JP6737321B2 - Method for manufacturing seamless steel pipe - Google Patents

Description

The present invention relates to a method for manufacturing a seamless steel pipe by the Mannesmann-Plug mill method. In particular, the present invention relates to a method for producing a seamless steel pipe capable of preventing the occurrence of external surface defects and obtaining a seamless steel pipe having excellent external surface quality.

Generally, the Mannesmann-Plug mill method is one of the methods for producing a seamless steel pipe. In this method, as shown in FIG. 3, a raw material 1 which is a steel slab (for example, a round billet) is heated in a heating furnace 2 and a piercer mill 3 presses a plug 4 on the raw material 1 after heating, The hollow shell 5 is formed. Subsequently, the hollow shell 5 (also referred to as a material to be rolled) 5 is formed and rolled into a tubular body 9 by a rolling machine having an elongator 6, a plug mill 7 (for drawing), and a reeler 8 (for polishing tube). Specifically, the plug 4 is inserted into the hole (hollow hole) of the hollow shell 5 as in the case of drilling, expanded by the elongator 6, and stretch-rolled by the plug mill 7 (hereinafter also referred to as plug-mill rolling. Then, the tube is polished with the reeler 8.

The obtained tubular body 9 after the forming and rolling is further passed through a reheating furnace 10 and then narrowed to a predetermined size by a sizing mill 11 which is a multi-stage drawing and rolling machine. After that, the product is cut into a certain length (for example, about 14 m) by a cutting machine (not shown).

In the above-described manufacturing process, in the plug mill 7, a pair of hole-shaped rolls (main rolls) and inner surface tools (plugs) are used, and rolling is usually performed in two or more passes. In each pass and the following pass, rolling is performed by shifting the circumferential position of the material to be brought into contact with the caliber bottom of the main roll by 90 degrees. When performing this rolling, the main roll has a free deformation part (flange part) in the region between the upper roll and the lower roll, and the width expansion amount of the material to be rolled in the free deformation part changes depending on the rolling conditions. .. In particular, when the amount of width expansion of the free-deformation part was excessive, the outer surface defect could occur due to the width expansion part falling down in the reeler mill in the subsequent step.

As a plug mill rolling method, for example, Patent Literature 1 describes a technique of using a plug having an appropriate diameter to eliminate a local large rolling reduction region and suppress uneven thickness and flaws.

JP, 2014-166649, A

2. Description of the Related Art In recent years, seamless steel pipes required for development of oil fields and gas fields, heat exchangers, pipes for chemical plants, and structural members have been increasingly required to be thin or long. When thinning or lengthening is required, the rolling reduction and the diameter reduction rate in the plug mill rolling increase, and the width expansion during rolling tends to increase. As a result, an excessive amount of width spread causes a problem of external surface flaw.

For the problem of the occurrence of external defects, conventionally, in order to keep the reduction rate and the diameter reduction rate below a certain level, by holding a plurality of billet diameters before the plug mill rolling and the tool size of the mill to be used for rolling, We make products according to the desired product size. However, from the viewpoint of shortening the tool change time and expanding the range of support for changing the final product size after manufacturing the ingot and determining the initial size (easiness of changing the final product size), a single tool size Alternatively, it is desirable to expand the range of product sizes that can be produced separately with a single initial slab size. Therefore, there is a need for a diameter reduction and a large reduction rolling with respect to the rolling of the plug mill, that is, a need for a plug mill rolling that can reduce the width expansion amount even under the rolling conditions in which the reduction ratio and the diameter reduction ratio are large.

Further, in the above-mentioned conventional plug mill rolling, the width expansion amount of the free deformation portion becomes excessive depending on the rolling conditions, and the reeler mill in the subsequent step causes it to fall and result in an external flaw, which causes a problem of lowering the yield.

The method described in Patent Document 1 is a technique that is effective in greatly reducing internal defects, but does not consider external defects.

