Steel sheet pile and its manufacturing method

Abstract

To provide a sheet pile of which machinability during a drilling process is improved, while having the same mechanical characteristics compared with a conventional steel pile and a method for manufacturing the same.SOLUTION: A steel pile comprising, by mass%, C: 0.05 to 0.18%, Si: 0.05 to 0.55%, and Mn: 1.00 to 1.65%, and the reminder of Fe and inevitable impurities, P and S as the reminder, P: 0.025% or less and S: 0.020% or less, a microstructure in a web has a texture of a total area rate of ferrite and perlite: 95% or more, average Vickers hardness Hv is 150 or more and 250 or less and a standard deviation σHv in the web is 10 or less.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a steel sheet pile that is used as a permanent or temporary structure in the civil engineering and construction fields, and a manufacturing method thereof. [Background technology]

[0002] Steel sheet piles are required to have high strength and toughness because they are subjected to high loads when used for quays and retaining walls. For example, steel sheet piles with a yield strength (YP) of 290 MPa or more or 390 MPa or more are used.

[0003] Steel sheet piles come in a variety of cross-sectional shapes, including hat-shaped, U-shaped and straight, and a common construction method is the tie rod type.

[0004] The tie rod type is a construction method that stabilizes the wall by connecting the steel sheet pile and the retaining work with a tie rod. Since the elastic deformation of the steel sheet pile itself is restrained by the tie rod, depending on the environment, a steel sheet pile with a strength of YP430MPa or more may be required.

[0005] Against this background, research and development of high-strength and high-toughness steel sheet piles is being carried out. That is, Patent Document 1 proposes a steel sheet pile having a YP of 440 MPa or more and high toughness by adding Nb to the composition.

[0006] In addition, Patent Document 2 proposes a steel sheet pile having a composition in which both Nb and V are added, and by controlling the rolling reduction rate at 1000°C or less, the average grain size of ferrite, the area ratio of island martensite, and the number density of precipitates are optimized to achieve a YP of 440 MPa or more and high toughness. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2018-83963 A [Patent Document 2] JP 2018-90845 A Summary of the Invention [Problem to be solved by the invention]

[0008] On the other hand, the above conventional technologies had the following problems to be solved. The steel sheet piles used in the tie rod system may have a structure in which holes are drilled in the web beforehand when the steel sheet pile and the bracing work are connected with the tie rod, and the tie rod is passed through the hole. Therefore, in addition to the above-mentioned strength and toughness, excellent machinability during drilling is required. In the technologies proposed in the above Patent Documents 1 and 2, a composition containing Nb and V is used, and Nb and V are precipitated as carbonitrides to achieve both high strength and toughness, but the microstructure control of the web part is insufficient, and the poor machinability of the web part reduces the tool life during drilling.

[0009] The present invention is intended to solve the above-mentioned problems, and aims to provide a steel sheet pile and a manufacturing method thereof that have improved machinability during drilling while maintaining equivalent mechanical properties compared to conventional steel sheet piles. [Means for solving the problem]

[0010] In order to obtain a steel sheet pile with excellent machinability, particularly during drilling, the inventors prepared steel materials with different contents of C, Si, Mn, P, and S, and conducted intensive research, focusing on temperature control during hot rolling and management of cumulative rolling reduction. As a result, they found a way to provide a steel sheet pile with excellent machinability by optimizing the rolling conditions to uniformly refine the structure of the web portion and suppress hard phases such as martensite and bainite, thereby completing the present invention. The gist of the present invention is as follows.

[0011] 1. In mass percent, C: 0.05-0.18%, Si: 0.05 to 0.55% and Mn: 1.00-1.65% and the balance being Fe and unavoidable impurities, the unavoidable impurities being P: 0.025% or less and S: 0.020% or less, A steel sheet pile having a microstructure in a web with a total area ratio of ferrite and pearlite of 95% or more, an average Vickers hardness Hv of the web being 150 or more and 250 or less, and a standard deviation σHv of 10 or less.

