CN105229190A - The High Strength Steel of excellent in fatigue characteristics and manufacture method thereof - Google Patents

Abstract

The present invention relates to a thick steel plate having a plate thickness of 30 mm or more, and provides a high-strength steel material excellent in fatigue properties and a manufacturing method thereof. A high-strength steel material excellent in fatigue properties, the composition of which contains C: 0.10-0.20%, Si: 0.50% or less, Mn: 1.0-2.0%, P: 0.030% or less, S: 0.0005-0.0040%, Sol.Al: 0.002 to 0.07%, Ca: 0.0005 to 0.0050%, and the remainder is composed of Fe and unavoidable impurities. The metal structure is ferrite as the main phase and bainite and pseudo-pearlite as the second phase.

Description

High-strength steel material excellent in fatigue properties and manufacturing method thereof

technical field

The present invention relates to a thick steel plate with a plate thickness of 30 mm to 50 mm, and relates to resistance to fatigue crack generation and fatigue crack development suitable for welded structures such as ships, marine structures, bridges, buildings, tanks, etc. that strongly require structural safety A high-strength steel material with excellent properties and a method for producing the same.

Background technique

Steel materials used in ships, marine structures, bridges, tanks and other structures, in addition to excellent mechanical properties such as strength and toughness, and excellent weldability, must have a Structural safety of structures.

Excellent fatigue properties are required for repeated loads and repeated vibrations. In particular, in order to prevent final failure such as component fracture, it is considered effective to suppress the occurrence and growth of fatigue cracks that steel materials have.

In the case of a normal welded structure, the edge of the welded seam tends to become a stress concentration part, and tensile residual stress due to welding also acts, so it often becomes a source of fatigue cracks. As a countermeasure against this, it is known to perform welding without welding rod on the edge of the welded seam, or to introduce compressive residual stress by shot peening.

However, the welded structure has a large number of weld seam portions, and also has a large cost burden. Therefore, these methods are not suitable for implementation on an industrial scale, and the improvement of the fatigue resistance of welded structures is often achieved by the improvement of the fatigue properties of the steel materials used.

Non-Patent Document 1 discusses the fatigue crack propagation behavior of two types of steel materials produced by repeatedly subjecting a steel with a limited composition to special heat treatment on a laboratory scale. In this document, the following matters are examined and described in detail: For making the hard phase (Vickers hardness: 565, the fraction of the phase: 36.4%, the average size of the phase: 149 μm) in the soft phase (Vickers hardness: 148 ) and steel material B in which a soft phase (Vickers hardness: 149) is surrounded by a hard phase (Vickers hardness: 546, phase fraction: 39.2%) As a result of examining fatigue crack propagation, the fatigue crack propagation speed of the steel material B was significantly reduced.

Patent Document 1 describes a steel plate having an effect of suppressing the growth of fatigue cracks, which is characterized in that the matrix of the hard part and the soft parts dispersed in the matrix constitute a metal structure, and the difference in hardness between the two parts is represented by Vickers hardness Count as 150 or more.

Patent Document 2 describes a thick steel plate excellent in fatigue strength, characterized in that the metal structure is composed of a structure including ferrite and a hard second phase, and the above-mentioned hard second phase in the cross-sectional structure parallel to the steel plate surface The area fraction of the two phases is 20-80%, the Vickers hardness is 250-800, the average equivalent circle diameter is 10-200 μm, and the maximum interval between the hard second phases is 500 μm or less.

Patent Document 3 describes a steel plate excellent in resistance to fatigue crack growth, characterized in that the metal structure is a bainite structure of 60 to 85% in area ratio and a martensite structure of 0 to 5% in total. And pearlite structure, the rest is ferrite structure.

prior art literature

patent documents

Patent Document 1: Japanese Patent No. 2962134

Patent Document 2: Japanese Patent No. 3860763

Patent Document 3: Japanese Patent No. 4466196

non-patent literature

Non-Patent Document 1: H.SUZUKIANDA.J.MCEVILY: MetallurgicalTransactionsA, Volume10A, P475-481, 1979

Contents of the invention

However, the steel described in Non-Patent Document 1 requires five stages of heat treatment, and it is unrealistic from the standpoint of cost and productivity for process production on an industrial scale. In addition, ductility decreases contrary to fatigue crack propagation characteristics, so that such steel cannot be applied to structures.

