JP2005336514A - Steel sheet having excellent fatigue crack propagation resistance and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet having excellent fatigue crack resisting performance enhanced in resistance to that cracks inherent in a weld zone of a welded structure or the liken progress by being applied with repeated stress, and to provide its production method. <P>SOLUTION: The steel sheet has a composition containing, by mass, 0.01 to 0.1% C, 0.03 to 0.6% Si, 0.3 to 2% Mn, 0.001 to 0.1% solAl and 0.0005 to 0.008% N, and, if required, comprising one or more kinds of elements selected from at least 1 group among 1 to 3 groups, and the balance Fe with impurities, and has a metallic structure composed of a bainitic structure by ≥30%, in area ratio, a martensitic structure and a pearlitic structure by ≤5% in total, and the balance ferritic structure. The production method uses the steel sheet. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a steel plate excellent in fatigue crack propagation resistance in the atmosphere and in a corrosive environment, which is suitable as a material for structures such as civil engineering and building structures, hulls, marine structures, and line pipes.

  Steel materials used in ships, offshore structures, bridges, buildings, tanks or automobiles are required to have excellent mechanical properties such as strength and toughness and excellent weldability. . In particular, fatigue characteristics are extremely important in the strength design of structures among mechanical properties.

  Patent Document 1 discloses an invention of a method for producing a high-strength hot-rolled steel sheet having excellent fatigue characteristics. This invention defines a metal structure to increase fatigue strength or fatigue limit ratio even with the same tensile strength. Steel with P and Cu added is made of ferrite and bainite, and the hardness of the ferrite portion is 120 Hv or more. If so, it is shown that the workability is excellent and the fatigue limit is improved.

  Patent Document 2 discloses an invention relating to a high-strength hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability. In the present invention, in the high-strength hot-rolled steel sheet comprising ferrite and a second phase (such as pearlite, bainite, martensite, and retained austenite), each content of Si, P, Mn, and Cr is defined. The fatigue limit ratio is improved by controlling the hardness of the ferrite to a certain hardness determined from the amount of the second phase, with a thickness of 200 to 600 Hv and a volume ratio of 5 to 10%.

  In the inventions described in these documents, the fatigue limit is improved. However, the fatigue limit or the fatigue limit ratio is usually obtained from an SN curve by a fatigue test of rotational bending, and in the case of a thin plate, plane bending. It is done. Except for specific cases, these test pieces are smoothed as much as possible, so that they cannot be used as a reference when there are wrinkles or cracks in the material.

  In general, the process in which fatigue fracture occurs can be broadly divided into two processes: the occurrence of a crack at the stress concentration part and the subsequent development of the fatigue crack. The limit ratio values are unclear how they affect crack initiation and propagation.

  In welded structures, there are many weld toes as stress concentration parts, and it is almost impossible technically to prevent the occurrence of fatigue cracks, and it is not economically advantageous. . Therefore, it is necessary to greatly extend the crack growth life from the state where the crack already exists.

Therefore, it is important to make the crack growth rate as slow as possible.
As a countermeasure when designing a structure, by distributing the load so that stress is not concentrated and giving a sufficient margin in strength, even if a crack occurs, it will not cause a fatal failure. It is possible to do so. However, providing a sufficient margin for strength is economically constrained, and if possible, it is desirable to slow the progress of fatigue cracks in the steel itself, that is, to increase the crack propagation resistance. However, a technique for improving the fatigue crack growth resistance of this material has not been studied so far.

  Patent Document 3 describes a method for producing a high-strength hot-rolled steel sheet excellent in both fatigue strength and fatigue crack propagation resistance. The content of P and Cu is regulated, and the ferrite crystal grain size is 5 to 25 μm. It is disclosed that fatigue strength and fatigue crack propagation resistance are improved by using a two-phase structure with a volume fraction of the second phase of 10 to 30%. However, the fatigue crack propagation resistance referred to in this document is a lower limit stress intensity factor range (ΔKth) in the fatigue crack growth described later, and increases the lower limit stress intensity factor value at which the fatigue crack propagates. Although effective, it is not an effective method for reducing the fatigue crack growth rate.

