JPWO2014208082A1 - High strength steel material with excellent fatigue characteristics and method for producing the same - Google Patents

Abstract

A high-strength steel material excellent in fatigue characteristics and a method for producing the same are provided for a thick steel plate having a thickness of 30 mm or more. The component composition is mass%, C: 0.10 to 0.20%, Si: 0.50% or less, Mn: 1.0 to 2.0%, P: 0.030% or less, S: 0.00. 0005-0.0040%, Sol. Al: 0.002 to 0.07%, Ca: 0.0005 to 0.0050%, consisting of the balance Fe and inevitable impurities, the metal structure is ferrite of the main phase, bainite of the second phase and High-strength steel material with excellent fatigue properties that is pseudo-pearlite.

Description

  The present invention relates to a thick steel plate having a thickness of 30 mm or more and 50 mm or less, and is suitable for welded structures such as ships, marine structures, bridges, buildings, tanks, and the like that are strongly required for structural safety, and is suitable for fatigue crack generation and fatigue crack propagation. The present invention relates to a high-strength steel material excellent in resistance and a method for producing the same.

  Steel materials used in ships, offshore structures, bridges, tanks, and other structures have excellent mechanical properties such as strength and toughness and weldability, as well as repeated loads, wind, and earthquakes during normal operation. The structure must have structural safety against repeated vibration caused by the above.

  Excellent fatigue characteristics are required for repeated loads and vibrations. In particular, it is considered effective to suppress the occurrence and progress of fatigue cracks of steel materials in order to prevent ultimate failure such as member breakage.

  In the case of a general welded structure, the weld toe portion tends to be a stress concentration portion, and tensile residual stress due to welding also acts, so that it often becomes a source of fatigue cracks. As a preventive measure, it is known to sew and weld a weld toe or to introduce compressive residual stress by shot peening.

  However, the welded structure has a large number of weld toes and is expensive. For this reason, these methods are unsuitable for implementation on an industrial scale, and the improvement of the fatigue resistance characteristics of the welded structure is often achieved by improving the fatigue characteristics of the steel material itself used.

  Non-Patent Document 1 discusses the fatigue crack propagation behavior of two types of steel materials manufactured by repeatedly performing special heat treatments on a lab scale with steel of limited components. This document describes a steel material A in which a hard phase (Vickers hardness: 565, phase fraction: 36.4%, average phase size: 149 μm) is uniformly dispersed in a soft phase (Vickers hardness: 148), and a hard phase As a result of investigating the fatigue crack propagation property with the steel material B in which the soft phase (Vickers hardness: 149) is surrounded by a mesh with a phase (Vickers hardness: 546, phase fraction: 39.2%), the steel material B However, it is stated that the fatigue crack propagation speed is greatly reduced along with detailed considerations.

  Patent Document 1 has a fatigue crack progress suppressing effect characterized in that a metal structure is composed of a hard part base and a soft part dispersed in the base, and the hardness difference between the two parts is 150 or more in terms of Vickers hardness. The steel plate which has is described.

  In Patent Document 2, the metal structure is composed of a structure including ferrite and a hard second phase, and the hard second phase in a cross-sectional structure parallel to the steel sheet surface has an area fraction of 20 to 80%, Vickers hardness. : A thick steel plate having excellent fatigue strength, characterized in that the average equivalent circle diameter is 250 to 800, the average equivalent circle diameter is 10 to 200 μm, and the maximum distance between the hard second phases is 500 μm or less.

  Patent Document 3 discloses fatigue crack resistance, characterized in that the metal structure is a bainite structure with an area ratio of 60 to 85%, a total of 0 to 5% martensite structure and pearlite structure, and the balance is a ferrite structure. Are described.

Japanese Patent No. 2962134 Japanese Patent No. 3860763 Japanese Patent No. 4466196

H.SUZUKI AND A.J.MCEVILY: Metallurgical Transactions A, Volume 10A, P475〜481, 1979

  However, the steel described in Non-Patent Document 1 requires five stages of heat treatment, and it is not realistic from the viewpoint of cost and productivity to perform process production on an industrial product scale. In addition, the ductility is reduced contrary to the fatigue crack propagation characteristics, and such steel cannot be applied to structures.

