EP0159386B1

EP0159386B1 - Method of controlling mill pacing

Info

Publication number: EP0159386B1
Application number: EP84104769A
Authority: EP
Inventors: Kazuyuki Sakurada; Takanori Fujiwara; Yutaka Funyu
Original assignee: Kawasaki Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1984-04-27
Filing date: 1984-04-27
Publication date: 1989-03-29
Also published as: DE3477475D1; EP0159386A1

Description

This invention relates to a method of controlling mill pacing as defined in the precharacterizing part of the claim.
In a rolling equipment for seamless steel pipes, shapes and the like, a multiplicity of pieces to be rolled are present in a rolling line consisting of a heating furnace and a plurality of rolling mill stands. Particularly, as compared with plate rolling, in seamless steel pipe rolling, the number of rolling mill stands under control as well as the number of pipes is numerous. Therefore it has been very difficult to reduce the cycle time. In the aforesaid rolling equipment of the prior art the techniques of controlling the respective rolling mill stands and the heating furnace have relied on the experience and the skill of operators. Therefore it has been impossible to improve the productivity to an ideal extremity.
A method of the kind given above is known from JP-A-54/127812. This method contemplates that the relations between the elapsed time and the temperature of piercing are sought for the heating furnace and the piercer to thereby control the furnace temperature; this method is restricted to the piercing stage, such that the conditions for a group of mills downstream of the piercer are not taken into account.
The object of the present invention is to provide a method of controlling a mill pacing of the kind given above wherein the operation conditions of a rolling line as a whole is optimized to improve the productivity.
This object is achieved by a method of controlling mill pacing in a rolling equipment according to a progressively revisable schedule wherein a plurality of rolling mill stands are provided downstream of a heating furnace in which a plurality of pieces rest on a furnace hearth moving together with the pieces for successive extraction of said pieces from said furnace, said method being characterized by performing for each piece in turn at position n in the schedule the steps of

a) estimating a maximum allowable waiting time r_x for said piece n at the entry to each mill stand x;
b) calculating an expected waiting time h_x at the entry to mill stand x from the following equation:

wherein MCTx(i) is an estimated rolling cycle time (MCT) at a rolling mill stand x for a piece i counted in scheduled sequence from the most recent piece (1) to begin rolling, T_n is a scheduled extraction time of a piece n in the schedule for extraction from the heating furnace and T₁ is an actual time at which the most recent piece to begin rolling in rolling mill stand x was extracted from the heating furnace;
c) calculating a difference f_x between the predetermined maximum allowable waiting time r_x and the expected waiting time h_x from the following equation:
d) determining the maximum difference F for each of the rolling mills x from 1 to m from the following equation:
e) establishing that the maximum allowable waiting time r_x is met if F≤0 and is not met if F>0;
f) continuing with step (g), if r_x is met or estimating another maximum allowable waiting time r_x on the basis of a revised rolling schedule and repeating the steps (a) through (f) if r_x is not met;
g) calculating a furnace neck extraction cycle time CT_m for each particular piece m from m=1 to n as a function of the type of material of piece m, a diameter D_m of the piece m and a length L_m of the piece m from the following equation:
h) determining a critical heating cycle time HCT necessary for heating to a predetermined condition each piece m=₁ to n to be extracted from the heating furnace using the following equation:
i) determining if the critical heating cycle time HCT is compatible with the scheduled time interval between the extraction of the last piece and the next following piece; and
j) extracting the piece at the critical heating cycle time HCT if the critical heating cycle time is not greater than the time interval between the extraction of the last piece and the next following piece or estimating another maximum allowable waiting time r_x on the basis of a revised rolling schedule and repeating steps (a) through (j) if the critical heating time HCT is greater than said time interval.

This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a diagram of control system showing an embodiment, in which the present invention is applied to a rolling equipment for seamless steel pipes;
Fig. 2 is a flow chart showing control steps of a calculating means in the above embodiment;
Fig. 3 is an explanatory view showing the definition of a rolling cycle time in the above embodiment;
Fig. 4A is a histogram showing the relationship between the loss time and the number of pieces in the equipment under critical condition in a conventional system; and
Fig. 4B is a histogram showing the relationship between the loss time and the number of pieces in the equipment under critical condition in the method according to the present invention.

