Fixed guide shoe, inclined rolling mill equipped therewith, and method for manufacturing seamless steel pipes

Abstract

To provide a fixed guide shoe excellent in heat crack resistance and excellent in productivity of a seamless steel pipe.SOLUTION: A fixed guide shoe installed in an inclined rolling mill, the fixed guide shoe having a component composition containing, by mass percent, C:0.35-0.45%, Si:0.70-1.3%, Mn:0.2-0.7%, and Cr:5.0-10%, and a remainder consists of Fe and an unavoidable impurity and a tissue having areal ratio of carbide is 6% or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a fixed guide shoe installed in an inclined rolling mill, and particularly to a fixed guide shoe having excellent heat crack resistance and excellent productivity of seamless steel pipes.

Background Art

[0002] FIG. 1 shows a schematic diagram showing the steps of the Mannesmann - plug mill method, which is one of the methods for manufacturing seamless steel pipes. As shown in FIG. 1, after heating a billet in a rotary heating furnace, it is pierced with a piercer to obtain a hollow tube. Then, the hollow tube is subjected to stretch rolling by an elongator and a plug mill to reduce the wall thickness of the hollow tube, and then the inner surface of the hollow tube is flattened by tube rubbing with a reel. Then, after heating the tube - rubbed hollow tube in a reheating furnace, it is subjected to sizing rolling by a sizing mill to a predetermined outer diameter, thereby manufacturing a seamless steel pipe.

[0003] FIG. 2 is an example of an inclined rolling mill used for manufacturing seamless steel pipes. FIG. 2(a) is a schematic diagram showing the configuration of a piercer, and FIG. 2(b) is a schematic diagram showing the configuration of an elongator. The piercer and the elongator each have two rolls whose rotating axes are inclined with respect to each other and a plug positioned therebetween. And, by the plug, the piercer rolls the billet and the elongator rolls the hollow tube, respectively.

[0004] In such a piercer or elongator, in order to prevent the billet or hollow tube from swinging during rolling, a fixed guide shoe (fixed guide shoe) or a disc - roll type guide shoe (disc - roll type guide shoe), which is a pipe guide, is installed. FIG. 3 is an example of an inclined rolling mill equipped with a fixed guide shoe, and is a front view of the region where the rolls and the plug of the inclined rolling mill (piercer or elongator) exist as viewed from the rolling direction.

[0005] During rolling, the fixed guide shoe comes into contact with the rolled material, which is heated to a high temperature. Therefore, the fixed guide shoe is rapidly heated by heat conduction from the rolled material during rolling, and then rapidly cooled by cooling water after rolling. Repeated rapid heating and cooling causes heat cracks in the fixed guide shoe, leading to cracking or breakage, which necessitates replacement with a new fixed guide shoe, reducing the productivity of seamless steel pipes.

[0006] Methods for suppressing heat cracks in guide shoes have been investigated in the past. For example, Patent Document 1 discloses a guide shoe for seamless pipe drilling and rolling, which has a composition comprising, by weight percentage, C: 0.5~2.0%, Si: 0.1~3.0%, Mn: 5~20%, Cr: 20~40%, and Ni: 10~20%, or further comprising one or more of Mo: 0.10~6.0%, W: 0.10~6.0%, Nb: 0.01~3.0%, V: 0.02~3.0%, B: 0.001~0.2%, and N: 0.005~0.2%, with the remainder being substantially Fe. Furthermore, Patent Document 2 discloses a heat treatment method for guide shoes for seamless steel pipe rolling, in which tool steel consisting of C: 0.2-0.5%, Si: ≤1.6%, Mn: ≤1.1%, Ni: 30-60%, Cr: 25-31%, P: ≤0.02%, S: ≤0.02%, with the remainder being Fe and unavoidable impurities is heat-treated in an atmospheric environment at 1200-1280°C for 30-180 minutes, followed by heat-treated in an atmospheric environment at 700-900°C for 7-10 hours. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 5-17849 [Patent Document 2] Japanese Patent Publication No. 2007-308781 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] While the technologies described in Patent Documents 1 and 2 above are excellent at suppressing heat cracks in guide shoes, the guide shoes proposed in these documents have a high content of alloying elements, which results in high manufacturing costs.

