KR20160055193A - Process for manufacturing high-carbon electric resistance welded steel pipe, and automobile part - Google Patents

Abstract

Provided is a high carbon carbon steel welded steel pipe having an excellent weldability. And a balance of Fe and unavoidable impurities, wherein the composition contains 0.30 to 0.60% of C, 0.05 to 0.50% of Si, 0.30 to 2.0% of Si, 0.50% or less of Al and 0.0100% or less of N, The hot-rolled steel pipe is formed into a substantially cylindrical shape by cold working, and the hot-rolled steel pipe is used as the hot-rolled steel pipe. The hot rolled steel pipe is subjected to cold- The steel sheet is immediately reheated or cooled to reheat the steel sheet and subjected to hot shrinkage rolling at a temperature of 850 DEG C or higher at a shrinkage ratio of 10% or more. As a result, it is possible to obtain a high carbon carbon steel welded steel pipe in which the occurrence of defects in the welded portion of the rolled steel is suppressed, thereby making the rolled steel welded portion excellent in reliability and the reliability remarkably improved. In addition, the reliability of automobile parts is improved by using these high-carbon steel-plated steel pipes as materials.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing a high carbon steel welded steel pipe,

TECHNICAL FIELD The present invention relates to a method for manufacturing a high carbon welded welded steel pipe suitable for a hollow machine part such as an automobile and an automobile part.

TECHNICAL FIELD The present invention relates to a method for manufacturing a high carbon electric resistance welded steel pipe or tube suitable for hollow mechanical parts such as automobiles. Particularly, the present invention relates to improvement of reliability of an electric resistance weld.

Recently, improvement of fuel efficiency of automobile is strongly demanded from the viewpoint of preservation of global environment, and weight saving of automotive body is strongly oriented. Therefore, a hollow material has been used as a material for automobile parts in place of the solid material used in the past. As a hollow material for parts requiring heat treatment for use in automobiles and the like, it is a high-carbon steel for mechanical structural use, especially because of its high dimensional accuracy and low surface decarburization. The use of carbon steel welded steel pipes has been studied.

However, in a high-carbon steel material for mechanical structure, since the amount of carbon is increased, strength is increased, elongation is lowered, and segregation tends to become strong. Therefore, in a segregated portion in which C, Mn, and P are strongly segregated, deterioration of hot workability at high temperatures becomes remarkable, so that it becomes difficult to weld the entire steel itself, Defects such as hot cracking are frequently caused in the steel pipe, thereby causing problems in workability as a steel pipe.

With respect to such a problem, Patent Document 1, for example, discloses a ferritic stainless steel which comprises 0.4 to 0.8% of C, 0.15 to 0.35% of Si, 0.3 to 2.0% of Mn, 0.030% or less of P, 0.035% or less of S And 0.035% or less of Al, 0.05 to 0.15% of Mo is further added, and the balance Fe and inevitable impurities are contained in a steel pipe of mechanical structural high carbon steel. According to the technique described in Patent Document 1, the addition of Mo makes it possible to significantly improve the workability in the hot state at 1000 ° C or higher, and it becomes a steel-made steel pipe for machine structural high carbon steels excellent in hot workability.

Patent Document 2 discloses a ferritic stainless steel which contains 0.3 to 0.6% of C, 0.15 to 0.35% of Si, 0.3 to 1.5% of Mn, 0.012% or less of P, 0.035% or less of S and 0.035% or less of Al A high carbon steel slab obtained by hot rolling a high carbon steel slab adjusted to a low level, in which the P concentration of the center segregation part, which is continuous cast, satisfies a specific relationship with the C concentration, There is disclosed a method of manufacturing a high-strength, high-carbon steel, seamless steel pipe in which a steel pipe is manufactured using a hot-rolled coil as a material. According to the technique described in Patent Document 2, cracks at a high temperature during welding are suppressed, and the yield is improved. Further, according to the technique described in Patent Document 2, the possibility of occurrence of embrittlement cracks in the segregation portion is low even when severe processing such as bulge forming is performed, and the workability of the high-carbon steel sewing steel pipe is improved .

