TW200406494A - 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- 0.32 γ (γ 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

200406494 (1) Description of the invention: Technical field of the invention The present invention relates to two-wheeled vehicles made of stainless steel, such as bicycles, motorcycles, and wheelchairs, which are excellent in corrosion resistance and bending resistance in welding heat-affected zones. Structural parts. (II) Prior technology Tire wheel rims and frames of two-wheeled vehicles such as bicycles, motorcycles and wheelchairs are often exposed to corrosive atmosphere. In order to extend their life, the materials of these structural parts are changed from galvanized steel to ferritic stainless steel, such as · SUS 3 04. The weldability of this material will also be good because structural parts are usually manufactured by welding. JP61-73866A proposes a method in which the toughness, ductility, and corrosion resistance of the welded part are improved by adding two elements of Ti and Nb at a ratio of 0.1 to 0.3% by mass and 0.15 to 0.3% by mass, respectively. Ferrite stainless steel containing 10 to 20% by mass of Cr without bending. However, the proposed non-mirror steel has the disadvantage that the Ti inclusions often cause T i streaks due to the addition of T i in a high proportion. · JP62-1648 57 proposes another ferritic stainless steel for tire rims, in which an austenite composition such as Ni, Mn or Cu is added to a mass of 12.5 to 17 in weight In the ferritic stainless steel of% Cr, the content of C and N is reduced at the same time to improve the intergranular corrosion resistance in the welded part without the need for stabilizing elements such as T i or Nb for toughness and workability. It also reported that CE 値 was controlled within a predetermined range for the generation of a martensite phase. However, the stainless steel has poor strength because the total content of C and N is reduced to 0.4 mass% or less, which has nothing to do with the formation of martensite. The steel can be strengthened by adding alloying elements, but adding alloying elements increases the cost of the steel. Iii) Summary of the invention The object of the present invention is to provide an inexpensive structural part made of a stainless steel plate or pipe with improved weldability, strength, bending resistance, and corrosion resistance in both the heat-affected zone and the metal portion of the base. , Such as tire rims and frames for two-wheeled vehicles, such as bicycles, motorcycles, and wheelchairs. The invention provides a structural part made of a ferritic / martensitic duplex stainless steel plate. To achieve this purpose, stainless steel has a maximum of 0.04% by mass of C, a maximum of 2.0% by mass of Si, a maximum of 2.0% by mass of Mn, 10.0 to 20.0% by mass of Ci ·, and a maximum of 4.0% by mass of Ni, Cu up to 3.0% by mass, N up to 0.12% by mass, and B which can be selected up to 0.015% by mass, Mo up to 3.0% by mass, Ti up to 0.10% by mass, and up to 0.40 One or more of Nb by mass, V by up to 0.30 mass%, and a chemical composition consisting of iron except for inevitable impurities. # By controlling the ratio of martensite to C and N content (γ), the stainless steel will be represented by the formula St = 100C + 30N— 0.32r (where 7 represents the proportion of martensite phase in the dual-phase annealed state (vol%)) The limited sensitization index (S t) is adjusted to -31 to -7, and has a dual phase composed of 5 to 75% by volume of ferrite and 25 to 95% by volume of martensite. structure. Tire rims are usually made of stainless steel plates, while frame parts are usually made of stainless steel pipes. In any case, adjust the surface hardness of the stainless steel plate or tube, -6-200406494 to HV270 or higher except for the welded part. The stainless steel plate used as a structural part is prepared as follows: a steel strip having a specific composition is cold-rolled and annealed in a continuous annealing furnace in a continuous annealing furnace at a temperature of 850 to 1100 ° C in a ferrite / martensite dual-phase region. The lower heating steel strip is subjected to two-phase annealing as a final heat treatment and the heating steel strip is subsequently cooled. The inventors have investigated and examined the performance of stainless steel plates necessary for structural parts of two-wheeled vehicles, such as bicycles, motorcycles, and wheelchairs, in particular from the aspects of corrosion resistance and bending resistance of welded heat-affected zones. The following description is for bicycle parts' but the same performance is of course necessary for motorcycles and wheelchairs. The main structural parts of bicycles are tire rims and frames. In order to stabilize the shape of the car, chasing some parts will keep the original shape without deformation. Tire