JP4924047B2 - Manufacturing method of steel material having excellent fatigue crack propagation characteristics with absolute value of surface residual stress of 150 N / mm 2 or less - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a steel material having excellent fatigue crack propagation resistance, and particularly to a steel plate having a small residual stress and suitable for a thick steel plate used for materials such as a hull, an offshore structure, a bridge, a building structure, a construction machine, and an industrial machine. .

In recent years, with the increase in size of steel structures, the application of accelerated cooling type high-strength steel sheets is expanding. Generally, when a steel plate is applied to a steel structure, it is assembled into a desired structure by welding after cutting or die cutting. When these slitting and die cutting are applied to high-speed steel sheets of accelerated cooling type, deformation such as warpage is a problem. This is thought to be due to the residual stress generated during the accelerated cooling process.
Conventionally, methods for suppressing residual stress include water volume adjustment and cold leveler correction during accelerated cooling, but they have not completely eliminated residual stress.
On the other hand, when a welded structure is subjected to repeated stresses in the operating environment, the stress concentrates on large discontinuities such as weld toes, causing fatigue cracks to develop and eventually penetrate.・ It may break and cause a major accident.

  In many cases, the life of a welded structure is determined by the progress of fatigue, and from the viewpoint of reducing the life cycle cost, suppression of fatigue fracture is desired. In addition, the destruction of welded structures such as hulls, offshore structures, bridges, etc., exposes human lives to danger, and from the viewpoint of safety, it is required to suppress the occurrence and progression of fatigue cracks.

  Conventionally, as a means to suppress the occurrence of fatigue cracks, the welding toe has a smooth shape with no discontinuities, and a welding method has been devised to avoid stress concentration. Therefore, it takes a lot of time, which causes problems such as reduced construction efficiency and increased manufacturing costs.

  In addition, when the design of a structure is complicated, stress concentration is unavoidable only by construction measures, and it is not always possible to take effective measures to suppress the occurrence of fatigue cracks.

  For such problems, when fatigue cracks occur in steel, it is effective to suppress the growth of fatigue cracks by slowing the propagation speed of fatigue cracks. If the fatigue crack propagation speed is slow, even if a fatigue crack occurs, the crack can be found and repaired by periodic inspections before the structure breaks down.

If the fatigue crack propagation rate of steel can be slowed, the frequency of periodic inspections, that is, the frequency of repairs can be reduced, which is advantageous in terms of the life cycle cost of the steel.
In response to such demands, Patent Documents 1 to 5 propose steel sheets and manufacturing methods for reducing the fatigue crack propagation rate.

  Patent Document 1 and Patent Document 2 describe a method of dispersing a hard second phase such as bainite and martensite in a ferrite matrix.

  Patent Document 3 describes a method for reducing the crack propagation speed in the plate thickness direction by controlling the crystal orientation of ferrite.

  Patent Document 4 describes a method of improving fatigue characteristics by reducing the ferrite grain size to 1 to 3 μm.

  Although the toughness can be improved at the same time by refining the crystal grains, it is necessary to perform rolling with a large cumulative reduction ratio of 50% or more in the austenite / ferrite two-phase region, which is lower than the normal hot rolling temperature. There is.

Patent Document 5 describes a technique for improving fatigue crack propagation characteristics by increasing the content of Si or Al so that retained austenite is contained in the steel.
Japanese Patent Laid-Open No. 10-60575 JP-A-11-310846 JP-A-8-199286 JP 2002-363644 A JP 2004-76156 A

  However, since the techniques described in Patent Document 1 and Patent Document 2 have high residual stress, deformation after line cutting and die cutting becomes a problem. In addition, the propagation of fatigue cracks may not be sufficiently suppressed, and there is a concern that toughness may be deteriorated.

  Since the technique described in Patent Document 3 has a high residual stress, there is a concern that deformation after line cutting and die cutting becomes a problem and fatigue crack propagation characteristics that propagate in directions other than the sheet thickness direction cannot be improved. is there.

