JP2007262469A - Steel pipe and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel pipe whose fatigue strength is more improved than that of the conventional one. <P>SOLUTION: Regarding the steel pipe comprising, by mass, 0.3 to 0.1% C, ≤3.0% Si and 2.0% Mn, and the balance Fe with inevitable impurities, and in which at least a part is subjected to quenching, in the quenched structure, the average grain size of old austenitic grains is ≤12 μm, and also, the maximum grain size is ≤4 times the average grain size. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel pipe and a method for manufacturing the same, which are at least partially provided with a hardened portion by induction hardening.

Conventionally, for example, a drive shaft for an automobile is processed as a drive shaft by subjecting a hot rolled steel bar to hot forging, further cutting, cold forging, etc., and processing it into a predetermined shape, followed by induction hardening and tempering. In general, the fatigue strengths such as torsional fatigue strength, bending fatigue strength, rolling fatigue strength, and sliding rolling fatigue strength, which are important characteristics, are ensured.
On the other hand, in recent years, there is a strong demand for weight reduction of automotive parts due to environmental problems, and from this point of view, replacement of a drive shaft from a solid steel bar to a hollow steel pipe has been studied.

  As the steel pipe used for the drive shaft, use of an electric resistance welded steel pipe, a pressure welded steel pipe, a forged steel pipe or the like can be considered. In order to employ these steel pipes for the drive shaft, resistance to a load repeatedly applied to the drive shaft as the automobile starts suddenly, particularly excellent fatigue strength is required.

Here, regarding a hollow drive shaft using an electric resistance welded steel pipe, Patent Document 1 discloses that the strength is increased by induction hardening. For example, in order to improve the torsional fatigue strength, it is conceivable to increase the quenching depth by induction quenching. However, even if the quenching depth is increased, the fatigue strength is saturated at a certain depth. Therefore, simply performing induction hardening has left a problem in that it cannot sufficiently meet the recent demand for fatigue strength of automobile parts.

Further, in Patent Document 2, in order to improve the fatigue strength of the steel pipe used for the drive shaft, in order to suppress the grain growth by MnS and improve the fatigue strength, the heating temperature of the slab during the hot rolling is used. Is set to 1350 ° C to 1420 ° C. However, the demand for torsional fatigue strength in recent years has not been sufficiently met.
JP 2004-162125 A JP 2003-321713 A

  The present invention has been developed in view of the above-described present situation, and an object thereof is to propose a steel pipe having a further improved fatigue strength as compared with the conventional manufacturing method.

Now, in order to effectively improve the above-described fatigue characteristics, the inventors have conducted intensive studies particularly on the induction-quenched structure.
As a result, focusing on the grain size of the prior austenite grains in the induction-quenched structure, and reducing the average grain size of the prior austenite grains, fatigue characteristics such as torsional fatigue strength, bending fatigue strength, and rolling fatigue strength are improved. I came to find out.

That is, the gist configuration of the present invention is as follows.
(1) C: 0.3 to 1.5 mass%,
Si: 3.0mass% or less and
Mn: 2.0 mass% or less, the balance is made of Fe and inevitable impurities, and further has a hardened portion by induction hardening, and the hardened portion has an average diameter of prior austenite grains of 12 μm or less and a maximum grain size of average grains A steel pipe excellent in fatigue characteristics, characterized by being 4 times or less in diameter.

(2) The steel pipe further includes
Al: The steel pipe according to (1) above, containing 0.25 mass% or less.

(3) The steel pipe is further
Cr: 2.5 mass% or less,
Mo: 1.0mass% or less,
Cu: 1.0 mass% or less,
Ni: 2.5 mass% or less,
Co: 1.0mass% or less,
The steel pipe according to (1) or (2) above, which contains one or more selected from V: 0.5 mass% or less and W: 1.0 mass% or less.

(4) The steel pipe further includes
Ti: 0.1 mass% or less,
Nb: 0.1 mass% or less,
Zr: 0.1 mass% or less,
B: 0.01 mass% or less,
Ta: 0.5 mass% or less,
Hf: 0.5 mass% or less and
The steel pipe according to (1), (2) or (3) above, containing one or more selected from Sb: 0.015 mass% or less.

(5) The steel pipe is further S: 0.1 mass% or less,
Pb: 0.1 mass% or less,
Bi: 0.1 mass% or less,
Se: 0.1 mass% or less,
Te: 0.1 mass% or less,
Ca: 0.01 mass% or less,
Mg: 0.01 mass% or less and
REM: The steel pipe according to any one of (1) to (4) above, which contains one or more selected from 0.1 mass% or less.

(6) Using as a raw material a steel pipe containing at least 10% by volume of one or both of a fine bainite structure and a fine martensite structure, a temperature increase rate of 400 ° C./s or more reaches at least a part of the steel pipe. A method for producing a steel pipe, characterized by performing high-frequency heating at a temperature of 1000 ° C. or less once or more.

(7) In the above (6), the raw material has a hot working step in which the total working rate at 800 to 1000 ° C. is 80% or more, and a temperature range of 700 to 500 ° C. after the hot working step is 0.2 ° C. Cooling step of cooling at a cooling rate of at least / s, and further, processing of 20% or more is performed in a temperature range of less than 700 to 800 ° C. before the cooling step, or the A ₁ transformation point or less after the cooling step And a second processing step of performing a processing of 20% or more in the temperature range of the steel pipe manufacturing method.

