JP5867474B2 - Manufacturing method of high carbon ERW welded steel pipe with excellent reliability of ERW welds - Google Patents

Description

  The present invention relates to a method for manufacturing a high carbon electric resistance welded steel pipe suitable for hollow machine parts such as automobiles, and more particularly to an improvement in reliability of an electric resistance welded portion.

  In recent years, from the viewpoint of the preservation of the global environment, there has been a strong demand for improvement in fuel efficiency of automobiles, and weight reduction of automobile bodies is strongly directed. For this reason, a hollow material has been used instead of the solid material that has been conventionally used. As a hollow material for parts that require heat treatment used in automobiles, etc., the use of high-carbon ERW welded steel pipe, which is a high-carbon steel material for machine structures, is particularly good because of its good dimensional accuracy and low surface decarburization. It is being considered.

  However, the high carbon steel material for mechanical structures has a tendency to increase the strength, increase the elongation, decrease segregation, and increase the carbon content. For this reason, in segregated parts where C, Mn, P, etc. are strongly segregated, the hot workability at high temperatures is significantly reduced, making ERW welding itself difficult, or causing ERW welding due to segregated parts. In this part, defects such as hot cracking frequently occurred, and there was a problem in workability as a steel pipe.

  For such problems, for example, in Patent Document 1, C: 0.4 to 0.8%, Si: 0.15 to 0.35%, Mn: 0.3 to 2.0%, P: 0.030% or less, S: 0.035% or less, Al : High-carbon steel ERW steel pipe for machine structure comprising 0.035% or less, Mo: 0.05-0.15% added, and the balance Fe and inevitable impurities. In the technique described in Patent Document 1, by adding Mo, the workability in hot work at 1000 ° C. or higher can be greatly improved, and the high carbon steel ERW steel pipe for machine structures having excellent hot workability and It is going to be.

  Patent Document 2 includes C: 0.3 to 0.6%, Si: 0.15 to 0.35%, Mn: 0.3 to 1.5%, P: 0.012% or less, S: 0.035% or less, Al: 0.035% or less, and continuous. Manufactured ERW steel pipes using high-carbon hot-rolled coils obtained by hot rolling high carbon slabs that have been adjusted to a low level that satisfies the specific relationship between the P concentration of the cast center segregation and the C concentration. A process for producing a high workability, high carbon ERW steel pipe is described. According to the technique described in Patent Document 2, high-temperature cracking at the time of ERW welding is suppressed, yield is improved, and embrittlement cracking occurs in the segregated portion even when subjected to severe processing such as bulge forming. The possibility is low, and the workability of high-carbon ERW steel pipes will be improved.

  Patent Document 3 discloses a low level in which high carbon steel containing C: 0.30 to 0.60% and P: 0.012% or less is continuously cast, and the P concentration in the central segregation portion satisfies a specific relationship in relation to the C concentration. A high-carbon steel slab adjusted to a high carbon hot-rolled coil obtained by hot-rolling this high-carbon steel slab as a raw material, and then forming a cylindrical open pipe by a group of forming rolls, then both edges of the open pipe , Preferably 2 to 4 mm wider than usual, preheated to 800 to 1000 ° C., welded by electro-welding, and then air-cooled by ERW A method is described. According to the technique described in Patent Document 3, high temperature cracking during ERW welding is suppressed, yield is improved, hardness of the ERW welded portion is reduced, and severe processing such as bulge forming is applied. However, it is possible to prevent cracking at the welded portion and improve the workability of the high carbon ERW steel pipe.

Patent Document 4 discloses that an electric resistance welded steel pipe having a composition containing C: 0.03 to 0.30%, Si: 0.50 to 3.00%, and Mn: 0.30 to 3.00% is subjected to electric resistance welding, and then the weld is heated to 800 to 1000 ° C. A method for heat treatment of an electric resistance welded portion is described in which, after heating, it is rapidly cooled at 20 to 200 ° C./s from the Ar ₃ transformation point or higher to leave residual austenite in the electric resistance welded portion to improve the workability of the electric resistance welded portion ing. According to the technique described in Patent Document 4, the ductility of the ERW welded portion is improved, and an ERW steel pipe that can withstand severe processing such as hydroforming is obtained.

