JP6874286B2 - Manufacturing method of steel pipe - Google Patents

Description

The present invention relates to a method for producing a steel pipe, and more particularly to a production method including a quenching step.

Steel pipes may be required to have high fatigue strength depending on the application. For example, a steel pipe used for a stabilizer of an automobile is repeatedly subjected to a torque load, and it is necessary that the steel pipe for this purpose is not damaged by such a torque load.

When the steel pipe is heat-treated in the atmosphere, the surface layer portion of the steel pipe is oxidized and decarburized, which reduces the hardness of the portion and the fatigue strength. Even if the hardness of the surface layer portion of the outer surface of the steel pipe is reduced, fatigue strength can be ensured by performing shot peening and blasting treatment. However, depending on the size and shape of the steel pipe, such treatment cannot be applied to the surface layer portion of the inner surface. Therefore, it is necessary to suppress decarburization of the inner surface of the steel pipe.

Patent Document 1 discloses a method for manufacturing a steel pipe for hollow parts, which has improved fatigue strength by suppressing the occurrence of a decarburized layer. In this method, first, a solid carburizing agent containing one or more of charcoal, graphite, coal, and coke is inserted into the steel pipe. Then, plugs are attached to both ends of the steel pipe, the steel pipe is induced to be heated, and the diameter is reduced and rolled hot. The solid carburizing agent is, for example, in the form of powder and is inserted into the inside of the steel pipe by air blowing. It is described in Patent Document 1 that this method can suppress the formation of a decarburized layer on the inner surface of the steel pipe and improve the fatigue strength.

Patent Document 2 discloses a method for preventing decarburization of the inner surface of a steel pipe for bearings. In this way, when subjected to a spheroidizing annealing to a bearing steel, carbon material is sealed in the steel bearing, then, both ends of a steel pipe is sealed, steel is annealed at a temperature of 600 ° C. to A _c3 point .. The carbon material is inserted into the steel pipe by, for example, air blowing. It is described in Patent Document 2 that decarburization in a steel pipe can be sufficiently prevented by this method.

Japanese Unexamined Patent Publication No. 2010-189758 Japanese Unexamined Patent Publication No. 5-339369

However, even with these methods, it may not be possible to increase the fatigue strength of the steel pipe after quenching.
Therefore, an object of the present invention is to provide a method for manufacturing a steel pipe, which can improve the fatigue strength.

The method for producing a steel pipe according to the embodiment of the present invention includes a filling step and a quenching step. In the filling step, the inside of the steel pipe is filled with powder. In the quenching step, the steel pipe filled with the powder is quenched. The melting point of the powder is higher than the maximum temperature reached of the steel pipe in the quenching step.

According to the manufacturing method of the present invention, a steel pipe having high fatigue strength can be manufactured.

FIG. 1A is an electron microscope image of a cut surface of a steel pipe hardened under condition 1. FIG. 1B is an electron microscope image of a cut surface of a steel pipe hardened under condition 2. FIG. 1C is an electron microscope image of a cut surface of a steel pipe hardened under condition 3, showing a portion where the inner surface is maintained flat. FIG. 1D is an electron microscope image of a cut surface of a steel pipe hardened under condition 3, showing a portion where the inner surface is roughened. FIG. 1E is an electron microscope image of a cut surface of a steel pipe hardened under condition 4. It is a figure which shows the relationship between the depth from the inner surface of a steel pipe, and the hardness of the sample which hardened under conditions 2-4. It is a top view which shows the shape and size of the sample used for a fatigue test.

As used herein, "filling" a steel pipe with powder means making the volume of the powder occupy 90% or more of the effective internal volume of the steel pipe. "Effective internal volume" refers to the volume of the portion of the internal space of the steel pipe where powder can exist. As will be described later, when the end portion of the steel pipe filled with powder is sealed and hardened, the volume of the space occupied by the sealing material in the steel pipe is not included. "Powder volume" refers to the bulk of the powder and includes the volume of space between the particles that make up the powder.

"Melting point of powder" means the upper limit of the temperature at which the powder can exist in the solid phase, and when the powder changes from the solid phase to the gas phase without going through the liquid phase, the temperature from the solid phase to the gas phase. To say.

