JP4436419B2 - Hot-rolled steel sheet for machine structural steel pipes with excellent fatigue characteristics and bending formability and its manufacturing method - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet for machine structural steel pipes excellent in fatigue characteristics and bend formability, and a method for producing the same. Examples of the mechanical structural steel pipe of the present invention include an automobile structural steel pipe and an automobile undercarriage part steel pipe.

  Steel for machine structure has been widely used in the industry as a material for members and parts used in general machinery, industrial machinery, construction machinery, transportation machinery (automobiles, transportation vehicles, etc.) and the like. One of the steel materials includes a cylindrical hollow body structural steel pipe and a base steel plate for the mechanical structural steel pipe (for example, a hot rolled steel sheet (including a hot rolled steel strip)). The steel pipes and the steel sheets for steel pipes (for example, hot-rolled steel sheets (including hot-rolled steel strips)) are required to have bend formability for fatigue characteristics due to repeated loads during use and for member and part processing. Among machine structural steel pipes, automobile structural steel pipes are also applied to safety members of automobiles, so that not only bend formability but particularly excellent fatigue characteristics are required. One example of this automobile structural steel pipe is an automobile undercarriage part steel pipe. Examples of the automobile underbody parts include an axle beam disposed between the left and right wheels of the automobile and a suspension member around the axle beam. Since these are repeatedly subjected to an impact load, a torsional load, and the like during traveling, high strength and high fatigue characteristics are required. In addition, automobile structural members, particularly automobile undercarriage parts, are often processed into complex part shapes and require high formability. In particular, extremely high bend formability is required for members and parts subjected to bend forming with a small bending radius R.

  For example, an automobile axle beam, which is an automobile underbody component, is obtained by a method of pressing while applying liquid pressure to the inner surface of a cylindrical workpiece (for example, a steel pipe) as disclosed in Patent Document 1 and the method. An axle beam having a modified cross-section cylindrical body has been proposed. As this axle beam material, steel materials having high fatigue characteristics and high bending formability (for example, machine structural steel pipes, automobile structural steel pipes, automobile undercarriage parts steel pipes, and steel plates of these steel pipes) are desired, Steel pipes and steel plate for steel pipes (including steel strips) that sufficiently satisfy fatigue characteristics and bending formability have not been obtained. Therefore, the present situation is that the fatigue characteristics and strength of members and parts are improved to a desired level by performing hardening heat treatment such as quenching and annealing after pressing the steel pipe. When such heat treatment is carried out, the cost of members and parts increases, but of course, the shape of the members and parts may change due to the heat treatment, and additional correction may be required, In the case of softening, there has been a problem that additional reinforcing means (for example, surface hardening treatment) may be required. For this reason, machine structural steel pipes (for example, automobile structural steel pipes, automobile undercarriage parts steel pipes, etc.) excellent in fatigue characteristics and bendability and steel plates (including steel strips) used as the steel pipe materials are highly anticipated in the industry. .

Moreover, it shows in patent document 2 as steel materials (welding steel pipe, steel strip for welded steel pipe raw materials) which balance fatigue characteristics and workability required for various shaping | molding (workability, such as a bending, a hydraulic pressure, a pipe expansion, and a contraction pipe). An invention has been proposed. The invention steel material of Patent Document 2 is a material that produces a large amount of ferrite structure in which ultra-fine (Ti, Mo) composite carbide having a specific element ratio is dispersed and precipitated. Moreover, since the area fraction of the ferrite structure which is a soft phase is as high as 60 to 100% and the particle size of the precipitate is extremely small, the formability is excellent. On the other hand, since the invention steel material of Patent Document 2 has a large amount of soft ferrite structure, the pearlite and bainite of the remaining structure become hard, and the hard structure becomes a structure mixed with ferrite. In such a structure, a hard structure cannot sufficiently withstand processing during severe bending and cracking may occur. Furthermore, when manufacturing a mechanical structural member, an automobile structural member, or an automobile undercarriage part from a steel pipe, there is a possibility that micro voids that do not cause cracking exist in the steel even if cracking does not occur in bending. When a steel pipe is used as a member or part in a mechanical structure, an automobile structure, or the like and a fatigue load is applied, fatigue cracks may be generated and propagated from microvoids generated by bending during the manufacture of the member or part.
JP 2001-321846 A JP2003-321748

  As mentioned above, mechanical structures (for example, automobile structures, automobile undercarriage parts, etc.) steel pipes and steel sheets for steel pipes (especially hot-rolled steel sheets) that have been able to sufficiently satisfy both fatigue characteristics and bend formability have been obtained so far. Absent. In general, the fatigue properties of steel materials tend to be superior when the strength of the steel material itself or a member or part processed from the steel material is high, and the bending formability of the steel material tends to be superior when the strength is low. For this reason, it is very difficult to achieve compatible characteristics. An object of the present invention is to provide a hot-rolled steel sheet for machine structural steel pipes, a hot-rolled steel sheet for automobile structural steel pipes, or a hot-rolled steel sheet for automobile undercarriage parts steel pipes, and a method for producing these steel sheets, which has excellent fatigue characteristics and bend formability. It is to provide.

  The inventors of the present invention have a hot-rolled steel sheet for machine structural steel pipes having excellent fatigue characteristics and excellent bendability (for example, hot-rolled steel sheets for automobile structural steel pipes, hot-rolled steel sheets for automobile undercarriage parts steel pipes, and those steel sheets). In order to obtain a steel pipe, the formation of microvoids at each thickness portion of the hot-rolled steel sheet and the process of fatigue crack initiation and propagation were continuously investigated from before and after bending forming until fatigue load loading. As a result, the present inventors have found that fatigue cracks in steel pipe mechanical structural members, for example, steel pipe automobile structural members, steel pipe automobile undercarriage parts, surface layer portion of the steel sheet (0.05 mm depth from the steel surface). In the case where there is a surface coating layer, it was newly found that it occurs within a depth of 0.05 mm from the interface of the railway. Also, when manufacturing the above steel pipe members and steel pipe parts, the work cracks when bending the steel pipe are the steel surface layer part with the highest work strain (within 0.05mm depth from the steel surface, with a surface coating layer) Was newly found to occur within a depth of 0.05mm from the interface of the steel. Furthermore, it has been newly found that microvoids generated in the steel surface layer portion during bending forming promote the generation and progress of fatigue cracks when used as members or parts after processing. As a result of various investigations focusing on this knowledge and focusing on the microstructure, hardness, structure crystal morphology, grain boundary precipitate presence state, etc. of the steel surface layer portion, the present inventors have found that fatigue characteristics under specific conditions It has been newly found that a hot rolled steel sheet for machine structural steel pipes, a hot rolled steel sheet for automobile structural steel pipes, or a hot rolled steel sheet for automobile undercarriage parts steel pipes, which are excellent in bending property and bendability. In other words, a steel pipe steel sheet having the desired characteristics can be obtained by controlling the structure of the steel surface layer portion to be uniform and fine, controlling it to be moderately hard, and further reducing the grain boundary precipitated carbide. In addition, the inventors have studied a method for obtaining the above-mentioned specific conditions in an industrially advantageous manner, and the present inventors have obtained a suitable component system and manufacturing process. The specific condition of the surface layer portion is sufficiently effective for a steel pipe base steel plate having a steel plate thickness of 0.7 to 20 mm, and the steel plate of the present invention includes a steel strip.

