KR20180005745A - Thin steel plate having excellent fatigue properties and production method therefor - Google Patents

Abstract

The steel sheet according to the present invention satisfies a predetermined chemical composition and is measured at an observation position at a depth of 3 mm from the steel sheet surface in a longitudinal section parallel to the rolling direction, , And the precipitate satisfies the following requirement (A). (a) The metal structure is composed of a bainite structure and a residual portion structure, and the bainite fraction of the whole structure is 80% or more by area. (b) When the crystal grains of bainite are determined based on the diagonal grain boundaries of the adjacent crystals having an azimuth difference of 15 degrees or more, the average length of the crystal grains in the plate thickness direction is 7 mu m or less. (c) The circle-equivalent diameter of the residual portion structure is 3.0 탆 or less. (d) the percentage of pearlite in the remaining part of the structure is 80% by area or more. (A) Nb, Ti, and one the at least one circle-equivalent diameter of less than or equal to the number of 20㎚ precipitate containing 100 of the V / ㎛ ² Or more.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate having excellent fatigue characteristics and a manufacturing method thereof. BACKGROUND OF THE INVENTION < RTI ID = 0.0 &

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate used as a structural material of a ship, a building, a bridge, a construction machine, etc., and a manufacturing method thereof. More specifically, the present invention relates to a steel sheet having excellent fatigue characteristics at a tensile strength of 490 MPa or more and less than 650 MPa, and a manufacturing method thereof.

In large structures such as ships, buildings, bridges, and construction machines, the structures are becoming larger in size, and in the case of breakage, the damage is large. Therefore, reliability of the structural members is required. It has been conventionally known that most of the causes of breakage in large structures are fatigue fractures, and a variety of endothelial fracture techniques have been developed. However, there are still a few cases where breakage has been caused by fatigue fracture.

In general, stress concentration has been alleviated by structural devising in areas where fatigue damage is likely to occur in large structures. However, in such structures, manufacturing costs are often increased by the addition of members or the use of high strength steels. Therefore, there is a demand for a technique for improving the fatigue characteristics of the steel itself.

For example, Non-Patent Document 1 discloses the effects of various influencing factors imparted to fatigue characteristics, and fatigue characteristics are improved by solid solution strengthening, precipitation strengthening, grain refinement, and second phase strengthening. In dislocation strengthening, however, It is difficult to obtain an improvement in fatigue characteristics. The process of fatigue failure can be divided into (1) the process until cracks are generated due to repetitive loading, and (2) the process from failure to crack propagation. Among the improvement factors of the fatigue characteristics described above, it is effective to suppress the accumulation of dislocations in the process of the above (1), and it is considered that solid solution strengthening, precipitation strengthening, grain refinement, etc. are effective. On the other hand, in the process (2), since it is effective to inhibit the progress of the crack, it is considered that the grain refinement and the second phase strengthening are effective.

On the other hand, Patent Document 1 proposes that a two-phase structure of fine ferrite and hard martensite is defined and the difference in hardness is specified to suppress the crack propagation speed and to improve the fatigue life after cracking. However, in this technique, it is easy to imagine that not only a large amount of potential is introduced by the quenching treatment but also the potential is introduced again even when the second phase is transformed from austenite to martensite. And it is difficult to stably improve the overall service life. It is also expected to precipitate carbonitrides by the addition of Nb and V. However, in the case of rapid cooling as in the quenching treatment, it is difficult to obtain such precipitates stably and to obtain improved fatigue characteristics by precipitation strengthening .

Patent Document 2 proposes a technique of reducing the crack propagation speed by making a steel structure of a mixture of fine ferrite and bainite. By using this technique, it is expected that fatigue life after crack generation can be improved even in fatigue fracture. However, no consideration is given to the fatigue characteristics until the occurrence of cracks, and improvement in fatigue characteristics can not be expected.

Patent Document 3 proposes to improve fatigue strength by depositing carbide in a ferrite structure. However, since fatigue characteristics after cracking are not described, and since they are intended for thin steel sheets, it is difficult to satisfy other properties required for large structures such as toughness.

The inventors of the present invention investigated the total life time until fatigue failure and the ratio of the shear lifetime up to the generation of cracks and the life of the rear end up to failure after the occurrence of cracks. As a result, the shear lifetime until the occurrence of cracks accounts for about 50% of the entire life span to the fatigue failure, and the ratio of the shear lifetime until the occurrence of cracks increases as the stress level is lowered Proved. From this, it is necessary to improve not only the fatigue characteristics after the occurrence of cracks but also the fatigue characteristics until the occurrence of cracks, in order to prolong the overall service life to fatigue failure. Particularly, in the vicinity of the fatigue limit, there is a tendency that the ratio of the shear lifetime until the occurrence of cracks increases.

As described above, although a technique for improving the fatigue characteristic is required in a large structure, a technique for reducing the crack propagation speed (a technique for improving crack propagation characteristics) is required. This is because, even if a fatigue crack occurs, if the crack propagation speed is low, it is possible to repair and repair the damaged part until reaching failure. Therefore, there is a demand for a steel sheet which improves the fatigue characteristics (fatigue strength) of the steel itself and improves the fatigue crack growth characteristics (decreases the rate of advance of fatigue cracks).

In addition, crack propagation of the above-described fatigue characteristics is a region having a short crack length particularly shortly after the occurrence of cracking, and therefore, it is necessary to perform evaluation with the test piece collected from the surface layer. On the other hand, the crack propagation in the crack propagation characteristic is a region in which the crack grows and stably advances (so-called long crack), and is influenced not by the surface layer but by the structure inside the steel. Therefore, in order to produce a steel sheet excellent in crack propagation characteristics as well as fatigue characteristics, it is necessary to control not only the texture of the surface layer but also the texture form inside the steel.

Japanese Unexamined Patent Application Publication No. 10-60575 Japanese Patent Application Laid-Open No. 2011-195944 Japanese Patent Application Laid-Open No. 2009-84643

Abe et al., "Iron and Steel" 70th year (1984) No. 10 1459-1466

SUMMARY OF THE INVENTION The present invention has been made in view of the circumstances as described above, and its object is to provide a steel sheet having improved fatigue characteristics up to cracking and fatigue characteristics after cracking, And to provide a useful method for doing so.

