JP2008007834A - Manufacturing method of steel material superior in fatigue crack propagation resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a steel material superior in fatigue crack propagation resistance, little in crack propagation anisotropy and suitable for welded structures such as ships, marine structures, bridges, buildings and tanks for which structural safety is strongly required. <P>SOLUTION: The steel material has a composition comprising, by mass%, 0.04-0.20% C, 0.05-0.50% Si, 0.5-1.8% Mn, 0.05% or less P, 0.02% or less S, and at needed, one or more elements selected from the group consisting of Cu, Ni, Cr, Mo, Nb, V, Ti and B, and essentially balance Fe, and a specified microstructure mainly comprising a two-phase microstructure of ferrite and perlite in which the area ratio of the ferrite is 65-85%. The steel material is obtained by heating the steel having the above-mentioned composition to a temperature of 1,000-1,300°C, rolling it with a cumulative reduction of 50% or more at a temperature of Ar<SB>3</SB>point or higher, and finishing the rolling at a temperature of Ar<SB>3</SB>point or higher, then acceleratedly cooling it from a temperature range from Ar<SB>3</SB>to Ar<SB>3</SB>-60°C to a temperature of 450-650°C at a cooling speed of 10°C/s or higher. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a method for producing a steel material having excellent fatigue crack propagation resistance, and particularly, a welded structure having a small crack propagation anisotropy and strongly demanding structural safety such as a ship, an offshore structure, a bridge, a building, and a tank. It is related with what is suitable for.

  Steel materials used in structures such as ships, offshore structures, bridges, buildings, tanks, etc. have excellent mechanical properties such as strength and toughness and weldability, as well as repeated loads and winds during normal operation. In addition, the structural safety of the structure must be ensured against repeated vibration caused by earthquakes.

  It is required to have excellent fatigue characteristics for repeated loads, and in order to prevent ultimate failure such as breakage of members in particular, it is effective to improve the propagation resistance of fatigue cracks possessed by steel materials. Think.

  In the case of general welded structures, the weld toe tends to be a stress concentration part, and tensile residual stress due to welding also acts and often becomes a source of fatigue cracks. It is known to introduce compressive residual stress by tanning welding or shot peening.

  However, since there are a large number of weld toes in the welded structure and the burden is high in cost, these methods are unsuitable for implementation on an industrial scale, and the fatigue resistance characteristics of the welded structure are It is often achieved by improving the fatigue crack propagation characteristics of the steel material used.

  These steel structures are often welded freely from various directions to the steel sheet, for example, from various directions with respect to the rolling direction, and therefore the direction of fatigue crack initiation and propagation is also various, It is desirable that the fatigue crack propagation resistance performance of the steel sheet is high regardless of the direction in the steel.

  Patent Document 1 relates to a steel plate for a tanker, and the structure thereof is composed of a mixed structure of a first phase of ferrite and a second phase of bainite and / or pearlite. It is described that it has excellent fatigue crack growth characteristics.

  Patent Document 2 discloses a steel plate having an effect of suppressing fatigue crack growth, characterized in that the structure is composed of a base of a hard part and a soft part dispersed in the base, and the difference in hardness between the two parts is 150 or more in terms of Vickers hardness. Is described.

In Patent Document 3, the steel structure of the cross section is ferrite and bainite, the ferrite phase has an area ratio of 38% or more and 52% or less, the hardness of the ferrite phase portion is 80HV0.02-150HV0.02, and The tensile strength with excellent fatigue crack growth resistance is 55 kgf / characteristically characterized in that the boundary between the ferrite phase and the bainite phase exists at a density of 50 to 300 locations / mm on a straight line drawn at an arbitrary location in the cross section. A ferrite-bainite duplex steel of mm ² or more is described.

Patent Document 4 discloses a ferrite matrix in which the distance from the interface between the second phases in the fatigue crack propagation direction to the interface to the next second phase is 25 μm or less, and the cross-sectional structure in the plate thickness direction is 60 to 90% in area ratio. The second phase hardness: Hv (SP) and the ferrite hardness: Hv (F) satisfy the value represented by the formula, and the aspect ratio of the second phase: 1 ( A steel material having excellent fatigue crack propagation characteristics, characterized in that the long axis length) / d (short axis length) is 1 / d> 3.42, is described.
Japanese Patent No. 2785643 Japanese Patent No. 2962134 Japanese Patent No. 3489243 Japanese Patent No. 3434434

  However, the steel sheets according to the inventions described in Patent Documents 1, 2, and 3 are investigated by a fatigue crack propagation test using a compact test piece whose thickness or thickness is reduced.