[0009] In light of the above-mentioned problems, the objective of the present invention is to provide a fixed guide shoe that has excellent heat crack resistance and excellent productivity for seamless steel pipes. [Means for solving the problem]

[0010] The inventors investigated the cause of heat cracks occurring in fixed guide shoes as shown in Figure 3. As a result, they found that Cr carbide, an M7C3 type carbide precipitated on the fixed guide shoe, serves as the propagation path for heat cracks, and that repeated rapid heating and cooling generates thermal stress that acts as the starting point for heat cracks.

[0011] Therefore, the inventors considered that by suppressing Cr carbide formation, the propagation path of heat cracks could be eliminated, and by using a fixed guide shoe with a low coefficient of thermal expansion, the thermal stress that serves as the initiation point for heat cracks could be reduced. The present invention is based on the above findings, and its gist is as follows.

[0012] [1] A fixed guide shoe installed on an inclined rolling mill, The composition, by mass%, consists of C: 0.35-0.45%, Si: 0.70-1.3%, Mn: 0.2-0.7%, Cr: 5.0-10%, with the remainder being Fe and unavoidable impurities. A fixed guide shoe having a structure in which the area ratio of carbides is 6% or less. [2] The fixed guide shoe according to [1], wherein the component composition further contains one or more selected from Mo: 0.9 to 1.5%, Ni: 2.0% or less, and V: 0.5 to 1.6% by mass. [3] The coefficient of linear expansion at 800°C is 1.5 × 10 -5The fixed guide shoe described in [1] or [2] and having a temperature of 1 / °C or lower. [4] A skew rolling mill comprising the fixed guide shoe described in any one of [1] to [3]. [5] A method for manufacturing a seamless pipe using the skew rolling mill described in [4].

Advantages of the Invention

[0013] According to the present invention, it is possible to provide a fixed guide shoe having excellent heat crack resistance and excellent productivity for seamless pipes.

[0014] According to the present invention, when manufacturing a seamless pipe using a skew rolling mill equipped with a fixed guide shoe, it is possible to suppress or reduce the occurrence of heat cracks that cause loss or breakage of the fixed guide shoe, reduce the replacement frequency of the fixed guide shoe, and increase the productivity of the seamless pipe. In addition, by using an inexpensive fixed guide shoe with a reduced content of alloy elements such as Cr and Ni, the manufacturing cost of the seamless pipe can be reduced and the productivity of the seamless pipe can be increased.

Brief Description of the Drawings

[0015] [Figure 1] FIG. 1 is a schematic diagram showing the process of the Mannesmann plug mill method. [Figure 2] FIG. 2 is a schematic diagram showing the configuration of a piercer and an elongator. [Figure 3] FIG. 3 is a front view of a region where rolls and plugs of a skew rolling mill equipped with a fixed guide shoe are present, as viewed from the rolling direction. [Figure 4] FIG. 4 is an explanatory diagram for explaining the sampling positions of a sample for microstructure observation and a sample for linear expansion coefficient measurement.

Modes for Carrying Out the Invention

[0016] Hereinafter, the present invention will be described in detail.

[0017] The stationary guide shoe of the present invention has the following component composition. Hereinafter, % indicating the content of each component means mass % unless otherwise specified.

[0018] C: 0.35 - 0.45% C is an important element that affects the strength characteristics of the stationary guide shoe. In order to obtain good strength characteristics, the C content should be 0.35% or more. By this, appropriate strength characteristics can be obtained. On the other hand, if the C content is excessive, Cr carbides will precipitate excessively, leading to a decrease in heat crack resistance. Therefore, the C content should be 0.45% or less.