The gist of the present invention is (1) % by mass, C: 0.05 to 0.19, Si: 0.05 to 1.0, Mn: 0.3 to 2.5, P: 0.03 or less, S: 0.025 or less, Ti: 0.005 to 0.1, Cr: 0.03 -1.0, Sol. Al: 0.005 to 0.1, N: 0.0005 to 0.01, B: 0.0001 to 0.01, and satisfying the formula (A) , the surface layer portion of the steel composed of the remaining Fe and inevitable impurities , 80% or more of the area fraction of the microstructure is Fatigue characterized by being bainite, having a Vickers hardness Hv of 210 or more and 300 or less, an average value of the major axis length of bainite being 5 μm or less, and an average grain boundary carbide particle size of 0.5 μm or less. Hot-rolled steel sheet for machine structural steel pipes with excellent properties and bending formability.
3C ≦ 0.27Mn + 0.2Cr + 0.05Cu + 0.11Ni + 0.25Mo ≦ 3C + 0.3 (A)
However, the values of C, Mn, Cr, Cu, Ni, and Mo in the formula (A) are mass%.
In addition, the surface layer part of the steel of this invention is a site | part within 0.05 mm in the thickness direction from the steel surface of a steel plate (a steel strip is included). However, the Vickers hardness Hv is a value at a depth position of 0.05 mm in the thickness direction from the steel surface of the steel plate (including the steel strip).

(2) The steel of (1 ) above is further mass%,
As a group of bainite generation promoting elements,
Cu: 0.005-1.0, Ni: 0.005-1.0, Mo: 0.02-1.0,
As crystal and grain boundary carbide refined element group,
Nb: 0.003-0.2, V: 0.001-0.2,
As an inclusion form control element group,
Ca: 0.0001 to 0.02, Mg: 0.0001 to 0.02, Zr: 0.0001 to 0.02, REM: 0.0001 to 0.02
It is a steel selected from one or two or more element groups, and containing one or more elements in each selected element group and satisfying the formula (A) Hot-rolled steel sheet for machine structural steel pipes with excellent fatigue characteristics and bending formability.
3C ≦ 0.27Mn + 0.2Cr + 0.05Cu + 0.11Ni + 0.25Mo ≦ 3C + 0.3 (A)
However, the values of C, Mn, Cr, Cu, Ni, and Mo in the formula (A) are mass%.

(3) A hot-rolled steel sheet for a machine-structured steel pipe excellent in fatigue characteristics and bending formability, wherein the machine-structured steel pipe is an automobile structure steel pipe in the above (1) or (2 ).

(4) A hot-rolled steel sheet for machine structural steel pipes having excellent fatigue characteristics and bending formability, characterized in that the automobile structure is an automobile underbody part in (3 ) above.

(5) By mass%, C: 0.05 to 0.19, Si: 0.05 to 1.0, Mn: 0.3 to 2.5, P: 0.03 or less, S: 0.025 or less, Ti: 0.005 to 0.1, Cr: 0.03 to 1.0, Sol. Al: 0.005-0.1, N: 0.0005-0.01, B: 0.0001-0.01, satisfying the formula (A), and heating the steel slab composed of the remaining Fe and inevitable impurities to 1070 ° C or higher and 1300 ° C or lower, and then finish rolling temperature Fatigue properties and bend formability, characterized in that the steel is hot-rolled at a temperature of 850 ° C to 1070 ° C and cooled to 300 ° C or less at the cooling rate Vc (° C / sec) shown in equation (B) after finish rolling The manufacturing method of the hot-rolled steel plate for machine-structure steel pipes excellent in.
3C ≦ 0.27Mn + 0.2Cr + 0.05Cu + 0.11Ni + 0.25Mo ≦ 3C + 0.3 (A)
1.2 / C ≦ Vc ≦ 1.8 / C (B)
However, the values of C, Mn, Cr, Cu, Ni, and Mo in the formulas (A) and (B) are mass%.

(6) By mass%, C: 0.05 to 0.19, Si: 0.05 to 1.0, Mn: 0.3 to 2.5, P: 0.03 or less, S: 0.025 or less, Ti: 0.005 to 0.1, Cr: 0.03 to 1.0, Sol. Al: 0.005-0.1, N: 0.0005-0.01, B: 0.0001-0.01, and further
As a group of bainite generation promoting elements,
Cu: 0.005-1.0, Ni: 0.005-1.0, Mo: 0.02-1.0
As crystal and grain boundary carbide refined element group,
Nb: 0.003-0.2, V: 0.001-0.2,
As an inclusion form control element group,
Ca: 0.0001 to 0.02, Mg: 0.0001 to 0.02, Zr: 0.0001 to 0.02, REM: 0.0001 to 0.02
Selected from one or two or more element groups, containing one or more elements in each selected element group, satisfying formula (A), and comprising the balance Fe and inevitable impurities After heating the steel slab to 1070 ° C or higher and 1300 ° C or lower, it is hot-rolled at a finish rolling temperature of 850 ° C or higher and 1070 ° C or lower. A method for producing a hot-rolled steel sheet for a machined steel pipe excellent in fatigue characteristics and bending formability, characterized by cooling to 300 ° C or lower.
3C ≦ 0.27Mn + 0.2Cr + 0.05Cu + 0.11Ni + 0.25Mo ≦ 3C + 0.3 (A)
1.2 / C ≦ Vc ≦ 1.8 / C (B)
However, the values of C, Mn, Cr, Cu, Ni, and Mo in the formulas (A) and (B) are mass%.

(7) In the method according to (5) or (6) above, the method of manufacturing a hot-rolled steel sheet for a mechanical structure steel pipe having excellent fatigue characteristics and bending formability, wherein the mechanical structure steel pipe is an automobile structural steel pipe .