The steel sheet of the present invention which can solve the above problems contains 0.03 to 0.12% of C, 0.3 to 0.3% of Si, 1.0 to 2.0% of Mn, and 0.0005 to 0.005% of B, At least one selected from Cu: 0.1 to 1.0% and Ni: 0.1 to 1.0%; V: more than 0% to 0.05%; Nb: more than 0% to 0.05% And the balance elements satisfy the relation of the following formulas (1) to (3), and the remaining amount is a post-steel sheet of iron and inevitable impurities,

(A) to (d) when the metal structure is measured at an observation position at a depth of 3 mm from the surface of the steel sheet in a longitudinal section parallel to the rolling direction and the precipitate satisfies the following requirement (A) .

([Nb], [Ti] and [V] represent the content of Nb, Ti and V, respectively, on the basis of mass% in the steel sheet.)

([Cu], [Ni] and [Si] represent the content of Cu, Ni and Si in the steel sheet, respectively, on the basis of mass%).

([Mn], [Cr] and [Mo] represent the content of Mn, Cr and Mo in the steel sheet, respectively, on the basis of mass%).

(a) The metal structure is composed of a bainite structure and a residual portion structure, and the bainite fraction of the whole structure is 80% or more by area.

(b) When the crystal grains of bainite are determined based on the diagonal grain boundaries of the adjacent crystals having an azimuth difference of 15 degrees or more, the average length of the crystal grains in the plate thickness direction is 7 mu m or less.

(c) The circle-equivalent diameter of the residual portion structure is 3.0 탆 or less.

(d) the percentage of pearlite in the remaining part of the structure is 80% by area or more.

(A) Nb, Ti, and one the at least one circle-equivalent diameter of less than or equal to the number of 20㎚ precipitate containing 100 of the V / ㎛ ² Or more.

The " circle equivalent diameter " means the diameter (circle equivalent diameter) when converted into a circle having the same area. In addition to the basic components (C, Si, Mn, Cu, Ni, B, V, Nb and Ti) of the steel sheet according to the present invention, among the elements specified by the above formula (3) (Hereinafter, these values are referred to as " Kp value "), and when these elements are not included, the left side value of the expression (3) , The Kp value may be calculated from the above equation (3).

(I) Ca: more than 0% to not more than 0.005%, (ii) Al: more than 0% to not more than 0.10%, (iii) N: more than 0% to less than 0.010% (Iv) Cr: not less than 0% and not more than 2%, (v) Mo: not less than 0% and not more than 1%, and the like.

The inventors of the present invention found that the structural form of the 1/4 position (also referred to as the t / 4 position) of the plate thickness on the longitudinal section parallel to the rolling direction as a result of performing the crack propagation test and the structural observation on the various steel sheets, (e) to (g), it was found that a steel sheet excellent in crack propagation characteristics as well as fatigue characteristics was obtained.

(e) The metal structure is composed of a bainite structure and a remaining portion structure harder than bainite, and the bainite fraction in the whole structure is 80% or more by area.

(f) When the crystal grains of bainite are determined based on the diagonal grain boundaries of the adjacent crystals having an azimuthal difference of 15 degrees or more, the average length of the crystal grains in the plate thickness direction is 7 占 퐉 or less.

(g) The circle-equivalent diameter of the above-mentioned hard residual portion structure is 3 탆 or less.

On the other hand, the production method of the present invention, which has been able to achieve the above object, is characterized in that the above-mentioned steel sheet having the chemical composition is hot-rolled under the following conditions. In this production method, the cooling stop temperature is preferably 550 DEG C or higher.

Heating temperature: 1000 to 1200 DEG C

Cumulative rolling reduction of the entire hot rolling process: 70% or more

In the finish rolling, Ar ₃ transformation point + 150 ℃ to Ar ₃ transformation point + cumulative rolling reduction in the temperature range of 50 ℃: 50%

Finish rolling finish temperature: Ar ₃ transformation point + 30 ° C or more

Average cooling rate from finish rolling finish temperature to 600 占 폚: 10 占 폚 / sec or less

According to the present invention, since the texture and the precipitate are appropriately controlled together with the chemical composition of the steel sheet after the formation of the steel sheet, fatigue characteristics (crack generation suppression characteristics) until crack generation and fatigue characteristics (crack propagation prevention property) It is possible to realize a post-steel sheet having an excellent fatigue characteristic.

1 is a schematic view showing a test piece used for measurement of fatigue characteristics.
2 is a schematic explanatory diagram showing the shape of a compact test piece used for measurement of crack propagation velocity.

Means for securing the fatigue life after the occurrence of fatigue crack first was examined by the present inventors. Since the fatigue life (rear end life) after the occurrence of cracks contributes greatly to the progress of cracking as described above, it is possible to reduce the grain size of the main body of the structure and the residual portion structure (second phase) other than the main body, . With respect to the refinement of the crystal grains, the crystal grains can be miniaturized by making the structure of the steel material a bainite structure or a martensite structure. Further, by making the residual portion structure into a pearlite main body which does not cause shear deformation at the time of transformation, deterioration of the shear life can be prevented.

The inventors of the present invention investigated the influence of the tissue morphology on the fatigue characteristics of various steel plates. As a result, it has been found that a steel sheet having fatigue characteristics superior to that of the conventional steel sheet can be obtained by controlling the structure in the thickness direction section (longitudinal section parallel to the rolling direction) at a depth of 3 mm from the surface of the steel sheet as follows. Here, in the case of a structure having a depth of 3 mm from the surface of the steel sheet, it is necessary to control the structure in the vicinity of the surface layer because a crack is generated on the surface of a steel sheet in a normal steel sheet. This is because the fatigue characteristics can not be improved even if the structure at the t / 2 position (t: plate thickness) is controlled.

(group)

When the bainite crystal grains are determined on the basis of the diagonal grain boundaries in which the bainite fraction occupied in the metal structure is 80% or more by area and the azimuthal difference of adjacent crystals is 15 or more, the average length of the crystal grains in the plate thickness direction , Sometimes referred to as " effective crystal grain diameter ") is 7 mu m or less. The bainite fraction is preferably at least 85% by area, more preferably at least 90% by area. The effective crystal grain diameter is preferably 6 占 퐉 or less, and more preferably 5 占 퐉 or less.

The structure of the steel sheet according to the present invention is composed of a structure mainly composed of bainite as described above in order to satisfy the strength class of not less than 490 MPa and less than 650 MPa and to secure excellent fatigue characteristics. Here, the term " bainite " includes structures such as upper bainite, lower bainite, acicular ferrite and granular bainitic ferrite.

