The present invention relates to a method of manufacturing a seamless pipe or tube using a mandrel mill that is capable of not only enhancing a elongation ratio of a pipe or tube, but also suppressing occurrence of a pinhole defect and a method for manufacturing the seamless pipe or tube using the same.

As a mandrel mill, which is a piece of equipment for manufacturing a seamless pipe or tube, there have been conventionally used a 2-roll type mandrel mill in which two grooved rolls opposed are disposed in each rolling stand, and between adjacent rolling stands, the rolling directions of the grooved rolls are alternately arranged, being 90° shifted, and also a 3-roll type mandrel mill in which three grooved rolls are disposed in each rolling stand such that the angle formed by the rolling directions of the grooved rolls is 120°, and between adjacent rolling stands, the rolling directions of the grooved rolls are alternately arranged, being 60° shifted. Hereinafter, "pipe or tube "is referred to as "pipe" when deemed appropriate.

US 3,842,635 discloses a tube rolling mill for producing tubing having two sets of three rolls and which relates to method of manufacturing a seamless tube according to the preamble of claim 2.

JP 7214109 A discloses a method of manufacturing a seamless steel tube according to the preamble of claim 1, using a two-roll mill of plural stands and a four-roll mill in the stand of the final stage.

JP 9201605 A discloses a mill having using rolls with plural grooves on the roll. This rolling mill is not related to a mandrel mill.

DE 1017122 B a high roll diameter ratio is described for a single roll assembly.

Herein, in order to enhance the efficiency of manufacturing a seamless pipe, it is desirable that the elongation ratio of a pipe for a mandrel mill (i.e., the ratio of wall thickness of the pipe on the inlet side of the mandrel mill to that on the outlet side thereof) be enhanced as much as possible. This is because, if the elongation ratio of the pipe is enhanced, a pipe with a longer length can be obtained by drawing and rolling, and thus more product stocks can be obtained in one step of drawing and rolling.

In order to enhance the elongation ratio of the pipe for a mandrel mill, it can be conceived, for example, that the wall thickness rolling reduction rate for each of the rolling stands (especially for the upstream rolling stands) is set at a high value. However, if the wall thickness rolling reduction rate is set at too high a value, there arises a possibility that a through-hole called a pinhole defect occurs in a portion of the pipe that is opposed to the flange of the grooved roll, resulting from such a cause as the flow (metal flow) of the pipe material which is rolled at the groove bottom of the grooved roll being not sufficiently spread to the flange side of the grooved roll. Alternatively, for enhancing the elongation ratio of the pipe, it can be conceived to increase the number of rolling stands without setting the wall thickness rolling reduction rate for each of the rolling stands at too high a value. However, increasing the number of rolling stands will increase the equipment cost in proportion, and also require increased manpower for maintenance, thus a practical number of rolling stands is said to be about 5 to 8. Thus, with the conventional mandrel mill, for example, a mandrel mill provided with five rolling stands, the elongation ratio of the pipe is generally held to under 4. Therefore, a mandrel mill which allows the elongation ratio of the pipe to be further enhanced for increasing the efficiency of manufacturing a seamless pipe has been demanded.

The present invention has been made in view of such a problem with the prior art, and it is an object of the present invention to provide methods of manufacturing a seamless pipe using a mandrel mill that is capable of not only enhancing the elongation ratio of the pipe, but also suppressing a pinhole defect from occurring.

In order to solve the aforementioned subject, the present invention provides methods of manufacturing a seamless pipe or tube according to claim 1 or claim 2.

In order to solve the aforementioned subject, the present inventor has made a keen study, and has found the fact that, if the roll diameter for the first rolling stand and the second rolling stand is increased to a predetermined value or over (accordingly, if the roll diameter ratio is increased to a predetermined value or over), not only the elongation ratio of the pipe or tube can be enhanced, but also occurrence of a pinhole defect can be suppressed. The present invention has been completed on the basis of such a new finding of the present inventor.