(8) A method for producing a hot-rolled steel sheet for a machine-structured steel pipe excellent in fatigue characteristics and bending formability, characterized in that the automobile structure is an automobile undercarriage part in the method described in (7) above.
However, in the above (5), 6, (7), and (8) , in the steel surface layer portion, 80% or more of the area fraction of the microstructure is bainite, and the Vickers hardness Hv is 210 or more. For machine structural steel pipes, automobile structural steel pipes, or automobile undercarriage parts steel pipes with an average value of 300 or less, average length of bainite of 5 μm or less, and an average grain boundary carbide particle size of 0.5 μm or less It is a manufacturing method suitable for hot-rolled steel sheets (including steel strips). The Vickers hardness Hv is a value at a depth position of 0.05 mm from the steel surface.

The hot-rolled steel sheet for machine structural steel pipes of the present invention, the hot-rolled steel sheet for automobile structural steel pipes, or the hot-rolled steel sheet for automobile undercarriage parts steel pipes is a surface layer portion of steel (part from the steel surface to a depth of 0.05 mm, but the ground When there is a scale layer or a function-imparting coating layer (plating layer, thermal spray layer, nitriding layer, etc.) on the iron surface, the part up to 0.05mm deep from the iron surface interface, but the Vickers hardness Hv is Since the following requirements (a), (b), (c), and (d) are all satisfied at a depth of 0.05 mm from the interface, both fatigue characteristics and bend formability are excellent.
(a) The bainite structure is 80% or more (100% or less) of the microstructural area fraction.
(b) Vickers hardness Hv is 210 or more and 300 or less.
(c) The average long axis length of bainite is 5 μm or less.
(d) The average grain boundary carbide particle size is 0.5 μm or less.
However, as for the hardness of requirement (b), it is difficult to obtain a Vickers indentation in the vicinity of the steel surface (or the interface), so the Hv value at a depth of 0.05 mm from the steel surface is set to the surface layer Vickers hardness of the steel. Hv should be used.

  The hot-rolled steel sheet for steel pipes (including steel strips) of the present invention is less prone to fatigue cracks, has a very slow fatigue crack growth rate, and is excellent in fatigue characteristics. The reason for this is that, according to requirement (a), the structure of the steel surface layer is almost uniform and bainite structure, so fatigue damage is not localized, and Hv is 210 or more as in requirement (b). The steel surface layer that is the starting point is hard and fatigue cracks are unlikely to occur. Furthermore, since the average value of the long axis length of bainite is 5 μm or less as in requirement (c), the area of the grain boundary that increases fatigue crack growth resistance is large, and in addition, the average grain boundary as in requirement (d). Since the carbide grain size is as fine as 0.5 μm or less, the grain boundary strength is high and the fatigue crack growth resistance is high. Since it is difficult for microvoids to form in the steel surface layer after bending the steel pipe, the generation and propagation of fatigue cracks caused by these microvoids can be suppressed.

  Moreover, the hot-rolled steel sheet for each steel pipe of the present invention is excellent in bending formability. The reason is that the requirements (a) and (b) are satisfied, so the structure of the steel surface layer portion is almost uniform, the hardness of the steel surface layer portion is Hv300 or less, and the hardness is not excessively hard. Further, since the requirement (c) and the requirement (d) are satisfied, the crystal of the steel surface layer portion that becomes the starting point of cracking during bending is fine and tough, and the strength of the grain boundary is high due to the fine grain boundary carbide. In addition, when the requirements (a), (b), (c), and (d) are combined, it is possible to prevent generation of microvoids in the steel surface layer portion that does not lead to work cracking.

  The above requirements (a), (b), (c), (d) are all the requirements for the steel surface layer part, but other than the steel surface layer part (for example, the center part of the plate thickness, 1/4 part, etc.) ) May be satisfied. Even if one or more of the above-mentioned limiting requirements (a), (b), (c), (d) are satisfied at the central part or ¼ part of the thickness of the steel sheet for steel pipes, the present invention It does not deviate from.

  Therefore, the hot-rolled steel sheet (including steel strip) obtained by the present invention is excellent in both fatigue characteristics and bending formability, which are contradictory characteristics. Therefore, mechanical structural members, automotive structural members, Suitable as material for undercarriage parts (for example, steel pipe or steel sheet for steel pipe). For example, as the material of the irregular cross-section cylindrical part shown in [Technical field to which the invention belongs] in Patent Document 1, the hot-rolled steel sheet (including steel strip) of the present invention and a steel pipe made of the steel sheet (including steel strip) can be applied. Since the steel sheet (including steel strip) of the present invention has sufficient bending formability as a material, it is bent even when the mechanical structural member, for example, an automobile structural member or an automobile undercarriage part has a small bending radius R. Sometimes molding cracks can be prevented. Further, although bending does not lead to cracking, the formation of local microvoids in the steel surface layer that lowers the fatigue properties of the member or part can be suppressed. It has sufficient fatigue properties as a component or member. For this reason, the steel pipe which consists of the steel plate of this invention, and the member or component which consists of the steel pipe can abbreviate | omit heat processing, such as hardening after hardening and high intensity | strength. By omitting the heat treatment, the heat treatment cost can be reduced. Furthermore, there are many advantages such as not being able to prevent the oxide scale from adhering during heat treatment and not deteriorating the appearance quality of the member or component, and also preventing the shape change caused by the heat treatment.

The surface layer portion of the steel of the present invention is a portion from the steel surface to a depth of 0.05 mm, but the requirement (b) may be a 0.05 mm depth position from the steel surface. Even if the center part of steel plate thickness, 1/4 structure, hardness, crystal grain size, and precipitates change, fatigue characteristics and bending formability are hardly changed.
As described above, the present inventors have found that a steel surface layer portion is formed by bending a steel pipe in a member or part obtained by bending a raw steel pipe even when a minute void (for example, a void diameter of 1 μm or less) exists in the steel. It was newly found that only the fine voids formed on the steel affect the deterioration of fatigue characteristics.
According to the present invention, a steel pipe made of a hot rolled steel sheet having a thickness of 0.7 to 20 mm and a hot rolled steel sheet (including a steel strip) having the thickness is possible, and the tensile strength is 590 MPa class, 685 MPa class, 780 MPa class, 865 MPa. It is suitable for a hot-rolled steel sheet for a high-grade steel pipe.