[0019] Si: 0.70 - 1.3% Si is an element effective in enhancing the strength characteristics of the stationary guide shoe. To obtain this effect, the Si content should be 0.70% or more. On the other hand, if the Si content becomes excessive, it may embrittle during hot working and become inappropriate as a tool. Therefore, the Si content should be 1.3% or less.

[0020] Mn: 0.2 - 0.7% Mn is effective in enhancing the strength characteristics. To obtain this effect, the Mn content should be 0.2% or more. On the other hand, if the Mn content becomes excessive, retained austenite will remain and the fatigue strength will decrease. Also, Mn is known as a carbide-forming element, and if the added amount of Mn becomes excessive, the precipitation amount of Cr carbides will increase and the heat crack resistance will decrease. Therefore, the Mn content should be 0.7% or less.

[0021] Cr: 5.0 - 10% Cr is an element with excellent high-temperature strength characteristics. To obtain this effect, the Cr content should be 5.0% or more. On the other hand, if the Cr content is too high, Cr carbides, which are M7C3 type carbides, will precipitate excessively, leading to a decrease in heat crack resistance. Therefore, the Cr content should be 10% or less.

[0022] The above constitutes the basic components (essential components) of the fixed guide shoe of the present invention. In the above component composition, the remainder other than the basic components can be Fe and unavoidable impurities.

[0023] Furthermore, the above-mentioned component composition may also contain one or more elements selected from Mo, Ni, and V within the following ranges.

[0024] Mo: 0.9~1.5% Mo is effective in improving heat resistance. To obtain this effect, if Mo is included, the Mo content should ideally be between 0.9% and 1.5%.

[0025] Ni: 2.0% or less Ni is effective in improving toughness. To obtain this effect, if Ni is included, the Ni content should preferably be 2.0% or less. Furthermore, if Ni is included, a Ni content of 0.01% or more is preferable.

[0026] V: 0.5~1.6% V is effective in improving strength characteristics. To obtain this effect, if V is included, the V content is preferably 0.5 to 1.6%.

[0027] Carbides in the microstructure of fixed guide shoes: less than 6% by area percentage M7C3 carbides provide excellent wear resistance through precipitation, but as the amount of carbide precipitation increases, they develop into a network, becoming a pathway for heat crack propagation and causing a decrease in heat crack resistance. Therefore, the fixed guide shoe shall have a microstructure in which the area ratio of M7C3 carbides is 6% or less. The microstructure of the fixed guide shoe shall be observed as follows: A sample for microstructure observation, measuring 30 mm × 15 mm × 10 mm, is taken at a depth of 10 mm from the surface of the fixed guide shoe (the surface that contacts the rolled material) (see Figure 4). After polishing the surface parallel to the above surface, the metallized microstructure revealed by etching with a virela is observed with an optical microscope (magnification: 200x). At this time, the carbides appear white, and the area ratio (area %) of the white area relative to the entire observation area is calculated by image analysis. Metallographic photographs of five fields of view are taken, and the area ratio of the carbides in each is measured, and the average value is taken as the area ratio of carbides in the microstructure of the fixed guide shoe.

[0028] Coefficient of linear expansion of fixed guide shoe at 800°C: 1.5 × 10 -5 Below 1 / ℃ The thermal stress that initiates heat cracks increases with increasing coefficient of thermal expansion. Therefore, to reduce thermal stress, the coefficient of thermal expansion of the fixed guide shoe is set to 1.5 × 10⁻⁶. -5 It is desirable that the coefficient of thermal expansion be 1 / °C or less. The coefficient of thermal expansion is measured as follows: A sample measuring the coefficient of thermal expansion is taken from a depth of 10 mm (see Figure 4) from the surface of the fixed guide shoe (the surface that contacts the rolled material), with a diameter of 4 mm and a length of 10 mm. The sample is heated at a heating rate of 100°C / sec using a Formaster testing machine, and the change in the length of the sample is measured to calculate the coefficient of thermal expansion at 800°C.

