TWI275649B - Deflection-resistant stainless steel-made structural members of a two-wheeled vehicle - Google Patents

Abstract

A structural member of a two-wheeled vehicle, e.g. bicycles, motorcycles or wheelchairs, is made of a ferrite/martensite dual-phase stainless steel sheet or pipe. The stainless steel consists of C up to 0.04 mass %, Si up to 2.0 mass %, Mn up to 2.0 mass %, 10.0-20.0 mass % of Cr, Ni up to 4.0 mass %, Cu up to 3.0 mass %, N up to 0.12 mass %, optionally one or more of B up to 0.015 mass %, Mo up to 3.0 mass %, Ti up to 0.10 mass %, Nb up to 0.40 mass % and V up to 0.30 mass %, and the balance being essentially Fe. The ferrite/martensite dual-phase is controlled in the manner such that a sensitizing index St defined by the formula of St=100C+30N-0.32gamma (gamma represents a ratio (vol.%) of martensite in a dual-phase annealed state at a room temperature) is controlled within a range of from -31 to -7. Moreover, the steel sheet has surface hardness of HV 270 or more.

Description

1275649 玖, invention description: _. (two inventions. belongs to the branch of the excellent chair, etc. and the frame, such as the usual % and make the bend in the non-inclusions ^ Cu C and toughness to produce C and this with the system of the present invention Two-wheeled vehicles made of stainless steel for corrosion resistance and bending resistance of welding heat affected zone, such as bicycle, motorcycle and wheel structural parts. (2) Previous branches of the two-wheeled vehicles, such as bicycles, motorcycles and wheelchairs Tire rims are often exposed to the uranium atmosphere. In order to extend their life, the material of the structural parts is changed from galvanized steel to ferritic stainless steel, SUS 3 04. The weldability of this material will also be good. Because the structural parts are manufactured by welding. JP6 1 - 7 3 866A proposes to separately add Ti and Nb elements by a ratio of 0.1 to 0.3 mass 0.115 to 0. 3 mass%, that is, weld toughness, The ductility and corrosion resistance are improved and are contained in the case of welding at 10 to 20% by mass (: "ferritic stainless steel. However, the proposed stainless steel has a disadvantage in that it is added by a high proportion of Ti. i things often cause T i streaks. JP62-164857 proposes another type of ferritic steel for tire rims, in which austenite constituents such as N i, Mll ] are added to the appropriate ratio to 12.5~17 In heavy-duty ferritic stainless steels, the N content is simultaneously reduced to improve the uranium resistance between the grains in the welded portion without the need for stabilization and workability stabilizing elements such as Ti or Nb. It is also reported as The martensite phase controls the CE値 within a predetermined range, but the stainless steel has a bad strength due to a decrease in the total amount of N to 0·〇4% by mass or less, and the 1275649 martensite is irrelevant. The steel can be added by adding an alloy. The element is strengthened, but the addition of alloying elements increases the cost of the steel. SUMMARY OF THE INVENTION It is an object of the present invention to provide improved weldability, strength and bending resistance, and corrosion resistance at both the weld heat affected zone and the base metal portion. Cheap structural parts made of stainless steel sheets or tubes, such as two-wheeled vehicles, such as bicycle rims and frames for bicycles, motorcycles and wheelchairs. The present invention proposes a knot made of a ferritic/martensitic duplex stainless steel sheet. The stainless steel for this purpose has a c of up to 0.04% by mass, up to 2.0% by mass of Si, up to 2.0% by mass of Μη, 10.0 to 20.0% by mass of Cr, up to 4.0% by mass of Ni, Up to 3.0% by mass of Cu, up to 0.12% by mass of N, and up to 0.015% by mass of B, up to 3.0% by mass of Mo, up to 0.10% by mass of Ti, up to 0.40% by mass of Nb One or more of V up to 0.30% by mass, and the balance is a chemical composition composed of iron in addition to unavoidable impurities. The ratio of the martensite to the C and N content (γ) is determined by the formula St=100C+3 0N—0.32r (where r represents the proportion (vol%) of the martensite phase in the duplex annealing state) The defined sensitization index (St) is adjusted to a range of -3 1 to -7 ,, having a double composed of 5 to 75 vol% of ferrite and 25 to 95 vol% of martensite. Phase structure. Tire rims are usually made of stainless steel sheets, while frame parts are usually made of stainless steel tubes. In any case, adjust the surface hardness of the stainless steel plate or tube, -6 - 1275649, to the HV270 or higher, except for the welded portion. The stainless steel sheet used as the structural part is prepared as follows: a steel strip having a specific composition is cold rolled and then double-annealed in a continuous annealing furnace through a ferritic/martensitic double-phase zone at 850 to 1 100T: The heated steel strip is subjected to two-phase annealing as a final heat treatment and then the heated steel strip is cooled. The present inventors have studied and tested the performance of stainless steel