Method for electric resistance welded steel tube

Abstract

A method for manufacturing an electric resistance welded steel tube including: forming a steel tube material into an almost cylindrical open pipe, the steel tube material being a steel sheet wherein Ti and N satisfy (N/14)<(Ti/47.9); forming an electric resistance welded steel tube by bonding ends of the open pipe to each other by induction resistance welding with heat input controlled so that the bond width is 30 to 65 m; heating the electric resistance welded steel tube to a temperature equal to or higher than the Ac.sub.3 transformation temperature; and diameter-reducing rolling the heated electric resistance welded steel tube with rolling reduction expressed by an outer diameter ratio greater than (125/the bond width before diameter-reducing rolling (m))100% such that the bond width is 25 m or less.

Claims

1. A method for manufacturing an electric resistance welded steel tube for heat treatment, comprising: forming a steel sheet into an almost cylindrical open pipe, the steel tube material being a steel sheet having a composition constituted by, in mass percent: TABLE-US-00004 C: 0.15 to 0.40% Si: 0.05 to 0.50% Mn: 0.30 to 2.00% Al: 0.01 to 0.10% Ti: 0.001 to 0.04% B: 0.0005 to 0.0050%, and N: 0.0010 to 0.0100% with Ti and N satisfying (N/14)<(Ti/47.9) as well as Fe and unavoidable impurities as the balance; forming an electric resistance welded steel tube by bonding lengthwise ends of the open pipe to each other by electric resistance welding with heat input controlled so that the bond width will fall within the range of 30 to 65 m; heating the electric resistance welded steel tube to a temperature equal to or higher than the Ac.sub.3 transformation temperature; and diameter-reducing rolling the heated electric resistance welded steel tube with rolling reduction expressed by an outer diameter ratio being (125/the bond width before diameter-reducing rolling (m))100% or greater such that the bond width will be 25 m or less.

2. The method according to claim 1, wherein the composition further comprises one or more selected from, in mass percent, Cr: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, Ni: 1.0% or less, and Cu: 1.0% or less.

3. The method according to claim 2, wherein the composition further contains either or both of, in mass percent, Nb: 0.2% or less and V: 0.2% or less.

4. The method according to claim 1, wherein the composition further contains either or both of, in mass percent, Nb: 0.2% or less and V: 0.2% or less.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a graph showing a relationship between the hardness of an electric resistance welded portion reached after quenching and tempering and the bond width of the electric resistance welded portion.

(2) FIG. 2 is a graph showing a relationship between fatigue endurance and the ratio of the hardness of an electric resistance welded portion to that of the base metal.

(3) FIG. 3 is a graph showing a relationship between the frequency of welding defects and the bond width of an electric resistance welded portion.

(4) FIG. 4 is a graph showing a relationship between the ratio of the bond width before diameter-reducing rolling to that reached after diameter-reducing rolling and the diameter-reducing rolling reduction.

(5) FIG. 5 is a schematic diagram of quenching treatment conditions used in an example.

(6) FIG. 6 is a schematic diagram of other quenching treatment conditions used in the example.

DETAILED DESCRIPTION

(7) An electric resistance welded steel tube is an electric resistance welded steel tube having an electric resistance welded portion whose bond width is 25 m or less. When the bond width of the electric resistance welded portion is increased above 25 m, the width of a low carbon layer is accordingly increased; as a result, the hardness after quenching of the electric resistance welded portion becomes much lower than that of the base metal as shown in FIG. 1 after the electric resistance welded steel tube is quenched by rapid heating using electric heating or the like, leading to a decreased durability of resultant quenched parts for hollow stabilizers or the like. Note that the bond width mentioned above is the width of a zone (a layer) located on the plane perpendicular to the tube axis direction and producing no segregation lines after metal flow etching.

(8) Also, the electric resistance welded steel tube is excellent in terms of flatness. The expression excellent in terms of flatness used above means that, for example, flatness measurement according to the general rules specified in 8.4 of JIS G 3445 (flatness measurement), in which a test specimen is compressed until the distance between two flat plates reaches double the thickness of the test specimen, results in no cracks of electric resistance welded portions. In this flatness measurement, a test specimen (a steel tube) is positioned so that the electric resistance welded portion thereof is perpendicular to the direction of compression.

