Steel sheet pile and method of manufacturing the same

Abstract

The present invention stably provides a high-strength and high-toughness steel sheet pile at a high productivity. The steel sheet pile has a prescribed composition, a microstructure of a ferrite matrix having an average grain size of 15 μm or less and a maximum grain size of 40 μm or less, an island-shaped martensite occupying an area ratio of 1.0% or less in the microstructure, a yield strength of 440 MPa or more, and a vTrs of -10°C or less.

Description

Technical Field

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

[0002] Sheet piles are used in applications such as wharves and retaining walls to withstand high loads, thus requiring high strength and toughness. For example, sheet piles with a yield strength (YP) of 290 MPa or higher, or 390 MPa or higher, can be used. On the other hand, in harsher 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 method is to add alloying elements and roll in the non-recrystallized region of austenite. However, in the manufacture of steel sheet piles with complex shapes, from a formability point of view, the goal is to roll and form at high temperatures with lower deformation resistance, which sometimes limits the addition of alloys that can increase deformation resistance.

[0004] The JIS standard (SYW) for sheet piles specifies the Charpy energy absorption at 0°C. However, sheet piles are used even in environments below 0°C, such as during Japan's cold season, so it is anticipated that sheet piles with higher toughness will be required in the future.

[0005] Against this backdrop, research and development of high-strength and high-toughness steel sheet piles were carried out.

[0006] Patent document 1 proposes a steel sheet pile with high toughness and YP of over 440MPa by using a composition with added Nb.

[0007] In addition, Patent Document 2 proposes a steel sheet pile that optimizes the average grain size of ferrite, the area ratio of island martensite, and the number density of precipitates by using a composition that simultaneously adds Nb and V and controlling the reduction rate below 1000°C, thereby achieving a YP of over 440MPa and high toughness.

[0008] Patent document 3, similar to patent document 2, proposes a steel sheet pile with the addition of both Nb and V, sets the cumulative reduction rate below 900°C to 90% or more, and optimizes the average particle size of ferrite and the number density of precipitates, thereby obtaining a YP of 460 MPa or more and high toughness.

[0009] On the other hand, Patent Document 4 proposes a sheet pile that achieves a YP of over 340 MPa and high toughness by limiting the Nb content in unavoidable impurities to below 0.005%.

[0010] Patent document 5 or 6 proposes a steel sheet pile that achieves a YP of 440MPa or higher and a vTrs of -10℃ or lower by water cooling at a specified position during or after hot rolling.

[0011] Existing technical documents

[0012] Patent documents

[0013] Patent Document 1: Japanese Patent Application Publication No. 2018-83963

[0014] Patent Document 2: Japanese Patent Application Publication No. 2018-90845

[0015] Patent Document 3: Japanese Patent Application Publication No. 2014-118629

[0016] Patent Document 4: Japanese Patent Application Publication No. 2002-294392

[0017] Patent Document 5: Japanese Patent Application Publication No. 2007-332414

[0018] Patent Document 6: Japanese Patent Application Publication No. 2008-221318 Summary of the Invention

[0019] Patent documents 1, 2, and 3 describe techniques for obtaining high-strength and high-toughness steel sheet piles by adding Nb to the composition. However, regardless of whether it is in a solution-treated or precipitated state, Nb tends to increase the deformation resistance during hot rolling, requiring strict shape control during hot rolling. Furthermore, in the techniques described in patent documents 2 and 3, the ferrite grain size is specified as an average value. However, Nb inhibits the recrystallization of austenite, sometimes resulting in deviations in the precipitation state, leading to mixed ferrite grains with diameter differences. Therefore, consistently achieving high toughness is also required.

[0020] On the other hand, Patent Document 4 proposes a sheet pile that promotes complete recrystallization of austenite and obtains a uniform microstructure by limiting the rolling temperature and the reduction rate of the final pass, thereby obtaining a sheet pile with YP of 340 MPa or higher and high toughness. However, the YP is less than 440 MPa, requiring further improvement in YP.

[0021] In patent documents 5 or 6, water cooling of a specified section is required to obtain sheet piles with a YP of 440 MPa or higher and a vTrs of -10°C or lower. Therefore, shape changes such as bending and warping become unavoidable issues.