[0012] 2. The composition of the components is further expressed in mass% as follows: Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.30% or less Al: 0.10% or less, Ca: 0.0050% or less, V: 0.50% or less, Nb: 0.50% or less, Ti: 0.50% or less B: 0.0010% or less and REM: 0.010% or less 2. The steel sheet pile according to claim 1, containing one or more of the following:

[0013] 3. In mass percent, C: 0.05-0.18%, Si: 0.05 to 0.55% and Mn: 1.00-1.65% and the balance being Fe and unavoidable impurities, and the unavoidable impurities being P: 0.025% or less and S: 0.020% or less, a steel material is heated to 1200 to 1350°C, hot-rolled with a cumulative reduction of 10% or more from 900°C to Ar3 points, and a reduction per pass of 10% or less and a cumulative reduction of 20% or less at Ar3 point or less, and a web portion is cooled (after Ar3 point or completion of rolling) at an average cooling rate of 0.2 to 1.5°C / s to 500°C, the microstructure in the web being a structure in which the total area ratio of ferrite and pearlite is 95% or more, and the average Vickers hardness Hv in the web is 150 to 250 and the standard deviation σHv is 10 or less.

[0014] 4. The composition of the components is further expressed in mass% as follows: Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.30% or less Al: 0.10% or less, Ca: 0.0050% or less, V: 0.50% or less, Nb: 0.50% or less, Ti: 0.50% or less B: 0.0010% or less and REM: 0.010% or less 4. The method for manufacturing a steel sheet pile according to claim 3, comprising one or more of the following: Effect of the Invention

[0015] According to the present invention, a steel sheet pile having excellent machinability and mechanical properties equivalent to those of the conventional steel sheet pile can be provided stably and with high productivity. Therefore, the speed of construction using the steel sheet pile can be increased, which is very useful in industry. [Brief description of the drawings]

[0016] [Figure 1] FIG. 2 is a diagram showing an example of a cross-sectional shape of a steel sheet pile. [Diagram 2] FIG. 1 is a diagram showing a typical groove shape in a hot rolling process of a hat-shaped steel sheet pile. [Diagram 3] FIG. 1 is a graph showing the relationship between average Vickers hardness and its standard deviation, and machinability. [Figure 4] FIG. 1 is a graph showing the relationship between the average cooling rate and machinability from the Ar3 point to 500° C., or from the end of rolling to 500° C. when the rolling end temperature is less than the Ar3 point. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] [Chemical composition] First, the reasons for limiting the composition of the steel sheet pile of the present invention will be described. In the following description, the "%" of the content of each element means "mass%" unless otherwise specified.

[0018] C: 0.05 to 0.18% C is an essential element for ensuring the strength of the base material, and must be added at 0.05% or more. On the other hand, if the C content exceeds 0.18%, bainite containing island martensite is generated, and the hardness variation in the thickness direction of the web increases, resulting in a shortened tool life during cutting and a decrease in toughness. Therefore, the C content is set to 0.05 to 0.18%. Furthermore, the C content is preferably set to 0.10% or more. Moreover, the C content is preferably set to 0.16% or less.

[0019] Silicon: 0.05 to 0.55% Silicon is an element that increases the strength of the base material through solid solution strengthening, and therefore must be contained at 0.05% or more. On the other hand, if the silicon content is excessive, it bonds with oxygen to produce SiO2. SiO2 is present as hard particles between friction surfaces during cutting, causing aggressive wear and shortening tool life. Therefore, the silicon content is set to 0.55% or less. Furthermore, the silicon content is preferably set to 0.10% or more. Furthermore, the silicon content is preferably set to 0.50% or less.

[0020] Mn: 1.00-1.65% Mn, like Si, is a relatively inexpensive element that has the effect of increasing the strength of steel, and is a necessary element for increasing strength. However, when the Mn content is less than 1.00%, the effect is reduced. On the other hand, when the Mn content exceeds 1.65%, it promotes the formation of upper bainite containing island martensite, which not only reduces the tool life during cutting but also causes a decrease in toughness. Therefore, the Mn content is set to 1.00 to 1.65%. Furthermore, the Mn content is preferably set to 1.10% or more. Moreover, the Mn content is preferably set to 1.60% or less.