(9) Incidentally, the following describes the reasons for the specified composition of the electric resistance welded steel tube.

(10) C: 0.15 to 0.40%

(11) C is a useful element, which increases the strength of steel when mixed into the steel and, after the steel is tempered, precipitates as carbides or carbonitrides increasing the strength of the steel. Our tubes should contain C at a content ratio of 0.15% or more such that a resultant steel tube can have a desired strength and that resultant quenched parts for hollow stabilizers can also have a desired strength. However, C contained at a content ratio higher than 0.40% would result in a reduced toughness after quenching. Therefore, the content ratio of C is limited to the range of 0.15 to 0.40%. Preferably, the content ratio of C is in the range of 0.20 to 0.35%.

(12) Si: 0.05 to 0.50%

(13) Si is an element that acts as a deoxidizing agent and should be contained at a content ratio of 0.05% or more to perform this action. However, when the content ratio of Si is higher than 0.50%, the deoxidizing action saturates without offering effects corresponding to the high content ratio, and thus a cost issue rises. Furthermore, such a high content ratio of Si would make inclusions more likely to occur during the electric resistance welding process, thereby affecting the soundness of an electric resistance welded portion. Therefore, the content ratio of Si is limited to the range of 0.05 to 0.50%. Preferably, the content ratio of Si is in the range of 0.10 to 0.30%.

(14) Mn: 0.30 to 2.00%

(15) Mn is an element that increases the strength of steel when mixed into the steel and improves the hardenability of the steel. Mn should be contained at a content ratio of 0.30% or more such that a desired strength will be ensured. However, Mn contained at a content ratio higher than 2.00% would result in the formation of residual austenite (), which reduces toughness after quenching. Therefore, the content ratio of Mn is limited to the range of 0.30 to 2.00%. Preferably, the content ratio of Mn is in the range of 0.30 to 1.60%.

(16) Al: 0.01 to 0.10%.

(17) Al is an element that not only acts as a deoxidizing agent but also has an effect of ensuring the effective amount of solid-solution B in improving hardenability via the fixation of N. Al should be contained at a content ratio of 0.01% or more to obtain this effect. However, Al contained at a content ratio higher than 0.10% would result in the formation of more inclusions, thereby shortening fatigue life in some cases. Therefore, the content ratio of Al is limited to the range of 0.01 to 0.10%. Preferably, the content ratio of Al is in the range of 0.02 to 0.05%.

(18) B: 0.0005 to 0.0050%

(19) B is an effective element that improves the hardenability of steel. Furthermore, B has the action of reinforcing grain boundaries and an effect of preventing cracks from occurring. B should be contained at a content ratio of 0.0005% or more to obtain these effects. However, when the content ratio of B is higher than 0.0050%, the effects described above saturate, and thus a cost issue rises. Furthermore, a content ratio of B higher than 0.0050% would result in the formation of coarse B-containing deposits, which reduces toughness in some cases. Therefore, the content ratio of B is limited to the range of 0.0005 to 0.0050%. Preferably, the content ratio of B is in the range of 0.0010 to 0.0025%.

(20) Ti: 0.001 to 0.040%

(21) Ti acts as a N-fixing element and has an effect of ensuring the effective amount of solid-solution B to improve hardenability. Furthermore, Ti precipitates as fine carbide particles, which prevent crystal grains from coarsening during welding and heat treatment, thereby contributing to improvement in toughness. Ti should be contained at a content ratio of 0.001% or more to obtain these effects. However, Ti contained at a content ratio higher than 0.040% would make inclusions much more likely to occur, thereby resulting in a reduced toughness. Therefore, the content ratio of Ti is limited to the range of 0.001 to 0.040%. Preferably, the content ratio of Ti is in the range of 0.020 to 0.030%.

(22) N: 0.0010 to 0.0100%

(23) N is an element that binds to alloy elements existing in steel to form nitrides and carbonitrides, thereby contributing to maintenance of strength during quenching. N should be contained at a content ratio of 0.0010% or more to obtain this effect. However, N contained at a content ratio higher than 0.0100% would allow nitrides to coarsen, thereby resulting in a reduced toughness and a shortened fatigue life. Therefore, the content ratio of N is limited to the range of 0.0010 to 0.0100%.

(24) The content ratios of Ti and N, falling within the ranges described above, satisfy the following equation:
(N/14)<(Ti/47.9).

