KR100709600B1 - Steel product excellent in characteristics of resistance to fatigue crack extension and method for production thereof - Google Patents

Abstract

Provided are steels having excellent fatigue crack propagation characteristics having a structure mainly composed of a hard phase, and a method of manufacturing the same. The structure is mainly composed of ferrite / bainite, and the half width of the X-ray diffraction intensity from the (110) plane is assumed to be 0.13 degrees or more. Specifically, the steel composition is C: 0.01 to 0.10%, Si: 0.03 to 0.60%, Mn: 0.5 to 2.0%, sol.Al: 0.005 to 0.10%, N: 0.0005% to 0.008%, and Ft (3Mn + Cu + 1.5Cr + 1.8Ni + 1.5Mo): 4.0 to 6.0%, or 0.0030% or less of B, Ft: 3.5 to 5.5%, and at least one of Cu, Ni, Cr, and Mo do. It is prepared by quenching after hot rolling.

Description

STEEL PRODUCT EXCELLENT IN CHARACTERISTICS OF RESISTANCE TO FATIGUE CRACK EXTENSION AND METHOD FOR PRODUCTION THEREOF}

The present invention relates to steel materials such as thick steel sheets used as structural materials for hulls, civil constructions, construction machinery, hydraulic steel pipes, marine structures, line pipes, and the like, particularly steel materials having excellent fatigue crack propagation characteristics of 490 MPa class and methods for producing the same. It is about.

In recent years, the tendency of a welded structure to be enlarged is remarkable, and high strength and weight reduction are calculated | required. However, when the high strength steel is used, the design stress increases, so that fatigue breakage easily occurs in the welded portion, and the improvement is an important problem. In the case of thick steel sheets such as structural steels, welding is generally performed, so that fatigue cracks generated and developed in the welded portions can be stopped by the steel, which is effective for extending the fatigue life of the structure. For this reason, the steel plate which has a fatigue crack growth suppression effect is proposed variously.

Japanese Laid-Open Patent Publication No. 7-90478 discloses a steel sheet having good fatigue crack growth resistance and a method of manufacturing the same. The steel sheet has a structure in which a stripe-shaped hard second phase extending in the rolling direction is interspersed at a ratio of 5 to 50% in an area ratio in the soft base phase.

In addition, a soft phase exists as a mother phase, and a hard 2nd phase is a stripe state, it extends in a steel plate rolling direction, and suppresses crack growth. In this method, however, the growth inhibition effect of the fatigue crack is only in the plate thickness direction, and the growth inhibition effect in the other direction is small.

In addition, Japanese Patent Laid-Open No. 6-271985 discloses an island composed mainly of one or two or more of ferrite, pearlite, and bainite, and having an average existence interval of 20 µm or less and an average flatness ratio of 5 or more. A steel sheet excellent in fatigue crack propagation characteristics in which shape martensite is present in a proportion of 0.5 to 5% by volume is disclosed. However, the presence of island martensite having a large average flat ratio in high strength steel causes deterioration of toughness.

Japanese Laid-Open Patent Publication No. 7-242992 is composed of a base of which the tissue is a hard part, and a soft part of which is dispersed in the base, and the fatigue crack growth suppression is characterized in that the hardness difference between the two parts is Vickers hardness of 150 or more. A steel sheet having an effect is disclosed. However, in order to obtain a hardness difference of 150 or more, it is necessary to stably suppress the strength to 490 MPa class in thin materials and the like, which is not necessarily easy.

The present invention has been made to solve these problems, and its object is to be used as structural materials such as hulls, civil constructions, construction machinery, hydraulic steel pipes, offshore structures, line pipes, etc. It is to provide this excellent steel and its manufacturing method.

MEANS TO SOLVE THE PROBLEM The present inventors obtained various knowledge about the relationship between the crystal structure of steel and the fatigue crack growth resistance, and acquired the following knowledge.

That is, the dislocation density in the structure of the hard phase (bainite, martensite, temper martensite, etc.) affects the fatigue crack growth rate, and in particular, when the dislocation density is sufficiently high, the hard phase mainly Also in a steel having a structure to be obtained, a high strength steel having excellent fatigue resistance due to high fatigue crack growth resistance can be obtained. Even in a steel having such a hard phase as the main structure, the fatigue crack growth resistance is excellent in that the hardness of the hard phase having a high dislocation density decreases in the course of the cyclic deformation of iron, so that the opening load at the tip of the fatigue crack decreases. It is thought to contribute to deterioration.

Such a high dislocation density tissue is obtained as a tissue transformed at low temperature. Moreover, since the structure transformed at low temperature and high in dislocation density contains many lattice deformation, the width | variety of a peak becomes wide in the diffraction intensity distribution at the time of an X-ray-diffraction test. Therefore, sufficient fatigue crack growth resistance is the half width of the diffraction intensity obtained by the X-ray diffraction test (the intensity is the distribution width in 1/2 of the peak intensity, and the unit is "degree"). It can be obtained when it is over a certain value.

