JP7111092B2 - Seamless steel pipe rolling plug, method for manufacturing seamless steel pipe rolling plug, seamless steel pipe rolling plug mill, method for rolling seamless steel pipe, and method for manufacturing seamless steel pipe - Google Patents

Description

The present invention relates to a mandrel (hereinafter referred to as a "plug") used in Mannesmann plug mill rolling of seamless steel pipes, and in particular, it produces a hard oxide layer with excellent wear resistance on the surface of the mandrel to extend the life of the plug. The present invention relates to a plug for seamless steel pipe rolling, which has been improved to reduce manufacturing costs.

The plugs used in the plug mill rolling in the conventional method for manufacturing seamless steel pipes have, in mass%, C: 1.0 to 2.0%, Cr: 12 to 20%, and Ni: 1.0 to 2.5%. , Mo and W (Mo+W): 1.0 to 2.5%, with the balance being mainly Fe. A plug made of such a high-Cr-based high-alloy steel is excellent in high-temperature strength and high-temperature oxidation resistance, and is particularly suitable as a plug used under severe conditions such as plug mill rolling.

However, even plugs made of such materials cannot be used due to surface wear, local gouging, seizure, etc. due to plug damage due to high pressure and high temperature in actual operation when the number of rolled pieces is small. There was a problem that it often becomes Since these damages cause defects on the inner surface of the product and poor dimensional accuracy, the development of a plug with excellent wear resistance is desired in order to improve the life of the plug.

In general, in order to improve wear resistance, it is necessary to increase the strength of the surface of the plug. , a plug having a hardness of 900 HV or more is disclosed.

For example, Patent Document 2 discloses that an oxide layer is formed on the surface of a plug material by heating in an oxidizing atmosphere, and then a temperature range over a temperature range of at least 350° C. is cooled at a cooling rate equal to or higher than that of air cooling. A method is disclosed in which a first heat treatment is performed in which the plug material is cooled at room temperature, and then a second heat treatment is performed in which the plug material is heated again in an oxidizing atmosphere to form an oxide layer again on its surface. As a result, of the oxide layer formed by the first heat treatment, the portion where the formation of the Ni-enriched base iron particles is insufficient and the adhesion to the base iron is poor is left to cool after the first heat treatment. During the cooling process at the above cooling rate, the oxide layer is peeled off and then heat-treated again to form a new oxide layer. As a result, an oxide layer is formed at the interface between the oxide layer and the base iron as if it were nailed (anchor effect). Due to the anchoring effect obtained by this secondary heat treatment, the adhesion between the oxide layer and the base iron is improved, and the oxide layer protects the plug and extends the life of the plug.

JP-A-57-198243 JP-A-61-281819

In recent years, there has been a tendency to increase the alloying of materials to be rolled for the production of seamless steel pipes, which has led to an increase in the rolling load, and furthermore, there has been a tendency to increase the rolling speed for the purpose of improving productivity. Such increase in hardness due to the addition of a large amount of high alloy to the material to be rolled and the increase in rolling speed have resulted in an increase in the unit price of the plug. The technology described in Patent Document 1 contains a large amount of alloying elements for the purpose of increasing strength, but there is also a need to establish a technology that can improve wear resistance without containing a large amount of alloying elements. rice field.

In addition, as in the technique described in Patent Document 2, performing heat treatment twice for the purpose of achieving high adhesion between the oxide layer and the base iron in the manufacture of the plug requires time, and the time required for manufacture is reduced. cost increase.

Furthermore, by forming an oxide layer on the plug by heat treatment, it is possible to suppress the temperature rise of the plug, prevent seizure, and improve the life of the plug by lowering the coefficient of friction. However, although the addition of alloying elements for the purpose of increasing the strength improves the impact resistance of the plug, it often impairs the adhesion to the oxide layer. For example, when a large amount of Co or Cr is added, the oxide layer peels off when it comes into contact with the material to be rolled, causing seizure and erosion. When the erosion is transferred to the material to be rolled, scratches are generated on the inner surface of the material to be rolled.

Moreover, it is difficult to improve the wear resistance of the oxide layer by improving only the adhesion of the oxide layer as in the technique described in Patent Document 2. Under a severe rolling environment, the oxide layer wears and becomes thin, so that the oxide layer does not sufficiently suppress the temperature rise of the plug body, making it difficult to suppress deformation of the plug.

Thus, under severe rolling conditions, it was difficult to obtain a cost-effective, long-life plug because it did not have sufficient wear resistance.
Also, when the addition of alloy is small and the high-temperature strength is low, the temperature of the plug is increased during rolling and the plug is easily deformed. However, if a large amount of alloy is added in order to increase the high-temperature strength, the oxide layer becomes thin or the adhesion becomes weak, and the temperature of the plug tends to rise. On the other hand, if the alloy is reduced and the oxide layer is thickened, the original strength is insufficient and deformation occurs.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides a technology for a plug for seamless steel pipe rolling that has improved wear resistance and achieves sufficient suppression of deformation without performing multiple heat treatments. for the purpose.