(8) The method for producing a steel pipe according to (6) or (7), wherein a residence time of 800 ° C. or more in one high-frequency heating is 5 seconds or less.

(9) In any one of the above (6) to (8), the steel pipe is
C: 0.3-1.5 mass%
Si: 3.0mass% or less and
Mn: The manufacturing method of the steel pipe characterized by being a composition which contains 2.0 mass% or less and consists of remainder Fe and an unavoidable impurity.

(10) In the above (9), the steel pipe further comprises:
A method for producing a steel pipe, comprising Al: 0.25 mass% or less.

(11) In the above (9) or (10), the steel pipe further comprises:
Cr: 2.5 mass% or less,
Mo: 1.0mass% or less,
Cu: 1.0 mass% or less,
Ni: 2.5 mass% or less,
Co: 1.0mass% or less,
The manufacturing method of the steel pipe characterized by containing 1 type (s) or 2 or more types selected from V: 0.5 mass% or less and W: 1.0 mass% or less.

(12) In the above (9), (10) or (11), the steel pipe further comprises:
Ti: 0.1 mass% or less,
Nb: 0.1 mass% or less,
Zr: 0.1 mass% or less,
B: 0.01 mass% or less,
Ta: 0.5 mass% or less,
Hf: 0.5 mass% or less and
Sb: The manufacturing method of the steel pipe characterized by containing 1 type, or 2 or more types selected from below 0.015 mass%.

(13) In any one of the above (9) to (12), the steel pipe is further S: 0.1 mass% or less,
Pb: 0.1 mass% or less,
Bi: 0.1 mass% or less,
Se: 0.1 mass% or less,
Te: 0.1 mass% or less,
Ca: 0.01 mass% or less,
Mg: 0.01 mass% or less and
REM: The manufacturing method of the steel pipe characterized by including 1 type or 2 types or more selected from 0.1 mass% or less.

  According to the present invention, it is possible to stably obtain a steel pipe excellent in all fatigue characteristics such as bending fatigue characteristics, rolling fatigue characteristics, and sliding rolling fatigue characteristics as well as torsional fatigue characteristics. This is very effective for reducing the weight of components.

The present invention will be specifically described below.
The steel pipe of the present invention can be applied as a drive shaft, an input shaft, an output shaft, a crankshaft and the like for automobiles, and has a hardened layer that has been hardened particularly on a part or the whole where fatigue strength is required. It is important that the quenched structure has a prior austenite particle size of 12 μm or less.

The following describes the research results that led to this finding.
The steel material having the composition shown in the following steel a or steel b is melted in a 150 kg vacuum melting furnace, the steel material is hot-rolled according to various hot working conditions to form a steel strip having a width of 68 mm, The steel strip is formed into a tubular shape with a forming roll, the edge portion of the tubular steel strip is heated, and both edge portions are welded together by abutting the edge portions together using a pressure roll. A steel pipe was manufactured. Next, after cutting this ERW steel pipe to a predetermined length, the surface is machined and partly cold rolled to adjust the diameter, and at the same time, the spline part is rolled, and FIG. A tubular test piece 1 having a spline portion 2 having the dimensions and shape shown was produced.
[Steel a] C: 0.48 mass%, Si: 0.55 mass%, Mn: 0.78 mass%, P: 0.011 mass%, S: 0.019 mass%, Al: 0.024 mass%, N: 0.0043 mass%, balance Fe and Inevitable impurities.
[Steel b] C: 0.48 mass%, Si: 0.51 mass%, Mn: 0.79 mass%, P: 0.011 mass%, S: 0.021 mass%, Al: 0.024 mass%, N: 0.0039 mass%, Mo: 0.45 mass %, Ti: 0.021 mass%, B: 0.0024 mass%, remaining Fe and inevitable impurities.

  Next, after heating and quenching the test piece under various conditions using an induction hardening apparatus having a frequency of 10 to 200 kHz, tempering was performed at 170 ° C. for 30 minutes using a heating furnace. Thereafter, the torsional fatigue strength was evaluated.

The torsional fatigue strength was evaluated by the torque value (N · m) when the number of repetitions of fracture was 1 × 10 ⁵ times in the torsional fatigue test of the test piece. In the torsional fatigue test, a hydraulic fatigue tester is used, and as shown in FIG. 2 (a), the spline portions 2a and 2b are incorporated in the disc-shaped grippers 3a and 3b, respectively, and between the grippers 3a and 3b. Frequency: performed by repeatedly applying torsional torque at 1 to 2 Hz.

Moreover, about the same test piece, the structure | tissue of the hardened layer was observed using the optical microscope, and the prior austenite grain average diameter and the largest old austenite grain size were calculated | required.
The average austenite grain size is measured 400 times (area of 1 field: 0.25 mm x 0.225 mm) to 1000 times (area of 1 field of view: 0.10 mm x 0.09 mm) using an optical microscope. Five fields of view were observed for each of the 1/5 position, 1/2 position, and 4/5 position, and the average austenite grain diameter at each position was measured. The maximum value was defined as the average austenite grain diameter. The hardened layer thickness was a depth region from the surface until the area ratio of the martensite structure was reduced to 98%.