  Also in Patent Documents 1 and 2, since heat treatment is applied before and after hot rolling, it is not desirable in terms of efficiency in process production. For example, in Patent Document 2, diffusion heat treatment-hot rolling-two-phase region heat treatment is performed in order to improve the characteristics of a thick material.

  In Patent Document 3, a 15 mm thick material having a relatively small thickness is targeted, and a thick material having a thickness of 30 mm or more is not supported. In order to ensure the strength of the thick material, it is necessary to add an alloying element such as C. However, in Patent Document 3, the maximum amount of C is 0.1%, and there is a concern about insufficient strength when the thickness is increased.

  In any of the above-described inventions, either improvement of fatigue crack generation or fatigue crack growth is attempted, and a steel plate having both characteristics has not been studied. The suppression of the occurrence of fatigue cracks is improved by increasing the fatigue strength, that is, increasing the yield stress of the base steel plate. However, the higher the strength of the steel, the greater the stress concentration at the tip of the fatigue crack, which promotes fatigue crack growth.

  Then, an object of this invention is to provide the steel material excellent in the resistance of fatigue crack generation | occurrence | production and fatigue crack progress, and its manufacturing method regarding the thick steel plate of plate thickness 30mm or more and 50mm or less.

  The inventors of the present invention have made extensive studies to achieve the above-mentioned problems, and have obtained the following knowledge about high-strength thick steel sheets having excellent fatigue characteristics even with thick steel sheets having a thickness of 30 mm to 50 mm.

  1. In order to simultaneously improve both the fatigue crack initiation and fatigue crack growth characteristics of thick steel plates exceeding 30 mm thick, a mixed structure composed of ferrite of the main phase, bainite of the second phase and pseudo-pearlite; It is important to. Such a structure can be realized by manufacturing within an appropriate range of conditions. In the present invention, by containing 0.10% or more of C, it is possible to stably achieve high strength by increasing the area fraction of the second phase.

2. Furthermore, in a high-strength thick material, sulfide control by Ca addition works effectively in order to ensure fatigue characteristics. Ca fixes S by forming CaS, and produces | generates a composite inclusion with MnS. When MnS is present alone, it is elongated during rolling and becomes a starting point of fracture. However, by using CaS as a complex inclusion with MnS, fine dispersion is achieved in the parent phase, and resistance to fatigue crack initiation and fatigue crack growth is improved.
The present invention has been made by further studying the above knowledge, and the gist thereof is as follows.

  [1] Component composition is mass%, C: 0.10 to 0.20%, Si: 0.50% or less, Mn: 1.0 to 2.0%, P: 0.030% or less, S : 0.0005 to 0.0040%, Sol. Al: 0.002 to 0.07%, Ca: 0.0005 to 0.0050%, consisting of the balance Fe and inevitable impurities, the metal structure is ferrite of the main phase, bainite of the second phase and High-strength steel material with excellent fatigue properties that is pseudo-pearlite.

  [2] The component composition further includes one or two kinds selected from Ti: 0.003 to 0.03% and Nb: 0.005 to 0.05% by mass%. The high-strength steel material excellent in fatigue characteristics as described in [1].

  [3] The component composition is further mass%, Cr: 0.1 to 0.5%, Mo: 0.02 to 0.3%, V: 0.01 to 0.08%, Cu: 0.00. The high strength steel material having excellent fatigue properties according to [1] or [2], comprising at least one selected from 1 to 0.6% and Ni: 0.1 to 0.5% .

  [4] The component composition further includes O: 0.0040% or less, and satisfies the following formula (1): Excellent fatigue properties according to any one of [1] to [4] High strength steel.

0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S≦0.8 (1)
However, Ca, O, S in Formula (1) represents content (mass%) of each component.

[5] [1] or a steel material having a chemical composition according to any one of [4], after heating to: 950 ° C., subjected to cumulative rolling reduction of 50% or more rolling at Ar ₃ point or more, Ar ₃ A method for producing a high-strength steel material having excellent fatigue characteristics, characterized by accelerated cooling at a cooling rate of 5 ° C / s or higher from a temperature range of -60 ° C or higher to a temperature range of 600 ° C or lower and 350 ° C or higher.