  The technique described in Patent Document 4 has a concern that the load on the rolling mill is increased, the occupation time of the rolling mill is increased, and the rolling efficiency is reduced. Since the technique described in Patent Document 5 increases the content of Si and Al, there is a concern that the toughness of the base material and the weld heat affected zone deteriorates.

  Accordingly, an object of the present invention is to provide a steel sheet having a small residual stress and excellent fatigue crack propagation characteristics and a method for manufacturing the same, which solve the above-described problems of the prior art.

In order to achieve the above-mentioned problems, the present inventors diligently studied various factors affecting residual stress, fatigue crack propagation resistance and mechanical characteristics, and obtained the following knowledge.
(1) In order to improve fatigue crack propagation resistance, the steel sheet is composed of a ferrite phase that defines the upper limit of hardness as a soft phase and a tempered martensite phase that defines the lower limit of hardness as a hard phase. Furthermore, it is important to control the area fraction of the mixed tissue.
(2) In order to maximize the fatigue crack propagation characteristics by this mixed structure control, strict component adjustment is indispensable, and it does not increase the hardness of the ferrite phase. It is important to add Cr that promotes site formation.
(3) Furthermore, when it is combined with the addition of at least one of Mo or V having a high tempering and softening resistance, even better results can be obtained.
(4) In addition, after hot rolling the steel material whose components have been adjusted as described above, a two-phase reheating process in which the heating temperature and the cooling rate are optimized, and the heating temperature and the holding time are optimized. By performing the reversion treatment, the above-mentioned microstructure requirement can be achieved, excellent fatigue crack propagation characteristics and mechanical characteristics can be obtained, and further, the residual stress can be reduced.

The present invention has been completed based on the above findings and further studies, that is, the present invention,
1. Steel composition is mass%,
C: 0.05-0.30%,
Si: 0.03 to 0.35%,
Cr: 0.05 to 2.0%,
P: 0.03% or less S: 0.003% or less Al: A slab or steel slab containing 0.1% or less, the balance being Fe and inevitable impurities, after hot rolling, and reheating After quenching treatment, reheat to 2-phase region temperature range of Ac ₁ transformation point + 10 ° C-790 ° C, quench at average cooling rate 5-60 ° C / s, hold at 400-650 ° C for 10 minutes or more and temper A mixed structure of a ferrite structure having a Vickers hardness of 85 to 130 and a tempered martensite phase having a Vickers hardness of 15 to 85% and a 340 to 440 or less Vickers hardness. A method for producing a steel material having excellent fatigue crack propagation characteristics with an absolute value of surface residual stress of 150 N / mm ² or less .
2. When the steel composition is mass%,
Mo: 0.05-1.0%,
V: 0.01 to 0.3%
Tempering of 340 to 440 or less with a Vickers hardness of 15 to 85% Vickers hardness and a ferrite structure having a Vickers hardness of 85 to 130 and a metal structure described in 1 containing 1 type or 2 types of A method for producing a steel material that is a mixed structure of martensite phase and has excellent fatigue crack propagation resistance with an absolute value of surface residual stress of 150 N / mm ² or less .
3. When the steel composition is mass%,
Mn: 1.2% or less Cu: 0.8% or less Ni: 1.0% or less Nb: 0.1% or less Ti: 0.03% or less B: 0.005% or less Ca: 0.005% or less REM : 0.02% or less Mg: 0.005% or less of 1 or 2 containing one or more , the metal structure is a Vickers hardness of 85 to 130 ferrite phase, and the area fraction A method for producing a steel material excellent in fatigue crack propagation resistance having an absolute value of surface residual stress of 150 N / mm ² or less, which is a mixed structure of tempered martensite phase of 340 to 440 with a Vickers hardness of 15 to 85% .

  According to the present invention, it is possible to stably produce a thick steel plate having excellent fatigue crack propagation resistance, greatly contributing to the improvement of the reliability of the steel structure and the reduction of the life cycle cost, and a remarkable industrial effect. Play.

