JP2008188641A - High-strength welded steel tube of excellent welding softening resistance and excellent fatigue characteristic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a welded steel tube capable of improving the fatigue characteristic by reducing a softened area and a softening ratio of a weld heat affected zone of the welded steel tube manufactured of a high-strength steel plate subjected to the work reinforcement as a material. <P>SOLUTION: A cold-rolled steel plate as a material is formed of a low-alloy steel containing Ti, Nb and B, and further containing 0.01-0.30 mass% Mo as necessary with C equivalent defined by the formula (4) C equivalent = C+1/6Mn+1/24Si+5B+1/4Mo to be 0.25-0.6 mass%. A welded steel tube is formed of the steel plate satisfying an inequality ΔHV × width of softened part ≤150, where ΔHV of the weld softened part and the width of the softened part are defined as (1) ΔHV of the weld softened part = hardness of base material - hardness of the mostly softened part of the weld, and (2) the width of softened part = the width (mm) of the softened part in which its hardness is lower than that of the base material by the welding heat input. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-strength welded steel pipe excellent in weld softening resistance and fatigue characteristics, which is used for structural members and reinforcing materials such as automobiles and bicycles, which are high-strength and inexpensive.

In welded steel pipes used for structural members and reinforcing members such as automobiles and bicycles, predetermined strength and fatigue characteristics are required. Also, in order to reduce the weight required for applications such as automobiles and bicycles, it is important that the mechanical strength is high and that the desired strength level is satisfied even if the thickness is reduced. Moreover, it is desirable to increase the strength by an inexpensive method.
Examples of the strengthening mechanism of steel include a solid solution strengthening method, a transformation structure strengthening method, and a work strengthening method. However, in the solid solution strengthening method and the transformation structure strengthening method, it is necessary to add a large amount of special alloy elements such as Si and Mn, and the strength increases with the added amount, but the steel material cost inevitably increases.

On the other hand, the work strengthening method such as cold rolling is an efficient method in that high strength can be achieved at low cost. High-strength steel that is low-carbon steel, hot-rolled steel strip from which hot-rolled scale has been removed by pickling is rolled for one pass at a rolling reduction of 10 to 50%, and then both ends in the width direction of the obtained steel strip are welded. A welded steel pipe is proposed in Patent Document 1.
The present inventors also cold rolled a C-Mn-based hot-rolled steel sheet at a reduction rate of 10 to 75%, and welded both ends in the width direction of the steel strip as it was cold-rolled. Patent Document 2 proposes a method for manufacturing a steel pipe.
JP 2002-327245 A JP 2005-29882 A

However, as disclosed in Patent Document 2 mentioned above, the maximum in the case of producing a high-strength welded steel pipe from a steel sheet work-hardened by cold rolling a C-Mn hot-rolled steel sheet at a high pressure reduction rate. The problem is that even if high strength can be achieved easily, softening occurs in the heat affected zone due to heat input during welding. In addition, various types of welding such as arc welding and TiG welding are performed on structural members used in automobiles, bicycles, etc., but welding softening also occurs when the amount of heat input during the welding is large. If a weld softening zone is generated in the weld heat affected zone, the strength and fatigue resistance of this member will be greatly reduced as compared to a zone where there is no welding heat affected zone. Even in steel sheets that have been strengthened by other strengthening methods, a decrease in strength is observed due to the heat effect during welding, but weld softening in work-hardened steel materials is more noticeable than steel materials by other strengthening methods.
As a measure for welding softening, elements such as Ti and Nb are usually added to the steel sheet in combination, and the softening of the weld heat affected zone is suppressed as much as possible by optimizing the C equivalent. However, it is insufficient for solving the problem that the strength and fatigue characteristics of a welded steel pipe manufactured using work-hardened steel as a raw material are lowered.