Regarding Patent Documents 1 and 2, since heat treatment is applied before and after hot rolling, it is not preferable in terms of efficiency in process production. For example, in Patent Document 2, in order to improve the properties of thick-walled materials, diffusion heat treatment-hot rolling-two-phase domain heat treatment is performed.

In Patent Document 3, the relatively small thickness of 15 mm is used as an object, and it does not deal with thick-walled materials having a thickness of 30 mm or more. In order to ensure the strength of thick-walled materials, it is necessary to add alloying elements such as C. However, in Patent Document 3, the amount of C is at most 0.1%, and there is a possibility that the strength may be insufficient in the case of thickening.

In addition, any of the above-mentioned inventions seeks to improve either one of fatigue crack generation and fatigue crack growth, and no study has been made on a steel plate having both characteristics. 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 the growth of the fatigue crack.

Therefore, the present invention relates to a thick steel plate having a plate thickness of 30 mm to 50 mm, and an object of the present invention is to provide a steel material excellent in fatigue crack generation and fatigue crack growth resistance and a manufacturing method thereof.

The inventors of the present invention have conducted intensive studies to achieve the above-mentioned problems, and have obtained the following knowledge about a high-strength thick steel plate having excellent fatigue properties even with a thick steel plate of 30 mm to 50 mm in thickness.

1. For thick steel plates with a plate thickness greater than 30mm, in order to simultaneously improve the two characteristics of fatigue crack resistance and fatigue crack development resistance, it is important to make ferrite as the main phase and Bainite as the second phase. A mixed structure composed of body and pseudo-pearlite. Such organization can be achieved by fabrication under a suitable range of conditions. In the present invention, by containing 0.10% or more of C, it is possible to stably achieve high strength due to an increase in the area fraction of the second phase.

2. Furthermore, for high-strength thick-walled materials, in order to ensure fatigue properties, sulfide control by Ca addition works effectively. Ca fixes S by forming CaS, and generates composite inclusions with MnS. When MnS exists alone, it is elongated during rolling and becomes the starting point of destruction. However, by making CaS a composite inclusion with MnS and finely dispersing it in the matrix, the resistance to fatigue crack generation and fatigue crack growth is improved.

The present invention has been made by further studying the above-mentioned knowledge, and the gist thereof is as follows.

[1] A high-strength steel material excellent in fatigue properties, the composition of which contains C: 0.10 to 0.20%, Si: 0.50% or less, Mn: 1.0 to 2.0%, P: 0.030% or less, and S: 0.0005% to 0.0040%, Sol.Al: 0.002～0.07%, Ca: 0.0005～0.0050%, the rest is composed of Fe and unavoidable impurities, the metal structure is the main phase of ferrite and the second phase of bainite and pseudo-pearlite body.

[2] The high-strength steel material excellent in fatigue properties according to [1], wherein the component composition further contains one selected from Ti: 0.003 to 0.03%, Nb: 0.005 to 0.05%, or Two kinds.

[3] The high-strength steel material excellent in fatigue properties according to [1] or [2], wherein the component composition further contains, in mass %, Cr: 0.1-0.5%, Mo: 0.02-0.3%, One or more of V: 0.01 to 0.08%, Cu: 0.1 to 0.6%, and Ni: 0.1 to 0.5%.

[4] The high-strength steel material excellent in fatigue properties according to any one of [1] to [4], wherein the composition further contains O: 0.0040% or less and satisfies the following formula (1).

0<(Ca-(0.18+130×Ca)×O)/1.25/S≤0.8···(1)

However, Ca, O, and S in formula (1) represent the content (mass %) of each component.