This invention is completed based on these knowledge, The summary is the steel material excellent in the fatigue crack growth resistance of the base material of following (1)-(8), and its manufacturing method as described in (9)-(13). .

(1) The structure is composed mainly of ferrite and bainite, and the area ratio of pearlite is 10% or less, and the half width of the X-ray diffraction intensity from the (110) plane is 0.13 degrees or more. Steel with excellent crack propagation resistance.

(2) The structure is mainly composed of ferrite and bainite, the area ratio of pearlite is 10% or less, and the steel component (mass%) satisfies the following formulas (1) and (2): Steel with excellent fatigue crack propagation characteristics.

 6? 20 x C + 5 x Si + 10 x Mn? (One)

 0.01? C / Mn? (2)

(3) The structure is mainly composed of ferrite and bainite, the area ratio of pearlite is 10% or less, the half width of the X-ray diffraction intensity from the (110) plane is 0.13 degrees or more, and the steel component (mass% ) Is a steel material excellent in fatigue crack propagation characteristics, characterized by satisfying the following formulas (1) and (2).

6? 20 x C + 5 x Si + 10 x Mn? (One)

0.01? C / Mn? (2)

(4) The chemical composition of the steel is, in mass%, C: 0.01% or more, 0.10% or less, Si: 0.03% or more, 0.60% or less, Mn: 0.5% or more, 2.0% or less, sol.Al: more than 0.005%, Fatigue crack growth in any one of said (1)-(3) characterized by including 0.10% or less, N: 0.0005% or more, 0.008% or less, and remainder consists of Fe and an unavoidable impurity. Steel with excellent properties.

(5) The chemical composition of the steel is, in mass%, C: 0.01% or more, 0.10% or less, Si: 0.03% or more, 0.60% or less, Mn: 0.3% or more, 2.0% or less, so1.Al: more than 0.005%, 0.10% or less, N: 0.0005% or more, 0.008% or less, B: 0.0003 to 0.0030%, and the balance is composed of Fe and unavoidable impurities, according to any one of (1) to (3) above. Steel with excellent fatigue crack propagation characteristics.

(6) The chemical composition of the steel is, in mass%, Nb: 0.005% or more, 0.08% or less, Ti 0.005% or more, 0.03% or less, V: 0.005% or more, 0.080% or less, one or more kinds The steel material which contains and is excellent in the fatigue crack growth characteristics as described in said (4) or (5) which satisfy | fills following formula (3).

0.01? C / (Mn + 20 Nb + 10 Ti + 5 V)? (3)

(7) The chemical composition of the steel is, in mass%, Cu: less than 0.7%, Ni: 3.0% or less, Cr: less than 1.0%, Mo: 0.80% or less, W: 0.05 to 0.50%, 1 type The steel material which contains the above and is excellent in fatigue-fatigue propagation characteristic in any one of said (4)-(6) which satisfy | fills following formula (4).

0.01? C / (Mn + 1/10 Cu + 1/2 Ni + 1/4 Cr + Mo + 20 Nb + 10 Ti + 5 V)? (4)

(8) The chemical composition of the steel further comprises, by mass%, Ca: 0.007% or less, Mg: 0.007% or less, Ce: 0.007% or less, Y: 0.5% or less, Nd: 0.5% or less, REM: 0.05% or less. Steel material excellent in the fatigue crack growth characteristics in any one of said (4)-(7) which contains 1 or more types of groups.

(9) a heating step of heating the cast slab having the chemical composition according to any one of (4) to (8) to 1000 ° C to 1250 ° C, a hot rolling step of performing hot rolling on the heated slab, A cooling step of cooling the steel material subjected to the hot rolling is provided. In the cooling step, accelerated cooling is performed at an average cooling rate of 5 to 25 ° C / s between 650 ° C and 400 ° C, and the accelerated cooling is performed. The method of manufacturing a steel having excellent fatigue crack growth resistance, wherein the cooling is terminated at a temperature of 400 ° C. or lower, and then the reheating temperature range is 70 ° C. or lower.

(10) a heating step of heating the casting slab having the chemical component according to any one of (4) to (8) to 1000 ° C to 1250 ° C, a hot rolling step of performing hot rolling on the heated slab, Thereafter, there is provided a step of cooling the product without performing accelerated cooling, a heating step of reheating at Ac ₁ point + 50 ° C or more, and a cooling step of cooling the reheated steel material, wherein the cooling step includes 650 ° C to 400 ° C. Accelerated cooling with an average cooling rate between 5 ° C. and 25 ° C./s is performed, and the cooling is stopped at a temperature of 400 ° C. or lower, and then the cooling is terminated while the recuperation temperature range is 70 ° C. or lower. A method for producing a steel having excellent fatigue crack growth resistance, characterized by the above-mentioned.