  The present invention has been devised to solve such problems, adding B in addition to Ti and Nb to a C-Mn steel sheet, further adding Mo as required, An object is to provide a high-strength welded steel pipe excellent in weld softening resistance and fatigue characteristics characterized by optimizing the C equivalent and suppressing the softening region and softening ratio of the weld heat-affected zone to be small.

The high strength welded steel pipe excellent in weld softening resistance and fatigue characteristics of the present invention is a welded steel pipe manufactured by welding the ends of cold-rolled steel sheets formed into an open pipe shape to achieve the purpose. When the ΔHV and the softened portion width of the weld softened portion in the vicinity of the weld metal of the joint portion are defined as the following formulas (1) and (2), respectively, ΔHV × softened portion width of the weld softened portion ≦ 150.
ΔHV of welded softened portion = base metal hardness−hardness of welded softest portion (1)
Softened part width = width of the part softened from the base metal by welding heat input (mm) (2)

As the cold-rolled steel sheet, C: 0.01 to 0.20% by mass, Si: 1.5% by mass or less, Mn: 2.5% by mass or less, P: 0.05% by mass or less, S: 0.02 mass% or less, acid-soluble Al: 0.005-0.10 mass%, B: 0.0005-0.005 mass%, Ti: 0.01-0.15 mass%, and Nb : 0.01 to 0.10% by mass, further containing Mo: 0.01 to 0.30% by mass as necessary, the balance being Fe and inevitable impurities, and the following (3) or (4 It is preferable to have a component composition in which the C equivalent defined by the formula is adjusted to 0.25 to 0.6% by mass.
C equivalent = C + 1 / 6Mn + 1 / 24Si + 5B (3)
C equivalent = C + 1 / 6Mn + 1 / 24Si + 5B + 1 / 4Mo (4)

  In order to suppress deterioration of fatigue characteristics of a high-strength welded steel pipe, the present invention firstly requires that the softening rate and the size of the softened portion in the weld heat affected zone be extremely small. When the width is defined as in the above formulas (1) and (2), ΔHV of the weld softened portion × softened portion width ≦ 150. Furthermore, means for controlling the softening rate and size of the weld softened part are the steel composition and welding conditions. Therefore, by adding B to C-Mn steel in addition to Ti and Nb, and further adding Mo as necessary, and optimizing the C equivalent, the welding heat-affected zone can be improved even under a wide range of welding conditions. It is possible to provide a welded steel pipe that can suppress the softening rate and the softened portion to be small and has improved fatigue characteristics.

  Using the hot-rolled steel sheet of such a component composition as a base material, and further increasing the strength by applying cold working by ordinary cold rolling, and at the time of manufacturing a welded steel pipe, which is a compositional characteristic of the material steel sheet Softening of the weld heat affected zone and softening of the weld heat affected zone when used as a structural material can be suppressed. Since high-priced alloy elements are not used in large quantities, the present invention can provide a high-strength welded steel pipe excellent in weld softening resistance and fatigue characteristics at low cost.

Next, the reason why the softening rate and the softened portion width of the weld heat affected zone in the present invention are limited will be described.
ΔHV of weld softened portion × width of softened portion ≦ 150
When a welded steel pipe is used as a structural member, if a high stress is generated in the welded part depending on the shape of the member or the load load, a crack due to fatigue may occur and propagate based on the weld softened part. In order to suppress this, it is preferable to make the degree and area of softening of the weld softened portion as small as possible. Details will be given in the description of the examples, but the inventors manufactured welded steel pipes with various alloy compositions and welding conditions, and evaluated the weld softened part, the welded part's strength characteristics, and the fatigue characteristics. It has been found that when ΔHV × softened part width of the softened part exceeds 150, the strength characteristics are remarkably lowered and the fatigue resistance characteristics are also deteriorated. For this reason, the ΔHV × softened portion width is limited to 150 or less.
In addition, as shown in FIG. 1, ΔHV of the weld softened portion is a value obtained by subtracting the hardness of the weld softened portion from the hardness of the base metal, and the softened portion width is softened from the base metal by welding heat input. The width of the part (unit is mm).