  In this case, the obtained fatigue crack propagation rate is a characteristic in the longitudinal direction or width direction of the steel sheet, and the crack propagation direction in any direction (hereinafter referred to as crack propagation difference) is not specified within the steel sheet. There is no description about improving the directionality.

  In Patent Document 4, a fatigue test is performed using an out-of-plane gusset-type joint test piece. The structure of the steel sheet according to the invention described in Patent Document 4 aims to suppress the propagation speed in the thickness direction. It is clear.

  Furthermore, since the steel plates according to the inventions described in Patent Documents 1 to 4 introduce bainite or martensite as a hardened phase, there is a concern about deterioration of ductility and bending workability.

  Accordingly, an object of the present invention is to provide a steel material having small crack propagation anisotropy, excellent fatigue crack propagation resistance, excellent ductility and bending workability, and a method for producing the same.

  The inventors of the present invention have studied in detail the influence of microstructure on fatigue crack propagation from the relationship between the crack propagation direction in three-dimensional space and the microstructure morphology in the two-phase structure of ferrite and pearlite. When the specific amount of ferrite in the two-phase structure is studied and the pearlite average interval (L, T plane), the ferrite average particle size (L, T plane) and the pearlite lump shape (L-T-Z space) are specified, It was newly found that a steel material with small crack propagation anisotropy and excellent fatigue crack propagation resistance can be obtained.

The present invention has been made by further studying the obtained knowledge. In mass%, C: 0.04 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.5 to 1.8%, P: 0.05% or less, S: 0.02 % or less, the steel balance being substantially Fe, 1000 ° C. or higher, then heated to 1300 ° C. or less, after completion of the rolling at a cumulative reduction of 50% or more of the rolling performed Ar ₃ point or more by Ar ₃ or more points From the temperature range of Ar ₃ to Ar ₃ −60 ° C. to 650 ° C. or lower and 450 ° C. or higher, accelerated cooling is performed at 10 ° C./s or higher, and the microstructures of ferrite and pearlite having the characteristics of (1) to (3) A method for producing a steel material having excellent fatigue crack propagation resistance, comprising a ferrite having an area ratio of 65 to 85% in a phase structure.
(1) L-plane and T-plane pearlite average spacing: 15-30 μm
(2) L-plane and T-plane ferrite average particle diameter: 10 to 20 μm
(3) Perlite lump shape: L (L) ≦ 3Z (L), T (T) ≦ 3Z (T)
Here, L (L): L direction average length on the L plane, Z (L): Z direction average length on the L plane, T (T): T direction average length on the T plane, Z ( T): Z-direction average length on the T plane Further, as a steel component, in mass%, Cu: 0.4% or less, Ni: 0.8% or less, Cr: 0.4% or less, Mo: 0.4% or less, Nb: 0.05% or less, V: 0.10% or less, Ti: 0.03% or less, B: 0.003% or less of one type or two or more types of steel material having excellent fatigue crack propagation resistance according to 1, Production method.

  According to the present invention, a steel material that always has high fatigue crack propagation resistance, small crack propagation anisotropy, and excellent fatigue crack propagation resistance regardless of the direction of fatigue cracks that occur and propagate in steel. Even if fatigue cracks have occurred over time from stress-concentrated parts or welded parts, it is possible to delay the subsequent propagation and increase the safety of steel structures. Very useful.

  The component composition, production conditions and microstructure of the present invention will be described in detail.

[Ingredient composition] In the description, “%” means “mass%”.
C
C is added in an amount of 0.04% or more to ensure strength. If it exceeds 0.20%, weldability is impaired, so 0.04 to 0.20%, preferably 0.06 to 0.18% is added.

Si
Si is added in an amount of 0.05% or more to ensure deoxidation and strength. If added over 0.50%, weldability and toughness deteriorate, so 0.05 to 0.50%, preferably 0.10 to 0.40%.

Mn
Mn is added in an amount of 0.5% or more in order to ensure strength and toughness by increasing hardenability. If it exceeds 1.8%, the weldability deteriorates, so 0.5 to 1.8%, preferably 0.8 to 1.6% is added.

