Steel sheet pile and method of manufacturing the same

Abstract

The present invention provides a high-strength and high-toughness steel sheet pile with stability and high productivity. The steel sheet pile has a prescribed composition; a microstructure in which the area fraction of ferrite is 70% or more and the area fraction of island-shaped martensite is 1.0% or less; a microstructure in which the area fraction of ferrite is 70% or more and the area fraction of island-shaped martensite is 1.0% or less, the total area fraction of V carbonitride, Nb carbonitride, and V and Nb complex carbonitride having a particle diameter of 10 nm or less is 2.6% or more, the area fraction of V carbonitride, Nb carbonitride, and these complex precipitates having a prescribed particle diameter d (nm) is 0.3% or more; and the average particle diameter of the ferrite is 15 μm or less and the maximum particle diameter is 40 μm or less.

Description

Technical Field

[0001] This invention relates to a steel sheet pile suitable for permanent or temporary structures in the fields of civil engineering and construction, and a method for manufacturing the same. Background Technology

[0002] Sheet piles are subjected to high loads when used in applications such as bank walls and retaining walls, thus requiring high strength and toughness. For example, sheet piles with a yield strength (hereinafter referred to as YP) of 290 MPa or higher, and even 390 MPa or higher, can be used. Furthermore, in more severe environments, sheet piles with a strength of YP of 440 MPa or higher are sometimes required.

[0003] In manufacturing high-strength and high-toughness steel products, the common practice is to add alloying elements or roll in the austenite non-recrystallization temperature range. However, in the manufacture of steel sheet piles with complex shapes, from a formability point of view, rolling and forming at high temperatures with lower deformation resistance is desirable, thus limiting the alloys that could increase deformation resistance.

[0004] While the JIS standard (SYW) specifies the Charpy energy absorption at 0°C, sheet piles are also used in environments below 0°C, such as during Japan's cold season. Therefore, it is foreseeable that there will be a greater demand for high-toughness sheet piles in the future.

[0005] Against this backdrop, research and development of high-strength and high-toughness steel sheet piles are underway.

[0006] Patent document 1 proposes a steel sheet pile that achieves a YP of over 440MPa and high toughness by adding more than 0.05% Nb to its composition.

[0007] Patent document 2 proposes a steel sheet pile that achieves a YP of over 440 MPa and high toughness by setting the composition to include more than 0.030% Nb along with V, controlling the reduction rate below 1000℃, and optimizing the average grain size of ferrite, the area ratio of island martensite, and the number density of precipitates.

[0008] Patent document 3 proposes a steel sheet pile, which is composed of low carbon steel with C of 0.005 to 0.030% and Nb and B added, thus achieving high strength, high toughness and excellent underwater weldability.

[0009] In addition, Patent Document 4 proposes a steel sheet pile with one or two V or Nb components, controlling the reduction rate in the non-recrystallization temperature range below 900°C, and accelerating cooling after rolling to achieve high toughness.

[0010] On the other hand, in Patent Document 5, a sheet pile is proposed that achieves a YP of over 340MPa and high toughness by limiting the Nb in unavoidable impurities to below 0.005%.

[0011] Patent documents 6 and 7 propose a steel sheet pile that achieves a YP of over 440MPa and a vTrs of -10℃ by water cooling at a specified position during or after hot rolling.

[0012] Existing technical documents

[0013] Patent documents

[0014] Patent Document 1: Japanese Patent Application Publication No. 2018-83963

[0015] Patent Document 2: Japanese Patent Application Publication No. 2018-90845

[0016] Patent Document 3: Japanese Patent Application Publication No. 2000-17378

[0017] Patent Document 4: Japanese Patent Application Publication No. 2006-249513

[0018] Patent Document 5: Japanese Patent Application Publication No. 2002-294392

[0019] Patent Document 6: Japanese Patent Application Publication No. 2007-332414

[0020] Patent Document 7: Japanese Patent Application Publication No. 2008-221318 Summary of the Invention

[0021] The technologies described in Patent Documents 1 and 2 obtain high-strength and high-toughness steel sheet piles by setting the composition to include more than 0.030% Nb. However, Nb, whether in solid solution or precipitated state, tends to increase the deformation resistance during hot rolling. Therefore, strict shape control is required during hot rolling.

[0022] In the technology described in Patent Document 3, since the carbon content is less than 0.005 to 0.030%, the decarburization process during steel smelting becomes longer, resulting in low productivity in the refining process.

[0023] Furthermore, in the technology described in Patent Document 4, in order to obtain steel sheet piles with high toughness, the reduction rate in the non-recrystallization temperature region needs to be more than 20%, the shape during hot rolling needs to be strictly controlled, and a specified accelerated cooling is required after rolling. Therefore, it is impossible to avoid problems such as bending and warping.

[0024] On the other hand, in the technology described in Patent Document 5, a steel sheet pile is proposed that promotes the complete recrystallization of austenite by limiting the rolling temperature and the reduction rate of the final pass, thereby obtaining a uniform microstructure and achieving a YP of over 340MPa and high toughness. However, the YP is less than 440MPa, and further improvement of YP is needed.

[0025] In the technology described in Patent Document 6 or 7, in order to obtain sheet piles with YP440MPa or higher and vTrs below -10℃, a specified portion of water cooling is required. Therefore, it is impossible to avoid shape changes such as bending and warping.

[0026] This invention addresses the aforementioned problems and aims to provide a high-strength and high-toughness steel sheet pile in a stable and highly productive manner. Here, high strength refers, for example, to a YP of 440 MPa or higher, and high toughness refers to a fracture transition temperature (hereinafter referred to as vTrs) of -10°C or lower with a ductility fraction of 50%.