[0029] The microstructure of the fixed guide shoe of the present invention preferably consists mainly of martensite, with retained austenite accounting for less than 1.0% by area. In this invention, the main phase refers to the phase accounting for 50% or more by area. The area ratio of the main phase is preferably 60% or more, and more preferably 75% or more. The microstructure of the fixed guide shoe of the present invention can be determined by cutting a sample for microstructure observation from a location 10 mm deep from the surface of the fixed guide shoe (the surface that contacts the rolled material) (see Figure 4), and observing the surface of the sample by a known method.

[0030] The fixed guide shoe of the present invention is not particularly limited, but can be manufactured by preparing molten steel having the above-mentioned component composition and casting it into a predetermined fixed guide shoe shape by the V-process method. [Examples]

[0031] Next, embodiments of the present invention will be described in detail. However, the present invention is not limited by the embodiments described below, and can be modified as appropriate within the scope that is consistent with the spirit of the present invention, and all of these are included within the technical scope of the present invention.

[0032] Molten steel having the component composition shown in Table 1 was prepared, and a fixed guide shoe was manufactured by casting it using the V-process method. The fixed guide shoe was then placed on a piercing mill (inclined rolling mill), and a billet with an outer diameter of 230 mm and material API 5CTL80 was heated to 1290°C and pierced and rolled. Separately, a sample for microstructure observation was taken from a depth of 10 mm from the surface of the manufactured fixed guide shoe, and the area ratio of carbides was measured according to the method described above. In addition, a sample with a diameter of 4 mm and a length of 10 mm was taken from a depth of 10 mm from the surface of the fixed guide shoe (the surface that contacts the rolled material) according to the method described above, and the coefficient of linear expansion was measured. Carbides were determined by image analysis. The coefficient of linear expansion was calculated by measuring the change in length of a sample with a size of 4 mm in diameter and a length of 10 mm using a Formaster tester at a heating rate of 100°C / sec, and then calculating the coefficient of linear expansion at 800°C.

[0033] Heat crack resistance was assessed by visually inspecting the surface of the fixed guide shoe after the drilling and rolling process described above. Those that showed no heat cracks were judged to have excellent heat crack resistance.

[0034] As can be seen from Table 1, by satisfying the conditions of the present invention, a fixed guide shoe with excellent heat crack resistance can be obtained. This suppresses or reduces the occurrence of heat cracks that cause cracking or fracture of the fixed guide shoe, reduces the frequency of replacement of the fixed guide shoe, and increases the productivity of seamless steel pipes. Furthermore, by using an inexpensive fixed guide shoe with reduced content of alloying elements such as Cr and Ni, the manufacturing cost of seamless steel pipes is reduced and the productivity of seamless steel pipes is increased.

[0035] [Table 1]

Claims

1. A fixed guide shoe installed on an inclined rolling mill, The composition is as follows, by mass%, containing C: 0.35-0.45%, Si: 0.70-1.3%, Mn: 0.2-0.7%, Cr: 5.0-10%, Ni: 0.01-2.0%, with the remainder being Fe and unavoidable impurities. A fixed guide shoe having a structure in which the area ratio of martensite is 50% or more and the area ratio of carbides is 6% or less.

2. The fixed guide shoe according to claim 1, wherein the component composition further contains one or two selected from Mo: 0.9 to 1.5% and V: 0.5 to 1.6% by mass.

3. The coefficient of linear expansion at 800°C is 1.5 × 10⁻⁶ －５ A fixed guide shoe according to claim 1, wherein the temperature is 1 / °C or less.

4. The coefficient of linear expansion at 800°C is 1.5 × 10⁻⁶ －５ A fixed guide shoe according to claim 2, wherein the temperature is 1 / °C or less.

5. An inclined rolling mill equipped with a fixed guide shoe according to any one of claims 1 to 4.

6. A method for manufacturing seamless steel pipes using the inclined rolling mill described in claim 5.

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