  [6] The cooling rate is equal to or lower than the cooling rate when the cooling curve in the CCT diagram of the steel material having the component composition described in any of [1] to [3] is applied to the ferrite transformation nose. The method for producing a high-strength steel material having excellent fatigue properties as described in [5].

[7] The method for producing a high-strength steel material having excellent fatigue characteristics according to [5] or [6], further comprising tempering at a temperature of Ac ₁ point or less after the accelerated cooling.

  According to the present invention, a steel material excellent in fatigue crack resistance and fatigue crack growth resistance and a method for producing the same can be obtained. For example, even if fatigue cracks occur over time from stress-concentrated parts or welded parts, it is possible to delay the subsequent propagation and enhance the safety of the steel structure, which is extremely useful industrially.

FIG. 1 is a schematic diagram showing a CCT diagram (continuous cooling transformation diagram) of a steel material.

  The composition of the present invention, the production conditions and the definition of the metal structure will be described in detail.

1. About component composition Below, the component composition of this invention is demonstrated. In addition,% in a component composition shall be mass% altogether.

C: 0.10 to 0.20%
C must have a content of 0.10% or more in order to obtain the strength required for structural steel. However, if the content exceeds 0.20%, the weldability is impaired, so the C content is in the range of 0.10 to 0.20%. Preferably it is 0.10 to 0.18% of range. More preferably, it is 0.11 to 0.17% of range.

Si: 0.50% or less Si is an element useful as a deoxidizing element, and exhibits its effect when contained in an amount of 0.01% or more. However, if the content exceeds 0.50%, the toughness of the base metal and the weld heat-affected zone is remarkably lowered and the weldability is remarkably lowered. For this reason, the amount of Si shall be 0.50% or less. Preferably it is 0.05 to 0.40% of range.

Mn: 1.0-2.0%
Mn is added from the viewpoint of securing the strength of the base material. However, if the content is less than 1.0%, the effect is not sufficient. Moreover, when it contains exceeding 2.0%, hardenability will be raised excessively and the toughness of a heat affected zone will be reduced remarkably. For this reason, the amount of Mn shall be 1.0 to 2.0% of range. Preferably it is 1.0 to 1.8% of range. More preferably, it is 1.0 to 1.6% of range.

P: 0.030% or less When P exceeds 0.030%, the toughness of the base material and the heat-affected zone is significantly reduced. Therefore, the P content is 0.030% or less. Preferably it is 0.02% or less.

S: 0.0005 to 0.0040%
S is required to be 0.0005% or more in order to produce required CaS or MnS. If it exceeds 0.0040%, the toughness of the base material is deteriorated. Therefore, the S content is in the range of 0.0005 to 0.0040%. Preferably it is 0.001 to 0.0035% of range. More preferably, it is 0.001 to 0.0030% of range.

Sol. Al: 0.002 to 0.07%
Sol. Al is required to be 0.002% or more in view of deoxidation of steel, and is preferably contained in an amount of 0.01% or more. However, if it exceeds 0.07%, the toughness of the base material is lowered. Therefore, Sol. The Al content is in the range of 0.002 to 0.07%. Preferably it is 0.005 to 0.07% of range. More preferably, it is 0.01 to 0.06% of range.

Ca: 0.0005 to 0.0050%
Ca forms CaS to chemically fix S and generate complex inclusions with MnS. When MnS is present alone, it is elongated during rolling and becomes a starting point of fracture. However, a composite inclusion with MnS finely disperses in the parent phase and improves the resistance to fatigue cracking. In order to exhibit such an effect, it is necessary to contain at least 0.0005% or more. However, even if the content exceeds 0.0050%, the effect is saturated. For this reason, Ca amount is taken as 0.0005 to 0.0050% of range. Preferably it is 0.001 to 0.0040% of range. More preferably, it is 0.001 to 0.0030% of range.