As a result of intensive studies to solve the above problems, the inventors of the present invention have found that even if the plug material is heat-treated only once, the component composition of the plug material can be precisely controlled, and the The oxide layer is made to have a predetermined thickness or more, and the spinel type oxide is the main component and has specific physical properties. I found that I could secure enough time until.
Further, the inventors have also found that sufficient suppression of deformation can be realized by setting the Vickers hardness within a predetermined range under a predetermined composition.

The present invention is based on the above findings, and has the following features.
[1] A seamless steel pipe rolling plug comprising a plug material and an oxide layer formed on the surface of the plug material,
The plug material is
% by mass, C: 1.0 to 2.0%, Si: 0.3 to 1.5%, Mn: 0.3 to 1.5%, Al: 0.01 to 0.1%, Ni: Contains 0.01-5.0%, Cr: 12-20%, one or two of Mo and W (Mo+W): 0.1-3.0%, V: 0.01-0.1% and the balance has a component composition consisting of Fe and inevitable impurities,
The oxide layer has a spinel-type oxide at a volume fraction of 50% or more and has a layer thickness of 0.4 mm or more,
A seamless steel pipe rolling plug having a Vickers hardness of 500 to 700 HV at a position 10 mm in the thickness direction from the surface of the plug material.
[2] A method for manufacturing a seamless steel pipe rolling plug according to [1] above,
The plug material is maintained at 1000 to 1200°C for 1 to 3 hours in an atmosphere with an oxygen concentration of 0.3% or less and a carbon monoxide concentration of 4 to 8% in volume fraction, and then cooled to 800 to 900°C. and holding for 1 to 3 hours to perform heat treatment to form an oxide layer on the surface of the plug material.
[3] A seamless steel pipe rolling plug mill comprising the seamless steel pipe rolling plug according to [1].
[4] A method for rolling a seamless steel pipe using the seamless steel pipe rolling plug according to [1].
[5] A method for producing a seamless steel pipe using the seamless steel pipe rolling plug according to [1].
The thickness direction mentioned above refers to the direction perpendicular to the tangent line at each position on the surface of the plug material.

ADVANTAGE OF THE INVENTION According to this invention, the technology of the plug for seamless steel pipe rolling which improved wear resistance and implement|achieved deformation|transformation suppression is provided, even if it does not heat-process several times.
As a result, it is possible to obtain a long-life plug with good cost performance.
Specifically, according to the present invention, the steel composition and hardness are controlled to suppress deformation due to impact applied to the plug during rolling. Furthermore, even with a single heat treatment, an oxide layer with excellent wear resistance is formed, and a long-life plug can be obtained in which the base iron is less likely to be exposed on the surface even after a severe rolling process.
In addition, even if the unit price is high and a large amount of alloying elements for the purpose of high strength are not contained, it is possible to make the plug excellent in strength and wear resistance, and it is possible to reduce the unit price of the plug. .

FIG. 1 is a schematic diagram for explaining the seamless steel pipe rolling plug of the present invention. Fig. 2 shows the relationship between the thickness of the oxide layer and the maximum temperature (temperature just below the oxide layer) at the interface (plug material surface) between the oxide layer and the base iron when a high-temperature object is brought into contact with the oxide layer. is a graph showing FIG. 3 is a graph showing the transformation temperature range of steel having the chemical composition shown in Table 1, obtained by Formaster. FIG. 4 is a graph showing the relationship between yield stress and temperature obtained by measuring the hot deformation resistance of steels having the chemical compositions shown in Table 1. In FIG. FIG. 5 is the stress-strain curve of the blank tube used for the finite element analysis. FIG. 6 is a cross-sectional photograph of a plug showing the degree of cracking when stresses of 200 MPa, 400 MPa, and 600 MPa were applied to the plug. FIG. 7 is a cross-sectional photographic view of a cut plug sample after heating. FIG. 8 shows the result of contacting a plug sample having no oxide layer and having a chemical composition within the range of the present invention with a material to be rolled heated to 1000° C. for 10 seconds (time equivalent to plug mill rolling). , and a photographic view of the heat-affected zone obtained by subsequently corroding the side surface of the sample.

The present invention will be described with reference to the drawings. It should be noted that the present invention is not limited by this embodiment.

A plug for seamless steel pipe rolling according to the present invention is a plug for seamless steel pipe rolling comprising a plug material and an oxide layer formed on the surface of the plug material. 1.0-2.0%, Si: 0.3-1.5%, Mn: 0.3-1.5%, Al: 0.01-0.1%, Ni: 0.01-5. 0%, Cr: 12 to 20%, one or two of Mo and W (Mo + W): 0.1 to 3.0%, V: 0.01 to 0.1%, the balance being Fe and The oxide layer has a component composition consisting of unavoidable impurities, and the oxide layer has a spinel-type oxide at a volume fraction of 50% or more, and has a layer thickness of 0.4 mm or more and a thickness of 10 mm from the plug material surface in the thickness direction. The position satisfies the Vickers hardness of 500 to 700HV.
The thickness direction mentioned above refers to the direction perpendicular to the tangential line at each position on the surface of the plug material.

