When a material to be rolled is rolled by a tandem rolling mill comprising a plurality of rolling mills (stands), such as a finishing mill in a hot rolling line, the operation of each stand is determined such that the sheet thickness, sheet width and the like of the material to be rolled on an exit side of a final stand meet a target condition. This operational condition of each stand is called a draft schedule (pass schedule) and has a large influence on the product quality, productivity and the like. It is therefore required to determine a proper draft schedule in accordance with the product.
The draft schedule of the tandem finishing mill in the hot rolling line is usually determined in a way that a rolling load is smaller in a stand in the latter stage (on a downstream side in a traveling direction of the material to be rolled), which is closer to a final product stage, in order to reduce roughness on the surface of a work roll and maintain favorable surface properties of a product. There is a rolling characteristic that even if the same rolling reduction is set in a stand in the earlier stage (on an upstream side in the traveling direction of the material to be rolled) and in the stand in the latter stage, a large rolling load is needed in the latter-stage stand which rolls a material to be rolled with a small sheet thickness. Therefore, in an ordinary draft schedule, rolling reduction is smaller in the latter-stage stand.
On the other hand, a steel material to be used for automobiles, structural materials, and the like is required to have excellent mechanical properties such as strength, workability, and toughness. In order to enhance these mechanical properties comprehensively, it is effective to refine the crystal grains of a hot-rolled steel sheet. If the crystal grains of the hot-rolled steel sheet are refined, it is possible to manufacture a high-strength hot-rolled steel sheet having excellent mechanical properties even if the amount of alloy elements added is reduced.
As a method for refining the crystal grains of the hot-rolled steel sheet, it is known that large reduction rolling (finish rolling in which the rolling reduction in the latter-stage stand is increased) is carried out especially in the latter stage of hot finish rolling to cause large deformation in the austenite grains and to increase a dislocation density, thereby obtaining refined ferrite grains after cooling. In order to manufacture a hot-rolled steel sheet having fine crystal grains (hereinafter, referred to as “fine-grained steel”) by this method, it is necessary to increase rolling reduction in the latter-stage stand of the tandem finishing mill in the hot rolling line more than in conventional cases. Accordingly, in order to manufacture the fine-grained steel, it is necessary to determine a draft schedule different from the conventional ones and to control operation of the tandem finishing mill differently from the conventional cases.
Further, especially when carrying out large reduction rolling on a hard material that has a large deformation resistance at a time of being rolled, a rolling load becomes significantly large, and a gap between the upper and lower work rolls due to the elastic deformation of the rolling mill (hereinafter, the gap being referred to as a “rolling mill gap”) also becomes large. Therefore, in order to obtain a target exit side sheet thickness, that is, in order to accord the rolling mill gap under the imposition of the rolling load with the target sheet thickness, the gap before the imposition of the rolling load needs to be set small in advance. When the rolling load is large and the target sheet thickness is small, the pre-set gap theoretically becomes minus. In an actual situation, the upper and lower work rolls are contacted with each other (hereinafter, this state is referred to as a “kiss roll”.) and are further tightened by a screw-down device to be given a load; and the rolling mill is elastically deformed in advance. In usual hot rolling, the kiss roll itself is rarely needed and the load is minute, so there will not be a problem. However, in the case of the above mentioned fine-grained steel rolling, a tremendously large kiss roll load is generated, thus causing troubles in equipment maintenance. For example, a roll drive system component breaks due to torque circulation attributed to a minute difference in a circumferential speed of the upper and lower work rolls; or when the axes of the upper and lower work rolls are crossed or skewed in the horizontal plane, a roll bearing breaks due to an axial force (hereinafter referred to as a “thrust force”) between the rolls. Both of these are caused by direct contact of the upper and lower work rolls, and do not occur if there is a material being rolled between the work rolls, that is, during rolling.
In order to protect the rolling mill, it is necessary to take measures to inhibit the torque circulation or the thrust force even when the kiss roll occurs, or to reduce the kiss roll load itself. However, limiting the pre-tightening in order to reduce the kiss roll load makes it impossible to obtain a target sheet thickness, therefore requiring special operational control of the rolling mill.
As a measure to solve the above problems, Non-Patent Document 1 for example discloses a method in which a lubricant is applied to rolls during kiss roll to reduce a friction force between the rolls. Further, as a technique related to operational control of a rolling mill, Patent Document 1 for example discloses a hot finish rolling method wherein in a hot finishing mill constituted by a plurality of stands, a gap in at least one stand among the continuously arranged stands is enlarged, the method comprising: a first step of starting modification of the gap in the stand when a front end portion of the sheet being rolled that is transported reaches the work rolls of the stand whose gap is to be modified; a second step of rolling the front end portion of the sheet being rolled, into a tapered shape by carrying out the gap modification continuously over time that has been started in the first step, until a preset gap is achieved; and a third step of rolling a constant portion of the sheet being rolled in a constant thickness by keeping the gap constant, after the modification into the preset gap has been done in the second step.