[0027] To ensure high toughness, an effective approach is to reduce the number of breakage units based on grain refinement. One method for grain refinement is to use precipitates present within or at grain boundaries of old austenite grains as nuclei for ferrite formation. The inventors investigated the mechanism of precipitate-based grain refinement and found that for the precipitates to contribute to ferrite grain refinement, they need to have a certain particle size. Furthermore, they found that this critical particle size tends to decrease with decreasing ferrite phase transformation initiation temperature.

[0028] In addition, to ensure high strength, precipitation dispersion enhancement based on precipitates is one of the effective methods. If the particle size of the precipitates is assumed to be the same area fraction, the finer the particle size of the precipitates, the more significant the increase in strength.

[0029] To ensure high toughness, precipitates with a certain particle size are required; conversely, fine precipitates are effective in ensuring high strength. These two aspects are contradictory, and historically, it has been difficult to achieve both high strength and high toughness simultaneously by utilizing precipitates. One solution is to lower the ferrite phase transformation initiation temperature based on accelerated cooling, but this may lead to warping and is therefore not preferred. Alternatively, aggressive rolling within the non-recrystallization temperature range is also an effective method, but requires strict shape control.

[0030] For achieving precipitate-based grain refinement, a composition containing vanadium (V) is effective. V precipitates in austenite as vanadium carbonitrides at relatively high temperatures and has an integration interface with ferrite, thus contributing significantly to ferrite formation nuclei. Furthermore, to ensure high strength, a composition containing nitrogen (Nb) is effective. Nb precipitates in austenite primarily through strain induction as Nb carbonitrides with fine particle sizes on the order of nanometers, thus expecting precipitate-based dispersion strengthening.

[0031] Based on these viewpoints, the inventors investigated the V to Nb composition ratio for precipitate control in composite additions of V and Nb. The results showed that there exists a suitable range for the V to Nb composition ratio to achieve both grain refinement through the use of V carbonitride compounds or composite V and Nb carbonitride compounds as ferrite nuclei and precipitate-based dispersion strengthening.

[0032] Furthermore, simply optimizing the V and Nb composition ratio is insufficient. Therefore, rolling conditions that ensure both high productivity and high strength and toughness were explored. The results showed that rolling at temperatures directly above the non-recrystallization temperature range is highly effective. Specifically, strain-induced precipitation of Nb carbonitrides or V / Nb composite carbonitrides is crucial. This alleviates the limitations of conventional rolling conditions over a wide temperature range and the reduction rate (hereinafter also referred to as CR rate) in the non-recrystallization temperature range of austenite, without requiring accelerated cooling that influences bending and warping.

[0033] The inventors have discovered that, in order to effectively improve strength and toughness, it is necessary to have precipitates with fine particle sizes that contribute to the increase in strength and precipitates with coarse particle sizes that contribute to the formation of ferrite nuclei, with an area ratio of a certain value or higher.

[0034] Based on the above, the inventors have discovered a method for providing high-strength steel sheet piles with a YP of over 440 MPa and a vTrs of -10°C or lower through dispersion strengthening based on V carbonitrides, Nb carbonitrides, or composite precipitates thereof, and uniform micro-refinement of the microstructure. The main points of this invention are as follows.

[0035] 1. A steel sheet pile, having the following composition and microstructure,

[0036] The composition of the ingredients is such that, in mass percent, it contains C: 0.05–0.18%, Si: 0.05–0.55%, Mn: 1.00–1.65%, sol.Al: 0.080% or less, V: 0.005–0.250%, Nb: 0.005% or more and less than 0.030%, and N: 0.0010–0.0060%, and the remainder is Fe and unavoidable impurities, wherein the unavoidable impurities P, S, and B are P: 0.025% or less, S: 0.020% or less, and B: 0.0003% or less.

[0037] The microstructure is characterized by a ferrite area fraction of 70% or more and an island martensite area fraction of 1.0% or less, a total area fraction of V carbonitrides, Nb carbonitrides, and V and Nb composite carbonitrides with a particle size of 10 nm or less of 2.6% or more, and a total area fraction of V carbonitrides, Nb carbonitrides, and V and Nb composite precipitates with a particle size d (nm) satisfying the following formula (2) of 0.30% or more.

[0038] The average particle size of the ferrite is less than 15 μm and the maximum particle size is less than 40 μm.

[0039] The yield strength is above 440 MPa and vTrs is below -10℃.

[0040] -0.010≤[%Nb]-0.1[%V]≤0.020……(1)

[0041] Here, [%V] and [%Nb] represent the V and Nb content (mass%) in the steel, respectively.

[0042] d≥5[(Ae3-Ar3) / Ae3] -0.63 ……(2)

[0043] Here, Ae3: the ferrite phase transition initiation temperature during equilibrium phase transition (°C).

[0044] Ar3: Ferrite phase transformation initiation temperature upon cooling (°C)

[0045] It should be noted that the above Ae3 and Ar3 are derived based on equations (3) and (4) shown below.

[0046] [Number 1]

[0047]

[0048] Ar3=910-310[%C]-80[%Mn]-20[%Cu]-15[%Cr]-55[%Ni]-80[%M0]……(4)

[0049] Here, [%C], [%Si], [%Mn], [%Cu], [%Cr], [%Ni] and [%Mo] refer to the contents (mass%) of C, Si, Mn, Cu, Cr, Ni and Mo in the steel, respectively.

