JP6211946B2 - Thick steel plate with excellent fatigue characteristics and method for producing the same - Google Patents

Description

  The present invention mainly relates to a thick steel plate used as a structural material for ships, buildings, bridges, construction machines, and the like, and a method for manufacturing the same. More specifically, the present invention relates to a thick steel plate having a tensile strength of 490 MPa or more and less than 650 MPa and excellent fatigue characteristics, and a method for producing the same.

  In large structures such as ships, buildings, bridges, and construction machinery, the size of the structure is increasing, but the structural members are required to have higher reliability due to the magnitude of damage in the event of damage. It has been. It has been known that many of the causes of damage in large structures are fatigue failure, and various anti-fatigue failure technologies have been developed, but there are still examples of damage caused by fatigue failure. Not a few.

  In general, stress concentration has been alleviated by applying structural measures in areas where fatigue damage is likely to occur in large structures. However, in such structures, manufacturing costs increase due to the addition of members and the use of high-strength steel materials. There are many cases. Therefore, a technique for improving the fatigue characteristics of the steel material itself is required.

  For example, Non-Patent Document 1 shows the effect of various influencing factors on fatigue properties. The fatigue properties are improved by solid solution strengthening, precipitation strengthening, grain refinement, and second phase strengthening. It is said that it is difficult to improve fatigue characteristics because of the increase in movable dislocations. The process of fatigue fracture can be divided into (1) a process until a repeated load is applied and a crack is generated, and (2) a process until the generated crack progresses to a fracture. Of the improvement factors of fatigue characteristics shown above, it is effective to suppress the accumulation of dislocations in the process (1), and solid solution strengthening, precipitation strengthening, grain refinement, etc. are effective. Conceivable. On the other hand, in the above process (2), it is effective to prevent the crack from progressing, and therefore it is considered that crystal grain refinement and 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 used, and the crack growth rate is suppressed by defining the hardness difference to improve the fatigue life after the occurrence of cracks. Yes. However, in this technique, in addition to the introduction of a large amount of dislocations due to the quenching treatment, further dislocations are introduced when the second phase transforms from austenite to martensite, so the life until cracking is reduced. It can be easily imagined, and it is difficult to stably improve the total lifetime. Also, precipitation of carbonitrides is expected by the addition of Nb and V. However, when cooling is performed rapidly as in the quenching process, such precipitates are stably secured and fatigue due to precipitation strengthening is achieved. It is difficult to obtain improved characteristics.

  Patent Document 2 proposes a technique for reducing the crack growth rate by making the steel structure a mixed structure of fine ferrite and bainite. By using this technology, it can be expected to improve the fatigue life after cracking even in fatigue fracture, but no consideration is given to the fatigue properties until cracking, and no significant improvement in fatigue properties can be expected. .

  Patent Document 3 proposes improving the fatigue strength by precipitating carbides in the ferrite structure. However, there is no description about the fatigue characteristics after the occurrence of cracks, and it is intended for thin steel sheets, and it is difficult to satisfy other characteristics required for large structures such as toughness.

  The present inventors investigated the ratio of the life before the crack until the occurrence of crack and the ratio of the life after the crack until the fracture until the fatigue failure. As a result, the pre-stage life until crack initiation accounts for about 50% of the total life until fatigue failure, and the proportion of pre-stage life until crack initiation increases as the stress level is lowered and the total life becomes longer. Turned out to be. For this reason, in order to increase the total life until fatigue failure, 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 particular, in the vicinity of the fatigue limit, there is a tendency that the ratio of the previous stage life until the occurrence of a crack increases.

  As described above, a technique for improving fatigue characteristics is required for large structures, but a technique for reducing the crack growth rate (a technique for improving crack growth characteristics) is also required. This is because even if a fatigue crack occurs, if the crack growth rate is low, it is possible to find and repair the damaged part before failure. Therefore, there is a demand for a steel sheet that improves the fatigue characteristics (fatigue strength) of the steel material itself and improves the fatigue crack growth characteristics (decreases the fatigue crack growth rate).

  Note that the crack growth with the above-mentioned fatigue characteristics is a region where the crack length is particularly short immediately after the occurrence of the crack, and therefore it was necessary to evaluate with a test piece taken from the surface layer. On the other hand, the crack growth in the crack growth characteristics is a region where the crack grows and stably propagates (so-called long crack), and is influenced by the structure inside the steel material, not the surface layer. Therefore, in order to manufacture a steel sheet that is excellent in crack growth characteristics in addition to fatigue characteristics, it is necessary to control not only the surface layer structure but also the structure inside the steel material.

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

Japanese Patent Laid-Open No. 10-60575 JP 2011-195944 A JP 2009-84643 A

  The present invention has been made in view of the circumstances as described above, and its purpose is to improve both the fatigue characteristics until crack initiation and the fatigue characteristics after crack initiation, and is a thick steel plate excellent in unprecedented fatigue characteristics. And providing a useful method for producing such thick steel plates.

The thick steel plate of the present invention that can solve the above problems is C: 0.03 to 0.12% (meaning “mass%”; the same applies hereinafter), Si: 0.3% or less (not including 0%) , Mn: 1.0 to 2.0%, B: 0.0005 to 0.005%, respectively, and Cu: 0.1 to 1.0% and Ni: 0.1 to 1.0% One or more selected, V: 0.05% or less (not including 0%), Nb: 0.05% or less (not including 0%), and Ti: 0.05% or less (not including 0%) 1) or more selected from the group consisting of: and these elements satisfy the relationship of the following formulas (1) to (3), the balance being a steel plate of iron and unavoidable impurities:
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, the metal structure satisfies the following requirements (a) to (d), and the precipitate satisfies the following requirements (A). It is characterized by satisfaction.
0.01 ≦ [Nb] +2 [Ti] +2 [V] ≦ 0.10 (1)
([Nb], [Ti] and [V] indicate the contents (mass%) of Nb, Ti and V in the steel sheet, respectively.)
0 ≦ ([Cu] + [Ni]) − 2 [Si] ≦ 1.0 (2)
([Cu], [Ni] and [Si] indicate the contents (mass%) of Cu, Ni and Si in the steel sheet, respectively.)
[Mn] +1.5 [Cr] +2 [Mo] ≦ 2.4 (3)
([Mn], [Cr] and [Mo] indicate the contents (mass%) of Mn, Cr and Mo in the steel sheet, respectively.)
(A) A metal structure is comprised from a bainite structure and a remainder structure, and a bainite fraction is 80-96 area% in all the structures.
(B) When bainite crystal grains are determined based on a large-angle grain boundary in which the orientation difference between adjacent crystals is 15 ° or more, the average length of the crystal grains in the plate thickness direction is 7 μm or less.
(C) A circle equivalent diameter of the remaining tissue is 3.0 μm or less.
(D) The pearlite fraction is 80 area% or more in the remaining structure.
(A) The number of precipitates having an equivalent circle diameter of 20 nm or less containing at least one of Nb, Ti and V is 100 / μm ² or more.