Patent Document 4 discloses a fatigue crack growth rate evaluation method for steel materials in which 20% or more of the structure is bainite. However, the invention described in this document relates to the evaluation method, and mechanical properties such as strength and toughness of steel materials are not considered, and it is appropriate to apply to civil engineering and building structures, hulls and marine structures. I can't say that.
JP-A-4-276016 JP-A-4-329848 JP-A-4-337026 JP 2001-41868

An object of the present invention is to provide a steel sheet excellent in fatigue fracture resistance with improved resistance against cracks inherent in welded parts of various welded structures and the like, and the method of manufacturing the same. It is to provide. Specifically, the fatigue crack growth rate obtained under the condition that ΔK, which is the difference between the maximum value and the minimum value of the stress intensity factor K in one cycle of the repeated load described later, is 20 MPa√m and the stress ratio is 0.1. Is set to 3.2 × 10 ⁻⁵ mm / cycle or less.

As a result of repeating various experiments and studies in order to solve the above-mentioned problems, the present inventors have invented a steel plate excellent in fatigue crack growth resistance and a method for producing the same as described below.
(1) The steel sheet is as follows.

  1) By mass%, C: 0.01 to 0.1%, Si: 0.03 to 0.6%, Mn: 0.3 to 2%, sol. Al: 0.001 to 0.1%, N: 0.0005 to 0.008% contained, the balance having a chemical composition composed of Fe and impurities, and a metal structure having a bainite structure with an area ratio of 30% or more, a total of 0 to 5% martensite A steel sheet characterized by a structure and a pearlite structure, and the balance being a ferrite structure.

2) The above steel sheet may further contain one or more components selected from at least one of the following first to third groups.
First group:
In mass%, Cu: 0.05-1%, Ni: 0.05-1%, Cr: 0.05-1%, Mo: 0.05-0.8% and W: 0.05-0. 5%.

Second group:
In mass%, Nb: 0.005 to 0.08%, Ti: 0.005% to 0.03%, V: 0.005 to 0.08% and B: 0.0005 to 0.003%.

Group 3:
By mass%, Ca: 0.0005-0.007%, Mg: 0.0005-0.007% and REM: 0.0005-0.05%.

(2) The manufacturing method is as follows.
1) A method for producing a steel sheet in which a steel slab having the above chemical composition is heated in a temperature range of 1000 ° C. to 1250 ° C., then hot-rolled and cooled, and shape correction is performed after cooling. In this case, at least a temperature range of 650 ° C. to 500 ° C. is a method for producing a steel sheet that is accelerated and cooled at an average cooling rate of 5 ° C./s or more.

2) After the steel slab having the above chemical composition is heated within a temperature range of 1000 ° C. to 1250 ° C., it is hot-rolled, and after hot rolling, it is reheated to a temperature of Ac ₁ point or higher and cooled. A method of manufacturing a steel sheet that performs shape correction after cooling, wherein the temperature range of at least 650 ° C. to 500 ° C. is accelerated cooling with an average cooling rate of 5 ° C./s or more.

3) In the manufacturing method according to 1) or 2) above, a method for manufacturing a steel sheet that is further tempered by cooling to 450 ° C. or lower after cooling.
4) The method for producing a steel sheet according to any one of 1) to 3), wherein the plastic deformation rate of the steel sheet in shape correction is corrected to 0.3 to 0.87.

  5) The apparatus used for shape correction is a roller leveler, and the plastic deformation rate (η) of the steel sheet by the third roll from the roller leveler side of the roller leveler obtained from the following formula is corrected as 0.3 to 0.87, 1) to above. The manufacturing method of the steel plate in any one of 3).

η = 1−2ρ _i σ _y / Et
Where ρ _{i is the} radius of curvature of the steel sheet in the third roll from the roller leveler entry side, σ _{y is the} two-dimensional yield stress, σ _y = 1.15 × σe (σe is the yield stress expressed in ordinary steel), E: Longitudinal elastic modulus, t: thickness.

(3) The structure is as follows.
A structure using any one of the above steel materials or a steel material produced by the production method according to any one of the above.

FIG. 1 is a diagram showing the relationship between the fatigue crack growth rate and the stress intensity factor.
As described above, the process in which fatigue fracture occurs can be broadly divided into two processes: the generation of a crack in the stress concentration portion and the subsequent growth of the fatigue crack. When the crack growth is handled mechanically in a state where a fatigue crack has already occurred, the stress ratio of repeated stress (R = σmin / σmax: ratio of maximum stress to minimum stress in one cycle) is constant , Fatigue crack growth rate (da / dN: amount of crack growth per cycle) and stress intensity factor range (ΔK = Kmax−Kmin: difference between maximum value and minimum value of stress intensity factor K during one cycle) 1), there is a relationship as shown in FIG.