[0050] 2. The sheet pile according to claim 1 above, wherein the above-mentioned composition further contains, by mass %, one or more of the following: Cu: less than 0.50%, Ni: less than 0.50%, Cr: less than 0.50%, Mo: less than 0.30%, Ca: less than 0.0050%, Ti: less than 0.025%, and REM: less than 0.005%.

[0051] 3. A method for manufacturing steel sheet piles, comprising heating a steel billet having the composition described in 1 or 2 above to 1200°C to 1350°C, hot rolling including rough rolling, intermediate rolling and finish rolling, under the conditions of a cumulative reduction rate of 20% or more at 900°C to 1000°C, a reduction rate of 10% or more and less than 20% in the non-recrystallization temperature region of austenite, and an intermediate rolling end temperature of 650°C to 900°C, wherein the yield strength of the steel sheet pile is 440 MPa or more and vTrs is -10°C or less.

[0052] Invention Effects

[0053] According to the present invention, high-strength and high-toughness steel sheet piles with YP of over 440 MPa and vTrs of -10°C can be stably and productively provided, which is of great industrial value. Attached Figure Description

[0054] Figure 1 This is a diagram showing the cross-sectional shape of a steel sheet pile.

[0055] Figure 2 This is a diagram showing a representative pore membrane during the hot rolling process of cap-shaped steel sheet piles.

[0056] Figure 3 This is a graph showing the relationship between the area ratio of precipitates satisfying equation (2) and vTrs.

[0057] Figure 4 A graph showing the relationship between the area ratio and yield strength of precipitates with a size of less than 10 nm. Detailed Implementation

[0058] As a form of steel sheet pile, Figure 1 The cap-shaped sheet pile 1 shown in (a) is a typical example. Additionally, it also has... Figure 1 The straight sheet piles 9 shown in (b) are all mediated by Figure 2The porous membrane shown is given its final shape. Its morphology and manufacturing steps will be described in detail later.

[0059] <Sheet Piles>

[0060] The composition and microstructure of the steel sheet pile of the present invention will be described in detail below.

[0061] [Ingredients]

[0062] First, the reasons for limiting the composition of the steel sheet piles of the present invention will be explained. It should be noted that, unless otherwise specified, all expressions of element content in the following description, expressed in "%", refer to "mass %".

[0063] In addition, in this specification, V carbonitride compounds are also referred to as "V precipitates" or "V(C,N)", Nb carbonitride compounds are also referred to as "Nb precipitates" or "Nb(C,N)", and V and Nb composite carbonitride compounds are also referred to as "their (composite) precipitates" or "(V,Nb)(C,N)".

[0064] C: 0.05~0.18%

[0065] Carbon (C) is an essential element for stably ensuring the strength and toughness of the base material in steel by combining with V, Nb, and N and precipitating as carbonitridium compounds such as V(C,N), Nb(C,N), or (V,Nb)(C,N). It needs to be added at least 0.05%. On the other hand, if it exceeds 0.18%, bainite containing island-like martensite is formed. The increase in island-like martensite significantly reduces toughness, and the excess precipitates further reduce toughness. Therefore, in this invention, the C content is set to 0.05% to 0.18%. Moreover, the C content is preferably 0.10% or more. Furthermore, the C content is preferably 0.16% or less.

[0066] Si: 0.05–0.55%

[0067] Si is an element that increases the strength of the base material through solid solution strengthening, and its content needs to be 0.05% or more. On the other hand, if the Si content is excessive, it will promote the formation of island martensite, which reduces toughness; therefore, the Si content is set to 0.55% or less. Thus, the Si content is set to 0.05 to 0.55%. Furthermore, the Si content is preferably set to 0.10% or more. In addition, the Si content is preferably 0.50% or less.

[0068] Mn: 1.00~1.65%

[0069] Like Si, manganese (Mn) is a relatively inexpensive element that improves the strength of steel, making it a desirable element for achieving high strength. However, if the Mn content is less than 1.00%, this effect diminishes. On the other hand, if it exceeds 1.65%, it promotes the formation of upper bainite containing island-like martensite, resulting in a significant loss of toughness. Therefore, the Mn content is set to 1.00–1.65%. Furthermore, the Mn content is preferably 1.10% or more. Additionally, the Mn content is preferably 1.60% or less.

[0070] sol.Al: 0.080% or less

[0071] Al is an element added as a deoxidizer. However, the deoxidizing effect of Al saturates when the sol.Al content exceeds 0.080%, therefore sol.Al is set to 0.080% or less. There is no particular limitation on the lower limit, but for deoxidation, it is preferable to set it to 0.001% or more. More preferably, it is 0.003% or more. Furthermore, it is preferably 0.060% or less.

[0072] V: 0.005~0.250%

[0073] V (V) precipitates in austenite as V(C,N) or (V,Nb)(C,N) during rolling or cooling, contributing to the formation of ferrite nucleation sites and refining grain size, making it an important element. Furthermore, V enhances the strength of the base metal through dispersion strengthening as precipitates, making it an element necessary to ensure both strength and toughness. To achieve these effects, the V content needs to be 0.005% or more. On the other hand, if the V content exceeds 0.250%, precipitation embrittlement is promoted, significantly impairing the toughness of the base metal. Therefore, the V content is set to 0.005–0.250%. More preferably, the V content is 0.075% or more, and even more preferably over 0.080%. Additionally, the V content is preferably 0.200% or less.