sheets necessary for two-wheeled vehicles, such as bicycles, motorcycles, and wheelchair structural parts, in particular, from the contact resistance and bending resistance of the welded heat affected zone. The following instructions are for bicycle parts, but the same performance is of course necessary for motorcycles and wheelchairs. The main structural components of the bicycle are the tire rim and the frame. In order to stabilize the state of cycling, these parts will remain in their original shape without deformation. A tire rim that is close to the road surface when riding is exposed to atmospheric corrosion and is also subject to friction with the rubber brake. Pebble or the like collides with the tire rim and frame. In view of these practical conditions, the tire rim and frame must have good rigidity, corrosion resistance, wear resistance and resistance to dents. Since the semi-stable austenitic stainless steel having a strain-induced martensite and austenitic metallurgical structure is required for rigidity and dent resistance which is required for this purpose, the inventors have studied the ferrite/martensitic duplex. For the practicality of stainless steel, the steel contains up to 0.04% by mass of C, up to 0.12% by mass of N, and 10·0 to 20.0% by mass of Cr. The rigidity, the dent resistance, and the corrosion resistance suitable for the purpose are attained by appropriately controlling the proportion of martensite in the two-phase annealing state and the sensitization index S t with respect to the C and N contents. Rigidity (bending resistance) is improved by hardening the stainless steel to a Vickers hardness of 270 or 1275649 and fine-tuning the ferrite/martensitic duplex to increase the Young's modulus. The martensite phase is produced to increase strength and improve resistance to dents and corrosion. When the stainless steel plate is welded, it is heated by welding up to 600~900 °C. In such high temperature regions, chromium carbonitride precipitates from the steel matrix and returns to the Cr depletion zone. The C 1: depleted zone adversely promotes sensitization and reduces corrosion resistance. The ferrite and martensite phases are b · c · c · (body-centered cubic) crystal structures in which C and N are substantially insoluble and the carbonitrides have a diffusion rate greater than Lu f · c · c · (face-centered cubic) The austenite phase of the crystal structure is likely to precipitate. The martensite is transformed in the opposite direction into an austenite phase in which a considerable amount of C and N are dissolved at a high temperature compared to ferrite or martensite. The carbonitride which has once precipitated in the ferrite or martensite phase dissolves again in the austenite phase upon reverse transformation. Precipitation of the C r depletion zone and reduced sensitivity of carbonitrides can also be avoided by rapidly heating the stainless steel to the high temperature austenite zone for complete reverse transformation in a short time. The reverse transformation is accomplished before the carbonitride precipitation by the kinetic theory. That is, when the * stainless steel is heated up to the austenite region, it passes through the temperature zone during the heating phase, in which the carbonitride precipitates in the ferrite or martensite, but the precipitation of the carbide starts after a certain incubation period. In the subsequent cooling process, the austenite phase in which C is dissolved is converted into martensite without precipitation of carbides. Therefore, the martensite phase is strengthened. The state of the C and N atoms in the reverse conversion means that the sensitization is accelerated when the C and N contents are increased, and the ratio of the reversely transforming horses to 8 to 1275649 when in the two-phase annealing state (in other words, in the room) When the proportion of martensite which can be reversely transformed at a temperature is increased, sensitization is suppressed. In this sense, in order to suppress the precipitation of chromium carbides and the generation of C r depletion regions, the ratio of reverse transformation of martensite at room temperature is controlled to a specific relationship with C and N so that it is elevated. The C and N atoms are dissolved in the reverse transformed austenite at a temperature.丄4) Embodiment A stainless steel used as a structural part, such as a tire rim and a frame, contains a predetermined proportion of various alloying elements as follows: C up to 0.04 mass% C is an austenite effective for strengthening the martensite phase Composition. The proportion of martensite produced by heating the stainless steel at a temperature higher than Ac^ and then cooling it to room temperature is controlled by the c content. The effect of C on the martensite phase strength and the martensite ratio is shown when C is 0.01% by mass or more. However, an excess of C greater than 0. 04% by mass causes precipitation of chromium carbides at the grain boundary during the cooling phase of the duplex annealing or at the hardening and reduces the intergranular uranium resistance.

