WO2009133965A1 - Hot-rolled steel plate for steel tubes for machine structural purposes which is excellent in fatigue characteristics and bending formability and process for production of the plate - Google Patents

Abstract

A hot-rolled steel plate for steel tubes for machine structural purposes which is excellent in fatigue characteristics and bending formability; and a process for production of the plate.  In the surface layer of the hot-rolled steel plate for steel tubes for machine structural purposes, bainite accounts for 80% or above of the microstructure; the Vickers hardness (Hv) is 210 to 300; the average major-axis length of bainite is 5μm or below; and the mean particle diameter of grain boundary carbides is 0.5 μm or below.  Since the surface layer from which crack starts because of fatigue or bending is constituted of a uniform fine bainite-base structure, the plate is excellent in both fatigue characteristics and bending formability.

Description

Title of Invention Hot rolled steel sheet for machine structural steel pipes with excellent fatigue characteristics and bend formability and manufacturing method thereof Technical Field

 The present invention relates to a hot-rolled steel sheet for machine structural steel pipes having excellent fatigue characteristics and bending 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. Background art

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

 For example, an automobile axle beam, which is an automobile underbody part, 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 shown in Patent Document 1, and obtained by the method. There has been proposed an axle beam having a modified cross-section cylindrical body. As this beam material, steel materials with high fatigue characteristics and high bendability (for example, machine structural steel pipes, automobile structural steel pipes, automobile undercarriage parts steel pipes, and steel plates made of these steel pipes) are desired. At present, steel pipes and steel plate materials (including steel strips) that sufficiently satisfy fatigue characteristics and bendability are not obtained. Therefore, after the steel pipe is pressed, hardening heat treatment such as quenching and annealing is performed to improve the fatigue characteristics and strength of the parts to the desired level. When such a heat treatment is carried out, the cost of the parts increases as a matter of course. However, since the heat treatment is performed, the shape of the parts and parts may change and additional correction may be required. There is a problem that additional strengthening means (for example, surface hardening treatment) may be required when the parts are softened. For this reason, machine-structured steel pipes (for example, automobile structural steel pipes, automobile undercarriage parts steel pipes, etc.) with excellent fatigue characteristics and bendability, and steel plates (including steel strips) that serve as steel pipe materials are highly anticipated in the industry. Yes.

In addition, as a steel material (welded steel pipe, steel strip for welded steel pipe material) having both fatigue characteristics and workability required for various forming (workability of bending, hydraulic pressure, pipe expansion, contraction pipe, etc.), Patent Document 2 The invention shown in (2) has been proposed. The invention steel material of Patent Document 2 is a material in which a large amount of ferrite structure in which ultra-fine and specific element ratio (Ti, Mo) composite carbides are dispersed and precipitated is produced. In addition, the area ratio of the ferrite structure, which is a soft phase, is 60 to 100%. It is high and the particle size of the precipitate is very small, so it has excellent moldability. On the other hand, the invention steel material of Patent Document 2 has a large amount of soft ferrite structure, so the remaining part of the structure and the balance become hard, and the hard structure becomes mixed with ferrite. . In such a structure, a hard structure cannot sufficiently tolerate heat during severe bending and cracking may occur. Furthermore, when manufacturing machine structural members, automobile structural members, and automobile undercarriage parts from steel pipes, there may be microvoids in the steel that do not cause cracking even if cracking does not occur in bending. There is. When steel pipes are used as components in parts such as mechanical structures and automobile structures and fatigue loads are applied, fatigue cracks may be generated from the microvoids created by bending during the production of parts. Prior art documents

Patent Literature

 Patent Document 1 Japanese Patent Application Laid-Open No. 2001-321846

 Patent Document 2 JP 2003-321748 A Summary of the Invention

Problems to be solved by the invention

As described above, mechanical structures (for example, automobile structures, automobile undercarriage parts, etc.) Steel pipes and material steel plates for steel pipes (especially hot-rolled steel sheets) that have been able to achieve both sufficient fatigue properties and bendability have been obtained so far. Absent. In general, the fatigue properties of steel materials tend to be better when the strength of the steel material itself or a member or part processed from the steel material is higher, and the bending formability of the steel material tends to be better when the strength is lower. For this reason, it is very difficult to achieve conflicting characteristics. An object of the present invention is to provide a hot rolled steel sheet for machine structural steel pipes and a hot rolled steel for automobile structural steel pipes having excellent fatigue characteristics and excellent bendability. It is to provide hot-rolled steel sheets for steel plates or automobile undercarriage parts steel pipes and a method for producing these steel sheets. Means for solving the problem

 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 automotive structural steel pipes, hot-rolled steel sheets for automobile undercarriage parts steel pipes, and these steel sheets). In order to obtain a steel pipe, the generation of microvoids and fatigue cracks at each thickness part of the hot-rolled steel sheet and the process of fatigue crack growth / progression were continued from before and after bending to the fatigue load. investigated. 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 (depth 0. 0 from the steel surface). It was newly found that it occurs at a depth of 0.05 mm or less from the surface of the railway when there is a surface coating layer within 05 mm. In addition, when manufacturing the above-mentioned steel pipe parts and steel pipe parts, the work cracks when bending the steel pipe are the surface layer of the steel material with the highest processing strain (depth within 0.05 mm from the steel surface, surface coating layer It was newly found that it occurs at a depth within 0.05 mm from the interface of the railway.