[0022] The present invention addresses the aforementioned problems and aims to reliably provide high-strength and high-toughness steel sheet piles with high productivity. Here, high strength refers to, for example, a YP of 440 MPa or higher, and high toughness refers to a vTrs of -10°C or lower. Furthermore, vTrs is the section transition temperature (temperature at which the ductility ratio is 50%) determined by Charpy impact testing based on JIS Z2242, and is hereinafter also referred to as the section transition temperature at which the ductility ratio is 50%.

[0023] To obtain steel sheet piles with high strength and superior toughness, the inventors used V as an essential component instead of Nb. They conducted in-depth research focusing not only on temperature control and cumulative reduction during hot rolling, but also on the average reduction per pass in the high-temperature region. The results showed a method for optimizing rolling conditions to achieve uniform and fine microstructure, and utilizing the dispersion strengthening effect of V precipitates to provide steel sheet piles with high strength (YP) above 440 MPa and high toughness (vTrs) below -10°C.

[0024] The main points of this invention are as follows.

[0025] 1. A steel sheet pile having the following composition, comprising, by mass%, C: 0.05-0.18%, Si: 0.05-0.55%, Mn: 1.00-1.65%, sol.Al: 0.080% or less, V: 0.050-0.300%, and N: 0.0010-0.0060%, with the remainder being Fe and unavoidable impurities, wherein the unavoidable impurities are P: 0.025% or less, S: 0.020% or less, and B: 0.0003% or less.

[0026] The microstructure is predominantly ferrite, with an average ferrite grain size of less than 15 μm and a maximum grain size of less than 40 μm. Island-like martensite occupies less than 1.0% of the area in this microstructure.

[0027] The yield strength is above 440 MPa and vTrs is below -10℃.

[0028] 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%, Nb: less than 0.005%, Ti: less than 0.025%, and REM: less than 0.005%.

[0029] 3. A method for manufacturing steel sheet piles, used to manufacture steel sheet piles with the following characteristics: the microstructure is mainly ferrite, the average grain size of the ferrite is less than 15 μm and the maximum grain size is less than 40 μm, the area fraction of island martensite in the above microstructure is less than 1.0%, the yield strength is greater than 440 MPa and the vTrs is less than -10℃.

[0030] In the manufacturing method of steel sheet piles, a steel billet having the following composition is heated to 1200℃~1350℃, wherein the composition, by mass%, contains C: 0.05~0.18%, Si: 0.05~0.55%, Mn: 1.00~1.65%, sol.Al: 0.080% or less, V: 0.050~0.300%, and N: 0.0010~0.0060%, with the remainder being Fe and unavoidable impurities. The unavoidable impurities, P, S, and B, are present in the following proportions: P: 0.025% or less, S: 0.020% or less, and B: 0.0003% or less.

[0031] The steel billet is hot-rolled, and in this hot rolling process,

[0032] The average reduction rate per pass at temperatures between 900℃ and 1150℃ is over 10%.

[0033] The cumulative reduction rate at 800℃~1150℃ is over 60%, and the end temperature of intermediate rolling is 650℃~900℃.

[0034] Here, intermediate rolling refers to the rolling process from rough rolling to finish rolling. In intermediate rolling, the part that will become the web is pressed down along the thickness direction to adjust the thickness.

[0035] 4. The method for manufacturing steel sheet piles according to 3 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%, Nb: less than 0.005%, Ti: less than 0.025%, and REM: less than 0.005%.

[0036] According to the present invention, it is possible to reliably provide high-strength and high-toughness steel sheet piles with YP440MPa and vTrs below -10°C at high productivity, which is of great industrial value. Attached Figure Description

[0037] Figure 1 This is a diagram showing the cross-sectional shape of a steel sheet pile.

[0038] Figure 2 This is a diagram showing a representative die type in the hot rolling process of cap-shaped sheet piles. Detailed Implementation

[0039] [Ingredients]

[0040] First, the reasons for limiting the composition of the steel sheet pile according to one embodiment of the present invention will be explained. It should be noted that, unless otherwise specified, the "%" of the content of each element in the following description refers to "mass %".

[0041] C: 0.05~0.18%

[0042] Carbon (C) combines with V and N in steel and is required to ensure the strength of the base material as a stable component of V(C, N), and needs to be added at a concentration of 0.05% or more. On the other hand, if the C content exceeds 0.18%, bainite containing island-like martensite is formed, and the increase in island-like martensite leads to a significant decrease in toughness. In addition, excessive precipitates further reduce toughness. Therefore, 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.