P
P is an impurity and degrades toughness. Therefore, its content is preferably as small as possible, and is 0.05% or less, preferably 0.03% or less in terms of manufacturing cost.

S
Since S is an impurity and degrades toughness, the content is preferably as small as possible, and is 0.02% or less, preferably 0.01% or less in terms of manufacturing cost.

  The above is the basic component composition of the steel according to the present invention. When further improving the strength, toughness, weldability, or imparting weather resistance, Cu, Ni, Cr, Mo, Nb, V, Ti, B Add one or two or more.

Cu
Cu increases the strength by solid solution and improves the weather resistance, so it is added according to the desired properties. When added, if it exceeds 0.4%, the weldability is impaired, and flaws are likely to occur during the production of the steel material, so 0.4% or less, preferably 0.3% or less.

Ni
Ni improves low-temperature toughness and weather resistance, and improves hot brittleness when Cu is added, so it is added according to desired characteristics. When added, if it exceeds 0.8%, the weldability is impaired and the steel material cost increases, so the content is made 0.8% or less, preferably 0.6% or less.

Cr
Cr increases the strength and improves the weather resistance, so it is added according to the desired properties. When added, if it exceeds 0.4%, weldability and toughness are impaired, so 0.4% or less, preferably 0.3% or less.

Mo
Since Mo increases strength, it is added according to desired characteristics. When added, if it exceeds 0.4%, weldability and toughness are impaired, so 0.4% or less, preferably 0.2% or less.

Nb
Nb suppresses austenite recrystallization during rolling to achieve finer grains, and at the same time, precipitates during air cooling after accelerated cooling and increases strength. Therefore, Nb is added according to desired characteristics. When added, if it exceeds 0.05%, the toughness is impaired, so 0.05% or less, preferably 0.03% or less.

V
V precipitates during air cooling after accelerated cooling and increases the strength, so it is added according to the desired characteristics. When added, if it exceeds 0.05%, weldability and toughness are impaired, so 0.05% or less, preferably 0.03% or less.

Ti
Ti increases strength and improves weld toughness, so it is added according to desired properties. When adding, if it exceeds 0.03%, the steel material cost increases, so 0.03% or less, preferably 0.02% or less.

B
B increases the hardenability and increases the strength, so it is added according to the desired properties. When adding, if it exceeds 0.003%, weldability deteriorates, so 0.003% or less, preferably 0.002% or less.

[Production conditions]
Steel components according to the steel according to the present invention described above, 1000 ° C. or higher, then heated to 1300 ° C. or less to complete the rolling at a cumulative reduction of 50% or more of the rolling performed Ar ₃ point or more by Ar ₃ or more points Then, it is obtained by accelerated cooling at 10 ° C./s or more from Ar ₃ to Ar ₃ −60 ° C. to 650 ° C. or less and 450 ° C. or more.

1. Heating temperature The heating temperature is set to 1000 ° C. or higher in order to secure the rolling temperature. If the temperature exceeds 1300 ° C, the crystal grains of the steel become coarse, so the upper limit is made 1300 ° C or less.

2. Rolling conditions When the rolling end temperature is lower than the Ar ₃ point, two-phase region rolling occurs, and the pearlite extends in the rolling direction, causing anisotropy in the fatigue crack propagation rate. Further, when the cumulative rolling reduction at ₃ or more points of Ar is less than 50%, refinement of ferrite grains and refinement of pearlite intervals through refinement of austenite grains cannot be achieved. The rolling is desirably performed in the austenite recrystallization region so as not to cause anisotropy.

3. Accelerated cooling conditions The accelerated cooling start temperature is set to ₃ points or less in order to suppress the formation of bainite that reduces ductility and bending workability. Further, Ar ₃ point to 60 ° C. is used in order to suppress generation of band-like pearlite that causes anisotropy in fatigue crack propagation characteristics.

  The accelerated cooling stop temperature is set to 650 ° C. or lower and 450 ° C. or higher in order to cause pearlite transformation of untransformed austenite and to disperse the pearlite finely and uniformly in the steel material.

  When the accelerated cooling stop temperature exceeds 650 ° C., band-like pearlite that causes anisotropy in fatigue crack propagation characteristics is generated. Further, when the accelerated cooling stop temperature is lower than 450 ° C., bainite is generated.

  The cooling rate is set to 10 ° C./s or more in order to prevent coarsening of ferrite that deteriorates fatigue crack propagation characteristics during cooling and coarsening of the pearlite interval through the ferrite.