Namely, the first means of the present invention provides a mandrel mill comprising a plurality of rolling stands in which two grooved rolls are disposed, respectively, the mandrel mill being characterized in that the roll diameter ratio for the first rolling stand and the second rolling stand is set at a value of 4.6 or over.

According to the present invention, the "first rolling stand" in the first means refers to a rolling stand which is disposed first when counted from the inlet side of the mandrel mill. Likewise, the "second rolling stand" in the first means refers to a rolling stand which is disposed second when counted from the inlet side of the mandrel mill. In addition, the "roll diameter ratio" for the first means refers to a ratio expressed by D_R/D_C, where D_R is the roll diameter (the smallest roll diameter) at the groove bottom of each grooved roll which is disposed in the rolling stand, and Dc is the distance between the groove bottoms of the grooved rolls.

Also, the second means of the present invention provides a mandrel mill comprising a plurality of rolling stands in which three grooved rolls are disposed, respectively, the mandrel mill being characterized in that the roll diameter ratio for the first rolling stand and the second rolling stand is set at a value of 2.8 or over.

The meanings of the "first rolling stand" and the "second rolling stand" for the second means are the same as those for the first means that are described above. In addition, the "roll diameter ratio" according to the present invention for the second means refers to a ratio expressed by D_R/D_C, where D_R is the roll diameter (the smallest roll diameter) at the groove bottom of each grooved roll which is disposed in the rolling stand, and D_C/2 is the distance between the groove bottom of each grooved roll and the pass center.

According to the present invention, not only the elongation ratio of the pipe or tube can be enhanced, but also occurrence of a pinhole defect can be suppressed. Therefore, the efficiency of manufacturing a seamless pipe or tube can be increased without causing poor-quality rolling.

• Figure 1A is a longitudinal sectional view illustrating a schematic configuration of a rolling stand constituting a two-roll type mandrel mill according to a first embodiment of the present invention;
• Figure 1B is a graph obtained by plotting the lower limit values of roll diameter ratio with which no pinhole defects occurred in a pipe when drawing and rolling thereof was performed with the wall thickness rolling reduction rate of the pipe and the roll diameter ratio being set at various values in the first rolling stand and the second rolling stand of the mandrel mill according to the first embodiment of the present invention;
• Figure 2A is a longitudinal sectional view illustrating a schematic configuration of a rolling stand constituting a three-roll type mandrel mill according to a second embodiment of the present invention; and
• Figure 2B is a graph obtained by plotting the lower limit values of roll diameter ratio with which no pinhole defects occurred in a pipe when drawing and rolling thereof was performed with the wall thickness rolling reduction rate of the pipe and the roll diameter ratio being set at various values in the first rolling stand and the second rolling stand of the mandrel mill according to the second embodiment of the present invention.

Hereinbelow, embodiments of the present invention will be explained with appropriate reference to the attached drawings.

Figure 1A is a longitudinal sectional view illustrating a schematic configuration of a rolling stand constituting a two-roll type mandrel mill which could be used in the method according to a first embodiment of the present invention. As shown in Figure 1A, a mandrel mill comprises a plurality of rolling stands in which two grooved rolls R11 and R12 are disposed. And, the mandrel mill is characterized in that the roll diameter ratio D_R/D_C for the first rolling stand and the second rolling stand is set at a value of 4.6 or over.

In other words, as shown in Figure 1A, if the roll diameter (the smallest roll diameter) at the groove bottom B of each grooved roll R11, R12 which is disposed in the first rolling stand is D_R, and the distance between the groove bottoms B of the grooved rolls R11, R12 is D_C, the roll diameter ratio expressed by D_R/D_C is set at 4.6 or over. This description is also applicable to the second rolling stand. For the other rolling stands (for example, a third rolling stand to a fifth rolling stand in case where the mandrel mill comprises five rolling stands in total), there is no need for setting the roll diameter ratio at the above-mentioned value, but the roll diameter ratio may be set at a value which has been conventionally used (for example, 3 or under). Hereinbelow, the reason why the roll diameter ratio D_R/D_C for the first rolling stand and the second rolling stand is set at a value of 4.6 or over will be explained with reference to Figure 1B.