[5] A method for producing high-strength steel excellent in fatigue properties, comprising heating a raw steel material having the composition described in any one of [1] to [4] to 950 to 1250° C., and then Rolling with a cumulative reduction ratio of 50% or more at the Ar ₃ point or higher, accelerated cooling from the Ar ₃ point-60°C or higher temperature range to the 350-600°C temperature range at a cooling rate of 5°C/s or higher.

[6] The method for producing a high-strength steel material excellent in fatigue properties according to [5], wherein the cooling rate is a steel material having the composition described in any one of the above [1] to [3] The cooling curve in the CCT diagram of is below the cooling rate when the ferrite transformation protrudes.

[7] The method for producing a high-strength steel material excellent in fatigue properties according to [5] or [6], wherein after the accelerated cooling, tempering is further performed at a temperature of Ac ₁ point or lower.

According to the present invention, a steel material excellent in resistance to fatigue crack occurrence and fatigue crack growth and a method for producing the same can be obtained. For example, even if fatigue cracks occur from stress concentration parts, welded parts, etc. over the years, the subsequent propagation can be delayed to improve the safety of steel structures, which is extremely useful industrially.

Description of drawings

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

detailed description

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

1. About composition

Hereinafter, the component composition of this invention is demonstrated. In addition, all the % in a component composition shall be mass %.

C: 0.10 to 0.20%

C requires a content of 0.10% or more in order to obtain the strength required as steel for structural use. However, if the content exceeds 0.20%, weldability will be impaired, so the amount of C is made into the range of 0.10 to 0.20%. It is preferably in the range of 0.10 to 0.18%. More preferably, it is the range of 0.11 to 0.17%.

Si: 0.50% or less

Si is an element useful as a deoxidizing element, and its effect is exerted by containing 0.01% or more. However, if the content is more than 0.50%, the toughness of the base metal and the weld heat-affected zone will be significantly lowered, and the weldability will be significantly lowered. Therefore, the amount of Si is made 0.50% or less. Preferably it is in the range of 0.05 to 0.40%.

Mn: 1.0-2.0%

Mn is added from the viewpoint of ensuring the strength of the base material. However, when the content is less than 1.0%, the effect is not sufficient. In addition, if the content is more than 2.0%, the hardenability will be increased excessively, and the toughness of the heat-affected zone will be significantly lowered. Therefore, the amount of Mn is set in the range of 1.0 to 2.0%. Preferably it is in the range of 1.0 to 1.8%. More preferably, it is the range of 1.0 to 1.6%.

P: 0.030% or less

If the P content exceeds 0.030%, the toughness of the base material and the heat-affected zone will be significantly reduced. Therefore, the amount of P is made 0.030% or less. Preferably it is 0.02% or less.

S: 0.0005～0.0040%

S needs to be 0.0005% or more in order to form desired CaS or MnS, and if the content exceeds 0.0040%, the toughness of the base material will be deteriorated. Therefore, the amount of S is set in the range of 0.0005 to 0.0040%. Preferably it is in the range of 0.001 to 0.0035%. More preferably, it is the range of 0.001 to 0.0030%.

Sol.Al: 0.002～0.07%

Sol.Al needs to be 0.002% or more for deoxidation of steel, preferably 0.01% or more. However, if the content exceeds 0.07%, the toughness of the base material will decrease. Therefore, the amount of Sol.Al is set in the range of 0.002 to 0.07%. It is preferably in the range of 0.005 to 0.07%. More preferably, it is the range of 0.01-0.06%.

Ca: 0.0005～0.0050%

Ca chemically immobilizes S by forming CaS to generate composite inclusions with MnS. When MnS exists alone, it is elongated during rolling and becomes the starting point of destruction. However, by forming composite inclusions with MnS and finely dispersing in the matrix phase, resistance to fatigue crack generation and fatigue crack growth is improved. In order to exhibit this effect, it is necessary to contain at least 0.0005% or more. However, even if the content is greater than 0.0050%, the effect is saturated. Therefore, the amount of Ca is set in the range of 0.0005% to 0.0050%. Preferably it is in the range of 0.001 to 0.0040%. More preferably, it is the range of 0.001 to 0.0030%.