In view of the above problems, it is an object of the present invention to provide a method for manufacturing a seamless steel pipe that can prevent the occurrence of external surface defects and obtain a seamless steel pipe having excellent external surface quality.

In order to achieve the above-mentioned object, the inventors of the present invention have made earnest studies on improvement of the setup of plug mill rolling and suppression of the width expansion amount by the setup, and have obtained the following findings.

The width widening can be interpreted as a phenomenon in which the material goes out of the roll caliber due to an excessive length in the circumferential direction of the tube. Therefore, in order to suppress the amount of width expansion, it was conceived that it is effective to promote extension in the longitudinal direction during plug mill rolling and to inhibit extension in the circumferential direction.

Therefore, as a result of investigation of the width expansion amount by rolling experiments and calculations (FEA: finite element analysis), the inventors of the present invention determined that it was a factor for suppressing the elongation of the material in the longitudinal direction between the opposing main roll and plug. It was found that there are restraints due to contact with the main roll on the outer surface of the pipe, and restraint due to contact with the plug on the inner surface of the pipe.

Then, as a result of further studies, the diameter of the main roll, the outer diameter and the wall thickness of the tube before and after the stretching and rolling, by controlling the rolling conditions so as to satisfy a predetermined condition, the elongation in the longitudinal direction It was found that the width expansion amount can be suppressed by being promoted.

The present invention has been made on the basis of the above findings, and has the following gist.

1. A method for producing a seamless steel pipe by the Mannesmann-Plug mill method,
Hollow a steel slab with a piercer mill to make a hollow shell,
Expand the hollow shell with an elongator,
When performing stretch rolling with a plug mill having a pair of upper and lower main rolls and a pair of upper and lower return rolls,
A method for producing a seamless steel pipe, wherein the stretching rolling is performed under the condition that E defined by the following formula (1) is 0 or less.
E=(0.390×π(φ _E −φ _P )−45.1)÷(0.113×(φ _P ×t _E ÷t _P ))+(D÷1000) (1)
here,
D: Diameter of main roll (mm)
φ _E : Outer diameter of the hollow shell before stretching and rolling (mm)
φ _P : Outer diameter of the hollow shell after drawing and rolling (mm)
t _E : wall thickness of the hollow shell before stretching and rolling (mm)
t _P : wall thickness of the hollow shell after stretch rolling (mm)

2. The method for producing a seamless steel pipe according to 1 above, wherein the diameter D of the main roll is 500 mm or more and 1300 mm or less.

According to the present invention, in manufacturing a seamless steel pipe by the Mannesmann-Plug mill method, it is possible to prevent the occurrence of external surface defects and obtain a seamless steel pipe having excellent external surface quality.

It is a schematic diagram explaining the plug mill rolling of this invention, FIG.1(A) is a side surface sectional view during plug mill rolling, FIG.1(B) is the sectional view on the AA line in FIG.1(A). 6 is a graph illustrating the relationship between the width expansion amount and the width expansion amount obtained by a regression equation in the embodiment of the present invention. It is a schematic diagram explaining a manufacturing process of a general seamless steel pipe.

Hereinafter, the present invention will be described in detail. The present invention is not limited to this embodiment.

First, the technical idea of the present invention will be described. FIG. 1 is a schematic diagram for explaining stretch rolling of a seamless steel pipe using a plug mill (hereinafter, also referred to as plug mill rolling). Note that the plug mill 7 has a pair of upper and lower main rolls 7a and a pair of upper and lower return rolls 7b as shown in FIG. 3, but in the present invention, attention is paid to the fact that the rolling with the main rolls 7a affects the external surface flaw. did. Therefore, FIGS. 1A and 1B are sectional views showing the state of the plug mill rolling in the main roll 7a.