rims approaching the road surface during cycling are exposed to atmospheric corrosion and are also subject to friction with rubber brakes. Pebble or the like hit the tire rim and frame. Considering these practical conditions, tire rims and frames must have good rigidity, corrosion resistance, abrasion resistance, and dent resistance. Because the semi-stable austenitic stainless steel with strain-induced martensite and austenite metallurgical structures has poor rigidity and dent resistance for this purpose, the inventors have investigated the ferrite / martensite dual The practicality of the phase stainless steel is that 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. By appropriately controlling the proportion of martensite in the two-phase annealed state and the sensitization index S t with respect to the C and N contents, rigidity, dent resistance and corrosion resistance suitable for the purpose are achieved. Rigidity (bending resistance) is improved by hardening the stainless steel to a Vickers hardness of 2 70 or -7- 200406494 or higher and by recrystallizing the ferrite / martensite dual phase to increase the Young's modulus. The generation of a martensite phase results in increased strength and improved sag and corrosion resistance. When the stainless steel plate is welded, it is heated by welding up to 600 ° ~ 900 ° C. In such high temperature regions, chromium carbonitrides are precipitated from the steel matrix and come back to create a depleted region. The C r depleted region disadvantageously 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 f · c · c · (face-centered cubic) crystals The austenite phase of the structure is likely to precipitate. The martensite is transformed into the austenite phase in the opposite direction, where a relatively large amount of C and N is dissolved at a high temperature compared to ferrite or martensite. The carbonitrides that were once precipitated in the ferrite or martensite phase are again dissolved in the austenite phase during reverse transformation. By rapidly heating the stainless steel to a high-temperature austenite region so that complete reverse conversion in a short period of time can also avoid the precipitation of carbonitrides that cause Cr depleted regions and reduced sensitivity. The kinetic theory is a good explanation for the completion of reverse conversion before carbonitride precipitation. That is, when stainless steel is heated up to the austenite region, 'passes the temperature region during the heating stage, where carbonitrides precipitate in ferrite or martensite' but the precipitation of carbides begins after a certain holding period. In the subsequent cooling step, the austenite phase in which C is dissolved is transformed into martensite without carbide precipitation. Therefore, the martensite phase is strengthened. The state of c and N atoms during reverse transformation means that the sensitization is accelerated when the c and N content is increased ', and the proportion of martensite is reversed in the two-phase annealed state (in other words, in the chamber When the proportion of martensite that can be reversely transformed at temperature is increased, the sensitization is suppressed. In this sense, in order to suppress the precipitation of chromium carbides and the generation of Cr depleted regions, the proportion of martensite reversely transformed at room temperature is controlled to a specific relationship with C and N, so that the C and N atoms are dissolved in reverse-converted austenite at temperature. (IV) Embodiment A stainless steel used as a structural part, such as a tire rim and a frame, contains various different alloying elements in a predetermined proportion as follows: C is up to 0.04 mass% C is an austenite effective for strengthening the martensite phase Structure. The proportion of martensite produced by heating the stainless steel at a temperature higher than A c i and then cooling it to room temperature is controlled by the c content. The effect of C on the strength of the martensite phase and the proportion of martensite is shown when C is 0.01% by mass or more. However, an excessive C of more than 0.04 mass% causes precipitation of chromium carbides at the cooling stage of the two-phase annealing or at the grain boundaries at the time of hardening and reduces the intergranular corrosion resistance. S i is up to 2.0% by mass. S i is an element added as a deoxidizing agent during steelmaking and improves the age hardenability of the steel sheet by increasing strain aging. When the Si content increases, the martensite phase hardens, the austenite phase solidifies and solidifies, and the cold-worked steel sheet is strengthened. However, excess Si of more than 2.0% by mass causes thermal cracking and damage during production. The upper limit of the Si content is preferably set to 1.5% by mass.