The above is the basic chemical composition of the present invention, and the rest is composed of Fe and unavoidable impurities. Furthermore, for the purpose of improving strength and toughness, one or more selected from Ti and Nb may be contained as an optional element.

Ti: 0.003～0.03%

In order to further improve toughness, Ti may be contained. Ti forms TiN during heating before rolling, refines the austenite grain size, and improves toughness. When the content is less than 0.003%, the effect is insufficient, and even if the content exceeds 0.03%, the effect is saturated. Therefore, when Ti is contained, the amount of Ti is preferably in the range of 0.003 to 0.03%.

Nb: 0.005～0.05%

In order to improve the strength, Nb may be contained. When the content is less than 0.005%, the effect is not sufficient, and when it is more than 0.05%, the toughness is reduced. Therefore, when Nb is contained, its amount is preferably in the range of 0.005 to 0.05%. More preferably, it is the range of 0.003 to 0.030%.

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

Cr: 0.1-0.5%

When Cr is less than 0.1%, the effect is not sufficient, and when the content exceeds 0.5%, weldability will deteriorate. Therefore, when Cr is contained, the amount of Cr is preferably in the range of 0.1 to 0.5%. More preferably, it is the range of 0.1 to 0.4%.

Mo: 0.02 to 0.3%

When Mo is less than 0.02%, the effect is not sufficient, and when the content exceeds 0.3%, the weldability is remarkably reduced. Therefore, when Mo is contained, the amount of Mo is preferably in the range of 0.02 to 0.3%. More preferably, it is the range of 0.02 to 0.20%.

V: 0.01～0.08%

When V is less than 0.01%, the effect is not sufficient, and when the content exceeds 0.08%, the toughness is remarkably lowered. Therefore, when V is contained, the amount of V is preferably in the range of 0.01 to 0.08%. More preferably, it is the range of 0.01 to 0.07%.

Cu: 0.1-0.6%

When Cu is less than 0.1%, the effect is insufficient, and when the content exceeds 0.6%, the possibility of Cu cracking increases. Therefore, when Cu is contained, the amount of Cu is preferably in the range of 0.1 to 0.6%. More preferably, it is the range of 0.1 to 0.3%.

Ni: 0.1 to 0.5%

When the content of Ni is less than 0.1%, the effect is insufficient, and when the content exceeds 0.5%, the cost of steel materials will increase significantly. Therefore, when Ni is contained, the amount of Ni is preferably in the range of 0.1 to 0.5%. More preferably, it is the range of 0.1 to 0.4%.

In addition to the above composition, the high-strength steel material of the present invention preferably contains O at 0.0040% or less.

O: 0.0040% or less

If the O content exceeds 0.0040%, the toughness will deteriorate, so it 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 (mass %) of each component.

Ca, O, and S need to be contained so that the above formula (Ca-(0.18+130*Ca)*O)/1.25/S satisfies the relationship of greater than 0 and 0.8 or less. In this case, it is in the form of a composite sulfide in which MnS is deposited on CaS. When MnS exists alone, it is elongated during rolling and becomes the starting point of destruction. However, by making CaS a composite inclusion with MnS, it is finely dispersed in the parent phase, and the occurrence of fatigue cracks is suppressed. If the value of (Ca-(0.18+130×Ca)×O)/1.25/S is greater than 0.8, MnS is not formed, and O and S are all crystallized in the form of Ca-based sulfur oxides. Therefore, the size becomes coarse, and the stress concentration at the parent phase/inclusion interface becomes large, making it difficult to ensure fatigue strength. When (Ca-(0.18+130×Ca)×O)/1.25/S is 0 or less, CaS does not crystallize, so S is precipitated in the form of MnS alone, and this MnS is elongated by rolling during steel plate manufacturing, and does not Maintain fine dispersion. Therefore, (Ca-(0.18+130×Ca)×O)/1.25/S is set to be in the range of more than 0 and 0.8 or less.