The above temperature is the surface temperature of the steel material, and the cooling rate is the average cooling rate in the thickness direction of the steel material. The Ar ₃ point is determined by Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (where the element symbol represents the content in mass% of each element in the steel). be able to.

  A steel sheet having the following microstructure can be obtained by a combination of the above-described component composition and manufacturing conditions. The resulting microstructure is composed mainly of a two-phase structure of ferrite and pearlite, and has a small crack propagation anisotropy and fatigue crack propagation due to the characteristics of the area ratio and form on the L, T, and Z planes. Has excellent resistance characteristics.

[Microstructure]
In the present invention, the obtained microstructure is divided into three-dimensional microstructures represented by the area ratios of the ferrite and pearlite of the steel material and the L, T, and Z surfaces (defined in FIG. 1) of these structures. In pearlite spacing, ferrite grain size, and pearlite three-dimensional shape.

1. Considering the area ratio ductility and bending workability of ferrite and pearlite, the microstructure of the microstructure is a two-phase structure composed of ferrite and pearlite. When the ferrite area ratio is less than 65%, ductility and bending workability are lowered. On the other hand, when it exceeds 85%, sufficient strength cannot be obtained.

  When the pearlite area ratio is less than 10%, the effect of improving the fatigue crack propagation characteristics described later is not exhibited. On the other hand, when it exceeds 30%, weldability, ductility, and bending workability deteriorate.

  In the present invention, bainite and martensite are allowed to be mixed in as the remaining structure. However, in consideration of ductility and bending workability, the area fraction of the remaining structure is preferably 5% or less.

2. Perlite average interval between L plane and T plane The pearlite interval is the effective arrangement of pearlite in the direction in which fatigue cracks propagate, and this pearlite controls the formation of a plastic zone at the crack tip, resulting in wraparound to pearlite. In order to induce crack bending, the average interval is set to 15 μm or more.

  However, if the average interval exceeds 30 μm, the frequency of wraparound decreases and the crack propagation resistance decreases, so the thickness is set to 15 to 30 μm. It is desirable to secure an average interval for any surface of the steel sheet so as not to cause crack propagation anisotropy, and it is defined by the L and T planes as representatives.

3. The ferrite average particle diameter of ferrite on the L face and T face is set to 20 μm or less so that the number of times the fatigue crack collides with the ferrite grain boundary is sufficient to obtain the effect of reducing the crack growth rate.

  However, if the particle size is too small, the formation of the plastic region between the above-mentioned pearlites is hindered, so that it is 10 μm or more, and 10 to 20 μm. In order not to cause anisotropy of the average grain size crack propagation, it is desirable to secure an arbitrary surface of the steel sheet, and the L plane and the T plane are representatively defined.

4). Three-dimensional shape of pearlite massIn the present invention, the fatigue crack propagation resistance is controlled by controlling the shape change of the plastic zone at the crack tip that occurs when the fatigue crack approaches the pearlite, and by utilizing the flexural progress of the crack due to the pearlite wrapping around the fatigue crack. It has improved. The three-dimensional shape of pearlite is defined as follows so that this effect exhibits good fatigue crack propagation resistance in all directions without causing anisotropy in the crack propagation direction.

That is, the main purpose of the present invention is to provide equally excellent fatigue crack propagation resistance characteristics with respect to fatigue crack propagation in various directions with respect to the steel sheet, which is a concern in actual structures. The pearlite three-dimensional shape is defined by equations (1) and (2) so that the fatigue crack propagation characteristics with respect to the direction are equal.
L (L) ≦ 3Z (L) (1) Formula T (T) ≦ 3Z (T) (2) where L (L) is the average length in the L direction on the L plane, Z (L) represents the average length in the Z direction on the L plane, T (T) represents the average length in the T direction on the T plane, and Z (T) represents the average length in the Z direction on the T plane. 2 to 4 show the effect of the microstructure satisfying the above-mentioned regulations 1 to 3.

  FIG. 2 shows the relationship between the average pearlite interval and (actual crack propagation length) / (linear distance in the crack propagation direction), and the ferrite area ratio, pearlite area ratio, ferrite average particle diameter, and pearlite lump shape are within the scope of the present invention. Then, it was obtained from the result of fatigue crack propagation test (test condition: ΔK = 20 MPa√m constant) in the T direction of steel with only the pearlite average interval changed.