JP 04-263039 A Japanese Patent Laid-Open No. 11-156433 Japanese Patent Laid-Open No. 11-226634 JP-A-11-323442

  Recently, particularly from the viewpoint of ensuring the safety of automobiles and the like, parts such as automobiles are strictly required to maintain high reliability. In particular, when an ERW welded steel pipe is used as a material for parts, it is required to have an ERW welded portion having higher reliability than conventional ones. However, the techniques described in Patent Documents 1 to 4 have a problem that the reliability requirement value represented by the fatigue strength of the electric resistance welded portion cannot be sufficiently satisfied.

  An object of this invention is to solve this problem and to provide the high carbon electric resistance welded steel pipe which has the electric resistance weld part excellent in reliability. Here, “excellent in reliability” refers to a case where there is no defect affecting the fatigue strength in the ERW weld. Specifically, based on a notch of depth 0.2mm × length 12.5mm as a reference, the number of repetitions was 0, and the torsional stress τ of the outer surface was 350MPa in the ultrasonic flaw test conducted with 6db sensitivity enhancement: This shall mean the case where no cracks occur in the torsional fatigue test up to 2 million times.

  In order to achieve the above-mentioned object, the present inventors diligently investigated the cause of the low reliability of the conventional high carbon electric resistance welded steel pipe. As a result, it has been found that conventional high carbon ERW welded steel pipes tend to have defects such as cracks remaining in ERW welds. Conventional high carbon electric resistance welded steel pipes are usually subjected to cold sizer drawing and bending correction after the completion of electric resistance welding because of the necessity of adjusting to a predetermined dimension and shape with high accuracy. It is considered that the bend correction caused cracks in the ERW welds that had been hardened by ERW welding, resulting in decreased reliability.

  Therefore, in the case of high carbon ERW welded steel pipe, it is considered that after ERW welding is completed, only the ERW welded portion is normalized, and thereafter, cold work such as sizer drawing and bending correction is performed. . However, even with this method, sufficient reliability cannot be improved. The cause of this is not clear at this time, but it is presumed that there is a high possibility that a shrinkage defect is related. This is because in ERW welding in low carbon steel, shrinkage defects are usually prevented by squeezing the weld with a squeeze roll, but in ERW welding of high carbon steel, the melting point is low, It is considered that the melted portion remains until after passing through the squeeze roll, and shrinkage defects are likely to occur.

For this reason, the present inventors have not only improved the ductility by simply performing heat treatment on the ERW welded part, but also improving the ductility by improving the reliability of the high carbon ERW welded steel pipe. It came to mind that it is necessary to perform processing (drawing rolling) that crushes shrinkage defects occurring in the welded portion.
As a result of further studies, the present inventors have minimized the cold working such as straightening immediately after ERW welding to further improve the reliability of high carbon ERW welded steel pipes. Furthermore, it was found that it is effective to reheat and perform hot reduction rolling at a temperature reduction rate of 10% or more in a temperature range of 850 ° C. or more. It has also been found that the use of induction heating can reduce the heating time and can suppress decarburization during reheating.

The experimental results on which the present invention is based will be described.
A high-carbon steel plate (thickness: 7.9 mm) with a composition containing 0.37% C-0.25% Si-1.50% Mn-0.025% Al-0.004% N-0.02% Ti-0.002% B in mass% is used as the material steel plate. Then, it was cold-formed into a substantially cylindrical shape using a plurality of rolls, but the opposed end faces were butted together and electro-welded to form an electric-welded steel pipe (outer diameter 89.1 mmφ). After the electric seam welding, cold drawing was performed at a drawing ratio of 0 to 1.2% using a sizer rolling machine in the cold. The obtained ERW welded pipe was subjected to ultrasonic flaw detection, particularly with respect to the ERW weld, and the number of defects (number of defects) was measured. Ultrasonic flaw detection was performed with a 6db sensitivity improvement based on a notch having a depth of 0.2 mm and a length of 12.5 mm. The obtained results are shown in FIG. As can be seen from FIG. 1, when the drawing ratio of cold drawing rolling exceeds 0.8%, the occurrence of defects becomes significant.