(25) The content ratios of Ti and N not satisfying the equation shown above would destabilize the amount of solid-solution B during quenching and thus are unfavorable.

(26) The components described above are basic components; however, besides these basic components, one or more selected from Cr: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, Ni: 1.0% or less, and Cu: 1.0% or less and/or either or both of Nb: 0.2% or less and V: 0.2% or less may be contained as needed.

(27) One or more selected from Cr; 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, Ni: 1.0% or less, and Cu: 1.0% or less Cr, Mo, W, Cu, and Ni are all elements having the action of improving the hardenability of steel; one or more of them may be contained as needed.

(28) Cr not only improves hardenability but also has the action of increasing strength via the formation of fine carbides thereof, through which Cr contributes to maintenance of a desired strength. Cr is contained desirably at a content ratio of 0.05% or more to obtain these effects; however, when the content ratio of Cr is higher than 1.0%, the effects described above saturate, and thus a cost issue rises. Furthermore, such a high content ratio of Cr would make inclusions more likely to occur during the electric resistance welding process, thereby affecting the soundness of an electric resistance welded portion. Therefore, the content ratio of Cr is preferably limited to 1.0% or less. More preferably, the content ratio of Cr is in the range of 0.10 to 0.30%.

(29) Mo not only improves hardenability but also has the action of increasing strength via the formation of fine carbides thereof, through which Mo contributes to maintenance of a desired strength. Mo is contained desirably at a content ratio of 0.05% or more to obtain these effects; however, when the content ratio of Mo is higher than 1.0%, the effects described above saturate, and thus a cost issue rises. Furthermore, such a high content ratio of Mo would lead to the formation of coarse carbide particles, thereby resulting in a reduced toughness in some cases. Therefore, the content ratio of Mo is preferably limited to 1.0% or less. More preferably, the content ratio of Mo is in the range of 0.10 to 0.30%.

(30) W is an element that not only improves hardenability but also has the action of balancing hardness and toughness during thermal refining. W is contained desirably at a content ratio of 0.05% or more to obtain these effects. However, when the content ratio of W is higher than 1.0%, the effects saturate, and thus a cost issue rises. Therefore, the content ratio of W is preferably limited to 1.0% or less. More preferably, the content ratio of W is in the range of 0.10 to 0.30%.

(31) Ni is an element that not only improves hardenability but also contributes to improvement in toughness and is contained desirably at a content ratio of 0.05% or more to obtain these effects; however, when the content ratio of Ni is higher than 1.0%, the effects described above saturate, and thus a cost issue rises. Furthermore, such a high content ratio of Ni would result in a reduced workability. Therefore, the content ratio of Ni is preferably limited to 1.0% or less. More preferably, the content ratio of Ni is in the range of 0.10 to 0.50%.

(32) Cu is an element that not only improves hardenability but also increases resistance for hydrogen embrittlement and is contained desirably at a content ratio of 0.05% or more to obtain these effects; however, when the content ratio of Cu is higher than 1.0%, the effects described above saturate, and thus a cost issue rises. Furthermore, such a high content ratio of Cu would result in a reduced workability. Therefore, the content ratio of Cu is preferably limited to 1.0% or less. More preferably, the content ratio of Cu is in the range of 0.10 to 0.30%. Either or both of Nb: 0.2% or less and V: 0.2% or less

(33) Nb and V are elements that contribute to an increase in strength via the formation of carbides thereof and may be contained as needed. Nb and V are contained desirably at a content ratio of 0.01% or more and 0.01% or more, respectively, to obtain this effect; however, when Nb or V is contained at a content ratio higher than 0.2%, the effect saturates, and thus a cost issue rises. Therefore, the content ratio of Nb and V is preferably limited to 0.2% or less and 0.2% or less, respectively.

(34) The balance, namely, the content other than the components described above, is constituted by Fe and unavoidable impurities. Acceptable unavoidable impurities are P: 0.020% or less, S: 0.010% or less, and O: 0.005% or less.

(35) P is an element that affects weld cracking resistance and toughness, and the content ratio thereof is preferably controlled to be 0.020% or less in manufacturing of hollow stabilizers. More preferably, the content ratio of P is 0.015% or less.