As a result of studies by the present inventors, it has been clarified that the fatigue strength of the steel pipe after quenching is not increased by the methods of Patent Documents 1 and 2. The reason is as follows. In the methods of Patent Documents 1 and 2, the solid carburizing agent or carbon material (hereinafter referred to as "carbon material or the like") is not filled inside the steel pipe. That is, there are many spaces in the steel pipe after the carbon material or the like is inserted, in which the carbon material or the like does not exist.

Therefore, on the inner surface of the steel pipe, a portion that comes into contact with the carbon material or the like and a portion that does not come into contact occur. When the steel pipe in this state is hardened, oxidation may proceed and decarburization may occur in the portion where the carbon material or the like is not in contact. Further, even if the decarburization of the inner surface of the steel pipe is suppressed, there is a difference in the cooling rate between the portion where the carbon material or the like contacts and the portion where the carbon material does not contact during cooling, and uneven burning occurs on the surface layer portion of the inner surface of the steel pipe. Due to these factors, a portion having a low hardness is generated on the surface layer portion of the inner surface of the steel pipe. Since that part can be the starting point of fracture, the fatigue strength is reduced.

Therefore, the present inventors have come up with the idea of performing quenching in a state where the steel pipe is filled with powder. The method for producing a steel pipe according to the embodiment of the present invention includes a filling step and a quenching step. Hereinafter, each step will be described in detail.

<Filling process>
In this step, the inside of the steel pipe is filled with powder. The shape and size of the steel pipe are not limited. The steel pipe may be, for example, a welded pipe or a seamless pipe. The material of the steel pipe is not particularly limited. The material of the steel pipe can be, for example, carbon steel for machine structure specified by JIS such as S25C to S55C. The steel pipe may be a steel containing Cr, for example, JIS, SCM435, SCM440, JIS G 4051-1979, SNC236, or SNCM65. Further, in order to improve the hardenability, one or more of optional additive elements such as Ni, Ti, Al, B, Ca, V and W are added to the carbon steel for mechanical structure in total. It may be contained in an amount of 0% by mass or less.

The average particle size of the particles constituting the powder is preferably 1 mm or less. The average particle size can be the average value of the results measured three times by the laser diffraction method based on JIS8825: 2013. The reason why the average particle size is preferably 1 mm or less is that when the average particle size exceeds 1 mm, the proportion of the portion of the inner surface of the steel pipe that does not come into contact with the powder increases, and the fatigue strength after quenching may decrease. Because there is. For the purpose of improving fatigue strength, there is no lower limit to the average particle size of the particles. However, if the average particle size is less than 20 μm, the powder tends to scatter and is difficult to handle. Therefore, the average particle size is preferably 20 μm or more.

The method of filling the steel pipe with powder is not particularly limited. For example, when the steel pipe is a straight pipe, the powder may be inserted into the steel pipe through the opening at the upper part of the steel pipe with the steel pipe directed along the vertical direction. When the steel pipe has a curved portion or a bent portion, for example, powder is inserted into the steel pipe from one end of the steel pipe, and then the inserted powder is pushed from one end to the other end of the steel pipe. By repeating the operation, the steel pipe can be filled with powder.

<Quenching process>
In this step, the powder-filled steel pipe is hardened. Quenching conditions are not particularly limited. For example, the steel pipe may be heated to (A3 transformation point +50) ° C. to 1050 ° C. and then water-cooled or oil-cooled. The hardened steel pipe may have, for example, a martensite structure in substantially all parts.

Since the steel pipe is filled with powder, the amount of oxygen present as a gas in the steel pipe is small. Therefore, when the temperature of the steel pipe is high due to heating for quenching or the like, the inner surface of the steel pipe is less likely to be oxidized. As a result, decarburization is unlikely to occur on the inner surface. Further, since the steel pipe is filled with powder, the unevenness of the cooling rate of the steel pipe is reduced during quenching. Therefore, the inner surface of the steel pipe has less quenching unevenness. Therefore, in the steel pipe manufactured by the method of the present invention, a portion having a low hardness is unlikely to occur. Therefore, this steel pipe has high fatigue strength.