  The above is the basic chemical component of the present invention, and the balance consists of Fe and inevitable impurities. Furthermore, you may contain 1 or more types chosen from Ti and Nb as a selection element in order to improve intensity | strength and toughness.

Ti: 0.003 to 0.03%
In order to further improve toughness, Ti can be contained. Ti generates TiN during heating before rolling, refines the austenite grain size, and improves toughness. If the content is less than 0.003%, the effect is not sufficient, and even if the content exceeds 0.03%, the effect is saturated. For this reason, when it contains Ti, it is preferable to make Ti amount into the range of 0.003-0.03%.

Nb: 0.005 to 0.05%
In order to improve the strength, Nb can be contained. If the content is less than 0.005%, the effect is not sufficient, and if it exceeds 0.05%, the toughness is lowered. For this reason, when it contains Nb, it is preferable to make the quantity into 0.005 to 0.05% of range. More preferably, it is 0.003 to 0.030% of range.

  In addition to the above composition, the high-strength steel material of the present invention may further contain one or more selected from Cr, Mo, V, Cu, and Ni as selective elements for the purpose of improving the strength.

Cr: 0.1 to 0.5%
If Cr is less than 0.1%, the effect is insufficient, and if it exceeds 0.5%, the weldability is lowered. For this reason, when it contains Cr, it is preferable to make Cr amount into the range of 0.1 to 0.5%. More preferably, it is 0.1 to 0.4% of range.

Mo: 0.02-0.3%
If Mo is less than 0.02%, its effect is insufficient, and if it exceeds 0.3%, the weldability is remarkably lowered. For this reason, when it contains Mo, it is preferable to make Mo amount into the range of 0.02-0.3%. More preferably, it is 0.02 to 0.20% of range.

V: 0.01 to 0.08%
If V is less than 0.01%, the effect is insufficient, and if it exceeds 0.08%, the toughness is remarkably lowered. For this reason, when it contains V, it is preferable to make V amount into the range of 0.01 to 0.08%. More preferably, it is 0.01 to 0.07% of range.

Cu: 0.1 to 0.6%
If Cu is less than 0.1%, the effect is not sufficient, and if it exceeds 0.6%, the concern about Cu cracking increases. For this reason, when it contains Cu, it is preferable to make Cu amount into the range of 0.1 to 0.6%. More preferably, it is 0.1 to 0.3% of range.

Ni: 0.1 to 0.5%
If the Ni content is less than 0.1%, the effect is not sufficient. If the Ni content exceeds 0.5%, the cost of the steel material is significantly increased. For this reason, when it contains Ni, it is preferable to make Ni amount into the range of 0.1 to 0.5%. More preferably, it is 0.1 to 0.4% of range.

  The high-strength steel material of the present invention preferably contains O in an amount of 0.0040% or less in addition to the above component composition.

O: 0.0040% or less If O exceeds 0.0040%, the toughness deteriorates, so the content is made 0.0040% or less.

  The high-strength steel material of the present invention preferably further satisfies the following formula (1).

0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S≦0.8 (1)
However, Ca, O, and S in the formula represent the content (% by mass) of each component.

  Ca, O, and S must be contained so that the above formula (Ca− (0.18 + 130 × Ca) × O) /1.25/S satisfies the relationship of 0 to 0.8. In this case, it becomes the form of the composite sulfide in which MnS is deposited on CaS. When MnS is present alone, it is elongated during rolling and becomes a starting point of fracture. However, by using CaS as a complex inclusion with MnS, fine dispersion is performed in the parent phase, thereby suppressing the occurrence of fatigue cracks. When the value of (Ca− (0.18 + 130 × Ca) × O) /1.25/S exceeds 0.8, MnS is not generated, and O and S are all crystallized as Ca oxysulfide. For this reason, the size becomes coarse, and the stress concentration at the matrix / inclusion interface becomes large, and it becomes difficult to ensure the fatigue strength. When (Ca− (0.18 + 130 × Ca) × O) /1.25/S is 0 or less, since CaS does not crystallize, S precipitates in the form of MnS alone, and this MnS is produced in the steel sheet. It is stretched by rolling at the time and fine dispersion is not maintained. Therefore, (Ca− (0.18 + 130 × Ca) × O) /1.25/S is set to a range of 0 to 0.8.