Patent Document 3 discloses a method of continuously casting high carbon steels containing 0.30 to 0.60% of C and 0.012% or less of P at a percentage by mass, to determine whether the P concentration of the center segregated portion satisfies a specific relationship in relation to the C concentration A high-carbon steel slab adjusted to a low level and a hot-rolled coil of a high carbon steel obtained by hot rolling the slab of a high carbon steel as a material, both ends of the open pipe are pre-heated to 800 to 1000 占 폚 with a heating width of 2 to 4 mm which is generally wider than usual, There is disclosed a method for producing a high carbon steel, galvanized steel pipe having high airtight mechanical structure for air cooling. According to the technique described in Patent Document 3, high-temperature cracks at the time of welding are suppressed, yield is improved, and the hardness of the full-joint welding portion is reduced. Thus, even when severe processing such as bulge- And it is said that the workability of the high strength carbon steel pipe is improved.

Patent Document 4 discloses a method of welding a seamless steel pipe having a composition containing 0.03 to 0.30% of C, 0.50 to 3.0% of Si and 0.30 to 3.00% of Mn, and heating the welded portion to 800 to 1000 占 폚 , Ar ₃ There is disclosed a heat treatment method of a seam welding portion in which retained austenite remains in a seam welding portion to quench at a temperature of 20 to 200 캜 / s from a transformation point or higher to enhance the workability of the seam welding portion. According to the technique described in Patent Document 4, the ductility of the welded portion of the seam is improved, and it is said that the steel pipe is able to withstand rigid processing such as hydroforming.

Japanese Laid-Open Patent Publication No. 04-263039 Japanese Patent Application Laid-Open No. 11-156433 Japanese Patent Application Laid-Open No. 11-226634 Japanese Laid-Open Patent Publication No. 11-323442

In recent years, particularly from the viewpoint of ensuring safety of automobiles and the like, it is strictly required to maintain high reliability in parts such as automobiles. Particularly, in the case of using a seamless-welded steel pipe as a material for a component, it is required that the seamless-welded steel pipe has an electrode-welded portion having higher reliability than the conventional one. However, in the technologies described in Patent Documents 1 to 4, there is a problem that the requisite performance for reliability, which is represented by the fatigue strength of the welded joint, can not be sufficiently satisfied.

It is an object of the present invention to solve the above problems and provide a method for manufacturing a seamless welded steel pipe of high carbon steel having an excellent welding reliability. The term " excellent in reliability " as used herein refers to a case where there is no defect that affects the fatigue strength in the weld zone. Specifically, a notch having a depth of 0.2 mm and a length of 12.5 mm is used as a reference, and the ultrasonic flaw detection test is based on the UA of JIS G 0582, "Ultrasonic Testing of Steel Tubes" , The sensitivity was increased to 6 db (decibel) sensitivity enhancement in order to detect fine defects and the number of repetitions was 0, and the torsional stress τ of the outer surface was 350 MPa. It is assumed that cracks do not occur in the torsion fatigue test up to 2,000,000 times.

In order to achieve the above object, the inventors of the present invention have extensively studied the cause of low reliability of conventional high-carbon steel seamless welded steel pipes. As a result, it has been found that defects such as cracks are likely to remain in the seam welding portion in the conventional high-carbon steel seamless welded steel pipe. In conventional high-carbon steel seamless-welded steel pipes, sizing and straightening are usually performed in cold after completion of full-stroke welding in order to adjust the precision to a predetermined size. It is conceivable that cracking is caused in the welded portion of the rolled-joint by the reducing rolling or the bending correction, which is hardened by the welding by the welding, and the reliability is lowered.