Cr is 10.0 ~ 20.0% by mass

Cr is a basic element of corrosion resistance. The Cr content is set to 10.0 mass% or more to impart the required corrosion resistance for the purpose of stainless steel. However, an excessive Cr of -9-200406494 to 20.0-mass% not only deteriorates the toughness of the stainless steel, but also requires the addition of austenite constituents such as C, N, Ni, Mn to generate martensite grains. And Cu. The addition of austenite constituents increases the cost of the steel and disadvantageously stabilizes the austenite phase at room temperature. The C r content is preferably controlled within a range of 13.5 to 18.5 mass%.

Ni, Mn, and Cu as austenite constituents are necessary to produce a ferrite / austenite dual-phase structure (converted into a ferrite / martensite structure at room temperature) at high temperatures. Because the proportion of martensite becomes larger as the contents of Ni, Mn, and Cu increase, the stainless steel plate becomes harder. However, excessive addition of more than 4.0% by mass of Ni, more than 2.0% by mass of Mn, and more than 3.0% by mass of Cu hinder the conversion of austenite to martensite, and cause austenite to exist at room temperature, resulting in undesirable Strength of. In this sense, it is preferable to control the contents of Ni, Mn, and Cu, respectively, in a range of 0.5 to 3.0 mass%, 0.01 to 2.0 mass%, and 0.05 to 2.5 mass%. . N is up to 0.12% by mass. N is the same austenite structure 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 grain boundaries during the dual-phase annealing cooling stage or at the time of hardening adversely reduce the 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 mass% (preferably 0.08 mass%). B is up to 0.015% by mass. B is an option to prevent strip edge cracks caused by the difference in deformation resistance between ferrite and 200406494 austenite phase in the elevated temperature zone of hot rolled steel. Alloying elements. However, an excessive amount of B exceeding 0.015% by mass promotes the generation of low-melting-point boride that is detrimental to hot workability and heat crack resistance during welding.

Mo is up to 3.0 mass%.

Mo is also an optional element effective for corrosion resistance, but excess Mo of more than 3.0 mass% reduces hot workability and increases the cost of steel. The upper limit of the Mo content is preferably 2.0 mass.

Ti reaches up to 0.10% by mass, Nb reaches up to 0.40%, and V_ reaches up to 0.30% by mass. Ti, Nb, and V are optional elements that stabilize C and N into carbonitrides and improve the corrosion resistance of the welding heat-affected zone. These elements are effective for minimizing crystal particles and strengthening stainless steel. However, an excess T i of more than 0.1 mass% may cause the generation of titanium clusters and surface defects, and an excessive Nb of more than 0.40 mass% promotes the generation of a low-melting alloyed layer or oxide and reduces the stainless steel Resistance to welding hot cracking, and excessive V higher than 0.30% by mass extremely increase the high temperature strength of the stainless steel plate and cause trouble in the production process. In addition to the above elements, under the alloying design to form a ferrite / martensite dual-phase structure at room temperature, one or more ferrite constituents such as aluminum can be added. One or more of Y, Ca, and REM (rare earth metals) may also be added to improve corrosion resistance and hot workability as long as the addition of these elements does not reduce the strength of the stainless steel sheet. Stainless steel plates are additionally specified by the following sensitization index and surface hardness: The sensitization index is in the range of -3 1 ~ -7-11- 200406494 When the stainless steel plate is exposed to high temperature atmosphere during welding, chromium carbides are in the steel Precipitation in the matrix. The precipitation of chromium carbide means the generation of C 1 · depleted regions and the reduction of the final corrosion resistance. The factors that promote sensitization are the C and N content, while the factors that inhibit sensitization are the ratio of reversely transformed austenite in the two-phase annealing state, that is, the ratio of reversely transformed martensite at room temperature r (volume% ). In short, stainless steel is less sensitized by appropriately controlling the sensitization index indicating the relationship between the martensite ratio and the C and N content at room temperature, but improves the corrosion resistance. The present inventors have found that the sensitization index S t is typically defined by unsatisfactory tests by the spring formula S t = 100C + 3 0N— 0.32 r, and the sensitization is achieved by setting the sensitization index S t to not greater than -7 And suppressed. However, a sensitization index of less than -3 1 means that the content of C and N is reduced to a level that is not sufficiently hard to HV270. The preferred sensitization index is in the range of -28 to -10. For example, by annealing the hot-rolled steel sheet at 780 ° C for 12 hours, cooling it in a furnace as it is, cold-rolling it at a compression ratio of 80%, and annealing the cold-rolled steel sheet at 95 ° C for 1 hour, and then in the open air. Cooling produced a stainless steel plate with a martensitic control ratio. · Base metals with surface hardness HV270 or higher In addition to the martensite ratio, the hardness of stainless steel plates is controlled by the C and N content. In order to provide light structural parts with good elasticity, the stainless steel plate must have a surface hardness of HV270 or higher (preferably HV300 or higher) in a portion away from the heat-affected part 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. At room temperature, the proportion of martensite is not less than 25% by volume (preferably 40% by volume-1 2-200406494). It is necessary for the surface hardness to be HV270 or higher. Such a martensite ratio is used to give the two-wheeled vehicle structural parts resistance to dent And abrasion resistance are also effective. Other features of the present invention will be clearly understood from the following examples. Example 1 Some steels with the chemical composition shown in Table 1 were melted in a vacuum furnace, cast into slabs, and hot rolled to thickness It is 4.5 mm, annealed at 78CTC for 12 hours in the furnace and then cooled as it is. The annealed steel sheet is cooled, cold rolled to a thickness of 1.5 mm, annealed at 800 ° C for 1 minute, cooled in the open air, and cold rolled again to the final thickness. 0.5mm, and then annealed at 95 ° C for 1 minute. # Observe the martensite by inspecting each annealed steel sheet in a field of view 200 // mx 200 / zm in the thickness direction through a microscope. 10 times Observe each steel plate, calculate and average the martensitic volume ratio. The test blocks sampled from each of the two-phase annealed steel plates are plastically formed and TIG welded to prepare a tire rim having the shape shown in Fig. 1. By not using a welding wire, Butt welding TIG welding under the following conditions: Welding current is 70A, welding torch moving speed is 300mni / min, the volume of argon β as sealing gas is 10 liters / min, and crane electrode diameter is 1.6mm. Welding by honing Straighten the welded part and finish it with # 4 0 0 abrasive paper together with the base metal part. The size of the sample taken from the welded and finished steel plate is 100mm x 15 0mm test block and pass the CASS specified in J.IS H8 5 02 Test (therefore, the test block was immersed at 35 ± 2 ° C with a pH of 3.0 ~ 3.1 (5% NaCl + 0.26g / l 200406494

