JP5088631B2 - Machine-structured steel pipe with excellent fatigue characteristics and bending formability and its manufacturing method - Google Patents

Description

  The present invention relates to a machine structural steel pipe excellent in fatigue characteristics and bending formability suitable for use as, for example, an automobile structural steel pipe, an automobile undercarriage part steel pipe, and the like, and a method for manufacturing the same.

  Machine structural steel pipes have been widely used in the industry as materials for members and parts used in general machines, industrial machines, construction machines, transport machines (automobiles, transport vehicles, etc.) and the like. One of the steel materials is a cylindrical hollow body mechanical structure steel pipe. The steel pipe is required to have bend formability for fatigue characteristics due to repeated loading during use and for processing of members and parts. 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.

  Patent Document 1 describes a uniform and fine martensitic steel excellent in fatigue characteristics that has been partially quenched and a method for producing the same. However, just having some excellent fatigue properties is not enough for automobile parts. In addition, high C martensite is too hard to ensure sufficient formability.

Patent Document 2 describes a high-strength steel pipe excellent in workability and a manufacturing method thereof. In Patent Document 2, a structure mainly composed of bainite and martensite is used, and the r value is controlled by cold rolling annealing to improve the formability. However, since it is not a uniform and fine structure, it has sufficient fatigue characteristics. I can not say.
Japanese Unexamined Patent Publication No. 2007-262469 JP 2004-225131 A

  As described above, there is no steel pipe having sufficient bending formability and fatigue characteristics to date. Accordingly, an object of the present invention is to provide a machine-structured steel pipe excellent in fatigue characteristics and bend formability that achieves both bend formability and fatigue characteristics, and a method for producing the same.

  As a result of repeated studies to solve the above-mentioned problems, the present inventor uses a low C (0.03-0.15%) steel material, uniformly disperses C as a lower bainite or martensite structure by hot rolling, Uniform and sufficient diameter reduction with 4rollSZ (4 roll sizing) in pipe making with a total diameter reduction of 0.2% to 0.6% strain enables uniform and sufficient nucleation sites during reverse transformation in induction hardening. The present inventors have found that by making the entire surface a uniform and fine full martensite structure, it is possible to obtain a mechanically-structured steel pipe excellent in fatigue characteristics and bend formability that achieves both bend formability and fatigue characteristics.

The mechanical structural steel pipe excellent in the fatigue characteristics and bending formability of the present invention made based on the above knowledge is C% 0.03-0.15, Si: 0.05-1.0, Mn: 0.3-2.5, P: 0.03 in mass%. Below, S: 0.025 or less, Ti: 0.005-0.1, Sol.Al: 0.005-0.1, N: 0.0005-0.01, B: 0.0001-0.01, which consists of steel consisting of the remainder Fe and inevitable impurities , 80% or more of the area fraction is martensite, the average block diameter of the martensite structure is 3 μm or less, the maximum block diameter is 1 to 3 times the average block diameter, and the maximum hardness at 10 g Vickers Is not less than 1 and not more than 1.2 times the average hardness.

Further, as in claim 2 , the mechanical structural steel pipe of the present invention further includes Cu: 0.005-1.0, Ni: 0.005-1.0, Mo: 0.02-1.0, Cr: 0.03-1.0, block as martensite formation promoting element group. 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: 0.0001 to 0.02, preferably selected from one or more element groups, and preferably contains one or more elements in each selected element group.

Moreover, the manufacturing method of the mechanical structure steel pipe excellent in the fatigue characteristics and the bending formability of the present invention according to claim 3 is mass%, C: 0.03 to 0.15, Si: 0.05 to 1.0, Mn: 0.3 to 2.5, P: 0.03 or less; S: 0.025 or less; Ti: 0.005-0.1; Sol. After finishing, it was hot rolled to a finish rolling temperature of 850 ° C or higher and 1070 ° C or lower. After finishing rolling, it was cooled to 300 ° C or lower at a cooling rate of 8 ° C / sec or higher to form a hot rolled coil, and then piped. Later in the shaping process, the diameter is reduced so that the total diameter reduction strain in the 4-roll sizing is 0.2% or more and 0.6% or less, and then heated to 850 ° C or more and 1050 ° C or less at a high temperature rise rate of 35 ° C / sec or more. And cooling at a cooling rate of 100 ° C./sec or more.