Thus, in the case of high-carbon steel seamless-welded steel pipes, it is conceivable to normalize only the welded joints after the completion of the welding, and then to perform cold sizing, bending correction and the like. However, also with this method, a sufficient improvement in reliability can not be obtained. The reason for this is unclear at this point, but it is likely that defects such as shrinkage cavities are likely to be involved. That is, in general, in the welding of low carbon steels, the welded portion is usually squeezed by squeeze roll to prevent shrinkage hole defects. However, since the melting point is lowered in the welding of high carbon steels, It may be considered that the melting section remains until after passing, and the shrinkage cavity-like defect tends to occur easily.

In view of this, the inventors of the present invention have found that, in order to further improve the reliability of the welded steel pipe of high carbon steel, it is necessary not only to improve the ductility by performing a simple heat treatment on the welded portion of the welded joint, It is necessary to perform the same processing (reducing process) as that of the above-mentioned process (reduction process).

As a result of further studies, the inventors of the present invention have found that, in order to further improve the reliability of the welded seamless steel pipe of high carbon steel, after cold working such as calibration immediately after welding is minimized, It has been found effective to carry out hot shrinkage rolling at a reducing rate of diameter of 10% or more at a temperature range of 850 占 폚 or more. It has also been recognized that, during reheating, it is possible to shorten the heating time by using induction heating and to suppress decarburization.

Experimental results that are the basis of the present invention will be described.

Carbon steel sheet having a composition of 0.37% by weight, 0.25% by weight of Si, 1.50% by weight of Mn, 0.025% by weight of Al, 0.004% by weight of N, 0.02% by weight of Ti and 0.002% Mm) was used as a material steel plate and cold-formed into a substantially cylindrical shape by using a plurality of rolls. The opposite ends of the steel plates were brought into contact with each other and were subjected to full-joint welding to form a seamless steel pipe (outer diameter 89.1 mm?). After cold-rolled, cold-reduced rolling was carried out in a cold sizer rolling mill at a reducing rate of 0 to 1.2%. The number of defective portions (number of defects) was measured by performing ultrasonic inspection on the obtained seamless-welded steel pipe, particularly with respect to the welded portion of the rope. The ultrasonic flaw detection was performed by increasing the sensitivity of 6 db 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.

It can be seen from Fig. 1 that when the re-usable rate of the cold-reduced rolling exceeds 0.8%, the occurrence of defects becomes remarkable.

After cold rolling, cold reductive rolling at a re-reducing rate of 0.1% was performed, and then immediately reheated to 980 占 폚 to change the diameter reduction rate at a temperature range of 850 占 폚 or more to 0 to 35% And subjected to shaper-diameter rolling. The obtained welded steel pipe was ultrasonically inspected for the welded portion of the welded joint, and the number of defective parts (the number of defects) was measured. The conditions of the ultrasonic test were the same as those after welding. The obtained results are shown in Fig. From Fig. 2, it can be seen that the occurrence of defects in the welded portion of the rolled steel is remarkable in the hot shrinkage rolling with a reduction rate of less than 10%, and the occurrence of defects is remarkably reduced when the reduction rate exceeds 10%.

The present invention has been completed on the basis of this recognition with further examination. That is, the gist of the present invention is as follows.

(1) A method for producing a seamless steel pipe in which the steel sheet is formed into a substantially cylindrical shape by cold working, and the opposite ends of the steel sheet are brought into contact with each other to be subjected to complete welding, , A balance of Fe and unavoidable impurities, and a high-carbon steel sheet having a composition of 0.30 to 0.60% of C, 0.05 to 0.50% of Si, 0.30 to 2.0% of Mn, 0.50% or less of Al and 0.0100% or less of N, After cold rolling, cold reductive rolling with a reduction ratio of 0.8% or less is carried out and then immediately reheated or cooled to reheat the steel and subjected to hot shrinkage rolling at a temperature of 850 DEG C or higher at a reduction rate of 10% Welded steel pipe excellent in the reliability of the whole-jointed welded steel pipe.

(2) The steel sheet according to item (1), further comprising, in mass%, 1.0% or less of Cu, 1.0% or less of Ni, 1.2% or less of Cr, 1.0% or less of Mo, By weight, based on the total weight of the carbon steel pipe.