[0043] Si: 0.05–0.55%

[0044] Si is an element that enhances the strength of the base material through solid solution strengthening, and it needs to contain at least 0.05%. On the other hand, if the Si content is excessive, it promotes 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 0.10% or more. Additionally, the Si content is preferably 0.50% or less.

[0045] Mn: 1.00~1.65%

[0046] Like silicon, manganese (Mn) is a relatively inexpensive element that can improve the strength of steel and is required for high-strength applications. However, its effect diminishes if the Mn content is less than 1.00%. On the other hand, if the Mn content exceeds 1.65%, it promotes the formation of upper bainite containing island-like martensite, significantly impairing 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.

[0047] sol.Al: 0.080% or less

[0048] Al is an element added as a deoxidizer. However, the effect of Al as a deoxidizer saturates when it exceeds 0.080% in terms of sol.Al. Therefore, it is preferred to contain 0.080% or less of sol.Al. It is more preferable to contain 0.060% or less of sol.Al. There is no particular limitation on the lower limit of sol.Al, but for deoxidation, it is preferable to contain 0.001% or more of sol.Al. It is more preferable to contain 0.003% or more of sol.Al.

[0049] V: 0.050~0.300%

[0050] V is an important element that, during rolling or cooling, acts as a nucleation site for V(C,N) precipitation in austenite, contributing to ferrite formation and refining grain size. Furthermore, V enhances the strength of the base metal through dispersion strengthening as precipitates, making it essential for ensuring both strength and toughness. To maximize these effects, the V content should be 0.050% or higher. Conversely, if the V content exceeds 0.300%, precipitation embrittlement is encouraged, significantly impairing the toughness of the base metal. Therefore, the V content is set between 0.050% and 0.300%. More preferably, the V content is 0.075% or higher. More preferably, it exceeds 0.080%. Additionally, the V content is preferably 0.200% or lower.

[0051] N: 0.0010~0.0060%

[0052] Nitrogen (N) combines with v and c in steel and is a useful element for improving the strength of the base material as a V(C,N) compound, requiring a content of 0.0010% or more. However, if the N content exceeds 0.0060%, the formed carbonitrides become coarse, significantly impairing the toughness of the base material. Therefore, the N content is set to 0.0010–0.0060%. Furthermore, the N content is preferably 0.0015% or more. Additionally, the N content is preferably 0.0055% or less.

[0053] The above is a basic component composition of the steel sheet pile according to one embodiment of the present invention, but it may contain one or more of the following elements as needed.

[0054] Cu: less than 0.50%

[0055] Cu is an element that can further increase the strength of steel through solid solution strengthening. To achieve this effect, it is preferable that the Cu content is 0.01% or more. However, when the Cu content exceeds 0.50%, Cu cracks are more likely to occur. Therefore, when Cu is included in the composition of steel, it is preferable to set the Cu content to 0.50% or less.

[0056] Ni: below 0.50%

[0057] Like Cu, Ni is an element that, when dissolved in steel, does not degrade its ductility or toughness, thus achieving high strength. To obtain this effect, a Ni content of 0.01% or more is preferred. In particular, adding Ni in combination with Cu helps suppress Cu cracking. Therefore, adding Ni in combination with Cu is preferred. On the other hand, an excess of Ni can promote the formation of island martensite. Furthermore, Ni is an expensive element. Therefore, a Ni content of 0.50% or less is preferred.

[0058] Cr: less than 0.50%

[0059] Cr is an element that further enhances the strength of steel through solid solution strengthening. To achieve this effect, a Cr content of 0.01% or more is preferred. On the other hand, if the Cr content is excessive, it promotes the formation of island martensite. Therefore, the Cr content is preferably 0.50% or less.

[0060] Mo: 0.30% or less

[0061] Mo is an element that can further increase the strength of steel through solid solution strengthening. To achieve this effect, a Mo content of 0.01% or more is preferred. On the other hand, if the Mo content is excessive, it promotes the formation of island martensite. Therefore, the Mo content is preferably 0.30% or less.

[0062] Ca: below 0.0050%

[0063] Ca combines with S and O to reduce MnS in steel. This improves the toughness and ductility of the steel. To achieve this effect, a Ca content of 0.0005% or more is preferred. On the other hand, if the Ca content exceeds 0.0050%, cleanliness decreases, and toughness tends to decrease. Therefore, a Ca content of 0.0050% or less is preferred.

