structure mainly of fine ferrite grains. BACKGROUND ART In hot-rolling a thin steel sheet by a continuous hot-rolling mill (finishing mill) formed by arranging five to seven rolling stands in a tandem arrangement, a pass schedule is designed, in most cases, to place a maximum rolling load on the second or the third rolling stand. Such a pass schedule is recommended because a) the workpiece fed to the first rolling stand is thick and the workpiece is not pushed from the upper side into the first rolling stand, and hence the leading end of the thick workpiece cannot be gripped between the rolls if rolling force is excessively high, and b) the workpiece tends to meander and the shape of the workpiece, such as flatness, is deteriorated if an excessively high rolling force is exerted on the workpiece by the lower rolling stands at a low...

AI Technical Summary

Benefits of technology

In this continuous hot-rolling mill, the drive motors of the two or more differential or very-small-diameter rolling stands of the lower rolling unit have a capacity greater than that of any one of the drive motors of the rolling stands disposed upstream with respect to the differential or very-small-diameter rolling stands. Therefore, the lower rolling unit, which influences metallographic structure greatly, is able to achieve high-draft rolling. Since the lower rolling unit of this tandem rolling mill has the two or more differential or very-small-diameter rolling stands, the tandem rolling mill is able to roll a thin sheet at a high draft without causing the thin sheet to meander or deteriorating the shape of the sheet because the tandem rolling mill of this type is capable of achieving rolling at a high draft (and a large strain) by using a comparatively low rolling force. When the sheet is rolled by a low rolling force, lateral force (thrust) exerted on the sheet is low and hence the sheet is caused to meander scarcely, and the adverse effect of rolling on the shape of the sheet, such as edge drop, can be reduced because the flat deformation of the work rolls is reduced. Since problems relating to the passage and shape of the sheet arise scarcely, the lower rolling unit is able to raise the draft considerably according to the capacity of the drive motor and to roll the sheet at a cumulative strain of 0.9 or above. Thus, the tandem rolling mill is capable of producing a hot-rolled, fine-grained steel sheet having fine structures mainly of fine ferrite grains.

The lower rolling unit of the continuous hot-rolling mill has a satisfactory control characteristic relating to controlling the passage and shape of the sheet. Since the rolling stand having the CVC function is able to change and control roll gap shape in a wide range, the bending and the crowning of the rolls due to thermal expansion can be prevented to control the shape of the sheet effectively. Accordingly, the passage of the sheet through the lower rolling unit can be stabilized. When the lower rolling unit includes the CVC rolling stand, the passage and shape of the sheet can be directly and finely controlled at a stage near the delivery of a finished sheet. When the upper rolling unit includes the CVC rolling stand, the comparatively thick sheet can be controlled in a wide control range.

It is effective to increase the strain ε (or the draft) toward the last rolling stand to produce a hot-rolled, fine-grained steel sheet by rolling the steel sheet by the lower rolling unit at high drafts. The influence of the rolling action of the upper rolling stand on the metallographic structure of the sheet is lower than that of the last rolling stand or the rolling stand near the last rolling stand. Therefore it is advantageous to operate the last rolling stand or the rolling stand near the last rolling stand at a high draft to produce a sheet having the same metallographic structure without greatly increasing the average draft of drafts for all the rolling stands. Therefore, the continuous hot-rolling mill is capable of producing a hot-rolled, fine-grained steel sheet efficiently, particularly in respect of equipment cost and energy consumption.

As mentioned above, in view of the influence of rolling action on the metallographic structure, the upper rolling stands of the continuous hot-rolling mill do not need to roll the steel sheet at particularly high draft in producing a hot-rolled, fine-grained steel sheet. On the other hand, it is preferable that the lower rolling stands including the last rolling stand roll the steel sheet at high drafts to increase the cumulative strain to 0.9 or above. Therefore, the continuous hot-rolling mill including the two or more lower rolling stands, namely, the differential or very-small-diameter rolling stands, provided with the drive motors having the capacities considerably larger than those of the motors of the upper rolling stands is a reasonable, viable continuous hot-rolling mill capable of smoothly producing a hot-rolled, fine-grained steel sheet. Since the rolling stands of the upper rolling unit that roll the steel sheet at low drafts are provided with the drive motors having capacities considerably smaller than those of the drive motors of the rolling stands of the lower rolling unit, the continuous hot-rolling mill is economically advantageous. It is particularly advantageous in equipment cost and energy consumption to provide the last rolling stand with a motor having the maximum capacity for the aforesaid reasons.

The water-curtain cooling means are disposed not only at the exit of the last rolling stand but also at the respective exits of the plurality of rolling stands of the lower rolling unit. Thus, heat generated in the steel sheet by rolling at the rolling stands including the last rolling stand is removed effectively to maintain the steel sheet at a proper temperature, and the sheet is cooled strongly immediately after rolling by the last rolling stand to stop the growth of grains of the fine structure. Since the water-curtain cooling means pour cooling water over the entire width of the rolled steel sheet, the rolled steel sheet can be uniformly cooled.

Problems solved by technology

The conventional continuous hot-rolling mill has difficulties in producing a hot-rolled, fine-grained steel sheet having fine ferrite structures and high mechanical properties.

However, the conventional continuous hot-rolling mill having the rolling stands provided with motors having the capacities explained above has difficulty in carrying out such a rolling method.

Even if the lower rolling stands are provided with motors each having a relatively large capacity as mentioned in ii and iii), the lower rolling stands do not have a rolling capacity to roll the steel sheet at a high draft by using a high rolling force.

Even if the motor of the lower rolling stand has an excess capacity and the lower rolling stand is able to roll the steel sheet at a sufficiently high draft, it is impossible to solve problems relating to the meandering and shape deterioration of thin steel sheets

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