  The “circle equivalent diameter” means a diameter (circle equivalent diameter) when converted into a circle having the same area. Further, among the elements defined by the above formula (3), other than the basic components (C, Si, Mn, Cu, Ni, B, V, Nb, Ti) of the thick steel plate of the present invention, it is contained as necessary. (For example, Cr, Mo), but when these elements are not included, the value on the left side of the equation (3) (hereinafter, this value is referred to as “Kp value”) is assumed to be absent. When calculating and including these elements, the Kp value may be calculated from the above equation (3).

  In the thick steel plate of the present invention, if necessary, (a) Ca: 0.005% or less (not including 0%), (b) Al: 0.10% or less (not including 0%), (c ) N: 0.010% or less (not including 0%), (d) Cr: 2% or less (not including 0%), (e) Mo: 1% or less (not including 0%), etc. It is also useful to improve the properties of the thick steel plate depending on the type of element contained.

As a result of conducting a crack growth test and a structure observation on various steel sheets, the present inventors have determined the structure form at a position of 1/4 of the sheet thickness in the longitudinal section parallel to the rolling direction by the following (e) to (g It was found that by controlling so as to satisfy the requirement of (), a steel sheet having excellent crack growth characteristics in addition to fatigue characteristics can be obtained.
(E) The metal structure is composed of a bainite structure and a remaining structure harder than the bainite, and the bainite fraction in the entire structure is 80 to 96 area% .
(F) When bainite crystal grains are determined based on a large-angle grain boundary in which the orientation difference between adjacent crystals is 15 ° or more, the average length of the crystal grains in the plate thickness direction is 7 μm or less.
(G) The equivalent circular diameter of the hard residual structure is 3 μm or less.

On the other hand, the production method of the present invention that has achieved the above object is characterized by hot rolling a steel slab having the chemical composition as described above under the following conditions. In this manufacturing method, the cooling stop temperature is preferably 550 ° C. or higher.
(A) Heating temperature: 1000 to 1200 ° C
(B) Heating rate of the steel slab: 3 to 10 ° C / min
(C) Cumulative rolling reduction in all hot rolling processes: 70% or more
(D) Cumulative rolling reduction in the temperature range of Ar ₃ transformation point + 150 ° C. to Ar ₃ transformation point + 50 ° C. during finish rolling: 50% or more
(E) Finish rolling end temperature: Ar ₃ transformation point + 30 ° C or higher temperature
(F) Average cooling rate from finish rolling finish temperature to 600 ° C: 10 ° C / second or less
Moreover, in order to satisfy said (e)-(g), it manufactures on the following conditions.
(G) Heating temperature: 1000 to 1200 ° C
(H) Heating rate of the steel slab: 3 to 10 ° C./min
(I) Cumulative rolling reduction in all hot rolling processes: 80% or more
(J) Cumulative rolling reduction in the temperature range of Ar ₃ transformation point + 150 ° C. to Ar ₃ transformation point + 50 ° C. during finish rolling : less than 70%
(K) Finishing rolling end temperature: Ar ₃ transformation point + temperature of 30 ° C or higher
(L) Average cooling rate from finish rolling finish temperature to 600 ° C: 10 ° C / second or less

  According to the present invention, since the structure and precipitates are appropriately controlled together with the chemical composition of the thick steel plate, fatigue characteristics until crack initiation (cracking suppression characteristics) and fatigue characteristics after crack initiation (crack growth) In addition, it is possible to improve the suppression property) and to realize a thick steel plate having extremely excellent fatigue properties.

FIG. 1 is a schematic view showing a test piece used for measurement of fatigue characteristics. FIG. 2 is a schematic explanatory view showing the shape of a compact test piece used for measuring the crack growth rate.

  The present inventors first examined a means for ensuring the fatigue life after the occurrence of fatigue cracks. As described above, the fatigue life after crack initiation (post-stage life) greatly contributes to the growth of cracks, so that the grain structure in the main structure is refined and the remaining structure other than the main structure (second phase) ) Is effective. Among these, regarding the refinement of crystal grains, the crystal grains can be refined by changing the structure of the steel material to a bainite structure or a martensite structure. Moreover, the fall of a former stage lifetime can be prevented by making the remainder structure | tissue into the pearlite main body which does not produce a shear deformation at the time of transformation.

  The present inventors investigated the influence of the structure morphology on fatigue properties for various thick steel plates. As a result, by controlling the structure of the cross section in the thickness direction (longitudinal cross section parallel to the rolling direction) at a depth of 3 mm from the surface of the steel sheet as follows, a thick steel sheet with fatigue characteristics superior to that of the prior art is obtained. I found out that Note that the structure having a depth of 3 mm from the surface of the steel sheet is the target for the normal thick steel sheet, and it is necessary to control the structure in the vicinity of the surface layer because the crack is generated on the surface of the steel sheet. This is because fatigue characteristics cannot be improved even if the structure at the / 4 position or t / 2 position (t: plate thickness) is controlled.

(Organization)
When the bainite fraction occupying 80 to 96 area% in the metal structure and the orientation difference between adjacent crystals is determined based on a large-angle grain boundary of 15 ° or more, the thickness direction of the crystal grains is determined. The average length (hereinafter, sometimes referred to as “effective crystal grain size”) is 7 μm or less. The bainite fraction is preferably 85 area% or more, more preferably 90 area% or more. The effective crystal grain size is preferably 6 μm or less, more preferably 5 μm 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 490 MPa or more and less than 650 MPa and to ensure excellent fatigue characteristics. Here, “bainite” includes structures such as upper bainite, lower bainite, acicular ferrite, and granular bainitic ferrite.

  Bainite has a smaller crystal grain size than ferrite, etc., and improves both the life before cracks (previous life) and the life after cracks (second life). Let Although martensite has a fine structure, a large amount of dislocations are introduced by shear deformation that occurs during the martensite transformation, and the life of the previous stage is impaired, so that the overall fatigue characteristics cannot be improved. The reason why the crystal grain size (effective crystal grain size) is defined by the length in the thickness direction (cutting length) is that the progress of fatigue cracks generated on the steel sheet surface is taken into consideration.