In the present invention, the metal structure, component composition and production conditions are defined. The reasons for these limitations will be explained in detail below.
[Metal structure]
In the present invention, not only the hardness and dispersion amount of the hard phase but also the hardness of the soft phase is specified. In order to stably achieve excellent fatigue crack propagation characteristics and mechanical properties, the hard phase in the metal structure is made into a tempered martensite phase with an area fraction of 15 to 85% and a Vickers hardness of HV340 or more, and The mixed structure is a ferrite phase in which the Vickers hardness of the soft phase is limited to HV130 or less.

  Reducing the hardness of the ferrite phase decreases the fatigue crack growth rate by expanding the strain region at the tip of the fatigue crack and suppressing work hardening during repeated load strain. In addition, when the main crack that has propagated through the ferrite phase is tempered and reaches very close to the martensite phase, a microcrack is generated from the main crack, and the main crack is bent and branched to reduce the fatigue crack growth rate.

  In order to obtain this effect, the Vickers hardness of the ferrite phase is limited to the range of HV85 to 130. In addition, Preferably, it is set as HV95 or more and 120 or less.

  Further, the Vickers hardness HV of the tempered martensite phase is limited to 340 or more and 440 or less. In addition, Preferably, it is set as HV350-420. In the case of an as-quenched martensite phase, the ductility and toughness of the base metal deteriorate, so tempering is performed to obtain a tempered martensite phase.

  If the area fraction of the tempered martensite phase is less than 15% or more than 85%, the effect of delaying fatigue crack propagation as described above cannot be obtained, so the area fraction of the tempered martensite phase is It is limited to a range of 15 to 85%. In addition, Preferably, it is 20 to 80%.

  In addition, the hardness is a range of 0.098 N (10 gf) to 0.98 N (100 gf), preferably a ferrite phase and a tempered martensite phase of a hardness test piece using a micro Vickers hardness meter, preferably It is defined by the value obtained at 0.49 N (50 gf). In the case of this condition, it is possible to ignore errors due to test conditions.

  For each of the ferrite phase and the tempered martensite phase, the hardness of at least 10 grains is measured, and the average value is taken as the hardness of each phase.

  In the steel of the present invention, it is allowed to mix a small amount of a structure such as bainite and pearlite as the third phase other than the ferrite phase and the tempered martensite phase within a range not deteriorating the fatigue crack propagation characteristics.

  The area fraction of the third phase is preferably small, and the structure of bainite and pearlite is preferably 5% or less in terms of area fraction.

Next, the reasons for limiting the component composition of the steel of the present invention will be specifically described.
[Ingredient composition]
Unless otherwise specified, the “%” label for ingredients means mass%.
C: 0.05-0.30%
C is an element useful for increasing the strength of steel and ensuring the strength necessary for structural steel. Further, in order to obtain the second phase structure of the tempered martensite phase having a Vickers hardness of 340 or more, the content of 0.05% or more is required.

  On the other hand, if the content exceeds 0.30%, the HAZ toughness and weld crack resistance deteriorate, and the toughness of the base material deteriorates. For this reason, C is limited to a range of 0.05 to 0.30%. In addition, Preferably, it is 0.08 to 0.25%.

Si: 0.03-0.35%
Si acts as a deoxidizer and suppresses the formation of cementite, thereby concentrating C into austenite and promoting martensite formation during quenching. Furthermore, in order to increase the temper softening resistance of the martensite phase during tempering, the content should be 0.03% or more.

  On the other hand, if the content exceeds 0.35%, the hardness of the ferrite phase is increased, and the fatigue crack propagation resistance is decreased. In addition, the toughness of the base material deteriorates, and the weldability and HAZ toughness deteriorate significantly. For this reason, Si is limited to the range of 0.03 to 0.35%. In addition, Preferably, it is 0.05 to 0.25%.

Cr: 0.05-2.0%
Cr is an important alloying element in the present invention. Even if it is added in a large amount, the effect on the Ar ₃ transformation point is small, and the same body-centered cubic structure as α-Fe has an atomic radius close to that of Fe. Very small and does not increase the hardness of the ferrite phase. On the other hand, at the time of quenching from the austenite region, the hardenability of austenite is increased and the formation of a martensite phase as a second phase structure is promoted.