[0021] The above is the basic composition of the steel sheet pile according to one embodiment of the present invention, with the balance being inevitable impurities and Fe. In addition, the above basic composition may contain one or more of the following elements as necessary. Cu: 0.50% or less Cu is an element that has the effect of strengthening the matrix by solid solution strengthening. In order to obtain such an effect, it is preferable that Cu is contained in an amount of 0.01% or more. On the other hand, if the Cu content is excessive, the hardness of the matrix increases excessively, shortening the tool life during cutting. Therefore, it is preferable that the Cu content is 0.50% or less.

[0022] Ni: 0.50% or less Ni is an element that has the effect of strengthening the matrix by solid solution strengthening, similar to Cu. In order to obtain such an effect, it is preferable that Ni is contained at 0.01% or more. On the other hand, if the Ni content is excessive, the hardness of the matrix increases excessively, shortening the tool life during cutting. Therefore, it is preferable that the Ni content is 0.50% or less.

[0023] Cr:0.50% or less Cr is an element that has the effect of strengthening the matrix by solid solution strengthening. To obtain such an effect, it is preferable that Cr is contained in an amount of 0.01% or more. On the other hand, if the Cr content is excessive, the hardenability is increased and bainite containing island martensite is generated, and the hardness variation in the thickness direction of the web portion increases, thereby reducing the tool life during cutting. Therefore, the Cr content is preferably 0.50% or less.

[0024] Mo: 0.30% or less Mo is an element that has the effect of strengthening the matrix by solid solution strengthening. To obtain such an effect, it is preferable that Mo is contained in an amount of 0.01% or more. On the other hand, if the Mo content is excessive, the hardenability is increased and bainite containing island martensite is generated, and the hardness variation in the thickness direction of the web portion increases, thereby reducing the tool life during cutting. Therefore, the Mo content is preferably 0.30% or less.

[0025] Al: 0.10% or less Al is an element added as a deoxidizer. If the Al content exceeds 0.10%, a large amount of oxide inclusions is generated, and the machinability is deteriorated, thereby shortening the tool life. Therefore, it is preferable that Al is contained at 0.10% or less. Furthermore, it is more preferable that Al is contained at 0.060% or less. On the other hand, although there is no particular restriction on the lower limit of Al, it is preferable that Al is contained at 0.001% or more for deoxidation. It is more preferable that Al is contained at 0.003% or more.

[0026] Ca:0.0050% or less Ca has the effect of improving the stability of oxides and sulfides in sulfide-based inclusions and extending tool life. To obtain such an effect, it is preferable that Ca is contained in an amount of 0.0010% or more. On the other hand, if the Ca content is excessive, coarse oxides are formed and toughness is reduced. Therefore, it is preferable that the Ca content is 0.0050% or less.

[0027] V: 0.50% or less V precipitates in austenite as V(C,N) during rolling or cooling to strengthen the base material. To obtain such an effect, it is preferable that V is contained at 0.01% or more. On the other hand, if the V content is excessive, V(C,N) precipitates excessively, increasing hardness and shortening tool life during cutting. Therefore, it is preferable that the V content be 0.50% or less.

[0028] Nb: 0.50% or less Nb precipitates in austenite as Nb(C,N) during rolling or cooling to strengthen the base material. To obtain such an effect, it is preferable that the Nb content is 0.01% or more. On the other hand, if the Nb content is excessive, not only will the hardness increase due to the excessive precipitation of Nb(C,N), but it will also promote the increase in the amount of bainite containing island martensite produced, shortening the tool life during cutting. Therefore, it is preferable that the Nb content be 0.50% or less.