  The ΔHV of the weld softened portion and the width of the softened portion are greatly affected by the composition of the steel sheet and the amount of heat input to the material by welding. The composition of the component disclosed in claim 2 or 3 needs to be adopted as the composition of the steel sheet. Moreover, since the ΔHV and the softened portion width of the weld softened portion increase as the heat input to the material by welding increases, one of the specific means for reducing the ΔHV × softened portion width to 150 or less is the amount of heat input by welding. Adjustment is generally performed by adjusting the welding current and welding speed. The greater the welding current or the lower the welding speed, the greater the heat input by welding.

Next, the reasons for limiting the action and content of the chemical components of the base steel in the present invention will be described individually.
C: 0.01 to 0.20% by mass
C is an alloy component effective for increasing the strength of the steel sheet. The strengthening effect by C is seen at 0.01 mass% or more. However, inclusion of an excessive amount of C greatly affects the hardenability, degrades the workability of the welded part, and causes cracking. From the viewpoint of ductility and weld toughness, the upper limit value was set to 0.20% by mass.

Si: 1.5% by mass or less Si is an effective alloy component for improving the strength, and the effect of adding Si is seen at 0.05% by mass or more. However, if added over 1.5% by mass, the cold workability and surface properties are likely to deteriorate although the strength increases.

Mn: 2.5% by mass or less Mn is an alloy component that contributes to strength improvement. The strength improvement effect by Mn is seen at 0.30% by mass or more, and becomes more remarkable as the Mn content increases. However, the excessive amount of Mn significantly deteriorates the weldability. Furthermore, Mn is an element that improves the hardenability, and increases the C equivalent, thereby degrading the workability of the welded part and causing cracks. In this respect, the Mn content is preferably as low as possible. In this component system, the upper limit value is set to 2.5% by mass. Preferably it is 1.0-2.0 mass%.

P: 0.05 mass% or less If it contains more than 0.05 mass%, low temperature toughness will fall. Therefore, the lower the P content, the better. In this component system, the upper limit value was set to 0.05 mass%. Preferably it is 0.02 mass% or less.

S: 0.02 mass% or less Since S is a component harmful to hot workability and cold workability, it is preferable to reduce the content thereof as much as possible. Usually, as long as it is unavoidably contained: 0.02% by mass or less, there is no adverse effect on the properties of the welded steel pipe. Preferably it is 0.005 mass% or less.

Acid-soluble Al: 0.005 to 0.10% by mass
Al is an alloy component added as a deoxidizer in the steelmaking stage. In order to obtain a sufficient deoxidation effect, it is necessary to add 0.005% by mass or more as acid-soluble Al. The Al deoxidation effect is saturated at 0.10% by mass of acid-soluble Al, and addition beyond this causes an increase in steel material cost. Preferably, the amount of Al added is in the range of 0.02 to 0.04% by mass.

Ti: 0.01-0.15 mass%
Ti is a component that fixes solute C, S, and N in steel as precipitates, and is an effective component for increasing the strength of a steel sheet by precipitation strengthening. The precipitate suppresses the recovery of processing strain introduced into the weld heat affected zone, and prevents softening of the weld heat affected zone by solid solution and reprecipitation during welding heating. Further, by fixing N as TiN, it is an effective component in preventing the reduction of the effective B amount described later. The effect of addition of Ti on increasing the strength is seen at 0.01% by mass or more, but excessive addition exceeding 0.15% by mass leads to an increase in production cost. The Ti content is preferably selected in the range of 0.01 to 0.05 mass%.