  In this figure, in the region where ΔK shown as IIa is small, even if there is a crack, the growth rate is small, and at a certain lower limit ΔKth, the fatigue crack growth rate da / dN decreases rapidly, and the crack growth is Virtually disallowed. This ΔKth is referred to as the lower limit stress intensity factor range, and if the stress is less than this range, no progress occurs even in the presence of a crack.

The area indicated as IIb is the region where the crack progresses due to the separation of the slip surface at the crack tip. The striation formed in this region is observed as a typical fatigue fracture surface. In this region IIb, the formula known as the Paris rule
da / dN = C (ΔK) ^m
Is established. Here, C and m are constants depending on the material, environment, stress ratio, and the like.

  In the region indicated as IIc, the fracture is close to normal fracture due to tensile stress, i.e., static fracture that shows microscopic metallographic aspects such as cleavage, intergranular cracking, or coalescence of microvoids. The speed of development is significantly accelerated.

Here, a method for measuring the fatigue crack growth rate will be described.
FIG. 6 is a diagram for explaining a method of measuring the fatigue crack growth rate. As shown in the figure, the value of da / dN is obtained when the stress intensity factor range ΔK is approximately 18, 22, 26, 30, 34 MPa√m. Next is Paris
da / dN = C (ΔK) ^m
Is used to create a logarithmic graph from the relationship between five da / dN and ΔK, and the values of C and m are obtained from linear approximation. Then, da / dN when ΔK = 20 MPa√m is obtained by interpolation, and the target value of the present invention is 3.2 × 10 ⁻⁵ mm / cycle or less.

  The present inventors paid attention to slowing the crack growth in the region IIb of FIG. 1, and conducted various tests on the influence of the material on the fatigue crack growth rate da / dN in the region and the manufacturing method. As a result of repeated studies, the following knowledge was obtained.

1) The fatigue crack growth rate da / dN depends on the structure, and decreases when the bainite structure is 30% or more. In particular, da / dN becomes the smallest at 60 to 85%.
2) If shape correction is performed under appropriate conditions, da / dN is further reduced.

   3) Since the martensite structure and the pearlite structure are hard and brittle, the progress of fatigue cracks cannot be suppressed at the phase boundary, and when these structures exceed 5% in area ratio, da / dN deteriorates.

Regarding the above 1), the present inventors have confirmed that when a fatigue crack propagates and encounters a bainite phase, the crack propagates while retaining at the grain boundary or bending to avoid the bainite structure. .
FIG. 2 is a perspective view of a laminated CT specimen made of a ferrite single-phase steel and a bainite single-phase steel, used for examining the influence of the bainite phase on fatigue crack growth.

  The inventors of the present invention produced a laminated CT test piece made of a ferritic single phase steel (F) and a bainite single phase steel (B) as shown in FIG. Processed to be perpendicular to the boundary. Using this specimen, the fatigue crack growth rate at ΔK = 25 MPa√m was measured under the condition of a stress ratio of 0.1.

  FIG. 3 is a diagram showing the relationship between the fatigue crack length and the fatigue crack growth rate (propagation rate) obtained from the measurement results. As shown in the figure, it is clear that when the crack propagates from the ferrite phase (F) to the bainite phase (B), the growth rate is greatly suppressed, and the crack is retained and bent at the boundary of the bainite phase. It was suggested that this is affecting.

  In addition, it is known that bainite softens when subjected to repeated deformation as in a fatigue crack growth test. This is because the dislocations introduced by transformation coalesce and disappear due to repeated deformation, thereby relaxing the strain accumulated at the fatigue crack tip. In other words, it is considered that bainite is effective in suppressing crack growth because the crack growth driving force decreases due to work softening characteristics.

Therefore, the present inventors conducted the following tests.
A steel slab having a chemical composition of C: 0.08%, Si: 0.25%, Mn: 1.4%, Nb: 0.02%, Ti: 0.01%, sol.Al: 0.025%, N: 0.004% is heated to 1150 ° C. After hot rolling, accelerated cooling was performed to obtain four types of bainite-ferrite steel sheets having different bainite fractions. The da / dN at ΔK = 20 MPa√m of these steel plates was measured under the condition of a stress ratio of 0.1.