(11) A method for producing a steel having excellent fatigue crack growth resistance, wherein the reheating and cooling steps according to (10) are performed two or more times.

(12) The production method according to (9), further comprising a heating step of reheating the Ac ₁ point + 50 ° C or more after the completion of the cooling, and a cooling step of cooling the reheated steel. In a cooling process, accelerated cooling which makes the average cooling rate between 650 degreeC-400 degreeC into 5-25 degreeC / s is performed, the said cooling is stopped at the temperature of 400 degreeC or less, and a reheat temperature width is 70 degreeC after that. Cooling is complete | finished so that it may become the following. The manufacturing method of the steel excellent in fatigue crack growth resistance.

(13) In addition to the manufacturing method of any one of (9)-(12), it heats again at temperature below Ac ₁ point, and tempers, The manufacturing method of the steel excellent in fatigue crack growth resistance characterized by the above-mentioned.

Although the performance of the steel material excellent in fatigue crack growth resistance which concerns on this invention is not specifically limited, The fatigue performance of fatigue crack growth is 3x10 <^-5> mm / cycle or less in preferable performance.

Furthermore, in the case of the steel material whose toughness is important, it has the characteristic that the absorption energy in impact test is 100 J or more in vE- ₂₀ .

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram for demonstrating the half width measurement method of X-ray diffraction, and FIG. 1 (a), (b) is a graph which shows the diffraction intensity in (110) plane, respectively.

FIG.2 (a) is a schematic diagram which shows the outline of a servo pulser fatigue test apparatus, and FIG.2 (b) is a schematic diagram which shows the shape of a fatigue test piece.

The reason for limiting the structure and chemical composition of the steel according to the present invention is as follows.

Structure: In order to obtain high strength easily, the steel material which concerns on this invention consists mainly of ferrite and bainite. The bainite includes tissues such as upper bainite, lower bainite, acicular ferrite, granular bainite, and the like.

"Mainly" means that the structural ratio of the structure of ferrite and bainite in the structure of the steel is 90% or more in terms of area ratio in total. The remaining structure is not particularly limited, and may be a structure usually observed, such as a pearlite or a pseudo pearlite structure.

Half-width of X-ray diffraction: Half-width is the value which expressed the distribution width of the part whose diffraction intensity becomes 1/2 of a peak intensity in diffraction angle in X-ray diffraction intensity distribution. The half width becomes small with the structure produced | generated at high temperature and small dislocation density. The larger the half width structure, the larger the dislocation density and the better the fatigue crack growth resistance.

The crystal plane performing X-ray diffraction was the (110) plane for the most commonly used reason. The half width defined in the present invention is that the half width of the diffraction intensity at the (110) plane is 0.13 degrees or more in order to obtain good fatigue crack growth resistance. In the case of 490 MPa grade steel, it is preferable to set it as 0.13-0.24 degree | times from a viewpoint of balance, such as strength.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram explaining the half width analysis method in the data of X-ray-diffraction intensity. 1A and 1B are graphs showing diffraction intensities on the (110) plane, respectively. As shown in Fig. 1 (a), the half width represents the distribution width at an angle where the diffraction intensity is 1/2 of the highest intensity value at the peak of the diffraction intensity. As shown in Fig. 1 (b), when the peak is divided into two, the value of 1/2 of the higher peak is taken.

The half-value width, if it appears in the diffraction pattern a peak of the Kα ₁ and Kα ₂ independently, the value of the Kα _1, when the value of the displayed superimposed Kα ₁ and Kα ₂ measured by the total width. In addition, the said half width measurement shall be performed in the surface parallel to a rolling surface in the site | part which entered 1 mm inside from the steel surface in the thickness direction.

(1) Value of equation: 6 to 30

Formula (1) shows the ratio of the bainite structure as a hard phase, and when the value of (1) is less than 6, the ratio of bainite in the ferrite + bainite structure is not sufficient, and the production conditions of the present invention Even if a steel sheet is manufactured, an appropriate half width cannot be obtained and favorable fatigue strength propagation resistance cannot be obtained.

On the contrary, when the value of Formula (1) exceeds 30, when the strength is set to 490 MPa class, the ferrite structure in the ferrite + bainite structure must be increased, and in this case, good fatigue crack growth resistance cannot be obtained.

(2), (3), (4) Formula: 0.01 or more and 0.10 or less

These formulas indicate the hardness of bainite structure. If the value is less than 0.01, the hardness of bainite structure becomes insufficient, and good fatigue crack growth resistance cannot be obtained. On the contrary, when it exceeds 0.10, the cooling rate dependence of transformation progress becomes large, and it is not easy to acquire uniform fatigue crack growth resistance in the whole steel plate.