As shown in FIG. 1(A), in the plug mill rolling, a pair of main rolls (referred to as hole-shaped rolls) arranged vertically on the outer side of the hollow shell 5 (hereinafter, sometimes referred to as rolled material). 7a) and a plug 4 inserted inside the tube. Here, the main roll arranged on the upper side is called an upper roll, and the main roll arranged on the lower side is called a lower roll. Usually, in the plug mill rolling, a hollow shell having a substantially circular cross section, which is thinned by the expansion by the elongator (elongator rolling) prior to this, is used as a starting material, and after the first pass rolling, it is returned to the inlet side again and the second pass is passed. Roll it. The rolling of the second pass is performed by changing the rolling position by 90 degrees in the circumferential direction from the rolling position of the first pass.

Here, as shown in FIG. 1(B), a main roll 7a having a substantially circular arc-shaped cross section and a plug 4 having a circular cross section are used. The upper main roll 7a and the lower main roll 7a are arranged at a predetermined interval to form the flange portion 12. The caliber shape of the main roll 7a used in the plug mill rolling is roughly classified into one having a straight relief and one having a circular relief, but the invention is not particularly limited thereto.

In the present invention, the width expansion caused by the plug mill rolling was considered to occur because the circumferential length of the rolled steel pipe was excessively larger than the target length.

Therefore, as a result of investigating by FEA (Finite Element Analysis) using general-purpose software Abaqus standard, and investigating the relationship between the factors considered to be related to the circumference and the width spread amount, the following three factors are spread width It was found that the quantity was greatly affected.
(1) Circumferential difference of steel pipe before and after drawing and rolling: π(φ _E −φ _P ).
(2) Product of reduction ratio of steel pipe before and after stretching and rolling and outer diameter after stretching and rolling: φ _P ×t _E ÷t _P
(3) Diameter of main roll: D
here,
φ _E : Outer diameter of the hollow shell before stretching and rolling (mm)
φ _P : Outer diameter of the hollow shell after drawing and rolling (mm)
t _E : wall thickness of the hollow shell before stretching and rolling (mm)
t _P : wall thickness of the hollow shell after stretch rolling (mm)

In the present invention, the diameter of the main roll refers to the diameter of the bottom portion of the main roll. The bottom part is the bottom part of the caliber (groove) provided on the outer peripheral surface of the main roll, in other words, the part where the diameter of the main roll is the smallest. Usually, the bottom portion corresponds to the widthwise center of the main roll, as shown in FIG.

In the present invention, the outer diameter of the hollow shell before stretching and rolling refers to the outer diameter of the hollow shell that is subjected to the stretching and rolling by the plug mill, that is, the hollow shell after expanded by the elongator. Further, the outer diameter of the hollow shell after stretch rolling refers to the outer diameter of the hollow shell at the time when the stretch rolling by the plug mill is completed. The “outer diameter” here means the diameter of the tube at a position facing the bottom portion of the main roll, that is, the diameter in the vertical direction of FIG. 1(B). The diameter can be measured using a laser outer diameter measuring device or the like.

In the present invention, the wall thickness of the hollow shell before stretching and rolling refers to the wall thickness of the hollow shell that is subjected to the stretching and rolling by the plug mill, that is, the wall thickness of the hollow shell after expanded by the elongator. Further, the wall thickness of the hollow shell after stretch rolling refers to the wall thickness of the hollow shell at the time when the stretch rolling by the plug mill is completed. Here, the "wall thickness" is calculated from the volume of the used billet, the outer diameter of the hollow shell measured by the above method, and the length in the longitudinal direction of the hollow shell measured by a laser displacement meter or the like. Use average wall thickness.

Among the factors (1) to (3), both (1) and (2) are parameters that contribute to the change in length in the circumferential direction due to the plug mill rolling. As a result of examination by FEA, it was found that the width expansion amount of the steel pipe can be approximated by a linear expression of these two variables.

Further, in order not to increase the peripheral length after the plug mill rolling, it is necessary to change the volume of the thinned portion into the elongation in the longitudinal direction of the steel pipe, not in the circumferential direction of the steel pipe. As a result of study, by controlling the diameter D of the main roll 7a used for rolling to reduce the contact length between the caliber bottom of the main roll and the material to be rolled, and to reduce the longitudinal constraint of the material to be rolled, It has been found that the elongation of the circumference can be increased and the increase of the circumference can be suppressed.