In the present invention, the reasons for limiting the requirements (a) to (d) relating to the structure, hardness, crystal grain size, and grain boundary carbide of the surface layer portion of steel will be described below.
FIG. 1 and FIG. 2 show the relationship between the bainite microstructure fraction of the steel surface layer and the fatigue limit, which is an index of fatigue characteristics, and the limit bending rate, which is an index of bending formability. FIG. 1 and FIG. 2 show the results when the requirements (b), (c), and (d) of the surface layer portion of the steel are satisfied in the vicinity of the range boundary. In addition, the bainite structure surface fraction is the value of the steel surface layer part (within 0.05 mm depth from the iron-iron interface).
As shown in FIG. 1, the fatigue limit is rapidly improved until the area fraction of bainite reaches 80%, and becomes an extremely high value of 400 MPa or more when the area fraction of bainite is 80%. Further, as shown in FIG. 2, the critical bending strain is rapidly increased to an area fraction of bainite up to 80%, and increases to 35% or more at 80%. The structure area fraction of bainite is preferably close to 100%, and may be 100%, but the remaining structure of bainite contains 20% or less of one or more of ferrite, pearlite, martensite, and retained austenite. Even in this case, the effect of the present invention can be sufficiently obtained when the surface layer portion of the steel satisfies the requirements (b), (c) and (d). For this reason, the structural area fraction of bainite, which is the requirement (a) of the steel surface layer portion, needs to be 80% or more.
When a phase softer than bainite is excessively present in the steel surface layer, microvoids and fatigue cracks are likely to occur in the soft phase. Further, when a phase harder than bainite is excessively present in the steel surface layer portion, microvoids and fatigue cracks are likely to occur at the interface between the hard phase and the bainite phase or in the vicinity of the interface. Phases softer than bainite include ferrite, pearlite, and stable retained austenite, and phases harder than bainite include martensite and unstable retained austenite that generates work-induced martensite.

Steel surface layer Vickers hardness Hv is moderate between 210 and 300, so it is for members or parts even though the steel surface layer where the fatigue crack starts as a member or part is relatively hard. Microvoids do not occur due to the combination of requirements (c) and (d) below when bending materials (hot-rolled steel sheets for steel pipes), and even when subjected to fatigue loads during use as members or parts It is difficult for fatigue cracks due to microvoids to occur and propagate in the steel surface layer.
If the Vickers hardness Hv is less than 210, even if the surface area fraction of the bainite in the surface layer is 80% or more, the surface layer is too soft and fatigue cracks are generated in the surface layer when subjected to fatigue load. Because it is easy, the fatigue limit is considerably lowered. On the other hand, when the Vickers hardness Hv exceeds 300, the bending formability is extremely lowered. For this reason, the Vickers hardness Hv, which is the requirement (b) of the surface layer portion of steel, needs to be 210 or more and 300 or less.

When the average value of the major axis length of bainite is 5 μm or less, the area of the crystal grain boundary that increases the fatigue crack propagation resistance is large, and even a bainite-based structure is excellent in bending formability.
FIG. 3 and FIG. 4 show the relationship between the average value of the long axis length of bainite in the surface layer portion of the steel, the fatigue limit that is an index of fatigue characteristics, and the limit bending rate that is an index of bending formability. The long axis length of bainite is the value of the steel surface layer portion (within 0.05 mm depth from the iron-iron interface).
As shown in FIG. 3, when the average value of the ellipse major axis length exceeds 5 μm, the fatigue crack growth resistance is remarkably reduced, so that the fatigue limit is rapidly reduced from 400 MPa as the average value is increased. The average value is preferably small, but if it is 5 μm, the effect of crystal refinement can be obtained. Further, as shown in FIG. 4, when the average value of the major axis length of the bainite exceeds 5 μm, the bend formability is remarkably deteriorated because the area of the crystal grain boundary is small. Therefore, the average value of the major axis length of bainite in the surface layer portion of steel needs to be 5 μm or less. Incidentally, even if the average value of the major axis length is extremely fine, for example, 0.01 μm, the fatigue characteristics are not adversely affected. The major axis length of the bainite may be the major axis length in the crystal elongation direction of the bainite structure (for example, the main rolling direction in the plate thickness section of the steel plate).

When the average grain boundary carbide grain size of the steel surface layer is 0.5 μm or less, the grain boundary strength is high and the fatigue crack growth resistance is high, so the fatigue characteristics are excellent.
FIG. 5 and FIG. 6 show the relationship between the average grain size of grain boundary carbides in the surface layer portion of steel, the fatigue limit that is an index of fatigue characteristics, and the limit bending rate that is an index of bending formability. The average grain boundary carbide particle size is the value of the steel surface layer portion (within 0.05 mm depth from the base iron interface).
When the average particle size exceeds 0.5 μm, voids are generated along the grain boundaries of bainite on the surface layer of the steel during the bending test, and cracks develop along the voids during the fatigue test. And fatigue properties are drastically reduced. The average grain size of the grain boundary carbide is preferably smaller, but even if it is extremely fine, for example, 0.005 μm, in the members and parts made of the steel pipe using the steel plate of the present invention, the bending formability is not impaired. Good fatigue properties can be obtained. Since the strength of the crystal grain boundary is high at the time of bending, it is difficult to break, and the strength of the crystal grain boundary is also high at the time of fatigue test, so the resistance to crack propagation is high.

Below, the method of calculating | requiring the bainite surface fraction of a surface layer, hardness, a crystal grain size, and a grain-boundary carbide particle size is described with reference.
The structure area fraction of bainite is corroded with a 3% nital solution after embedding and polishing the plate thickness cross section, and the surface layer portion of the steel (part from the steel surface to a depth of 0.05 mm by the optical microscope at 400 times, however, If there is a scale layer or coating layer (plating layer, thermal spray layer, nitriding layer, etc.) on the surface of the ground iron, the microstructure of the portion from the ground surface to 0.05 mm deep) is observed, and bainite The area ratio of the part was obtained by quantification.
The Vickers hardness is not appropriate on the surface because it is easily affected by the scale layer and the coating layer on the base iron other than steel. For this reason, the hardness of the surface layer part of the steel sheet of the present invention is 50 gf with a micro Vickers measuring machine (based on JIS Z 2244) at a depth of 0.05 mm from the steel ground interface where the correct hardness can be measured. The load (test force 0.4903N) was measured.
The average long axis length of bainite is determined by observing the crystal orientation distribution image by EBSP (Electron Back Scattering Pattern) method, specifying the grain boundary of the crystal from the hue difference due to the difference in crystal orientation, The grain size in the major axis length direction) was measured and the average value was calculated.
The average grain boundary carbide grain size is corroded with a 3% nital solution after embedding the plate thickness cross section, observed with a scanning electron microscope at a magnification of 2000 times, and present at grain boundaries in the range of 50 μm × 50 μm The particle size of the carbide to be measured was measured in five fields of view, and the average value was obtained.