The reason for more specifically defining the chemical composition of the steel in a suitable embodiment of the present invention is as follows.

C: An element effective for increasing the strength of the steel, and in order to obtain the strength of the steel, 0.01% or more is contained. However, since the toughness deteriorates when it exceeds 0.10%, C content is made into 0.10% or less in order to avoid this. More preferably, it is 0.03 to 0.07%.

Si: An element effective for deoxidation of steel and containing 0.03% or more in order to obtain the effect. However, if it contains more than 0.60%, formation of M-A tissue is promoted. Here, the M-A structure is a kind of island-like martensite formed in the bainite structure, and is an M-A transformation product containing residual austenite. It is known that M-A structure is very hard and deteriorates toughness easily. Therefore, in order to avoid toughness deterioration, Si content is made into 0.60% or less. More preferably, they are 0.3% or more and 0.5% or less.

Mn: An element effective for improving hardenability, and is contained in an amount of 0.5% or more in order to improve strength and resistance to fatigue crack growth. On the other hand, if it exceeds 2.0%, the toughness deteriorates, so the Mn content is made 2.0% or less.

However, when it contains B as mentioned later, you may make Mn: 0.3% or more and 2.0% or less.

so1.Al: Al is an element necessary for deoxidation together with Si and contains more than 0.005% of so1.Al to obtain the effect. On the other hand, when the so1.Al content exceeds 0.10%, the M-A ratio (the ratio of M-A structure present) increases and the toughness deteriorates. To avoid this, the so1.Al content is 0.10% or less.

N is combined with A1 or Ti to form a precipitate, which contributes to the fine graining of the austenite particles, thereby improving toughness. In order to acquire this effect, N is contained 0.0005% or more. On the other hand, when N content exceeds 0.008%, M-A ratio will increase and toughness will deteriorate. In order to avoid this, N content is made into 0.008% or less.

B: Although not an essential element, B has an effect of significantly increasing hardenability, and is effective in improving strength and resistance to fatigue crack growth. Therefore, you may contain in order to acquire these effects further. In order to acquire the said effect, it is effective to contain 0.0003% or more. However, when B exceeds 0.0030%, toughness deteriorates, so the upper limit is preferably made 0.0030%. When it contains B, the minimum of Mn can be made into 0.3%.

Nb: Although not an essential element, it has a function of improving toughness through a fine granulation action. In addition, since the hardenability is increased, it is effective for improving strength and suppressing fatigue crack growth. Therefore, you may contain in order to acquire these effects. In this case, it is preferable to contain Nb 0.005% or more. On the other hand, when the content exceeds 0.08%, the toughness deteriorates, so the upper limit is made 0.08%. More preferably, it is 0.06% or less.

Ti: Although not an essential element, it is effective for improving strength and suppressing fatigue crack growth, and may be included in order to obtain these effects. In order to acquire the said effect, it is preferable to contain 0.005% or more. On the other hand, when it exceeds 0.03%, toughness deteriorates, so the upper limit is preferably made 0.03%.

V: Although not an essential element, since it is effective for improving strength, it may be included in order to obtain these effects. When it contains, in order to acquire the said effect, it is preferable to contain 0.005% or more. On the other hand, if it exceeds 0.080%, toughness deteriorates, so the upper limit is preferably 0.080%.

Cu: Although not an essential element, since it has the effect | action which raises the strength of steel, you may contain it for the purpose. In order to acquire the effect, it is preferable to contain 0.3% or more. However, when the content is 0.7% or more, the toughness of the steel is deteriorated. Therefore, even if it contains, the upper limit is made less than 0.7%. Preferably less than 0.5%.

Ni: Although not an essential element, it has the effect of increasing the strength of the steel and is also effective for suppressing fatigue crack growth. Therefore, you may contain in order to acquire these effects. In order to acquire the effect, it is preferable to contain 0.2% or more. However, if the content is more than 3.0%, the effect of increasing the strength and suppressing fatigue crack growth suitable for increasing the cost is not seen, so even when it is contained, the upper limit is made 3.0%.

Cr: Although not an essential element, it has the effect of increasing the strength of the steel and is also effective for suppressing fatigue crack growth. Therefore, you may contain in order to acquire these effects. In that case, it is preferable to contain 0.3% or more. However, when it contains excessively, toughness will deteriorate, so even if it contains, it is preferable to set it as less than 1.0%.

Mo: Although not an essential element, it has the effect of increasing the strength of the steel and is also effective for suppressing fatigue crack growth. Therefore, you may contain in order to acquire these effects. In that case, it is preferable to contain 0.3% or more. However, since excessive toughness deteriorates toughness, it is preferable to make it 0.80% or less even if it contains.