[0029] Ti: 0.50% or less Ti precipitates in austenite as Ti(C,N) to strengthen the base material. To obtain such an effect, it is preferable that Ti is contained at 0.001% or more. On the other hand, if the Ti content is excessive, Ti(C,N) precipitates excessively, increasing hardness and shortening tool life during cutting. Therefore, it is preferable that the Ti content is 0.50% or less.

[0030] B: 0.0010% or less B segregates on grain boundaries and strengthens the parent phase. To obtain such an effect, it is preferable that the B content is 0.0001% or more. On the other hand, if the B content is excessive, it promotes the formation of bainite containing island martensite, and the hardness variation in the thickness direction of the web increases, shortening the tool life during cutting, and also reducing toughness due to grain boundary embrittlement. Therefore, it is preferable that the B content is 0.0010% or less.

[0031] REM: 0.010% or less REM (rare earth elements) improve the stability of oxides and sulfides in sulfide-based inclusions, and have the effect of extending tool life. To obtain such an effect, it is preferable that the REM content is 0.001% or more. On the other hand, if the REM content is excessive, coarse oxides are formed and toughness is reduced. Therefore, it is preferable that the REM content is 0.010% or less.

[0032] In the composition of the steel sheet pile according to one embodiment of the present invention, the balance other than the above elements is Fe and inevitable impurities. In addition, when the content of the elements related to the above-mentioned optional added components is less than the respective preferable lower limit value, the element is treated as an inevitable impurity (contained as an inevitable impurity). Among the inevitable impurities, the following upper limits are set for P, S, N, and O.

[0033] P:0.025% or less P exists in steel as an inevitable impurity. However, if the P content is excessive, it dissolves in ferrite and the hardness of ferrite increases, impairing machinability, shortening tool life, and reducing the toughness of the steel, so the P content is set to 0.025% or less. The P content is preferably 0.020% or less, and more preferably 0.015% or less. On the other hand, the lower limit of the P content is not particularly limited and may be 0%, but since P is an element that is usually inevitably contained in steel as an impurity, it may be more than 0% industrially. Note that excessively reducing the P content leads to an increase in refining time and an increase in cost, so the P content is preferably 0.001% or more.

[0034] S: 0.020% or less Like P, S is contained in steel as an inevitable impurity and also exists as A-type inclusions. A-type inclusions improve the machinability of steel. However, if the S content is excessive, it dissolves in ferrite and the hardness of ferrite increases, impairing machinability and shortening tool life, as well as reducing the toughness of steel, so the S content is set to 0.020% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0%, but since S is usually an element that is inevitably contained in steel as an impurity, it may be more than 0% industrially. Note that excessively reducing the S content leads to an increase in refining time and an increase in costs, so the S content is preferably 0.001% or more.

[0035] N: 0.0100% or less N exists in steel as an inevitable impurity. However, if the N content is excessive, the amount of carbonitrides produced increases, and the hardness of the base material increases, impairing machinability and reducing the tool life and toughness of the steel, so the N content is preferably 0.0100% or less. The N content is more preferably 0.0080% or less, and even more preferably 0.0060% or less.

[0036] О:0.0050% or less O exists in steel as an unavoidable impurity. However, if the O content is excessive, the amount of oxides increases and the toughness of the steel decreases, so the O content is preferably 0.0050% or less. The O content is more preferably 0.0040% or less, and further preferably 0.0020% or less.

[0037] Next, the microstructure of the web of the steel sheet pile according to one embodiment of the present invention will be described. In addition, since the properties targeted by the present invention can be obtained as long as the microstructure of the web satisfies the conditions described below, the microstructure of the portion other than the web is not particularly limited in the steel sheet pile according to one embodiment of the present invention.

[0038] It is essential that the area ratio of ferrite and pearlite in the web microstructure satisfies the following conditions. [Area ratio of ferrite and pearlite: 95% or more] In the microstructure of the web of the steel sheet pile, by suppressing hard phases such as bainite and martensite, it is possible to satisfy the average Vickers hardness Hv or its standard deviation σHv described later. The breakdown of the area ratio of ferrite and pearlite is not particularly limited. In addition, the structure is not particularly limited as long as the area ratio of the structure excluding ferrite and pearlite is less than 5%. Examples of the structure excluding ferrite and pearlite include bainite and martensite. The area ratio can be measured according to the measurement method described in the examples described later.