  The fatigue characteristics and bend formability of the hot-rolled steel sheet for steel pipes may be evaluated by performing a bending fatigue test and a bend form test. In the bending fatigue test, a test piece having a parallel portion at the center in the longitudinal direction was sampled from the steel sheet comprising the surface layer portion, and the test was performed by changing the stress conditions with both swings at a frequency of 30 Hz to obtain the fatigue limit. In the bending test, a steel plate with a thickness of t = 1.2 mm was pushed with a V-shaped punch with various curvature radii R at the tip, and the limit R at which cracking occurred was investigated. The magnitude of the surface bending strain (limit bending strain) was calculated and evaluated. Judgment criteria to be good were to achieve both fatigue characteristics with a fatigue limit of 400 MPa or more and bend formability with a limit bending strain of 35% or more.

Next, a steel composition suitable for simultaneously satisfying the structure, hardness, grain size, and grain boundary carbide grain size of the steel surface layer of the present invention will be described. Although the following steel composition may be the composition of the whole steel plate as usual, only a surface layer part may be this steel composition.
C is preferably 0.05% or more in order to obtain a strength level (for example, 590 MPa class, 690 MPa class, 780 MPa class, 865 MPa class) required for the steel sheet. On the other hand, if it exceeds 0.19%, the formation control of the bainite is difficult, the formation of hard martensite increases, the surface layer hardness becomes excessive, the bend formability may be impaired, and the toughness is reduced, resulting in fatigue characteristics. Affects. A preferable range of C is 0.05 to 0.19%.
It is effective to contain 0.05% or more of Si as a deoxidizing element for suppressing coarse oxides that hinder bending workability and fatigue characteristics. On the other hand, if over 1.0% is added, there is a possibility that defects due to SiO ₂ may occur in the welded part during steel pipe production (for example, during ERW welding). Therefore, a preferable range of Si is 0.05 to 1.0%.
Mn is effective for securing hardenability and obtaining a bainite structure, and for that purpose, 0.3% or more is desirable. If it exceeds 2.5%, defects due to MnO ₂ and center segregation due to MnS become prominent. A preferable range of Mn is 0.3 to 2.5%.
P tends to concentrate at the grain boundaries, and if it exceeds 0.03%, the fatigue strength of the grain boundaries may be reduced. For this reason, P is preferably 0.03% or less. On the other hand, if S exceeds 0.025%, coarse MnS may be formed to impair bending formability and fatigue characteristics. For this reason, S is preferably 0.025% or less.

  Ti is effective in suppressing the coarsening of the austenite grain size and achieving finer crystals of the bainite structure in the surface layer portion of the steel. In order to acquire this effect, it is desirable to contain 0.005% or more. On the other hand, if it exceeds 0.1%, the above-mentioned crystal refining effect is almost saturated, and coarse TiN may be generated to reduce fatigue characteristics and bending formability. For this reason, Ti is preferably 0.005 to 0.1%.

  Cr is effective for improving hardenability and obtaining a fine bainite structure, and is also effective for refining grain boundary carbides. In order to obtain these effects, the Cr content is preferably 0.03% or more. On the other hand, if the Cr content exceeds 1.0%, the hard martensite surface area fraction in the surface layer of the steel is greatly increased, making it difficult to obtain the desired bainite structure area fraction. There is. Further, the coarsened carbides increase, and the effect of improving the bending formability and fatigue characteristics due to the increase in grain boundary strength may be reduced, or conversely, the bending formability and fatigue characteristics may be hindered. Cr is preferably 0.03 to 1.0%.

  Sol. Al and N produce AlN in the manufacturing process of steel materials (for example, hot-rolled steel sheets for steel pipes and steel pipes) that are the materials of the above-mentioned members and parts, and suppress the coarsening of austenite grains, thereby reducing the grain size of the bainite structure. It is effective to promote the conversion. If the Al content is less than 0.005%, the effect is not always sufficient. If the Al content exceeds 0.1% and N exceeds 0.01%, the cleanliness of the steel decreases and coarse AlN is generated, resulting in bending formability and / or fatigue properties. May decrease. N is sufficient to be 0.0005% or more in order to utilize the effect of suppressing the coarsening of the austenite grains of AlN. Sol. Al is desirably 0.005 to 0.1%, and N is desirably 0.0005 to 0.01%.

  B is an extremely effective element for improving the hardenability of the steel and obtaining a fine bainite structure. If B is less than 0.0001%, the effect is not necessarily sufficient, and if it exceeds 0.01%, coarse boride ( Boron carbides, boride carbides, boride carbonitrides, etc.) are easily generated and hardenability is impaired. Also, when bending and fatigue loads are applied, cracks and microvoids are also generated. easy. B is preferably 0.0001 to 0.01%.

Next, the relational expression (A) of the steel composition effective in finely controlling the average grain size of bainite and grain boundary carbide, which is important in the present invention, will be described. The present inventors paid attention to the bainite forming action of B (boron), and suitable bainite forming parameters for obtaining a fine bainite structure, BP = 0.27Mn + 0.2Cr + 0.05Cu + 0.11Ni + 0.25Mo and the formula (A) Newly found. Since this formula also utilizes the bainite formation promoting action of B, it is effective when B is contained in the steel (at least the surface layer part of the steel) by 0.0001 to 0.01%.
3C ≦ 0.27Mn + 0.2Cr + 0.05Cu + 0.11Ni + 0.25Mo ≦ 3C + 0.3 (A)
However, the values of C, Mn, Cr, Cu, Ni, and Mo in the formula (A) are mass%.
In formula (A), 0.27Mn, 0.2Cr, 0.05Cu, 0.11Ni, and 0.25Mo mean 0.27 × Mn, 0.2 × Cr, 0.05 × Cu, 0.11 × Ni, and 0.25 × Mo, respectively.
When the BP is less than 3C, as shown in FIG. 7 and FIG. 8, the bainite elliptical long axis length and the average grain boundary carbide particle size increase abruptly, and the upper limit requirement limit of the surface layer portion of the present invention is reached. The bainite ellipse major axis length exceeds 5 μm and the average grain boundary carbide particle size exceeds 0.5 μm. In addition, when the BP of the formula (A) exceeds 3C + 0.3, martensite is excessively generated in the surface layer portion, and the bainite structural area fraction tends to be less than 80%, and the Vickers hardness Hv is May exceed 300.

In addition to the basic steel composition, one or more element groups are selected from the following element groups I, II, and III, and one or more of the selected element groups are selected. It is possible to contain these elements.
I. As a bainite production promotion element group, Cu: 0.005-1.0%, Ni: 0.005-1.0%, Mo: 0.02-1.0%.
II. As grain refiner element group, Nb: 0.003~ 0.2%, V : 0.001~ 0.2%. III. Inclusion form control element group, Ca: 0.0001-0.02%, Mg: 0.0001-0.02%, Zr: 0.0001-0.02%, REM: 0.0001-0.02 %.