Furthermore, it was newly found that the micro-point generated on the steel surface layer during bending forming promotes the generation and development of fatigue cracks when used as a member or part after processing. As a result of various investigations focusing on this knowledge and focusing on the microstructure, hardness, crystal morphology of the structure, the presence of grain boundary precipitates, etc., the present inventors have found fatigue under specific conditions. Newly discovered that hot-rolled steel sheets for machine structural steel pipes, hot-rolled steel sheets for automotive structural steel pipes, or hot-rolled steel sheets for automobile undercarriage parts steel pipes with excellent characteristics and excellent bend formability can be obtained. It is. In other words, it has the desired characteristics by controlling the structure of the steel surface layer to a uniform fine grain structure main body, making it hard to a medium level, and further reducing the grain boundary precipitated carbides. A steel sheet for steel pipe is obtained. In addition, by studying a method for industrially obtaining the specific conditions, the present inventors have obtained a suitable component system and production process. This specific condition of the surface layer portion is sufficiently effective for a steel pipe material steel plate having a steel plate thickness of 0.7 to 20, and the steel plate of the present invention includes a steel strip.

 The gist of the present invention is as follows: (1) In the surface layer portion of steel, 80% or more of the surface area of the microstructure is Paynite, Vickers hardness Hv is 210 or more and 300 or less, and the major axis length of Paynai 卜A hot-rolled steel sheet for machine structural steel pipes with excellent fatigue characteristics and bend formability, characterized in that the average value of is less than 5 m and the average grain size of grain boundary carbides is less than 0.5 m. The surface layer of the steel of the present invention is a portion within 0.05 mm in the thickness direction from the steel surface of the steel plate (including steel strip). However, the Vickers hardness Hv is the value at a depth of 0.05 mm in the thickness direction from the steel surface of the steel plate (including steel strip).

 (2) The steel of (1) above is 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% Ti: 0.005-0.1%, Cr: 0.03-1.0%, So 1, AI: 0.005-0.1%, N: 0.0005-0.01%, B: 0.0001-0.01% A hot-rolled steel sheet for machine structural steel pipes with excellent fatigue characteristics and bend formability, characterized by being a steel composed of the balance Fe and inevitable impurities.

 3C≤0.27Mn + 0.2Cr + 0.05Cu + 0.llNi + 0.25Mo≤3C + 0.3 ... A> However, the values of C, Mn, Cr, Cu, Ni, and Mo in <A> are mass%.

 (3) The steel of (2) above is further mass%,

 As a group of elements for promoting the formation of paynite,

 Cu: 0.005-1.0%, Ni: 0.005-1.0%, Mo: 0.02-1.0%, As a group of refined elements of crystals and grain boundary carbides,

Nb: 0.003 to 0.2%, V: 0.001 to 0.2%, W: 0.001 to 0.1%, 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% A hot-rolled steel sheet for machine structural steel pipes with excellent fatigue characteristics and bending formability, characterized by being a steel that contains one or more of the above elements and satisfies the formula <A>.

 3C≤0.27Mn + 0.2Cr + 0.05CU + 0. llNi + 0.25Mo≤3C + 0.3 ... A> However, the values of C, Mn, Cr, Cu, Ni, and Mo in <A> are mass%.

 (4) In the above (1), (2) or (3), the hot rolled steel sheet for machine structural steel pipes with excellent fatigue characteristics and bending formability, characterized in that the mechanical structural steel pipe is an automobile structural steel pipe .

 (5) 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 undercarriage part in (4) above.

 (6) By mass%, C: 0.05 to 0.19%, Si: 0.05 to 0%, Mn: 0.3 to 2.5%, P: 0.03% or less, S: 0.025% or less, Ti: 0.005 to 0.1%, Cr: A steel slab containing 0.03 to 1.0%, Sol. A1: 0.005 to 0.1%, N: 0.0005 to 0.01%, B: 0.0001 to 0.01%, satisfying the A> formula, and the balance Fe and inevitable impurities. After heating to ° C or higher and 1300 ° C or lower, hot rolling at a finish rolling temperature of 850 ° C or higher and 1070 ° C or lower is performed, and after finish rolling Cooling rate Vc (° C / sec) shown in formula <B> A method for producing hot-rolled steel sheets for machine structural steel pipes with excellent fatigue characteristics and bendability, which is characterized by cooling to 300 ° C or lower.

 3C≤0.27Mn + 0.2Cr + 0.05CU + 0.llNi + 0.25Mo≤3C + 0.3 ... A>

1.2 / C≤Vc≤ 1.8 / 0

However, the values of C, Mn, Cr, Cu, Ni, and Mo in the formulas <A> and B> are mass%.

(7) The steel slab of (6) is in mass%, and As a group of elements for promoting the formation of paynite,

 Cu: 0.005-1.0%, Ni: 0.005-1.0%, Mo: 0.02-1.0%, As a group of refined elements of crystals and grain boundary carbides,

 Nb: 0.003 to 0.2%, V: 0.001 to 0.2%, W: 0.001 to 0.1%, Inclusion form control element group,

 Each element selected from one or more element groups of Ca: '0.0001-0.02%, Mg: 0.0001-0.02%, Zr: 0.0001-0.02%, REM: 0.0001-0.02% A method for producing a hot-rolled steel sheet for a machine structural steel pipe having excellent fatigue characteristics and bending formability, characterized by containing one or more elements in the group.

 (8) The method according to (6) or (7), wherein the mechanical structure steel pipe is an automobile structural steel pipe, and the method for producing a hot rolled steel sheet for a mechanical structure steel pipe having excellent fatigue characteristics and bending formability .

 (9) 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 (8) above.