[0064] Nb: below 0.005%

[0065] During rolling, Nb precipitates as Nb(C,N) in austenite, which has the effect of suppressing austenite recrystallization and refining grain size. To obtain this effect, it is preferable to have an Nb content of 0.001% or more. On the other hand, whether in solution or precipitated state, Nb tends to increase the deformation resistance during hot rolling. In particular, as will be described later, when the average reduction per pass in the recrystallization temperature region is set to 10% or more, it is advantageous to set the Nb content to 0.005% or less. Therefore, when Nb is present in the composition of steel, it is preferable to set the Nb content to 0.005% or less.

[0066] Ti: below 0.025%

[0067] Ti has the effect of precipitating as TiN in austenite and refining the grain size. To achieve this effect, it is preferable to have a Ti content of 0.001% or more. On the other hand, if the Ti content is excessive, the precipitated TiN becomes coarse, resulting in coarse grains and thus often reducing toughness. Therefore, it is preferable to set the Ti content to 0.025% or less.

[0068] REM: below 0.005%

[0069] Like Ca, rare earth elements (REMs) combine with sulfur (S) and oxygen (O) to reduce MnS in steel, thus improving its toughness and ductility. To achieve this effect, a REM content of 0.001% or higher is preferred. On the other hand, if the REM content exceeds 0.005%, cleanliness decreases, and toughness often decreases. Therefore, it is preferable to set the REM content to 0.005% or less.

[0070] Regarding the composition of the sheet pile according to one embodiment of the present invention, the remainder other than the elements mentioned above consists of Fe and unavoidable impurities. It should be noted that for any of the above-mentioned added elements, if their content is less than the preferred lower limit, the element is treated as an unavoidable impurity (included as an unavoidable impurity). Furthermore, the total amount of unavoidable impurities can be the amount to which they are unavoidably mixed into the steel by general manufacturing methods, for example, preferably 0.050% or less, more preferably 0.040% or less. For P, S, and B among the unavoidable impurities, the upper limit of their content is set as follows.

[0071] P: below 0.025%

[0072] Phosphorus (P) is an unavoidable impurity in steel. However, if the P content is excessive, the toughness of the steel decreases; a P content of 0.025% or less is preferable. Lower P content is better, ideally 0%, but excessive reduction in P content can lead to decreased productivity due to prolonged refining processes. Therefore, a P content of 0.005% or more is more preferable.

[0073] S: below 0.020%

[0074] Like phosphorus (P), sulfur (S) is present in steel as an unavoidable impurity, and exists as Al-series inclusions. Excessive S content leads to an excessive increase in inclusions, reducing the steel's toughness. Therefore, the S content is set to 0.020% or less. Lower S content is preferred, ideally 0%, but excessive reduction in S content can lead to decreased productivity due to prolonged refining processes. Therefore, an S content of 0.002% or more is more preferable.

[0075] B: Below 0.0003%

[0076] Boron (B) is an element that segregates at grain boundaries in steel, increasing grain boundary strength. When using low-quality raw materials, the steel may contain more than 0.0003% B. In this case, coarse grain boundary precipitates form, hardenability increases, thereby promoting the formation of island martensite and reducing toughness. Therefore, the B content is set to 0.0003% or less. Furthermore, the B content is preferably 0.0002% or less. It should be noted that a lower B content is more preferred, and can be as low as 0%.

[0077] Next, the microstructure of a sheet pile according to one embodiment of the present invention will be described. Here, the web of the sheet pile is defined as the representative part of the microstructure of the sheet pile according to one embodiment of the present invention. Among all parts of the sheet pile, the web has the lowest workability, the coarsest structure, and is the most difficult to ensure in terms of strength and toughness. Therefore, the microstructure of the web is defined as the representative part. It should be noted that if the microstructure of the web meets the conditions described later, the characteristics targeted in this application can be obtained. Therefore, in the sheet pile of one embodiment of the present invention, there is no particular limitation on the microstructure of parts other than the web. In addition, if the microstructure of the web meets the conditions described later, it can be said that the same microstructure is highly likely to be obtained even in parts other than the web.

[0078] For this microstructure to be effective, it is important that the area fraction of ferrite, the average grain size, the maximum grain size, and the area fraction of island martensite satisfy the following conditions.