CuCl2 + acetic acid) solution). The test block was inspected after the 200-hour CASS test to detect rust in the welding HAZ. The evaluation of the results is illustrated in Figure 2. 〇: Test piece without rust X: Test piece with rust The inventors have investigated the effect of rust on the martensite ratio r and 100C + 3ON, and passed the line 100C + 20N — 0 = -7 shown in FIG. 2 The marks 0 and X are clearly distinguished from each other. The proof of the result in Fig. 2 ′ In order to prevent the welded part from being corroded, the sensitization index S t (specified by the formula 100C + 20N- 0.32r) should be less than -7. However, when St is less than -31, due to the lack of C and N, the steel plate is weakened and the hardness is reduced to HV270. 观 It is clear from the comparison that • 32 r shows that S t = decreases less than 200406494. Table 1: Stainless steel in Example 1 Chemical composition (mass%) steel number C Si Mn 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 stands for martensite at room temperature Proportion (vol%). Example 2 Some steels having the chemical composition shown in Table 2 were melted in a vacuum furnace, cast into slabs, hot rolled to a thickness of 4.5 mm, annealed in a furnace at 7 80 ° C for 12 hours and cooled as it was. Pickled and annealed steel sheets were cold rolled to a thickness of 1.5 mm, annealed at 800 ° C for 1 minute, cooled in the open air, cold rolled again to a thickness of 0.5 mm, and finally annealed at 1 30 ° C for 1 minute. The steel K in Table 2 corresponds to SUS 4 3 0LX, with the exception of being annealed at 1000 ° C for 1 minute in both hot and cold rolled states. The test blocks sampled from various stainless steel plates were plastically formed into a tire rim having the shape shown in Fig. 1 and TIG-welded into a steel pipe having a diameter of 30 mm. TIG welding with welding current of 150A and welding torch moving speed of 500mm / min 200406494. The welded part was rectified by a honing bead and then finished with # 400 abrasive paper together with the base metal part. The bending resistance is evaluated as a permanent strain by a bending test, whereby the height h will be along the L direction. The test body R is made into a half-size shape similar to that of a tire rim (shown in FIG. 3). A 50 kg weight w is loaded on the test body R and then the load is removed. Then, the height hi of the test body r in the L direction is measured and compared with the original Height h. Compare to calculate permanent strain. The permanent strain 値 smaller than 1 mm was evaluated as good bending resistance (0). 200406494 Table 2: Chemical composition (mass%) of stainless steel in Example 12 Steel 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 .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 clear 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 Case G 0.033 1.23 1.45 2.04 16.43 0.98 0.032 0.00 0.00 0.00 0.000 0. 00 < 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 0.01 0.005 0.04 for 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 than 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 number of underline is in the present invention Outside the predetermined range. Steel J has the chemical composition specified in the present invention, but its S t 値 is greater than -7. Each steel sheet was examined to determine the proportion of martensite in the two-phase annealed state at room temperature. Substitute the measured 値 for S t = 1 0 0 C + 3 0 N — 0. 3 2 7 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 can be understood that any tire rim and any steel pipe base metal made from the steel of the present invention has a hardness of H V270 or higher, while rust does not occur in both the base metal and the welded parts. Tire rims with a permanent strain of less than 1 mm are suppressed -17- 200406494. Comparative steel reeds to K have a chemical composition or sensitization index S t outside the range specified in the present invention. The comparative steel reeds and I contained excessive C and excessive C + N, respectively, and thus their welding HAZ had poor corrosion resistance. Comparative steel; [The tire rim and steel tube produced were not composed of a martensite / ferrite dual phase structure and had poor hardness. The permanent strain is clearly greater than 1 mm. Comparative steel: Because of insufficient Cr content, the corrosion resistance and bending resistance are also poor. Comparative steel K has poor corrosion resistance in its heat-affected zone due to its higher sensitization index S t. The decrease in corrosion resistance proves the development of sensitization caused by welding heat. Compared with steel L, which corresponds to S U S 4 3 0 L X, the bending resistance is poor. 200406494 Table 3: Test results of test blocks