(3) The steel according to the above (1) or (2), further comprising one or more than one selected from the group consisting of Ti in an amount of 0.04% or less, Nb in an amount of 0.2% Wherein the welded steel pipe is made of a high-carbon steel.

(4) The method for producing a seamless carbon steel welded steel pipe according to any one of (1) to (3), further comprising, in mass%, B: 0.0005 to 0.0050% in addition to the above composition.

(5) The method for producing a seamless carbon steel pipe of high carbon steel according to any one of (1) to (4), wherein the reheating is heating by a high frequency induction heating means.

(6) An automobile part manufactured by using a high carbon steel sewage pipe manufactured by using the method for manufacturing a seamless carbon steel pipe of high carbon steel according to any one of (1) to (5).

(7) The automatic transmission according to (6), wherein the automobile part is a front fork, a rack bar, a drive shaft, a tie rod, a stator shaft, Automotive parts, which are camshafts.

According to the present invention, it is possible to obtain a seamless steel welded steel pipe of high carbon steel having a seamless welding portion with suppressed occurrence of defects and excellent reliability. As a result, according to the present invention, the reliability of the welded seamless steel pipe of high carbon steel is remarkably improved. Further, according to the present invention, it is possible to manufacture various kinds of automobile parts such as a hollow part made of a seamless carbon steel pipe of high carbon steel as a raw material such as a front fork, a rack bar, a drive shaft, a tie rod, a stator shaft, The reliability of the apparatus is also improved.

FIG. 1 is a graph showing the influence of the re-duty ratio of the cold-reducing steel rolling on the number of defects in the weld zone.
Fig. 2 is a graph showing the influence of the reduction rate of hot sharply reduced rolling on the number of defects in the seamless weld.

(Mode for carrying out the invention)

The present invention relates to a method for producing a seamless carbon steel pipe of high carbon steel. In the present invention, a high-carbon steel sheet is used as the material steel sheet, and a method of manufacturing a commercially available seamless-welded steel pipe is applied. The term " steel plate " used herein also includes a steel strip.

First, the reason for limiting the composition of the high carbon steel sheet which is a steel sheet will be described. Hereinafter, unless otherwise stated, mass% is simply expressed as%.

In the present invention, the steel sheet made of the material steel sheet contains 0.30 to 0.60% of C, 0.05 to 0.50% of Si, 0.30 to 2.0% of Mn, 0.50% or less of Al and 0.0100% or less of N. The steel sheet as the material steel sheet in the present invention is preferably one or more selected from the group consisting of 1.0% or less of Cu, 1.0% or less of Ni, 1.2% or less of Cr, 1.0% or less of Mo and 1.5% . In the present invention, the steel sheet made of the material steel sheet may contain one or more selected from among 0.04% or less of Ti, 0.2% or less of Nb, and 0.2% or less of V. Further, the steel sheet made of the material steel sheet in the present invention may contain B: 0.0005 to 0.0050%. The remainder other than the essential and optional components are Fe and inevitable impurities. Further, in order to improve the reliability of the rolled-up welding portion, it is preferable that the thickness of the material steel plate is 8 mm or less from the viewpoint of discharging the oxide from the rolled-up welding portion.

C: 0.30 to 0.60%

C is an element which is dissolved by heating or as carbide or carbonitride and contributes to an increase in strength. In order to obtain such effects and secure the desired strength of the steel pipe and the strength of the steel pipe after the heat treatment, the C content is set to 0.30% or more. The " desired steel pipe strength " as used herein refers to a tensile strength TS: 1200 MPa or more. On the other hand, if the C content exceeds 0.60%, the toughness after the heat treatment is lowered. For this reason, the C content is limited to the range of 0.30 to 0.60%.

Si: 0.05 to 0.50%

Si is an element acting as a deoxidizer. In order to obtain such an effect, the Si content should be 0.05% or more. On the other hand, if the Si content exceeds 0.50%, the effect becomes saturated and becomes economically disadvantageous, and the generation of inclusions is promoted at the time of the complete welding, adversely affecting the soundness of the fully welded portion. Therefore, the Si content is limited to the range of 0.05 to 0.50%. Further, it is preferably 0.10 to 0.30%.