[0074] Nb: ≥0.005% and <0.030%

[0075] Nb precipitates in austenite at a size on the order of several nanometers, primarily through strain-induced precipitation during rolling, as Nb(C,N) or (V,Nb)(C,N). This controls austenite recrystallization and refines the grain size. Furthermore, Nb enhances the strength of the base material through dispersion strengthening as a precipitate, making it an essential element for ensuring both strength and toughness. To achieve this effect, an Nb content of 0.005% or more is required, preferably 0.010% or more. On the other hand, while Nb increases the deformation resistance in hot zones, a content of 0.030% or more leads to precipitation embrittlement and reduced toughness. Therefore, the Nb content is set to less than 0.030%, preferably 0.025% or less.

[0076] N: 0.0010~0.0060%

[0077] Nitrogen (N) is a useful element in steel that combines with V, Nb, and C to improve the strength and toughness of the base material in the form of V(C,N), Nb(C,N), or (V,Nb)(C,N), and its content needs to be 0.0010% or more. However, if the N content exceeds 0.0060%, precipitation embrittlement significantly impairs the toughness of the base material. Therefore, in this invention, the N content is set to 0.0010 to 0.0060%. Furthermore, the N content is preferably 0.0015% or more. Additionally, the N content is preferably 0.0055% or less.

[0078] Furthermore, in this invention, it is important that, based on the various elements satisfying the above-mentioned ranges, for V and Nb, when the contents (mass%) of V and Nb are set as [%V] and [%Nb] respectively, the relationship of equation (1) described later is satisfied.

[0079] -0.010≤[%Nb]-0.1[%V]≤0.020……(1)

[0080] The inventors evaluated the strength and toughness of various sheet piles with steel compositions within the aforementioned range. Their results showed that a balance between the amounts of precipitated V(C,N), Nb(C,N), or (V,Nb)(C,N) is crucial for achieving excellent strength and toughness. Specifically, when the value calculated from [%Nb] - 0.1[%V] is less than -0.010, indicating excessive V, V precipitates coarsely in large quantities at high temperatures in the form of V(C,N) or (V,Nb)(C,N), while the strain-induced precipitation of fine Nb(C,N) or (V,Nb)(C,N) decreases, thus reducing strength.

[0081] That is, by controlling the value calculated by the above formula as a parameter based on the content of V and Nb to be above -0.010, V helps to form nucleation sites for ferrite, making the microstructure uniformly finer. This ensures sufficient size and amount of precipitated V(C,N) or (V,Nb)(C,N) to contribute to improved toughness, and also ensures sufficient amount of finely precipitated Nb(C,N) or (V,Nb)(C,N) to contribute to increased strength. On the other hand, when the value [%Nb]-0.1[%V] exceeds 0.020, i.e., when Nb is excessive, it becomes a nucleation site for ferrite, which contributes to improved toughness. However, sufficient size and amount of precipitated V(C,N) or (V,Nb)(C,N) decreases, resulting in a loss of toughness in the base material. Therefore, in this invention, the value calculated by [%Nb]-0.1[%V] is set to the range of -0.010 to 0.020. It should be noted that the value calculated by the above formula is preferably -0.005 or above. Furthermore, it is preferable to have a value of 0.015 or less.

[0082] In the chemical composition of this invention, the remaining portion besides the elements mentioned above consists of Fe and unavoidable impurities. Among these unavoidable impurities, upper limits are set for the content of P, S, and B as shown below.

[0083] P: below 0.025%

[0084] Polymer (P) is an unavoidable impurity in steel, but if the P content is excessive, the toughness of the steel will decrease. Therefore, the P content should be below 0.025%. The lower the P content, the better; it can be as low as 0%. Excessive reduction in P content will lead to a decrease in productivity due to the longer refining process. Therefore, the P content is preferably above 0.005%.

[0085] S: below 0.020%

[0086] Like phosphorus, sulfur (S) is present in steel as an unavoidable impurity and exists as Al-series inclusions. If the S content is excessive, the amount of Al-series inclusions increases excessively, reducing the toughness of the steel. Therefore, the S content is set to 0.020% or less. Lower S content is preferred; it can be 0%, but excessive reduction in S content leads to decreased productivity due to prolonged refining processes. Therefore, the S content is preferably 0.002% or more.

[0087] B: Below 0.0003%

[0088] Boron (B) is an effective element that segregates at grain boundaries in steel, increasing grain boundary strength. However, when using low-quality raw materials, the steel may contain more than 0.0003% B. In such cases, coarse grain boundary precipitates form, hardenability increases, which promotes the formation of island martensite and reduces toughness. Therefore, the B content is set to 0.0003% or less, and preferably 0.0002% or less.

[0089] The above are the basic components of this invention, but as needed, it may contain one or more of the following elements.

[0090] Cu: less than 0.50%,

[0091] Ni: below 0.50%,

[0092] Cr: less than 0.50%,

[0093] Mo: 0.30% or less,

[0094] Ca: below 0.0050%,

[0095] Ti: less than 0.025% and REM: less than 0.005%.

[0096] Cu: less than 0.50%

[0097] Cu is an element that can further increase the strength of steel through solid solution strengthening. To achieve this effect, it is preferable to have a Cu content of 0.01% or more. However, if the content exceeds 0.50%, Cu cracking is likely to occur. Therefore, when Cu is included in the composition of steel, it is preferable to set the content to 0.50% or less.

[0098] Ni: below 0.50%

[0099] Like Cu, Ni is an element that can be dissolved in steel to achieve high strength without deteriorating ductility or toughness. To achieve this effect, a Ni content of 0.01% or more is preferred. In particular, Ni is preferably added in combination with Cu to suppress Cu cracking. On the other hand, excessive Ni content promotes the formation of island martensite, and since Ni is a high-valence element, the Ni content is preferably 0.50% or less.