FIG. 1 is a schematic diagram for explaining the seamless steel pipe rolling plug of the present invention. A plug 1 of the present invention is provided in a rolling mill 101 and supported at its rear end by a bar 3, as shown in FIG. The plug 1 of the present invention is used to reduce the thickness of a hollow S fed between a pair of rolling arc-shaped (caliber) rolls 2a and 2b to a predetermined thickness. Through subsequent processes, it becomes a seamless steel pipe.
The shape of the plug 1 and the plug material of the present invention is not particularly limited. For example, as shown in FIG. can be used.

The shape of the plug 1 of the present invention is not particularly limited, and the size is less than φ40 mm in plug outer diameter, so that when the oxide layer wears, the dimensional accuracy of the mother tube is likely to be affected. Therefore, it is preferable that the outer diameter of the plug (maximum outer diameter of the plug when viewed in cross section perpendicular to the rolling direction) is φ40 mm or more.

<plug material>
Next, the effect of the chemical composition of the alloy of the plug material of the present invention and the reason for limiting the content will be described. Hereinafter, percentages indicating the components of alloys are percentages by mass.

C: 1.0-2.0%
C combines with Cr, Mo, V, W, etc. to form a high-hardness composite carbide to improve wear resistance. In addition, it also partially dissolves in the austenite of the matrix and contributes to the effect of increasing the strength by the martensite formed by the quenching operation. C thus has the effect of improving wear resistance and hardness and strength from room temperature to high temperature. If the C content is less than 1.0%, the amount of carbides crystallized is small, so high wear resistance cannot be obtained. On the other hand, if the C content exceeds 2.0%, the amount of crystallization of carbides becomes excessive and the steel becomes extremely sensitive to thermal shock, resulting in early cracking. Therefore, the C content should be 1.0 to 2.0%.
In addition, the C content is preferably 1.3 to 1.5% from the viewpoint of improving the balance between the amount of crystallized carbide and thermal shock resistance.

Si: 0.3-1.5%
Si is contained in order to deoxidize the molten metal and improve the fluidity of the molten metal. Also, when Si is added, the amount of wustite in the oxide layer is reduced and the amount of magnetite, which is a spinel type oxide, is increased. If the Si content is less than 0.3%, the effect cannot be obtained. On the other hand, when the Si content exceeds 1.5%, the toughness is lowered. Also, if Si is added excessively, a layer of SiO ₂ is formed at the interface between the oxide and the base iron, resulting in a decrease in impact resistance. Therefore, the Si content should be 0.3 to 1.5%. Also, the Si content is preferably 0.4 to 0.7%.

Mn: 0.3-1.5%
Mn is added together with Si for deoxidation. If the Mn content is less than 0.3%, a sufficient deoxidizing effect cannot be obtained. On the other hand, when the Mn content is more than 1.5%, the toughness is lowered. Therefore, the Mn content should be 0.3 to 1.5%. Furthermore, considering the range in which the spinel-type oxide layer increases, the Mn content is preferably 0.6 to 1.2%.

Al: 0.01-0.1%
Al is added together with Si and Mn for deoxidation. If the Al content is less than 0.01%, a sufficient deoxidizing effect cannot be obtained. On the other hand, when the Al content is more than 0.1%, the toughness is lowered. Therefore, the Al content should be 0.01 to 0.1%.

Ni: 0.01-5.0%
Ni improves the hardenability of the matrix, and if the Ni content is less than 0.01%, there is no effect of addition. On the other hand, such an effect of improving the hardenability is saturated when the Ni content is 5.0%. Furthermore, Ni contributes to the improvement of the adhesion of the oxide layer through the inclusion of Ni-enriched ferrous particles in the oxide layer. Causes deterioration of heat treatability due to the formation of austenite. Therefore, the Ni content should be 0.01 to 5.0%. Also, the Ni content is preferably 0.6 to 3.0%.

Cr: 12-20%
Cr gives high-temperature strength as plugs for plug mill rolling used under high temperature and pressure, and provides corrosion resistance to plugs often used in corrosive environments contained in lubricants and the like. In order to obtain these effects, it is necessary to set the Cr content to 12% or more. On the other hand, if the Cr content exceeds 20%, it becomes difficult to make the oxide layer sufficiently thick when heat treatment is performed, and a ferrite phase is generated to impair the heat treatment properties. Therefore, the Cr content is set to 12 to 20%. Further, the Cr content is preferably 13 to 17% in order to reduce the corundum-type oxide layer and to contain more spinel-type oxide.