Bainite can make the crystal grain diameter finer than ferrite and the like, and improves both the life (shear life) until the crack occurs and the life (posterior life) after the crack progresses. Although martensite has a fine structure, a large amount of dislocations are introduced due to shear deformation occurring at the time of martensitic transformation, and the shear lifetime is impaired, so that the improvement of the fatigue characteristics as a whole can not be obtained. The reason why the crystal grain diameter (effective crystal grain diameter) is defined as the length in the thickness direction (cut length) is that the fatigue crack progresses on the surface of the steel sheet are considered.

In order to obtain a structure mainly composed of bainite, in addition to the addition of alloying elements such as C and Mn, quenching after rolling is often performed. However, if the cooling rate is high, it is difficult to secure a desired precipitate (to be described later). Therefore, in the present invention, it is necessary to add B (to be described later) in addition to C and Mn, which are usually added to the steel sheet. B has an effect of suppressing the ferrite transformation, it is possible to secure a structure mainly composed of bainite stably even if the cooling rate is relatively low.

(Tissue of remaining part)

It is necessary that the structure (residual portion structure) other than the bainite structure is set to 3.0 탆 or less in circle equivalent diameter and the pearlite fraction occupied in the remaining portion structure is 80% or more by area. The reason why the circle-equivalent diameter of the residual portion structure is 3.0 占 퐉 or less is that if the average size of the remaining portion structure exceeds 3.0 占 퐉, the other characteristics such as toughness may be significantly lowered. The residual portion structure can also be expected to improve the post-stage service life by making the residual portion structure a hard martensite or a martensite-austenite mixed structure (MA). On the other hand, in these structures, since a large amount of movable potential is introduced into the bainite structure at the time of transformation, there is a fear that the shear life is greatly lowered and the whole service life is lowered. On the other hand, by making the residual portion structure into a pearlite main body which does not cause shear deformation at the time of transformation, it is possible to prevent the deterioration of the crack generation life. The preferable upper limit of the circle equivalent diameter of the residual portion structure is 2.5 占 퐉 or less (more preferably 2.0 占 퐉 or less), and the lower limit is preferably 0.5 占 퐉 or more.

Further, the residual portion structure may include a soft texture such as a part of ferrite. In order to exhibit the effect, the remaining part of the structure is preferably 3% by area or more (more preferably 5% by area or more).

The present inventors also investigated the influence of the precipitates on the fatigue characteristics. As a result, it has been found that a post-steel sheet exhibiting better fatigue characteristics than the conventional steel sheet can be obtained by controlling the precipitates in longitudinal cross-sections parallel to the rolling direction at a depth of 3 mm from the surface of the steel sheet as follows.

(Precipitate)

In order to improve the fatigue characteristics of the steel after the steel sheet, precipitates (carbides and carbonitrides) having a circle-equivalent diameter of 20 nm or less including at least one of Nb, Ti and V must be finely dispersed in the steel sheet. For strengthening by precipitation (precipitation strengthening), it is generally effective to disperse the precipitate more finely and in a large amount. When the precipitate is not sheared by dislocation, the effect of precipitation strengthening becomes larger when the precipitate size is finer. The reason why the circle equivalent diameter of the precipitate (carbide and carbonitride) is specified to be 20 nm or less is that it suppresses generation of coarse precipitates that degrade other properties such as toughness while utilizing precipitation strengthening.

However, if the precipitate becomes too fine, precipitates are sheared by the dislocation, so precipitation strengthening becomes difficult to obtain. The critical size at which the precipitate shears is different depending on the kind of the precipitate, and the critical size of the precipitate containing at least one of Nb, Ti, and V is about 5 nm. As a result, even a finer precipitate contributes to precipitation strengthening, so precipitation strengthening can be effectively utilized by finely dispersing and increasing the water density. In order to secure a predetermined amount of precipitate, it is general to add a large amount of alloy, or to perform heat treatment such as tempering. However, this method not only increases the material cost and the manufacturing cost, but also precipitates the precipitates, thereby failing to improve the fatigue characteristics, and possibly deteriorating other properties such as toughness.

The inventors of the present invention have been able to secure a fine precipitate at a low cost and stably by designing alloys in consideration of the ability to form precipitates by C (this is called " C activity "). From this point of view, it is necessary to appropriately control the addition amount of the elements constituting the precipitate. Specifically, it is necessary that the content of Nb, Ti and V satisfy the relation (1).

([Nb], [Ti] and [V] represent the content (mass%) of Nb, Ti and V in the steel sheet, respectively)

In the present invention, the chemical composition of the steel sheet for improving fatigue characteristics after cracking is described. In the post-steel sheet of the present invention, a predetermined amount of bainite structure is ensured by suitably adding alloying elements such as C, Mn, and B, and at the same time, the amount of Mn, Cr, It is possible to control the addition amount of Nb, Ti and V which are constituent elements of the precipitate appropriately and also to adjust the relation of the addition amounts of Cu, Ni and Si affecting the C activity to disperse fine precipitates By precipitation, a post-steel sheet exhibiting excellent fatigue characteristics can be realized. From this point of view, each component is adjusted as follows. Here, the content (%) of each chemical component is based on mass% unless otherwise specified. Also, in the present specification, the percentage (mass%) based on mass is equal to the percentage (% by weight) based on weight. In addition, the content of each chemical component may be expressed as "X% or less (not including 0%)" and "0% or more and X% or less".

(C: 0.03 to 0.12%)

C is an important element for securing the strength of the base material (steel sheet), and is an element constituting the precipitate. In order to effectively exhibit this effect, the C content is set to 0.03% or more. The amount of C is preferably 0.04% or more, and more preferably 0.05% or more. On the other hand, when the amount of C becomes excessive, the residual portion structure is coarse and excessively occurs, and the fatigue characteristics are deteriorated. Therefore, the amount of C was set at 0.12% or less. The amount of C is preferably 0.10% or less, and more preferably 0.08% or less.

[Si: not more than 0.3% (not including 0%)]

Si is an element necessary for securing the strength of the base material (steel sheet), and at the same time, it is an element that lowers the C activity. Therefore, the addition amount of Si needs to be 0.3% or less. The amount of Si is preferably 0.25% or less, and more preferably 0.2% or less. In order to exert such action, the amount of Si is preferably 0.01% or more, and more preferably 0.05% or more.

(Mn: 1.0 to 2.0%)

Mn is an important element for ensuring quenching in order to obtain a bainite structure. In order to effectively exhibit such action, the Mn content should be 1.0% or more. The amount of Mn is preferably 1.2% or more, and more preferably 1.4% or more. However, when the amount of Mn is excessive, the pearlite fraction occupied in the remaining part of the structure is lowered, so that sufficient fatigue characteristics can not be obtained. Therefore, the Mn content should be 2.0% or less. The amount of Mn is preferably 1.8% or less, and more preferably 1.6% or less.