W is an effective element in order to raise the base material strength and to improve corrosion resistance. To obtain this effect, at least 0.05% is added. However, exceeding 0.50% causes deterioration of toughness.

Ca contributes to toughness improvement through tissue refinement. However, when it exceeds 0.007%, Ca inclusion amount becomes excess and toughness deteriorates on the contrary. Therefore, Ca amount is made into 0.007% or less. Moreover, the range of preferable addition amount is 0.0015% or more and 0.0030% or less.

Mg contributes to toughness improvement through tissue refinement. However, when it exceeds 0.007%, Mg inclusion amount will become excess and toughness will rather deteriorate. Therefore, the amount of Mg is made into 0.007% or less. Moreover, the range of preferable addition amount is 0.0005% or more and 0.0030% or less.

Ce contributes to toughness improvement through tissue refinement. However, when it exceeds 0.007%, Ce inclusion amount becomes excess and toughness deteriorates rather. Therefore, the amount of Ce is made into 0.007% or less. Moreover, the range of preferable addition amount is 0.0005% or more and 0.0030% or less.

Y contributes to toughness improvement through tissue refinement. However, when it contains more than 0.5%, Y inclusion amount will become excess and toughness will rather deteriorate. Therefore, Y amount is made into 0.5% or less. Moreover, the range of preferable addition amount is 0.01% or more and 0.05% or less.

Nd contributes to toughness improvement through tissue refinement. However, when it contains more than 0.5%, Nd inclusion amount will become excess and toughness will rather deteriorate. Therefore, Nd amount is made into 0.5% or less. Moreover, the range of preferable addition amount is 0.01% or more and 0.05% or less.

REM contributes to toughness improvement through tissue refinement. However, when it exceeds 0.05%, Nd inclusion amount will become excess and toughness will rather deteriorate. Therefore, the amount of REM is made 0.05% or less. Moreover, the range of preferable addition amount is 0.005% or more and 0.03% or less.

Remainder of the chemical composition in this invention is Fe and an unavoidable impurity. P and S are exemplified as one kind of impurities, and P and S are preferably limited to 0.015% or less and 0.005% or less, respectively.

The means for producing a steel material excellent in fatigue crack growth resistance according to the present invention is not particularly limited, and as long as desired characteristics are obtained, using a known hot rolling facility or a known hot rolling facility and a known heat treatment facility, You may manufacture. However, the method described below is suitable for the manufacturing conditions.

Hot-rolling is performed after the casting slab which has a chemical composition concerning this invention is heated to 1000 degreeC-1250 degreeC. Subsequently, when cooling it, in the cooling process of the obtained hot rolled steel, accelerated cooling which makes the average cooling rate between 650 degreeC-400 degreeC more than 5 degreeC / s, Preferably it is more than 5 degreeC / s and 25 degrees C / s or less is performed. The accelerated cooling is stopped at a temperature of 400 ° C. or lower, and then the cooling is terminated so that the recuperation temperature width is 70 ° C. or lower.

Here, the recuperation temperature range means a difference between the temperature attained when cooling is stopped and the temperature at which the surface temperature rises and is stabilized by heat inside the steel sheet after cooling stop. Specifically, it is the difference between the temperature measured immediately after exiting the water cooling device and the temperature measured at 20 to 50 seconds by the plate thickness thereafter.

If the heating temperature of the cast slab does not satisfy 1000 ° C., the ferrite rate is increased to increase the rate of crack growth. If the temperature exceeds 1250 ° C, the structure becomes rough and large, and the toughness deteriorates.

According to the present invention after hot rolling, accelerated cooling is performed. At this time, if the average cooling rate between 650 ° C and 400 ° C during the cooling process does not satisfy 5 ° C / s, the ferrite rate is increased and the crack growth speed is also increased. . Preferably it is 25 degrees C / s or less. When the accelerated cooling stop temperature exceeds 400 ° C, the ferrite rate is increased and the speed of growth is increased. Preferably it is 350 degreeC or more.

When the recuperation temperature range from the accelerated cooling stop to the end of cooling exceeds 70 ° C., the dislocation density decreases and the growth speed increases. In order to reduce the reheating temperature width, it is preferable to reduce the temperature difference between the steel plate surface layer and the central portion during cooling, and to terminate at least the phase transformation of the surface layer portion at the end of cooling.

In addition, such accelerated cooling may be carried out by reheating after cooling once after hot rolling.

In order to reduce the temperature difference between the steel sheet surface layer and the central portion, the cooling rate at the rear end may be larger than the front end of the cooling band. Moreover, in order to complete phase transformation of the surface layer part at the time of accelerated cooling stop, it is preferable to make the stop temperature of accelerated cooling into 400 degrees C or less.