 However, in the above (6), (7), (8), (9), in the surface layer of steel, 80% or more of the area fraction of the microstructure is the paynite, and the Vickers hardness Hv 210 or more and 300 or less, the average value of the long axis length of bainite is 5 or less, and the average grain boundary carbide particle size is 0.5 2 m or less. Or it is a manufacturing method suitable for hot-rolled steel sheets (including steel strips) for automobile undercarriage parts steel pipes. Vickers hardness Hv is a value at a depth of 0.05 mm from the steel surface. The invention's effect

Hot rolled steel sheet for machine structural steel pipe of the present invention, Hot rolled steel sheet for automobile structural steel pipe, Alternatively, the hot-rolled steel sheet for steel pipes for automobile undercarriage parts is the surface layer of steel (part from the steel surface to a depth of 0.05 mm, but the scale layer and functional coating layer on the surface of the steel surface (metallized layer, sprayed layer) If there is a nitriding layer, etc.), the following requirements (a), (b) in the part from the iron surface interface to 0.05mm depth, where the picker hardness Hv is 0.05mm depth position from the metal surface interface) ), (C), and (d) are all satisfied, both fatigue characteristics and bend formability are excellent.

 (a) More than 80% (100% or less) of the microstructure area fraction is a bait organization.

 (b) Pickers hardness Hv is 210 or more and 300 or less.

 (c) The average long axis length of bainite is 5 m or less.

 (d) Average grain boundary carbide particle size is 0 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 steel interface). Therefore, the Hv value at a depth of 0.05 dragon depth from the steel surface is set to Vickers hardness of the steel surface layer. It should be Hv.

The hot-rolled steel sheet for steel pipes of the present invention (including steel strips) is less prone to fatigue cracks, and the fatigue crack growth rate is extremely slow and has excellent fatigue properties. The reason is that, according to requirement (a), the structure of the steel surface layer is almost uniform and bainitic, so fatigue damage is not localized, and because Hv is 210 or more as in requirement (b), fatigue occurs. The steel surface layer where the crack starts is hard and fatigue cracks are less likely to occur. Furthermore, since the average value of the major axis length of the bait is 5 lim or less as in requirement (c), the area of the grain boundary that increases fatigue crack growth resistance is large, and in addition, as in requirement (d) In addition, the grain boundary carbide average grain size is as fine as 0.5 ^ m or less, so the grain boundary strength is high and fatigue crack growth resistance is high. After bending the steel pipe, it is difficult for microvoids to form in the steel surface layer, so it is possible to suppress the occurrence and propagation of fatigue cracks caused by these microvoids. The hot-rolled steel sheet for steel pipes of the present invention is also excellent in bend formability. The reason is that the requirements (a) and (b) are satisfied, so the structure of the steel surface layer is almost uniform, the hardness of the steel surface layer is Hv300 or less, and the hardness is too high. Furthermore, since the requirement (c) and the requirement (d) are satisfied, the crystal of the steel surface layer which is the starting point of cracking during bending is fine and has high toughness, and the grain boundary strength is high due to the fine grain boundary carbide. In addition, when requirements (a), (b), (c), and (d) are combined, it is possible to prevent the formation of microvoids in the steel surface layer that do not lead to work cracking.

The above requirements (a), (b), (c), (d) are all limited requirements for the steel surface layer, but other than the steel surface layer (for example, the center of the plate thickness, 1 Z 4 parts, etc. You may be satisfied with Even if one or more of the above requirements (a), (b), (c), (d) are satisfied at the center of the plate thickness of steel sheet for steel pipes or 1 Z 4 parts, 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 pipes or steel sheets for steel pipes). For example, as a material for a modified 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) are applicable it can. Since the steel sheet (including steel strip) of the present invention has sufficient bending formability as a raw material, the above-mentioned mechanical structural member, for example, an automobile structural member or an automobile undercarriage part has a portion with a small bending radius R. Forming cracks can be prevented during bending. Also, bending does not lead to cracking, but it can suppress the formation of local microvoids in the steel surface layer that degrade the fatigue properties of the member or part. It has sufficient fatigue characteristics as a component or member. Therefore, the steel pipe made of the steel plate of the present invention, and the member or part made of the steel pipe are Heat treatment such as hardening or strengthening can be omitted. By omitting the heat treatment, the heat treatment cost can be reduced. Furthermore, there are many advantages such as not only that the appearance scale of the member or component can be prevented from being adhered by the oxidation scale during the heat treatment, but also the shape change caused by the heat treatment can be prevented. Brief Description of Drawings

 FIG. 1 is a graph showing the relationship between the microstructure area ratio of the bainite and fatigue characteristics in the present invention.

 FIG. 2 is a graph showing the relationship between the texture area ratio of the vine and the bend formability in the present invention.

 FIG. 3 is a graph showing the relationship between the average value of the long axis length of the bait and the fatigue characteristics in the present invention.

 FIG. 4 is a graph showing the relationship between the average value of the long axis length of the bait and the bending formability in the present invention.

 FIG. 5 is a graph showing the relationship between the grain boundary carbide average grain size and the fatigue characteristics in the present invention.

 FIG. 6 is a graph showing the relationship between the grain boundary carbide average particle size and the bending formability in the present invention.

 FIG. 7 is a diagram showing the relationship between the <A> formula BP and the long axis length of the bin according to the present invention.

 FIG. 8 is a graph showing the relationship between <A> formula BP and the grain boundary carbide average particle size in the present invention.

 FIG. 9 is a schematic diagram showing the heat history of hot rolling-cooling in the present invention.

FIG. 10 is a diagram showing the relationship between Vc and the bayonet texture ratio in the present invention. FIG. 11 is a diagram showing the relationship between Vc and hardness in the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

 With regard to the force requirement (b) in which 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, it may be a position of 0.05πιηι depth from the steel surface. Fatigue properties and bendability are almost the same even when the center of the steel plate thickness, 1 to 4 structure, hardness, grain size, and precipitates change.