Figure 1B is a graph obtained by plotting the lower limit values of roll diameter ratio with which no pinhole defects occurred in a pipe when drawing and rolling thereof was performed with the wall thickness rolling reduction rate of the pipe and the roll diameter ratio being set at various values in the first rolling stand and the second rolling stand of the mandrel mill. In other words, the graph shown in Figure 1B indicates that, if the wall thickness rolling reduction rate and the roll diameter ratio are set at a value in the region under an approximate straight line L shown in Figure 1B, a pinhole defect will occur in the pipe, while, if the wall thickness rolling reduction rate and the roll diameter ratio are set at a value in the region above the approximate straight line L, no pinhole defect will occur in the pipe. The wall thickness rolling reduction rate, according to the present invention, which is represented by the abscissa in Figure 1B, is a rate defined by the following formula (1): $$\begin{array}{l}\mathrm{Wall\; thickness\; rolling\; reduction\; rate}\\ =\left({\mathrm{t}}_{\mathrm{i}\mathrm{1}}-{\mathrm{t}}_{\mathrm{o}\mathrm{2}}\right)/{\mathrm{t}}_{\mathrm{i}\mathrm{1}}=\mathrm{1}-{\mathrm{t}}_{\mathrm{o}\mathrm{2}}/{\mathrm{t}}_{\mathrm{i}\mathrm{1}}\end{array}$$ where t_i1 is the wall thickness of the pipe on the inlet side of the first rolling stand and t_o2 is the wall thickness of the pipe on the outlet side of the second rolling stand.

Herein, t_i1/t_o2, which is a reciprocal of t_o2/t_i1 given on the right side of the above formula (1), provides a value corresponding to the elongation ratio of the pipe for the first rolling stand and the second rolling stand. If this t_i1/t_o2 is set at a value of at least 4, the value of the elongation ratio of the pipe (the ratio of wall thickness of the pipe on the inlet side of mandrel mill to that on the outlet side thereof) for the entire mandrel mill will be 4 or over, which is larger than conventional. When t_i1/t_o2 = 4, the wall thickness rolling reduction rate defined by the above formula (1) will be 0.75. And, as can be seen from Figure 1B, when the wall thickness rolling reduction rate = 0.75, setting the roll diameter ratio D_R/D_C at a value of 4.6 or over will allow occurrence of a pinhole defect to be suppressed. In other words, if the roll diameter ratio D_R/D_C is set at a value of 4.6 or over, not only the elongation ratio of the pipe for the first rolling stand and the second rolling stand can be enhanced to 4 (accordingly, the elongation ratio of the pipe for the entire mandrel mill can be enhanced to a value of 4 or over, which is larger than conventional), but also occurrence of a pinhole defect can be suppressed.

From the reason as described above, the roll diameter ratio D_R/D_C for the first rolling stand and the second rolling stand of the mandrel mill is set at a value of 4.6 or over. Thereby, with the mandrel mill, not only the elongation ratio of the pipe can be enhanced, but also occurrence of a pinhole defect can be suppressed. With the roll diameter ratio D_R/D_C being increased (the roll diameter D_R being increased), the length of contact made between the pipe which is being subjected to drawing and rolling and the grooved roll R11, R12 (the contact length of the pipe along the direction of axis thereof) is increased. It can therefore be considered that, even if the elongation ratio (the wall thickness rolling reduction rate) is set at a high value, the flow (metal flow) of the pipe material is sufficiently spread to the flange side of the grooved roll R11, R12 during the drawing and rolling, whereby occurrence of a pinhole defect can be suppressed.

Figure 2A is a longitudinal sectional view illustrating a schematic configuration of a rolling stand constituting a three-roll type mandrel mill which could be used in the method according to a second embodiment of the present invention. As shown in Figure 2A, a mandrel mill comprises a plurality of rolling stands in which three grooved rolls R21 R22 and R23 are disposed. And, the mandrel mill is characterized in that the roll diameter ratio D_R/D_C for the first rolling stand and the second rolling stand is set at a value of 2.8 or over.