  FIG. 4 is a diagram showing the relationship between the bainite structure separation and the fatigue crack growth rate da / dN obtained from the measurement results. As shown in the figure, when the bainite fraction is 30% or more, da / dN becomes small, and it has been found that it becomes the smallest particularly at 60 to 85%.

In many cases, shape correction is performed by a roller leveler or a press after rolling or heat treatment.
In particular, the following experiments conducted by the present inventors show that correction using a roller leveler is effective not only for shape correction but also for reducing the fatigue crack growth rate, and that the plastic deformation rate is an important management item. It came.

  Steel slab having the composition of C: 0.05%, Si: 0.20%, Mn: 1.45%, Cu: 0.2%, Ni: 0.1%, Nb: 0.02%, sol.Al:0.030%, N: 0.004% at 1150 ° C After heating, the steel was hot worked at a finishing temperature of 880 ° C., then accelerated cooling was started from 800 ° C., and the cooling was stopped at 100 ° C. or lower to obtain a steel plate having a thickness of 15 mm.

  The steel sheet is passed through a roller leveler with different rolling conditions, and the specimen is taken from the obtained steel sheet to measure the fatigue crack growth rate and the fatigue crack growth of the steel sheet before correction. The speed was also measured and compared with steel sheets before and after straightening. The fatigue crack growth rate is that when ΔK = 20 MPa√m under the condition of a stress ratio of 0.1.

  FIG. 5 is a diagram showing the relationship between the plastic deformation rate and the crack growth rate change ratio. The fatigue crack growth rate ratio in the figure is obtained by dividing da / dN before correction by da / dN after correction. That is, if this ratio exceeds 1, da / dN is increased by correction, and if it is less than 1, da / dN is decreased. In addition, the plastic deformation rate of the leveler is set by the third roll counted from the leveler entrance side. In general, in the case of a roller leveler, the amount of reduction is reduced in an inclined manner from the entry side to the exit side, and the roll interval is corrected so as to be tapered from the so-called entry side to the exit side. Therefore, the load on the entrance side roll is greater than that on the exit side. This is because when considering material mechanics, the first and second rolls have a small number of fulcrum points, and therefore the third roll is the most loaded. The calculation method of the plastic deformation rate by this third roll is shown below.

The plastic deformation rate η can be obtained by the following equation.
η = 1−2ρ _i σ _y / Et
Where ρ _{i is the} radius of curvature of the steel sheet at the i-th roll from the roller leveler entry side, σ _{y is the} two-dimensional yield stress, σ _y = 1.15 × σe (σe is the yield stress expressed in ordinary steel materials), E is the longitudinal Elastic modulus, t: plate thickness.
The longitudinal elastic modulus changes depending on the temperature of the steel plate. For example, Table 2 on page A4-6 of A4 knitting material mechanics in mechanical engineering manual shows the relationship between temperature and longitudinal elastic modulus of carbon steel (C: 0.25% or less), and it is desirable to use this value.

  FIG. 5 shows that the crack growth rate change ratio becomes smaller by optimizing the plastic deformation rate than before the straightening process. This is considered to be because the dislocation inside the test piece is changed to a dislocation that is easily moved during the fatigue test by the straightening process. Usually, dislocations are introduced into the structure in a large amount by rolling. However, these dislocations are entangled with each other and hardly move during a fatigue test, and even during repeated deformation, dislocations do not disappear and repeated softening does not occur. On the other hand, by performing correction with a leveler, dislocations in the tissue are easy to move, coalescence and annihilation of dislocations due to repeated deformation increase, the amount of repeated softening increases, and as a result, the rate of progress decreases. it is conceivable that.

  According to the present invention, a steel plate having a high resistance to fatigue crack growth is obtained, and this steel plate is used as a steel structure used under repeated loads such as ships, offshore structures, bridges, buildings, tanks and automobiles. By applying this, the safety can be increased, the life of the structure can be extended, and the amount of steel used can be reduced.

The reason for limitation of each condition in the steel plate and manufacturing method of the present invention will be described in detail.
In addition,% which shows content of each element shown below is the mass%.

Chemical composition of steel sheet C: 0.01 to 0.1%
In order to ensure strength and generate an appropriate amount of bainite phase, it is necessary to control the content. If the content is less than 0.01%, the amount of bainite is insufficient and the resistance to crack propagation cannot be increased. On the other hand, if the content exceeds 0.1%, weldability deteriorates. Therefore, the C content is set to 0.01 to 0.1%. Desirably, it is 0.02 to 0.08%.