(B: 0.0005 to 0.005%)

B is an element that improves the quenching property, and is an element that inhibits ferrite transformation to facilitate generation of bainite structure. In order to exhibit such an effect, B must be contained in an amount of 0.0005% or more. The amount of B is preferably 0.001% or more, and more preferably 0.002% or more. However, if the B content is excessive, sufficient precipitates can not be obtained and the fatigue property improving effect can not be obtained. Therefore, the B content should be 0.005% or less. The preferable upper limit of the B content is 0.004% or less, and more preferably 0.003% or less.

(At least one selected from Cu: 0.1 to 1.0% and Ni: 0.1 to 1.0%

Cu and Ni are necessary elements for increasing the C activity and generating fine precipitates. In order to exhibit such action, it is necessary to contain at least one of them at 0.1% or more. It is preferably at least 0.15%, more preferably at least 0.2%. However, if the content of Cu and Ni is excessively large, the precipitates are coarse and the fatigue characteristics are not improved, and other properties such as toughness are deteriorated. From this point of view, it is necessary to set the total amount to 1.0% or less, preferably 0.8% or less, and more preferably 0.6% or less.

Since Cu, Ni and Si are elements affecting C activity, it is necessary to control the content of these Cu, Ni and Si so as to satisfy the following relationship (2) in order to stably obtain fine precipitates.

([Cu], [Ni] and [Si] represent the content (mass%) of Cu, Ni and Si in the steel sheet, respectively.

If the value of (Cu) + [Ni] - 2 [Si] (hereinafter referred to as "CA value") is less than 0 (%), C activity is insufficient and sufficient precipitates can not be obtained, Is not obtained. If the CA value exceeds 1.0 (%), the C activity becomes excessive, and coarse precipitates are formed on the contrary, fatigue characteristics are not improved, and other characteristics may be deteriorated. The preferable lower limit of the CA value is 0.1 (%) or more, and more preferably 0.15 (%) or more. Further, the preferable upper limit of the CA value is 0.9 (%) or less, and more preferably 0.7 (%) or less.

[V: one kind selected from the group consisting of not more than 0.05% (not including 0%), Nb: not more than 0.05% (not including 0%) and Ti: not more than 0.05% More than]

V, Nb and Ti are elements necessary for generating precipitates in addition to securing the bainite structure by improving the hardenability. However, if it is contained in excess, the precipitates become coarse and a sufficient fatigue-improving effect can not be obtained. Therefore, it is necessary to limit the fatigue limit to 0.05% or less. The preferable lower limit of these elements is determined spontaneously in the relation of the above-mentioned expression (1).

These elements need to satisfy the relation of the above-mentioned formula (1). When the value of [Nb] +2 [Ti] +2 [V] (hereinafter referred to as "PR value") is less than 0.01%, sufficient precipitates can not be obtained and improvement in fatigue characteristics can not be obtained. The PR value is preferably 0.03 (%) or more, and more preferably 0.05 (%) or more. When the PR value becomes too large, the precipitates become coarse and the fatigue characteristics are deteriorated. The PR value is preferably 0.08 (%) or less, and more preferably 0.06 (%) or less.

In order to prevent the deterioration of the crack generation life by using the residual portion structure as the structure of the pearlite main body, it is necessary to appropriately control the relationship of the contents of Mn, Cr and Mo. Mn, Cr and Mo must satisfy the relation of the following expression (3).

[[Mn], [Cr] and [Mn] represent the contents (mass%) in the steel sheets of Mn, Cr and Mo, respectively.

If the value of [Mn] +1.5 [Cr] +2 [Mo] (hereinafter referred to as " Kp value ") exceeds 2.4 (%), the pearlite fraction in the remaining- So that sufficient fatigue characteristics can not be obtained. The preferable upper limit of the Kp value is 2.2 (%) or less, and more preferably 2.0 (%) or less. The lower limit of the Kp value is preferably 1.0 (%) or more, and more preferably 1.3 (%) or more.

The basic components of the post-steel sheet of the present invention are as described above, and the remaining portion is substantially iron. However, it is a matter of course that inevitable impurities (for example, P, S, etc.) brought in due to the conditions of raw materials, materials, and manufacturing facilities are included in the steel. Further, in the post-steel sheet of the present invention, it is also effective to positively contain the following elements, and the characteristics of the post-steel sheet are further improved according to the types of elements contained therein.

[Ca: 0.005% or less (not including 0%)]

Ca is an element that reduces the anisotropy of the shape of inclusions (such as MnS) in the steel, and is an element effective in preventing inclusions from becoming a starting point of fracture and improving fatigue characteristics. In order to exhibit such an effect effectively, it is preferable that the content is 0.0005% or more. More preferably, it is 0.001% or more. However, when the Ca content is excessive, the purity of the steel is lowered, and the fatigue characteristics are rather deteriorated, and other properties such as toughness may be deteriorated. Therefore, the Ca content is preferably 0.005% or less, and more preferably 0.003% or less.

[Al: 0.10% or less (not including 0%)]

Al is an element useful as a de-oxidizing material. In order to exhibit such action, it is preferable to contain Al in an amount of 0.01% or more. More preferably, it is 0.02% or more. However, when the Al content is excessive, not only the fatigue characteristics but also the toughness are deteriorated. Therefore, the Al content is preferably 0.10% or less, more preferably 0.04% or less.

[N: not more than 0.010% (not including 0%)]

N has an effect of enhancing the strength by solid solution strengthening, so that N is positively included as needed. However, when the N content is excessive, coarse nitride is generated and the fatigue characteristics are deteriorated. From this viewpoint, the N content is preferably 0.010% or less, and more preferably 0.008% or less. In order to effectively exhibit the effect of N, the content thereof is preferably 0.0035% or more, and more preferably 0.0040% or more.

[Cr: 2% or less (not including 0%)]

Cr is an element having the same effect as Mn, and a bainite structure can be stably obtained. In order to exhibit such an action, Cr is preferably contained in an amount of 0.1% or more. More preferably, it is 0.5% or more. However, if the Cr content is excessive, the pearlite fraction in the residual portion structure may be reduced and the fatigue characteristics may be deteriorated. Therefore, the Cr content is preferably 2% or less. A more preferable upper limit of the Cr content is 1.7% or less.