  Furthermore, during tempering, it has the effect of increasing the temper softening resistance of the martensite phase and is effective in reducing the fatigue crack propagation rate. In this invention, in order to acquire this effect, 0.1% or more of content is required.

  On the other hand, if the content exceeds 2.0%, the weld crack resistance and the HAZ toughness deteriorate significantly. For this reason, Cr is limited to a range of 0.05 to 2.0%. In addition, Preferably, it is 0.1 to 1.5%.

P: 0.03% or less P is an element that increases the strength of steel and deteriorates toughness, and particularly deteriorates the toughness of welds. Therefore, it is desirable to reduce it as much as possible. If P is contained in excess of 0.03%, this tendency becomes remarkable, so the upper limit is set. In addition, excessive P reduction raises the refining cost and is economically disadvantageous, so 0.003% or more is desirable.

S: 0.0050% or less S is an element that deteriorates the toughness of the base metal and the welded portion, and is desirably reduced as much as possible. If S is contained in excess of 0.0050%, this tendency becomes remarkable, so the upper limit is set.

Al: 0.1% or less Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process of high-strength steel. Also, N in the steel is fixed as AlN, which contributes to improvement of the toughness of the base metal. On the other hand, if the content exceeds 0.1%, the toughness of the base metal decreases, and it is mixed into the weld metal during welding to deteriorate the toughness. For this reason, Al is limited to 0.1% or less. In addition, Preferably it is 0.01 to 0.07%.

  In the present invention, in addition to the basic component system described above, Mo: 0.05 to 1.0%, V: 0.01 to 0.3%, Mn: 1.2% or less, Cu, depending on the desired characteristics : 0.8% or less, Ni: 1.0% or less, Nb: 0.1% or less, Ti: 0.03% or less, B: 0.005% or less, Ca: 0.005% or less, REM: 0 One or two or more selected from 0.02% or less and Mg: 0.005% or less can be contained.

Mo: 0.05-1.0%
Mo enhances the hardenability of austenite during quenching, promotes the formation of martensite phase as a second phase structure, and significantly reduces temper softening of the martensite phase by generating carbides during tempering. It is effective in reducing the fatigue crack propagation rate. In order to exhibit this effect, addition of 0.05% or more is necessary. On the other hand, if added over 1.0%, the toughness is adversely affected. For this reason, when adding Mo, it limits to 0.05 to 1.0% of range.

V: 0.01 to 0.3%
V increases the hardenability of austenite during quenching, promotes the formation of martensite phase as a second phase structure, and significantly reduces temper softening of the martensite phase by generating carbides during tempering. It is effective in reducing the fatigue crack propagation rate.

  In order to obtain this effect, addition of 0.01% or more is necessary. On the other hand, if added over 0.3%, the toughness is adversely affected. For this reason, when adding V, it limits to 0.01 to 0.3% of range.

Mn: 1.2% or less Mn has the effect of increasing the strength of steel. On the other hand, if the content exceeds 1.2%, the ferrite phase hardness increases and the fatigue crack propagation delay effect deteriorates. For this reason, when adding Mn, it is limited to 1.2% or less.

Cu: 0.8% or less Cu is an element that can increase strength while maintaining high toughness, has little influence on HAZ toughness, is useful for increasing strength, and can be selected as necessary. Can be contained.

  On the other hand, when the content exceeds 0.8%, hot brittleness is caused to deteriorate the surface properties of the steel sheet, the hardness of the ferrite phase is increased, and the fatigue crack propagation delaying effect is reduced. For this reason, when adding Cu, it limits to 0.8% or less.

Ni: 1.0% or less Ni is an element that can increase strength while maintaining high toughness, has little effect on HAZ toughness, is useful for increasing strength, and can be selected as necessary. Can be contained. However, even if the content exceeds 1.0%, the effect is saturated, an effect commensurate with the content cannot be obtained, it becomes economically disadvantageous, the hardness of the ferrite phase increases, and fatigue crack propagation Delay effect is reduced. For this reason, when adding Ni, it limits to 1.0% or less.