Si up to 2.0% by mass S i is an element added as a deoxidizer during steel making, and the age-effective hardenability of the steel sheet is improved by improving strain aging. When the Si content increases, the martensite phase hardens, the austenite phase solid solution hardens, and the cold worked steel sheet is strengthened. However, an excess of S i of more than 2.0% by mass causes thermal cracking and damage during the preparation. The upper limit of the Si content is preferably set to 1.5% by mass.

Cr is 10.0 to 20.0% by mass

Cr is an essential element of uranium resistance. The Cr content is set to 1 〇. 〇% by mass or more, and the uranium resistance required for the purpose of imparting stainless steel. However, an excess of cr of -9 to 1275649 at 20.0-% by mass not only deteriorates the toughness of the stainless steel, but also requires the addition of an austenite composition for the production of martensite grains, such as C, N, N i ' Μ Π C11 〇 Adding an austenitic composition increases the cost of steel and disadvantageously stabilizes the austenite phase at room temperature. The Cr content is preferably controlled within the range of 13.5 to 18.5 mass%.

Ni ' Mu and Cu as austenite constituents are necessary for producing a ferrite/austenite duplex structure at high temperatures (converted to a ferrite/martensitic structure at room temperature). Because the proportion of martensite becomes larger as the contents of Ni, Μη and Cu increase, the stainless steel plate hardens. However, excessive addition of more than 4. 〇 mass % of Ni, 高于 〇 mass % of Μη and more than 3.0% by mass of Cu hinder transformation of austenite into martensite, and cause austenite to exist at room temperature Body, resulting in poor strength. In this sense, it is preferred to control the contents of N i, Μη, and Cu, respectively, in 〇·50~3. 〇 mass%, 0.01 to 2 · 0 mass%, and 0. 02~2 · 5 mass% range Inside. N up to 0.12% by mass N is the same austenite composition as C, although its effect on the strength of the steel sheet is slightly weaker than C. The proportion of martensite produced by heating the stainless steel at a temperature higher than Ac i and then cooling it at room temperature is controlled by the N content. However, N is more susceptible to sensitization of stainless steel than C, and nitrides precipitated at the grain boundaries during the two-phase annealing cooling stage or at the time of hardening disadvantageously reduce corrosion resistance. In addition, excessive N causes internal defects such as pores. In this sense, the upper limit of the N content is set to 0.12% by mass (preferably 0.08% by mass). B up to 0.015 mass% B is to prevent the edge crack of the strip caused by the difference in deformation resistance between the ferrite and the -10- 1275649 austenite phase in the elevated temperature zone of the hot rolled steel sheet. An alternative alloying element. However, an excess b of more than 0.015 mass% promotes the generation of low melting point boride which is detrimental to hot workability and heat crack resistance at the time of welding.

Mo is up to 3.0% by mass.

Mo is also an optional element that is effective for uranium resistance, but higher than 3. 〇 The excess Mo of mass % reduces hot workability and increases the cost of steel. The upper limit of the Mo content is preferably 2.0 mass.