Si: 0.03-0.6%
Si is added for the purpose of deoxidation and increasing strength. If the content is less than 0.03%, the effect is not sufficient. If the content exceeds 0.6%, island-shaped martensite is formed in the bainite structure, resulting in deterioration of toughness and surface properties. The amount was 0.03 to 0.6%. In addition, as a suitable range, it is 0.1 to 0.5%.

Mn: 0.3-2%
Mn is an element necessary for guaranteeing strength as a structural steel and generating a stable bainite phase. If it is less than 0.3%, the effect is not sufficient, and if it exceeds 2%, weldability and toughness deteriorate. Preferably it is 0.5% or more.

A desirable range in which good effects can be stably obtained is 0.8 to 1.8%.
Sol.Al: 0.001 to 0.1%
Al is added during steelmaking for the purpose of deoxidation. If the content is less than 0.001%, deoxidation is insufficient and internal defects increase in the steel ingot before rolling, and if it exceeds 0.1%, the toughness deteriorates. Preferably it is 0.01% or more.

Therefore, the Al content is set to 0.001% to 0.1%. In addition, since an effect will be saturated even if it adds more than a certain amount, 0.01 to 0.05% is desirable.
N: 0.0005 to 0.008%
N combines with Al and Ti to form precipitates, contributes to the finer austenite grains, and has the effect of improving toughness. In order to acquire this effect, it is necessary to contain N 0.0005% or more. On the other hand, if the N content exceeds 0.008%, the island-like martensite ratio increases and the toughness deteriorates, so the upper limit was made 0.008%.

Cu, Ni, Cr, Mo, W:
These elements are effective in improving the strength of steel and suppressing fatigue crack growth, and are also effective in improving corrosion resistance. Therefore, it is effective in suppressing fatigue crack growth even in a corrosive environment such as in sour crude oil, and is contained if necessary. In the case of inclusion, it is 0.05 to 1% for Cu, Ni and Cr, 0.05 to 0.8% for Mo, and 0.05 to 0.5% for W. Below these lower limits, a sufficient effect cannot be obtained. On the other hand, when these upper limits are exceeded, Cu causes cracking during hot rolling, Ni and Cr cause deterioration of weldability, and Mo and W cause deterioration of toughness.

Ti, Nb, V, B:
These elements have the effect of increasing the strength and suppressing the fatigue crack growth rate, and are contained if necessary.

  Ti and Nb can improve strength by precipitation hardening. Further, the austenite grain size can be controlled by a combination with rolling conditions and heat treatment conditions, and further, the fatigue crack growth rate is suppressed by introducing dislocations by improving hardenability. In order to obtain these effects sufficiently, it is necessary to contain both 0.005% or more of Ti and Nb. On the other hand, if the amount is too large, the toughness of the steel deteriorates, so the upper limit of the content of both was set to 0.1% or less. Preferably, both are 0.01 to 0.05%. V and B have the effect of increasing toughness in addition to increasing the strength of steel. Moreover, these elements contribute to the improvement of fatigue crack growth rate by increasing the dislocation density in the structure by improving the hardenability. In particular, B is effective because it has a great effect of lowering the transformation point by improving hardenability. In order to obtain these effects sufficiently, it is necessary to contain V in an amount of 0.005% or more and B in an amount of 0.0005% or more. On the other hand, if V exceeds 0.08% and B exceeds 0.003%, the toughness deteriorates. Preferable content is 0.02 to 0.06% for V and 0.0008 to 0.002% for B.

Ca, Mg, REM (rare earth elements):
These elements refine the structure and are effective in improving toughness, and are contained if necessary. In order to obtain these effects sufficiently, it is necessary to contain 0.0005% or more of each element. However, since the toughness deteriorates if added excessively, the upper limit of Ca and Mg is 0.007%, and the upper limit of REM is 0.05%. Therefore, Ca: 0.0005 to 0.007%, Mg: 0.0005 to 0.007%, and REM: 0.0005 to 0.05%.

  P and S are both impurity elements that deteriorate toughness, and the smaller the better. In the steel of the present invention, it is desirable that the P and S contents be 0.02% and 0.01% or less, respectively, as a limit that does not have a noticeable effect.

(2) Metal structure Bainitic structure As a development goal of a steel plate with excellent fatigue crack growth resistance, da / dN was set to 3.2 or less under the condition of ΔK = 20 MPa√m and stress ratio of 0.1. Thus, it was found that da / dN was the smallest in the vicinity of 80% in terms of the bainite fraction.