[0100] Cr: less than 0.50%

[0101] Cr is an element that can further increase the strength of steel through solid solution strengthening. To achieve this effect, it is preferable that the Cr content is 0.01% or more. On the other hand, if the Cr content is excessive, it will promote the formation of island martensite, so the Cr content is preferably 0.50% or less.

[0102] Mo: 0.30% or less

[0103] Mo is an element that can further increase the strength of steel through solid solution strengthening. To achieve this effect, it is preferable to contain 0.01% or more Mo. On the other hand, if the Mo content is excessive, it will contribute to the formation of island martensite, so the Mo content is preferably 0.30% or less.

[0104] Ca: below 0.0050%

[0105] Ca reduces MnS in steel by combining with S and O, thereby improving the toughness and ductility of the steel. To achieve this effect, it is preferable to have a Ca content of 0.0005% or more. On the other hand, if the Ca content exceeds 0.0050%, the cleanliness decreases and the toughness gradually decreases; therefore, it is preferable to set the Ca content to 0.0050% or less.

[0106] Ti: below 0.025%

[0107] Ti, in the form of TiN, has the effect of precipitating in austenite and refining the grain size. To achieve this effect, it is preferable to contain 0.001% or more Ti. On the other hand, if the Ti content is excessive, the precipitated TiN becomes coarse, and the grains become coarse, thus gradually reducing the toughness. Therefore, it is preferable to set the Ti content to 0.025% or less.

[0108] REM: below 0.005%

[0109] Like Ca, REM (rare earth elements) can reduce MnS in steel by combining with S and O, thereby improving the steel's toughness and ductility. To achieve this effect, a REM content of 0.001% or higher is preferred. On the other hand, if the Ca content exceeds 0.005%, the cleanliness decreases, and the toughness gradually decreases; therefore, it is preferable to set the REM content to 0.005% or less.

[0110] Next, the microstructure of the sheet pile of the present invention will be described. It should be noted that in the present invention, only the microstructure of the web portion of the sheet pile needs to be specified. This is because the web portion has the lowest workability, the coarse structure, and the most difficult aspect to ensure strength and toughness. Therefore, if the microstructure conditions described later are met in the web portion, then the microstructure conditions described later will also be met in other portions.

[0111] It is important that the area fraction of ferrite is above 70% and the area fraction of island martensite is below 1.0%, and that the average grain size of ferrite is below 15 μm and the maximum grain size is below 40 μm.

[0112] [Ferrite Body Structure]

[0113] The microstructure of sheet piles is predominantly ferrite. A ferrite-dominant microstructure refers to a microstructure where the ferrite area fraction is 70% or more. When the ferrite area fraction is less than 70%, the hard phase increases, and the toughness decreases. From the perspective of ensuring strength, the upper limit of the ferrite area fraction is preferably less than 90%. It should be noted that there is no particular limitation on the second phase, but examples include bainite microstructure containing pearlite, island martensite, and martensite. The area fraction of island martensite is defined later.

[0114] [The average grain size of ferrite is less than 15 μm, and the maximum grain size is less than 40 μm.]

[0115] In the microstructure of sheet piles, the average grain size of ferrite is set to 15 μm or less, and the maximum grain size is set to 40 μm or less. When the average grain size of ferrite is greater than 15 μm or the maximum grain size is greater than 40 μm, YP decreases, and it becomes difficult to ensure toughness. Furthermore, to obtain high strength and toughness, it is preferable that the average grain size of ferrite is 12 μm or less, and the maximum grain size is 30 μm or less. It should be noted that the average grain size and maximum grain size of ferrite can be determined according to the measurement methods described in the examples below.

[0116] The lower limit of the average grain size of ferrite is not particularly limited, but from the viewpoint of ensuring tensile strength, it is preferably 5 μm or more, and more preferably 10 μm or more. Similarly, the lower limit of the maximum grain size of ferrite is not particularly limited, but from the viewpoint of ensuring tensile strength, it is preferably 20 μm or more, and more preferably 25 μm or more.

[0117] [The area ratio of island martensite is less than 1.0%]

[0118] In the microstructure of sheet piles, the area fraction of island martensite is set to 1.0% or less. If the area fraction of island martensite exceeds 1.0%, it is difficult to ensure toughness. To obtain higher toughness, it is preferable to set the area fraction of island martensite to 0.5% or less. The lower the area fraction of island martensite, the better; it can be 0%, so no specific lower limit is set. It should be noted that the area fraction of island martensite can be determined according to the measurement method described in the embodiments below.

[0119] Furthermore, in order to obtain steel sheet piles with high strength and high toughness, dispersion strengthening based on precipitates and uniform micro-refinement based on precipitate structure are effective. Therefore, the particle size and area ratio of the precipitates were studied.

[0120] That is, steel billets having various compositions within the composition range described above according to the present invention are hot-rolled and formed into various sheet piles, and then air-cooled (cooling rate: 0.50℃ / s). Samples are taken from each air-cooled sheet pile, surface-treated by electrolytic grinding, and observed using a transmission electron microscope (TEM) to determine the particle size and area fraction of precipitates composed of V carbonitrides, Nb carbonitrides, and V and Nb composite carbonitrides. It should be noted that the determination of the particle size and area fraction of the precipitates is based on the method described later.