Ti is up to 0.10% by mass, Nb is up to 0.40%, and V# is up to 0.30% by mass. T i, Nb and V are optional elements which stabilize C and N into carbonitrides and improve the corrosion resistance of the weld heat affected zone. These elements are effective for minimizing crystal grains and reinforcing stainless steel. However, an excessive T i of more than 0.1% by mass causes generation of titanium clusters and surface defects, and an excess of Nb of more than 0.40% by mass promotes generation of a low-melting alloyed layer or oxide and reduces soldering resistance of stainless steel. The thermal cracking performance, and the excessive V extreme of more than 0.3% by mass, increase the high temperature strength of the stainless steel sheet and cause trouble in the production process. ® In addition to the above elements, in addition to the alloying design for the formation of a ferrite/martensitic two-phase structure at room temperature, one or more ferrite constituents such as aluminum may be additionally added. It is also possible to add one or more of Y, C a and REM (rare earth metal) to improve corrosion resistance and hot workability as long as the addition of these elements does not lower the strength of the stainless steel sheet. The stainless steel plate is additionally specified by the following sensitization index and surface hardness: Sensitization index is in the range of -3 1 - 7 - 11 - 7575649 When the stainless steel plate is exposed to a high temperature atmosphere during welding, the carbide of chromium is in the steel matrix. precipitation. Precipitation of chromium carbide means the production of Cr depleted zone, sensitization and a reduction in ultimate corrosion resistance. The factors that promote sensitization are the C and N contents, and the factors that inhibit sensitization are the proportion of reverse transformed austenite in the two-phase annealed state, that is, the ratio r (% by volume) of reverse-transformed martensite at room temperature. ). In summary, stainless steel is less sensitized by appropriately controlling the sensitization index indicating the relationship between the martensite ratio at room temperature and the C and N contents, but the corrosion resistance is improved. The present inventors have found that the sensitization index S t is typically defined by the test St=100C+3 0N-0.32r, and the sensitization is suppressed by the sensitization index S t not more than -7. . However, a sensitization index of less than -3 1 means that the contents of C and N are reduced to an extent that the hardness is not HV270. The preferred sensitization index is in the range of -28 to -10. For example, by annealing a hot-rolled steel sheet at 780 ° C for 12 hours, it is cooled as it is in a furnace, cold-rolled at a compression ratio of 80%, and the cold-rolled steel sheet is annealed at 950 ° C for 1 hour. Then, it is cooled in the open air to obtain a stainless steel plate having a martensite control ratio. Base metal having a surface hardness of HV2 70 or higher In addition to the martensite ratio, the hardness of the stainless steel sheet is controlled by the C and N contents. In order to provide a light structural part with good elasticity, the stainless steel plate must have a surface hardness of HV 270 or higher (preferably HV3 00 or higher) in the portion affected by the heat of welding. When the surface hardness is less than HV270, the use of thick structural parts is not conducive to the production of two-wheeled vehicles, resulting in heavy products. The martensite ratio at room temperature is not less than 25% by volume (preferably 40% by volume) -12 to 1275649. It is necessary to have a surface hardness of ΗV 2 70 or higher. Such a martensite ratio is also effective for imparting dent resistance and wear resistance to structural parts of a two-wheeled vehicle. Other features of the invention will be apparent from the following examples. Example 1 Some steels having the chemical composition shown in Table 1 were melted in a vacuum furnace, cast into slabs, hot rolled to a thickness of 4.5 mm, annealed in a furnace at 780 ° C for 12 hours and then cooled as it is, and the annealed steel sheets were annealed. After cooling, cold rolling to a thickness of 1.5 ΙΏΠ1, annealing at 800 ° C for 1 minute, cooling in the open air, cold rolling again to a final thickness of 0 · 5 mm, and then annealing at 950 ° C for 1 minute. Each duplex annealed steel sheet was observed by a microscope in a field of view of 200 // mx 200 // m in the thickness direction to detect martensite. Each steel plate was observed 10 times, and the volume ratio of martensite was calculated and averaged. The test piece sampled from each of the duplex annealed steel sheets was plastically formed and TIG welded to prepare a tire rim having the shape shown in Fig. 1. TIG welding was carried out by butt welding without using a wire under the following conditions: a welding current of 70 A, a torch moving speed of 300 mm/min, a volume of argon as a sealing gas of 10 liter/min, and a tungsten electrode diameter of 1.6 mm. . The welded portion was trimmed by honing the bead and finished with the base metal portion using #400 abrasive paper. The size of the test piece from the welded and finished steel plate was 100 mm x 150 mm and passed the CASS test specified in J.IS H8502 (thus, the test piece was immersed at 35 ± 2 ° C and the pH was 3 · 0~3 . 1 (5% NaCl + 0 · 26g / 1 1275649