  In addition, after ERW welding, after cold-rolling with a drawing ratio of 0.1%, immediately reheat to 980 ° C and change the diameter reduction rate in the temperature range of 850 ° C or higher to 0-30%. Hot reduction rolling was performed. The obtained ERW welded steel pipe was subjected to ultrasonic flaw inspection at the ERW weld and the number of defects (number of defects) was measured. The conditions for ultrasonic flaw detection were the same as those after ERW welding. The obtained results are shown in FIG. From FIG. 2, it can be seen that in hot reduction rolling with a diameter reduction ratio of less than 10%, the occurrence of defects in the ERW welds is significant, and when it exceeds 10%, the occurrence of defects is significantly reduced.

The present invention has been completed on the basis of such findings and further studies. That is, the gist of the present invention is as follows.
(1) After forming the material steel plate into a substantially cylindrical shape by cold working, the opposite end surfaces are butted together, and in the method for producing an ERW welded steel pipe by ERW welding, the material steel plate is High in composition with the balance consisting of the balance Fe and unavoidable impurities, including C: 0.30-0.60%, Si: 0.05-0.50%, Mn: 0.30-2.0%, Al: 0.50% or less, N: 0.0100% or less. A carbon steel sheet is subjected to cold drawing rolling with a drawing ratio of 0.8% or less after the aforementioned ERW welding, and then immediately reheated or cooled and reheated, and in a temperature range of 850 ° C. or higher, the diameter reduction ratio: A method for producing a high carbon electric resistance welded steel pipe, characterized by performing hot shrinking rolling of 10% or more to obtain an electric resistance welded portion having excellent reliability.

(2) In (1), in addition to the above composition, by mass%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.2% or less, Mo: 1.0% or less, W: 1.5% or less The manufacturing method of the high carbon electric resistance welded steel pipe characterized by including 1 type, or 2 or more types chosen from these.
(3) In (1) or (2), in addition to the above-mentioned composition, by mass%, one or two selected from Ti: 0.04% or less, Nb: 0.2% or less, V: 0.2% or less The manufacturing method of the high carbon electric resistance welded steel pipe characterized by including a seed | species or more.

(4) In any one of (1) to (3), in addition to the above composition, the method further comprises B: 0.0005 to 0.0050% by mass%, and a method for producing a high carbon electric resistance welded steel pipe.
(5) The method for producing a high carbon electric resistance welded steel pipe according to any one of (1) to (4), wherein the reheating is heating by high frequency induction heating means.
(6) A method of manufacturing an automobile part, wherein an automobile part is manufactured using a high-carbon electric-welded steel pipe as a raw material, wherein the raw material is a high-carbon electric-welded steel pipe according to any one of ( 1) to (5) A method of manufacturing an automobile part, wherein the method is a high carbon electric resistance welded steel pipe manufactured using the method.

(7) The method of manufacturing an automobile part according to (6), wherein the automobile part is any one of a front fork, a rack bar, a drive shaft, a tie rod, a stator shaft, and a cam shaft.

  According to the present invention, the occurrence of defects is suppressed and an electric resistance welded portion having excellent reliability is obtained, the reliability of the high carbon electric resistance welded steel pipe is remarkably improved, and a remarkable industrial effect is achieved. In addition, according to the present invention, the reliability of various automobile parts such as hollow parts made of high carbon ERW welded steel pipes, such as front forks, rack bars, drive shafts, tie rods, stator shafts, camshafts, etc. is also improved. There is also an effect.

It is a graph which shows the influence of the drawing rate of cold drawing rolling on the defect generation number of an electric-welding weld part. It is a graph which shows the influence of the diameter reduction rate of hot diameter reduction rolling on the defect generation number of an ERW weld part.