  In order to obtain a structure mainly composed of bainite, usually, in addition to the addition of alloy elements such as C and Mn, a rapid cooling treatment after rolling is often performed. However, when the cooling rate is high, it is difficult to secure a desired precipitate (described later). Therefore, in this invention, it is necessary to add B in addition to C, Mn, etc. which are normally added to a steel plate (it mentions later). Since B has an effect of suppressing the ferrite transformation, a structure mainly composed of bainite can be secured stably even if the cooling rate is relatively low.

(Remaining organization)
It is necessary that the structure (remaining structure) other than the bainite structure is 3.0 μm or less in terms of the equivalent circle diameter, and the pearlite fraction in the remaining structure is 80 area% or more. The reason why the equivalent circle diameter of the remaining structure is set to 3.0 μm or less is that when the average size of the remaining structure exceeds 3.0 μm, other characteristics such as toughness may be greatly deteriorated. The remaining structure can be expected to improve the life of the latter stage by making the remaining structure hard martensite or a martensite-austenite mixed structure (MA). On the other hand, in these structures, since a large amount of movable dislocations is introduced into the bainite structure during transformation, there is a possibility that the life of the previous stage is greatly reduced and the entire life is also reduced. On the other hand, it is possible to prevent a decrease in crack generation life by making the remaining structure a main body of pearlite that does not cause shear deformation during transformation. A preferable upper limit of the equivalent circle diameter of the remaining tissue is 2.5 μm or less (more preferably 2.0 μm or less), and a preferable lower limit is approximately 0.5 μm or more.

  The remaining structure may partially contain a soft structure such as ferrite. The remaining structure is preferably 3 area% or more (more preferably 5 area% or more) in order to exert the effect.

  The inventors also investigated the effect of precipitates on fatigue properties. As a result, at a position 3 mm deep from the surface of the steel plate, a thick steel plate that exhibits fatigue characteristics superior to conventional ones can be obtained by controlling the precipitates in the longitudinal section parallel to the rolling direction as follows. I found.

(Precipitate)
In order to improve the fatigue characteristics of the thick steel plate, it is necessary to finely disperse precipitates (carbide and carbonitride) having an equivalent circle diameter of 20 nm or less including at least one of Nb, Ti and V in the steel plate. In general, for strengthening by precipitation (precipitation strengthening), it is effective to finely disperse the precipitate in a large amount. In addition, when the precipitate is not sheared by dislocation, the effect of precipitation strengthening is greater when the precipitate size is finer. The reason why the equivalent circle diameter of the above-mentioned precipitates (carbides and carbonitrides) is defined as 20 nm or less is to suppress the formation of coarse precipitates that deteriorate other properties such as toughness while utilizing precipitation strengthening. From the viewpoint of doing.

  However, if the precipitate becomes too fine, the precipitate is sheared by dislocation, so that it is difficult to obtain precipitation strengthening. The critical size at which the precipitate is sheared varies depending on the type of the precipitate, and the precipitate containing at least one of Nb, Ti and V has a critical size of about 5 nm. For this reason, since even finer precipitates contribute to precipitation strengthening, precipitation strengthening can be effectively utilized by finely dispersing them and increasing the number density. In order to ensure a predetermined amount of precipitates, it is common to add a large amount of alloy or to perform heat treatment such as tempering. However, such a method not only increases the material cost and the manufacturing cost, but also not only does the precipitate coarsen and the improvement of the fatigue characteristics is not obtained, but also there is a risk of greatly reducing other characteristics such as toughness. .

The inventors of the present invention designed the alloy by paying attention to the ability to form precipitates by C (this is called “C activity”) so that fine precipitates can be secured stably at low cost. did. From this point of view, it is necessary to appropriately control the amount of the elements constituting the precipitate, and specifically, the following contents of Nb, Ti and V must satisfy the relationship of the formula (1).
0.01 ≦ [Nb] +2 [Ti] +2 [V] ≦ 0.10 (1)
([Nb], [Ti] and [V] indicate the contents (mass%) of Nb, Ti and V in the steel sheet, respectively.)

  In the present invention, the chemical component composition of the steel sheet that improves the fatigue characteristics after cracking will be described. In the steel plate of the present invention, a predetermined amount of bainite structure is ensured by appropriately adding alloy elements such as C, Mn, and B, and at the same time, the subsequent life is improved by appropriately adjusting the amount of addition of Mn, Cr, and Mo. The bainite main structure to be obtained is obtained, and the addition amount of Nb, Ti and V, which are constituent elements of the precipitate, is appropriately limited, and the relationship between the addition amounts of Cu, Ni and Si that affect the C activity is adjusted. By thickly depositing fine precipitates, a thick steel plate exhibiting excellent fatigue characteristics can be realized. From this viewpoint, each component is adjusted as follows.

(C: 0.03-0.12%)
C is an important element for securing the strength of the base material (steel plate), and is an element constituting the precipitate. In order to effectively exhibit such effects, the C content is determined to be 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 is excessive, the remaining structure is coarse and excessively generated, so that fatigue characteristics are deteriorated. Therefore, the C amount is set to 0.12% or less. The amount of C is preferably 0.10% or less, and more preferably 0.08% or less.

(Si: 0.3% or less (excluding 0%))
Si is an element necessary for ensuring the strength of the base material (steel plate), but at the same time, it is an element that lowers the C activity, so that the amount added must be 0.3% or less. The amount of Si is preferably 0.25% or less, and more preferably 0.2% or less. In addition, in order to exhibit the above effects, the Si amount is preferably 0.01% or more, more preferably 0.05% or more.

(Mn: 1.0-2.0%)
Mn is an important element in securing hardenability in order to obtain a bainite structure. In order to exhibit such an action effectively, the amount of Mn needs to be 1.0% or more. The amount of Mn is preferably 1.2% or more, more preferably 1.4% or more. However, when the amount of Mn is excessive, the pearlite fraction in the remaining structure is reduced, so that sufficient fatigue characteristics cannot be obtained. Therefore, the amount of Mn needs to be 2.0% or less. The amount of Mn is preferably 1.8% or less, more preferably 1.6% or less.

(B: 0.0005 to 0.005%)
B is an element that improves hardenability, and is an element that suppresses ferrite transformation and easily causes a bainite structure. In order to exhibit such an effect, B needs to be contained by 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 cannot be obtained and the effect of improving fatigue characteristics cannot be obtained, so 0.005% or less is necessary. The upper limit with preferable B content is 0.004% or less, More preferably, it is 0.003% or less.