One or two of Mo and W (Mo + W): 0.1 to 3.0%
Mo and W form a solid solution in the matrix to improve high-temperature properties such as high-temperature softening resistance, and combine with C during tempering to form fine composite carbides to improve wear resistance. Furthermore, since Mo and W have a more noble bond with O than Fe, selective oxidation occurs when the oxide layer is formed, and the interface between the oxide layer and the base iron looks like a nail. It becomes a shape, and high adhesion is obtained by the anchor effect. These effects can be obtained by setting the total content of Mo and W (Mo+W) to 0.1% or more.
On the other hand, since Mo and W are ferrite-forming elements, increasing their contents narrows the composition range in which austenite is formed. Moreover, even if the amounts of C and Ni are increased in order to compensate for the narrowness of the composition range, the content of each element is subject to the aforementioned limitations. Therefore, the total content of Mo and W (Mo+W) is set to 3.0% or less. For the above reasons and from the economic point of view, one or two types of Mo and W are contained, and the total content of Mo and W (Mo+W) is set to 0.1 to 3.0%.

V: 0.01-0.1%
Addition of a small amount of V forms carbides or nitrides, which contributes to refinement of crystal grains and improves toughness. This effect can be obtained by setting the V content to 0.01% or more.
On the other hand, when the V content exceeds 0.1%, the above effect is saturated and a hard phase is formed, resulting in a decrease in toughness. Therefore, the V content should be 0.01 to 0.1%.

The above are the basic components contained in the plug material of the present invention, and the balance other than the above consists of Fe and unavoidable impurities.

<Oxide layer>
Next, the oxide layer formed on the plug of the present invention will be described.

(Layer thickness: 0.4 mm or more)
Figure 2 shows the thickness of the oxide layer and the maximum reach of the thermocouple placed at the interface (plug material surface) between the oxide layer and the base iron when a high temperature object of 1000°C is brought into contact with the oxide layer for 10 seconds. It is a graph which shows the relationship with the temperature (temperature directly under the oxide layer). The 10 second contact time mentioned above simulates the time the plug is exposed to high temperatures. As shown in FIG. 2, the thicker the oxide layer, the lower the maximum temperature reached. Also, the maximum temperature is lowered by about 55° C. each time the oxide layer is thickened by 0.1 mm (100 μm).
As a specific example, when the thickness is relatively thick at 0.7 mm (700 μm), the maximum temperature is about 650° C., while when it is thin at less than 0.4 mm (400 μm), the maximum temperature exceeds 800° C. can be seen from FIG.
Thus, in plug rolling, the oxide layer has the effect of suppressing the temperature rise of the surface of the plug material (and the temperature rise of the surface of the material to be rolled), and the thicker the oxide layer, the better.

FIG. 3 is a graph showing the transformation temperature range of steel having the chemical composition shown in Table 1, obtained by Formaster.

The component composition shown in Table 1 is an example of the component composition of the plug material of the present invention described above. As shown in FIG. 3, it can be seen that there is an austenite transformation point around 800°C.

Moreover, FIG. 4 shows the relationship between the yield stress obtained by measuring the hot deformation resistance of the steel having the composition shown in Table 1 and the temperature. The strain rate at this time is 1/s.
When the temperature exceeds 800°C, which is the austenite transformation point, the hot deformation resistance is greatly reduced. Moreover, there is a correlation between the temperature of the steel material (plug) and its yield stress, and the higher the temperature, the lower the yield stress.
However, when the plug and the material come into contact with each other, as described above, when the oxide layer has a thickness of 0.7 mm, the maximum temperature reached only rises to about 650° C. (see FIG. 2 again). ), then the plug will have a yield stress of the order of 1200 MPa as shown in FIG. On the other hand, when the maximum temperature reaches 800° C., which is the austenite transformation temperature, the yield stress of the plug is about 400 MPa.

Next, we attempted to analyze the stress applied to the plug during rolling by the finite element method. Table 2 shows analysis conditions. Table 3 shows the rolling conditions.

FIG. 5 shows the stress-strain curve of the blank tube used for the finite element analysis. As shown in Table 2 above, the plug was modeled as an elasto-plastic body for simplification of calculation and analyzed. The analysis results revealed that the stress applied to the plug was about 400-600 MPa.

FIG. 6 is a cross-sectional photograph of a plug showing the degree of cracking when stresses of 200 MPa, 400 MPa, and 600 MPa were applied to a plug without an oxide layer formed thereon.
The plug material used for the plug had the component composition shown in Table 1. This plug was heated to 800°C, various stresses were applied at that time, then cooled, cut to observe the cross section, Corrosion was applied to facilitate observation of the degree of cracking. In FIG. 6(a), a stress of 200 MPa was applied to the plug, (b) a stress of 400 MPa, and (c) a stress of 600 MPa. It was found that when the plug reaches about the austenite transformation point, the plug is damaged when a stress exceeding 400 MPa is applied to the plug.