[0121] For the various sheet piles described above, Charpy impact test specimens were taken from each sheet pile to determine the fracture transition temperature (vTrs) at 50% ductility section ratio. It should be noted that the vTrs determination was performed according to the method described in the examples below.

[0122] Figure 3The figure shows the relationship between the measured results of vTrs and the area ratio of the precipitates mentioned above. Here, the precipitates can be rearranged according to equation (2) described later.

[0123] d≥5[(Ae3-Ar3) / Ae3] -0.63 ……(2)

[0124] Here, d, Ae3, and Ar3 are the particle size (nm) of the precipitate, the ferrite phase transformation initiation temperature (°C) during equilibrium phase transformation, and the ferrite phase transformation initiation temperature (°C) during air cooling, respectively. Ae3 and Ar3 are obtained by equations (3) and (4) shown below, respectively.

[0125]

[0126] Ar3=910-310[%C]-80[%Mn]-20[%Cu]-15[%Cr]-55[%Ni]-80[%M0]……(4)

[0127] Here, [%C], [%Si], [%Mn], [%Cu], [%Cr], [%Ni] and [%Mo] represent the contents (mass%) of C, Si, Mn, Cu, Cr, Ni and Mo in the steel, respectively.

[0128] Figure 3 The relationship between the area fraction of V carbonitrides, Nb carbonitrides, and these composite precipitates having precipitate particle sizes satisfying formula (2) and the fracture transition temperature (°C) (hereinafter "vTrs") is shown. It can be seen that when the total area fraction of V carbonitrides, Nb carbonitrides, and these composite precipitates having precipitate particle sizes satisfying formula (2) is 0.30% or more, vTrs-10°C or less. This total area fraction is preferably 0.35% or more.

[0129] There is no particular upper limit to the total area ratio, but from the viewpoint of suppressing excessive precipitation embrittlement, it is preferably below 1.00%.

[0130] Furthermore, for each of the aforementioned sheet piles, tensile test specimens were taken from each sheet pile to determine YP (0.2% endurance). It should be noted that the determination of YP was based on the method described in the examples below. Regarding the results of this determination, Figure 4 The relationship between the particle size and area fraction of the precipitates extracted based on the measurement results is shown for V carbonitride compounds, Nb carbonitride compounds, and the area fraction of these composite precipitates with a particle size of less than 10 nm. The reason for focusing on the area fraction of V carbonitride compounds, Nb carbonitride compounds, and their composite precipitates with a particle size of less than 10 nm is that the Orovan stress involved in precipitation strengthening is inversely proportional to the particle size of the precipitates, thus becoming an indicator for precipitation strengthening. According to... Figure 4 It is known that by ensuring that the total area fraction of V carbonitrides, Nb carbonitrides, and their composite precipitates with a particle size of 10 nm or less is 2.6% or more, a YP of 440 MPa or higher is achieved. This total area fraction is preferably 4.0% or more.

[0131] There is no particular upper limit to the total area ratio, but it is preferably below 10.0% from the viewpoint of suppressing excessive precipitation embrittlement.

[0132] <Manufacturing Methods of Steel Sheet Piles>

[0133] Next, the manufacturing method of the steel sheet pile of the present invention will be described.

[0134] Steel sheet piles are manufactured by hot rolling, which includes rough rolling, intermediate rolling, and finish rolling of steel billets such as slabs with the above-mentioned components after heating in a heating furnace.

[0135] Figure 1 (a) shows a hat-shaped sheet pile 1 as a typical example of a sheet pile. The hat-shaped sheet pile 1 has a base plate 2, a pair of flanges 3 and 4 extending obliquely from both ends of the base plate 2, arms 5 and 6 extending parallel to the base plate 2 from the side opposite to the two flanges 3 and 4, and claws 7 and 8 located at both ends of the arms 5 and 6.

[0136] Taking the manufacture of this cap-shaped sheet pile as an example, after heating the steel billet, it undergoes rough rolling, intermediate rolling, and finish rolling processes, ultimately passing through... Figure 2 The sheet pile is formed using a perforated membrane. Specifically, after the steel billet is rolled multiple times in the initial rough rolling, it is finally formed into a general shape by passing through the perforated membrane 13. Next, in the intermediate rolling, the thickness of the portions forming the base plate 2, flanges 3 and 4, arms 5 and 6, and claws 7 and 8 is adjusted while finally passing through the perforated membrane 14. Furthermore, in the finish rolling, shape control, mainly including claw bending, is performed, and finally, the sheet pile is formed into the final product shape by passing through the perforated membrane 15.

[0137] It should be noted that sheet piles other than the cap-shaped sheet piles shown above, such as... Figure 1 As shown in (b) the straight sheet pile 9, sheet piles with different base plate thicknesses and claw shapes may have variations in the number of hot rolling passes and rolling temperatures, but there are no fundamental differences in manufacturing processes based on rough rolling, intermediate rolling, and finish rolling (including claw bending). All of these are included in the manufacturing method of the present invention. Here, Figure 1 In the straight sheet pile 9 shown in (b), the straight section between the left and right claws 11 and 12 is used as the base plate 10.

[0138] It is important that the process is carried out under the following conditions: the steel billet is heated to 1200℃~1350℃, the cumulative reduction rate of hot rolling at 900℃~1000℃ is 20% or more, the reduction rate of the non-recrystallized region of austenite (hereinafter also referred to as CR rate) is 10% or more and less than 20%, and the finishing temperature of intermediate rolling is 650℃~900℃.