Nb: 0.01 to 0.10% by mass
Nb reacts with C in the same manner as Ti to produce precipitates, and is an effective component for increasing the strength of steel sheets by precipitation strengthening. Nb also refines the metal structure and improves the strength of the steel material. Further, in the welded portion, the recovery of processing strain is suppressed similarly to Ti, and the weld heat affected zone is prevented from being softened by solid solution and reprecipitation during welding heating. The effect of Nb addition is observed at 0.01% by mass or more, but when it exceeds 0.10% by mass, the effect is saturated and the manufacturing cost increases. The Nb content is preferably selected in the range of 0.01 to 0.05% by mass.

B: 0.0005-0.005 mass%
B is a characteristic component in the present invention. It is an extremely effective component for enhancing the hardenability of steel and increasing its strength. Moreover, the combined addition with Ti and Nb exerts an effect of suppressing recovery of processing strain introduced into the weld heat affected zone, narrows the softened portion width of the weld heat affected zone, and significantly reduces the softening rate. be able to.
Those effects are not recognized to be less than 0.0005% by mass, and if the content exceeds 0.005% by mass, the workability is remarkably lowered although the strength is increased. Therefore, the B addition amount is set to a range of 0.0005 to 0.005% by mass. Preferably it is the range of 0.001 to 0.003 mass%.

Mo: 0.01 to 0.30% by mass
It is an alloy component that is added as necessary, and is a component that contributes to increasing the strength and improving the toughness of the weld. The addition effect of Mo is seen at 0.01 mass% or more. The effect of improving properties is more remarkable as the amount added is increased, but the effect is saturated by the addition of 0.30% by mass. On the other hand, the addition of more causes an increase in production cost. When Mo is added, it is selected in the range of 0.01 to 0.30% by mass, preferably in the range of 0.05 to 0.1% by mass.

C equivalent: 0.25 to 0.6% by mass
The C equivalent is an index that greatly affects the suppression of softening of the weld heat affected zone. In the present invention, the C equivalent is determined by using Equation (3) when Mo is not included and by Equation (4) when Mo is included. The C content is a major factor, but besides C, the contents of Mn and Si added to improve the strength, and the content of B and Mo which are effective in suppressing the softening rate and the width of the softened portion are also factors.
As described above, when heat input by welding is applied to a cold-rolled steel sheet that has been strengthened by work strengthening, there is a problem that a remarkable weld softening region occurs in a heat-affected zone near the weld. At this time, the degree of softening is such that the higher the C equivalent, the smaller the softening rate and softening width. Therefore, in order to satisfy the properties of the welded steel pipe, the C equivalent needs to be 0.25% by mass or more. However, if the amount is increased to exceed 0.6% by mass, the welded portion is markedly cured, and not only the workability of the welded portion is remarkably impaired, but also causes a weld crack. Therefore, it is preferable to design the components so that the C equivalent is adjusted to a range of 0.25 to 0.6 mass%.
C equivalent = C + 1 / 6Mn + 1 / 24Si + 5B (3)
C equivalent = C + 1 / 6Mn + 1 / 24Si + 5B + 1 / 4Mo (4)

Then, the manufacturing method of the welded steel pipe of this invention is demonstrated easily.
In order to stabilize the hot rolling hot strength, after hot rolling at a finishing temperature not lower than the Ar ₃ transformation point, winding is performed in a temperature range of 450 ° C. or higher and 600 ° C. or lower, and the strength is increased by transformation. When the finishing temperature is below the Ar ₃ transformation point, the thickness accuracy is likely to deteriorate due to gauge hunting, width drawing, or the like in which the variation in hot strength accompanying transformation is large and the thickness changes greatly in the rolling direction. The higher the coiling temperature, the better the ductility of the steel strip. However, when coiled in a temperature range exceeding 600 ° C., the strength is significantly reduced due to the formation of iron-based carbides. The strength increases as the coiling temperature decreases, but if it is coiled at an excessively low temperature of less than 450 ° C, it becomes hard due to the low-temperature transformation phase, and the sheet thickness accuracy during cold rolling and the shape after cold rolling deteriorate. It becomes easy to do.
Therefore, the hot rolling is preferably performed under the conditions of finishing temperature: Ar ₃ transformation point or higher and winding temperature: 450 ° C. or higher and 600 ° C. or lower.