The present invention is a method for producing a high carbon electric resistance welded steel pipe. The material steel plate is made of a high carbon steel plate, and a conventional method for producing an electric resistance welded steel pipe is applied to obtain a high carbon electric resistance welded steel pipe. The “steel plate” mentioned here includes a steel strip.
First, the reasons for limiting the composition of the high-carbon steel sheet, which is a raw steel sheet, will be described. Hereinafter, unless otherwise specified, mass% is simply referred to as%.

  The steel plate used as the material steel plate in the present invention includes C: 0.30 to 0.60%, Si: 0.05 to 0.50%, Mn: 0.30 to 2.0%, Al: 0.50% or less, N: 0.0100% or less, or, further, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.2% or less, Mo: 1.0% or less, W: 1.5% or less, and / or Ti: 0.04% or less, Nb: 0.2% or less, V: one or more selected from 0.2% or less, and / or B: 0.0005-0.0050%, high carbon with a composition comprising the balance Fe and inevitable impurities It is a steel plate. In order to improve the reliability of the electric resistance welded portion, the thickness of the material steel plate is preferably 8 mm or less from the viewpoint of discharging oxide from the electric resistance welded portion.

C: 0.30 ~ 0.60%
C is an element that contributes to an increase in strength by solid solution or precipitation as carbide or carbonitride. In order to obtain such an effect and to secure a desired steel pipe strength and a steel pipe strength after heat treatment, it is necessary to contain 0.30% or more. Here, “desired steel pipe strength” refers to a tensile strength TS of 1200 MPa or more. On the other hand, if the content exceeds 0.60%, the toughness after heat treatment decreases. For this reason, C was limited to the range of 0.30 to 0.60%.

Si: 0.05-0.50%
Si is an element that acts as a deoxidizer. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if the content exceeds 0.50%, the effect is saturated and economically disadvantageous, and the formation of inclusions is promoted during ERW welding, which adversely affects the soundness of ERW welds. For this reason, Si was limited to the range of 0.05 to 0.50%. In addition, Preferably it is 0.10 to 0.30%.

Mn: 0.30 to 2.0%
Mn is an element that contributes to increase in strength and improve hardenability by solid solution. In order to obtain such an effect, a content of 0.30% or more is required. On the other hand, if the content exceeds 2.0%, retained austenite is formed, and the toughness after tempering is lowered. For this reason, Mn was limited to the range of 0.30 to 2.0%. In addition, Preferably it is 0.8 to 1.6%.

Al: 0.50% or less
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.01% or more. On the other hand, if the content exceeds 0.50%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous and promotes the formation of inclusions during ERW welding, and the integrity of ERW welds. Adversely affect. For this reason, Al was limited to the range of 0.50% or less. In addition, Preferably it is 0.02 to 0.04%.

N: 0.0100% or less
N is a useful element for forming nitrides or carbonitrides and ensuring strength after heat treatment (tempering). In order to acquire such an effect, it is desirable to contain 0.0005% or more, but when it contains more than 0.0100%, coarse nitrides are formed, and the toughness and fatigue life may be reduced. For this reason, N was limited to 0.0100% or less. In the case where N contains Ti, the following formula is used in relation to the Ti content:
N / 14 ≤ Ti / 47.9
(Where N, Ti: content of each element (mass%))
It is desirable to adjust so as to satisfy.

  The above components are basic components. In the present invention, in addition to this basic composition, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.2% or less, Mo: 1.0% as necessary. In the following, one or more selected from W: 1.5% or less, and / or Ti: 0.04% or less, Nb: 0.20% or less, V: 0.20% or less Two or more kinds and / or B: 0.0005 to 0.0050% may be selected and contained.

One or more selected from Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.2% or less, Mo: 1.0% or less, W: 1.5% or less
Cu, Ni, Cr, Mo, and W are all elements that contribute to increasing the strength and improving the hardenability, and can be selected as necessary and contained in one or more.
Cu is an element that dissolves and contributes to increasing strength and improving hardenability, as well as improving toughness, delayed fracture resistance, and corrosion fatigue resistance. In order to acquire such an effect, it is desirable to contain 0.05% or more. On the other hand, if the content exceeds 1.0%, the above-described effects are saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous and processability is lowered. For this reason, when it contains, it is preferable to limit Cu to 1.0% or less. In addition, More preferably, it is 0.05 to 0.25%.