Fig. 1 is a diagram of control system, showing an embodiment, in which the present invention is applied to a rolling equipment for seamless steel pipes. More specifically, this rolling equipment is a rolling line for seamless steel pipes according to the Mannessmann-plug mill system, wherein a piece, which has been heated in a heating furnace 11, is pierced and rolled in a piercer 12, rolled for pipe-expanding and elongating the piece in an elongator 13, rolled to a length of a pipe having a wall thickness substantially equivalent to the product wall thickness in a plug mill stand 14, the inner and outer surfaces of the pipe-shaped piece are polished by two reelers 15and the wall thickness reduction and pipe-expanding to a certain extent are effected, and the piece is finished to be a pipe having an outer diameter of a predetermined value in a sizer 16, thus enabling to provide a final product. In addition, the heating furnace 11 is of a rotary furnace hearth type, in which a plurality of pieces are rested on the furnace hearth moving together with the pieces, such that they can be successively extracted.
In the above rolling equipment, the heating furnace 11 is controlled by a heating furnace control device 21 and the mill stands 12 through 16 are controlled by respective rolling mill control devices 22 through 26. An actual result of extraction of the piece obtained in the heating furnace control device 21 and actual results of rolling of the pieces obtained in the respective rolling mill control devices 22 through 26 are transmitted to a calculating means 27. As will be described below, the calculating means 27 determines the optimal extraction time in the heating furnace 11 and makes it possible to transmit the extraction time to the heating furnace control device 21.
The operation of the calculating means 27 is shown in Fig. 2. More specifically, the calculating means 27 calculates and estimates respective rolling cycle times MCT between the start of rolling in the respective rolling mill stands 12 through 16 of a piece to be subsequently extracted from the heating furnace 11 and the completion of the preparation for rolling of the following piece on the basis of a rolling schedule. Here, the rolling cycle time MCT is defined as a period of time between the time of starting the rolling and time of completing the preparation for the rolling as indicated by a rolling starting signal P₁ and a following piece rolling readiness signal p.2 in Fig. 3. Then, after pierced pieces of the same batch with similar material quality, dimensions and the like as the piece to be subsequently extracted have been passed through the mill stands 12 through 16, respective estimated cycle times MCT are calculated on the basis of actual result cycle times MCT collected in the respective rolling mill control devices 22 through 26. Whereas, before pierced pieces of the same batch as the subsequent pieces have not yet been passed through the mill stands 12 through 16, the respective estimated rolling cycle times MCT are calculated on the basis of rolling speeds, rolling lengths, required idle times and the like in the respective mill stands 12 through 16. More specifically, the aforesaid rolling cycle time MCT is the truly required rolling cycle time in the rolling mill stand under the control. The period of time between the completion of preparation for the following piece and the start of rolling is a loss time since the rolling mill stand under the control then remains idle. Therefore, it is desirable that the loss times be eliminated in all of the rolling mill stands; however, the loss times cannot be eliminated at each of the rolling mill stands, because the rolling cycle times MCT are different from one rolling mill stand to another. However, if the loss time is eliminated in the rolling mill stand (equipment under critical condition) having the largest rolling cycle time MCT, then it becomes possible to improve the productivity to the utmost under given conditions. In other words, this means that the subsequent piece should be extracted from the heating furnace 11 according to the rolling cycle time of one of the mill stands 12 through 16, which has the largest rolling cycle time MCT. In consequence, the calculating means 27 adds the largest value of the rolling cycle times MCT relating to the aforesaid subsequent piece to be extracted, i.e. an assumed extraction cycle time to the latest actual extraction time of the heating furnace 11 transmitted from the heating furnace control device 21, to thereby estimate an extraction time of the subsequent piece to be extracted.
Here, if the estimated extraction cycle time determined from the respective rolling cycle times MCT as described above is too short, then the loss times in the respective mill stands 12 through 16 are decreased, however, there occur some cases where materials are retarded immediately before the mill stands 12 through 16, thus leading to deteriorated product quality and considerably worn rolling tools such as reduction rolls due to lowered temperature of the piece. Therefore, the calculating means 27 previously determines allowable retard periods of time for the pieces immediately before the mill stands 12 through 16, confirms that the presupposed retard periods of time of the subsequent pieces to be extracted immediately before the mill stands 12 through 16 are within the range of the aforesaid allowable retard periods of time, and when the estimated retard periods of time are beyond the range of the allowable retard periods of time, the calculating means 27 performs again the calculation and estimation of respective rolling cycle times MCT and the provisional calculation of extraction time.