Based on the above results, and by conducting experiments under other conditions, it was found that in the present invention, it is necessary to keep the plug surface temperature from exceeding 800°C. was found to be required to be 0.4 mm or more.
Therefore, in the present invention, the thickness of the oxide layer formed on the plug is set to 0.4 mm or more.
Although the upper limit is not particularly limited, when the thickness of the oxide layer is 1.0 mm or more, the ratio of hematite in the oxide layer tends to increase. In addition, if the thickness of the oxide layer is 1.0 mm or more, 2.0 mm (=1.0 mm×2 (both ends of the diameter)) or more of the outer diameter of the plug becomes the oxide layer, and when the oxide layer is worn, In addition, there is a concern that the dimensional accuracy of the mother tube that is subsequently rolled is reduced. Therefore, the thickness of the oxide layer is preferably less than 1.0 mm.

(Spinel type oxide: 50% or more)
Next, the composition in the oxide layer will be explained.
The oxide layer produced in the atmospheric furnace is thick, and the existence of this thick oxide layer makes it possible to lengthen the distance between the base iron surface of the plug and the inner surface of the blank tube, thereby suppressing the increase in temperature of the plug. However, when an oxide layer is formed in an atmospheric furnace, the composition ratio of hematite increases due to the high oxygen concentration in the furnace. Although hematite is hard, it has low adhesion and is easy to peel off when impact is applied. Therefore, the oxide layer is easily peeled off during severe rolling, such as plug mill rolling, where there is no gap between the plug, the tube and the roll. put away.

In order to carry out plug mill rolling in a severe environment, the oxide layer on the surface of the plug must have excellent wear resistance. Oxides with high wear resistance include spinel-type oxides, and the spinel-type oxide of iron is magnetite.

Here, the plug material having the component composition shown in Table 1 above was heated in an atmosphere having an oxygen concentration of 0.3% or less and a carbon monoxide concentration of 4 to 8% by volume fraction. was held at 1185° C. for 2 hours, then cooled to 880° C. and held for 2 hours to form an oxide layer on the surface.

FIG. 7 is a SEM cross-sectional photograph obtained by cutting a sample of the heated plug thus obtained and observing it at a magnification of 200 times.
First, an X-ray diffraction analysis was performed on a powdered sample obtained by exfoliating the oxide layer, and the volume fraction of the spinel-type oxide in the oxide of the entire oxide layer was obtained.
In addition, an energy dispersive X-ray spectrometer was used to analyze the position where the component composition changed, and the obtained volume fraction was compared. In addition, considering that the oxide layer (hematite-magnetite-wustite) is configured so that the oxygen concentration decreases from the outer surface toward the base iron, in the cross-sectional photograph in FIG. iron (plug material)), FeO (wustite), Fe ₃ O ₄ (magnetite), and Fe ₂ O ₃ (hematite).

Next, in order to evaluate the wear resistance of the spinel-type oxide layer (magnetite in iron), a hot roll wear/thermal shock testing device (device described in JP-A-2000-88727) was used to evaluate the wear resistance of the plug mill. A simulated rolling experiment was conducted to measure the time required for hematite and magnetite to reach the same wear amount. Specifically, in the above simulation test, a test piece of a sample having an oxide layer formed on a rotating body called a mating piece was used. The mating piece can be heated by installing a coil around it, and the temperature was set to be constant even during the test. The size of the test piece was 30 mm long×5 mm wide×15 mm high, and the size of the mating piece was φ190 mm×15 L mm.
A sample for wear resistance evaluation was prepared by holding a material having the composition shown in Table 1 above at 1185° C. for 2 hours using an air atmosphere furnace.
Since the outermost surface of the sample is hematite, the time until 0.2 mm, which is the thickness of the hematite layer, is worn away was measured.

Further, after the heat treatment, the hematite was exfoliated by shot blasting, and the magnetite layer was processed so that the outermost surface was formed. The hematite layer was worn by 0.2 mm in about 4 seconds, whereas the magnetite layer was worn by 0.2 mm in about 5 minutes. This indicates that magnetite is more wear resistant than hematite.

Regarding the volume fraction of this magnetite, that is, spinel-type oxide in the oxide layer, when the volume fraction is less than 50%, the ratio of corundum-type oxide is greater and the oxide layer is easily peeled off. put away. As a result, a problem arises in that the temperature of the base iron rises and deformation of the plug cannot be suppressed.
Therefore, in the present invention, the oxide layer formed on the plug should contain a spinel oxide at a volume fraction of 50% or more.
On the other hand, if the oxide layer contains too much spinel-type oxide (magnetite), cracks occurring in the oxide layer tend to propagate to the base iron. % is present. That is, preferably, the spinel-type oxide has a volume fraction of 90% or less. As a result, the adhesion to the base iron can be further improved. Furthermore, in consideration of both wear resistance and adhesion to the base steel, it is more preferable to have a spinel-type oxide in a volume fraction of 50 to 80%.

The volume fraction of the spinel-type oxide layer can be obtained by performing X-ray diffraction analysis on a powdered sample obtained by exfoliating only the oxide layer, which is the outermost surface portion of the plug. , refers to the volume fraction of the spinel-type oxide layer in the oxide of the entire oxide layer.