On the other hand, the maximum prior austenite grain size is 400 times (area of 1 field of view: 0.25 mm x 0.225 mm), and the area corresponding to 5 fields of view is measured at each position in the cured layer thickness direction. The value obtained by the following formula from the particle size distribution inside was taken as the maximum particle size.
Maximum particle size = average particle size + 3σ (σ: standard deviation)

  For the measurement of prior austenite grains, for the cross section cut in the thickness direction of the hardened layer, 11 g of sodium dodecylbenzenesulfonate: ferrous chloride in a picric acid aqueous solution in which 50 g of picric acid was dissolved in 500 g of water. : 1 g and oxalic acid: 1.5 g added were allowed to act as a corrosive solution to reveal the prior austenite grain boundaries.

  First, FIG. 3 shows the relationship between the average prior austenite grain size and torsional fatigue strength. As shown in FIG. 3 (a), it was recognized that the fatigue strength increases as the average particle size decreases. However, when the prior austenite particle size is as small as 12 μm or less, there may be a difference in fatigue strength even when the particle size is similar, and this cause depends on the particle size distribution, particularly the maximum particle size. I found out. As a result of further intensive studies on this point, it has been found that when the maximum particle size is 4 times or less than the average particle size, the effect of improving the fatigue strength by refining the average particle size becomes significant. In each plot shown in FIG. 3A, when the maximum particle size / average particle size is 4 or less, a white square or rhombus, and when the maximum particle size / average particle size is more than 4, black squares or What was re-plotted as a rhombus is shown in FIG.3 (b).

Thus, the factors that affect the fatigue strength by the average particle size and the maximum particle size are estimated as follows.
Impurity elements that cause fatigue failure tend to segregate at the prior austenite grain boundaries. Therefore, as the grain size of the prior austenite grain boundary becomes finer, the segregation area increases, the impurity concentration at each segregation site decreases, and the fracture strength increases. Further, the stress concentration on the prior austenite grain boundaries due to notches or the like is dispersed as the grain size becomes finer, and the stress acting on the individual grain boundaries decreases, resulting in an increase in fatigue strength. It is estimated that such an effect is influenced not only by the average particle diameter but also by the maximum particle diameter. That is, in the vicinity of a large grain, since the grain boundary area is small, the concentration of impurities is likely to proceed. Furthermore, it is considered that stress is hardly dispersed.

Thus, when there exists a large grain exceeding 4 times the average grain diameter, it is presumed that the possibility of lowering the fatigue strength due to the above action increases.
In particular, when the maximum grain size of the prior austenite grains is 20 μm or less, a large improvement in fatigue strength can be expected stably over a wide range of component shapes. More preferably, the average particle size is 5 μm or less. More preferably, the average particle size is 4 μm or less.

  Next, FIG. 4 is a graph showing the influence of the average prior austenite grain size of the hardened layer and the maximum prior austenite grain size / average prior austenite grain size on the torsional fatigue strength. It can be seen that when the average prior austenite particle size is 12 μm or less, the fatigue strength can be remarkably improved by setting the maximum prior austenite particle size / average prior austenite particle size to 4 or less. It can also be seen that when the average austenite particle size is 5 μm or less, and further 3 μm or less, the fatigue strength improvement effect due to the maximum old austenite particle size / average prior austenite particle size being 4 or less becomes even more remarkable.

FIG. 5 shows the effects of the cold working rate, the highest temperature reached during high-frequency heating (heating temperature), and the rate of temperature increase on torsional fatigue strength. FIG. 5 shows that excellent fatigue characteristics can be obtained under conditions where the cold working rate is 25% or more, the maximum temperature reached during induction hardening is 1000 ° C. or less, and the temperature rising rate is 400 ° C./s or more.
Table 1 shows the test results used to obtain the above-described FIGS.

  Here, in order to set the average grain size of prior austenite grains to 12 μm or less and the maximum grain size to 4 times or less the average grain size, a uniform fine bainite structure and / or martensite structure is used in the structure before induction hardening. It is advantageous to have a method of containing the above. This method will be described below.

  That is, regarding the structure before induction hardening, the structure fraction of the bainite structure and / or martensite structure is set to 10 vol% or more, preferably 25 vol% or more. If there are many bainite or martensite structures in the pre-quenching structure, the area of the ferrite / carbide interface that is the nucleation site of austenite increases during quenching heating because the bainite structure or martensite structure is a structure in which carbides are finely dispersed. And since the produced austenite refines | miniaturizes, it contributes effectively in refining the prior austenite grain size of a hardening hardening layer. The grain boundary strength is increased and the fatigue strength is improved by making the austenite grain size finer during quenching heating.

  To achieve a uniform fine bainite structure and / or martensite structure fraction of 10 vol% or more, hot working that the total processing rate at 800 to 1000 ° C of steel with the component composition described below is 80% or more is required. And after this hot working, the temperature range of 700 to 500 ° C. may be cooled at a cooling rate of 0.2 ° C./s or more. This is because when the total processing rate at 800 to 1000 ° C. is less than 80%, a sufficiently uniform fine bainite structure or martensite structure cannot be obtained. Further, if the temperature range of 700 to 500 ° C. is not cooled at a cooling rate of 0.2 ° C./s or more after hot working, the bainite structure and / or martensite structure cannot be made 10 vol% or more in total.

  In addition, the finish rolling at the time of hot rolling in the manufacturing process of the steel plate used as the raw material for pipe making is used suitably for the hot working here.

