GB2089702A - Method of manufacturing stainless steel pipes - Google Patents

Abstract

A pipe is formed by passing a billet 2a sequentially through a two-roll Mannesmann piercing mill 3, a three- roll mandrel elongator 4, a multiple- stand continuous pipe rolling mill 6 of the mandrel bar restraining type and a high-reduction sizer 8. The sizer 8 is arranged on the exit side of the rolling mill 6, so that the shell 2c being rolled by the rolling mill is gripped by the sizer 8 which in turn withdraws the shell 2d being rolled from the mandrel bar 7 of the rolling mill 6, in such a manner that a tensile stress acts on the shell 2d extending between the rolling mill exit and the sizer entry during the simultaneous rolling of the shell by the rolling mill 6 and by the sizer 8. <IMAGE>

Description

SPECIFICATION Method of manufacturing seamless steel pipes The present invention relates to methods of manufacturing seamless steel pipes and more particularly to an improved method which employs a novel line arrangement covering a sequence of operations including from piercing to sizing thereby manufacturing product seamless steel pipes of wide range of sizes from starting material of the same size with high dimensional accuracy and efficiency.

The seamless steel pipe manufacturing methods heretofore known in the art may be divided broadly into two processes, that is, the Mannesmann mandress mill process used for producing small-diameter seamless steel pipes of 27.2 mm (1 inch) to 139.8 mm (51/2 inches) in outside diameter and the Mannesmann plug mill process used for producing medium-diameter seamless steel pipes of 152.4 mm (6 inches) to 406.4 mm (16 inches) in outside diameter.The former mandrel mill process produces a seamless steel pipe by rolling the starting material billet from a heating furnace through a Mannesmann piercing mill, a mandrel mill, a reheating furnace and a stretch reducer in this order, and the application of this process to the larger-diameter products is limited by the fact that the mandrel bar must be increased in length or the temperature drop due to the reduction in pipe wall thickness increases with increase in the outside diameter of pipes to be produced.On the other hand, the latter plug mill process produces a seamless steel pipe by rolling the similar billet through a Mannesmann piercing mill, a Mannesmann type elongator (second piercing mill), a plug mill, a reeler and a sizer, and the efficiency of this process deteriorates with decrease in the outside diameter of pipes to be produced by limitations due to the rolling length and the handling time.

As a result, there has existed in fact no process which is capable of efficiently producing pipes in the range of outside diameters (4 to 10 inches) which is intermediary between those of the two processes.

Also, with the above-mentioned prior art processes it has been known that the dimensional accuracy, particularly the accuracy of wall thickness of the produced seamless steel pipes is deteriorated by various factors such as the occurrence of a circumferential thickness deviation in the piercing mill, the reduced thickness deviation correcting effect of the elongator and the occurrence of a thickness deviation due to the elongator itself or the occurrence of a longitudinal thickness deviation in the mill, the butt thickness deviation, the occurrence of a pipe end thickness deviation in the stretch reducer, etc., in the case of the mandrel mill process. Thus, the resulting accuracy has been as low as + 12.5% with respect to the nominal wall thickness.

Moreover, where a multiple-stand continuous seamless steel pipe rolling mill is used in which the mandrel bar is restrained with respect to the axial relative movement during the rolling of a seamless steel pipe by the mandrel mill, it is necessary to use a separate equipment such as an extracting mill at a position downstream of the rolling mill so as to withdraw the rolled steel pipe (shell) during or after the rolling operation and usually the extracting mill of the loaded type with three stands or so is used. Thus, there is a disadvantage that it increasingly becomes difficult to provide a sufficient extracting force with increase in the length of the pipe to be rolled and this makes difficult particularly the rolling of the light-wall pipes.Moreover, the fact that the length of the table from the rolling mill to the following operation or the sizer is increased gives rise to another disadvantage that it is impossible to prevent the occurrence of such problems as an increase in the overall length of the mill line and a decrease in the temperature of the shell due to the increased transfer time up to the sizer, and thus it is obliged to arrange a reheating furnace between the rolling mill and the sizer.

It is therefore an object of the present invention to provide a seamless steel pipe manufacturing method which replaces as a third process the above-mentioned two prior art processes and which overcomes all of the foregoing deficiencies in the prior art.

It is another object of the invention to provide an improved such method which employs a multiple-stand continuous mandrel mill as a rolling mill.