Assuming that the C (carbon) content of the steel pipe is A mass%, the powder filled inside the steel pipe in the filling step preferably contains 1.1 × A mass% or less of carbon. In this case, when the steel pipe is heated, the fatigue strength can be increased by carburizing the surface layer portion of the inner surface of the steel pipe to improve the hardness. If the C content of the powder exceeds 1.1 × A mass%, C may be excessively diffused from the powder to the surface layer portion on the inner surface of the steel pipe. In this case, the C content of the surface layer portion of the inner surface of the steel pipe becomes high, and cracks are likely to occur. In order to increase the fatigue strength by carburizing, the C content of the powder is preferably 0.5 × A mass% or more, for example.

The powder filled inside the steel pipe in the filling step preferably contains Fe (iron) powder. The Fe content in the powder is preferably sufficiently high, for example, 90% by mass or more. When the steel pipe is heated, element diffusion can occur from the powder to the surface layer on the inner surface of the steel pipe. When the Fe content in the powder is sufficiently high, the composition fluctuation of the surface layer portion due to such diffusion is unlikely to occur. The powder may be iron powder. When the powder contains Fe and C, the powder may be a mixture of iron powder and carbon powder, or may be steel powder (for example, carbon steel powder).

The production method of the present invention preferably further includes a step of sealing the end of the powder-filled steel pipe before the quenching step. This step may be carried out, for example, by sealing one end of the steel pipe, filling the inside of the steel pipe with powder, and then sealing the other end of the steel pipe. As a result, even if the steel pipe is tilted from a horizontal state during quenching, the filled powder can be prevented from flowing out of the steel pipe. The sealing material used for sealing may be present in a region including the inside of the steel pipe near the end of the steel pipe. As the sealing material, for example, clay can be used.

In order to confirm the effect of the present invention, the steel pipe was hardened under various conditions and evaluated. For the test, an electrosewn steel pipe of chromium molybdenum steel (SCM430) having the chemical composition shown in Table 1 was used. This steel pipe was a straight pipe having an outer diameter of 25.5 mm, a thickness of 4 mm, and a length of 150 mm. The inner surface of this steel pipe was buffed and flat.

Quenching was carried out under the following six conditions. In the following, the predetermined temperatures were all set to 950 ° C.

Condition 1: The steel pipe is heated in an atmospheric furnace and reaches a predetermined temperature without filling the steel pipe with anything (however, air was present. Hereinafter, the same applies when "nothing is filled"). After that, it was immersed in water without substantially holding it at that temperature.
Condition 2: Without filling the steel pipe with anything, the steel pipe was heated in an atmospheric furnace, reached a predetermined temperature, held for 10 minutes, and immersed in water.

Condition 3: The steel pipe was immersed in a salt bath (molten salt bath) and heated without filling the steel pipe with anything, and after reaching a predetermined temperature, the steel pipe was held for 10 minutes. In the molten salt, the molten salt was present in the steel pipe. Then, the steel pipe was pulled up from the salt bath and immersed in water.

Condition 4: One end of the steel pipe is sealed with clay, and a powder obtained by mixing iron powder and carbon powder at a ratio of 99.72: 0.28 (mass ratio) inside the steel pipe is mixed with 90 of the effective internal volume of the steel pipe. After filling in% or more, the other end of the steel pipe was sealed with clay. The steel pipe in this state was heated in an atmospheric furnace, and after reaching a predetermined temperature, it was held for 10 minutes and immersed in water.

Condition 5: One end of the steel pipe was sealed with clay, Styrofoam (Styrofoam (registered trademark)) was inserted into the inside of the steel pipe, and then the other end of the steel pipe was sealed with clay. The steel pipe in this state was heated in an atmospheric furnace, and after reaching a predetermined temperature, it was held for 10 minutes and immersed in water. The amount of Styrofoam inserted was such that the amount of carbon per unit volume was the same as in Condition 4 (0.02 g / cm ³ ). The melting point of Styrofoam used was about 170 ° C., which was lower than the maximum temperature reached at the time of quenching, which was 950 ° C. During quenching and raising the temperature, molten styrofoam flowed out of the steel pipe from the gap formed in the sealing portion at the end of the steel pipe.