[Mo: 1% or less (not including 0%)]

Mo is an element having the same effect as Mn, and a bainite structure can be stably obtained. In order to exhibit such an action, Mo is preferably contained in an amount of 0.005% or more, more preferably 0.01% or more, and still more preferably 0.03% or more. However, if the Mo content exceeds the above range, the pearlite content in the residual portion structure is reduced to lower the fatigue characteristics, so that the content thereof is preferably 1% or less. A more preferable upper limit of the Mo content is 0.7% or less, and more preferably 0.5% or less.

Since the precipitate interferes with the movement of dislocations, the precipitation strengthening suppresses the accumulation of dislocations due to cyclic loading of fatigue and improves fatigue characteristics. However, not only the fatigue characteristics but also the toughness are important factors in the steel sheets used in ships, bridges and offshore structures, and coarse precipitates may deteriorate toughness. As a result, it is necessary to disperse fine precipitates, but if the size of the precipitates becomes small, they are sheared by dislocations, and a strengthening effect due to the precipitates can not be obtained. However, even if a precipitate containing Nb, Ti, V or the like is a very fine precipitate of about 5 nm, it is not sheared by dislocations, and therefore fatigue characteristics can be improved without deteriorating other characteristics.

As described above, in order to improve the fatigue characteristics of the steel sheet, it is necessary to disperse precipitates (carbides and carbonitrides) having a circle equivalent diameter of 20 nm or less and at least one of Nb, Ti and V in the steel sheet . For strengthening by precipitation (precipitation strengthening), it is generally effective to disperse the precipitate more finely and in a large amount. From this point of view, it is necessary that the circle-equivalent diameter is 20 nm or less and the number of precipitates containing at least one of Nb, Ti and V is 100 pieces / 占 퐉 ² or more at a depth of 3 mm from the surface of the steel sheet . The number of precipitates is preferably 150 pieces / 탆 ² or more, more preferably 200 pieces / 탆 ² Or more.

The thickness of the post-steel sheet according to the present invention is not particularly limited. However, when the thickness of the post-steel sheet is small, the contribution of improving the life of the post-grooved core is reduced. From this viewpoint, the plate thickness is preferably 6 mm or more, and more preferably 10 mm or more. In addition, the post-steel sheet of the present invention exhibits a post-steel sheet excellent in fatigue characteristics unconventional by making a plurality of fatigue property improving factors compatible as described above.

The steel sheet of the present invention satisfies the above requirements and its production method is not particularly limited. However, in a series of manufacturing steps of a steel sheet in which hot rolling is performed after casting steel with a solvent, (For example, slab) having the chemical composition as described above is used to obtain a precipitate to be improved by heating temperature before hot rolling, cumulative rolling reduction during hot rolling, finish rolling temperature, finish rolling reduction ratio, The rolling finish temperature, the cooling rate after hot rolling, and the cooling stop temperature are preferably controlled as follows.

Heating temperature: 1000 to 1200 DEG C

Cumulative rolling reduction of the entire hot rolling process: 70% or more

In the finish rolling, Ar ₃ transformation point + 150 ℃ to Ar ₃ transformation point + cumulative rolling reduction in the temperature range of 50 ℃: 50%

Finish rolling finish temperature: Ar ₃ transformation point + 30 ° C or more

Average cooling rate from finish rolling finish temperature to 600 占 폚: 10 占 폚 / sec or less

Prior to hot rolling, the steel strip is heated to a temperature range of 1000 to 1200 캜. Preferably 1050 DEG C or more. The cumulative rolling reduction of the entire hot rolling process is at least 70%. It is preferably at least 75%. In order to reduce the texture size of bainite, it is necessary to apply sufficient pressure reduction in the non-recrystallized temperature region. Sufficient shrinkage is carried out by heating in a temperature range of 1000 to 1200 占 폚 so as to ensure a cumulative rolling reduction of 70% or more during hot rolling while preventing coarsening of crystal grains. Further, in order to obtain a bainite structure, it is preferable that an Ar ₃ transformation point + 150 ° C to Ar ₃ The cumulative reduction ratio in the temperature range of the transformation point + 50 占 폚 (the non-recrystallization temperature region) is 50% or more (preferably 60% or more). Further, the temperature (the finish rolling end temperature) to end the finish rolling is, in order to obtain a bainite structure, Ar ₃ transformation point + can be appropriately set up not less than 30 ℃, as much as possible to achieve the desired cumulative rolling reduction of, the Ar ₃ transformation point + 30 < 0 > C. The preferable upper limit of the finish rolling finish temperature is the Ar ₃ transformation point of the steel sheet + 130 캜 or less (more preferably, the Ar ₃ transformation point + 1000 캜 or less).

The heating rate at the time of heating the steel sheet is not particularly limited, but is preferably about 3 to 10 占 폚 / min. If the heating rate is less than 3 ° C / min, coarse carbides are liable to precipitate in the heating step of the slab, and the alloying elements are segregated because they can not be sufficiently solidified in the subsequent steps, so that it is difficult to finely disperse the desired precipitates . On the other hand, if the heating rate exceeds 10 DEG C / min, the component segregation in the slab is not sufficiently solved, and it becomes difficult to finely disperse the desired precipitate.

The " cumulative reduction factor " is a value calculated from the following formula (4). The Ar ₃ transformation point adopts a value obtained by the following equation (5) (the values shown in Tables 1 and 2 described later are also the same).

(4) where t ₀ is the rolling start thickness (mm) of the steel strip when the temperature at the 3 mm position from the surface is in the rolling temperature range, t ₁ is the temperature at the 3 mm position from the surface in the rolling temperature range (Mm), and t ₂ represents the thickness of the slab (for example, slab) before rolling. ]

[C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo] and [Nb] (Mass%) of Nb.

After completion of the hot rolling, cooling is carried out at an average cooling rate of at least 10 占 폚 / sec (preferably 8 占 폚 / sec or less) at a finish rolling finish temperature of at least 600 占 폚, (Cooling stop temperature) is 550 DEG C or higher, it is possible to finely disperse the precipitate and to make the residual portion structure into a pearlite-based structure. The reason why the cooling range is changed from "finish rolling finish temperature to 600 ° C" is because it is a temperature region in which precipitates are precipitated in addition to the formation of a horny structure. Further, in the steel sheet of the present invention, even if the average cooling rate is 10 DEG C / sec or less, the bainite structure can be miniaturized while suppressing the formation of ferrite.