2. About the metal structure

In order to achieve a high tensile strength of 510 MPa or more, the metallic structure is substantially a mixed structure of ferrite, bainite, and pseudo-pearlite. In essence, the mixed structure of ferrite, bainite and pseudo-pearlite is a structure whose total area fraction is 95% or more, and the remainder contains 5% or less of martensite, One or more of island martensite, retained austenite, and the like.

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

For a thick-walled material with a plate thickness of 30 mm to 50 mm, it is preferable to disperse 15% or more of the total area fraction of bainite and pseudo-pearlite as the second phase in order to achieve high strength and improvement of fatigue properties. By setting the area fraction to 15% or more, improvements in the strength and fatigue strength of the base material can be expected. In addition, pseudo-pearlite is pearlite (thin-layered pearlite) dispersed in layers relative to carbides and ferrite phases, and the shape of the thin layer collapses and the carbides bend, or they are dispersed into massive carbides. The main structure may contain a part of lamellar carbides (40% or less in area fraction relative to the total amount of carbides). It is considered that when the form of carbide is massive, the stress concentration at the parent phase/second phase interface is reduced compared to the case of thin layer, the occurrence of fatigue cracks is suppressed, and the fatigue strength is improved.

3. About the manufacturing method

Preferably, the steel having the above composition is melted by a conventional method using a smelting mechanism such as a converter or an electric furnace, and is produced into a steel material such as a billet by a conventional method such as a continuous casting method or an ingot-casting method. In addition, the melting method and the casting method are not limited to the above-mentioned methods. In addition, from the viewpoint of economic efficiency, it is preferable to perform casting of steel sheets by a steelmaking process by a converter method and by a continuous casting process. Thereafter, it is rolled into the desired shape for properties. The production conditions of the present invention are shown below.

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

(1) Heating temperature: 950～1250℃

When performing hot rolling, the steel sheet needs to be completely austenitized, so the heating temperature is set to 950° C. or higher. On the other hand, when the steel sheet is heated to a temperature higher than 1250°C, the austenite grains begin to coarsen, which adversely affects the toughness, so the heating temperature is set in the range of 950 to 1250°C. From the viewpoint of toughness, the range of the preferable heating temperature is 1000°C to 1100°C.

(2) Cumulative reduction rate at Ar ₃ point or more: 50% or more

In rolling, in order to maintain fine crystal grains and improve toughness, processing strain is introduced in the temperature range above the Ar ₃ point. When the cumulative reduction ratio is set to 50% or more, ferrite grains after transformation are sufficiently refined to improve toughness. Therefore, the cumulative rolling reduction during rolling is set to 50% or more at the Ar ₃ point or more. It should be noted that the Ar ₃ point is obtained 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 the content (mass %) of each element, and it is set to 0 when not contained.

If the hot rolling temperature is lower than the ferrite transformation start temperature, ferrite will be formed during the reduction and the strength will decrease. Therefore, the hot rolling end temperature should be at least Ar ₃ point or higher.

(3) Cooling start temperature: above Ar ₃ point -60°C

If the cooling start temperature is too low, the amount of ferrite formed in the early stage of accelerated cooling increases and the strength decreases. Therefore, cooling is started from a temperature above Ar ₃ -60°C.

(4) Cooling rate: 5°C/s or more

Continue to implement accelerated cooling after hot rolling. By setting the cooling rate at 5° C./sec or more, the structure is not coarsened, but a fine-grained structure can be obtained, and the targeted excellent strength, toughness, and fatigue properties can be obtained. When the cooling rate is less than 5° C./sec, the structure is coarsened and the ferrite fraction is increased, so that the target base metal strength, fatigue strength, and fatigue crack growth resistance cannot be obtained. In addition, the upper limit of the cooling rate is preferably equal to or lower than the cooling rate when the cooling curve in the CCT diagram of the steel material having the above-mentioned composition is in a protruding part of ferrite transformation. When the cooling rate is higher than the cooling rate at the protruding part of ferrite transformation, the bainite fraction becomes high, and the target fatigue crack growth resistance, ductility and toughness of the base material cannot be obtained. In order to obtain the desired structure, the plate thickness is preferably 30 mm to 50 mm within this cooling rate range.