[0039] Next, the average Vickers hardness Hv and its standard deviation σHv in the web of the steel sheet pile are described. FIG. 3 shows the relationship between the average Vickers hardness Hv and its standard deviation σHv and machinability. The machinability is evaluated using the total number of holes until the drill becomes unable to cut by drilling. In FIG. 3, excellent machinability (total number of holes ≧350) is indicated by ○, and poor machinability (total number of holes <350) is indicated by ×. The machinability, that is, the superiority or inferiority, is evaluated by measuring the total number of holes that can be drilled according to the measurement method described in the examples below. As shown in FIG. 3, in addition to the area ratio of ferrite and pearlite, it is essential that the average Vickers hardness Hv and its standard deviation σHv satisfy the following conditions for the microstructure of the web portion.

[0040] [Average Vickers hardness Hv: 150 to 250] In the microstructure of the web, the average Vickers hardness Hv is set to 150 or more and 250 or less. If the average Vickers hardness Hv is less than 150, the soft parts increase and the processing heat generated during cutting increases, resulting in a shortened tool life. It is preferably 160 or more. On the other hand, if the average Vickers hardness Hv exceeds 250, the amount of wear during cutting increases and the tool life shortens. The average Vickers hardness Hv can be measured according to the measurement method described in the examples below.

[0041] [Standard deviation of Vickers hardness σHv: 10 or less] In the microstructure of the web, the standard deviation σHv of the Vickers hardness is set to 10 or less. The fact that there is variation in the Vickers hardness in the thickness direction of the web means that soft and hard parts are repeatedly exposed at the wear interface (cutting surface) during cutting. This repetition of soft and hard parts increases the load on the tool and causes a decrease in tool life.

[0042] Next, a method for manufacturing a steel sheet pile according to an embodiment of the present invention will be described. The steel sheet pile is manufactured by heating a steel material such as a slab having the above-mentioned composition in a heating furnace, and then subjecting the steel material to hot rolling including rough rolling, intermediate rolling and finish rolling. Fig. 1(a) shows a hat-shaped steel sheet pile 1, which is a typical example of a steel sheet pile. The hat-shaped steel 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 the opposite side of the flanges 3 and 4 from the web 2, and claws 7 and 8 at both ends of the arms 5 and 6.

[0043] Taking the manufacturing of the hat-shaped steel sheet pile as an example, after heating the steel material, it is finally shaped by passing through a groove as shown in Fig. 2 in each of the rough rolling, intermediate rolling and finish rolling. Specifically, in the first rough rolling, the steel material is rolled several times, and finally passes through a groove 13 to form the rough shape of the steel sheet pile. In the subsequent intermediate rolling, the thicknesses of the parts that will become the web 2, flanges 3 and 4, arms 5 and 6, and claws 7 and 8 are adjusted, and finally passes through a groove 14. Furthermore, in the finish rolling, shape control, mainly including claw bending, is performed, and finally passes through a groove 15 to form the final product shape.

[0044] Thus, hot rolling includes rough rolling, intermediate rolling, and finish rolling. Among these, rough rolling gives the steel sheet pile its general shape. Intermediate rolling refers to rolling after rough rolling and before finish rolling (in the above example, claw bending (rolling)), and in intermediate rolling, the part that will become the web is mainly rolled down in the thickness direction to adjust the thickness. In finish rolling, the final shape is controlled, and in the above example, claw bending is included in finish rolling.