(36) S, existing as sulfide inclusions in steel, is an element that reduces the workability, toughness, and fatigue life of steel tubes while increasing the reheat crack sensitivity of the steel tubes, and the content ratio thereof is preferably controlled to be 0.010% or less in manufacturing of hollow stabilizers. More preferably, the content ratio of S is 0.005% or less.

(37) O, existing mainly as oxide inclusions in steel, reduces the workability, toughness, and fatigue life of steel tubes, and thus the content ratio thereof is preferably controlled to be 0.005% or less in manufacturing of hollow stabilizers. More preferably, the content ratio of O is 0.002% or less.

(38) Incidentally, the following describes a preferred method for manufacturing of the electric resistance welded steel tube.

(39) A steel sheet having the composition described above is used as a steel tube material. There need be no particular limitations on the method for producing the steel sheet; it may be any known production method. The term steel sheet includes steel strips. There need be no particular limitations also on the kind of steel sheet, such as a hot-rolled steel sheet or a cold-rolled steel sheet; however, a hot-rolled steel sheet is preferable also in terms of material cost. In addition, most steel sheets for hollow stabilizers have a thickness of 2 mm or more, and thus the use of a cold-rolled steel sheet is impractical considering cost efficiency.

(40) A steel tube material, namely, a steel sheet, is formed into an almost cylindrical open pipe preferably by continuous forming, and then the ends of the open pipe are bonded to each other by electric resistance welding based on induction resistance welding; as a result, an electric resistance welded steel tube is formed. The heat input used during this electric resistance welding process is controlled so that the bond width falls within the range of 30 to 65 It is preferable that the control of the heat input is achieved by adjusting voltage and current used in induction resistance welding, welding speed, the amount of upset, and so forth and that repeated measurement of the bond width under various electric resistance welding conditions has determined the induction resistance welding conditions that provide a desired bond width.

(41) As shown also in FIG. 3, a bond width smaller than 30 m means that the heat input is too small, which causes many welding defects and decreases in flatness. However, when the heat input is increased to the extent that the bond width is greater than 65 m, oxidization during welding is promoted and oxides become more likely to remain, thereby causing welding defects to occur frequently; furthermore, diameter-reducing rolling with a greater rolling reduction is needed to reduce the width of a low carbon layer to one causing no decreases in hardness after quenching of an electric resistance welded portion, but a relatively great diameter limits the applications thereof. Therefore, the bond width reached after electric resistance welding is limited to the range of 30 to 65 m. More preferably, the bond width is in the range of 35 to 45 m.

(42) Then, the entire body of the electric resistance welded steel tube with the adjusted bond width is heated to a temperature equal to or higher than the Ac.sub.3 transformation temperature and then subjected to diameter-reducing rolling such that the bond width (the low carbon layer) of the electric resistance portion is mechanically reduced. Immediately after electric resistance welding, the electric resistance welded portion has a greater hardness than the base metal because of welding heat applied during the electric resistance welding process; thus, the electric resistance welded portion heated to a temperature lower than the Ac.sub.3 transformation temperature cannot be effectively deformed during the subsequent diameter-reducing process, in other words, the bond width (the low carbon layer) of the electric resistance portion cannot be effectively reduced. The heating temperature is preferably 1000 C. or lower considering surface decarburizetion. In addition, localized heating of an electric resistance welded portion would cause the electric resistance welded portion to swell during diameter-reducing rolling and, accordingly, the resultant steel tube to have an unfavorable shape. Therefore, the electric resistance welded steel tube is heated as a whole to a temperature equal to or higher than the Ac.sub.3 transformation temperature.

(43) The rolling reduction for diameter-reducing rolling expressed by an outer diameter ratio is greater than (125/the bond width before diameter-reducing rolling (m))100%. When the rolling reduction is lower than the value described above, the desired bond width (width of a low carbon layer), namely, 25 m or less, cannot be obtained. The higher the rolling reduction for diameter-reducing rolling is, the narrower the resultant bond width (width of a low carbon layer) is. There need be no particular limitation on the upper limit; however, considering the number of stands a diameter-reducing roller has, the upper limit is usually approximately 75%.