[0079] [Ferrite Body Structure]

[0080] 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. If the ferrite area fraction is less than 70%, the hard phase increases, leading to a decrease in toughness. From the viewpoint of ensuring strength, the upper limit of the ferrite area fraction is preferably less than 90%. It should be noted that there are no particular limitations on the remaining microstructure other than ferrite, but examples include bainite microstructure containing pearlite, island martensite, and martensite. The total area fraction of the remaining microstructure other than ferrite is preferably 30% or less. Furthermore, the total area fraction of the remaining microstructure other than ferrite is preferably more than 10%. The area fraction of island martensite needs to be limited as described later. It should be noted that the area fraction of each phase can be measured according to the measurement method described in the examples below.

[0081] [The average grain size of ferrite is less than 15 μm and the maximum grain size is less than 40 μm.]

[0082] 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, it is difficult to ensure toughness. It should be noted that, to obtain excellent 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. Furthermore, it is more preferable that the average grain size of ferrite is 10 μm or less and the maximum grain size is 25 μm or less. It should be noted that the lower limits of the average and maximum grain sizes of ferrite are not particularly limited. In addition, the average and maximum grain sizes of ferrite can be measured according to the measurement methods described in the examples below.

[0083] [The area ratio of island martensite is less than 1.0%]

[0084] In the microstructure of sheet piles, the area fraction of island martensite is set to 1.0% or less. When the area fraction of island martensite exceeds 1.0%, it is difficult to ensure toughness. To obtain superior toughness, it is preferable to set the area fraction of island martensite to 0.5% or less. A lower area fraction of island martensite is more preferred, and can be 0%, therefore a lower limit is not specifically set. It should be noted that the area fraction of island martensite can be measured according to the measurement method described in the examples below.

[0085] Next, a method for manufacturing steel sheet piles according to one embodiment of the present invention will be described.

[0086] Steel sheet piles are manufactured by heating steel billets, such as slabs, with the above-mentioned components in a heating furnace, and then hot rolling, which includes rough rolling, intermediate rolling, and finish rolling.

[0087] Figure 1 (a) shows a cap-shaped sheet pile 1 as a typical example of a sheet pile. The cap-shaped sheet pile 1 has a web 2, a pair of flanges 3 and 4 extending obliquely from both ends of the web 2, arms 5 and 6 extending parallel to the web 2 from opposite sides of the web 2 of the two flanges 3 and 4, and claws 7 and 8 located at both ends of the arms 5 and 6.

[0088] Taking the manufacturing of this cap-shaped steel sheet pile as an example, the steel billet is heated and then subjected to rough rolling, intermediate rolling, and finish rolling, ultimately passing through... Figure 2 The sheet pile is formed using the die pattern shown. Specifically, after rolling the steel billet multiple times in the initial rough rolling, it is finally passed through die 13 to form the approximate shape of the sheet pile. Then, in the intermediate rolling, the thickness of the portions forming the web 2, flanges 3 and 4, arms 5 and 6, and claws 7 and 8 is adjusted, and it is finally passed through die 14. Furthermore, in the finish rolling, shape control, including claw bending, is mainly performed, and it is finally passed through die 15 to form the final product shape.

[0089] Thus, hot rolling includes roughing, intermediate rolling, and finishing. Roughing provides the approximate shape of the sheet pile. Intermediate rolling refers to the rolling process from roughing to finishing (in the example above, claw bending (rolling)). In intermediate rolling, the portion that will become the web is pressed down along the thickness direction to adjust the thickness. Finishing is where the final shape control is performed, including claw bending in the example above.

[0090] It should be noted that sheet piles other than the cap-shaped sheet piles shown above, for example... Figure 1 Like the straight sheet pile 9 shown in (b), sheet piles with differences in web thickness and claw shape may sometimes have differences in the number of hot rolling passes and rolling temperature, but there is no fundamental difference in manufacturing them through rough rolling, intermediate rolling, and finish rolling (including claw bending). All of these are included in the sheet pile manufacturing method of one embodiment of the present invention. Here, Figure 1 In the straight sheet pile 9 shown in (b), the straight portion located between the left and right claw portions 11 and 12 is used as the web 10.

[0091] Furthermore, in hot rolling, after heating the billet to 1200℃~1350℃, the average reduction rate per pass from 900℃~1150℃ should be 10% or more, and the cumulative reduction rate from 800℃~1150℃ should be 60% or more. It is also important that the finishing temperature for intermediate rolling be 650℃~900℃. It should be noted that all the temperature specifications below are based on the surface temperature of the billet and the rolled material. The temperature of the billet and the rolled material can be measured using a radiation thermometer.