  Cu, Ni, and Mo of the bainite formation promoting element group of the element group I are all effective in improving the hardenability and generating a bainite structure. The contribution of each element of group I is as in the above formula (A). When Cu, Ni, and Mo are less than 0.005%, less than 0.005%, and less than 0.02%, respectively, the bainite formation promoting action of each element is not sufficiently obtained. On the other hand, when Cu, Ni, and Mo are more than 1.0%, more than 1.0%, and more than 1.0%, a hard phase tends to be generated in a large amount in the surface layer portion of the steel. It is difficult to satisfy 80% or more of the structural fraction of bainite, which is a constituent requirement. Cu can be contained in the steel in the range of 0.005 to 1.0%, Ni in the range of 0.005 to 1.0%, and Mo in the range of 0.02 to 1.0%.

Both the crystal of element group II and Nb and V of grain boundary carbide refined element group are effective for producing fine bainite and fine grain boundary carbide, which are constituent elements of the present invention. For this purpose, it is desirable that Nb is 0.003% or more and V is 0.001% or more . If Nb is more than 0.2% and V is more than 0.2% , coarse carbides are likely to be formed in the steel and become the starting point of cracking during bending molding, or the processing strain during molding is localized near the coarse carbides. There is a concern that the surface quality of the material, member, or part may be lowered, the fatigue damage may be localized during use of the member or part after being processed, and the fatigue characteristics may be lowered. Therefore, V can be contained in the steel in the range of 0.003 to 0.2 % and Nb in the range of 0.001 to 0.2% .

  As the inclusion form control element group of the element group III, Ca can be contained in 0.0001 to 0.02%, Mg in 0.0001 to 0.02%, Zr in 0.0001 to 0.02%, and REM in 0.0001 to 0.02%. Ca, Mg, Zr, and REM all have the effect of improving the formability by controlling the form of sulfide. In order to make use of this action, it is desirable to contain 0.0001% or more of Ca, 0.0001% or more of Mg, 0.0001% or more of Zr, and 0.0001% or more of REM. When these elements are excessively contained, a composite compound with coarse sulfides or clustered oxides of these elements may be formed, and conversely, the bending formability and fatigue characteristics may be lowered. For this reason, it is desirable to contain Ca at 0.0001 to 0.02%, Mg at 0.0001 to 0.02%, Zr at 0.0001 to 0.02%, and REM at 0.0001 to 0.02%.

Next, the suitable manufacturing method of the hot-rolled steel plate used as the raw material of the member or component of this invention is described.
In mass%, C: 0.05 to 0.19, Si: 0.05 to 1.0, Mn: 0.3 to 2.5, P: 0.03 or less, S: 0.025 or less, Ti: 0.005 to 0.1, Cr: 0.03 to 1.0, Sol. A steel slab made of Al: 0.005 to 0.1, N: 0.0005 to 0.01, B: 0.0001 to 0.01 and satisfying the formula (A) and the balance Fe and inevitable impurities is used. The reasons for limiting the steel composition and the formula (A) are as described in the hot rolled steel sheet of the present invention.

The steel slab having the above composition is heated to 1070 ° C. or higher and 1300 ° C. or lower, and then subjected to hot rolling at a finish rolling temperature of 850 ° C. or higher and 1070 ° C. or lower. Fine bainite structure and fine average grain boundary carbide grains It is effective to obtain the diameter.
When steel slabs are heated to 1070 ° C or higher, fine grain boundary carbides can be advantageously obtained in hot-rolled steel sheets by dissolving solid carbide, nitrogen compounds and carbonitride compounds precipitated in the molten steel solidification process. . When a steel slab is heated to over 1300 ° C, AlN precipitates coarsely in the hot rolling process or in the cooling process after rolling, or borides (boron carbide, boron nitride, charcoal, which inhibit the effect of improving the hardenability of B) Boron nitride) may be formed, which is not desirable. The heating during the hot rolling of the steel slab is preferably from 1070 ° C to 1300 ° C.
In order to produce a large amount of fine bainite, the hot rolling is desirably performed in a temperature range of 850 ° C. or higher, which is substantially an austenite single phase and a recrystallization region. On the other hand, if it exceeds 1070 ° C., bainite is likely to be coarse and the crystal may be difficult to control finely. Therefore, the finish rolling temperature of hot rolling is preferably 850 ° C. or higher and 1070 ° C. or lower.

Furthermore, it is effective to produce fine bainite and fine grain boundary carbide by controlling the cooling of the steel sheet after hot rolling to 300 ° C. or less with the cooling rate Vc (° C./s) of the formula (B).
When the cooling rate Vc (° C./s) exceeds 1.8 / C (C represents mass% of C), as shown in FIG. 10 and FIG. The bainite structural area fraction of the part tends to be less than 80%. The martensite surface fraction of the surface layer portion is remarkably increased and the desired bainite surface fraction cannot be obtained, and the surface layer hardness Hv exceeds 300, so that sufficient bending formability cannot be obtained. When Vc is less than 1.2 / C (C represents mass% of C), as shown in FIGS. 10 and 11, a large amount of a phase softer than bainite is generated, and the bainite area fraction of the surface layer portion is increased. It tends to be less than 80%. Formation of ferrite and pearlite in the surface layer is remarkably increased, and the desired surface area fraction of bainite cannot be obtained, and the surface layer hardness Hv is less than 210, which is easy to generate and grow cracks from the surface layer part under fatigue load. Fatigue characteristics cannot be obtained. The Vickers hardness Hv is a value of the steel surface layer portion (position at a depth of 0.05 mm from the base iron interface).
Even when Vc is within the invention range, when the cooling stop temperature shown in FIG. 9 exceeds 300 ° C., the grain boundary carbide is not sufficiently refined and fine bainite cannot be obtained sufficiently. Therefore, it is desirable that the hot-rolled steel sheet having the above steel composition is controlled to be cooled to Vc satisfying the formula (B) of the present invention up to 300 ° C. or less. Even if the controlled cooling at Vc according to the present invention is stopped at a temperature within the range from room temperature to 250 ° C, the hot rolled steel sheet coil is maintained at a temperature range of 300 ° C or lower (for example, stacking of hot rolled coils) deviates from the present invention. Not what you want. Further, even when a simple heat treatment in which the temperature of the steel sheet surface layer rises in a temperature range of 300 ° C. or less is added to the steel sheet or the steel pipe made of the steel sheet by surface care or removal of residual stress, it does not depart from the present invention.