According to a preferred embodiment of the present invention, after the cooling after the accelerated cooling is completed or after the end of cooling without ending the accelerated cooling, it is reheated to Ac ₁ point + 50 degreeC or more, and accelerated cooling is performed. In this case, reheating to Ac ₁ point + 50 ° C or higher does not sufficiently cause austenite transformation below Ac ₁ point + 50 ° C, so that the fraction of tissues transformed by subsequent cooling decreases, resulting in a structure having excellent fatigue characteristics. Can't get enough Therefore, the reheating temperature were as Ac ₁ point + 50 ℃. Preferable heating temperature is Ac <_3> or more.

The cooling conditions are as described above. This condition does not change even if it is on-line accelerated cooling or off-line accelerated cooling.

On the other hand, when the tempering temperature exceeds Ac ₁ point, austenite transformation occurs, leading to a decrease in cyclic softening and a decrease in strength and toughness. Therefore, tempering temperature was made into Ac ₁ point or less. Moreover, as for tempering temperature, 550 degreeC or less is preferable.

In addition, the process of reheating and cooling by Ac ₁ point +50 degreeC or more may be performed twice or more as needed. By repeating the process of reheating and cooling more than Ac <_1> +50 degreeC twice or more, the structure after cooling becomes fine and strength and toughness improve.

The present invention relates to a steel material, but the steel material at this time contains many types of materials such as a plate, a pipe, a bar, a mold, a wire, and the like.

Example

The steel of chemical composition shown in Table 1 was vacuum-dissolved in a laboratory to a slab having a thickness of 100 to 160 mm, hot rolled under various conditions, and then cooled under various conditions to a thick steel sheet having a thickness of 12 to 40 mm. I did. Hot rolling conditions and cooling conditions are shown in Table 2.

 TABLE 1

TABLE 2

The structure, the half width of X-ray diffraction, tensile strength, toughness, and fatigue crack growth rate of the obtained steel sheet were examined by the following method.

The structure of the steel was measured at 10 o'clock for one sample by optical microscopic observation on the surface of the sample which was polished at a portion corresponding to 1/4 of the plate thickness and etched with a 2% nital corrosion solution. And the structure of the said steel plate was determined with the average of ten measured values.

The half width of the X-ray diffraction sampled a 25 mm square sample, electrolytically polished the surface parallel to the rolling surface of 1 mm inside from the surface in the thickness direction, and made it the measurement surface.

X-ray measurement was performed using the RU-200 made from Chemical Industries, Ltd .. Using a cobalt source, the output was 30 kV and 100 mA. A range of 20 mm in diameter was assumed in a 25 mm square test piece.

Tensile test pieces were taken in parallel with the JIS 14A tensile test piece in the rolling direction at the center of the plate thickness, and used for the tensile test. Toughness was taken in parallel in the rolling direction from the sheet thickness center part No. 4 suffie impact test piece prescribed | regulated to JIS-Z2202, the sruff impact test was done, and the shock absorption energy (vE-20, the unit is J) was calculated | required.

The fatigue crack growth rate was measured by the fatigue test method using the servo pulser apparatus shown in FIG.2 (a), and the CT test piece 1 shown in FIG.2 (b). In the apparatus shown in Fig. 2 (a), reference numeral 1 is a CT test piece, 2 is a test solution tank, 3 is a solution circulation pump, 4 is a load cell, 5 is a hydraulic cylinder, 6 is a hydraulic source, and 7 is a servovalve. 8 denotes a waveform generator, 9 denotes a load controller, and 10a and 10b denote load bars, respectively. The cutout 10 of 2.5 mm is given to the CT test piece 1 (60x62.5 mm, thickness 12.5 mm) shown to FIG. 2 (b), and the load rod 10a and the hole part 12 of the upper and lower parts are carried out. Attach (10b).

By this apparatus, a cyclic stress is loaded to the notch tip part in the test solution tank 2 via the load rods 10a and 10b by the hydraulic cylinder 5 to the test piece 1. The test piece was extract | collected so that the longitudinal direction of a notch may be parallel to a rolling perpendicular | vertical direction from a sheet thickness center part in thickness direction.

Fatigue test conditions were as follows.

f (repeat rate) = 20 Hz

R (stress ratio) = 0.1

T (test temperature) = room temperature

The test atmosphere is waiting

As a result of the fatigue crack propagation test, the fatigue crack propagation rate was evaluated in any of the test pieces in the medium ΔK region (ΔK: difference between the maximum stress intensity factor and the minimum stress intensity factor in the stress intensity factor range). The middle ΔK region in this test was equivalent to the second region of fatigue crack growth (15 to 30 MPa √m).

Paris side [Trans. ASTM, Ser. D. 85.523 (1963)],

da / dN = C (ΔK) ^m , where ^ΔK : kN / mm ^3/2 ,

da / dN: mm / cycle

This proved to be true.

