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

Abstract

To provide a fixed guide shoe excellent in strength characteristics and excellent in seizure resistance.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.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a fixed guide shoe installed in an inclined rolling mill, and more particularly to a fixed guide shoe having excellent seizure resistance.

Background Art

[0002] Fig. 1 shows a schematic diagram showing the process 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 polishing with a reel. Thereafter, after heating the polished hollow tube in a reheating furnace, it is subjected to sizing rolling to a predetermined outer diameter by a sizing mill, thereby manufacturing a seamless steel pipe.

[0003] Fig. 2 shows 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 disk roll type guide shoe (disk roll type guide shoe), which is a pipe guide, is installed. Fig. 3 shows 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) are present as viewed from the rolling direction.

[0005] From the standpoint of yield and manufacturing cost, it is desirable to reduce and suppress external defects in seamless steel pipes. An example of external defects in seamless steel pipes is defects caused by grease adhering to the surface of tools such as guide shoes scratching the billet or raw pipe during drilling or rolling.

[0006] In the past, methods have been considered to suppress and reduce external defects in billets and raw pipes caused by the seizing of tools such as guide shoes. For example, Patent Document 1 proposes a method to suppress seizing of the guide shoe surface by placing a high-pressure nozzle below the pass line of the rolled material and spraying cooling water from the high-pressure nozzle toward the contact surface between the fixed guide shoe and the rolled material to cool the guide shoe. Patent Document 2 also proposes a method to suppress seizing by suppressing excessive contact between the rolled material and the fixed guide shoe by changing the shape of the plug, which is an internal tool of the inclined rolling mill, and the rolled material, thereby keeping the contact area between the plug and the rolled material within a predetermined range. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2012-121059 [Patent Document 2] Japanese Patent Application Publication No. 10-58014 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] In the technology disclosed in Patent Document 1, it is necessary to install new cooling equipment for the fixed guide shoe around the inclined rolling mill in order to suppress seizure of the fixed guide shoe. In addition, in the technology disclosed in Patent Document 2, it is necessary to prepare plugs of various shapes as needed in order to suppress seizure of the fixed guide shoe, which leads to the problem of complicated tool handling.

[0009] In light of the above-mentioned problems, the object of the present invention is to provide a fixed guide shoe that has excellent strength characteristics and excellent seizure resistance. [Means for solving the problem]

[0010] The inventors investigated the cause of seizure in a fixed guide shoe as shown in Figure 3 and found that it is due to the bonding effect between the fixed guide shoe and the rolled material, and the vigorous sliding between the surface of the fixed guide shoe and the rolled material. The bonding effect is the effect of increased adhesion when two metals of the same type slide against each other.

[0011] Therefore, the inventors considered that seizure of the fixed guide shoe could be suppressed by inhibiting the bonding effect between the fixed guide shoe and the rolled material, and by causing plastic flow in the surface layer of the fixed guide shoe. This is because suppressing the bonding effect can suppress the transfer of the rolled material to the fixed guide shoe, thereby suppressing the starting point of seizure. Furthermore, the plastic flow of the surface layer of the fixed guide shoe can reduce the shear stress on the rolled material that is generated by the sliding between the fixed guide shoe and the rolled material. This is because reducing the shear stress can suppress the growth of the rolled material that has transferred to the fixed guide shoe into seizure. 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, A fixed guide shoe having a composition in mass percent 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. [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] A fixed guide shoe according to [1] or [2], wherein the Vickers hardness at 800°C is 25HV0.1 or more and 50HV0.1 or less. [4] An inclined rolling mill equipped with a fixed guide shoe as described in any of [1] to [3] above. [5] A method for manufacturing seamless steel pipes using the inclined rolling mill described in [4] above. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a fixed guide shoe that has excellent strength characteristics and excellent seizure resistance.