Mn: 0.30 to 2.0%

Mn is an element contributing to enhancement of strength and improvement of quenching property by solid-solution treatment. To obtain such an effect, the Mn content should be 0.30% or more. On the other hand, if the Mn content exceeds 2.0%, the retained austenite is formed and the toughness after the tempering treatment is lowered. For this reason, the Mn content is limited to the range of 0.30 to 2.0%. Further, it is preferably 0.8 to 1.6%.

Al: 0.50% or less

Al is an element acting as a deoxidizer. In order to obtain such an effect, the Al content is preferably 0.01% or more. On the other hand, when the Al content exceeds 0.50%, the effect becomes saturated and an effect suitable for the content can not be expected, which is economically disadvantageous. In addition, the occurrence of inclusions at the time of welding is accelerated, It goes crazy. Therefore, the Al content is limited to a range of 0.50% or less. Further, it is preferably 0.02 to 0.04%.

N: 0.0100% or less

N is a useful element for forming nitride or carbonitride and securing strength after heat treatment (tempering). In order to obtain such an effect, it is preferable to contain 0.0005% or more. When the N content exceeds 0.0100%, a coarse nitride is formed, and the toughness or endothelial life sometimes deteriorates. Therefore, the N content is limited to 0.0100% or less. In the case where N contains Ti, it is preferable to adjust N so as to satisfy the following equation in relation to the Ti content.

N / 14? Ti / 47.9

(Where N, Ti: content of each element (mass%))

The above-mentioned component is a basic component of a steel sheet to be a steel sheet. In the present invention, in addition to this basic composition, one or more selected from among Cu: not more than 1.0%, Ni: not more than 1.0%, Cr: not more than 1.2%, Mo: not more than 1.0% And at least one selected from the group consisting of at least one of Ti, at most 0.04% of Ti, at most 0.20% of Nb, at most 0.20% of V, and at most 0.205% of B, and / Maybe.

At least one selected from the group consisting of Cu: not more than 1.0%, Ni: not more than 1.0%, Cr: not more than 1.2%, Mo: not more than 1.0%, W: not more than 1.5%

Cu, Ni, Cr, Mo, and W all contribute to an increase in strength and an improvement in hardenability. The steel sheet to be the material steel sheet may contain one or more kinds selected from Cu, Ni, Cr, Mo and W, if necessary.

Cu is an element that improves toughness, delayed fracture resistance, and corrosion fatigue resistance as well as contributing to an increase in strength and an improvement in quenchability by solidification. In order to obtain such an effect, the Cu content is preferably 0.05% or more. On the other hand, if the Cu content exceeds 1.0%, the above effect is saturated and the effect that is suitable for the content can not be expected, which is economically disadvantageous and the workability is lowered. Therefore, in the case of containing Cu, it is preferable that the content of Cu is limited to 1.0% or less. More preferably, it is 0.05 to 0.25%.

Ni is an element contributing to an increase in strength and an improvement in quenching property by employment and also to an improvement in toughness, delayed fracture resistance and internal fatigue characteristics. In order to obtain such an effect, the Ni content is preferably 0.05% or more. When the Ni content exceeds 1.0%, the above effect is saturated and the effect that is suitable for the content can not be expected, which is economically disadvantageous, and the workability is lowered. Therefore, when Ni is contained, the Ni content is preferably limited to 1.0% or less. More preferably, it is 0.05 to 0.25%.

Cr contributes to an increase in strength and an improvement in quenching property by solidification of Cr, and also contributes to an increase in strength by precipitation strengthening by generating fine carbides. In order to obtain such an effect, the Cr content is preferably 0.1% or more. On the other hand, if the Cr content exceeds 1.2%, the effect becomes saturated and the effect suitable for the content can not be expected, which is economically disadvantageous. In addition, inclusion is liable to occur at the time of welding, It goes crazy. Therefore, when Cr is contained, the Cr content is preferably limited to 1.2% or less. More preferably, it is 0.1 to 0.5%.