  If (actual crack propagation length) / (straight distance in the crack propagation direction) is large here, cracks and wraparound often occur in the pearlite part, resulting in an increase in the actual propagation distance and irregularities in the fracture surface. It means that there is an increase.

  (Actual crack propagation length) is defined by the total length of the crack propagated, and when the crack is meandering, the length is along the meandering. When the crack is branched, the length is the length of the main crack (the crack having the longest propagation length).

  (Linear distance in the crack propagation direction) is defined as the length obtained when a crack is projected in the crack propagation direction. When the crack is meandering, the crack propagation direction is meandering, the crack progressing direction, and the length obtained by projecting the crack in that direction. When the crack is branched, the crack propagation direction is the direction in which the main crack progresses, and the distance obtained by projecting the main crack in that direction.

  The object of the present invention is to set the value of (actual crack propagation length) / (linear distance in the crack propagation direction) to 1.1 or more. From the figure, this value is achieved at an average pearlite interval of 15 to 30 μm. It is recognized that

  FIG. 3 shows the relationship between (actual crack propagation length) / (linear distance in the crack propagation direction) and fatigue crack propagation speed obtained in the test performed in FIG.

  As a result of the increase in the actual propagation distance and the increase in the fracture surface irregularities due to the control of the pearlite average interval described above, this value is 1.1 or more as the (actual crack propagation length) / (linear distance in the crack propagation direction) increases. In this case, a decrease in fatigue crack propagation rate, that is, an improvement in fatigue crack propagation resistance is observed.

  FIG. 4 shows L (L) / Z (L): ratio of L-direction average length on the L plane to Z-direction average length on the L plane, T (T) / Z (T): T The ratio of the average length in the T direction in the plane to the average length in the Z direction in the T plane, and these and the speed (Z) / speed (L): the ratio of the fatigue crack propagation speed in the Z direction to the crack propagation speed in the L direction, Velocity (Z) / Velocity (T): Shows the relationship between the fatigue crack propagation speed in the Z direction and the crack propagation speed in the T direction.

  The results shown in FIG. 4 indicate that the fatigue crack propagation test (test condition: ΔK =) of steel in which the ferrite area ratio, pearlite area ratio, ferrite average particle diameter, and pearlite average interval are within the scope of the present invention, and only the pearlite lump shape is changed. It is obtained from the result at 20 MPa√m constant).

  By setting L (L) / Z (L) and T (T) / Z (T) to 3 or less, fatigue crack propagation characteristics with less directionality (small anisotropy) can be obtained. Equally excellent fatigue crack propagation resistance is realized for fatigue cracks that propagate in various directions with respect to the steel sheet.

  A steel sheet having a thickness of 12 to 50 mm was prepared under the conditions shown in Table 2 using steel pieces having the composition shown in Table 1, and the microstructure observation, mechanical properties, and fatigue crack propagation characteristics of the obtained steel sheet were investigated. .

  The structure observation was carried out using a sample obtained by polishing a sample collected from an arbitrary location at a thickness of / 4 on the T and L surfaces etched with 2% nital etchant.

  The area ratio of ferrite, the area ratio of pearlite, the average interval of pearlite, the average particle diameter of ferrite, and the shape of pearlite lump were measured by optical microscope observation. These values were carried out for 5 samples per sample, and were obtained as an average value in the total field of view.

  Fatigue crack propagation characteristics were examined by a fatigue crack propagation test when a crack propagates in the L direction and the T direction using a full-thickness CT test piece (a plate thickness exceeding 25 mm is a 25 mmt single-sided thinned CT test piece). The fatigue crack propagation characteristics in the case where cracks propagate in the Z direction were investigated using a full-thickness three-point bending test piece (thickness on one side is reduced by 25 mm when the plate thickness exceeds 25 mm).

The test conditions were a stress ratio of 0.1, a frequency of 20 Hz, and a room temperature atmosphere. In the examples, the fatigue crack propagation rate when the stress intensity factor K = 20 MPa√m is set to 5.0 × 10 ⁻⁸ m / cycle or less regardless of the crack propagation direction.

  The tensile strength was obtained by a tensile test using a full thickness test piece of JISZ2201 1A collected in the T direction. In the examples, TS was 400 MPa or more, and the elongation was 20% or more.