In cold rolling after cold rolling pickling, it is preferable to set the cold rolling rate to 10% or more in order to increase the strength of the steel strip by work strengthening. If it is less than 10%, the increase in strength is small. A cold rolling rate of 10% or more is also effective in ensuring plate thickness accuracy. However, although the strength increases as the cold rolling rate increases, an excessively large cold rolling rate increases the manufacturing cost, so the upper limit of the cold rolling rate is set to 75%.
Thus, it is preferable to perform cold rolling after pickling at a cold rolling rate of 10 to 75%.

In the pipe forming process hot-rolled sheet, Ti and Nb are strengthened by precipitation by fixing C, N, S and the like in the steel as precipitates. Further, the cold-rolled steel strip, which has been strengthened by work strengthening in the cold rolling process, is cut into a predetermined width without going through the annealing process and sent to the pipe making line. In a pipe making line, a steel strip is roll-formed or roll-less shaped and processed into an open pipe shape, and both ends of the steel strip in the width direction are joined by high-frequency welding or the like to produce a welded steel pipe having a predetermined diameter.

In addition, a steel strip cut to a predetermined width is cut in the longitudinal direction to form a cut plate, and both end portions of the cut plate are welded by various welding methods such as plate winding, TIG welding, MIG welding, laser welding and the like. A single tube of diameter can also be produced.
At this time, in order to obtain a high-strength welded steel pipe excellent in weld softening resistance and fatigue properties according to the present invention, the ΔHV of the weld softened part in the vicinity of the weld metal of the joint to be obtained at the time of welding in this pipe forming process. X The softened part width needs to be 150 or less. Here, as described above, one of the specific means for controlling the ΔHV of the weld softened portion and the width of the softened portion is the adjustment of the heat input during the welding, and the adjustment of the heat input is performed by adjusting the welding current and the welding speed. This is generally done by adjustment. As the welding current increases or the welding speed decreases, the heat input by welding increases, and accordingly, ΔHV and the width of the softened portion of the weld softened portion increase. Therefore, in order to set the ΔHV × softened portion width to 150 or less, the welding conditions are selected so that welding is performed with as little heat input as possible within a range that does not fall below the heat input capable of forming a sound weld metal in the weld joint. There is a need.

Example 1:
A steel slab having the composition shown in Table 1 is heated to 1230 ° C., finish-rolled at a temperature equal to or higher than the Ar ₃ transformation point, wound at 560 ° C., and hot-rolled to form a hot-rolled steel strip having a thickness of 3.2 mm. Manufactured. After pickling each hot-rolled steel strip, cold rolling was subsequently performed at a rolling rate of 10 to 60% to prepare a steel strip for pipe making.

Among the prepared steel strips for pipe making, prepare test pieces that were cold-rolled with steel No. 5 and No. 10 with a rolling rate of 60%, and change the welding speed while maintaining a flat plate shape. The relationship between ΔHV of the weld softened part, the softening rate, and the softened part width with respect to the heat input was evaluated. At this time, the welding method was the MIG welding method, and the welding conditions were a voltage of 14 V, a current of 70 A, and a welding speed of 60 to 85 mm / min. Here, the heat input amount is changed by changing the welding speed, but it may be performed by a welding current.
Regarding the softening rate and softening width of the weld heat affected zone, the cross section of the weld zone is embedded and the hardness of the heat affected zone is determined according to JIS Z3101 “Maximum hardness test method for weld heat affected zone”. It measured with the meter, and calculated | required (DELTA) HV and the softening rate of the weld softening part by following Formula. Moreover, the softened part width | variety was calculated | required on the basis of the relationship shown in FIG.
ΔHV of weld softened portion = hardness of base metal−hardness of weld softened portion Softening rate (%) = [(base material hardness−hardness of weld softened portion) / base material hardness] × 100 (2 )
The results are shown in Table 2.