2. Metal structure In order to increase the tensile strength of 510 MPa or more, the metal structure is substantially a mixed structure of ferrite, bainite, and pseudopearlite. Substantially the mixed structure of ferrite, bainite and pseudo-pearlite has a total area fraction of 95% or more, and the balance is one or two of martensite, island martensite, retained austenite, etc. This is a structure containing 5% or less of the above by area fraction.

  The main phase is a structure having an area fraction exceeding 50%, and the ferrite of the main phase preferably has an area fraction of ferrite of 55% or more. The second phase is a structure having an area fraction of less than 50%.

  In order to increase the strength and improve the fatigue characteristics of a thick material having a plate thickness of 30 mm or more and 50 mm or less, it is desirable to disperse bainite and pseudo pearlite as the second phase in an area fraction of 15% or more in total. By setting the area fraction to 15% or more, the strength of the base material and the fatigue strength are expected to be improved. In addition, pseudo pearlite is a structure mainly composed of massive carbides in which the lamellar shape collapses and the carbides are curved or dispersed in lumps, whereas pearlite (lamellar pearlites) in which the carbide and ferrite phases are dispersed in layers. There are also some lamellar carbides (40% or less in area fraction with respect to the total amount of carbides). When the carbide is in the form of a lump, it is considered that the stress concentration at the interface between the parent phase and the second phase is reduced as compared with the lamellar shape, the occurrence of fatigue cracks is suppressed, and the fatigue strength is improved.

3. About the production method Steel having the above composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and a steel material such as a slab is formed by a conventional method such as a continuous casting method or an ingot-bundling method. It is preferable to do. The melting method and the casting method are not limited to the methods described above. From the viewpoint of economy, it is desirable to cast a steel piece by a steelmaking process by a converter method and a continuous casting process. Then, the performance is rolled into a desired shape. The production conditions of the present invention are shown below.

  The steel temperature condition defined in the present invention refers to the average temperature in the steel slab or steel plate thickness direction. The average temperature in the plate thickness direction is obtained by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions, and the like. For example, the average temperature in the plate thickness direction can be obtained by calculating the temperature distribution in the plate thickness direction using the difference method.

(1) Heating temperature: 950 to 1250 ° C
In performing hot rolling, since it is necessary to completely austenite the steel slab, the heating temperature is set to 950 ° C. or higher. On the other hand, if the steel slab is heated to a temperature exceeding 1250 ° C., coarsening of austenite grains starts and adversely affects toughness, so the heating temperature is set to a range of 950 to 1250 ° C. From the viewpoint of toughness, a preferable heating temperature range is 1000 ° C to 1100 ° C.

(2) Cumulative rolling reduction at ₃ or more points of Ar: 50% or more In rolling, work strain is introduced in a temperature range of ₃ or more points of Ar in order to maintain fine crystal grains and improve toughness. By setting the cumulative reduction ratio to 50% or more, the ferrite crystal grains after transformation are sufficiently refined, and toughness is improved. Accordingly, the cumulative rolling reduction during rolling is set to 50% or more at Ar ₃ points or more. Incidentally, Ar ₃ point is calculated by the following formula (2).
Ar ₃ = 910-310 [% C] -80 [% Mn] -20 [% Cu] -55 [% Ni] -15 [% Cr] -80 [% Mo] (2)
Here, each element symbol means content (mass%) of each element, and is set to 0 when not contained.
When the hot rolling temperature is lower than the ferrite transformation start temperature, ferrite is generated during the reduction and the strength is lowered. Therefore, the hot rolling end temperature is set to at least Ar ₃ points or more.

(3) Cooling start temperature: Ar ₃ points −60 ° C. or more If the cooling start temperature is too low, the ferrite generation amount increases and the strength decreases in the stage before accelerated cooling. For this reason, cooling is started from a temperature of Ar ₃ −60 ° C. or higher.