[0045] In addition, in steel sheet piles other than the hat-shaped steel sheet piles shown above, for example, in the case of a steel sheet pile having a different product shape including a web thickness and a claw portion, such as the straight steel sheet pile 9 shown in Fig. 1(b), there may be a difference in the number of rolling passes and the rolling temperature in hot rolling, but there is no fundamental difference in that they are manufactured by rough rolling, intermediate rolling, and finish rolling (including claw bending), and all of them are included in the manufacturing method of a steel sheet pile according to one embodiment of the present invention. Here, in the straight steel sheet pile 9 shown in Fig. 1(b), the straight part located between the left and right claw portions 11 and 12 is called the web 10.

[0046] In hot rolling, it is essential to heat the steel material to 1200°C to 1350°C and perform hot rolling with a cumulative reduction of 10% or more between 900°C and Ar3, and a reduction per pass of 10% or less and a cumulative reduction of 20% or less below Ar3. Note that the following temperature specifications are all based on the surface temperature of the steel material or the material being rolled. The temperatures of the steel material or the material being rolled can be measured using a radiation thermometer.

[0047] [Heating temperature of steel material: 1200℃~1350℃] When hot rolling, the steel material needs to be heated to 1200°C to 1350°C. If the heating temperature is less than 1200°C, the hot deformation resistance increases and the rolling rolls may break. On the other hand, if the heating temperature exceeds 1350°C, the crystal grains become coarse, and the structure after rolling and cooling becomes coarse grained, which increases the variation in the macroscopic structure, causing variation in Vickers hardness and shortening the tool life.

[0048] [Cumulative rolling reduction at 900℃ to Ar3: 10% or more] Rolling in the unrecrystallized region of austenite at 900°C to Ar3 introduces deformation bands into the austenite, which act as nuclei for the formation of ferrite, resulting in a finer structure and a smaller macroscopic structure variation, which in turn reduces the variation in Vickers hardness and extends the tool life. To achieve this effect, the cumulative reduction at 900°C to Ar3 must be 10% or more. If the cumulative reduction at 900°C to Ar3 is less than 10%, the variation in Vickers hardness increases, making it difficult to keep the standard deviation σHv at 10 or less.

[0049] [Rolling reduction per pass below Ar3: 10% or less and cumulative rolling reduction below Ar3: 20% or less] In the two-phase region of ferrite and austenite below the Ar3 point or in the region of ferrite and pearlite where transformation from austenite is completed, rolling generates strain in the structure, mainly increasing the surface hardness. If the rolling reduction rate per pass below the Ar3 point exceeds 10%, even if the cumulative rolling reduction rate below the Ar3 point is 20% or less, strain is introduced into the inner thickness of the web, increasing the hardness, and making it difficult to achieve an average Vickers hardness Hv of 250 or less. Even if the rolling reduction rate per pass below the Ar3 point is 10% or less, if the cumulative rolling reduction rate below the Ar3 point exceeds 20%, it becomes difficult to achieve an average Vickers hardness Hv of 250 or less. It is preferable that the rolling reduction rate per pass below the Ar3 point is 5% or less and the cumulative rolling reduction rate below the Ar3 point is 10% or less. More preferably, both are 0%.

[0050] Figure 4 shows the relationship between the average cooling rate from Ar3 to 500°C, or from the end of rolling to 500°C when the rolling end temperature is less than Ar3, and the machinability of the web of the steel sheet pile that satisfies the composition of the present invention and the rolling conditions described above. It was found that not only the rolling conditions described above but also the average cooling rate have a large effect on the machinability, and it is essential to cool the web at 0.2 to 1.5°C / s. Note that the following temperature specifications are all based on the surface temperature of the steel material or the material being rolled. The temperatures of the steel material or the material being rolled can be measured by a radiation thermometer.

[0051] [(At Ar3 point or after rolling) Cool the web at an average cooling rate of 0.2 to 1.5°C / s up to 500°C] It is essential to control the cooling rate of the web from the Ar3 point to 500°C, which is the temperature range where the transformation of austenite progresses and is completed, or from the end of rolling to 500°C when the rolling end temperature is less than the Ar3 point. If the average cooling rate from the Ar3 point or the end of rolling to 500°C is less than 0.2°C / s, the structure becomes coarse, causing a decrease in Vickers hardness and an increase in variation, making it difficult to achieve an average Vickers hardness Hv of 150 or more and a standard deviation σHv of 10 or less. If the average cooling rate from the Ar3 point or the end of rolling to 500°C exceeds 1.5°C / s, hard phases such as bainite and martensite increase, making it difficult to achieve an average Vickers hardness Hv of 250 or less. There is no restriction on the cooling method as long as the specified average cooling rate is satisfied. Examples of the cooling method include mist cooling and water cooling.