EXAMPLES

(44) Hot-rolled steel sheets having compositions shown in Table 1 were used as steel tube materials. These steel tube materials were cold-formed into almost cylindrical open pipes, and then the ends of each of the open pipes were bonded to each other by electric resistance welding based on induction resistance welding; as a result, electric resistance welded steel tubes (mother tubes) were formed. Repeated measurement of the bond width under various electric resistance welding conditions (width variously adjusted heat input) had determined standard conditions. With reference to the standard conditions, the welding conditions were variously changed as shown in Table 2, and the bond width of the mother tubes was controlled as shown in Table 2. Subsequently, these electric resistance welded steel tubes (mother tubes) were heated in their entireties under the conditions shown in Table 2 and then subjected to diameter-reducing rolling with the rolling reductions shown in Table 2. In this way, product tubes were obtained.

(45) Test specimens for structure observation each including an electric resistance welded portion were sampled from the obtained mother tubes and product tubes. The test specimens were observed for their structures and evaluated for the bond width of the electric resistance welded portion. Also, the obtained mother tubes and product tubes were tested for welding quality on the basis of the presence/absence of cracks determined by flatness measurement.

(46) Test specimens for hardness measurement each including an electric resistance welded portion were also sampled from the obtained electric resistance welded steel tubes. The test specimens were quenched by rapid heating under the conditions shown in FIGS. 5 and 6, and then tempered under the conditions shown in Table 3. Subsequently, the test specimens were evaluated in hardness measurement for the hardness after quenching and the hardness after quenching and tempering of the electric resistance welded portion. Also, test materials for fatigue test (length in the tube axis direction: 250 mm) were sampled from the obtained product tubes. After being subjected to quenching treatment by rapid heating shown in FIG. 5 or FIG. 6 followed by tempering treatment shown in Table 2, the test materials for fatigue test proceeded with torsion fatigue test. The test methods were as follows:

(47) (1) Structure Observation From each of the mother tubes and product tubes, a cross section that includes an electric resistance welded portion and is perpendicular to the tube axis direction was cut out. After being polished and corroded with a metal flow etching (5% picric acid+a surface acting agent) solution, the cross sections proceeded with observation of the structure thereof under a light microscope (magnification ratio: 400). Then, the maximum width of a zone (a layer) in the cross-sectional structure producing no segregation lines was measured; the obtained widths were used as widths of bond.

(48) (2) Flatness Measurement Seven test specimens for flatness measurement (length: 150 mm) were sampled from each of the mother tubes and product tubes. After being subjected to a normalization treatment at 900 C.10 min, the test specimens were tested for cracks in the electric resistance welded portion thereof in flatness measurement according to the general rules specified in 8.4 of JIS G 3445. Note that, in flatness measurement, each of the test specimens was compressed until the distance between two flat plates reaches double the thickness of the test specimen. Also, each of the test specimens was positioned so that the electric resistance welded portion thereof was perpendicular to the direction of compression.

(49) (3) Hardness Measurement From the mother tubes and product tubes, test specimens for electric resistance welded portion hardness measurement (size: the thickness of the pipe10 mm in length) and test specimens for base metal hardness measurement having the same size as the test specimens for electric resistance welded portion hardness measurement were sampled. The test specimens were subjected to quenching treatment by rapid heating shown in FIGS. 5 and 6, and then the Vickers hardness Hv0.5 of the electric resistance welded portions and the base metals was measured using a Vickers hardness meter (load: 5 N). The measurement began with outer surfaces and proceeded by a pitch of 0.15 mm. The obtained values were arithmetically averaged, and the obtained averages were used as the electric resistance welded portion hardness and the base metal hardness of the steel tubes.

(50) (4) Fatigue Test Test materials for fatigue test (length in the tube axis direction: 250 mm) were sampled from the product tubes. After being subjected to quenching treatment by rapid heating shown in FIGS. 5 and 6 followed by tempering treatment under the conditions shown in Table 2, the test materials for fatigue test proceeded with torsion fatigue test. In fatigue test, fatigue endurance was determined in a two-sided torsion fatigue test according to the general rules specified in JIS Z 2273 with the repetition number of cycles being 10.sup.6.

(51) The obtained results are shown in Tables 2 and 3.