Furthermore, regarding the influence of the main roll diameter on the width expansion amount, by comparing the actual rolling result and the FEA calculation result, the regression coefficient of (φ _P ×t _E ÷t _P ) of the regression expression of the width expansion amount It has been found that the accuracy of the calculated width spread amount can be improved, assuming that is proportional to the diameter (D) of the main roll in (3) above.

Specifically, the width expansion amount Q can be approximated by the following linear expression (a).
Q=P+A×π(φ _E −φ _P )+C×(φ _P ×t _E ÷t _P )×(D÷1000) (a)
Here, the meaning of each symbol in the above formula is as follows.
P: intercept A: regression coefficient of π (φ _E −φ _P )C: regression coefficient of (φ _P ×t _E ÷t _P )×(D ÷1000) D:diameter of main roll (mm)
φ _E : Outer diameter of the hollow shell before stretching and rolling (mm)
φ _P : Outer diameter of the hollow shell after drawing and rolling (mm)
t _E : wall thickness of the hollow shell before stretching and rolling (mm)
t _P : wall thickness of the hollow shell after stretch rolling (mm)

The regression coefficients A and C in the above equation (a) and the intercept P can be determined by performing FEA under various conditions to obtain the width spread amount Q and performing multiple regression analysis using the result.

In the actual manufacturing process, when the width expansion amount Q exceeds a certain value, external surface defects occur. Therefore, when the upper limit of the wide rising amount Q outer surface flaw is not generated and Q _C, conditions under which the outer surface flaws are not generated can be expressed by the following equation (b).
P+A×π(φ _E −φ _P )+C×(φ _P ×t _E ÷t _P )×(D÷1000)≦Q _C (b)

Q _C in the above equation (b), in fact, Mannesmann under various conditions - manufactures of seamless steel pipes by plug mill method, it can be determined by confirming the presence or absence of the occurrence of outer surface flaws.

Therefore, the (b) a _{Q C} and transposing to the left side in the equation, both sides is divided by _{C × (φ P × t E} ÷ t P), by placing further _Q C -P is B, the following (c) The formula is obtained.
(A×π(φ _E −φ _P )−B)÷(C×(φ _P ×t _E ÷t _P ))+(D÷1000)≦0 (c)

Therefore, it can be understood that the outer surface flaw can be prevented by performing the plug mill rolling under the condition that E defined by the following equation (d) is 0 or less.
E=(A×π(φ _E −φ _P )−B)÷(C×(φ _P ×t _E ÷t _P ))+(D÷1000) (d)

That is, when E exceeds 0, the width expansion amount in the plug mill rolling becomes excessive, and as a result, the widened portion collapses in the reeler mill in the subsequent step and becomes a defect, and the desired outer surface quality cannot be obtained.

Incidentally, A in the above equation _{(d), B (= Q C -P)} , and C is a parameter affected by the size and composition of the tubes obtained by drawing and rolling. Therefore, by determining these parameters in consideration of the actual operating conditions, it is possible to accurately determine the conditions for the plug mill rolling in which outer surface defects do not occur.

Next, the method for manufacturing the seamless steel pipe of the present invention will be described in detail.

The present invention is a method for producing a seamless steel pipe by the Mannesmann-plug mill method, in which a steel slab is perforated with a piercer mill to form a hollow shell, the hollow shell is expanded with an elongator, and then a pair of upper and lower main rolls. And, the drawing and rolling is performed by a plug mill having a pair of upper and lower return rolls.

The steel slab is not particularly limited, and any one can be used. For example, a steel slab can be obtained by melting and casting molten steel having a predetermined composition. The melting is not particularly limited, but can be performed using, for example, a converter, an electric furnace, a vacuum melting furnace, or the like. The casting can be performed by, for example, a continuous casting method, an ingot-slump rolling method, or the like. Further, the steel slab obtained by further hot rolling the slab obtained by the casting may be used as the steel slab.