It is still another object of the invention to provide such method which eliminates the provision of an extracting mill and a reheating furnace and which reduces the temperature drop during the rolling up to the sizing.

Thus, in accordance with one form of this invention the seamless steel pipe manufacturing method is so designed that a sizer is connected to the exit side of a multiple-stand continuous seamless steel pipe rolling mill ofthe mandrel bar restraining type so that the steel pipe being rolled by the rolling mill is gripped by the sizer and the steel pipe being rolled is withdrawn by the sizer from the mandrel bar of the rolling mill while simultaneously rolling the pipe by the sizer such that a tensile stress acts on the pipe or shell in the portion between the exit 6f the rolling mill and the entry end of the sizer. Thus, in this condition the tension that acts on the shell has a continuous relation in the portion extending from the rolling mill to the sizer.

In accordance with another form of the invention, the method is designed so that after a heated raw material billet has been pierced and rolled by a two-roll Mannesmann piercing mill with a stretch ratio of ss p S 3, the shell is subjected to the mandrel rolling by a three-roll mandrel elongator, and while performing a wall thickness reducing rolling by a multiple-stand continuous pipe rolling mill of the mandrel bar restraining type, a high degree of outside diameter reduction is applied to the shell while pulling it by a sizer connected to the exit side of the rolling mill with a minimum force of the order which is sufficient to withdraw the mandrel bar from within the shell being rolled.

In accordance with the invention, the piercing mill is not caused to share a large rolling reduction but it shares a rolling reduction such that the stretch ratio is less than those of the prior art processes and the elongator comprises a three-roll mill of the mandrel bar type. The following rolling is effected by a multiple-stand rolling mill of the mandrel bar restraining type, and the sizer applies a high degree of outside diameter reduction to the pipe with the minimum required stretch, thus cooperatively improving the dimensional accuracy, particularly the wall thickness accuracy of product as high as + 5% and remarkably improving the inner surface properties of product by virtue of the double-stage mandrel bar rolling, making the product well suited for use as an oil well pipe such as a casing pipe for oii well casing applications.Also, since the shares of working by the various mills can be combined rationally so as to allow display of the merits of these mills, it is possible to produce seamless steel pipes of good quality with a high yield by using inexpensive continuously cast billets as starting material, and by limiting the stretch of the sizer to the minimum requirement, it is possible to minimize the thickness deviation over the entire length as well as the longitudinal increase of the wall thickness with the resulting very remarkable improvement in the yield.

Further, in accordance with the invention the sizer is arranged just in the rear of the rolling mill such that the rolling of the shell is effected by the two mills side by side, with the result that the sizer provides a force sufficient to withdraw the shell from the mandrel bar during the rolling operation of the rolling mill and that the entire line length is reduced by more than several tens meters thus reducing the pass time between the mills and thereby reducing the temperature drop, eliminating the need to reheat the shell before the sizing and greatly contributing to the saving of energy.

The above and other objects and advantages of this invention will be made more apparent by the following detailed description taken in conjunction with the accompanying drawings.

Figure 7 is a perspective view for explaining the various operations according to a seamless steel pipe manufacturing method of this invention.

Figures2a, 2b and 2e are diagrams useful in explaining the working relation between a rolling mill and a sizer.

Referring to Figure 1, numeral 1 designates a heating furnace, and a starting material billet 2a is formed into a hollow shell 2b by a two-roll Mannesmann piercing mill 3. The reduction share of the piercing mill 3 is selected so that the stretch ratio Fp of the billet is less than 3, preferably 2.1 to 2.4, thus allowing full play to the specific characteristics of this type of piercing mill that it is possible to effect the piercing and rolling with a high degree of accuracy and efficiency to produce the pipes of about the medium wall thickness.When the hollow shell 2b is conveyed to a three-roll mandrel elongator 4 so that the rolling process with a stretch ratio = = 1.5 to 1.8 by a mandrel bar 5 is effected, high thickness deviation correcting effect of the three-roll mandrel bar elongator and the improved pipe inner surface properties due to the mandrel bar rolling are both ensured. The resulting shell 2c passing through the three-roll mandrel elongator 4 is fed into a multiple-stand continuous rolling mill 6 of the mandrel bar restraining type so that the shell 2c is rolled to obtain the desired wall thickness by providing a large stretch ratio (M = up to 5 or so) due to rolling.The mill 6 includes a mandrel bar 7 whose axial movement other than the stroke required for rolling is restrained at its tail end by a restraining mechanism 9, so that it is possible to roll long material by the relatively short mandrel bar 7 and the relative speeds of the mandrel bar7 and the shell 2carve maintained constant in the pipe lengthwise direction thus preventing large variations in the rolling condition and reducing the longitudinal thickness deviation. Of course, the inner surface properties of the shell are improved.