Condition 6: One end of the steel pipe was sealed with clay, and the inside of the steel pipe was filled with silicone sealant at 90% or more of the effective internal volume of the steel pipe, and then the other end of the steel pipe was sealed with clay. The steel pipe in this state was heated in an atmospheric furnace, and after reaching a predetermined temperature, it was held for 10 minutes and immersed in water. The melting point of the silicone sealant used was about 200 ° C., which was lower than the maximum temperature reached at the time of quenching, which was 950 ° C. At the time of quenching and raising the temperature, the molten silicone sealant flowed out of the steel pipe from the gap formed in the sealing portion at the end of the steel pipe.

At the time of quenching, under condition 4, the steel pipe was filled with the mixed powder from the start of heating to the end of cooling. That is, there was no period in which substantially only air was present in the steel pipe. On the other hand, under conditions 1 and 2, substantially only air was present in the steel pipe until it was immersed in water for cooling. Under condition 3, there was a period in which substantially only air was present in the steel pipe before the steel pipe was pulled out of the molten salt and immersed in water. Under conditions 5 and 6, most of the inside of the steel pipe was occupied by air after the molten styrofoam or silicone sealant flowed out of the steel pipe at the time of temperature rise.

All the hardened steel pipes were tempered at a low temperature of 200 ° C. for 1 hour, and then the outer surface of the steel pipes was shot peened and blasted.

The production method including quenching of condition 4 is an embodiment of the present invention, and the production method including quenching of conditions 1, 3, 5, and 6 is a comparative example that does not satisfy the requirements of the production method of the present invention. is there.
As a reference example, a sample was prepared by removing the decarburized layer of the inner surface surface layer by cutting with respect to the sample prepared under the same conditions as the sample (steel pipe) of Test No. 2. The removal thickness was about 0.2 mm.

Then, a sample including the inner surface of the steel pipe was collected from each of these steel pipes, and microstructure observation, hardness measurement, and fatigue test were performed. Table 2 shows the quenching conditions, hardness measurement, and fatigue test results.

The structure was observed by obtaining an electron microscope image of the cut surface perpendicular to the inner surface of the steel pipe. 1A to 1E show electron microscopic images of the samples hardened under each of the conditions 1 to 4 (samples of test numbers 1 to 4, respectively). Most of the samples were martensitic.

FIG. 1A is an electron microscope image of the steel pipe of test number 1. In this image, the upper end of the cut surface corresponds to the inner surface of the steel pipe (the same applies to FIGS. 1B to 1E). The inner surface of this steel pipe was maintained in a flat state, but texture unevenness occurred, and particles significantly smaller than the particles of the main size were present. It is probable that the tissue unevenness occurred because the temperature was not maintained at the maximum temperature (950 ° C.) at the time of quenching.

FIG. 1B is an electron microscope image of the steel pipe of test number 2. No tissue unevenness occurs in this steel pipe. However, the inner surface was rough. It is considered that this is because the inner surface of the steel pipe was oxidized and decarburized.

1C and 1D are electron microscope images of the steel pipe of test number 3. This steel pipe had a uniform particle size. The inner surface of the steel pipe was mostly maintained in a flat state as shown in FIG. 1C, but was partially roughened as shown in FIG. 1D. The roughening of the inner surface of the steel pipe is considered to be due to oxidation and decarburization.

FIG. 1E is an electron microscope image of the steel pipe of test number 4. In this steel pipe, the inner surface was flat and had a uniform particle size.

The hardness was measured at 6 points for each sample under each quenching condition as Vickers hardness with a load of 10 gf (0.098 N). Table 2 shows the lowest value (minimum value) of the measured hardness values for the samples under each quenching condition.

In the sample of Reference Example (Test No. 7), the surface layer portion on the inner surface is removed as described above. As a result, it is considered that the decrease in hardness due to uneven burning and decarburization does not occur substantially. It can be said that the sample of test number 4 has almost the same hardness as the sample of the reference example.

The sample of test number 1 has a slightly lower minimum hardness than the sample of the reference example. This is considered to be related to the fact that the steel pipe has uneven structure. The samples of test numbers 2 and 3 have a lower minimum hardness than the samples of test number 1. The sample of test number 3 had the same hardness as the sample of the reference example at most of the measurement points, but had a low hardness at some measurement points, that is, locally. .. It is considered that this is because the inner surface of the steel pipe of test number 2 and a part of the steel pipe of test number 3 was decarburized.