(One or more selected from Cu: 0.1 to 1.0% and Ni: 0.1 to 1.0%)
Cu and Ni are elements necessary for increasing the C activity and generating fine precipitates. In order to exert such an effect, it is necessary to contain at least one of 0.1% or more. Preferably it is 0.15% or more, More preferably, it is 0.2% or more. However, if the Cu and Ni contents are excessive, the precipitates become coarse and fatigue characteristics cannot be improved, and other characteristics such as toughness are deteriorated. From such a viewpoint, it is necessary that both be 1.0% or less, preferably 0.8% or less, more preferably 0.6% or less.

Since Cu, Ni, and Si are elements that affect the C activity, in order to stably obtain fine precipitates, these contents should be set so as to satisfy the relationship of the following formula (2). Need to control.
0 ≦ ([Cu] + [Ni]) − 2 [Si] ≦ 1.0 (2)
([Cu], [Ni] and [Si] indicate the contents (mass%) of Cu, Ni and Si in the steel sheet, respectively.)

  When the value of ([Cu] + [Ni])-2 [Si] (hereinafter referred to as “CA value”) is less than 0 (%), the C activity is insufficient, and sufficient precipitates cannot be obtained. The fatigue characteristics cannot be improved. On the other hand, if the CA value exceeds 1.0 (%), the C activity is excessive and coarse precipitates are formed, and not only the fatigue characteristics cannot be improved, but other characteristics may be deteriorated. . The minimum with a preferable CA value is 0.1 (%) or more, More preferably, it is 0.15 (%) or more. Moreover, the upper limit with preferable CA value is 0.9 (%) or less, More preferably, it is 0.7 (%) or less.

(V: 0.05% or less (not including 0%), Nb: 0.05% or less (not including 0%) and Ti: 0.05% or less (not including 0%) One or more)
V, Nb, and Ti are elements necessary for producing precipitates in addition to securing a bainite structure by improving hardenability. However, if it is excessively contained, the precipitates become coarse and a sufficient fatigue improvement effect cannot be obtained. Therefore, it is necessary to limit each to 0.05% or less. The preferable lower limit of these elements is naturally determined by the relationship of the above formula (1).

  These elements need to satisfy the relationship of the above formula (1). If the value of [Nb] +2 [Ti] +2 [V] (hereinafter referred to as “PR value”) is less than 0.01 (%), sufficient precipitates cannot be obtained and fatigue characteristics cannot be improved. . The PR value is preferably 0.03 (%) or more, more preferably 0.05 (%) or more. If the PR value becomes too large, the precipitates are coarsened and the fatigue characteristics are deteriorated. The PR value is preferably 0.08 (%) or less, more preferably 0.06 (%) or less.

In order to make the remaining structure a pearlite-based structure and prevent a decrease in crack generation life, it is necessary to appropriately control the relationship among the contents of Mn, Cr and Mo. Mn, Cr and Mo must satisfy the relationship of the following formula (3).
[Mn] +1.5 [Cr] +2 [Mo] ≦ 2.4 (3)
([Mn], [Cr] and [Mn] indicate the contents (mass%) of Mn, Cr and Mo in the steel sheet, respectively.)

  When the value of [Mn] +1.5 [Cr] +2 [Mo] (hereinafter referred to as “Kp value”) exceeds 2.4 (%), the pearlite fraction in the remaining structure decreases, and cracks occur. Sufficient fatigue characteristics cannot be obtained because the generation life is greatly reduced. The upper limit with this preferable Kp value is 2.2 (%) or less, More preferably, it is 2.0 (%) or less. Moreover, the minimum with preferable Kp value is 1.0 (%) or more, More preferably, it is 1.3 (%) or more.

  The basic components in the thick steel plate of the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that unavoidable impurities (for example, P, S, etc.) brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. are contained in the steel. In the thick steel plate of the present invention, it is also effective to positively contain the following elements, and the properties of the thick steel plate are further improved according to the type of the contained element.

(Ca: 0.005% or less (excluding 0%))
Ca is an element that reduces the anisotropy of the shape of inclusions (for example, MnS, etc.) in steel, and is an element that is effective in preventing inclusions from becoming the starting point of fracture and improving fatigue properties. . In order to exhibit such an action effectively, it is preferable to contain 0.0005% or more. More preferably, it is 0.001% or more. However, when the Ca content is excessive, not only the cleanliness in the steel is lowered and the fatigue properties are deteriorated, but other properties such as toughness may be lowered. Therefore, the Ca content is preferably 0.005% or less, and more preferably 0.003% or less.

(Al: 0.10% or less (excluding 0%))
Al is an element useful as a deoxidizer, and it is preferable to contain 0.01% or more in order to exhibit such an action. More preferably, it is 0.02% or more. However, when the Al content is excessive, in order to reduce not only fatigue properties but also toughness and the like, the content is preferably 0.10% or less, more preferably 0.04% or less.

(N: 0.010% or less (excluding 0%))
Since N has an effect of improving strength by solid solution strengthening, it is positively incorporated as necessary. However, when the N content is excessive, coarse nitrides are formed and the fatigue characteristics are deteriorated. From such a viewpoint, the N content is preferably set to 0.010% or less, and more preferably 0.008% or less. In order to effectively exhibit the effect of N, the content is preferably 0.0035% or more, more preferably 0.0040% or more.

(Cr: 2% or less (excluding 0%))
Cr is an element having the same effect as Mn, and can stably obtain a bainite structure. In order to exert such an effect, it is preferable to contain Cr by 0.1% or more. More preferably, it is 0.5% or more. However, if the Cr content is excessive, the pearlite fraction in the remaining structure may be reduced and the fatigue characteristics may be deteriorated, so 2% or less is preferable. The upper limit with more preferable Cr content is 1.7% or less.

(Mo: 1% or less (excluding 0%))
Mo is an element having an effect similar to that of 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 further preferably 0.03% or more. However, if the Mo content is excessive, the pearlite fraction in the remaining structure is reduced and the fatigue characteristics are lowered, so the content is preferably 1% or less. The upper limit with more preferable Mo content is 0.7% or less, More preferably, it is 0.5% or less.

  Since precipitates hinder the movement of dislocations, precipitation strengthening suppresses the accumulation of dislocations due to repeated fatigue loading and improves fatigue properties. However, in thick steel plates used in ships, bridges, offshore structures, etc., not only fatigue properties but also toughness is an important factor, and coarse precipitates may reduce toughness. Therefore, it is necessary to disperse fine precipitates. However, when the size of the precipitates is reduced, the precipitates are sheared by dislocation, and the strengthening effect by the precipitates cannot be obtained. However, precipitates containing Nb, Ti, V, etc. are not sheared by dislocations even if they are very fine precipitates of about 5 nm, and it is possible to improve fatigue properties without degrading other properties. is there.