(52) TABLE-US-00001 TABLE 1 Relation between Steel Chemical components (mass %) N and Ti No. C Si Mn P S Al N O Ti B Cr Ca Nb Cu satisfied? A 0.24 0.23 0.54 0.012 0.0020 0.018 0.0035 0.0010 0.016 0.0023 0.29 0.0001 B 0.19 0.39 1.58 0.011 0.0010 0.033 0.0032 0.0013 0.013 0.0012 0.015 0.15 C 0.35 0.28 1.32 0.010 0.0009 0.033 0.0037 0.0006 0.014 0.026 0.0002 *(N/14) < (Ti/47.9) is satisfied: ; other cases: x

(53) TABLE-US-00002 TABLE 2 Mother tube: Diameter-reducing Electric resistance size rolling conditions Steel welding conditions Outer Mother Heating Finishing tube Steel Heat Welding diameter Thickness tube: bond temperature temperature No. No. input* Speed* (mm) (mm) width (m) ( C.) ( C.) 1 A +2.4% 30% 88 2.6 80 980 780 2 A +1.6% Standard 88 2.6 65 980 780 3 A +0.8% Standard 88 2.6 55 980 780 4 A Standard Standard 88 2.6 50 980 780 5 A 0.8% Standard 88 2.6 40 980 780 6 A 1.6% Standard 88 2.6 30 980 780 7 A 3.2% Standard 88 2.6 10 980 780 8 A 4.0% Standard 88 2.6 2 980 780 9 A Standard Standard 88 2.6 50 980 780 10 B Standard Standard 88 6.5 42 980 780 11 C Standard Standard 88 5.5 37 980 780 Diameter-reducing rolling conditions Product Predetermined tube: size Steel Rolling rolling Outer Product tube reducttion reduction** diameter Thickness tube: bond No. ( C.) (%) (mm) (mm) width (m) Remarks 1 60 69 35.1 2.6 32 Comparative Example 2 62 62 33.4 2.6 25 Example 3 60 55 35.1 2.6 22 Example 4 60 50 35.1 2.6 20 Example 5 60 38 35.1 2.6 16 Example 6 60 17 35.1 2.6 12 Example 7 60 0 35.1 2.6 4 Comparative Example 8 60 0 35.1 2.6 1 Comparative Example 9 40 50 50.8 2.6 30 Comparative Example 10 70 40 26.5 6.0 13 Example 11 51 32 42.7 5.0 18 Example *Heat input: A deviation (%) from the standard Welding speed: A deviation (%) from the standard **1-25/(Bond width of a mother tube (m))) 100

(54) TABLE-US-00003 TABLE 3 Mother tube: electric Heat treatment resistance welded portion Cracks in flatness conditions Hardness as Steel measurement Tempering Hardness as quenching tube Mother Product Quenching temperature quenching and tempering No. tube Tube conditions ( C.) Hv0.5 Hv0.5 1 Found Found FIG. 5 350 260 260 2 Not found Not found FIG. 5 350 261 261 3 Not found Not found FIG. 5 350 260 260 4 Not found Not found FIG. 5 350 260 260 5 Not found Not found FIG. 5 350 260 260 6 Not found Not found FIG. 5 350 262 262 7 Found Found FIG. 5 350 447 390 8 Found Found FIG. 5 350 499 414 9 Not found Not found FIG. 6 350 10 Not found Not found FIG. 5 350 260 260 11 Not found Not found FIG. 5 400 384 317 Product tube: electric resistance welded portion Hardness ratio Hardness as (%) (electric Steel Hardness as quenching resistance Fatigue tube quenching and tempering welded portion/ endurance** No. Hv0.5 Hv0.5 base metal) (MPa) Remarks 1 261 261 62 180 Comparative Example 2 380 360 86 250 Example 3 392 365 87 250 Example 4 399 369 88 250 Example 5 417 376 90 250 Example 6 437 385 92 250 Example 7 485 407 97 200 Comparative Example 8 508 418 99 200 Comparative Example 9 360 343 82 240 Comparative Example 10 408 372 93 250 Example 11 580 406 94 280 Example **Fatigue endurance measured after 10.sup.6 cycles

(55) Our examples all had no cracks after flatness measurement, were excellent in terms of flatness, and experienced no significant decrease in the hardness after quenching of the electric resistance welded portion or no decrease in fatigue endurance. However, the comparative examples, which had a bond width of the electric resistance welded portion outside our range, experienced a significant decrease in the hardness after quenching of the electric resistance welded portion and in fatigue endurance, had many welding defects, or experienced a further decrease in fatigue endurance.