The percentage of martensite in the HV duplex annealed steel grade (volume%) Sensitivity index St CASS test Corrosion resistance Bending resistance injection tire rim frame ⑴ (2) (1) ⑵ A 281 78 -21.3 〇〇〇 〇〇B 382 88 -24.4 〇 〇〇〇 C 306 90. -25.2 〇 〇 〇 〇 D D 347 50 -11.1 〇 〇 〇 〇 〇 Real E 307 60 -15.7 〇 〇 〇 F F 277 62 -17.0 〇〇〇〇〇〇 Case G 335 80 -21.3 〇〇〇〇〇〇 392 70 -15.5 XXXX 〇I 384 58 -9.1 XXXX 〇 J 168 0 0.5 XXXXX ratio K 295 32 -4.1 〇X 〇X 〇 L 173 0 1.7 〇〇〇〇〇

(1) Corrosion resistance of the base metal part (2) Corrosion resistance of the welded part According to the present invention described above, the proportion of martensite that can be reversely converted into austenite during dual-phase annealing is controlled to be equal to that of ferrite / martensite Specific relationship of C and N content in bulk duplex stainless steel plate without reducing C and N content too much. Due to this specific relationship, two-wheeled vehicle tire steel reeds and frames made from the stainless steel plate are excellent in both corrosion resistance and bending resistance without sensitization in the welding heat affected zone. 200406494 (V) Brief description of the drawings Figure 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 content and the martensite ratio. FIG. 3 is a view explaining a bending test.

Claims (1)

200406494 Scope of patent application: 1 · A bending-resistant structural part of a two-wheeled vehicle, the part of which is made of a nonferrous steel sheet containing: up to 0.04% by mass of C, up to 20% by mass of Si, and up to 2 0% by mass of Mn, 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 the balance in addition to unavoidable impurities In addition, 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% by volume of ferrite and 25 to 95% by volume of martensite, provided that: By controlling the ratio of martensite to C and N content 7, the sensitization index St defined by the formula St = 100C + 30N-0.327 is adjusted to a range of 31 to -7, where r represents the horse in the two-phase annealed state. Volume percentage of the austenite phase. 2. The anti-bending structural part according to item 1 of the patent application scope, wherein the chemical composition additionally contains up to 0.015% by mass of B, up to 3.0% by mass of Mo, up to 0.10% by mass of Ti, and up to 0.40% by mass Nb, one or more of V up to 0.30 mass%. 3. The anti-bending structural part according to the scope of patent application item 1, wherein the structural part is a welded tire rim or frame.