  The hot-rolled steel sheet of the present invention is effective as a steel sheet for machine structural use, for example, a steel sheet for automobile structural use, a steel sheet for automobile undercarriage parts steel pipe.

  The hot-rolled steel sheet of the present invention is effective when applied to steel pipes for machine structures, for example, steel pipes for automobile structures, steel pipes used for automobile undercarriage parts, but both fatigue characteristics and bend formability are also required. For example, it is needless to say that the present invention is effective even when applied to steel pipe parts of transport vehicles such as airplanes and railways that are transport machines. Moreover, the steel sheet for machine structural steel pipes excellent in fatigue characteristics and bend formability of the present invention is adjusted within the scope of the present invention by adjusting the components and hot rolling conditions to increase the surface layer hardness within the scope of the present invention. Therefore, it is possible to obtain a hot-rolled steel sheet or steel pipe for steel pipes that can be applied to machine structural members / parts where fatigue characteristics are particularly important, and by making the surface layer surface hardness low within the scope of the present invention. In particular, it is possible to provide a hot-rolled steel sheet for steel pipes and steel pipes that can be applied to machine structural members and parts where bending formability is particularly important.

  Steels having the components shown in Table 1 were made into 30 kg ingots in a vacuum melting furnace. The steel ingot was heated and then hot rolled to a thickness of 4 mm and 1.2 mm, and then cooled to obtain a hot rolled steel sheet. During cooling, the amount of cooling water was controlled so that the cooling rate was the same for the 4 mm thick material and the 1.2 mm thick material. For this reason, the structure of the surface layer portion of 4 mm material and 1.2 mm material, hardness Hv, crystal grain size of bainite, and grain boundary carbide diameter were almost the same. Table 2 shows the structure of the steel surface layer, the Vickers hardness Hv, the crystal grain size of bainite, and the grain boundary carbide grain size obtained with a 4 mm thick steel plate. In Tables 1 and 2, those that depart from the requirements of the present invention are underlined. In Table 1, the blank for the selected element indicates no addition.

  The area fraction of bainite was obtained by embedding and polishing the plate thickness section, corroding with a 3% nital solution, observing the microstructure with an optical microscope at 400 times, and quantifying the area ratio of the bainite portion. The Vickers hardness was measured by measuring the hardness at a position of 0.05 mm from the plate surface with a load of 50 gf (test force 0.49 N) with a micro Vickers measuring instrument (based on JIS Z 2244). The average long axis length of bainite is determined by observing the crystal orientation distribution image by EBSP (Electron Back Scattering Pattern) method, specifying the grain boundary of the crystal from the hue difference due to the difference in crystal orientation, The grain size in the major axis length direction) was measured and the average value was calculated. The average grain boundary carbide particle size is determined by embedding and polishing the plate thickness section from which the scale on the steel surface has been removed with acid, and then corroding with 3% nital solution. The microstructure of five fields of view was observed at a magnification of 2000, and the equivalent circle diameter of carbides present at the grain boundaries in the range of 50 μm × 50 μm was measured, and the average value was determined.

  In order to evaluate fatigue characteristics and bend formability of the obtained plate, a bending fatigue test and a bend form test were performed. In the bending fatigue test, a t = 4 mm plate was tested with a test piece having a parallel portion in the central portion in the longitudinal direction of the plate at a frequency of 30 Hz with both swings, changing the stress conditions, and determining the fatigue limit. In the bending test, a t = 1.2 mm plate was pushed with a V-shaped punch with various Rs at the tip, and the limit R at which cracking occurred was investigated, and the bending strain on the outermost surface at that time was investigated. The size of was calculated and evaluated. The target value is to achieve both fatigue characteristics with a fatigue limit of 400 MPa or more and bending formability with a limit bending strain of 35% or more. Table 2 shows the absolute value and evaluation of the test results for the fatigue limit and the critical bending strain. The fatigue limit was evaluated by ◯ for 400 MPa or more and x for less than 400 MPa. Further, the evaluation of the critical bending strain was indicated by ◯ for 35% or more and by × for less than 35%.

The test results are shown in Table 2. The steel sheet of the present invention is substantially composed of a bainite structure, has a small structure variation, has an appropriate surface hardness, has a fine bainite ellipse major axis length average value, an average grain boundary carbide grain size, and a high grain boundary strength. Therefore, as shown in Table 2, the product of the present invention can achieve both fatigue characteristics with a fatigue limit of 400 Mpa or more and bending formability with a limit bending strain of 35% or more.

  On the other hand, the structure area fraction of bainite, Vickers hardness, average value of bainite elliptical long axis length, average grain boundary carbide particle size, component or part of hot rolling-cooling conditions deviate from the scope of the present invention. In the comparative example, the fatigue characteristics and the bending formability cannot be compatible. In Comparative Example 1, since the amount of C is too large, the surface layer portion has a Vickers hardness of more than 300 and is too hard, the average grain boundary carbide particle size is too large, and the moldability is insufficient. In Comparative Example 2, the hardenability is insufficient due to the small amount of addition of B, the structure area fraction of bainite is too low and the structure area fraction of ferrite is high, and sufficient surface layer hardness cannot be obtained. . In addition, since the average long axis length of bainite and the average grain boundary carbide particle size are not sufficiently fine at the surface layer portion, fatigue characteristics are insufficient. Since Comparative Example 3 does not satisfy the formula (A) because Mn is small, the structure area fraction of bainite is too small, the structure area ratio of ferrite is high, and sufficient surface layer portion hardness cannot be ensured. Moreover, since the average long axis length of bainite and the average grain boundary carbide particle size are not sufficiently refined, the fatigue characteristics are insufficient. In Comparative Example 4, since the heating temperature is low, the precipitation of the precipitate in the steel is not sufficiently formed, the bainite crystal of the additive element and the grain boundary carbide is not sufficiently refined, the length of the bainite The average axis length and the average grain boundary carbide particle size could not be sufficiently refined. As a result, the fatigue characteristics were insufficient. In Comparative Example 5, since the finish rolling temperature was low, the steel surface layer portion was a structure mainly composed of ferrite, and sufficient hardness could not be secured, resulting in insufficient fatigue characteristics. In Comparative Example 6, since the cooling rate is slow at less than 1.2C, the structure area fraction of bainite is too small and the structure area fraction of ferrite increases, and the average long axis length of bainite and the average grain boundary carbide particle size However, the fatigue characteristics were not sufficient. In Comparative Example 7, since the bainite transformation occurred at a high temperature because the cooling stop temperature was high, the average grain boundary carbide particle size became coarse, so that the fatigue characteristics were insufficient.