  Ni is an element that contributes to the improvement of toughness, delayed fracture resistance, and corrosion fatigue resistance in addition to solid solution that contributes to increasing strength and improving hardenability. In order to obtain such an effect, the content is preferably 0.05% or more. However, even if the content exceeds 1.0%, the above effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. In addition, workability is reduced. For this reason, when it contains, it is preferable to limit Ni to 1.0% or less. In addition, More preferably, it is 0.05 to 0.25%.

  Cr dissolves and contributes to increasing strength and improving hardenability, and further generates fine carbides and contributes to increasing strength by precipitation strengthening. In order to acquire such an effect, it is desirable to contain 0.1% or more. On the other hand, even if the content exceeds 1.2%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous and easily causes inclusions during ERW welding. Adversely affects the health of For this reason, when contained, Cr is preferably limited to 1.2% or less. In addition, More preferably, it is 0.1 to 0.5%.

  Mo dissolves and contributes to increasing the strength and improving the hardenability, and further, forming fine carbides to contribute to increasing the strength by precipitation strengthening. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, even if the content exceeds 1.0%, the effect is saturated and an effect commensurate with the content cannot be expected, which is disadvantageous economically, and coarse carbides are formed and the toughness may be lowered. For this reason, when contained, Mo is preferably limited to 1.0% or less. In addition, More preferably, it is 0.10 to 0.30%.

  W has a function of improving the balance between hardness and toughness after heat treatment in addition to solid solution and contributing to increase in strength and improvement in hardenability. In order to ensure such an effect, it is desirable to contain 0.01% or more. On the other hand, if the content exceeds 1.5%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, when contained, W is preferably limited to 1.5% or less. In addition, More preferably, it is 0.10 to 0.30%.

One or more selected from Ti: 0.04% or less, Nb: 0.20% or less, V: 0.20% or less
Ti, Nb, and V are all elements that contribute to increasing the strength by forming fine carbides, and can be selected as necessary and contained in one or more.
In addition to the above-described action, Ti is an element having an action of securing solid solution B effective for improving hardenability by binding N and fixing N. Further, Ti forms fine nitrides, has an action of suppressing the coarsening of crystal grains during heat treatment and electric resistance welding, and contributes to improvement of toughness. In order to acquire such an effect, it is desirable to contain 0.001% or more. On the other hand, if the content exceeds 0.04%, the amount of inclusions may increase and the toughness may decrease. For this reason, when it contains, it is preferable to limit Ti to 0.04% or less. In addition, when Ti is contained, the following formula in relation to the N content:
N / 14 ≤ Ti / 47.9
(Where N, Ti: content of each element (mass%))
It is desirable to contain so that it may satisfy. In addition, More preferably, it is 0.01 to 0.03%.

  Nb has the effect of improving the toughness and delayed fracture resistance by forming fine carbides during tempering and contributing to an increase in strength, and by refining the structure after heat treatment. In order to acquire such an effect, it is desirable to contain 0.001% or more. On the other hand, even if the content exceeds 0.20%, the above effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, when it contains, it is preferable to limit Nb to 0.20% or less. In addition, More preferably, it is 0.01 to 0.02%.

  V contributes to an increase in strength by forming fine carbides during tempering. In order to ensure such an effect, it is desirable to contain 0.001% or more. On the other hand, even if the content exceeds 0.20%, the above effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when contained, V is preferably limited to 0.20% or less. In addition, More preferably, it is 0.01 to 0.08%.

B: 0.0005-0.0050%
B is an effective element that improves hardenability with a small amount of content, improves the balance between hardness and toughness after heat treatment, and strengthens the grain boundaries to improve the fire cracking resistance. Can be contained. In order to acquire such an effect, 0.0005% or more needs to be contained. On the other hand, even if the content exceeds 0.0050%, the above effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous, and coarse B-containing precipitates are produced, resulting in a decrease in toughness. For this reason, when it contains, it is preferable to limit B to 0.0005 to 0.0050% of range. In addition, More preferably, it is 0.002 to 0.003%.