 As described above, the present inventors are not able to bend steel pipes in parts that are formed by bending a steel pipe even when there are micro-voids in the steel (for example, a void diameter of l ^ m or less). It was newly found that only the fine voids formed on the steel surface by forming have an effect on the deterioration of fatigue properties.

 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) of that thickness is possible, and the tensile strength is 590 MPa class, 685 MPa class, 780 MPa. In the present invention, in the present invention, the requirements (a) to (d) for the structure, hardness, crystal grain size, and grain boundary carbide of the steel surface layer are limited. The reasons for this are described below. Figures 1 and 2 show the relationship between the texture fraction of the steel surface layer and the fatigue limit, which is an indicator of fatigue properties, and the limit bending rate, which is an indicator of bending formability. Figures 1 and 2 show the results when the requirements (b), (c), and (d) for the steel surface layer are satisfied near the boundary of the range. The Paynite structure area fraction is the value in the steel surface layer (within 0.05 mm depth from the steel interface).

As shown in Fig. 1, the fatigue limit sharply increases until the bainite area ratio reaches 80%, and the area ratio of Paynai 卜 reaches 80MPa and is extremely high at 400MPa or more and is almost stabilized. . As shown in Fig. 2, the critical bending strain is , _{Λ rt} Payt area ratio increases rapidly to 80% and increases to over 35% at 80%. The organization ratio of paynite is preferably close to 100%, and it may be 100%, but as the remaining organization of bainite, ferritic, partite, martensite, one type of residual austenite or two or more types Even when the total content is 20% or less, the effect of the present invention can be sufficiently obtained if the surface layer of the steel satisfies the requirements (b), (c) and (d). For this purpose, the surface area fraction of the steel surface layer requirement (a) must be 80% or more.

 If a phase softer than the Paynite is excessively present in the steel surface layer, micropoids and fatigue cracks are likely to occur in the soft phase. In addition, if a phase harder than the paynite is excessively present in the steel surface layer, fatigue cracks are likely to occur in the microvoids at or near the interface between the hard phase and the paynite phase. Phases that are softer than bainite include ferrite, paleite, and stable residual austenite, and phases that are harder than bainite are martensite and unstable residues that generate processing-induced martensite. There are austenites.

 Steel surface layer Vickers hardness Hv is moderate to 210 or more and 300 or less, so even though the hardness of the steel surface layer part that is the starting point of fatigue cracks as a member or part is relatively hard, When bending a component material (hot-rolled steel sheet for steel pipes), micro-powders do not occur due to the combination of the following requirements (c) and (d), and a fatigue load is applied when used as a member or component. Even if it is received, fatigue cracks due to microvoids are generated and progressed less easily in the steel surface layer.

When the Vickers hardness Hv is less than 210, even if the surface area of the Paynai 卜 is 80% or more, the surface layer is too soft and fatigue cracks are generated in the surface layer when subjected to fatigue load. The fatigue limit is considerably lowered because it is easy to do. Also, when the picker hardness Hv is over 300, the bending formability is extremely high. It will decline. For this purpose, the Bickers hardness Hv, which is the requirement (b) of the surface layer of steel, needs to be 2 10 or more and 300 or less.

 When the average long axis length of bainite is 5 zm or less, the area of the grain boundary that increases the fatigue resistance of fatigue cracks is large, and even the structure mainly composed of bainite is excellent in bending formability.

 Figures 3 and 4 show the relationship between the mean value of the major axis length of the steel surface layer and the fatigue limit, which is an indicator of fatigue properties, and the limit bending rate, which is an indicator of bending formability. The long axis length of the bainite is the value of the steel surface layer (depth within 0.05 mm from the interface).

 As shown in Fig. 3, when the average value of the ellipse major axis length exceeds 5 m, the fatigue crack growth resistance is remarkably reduced, so that the fatigue limit decreases rapidly from 400 MPa as the average value increases. To do. The average value is preferably small, but if it is 5, the effect of crystal refinement can be obtained. In addition, as shown in Rotation 4, when the average value of the major axis length of the bainite exceeds 5, the area of the crystal grain boundary is small, so that the bending formability is remarkably lowered. Therefore, the average value of the major axis length of the bainite in the surface layer of steel must be 5 m or less. Incidentally, even if the average value of the major axis length is extremely fine, for example, 0.0 1 m, 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 bainitic structure (for example, the main rolling direction in the plate thickness section of the steel sheet).

 When the grain boundary carbide average grain size of the steel surface layer is 0.5 m or less, the grain boundary strength is high and the fatigue crack growth resistance is high, so the fatigue characteristics are excellent.

Figures 5 and 6 show the relationship between the average grain size of grain boundary carbides in the steel surface layer and the fatigue limit, which is an index of fatigue properties, and the limit bending rate, which is an index of bending formability. The average grain boundary carbide grain size is the value of the steel surface layer part (within 0.05 mm depth from the interface). When the average grain size exceeds 0.5 / m, voids are generated along the grain boundaries of the Painay 卜 in the surface layer of the steel during the bending test, and cracks propagate along the voids during the fatigue test. For this reason, the bending formability and fatigue characteristics are rapidly reduced. Although the average grain size of the grain boundary carbide is preferably small, even if it is extremely fine, for example, 0.005 ^ m, in the member and parts made of the steel pipe using the steel plate of the present invention, the bending formability is impaired. And good fatigue properties can be obtained. Since the crystal grain boundary strength is high during bending, it is difficult to break, and even during fatigue testing, the crystal grain boundary strength is high, so the resistance to crack growth is high.

 Hereinafter, the method for obtaining the bainitic area fraction, hardness, crystal grain size, and grain size of grain boundary carbide in the present invention will be described as a reference.