[0092] Heating temperature of steel billet: 1200℃~1350℃

[0093] 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 in the steel composition becomes insufficient. As a result, the precipitates become coarse, making it difficult to ensure strength and toughness. In addition, the resistance to hot deformation increases, which may lead to rolling fracture. On the other hand, if the heating temperature exceeds 1350℃, the grains become coarse, making it difficult to ensure toughness. Furthermore, the heating time increases, reducing productivity. Therefore, the heating temperature of the steel billet is 1200℃~1350℃. The preferred heating temperature for the steel billet is 1250℃~1350℃.

[0094] [The average reduction rate for each pass at temperatures between 900℃ and 1150℃ is over 10%]

[0095] It is important that the average reduction per pass (hereinafter referred to as average reduction) at 900℃ to 1150℃ is 10% or more. By setting the average reduction to 10% or more, the recrystallization of austenite is promoted, resulting in uniform and fine grains. As a result, YP and toughness are significantly improved. The average reduction is preferably 12% or more.

[0096] In addition, the average reduction rate can be calculated according to the following formula (1).

[0097] R = 100{1 - (T) f / T s ) 1 / n}……(1)

[0098] Here, R represents the average reduction rate (%), and T... f and T s The values ​​(mm) represent the thickness of the rolled material (relative to the web) at 900°C and 1150°C, respectively, where n is the number of rolling passes between 900°C and 1150°C. It should be noted that this number of rolling passes is counted for rolling passes where at least one of the rolling start temperature and rolling end temperature is in the range of 900°C to 1150°C. For example, a rolling pass with a rolling start temperature of 1160°C and a rolling end temperature of 1100°C is counted as one rolling pass. Furthermore, the aforementioned plate thickness can be controlled by the roll gap of the rolling mill.

[0099] However, it is believed that in rolling processes performed by multiple mills, such as continuous rolling, austenite is rolled continuously without time for recrystallization, resulting in strain accumulation. Therefore, in such continuous rolling, the process from bite-in to bite-out is considered one pass.

[0100] Furthermore, from the viewpoint of shape control, the average reduction rate is preferably 20% or less. In addition, when the average reduction rate is less than 10%, the recovery or partial recrystallization of austenite becomes significant, the grains become coarse and mixed, and it may be difficult to ensure toughness.

[0101] It should be noted that if hot rolling occurs at temperatures exceeding 1150°C, the austenite grains grow large and the micronization effect is small; therefore, no specific reduction rate is specified for this temperature range. Furthermore, if the desired cumulative reduction rate is achieved within the 800°C–1150°C temperature range described later, no specific reduction rate is required for temperatures below 900°C.

[0102] [The cumulative reduction rate at 800℃~1150℃ is over 60%]

[0103] In addition to the above, it is important to set the cumulative reduction rate (hereinafter also referred to as cumulative reduction rate) at 800℃ to 1150℃ to be 60% or more. If the cumulative reduction rate is less than 60%, the average grain size of the ferrite in the final microstructure will be larger than 15μm, making it difficult to ensure toughness. The cumulative reduction rate is preferably 70% or more. Furthermore, no upper limit is set for the cumulative reduction rate, but from the viewpoint of the risk of roll breakage, a cumulative reduction rate of 90% or less is preferred.

[0104] In addition, the cumulative reduction rate can be calculated according to the following formula (2).

[0105] R'=100{1-(T L / T s )}……(2)

[0106] Here, R' represents the cumulative reduction rate (%), and T... L and T s These are the thicknesses (mm) of the rolled material (at the position corresponding to the web) at 800℃ and 1150℃, respectively.

[0107] [The finishing temperature of intermediate rolling is 650℃~900℃]

[0108] The end temperature of the intermediate rolling process that forms the web and flange (in other words, the end temperature of the final rolling pass in the intermediate rolling process) is 650°C to 900°C. If the end temperature of the intermediate rolling process exceeds 900°C, it becomes difficult to meet either of the two rolling conditions mentioned above, resulting in a final microstructure where the average grain size of ferrite is greater than 15 μm or the maximum grain size is greater than 40 μm, making it difficult to ensure toughness. On the other hand, if the end temperature of the intermediate rolling process is less than 650°C, the rolling load in the intermediate rolling process becomes higher, increasing the risk of roll breakage in the intermediate rolling mill.