The reason why the cooling stop temperature is set to 550 占 폚 or more is that when the cooling is performed at the cooling rate up to below 550 占 폚, the residual portion structure becomes MA or martensite. In order to stabilize the remaining amount structure into pearlite, It is necessary to stop the cooling. However, if the temperature is less than 550 ° C, cooling by cooling may be performed. From this viewpoint, the preferable lower limit of the average cooling rate is 1 占 폚 / second or more (more preferably 1.5 占 폚 / second or more).

As described in relation to the fatigue characteristic, the suppression of the crack propagation is effective in refining the crystal grains in the structure to be the main body and hardening by the hard second phase. However, in general hot rolling, even if forced cooling is performed on the steel sheet after rolling, the internal cooling rate is slower than that of the surface layer of the steel sheet. Even if the surface layer has a fine bainite structure, And the crack propagation speed increases. Therefore, in order to obtain a steel sheet excellent in fatigue characteristics and excellent in crack propagation characteristics, it is preferable to control the texture form of the steel sheet (steel sheet) while controlling the texture of the surface layer.

(Tissue inside the steel plate)

When the bainite fraction in the metal structure at the position of t / 4 of the plate thickness is ensured to be 80% or more by area and the crystal grains of bainite are determined based on the diagonal grain boundaries in which the adjacent crystal orientation difference is 15 or more, (Average effective crystal diameter) in the plate thickness direction is 7 占 퐉 or less. The bainite fraction is preferably at least 85% by area, more preferably at least 90% by area. The average effective crystal grain diameter is preferably 6 占 퐉 or less, and more preferably 5 占 퐉 or less.

(Remaining part of steel sheet)

It is preferable that the hard residual portion structure (hard residual portion structure) of the structure other than the bainite structure (remaining portion structure) is 3 탆 or less in circle equivalent diameter and harder than bainite. The reason why the circle-equivalent diameter of the hard residual portion structure is set to 3 탆 or less is that if the average size of the remaining portion structure exceeds 3 탆, the other characteristics such as toughness may be largely lowered. The residual portion structure basically includes martensite and MA, and these hard residual portions can reduce the crack propagation speed.

In order to secure this type of structure, it is desirable to control the cumulative reduction ratio in the hot rolling and the reduction ratio in the non-recrystallization temperature region as follows.

Cumulative rolling reduction of the entire hot rolling process: 80% or more

Reduction ratio in non-recrystallization temperature range: less than 70%

It is necessary to increase the cumulative rolling reduction rate in the hot rolling step in order to make the texture size at a position that is 1/4 of the plate thickness to be 7 占 퐉 or less, and the rolling reduction rate is preferably 80% or more. If the cumulative reduction factor is insufficient, the structure at the position where the surface layer becomes finer becomes even smaller than 1/4 of the plate thickness, and the crack propagation speed is not sufficiently lowered. More preferably, it is 85% or more.

In addition, if the reduction rate in the non-recrystallization temperature region is increased, the ferrite nucleation site is increased and the ferrite transformation is likely to occur, and the bainite fraction is lowered, so that sufficient crack propagation rate inhibiting effect can not be obtained. Therefore, in order to secure a bainite fraction of not less than 80% by area in the steel sheet, it is necessary not to excessively reduce the rolling in the non-recrystallization temperature region. In this respect, it is desirable that the cumulative rolling reduction in the non-recrystallization temperature region is less than 70%.

[Example]

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited to the following Examples, but may be appropriately changed within the scope of the present invention. All of which are included in the technical scope of the present invention.

(Example 1)

(Steel types A to Z) shown in the following Tables 1 and 2 were cast and molded in accordance with a conventional casting method, and then subjected to casting under various conditions (rolling conditions No. a to p) shown in Table 3 below Followed by hot rolling to obtain a steel sheet having a thickness of 18 to 20 mm. Note that, in Table 3, a "non-recrystallization temperature region rolling reduction" is, Ar ₃ transformation point + 150 ℃ to a reduction rate (cumulative rolling reduction) in the temperature range of Ar ₃ transformation point + 50 ℃. In addition, the "non-recrystallization temperature zone reduction ratio" shown in Table 3 is a design value, and when the finishing rolling finishing temperature is higher than the Ar ₃ transformation point + 150 ° C. (ie, rolling is completed before reaching the non-recrystallization temperature region) The temperature region reduction rate is 0% (i.e., no reduction in the non-recrystallization temperature region) (for example, Test No. 26 in Table 5).

[Table 1]

[Table 2]

[Table 3]

 For the steel sheet, the bainite fraction of the steel sheet, the effective crystal grain diameter, the size of the second phase (residual portion structure), the pearlite fraction in the remaining portion structure, the tensile strength, the fatigue characteristics, the number density and the size . Further, in all the measurements, the test piece was sampled so that the position 3 mm from the surface layer of the steel sheet became the evaluation position.

(Bainite fraction)

A sample was cut out so that a surface parallel to the rolling direction of the steel sheet at a depth of 3 mm from the surface of the steel sheet and perpendicular to the surface of the steel sheet was exposed and polished using a wet emery of # 150 to # 1000, Polished surface was polished using a diamond abrasive as an abrasive. This specular specimen was etched with a 2% nitric acid-ethanol solution (Na dissolution solution), and a field of view of 150 탆 x 200 탆 was observed at an observation magnification of 400 times and the bainite fraction (area%) was determined by image analysis. The bainite fraction of the five field of view in total was determined, and the average value was employed.

(Effective grain diameter)

In the cross section parallel to the steel sheet rolling direction at a depth of 3 mm from the surface layer of the steel sheet, the effective grain diameter (the grain size of the grain boundary) was measured by an SEM (Scanning Electron Microscope) -EBSP (Electron Backscatter Pattern Analysis) Diameter) was measured. Specifically, an EBSP apparatus (trade name: "OIM") manufactured by TEX SEM Laboratories was used in combination with SEM, and the effective crystal grain diameter was measured by using a grain boundary system with a boundary angle of 15 ° or greater. The measuring conditions at this time were a measuring area of 200 占 퐉 占 200 占 퐉, a measuring step of 0.5 占 퐉 apart, and a measuring point having a Confidence Index less than 0.1, which indicates the reliability of the measuring bearing, was excluded from the analysis object. For the grain boundaries thus obtained, cut lengths at 100 points in the plate thickness direction were measured, and the average value was defined as the effective crystal grain diameter. However, when the effective crystal grain diameter was 2.0 탆 or less, it was judged to be measurement noise and excluded.