The specific oxide layer formed on the plug of the present invention as described above can be obtained by controlling the component composition of the plug material and by subjecting the plug material to heat treatment under specific conditions.

According to the present invention, the oxide layer of the heat-treated plug mainly contains a spinel-type oxide with excellent wear resistance. The residual rate of the substance layer is high, and the life of the plug can be extended.

< Vickers hardness of 500 to 700 HV at a position 10 mm in the thickness direction from the surface of the plug material >
As a result of analyzing the plug mill rolling under the conditions shown in Tables 2 and 3 above, the maximum stress applied to the plug was approximately 1000 MPa at the moment the plug contacted the blank tube. Regarding the relationship between hardness and tensile strength, when the plug has a hardness of 300 HV, it has a tensile strength of about 1000 MPa.

FIG. 8 shows the result of contacting a plug sample having no oxide layer and having a chemical composition within the range of the present invention with a material to be rolled heated to 1000° C. for 10 seconds (time equivalent to plug mill rolling). , and a photographic view of a heat-affected zone obtained by subsequently corroding the side surface of the sample.

As shown in FIG. 8, when the oxide layer is not formed, the temperature rises to around 800° C., which is the austenite transformation point, because the heat-affected zone extends from the surface of the plug material to about 8 mm in the thickness direction. Based on this result and the results of other investigations, if the component exhibits 300 HV or more at a position 10 mm in the thickness direction from the surface of the plug material at 800° C., damage to the plug can be suppressed even if the oxide layer is peeled off.

Therefore, samples having room temperature hardnesses of 700 HV and 500 HV were prepared, heated to a predetermined temperature by high frequency, and then rapidly cooled with argon gas, and the hardness of the samples was investigated for each temperature. In all samples, the hardness peaks at 800°C and recovers, but this is hardness recovery due to martensite transformation due to cooling. It is assumed that the actual hardness at 800°C or higher is equal to or lower than the hardness at 800°C. As a result of further investigation based on this result, it was found that if the normal temperature hardness is 500 HV, 300 HV can be achieved at 800°C. Further, if the hardness exceeds 700 HV, plug processing becomes difficult. Therefore, in the present invention, the Vickers hardness of 500 to 700 HV is satisfied at a position 10 mm in the thickness direction from the surface of the plug material.
Here, room temperature hardness refers to Vickers hardness at room temperature (25° C.) measured based on JIS Z 2244:2009.
Moreover, the thickness direction refers to a direction perpendicular to a tangential line to the surface of the plug material.

As described above, when a plug material having a predetermined chemical composition is subjected to a predetermined heat treatment, a dense and strong oxide layer having a Vickers hardness of 500 to 700 HV is obtained at a position 10 mm in the thickness direction from the surface of the plug material. can be obtained, and plug mill rolling can be performed at low cost and high production.

<Manufacturing method of plug>
Next, a method for manufacturing a plug for seamless steel pipe rolling according to the present invention will be described.
The plug for seamless steel pipe rolling according to the present invention is produced by dissolving a plug material having the above-described chemical composition in an atmosphere having an oxygen concentration of 0.3% or less and a carbon monoxide concentration of 4 to 8% in terms of volume fraction. After holding at 1200° C. for 1 to 3 hours, the plug material is cooled to 800 to 900° C. and held for 1 to 3 hours to perform heat treatment to form an oxide layer on the surface of the plug material.
The plug material can be manufactured into a desired shape by a conventionally known method.

The resulting plug material is first held at 1000-1200° C. for 1-3 hours in order to obtain the desired properties. When the holding temperature at this time exceeds 1200° C., the effect is saturated.
At this time, the oxygen concentration in the atmosphere is set to 0.3% or less and the carbon monoxide concentration is set to 4 to 8% by volume fraction. When the oxygen concentration exceeds 0.3%, the thickness of the oxide layer is insufficient.
Also, if the carbon monoxide concentration is less than 4%, the amount of hematite becomes too large. On the other hand, when the carbon monoxide concentration exceeds 8%, the thickness of the oxide layer becomes insufficient.
On the other hand, when the holding time is less than 1 hour, the amount of the oxide layer produced is insufficient, and the temperature of the plug cannot be uniformed, resulting in unevenness in the thickness of the oxide layer. On the other hand, after 3 hours, the effect saturates.
Therefore, first, the obtained plug material is held at 1000 to 1200° C. for 1 to 3 hours in an atmosphere having an oxygen concentration of 0.3% or less and a carbon monoxide concentration of 4 to 8% in volume fraction.
In this atmosphere, the remaining gas component is not particularly limited, but generally an inert gas (nitrogen, argon, etc.) may be used.

Subsequently, the plug material after being held is heated to 800 to 900° C. (furnace cooling) as tempering treatment and held for 1 to 3 hours to form a desired oxide layer on the surface of the plug material.