The rolling mill 6 is followed by a sizer 8 arranged in succession so that when the shell 2c is being rolled by the rolling mill 6, a rolled shell portion 2d of the shell 2c is pulled by the sizer 8 with a moderate applied stretch and thus a force acts which withdraws downstream the shell portion 2d from the mandrel bar 7. As a result, the rolling mill 6 is no longer required to maintain as large a clearance as heretofore required between the mandrel bar 7 and the shell inner surface in the final rolling area near its exit end, so that the rolling mill 6 is provided with the rolling condition which minimizes the circumferential thickness deviation and the production of lighter-wall pipes is made possible.

The sizer 8 provide a high degree of outside diameter reduction under the application of the above-mentioned minimum requirement stretch sufficient for withdrawing the shell 2d from the mandrel bar 7 and its conditions are set so that its stretch ratio us becomes 1.1 to 1.3. By this maintaining the stretch at a low value, it is possible to produce pipes of different outside diameters from the same hollow shell while ensuring a high degree of pipe dimensional accuracy and it is also possible to produce pipes of the same diameter with the reduced thickness deviation over the entire length.

The following Table shows a detailed exemplary distribution of the amounts of working according to the seamless steel pipe manufacturing method of this invention as well as those of the prior art methods. conventional conventional invention invention process process process process (mandrel mill) (plug mill) A B

starting material outside diameter (mm) 170 175 215 260 billet unit weight (Kg/m) 178 189 285 417 piercer exit outside diameter (mm) 176 185 220 265 (2-roll Mannesmann) exit wall thickness (mm) 16.0 18 27 31 unit weight (Kg/mm) 63 74 129 179 stretch ratio ( p) 2.82 2.56 2.21 2.33 elongator exit outside diameter (mm) 200 190 235 (3-roll mandrel type exit wall thickness (mm) 9 18 22 in invention unit weight (Kg/m) # 42 76 116 process A & B) stretch ratio ( E) 1.76 1.7 1.54 mandrell mill exit outside diameter (mm) 148 exit wall thickness (mm) 4.0 # unit weight (Kg/m) 14 stretch ratio ( m) 4.5 # plug mill exit outside diameter (mm) 193 # exit wall thickness (mm) # 6 unit weight (Kg/m) 28 stretch ratio ( pl) 1.5 invention invention conventional conventional process process process process A B (mandrel mill) (plug mill)

multiple-stand exit outside diameter (mm) 155 195 continuous pipe exit wall thickness (mm) 4.0 5.7 rolling mill of bar unit weight (Kg/m) # 15 27 restraining type stretch ratio ( M) # 5.06 4.3 reducer exit outside diameter (mm) 114.3 exit wall thickness (mm) 4.5 unit weight (Kg/m) 12 stretch ratio ( R) 1.17 reeler exit outside diameter (mm) 200 # # exit wall thickness (mm) 5.6 unit weight (Kg/m) 27 stretch ratio ( t) 1.04 sizer exit outside diameter (mm) 177.8 114.3 177.8 exit wall thickness (mm) 5.87 4.5 5.87 unit weight (Kg/m) 25 12 25 stretch ratio ( s) 1.08 1.25 1.12 As will be seen from the Table, in accordance with the present invention the same line arrangement permits the rolling and processing of both the larger outside diameter pipe of 114.3 mm produced by the conventional mandrel mill process and the smaller outside diameter pipe of 177.8 mm produced by the conventional plug mill process and the method of this invention should be noticed as a high efficiency process which is adapted to produce pipes of sizes in the range which is intermediate between those of the conventional processes. Also, in contrast to the overall stretch ratio of 14.8 according to the conventional mandrel mill process and the stretch ratio of 7.56 according to the conventional plug mill process, the processes A and B of this invention provide the stretch ratios of 23.7 and 16.7 which are respectively about two times the ratios of the corresponding conventional processes.Thus, the working ratio leaves a large margin so that the concentration of size of the starting material is possible and in particular the present invention is well suited for producing products of various sizes from the continuously cast billets. Moreover, the billet is continuously processed without being subjected to any intermediate reheating and thus the thermal loss can be reduced to a minimum.