(Size and pearlite fraction of the residual portion structure)

The size of the residual portion structure and the pearlite fraction in the residual portion texture were obtained by cutting the sample by the same method as the measurement of the bainite fraction, polishing and etching, observing the sample at a magnification of 1000 times with SEM, The size (circle equivalent diameter) of the residual portion structure and the pearlite fraction in the remaining portion structure were determined. Both adopted an average of 5 fields of view.

(The tensile strength)

Tensile test specimens having a thickness of 4 mm and a gauge distance of 35 mm were taken from the surface layer depths of 2 to 6 mm from each steel sheet and tensile strength TS was measured by performing tensile test according to JIS Z2241 (2011).

(Fatigue characteristics)

As to the fatigue characteristics, a steel material having a thickness of 4 mm was cut out from a position of 2 to 6 mm in thickness of the surface layer of the plate thickness, and a test piece as shown in Fig. 1 was produced. Further, the surface of the test piece was polished to # 1200 with an emery paper to remove the influence of the surface state. The obtained test pieces were subjected to a fatigue test under the following conditions using an electric hydraulic servo type fatigue tester manufactured by Instron.

Test environment: room temperature, atmospheric

Control method: Load control

Control waveform: sinusoidal wave

Stress ratio: R = -1

Test speed: 20Hz

End of test cycle: 5,000,000 times

The fatigue property is affected by the tensile strength. To eliminate the influence, the fatigue limit ratio of 5,000,000 times was obtained, and it was determined that the fatigue limit ratio of 5,000,000 times exceeded 0.51. The fatigue limit ratio of 5,000,000 times is the value obtained by dividing the fatigue strength of 5,000,000 times by the tensile strength, and the fatigue strength of 5,000,000 cycles was determined as follows. The fatigue test was carried out with a stress amplitude at which the value of (? A / TS) obtained by dividing the stress amplitude? A by the tensile strength TS in each of the test pieces was 0.51. Quot; and " " in the following Tables 4 and 5). Further, the fatigue test was carried out with a stress amplitude at which the value of (sigma a / TS) was 0.53, and the samples which were not broken at the time of reaching five million cycles were shown as "? &Quot; in the following Tables 4 and 5.

(Number density and size of precipitates)

From the sample taken from the position 3 mm from the surface of the steel sheet, the test specimen produced by the extraction replica method was observed with a transmission electron microscope (TEM: transmission electron microscope) at an observation magnification of 150000, an observation field of view (750 nm x 625 nm, The area of the precipitate containing any of Nb, Ti and V in the visual field was measured by image analysis, and the circle equivalent diameter of each precipitate was calculated from this area. Further, Nb, Ti, V was determined by EDX (Energy Dispersive X-ray spectrometry). Converting into 1 μm ² of the precipitate having a circle-equivalent diameter of 20 nm or less, did.

The results are shown in Tables 4 and 5 below.

[Table 4]

[Table 5]

From these results, it can be considered as follows. That is, 1 to 19 (Table 4) satisfy the requirements (texture, precipitate) defined in the present invention because the chemical composition of steel and the production conditions are appropriately controlled, and excellent fatigue characteristics are exhibited.

On the other hand, 20 to 41 (Table 5), fatigue characteristics were deteriorated because at least one of the chemical composition and the manufacturing conditions of the steel sheet was inadequate. Among them, 20, the heating rate of the slab was excessively high (rolling condition No. g), the density of the precipitates was small, and the fatigue characteristics were deteriorated. Test No. 21, the heating rate of the slab was excessively slow (rolling condition No. h), the density of the precipitates was small, and the fatigue characteristics deteriorated.

Test No. 22 shows that the heating temperature before hot rolling becomes too high (rolling condition No. i), and the predetermined tensile strength is not achieved (other properties are not evaluated). Test No. 23, the heating temperature before hot rolling became too low (rolling condition No. j), the bainite fraction decreased, the effective crystal grain diameter became large, the precipitate number density became small, and the fatigue characteristics deteriorated.

Test No. 24, the cumulative reduction ratio at the time of hot rolling became too small (rolling condition No. k), the effective crystal grain diameter became large, and the fatigue characteristics deteriorated. Test No. 25 exhibited a reduction in rolling reduction in the non-recrystallization temperature region (rolling condition No. 1), the effective crystal grain diameter became excessively large and the residual portion size became large, and the fatigue characteristics deteriorated.

Test No. 26 shows that the finish rolling finish temperature is excessively high (rolling condition No. m: reduction rate in the non-recrystallization temperature region is substantially 0%), the tensile strength is lowered, the effective crystal grain diameter is increased, And the fatigue characteristics deteriorated. Test No. 27, the finish rolling finish temperature was too low (rolling condition No. n), the bainite fraction decreased, the precipitate number density also became small, and the fatigue characteristics deteriorated.

Test No. 28, the cooling rate after hot rolling became faster (rolling condition No. o), the precipitate number density was not achieved (the fine precipitates did not disperse), and the fatigue characteristics deteriorated (tensile strength also increased). Test No. 29, the cooling stopping temperature was lowered (rolling condition No. p), and the pearlite fraction in the residual amount structure became smaller, and the fatigue characteristics deteriorated.

Test No. 30 is an example using a steel piece (steel grade O) having an excess of C content, and the tensile strength is too high (other characteristics are not evaluated). Test No. 31 is an example using a steel strip (grade P) having a low C content, and a predetermined tensile strength is not achieved (other characteristics are not evaluated).

Test No. 32 is an example using a steel piece (steel grade Q) having an excess of Si content, and the density of precipitates was not achieved (fine precipitates were not dispersed), and fatigue characteristics were deteriorated. Test No. 33 is an example using a steel having a low Mn content (steel type R), and the bainite fraction was lowered and fatigue characteristics were deteriorated. Test No. 34 is an example using a steel piece (steel grade S) having a large amount of Mn, and the pearlite fraction was insufficient in the residual amount structure, and the fatigue characteristics deteriorated.

Test No. 35 is an example using a steel piece (steel type T) having a low CA value due to insufficient content of Cu and Ni, and the density of precipitates was reduced and the fatigue characteristics were deteriorated. Test No. 36 is an example using a steel plate (steel type U) having a large CA value (Ni content is excessive), the coarse precipitates are increased, the precipitate number density is decreased, and the fatigue characteristics are deteriorated ).

Test No. 37 is an example using a steel plate (steel type V) having a small PR value, and the density of precipitates is reduced, and the fatigue characteristics are deteriorated. Test No. 38 shows an example in which a piece of a steel having a large PR value (steel type W) was used, the density of precipitates was reduced, and the fatigue characteristics deteriorated.