(4) Cooling rate: 5 ° C./s or more Accelerated cooling is performed following hot rolling. By setting the cooling rate to 5 ° C./s or more, a fine-grained structure can be obtained without coarsening the structure, and excellent excellent strength, toughness and fatigue characteristics can be obtained. If the cooling rate is less than 5 ° C./s, the structure becomes coarse and the ferrite fraction becomes large, and the target base material strength, fatigue strength, and fatigue crack growth resistance cannot be obtained. Moreover, as an upper limit of a cooling rate, it is preferable that it is below the cooling rate when the cooling curve in the CCT figure of the steel raw material which has said component composition starts to a ferrite transformation nose. If the cooling rate exceeds the cooling rate applied to the ferrite transformation nose, the bainite fraction becomes high, and the target fatigue crack resistance, ductility and toughness of the base material cannot be obtained. In order to obtain a desired structure, the plate thickness is preferably 30 mm to 50 mm within this cooling rate range.

  The CCT diagram (continuous cooling transformation diagram) shows a plurality of φ8 × 12mm cylindrical samples taken from steel materials having the above composition, and processes corresponding to rolling and various cooling rates using a hot-working reproduction test device. It is prepared by the usual method of examining the transformation temperature by measuring the expansion of the test piece at the same time by performing the heat treatment with the cooling pattern in FIG. As shown in FIG. 1, a constant cooling rate curve passing through the ferrite transformation nose of the obtained CCT diagram (the fastest cooling rate in the ferrite transformation region) (the horizontal axis (time) is logarithmic in the CCT diagram) Therefore, the cooling rate is obtained. In the present invention, pseudo pearlite is generated and the fatigue strength is improved by cooling at a cooling rate of 5 ° C./s or less when the cooling curve in the CCT diagram is applied to the ferrite transformation nose.

(5) Cooling stop temperature: 600-350 ° C
By setting the cooling stop temperature to 600 ° C. or lower and 350 ° C. or higher, a desired structure obtained by hot rolling and subsequent cooling can be formed. If the cooling stop temperature is higher than 600 ° C, the amount of bainite or pseudopearlite dispersed becomes insufficient, and if it is lower than 350 ° C, it becomes difficult to ensure ductility and toughness. The cooling stop temperature is more preferably 550 ° C. or lower and 400 ° C. or higher.

(6) Tempering temperature: Ac ₁ point or less When the shape correction, ductility, and toughness of the steel material are required, tempering can be performed at less than Ac ₁ point after accelerated cooling. When the tempering temperature exceeds ₁ Ac, island-shaped martensite is generated and the toughness deteriorates. In addition, Ac ₁ point is calculated | required by following formula (3).
Ac ₁ = 723-11 [% Mn] +29 [% Si] -17 [% Ni] +17 [% Cr] (3)
Here, each element symbol means content (mass%) of each element, and is set to 0 when not contained.

  For steel slabs having the composition shown in Table 1, specimen steels having a thickness of 30 to 50 mm were produced under the production conditions shown in Table 2, and the metal structure observation, mechanical properties and fatigue strength, fatigue crack growth of the obtained steel plates were obtained. The characteristics were investigated. In addition, the cooling rate when the cooling curve in the CCT diagram (continuous cooling transformation diagram) is applied to the ferrite transformation nose is obtained by collecting a plurality of φ10 × 12 mm cylindrical samples from steel materials having the composition shown in Table 1. It was created and obtained by the usual method of investigating the transformation temperature by measuring the expansion of the test piece at the same time by processing and heat treatment with processing corresponding to rolling and cooling patterns at various cooling rates in the processing reproduction test equipment .

  The structure observation was performed at a thickness 1/4 position of the cross section in the rolling direction (L cross section) etched with a 3% nital etchant using a sample obtained by polishing a sample collected from an arbitrary location. Moreover, the area ratio of ferrite, bainite, and pseudo pearlite was measured by observation with an optical microscope. These values were carried out for 5 samples per sample, and were obtained as an average value over the total field of view.

  Tensile properties were measured in the direction perpendicular to the rolling direction (C direction) using a test piece (NKV1 test piece) with a total thickness of 200 mm between the gauge points in accordance with the provisions of NK class K. And tensile properties were determined.