In the Mannesmann-Plug mill method, the steel slab is hot formed to produce a seamless steel pipe. In order to perform hot forming, it is sufficient to heat the steel slab prior to drilling with a piercer mill described later. The heating can be performed by any method, and for example, it is preferable to use a heating furnace.

The heating temperature in the heating is not particularly limited. However, if the heating temperature of the steel slab is less than 1100° C., the deformation resistance becomes excessive, and as a result, rolling defects are likely to occur and the tool life is shortened. Therefore, the heating temperature is preferably 1100°C or higher, more preferably 1150°C or higher. On the other hand, if the heating temperature is higher than 1350° C., rolling is possible, but energy loss in the heat treatment is large, which is not desirable from the economical viewpoint. Therefore, the heating temperature is preferably 1350° C. or lower, and more preferably 1300° C. or lower.

Then, the steel slab is perforated with a piercer mill to form a hollow shell. The piercing by the piercer mill can be carried out under any condition without particular limitation, and for example, it may be carried out according to a conventional method.

Next, the hollow shell is expanded with an elongator. The tube expansion by the elongator can be performed under arbitrary conditions without particular limitation, and for example, it may be performed according to a conventional method.

After pipe expansion by the elongator, stretch rolling is performed by a plug mill. As the plug mill, a plug mill including a pair of upper and lower main rolls and a pair of upper and lower return rolls is used.

In the present invention, the stretching rolling is performed under the condition that E defined by the following formula (1) is 0 or less.
E=(0.390×π(φ _E −φ _P )−45.1)÷(0.113×(φ _P ×t _E ÷t _P ))+(D÷1000) (1)
here,
D: Diameter of main roll (mm)
φ _E : Outer diameter of the hollow shell before stretching and rolling (mm)
φ _P : Outer diameter of the hollow shell after drawing and rolling (mm)
t _E : wall thickness of the hollow shell before stretching and rolling (mm)
t _P : wall thickness of the hollow shell after stretch rolling (mm)

By performing the stretch rolling under the above conditions, it is possible to suppress the width expansion amount caused by the stretch rolling and significantly reduce the occurrence of external surface flaws. Further, according to the present invention, since the range of appropriate rolling conditions can be set, it is possible to expand the range of product sizes that can be produced separately from a single billet size/tool size. As a result, it is possible to shorten the tool change time and expand the range of product size change before rolling. In addition, E is preferably −0.050 or less, and more preferably −0.100 or less. On the other hand, the lower limit of E is not particularly limited, but may be, for example, −0.800 or more.

The process after the stretching and rolling is not particularly limited. For example, after stretching and rolling, constant diameter rolling can be performed with a reeler using a polishing tube and a sizing mill. Reheating may be performed before the constant diameter rolling. Further, after the constant diameter rolling, for example, it may be allowed to cool. Further, if necessary, heat treatment may be performed to obtain desired strength and the like.

Next, the parameters A, B, and C in the above equation (1) will be described.

Next, the values of the parameters P, A, and C in the above equation (a) were determined on the assumption that a small-diameter size seamless steel pipe is manufactured, which is a condition in which width expansion is particularly likely to occur. Specifically, multiple regression analysis was performed on a steel pipe having a thickness after plug mill rolling of 7 mm to 40 mm in wall thickness and 190 mm to 270 mm in outer diameter, and each coefficient was calculated. As a result, P=1.079, A=0.00390, and C=0.00113 were obtained.

FIG. 2 shows the relationship between the width expansion amount (horizontal axis) calculated by FEA and the width expansion amount (vertical axis) obtained by the equation (a). In FIG. 2, ◯ indicates that external flaws did not occur, and x indicates that external flaws occurred. As can be seen from these results, when the width expansion amount Q exceeds 1.530, external surface defects occur. These _results were obtained Q C = 1.530, the B = 0.451. By substituting A, B, and C obtained above into the equation (d), and further multiplying the denominator and the numerator by 100, the equation (1) was obtained. By performing the stretching rolling under the condition that E defined by the equation (1) is 0 or less, the width expansion amount caused by the stretching rolling becomes small. As a result, it was found that the occurrence of external defects can be significantly reduced.