  As for toughness, a fracture surface transition temperature vTrs (° C.) was obtained by a Charpy impact test. A Charpy impact test piece (JISZ2202) was taken in parallel with the rolling direction from a thickness of / 4 (plate thickness of less than 25 mm was thickness / 2). In the examples, vTrs of −20 ° C. or less was taken as an example of the present invention.

  Table 3 shows the structure observation results, and Table 4 shows the tensile, toughness, and fatigue crack propagation test results. The plate number No. in which the components, the production method and the structure are within the scope of the present invention. 1-No. It is confirmed that the steel plate No. 8 exhibits excellent fatigue crack propagation resistance in any direction and is excellent in strength, ductility, and toughness.

  On the other hand, No. C, Si, and Mn exceed the scope of the present invention. Since the steel plate No. 9 is a high component, the ferrite fraction and the pearlite average interval are below the specified values of the present invention, and the pearlite fraction exceeds the specified values of the present invention. For this reason, fatigue crack propagation resistance is inferior and ductility and toughness are low.

The heating temperature exceeds the specified value of the present invention, and the rolling reduction rate of Ar ₃ points or more falls below the specified value of the present invention. Steel plate No. 10 has high hardenability and has a ferrite / bainite structure. For this reason, ductility and toughness are low.

The accelerated cooling start temperature is higher than the Ar ₃ point. Since the steel plate No. 11 has a bainite single phase structure, the ductility is low and the fatigue crack propagation resistance is poor. The rolling end temperature is lower than the Ar ₃ point (two-phase rolling), and the accelerated cooling start temperature is lower than Ar ₃ -60 ° C. Steel No. 12 has a ferrite grain size and an average pearlite interval below this specified value, and the pearlite lump shapes are L (L)> 3Z (L) and T (T)> 3Z (T).

  For this reason, anisotropy occurs in the fatigue crack propagation resistance, and the fatigue crack propagation rates in the L direction and the T direction are significantly deteriorated compared to the Z direction. No. in which the cooling rate during accelerated cooling is lower than this specified value. No. 13 steel sheet, No. in which the stop temperature during accelerated cooling exceeds this specified value. In the steel plate No. 14, the ferrite grain size and the pearlite average interval exceed the specified values.

  For this reason, it is inferior to fatigue crack propagation resistance and has low strength and toughness. No. of acceleration cooling stop temperature below this specified value. The steel plate of 15 has a ferrite / bainite structure. For this reason, ductility and toughness are low.

The figure explaining the L, T, Z surface and L, T, Z direction of the steel materials prescribed | regulated by this invention. The figure which shows the relationship of a pearlite average space | interval and (actual crack length) / (linear distance of a crack propagation direction). The figure which shows the relationship between (actual crack length) / (linear distance of a crack propagation direction) and fatigue crack propagation velocity. The figure which shows the relationship between a pearlite lump shape and a fatigue crack propagation rate ratio.

Claims (2)

In mass%, C: 0.04 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.5 to 1.8%, P: 0.05% or less, S: 0.02 % or less, the steel balance being substantially Fe, 1000 ° C. or higher, then heated to 1300 ° C. or less, after completion of the rolling at a cumulative reduction of 50% or more of the rolling performed Ar ₃ point or more by Ar ₃ or more points From the temperature range of Ar ₃ to Ar ₃ −60 ° C. to 650 ° C. or lower and 450 ° C. or higher, accelerated cooling is performed at 10 ° C./s or higher, and the microstructures of ferrite and pearlite having the characteristics of (1) to (3) A method for producing a steel material having excellent fatigue crack propagation resistance, comprising a ferrite having an area ratio of 65 to 85% in a phase structure.
(1) L-plane and T-plane pearlite average spacing: 15-30 μm
(2) L-plane and T-plane ferrite average particle diameter: 10 to 20 μm
(3) Perlite lump shape: L (L) ≦ 3Z (L), T (T) ≦ 3Z (T)
Here, L (L): L direction average length on the L plane, Z (L): Z direction average length on the L plane, T (T): T direction average length on the T plane, Z ( T): Average length in the Z direction on the T plane Further, as a steel component, in mass%, Cu: 0.4% or less, Ni: 0.8% or less, Cr: 0.4% or less, Mo: 0.4% or less, Nb: 0.05% or less, It has excellent fatigue crack propagation resistance according to claim 1, characterized by containing one or more of V: 0.10% or less, Ti: 0.03% or less, B: 0.003% or less. Steel manufacturing method.

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