Detection in CuCl2 + acetic acid) solution. After the 200 hour CASS test, the test block was observed to detect rust in the heat affected zone of the weld. The results were evaluated as follows and illustrated in Fig. 2. 〇: Test block without rust X: rusted test block The inventors have studied the effect of the martensite ratio r and the 〇oc + 30N 値 on the appearance of rust, and passed the 100C + 20N shown in Fig. 2 — 0 . 32 7 = -7 lines make the marks 〇 and X distinct from each other. The results in Fig. 2 demonstrate that the sensitization index S t (defined by the formula S t = 100C + 2 0N - 〇.32r) should be less than -7 in order to prevent the welded portion from being corroded. However, when St decreases by less than -3 1, the hardness of the steel plate becomes weaker than HV270 〇1275649 due to the shortage of c and N. Table 1 = Chemical composition of stainless steel in Example 1 (% by mass) Steel No. C Si Μη Ni Cr Cu N Mo Ti Nb 100C +30N r 1 0.055 0.23 0.03 0.98 19.23 2.23 0.120· 0.97 0.04 0.05 9.10 50 2 0.015 0.45 0.98 0.82 17.54 1.67 0.015 0.00 0.00 0.23 1.95 20 3 0.034 0.78 0.33 0.56 18.23 0.13 0.118 0.00 0.07 0.34 6.94 42 4 0.055 0.65 0.76 0.98 15.23 0.98 0.020 2.23 0.06 0.02 6.10 40 5 0.040 0.30 0.55 3.23 18.79 1.23 0.030 0.00 0.00 0.38 4.90 88 6 0.035 0.87 0.31 3.21 18.65 0.88 0.020 0.00 0.02 0.12 4.10 48 7 0.020 1.23 1.76 2.00 17.34 1.23 0.020 1.95 0.03 0.01 2.60 43 8 0.033 0.34 1.24 1.43 18.98 0.45 0.090 0.38 0.04 0.23 6.00 72 9 0.022 1.33 0.27 0.99 16.77 1.89 0.035 0.00 0.02 0.00 3.25 55 r represents the proportion (% by volume) of martensite at room temperature. Example 2 Some steels having the chemical compositions shown in Table 2 were melted in a vacuum furnace, cast into a sheet, hot rolled to a thickness of 4.5 mm, annealed at 780 ° C for 1 2 hours in an oven, and cooled as it was. The pickled annealed steel sheet was cold rolled to a thickness of 1.5 m, annealed at 800 ° C for 1 minute, cooled in the open air, cold rolled again to a thickness of 0 · 5 mni and then finally annealed at 1030 ° C for 1 minute. The steel K in Table 2, corresponding to SUS 430LX, was annealed at 1 000 ° C for 1 minute in the hot rolling and cold rolling conditions. Test pieces sampled from various stainless steel sheets were plastically formed into a tire reed having the shape shown in Fig. 1 and welded by TIG into a steel pipe having a diameter of 30 mm. TIG welding 1275649 was performed with a welding current of 1 50 A and a torch moving speed of 500 mm/min. The welded portion was trimmed by honing the bead and then finished with the base metal portion using #400 abrasive paper. The bending resistance was evaluated by a bending test by a permanent strain, whereby the height h in the L direction was obtained. The test body R is made into a half-size shape of the tire rim (shown in Fig. 3), 50 kg of weight W is loaded on the test body R and then the load is removed, and then the height of the test body R along the .L direction is measured and Original height h. Compare to calculate permanent strain. A permanent strain 値 of less than 1 mm was evaluated as good bending resistance (〇).

-16- 1275649 Table 2: Chemical composition of stainless steel in Example 2 (% by mass) Steel type C Si Mn Ni Cr Cu N Mo Ti Nb BV Note A 0.013 0.22 1,30 1.20 14.23 1.43 0.080 0.32 0.04 0.04 0 . 000 0.02 B 0.029 0.47 0.31 2.43 16.59 0.48 0.029 0.05 0.01 0.02 0.007 0 . 06 Hair C 0.027 0.39 0.29 2.56 16.38 0.03 0.031 0.08 0.00 0.01 0.005 0.07 Ming D 0.039 0.23 1.76 1.88 18.56 0.97 0.033 1.95 0.00 0.01 0.012 0.07 Real E 0.033 0.34 1.24 2.34 16.66 0.33 0.010 0.38 0.00 0.32 0.003 0.12 Application F 0.009 0.54 0.78 0.97 15.78 2.17 0.066 0.88 0.08 0.01 0.006 0.23 Example G 0.033 1.23 1.45 2.04 16.43 0.98 0.032 0.00 0.00 0.00 0.000 00 1 Η 0.066 0.55 0.29 1.95 16.35 0.06 0.009 0.08 0.01 0.01 0.006 0.02 I 0.055 0.55 0.25 0.11 16.29 0.15 0.132 0.23 0.01 0.G1 0.005 0.04 to J 0.003 1.78 0.31 0.19 9.45 0.76 0.008 0.22 0.00 0.07 0.000 0.08 Ratio K 0.032 0.34 0.87 0.23 18.23 0.43 0.098 0.26 0.08 0.14 0.002 0.04 Example L 0.013 0.44 0.25 0.19 17.11 0.06 0.012 0.05 0.01 0.38 0.001 0.02 The underlined number is the rule of the invention. Outside the range.