In addition, the CCT diagram (continuous cooling phase transformation diagram) is made by the following general method: collect a plurality of φ8×12mm cylindrical samples from the steel with the above composition, and use the temperature corresponding to rolling in the hot processing reproduction test device Processing and cooling mode at various cooling rates Perform processing heat treatment while measuring the expansion of the test piece to investigate the phase transition temperature. As shown in Figure 1, the curve of the constant cooling rate of the ferrite transformation protrusion (the side with the fastest cooling rate in the region where the ferrite transformation occurs) is obtained from the obtained CCT diagram (the CCT diagram is due to the horizontal The axis (time) is logarithmic, so it becomes the cooling rate of the curve). In the present invention, pseudo pearlite is formed by cooling at a cooling rate of 5° C./s or more and not higher than the cooling rate when the cooling curve in the CCT diagram is at the protruding part of ferrite transformation, and the fatigue strength is improved.

(5) Cooling stop temperature: 600～350℃

By setting the cooling stop temperature to 350°C to 600°C, a desired structure obtained by hot rolling and subsequent cooling can be formed. If the cooling stop temperature is higher than 600°C, the dispersion amount of bainite and pseudo-pearlite will become insufficient, and if it is lower than 350°C, it will be difficult to ensure ductility and toughness. The cooling stop temperature is more preferably 400°C to 550°C.

(6) Tempering temperature: below Ac ₁ point

When it is necessary to correct the shape of the steel or to improve the ductility and toughness, it can be tempered at a point less than Ac ₁ after accelerated cooling. If the tempering temperature is higher than the Ac ₁ point, insular martensite is formed 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 the content (mass %) of each element, and it is set to 0 when not contained.

Example 1

For steel sheets with the composition shown in Table 1, test steels with a thickness of 30 to 50 mm were produced under the manufacturing conditions shown in Table 2, and the metal structure observation, mechanical properties, fatigue strength, and fatigue cracks of the obtained steel sheets were investigated. development characteristics. In addition, the cooling rate when the cooling curve in the CCT diagram (continuous cooling phase transformation diagram) is in the protruding part of the ferrite transformation is obtained by the following general method: from steel materials having the composition shown in Table 1 A plurality of cylindrical samples of φ10×12mm were collected, processed and heat-treated in a thermal processing reproduction test device in a processing corresponding to rolling and cooling modes at various cooling rates, and at the same time measuring the expansion of the test piece to investigate the phase transition temperature.

[Table 1]

[Table 2]

Microstructure observation was carried out at a position 1/4 of the thickness of the rolling direction cross section (L cross section) etched with a 3% nital etching solution using a sample obtained by grinding a sample collected from an arbitrary position. In addition, the area ratios of ferrite, bainite, and pseudo-pearlite were measured by optical microscope observation. These values were carried out in five fields of view for one sample, and obtained as an average value of the total fields of view.

Tensile properties are obtained by using a test piece (NKV1 No. test piece) with a total thickness of 200 mm in the rolling direction and a perpendicular direction (C direction), and carrying out a tensile test in accordance with the provisions of Chapter K of NK Classification. Tensile properties.

For toughness, the 2mm V-notch Charpy impact test piece (NKV4 test piece) is collected from the plate thickness 1/4 position parallel to the rolling direction, and the Charpy impact test is carried out in accordance with the provisions of NK Class K. The average value (vE-40(J)) of three at -40 degreeC was evaluated.