[0139] Heating temperature of steel billet: 1200℃~1350℃

[0140] During hot rolling, the steel billet needs to be heated to 1200℃~1350℃. If the heating temperature is below 1200℃, the solid solution of V and Nb in the steel composition becomes insufficient, the precipitates become coarse, making it difficult to ensure strength, and the deformation resistance in the hot zone may increase, leading to damage to the rolling rolls. On the other hand, if the heating temperature exceeds 1350℃, the grains become coarse, making it difficult to ensure toughness, and the heating time increases, reducing productivity. Therefore, the heating temperature of the steel billet is 1200℃~1350℃, preferably above 1250℃.

[0141] [The cumulative reduction rate at 900℃~1000℃ is over 20%]

[0142] Importantly, the cumulative reduction rate at 900°C to 1000°C is 20% or more. By setting the reduction rate directly above the non-recrystallization temperature region to 20% or more, strain-induced precipitation of Nb carbonitrides or Nb-V composite precipitates at the nanometer level occurs in austenite, significantly increasing YP. Preferably, it is 25% or more. It should be noted that there is no particular limitation on the upper limit, but from a manufacturing point of view, it is preferably 30% or less.

[0143] [CR rate is above 10% and below 20%]

[0144] Building upon the above, it is crucial that the CR rate be 10% or higher. If the CR rate is less than 10%, the final microstructure becomes coarse-grained, with the average ferrite grain size exceeding 15 μm or the maximum grain size exceeding 40 μm, making it difficult to ensure toughness. Furthermore, due to the increased rolling load, strict shape control is required, therefore the CR rate should be less than 20%. Preferably, it should be 13% or higher and less than 18%. Here, the CR rate can be adjusted by increasing or decreasing the roll gap during the rolling process.

[0145] [The finishing temperature of intermediate rolling is 650℃~900℃]

[0146] The finishing temperature of the intermediate rolling process for forming the base plate and flange described above is 650°C to 900°C. If the temperature exceeds 900°C, it becomes difficult to meet either of the two rolling conditions mentioned above, resulting in an average ferrite grain size larger than 15 μm or a maximum grain size larger than 40 μm in the final microstructure, making it difficult to ensure toughness. Preferably, the temperature is below 850°C. On the other hand, if the temperature is below 650°C, the rolling load of the intermediate rolling process becomes high, increasing the risk of damage to the rolling rolls in the intermediate rolling mill. Preferably, the temperature is above 700°C. It should be noted that, as described above, intermediate rolling refers to the rolling process from the rough rolling of the sheet pile to the claw bending rolling process, where the portion that mainly forms the base plate is pressed down in the thickness direction.

[0147] The sheet piles of the present invention do not require accelerated cooling during rolling or after claw bending rolling, which aims to improve strength and toughness. Accelerated cooling is not preferred in production because it causes shape changes such as bending and warping. Therefore, air cooling is preferred after claw bending rolling. It should be noted that from the viewpoint of shape control during rolling, unavoidable cooling such as water or water mist from the cooling bed will not affect the characteristics of the sheet piles of the present invention.

[0148] By performing composition adjustment, rolling, and cooling based on the above conditions, it is possible to obtain sheet piles with high strength of YP440MPa or higher and excellent mechanical properties such as vTrs below -10°C. It should be noted that the sheet piles targeted in this invention are independent of their cross-sectional shape, including cap-shaped, U-shaped, combinations thereof, and straight shapes, and the thickness of the base plate and the shape of the claws are not particularly limited.

[0149] Example

[0150] The structure and effects of the present invention will be described in more detail below with reference to embodiments. It should be noted that the present invention is not limited to the following embodiments and can be appropriately modified within the scope suitable for the spirit of the present invention; all such modifications are included within the scope of the present invention.

[0151] Using a continuous casting machine, prepare steel billets with the compositions shown in Tables 1-1 and 1-2, and heat and hot roll them under the conditions shown in Tables 2-1 and 2-2 to manufacture... Figure 1 The hat-shaped sheet pile shown has a base plate 2, a pair of flanges 3 and 4 extending from both ends of the base plate 2 in a direction that extends horizontally and horizontally in parallel with the base plate 2, and claws 7 and 8 located at both ends of the arms 5 and 6.

[0152]

[0153]

[0154] For the obtained sheet piles, the microstructure, precipitates, tensile and toughness tests were performed. The evaluation methods are explained below.

[0155] <Observations on Microstructures>

[0156] Test specimens were taken from the bottom 1 / 4 thickness position of the sheet pile base plate for microstructural observation. The specimens were ground and etched with nitric acid and ethanol before observation. Using an optical microscope, the thickness-direction section of the base plate was observed at 100x magnification to identify the microstructure. In an 800μm × 600μm field of view, ferrite, pearlite, bainite, and martensite were converted to three gray levels (white, black, and gray) based on image analysis and distinguished, yielding the area fraction of each microstructure. Furthermore, the average grain size of ferrite was calculated using image analysis based on a watershed algorithm. The area of ​​each ferrite grain in the field of view was calculated, and the circular equivalent diameter of each grain was taken as the ferrite grain size, which was then averaged within the field of view. The maximum ferrite grain size was the largest of the circular equivalent diameters within the field of view. Furthermore, for the observation of island martensite, the same test piece as above was subjected to two-stage etching treatment of electrolytic corrosion and nitric acid ethanol to dissolve cementite. Using a scanning electron microscope (SEM), more than 10 fields of view were randomly observed at a magnification of about 1000x. Through the same image analysis as above, the area ratio of island martensite was calculated.