Steel J has the chemical composition specified by the present invention, but its s t値 is greater than -7. Each of the steel sheets was examined to determine the proportion of martensite in the two-phase annealed state at room temperature. The measured enthalpy is substituted for S t = 1 00C + 3 ON - 0 . 3 2 r The r of the formula is used to calculate the sensitization index s t . The surface hardness, permanent strain, and corrosion resistance were measured or evaluated in the same manner as in Example 1. The results are shown in Table 3. It will be understood that any tire rim and any steel tube base metal made from the steel of the present invention have a hardness of HV 270 or higher, and rust does not occur in both the base metal and the welded portion. The tire rim has a permanent strain less than 丨mm that is suppressed by -17-1267549. The comparative steel to K has a chemical composition or sensitization index S t outside the range specified by the present invention. Comparative steel crucibles and I contain an excess of C and an excess of C + N, respectively, so their weld heat affected zone has poor corrosion resistance. Comparative steel; [The resulting tire rim and steel tube are not composed of a martensitic/ferritic two-phase structure and have poor hardness. Permanent strain is greater than 1 mm. Comparative steel J is also poor in corrosion resistance and bending resistance due to insufficient Cr content. Comparative steel K has poor corrosion resistance in its weld heat affected zone due to its high sensitization index S t . The decrease in corrosion resistance proves the development of sensitization caused by welding heat 〇 Comparative steel L, corresponding to SUS 43 0 LX, has poor bending resistance. 1275649 ^ 3 = Test results of the test block

Steel hardness HV proportion of martensite in double-phase annealed state (% by volume) Sensitivity index St CASS test corrosion resistance and bending resistance Tire steel ring frame (1) (2) (1) (2) A 281 78 - 21.3 〇〇〇〇〇B 382 88 -24.4 〇〇〇〇〇发 C 306 90 -25.2 D明 D 347 50 -11.1 〇〇〇〇〇实 E 307 60 -15.7 〇〇〇〇〇 F 277 62 -17.0 Example G 335 80 -21.3 〇〇〇〇〇Η 392 70 -15.5 XXXX 〇I 384 58 -9.1 XXXX 〇 to J 168 0 0.5 XXXXX than a K 295 32 -4.1 〇X 〇X Example L 1 173 0 1.7 〇〇〇〇 X

(1) uranium resistance of the base metal portion (2) corrosion resistance of the welded portion According to the present invention described above, the proportion of martensite which can be reversely transformed into austenite during double phase annealing is controlled with ferrite/horse The specific relationship between the C and N contents in the duplex stainless steel sheet without reducing the C and N contents too much. Due to this particular relationship, the two-wheeled vehicle tire rim and frame made of the stainless steel sheet are excellent in both corrosion resistance and bending resistance without sensitization of the weld heat affected zone. -19 1275649 (5) Brief description of the drawings Fig. 1 is a cross-sectional view illustrating the shape of a tire rim product. Fig. 2 is a graph showing the corrosion resistance of the welded portion with respect to the C and N contents and the martensite ratio. Figure 3 is a view illustrating the bending test.

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Claims (1)

1275649 Pickup, patent application scope: 1 · A two-wheeled vehicle with anti-bending structural parts made of a stainless steel plate containing: up to 0.04% by mass of C, up to 2.0% by mass of Si, up to 2 · 0 mass % Μη, 10. 0~20 · 0 mass% of Cr, up to 4.0% by mass of Ni, up to 3.0% by mass of Cu, up to 0.12% by mass of N, and the balance is unavoidable In addition to impurities, it is a chemical composition composed of iron; its surface hardness is HV270 or higher; and a two-phase structure composed of 5 to 75 vol% of ferrite and 25 to 95 vol% of martensite, provided that By controlling the martensite ratio 7 relative to the C and N contents, the sensitization index S t defined by the formula St = 100C + 30N - 0.32r is adjusted to a 3 of 3 1 〜 7 , where 7 represents a duplex The volume percentage of the martensite phase in the annealed state. 2 · The anti-bending structural part of the patent application scope 1 'in which the chemical composition additionally contains B up to 0.015 mass%, Mo up to 3 mass%, Ti up to 〇1. mass%, Up to 40% by mass of one or more of Nb's up to 0.30% by mass of v. 3. The anti-bending structure climbing member of claim 1 is in which the structural part is a welded tire rim or frame.