  As for toughness, a 2 mm V notch Charpy impact test piece (NKV4 test piece) was sampled in parallel to the rolling direction from the 1/4 thickness position, and a Charpy impact test was conducted in accordance with the provisions of NK classification K. The evaluation was performed using the average value (vE-40 (J)) of three samples at a temperature of -40 ° C.

  Fatigue strength was evaluated using a round bar tensile test piece having a diameter of φ12 mm × distance between gauges of 24 mm, and a value at a stress load of 1 million times. In accordance with JIS Z2273, the test piece was sampled from a position where the plate thickness was 50 mm and the thickness of the plate was 30 mm, and from the position where the plate thickness was 30 mm.

The fatigue crack growth characteristics were investigated in a fatigue crack growth test when a crack propagates in the C direction using a CT test piece having a plate thickness of 25 mm in accordance with ASTM E647. The specimens were sampled from a 1/4 thickness position for a 50 mm thick material and from a 1/2 thickness position for a 30 mm thick material. The test conditions were a stress ratio of 0.1 and room temperature atmosphere, and the fatigue crack growth rate was evaluated when the stress intensity factor range (ΔK) was 25 MPa · m ^1/2 .

  The test results are shown in Table 3.

The test results are: Yield stress YS: 390 N / mm ² or more, Tensile strength TS: 510 N / mm ² or more, Elongation: 19% or more, vE-40: 100 J or more, Fatigue strength: 340 Mpa or more, Fatigue crack growth rate: 1 0.0 × 10 ⁻⁷ (m / cycle) or less was determined as a pass / fail criterion.

From Table 3, No. 1 is an example of the present invention. In 1-1 to 8-1, it was confirmed that the yield stress YS was 390 N / mm ² or more and the tensile strength TS was 510 N / mm ² or more, and excellent base material characteristics were obtained. In addition, the steel of the present invention has excellent fatigue properties with a fatigue strength of 340 MPa or more and a fatigue crack growth rate of 1.0 × 10 ⁻⁷ (m / cycle) or less. On the other hand, No. which is a comparative example in which chemical components and production conditions are outside the scope of the present invention. 9-1 to 12-1 are inferior in any one or more of the above characteristics.

  About the steel slab of the component composition shown in Table 4, sample steel with a plate thickness of 30 to 50 mm was produced under the manufacturing conditions shown in Table 5, and the metal structure observation, mechanical properties and fatigue strength, fatigue crack growth of the obtained steel plate The characteristics were investigated. The cooling rate when the cooling curve in the CCT diagram (continuous cooling transformation diagram) is applied to the ferrite transformation nose was obtained by collecting a plurality of φ10 × 12 mm cylindrical samples from steel materials having the composition shown in Table 4 It was created and obtained by the usual method of investigating the transformation temperature by measuring the expansion of the test piece at the same time by processing and heat treatment with processing corresponding to rolling and cooling patterns at various cooling rates in the processing reproduction test equipment .

  The structure observation was performed at a thickness 1/4 position of the cross section in the rolling direction (L cross section) etched with a 3% nital etchant using a sample obtained by polishing a sample collected from an arbitrary location. Moreover, the area ratio of ferrite, bainite, and pseudo pearlite was measured by observation with an optical microscope. These values were carried out for 5 samples per sample, and were obtained as an average value over the total field of view.

  Tensile properties were measured in the direction perpendicular to the rolling direction (C direction) using a test piece (NKV1 test piece) with a total thickness of 200 mm between the gauge points in accordance with the provisions of NK class K. And tensile properties were determined.

  As for toughness, a 2 mm V notch Charpy impact test piece (NKV4 test piece) was sampled in parallel to the rolling direction from the 1/4 thickness position, and a Charpy impact test was conducted in accordance with the provisions of NK classification K. The evaluation was carried out using the average value (vE-40 (J)) of three at a temperature of -40 ° C.