More specifically, the aforesaid allowable retard periods of time will be determined in the following manner. To cite a seamless steel pipe for example, a theoretical radiation calculation formula (the relationship of the lowered value of the temperature of a pipe material with the elapsed time) on the dimensions (outer diameter and wall thickness) of the pipe materials in the respective mill stands 12 through 16 is sought, while, the relationships between the temperature of the pipe material and defects on the inner surface of the pipe material and between the temperature of the pipe material and wear of the rolling tools, etc. are sought on the basis of the actual results, and it is determined within what maximum retard period of time the rolling can be started in order to roll the pipe material within the range of suitable temperature.
Furthermore, the calculation of the estimated retard periods of time immediately before the mill stands 12 through 16 and the confirmation of that the estimated retard periods of time are within the range of the allowable retard period of time are carried out in the following manner. Namely, if an estimated rolling cycle time MCT of a piece No. i counted from the latest piece (1) to be started for rolling in a rolling mill No. x is made to be MCTx(i), a target extraction time of the subsequent piece (n) to be extracted from the heating furnace 11 is made to be Tn and an actual result time when the latest piece to be started for rolling in the rolling mill stand No. x is extracted from the heating furnace 11 is made to be T₁, then an estimated retard period of time h_x of the subsequent material to be extracted immediately before the rolling mill No. x is sought through the following equation.
Subsequently, an allowable retard period of time in the rolling mill stand No. x is made to be r_x and a difference f_x between r_x and the estimated retard period of time h_x is sought through the following equation.
Further, calculation is made through the following equation for the rolling mill stands from No. 1 to the final one (No. m). And, if FZO, then it is judged that extraction can be made, and, if F>0, then it is judged that extraction cannot be made.
As described above, when the subsequent pieces to be extracted are conveyed to the mill stands 12 through 16, the calculating means 27 calculates and estimates the periods of time for awaiting the rolling immediately before the mill stands 12 through 16, i.e. retard periods of time.
When the retard periods of time are within the range of allowable values for all of the rolling mill stands, it is judged that extraction can be made, and when the retard period of time exceeds the allowable retard period of time in any one of the rolling mill stands, it is judged that extraction cannot be made.
However, when the subsequent piece is extracted from the heating furnace 11 as described above, if the extraction cycle time is short, then such a case occurs in the heating furnace 11 that the piece is not satisfactorily and uniformly heated, thus causing undesirable influence to the product quality. Then, the calculating means 27 calculates a heating critical cycle time HCT sufficient for applying heat of a predetermined value to the respective pieces including the subsequent piece to be extracted and the latest piece to be extracted, which are present in the heating furnace 11, on the basis of the rolling schedule. More specifically, the calculating means 27 calculates the heating critical cycle time sufficient for applying heat of the predetermined value to the respective pieces being present in the heating furnace 11 through the following equation.
The CT_m above is sought through the equation (5) which will be shown below.
where CT_m is a furnace neck extraction cycle time for a piece of batch No. m counting in the order of extraction, D_m a diameter of the piece of lot No. m, and L_m a length of the piece of lot No. m.
Further, when the subsequent piece is extracted at the assumed extraction time, the calculating means 27 judges whether or not the aforesaid heating critical cycle time HCT is secured between the subsequent piece to be extracted and its preceding piece to be extracted. When it is judged that the heating critical cycle time HCT can be secured, the calculating means 27 transmits the aforesaid assumed extraction time to the heating furnace control device 21 as the optimal extraction time for the subsequent piece to be extracted. When it is judged that the aforesaid heating critical cycle time HCT is not secured, the calculating means 27 repeats the calculation and estimation of the respective rolling cycle times MCT, assumption of the extraction time, confirmation that the estimated retard period of time is within the allowable retard period of time and confirmation that the heating critical cycle time can be secured.
In addition, the rolling start signal necessary for determining the rolling cycle times MCT in the mill stands 12 through 16 and the subsequent piece rolling readiness signal are determined as shown in Table 1.
Figs. 4A and 4B are histograms showing the relationship between the loss time and the number of pieces N in the equipment under critical condition in the operating system according to the prior art and in the operating system in the method according to the present invention, respectively, when the pieces each having a diameter of 230 mm and a length of 1,535 mm are loaded into the heating furnace in two rows and rolled into steel pipes each having an outer diameter of 273.6 mm, a wall thickness of 6.35 mm and a length of 11,705 mm. As apparent from Figs. 4A and 4B, an average loss time in the operating system according to the prior art is 1.08 sec, whereas an average loss time in the operating system performing the method according to the present invention is 0.33 sec, thus remarkably improving the productivity.
The present invention is applicable not only to the production line for seamless steel pipes in accordance with the Mannesmann-plug mill system but also to rolling equipment for seamless steel pipes according to the Mandrel mill system, Assel mill system and the like. Further, the present invention is applicable to rolling equpiment for rolling bar steel, wire steel and the like. Furthermore, the present invention is applicable irrespective of the number of rolling mill stands, and on the contrary, the larger the number of rolling mill stands is, the greater are the results which can be attained.