Nb: 0.1% or less Nb is an element that contributes to strength improvement, but inclusion exceeding 0.1% degrades the base metal toughness and the HAZ toughness. For this reason, when adding Nb, it limits to 0.1% or less.

Ti: 0.03% or less Ti contributes to strength improvement, and also has a strong affinity with N, precipitates as TiN during solidification, and suppresses coarsening of austenite grains in HAZ, thereby contributing to toughening of HAZ. . On the other hand, if it exceeds 0.03%, the toughness of the base metal is deteriorated. For this reason, when adding Ti, it is desirable to limit to 0.03% or less.

B: 0.0050% or less B has an effect of increasing the strength of steel through improving hardenability. On the other hand, if the content exceeds 0.0050%, the hardenability is remarkably increased and the toughness and ductility of the base metal are deteriorated. For this reason, when adding B, it limits to 0.0050% or less.

Ca: 0.005% or less Ca is a useful element that improves toughness through refinement of crystal grains, but if added over 0.005%, the effect is saturated. The upper limit is 0.005%.

REM: 0.02% or less REM is an element that contributes to the improvement of toughness, but the effect is saturated even if contained over 0.02%, so when added, the upper limit is 0.02% .

Mg
Mg is a useful element that improves toughness through refinement of crystal grains, but the effect is saturated even if contained over 0.005%, so when added, the upper limit is 0.005% And

The balance other than the components described above is Fe and inevitable impurities. Next, the manufacturing conditions of the present invention will be described.
[Production conditions]
In the description, “° C.” related to the temperature means a temperature of 1/2 t part thickness unless otherwise specified.
Reheating temperature The steel according to the present invention is obtained by melting a molten steel having the above composition by a generally known method such as a converter, an electric furnace, a vacuum melting furnace or the like, and the obtained steel material is preferably 1000 ° C to 1300 ° C. Reheat to.

  If the reheating temperature is less than 1000 ° C, the deformation resistance in hot rolling becomes high and the amount of rolling reduction per pass cannot be made large. Therefore, the number of rolling passes increases and the rolling efficiency decreases, and the steel material The casting defect in (slab) may not be crimped.

On the other hand, when the reheating temperature exceeds 1300 ° C., surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases. For this reason, it is preferable to make the reheating temperature of a steel raw material into the range of 1000-1300 degreeC.
Hot rolling conditions The reheated steel material is hot rolled so that a desired plate thickness and shape can be satisfied. In the hot rolling, it is preferable that the rolling end temperature is not less than the Ar ₃ transformation point. The cooling after hot rolling may be air cooling or accelerated cooling.

In the case of an extremely thick steel plate having a plate thickness exceeding 80 mm, it is desirable to secure at least one rolling pass at which the rolling reduction per pass is 15% or more for zaku pressure bonding. When the rolling end temperature is less than the Ar ₃ transformation point, the deformation resistance becomes too high, the rolling load increases, and the burden on the rolling mill increases.

The correlation with the chemical composition of Ar ₃ points can be roughly organized by the following equation (1).
Ar ₃ = 868-396C + 25Si-68Mn-21Cu-36Ni-
25Cr-30Mo (1)
(However, C, Si, Mn, Cu, Ni, Cr, Mo: content of each alloy element (mass%))
Heat treatment In the present invention, the two-phase region reheating quenching treatment and tempering treatment after hot rolling is an important process in order to achieve both a reduction in residual stress and fatigue crack propagation resistance.

In the two-phase region reheating and quenching treatment, water-cooling is performed at an average cooling rate of 5 to 60 ° C./s after reheating and holding in the two-phase region temperature range of Ac ₁ transformation point + 10 ° C. to 790 ° C.

In the case of two-phase reheating quenching, if the Ac ₁ transformation point is less than + 10 ° C, the fraction of austenite phase is low, so the martensite fraction in the microstructure after quenching is small and the fatigue crack propagation characteristics deteriorate. , Ac ₁ transformation point + 10 ° C. or higher.