The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities include P: 0.020% or less, S: 0.010% or less, and O: 0.005% or less.
P: 0.020% or less
P is an element that adversely affects weld crack resistance and toughness, and it is desirable to reduce it as much as possible within a range of 0.020% or less. However, excessive reduction raises the refining cost, so 0.0005% or more is desirable. More preferably, it is 0.010% or less.

S: 0.010% or less
S exists as a sulfide inclusion in steel and has an adverse effect on workability, toughness and fatigue life, and increases reheat cracking susceptibility. It can be reduced as much as possible within a range of 0.010% or less. desirable. However, excessive reduction raises the refining cost, so 0.0005% or more is desirable. More preferably, it is 0.001% or less.

O: 0.005% or less
O (oxygen) exists as an oxide inclusion in steel and adversely affects workability, toughness, and fatigue life. For this reason, it is desirable to reduce O (oxygen) within the range of 0.005% or less as much as possible. More preferably, it is 0.002% or less.
In the present invention, the high carbon steel plate having the above composition is used as the raw steel plate, but the manufacturing method of the raw steel plate is not particularly limited. Any of the usual methods for producing hot-rolled steel sheets can be applied. The raw steel plate is slit to a predetermined width and formed into a substantially cylindrical shape in the cold, preferably continuously using a plurality of forming rolls. It shall be a sewn welded steel pipe.

  In the present invention, after the ERW welding is performed to form an ERW welded steel pipe, the ERW welded steel pipe is preferably subjected to cold drawing rolling with a sizer rolling mill in order to prevent shape defects. In the present invention, the drawing ratio of cold drawing is limited to 0.8% or less. When the drawing ratio exceeds 0.8%, defects such as cracks occur in the ERW weld, and the reliability of the ERW weld decreases. For this reason, the drawing ratio of the cold drawing rolling performed after the ERW welding is limited to 0.8% or less. In addition, Preferably it is 0.01 to 0.1%. It is preferable not to perform cold drawing rolling (drawing ratio 0%) for the occurrence of defects in ERW welds, but if cold drawing rolling is not performed, the probability that a tube shape will be defective increases. .

The ERW welded steel pipe subjected to cold drawing with a drawing ratio of 0.8% or less is immediately reheated or cooled to room temperature and then reheated. Although the reheating temperature is not particularly limited, it is preferably set to a temperature at which hot reduction rolling can be performed to give a reduction ratio of 10% or more in a temperature range of 850 ° C. or higher, that is, 900 to 1050 ° C. .
In the present invention, the hot reduction rolling is reheated to the austenite region, and the electric seam welded portion is made tough and the defects generated in the electric seam welded portion are crushed and made harmless. To improve reliability. If the finish rolling temperature of hot reduction rolling is less than 850 ° C., crimping of shrinkage defects becomes insufficient, and the desired defect cannot be rendered harmless. In addition, the finish rolling temperature of hot reduction rolling is preferably 900 ° C. or higher. In addition, the upper limit of the finish rolling temperature of hot diameter reduction rolling is 1000 ° C. which can prevent the coarsening of the structure.

  Further, if the reduction ratio of the hot reduction rolling is less than 10% in the temperature range of 850 ° C. or higher, the reduction ratio is insufficient and the desired defect cannot be rendered harmless. For this reason, the reduction ratio of hot reduction rolling is limited to 10% or more. In addition, Preferably it is 30% or more. The upper limit of the diameter reduction rate of hot diameter reduction rolling is determined according to the desired size and shape.

  A high carbon hot-rolled steel sheet (thickness: 7.8 mm) having the composition shown in Table 1 was used as the raw steel sheet. After slitting these steel sheets into a predetermined width and forming them into a substantially cylindrical open tube with multiple rolls in the cold, the opposing end faces are butted together by electro-welding, and the outer diameter is 89.1 mmφ x meat A 7.9 mm thick ERW welded steel pipe (mother pipe) was used. The ERW welded steel pipe was subjected to cold drawing with the drawing ratios shown in Table 2 and adjusted so as to have a predetermined size and shape using a sizer mill after ERW welding. Immediately after the cold-drawing rolling, it is heated to the temperature shown in Table 2 by induction heating means, subjected to hot-reduction rolling under the conditions shown in Table 2 with a hot-reduction mill, and air-cooled after hot-reduction rolling. An electric resistance welded steel pipe having an outer diameter of 42.7 mmφ and a thickness of 8.0 mm was used.