As described above, in order to improve the fatigue characteristics of the thick steel plate, a precipitate (carbide and carbonitride) having an equivalent circle diameter of 20 nm or less and containing at least one of Nb, Ti and V is included in the steel plate. Need to be distributed. In general, for strengthening by precipitation (precipitation strengthening), it is effective to finely disperse the precipitate in a large amount. From such a viewpoint, it is necessary that the equivalent circle diameter is 20 nm or less and the number of precipitates including at least one of Nb, Ti and V is 100 / μm ² or more at a position 3 mm deep from the steel sheet surface. The number of precipitates is preferably 150 / μm ² or more, and more preferably 200 / μm ² or more.

  The plate thickness of the thick steel plate according to the present invention is not particularly limited, but when the plate thickness is small, the contribution of improving the crack growth life is reduced. From such a viewpoint, the plate thickness is preferably 6 mm or more, and more preferably 10 mm or more. Moreover, the thick steel plate of this invention exhibits the thick steel plate which was excellent in the fatigue characteristic which is not before by making a several fatigue characteristic improvement factor compatible as mentioned above.

The thick steel plate of the present invention satisfies the above-mentioned requirements, and its manufacturing method is not particularly limited. In the steel plate series manufacturing process of hot rolling after melting and casting the steel, In order to obtain precipitates with improved properties, steel slabs having the chemical composition described above (for example, slabs) are used, the heating temperature before hot rolling, the cumulative rolling reduction during hot rolling, and finish rolling. It is preferable to control the temperature, finish rolling reduction, finish rolling end temperature, cooling rate after hot rolling, and cooling stop temperature as follows.
Heating temperature: 1000-1200 ° C
Cumulative rolling reduction in all hot rolling processes: 70% or more Cumulative rolling reduction in the temperature range of Ar ₃ transformation point + 150 ° C. to Ar ₃ transformation point + 50 ° C. during finishing rolling: 50% or more Finish rolling finishing temperature: Ar ₃ Transformation point + 30 ° C or higher Average cooling rate from finish rolling finish temperature to 600 ° C: 10 ° C / second or less

Prior to hot rolling, the steel slab is heated to a temperature range of 1000 to 1200 ° C. Preferably it is 1050 degreeC or more. The cumulative rolling reduction in the all hot rolling process is 70% or more. Preferably it is 75% or more. In order to reduce the structure size of bainite, it is necessary to apply sufficient reduction in the non-recrystallization temperature range. Sufficient reduction is performed by heating to a temperature range of 1000 to 1200 ° C. so as to ensure a cumulative reduction ratio of 70% or more during hot rolling while preventing coarsening of crystal grains. Further, in order to obtain a bainite structure, the cumulative rolling reduction in the temperature range (non-recrystallization temperature range) of Ar ₃ transformation point + 150 ° C. to Ar ₃ transformation point + 50 ° C. during finish rolling of the steel sheet is set to 50% or more. (Preferably 60% or more). In addition, in order to obtain the bainite structure, the temperature at which finish rolling is finished (finish rolling finish temperature) can be appropriately set as long as it is Ar ₃ transformation point + 30 ° C. or higher. It may be higher than Ar ₃ transformation point + 30 ° C. The upper limit of the finish rolling end temperature is preferably the Ar ₃ transformation point of the steel sheet + 130 ° C. or less (more preferably Ar ₃ transformation point + 100 ° C. or less).

  Although it does not specifically limit about the heating rate when heating a steel plate, It is preferable to set it as about 3-10 degreeC / min. If the heating rate is less than 3 ° C./min, coarse carbides are likely to precipitate in the heating stage of the slab, and the alloy elements are segregated because they cannot be sufficiently dissolved in the subsequent process. It becomes difficult to finely disperse the precipitate. Conversely, when the heating rate exceeds 10 ° C./min, component segregation in the slab is not sufficiently eliminated, and it becomes difficult to finely disperse the desired precipitate.

The “cumulative rolling reduction” is a value calculated from the following equation (4). The Ar ₃ transformation point employs a value obtained by the following equation (5) (the values shown in Tables 1 and 2 described later are also the same).
Cumulative rolling reduction = (t ₀ −t ₁ ) / t ₂ × 100 (4)
[In the formula (4), t ₀ is the rolling start thickness (mm) of the steel slab when the temperature at the position 3 mm from the surface is in the rolling temperature range, and t ₁ is the temperature at the position 3 mm from the surface in the rolling temperature range. The rolling end thickness (mm) of the steel slab at a certain time, and t ₂ indicate the thickness of the steel slab (eg, slab) before rolling. ]
Ar ₃ transformation point = 910−230 × [C] + 25 × [Si] −74 × [Mn] −56 × [Cu] −16 × [Ni] −9 × [Cr] −5 × [Mo] −1620 × [Nb] (5)
However, [C], [Si], [Mn], [Cu], “Ni”, [Cr], [Mo] and [Nb] are C, Si, Mn, Cu, Ni, Cr, Mo and The Nb content (% by mass) is shown.

  After the hot rolling is completed, cooling is performed from the finish rolling finish temperature to at least 600 ° C. at an average cooling rate of 10 ° C./second or less (preferably 8 ° C./second or less), preferably a stop temperature at the average cooling rate. By setting the (cooling stop temperature) to 550 ° C. or higher, the remaining structure can be made a pearlite-based structure while finely dispersing the precipitate. The reason why the cooling range is “from the finish rolling finish temperature to 600 ° C.” is that it is a temperature range where precipitates precipitate in addition to the formation of the matrix structure. In the thick steel plate of the present invention, even if the average cooling rate is 10 ° C./second or less, the bainite structure can be refined while suppressing the generation of ferrite.

  The cooling stop temperature was set to 550 ° C. or higher when cooling at the above cooling rate to less than 550 ° C., the remaining structure becomes MA or martensite, and 550 ° C. or higher is required to stably make the remaining structure pearlite. This is because the cooling must be stopped. However, if it is less than 550 degreeC, you may cool by standing_to_cool. From such a viewpoint, the preferable lower limit of the average cooling rate is 1 ° C./second or more (more preferably 1.5 ° C./second or more).