As described above, the condition of the above formula (1) can be suitably used when the size after the plug mill rolling is 7 mm to 40 mm in wall thickness and 190 mm to 270 mm in outer diameter. This is because it is difficult to accurately predict the presence or absence of external surface flaws by using the equation (3), since the easiness of width widening changes when the value is out of this range.

Next, the composition of the steel to which the present invention can be preferably applied will be described. In general, the smaller the alloy content of steel, the more likely it is that the flange portion will widen. The condition of the formula (1) in the present invention is particularly effective for carbon steel or low alloy steel which is apt to widen. Therefore, in the present invention, it is preferable to use carbon steel or low alloy steel as the steel material.

Further, in the present invention, as the above steel material, in mass%,
C: 0.05 to 0.43%,
Si: 0.05 to 0.50%,
Mn: 0.03 to 1.80%,
P: 0 to 0.03%,
S: 0 to 0.015%,
Al: 0 to 0.06%,
N: 0 to 0.015%,
Cu: 0 to 1.0%,
Ni: 0 to 2.0%,
Cr: 0 to 9.5%,
Mo: 0 to 1.0%,
V: 0 to 0.3%,
Including Nb:0 to 0.06% and Ti:0 to 0.03%,
It is preferable to use a steel material having a component composition consisting of the balance Fe and unavoidable impurities.

Further, in the present invention, as the steel material, in mass%,
C: 0.05 to 0.43%,
Si: 0.05 to 0.50%,
Mn: 0.03 to 1.80%,
P: 0 to 0.03%,
S: 0 to 0.015%,
Al: 0 to 0.06%,
N: 0 to 0.015%,
Cu: 0 to 1.0%,
Ni: 0 to 2.0%,
Cr: 0 to 2.5%,
Mo: 0 to 1.0%,
V: 0 to 0.3%,
Including Nb:0 to 0.06% and Ti:0 to 0.03%,
It is more preferable to use a steel material having a component composition consisting of the balance Fe and unavoidable impurities.

In addition, in the above component composition, the element with the lower limit of content being 0 is an element which can be added arbitrarily, and is not necessarily required to be contained. In addition, since the composition of the steel does not change during the manufacturing process of the seamless steel pipe, the composition of the finally obtained seamless steel pipe is equal to the composition of the steel material to be used, and therefore the above composition may be used. preferable.

The diameter D of the main roll may satisfy the condition of the above formula (1). However, if D is 1300 mm or less, the contact length between the caliber bottom portion of the main roll and the material to be rolled can be further reduced, and the width expansion amount in stretch rolling can be further reduced. Therefore, D is preferably 1300 mm or less, more preferably 1100 mm or less, and further preferably 1035 mm or less. On the other hand, although the lower limit of D is not particularly limited, if D is 500 mm or more, the rigidity of the main roll is improved and the bending during rolling can be prevented, so that the accuracy of stretch rolling is improved. Therefore, it is preferable that D is 500 mm or more.

Hereinafter, the operation and effect of the present invention will be described using examples. The present invention is not limited to the examples below.

Molten steel having the composition shown in Table 1 was melted, further subjected to degassing treatment, and subsequently, a steel slab (billet) of φ: 210 mm × length: 6000 mm was manufactured by the ingot-making method, and was air-cooled to room temperature.

Next, the billet was heated to the heating temperature shown in Table 2 in a heating furnace, then pierced and rolled by a piercer to make a hollow shell, and then the hollow shell was expanded by an elongator (thinning/expansion rolling).