As can be seen from the results shown in Table 2, if the welding speed is decreased to increase the heat input to the material, the ΔHV and the softening rate of the weld softened portion increase regardless of the material. Also, the width of the softened portion is increased. Thus, the welding condition is one of means for controlling ΔHV × softened part width of the weld softened part.
Moreover, when welding to steel No. 5 and No. 10 under the same welding conditions, No. 5 (steel of the present invention) has a ΔHV, softening rate, softening of the weld softened part than No. 10 (comparative steel). Small in any part width. Thus, the steel composition is also one of means for controlling ΔHV × softened portion width, and it can be seen that the steel of the present invention can suppress ΔHV, the softening rate, and the softened portion width of the weld softened portion.

On the other hand, it is difficult to select a welding condition satisfying ΔHV × softening portion width ≦ 150 in a steel in which ΔHV, softening rate, and softening portion width are likely to be large like steel No. 10 of the comparative example. This is due to the following reason. Even if the steel tends to have a large ΔHV, softening rate, and softened part width, it is possible to reduce the ΔHV × softened part width by reducing the heat input of the material by welding. However, welded steel pipes are manufactured by joining the ends of cold-rolled steel sheets formed into a tubular shape and joining them together by welding. Therefore, if the heat input is insufficient, penetration of the joints becomes insufficient, and a sound weld metal is produced. Problems such as not being formed. Therefore, if the welding conditions for supplying the heat input necessary for forming a sound weld are adopted, depending on the steel composition, the heat input may exceed the heat input by setting the ΔHV × softened part width of the weld softened part to 150 or less. There is.
That is, by adopting the steel composition of the present invention and appropriately selecting the welding conditions, it is possible to achieve ΔHV × softened portion width of the weld softened portion of 150 or less.

Example 2:
Next, after the steel strip for pipe making prepared in Example 1 was slit, it was processed into an open pipe by roll forming, and both ends in the width direction of the steel strip were welded to produce a welded steel pipe having a diameter of 31.8 mm. At this time, the welding method was MIG welding, and the welding conditions were a voltage of 14 V, a current of 70 A, and a welding speed of 70 mm / min.
The ΔHV, softening rate, and softened part width of the weld heat affected zone were determined in the same manner as in Example 1. However, steel no. The welded steel pipe manufactured from No. 6 cracked from the welded part when the welded steel pipe was cut to embed the cross section of the welded part. This indicates that the material of steel No. 6 having a large C equivalent has a brittle weld.
In addition, about the steel plate obtained by cold rolling, the tensile test at room temperature according to JISZ2241 was done using the No. 5 test piece of JISZ2201, and the tensile strength of the base material was measured. These results are shown in Table 3.

Among each test piece of Table 3, about the test piece (No. 1, 3, 5, 7, 9, 13, 15, 16, 17, 18) whose rolling rate is 60%, C equivalent and welding softening part The relationship of the softening rate is shown in FIG. Since these have a high cold rolling rate of 60%, all the test pieces are high strength materials having a tensile strength exceeding 900 N / mm ² .
As described above, when heat input by welding is applied to a cold-rolled steel sheet that has been strengthened by work strengthening, there is a problem that a remarkable weld softening region occurs in the heat-affected zone near the weld. At this time, if it is steel of this invention and C equivalent is the range of 0.25-0.6, even if it is the steel plate fully strengthened, a softening rate can be restrained small.