 The structure area fraction of bainite is corroded with 3% nital solution after embedding and polishing the plate thickness cross section, and the surface layer of the steel (part from the steel surface to a depth of 0.05mm, but 400 times with an optical microscope) If there is a scale layer or coating layer on the surface of the iron surface (such as a plating layer, thermal spray layer, or nitriding layer), observe the microstructure of the surface from the surface of the iron surface to 0.05 mm deep) The area ratio of the G portion was quantified.

 The Vickers hardness is not appropriate on the surface because it is easily affected by the scale layer and coating layer on the steel other than steel. For this reason, the hardness of the surface layer of the steel sheet of the present invention is measured at a depth of 0.05 mm from the steel ground interface where the correct hardness can be measured, using a micro pick-up measuring machine (based on JIS Z 2244) And measured with a load of 50 gf (test force 0.4903 N).

The average long axis length of Paynite is obtained by observing the crystal orientation distribution image by EBSP (Electron Back Scattering Pattern) method, identifying the grain boundary of the crystal from the hue difference due to the difference in crystal orientation, The average grain size was calculated by measuring the grain size in the major axis length direction in the drawing direction (the main rolling direction in the plate thickness section of the steel sheet). , ν Grain boundary carbide average particle size is 50 m x 5 by observing the mouth structure at 2000 times with a scanning electron microscope after corroding and polishing the plate thickness cross section and then corroding with 3% nitrite solution. The particle size of the carbide existing at the grain boundary in the range of 0 im is 5

Measured in the field of view, the average value was obtained.

 The fatigue properties and bendability of hot-rolled steel sheets for steel pipes can be evaluated by conducting a bending fatigue test and a bend forming test. In the bending fatigue test, specimens with a parallel part in the longitudinal center were collected from the steel sheet consisting of the above surface layer part, and the test was performed by changing the stress conditions with both swings at a frequency of 30 Hz, and the fatigue limit was obtained. . 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 bending strain on the outermost surface (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 average grain size of the steel surface layer of the present invention will be described. The following steel composition may be the composition of the entire steel sheet as usual, but only the surface layer may have this steel composition.

 C is preferably 0.05% or more in order to obtain the strength level required for the steel sheet (for example, 590MPa class, 690MPa class, 780MPa class, 865MPa class). One

On the other hand, if it exceeds 0.19%, it is difficult to control the production of the bainite, the formation of hard martensite increases, the surface hardness becomes excessive, the bend formability may be impaired, and the toughness decreases. Affect fatigue properties

. 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, it will dissolve during steel pipe production (for example, during electric welding). It may occur defects Si0 ₂ due to the contact portion. Therefore, the preferable range of Si is 0.05 to 1.0%.

Mn is effective in securing hardenability and obtaining a bainitic structure, and for that purpose 0.3% or more is desirable. More than 2.5%, the Mn0 prayer center polarized by by that defect and MnS ₂ becomes remarkable. 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. In addition, if S exceeds 0.025%, coarse MnS may be formed, which may 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 refinement of the crystal structure in the steel structure of the steel surface layer. To obtain 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, resulting in deterioration of fatigue characteristics and bending formability. For this reason, Ti is preferably 0.005 to 0.1%.

 Cr is effective for improving the hardenability and obtaining a fine bainite structure, and is also effective for reducing 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 surface area of hard martensite is greatly increased in the steel surface layer, making it difficult to obtain the desired surface area of the Painai. There is a case. In addition, the amount of coarse carbides increases, and the effect of improving the bending formability and fatigue characteristics due to the increase in grain boundary strength may decrease, or conversely, the bending formability and fatigue characteristics may be hindered. Cr is preferably 0.03 to 1.0%.

Sol.Al and N produce austenite grains by producing A1N in the manufacturing process of steel materials (such as hot-rolled steel sheets for steel pipes and steel pipes) that are the materials of the above-mentioned parts. This is effective in preventing grain coarsening and promoting the refinement of crystal grains in the paynite structure. If A1 is less than 0.005%, the effect is not always sufficient.If A1 is 0.1% and Ν exceeds 0.01%, the cleanliness of the steel is reduced and coarse A1N is formed, resulting in bending formability and Ζ or fatigue properties. May decrease. It is sufficient that the wrinkle is 0.0005% or more to use the effect of suppressing the coarsening of the austenite grains of A1N. Sol. Al is preferably 0.005 to 0.1%, and N is preferably 0.0005 to 0.01%.

 B is an extremely effective element for improving the hardenability of steel and obtaining a fine bainitic structure. If B is less than 0.0001%, the effect is not always sufficient. If it exceeds 0.01%, coarse borides (borated carbides, boronitrides, borocarbonitides, etc.) are likely to be formed and hardenability is increased. In addition, it tends to be the starting point of cracks and microvoids when bending or when fatigue load is applied. B is preferably 0.0001 to 0.01%.

 Next, the relational expression <A> of the steel composition, which is important in the present invention and is effective in finely controlling the Painai grain and the average grain size of the grain boundary carbide, will be described. The present inventors paid attention to the formation of B (boron) bainite, and suitable bainite generation parameters for obtaining a fine bainitic structure, BP = 0.27Mn + 0.2Cr + 0.05Cu + 0, llNi + Newly found 0.25Mo and A> formula. Since this formula also uses the promotion of B basin ナ formation, it is effective when B is contained in steel (at least in the surface layer of steel) by 0.0001-0.01%.

 3C≤0.27Μη + 0 · 2Cr + 0.05Cu + 0. llNi + 0.25Mo≤3C + 0.3 ... A> However, the values of C, Mn, Cr, Cu, Ni, and Mo in <A> are mass%.

 In the formula <A>, 0.27Mn, 0.2Cr, 0.05Cu, 0, llNi, 0.25Mo means 0, 27XMn, 0.2XCr, 0.05XCu, 0. llXNi> 0.25 XMo, respectively.