[0109] It should be noted that, as mentioned above, intermediate rolling refers to the rolling process from rough rolling to finish rolling (claw bending forming (rolling) in the example above). Furthermore, in intermediate rolling, the portion that will primarily become the web is pressed down along the thickness direction to adjust its thickness. In finish rolling, final shape control is performed. That is, finish rolling refers not only to the final pass of hot rolling but also to the rolling process that performs final shape adjustments after intermediate rolling. Finish rolling is performed from a design perspective and does not significantly affect properties; therefore, the conditions for finish rolling are not specifically defined.

[0110] The method for manufacturing sheet piles according to one embodiment of the present invention does not require hot rolling (roughing, intermediate rolling, and finishing rolling) aimed at improving strength and toughness, nor accelerated cooling after hot rolling (finish rolling (claw bending rolling)). Accelerated cooling causes shape changes such as bending and warping, which is therefore undesirable in production. Therefore, air cooling is preferred after hot rolling (finish rolling (claw bending rolling)). It should be noted that from the viewpoint of shape control during rolling, unavoidable cooling methods involving water or water mist on the cooling bed do not affect the properties of the sheet piles.

[0111] By adjusting the composition, rolling, and cooling according to the above conditions, high strength of YP440MPa or higher and excellent mechanical properties of vTrs below -10°C can be obtained in the sheet pile. It should be noted that there are no particular limitations on manufacturing conditions other than those described above, and conventional methods can be used. For example, the number of rolling passes for roughing, intermediate rolling, and finishing rolling is preferably 5 to 20 passes, 1 to 5 passes, and 1 to 3 passes, respectively. Furthermore, rolling passes at 900°C to 1150°C include, for example, rolling passes based on roughing and intermediate rolling. Rolling passes at 800°C to 1150°C include, for example, rolling passes based on roughing and intermediate rolling, and may arbitrarily include rolling passes based on finishing rolling. The finishing rolling end temperature is preferably 550 to 700°C. Additionally, the sheet pile of one embodiment of the present invention includes cap-shaped, U-shaped, combinations thereof, and straight types, etc., regardless of the cross-sectional shape, and the web thickness and the shape of the claws are not particularly limited.

[0112] Example

[0113] The structure and effects of the present invention will be described in more detail below with reference to embodiments. However, the present invention is not limited to the embodiments described below, and appropriate modifications may be made within the scope that conforms to the spirit of the present invention, all of which are included within the scope of the present invention.

[0114] Using a continuous casting machine, steel billets with the steel composition shown in Table 1 (the remainder being Fe and unavoidable impurities) are prepared, heated and hot-rolled according to the conditions shown in Table 2 to manufacture cap-shaped steel sheet piles. These cap-shaped steel sheet piles have... Figure 1 The web 2 shown, a pair of flanges 3 and 4 extending obliquely from both ends of the web 2, arms 5 and 6 extending in a direction parallel to the web 2 to the left and right, and claws 7 and 8 located at both ends of arms 5 and 6. It should be noted that cooling after hot rolling is carried out by air cooling. Furthermore, for conditions other than those described above, conventional methods are followed.

[0115]

[0116] The obtained sheet piles were subjected to microstructure observation, tensile testing, and toughness testing. The evaluation methods are explained below.

[0117] <Observations on Microstructures>

[0118] Test specimens were taken from the web of the sheet pile at a point one-quarter of its thickness for microstructural observation. The specimens were ground and etched using nitric acid and ethanol before observation. Using an optical microscope, the microstructure was identified by observing a 100x cross-section along the thickness of the web. In an 800μm × 600μm field of view, the watershed algorithm was used to analyze the image, converting ferrite, pearlite, bainite, and martensite into three grayscale values ​​(white, black, and gray) to distinguish them, thus obtaining the area fraction of each microstructure. Furthermore, the average grain size of ferrite was also calculated using the watershed algorithm, determining the area of ​​each ferrite grain in the field of view. The equivalent circle diameter of each grain was set as the ferrite grain size, and the average value within the field of view was calculated. The maximum ferrite grain size was the largest of the equivalent circle diameters within the field of view. It should be noted that the average grain size of ferrite was calculated using only grains with a diameter of 3 μm or larger (equivalent to a circle diameter) that could be confirmed within the aforementioned field of view. Furthermore, for the observation of island-shaped martensite, cementite was dissolved by performing a two-step etching process—electrolytic etching and nitric acid ethanol etching—on the same test specimen as described above. At least 10 fields of view were randomly observed using SEM at approximately 1000x magnification, and the area ratio of the island-shaped martensite was determined through the same image analysis described above.