In other words, as shown in Figure 2A, if the roll diameter (the smallest roll diameter) at the groove bottom B of each grooved roll R21 to R23 which is disposed in the first rolling stand is D_R, and the distance between the groove bottom B of each grooved roll R21 to R23 and the pass center O (the pass line center for the pipe as the workpiece) is D_C/2, the roll diameter ratio expressed by D_R/D_C is set at 2.8 or over. This description is also applicable to the second rolling stand. For the other rolling stands (for example, a third rolling stand to a fifth rolling stand in case where the mandrel mill comprises five rolling stands in total), there is no need for setting the roll diameter ratio at the above-mentioned value, but the roll diameter ratio may be set at a value which has been conventionally used (for example, 3 or under). Hereinbelow, the reason why the roll diameter ratio D_R/D_C for the first rolling stand and the second rolling stand is set at a value of 2.8 or over will be explained with reference to Figure 2B.

Figure 2B is a graph obtained by plotting the lower limit values of roll diameter ratio with which no pinhole defects occurred in a pipe when drawing and rolling thereof was performed with the wall thickness rolling reduction rate of the pipe and the roll diameter ratio being set at various values in the first rolling stand and the second rolling stand of the mandrel mill. In other words, the graph shown in Figure 2B indicates that, if the wall thickness rolling reduction rate and the roll diameter ratio are set at a value in the region under an approximate straight line L shown in Figure 2B, a pinhole defect will occur in the pipe, while, if the wall thickness rolling reduction rate and the roll diameter ratio are set at a value in the region above the approximate straight line L, no pinhole defect will occur in the pipe. The wall thickness rolling reduction rate, which is represented by the abscissa in Figure 2B, is a rate defined by the above formula (1), as the first embodiment.

And, as can be seen from Figure 2B, when the wall thickness rolling reduction rate = 0.75, setting the roll diameter ratio D_R/D_C at a value of 2.8 or over will allow occurrence of a pinhole defect to be suppressed. In other words, if the roll diameter ratio D_R/D_C is set at a value of 2.8 or over, not only the elongation ratio of the pipe for the first rolling stand and the second rolling stand can be enhanced to 4 (accordingly, the elongation ratio of the pipe for the entire mandrel mill can be enhanced to a value of 4 or over, which is larger than conventional), but also occurrence of a pinhole defect can be suppressed.

From the reason as described above, the roll diameter ratio D_R/D_C for the first rolling stand and the second rolling stand of the mandrel mill according to the present embodiment is set at a value of 2.8 or over. Thereby, with the mandrel mill according to the present embodiment, not only the elongation ratio of the pipe can be enhanced, but also occurrence of a pinhole defect can be suppressed. With the roll diameter ratio D_R/D_C being increased (the roll diameter D_R being increased), the length of contact made between the pipe which is being subjected to drawing and rolling and the grooved roll R21, R22 and R23 (the contact length of the pipe along the direction of axis thereof) is increased. It can therefore be considered that, even if the elongation ratio (the wall thickness rolling reduction rate) is set at a high value, the flow (metal flow) of the pipe material is sufficiently spread to the flange side of the grooved roll R21, R22 and R23 during the drawing and rolling, whereby occurrence of a pinhole defect can be suppressed.

With the mandrel mill, if the roll diameter ratio D_R/D_C for the first rolling stand and the second rolling stand is set at too high a value (the roll diameter D_R is set at too high a value), grooved rolls having a large diameter will be required, resulting in an increased equipment cost. In addition, the distance between rolling stands (the distance from the first rolling stand to the second rolling stand and that from the second rolling stand to the third rolling stand) will be increased, resulting in the length of the non-steady portion of the pipe end (the portion of the pipe end that is subjected to drawing and rolling with both pipe ends being not constrained by the grooved rolls) being increased, which may deteriorate the quality of the pipe. Further, with the distance between rolling stands being increased, a continuous length of mandrel bar will be required, resulting in the equipment cost being increased. In order to avoid the above-mentioned undesirable effects, it is preferable that the roll diameter ratio D_R/D_C for the first rolling stand and the second rolling stand be set at a value as close to the lower limit value as possible.