As described above, according to the present invention, a predetermined Vickers hardness is satisfied, and deformation due to impact applied to the plug during rolling is suppressed. Furthermore, even with a single heat treatment, a thick oxide layer having high strength and excellent wear resistance is formed. As a result, the life of the plug can be improved.
In addition, since the unit price is high and the desired effect can be obtained even if the content of the alloy element for the purpose of increasing the strength is reduced, the unit cost of the plug can be reduced.

Further, in the present invention, there are provided a seamless steel pipe rolling plug mill equipped with the seamless steel pipe rolling plug described above, a seamless steel pipe rolling method using the seamless steel pipe rolling plug described above, and a seamless steel pipe rolling method described above. Also provided is a method of manufacturing a seamless steel pipe using a plug for steel.

The present invention will be further described below based on examples.
Table 4 shows the chemical compositions of the plug materials of the plugs used in this example (Steel Nos. A to X).
Steel no. For the plugs obtained from A to U, the plug materials having each component composition were heated at 1185° C. for 2 hours in an atmosphere with an oxygen concentration of 0.3% or less and a carbon monoxide concentration of 4 to 8% by volume fraction. After holding for a period of time, the plugs were cooled to 880° C. (furnace cooling) and held for 2 hours to perform heat treatment to form an oxide layer on the surface of the plug material (Plug Nos. 1 to 21).
Also, steel no. Plug no. Regarding No. 22, the plug material is kept at 1185°C for 2 hours in an atmosphere having an oxygen concentration of 0.3% or less and a carbon monoxide concentration of 20 to 25% by volume, and then cooled to 880°C (furnace A heat treatment was performed to form an oxide layer on the surface of the plug material by cooling and holding for 1.5 hours.
Also, steel no. Plug no. Regarding No. 23, the plug material was held at 1150°C for 2 hours in an atmosphere with an oxygen concentration of 10 to 15% and a carbon monoxide concentration of 6 to 7% by volume, and then cooled to 850°C (furnace cooling). ) and held for 1.5 hours, heat treatment was performed to form an oxide layer on the surface of the plug material.
Also, steel no. Plug no. Regarding No. 24, the plug material is held at 1200° C. for 1 hour in an atmosphere having an oxygen concentration of 0.3% or less and a carbon monoxide concentration of 5 to 6% by volume fraction, and then cooled to 700° C. (furnace A heat treatment was performed to form an oxide layer on the surface of the plug material by cooling it down and holding it for 3 hours.

[Analysis of oxide layer]
Samples having the composition shown in Table 4 were prepared separately from the plugs to be rolled by the plug mill, and subjected to the heat treatment described above. The size of the used sample was 150 mm square and 30 mm thick.
Then, the heat-treated sample was cut, and the thickness of the oxide layer (scale thickness) was measured by dimensional measurement using a digital microscope.
In addition, after the heat treatment for forming the oxide layer, a Vickers hardness tester (load: 3 kgf) was used at a 0.5 mm pitch at a position of 10 mm in the thickness direction from the outer surface of the plug, and was measured at room temperature based on JIS Z 2244:2009. 25° C.) was measured for Vickers hardness.

X-ray diffraction was also performed to assess the composition of these samples. The peak area of each compound was determined from the measured X-ray diffraction pattern and divided by the relative sensitivity coefficient to obtain the volume fraction of the spinel type in the oxide layer. More specifically, only the oxide layer was peeled off from the sample, and the powdered oxide powder was measured.

[evaluation]
Using each of the above plugs, the same material to be rolled was subjected to hot plug mill rolling until the life of the plug. The material to be rolled used had an outer diameter of φ223 mm, a thickness of 16.99 mm, and a length of 8 m, and the material standard was 0.2 mass % C-13 mass % Cr.
The above rolling was carried out under rolling conditions such that the amount of reduction was 30%. The temperature of the material to be rolled was 950 to 1000° C. as the temperature of the outer surface measured by a radiation thermometer. Once the plug was rolled, it was water-cooled, and when the outer surface temperature of the plug became 50° C. or lower, the water-cooling was continued for 10 minutes, and the plug was reused.

The plug life was determined according to the following criteria from the viewpoint of evaluating whether good wear resistance and deformation suppression were achieved.

(Abrasion resistance evaluation)
After rolling, if the thickness of the oxide layer measured by a non-contact thickness measuring device (37DL PLUS, manufactured by Nihon Panametrics Co., Ltd.) using an ultrasonic sensor is less than 0.1 mm, it is assumed that the plug is worn and oxidized. When the thickness of the material layer was 0.1 mm or more, it was determined that the plug was not worn.

(deformation evaluation)
After rolling, the dimensions are measured using a 3D scanner. If a volume change of 10% or more is found, the plug is considered to be deformed, and if the volume change is less than 10%, the plug is not deformed. did. In this evaluation, the presence or absence of deformation was determined after confirming that the surface temperature of the plug had reached room temperature (25° C. or lower) using a contact thermometer.