After that, stretch rolling with a plug mill (plug mill rolling) was performed under various conditions. The plug mill rolling was performed with the steel pipe size on the inlet side, the steel pipe size on the outlet side, and the main roll diameter shown in Table 2. Table 2 shows the E value calculated from each condition using the equation (1).

Then, the tube was rolled by a reeler and the standard rolling was performed by a sizing mill. After the standard rolling, it was allowed to cool to obtain seamless steel pipes of various sizes.

With respect to the obtained seamless steel pipe, the external surface flaw was measured by the following method, and the external surface quality was evaluated. The evaluation results are also shown in Table 2.

(Evaluation of external quality)
The flaws on the outer surface along the entire length of the obtained seamless steel pipe were detected by magnetic particle flaw detection (after applying fluorescent magnetic powder, and then visually radiated with ultraviolet rays) to measure the depth of cracks on the outer surface flaws. The crack depth was measured by cutting along a plane perpendicular to the longitudinal direction of the steel pipe so as to include the crack portion and observing with an optical microscope. Then, the outer surface quality was evaluated according to the following criteria. Here, the case of the symbols ◯ and ◎ was evaluated as being excellent in appearance quality, and the case of the symbol x was evaluated as being inferior in external surface quality.
・Symbol x: Cracks with a maximum depth of 1.0 mm or more ・Symbol ○: Cracks with a maximum depth of 0.5 mm or more and less than 1.0 mm ・Symbol ◎: Cracks with a maximum depth of less than 0.5 mm, or no crack

As can be seen from the results shown in Table 2, in the invention examples satisfying the conditions of the present invention, it was possible to obtain a seamless steel pipe having an excellent outer surface quality. Above all, in the invention examples in which the diameter D of the main roll was small, a more excellent outer surface quality was obtained. On the other hand, in the comparative example not satisfying the conditions of the present invention, the width spread in the plug mill rolling was excessive, and a flaw having a maximum depth of 1.0 mm or more occurred on the outer surface of the seamless steel pipe.

1 Material 2 Heating Furnace 3 Piercer Mill 4 Plug 5 Hollow Element Pipe 6 Elongator 7 Plug Mill 7a Main Roll 7b Return Roll 8 Reeler 9 Tubular Body 10 Reheating Furnace 11 Sizing Mill 12 Flange Part

Claims (2)

A method for producing a seamless steel pipe by the Mannesmann-Plug mill method,
Hollow a steel slab with a piercer mill to make a hollow shell,
Expand the hollow shell with an elongator,
When performing stretch rolling with a plug mill having a pair of upper and lower main rolls and a pair of upper and lower return rolls,
The steel slab, in mass%,
C: 0.05 to 0.43%,
Si: 0.05 to 0.50%,
Mn: 0.03 to 1.80%,
P: 0 to 0.03%,
S: 0 to 0.015%,
Al: 0 to 0.06%,
N: 0 to 0.015%,
Cu: 0 to 1.0%,
Ni: 0 to 2.0%,
Cr: 0 to 9.5%,
Mo: 0 to 1.0%,
V: 0 to 0.3%,
Nb: 0 to 0.06%, and
Ti:0-0.03% is included,
It has a composition of the balance Fe and unavoidable impurities,
A method for producing a seamless steel pipe, wherein the stretch rolling is carried out under the condition that E defined by the following formula (1) is 0 or less , t _P is 7 to 40 mm, and φ _P is 190 to 270 mm .
E=(0.390×π(φ _E −φ _P )−45.1)÷(0.113×(φ _P ×t _E ÷t _P ))+(D÷1000) (1)
here,
D: Diameter of main roll (mm)
φ _E : Outer diameter of hollow shell before drawing and rolling (mm)
φ _P : Outer diameter of hollow shell after drawing and rolling (mm)
t _E : Thickness of hollow shell before stretching and rolling (mm)
t _P : Wall thickness of hollow shell after stretch rolling (mm) The method for producing a seamless steel pipe according to claim 1, wherein the diameter D of the main roll is 500 mm or more and 1300 mm or less.

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