Mo contributes to the increase of strength and the improvement of quenching property by employment and also to the increase of strength by precipitation strengthening by producing fine carbide. In order to obtain such an effect, it is preferable that the Mo content is 0.01% or more. On the other hand, if the Mo content exceeds 1.0%, the effect becomes saturated and an effect suitable for the content is not expected, which is economically disadvantageous, and coarse carbide is formed and toughness is lowered in some cases. Therefore, when Mo is contained, it is preferable to limit the Mo content to 1.0% or less. More preferably, it is 0.10 to 0.30%.

W has an effect of improving the balance between hardness and toughness after heat treatment, in addition to contributing to the increase of strength and the improvement of quenching property by employment. In order to secure such effect, it is preferable to set the W content to 0.01% or more. On the other hand, if the W content exceeds 1.5%, the effect becomes saturated and an effect suitable for the content can not be expected, which is economically disadvantageous. Therefore, in the case of containing W, W is preferably limited to 1.5% or less. More preferably, it is 0.10 to 0.30%.

Ti: not more than 0.04%, Nb: not more than 0.20%, V: not more than 0.20%

Ti, Nb, and V are all elements that contribute to the increase in strength by forming a fine carbide, and may contain one or two or more of them if necessary.

Ti is an element having an action to secure solubility B effective for improving quenching by fixing N by bonding with N in addition to the above action. Further, Ti has a function of forming a fine nitride and suppressing coarsening of crystal grains during heat treatment or welding, and contributes to improvement of toughness. In order to obtain such an effect, the Ti content is preferably 0.001% or more. On the other hand, if the Ti content exceeds 0.04%, the inclusions increase and the toughness may decrease. Therefore, in the case of containing Ti, the Ti content is preferably limited to 0.04% or less. When Ti is contained, it is preferable that Ti contains the following formula in relation to the N content. More preferably, it is 0.01 to 0.03%.

N / 14? Ti / 47.9

(Where N, Ti: content of each element (mass%))

Nb has a function of forming fine carbides at the time of tempering to contribute to the increase in strength and also to improve the toughness and delayed fracture resistance by making the structure after heat treatment finer. In order to obtain such an effect, the Nb content is preferably 0.001% or more. On the other hand, when the Nb content exceeds 0.20%, the above effect is saturated and the effect suitable for the content is not expected, which is economically disadvantageous. Therefore, when Nb is contained, Nb is preferably limited to 0.20% or less. More preferably, it is 0.01 to 0.02%.

V forms fine carbides at the time of tempering and contributes to the increase in strength. In order to secure such effect, it is preferable that the V content is 0.001% or more. On the other hand, if the V content exceeds 0.20%, the above-mentioned effect becomes saturated and an effect suitable for the content can not be expected, which is economically disadvantageous. Therefore, when V is contained, the V content is preferably limited to 0.20% or less. The more preferable range is 0.01 to 0.08%.

B: 0.0005 to 0.0050%

B improves the quenching property with a trace amount and improves the balance between hardness and toughness after heat treatment. In addition, B enhances the grain boundary to improve quenching crack resistance. B may be contained as needed. In order to obtain such an effect, the B content is set to 0.0005% or more. On the other hand, if the B content exceeds 0.0050%, the above effect is saturated and an effect suitable for the content is not expected, which is economically disadvantageous, and tough B-containing precipitates are produced and the toughness is lowered. Therefore, when B is contained, the B content is preferably limited to a range of 0.0005 to 0.0050%. More preferably, it is 0.002 to 0.003%.

The balance other than the above-mentioned components is composed of Fe and inevitable impurities. As the inevitable impurities, 0.020% or less of P, 0.010% or less of S and 0.005% or less of O can be allowed.

P: not more than 0.020%

P is an element that adversely affects the weld crack resistance and toughness. The P content is preferably reduced as much as possible within a range of 0.020% or less. However, excessive reduction of the P content raises the refining cost, so the P content is preferably 0.0005% or more. More preferably, it is 0.010% or less.