From this, in this invention, the fatigue crack growth characteristic was evaluated by the crack growth rate da / dn (mm / cycle) in (DELTA) K = 20MPa (s) m of (DELTA) K area | region.

Table 3 shows the results of the above investigations, measurements and fatigue tests. In Table 3, code | symbol B of the subject tissue (structure which occupies 90% or more by area ratio) column means bainite, M is martensite, F is ferrite, P is pearlite.

TABLE 3

As shown in Table 3, the steel sheets of Test Nos. 1 to 17 in which the structure and the half width satisfy the conditions specified by the present invention have a slow fatigue crack growth rate of 4 × 10 ⁻⁵ mm / cycle or less. It has very good fatigue crack growth resistance. On the other hand, the steel plates of Test Nos. 18 to 35 had a strength of 490 MPa or more (TS ≧ 620 MPa, YS ≧ 500 MPa), or the absorbed energy did not satisfy 100J. If the half width of the structure or X-ray diffraction, which is the main body, is out of the range defined by the present invention, the fatigue crack growth rate exceeds 4 × 10 ⁻⁵ mm / cycle, and desired fatigue crack growth resistance cannot be obtained.

Since the steel material which concerns on this invention has favorable fatigue crack growth resistance, and mainly uses a hard structure, it is easy to raise the strength of steel. Moreover, it can also be equipped with the outstanding toughness by adjustment of a chemical composition. Therefore, it is suitable for thick steel plates of any thickness used as structural materials such as hulls, civil constructions, construction machinery, hydraulic steel pipes, marine structures, line pipes, and the like. Moreover, since the steel material of this invention can be manufactured easily by the cooling control after hot rolling, industrial value is large.

Claims (19)