Furthermore, the manufacturing method of the mechanical structure steel pipe excellent in the fatigue characteristics and bend formability of the present invention according to claim 4 is mass%, C: 0.03 to 0.15, Si: 0.05 to 1.0, Mn: 0.3 to 2.5, P: 0.03 or less, S: 0.025 or less, Ti: 0.005-0.1, Sol.Al: 0.005-0.1, N: 0.0005-0.01, B: 0.0001-0.01, and further as a martensite formation promoting element group, Cu: 0.005- 1.0, Ni: 0.005-1.0, Mo: 0.02-1.0, Cr: 0.03-1.0, Nb: 0.003-0.2, V: 0.001-0.2, W: 0.001-0.1, inclusion morphology control Each selected element group selected from one or more element groups of Ca: 0.0001 to 0.02, Mg: 0.0001 to 0.02, Zr: 0.0001 to 0.02, REM: 0.0001 to 0.02 After heating a steel slab containing one or more of these elements and the balance Fe and inevitable impurities to 1070 ° C to 1300 ° C, the final rolling temperature is set to 850 ° C to 1070 ° C. After rolling, finish rolling and cooling to 8 ° C / sec or more to 300 ° C or less to form a hot-rolled coil, and after pipe forming, the total diameter reduction strain in 4-roll sizing is 0.2% to 0.6% in the shaping process The diameter is reduced to the following, and then heated at a high frequency at a temperature rising rate of 35 ° C./sec or higher to 850 ° C. or higher and 1050 ° C. or lower, and cooled at a cooling rate of 100 ° C./sec or higher.

  The mechanical structural steel pipe excellent in fatigue characteristics and bend formability of the present invention has a uniform and fine martensite structure on the entire surface and is low C, so the strength is not too hard and not too soft. Excellent bend formability and fatigue characteristics.

  The steel pipe of the present invention is excellent in bending formability. That is, the structure of the steel pipe of the present invention is a martensite structure in which 80% or more is uniform (ratio of average hardness and maximum hardness at 10 g Vickers is 1 or more and 1.2 or less), so there is no particularly soft part, Strain during bending is not localized. This is because various bendability of low C martensite steel was observed. Strain is reduced in hard places, strain is increased in soft places, and if the difference in hardness is large, the difference in strain is increased, leading to cracks. Was found from the observation.

  The steel pipe of the present invention is excellent in fatigue characteristics. That is, the structure of the steel pipe of the present invention is a martensite structure in which 80% or more is uniform (ratio of average hardness and maximum hardness at 10 g Vickers is 1 or more and 1.2 or less), so there is no particularly soft part, Fatigue damage is not localized. Further, since the average block diameter is 3 μm or less and the maximum block diameter is uniform and fine, 3 times or less of the average block diameter, the area of the block boundary is large everywhere and the fatigue crack propagation resistance is high. This was observed from various observations of fatigue damage in low-C martensitic steel, which was found to be caused by cracks propagating along the block boundary. It was done.

  In addition, since the steel pipe of the present invention is martensite during induction hardening, the characteristics are further improved as compared with the case of martensite during hot rolling. In other words, when a martensite structure is used during hot rolling (hot rolling → pipe making → bending by user), processing distortion occurs when the pipe is formed after hot rolling, and the bending formability when forming into parts, etc. accordingly. Is inferior. In addition, the heat-affected zone is blunted during electro-sewing welding, and the fatigue characteristics of that portion are reduced. On the other hand, when it is made martensite by induction hardening as in the present invention (hot rolling → pipe making → induction hardening → bending by the user), the strain entered in the pipe making is released during induction hardening, There is no decrease in bending formability due to processing strain, and the heat-affected zone during ERW welding has a uniform and fine martensite structure during induction hardening, and therefore has bending formability and fatigue characteristics equivalent to the base material.

  Therefore, the steel pipe obtained by the present invention is excellent in both fatigue characteristics and bend formability, which are contradictory characteristics. Therefore, as a steel pipe for machine structural members, automobile structural members, and undercarriage parts of automobiles, both of which are required Suitable.

  Moreover, according to the manufacturing method of the mechanical structure steel pipe of this invention described in Claim 4, 5, the mechanical structure steel pipe which made the above-mentioned fatigue characteristic and bending formability compatible can be manufactured.