On the other hand, when not tempered after holding at a temperature of greater than 790 ° C., be subjected to a tempering treatment, since sufficient residual stress is not reduced, it reheated the two-phase region, Ac ₁ transformation point + 10 ℃ ~790 ℃, preferably , Ac ₁ transformation point + 15 ° C to 785 ° C. In addition, the Ac ₁ point can be roughly correlated with the chemical composition by the equation (2).
_{Ac 1 = 723-14Mn + 22Si-14.4Ni} + 23.3Cr (2)
(However, C, Si, Mn, Ni, Cr: Content of each alloy element (mass%))
The holding time in this temperature range is 5 min. In order to make the temperature uniform in the steel sheet and to suppress variation in characteristics. The above is preferable. When the holding time is 1 hr or longer, the toughness of the base material deteriorates due to the coarsening of the austenite grains, so that it is preferably within 1 hr.

  The above-described two-phase reheating quenching process effectively promotes the concentration of C to austenite and the dilution softening of the ferrite ground, resulting in a hard martensite and soft ferrite. This improves the fatigue crack propagation characteristics.

  When the average cooling rate after heating in a two-phase reheating quenching process is 5 ° C / s or less, a pearlite or bainite phase is formed as a hard phase, and the target martensite structure cannot be achieved, and fatigue crack propagation resistance characteristics Deteriorates.

  On the other hand, when the temperature is 60 ° C./s or higher, the residual stress is not sufficiently reduced even if tempering is performed, so the average cooling rate is limited to the range of 5 to 60 ° C./s, preferably 8 to 55 ° C./s. To do. In addition, an average cooling rate shall mean the cooling rate to holding temperature -50 degreeC-400 degreeC in a board thickness direction 1 / 2t part.

  In the present invention, the steel sheet is subjected to a reheat tempering treatment after the two-phase region reheating quenching treatment. Residual stress can be reduced without deteriorating fatigue crack propagation characteristics by tempering by holding at reheating temperature of 400-650 ° C for 10 minutes or more and then air cooling, improving toughness and ductility of base metal To do.

  When the tempering temperature is less than 400 ° C., the residual stress generated by the two-phase quenching is not sufficiently eliminated, and the toughness and ductility of the base material may be deteriorated.

  On the other hand, when the temperature exceeds 650 ° C., the hardness of the tempered martensite phase decreases and the effect of delaying fatigue crack propagation decreases, so the tempering temperature is 400 ° C. to 650 ° C., preferably 420 ° C. to 630 ° C. The range.

  If the holding time is less than 10 minutes, the effect of reducing the residual stress cannot be obtained. The upper limit of the holding time is not specified, but if it is 1 hour or more, the hardness of the tempered martensite phase is lowered and the effect of delaying fatigue crack propagation is lowered.

  In the present invention, reheating and normalizing or quenching may be performed between hot rolling and two-phase region reheating and quenching.

In the case of a thick steel plate, the structure in the thick steel plate is further homogenized and refined by being reheated and held in a temperature range above the Ac ₃ transformation point. Although the upper limit of the heating temperature is not specified, the surface property of the steel sheet deteriorates when it is 1100 ° C. or higher.

  Further, although the holding time is not specified, it is preferably within 1 hr since the toughness of the base material deteriorates due to coarsening of austenite grains when it is 1 hr or more.

  Thickness of the plate thickness shown in Table 2 by hot rolling-reheating quenching-tempering of steel materials prepared in the composition shown in Table 1 by the converter-ladle refining-continuous casting method. A steel plate was used.

  For the investigation of the texture fraction, a sample for microstructural observation was taken on the cross section parallel to the rolling direction of each steel plate obtained, and after taking Nital corrosion, the optical microstructure was photographed, and the texture fraction was analyzed using an image analyzer. The rate was determined.

  The scope of the present invention is a steel material having a tensile strength of 440 N / mm 2 or more and a total elongation of 22% or more, and the toughness is absorbed at −20 ° C. by a Charpy impact test using a 2 mm V notch test piece according to JISZ2242 (2006). The steel material has energy of 100 J or more.