The sample of the reference example had good fatigue characteristics as well as hardness as described later. However, the removal of the inner surface layer portion including the decarburized layer causes a significant increase in man-hours and costs with respect to the manufacturing methods of test numbers 1 to 6, and it is not realistic to carry out the removal at the time of mass production of steel pipes.

The hardness of the samples quenched under conditions 2 to 4 (samples of test numbers 2 to 4, respectively) was measured by changing the depth position from the inner surface. From the results, the relationship between the depth from the inner surface and the hardness was clarified. FIG. 2 shows the relationship between the depth from the inner surface and the hardness of each sample. When the hardness is measured at a plurality of points at the same depth, FIG. 2 shows the maximum value of the hardness at that depth.

As shown in FIG. 2, in a portion deeper than the inner surface by 0.6 mm or more, these samples show almost the same values as each other. On the other hand, in the shallower portion, the hardness (maximum value) of the sample quenched under condition 4 was significantly higher than that of the sample quenched under conditions 2 and 3.

The shape of the sample used in the fatigue test (hereinafter referred to as "fatigue test sample") was a substantially rectangular flat plate, and the longitudinal direction of the fatigue test sample was the same as the longitudinal direction of the steel pipe. FIG. 3 is a plan view showing the shape and size of the fatigue test sample. As shown in FIG. 3, the fatigue test sample has a groove-shaped curved inner surface (the inner surface in the steel pipe) at the center of the surface in the width direction along the longitudinal direction of the fatigue test sample. Had s. The maximum depth of the inner surface s was 0.1 mm.

The fatigue test was performed by repeatedly twisting the fatigue test sample around its longitudinal axis. The maximum stress during twisting was 500 MPa or 400 MPa. As shown in Table 2, the sample of test number 4 did not break by twisting ^{2.00 × 10 6 times in any of the tests performed at the maximum stress.} That is, the sample of test number 4 had a significantly longer breaking life than the samples of test numbers 1, 3, 5 and 6. As with the sample of Test No. 4, the sample of the reference example did not break by twisting ^{2.00 × 10 6 times in any of the tests performed at the maximum stress.}

In the sample hardened under condition 4, the amount of oxygen in the vicinity of the inner surface of the steel pipe was kept uniform and low because the steel pipe was filled with a mixed powder of iron powder and carbon powder at the time of baking, and the inner surface of the steel pipe was hardened. However, it hardly oxidized and decarburization did not occur substantially. Further, since the mixed powder was filled in the steel pipe, the cooling rate became uniform during quenching, and uneven baking did not occur. Due to these factors, the starting point of fracture was unlikely to occur in this steel pipe, and the fracture life was long. That is, the fatigue strength was high.

In the samples of test numbers 1 to 3, as described above, at least a part of the inner surface was decarburized by oxidation. In these samples, it is considered that the fracture occurred starting from the decarburized layer. In the samples of test numbers 5 and 6, oxygen flowed into the steel pipe due to the outflow of styrofoam and silicone sealant melted during quenching and heating, decarburization occurred on the surface layer of the inner surface of the steel pipe, and the decarburized layer was the starting point. It is probable that the destruction occurred.

Claims (3)

The filling process of filling the inside of the steel pipe with powder,
Including a quenching step of quenching a steel pipe filled with the powder.
The melting point of the powder is higher than the maximum temperature reached by the steel pipe in the quenching step.
The powder has an Fe content of 90% by mass or more, and is a mixture of Fe powder and carbon powder.
A method for producing a steel pipe, wherein the C content of the steel pipe is A mass%, and the powder contains 0.5 × A mass% to 1.1 × A mass% of carbon. The method for manufacturing a steel pipe according to claim 1.
A method for producing a steel pipe, wherein the average particle size of the powder is 1 mm or less. The method for manufacturing a steel pipe according to claim 1 or 2.
A method for producing a steel pipe, further comprising a step of sealing an end portion of the steel pipe filled with the powder before the quenching step.

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