[0119] Tensile Test

[0120] Take a tensile test specimen of JIS1A specified in JIS Z2201 from the position 1 / 4 of the web thickness of the sheet pile, with the tensile direction as the length direction, and perform a tensile test according to JIS Z2241 to determine the yield point (YP) and tensile strength (TS).

[0121] <Toughness Test>

[0122] A 2mm V-notch Charpy impact test specimen, as specified in JIS Z2202, is taken from the web of the sheet pile at a position one-quarter of the web thickness. Charpy impact testing is then conducted according to JIS Z2242. It should be noted that the impact test is carried out within a temperature range of -80 to 40°C. The absorbed energy at 0°C (vE0) and the section transition temperature (vTrs) at 50% ductility are determined.

[0123] Table 2 also shows the results of the above investigation. The test results (No. 1 to 17 in Table 2) of the steel sheet piles of the invention examples, manufactured using suitable steel with the specified composition and under specified manufacturing conditions, all met the desired characteristics (yield strength YP: 440 MPa or higher, section transition temperature vTrs with a ductility ratio of 50%: -10°C or lower). Furthermore, the invention examples all confirmed that no large bending, warping, or other shape changes occurred, and that stable manufacturing at high productivity was possible.

[0124] On the other hand, comparative examples (No. 18 to 38 in Table 2) that do not meet the specified composition, manufacturing conditions, or either of the above, do not meet the required characteristics in either yield strength or section transition temperature (vTrs) with a 50% section area ratio.

[0125]

[0126] Symbol Explanation

[0127] 1: Hat-shaped steel sheet piles

[0128] 2: Web

[0129] 3: Flange

[0130] 4: Flange

[0131] 5: Arms

[0132] 6: Arms

[0133] 7: Claws

[0134] 8: Claws

[0135] 9: Straight steel sheet piles

[0136] 10: Web

[0137] 11: Claws

[0138] 12: Claws

[0139] 13: Drill hole pattern for the final pass of rough rolling of cap-shaped steel sheet piles

[0140] 14: The final pass die shape in the intermediate rolling of cap-shaped sheet piles

[0141] 15: Drill hole pattern for the final pass of finish rolling of cap-shaped steel sheet piles

Claims

1. A steel sheet pile having the following composition, comprising, by mass%, C: 0.05–0.18%, Si: 0.05–0.55%, Mn: 1.00–1.65%, sol.Al: 0.080% or less, V: 0.050–0.300%, and N: 0.0010–0.0060%, with the remainder being Fe and unavoidable impurities, wherein the unavoidable impurities are P: 0.025% or less, S: 0.020% or less, and B: 0.0003% or less. The microstructure of the web is predominantly ferrite, meaning that the ferrite area fraction is greater than 70%, the average ferrite grain size is less than 15 μm and the maximum grain size is less than 40 μm, and the island martensite accounts for less than 1.0% of the area fraction in the microstructure. The yield strength of the web is above 440 MPa and vTrs is below -10℃.

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%, Nb: less than 0.005%, Ti: less than 0.025%, and REM: less than 0.005%.

3. A method for manufacturing sheet piles, used to manufacture sheet piles in which the microstructure of the web is a ferrite-dominated microstructure, wherein the ferrite-dominated microstructure refers to a microstructure in which the area fraction of ferrite is 70% or more, the average grain size of ferrite is 15 μm or less and the maximum grain size is 40 μm or less, the area fraction of island martensite in the microstructure is 1.0% or less, and the yield strength of the web is 440 MPa or more and vTrs is -10℃ or less. In the manufacturing method of the steel sheet pile, a steel billet having the following composition is heated to 1200℃~1350℃, wherein the composition, by mass%, contains C: 0.05~0.18%, Si: 0.05~0.55%, Mn: 1.00~1.65%, sol.Al: less than 0.080%, V: 0.050~0.300%, and N: 0.0010~0.0060%, with the remainder being Fe and unavoidable impurities. The unavoidable impurities, P, S, and B, are present in the following proportions: P: less than 0.025%, S: less than 0.020%, and B: less than 0.0003%. The steel billet is hot-rolled, with an average reduction rate of over 10% per pass at temperatures between 900℃ and 1150℃. The cumulative reduction rate at 800℃~1150℃ is over 60%, and the end temperature of intermediate rolling is 650℃~900℃.

4. The method for manufacturing steel sheet piles according to claim 3, 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%, Nb: less than 0.005%, Ti: less than 0.025%, and REM: less than 0.005%.

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