The structure is composed of ferrite and bainite, the area ratio of pearlite is 10% or less, and the half width of the X-ray diffraction intensity from the (110) plane is 0.13 degrees or more. Steel. The structure is composed of ferrite and bainite, and the area ratio of pearlite is 10% or less, and the steel component (mass%) satisfies the following formulas (1) and (2). This excellent steel.  6? 20 x C + 5 x Si + 10 x Mn? (One) 0.01? C / Mn? (2) The structure is composed of ferrite and bainite, the area ratio of pearlite is 10% or less, the half width of the X-ray diffraction intensity from the (110) plane is 0.13 degrees or more, and the steel component (mass%) is ), (2) The steel material excellent in fatigue crack propagation characteristics characterized by satisfying the formula. 6? 20 x C + 5 x Si + 10 x Mn? (One) 0.01? C / Mn? (2) The method according to any one of claims 1 to 3, The chemical composition of the steel is, by mass%, C: 0.01% or more, 0.10% or less, Si: 0.03% or more, 0.60% or less, Mn: 0.5% or more, sol.Al: more than 0.005%, 0.10% or less , N: 0.0005% or more, 0.008% or less, and the balance is excellent in fatigue crack growth characteristics, characterized in that the remainder is composed of Fe and unavoidable impurities. The method according to any one of claims 1 to 3, The chemical composition of the steel is, by mass%, C: 0.01% or more, 0.10% or less, Si: 0.03% or more, 0.60% or less, Mn: 0.3% or more, so1.Al: more than 0.005%, 0.10% or less , N: 0.0005% or more, 0.008% or less, B: 0.0003 to 0.0030%, and the balance is excellent in fatigue crack growth characteristics, characterized in that the balance is made of Fe and unavoidable impurities. The method of claim 4, wherein The chemical composition of the steel also contains, by mass%, one or more of the group consisting of Nb: 0.005% or more, 0.08% or less, Ti 0.005% or more, 0.03% or less, V: 0.005% or more and 0.080% or less. Moreover, the steel material excellent in the fatigue-fatigue propagation characteristic which satisfy | fills following (3) Formula. 0.01? C / (Mn + 20 Nb + 10 Ti + 5 V)? (3) The method of claim 4, wherein The chemical composition of the steel contains, in mass%, one or more of the group consisting of Cu: less than 0.7%, Ni: 3.0% or less, Cr: less than 1.0%, Mo: 0.80% or less, and W: 0.05 to 0.50%. And the steel material excellent in the fatigue crack growth characteristics which satisfy | fills following (4) Formula. 0.01? C / (Mn + 1/10 Cu + 1/2 Ni + 1/4 Cr + Mo + 20 Nb + 10 Ti + 5 V)? (4) The method of claim 4, wherein The chemical composition of the steel is also in mass%, Ca: 0.007% or less, Mg: 0.007% or less, Ce: 0.007% or less, Y: 0.5% or less, Nd: 0.5% or less, REM: 0.05% or less, Steel material with excellent fatigue crack propagation properties containing one or more kinds. A heating step of heating the cast slab having the chemical composition according to claim 4 to 1000 ° C to 1250 ° C, a hot rolling step of performing hot rolling on the heated slab, and a cooling step of cooling the steel material subjected to the hot rolling. In the said cooling process, accelerated cooling which makes the average cooling rate between 650 degreeC-400 degreeC into 5-25 degreeC / s, stops the said accelerated cooling at the temperature of 400 degreeC or less, after that, Cooling is complete | finished so that reheat temperature range may be 70 degrees C or less, The manufacturing method of the steel excellent in fatigue crack growth resistance characterized by the above-mentioned. A heating step of heating the cast slab having the chemical component according to claim 4 to 1000 ° C to 1250 ° C, a hot rolling step of performing hot rolling on the heated slab, and a step of cooling the room without accelerated cooling thereafter; And a heating step of reheating to Ac ₁ point + 50 ° C or higher, and a cooling step of cooling the reheated steel material, wherein the average cooling rate between 650 ° C and 400 ° C is 5-25 ° C. Steel material excellent in fatigue crack growth resistance characterized by performing accelerated cooling at / s, stopping the cooling at a temperature of 400 ° C. or less, and then terminating the cooling so that the recuperation temperature range is 70 ° C. or less. Method of preparation. The reheating and cooling process of Claim 10 is performed twice or more, The manufacturing method of the steel excellent in fatigue crack growth resistance. In the production method described in claim 9, wherein in the heating step, and a cooling step of conducting cooling to the said re-heating the steel material, the cooling step to end the cooling, and then again re-heated to the Ac ₁ point + more than 50 ℃, Accelerated cooling is performed at an average cooling rate of 5 to 25 ° C / s between 650 ° C and 400 ° C, and the cooling is stopped at a temperature of 400 ° C or less, and then the recuperation temperature width is 70 ° C or less. A method for producing a steel having excellent fatigue crack growth resistance, wherein cooling is completed. In addition to the manufacturing method of Claim 9, it heats again at the temperature below Ac ₁ point, and tempers, The manufacturing method of the steel excellent in fatigue crack growth resistance characterized by the above-mentioned. The method of claim 5, The chemical composition of the steel also contains, by mass%, one or more of the group consisting of Nb: 0.005% or more, 0.08% or less, Ti 0.005% or more, 0.03% or less, V: 0.005% or more and 0.080% or less. Moreover, the steel material excellent in the fatigue-fatigue propagation characteristic which satisfy | fills following (3) Formula. 0.01? C / (Mn + 20 Nb + 10 Ti + 5 V)? (3) The method of claim 5, The chemical composition of the steel contains, in mass%, one or more of the group consisting of Cu: less than 0.7%, Ni: 3.0% or less, Cr: less than 1.0%, Mo: 0.80% or less, and W: 0.05 to 0.50%. And the steel material excellent in the fatigue crack growth characteristics which satisfy | fills following (4) Formula. 0.01? C / (Mn + 1/10 Cu + 1/2 Ni + 1/4 Cr + Mo + 20 Nb + 10 Ti + 5 V)? (4) The method of claim 5, The chemical composition of the steel is also in mass%, Ca: 0.007% or less, Mg: 0.007% or less, Ce: 0.007% or less, Y: 0.5% or less, Nd: 0.5% or less, REM: 0.05% or less, Steel material with excellent fatigue crack propagation properties containing one or more kinds. A heating step of heating the cast slab having the chemical composition according to claim 5 to 1000 ° C to 1250 ° C, a hot rolling step of performing hot rolling on the heated slab, and a cooling step of cooling the steel material subjected to the hot rolling. In the said cooling process, accelerated cooling which makes the average cooling rate between 650 degreeC-400 degreeC into 5-25 degreeC / s, stops the said accelerated cooling at the temperature of 400 degreeC or less, after that, Cooling is complete | finished so that reheat temperature range may be 70 degrees C or less, The manufacturing method of the steel excellent in fatigue crack growth resistance characterized by the above-mentioned. A heating step of heating the cast slab having the chemical component according to claim 5 to 1000 ° C to 1250 ° C, a hot rolling step of performing hot rolling on the heated slab, and a step of cooling the room without accelerated cooling thereafter; And a heating step of reheating to Ac ₁ point + 50 ° C or higher, and a cooling step of cooling the reheated steel material, wherein the average cooling rate between 650 ° C and 400 ° C is 5-25 ° C. Steel material excellent in fatigue crack growth resistance characterized by performing accelerated cooling at / s, stopping the cooling at a temperature of 400 ° C. or less, and then terminating the cooling so that the recuperation temperature range is 70 ° C. or less. Method of preparation. In addition to the manufacturing method of Claim 10, it heats again at the temperature of Ac ₁ or less and tempers, The manufacturing method of the steel excellent in fatigue crack growth resistance characterized by the above-mentioned.

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