When the BP is less than 3C, as shown in FIG. 7 and FIG. 8, the length of the veneer ellipse major axis and the average grain size of grain boundary carbides suddenly increase. The upper limit Painai, which is the limiting requirement of the layer part, exceeds the ellipse major axis length of 5 and the average grain boundary carbide grain size of 0.5 ^ m. In addition, if the BP in the formula (A) exceeds 3C + 0.3, martensite is excessively generated in the surface layer, and the surface area fraction of the beanite tends to be less than 80%, and the picker hardness is low. Hv may exceed 300.

 In addition to the basic steel composition, one or more element groups are selected from the following element groups [I], [II], [III], and It is possible to contain one or more elements.

 [I] As a group of bainite formation promoting elements, Cu: 0.005-1.0%, Ni: 0.005-1.0%, Mo: 0.02-1.0%.

 [II] As a crystal refinement element group, Nb: 0.003 to 0.2%, V: 0.001 to 0.2%, W: 0.001 to 0.1%.

 [III] 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%.

 Cu, Ni, and Mo of the element group [I] that promotes the formation of bainite all improve the hardenability and are effective for the formation of the Painai cocoon structure. The contribution of each element of the [I] group is as indicated by the formula A>. When Cu, Ni, and Mo are less than 0.005%, less than 0.005%, and less than 0.02%, respectively, it is difficult to obtain the bainite formation promoting effect of each element. On the other hand, if Cu, Ni and Mo are more than 1.0%, more than 10% and more than 1.0%, a large amount of hard phase is likely to be formed in the surface layer of steel. It is difficult to satisfy the organization ratio of Paynite above 80%. 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%.

The crystal of the element group [II] and the Nb, V, and W of the grain boundary carbide refinement element group are the fine bainite and the fine grain boundary, which are constituent elements of the present invention. Effective for the formation of carbides. For this purpose, it is desirable that Nb is 0.003% or more, V is 0.001% or more, and W is 0.001% or more. Also, if Nb exceeds 0.2%, V exceeds 0.2%, and W exceeds 0.1%, coarse carbides are likely to form in the steel, becoming the starting point of cracking during bending, or forming near the coarse carbides. There is a concern that the processing strain is localized and the surface quality of the material and the member is deteriorated, and the fatigue damage is deteriorated by inducing the localization of the fatigue damage during the use of the member and the component after processing. Therefore, it is desirable that V is 0.003 to 0.2%, Nb is 0.001 to 0.2%, and W is 0.001 to 0.1%.

 As the inclusion form control element group of the element group [III], Ca may contain 0.0001 to 0.02%, Mg may contain 0.0001 to 0.02%, Zr may contain 0.0001 to 0.02%, and REM may contain 0.0001 to 0.02%. Is possible. Ca, Mg, Zr, and REM all have the effect of improving formability by controlling the morphology of sulfides. In order to utilize 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 on the contrary, 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, a preferred method for producing a hot-rolled steel sheet as a material for the member or part of the present invention will be described. In mass%, C: 0, 05 to 0.19%, Si: 0.05 to 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. A1: 0.005 to 0.1%, N: 0.000 5 to 0, 01%, B: 0.0001 to 0.01%, and satisfies the formula <A>. Remaining Fe and inevitable impurities A steel slab consisting of The reason for limiting the steel composition A> formula is 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 or higher and 1070 ° C or lower. Effective for obtaining grain size of grain boundary carbide.

 When steel slabs are heated above 1070, fine grain boundary carbides are advantageously obtained in hot-rolled steel plates by solidifying the carbides, nitrogen compounds, and carbonitride compounds precipitated in the molten steel solidification process in the steel. I can do it. When steel slabs are heated to over 1300 ° C, A 1 N precipitates coarsely in the hot rolling process or in the cooling process after rolling, and borides that inhibit the effect of improving the hardenability of boron (boron carbide, Boron nitride and boron carbonitride) are not desirable. Heating during hot rolling of steel slabs is preferably 1070 ° C or more and 1300 ° C or less.

 In order to produce a large amount of fine bainite, hot rolling is preferably performed in a temperature range of 850 ° C or higher, which is almost an austenite single phase and a recrystallization region. On the other hand, at temperatures above 1070 ° C, bainites tend to be coarse, and crystals 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, controlling the steel sheet cooling after hot rolling to 300 ° C or less with the cooling rate Vc (° CZ s) of the <B> formula is effective for producing fine grain and fine grain boundary carbides. is there.

When the cooling rate Vc (° C / s) exceeds 1.8Z C (C represents mass% of C), there are many harder phases than bainite as shown in Fig. 10 and Fig. 11. And the surface area of the surface area is less than 80%. The martensite surface fraction of the surface layer is remarkably increased and the desired vinyl surface fraction cannot be obtained, and the surface layer hardness Hv exceeds 300 and sufficient bending formability cannot be obtained. When Vc is less than 1.2 / C (C is the mass% of C), a softer phase than bainite is obtained as shown in Fig. 10 and Fig. 11. It is generated in large quantities and the surface area fraction of the surface layer tends to be less than 80%. The surface area of ferrite and cocoon is greatly increased and the desired surface area ratio of the ridge is not obtained, and the surface hardness Hv is less than 2 10 and cracks are generated from the surface layer due to fatigue load. It is easy to generate and grow, and sufficient fatigue characteristics cannot be obtained. Note that the Vickers hardness Hv is the value of the steel surface layer (depth 0.05mm from the steel interface).