Claims

A method of controlling mill pacing in a rolling equipment according to a progressively revisable schedule wherein a plurality of rolling mill stands (12-16) are provided downstream of a heating furnace (11) in which a plurality of pieces rest on a furance hearth moving together with the pieces for successive extraction of said pieces from said furnace (11), characterized by performing for each piece in turn at position n in the schedule the steps of
a) estimating a maximum allowable waiting time r), for said piece n at the entry to each mill stand x;

b) calculating an expected waiting time h_x at the entry to mill stand x from the following equation:
wherein MCTx(i) is an estimated rolling cycle time (MCT) at a rolling mill stand x for a piece i counted in scheduled sequence from the most recent piece (1) to begin rolling, T_n is a scheduled extraction time of a piece n in the schedule for extraction from the heating furnace (11) and T₁ is an actual time at which the most recent piece to begin rolling in rolling mill stand x was extracted from the heating furnace (11);

c) calculating a difference f_x between the predetermined maximum allowable waiting time r_x and the expected waiting time h_x from the following equation:

d) determining the maximum difference F for each of the rolling mills x from 1 to m from the following equation:

e) establishing that the maximum allowable waiting time r_x is met if F≤0 and is not met if F>0;

f) continuing with step (g), if r_x is met or estimating another maximum allowable waiting time r_x on the basis of a revised rolling schedule and repeating the steps (a) through (f) if r_x is not met;

g) calculating a furnace neck extraction cycle time CT_m for each particular piece m from m=1 to n as a function of the type of material of piece m, a diameter D_m of the piece m and a length L_m of the piece m from the following equation:

h) determining a critical heating cycle time HCT necessary for heating to a predetermined condition each piece m=1 to n to be extracted from the heating furnace (11) using the following equation:

i) determining if the critical heating cycle time HCT is compatible with the scheduled time interval between the extraction of the last piece and the next following piece; and

j) extracting the piece at the critical heating cycle time HCT if the critical heating cycle time is not greater than the time interval between the extraction of the last piece and the next following piece or estimating another maximum allowable waiting time r_x on the basis of a revised rolling schedule and repeating steps (a) through (j) if the critical heating time HCT is greater than said time interval.