  As described in relation to the fatigue characteristics, the suppression of crack growth is effective by refining crystal grains in the main structure or strengthening by a 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 the surface layer of the steel sheet, even if the surface structure is a fine bainite structure, the internal structure The size becomes coarse or the bainite fraction decreases and the crack growth rate increases. Therefore, in order to obtain a steel sheet having excellent fatigue characteristics and excellent crack growth characteristics, it is preferable to control the structure form of the surface layer and the structure form inside the steel material (steel sheet).

(Structure inside the steel plate)
When the bainite fraction occupying 80 to 96 area% of the metal structure at the t / 4 position of the plate thickness is secured, and the crystal grains of bainite are determined based on the large-angle grain boundary where the adjacent crystal orientation difference is 15 ° or more, The average length of the crystal grains in the plate thickness direction (average effective crystal grain size) is 7 μm or less. The bainite fraction is preferably 85 area% or more, more preferably 90 area% or more. The average effective crystal grain size is preferably 6 μm or less, more preferably 5 μm or less.

(Remaining structure inside the steel plate)
Of the structures (remaining structures) other than the bainite structure, the hard remaining structure (hard remaining structure) is preferably 3 μm or less in terms of the equivalent circle diameter, and is preferably harder than bainite. The reason why the equivalent circle diameter of the hard residual structure is set to 3 μm or less is that when the average size of the residual structure exceeds 3 μm, other characteristics such as toughness may be greatly deteriorated. The remaining structure basically includes martensite and MA, and these hard remaining structures can reduce the crack growth rate.

In order to ensure such a structure form, it is preferable to control the cumulative reduction ratio during hot rolling and the reduction ratio in the non-recrystallization temperature range as follows.
Cumulative rolling reduction in all hot rolling processes: 80% or more Rolling reduction in non-recrystallization temperature range: less than 70%

  In order to make the structure size at the 1/4 thickness position 7 μm or less, it is necessary to increase the cumulative rolling reduction during the hot rolling process, and the rolling reduction is preferably 80% or more. If this cumulative rolling reduction is insufficient, even if the surface layer structure becomes fine, the structure at the ¼ thickness position does not become sufficiently fine, and the crack growth rate does not sufficiently decrease. More preferably, it is 85% or more.

  Further, when the rolling reduction in the non-recrystallization temperature region increases, ferrite nucleation sites increase and ferrite transformation easily occurs, and the bainite fraction decreases, so that a sufficient crack growth rate suppressing effect cannot be obtained. Therefore, in order to secure a bainite fraction of 80% by area or more in the steel plate, it is necessary not to excessively reduce the unrecrystallized temperature range. For these reasons, it is preferable that the cumulative rolling reduction in the non-recrystallization temperature range is less than 70%.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Example 1
The steels (steel types A to Z) having the chemical composition shown in Tables 1 and 2 below are melted and cast according to a normal melting method, and then subjected to various conditions (rolling conditions No. a to p) shown in Table 3 below. Then, hot rolling was performed to obtain a steel plate having a thickness of 18 to 20 mm. In Table 3, the “non-recrystallization temperature range reduction ratio” is the reduction ratio (cumulative reduction ratio) in the temperature range of Ar ₃ transformation point + 150 ° C. to Ar ₃ transformation point + 50 ° C. In addition, “Non-recrystallization temperature range reduction ratio” shown in Table 3 is a design value, and the finish rolling finish temperature becomes higher than Ar ₃ transformation point + 150 ° C. (that is, rolling is completed before reaching the non-recrystallization temperature range). In this case, the unrecrystallization temperature range reduction rate is 0% (that is, no reduction in the non-recrystallization temperature range) (for example, test No. 26 in Table 5).

  For each steel sheet, the bainite fraction of the steel sheet, the effective crystal grain size, the size of the second phase (remaining structure), the pearlite fraction in the remaining structure, the tensile strength, the fatigue properties, and the number density of fine precipitates for each steel sheet And the size was measured. In any measurement, the test piece was collected so that the evaluation position was 3 mm from the surface layer of the steel plate.

(Bainite fraction)
A sample was cut out so that a plane parallel to the rolling direction of the steel sheet at a depth of 3 mm from the steel sheet surface and perpendicular to the steel sheet surface was exposed, and this was polished using wet emery paper from # 150 to # 1000 Then, mirror polishing was performed using a diamond abrasive as an abrasive. After this mirror surface test piece was etched with a 2% nitric acid-ethanol solution (a nital solution), a 150 μm × 200 μm visual field was observed at an observation magnification of 400 times, and the bainite fraction (area%) was measured by image analysis. A total of 5 bainite fractions were obtained and the average value was adopted.

(Effective crystal grain size)
In a cross section parallel to the rolling direction of the steel sheet at a depth of 3 mm from the steel sheet surface layer, the effective crystal grain size (large angle) is measured by SEM (Scanning Electron Microscape) -EBSP (Electron Backscatter Pattern). Grain boundary diameter) was measured. Specifically, using an EBSP device (trade name: “OIM”) manufactured by TEX SEM Laboratories in combination with SEM, the effective crystal grain size is measured with a boundary having an inclination (crystal orientation difference) of 15 ° or more as the grain boundary. did. The measurement conditions at this time are: measurement area: 200 μm × 200 μm, measurement step: 0.5 μm interval, and measurement points whose confidence index (Confidence Index) indicating the reliability of the measurement direction is smaller than 0.1 are analyzed. Excluded. With respect to the crystal grain boundaries thus obtained, the cutting lengths at 100 locations in the plate thickness direction were measured, and the average value was taken as the effective crystal grain size. However, an effective crystal grain size of 2.0 μm or less was judged as measurement noise and excluded.

(Remaining tissue size and pearlite fraction)
The size of the remaining structure and the pearlite fraction in the remaining structure were cut out by a method similar to the measurement of the bainite fraction, polished and etched, and then observed with an SEM at an observation magnification of 1000 times. The size of the remaining structure (equivalent circle diameter) and the pearlite fraction in the remaining structure were determined by analysis. Both adopted the average value of 5 fields of view.

(Tensile strength)
Tensile strength TS was measured by collecting a tensile test piece having a plate thickness of 4 mm and a gauge distance of 35 mm from a surface layer depth of 2 to 6 mm, and performing a tensile test according to JIS Z2241 (2011).

(Fatigue properties)
The fatigue characteristics were obtained by cutting out a steel material having a thickness of 4 mm from the 2-6 mm position of the plate thickness surface layer and preparing a test piece as shown in FIG. The surface of the test piece was polished to # 1200 with emery paper to remove the influence of the surface state. About the obtained test piece, the fatigue test was done on condition of the following using the electrohydraulic servo type fatigue testing machine made from Instron.