  Therefore, in order to make da / dN 3.2 or less, it is necessary to make the bainite fraction 30% or more as can be seen from FIG. Preferably it is 35 to 90%. The upper limit is not specified but is preferably 92% or less.

Martensite structure and pearlite structure Martensite and pearlite are preferably as few as possible because they are hard and brittle and cannot suppress the growth of fatigue cracks at the phase boundary. If the total of these exceeds 5%, da / dN deteriorates, so 0 to 5% was set. The balance consists of a ferrite structure.

(3) Manufacturing method
(i) Steel slab heating temperature, hot rolling:
Although heating temperature was 1000-1250 degreeC, this temperature is the center temperature of a steel piece (slab). What is necessary is just to determine the temperature setting and in-furnace time of each zone of a heating furnace so that this may become the said temperature range by heat-transfer calculation. If it is less than 1000 ° C., the ferrite rate increases and the crack growth rate increases. Moreover, when it exceeds 1250 degreeC, a structure | tissue will become coarse and toughness will deteriorate. The hot rolling may be performed by a normal method, and the finishing temperature is not particularly specified, but it is desirable to finish to the required thickness at a temperature sufficiently higher than the Ar ₃ point. In each pass during rolling, particularly in the finish rolling process, it is desirable that the rolling reduction be 10% or more. This is to make the bainite fraction of the metal structure 30% or more.

(ii) Accelerated cooling:
Accelerated cooling refers to forcibly cooling a steel sheet using a cooling medium such as water.
a) A so-called TMCP type manufacturing method in which accelerated cooling is performed immediately after hot rolling.
The reason why the forced cooling rate in the temperature range of at least 650 to 500 ° C. is set to 5 ° C./s or more is that if it is less than 5 ° C./s, the ferrite rate increases and the crack growth rate increases. The reason why the forced cooling stop temperature is set to 500 ° C. or less is that when the forced cooling stop temperature is higher than 500 ° C., the ferrite rate increases and the crack growth rate increases. A preferable forced cooling stop temperature range is 450 ° C. or lower, and more preferably 400 ° C. or lower.

b) A manufacturing method in which the steel sheet is heated again after quenching without accelerated cooling immediately after rolling and then quenched by forced cooling.
This method is a method of cooling after hot rolling and performing reheating, forced cooling and shape correction in another line, and the reasons for limiting the forced cooling conditions and the forced cooling stop temperature are the same as in the case of a) above.

In the methods a) and b), the lower limit of the forced cooling stop temperature is not limited, and forced cooling may be performed to room temperature.
The reason why the reheating temperature is set to Ac ₁ point or more is that if the temperature is less than Ac ₁ point, transformation due to quenching does not occur, and thus a steel material having a target bainite fraction of 30% or more cannot be obtained.

(iii) Tempering temperature:
Tempering is applied when strength adjustment and toughness improvement are required. When tempering is performed, the tempering temperature is set to 450 ° C. or lower, and the lower limit is not limited, but is preferably set to 300 ° C. or higher to obtain the effect.

  The inventors have a composition of C: 0.06%, Si: 0.30%, Mn: 1.50%, Cu: 0.2%, Ni: 0.1%, Nb: 0.02%, sol.Al: 0.03%, N: 0.003%. After heating the steel piece to 1150 ° C. and hot-working at a finishing temperature of 870 ° C., using accelerated steel from 810 ° C. and stopping cooling at 400 ° C., using a 25 mm thick steel material The effect of tempering temperature on da / dN was investigated.

FIG. 7 is a diagram showing the relationship between the tempering temperature and the fatigue crack growth rate obtained as a result of the test.
As shown in the figure, it was revealed that the crack growth rate deteriorates rapidly at 450 ° C. or higher. The cause is not clear, but it is thought that dislocations in bainite disappear and the effect of suppressing crack growth decreases.

(iv) Shape correction:
The apparatus used for shape correction is not particularly limited, but a roller leveler method is particularly desirable. According to the inventors' experiment, in the case of a roller leveler, the plastic deformation rate is an important management item.

  The relationship between the plastic deformation rate and the crack growth rate is as shown in FIG. 5 obtained from the test results. If the plastic deformation rate is 0.3 to 0.87, the rate of change in speed compared to that before straightening. It turns out that becomes small. For example, when the speed change ratio is 0.9 or less, the plastic deformation rate of the leveler is preferably 0.3 to 0.82.