  Hereinafter, preferred embodiments of the present invention will be described in detail. First, the reasons for limiting the structure, block diameter, and 10 g Vickers hardness in the present invention will be described below.

  FIG. 1 and FIG. 2 show the relationship between the area fraction of the microstructure of a steel pipe and the fatigue limit, which is an index of fatigue characteristics, and the limit bending rate, which is an index of bending formability. However, since the fatigue characteristics and the bending formability differ depending on the strength, the figure shows the vertical axis as a value arranged by the yield strength YS. From these figures, it can be seen that when the martensite surface fraction is 80% or more, the fatigue characteristics and the bending formability are excellent. The reason why the area ratio of martensite is less than 80% and the characteristics are not sufficient is that when the soft phase increases, the portion is preferentially damaged by fatigue, and when bending, the soft portion and the hard martensite portion This is considered to be because the difference in strain increases and stress concentrates on that part, leading to cracking.

  Next, FIG. 3 and FIG. 4 show the relationship between the block diameter of the martensite structure and the fatigue characteristics. From these figures, it can be seen that sufficient fatigue characteristics are exhibited when the average block diameter is 3 μm or less, and sufficient fatigue characteristics are exhibited when the maximum block diameter is 1 to 3 times the average block diameter. The reason why the fatigue properties are not sufficient outside of these conditions is considered to be that, as described above, when the block diameter increases, the area of the block boundary decreases, and the crack propagation resistance due to fatigue decreases.

  Next, FIG. 5 and FIG. 6 show the relationship between the 10 g Vickers hardness of the martensite structure, fatigue characteristics, and bend formability. From these figures, it can be seen that when the maximum hardness at 10 g Vickers is 1 to 1.2 times the average hardness, sufficient fatigue properties and bending formability are exhibited. The reason why the fatigue characteristics are not sufficient outside of these conditions is considered to be because the soft part preferentially undergoes fatigue damage when there is variation in hardness as described above. In addition, as described above, the reason why the bend formability is not sufficient outside of these conditions is that, as described above, if there is a variation in hardness, the difference in strain between the soft part and the hard martensite part becomes large at the time of bend forming. This is thought to be due to stress concentration in the part and cracking.

The method for obtaining the martensite area fraction, block diameter, and 10 g Vickers hardness is shown below.
The martensite surface area ratio is determined by burying and polishing the plate thickness section, corroding with 3% nital solution, observing the microstructure of the steel at 400 times with an optical microscope, and quantifying the area ratio of the martensite portion. Asked.

  For the block diameter, the crystal orientation distribution image is observed by the EBSP (Electron Back Scattering Pattern) method, the block boundary is identified from the hue difference due to the difference in crystal orientation, and the longest diagonal length among the blocks observed from the image (Fig. 7). Further, at least 50 block diameters were randomly measured, and an average block diameter and a maximum block diameter were obtained from the results.

  10g Vickers hardness is measured by embedding and polishing the plate thickness cross section, then measuring the load at 10gf with a micro Vickers hardness measuring machine and randomly measuring only the martensite structure part at least 50 points. Asked.

  The fatigue characteristics and bend formability of the hot-rolled steel sheet for steel pipes may be evaluated by performing a plane bending fatigue test and a bend forming test. In the plane bending fatigue test, an arc-shaped steel plate was cut out from a steel pipe and straightened so that it became a flat plate, and then 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 flat plate is sampled from a steel pipe by the same method, and it is pushed with a V-shaped punch with various curvature radii R at the tip, and the size of the limit R where cracking occurs is investigated. The magnitude of bending strain (limit bending strain) on the outermost surface was calculated and evaluated. Judgment criteria for good fatigue properties were fatigue limit / YS of 0.7 or more, and bending formability of limit bending strain × YS of 250 MPa or more.

Next, a steel composition suitable for simultaneously satisfying the martensite surface fraction, block diameter and 10 g Vickers hardness will be described.
C is preferably 0.03% or more in order to obtain the required strength level (for example, 590 MPa class, 690 MPa class, 780 MPa class, 980 MPa class, 1200 MPa class). On the other hand, if it exceeds 0.15%, the strength is increased and the bending formability may be impaired, and the toughness is lowered to affect the fatigue characteristics. A preferable range of C is 0.03 to 0.15%.
It is effective to contain 0.05% or more of Si as a deoxidizing element for suppressing coarse oxides that hinder bending formability and fatigue characteristics. On the other hand, if over 1.0% is added, there is a possibility that a defect caused by SiO ₂ may be generated in the welded part during ERW welding in pipe making. Therefore, the preferable range of Si is 0.05 to 1.0%.
Mn is effective for securing hardenability and obtaining a martensite 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 coarsening of the austenite grain size and achieving finer block diameter. 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 effect of miniaturization 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%.