Further, in order to refine the average grain size and the maximum grain size of the prior austenite in the hardened layer after induction hardening, 20% or more processing is performed in a temperature range of less than 800 ° C. before induction hardening (second) Processing step). Processing in a temperature range of less than 800 ° C may be performed in the hot working process before cooling at the cooling rate (temperature range of less than 700 to 800 ° C), or may be separately cold worked after cooling, or it may be subjected to re-heating to warm working at a ₁ transformation point or lower. The processing rate at less than 800 ° C is more preferably 30% or more.
Examples of the processing method include diameter reduction processing after pipe forming, drawing processing, rolling processing, and shots.

Next, a suitable steel component for obtaining such a pre-structure will be described.
C: 0.3-1.5mass%
C is an element having the greatest influence on the hardenability, and contributes to the improvement of fatigue strength by increasing the hardness and depth of the hardened hardened layer. However, if the content is less than 0.3 mass%, the quench hardened layer depth must be drastically increased in order to ensure the required fatigue strength. Add 0.3mass% or more because it becomes difficult to generate the structure. On the other hand, when the content exceeds 1.5 mass%, the grain boundary strength decreases, and accordingly, the fatigue strength also decreases, and the machinability, cold forgeability and fire cracking resistance also decrease. For this reason, C was limited to the range of 0.3 to 1.5 mass%. Preferably it is the range of 0.4-0.6 mass%.

Si: 3.0mass% or less
Si not only acts as a deoxidizer, but also contributes to improving the strength effectively. However, if the content exceeds 3.0 mass%, machinability and forgeability are reduced, so the Si amount is 3.0 mass. % Or less is preferable.
In addition, it is preferable to set it as 0.05 mass% or more for an intensity | strength improvement.

Mn: 2.0 mass% or less
Mn is added because it is a useful component for improving the hardenability and ensuring the depth of the hardened layer at the time of quenching. If the content is less than 0.2 mass%, the effect of addition is poor, so 0.2 mass% or more is preferable. More preferably, it is 0.3 mass% or more. On the other hand, when the amount of Mn exceeds 2.0 mass%, the retained austenite after quenching increases, and on the contrary, the surface hardness decreases, and as a result, the fatigue strength decreases. Therefore, Mn is preferably 2.0 mass% or less. It should be noted that if the content of Mn is large, the base material is hardened and there is a tendency to be disadvantageous in machinability. Therefore, the content is preferably 1.2 mass% or less. More preferably, it is 1.0 mass% or less.

Furthermore, in addition to the above basic components, the following Al can be added.
Al: 0.25 mass% or less
Al is an element effective for deoxidation. Moreover, it is an element useful also in refine | miniaturizing the particle size of a hardening hardening layer by suppressing the austenite grain growth at the time of quenching heating. However, even if the content exceeds 0.25 mass%, the effect is saturated, and a disadvantage that causes an increase in the component cost occurs. Therefore, Al is preferably contained in a range of 0.25 mass% or less. More preferably, it is in the range of 0.001 to 0.10 mass%.

As mentioned above, although the basic component and Al were demonstrated, in this invention, the 1 type (s) or 2 or more types of the 6 components described below can be contained suitably.
Cr: 2.5 mass% or less
Cr is effective for improving the hardenability and is a useful element for securing the hardening depth. However, if contained excessively, the carbide is stabilized to promote the formation of residual carbide, and the grain boundary strength is lowered to deteriorate the fatigue strength. Therefore, it is desirable to reduce the Cr content as much as possible, but up to 2.5 mass% is acceptable. Preferably it is 1.5 mass% or less.

Mo: 1.0 mass% or less
Mo is an element useful for suppressing the growth of austenite grains, and for that purpose, it is preferable to contain it at 0.05 mass% or more, but if added over 1.0 mass%, machinability is deteriorated. , Mo is preferably 1.0 mass% or less.

Cu: 1.0 mass% or less
Cu is effective in improving the hardenability, and also dissolves in ferrite, and this solid solution strengthening improves fatigue strength. Furthermore, by suppressing the formation of carbides, a decrease in grain boundary strength due to carbides is suppressed, and fatigue strength is improved. However, if the content exceeds 1.0 mass%, cracking occurs during hot working, so addition of 1.0 mass% or less is preferable. In addition, More preferably, it is 0.5 mass% or less. In addition, since addition effect less than 0.03 mass% has a small effect of improving hardenability and suppressing the decrease in grain boundary strength, it is desirable to add 0.03 mass% or more.

Ni: 2.5 mass% or less
Since Ni is an element that improves hardenability, Ni is used when adjusting hardenability. Moreover, it is an element which suppresses the production | generation of a carbide | carbonized_material and suppresses the fall of the grain boundary strength by a carbide | carbonized_material, and improves fatigue strength. However, Ni is an extremely expensive element, and adding more than 2.5 mass% increases the cost of the steel material. Therefore, it is preferable to add less than 2.5 mass%. In addition, since the effect of improving hardenability and the effect of suppressing the decrease in grain boundary strength are small when added at less than 0.05 mass%, it is desirable to add 0.05 mass% or more. Furthermore, it is preferably 0.1 to 1.0 mass%.

Co: 1.0 mass% or less
Co is an element that suppresses the formation of carbides, suppresses a decrease in grain boundary strength due to carbides, and improves fatigue strength. However, Co is an extremely expensive element, and if added in excess of 1.0 mass%, the cost of the steel material increases, so 1.0 mass% or less is added. In addition, since addition of less than 0.01 mass% has a small effect of suppressing the decrease in grain boundary strength, it is desirable to add 0.01 mass% or more. More preferably, it is 0.02 to 0.5 mass%.