The total length (about 10,000 m) of the ERW welded steel pipe obtained was subjected to ultrasonic flaw detection, and the presence of detected defects and the number of defects (converted per 10000 m in length) were investigated. Ultrasonic flaw detection was performed with a 6 dB sensitivity improvement based on a notch having a depth of 0.2 mm and a length of 12.5 mm.
In addition, after collecting the test material from the obtained ERW welded pipe and performing cold drawing to an outer diameter of 36.7mmφ x thickness of 7.2mm, normalization treatment (air cooling after heating at 945 ° C) and quenching treatment (950 (Cooling after water cooling) was performed, and a torsional fatigue test piece (length: 500 mm) was collected and subjected to a torsional fatigue test. In the torsional fatigue test, 10 specimens were tested with an outer surface torsional stress τ of 350 MPa and repeated up to 2 million times to measure the occurrence ratio (%) of ERW cracks. From these results (results of ultrasonic flaw detection and torsional fatigue test), the reliability of ERW welds was evaluated. Reliability was evaluated by setting the case where the number of defects in ultrasonic flaw detection was 0 and no cracking occurred in the torsional fatigue test as ○, and other cases as ×.

  The obtained results are shown in Table 3.

  In all of the examples of the present invention, the occurrence of defects in the ERW welds is small, and the occurrence of cracks in the ERW welds is also reduced in the torsional fatigue test. On the other hand, the comparative example which is out of the scope of the present invention has a large number of defects in the electric resistance welded portion, and cracks are generated in the electric resistance welded portion even in the torsional fatigue test.

Claims (7)

After forming the material steel plate into a substantially cylindrical shape by cold working, in the manufacturing method of the ERW welded steel pipe to make the ERW welded steel pipe by butt-welding the opposing end faces to each other,
The material steel plate in mass%,
C: 0.30 to 0.60%, Si: 0.05 to 0.50%, Mn: 0.30 to 2.0%, Al: 0.50% or less, N: 0.0100% or less, and a high carbon steel plate having a composition comprising the balance Fe and inevitable impurities,
After the ERW welding, after cold drawing rolling with a drawing ratio of 0.8% or less, immediately reheat or cool and reheat, and at a temperature range of 850 ° C. or higher, the diameter reduction ratio is 10% or more. A method for producing a high carbon electric resistance welded steel pipe, characterized in that hot shrinking rolling is performed to make an electric resistance welded portion excellent in reliability.   In addition to the above composition, one or two selected from the following by mass: Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.2% or less, Mo: 1.0% or less, W: 1.5% or less The method for producing a high carbon electric resistance welded steel pipe according to claim 1, comprising at least a seed.   The composition further comprises one or more selected from Ti: 0.04% or less, Nb: 0.2% or less, and V: 0.2% or less in mass% in addition to the composition. The manufacturing method of the high carbon electric resistance welded steel pipe of 1 or 2.   The method for producing a high carbon electric resistance welded steel pipe according to any one of claims 1 to 3, further comprising B: 0.0005 to 0.0050% by mass% in addition to the composition.   The method for producing a high carbon electric resistance welded steel pipe according to any one of claims 1 to 4, wherein the reheating is heating by high frequency induction heating means. An automobile part manufacturing method for manufacturing an automobile part using a high carbon electric resistance welded steel pipe as a raw material, wherein the raw material is manufactured using the method for manufacturing a high carbon electric resistance welded steel pipe according to any one of claims 1 to 5. method for producing automotive parts, which is a high-carbon seam welded steel pipe which is. The method of manufacturing an automobile part according to claim 6, wherein the automobile part is one of a front fork, a rack bar, a drive shaft, a tie rod, a stator shaft, and a camshaft.

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