  Sol.Al and N are effective for generating AlN in the manufacturing process and suppressing the coarsening of the austenite grains and promoting the refinement of the block diameter. If Al is less than 0.005%, the effect is not always sufficient. If Al exceeds 0.1% and N exceeds 0.01%, the cleanliness of the steel decreases and coarse AlN is generated, resulting in a decrease in bending formability and / or fatigue properties. There is a case. 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 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 the steel and obtaining a fine martensite structure. If B is less than 0.0001%, the effect is not necessarily sufficient, and if it exceeds 0.01%, coarse boride (Boride carbide, boride carbide, boride carbonitride, etc.) are easily generated and hardenability is deteriorated. Also, when bending or when fatigue load is applied, cracks and microvoids are also generated. Easy to be. B is preferably 0.0001 to 0.01%.

In addition to the basic steel composition, one or more element groups are selected from the following element groups (1), (2), (3), and It is possible to contain one or more elements.
(1) As a bainite formation promoting element group, Cu: 0.005-1.0%, Ni: 0.005-1.0%, Mo: 0.02-1.0% , Cr: 0.03-1.0%.
(2) As a block diameter refinement element group, Nb: 0.003-0.5%, V: 0.001-0.5%.
(3) 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%.

  Cu, Ni, Mo, and Cr of the martensite generation promoting element group of the element group (1) are all effective in improving the hardenability and generating a martensite structure. (When Cu, Ni, Mo, and Cr are less than 0.005%, less than 0.005%, less than 0.02%, and less than 0.03%, respectively, the martensite formation promoting action of each element is not sufficiently obtained. When Cu, Ni, Mo, and Cr are over 1.0%, over 1.0%, over 1.0%, and over 1.0%, the martensite formation promoting action is saturated, and an effect commensurate with the added amount cannot be expected. 0.005 to 1.0%, Ni can be contained in the steel in the range of 0.005 to 1.0%, Mo can be contained in the range of 0.02 to 1.0%, and Cr can be contained in the range of 0.03 to 1.0%.

  The element group (2) Nb, V, and W in the block diameter refinement element group are all effective for suppressing the austenite grains from coarsening and promoting the refinement of the block diameter. 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. When Nb exceeds 0.2%, V exceeds 0.2%, and W exceeds 0.1%, coarse carbides are likely to form in the steel, which becomes the starting point of cracking during bending, and there is processing strain during molding near the coarse carbides. There is a concern that the surface quality of the material, member, or part is lowered by localization, the fatigue damage is induced during use after processing the member or part, and the fatigue characteristics are lowered. Therefore, V can be contained in the steel in the range of 0.003 to 0.2%, Nb in the range of 0.001 to 0.2%, and W in the range of 0.001 to 0.1%.

  As the inclusion form control element group of the element group (3), 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%. is there. Ca, Mg, Zr, and REM all have the effect of improving the formability by controlling the form of sulfide. 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 conversely, the bending formability and fatigue characteristics may be lowered. Therefore, it is desirable that Ca is contained at 0.0001 to 0.02%, Mg is 0.0001 to 0.02%, Zr is 0.0001 to 0.02%, and REM is contained at 0.0001 to 0.02%.

Next, the suitable manufacturing method of the steel pipe of this invention is described.
In the present invention, by mass%, C: 0.03-0.15, Si: 0.05-1.0, Mn: 0.3-2.5, P: 0.03 or less, S: 0.025 or less, Ti: 0.005-0.1, Sol.Al: 0.005-0.1 , N: 0.0005 to 0.01, B: 0.0001 to 0.01, and a steel slab. The reason for limiting the steel composition is as described above.