[0052] By adjusting the composition, rolling and cooling according to the above conditions, the steel sheet pile can have mechanical properties equivalent to those of the conventional steel sheet pile and excellent machinability of the web portion. The manufacturing conditions other than those described above are not particularly limited, and may be in accordance with the usual method. For example, the number of rolling passes in the rough rolling, intermediate rolling and finish rolling are preferably 5 to 20 passes, 1 to 5 passes and 1 to 3 passes, respectively. The finish rolling temperature is preferably 550 to 700°C. The steel sheet pile according to one embodiment of the present invention includes hat-shaped, U-shaped, combinations thereof and straight shapes regardless of its cross-sectional shape, and the web thickness and the shape of the claw portion are not particularly limited.

[0053] The above Ar3 point can be determined according to the following formula (1). Ar3=910-310×[%C]+25×([%Si]+2×[%Al])-80×[Mneq] ···(1) Here, [Mneq] is a value calculated by the following formula (2). [Mneq]=[%Mn]+[%Cr]+[%Cu]+[%Mо]+[%Ni] / 2···(2) In the above formulas (1) and (2), [%M] means the content (mass%) of element M in steel. Here, when calculating Ar3 in the above formulas (1) and (2), the content of element M that is not intentionally contained is calculated using the content (analytical value) of element M contained as an unavoidable impurity. EXAMPLES

[0054] The configuration and effects of the present invention will be described in more detail below with reference to the following examples. However, the present invention is not limited to the following examples, and can be modified as appropriate within the scope of the present invention, and all of these are included in the technical scope of the present invention.

[0055] A steel material having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was prepared in a continuous casting machine, and was heated and hot-rolled under the conditions shown in Table 2 to produce a hat-shaped steel sheet pile having a web 2, a pair of flanges 3 and 4 extending in an inclined manner from both ends of the web 2, arms 5 and 6 extending in a direction spreading out to the left and right in parallel to the web 2, and claws 7 and 8 at both ends of the arms 5 and 6, as shown in Fig. 1. Conditions other than those mentioned above were in accordance with normal methods.

[0056] [Table 1]

[0057] The microstructure of the steel sheet pile was observed, the hardness was measured, the machinability was evaluated, and the toughness was tested. Each evaluation method is described below.

[0058] <Observation of Microstructure> A test piece was taken from the web of the steel sheet pile at 1 / 4 of the web thickness along the L-section along the rolling direction, and the microstructure was observed. The surface of the test piece was polished and etched with nital prior to observation. The type of structure was identified by observing the cross section of the web thickness direction at 100 times magnification using an optical microscope, and the ferrite, pearlite, bainite and martensite were differentiated by converting them into three gradations of white (ferrite), black (pearlite) and gray (bainite and martensite) using image analysis with a watershed algorithm in a field of view of 800 μm x 600 μm, and the area ratio of each structure was obtained.

[0059] <Hardness measurement> The Vickers hardness was measured at 98 N and 0.5 mm pitch in the region from the front surface to the back surface at the center position of the cross-sectional width of the web portion 2 in Fig. 1. The average Vickers hardness Hv and its standard deviation σHv were calculated from the obtained Vickers hardness values.

[0060] <Machinability evaluation> Plate material was taken from the center of the cross-sectional width of the web portion 2 in Figure 1 in an area of ​​100 mm x 100 mm including the entire thickness, and the machinability by drilling was evaluated by a drill cutting test. The cutting test was performed using a 6 mmΦ straight drill made of JIS high-speed tool steel SKH51 at a feed rate of 0.25 mm / rev and a rotation speed of 700 rpm to drill through holes, and the total number of holes (pieces) until the drill became unable to cut was evaluated. A total of 350 or more holes was considered to be excellent in tool life.