It is the figure which showed the relationship between the structure area fraction of bainite, and a fatigue characteristic. It is the figure which showed the relationship between the structure area fraction of bainite, and bending formability. It is the figure which showed the relationship between the long axis length average value of a bainite, and a fatigue characteristic. It is the figure which showed the relationship between the long axis length average value of bainite, and bending formability. It is the figure which showed the relationship between an average grain boundary carbide particle size and fatigue characteristics. It is the figure which showed the relationship between an average grain boundary carbide particle size and bending formability. (A) It is the figure which showed the relationship between Formula BP and the long-axis length of bainite. (A) It is the figure which showed the relationship between Formula BP and an average grain boundary carbide particle size. It is a schematic diagram which shows the heat history of hot rolling and cooling. It is the figure which showed the relationship between Vc and the structure area fraction of a bainite. It is the figure which showed the relationship between Vc and hardness.

Claims (8)

In mass%, C: 0.05 to 0.19, Si: 0.05 to 1.0, Mn: 0.3 to 2.5, P: 0.03 or less, S: 0.025 or less, Ti: 0.005 to 0.1, Cr: 0.03 to 1.0, Sol. Al: 0.005-0.1, N: 0.0005-0.01, B: 0.0001-0.01, satisfying (A), and in the surface layer portion of steel composed of the remaining Fe and inevitable impurities , 80% or more of the area fraction of the microstructure is bainite Fatigue characteristics, characterized in that Vickers hardness Hv is 210 or more and 300 or less, the average value of the long axis length of bainite is 5 μm or less, and the average grain boundary carbide particle size is 0.5 μm or less. Hot-rolled steel sheet for machine structural steel pipes with excellent bend formability.
3 C ≦ 0.27Mn + 0.2Cr + 0.05Cu + 0.11Ni + 0.25Mo ≦ 3C + 0.3 (A)
However, the values of C, Mn, Cr, Cu, Ni, and Mo in the formula (A) are mass%. The steel according to claim 1 is further in mass%,
As a group of bainite generation promoting elements,
Cu: 0.005-1.0, Ni: 0.005-1.0, Mo: 0.02-1.0,
As crystal and grain boundary carbide refined element group,
Nb: 0.003-0.2, V: 0.001-0.2,
As an inclusion form control element group,
Ca: 0.0001 to 0.02, Mg: 0.0001 to 0.02, Zr: 0.0001 to 0.02, REM: 0.0001 to 0.02
Characterized in that the steel is selected from one or two or more element groups, contains one or more elements in each selected element group, and satisfies the formula (A) Hot-rolled steel sheet for machine structural steel pipes with excellent fatigue properties and bending formability.
  3C ≦ 0.27Mn + 0.2Cr + 0.05Cu + 0.11Ni + 0.25Mo ≦ 3C + 0.3 (A)
However, the values of C, Mn, Cr, Cu, Ni, and Mo in the formula (A) are mass%. The hot-rolled steel sheet for machine structure steel pipes according to claim 1 or 2, wherein the machine structure steel pipe is an automobile structure steel pipe and has excellent fatigue characteristics and bending formability.   The hot-rolled steel sheet for a machine-structure steel pipe excellent in fatigue characteristics and bending formability, wherein the automobile structure is an automobile underbody part according to claim 3. In mass%, C: 0.05 to 0.19, Si: 0.05 to 1.0, Mn: 0.3 to 2.5, P: 0.03 or less, S: 0.025 or less, Ti: 0.005 to 0.1, Cr: 0.03 to 1.0, Sol. Al: 0.005-0.1, N: 0.0005-0.01, B: 0.0001-0.01, satisfying the formula (A), and heating the steel slab composed of the remaining Fe and inevitable impurities to 1070 ° C or higher and 1300 ° C or lower, and then finish rolling temperature Fatigue properties and bend formability, characterized by performing hot rolling at a temperature of 850 ° C to 1070 ° C and cooling to 300 ° C or less at the cooling rate Vc (° C / sec) shown in equation (B) after finish rolling The manufacturing method of the hot-rolled steel plate for machine-structure steel pipes excellent in.
  3C ≦ 0.27Mn + 0.2Cr + 0.05Cu + 0.11Ni + 0.25Mo ≦ 3C + 0.3 (A)
  1.2 / C ≦ Vc ≦ 1.8 / C (B)
However, the values of C, Mn, Cr, Cu, Ni, and Mo in the formulas (A) and (B) are mass%. In mass%, C: 0.05 to 0.19, Si: 0.05 to 1.0, Mn: 0.3 to 2.5, P: 0.03 or less, S: 0.025 or less, Ti: 0.005 to 0.1, Cr: 0.03 to 1.0, Sol. Al: 0.005-0.1, N: 0.0005-0.01, B: 0.0001-0.01, and further
As a group of bainite generation promoting elements,
Cu: 0.005-1.0, Ni: 0.005-1.0, Mo: 0.02-1.0,
As crystal and grain boundary carbide refined element group,
Nb: 0.003-0.2, V: 0.001-0.2,
As an inclusion form control element group,
Ca: 0.0001 to 0.02, Mg: 0.0001 to 0.02, Zr: 0.0001 to 0.02, REM: 0.0001 to 0.02
Selected from one or more element groups, containing one or more elements in each selected element group, and satisfying formula (A), from the remaining Fe and inevitable impurities The steel slab is heated to 1070 ° C or higher and 1300 ° C or lower, and then subjected to hot rolling at a finish rolling temperature of 850 ° C or higher and 1070 ° C or lower. After finish rolling, the cooling rate Vc (° C / sec) shown in the formula (B) A method for producing a hot-rolled steel sheet for a steel pipe for a mechanical structure having excellent fatigue characteristics and bending formability, characterized by cooling to 300 ° C. or lower at a low temperature.
  3C ≦ 0.27Mn + 0.2Cr + 0.05Cu + 0.11Ni + 0.25Mo ≦ 3C + 0.3 (A)
  1.2 / C ≦ Vc ≦ 1.8 / C (B)
However, the values of C, Mn, Cr, Cu, Ni, and Mo in the formulas (A) and (B) are mass%. 7. The method for producing a hot-rolled steel sheet for machine structural steel pipes according to claim 5 or 6, wherein the mechanical structural steel pipe is an automobile structural steel pipe and has excellent fatigue characteristics and bending formability. 8. The method of manufacturing a hot-rolled steel sheet for machine structural steel pipes according to claim 7, wherein the automobile structure is an automobile undercarriage part and has excellent fatigue characteristics and bend formability.

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