  A steel slab having the above composition is heated to 1070 ° C. or higher and 1300 ° C. or lower, and then subjected to hot rolling with a finish rolling temperature of 850 ° C. or higher and 1070 ° C. or lower to obtain a pre-structure for making the steel pipe of the present invention. It is effective. When the steel slab is heated to 1070 ° C. or higher, carbides, nitrogen compounds, and carbonitride compounds precipitated in the molten steel solidification process are dissolved in the steel, so that the non-uniformity of the components in the steel is extremely reduced. When the steel slab is heated to over 1300 ° C, AlN 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 B (boron carbide, boron nitride, carbon Boron nitride) may be formed, which is not desirable. The heating temperature in hot rolling of the steel slab is preferably from 1070 ° C to 1300 ° C.

  In order to obtain martensite with a fine block diameter after induction hardening, it is advantageous to make the structure as fine as possible even after hot rolling, which is a structure before induction hardening. For that purpose, it is desirable to carry out in a temperature range of 850 ° C. or higher, which is almost an austenite single phase and a recrystallization range. On the other hand, if it exceeds 1070 ° C., the austenite grain size becomes extremely coarse. Therefore, the finish rolling temperature of hot rolling is preferably 850 ° C. or higher and 1070 ° C. or lower.

Cooling the steel slab having the above composition to a temperature of 300 ° C. or less at a cooling rate of 8 ° C./sec or more after finish rolling is effective for obtaining a pre-structure for making the steel pipe of the present invention.
In order to obtain martensite with a uniform block diameter and hardness after induction hardening, it is advantageous to make the structure as uniform as possible even after hot rolling, which is a structure before induction hardening. In particular, reverse transformation during induction hardening occurs from the place where C exists, and therefore it is desirable to disperse C as uniformly as possible after hot rolling. For this purpose, it is advantageous to form a martensite structure or a lower bainite structure after hot rolling. For this purpose, it is effective to cool to 300 ° C. or less at a cooling rate of 8 ° C./sec or more after finish rolling. If the cooling rate after rolling is less than 8 ° C./sec, ferrite may precipitate and form a non-uniform structure. On the other hand, when the cooling stop temperature exceeds 300 ° C., the upper bainite structure may be formed, and C may segregate at the grain boundaries or may become a ferrite pearlite structure, resulting in a non-uniform structure.

  After hot rolling, pipe making is performed. After normal pipe making from hot-rolled coil under the hot-rolling conditions, the total shrinkage strain in 4rollSZ (4 roll sizing: pipe making using 4 sizing rolls) is 0.2% or more in the molding process It is effective to reduce the diameter to 0.6% or less in order to obtain the steel pipe of the present invention. The reverse transformation during induction hardening is likely to occur from the location where dislocations exist among the locations where C exists, so it is better to disperse the dislocations evenly throughout the steel pipe, which has a uniform fine block diameter and uniform hardness. It is advantageous to obtain site steel.

Generally, in the sizing process, 2rollSZ with a roll division number of 2 and 4rollSZ with a roll division number of 4 are used, but in 2rollSZ, dislocations are concentrated at the 0 ° and 180 ° positions of the steel pipe. There are almost no dislocations at the 90 ° and 270 ° positions. On the other hand, with 4rollSZ, it is possible to introduce dislocations over the entire surface of the steel pipe.
If the total reduced diameter strain is less than 0.2%, sufficient dislocations cannot be entered, and if the total reduced diameter strain exceeds 0.6%, sufficient dislocation is introduced, but it becomes a considerably strong work, so that processing cracks occur. Or uneven thickness may occur, both of which are not preferred. Therefore, it is desirable that the total diameter reduction strain is 0.2% or more and 0.6% or less.

Induction hardening is performed after pipe making to obtain the steel pipe of the present invention. The steel pipe after the shaping process is heated from 850 ° C. to 1050 ° C. at a heating rate of 35 ° C./sec or higher, and induction-quenched at a cooling rate of 100 ° C./sec or higher.
If the rate of temperature rise is less than 35 ° C./sec, the growth of the austenite grains reversely transformed first proceeds, and a large difference occurs in the block diameter with the austenite grains reversely transformed later. When the heating temperature is less than 850 ° C., there is a possibility that a site that cannot be reversely transformed due to the low temperature is generated, resulting in an uneven structure. When the heating temperature is 1050 ° C. or higher, austenite grains grow and the block diameter after quenching becomes coarse. If the cooling rate is less than 100 ° C./sec, there is a possibility that 80% or more cannot be made into a martensite structure due to insufficient cooling rate. From the above, it is desirable that the induction hardening is performed by heating from 850 ° C. to 1050 ° C. at a temperature increase rate of 35 ° C./sec or more and a cooling rate of 100 ° C./sec or more.