  JIS No. 4 tensile test piece or JIS No. 5 full-thickness tensile test piece is taken from 1/2 position of the obtained steel plate thickness, and tensile test is performed according to the standard of JISZ2241 (2006). investigated.

In addition, V-notch test specimens were collected from the obtained steel plate thickness 1/2 position in accordance with JISZ2202 (2006), and Charpy impact test was conducted in accordance with JISZ2242 (2006). , The absorbed energy (vE _-20 ) at -20 ° C was obtained, and the base metal toughness was investigated.

  To investigate the fatigue crack propagation characteristics, CT specimens based on ASTM E647 are collected from each steel plate so that the load direction is parallel to the rolling direction, and the fatigue crack propagation test is performed by the crack gauge method to determine the propagation speed. It was.

The steel material having excellent fatigue crack propagation characteristics in the present invention is a steel material having a fatigue crack propagation rate (da / dN) of 8 × 10 ⁻⁹ (m / cycle) or less at a stress intensity factor ΔK: 15 MPa / √m. .

The residual stress was investigated using the X-ray stress measurement method for the surface portion of each steel plate. The steel sheet having a small residual stress in the present invention is a steel material having an absolute value of surface residual stress of 150 N / mm ² or less.

The obtained results are shown in Table 3. In all of the examples of the present invention, the fatigue crack propagation rate (da / dN) at a stress intensity factor ΔK: 15 MPa / √m is extremely slow as 8 × 10 ⁻⁹ (m / cycle) or less, and excellent fatigue crack resistance. Has propagation characteristics.

In addition, the absolute value of the residual stress on the steel sheet surface is 150 N / mm ² or less, and the residual stress is extremely small. Furthermore, it was confirmed that it has a base material characteristic of tensile strength of 440 N / mm ² or more, total elongation of 22% or more, and high strength, high ductility, and high toughness of absorbed energy vE-20> 100 J at −20 ° C. .

  On the other hand, in a comparative example that is out of the scope of the present invention, one or more of the fatigue crack propagation characteristics, residual stress, and mechanical characteristics do not satisfy the target value.

Claims (3)

Steel composition is mass%,
C: 0.05-0.30%,
Si: 0.03 to 0.35%,
Cr: 0.05 to 2.0%,
P: 0.03% or less S: 0.003% or less Al: A slab or steel slab containing 0.1% or less, the balance being Fe and inevitable impurities, after hot rolling, and reheating After quenching treatment, reheat to 2-phase region temperature range of Ac ₁ transformation point + 10 ° C-790 ° C, quench at average cooling rate 5-60 ° C / s, hold at 400-650 ° C for 10 minutes or more and temper A mixed structure of a ferrite structure having a Vickers hardness of 85 to 130 and a tempered martensite phase having a Vickers hardness of 15 to 85% and a 340 to 440 or less Vickers hardness. A method for producing a steel material having excellent fatigue crack propagation characteristics with an absolute value of surface residual stress of 150 N / mm ² or less . When the steel composition is mass%,
Mo: 0.05-1.0%,
V: 0.01 to 0.3%
The metal structure described in claim 1 containing one or two of the above, a ferrite phase having a Vickers hardness of 85 or more and 130 or less, and a Vickers hardness of 15 to 85% in a Vickers hardness of 340 or more and 440 or less. A method for producing a steel material that is a mixed structure of a tempered martensite phase and has excellent fatigue crack propagation characteristics with an absolute value of surface residual stress of 150 N / mm ² or less . When the steel composition is mass%,
Mn: 1.2% or less Cu: 0.8% or less Ni: 1.0% or less Nb: 0.1% or less Ti: 0.03% or less B: 0.005% or less Ca: 0.005% or less REM : 0.02% or less Mg: 0.005% or less of one type or two or more types , wherein the metal structure has a Vickers hardness of 85 to 130 ferrite phase, Excellent fatigue crack propagation resistance with an absolute value of surface residual stress of 150 N / mm ² or less, which is a mixed structure of tempered martensite phase of 340 to 440 with a Vickers hardness of 15 to 85% . Steel manufacturing method .