In the present invention, correction by this leveler is particularly important. This is because the initial dislocation generated by rolling is changed to a dislocation that is easily moved during the fatigue test.
Although a large amount of dislocations is introduced into the structure by rolling, these dislocations are entangled with each other and hardly move during a fatigue test, and even during repeated deformation, dislocations do not disappear and repeated softening does not occur. On the other hand, by performing correction with a leveler, dislocations in the tissue are easy to move, and coalescence and annihilation between dislocations due to repeated deformation increase, and the amount of repeated softening increases, resulting in a slow progress rate. it is conceivable that.

  Using a steel slab (slab) having the chemical composition shown in Table 1, a hot-rolled steel sheet having a thickness of 15 mm was manufactured under the manufacturing conditions shown in Table 2.

  Various test pieces were collected from the obtained steel sheet, and subjected to optical microscope structure observation, tensile test and Charpy test. Further, the CT specimen shown in FIG. 8 was collected and subjected to a fatigue crack growth test in accordance with ASTM standard E 647. The results are shown in Table 3.

Observation of the metal structure, tensile strength, toughness, and fatigue crack growth rate were evaluated by the following methods.
The metal structure was obtained by polishing a cross section of a sample taken from a portion corresponding to 1/4 of the plate thickness, and measuring the fraction of bainite and pearlite by optical microscope observation on the surface etched with 2% nital corrosive solution. . Ten visual fields were measured for one sample, and the average of the ten measurements was taken as the bainite fraction and pearlite fraction of the steel sheet.

  As the tensile test piece, a JIS 14B tensile test piece was sampled at right angles to the rolling direction and subjected to a tensile test. As for toughness, No. 4 Charpy impact test piece specified in JIS-Z2202 is taken in parallel to the rolling direction from the center of the plate thickness and subjected to Charpy impact test, and impact absorption energy (vE-20, unit is J). Asked.

The fatigue crack growth rate was measured by a fatigue test method using a CT test piece and an electrohydraulic closed loop fatigue test apparatus shown in FIG.
FIG. 8 is a view showing a CT test piece, FIG. 8A is a front view, and FIG. 8B is a side view.

  FIG. 9 is a side view showing an electrohydraulic closed loop fatigue testing apparatus. In the apparatus shown in FIG. 9, reference numeral 1 is a CT test piece, 2 is a load cell for load measurement, 3 is a hydraulic cylinder, 4 is a hydraulic source, 5 is a hydraulic valve, 6 is a waveform generator, 7 is a load controller, 8a And 8b show load bars respectively. In the CT test piece shown in FIG. 8, a fatigue precrack having a length of 2.5 mm is introduced at the tip of the notch, and load rods 8a and 8b are mounted in the upper and lower holes.

  With this apparatus, the CT test piece 1 is repeatedly subjected to stress from the hydraulic cylinder 3 via the load rods 8a and 8b to the fatigue precrack tip. The specimens were taken from the center of the plate thickness in the thickness direction so that the longitudinal direction of the fatigue crack was perpendicular to the rolling direction, and the front and back surfaces were removed by 0.5 mm and mirror polished.

The fatigue test conditions were as follows.
f (repetition rate) = 20Hz
R (stress ratio) = 0.1
T (test temperature) = room temperature The test atmosphere is in the atmosphere. As a result of fatigue crack growth test, in each specimen, the medium ΔK region (ΔK: the maximum stress intensity factor and the minimum stress intensity factor in the stress intensity factor range) Fatigue crack growth rate was evaluated. The middle ΔK region (15 to 34 MPa√m) in this test corresponded to the IIb region of fatigue crack growth. That is, the Paris rule [Trans. ASTM, Ser. D. 85, 523 (1963)]
da / dN = C (ΔK) ^m
△ K: MPa√m, da / dN: mm / cycle
Was found to hold.

Therefore, in the present invention, the fatigue crack growth characteristics were evaluated by the crack growth rate da / dN (mm / cycle) at ΔK = 20 MPa√m in the ΔK region.
Tables 2 and 3 show the results of the above tests. As shown in these tables, steel types A to L whose structure and components satisfy the conditions specified in the present invention have a fatigue crack growth rate as slow as 3.2 × 10 ⁻⁵ mm / cycle or less, and extremely excellent fatigue It had crack growth resistance. On the other hand, steel plates with steel grades M to Q have an absorbed energy of less than 100 J or a fatigue crack growth rate exceeding 4 × 10 ^-5 mm / cycle, and the desired fatigue crack growth resistance. It was not obtained.