V: 0.5 mass% or less V combines with C and N in steel and acts as a precipitation strengthening element. Moreover, it is an element which improves temper softening resistance, and improves fatigue strength by these effects. However, since the effect is saturated even if it contains exceeding 0.5 mass%, it is preferable to make it 0.5 mass% or less. In addition, since the improvement effect of fatigue strength is small when added less than 0.01 mass%, it is desirable to add at 0.01 mass% or more. More preferably, it is 0.03-0.3 mass%.

W: 1.0 mass% or less W is an element useful for suppressing the growth of austenite grains. For that purpose, W is preferably contained in an amount of 0.005 mass% or more. Therefore, W is preferably set to 1.0 mass% or less.

Furthermore, in the present invention, Ti: 0.1 mass% or less, Nb: 0.1 mass% or less, Zr: 0.1 mass% or less, B: 0.01 mass% or less, Ta: 0.5 mass% or less, Hf: 0.5 mass% or less, and Sb: One or two abnormalities selected from 0.015 mass% or less can be contained.
Ti: 0.1 mass% or less
Ti combines with N mixed as an unavoidable impurity to prevent B from becoming BN and prevent the B hardenability improving effect from being burned out, and has the effect of fully exhibiting the B hardenability improving effect. In order to acquire this effect, it is preferable to contain 0.005 mass% or more, but if it contains more than 0.1 mass%, a large amount of TiN is formed. In order to cause a significant decrease, Ti is preferably 0.1 mass% or less. Preferably it is the range of 0.01-0.07 mass%.

Nb: 0.1 mass% or less
Nb not only has an effect of improving hardenability, but also combines with C and N in steel and acts as a precipitation strengthening element. It is also an element that improves tempering and softening resistance, and these effects improve fatigue strength. However, since the effect is saturated even if it contains exceeding 0.1 mass%, it is preferable to make it 0.1 mass% or less. In addition, since the improvement effect of precipitation strengthening action and tempering softening resistance is small when less than 0.005 mass% is added, it is desirable to add 0.005 mass% or more. More preferably, it is 0.01-0.05 mass%.

Zr: 0.1 mass% or less
Zr not only has an effect of improving hardenability but also combines with C and N in steel and acts as a precipitation strengthening element. Moreover, it is an element which improves tempering softening resistance, and improves fatigue strength by these effects. However, since the effect is saturated even if it contains exceeding 0.1 mass%, it is preferable to make it 0.1 mass% or less. In addition, since the improvement effect of precipitation strengthening action and tempering softening resistance is small when less than 0.005 mass% is added, it is desirable to add 0.005 mass% or more. Furthermore, Preferably it is 0.01-0.05 mass%.

B: 0.01 mass% or less B is a useful element that not only improves fatigue properties by grain boundary strengthening but also improves strength, and is preferably added in an amount of 0.0003 mass% or more, but exceeds 0.01 mass%. Even so, since the effect is saturated, it was limited to 0.01 mass% or less.

Ta: 0.5 mass% or less
Ta is effective for delaying the change in microstructure, and has an effect of preventing deterioration of fatigue strength, particularly rolling fatigue. Therefore, Ta may be added. However, even if the content exceeds 0.5 mass% and the content is increased, it does not contribute to further improvement in strength, so the content is set to 0.5 mass% or less. In addition, in order to express the improvement effect of fatigue strength, it is preferable to set it as 0.02 mass% or more.

Hf: 0.5 mass% or less
Hf is effective for delaying the change in microstructure, and has an effect of preventing deterioration of fatigue strength, particularly rolling fatigue, so it may be added. However, even if the content exceeds 0.5 mass% and the content is increased, it does not contribute to improvement of the strength any more, so the content is made 0.5 mass% or less. In addition, in order to express the improvement effect of fatigue strength, it is preferable to set it as 0.02 mass% or more.

Sb: 0.015 mass% or less
Sb is effective for delaying the microstructure change, and has an effect of preventing deterioration of fatigue strength, particularly rolling fatigue, and therefore may be added. However, if the content exceeds 0.015 mass% and the content is increased, the toughness deteriorates, so 0.015 mass% or less, preferably 0.010 mass% or less. In addition, in order to express the fatigue strength improvement effect, it is preferable to set it as 0.005 mass% or more.

Furthermore, in the present invention, S: 0.1 mass% or less, Pb: 0.1 mass% or less, Bi: 0.1 mass% or less, Se: 0.1 mass% or less, Te: 0.1 mass% or less, Ca: 0.01 mass% or less, Mg : 0.01 mass% or less and REM: 0.1 mass% or less can be contained.
S: 0.1 mass% or less S is a useful element that forms MnS in steel and improves the machinability, but if it exceeds 0.1 mass%, it segregates at the grain boundary and lowers the grain boundary strength. , S was limited to 0.1 mass% or less. Preferably it is 0.04 mass% or less.

Pb: 0.1 mass% or less
Bi: 0.1 mass% or less
Both Pb and Bi can be added for this purpose because they improve machinability by melting, lubrication and embrittlement during cutting. However, adding Pb: 0.1 mass% and Bi: exceeding 0.1 mass% not only saturates the effect, but also increases the component cost. In order to improve machinability, it is preferable to contain Pb in an amount of 0.01 mass% or more and Bi in an amount of 0.01 mass% or more.

Se: 0.1 mass% or less
Te: 0.1 mass% or less
Se and Te combine with Mn to form MnSe and MnTe, respectively, which act as a chip breaker to improve machinability. However, if the content exceeds 0.1 mass%, the effect is saturated and the component cost is increased, so that the content is 0.1 mass% or less. In order to improve machinability, it is preferable to contain 0.003 mass% or more in the case of Se and 0.003 mass% or more in the case of Te.