Test No. 39 is an example using a steel billet having a large Kp value (steel grade X). The bainite fraction is reduced, the effective crystal grain diameter is increased, and the pearlite fraction in the residual amount structure is decreased.

Test No. 40 is an example using a (low-content added) (unfilled) slab (steel grade Y), the bainite fraction decreased, the effective crystal grain diameter became large, and the fatigue characteristics deteriorated. Test No. 41 is an example using a steel sheet having a large B content (steel type Z), the CA value is also small, the bainite fraction is decreased, and the fatigue characteristics are deteriorated (tensile strength is also increased).

(Example 2)

Test No. shown in Table 4 For each of the steel sheets 1 to 19, the bainite fraction at the position that is 1/4 of the plate thickness, the effective crystal grain diameter, and the size of the second phase were evaluated in the same manner as in the method shown in Example 1. The sampling method of the test piece is the same as the above, except that the position is set to 1/4 of the plate thickness. Further, for these steel sheets, the crack propagation rate was measured by the following method.

(Crack propagation speed)

A compact test specimen was used in accordance with ASTM E647, and a fatigue crack growth test was conducted under the following conditions in an electric-hydraulic servo fatigue tester to measure the crack propagation speed. The compact test specimen was taken from a position where the plate thickness was one-half of that of the plate, and the specimen shown in Fig. 2 was used. In addition, the crack length was determined by the compliance method.

Test environment: room temperature, atmospheric

Control method: Load control

Control waveform: sinusoidal wave

Stress ratio: R = -1

Test speed: 5 to 20 Hz

At this time, the value obtained when the stable growth region ΔK = 20 (MPa · m ^1/2 ) established by the Parrish rule defined by the following formula (6) was evaluated as a representative value. ^{ΔK = 20 (MPa · m 1} /2) was that the crack growth rate is excellent in crack propagation characteristics being below 5.0 × 10 ^-5 mm / cycle when the.

(1) where a is the crack length, n is the number of repeats, and C and m are constants determined under the conditions of the material and the load.

The results are shown in Table 6 below.

[Table 6]

From this result, it can be considered as follows. That is, 1, 3 to 6, and 9 to 19 satisfy satisfactory requirements (texture, precipitate) in the steel sheet and have excellent fatigue crack growth characteristics (cracks And the advancing speed is 5.0 × 10 ^-5 mm / cycle or less).

On the contrary, 2 and 8, the cumulative rolling reduction in the entire hot rolling process was small (rolling condition No. c), the effective crystal grain diameter became large, and the fatigue crack growth characteristics deteriorated. In addition, 7, the rolling reduction (cumulative rolling reduction) in the non-recrystallization temperature region was increased (rolling condition No. f), and the bainite fraction was lowered and the fatigue crack growth characteristics deteriorated.

Although the present invention has been described in detail with reference to specific embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention.

The present application is based on Japanese patent application (Japanese Patent Application No. 2013-195590) filed on September 20, 2013 and Japanese Patent Application filed on January 30, 2014 (Japanese Patent Application No. 2014-015524 ), All of which are cited by reference.

Claims (4)

Wherein the steel sheet further contains 0.1 to 1.0% of Cu, 0.1 to 1.0% of Cu, and 0.1 to 1.0% of Cr, And at least one element selected from the group consisting of V: more than 0% to 0.05%, Nb: more than 0% to 0.05%, and Ti: more than 0% to 0.05%
, And more preferably more than 0% and not more than 1% of Mo and not more than 0.005% of Ca, more than 0% and not more than 0.10% of Al, not less than 0.0035% and not more than 0.010% And at least one member selected from the group consisting of
Wherein the elements satisfy the relationship of the following formulas (1) to (3), the balance being iron and inevitable impurities after the steel sheet,
(A) to (D) and the precipitate satisfies the following requirement (A) when measured at an observation position at a depth of 3 mm from the steel sheet surface in a longitudinal section parallel to the rolling direction: Wherein the steel sheet has excellent fatigue characteristics.

([Nb], [Ti] and [V] represent the contents of Nb, Ti and V in the steel sheet on the basis of mass%).

([Cu], [Ni] and [Si] represent the content of Cu, Ni and Si in the steel sheet, respectively, on the basis of mass%).

([Mn], [Cr] and [Mo] represent the content of Mn, Cr and Mo in the steel sheet, respectively, on the basis of mass%).
(a) The metal structure is composed of a bainite structure and a residual portion structure, and the bainite fraction of the whole structure is 80% or more by area.
(b) When the crystal grains of bainite are determined based on the diagonal grain boundaries of the adjacent crystals having an azimuthal difference of 15 degrees or more, the average length of the crystal grains in the plate thickness direction is 7 mu m or less.
(c) The circle-equivalent diameter of the residual portion structure is 3.0 탆 or less.
(d) the percentage of pearlite in the remaining part of the structure is 80% by area or more.
(A) The number of precipitates having a circle-equivalent diameter of 20 nm or less including at least one of Nb, Ti and V is 100 / 탆 ² or more. The steel sheet according to any one of claims 1 to 4, wherein the metal structure satisfies the following requirements (e) to (g) when a position that is a 1/4 position of the plate thickness is observed on a longitudinal section parallel to the rolling direction: After the steel sheet with excellent characteristics.
(e) The metal structure is composed of a bainite structure and a remaining portion structure harder than bainite, and the bainite fraction in the whole structure is 80% or more by area.
(f) When the crystal grains of bainite are determined based on the diagonal grain boundaries of the adjacent crystals having an azimuthal difference of 15 degrees or more, the average length of the crystal grains in the plate thickness direction is 7 占 퐉 or less.
(g) The circle-equivalent diameter of the above-mentioned hard residual portion structure is 3 탆 or less. A method for producing a steel sheet having excellent fatigue characteristics, characterized by hot-rolling the steel sheet according to claim 1 under the following conditions.
Heating temperature: 1000 to 1200 DEG C
Cumulative rolling reduction of the entire hot rolling process: 75% or more
In the finish rolling, Ar ₃ transformation point + 150 ℃ to Ar ₃ transformation point + cumulative rolling reduction in the temperature range of 50 ℃: 50%
Finish rolling finish temperature: Ar ₃ transformation point + 30 ° C or more
Average cooling rate from finish rolling finish temperature to 600 占 폚: 10 占 폚 / sec or less The method according to claim 3, wherein the cooling stop temperature is 550 ° C or more, and the fatigue characteristic is excellent.

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