(Judgment criteria)
○ (accepted): It was determined that the plug was neither worn nor deformed even after rolling five times or more.
⊚ (acceptable (better)): It was judged that the plug was neither worn nor deformed even after rolling 10 times or more.
x (failed): It was judged that at least one of wear and deformation occurred in the plug after rolling less than 5 times.

Table 5 shows the results.

All the examples of the present invention had high Vickers hardness in the range of 500 to 700 HV. Furthermore, the thickness of the oxide layer was 0.4 mm or more, and the volume fraction of the spinel-type oxide layer was 50% or more. The plugs of the present invention were excellent in wear resistance and were able to achieve sufficient suppression of deformation. As a result, it turned out to be a long-life plug with good cost performance.
Among the examples of the present invention, in particular, plug No. 1 in which the volume fraction of the spinel-type oxide layer is 70% or more. 1, 2, 5 gave better results. Moreover, the plug No. In No. 8, the volume fraction of the spinel-type oxide layer is 60% or more, the Vickers hardness is 600 HV or more, and the thickness of the oxide layer is 0.55 mm or more, so superior results were obtained. .

On the other hand, in comparative examples outside the scope of the present invention, the hardness after heat treatment is less than 500 HV or more than 700 HV, the thickness of the oxide layer is less than 0.4 mm, or the volume fraction of the spinel-type oxide layer is 50 % could not be secured. Further, the plugs of the comparative examples could not obtain desired effects in at least one of wear resistance and deformation suppression.

Specifically, first, the plug No. In Steel No. 16 (Steel No. P), the C content was less than the lower limit of the range of the present invention, and the volume change was 10% or more after rolling less than 5 times.
Plug No. In Steel No. 17 (Steel No. Q), the Cr content was less than the lower limit of the range of the present invention, and the volume change was 10% or more after rolling less than 5 times.
Plug No. In No. 18 (Steel No. R), the Cr content exceeded the upper limit of the range of the present invention, and the thickness of the oxide layer became less than 0.1 mm after rolling less than 5 times.
Plug No. Steel No. 19 (Steel No. S) has a C content exceeding the upper limit of the range of the present invention, a content of Mo + W exceeding the upper limit of the range of the present invention, and a volume change after rolling less than 5 times. It was 10% or more.
Plug No. In No. 20 (Steel No. T), the Si content exceeds the upper limit of the range of the present invention, the Cr content exceeds the upper limit of the range of the present invention, and the oxide layer is formed by rolling less than 5 times. The thickness was less than 0.1 mm and the volume change was 10% or more.
Plug No. In Steel No. 21 (Steel No. U), the Mn content exceeded the upper limit of the range of the present invention, and the thickness of the oxide layer became less than 0.1 mm after rolling less than 5 times.
Plug No. No. 22 (Steel No. V) had a steel composition within the range of the present invention, but the concentration of carbon monoxide during heat treatment was high and the formation of an oxide layer was small. The layer thickness was less than 0.1 mm.
Plug No. In No. 23 (steel No. V), the steel composition was within the range of the present invention, but the oxygen concentration during heat treatment was high and the oxide layer was formed thickly, but the ratio of magnetite was small. After rolling, the thickness of the oxide layer became less than 0.1 mm, and the volume change was 10% or more.
Plug No. No. 24 (steel No. V) had a steel composition within the range of the present invention, but the temperature during heat treatment was low and the desired hardness could not be obtained. It was 10% or more.

1 plug 2a, 2b rolling roll 3 bar 101 rolling mill S hollow

Claims (5)

A seamless steel pipe rolling plug comprising a plug material and an oxide layer formed on the surface of the plug material,
The plug material is
% by mass, C: 1.0 to 2.0%, Si: 0.3 to 1.5%, Mn: 0.3 to 1.5%, Al: 0.01 to 0.1%, Ni: Contains 0.01-5.0%, Cr: 12-20%, one or two of Mo and W (Mo+W): 0.1-3.0%, V: 0.01-0.1% and the balance has a component composition consisting of Fe and inevitable impurities,
The oxide layer has a spinel-type oxide at a volume fraction of 50% or more and has a layer thickness of 0.4 mm or more,
A seamless steel pipe rolling plug having a Vickers hardness of 500 to 700 HV at a position 10 mm in the thickness direction from the surface of the plug material. A method for manufacturing a seamless steel pipe rolling plug according to claim 1,
The plug material is maintained at 1000 to 1200°C for 1 to 3 hours in an atmosphere having an oxygen concentration of 0.3% or less and a carbon monoxide concentration of 4 to 8% by volume, and then cooled to 800 to 900°C. A method for producing a plug for seamless steel pipe rolling, wherein heat treatment is performed to form an oxide layer on the surface of the plug material by holding for 1 to 3 hours. A seamless steel pipe rolling plug mill comprising the seamless steel pipe rolling plug according to claim 1 . A method for rolling a seamless steel pipe using the seamless steel pipe rolling plug according to claim 1 . A method for producing a seamless steel pipe using the seamless steel pipe rolling plug according to claim 1 .

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