Ca: 0.01 mass% or less
REM: 0.1 mass% or less
Ca and REM each form a sulfide with MnS, which improves the machinability by acting as a chip breaker. However, even if Ca and REM are added in amounts exceeding 0.01 mass% and 0.1 mass%, respectively, the effect is saturated and the component cost is increased. In order to improve the machinability, it is preferable to contain 0.0001 mass% or more of Ca and 0.0001 mass% or more of REM.

Mg: 0.01 mass% or less
Mg is not only a deoxidizing element but also serves as a stress concentration source and has an effect of improving machinability, and can be added as necessary. However, if added excessively, the effect is saturated and the component cost increases, so the content was set at 0.01 mass% or less. In order to improve machinability, Mg is preferably contained in an amount of 0.0001 mass% or more.

  The balance other than the elements described above is preferably Fe and inevitable impurities. Examples of the inevitable impurities include P, O, and N. P: 0.10 mass%, N: 0.01 mass%, and O: 0.008 mass%, respectively. Can be tolerated.

Next, the manufacturing method of this invention is demonstrated with reference to FIG.
A steel material adjusted to the above-described predetermined component composition, for example, a steel slab, is heated to a predetermined temperature in a heating furnace, and then a steel strip having a constant width is formed by hot rolling. Then, as shown in FIG. 6, the steel strip 11 is fed into a group of forming rolls 12 at a constant speed with the steel strip surface horizontal. As the forming roll 12, for example, an edge bend roll 13 that bends both ends in the steel strip width direction sequentially from the entry side, a center bend roll 14 that bends the central portion, and an end forming cage. A roll 15 and a fin pass roll 16 for finish molding are arranged in series. The steel strip 11 formed into a cylindrical shape by these forming rolls 12 is continuously heated at its end in the width direction (hereinafter simply referred to as a butt portion or an edge portion) by an induction coil 17 or the like, and is squeezed by a squeeze roll 18. It is pressed, pressure-bonded and welded to form a temporary tube body 19. In the tube body 19, a bead (not shown, but a bead shaped material generated in a normal welded portion) generated on the inner and outer surfaces by the above-described welding is removed by bead cutting means 20. Furthermore, the flaw inspection by the ultrasonic flaw detector 21 and the heat treatment by the annealing device such as the seam annifer 22 are performed to anneal the welded portion (also referred to as the seam portion), and then the water jet nozzle 23 is sequentially cooled. Thereafter, the size is adjusted by a drawing mill 24 such as a stretch reducer or a sizer, and then cut to a desired length by a cutter 25 on a delivery line for discharging, so that an ERW steel pipe having desired characteristics is obtained. .
Further, after the above-described steps, hot reduction / thinning rolling or cold drawing may be performed as necessary.
Moreover, when it welds or forges instead of the said welding, it will become a pressure welded steel pipe or a forge welded steel pipe.

  Finally, induction quenching is performed on at least a part of the steel pipe thus obtained under the condition of heating temperature: 800 to 1000 ° C. At least a part of this is a portion where fatigue strength is required.

In this series of steps, (1) Hot working is performed with a total working rate at 800-1000 ° C of 80% or more, and 700-500 ° C temperature range is cooled to 0.2 ° C / s or more after hot processing. cooled at a rate further, either through the processing and cooling history subjected to 20% or more of the processing by a ₁ transformation point of the temperature range, or (2) 800-1000 total working ratio at ℃ 80% or more, 700 After hot working with a total working rate of 20% or more at less than ~ 800 ° C, after the hot working, the temperature range of 700-500 ° C is cooled at a cooling rate of 0.2 ° C / s or more through a cooling history By adopting the induction hardening conditions described below for the manufactured steel pipe, it becomes possible to obtain a quenched structure with a prior austenite grain size of 12 μm or less.

As the above hot working, the above-described hot rolling can be suitably employed, and finish rolling in the hot rolling process is particularly suitable. Further, processing at temperatures below zone A ₁ transformation point, processing by rolling mill 24 stop described above, or reduced diameter reduction meat rolling or cold drawing performed as required can be preferably employed.
In the following, each regulation will be described in detail, taking as an example the case where an ERW steel pipe is manufactured from a hot-rolled steel sheet.

[Processing conditions]
As for the processing conditions, it is necessary to adopt either (1) or (2) described below.
(1) Hot working, specifically, when producing a hot-rolled steel sheet as a raw material for pipe making, the total working rate at 800 to 1000 ° C. during hot rolling is 80% or more. The total processing rate can be adjusted by controlling the rolling reduction. Preferably, the finish rolling mill entry side temperature and finish mill exit side temperature are in the range of 800 to 1000 ° C. during finish rolling, and the finish rolling mill entry side sheet thickness and finish sheet thickness are reduced to 80% or more. Should be set. After hot rolling, the coil is wound at a cooling rate of 700 to 500 ° C in a temperature range of 0.2 ° C / s or more and a winding temperature of 500 ° C or less. The hot-rolled steel sheet thus obtained is used as a raw material to manufacture an ERW pipe in the above-described process. Then, as shown in FIG. 6, or for machining of more than 20% reducing mill 24 after the tube 19, or, in a temperature range below the A ₁ transformation point reduction diameter reduction meat rolling above Do as 20% or more. The total processing rate of the total of rolling by the drawing mill 24 and reduction in diameter reduction may be 20% or more. By adopting the above processing conditions at the time of hot rolling and processing after pipe forming, a uniform bainite and / or martensite structure can be obtained, and austenite grains are fine during subsequent induction hardening heating. Turn into. As the processing in the temperature range below the A ₁ transformation point, change course processing may be adopted shots like.