[0014] According to the present invention, external surface defects of the rolled material (billet, raw pipe) caused by seizure of the fixed guide shoe can be suppressed or reduced. Furthermore, according to the present invention, there is no need to install cooling equipment for the fixed guide shoe in order to suppress seizure of the fixed guide shoe, nor is there a need to prepare plugs of various shapes, and a method for manufacturing seamless steel pipes that is easy to handle with tools can be provided. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 is a schematic diagram illustrating the process of the Mannesmann-Plug Mill method. [Figure 2] Figure 2 is a schematic diagram showing the configuration of the piercer and elongator. [Figure 3] Figure 3 is a front view from the rolling direction of the region where the rolls and plugs of an inclined rolling mill equipped with fixed guide shoes are located. [Figure 4] Figure 4 is an explanatory diagram illustrating the sampling locations for tissue observation samples and Vickers hardness measurement samples. [Modes for carrying out the invention]

[0016] The present invention will be described in detail below.

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

[0018] C: 0.35 - 0.45% C is an important element that affects the strength characteristics of the stationary guide shoe. 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, the strength characteristics will also become excessive and the seizure resistance will decrease. 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. 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, the bimetallic effect with the rolled material will increase and the seizure resistance will decrease. Therefore, the Cr content should be 10% or less.

[0022] The above are the basic components (essential components) in the component composition of the stationary guide shoe of the present invention. In the above-mentioned component composition, the remainder other than the above 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] Vickers hardness at 800°C: 25HV0.1 or higher, 50HV0.1 or lower The Vickers hardness at 800°C (hereinafter also referred to as high-temperature Vickers hardness) is an important indicator for the wear resistance and seizure resistance of a fixed guide shoe. To ensure excellent strength characteristics and wear resistance, it is desirable that the high-temperature Vickers hardness be 25HV0.1 or higher. On the other hand, if the high-temperature Vickers hardness is too high, the surface layer of the fixed guide shoe will not undergo plastic flow easily, and the seizure resistance may decrease due to vigorous sliding between the fixed guide shoe and the rolled material. Therefore, it is desirable that the high-temperature Vickers hardness be 50HV0.1 or lower. In this invention, the Vickers hardness of the fixed guide shoe at 800°C is the Vickers hardness at a depth of 10 mm from the surface of the fixed guide shoe. Vickers hardness is measured by cutting a sample from a point 10 mm deep from the surface of the fixed guide shoe (the surface that contacts the rolled material) (see Figure 4), heating it at 800°C for 5 minutes, and then striking the surface of the sample with an indenter under a load of 100 gf (HV 0.1).

[0028] 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.

[0029] 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]

[0030] 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.

[0031] A fixed guide shoe was manufactured by preparing molten steel having the component composition shown in Table 1 and 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 made of API 5CTL80 material was heated to 1290°C and subjected to piercing rolling. Separately, a sample was taken from a depth of 10 mm from the surface of the manufactured fixed guide shoe, and its Vickers hardness (high-temperature Vickers hardness) at 800°C was measured. High-temperature Vickers hardness was measured by heating a 5.0 mm × 4.5 mm × 10.0 mm sample at 800°C for 5 minutes, and then striking the surface with an indenter under a load of 100 gf. In this example, samples with a high-temperature Vickers hardness of 25 HV 0.1 or higher were evaluated as having excellent strength characteristics.

[0032] To check for seizure of the fixed guide shoe after rolling, the contact surface between the fixed guide shoe and the billet was analyzed for its components using SEM-EDX. If a component different from that of the fixed guide shoe was detected on its surface, it was determined that seizure had occurred.

[0033] As can be seen from Table 1, by satisfying the conditions of the present invention, a fixed guide shoe with excellent strength characteristics and excellent seizure resistance can be obtained. This makes it possible to suppress or reduce external surface defects of the rolled material (billet, raw pipe) caused by seizure of the fixed guide shoe. Furthermore, for example, it is possible to provide a method for manufacturing seamless steel pipes that does not require the separate installation of cooling equipment to suppress seizure of the fixed guide shoe, does not require the separate preparation of plugs of various shapes, and allows for easy handling of tools.

[0034] [Table 1]

Claims

1. A fixed guide shoe installed on an inclined rolling mill, The composition consists of, by mass%, 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. It has a steel structure in which the area ratio of martensite is 50% or more, A fixed guide shoe having a Vickers hardness of 25 HV 0.1 or higher and 50 HV 0.1 or lower at 800°C.

2. The fixed guide shoe according to claim 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 0.75% by mass.

3. An inclined rolling mill equipped with a fixed guide shoe according to claim 1 or 2.

4. A method for manufacturing seamless steel pipes using the inclined rolling mill described in claim 3.

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