Figures 2a, 2b and 2c show in due order the conditions in the rolling mill 6 and the sizer 8 during the rolling operation. More specifically, Figure 2 shows the conditions prior to the start of the rolling operation and Figure 2b shows the conditions during the rolling operation. Figure 2c shows the conditions after the shell has passed through the rolling mill. In the Figures, numeral 6 designates the multiple-stand continuous seamless steel pipe rolling mill, and 7 the rolling mill mandrel bar whose axial movement other than the stroke required for the rolling is restrained at its rear end by the restraining mechanism 9.Numeral 8 designates the roll type sizer arranged to immediately follow the exit side of the rolling mill 6, and positioned between the exit end of the rolling mill 6 and the entry end of the sizer 8 is a tension detector 10 for measuring the tension between the mills.

As shown in Figure 1 a, with the mandrel bar 7 being inserted, the hollow shell 2c is conveyed to the entry side of the rolling mill 6 so that the shell 2c is subjected successively to the mandrel rolling by the respective stands of the rolling mill 6 and then it is passed to its exit end. During the rolling operation, the mandrel bar 7 is held at its rear end by the restraining mechanism 9 and the rolling is effected with the mandrel bar 7 being restrained. The rolled shell 2d emerging from the exit end of the rolling mill 6 is directly passed into the entry stand of the sizer 8 so that the forward end shell 2d of the hollow shell 2c is simultaneously subjected to rolling reduction while the hollow shell 2c is being rolled by the rolling mill 6.Figure 2c shows the hollow shell 2c, the shell 2d and a sized steel pipe 2e (it is needless to say that they form the individual portions of the single pipe) and the positional relation of the mandrel bar 7 with respect to the respective mills in the condition described so far. In this rolling condition, if a compressive stress is applied to the shell portion 2d existing between the rolling mill exit and the sizer entry, the shell will be deformed. Thus, while monitoring the tension between the mills by the tension detector 10, the roll speed is distributed so that a tensile force tending to stretch is produced in the sizer 8 and thus a tensile stress is always applied. In this case, however, if any excessive tensile stress is applied, when the tail end of the shell passes through the exit stand of the rolling mill 6 as shown in Figure 2c, the wall thickness will be appreciably increased in the shell tail end portion between the mills. As a result, it is preferable to limit the speed distribution such that the roll peripheral speed of the sizer entry stand is higher by several percent than that of the rolling mill exit stand.

Claims (7)

1. A method of manufacturing seamless steel pipes wherein a pierced shell which is being rolled by a multiple-stage seamless steel pipe rolling mill of a mandrel bar restraining type is gripped by a sizer arranged in succession to an exit side of said rolling mill, and said pierced shell being rolled is then withdrawn by said sizer from a mandrel bar of said rolling mill in such a manner that a tensile stress acts on said pierced shell extending between said rolling mill exit and an entry of said sizer while simultaneously effecting rolling of said shell by said rolling mill and rolling of said shell by said sizer.

2. A method according to Claim 1, wherein an intermill tension between the exit of said rolling mill and the entry of said sizer is measured to monitor said tensile stress.

3. A method of manufacturing seamless steel pipes comprising the steps of: piercing and rolling a heated starting material billet with a stretch ratio u p S 3 by a two-roll Mannesmann piercing mill; subjecting said pierced shell to mandrel rolling by means of a three-roll mandrel elongator; and then while subjecting said shell to wall thickness reducing rolling by a multiple-stand continuous pipe rolling mill of a mandrel bar restraining type, causing a high degree of outside diameter reduction of said rolled shell by a sizer arranged in succession to an exit side of said rolling mill by pulling said rolled shell with as small a force as possible but sufficient to withdraw a mandrel bar of said rolling mill from within said rolled shell.

4. A method according to Claim 3, wherein the reduction is distributed such that said stretch ratio ftp of said piercing mill is in the range from 2.1 to 2.4.

5. A method according to Claim 3, wherein said elongator performs said mandrel bar rolling with a stretch ratio uE of 1.5 to 1.8.

6. A method according to Claim 3, wherein the reduction is distributed such that said sizer provides a stretch ratio uS of 1.1 to 1.3.

7. A method of manufacturing seamless steel pipes substantially as hereinbefore described with reference to the accompanying Drawings.