[0157] <Observation of the precipitates>

[0158] Samples were taken from the bottom 1 / 4 thickness of the sheet pile base plate. The surface was treated using electrolytic polishing. Thirty fields of view were observed using a transmission electron microscope (TEM) at 200,000x magnification. For precipitates with a particle size greater than 1 nm, dark-field microscopy and an electrostatic precipitator (EDS) were used to identify V-carbonitrides, Nb-carbonitrides, and their composite precipitates. Each precipitate was counted, and the total area fraction of precipitates occupying the field of view was calculated. The precipitates were considered elliptical, and the average of the major and minor axes was taken as the particle size.

[0159] Tensile Test

[0160] Tensile test specimens of JIS1A specified in JIS Z2241 are taken from the bottom plate of the sheet pile at 1 / 4 of the bottom plate thickness, with the tensile direction being the longer direction. Tensile tests are performed based on JIS Z2241 to determine the yield point (YP) and tensile strength (TS).

[0161] <Toughness Test>

[0162] Charpy impact tests were conducted based on JIS Z2242, using 2mm V-notch Charpy impact test specimens (V-notch depth 2mm) taken from a position 1 / 4 of the thickness of the bottom plate of the sheet pile. It should be noted that the impact tests were conducted within a temperature range of -80 to 40°C, and the absorbed energy at 0°C (vE0) and the fracture transition temperature at 50% ductility ratio (vTrs) were determined.

[0163] The results of the above investigation are shown together in Tables 2-1 and 2-2. The test results of steel sheet piles made using suitable steel with a composition that meets the requirements of the present invention and manufactured by the manufacturing method of the present invention (No. 1 to 17 of Table 2-1) all meet the desired characteristics (yield strength YP: ≥ 440 MPa, fracture transition temperature vTrs with ductility section ratio of 50%: ≤ -10°C).

[0164] On the other hand, in comparative examples (Nos. 18 to 38, 41 of Table 2-2) where the steel composition of the sheet pile does not meet the conditions of the present invention or the conditions of the manufacturing method of the present invention, or neither, either the yield strength and the fracture transition temperature (vTrs) at 50% of the ductility section ratio do not meet the required characteristics.

[0165] In addition, the comparative examples that did not meet the conditions of the manufacturing method of the present invention (No. 39, 40, and 42 in Table 2-2) all had excessive rolling load during intermediate rolling, exceeding the roll's load-bearing capacity, and therefore the rolling was stopped.

[0166]

[0167]

[0168] Explanation of symbols

[0169] 1: Hat-shaped steel sheet piles

[0170] 2: Base plate

[0171] 3: Flange

[0172] 4: Flange

[0173] 5: Arms

[0174] 6: Arms

[0175] 7: Claws

[0176] 8: Claws

[0177] 9: Straight steel sheet piles

[0178] 10: Base plate

[0179] 11: Claws

[0180] 12: Claws

[0181] 13: The final pass of rough rolling for cap-shaped sheet piles.

[0182] 14: The final pass of intermediate rolling in cap-shaped sheet piles.

[0183] 15: The final pass of the finishing rolling of cap-shaped steel sheet piles.

Claims

1. A steel sheet pile, having the following composition and microstructure, The composition of the ingredients satisfies the following formula (1) and contains, in mass %: C：0.05~0.18%、 Si: 0.05~0.55% Mn: 1.00~1.65%, sol.Al: 0.080% or less, V：0.005~0.250%、 Nb: ≥0.005% and <0.030% and N: 0.0010~0.0060%, The remainder consists of Fe and unavoidable impurities, with the unavoidable impurities being P: less than 0.025%, S: less than 0.020%, and B: less than 0.0003%. The microstructure is characterized by a ferrite area fraction of 70% or more and an island martensite area fraction of 1.0% or less, and the total area fraction of V carbonitrides, Nb carbonitrides, and V and Nb composite carbonitrides with a particle size of 10 nm or less is 2.6% or more, and the total area fraction of V carbonitrides, Nb carbonitrides, and V and Nb composite carbonitrides with a particle size d satisfying the following formula (2) is 0.30% or more, wherein the particle size d is in nm. Furthermore, the average grain size of the ferrite is less than 15 μm and the maximum grain size is less than 40 μm. The sheet piles have a yield strength of 440 MPa or higher, a ductility ratio of 50%, and a fracture transition temperature of -10°C or lower. -0.010≤[%Nb]-0.1[%V]≤0.020 (1) in, [%V] and [%Nb] represent the mass percentages of V and Nb in the steel, respectively. d≥5[(Ae3-Ar3) / Ae3] －0.63 (2), Where Ae3 represents the ferrite phase transition initiation temperature at equilibrium, expressed in °C. Ar3: The temperature at which the ferrite phase transformation begins upon cooling, expressed in °C.

2. The sheet pile according to claim 1, wherein, The composition further contains, by mass%, one or more of the following: Cu: less than 0.50%, Ni: less than 0.50%, Cr: less than 0.50%, Mo: less than 0.30%, Ca: less than 0.0050%, Ti: less than 0.025%, and REM: less than 0.005%.

3. A method for manufacturing steel sheet piles, which is the method for manufacturing steel sheet piles according to claim 1 or 2, wherein a steel billet having the composition is heated to 1200°C to 1350°C, and hot rolling including rough rolling, intermediate rolling and finish rolling is carried out under the conditions of a cumulative reduction rate of 20% or more at 900°C to 1000°C, a reduction rate of 10% or more and less than 20% in the non-recrystallization temperature region of austenite, and the end temperature of the intermediate rolling is 650°C to 900°C.

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