It is a figure which shows the relationship between a fatigue crack growth rate and the stress intensity factor range. 1 is a perspective view of a multilayer CT specimen made of a ferrite single phase steel and a bainite single phase steel. FIG. It is a figure which shows the relationship between fatigue crack length and fatigue crack growth rate. It is a figure which shows the relationship between a bainite structure fraction and fatigue crack growth rate da / dN. It is a figure which shows the relationship between a plastic deformation rate and a crack growth rate change rate. FIGS. 6A, 6B and 6C are diagrams for explaining a method of measuring the fatigue crack growth rate. It is a figure which shows the relationship between tempering temperature and fatigue crack growth rate. They are a CT test piece front view and a side view. It is a side view which shows an electrohydraulic closed loop type fatigue test apparatus.

Explanation of symbols

1. CT specimen 2. 2. Load cell for load measurement Hydraulic cylinder4. Hydraulic source 6. 6. Waveform generator Load controller 8. Load rod

Claims (10)

  In mass%, C: 0.01 to 0.1%, Si: 0.03 to 0.6%, Mn: 0.3 to 2%, sol. Al: 0.001 to 0.1%, N: 0.0005-0.008% is contained, the balance has a chemical composition consisting of Fe and impurities, and the metal structure is a bainite structure with an area ratio of 30% or more, and a total martensite structure of 0-5% A steel sheet characterized by having a pearlite structure and the balance being a ferrite structure.   In place of a part of Fe, in mass%, Cu: 0.05 to 1%, Ni: 0.05 to 1%, Cr: 0.05 to 1%, Mo: 0.05 to 0.8% And W: 0.05-0.5% of 1 type or 2 types or more are contained, The steel plate of Claim 1 characterized by the above-mentioned.   In place of a part of Fe, further mass%, Nb: 0.005 to 0.08%, Ti: 0.005% to 0.03%, V: 0.005 to 0.08% and B: 0 The steel sheet according to claim 1 or 2, characterized by containing one or more of 0.0005 to 0.003%.   In place of a part of Fe, by mass%, Ca: 0.0005-0.007%, Mg: 0.0005-0.007% and REM: 0.0005-0.05% The steel plate according to any one of claims 1 to 3, comprising more than seeds.   The steel slab having the chemical composition according to any one of claims 1 to 4 is heated in a temperature range of 1000 ° C to 1250 ° C, then hot-rolled and then cooled, and the shape is corrected after cooling. A method for producing a steel plate, comprising the step of performing accelerated cooling at an average cooling rate of 5 ° C./s or more in a temperature range of at least 650 ° C. to 500 ° C. during the cooling. The steel slab having the chemical composition according to any one of claims 1 to 4 is heated in a temperature range of 1000 ° C to 1250 ° C, then hot-rolled, and then hot-rolled, Ac ₁ point or more A method of manufacturing a steel sheet which is reheated to a temperature and cooled, and is subjected to shape correction after cooling. In the cooling, at least a temperature range of 650 ° C. to 500 ° C. is accelerated at an average cooling rate of 5 ° C./s or more. A method for producing a steel sheet, comprising:   The manufacturing method according to claim 5 or 6, wherein after accelerating cooling, the steel plate is further heated to 450 ° C or lower to be tempered.   The method of manufacturing a steel sheet according to any one of claims 5 to 7, wherein the shape correction is performed by setting the plastic deformation rate of the steel sheet to 0.3 to 0.87. Shape correction is performed with a roller leveler, and the shape correction is performed by setting the plastic deformation rate (η) of the steel sheet by the third roll from the steel sheet entrance side of the roller leveler obtained from the following formula to 0.3 to 0.87. The manufacturing method of the steel plate in any one of Claims 5-7.
η = 1−2ρ _i σ _y / Et
Where ρ _{i is the} radius of curvature of the steel sheet in the third roll from the roller leveler entry side, σ _{y is the} two-dimensional yield stress, σ _y = 1.15 × σe (σe is the yield stress expressed in ordinary steel), E: Longitudinal elastic modulus, t: thickness.   A structure using the steel material according to any one of claims 1 to 4 or the steel material produced by the production method according to any one of claims 5 to 9.

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