 Even when the above-mentioned Vc is within the range of the invention, when the cooling stop temperature shown in FIG. 9 is more than 300, the grain boundary carbides are not sufficiently refined, and a fine grain 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 with Vc of the present invention is stopped at a temperature within room temperature to 250 ^: and the hot-rolled steel sheet coil is held in a temperature range of 300 ° C or lower (for example, stacking of hot-rolled coils), the present invention It does not deviate from. Further, even when a simple heat treatment that increases the temperature in the temperature range of the steel sheet surface force of 00 ° C. or less is applied to the steel plate or the steel pipe made of the steel plate by surface care or residual stress removal, 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 steel pipes, for example, automobile structural steel pipes and automobile undercarriage parts steel pipes.

The hot-rolled steel sheet of the present invention is effective when applied to machine structural steel pipes such as automobile structural steel pipes and steel pipes used for undercarriage parts of automobiles, but also requires both fatigue characteristics and bend formability. For example, it goes without saying that it is effective even when applied to steel pipe parts of transport vehicles such as airplanes and railways that are transport machines. Further, the steel sheet for machine structural steel pipes with excellent fatigue characteristics and bend formability according to the present invention is adjusted within the scope of the present invention by adjusting the components and hot rolling conditions so as to increase the surface layer hardness within the scope of the present invention. Therefore, it is possible to make hot-rolled steel sheets and steel pipes for steel pipes that can be applied to machine structural members and parts where fatigue characteristics are particularly important. By making it lower within the scope of the invention, it is possible to provide a hot-rolled steel sheet for steel pipes and steel pipes that can be applied to machine structural members / parts where bending formability is particularly important. Example

Steels with the components shown in Table 1 and Table 2 (Table 1 continued) were made into 30kg steel ingots in a vacuum melting furnace. After heating the steel ingot, it was hot-rolled to a sheet thickness of 4 dragons and 1.2 m, 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 4 mm thick material and 1.2 mm thick material. For this reason, the surface layer structure of 4 nun material and 1.2 mm material, hardness Hv, grain size and grain boundary carbide average diameter of the Painai steel were almost the same. Table 3 and Table 4 (Table 3 continued) show the steel surface layer structure, Vickers hardness Ην, grain size of grainy carbides and average grain size of grain boundary carbides obtained with 4 nun plate thickness. . Tables 1 to 4 that deviate from the requirements of the present invention are underlined. In Tables 1 and 2, the blank for the selected element indicates no addition.

table 1

Table 2 (continued from Table 1)

The area fraction of Paynai 卜 was eroded by embedding and polishing the thickness of the plate thickness, then corroded with a 3% Nayar solution, observing the microstructure at 400 times with an optical microscope, and quantifying the area ratio of the bainite portion. Asked. The Pitzers hardness was measured with a micro Vickers measuring instrument (based on JI S Z 2244) at a load of 50 gf (test force 0.49N) from the plate surface. For the average long axis length of the paynite, observe the crystal orientation distribution image by the EBSP (Electron Back Scattering Pattern) method, identify the grain boundary of the crystal from the hue difference due to the difference in crystal orientation, and then extend the crystal elongation. The grain size in the major axis length direction in the direction (main rolling direction in the plate thickness section of the steel plate) was measured, and the average value was calculated. The average grain size of the grain boundary carbides is obtained by embedding and polishing a plate thickness cross section obtained by removing the scale on the steel surface with an acid, then corroding with a 3% nital solution, and using a scanning electron microscope within a depth of 0.05 mm from the steel surface. The microstructure of 5 fields was observed at 2000 times, and the circle equivalent particle diameter of the carbide existing at the grain boundary was measured in the range of 50 mX50 m, and the average value was obtained.

In order to evaluate the 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 at the center in the longitudinal direction of the plate at a frequency of 30 Hz, with both vibrations, under different stress conditions, and the fatigue limit was obtained. Also, in the bending forming test, a t = 1.2 mni plate was pushed in with a V-shaped punch with various curvature radii R at the tip, and the size of R at which cracking occurred was investigated, and the outermost surface at that time was investigated. The magnitude of the bending strain 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 values and evaluation of the test results for fatigue limit and critical bending strain. The fatigue limit was evaluated as ◯ for 400 MPa or more and X for less than 400 MPa. In addition, the evaluation of the critical bending strain was indicated by ◯ for 35% or more and X for less than 35%. Table 3

Table 4 (Table 3 continued)

Tables 3 and 4 show the test results. The steel sheet of the present invention has a substantially small variation in structure from the bainitic structure, has an appropriate surface hardness, and has an average bainite ellipse long axis length average value, average grain boundary carbide grain size, and grain boundary strength. Is expensive. Therefore, as shown in Table 3 and Table 4, 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 compositional area fraction of Bainite, Pitzka's hardness, Beiny 卜 ellipse major axis length average value, average grain boundary carbide particle size, component or part of hot rolling and cooling conditions are included in the present invention. In the comparative example deviating from the above range, it is impossible to achieve both fatigue characteristics and bend formability. In Comparative Example 1, since the amount of C is too large, the surface hardness of the surface layer portion is over 300, too hard, the grain boundary carbide average particle size is too large, and the moldability is insufficient. In Comparative Example 2, the hardenability is inadequate due to the small amount of B added, and the surface area fraction of the ferrite is too low, and the surface area fraction of the ferrite is high, so that the surface layer hardness is sufficient. It cannot be obtained. In addition, the average long axis length of bainite and the average grain boundary carbide particle size are not sufficiently fine at the surface layer, so that the fatigue characteristics are insufficient. In Comparative Example 3, since <M> is not satisfied because Mn is small, the structure area fraction of the bainite is too low, the structure area fraction of the ferrite is high, and sufficient surface layer hardness cannot be secured. . In addition, the average long axis length of the bainite and the average grain size of grain boundary carbide are large and not sufficiently refined, so that the fatigue characteristics are insufficient. In Comparative Example 4, since the heating temperature is low, the precipitates in the steel are not sufficiently solutioned, and the additive elements are not sufficiently refined in the vain 卜 crystals and grain boundary carbides. The average long axis length and average grain size of grain boundary carbide 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 part was a structure mainly composed of ferrite, so that sufficient hardness could not be secured and fatigue characteristics were insufficient. Comparative Example 6 shows the cooling rate Since the degree is low at less than 1.2 C, the structure ratio of the ferrite is too low and the structure ratio of the ferrite increases, and the average length of the long axis of the vein and the average grain boundary carbide grain The diameter was too large to be sufficiently miniaturized, and the fatigue characteristics were insufficient. In Comparative Example 7, because the cooling stop temperature is high, bainite transformation occurs at a high temperature, and the average grain size of grain boundary carbides becomes coarse, so that the fatigue characteristics are insufficient.