  Fatigue strength was evaluated using a round bar tensile test piece having a diameter of φ12 mm × distance between gauges of 24 mm, and a value at a stress load of 1 million times. In accordance with JIS Z2273, the test piece was sampled from a position where the plate thickness was 50 mm and the thickness of the plate was 30 mm, and from the position where the plate thickness was 30 mm.

The fatigue crack growth characteristics were investigated in a fatigue crack growth test when a crack propagates in the C direction using a CT test piece having a plate thickness of 25 mm in accordance with ASTM E647. The specimens were sampled from a 1/4 thickness position for a 50 mm thick material and from a 1/2 thickness position for a 30 mm thick material. The test conditions were a stress ratio of 0.1 and room temperature atmosphere, and the fatigue crack growth rate was evaluated when the stress intensity factor range (ΔK) was 25 MPa · m ^1/2 .

  The test results are shown in Table 6.

The test results are: Yield stress YS: 390 N / mm ² or more, Tensile strength TS: 510 N / mm ² or more, Elongation: 19% or more, vE-40: 100 J or more, Fatigue strength: 340 Mpa or more, Fatigue crack growth rate: 8 .Times.10.sup.- ⁸ (m / cycle) or less was determined as a pass / fail criterion.

From Table 6, No. which is an example of the present invention. In 1-2 to 2-2, it was confirmed that the yield stress YS was 390 N / mm ² or more and the tensile strength TS was 510 N / mm ² or more and excellent base material characteristics were obtained. In addition, the steel of the present invention has a fatigue strength of 340 MPa or more, a fatigue crack growth rate of 8.5 × 10 ⁻⁸ (m / cycle) or less, and excellent fatigue characteristics. It can be said that when the formula (1) is greater than 0 and less than or equal to 0.8, a high-strength steel material superior in fatigue crack growth resistance can be obtained. On the other hand, No. which is a comparative example in which chemical components and production conditions are outside the scope of the present invention. 9-2 to 16-2 are inferior in any one or more of the above characteristics.

Claims (7)

  The component composition is mass%, C: 0.10 to 0.20%, Si: 0.50% or less, Mn: 1.0 to 2.0%, P: 0.030% or less, S: 0.00. 0005-0.0040%, Sol. Al: 0.002 to 0.07%, Ca: 0.0005 to 0.0050%, consisting of the balance Fe and inevitable impurities, the metal structure is ferrite of the main phase, bainite of the second phase and High-strength steel material with excellent fatigue properties that is pseudo-pearlite.   The component composition further contains one or two kinds selected from Ti: 0.003 to 0.03% and Nb: 0.005 to 0.05% by mass%. 1. A high-strength steel material excellent in fatigue characteristics as described in 1.   In addition, the component composition is, in mass%, Cr: 0.1 to 0.5%, Mo: 0.02 to 0.3%, V: 0.01 to 0.08%, Cu: 0.1 to 0 The high-strength steel material excellent in fatigue characteristics according to claim 1 or 2, characterized by containing at least one selected from .6% and Ni: 0.1 to 0.5%. The component composition further contains O: 0.0040% or less and satisfies the following formula (1): The high-strength steel material excellent in fatigue characteristics according to any one of claims 1 to 3.
0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S≦0.8 (1)
However, Ca, O, S in Formula (1) represents content (mass%) of each component. A steel material having the component composition according to any one of claims 1 to 4 is heated to 950 to 1250 ° C, and then rolled at an Ar ₃ point or higher and a cumulative reduction ratio of 50% or higher, Ar ₃ point to -60 ° C or higher. A method for producing a high-strength steel material having excellent fatigue properties, characterized by accelerated cooling at a cooling rate of 5 ° C./s or more from a temperature range of 600 ° C. to 350 ° C.   The said cooling rate is below the cooling rate when the cooling curve in the CCT figure of the steel raw material which has the component composition in any one of the said Claim 1 thru | or 4 applies to a ferrite transformation nose. A method for producing a high-strength steel material having excellent fatigue properties as described in 1. The method for producing a high-strength steel material having excellent fatigue characteristics according to claim 5 or 6, further comprising tempering at a temperature of Ac ₁ point or less after the accelerated cooling.

Images