Test environment: Room temperature, in air Control method: Load control Control waveform: Sine wave Stress ratio: R = -1
Test speed: 20Hz
Number of test end cycles: 5000000 times

  The fatigue characteristics are affected by the tensile strength. In order to eliminate the influence, the fatigue limit ratio of 5 million times was obtained, and the fatigue limit ratio of 5 million times exceeded 0.51. The 5 million times fatigue limit ratio is a value obtained by dividing the 5 million times fatigue strength by the tensile strength, and the 5 million times fatigue strength was determined as follows. In each test piece, a fatigue test was performed with a stress amplitude at which the value of (σa / TS) obtained by dividing the stress amplitude σa by the tensile strength TS was 0.51, and the test piece that was not broken when reaching 5 million times passed ( The broken piece is indicated as “x” as a failure, and is indicated by “◯” in Tables 4 and 5 below. In addition, fatigue tests were performed at a stress amplitude at which the value of (σa / TS) was 0.53, and those that were not broken when reaching 5 million times were represented by “◎” in Tables 4 and 5 below.

(Number density and size of precipitates)
A test piece prepared by the extraction replica method from a sample collected from a position 3 mm from the surface of the steel sheet was observed with a transmission electron microscope (TEM) at an observation magnification of 150,000 times, an observation field of view 750 nm × 625 nm, and an observation part 5 fields of view. The area of the precipitate containing any of Nb, Ti, and V in the visual field was measured by image analysis, and the equivalent circle diameter of each precipitate was calculated from this area. The inclusion of Nb, Ti, or V was determined by EDX (Energy Dispersive X-ray spectrometry). The number density was determined by converting per 1 μm ^{2 of the} precipitate having an equivalent circle diameter of 20 nm or less.

  These results are shown in Table 4 and Table 5 below.

  From these results, it can be considered as follows. That is, test no. 1 to 19 (Table 4) satisfy the requirements (structure, precipitates) defined in the present invention because the chemical composition of the steel and the production conditions are appropriately controlled, and exhibit excellent fatigue properties. doing.

  On the other hand, test no. 20-41 (Table 5) resulted in inferior fatigue properties because at least one of the chemical composition and production conditions of the steel sheet was inappropriate. Of these, test no. In No. 20, the slab heating rate was too high (rolling condition No. g), the precipitate number density was small, and the fatigue characteristics deteriorated. Test No. In No. 21, the slab heating rate was too slow (rolling condition No. h), the precipitate number density was small, and the fatigue characteristics deteriorated.

  Test No. In No. 22, the heating temperature before hot rolling becomes too high (rolling condition No. i), and the predetermined tensile strength is not achieved (other characteristics are not evaluated). Test No. In No. 23, the heating temperature before hot rolling becomes too low (rolling condition No. j), the bainite fraction decreases, the effective crystal grain size increases, and the precipitate number density also decreases. Characteristics deteriorated.

  Test No. In No. 24, the cumulative rolling reduction during hot rolling was too small (rolling condition No. k), the effective crystal grain size was increased, and the fatigue characteristics were deteriorated. Test No. In No. 25, the reduction rate in the non-recrystallization temperature range was small (rolling condition No. 1), the effective crystal grain size was too large, the remaining structure size was large, and the fatigue characteristics deteriorated.

  Test No. No. 26, the finish rolling finish temperature is too high (rolling condition No. m: the rolling reduction in the non-recrystallization temperature range is substantially 0%), the tensile strength is reduced, and the effective crystal grain size is increased, In addition, the number density of the precipitates was reduced, and the fatigue characteristics were deteriorated. Test No. In No. 27, the finish rolling end temperature was too low (rolling condition No. n), the bainite fraction was lowered, and the precipitate number density was also reduced, so that the fatigue characteristics deteriorated.

  Test No. No. 28, the cooling rate after hot rolling is high (rolling condition No. o), the number density of precipitates is not achieved (fine precipitates are not dispersed), and the fatigue characteristics are deteriorated (the tensile strength is also low). Higher) Test No. In No. 29, the cooling stop temperature was low (rolling condition No. p), the pearlite fraction in the remaining structure was small, and the fatigue characteristics were deteriorated.

  Test No. No. 30 is an example using a steel piece (steel type O) with an excessive C content, and the tensile strength is too high (other properties are not evaluated). Test No. No. 31 is an example using a steel piece (steel type P) with a low C content, in which a predetermined tensile strength has not been achieved (other properties are not evaluated).

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

  Test No. No. 35 is an example using a steel slab (steel type T) having a small CA value due to insufficient Cu and Ni contents. The precipitate number density was reduced and the fatigue characteristics were deteriorated. Test No. 36 is an example using a steel slab (steel type U) with a large CA value (Ni content is excessive), coarse precipitates increase, precipitate number density decreases, and fatigue characteristics are reduced. Deteriorated (high tensile strength).

  Test No. No. 37 is an example using a steel slab (steel type V) having a small PR value. The number density of precipitates was reduced and the fatigue characteristics were deteriorated. Test No. No. 38 is an example using a steel slab (steel type W) having a large PR value, and the number density of precipitates was reduced, and the fatigue characteristics were deteriorated.

  Test No. 39 is an example using a steel piece (steel type X) with a large Kp value, the bainite fraction is reduced, the effective crystal grain size is increased, and the pearlite fraction in the remaining structure is reduced, The fatigue characteristics deteriorated.

  Test No. 40 is an example using a steel slab (steel type Y) with a low B content (not added). The bainite fraction decreased, the effective crystal grain size increased, and the fatigue characteristics deteriorated. Test No. No. 41 is an example using a steel slab (steel type Z) with a high B content, the CA value is also small, the bainite fraction is lowered, and the fatigue properties are deteriorated (the tensile strength is also high). ).

(Example 2)
Test No. shown in Table 4 About each steel plate of 1-19, it carried out similarly to the method shown in Example 1 about the bainite fraction of plate | board thickness 1/4 position, an effective crystal grain size, and the size of the 2nd phase. The method for collecting the test piece is the same as described above except that the position is 1/4 of the plate thickness. Moreover, about these steel plates, the crack growth rate was measured by the following method.