Claims

WO 2009/133965 PCT / JP2009 / 058732f 請求 "Claim 1 In the surface layer of steel, 80% or more of the area fraction of the microstructure is Paynite, and the pickers hardness Hv is 210 or more and 300 or less, The average value of the long axis length of bainite is 5 m or less, and the average grain size of grain boundary carbides is 0.5 m or less. Steel sheet according to claim 1 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 %: Ti: 0.005-0.1%, Cr: 0.03-1.0%, So 1.A1: 0.005-0.1%, N: 0.0005-0.01%, B: 0.0001-0.01%, and <A> The hot-rolled steel sheet for machined structure steel pipes with excellent fatigue characteristics and bending formability according to claim 1, characterized by satisfying the formula and comprising the balance Fe and inevitable impurities 3C≤0.27Mn + 0.2 Cr + 0.05Cu + 0. llNi + 0.25Mo ≤ 3C + 0.3 ... A> where C, Mn, Cr, Cu, Ni, and Mo in the formula <A> are mass%. In addition, by mass%, as a group of elements for promoting the formation of painite, Cu: 0.005 to 1.0%, Ni: 0.005 to 1.0%, Mo: 0.02 to 1.0%, as a group of refined elements of crystals and grain boundary carbides, Nb: 0.003 to 0.2%, V: 0.001 to 0.2%, W: 0.001 to 0.1%, Inclusion form control element group, Ca: 0.0001 to 0.02%, Mg: 0.0001 to 0.02%, Zr: 0.0001 to 0.02%, REM: From one or more elements in 0.0001 to 0.02% ηΛΤ / tn nnr 'n CQ ^ WO 2009/133965 PCT / JP2009 / 0587323 ΰ "Selected elements in each selected element group 1 A hot-rolled steel sheet for machined structural steel pipes with excellent fatigue characteristics and bendability, characterized by being a steel containing two or more seeds and satisfying the formula <Α>. 3C≤0.27Mn + 0.2Cr + 0.05Cu + 0. UNi + O.25Mo≤3C + 0.3 ... A> However, the value of C, Mn, Cr, Cu, Ni, Mo in the <A> formula is mass%. 4. The hot-rolled steel sheet for machine structural steel pipes according to claim 1 or claim 2 or claim 3, wherein the mechanical structural steel pipe is an automobile structural steel pipe and has excellent fatigue characteristics and bending formability. 5. A hot-rolled steel sheet for machine structural steel pipes having excellent fatigue characteristics and bending formability, wherein the automotive structural steel pipe is an automobile underbody part according to claim 4. Claim 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 ～ 1 · 0%, Sol. A1 ： 0.005 ～ 0, 1%, N ： 0.0005 ～ 0.01%, B ： 0.0001 ～ 0.01% and satisfy the formula <A>, the balance Fe and inevitable After the steel slab made of impurities is heated to 1070 ° C or higher and 1300 ° C or lower, it is hot-rolled to a finish rolling temperature of 850 ° C or higher and 1070 ° C or lower. A method for producing hot-rolled steel sheets for machine structural steel pipes with excellent fatigue properties and bendability, characterized by cooling to 300 ° C or lower at a cooling rate of Vc C / sec. 3C≤0.27Mn + 0.2Cr + 0.05Cu + 0.UNi + O.25Mo≤3C + 0.3〜 <A>

1.2 / C≤Vc≤1.8 / C ... B>

The values for C, Mn, Cr, Cu, Ni, and Mo in the formula <A> and <B> are mass%. rui / \ jr * iuu'3 / U5) 8 / di Claim 7

 The steel slab according to claim 6, wherein

 As a group of elements for promoting the formation of paynite,

 Cu: 0.005-1.0%, Ni: 0.005-1.0%, Mo: 0.02-1.0%,

 As refinement element group of crystal and grain boundary carbide,

 Nb: 0.003-0.2%, V: 0.001-0.2%, W: 0.001-0.1%,

 As inclusion group control element group,

 Ca: 0.0001 to 0.02%, Mg: 0.0001 to 0.02%, Zr: 0.0001 to 0.02%

REM: From one or more elements in 0.0001 to 0.02%

Contains one or more elements selected and within each selected element group

Machine structure steel pipe with excellent fatigue characteristics and bending formability

For producing hot-rolled steel sheets for use.

Claim 8

 The machine structural steel pipe according to claim 6 or 7, wherein the machine structural steel pipe is an automotive structural steel.

Mechanical structure with excellent fatigue characteristics and bending formability, characterized by being a tube

Manufacturing method of hot-rolled steel sheet for steel pipes.

Claim 9

 In claim 8, the automobile structure is an automobile undercarriage part.

Hot-rolled steel sheet for machine structural steel pipes with excellent fatigue characteristics and bending formability

Manufacturing method.