S: not more than 0.010%

S exists as a sulfide inclusion in the steel and adversely affects the workability, toughness and fatigue life, and increases reheat cracking sensitivity. The S content is preferably reduced as much as possible within a range of 0.010% or less. However, since the excessive reduction raises the refining cost, the S content is preferably 0.0005% or more. More preferably, it is 0.001% or less.

O: 0.005% or less

O (oxygen) exists as an oxide inclusion in the steel, and adversely affects the workability, toughness and fatigue life. Therefore, it is preferable that the O (oxygen) content is reduced as much as possible within the range of 0.005% or less. More preferably, it is 0.002% or less.

In the present invention, the high carbon steel sheet having the above composition is used as a material steel sheet. The production method of the material steel sheet is not particularly limited. Any of the usual methods for producing hot-rolled steel sheets can be applied. The material steel sheet is slit to a predetermined width and is continuously formed into a substantially cylindrical shape in a cold state, preferably using a plurality of forming rolls. Thereafter, the opposite end faces are brought into contact with each other, Welded steel pipe.

In the present invention, after cold-rolled steel pipe is welded by full-joint welding, cold rolled steel is subjected to cold-drawn steel pipe in order to prevent defect of shape. For this rolling, a sizer mill is preferably used. In the present invention, the re-duty ratio of cold-reducing rolling is limited to 0.8% or less. If the reeducing ratio exceeds 0.8%, defects such as cracks are generated in the rolled-on welded portion and the reliability of the rolled-up welded portion is deteriorated. For this reason, the re-duty ratio of the cold-reduced rolling performed after the welding is limited to 0.8% or less. Further, it is preferably 0.01 to 0.1%. It is preferable that the occurrence of defects in the rollover welding portion is not subjected to cold reduction rolling (0% of re-cutting). When the cold reduction rolling is not performed, the probability of occurrence of defects in a tube shape is increased. Further, the definition of the reeducing ratio is (perimeter before the sizing-length around the periphery after sizing) / length before the sizing x 100 (%).

Reduced Ratio: The cold-rolled steel pipe subjected to cold-reducing rolling of 0.8% or less is immediately reheated or reheated after being cooled to room temperature. The reheating temperature is not particularly limited. In the present invention, the reheating temperature is preferably 900 to 1050 占 폚 at which the hot shrinking can be performed at a temperature of 850 占 폚 or more in a temperature range of 10% or more.

In the present invention, the hot shrinking process reheats the steel sheet to the austenite region, achieves high toughness of the welded portion of the rolled steel sheet, eliminates the deterioration caused by the rolled steel sheet welded portion, For improvement. When the finishing rolling temperature of the hot sharps rolling is less than 850 ° C, compression bonding of shrinkage cavity defects becomes insufficient and deterioration of a desired defect can not be achieved. The finish rolling temperature of hot sharply reduced rolling is preferably 900 DEG C or higher. The upper limit of the finish rolling temperature of the hot sharpshield rolling is 1000 캜, which can prevent the coarsening of the structure.

If the reduction ratio of the hot shrinkage-rolling is less than 10% at a temperature range of 850 占 폚 or more, the shortening of the reduction rate is insufficient and deterioration of a desired defect can not be achieved. For this reason, the reduction rate of the hot sharply reduced rolling is limited to 10% or more. Further, it is preferably 30% or more. The upper limit of the diameter reduction ratio of the hot sharps rolling is determined according to a desired dimension shape. The diameter reduction rate is defined as (outer diameter before rolling-outer diameter after rolling) / outer diameter before rolling x 100 (%).