The fatigue strength was evaluated as a value when stress loading was repeated 1 million times using a round bar tensile test piece of φ12 mm×inter-mark distance 24 mm. For the test piece, according to JISZ2273, the material with a plate thickness of 50 mm is collected from the 1/4 position of the plate thickness, and the material with a plate thickness of 30 mm is collected from the 1/2 position of the plate thickness.

The fatigue crack growth characteristics were investigated in a fatigue crack growth test when cracks developed in the C direction using a CT test piece with a plate thickness of 25 mm in accordance with ASTME647. For the test piece, the material with a plate thickness of 50mm is collected from the 1/4 position of the plate thickness, and the material with a plate thickness of 30mm is collected from the 1/2 position of the plate thickness. The test conditions are carried out in the atmosphere with a stress ratio of 0.1 and room temperature to evaluate the fatigue crack growth rate at 25MPa·m ^1/2 within the range of the stress amplification factor (ΔK).

Table 3 shows the test results.

[table 3]

The test results are based on yield stress YS: 390N/mm ² or more, tensile strength TS: 510N/mm ² or more, elongation: 19% or more, vE-40: 100J or more, fatigue strength: 340Mpa or more, fatigue crack development Speed: 1.0×10 ^-7 (m/cycle) or less is used as a criterion for judging whether it is acceptable or not.

As confirmed from Table 3, Nos. 1-1 to 8-1, which are examples of the present invention, all have a yield stress YS of 390 N/mm ² or more and a tensile strength TS of 510 N/mm ² or more, and have excellent base material properties. In addition, the steel of the present invention has a fatigue strength of 340 MPa or more, a fatigue crack growth rate of 1.0×10 ^-7 (m/cycle) or less, and excellent fatigue characteristics. On the other hand, Nos. 9-1 to 12-1, which are comparative examples whose chemical components and production conditions deviate from the scope of the present invention, were inferior in any one or more of the above characteristics.

Example 2

For steel sheets with the composition shown in Table 4, test steels with a plate thickness of 30 to 50 mm were produced under the manufacturing conditions shown in Table 5, and the metal structure observation, mechanical properties, fatigue strength, and fatigue cracks of the obtained steel plates were investigated. development characteristics. In addition, the cooling rate when the cooling curve in the CCT diagram (continuous cooling phase transformation diagram) is in the protruding part of the ferrite transformation is obtained by the following general method: from steel materials having the composition shown in Table 4 A plurality of cylindrical samples of φ10×12mm were collected, processed and heat-treated in a thermal processing reproduction test device in a processing corresponding to rolling and cooling modes at various cooling rates, and at the same time measuring the expansion of the test piece to investigate the phase transition temperature.

[Table 4]

[table 5]

Microstructure observation was carried out at a position 1/4 of the thickness of the rolling direction cross section (L cross section) etched with a 3% nital etching solution using a sample obtained by grinding a sample collected from an arbitrary position. In addition, the area ratios of ferrite, bainite, and pseudo-pearlite were measured by optical microscope observation. These values were carried out in five fields of view for one sample, and obtained as an average value of the total fields of view.

Tensile properties are obtained by using a test piece (NKV1 No. test piece) with a total thickness of 200 mm in the rolling direction and a perpendicular direction (C direction), and carrying out a tensile test in accordance with the provisions of Chapter K of NK Classification. Tensile properties.

For toughness, the 2mm V-notch Charpy impact test piece (NKV4 test piece) is collected from the plate thickness 1/4 position parallel to the rolling direction, and the Charpy impact test is carried out in accordance with the provisions of NK Class K. The average value (vE-40(J)) of three at -40 degreeC was evaluated.

The fatigue strength was evaluated as a value when stress loading was repeated 1 million times using a round bar tensile test piece of φ12 mm×inter-mark distance 24 mm. For the test piece, according to JISZ2273, the material with a plate thickness of 50 mm is collected from the 1/4 position of the plate thickness, and the material with a plate thickness of 30 mm is collected from the 1/2 position of the plate thickness.