  The steel pipe of the present invention is effective when applied to steel pipes for machine structures such as automobile structural steel pipes and steel pipes used for automobile undercarriage parts, but also requires both fatigue characteristics and bend formability. Needless to say, the present invention is also effective when applied to steel pipe parts of transport vehicles such as aircraft and railways that are transport machines. In addition, the steel sheet for machine structural steel pipes with excellent fatigue characteristics and bendability of the present invention is particularly focused on fatigue characteristics by adjusting the components and hot rolling conditions within the scope of the present invention to increase the strength. It is optimal for machine structural members / parts to be manufactured, and by reducing the strength, it is optimal for machine structural members / parts in which bend formability is particularly important.

  Steels having the components (wt%) shown in Table 1 were melted and cast. After heating the steel ingot, perform hot rolling, pipe making and induction hardening under the conditions shown in Table 2 to make a steel pipe of φ114.3 × t2.0mm, and then cut out a part in an arc shape to become a flat plate After correction, it was processed into a test piece for bending and a test piece for plane bending fatigue. The plane bending fatigue test was performed by changing the stress conditions with both swings at a frequency of 30 Hz to determine the fatigue limit. In the bending test, the tip was pushed with a V-shaped punch having various radii of curvature R, the limit R where cracking occurred was investigated, and the bending strain on the outermost surface at that time (limit bending) (Strain) was calculated and evaluated. Judgment criteria for good fatigue properties were fatigue limit / YS of 0.7 or more, and bending formability of limit bending strain × YS of 250 MPa or more. Hot rolling conditions, pipe making conditions, induction hardening conditions, martensite surface fraction, average block diameter, maximum block diameter / average block diameter, maximum hardness / average hardness, and results of plane bending fatigue test and bending forming test Is as shown in Table 2. In Tables 1 and 2, parts that deviate from the scope of the present invention are underlined.

  As described above, the steel of the present invention has a martensite structure on the entire surface, a small variation in structure, and a sufficiently uniform and fine structure. Therefore, the fatigue limit / YS is not less than 0.7 and the critical bending strain. × Bendability with YS of 250 or more can be achieved.

On the other hand, in the comparative example in which part of the martensite surface fraction, average block diameter, maximum block diameter / average block diameter, maximum hardness / average hardness deviated from the scope of the present invention, fatigue characteristics and bend formability Can not be compatible.
In Comparative Example 1, since Ti is not included in the conditions, B cannot be effectively used, and the martensite surface fraction, average block diameter, maximum block diameter / average block diameter, maximum hardness / average hardness However, both the fatigue characteristics and the bending formability were insufficient.
In Comparative Example 2, since B is out of the condition, sufficient hardenability cannot be secured, martensite surface fraction, average block diameter, maximum block diameter / average block diameter, maximum hardness / The average hardness deviated from the scope of the present invention, and both fatigue characteristics and bend formability were insufficient.
In Comparative Example 3, since the cooling rate in hot rolling was slow, the structure after hot rolling was an uneven structure in which ferrite, pearlite, and bainite were mixed. Therefore, the maximum block diameter / average block diameter and the maximum hardness / average hardness deviated from the scope of the present invention, and both fatigue characteristics and bend formability were insufficient.
In Comparative Example 4, since the cooling stop temperature in hot rolling was high, the structure after hot rolling became a ferrite pearlite structure. Therefore, the maximum block diameter / average block diameter and the maximum hardness / average hardness deviated from the scope of the present invention, and both fatigue characteristics and bend formability were insufficient.
In Comparative Example 5, the amount of 4roll reduction in pipe making was not sufficient, so that a sufficient amount of strain to make uniform martensite could not be dispersed in the steel. Therefore, the maximum block diameter / average block diameter and the maximum hardness / average hardness deviated from the scope of the present invention, and both fatigue characteristics and bend formability were insufficient.
In Comparative Example 6, since the heating temperature at the time of induction hardening was high, austenite was coarsened and the average block diameter was also coarse, so that the fatigue characteristics were insufficient.
In Comparative Example 7, since the cooling rate during induction hardening was slow, the martensite surface fraction was low, and both fatigue characteristics and bending formability were insufficient.