Example

A hot-rolled steel sheet (sheet thickness: 7.8 mm) of high carbon steel having the composition shown in Table 1 was used as a material steel sheet. These material steel plates were slitted to a predetermined width and formed into a substantially cylindrical open pipe by a plurality of rolls in a cold state. Thereafter, the opposite end faces were subjected to face-to-face welding so as to form a seamless welded steel pipe (core pipe) having an outer diameter of 89.1 mm and a thickness of 7.9 mm. After welding, the cold-rolled steel pipe was subjected to cold-reducing rolling with a reducing rate as shown in Table 2 so as to have a predetermined size. After the cold reduction rolling, the steel sheet was immediately heated to the temperature shown in Table 2 by means of an induction heating means and subjected to hot sharply reduced rolling under the conditions shown in Table 2 by a hot sharps rolling mill. After hot sharply reduced rolling, Welded steel pipe.

Ultrasonic inspections were conducted on the total length (about 10000 m) of the entire welded joint of the obtained seamless welded steel pipe, and the presence or absence of defects detected and the number of defects (converted into 10000 m in length) were investigated. Ultrasonic flaw detection was performed with a 6 dB sensitivity increase based on a notch having a depth of 0.2 mm and a length of 12.5 mm.

The test material was taken from the obtained welded steel pipe and subjected to cold drawing to an outer diameter of 36.7 mm and a thickness of 7.2 mm and subjected to a normalizing treatment (air cooling after heating at 945 캜) and a quenching treatment (water quenching after heating at 950 캜) , And a torsional fatigue test piece (length: 500 mm) were taken and subjected to a torsional fatigue test.

In the torsional fatigue test, the occurrence rate (%) of occurrence of cracks in the welded portion of the seam was measured by performing tests up to the number of repetitions: 2,000,000 times, with the torsional stress τ of the outer surface being 350 MPa for 10 test pieces. From these results (the results of the ultrasonic test and the torsional fatigue test), the reliability of the welded joint was evaluated. The reliability was evaluated as "? &Quot; when the number of defects in the ultrasonic inspection was zero and no crack occurred in the torsion fatigue test, and the others were evaluated as " x ".

The obtained results are shown in Table 3.

In all of the examples of the present invention, the generation of defects in the welded portion of the rolled steel is small, and the occurrence of cracks in the rolled steel welded portion is reduced even in the torsional fatigue test. On the other hand, in the comparative example deviating from the scope of the present invention, since the number of defects in the welded portion of the welded seam is large, cracks are more likely to occur in the welded joint portion in the torsional fatigue test.

Claims (7)

A method of manufacturing a seamless steel pipe in which a steel sheet is formed into a substantially cylindrical shape by cold working, and the opposite end faces are then subjected to face-to-face welding to form a seamless steel pipe,
The above-mentioned material steel sheet, in mass%
Carbon steel sheet having a composition comprising 0.30 to 0.60% of C, 0.05 to 0.50% of Si, 0.30 to 2.0% of Mn, 0.50% or less of Al and 0.0100% or less of N and the balance Fe and inevitable impurities After cold rolling, cold reductive rolling with a reducing rate of 0.8% or less is performed, and then immediately reheated or cooled to reheat the steel, and at a temperature range of 850 DEG C or higher, A method of manufacturing a seamless carbon steel pipe with high carbon steel for rolling. The method according to claim 1,
In addition to the above composition, at least one selected from the group consisting of Cu: not more than 1.0%, Ni: not more than 1.0%, Cr: not more than 1.2%, Mo: not more than 1.0%, W: not more than 1.5% By weight based on the total weight of the welded steel pipe. 3. The method according to claim 1 or 2,
Further comprising, in mass%, at least one selected from the group consisting of Ti: not more than 0.04%, Nb: not more than 0.2%, and V: not more than 0.2% ≪ / RTI > 4. The method according to any one of claims 1 to 3,
In addition to the above composition, further contains, by mass%, B: 0.0005 to 0.0050%. 5. The method according to any one of claims 1 to 4,
Wherein said reheating is heating by means of high frequency induction heating means. An automobile part characterized in that it is manufactured by using a high carbon carbon steel pipe manufactured by using the method for manufacturing a high carbon alloy welded steel pipe according to any one of claims 1 to 5. The method according to claim 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.

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