(2) Hot working, specifically when manufacturing hot-rolled steel sheet as a material for pipe making, the total working rate at 800-1000 ° C during hot rolling is 80% or more, at 700-800 ° C Set the total processing rate to 20% or more. The total processing rate can be adjusted by controlling the rolling reduction. Preferably, the finish rolling mill inlet side temperature is set to 1000 ° C. to 800 ° C. during finish rolling, and 80% or more at the front stage stand of the finish rolling mill row (for example, the first to fifth stands in a continuous finishing mill consisting of 7 stands). Rolling at a reduction rate of Then, rolling is performed at a reduction rate of 20% or more at 700 to 800 ° C. in a rear stage stand (for example, the sixth and seventh stands in a continuous finishing mill having seven stands). Therefore, the rear stand can be rolled at 700 to 800 ° C. using an inter-stand cooling device. After rolling, the coil is wound at a cooling rate of 700 to 500 ° C in a temperature range of 0.2 ° C / s or more and a winding temperature of 500 ° C or less. The hot-rolled steel sheet thus obtained is used as a raw material to manufacture an ERW pipe in the above-described process. By adopting the above processing conditions at the time of hot rolling, a uniform fine bainite and / or martensite structure can be obtained, and austenite grains become fine and uniform during subsequent induction hardening heating.

[Induction hardening conditions]
The heating temperature is set to 800 to 1000 ° C., and the temperature is raised from 600 to 800 ° C. at a rate of temperature increase of 400 ° C./s or more. When the heating temperature is less than 800 ° C., the austenite structure is not sufficiently generated, and a cured layer cannot be obtained. On the other hand, when the heating temperature exceeds 1000 ° C., the growth rate of austenite grains is remarkably increased, and at the same time, the average grain size is increased. The maximum particle size exceeds 4 times the average particle size, leading to a decrease in fatigue strength.
Also, when the heating rate at 600 to 800 ° C. is less than 400 ° C./s, the growth of austenite grains is promoted and at the same time, the variation in grain size increases, and the maximum grain size is four times the average grain size. It becomes super and causes a fall of fatigue strength. This is presumably because when the rate of temperature increase is slow, reverse transformation from ferrite to austenite starts at a lower temperature, and uneven grain growth tends to occur depending on the location.

In addition, it is preferable that heating temperature shall be 800-950 degreeC, and it is preferable that the temperature increase rate of 600-800 degreeC is 700 degrees C / s or more. More preferably, it is 1000 ° C./s or more.
In addition, when the residence time of 800 ° C. or higher is increased during high-frequency heating, austenite grains grow, and as a result, the maximum particle size tends to be more than four times the average particle size. Therefore, the residence time of 800 ° C. or more is 5 seconds. The following is preferable.

  As the steel pipe of the present invention, a shaft simulating an automobile drive shaft, output shaft, and input shaft was manufactured. That is, a steel material having the composition shown in Table 2 was melted by a converter and made into a slab by continuous casting. The slab size was 300 × 400 mm. This slab was made into a hot-rolled steel sheet by hot rolling. At this time, the rolling reduction in the temperature range of 800 to 1000 ° C. during finish rolling is as shown in Table 3, the cooling rate in the temperature range of 700 to 500 ° C. is as shown in Table 3, and after cooling, It wound up at below ℃. The obtained hot-rolled steel sheet was used as a raw material, and was formed into a tubular shape in the above-described pipe making process.

  Then, a tubular test piece having the dimensions and shape shown in FIG. 1 was produced from the tube by cold rolling and cutting. Table 3 shows the cold working rate with respect to the parallel portion of the test piece. Here, the processing rate is a cross-sectional reduction rate.

  This steel pipe (tubular test piece) was quenched under the conditions shown in Table 3 using an induction hardening apparatus with a frequency of 15 kHz, and then tempered at 170 ° C. for 30 minutes using a heating furnace. Then, the torsional fatigue strength was investigated. Here, with respect to some tubular test pieces, tempering was omitted and the torsional fatigue strength was investigated.

The torsional fatigue strength was evaluated by the torque value (N · m) when the number of repetitions of fracture was 1 × 10 ⁵ times in the torsional fatigue test of the shaft. In the torsional fatigue test, a hydraulic fatigue tester is used. As shown in FIG. 2, the spline portions 2a and 2b are incorporated in the disc-shaped grippers 3a and 3b, respectively, and the frequency is 1 to 2 Hz between the grippers 3a and 3b. And repeatedly applying a torsional torque.

  In addition, for the same shaft, the hardened layer was a corrosive solution containing picric acid as a main component (water: 500 g of picric acid: 50 g of picric acid dissolved in an aqueous solution of picric acid: 11 g of sodium dodecylbenzenesulfonate, ferrous chloride : 1 g and oxalic acid: 1.5 g added), the structure was observed using an optical microscope, and the average particle size of prior austenite grains was determined. The average particle size was measured in the same manner as described above.