The fatigue crack growth characteristics were investigated in a fatigue crack growth test when cracks developed in the C direction using a CT test piece with a plate thickness of 25 mm in accordance with ASTME647. For the test piece, the material with a plate thickness of 50mm is collected from the 1/4 position of the plate thickness, and the material with a plate thickness of 30mm is collected from the 1/2 position of the plate thickness. The test conditions are carried out in the atmosphere with a stress ratio of 0.1 and room temperature to evaluate the fatigue crack growth rate at 25MPa·m ^1/2 within the range of the stress amplification factor (ΔK).

Table 6 shows the test results.

[Table 6]

The test results are based on yield stress YS: 390N/mm ² or more, tensile strength TS: 510N/mm ² or more, elongation: 19% or more, vE-40: 100J or more, fatigue strength: 340Mpa or more, fatigue crack development Speed: 8.5×10 ^-8 (m/cycle) or less is used as a criterion for judging whether it is acceptable or not.

As confirmed from Table 6, Nos. 1-2 to 8-2, which are examples of the present invention, all have yield stress YS of 390 N/mm ² or more and tensile strength TS of 510 N/mm ² or more, and have excellent base material properties. 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 0.8 or less, a high-strength steel material that is more excellent in fatigue crack growth resistance can be obtained. On the other hand, Nos. 9-2 to 16-2, which are comparative examples whose chemical components and production conditions deviate from the scope of the present invention, were inferior in any one or more of the above-mentioned characteristics.

Claims (7)

1. the High Strength Steel of an excellent in fatigue characteristics, in mass %, become to be grouped into containing C:0.10 ~ 0.20%, below Si:0.50%, Mn:1.0 ~ 2.0%, below P:0.030%, S:0.0005 ~ 0.0040%, Sol.Al:0.002 ~ 0.07%, Ca:0.0005 ~ 0.0050%, rest part is made up of Fe and inevitable impurity, and metal structure is the ferrite of principal phase and the bainite of the 2nd phase and pseudopearlite.

2. the High Strength Steel of excellent in fatigue characteristics as claimed in claim 1, is characterized in that, in mass %, becomes to be grouped into further containing the one be selected from Ti:0.003 ~ 0.03%, Nb:0.005 ~ 0.05% or two kinds.

3. the High Strength Steel of excellent in fatigue characteristics as claimed in claim 1 or 2, it is characterized in that, in mass %, become to be grouped into further containing be selected from Cr:0.1 ~ 0.5%, Mo:0.02 ~ 0.3%, V:0.01 ~ 0.08%, Cu:0.1 ~ 0.6%, Ni:0.1 ~ 0.5% more than one.

4. the High Strength Steel of the excellent in fatigue characteristics according to any one of claims 1 to 3, is characterized in that, becomes to be grouped into further containing below O:0.0040%, and meets following formula (1),

0＜(Ca-(0.18+130×Ca)×O)/1.25/S≤0.8···(1)，

Wherein, Ca, O, S in formula (1) represent the content of each composition, and wherein, the content of each composition in mass %.

5. a manufacture method for the High Strength Steel of excellent in fatigue characteristics, is characterized in that, after the steel raw material that the one-tenth had according to any one of Claims 1 to 4 is grouped into is heated to 950 ~ 1250 DEG C, at Ar ₃point is above carries out the rolling that accumulation draft is more than 50%, with the speed of cooling of more than 5 DEG C/sec from Ar ₃the temperature province of the temperature province accelerating cooling to 350 of point more than-60 DEG C DEG C ~ 600 DEG C.

6. the manufacture method of the High Strength Steel of excellent in fatigue characteristics as claimed in claim 5, it is characterized in that, described speed of cooling is below the speed of cooling of the cooling curve had in the CCT figure of the steel raw material that the one-tenth according to any one of described Claims 1 to 4 is grouped into when being in ferrite transformation protuberance.

7. the manufacture method of the High Strength Steel of the excellent in fatigue characteristics as described in claim 5 or 6, is characterized in that, after described accelerating cooling, further with Ac ₁the following temperature of point carries out temper.