It is the figure which showed the relationship between the structure area fraction of martensite, and a fatigue characteristic. It is the figure which showed the relationship between the structure area fraction of martensite, and bending formability. It is the figure which showed the relationship between the average block diameter of a martensite, and a fatigue characteristic. It is the figure which showed the relationship between the block diameter variation of martensite, and a fatigue characteristic. It is the figure which showed the relationship between the 10g Vickers hardness variation of martensite, and a fatigue characteristic. It is the figure which showed the relationship between the 10g Vickers hardness variation of martensite, and bending formability. It is the figure which defined the block diameter of the martensite.

Claims (4)

% By mass
C: 0.03-0.15,
Si: 0.05 to 1.0
Mn: 0.3 to 2.5,
P: 0.03 or less,
S: 0.025 or less,
Ti: 0.005-0.1,
Sol.Al: 0.005-0.1,
N: 0.0005 to 0.01
B: 0.0001-0.01,
80% or more of the area fraction of the microstructure is martensite, the average block diameter of the martensite structure is 3 μm or less, and the maximum block diameter is the average block A machine-structured steel pipe with excellent fatigue characteristics and bending formability, characterized by having a diameter of 1 to 3 times the diameter and a maximum hardness at 10 g Vickers of 1 to 1.2 times the average hardness. Steel is further mass% as martensite formation promoting element group,
Cu: 0.005-1.0,
Ni: 0.005-1.0,
Mo: 0.02-1.0,
Cr: 0.03-1.0,
As block diameter refinement element group,
Nb: 0.003-0.2,
V: 0.001-0.2,
W: 0.001-0.1,
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
2. The fatigue property and bending according to claim 1 , wherein the fatigue characteristics and bending are selected from one or two or more element groups and containing one or more elements in each selected element group. Mechanical structural steel pipe with excellent formability. % By mass
C: 0.03-0.15,
Si: 0.05 to 1.0
Mn: 0.3 to 2.5,
P: 0.03 or less,
S: 0.025 or less,
Ti: 0.005-0.1,
Sol.Al: 0.005-0.1,
N: 0.0005 to 0.01
B: 0.0001-0.01,
After heating the steel slab composed of the remaining Fe and inevitable impurities to 1070 ℃ or more and 1300 ℃ or less, it is hot-rolled to finish rolling temperature of 850 ℃ or more and 1070 ℃ or less. After cooling to 300 ° C or lower to form a hot-rolled coil, after pipe forming, the diameter is reduced so that the total diameter-reducing strain in the 4-roll sizing is 0.2% or more and 0.6% or less in the shaping process. A method of manufacturing a mechanically-structured steel pipe excellent in fatigue characteristics and bending formability, characterized by heating at a temperature rate of 35 ° C / sec or higher to 850 ° C or higher and 1050 ° C or lower and cooling at a cooling rate of 100 ° C / sec or higher. % By mass
C: 0.03-0.15,
Si: 0.05 to 1.0
Mn: 0.3 to 2.5,
P: 0.03 or less,
S: 0.025 or less,
Ti: 0.005-0.1,
Sol.Al: 0.005-0.1,
N: 0.0005 to 0.01
B: 0.0001 to 0.01
In addition, as a martensite generation promoting element group,
Cu: 0.005-1.0,
Ni: 0.005-1.0,
Mo: 0.02-1.0,
Cr: 0.03-1.0,
As block diameter refinement element group,
Nb: 0.003-0.2,
V: 0.001-0.2,
W: 0.001-0.1,
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
A steel slab selected from one or two or more element groups, containing one or more elements in each selected element group, and the remainder consisting of Fe and inevitable impurities is 1070 ° C or higher and 1300 ° C. After heating to ℃ ℃ or less, hot rolling to finish rolling temperature of 850 ℃ or more and 1070 ℃ or less was performed, and after finishing rolling, it was cooled to 300 ℃ or less at 8 ℃ / sec or more to make a hot rolled coil, and then piped Later in the shaping process, the diameter is reduced so that the total diameter reduction strain in the 4-roll sizing is 0.2% or more and 0.6% or less, and then heated to 850 ° C or more and 1050 ° C or less at a high temperature rise rate of 35 ° C / sec or more. And a method of manufacturing a machine-structured steel pipe excellent in fatigue characteristics and bending formability, characterized by cooling at a cooling rate of 100 ° C./sec or more.

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