[0061] <Toughness test> A 2mm V-notch Charpy impact test specimen, as specified in JIS Z2202, was taken from the 1 / 4 position of the web thickness of the steel sheet pile web, and a Charpy impact test was conducted in accordance with JIS Z2242. The impact test was conducted at 0°C to determine the absorbed energy (vE0) at 0°C. A vE0 of 43J or more is considered to have high toughness.

[0062] The results of the above investigation are also shown in Table 2. The test results (No. 1 to 16 in Table 2) of the steel sheet piles of the invention examples, which were manufactured under the specified manufacturing conditions using suitable steels satisfying the specified composition, all satisfied the requirement of 350 or more total holes before the drill became unable to cut in the machinability evaluation. In addition, no significant deformation of shape such as bending or warping occurred in any of the examples of the invention, and it was confirmed that they can be manufactured stably and with high productivity.

[0063] On the other hand, the comparative examples (Nos. 17 to 31 in Table 2), which did not satisfy the specified component composition, did not satisfy the specified manufacturing conditions, or did not satisfy any of the above, did not satisfy the total number of holes to be drilled before the drill became unable to cut, which was 350 or more, in the machinability evaluation.

[0064] [Table 2] [Explanation of symbols]

[0065] 1: Hat-shaped steel sheet pile 2: Web 3: Flange 4: Flange 5: Arm 6: Arm 7: Claw part 8: Claw part 9: Straight steel sheet pile 10: Web 11: Claw part 12: Claw part 13:Last pass groove shape in rough rolling of hat-shaped steel sheet pile 14:Last pass groove shape in intermediate rolling of hat-shaped steel sheet piles 15:Last pass groove shape in finish rolling of hat-shaped steel sheet piles

Claims

1. In mass percent, C: 0.05-0.18%, Si: 0.05 to 0.55% and Mn: 1.00-1.65% and the balance being Fe and unavoidable impurities, the unavoidable impurities being P: 0.025% or less and S: 0.020% or less, A steel sheet pile having a microstructure in a web with a total area ratio of ferrite and pearlite of 95% or more, an average Vickers hardness Hv of the web being 150 or more and 250 or less, and a standard deviation σHv of 10 or less.

2. The composition further includes, in mass%, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.30% or less, Al: 0.10% or less, Ca: 0.0050% or less, V: 0.50% or less, Nb: 0.50% or less, Ti: 0.50% or less B: 0.0010% or less; REM: 0.010% or less The steel sheet pile according to claim 1, comprising at least one of the following:

3. In mass percent, C: 0.05-0.18%, Si: 0.05 to 0.55% and Mn: 1.00-1.65% The steel material has a composition containing 0.025% or less P and 0.020% or less S, and the balance is Fe and unavoidable impurities, and the unavoidable impurities are P: 0.025% or less and S: 0.020% or less. ３ The cumulative rolling reduction at the point is 10% or more, Ar ３ The hot rolling is performed at a rolling reduction rate of 10% or less per pass and a cumulative rolling reduction rate of 20% or less at the rolling point or less. ３ The method for manufacturing a steel sheet pile includes cooling at an average cooling rate of 0.2 to 1.5 °C / s (after the rolling or after the rolling is completed) to 500 °C, the microstructure in the web has a total area ratio of ferrite and pearlite of 95% or more, and the average Vickers hardness Hv in the web is 150 to 250 and the standard deviation σHv is 10 or less.

4. The composition further includes, in mass%, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.30% or less, Al: 0.10% or less, Ca: 0.0050% or less, V: 0.50% or less, Nb: 0.50% or less, Ti: 0.50% or less B: 0.0010% or less; REM: 0.010% or less The method for manufacturing a steel sheet pile according to claim 3, further comprising:

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