Example 3:
Moreover, about each test piece of Table 3, the steel strip for pipe making is used about the test piece (No.1,3,5,7,9,13,15,16,17,18) whose rolling rate is 60%. The end surfaces were attached to each other while being flat and welded, and then a JIS No. 5 test piece was cut out to produce a tensile test piece with a weld. The welding conditions were a MIG welding method with a voltage of 14 V, a current of 70 A, and a welding speed of 70 mm / min. The tensile strength of the welded portion was evaluated using the prepared tensile test piece with welded portion, and the strength reduction rate of the tensile strength was determined by the following formula.
Strength reduction rate (%) = [(tensile strength of cold-rolled material−tensile strength of welded portion) / tensile strength of cold-rolled material] × 100
In addition, in order to evaluate the fatigue characteristics, a single-swing tensile fatigue test specimen was prepared, and in accordance with JIS Z3103 “One-sided tensile fatigue test method for arc welded joints” and JIS Z2273 “General fatigue test method for metal materials”. The fatigue limit was measured to determine the fatigue limit ratio, and a fatigue limit ratio of 0.5 or higher was accepted. Table 4 shows the strength reduction rate and fatigue limit ratio of the welds evaluated in this way. Further, FIG. 3 shows the relationship between ΔHV × softened portion width and fatigue limit ratio.
In addition, since test piece No. 10 (steel No. 6) had a weak welded part and poor workability, the strength reduction rate and fatigue limit ratio were not evaluated.

As shown in Table 4, the welded portion of the steel of the present invention has a smaller ΔHV × softened portion width of the weld softened portion and a lower strength reduction of the welded portion than the comparative steel. Further, the fatigue limit ratio is higher than that of the comparative steel, indicating that the fatigue resistance characteristics of the welded portion of the steel of the present invention are excellent.
As described above, even when a material to which work strengthening is applied is used, a high-strength welded steel pipe excellent in weld softening resistance and fatigue characteristics can be obtained by the present invention.

The figure explaining the standard which determines the softening part width | variety and (DELTA) HV of a welding heat affected zone The figure which showed the relationship between C equivalent and the softening rate of a weld softened part Diagram showing the effect of ΔHV × softening width on the fatigue limit ratio

Claims (3)

A welded steel pipe manufactured by welding the ends of a cold-rolled steel sheet formed into an open pipe shape by welding, and the ΔHV and the width of the softened part in the vicinity of the weld metal of the joint are as follows: A high strength welded steel pipe excellent in weld softening resistance and fatigue characteristics, characterized in that ΔHV × softened part width ≦ 150 of the weld softened part when defined as in formulas (1) and (2).
ΔHV of welded softened portion = base metal hardness−hardness of welded softest portion (1)
Softened part width = width of the part softened from the base metal by welding heat input (mm) (2) Cold-rolled steel sheet has C: 0.01 to 0.20 mass%, Si: 1.5 mass% or less, Mn: 2.5 mass% or less, P: 0.05 mass% or less, S: 0.02 mass% % Or less, acid-soluble Al: 0.005 to 0.10% by mass, B: 0.0005 to 0.005% by mass, Ti: 0.01 to 0.15% by mass, and Nb: 0.01 to 0 .10 mass%, the balance is composed of Fe and inevitable impurities, and the component composition adjusted so that the C equivalent defined by the following formula (3) is 0.25 to 0.6 mass% The high-strength welded steel pipe excellent in weld softening resistance and fatigue characteristics according to claim 1, which is a cold-rolled steel sheet.
C equivalent = C + 1 / 6Mn + 1 / 24Si + 5B (3) Cold-rolled steel sheet has C: 0.01 to 0.20 mass%, Si: 1.5 mass% or less, Mn: 2.5 mass% or less, P: 0.05 mass% or less, S: 0.02 mass% %, Acid-soluble Al: 0.005-0.10 mass%, B: 0.0005-0.005 mass%, Ti: 0.01-0.15 mass%, Nb: 0.01-0. 10% by mass, and Mo: a component containing 0.01 to 0.30% by mass and adjusted so that the C equivalent defined by the following formula (4) is 0.25 to 0.6% by mass The high-strength welded steel pipe excellent in weld softening resistance and fatigue characteristics according to claim 1, which is a cold-rolled steel sheet having a composition.
C equivalent = C + 1 / 6Mn + 1 / 24Si + 5B + 1 / 4Mo (4)

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