(Crack growth rate)
In accordance with ASTM E647, using a compact test piece, a fatigue crack growth test was performed using an electrohydraulic servo fatigue tester under the following conditions, and the crack growth rate was measured. In addition, the compact test piece was extract | collected from the position of 1/2 of plate | board thickness, and the thing of the shape shown in FIG. 2 was used. Moreover, the compliance method was used for the crack length.
Test environment: Room temperature, in air Control method: Load control Control waveform: Sine wave Stress ratio: R = -1
Test speed: 5-20Hz

At this time, the value in the stable growth region ΔK = 20 (MPa · m ^1/2 ) where the Paris law defined by the following equation (6) is satisfied was evaluated as a representative value. A crack having a crack growth rate of 5.0 × 10 ⁻⁵ mm / cycle or less when ΔK = 20 (MPa · m ^1/2 ) is considered to have excellent crack growth characteristics.
da / dn = C (ΔK) ^m (6)
[In formula (1), a: crack length, n: number of repetitions, C, m: constants determined by conditions such as material and load, respectively. ]

  These results are shown in Table 6 below.

From these results, it can be considered as follows. That is, test no. 1, 3-6, and 9-19 satisfy the preferable requirements (structure, precipitates) inside the steel sheet because the chemical composition and production conditions of the steel are appropriately controlled, and excellent fatigue crack growth Properties (crack growth rate is 5.0 × 10 ⁻⁵ mm / cycle or less) are obtained.

  In contrast, test no. In Nos. 2 and 8, the cumulative rolling reduction in the all-hot rolling process was reduced (rolling condition No. c), the effective crystal grain size was increased, and the fatigue crack growth characteristics were deteriorated. In addition, Test No. In No. 7, the rolling reduction (cumulative rolling reduction) in the non-recrystallization temperature range was large (rolling condition No. f), the bainite fraction was lowered, and the fatigue crack growth characteristics were deteriorated.

Claims (11)

C: 0.03 to 0.12% (meaning “mass%”; the same applies hereinafter), Si: 0.3% or less (excluding 0%), Mn: 1.0 to 2.0%, B: One or more selected from 0.0005 to 0.005% and Cu: 0.1 to 1.0% and Ni: 0.1 to 1.0%, and V: 0.05% or less (Not including 0%), Nb: not more than 0.05% (not including 0%) and Ti: not less than 0.05% (not including 0%) , These elements satisfy the relationship of the following formulas (1) to (3), the balance is a steel plate of iron and inevitable impurities,
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, the metal structure satisfies the following requirements (a) to (d), and the precipitate satisfies the following requirements (A). Thick steel plate with excellent fatigue characteristics characterized by satisfaction.
0.01 ≦ [Nb] +2 [Ti] +2 [V] ≦ 0.10 (1)
([Nb], [Ti] and [V] indicate the contents (mass%) of Nb, Ti and V in the steel sheet, respectively.)
0 ≦ ([Cu] + [Ni]) − 2 [Si] ≦ 1.0 (2)
([Cu], [Ni] and [Si] indicate the contents (mass%) of Cu, Ni and Si in the steel sheet, respectively.)
[Mn] +1.5 [Cr] +2 [Mo] ≦ 2.4 (3)
([Mn], [Cr] and [Mo] indicate the contents (mass%) of Mn, Cr and Mo in the steel sheet, respectively.)
(A) A metal structure is comprised from a bainite structure and a remainder structure, and a bainite fraction is 80-96 area% in all the structures.
(B) When bainite crystal grains are determined based on a large-angle grain boundary in which the orientation difference between adjacent crystals is 15 ° or more, the average length of the crystal grains in the plate thickness direction is 7 μm or less.
(C) A circle equivalent diameter of the remaining tissue is 3.0 μm or less.
(D) The pearlite fraction is 80 area% or more in the remaining structure.
(A) The number of precipitates having an equivalent circle diameter of 20 nm or less containing at least one of Nb, Ti and V is 100 / μm ² or more.   The thick steel plate according to claim 1, further comprising Ca: 0.005% or less (not including 0%).   Furthermore, the thick steel plate of Claim 1 or 2 which contains Al: 0.10% or less (0% is not included).   Furthermore, N: 0.010% or less (0% is not included) The thick steel plate in any one of Claims 1-3.   Furthermore, Cr: 2% or less (0% is not included) The thick steel plate in any one of Claims 1-4.   Furthermore, Mo: 1% or less (0% is not included) The thick steel plate in any one of Claims 1-5. The metal structure satisfies the following requirements (e) to (g) when observing a position that is a quarter of the plate thickness in a longitudinal section parallel to the rolling direction. The thick steel plate according to any one of the above.
(E) The metal structure is composed of a bainite structure and a remaining structure harder than the bainite, and the bainite fraction in the entire structure is 80 to 96 area% .
(F) When bainite crystal grains are determined based on a large-angle grain boundary in which the orientation difference between adjacent crystals is 15 ° or more, the average length of the crystal grains in the plate thickness direction is 7 μm or less.
(G) The equivalent circular diameter of the hard residual structure is 3 μm or less. The steel plate according to any one of claims 1 to 6, which is excellent in fatigue properties, characterized by hot-rolling a steel slab having a chemical composition according to any one of claims 1 to 6 under the following conditions. Production method.
(A) Heating temperature: 1000 to 1200 ° C
(B) Heating rate of the steel slab: 3 to 10 ° C / min
(C) Cumulative rolling reduction in all hot rolling processes: 70% or more
(D) Cumulative rolling reduction in the temperature range of Ar ₃ transformation point + 150 ° C. to Ar ₃ transformation point + 50 ° C. during finish rolling: 50% or more
(E) Finish rolling end temperature: Ar ₃ transformation point + 30 ° C or higher temperature
(F) Average cooling rate from finish rolling finish temperature to 600 ° C: 10 ° C / second or less   The method for producing a thick steel plate according to claim 8, wherein the cooling stop temperature is 550 ° C or higher.   The method for producing a thick steel plate according to claim 7, wherein the steel slab having the chemical composition according to any one of claims 1 to 6 is hot-rolled under the following conditions and has excellent fatigue characteristics.
(G) Heating temperature: 1000 to 1200 ° C
(H) Heating rate of the steel slab: 3 to 10 ° C./min
(I) Cumulative rolling reduction in all hot rolling processes: 80% or more
(J) Ar during finish rolling ₃ Transformation point + 150 ° C to Ar ₃ Cumulative rolling reduction in the temperature range of transformation point + 50 ° C: less than 70%
(K) Finishing rolling finish temperature: Ar ₃ Transformation point + temperature above 30 ° C
(L) Average cooling rate from finish rolling finish temperature to 600 ° C: 10 ° C / second or less The method for producing a thick steel plate according to claim 10, wherein the cooling stop temperature is 550 ° C or higher.