Furthermore, the same shaft was also investigated for fire cracking resistance.
This anti-fire cracking resistance was evaluated by the number of occurrence of fire cracks when the C-cross section of the spline part after induction hardening was cut and polished and observed with an optical microscope (magnification: 100 to 200 times).
The results obtained are also shown in Table 3.

  As is clear from Table 3, any shaft having a quenched structure with an average grain size of prior austenite grains of 12 μm or less can obtain high torsional fatigue strength and excellent anti-cracking resistance of 0. did it.

  On the other hand, any shaft having a quenched structure in which the average grain size of the prior austenite grains is not less than 12 μm has low fatigue strength.

It is a front view of a typical shaft. It is a figure which shows the point of the torsional fatigue test of a shaft. It is a graph which shows the relationship between a prior-austenite grain average diameter and torsional fatigue strength. It is a graph which shows the influence of the average prior austenite particle size of a hardened layer and the maximum prior austenite particle size / average prior austenite particle size on torsional fatigue strength. It is a graph which shows the influence of the cold work rate, the highest attained temperature (heating temperature) at the time of high frequency heating, and a temperature increase rate which influences torsional fatigue strength. It is a figure which shows the manufacture procedure of an electric resistance steel pipe.

Explanation of symbols

1 Test piece 2 Spline part 3a, 3b Grab

Claims (13)

C: 0.3-1.5 mass%
Si: 3.0mass% or less and
Mn: 2.0 mass% or less, the balance is made of Fe and inevitable impurities, and further has a hardened portion by induction hardening, and the hardened portion has an average diameter of prior austenite grains of 12 μm or less and a maximum grain size of average grains A steel pipe excellent in fatigue characteristics, characterized by being 4 times or less in diameter. The steel pipe further includes
The steel pipe according to claim 1, containing Al: 0.25 mass% or less. The steel pipe further includes
Cr: 2.5 mass% or less,
Mo: 1.0mass% or less,
Cu: 1.0 mass% or less,
Ni: 2.5 mass% or less,
Co: 1.0mass% or less,
The steel pipe according to claim 1 or 2, comprising one or more selected from V: 0.5 mass% or less and W: 1.0 mass% or less. The steel pipe further includes
Ti: 0.1 mass% or less,
Nb: 0.1 mass% or less,
Zr: 0.1 mass% or less,
B: 0.01 mass% or less,
Ta: 0.5 mass% or less,
Hf: 0.5 mass% or less and
The steel pipe according to claim 1, 2 or 3, comprising one or more selected from Sb: 0.015 mass% or less. The steel pipe is further S: 0.1 mass% or less,
Pb: 0.1 mass% or less,
Bi: 0.1 mass% or less,
Se: 0.1 mass% or less,
Te: 0.1 mass% or less,
Ca: 0.01 mass% or less,
Mg: 0.01 mass% or less and
The steel pipe according to any one of claims 1 to 4, comprising one or more selected from REM: 0.1 mass% or less.   A steel pipe containing a total of 10% by volume or more of one or both of a fine bainite structure and a fine martensite structure is used as a raw material, and at least part of the steel pipe has a temperature increase rate of 400 ° C./s or more and an ultimate temperature of 1000 ° C. The manufacturing method of the steel pipe characterized by performing the following high frequency heating 1 times or more. In Claim 6, the said raw material is a hot working process in which the total working rate in 800-1000 degreeC becomes 80% or more, and the temperature range of 700-500 degreeC after this hot working process is 0.2 degreeC / s or more. A cooling step of cooling at a cooling rate, and further, processing of 20% or more is performed in a temperature range of less than 700 to 800 ° C. before the cooling step, or in a temperature range of A ₁ transformation point or less after the cooling step. And a second processing step of performing processing of 20% or more.   8. The method of manufacturing a steel pipe according to claim 6, wherein a residence time of 800 ° C. or more in one high-frequency heating is set to 5 seconds or less. The steel pipe according to any one of claims 6 to 8,
C: 0.3-1.5 mass%
Si: 3.0mass% or less and
Mn: The manufacturing method of the steel pipe characterized by being a composition which contains 2.0 mass% or less and consists of remainder Fe and an unavoidable impurity. The steel pipe according to claim 9, further comprising:
A method for producing a steel pipe, comprising Al: 0.25 mass% or less. The steel pipe according to claim 9 or 10, further comprising:
Cr: 2.5 mass% or less,
Mo: 1.0mass% or less,
Cu: 1.0 mass% or less,
Ni: 2.5 mass% or less,
Co: 1.0mass% or less,
The manufacturing method of the steel pipe characterized by containing 1 type (s) or 2 or more types selected from V: 0.5 mass% or less and W: 1.0 mass% or less. The steel pipe according to claim 9, 10 or 11, further comprising:
Ti: 0.1 mass% or less,
Nb: 0.1 mass% or less,
Zr: 0.1 mass% or less,
B: 0.01 mass% or less,
Ta: 0.5 mass% or less,
Hf: 0.5 mass% or less and
Sb: The manufacturing method of the steel pipe characterized by containing 1 type, or 2 or more types selected from below 0.015 mass%. In any one of Claims 9 thru | or 12, The said steel pipe is further S: 0.1 mass% or less,
Pb: 0.1 mass% or less,
Bi: 0.1 mass% or less,
Se: 0.1 mass% or less,
Te: 0.1 mass% or less,
Ca: 0.01 mass% or less,
Mg: 0.01 mass% or less and
REM: The manufacturing method of the steel pipe characterized by including 1 type or 2 types or more selected from 0.1 mass% or less.

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